journal contacts acta imeko issn: 2221-870x september 2022, volume 11, number 3, 1 2 acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 1 journal contacts about the journal acta imeko is an e-journal reporting on the contributions on the state and progress of the science and technology of measurement. the articles are mainly based on presentations presented at imeko workshops, symposia and congresses. the journal is published by imeko, the international measurement confederation. the issn, the international identifier for serials, is 2221-870x. about imeko the international measurement confederation, imeko, is an international federation of actually 42 national member organisations individually concerned with the advancement of measurement technology. its fundamental objectives are the promotion of international interchange of scientific and technical information in the field of measurement, and the enhancement of international co-operation among scientists and engineers from research and industry. addresses principal contact prof. francesco lamonaca university of calabria department of computer science, modelling, electronic and system science via p. bucci, 41c, vi floor, arcavacata di rende, 87036 (cs), italy e-mail: editorinchief.actaimeko@hunmeko.org support contact dr. dirk röske physikalisch-technische bundesanstalt (ptb) bundesallee 100, 38116 braunschweig, germany e-mail: dirk.roeske@ptb.de editor‐in‐chief francesco lamonaca, italy founding editor‐in‐chief paul p. l. regtien, netherlands associate editor dirk röske, germany copy editors egidio de benedetto, italy silvia sangiovanni, italy layout editors dirk röske, germany leonardo iannucci, italy domenico luca carnì, italy editorial board leopoldo angrisani, italy filippo attivissimo, italy eulalia balestieri, italy eric benoit, france paolo carbone, italy lorenzo ciani, italy catalin damian, romania pasquale daponte, italy luca de vito, italy sascha eichstaedt, germany ravi fernandez, germany luigi ferrigno, italy edoardo fiorucci, italy alistair forbes, united kingdom helena geirinhas ramos, portugal sabrina grassini, italy leonardo iannucci, italy fernando janeiro, portugal konrad jedrzejewski, poland andy knott, united kingdom yasuharu koike, japan dan kytyr, czechia francesco lamonaca, italy aime lay ekuakille, italy massimo lazzaroni, italy fabio leccese, italy rosario morello, italy michele norgia, italy franco pavese, italy pedro miguel pinto ramos, portugal nicola pompeo, italy sergio rapuano, italy renato reis machado, brazil álvaro ribeiro, portugal gustavo ripper, brazil dirk röske, germany maik rosenberger, germany alexandru salceanu, romania constantin sarmasanu, romania lorenzo scalise, italy emiliano schena, italy michela sega, italy enrico silva, italy pier giorgio spazzini, italy krzysztof stepien, poland ronald summers, uk marco tarabini, italy tatjana tomić, croatia joris van loco, belgium zsolt viharos, hungary bernhard zagar, austria davor zvizdic, croatia mailto:editorinchief.actaimeko@hunmeko.org mailto:dirk.roeske@ptb.de acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 2 section editors (vol. 7 11) yvan baudoin, belgium piotr bilski, poland francesco bonavolonta, italy giuseppe caravello, italy carlo carobbi, italy marcantonio catelani, italy mauro d’arco, italy egidio de benedeto, italy alessandro depari, italy alessandro germak, italy istván harmati, hungary min-seok kim, korea bálint kiss, hungary momoko kojima, japan koji ogushi, japan vilmos palfi, hungary jeerasak pitakarnnop, thailand md zia ur rahman, india fabio santaniello, italy jan saliga, slovakia emiliano sisinni, italy ciro spataro, italy oscar tamburis, italy zafar taqvi, usa jorge c. torres-guzman, mexico ioan tudosa, italy ian veldman, south africa rugkanawan wongpithayadisai, thailand claudia zoani, italy application of butterworth high pass filter as an approximation of wood anderson seismometer frequency response to earthquake signal recording acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 379 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 379 382 application of butterworth high pass filter as an approximation of wood anderson seismometer frequency response to earthquake signal recording hamidatul husna matondang1, endra joelianto2, sri widiyantoro3 1 bmkg, jakarta, indonesia, hamidatul.husna@bmkg.go.id 2 itb, bandung, indonesia, ejoel@tf.itb.ac.id 3 itb, bandung, indonesia, sriwid@geoph.itb.ac.id abstract: the method for generating maximum amplitude and signal to noise ratio values by using second order high pass butterworth filter on local seismic magnitude scale calculations is proposed. the test data are signals from local earthquake that have been occurred in sunda strait on april 8th 2012. based on the experimental results, a 8 hz cutoff frequency and a gain of 2200 of second order butterworth high pass filter as an approach to simulating the frequency response of wood anderson seismometer can provide maximum amplitude value, snr, and the magnitude better than simulated wood anderson frequency response. keywords: high pass butterworth filter; wood anderson seismometer; frequency response simulation; instrument correction 1. introduction initially, the calculation of the local seismic magnitude scale was based on the calculation of the magnitude scale obtained from wood anderson's seismometer recordings. however, the wood anderson seismometer is a short period instrument that has analog recording, in which, the analog recording speed on paper is limited. this resulted in an earthquake recording on the recording station that was close to the location of the earthquake experiencing clipping. therefore, recording earthquake signals from the wood anderson seismometer is no longer used in the calculation of local magnitude scales. thus, the study of local magnitude has been shifted to the use of digital recording. in order to produce digital recordings as if coming from a wood anderson seismometer, the digital recordings are processed to simulate the wood anderson seismometer frequency response. the recording of the wood anderson frequency response simulation provides more accurate data from the original instrument, because there is no clipping on earthquake recordings [1]. however, in practice, the butterworth filter application is still used to eliminate microseismic noise [2]. considerable research has been done on simulating seismometer responses. among them, designing a simulation of the frequency response of wood anderson seismometer from recursive filters with differential equations has been considered in [3]. the seismometer frequency response is considered as a second order high pass filter. if the filter is applied to an earthquake signal, the 2nd order butterworth high pass filter with a cutoff frequency of 2 hz can approximate the local magnitude in 0.1 magnitude units [1]. the seismometer frequency response approach can also be obtained from a 5th order butterworth low pass filter cascaded with a 3rd order butterworth high pass filter [4]. these filters are used to correct short period and broadband seismometer responses. pole and zero determination was provided for recursive filters used as a correction for seismometer instruments [5]. different from previous studies, this paper tests earthquake signals using a second order butterworth high pass filter with a cutoff frequency of 0.1 to 12 hz. the test data used in this paper are local earthquake signals and noise signals recorded by four earthquake recording stations. the do the authors mean to correct short period and broadband seismometer responses recording stations are cigelis jawa indonesia (cgji), lembang (lem), cisompet (cisi) and karang pucung jawa indonesia (kpji). local earthquake recorded signals used are the ones related to the earthquake that have occurred in the sunda strait on 8th april 2012 at 08: 06: 47.1 am utc+07.00 with a strength of 4.6 magnitudes, at coordinates longitude 105.859998 and latitude -6.94 with depth of 100 km. while noisy signals were taken in the morning at 03.00 am utc+07.00, noon at 12.00 pm utc+07.00 and at night at 10.00 pm utc+07.00 [6]. http://www.imeko.org/ mailto:hamidatul.husna@bmkg.go.id mailto:ejoel@tf.itb.ac.id mailto:sriwid@geoph.itb.ac.id acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 380 from the test results, the gain value and cut off frequency of the filter are configured considering the recorded data characteristic. the filter is then applied to the signal. the results of the filter application are expected to produce higher amplitude and signal to noise ratio (snr) values than the simulated wood anderson’s seismometer frequency response. the butterworth filtered results are used as an approach to simulate earthquake recordings that would be obtained from a typical frequency response of a wood anderson's seismometer. 2. proposed methodology the design is based on the similarity of responses between the butterworth high pass filter and the wood anderson seismometer which is a second order system [7]. the filter is applied to earthquake signals that have been used for local seismic magnitude scale calculations. from the filtered signal, the maximum amplitude is obtained in the s signal phase and signal to noise ratio (snr). for comparison, the maximum amplitude and snr of filtered earthquake signals are compared with the maximum amplitude and snr of the earthquake signal on a simulated wood anderson seismogram. the research flow chart is shown in figure 1. start data collection catalogue data of bmkg waveform data conversion of seed to sac resampling determination of the butterworth highpass filter coefficient application of high pass butterworth filter frequency response design results component rotation readings of maximum amplitude and snr calculation component rotation finished application of high pass butterworth filter frequency response design results readings of maximum amplitude and snr calculation reference figure 1: flowchart of the method 3. analysis the results of the butterworth filter test applied to bmkg data show an almost similar amplitude value for each earthquake recording station. the average maximum amplitude value for the four recording stations is 24,000,000 nm, produced by the butterworth filter with a gain of 3000 and a cutoff frequency of 0.1 hz. while the minimum amplitude is obtained from the gain with a value of 1000 and the cut-off frequency around 0.1 hz. see figure 2. figure 2: cut off frequency, gain and maximum amplitude curves it is also known that the frequency response of second order butterworth filter with gains of 2000, 2200, and 2300 have similarities with frequency response of wood anderson’s seismometer. however, the second-order butterworth filter frequency response with a gain of 2200 and an 8 hz cut-off frequency that is close to the maximum amplitude of wood anderson's seismometer. the second-order butterworth filter frequency response with a gain of 1000 has a magnitude smaller than the magnitude generated by the wood anderson seismometer. whereas the second order butterworth filter frequency response with a gain of 3000 has a magnitude greater than the seismic magnitude generated by the wood anderson seismometer. see figure 3. table 1: signal to noise ratio of wood anderson seismometer and butterworth filter simulated the largest maximum amplitude value of each recording station is obtained from the butterworth filter simulation results, while the results from the station simulated wood anderson butterworth filtered signal 3:00 am (%) 12:00 pm (%) 22:00 pm (%) 3:00 am (%) 12:00 pm (%) 22:00 pm (%) cgji 6,7 3,9 1,8 5,1 6,2 6,6 lem 4,9 1,5 1,1 -1,3 -1,3 1,3 cisi 2,1 1,4 4,1 4,6 3,7 5,2 kpji -9,8 -7,8 2,9 3,0 2,8 3,1 http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 381 wood anderson seismometer frequency response simulation have lower values. in calculating the signal to noise ratio values, the butterworth filter frequency response simulation gives an increase in signal to noise ratio values at each recording station, but at the lem and kpji recording stations snr decreases in the morning. this is caused by noise due to human activities. at the kpji station, the percentage of snr values decreased in the signal from the wood anderson seismometer frequency response simulation results. meanwhile, the signal from the simulation results using the butterworth filter is increased. however, this does not affect the results of local magnitude calculations. simulation of butterworth filter frequency response produces local magnitude values close to the seismic magnitude value released by bmkg of 4.6 [8]. while the local magnitude value is generated by wood anderson's frequency response seismometer simulation is 4.5. it can also be seen that the local magnitude of the signal from the butterworth filter simulation results has a more consistent value. figure 3: comparison of wood anderson seismometer frequency response with butterworth filter cut off frequencies 7 to 9 hz 4. summary in the paper, an earthquake signal processing algorithm was designed so that the signal was presented in physical displacement units. the second-order butterworth filter with a gain of 2200 and a cut off frequency of 8 hz had a maximum amplitude value and a higher snr than the wood anderson seismometer. hence, the butterworth filter can be used as a wood anderson seismometer approach. this research has the opportunity to be further developed so that it can be applied to the calculation of earthquake magnitude. however, it is necessary to calculate the local magnitude more accurately. in addition, it is necessary to have a greater number of observation stations. http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 382 5. references [1] havskov. j, and ottemoler. l, “routine data processing in earthquake seismology”, springer, pp. 14-160, 2010. [2] ottemoller. l, and sargeant. s.t, “a local magnitude scale ml for the united kingdom”, bulletin of the seismological society of america, vol 103, pp. 2884-2893, 2013. [3] kanamori. h, maechling. p, and hauksson. e, “continuous monitoring of ground-motion parameters”, bulletin of the seismological society of america, vol. 89, pp. 311-316, 1999. [4] haney, m.m, power, j, west, m, and michaels. p, “causal instrument corrections for short period and broadband seismometers”, seismological research letters, vol. 83, pp. 834-845, 2012. [5] anderson. j. f, lees. j. m, “instruments corrections by time-domain deconvolution”, seismological research letters, vol. 85, pp. 197-201, 2014. [6] http://202.90.198.92/arclink/query?sesskey [7] hutton. l.k, and boore. d.m, “the ml scale in southern california”, bulletin of the seismological society of america , vol. 77, pp. 2074-2094, 1987. [8] http://172.19.3.51 http://www.imeko.org/ http://202.90.198.92/arclink/query?sesskey announcement of acta imeko second issue 2022 acta imeko issn: 2221-870x may 2022, volume 11, number 2, 1 acta imeko | www.imeko.org may 2022 | volume 11 | number 2 | 1 announcement of acta imeko second issue 2022 francesco lamonaca1 1 department of department of computer science, modeling, electronics and systems engineering (dimes), university of calabria, ponte p. bucci, 87036, arcavacata di rende, italy section: editorial citation: francesco lamonaca, announcement of acta imeko second issue 2022, acta imeko, vol. 11, no. 2, article 1, may 2022, identifier: imeko-acta11 (2022)-02-01 received may 4, 2022; in final form may 4, 2022; published may 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: francesco lamonaca, e-mail: editorinchief.actaimeko@hunmeko.org dear readers, the second issue of volume 11 is started. for acta imeko, this is the first editorial announcing the start of an issue, since from now the submitted papers will be published as soon as they are ready. this new publication policy is in line with the actions towards the speed-up of the publication time and the attracting of high-value papers. this issue will include the papers of the general issue together with the special issue segment of selected papers from the two events organized by tc17, the imeko technical committee on robotic measurement and managed by prof zafar taqvi, research fellow at the university of houston clear lake in texas. annually tc17 organizes "international symposium on measurements and control in robotics" (ismcr), a full-fledged event, focusing on various aspects of international research, applications, and trends of robotic innovations for benefit of humanity, advanced human-robot systems, and applied technologies, e.g. in the allied fields of telerobotics, telexistance, simulation platforms, and environment, and mobile work machines as well as virtual reality (vr), augmented reality (ar), and 3d modelling and simulation. during the imeko congress years, tc17 organizes only "topical events." in 2021, tc17 organized two virtual topical events, both following the covid-19 restrictions. ismcr2021 had a theme "virtual media technologies for the post covid19 era" and the other tc17-vrise was a jointly organized event with the theme "robotics for risky interventions and environmental surveillance.” these symposia are forums for the exchange of recent research results and futuristic ideas in robotics technologies and applications. it is of interest to a wide range of participants from government agencies, relevant international institutions, universities, and research organizations, working with futuristic applications of automated vehicles. the presentation is also of interest to the media as well the general public. the papers in the special issue segment were specially selected from the above two events. we are sure that acta imeko readers will found this special issue a further source of ideas for their specific research fields. furter information about the published papers will be given in june, as usual, in the introductory note. we hope that you will enjoy your readings and that you can confirm acta imeko as your main source to find new solutions and ideas and a valuable resource for spreading your results. zafar taqvi chairperson of imeko tc17 francesco lamonaca editor in chief mailto:editorinchief.actaimeko@hunmeko.org development of a method for the determination of the pressure balance piston fall rate acta imeko june 2014, volume 3, number 2, 44 – 47 www.imeko.org development of a method for the determination of the pressure balance piston fall rate lovorka grgec bermanec 1, davor zvizdic1, vedran simunovic2 1 faculty of mechanical engineering and naval architecture, laboratory for process measurement, 10000 zagreb, croatia 2 faculty of mechanical engineering and naval architecture, laboratory for length measurement, 10000 zagreb, croatia section: research paper keywords: pressure balance; fall rate measurement; sobel filter citation: lovorka grgec bermanec , marin martinaga1, davor zvizdic, vedran simunovic, development of method for determination of pressure balance piston fall rate, acta imeko, vol. 3, no. 2, article 11, june 2014, identifier: imeko-acta-03 (2014)-02-11 editor: paolo carbone, university of perugia received april 15th, 2013; in final form june 22nd, 2013; published june 2014 copyright: © 2014 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: (none reported) corresponding author: lovorka g.bermanec, e-mail: lovorka.grgec@fsb.hr 1. introduction the determination of the pressure balance piston fall rate is important for to several reasons. as an internal measure for quality assurance it indicates some deformation or changes in effective area [1], and in the ''cross-float'' calibration of other pressure balances where the fall rate is obtained when the two balances are connected and is compared with the natural fall rate. if the fall rates differ, small masses can be added to or subtracted from one of the pressure balance, and the measurements should be repeated until the fall rates agree. [2]. for periodical determination of the lpm standard unit fall rates and for internal quality assurance it was necessary to develop a simple, efficient, repeatable and precise enough method. since there is no standard procedure for this measurement, there was no limitation in selecting equipment. piston rate of fall is usually determined with laser sensors or expensive optic. further equipment that was also taken into consideration in this work comprises eddy current sensors and different cameras. a simple camera has been chosen for analysing measurement possibilities regarding accuracy, accessibility and price. 2. measurement method and calculation procedure measurements were performed with an amateur camera (nikon digital camera) equipped with appropriate lenses. a plane parallel gauge block with 1.5 mm thickness was used to relate relative motion to real displacement in millimetres. before the measurement, while the piston was in stand-up position, a snap of the standard gauge block was taken. pictures were analysed using matlab software which has inbuilt and predefined functions for various filters. in this measurement a sobel filter was used. this filter is often used for edge detection. edge detection enables to follow the relative movement of the pressure balance edges through continuous pictures. after implementation of the sobel filter, a simple method for transforming real thickness into pixel numbers was applied. from the number of pixels the movement between pictures can be calculated and converted into millimetres. two different results were obtained. for two consecutive measurements on the same gauge, the thickness of the standard gauge block was found to be 16 pixels and 15 pixels, abstract this paper describes a laboratory method for the determination of the pressure balance piston fall rate using a simple camera-based optical system with internally developed software. measurements were carried out on three standard piston/cylinder units in the croatian national pressure laboratory (lpm) using gas and oil as transmitting medium. measurement equipment, procedure and fall rate results for three sets of measurements are given, as well as an evaluation of the measurement uncertainty. results were compared with other relevant measurements. acta imeko | www.imeko.org june 2014 | volume 3 | number 2 | 44 respectively. this shows that a resolution of the described equipment of at least 0.1 mm was achieved. the interval between two consecutive pictures was somewhat longer than one minute, so an adequate number of pictures can be acquired to achieve good accuracy. for the analysis of each measurement result, picture intervals of 3 seconds were used, which means 20 measurement points per minute. the initial concept was to take 60 measurement points in a minute to precisely follow the movement of the pressure balance piston. due to computer and camera limitations, the smallest possible interval between pictures with this equipment was 3 seconds. to avoid any contact with the camera, all adjusting parameters and the start of the photographing process were controlled by a computer connected to the camera. every setting was adjusted by appropriate software that allowed camera control via a cable. in this way a fixed position of the camera was assured, which is critical for the applied measurement method. after all the photographs were taken and have passed the sobel filter (figure 1), a series of 20 pictures for each measurement was obtained with information about the relative movement in pixels. to avoid accidental movement of the camera or imperfections of edges visible on the pictures, the x-axis was kept constant. in this way, possible distortions of the pictures are constant over the whole y-axis movement, avoiding errors. the relative motion in pixels on the y-axis was converted into mm for every two consecutive pictures. measurements were performed on three different effective areas of the piston/cylinder, including oil and gas pressure balances (figure 2). the oil pressure balance was designed by budenberg with a double piston for 600 bar and 60 bar loads (figure 3 and figure 4). the gas pressure balance that was used in this work was a dhi pg7601 with 3 bar load. the dhi standard pressure balance is equipped with an internal fall rate sensor so we had an opportunity to compare results obtained between the proposed method and those obtained from the dhi pressure balance. the fall rate of the oil unit was compared with the last calibration certificates from the physikalisch-technische bundesanstalt (ptb). 3. fall rate results and measurement uncertainty evaluation in this paragraph the results for the three standard piston/cylinder units as well as the measurement uncertainty evaluation are presented. the fall rate measurement uncertainty, uf, was evaluated as type b uncertainty [3] taking into account the gauge block uncertainty, the camera resolution and the time measurements as the major influence quantities. 2 2 2= + +f g r tu u u u (1) where: ug uncertainty of the plane parallel gauge block ur uncertainty due to resolution ut -uncertainty due to the time measurement figure 2. determination of oil piston fall rate at 600 bar load. figure 1. computer image of the pressure balance and the plane parallel gauge block with applied sobel filter. figure 3. start of measurement (first photograph) of the budenberg pressure balance with 600 bar load. figure 4. start of the measurement (computer image) of the budenberg pressure balance with 600 bar load after application of the sobel filter. acta imeko | www.imeko.org june 2014 | volume 3 | number 2 | 45 3.1. oil operated system up to 600 bar measurements performed on the budenberg standard pressure balance with 600 bar load has a high rate of fall, as expected. this was clearly visible without any equipment. results can be compared with results given in the calibration certificate obtained from the ptb. these results were (3.0 ± 0.5) mm/min. results from the lpm first unit are given in table 1. from this result it can be seen that the fall rate is too large for a pressure balance classified in the accuracy class of 0.02. the maximum piston fall rate defined in [1] is 1.5 mm/min. the uncertainty estimation is given in table 2, only for the first piston/cylinder unit, although it is calculated for each unit separately with different sensitivity coefficient for the time measurement. 3.2. oil operated system up to 60 bar the second measurement is performed on the same budenberg oil unit but using a low pressure range up to 60 bar maximum load. results from the ptb for this unit were (0.26 ± 0.10) mm/min. results obtained from the picture analysis are shown in table 3. good agreement between the results of ptb and lpm can be observed. table 3. determination of oil piston fall rate at 60 bar load. table 4. determination of dhi pg7601 gas piston fall rate at 3 bar load. table 1. determination of oil piston fall rate at 600 bar load. table 2. fall rate uncertainty evaluation. influence quantity uncertainty of the influence quantity factor sensitivity coefficient contribution to the standard uncertainty plane parallel gauge block 0.1 µm 3 1 0.06 µm resolution 0.1 mm 3 1 57.8 µm time 0.5 s 3 0.06 mm/s 17.3 µm fall rate uncertainty uf 60 µm expanded fall rate measurement uncertainty (k=2) uf=2∙uf 0.12 mm acta imeko | www.imeko.org june 2014 | volume 3 | number 2 | 46 3.3. gas operated standard system up to 3 bar third set of measurements was performed on the gas operated dhi pressure balance with a maximum load of 3 bar. this unit is equipped with an internal fall rate sensor and all the results were directly compared. results obtained after pictures analyses are shown in table 4. in this measurement, the relative movement was 0.094 mm for one pixel, and the internal fall rate sensor has a precision of 0.1 mm. this prevented direct comparison. as it can be seen from the results in table 3, the pressure balance fall rate is 0.56 mm/min, and the internal fall rate sensor started changing its value from 0.5 mm to 0.6 mm after one minute and three seconds. the maximum piston fall rate for gas operated systems according to the oiml document is 1 mm/min. comparing the results in all three cases with results from the calibration certificates, as well as from comparison with the internal fall rate sensor in the dhi pressure balance, it can be concluded that new method is sufficiently accurate for further development. 4. conclusions an internal laboratory method for the determination of the fall rate was developed in the lpm, with a target uncertainty of 0.1 mm/min, using a camera based optical system. the advantages of the proposed method are a simple and cheap measurement equipment. measurement results obtained with the proposed method show good agreement with other relevant measurements. disadvantages are found in the choice of lenses. further development of the method focuses on the automation of the measurements. references [1] 1994 oiml regulation r110, edition 1994(e) pressure balances (paris: organisation international de m´etrologie l´egale) [2] dadson r.s., lewis s.l., peggs g.n., the pressure balance: theory and practice, ed.1., hmso, london, 1982. [3] iso guide to the expression of uncertainty in measurement, [4] geneva: iso, 1995. acta imeko | www.imeko.org june 2014 | volume 3 | number 2 | 47 development of a method for the determination of the pressure balance piston fall rate laser-doppler-vibrometer calibration by laser stimulation acta imeko issn: 221-870x december 2020, volume 9, number 5, 357-360 laser-doppler-vibrometer calibration by laser stimulation h. volkers1 and th. bruns2 1physikalisch-technische bundesanstalt, braunschweig und berlin, germany, henrik.volkers@ptb.de 2physikalisch-technische bundesanstalt, braunschweig und berlin, germany, thomas.bruns@ptb.de abstract a new set-up for primary laser vibrometer calibration was developed and tested at the acceleration laboratory of ptb. contrary to existing set-ups, this configuration makes use of electro-optical excitation. while avoiding the limitations imposed by mechanical motion generators in classic set-ups, the new method still encompasses all components of commercial laser vibrometers in the calibration and thus goes beyond the current capabilities of the purely electrical excitation schemes. keywords: primary calibration, laser doppler vibrometer, ldv calibration 1 introduction laser doppler vibrometers (ldv) are great tools for all kind of vibration measurements, especially for the use as a primary reference for calibration of accelerometers as described in the standards [1, 2]. if pd x(t) y(t) demodulator/ controller u pd_raw (t) f aom u x,v,a (t) ldv figure 1: schematic signal chain of a laser doppler vibrometer figure 1 shows a schematic diagram of the signal flow in an ldv. at the measurement point, the motion quantity x(t) is measured via a laser beam that passes an interferometer (if) including an acoustooptical modulator (aom) and finally illuminates a photodetector (pd) with an intensity modulated by interference according to i(t) = i0 ·sin(2π faom ·t + 4π x(t) λ + τ0)+ b + enoise (1) with faom as the frequency of the aom, λ the wavelength of the laser, τ0 a constant delay due to the time of flight of the laser light, a constant bias intensity b typical for interference and a noise component enoise, e.g. from stray ambient light. the voltage output updraw of the photo detector follows the intensity with a certain additional delay and additional noise from embedded amplifiers and feeds into the demodulation stage of the ldv controller. the internal processing of a commercial ldv is generally unknown to the user but probably follows an arctangents demodulation scheme as described in [3]. the demodulated analog (voltage) output of the instrument is then characterized by its complex transfer functions s in the frequency domain: sux( f ) = ux( f ) x( f ) (2) where the signal delay is characterized by the phase of this complex quantity. 2 existing methods existing calibration methods and set-ups either follow the classical approach of measuring a mechanical vibration [4, 5] or use an electrical excitation of the ldv controller and thus simulate the optical laser head. for the former set-up a typical primary accelerometer calibration system is utilized where a reference ldv measures the motion of a shaker’s armature to provide a reference acceleration signal. the measurement beam of the device under test (dut) is coaligned to the reference beam using a beam splitter and points at the same spot on the armature surface. in the optimized set-up [6] only a single laser beam needs to be adjusted, hence, inaccuracies of the co-alignment are avoided. this method provides a significant smaller uncertainty for ldv calibration than for the classical accelerometer calibration. however, it still suffers from the mechanical limitations of the utilized shaker system in terms of a limited frequency and amplitude range and non-ideal motion. the second method substitutes the laser head of the dut with an electrical signal generator. the signal generator provides the frequency modulated input to acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 357 henrik.volkers@ptb.de thomas.bruns@ptb.de if pd pol λ/4 demodulator/ controller u pd_ref (t) f aom u x,v,a (t) ld signal generator pd adc1 adc2 u am (t) ldv bs figure 2: schematic of a ldv calibration setup with amplitude-modulated laser diode stimulation the ldv controller of the dut, simulating the photo diode output of the laser head. by providing a corresponding fm signal, a very wide variety of virtual motion patterns can be simulated under almost ideal conditions. by simultaneous sampling of the generator signal and the dut output a precise determination of the complex transfer function of the dut is possible. this method, however, requires good knowledge of the working principle of the hardware in order to provide adequate signal levels and carrier frequencies to the controller. in addition, it is not able to account for any signal preconditioning performed within the original laser head. on the other hand it suffers from far less limitations by avoiding any mechanical components[7]. 3 stimulation by amplitude modulated laser source the basic idea of the new set-up evolves from the eq. (1) and figure 1 and is shown in figure 2. the photo diode, being the central sensing part of the ldv, cannot distinguish the cause of an intensity variation. whether it is the result of optical interference or simply is an intensity variation caused by a modulated external light source makes no difference. if an appropriate light source with a suitable amplitude modulation following eq. (1) is targeted at its sensing element, the ldv response will be identical to the respective interference caused by real motion. this approach combines the benefits of those existing set-ups described above, while avoiding the disadvantages. while ommiting any mechanical moving components, that may limit the scope or accuracy, it still includes the potential preprocessing of the laser head in the calibration. all requested excitations for the calibration of ldvs can be provided by electrooptical means. in the new calibration set-up at ptb (c.f. figure 2), the light source is a common 10 mw laser diode (ld) of a wavelength of 635 nm, well matching the ldv’s he-ne wavelength. the bias current is adjusted such that the mean beam power entering the ldv is less than 1 mw, matching approximately the typical output power of the ldv’s laser. an non-polarising beam splitter (bs) separates about 50 % of the light which is directed to a reference photo diode (femto hca-s-400m) of a bandwidth of 400 mhz and a known time delay [8]. an rf generator with phase modulation capability provides the modulation current for the laser diode. the modulation depth is adjusted in a range of 30% to 50% and monitored by the reference photo diode. a polarization filter (pol) and a λ /4 plate between ld and ldv ensure that circular polarized laser light enters the interferometer. the majority of the emitted light from the ld is already linear polarized, however, for the first set-up, the orientation of the polarity is unknown, hence, a polarization filter eases the initial orientation of the λ /4 plate. not shown in figure 2 is the collimator lens of the laser diode and two aperture plates used to ease the alignment of the stimulating laser diode unit with the ldv’s beam line. 4 signal generation and processing the data acquisition system is based on a pxi system controlled by a labview program and provides two synchronized adc channels for the acquisition of the reference signal and the dut output. while the final stage of the set-up is supposed to include an arbitrary waveform generator and the ability to phase lock the generator or the whole pxi system to the ldv carrier frequency, the preliminary results given here were acquired by utilizing an rf generator (type agilent e4400b). the system is similar to the acquisition systems already used and validated for the primary acceleration calibration facilities at ptb. the synchronous data acquisition was performed at 200 ms/s to account for the high carrier frequency. the demodulation of the reference followed the validated arctangents demodulation of ptb’s national primary calibration standards. finally, the evaluation of the simulated motion used the usual three-parameter sine-approximation. 4.1 determination of time delays figure 3 shows the involved time delays of the calibration set-up depicted in figure 2. all components except hldv are assumed to have a true time delay, i.e. the time delay is constant within the frequency ranges observed. the time delay τcor = φcor ω , with ω = 2π f (3) acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 358 ld pd τ r1 τ m1 ldv head τ m2.1 τm2.2 ldv controller τm2.3 hcontr adc2 adc1 τm3 τ r2 τr3 δτadc hldv figure 3: time delays of the signal chains to correct the measured transfer function hmeas to get hldv hldv(ω) = |hmeas(ω)|·e jω(φmeas−φcor) (4) is calculated as τcor = τr1 −τm1 + τr2 + τr3 −τm3 + ∆τadc (5) with the values: τr1 −τm1 = −545(5)mm·cair =−1.82(3)ns(6) cair = 2.9971×108 m/s being the difference of the laser beam lengths measured from the reference surface of the ldv head stated in the manual. its uncertainty covers the unknown delays of the λ /4 plate and the polarization filter. the photo diode delay τr2 was measured in [8] as τr2 = 3.10(8)ns. (7) the time delay difference between the two simultaneously sampled channels, each with an rg58 cable of 2 m length, representing the term τr3−τm3 + ∆τadc, is measured by feeding both cable ends with a common signal from the generator via a power splitter minicircuits zfsc-2-4. by doing multiple measurements with swapped cables at the power splitter and adc inputs we found τr3 −τm3 + ∆τadc = 0.15(5)ns (8) including variances due to remounting. putting the results into (5) gives τcor = 1.43(10)ns (9) the measurement of a time delay was performed in a two-step process. in the first step, a signal with a carrier frequency of 40 mhz and an fm sine modulation of 500 hz and a modulation depth of 3.15 mhz was applied and a coarse time delay was determined by the maxima of a computed cross correlation, see figure 4. in the final step the modulation was turned off and the phase difference of the 40 mhz carrier signal was measured by applying a three parameter sine approximation. this process was chosen to be able to measure time delays of heterodyne signal outputs of ldv controllers with a time delay greater than one period of the carrier signal (25 ns in this case) due to internal signal processing like amplitude stabilisation or filter. 800 -800 0 a m p li tu d e 800 -800 0 a m p li tu d e 800 -800 0 a m p li tu d e 0 0 80n 120n 160n40n0-40n-80n -10µ -8µ -6µ -4µ -2µ 2µ 4µ 6µ 8µ 10µ -0.2m-0.4m-0.6m-0.8m-1m 0.2m 0.4m 0.6m 0.8m 1m time in s figure 4: cross correlation of the two adc channels at a sample rate of 200 mhz, both fed with a 40 mhz fm signal modulated with a 500 hz sine of 3.15 mhz modulation depth, on different time scales, with one of the two connection cables being about 8 m longer, resulting in a delay of 41.90(5)ns. 5 results first tests with a ldv controller polytec ofv5000-ku and a laser head ofv 353, connected via a 5 m cable, were performed and figure 5 shows a measured frequency response obtained with the new setup. the ldv is based on a carrier frequency faom of 40 mhz. the ldv controller settings were: • velocity decoder: vd-01 • range: 1 m/s/v • max. frequency: 50 khz • tracking filter: off • low pass filter: 100 khz • high pass filter: off the frequency range in terms of the simulated vibration was 100 hz to 30 khz and an amplitude of 1 m/s leading to a modulation depth of 3.159 mhz. the plots show the relative deviation in magnitude and the absolute phase of the analog velocity output in relation to the demodulated intensity stimulus on a linear frequency scale. the nearly linear phase corresponds to a delay of about 7.52 µ s. 6 outlook instead of a photo diode with known delay, a known laser source would obviate the beam splitter and photo acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 359 0 5000 10000 15000 20000 25000 30000 -4.0% -3.5% -3.0% -2.5% -2.0% -1.5% -1.0% -0.5% 0.0% 0.5% -90 -80 -70 -60 -50 -40 -30 -20 -10 0 magnitude phase frequency in hz re l. d ev ia ti o n m ag n it u d e p h as e in 1 ° figure 5: transfer function of a ldv analog velocity output at 1 v/(m/s) diode, simplifying the set-up. it is a classical chickenor-egg dilemma; in our case we had the known pd first to relate an optical signal to an electrical signal. the measurement uncertainty budget is still under investigation, the absence of mechanical excitation are expected to significantly reduce the uncertainties, leaving the absolute ac voltage measurement as the main contributor for magnitude uncertainty, while the phase uncertainties are expected to be better than 0.01° for frequencies up to 100 khz. acknowledgment the authors like to thank dr. siegmund of polytec for some inspiring walks and talks. 7 references [1] iso 16063-11:1999 methods for the calibration of vibration and shock transducers — part 11: primary vibration calibration by laser interferometry, iso, geneva, switzerland, 1999 [2] iso 16063-13:2001 methods for the calibration of vibration and shock transducers — part 13: primary shock calibration using laser interferometry [3] iso 16063-41:2011 methods for the calibration of vibration and shock transducers — part 41: calibration of laser vibrometers [4] u buehn et al., "calibration of laser vibrometer standards according to iso 16063-41", xviii imeko world congress 2006, rio de janeiro, brazil, september, 2006 https://www.imeko.org/publications/wc2006/pwc-2006-tc22-007u.pdf. [5] th. bruns, f. blume, a. täubner, "laser vibrometer calibration at high frequencies using conventional calibration equipment", xix imeko world congress, september 6-11, 2009, lisbon, portugal http://www.imeko2009.it.pt/papers/fp_495.pdf [6] f. blume, a. täubner, u. göbel, th. bruns, "primary phase calibration of laser-vibrometers with a single laser source", metrologia, 2009, vol. 46, n. 5, https://dx.doi.org/10.1088/0026-1394/46/5/013 [7] m. winter, h. füser, m. bieler, g. siegmund, c. rembe, "the problem of calibrating laser-doppler vibrometers at high frequencies", aip conference proceedings 1457, 165 (2012) https://doi.org/10.1063/1.4730555 [8] th. bruns, f. blume, k.baaske, m. bieler, h. volkers "optoelectronic phase delay measurement for a modified michelson interferometer", measurement, 2013, vol.46, n. 5, https://doi.org/10.1016/j.measurement.2012.11.044 acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 360 https://www.imeko.org/publications/wc-2006/pwc-2006-tc22-007u.pdf https://www.imeko.org/publications/wc-2006/pwc-2006-tc22-007u.pdf http://www.imeko2009.it.pt/papers/fp_495.pdf https://dx.doi.org/10.1088/0026-1394/46/5/013 https://doi.org/10.1063/1.4730555 https://doi.org/10.1016/j.measurement.2012.11.044 introduction existing methods stimulation by amplitude modulated laser source signal generation and processing determination of time delays results outlook references comment to: l. mari “is our understanding of measurement evolving?” acta imeko issn: 2221-870x june 2022, volume 11, number 2, 1 3 acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 1 comment to: l. mari “is our understanding of measurement evolving?” franco pavese1 1 independent scientist, research director in metrology (formerly, at cnr then inrim, torino), 10139 torino, italy section: technical note keywords: metrology; measurement science; measurement process; informational component citation: franco pavese, comment to: l. mari “is our understanding of measurement evolving?”, acta imeko, vol. 11, no. 2, article 19, june 2022, identifier: imeko-acta-11 (2022)-02-19 section editor: francesco lamonaca, university of calabria, italy received february 7, 2022; in final form june 1, 2022; published june 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: franco pavese, e-mail: frpavese@gmail.commailto:frpavese@gmail.com 1. introduction after having read the recently published paper [1], i am feeling, dictated perhaps by being a long-since metrologist, a compulsory need to write the comments below, more from some sense of surprise than for a less appreciation for its author. the comments concern the following quoted parts (where added section numbers are the ones used here below): (2.) – “doubt: isn’t metrology a ‘real’ science ? … metrology is a social body of knowledge” (original italics); (3.) – “measurements are aimed at attributing values to properties: since values are information entities, any measurement must then include an informational component”; – “listing some necessary conditions that characterize measurement, and that plausibly are generally accepted, is not a hard task: measurement is … (iv) that produces information in the form of values of that property. indeed, (iv) characterises measurement as an information process”. (4.) – “however, not any such process is a measurement, thus acknowledging that not any data acquisition is a measurement. we may call “property evaluation” a process fulfilling (i)-(iv). what sufficient conditions characterize measurement as a specific kind of property evaluation? the answer does not seem as easy” (blank lines added for clarity) … “the possible evolutionary perspectives of measurement can be considered along four main complementary, though somewhat mutually connected, dimensions: – measurable entities as quantitative or non-quantitative properties …; – measurable entities as physical or non-physical properties; – measuring instruments as technological devices or human beings; – measurement as an empirical or an informational process, and therefore the relation between measurement and computation …”. 2. “isn’t metrology a ’real’ science?” my position is that metrology is a part of measurement science, a process intended to share common ways to transmit the knowledge to the community that is not limited to a single generation of scientists and practitioners, and to obtain the necessary consensus. it is metrology the part that defines the meaning of the terms “precision” and “accuracy” by introducing the concept of uncertainty, otherwise not necessarily embedded in the meaning of measurement, as it happened, e.g., in times anterior to modern science. another mandatory requirement of metrology is the need of a multiplicity of the measurements in order to obtain data comparable with each other, so requiring that they are abstract the present contribution is a comment which addresses the paper published in this journal “is our understanding of measurement evolving?” and authored by luca mari. this technical note concerns specific parts of that paper, namely the statements: “doubt: isn’t metrology a ‘real’ science? … metrology is a social body of knowledge”, “measurements are aimed at attributing values to properties: since values are information entities, any measurement must then include an informational component” and “what sufficient conditions characterise measurement as a specific kind of property evaluation?”, and discusses alternatives. mailto:frpavese@gmail.com mailto:frpavese@gmail.com acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 2 traceable—to a common denominator— to each other, in order to ensure that all the scientists are “on the same page”. the reason for this is that data, numerical or not, are certainly to be considered facts—as opposed to inferences—but not necessarily and usually as un-equivocal as one would ask for. restricting the case to numerical data—e.g., rarely the published ones are the “rough” instrumental indications (as recognized, e.g., by the vim [2])—metrology is the science dealing with correctly setting rules for their elaboration. metrological competence is also necessary to implement the increase in precision of the measurements, the latter being a normal goal of science as one of the ways for increasing knowledge. yet, in order to allow sharing the results, “good-practice” rules must be installed, eventually ordered in protocols and conventions, only apparently a non-scientific stage of the measurement process. the latter requires instead scientific competence and sharing meanings and, consequently, a language (also across local ones). at the end of this process, language is stored into written rules (“scripta manent, verba volant”) —notation being a specific symbolic language—a feature common to every frame of shared knowledge, called consensus (“pacta sunt servanda”), in the lack of human possibility of knowing truth. on the other hand, consensus cannot replace or contrast current knowledge, but its function is merely a notary one (an issue becoming today more and more critically important and subject to misuse). in the above respect, my opinion is that measurement science may be compared to the dna of the observational science, i.e. the body of knowledge, with metrology its rna as presently interpreted in biology, i.e. the tool for implementation of its basic principles and rules. 3. “any measurement must then include an informational component” i agree that a measurement result, numerical or not, is additional information, assumed to be useful for increasing the knowledge level—otherwise there would be no reasons to perform measurements. however, after shannon having extended the scientific meaning of the term measurement, and especially in the present times dominated by informatics, i suggest that assigning a specific further meaning to the term information becomes necessary, as i do not consider it anymore a generic or un-equivocal one. i consider it sufficient here to point out that one meaning of information is the one carried by a value measured according to the scientific definition of measurement, i.e., a “material” information concerning the “external world” as perceived by the humans and their built apparatuses—made to supplement the human limited (standard) sensorial capabilities. instead, i consider a different meaning of information what scientists (and anybody else) elaborate or communicate (the information born from) thoughts of their mind. the difference is the usual one, between the case of becoming informed by our senses about a feature of a world’s phenomenon, and the case of having a personal thought—or based on an inter-subjective one, basically without difference when concerning this issue. in other words, in all cases “information” is a concept from human mind, while a ”measured value” for a human is an external fact, then assimilated through subsequent mind inference. this difference is substantial and does not produce, in my opinion, any “evolutionary perspective” (italics added) in measurement science— at least, when the perspective does not concern the human mind. in this sense and meaning, i respectfully disagree with the above clause (3.) (iv), because it is not an information process in the current sense, and especially not according to the procedures used today in information science. let me introduce here a bit of humour, by citing an extreme case that recently occurred to me in this subject matter. it is popular in this period for an author to pretend to have found an information method to check if the current scientific evaluation of measurement uncertainty leads to correct estimates, e.g., in the case of the uncertainty associated to the value of the universal constants of physics (for the planck one see [3]). his method is said to be based on the information content. i had a short correspondence with him to understand the way he implements the information process and gets his results, until i discovered that he is considering as the information content of a given physical constant value the number of times that the value is cited in the reference document of the si, that being the “firm” basis of the rest of his computations … 4. “what sufficient conditions characterize measurement as a specific kind of property evaluation?” the author indicates as “evolutionary perspective” basically an extension to a wider meaning of the term “measurement”, namely to categories of observations that historically were not comprised in the current definition of measurement, namely the non-quantitative and the informational ones, a goal having attracted more attention in recent times. it seems to me that, for that purpose, it would be simpler to use a term different from “measurement”, e.g., the one used by the author itself, “evaluation” for the non-quantitative case. it does not seem to me a diminutio, being used simply for indicating a previous stage of the process, even in the quantitative case [4— or “representation”, if one prefers to avoid any misunderstanding about the existence of a possible quantification. concerning instead the issue of measurement vs. computation, i think that “computation” has always been in modern science part of the elaboration of the numerical (or logical) data obtained from observations (here the theoretical case is not considered), and that recently the elaboration is done prevalently via automatic computing. this fact has induced the development of a new discipline in science: informatics. arising from its nature, its most important influence in science has been an exponential increase in time of the development of new (machine) languages, obviously having for their roots in the human ones, thus also concerning the organization of the numerical knowledge for it elaboration and use. in that sense, i see informatics as a marginal follow up of the measurement process, not an integral stage of it—as also “simulation” and “extrapolation” are, both based on models, so actually pertaining to the theoretical frame. 5. conclusions i conclude by saying that i am aware and i had direct experience long since that there are gaps between disciplines—i contributed, since 30 years ago, to start a conference series just with the main goal, at that time, to “increase the extent of cooperation by calling scientists from both the mathematical and acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 3 the metrological fields to meet and exchange experiences” [5], later extended to computational science. these gaps include language: metrology has developed its own idiom that, in my opinion, is satisfactorily summarized in [2]—for a more extended discussion on this issue see [6]. different meanings may be assigned to terms in other discipline idioms, namely those of philosophy of science, or different terms may be used. consequently, it may easily happen that it becomes hard to overcome basic misunderstandings in inter-discipline conversations. on the other hand, this diversity is a richness of science. [7] references [1] l. mari, is our understanding of measurement evolving? acta imeko (2021) 10 (4), pp. 209–213. issn: 2221-870x. doi: 10.21014/acta_imeko.v10i4.1169 [2] jcgm 102:2008. international vocabulary of metrology – basic and general concepts and associated terms (vim) 3rd edition, jcgm (2012) pp. 108 [3] b. menin, the boltzmann constant: evaluation of measurement relative uncertainty using the information approach. journal of applied mathematics and physics (2019) 7, 486–504. doi: 10.4236/jamp.2019.73035 [4] f. pavese, measurement in science: between evaluation and prediction. in advanced mathematical and computational tools in metrology and testing xii (f. pavese, a. b. forbes, n. f. zhang, a. g. chunovkina, eds) series on advances in mathematics for applied sciences 90 (2021), world scientific publishing co, singapore, pp. 346–363. isbn 978-981-124-237-3 (hardcover), 978-981-124-239-7 (ebook) [5] advanced mathematical tools in metrology, workshop (p. ciarlini, m. g. cox, r. monaco, f. pavese, eds., 1993, turin, italy) series on advances in mathematics for applied sciences 16 (1994), world scientific publishing co, singapore. isbn 981-021758-7. see present denomination in ref. 4 above [6] f. pavese, from vim3 toward the next edition of the international dictionary of metrology, acqual, in special issue in memory of paul de bièvre, 2022 (in press) [7] f. pavese, p. de bièvre, fostering diversity of thought in measurement. in advanced mathematical and computational tools in metrology and testing x, vol.10 (f. pavese, w. bremser, a. g. chunovkina, n. fischer, a.b. forbes, eds.), series on advances in mathematics for applied sciences vol 86, world scientific, singapore, 2015, pp 1–8. isbn: 978-981-4678-61-2, isbn: 978-981-4678-62-9(ebook) https://doi.org/10.21014/acta_imeko.v10i4.1169 https://doi.org/10.4236/jamp.2019.73035 introductory notes for the acta imeko special issue on the xxix italian national congress on mechanical and thermal measurements acta imeko issn: 2221-870x december 2021, volume 10, number 4, 6 7 acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 6 introductory notes for the acta imeko special issue on the xxix italian national congress on mechanical and thermal measurements alfredo cigada1, roberto montanini2 1 dipartimento di meccanica., politecnico di milano, via la masa 120156 milano, italy 2 dipartimento di ingegneria, università degli studi di messina, c.da di dio 98166 villaggio sant’ agata, messina, italy section: editorial citation: alfredo cigada, roberto montanini, introductory notes for the acta imeko special issue on the xxix italian national congress on mechanical and thermal measurements, acta imeko, vol. 10, no. 4, article 4, december 2021, identifier: imeko-acta-10 (2021)-04-04 received december 14, 2021; in final form december 14, 2021; published december 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: alfredo cigada, e-mail: alfredo.cigada@polimi.it dear readers, there is no doubt that measurements are playing a fundamental role in our everyday life. hot topics like the internet of things, industry 4.0, measurements for health or smart structures can not exist without a massive and pervasive presence of sensors and, more generally speaking, of measurement systems, meaning that data management and their analysis, both led by the interpretation by numerical models and by data driven approaches, are to be considered research trends of paramount interest for measurements. the reason why mechanical measurements are considered a science on its own has strong roots, not always fully understood, as data quality assessment, a preliminary step to any model, requires skills in a broad range of engineering topics, from sensors to mechanics, to materials and data science, just to mention some: this wide knowledge makes the cultural background of the expert in mechanical measurements one of the largest in engineering. all these aspects make the yearly italian meeting of “forum nazionale delle misure” a milestone for the discussion and update about the new trends in measurements, a science running very fast: this is also the occasion to relate to the colleagues mainly dealing with electric and electronic measurements, to widen the horizons and cultural exchange, making the occasion very rich for both a review on the present activities and a stimulus for the future. the pandemic has created a discontinuity in the meeting long lasting tradition, started in 1986: in 2020 the forum took place online, and the in-person meeting was organized again in september 2021 at giardini naxos, a wonderful location close to messina. as usual, part of the workshop has consisted in joint meetings between scientists from the mechanical and thermal measurements and the electrical and electronic measurements academic groups, in an interesting cultural discussion on the shared topics, faced with different viewpoints, and then on other topics, more peculiar to each group, in which the common language of metrology has a fundamental role to define the measurements quality. this special issue collects a selection of 12 papers presented during the three days of the “forum”: the authors have been asked to review their work and prepare an extended version, fit for the publication on acta imeko, a reference journal for measurements. the remarkable transversality of the mechanical measurements field is witnessed by the significant heterogeneity of the topics covered in each of the twelve selected works. the paper entitled ‘skin potential response for stress recognition in simulated urban driving’ by zontone et al. addresses the problem of stress conditions arising in car drivers using machine learning techniques based on skin potential response (spr) signals recorded from each hand of the test subjects. results showed that, in a situation without traffic, the test individuals are less stressed, confirming the effectiveness of the proposed minimally invasive system for detection of stress in drivers. in the paper ‘human identification and tracking using ultrawideband-vision data fusion in unstructured environments’ the research group of the university of trento, led by prof. mariolino de cecco, faced the problem of the cooperation between automated guided vehicles (agv) and the operator, to solve two crucial functions of autonomy: operator identification, and tracking. using sensor fusion, the authors were able to improve the accuracy and goodness of the final tracking, reducing uncertainty. mailto:alfredo.cigada@polimi.it acta imeko issn: 2221-870x december 2021, volume 10, number 4, 7 8 acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 7 the third paper, authored by giulietti et al., deals with the continuous monitoring of cement-based structures and infrastructures, to optimize their service life and reduce maintenance costs. the proposed approach is based on electrical impedance measurements. data can be made available on a cloud through wi-fi network or lte modem, hence they can be accessed remotely, via a user-friendly multi-platform interface. the paper ‘validation of a measurement procedure for the assessment of the safety of buildings in urgent technical rescue operations’ saw the collaboration between the research group of the university of l’aquila and the fire, public rescue and civil defense department of the italian ministry of the interior. the work provides a preliminary contribution to the draft of standard procedures for the adoption of total stations by rescuers in emergency situations, so as to offer a reliable and effective support to their assessment activities. in the paper ‘a comparison between aeroacoustic source mapping techniques for characterization of wind turbine blade models with microphone arrays’, g. battista et al. deal with the problem of characterizing the aeroacoustic noise sources generated by a rotating wind turbine blade, in order to provide useful information for tackling noise reduction. this paper discusses a series of acoustic mapping strategies that can be exploited in this kind of applications, based on laboratory tests carried out in a semianechoic room on a single-blade rotor. the research group of the university of padua, led by prof. stefano debei, presented a paper entitled ‘occupancy grid mapping for rover navigation based on semantic segmentation’, dealing with obstacle perception in a planetary environment by means of occupancy grid mapping. to evaluate the metrological performances of the proposed method, the esa katwijk beach planetary rover dataset have been used. the paper, ‘characterization of glue behaviour under thermal and mechanical stress conditions’ by caposciutti et al., explores the behaviour of the glued interface commonly used to fix the electronics to the box housing, as it undergoes daily or seasonal thermal cycles combined to mechanical stress. to carry out the study, the authors prepared some parallel plates capacitors by using glue as a dielectric material. non-linear behaviour of the capacitance vs. temperature as well as effects of thermal cycles on the glue geometry were investigated. the research group of the university of perugia, led by prof. gianluca rossi, presented an optical-flow-based motion compensation algorithm to be used in thermoelastic stress analysis to account for rigid-displacements that can occur during loading. the proposed approach is based on measuring the displacement field of the specimen directly from the thermal video. the blurring and edge effects produced by the motion were almost completely eliminated, making it possible to accurately measure the stress field, especially in areas around geometrical discontinuities. thermoelasticity and aruco markers were also employed by l. capponi et al. to validate a numerical model of the inspection robot mounted on the new san giorgio's bridge on the polcevera river in genova. an infrared thermoelasticity-based approach was used to measure stress-concentration factors while aruco fiducial markers were exploited to assess the natural frequencies of the robot inspection structure. a completely different field of application concerns the paper ‘doppler flow phantom failure detection by combining empirical mode decomposition and independent component analysis with short time fourier transform’, that reports some very last results obtained by the research group led by prof. sciuto at the university of roma tre. the paper aims at providing an improvement of a previously proposed method for doppler flow phantom failures detection, combining application of empirical mode decomposition (emd), independent component analysis (ica) and short time fourier transform (stft) techniques on pulsed wave (pw) doppler spectrograms. the paper ‘comparison between 3d-reconstruction optical methods applied to bulge-tests through a feed-forward neural network’ was originated from the collaboration between the research groups of mechanical and thermal measurements of the university of messina and of the university of catania. the aim of the work was to compare two different 3d reconstruction techniques, epipolar geometry and digital image correlation, to measure the deformation field of hyperelastic membranes under plane and equibiaxial stress state. a ffnn neural network was then used to assess accuracy of the two experimental approaches using a laser sensor as reference. finally, the paper ‘development and characterization of a self-powered measurement buoy prototype by means of piezoelectric energy harvester for monitoring activities in a marine environment’, written by the research unit of the university of messina, led by prof. roberto montanini, addresses a series of interesting topics: among the others measurements for the sea and energy harvesting from the sea waves: the paper focuses on this aspect with an innovative approach. we gratefully acknowledge all the authors who have contributed to this special issue, as well as all the reviewers and a special thanks goes to prof. francesco lamonaca, editor in chief of acta imeko for his tireless and patient help which has made possible this special issue. we are proud of having served as guest editors for this issue, hoping that this will help spreading the culture of measurements. alfredo cigada and roberto montanini guest editors accuracy – review of the concept and proposal for a revised definition acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 414 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 414 418 accuracy – review of the concept and proposal for a revised definition c. müller-schöll1 1 mettler-toledo international inc., greifensee, switzerland, christian.mueller-schoell@mt.com abstract: accuracy as a concept is widely used in metrology. it has undergone a historical development and is defined and used differently in different documents, communities and usage situations. present existing definitions are sometimes not clear, not unambiguous and sometimes even not useful. this paper explains the difficulties regarding present definitions (section 3 and 4), throws a spotlight on present day use of the concept (section 5), clarifies the question if accuracy is a qualitative concept (section 6) and finally presents conceptual ideas and a proposal for a new wording of a sustainable definition for accuracy (section 7). this paper is intended to support the highly estimated work of jcgm wg2. keywords: accuracy; terminology; metrology 1. introduction, motivation language is subject to ongoing change, due to its usage and due to its users. the term “accuracy” in the context of metrology has undergone changes in its definition, in its use and in its understanding and application in the past. there is still today uncertainty about its proper use and its proper meaning, and there is more than one normative document defining accuracy, using different words. it might be time to reflect on the language and it might be time for a clarification resulting in a clear and (for the field of metrology) universally applicable definition. it turns out that two distinguishable concepts are in use: one is taken from the international vocabulary of metrological terms and concepts [1] henceforth referred to as "the vim", the other one is more prevalent in standard documents published by the international organization for standardization (iso)1. 2. review of the vim definition2 2.1. text review the definition of accuracy according to [1] reads: 1 the vim is not considered an "iso document" here, although it is also published as "iso guide 99" under an iso name. "2.13 (3.5) measurement accuracy; accuracy of measurement; accuracy closeness of agreement between a measured quantity value and a true quantity value of a measurand note 1 the concept ‘measurement accuracy’ is not a quantity and is not given a numerical quantity value. a measurement is said to be more accurate when it offers a smaller measurement error. note 2 the term “measurement accuracy” should not be used for measurement trueness and the term “measurement precision” should not be used for ‘measurement accuracy’, which, however, is related to both these concepts. note 3 ‘measurement accuracy’ is sometimes understood as closeness of agreement between measured quantity values that are being attributed to the measurand." additionally, an "annotation" can be found in the internet-version of the vim, reading: "annotation (informative) [9 june 2016] historically, the term "measurement accuracy" has been used in related but slightly different ways. sometimes a single measured value is considered to be accurate (as in the vim3 definition), when the measurement error is assumed to be small. in other cases, a set of measured values is considered to be accurate when both the measurement trueness and the measurement precision are assumed to be good. sometimes a measuring instrument or measuring system is considered to be accurate, in the sense that it provides accurate indications. care must therefore be taken in explaining in which sense the term "measurement accuracy" is being used. in no case is there an established methodology for assigning a numerical value to measurement accuracy." 2.2. text analysis the vim definition uses the concept “closeness of agreement”, however, the term “closeness” is not defined and therefore is subject to interpretation. the definition speaks of the closeness of agreement of “a” measured quantity value, so it applies to a single measured value. note 2 mentions a "relation" between accuracy and precision and between accuracy and trueness, but leaves open what kind of relation that is. although accuracy being "not a quantity" [1], a comparative statement like “more accurate" is possible according to note 1. it is left open how two things can be judged “more” and “less”, when being not a quantity. 2 in the following paragraphs we investigate some definitions of the term accuracy as examples. other definitions might exist. http://www.imeko.org/ mailto:christian.mueller-schoell@mt.com acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 415 a relation between "measurement accuracy" and “measurement error” is mentioned in note 1. “measurement error” in turn is given a relation to “quantity value” in its vim-definition (2.16), meaning that both “measurement error” and “quantity value” are of the same kind. “quantity value” in turn is used as the first fraction/component of a “measurement result” (vim definition 2.9) where measurement result consists of a first fraction/component "value" and a second fraction/component "uncertainty associated to it". consequently, measurement error is clearly a one-dimensional property (it is a one-dimensional quantity value) and not a two dimensional vector like "measurement result" (with its two fractions/dimensions/components "value" and "uncertainty"). the "annotation" (2016) that can be found in the internet-version of the vim clearly points at ambiguities of interpretations of the definition, calling them "historical". but in the text of the annotation no hint is given that either of the two interpretations given there is considered outdated, so they still both exist. the two interpretations a) either relate "a" measured value to its error (note "a" value, not "values"), so "smaller error = more accurate" b) or relate "a set of measured values" to both its trueness and its precision (where both the trueness and the precision need to be "good" for the set to be considered "accurate"). but this is not consistent in itself, because even a single value can be attributed both a trueness and also a precision (!) if the distribution is known from which this single value is drawn (type a uncertainty of a single measured value). so the relation of "a" measured value to its trueness and its precision is missing here. the npl has published interpretations referencing to the vim-definition where accuracy is independent from precision [2] and where a set of measurements (!) is said to have "high accuracy" while having "low precision" at the same time. the consistency with case b) mentioned above appears unclear or not being given. 2.3. conclusion regarding vim with regards to the vim definition, it is remains unclear how accuracy is defined and how accuracy can be expressed. it is said to be "not a quantity", but vim does clearly not state it be "qualitative". accuracy can be judged "more accurate [than]" but accuracy cannot be assigned a value. it is unclear how this goes together. accuracy is stated to relate to "error" (smaller error = more accurate), however an additional property like dispersion or variance or uncertainty is not an explicitly mentioned part of this definition (only in the notes). on the other hand, accuracy is said to be "related to both" trueness and precision (note 2), where "precision" (vim 2.15) is clearly not related to error, but to dispersion. this is a contradiction in the vim in the view of the author. since the idea of accuracy being applied to more than one value and the idea of dispersion of values are not visible in the vim definition text (not considering the non-normative annotation 2), the commonly used dartboard model with dispersing hits cannot be considered an appropriate illustration for the definition of accuracy according to the vim. accuracy and precision are on the same hierarchical level in this concept (they are like apples and pears). within this concept it is possible that a result can be attributed e.g. “accurate but not precise” which can be found in publications (e.g. [2]). 3. review of the "iso definition" 3.1. text review iso (iso 5725-1:1994) the title of the standard iso 5725-1 [3] is: “accuracy (trueness and precision) of measurement methods and results". this standard gives definitional phrases in two different locations: one rather explanatory (yet definitional in character) in section 0.1, and a definition in section 3.6. (in chapter 3 named "definitions"): section 0.1 of iso 5725-1 uses two terms "trueness" and "precision" to describe the accuracy of a measurement method. "trueness'' refers to the "closeness of agreement between the arithmetic mean of a large number of test results and the true or accepted reference value". "precision" refers to the "closeness of agreement between test results". additionally, we find a so-declared "definition" in the same standard: 3.6 accuracy: the closeness of agreement between a test result and the accepted reference value. note 2 the term accuracy, when applied to a set of test results, involves a combination of random components and a common systematic error or bias component [emphasis by the author]. 3.2. text analysis – iso 5725-1:1994 section 0.1 of iso 5725-1:1994 according to section 0.1 of [3], it is both “trueness” and “precision” that are used to form accuracy. the text of 0.1 refers to accuracy as a property of a measurement method (and not of a single result and not of a set of results). this is a clear, yet understandable difference to the vim concept since the focus of [3] is methods. both trueness and precision are explained using the words “a large number of” results or at least “results” (in plural form) in [3]. according to this, accuracy, as explained here, cannot be applied to a single measurement result, since the language does not cover a single result (again in contradiction to vim). http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 416 precision is not the same as uncertainty. so, accuracy – according to this concept – is not related to what is usually perceived as being the quality of a "measurement result" (vim 2.9) which consists (vim 2.9 note 2) of "a measured quantity value and a measurement uncertainty" [emphasis by author] and is thus a two-dimensional statement. the two dimensions of "measurement result" introduced in this definition are: • the first dimension, either the measured quantity value or the measurement error (with reference to a reference value) calculated from it and, • a second dimension characterizing the dispersion which is the uncertainty associated with the first component. the wording of 0.1 of [3] is in contradiction with the title of the standard, which announces both the accuracy of "methods and results". the accuracy of results is neither explained nor even mentioned here. section 3.6 of iso 5725-1:1994 in section 3.6 of the same standard, a definition is given for accuracy. this definition is the same as the vim definition which refers to “a” test result (singular form) and to a “test result”. methods are not mentioned in the definition. however, a note (note 2) has been added, broadening the scope to “a set of … results”. this circumstance in the note is inconsistent with the definition. the note is not in alignment with the definition and a definition that can be applied to the accuracy of a method is missing in the whole standard despite its title. additionally, the restriction "when applied to a set of results" is misleading (or maybe even wrong), since also a single value can be assigned a dispersion originating from the population this single value was drawn from (as explained above). according to 3.6 of [3], accuracy embeds both trueness and precision. this defines a hierarchy: trueness and precision are on the same level and accuracy combines both, defining a parent hierarchical level. (accuracy is like fruits and trueness and precision are like apples and pears.) a two-dimensional dartboard model is frequently used to visualize the concept depicting trueness and precision with a set of hits (by means of centering of the mean value and spread of the hits). 3.3. conclusion regarding iso 5725-1:1994 it seems that in the definition of accuracy given in [3], the authors intended to take both, a "deviation component" and a "spread component" into account. however, instead of measurement uncertainty, the precision was chosen to describe what was missing when accuracy "at one time" (quoted from section 0.6) contained only what we today call trueness. nevertheless, the different statements within the standard are hard to match (if not impossible to match). note 2 broadens the scope of the definition to "a set of results" which is not consistent with the mere text of the definition. 3.4. text review iso iwa 15 iso iwa 15 [4] reproduces the vim definition for accuracy. however, in a specific definition of "uncertainty" (section 3.1.3), we find the following phrase in a note: "uncertainty is inversely related to accuracy, and is a quantity value." this is one of the rare occasions that in the defining literature uncertainty comes explicitly into play in a relation to the concept of accuracy. nevertheless, it is neither detailed, how this "relation" looks like nor how a relation would lead from a non-quantitative accuracy to a quantitative uncertainty nor if and how "inversely" is to be understood mathematically. chapter 5.1 of [4] reads: "accuracy may be improved by improving precision and trueness." this hints again at the concept of [3] where accuracy is (whatever kind of) a combination trueness and precision and accuracy is on a higher hierarchical level than trueness and precision (please note that at this point, no connection to "uncertainty" is made). however, immediately afterwards, we read "accuracy, precision and trueness are conceptual terms. quantitative expressions of these concepts are given in terms of uncertainty, random error and systematic error, respectively." this is clarifying accuracy: accuracy is said to be quantified by uncertainty, which in turn is said to be combined of systematic and random errors. the latter of course is wrong since uncertainty according to the concept of the gum [5] is not just a combination of random and systematic "errors". (in order to clearly separate these conceptual areas, the gum has introduced "type a" and "type b" uncertainties and does not support the wording quoted from [4] above.) later in [4], accuracy is even expressed as a quantity value (e.g. in table 2, column name: "typical accuracy", column content is values). this is in clear contradiction to the definition given in the same document. and also later in the standard (section b.6.1.4) "accuracy" and "precision" are mentioned on the same hierarchical level, which again is an internal contradiction in the same document and in section b.7.1 systematic error is equalized to accuracy which also contradicts internally in the document. 3.5. conclusion regarding iso iwa 15 also [4] does not clarify the matter. it adds ideas of uncertainty to the concept of accuracy, however, also appears not fully internally consistent. 4. current use of the term accuracy a recent high-level metrology publication adds an interesting view to the discussion: it is the paper titled "evaluation of the accuracy, consistency, and http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 417 stability of measurements of the planck constant…" by a possolo, s schlamminger et al. [6]. in section 3 of the paper (section named "accuracy"!), the authors detail on the "accuracy requirements" of the ccm regarding the re-definition of the kilogram. in fact, the requirements made by the ccm are not given this specific name ("accuracy requirements") in the original ccm document [7]. in the publication, these requirements and the values that are compared to them are solely expressed in terms of uncertainties. (it is obvious that "errors" are no subject in this field of work.) the only accuracy measure (quantification) the authors take into consideration (and relate to the title of the publication) is uncertainty. this is an encouraging indicator that according to present recognized metrologists, uncertainty must be at least part of the concept of accuracy. 5. qualitative or quantitative? note 1 to the vim definition of accuracy states "the concept ‘measurement accuracy’ is not a quantity and is not given a numerical quantity value.". this statement (being "not a quantity") obviously tempts a number of authors to make statements of accuracy being "qualitative", e.g. [2] and [8]. however, this is not what is said in the referred vim text and is probably also not the intention of the wording of the vim. there is a fine yet significant difference between being "not a quantity" and being "qualitative": accuracy is said to be "not a quantity" [1] and not being given "a quantity value". this, according to the understanding of the author, wants to express that – in in simple words – there is no globally accepted number scale and no unit to express accuracy to make accuracy a metrologically traceable or metrologically comparable property (= a quantity). this note statement should probably separate the concept of accuracy from concepts like "quantity" or "uncertainty", where there is a common, global understanding how to quantify these. however, there is clearly no statement and no hint anywhere in the vim definition that accuracy be qualitative! common usage of "accuracy" requests comparative statements like "more" or "less" accurate which indicates the necessity to give accuracy "some kind of number" (a value), at least in a given situation or for a given purpose. this gives room for a user to apply a purpose oriented algorithm to get and assign an indicative accuracy number which allows comparative statements in a given situation. 6. synthesis, proposal for a solution it appears that the concept "accuracy" as we find it today in the vim and in iso documents is a leftover of an ongoing, not yet completed, development. it is stated that "at one time" ([3], 0.6) the concept of accuracy, being perceived as one-dimensional (only related to measurement error), was amended by the concept of precision as additional information to take account of the possible dispersion of values as a second dimension. probably, the concept of uncertainty was not yet fully established at that time. however today, according to the vim, it is exactly measurement uncertainty that is "characterizing the dispersion of the quantity values being attributed to the measurand" ([1] 2.26), which is exactly what should be used if a "dispersion dimension" is to be considered in the concept of accuracy in addition to a "trueness dimension". in addition, it is historically obvious and intentional, that there is not one metrologically traceable and metrologically comparable ([1] 2.46) way of quantifying accuracy. yet, it is necessary in practice that accuracy can be quantified in order to compare or rank. these quantifications may be done using an algorithm (e.g. a mathematical equation) which may follow a purpose given by the specific situation. therefore, we propose the introduction of a new definition for “accuracy” which should take into account: • the modern understanding of a measurement result, consisting of a "measured quantity value and a measurement uncertainty" (vim 2.9, note 2), • backwards compatibility still allowing to combine accuracy from a combination of trueness and precision, • the possibility of applying the concept of accuracy to a single value and to a set of values and to a method and to a procedure, • clarifying that "not a quantity" does not mean "being qualitative", • the fact that there may be (various possible) ways of assigning values to accuracy for making comparative statements. this could be realized with the following wording which is submitted to further discussion and consideration: http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 418 accuracy (measurement accuracy, accuracy of measurement) term to describe the closeness of agreement between one or several measurement results and a true quantity value. accuracy consists of a combination of a trueness property and a dispersion property. any algorithm to combine these two to yield a quantification, may follow its intended purpose. quantifications of accuracy which origin from the same algorithm may offer comparability (statements like "more accurate" or "less accurate" are then possible). note 1: the trueness property is preferably the measurement error (vim 2.16) and the dispersion property is preferably the measurement uncertainty (vim 2.26). note 2: accuracy according to this definition can be applied, if the necessary information is available, to a set of measurement results, to a single measurement result and to a measurement method and to a measurement procedure. note 3: possible algorithms can be summations of e.g. measurement error and measurement uncertainty in quadrature or considering the absolute of measurement error and the quadrature of measurement uncertainty etc. also a two-dimensional dartboard model can used to illustrate the concept, see figure 1: figure 1: accuracy dart board model; x-axis: improving dispersion, y-axis: improving trueness the x-axis is labelled "improving dispersion", the y-axis is labelled "improving trueness". it is left to the user whether "dispersion" is substituted by "precision" or by "uncertainty". the definition encompasses both concepts, which is no problem due to the fact that according to the definition proposed above, accuracy does neither deliver mathematically unambiguous nor metrologically traceable or comparable figures. it is admitted that the above proposed definition is rather lengthy and it would be desirable that definitions were brief and clear without "notes". however, given the history of this concept and the ambiguities explained in this paper, it appears to be necessary to use more words to connect the new wording to its history and to clarify misconceptions. 7. summary in this paper, we have analysed current use of the concept "accuracy". as means of example, we investigated three normative documents in which accuracy is defined. we have detected various incompatibilities, even within the same document but also in the community using this concept. we tried to understand the historical development that might have affected the understanding of this concept and conclude with a synthesis proposal for a new definition that encompasses historical as well as modern concepts and thus offers full backwards-compatibility. 8. references [1] jcgm 200:2012: international vocabulary of metrology – basic and general concepts and associated terms (vim) 3rd edition 2008 version with minor corrections (including "vim definitions with informative annotations", last updated 29 april 2017), accessed 06/2020 [2] npl: the difference between accuracy & precision https://www.npl.co.uk/skills-learning/outreach/school-posters/npl-schools-poster-_accuracy-precision-v7-hr-nc.pdf (accessed 06/2020) [3] international organization for standardization iso 5725-1:1994 accuracy (trueness and precision) of measurement methods and results, geneva. switzerland [4] international organization for standardization iso iwa 15:2015 specification and method for the determination of performance of automated liquid handling systems, geneva. switzerland [5] jcgm 100:2008. evaluation of measurement data – guide to the expression of uncertainty in measurement (gum 1995 with minor corrections) [6] a possolo, s schlamminger et al. evaluation of the accuracy, consistency, and stability of measurements of the planck constant… metrologia 55 29 [7] recommendation of the consultative committee for mass and related quantities submitted to the international committee for weights and measures; recommendation g 1 (2013) on a new definition of the kilogram https://www.bipm.org/cc/ccm/allowed/14/31a_recommendation_ccm_g1(2013).pdf (accessed 06/2020) [8] nist engineering statistics handbook https://www.itl.nist.gov/div898/handbook/mpc/section1/mpc113.htm (accessed 01/2020) http://www.imeko.org/ https://www.npl.co.uk/skills-learning/outreach/school-posters/npl-schools-poster-_-accuracy-precision-v7-hr-nc.pdf https://www.npl.co.uk/skills-learning/outreach/school-posters/npl-schools-poster-_-accuracy-precision-v7-hr-nc.pdf https://www.npl.co.uk/skills-learning/outreach/school-posters/npl-schools-poster-_-accuracy-precision-v7-hr-nc.pdf https://www.bipm.org/cc/ccm/allowed/14/31a_recommendation_ccm_g1(2013).pdf https://www.bipm.org/cc/ccm/allowed/14/31a_recommendation_ccm_g1(2013).pdf https://www.bipm.org/cc/ccm/allowed/14/31a_recommendation_ccm_g1(2013).pdf https://www.itl.nist.gov/div898/handbook/mpc/section1/mpc113.htm https://www.itl.nist.gov/div898/handbook/mpc/section1/mpc113.htm introductory notes for the acta imeko special issue on the 24th imeko technical committee 4 international symposium and the 22nd international workshop on analogue-to-digital conversion and digital-to-analogue conversion modelling and testing acta imeko issn: 2221-870x june 2021, volume 10, number 2, 1 3 acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 1 introductory notes for the acta imeko special issue on the 24th imeko technical committee 4 international symposium and the 22nd international workshop on analogue-to-digital conversion and digital-to-analogue conversion modelling and testing giuseppe caravello1, ciro spataro1 1 università degli studi di palermo, viale delle scienze, 90128 palermo, italy section: editorial citation: giuseppe caravello; ciro spataro, introductory notes for the acta imeko special issue on the 24th imeko technical committee 4 international symposium and the 22nd international workshop on analogue-to-digital conversion and digital-to-analogue conversion modelling and testing, acta imeko, vol. 10, no. 2, article 1, june 2021, identifier: imeko-acta-10 (2021)-02-01 editor: francesco lamonaca, university of calabria, italy received may 25, 2021; in final form may 25, 2021; published june 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution and reproduction in any medium, provided the original author and source are credited. corresponding authors: giuseppe caravello, e-mail: giuseppe.caravello02@unipa.it ciro spataro, e-mail: ciro.spataro@unipa.it dear readers, measurement has always been a tool by which we can observe the world around us. this concept was once again confirmed during the 24th imeko technical committee 4 (tc4) international symposium, which showed how topics related to the world of measurement range across many fields of knowledge. the imeko tc4 international symposium is one of the most important events in the fields concerned with the theoretical and practical aspects of the measurement of electrical quantities and related instrumentation. it involves institutions and academia in a discussion of the state of the art and issues that require a joint approach by engineers, academics and other experts of measurement, instrumentation, testing and metrology. the 24th edition of the symposium was originally planned to be held in palermo, italy; however, due to the covid-19 emergency, the committee was forced to organise the event as a virtual conference. we do hope that, soon, there will be another chance to host you all in palermo. the virtual symposium was organised to make an online conference not so different from a live event. it was challenging to set up a web platform to enable live presentations, and we thank the organising team, who addressed this issue professionally. it was also challenging to pursue imeko tc4’s standard mission of creating an international platform where experts from academia and industry can consider the measurement of electrical quantities, emphasising both theoretical and practical aspects of research in the field. as in the last editions, the 2020 symposium covered a large number of engineering fields, from digitalisation to renewable energy and from acoustic and mechanical measurements to biomedical and chemical fields. the large space devoted to quantum metrology was a novelty this year. thanks to the help of many physicist colleagues, it was, in fact, possible to propose both a special session and a plenary talk on the subject. in this issue, quantum metrology is represented by martina marzano et al., who, in the paper ‘design and development of a coaxial cryogenic probe for precision measurements of the quantum hall effect in the ac regime’, describe and characterise a cryogenic probe able to perform very accurate measurements in the alternating current (ac) regime with impedance bridges. the characterisation results show that the probe can be usefully employed to reach the quantisation condition in hall devices, performing sensitive direct current (dc) measurements. three papers in the issue concern metrological characterisation. a first contribution, ‘metrological characterisation of current transformers calibration unit for accurate measurement’ by valentyn isaiev et al., proposes an approach to simulate the errors generated by current transformers with the aim of characterising the performances of the ac comparators commonly used to calibrate the transformers. the results show mailto:giuseppe.caravello02@unipa.it mailto:ciro.spataro@unipa.it acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 2 that the proposed approach can achieve several tenths of µa/a when calibrating a commercial transformer calibration unit under ordinary laboratory conditions. the paper by stefano sorti et al., ‘metrological characterisation of rotating-coil magnetometer systems’ characterises a rotating-coil magnetometer for the measurement of integral magnetic-field harmonics in accelerator magnets. the authors focus their attention on modelling the mechanical components of the device to predict the transducer response in both static and dynamic conditions. sioma baltianski, in the paper ‘bias-induced impedance effect of the current-carrying conductors’, presents previously unstudied properties of current-carrying conductors utilising impedance spectroscopy. the methodology is based on the superposition of test signals and bias affecting the objects under study. the work shows that the studied objects have an additional low-frequency impedance that can be either capacitive, inductive or both, depending on the current density and the properties of the material. as in the last editions, a high number of papers in the symposium concerned sensors and actuators. a special session was dedicated to the topic, which, in this special issue, is represented by five papers. a first contribution, by giovanni gugliandolo et al., ‘on the design and characterisation of a microwave microstrip resonator for gas sensing applications’, deals with relative humidity monitoring. the proposed solution is based on a one-port microwave gas transducer developed by coupling a microstrip resonator for electromagnetic wave propagation. the developed transducer can be applied to detect different target gases by selecting an appropriate sensing material tailored to the specific sensing application. the paper by federica vurchio et al., ‘comparative evaluation of three image analysis methods for angular displacement measurement in a mems microgripper prototype: a preliminary study’, compares measurements performed using different methods for the angular displacement of a comb drive in a microelectromechanical system (mems) gripper prototype for biomedical applications. the angular displacement was measured by means of two novel automatic procedures based on an image analysis method. the performances of the proposed procedures were compared with those of a semi-automatic method. the contribution by antonino quattrocchi et al., ‘pmmacoated fiber bragg grating sensor for measurement of ethanol in liquid solution: manufacturing and metrological evaluation’, explores the possibility of measuring the concentration of ethanol in aqueous solutions by using polymethyl methacrylate (pmma) as a coating material for a single-mode fibre bragg grating sensor. a prototype of this sensor was developed and compared to traditional sensors. the paper by lorenzo ciani et al., ‘design optimisation of a wireless sensor node using a temperature-based test plan’, deals with the design optimisation of a sensor node in a wireless mesh network under temperature stress. since there is not a specific standard for this kind of system, a customised test plan was developed in this work. tommaso addabbo et al., in the paper ‘solar energy harvesting for lorawan-based pervasive environmental monitoring’, propose the architecture of a self-powered lowpower wide-area network sensor node for the pervasive measurement of particulate matter concentrations in urban areas. to validate the effectiveness of the proposed solution, various field tests were carried out with the integrated environmental monitoring device. three contributions in the issue deal with biomedical measurement. imran ahmed et al., in the paper ‘iomt-based biomedical measurement systems for healthcare monitoring: a review’, present an extended overview of recent activities towards the development of internet of medical things (iomt)-based biomedical measurement systems for various healthcare applications. several approaches that are used in the development of these systems are presented and discussed, and metrological aspects related to accuracy, reliability and calibration requirements are considered. a second contribution, by giorgia fiori et al., ‘a preliminary study on an image analysis based method for lowest detectable signal measurements in pulsed wave doppler ultrasounds’, proposes and validates a novel image-analysis-based method for the estimation of the lowest detectable signal in the spectrogram of blood flow velocity. the paper by marius-vasile ursachianu et al., ‘experimental study on sar reduction from cell phones’, deals with the measurement of the specific absorption rate (sar) relative to the emissions generated by three different generations of mobile phone. the test results quantify the dependence of the measured values on the positioning of the antenna, the size of the device, the relative position to the human head and the presence of protective cases. in the palermo symposium, the topic of renewable energy was highlighted, and it is represented here by three papers. the first contribution, by marco balato et al., ‘buck-based dmppt emulator: a helpful experimental demonstration unit’, deals with the troublesome question of the efficiency reduction in mismatched photovoltaic systems. in particular, the paper describes the realisation and use of a buck-based distributed maximum power point tracking (dmppt) emulator able to fully understand the advantages offered by the dmppt approach in various operative conditions. alessio carullo et al., in the paper ‘an innovative correction method of wind speed for efficiency evaluation of wind turbines’, propose an innovative statistical method to evaluate the average efficiency of wind turbines by correcting the wind speed at the entrance of the rotor assessed by a nacelle anemometer. the measured values, in fact, were systematically lower than the actual ones. the proposed statistical approach, unlike those already presented in the literature, does not need data measured from a meteorological station but is based only on the power curve declared by the turbine manufacturer. the aim of the paper by davide aloisio et al., ‘comparison of machine learning techniques for soc and soh evaluation from impedance data of an aged lithium ion battery’, is the determination of the state of charge (soc) and the state of health (soh) of batteries. the proposed approach, which was applied on an aged lithium-ion battery, was based on different machine learning techniques, and the performances of these were compared. the topic of electromagnetic compatibility is treated by andrea mariscotti et al. their contribution, ‘review of models and measurement methods for compliance of electromagnetic emissions of electric machines and drives’, discusses the problem of electromagnetic compliance for electric machinery and power drives by introducing and reviewing the normative references for electromagnetic emissions, the available modelling approaches and their accuracy and the measurement methods. the influence acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 3 of the setup, environment and behaviour of various types of machines is also considered. energy monitoring is the subject of the paper by barbara cannas et al., ‘nilm techniques applied to a real-time monitoring system of the electricity consumption’. the work presents a low-frequency non-intrusive load monitoring (nilm) system suitable for a typical domestic user. the system is able to disaggregate and keep track of a device’s consumption by analysing low frequency aggregate data. andrea mariscotti, in the paper ‘power quality metrics for dc grids with pulsed power loads’, analyses the interaction between pulsed power loads and a dc grid and then focuses on metrics to quantitatively describe such interactions and on the impact on dc grid operation and power quality. the contribution by jakub svatos et al., ‘system for an acoustic detection, localisation and classification’, presents a system able to detect, localise and classify gunshots. the system consists of sensor units that continuously monitor acoustic events and an advanced digital signal processing technique that analyses the acoustic data. lastly, among the papers of the 22nd imeko international workshop on analogue-to-digital conversion (adc) and digital-to-analogue conversion (dac) modelling and testing, the paper by tomasz kowalski et al., ‘design, characterisation, and digital linearisation of an adc analogue front-end for gamma spectroscopy measurements’, deals with the design, characterisation and performance assessment of an analogue front end for use in the polish national centre for nuclear research’s gamma spectrometry system. in-field experiments involving actual spectrometry measurements validated the designed front end. we would like to conclude these brief introductory notes by thanking, first, the authors for their interesting and high-quality papers and the reviewers for their indispensable and professional contribution. moreover, we would like to thank our past editor in chief, dušan agrež, who started the production of this special issue with us, our current editor in chief, francesco lamonaca, who has undertaken his new assignment with the same professionalism and competence as his predecessors, and the imeko tc4 chairperson, alexandru sălceanu, for his invaluable support and advice. it was a great honour for us to act as guest editors, and we hope that the readers will find this issue of acta imeko useful and inspiring. giuseppe caravello and ciro spataro guest editors https://www.google.it/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahukewjqnugohelwahxgwaihhzkwbsaqfnoecayqaa&url=https%3a%2f%2fwww.apavital.ro%2fuploads%2fpages%2f383292%2fdeclaratie_de_avere_-_2019_-_salceanu%2520alexandru.pdf&usg=aovvaw3cgrljkj_djc2ujyoywzql https://www.google.it/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahukewjqnugohelwahxgwaihhzkwbsaqfnoecayqaa&url=https%3a%2f%2fwww.apavital.ro%2fuploads%2fpages%2f383292%2fdeclaratie_de_avere_-_2019_-_salceanu%2520alexandru.pdf&usg=aovvaw3cgrljkj_djc2ujyoywzql towards the development of a cyber-physical measurement system (cpms): case study of a bioinspired soft growing robot for remote measurement and monitoring applications acta imeko issn: 2221-870x june 2021, volume 10, number 2, 104 110 acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 104 towards the development of a cyber-physical measurement system (cpms): case study of a bioinspired soft growing robot for remote measurement and monitoring applications stanislao grazioso1, annarita tedesco2, mario selvaggio3, stefano debei4, sebastiano chiodini4 1 department of industrial engineering, university of naples federico ii, naples, italy 2 ims laboratory, university of bordeaux, bordeaux, france 3 department of electrical engineering and information technology, university of naples federico ii, naples, italy 4 department of industrial engineering, university of padova, padova, italy section: research paper keywords: 4.0 era; soft growing robots; remote monitoring; monitoring systems; remote sensing citation: stanislao grazioso, annarita tedesco, mario selvaggio, stefano debei, sebastiano chiodini, towards the development of a cyber-physical measurement system (cpms): case study of a bioinspired soft growing robot for remote measurement and monitoring applications, acta imeko, vol. 10, no. 2, article 15, june 2021, identifier: imeko-acta-10 (2021)-02-15 section editor: francesco lamonaca, university of calabria, italy received may 4, 2021; in final form may 11, 2021; published june 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: stanislao grazioso, e-mail: stanislao.grazioso@unina.it 1. introduction the 4.0 era is characterized by an innovative multidisciplinary approach which addresses technical challenges by seeking transverse solutions to both technological and methodological problems [1]-[8]. the most effective expression of the 4.0 paradigm is represented by cyber-physical systems (cpss), i.e., smart systems that include engineered interacting networks of physical and computational components, able to monitor and control the physical environment [9]-[14]. from this definition, it appears that measurement and monitoring systems (mmss) are essential for the implementation of cpss. however, mmss are generally considered as subordinate elements of a cps: they provide source of information for the cps (i.e., connection to the physical world) but do not participate in any higher-level actions of the cps (i.e., conversion, cyber, cognition, configuration) [15]. mmss are historically seen as responsible for sensing the conditions from the physical environment, rather than as providers of higher-level intelligent information. however, the suitable adoption of the enabling technologies can reshape the role of mmss in the 4.0 era. this requires a fundamental change of perspective, in which the enabling technologies stop being employed as external superstructures for mmss, and become embedded solutions, intrinsically present in the architecture of the mms, and fully effective through an adequate metrological configuration. this approach emphasizes the holistic nature of monitoring and paves the way for 4.0 transition-driven monitoring systems. through the wise adoption of the 4.0 enabling technologies, mmss can turn into self-aware, abstract the most effective expression of the 4.0 era is represented by cyber-physical systems (cpss). historically, measurement and monitoring systems (mmss) have been an essential part of cpss; however, by introducing the 4.0 enabling technologies into mmss, a mms can evolve into a cyber-physical measurement system (cpms). starting from this consideration, this work reports a preliminary case study of a cpms, namely an innovative bioinspired robotic platform that can be used for measurement and monitoring applications in confined and constrained environments. the innovative system is a “soft growing” robot that can access a remote site through controlled lengthening and steering of its body via a pneumatic actuation mechanism. the system can be endowed with different sensors at the tip, or along its body, to enable remote measurement and monitoring tasks; as a result, the robot can be employed to effectively deploy sensors in remote locations. in this work, a digital twin of the system is developed for simulation of a practical measurement scenario. the ultimate goal is to achieve a self-adapting, fully/partially autonomous system for remote monitoring operations to be used reliably and safely for the inspection of unknown and/or constrained environments. mailto:stanislao.grazioso@unina.it acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 105 self-conscious, self-maintained entities, able to generate highly valued insights, just like cpss. this means that mmss can evolve into cyber-physical measurement systems (cpmss), thus becoming a proactive expression of the 4.0 era, strengthening not only the role of measurement, but the performance of the overall 4.0 ecosystem. starting from these considerations, this work introduces a definition for the cpmss and presents a preliminary case study, i.e., a “soft growing” robot for measurement and monitoring applications in constrained and confined remote environments. this system is a concrete example of cpms for its ability to sense itself within the environment (i.e., self-awareness), to adapt itself to the surrounding environment thanks to its softness and growth and steering capabilities (i.e., self-configure), and to autonomously predict navigation path towards the inspection target (i.e., selfpredict). embedding multiple 4.0 enabling technologies in one measurement and monitoring system represents a promising solution for achieving the cpms capabilities. this work is an extension of our previous conference paper [16], where the general idea was preliminarily introduced. this paper is organized as follows. in section 2, the state-of-the-art of robotic technologies is presented, with focus on measurement and monitoring remote applications in difficult-to-reach environments. then, in section 3, the definition for the cpmss is introduced. section 4 relates to the design and implementation of the soft growing robot and shows a simulated measurement and monitoring scenario of the cpms in a remote location. finally, in section 6, conclusions are drawn, and the future work is outlined. 2. background as well known, robotic technologies are largely used for carrying out remote measurement tasks, especially in environments that are hazardous, dangerous or difficult to reach for humans [17], [18]. nevertheless, there are several applications in which traditional rigid-bodied robot technologies cannot be used. this occurs, for example, when measurements must be carried out in confined, constrained, or unknown environments (e.g., inspection of difficult-to-reach industrial environments, exploration of archaeological sites which are often inaccessible and fragile). one technological solution suitable to perform this kind of tasks is represented by soft continuum robots [19], i.e. robots with a continuously deformable mechanical structure, whose design is inspired from the principles of shaping, movements, sensing and control of soft biological systems [20]. in the literature, there are several examples of soft continuum robots for remote measurement applications, in different fields: space, airlines, nuclear, marine (inspection and maintenance), medical (minimally invasive surgery), and so on [21]. one limitation of soft continuum robots is their limited workspace, as they usually have a fixed base and a pre-established length; this can be a problem for tasks that require inspection and exploration of large environments. to overcome this issue, soft continuum robots can be endowed with locomotion capabilities, by using tethered/untethered fluidic or cable-driven actuators, taking inspiration from the animal movements (snake, earthworms, caterpillars) [22]-[24]. however, this solution involves a relative movement between the robot and the environment, which can lead to low energy efficiency of the robot when high sliding friction is present. a recent design solution of soft continuum robots achieves enhanced mobility through growth rather than locomotion, taking inspiration from the growing process of plants and vines [25]. these robots, referred to as soft growing robots, achieve mobility by everting new material at the tip: this enables lengthening without relative movements between the robot’s body and the environment. with soft growing robots, the inspection and exploration length of remote environments is therefore limited only by the amount of robot’s body material that can be transported on the field. although different mechanisms have been used to enable this form of apical extension [26], recently, pneumatically-driven solutions have achieved great potentials [27]. in these systems, the growing process is implemented by pressurizing a fluid (typically, air) inside a chamber created by a self-folded cylindrical body, which unfolds by everting new material from the tip; this enables the forward growth of the robot's body through lengthening. while growing, the robot's body can be curved/steered by additional actuators distributed along its body (e.g., pneumatic artificial muscles [28], artificial tendons, etc.). the contraction of these additional actuators on one side causes bending (or kinking) along that direction. an example of the growing and steering processes of soft growing robots is shown in figure 1. 3. cyber-physical measurement system the concept of cpms is built on the 5c architecture [10] used for cps, as shown in figure 2. it consists of the following five levels: 1. connection layer: connected with the physical world where the measurements are collected, and the sensing is performed. 2. conversion layer: responsible for very first processing for endowing the system with self-awareness capabilities (i.e., reconstruction of its internal state). 3. cyber layer: responsible for the development of the digital twin model of the measurement system and for figure 1. example of the concept of a soft growing robot that achieves enhanced mobility through growth (top image). concept of growth by eversion of material rolled onto a spool (central image); and the concept of curving by pressurization, with contraction (bottom image) of soft actuators placed and sealed on the main body. in the example, a camera is placed on the tip of the robot. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 106 endowing the system with self-comparison capabilities (i.e., self-awareness within the network). 4. cognition layer: responsible for cognition and reasoning, i.e., high-level models and algorithms to endow the measurement system with decision support capabilities. 5. configuration layer: starting from the knowledge generated by the cognitive level, this layer generates corrective actions, as adaptation, and reactiveness to the environments. a cpms can be defined as a novel form of mms which, in addition to collecting data from the physical world, is able to provide higher-level information thanks to the use of suitable models and 4.0 enabling technologies. similarly to a cps, a cpms has knowledge of its state in time and space (self-awareness) and with respect to other systems in the network (self-comparison); it is capable of enforcing actions for its own maintenance (selfmaintain), predicting its own evolution in time and space (selfpredict), and adapting to the environment (self-configure). drawing a comparison with the master-slave architecture, historically mmss represented the slaves (mostly dedicated to data collection) of a master (i.e., the cps itself). by evolving into cpmss, instead, mmss become cpss among cpss; eventually, the current master-slave relationship between mmss and cpss turns into a peer-to-peer cooperation. 4. case study: the soft growing robot this section addresses the design and implementation of the soft growing robot, as a preliminary example of cpms to be used for applications within confined and constrained remote environments. first, the design requirements of the system are presented. successively, its major components (i.e., the robotic platform and the electronic control unit) are described in detail. finally, an example of a practical measurement scenario through a simulated digital twin of the system is presented. the architecture of the soft growing robot is shown in figure 3. 4.1. requirements the requirements of the soft growing robot are the following: • access within environments with small cross sections (with minimum dimension equal to 100 mm); • high inspection/exploration length (up to 10 m) while maintaining portability; • controllable growth; • steering/curving capability; and • human situation awareness. the long-term goal is to develop the first soft growing robot endowed with model-based strategies [29] for planning [30], control and navigation to accomplish remote measurement tasks. these requirements give the possibility to develop a generalpurpose platform for inspection and exploration usable in a wide range of scenarios. 4.2. robotic platform the robotic platform is mainly constituted by the robot base (where the body material is stored) and the robot body (which grows and accesses the remote environment). the cad model of the robotic platform is shown in figure 4. the robot base is the container of the unfolded robot body and represents the pressurized vessel when the robot is in figure 2. description of a cpms according to the 5c architecture. figure 3. architecture of the proposed soft growing robotic system (dimensions not in scale). acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 107 operation. it is formed by an acrylic cylinder with two end caps (qc-108 qwik cap, fernco inc., davison, mi). the spool for rolling out the robot material is driven by a dc motor (6408k27, mcmaster-carr inc., douglasville, ga) with a magnetic encoder which allows growing/retracting the robot body and measuring its current length. the vessel of the robot base is a cylinder with diameter d equal to 25 cm and height h equal to 50 cm. the main body of the soft growing robot is made of an airtight tube which is flexible but not stretchable. during the eversion process, the material should slide relative to itself with negligible friction; and should guarantee major durability for field applications. to this end, a double side silicon-coated ripstop nylon (rockywoods fabric llc, loveland, co) is used as a material for the main body of the robot. the soft robot body is rolled up and fixed from one hand around a spool inside the base vessel: when pressurized, the material is pushed outside the robot base through an opening and everts from the tip of the robot. when fully extended, the robot body achieves maximum dimensions corresponding to a diameter p of 10 cm and a maximum length l of 10 m. the forward growth is controllable by finding a suitable balance between the desired air pressure inside the vessel and the desired spool rotation, and thus, motor angular velocity about the axis of the spool. for guaranteeing a reversible steering/curving of the robot body, soft pneumatic actuators (made of the same material of the robot’s body) are placed along the entire length of the robot [28]: the steering/curving control is guaranteed by a suitable pressurization of these additional actuators, considering models of curvature/deformations of the robot’s shape [31]. during the retraction process of the robot’s body, to avoid structural kinking, suitable mechanisms to drive the retraction should be foreseen [32]. 4.3. electronic control unit the electronic control unit is composed by two sub-systems: one for generating the desired air pressure for pressurization of the vessel, and one for generating the desired voltage for the dc motor for growth/retraction of the robot body. the pneumatic circuit regulates the air pressure by pulse-width modulation (pwm), which involves the controlled timing of the opening and closing of solenoid valves (sy114-5lou, smc) through a mosfet board (based on irf540 mosfets, stmicroelectronics), with pressure sensors (asdxavx100pgaa5, honeywell) providing feedback. the pneumatic circuit is an essential component of the robot, as it is responsible for the growth process (one pneumatic tube for the main tube of the robot body) and the steering of the robot body (one pneumatic tube for each of the serial soft actuators placed along the robot body). this pwm-based pneumatic homemade circuit substitutes more expensive solutions based on professional closed-loop pressure regulators. the prototype of the developed electronic control unit (related to the compressed air circuit) is shown in figure 5. 4.4. example of a practical measurement scenario the practical measurement scenario consists of a human operator performing visual inspection in a remote site through the proposed soft growing robot, endowed with a tip-mounted camera. an input device is used by the human operator to impart growing/steering commands. a digital twin of the measurement system is built in v-rep including the model of the robot (modelled as a growing constant curvature robot [33]), the model of the cameras as well as the model of the environment. the constant curvature assumption is reasonable when artificial pneumatic muscles are used to steer the robot, as shown in [30]. snapshots of the measurement scenario are shown in figure 6, where we can see the remotely operated soft growing robot approaching and inspecting a target (red box) within the simulated remote site. the soft growing robotic platform represents a novel technology for the inspection of confined and constrained remote environments that are not accessible by current technologies. furthermore, it represents a suitable platform for enabling measurement applications in large, gps-denied environments. in addition to industrial inspections, this robotic platform may be used for exploration purposes or even for search and rescue applications after accidents, earthquakes, and collapses of buildings. finally, an additional application is the sensor delivery in difficult-to-reach remote sites. figure 4. cad representation of the robotic platform. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 108 5. conclusion and future work in this work, a definition for cpms was introduced and a soft growing robot was presented as a case study. a cpms can be considered as a 4.0-oriented evolution of traditional mms; as a matter of fact, thanks to the adoption of 4.0 enabling technologies, a mms is not only seen as a system for collecting data, but also for data processing and interpretation. the soft growing robot as a cpms is intended to be used for remote measurement and monitoring applications in constrained and confined environments. the proposed system consists of a robot, to be placed outside the remote site, and a soft body that accesses the site through growth, with the possibility of controlling length and steering. at the tip of the system, a sensor can be placed to enable remote measurement tasks, or a sensor can be deployed through the robot body when the target is reached. in this work, we have considered a tip-mounted camera in the system. the benefits of using soft growing robots for remote measurement and monitoring applications are access within small-scaled sections; high inspection/exploration length; transportability; controllable growth and steering; safe interaction with the environment; and ability to perform sensor deployment. to get closer to a full definition of cpms, future work will be dedicated to embedding additional 4.0 enabling technologies (e.g., artificial intelligence algorithms) in the cmps, for endowing monitoring system with autonomous planning/navigation capabilities. also, effort will be dedicated to motion analysis and control in highly constrained situations. additionally, a set of practical applications cases will be identified, to experiment the system and assess its metrological performance. finally, suitable sensing technologies and processing strategies will be developed to enhance the metrological performance of the system (e.g., in terms of resolution, reliability and accuracy of interaction with the environment). the ultimate goal is to achieve a self-adapting, fully autonomous system for remote monitoring operations to be used reliably and safely for the inspection of unknown and/or constrained and confined environments. figure 5. picture of the electronic control unit (pneumatic board). figure 6. example scenario for remote inspection 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https://doi.org/10.1089/soro.2018.0047 acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 110 [30] m. selvaggio, l. ramirez, n. naclerio, b. siciliano, e. hawkes, an obstacle-interaction planning method for navigation of actuated vine robots, in proc. of 2020 ieee international conference on robotics and automation (icra), paris, france, may 31aug. 31, 2020, pp. 3227–3233. doi: 10.1109/icra40945.2020.9196587 [31] s. grazioso, g. di gironimo, b. siciliano, from differential geometry of curves to helical kinematics of continuum robots using exponential mapping, in proc. of international symposium on advances in robot kinematics, bologna, italy, 01-05 july 2018, pp. 319-326. doi: 10.1007/978-3-319-93188-3_37 [32] m. m. coad, r. p. thomasson, l. h. blumenschein, n. s. usevitch, e. w. hawkes, a. m. okamura, retraction of soft growing robots without buckling, ieee robotics and automation letters 5(2) (2020), pp. 2115-2122. doi: 10.1109/lra.2020.2970629 [33] s. grazioso, g. di gironimo, b. siciliano, analytic solutions for the static equilibrium configurations of externally loaded cantilever soft robotic arms, in proc. of 2018 ieee international conference on soft robotics(robosoft), livorno, italy, 24-28 april 2018, pp. 140-145. doi: 10.1109/robosoft.2018.8404910 https://doi.org/10.1109/icra40945.2020.9196587 https://doi.org/10.1007/978-3-319-93188-3_37 https://doi.org/10.1109/lra.2020.2970629 https://doi.org/10.1109/robosoft.2018.8404910 validation of a measurement procedure for the assessment of the safety of buildings in urgent technical rescue operations acta imeko issn: 2221-870x december 2021, volume 10, number 4, 140 146 acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 140 validation of a measurement procedure for the assessment of the safety of buildings in urgent technical rescue operations maria alicandro1, giulio d’emilia2, donatella dominici1, antonella gaspari3, stefano marsella4, marcello marzoli4, emanuela natale2, sara zollini1 1 diceaa, università dell’aquila, via g. gronchi 18, 67100 l’aquila, italy 2 diiie, università dell’aquila, via g. gronchi 1867100, l’aquila, italy 3 dmmm, politecnico di bari, via orabona 4, 70125 bari, italy 4 dipartimento dei vigili del fuoco del soccorso pubblico e della difesa civile, ministero dell’interno, piazzale viminale 1, 00184 roma, italy section: research paper keywords: validation; measurement uncertainty; calibration; total station; building monitoring; technical rescue citation: maria alicandro, giulio d’emilia, donatella dominici, antonella gaspari, stefano marsella, marcello marzoli, emanuela natale, sara zollini, validation of a measurement procedure for the assessment of the safety of buildings in urgent technical rescue operations, acta imeko, vol. 10, no. 4, article 23, december 2021, identifier: imeko-acta-10 (2021)-04-23 section editor: roberto montanini, università di messina and alfredo cigada, politecnico di milano, italy received july 26, 2021; in final form december 6, 2021; published december 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: emanuela natale, e-mail: emanuela.natale@univaq.it 1. introduction seismic-damage prevention is one of the main goals of researchers in the management of historical buildings, and several authors have dealt with the appraisal and the inventory of the building’s heritage [1]-[7]. a particular application concerns the evaluations required during urgent technical rescue operations. besides the primary target to search and rescue survivors, soon after an earthquake most resources are spent to evaluate the level of damages suffered by buildings and infrastructures, both in the immediate aftermaths of the event, to support the logistics of the rescue itself, to assess the level of safety of the rescue operations and the road network, and in the following phases, to implement provisional measures able to secure buildings and, in particular, the cultural heritage [8]-[12]. such assessment is quite challenging and critical, both from the technical and the logistic point of view, due to the high number of buildings (up to thousands), good part of which could be listed as cultural heritage. moreover, recent earthquakes were followed by 1-3 months aftershocks with similar intensity, causing a waste of most of the spent effort, which imposed to repeat the assessments anew. up to now, in italy as well as worldwide, this task has been carried out thanks to the expertise of fire fighters or other technicians, who had to assess the residual safety of the buildings on the only basis of a visual inspection. such assessment is obviously subjective, even if carried out by applying severe operational procedures, which foresee the analysis of the damages against well-defined schemas. abstract this work would like to provide a preliminary contribution to the draft of standard procedures for the adoption of total stations by rescuers in emergency situations, so as to offer a reliable and effective support to their assessment activities. in particular, some considerations will be made regarding the effect of the number and positioning of monitoring points on the tilt determination of a building façade, in order to set up simplified procedures, which are quick and easy to implement in emergency situations, at the same time guaranteeing the reliability of the results. two types of building will be taken into account as test cases, which have different characteristics in terms of height, distance and angle with respect to the total station. some considerations will be made about the aspects to be explored in future work, for the calibration of the method as a whole and the definition of all the steps of a procedure for the evaluation of the safety of a building. mailto:emanuela.natale@univaq.it acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 141 to improve the efficiency and reduce the subjectivity of such assessments, the authors propose to employ a dedicated survey system designed to support rescue operations and the implementation of provisional measures. such system is now possible, thanks to the recent availability of dedicated, userfriendly human-machine interfaces, able to hide the complexity of the employed tools, while maintaining the scientific value of the retrieved data. in fact, such user-friendly interfaces make them accessible by untrained first responders, which are employed on the field in the first phases of the rescue operations [8]-[12]. systems and tools make less subjective the assessment of building residual safety, and, in particular, could decisively improve the following: • fast execution of accurate surveys of damaged buildings, so as to reduce the exposure risk of rescuers in the first phase of the emergency; • detailed design of provisional measures; • quantitative monitoring of building damage evolution over time; • quantitative monitoring of provisional measures evolution over time, to assess their residual efficacy, in particular following aftershocks. available technologies offer several instruments able to reach the aim. e.g., most common methods to monitor and assess cultural heritage building damage are based on satellite systems, photogrammetry, laser scanning, infrared thermography [13][17]. 3d reconstruction techniques of buildings, based on unmanned aerial vehicle (uav) tilt photography, have the advantages of multi-angle and three-dimensional, but they are very time-consuming and high levels of accuracy are not easy to achieve [18]-[21]. approaches are proposed, for identifying intact and collapsed buildings via multi-scale morphological profiles with multi-structuring elements from post-earthquake satellite imagery, or using synthetic aperture radar (sar) techniques; however, these methodologies do not examine in detail the structural characteristics of individual buildings [22]-[24]. total stations are remote sensing tools, which can be easily used both indoor and outdoor, thanks to the easy installation and real-time output, rain and wind-proof, wide range of operating temperatures and insensitivity to light conditions. as for total stations used without reflective targets, point distance is measured remotely, at safe distance from the target building, so that surveys are completely not-invasive and safe for the operators. as such, they consent to reach a good balance between precision and data quality, easy-to-use, cost, easy to realtime processing. there are studies in the literature that refer to indoor calibration of total stations [25]. to obtain reliable outcomes, it is crucial to standardise procedures which take into proper account the constraints imposed by the deployment on the field in emergency situations. in fact, such environment implies numerous operational issues, due to fast-evolving scenarios, with unpredictable intrusions by rescuers and vehicles. in particular: • the impossibility to measure some target points, especially the lowest, due to the interposition of obstacles (rescuers and rescue vehicles) between instrument and target; • the need to deploy as fast as possible, and to survey the minimal number of target points, needed to obtain outcomes with sufficient accuracy; • the possibility of accidental or voluntary movements of the instruments (e.g., when caterpillars have to move in between), for which it is necessary to define stable reference points, in order to correctly acquire series of data carried out from different positions. in the process of simplifying and speeding up procedures for using instrumentation in specific applications, validation techniques are needed to ensure the reliability of the results [26] [29]. in this paper the authors, as a preliminary contribution, will report their considerations regarding the impact on the tilt determination of a building façade, of the number and positioning of reference and monitoring points. this work would like to contribute to the draft of standard procedures for the adoption of total stations by rescuers in emergency situations, so as to offer a reliable and effective support to their assessment activities. section 2 will describe the instrumentation used for these tests and the lay-out of the site; the position of the measuring points is also discussed and the measuring set-up able to reduce step by step the monitoring points on which data processing is carried out. in section 3, the results are presented and analysed with reference to procedure simplification purposes. conclusions and future work will end the paper. 2. materials and methods the test area is located in fossa, a small village next to l’aquila, in central italy (figure 1), hits by 2009 earthquake [15]. the area is a square surrounded by three buildings and a tower. reference targets have been materialised on two buildings, while monitoring points have been located on the other buildings, the “house” and the “tower” in figure 2. these buildings have been chosen as test cases for the analyses, as they have different characteristics from the point of view of height, that implies different distance and inclination angle of the highest points with respect to the position of the measurement system, which is more or less halfway between the two buildings. the safer system has been used to perform the measurements. the safer system has been designed to estimate and monitoring any critical movements of structures and civil works. the operating modes and functions have been developed to make the system as suitable as possible for the emergency activities of the fire fighters. it is composed by (figure 3): • a total station leica geosystems (tps), model ts16 imaging, tool used extensively in monitoring activities by measuring azimuthal and zenithal angles and distances from the instrumental centre to the measured point with high precision. these measures (polar figure 1. test area location [image credit: google earth]. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 142 coordinates) are transformed by the system into cartesian coordinates (x, y, z); • software safer, in order to manage the total station; • a tablet pc, with the software installed on it, able to communicate with the leica total station (tps) both by wireless and with dedicated cabling. by measuring the 3d coordinates, it is possible to carry out monitoring and control activities of slow or relatively fast movements over time both in external and internal environments, during day and night. all measurements are carried out automatically in order to exclude any errors by the operators. the measurement results are immediately processed in the field and displayed in graphical and tabular form, helping the operator to fast interpretate the phenomenon in progress and make immediate decisions. the safer system is designed to provide spatial information (3d coordinates) of punctual points and the operational scene is schematised in figure 4. the points on the building considered to be stable are called reference points while the ones on the building to be monitored are called monitoring points. the safer system is positioned at the centre in order to have maximum visibility. all the preliminary operations, required by the standard procedures for using this kind of equipment (fixing the tripod to the ground, levelling), have been correctly carried out. the measurement by the total station can be carried out in two different ways, by using: infrared ray laser beam. surveying by infrared ray needs a reflective target, the prism. using the prism allows more accurate measurements and guarantees the achievement of greater distances. the survey by laser beam is adopted to detect points that are difficult to access by the operator, who must physically place the prisms. in this case, in fact, it is not necessary to adopt any external reflecting system. reference points have to be defined to have a stable reference, and they must be fixed according to an optimal geometry (not too close or aligned) and on fixed points, not susceptible to movement: these aspects will be taken into account in the procedure definition. seven reference points have been placed in independent positions with respect to the monitored buildings. at the reference points, prisms are used; at the monitoring points, white/black targets (figure 5) are positioned using epoxy resins, to have reference positions, easily identifiable, on which to impose, in perspective, known displacements for the evaluation of the metrological characteristics of the instrument and to obtain useful information for the optimization of the procedure. the latter targets have reflectivity characteristics similar to those of a common building wall. a coordinate system has been defined with reference to the first building, the house, in such a way that the x-axis direction is obtained by projecting on the horizontal plane the points taken on the façade, the z-axis is vertical and its origin is on the horizontal plane at the height of the instrument, the y-axis enters the wall of the house. figure 6 shows the point clouds acquired on the façades of the two buildings and the defined reference systems. for each monitoring point, 20 repeated measurements have been carried out. the monitoring points have been named as indicated in figure 7. on the basis of the acquired measurements, after elimination of outliers, the following analysis are carried out, using the matlab software: a) b) figure 2. monitored buildings: a) house; b) tower. figure 3. safer system used for the analysis. figure 4. safer system operational scene [30]. a) b) figure 5. targets used for monitoring and reference points, respectively: a) black/white target; b) prism. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 143 1. standard deviations of the measured values of the coordinates of the monitoring points are calculated, on the basis of 20 repeated measurements. 2. the selected points (ml 1-12 for the house, ml 14-19, for the tower) are processed by means of a least squares regression (first degree polynomial model) and the inclination angles of the façades are determined with respect to the horizontal plane. these values are considered as a reference, with respect to which to evaluate other configurations of the monitoring points. 3. the angle of inclination is calculated with reference to the configurations described in figure 8 and figure 9, that is: excluding points along vertical lines from the analysis (configurations b, c, d and e for house; configurations b’ and c’ for tower) excluding the points along the lowest line, only in the case of house (configuration f) considering only extreme points (configuration g and h for house; configuration d’ for tower). 3. results figure 10 and figure 11 show the calculated standard deviations for the monitoring points on the house and the tower: these values do not exceed 5 mm. the selected points (ml 1-12 for the house, ml 14-19, for the tower) are processed by means of a least squares regression, according to a first degree polynomial model, and the inclination angles of the façades are determined with respect to the horizontal plane. the obtained results, in terms of inclination angles and signs of the direction cosines (cos(ry) and cos(rz)) of the normal to the plane, are summarized in table 1. the sign of the direction cosines indicates if the façade is inclined towards the outside or inside of the building: taking into account figure 6, in the case of the house, if the signs of cos(rz) and cos(ry) are concordant, the façade is inclined toward the outside, if discordant toward the inside; in the case of tower, on the contrary, if the signs are concordant, the façade is inclined toward the inside. the least squares regression has been also performed for the configurations of figure 8 and figure 9, and the inclination differences with respect to the reference case have been calculated (figure 12 and figure 13). furthermore, the displacement in the horizontal direction of the highest point in both the two buildings has been evaluated (figure 14 and figure 15). the following observations can be made: • for the house, variations in the angle of inclination up to 0.18°, compared to the reference case, may result, by changing the number of points chosen for processing; the displacements evaluated in the horizontal direction, at the height of the highest point, can reach about 20 mm. • for the tower, variations in the angle of inclination up to 0.062°, compared to the reference case, may result, by changing the number of points chosen for processing; the displacements evaluated in the horizontal direction, at the height of the highest point, can reach about 25 mm. • the results show, for both buildings, that the acquisition of only the extreme monitoring points allows to obtain results comparable to those of the reference case. these results appear promising from the point of view of the possibility of simplification, also considering that the standard deviation of the measurements is lower with respect to the calculated displacements. • it must be noticed that the standard deviation of the measurements is greater in some areas of the façade of the house, and this does not seem to be directly related to parameters such as distance and angle of positioning of the total station with respect to the building. this aspect will need to be explored in future work. figure 6. point clouds acquired on the façades of the buildings: in red the points on the house; in blue the points on the tower. a) b) figure 7. identification codes of the monitoring points for: a) house; b) tower. table 1. results of the least squares regression. house tower inclination angle 89.99 ° 89.93 ° signs of cos(ry) and cos(rz) concordant concordant acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 144 a) b) c) d) e) f) g) h) figure 8. configurations of measuring point for the house: a) “ref” (reference); b) “config 2”; c) “config 3”; d) “config 4”; e) “config 5”; f) “config 6”; g) “config 7”; h) “config 8”. a) b) c) d) figure 9. configurations of measuring point for the tower: a’) “ref” (reference); b’) “config 2’”; c’) “config 3’”; d’) “config 4’”. figure 10. standard deviation of the coordinates of the monitoring points for the house. figure 11. standard deviation of the coordinates of the monitoring points for the tower. figure 12. inclination difference with respect to the reference for the house. figure 13. inclination difference with respect to the reference for the tower. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 145 4. conclusions in this paper some considerations have been made regarding the effect on the tilt determination of a building façade of the number and positioning of the monitoring points. the study has been conducted considering two buildings with different characteristics as test cases. the results show, for both buildings, that the acquisition of only part of the monitoring points can allow to obtain results comparable to those of the reference case. these results appear promising from the point of view of the possibility of simplification, also considering that the standard deviation of the measurements do not exceed 5 mm, and it is lower than the calculated displacements (20 mm – 25 mm). in future work monitoring targets will be subject to known displacements to calibrate the method on the whole. furthermore, the causes of increased standard deviation of the measurements carried out in specific areas will be investigated. acknowledgments work carried out with the co-financing of the storm research and development project, funded by the 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online [accessed 14 december 2021] https://leica-geosystems.com/ https://doi.org/10.21014/acta_imeko.v10i1.847 https://doi.org/10.1088/1755-1315/783/1/012081 https://doi.org/10.3390/rs12162621 https://doi.org/10.5194/isprs-annals-v-5-2020-61-2020 https://doi.org/10.1080/10903127.2018.1528323 https://doi.org/10.1080/19475705.2020.1818637 https://doi.org/10.1109/access.2020.3007255 https://doi.org/10.5194/isprs-archives-xliii-b1-2020-587-2020 https://doi.org/10.21014/acta_imeko.v4i4.239 https://doi.org/10.21014/acta_imeko.v4i4.239 https://doi.org/10.1016/j.measurement.2010.08.016 https://doi.org/10.1109/i2mtc50364.2021.9459924 https://doi.org/10.1016/j.measurement.2017.12.034 https://doi.org/10.21014/acta_imeko.v4i4.239 https://leica-geosystems.com/ image analysis for the sorting of brick and masonry waste using machine learning methods acta imeko issn: 2221-870x june 2023, volume 12, number 2, 1 5 acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 1 image analysis for the sorting of brick and masonry waste using machine learning methods elske linß1, jurij walz1, carsten könke1 1 materialforschungsund -prüfanstalt at the bauhaus-university of weimar (mfpa), coudraystraße 9, 99423 weimar, germany section: research paper keywords: optical sorting of building material; masonry waste; image analysis; classification; machine learning citation: elske linß, jurij walz, carsten könke, image analysis for the sorting of brick and masonry waste using machine learning methods, acta imeko, vol. 12, no. 2, article 15, june 2023, identifier: imeko-acta-12 (2023)-02-15 section editor: eric benoit, université savoie mont blanc, france received july 8, 2022; in final form february 27, 2023; published june 2023 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work is supporting by the tmwwdg of the free state of thuringia, germany. corresponding author: elske linß, e-mail: elske.linss@mfpa.de 1. introduction and state of the art currently, 20-23 million tons of masonry building materials (mortar and plaster, lightweight concrete, aerated concrete, sandlime bricks, and bricks) are produced annually in germany. the quantity of bricks produced, including roof tiles, is approx. 1015 million tons [1]. current guiding strategies and ambitious environmental policy goals increasingly call on manufacturers of mineral building products to introduce material cycles [2], [3]. pure brick aggregates are currently used in sports field construction, in vegetation applications, in road construction as a proportionate component of frost protection and gravel base courses, and in building construction as recycled aggregate for concrete production [1]. pure brick recycled aggregates (figure 1), which are made from low-dense waste and can be obtained from masonry waste, can be returned to brick production as recycled material after being ground again. a prerequisite for this, however, is that no mortar adhesions or other impurities may be present [4]-[6]. based on the investigations in the projects [5], [7], a leaflet on the use of recycled bricks in the brick industry was drafted. it recommends pure hard-fired material, pure/hard material, lowfired material, and masonry waste as suitable feed material for the production of roof tiles, facing bricks, and vertically perforated bricks. depending on the type of brick, up to 25 wt.-% can be reused [6], [7]. after grinding, masonry waste can also be used as a cement composite material in the cement industry [8]. here it is necessary to be able to distinguish and separate low-fired and high-fired brick types, mortar, concrete, and other components from each other. figure 1. masonry waste. abstract this paper describes different machine learning methods for recognizing and distinguishing brick types in masonry debris. certain types of bricks, such as roof tiles, facing bricks and vertically perforated bricks can be reused and recycled in different ways if it is possible to separate them by optical sorting. the aim of the research was to test different classification methods from machine learning for this task based on high-resolution images. for this purpose, image captures of different bricks were made with an image acquisition system, the data was pre-processed, segmented, significant features selected and different ai methods were applied. a support vector machine (svm), multilayer perceptron (mlp), and k-nearest neighbour (k-nn) classifier were used to classify the images. as a result, a recognition rate of 98 % and higher was achieved for the classification into the three investigated brick classes. mailto:elske.linss@mfpa.de acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 2 optical single-grain sorting methods offer a chance for much differentiation even of very similar materials [10, 11, and 12]. optical analysis and sorting methods are not yet used on a large scale in the sorting of construction and demolition waste. the optical sorting methods are essentially based on innovative detection routines that can be integrated into corresponding software (figure 2). the aim of the investigations is the development of a recognition routine for the differentiation of different brick types for single-variety brick and masonry rubble. features from the high-resolution image and spectrum and evaluation algorithms from machine learning will be used for the recognition task. the main focus is in a first step on particles with a size of 8/16 mm. later it will be extended on particle sizes 2/4 and 4/8 mm. very important is the sorting out impurities, like mortar, gypsum, concrete and so on. the fundamentals are being created in order to develop more effective sorting processes in the recycling of residual brick masses in the future. this paper investigates the recognition of types of brick by using rgb images and innovative methods of machine learning are described. this provides the basis for the development of optical sorting procedures. in the investigations, recognition routines and algorithms are to be tested for the correct differentiation and separation of various brick types based on image features. 2. investigations figure 3 gives an overview of the test procedure. 2.1. materials and categories at the mfpa, initial preliminary investigations were carried out to distinguish between brick types on the basis of visual image information. in the investigations, different brick products were distinguished, which can be assigned to the following brick categories: • category ii: roof tiles and facing bricks, • category iii: vertically perforated bricks and • category v: other. at first, a sample collection of a representative nationwide cross-section of brick varieties and adhering mineral building materials were done. the brick samples in the different categories are composed of different new (unused) and recycled (used) bricks without adherents. furthermore, other relevant building materials such as aerated concrete, concrete, mortar, sand-lime bricks, etc. (new and recycled) have also been included as impurities. the measured material parameters water absorption after 24 h and bulk density according to din 4226-101 for all samples included in the investigations are shown in the diagram (figure 4). a total of 7,765 image recordings of samples were made. category i (high-fired material) is still missing from these investigations, as too little material was available for examination. category iv (brick waste from recycling plant) was also excluded at this point, as the samples were not homogeneous and can therefore belong to different categories. in future works, both categories will be added to the data set. the particle size is 8-16 mm. the brick samples listed in table 1 were divided into three classes, which differ in bulk density. figure 2. principle of optical single-grain sorting method. figure 3. overview of the experimental programme. figure 4. water adsorption versus bulk density for all samples. table 1. example images for the investigated three brick categories. category objects per category example images ii roof tiles and facing bricks 3,203 iii vertically perforated bricks 2,314 v others 2,248 acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 3 2.2. image acquisition and used software a data set of high-resolution images of the brick particles was created. the brick samples were examined under different conditions. on the one hand, three different types of lighting and, on the other hand, different combinations of features were used. this allows the influence of the illumination on the analysis results to be investigated. figure 5 shows the used image acquisition system “qualileo”. it consists of an rgb matrix camera with a 12 mp sensor and a step-less adjustable lighting system (figure 6). 2.3. investigations and results the provided algorithms are analysed using the presented data set with individual parameters and the average recognition rate (rr) captured. the achieved rr and standard deviation (stdev) are used to compare all of the results. all of the researches were carried out in the halcon programming environment. the investigation’s classifier have different setting parameters. the distance-based nearest k-neighbours value has been set to 5 in the k-nn classifier. the rbf-kernel was used with svm and the γ parameter was set to 0.02. also the one-versusall classification mode was used. with this setting a multi-class problem is reduced to a binary decision each class is compared to all the other classes. for mlp is used a softmax activation function. it performs best at classification tasks with multiple independent classification outputs. in the hidden layer, the number of hidden units is set to 15. in this study, the data set is split into two parts: 80 % and 20 %. the classifier are trained with a major part of the data set, while the classifier are tested with a smaller fraction of the data. 2.3.1. results before selection and combination of features at the beginning, the typical feature groups were analysed. table 2 shows a summary of these results. the first result was that lighting 2 (half of a ring light that simulates light from the side) always produced significantly worse results, which is why the following only presented the results for lighting l1 (fully ring light on) and l3 (incident light). this data set showed that colour features alone are the most important feature for this recognition task. the recognition rates achieved are always above 95 % for all data sets and classifiers, as shown in table 2. the region features have only little impact on recognition. additionally, the texture features prove more effective with over 80 % recognition rates for svm and mlp. the k-nn classifier had the lowest recognition results. aside from colour features, all other accuracy is insufficient. this is because the k-nn algorithm is more vulnerable to redundant features than the robust classifiers such as svm and mlp. this research found as well that using all of the possible number of features does not always result in a better accuracy, or that the improvement is minor because many of the features are redundant. besides that, with a large number of features, the classifier’s training time increase. a further improvement is to be achieved with feature selection. only the most important features for the study’s problem are calculated by this algorithm. 2.3.2. results after selection and combination of features the best results by feature selection are shown in figure 7 and figure 8. the very high recognition rates for all classifiers are remarkable. figure 5. used image acquisition system qualileo. figure 6. step-less adjustable lighting system on the qualileo. table 2. recognition rates (rr) for different classifiers learned by various numbers and kinds of features for different lightings (l1 = lighting 1, l3 = lighting 3). features svm (rr in %) mlp (rr in %) k-nn (rr in %) l1 l3 l1 l3 l1 l3 18 colour 96.27 97.04 98.30 98.23 95.91 95.24 32 region 36.62 37.71 44.98 46.33 35.26 37.52 195 texture 86.16 83.78 84.94 81.92 65.83 64.74 382 gray 78.38 77.48 74.13 85.14 42.92 44.08 432 all 80.05 79.99 93.89 94.02 58.42 60.01 figure 7. results of the reached recognition rates for the different brick types using different classifiers on l1 data set. category ii roof tiles and facing bricks category iii vertically perforated bricks category v others acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 4 the very high recognition rates for all classifiers are remarkable. also, there is no significant difference in the detection rate in the different lighting settings. both ring light images and incident light images have very similar results. the best result was achieved by the mlp with 98.51 % for lighting 1. with svm lighting l3 an average recognition rate of 97.92 % was reached. with a rr of 96.40 % a slightly lower result is achieved by k-nn classifier. however, the difference is marginal. a look at the individual categories also shows that category iii is recognised with an over 99 % recognition rate with both svm and mlp. not far from this result, k-nn is just below. category ii is always recognised equally well by svm and mlp (close to 98 %). here k-nn is clearly worse in the recognition (about 96 %). category v shows a lower recognition rate between 96 % and 97 % for all classifiers, but again k-nn shows the lowest results of all. compared to the results without feature selection and combination, no large increase in the recognition rates was observed. with svm and k-nn, the detection rate increased by only about 1 %, with mlp it remained at about the same rate. finally, figure 9 shows the importance of the feature selection procedure in classical classification classifiers. because it is not useful to use as many features as possible for classification, but only the most important ones. this can be seen as the score progresses. the rr increases with the number of features until it reaches its maximum value. after that, the rr decreases as more characteristics are added. because a lot of the features are redundant and have a lot of noise in them. as a feature selection method halcon presents a greedy method. in this algorithm the currently most promising feature is added to the feature vector. after that, it is evaluated whether one of the newly added features is unnecessary. summarised, it is important to note that the high recognition rates after feature selection were achieved with a relatively small number of features (table 3). the average rr is over 95 % for all classifier and both data sets l1 and l3. hence the different settings on light had only low effect on the result of the optimised classifier. additionally, the standard deviation is low, indicating a stable classification process. this simplifies the future implementation of the application and reduces the required computing effort and time. 3. conclusion and future work in this work, a method for recognizing different kinds of the brick using image processing and machine learning was investigated. a very good differentiation of the selected brick categories roof tiles and facing bricks or vertically perforated bricks could be demonstrated. with an overall recognition rate of about 98 %, the three categories are separated. in the future, the data set will be extended by additional categories, and further classifier methods (also deep learning) will be tested. the algorithms obtained are to be further used for the development of optical sorting methods. acknowledgement the results were developed within the framework of the research group "sensor technology for products and processes" at mfpa weimar, which is funded by the free state of thuringia. the investigations will be continued in an aif-igf research project. we would like to express our sincere thanks for the funding from the thuringian ministry of economics, science and digital society, and the federal ministry of economics and climate protection. the responsibility for the research content lies with the authors. references [1] a. müller, i. martins, recycling of building materials: generation processing – utilization, springer vieweg (2022). doi: https://doi.org/10.1007/978-3-658-34609-6 [2] bundesverband der deutschen ziegelindustrie e. v., roadmap for a greenhouse gas neutral brick and roof tile industry in germany, futurecamp climate gmbh (2021). online [accessed 26 may 2023] figure 8. results of the reached recognition rates for the different brick types using different classifiers on l3 data set. category ii roof tiles and facing bricks category iii vertically perforated bricks category v others figure 9. mean recognition rate (rr) of the mlp classifier (on lighting 1 data set) in dependence of the feature. table 3. mean value of recognition rates (rr) and standard deviation (sd) for all classifiers and different lightings (l1 = lighting 1, l3 = lighting 3). classifier l1 (ring light) l3 (incident light) svm (rr and sd in %) rr sd features 97.85 0.42 23 97.92 0.42 23 mlp (rr and sd in %) rr sd features 98.51 0.16 20 98.37 0.24 19 k-nn (rr and sd in %) rr sd features 96.40 0.54 18 96.05 0.31 13 https://doi.org/10.1007/978-3-658-34609-6 acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 5 https://cerameunie.eu/topics/cerame-uniesectors/sectors/roadmap-for-a-greenhouse-gas-neutral-brickroof-tile-industry-in-germany/ [3] mehr recycling von bauund abbruchabfällen in europa notwendig online [accessed 3 january 2020] [in german] https://www.recyclingmagazin.de/2016/10/02/352716 [4] m. landmann, a. müller, u. palzer, b. leydolph, leistungsfähigkeit von aufbereitungsverfahren zur rückgewinnung sortenreiner 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[in german] [13] f. rosenblatt, the perceptron: a probabilistic model for information storage and organization in the brain, psychological review, 6 (1958), p. 386-408. doi: 10.1037/h0042519 https://cerameunie.eu/topics/cerame-unie-sectors/sectors/roadmap-for-a-greenhouse-gas-neutral-brick-roof-tile-industry-in-germany/ https://cerameunie.eu/topics/cerame-unie-sectors/sectors/roadmap-for-a-greenhouse-gas-neutral-brick-roof-tile-industry-in-germany/ https://cerameunie.eu/topics/cerame-unie-sectors/sectors/roadmap-for-a-greenhouse-gas-neutral-brick-roof-tile-industry-in-germany/ https://www.recyclingmagazin.de/2016/10/02/352716 https://www.mvtec.com/doc/halcon/1712/de/toc_regions_features.html https://www.mvtec.com/doc/halcon/1712/de/toc_regions_features.html https://doi.org/10.1037/h0042519 the contribution of colour measurements to the archaeometric study of pottery assemblages from the archaeological site of adulis, eritrea acta imeko issn: 2221-870x march 2022, volume 11, number 1, 1 8 acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 1 the contribution of colour measurements to the archaeometric study of pottery assemblages from the archaeological site of adulis, eritrea abraham zerai gebremariam1,2, patrizia davit3, monica gulmini3, lara maritan4, alessandro re1,2, roberto giustetto2,5, serena massa6, chiara mandelli6, yohannes gebreyesus7, alessandro lo giudice1,2 1 department of physics, university of turin, via pietro giuria 1, 10125 turin, italy 2 national institute of nuclear physics, turin section, via pietro giuria 1, 10125 turin, italy 3 department of chemistry, university of turin, via pietro giuria 7, 10125 turin, italy 4 department of geosciences, university of padua, via giovanni gradenigo 6, 35131 padua, italy 5 department of earth sciences, university of turin, via valperga caluso n. 35, 10125 turin, italy 6 department of archaeology, catholic university of the sacred heart of milan, largo gemelli 1, 20123 milan, italy 7 northern red sea regional museum of massawa, p.o. box 33, massawa, eritrea section: research paper keywords: colorimetry; pottery; fabric; adulis citation: abraham zerai gebremariam, patrizia davit, monica gulmini, lara maritan, alessandro re, roberto giustetto, serena massa, chiara mandelli, yohannes gebreyesus, alessandro lo giudice, the contribution of colour measurements to the archaeometric study of pottery assemblages from the archaeological site of adulis, eritrea, acta imeko, vol. 11, no. 1, article 17, march 2022, identifier: imeko-acta-11 (2022)-01-17 section editor: fabio santaniello, university of trento, italy received march 7, 2021; in final form march 15, 2022; published march 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this project has received funding from the european union’s horizon 2020 research and innovation programme under the marie skłodowska-curie grant agreement no 754511 (phd technologies driven sciences: technologies for cultural heritage – t4c) corresponding author: abraham zerai gebremariam, e-mail: abraham.zeraigebremariam@unito.it 1. introduction colour is an important characteristic of archaeological materials and yet quantitative and reproducible measurements are needed for a meaningful analysis and classification of artefacts. review of works on the colorimetric study of ancient ceramics pinpoints to the versatility of this method over traditional and yet solely qualitative description of colour using the munsell colour charts [1]. the international commission on illumination; l*, a*, b* (cielab) colour space has been mentioned as a suitable system aimed at standardizing and comparing colorimetric features, and its utility demonstrated in many studies of different ceramic objects. colorimetric surveys on pottery objects that range from the evaluation of colour of the exposed interior and exterior surfaces as well as their core, and colour measurements on powdered samples and based up on the collection of cielab colour data from digital abstract colorimetric evaluation was applied on archaeological pottery from the ancient port city of adulis in the red sea coast of eritrea. pottery samples belong to the ayla-aksum typology, late roman amphora 1 and dolia classes, which had never been analyzed by means of this approach. the survey consisted of colorimetric measurements from different parts of the ceramic bodies, to comprehend how these data could be related to the overall fabric classification. differences in the colorimetric parameters provided helpful information on both technological manufacturing processes and fabric classification. subtle variations in the colour coordinates were detected and aptly interpreted, so as to ascribe the related differences. such an approach proved that the information provided by colour measurements can be partially correlated to observations from stereomicroscopy and optical microscopy, allowing a more in-depth description of the fabrics in the study of archaeological pottery. mailto:abraham.zeraigebremariam@unito.it acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 2 photographs, have been extensively reported in the literature [2], [3]. in general, the archaeometric study of ancient pottery mainly aims at locating the production centres, the specific technology involved in pottery production and understanding the distribution patterns. provenance is often tackled by determining the chemical and/or mineralogical compositions. and by comparing them with the composition of reference groups. mirti and davit (2004) demonstrated that colour measurements may help in assessing provenances and/or production technologies. the general agreement is that sherds made from different clays or even from the same clay but processed in different ways would show different colour curves, while ceramic materials obtained from a single clay and processed following a similar procedure should display a similar curve [4]. colour measurements on archaeological pottery have been applied particularly for the evaluation of the original firing conditions [4][16]. on the other hand, the colour of ceramic bodies is not only affected by temperature, but also by firing duration and atmosphere, as well as by the final porosity, the mineralogical composition of the raw materials used and the nature of inclusions. experimental analyses, aimed at evaluating the colour change with temperature by re-firing, allowed the detection of colour alterations in a specific clay body, and thus enabling an evaluation of the possible behaviour of different sherds. [3-4]. it is also noted that colorimetry, when coupled to other characterization techniques, can provide important results for examining raw materials and artefacts variability. the nucleation, growth, and grain size of hematite during firing as well as the redox reactions inherent to firing process and the influence of mineralogical composition, are all parameters that contribute to colour in ceramics, the consequences of which can be inferred by using mineralogical, micro-structural and chemical approaches [7]-[10] and [13]-[16]. therefore, the complexity of mineralogical and physical-chemical factors influencing the colour of ceramics requires that colorimetry should be coupled to other archaeometric techniques. in this respect, the dynamics of firing and changes of microstructures affecting colour variations in ceramics can be understood through detailed analytical approaches, colorimetry can be useful to determine colour parameters of the paste allowing preliminary fabric determination. in this study, pottery assemblages from the archaeological site of adulis, the primary port of the aksumite empire in late antiquity in the red sea coast of eritrea, were investigated by means of colorimetry. the site of adulis was principally involved in the major developments in the history of the northern horn of africa from the first millennium ce [17]-[19]. the long standing trade relations and cultural interaction with the romans and the mediterranean is attested in later periods during first millennium ce, through the red sea. comparative analysis of architectural and ceramic typological sequences attest that the site was continuously inhabited from the first-second up to the sixth and early seventh c. ce and intensely occupied in the 5th 6th c. ce [18], [19]. the ayla-aksum, late roman amphora 1 as well as dolia samples considered in this colorimetric survey indicate imports from the mediterranean world in the later phases of the occupation of the site. studies on these pottery assemblages from the northern horn of africa are rare [20]-[22] and colorimetric studies have never been applied previously. in this work, we highlight the evaluation of colorimetric parameters on different parts of ceramic bodies and their treatment in the form of powdered samples to assess the usefulness of colorimetry in pottery studies. the aim of this survey is to establish a preliminary fabric classification, possibly to be used for provenance determination. the results and limits of colorimetric evaluation are also reported here. the following sections provide a look into colorimetric assessment (considering different sampling procedures) and the use of such data to define the variability existing among the different classes. 2. materials and methods the colorimetric survey adopted for this study included different sampling procedures to evaluate the colorimetric parameters on archaeological pottery representing the ayla-aksum, late roman amphora 1 and dolia classes. all the samples considered in this survey were collected in the 2019 fieldwork of the ongoing italian-eritrean excavations at adulis [23]-[25]. on one hand, a set of samples was selected among a collection of 49 small sherds under archaeometric investigations: part of them were in fact not represented in this survey for being too small or showing colour variations after possibly post-depositional alterations. for the examined samples, colorimetric evaluation was done both on the interior and exterior surfaces of the ceramic bodies. the analysed surface was about 8 mm in diameter; thus, the detected coordinates relate to an average area of about 50 mm2. figure 1. general view and stereo-microscope images of some ayla-aksum: (a), (e) sample 1.3(2); (b), (f) sample 2.0, late roman amphora 1 (c), (g) sample 3.9 and dolia sample (d), (h) sample 4.9. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 3 table 1. all colorimetric values (l*, a*, b* respectively) for exterior and interior surfaces as well as cross-sections and powders on ayla – aksum and late roman 1 typologies. typology sample powder exterior interior section ayla aksum 1.1 64.16; 10.84; 17.28 ------ 1.2 70.06; 1.88; 14.99 62.20; 3.14; 18.12 61.11; 4.11; 19.23 45.80; 3.36; 11.95 47.64; 2.93; 11.11 -- 1.3(1) 66.96; 6.21; 17.60 55.27; 7.77; 21.53 55.30; 7.82; 21.71 56.87; 6.03; 18.54 53.37; 6.15; 17.69 -- 1.3(2) 65.21; 8.41; 17.25 54.62; 6.10; 17.61 54.11; 5.89; 16.94 53.10; 5.70; 17.61 52.77; 7.90; 18.43 52.85; 7.95; 19.14 54.44; 8.26; 19.51 62.05; 10.65; 19.22 60.25; 10.65; 18.94 59.39; 10.52; 18.78 1.4(1) 68.95; 5.47; 16.53 ------ 1.4(2) 75.10; 1.86; 15.09 63.34; 5.67; 17.02 61.71; 5.39; 17.98 63.21; 5.16; 18.82 66.23; 4.32; 19.75 66.45; 4.74; 20.23 71.91; 1.52; 15.70 67.71; 1.59; 17.04 1.5 76.53; 2.74; 20.06 ------ 1.6 61.39; 15.35; 20.43 ------ 1.7(1) 63.13; 12.57; 18.44 ------ 1.7(2) 68.47; 3.92; 15.35 ------ 1.8 60.49; 14.40; 19.61 ------ 2.0 69.92; 6.89; 17.29 66.61; 5.15; 19.63 69.84; 3.98; 19.98 68.91; 3.83; 19.42 70.78; 3.36; 19.16 68.65; 3.52; 19.06 -- 3.3 71.59; 1.04; 11.49 ------ 3.4 67.73; 1.27; 14.16 ------ 3.5 69.19; 9.18; 17.87 ------ 3.6 68.51; 10.71; 18.45 66.54; 4.69; 19.69 61.90; 4.18; 18.99 63.35; 4.95; 22.04 61.92; 5.96; 20.94 60.44; 5.48; 20.37 -- 3.7 58.55; 19.19; 22.41 ------ 3.8 57.24; 16.01; 19.36 ------ c01 65.68; 8.50; 18.00 ------ c04 67.38; 5.97; 17.24 ------ c05 67.81; 9.43; 18.40 ------ late roman 1 1.9 60.50; 9.70; 16.72 44.77; 5.97 17.34 43.13; 8.32; 17.84 47.34; 8.39; 18.62 51.60; 7.82; 20.66 49.25; 7.93; 16.37 49.69; 8.13; 17.91 -- 2.1 65.58; 6.11; 15.48 53.48; 7.91; 18.10 52.82; 8.30; 17.85 50.37; 8.96; 18.32 45.14; 7.22; 15.89 48.91; 7.64; 17.82 -- 2.2 67.28; 4.56; 13.36 ------ 2.3 65.58; 6.11; 15.48 54.16; 10.87; 21.16 55.82; 11.88; 22.28 55.49; 12.91; 23.16 51.55; 13.11; 24.90 52.26; 13.83; 24.82 53.19; 13.54; 24.78 58.75; 9.16; 18.02 58.38; 9.58; 18.26 58.22; 9.08; 17.70 2.4 63.69; 12.86; 20.75 ------ 3.0 67.49; 9.55; 18.06 58.75; 5.69; 19.91 58.39; 6.38; 18.95 64.28; 6.87; 21.27 63.50; 6.67; 20.81 -- 3.1 67.97; 8.27; 17.38 64.58; 7.63; 21.78 61.31; 8.23; 21.86 63.10; 6.39; 21.61 61.15; 6.68; 21.58 -- 3.2 67.91; 10.44; 18.81 ------ 3.9 64.34; 3.64; 12.06 ------ 4.1 68.64; 6.54; 16.65 58.44; 7.15; 17.93 58.69; 7.08; 17.20 58.67; 7.19; 19.44 56.15; 7.33; 18.59 58.98; 7.68; 19.15 62.56; 8.25; 20.00 62.01; 8.44; 21.01 59.95; 8.66; 20.50 c02 67.28; 9.89; 18.40 ------ c03 67.02; 5.86; 15.75 ------ c06 62.50; 6.36; 14.56 ----58.96; 8.96; 20.01 57.92; 9.61; 19.84 58.22; 9.58; 20.59 c07 65.99; 7.09; 15.06 ----46.31; 9.32; 17.89 47.21; 10.74; 18.50 47.38; 9.91; 18.43 c08 65.92; 6.97; 14.86 ----46.79; 9.10; 17.94 49.51; 8.56; 18.10 48.05; 9.62; 19.02 c09 71.15; 4.76; 13.40 ----46.62; 9.37; 16.14 44.70; 9.26; 15.02 45.52; 9.55; 15.66 acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 4 for some sherds the thickness was also adequate to evaluate colour parameters on their cross-sections. the cleanest areas on the surfaces were selected, to estimate at best the true colour of the ceramic body. in all cases, depending on the dimension of the fragment and on the suitability of the analysed surfaces, 2 or 3 measurements were carried out on different areas of the surfaces to estimate the spread of the data on every considered sample. finally, colorimetric measurements were performed on the powders of all samples representing the ayla-aksum, late roman amphora 1 and dolia classes. a fragment was cut from each sample, polished to avoid contaminations, and crushed using an agate mortar and pestle to obtain 100 mg of powders. by doing this, the resulting powders represent a mix of the components of the exterior, interior and cross-section parts of the ceramic bodies, including temper grains, which can also affect colour measurements due to compositional variability and differences in particle size [10]. the measurements were performed by inserting these powders in specific cylindrical cells of optical fused silica with transmittance ˃ 95 % and no features in the whole visible range of the spectrum. a minolta cm-508i portable spectrophotometer was used, equipped with a pulsed xenon arc lamp and an integrating sphere to diffusely illuminate the specimen surface, which was viewed at an angle of 8° to the normal (d/8 geometry). the light reflected by the sample surface (specular component included) was detected by a silicon photodiode array, which allowed obtaining the reflectance spectrum in the 400 nm – 700 nm range, with wavelength pitches of 20 nm. the spectrophotometer was calibrated to provide the mean values of three consecutive measurements. colour coordinates were expressed in the cie l*a*b* system, using the illuminant d65 (average solar light) and a 10˚ viewing angle. in this system, the l*coordinate is related to colour lightness, while a* and b* are each determined by both hue and saturation respectively [4], [6]. in figure 1, representative ayla-aksum, late roman amphora 1 and dolia samples (scale in cm) and the stereo-microscopic images of their fresh cut are shown. it is worth noting that many factors could contribute to the occurrence of inconsistencies in the data both within the same sample and between different samples, when untreated archaeological materials are analysed. phenomena such as imperfect geometries of the analysed surfaces, presence of contaminants (not easily detectable to the naked eye), alterations due to post-depositional processes, and porosity can contribute to this variability in the obtained measurements (see figure 1). 3. results and discussion 3.1. colorimetric measurements on exterior, interior and cross sections table 1 and table 2 report colorimetric coordinates for all measured samples while in table 3 the largest values for ∆eab (∆eab,max) of each sample are indicated, computed according to the following equation: δ𝐸ab,max = √(𝑎1 ∗ − 𝑎2 ∗ )2 + (𝑏1 ∗ − 𝑏2 ∗)2 , (1) table 2. all colorimetric values (l*, a*, b* respectively) for exterior and interior surfaces as well as cross-sections and powders on dolia typology. typology sample powder exterior interior section dolia 1.0 54.33; 11.66; 13.16 47.46; 12.55; 20.16 47.42; 12.70; 20.20 48.80; 13.01; 20.75 49.76; 12.11; 17.82 49.24; 12.29; 17.32 48.76; 12.61; 17.56 46.20; 15.06; 18.40 47.51; 15.54; 18.76 47.64; 14.99; 17.93 4.0 59.74; 19.19; 22.41 57.39; 9.53; 22.41 54.48; 9.01; 20.38 55.54; 8.82; 20.30 49.81; 15.14; 19.52 50.40; 15.34; 20.34 50.70; 15.34; 21.16 -- 4.8 66.75; 4.66; 12.72 59.01; 6.75; 16.66 55.63; 7.02; 17.79 56.02; 7.76; 18.99 58.46; 6.53; 17.86 52.99; 6.68; 18.90 55.65; 5.74; 16.75 59.10; 6.11; 15.35 59.11; 5.79; 15.32 59.12; 6.26; 15.24 4.9 61.15; 6.96; 14.28 43.06; 7.87; 18.66 48.06; 8.65; 19.25 47.06; 8.24; 16.31 50.67; 8.75; 17.63 47.06; 8.24; 17.63 52.44; 12.23; 19.43 55.23; 11.18; 20.83 table 3. association to fabric groups identified by petrography (fg) and maximum values of eab,max from colorimetry for each sample. the average for fg is also shown when there is more than one measurement. ayla aksum fg sample exterior interior in-out a 1.2 1.3(1) 1.4(2) 1.47 0.19 1.87 0.94 0.86 0.64 8.21 3.34 1.87 b 1.3(2) 2.0 3.6 0.70 1.22 3.15 1.14 0.54 0.75 4.35 1.85 3.15 a average 1.2 0.8 4.5 b average 1.7 0.8 3.1 late roman fg sample exterior interior in-out a 1.9 2.74 4.29 4.29 b 2.1 2.3 3.0 3.1 1.07 2.86 1.18 0.61 1.98 0.72 0.50 0.29 2.99 4.71 2.37 1.86 c 4.1 2.24 0.66 2.24 b average 1.4 0.9 3.0 dolia fg sample exterior interior in-out a 1.0 4.0 0.75 2.23 0.56 1.65 3.50 6.58 a1 4.8 2.54 2.35 3.02 b 4.9 0.98 1.42 2.97 a average 1.49 1.11 5.04 acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 5 where the differences between a1* and a2*, b1* and b2* represent the colorimetric coordinates showing the maximum differences among the set of measurements for the exterior and interior surfaces, for each sample. lightness (l*) is reputed to be a less suitable parameter than hue and saturation (a* and b*) and, due to this, it was not considered in this formulation [3]. the reported values of ∆eab,max are quite low in each dataset, indicating that the surface colours of both the interior and exterior surfaces of the pottery samples appear to be quite homogenous. besides, it seems that data from the interior surfaces are less spread than those from the exterior ones. for example, the average values of ∆eab,max are 1.2-1.7 for exterior surfaces of the ayla-aksum -depending on the fabricand 0.8 for their interior ones, respectively. to highlight this behaviour, the bivariate plot of l*, a* and b* parameters for all measurements is shown in figure 2 for a selected number of samples (one for each fabric). it is quite evident that differences can arise in colour coordinates depending on which side of the ceramic body surface is being measured (table 3). for example, in ayla-aksum samples ∆eab,max is lower than 3.15 for the exterior and less than 1.14 for the interior surface. a similar behaviour is observed in late roman amphora 1 (with the sole exception of an anomalous interior value for sample 1.9) and dolia samples. the measurements on cross-section (not reported in table 3 but shown in figure 2) are particularly homogeneous for all classes. this is probably due to favourable experimental conditions, related to the uncontaminated and flat surfaced of the sections. when comparisons are made based on the values obtained for the exterior and interior surfaces, cross-sections, and powdered samples the colorimetric values for crosssections seem to be closer to those obtained for the powders in many instances. however, some discrepancies in the colorimetric measurements of the cross-sections could also be related to irregular geometries. figure 2. colorimetric values for selected samples (left); all colorimetric values for exterior and interior surfaces as well as cross-sections and powders (centre and right). acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 6 moreover, in almost all cases, it clearly appears that the colour for the exterior and interior surfaces of the ceramic bodies are darker, i.e., the values of l* are lower than those obtained for their powders. such a survey confirms that discrepancies in measurements on both the exterior and interior surfaces – and, in some cases, on the cross-sections – are quite common when untreated archaeological samples are concerned. this sampling problem can be linked to different phenomena. further treatment of the samples, either by refiring them at different temperatures to evaluate colour changes or by extracting powders from their fragments, however, can sometimes compensate for measurement inconsistencies. 3.2. colorimetric measurements on powdered samples for colorimetric measurements on powders, 21 ayla-aksum, 15 late roman amphora 1 and 4 dolia samples were considered. in particular, the dispersion of the colorimetric values within a specific class of pottery was considered. such a dispersion for ayla-aksum samples (figure 3) clearly indicates that colorimetric information can be useful for discriminating different fabrics within a given class. data dispersion in these samples indicates a colour variation from creamy (lower a* value) to a reddish hue (higher a* values). the former is typical of samples belonging to the fabric a of the ayla-aksum amphorae; the latter pertains to the fabric b. such a differentiation might be related to different technologies of production (morphological and/or microstructural changes) and perhaps compositional differences too. the distinction between these samples enabled by colorimetric parameters further complements the observation of different fabrics identified for the ayla-axum amphorae by petrography. in this respect, colorimetry can thus be useful to allow preliminary fabric classification when coupled to typological classification, petrography, and stereo microscopy. it should be noted, however, that the colour parameters defined for a homogenised paste (powders) only allows a preliminary fabric determination for each sample, rather than providing information about its composition or provenance. however, in some cases the fabric classification extrapolated from petrography does not match the distinction made by colorimetric evaluation. this was observed in the trends discerned from the colorimetric evaluation of powder samples from the late roman amphora 1 and dolia classes (figure 4). such evidence might indicate that, although the original clay used for manufacturing should be similar, the addition of one or more tempers might account for fabric variability, as defined by petrographic analyses. in this respect the mix design in the production of the ceramics can contribute to colour changes [10]. therefore, in order to make reliable deductions it is strictly necessary to link colorimetric observations to textural, microstructural and chemical studies. the results of this study prove that while colorimetry can assist in defining the fabrics and typological classifications, it certainly needs to be complemented with other approaches in order for more detailed information to be achieved. furthermore, colorimetry can be significantly useful to make deductions about the inter-fabric variability of samples belonging to specific classes. the mere collection of colorimetric parameters for each sample considered in this study could not (if considered per se) exclusively ascribe sharp distinctions, when comparison is made among fabrics defined for ayla-axum, late roman amphora 1 and dolia classes. this limitation is characteristic of colorimetric evaluations as can be seen from figure 4, where the overlapping of the colorimetric data for different samples belonging to the three classes of pottery considered in this study prevents any feasible conclusions. nevertheless, the importance of this survey is relevant, as it allowsin many cases a preliminary distinction of fabric variability within a specific typological class of pottery. 4. conclusions colorimetric observations were done on ayla-aksum, late roman amphora 1 and dolia pottery samples collected from excavations at adulis in order to check potential grouping with figure 3. colorimetric values for powdered samples. colours refer to the fabric classification as reported in table 3. figure 4. colorimetric values for powdered samples (ayla-aksum, late roman and dolia classes. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 7 respect to the fabric description obtained by means of stereomicroscopy and petrography. in this respect, this study showed that it is possible to correlate colorimetric values to specific fabric attributions achieved by means of traditional microscopy approaches. different sampling procedures were adopted to collect colorimetric data, in order to thoroughly understand the limitations and strengths of such an approach. the exterior and interior surfaces of the ceramic bodies, as well as their cross-sections were considered for the untreated samples, while powders were extracted from many samples in order to obtain homogenous samples for the survey. subtle differences in colour measurements due to these different sampling conditions were interpreted as potential discriminating parameters in order to establish fabrics. data variability collected on the exterior or interior surfaces of the ceramic bodies, as well as on their cross-sections or powders, is an indication that various phenomena should be considered while interpreting these data, particularly on untreated samples. the ∆eab,max computation allowed us to understand subtle variations and/or inconsistencies in colorimetric measurements on the exterior and interior surfaces, as well as on the cross-sections. marked differences in these values have been considered in order to make extrapolations, while feeble differences between samples belonging to the same fabric can hardly be used to postulate a differentiation between them. yet, the limited number of analysed samples, coupled to the detected measurement discrepancies (due to several factors, such as imperfect geometries of the surfaces, post-depositional alterations and perhaps also porosity), make it necessary to increase the statistical reliability pertaining to the colorimetric approach applied to the classes considered in this study. however, in many cases the attribution of samples to distinct groups was possible based on colorimetric evaluation, which proved to be consistent with previously determined classification by petrography. this observation pinpoints that colorimetric measurements can be useful to complement petrographic studies for in-depth fabric description. the correspondence of the colorimetric groupings with petrographic observations is a further indicator that the information from colorimetry can be useful to support provenance and/or technological studies on archaeological pottery. on the other hand, when colorimetric information cannot parallel petrographic information (as seen in a few cases in this study), a detailed textural and chemical study as well as a micro-structural understanding of the ceramic body becomes necessary. in conclusion, different parts of the ceramic body offer a variety of sampling decisions for colorimetric evaluation with non-invasive procedures, and thus the objective comparison of colour through a quantitative analysis can overcome issues related to typological classification. moreover, this survey showed that colorimetric measurements could be useful, at least in some cases, to ascribe preliminary fabric determination when coupled to complementary techniques – such as optical microscopy. such an information could be used to understand – at least partly – the technological processes. as well as to help in tracing the provenance of a given artefact (assuming that objectively attained colour measurements could be correlated to information obtained from these complementary techniques). the inherent limitations of this approach have also been highlighted, particularly to deduce colour variations due to mineralogical, chemical, and micro-structural differences a subject that needs to be dealt with, in the future direction of this research. acknowledgments the authors wish to warmly thank the commission of culture and sports of the state of eritrea, northern red sea museum of massawa, and centro ricerche sul deserto orientale (ce.r.d.o.) for supporting this research. we acknowledge here also the funding from the european union’s horizon 2020 research and innovation programme under the marie skłodowska-curie grant agreement no 754511 (phd technologies driven sciences: technologies for cultural heritage – t4c. references [1] m. giardino, r. miller, r. kuzio, d. muirhead, analysis of ceramic colour by spectral reflectance, american antiquity, 63(3), (1998), pp. 477-483. [2] j. r. mcgrath, m. beck, m. e. hill, jr. replicating red: analysis of ceramic slip colour with cielab colour data, journal of archaeological science: reports14(2017), pp.432-438. doi: 10.1016/j.jasrep.2017.06.020 [3] p. mirti, p. davit, new developments in the study of ancient pottery by colour measurement, journal of archaeological science 31(2004), pp. 741–751. doi: 10.1016/j.jas.2003.11.006 [4] p. mirti, on the use of colour coordinates to evaluate firing temperatures of ancient pottery, archaeometry 40 (1998), pp. 45– 57. doi: 10.1111/j.1475-4754.1998.tb00823.x [5] m. daszkiewicz, l. maritan, experimental firing and re-firing, in: the oxford handbook of archaeological ceramic analysis, a. m. w. hunt (ed.), oxford handbooks in archaeology, 2016, isbn: 9780199681532. [6] m. bayazit, i. işık, a. issi, e. genç, spectroscopic and thermal techniques for the characterization of the first millennium ad potteries from kuriki, turkey, ceramics international 40(9) (2014), pp. 14769-14779. doi: 10.1016/j.ceramint.2014.06.068 [7] m. j. feliu, m.c. edreira, j. martin, application of physical– chemical analytical techniques in the study of ancient ceramics, analytica chimica acta 502 (2004), pp. 241–250. doi: 10.1016/j.aca.2003.10.023 [8] l. nodari, e. marcuz l. maritan c. mazzoli, u. russo, hematite nucleation and growth in the firing of carbonate-rich clay for pottery production, journal of the european ceramic society 27 (2007), pp. 4665–4673. doi: 10.1016/j.jeurceramsoc.2007.03.031 [9] c. gernminario, g. cultrone, a. de bonis, f. izzo, a. langella, m. mercurio, v. morra, a. santoriello, s. siano, c. grif, the combined use of spectroscopic techniques for the characterization of late roman common wares from benevento (italy), measurement 114(2018), pp. 515-525. doi: 10.1016/j.measurement.2016.08.005 [10] de bonis, g. cultrone. c. grif, a. langella, a. leone, m. mercurio, v. morra, different shades of red: the complexity of mineralogical and physico-chemical factors influencing the colour of ceramics, ceramics international 43 (2017), pp.8065–8074. doi: 10.1016/j.ceramint.2017.03.127 [11] y. yang, m. feng, x. ling. z. mao, c. wang, x, sun, m. guo, microstructural analysis of the colour-generating mechanism in ru ware, modern copies, and its differentiation with jun ware, journal of archaeological science 32 (2005), pp.301–310. doi: 10.1016/j.jas.2004.09.007 [12] j. molera, t. pradell, m. vendrell-saz, the colours of ca-rich ceramic pastes: origin and characterization, applied clay science 13 (1998), pp.187–202. doi: 10.1016/s0169-1317(98)00024-6 https://doi.org/10.1016/j.jasrep.2017.06.020 https://doi.org/10.1016/j.jas.2003.11.006 https://doi.org/10.1111/j.1475-4754.1998.tb00823.x https://doi.org/10.1016/j.ceramint.2014.06.068 https://doi.org/10.1016/j.aca.2003.10.023 https://doi.org/10.1016/j.jeurceramsoc.2007.03.031 https://doi.org/10.1016/j.measurement.2016.08.005 https://doi.org/10.1016/j.ceramint.2017.03.127 https://doi.org/10.1016/j.jas.2004.09.007 https://doi.org/10.1016/s0169-1317(98)00024-6 acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 8 [13] l. nodari, l. maritan, c. mazzoli, u. russo, sandwich structures in the etruscan-padan type pottery, applied clay science 27(2004) pp. 119-128. doi: 10.1016/j.clay.2004.03.003 [14] v. valanciene, r. siauciunas, j. baltusnikaite, the influence of mineralogical composition on the colour of clay body, journal of european ceramic society 30(2010), pp. 1609-1617. doi: 10.1016/j.jeurceramsoc.2010.01.017 [15] r. mentesana, v. kilkogolu, s. todaro, p. m. day, reconstructing change in firing technology during the final neolithic-early bronze age transition in phaistos, crete. just the tip of the iceberg? journal of archaeological and anthropological sciences 11 (2019), pp. 871-894. doi: 10.1007/s12520-017-0572-8 [16] y. maniatis, the emergence of ceramic technology and its evolution as revealed with the use of scientific techniques, in: from mine to microscope: advances in the study of ancient technology. a. shortland, i. freestone, t. rehren (editors), oxbow books, 2009, eisbn: 978-1-78297-279-2, pp. 11-28. [17] c. zazzaro, e. cocca, a. manzo, towards a chronology of the eritrean red sea port of adulis (1st – early 7th century ad). journal of african archaeology 12 (1) (2014), pp.43-73. doi: 10.3213/2191-5784-10253 [18] peacock, d. & blue, l. (eds.) the ancient red sea port of adulis, eritro-british expedition, 2004–5. oxbow books, oxford, 2007, isbn: 9781842173084 [19] c. zazzaro, the ancient red sea port of adulis and the eritrean coastal region, bar international series, vol. 2569, oxford, 2013, isbn 978 1 4073 1190 6. [20] r. k. pedersen, the byzantine-aksumite period shipwreck at black assarca island, eritrea, azania xliii (2008), pp.77-94. doi: 10.1080/00672700809480460 [21] m. m. raith, r. hoffbauer, h. euler, p. a. yule, k. damgaard, the view from zafar an archaeometric study of the aqaba pottery complex and its distribution in the 1st millennium ce, zora 6 (2013), pp. 320-350. [22] s. massa, a. de bonis, v. morra, v. guarino, in s. massa (ed.), adulis project 2015 report (2015), pp. 85-90 (unpublished). [23] adulis project 2018 report (unpublished), s. massa (ed) [24] adulis project 2019 report (unpublished), s. massa (ed). [25] adulis project 2020 report (unpublished), s. massa (ed) https://doi.org/10.1016/j.clay.2004.03.003 https://doi.org/10.1016/j.jeurceramsoc.2010.01.017 https://doi.org/10.1007/s12520-017-0572-8 https://doi.org/10.3213/2191-5784-10253 https://doi.org/10.1080/00672700809480460 a strategy to control industrial plants in the spirit of industry 4.0 tested on a fluidic system acta imeko issn: 2221-870x june 2022, volume 11, number 2, 1 7 acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 1 a strategy to control industrial plants in the spirit of industry 4.0 tested on a fluidic system laura fabbiano1, paolo oresta1, rosario morello2, gaetano vacca1 1 dmmm, politecnico di bari university, bari, italy 2 diies, university mediterranea of reggio calabria, italy section: research paper keywords: industry 4.0; predictive maintenance; prognostic approach; plant operation simulator; fluidic thrust plant citation: laura fabbiano, paolo oresta, rosario morello, gaetano vacca, a strategy to control industrial plants in the spirit of industry 4.0 tested on a fluidic system, acta imeko, vol. 11, no. 2, article 31, june 2022, identifier: imeko-acta-11 (2022)-02-31 section editor: francesco lamonaca, university of calabria, italy received november 9, 2021; in final form february 21, 2022; published june 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: laura fabbiano, e-mail: laura.fabbiano@poliba.it 1. introduction the costs of maintenance are a consistent amount of the total operating costs in industrial production. moreover, several extra costs come from the profit loss due to the undesired failure of the plant. they can be dropped down with a failure prediction that preserves the devices through just on-time maintenance. these two classes of problems can be solved by the implementation of preventive maintenance in the perspective of the smart industry, [1]-[3], for which all the elements of a factory work in a completely, collaborative and integrated way and deal in real-time with the timely changes of the workflow. concerning costs reduction in force of the just on-time failure detection, the goal of the smart industry is to implement all the fundamental measurement procedures for continuous real-time monitoring (real-time condition monitoring) and coordinated monitoring of all plant elements, [4]-[6]. the keywords of the fourth industrial revolution are, therefore "preventive maintenance", "intelligent and real-time orchestration" and "synchronization of physical and digital processes". in literature, there are crucial new insights about the continuous screening of the devices that made possible the detection of incipient failure, [7]-[9]. among others, the prognostic approach is mandatory to obtain an accurate and coding of incipient machine failures, [10]-[12]. the detection of the degradation and damage of a plant component is the goal of the predictive approach. the condition-based maintenance (cbm) specifications [13], [14] inspired by the prognostic approach has been applied by the authors to the case of a simple fluidic thrust system by using a mathematical approach. it consists of a pump-motor block with inverter and a control valve, which represents the load acting on the p-m block and attributable to the network supplied by it. the approach uses a numerical simulator to manage and analyse in real-time all the characteristic parameters (sensed or mathematically predicted) of each system components to get and to advise concerning likely incipient anomalies of the monitored components. abstract the goal of the paper is to propose a strategy of automating the control of wide spectrum industrial processes plants in the spirit of industry 4.0. the strategy is based on the creation of a virtual simulator of the operation of the plants involved in the process. through the digitization of the operational data sheets of the various components, the simulator can provide the reference values of the process control parameters to be compared with their actual values, to decide the direct inspection and/or the operational intervention on critical components before a possible failure. as example, a simple fluidic thrust plant has been considered, which a mathematical model (simulator) for its optimal operating conditions has been formulated for, by using the digitalized real operational data sheets of its components. the simple thrust system considered consists of a centrifugal pump driven by a three-phase electric motor, an inverter to regulate the rotation of the motor and a proportional valve that simulates the external load acting on the pump. as results, the operational data sheets and principal characteristics of the pump have been reproduced by means of the simulator here developed, showing a very good agreement. mailto:laura.fabbiano@poliba.it acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 2 the virtual simulator is based on the digital reconstruction of the data sheets of all the more sensitive components of the system to allow the digital prediction of the optimal point of system working conditions. these data provided by model are compared with the real-time instantaneous acquisitions of the correspondent main operating parameters. the eventual discrepancy between virtual and actual values of the monitored parameter identifies which of the components of the system may be in critical conditions, thus dropping down its start and stop time responsible of the maintenance and service interruption costs. the proposed procedure, based on the digitization of the technical data sheets of the plant devices can be easily extended to most of the systems operating in industrial processes, thus allowing to control the entire system in real-time and detecting the dangerous failure scenarios. 2. plant specifications the system here simulated is a simple fluidic thrust plant and consists of a centrifugal pump (calpeda nm4 25/12a/a, [15]) driven by an electric asynchronous motor whose rotation speed is regulated by an inverter, and a control valve simulating the supplied network. the operational point of such system is determined by the coupling of the pump with the valve. the technical and geometric characteristics of each component are known. those of the pump are here reported: 𝑛 = 1450 rpm rotational speed of reference 𝑃 = 0.25 kw absorbed power at 1450 rpm 𝑄 = 1 ÷ 6 m3/h flow rate working range 𝐻𝑢 = 6.1 ÷ 3.3 m head range 𝐷2 = 131.5 mm external diameter of the impeller 𝛽2 = 157.5 ° angle of blade exit 𝑙2 = 4 mm blade height 𝑧 = 7 number of blades 𝜁 = 1 − 𝑧𝑠 π 𝐷2 = 0.95 blade bulk coefficient a user interface has been set up to manage the procedure of analysis and acquisition of data. specifically, that interface has been created on labview platform and is divided into several sections; each section manages and analyses the different parts of the plant, figure 1. in the control section you find the commands to change the opening degree of the valve and the inverter frequency. the target section enforces the rotation number of the engine. once the frequency is set by the control panel, it automatically varies until the system reaches a stable point of operation where the number of revolutions of the pump coincides with the value imposed in the target panel. the transient panel shows the indicators of instant monitoring of the plant. in particular, the quantities acquired by the transducers (number of revolutions, mechanical torque, flow rate, head) and some derived quantities such as instantaneous absorbed power are represented. the results panel collects in graphic form (three different plots) the trends of the pump head (characteristic curve), efficiency and absorbed power as a function of the flow rate. 3. virtual simulator the mathematical model of the plant consists of: 1. valve model; 2. pump motor block model. most of the geometric and operating data have been inferred from the technical specifications of the components. 3.1. valve model the control valve (here a proportional solenoid valve) allows the modulation of the circuit external characteristic (valve characteristic), [16]. that is, by acting on the closing/opening control devise, the relationship between the pressure drops and the flow rate flowing through the valve changes, so determining a new operating point as intersection with the internal characteristic (pump operating curve). from bernoulli's equation the relationship to the basis of the valve operation comes out as: 𝑄 = [𝑐𝑣max ℎ + 𝑐𝑣min (1 − ℎ)]√ 𝐻𝑣 𝐻𝑣 ∗ , (1) where: • 0 < ℎ < 1 degree of opening of the valve (or shutter stroke); • 𝑐𝑣max = 0.005 m 3/h and 𝑐𝑣min = 10 −6 m3/h flow rates (or efflux coefficients) minimum and maximum estimated by the technical data sheet of the pump; • 𝐻𝑣 → static pressure drop through the valve (m); • 𝐻𝑣 ∗ → 1 𝑝𝑠𝑖 static pressure drop through the valve (m). it represents the linear operational limit of the valve itself. from the previous relationship we get: 𝐻𝑣 = 𝑄 2 𝐻𝑣 ∗ [𝑐𝑣max ℎ + 𝑐𝑣min (1 − ℎ)] 2 (2) if a linear valve is considered, its characteristic coefficient 𝐾𝑣 (ℎ) reads as: 𝐾𝑣 (ℎ) = 𝐻𝑢 ∗ [𝑐𝑣𝑚𝑎𝑥 ℎ + 𝑐𝑣𝑚𝑖𝑛 (1 − ℎ)] 2 , (3) so, making possible to rewrite the previous relation as: 𝐻𝑣 = 𝑄 2𝐾𝑣 (ℎ) (4) the efflux coefficient of the valve is an increasing monotonous function of the shutter stroke, 𝑐𝑣 (ℎ), and it can be expressed in dimensionless form if the following quantities are considered: • the relative efflux coefficient: 𝜑 = 𝑐𝑣 /𝑐max • the intrinsic rangeability, ratio between the maximum and the minimum values of the efflux coefficient: 𝑟 = 𝑐𝑣max /𝑐𝑣min figure 1. user interface in labview platform. acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 3 examples of valve characteristics expressed in terms of ℎ are shown in figure 2 for linear, equal percentage, fast-opening and quadratic valves: for the linear characteristic, it holds the relation: 𝜑(ℎ) = ℎ + 1 𝑟 (1 − ℎ) (5) in the plant operating model (simulator), the characteristic coefficient of the valve can be rewritten as, from (4): 𝐾𝑣 (ℎ) = 𝐻𝑣 ∗ 𝑐𝑣 (ℎ) 2 = 𝐻𝑣 ∗ 𝑐𝑣 max 2 𝜑(ℎ)2 (6) this relation for 𝐾𝑣 (ℎ) keeps holding even for other kind of valves, if the right formula for 𝜑(ℎ) is introduced in it. 3.2. motor pump block the 'pump-motor' block, figure 3, represents the virtual simulator of a centrifugal pump and its electric control motor. this block has 2 inputs and 4 outputs. the 2 inputs are, respectively: 1. the coefficient of the valve, whose operating characteristic is enclosed in the 'valve' model and is provided by it. its regulation will result in the variation of the number of revolutions of the pump, 𝑛 2. the magnetic field frequency, parameter recalled by the control unit, which allows to act in feedback on it, in order to restore the number of revolutions to a target and constant value during the process of digital reconstruction of the characteristic curves of the pump. the 4 outputs represent the virtual sensors of the p-m block suitable for the detection of: 1. q flow rate (m³/h) 2. 𝐻𝑢 head provided by the pump (m) 3. 𝑛 number of engine revolutions (rpm) 4. 𝐶𝑚 drive torque, (n m). they are recalled by the supervisor function (see figure 1) and made visible in the transient panel, where the instant monitoring indicators of the system are reported. in particular, the quantities acquired by the transducers (for a number of revolutions, mechanical torque, flow rate, head) and some derived quantities such as the instantaneous absorbed power are shown. in the model there are some geometric parameters directly available from the technical characteristics of the pump; other necessary parameters are inferred from the characteristic curves and the power absorbed by the pump, for a fixed pump revolution. the internal characteristic of the pump at the revolution 𝑛0 = 450 rpm can be expressed in polynomial form as follows: 𝐻𝑢 = −0.0893 𝑄 2 + 0.0706 𝑄 + 6.104 (7) obtained by best square polynomial regression of the pump, reported in the table 1 and shown in figure 4. it is possible to generalize this relation and make it valid for any number of revolutions, as: 𝐻𝑢 = 𝑎 𝑄 2 + 𝑏 𝑛 𝑛0 𝑄 + 𝑐 ( 𝑛 𝑛0 ) 2 (8) where 𝑎, 𝑏 and 𝑐 are the coefficients of the specific previous relation. substituting the expression of the valve characteristic (4) in (8), in the steady state operating condition of the system, we obtain: [𝑎 − 𝐾𝑣 (ℎ)]𝑄 2 + 𝑏 𝑛 𝑛0 𝑄 + 𝑐 ( 𝑛 𝑛0 ) 2 = 0. (9) by expressing then 𝑄 as a function of the geometrical and operating parameters of the pump as: 𝑄 = 𝐾𝑄 𝜑 𝑛 , (10) where 𝐾𝑄 = 𝜂𝜈 𝜁 π 2 𝑙2 𝐷2 2, and substituting it in the equation that regulates the operation of the plant (9) we get the new relation: 𝐾𝜙0 2 [𝑎 − 𝐾𝑣 (ℎ)]𝜙 2 + 𝑏𝐾𝜙1 𝜙 + 𝐾𝜙 2 = 0 , (11) where 𝐾𝜙0 = 𝐾𝑄 2 𝑛0 2[𝑎 − 𝐾𝑣 (ℎ)], 𝐾𝜙1 = 𝑏 𝐾𝑄 𝑛0, 𝐾𝜙2 = 𝑐, and 𝜙 the flow parameter. the equality of the head provided by the pump with the pressure drop introduced by the control valve is not sufficient to describe the operation of the experimental system. we need the figure 2. valve characteristic curves in terms of the flow rate coefficient, 𝜙 as function of the opening position, h. the curves show the cases of linear, parabolic, equal percentage and fast opening design. figure 3. inputs and outputs of the pump-motor block. figure 4. pump performance curve at 𝑛 = 1450 rpm. comparison between simulation and actual operational point. table 1. pump data. q in m³/h 1 1.2 1.5 1.8 2.4 3 3.6 4.2 4.8 5.4 6 h in m 6 6.05 6 5.9 5.8 5.5 5.2 4.8 4.4 3.9 3.3 0 2 4 6 8 0 2 4 6 8 h u [ m ] q [m 3 / h] best-fit data sheet acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 4 power conservation law, for which the power supplied by the motor equals the power absorbed by the pump. the last one is expressed by: 𝑃𝑎𝑝 = 𝛾 𝑄 𝐻𝑢 𝜂𝑦 𝜂𝑣 𝜂𝑚 = 𝜌 𝑄 𝐿𝑖 𝜂𝑣 𝜂𝑚 = 𝜌 𝐾𝑄 𝜙 𝜓 π 𝐷 2 𝑛2 2 𝜂𝑣 𝜂𝑚 (12) in which 𝜓 = 2(1 − 𝜙 cotg𝛽2) − (2 π sin 𝛽2)/𝑧 and 𝜂𝑣 𝑒𝑑 𝜂𝑚 are the volumetric and mechanical efficiencies to be discussed in the paragraph of the results, where the methods to evaluate their values during the actual operation of the plant for its monitoring will be described. both could result indices of possible anomalies. with reference to the expression of the power supplied by the motor, 𝑃𝑚, the mechanical torque, 𝐶𝑚, can be expressed. the mechanical torque 𝐶𝑚 of the electric motor can be expressed as a function of the slip, 𝑠, according to the following relation: 𝐶𝑚 = 2𝐶𝑚 max 𝑠max ∙ 𝑠 (𝑠max 2 + 𝑠2) (13) in which 𝐶𝑚max is the maximum torque available for the slip value 𝑠 = 𝑠max, available from the relative data sheet and equal to 𝐶𝑚 = 50 n m and 𝑠 = 0.2. the slip is representative of the difference between the speed of rotation of the rotor, i.e., the shaft and the speed of rotation of the magnetic field, 𝑛𝑠,that is: 𝑠 = 𝑛𝑠 − 𝑛 𝑛𝑠 , (14) where: 𝑛 = 60𝑓 𝑝 (1 − 𝑠) (15) with 𝑝 = 2, motor polar couples. the mechanical characteristic of the motor, 𝐶𝑚, is represented as function of the revolution of the shaft in figure 5 for 3 different values of frequency. this characteristic shows that when the slip is roughly zero the torque reaches its maximum value, and the motor speed is close to the synchronism speed 𝑛𝑠. the efficiency 𝜂 of the three-phase asynchronous motor can be calculated with the well-known formula: 𝜂 = 𝑃𝑟 𝑃𝑎 , (16) where 𝑃𝑟 is the mechanical power supplied to the rotor and 𝑃𝑎 is the electrical power provided by the stator. equating the power absorbed by the pump to the power supplied by the motor, we obtain: 𝜌 𝐾𝑄 𝜙 𝜓 π 𝐷 2 𝑛2 2 𝜂𝑣 𝜂𝑚 = (2𝐶𝑚 max 𝑠max ∙ 𝑠 𝑠max 2 + 𝑠2 ) 2 π 𝑛 (17) and plugging into it the expression of the slip 𝑠, we get: 𝜌 𝐾𝑄 𝜙 𝜓 π 𝐷2 2 ( 𝑝 𝑓 ) 2 𝑛4 − 𝜌 𝐾𝑄 𝜙 𝜓 π 𝐷2 2 2 𝑝 𝑓 𝑛3 + 𝜌 𝐾𝑄 𝜙 𝜓 π 𝐷2 2(𝑠max + 1) 𝑛 2 + 8 𝐶𝑚 max 𝑠max 𝜂𝑣 𝜂𝑚 𝑝 𝑓 𝑛 − 8 𝐶𝑚 max 𝑠max 𝜂𝑣 𝜂𝑚 = 0. (18) the previous equation can be rewritten in the easier-to-read following way: 𝐾𝑛0 𝑛 4 + 𝐾𝑛1 𝑛 3 + 𝐾𝑛2 𝑛 2 + 𝐾 𝑛3 𝑛 + 𝐾𝑛4 = 0, (19) with obvious meaning of the symbols. the former equation represents the main equation of the plant operating model. such an equation can be solved iteratively by the newton-raphson method, for example, to provide the revolution speed of the system for a fixed working condition. the iterations, in the present case, have been interrupted when the percentage error on 𝑛 was less than 10−5. the described set of model equations of the thrust system components constitute the simulator of its working operation. 4. results the volumetric efficiency of the pump changes according to the operating regime. it is negligible for null flow rates in the case of a control valve fully closed. as the flow rate increases, the volumetric efficiency increases up to the plateau value that is almost constant for a wide range of the pump flow rate, in steady state operation. the data sheet values of it are shown in figure 6 here below. in the case of high flow rate, the following procedure aims to calculate the values of the mechanical and the volumetric efficiencies, 𝜂𝑚, 𝜂𝑣∞ (asymptotic value), respectively. the pressure parameter, 𝜓, can be computed, in the following two ways: 𝜓 = 2 𝑔 𝐻𝑢 𝑢2 2 𝜂𝑝 𝜂𝑚𝜂𝑦 (20) or 𝜓 = 2(1 + 𝜙 cotg 𝛽2) − 2 π sin𝛽2 𝑧 . (21) equating the above equations and rewriting the flow parameter as a function of the flow rate, 𝜙 = 𝜙(𝑄), we get: figure 5. characteristics of the asynchronous motor in terms of torque as function of revolution, for different frequency values. figure 6. pump volumetric efficiency experimental data. acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 5 2 𝑔 𝐻𝑢 𝑢2 2 𝜂𝑝 = 2 cotg 𝛽2 𝜂𝑣 𝜉 π 2 𝑙2 𝐷2 2 𝑛 1 𝜂𝑚 𝜂𝑣 𝑄 + 2 𝜂𝑚 𝜂𝑣 (1 − π sin 𝛽2 𝑧 ) . (22) from the experimental data of the pump calpeda nm4 25/12a/a, it is observed that for flow rates greater than a threshold value (0.001 m3/s), the term 2 𝑔 𝐻𝑢 /(𝑢2 2 𝜂𝑝) is a linear function of 𝑄, figure 7. in the linearity range, the coefficients of the previous equation can be calculated by best fitting the pump data sheet discrete values, figure 8, and therefore it is possible to estimate the unknown values of the mechanical efficiency 𝜂𝑚 and the volumetric efficiency 𝜂𝑣 . specifically, the mechanical efficiency is equal to 𝜂𝑚 = 0.955 and it is assumed constant for the whole operating regime, whereas the volumetric efficiency obtained from this procedure represents the asymptotic limit only in the range of high flow rates and it is equal to 𝜂𝑣∞ = 0.719. known the value of the mechanical efficiency, from the (22), 𝜂𝑣 can be made explicit as function of the flow rate only, such as 𝜂𝑣 = 𝜂𝑣(𝑄). the relationship 𝜂𝑣 = 𝜂𝑣 (𝑄) is well described by the following relation: 𝜂𝑣 = 𝜂𝑣∞ (1 − e − 𝑄 𝜏 ) , (23) where the value of 𝜏 = 3.83 ∙ 10−4𝑚3/𝑠 is obtained from the best-fit polynomial of the available data operation data of the pump with 𝜂𝑣∞ = 0.719. the following figure shows the good agreement between the theoretical prediction and pump data over the whole range of the flow rates. the simulator function uses this model of the volumetric efficiency and the prediction of the mechanical efficiency. it provides a good agreement of the control parameters with the characteristic data of the pump, in terms of absorbed power 𝑃𝑎 , efficiency 𝜂𝑝 and pressure head 𝐻𝑢 . through the simulator, described in the previous section, it is possible to digitally reconstruct the pump characteristic curves, figure 10, figure 11 and figure 12. by using the digitalized technical and operating data of the system components and setting a value of the frequency of the inverter regulating the number of revolutions of the pump-motor block, this one can be iteratively determined from (19), and then the outputs of the pump-motor are calculated from its model, i.e. the flow rate, the head, the torque 𝐶𝑚 and the efficiency of the pump 𝜂𝑝, which constitute the virtual values with which to compare the physical ones acquired (and calculated, such as mechanical and volumetric efficiencies) by the sensors prepared for that purpose in the system. so, the entire performance curves of the pump can be reconstructed and digitalized just varying the opening degree of the valve for each of the desired discrete values of the frequency (revolution speed of the pump), and by reiterating the use of the simulator equations as reported in the sections. figure 7. experimental values for the left-hand side term in (22) as a function of the flow rate. figure 8. linear regression of experimental data for the left-hand side term of (22) as function of the flow rate. figure 9. least square regression of 𝜂𝑣 experimental data. figure 10.expected pump efficiency from the simulator compared with actual operational points. 0 1 2 3 0 0.0005 0.001 0.0015 0.002 0.0025 2 g h u /( h p u 2 ) q [m 3 / s] data sheet acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 6 the figure 13 shows the pump performance curves evaluated by the simulator for 3 revolution speed values, as example of its application. the operating parameter values predicted by the models, including the mechanical and volumetric efficiency, are the reference targets for eventual anomalies detection when compared with the relative acquired values. the eventual significative discrepancy between the predicted and sensed values of each characteristic quantity can indicate an incipient criticality for the component whom it refers to. in these conditions it is possible to report and provide for the irregularity of the operation with an active control on the system. in addition, it is even possible to understand the origin of the disagreement between the real and expected values of mechanical or volumetric efficiency. for example, with reference to their possible discrepancy it is possible to suspect a critical condition of the bearings or seals of the pump-motor block. 5. discussion and conclusion the intent of the authors was to propose a strategy for the automation of the monitoring of wide-ranging industrial processes, in the spirit of the "smart industry", in order to reduce the risks of sudden default of the activities caused by the possible criticality of the most delicate components, to reduce the time to identify the criticality, to reduce the maintenance time necessary to restore normal operating conditions, thus reducing the associated costs. the strategy is based on the creation of a virtual simulator of the operation of the various plants involved in the process which, through the digitization of the data sheets of the various components, can provide the reference values of the process control parameters. those values are so compared with the values acquired by the measuring chains predisposed for the purpose, to allow the operator the reporting of any suspected anomaly in place and to quickly intervene to restore optimal operating conditions through targeted maintenance. to this end and by way of an example, we have proposed the formulation of a simulator of a simple fluidic thrust system, consisting of a pump-motor block with inverter and a regulation valve, which represents the load acting on the pm block and attributable to the network it supplies. the simulator allows to manage and analyze in real time all the characteristic parameters, acquired and/or calculated, of each of the monitored system components, during normal operation, to determine if there are conditions of possible incipient anomalies on the components under observation. this operation allows to compare, through realtime acquisitions, the instantaneous values of the characteristic operating parameters with those provided by the simulator, corresponding to those that should be in the optimal operating conditions of the system. so, it is possible to identify which characteristic parameter of the various monitored components of the system reveals a more discrepant value from the optimal one, thus denouncing a possible critical condition of the component to which it refers to. in this way, the operator can decide to intervene on the component in time, thus minimizing the intervention times and therefore the maintenance and restoration costs of the normal operation of the system. is opinion of the authors that the identified procedure, based on the digitization of the technical data sheets of the plant components, is extendable to most of the operating plants of an industrial process, thus allowing the entire process to be controlled in real time. references [1] r. a. luchian, g. stamatescu, i. stamatescu, i. fagarasan, d. popescu, iiot decentralized system monitoring for smart industry applications, 2021 29th mediterranean conference on control and automation (med), 2021, pp. 1161-1166. doi 10.1109/med51440.2021.9480341 [2] l. fabbiano, g. vacca, g. dinardo, smart water grid: a smart methodology to detect leaks in water distribution networks, figure 11. expected pump manometric head from the simulator compared with actual operational points. figure 12. expected pump absorbed power from the simulator compared with actual operational points. figure 13. expected pump performance curves from the simulator for three revolution speed (red lines) and four 𝜂𝑝 iso-lines. https://doi.org/10.1109/med51440.2021.9480341 acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 7 measurement, vol. 151, (2020). doi: 10.1016/j.measurement.2019.107260 [3] l. ardito, a. messeni petruzzelli, u. panniello, a. garavelli, towards industry 4.0: mapping digital technologies for supply chain management-marketing integration, business process management journal, vol. 25, (2019), no. 2, pp. 323-346. doi: 10.1108/bpmj-04-2017-0088 [4] a. j. isaksson, i. harjunkoski, g. sand, the impact of digitalization on the future of control and operations, comput. and chem. eng., 114, (2018), pp. 122–129. doi: 10.1016/j.compchemeng.2017.10.037 [5] g. dinardo, l. fabbiano, g. vacca, a smart and intuitive machine condition monitoring in the industry 4.0 scenario. measurement, 126, (2018), pp. 1–12. doi: 10.1016/j.measurement.2018.05.041 [6] m. short, j. twiddle, an industrial digitalization platform for condition monitoring and predictive maintenance of pumping equipment. sensors, 19, (2019), 3781. doi: 10.3390/s19173781 [7] p. girdhar, c. scheffer, predictive maintenance techniques: part 1 predictive maintenance basics, in practical machinery vibration analysis and predictive maintenance, p. girdhar and c. scheffer, eds., oxford, newnes, (2004), pp. 1-10. [8] p. girdhar and c. scheffer, predictive maintenance techniques: part 2 vibration basics, in practical machinery vibration analysis and predictive maintenance, p. girdhar and c. scheffer, eds., oxford, newnes, (2004), pp. 11-28. [9] m. caciotta, v. cerqua, f. leccese, s. giarnetti, e. de francesco, e. de francesco, n. scaldarella, a first study on prognostic system for electric engines based on envelope analysis, in ieee metrology for aerospace (2014), benevento, italy, 29-30 may 2014, pp. 362-366. doi: 10.1109/metroaerospace.2014.6865950 [10] t. van tung, y. bo-suk, machine fault diagnosis and prognosis: the state of the art, international journal of fluid machinery and systems 2.1, (2009), pp. 61-71. doi: 10.5293/ijfms.2009.2.1.061 [11] li zhe, yi wang, ke-sheng wang, intelligent predictive maintenance for fault diagnosis and prognosis in machine centers: industry 4.0 scenario, advances in manufacturing 5.4, (2017), pp. 377-387. doi: 10.1007/s40436-017-0203-8 [12] e. petritoli, f. leccese, g. schirripa spagnolo, new reliability for industry 4.0: a caste study in cots-based equipment, ieee international workshop on metrology for industry 4.0 & iot (metroind4.0&iot), rome, italy, 7-9 june 2021, pp. 27-31. doi: 10.1109/metroind4.0iot51437.2021.9488555 [13] e. quatrini, f. costantino, g. di gravio, r. patriarca, conditionbased maintenance an extensive literature review, machines, (2020), 8, 31, pp. 1-28. doi: 10.3390/machines8020031 [14] a. k. s. jardine, d. lin, d. banjevic, a review on machinery diagnostics and prognostics implementing condition-based maintenance, mechanical systems and signal processing, vol. 20, (2006), pp. 1483-1510. doi: 10.1016/j.ymssp.2005.09.012 [15] caldepa website. online [accessed 03 november 2021] https://pump-selector.calpeda.com/pump/23 [16] f. fornarelli, a. lippolis, p. oresta, a. posa, computational investigation of a pressure compensated vane pump, energy procedia, volume 148, 73rd conference of the italian thermal machines engineering association, pisa, italy, 12 september 2018, pp 194-201. doi: 10.1016/j.egypro.2018.08.068 https://doi.org/10.1016/j.measurement.2019.107260 https://doi.org/10.1108/bpmj-04-2017-0088 https://doi.org/10.1016/j.compchemeng.2017.10.037 https://doi.org/10.1016/j.measurement.2018.05.041 https://doi.org/10.3390/s19173781 https://doi.org/10.1109/metroaerospace.2014.6865950 http://dx.doi.org/10.5293/ijfms.2009.2.1.061 https://doi.org/10.1007/s40436-017-0203-8 https://doi.org/10.1109/metroind4.0iot51437.2021.9488555 https://doi.org/10.3390/machines8020031 https://doi.org/10.1016/j.ymssp.2005.09.012 https://pump-selector.calpeda.com/pump/23 https://doi.org/10.1016/j.egypro.2018.08.068 dynamic parameter identification method for robotic arms with static friction modelling acta imeko issn: 2221-870x september 2021, volume 10, number 3, 44 50 acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 44 dynamic parameter identification method for robotic arms with static friction modelling dániel szabó1, emese gincsainé szádeczky-kardoss 1 1 department of control engineering and information technology, budapest university of technology and economics, budapest, hungary section: research paper keywords: identification; dynamics; manipulator citation: dániel szabó, emese gincsainé szádeczky-kardoss, dynamic parameter identification method for robotic arms with static friction modelling, acta imeko, vol. 10, no. 3, article 8, september 2021, identifier: imeko-acta-10 (2021)-03-08 section editor: bálint kiss, budapest university of technology and economics, hungary received january 15, 2021; in final form september 6, 2021; published september 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: dániel szabó, e-mail: szabo.daniel@iit.bme.hu 1. introduction automation is playing an increasingly important role in industry, where manipulators are used to solve various types of problems. precise control algorithms are required to meet the need for speed and accuracy, and an accurate dynamic model is needed to use these control algorithms, such as the computed torque method [1]. although the robot manufacturer can provide the necessary values, they are usually either inaccurate or nonexistent; thus, the dynamics of the manipulator have to be determined through an identification process. an independently identifiable parameter vector and a regression matrix are required to create an identifiable model. the barycentric parameters [2] or the modified recursive newton–euler formula [3] can be used to solve this problem. in general, the friction effect of the joints is not negligible, hence the dynamic model of the robot, determined according to the structure of the manipulator, has to be extended with the friction model. there are plenty of existing friction models that can be used in the identification of the dynamic model, but there has to be a trade-off between model accuracy and computational complexity. in the simplest case, only the coulomb and viscous friction effects need to be considered in the dynamic model [4]. although this can be sufficient in some cases, the stiction effect in the joint remains unmodelled, and the nonlinearity has to be compensated for by the controller. a dynamic friction model can be applied if a high level of precision is needed in the model, but in this case, the model cannot be converted into a linear-in-parameters model [5]. this method can be used only if the dynamic model is known and only friction compensation is needed. discontinuities of the sign function can be eliminated by using the arctangent function to approximate the step at which the velocity changes its sign. the continuity of this function is necessary when using the identified model in the controller for a smooth control signal when the velocity reaches zero. in [6], both static and dynamic friction identification were presented but only for cases where the other dynamic parameters were known. it is possible to convert the static friction model into a linear-in-parameters form. the optimisation of the excitation trajectory is important for generating a well-conditioned regression matrix. the finite fourier series [7] and the polynomial functions [8] are used frequently as excitation trajectories. abstract this paper presents an identification method for robotic manipulators. it demonstrates how a dynamic model can be constructed with the help of the modified newton–euler formula. to model the friction of the joints, static friction modelling is used, in which the friction behaviour depends only on the actual velocity of the given joint. with these techniques, the model can be converted into a linear-inparameters form, which can make the identification process easier. two estimators are introduced to solve the identification problem, the least-squares and the weighted least-squares estimators, and the determination of the independently identifiable parameter vector to make the regression matrix maximal column rank is presented. the frobenius norm is used as the condition of the regression matrix to optimise the excitation trajectories, and the form of the trajectories has been selected from the finite fourier series. the method is tested in a simulated environment to achieve a three-degrees-of-freedom manipulator. mailto:szabo.daniel@iit.bme.hu acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 45 the proper estimator for a problem can be selected by investigating the rate of the noise corrupting the measurements [7]. in this study, the least-squares (ls) and weighted-ls (wls) estimation methods were assessed. as was shown in [9], the maximum likelihood estimation gives better results in simulations when there are corrupted joint value measurements, but usually, in real-life scenarios, this noise is negligible compared with the noise of the torque measurements. in [10], the dynamics of the payload were determined by an identification process that measured actuator currents. in this method, the dynamic parameters of the manipulator were known. the present study compares two methods to estimate the parameters of a robot dynamic model with static friction in each joint. it demonstrates how the reduced row echelon form of the regression matrix can be used to find the independently identifiable variables. the results in [9] are extended in this paper by presenting a method incorporating the friction model into the identification process. the paper is organised as follows. in section 2, the form of the dynamic model is introduced, and this section describes how the modified newton–euler formula and the static friction model are used to obtain its linear-in-parameters form. section 3 describes the configuration of the measurements, and the method to determine the independently identifiable parameters is explained in section 4.1. the ls and the wls estimators are then described. after that, section 0 introduces the optimisation of trajectory parameters and the criterion for this optimisation method. the results of the simulation are in section 6, and finally, the conclusions are set out in section 7. 2. model evaluation 2.1. dynamics of the manipulators a manipulator can be modelled with an open kinematic chain, which is composed of links connected by joints. the denavit– hartenberg parameters can be used to define the kinematics of a given manipulator [11], and the homogeneous transformation matrices can be defined to calculate the actual position and orientation of the end effector. the dynamic model of a manipulator is given by the nonlinear multiple-input and multiple-output function, which clarifies the relationship between the joint positions, their firstand secondorder derivatives and the joint torques/forces. the following equation shows this relationship: 𝝉 = 𝑯(𝒒)�̈� + 𝒉(𝒒,�̇�) + 𝒄(𝒒), (1) where 𝐪 denotes the 𝑛-by-1 vector of the joint variables (for an 𝑛 degree-of-freedom manipulator), 𝛕 is the vector of joint torques (or forces), 𝐇 is the 𝑛-by-𝑛 symmetric, positive-definite joint-space inertia matrix, the coriolis and centrifugal forces are modelled by vector 𝐡 and the gravitational effects are given by vector 𝐜. although the model in (1) provides a good physical interpretation of the system, it cannot be used effectively during the identification process. to solve this problem, the linear-inparameters form of the model can be used: 𝝉 = 𝜱(𝒒,�̇�, �̈�)𝜣, (2) where 𝛷 denotes the regression matrix and 𝛩 is the parameter vector. the values of the inertia matrix, the mass of the link (𝑚𝑗) and the first moment of the links are contained in 𝛩: 𝛩 = [𝛩1 𝑇 𝛩2 𝑇 … 𝛩𝑛 𝑇]𝑇 𝛩𝑗 = [𝐼𝑥𝑥 𝑗 𝐼𝑦𝑦 𝑗 𝐼𝑧𝑧 𝑗 𝐼𝑥𝑦 𝑗 𝐼𝑥𝑧 𝑗 𝐼𝑦𝑧 𝑗 … 𝑚𝑗 𝑚𝑗𝑠𝑥 𝑗 𝑚𝑗𝑠𝑦 𝑗 𝑚𝑗𝑠𝑧 𝑗 ] 𝑇 , (3) where 𝐼𝑘𝑙 𝑗 parameters are the entries of the symmetric inertia matrix and 𝐬𝑗 = [𝑠𝑥 𝑗 𝑠𝑦 𝑗 𝑠𝑧 𝑗 ] 𝑇 denotes the position of the centre of mass of the 𝑗-th link measured in the frame belonging to the 𝑗-th joint. to solve the identification problem, it is necessary to calculate the regression matrix. this can be achieved, for example, with the lagrangian formula, which provides a good physical view of the system; but it is computationally expensive. hence, a better solution is the newton–euler formula, which has a computational complexity of 𝒪(𝑛) [3]. first, using the newton-euler formula, the kinematics of each frame of the joints are calculated and transformed into the frame of the end effector. this is done with forward recursion, and the velocities, angular velocities, accelerations and angular accelerations are determined for each frame. then, the forces and moments are transformed with backward recursions from the end-effector frame to the base frame. the applied torque/force of the given joint is determined by the following equation in a given step of the backward recursion: 𝛕𝑗 = { 𝐧𝑗 𝑇 (𝐀𝑗 𝑇 𝐳0) rotational 𝐟𝑗 𝑇 (𝐀𝑗 𝑇 𝐳0) translational , (4) where 𝐧𝑗 and 𝐟𝑗 are the moment and the force exerted on link 𝑗 by link 𝑗-1 and 𝐀𝑗 is the orthonormal rotation matrix that describes the rotation between the 𝑗-th and the 𝑗-1-th frame; 𝐳0 is the base vector of the axis of rotation (or translation), which is selected by the convention [0 0 1]𝑇. this results in linear terms in the inertias and the masses, while the centre-of-mass vector will be represented in either linear or quadratic terms. these quadratic terms can be eliminated by transforming the inertia tensor: let 𝒞𝑖 denote the coordinate system for the 𝑖-th link and 𝒞𝑖 ∗ denote the frame of the centre of mass of the 𝑖-th link; let 𝐀𝑖,𝑖∗ be the orthonormal rotation matrix between 𝒞𝑖 and 𝒞𝑖 ∗. if 𝐈𝑖 is the inertia tensor around the centre of mass, then the inertia tensor 𝐈′𝑖 in 𝒞𝑖 is 𝐈′𝑖 = 𝐀𝑖,𝑖∗𝐈𝑖𝐀𝑖,𝑖∗ 𝑇 + 𝑚𝑖(𝐬𝑖 𝑇𝐬𝑖𝐈(3𝑥3) − 𝐬𝑖𝐬𝑖 𝑇), (5) where 𝐈(3𝑥3) is the 3-by-3 identity matrix. 2.2. friction modelling the model in (1) must be extended with the torque vector 𝝉𝒇 to model the effect of the friction: 𝝉 − 𝝉𝒇 = 𝑯(𝒒)�̈� + 𝒉(𝒒,�̇�) + 𝒄(𝒒). (6) there are several methods to model the vector of the friction torques. in this study, static friction modelling was used, including stiction, coulomb and viscous friction effects, and the striebeck effect, which was represented using the arctangent function [5]: 𝝉𝒇 = 𝝉𝒔𝑆0(𝒗) + 𝝉𝒔𝒄 2 π arctan(𝒗𝛿) + 𝝉𝒗 , (7) acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 46 where 𝒗 denotes the vector of velocities (thus, the first-order derivative of the joint variables), 𝝉𝒔 represents stiction, 𝝉𝒔𝒄 is the difference between coulomb friction and stiction, 𝛿 represents the shape of the striebeck effect, 𝝉𝒗 denotes the coefficient of the viscous friction and the function 𝑆0 is used as the approximation of the 𝑠𝑖𝑔𝑛(𝒗) function: 𝑆0(𝒗) = 2 π arctan(𝒗𝐾𝑣), (8) where 𝐾𝑣 defines the shape of the function. in this case, the friction model can be transformed into a linear-in-parameters form, where the regression matrix and the parameters belonging to one joint are the following: 𝛷fric 𝑗 = [ 2 π arctan(𝒗𝐾𝑣), 2 π arctan(𝒗𝛿) ,𝑣] 𝛩fric 𝑗 = [𝜏𝑠 𝑗 , 𝜏𝑠𝑐 𝑗 , 𝜏𝑣 𝑗 ] 𝑇 . (9) 3. measurement setup simulations were used to perform the measurements in a matlab simulink environment, but the proposed method could also be used with real measurements. to model the dynamics and the friction effect, the simscape toolbox was applied. at this stage, control of the manipulator was performed by computing the torques automatically using the simulation; furthermore, the torque measurements were corrupted with independent zero-mean gaussian noise. the measurements were repeated with different variances for the specific joints, and the joint torques were corrupted with independent zero-mean gaussian noise when each joint had different variances. the joint angles were also corrupted with noise, but this was negligible compared with the noise of the torque measurements [9]. 4. identification process 4.1. determination of the independent variables according to (2) and (9), the dynamics of the manipulator can be written in a linear-in-parameters form, but to use it during the identification process, some changes have to be applied because of the parameter vector described in (3). with these parameter vectors, the column-rank of the regression matrix 𝛷 will not be maximal. this means that unidentifiable parameters or parameters that are not independently identifiable also exist. three parameter types can be defined [12]: 1. if the 𝑖-th column of 𝛷 is a null vector, then the 𝑖-th parameter is unidentifiable and the dynamics are unaffected. in this case, this parameter and the 𝑖-th column of 𝛷 can be removed. 2. if the 𝑖-th column of 𝛷 is not null and can be expressed as a linear combination of the other columns, then the 𝑖-th parameter can be identified only in a linear combination with other parameters. 3. if the 𝑖-th column of 𝛷 is not null and cannot be expressed as a linear combination of the other columns, then the 𝑖-th parameter is independently identifiable. first, all the columns of 𝛷 with only zeros should be eliminated; the number of independent parameters is then equal to the rank of 𝛷. these parameters can be expressed in a linear combination of the original parameters. to obtain the coefficients for the linear combinations, the reduced row echelon form of 𝛷 is required: [𝛷𝑟𝑟𝑒𝑓, 𝐣𝐋] = 𝑟𝑟𝑒𝑓(𝛷), (10) where 𝐣𝐋 denotes the indexes of the columns with the leading values 1 in the reduced row echelon form. these indexes will give a basis in the columns of 𝛷 by indexing out the column of the regression matrix with 𝐣𝐋, and 𝐣𝐊 is the vector of the indexes that are not contained by 𝐣𝐋. let 𝐋 and 𝐊 be the matrices with columns indexed by 𝐣𝐋 and 𝐣𝐊 from 𝛷, respectively. in this way, a linear combination can be defined to express each column of 𝐊 with the columns of 𝐋, and the coefficients can be determined by the columns of 𝛷𝑟𝑟𝑒𝑓 indexed by 𝐣𝐊: 𝐊(: , 𝑖) = 𝛷𝑟𝑟𝑒𝑓(1, 𝐣𝐊(𝑖))𝐋(: ,1) + ⋯+ +𝛷𝑟𝑟𝑒𝑓(𝑚,𝐣𝐊(𝑖))𝐋(: ,𝑚), (11) where 𝑚 is the rank of 𝛷, i.e. the number of independent parameters. the matrix 𝐊 can therefore be expressed as 𝐊 = 𝐋𝐁, with the help of 𝐁 defined as: 𝐁 = 𝛷𝑟𝑟𝑒𝑓(1:𝑚,𝐣𝐊). (12) two vectors of the parameters are defined by indexing them with 𝐣𝐋 and 𝐣𝐊: 𝛩𝐿 = 𝛩(𝐣𝐋) 𝛩𝐾 = 𝛩(𝐣𝐊) . (13) thus, a new form of the model can be introduced: 𝛕 = 𝛷𝛩 = [𝐋 𝐊][ 𝛩𝐿 𝛩𝐾 ] = = 𝐋[𝐼(𝑚𝑥𝑚) 𝐁][ 𝛩𝐿 𝛩𝐾 ] = 𝐋𝛩∗, (14) where 𝛩∗ = 𝛩𝐿 + 𝐁𝛩𝐾. consequently, 𝛩𝐿(𝑖) is an independently identifiable parameter if, and only if, the 𝑖-th row of 𝐁 contains only zeros. 4.2. ls estimation if the torques of the actuators are corrupted with zero-mean gaussian noise (𝐧𝜏) with the same standard deviation, the following measurement model can be defined: 𝛕𝑚(𝑘) = 𝛕(𝑘) + 𝐧𝜏(𝑘) 𝑒𝜏𝑖(𝛩 ∗,𝑘) = 𝛕𝑚𝑖(𝑘) − 𝛕𝑖(𝛩 ∗,𝑘) = = 𝛕𝑚𝑖(𝑘) − 𝐋(𝑘, :)𝛩 ∗, (15) where 𝛕𝑚 is the measured value of the torque vector, the error of the given joint is denoted by 𝑒𝜏𝑖 and 𝑘 denotes the index of the measurement. with this measurement model, the standard ls estimator can be used. the minimisation criterion of the estimator is the following: 𝑉𝐿𝑆 = ∑∑( 𝑛 𝑖=1 𝑁 𝑘=1 𝑒𝜏𝑖(𝛩 ∗,𝑘))2, (16) where 𝑁 is the number of measurements. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 47 the solution to the problem can be given in a closed form with the help of the moore–penrose pseudoinverse of 𝐋: �̂�𝐿𝑆 = 𝐋 +𝛕𝑚 = (𝐋 𝑇𝐋)−1𝐋𝑇𝛕𝑚. (17) 4.3. wls estimation if noises with different standard deviations (𝜎𝜏𝑖) are corrupting the measurements, the wls estimator has to be applied. here the error of the measurements in the optimisation criterion are weighted with the reciprocal of the variation of the noise for a given joint: 𝑉𝑊𝐿𝑆 = ∑∑ (𝑒𝜏𝑖(𝛩 ∗,𝑘))2 𝜎𝜏𝑖 2 𝑛 𝑖=1 𝑁 𝑘=1 . (18) consequently, the problem can be solved as �̂�𝑊𝐿𝑆 = (𝐋 𝑇𝚺−1𝐋)−1𝐋𝑇𝚺−1𝛕𝑚, (19) where 𝚺 is the diagonal covariance matrix of the noise. 5. trajectory optimisation the quality of the identification is dependent on the condition of the 𝐋 matrix, which can be optimised by using the correct excitation trajectories. there are several criteria to perform this. 5.1. optimisation criteria in several papers, the optimisation criterion to find the optimal excitation trajectories is the condition number of 𝐋𝑇𝐋 [13], while in [14], the maximisation of the minimum singular value of 𝐋𝑇𝐋 is used as the optimisation criterion. however, faster convergence can be achieved if the frobenius norm is used as the criterion [8]: condf(𝐀) = ||𝐀||f||𝐀 −1||f ||𝐀||f = (∑∑𝐀𝑖𝑗 2 𝑝 𝑗=1 𝑝 𝑖=1 ) 1/2 , (20) where 𝐀 is an 𝑝-by-𝑝 nonsingular matrix. hence, the optimisation problem can be written in the following form: min 𝛅 condf(𝐋 𝑇𝐋) such that { 𝐪min < 𝐪 < 𝐪max �̇�min < �̇� < �̇�max �̈�min < �̈� < �̈�max 𝐪(0) = 𝐪0 𝐪(𝑡𝑓) = 𝐪𝑓 �̇�(0) = 0 �̇�(𝑡𝑓) = 0 �̈�(0) = 0 �̈�(𝑡𝑓) = 0 , (21) where 𝛅 vector contains the parameters of the trajectory and the 𝐪𝑚𝑖𝑛,𝐪𝑚𝑎𝑥, etc. vectors are the minimal and maximal joint values, velocities and accelerations, respectively. by using equality constraints, the initial (𝐪0) and final (𝐪𝑓) conditions can be taken into consideration. the initial and final velocities and accelerations are selected as zero. the optimisation problem in (21) was solved in matlab using the fmincon function with the interior-point method as the solving algorithm. 5.2. trajectory parametrisation the form of the applied trajectories can be expressed as finite fourier series. the definitions of the parametrisation for the joint variables and their firstand second-order derivatives are the following [15]: 𝑞𝑖(𝑡) = ∑ 𝑎𝑙 𝑖 𝜔𝑓𝑙 𝑀 𝑙=1 sin(𝜔𝑓𝑙𝑡) − 𝑏𝑙 𝑖 𝜔𝑓𝑙 cos(𝜔𝑓𝑙𝑡), �̇�𝑖(𝑡) = ∑𝑎𝑙 𝑖 𝑀 𝑙=1 cos(𝜔𝑓𝑙𝑡) + 𝑏𝑙 𝑖sin(𝜔𝑓𝑙𝑡), �̈�𝑖(𝑡) = ∑− 𝑀 𝑙=1 𝑎𝑙 𝑖𝜔𝑓𝑙sin(𝜔𝑓𝑙𝑡) + 𝑏𝑙 𝑖𝜔𝑓𝑙cos(𝜔𝑓𝑙𝑡), (22) where 𝜔𝑓 is the fundamental frequency of the fourier series. the parameter vector can therefore be defined by an 𝑛 × 𝑀 matrix ([𝑎1 1 … 𝑎𝑀 𝑛 𝑏1 1 … 𝑏𝑀 𝑛]) , where 𝑛 is the number of joints of the manipulator and 𝑀 denotes the number of harmonics in the fourier series. 6. experimental results the implementation of the identification process was performed in matlab, while the dynamics and the friction modelling were evaluated in matlab simulink (section 3). the structure of the three-degrees-of-freedom manipulator used in this study is depicted in figure 1, and its denavit– hartenberg parameters are introduced in table 1. the matlab symbolic toolbox was used to determine the dynamics of the manipulator. the simulated trajectories are shown in figure 2. these trajectories were designed by the method described in section 0. the results of the identification and the symbolic expressions of the final 𝛩∗ parameter vector are shown in table 2. it can be seen that 24 identifiable variables were determined. figure 1. model of the three-degrees-of-freedom manipulator, defined by the denavit–hartenberg parameters in table 1. table 1. the denavit–hartenberg parameters of the modelled three-degreesof-freedom manipulator 𝒊 𝜽𝒊 𝐢𝐧 rad 𝒅𝒊 𝐢𝐧 m 𝒂𝒊 𝐢𝐧 m 𝜶𝒊 𝐢𝐧 rad 1 𝑞1 0.3 0 π 2 2 𝑞2 + π 2 0 0.5 0 3 𝑞3 0 0.5 0 acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 48 figure 2. measurements with optimised excitation trajectories, 𝜎𝜏 = [0.5,0.3,0.7] n·m. table 2. results of the estimations (𝜇: mean, 𝜎2: variance of the estimators) symbolic expression exact values ls wls 𝜇 𝜎2 𝜇 𝜎2 𝐼𝑥𝑥 (2) − 𝐼𝑦𝑦 (2) − 𝑚(2)𝑠𝑥 (2) 2 0.059 0.059 8.27 ⋅ 10 −04 0.059 6.25 ⋅ 10−04 𝐼𝑥𝑥 (3) − 𝐼𝑦𝑦 (3) − 𝑚(3)𝑠𝑥 (3) 2 0.059 0.057 8.09 ⋅ 10 −04 0.058 6.18 ⋅ 10−04 𝐼𝑥𝑦 (2) 0.000 0.002 3.23 ⋅ 10−04 0.001 2.27 ⋅ 10−04 𝐼𝑥𝑦 (3) 0.000 −0.000 2.04 ⋅ 10−04 −0.000 1.47 ⋅ 10−04 𝐼𝑦𝑦 (1) + 𝐼𝑦𝑦 (2) + 𝐼𝑦𝑦 (3) + 𝑚(2)𝑠𝑥 (2) 2 + 𝑚(3)𝑠𝑥 (3) 2 −0.117 −0.115 5.25 ⋅ 10 −04 −0.115 4.30 ⋅ 10−04 𝐼𝑥𝑧 (2) − 𝑚(2)𝑠𝑧 (2) 2 − 𝑚(3)𝑠𝑧 (3) 2 0.000 0.001 2.06 ⋅ 10 −04 0.000 1.50 ⋅ 10−04 𝐼𝑥𝑧 (3) − 𝑚(3)𝑠𝑧 (3) 2 0.000 −0.001 1.60 ⋅ 10 −04 −0.000 1.14 ⋅ 10−04 𝐼𝑦𝑧 (2) 0.000 −0.001 1.32 ⋅ 10−04 −0.000 1.06 ⋅ 10−04 𝐼𝑦𝑧 (3) 0.000 0.001 7.56 ⋅ 10−05 0.001 5.49 ⋅ 10−05 𝐼𝑧𝑧 (2) + 𝑚(2)𝑠𝑥 (2) 2 −0.059 −0.060 3.68 ⋅ 10 −04 −0.060 2.68 ⋅ 10−04 𝐼𝑧𝑧 (3) + 𝑚(3)𝑠𝑥 (3) 2 −0.059 −0.057 3.30 ⋅ 10 −04 −0.058 2.21 ⋅ 10−04 𝑚(2) + 2𝑚(2)𝑠𝑥 (2) − 2𝑚(3)𝑠𝑥 (3) 1.414 1.415 1.50 ⋅ 10 −04 1.414 6.01 ⋅ 10−05 𝑚(3) + 2𝑚(3)𝑠𝑥 (3) 0.707 0.706 3.82 ⋅ 10 −05 0.707 1.42 ⋅ 10−05 𝜏𝑠 (1) 2.000 1.998 7.97 ⋅ 10 −04 1.998 7.80 ⋅ 10−04 𝜏𝑠 (2) 1.000 0.999 1.42 ⋅ 10 −04 0.999 1.41 ⋅ 10−04 𝜏𝑠 (3) 2.000 1.998 2.36 ⋅ 10 −03 1.998 2.34 ⋅ 10−03 𝜏𝑣 (1) 0.500 0.505 8.26 ⋅ 10 −03 0.506 7.89 ⋅ 10−03 𝜏𝑣 (2) 0.277 0.243 9.26 ⋅ 10 −04 0.244 8.08 ⋅ 10−04 𝜏𝑣 (3) 0.030 0.034 1.60 ⋅ 10 −02 0.035 1.54 ⋅ 10−02 𝜏𝑠𝑐 (1) −0.300 −0.302 1.45 ⋅ 10 −02 −0.303 1.39 ⋅ 10−02 𝜏𝑠𝑐 (2) −0.200 −0.176 1.67 ⋅ 10 −03 −0.177 1.65 ⋅ 10−03 𝜏𝑠𝑐 (3) −0.300 −0.306 3.56 ⋅ 10 −02 −0.307 3.45 ⋅ 10−02 𝑚(2)𝑠𝑦 (2) 0.000 0.000 3.82 ⋅ 10−06 0.000 2.24 ⋅ 10−06 𝑚(3)𝑠𝑦 (3) 0.000 0.000 4.67 ⋅ 10−06 0.000 2.37 ⋅ 10−06 acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 49 the dynamics have some independently identifiable parameters (𝐼𝑥𝑦 (2) , 𝐼𝑥𝑦 (3) , 𝐼𝑦𝑧 (2) , 𝐼𝑦𝑧 (3) , 𝑚(2)𝑠𝑦 (2) , 𝑚(3)𝑠𝑦 (3) ) and some unidentifiable variables (e.g. the parameters of the inertia matrix of the first segment), which do not influence the robot dynamics. all the friction parameters are independently identifiable, and the other parameters can be identified in a linear combination with more parameters. it can be seen from the results that the ls and the wls estimators performed well. with this method, the friction parameters were also identified with only one measurement configuration, which simplifies the measurement process. figure 3 depicts the efficiency of the wls estimator in the case of the non-optimised trajectories, using a finite set of fourier series as excitation. the difference between the approximated and the nominal value of the torque vector is negligible. 7. conclusion this paper presents two methods to solve the identification problem of the dynamic model of robotic manipulators. by modelling only the static friction behaviour, the whole dynamic model can be expressed in linear-in-parameters form. in this form, both ls and wls estimators can be used effectively to approximate the unknown parameters of the robotic arm. as can be seen in table 2, all the friction parameters can be identified independently, hence the method can also be used when the dynamic parameters are known, but only for friction compensation. the advantage of this method is that only one measurement configuration is needed to obtain all the desired parameters, but it is not required for moving only one joint at a time. these results can be used in advanced control algorithms. 8. acknowledgement the research reported in this paper, carried out at the budapest university of technology and economics, was supported by the ‘tkp2020, institutional excellence programme’ of the national research development and innovation office in the field of artificial intelligence (bme iemi-sc tkp2020). references [1] r. middletone, g. goodwin, adaptive computed torque control for rigid link manipulators, proc. of the 25th ieee conference on decision and control, athens, greece, 10 – 12 december 1986, pp. 68-73. doi: 10.1016/0167-6911%2888%2990033-3 [2] b. raucent, g. campion, g. bastin, j.-c. samin, p. y. willems, identification of the barycentric parameters of robot manipulators from external measurements, automatica 28 (1992) pp. 10111016. doi: 10.1016/0005-1098(92)90155-9 [3] p. khosla, t. kanade, an algorithm to estimate manipulator dynamics parameters, int. j. robotics and automation, 2(3) (1987), pp. 127-135. [4] m. m. olsen, h. g. petersen, a new method for estimating parameters of a dynamic robot model, ieee transactions on robotics and automation 17 (2001) pp. 95-100. doi: 10.1109/70.917088 [5] m. r. kermani, r. v. patel, m. moallem, friction identification and compensation in robotic manipulators, ieee transactions on instrumentation and measurement 56 (2007) pp. 2346-2353. doi: 10.1109/tim.2007.907957 [6] m. indri, s. trapani, a framework for static and dynamic friction identification for industrial manipulators, ieee/asme transactions on mechatronics (2020), vol. 25, no. 3, pp. 15891599. doi: 10.1109/tmech.2020.2980435 [7] j. swevers, c. ganseman, d. b. tukel, j. de schutter, h. van brussel, optimal robot excitation and identification, ieee transactions on robotics and automation 13 (1997) pp. 730-740. doi: 10.1109/70.631234 [8] m. gautier, w. khalil, exciting trajectories for the identification of base inertial parameters of robots, international journal of robotics research 11 (1992) pp. 362-375. doi: 10.1109/cdc.1991.261353 [9] d. szabó, e. g. szádeczky-kardoss, parameter estimation process for the dynamic model of robotic manipulators, proc. of the 23rd figure 3. comparison between the measured torques (𝝉𝒎), the value of the torque vector without noise (𝝉𝟎) and the estimated torque vector calculated from the identified model (�̂�) using the wls estimator. the measured noise of 𝝉𝒎 is the same as that of figure 2 (𝜎𝜏 = [0.5,0.3,0.7] nm). the bottom graph shows the absolute value of the difference between 𝝉𝟎 and �̂�. https://doi.org/10.1016/0167-6911%2888%2990033-3 https://doi.org/10.1016/0005-1098(92)90155-9 https://doi.org/10.1109/70.917088 https://doi.org/10.1109/tim.2007.907957 https://doi.org/10.1109/tmech.2020.2980435 https://doi.org/10.1109/70.631234 https://doi.org/10.1109/cdc.1991.261353 acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 50 international symposium on measurement and control in robotics (ismcr), budapest, hungary, 15 – 17 october 2020, pp. 1-6. [10] y. dong, t. ren, k. chen, d. wu, an efficient robot payload identification method for industrial application, industrial robot: an international journal (2018), vol. 45 no. 4, pp. 505-515. doi: 10.1108/ir-03-2018-0037 [11] m. w. spong, s. hutchinson, m. vidyasagar, robot modeling and control, john wiley & sons, inc., 2006. [12] l. zollo, e. lopez, l. spedaliere, n. garcia aracil, e. guglielmelli, identification of dynamic parameters for robots with elastic joints, advances in mechanical engineering 7 (2015) p. 843186. doi: 10.1155/2014/843186 [13] f. pfeiffer and j. holzl, parameter identification for industrial robots, proceedings of 1995 ieee international conference on robotics and automation, 1995, vol. 2, pp. 1468-1476. doi: 10.1109/robot.1995.525483 [14] j. swevers, b. de moor, h. van brussel, stepped sine system identification, errors-in-variables and the quotient singular value decomposition, mechanical systems and signal processing 6 (1992) pp. 121-134. doi: 10.1016/0888-3270(92)90060-v [15] k.-j. park, fourier-based optimal excitation trajectories for the dynamic identification of robots, robotica 24 (2006) p. 625. doi: 10.1017/s0263574706002712 https://doi.org/10.1108/ir-03-2018-0037 https://doi.org/10.1155/2014/843186 https://doi.org/10.1109/robot.1995.525483 https://doi.org/10.1016/0888-3270(92)90060-v https://doi.org/10.1017/s0263574706002712 x-rays investigations for the characterization of two 17th century brass instruments from nuremberg acta imeko issn: 2221-870x september 2022, volume 11, number 3, 1 7 acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 1 x-rays investigations for the characterization of two 17th century brass instruments from nuremberg michela albano1,2,3, giacomo fiocco1,4, daniela comelli2, maurizio licchelli5, claudio canevari3, francesca tasso6, valentina ricetti6, pacifico cofrancesco5, marco malagodi1,3 1 arvedi laboratory of non-invasive diagnostics, cisric, university of pavia, via bell’aspa 3, 26100 cremona, italy 2 department of physics, polytechnic of milan, piazza leonardo da vinci 32, 20133, milano, italy 3 department of musicology and cultural heritage, università degli studi di pavia, corso garibaldi 178, 26100 cremona, italy 4 department of chemistry, university of turin, via pietro giuria 5, 10125, torino, italy 5 department of chemistry, university of pavia, via torquato taramelli 12, 27100 pavia, italy 6 museum of musical instruments, castello sforzesco, milano, italy section: research paper keywords: xrf spectroscopy; x-ray radiography; brass, musical instrument; nuremberg citation: michela albano, giacomo fiocco, daniela comelli, maurizio licchelli, claudio canevari, francesca tasso, valentina ricetti, pacifico cofrancesco, marco malagodi, x-rays investigations for the characterization of two 17th century brass instruments from nuremberg, acta imeko, vol. 11, no. 3, article 19, september 2022, identifier: imeko-acta-11 (2022)-03-19 section editor: francesco lamonaca, university of calabria, italy received march 3, 2021; in final form august 25, 2022; published september 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: marco malagodi, e-mail: marco.malagodi@unipv.it 1. introduction in this work, two brass natural horns held in the storage of the museum of castello sforzesco in milan were investigated. one of the reasons for the extraordinary relevance of these objects is related to the makers: they are two of the most important components of the haas family, among the most influential ones in nuremberg (germany), between the 17th and the 18th century. during this period, the city was the capital of brass instrument making, and the families organized into guilds were exporting the instruments throughout europe dominating the field for several generations [1], [2]. here the craftsmanship has been improved so much to become a reference for the future and for contemporary makers. brass, an alloy of copper (cu) and zinc (zn), has always been considered the most suitable material for the construction of brasswind musical instruments. the selection of the chemical composition of such an alloy as well as the manufacturing process used are extremely important in determining the properties of the finished instrument. in particular, the amount of zn can affect the physical and chemical properties of the alloy such as malleability, durability, and resistance to corrosion [3], as well as the mechanical properties and the timbre of the finished instrument [4]. and if those are all crucial aspects for the makers of a new instrument, they are also essential for the modern makers who attempt to reproduce early brass wind musical instruments with the aim to find their original sound and style [5]-[8]. due to the peculiarity of the historical and technological context, and to the poor information on the chemical composition of brass alloys before the xvii century, interest has been growing around ancient nuremberg brass since the last century. the reference composition of the alloy was defined by study through x-ray fluorescence spectroscopy of 273 nuremberg jetons produced during the period from 1475 to abstract a recent finding at the castello sforzesco in milan of two brass natural horns from the end of the 17th century and assigned to the haas family from nuremberg brought to light new information about this class of objects. the instruments were heavily damaged, but their historical value was great. in this study, a multidisciplinary approach mainly based on non-invasive analytical techniques and including x-rays investigations (x-ray radiography, x-ray fluorescence and x-ray diffraction) was used. the present study was aimed at: i) pointing out the executive techniques for archaeometric purposes; ii) characterizing the morphological and the chemical features of materials; and iii) identifying and mapping the damages of the structure and the alterations of the surface. mailto:marco.malagodi@unipv.it acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 2 1888: considering the nuremberg trade laws that permitted the use of the sole local row supplies, the material used to forge the jeton must have been the same used by brasswind instrument makers [9]. nuremberg brass features include great purity, notwithstanding such a rare and precious alloy was commonly obtained by recycling any scraps believed to contain copper or zinc. further studies confirmed its high purity degree and the great compositional difference from other similar alloys coming from different geographical areas [4], [10]. the present study aims at contributing to the characterization of the materials used in the brass instruments’ craftsmanship. moreover, mainly non-invasive techniques have been used for documenting the executive and decorative details (e.g., diameters of tubes, depths of joints or jointing methods) and for evaluating the conservation state of instruments held in museum collections. this approach represents an essential support to define the best preservation and maintenance practices [11]-[14]. in particular, the diagnostic campaign was accomplished through photographic [15] and stereoscopic documentation of the surface details, x-ray radiography (rx), and x-ray fluorescence spectroscopy (xrf). small amounts of powder were detached from selected areas and analyzed by x-ray diffraction (xrpd). moreover, fourier-transformed infrared (ftir) spectra were collected in reflection mode where the surface was suitable to permit the correct acquisition geometry [16]. among the techniques, xrf generally affords the chemical characterization of the alloy disclosing plenty of information, even suggesting clues on the original parts, replacements, and conservation issues [17]-[19]. the realization of two 3d models acquired by a laser scanner and the prototyping of an augmented reality application for the museum exposition, detailed in previous work [20], complemented the investigation by fulfilling the collaboration with the museum. the main objectives of this investigation were: i) pointing out the executive techniques; ii) characterizing the morphological and the chemical features; and iii) identifying and mapping the structural damages and the alterations of the surface. 2. materials and methods the investigated instruments are shown in figure 1. the oldest one (figure 1a), cataloguing number inv.878, was made by johann wilhelm haas (1649-1723) who represents the most important member of the family and for what concerns the model, it attests the early construction of this horn-type in nuremberg, and it is probably dated back to the late 17th century. the second one (figure 1b), cataloguing number inv.877, is datable to the 1720s and was made by wolf wilhelm haas (16811760), son of johann. the photographic documentation under visible light was acquired with a nikon d4 full-frame digital camera equipped with a 50 mm f.1.4 nikkor objective using a softbox led lamp. the magnification of the surface details was performed with an olympus stereomicroscope equipped with an olympus hd dp73 camera. the images were recorded through stream essentials software. x-ray radiographic investigation was carried out by means of an x-ray generating industrial control machines cp 120b (settings: 110 kv, 1 μa of 100 s exposure time) and a photosensitive radiographic plate dürr ndt gmbh & co. scanned with cr35ndt dürr ndt. xrf analysis was exploited by elio portable x-ray fluorescence spectrometer by xglab (milan, italy), with an analytical spot diameter of 1.3 mm [21]. the x-ray source worked with a rh anode. x-ray fluorescence spectra were collected on 2048 channels with the following set parameters: 40 kv, 60 μa, and 120 s. data were processed by elio 1.6.0.29 software. with the aim to minimize the errors relative to the net area, the dead time of the detector was also kept constant varying the current intensity around the declared value. a small amount of few samples of crystalline phases were analyzed by xrpd. xray diffraction patterns were collected on powder samples taken from areas chosen by the conservators. x-ray powder diffraction analyses were performed by a d5005 instrument by bruker (germany). the measuring conditions were cukα radiation, figure 1. visible photographic images of a) inv.878 by johann wilhelm haas (1649-1723) and b) inv.877 by wolf wilhelm haas (1681-1760); x-rays radiographic images of inv.878 c) and inv.877 d) instruments. acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 3 λ = 1.54060 å, 2θ angle ranging from 3° to 80° with a scan rate of 0.012°/min. a bruker silicon zero background sample holder was used to allow measurements with very small amount of sample. the diffraction patterns were analyzed with eva and match software for the identification of the phases. ftir spectroscopic analyses were performed using the alpha portable spectrometer (bruker) equipped with the r-alpha module. the working distance of about 15 mm with a beam diameter of 5 mm, enabled non-contact examinations. spectra were collected between 7500 and 375 cm-1 at the resolution of 4 cm-1, with an acquisition time of 1 min. for the investigation of absorption bands, due to the occurrence of the transflection, reflection infrared spectra were observed and are shown hereby in pseudoabsorbance. data were processed using opus 7.2 software. 3. results and discussion the horn made by wolf bears the inscription "macht / johann / haas / nurnberg" and the initials "iwh" above a leaping hare turning backwards its head, while the other one by the father johann, bears the hare keeping its face forwards [22]. the bells and the tubes were detached, some parts were lacking, and the surface suffered from different kinds of alteration. in both instruments, numerous deformations of the lamina, macroscopic cracks, and areas of soldering and reparations were highlighted and a magnification of some of their visible traces on the external surface has been documented by stereomicroscope. the study of the structural features gave access to the manufacturing processes, to identifying the damages, the presence of some reparations or restorations. even the alterations commonly found on the inner surface of tubes caused by the playing were put into evidence [23]. rx investigation suggested that the inv.878 (figure 1c) was characterized by a lamina with a uniform thickness joint by a nipped tooth joint between the edges (figure 2a). two important elements referred to the peculiar executive technique were highlighted for this instrument through rx: (i) the presence of the “gusset” (figure 1c) which is a “v” shaped lamina inserted with nipped tooth joints on both sides added to prevent too much stretching of the edge during the hammering to obtain a larger bell; (ii) the difference in the optical density around the ring that divides the tube into two parts and that seems to join laminas of different thickness. otherwise, the bell of the inv.877, which showed a unique metal joint (figure 1d), was made of a uniform lamina, probably thinner, and characterized by numerous cracks. moreover, the presence of the metal thread detailed in figure 2b, confirmed a traditional making technique of the garland that was the standard bell finishing method of early brasses before the industrial revolution: sheets of brass were cut in a “y” shape, folded, brazed, and hammered over a mandrel, the garland was then fitted over the edge without soldering [24]. on the outside of the bell, the position of the original support between the bell and the tube (the stay) is proven by a leftover. for this instrument, even the radiography (figure 1d) suggested the use of the traditional executive technique employed up to the first part of the xix century [24]. the tube was a single folded element probably obtained by flattering and hammering a brass lamina (cold-worked) then followed by soft annealing. with this procedure, the required shapes (bells and tubes) were produced [25]. the xrf results relating to the alloys (table 1) revealed that both the horns were manufactured from a yellow brass which is a kind of brass that generally contains 23 41 % of zinc as the major alloying element and may contain up to 3 % of lead and up to 1.5 % of tin as additional alloying elements [13]. in the studied objects, a slightly variable cu: zn ratio was detected for the tubes and the bells, respectively. the quantity of cu ranges between 68% and 75%, zn between 21% and 28%, and variable additional alloying elements such as iron (fe), nickel (ni), and lead (pb) in less than 1% amount, were also distinguished. generally, yellow brass and in particular the α-brass (with up to about 32.5 wt. % zinc) are ductile, easily cold-worked, can be rolled or hammered into thin sheets, and have good corrosion resistance in a salt-water atmosphere [26], and for these reasons, it was extensively employed for the making of historical brasses [10], [27]. the presence of several trace elements was the proof of the traditional cementation method employed by the early craftsmen to obtain brass, which included the use of calamine (a high-grade zinc carbonate ore). this process involved many different raw materials instead of melting the two principal elements of copper and zinc directly into each other as occurred in the modern processes [10], [28], [29]. among the trace elements commonly found in the historical brass, pb is the most critical since it could affect the mechanical properties of the material, reducing the brass shear strength and making it prone figure 2. details magnification by stereomicroscope: a) nipped tooth joint (inv. 878); b) metal thread in the garland (inv.877); c) corrosion morphology with pustules (inv. 878); d) reddish surface inside the bell (inv.878); e) reparation with patch and soldering (inv.878); f) white dusty leftovers inside punching (inv.877); g) translucent green matter along the tubes (inv. 877); h) possible residue of surface treatment (inv.878). acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 4 to crack [30], [31]. in this case, the pb amount is limited and the observed embrittlement and cracking of the laminas could be imputed to mechanical stress also probably promoted by a selective corrosion and/or dezincification process that commonly affect the ancient brass object [31]-[35] whereas the presence of calcium (ca) detected widespread on the surface with variable and apparently random amounts must be considered independent of the brass composition and probably introduced by the environment or by cleaning procedures. in fact, a paste of precipitated chalk and water was often used with a soft cloth to remove the residue of corrosion products after the mechanical removal of dust and dirt [13], [27]. among the trace elements, sulphur (s) was detected almost everywhere. although it is not reported in the xrf results tables (table 1 and table 2) due to the low sensitivity of the technique to this element and the impossibility to provide affordable % amounts, more intense signals s were detected in correspondence of the brown and black pustules of corrosion spread on all over the surface (figure 2c). the elemental composition of this kind of corrosion did not differ from the composition of the sound brass lamina. a low amount of powder sampled and examined through xrpd oddly revealed an unexpected amorphous nature. nevertheless, the presence of sulphates as corrosion products was occasionally confirmed by the ftir analysis, elsewhere, possibly in combination with acetates and carbonates. accordingly, in figure 3, two selected spectra acquired on the bells of the two horns (black and grey spectra) show the presence of a mixture of the inorganic compounds mentioned above. with regard to the spectral features at higher wavenumbers, the signal in the range between 3700 and 3300 cm-1 could be assigned to the oh stretching vibration. instead, observing the fingerprint region, the main features of three main groups were recognized. the presence of a copper carbonate-based (i.e. malachite cuco3cu(oh)2) was identified through the absorption bands centred around 1575, 1495, 1420, 1395 cm-1, all caused by the co3 antisymmetric stretching vibrations; and those around 1095 cm-1, 820, 750 and 715 cm-1 generated by the co3 symmetric stretching [36]. moreover, the bands peaked around 1050 cm-1 and 880 cm-1, ascribed to the oh bending support the attribution. the presence of copper sulphate (i.e. brochantite cuso43cu(oh)2) could be hypothesized for the bands centred around 1120 cm-1, 980 cm-1, 620 cm-1 all of them reasonably assigned to the stretching vibration of the so42ion; finally, the signals around 450 cm-1 could be ascribed to the cu–o vibrations [36]. finally, also the characteristic bands of acetate (i.e. cu(ch3coo)2h2o, zn(ch3cooh)2h2o) were identified in the main bands around 1625 and 1425 cm-1 related to the copper carboxylates and their antisymmetric and symmetric stretching vibrations; and around 1455, 1355 and 1030 cm-1 possibly ascribable to ch3 deformations [36], [37]. all those materials were supposed to be alteration products due to the interaction with the environment. in addition, the presence of the organic fraction (i.e. natural resins and oils) could be tentatively recognized through the strong sharp carbonyl band around 1740 cm-1 [38] and the quite strong hydrocarbon stretches for both methylene groups around 2925 (asymmetric) and 2850 (symmetric) cm-1, and methyl groups around 2962 and 2872 cm-1. an outstanding result must be reported for the composition of both the instruments' bells: if the alloy of the tube and the outside of the bells are comparable (the same in inv.877), one of the insides of the bells was characterized by a higher amount of cu that corresponded to a more reddish surface with respect to the others (figure 2d). generally, cu enrichment of ancient brass surfaces was commonly due to the selective corrosion of the zn or an underwent dezincification process triggered by handling or by the contact with acids [13]. in this case, the homogeneity of the surface prompted the hypothesis of a treatment done on purpose to diversely decorate the bell: finishing it with a dull coating or a fine decoration was a common practice [3]. the soldering and the patches (figure 2e) were characterized by a coarse aspect that could be correlated to later restorations. xrf investigation shows that the nipped tooth joint was lacking the brazing as expected, and probably only the stay of the inv. 878 results to be brazing of a comparable composition with the brass alloy. the other solder joints are soft solder assembled with pbsn solder containing a variable ratio of the elements (table 2). the use of this kind of solder is largely documented in ancient copper-based objects and the proportions of the elements can vary from 30% pb (and 70% sn), to as much as 98% pb (and only 2% sn) [26], [39]-[42]. the results about the soldering and the similarity between the alloys of the mouthpipes joint in both table 1. chemical composition of the constitutive alloy of the tube and of the outside and the inside of the bell for both the instruments, namely inv.877 and inv.878. three points for each area were acquired. the mean and the standard deviation of the wt% values are shown for the detected elements. ca (wt%) cu (wt%) fe (wt%) ni (wt%) pb (wt%) zn (wt%) in v. 8 7 8 tube 2.0 ± 0.4 68.9 ± 1.1 0.6 ± 0.0 0.4 ± 0.0 0.3 ± 0.0 27.9 ± 0.6 bell out 1.1 ± 0.3 70.7 ± 0.6 0.6 ± 0.0 0.3 ± 0.0 0.4 ± 0.1 27.0 ± 0.8 bell in 2.7 ± 0.7 74.8 ± 0.6 0.6 ± 0.1 0.3 ± 0.0 0.4 ± 0.4 21.1 ± 1.7 in v. 8 7 7 tube 2.7 ± 2.7 68.5 ± 2.5 0.6 ± 0.2 0.3 ± 0.0 0.2 ± 0.2 27.7 ± 0.6 bell out 2.6 ± 0.4 68.5 ± 0.8 0.4 ± 0.1 0.3 ± 0.0 0.3 ± 0.1 27.8 ± 0.6 bell in 3.4 ± 2.3 72.2 ± 0.5 0.4 ± 0.0 0.4 ± 0.0 0.4 ± 0.1 23.2 ± 2.0 figure 3. reflection ftir spectra in pseudo-absorbance collected from inv.878 and inv.877. marker bands of copper carbonate-based (*), acetates (◆), sulphates (▽), organic matter (■), and zeolites (+) are highlighted. acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 5 the instruments could prove later restorations supporting the hypothesis of the conservator according to which the mouthpipes underwent replacement. concerning the patches, each of them showed a different chemical composition: sn-based (tube inv.878), or cu, sn and pb, or pb and sn alloy (table 2) probably denoting different periods and restorers. about the decoration in the middle of the tubes with a ring shape, the composition of the inv.878 one resulted to be made of an alloy of a comparable composition of the tube. differently, in the latter (inv.877) the cu/zn alloy was enriched with pb and sn (more than 1%) and for this reason, it could be considered a later supplement, processed differently by the laminas the instrument was made up of. in fact, pb is known to be added to facilitate the shape modelling or the pouring of the alloy into the mould, or the soldering of the element. furthermore, white dusty and adherent deposit was observed as a patina and agglomerated inside the engravings or punches (figure 2f), and often in correspondence with the reparations. the xrf analysis detected a certain amount of ca (ranging from 6% up to 26%), probably a cleaning residue; xrpd examination even confirmed the presence of pure quartz often found among the ingredients of the modern polish for metal surfaces. in other areas of the tubes, spots of white ink with titanium (ti) based composition were also identified: an accidental stain of wall painting could not be excluded. with a similar distribution, a green translucent matter was found in the correspondence of small areas where the surface was more worn-out along the tubes (figure 2g). if xrf could not highlight significant information except for a more intense peak of silicon (si), xrpd examination clearly identified siliceous faujasite (na2, ca, mg)35[al7si17o48] 32(h2o) and, more generally, other zeolites, probably ascribable to polish supposed to be a synthetic mixture of lubricant and small dispersed particles (zeolites). this hypothesis is also endorsed by ftir results reported in figure 3 (green spectrum). here, some of the most characteristic bands of zeolites were identified [43], [44]: in particular, the bands that range from 3750 to 3450 cm-1 are ascribable to si-oh, si-oh-al and –oh hydroxyl groups whereas the bands between 1200 and 450 cm-1 could be assigned to si-o-al, si-o-si, si-o, si-al species. in particular, the bands ranging between 800 and 600 cm-1 in the green spectrum in figure 3 is probably due to the si-o-m where m is the ion metal species (na, ca, mg). moreover, concerning the hypothesized synthetic fraction, in the same spectrum the bands of the acetates and carbonates above-described and ascribed to the alteration products of the alloy, were accompanied to sharper bands in correspondence of the ch stretching vibration in the ranges 3200 – 2800 cm-1, to the bands ascribed to the coovibration in the range 1590-1520 cm-1, and to the bending vibration of the ch2 around 720 cm-1 which could suggest the presence of a synthetic mixture or a metal soaps originated from degradation or used as lubricants, probably, for maintenance or restoration practice [45]. finally, the documentation of a superimposed layer characterized by red particles suggested the presence of surface treatments (figure 2h). in fact, brass tends to oxidize quickly when exposed to air and as a common practice, a varnish was applied to give shine and protection to the metal alloy [3], [39]. unexpectedly, neither the xrf nor the ftir analysis even if confirming the presence of some organic matter could clearly prove the presence of such a coating. table 2. chemical composition of the selected area of interest in both the instruments, namely inv.878 and inv.877. a brief description of the measuring area and the relative results expressed in wt% are reported. ca (wt%) cu (wt%) fe (wt%) ni (wt%) pb (wt%) sn (wt%) ti (wt%) zn (wt%) in v. 8 7 8 soldering and reparations mouthpipe soldering 12.1 14.5 1.0 0.1 50.7 15.9 5.7 stay soldering 5.7 66.8 0.7 0.4 0.3 26.1 tube ring 1.5 70.6 0.5 0.4 0.5 26.5 patch 18.4 4.9 1.7 2.5 71.8 0.8 bell out soldering 13.6 2.0 1.0 21.9 61.0 0.5 others tube green 1.8 69.5 0.6 0.4 0.2 27.7 white powdery 6.3 59.6 5.3 0.3 0.7 27.8 bell out black corrosion 2.4 69.5 0.7 0.3 0.5 26.7 bell in black corrosion 1.7 75.5 0.6 0.3 1.0 20.9 in v. 8 7 7 soldering and reparations mouthpipe soldering 15.7 0.9 3.2 50.5 29.3 0.4 soldering 11.6 11.8 1.1 55.1 15.8 4.7 tube patch 3.3 2.9 0.1 30.9 62.4 0.5 ring 57.3 0.9 0.3 7.4 9.9 24.2 bell out patch 10.7 48.7 0.5 0.3 13.5 6.6 19.8 patch_ patina 14.0 22.8 6.1 0.1 32.1 12.5 12.4 joint 3.7 60.4 0.5 0.4 0.4 1.8 32.9 bell in joint 7.0 62.0 0.7 0.4 0.6 1.0 28.3 others tube green 6.4 63.2 0.5 0.3 0.2 29.4 white ink/matter 47.2 25.8 0.7 0.2 15.0 11.1 white powdery 26.0 52.3 0.4 0.3 0.2 20.8 black corrosion 8.8 65.9 1.2 0.3 23.8 acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 6 4. conclusions the non-invasive approach presented in the paper, mainly based on x-ray techniques, allowed us to obtain a map of the damages and the fragile areas of the two investigated brass horns. besides, it provides the conservator with some tools to better define the conservation state of the objects: complete documentation was provided for both the inner structure, the morphology, and the surface aspect with high magnification of the most significant details. moreover, information about the craftsmanship of the brass wind instrument in nuremberg during the 17th century was disclosed with a hint to the evolution of the technique during a period of about a century in which the traditional procedures seem to be kept. the in-depth chemical characterization of the instruments highlighted the original and the superimposed parts. in many cases, the superimposed matter and the reparations seem to have the same nature, supporting the idea that the instruments were kept together in the last part of their history. furthermore, discriminating the original parts enabled the opportunity to produce a replica of the objects to hopefully retrieve the original aspect and the timbre. unfortunately, further knowledge of the cold working and annealing of the laminas could only come from the microstructural investigation of the alloy. the availability of material for further investigations will be able to answer the questions left open by this study about the main corrosion products and the surface treatments that turned red on the inside of the bells, as useful and precious support for conservators and restorers in the planning of the preliminary preservation strategies. acknowledgement the authors would like to thank prof. renato meucci, for the valuable collaboration and for its worthy studies about 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https://www.thehenryford.org/docs/default-source/default-document-library/the-henry-ford-brass-amp-bronze-conservation.pdf/?sfvrsn=2 https://www.thehenryford.org/docs/default-source/default-document-library/the-henry-ford-brass-amp-bronze-conservation.pdf/?sfvrsn=2 https://doi.org/10.1016/j.phpro.2009.11.113 https://doi.org/10.1007/978-3-319-90617-1_10 arel – augmented reality–based enriched learning experience acta imeko issn: 2221-870x september 2022, volume 11, number 3, 1 5 acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 1 arel – augmented reality–based enriched learning experience a v geetha1, t mala2 1 research scholar, department of information science and technology, college of engineering, anna university, india 2 associate professor, department of information science and technology, college of engineering, anna university, india section: research paper keywords: augmented reality; learning technologies; education; vuforia citation: a v geetha, t mala, arel – augmented reality–based enriched learning experience, acta imeko, vol. 11, no. 3, article 12, september 2022, identifier: imeko-acta-11 (2022)-03-12 section editor: zafar taqvi, usa received march 30, 2022; in final form july 30, 2022; published september 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: a v geetha, e-mail: geethu15@gmail.com 1. introduction within few months, the pandemic of coronavirus illness 2019 (covid-19) caused by the novel virus sars-cov-2 has forced enormous changes in the way businesses and other sectors operate. according to world economic forum, 1.2 billion children in 186 countries were affected by school closures, as of march 2021 [1]. moreover, the new wave of cases in several regions of the world impacts the return towards normalcy. herd immunity and vaccines provide only temporary relief to regions affected by the new virus strains. thus, online learning has evolved into a viable alternative to traditional classroom-based learning, with instruction delivered remotely and using digital platforms. even before the covid era, there is a steady increase in the growth rate and adoption of technology in education. according to globenewswire, it is estimated that the online education market will reach $350 billion by 2025. in addition, concerning the response to covid-19, several online education platforms such as dingtalk, have scaled their cloud services by more than 100,000 servers [2]. augmented and virtual reality (ar/vr), the emerging technology trend, can improve the online learning experience by increasing engagement and retention. vr headsets such as google cardboard (gc) have made the technology accessible to most of the world's population. vr and ar based online learning platforms offer experiential learning, where the students learn through experience rather than through traditional methods such as rote learning. some of the benefits of experiential learning are accelerated learning, engagement, understanding of complex concepts easily. traditional educational methods are progressively becoming digital, due to technological advancements. mobile augmented reality (ar) is the superimposition of the virtual objects over reality. ar is widely used in many field such as manufacturing, robotics, behavioral treatment, aircraft engineering design and so abstract in present era, teaching occur either on a chalkboard or on a projected power point presentation on the wall. traditional teaching methods such as blackboards and power point presentations are being phased out in favor of enriched learning experiences provided by emerging edtech. with the closure of schools due to covid-19, the demand for online educational platforms has also increased. furthermore, some of the recent trends in edtech include personalized learning, gamification and immersive learning with extended reality (xr) technologies. due to its immersive experience, xr is a pioneering technology in education, with multiple benefits including greater motivation, a positive attitude toward learning, concrete learning of abstract concepts, and so on. existing augmented reality (ar) based education applications often rely on unimodal input such as marker-based trigger to launch the educational content. hence, this work proposes a multi-modal interface to enable the content delivery through marker and speech recognition-based content delivery. additionally, the proposed work is designed as mobile based ar platform with the regional language support to increase the ubiquitous accessibility of the ar content. thus, the proposed mobile ar based enriched learning (arel) platform provides a multimodal mobile based educational ar platform for primary students. based on the feedback received after the usage, it is observed that arel improves the learning experience of the students. mailto:geethu15@gmail.com acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 2 on [3]. the spectrum of extended reality (xr), which refers to the spectrum of real-virtual environments. xr has gradually evolved in the realm of education, revolutionizing pedagogical practices as well as students' educational experiences by facilitating the understanding of complicated aspects in education through the visual depiction of images based on realworld data [4]. specifically, ar and vr has been extensively used in many educational applications. for example, in [5], a solar system mobile app is designed to test the knowledge retention of college students. in [6], a collaborative augmented reality system is used to deliver geometrical concepts in mathematics, which are abstract can be easily illustrated using ar platform. as a result, ar aids in the comprehension of challenging concepts of the learning module. in [7], the work presents a gesture-based ar system to aid in understanding the anatomical structures of the human body. [8] focuses on the concept of comic-based education through markerless ar for improving metacognitive abilities of the students. thus, there are variety of approaches for designing the educational apps based on ar and vr such as head mounted displays, markerless systems, and gesture-based systems. in addition, mobile ar systems are effective because they allow for portability and easy access. therefore, the proposed arel system is a mobile ar-based learning platform in which students can scan the contents of their books to discover videos that appear magically over the pages, transforming a plain textbook into a book with dynamic information. furthermore, arel delivers the contents in regional language of the students to improve the engagement with the app. arel is made up of a collection of modules such as speech recognition system, image tracking and registration module that take advantage of mobile sensors and computational power. the application is developed using unity engine and vuforia sdk. the mobile application interfaces with the vuforia cloud target recognition system via a client-server architecture. to grab students' attention and increase their learning experience, the content in the book is enriched with augmented graphics, animations, and other edutainment features. the mobile camera is linked to the scene's virtual camera as soon as the program is launched. once the target image is recognized with the help of the vuforia image database, using pattern recognition algorithms, the corresponding output view is rendered using the display unit. as a result, the output view consists of virtual objects laid out over the real-time objects. the triggered audio output explains the concepts that are scanned, which in turn improves the experience of the learning. the learning module also consists of multiple-choice questions on the contents taught, to assess the understanding of the learned material. based on the feedback received after the usage, it is observed that arel positively improves the learning experience. 2. methodology arel is designed as a multi-modal ar interface for students to deliver mobile-based learning. based on the survey of various literature related to ar based educational platform design, it is observed that ar-based learning platform improves engagement and comprehension of difficult concepts. ar also provides an experiential learning experience rather than traditional methods such as rote learning or instructor-led learning. arel is designed as a mobile-based ar system for increasing the accessibility of ar based learning for students. thus, arel complements and improves the online learning solutions or protocols developed during the covid era. 2.1. development model arel is developed using the rapid application development (rad) model, as it accelerates the system development. rad model is appropriate when the product development time is less, and the project requires high component reusability and modularity. figure 1 illustrates the rad model followed in the work. the rad model involves four development phases, and they are briefly explained as follows: • requirement gathering: in this phase, the objectives and requirements for the products are gathered based on the technical review. thus, this phase aids in the understanding of the project goals and expectations. • user description: to design the component, the developer collects the description of the component design from its user. based on the design from the user, a prototype of the application is developed, which further reviewed, and the design is updated. • implementation: in this phase, the developer implements the requirements and perform testing on the product. for a typical ar application, this phase involves: ui interface design, creation of 3d objects, coding and testing of the product. • evaluation: once the product is completely developed and tested whether the user expectations are met. once the product is successfully evaluated, the project reaches the users. 2.2. software used • unity: unity is a game development engine which is used to create games for 3d environments such as vr and ar. unity supports scripting through c# [9]. • the applications developed using unity can be exported to platforms such as ios, android, or desktop platforms like windows. unity provides a comprehensive framework for adding interactive animations, audio and physics based logical simulations for natural and close to real interactions. therefore, arel is designed used unity engine. • vuforia software development kit (sdk): vuforia is an sdk supported by unity and enables creation of ar applications for mobile [10]. it uses computer vision-based technologies to track image targets, object-targets, or area targets for marker-based ar application. • upon recognition, the virtual object is placed relative to the marker and the virtual camera position. vuforia is integrated figure 1. rad development model. acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 3 to the unity engine for developing the ar concepts of arel. • google speech-to-text api: the google speech to text api [11] enables integration of the speech recognition ability into to variety of applications. upon sending a voice audio, it sends a transcript of the audio from the service. it uses sophisticated deep learning models ranging from long term short term-recurrent neural networks to sophisticated speech recognition algorithms to perform accurate recognition. 3. augmented reality based enriched learning the objectives of the arel system are as follows: • to design a mobile-based ar system which increases the accessibility of ar based learning for students • to design an ar platform that can act as teaching aid for the students • to complement and improve the online experience through ar • to provide multi-modal interface and regional language support. the objectives are achieved in arel through its multi-modal interfaced content delivery methods. arel consists of two modes of content delivery as follows: a) image target-based content delivery b) speech-to-text based content delivery. this is illustrated in figure 2, where the application receives input from the camera and microphone to deliver the content via ar. 3.1. image target-based content delivery the ar interface of the system is developed using vuforia sdk and uses its image target database system for processing the image targets. the image-targets from the children’s textbook is created and the processed in the target database system of vuforia. it is then integrated with the application through unity gaming engine. upon scanning these image targets, the virtual camera performs an image recognition based on the features available in the target database. once a matching image target is found, relevant content is displayed as ar content. figure 3 illustrates the image-target based content delivery. the output view consists of video content or 3d objects (with audio description) laid out over the real-time objects. 3.2. speech-to-text based content delivery arel also supports speech-to-text based content delivery. the audio samples received from the microphone is preprocessed using noise cancellation and the speech input is sent to the speech-to-text service. speech processing involves several steps, including analysis, feature extraction, modelling, and testing. the feature extraction process extracts unique features of the audio using mel frequency cepstral coefficients (mfcc) technique. upon recognizing the sample, the speech input is converted to text. if the text matches any speech commands, then the appropriate ar content is displayed. the detailed steps for the content delivery here is as follows: 1) record a short audio from the user’s microphone 2) convert the audio into wav format 3) upload the file into the google server 4) once the uploaded file is processed, it receives the output from the json file. 5) process the json file with the text command, the corresponding number is displayed figure 4 depicts the working of the speech to text-based content delivery. algorithm 1 depicts the process involved in the speech-to-text based content delivery. algorithm 1: speech-to-text based content delivery input: microphone audio (a) output: ar content based on speech 1: 𝐶:=command_words 2: w:=convert_audio_to_wav(a) 3: text_from_speech:=speech_to_text_api(w) 4: if text_from_speech ∈ 𝐶 then 5: command= text_from_speech ∩ 𝐶 6: load_content(command) 7: end if 4. results and discussion arel provides an ar based multi-modal audio-visual ar based learning with regional language support. to evaluate the usability and the learning experience of the arel system, an observation study of the prototype system is made in a primary school in chennai, india. participants of this study range from 58 years. the students were instructed on how to use the app over their physical book. during the experiment, the children were asked to try both the speech mode of learning and image targetbased mode. the screenshots of the image target-based content delivery are shown in figure 5. the screenshots of the speechbased content delivery are illustrated in figure 6. during the analysis of the arel experiment, the children were tested for the concepts presented. it is observed that after figure 2. architecture of the arel system. figure 3. image target-based content delivery. acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 4 the usage of the arel the children improved and answered correctly. post the experiment, the parents of the children were asked to provide feedback on the usability, learning retention, learning engagement and overall experience. the result of the survey is aggregated and tabulated, as shown in the figure 7, where 1 represents the lowest rating such as unusable app or poor learning retention or improper learning engagement. overall experience of 10 represents a user-friendly design and development with parameters related to learning are score high. the average usability rating of the app is 7, which represents the user-friendliness score of using the application. as the application supports both voice interaction and image-based interaction, it aids in better exploring their book. the children were excited to see the virtual content appearing in real-time over their book. according to the results of the experiment, the multi-modal user interface with image-target and voice-based interactions, as well as the augmented reality display integrating real and virtual items, functions as a natural immersive experience for children. as the children try out the various interactions of the same learnable content, the learning retention got improved, as indicated by the tabulated score in figure 7. the children expressed a strong willingness to explore the application, indicating that it might be used as a fun and engaging learning tool. this is also indicated by an average learning engagement score of 8.33 from the survey. the overall experience of the mobile ar platform is at 8, which indicates the learning experience and usability experience of the children is positive and improved. 5. conclusions the development and evaluation of a mobile ar based enriched learning experience for learning language and math are reported in this work. one of the benefits of an ar learning experience over a standard book is that other intriguing aspects like animation, virtual objects, sound, and video may be included while the physical book is still present. the findings of the study suggest that the existence of such aspects during the learning process generate excitement, learning engagement, and enjoyment. the findings are supported by the answers to our survey questions to the parents of the children. the findings also show that the multi-modal interface of real and virtual things provides a natural immersive experience as well as an engaging and exciting learning tool for this age range. however, after repeated usage of the same book, children may become bored with arel if they can guess what things will appear. therefore, as part of future work, including a surprise aspect in the application could make it more enjoyable and engaging. while each image target-based marker has multiple types of visual content that could be presented, randomising the presentation of such content could surprise the child and can be included as future enhancement. furthermore, learning analytics of user engagement and learning retention can be utilised to evaluate the user experience, and personalised content for each student will be applied in future work. references [1] the rise of online learning during the covid-19 pandemic |world economic forum. onlinbe [accessed 16 august 2022] https://www.weforum.org/agenda/2020/04/coronaviruseducation-global-covid19-online-digital-learning/ [2] online education market study 2019 | world market projected. online [accessed 16 august 2022] https://www.globenewswire.com/newsrelease/2019/12/17/1961785/0/en/online-education-market figure 4. speech-to-text based content delivery. figure 5. screenshots from image target-based content delivery. figure 6. screenshots from image target-based content delivery. figure 7. feedback for arel. https://www.weforum.org/agenda/2020/04/coronavirus-education-global-covid19-online-digital-learning/ https://www.weforum.org/agenda/2020/04/coronavirus-education-global-covid19-online-digital-learning/ https://www.globenewswire.com/news-release/2019/12/17/1961785/0/en/online-education-market-study-2019-world-market-projected-to-reach-350-billion-by-2025-dominated-by-the-united-states-and-china.html https://www.globenewswire.com/news-release/2019/12/17/1961785/0/en/online-education-market-study-2019-world-market-projected-to-reach-350-billion-by-2025-dominated-by-the-united-states-and-china.html acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 5 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https://cloud.google.com/speech-to-text/docs/transcribe-client-libraries#before-you-begin https://cloud.google.com/speech-to-text/docs/transcribe-client-libraries#before-you-begin measurement of the structural behaviour of a 3d airless wheel prototype by means of optical non-contact techniques acta imeko issn: 2221-870x september 2022, volume 11, number 3, 1 8 measurement of the structural behaviour of a 3d airless wheel prototype by means of optical non-contact techniques antonino quattrocchi1, damiano alizzio1, lorenzo capponi2, tommaso tocci3, roberto marsili3, gianluca rossi3, simone pasinetti4, paolo chiariotti5, alessandro annessi6, paolo castellini6, milena martarelli6, fabrizio freni1, annamaria di giacomo1, roberto montanini1 1 department of engineering, university of messina, messina, italy 2 aerospace department, university of illinois urbana-champaign, urbana, illinois, usa 3 department of engineering, university of perugia, perugia, italy 4 department of mechanical and industrial engineering, university of brescia, brescia, italy 5 department of mechanical engineering, politecnico di milano, milan, italy 6 department of industrial engineering and mathematical sciences, polytechnic university of marche, ancona, italy section: research paper keywords: additive manufacturing; digital image correlation; thermoelastic stress analysis; finite element modelling; contact pressure mapping citation: antonino quattrocchi, damiano alizzio, lorenzo capponi, tommaso tocci, roberto marsili, gianluca rossi, simone pasinetti, paolo chiariotti, alessandro annessi, paolo castellini, milena martarelli, fabrizio freni, annamaria di giacomo, roberto montanini , measurement of the structural behaviour of a 3d airless wheel prototype by means of optical non-contact techniques, acta imeko, vol. 11, no. 3, article 13, september 2022, identifier: imeko-acta11 (2022)-03-13 section editor: francesco lamonaca, university of calabria, italy received march 17, 2022; in final form june 23, 2022; published september 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was supported by the ministry of education, university and research in italy (miur) under the research project of national interest (prin2015) “experimental techniques for the characterization of the effective performances of trabecular morphology structures realized in additive manufacturing”. corresponding author: antonino quattrocchi, e-mail: antonino.quattrocchi@unime.it 1. introduction additive manufacturing (am) or, more commonly, 3d printing is an emerging manufacturing technique, applied in several fields of industry [1], [2]. it allows to achieve some important goals such as weight reduction, development of complex shapes and use of a wide variety of materials [3]. currently, am is applied to exploit the design flexibility of numerical topology optimization tools and to lead to the creation of innovative samples [4]. moreover, am is not only adopted to produce prototypes, but it can target mass production. the latter abstract additive manufacturing (am) is becoming a widely employed technique also in mass production. in this field, compliances with geometry and mechanical performance standards represent a crucial constrain. since 3d printed products exhibit a mechanical behaviour that is difficult to predict and investigate due to the complex shape and the inaccuracy in reproducing nominal sizes, optical non-contact techniques are an appropriate candidate to solve these issues. in this paper, 2d digital image correlation and thermoelastic stress analysis are combined to map the stress and the strain performance of an airless wheel prototype. the innovative airless wheel samples are 3d-printed by fused deposition modelling and stereolithography in poly-lactic acid and photopolymer resin, respectively. the static mechanical behaviour for different wheel-ground contact configurations is analysed using the aforementioned non-contact techniques. moreover, the wheel-ground contact pressure is mapped, and a parametric finite element model is developed. the results presented in the paper demonstrate that several factors have great influence on 3d printed airless wheels: a) the type of material used for manufacturing the specimen, b) the correct transfer of the force line (i.e., the loading system), c) the geometric complexity of the lattice structure of the airless wheel. the work confirms the effectiveness of the proposed non-contact measurement procedures for characterizing complex shaped prototypes manufactured using am. mailto:antonino.quattrocchi@unime.it acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 2 purpose implies compliance with many restrictions due to the need to satisfy the production standards [5]. functional quality is generally associated with the structural response during the application of static and dynamic loads. however, in 3d printed lattice structures, the mechanical response can be significantly different than expected [6]. the causes of this anomaly can be identified in the geometric complexity and in the inaccurate reproduction of the nominal shape by the am process [7]. although finite elements numerical analyses have been performed [8], [9], experimental measurement of the actual structural response of lattice components has a relevant interest for am technology [10], [11]. indeed, traditional techniques [12] are not suitable for investigating 3d printing lattice parts, for example, due to the complex structures, non-conventional material, and small size. non-contact methods represent powerful alternatives, providing remarkable results despite the demanding technological requirements [13], [14]. the state-ofthe-art on this topic collects only few works, which often do not investigate in depth the mechanical behaviour of the product obtained. despite numerical finite elements analyses provide a good overview [15], the experimental mechanical characterization has been generally limited to the estimation of conventional stress-strain curves, carried out by static and dynamic compression tests [16], [17]. other ancillary analyses were performed for the evaluation of the porosity of the material by scanning electron microscopy [18] and for the investigation of the composition of the crystalline structure of the produced materials by means of backscattering electron diffraction [19]. one of the first examples of experimental mechanical characterization by non-contact methods was discussed by brenne et al. [20]. they mapped the strain behaviour of a lattice, heat-treated titanium alloy specimen, subjected to both uniaxial and bending loads, using 2d digital image correlation (dic) [21], [22] and electron backscatter diffraction. the results clarified the effect of the heat-treatment and the weaknesses of the structure, but, unfortunately, the quality of the 2d dic images [23], [24] did not allow to investigate the performance of the individual beam and to carry out some quantitative results. a more detailed inspection was provided by vanderesse et al. [25]. they investigated the strain behaviour of porous lattice materials with body cubic-centred reinforced, and diamond mesostructures subjected to quasi-static compression up to failure. the strain maps, obtained using dic, showed the localization of the most strained areas before and after the sample failure, highlighting a diffuse distribution, strongly depending on the analyzed reticular structure. in fact, it was generally noted that some struts exhibited a critical behavior that quickly leaded to their collapse, while other ones were slightly strained. allevi et al. [26] performed a feasibility study of the thermoelastic stress analysis (tsa) [27], [28] on a titanium based-alloy space bracket, made by electron beam melting. their results showed the same load trends that can be identified on larger scales, but also especially small and unexpected peaks in the tsa output and theoretical outcomes calculated by finite elements analysis. quattrocchi et al. [29] evaluated the mechanical behavior of a lumbar transforaminal interbody fusion cage implant, made by a 3d printing process adopting medical grade titanium. although these devices have a trabecular structure, useful for bone development, tsa allowed to identify that at the small scale the complex geometry of the specimen determines local differences in the stress distribution with intensification of the loads at the trabecular knots. finally, allevi et al. [30], [31] developed experimental protocols based on advanced non-contact measurement techniques to qualify the full mechanical behaviour of lattice structures. they employed different techniques, such as 2d vision systems, 2d dic, tsa and laser doppler vibrometry (ldv), to investigate morphological characteristics, to map local stress-strain fields, and to analyse the modal behaviour of simple lattice structures. this paper is the extended version of the one presented at the ieee i2mtc 2020 [32] and focuses on the evaluation of local stress-strain mapping. consequently, the aim of this work is to measure the mechanical behaviour of an airless wheel [33], [34] with a complex lattice structure, obtained through am, by adopting non-contact optical techniques. indeed, unlike traditional methods which are inefficient and even inapplicable in such conditions, non-contact optical techniques are an appropriate candidate for obtaining full-field information without altering the object of the study. two airless wheel samples, which were manufactured using different printing technologies (fused deposition modelling, fdm, and stereolithography, sla) and materials (poly-lactic acid, pla, and photopolymer resin, ppr), have been investigated. a measurement procedure, based on dic and tsa, has been applied to achieve combined full-field strain and stress measurements of the different wheel-ground contact configurations. furthermore, the wheel-ground contact pressure (cp) has been mapped estimating the load transfer. finally, a parametric finite elements model has been developed and compared to the results obtained from the experimental approaches. 2. materials and methods 2.1. airless wheel prototypes the airless wheel prototype (figure 1) was specifically manufactured according to [4]. the geometry is designed following a regular pattern of fixed angular amplitude (36◦). the pattern is then extruded in the axial direction of the same wheel. the lattice structure is obtained by connecting the intersection points of four circular crowns of equal width along the diameter and ten circular sectors of the wheel through a “zig-zag” criterion. the thickness of each trabecula is always kept the same. while fdm allows to obtain a model by subsequent layering of fused material, sla adopts a 3d printing method based on a photochemical process. a laser beam is focused on a liquid ppr to enable the wiring and solidification processes of the monomers on a building platform. the raw structure is washed in isopropyl alcohol; the supports useful for the realization of the product are removed and the grafting points of the same supports are smoothed. finally, a post-cure process is performed to complete the solidification process and to improve the figure 1. frontal view of the airless wheel prototype with lattice morphology. acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 3 mechanical properties of the printed structure. the sla sample (figure 2 a) was realized by a form 2 printer (formlabs inc., usa) with a standard ppr, while the post-cure process was performed by exposing the raw sample to uv light (wavelength of 405 nm) for 30 min at 60 °c (15 min for each side). instead, the fdm sample (figure 2 b) was obtained by ultimaker 2+ printer (ultimaker b.v., utrecht; nl) with pla. in this case, the post-cure is not necessary. table 1 reports the parameters adopted for the 3d printing processes. 2.2. experimental setup the mechanical behaviour was studied using an experimental setup consisting of a loading system, a digital camera with lighting projectors for dic measurements, an infrared (ir) camera suited to tsa measurements and an acquisition system with a piezoresistive sensor for wheel-ground cp estimation (figure 3). a rubber tread was not considered yet for the tests performed and discussed in this paper. the stress and the strain fields were mapped employing specific load stages on the wheel. 2d dic was performed applying a static load, while tsa was implemented by a dynamic load with harmonic trend. the load was transferred to the wheel by a dedicated apparatus (figure 4), i.e., a shaft and a loading frame specifically designed and manufactured. the airless wheel was locked by a bolted connection. this arrangement prevented the rotation of the wheel around its axis. the load was then applied in the vertical direction (y-axis). the loading condition was driven by an actuator and the effective load provided was measured through a load cell. more in detail, two slightly different loading systems were used: an electromechanical material testing machine (mod. electroplus e3000, instron company, norwood, ma, usa) equipped with a calibrated 5 kn load cell was employing during the tests on the sla wheel, while an electrodynamic shaker (mod. lds v650, brüel & kjær, nærum, denmark) with a dedicate power amplification unit was used for the tests on the fdm wheel. the morphology of the lattice structure of the airless wheel does not guarantee the same the wheel hub stress and strain distributions because these depend on the topology occurring on the angular portion of the wheel corresponding to the wheel contact patch. consequently, three different contact configurations have been analyzed: mixed, rhombic and trapezoidal; this labelling refers to the frontal geometric topology of the lattice region. (figure 5). 2d dic measurements required a preliminary preparation of the target surface (frontal view) of the wheel in order to create a suitable random speckle pattern [35]. this was done by spraying a matt white paint, hence obtaining a high contrast surface given the natural black colour of the wheel material. the images were taken using a nikkor af micro 200 mm lens mounted on a a) b) figure 2. 3d printed samples manufactured by means of: a) sla and b) fdm technologies. figure 3. schematic representation of the experimental setup used for dic and tsa measurements; cps = contact pressure sensor. table 1. 3d printing parameters used for the airless wheel prototypes. am technique sla fdm material black v4 pla red support points size in mm 0.6 layer thickness in µm 50 100 printing time in h 9.15 23.33 number of layers 1003 258 material volume or weight 72 ml 56 g orientation in ° 15 0 figure 4. loading apparatus for dic and tsa measurements. a) b) c) figure 5. frontal view of the wheel lattice structure for different wheelground contact configurations: a) rhombic, b) mixed and c) trapezoidal. acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 4 nikon d3100 digital camera (for the tests on the sla-printed wheel) and a canon ef 200m lens with canon eos 7d digital camera (for the tests on the fdm-printed wheel). the two imaging systems approximately ensure the same field of view (fov) but with a different spatial resolution. a diffuse illumination was obtained by exploiting led light. this was adopted to improve image quality and further increase contrast. particular attention was paid to checking the image quality and to verifying that no saturation phenomena occurred in the image. dic tests were carried out at different load levels synchronizing the frame rate to the load increase (i.e., 1 frame recorded every 1 n load increase step). the post-processing of dic tests was performed off-line, using the gom correlate pro software (gom gmbh, braunschweig, germany). table 2 reports the details of the two image systems and the load configuration for the 2d dic measurement. the wheel-ground cp was acquired during the dic tests using a flexible, thin and piezoresistive sensor (mod. 5040n), wired to a specific acquisition system (evolution™ handle, tekscan, south boston, ma, usa) and opting for a sampling frequency of 1 hz. the characteristics of the cp system are described in table 3. tsa was carried out by working in lock-in mode, i.e. by synchronizing ir system frame rate to the harmonic load applied. a preliminary study was conducted to determine the correct excitation parameters. this made it possible to obtain quasiadiabatic conditions and a high signal to noise ratio (snr) thermoelastic signal. these parameters are highly related to the thermal and mechanical properties of the printing material, more specifically to the heat capacity and stiffness. consequently, the working parameters are slightly different for the sla and the fdm prototypes. tests were performed with specific pre-load and overload levels but employing the same sampling frequency of 100 hz. the ir images were taken using a flir a6751sc camera (sla-printed wheel) and a flir sc7600 one (fdmprinted wheel). both ir cameras have a spatial resolution of 640  512 pixel and an insb detector with a thermal resolution of 20 mk at the room temperature. table 4 summarizes the characteristics of the two imaging systems and the load configuration used for the tsa measurements. 3. results and discussion 3.1. 2d dic measurements as an example, figure 6 displays the full-field maps of the displacements for the airless wheels along the vertical direction (y-axis). for the ppr sample (sla-printed), the displacement is uniformly high on the beams and nodes of the angular portion of the wheel when the wheel-ground contact takes place in the table 2. details of the systems configuration used for 2d dic measurements. am technique sla fdm camera nikon d3100 canon eos 7d focus distance in mm 610 550 f-stop f/7.1 f/4.5 exposition time in s 1/40 1/30 iso* sensitivity 2000 6400 focal distance in mm 200 100 images resolution in pixel × pixel 4608 × 3072 1920 × 1080 loads in n 0 to 200 0 to 250 * international organization for standardization (iso) table 3. details of the measurement systems. cp technique sla software i-scan 5040n sensing area (l × w × d) in mm 44.0 × 44.0 × 0.1 5040n sensing resolution in sensel/cm2 100.0 image scale (8 bit) in dl 0-255 images resolution in pixel × pixel 430×430 loads in n 0 to 200 table 4. details of the systems configuration used for tsa measurements. am technique sla fdm thermal camera flir titanium sc7200 flir a6751sc software altair li flir research ir + matlab images resolution in pixel × pixel 640 × 512 640 × 512 filter low-pass median load frequency in hz 5 10 15 5 10 15 preload in n 20 50 80 50 75 100 peak-peak load in n 5 10 15 20 40 60 a) b) c) d) figure 6. displacement along the vertical direction (y-axis) for the airless wheel in different contact configurations: a) rhombic, b) mixed and c) trapezoidal for the ppr sample at 200 n and d) trapezoidal for the pla one at 250 n; specifically, c) and d) have two distinct scales, different by an order of magnitude. a) b) c) d) figure 7. strain along the vertical direction (y-axis) for the airless wheel in different contact configurations: a) rhombic, b) mixed and c) trapezoidal for the ppr sample at 200 n and d) trapezoidal for the pla one at 250 n; specifically, c) and d) have two distinct scales, different by a factor three. acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 5 rhombic and mixed configurations (figure 6 a and figure 6 b). the trapezoidal configuration (figure 6 c) shows a displacement mainly concentrated on the external ring of the wheel rather than on the beams (i.e., the weakest region), considering the unloaded stage. furthermore, for the rhombic and trapezoidal configurations (figure 6 a and figure 6 c), the displacements appear to be symmetrical if comparing the loading line with the other sectors that seem to be relatively unloaded. finally, by taking into account the trapezoidal configurations of the two types of samples (figure 6 c and figure 6 d), they exhibit the same trend, even if the trend of the pla one (fdm-printed) has a significant reduction. indeed, even though the pla sample was subjected to a greater load (25 % more with respect to the ppr sample) the maximum displacement is an order of magnitude less than that of the ppr wheel. figure 7 reports the full-field maps of the measured strain for the airless wheels along the vertical direction (y-axis). in each contact configuration investigated, the strain is well distributed on the lattice structure but concentrated in the nodes. the strain analysis demonstrates that flexural effects prevail, with maximum positive values (i.e. tensile strain) at the vertexes of the beams and maximum negative values (i.e. compressive strain) along the interconnecting segments. a marked symmetry can be highlighted on the trapezoidal and rhombic configurations (figure 7 a and figure 7 c). only for the trapezoidal configuration, the most stressed region corresponds to the sector along the loading line. specifically, for the ppr sample (figure 7 c), the external ring of the wheel shows a high strain value on the contact area. indeed, it is also very clear the elastic behavior of the connecting beams of the lattice structure produced by relatively high loads. contrarily, although the trend is similar to the one measured in the former case, the pla sample (figure 7 d) shows a maximum strain greater than the one of the ppr sample. this could be a consequence of the specific material used for manufacturing and loading the wheel. indeed, the shaker seemed to be more inefficient in transferring the load with respect to the electromechanical material testing machine, which appeared to be more rigid. the lower level of the effective load transferred to the wheel is reflected in a higher level of noise, which sometimes prevented an accurate identification of the strains. figure 8 exhibits the trend of y-strain along the trabeculae, from the external ring to the wheel hub, in the trapezoidal configuration for the ppr sample. as already discussed for figure 7, figure 8 highlights the flexural effects on the trabeculae. indeed, the maximum positive values correspond to the intersections of the trabeculae (points b, d, f and h), while the maximum negative values (close to points c, e, g and i) to the centreline of the individual trabeculae. furthermore, the considered trabeculae, two for the trapezoidal sector (b-d and d-f) and two for the rhombic sector (f-h and h-i), present approximately a similar trend. this can be sufficiently considered the same for all trabeculae of the wheel. 3.2. tsa measurements in tsa measurements, the lock-in process allows the entire sequence of the recorded ir images to be condensed into only two images that represent the amplitude (figure 9) and the phase figure 8. y-strain along the trabeculae (“white line” in strain map) for the ppr sample in trapezoidal configuration at 200 n. a) b) c) d) figure 9. amplitude of the tsa signal for the airless wheel in different contact configurations: a) rhombic, b) mixed and c) trapezoidal for the ppr samples and d) trapezoidal for the pla ones (harmonic excitation at 5 hz, with 15 n peak-to-peak load and 80 n preload). a) b) c) d) figure 10. phase of the tsa signal for the airless wheel in different contact configurations: a) rhombic, b) mixed and c) trapezoidal for the ppr samples and d) trapezoidal for the pla ones (harmonic excitation at 5 hz, with 15 n peak-to-peak load and 80 n preload). acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 6 (figure 10) of the thermoelastic signal. as well known, the first one shows differential temperatures δt that are linked to the sum of the principal components of the stress tensor. also in this case, as already highlighted for the strain measurements, the loading apparatus used for transferring the load to the airless wheel hub plays a key role in determining the effective stress level to which the structure is subjected to. however, for both the ppr and the pla samples, tsa guarantees the correct identification of the most stressed regions, which are found to be located at the vertex connections of the beams and to be confined to the lower sectors of the wheel. tsa measurements were performed out of the frontal view of the wheel, i.e. by tilting the optical axis of the ir camera with respect to the wheel axis. more in detail, camera-wheel relative angles of 35 ° and 45 ° were used for the ppr and pla samples respectively. this configuration was introduced because the frontal view was subject to large edge effects due to the thin shaped beams. specifically, for the ppr sample, the rhombic and mixed configurations (figure 9 a and figure 9 b) exhibit a wide load distribution, marked at the vertex connections of the beams and at the sectors crossed by the loading line. contrarily, in the trapezoidal configuration (figure 9 c) the most stressed areas are reduced and concentrated in the lower sectors of the wheel. comparing the trapezoidal configurations of the ppr with that of the pla, the latter (figure 9 d) presents a more limited load amplitude. phase distribution (figure 10) is in line with the expected distributions for all the wheel-test configurations. 3.3. cp mapping the wheel-ground contact influences both the motion transfer and the wear of the rubber tread. therefore, the evaluation of the cps is useful for estimating both the functionality and stability of a vehicle. as an example, figure 11 shows the increment of the cps for the trapezoidal configuration of the ppr wheel. the cp maps show quite a good symmetry: the small dissimilarities are due to the connection of the airless wheel to the loading apparatus and the relative geometric tolerances. it is evident that the lattice structure has a decisive influence on the redistribution of the load. indeed, the beams, as previously emphasized, are the most rigid part of the airless wheel, hence the greatest cps are estimated in correspondence with their mark on the ground. this well matches the results obtained on the strain analysis when the wheel is tested in the same topology arrangement (figure 7 c). figure 12 compares the cp maps of the three different configurations at the maximum load (i.e. 200 n) on the ppr wheel. also in this case, it is possible to identify a good symmetry, except for the mixed configuration. the largest contact area is that of the trapezoidal configuration, while the average cp is greater for the mixed one. finally, the rhombic configuration shows the smallest contact area, but also the best symmetry. the maximum value of the cp is highlighted in correspondence with the beam of the mixed configuration. in both cases, mixed and rhombic configuration, the buckling effect would seem to be negligible due to the short arc of the external ring of the wheel between two successive beams. 3.4. finite elements analysis as a further analysis, a finite elements model (figure 13) of the airless wheel was developed. this model was created to calculate the displacements and the strains of the wheel in ideal conditions (in terms of both geometries, loads and constraints). the structure was meshed with about 2.5 million of solid tetrahedral elements, considering a fixed support at the wheel hub and a vertical static force at the wheel-ground interface. furthermore, to improve the computation resolution, the mesh quality was refined on those areas where stress and strain are expected to be more relevant (figure 14). as an example, figure 15 displays the maps relative to the computation of the y-displacement and the y-strain, for the pla airless wheel printed in fdm. the numerical simulation highlights the same trends observed in the experimental results. the computed strain closely resembles the real one, showing that the interconnection segments of the lattice structure are mainly subjected to bending, exhibiting both tensile and compression stresses. hence, the presented model might be used to analyze alternative configurations of the lattice geometry, laying the foundations for a structural optimization of the topology of the whole airless wheel. figure 11. cp map of the trapezoidal configuration for the ppr wheel. a) b) c) figure 12. comparison between the cp maps for the configurations: a) rhombic, b) mixed and c) trapezoidal of the ppr wheel at 200 n. figure 13. finite elements model of the airless wheel. acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 7 4. conclusions this paper addresses the applicability of non-contact techniques, such as dic and tsa, to measure the actual stressstrain field of complex lattice structures printed in am. the chosen structure is based on the innovative airless wheel concept. this type of design could be widely applied for different vehicle models, taking advantage of its specific features. globally, a puncture-proof wheel improves safety and reliability by reducing its replacement due to critical events and preventing the rapid decrease in vehicle stability. furthermore, the mechanical response of a such structure could be enhanced by increasing the adhesion to the road surface and limiting the tread wear. specifically, the use of an airless wheel could be heterogeneous. for example, it could be a good solution for skateboards, exploiting the high deformability of the lattice structure, or for aircrafts and aerospace vehicles thanks to the possibility of reducing weight maintaining the same performance. in summary, the airless wheel should be of great interest to the next generation of full-electric cars. an airless wheel prototype, manufactured by different 3d printing technologies (fdm and sla) and made with different polymeric materials (pla and ppr) has been tested by employing dic and tsa techniques. the wheel-ground interaction has been studied by mapping the cps. a parametric finite elements model of the same wheel has been developed. furthermore, the experimental tests have been performed in two separated laboratories also using specific loading systems and instrumentation. the results achieved have demonstrated some critical aspects that should be considered in the characterization of such a system: the type of material used for manufacturing the sample, the loading system and the topology of the lattice structure. in this sense, considering a numerical model previously validated at least in terms of contact pressure distribution, can help in dealing with these issues. indeed, the proposed model might be used to analyze alternative configurations of the lattice geometry, laying the foundations for a structural optimization of the lattice topology with specific objective functions (e.g., uniform stress distribution or minimum weight). according to these outcomes, the present study has then confirmed the effectiveness of the non-contact techniques (dic and tsa) for measuring the spatial distribution of both strain and stress fields in functional and complex structures obtained from am. specifically, these investigation techniques have shown that the mechanical response of a lattice structure exhibits considerable complexity. in fact, although strain and stress are well distributed over almost all regions, concentrations have been identified in correspondence of the nodes of the trabeculae and in the lower sectors of the wheel. for example, this suggests moving towards a topological optimization that involves increasing the thickness of the trabeculae at the nodes and along the external ring, thinning the section of the trabeculae at their centerline and reducing the excess of material closer to the wheel hub. references [1] t. d. ngo, 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adaptive learning acta imeko issn: 2221-870x march 2022, volume 11, number 1, 1 7 acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 1 spectrum sensing using energy measurement in wireless telemetry networks using logarithmic adaptive learning nagesh mantravadi1, md zia ur rahman1, sala surekha1, navarun gupta2 1 department of electronics and communication engineering, koneru lakshmaiah education foundation, k l university, vaddeswaram, guntur, andhra pradesh, 522502, india 2 department of electrical engineering, university of bridgeport, bridgeport, ct 06604, usa section: research paper keywords: adaptive algorithm; cognitive radio; energy measurement; noise uncertainty; threshold point citation: nagesh mantravadi, md zia ur rahman, sala surekha, navarun gupta, spectrum sensing using energy measurement in wireless telemetry networks using logarithmic adaptive learning, acta imeko, vol. 11, no. 1, article 34, march 2022, identifier: imeko-acta-11 (2022)-01-34 section editor: md zia ur rahman, koneru lakshmaiah education foundation, guntur, india received december 28, 2021; in final form february 18, 2022; published march 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: sala surekha, e-mail: surekhasala@gmail.com 1. introduction radio frequencies are limited natural resources they are controlled by government authorities, in a particular band the primary users can exclusively use licensed spectrum even the secondary users are unoccupied as these unlicensed users are avoided for use. due to the enormous growth in wireless communication applications the usage of radio frequencies is increasing rapidly. to overcome this spectrum utilization issues, cognitive radio systems are emerged with new technology [1]-[3] in wireless communications. enormous efforts are used for enhancing the usage of efficiency of cognitive radio systems, there is huge change in the usage of frequencies, time and space domain and then secondary users are allowed without creating any interference to the primary users. to overcome these interferences [4], [5] in wireless communications and for increasing spectrum utilization ieee 802.22 wireless frequency band can be used. with the help of these frequency bands, the data will be transmitted without causing interference in health care monitoring wireless application by using zigbee, wi-fi, ad hoc networks with operating frequency less than 3 mhz. orthogonal frequency division multiplexing (ofdm) is the eminent method used in wireless communication systems. channel estimation issues raised in ofdm based cognitive radio system are discussed in [6]-[8], this is because of mean square error (mse) of channel estimations. the unknown noise components effecting the spectrum sensing also studied. performance evaluation of channel estimation techniques for imperfect channels is analysed for correlated channels of cognitive radio system, then mutual information is considered at the input and output to provide better communication sensing and channel uncertainty. the receiver's minimal mean square error is then computed to obtain the fading coefficients of the fading channel, as well as the anticipated attainable rate for gaussian signalling and linear modulation schemes, assuming abstract to identify primary user signals in cognitive radios spectrum sensing method is used. due to statistical variances in received signal, noise is present in primary user signals, this noise powers are varied due to random nature of noise signals and leads to noise uncertainty problem in the performance of energy detection. the task of energy measurement and further detecting the unused frequency spectrum is a key task in cognitive radio applications. for avoiding these problems, least logarithmic absolute difference (llad) algorithm is proposed in which noise powers are adjusted at sensing point of licensed users. with help of proposed method, estimated noise signals are eliminated. sign regressor version of llad algorithm is considered due to it reduces computational complexity and convergence rate is improved. further probability of detection (pod), probability of false alarm (pofa) is estimated to know threshold value. from results, it is clear that good performance in terms of pofa versus pod in range of low signal to noise ratio in multiple nodes. therefore, the proposed energy measurement-based spectrum sensing method is useful in remote health care monitoring, medical telemetry applications by sharing the un-used spectrum. mailto:surekhasala@gmail.com acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 2 interference and channel estimation errors occur at primary users [9], [10]. spectrum sensing is used to avoid difficulties with spectrum underutilization and interference. the most commonly used detection techniques are wavelet detection, cyclostationary detection, energy detection, and covariance detection. the first three techniques require prior information of the principal user signal, frequency components, interference, and noise variance, but the fourth approach requires no prior data and is hence the most often used spectrum sensing method. in addition, for energy sensing, circuit implementation and computing complexity are minimal. spectrum sensing [11]-[17] performance is assessed in terms of false alarm and detection probabilities; it assumes that the primary user is idle or active during the spectrum sensing period, and then spectrum holes are detected in the spectrum frequency range based on this. there is a compromise between probability detection and false alarm, however by utilizing these factors, we can see if there was any interference among the primary and secondary users. in [18] sensing trade-off in cognitive radios is studied when numerous primary users arrive randomly. the trade-off is also caused by the performance of cognitive radio spectrum utilization and spectrum sensing, which is based mostly on primary user activity. a numerical technique was used to investigate this trade-off utilizing cooperative spectrum sensing. the spectrum potential for successful communication between a transmitter and receiver of cognitive networks is being investigated. also studied about when secondary user sensing time increases, false alarm probability reduced, it means there is high chances to secondary users are having to access idle channel, but less chance to transmit because of transmission time is limited. main objective is efficiently use spectrum utilization with optimum sensing for improving spectrum opportunities of cognitive radio networks. practically cognitive radios are having channel sensing errors because of miss detection and false alarms these problems will affect channel estimation [19] design also quality channel estimation error problems, receiver operating characteristics are also analysed for these parameters. further cooperative cognitive radio is considered for analysis of fading scenarios occurred with improved energy detection method. for this nakagami multipath fading is considered with power 2 based energy detection spectrum sensing [20], [21], then performance is improved in cognitive radio networks in terms of detection probability parameters and their operating curves also analysed. in spectrum sensing, there after spectrum allocation one of the promising methods is energy detection. in this work a new energy measurement methodology is proposed based on least logarithmic absolute difference algorithm. this methodology of energy detection and measurement is a key task in measurement technology as well as in cognitive radio-based communication systems. to overcome noise uncertainty problems double threshold-based spectrum sensing is studied for improving energy detection. this cognitive radio concept is widely used in health care applications, but interferences occurred due to wireless devices will affect the performance. to avoid these interferences occurred with health care, novel cognitive radio method is used by proposing modified normalized least mean square algorithm (mnlms) for hospital environment applications so that errors/interferences occurred with medical devices are removed and their performance is evaluated using matlab simulations. 2. system model spectrum sensing is mostly used method for detecting spectrum holes of cognitive radio network. by using this method, we can decide primary user is absent or present. energy detection block diagram is shown in figure 1. by using hypothesis testing, detection problem is considered as 𝑇0: 𝑧(𝑡) = 𝑤(𝑡) 𝑇1: 𝑧(𝑡) = 𝑤(𝑡) + 𝑠(𝑡) , (1) where 𝑧(𝑡) is received sample signal, 𝑤(𝑡) is noise effect of transmitted signal, 𝑠(𝑡) is primary transmitted signal with 𝑡 = 1, … , 𝑇 length carried for identifying spectrum. energy detection method [22] is considered for detecting spectrum holes, energy level is measured, then estimated noise variance by placing detection threshold value. then secondary user decides statistics of energy detection as 𝐷 = ∑[𝑧(𝑡)]2 𝑇 𝑡=1 . (2) decision static have central chi square distribution with t degrees of freedom when primary user signal is absent. it is having non central chi square distribution decision statistics when primary user is present. central limit theorem with gaussian approximation is used if detection samples are greater than 250, then mean and variance of decision static of primary user signal given as 𝐷 ~ 𝑇(𝑇𝜎𝑤𝑡 2 , 2𝑇𝜎𝑤𝑡 4 ) for h1 𝐷 ~ 𝑇(𝑇((𝜎𝑠𝑡 2 + 𝜎𝑤𝑡 2 , 2𝑇((𝜎𝑠𝑡+ 2 𝜎𝑤𝑡 2 )2) for h0 . (3) in testing 𝑇0 and 𝑇1, false alarm and misdetection errors are occurred due to false identification of 𝑇0 and 𝑇1 . energy detector performance is measured with probability of these two errors. probability false alarm occurred due to showing of wrong spectrum band occupancy, probability of miss detection is due to it shows as primary user absence, but actually it is present, it is also called as probability detection. probability false alarm, probability detection evaluated as figure 1. energy detector block diagram. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 3 𝑃ofa = 𝑄 ( 𝛿𝑑 − 𝑇𝜎𝑤𝑡 2 √2 𝑇 𝜎𝑤𝑡 4 ) (4) 𝑃od = 𝑄 ( 𝛿𝑑 − 𝑇(𝜎𝑠𝑡 2 + 𝜎𝑤𝑡 2 ) √2 𝑇(𝜎𝑠𝑡+ 2 𝜎𝑤𝑡 2 )2 ) , (5) where 𝛿𝑑 = 𝜎𝑤𝑡 2 (𝑄−1(𝑃ofa)√2𝑇 + 𝑇) is the threshold. 2.1. least logarithmic absolute difference algorithm least logarithmic absolute difference (llad) technique elegantly and gradually adapts conventional cost function depends on amount of error in its implementation. in impulse-free noise environments, lms and llad algorithm exhibits likely convergence behaviour, while llad algorithm is robust against impulsive interference and exceeds sign algorithm [23], [24]. flowchart for llad algorithm is as shown in figure 2. mathematical modeling 𝑇 is filter length, 𝜇 is step size parameter is considered. let the tap input be x(𝑛) and filter length 𝑇 is moderate to large, w(0) = 0 is considered as initial condition, x(𝑛) is 𝑇 -by1 tap input vector to filter 𝑛2 at time 𝑛 as [x(𝑛), x(𝑛 − 1), … , x(𝑛 − 𝑇 + 1)]t (6) w(𝑛) is tap weight vector, d(𝑛) is desired response at time 𝑛, ωo is an unknown vector, (. ) t is the transpose of (. ). to be computed: w(𝑛 + 1) is to be computed tap-weight vector at time 𝑛 + 1. computation an unknown vector ωo is represented with linear model as d(𝑛) = ωo t x(𝑛) + n𝑡 (7) the instantaneous estimate of gradient vector j is written as: ∇’j(n) = − 2 x(𝑛) d∗(𝑛) + 2 d(𝑛) xt(𝑛) w(𝑛) (8) where x(𝑛) is the input tap vector, w(𝑛) is tap weight vector. w(𝑛) is random vector depends on x(𝑛) with its taps stored in a row vector given by [𝑤(𝑛), w(𝑛 − 1), … , w(𝑛 − 𝑇 + 1)]t (9) output of the filter y(𝑛) = [w0(𝑛) x(𝑛) + ⋯ + w𝐿−1(𝑛) x(𝑛 − 𝑀 + 1)] = wt(𝑛) x(𝑛). (10) expression for estimation error is given by e(𝑛) = d(𝑛) − wt(𝑛) x(𝑛). (11) where the term wt(𝑛) x(𝑛) is inner product of w(𝑛) and x(𝑛). the normalized error cost function introduced using logarithmic function is given by figure 2. spectrum sensing using llad. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 4 j(e(𝑛)) = f(e(𝑛)) − 1 𝛼 ln (1 + 𝛼 f(e(𝑛))) (12) based on steepest descent method, general weight update recursion is given by w(𝑛 + 1) = w(𝑛) − 𝜇 ∇j(𝑛) (13) new recursive relation for ∇j(𝑛) is written as ∇j(𝑛) = 𝐸{∇|e(𝑛)|2} = 𝐸{e(𝑛)∇e∗(𝑛)} ∇e∗(𝑛) = −x∗(𝑛) (14) thus, resultant expression for gradient vector is given by ∇j(𝑛) = −𝐸{e(𝑛) x∗(𝑛)} (15) also, first gradient of the relation in is given by δw. f(e(𝑛)) the signum representation is given below: sign{x(𝑛)} = { 1: x(𝑛) > 0 0: x(𝑛) = 0 −1: x(𝑛) < 0 } (16) step size boundary for convergence of mean square for lms algorithm is given by 0 < 𝜇 < 2 x𝑇 (𝑛) x(𝑛) (17) by substituting estimate of ∇’j(𝑛) in steepest descent algorithm, new recursive relation for updating tap weight vector is w(𝑛 + 1) = w(n) + 𝜇 x(𝑛)[d∗(𝑛) − xt(𝑛) ∙ w(𝑛)]. (18) to provide robustness against impulsive interferences, a cost function is introduced with normalized error by use of logarithmic function stated as 𝐹(e(𝑛)) = 𝐸 [(e(𝑛)) 2 ] = 𝐸[|e(𝑛)|] . (19) thus, the stochastic gradient update is given by w(𝑛 + 1) = w(𝑛) + 𝜇 ∙ x(𝑛) ∂f(e(𝑛)) ∂e(𝑛) [ 𝛼 f(e(𝑛)) 1 + 𝛼 f(e(𝑛)) ] , (20) where 𝛼 > 0 is a design parameter also f(e(𝑛)) as the conservative cost function for error signal e(𝑛). for |𝛼 f(e(𝑛))| ≤ 1, applying maclaurin series using natural algorithm, equation (19) gives j(e(n)) = f(e(𝑛)) − 1 𝛼 (𝛼 f(e(𝑛)) − 𝛼 2 f2(e(𝑛))). (21) for low values of f(e(𝑛)), it is an infinite combination for conventional cost functions. for smaller values of error, cost function j(e(𝑛)) resemble f(e(𝑛)) for instance as f(e(𝑛)) − 1 𝛼 ln (1 + 𝛼 𝐹(𝑒(𝑛))) → f(e(𝑛)) (22) thus general update expression of stochastic gradient is stated as w(𝑛 + 1) = w(𝑛) + 𝜇 ∙ x(𝑛) ∙ ∂f(e(𝑛)) ∂e(𝑛) [ 𝛼 f(e(𝑛)) 1 + 𝛼 f(e(𝑛)) ] (23) norm is power of least probable error ais at convex cost function, signed algorithm delivers slow rate of convergence. for f(e(𝑛)) = e[|e(𝑛)|] in (23), then resultant expression is w(𝑛 + 1) = w(𝑛) + 𝜇 x(𝑛) sign(e(𝑛)) [ 𝛼 (|e(n)|) 1 + 𝛼 (|e(n)|) ] . (24) then the weight update relation of the llad algorithm becomes w(𝑛 + 1) = w(𝑛) + 𝜇 [ 𝛼 x(𝑛) e(𝑛)) 1 + 𝛼 (|e(𝑛)|) ] (25) to reduce computational difficulty of lms, signed variants are preferable. sign regressor version offers low computational complexity with a smaller number of multiplications among all signed variants. here, sign regressor llad (srllad) algorithm by applying sign function on each element. is remains obtained as recursion of lms for altering input tap vector. 2.2. sign based least logarithmic absolute difference (llad) algorithms llad algorithm is generalized version of higher order adaptive filter. combination of llad in (25) with three types of sign variants results in srllad, sllad and ssllad algorithms respectively. hence weight update relations of signed llad based variants are given as follows. w(𝑛 + 1) = w(𝑛) + 𝜇 sign{x(𝑛)} sign {e(𝑛) [ 𝛼 (|e(𝑛)|) 1 + 𝛼 (|e(𝑛)|) ]} (26) w(𝑛 + 1) = w(𝑛) + 𝜇 x(𝑛) sign {e(𝑛) [ 𝛼 (|e(𝑛)|) 1 + 𝛼 (|e(𝑛)|) ]} (27) w(𝑛 + 1) = w(𝑛) + 𝜇 sign{x(𝑛)} sign {e(𝑛) [ 𝛼 (|e(𝑛)|) 1 + 𝛼 (|e(𝑛)|) ]}. (28) 3. results and discussion the proposed llad algorithm method for spectrum sensing is used to assess performance. the spectrum sensing of the transmitter of primary user and receiver of secondary user is evaluated. spectrum sensing simulations are run for 5000 samples, t is filter length it is chosen as 10 and distinct signals are acquired for each noise sample to process. for improved results, a received signal attenuation factor was introduced, and then the performance of energy detection was evaluated under various snr situations. because of the noise levels in the sensing spectrum, the main goal of developing an adaptive filter is based on spectrum sensing approach. due to incorrect detection of test acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 5 data, this spectrum sensing may result in probability detection and missed detection probability mistakes. due to incorrect detection of test data, this spectrum sensing may result in probability detection and missed detection probability errors. the proposed technique adjusts the threshold value for each sensing event and then adjusts the noise power accordingly. when the size of the receiving antenna, the averaging of eigen values rises, resulting in a reduction in estimate error. the performance of noise power estimation is measured in terms of mean square error (mse). when the antenna size and quantity of samples are increased, the steady state error reduces. the sensitivity of adaptive filter-based energy detection is measured in terms of probability detection, which is a function of snr and a constant false alarm probability. the proposed llad algorithm performs better when probability detection is used. noise uncertainty is not a problem for this proposed strategy. this detecting capability successfully identifies spectrum gaps, allowing for prevented spectrum reuse opportunities. to accomplish probability detection (pod) and false alarm probability (pofa) concurrently, snr values are considered as minimal and number of samples, noise uncertainty relationship is adopted. to obtain greater probability detection without an adaptive method for spectrum sensing, noise uncertainty increases for minimal snr levels. even though there is no influence on how many received signal samples are examined for sensing spectrum, some restrictions on snr for performance of probability detection beyond these limit false alarm values are sacrificed for values bigger than zero of noise uncertainty factor. for noise certainty levels greater than zero for every increasing snr value, a larger number of samples are required for improved detection probability. it is obvious that raising snr values improves detection sensitivity for noise circumstances in the proposed technique. theoretical values for probability detection of different snr levels for the basic energy detection technique and suggested energy detection using llad are determined using (4) and (5). computational complexity of various lms based adaptive algorithms shown in table 1. with the suggested strategy, probability detection produces superior results, as shown in table 2. simulation curves for pofa versus pod for various snr values are provided in figure 3. for low snr levels, detection probability performance is better, as shown in table 2 and figure 3. signal correlations create propagation fading in wireless communications, and their influence on the receiving antenna generates correlation losses in the received signal. the performance of the energy detector is initially unaffected by signal correlations, and noise power is evaluated using eigen values. the antenna correlation effect is avoided while calculating noise power estimate by using one primary user signal when calculating eigen values of antenna correlation effect on bigger eigen values compared to small eigen values. the main goal of the proposed llad is to improve energy detection accuracy when used in low snr and noise-uncertainty situations. the detection probability is then improved with constant false alarm probability and low snr values. threshold values are evaluated for energy detectors to perceive changes in noise power in order to minimize difficulties with noise uncertainties, therefore threshold values are adapted to compute table 1. computational complexities of various sign lms based adaptive. s.no algorithm multiplications additions divisions 1 lms t+1 t+1 nil 2 llad t+4 t+2 1 3 srllad 4 t+2 1 4 sllad t+3 t+2 1 5 sslad t+2 t+2 1 figure 3. pofa versus pod for different snr values. figure 4. convergence curves for sign based llad adaptive algorithms. table 2. performance comparison of eigen value based spectrum sensing, proposed llad and its sign variants. snr (db) 0 -5 -10 -15 -20 pfa pd pfa pd pfa pd pfa pd pfa pd eigen value-based spectrum sensing 0.4 0.6789 0.3 0.6989 0.3 0.7125 0.3 0.7859 0.3 0.8752 llad 0.2520 0.7824 0.3 0.8997 0.3 0.9581 0.3 0.9785 0.3 0.9969 sr-llad 0.26 0.6241 0.2 0.6805 0.2 0.7825 0.2 0.8628 0.2 0.8853 s-llad 0.3821 0.5645 0.4 0.7192 0.4 0.7403 0.4 0.7893 0.4 0.7990 ss-llad 0.42 0.4561 0.5 0.4985 0.5 0.5782 0.5 0.5981 0.5 0.6782 acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 6 noise power estimation. when compared to spectrum sensing only by using energy detection, it provides improved results for the llad energy detection. the proposed approach for energy detection overcomes noise uncertainty difficulties, but it is also necessary to assess the signal correlation because it is based on noise power estimations with eigen values. if signal correlation occurs, the performances of energy detection and noise power estimates have an impact on spectrum sensing. convergence becomes delayed by applying signum function. according to the illustrations, sra variant is just lower than its non-signed variant. figure 4 shows llad algorithm and its sign variants have a higher convergence rate than lms. hence srllad algorithm is preferred when compared to llad and lms algorithms. when the proposed llad algorithm exceeds the lower bound stability constraint [25], the normalized mean square deviation noise variance diverges. as a result of this equation normalized mean square deviation provides higher stability for the proposed strategy. normalized mean square deviation analysis yields higher convergence for modified normalized median lms than nlms and lms for small step size noisy inputs. because of instabilities, the standard modified normalized lms method diverged at large step size values. in health care monitoring applications for remote patients, spectrum sensing with the proposed llad algorithm is utilized to reduce noisy inputs and interferences caused by wireless networks. the information of the patients is transmitted to doctors through cognitive systems by arranging wearable devices to patient body. the information is transmitted and received concurrently from both channels. in this case primary user of cognitive system takes higher priority over the secondary user. the data is accessible to secondary users, but not to primary users. this proposed llad algorithm eliminates interference and disturbances that arise in this situation. medical equipment’s are more sensitive than normal electric devices in health care accept. in these primary users are telemetry applications and secondary users are hospital information applications. by predicting spectrum holes the cognitive radio controller optimizes the performance of the system, cognitive radio controller regulates channel access, and probabilities are determined. for many data channels the loss and delay probabilities of cognitive radio system are enhanced. 4. conclusions the main objective of this paper is to reduce noise uncertainties during spectrum sensing using the proposed llad algorithm. probability detection and false alarm probability are errors that arise as a result of spectrum sensing at the receiver, and their expressions are derived. after that, the impact of noise uncertainty on threshold parameter selection is investigated. the mean and mean square deviation analysis ensures stability for noisy primary user inputs. we considered variable step size for improving the stability of the proposed spectrum sensing technique. the llad algorithm's performance is enhanced by a lower steady-state error rate and better convergence. in medical telemetry, this cognitive radio idea is utilized to minimize echo cancellations and signal correlations with noisy inputs at the primary user. as a consequence, we achieve improved outcomes in terms of noise uncertainty stability, convergence rate, and steady state error rate. the estimation of a false alert and the probability of detection were both computed as estimation parameters. references [1] t. düzenli, o. akay, a new spectrum sensing strategy for dynamic primary users in cognitive radio, ieee communications letters, vol. 20, no. 4, april 2016, pp. 752-755. doi: 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least-mean-square algorithm for noisy inputs, ieee transactions on signal processing, vol. 65, no. 11, 1 june 2017, pp. 2949-2961. doi: 10.1109/tsp.2017.2675865 https://doi.org/10.1109/tvt.2015.2413787 https://doi.org/10.21014/acta_imeko.v6i3.454 https://doi.org/10.1109/lgrs.2019.2895112 https://doi.org/10.1109/jiot.2019.2927089 https://doi.org/10.1109/taes.2018.2890354 https://doi.org/10.1109/tvt.2014.2381648 https://doi.org/10.1109/i2mtc43012.2020.9129107 https://doi.org/10.1109/access.2018.2848201 https://doi.org/10.1109/access.2018.2866303 https://doi.org/10.1109/tsp.2017.2675865 solar energy harvesting for lorawan-based pervasive environmental monitoring acta imeko issn: 2221-870x june 2021, volume 10, number 2, 111 118 acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 111 solar energy harvesting for lorawan-based pervasive environmental monitoring tommaso addabbo1, ada fort1, matteo intravaia1, marco mugnaini1, lorenzo parri1, alessandro pozzebon1, valerio vignoli1 1 department of information engineering and mathematics, university of siena, via roma 56, 53100 siena, italy section: research paper keywords: solar; energy harvesting; lorawan; environmental monitoring; particulate matter citation: tommaso addabbo, ada fort, matteo intravaia, marco mugnaini, lorenzo parri, alessandro pozzebon, valerio vignoli, solar energy harvesting for lorawan-based pervasive environmental monitoring, acta imeko, vol. 10, no. 2, article 16, june 2021, identifier: imeko-acta-10 (2021)-02-16 section editor: giuseppe caravello, università degli studi di palermo, italy received january 18, 2021; in final form may 5, 2021; published june 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: alessandro pozzebon, e-mail: alessandro.pozzebon@unisi.it 1. introduction energy self-sufficiency is one of the crucial requirements for the realisation of efficient real-time distributed monitoring infrastructures, in wide range of application fields, from environmental [1], [2] and cultural heritage monitoring [3], [4] to the aerospace [5] and the smart industry [6], [7] domains. indeed, when developing a large number of wirelessly connected sensing devices, energy self-sufficiency means their usability as deployand-forget items, where any kind of manual intervention is reduced at minimum. besides reducing power consumption, energy self-sufficiency mainly requires the presence of a continuous or semi-continuous source of energy that, when these devices are expected to be employed in motion, cannot be the power grid. for this reason, the only way to ensure the continuous presence of the adequate amount of energy to allow the sensing device functioning is the so-called energy harvesting, i.e., the presence of a source of renewable energy on-board. among the various possible sources of energy, the most exploited is the solar one: indeed, several monitoring platforms have been provided with solar cells that are used to recharge onboard batteries or super-capacitors. nevertheless, such a solution has to face in several cases limitations due to the large dimensions of the solar cells, to the limited amount of achievable power or to the inadequate exposition of the sensing device. another factor influencing the performances of the energy harvesting system is the complexity of the sensing platform: indeed, the more power-hungry are the components, the more difficult is the design of the harvesting solution. the aim of this paper is to propose the characterisation of a small scale solar-based energy harvesting system, designed to identify an efficient trade-off among dimensions and power efficiency. in order to demonstrate the validity of the proposed solution, it has been embedded in a wireless sensing device thought to be employed for distributed real-time environmental monitoring: in particular, the sensing device is provided with sensors for the measurement of particulate matter (pm), with a gps module for localisation and tracking purposes and with long range wide area network (lorawan) connectivity. such a device is expected to be employed within a smart city context. while several city-scale air quality wireless monitoring abstract the aim of this paper is to discuss the characterisation of a solar energy harvesting system to be integrated in a wireless sensor node, to be deployed on means of transport to pervasively collect measurements of particulate matter (pm) concentration in urban areas. the sensor node is based on the use of low-cost pm sensors and exploits lorawan connectivity to remotely transfer the collected data. the node also integrates gps localisation features, that allow to associate the measured values with the geographical coordinates of the sampling site. in particular, the system is provided with an innovative, small-scale, solar-based powering solution that allows its energy self-sufficiency and then its functioning without the need for a connection to the power grid. tests concerning the energy production of the solar cell were performed in order to optimise the functioning of the sensor node: satisfactory results were achieved in terms of number of samplings per hour. finally, field tests were carried out with the integrated environmental monitoring device proving its effectiveness. mailto:alessandro.pozzebon@unisi.it acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 112 infrastructures can be found in literature [8], [9], [10], this paper focuses on the realisation of a different typology of data acquisition architecture. indeed, in the proposed solution, the sensor nodes are expected to be provided with localisation features [11] and then to be deployed on means of public transport. this approach is especially relevant since it allows to acquire data in a more pervasive way, bringing the measurement instrumentation in almost every spot of a city. at the same time, the scope of the paper is to propose a device to be employed in a “plug-and-play” fashion: the use of the photovoltaic source for the powering of the system goes specifically in this direction. indeed, while on means of public transportation several sources of energy may be available, the connection of a new device may require structural modifications on the vehicle itself to wire the sensor node to the power source. such modifications may be even more cumbersome if the sensor node is expected to be deployed outside the vehicle, as in the case of acquisition of environmental parameters. conversely, the design of a totally autonomous system may allow its deployment without any kind of intervention on the vehicle: in its final configuration, the sensor node may be attached for example with a magnet to the vehicle chassis. the choice of long range (lora) as the transmission technology comes from its ability to provide possibly the best compromise between performances and costs within the smart city scenario [12], [13]. indeed, the long transmission ranges allow to cover a large area with a relatively small number of gateways. at the same time, thanks to the lorawan protocol, a large number of end devices can be simultaneously managed thanks to multi-channel and gateway redundancies. on the other side, costs are kept very low since no fee is required for the transmitting devices: such aspect may be crucial when the number of devices to be deployed is expected to grow. at the same time, the same lorawan network may be exploited also for other activities, thus further reducing the costs. if compared with competing technologies, the benefits coming from the adoption of lorawan can be better underlined. starting from local area technologies like zigbee, bluetooth or wifi, their short transmission range obviously prevents for using them for monitoring at a city scale since too many gateways may be required. moving to wide area technologies, cellular ones are of course more reliable than lora. however, they require a subscription for each device and this cost may be unsustainable with a growing number of devices: conversely, lorawan may easily scale since no cost is required for connection, and the price of lora modules is in the order of few euros, notably lower than its concurrent cellular technology, i.e., nb-iot. the same limitation is applied to the other well-known sub-ghz technology, sigfox. indeed, such technology too requires the payment of a subscription for each device. at the same time, the limitations that may come from the usage of lorawan are not crucial for the proposed application scenario. indeed, the 1% duty-cycle limitation is not influent on the acquisition of pm values that can be performed every 10-15 minutes, while the limited reliability of the connection may lead to the loss of some packets that are not relevant too for the purpose of the proposed system. the rest of the paper is structured as follows: in section 2 some details related to the monitoring of pm concentrations are provided, while section 3 focuses on the state of the art related to solar-based energy harvesting solutions. section 4 provides a description of the overall sensor node architecture while section 5 is devoted to the design of the solar harvesting system. section 6 provides some field test results while in section 7 some conclusive remarks are presented. 2. particulate matter monitoring the term "particulate matter" (pm) encompasses a wide range of solid, organic, and inorganic particles and liquid droplets that are commonly found in air. in general, pm is composed of a wide range of different elements that change according to the specific environmental features [14], [15], but include sulphate, nitrates, ammonia, sodium chloride, black carbon, mineral dust, and water. pm is classified according to the dimensions of the single particles: we speak then of pm10 when the diameter of the particles is lower than 10 micron (dpm10 < 10 µm) and of pm2.5 when the diameter is lower than 2.5 micron (dpm2.5 < 2.5 µm). both typologies of pm can be easily inhaled by human beings, and a chronic exposition to this kind of pollutants can bring to the emergence of cardiovascular and respiratory diseases. in particular [16], pm10 can penetrate inside lungs, while pm2.5 can penetrate the lung barrier and enter the blood system, with even more harmful effects. for this reason, world health organisation has defined two thresholds for each type of particulate, that can be seen in table 1 [16], that should not be overcome to safeguard the citizens' health. pm levels are usually measured by public bodies which collect the data by means of fixed monitoring stations deployed in a limited number of spots: in general, only one or few monitoring stations are present in medium to large-sized cities. moreover, data collected by these stations refer only to the area of the city where they are deployed, while they cannot provide a pervasive feedback on the pm levels in other parts of the city. this fact is mainly due to the high cost of this monitoring stations that prevents from deploying them in a large number across a large territory. nevertheless, some low-cost pm sensors are currently available on the market: while their accuracy level is not comparable with the fixed monitoring stations, they can still provide an interesting feedback on the level of pm, in particular, for what concerns the overcoming of the daily and yearly thresholds. moreover, these devices are characterised by small dimensions, and can be then integrated on portable data acquisition platforms that can be provided with the adequate connectivity to transfer the acquired data in real time to a remote data management centre. by deploying a large quantity of this kind of devices, a pervasive monitoring infrastructure can be then set up across a whole urban centre, thus perfectly fulfilling the paradigm of the smart city [17], [18]. 3. solar energy harvesting in the last decades, due to the steadily growing number of power requiring devices and to the subsequent technology sustainability issues, light energy harvesting has attracted tremendous interest and has aroused a great research effort in the scientific community, resulting in a plethora of solar cell typologies [19], [20], [21], [22], each with different optical and mechanical properties, performances and cost. table 1. particulate matter thresholds. 24-hour mean annual mean pm2.5 25 mg/m3 10 mg/m3 pm10 50 mg/m3 20 mg/m3 acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 113 some studies [23], [24], [25], [26], [27], [28], [29] aim to enhance the performances under particular light spectrum conditions, mainly low-intensity indoor lighting, choosing materials with suitable absorption spectra. other researches focus on extremely improving the efficiency by realising multijunction structures capable of absorbing energy in a wide frequency range [30], [31]. crystalline (monocrystalline and polycrystalline) silicon [32] is surely the dominant technology for solar cells, representing a good compromise between performance and cost [20]. excellent efficiencies over 25% are achieved by monocrystalline silicon technology [19], thanks to efficiency improving strategies, such as carrier recombination reduction through contact passivation [33], [34], [35], [36]. even though these latest efficiency enhancing techniques are not commonly findable in the marketplace yet, monocrystalline silicon solar cells remain the preferable solution for powering a small outdoor electronic utility, like the application presented in this paper, with the minimum encumbrance and at a very reasonable cost. 4. sensor node structure the sensor node purpose is to periodically sample the amount of pm in the air and transmit this information over a lora radio channel. in addition, also the gps position of the node has to be acquired every time a sample is collected. the sensor is powered by a battery that is recharged by means of a crystalline silicon solar cell. the structure of the system is shown in figure 1. the main blocks that compose the node are the communication and control unit (ccu), the particle sensor, the gps module, a battery and a step-up dc-dc converter to manage the energy coming from the solar cells. the ccu has been developed ad-hoc (see figure 2) and hosts a low power stm32l073 microcontroller (mcu) by stmicroelectronics, a lora transceiver (rfm95 by hoperf) and a power management electronics to supply internal devices and charge a li-ion battery. the power from the solar cells is elevated and stabilised by a step-up dc-dc converter (ltc3105 on an evaluation board) that hosts a start-up controller (from 250 mv) and a maximum power point controller (mppc) that enables operation directly from low voltage power sources such as photovoltaic cells. the mppc set point can be selected depending on the solar cells used. if energy from solar cell is available, the battery charger (stc4054 by stmicroelectronics) will recharge the battery. the mcu and the radio module are supplied by a 2.5 v ldo regulator, the voltage level of the battery is controlled by an adc channel on the mcu and a voltage divider. the particle sensor (hpma115s0 by honeywell), shown in figure 3 requires 5 v to operate: this power source is generated by another ltc3105 module directly from the battery. this latter can be powered off by a specific shut down line from the mcu. the gps module (mtk3339 by adafruit) requires a power voltage of 3.3 v, that is available as an output of the particle sensor. since the sensor node is expected to operate continuously without the need of connection to the power grid, a power strategy based on a strict duty-cycling was adopted. in particular, the system is expected to perform data sampling and transmission, and then to be put in sleep mode according to an adaptive duty-cycling policy that will be described in detail in section 5. 5. energy harvesting and power management the sensor node is powered with a battery charged by a small solar harvester. the aim of this section is to foresee the maximum feasible duty cycle of the sensor node operations for not draining completely the battery, given the energy collected by the harvester. this problem can be formulated as the following condition: 𝑊𝐻 ≥ 𝑊𝛿 (𝛿) (1) figure 1. sensor node internal structure. figure 2. communication and control unit figure 3. honeywell hpma115s0 particulate matter sensor. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 114 where 𝑊h is the energy harvested over a day, while 𝑊𝛿 (𝛿) is the sensor node energy consumption over a day, which in fact depends on the duty cycle 𝛿. respecting condition (1) means to guarantee energy self-sufficiency to the sensor device, with no need for battery replacement. therefore, the first step is to assess the sensor node energy requirements, i.e. the quantity 𝑊𝛿 (𝛿). let us indicate with 𝑡0 the time (in seconds) necessary for a complete operating sequence, made up of mcu acquisition, data transmission via lora and gps localisation. the maximum number of operating sessions in an hour is given by: 𝑁 = ⌊ 3600 𝑡0 ⌋ . (2) in our case we have 𝑡0 = 10 𝑠 and thus 𝑁 = 360. let 𝑛 be the desired number of operating sessions in an hour. then the duty cycle is plainly given by: 𝛿 = 𝑛 𝑁 . (3) now, let 𝑊0 be the energy required for a single operating sequence. considering the sensor and analog front-end powering, the microcontroller consumption in run mode, the lora module consumption in stand-by and transmission modes, and the gps consumption, we obtained 𝑊0 ≈ 0.29 mw h. using equation (3), the daily energy consumption for a fixed duty cycle is given by: 𝑊𝛿 (𝛿) = 24 ∙ 𝑊0 ∙ 𝑛 = 24 ∙ 𝑊0 ∙ 𝛿 ∙ 𝑁. (4) substituting 𝑊𝛿 (𝛿) with 𝑊𝐻 in equation (4) yields the maximum feasible duty cycle: 𝛿max = 𝑊h 24 ∙ 𝑊0 ∙ 𝑁 . (5) the harvested energy 𝑊h varies dramatically during the year and obviously is strongly dependent on the weather conditions. in our previous work [37], we proposed a theoretical evaluation of the quantity 𝑊h over the year, calculating approximately the energy produced by a reference monocrystalline silicon solar cell [34]. in this paper, we present some recent experimental results on the energy harvested by a commercial monocrystalline silicon solar cell for outdoor use, produced by seeed studio (figure 4). the producers attest that this solar module is capable of operating at a voltage of 5.5 v and a current of 100 ma, resulting in a maximum power point (mpp) of 0.55 w1. the cell surface is 70 × 55 mm2. the measurements were performed in siena, italy, during sunny or partially cloudy days in the first half of january 2021 (a complete characterisation of the solar cell behaviour throughout the whole year would require a long measuring campaign, and data are not available to date). in the next section, the employed measurement system for the solar cell 1 the producers do not mention explicitly the working conditions under which their solar modules were characterised. usually, solar cell performances are evaluated under standard test conditions (i.e. 25 ℃ temperature, 1000 𝑊 𝑚2⁄ solar irradiance, 1.5 air mass). characterisation2 is described; section 5.2 shows the results of the measurements. 5.1. solar cell characterisation method the circuitry employed for characterising the performances of the selected solar module is based on a solution proposed by analog devices. figure 5 shows the circuit schematic. referring to figure 5, the solar module to be characterised is connected to the ports labelled pv+ and pvon the left. the lower branch is the voltage sensing part, made up of a simple voltage divider followed by an operational amplifier (oa) in noninverting configuration. the overall voltage gain 𝐺𝑉 is given by: 𝐺𝑉 = 𝑅2 𝑅1 + 𝑅2 ∙ (1 + 𝑅4 𝑅3 ) . (6) the r4 resistor is actually short-circuited because the solar module already outputs a voltage in the order of some volts. the upper branch is the current sensing part: the current is converted into a voltage through the 1 ω resistor and then amplified by 2 the term characterisation may be object of misunderstanding. in this context, we are only interested in evaluating the maximum power deliverable by the solar cell, we do not want to determine its parameters (open-circuit voltage, short-circuit current, fill factor) from the currentvoltage curve. therefore, in the following, the term “characterisation” is referred to the evaluation of the cell maximum deliverable power. figure 4. selected monocrystalline solar module. figure 5. solar cell characterisation circuit. for the varying elements, the value used for the experiments presented in this work is reported in brackets. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 115 another non-inverting amplifier. thus, the overall current gain 𝐺𝐼 is: 𝐺𝐼 = 1 + 𝑅11 𝑅10 . (7) this circuit is capable of scanning the current-voltage (iv) curve of the connected solar cell when mosfet q1 and mosfet q2 are conveniently driven. in detail, before starting the acquisition (idle state), q1 is on and q2 is on and therefore the solar cell is in short-circuit conditions, while the oa outputs are both zero. the measurement starts when q1 and q2 are switched off: when this happens, the current instantaneously flows through the capacitor c and the current sensing 1 ω resistor. the voltage across the solar cell is still zero (short-circuit conditions), but now the short-circuit current is visible amplified on the current sensing oa output. the capacitor starts to charge towards the cell open-circuit voltage, which is actually never reached as an effect of the presence of the voltage divider. in this way, the solar cell iv characteristic is scanned and, in particular, the mpp is certainly touched at some points. then, after the iv transient, q2, which has a drain dissipative power resistor to limit the current, is switched on again. finally, also q1 is switched on to return back into the initial idle state. the two oa outputs are sampled by a stm32l432kc microcontroller. the microcontroller acquires 500 samples (250 for the voltage and 250 for the current on two different adc channels) at 125 ksps per channel. the covered time interval is therefore 2 ms, which is sufficient for sampling the signals of interest with a satisfying time resolution. the adc has 12 bits and 3.3 v full-scale. the stm32l432kc also powers the two oa through an on-board 5 v output and drives q1 and q2 through 3.3 v tolerant general purpose input/output (gpio) pins. a raspberry pi powers the stm32l432kc and collects the data from it in javascript object notation (json) format. the data are available remotely on a web database. this solution for solar cell characterisation presents some problems (as already mentioned, the open-circuit voltage is unreachable), but is more than sufficient for our application, since we are not interested in drawing the entire iv curve, but only in evaluating the maximum power deliverable by the cell. furthermore, this solution exhibits two fundamental advantages: first, it is extremely low-cost; second, it is portable, as it exploits the response of the solar cell itself to a load impedance variation, and thus there is no need for a precision voltmeter or a signal generator (or for a more complex and power consuming current generator circuit), usually present for setting the cell current and measuring the voltage in more common solar cell characterisation methods. 5.2. solar cell characterisation results figure 6 shows the current and voltage variations on the cell during an acquisition cycle (i.e. the iv transient provoked by switching off q1 and q2, as explained in the previous section). as it can be seen, the cell passes from being short-circuited to an open-circuit condition. the resulting power curve (figure 7) assumes the bell shape typical of a solar cell. the specific curves in figure 6 and figure 7 were obtained in laboratory testing the circuit under a white led (3500 k colour temperature) and are only meant to demonstrate qualitatively the circuit functionality. the measurements were performed in siena, italy, throughout sunny or partially cloudy days in january 2021. the characterisation system was placed in a realistic position, exposed to the sun during most of the day but with some trees and other obstacles likely present in the actual sensor usage. an acquisition was performed every minute. as an example, figure 8 shows the cell maximum achievable power measured on 27th january 2021. the wells are due to the presence of obstacles (e.g. around midday some trees covered the solar module). the total energy collected during the examined days (that is 𝑊h, see the introduction of section 5) oscillates between a minimum of 400 mw h in partially cloudy days and a maximum of 1 w h. considering on average 𝑊h ≈ 700 mw h, the corresponding maximum feasible duty cycle, calculated through equation (5), is: 𝛿max ≈ 30% . (8) this result, put into equation (3), corresponds to about 100 sensor node acquisitions per hour, which is more than decent, considering that december and january are the worst months of the year for solar energy harvesting. however, it is clear that this performance assessment is not valid for heavily cloudy or rainy days, in which the energy production falls sharply (the energy produced on 9th january 2021 and 10th january 2021, which were completely sunless, was 20 mw h in total). for this reason, we are driving the future development of the powering of the sensor node towards a multi-source energy harvesting approach, figure 6. solar cell current and voltage variations during an iv acquisition cycle: laboratory test under white led. figure 7. power curve corresponding to current and voltage reported in figure 6. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 116 adding, along with the solar harvester, a piezoelectric harvester for collecting energy also from mechanical vibrations and even from rainfall [38]. 6. tests and measurements the performances of the system were tested in a real environment: in particular, the sensor node was placed for 1 week on the facade of the department of information engineering and mathematics of the university of siena, italy (see figure 9), acquiring pm10 and pm2.5 values each 15 minutes. during each sampling, 10 values were acquired and then their mean value was transmitted by means of lorawan protocol to a lorawan gateway positioned inside the building. the pm sensor was characterised only in static conditions (no significant vibration is present) since the idea in the real scenario is to acquire the measurement only when the vehicle is still. indeed, in its final configuration the node is expected to integrate an accelerometer that will be exploited to detect whether the vehicle is moving or not. the lorawan transmission is then performed too in this phase since the whole data acquisition and transmission task requires less than 2 s, avoiding thus possible additional issues due to the set up of a radio channel in nonstationary conditions. in order to verify the operation of the system, the sampled values were compared with the ones available on the website of the regional environmental protection agency of tuscany region (arpat), which owns a set of fixed monitoring stations deployed across the whole territory of tuscany. in particular, daily average values are available on arpat website: for this reason, for the acquired values the daily mean value was calculated. the values were compared with the ones acquired by the fixed station positioned in viale bracci, siena, italy, which is the closest one to the university building, at a distance of 2.5 km. a deployment close to this fixed station was not possible due to security reasons. figure 10 shows the daily mean values of pm10 concentrations measured at the fixed station by arpat and by the system described in this work, positioned on the university building. even if the two values are notably different, this is due to the deployment site: while the arpat fixed station is positioned close to the very busy road that leads to the siena hospital, the university building is positioned in a limited traffic area located in a peripheral part of the historic centre of siena. nevertheless, the effectiveness of the system can be noticed, since the trends of the two values all week long are almost similar: in particular, the values measured by the system are always almost half of the ones provided by arpat. an important comment has to be done: a low-cost sensor has been used in the realisation of the system, and its accuracy level cannot be compared with the professional, and then very expensive, measurement platforms used by arpat. nevertheless, looking at the values measured by the system, it is evident that the proposed solution can still be useful to collect data about pm in a more pervasive way, even if with a lower level of accuracy. in this sense, the proposed solution is not expected to replace the existing fixed measurement stations but mainly as system to enrich the knowledge about the different levels of pm that may be recorded in correspondence of different environmental conditions. figure 8. solar cell maximum power production on 27th january 2021 in siena, italy. figure 9. sensor node testing setup. figure 10. comparison between daily mean pm10 concentrations provided by arpat and measured by the system. figure 11 : geographical data visualisation. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 117 following the system characterisation performed in a controlled environment (i.e., the department facade), a geographical data acquisition campaign was carried out, measuring pm2.5 and pm10 concentrations along the roads of a wide area within the historic centre of siena. for this purpose, a dragino lorawan gateway was placed on the front facade of the department building, in the same spot that was used for the deployment of the sensor node in the previous experimentation. pm measurements were associated with the latitude and longitude values acquired by the gps module: these values were then used to set up a data visualisation tool by means of google maps services. the measured values show an increase in the narrower alleys where vehicular traffic was more consistent. a screenshot of the acquired data visualisation tool, with the measurements related to one of the positions, is shown in figure 11. blue markers represent the spots where measurements were acquired while the red star shows the gateway position. 7. conclusions the aim of this paper was to propose the architecture of a self-powered lorawan sensor node for the pervasive measurement of pm concentrations in urban areas. according to the presented results, the system is able to operate autonomously exploiting an energy harvesting system based on the use of a small low-cost monocrystalline solar cell. in particular, the experimentation carried out demonstrated how the energy provided by the solar harvester is sufficient to guarantee around a hundred samplings per hour during winter, when solar energy production is at its minimum. moreover, the addition of a mechanical vibration energy harvester is under evaluation as a future development, to enable a multi-source energy harvesting approach which would improve the energy production during sparsely lit days. at the same time, the system can sample the pm concentrations by means of a low-cost sensor, transmitting them to a lorawan gateway together with the geographic coordinates of the sampling location. by positioning the measurement system on means of public transport and combining these two data, pm levels may be measured across a large area and the level differences related to different areas of an urban centre may be identified. moreover, the energy selfsufficiency feature may allow an easy deployment 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network based kinetic gas molecule optimization acta imeko issn: 2221-870x june 2022, volume 11, number 2, 1 -7 acta imeko | www.imeko.org june 2022| volume 11 | number 2|1 video-based emotion sensing and recognition using convolutional neural network based kinetic gas molecule optimization kasani pranathi1, naga padmaja jagini2, satish kumar ramaraj3, deepa jeyaraman4 1 dept of information technology, vr siddhartha engineering college, kanuru, vijayawada-520007 andhra pradesh, india 2 dept of computer science and engineering, vardhaman college of engineering, hyderabad-501218, telangana , india 3 sengunthar college of engineering, tiruchengode, tamilnadu, india 4 dept of electronics and communication engineering, k. ramakrishnan college of technology, trichirapalli-621112, tamilndadu, india section:research paper keywords: artificial intelligence; convolutional neural network; kinetic gas molecule optimization; images; video-based emotion recognition citation: kasani pranathi, naga padmaja jagini, satish kumar ramaraj, deepa jeyaraman, video-based emotion sensing and recognition using convolutional neural network based kinetic gas molecule optimization, acta imeko, vol. 11, no. 2, article 13, june 2022, identifier: imeko-acta-11 (2022)-02-13 section editor: francesco lamonaca, university of calabria, italy received october 2, 2021; in final form june 3, 2022; published june 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: kasani pranathi, e-mail: pranathivrs@gmail.com 1. introduction emotional recognition is considered a major theme in machine learning and artificial intelligence (ai) [1] in recent years. the huge upsurge in the creation of advanced interaction technologies between humans and computers has further encouraged progress in this area [2]. facial actions convey emotions that transmit the character, the mood, and the intentions of a person, in turn. emotions and moods swiftly lead to the identification of the human mind. the psychologist says that emotions are mostly short and that mood is milder than emotion [3]. human emotions can be detected in different ways, such as verbal or voice responses, physical reactions or the languages of the body, autonomous responses, and so on [4]. in a person, the basic types of emotions are pleasing, normal, surprised, frightened, angry, disgusting or sad. while other expressions like dislike, amusement, or pride, dislike and honesty in humans are very difficult to find by expression of the face [5][6] is easy to detect emotions like happiness, normal, disgust, and fear. as we know, people identify emotion by combining difficult multimodal information and tend only to pay attention to significant information in various ways. for example, some people always talk while they keep a smile, while others talk loudly but not angrily [7]. consequently, we deliberate those human beings do not detect emotions based on modal alignment. the objective of emotional awareness can be generally achieved by means of visual or sound techniques. the field of humancomputer interaction has changed by artificial intelligence, providing many machine learning techniques to achieve our goal abstract human facial expressions are thought to be important in interpreting one's emotions. emotional recognition plays a very important part in the more exact inspection of human feelings and interior thoughts. over the last several years, emotion identification utilizing pictures, videos, or voice as input has been a popular issue in the field of study. recently, most emotional recognition research focuses on the extraction of representative modality characteristics and the definition of dynamic interactions between multiple modalities. deep learning methods have opened the way for the development of artificial intelligence products, and the suggested system employs a convolutional neural network (cnn) for identifying real-time human feelings. the aim of the research study is to create a real-time emotion detection application by utilizing improved cnn. this research offers information on identifying emotions in films using deep learning techniques. kinetic gas molecule optimization is used to optimize the fine-tuning and weights of cnn. this article describes the technique of the recognition process as well as its experimental validation. two datasets such as video-based and image-based datasets, which are employed in many scholarly publications, are also investigated. the results of several emotion recognition simulations are provided, along with their performance factors. mailto:pranathivrs@gmail.com acta imeko | www.imeko.org june 2022| volume 11 | number 2|2 [7]. the extraction of representative modal features using profound learning technology has grown easier. for example, the neural network [8] includes advanced cnn convolutional neural network (alexnet, vgg, resnet, senets, etc.) tuning process is extremely useful for capturing features of fine-grained facial expression; and long short-term memory unit (lstms) are another profound learning technology to store info with short-term interaction of time-step memory functionality [9]. on the basis of these technology of learning, there is much work to be done to define dynamic multimodal interactions by matching the relevance of each lstm memory unit. video-based emotional recognition is multidisciplinary, covering areas such as psychology, affective computing and interaction between humans and computers. the main element of the message is the expression of the face which makes up 55% of the overall impression. in order to create an appropriate model for the recognition of video emotions, proper feature frames of face expression must be provided within the scope. deep learning offers a diversity in terms of accuracy, learning rate and forecasting, rather than using standard techniques. cnn has offered support and platform for the analysis of visual imaging, among the in-depth learning methodologies. convolution is the basic application of a filter in an action to an input that results. reusing a related filter to an input creates an enactment map known as a feature map that shows the areas and the quality of, for instance, an identified element in an image. the growth of neural systems of convolution is thus the ability to learn skills with an enormous number of filters equating explicitly to a training dataset according to the needs of, for example, an image characterisation. the result is deeply explicit highlights which are distinguishable in the input images anywhere. deep learning accomplished a major achievement in the recognition of emotions and cnn is the renowned profound way of learning with exceptional image processing performance. this work aims to develop video-based emotional awareness through optimal cnn, as detailed in the next section. the remaining part of the paper is arranged: the related study on video emotional recognition is presented in section 2. section 3 provides an explanation for the suggested optimized cnn. section 4 discusses the validation of the methodology suggested with its current techniques. finally, section 5 presents the conclusion of this study with its future work. 2. literature review a number of disciplines, such as spam filtering, audio recognition, facial identification, classifying documents and processing of natural languages are addressed by machine learning algorithms. classification is one of the most frequently used domains of machine learning. video-based face-motion research in the computer vision community recently attracted notice. different kinds of input data, including facial expressions, voice, physiological indicators, and corporal motions, are utilised in emotional recognition. the work of michel healy[10], who describes a video feeding system in real time and uses a support vector machine for fast and reliable classification, offers several ways to the detection of emotion by facial expressions. the 68point facial features used in [10] are symbols. the application was taught to detect six emotions by monitoring changes in the expressions of the face. the work of dennis maier [11] currently uses neural networks via tensorflow to train image features and then achieves classification through fully connected neural layers. the advantage of image features over facial landmarks is the larger information space, where the spatial formation of landmarks gives a viable method for analysing facial expressions. however, this is also accompanied by a higher computing power requirement. the structure provides for an outsourced classification service that runs on a server with a gpu. images of faces are brought to the service in real time, which can perform a classification within a few milliseconds. in the future, this approach will be extended to include text and audio features and conversation context to boost accuracy. another approach uses cnns with tensorflow. an example of using tensorflow.js with the sense-care km-ep is discussed in [10], which deploys a web browser and node server. 93% rely on nonverbal’s (facial expressions as 55%, sound: 38%) and 7% rely on verbal language in terms of human emotional understanding. that is why various efforts have been carried out to recognize facial expression (fer) and acoustic emotion (aer) tasks. most of these works use deep learning (dl) skills to extract computer features to get recognition of high emotions. pramerdorfer et al., [12] used and confirmed contemporary dnn (vgg, resnet, inception) architectures to extract aspects of facial expression to enhance fer performance. on the other hand, the most typically employed features include pitch, log-mel filters, and cepstral coefficients for mfccs, as far as aer tasks are concerned. huang et al. [13] used four kinds to extract more complete emotional characteristics using log-mel. multiple spatial-time fusion feature framework (msff) was proposed by lu et al. [14]. they have improved the pretrained model for photos of facial expression to draw on facial expression characteristics and have applied the vgg-19 and blstm models to extract audio emotional aspects. however, the interactions between different modes were not taken into account. zadeh et al. [15], on the other hand, considered consistency and attributes complementary to the diverse modal information, proposing a memory fusion network which model modal and multimodal interactions through time to capture more efficacious emotional characteristics in the cmu-mosi dataset. liang et al. [16] have presented the neural model dynamic fusion graph to shape multimodal interactions, to capture one, two and three modal interactions, and, based on the importance, dynamically adjust multimodal dynamics of individual fusions. although [16] is able to dynamically collect interactions in several modalities, different modalities must be aligned with the word utterance time interval through the average of their modalities. the word-based alignment technique can nonetheless miss the chance to capture more active relationships between modes. 3. proposed methodology this section provides a description of the overall architecture for the development of a deep learning algorithm as a videobased emotional recognition model. in addition, architectural diagrams are briefly described together with various operations before and after processing. the system overview with cnn displays the suggested training and testing method in figure 1. the video input must pass through a number of procedures before cnn takes action. 3.1. pre-processing this is the first procedure used for the video sample input. emotions are typically classified as happiness, sadness, anger, pride, fear and surprise. frames should therefore be removed from the video input. the number of frames depends on complexity and computational time for different researchers. acta imeko | www.imeko.org june 2022| volume 11 | number 2|3 the pictures are transformed to the greyscale. the frame is rather black and white or grey monochrome after grey scaling. the contrast with low intensity leads to grey and white with high intensity. the histogram equalization of the frames is monitored by this step. histogram is a computer image management strategy to improve photographic contrast. this is achieved by extending, for instance by loosening the intensity of the image, the most successive intensity estimates. the intensity of a picture is shown by the histogram and it is the number of pixels for each intensity value deliberated in simple terms. 3.2. face detection emotions are usually characterized by the face. it is therefore important to detect the face for processing and recognition. many face detection algorithms are used by several investigators such as opencv, dlib, eigenfaces, the local histograms of binary patterns (lbph) and viola-jones (vj). conventional procedures included face recognition work in which facial highpoints are distinguished from the face image by extracting highlights or milestones. the calculation may, for example, survey the shape and size of the eyes, nose size and their relative position with the eyes in order to delete face highlights. the cheekbones and mastic may also be dissected. these highlights extracted would be used to view different images with matching features. the industry has gone deeply into learning throughout the years. cnn was recently used to improve the accuracy of calculations for facial recognition. these controls accept an image as information and concentrate on a very complex arrangement of features. these features include facial width, facial stature, nose width, lips, eyes, width proportion, skin shading, and surface. basically, a cnn divides a huge sum of highlights from a picture. this is then synchronised with the highlights in the database 3.3. image cropping and resizing during this phase, the face of the facial detection procedure is trimmed so that the facial image looks broader and clearer. cropping is the ejection from the photographic or graphical image of unwanted exterior parts. the technique often consists of expelling a section of the outermost regions of a picture, expelling the image's incidental waste, improving its surroundings, changing its perspective, or highlighting and disentangling the subject. the size of the images varies after the frames have been cropped. those photographs are therefore subject to resize, say 80 to 80 pixels for instance, in order to achieve homogeneity. a digital picture is only a quantity of information displaying a variety of red, green and blue pixels in a certain location. we notice these pixels more often than not as smaller than normal pixels on the pc screen wedged together. the frame size determines how long it takes to process. resizing is therefore very important if processing time is to be reduced. in addition, techniques of better resizing should be used to maintain image attributes following resizing. whether the features represent the expression well or not depends on the accuracy of the classification. the optimization of the selected features, therefore, improves classification precision automatically. 3.4. classification in this section, the classification includes learning rate optimization for cnn using kinetic gas molecule optimization (kgmo) is described briefly. initially, the cnn is explained as follows: cnn is a neural transmission network with several layer feeders, comprising several sorts of layers, including convolution layer and relu, pooling layers and fully connected output layers. figure 2 shows the architecture of cnn, which is intended to recognize visual characteristics such as borders and forms. 3.4.1. cnn cnn employs the vector x of the trained samples as the input for the associated target group y to support the back propagation technique for training information. learning is performed by comparing the desired target with each cnn output, and a learning error occurs in the difference between the two. taking mathematical responsibility for the future cnn, 𝐸(𝜔) = 1 2 ∑ ∑(𝑜j,p l − 𝑦j,p) 2 𝑁ι j=1 . 𝑝 ρ=1 (1) our goal is the cost function lessening of 𝐸(𝜔), discovery a minimizer �̃� = �̃�1, �̃�2, … , �̃�v ϵ ℝv, where v = ∑ weightnum (𝑘)lk=1 and indicate that the space of weight ℝ v is equal to the number of weights (weigtnum(. )) of the cnn network at each k layer of total l layers. ∇𝐸i(𝜔i) = ( ∂𝐸i ∂𝜔i 1 , … , ∂𝐸i ∂𝜔i v) (2) 𝜔i+1 = 𝜔i − 𝑛 ∇𝐸i(𝜔i) , (3) where 𝑛 is the learning rate (step) value. cnn is adapted to the video-based emotion detection, but how fast cnn is adapted can be controlled by this learning rate. more training epochs are acquired for smaller learning rates that give only small changes to the weights during each update. on the other hand, fewer training epochs are required for larger learning rates. specifically, the learning rate is a configurable hyper-parameter used in the training of neural networks that has a small positive value, often in the range between 0.0 and 1.0. to find the optimized learning rate value, this work uses the kgmo algorithm. here, the 𝑛 has figure 1. proposed workflow (top) and proposed training (bottom) testing processes. figure 2. typical architecture of some of the pre-trained deep cnn networks used in the study acta imeko | www.imeko.org june 2022| volume 11 | number 2|4 been designated with the assistance of the kgmo technique, which is explained as follows. 3.4.2. kgmo the boyle laws introduced gas, which are based on unproven descriptions to label gas glimmer's macroscopic plants. the motion of the gas molecule is based on certain characteristics such as pressure, temperature and gas volume. five suggestions and a cinematic molecular system of spectacular blasts were used to indicate the fauna of the gas molecules: • a gas involves the movement in straight-line movement of tiny molecules. the processing of the movement presents according to newton's law. • there is no capacity for the gas glimmer. it's like a subject. • no repulsive or attracting force exists between molecules, the average molecular kinetic energy of 3 k t/2 is described as t and boltzmann constant is described as k, income which has 1.38 × 10-23 m2 kg s-2 k-1. take on, that the scheme contains 𝑀particles. the locality of 𝑗𝑡ℎ the agent is labelled by (4) 𝑌𝑗 = (𝑦𝑗 1 , … … 𝑦 𝑗 𝑑, … … 𝑦 𝑗 𝑚) for (𝑗 = 1, 2, … , 𝑀), (4) where 𝑌𝑗 𝑑 defines the position of the 𝑗𝑡ℎ agent, that in 𝑑th dimension. the velocity of the 𝑗𝑡ℎ agent, it stated as below in (5) 𝑉𝑗 = (𝑣𝑗 1 , … . . , 𝑣𝑗 2 , … . . 𝑣𝑗 𝑚) for (𝑗 = 1, 2, … , 𝑀), (5) where 𝑉𝑗 𝑑 represents the 𝑗𝑡ℎ agent velocity, that in the 𝑑𝑡ℎ dimension. units are modified by the circulation of boltzmann. random element yield speed is related to the kinetic energy of the unit. equation (6) is the kinetic force of the atom as, 𝑘𝑗 𝑑(𝑢) = 3 2 𝑀𝑏 𝑇𝑗 𝑑 (𝑢), 𝐾𝑗 = (𝑘𝑗 1, … . . , 𝑘𝑗 𝑑, … . . 𝑘𝑗 𝑚) for (𝑗 = 1, 2, … , 𝑀), (6) where the number of atoms is represented 𝑀𝑏 as the boltzmann relentless and the high temperature of an 𝑗𝑡ℎ agent in the 𝑑𝑡ℎdimension at a time 𝑢 is characterized as 𝑇𝑗 𝑑(𝑢). the molecules rate is updated by (7) 𝑣𝑗 𝑑 (𝑢 + 1) = 𝑇𝑑 𝑗 (𝑢)𝑤𝑉𝑗 𝑑 (𝑢) + 𝐷1rand𝑗 (𝑢) (𝑔𝑏𝑒𝑠𝑡 𝑑 − 𝑦𝑗 𝑑 (𝑢)) + 𝐷2rand𝑗 (𝑢) (𝑝𝑏𝑒𝑠𝑡𝑗 𝑑 (𝑢) − 𝑦𝑗 𝑑 (𝑢)) , (7) where 𝑇𝑗 𝑑(𝑢) for the meeting molecules reduce the exponentially over time and is planned in (8). 𝑇𝑗 𝑑(𝑢) = 0.95 × 𝑇𝑗 𝑑(𝑢 − 1). (8) the mass 𝑚 of individual sub-division is a random number and it consumes a range of 0 < 𝑚 ≤ 1. once the mass is wellknown, the whole algorithm remains unchanged because only one type of gas is taken into consideration at any time. a random number shall be used to claim places in diverse types to produce various executions of the procedure. the constituent part is expressed as equation, based on the gesticulation equation (9), 𝑦𝑢+1 𝑗 = 1 2 𝑎𝑗 𝑑 (𝑢 + 1)𝑢2 + 𝑣𝑗 𝑑 (𝑢 + 1)𝑢 + 𝑦𝑗 𝑑 (𝑢), (9) where acceleration of the 𝑗𝑡ℎagent is in the 𝑑𝑡ℎ dimension exemplified as 𝑎𝑗 𝑑 . centered on the acceleration dismiss achieve in (10). 𝑎𝑑 𝑗 = (d𝑣𝑗 𝑑 ) d𝑢 . (10) built on the gas specks law is signified in (11). d𝑘𝑑 𝑗 = 1 2 𝑚(d𝑣𝑗 𝑑 )2 ⇒ d𝑣𝑗 𝑑 = √ 2(d𝑘𝑗 𝑑 ) 𝑚 . (11) from (10) and (11), the acceleration is mentioned in (12) 𝑎𝑑 𝑗 = √ 2(d𝑘𝑗 𝑑) 𝑚 d𝑢 . (12) the acceleration reckoning is re-written depends on the duration interval δ𝑢which is shown in (13) 𝑎𝑑 𝑗 = √ 2(δ𝑘j d) 𝑚 δ𝑢 . (13) in a unit time interval, the acceleration would be (14) 𝑎𝑑 𝑗 = √ 2(d𝑘𝑗 𝑑 ) 𝑚 . (14) from (9) and (14), the section of the particle is computed by uttered by (15) 𝑦𝑢+1 𝑗 = 1 2 𝑎𝑗 𝑑 (𝑢 + 1)δ𝑢2 + 𝑣𝑗 𝑑 (𝑢 + 1)δ𝑢 + 𝑦𝑗 𝑑 (𝑢) ⇒ 𝑦𝑢+1 𝑗 = 1 2 √ 2(δ𝑘𝑗 𝑑 ) 𝑚 (𝑢 + 1)δ𝑢2 + 𝑣𝑗 𝑑 (𝑢 + 1)δ𝑢 + 𝑦𝑗 𝑑 (𝑢). (15) in the past, the assumption is that the molecular mass is a random element in all rules exercising, but the execution is same in all particles. the location, which is expressed in (16). is sporadically reorganized to make the approach easier. 𝑦𝑢+1 𝑗 = √ 2(δ𝑘𝑗 𝑑 ) 𝑚 (𝑢 + 1) + 𝑣𝑗 𝑑 (𝑢 + 1) + 𝑦𝑗 𝑑 (𝑢) (16) the lowest appropriateness utility is firm by using resulting (17). 𝑝𝑏𝑒𝑠𝑡𝑗 = 𝑓(𝑦𝑗 ), if 𝑓(𝑦𝑗 ) < 𝑓(𝑝𝑏𝑒𝑠𝑡𝑗 ) 𝑔𝑏𝑒𝑠𝑡𝑗 = 𝑓(𝑦𝑗 ), if 𝑓(𝑦𝑗 ) < 𝑓(𝑔𝑏𝑒𝑠𝑡𝑗 ). (17) in the position(𝑥𝑗 𝑑 ) of that, each element is studied by consuming space that amid the current situation and the current position among in the space and 𝑔𝑏𝑒𝑠𝑡𝑗 . the next section will show the validation of proposed methodology with existing techniques. acta imeko | www.imeko.org june 2022| volume 11 | number 2|5 4. results and discussion all of our results were created on a single nvidia geforce rtx 2080 ti gpu. all the code was applied using pytorch2. 4.1. datasets description 4.1.1. image-based emotion recognition in the wild in this work, we have selected appropriate data sets to train the face extraction model. the data sets must address environments in the wild, in which numerous factors, such as obstruction, poses, lighting, etc., are uncontrolled. affectnet [17] and raf-db [18] are the most extensive datasets that meet these criteria by far. the photographs in the data sets are acquired using emotional keywords on the internet. experts note emotion labels to ensure trustworthiness. affectnet has two kinds of data, manual and automatic, with over 1,000,000 photos marked with 10 categories of emotions and dimensional emotions (valence and arousal). we only utilized photos in the manual group of seven fundamental categories of emotions. for example, we used 283901 training photos and 3500 validation images. the rafdb dataset comprises of approximately 30,000 facial photographs in basic emotional categories that have taken lighting variations, arbitrary poses, and occlusion under in-thewild situations. in this study, we selected 12,271 training images and 3068 evaluation images, all of them from the basic set of emotions. 4.1.2. video-based emotion recognition in the wild we used the afew dataset [19] to assess our work to determine face emotions in video clips. the video samples in the collection are obtained through uncontrolled occlusion, lighting and head positions from films and tv shows. each video clip was selected based on its label including emotional keywords that reflect the emotion shown by the main topic. using this information, we have assisted to deal with the challenge of temporality in the wild. we have used 773 trainings with the afew dataset and 383 validation video clips with labels for the seven basic types of emotion (anger, happiness, neutrality disgust, fear, sadness, and surprise). 4.2. evaluation parameters as quantitative measurements in this investigation, we employed precision (acc.) and the f 1 score. we also employed the average 𝑀𝑒𝑎𝑛𝐴𝑐𝑐 depending on the major diagonal of the standardized 𝑀norm confusion matrix, for the performing results to be evaluated as in [18]. these measurements are derived accordingly. 𝐴𝑐𝑐𝑢𝑟𝑎𝑐𝑦 = 𝑇𝑃 + 𝑇𝑁 𝑇𝑃 + 𝐹𝑃 + 𝑇𝑁 + 𝐹𝑁 (18) 𝐹1 = 2 𝑃𝑟𝑒𝑐𝑖𝑠𝑖𝑜𝑛. 𝑅𝑒𝑐𝑎𝑙𝑙 𝑃𝑟𝑒𝑐𝑖𝑠𝑖𝑜𝑛 + 𝑅𝑒𝑐𝑎𝑙𝑙 (19) 𝑀𝑒𝑎𝑛𝐴𝑐𝑐 = ∑ 𝑔𝑖,𝑗 𝑛 𝑖=1 𝑛 (20) 𝑆𝑡𝑑𝐴𝑐𝑐 = √ ∑ (𝑔𝑖,𝑗 − 𝑀𝑒𝑎𝑛𝐴𝑐𝑐 ) 2𝑛 𝑖=1 𝑛 , (21) where 𝑔𝑖,𝑗 ∈ diag(𝑀norm) is the ith diagonal value of the normalized confusion matrix 𝑀norm , 𝑛is the size of 𝑀norm , and tp, tn, fp, and fn, respectively, are true positive, false positive, true negative, and false negative. table 1 shows the confusion matrix for proposed methodology. 4.3. evaluation of proposed model with kgmo for two different datasets. in this section, the different cnn models include convnet, densenet and resnet are compared with and without kgmo techniques in terms of accuracy, f1-score and mean accuracy with standard metrics on two different datasets such as imagebased and video-based datasets. table 2 and figure 3 shows the results of projected model with kgmo on image-based datasets. in the accuracy experiments, the convnet, densenet and resnet without kgmo achieved 56.26 %, 61.51 % and 61.57 %, where these techniques are implemented with kgmo technique and achieved 81.23 %, 83.64 % and 87.22 %. these results proved that resnet with kgmo achieved better accuracy than other models. in the f1-score analysis, the convnet, densenet and resnet without kgmo achieved 56.38 %, 61.50 % and 61.46 %, where these techniques are implemented with kgmo technique and achieved 81.79%, table 1. confusion matrix of proposed model with kgmo on image-based emotion recognition evaluation. overall classification with manual id ground truth anger happiness neutrality disgust fear sadness surprise p re d ic te d anger 93.30 0.93 4.19 2.33 1.40 1.86 happiness 2.79 83.93 5.89 2.65 2.33 2.33 neutrality disgust 6.51 8.35 87.37 2.65 7.44 6.05 fear 3.26 2.72 0.93 88.19 4.19 3.72 sadness 2.79 3.14 4.36 1.86 86.28 7.91 surprise 1.26 1.69 72.56 2.65 11.16 84.70 table 2. validation of proposed model with kgmo on image-based emotion recognition evaluation. cnn-model acc (%) 𝑭𝟏 (%) 𝑴𝒆𝒂𝒏𝑨𝒄𝒄 ± 𝑺𝒕𝒅 convnet 56.26 56.38 56.23 ± 11.18 densenet 61.51 61.50 61.51 ± 10.40 resnet 61.57 61.46 61.57 ± 10.79 convnet-kgmo 81.23 81.79 77.08 ± 08.10 densenet-kgmo 83.64 83.81 76.96 ± 11.12 resnet-kgmo 87.22 87.38 82.45 ± 09.20 figure 3. graphical representation of proposed model with kgmo in terms of accuracy and f1-score on image-based dataset. acta imeko | www.imeko.org june 2022| volume 11 | number 2|6 83.81% and 87.38%. the mean accuracy of each technique without kgmo achieved nearly 56% to 61% with standard deviation of 11. but, when these techniques are implemented with kgmo, they achieved nearly 77 % to 82 % of mean accuracy with standard deviation of 9.20 on proposed model with kgmo. table 3 and figure 4 shows the results of proposed model, existing techniques by implementing with and without kgmo on video-based datasets. in the accuracy experiments, the convnet, densenet and resnet without kgmo achieved 51.70 %, 52.22 % and 54.05 %, where these techniques are implemented with kgmo technique and achieved 55.87 %, 56.14 % and 58.66 %. these results proved that resnet with kgmo achieved better accuracy than other models, however the proposed technique achieved less performance than image-based dataset. in the f1score analysis, the convnet, densenet and resnet without kgmo achieved 46.17 %, 48.26 % and 50.78 %, where these techniques are implemented with kgmo technique and achieved 52.76 %, 54.61 % and 58.50 %. the mean accuracy of each technique without kgmo achieved nearly 46 % to 48 % with standard deviation of 32. but, when these techniques are implemented with kgmo, they achieved nearly 52 % to 56 % of mean accuracy with standard deviation of 15.63 on proposed model with kgmo. the reason for the less performance is that it is difficult to identify the proper emotion recognition while the video is continuously playing. 4.4. comparative evaluation of proposed technique with existing techniques the following table 4 shows the comparative analysis of proposed technique with various existing techniques in terms of accuracy. figure 5 shows the graphical representation of proposed model on video-based dataset in terms of accuracy. the existing techniques [20]-[21] such as dsn and cnn features with lstm achieved nearly 49 % of accuracy. the existing fan [23] and caer-net [24] achieved only 51 % of accuracy, where the existing techniques achieved nearly 52 % to 53 % of accuracy. noisy student training with multi-level attention [26]-[30] achieved 55.17 % of accuracy, where the proposed model achieved 58.66 % of accuracy. the reason is that the cnn is implemented with optimization technique called kgmo for fine-tuning and weight optimization. 5. conclusion computer vision research on facial expression analysis has extensively studied in the past decades. in recognition of emotions, the success of this method has improved greatly over time. our work has shown the general architectural model for developing a deep learning recognition system. the objective is to examine preand post-process methods. using kgmo algorithm, the smoothing and weight of cnn is optimized. this report also included the data sets available for academics in this subject on pictures and video. in different study, the advancement in this area is measured by different performance measures. the testing is performed on the various datasets, in which resnet-kgmo has achieved an accuracy of 58.66 % in the video dataset, and an accuracy of 87.22 % for the imagebased dataset. there is a very attractive scope of future developments in this area. various deep learning multimodal and various architectures can be employed to increase the performance parameters. besides the realization of the feelings alone, the intensity level can be raised further. this can contribute to forecasting the intensity of the feeling. in future works multi-medians may also be used; for example, the construction of a model with multi-data sets can be used with video and audio. table 3. validation of proposed model with kgmo on video-based emotion recognition evaluation. cnn-model acc (%) 𝑭𝟏 (%) 𝑴𝒆𝒂𝒏𝑨𝒄𝒄 ± 𝑺𝒕𝒅 convnet 51.70 46.17 46.51 ± 34.38 densenet 52.22 48.26 47.33 ± 31.73 resnet 54.05 50.78 48.98 ± 32.28 convnet-kgmo 55.87 52.76 51.21 ± 29.87 densenet-kgmo 56.14 54.61 52.35 ± 25.53 resnet-kgmo 58.66 58.50 56.25 ± 15.63 table 4. comparative analysis of proposed model with existing techniques on video-based dataset author technique accuracy (%) vielzeuf et al. [20] (2018) max score selection with temporal pooling 52.20 fan et al. [21] (2018) deeply-supervised cnn (dsn) weighted average fusion 48.04 duong et al. [22] (2019) cnn features with lstm 49.30 li et al. [23] (2019) vgg-face features with bi lstm 53.91 meng et al. [24] (2019) frame attention networks (fan) 51.18 lee et al. [25] (2019) caer-net 51.68 kumar et al. 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https://doi.org/10.1016/j.measurement.2021.110170 https://doi.org/10.1016/j.measurement.2018.11.074 digital calibration certificate in an industrial application acta imeko issn: 2221-870x march 2023, volume 12, number 1, 1 6 acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 1 digital calibration certificate in an industrial application juho nummiluikki1, sari saxholm2, anu kärkkäinen2, sami koskinen1 1 beamex oy ab, ristisuonraitti 10, 68600 pietarsaari, finland 2 vtt mikes, tekniikantie 1, 02150 espoo, finland section: research paper keywords: calibration; digital calibration certificate; traceability; metrology; digitalization; quality infrastructure citation: juho nummiluikki, sari saxholm, anu kärkkäinen, sami koskinen, digital calibration certificate in an industrial application, acta imeko, vol. 12, no. 1, article 6, march 2023, identifier: imeko-acta-12 (2023)-01-06 section editor: daniel hutzschenreuter, ptb, germany received november 17, 2022; in final form march 22, 2023; published march 2023 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was supported by business finland [grant number 1811/31/2019]. corresponding author: sami koskinen, e-mail: sami.koskinen@beamex.com 1. introduction western europe was the third largest region in the measuring and control instruments market worth $137.9 billion in 2020, accounting for 19.6 % of the global measuring and control instruments market, preceded and followed by north america at 27.7 % and eastern europe at 5.6 % respectively. the western europe measuring and control instruments market is expected to grow to $188.59 billion by 2025. [1] 1.1. need and benefits of dcc since only reliable measurement data enables reliable decision making, regular calibration of measuring instruments and the metrological traceability of these results are essential. today calibration certificates are mostly paper documents or pdf files which require human operations in the calibration processes. with the help of the digital calibration certificate (dcc) [2], manual work with calibration certificates can be eliminated. this makes calibration processes faster and allows valuable human operators to concentrate on more valuable work. data transfer and uncertainty calculations related to calibration results can be performed automatically. effective processes, calculations without human errors, automated reporting procedures and use of calibration data have indirect economic effects on the industry through better product quality. in industrial production, the quality of products and processes is verified by measurements. efficient quality inspection is essential to avoid defect propagation in the manufacturing chain. the target is to minimize waste in production as well as to ensure long lifetime to minimize waste due to premature end-product failure [3]. fully digitalized cyberphysical manufacturing requires autonomous quality inspection and control processes as well as integration to digital design and digital quality certificates, including digital calibration certificates. these principles are also emphasized by sustainable development, where the production chain from raw material to recycling (or waste management) of the end-product can be digitally controlled with a digital twin. key challenges and a potential future role of metrology in the digital age are discussed by eichstädt et al [4]. the publication investigates algorithms, cyber-physical systems, fair data and metrology, and the role of metrology in the digital transformation in the quality infrastructure (dqi). the digital transformation of metrology is abstract rapid growth of automation in combination with digitalization generates increasing need for measurement data, provided by sensors, the interface between real and digital world. in industry 4.0 scheme the measurement data, including the calibration information, flows through the whole production chain in digital format. the present study demonstrated a fully digitalized environment for the calibration data generation, transfer, and usage in a proof of concept (poc) project. various stages of the poc and different information systems used by the partners aalto university, vtt mikes, beamex, vaisala and orion are discussed. the developed and tested digital calibration certificate (dcc) solution and its components are described. the major findings of the project include further need of dcc standardization and good practice subschemas. development of the dcc is ongoing worldwide and the big picture goes even beyond the dcc. it is not only a question of calibration certificates and related data transfer to digital and machine-readable format, but also how this data could be used effectively. mailto:sami.koskinen@beamex.com acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 2 a key enabler for dqi and adoption of advanced analysis methods like artificial intelligence (ai), machine learning (ml), and big data. 1.2. dcc as enabler for new innovations moving towards machine-readable calibration certificates aligns not only with the digitalization strategies of individual organizations but with more comprehensive initiatives such as the european digital strategy of the european commission (ec) [5]. providing a quality and traceability infrastructure for data-driven engineering in industry 4.0, the internet-of-things, autonomous systems, etc., also requires machine-actionable calibration information, i.e., access and interpretation by computer systems. the dcc information can be directly transferred into other digital processes to optimize process control and hence get the benefits of the calibration data online. thus, it can be utilized for digital workflows in measurement science, calibration, and industry. new digital metrology innovation can be found e.g., from sensor networks (iot) and in-situ selfor co-calibration techniques. although already widely used and rapidly increasing, sensor networks have not been addressed in a metrologically sound and systemic manner. empir project met4fof [6] developed an initial set of methods for the propagation of uncertainty through elementary pre-processing steps in ml for sensor network data. the project provided an agent-based approach to ml implementation and demonstrated the use of the d-si, dcc and semantic information for the automated metrological treatment of sensor networks. however, in realworld cases, and particularly in very large-scale sensor networks, practical methods, software frameworks, and guidelines for the automated uncertainty and data quality evaluation, and semantic descriptions are not available yet. the recently established funsnm project, financed by european partnership on metrology, will address on these issues. 1.3. background the international system of units (si) [7] provides a coherent foundation for the representation and exchange of measurement data. it also enables interoperability and reproducibility in all fields. the empir smartcom project [8] has defined digital si (d-si) [9], where a data format for reporting measurement results in terms of a numerical value, associated unit, and uncertainty statement is described. however, the correct interpretation of this information is only possible by understanding the content of the si system, and additionally the gum [10] about the uncertainty, the vim [11] about the metrological terms, and standards such as iso/iec 17025 [12] about the concept of metrological traceability and calibration. thus, these terms and concepts together with their meanings need to be represented in a machine-interpretable way, in order to make a dcc machine-readable in a wider sense, and thus enable the new services and possibilities based on digital calibration data. this publication presents results of a proof of concept (poc) study of fully digitalized environment for calibration data. this work originates from the empir smartcom project [8]. the doctoral thesis of mustapää [13] was partly done as a part of the project. however, the thesis conducted a broader investigation of metrology digitalization in industrial iot systems. the thesis also discussed methods for secure data exchange and provided means for establishing trust between organizations in a fully digital environment. furthermore, the master thesis of riska [14] provided insight into possibilities to establish a new dcc ecosystem and how it should be realized. it also investigated the benefits and disadvantages of either a closed or open dcc ecosystem. similar kinds of initiatives are currently studied globally. the most relevant to this work is the gemimeg ii [15] project. the project aims at safe and robustly calibrated metrological systems for the digital transformation enabling secure, consistent, legally compliant, and legally acceptable end-to-end availability of information for the implementation of reliable, networked measurement systems in germany [15]. the number of publications related to dcc has increased rapidly during the last years [16]. gadelrab and abouhogail [17] reviewed 17 research publications published in the period from 2018 to 2020 covering the topics “digital transformation in metrology” and “digital calibration certificates”. the publications report dcc requirements, conceptual studies, specific application implementations, security, etc., all relevant to this work as well. however, the authors are not aware of earlier publications that report poc results focusing on interoperability of dcc data over the whole value chain. 1.4. contributions of the present study many of the previous studies about the dcc and its applications have been rather focused on implementing the dcc in small scale. in the present study, we describe the results of a poc project that tested a fully digitalized environment for calibration data generation, transfer, and usage. the goal of this project was to test and identify barriers for large scale implementation of dcc in a whole calibration value chain. the members of the project consortium are organizations with a high number of calibration customers or calibration providers. implementing a dcc in this kind of environment brings new requirements. for example, receiving dccs through email from a small number of calibration providers might not be a problem, but receiving dccs from hundreds of different calibration providers needs to be highly organized and automated. one of the core benefits of the dcc is that it mitigates manual processing of data between organizations. to realize the full potential of the dcc, the sending and receiving process between the organizations should also be automated. in this project, a data transfer platform was developed and tested in order to automate dcc transfer between the calibration service provider and the calibration customer. another important aspect of the project was to test the dcc in an environment with several different information systems through the whole calibration chain as shown in fig. 1. implementing a dcc in a multi operator, each having their own system, environment requires a well standardized protocol in order to be interoperable between different systems. in this project, the interoperability of the data transfer protocol, i.e., the dcc, was tested by importing dccs generated by different systems into one calibration management system. the aim of this proof of concept (poc) was to implement the concept presented in [18] to demonstrate and investigate its feasibility. poc is an effective way to test dcc in practice and gain experience. in addition, special attention was paid to the digital authentication of the data, data integrity and fulfilment of metrological traceability. 2. methods in the project, two linked calibrations of a metrological traceability chain were chosen for the poc. the calibrated device acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 3 was a humidity and temperature probe type hmb75b, manufactured by vaisala oyj. the studied traceability chain covers the metrological traceability from the si to the end user working measurement standard. the chosen chain is not fully complete since it does not include the actual measurement results performed by the poc end user in their real process environment. the poc was executed with dcc version 2.4.0 [19]. the good practice template used in the project was collectively developed and agreed on by the partners in the poc. 2.1. project consortium in finland, a digital data ecosystem has been established with dcc in its focus. the group consists of research institutes, instrument manufacturers, calibration laboratories and end users, i.e., the whole value chain is represented. the common goal is efficient and extensive utilization of measurement and calibration data as well as increasing digitalization in this context. the pharmaceutical industry has been especially active in this field, and the current activities around the dcc are linked together through this sector. in the poc reported in the present study, the partners were aalto university, vtt mikes, beamex oy, vaisala oyj and orion oyj. aalto university was the project coordinator and developed the data transfer platform and cloud components for dcc management. vtt mikes is the national metrology institute (nmi) in finland and provided the expertise of calibration certificates and traceability to the project. beamex, an integrated calibration solution provider, was the calibration management system specialist and developer in the present project. vaisala manufactures innovative measurement solutions for weather, environment, and industrial processes. the instrument containing humidity and temperature sensors was a product of vaisala. they operated in the project as an accredited calibration service provider and a dcc receiving party. pharmaceutical company orion was the other dcc receiving party and the endpoint of the calibration chain. 2.2. implementation and test setup dcc management and exchange was tested by simulating a traceability chain which included three partners: vtt mikes, vaisala and orion. the poc concentrated on a humidity and temperature probe used by the calibration end user orion. the simulation concentrated on the temperature traceability chain. each partner in the traceability chain had a different calibration management system (cms). calibration certificate generation in vtt mikes is based on their proprietary in-house software together with microsoft office tools for data management and certificate templates. for their system environment, a manual dcc creation tool was estimated to be the easiest to implement in the project. in the future, the dcc creation can be implemented as a part of their current information management systems to increase automation in dcc generation, but for the poc this was decided to be out of scope. vaisala had a dual role in the poc as they both received a dcc from vtt mikes and sent a dcc to orion. vaisala calibration services use a proprietary in-house calibration management platform. on the dcc receiving end of vaisala, it was decided by the project consortium to test the manual receiving process. receiving of the dcc was based on a human readable format of the dcc as described in section 3.2. as the certificate creation process is already highly digitalized within vaisala’s calibration management platform, automated dcc generation was possible to test due to fully machine-readable data. automated dcc generation was tested as a part of the calibration management platform as explained in section 3.1. the dcc created by vaisala was received by the calibration end user orion. orion uses a commercial off-the-shelf calibration management system, beamex cmx. dcc import to the cms was tested as a part of the beamex cmx environment to enable automated import of calibration results from the dcc. the structure of the dcc import is presented in section 3.2. as each organization had their own cms, the data transfer needed to be arranged through the organization boarders. in the poc, the data transfer was tested through a data transfer platform that was integrated into the systems used by the partners in the traceability chain. the data transfer platform enabled system-tosystem integration of different cms systems eliminating the need for e.g., emails as a transfer method. 2.3. test procedure testing of the poc solution was carried out in three phases. in the first phase, generating and reading the dccs was tested with a peer-to-peer approach. the goal of the first testing phase was to demonstrate the capability of creating and reading a dcc with two separate approaches. first an example dcc was manually created using existing pdf certificates as a template. once the first version of the dcc was created utilizing notepad++, both the calibration service provider and the calibration customer reviewed and accepted the example. the example dcc compatibility with the dcc schema was also validated at this point. next, the dcc example was frozen, and a copy was distributed to both parties to develop an implementation for dcc creation or reading. after the development, a dcc was generated by the calibration service provider and sent to the receiving party of the calibration. at this point, the dccs were delivered manually through email and without digital signatures. next, the receiving party used the received dcc to import calibration results to their cms. in the calibration between vtt mikes and vaisala, the calibration results were not automatically imported to the cms. instead, the dcc was tested with the human-readable format as described in section 3.2. the next testing phase in the project was end-to-end testing. in the context of this project, it meant that both calibrations were executed following the required industry procedures and standards. at this time, the full data flow and all developed components were tested. the goal of the test phase was to establish a fully digital calibration process from generating a dcc in the calibration service provider’s cms to reviewing the calibration results at the calibration customer’s cms. the process included the following steps: 1. calibration service provider generates a dcc from calibration management system. 2. calibration service provider validates the dcc against the dcc xml schema. 3. calibration service provider adds a digital signature to the dcc. 4. calibration service provider uploads the dcc to the data transfer platform. 5. calibration service provider shares the dcc with the calibration customer. 6. calibration customer validates the digital signature in the dcc. 7. calibration customer imports the dcc to their calibration management system. acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 4 8. calibration customer reviews and accepts the calibration in their calibration management system. the final testing phase in the project was interoperability testing. the goal of the testing phase was to test the interoperability of all the dcc generation and reading methods with each other. the test was conducted by generating a dcc with both the manual web form and the automated cms dcc generation. these dccs were then read with both the human readable format and the automated dcc cms import. 2.4. data security for the calibration customer it is vital to ensure that the received calibration certificate is from the designated calibration service provider and it was not tampered by any third party. to ensure these objectives, the poc solution was designed with a focus on digital authentication of data and data integrity. digital authentication of data and data integrity were achieved with the combination of digital signature and user management. data integrity was ensured in the project with an xml advanced electronic signature (xades) [20] based signature solution. xades signatures are compliant with european eidas regulation [21] which defines requirements for legally binding digital signatures. digital signatures allow the calibration customer to validate that the received dcc was not modified in any way after it was accepted and authorized by the calibration service provider. in this project, digital signatures were tested as an organization level digital seals as described in section 3.1. in a production-ready implementation, the demo public key infrastructure (pki) should be replaced with another pki. requirements for such pki are described by e.g., pekka nikander et al [22]. some practical approaches for the pki are presented by e.g., tuukka mustapää [13]. in the poc solution, signing authorization was controlled through each organization’s existing user management. in this project, microsoft azure active directory was used as the user management solution as all participants used it. modern authentication solutions also have built-in audit trails that can record when and by whom a dcc was signed by. the project consortium considered data integrity as the most important aspect of data security in the calibration process. data confidentiality was also recognized as an important topic in the project. still many consortium members did not consider calibration data confidentiality critical to the business operations, since it’s difficult to misuse calibration data without the meta information involved in it. data confidentiality was ensured by using transport layer security (tls) encryption when uploading and downloading data from the data transfer platform. data was also stored encrypted at the data transfer platform. 3. results the tested solution created a fully digitalized environment for the calibration data generation, transfer, and usage. the different components and organizations in the traceability chain were integrated to each other through the data transfer platform. as presented in figure 1, the dcc generation tool in vaisala and the dcc receiving in orion’s cms were tested as a part of the organizations’ existing information technology environment. other components of the infrastructure were implemented as a part of the cloud-based data transfer platform. the calibration traceability chain was created through the dccs. each dcc includes all necessary information for traceability e.g., measurement data, deviation, uncertainty, and measurement references. 3.1. components for creation manual dcc creation: global adoption of the dcc format requires easy-to-use tools with low entry barrier. in the poc, a web form was developed, based on the good practice format of the dcc, for manual creation of dccs. the web form was developed using json gui technology and it was integrated to the data transfer platform. the web form enabled easy creation of dccs without any changes to the existing information systems. automated dcc creation: another approach to dcc creation was a fully automated dcc creation from the cms. the component was tested as a part of a laboratory calibration management platform. the component enabled creating a dcc automatically from the calibration management system database. fully automated dcc creation mitigates human errors in the dcc generation and removes unnecessary work. format validation: dcc offers a framework for the calibration data that needs to be specified to a use case in question. seamless interoperability requires that the use of the dcc schema and the good practice examples are commonly agreed and executed. to ensure this, dccs need to be verified to make sure that the data format follows the commonly agreed guidelines. format validation checks the data in the dcc against an xml schema, defined by the dcc schema and the use case specific good practice dcc, and highlights the possible differences. in the poc, the format validation was implemented as a part of the signature service. digital signature: calibration results need to be authorized by the calibration provider and secured during the transit. a cryptographical digital signature allows the calibration customer to trace the signature to the calibration provider and ensure that the data integrity of the calibration results is intact. in the poc, figure 1. data flow and components of the developed and investigated solution for digital calibration certificates (dcc). the color of the component indicates the organization that was responsible for the development. yellow and dark grey components were developed by aalto university, blue components by vaisala and green components by beamex. acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 5 xades based cryptographical digital seals were used. digital seals are digital signatures that identify a group of people instead of one individual person. in the poc, digital seals were used at organization level. each organization had their own private keys for signature creation traceable by the demo public key infrastructure (pki). 3.2. components for receiving signature validation: the calibration customer needs to ensure that the data included in the dcc is authenticated by the expected organization and has not been manipulated by third parties. this can be ensured by validating the digital signature in the dcc. in the poc, the signature validation service was implemented as a part of the data transfer platform. human readable format: the dccs need to be visualized to a human readable format to be used in manual processes. these can be e.g., manual transfer of data to a calibration management system or a manual review of calibration results. in the project, a human readable format was developed using xslt and javascript technologies. the human readable format was used to manually review and transfer calibration results to the calibration management system of the accredited calibration laboratory. the human readable format also included a user interface for validating the digital signature. dcc cms import: utilizing the calibration data in calibration management systems (cms) requires data extraction from the dcc xml document to the cms database. in the poc, a dcc data parser was developed to enable fully automated and error resistant data transfer process for the calibration customer. the dcc data parser was integrated to the customer’s cms (beamex cmx) as a separate driver that enabled import from local data storage or dcc data transfer platform. after the data import, the calibration result could be reviewed and accepted according to current calibration processes. finally, the calibration result data can be utilized in the cms as part of instrument analytics and asset management processes. 3.3. general components in the data transfer platform dcc sharing: the data transfer was implemented through a data transfer platform used by all the partners in the project. the data privacy of each organization was achieved though multitenancy. the data transit between organizations took place in the platform through transferring the dccs between tenants. the platform user management also allowed to add user and visibility rights to specific dccs for other organizations in the platform. dcc storage: the long-term storage of dccs was outside the scope of this poc but the platform approach enables several options. if both the calibration provider and the calibration customer use the platform for long-term dcc storing, there is no need for unnecessary data replication as the same document can serve both parties. either party can also take control of storing their own dccs. in case both parties have local storages for their dccs, the platform can be used only for data transfer and no data will be stored in the platform. user management: the user management in the tested solution was based on microsoft azure active directory (aad). in the poc, all the organizations used microsoft aad for user management. the data transfer platform was developed to support single sign-on (sso) through microsoft aad. this allowed all organizations to use their existing authentication methods to authorize users to sign, transfer and manage dccs in the data transfer platform. microsoft aad integration also allows each organization to have full control of the users that can access their organization’s data. as the platform’s user authentication is integrated as a part of the organization’s normal user authentication, the user rights to the platform can be added and removed automatically through normal employee onboarding and offboarding processes. 4. discussion the poc solution presented in the present study was designed for testing and demonstrating an example implementation of the dcc in an industrial application. the investigated solution and its components worked as expected in general and the test was considered successful by all consortium members. the poc solution was considered effective and user friendly for executing calibration data exchange. the cloud-based platform offered a low entry level for organizations with less software development capabilities. the development of the necessary components, excluding the custom components for generating and reading dcc from specific calibration management systems, as a part of the calibration data transfer platform lowered the effort to implement the dcc. all necessary integrations between the information systems were implemented through standardized application programming interfaces (apis). necessary integrations were user management integrations with the data transfer platform through aad and cms integrations with the data transfer platform. the presented solution gives guidelines for the development of a production-applicable solution. room for improvement was identified especially in the solution interfaces and in general robustness of the system. furthermore, a production-ready solutions will require further development of the solution infrastructure and data security, as was discussed in section 2.4. 4.1. challenges in the dcc implementation interoperability between different information systems requires a well-standardized and universal data format. the universal dcc format has been designed to provide a framework for representing calibration data. it enables various ways to insert measurement data and recursive data structures which challenge the receiving end machine-readability. need for interoperable machine-readability sets tight requirements not only to the dcc structure but also to the overall harmonization of the calibration certificates’ content, including common and fixed vocabularies, glossaries, and semantics content. as use case-specific needs and data formats can vary significantly, there needs to be a way to standardize the data in a more detail level. to achieve use case level standardization, several good practice examples are currently being developed. the goal of the good practice examples is to standardize the representation of data, e.g., with a specific measurement quantity. the poc was executed before widely adopted good practice examples were published, and there were several challenges to achieve an adequate level of interoperability. in the first tests, only the universal dcc schema was used to validate the created dccs. as a result, the cms data import failed to read both dccs created with the manual web form and the automated dcc creator. the reason was identified to be different data structures in the two dccs despite they both were compatible with the universal dcc schema. the finding highlights the importance of good practice examples and related good practice subschemas. the development of good practice subschemas should restrict acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 6 the use of the universal dcc schema to the level where use case specific requirements are fulfilled, and interoperability ensured. in addition to the need for more precise standardization, also other technical findings were identified. the current version of the digital si (d-si) does not support resolution for numerical values. for example, measurement result 1.00 will be saved as 1 in the xml which has implications e.g., to uncertainty calculations. furthermore, calibration date or calibration due date of the reference instrument used in the calibration do not have a dedicated field in the dcc. these findings were considered critical for the implementation of the dcc by the project consortium members. 4.2. remaining challenges and future work the development of good practice examples should be intensively coordinated between different working groups to avoid the risk of divergence. also, the total number of different good practice examples should be minimized, and same data structures applied to as many different use cases as possible, to ease the implementation of the dcc. every additional good practice format will increase the difficulty to implement the dcc especially for organizations that operate with several types of measurements. the authors see finding the balance between the number of good practices developed and ensuring standardized data structures in the dcc as the core challenge for the dcc. wide reformation, like digital transformation, in a quite traditional calibration field will not happen in a second. and unfortunately, all stakeholders do not have resources to be involved in the development work. this leads to a situation where stakeholders do not have easy access to the latest development work, knowledge, and capabilities needed for dcc implementation. dccs are currently developed mainly in a few large european metrology institutions and some forerunner industrial companies. support for the digital transformation and implementation of dccs is needed. especially end user needs in different industrial operations need to be considered. our future work will include executing a new dcc poc, exploring new possibilities to exploit dcc data, and validating business opportunities. the partners in the new poc are vtt mikes, beamex, lahti precision, orion, vaisala, and bayer. we will demonstrate mass and weighing instrument calibration cases. the poc will examine a complete metrological traceability chain from the si to the end user process. more information on the reported poc and the future work is available from the authors. references [1] the business research company, measuring and control instruments market covering other electrical equipment, electronic products and components; navigational, measuring, electro medical and control instruments; global summary 2021: covid 19 impact and recovery, 2021 [2] s. hackel, f. härtig, j. hornig, t. wiedenhöfer, the digital calibration certificate, ptb-mitteilungen, vol. 127, no. 4, 2017, pp. 75–81. doi: 10.7795/310.20170403 [3] j. lindström, p. kyösti, w. birk, e. lejon, an initial model for zero defect manufacturing, applied sciences, vol. 10, 2020, no. 13, 4570. doi: 10.3390/app10134570 [4] s. eichstädt, a. keidel, j. tesch, metrology for the digital age, measurement: sensors, vol. 18, 2021, 100232. doi: 10.1016/j.measen.2021.100232 [5] european comission, european commission digital strategy: a digitally transformed, user-focused and data-driven commission, 2018. online [accessed september 2020] https://ec.europa.eu/digital-singlemarket/en/content/european-digital-strategy [6] project met4fof: metrology for factory of the future. online [accessed january 2023] https://www.euramet.org/research-innovation/search-researchprojects/details/project/metrology-for-the-factory-of-the-future [7] bureau international des poids et mesures (bipm), si brochure, the international system of units, 2019 [8] project smartcom: communication and validation of smart data in iot-networks. online [accessed april 2022] https://www.ptb.de/empir2018/smartcom/project/ [9] d. hutzschenreuter, r. klobučar, p. nikander, t. elo, t. mustapää, p. kuosmanen, o. maennel, k. hovhannisyan, b. müller, l. heindorf, b. ačko, j. sýkora, f. härtig, w. heeren, t. wiedenhöfer, a. forbes, c. brown, i. smith, s. rhodes, i. linkeová, j. sýkora, v. paciello, digital system of units (d-si), zenodo, 2019. doi: 10.5281/zenodo.3522631 [10] jcgm 100:2008, evaluation of measurement data guide to the expression of uncertainty in measurement (gum), 2008 [11] jcgm 200:2012, international vocabulary of metrology – basic and general concepts and associated terms (vim), 2012 [12] iso/iec 17025:2017, general requirements for the competence of testing and calibration laboratories, 2017 [13] t. mustapää, digitalisation of metrology for improving trustworthiness and management of measurement data in industrial iot systems, doctoral thesis 139/2022, aalto university, 2022, isbn 978-952-64-0959-7 [14] k. riska, digital calibration certificate as part of an ecosystem, thesis of a master of engineering, novia uas, 2022. online [accessed 22 march 2023] https://urn.fi/urn:nbn:fi:amk-2022052211073 [15] gemimeg ii. online [accessed january 2023] https://www.digitaletechnologien.de/dt/navigation/en/programmeprojekte/akt uellestrategischeeinzelprojekte/gemimeg2/gemimeg2.html/ [16] c. r. h. barbosa, m. c. sousa, m. f. l. almeida, r. f. calili, smart manufacturing and digitalization of metrology: a systematic literature review and a research agenda, sensors, vol. 22, 2022, no. 16, 6114. doi: 10.3390/s22166114 [17] m. s. gadelrab, r. a. abouhogail, towards a new generation of digital calibration certificate: analysis and survey. measurement, vol. 181, 2021, 109611. doi: 10.1016/j.measurement.2021.109611 [18] j. nummiluikki, t. mustapää, k. hietala, r. viitala, benefits of network effects and interoperability for the digital calibration certificate management, 2021 ieee international workshop on metrology for industry 4.0 & iot, rome, italy, 2021, pp. 352–357. doi: 10.1109/metroind4.0iot51437.2021.9488562 [19] physikalisch-technische bundesanstalt, digital calibration certificate version 2.4.0. online [accessed july 2022] https://www.ptb.de/dcc/v2.4.0/ [20] xml advanced electronic signatures (xades). online [accessed 9 january 2023] https://www.w3.org/tr/xades/ [21] regulation (eu) no 910/2014 of the european parliament and of the council of 23 july 2014 on electronic identification and trust services for electronic transactions in the internal market and repealing directive 1999/93/ec. online [accessed 9 january 2023] https://eur-lex.europa.eu/legal-content /en/txt/?uri=uriserv:oj.l_.2014.257.01.0073.01.eng [22] p. nikander, t. elo, t. mustapää, p. kuosmanen, k. hovhannisyan, o. maennel, c. brown, j. dawkins, s. rhodes, i. smith, d. hutzschenreuter, h. weber, w. heeren, s. schönhals, t. wiedenhöfer, document specifying rules for the secure use of dcc covering legal aspects of metrology, zenodo, 2020. doi: 10.5281/zenodo.3664211 https://doi.org/10.7795/310.20170403 https://doi.org/10.3390/app10134570 https://doi.org/10.1016/j.measen.2021.100232 https://ec.europa.eu/digital-single-market/en/content/european-digital-strategy https://ec.europa.eu/digital-single-market/en/content/european-digital-strategy https://www.euramet.org/research-innovation/search-research-projects/details/project/metrology-for-the-factory-of-the-future https://www.euramet.org/research-innovation/search-research-projects/details/project/metrology-for-the-factory-of-the-future https://www.ptb.de/empir2018/smartcom/project/ https://doi.org/10.5281/zenodo.3522631 https://urn.fi/urn:nbn:fi:amk-2022052211073 https://www.digitale-technologien.de/dt/navigation/en/programmeprojekte/aktuellestrategischeeinzelprojekte/gemimeg2/gemimeg2.html/ https://www.digitale-technologien.de/dt/navigation/en/programmeprojekte/aktuellestrategischeeinzelprojekte/gemimeg2/gemimeg2.html/ https://www.digitale-technologien.de/dt/navigation/en/programmeprojekte/aktuellestrategischeeinzelprojekte/gemimeg2/gemimeg2.html/ https://doi.org/10.3390/s22166114 https://doi.org/10.1016/j.measurement.2021.109611 https://doi.org/10.1109/metroind4.0iot51437.2021.9488562 https://www.ptb.de/dcc/v2.4.0/ https://www.w3.org/tr/xades/ https://eur-lex.europa.eu/legal-content%20/en/txt/?uri=uriserv:oj.l_.2014.257.01.0073.01.eng https://eur-lex.europa.eu/legal-content%20/en/txt/?uri=uriserv:oj.l_.2014.257.01.0073.01.eng https://doi.org/10.5281/zenodo.3664211 characterization of laser doppler vibrometers using acousto-optic modulators acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 361 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 361 364 characterization of laser doppler vibrometers using acousto-optic modulators michael gaitan1, jon geist2, benjamin j. reschovsky3, ako chijioke4 1 nist, gaithersburg md, usa, michael.gaitan@nist.gov 2 nist, gaithersburg md, usa, jon.geist@nist.gov 3 nist, gaithersburg md, usa, benjamin.reschovsky@nist.gov 4 nist, gaithersburg md, usa, akobuije.chijioke@nist.gov abstract: we report on a new approach to characterize the performance of a laser doppler vibrometer (ldv). the method uses two acousto-optic modulators (aoms) to frequency shift the light from an ldv by a known quantity to create a synthetic velocity shift that is traceable to a frequency reference. results are presented for discrete velocity shifts and for sinusoidal velocity shifts that would be equivalent to what would be observed in an ideal accelerometer vibration calibration. the method also enables the user to sweep the synthetic vibration excitation frequency to characterize the bandwidth of an ldv together with its associated electronics. keywords: laser vibrometer; vibration calibration; acousto-optic modulator; 1. introduction following iso standard 16063-41 method for the calibration of vibration and shock transducers – part 41 calibration of laser vibrometers [1], laser doppler vibrometers (ldvs) are calibrated by a comparison-type measurement to a laser homodyne interferometer that is defined as the primary standard. not covered by iso 16063-41, but for the case where all the components of the ldv system and their associated uncertainties are known, methods can be employed for the direct determination of measurement uncertainty of the ldv [2], [3] or by using a combination of a heterodyne with a homodyne-quadrature configuration [4]. the technology for manufacturing commercial ldv systems has matured as well as their use in commercially available primary vibration calibration systems. these systems require calibration by the manufacturer over a periodic time interval that is typically one year and are traceable to the système international d'unités (si) through the manufacturer. from the end user perspective, the commercially manufactured ldv system is like a “black box”, meaning that the design and internal components of the ldv are not known in detail by the user. therefore, following an uncertainty determination approach described in [2] or [3] is not possible for such commercial black box systems especially if their internal workings are considered proprietary by the manufacturer. the only possibility provided by the standard for calibrating a such commercial ldv systems is therefore to follow iso 16063-41 and compare it to a primary heterodyne system, resulting in the ldv system being considered a secondary system. a challenge therefore remains in the adoption of cost-effective commercial ldv systems by national measurement institutes (nmis) who are responsible for direct determination of uncertainty. towards this end, we have recently reported on the calibration of laser heterodyne velocimeters using shock excitations and total distance travelled [5], [6]. one advantage of that method is that it characterizes the entire measurement system under the same conditions that would be used in an accelerometer shock calibration and could as well be included as part of the accelerometer shock calibration. however, a drawback of the method is that it does not characterize the frequency response and bandwidth of the ldv. the bandwidth of the excitation must not exceed the bandwidth of the ldv in order to produce accurate accelerometer shock calibrations. this drawback motivated us to develop a new method to characterize the bandwidth of the ldv system and resulted in the method that we present in this report. 2. experimental design figure 1 shows a diagram and photograph of the acousto-optic modulator (aom)-based ldv characterization system that we have developed. the laser light is first collimated using 300 mm and 30mm lenses to create a beam diameter that is compatible with the aperture of the aoms. the http://www.imeko.org/ mailto:michael.gaitan@nist.gov mailto:jon.geist@nist.gov mailto:benjamin.reschovsky@nist.gov mailto:akobuije.chijioke@nist.gov acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 362 beam passes through the 1st aom where it is downshifted in frequency f. next it passes through the 2nd aom where it is upshifted by a frequency f+. the beam is then reflected back along its path with a mirror, doubling the effect of the aoms, and delivering light to the ldv that is shifted in frequency by: 2𝛿 = 2 (𝑓 + 𝛿 − 𝑓). (1) the reason why we use two aoms to produce the frequency shift is that a single aom cannot generate frequency shifts of order 1 mhz or less as required. the velocity v reported by the ldv is related by the doppler equation with a frequency shift of 2𝛿 and the wavelength of the laser by the relationship: 𝑣 = ½ 𝜆 (2𝛿) = 𝜆 𝛿 , (2) where the laser wavelength 𝜆 = 632.81 nm [7]. 3. results the experimental results that we report were obtained using [8] a commercially available ldv system that includes a polytec ofv-503 sensor head, a ofv-5000 vibrometer controller, and a polytec data management system that were interfaced with the design shown in figure 1. the aoms (brimrose tef-110-50-633) were driven by two national instruments pxie-5650 sinusoidal radio frequency (rf) signal generators. an agilent 3458a digital multimeter was also used to measure root mean squared (rms) voltage for sinusoidal excitations. the signals from the rf signal generators were amplified using mini-circuits zhl-2010+ rf amplifiers connected to each of the aoms. the base frequency f for our results was selected to be 110 mhz, corresponding to the center frequency of the aoms. the zero frequency (dc) and transient velocity readings that we report were obtained using the polytec data management system. the rms readings that we report were obtained using the rms multimeter. 3.1. results for fixed frequency shift figure 2 shows the relationship between the frequency shift 𝛿, the reported velocity from the ldv, and the calculated velocity using the doppler equation (2). these results were obtained with vibrometer controller set to vd-09 with a corresponding amplification factor of 0.5 m/s/v. figure 1: diagram (upper) and photograph (lower) of the acousto-optic modulator (aom)-based ldv characterization system. http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 363 figure 2: plot of the velocity reported by the ldv as a function of frequency shift 𝛿 in blue and the corresponding calculated values using the doppler equation (2). the resulting dc voltage was sampled 2048 times at 204800 samples/s with no filtering using the polytec data management system. the data were averaged, and the standard deviation was determined. the ldv exhibited a 0.0012 m/s offset when directed onto a non-moving surface without the aoms in the beam path as well as when the frequency shift 𝛿 was set to zero. this offset was subtracted from the measured results depicted in the figure. the data in figure 2 is replotted in figure 3 in terms of the percent difference between the velocity reported by the ldv (with the offset subtracted) and the calculated velocity from the doppler shift equation (2). the data show a maximum percent difference of  0.04 % over the frequency range that was tested. figure 3: percent difference between the velocity reported by the ldv (with the offset subtracted) and the calculated velocity using the doppler shift equation (2). the manufacturer of the ldv reports a 1% uncertainty in their instrument specifications while the frequency shift that we can produce with the signal generators is orders of magnitude smaller in uncertainty. 3.2. results for sinusoidal excitation in this experiment the 2nd aom was excited using the national instruments pxie-5650 rf signal generator with a sinusoidal frequency modulation to create a synthesized vibration measurement. the goal of this experiment was to characterize the bandwidth of the ldv system. in this experiment, the root mean squared (rms) voltage from the analog output of the polytec ofv5000 vibrometer controller was measured using the rms multimeter and converted to rms velocity using the vd-09 gain factor of 0.5 m/s/v. the sinusoidal modulation at the 110 mhz base frequency was swept from 100 hz to 3 mhz. figure 4 shows that the ldv vibrometer controller has a uniform response up to 1 mhz and drops off beyond that frequency. figure 4: frequency response of the ldv using sinusoidal frequency modulation of the 2nd aom to create a synthesized vibration measurement condition. 3.3. results for velocity step function excitation in this experiment the 2nd aom was excited using an hp 83650b 10 mhz to 50 ghz rf swept signal generator to provide a capability to frequency modulate an arbitrary analog signal. an agilent 33250a arbitrary waveform generator signal generator was used to produce a 1 hz square wave alternating from 0 mv to 300 mv for frequency modulation to simulate a step function for synthesized velocity. the analog velocity signal from the vibrometer controller was digitized using the polytec data management system set at its maximum sampling rate of 204800 samples/s. the resulting response shown in figure 5 includes the effects of the ldv as well as the digital acquisition system, which would be expected to have a maximum bandwidth of 102400 hz. http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 364 4. summary our results show that this approach is immediately useful as a tool for characterizing dc, sinusoidal steady state, and transient response of an ldv as a system as a whole, together with its data acquisition and control electronics and amplifiers. the dc response we reported exhibited a maximum of  0.04 % difference between the measured value and the calculated value based on the doppler equation, which is in good agreement to what we had reported earlier using shock excitations and total distance travelled [5], and is well within the 1 % accuracy specified by the manufacturer. the ldv system bandwidth of 1 mhz determined by sinusoidal excitations is in good agreement to what the manufacturer specifies. lastly, the velocity step function experiment serves as an example that it is possible to create complex velocity profiles to test the response of the ldv together with its data acquisition, control electronics, and amplifiers. our future work is focused on further improvements on the system design and carrying out a full uncertainty analysis. one improvement that we envision in the design is measure the frequency shift 𝛿 with a photodiode rather than reading it from the signal generators driving the aoms. this could capture any offsets or fluctuations between the signal generators and the light entering the ldv, e.g. due to refractive index fluctuation. we anticipate that after further investigation this method can be used as a tool for primary calibration of laser doppler vibrometers. 5. references [1] iso standard 16063-41 method for the calibration of vibration and shock transducers – part 41 calibration of laser vibrometers, iso 1606341:2011(e). [2] g. siegmund, “sources of measurement error in laser doppler vibrometers and proposal for unified specifications”, proc. of spie vol. 7098, 70980y-1, 2008. [3] n. vlajic, a. chijioke, “traceable dynamic calibration of force transducers by primary means,” metrologia 53, s136–s148, 2016. [4] t. bruns, f. blume, a. täubner, “laser vibrometer calibration at high frequencies using conventional calibration equipment”, xix imeko world congress, lisbon, portugal, september 6-11, 2009. [5] m. gaitan, m. afridi, j. geist, “on the calibration of laser heterodyne velocimeters using shock excitations and total distance travelled”, imeko xxii world congress, belfast, 2018. [6] m. afridi, j. geist, m. gaitan, “primary calibration of the low frequency response of a laser heterodyne velocimeter used in a pendulumbased shock excitation system by si traceable distance measurement”, j res natl inst stan, in press. [7] iso standard 16063-11 methods for the calibration of vibration and shock transducers – part 11 primary vibration calibration by laser interferometry. [8] certain commercial equipment and instruments are identified in this article in order to describe the experimental procedure adequately. such identification is not intended to imply recommendation or endorsement by the national institute of standards and technology, nor is it intended to imply that the materials or equipment identified are necessarily the best available for the purpose. figure 5: response of the ldv system to a synthesized velocity step produced by frequency modulation of a 1 hz square wave on the 2nd aom. the time depicted on the x-axis has been magnified to observe the transient of the velocity step. http://www.imeko.org/ simulation study of the photovoltaic panel under different operation conditions acta imeko issn: 2221-870x december 2021, volume 10, number 4, 62 66 acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 62 simulation study of the photovoltaic panel under different operation conditions mohammed alktranee1, péter bencs1 1 department of fluid and heat engineering, faculty of mechanical engineering and informatics, university of miskolc, miskolc, hungary section: research paper keywords: photovoltaic; solar radiation; simulation; temperature citation: mohammed alktranee, péter bencs, simulation study of the photovoltaic panel under different operation conditions, acta imeko, vol. 10, no. 4, article 12, december 2021, identifier: imeko-acta-10 (2021)-04-12 section editor: francesco lamonaca, university of calabria, italy received march 28, 2021; in final form september 25, 2021; published december 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: mohammed alktranee, e-mail: mohammed84alktranee@gmail.com 1. introduction solar energy is the essential branch of renewable energy is used in different applications as an alternative source of conventional systems such as solar electrical generation, solar cooling systems, solar heating, etc. [1]. the photovoltaic (pv) panels are one solar energy technology used for electrical generation by pv cells made of semiconductor materials. the pv cells convert the sunlight (photons energy) into electrical energy, where about 15 % – 20 % of the sunlight converts to electricity, and the rest converts to heat which influences their performance [2]. different factors influence the pv panel performance; some are related to the balance of system (subsystems) failure, such as pv inverter [3]. others are regarding operation conditions, cell cracking problems due to wind or snow pressure, vibrations, and the dust that collects on the pv module, which causes a failure along its operating lifetime during a short lifetime [4]. the pv panels efficiency is significantly affected by the increase in the pv cells temperature that causes cell degradation and shorting of life expected of pv module with dropped output power [5]. several studies revealed the performance of the pv panel is affected by the operation temperature. increasing the pv cells temperature higher than 25 °c has negative effects on the pv cells yield, where decreases the open-circuit voltage and then the electrical output of the pv panel [6]-[8]. the pv cells performance and productivity mainly depend on the solar radiation values and ambient temperature. therefore, at an average temperature of 25 °c and solar radiation of 1000 w/m², the pv cells give a maximum yield generate as a reference power produced by the pv cells [9]. stander test condition (stc) is a concept defined as the performance of pv cells within a temperature of 25 °c and solar radiation of 1000 w/m². increasing the temperature of the pv panel temperature one degree higher than 25 °c leads to reducing the pv cells efficiency to 0.45 % [10]. therefore, various pv models have been developed to predict the pv modules power output under different operation conditions [11]. a thermal model was developed to simulate pv panels thermal and electrical performance under different operation conditions with and without cooling. the developed model was sequentially linked with electrical and radiation models to evaluate the pv abstract an increase in the temperature of the photovoltaic (pv) cells is a significant issue in most pv panels application. about 15–20% of solar radiation is converted to electricity by pv panels, and the rest converts to heat that affects their efficiency. this paper studies the effects of temperature distribution on the pv panel at different solar radiation values, temperatures under different operation conditions in january and july. a 3d model of the pv panel was simulated with ansys software, depending on the various values of temperatures and solar radiation values obtained using mathematic equations. the simulation results indicate that pv panel temperature lowered with solar radiation values lower in january, and the temperature was homogeneous on the pv panel surface. an increase in the solar radiation value and temperature in july led causes heating of the pv panel with observed a convergence of the maximum and average temperature of the panel. thus, the pv panel temperature increase is directly proportional to the solar radiation increase that causes lower performance. cooling the pv panel by passive or active cooling represents the optimum option to enhance their performance and avoid increasing the pv cells' temperature at temperature increase. mailto:mohammed84alktranee@gmail.com acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 63 panels performance. the simulation results show lower pv panel performance with absorbed solar radiation of 800 w/m² and ambient temperature 0 °c – 50 °c without cooling. meanwhile, use cooling has a slight increase of the pv panels at ambient temperature 25 °c and absorbed radiation from 200 w/m² to 1000 w/m² [11]. the ambient temperature and solar radiation are important parameters that influence the pv cells' conversion efficiency and power produced [12]. another 3d model was simulated by ansys software to evaluate temperature distribution on the pv panel at different climate conditions and thus analysing the pv panels thermal behaviour under operating conditions. the results indicate an increase in the pv panel temperature is associated with increased solar radiation intensity and ambient temperature [13]. another simulation was conducted of the pv panel at a constant temperature with various solar radiation values, vice versa to predict the pv model performance and compare it with the pv panel performance under stc. the simulation revealed a reduction of solar radiation values even with constant temperature effects on the voltage and current of the pv panel. on the other hand, the pv panel performance degraded at high temperatures even with different solar radiation values. thus, lowering the temperature of the pv panel contributes to an increase in output power [14]. the present work aims to calculate temperature distribution on the pv panel at different solar radiation values and ambient temperature then determine the optimum operation condition extent of the pv panel. the simulation depended on the layers properties of the pv panel, values of solar radiation, temperature, convective heat transfer coefficient of the model to evaluate temperature distribution and identify the appropriate extent of operating conditions of the pv panel. the second section will discuss the characteristics of the pv panel and the mathematic equation used to predict the solar radiation values and the important results obtained. the third section discussed the simulation results and compared it with other studies [15], [16] that simulated under similar conditions to investigate the effect of ambient temperature and solar radiation on the pv panel performance. 2. methodology the pv panel used type as-6p30 polycrystalline, consisting of glass covering, pv cells with two layers of ethylene vinyl acetate (eva), aluminium frame, and tedlar (pvf) layer, as shown in figure 1 below. table 1 below shows the material properties of the pv components inserted in the ansys software engineering data. the pv panel datasheet has adopted a reference for comparing the simulation results and the pv panel values under stc. 2.1. geometry simulation according to the manufacturer datasheet, the pv model’s geometry is built by solidworks software with 1640 × 992 × 35 mm³ model dimensions. the materials are defined by the ansys library data of all the pv panel layers, as shown in figure 1. the pv model has been imported to the ansys software to analyse the pv panels temperature distribution inserted in the software as variable values depending on the different values of beam solar radiation obtained using mathematical equations below. thus, according to stc, three solar radiation values have been used two of them according to january and july with an estimation of clear-sky radiation and the third solar radiation value under stc. the heat flux changes with time, which causes a change in the temperature that influences the pv panels performance. the temperature was fixed at 4 °c in january, 35 °c in july, and 25 °c in stc. the value of the convective heat transfer coefficient on the panel is 14.8, given in w/(m2 k) and calculated using equation (1) [18] ℎ = 5.7 + 3.8 𝑉𝑚 , (1) where 𝑉𝑚 is the wind speed value that adopted 2.4, given in m/s, according to miskolc city [19]. the mesh element size of the pv model mesh using ansys simulation software was 0.002 m, and the number of meshes elements was 278964 with high smoothing, as shown in figure 2 above, the time simulated was 43.1 min in all initial conditions, figure 3 show the simulation steps of the pv panel for of january, july and for stc values. 2.2. governing mathematical equations the mathematical equations were used to predict and estimate direct radiations transmittance (beam radiation) for miskolc city when a clear atmosphere on 7 january at solar time 11:30 am and 13 july with solar time 12:30 pm. to achieve that, it needs to find some parameters that help achieve that such angular figure 1. the layers of the pv panels. table 1. the layers properties of the pv panel [17]. material (layer)  kg/m3 k w/(m²·k) cp j/(kg·k) glass 3000 1.8 500 eva 960 0.35 2090 pv 2330 148 677 pvf 1200 0.2 1250 aluminium frame 2707 204 996 figure 2. depict the pv panel meshing. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 64 position of the sun (solar declination 𝛿) represents the angle between the line from the centre of the sun to the earth centre equator. the solar declination value is changeable because of the rotating the earth around the sun and the tilt of the earth on its axis of rotation found by using cooper's equation [20] 𝛿 = 23.45° sin (360 284 + 𝑛 365 ) , (2) where 𝑛 is the day of the year of that selected date (𝑛 = 7 for 7 january and 𝑛 = 194 for 13 july). the zenith angle is the incidence beam solar radiation angle between the vertical and the line to the sun use with horizontal surfaces and determined by the following equation cos 𝜃𝑧 = cos 𝜙 cos 𝛿 cos 𝜔 + sin 𝜙 sin 𝛿 , (3) with latitude 𝜙 between −90° ≤ 𝜙 ≤ 90° for miskolc, hungary 𝜙 is 48.1°, 𝜔 is hour angle which is negative at morning and afternoon positive. solar time depends on the sun's angular motion in the sky, which may not synchronize with local time [21]. the extra-terrestrial radiation incident 𝐺on is the quantity of solar energy received per unit of time at the mean distance between the sun to the earth. on the normal plane, the 𝐺oncan calculate by the equation (4) 𝐺o = 𝐺sc (1 + 0.033 cos 360 𝑛 365 ) , (4) and 𝐺sc is the solar constant. calculating the daily and hourly solar radiation received on a horizontal surface is useful under standard conditions. to calculate the beam radiation transmitted from the sun without scattering by clear atmospheres, consider the zenith angle and altitude of the atmosphere for four climate types from equation (5) [21] 𝜏𝑏 = 𝑎0 + 𝑎1e − 𝑘 cos 𝜃𝑧 (5) 𝑎0 ∗ = 0.4237 − 0.00821 (6 − 𝐴)2, (6) 𝑎1 ∗ = 0.5055 + 0.00595 (6.5 − 𝐴)2 , (7) 𝑘∗ = 0.2711 + 0.01858 (2.5 − 𝐴)2, (8) where a0, a1 are constant, and k represents the standard atmosphere at 23 km visibility that can be found by using the equation (6), (7), and (8) a represents the altitude of the observer in kilometres. hence, multiply the constant values by correction factors in table 2, 𝑎0 ∗  𝑟𝑜 , 𝑎1 𝑎1 ∗ and k  𝑟𝑘 could calculate the beam radiation transmitted. thus, it can determine the beam radiation for any zenith angle or altitude, even 2.5 km. the normal beam radiation in the clear sky can be found by multiplying the value of the beam radiation transmitted during clear atmospheres 𝜏𝑏 by the value of extraterrestrial radiation incident 𝐺on. the value results multiply by the zenith angle to obtain solar radiations value on the horizontal of panel 𝐺cb [22]. 3. results and discussion the pv panel has been simulated under different solar radiation values estimated by mathematical equations and various temperatures. for january, solar radiation values were 176 w/m2, for july was 735 w/m2 and under stc was 1000 w/m2. january's highest temperature was 4 °c, july highest temperature reaches 35 °c, and the pv panel temperature was under stc 25 °c. the results indicate that temperature distribution on the pv panels surface was high at the top of the pv panel while the aluminium frame's temperature was lower. the dark-blue colour refers to the minimum temperature on the panel, the bright-red colour refers to the maximum temperature on the panel, and other colours represent temperature variations. in january, when the temperature is low, the beam solar radiation is 176 w/m2. the temperature distribution on the pv panel is between a maximum temperature of 15.4 °c and a minimum of 11.9 °c, as shown in figure 4. thus, this solar radiation range influences pv panel performance represented by power output, without leaving any damage or overheating because of lower the pv panel temperature. at applied the july solar radiation 735 w/m2 when the temperature reaches 35 °c, the simulation shows that the pv panel is exposed to a maximum temperature of 84.6 °c, while the minimum temperature was 68.4 °c as shown in figure 5 above. increase the pv panel temperature by 10°c above the stc value causes a decrease in its efficiency. this extent of temperature causes other problems for the pv panel, such as overheating, which leads to burning some cells or reduced voltage, power, and the pv panel's output current; thus, the pv panel becomes inoperative [23]. figure 3. simulation steps of the pv panel. table 2. correction factors for climate types [20]. climate type 𝒓𝒐 𝒓𝟏 𝒓𝒌 tropical 0.95 0.85 1.02 mid latitude winter 1.03 1.01 1 subarctic summer 0.99 0.99 1.01 mid latitude summer 0.97 0.99 1.02 acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 65 under stc conditions of the pv panel 1000 w/m2 and 25 °c, observed rising the pv panel temperature until 92.5 °c, while the lowest temperature was 67.7 °c, as shown in figure 6 above. therefore, this temperature range is not suitable for pv panel operation due to the rising of the pv cells temperature and causes the same problems of july values as overheating pv cells or reduced conversion efficiency and production of the pv panel. the temperature increase of the pv panel is directly proportional to increasing solar radiation. due to the pv layers different properties, these layers temperature is different, as shown in the figures above; the pv panel temperatures surface is different from the aluminium frame. the maximum and average temperatures on the pv panel are converged somewhat at increasing the solar radiation, as shown in table 3 above, for july and stc values. the cooling of the pv panel to remove the excessive heat is important to continue its work under high temperatures, even with stc values. 4. conclusion the pv panel was simulated by ansys software according to the properties of the material of the pv layers, such as thermal conductivity, densities, and specific heat for each layer. the climatic conditions of miskolc city have depended on solar radiation, temperatures, and wind speed as a parameter in simulation. the solar radiation values have been estimated depending on the mathematical equations for january and july with an stc standard value. the pv panel showed a variable thermal behaviour with a change in solar radiations value, which became evident by the pv panel's distribution temperatures. thus, low solar radiation value and temperature influence the pv panels performance to the extent that it does not cause damage to the pv panel but reduces production. on the other hand, increasing solar radiation causes an increase in the pv panels temperature that causes reduce in power output with damage to the pv panel. a convergence of the maximum and average temperature was observed with increasing the solar radiation that causes the rising of pv cells temperature that requires cooling of the pv panel to remove the excessive heat. references [1] v. v. tyagi, n. a. a. rahim, n. a. rahim, j. selvaraj, progress in solar pv technology: research and achievement, renew sustain energy rev, 20 (2013), pp. 443-61. doi: 10.1016/j.rser.2012.09.028 [2] m. c. browne, b. norton, s. j. mccormack, heat retention of photovoltaic/thermal collector with pcm, sol energy, 133 (2016), pp. 533–48. doi: 10.1016/j. solener.2016.04.024 [3] loredana cristaldi, mohamed khalil, payam soulatiantork, a root cause analysis and a risk evaluation of pv balance of systems failures, acta imeko 6 (2007) 4, pp.113-120. doi: 10.21014/acta_imeko.v6i4.425 [4] loredana cristaldi, mohamed khalil, marco faifer, markov process reliability model for 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[9] skoplaki e, palyvos ja, on the temperature dependence of photovoltaic module electrical performance: a review of table 3. temperatures on the pv panel at different solar radiation. solar radiation w/m2 max temperature °c min temperature °c average temperature °c 176 15.8 11.4 15.2 735 84.6 66.3 82.1 1000 92.5 67.7 89.1 figure 4. at solar radiation 176 w/m2 and temperature 4 °c. figure 5. at solar radiation 734 w/m2 and temperature 35 °c. figure 6. at solar radiation 1000 w/m2 and temperature 25 °c. https://doi.org/10.1016/j.rser.2012.09.028 https://doi.org/10.1016/j.%20solener.2016.04.024 http://dx.doi.org/10.21014/acta_imeko.v6i4.425 http://dx.doi.org/10.21014/acta_imeko.v6i4.428 https://doi.org/10.1016/j.rser.2016.11.114 acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 66 efficiency/power correlations, solar energy 83(2009), pp. 614– 624. [10] solar facts: photovoltaic efficiency inherent and system. online [accessed 8 december 2021] https://www.solar-facts.com/panels/panel-efficiency.php [11] g. m. tina, s. scrofani, electrical and thermal model for pv module temperature evaluation, ieee, 2008. doi: 10.1109/melcon.2008.4618498 [12] cătălin george popovici, sebastian valeriu hudișteanu, theodor dorin mateescu, nelu-cristian cherecheș. efficiency improvement of photovoltaic panels by using air cooled heat sinks, energy procedia. 85 (2016) pp. 425 – 432. doi: 10.1016/j.egypro.2015.12.223 [13] leow w z, irwan y m, safwati i, irwanto m, amelia a r, syafiqah z, fahmi m i and rosle n, simulation study on photovoltaic panel temperature under different solar radiation using a computational fluid dynamic method. journal of physics, conference series, iop 1432 (2020). doi: 10.1088/1742-6596/1432/1/012052 [14] mohammed alktranee and péter bencs, test the mathematical of the photovoltaic model under different conditions by use matlab– simulink, journal of mechanical engineering research and developments vol 43 (2020), pp. 514-521. [15] marek jaszczur, qusay hassan, janusz teneta, ewelina majewska1, marcin zych, an analysis of temperature distribution in solar photovoltaic module under various environmental conditions, matec web of conferences 240, 04004 (2018). doi: 10.1051/matecconf/201824004004 [16] n. pandiarajan and ranganath muthu, mathematical modeling of photovoltaic module with simulink, international conference on electrical energy systems (icees 2011) (2011), pp. 3-5. doi: 10.1109/icees.2011.5725339 [17] s. armstrong, w. g. hurley, a thermal photovoltaic panels under varying atmospheric conditions, applied thermal engineering 30(2010), pp. 1488-1495. doi: 10.1016/j.applthermaleng.2010.03.012 [18] j. vlachopoulos, d. strutt, heat transfer, in spe plastics technicians toolbox 2 (2002), pp. 21-33. 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[22] hottel, h. c, a simple model for estimating the transmittance of direct solar radiation through clear atmospheres, solar energy 2 (1976), pp. 129-134. doi: 10.1016/0038-092x(76)90045-1 [23] bodnár, i., iski, p., koós, d., skribanek, á, examination of electricity production loss of a solar panel in case of different types and concentration of dust, advances and trends in engineering sciences and technologies iii (2019), pp. 313-318. doi: 10.1201/9780429021596-49 https://www.solar-facts.com/panels/panel-efficiency.php https://doi.org/10.1109/melcon.2008.4618498 https://doi.org/10.1016/j.egypro.2015.12.223 https://doi.org/10.1088/1742-6596/1432/1/012052 https://doi.org/10.1051/matecconf/201824004004 https://doi.org/10.1109/icees.2011.5725339 https://doi.org/10.1016/j.applthermaleng.2010.03.012 https://weather-and-climate.com/average-monthly-rainfall-temperature-sunshine,miskolc,hungary https://weather-and-climate.com/average-monthly-rainfall-temperature-sunshine,miskolc,hungary https://doi.org/10.1016/0038-092x(69)90047-4 https://doi.org/10.1016/0038-092x(76)90045-1 https://doi.org/10.1201/9780429021596-49 introductory notes for the acta imeko special issue on the 40th measurement day jointly organised by the italian associations gmee and gmtt acta imeko issn: 2221-870x december 2021, volume 10, number 4, 8 9 acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 8 introductory notes for the acta imeko special issue on the 40th measurement day jointly organised by the italian associations gmee and gmtt carlo carobbi1, nicola giaquinto2, gian marco revel3 1 department of information engineering, università degli studi di firenze, via s. marta 3, 50139 firenze, italy 2 department of electrical and information engineering, politecnico di bari, via e. orabona 4, 70125 bari, italy 3 department of industrial engineering and mathematical sciences, università politecnica delle marche, via brecce bianche 12, 60131 ancona, italy section: editorial citation: carlo carobbi, nicola giaquinto, gian marco revel, introductory notes for the acta imeko special issue on the 40th measurement day jointly organised by the italian associations gmee and gmtt, acta imeko, vol. 10, no. 4, article 5, december 2021, identifier: imeko-acta-10 (2021)-04-05 received december 13, 2021; in final form december 13, 2021; published december 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: carlo carobbi, e-mail: carlo.carobbi@unifi.it dear readers, the measurement day is an annual italian event, founded in 1982 by the late prof. mariano cunietti. with inspired intuition, cunietti conceived the event as a place where engineers, scientists, logicians, epistemologists, could talk to each other, investigating the very foundations of the act of measuring. at those times the event lasted two days and was a beloved appointment for many italian people passionate about measurements, including scholars, professionals, and simple amateurs, with any cultural background. in its long history, the format of the measurement day has evolved, adapting to the times, but always remaining a muchappreciated annual appointment for people interested in measurement. nowadays, it actually involves public discussion of invited presentations for half a day, and is mainly centred on advancement and updates in measurement technology and standards, including activities of international metrological bodies, contributions from calibration laboratories, discussions on historical perspectives and modern trends in the measurement world, etc. we can affirm that the measurement day has kept intact its vocation of bringing together people from different backgrounds, i.e. academia, industry, laboratories, accreditation bodies, etc., and is still a quite appreciated event by “measurers”. even if it is an event of national dimension, the measurement day involves high-level presentations of international level by recognized experts. therefore, we welcomed with pleasure, on its fortieth anniversary, the invitation of prof. lamonaca to devote a special section of acta imeko to the event. the 40th edition of the measurement day, organized in 2021 by the italian associations of electrical and electronic measurements (gmee) and of mechanical and thermal measurements (gmmt), was titled “comfort measurements between research, foundations, and industrial applications”. invited contributions have been from both young and expert researchers, in fields ranging from standardized measurements to psychology. below, we explain the rationale of the chosen theme for the 40th measurement day, and a brief account of the papers included in the special section. we chose the theme of the event starting from the consideration that the environment affects the well-being and the health and emotional state of people, and it is therefore of great importance – and even more in times of pandemics to guarantee the quality of the indoor life experience. for this purpose, it is essential to measure comfort accurately. but what exactly does it mean to measure comfort accurately? the question has no univocal answer, because we are not dealing with a physical quantity that can be defined regardless of the subject. a key recent trend is to integrate the measurement of actual physiological data, for example with wearable sensory devices, with environmental measurements. the standard predictive models need to be updated, as well as the data analysis methodologies. artificial intelligence has been successfully used in this area. comfort measurements do not only involve an update of hardware and software technologies: they require attention to the psychological aspects. how is the problem of measuring comfort (and in general, the problem of measurement) viewed in psychology and the humanities? how does the problem fit from a psychological perspective? the bipm has very recently circulated the “committee draft” of the new international vocabulary of metrology, the vim4. one of the purposes of vim4 is precisely to bring order mailto:carlo.carobbi@unifi.it acta imeko issn: 2221-870x december 2021, volume 10, number 4, 9 10 acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 9 to the measurement of “things” fundamentally different from the familiar physical quantities encoded in the international system, for example by introducing the definitions of measurement scale, ordinal scale, nominal scale. on what scale do the comfort measurements fit? how are they different from measurements of physical quantities, and what do these differences entail? the study of comfort measurement thus offers the opportunity to present some of the main innovations of the new vocabulary. as regards the papers of this acta imeko special section, there are five manuscripts that expand the scientific concepts and applications presented during the event, held online on the 30th of march 2021. in the manuscript ‘is our understanding of measurement evolving?’, authored by mari, an analysis is proposed, from an evolutionary perspective, trying to answer to some fundamental questions, such as: what kind of knowledge do we obtain from a measurement? what is the source of the acknowledged special efficacy of measurement? the measurement process is traditionally understood as a quantitative empirical process, but in the last decades measurement has been reconsidered in its aims, scope, and structure. comfort measurements are an application showing the importance and relevance of a reexamination of measurement fundamentals, as suggested in this paper. the paper ‘application of wearable eeg sensors for indoor thermal comfort measurements’, by mansi et. al., presents a measurement protocol and signal processing approach to use wearable eeg (electroencephalography) sensors for human thermal comfort assessment. results, reported from the experimental campaign, confirm that thermal sensation can be detected by measuring the brain activity with low-cost and wearable eeg devices. the paper entitled ‘impact of the measurement uncertainty on the monitoring of thermal comfort through ai predictive algorithms’, authored by morresi et al., proposes an approach to assess uncertainty in the measurement of human thermal comfort by using an innovative method that exploits a heterogeneous set of data, made by physiological and environmental quantities, and artificial intelligence (ai) algorithms. uncertainty estimation has been developed by applying the monte carlo method (mcm), given the complexity of the measurement method. the paper entitled ‘an iot measurement solution for continuous indoor environmental quality monitoring for buildings renovation’, by serroni et al., proposes an innovative iot sensing solution, the “comfort eye”, specifically applied for continuous and real-time indoor environmental quality (ieq) measurement in occupied buildings during the renovation process. ieq monitoring allows investigating the building’s performance to improve energy efficiency and occupant’s well-being at the same time. finally, considering the correlation between stress and discomfort, in the article ‘continuous measurement of stress levels in naturalistic settings using heart rate variability: an experience-sampling study driving a machine learning approach’ cipresso and colleagues repetitively measured physiological signals in 15 subjects in order to find a model of stress level in daily scenarios. using the experience sampling method, cipresso and colleagues were able to collect stressful moments reported by the 15 subjects through questionnaires. using a machine learning approach, the authors built a model to predict stressful situation based on physiological indexes that rely on cardiovascular measurements, that means only based on electrocardiogram or similar measures such as blood volume pulse extracted with a photopletismograph. carlo carobbi, nicola giaquinto and gian marco revel guest editors standards and affordances of 21st-century digital learning: using the experience application programming interface and the augmented reality learning experience model to track engagement in extended reality acta imeko issn: 2221-870x september 2022, volume 11, number 3, 1 6 acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 1 standards and affordances of 21st-century digital learning: using the experience application programming interface and the augmented reality learning experience model to track engagement in extended reality jennifer wolf rogers1, karen alexander2 1 people accelerator, the woodlands, texas, usa 2 xrconnected, pittsburgh, pennsylvania, usa section: technical note keywords: xapi; arlem; augmented reality; virtual reality; data transformation; capability development; training; learning; education citation: jennifer wolf rogers, karen alexander, standards and affordances of 21st-century digital learning: using the experience application programming interface and the augmented reality learning experience model to track engagement in extended reality, acta imeko, vol. 11, no. 3, article 7, september 2022, identifier: imeko-acta-11 (2022)-03-07 section editor: zafar taqvi, usa received march 1, 2022; in final form september 16, 2022; published september 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: karen alexander, phd, karen@xrconnected.com 1. introduction as the delivery of learning has been increasingly digitized over the past few decades, standards such as scorm, the sharable content object reference model, have helped to ensure interoperability across learning management systems (lms). but the 21st century has brought new, powerful tools for learning that enable educational and training experiences far richer than those available via a desktop or laptop computer. virtual, augmented, and mixed reality (vr, ar, mr) bring affordances to learning that far surpass what was previously available. immersive, embodied learning in 3d environments, with interactive 3d objects and collaborative engagements with teachers or other learners, will revolutionize education and training as we know it. for these reasons, new standards that can aid in the capture of data about a learner’s experience have arisen, notably the augmented reality learning experience model (arlem) and the experience application programming interface (xapi). with xapi and arlem, specific learner behaviour can be directly tracked and measured as it is shaped and/or changes in a specific interaction, thus permitting predictions of transfer from knowledge to demonstrable skill. adoption of these standards is key to avoiding silos of information and data around associated learner development and behaviour change encoded in different systems and formats that make communication across them difficult. in section 2 we will discuss how new technologies for learning demand new means of assessment. section 3 introduces xapi, its structure, and design principles for interoperable data structures. in section 4, we describe the augmented reality learning experience model, or arlem. a case study illustrates xapi in virtual reality in section 5. in section 6, the challenges and opportunities in using ar and vr and xapi in early abstract the development of new extended reality (xr) technologies for learning enables the capture of a richer set of data than has previously been possible. to derive a benefit from this wealth of data requires new structures appropriate to the new learning activities these immersive learning tools afford. the experience application programming interface (xapi) and the augmented reality learning experience model (arlem) standards have been developed in response, and their adoption will help ensure interoperability as xr learning is more widely deployed. this paper briefly describes what is different about xr for learning and provides an account of xapi and its structures as well as design principles for its use. the relationship between environmental context and arlem is explained, and a case study of a vr experience using xapi is examined. the paper ends with an account of some of the promises for collecting data from early childhood learning experiences and an unsuccessful attempt at a study using augmented reality with young children. mailto:karen@xrconnected.com acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 2 childhood education are outlined. in conclusion, we point toward possibilities for future developments in the fields of extended reality (xr) for training and interoperable standards for data collection. 2. new ways of learning, new ways of measuring among the affordances of virtual reality that make it such an effective tool for learning is the way that it engages the senses of proprioception and presence while permitting movement and gesture as the learner interacts with the virtual world and content within it. research has shown that large gestures and bodily movements enhance the acquisition and retention of knowledge, and vr is able to deliver experiences that allow such movement, unlike the small, hands-only interactions we have when using a computer keyboard interface for digital learning [1]. ar and mr deliver digital content within a real-world environment, and such content appears to the learner as present in space alongside them. the learner can modify the size of the virtual objects with which they are interacting or rotate them for a different view, walk around them, and experience the content in a fully embodied, present manner. xr technologies engage the body as it is inhabited in space and tap into embodied cognition. as researcher mina johnsonglenberg notes, “human cognition is deeply rooted in the body’s interactions with the world and our systems of perception.” but johnson glenberg also acknowledges that “for several decades, the primary interface in educational technology has been the mouse and keyboard; however, these are not highly embodied interface tools” [1]. xr permits the collection of data from bodily movements and interactions. one of the benefits of this can be seen in recent research that shows that the combination of physical movement and a gaming element in learning experiences for children not only enhances their ability to learn, but in fact also enhances cognitive function [2], [3]. with such new tools that can engage the body, “we need better learning analytics for real-time and real-world interaction” [4]. 3. the experience api 3.1. experience (xapi) data the experience (xapi) data is a modern data standard that is uniquely positioned to measure and track human behaviour in learning scenarios in a manner that more closely approximates their behaviour in everyday situations and experiences. the xapi differs from traditional, compliance-based, linear forced-choice methods of measuring learning (such as scoring responses to a finite series of multiple-choice questions and distractors). instead, it takes a more dynamic and analytical approach, acknowledging the often less-than-predictable nature of human response, and allowing learners to immerse themselves in a variety of contexts and respond organically (and, in some cases, automatically, without an intentional initiation of cognitive processing) to a variety of stressors and/or motivators in a manner that more closely approximates their true everyday behaviour in the real world. additionally, xapi provides a standard javascript object notation (json) syntax that facilitates the collection and subsequent analysis of these learner behaviours across learning scenarios and experiences, providing assurances around degree of predictability of a learner response over time, as well as the opportunity to correlate behaviour in practice/simulated environments with behaviour from the same individual/group of individuals, and potential real-world outcomes arising from that behaviour, in the physical world. 3.2. key considerations for structuring data collection using xapi though xapi is a broad standard and syntax aimed at increasing the efficacy of all modalities associated with learning experience, immersive environments are, in particular, wellsuited to this type of measurement. in xr experiences, there is a unique opportunity for learners to embody specific roles and interact with specific environments and objects, as well as demonstrate behaviours that are highly analogous to the physical world. when structuring xapi data collection in these immersive scenarios, it is important to consider the ways in which people, objects, and actions/behaviours might interact with one another, the outcomes and/or consequences that may arise from these interactions, and how these interactions and interdependencies might be captured. ideally, data collection will be structured using the base syntax to answer the following types of questions: ● what is the environmental context? ● who/what is present with the learner in this environment? ● how does the environment (and/or people or objects within it) trigger a learner behaviour? ● how may learner behaviour be described in such a way that it may be generalized across other immersive environments? is this behaviour tracked in the physical world in the form of metrics and/or key performance indicators (kpis)? how is it described/reported? ● does the environmental context change as a result of the learner’s behaviour(s)? is there a natural resolution to a specific problem/tension? if so, how is this described? 3.3. key design principles for human metrics in xr beyond the decisions driving specific ontologies and/or taxonomies to describe json statement syntax within specific interactions/scenarios, there are also key design principles to consider when designing an overall interoperable data structure meant to support and analyse this syntax: 1. human metrics design should be scalable. when constructing data schemas and frameworks to support xapi, it is imperative that human-centred design is table 1. overview of styles and font sizes used in this template. json syntax field field description adult learner example child learner example subject/ actor identifiers and/or descriptors for individual learners/groups jane jill verb action that learner takes in the scenario released donned object object and/or person that learner interacts with the pressure valve a coat context optional extension information to describe the context of the behaviour (e.g. location coordinates, physical barriers, emotional factors, etc.) …when operating parameters [began to trend out of limits] …when snow fell from the sky results optional extension information to describe results and/or resolution of the initial context, based upon learner action ….which resulted in pressure stabilization ….which resulted in health stats increasing acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 3 employed to ensure that the resulting metrics include verbs, results, and contexts that may be shared across a wide variety of xr scenarios/experiences for the same user(s) over time. in this case, the human/individual is the constant as they “travel” through a variety of xr simulations/scenarios and their behaviour in these different contexts must be described in a manner that lends itself to trend analysis and/or situational anomalies. additionally, as humans are not truly “compartmentalized” within a particular function/domain of their life, frameworks should be constructed in such a way that provides for longevity around the description of behaviour across broad ecosystems an individual may transverse, in both the physical and virtual world (e.g. familial networks, social networks, academic networks, professional networks, etc.). 2. actions/verb selection should minimize bias and/or interpretation, wherever possible. as the range of human behaviour is vast, and “appropriate” responses may vary across simulations/scenarios, every effort should be taken to avoid introduction of verbs into an ontology/taxonomy and/or individual interaction that imply some sense of judgement and/or right and wrong responses. instead, the verb portion of the syntax itself should merely describe the user behaviour in the statement itself, utilizing the results and context extensions where appropriate to contextualize the nature of appropriateness of the response in light of the conditions at a particular point in time in an interaction. 3. human metrics data should be interoperable and machine-readable. deviation from json format and xapi-specific context should be avoided, as both are open source standards that ensure that data recorded during immersive experiences/scenarios may be created, managed, and analysed broadly across existing and future platforms and technology, particularly those that pertain to measurement of human development, capabilities, and performance (e.g. student/workforce management systems, capability management and workforce planning systems, learning management systems, assessment management systems, performance management systems, etc.). additionally, metrics should be constructed in such a manner that lends them to be analysed by machine learning principles such as natural language processing and, in some cases, subsequently actioned further by artificial intelligence in subsequent interactions, as described below. 4. individual actors should be differentiated. appropriate and differentiated identifiers should be utilized in actor fields that, though anonymized where necessary, may be correlated back to a specific learner. this standardized practice ensures that interaction data is cumulative across a variety of immersive scenarios/simulations and that resulting data may be analysed for specific behavioural trends in a variety of contexts. additionally, this convention provides an affordance whereby user experience and/or nuances of a specific scenario’s context in future trials may be personalized to target specific growth/performance targets of the specific individual. 5. overall data sets, and the ontologies and/or taxonomies inherent within them, should be human-readable. to enable the proper analysis of human behaviour within specific contexts and the subsequent growth and performance of people over time, particularly as it pertains to informed practice for educational and human resources professionals responsible for guiding academic and career growth, it is imperative that virtual interaction descriptors are chosen that most closely approximate observable human behaviours in the physical world. 4. augmented reality learning experience model one recent iteration of the xapi standard as it pertains to xr specifically lies in the augmented reality learning experience model, otherwise known as arlem [5]. in this formulation, as described by secretan, wild, & guest, performance = action + competence. “an action is the smallest possible step that can be taken by a user in a learning activity,” they say, and “competence becomes visible when learners perform” [4]. accordingly, arlem aims to solve many of the challenges faced by colleagues in industrial and/or operational environments, whereby a worker's ability to take the right action at the right time is paramount. traditional measurement protocols were previously oriented toward providing flat, twodimensional “performance support” to these colleagues, in the form of viewing a schematic or standard operating procedure, watching a video of a colleague performing a process step, etc. and were centred around the passive “completion” of learning, which inferred only that an individual had accessed content and, in some cases, progressed through to the end. in some cases, additional measurement in the form of more formal assessment required learners to answer knowledge-based questions around the operational process. these assessments, often based on rote memorization, are incapable of measuring what actions a worker will take when faced with a particular situation. with the arlem and xapi standards, actions performed may now be prioritized over memorization of facts. 4.1. environmental triggers a hallmark of the arlem standard itself is its unique application of environmental “triggers” prompting human behaviour or experience; in this case, in the workplace specifically. by monitoring the environmental context and its operational parameters, arlem provides an association between situational context and appropriate supports provided to the end user to assist them in taking the appropriate action. 4.2. operational excellence key performance indicators utilizing the differentiators between environmental triggers and human activity, and further applying the results extension in the xapi syntax allows for a unique opportunity to assess the effectiveness of augmented reality in the flow or work. in some instances, iot data associated with operational processes and/or equipment that is trending out of limits/acceptable parameters may “trigger” an augmented reality support layer, comprised of appropriate schematics, videos, and even remote operations assistance from a geographically distant subject matter expert, over the real world and deployed to the correct individual in proximity to prompt human intervention to normalize the operational process. the specific iot parameters may be recorded in the “context” extensions in xapi syntax so that the acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 4 resulting human action can be understood in relation to operational kpis. furthermore, resolution of the iot data associated with the operational process, where available, may be recorded. 5. xapi in virtual reality an operational case study in light of the success with xapi in the arlem standard, this paper’s authors hypothesized that utilization of the xapi base syntax, associated extensions, and concept of operational triggers would also lend themselves quite well to measurement of learning experiences and other immersive experiences, such as virtual reality. to test this theory, an existing 360 video-based virtual reality interaction, centred around operational safety and risk in a heavy-manufacturing environment, and for which existing learning measurement data was available, was selected. in the selected vr scenario, an operator at a plant was given a job task to complete, and, in so doing, a set of dangers/risks in the environment were layered throughout the scenes (see figure 1). the scenario itself had been created to simulate the work environment and to increase plant operators’ ability to recognize and mitigate risks present in their working environment without overt prompts to do so. existing dashboards associated with the scenario were then examined to ascertain the likelihood of correlation with actual plant operator behaviour in the workplace. analysis of this information demonstrated that, though the immersive environment lent itself quite well to application of the xapi syntax, more traditional learning measurement methods, such as scoring of responses as right/wrong were employed, along with a scoring mechanism that subtracted points for each incorrect response (or lack thereof) within the simulation. elapsed time in the interaction was also reported (see figure 2). because the original measurement design did not approximate the type of behaviours observed, collected, and reported in the workplace, the effectiveness of the immersive learning experience, as compared to other traditional learning modalities, was difficult to determine across various learners and/or similar scenarios. as expected, the nature of the existing data did not give a great deal of information that would allow inference as to the immersive scenario’s ability to shape and/or change human behaviour in a manner that one could reasonably assume would make them safer and/or more productive in their actual jobs in the real world. 5.1. enhanced data set vr safety/risk in order to enhance the measurement of human interaction within the simulation in a manner that could be more closely correlated to human behaviour in the workplace, we obtained a sample of the historical data and, along with a careful analysis of the operational risks present in the design of the vr scenario, completed a data transform that allowed for xapi syntax to be expressed in a manner that identified environmental context, as well as subsequent user action/behaviour, and results of this behaviour in relation to the original “triggering” context itself (see figure 3). xapi-formatted json statements were then created, using existing available data fields, as well as additional data available and yet not previously recorded (see figure 4). these statements were subsequently loaded into a learning record store (lrs), and subsequent dashboards were created to demonstrate the relationships between environment and corresponding user behaviour, as well as relationships with existing competency frameworks related to health and safety and designed by the u.s. department of labor [6] (see figure 5). as compared to the original, more traditional measurement design, these dashboards contextualized human behaviour in conditions of varying risk in a manner similar to the way health and safety behaviours are measured in real operational environments and gave a clearer indication of human behaviour that might be demonstrated in these operational environments, based upon actions measured and tracked in the immersive simulation. 5.2. key findings the results of this case study, as well as reference implications of the arlem standard, as previously described by wild et al., demonstrate that it is possible to map additional information to existing xr datasets to infer “shaping of human behaviour”, as well as potentially correlate actions in simulated and/or augmented environments to those observed and recorded in the real world. furthermore, the original “triggering condition/context” metaphor suggested in the arlem standard was found to extend more broadly to all experiential-based learning experiences and work in harmony with the xapi data syntax associated with learning experiences. to enable effective measurement across scenarios and users, we created the following taxonomies for use in the xapi syntax, designed to be utilized broadly across xr scenarios measuring hazard identification and mitigation of operational risk: figure 1. selected vr scenario, designed to measure learner’s ability to identify and mitigate operational risk in a simulated environment. figure 2. measurement of non-xapi-compliant learner behaviour in a vr scenario. acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 5 1. stimulus (context) taxonomy to describe starting condition(s) within a scenario. 2. trigger (context) taxonomy to describe expected levels of perceived risk, competing human priorities (dilemmas), and/or intentional environmental distractor(s). 3. response (results)taxonomy to describe user roles and levels within the scenario, ending condition(s), correlation to open competency frameworks, haptics, eye gaze, biometrics. while these taxonomies were created to facilitate historical data transformation from an existing vr scenario into an xapicompliant dataset, it is acknowledged here that actual ontologies are more desirable. in order to promote interoperability and the broad reach of xapi as a measurement standard in xr learning scenarios, it is recommended that representative ontologies and/or taxonomies be developed that can inform widespread historical data transformation in the xr space, as well as serve as guides around measurement designs for additional immersive scenarios that are designed in future. these advancements, we believe, will provide a powerful measurement tool designed to effectively measure, describe, and perhaps even predict human behaviour in context in a manner that speaks to not only the knowledge that individuals possess, but also to their performed skill/competency level across a variety of contexts. this has major implications for the ways in which we measure and assess specific and generalized learnings in individuals over time. 6. designing augmented reality learning experiences for early childhood education: opportunities and challenges in a literature review of vr, ar, and mr in k-12 education, maas and hughes [7] noted that studies found increased attitude, motivation, engagement, performance/learning outcomes, and 21st century skills and competencies with the use of these xr technologies. but they also emphasized that it was challenging to find studies on xr in k-12 education in comparison to such research in higher ed. the reasons they theorized such research was difficult to find were the rapid development of the technology, which means that it has only recently become available; the lack of xr content for this age group; and a lack of resources, making xr unevenly available within countries and around the globe. with early childhood education, these challenges are even more daunting. when the current paper was first proposed, the intention was to discuss research on an early childhood ar application built on a platform compatible with xapi. that research was to be informed by work indicating that bodily engagement enhances learning outcomes and improves executive function. research on children’s learning by eng et al. using a modified vr game combined with a cognitive task suggests that the combination of “exergames” with a learning task not only improves performance on that task, but can actually improve cognitive ability and executive function [2], [3]. while eng et al. studied children using vr along with measurements in fmri as well as teacher assessments, in the case of the proposed new research, ar was to be used. augmented reality is much more accessible than virtual reality for early childhood and k-12 education, which increases the opportunities to study it. for example, tablets were already in use at the lab school where the study is expected to take place. more ar platforms and experiences are becoming compatible with xapi. thus, we still believe there is great promise for the study in development and for other such studies. in addition, arcompatible smartphones and tablets are also quite common in households in 2022, which means that many children would be able to participate in ar learning experiences from home. ironically, such ar learning experiences could greatly benefit the many children who have not been able to attend school in person in the time of covid and who have missed precious learning time at a key moment in their young lives. this proved challenging on many fronts. the challenges included ensuring that the design principle around differentiation of individual actors be followed. in practical terms, this would involve ensuring that each child be supplied with a device assigned to and associated with that particular child to avoid relying on young children to log into the device with the appropriate credentials, which would alleviate pressures on teachers in the classroom context. for young children who are just beginning to learn to read, audio prompts may need to be incorporated into the experience, or each student may need to be guided by the teacher, making the process resource intensive. the continuing effects of the pandemic also had a significant impact on the project because the early childhood education lab where the research was to be conducted experienced repeated closures and disruptions. as a result, the study has not yet formally begun. figure 3. measurement of learner behaviour in the same vr scenario, utilizing the xapi standard, in conditions of medium risk (context). figure 4. measurement of predicted results based upon learner behaviour in the same vr scenario, mirroring operational consequences in the workplace. figure 5. number of user actions in vr scenario showing evidence of competency in u.s. department of labor standard: “assessing material equipment, and fixtures for hazards”. acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 6 7. conclusions and next steps some of the early childhood learning that we would like to measure and that are appropriate for structuring with xapi include self-concept skills such as proprioception, awareness of others in space, and geometry and spatial sense skills such as recognition of 3d shapes and their persistent form when rotated, for example. learning of appropriate behaviours for self-care and social responses to others give situational triggers that are also targets for studies of the use of xapi with young children. in general, future steps include continuing to explore specific elements of xapi profiles appropriate for xr with regards to experiential learning, considering the relationship between knowledge-based concepts and experiential skills, and examining xapi profiles for xr across the talent pipeline (from pre-k to adult) in order to hypothesize about implications for the future of education. acknowledgement special thanks to colleagues at warp vr, namely guido helmerhorst and menno van der sman for contributing scenarios, historical data subsets, and transformation support. this effort would not have been possible without your spirit of innovation and commitment to open source-initiatives and practices. references [1] m. johnson-glenberg, immersive vr and education: embodied design principles that include gesture and hand controls, frontiers in robotics and ai, july 2018. doi: 10.3389/frobt.2018.00081 [2] c. m. eng, m. pocsai, f. a. fishburn, d. m. calkosz, e. d. thiessen, a. v. fisher, adaptations of executive function and prefrontal cortex connectivity following exergame play in 4to 5-year old children. [3] c. m. eng, d. m. calkosz, s. y. yang, n. c. williams, e. d. thiessen, a. v. fisher, doctoral colloquium enhancing brain plasticity and cognition utilizing immersive technology and virtual reality contexts for gameplay. 6th international conference of the immersive learning research network (ilrn), san luis obispo, ca, usa, 21-25 june 2020, pp. 395-398. doi: 10.23919/ilrn47897.2020.9155120 [4] j. secretan, f. wild, w. guest, learning analytics in augmented reality: blueprint for an ar / xapi framework, in tale 2019, yogyakarta, indonesia, 10-13 december 2019, pp. 1-6. doi: 10.1109/tale48000.2019.9225931 [5] ieee standard for augmented reality learning experience model, ieee std 1589-2020, april 2020, pp. 1–48. doi: 10.1109/ieeestd.2020.9069498. [6] credential finder. online [accessed 15 september 2022] https://credentialfinder.org/competencyframework/1928/enginee ring_competency_model_u_s__department_of_labor_(dol) [7] m. j. maas, j. m. hughes, virtual, augmented, and mixed reality in k-12 education: a review of the literature. technology, pedagogy, and education 29 (6) 2020. doi: 10.1080/1475939x.2020.1737210 https://doi.org/10.3389/frobt.2018.00081 https://doi.org/10.23919/ilrn47897.2020.9155120 https://doi.org/10.1109/ieeestd.2020.9069498 https://doi.org/10.1109/ieeestd.2020.9069498 https://credentialfinder.org/competencyframework/1928/engineering_competency_model_u_s__department_of_labor_(dol) https://credentialfinder.org/competencyframework/1928/engineering_competency_model_u_s__department_of_labor_(dol) https://doi.org/10.1080/1475939x.2020.1737210 microsoft word article 12 108-730-1-le.doc acta imeko december 2013, volume 2, number 2, 67 – 72 www.imeko.org acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 67 development of nanoparticle sizing system using fluorescence polarization terutake hayashi, masaki michihata, yasuhiro takaya, kok foong lee osaka university, 2-1 yamadaoka, suita, osaka 565-0871, japan section: research paper keywords: nanoparticle; fluorescence polarization; brownian motion; rotational diffusion coefficient; particle sizing citation: terutake hayashi, masaki michihata, yasuhiro takaya, kok foong lee, development of nanoparticle sizing system using fluorescence polarization, acta imeko, vol. 2, no. 2, article 12, december 2013, identifier: imeko-acta-02 (2013)-02-12 editor: paolo carbone, university of perugia received april 15th, 2013; in final form november 17th, 2013; published december 2013 copyright: © 2013 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: this work was supported in part by a grant-in-aid for scientific research from the ministry of education, culture, sports, science and technology of japan (grant-in-aid for exploratory research 21686015). corresponding author: terutake hayashi, e-mail: hayashi@mech.eng.osaka-u.ac.jp 1. introduction the integration of nanoparticles into devices produces unique electronic, photonic, and catalytic properties. it offers the prospect of both new fundamental scientific advances and useful technological applications. many of the fundamental properties of materials (e.g., optical, electrical, and mechanical) can be expressed as functions of the size, shape, composition, and structural order [1, 2]. it is important to evaluate nanoparticle sizes and maintain a constant state (size distribution, average size, shape uniformity). the size distribution of monodispersed nanoparticles in a solvent can be measured using dynamic light scattering [3，4]. however, the scattering method is inappropriate for detecting the sizes of particles in a mixture, which include both small monodispersed particles and large particles such as agglomerates. the signal intensity depends on the sixth power of the particle diameter. therefore, the scattering signal from monodispersed particles, compared to the signal from their agglomerates, is too small to detect. the size distribution of nanoparticles with a wide size distribution can be measured using transmission electron microscopy [5]. this requires a dried sample. however, it is difficult to evaluate both the average diameter and size distribution of nanoparticles because of the instability of their diameters in a solvent. to measure the sizes of nanoparticles with a wide size distribution in a solvent, we developed an optical microscopy system that enables fluorescence polarization (fp) measurement and optical observation. the fp method can trace the rotational diffusion constant of brownian motion in a fluorescent molecule. when a fluorophore is used to label nanoparticles, the rotational diffusion coefficient corresponds to the size of the nanoparticles. the system makes it possible to evaluate nanoparticle sizes over a wide range because the fluorescence signal intensity is independent of changes in the nanoparticle sizes. in this paper, we describe a fundamental experiment to verify the feasibility of using this method for different sizes of nanoparticles. 2. measurement of rotational diffusion coefficient and particle sizing 2.1. measurement of rotational diffusion coefficient we have developed an fp method [6, 7, 8] to measure nanoparticle sizes by measuring the rotational diffusion coefficient. when linearly polarized light is irradiated on a nanoparticle labeled with a fluorophore, a fluorescence signal abstract to measure the sizes of nanoparticles with a wide size distribution in a solvent, we developed an optical microscopy system that enables fluorescence polarization (fp) measurement and optical observation. this system allows the evaluation of nanoparticle sizes over a wide range because the fluorescence signal intensity is independent of changes in the nanoparticle sizes. in this paper, we describe a fundamental experiment to verify the feasibility of using this system for different sizes of nanoparticles. acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 68 initially maintains the polarization state of the irradiation light. as shown in figure 1, small particles will have a high degree of rotation due to brownian motion, and the polarization plane of the light emitted will vary greatly from that of the excitation light. in contrast, larger particles will have a low degree of rotation, so light is emitted in the polarized plane, similar to the excitation light. the fluorescence signal then depolarizes as time passes. the fluorescence anisotropy depends on the angle of the particle rotation. in addition, depolarization is rapid for particles with small volumes, whereas the polarization state is maintained longer in particles with large volumes. the fluorescence anisotropy r(t) is related to the rotational diffusion coefficient dr of a nanoparticle. the particle size can be calculated from dr on the basis of an analysis using the debye–stokes–einstein (dse) relationship [8, 9] when the particle shape is approximated to that of a sphere. here r(t) is defined by   // // 2 i i r t = i + i    (1) where the first and second subscripts refer to the orientation of the excitation and that of the emission, respectively. i∥ and i⊥ are the horizontally polarized (p-polarized) and vertically polarized (s-polarized) components of the fluorescence, respectively, when horizontally polarized light is irradiated. we can also express r(t) as a function of time t as       / / 0 t τ t θ er t = r r e + r i t = τ     (2) where i(t) is the instantaneous emission intensity normalized to a unit-integrated value. the parameter r reflects the extent of the rotational reorientation of the fluorophore. in this case, r is obtained by integrating the intensity-weighted r(t), as shown in eq. (3). the steady-state anisotropy r of a fluorophore undergoing isotropic rotational diffusion is related to the fluorescence lifetime,  and rotational correlation time [4]:     0 0 1 r r r = r t i t = +σ r r        (3) where r0 is a limiting value in the absence of rotation given by the relative orientation of the absorption and emission transition moments, and  is the ratio . measurements of the anisotropy decay can reveal a multiplicity of rotational correlation times reflecting heterogeneity in the size, shape, and internal motions of the fluorophore–nanoparticle conjugate. the rotational correlation times are determined in either the time domain or the frequency domain. the rotational diffusion coefficient can be precisely calculated from the fluorescence anisotropy in the frequency domain. when an excitation pulse is used as a forcing function, eq. (3) is transformed into a decay process in which the fluorescence lifetime  is no longer linked to the correlation time. the rotational correlation times are determined by measuring three parameters: , yac, and ydc. to define them, we measure the sinusoidal waveforms of both i∥ and i⊥ in the frequency domain. a series of i∥ and i⊥ are measured while the relative phase between the excitation light and the detector gain is adjusted as the excitation light and detector gain are modulated using the same radial frequency. after the sinusoidal signals of i∥ and i⊥ are acquired, each data set is processed to yield the frequency-dependent amplitudes and the phase shift between the excitation and emission light. the resulting polarized emission components are modulated at the same frequency but phase shifted with phase decay  relative to each other [10]. i∥ac and i⊥ac are the amplitudes of i∥ and i⊥ in the frequency domain, respectively. i∥dc and i⊥dc are the average values of i∥ and i⊥, respectively. these signals are characterized by the ratio // /// /ac ac ac dc dc dcy = i i , y = i i  . (4) furthermore, the fluorescence lifetime of the emission light can be measured in the frequency domain. the fluorescence lifetime  is determined by both a series of data sets for the excited light intensity, i()and the total light intensity of the emission light, i() the parameters , yac, and ydc are then related to parameters r , r0, r and  by              0 0 0 01 0 3 2 1 1 2 1 1 tan 1 1 2 1 2 r r r r r r r r                               , (5)                 22 2 0 0 22 2 0 0 1 2 1 2 1 2 1 1 1 ac r r r y r r r                     , (6)         0 0 1 2 1 2 1 1 dc r r y r r            , (7) 1 2 dc dc y r y    . (8) the rotational correlation time  is given by               1 2 2 2 22 2 2 2 2 2 1 tan 4 tan tan4 2 ac ac dc dc ac dc ac ac dc ac dc y y y y y y y y y y y                            (9) to define the rotational correlation time , the fluorescence lifetime  is required. the fluorescence lifetime  is determined by the phase decay and modulation m of the fluorescence, which is excited using light whose intensity i(t) varies sinusoidal with time [10]: figure 1. concept of fluorescence polarization method. acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 69    sini t a b t  (10) where  is the angular velocity of the modulated excitation light [10]. as a consequence of the finite duration of the excited state, the modulated fluorescence emission is delayed in phase by angle  relative to the excitation. in addition, the modulation of the fluorescence decreases. the intensity of the fluorescence is given by    sini t a b t    . (11) we acquire the phase decay  between the sinusoidal curve of the excitation light and the emitted fluorescence without the emission polarizer in the frequency domain by tan  . (12) 2.2. particle sizing using sphere model for brownian particles reorienting in a liquid, the debye model defines an exponential decay for the single-particle orientation time correlation function c and rotational correlation time   exp /c t   , (13) 1 6 rd   , (14) where dr is the rotational diffusion coefficient. dr is coupled with the shear viscosity  at temperature t using the dse relationship [8, 9]: b r h k t d v   (15) where kb is the boltzmann constant. the hydrodynamic volume vh is related to the particle volume  through a factor that depends on the shape of the reorienting particle and the boundary conditions. the particle volume  can be calculated by measuring the rotational diffusion coefficient dr when the relation between vh and  is formulated. for example, the formula for a sphere, 6 ,hv  is applied to the stick boundary conditions. the dse relationship is known to be effective at the molecular length scales of low-viscosity liquids. consequently, the hydrodynamic volume vh of a fluorophore can be determined using , yac, ydc, , and  on the basis of eqs. (14), eq. (15), and (12). when the sphere approximation is applied to a fluorophore–nanoparticle conjugate, the diameter can be calculated from vh. this reflects the excited state lifetime and intrinsic rotational diffusion properties of the fluorophore and the modulation frequency. 3. experimental setup we developed a rotational diffusion coefficient measurement system using an fp method, as shown in figure 2. an ar+ laser (wavelength: 488 nm) is the polarized light source, and it is coupled to an acousto-optic modulator (aom). before coupling to the aom, the polarization direction is oriented using a half-wave plate (1/2 wp) to improve the diffraction efficiency of the aom. high-speed light amplitude modulation (up to 80 mhz) can be achieved in the unit, which consists of the aom, lens (l), and iris. the polarization direction of the input signal is then oriented by a half-wave plate (1/2 wp) just as with the polarization direction of i//. the light is incident on the sample via a half mirror (hm) and objective lens. the sample is excited using linearly polarized light. the emission is relayed through the beam displacer, which divides the fluorescence polarization signals oriented parallel (i//) and perpendicular (i⊥) to the excitation beam polarizer. the fluorescence signals are finally relayed to the image intensifier. the image of the orthogonal components of the fluorescence signal is enhanced on the image intensifier and then relayed to a ccd. images of both the horizontally (i//) and vertically (i⊥) polarized components are analyzed to acquire the fluorescence anisotropy. the modulation of the gain of the image intensifier corresponds to the modulation of the input signal amplitude. the phase decay , is the phase difference between the incident light and the gain of the image intensifier. acquisition proceeds with a series of phase shifts to acquire a first-order photo bleaching compensation. a data series is processed to yield the frequencydependent amplitudes, along with the phase shift between the excitation light and the emitted lights in the two orthogonal polarization directions. the polarized emission components are modulated at the same frequency but phase shifted relative to each other. 4. experimental results 4.1. rotational correlation time for fluorophore to measure the particle sizes, precise measurement of the rotational diffusion coefficient, which corresponds to the rotational correlation time, is required, as shown in eq. (14). we performed fundamental experiments to verify the feasibility of ar-laser 488nm ½ wp l l aom iris m ½ wp p hm function generator function generator phase-locked  objective lens sample temperature controller emission filter tube lens beam displacer image intensifier relay lens ccd to computer figure 2. experimental setup. figure 3. ccd images of fluorescence signals. acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 70 precise measurement of the rotational correlation time for the fluorophore [11] without labeling gold nanoparticles. in the experiment, a beam displacer is used to divide the fluorescence light into the parallel and perpendicular components. as shown in figure 3, both components of the signal are observed in the ccd camera after the fluorescence light passes through the beam displacer. the upper component is polarized in the horizontal (h) direction, and the lower component is polarized in the vertical (v) direction. to check the efficiency of both sides, two measurements are required. first, the polarization angle of the excitation light is adjusted to the direction parallel to the horizontal direction, and an image is recorded, as shown in figure 3(a). the intensities of the horizontal and vertical signals are denoted by ihh and ihv, respectively. next, the polarization angle of the excitation light is adjusted to the direction parallel to the vertical direction, and another image is recorded. the intensities of the horizontal and vertical signals are also denoted by ivh and ivv, against the orthogonal direction of polarization respectively, as shown in figure 3(b). fifteen images are taken for each case. depending on the direction of polarization, the efficiency of the light passing through may differ. therefore, a calibration factor g is needed to measure i∥ and i⊥ from the two signal components. we calculated the g factor for ivh; thus, when excitation light in the v direction is used, ivv is used directly in our calculation, and ivh times g is used for the h direction of i∥. the g factor for ivh is obtained as follows: vv hv ivh vh hh i i g i i  . (16) the modulation frequency of the aom was set to 60 mhz for the following experiment. the ccd exposure time was 400 ms, and the micro channel plate voltage was 760 v. the modulated fluorescence signals by varying the phase  are shown in figure 4. , yac, and ydc are determined from the fitting curves of both fluorescence signals. first, the fluorescence anisotropy as a function of the time t is evaluated using the developed system. figure 5 shows the linear variation in the rotational correlation time versus the viscosity of the solution. three solutions were prepared for measuring the rotational correlation time of a fluorophore by mixing water with glycerin at 30 wt%, 50 wt%, and 60 wt%. the resulting viscosities were 2.5 mpa·s, 6.0 mpa·s, and 10.8 mpa·s at 293 k, respectively. we also used water, which has a viscosity of 1.0 mpa·s at 293 k, as a solution. the fluorophore was alexa fluore 488 (invitrogen corp.), which is the same size as fluorescein. the fluorescence lifetime  varies, with values of 4.1 ns in water, 3.8 ns in 30 wt% glycerin, 3.6 ns in 50 wt% glycerin, and 3.1 ns in 60 wt% glycerin. if we apply the sphere approximation for the fluorophore, the theoretical value of the rotational correlation time, which depends on the particle size, can be calculated from eqs. (14) and (15) as follows: 36 b d k t     . (17) the theoretical value is plotted according to the solution temperature t (293 k) and nanoparticle diameter d (1.1 nm). according to eq. (16), the rotational correlation time of the fluorophore agrees well with the value for a nanoparticle with a diameter of 1.1 nm under the nonslip boundary condition. this value is close to that of fluorescein, whose size is estimated to be 1.0 nm [12, 13]. the size difference is considered to be an effect of hydration of the fluorophore in the solution. from the above results, the rotational correlation time can be precisely measured using the developed system. 4.2. rotational diffusion coefficient for gold nanoparticle with figure 4. parallel and perpendicular polarized components of modulated fluorescence signal. 0 0.5 1 1.5 2 2.5 0 2 4 6 8 10 12 r ot at io na l c or re la tio n tim e( ns ) viscosity (mpa.s) theoretical value for 1.1nm figure 5. rotational correlation time of fluorophore.                               2 22 0 2 2 22 22 2 0 0 0222 2 0 0 1 1 2 2 2 5 2 4 4 1 2 1 2 1 1 9 1 2 1 2 5 4 1 2 1 1 ac ac ac ac y r r r r r r r r r y r r y r r r r r r r y r r                                                           (18)             2 22 22 2 2 2 2 0 0 0 0 0 0 0 2 1 2 1 9 2 2 5 1 1 2 6 1 ac ac ac r ac r y r y r r r y r r d y                   (19) acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 71 fluorescent dna probe to evaluate the diameter of a gold nanoparticle, the rotational diffusion coefficient of the nanoparticle with a fluorescent dna probe is evaluated using the developed system. gold nanoparticles with average diameters of 8.2 nm were prepared as the standard sample for measurement of the rotational diffusion coefficient. we evaluated the rotational diffusion coefficient of the gold nanoparticles directly with the fluorescent dna probe. the fluorescent probe was connected to the gold nanoparticles via double-strand dna consisting of adenine with a length of 23 bases. the length of the double-strand dna is estimated to be 9.6 nm, as shown in figure 6. the dna acts as a spacer to avoid quenching of the fluorophore [14, 15, 16], which is located near the gold nanoparticle. the 23-base double-strand dna is strong enough to avoid bending [17, 18] and to maintain the distance between the gold nanoparticle and fluorophore. figure 7 shows the rotational diffusion coefficient of the fluorophore and gold nanoparticle with the fluorescent dna probes versus the solution parameters, represented by t/. the rotational correlation time  is calculated using eq. (18) and the data for the parameters , yac, and ydc. the rotational diffusion coefficient can be calculated using the reciprocal of , as shown in eq. (19). table 1 shows the measurement parameters used to evaluate the rotational diffusion coefficient. as shown in figure 7, the rotational diffusion coefficient, which indicates the speed of rotational motion, increases linearly with t/. this relation agrees with eq. (17), which shows a linear relation to /t. the inclination of the graph for the rotational diffusion coefficient of the fluorescent dna probe is four times higher than that of the gold nanoparticle (average diameter, 8.2 nm) with the fluorescent dna probe. a large difference in the rotational correlation coefficient appears between the fluorescent dna probe and the gold nanoparticle with the probe. this enables us to estimate the size of a gold nanoparticle quantitatively using the inclination of the rotational diffusion coefficient against the viscosity of the temperature. to evaluate the resolution of the particle sizing obtained using the proposed method, further experiments for various diameters of gold nanoparticles are needed. we are now preparing samples of gold nanoparticles with average diameters of 5 nm, 10 nm, 15 nm, and 20 nm. 5. conclusions we developed a nanoparticle sizing system using an fp method. this system can precisely determine the rotational correlation time of nanoparticles with a fluorophore. the results indicate that we can determine the size of a nanoparticle using the dse relation when the particle shape approximates a sphere. we also investigated the rotational correlation time of a fluorophore with gold nanoparticles that were smaller than 10 nm. this indicates that the measurement results for the rotational correlation time of a fluorophore-labeled gold particle can be used to estimate the size of gold nanoparticles smaller than 10 nm. acknowledgment this research was supported in part by a grant-in-aid for scientific research from the ministry of education, culture, sports, science and technology of japan (grant-in-aid for exploratory research 21686015). references [1] t. gao, q. li, and t. wang, “sonochemical synthesis, optical properties, and electrical properties of core/shell-type zno nanorod/cds nanoparticle composites,” chem. mater., no. 17, pp. 887–892, 2005. [2] r. g. freeman, k. c. grabar, k. j. allison, r. m. bright, j. a. davis, a. p. guthrie, m. b. hommer, m. a. jackson, p. c. smith, d. g. walter, and m. j. natan, “self-assembled metal colloid monolayers: an approach to sers substrates,” science, vol. 267, no. 5204, pp. 1629–1632, 1995. [3] s. sun, c. b. murray, d. weller, l. folks, and a. moser, “monodisperse fept nanoparticles and ferromagnetic fept nanocrystal superlattices,” science, vol. 287, no. 5460, pp. 1989-1992, 2000. figure 6. distance between gold nanoparticle and fluorescent probe. table 1. measurement parameters. temperature [k] viscosity [mpas] probe probe + gold nanoparticle τ [ns] yac τ [ns] yac 293 1.002 2.0 1.225 1.0 1.457 298 0.890 1.199 1.435 303 0.797 1.172 1.427 308 0.719 1.151 1.402 313 0.653 1.135 1.396 figure 7. rotational diffusion coefficients for fluorophore and gold nanoparticle with fluorescent dna probe. acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 72 [4] r. pecora, “dynamic light scattering measurement of nanometer particles in liquids,” j. nanopart. res., vol. 2, issue 2, pp. 123-131, 2000. [5] l. c. gontard, d. ozkaya, and r. e. dunin-borkowski, “a simple algorithm for measuring particle size distributions on an uneven background from tem images,” ultramicroscopy, vol. 111, issue 2, pp. 101–106, 2011. [6] k. kinosita jr., s. kawato, and a. ikegami, “a theory of fluorescence polarization decay in membranes,” biophys. j., vol. 20, issue 3, pp. 289–305, 1977. [7] b. s. fujimoto and j. m. schurr, “an analysis of steady-state fluorescence polarization anisotropy measurements on dyes intercalated in dna,” j. phys. chem., vol. 91, no. 7, pp. 1947-1951, 1987. [8] f. stickel, e. w. fischer, and r. richert, “dynamics of glassforming liquids. ii. detailed comparison of dielectric relaxation, dc-conductivity, and viscosity data,” j. chem. phys., vol. 5, no. 104, pp. 2043–2055, 1996. 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[14] c. s. yun, a. javier, t. jennings, m. fisher, s. hira, s. peterson, b. hopkins, n. o. reich, and g. f. strouse, “nanometal surface energy transfer in optical rulers, breaking the fret barrier,” j. am. chem. soc., vol. 127, no. 9, pp. 3115–3119, 2005. [15] j. seelig, k. leslie, a. renn, s. kühn, v. jacobsen, m. van de corput, c. wyman, and v. sandoghdar, “nanoparticle-induced fluorescence lifetime modification as nanoscopic ruler: demonstration at the single molecule level,” nano lett., vol. 7, no. 3, pp. 685–689, 2007. [16] s. mayilo, m. a. kloster, m. wunderlich, a. lutich, t. a. klar, a. nichtl, k. kürzinger, f. d. stefani, and j. feldmann, “longrange fluorescence quenching by gold nanoparticles in a sandwich immunoassay for cardiac troponin t,” nano lett., vol. 9, no. 12, pp. 4558–4563, 2009. [17] d. porschke, “persistence length and bending dynamics of dna from electrooptical measurements at high salt concentrations,” biophys. chem., vol. 40, issue 2, pp. 169–179, 1991. [18] g. s. manning, “the persistence length of dna is reached from the persistence length of its null isomer through an internal electrostatic stretching force,” biophys. j., vol. 91, no. 10, pp. 3607–3616, 2006. measuring with confidence: leveraging expressive type systems for correct-by-construction software acta imeko issn: 2221-870x march 2023, volume 12, number 1, 1 5 acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 1 measuring with confidence: leveraging expressive type systems for correct-by-construction software conor mcbride1, georgi nakov1, fredrik nordvall forsberg1 1 department of computer and information sciences, university of strathclyde, glasgow, uk section: research paper keywords: type systems; correctness; metrology; programming languages citation: conor mcbride, georgi nakov, fredrik nordvall forsberg, measuring with confidence: leveraging expressive type systems for correct-byconstruction software, acta imeko, vol. 12, no. 1, article 15, march 2023, identifier: imeko-acta-12 (2023)-01-15 section editor: daniel hutzschenreuter, ptb, germany received november 19, 2022; in final form february 28, 2023; published march 2023 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: supported by the uk national physical laboratory measurement fellowship project “dependent types for trustworthy tools”. corresponding author: georgi nakov, e-mail: georgi.nakov@strath.ac.uk 1. introduction the digital transformation of metrology offers both challenges and opportunities. with increased software usage and complexity, there is also a need to increase trust in the computations performed — how do we know that the software is doing what is expected of it? computer science offers a wide range of formal methods and verification techniques to tackle this challenge. as always, there is a balance to be struck between how much additional effort is required from the user, and how useful the verification procedure can be. our main thesis is that a lot can be achieved by simply “bridging the semantic gap” between human and machine: current common practice is for computers to mindlessly execute instructions, without understanding for what purpose. this means that any verification must happen after the fact, by a separate process. what if, instead, there would be some way of communicating our intent as we are writing our software? then the machine could help us write it, rather than just tell us off for getting it wrong… we advocate for the use of type systems as a lightweight method to communicate intent. dependently typed programming languages are a new breed of languages which have type systems which are expressive and strong enough to use types to encode the meaning of programs to whatever degree of precision is needed. we can ensure that types can be automatically checked at compile-time, and so they provide machine-certification of the program's behaviour at low-cost. concretely, we therefore get both lightweight and machine-certified trust in the correctness of software. in the metrology domain, in particular, we can make good use of implicit tacit knowledge such as dimensional correctness to help the computer help us. this is work currently done by humans, but there is no reason why it could not be done automatically by a machine instead. as a small case study, we show that by turning informal comments about the expected input format of data into machine-readable form, we can not only check that given data conforms to the format, but automatically generate code for reading from disk and converting to appropriate units, thus eliminating a source of bugs and increasing trust in the software. our goals are similar to other software projects for calculation with physical quantities [1], [2], but we put additional emphasis on the use of types as a convenient method of communication between human and machine. thus, our focus is on how dependently typed programming languages can provide a sound basis for developing software for metrology rather than describing any such software in detail. abstract modern programming language type systems help programmers write correct software, and furthermore helps them write the software they actually intended to write. we show how expressive types can be used to encode dimension and units of measure information, which can be used to avoid dimensional mistakes and guide software construction, and how types can even help to generate code automatically, which eliminates a whole class of bugs. mailto:georgi.nakov@strath.ac.uk acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 2 2. type systems as lightweight formal methods in programming languages such as fortran or c, types such as floating-point numbers or integers are used to help the compiler with memory layout. a pleasant side effect is that basic errors such as trying to divide an integer by a string can be detected and reported at compile time, rather than at run time. while useful for avoiding disastrous results, this is quite a negative view on types: they are against errors, but are they also for something? types can be used to make an active contribution by offering guidance during the program construction process, not just criticism afterwards. we pay upfront by stating the type of the program we want to write, but are then paid back in the machine being able to use those types to offer suggestions for functions to call, or even generate code for boring parts of the program. the space within which we search for programs is correspondingly smaller and better structured. for such help to be meaningful, the types available need to be sufficiently expressive; it is usually not very instructive to be told that we need to for example supply an integer, nor is it going to be very helpful for the compiler to generate a floating-point number for us. as another example, consider a type whose elements are matrices. as given, this is again not very helpful — a matrix can, after all, be seen as a (structured) collection of numbers, and we just said that numbers in themselves do not carry much meaning. but we can refine our type of matrices to a type matrix(𝑛, 𝑚) which keeps track of the size 𝑛 × 𝑚 of the matrix: e.g., the type of matrix multiplication can be usefully expressed as matrix(𝑛, 𝑚) × matrix(𝑚, 𝑘) → matrix(𝑛, 𝑘) i.e., insisting that the sizes of the input matrices are compatible, and determining size of the output matrix. furthermore, if we were trying to write a program to implement matrix multiplication, the above type would give us helpful hints on what we need to produce. for another example, consider implementing a program that creates a block matrix by putting two given matrices next to each other. it is natural to give it type matrix(𝑛, 𝑚) × matrix(𝑛, 𝑘) → matrix(𝑛, 𝑚 + 𝑘) i.e., we insist that both input matrices have the same number of rows, and the number of columns in the output matrix is the sum of the number of columns in the input matrices. we see that computations such as 𝑚 + 𝑘 naturally arise in types — to do a proper job classifying such programs as meaningful, our systems must thus allow values and computations to occur in types. such type systems are called dependent type systems [3], as types can depend on values. they give us enough expressive power to meaningfully communicate our intentions to the compiler. 3. units of measure using types the metrology domain is perhaps especially well-suited for the use of types to guarantee correctness, because the prevalent use of dimensions (such as length and time) and units of measure (such as metres and seconds) in many respects play the same role as types: it is not dimensionally correct to add a metre and a second, just like it is not data type correct to add an integer and a string. it thus seems natural to use type systems to reduce dimension checking to type checking. indeed, many mainstream languages have support for units, implemented using a wide range of techniques, from static types to dynamic run-time checks (see bennich-björkman and mckeever's survey [4] for more): • microsoft's f# [5] has units of measure built-in to static type checking; • c++'s boost units library [6] uses templates to check units statically; • java has a proposed api adding classes for dimensioned quantities [7], but run-time casts are inevitable; • haskell's type system can now encode basic units of measure as a library [8] or a typechecker plugin [9]; • python libraries such as pint [10] cannot do static checking of dimensional correctness, but implement runtime checks instead. similarly, matlab has support for dynamic unit checking using the symbolic math toolbox [11]. many of these solutions however provide no static guarantees, or rely on rather ad-hoc extensions of the type system, often with unintelligible error messages as a result. encoding dimensions using dependent types is a more principled way to include dimension checking in a programming language. in the rest of this section, we briefly describe how we implemented a typechecker including dimensions [12]. first, how are we to represent dimensions themselves? following kennedy [13], we fix a set 𝐷 of fundamental dimensions (such as length l time t and mass m). we may multiply or divide dimensions (for example forming mass per time squared m/t2), and the order of dimensions do not matter (mass times length 𝑀 ⋅ 𝐿 is the same as length times mass 𝐿 ⋅ 𝑀). these considerations lead us to model dimensions as elements of the free abelian group over the set of fundamental dimensions 𝐷 [14]. for type checking, we need to be able to decide if two given dimensions are equal or not. this is made easier by a normal form for elements of the free group: we first (arbitrarily) impose a total order on the fundamental dimensions 𝐷 (for example mass before length before time 𝑀 < 𝐿 < 𝑇). any dimension may be given as a finite product of distinct fundamental dimensions, in the chosen order, raised to nonzero integral powers. hence to check equality of dimensions 𝑑 = ? 𝑑′, we can reduce 𝑑 and 𝑑′ to normal forms 𝑑 = mn0 ⋅ l𝑛1 ⋅ t𝑛2 , 𝑑′ = m𝑛0 ′ ⋅ l𝑛1 ′ ⋅ t𝑛2 ′ and then straightforwardly check equality of the exponents 𝑛𝑖 = ? 𝑛𝑖 ′ , rather than applying the group axioms directly. with equality of dimensions in place, the crucial step in making dimension checking part of type checking is to also allow abstract dimensions [15]: addition is not length-specific, but works in one arbitrary dimension, which can stand for any dimension in particular. similarly, multiplication and division of quantities multiplies and divides arbitrary dimensions respectively. by giving addition and multiplication these types and taking our refined notion of equality of dimensions into account, dimension checking simply becomes type checking. gundry [16] has shown that the property of programs still having most general types is retained in this setting. as discussed by e.g., hall [17], dimension checking seems to be more fundamental than “unit checking”. when dimensions are encoded in types, units can be introduced as “smart constructors” such as watt _w: r → 𝑄(m l2 t−3) — the type of this function says that for any real number 𝑟, we get a quantity acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 3 𝑟 w of dimension m l2 t−3. if this is the only way to introduce quantities, we can ensure that only meaningful expressions enter the system. similarly, by only allowing the extraction of an actual number at dimensionless types (which can for example be achieved by dividing a quantity by a unit constant), only physically meaningful information can flow out of the system. the described approach is agnostic in the concrete choice of fundamental dimensions; indeed, if one wants to distinguish between torque and energy, for example, which both have the same dimension m l2 t−2 in the international system of units (si) [18], then one could introduce separate dimensions for them. a more principled approach would be to extend our system utilising aspects from the m-layer representation of quantities [19], but we use a conceptually simple representation here, as our aim is to demonstrate how dimension checking can be internalised as type checking. 4. using types to automatically generate code types are not just a stick to be beaten with when one makes a mistake; they can also act as a carrot, for example by enabling code generation. as a simple demonstration of this principle, we have developed a program that automatically generates code for reading and validating input data based on type declarations. the implementation is available at https://github.com/gnakov/mgen. many metrology software packages come with careful descriptions of input formats in their documentation, usually describing what input is required (e.g., “thermal conductivities”), in what form (e.g., “an array with an entry for each layer”), and in what unit (e.g., “w m-2 k−1”). however, these are written for humans, not machines, and consequently the code to read the inputs and convert them to the internal units used, if applicable, is also written by humans. this is typically fiddly code, with perhaps nested loops, and many opportunities for off-by-one errors to slip in. our approach is instead to make the description of the input format formal, so that it can be understood by a machine, which can then write the code for reading the inputs. in practice, this requires minimal changes to the description — mostly ensuring that the required data is actually present. an example input description is displayed in figure 1. an input is declared with its name (for example ivals and nlayer, followed by a colon ‘:’ followed by its description, which is for the benefits of humans. an input is either a composite object (such as ivals), a scalar field (such as nlayer), or an array (such as kappas). details about inputs which are important for the machine are tagged with an @ symbol, such as if an input is a number (for example, nlayer is tagged as a @number), or an array of a certain length (for example, lams is tagged as an array of size @nlayer). later inputs can refer to earlier inputs for their description (for example, the description of lams refer to nlayer) — we are making full use of dependent types by allowing later entries to depend on earlier ones. each non-number field entry has a unit attached to it, again indicated by an @ symbol. these can either be attached to individual fields of an array (such as for the array kappas), or uniformly for the whole array (such as the array rs). also note that we allow si derived units such as watt w — we convert these to their standard form in terms of si base units internally. given an input description, we first validate that it is sensible: that array lengths are numbers, that field names are not repeated, and that each scalar field has a unit. this way, we can catch simple mistakes in the input description such as typos or undeclared input fields. after validating the input description, we can generate code for reading input data following it. we currently generate matlab and python code, but there is nothing particular about the choice of languages — it would be possible to cover most programming languages. for the input description from figure 1, we generate the following matlab code — the corresponding python code can be found in figure 2. function ivals = getinputsfromfile(fname); f1 = fopen(fname); c1 = textscan(f1, `%f`); src = c1{1}; fclose(f1); rptr = 1; ivals.nlayer = src[rptr]; rptr = rptr + 1; for i = 1:ivals.nlayer ivals.lams[i] = 1e3 * src[rptr+i]; end rptr = rptr + ivals.nlayer; ivals.kappas[1] = 1e-2 * src[rptr+1]; for i = 2:3 ivals.kappas[i] = 1e-3 * src[rptr+i]; end rptr = rptr + 3; for i = 1:3 ivals.rs[i] = 1e3 * src[rptr+i]; ivals.cps[i+3] = src[rptr+i+3]; end rptr = rptr + 6; ivals.tflash = 1e-3 * src[rptr]; ivals : nlayer : contains the @number of layers (2 or 3) in the sample lams : array of thermal conductivities of layer @nlayer (in @w m^-2 k^-1). kappas : contains radius of sample (in @cm) laser (in @mm) measuring (in @mm) rs : heat transfer coefficient for losses in @w m^-2 k^-1 from front face from rear face curved side face cps : specific heat capacities in @j kg^-1 k^-1 of the front face of the rear faced of the curved side face tflash : duration of laser flash in @ms figure 1. formal input data description. https://github.com/g-nakov/mgen https://github.com/g-nakov/mgen acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 4 we make sure to generate fresh variable names for the read pointer rptr, the file handle f1 and the file contents c1 and src. the rest of the names are guaranteed to be non-clashing, since we have validated the description. we then sequentially read the data, advancing the read pointer as we go along. we use the unit information to scale data into the units used internally in the program. note also that we have taken the opportunity to merge the loops for ivals.kappas and ivals.rs into a single loop. these are exactly the kind of code transformations that are easy to get wrong if done manually — in contrast, we can reason generically that this transformation will always be correct. as a result, the generated code looks like similar to code that one would write by hand, but without the risk of making, for example, an off-by-one error somewhere. another guiding principle for our code generation tool is convenience of use of the input data afterwards — the whole exercise would be pointless if the user manually would have to convert the data into a different data type after reading. hence, we make sure to store the data in a data type that is “natural” for the chosen language, and which allows for a direct access of the fields following the input data description. in matlab, structure arrays readily meet these requirements, but we have to take extra care to explicitly generate the needed classes in python. 5. conclusions and future work type systems could be a powerful tool in the digitalisation of metrology. by exploiting advances in dependent type systems, we have shown that we can strengthen our ability to reason about dimensional correctness, and also bridge the gap between human-readable semantic specifications of data, and the actual code representing it in a specific programming environment. crucially, we were able to reap these benefits with minimal additional costs we put to good use already existing typecheckers without having to rewrite the infrastructure in place from scratch. we have chosen a straightforward treatment of dimensions as elements of a free group, and units as constants; this choice does not accurately disambiguate for example radians rad = m m−1 and square radians sr = m2 m−2, even though they are of very different nature. however, we stress that this is not an inherent limitation in the methodology of using types for dimensions — dimensionless quantity ratios can if necessary be tracked separately in types, using the same principles as presented here, which we hope to do in the future. overall, the work reported here is part of a larger project to incorporate dependent types in matlab programs for correctness checking, including dimensional correctness. acknowledgements thanks to alistair forbes, keith lines and ian smith for discussions about this work. the authors would also like to thank the attendants of the first imeko tc6 international conference on metrology and digital transformation for the useful suggestions and comments on a presentation of this work, in particular the suggestion to extend code generation also to python. funding: supported by the uk national physical laboratory measurement fellowship project “dependent types for trustworthy tools”. 6. references [1] m. p. foster, quantities, units and computing, computer standards & interfaces 35 (2013), pp. 529–535. doi: 10.1016/j.csi.2013.02.001 [2] b. d. hall, software for calculation with physical quantities, 2020 ieee international workshop on metrology for industry 4.0 iot, rome, italy, 2020, pp. 458–463. doi: 10.1109/metroind4.0iot48571.2020.9138281 [3] a. bove, p. dybjer, dependent types at work, lernet alfa summer school 2008 revised tutorial lectures (2009), pp. 57–99. doi: 10.1007/978-3-642-03153-3_2 [4] o. bennich-björkman, s. mckeever, the next 700 unit of measurement checkers, 11th acm sigplan int. conference on class ivals(): __slots__ = ("lams", "kappas", "rs", "cps", "tflash", "nlayer") def __init__(self): self.lams = {} self.kappas = {} self.rs = {} self.cps = {} ivals = ivals() with open(fname, 'r') as f1: src = f1.readlines() rptr = 0 ivals.nlayer = int(src[rptr]) rptr = rptr + 1 ivals.tflash = 1e-3 * float(src[rptr]) rptr = rptr + 1 for i in range(0, 3): ivals.cps[i] = float(src[rptr + i]) ivals.rs[i+3] = 1e3 * float(src[rptr + i + 3]) rptr = rptr + 6 ivals.kappas[0] = 1e-2 * float(src[rptr + 0]) for i in range(1, 3): ivals.kappas[i] = 1e-3 * float(src[rptr+i]) rptr = rptr + 3 for i in range(0, ivals.nlayer): ivals.lams[i] = 1e3 * float(src[rptr + i]) figure 2. generated python code from the data description in figure 1. https://doi.org/10.1016/j.csi.2013.02.001 https://doi.org/10.1109/metroind4.0iot48571.2020.9138281 https://doi.org/10.1007/978-3-642-03153-3_2 acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 5 software engineering, boston, usa, 2018. pp. 121–132. doi: 10.1145/3276604.3276613 [5] a. kennedy, types for units-of-measure: theory and practice. central european functional programming school 2009, revised selected lectures (2010), pp. 268–305. doi: 10.1007/978-3-642-17685-2_8 [6] m. c. schabel, s. watanabe, boost c++ libraries, chapter 42 (boost.units 1.1.0), 2010. online [accessed 13 january 2023] https://www.boost.org/doc/libs/1_81_0/doc/html/boost_unit s.html [7] j.-m. dautelle, w. keil, o. santana, jsr 385: units of measurement, 2021. online [accessed 13 january 2023] https://unitsofmeasurement.github.io/pages/about.html [8] t. muranushi, r. eisenberg, experience report: type-checking polymorphic units for astrophysics research in haskell, 2014 acm sigplan symposium on haskell, gothenburg, sweden, 2014, pp. 31–38. doi: 10.1145/2633357.2633362 [9] a. gundry, a typechecker plugin for units of measure: domainspecific constraint solving in ghc haskell, 2015 acm sigplan symposium on haskell, vancouver, canada, 2015, pp. 11–22. doi: 10.1145/2804302.2804305 [10] pint developers, pint: makes units easy, 2022. online [accessed 13 january 2023] https://pint.readthedocs.io/ [11] mathworks, matlab units of measurement, 2022. online [accessed 13 january 2023] https://www.mathworks.com/help/symbolic/units-ofmeasurement.html [12] c. mcbride, f. nordvall forsberg, type systems for programs respecting dimensions, in: advanced mathematical and computational tools in metrology and testing xii. f. pavese, a. b. forbes, n. f. zhang, a. g. chunovkina (editors). world scientific, singapore, 2022, isbn 978-981-124-237-3, pp. 331– 345. doi: 10.1142/9789811242380_0020 [13] a. kennedy, programming languages and dimensions, ph.d. dissertation, university of cambridge, 1995. [14] c. c. sims, computation with finitely presented groups, cambridge university press, cambridge, 1994, isbn 978-051157-470-2. doi: 10.1017/cbo9780511574702 [15] m. wand, p. o'keefe, automatic dimensional inference, in: computational logic: essays in honor of alan robinson. j.-l. lassez, g. plotkin (editors), mit press, cambridge, massachusetts, 1991, isbn 978-0-262-12156-9, pp. 479–486. [16] a. gundry, type inference, haskell and dependent types, ph.d. dissertation, university of strathclyde, 2013. [17] b. d. hall, software representation of measured physical quantities, in: advanced mathematical and computational tools in metrology and testing xii. f. pavese, a. b. forbes, n. f. zhang, a. g. chunovkina (editors). world scientific, singapore, 2022, isbn 978-981-124-237-3, pp. 273-284. doi: 10.1142/9789811242380_0016 [18] bipm, the international system of units ('the si brochure') (ninth ed.). bureau international des poids et mesures, 2019. online [accessed 13 january 2023] http://www.bipm.org/en/si/si_brochure/ [19] b. d. hall, m. kuster, representing quantities and units in digital systems, measurement: sensors 23 (2022), 6 pp. doi: 10.1016/j.measen.2022.100387 https://doi.org/10.1145/3276604.3276613 https://doi.org/10.1007/978-3-642-17685-2_8 https://www.boost.org/doc/libs/1_81_0/doc/html/boost_units.html https://www.boost.org/doc/libs/1_81_0/doc/html/boost_units.html https://unitsofmeasurement.github.io/pages/about.html https://doi.org/10.1145/2633357.2633362 https://doi.org/10.1145/2804302.2804305 https://pint.readthedocs.io/ https://www.mathworks.com/help/symbolic/units-of-measurement.html https://www.mathworks.com/help/symbolic/units-of-measurement.html https://doi.org/10.1142/9789811242380_0020 https://doi.org/10.1017/cbo9780511574702 https://doi.org/10.1142/9789811242380_0016 http://www.bipm.org/en/si/si_brochure/ https://doi.org/10.1016/j.measen.2022.100387 primary shock calibration with fast linear motor drive acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 383 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 383 387 primary shock calibration with fast linear motor drive h. volkers1, h. c. schoenekess1, th. bruns1 1 physikalisch-technische bundesanstalt, braunschweig, germany, henrik.volkers@ptb.de abstract: this paper describes the implementation of a new, fast and precise linear motor drive for ptb’s primary shock calibration device. this device is used for monopole shock calibrations of accelerometers using the “hammer-anvil” principle according to iso 16063-13:2001 [1] and operates in a peak acceleration range from 50 m/s² to 5000 m/s². the main challenge of implementing this kind of shock generator is accelerating a hammer to velocities up to 5 m/s within distances of less than 70 mm. in this paper, a few helpful improvements are described which lead to an enhanced repeatability of pulse generation over the full shock intensity range as well as a substantial decrease of harmonic disturbing signals. keywords: primary shock calibration; hammeranvil principle; half-sine pulse; pulse transmission; ldv interferometry; linear motion drive; magnetic actuator 1 introduction ptb’s original shock exciter which was designed for low to medium acceleration intensities mainly consists of a mechanical spring unit, an airborne transmission element (the “hammer”) and an air-borne measuring unit (the “anvil”) to which the accelerometer to be calibrated is attached. the released spring pushes the hammer which subsequently strikes the anvil and accelerates it. exchangeable elastic pads (pulse shapers) between hammer and anvil allow certain shock shapes to be attained and disturbing oscillations to be reduced. using the spring drive unit, shock peak values can be varied between 100 m/s2 and 5000 m/s2, whereas the shock duration is between 8 ms and 1 ms. different sets of the modules (hammer, anvil and spring unit) are available, allowing different combinations of peak values and shock durations to be excited. the acceleration is measured by applying two laser doppler vibrometers (ldv). after a signal conditioning, both photo detector signals and the sensor measuring chain signal are simultaneously captured. following the signal demodulation, the primary measured acceleration peak and the sensor peak are used to determine the shock sensitivity according to 𝑆sh = 𝑢peak 𝑎peak (1) by definition [1], the shock sensitivity ssh is calculated as the quotient of the output charge peak value of the accelerometer and the peak value of the interferential measured shock acceleration (evaluation in the time domain). it is dependent on the impact spectrum, the duration of impact and the peak acceleration value. 2 the initial configuration the original driving mechanism to accelerate the hammer was realized by a spring-driven shaft with a free running length of up to 30 mm (c.f. figure 1). by manually setting the spring preload, the resulting hammer velocity was adjusted to generate the desired peak acceleration of the anvil. repeated loading and releasing of the spring was performed by motor-driven mechanics. a set of three spring modules with different stiffnesses provides the peak acceleration ranges needed. all of these spring units suffered from substantial drawbacks such as: the highest force acts at the start of motion, which results in a strong jerk. this introduces high frequency vibrations and excites resonances in the hammer and the anvil. the mechanical stop of the spring unit induces additional vibrations to the system. the force’s set-point has to be mechanically adjusted by hand. the repeatability of adjusted shock intensities is of only moderate quality. three different spring units have to be employed to cover the 100 m/s² to 5 km/s² range. shock intensities below 100 m/s² could not be achieved with the old spring units. http://www.imeko.org/ mailto:henrik.volkers@ptb.de acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 384 mechanical wear of the springs, locking knife edges and the release mechanism required frequent maintenance and readjustment. these were the main causes of poor repeatability. the calibration process involved a lot of manual interaction. the design and realization of this unit was the first of this type and was done in the 1990th of the last century [2, 3]. other nmis have set up similar devices in the meantime, sometimes with different drive methods [e.g. 4, 5]. to address the issues listed above, a project was started to update the driving unit specifically. the complete mechanical set-up of the calibration device is shown in figure 1. the manually adjustable spring unit is at the bottom. the air-borne hammer (the projectile) in the centre has a rectilinear motion distance of roughly 50 mm and is stopped after the impact with the anvil by mechanical dampers. the air-borne anvil (the target) at the top of the photograph has a freemotion distance of about 8 mm before it is halted by a pneumatic brake. figure 1: mechanics of the shock calibration device the impulse hammer element with a mass of approx. 0.25 kg hits the 0.25 kg anvil element with the mounted accelerometer (0.038 kg). the shock pulse is transmitted within a time that is shorter than 8 milliseconds. when the shortest duration (roughly 1 ms) is used, high accelerations can be achieved. the strongest spring with a maximum force of 400 n achieved more than 5 km/s² at the sensor’s reference surface, while the softest spring, with a minimum preload force of about 10 n, reached 100 m/s². via a suitably elastic pulse shaper at the tip of the hammer, a sin² acceleration time curve is generated and sensed by the accelerometer (c.f. fig. 2). several different pulse shapers made of rubber could be attached to the hammer to obtain the desired pulse duration and shape. figure 2: hammer and anvil heads with dut as discussed above, the mechanical spring unit proved to be a weak point in the old design. during the relaxation of the spring, strong vibrations were transmitted into the entire mechanical system, which led to disturbances. in addition, the knifeedge mechanics of the release mechanism suffered from wear and tear and caused a poor repeatability during calibration runs. the goal of this project was to overcome these issues using modern linear drive technology. 3 the new drive unit 3.1 requirements and design in order to ensure the full operation of the customer calibration service, it was necessary to have backstop conversion capability during the development and testing period. this resulted in dimensional design constraints. by applying laws of basic mechanics, the required mechanical specifications were easily derived from the involved moving masses, impact speeds and dimensions. subsequent market research covering mechanic, pneumatic, hydraulic and electro-magnetic actuators led to the decision to use an electric linear motor drive. the selected solution is a type ps01-23x160hhp-r linear motor with a pl01-12x350/310-hp magnetic slider as well as a type c1100 standard closed loop controller and a linmot® nti ag twohttp://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 385 phase power supply1 [6]. the package also includes software for the controller. this kind of drive was originally designed for so-called “fast pick and place” operation in industrial production lines. the use of such industrial equipment means that only moderate hardware costs are involved. in order to implement the drive, some modifications had to be applied to the calibration device such as: • construction of a damped stop for the runner of the linear motor (pusher). • a prolongation of the hammer and an increase of the distance between hammer and anvil air bearings. • an increase of the hammer’s mass (c.f. figure 3). the last two actions permitted the working length of the drive to be enlarged by about 72 mm. in the original set-up, similar hammer and anvil dimensions were chosen for an optimal momentum transfer during impact. this, however, also resulted in similar first modal resonances around 10 khz, which in turn favoured the propagation of disturbing oscillations during impact. the new hammer design reduces this effect by means of a (two times) heavier weight. table 1 summarizes the modified parameters. table 1: specifications of both impact generators change old state new state hammer weight 0.296 kg 0.595 kg anvil weight 0.296 kg 0.296 kg max. travel hammer 50 mm 72 mm max. travel anvil 4 to 7 mm 7 to 8 mm max. velocity hammer 5 m/s 5 m/s max. velocity anvil 5 m/s 6 to 7 m/s shock duration range 1 to 8 ms 1 to 8 ms max. drive force 400 n 138 n max. shock acceleration 5000 m/s² 5000+ m/s² these system modifications are a consequence of changing the hammer weight and travel parameters. the main improvements are found in the higher anvil velocity and, at the same time, the reduction of drive force to less than half. the latter greatly reduced the jerk and thus the issue of motion disturbances. 3.2 quasi-elastic central impact calculation the selected linear motor with a maximum propulsive force of fmot = 138 n serves as a new mechanical shock generator. the stroke s has been extended to 72 mm. the total mass to be accelerated consists of the runner’s 0.280 kg mass msl and the 1 commercial components are merely identified in this paper to adequately specify the experimental set-up. naming these products does not imply a recommendation by ptb, nor does it mass of the new air-borne hammer (mha = 0.595 kg). the mass of the anvil to be pushed by the hammer is man = 0.296 kg. the initial velocity reached by the accelerated hammer before impact is 𝑉𝐻𝑎 = √ 2∙𝐹𝑀𝑜𝑡∙𝑠 𝑚𝑠𝑙+𝑚𝐴𝑛 2 . ( 2) it attains up to 5 m/s. the moving hammer mha pushes the resting anvil man and transfers a large amount of its momentum to it. the mass ratio is mha/man = 2 and the anvil is initially at rest. applying the basic equations for momentum conservation, the speeds of the hammer and anvil after the impact become: 𝑉 ′𝐻𝑎 = 𝑚𝐻𝑎− 𝑚𝐴𝑛 𝑚𝐻𝑎+𝑚𝐴𝑛 ∙ 𝑉𝐻𝑎 (3) and 𝑉 ′𝐴𝑛 = 2 ∙ 𝑚𝐻𝑎 𝑚𝐻𝑎+𝑚𝐴𝑛 ∙ 𝑉𝐻𝑎 (4) or, with the given mass ratio: 𝑉 ′𝐻𝑎 = 1 3 ∙ 𝑉𝐻𝑎 and 𝑉 ′ 𝐴𝑛 = 4 3 ∙ 𝑉𝐻𝑎 (5) this transformation moves the anvil to a onethird higher speed than the hammer’s initial velocity. for an optimal hammer mass, one could insert (2) into the right hand side of (4) and solve for the maximum value. this gives an optimum hammer mass of 0.581 kg when all boundary conditions are considered. this is pretty close to the realized new hammer. figure 3 in the next section shows the new shock generator setup. 3.3 repeatability an additional advantage of this linear motor drive set-up is the option of preselecting an accurate target value by setting a potentiometer or using the software interface for digital adjustment. the controller of the linear motor can be parametrized and controlled via two serial interfaces by the supplied vendor software or a freely available labview driver which can be adapted to individual needs. for any given setting, the spread of the repeatedly realized acceleration peak values was drastically reduced compared to the original spring drive module. a comparison is depicted in figure 4. for both drives, the absolute range around the nominal value increases linearly with the shock imply that the equipment identified is necessarily the best available for the purpose. http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 386 intensity. however, for the new linear drive, the spread is more than an order of magnitude lower than before. figure 3: shock generator for 5000 m/s² with fine controllable linear motor drive figure 4: comparison of the statistical reach of acceleration intensity of spring drive vs. linear motor drive 3.4 signal quality the disturbances due to modal resonances of the anvil are reduced by the redesign of the hammer. while the resonance frequency of both the hammer and anvil was about 11.5 khz in the old setup, the first longitudinal resonance of the new hammer is now at 13 khz. because the design and materials were identical, this difference can be attributed to the elongated geometry and the solid design (no axial holes). this disparity greatly reduces the transmission of ringing vibrations from the hammer to the reference surface of the accelerometer. in combination with the largely reduced jerk in the driving motion itself, ringing is no longer an issue in the signal shape. as an example, figure 5 shows an accelerometer output signal (filtered at 100 khz) for an acceleration peak value of 1.97 km/s². figure 5: raw signal of a half-sine shock measurement by an endevco 2270 accelerometer 4 summary the primary monopole shock acceleration calibration facility at ptb has been successfully upgraded by the implementation of an industrial electric linear motor which was implemented as the driving unit for the hammer. the change in the driving mechanics and geometry involved adaptations to the hammer and anvil configuration, too. after successful implementation the following improvements were noticeable: 1. a strong reduction in disturbing vibrations due to a lower, but constant, force level generated by the new drive. 2. a very strong reduction in the transfer of mechanical ringing from the hammer to the anvil due to the weight disparity between the two. 3. great improvement of the repeatability of the shock intensity. http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 387 4. analog electrical or digital control of the set-point for the generated shock acceleration. 5. greatly reduced need for maintenance and operation is practically non-wear and has very little friction. 5 outlook based on the newly implemented drive a shift from manual operation to an automated performance of the low-intensity shock calibration seems feasible in the near future. the control options via a digital interface (rs-485) using labview or other programming languages together with the excellent repeatability allow, in principle, unsupervised operation. however, in the current set-up the pulse shaper remains as a component with unknown reproducibility. in order to achieve autonomous operation, some type of self-adjusting algorithm is necessary. first steps on the path towards such methods have been taken here and will be the topic of future publications. 6 references [1] iso 16063-13, https://www.iso.org/standard/27075.html [2] von martens, h.-j. taeubner a., wabinski w., link a., and. schlaak h.-j “laser interferometry as tool and object in vibration and shock calibrations”, proc. spie 3411, third international conference on vibration measurements by laser techniques: advances and applications, (1 june 1998); https://doi.org/10.1117/12.307700 [3] von martens h.-j., “ptb vibration and shock calibration”, royal swedish academy of engineering sciences (iva), 8 pages, 15-16 september 1993, stenungsund, sweden [4] nozato h., kokuyama w., ota a., “improvement and validity of shock measurements using heterodyne laser interferometer”, elsevier measurement, volume 77, pages 67-72, january 2016 [5] nozato h., usuda t., ota a., ishigami t. and kudo k., “development of shock acceleration calibration machine in nmij”, imeko 20th tc3, 3rd tc16 and 1st tc22 international conference cultivating metrological knowledge 27-30 november, 2007. merida, mexico. [6] linmot webpage https://linmot.com/ http://www.imeko.org/ https://www.iso.org/standard/27075.html https://doi.org/10.1117/12.307700 https://www.sciencedirect.com/science/article/pii/s0263224115004625#! https://www.sciencedirect.com/science/article/pii/s0263224115004625#! https://www.sciencedirect.com/science/article/pii/s0263224115004625#! https://www.sciencedirect.com/science/journal/02632241 https://www.sciencedirect.com/science/journal/02632241/77/supp/c https://linmot.com/ extended buffer zone algorithm to reduce rerouting time in biotelemetry systems using sensing acta imeko issn: 2221-870x march 2022, volume 11, number 1, 1 -7 acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 1 extended buffer zone algorithm to reduce rerouting time in biotelemetry systems using sensing bachina surendra babu1, satish kumar ramaraj2, karuganti phani rama krishna3, pinjerla swetha4 1 department of ece, bapatla engineering college, bapatla, guntur district, andhra pradesh, 522102, india, 2 department of medical electronics, sengunthar college of engineering, tiruchengode-637205, tamilnadu, india 3 dept of ece, pvp siddhartha institute of technology, vijayawada-520007, andhra pradesh, india 4 dept of ece, malla reddy college of engineering and technology, hyderabad,500100, telanagna, india section:research paper keywords: routing; buffer zone; measurement; rerouting time; virtual zone citation: bachina surendra babu, satish kumar ramaraj, karuganti phani rama krishna, pinjerla swetha, extended buffer zone algorithm to reduce rerouting time in biotelemetry systems using sensing, acta imeko, vol. 11, no. 1, article 26, march 2022, identifier: imeko-acta-11 (2022)-01-26 section editor: md zia ur rahman, koneru lakshmaiah education foundation, guntur, india received november 29, 2021; in final form march 4, 2022; published march 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: b. surendra babu, e-mail: surendrabachina@gmail.com 1. introduction a mobile adhoc network (manet) is a dynamic network made up of several nodes. an ad hoc is a self-contained system that operates without the assistance of a centralized authority. routing is a difficult operation outstanding to the flexibility of the nodes. the ad hoc changing architecture causes frequent route breakups. route failure has an impact on network connection. furthermore, the nodes are reliant on the partial battery power. a lack a power in any node can lead to network apportioning [1]. routing is the main function to guide communication across extensive networks. the basic duty of every routing is to find and preserve routes to the network destinations necessary. ad hoc network routing protocols are distributed into two types such as reactive and proactive and. proactive is the circumstance in which a node may send data to a certain terminus as soon as it receives it. a reactive routing protocol, on the other hand, determines a route as and when it is requested by a network node [2]. this article is about mobile adhoc networks that employ a proactive routing system. because of their dynamic topology, link breaks are a typical feature of mobile adhoc networks. in such circumstances, the routing protocol must seek alternate routes [3]. the rerouting interval is the period earlier new pathways are identified, and the rerouting time is the period of the rerouting interim [4]. stale routes exist across the broken connection during the rerouting interval. only when the routing protocol detects that the connection is damaged can rerouting take place. in fact, detecting the connection break accounts for a considerable portion of the rerouting time [5]. in summary, the rerouting time due to connection breakdowns is resolute by the time obligatory to complete the following processes: • link break detection. • network-wide linking-state notification of new pathways • draining of everything stale packets from output queue. the basic duty of every routing protocol is to find and preserve routes to the network endpoints necessary [6], [7]. ad abstract mobile adhoc network (manet) routing methods must deal with connection breakdowns caused by frequent node movement, measurement and a dynamic network topology. in these cases, the protocol must discover alternate routes. rerouting time refers to the lag that happens during this retransmission. researchers have proposed many ways to reduce rerouting period. one such technique is buffer zone routing (bzr), which divides a node transmission region into a safe zone adjacent to the node and a hazardous zone towards the end of the broadcast range. this technique, however, has rare gaps and restrictions, such as the ideal dimensions of the buffer zone, a rise in hop duration, network stress, and so on. this study offers a method to improve or expand buffer zone communication by grouping nodes inside the buffer zone into virtual zones based on their energy flat. when the routing decisions are made quickly, the energy consumption of the nodes is minimized. in the safe area of extended bzr, transfer time is reduced, and routing efficiency is increased. it solves issues in the present algorithm and fills holes in it, decreasing the time required for rerouting in manet. mailto:surendrabachina@gmail.com acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 2 hoc network routing protocols are separated into two classes such as, proactive and reactive [8]. this article is about manet environment that employ a proactive routing system. because of their dynamic topology, link breaks are a typical feature of manets. in such circumstances, the routing protocol must seek alternate routes. the rerouting interval is the period earlier new pathways are identified, and the redirecting time is the rerouting break period [9]. stale routes exist across the broken connection during the rerouting intermission. only when the routing protocol detects that the connection is damaged can rerouting take place. in fact, detecting the connection break accounts for a considerable portion of the rerouting time [10]. in summary, the rerouting period due to connection breakdowns is determined by the time essential to complete the following processes. in research work, bzr interact and update their routing tables once the nodes are live. in addition to the neighbouring information, the virtual zone information of the node in ebzr is also efficient in the routing table. this data is then used during the choice to route. finally, the routing could be achieved by continuously measuring the available zones, interference levels etc. the remainder of the paper is structured as trails. section ii first provides the different linked works. section iii describes the background of the optimized link state routing (olsr), the zone routing algorithm (zra), and link breakage and redirection time. the proposed method is obtainable in section iv, along with a comprehensive discussion and its benefits. simulation results are provided in section v, with the performance comparison charts of the standard olsr, buffer and virtual zone algorithm. lastly, in section vi, the scope for areas of future work is outlined out, with the conclusion. 2. related work this article is a continuation of [11] which examined mobile adhoc network routing protocols and metrics. it was discovered that rerouting time is a significant enactment measure. other authors, [12] offer an adaptive retry edge technique to decrease rerouting period. it also identifies stand in line as one of the key variables having a significant influence on rerouting time. the recommended remedy was also put into action and tested. g. jisha and s. dhanya [13] offer a review of different zone-based routing enhancements in mobile adhoc networks. other than the fundamental protocol implementation, this article performs a poll on ten other enhancements offered to zone routing protocol (zrp). it also analyses these enhancements and recommends apps that are best suited to them, [14]. this article implements a virtual zone-based routing enhancement technique that is described in this study. the authors present a novel zonebased routing algorithm for manet in [15]. to discover numerous stable routes among the source and destination nodes, the proposed technique integrates the concept of a buffer zone with the olsr procedure offer a trust-based computation method for improving the security of zone-based routing. when a lot of net nodes disobey and lose data packets, the performance of a vulnerable zrp is tested. the olsr is protracted in [16] to preserve node energy. in the qualnet simulator, the presentation of the projected system is evaluated in terms of control overheads, energy ingesting, end-to-end latency, and packet delivery ratio (pdr). the author from [17] have investigates the queuing problem and proposes methods to decrease queue stagnation. the author from [18] investigates the adaptive retry limit approach of eliminating the queueing problem in mobile adhoc networks. it also proposes a solution of asynchronous invocation, where the gaps in the adaptive retry limit approach are addressed. as a result, queueing is eliminated, and the rerouting time is reduced in mobile adhoc network routing. the simulation findings show that when the buffer zone communication is implemented in olsr, the rerouting time is decreased. when associated to regular olsr, the addition of a transmission buffer zone improves throughput. the use of a buffer zone is advantageous in both traffic situations of low and high [19]. 3. background 3.1. olsr the olsr is active in nature and routes for all network destinations are accessible immediately at each node. the open shortest path first is an optimisation of the pure link formal routing protocol (ospf). this method is linked to the multipoint relay idea (mpr). a multi-point relay minimizes the control messages' size [20]. the usage of mprs also reduces control traffic flooding. multipoint transmits forward control memos that give the benefit of lowering the sum of transmitted broadcast control memos. the olsr has two main features: neighbor detection and topology distribution. each node constructs routes with these two attributes for all known destinations. the hello and (topology control) tc are the two most significant messages in olsr: 1) hello messages: each node transmits hello messages on a regular basis to allow connection sensing, neighbour discovery and mpr selection signalling. the suggested hello messages emission intermission is 2 seconds and neighbour info time are 6 secs. a neighbour is deemed lost 6 secs after the neighbour received the last hello note. 2) tc messages: the link state (tc) messages are generated and transmitted via the system by every mpr, based on the information collected via hello messages. for tc communications, the optimum emission interval is 5 seconds and the hold duration is 15 seconds. 3.2. zone routing algorithm this method relies on the definition of nodes as safe or insecure, and whichever use as relay nodes if they are benign or avoid as relay nodes if they are insecure. in addition, if possible, traffic to hazardous nodes within the transmission region of the transmitting node should be tried through secure nodes. the signal power of the hello packets may be used as a criterion to identify in which nodes and what zones with different mobility speeds are regarded acceptable and unsafe. to support surrounding nodes in the routing of their unsafe neighbours, each link admission in the hello packets needs to be included in the zone status and communicated to other neighbours. a packet must not be routed to a relay node that has an unsafe neighbour as its destination. the routing table for every node is chief designed on the basis of the safety zone nodes. if this leads to dividing, it is included in the routing table to cross nodes in the dangerous zone. the buffer zone routing theory applies exclusively to the nodes in the harmless zone as realized in figure 1. the nodes in the dangerous bzr must only be utilized to forward if complete connectivity without them is impossible. as the neighbour defines the two-hop neighbour and topology set and no route modifications are allowed on the already specified routes. if a node is already shown as a acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 3 destination in the routing database, the afresh identified route to the similar destination will be ignored, even if it has fewer hops than its first route. in figure 2 the phases of the buffer zone routing method are depicted. 3.3. rerouting time one typical feature of ad hoc systems is that connections may break because of variations in radio circumstances, node mobility and other dynamic network. in these cases, the routing is meant to locate other paths. the period before novel paths is discovered is termed the recirculating time, and the recirculating time is called the recirculating time. take the experiment represented in figure 3, i.e., the period since the disruption of the link between a and c (1) to the re-establishment of connection via the intermediary node b (2). 3.4. queueing and rerouting time different conditions affect the timing of return in manet. queueing is one such scenario. the process of queuing packets in each layer is called queueing due to an enhanced transmission rate. in this circumstance, the packages are sequentially processed, which increases the redirection time. this problem has a layered solution, offering an adaptive retry limit to the mac layer. the retry limit shall be reduced by 1, for every packet with the same mac objective, which is lost by reaching the retry limit until every packet is transmitted once. once a packet is transferred effectively, the trial limit is reverted to its unique value that is equivalent to the former standard. two parameters significantly determine the latency associated with queuing, notably the size of the tail and the retry boundary. a large transmission queue size can lead to too many waste packages with stalled routing information. furthermore, too many waste retransmission attempts can result in a high retry number for these waste packages. the grouping of these factors could significantly extend the redirection time. figure 4 presents an overview of each layer's protocol stack and queues. 3.5. factors affecting rerouting time despite so many elements such node energy levels, transmission ranges, network structure, etc., affecting retraining figure 1. (a). communication area zones of node (b) safe node (c) unsafe node. figure 2. the zone routing algorithm. figure 3. rerouting time. figure 4. linux protocol stack. start clear routing table add routing to all neighbours (only in safe zone) if first time? add route to all topology tuplets with increasing hop count (only in safe zone) add two hop neighbours both are in safe zone exit acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 4 times, node speeds and traffic loads have a greater impact on the performance of this mobile adhoc network characteristic. node speed: reducing the node speed minimizes the amount of link breakage. the routing time grows as the node speed increases. with lowered speed, due to the lower node speed, the risk of a connected break due to an adjacent affecting out of the communication zone is lower. this makes it easy to adjust the threshold range higher to gain the same advantage while reducing the downside of greater track lengths. traffic load: if the whole network is overloaded by a huge number of unneeded broadcasts and an amplified packet loss, the successful transmission leaves a reduced part of the overall network capacity. in the event of link breakdowns, the combination of divisioning and decreased retransmissions increases the output at the lower levels. but the segmentation means that the packets will probably only reach a insufficient hops, exacerbating the injustice among the short and extended path traffic [20]. in short, the re-routing time is openly proportionate to the node speed and traffic load, which would increase the speed and traffic load of the node. 3.6. link breaks and rerouting time link breaks are the cause of the queueing scenario. when a node misplaces its transfer to its neighbour, the routing searches for the shortest alternative available path. in order to avoid such catastrophes, these connection disruptions have to be identified considerably sooner. the details of this preventive mechanism are outside the possibility of this paper. the standard technique to detect connection breakdowns for a routing is finished missed polling packets [21]. the olsr hello packets are then sent among one hop neighbours at the set time frequency to provide information about neighbourhood links and a method for detecting links. when no neighbour hello packet is received within a specific time interval (for example, within 6 seconds, the suggested olsr interval) the neighbour shall be deliberated inaccessible and a link to the neighbour shall be judged broken and invalid. another technique to identify connection failures is by delegating the routing protocol in the underlying link layer to a mechanism. a link break thru the link layer must be expressly notified of the routing protocol. the drawback of this link layer notification (lln) method may be the extra application complexity. the benefits, however, are that the connection layer usually detects the connection breaks earlier. used to detect interruptions through missing hello packets is the buffer zone or bza. the overall performance is vital to recognize the connection break in a timely manner because two negative impacts occur between both the physical connection breakdown and the routing protocol. main, the packets are marked with the next unachievable hop address in the queue of the interface. this means that at this time these packets are never reached and lost. second, these packets are sent numerous times to the mac layer until they are castoff. this will steal packets sent from other nodes of valuable intermediate time at a legal next hop address. 4. proposed solution 4.1. analysis there is a variance among both the buffer zone solution and regular olsr in the average sum of hops among a source and an endpoint. with the buffer zone solution, the number of hops per way is enhanced, as it prefers nodes in the safe zone as relay nodes. the hop length increase is the biggest drawback of the buffer zone key. chief, the hop length increases the number of transmissions required for the identical end-to-end streams and so reduces the overall capacity accessible per traffic stream. secondly, the danger increases as the links get longer that the topology info possessed by the transmitting nodes is incorrect. there is a greater likelihood that the ideal (both the actual and the tables presented) changes while the packet travels between both communication nodes. this increases the danger of a packet loop or a significant detour. an increased average path duration, an increase in packet reverse risk and an increased packet loop risk add to the likelihood of time-to-live (ttl) depletion. however, too much of a buffer zone indication to an unnecessarily larger mean sum of hops among the manet node pairs and a greater risk of network partitions. the bza is also averse to finding a way to predict the ideal size of the buffer zone, according to criteria such as network load and node mobility. finally, the buffer zone technique may be enhanced and extended to categorize a neighbour as safe or unsafe utilizing distance separate criteria [22]. these shortcomings and gaps allow the buffer zone algorithm to be enhanced or extended. 4.2. extending buffer zone algorithm this section describes the key characteristics of the bzr mechanism. they interact and update their routing tables once the nodes are live. then every node will know its individual neighbours, double and multiple. in addition to the neighbouring information, the virtual zone information of the node is also efficient in the routing table. this data is then used during the choice to route. virtual zones are animatedly constructed based on node similarities. a virtual zone generation could be carried out whenever the initial topology changes or can be carried out periodically. in virtual zones, the arrangement of notion of nodes within the safe zone is presented in figure 5. whenever a node leaves a virtual zone, it broadcasts it via the hello packet to its neighbours. the neighbours then update this evidence in their routing databases. as a result, the nodes are informed of their virtual zone at any given time. 4.3. exploring the benefits of the virtual zone algorithm as all nodes and other info on their routing table know all the information about the virtual zone, routed is optimized within the virtual zone and thereby reduces the overall number of hops and the redirection time to a minimum. this solution associations the benefits of the area routing algorithm with the benefits of virtual area induction. the energy equal of the nodes is also assessed in this approach apart from the distance from the nodes employed in the field routing algorithm alone. the energy consumption of the nodes is also reduced as routing decisions figure 5. nodes in virtual safe zone. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 5 are made quickly. this method increases routing efficiency and minimizes transfer time in the safe area. if the source and endpoint nodes are inside the similar virtual area and if both communication nodes are in a safe zone, it would be the worst scenario. if the neighbour or any of the multi-hop neighbours is in the simulated field, the source or destination would be average. 5. discussion after comprehending the upgrade to the present buffer zone, virtual districts may also be lengthy to the insecure zone, which would cut re-routing time ultimately. there are a few reasons behind this statement. although virtual area upsurges redirect time, it is an overhead to maintain the routing table and virtual area(s). secondly, introducing virtual areas in the buffer zone increases network traffic. it is known that the nodes communicate using hello packets concerning their virtual zone location. in order to retain virtual zone information, the number of packets delivered is increased. the packet size is also enhanced as it contains virtual zone information. given the node energy levels, the virtual zones are currently limited only to the safe zone. however, it is left open for future effort to extend them into the dangerous zone without hurting performance. another effort would be that the creation of virtual regions eventually leads to the zrp, which seeks to resolve proactive and reactive protocol constraints by incorporating the best features of both protocol technology and therefore can be characterised as a proactive or reactive hybrid routing protocol. the majority of communication between knots can be envisaged in a manet. when in the zone, the packet is transmitted proactively. the projected method is simply an allowance of the remaining area routing algorithm and is limited to the shortest path and proactive routing. the improvement is to lessen the time of return solely. this differs from the zrp, which employs reactive and proactive methods. 6. simulation results this section presents the simulation arrangement and then documents the findings. assessment charts between olsr standard, buffer zone and virtual zone are presented. the parameter settings for the simulator utilized throughout the simulations are provided in table 1. the simulations were conducted using the 2.34 version of ns-2 network simulator. the optimized link state routing protocol (olsr)[9] uses multihop routing with the ieee 802.11 scheme is used as the mac layer. each group node relayed packets to the other group nodes. the nodes were split into two equally huge groups. the traffic category was udp, with a constant bit rate of 64 bytes. figure 11 demonstrates the good performance results with a relative high node speed of 10 m/s and a little overall traffic load of 50 kbps. compared with the results of a virtual zone procedure at thresholds of less than 250 m with that of a standard olsr buffer zone algorithm, which equals the bza with a threshold of 250 meters, the increase would be 84 percent − 75 percent = 9 percent on a buffer-zone algorithm at 220 m. figure 6 shows that the main benefit of the algorithm virtual zone is that the loss of retransmission (ret) has been significantly decreased compared to the buffer zone ret loss and regular olsr. this again leads to further packages being lost because of the missing route (nrte loss), as shown in figure 7. most link breakage are then avoided because packets can still be received by nodes beyond the discard zone and reply with a recognition prior to the neighbour moving beyond a transmission radius. thus, the ret figure 6. ret loss (loss caused by mac extreme retransmissions reject). figure 7. nrte loss (loss affected by lack of route). figure 8. ttl loss (loss affected by exhausted time to live). table 1. parameter situations. parameters values radio-propagation tworayground interface queue fifo with droptail and priqueue interface queue size 300 packets antenna ideal omniantenna olsr hello interval 2 s maximum mac retries 7 traffic ttl 32 olsr hello timeout 6 s nominal transmission rate 2 mbps basic rate 1 mbps simulation period 500 s simulation period heuristic olsr tc interval 5 s olsr tc timeout 15 s acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 6 loss, also for the virtual zone approach, is decreased as seen in figure 6. this advantage is bigger than the advantage of a higher probability of partitioning, which results in a fully higher output than the olsr standard and buffer zone algorithm as shown in figure 11. it is noteworthy to look at the cost of this approach, since the reduction in ret loss has been recognized as the key advantage of the virtual zone method. figure 7 indicates that in terms of packets lost owing to a lack of routing, there is no transformation among the virtual area, buffer zone and normal olsr solutions. thus, the virtual zone algorithm is not increased compared to normal olsr and buffer zone by the odds of network partitioning. this is predicted because, if necessary, the buffer zone algorithm builds connections with neighbours in the buffer zone. the buffer zone solution and the virtual zone differ, however, in terms of the average sum of hops among a both nodes. as illustrated in figure 9, the sum of hops per path is augmented with the buffer zone approach, as the safety zone nodes as relay nodes are favoured. the hop longer is the biggest drawback of the buffer zone solution. this was lowered after the virtual zone was included into the safe zone. a higher average path length, more packet detour risk, and an increased packet loop risk add to the likelihood of exhaustion from time-to-live (ttl). indeed, because of the exhaustive time to live ratio (i.e., loss of ttl) of packets, the buffer zone technique is higher than the usual olsr, as shown in figure 8. figure 12 indicates that the virtual area method boosts the output, even under heavy traffic loads, as compared with the standard olsr and buffer zone results. however, compared with the virtual and buffer zone methods, they are more or less comparable. however, for such large traffic loads, the total profit of the virtual zone solution is inevitably lower. the reason for this is that the whole network is being undermined by a great increased packet loss which leaves the successful diffusion of traffic, whether by standard olsr traffic or traffic using a bza, with fewer parts of the total network capacity. in addition to a reduced average distance, the cost of the solution in the virtual zone also includes a higher routing burden for a higher payload of hello messages. this is because the virtual zone solution depends on the neighbouring nodes' zone status in hello messages being published. as in figure 10, the increase in routing load for 40 nodes at 10 m/s is about 3 kbps at a threshold of 250 m, which is rather low over the whole network capacity. 7. conclusion entering virtual zones in the buffer zone in the olsr improves the performance of the buffer zone alone in comparison with the olsr. when transmission takes place within virtual zones, the retraining time is also significantly decreased. this paper discusses the rationale of confining the virtual zone within the safe zone. there is also talk of the difference among the virtual zone and the area routing protocol. the approach proposed is simulated using ns 2.34 and experimentations are undertaken to demonstrate that the performance of the virtual zones is increased. comparison charts indicate that the redirection time is reduced when virtual zones are inducted. extending virtual areas to the dangerous area would be a stimulating piece of work. in addition to the node criterion supplied to construct a virtual zone, further criteria for the creation of virtual zones could be introduced in future. by improving both control traffic performance and delay of proposed zrp, the techniques can be applied to single or multiple channels of manets. it could also be proven that such additional criteria recover the routing presentation of the manet. although the routing speed with virtual areas is increased, maintaining the routing table is an upstairs due to the great mobility of the nodes. this creates the method difficult. as future work, a slight improvement in weight in the current technique is also left. the current approach merely limits the construction of the virtual zone to the safe zone of the bza. figure 9. average number of hops. figure 10. routing load. figure 11. goodput for 1 and 10 m/s at 50 kbps load. figure 12. goodput for 1 and 10 m/s at 500 kbps load. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 7 references [1] a. mohammed munawar, p. sheik abdul khader, a comprehensive analysis on mobile adhoc network routing metrics, protocols and simulators, proceedings of national conference on advances in computer applications, 28(2012), isbn: 978-93-80769-16-5. 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https://doi.org/10.21014/acta_imeko.v6i3.454 https://www.sciencedirect.com/science/article/abs/pii/s0263224121010393#! https://www.sciencedirect.com/science/article/abs/pii/s0263224121010393#! https://www.sciencedirect.com/science/article/abs/pii/s0263224121010393#! https://www.sciencedirect.com/science/article/abs/pii/s0263224121010393#! https://doi.org/10.1016/j.measurement.2021.110119 https://www.sciencedirect.com/science/article/abs/pii/s0263224121009994#! https://doi.org/10.1016/j.measurement.2021.110077 introductory notes for the acta imeko third issue 2022 acta imeko issn: 2221-870x september 2022, volume 11, number 3, 1 3 acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 1 introductory notes for the acta imeko third issue 2022 francesco lamonaca1 1 department of department of computer science, modeling, electronics and systems engineering (dimes), university of calabria, ponte p. bucci, 87036, arcavacata di rende, italy section: editorial citation: francesco lamonaca, introductory notes for the acta imeko third issue 2022, acta imeko, vol. 11, no. 3, article 1, september 2022, identifier: imeko-acta-11 (2022)-03-01 received september 28, 2022; in final form september 28, 2022; published september 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: francesco lamonaca, e-mail: editorinchief.actaimeko@hunmeko.org dear readers, the third issue 2022 of acta imeko collects contributions related to two events organized by imeko tc17, the imeko technical committee on robotic measurement [1]-[9], and, as usual, papers that do not relate to a specific event and are collected into the general track [10]-[18]. annually tc17 organizes the "international symposium on measurements and control in robotics" (ismcr), a full-fledged event, focusing on various aspects of international research, applications, and trends related to robotic innovations for benefit of humanity, advanced human-robot systems, and applied technologies, e.g. in the allied fields of telerobotics, telexistance, simulation platforms and environments, and mobile work machines as well as virtual reality (vr), augmented reality (ar) and 3d modelling and simulation. the introduction to the papers relatd to this event is given in the editorial authored by prof. zafar taqvi, organizer of this special issue. as editor in chief, it is my pleasure to give readers an overview of general track papers, with the aim of encouraging potential authors to consider sharing their research through acta imeko. additive manufacturing (am) is becoming a widely employed technique also in mass production. in this field, compliances with geometry and mechanical performance standards represent a crucial constrain. since 3d printed products exhibit a mechanical behaviour that is difficult to predict and investigate due to the complex shape and the inaccuracy in reproducing nominal sizes, optical non-contact techniques are an appropriate candidate to solve these issues. in the paper “measurement of the structural behaviour of a 3d airless wheel prototype by means of optical non-contact techniques”[10] by antonino quattrocchi et al., the 2d digital image correlation and thermoelastic stress analysis are combined to map the stress and the strain performance of an airless wheel prototype. the innovative airless wheel samples are 3d-printed by fused deposition modelling and stereolithography in poly-lactic acid and photopolymer resin, respectively. the static mechanical behaviour for different wheel-ground contact configurations is analysed using the aforementioned non-contact techniques. moreover, the wheel-ground contact pressure is mapped, and a parametric finite element model is developed. the results presented in the paper demonstrate that several factors have a great influence on 3d printed airless wheels: a) the material used for manufacturing the specimen, b) the correct transfer of the force line (i.e., the loading system), c) the geometric complexity of the lattice structure of the airless wheel. the work confirms the effectiveness of the proposed noncontact measurement procedures for characterising complexshaped prototypes manufactured using am. body impedance analysis (bia) is used to evaluate the human body composition by measuring the resistance and reactance of human tissues with a high-frequency, low-intensity electric current. nonetheless, the estimation of the body composition is influenced by many factors: body status, environmental conditions, instrumentation, and measurement procedure. valerio marcotuli et al., in “metrological characterization of instruments for body impedance analysis”, [11] present the results of a study about the effect of the connection cables, conductive electrodes, adhesive gel, and bia device characteristics on the measurement uncertainty. tests were initially performed on electric circuits with passive elements and on a jelly phantom simulating the body characteristics. results showed that the cables mainly contribute to increase the error on the resistance measurement, while the electrodes and the adhesive introduce a negligible disturbance on the measurement chain. the authors also propose a calibration procedure based on a multivariate linear regression to compensate for the systematic error effect of bia devices. sergio moltò et al. in the paper “uncertainty in mechanical deformation of a fabry-perot cavity due to pressure: towards best mechanical configuration” [12] present a study about the deformation of a refractometer used to achieve a quantum realization of the pascal. first, the propagation of the uncertainty in the pressure measurement due to mechanical deformation was assessed. then, deformation simulations were carried out with a cavity designed by the cnam (conservatoire national des arts mailto:editorinchief.actaimeko@hunmeko.org acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 2 et métiers). this step aims to corroborate the methodology used in the simulations. the assessment of modal components is a fundamental step in structural dynamics. while experimental investigations are generally performed through full-contact techniques, using accelerometers or modal hammers, the research proposed in the paper entitled “frequency response function identification using fused filament fabrication-3d-printed embedded aruco markers” [13], by lorenzo capponi et al, presents a non-contact frequency response function identification measurement technique based on aruco square fiducial markers displacement detection. a video of the phenomenon to be analyzed is acquired, and the displacement is measured through markers, using a dedicated tracking algorithm. the proposed method is presented using a harmonically excited fff 3d-printed flexible structure, equipped with multiple embedded-printed markers, whose displacement is measured by an industrial camera. comparison with numerical simulation and an established experimental approach is finally provided for the validation of the results. human movement modeling also referred to as motioncapture is a rapidly expanding field of interest for medical rehabilitation, sports training, and entertainment. motion capture devices are used to provide a virtual 3-dimensional reconstruction of human physical activities employing either optical or inertial sensors. using inertial measurement units and digital signal processing techniques offers a better alternative in terms of portability and immunity to visual perturbations when compared to conventional optical solutions. in the paper “lowcost real-time motion capturing system using inertial measurement units” [14], simona salicone et al. propose a cablefree, low-cost motion-capture solution based on inertial measurement units with a novel approach for calibration. the goal of the proposed solution is to apply motion capture to the fields that, because of cost problems, did not take enough benefit of such technology (e.g., fitness training centers). according to this goal, the necessary requirement for the proposed system is to be low-cost. therefore, all the considerations and all the solutions provided in this work have been done according to this main requirement. maximum-power extrapolation (mpe) techniques adopted for 4g and 5g signals are applied to systems using dynamic spectrum sharing (dss) signals generated by a base station and transferred to the measurement instruments through an air interface adapter to obtain a controlled environment. this allowed to focus the analysis on the effect of the frame structure on the mpe procedure, excluding the random effects associated to fading phenomena affecting signals received in real environments. the analysis presented by sara adda et al in the paper “experimental investigation in controlled conditions of the impact of dynamic spectrum sharing on maximum-power extrapolation techniques for the assessment of human exposure to electromagnetic fields generated by 5g gnodeb” [15] confirms that both the 4g mpe and the proposed 5g mpe procedure can be used for dss signals, provided that the correct number of subcarriers in the dss frame is considered. michela albano et al., in the paper entitled “x-rays investigations for the characterization of two 17th century brass instruments from nuremberg”, [16] propose a multidisciplinary approach mainly based on non-invasive analytical techniques and including x-rays investigations (x-ray radiography, x-ray fluorescence and x-ray diffraction) for the study of two brass natural horns from the end of the 17th recently found in castello sforzesco in milan (italy). these findings brought new information about this class of objects; actually, even though the instruments were heavily damaged, their historical value was great. the study proposed in the paper was aimed at: i) pointing out the executive techniques for archaeometric purposes; ii) characterizing the morphological and chemical features of materials; iii) identifying and mapping the damages of the structure and the alterations of the surface. in the paper “non-destructive investigation of the kyathos (6th-4th centuries bce) from the necropolis volna 1 on the taman peninsula by neutron resonance capture and x-ray fluorescence analysis” [17] nina simbirtseva et al. propose the method of neutron resonance capture analysis (nrca) to determine the elemental and isotope compositions of objects non-destructively, which makes it a suitable measurement tool for artefacts analysis without sampling. the method is currently being developed at the frank laboratory of neutron physics. nrca is based on the registration of neutron resonances in radiative capture and on the measurement of the yield of reaction products in these resonances. the potential of nrca at the intense resonance neutron source facility is demonstrated on the investigation of a kyathos from the necropolis volna 1 (6th-4th centuries bce) on the taman peninsula. in addition, x-ray fluorescence (xrf) analysis was applied to the same object. the elemental composition determined by nrca is in agreement with xrf data. a power system in which the generation units such as renewable energy sources and other types of generation equipment are located near loads, thereby, reducing operation costs and losses and improving voltage is referred to as ‘distributed generation’ (dg), and these generation units are named ‘distributed energy resources’. however, dgs must be located appropriately to improve the power quality and minimize power loss of the system. the objective of the paper entitled “performance enhancement of a low-voltage microgrid by measuring the optimal size and location of distributed generation” [18] by ahmed jassim ahmed et al., is to propose an approach for measuring the optimal size and location of dgs in a low voltage microgrid using the autoadd algorithm. the algorithm is validated by testing it on the ieee 33-bus standard system and, compared with previous studies, the algorithm proved its efficiency and superiority on the other techniques. a significant improvement in voltage and reduction in losses were observed when the dgs are placed at the sites selected by the algorithm. therefore, autoadd was used in finding the optimal size and location of dgs in the distribution system; then, the possibility of isolating the low voltage microgrid is discussed by integrating distributed generation units and the results showed the possibility of this scenario during faults time and intermittency of energy time. also in this issue, high quality and heterogeneous papers are presented, confirming acta imeko as the natural platform for disseminating measurement information and stimulating collaboration among researchers from many different fields. in particular, the technical note shows how acta imeko is the right place where different opinions and point of views can meet and compare, stimulating a fruitful and constructive debate in the scientific community of measurement science. i hope you will enjoy your reading. francesco lamonaca editor in chief acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 3 references [1] a. alsereidi, y. iwasaki, j. oh, v. vimolmongkolporn, f. kato, h. iwata, “experiment assisting system with local augmented body (easy-lab) in dual presence environment”, acta imeko, vol. 11, no. 3, pp. 1-6. [2] s. olasz-szabó, i. harmati, “path planning for data collection robot in sensing field with obstacles”, acta imeko, vol. 11, no. 3, pp. 1-6. [3] p. singh matharu, a. ashok ghadge, y. almubarak, y. tadesse, “jelly-z: twisted and coiled polymer muscle actuated jellyfish robot for environmental monitoring”, acta imeko, vol. 11, no. 3, pp. 1-7. [4] j. wolf rogers, k. alexander, “standards and affordances of 21stcentury digital learning: using the experience application programming interface and the augmented reality learning experience model to track engagement in extended reality”, acta imeko, vol. 11, no. 3, pp. 1-6. [5] e. e. cepolina, a. parmiggiani, c. canali, f. cannella, “disarmadillo: an open source, sustainable, robotic platform for humanitarian demining”, acta imeko, vol. 11, no. 3, pp. 1-8. [6] i. zobayed, d. miles, y. tadesse, “a 3d-printed soft orthotic hand actuated with twisted and coiled polymer muscles triggered by electromyography signals”, acta imeko, vol. 11, no. 3, pp. 1-8. [7] r. singh, s. mohapatra, p. singh matharu, y. tadesse, “twisted and coiled polymer muscle actuated soft 3d printed robotic hand with peltier cooler for drug delivery in medical management”, acta imeko, vol. 11, no. 3, pp. 1-6. [8] z. kovarikova, f. duchon, a. babinec, d. labat, “digital tools as part of a robotic system for adaptive manipulation and welding of objects”, acta imeko, vol. 11, no. 3, pp. 1-8. [9] a. v. geetha, t. mala “arel – augmented reality–based enriched learning experience”, acta imeko, vol. 11, no. 3, pp. 15. [10] a. quattrocchi, d. alizzio, l. capponi, t. tocci, r. marsili, g. rossi, s. pasinetti, p. chiariotti, a. annessi, p. castellini, m. martarelli, f. freni, a. di giacomo, r. montanini, “measurement of the structural behaviour of a 3d airless wheel prototype by means of optical non-contact techniques”, acta imeko, vol. 11, no. 3, pp. 1-8. [11] v. marcotuli, m. zago, a. p. moorhead, m. vespasiani, g. vespasiani, m. tarabini, “metrological characterization of instruments for body impedance analysis”, acta imeko, vol. 11, no. 3, pp. 1-7. [12] l s. moltó, m. a. sáenz-nuño, e. bernabeu, m. n. medina, “uncertainty in mechanical deformation of a fabry-perot cavity due to pressure: towards best mechanical configuration”, acta imeko, vol. 11, no. 3, pp. 1-8. [13] l. capponi, t. tocci, g. tribbiani, m. palmieri, g. rossi, “frequency response function identification using fused filament fabrication-3d-printed embedded aruco markers”, acta imeko, vol. 11, no. 3, pp. 1-6 [14] s. salicone, s. corbellini, h. v. jetti, s. ronaghi, “low-cost realtime motion capturing system using inertial measurement units”, acta imeko, vol. 11, no. 3, pp. 1-9. [15] s. adda, t. aureli, t. cassano, d. franci, m.d. migliore, n. pasquino, s. pavoncello, f. schettino, m. schirone “experimental investigation in controlled conditions of the impact of dynamic spectrum sharing on maximum-power extrapolation techniques for the assessment of human exposure to electromagnetic fields generated by 5g gnodeb”, acta imeko, vol. 11, no. 3, pp. 1-7. [16] m. albano, g. fiocco, d. comelli, m. licchelli, c. canevari, f. tasso, v. ricetti, p. cofrancesco, m. malagodi, “x-rays investigations for the characterization of two 17th century brass instruments from nuremberg”, acta imeko, vol. 11, no. 3, pp. 1-7. [17] n. simbirtseva, p. v. sedyshev, s. mazhen, a. yergashov, a. yu. dmitriev, i. a. saprykina, r. a. mimokhod, “non-destructive investigation of the kyathos (6th-4th centuries bce) from the necropolis volna 1 on the taman peninsula by neutron resonance capture and x-ray fluorescence analysis”, acta imeko, vol. 11, no. 3, pp. 1-6. [18] a. j. ahmed, m. h. alkhafaji, a.j. mahdi, “performance enhancement of a low-voltage microgrid by measuring the optimal size and location of distributed generation”, acta imeko, vol. 11, no. 3, pp. 1-8. integrating maintenance strategies in autonomous production control using a cost-based model acta imeko issn: 2221-870x september 2021, volume 10, number 3, 156 166 acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 156 integrating maintenance strategies in autonomous production control using a cost-based model robert glawar1, fazel ansari1,2, zsolt jános viharos3,4, kurt matyas2, wilfried sihn1,2 1 fraunhofer austria research gmbh, theresianumgasse 7, a-1040, vienna, austria, 2 tu wien, institute of management science, theresianumgasse 27, a-1040, vienna, austria 3 institute for computer science & control (sztaki), kendestr.13-17, h-1111, budapest, hungary 4 john von neumann university, izsáki u. 10, h-6000, kecskemét, hungary section: research paper keywords: maintenance; autonomous production control; production planning; cyber physical systems; industry 4.0 citation: robert glawar, fazel ansari, zsolt jános viharos, kurt matyas, wilfried sihn, integrating maintenance strategies in autonomous production control using a cost-based model, acta imeko, vol. 10, no. 3, article 22, september 2021, identifier: imeko-acta-10 (2021)-03-22 section editor: lorenzo ciani, university of florence, italy received february 9, 2021; in final form may 2, 2021; published september 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work has been supported by the european commission through the h2020 project epic (grant no. 739592) corresponding author: robert glawar, e-mail: robert.glawar@fraunhofer.at 1. introduction in today’s competitive market, manufacturing enterprises are faced with the challenge of achieving high productivity, short delivery times and a high level of delivery capability despite evershorter planning horizons, a large number of external planning changes and increasing planning complexity [1], [2]. this high degree of complexity in planning is no longer effectively and affordably manageable for humans [3]. on the one hand, there are high demands on flexibility and reaction times in planning and, on the other hand, high requirements regarding availability of production facilities, equipment and machines [4]. however, current systems for production planning and control (ppc) neither incorporate technical innovations nor social requirements and are therefore not able to meet the current challenges [5]. likewise, current maintenance processes and strategies are not sufficiently prepared for these challenges [6]. considering the advancement towards industry 4.0, new opportunities arise due to innovative technologies and approaches such as industrial internet of things (iiot) applications [7], horizontal and vertical communication within a production system by means of open platform communications unified architecture (opc ua) [8] or the use of artificial intelligence (ai) methods for data analysis, forecasting, optimization and planning [9]. the degree of autonomy of such a cyber physical production system (cpps) describes the ability to plan, control and initiate actions autonomously [10]. approaches to autonomous production control (apc) represent suitable ways to increase the degree of autonomy of a cpps [11], [12] therefore, apc represent suitable possibilities to deal with the aforementioned requirements [13]. however, these approaches are currently limited to lab research and are not ready for industrial applications [14]. most of the current approaches are based on idealised assumptions such as maximum availability (i.e. 95-98%) or do not take many abstract autonomous production control (apc) is able to deal with challenges, inter alia, high delivery accuracy, shorter planning horizons, increasing product and process complexity, and frequent changes. however, several state-of-the-art approaches do not consider maintenance factors contributing to operational and tactical decisions in production planning and control. the incomprehensiveness of the decision models and related decision support tools cause inefficiency in production planning and thus lead to a low acceptance in the manufacturing enterprises. to overcome this challenge, this paper presents a conceptual cost-based model for integrating different maintenance strategies in autonomous production control. the model provides relevant decision aspects and a cost function for different maintenance strategies using on a market-based approach. the present work thus makes a positive contribution to cope with the high demands on flexibility and response times in planning while at the same time ensuring high plant productivity. mailto:robert.glawar@fraunhofer.at acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 157 decisive factors such as maintenance strategies into account. for example, the question such as “how the current state of a production plant or machine can affect production control” is not taken into consideration [15]. exactly these factors, as exemplified, are decisive for the acceptance and implementation maturity of autonomous approaches in industrial companies. hence, the aim of the present work is to take a further step towards implementation maturity by integrating different maintenance strategies in apc. 2. maintenance in apc autonomous production control (apc) has the potential to deliver optimal and resource efficient processes as well as higher quality and variations of products than conventional, centralized decision-making systems [16] adaptive, decentralised production control can reduce planning efforts [17], enable shorter reaction times in planning [18] and create greater planning flexibility [19]. since in most cases not all decisions in a production system are made autonomously, a cpps typically includes a combination of hierarchical and heterarchical mechanisms for control [20]. since approaches to autonomous production control are able to deal quickly and flexibly with unplanned changes within the production system, they are used in the context of cpps to represent decision-making processes that require a high degree of responsiveness [21]. to ensure high level of acceptance among operational planning staff, it is particularly important that the underlying models comprehensively take relevant factors of the production system into account and thus making robust decisions [22]. current studies show that a large number of research activities are concerned with the development of approaches to autonomous production control [23]. current approaches focus on the description of the interactions between different parts of a production system from different perspectives. a typical task is to assign a waiting workpiece, which is to be processed in the course of a production job, to a machine or a workstation taking into account available resources, logistical parameters and the smoothing of the job load. existing apc approaches are usually either carried out as event-driven sequencing [18] or agent-based sequencing [24]. first examples that the agent-based simulation is a suitable possibility to realize apc by using multi-agent systems are shown by pantförder et al. (2017) [25]. the integration of such an approach in a production system on the basis of the upc-ua standard is shown by hoffmann et al. (2016) [26]. a key success factor for autonomous interaction in this context is the design of a of a robust system [27]. in order to reach such a design, different algorithms may be used to schedule the orders. in particular, genetic and evolutionary algorithms [28], swarm-based algorithms [29], [30], and market models [14] have been successfully used for apc. in addition, many current approaches to ppc rely on the application of methods of artificial intelligence. often, ml is applied, for example, to predict lead times predict and optimize resource utilization [31]. in addition, reinforcement learning is used to enable, for example, an autonomous order scheduling system [32]. however, as shown in table 1, few approaches deal with the integration of maintenance strategies in apc systems. for instance, erol and sihn (2017) presented a cloud-based architecture for intelligent production planning and control considering maintenance [33]. vallhagen et al. (2017) also presented a system and information infrastructure to enable optimized adaptive production control [34]. nevertheless, neither of these approaches explain which aspects of maintenance should be considered and how they should be integrated. in the approach presented by wang et al. (2018), the condition of production plants is automatically evaluated and thus the production sequence is intelligently adapted. system table 1. overview maintenance in autonomous production control. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 158 performance is improved by automatically evaluating the state of production systems and dynamically configuring processing paths for intelligent products and parts. while the implementtation as decentralized production control is proposed, the work deals in detail with a three-machine problem and neglects the dependencies on a higher-level production planning [35]. in summary, it can be concluded that none of the identified approaches includes a systematic integration of different maintenance strategies into autonomous production control. however, if this aspect is not taken into account, these approaches remain largely unsuitable for industrial application, as no valid decisions can be made in the case of unplanned outages or planned maintenance, and thus ultimately the acceptance of such approaches by operational planning staff is not given. in front of this background a comprehensive methodology for integrating maintenance strategies in autonomous production control has been presented by glawar et al. (2019) [20] and glawar et al. (2020) [36]. the core of this conceptual model, which is presented in detail in section 5, is a cost based model for the integrated planning. this model is laid out in section 4 based on the relevant aspects for the integration of maintenance in apc introduced in section 3. 3. relevant aspects for the integration of maintenance in apc for the integration of maintenance into apc, an important step is to clarify which maintenance aspects are relevant for the integration decision. for this purpose, an expert survey has been conducted including professionals from industrial sectors, namely semiconductor production, metal processing industry, condition monitoring and automotive industry as well as national and international academic experts. the aim of this survey was to discuss the following question with the experts: "how do you evaluate the individual aspects of maintenance with regard to their relevance for integration into production planning and control (ppc)?” the first step was to discuss which aspects of plant maintenance (aka industrial maintenance) are generally important for ppc and for which area of ppc a specific aspect is relevant. using the pair-wise comparison method, it was finally determined how relevant the individual aspects are for integration into the ppc. the results of this expert survey are presented in table 2. the essential aspects of maintenance are listed in the first column and evaluated with regard to their relevance for decision-making. in the second column the relevance for the integration into the different dimensions of ppc is shown. a significant finding is that the relevance for the consideration of the individual aspects for the ppc strongly depends on the general operational conditions, especially the degree of automation, production type and flexibility in case of a plant failure. a closer look at the results shows that some aspects are particularly relevant for integration into apc, while other aspects may have a positive influence on the quality of decisions but are not absolutely necessary for integration purpose. in addition, there are other aspects of plant maintenance which are particularly important for integration into mediumand longterm production planning as well as production controlling. these decision factors have not been further addresses during the course of the present work. 3.1. downtime & costs the time for a shutdown, in case of an occurring failure of the machine, can be estimated either on the basis of empirical knowledge, or calculated on the basis of historical shutdowns. it table 2. aspects and their importance for apc integration into ppc system evaluated by various domain experts [36]. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 159 is important to note that the downtime that occurs usually differs significantly depending on whether it is a planned or unplanned shutdown. since downtime costs correlate with the order situation, the lost contribution margin in the event of a downtime is usually used to calculate the downtime costs. penalties for delayed order completion are also taken into account, if applicable. together with the probability of failure, the downtime costs represent an important basis for decisionmaking in production control. 3.2. maintenance time & costs the time for a repair can either be estimated on the basis of manufacturer information and empirical knowledge or calculated on the basis of historical data (e.g., mean time to repair mttr). usually, when calculating repair costs, a distinction is made between internal and external repair costs, which are usually caused by external services. the underlying share of external services largely determines the repair time and costs. in the case of internal repair costs, the repair time is usually taken into account, taking into account the hourly rates of the personnel required for the repair, depending on their qualifications, and supplemented by the material costs for the necessary spare parts. this applies analogously to the occurring maintenance times & costs. the repair and maintenance costs calculated in this way are important factors in production control for deciding whether maintenance should be carried out or even brought forward, or whether the risk of a breakdown with subsequent repair should be taken. 3.3. spare parts availability the information on whether the spare parts required for repair and maintenance are basically available is decisive for the decision within the framework of production control as to whether maintenance is triggered or whether an order is produced on a system with a certain risk of failure. depending on the type and complexity of a machine as well as the organizational form of maintenance, spare parts availability represents a more or less important decision aspect. in the case that mechanical spare parts can be produced independently with relatively little effort or are outsourced to a service provider via a service contract, it may not be necessary to integrate this decision aspect into production control. 3.4. availability of maintenance capacity the information as to whether maintenance capacity is available for repair or maintenance is essential in the context of production control in order to make the decision as to whether this should be triggered. capacities can represent both internal personnel resources and external third-party services, which are usually not available in unlimited quantities. depending on the type of maintenance organization, this decision aspect is also more or less important. while a capacity check can be very important in a decentralized maintenance organization that has to manage with a narrowly limited capacity, it is less important in an organization that provides sufficient resources centrally. 3.5. availability of qualifications the qualifications required to perform a particular repair or maintenance task can also play a relevant role in the decision within the framework of production control. however, the significance of this decision aspect depends to a large extent on the complexity of the equipment as well as the available qualifications of the internal personnel resources. while only a small number of qualified personnel resources are generally available for highly complex plant components such as bionic components, the significance decreases for simple mechanical plant components, for which a large proportion of the available personnel resources are qualified. 3.6. planned maintenance orders and planned maintenance interval orders that are scheduled for the execution of maintenance are very relevant for the ppc as they tie up capacities. while internal maintenance tasks only reduce the capacity within a period but allow a certain flexibility with regard to sequence planning, externally performed tasks often represent a hard restriction for production control. the basis for these planned maintenance orders are often the defined intervals for (periodic) preventive maintenance. maintenance interval management thus represents a key success factor for medium-term production planning. it is crucial that this maintenance planning is coordinated with the expected fluctuation in production volumes in order to prevent equipment from being unavailable in a phase of particularly high order levels, while it could be maintained in a phase of low order levels. 3.7. probability of failure the probability of failure significantly determines the risk of a plant or machine failure during production and thus influences production control decisions. depending on the maintenance strategy applied, different approaches exist to calculate the probability of failure. in the simplest case, information from the manufacturer or internal empirical values (e.g. mean time between failure mtbf) are used to calculate the probability of failure. often, historical data based on statistical methods, such as the weibull distribution, can also be used to calculate the probability of failure. ideally, the probability of failure is determined based on the actual condition of the plant and a corresponding forecast for the next failure. 3.8. condition of the plant or machine components if the condition of a plant or machine can be reliably measured, estimated or calculated, it represents an essential decision-making factor for the pps. in the context of mediumterm production planning and spare parts planning, it is possible to react as soon as a component exhibits a critical condition, for example, by ensuring that the corresponding spare parts are available or by initiating planned maintenance. in the context of production control, the risk of failure can be taken into account based on the change in the condition of the machine for example in the context of sequence planning or machine assignment. 3.9. technical plant availability technical plant availability, which describes what proportion of the available operating time a plant is technically available, is a key aspect of medium-term production planning. depending on the availability, the plants are scheduled to a greater or lesser extent. technical plant availability also represents a hard restriction for the maximum possible production quantity. 4. development of a cost function for an integrated planning different algorithms can be used to determine the order of the orders within the autonomous production control. many of these algorithms use a cost function to prioritize or determine the production sequence. for example, when applying the market principle for apc, orders are allocated to individual acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 160 production units based on a cost function. in this paper, a cost function for autonomous production control is developed using the market principle for autonomous production control as an example: 𝐾pa = ∑ 𝐾s + ∑ 𝐾pf + ∑ 𝐾pv × 𝑋p + ∑ 𝐾tv × 𝑋t . (1) a possible formulation of the costs of a production order, as shown in (1), is presented by rötzer and schwaiger [37]. in this description, the costs of a production order (kpa) consist of individual location costs (ks) fixed process costs (kpf) variable process costs (kpv) and variable transport costs (ktv) as well as the production quantity (xp) and the number of transports (xt). transportation costs (ktv) represent the expenses for the necessary transports (xt) to move the workload to be produced within the production system. they include, for example, costs for material supply and provision, transports between different workplaces and production facilities, as well as expenses for intermediate, inward and outward storage of the produced worklist. fixed production costs (kpf). represent the portion of production costs that is independent of the amount of work in process (xp)-that is, fixed production costs are constant for each production order. this includes, for example, the expenses for setup between the production orders but also administrative costs for order processing. in comparison, variable production costs (kpv) depend on the amount of work produced. typical variable production costs are, for example, costs for material, auxiliary and operating supplies, expenses for the actual production depending on the processing time, as well as expenses for the necessary energy input during production. the variable process costs of a production thus consist of costs due to production backlog, production time, setup times, and the inventory necessary for production, as well as maintenance costs [38]. in this context, the availability of the machines and the delay of the end of the order are particularly relevant for the evaluation of a production order [39]. for this reason, it is necessary to explicitly add the maintenance-relevant factors in a cost function to describe the costs of a production order. since different maintenance strategies make different demands on production control, but also allow for different information, it is advisable to use different cost functions for the different maintenance strategies. the cost functions are successively designed to build on each other, so that it is possible to use them in a production system that uses different maintenance strategies for the different assets and their components. 4.1. reactive maintenance strategy the reactive maintenance strategy is characterized by the fact that the system components are operated until failure and therefore the failure probability and the associated costs are not relevant for decision-making. this is also reflected in the representation of the cost function of a production order under consideration of reactive maintenance krm, cf. (2). the cost function takes into account not only the sum of the fixed production costs kf, variable production costs kp, transport costs kt, as well as the current order load xp, and the number of transports xt, but also the maintenance cost ratio km. the maintenance cost ratio describes the maintenance costs per production quantity produced. the maintenance costs under consideration include the costs for maintenance and repair of the various elements of the production system, the costs for spare parts stocking, and external service costs. 𝐾rm = ∑ 𝐾f + ∑ 𝐾v × 𝑋p + ∑ 𝐾t × 𝑋t + ∑ 𝐾m × 𝑋p (2) 4.2. periodic preventive maintenance strategy in periodic preventive maintenance, measures are planned preventively either time-dependently, for example weekly, quarterly or annually, or load-dependently, for example after a certain number of operating hours or switching operations. hence, the cost function of a production order, taking into account preventive maintenance kpm, cf. (3). it also takes into account the risk of an unplanned production downtime rdtp, figure 1. costs of abased production order under consideration of multiple maintenance strategies [36]. costs of a product ion order under consideration of maintenance m aintenance strategies production planning production controlling reactive number of t ransport s fixed & variable product ion costs m aintenance cost ratio number of pieces produced preventive cbm pdm kf, kv km xp xt costs f or penalty payment s dow nt ime costs order backlog m aintenance planning forecast rul planned measures and degree of prevention determine dow ntime costs condition dow nt ime risk downtime costs depending on order backlog kdt kp c rul rdt acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 161 costs in the event of a downtime event kdt, as well as any costs for contractual penalties due to schedule variances kp. the costs in case of a downtime event are, for example, the lost contribution margin of the planned worklist in case of an unplanned downtime, as well as costs for repairs. these downtime costs are dependent on the current order load xp as shown in (4). the default risk in the case of periodic preventive maintenance rdtp, can be calculated based on historical failures. for this purpose, the probability density function fp(tslf) is used for integration, where tslf describes the time since last failure, cf. (5). normally, a normal distribution is assumed, cf. (6). while the normal distribution is calculated based on historical failures, the expected value is assumed by the mtbf. 𝐾pm = ∑ 𝐾f + ∑ 𝐾v × 𝑋p + ∑ 𝐾t × 𝑋t + ∑ 𝐾m × 𝑋p + ∑ 𝐾dt × 𝑅dtp + ∑ 𝐾p × 𝑅dtp (3) 𝐾dt = f(𝑋p ) (4) 𝑅dtp = ∫ fp(𝑇𝑆𝐿𝐹) d𝑇𝑆𝐿𝐹 𝑇𝑆𝐿𝐹 0 (5) 𝑓𝑝(𝑇𝑆𝐿𝐹) = 1 𝜎√2 π e − 1 2 ( 𝑇𝑆𝐿𝐹−𝑀𝑇𝐵𝐹 𝜎 ) 2 (6) 4.3. condition-based maintenance strategy the cost function of a production order under consideration of condition-based maintenance kcm, cf. (7), corresponds largely to the cost function of preventive maintenance and also includes the risk of an unplanned production downtime rdtc applying condition-based maintenance strategies. in this case, in which a maintenance task is planned depending on the actual condition of a component, rdtc is calculated by a condition-based function fc at the respective time of the condition determination tc and the determined condition c at this time, cf. (8). the determination of this function is usually based on empirical studies or on already known equations or manufacturer data. in many cases, especially if a complex empirical determination is not economical, it is sufficient to assign a fixed default risk rdtc to defined states c based on empirical knowledge. 𝐾cm = ∑ 𝐾f + ∑ 𝐾v × 𝑋p + ∑ 𝐾t × 𝑋t + ∑ 𝐾m × 𝑋p + ∑ 𝐾dt × 𝑅dtc + ∑ 𝐾p × 𝑅dtc (7) 𝑅dtc = 𝑓𝑐(𝑡𝑐; 𝐶) (8) 4.4. predictive maintenance strategy in predictive maintenance (pdm), maintenance tasks are planned depending on prognosis of remaining useful life (rul). the cost function of a production order, therefore, takes into account kpdm, cf. (9), the risk of an unplanned production downtime (rdrpdm). the failure risk is calculated analogous to the condition-based maintenance by a function fp which is determined by the rul i.e. the remaining degree of wear and tear of the machine component, cf. (10). this function must also be known or empirically determined. in (11), a determination of the rul using a weibull function is shown. here, t represents the characteristic life, beta the shape parameter and wi an influence factor to account for changing operating conditions of the weibull function. in summary, figure 1 shows the composition of the developed cost function depending on the applied maintenance strategy and visualizes the relationship between the individual cost factors 𝐾pdm = ∑ 𝐾f + ∑ 𝐾v × 𝑋p + ∑ 𝐾t × 𝑋t + ∑ 𝐾m × 𝑋p + ∑ 𝐾dt × 𝑅dtpdm + ∑ 𝐾p × 𝑅dtpdm (9) 𝑅dtpdm = fp(𝑅𝑈𝐿) (10) 𝑅𝑈𝐿 = e −( 𝑡 𝑇 × 𝑤𝑖 ) 𝛽 (11) 5. conceptual model for integrating maintenance strategies in apc the model for the integration of different maintenance strategies in apc is designed using three subsystems: i) a maintenance system, ii) a system for autonomous production control and iii) a system for production planning. in figure 2, these subsystems and their interrelations are shown in detail. the system for autonomous production control maps the level for machine-to-machine (m2m) communication of the apc model. it regulates the real-time communication of the different elements of a production system with the aim of autonomously determining a production sequence based on the requirements of the production control system (the production orders) and the current framework conditions of the production system. to achieve this goal, real-time communication between different machine agents (ma), work piece agents (wpa) and resource agents (ra) is necessary (information flow a). an ma represents the different machines and plants of a production system. wpas represent the open worklist within a production system. depending on the production environment, an open worklist can be a concrete workpiece, production lot or any clearly identifiable portion of the production quantity. an ra represents further elements of a production system which are of interest for the task of production control. depending on the production environment, these can be, for example, tools, workstations, measuring equipment, transport equipment and all other resources, which have a significant influence on the determination of the production sequence. the m2m communication between ma, wpa and ra takes place via a message transport system (mts), which communicates between the elements of the production system and an order agent (oa) via an agent management system (ams) and a directory facilitator (df). the ams manages the specific addresses of the individual agents (information flow b). in comparison, the df manages the specific attributes and properties of each individual agent (information flow c). examples of these attributes are the probability of failure, downtime costs, and repair and maintenance costs, which are communicated directly from the maintenance system to the df (information flow d). further attributes describe, for example, the ability of an agent to determine which possible production steps can be carried out at the respective ma or which processing times result from this. the mts distributes messages between the different agents and between agents and oa (information flow e). the mst transports information about the attributes and properties of the respective agents and production orders, which it receives from acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 162 df and oa. the mst transports this information from a specific address that it receives from ams to another specific address that is also provided by ams. the oa also receives information about spare parts availability, maintenance capacity availability, and available qualifications from the maintenance system (information flow f). with this information, taking into account the current production sequence, planned maintenance orders can be defined and confirmed to the maintenance system (information flow g). the operational control of these maintenance orders, as well as the control of production orders, takes place via communication between the various agents and df and ams using mts. the central task of the system for apc is to determine the production sequence based on real-time m2m communication. different scheduling models can be used to fulfil this task and to determine a sequential order for each of the different production orders provided by the oa. the production orders to be scheduled are typically created and managed by an enterprise resource planning (erp) or manufacturing execution system (mes). in the present work, a "marketplace-based" model is used to illustrate the integration of the system for apc. in this case, the oa receives a demand in the form of a production order from an erp or mes system. this demand is matched by a supply of capacities of the mas and ras representing the production capacities of the production system such as machine resources, work centre resources, tool resources or transport resources. the information necessary to describe the supply is provided to the oa by means of mst via the attributes figure 2. process model for the integrated planning of maintenance and apc [36]. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 163 of the production resources relevant for the production order in question, which are managed in the df. 𝑃 = 1 (𝑡i − 𝑡e) × (𝑡c − 𝑡i) × 𝐾min (12) using the information on supply and demand, oa is able to determine the priority of each production order, cf. (12). the priority p is calculated by taking into account the desired completion date tc, the possible order start time ti, the order receipt time te, and a priority factor kmin. the oa can calculate the priority factor necessary to determine the priority using the information received from the mts based on the information managed in the df. to determine the priority factor, the cost function, presented in this paper, is used here. 𝑝𝑚𝑖𝑛 = min[𝑝(𝑥)] ; 0 < 𝑥 < 𝑛𝑀𝐴 (13) due to the structure of the underlying cost function, the priority of a manufacturing order increases the higher the costs of the manufacturing order and therefore the priority factor. similarly, the greater the difference between the current time and the incoming order, the higher the priority. the higher the desired production duration of the worklist to be produced, the lower the priority of the underlying manufacturing order is calculated. since it is usually assumed that both the variable production costs and the risk of an unplanned downtime differ between the different machines and plants of a production system, it is necessary to calculate the priority factor for the number of possible ma (nma), and then determine the minimum of the possible priority factors (pmin), cf. (13). based on this minimum, the final step is to determine the sequence rank n of the production order to be produced on the assigned ma, cf. (14). for this purpose, the priority rank (p(i)) of the individual available production orders (pan), is determined in order of the minimum priority: 𝑁 = rank(p(i)) < min{𝑃𝐴𝑛 } . (14) based on the sequence rank n and the lead time tpt, which the oa can determine using the information it receives from df via the mts and the current time tact, the oa can determine the estimated time of completion tn, (15), and communicate this together with the defined production sequence to the production planning system (information flow j). for example, the oa provides this information to a mes via the mst or an alternative interface. 𝑡n = 𝑡act + ∑ 𝑡pt 𝑁 0 (15) 6. outlook and further research 6.1. evaluation of economic plausibility in further research of glawar et al. (2021) the presented model has been implemented and evaluated [40]. since an implementation in a real time environment is yet hard to realize an implementation using an agent-based simulation approach based on a real industrial use case in the automotive industry has been realized. on this basis, the conditions for a successful, and cost-effective implementation of the model in industry are derived. in this use-case the benefits of integrating maintenance strategies in apc can be described as follows [40]: a. an increase in on-time delivery of more than 9% by reducing schedule deviations due to backlogs of production jobs. since the condition of a machine is already taken into consideration during the production control, even a simultaneous failure of several machines has little impact on the adherence to delivery dates. b. a reduction in the cost of manual rescheduling of approximately € 29,500 per year, since in the event of an unplanned machine failure the sequence and machine assignment can be adjusted autonomously. c. increase of the uptime by using the potential of modern maintenance strategies by approx. 4% to over 96% and thus a reduction of maintenance costs of approx. € 52,000 per year. d. increase in productivity, which is defined as parts produced per hour, by over 5.6%. 6.2. integration into maintenance cost controlling the cost function developed in this thesis aims at integrating the relevant aspects of maintenance for autonomous production control. in a further step, this cost consideration can also be used as a basis for integration into maintenance cost controlling. such an analysis enables the formalization of the relationship between key figures such as the proportion of external costs or the maintenance ratio and the operational logistical targets such as lead time and adherence to delivery dates as well as the productivity of the production system. the maintenance ratio describes the maintenance costs incurred in relation to a period under consideration. it is therefore an essential component of production costs and can already be used in rough-cut planning and in sequencing. this makes it possible to consider the resulting effects at the tactical level and to derive measures for achieving an overall optimum, independent of a fixed defined maintenance budget. existing models for maintenance cost controlling such as the cost prove model [41] model planned and unplanned maintenance costs and attempt to derive optimization measures based on any deviation from a defined budget in order to achieve the ideal operating point between planned and unplanned measures for maintenance. in comparison, by taking into account the developed cost function depending on the current and future expected production program as well as the current risk of failure of the equipment necessary for the workers of this production program, the maintenance costs are dynamically adjusted. this creates transparency for the performance of maintenance by quantifying the benefit of concrete measures on the operating result in costs and thus justifying, for example, the exceeding of a target budget while ensuring adherence to schedules and productivity. an example of integration in maintenance cost controlling is shown in figure 3. maintenance cost controlling supplies relevant cost variables to production control, which pursues the goal of minimizing the costs for a production order while taking maintenance into account. if a deviation from the original maintenance budget occurs, the effect on productivity and ontime delivery is used in a mathematical reference model to optimize maintenance cost controlling. this results in new target costs for maintenance, which influences the initiation of planned measures depending on the risk of failure and the respective machine condition. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 164 in order to create such a mathematical reference model, it is expedient to differentiate between planned and unplanned cost factors, as explained by ansari [38] and to model these in order to achieve an overall optimum. 7. conclusions in the present paper a novel model for integrating maintenance strategies in autonomous production control has been presented. relevant decision aspects have been discussed and a cost function for an integrated planning using a marketbased approach have been laid out. it is based on the key elements of a cpps and their relations to establish a complete but fully, efficiently integrated component in ppc. essential findings are the identified and evaluated aspects of maintenance which are decision relevant for the integration into the production control. the most relevant aspects are taken into account in the developed cost function for integrated planning, thus providing a robust basis for the implementation of apc in industrial practice. only through clear guidelines on how autonomous control behaves in the event of a failure and how the case of an increased risk of failure is taken into account, a acceptance for the implementation of apc can be achieved. against this background, the developed cost-based model contributes to bringing approaches to apc a further step towards implementation maturity and thus provides an innovative approach to communication between production and maintenance planning both from a practical and a scientific point of view. however, further research questions remain open: 1) the implementation of the present process model in a real production environment and a corresponding evaluation of the benefits in industrial practice represent the logical next step. this requires a well-thought-out roadmap, since such an implementation deeply affects different areas of a production system. in addition, there is the challenge of preparing personnel for the new way of working using autonomous production control and providing appropriate qualification measures in good time. 2) the present model is limited to the mapping of a manufacturing area and negates at this point the dependencies with respect to the higher-level planning to the present or subsequent areas of the production system. in particular, the challenges to the integration of autonomous and human agents have to be addressed. 3) similarly, the impact of short-term production control at the operational level on the tactical and strategic levels, such as production controlling, poses exciting challenges for further research activities. in particular, integration into maintenance cost controlling, as outlined schematically in section 6.2 can offer a significant contribution to quantifying the contribution of maintenance to the achievement of operational targets in this context. 4) approaches that make use of artificial intelligence methods represent interesting alternatives for the relatively simple market-based model used in the present modelin this context, reinforcement learning approaches in particular represent an alternative which, in the view of the author, should be explored in this context in the future. 5) in order to be able to apply this approach easily and quickly to further use cases in the future, research should be conducted in the direction of automated parameter optimization, for example by means of simulation studies. acknowledgement this work has been supported by the european commission through the h2020 project epic (grant no. 739592). references [1] t. bauernhansl, r. miehe, industrielle produktion–historie, treiber und ausblick. fabrikbetriebslehre 1, springer vieweg, berlin, heidelberg, 2020, pp. 1-33. 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[41] f. ansari, meta-analysis of knowledge assets for continuous improvement of maintenance cost controlling. faculty of science and technology, thesis, university of siegen, 2014, 169 pp. https://doi.org/10.1016/j.promfg.2017.07.165 https://doi.org/10.1109/coase.2018.8560389 https://www.imeko.org/publications/tc10-2020/imeko-tc10-2020-037.pdf https://www.imeko.org/publications/tc10-2020/imeko-tc10-2020-037.pdf https://doi.org/10.1109/etfa.2013.6647980 https://doi.org/10.1109/codit.2013.6689647 frequency response function identification using fused filament fabrication-3d-printed embedded aruco markers acta imeko issn: 2221-870x september 2022, volume 11, number 3, 1 6 acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 1 frequency response function identification using fused filament fabrication-3d-printed embedded aruco markers lorenzo capponi1, tommaso tocci2, giulio tribbiani2, massimiliano palmieri2, gianluca rossi2 1 aerospace department, university of illinois at urbana-champaign, 61801 urbana, illinois 2 engineering department, university of perugia, 06122 perugia, italy section: research paper keywords: aruco; marker detection; non-contact measurement; structural dynamics citation: lorenzo capponi, tommaso tocci, giulio tribbiani, massimiliano palmieri, gianluca rossi, frequency response function identification using fused filament fabrication-3d-printed embedded aruco markers, acta imeko, vol. 11, no. 3, article 16, september 2022, identifier: imeko-acta-11 (2022)-03-16 section editor: francesco lamonaca, university of calabria, italy received july 24, 2022; in final form september 15, 2022; published september 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: lorenzo capponi, e-mail: lcapponi@illinois.edu 1. introduction when a flexible structure is excited at or close to one of its natural frequencies, resonance phenomenon occurs [1], [2]. in resonance operating conditions, most of the energy is released and the response vibration amplitudes significantly increases. this process generally leads to an increasing of the vibration fatigue damage [3]-[5]. due to this, the determination of the modal components (i.e., natural frequencies, mode shapes and damping) is fundamental in any structural dynamics approach, either numerical or experimental, in order to avoid potential critical conditions [2]. with this perspective, many experimental approaches have been developed in years that allows the determination of modal components [6]-[8]. the impact excitation using modal hammers and the shaker excitation are the two most used full-contact experimental approaches [2], [9]. in the last years, many image-based analysis have been introduced for the displacement measurements [10], [11] and consequently for the structural dynamics due to several operating advantages, e.g. high spatial density, full-field information, no sensors to be placed on the structure [12]. javh et al. [13] proved the hybrid modal-parameter identification of full-field mode shapes using a dslr camera for responses far above the camera's frame rate, employing the lucas-kanade optical flow algorithm [14]. gorjup et al. [15] researched on the full-field 3d operatingdeflection-shape (ods) identification using the frequency domain triangulation in the visible spectrum. capponi et al. [16] proposed a methodology based on the thermoelastic principle for the visual modal strain determination, that allowed the fatigue modal damage identification. one of the most promising approaches for deformation, displacement and motion detection involves markers, either physical or virtual [17]-[19]. virtual markers are often employed as they allow tracking objects in subsequent acquired frames without introducing physical targets [20], [21]. when virtual markers are not available, physical markers are employed [22], [23], and among them, the aruco marker library (aruco augmented reality university of cordoba) was found to be one of the most effective and robust to detection errors and occlusion [24]-[26]. elangovan et al. [27] used them for decoding contact forces exerted by adaptive hands, while sani and karamian [28] and lebedev et al. [29] employed them for drone quadrotor and uav autonomous navigation and landing, respectively. in relation to the use of fiducial markers for vibrations measurement, abdelbarr et al. [30] researched abstract the assessment of modal components is a fundamental step in structural dynamics. while experimental investigations are generally performed through full-contact techniques, using accelerometers or modal hammers, this research proposes a non-contact frequency response function identification measurement technique based on aruco square fiducial markers displacement detection. a video of the phenomenon to be analyzed is acquired, and the displacement is measured through markers, using a dedicated tracking algorithm. the proposed method is presented using a harmonically excited fused filament fabrication-3d-printed flexible structure, equipped with multiple embedded-printed markers, whose displacement is measured with an industrial camera. comparison with numerical simulation and an established experimental approach is finally provided for the results validation. mailto:paul@regtien.net acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 2 structural 3d displacement using aruco markers, while the study of kalybek et al. [31] provides one of the first evidence of the capability of optical vibration monitoring systems in modal identifications. recently, tocci et al. [32] presented an aruco marker-based vibration displacement technique, provided with an uncertainty analysis, based on acquisition parameters influence investigation. in this research, the aruco markers are employed for the determination of the frequency response function (frf) of a flexible structure using image-based analysis. in this study, the growing potential of 3d printing is exploited: the tested structure is realised in polylactic acid using fused filament fabrication 3d printing methodology. the employed markers are still realised using 3d printing methodology and they are generated as embedded in the structure during a unique printing job. an established experimental frf assessment technique and a numerical model are also provided for the results validation. the manuscript is organized as follows. in sec. 2 the theoretical background of structural dynamics and marker detection is given. in sec. 3, the proposed approach is presented and in sec. 4 the experimental campaign and the numerical model are described. sec. 5 gives the results while sec. 6 draws the conclusions. 2. theoretical background 2.1. structural dynamics flexible structures can be represented by n-degrees of freedom (dofs) systems using [8]: 𝑀 �̈�(𝑡) + 𝐷 �̇�(𝑡) + 𝐾 𝑥(𝑡) = 𝐷 �̇�(𝑡) + 𝐾 𝑦(𝑡), (1) where 𝑴, 𝑫 and 𝑲 are the mass, damping and stiffness matrices, while 𝒚(𝑡) and 𝒙(𝑡) are the excitation and the response displacements of the dofs, respectively. assuming a harmonic excitation 𝒚(𝑡) = 𝑌 𝑒 𝑖𝜔𝑡 and a response 𝒙(𝑡) = 𝑋 𝑒 𝑖𝜔𝑡 , eq. (1) can be written as[8]: (−ω2 + i ω 𝐷 𝑀−1 + 𝐾 𝑀−1) 𝑋(ω) = (i ω 𝐷 𝑀−1 + 𝐾 𝑀−1) 𝑌(ω). (2) from eq. (2), the displacement-response amplitude is obtained as [8]: α(ω) = 𝑋(ω) 𝑌(ω) = i ω 𝐷 𝑀−1 + 𝐾 𝑀−1 −ω2 + i ω 𝐷 𝑀−1 + 𝐾 𝑀−1 (3) where 𝜶(𝜔) defines the receptance matrix, that is also know as the frequency-response function (frf) from displacement to displacement [2], [8]. using the eigenvalues notation, 𝜶(𝜔), which relates the j-th response to the k-th excitation, can be written as [2], [8]: α𝑗𝑘 (ω) = ∑ ( 𝑅𝑗𝑘𝑟 i 𝜔 − 𝜆𝑟 + 𝑅 ∗𝑗𝑘𝑟 i 𝜔 − 𝜆 ∗𝑟 ) 𝑁 𝑟=1 , (4) where r is the eigenvalue index (i.e., the mode index), * stands for the complex conjugate notation, 𝑅𝑗𝑘𝑟 is the modal constant and 𝜆𝑟 is r-th eigenvalue [2]. in the experimental modal analysis, several approaches for the excitation and the measurement of the structural dynamics can be employed [2]. the frequency response function of a system can be experimentally determined using frf estimators [8]. when there is no input noise and the output noise is uncorrelated, the α̂(ω) is used: α̂(ω) = 𝑆𝑦�̂� (ω) 𝑆𝑦�̂� (ω) (5) where 𝑆𝑦�̂� (ω) is the cross-spectrum between the input excitation y and the output response x and 𝑆𝑦�̂� (ω) is the autospectrum of the input excitation y (the ^ symbol denotes the estimation from measurements), defined as: 𝑆𝑦�̂� (𝜔) = 1 𝑇 [𝑌∗̂(𝜔) �̂�(𝜔)], (6) 𝑆𝑦�̂� (𝜔) = 1 𝑇 [𝑌∗̂(𝜔) �̂�(𝜔)] (7) where t is the measurement time length, �̂�(ω) and �̂�(ω) are the spectra of the input excitation and of the response, respectively. for the experimental frf reconstruction, the modal parameters identification is required, and, for this purpose, different methods can be used [33], [34]. the preferred procedure in experimental modal analysis consists of using the least square frequency domain (lsfd) approach for the modal constants identification from the eigenvalues λ�̂� obtained through the least square complex frequency (lscf) and the stabilisation chart [35]. 2.2. aruco marker detection an aruco marker is a square-marker composed by a wide black border, that facilitates its detection in the image, and an inner binary-matrix, which determines its identification number [24], [25]. an example of aruco marker is presented in figure 1. the identification of an aruco marker in a captured frame requires several computational steps [32], that, as well as for the generation of the marker, are provided from the opencv python dedicated library. however, properly developed imageprocessing (e.g., filters and thresholds) can facilitate the pattern recognition. the marker detection is based on its 4 corners identification in each captured frame (see figure 1). from the corners, the spatial coordinates of the centre of the marker (𝑥𝑐 , 𝑦𝑐 ) are evaluated frame-by-frame during the acquisition [32]: 𝐶 = (𝑥c, 𝑦c) = �⃗� ⋅ ( 1 4 ∑ |𝑥r| 4 r=1 , 1 4 ∑ |𝑦r| 4 r=1 ) (8) where (𝑥𝑟 , 𝑦𝑟 ) are the coordinates of the r-th vertex and �⃗� is the calibration factor from pixel units to si units, defined as the ratio between the side length of the physical marker in si units 𝑑𝑆𝐼 and figure 1. example of an aruco marker from original dictionary: corners and reference system. acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 3 the average of the four side lengths (in pixels) of the captured marker in the fov 𝑑𝑝𝑥⃗⃗ ⃗⃗ ⃗⃗ ⃗ ∶ �⃗� = 𝑑𝑆𝐼 / 𝑑𝑝𝑥⃗⃗ ⃗⃗ ⃗⃗ ⃗ [m/pixels]. (9) the calibration factor is evaluated at each new acquired frame. in this way, if the marker is subjected to non-planar displacements or deformations during the measurement, the calibration factor is again estimated. the time-history of the centre the marker 𝐶(𝑡) is obtained by tracking it during the acquisition. 3. aruco marker-based frequency-response function identification with this study, a method for the experimental frequency response function identification is proposed. as discussed in sec. 2.1, the receptance matrix α̂(ω), estimated from experiments, can be determined using eq. (5). in eq. (6), the spectrum of the excitation input �̂�(ω) and of the structure response �̂�(ω) are determined from the aruco marker centre displacement time histories 𝐶𝑦 (𝑡) and 𝐶𝑥 (𝑡) (see eq. 8): 𝑌�̂� (𝜔) = ∫ 𝐶𝑦(𝑡) ∞ −∞   𝑒 −i𝜔𝑡 𝑑𝑡 (10) 𝑋�̂� (𝜔) = ∫ 𝐶𝑥 (𝑡) ∞ −∞   𝑒 −i𝜔𝑡 𝑑𝑡 (11) then, the receptance α̂(ω) is estimated using: α̂(ω) = 𝑌𝐶 ∗̂(ω) 𝑋�̂� (ω) 𝑌𝐶 ∗̂(ω) 𝑌�̂� (ω) (12) finally, frfs reconstruction using lsfd and lscf approaches is performed. 4. experimental research 4.1. setup in this research, a y-shaped specimen, shown in figure 2, was used [4], [12]. in particular, this geometry was chosen due to its structural dynamic properties. in fact, by using two steel weights (each of 360 g), fixed to each of the arms, the structural dynamics were adjusted to the research needs. the y-shaped sample was realised in white pla, using an ultimaker3 3d printer (100% infill and 0.1 mm of layer height). default values for other printing parameters were used. in the printing process, three 8x8 mm2 aruco markers were embedded in the last four layers of the y-sample geometry and printed using black pla material in one printing process (see figure 2). the sample was mounted on an electro-dynamical shaker (sentek l1024 with pa115 power amplifier), as shown in figure 3. on the shaker fixation, a fourth aruco marker (printed in b&w on a standard 80 g/m2 paper in 8x8 mm2) was rigidly glued for the input excitation measurement. the marker detection was performed using a flir backfly s 5 mp monochrome camera with sony imx250 sensor and fujinon 12 mm optic mounted. the resolution of the camera was settled at 1000 × 850 pixels and the frame rate at 160 fps. the setup consists also of a pcb-352c34 accelerometer, bonded on the shaker fixation for controlling and measuring the input excitation, and of a pcb-352c23/nc, fixed on one y-sample arm for the response measurement. the main specifications of the accelerometers used are shown in table 1. for the excitation, a sine-sweep of 0.5 g of constant amplitude from 5 hz to 80 hz was given to the shaker (close-loop control), with a sweep-rate of 16 oct/min (i.e., approximately 4 sweeps in 68 seconds). the sweep rate was carefully chosen in order to excite the natural frequencies of the sample. however, the measurement with the camera was limited at approximately 45 seconds due to hardware and memory limitations. 4.2. data acquisition the marker used in this research are from the aruco original library, identified as shown in figure 4. in particular, the markers with id1 and id7 are considered as input reference while the markers on the two arms (i.e., id2 and id5) as output displacement. the displacement of the four markers centre point, captured during the experiment and evaluated using eq. 8 and eq. 9, is shown in figure 5. figure 2. y-shaped specimen with installed sensors. table 1. technical specifications of accelerometers used. specifications pcb-352c34 pcb-352c23/nc sensitivity 100 mv/g (±10 %) 5 mv/g (±20 %) measurement range ±490 m/s² pk ±9810 m/s² pk frequency range (±5 %) 0.5 to 10000 hz 2 to 10000 hz resonant frequency ≥50 khz ≥70 khz broadband resolution 0.0015 m/s2 rms 0.03 m/s2 rms non-linearity ≥ 1 % ≥ 1 % transverse sensitivity ≥ 5 % ≥ 5 % figure 3. experimental setup. acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 4 once the displacement time-histories are obtained, the frfs can be evaluated through eq. 12, and, finally, reconstructed using lsfd and lscf approaches. similarly, the reference and the response accelerations time-histories are measured during the excitation (see figure 6) and the accelerance frf is evaluated [2]. 4.3. finite element model a finite element model of the y-shaped specimen is prepared using a commercial software. figure 7 shows the realized numerical model. the structure has been meshed using solid element with ten degree of freedom for node for a total of 89962 elements and 131580 nodes. to model the external masses attached to the free end of the structures, two-point mass, with a mass equal to 0.36 kg each one, are rigidly connected to the holes on the arms of the structure. moreover, to accurately replicate the experimental test, an additional mass of 1000 kg (namely large mass in figure 7) was connected to the constrain zone of the structure. the displacement along the y axis was not constrained, while all the other degree of freedom were fixed. in such a way it was possible to use the large mass method to evaluate the frequency response both in terms of displacement and in terms of acceleration. to calculate the numerical frequency response function (shown in sec. 5 the modal approach was used. for this reason, a modal analysis was necessary to obtain the natural frequencies of the system and the modal shapes in the point where the responses should be addressed. all the frequency response function shown in sec. 5 were obtained considering a percentage damping equal to 1% constant for each vibrating mode. 5. results four different frfs are obtained from the combination of the two markers as input and the two as output. however, as expected, the displacement measured with id1 and id7 markers is totally comparable and the same consideration can be performed for id2 and id5 markers, due to geometrical considerations. due to this, for the sake of clarity, only id1-id2 markers frf will be shown and considered in the further discussion. from the results in figure 8, the goodness of the numerical model is verified, comparing the frf obtained performing the accelerometer-base experiments. figure 4. aruco markers employed. figure 5. measured displacement of the detected markers. figure 6. reference and response accelerations. figure 7. fe model of the y-shaped specimen. figure 8. experimental and numerical acceleration frequency response functions comparison. acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 5 in the considered frequency range, three predominant natural frequencies are clearly identified at approximately 17 hz, 48 hz and 69 hz from both experiments and numerical model. finally, the comparison between the verified numerical model and the proposed approach is performed in terms of displacement frf (see figure 9). in the same considered frequency range, the same natural frequencies are identified using the aruco markers with high accuracy. the obtained natural frequencies are presented in table 2. a slightly decreasing of the third natural frequency is detected with respect to the numerical model. however, the experimental approaches give similar results, and this deviation can be attributed to the numerical model setting. 6. conclusions this study researches the modal components identification using a non-contact measurement approach based on aruco marker displacement detection. even though the established fullcontact methods are widely used for several research applications, the required instrumentation is expensive and delicate, and the experimental procedures are time consuming for a full-field comprehensiveness of the dynamics of a structure. on the other hand, the proposed method proved high accuracy on the assessment of natural frequencies of a structure with a relatively low computational effort and extremely lower budget sensors instrumentation: each aruco marker can be considered as a sensor, and if multiple markers are placed and detected in the field of view of the camera, more information on the dynamics of the structure can be easily provided. moreover, using the 3d printing technology, embedded sensors are demonstrated to be effective and reliable. further employment of aruco markers in structural dynamics will be investigated. references [1] d. benasciutti, fatigue analysis of random loadings. a frequencydomain approach, phd university of ferrara, department of engineering, 2004. 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[34] p. guillaume, p. verboven, b. cauberghe, s. vanlanduit, e. parloo, g. de sitter, frequency-domain system identification techniques for experimental and operational modal analysis, ifac proceedings volumes, vol. 36, no. 16, sep. 2003, pp. 1609– 1614. doi: 10.1016/s1474-6670(17)34990-x [35] b. peeters, h. van der auweraer, p. guillaume, j. leuridan, the polymax frequency-domain method: a new standard for modal parameter estimation?, shock and vibration, vol. 11, no. 3–4, 2004, pp. 395–409. doi: 10.1155/2004/523692 https://doi.org/10.1177/1475921718806895 https://doi.org/10.1007/978-94-007-1977-4 https://doi.org/10.1016/j.measurement.2014.09.063 https://doi.org/10.1016/j.imavis.2018.05.004 https://doi.org/10.1016/j.patcog.2014.01.005 https://doi.org/10.21014/acta_imeko.v10i4.1148 https://doi.org/10.1109/humanoids43949.2019.9035051 https://doi.org/10.1109/iconda.2017.8270408 https://doi.org/10.1007/978-3-030-60337-3_18 https://doi.org/10.1088/1361-665x/aa9450 https://doi.org/10.3390/s21041239 https://doi.org/10.1088/2631-8695/ac1fc7 https://doi.org/10.1016/s1474-6670(17)34990-x https://doi.org/10.1155/2004/523692 introductory notes for the acta imeko second issue 2021 general track acta imeko issn: 2221-870x june 2021, volume 10, number 2, 4 5 acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 4 introductory notes for the acta imeko second issue 2021 general track francesco lamonaca1 1 università della calabria, ponte p. bucci, 87036, arcavacata di rende, italy section: editorial citation: francesco lamonaca, introductory notes for the acta imeko second issue 2021 general track, acta imeko, vol. 10, no. 2, article 2, june 2021, identifier: imeko-acta-10 (2021)-02-02 received may 25, 2021; in final form may 25, 2021; published june 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: francesco lamonaca, e-mail: f.lamonaca@dimes.unical.it 1. introductory notes for the acta imeko general track this issue includes a general track aimed to collect contributions that do not relate to a specific event. as editor in chief, it is my pleasure to give readers an overview of these papers, with the aim of encouraging potential authors to consider sharing their research through acta imeko. elena fitkov-norris et al., in ‘are learning preferences really a myth? exploring the mapping between study approaches and mode of learning preferences’, present an interesting study on the presence of the conversion effect in the mapping related to the strength of students’ preferences for receiving information in a visual, auditory, reading/writing, or kinaesthetic modality and the study approaches they adopt when taking notes in class, learning new concepts, and revising for exams. this paper opens up the possibility of new measurement frontiers, as it stimulates research on the definition of new measurement methods and instruments for assessing and describing the approach taken by students to their studies. in ‘a colour-based image segmentation method for the measurement of masticatory performance in older adults’, lorenzo scalise et al. present a specific measurement method based on the automatic segmentation of two-coloured chewing gum and colour features using the k-means clustering algorithm. the proposed solution aims to quantify the mixed and unmixed areas of colour, separated from any background colour, in order to evaluate masticatory performance among older people with different dental conditions. this innovative measurement method will ameliorate people’s quality of life, especially the elderly. using the example of measurements of ion activity, oleksandr vasilevskyi in ‘assessing the level of confidence for expressing extended uncertainty: a model based on control errors in the measurement of ion activity’ proposes a method for estimating the level of confidence when determining the coverage factor based on control errors. based on information on tolerances and uncertainty, it is possible to establish a reasonable interval around the measurement result, within which most of the values that can be justified are assigned to the measured value. a novel design that changes the accelerometer mounting support of a commercial pneumatic shock exciter is described in ‘investigating the transverse motion of a pneumatic shock exciter using two different anvil mounting configurations’ by christiaan s. veldman. the aim is to reduce the transverse motion to which the accelerometer is subjected during shock excitation. the author describes the mounting support supplied by the manufacturer, the design changes made, and the measurement data to compare the transfer motions recorded using two different mounting designs. roberto de fazio et al., in ‘sensor-based mobile robot for harsh environments: functionalities, energy consumption analysis and characterisation’, illustrate the design of a semicustom wheeled mobile robot with an integrated high-efficiency monoor polycrystalline photovoltaic panel on the roof that supports the lithium ion batteries during specific tasks (e.g. navigating rough terrain, obstacles, or steep paths) in order to extend the robot’s autonomy. a new e-textile-based system for the remote monitoring of biomedical signals, named sweet shirt, is presented by armando coccia et al. in their paper ‘design and validation of an e-textile-based wearable system for remote health monitoring’. the system includes a textile sensing shirt, an electronic unit for data transmission, a custom-made android application for real-time signal visualisation, and desktop software for advanced digital signal processing. the device allows the acquisition of electrocardiographic, bicep electromyographic, and trunk acceleration signals. the study’s results show that the information contained in the signals recorded by the novel systems are comparable with those that can be obtained by a standard medical device used in a clinical environment. mailto:f.lamonaca@dimes.unical.it acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 5 valery mazin, in ‘measurements and geometry’, demonstrates the points of contact between measurements and geometry, which is done by modelling the main elements of the measurement process by the elements of geometry. it is shown in the study that the basic equation for measurements can be established based on the expression of a projective metric and represents its particular case. commonly occurring groups of functional transformations of the measured value are listed. in ‘towards the development of a cyber-physical measurement system (cpms): case study of a bioinspired soft growing robot for remote measurement and monitoring applications’, stanislao grazioso et al. report a preliminary case study of a cpms, namely an innovative bioinspired robotic platform that can be used for measurement and monitoring applications in confined and constrained environments. the innovative system is a ‘soft growing’ robot that can access a remote site through controlled lengthening and steering of its body via a pneumatic actuation mechanism. the system can be endowed with different sensors at the tip or along its body to enable remote measurement and monitoring tasks; as a result, the robot can be employed to effectively deploy sensors in remote locations. the heterogeneous topics of the papers submitted to the general track confirm acta imeko is the natural platform for disseminating measurement information and stimulating collaboration among researchers of many different fields but united by their common interest in measurement science and technologies. francesco lamonaca editor in chief comparison between 3d-reconstruction optical methods applied to bulge-tests through a feed-forward neural network acta imeko issn: 2221-870x december 2021, volume 10, number 4, 194 200 acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 194 comparison between 3d-reconstruction optical methods applied to bulge-tests through a feed-forward neural network damiano alizzio1, marco bonfanti2, guido garozzo3, fabio lo savio4, roberto montanini1, antonino quattrocchi1 1 dept. of engineering, university of messina, cont.da di dio, 98166 messina, italy 2 dept. of electric, electronic, informatics engineering, univ. of catania, via s. sofia, 54, 95100 catania, italy 3 zerodivision systems s.r.l. piazza s. francesco n. 1, 56127 pisa, italy 4 dept. of civil engineering and architecture, university of catania, via s. sofia, 54, 95100 catania, italy section: research paper keywords: bulge-test; creep; 3d-dic; epipolar geometry; neural network citation: damiano alizzio, marco bonfanti, guido garozzo, fabio lo savio, roberto montanini, antonino quattrocchi, comparison between 3dreconstruction optical methods applied to bulge-tests through a feed-forward neural network, acta imeko, vol. 10, no. 4, article 30, december 2021, identifier: imeko-acta-10 (2021)-04-30 section editor: roberto montanini, università di messina and alfredo cigada, politecnico di milano, italy received august 2, 2021; in final form december 4, 2021; published december 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: fabio lo savio, e-mail: flosavio@diim.unict.it 1. introduction the mechanical characterization of elastomers needs the knowledge of numerous hyperelastic constants describing their highly non-linear mechanical behaviour [1]-[6]. nowadays a lot of different techniques are used to determine the stress-strain curve, from the most traditional uniaxial tensile-compressive tests, through indentation and equibiaxial tests, up to the bulge tests. among these, the bulge test is a consolidated technique for the investigation of membranes subjected to an equibiaxial tension state [7], [8]. such method allows avoiding those edge damages commonly occurring when the specimen is stressed during other types of tests [9]. in bulge tests, a material thin sheet of uniform thickness is clamped between two circular flanges with cavities in the centre to cause, through the insufflation of fluid inside the test chamber, the sheet to deform plastically assuming a semi-spherical shape. since in isotropic materials the stress state is equibiaxial, the relation between the pressure-displacement and stress-strain curves is unique on the whole dome [10]. in light of the above, this technique, originally used for metallic materials, can be extended also to rubber-like materials, which show typically isotropic behaviour in absence of any previous calendering process which could generate a light transversally isotropy along calendering direction [11], [12]. under inflation, pressure and displacements are the testing parameters to be constantly monitored, these being the parameters necessary to determine the stress-strain curve. to achieve a faithful reconstruction of the dome, epipolar geometry [13]-[15] and 3d-digital image correlation (dic) [16][18] techniques were implemented. some of the authors have previously published a device with a mobile crosshead in order to subject an elastomeric membrane to the bulge test in force control (creep) [14], [19]. in the first of the two cited works [14], authors highlighted the 3d reconstructive technique of a hyperelastic specimen subjected to bulge-test, based on the epipolar geometry. in the abstract the mechanical behaviour of rubber-like materials can be investigated through numerous techniques that differ from each other in costs, execution times and parameters described. bulge test method proved helpful for hyperelastic membranes under plane and equibiaxial stress state. in the present study, bulge tests in force control were carried out on sbr 20% cb-filled specimens. 3d reconstructions of the dome were achieved through two different stereoscopic techniques, the epipolar geometry and the digital image correlation. through a feed-forward neural network (ffnn), these reconstructions were compared with the measurements by a laser triangulation sensor taken as reference. 3d-dic reconstruction was found to be more accurate. indeed, bias errors of the 3d-dic and epipolar techniques with respect to the relative reference values, under creep condition, were 0.53 mm and 0.87 mm, respectively. mailto:flosavio@diim.unict.it acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 195 second one [19], authors trained a feed-forward neural network (ffnn) on the data acquired using the previous technique, returning a predictive model that provides as output the dome apex height formed by the insufflated specimen. in the present paper, 5 specimens were bulge-tested in creep condition and, differently from previous works, a comparison between the stereoscopic reconstruction of the specimen dome based on epipolar geometry and 3d-dic was carried out. this comparison was preferred to be performed through a ffnn independently trained on the base of results from both the reconstructive techniques. the values of the dome apex height provided by the two ffnn models were, then, compared with the value revealed by using a laser triangulation measurement technique as reference. 2. theoretical background in the bulge test technique, some restrictive assumptions are required to be applied both on the tested material, such as isotropy and incompressibility, and on the geometry of the inflated sample, such as hemispherical shape and small thickness compared to the curvature radius. under these assumptions, the stress state can be thought as equibiaxial and plane and, according to the boyle-mariotte law (fit for thin-walled tanks), is defined by: 𝜎𝑐 = 𝜎𝑎 = 𝑝 ∙ 𝑅 2 𝑠 , (1) where 𝜎c and 𝜎a are, respectively, the circumferential and axial stress, p is the working inflation pressure, r is the curvature radius of the dome and s is the thickness at the dome apex. at the dome apex each meridian represents a main direction and, then, on its surface all the main strains are equal to each other (ε1= ε2= εeq). from the knowledge of the undeformed length l0 and the deformed length ld of a membrane finite element, these strains are given by: 𝜀1 = 𝜀2 = ln (𝑙𝑑 𝑙0)⁄ . (2) from the assumption of incompressibility of the material and in agreement with the well-known von mises equation [13]: 𝜀3 = ln (𝑠 𝑠0)⁄ , (3) where s0 is the undeformed specimen thickness. 3. 3d-reconstruction methods used the stereoscopic technique based on epipolar geometry is a 3d-reconstruction method allows determining the whole dome by identifying, via two cameras, the shifting of a grid printed on the surface of the sample as the dome inflates under creep bulge testing [20]. acquired images by two cameras are processed through apposite geometrical algorithms and different filters. the stereoscopic technique based on 3d-dic is an optical method adopted to measure full-field displacements and strains by applying the cross-correlation to measure shifts in digital images [21]. this method is effective at mapping displacements and deformations thanks to the contrast, necessary to correlate images, generated by the application of an airbrushed speckle pattern on the surface of the sample. the multi-camera system uses a single global axis system. a dedicate software, developed in matlab® environment, integrates a 2d-dic subset-based software with different camera calibration algorithms to reconstruct 3d surfaces from several acquisitions of stereo image pairs. furthermore, this software contains algorithms for computing and visualizing 3d displacements and strains. 4. experimental setup bulge tests in force control were performed by a home-made experimental setup already published in previous works of some of the authors [14], [19], [22]. a pneumatic circuit inflates a thin sample clamped between two flanges with adjustable flow rate (figure 1d). the device is equipped with a sliding crossbar, whose movements are proportional to the membrane inflation. creep phenomenon was obtained by keeping the pressure constant within the bulge chamber by means of a pressure regulator (0.1 bar resolution). in the creep test, after a transient inflation lasting 1 s, the sample was subjected to a constant pressure of 0.55 bar long enough for achieving a complete relaxation, ensuring at the same time that the strain fall within the linear and elastic field. the core of the stereoscopic acquisition setup (as shown in figure 1a) is a sliding crossbar, employed to translate two fixedfocus cameras (imaging source dmk23g445 monochromatic, equipped with fujifilm hf35ha-1b lens, having a frame rate up to 30 fps) in upward/downward direction so to follow the dome apex displacement due to the inflation and, thus, detect the equi-biaxial strain of the specimen. in this way, the dimensions of the captured images depend exclusively on the inflation of the dome and not on its approaching to the camera lenses. the crossbar shifting is made possible by converting the rotary motion of a stepper motor to a vertical translation of the crossbar through a driven shaft and a timing belt (figure 1b). the linear motion is controlled by an optical system rigidly connected to the crossbar and consisting of a laser diode (working as emitter) and a photodetector (hamamatsu si s5973-01 photodiode) placed laterally to the dome (figure 1c). when the laser beam is interrupted by the inflated dome, the system starts the crossbar shifting via a signal sent to a double h-bridge circuit. the shifting will be stopped once that the photodetector will be newly hit by the laser beam. the crossbar shifting takes place in steps of 0.125 mm, committing a focal length error of 0.03% and a negligible focusing error. system accuracy matches the size of the captured pixels (37.7 μm). since the photodiode diameter is greater than that of the laser spot (2 mm vs 1 mm), the laser beam does not affect the spatial resolution of the vertical motion (0.2 mm). a laser triangulation sensor (optoncdt 1302-200, having a resolution of 0.2% fso) was placed on the top of the device frame (figure 1a) and a target-lamina was fixed to the crossbar. these additions were made with the dual purpose of evaluating the uncertainty of the crossbar vertical shifting and indirectly obtaining the measurement of the height of the dome (as shown in figure 2) to be taken as reference value for both epipolar geometry and 3d-dic techniques. in other terms, the bias errors of the stereoscopic reconstructions will be calculated by comparing the laser measurement with the measurements from epipolar geometry and 3d-dic apparata. particular carefulness was payed for synchronizing, in pseudo real-time, the acquired images to data from pressure transducer and displacement sensor. for this purpose, a ni pxie-1073 chassis containing acquisition boards (pxie-6341 and pxi-8252), coupled to a pc in ni labview® environment, was used. this software also controls the compressed air supply and regulation system. an additional software (camera calibration matlab® toolbox), acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 196 using 3d calibration target on the pinhole model [23], was adopted to stereoscopically calibrate the cameras. to achieve the 3d reconstruction of the dome based on dic technique, an appropriate gom aramis 2m lt optical system (figure 3), also consisting of a double camera but mounted on a tripod, replaced the device with cameras/sliding crossbar used for the epipolar geometry acquisition. in this case, attention was taken during operations of calibration and focusing of cameras. 5. test samples the material tested in this paper was sbr 20% carbon blackfilled, an artificial elastomer widely used for several applications: from tires, through seals, up to shoe soles [24]-[26]. a set of 5 square specimens (180 × 180 mm2) were cut from a single 3 mm thick sheet of elastomer. each specimen was used for both stereoscopic techniques and, thus, presented two different patterns, one on each side, as shown in figure 4. on the first side (figure 4a), adopted for the epipolar reconstruction, a grid consisting of five concentric circles (hence named parallels), a small central circle whose centre corresponded to the top of the dome, and 73 equidistant rays (hence named meridians) radiating from the centre were silk-screened. on the second side (figure 4b), useful to carry out the 3d-dic reconstruction, a pattern of random spots of white paint was airbrushed (with nozzle diameter ϕ = 0.18 mm). in this way, the mean value of the diameter of the spots was found to be ϕ = 0.23 mm and their relative surface equals to 0.042 mm2. figure 1. (a) bulge test setup for epipolar geometry technique; (b) timing belt system; (c) optical system to regulate the vertical shifting; (d) cad of the bulge chamber cross-section (courtesy of authors [24]). figure 2. sketch of the laser sensor measurement (courtesy of authors [24]). figure 3. bulge test setup for 3d-dic technique (courtesy of authors [22]). acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 197 6. stereoscopic reconstructions and experimental tests in the epipolar technique, in order to acquire the whole grid, the cameras separation has not to overcome 10 % of working distance. thus, adopting a distance of 40 mm between the two cameras, the distance from the top of the sample (u) was set at 400 mm. moreover, a focal length of 10,610 pixels, corresponding to 400 mm, and a minimum focusing range from 250 mm to ∞ were set. for accurate displacement measurements, 33 samples at a sample rate of 1 khz were acquired for each image. acquisition time required is 1/30 s, corresponding to the camera frame rate. for each acquisition, the average of the 33 values, taken as best value, and the standard deviation of the series were computed. since the phase shift of a single frame was 1/30 s, the resulting synchronization uncertainty could be reasonably estimated in half of it. due to the slowness of the creep of the specific material tested in the experiments, one acquisition per minute was considered sufficient to capture the relevant information of the phenomenon. some pre-processing stages were needed to improve the image sharpness, as in the example shown in figure 5: the image was first converted to grayscale (figure 5a) and then filtered with the convolution (figure 5b) and median filters (figure 5c). the convolution filter was intended to refine the grid edges by recovering the light intensity lost due to the specimen strain. next, a smoothing filter (median filter) was applied in order to both reduce the noise introduced by the convolution filter and balance the image. in particular, the size of median filter was the 3 × 3 (kernel size) and the type of convolution filter chosen was the “sharpen filter”, in order to obtain a more sharpened image allowing a more accurate edge detection. for the reconstruction, the origin of reference system was set at the centre of the clamping flange, with the vertical z-axis passing through the midpoint between the focal centres of cameras, x-axis oriented to the right of the testing machine on the median plane and y-axis perpendicular to x and z-axes (figure 6) [19]. the sample was oriented so that the first meridian was along the x-axis. anticlockwise numeration was chosen for the 73 meridians. dome 3d reconstruction was performed using a customized matlab® algorithm able to fix the markers (figure 7) [14], corresponding to the intersections between the 73 meridians and the 5 parallels of the screen-printed grid on the specimen. moreover, this algorithm identifies with red and blue markers the light transitions from dark to light and vice versa, respectively (figure 7a), selects the homothetic markers (figure 7b) and compares the distance between two homothetic markers at two consecutive stress states (figure 7c). the 3d reconstruction was implemented also through 3ddic. the ccd sensors of the two cameras were of 1624 × 1236 pixels, allowing a spatial resolution of 0.092 mm/pixel. before acquiring, the optical setup was calibrated with respect to a measure volume equal to 150 mm × 110 mm × 110 mm, taken as reference and vertically set on the centre of the flange (z-axis), in order to ensure that the space gradually occupied by the dome figure 4. bulge test sample: (a) side for epipolar reconstruction; (b) side for 3d-dic reconstruction. in this sample, holes for clamping are already made (courtesy of authors [22]). figure 5. image pre-processing stages: (a) grayscale image; (b) convolutionfiltered image; (c) median-filtered image for recovering attenuated-light areas in red encircled of (b) (courtesy of authors [14]). figure 6. cartesian reference system (courtesy of authors [19]). figure 7. (a) light transitions: dark to light (red point) and light to dark (blue point); (b) homothetic markers; (c) comparison of the distance between two homothetic markers at two successive stress states (courtesy of authors [14]). acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 198 during creep expansion could be enclosed. to do that, a cp20 calibration table (175 × 140 mm²) was used, while the optics were adjusted on a depth of field equal to 5.6 mm. for post-processing needs, 35 stages in step of 300 s were acquired during the test. a facets size of 13 × 13 pixels with a relative distance of 8 pixels was chosen for computer processing of the dic meshes. each single creep bulge test lasted 180 minutes in order to ensure the complete relaxation of the material within the linear and elastic strains field. due to the temperature dependence of the tested elastomer, the temperature was constantly monitored and maintained at 20°c ± 0.5°c for the entire duration of the test. to use both stereoscopic methods it was necessary to analyse the two sides of each specimen individually. to avoid permanent deformation on the specimen between one test and another, the inflation pressure was kept constant at 0.55 bar that is a value well below the limit pressure. however, the nature of filled hyperelastic materials is such that even specimens from the same sheet can have different mechanical characteristics, especially when analyzed on opposite sides. therefore, based on the results obtained, it is legitimate to consider that the bias errors may be attributable to any differences in mechanical behavior between one side and the other of the specimen. 7. neural network architecture the neural network architecture used in this work was a ffnn (feed forward neural network) composed by 4 layers fully connected [19], [27]. the model had 2 n + 1 inputs (the cartesian coordinates (x, y) of the n points resulting from the intersection of meridians with parallels of the 3d image and the inflation pressure value) and n outputs: the height z relative of each point. the structural simplicity of such architecture (figure 8) allowed achieving a good quality of the outputs without a large waste of computational power and within training period relatively short. every layer had the sigmoid activation function and the hidden layers were composed by 30 neurons; only the last layer, the output layer, had a linear activation function. the model was trained with the dataset built using the cartesian points (x, y, z) from the 3d reconstruction of the dome and the inflation pressure during creep bulge test, and adopting a levenberg-marquardt algorithm as optimizer [28]. mse (mean square error) loss function and a technique of early stop were used in order to prevent the over-fitting of the model. 8. analysis of the results and conclusions figure 9 and figure 10 show the typical reconstruction of the dome at the end of the test, computed from epipolar geometry and from 3d-dic algorithm, respectively. figure 11a shows the plot of the inflating pressure as a function of the time, highlighting how this parameter, except for the instants in which the pressure regulator acts, remains constant during the whole test, as desired. from the comparison between the trends of the strain as function of the time resulting from the two stereoscopic techniques (figure 11b), as previously stated, it can be noted that the epipolar reconstruction can exploit a greater number of acquisitions respect the dic approach (180 vs 35). therefore, the timing resolution of the epipolar technique is better than the dic one. however, the spatial resolution is better in the latter, minimizing the error on the measurement of the dome apex height. in effect, while the 3d-dic spatial resolution is based on a points cloud, the epipolar spatial resolution is based on a limited homothetic markers set. a way to confirm the better quality of the 3d-dic reconstruction compared to that obtained with the epipolar geometry is based on the use of ffnn models. thus, two independent models were obtained by training the ffnn neural network on the acquired datasets. first dataset consists of the acquisitions on the 5 specimens analysed with the epipolar technique, the second one of those acquired on the same specimens with the 3d-dic technique. these models have as input the datasets, i.e. the set of the (x, y) coordinates of the acquired points and, as output, the height of the dome apex (z) from each acquisition. the output was used to estimate the bias error with respect to the reference laser value (hlaser) on the corresponding acquisition. figure 8. ffnn architecture. figure 9. epipolar geometric reconstruction of the dome (courtesy of authors [22]). figure 10. 3d-dic reconstruction of the dome. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 199 our results showed that the 3d-dic technique leads to smaller bias errors (table 1). with notations of figure 2, from the direct laser measurement (e), being fixed the working distance (u) at 400 mm (section 6), the indirect measurement of the dome apex height (hlaser) is geometrically given by: ℎ𝑙𝑎𝑠𝑒𝑟 = 𝑣 − 𝑚 − 𝑢 − 𝑒 , (4) where m = 75 mm and v = 680 mm. as previously stated, the measurements provided by the equation (4) were taken as references both for those obtained with the epipolar technique on one side of the specimen, and for those obtained with the 3d-dic technique on the other side. table 1 shows the best values of the dome apex height and the standard deviations (sd) of the measurements from the laser sensor (one for each sample side) and the two ffnn models after creep, where the coverage factor k for the uncertainty of each measurement was considered equal to 1. table 1 also shows the bias errors of each of the models with respect to its own reference value. each bias error is given by the difference, in absolute value, between the ffnn model value and the relative laser value. figure 12 plots gaussian distributions resumed in table 1. the small deviation observed between the values from ffnn models relative to dic and epipolar reconstructions, as shown in figure 12a and figure 12b is likely due to variations in the hyperelastic behavior of the specimens depending on the side analyzed and not to the inaccuracy of the experimental setup. this is confirmed by the corresponding variation in the reference values provided by the laser sensor. references [1] m. mooney, a theory of large elastic deformation, j appl phys, 11, 1940, 582-592. doi: 10.1063/1.1712836 [2] r. s. rivlin, large elastic deformations of isotropic materials iv. further developments of the general theory, phil trans r soc lond a, 241, 1948, 379-397. doi: 10.1098/rsta.1948.0024 [3] l. r. g. treloar, the physics of rubber elasticity, clarendon press, oxford, 1975. 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(b) trends of the strains from 3d-dic and epipolar geometry techniques. figure 12. comparison between gaussian distributions for the height of the dome apex after creep from (a) laser measurements and epipolar ffnn model and (b) laser measurements and 3d-dic ffnn model. table 1. comparison between epipolar and dic ffnn models respect to its own laser measurements. measurement units in mm. 𝒉𝐥𝐚𝐬𝐞𝐫 (𝐞𝐩𝐢𝐩. 𝐬𝐢𝐝𝐞) ± 𝑺𝑫 𝒉𝐥𝐚𝐬𝐞𝐫 (𝐃𝐈𝐂 𝐬𝐢𝐝𝐞) ± 𝑺𝑫 𝒉𝐞𝐩𝐢𝐩. ± 𝑺𝑫 𝒉𝐃𝐈𝐂 ± 𝑺𝑫 𝑩𝒊𝒂𝒔𝐞𝐩𝐢𝐩. 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2nd international conference on computer graphics and interactive techniques in australasia and southeast asia 2004, singapore, pp. 139-147. doi: 10.1145/988834.988859 [28] j. j. moré, the levenberg-marquardt algorithm: implementation and theory, numerical analysis, lnm 630 (2006), pp. 105-116. doi: 10.1007/bfb0067700 https://doi.org/10.1016/j.matdes.2010.05.057 https://doi.org/10.1557/jmr.1992.1553 https://doi.org/10.1117/12.2054257 https://doi.org/10.1016/0040-6090(81)90407-7 https://doi.org/10.1016/s0167-6636(03)00025-5 http://dx.doi.org/10.1016/j.polymertesting.2008.09.001 https://doi.org/10.1007/978-3-319-45781-9_122 https://doi.org/10.1007/s12289-009-0407-y https://doi.org/10.1063/1.5035022 https://doi.org/10.1007/978-1-4020-6239-1_135 https://doi.org/10.1007/978-1-4614-4436-7_20 https://doi.org/10.1016/j.polymertesting.2017.08.009 https://doi.org/10.1561/0600000007 http://hdl.handle.net/20.500.11769/496378 https://doi.org/10.1109/34.888718 https://doi.org/10.1016/j.polymertesting.2020.106548 http://dx.doi.org/10.1016/j.polymertesting.2019.106133 https://doi.org/10.1016/j.polymertesting.2019.01.005 https://doi.org/10.1145/988834.988859 https://doi.org/10.1007/bfb0067700 from electronic navigational chart data to sea-bottom models: kriging approaches for the bay of pozzuoli acta imeko issn: 2221-870x december 2021, volume 10, number 4, 36 45 acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 36 from electronic navigational chart data to sea-bottom models: kriging approaches for the bay of pozzuoli emanuele alcaras1, claudio parente2, andrea vallario2 1 international phd programme “environment, resources and sustainable development”, department of science and technology, parthenope university of naples, centro direzionale, isola c4, (80143) naples, italy 2 dist department of science and technology, parthenope university of naples, centro direzionale, isola c4, (80143) naples, italy section: research paper keywords: enc data; 3d modelling; ordinary and universal kriging interpolation; bathymetry; geospatial analysis citation: emanuele alcaras, claudio parente, andrea vallario, from electronic navigational chart data to sea-bottom models: kriging approaches for the bay of pozzuoli, acta imeko, vol. 10, no. 4, article 9, december 2021, identifier: imeko-acta-10 (2021)-04-09 section editor: silvio del pizzo, university of naples 'parhenope', italy received march 8, 2021; in final form december 10, 2021; published december 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: emanuele alcaras, e-mail: emanuele.alcaras@uniparthenope.it 1. introduction the knowledge of the seabed conformation allows not only safe navigation [1], but also the development of new frontiers for other kind of studies such as those on directions and intensity of currents [2], sensitivity of habitats and species [3], [4], and other activities such as maintenance dredging [5], coastal infrastructure protection [6] etc. it is therefore obvious that knowing the seabed and its variability is a fundamental requirement in marine studies. the hydrographic institute of the italian navy (istituto idrografico della marina militare iimm) represents the primary public authority in the study and analysis of the sea depths surrounding the italian peninsula [7]. it is, in fact, the only institution that can produce certified cartography for navigation in italy. the iimm conduces hydrographic surveys that permit to acquire water depth measurements. hydrographic survey can be carried out by using different techniques. single beam sonar (sbs) and multi beam sonar (mbs) determine the depth of any waterbody by using sound beams. particularly, they measure the time lag between transmitting and receiving a signal that travels through the water, springs back the seafloor, and returns to the sounder; the time lag is converted into a range using the known speed of sound [8]. sbs is a less expansive system that mbs but provides much lower spatial resolution [9]. a good level of information about seabed morphology can be extract by multispectral satellite images, even if only in shallow water (depths less than 20 meters) [10], [11]. the results of bathymetric survey are used for nautical charts that provide seabed morphology through depth points and contours [12]. available in digital form (raster or vector), nautical charts are legible and manageable by information systems supporting ship navigation, i.e. electronic charting systems (ecss) and electronic chart display and information systems (ecdiss) [13]. when they are in vector format and comply abstract electronic navigational charts (encs), official databases created by a national hydrographic office and included in electronic chart display and information system (ecdis), supply, among essential indications for safe navigation, data about sea-bottom morphology in terms of depth points and isolines. those data are very useful to build bathymetric 3d models: applying interpolation methods, it is possible to produce a continuous representation of the seafloor for supporting studies concerning different aspects of a marine area, such as directions and intensity of currents, sensitivity of habitats and species, etc. many interpolation methods are available in literature for bathymetric data modelling: among them kriging ones are extremely performing, but require deep analysis to define input parameters, i.e. semi-variogram models. this paper aims to analyze kriging approaches for depth data concerning the bay of pozzuoli. the attention is focused on the role of semi-variogram models for ordinary and universal kriging. depth data included in two encs, namely it400129 and it400130, are processed using geostatistical analyst, an extension of arcgis 10.3.1 (esri). the results testify the relevance of the choice of the mathematical functions of the semi-variogram: stable model supplies, for this case study, the best performance in terms of depth accuracy for both ordinary and universal kriging. mailto:emanuele.alcaras@uniparthenope.it acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 37 specific standard established by the international hydrographic organization (iho), digital nautical charts are named electronic navigational charts (encs). ecdis and enc are the two fundamental components of the electronic navigation: the first performs the functions of the hardware, the second supplies the relevant information for a voyage, including bathymetry in form of depth points and contour lines. regardless of the technique with which they are obtained and the source from which they are extracted, data concerning seabed morphology can be used to produce bathymetric model that, according to iho, can be defined, as ‘‘a digital representation of the topography (bathymetry) of the seafloor by coordinates and depths’’ [14]. when a point cloud dataset is available, i.e. single beam data or depth data derived from a nautical chart, a spatial interpolation process is necessary to generate a 3d model. starting from irregular spaced measured points, the depths in unsampled areas must be calculated, using appropriate grid spacing related to the accuracy of the input data [15]. the number of nodes which compose the grid is a variable which impacts on the accuracy of model: the more nodes there are, the better the accuracy will be [16]. spatial interpolation is a concept strongly linked with digital terrain models (dtms), introduced by miller & laflamme [17], or more generally with digital elevation models (dems). dems can be defined as the digital representation of the earth surface elevation referred to any given geodetic reference system [18], [19]: a 3-dimensional representation of a terrain surface consisting of three-dimensional coordinates (i.e. e, n, h or λ, ϕ, h) stored in digital form [20]. they can be produced in different way, i.e. using 3d stereoscopic viewing photogrammetric methods [21], [22] or interpolating height points and/or contour lines [23], [24]. interpolation methods that support dem generation can be used not only for terrain modelling, but for seabed modelling too. this can be indicated as digital depth model (ddm) because describes the variability of the distance between the sea surface and sea bottom [25]. several interpolation methods are offered by gis software to interpolate depth values, each of them has its own advantages and disadvantages [26], but in many cases, the most performing ones result kriging interpolators [24]. they cannot be applied in an automatic way but require to be tested, due to the specificity of the analysed surface morphology that may influence the results [27], and to be supervised by the user to set specific parameters. one of these parameters is the semi-variogram model, that is a graphical representation of the spatial correlation between the measurement points. an approximating function is used instead of the experimental semi-variogram [28], [29]. the aim of this article is to analyse kriging approach, specifically ordinary kriging and universal kriging methods, and demonstrate that the level of accuracy that can be achieved depends crucially on the choice of the mathematical model of the semi-variogram. this paper is organized as follows. section 2 focuses on the study area and dataset. section 3 describes: firstly, the main characteristics of the kriging approach based on semi-variance concept, with particular attention to the methodological aspects of ordinary and universal kriging, then the analysis process for ddm accuracy evaluation. section 4 introduces and discusses the results obtained in dependence of the choice of the mathematical function to fit the experimental semi-variogram (11 different models are compared). finally, section 5 presents our conclusions. 2. study area and dataset the area considered for this study concerns the bay of pozzuoli, located north-west of naples, in the campania region (italy), as shown in figure 1. in the bay of pozzuoli, the inner continental shelf, that extends between 0 – 40 m below sea level (b.s.l.), varies significantly, from a few hundred meters at its western side (baia) to 1.6 km at its eastern side (between bagnoli and nisida), reaching 1.8 km west of pozzuoli [30]. in the seabed morphology, gentle slopes prevail, and several terraced surfaces mostly oriented n130°e occur: those terraced areas present widths up to 1.5 km in the easternmost side of the bay and as small as 0.5 km in the west [31]. the area is of great interest for many purposes: as a consequence of the overall subsidence starting at the end of the roman period, the main part of the ancient coastal strip, including all the buildings and maritime structures, is nowadays submerged below the marine surface [32], giving life to the “underwater park of baia”, which is the subject of numerous figure 1. geo-localization of the bay of pozzuoli in the national context (top) and a rgb composition from sentinel-2 imagery of the bay of pozzuoli (bottom). acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 38 studies including those through underwater acoustics techniques [33], [34]. moreover, pozzuoli inland zone and pozzuoli bay are included in the active volcanic sector named “campi flegrei”, corresponding to a densely populated region with complex geological context [35]. as it usually happens in the case of volcanic areas, in addition to the calderas, several fault systems surround the craters in the gulf of pozzuoli [36]. particularly, it is characterized by at least one large caldera collapse structure which extends over an area of 8 km in diameter in the central sector and is associated with the eruption of the neapolitan yellow tuff (nyt), an ignimbrite deposit dated 15 ka b.p [37]. this exceptional environment, severely sacked over the years, has been included in a marine protected area since 2001 [38]. the seafloor of the gulf of pozzuoli has also been largely studied for sedimentological analysis [39], biological research [40], gravimetric measurements [41], sea-level changes [42], marine circulation [43], etc. furthermore, several bathymetric surveys have been conducted in the gulf of pozzuoli [31], [44]; therefore, the area presents excellent conditions for 3d modelling. depth data for this work are extracted from two encs produced by the iimm, in scale 1:30.000, identified as n° 129 and n° 130. the two sources are necessary as the area falls half in one and half in the other nautical chart. the original files are formed in accordance with the official standards established by the international hydrographic organization (s-57 iho) [14]. they are transformed in shape file for using them in arcgis version 10.3.1 (esri) [45]. encs are georeferred in wgs84 geodetic datum. the depths are referred to the mean lower low water (mllw), which is the average height of the lowest tide recorded at a tide station each day during a recording period [46]. encs are classified in six categories or “zones of confidence”, ranging from high accuracy to poor accuracy at the other end of the spectrum, and one category of “unassessed” [47]. the considered encs are classified d: it means that soundings are similarly sourced from historic surveys, but in this case those conducted with large distances between adjacent survey lines, or simply soundings collected on an opportunity basis by ships undertaking routine passage [48]. in absence of specific indications, we assume this dataset as characterized by nominal scale accuracy. this assumption is not in contradiction with our purpose, that is to establish the effectiveness of the kriging interpolators for bathymetric data in consideration of the map scale. firstly, we projected the dataset in the universal transverse of mercator (utm)/wgs84 zone 33 n (epsg code: 32633), and group the vertices of contour lines and the depth points in one shape file; formerly we select from them only ones that fall in the area shown in figure 2. this area extends within the following utm/wgs84 plane coordinates 33t zone: e1 = 422,600 m, e2 = 430,400 m, n1 = 4,514,200 m, n2 = 4,520,500 m. depth values range between 0 m and -142 m. however, from the same encs a less extended area was considered in a previous step of our research and a more limited dataset was extracted for bathymetric modelling applications; the first results were published in [49]. in this study, the resulting 3181 points are used as dataset for the application of the ordinary and universal kriging interpolation methods available in geostatistical analyst [50], an extension included in arcgis software. 3. methodology kriging is founded on the first law of geography introduced by waldo r. tobler's in 1969: "everything is related to everything else, but near things are more related than distant things" [51]. in other words, things closer together are more similar than things further away, so observations made on nearby points actually show less variability than observations made between distant points [52]. unlike to deterministic methods, kriging applies the geo-statistical model which includes the spatial correlation between sampled points and uses it to estimate the value at an unknown point [53]. kriging interpolation method assumes that the spatial variation of any continuous attribute is often too irregular to be modelled by a simple mathematical function. the variation can instead be better described by a stochastic surface [54]. in fact, kriging is included in stochastic interpolation approach, that is funded on the assumption that the link between the neighbouring points can be expressed by a statistical relationship, which may have no physical significance [55]. as a result, kriging approach can supply models that better represent and describe the territory variability: the method usually ensures a more reliable prediction of the values in the non-sampled points. in order to evaluate the variability of the points with increasing distances, the semi-variogram is adopted. the semivariogram is the diagram resulting from the representation of the semi-variance as a function of the distance between two points [56]. mathematically the semi-variance is given by [57]: 𝛾(ℎ) = 1 2 𝑛 ∑(𝑧(𝑥𝑖 ) − 𝑧(𝑥𝑖 + ℎ)) 2 𝑛 𝑖=1 , (1) where 𝛾(ℎ) is the value of the semi-variance at the distance ℎ; 𝑛 is the number of couples of points separated by ℎ; 𝑧 is the value of the depth; 𝑥𝑖 and 𝑥𝑖 + ℎ indicate the positions of each couple of points. to facilitate the procedure and make it faster, the pairs are grouped into lag bins. a good lag size has to be determined for grouping semi-variogram values. in this way, the size of a distance class into which pairs of locations are grouped permits to reduce the large number of possible combinations [58]. consequently, the semi-variance is calculated for all pairs of points that present distance within a specific range (e.g. 10 meters and 20 meters). mathematical models can be used to substitute the empirical semi-variogram, fitting the data in the best way: the standard model that finest approximates the empirical one has to be figure 2. the selected point dataset (in the green rectangle), extracted from enc depth information of the bay of pozzuoli, submitted to kriging interpolations. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 39 selected, in order to obtain a law that describes the trend of the random variable on the territory throughout the area covered by the samples [59]. this substitution permits to introduce in the kriging process semi-variogram values for lag distances that are not used in the empirical semi-variogram [60]. in kriging interpolation process, different weights are attributed to the measured values and chosen in such a way as to optimize the interpolating function [61]. to determine the weights, various approaches are adopted: this is a peculiar aspect that distinguishes different methods to implement kriging interpolation, as remarked in section 3.1 and section 3.2 for the considered ordinary kriging and universal kriging. the application of these methods requires to define some parameters as illustrated in section 3.3. because this article is aimed to analyse the role of the semivariogram model in ordinary kriging as well as in universal kriging, in section 3.4 the adopted approach to evaluate the accuracy of the resulting models is illustrated. 3.1. ordinary kriging ordinary kriging is the most widely used kriging method. it assumes the model [62]: 𝑧(𝑥0) = ∑ 𝜆𝑖 𝑧(𝑥𝑖 ) 𝑛 𝑖=1 (2) where 𝜆𝑖 are the kriging weights computed from a normal system of equations derived by minimization of the error variance. the function 𝑧(𝑥𝑖 ) is composed of a deterministic component µ and a random function 𝜀(𝑥𝑖 ) [63]: 𝑧(𝑥𝑖 ) = 𝜇 + 𝜀(𝑥𝑖 ). (3) the deterministic component is a constant value for each 𝑥𝑖 location in each search area. 3.2. universal kriging the universal kriging model assumes that the deterministic component can be expressed locally as a linear combination ∑ 𝑎𝑙 𝑓 𝑙 (𝑥)𝑙 of k known basis functions 𝑓 𝑙(𝑥) (generally polynomials) with unknown coefficients 𝑎𝑙 [64]. equation 3 can be re-written for this method as follow [65]: 𝑧(𝑥𝑖 ) = 𝜇(𝑥𝑖 ) + 𝜀(𝑥𝑖 ). (4) if compared with ordinary kriging this model is in general more difficult to implement [66]. 3.3. parameter settings to apply ordinary kriging as well as universal kriging, the user must define some parameters. first of all, the points involved in the estimation of depth value in each prediction location have to be established. according to the first law of geography, the user can take into account that the correlation of the measured values with the prediction value depends on the distance that separates dataset points from grid node and decreases as the distance increases. consequently, a search neighbourhood is necessary to exclude far points from the interpolation process to predict the depth at a specific location. the user defines shape and dimensions of the neighbourhood: in our experiments, an equal influence on the grid node is attributed to the surrounding points, so the same dimension of search is fixed for both semi-axis (isotropic model). the fixed value for the search radius defines the number of the points included in the neighbourhood. the search radius is not the only parameter to define. in addition, the user can divide the search area into sectors and ensure a minimum and a maximum number of surrounding points to be included in the interpolation process. however, the definition of the range of the surrounding point number is possible also in the case of a unique neighbourhood without sector division [67]. in our study, four sectors with an offset of 45° are used. an example of searching neighbourhood step for ordinary kriging application is shown in figure 3. the dialog box of the software for kriging application, usually permits to set also number and size of the lags to group semivariogram values. 3.4. cross-validation in order to evaluate the accuracy of each method, crossvalidation is adopted. it allows to define the level of accuracy of the predicted values by distinguishing between training set and validation set, the first used for model generation, the second for model evaluation [68]. there are several cross-validation approaches, among which one of the most adopted is the leaveone-out method. leave-one-out method is based on the removal of a point from the data to be interpolated, the use of the other points to estimate a value at the location of the removed point, and the performance test by means of the removed data [69]. to evaluate the performance of the selected interpolation method, the difference between the known value and the estimated value in each removed point is calculated [70]. for the experiments carried out in this study, the residuals are treated with a statistical approach, obtaining minimum, maximum, mean, standard deviation and root mean square error (rmse). 4. results and discussion in this study, ordinary and universal kriging are applied to the chosen dataset by varying all mathematical semi-variogram models available in geostatistical analyst. specifically, the following models are adopted: • gaussian model (gam), • circular model (cim), • exponential model (exm), • spherical model (spm), • tetraspherical model (tem), • pentaspherical model (pem), • stable model (stm), figure 3. searching neighbourhood step for ordinary kriging application. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 40 • j-bessel model (jbm), • k-bessel model (kbm), • rational quadratic model (rqm), and • hole effect model (hem). those models are described in the literature and some of them are very recurrent in kriging applications, so the readers could referrer to specific papers on this matter, e.g. [71]-[74]. to define the cell size of each model, the enc scale is considered. on this aspect several suggestions are available in literature. on the question of cell size definition for each raster map, waldo tobler [75] advised that the rule is: divide the denominator of the map scale by 1,000 to get the detectable size in meters; the resolution is one half of this amount. valenzuela and baumgardner recommended cell sizes ranging from 0.5 mm × 0.5 mm to 3 mm × 3 mm on the map when dealing with thematic maps in a raster-based gis [76]. relating grid resolution to cartographic concepts, tomislav hengl [77] proposed the following formulas for finding the right pixel size p (in meters) for dtm: 𝑝 ≤ 𝑆𝑁 ∙ 0.0025 m (5) 𝑝 ≥ 𝑆𝑁 ∙ 0.0001 m , (6) where sn is scale factor. he also suggested the following formula as a good compromise: 𝑝 = 𝑆𝑁 ∙ 0.0005 m . (7) in this application, considering also the enc poor accuracy (zoc = d) we fixed the cell at 30 m for the generated ddms. using all semi-variogram models available, eleven grids are generated for each kriging method. in the case of the ordinary kriging, a number of lag equal to 12 is chosen with a lag size of 290 m [78] and an isotropic search radius of 2300 m [67]. in the case of the universal kriging, a number of lag equal to 12 is chosen with a lag size of 72 m and an isotropic search radius of 570 m. in figure 4, 2d representations of 2 bathymetric models georeferred in utm-wgs84 plane coordinates, concerning the ordinary kriging applications are reported. particularly, the upper concerns the most performing model resulting from stm application, the lower concerns the worst performing model resulting from hem application. in the same way, in figure 5, 2d representations of 2 bathymetric models georeferred in utm-wgs84 plane coordinates, concerning the universal kriging applications are reported. also in this case, the upper concerns the most performing model resulting from stm application, the lower concerns the worst performing model resulting from hem application. in figure 6 to figure 9, four examples of semi-variogram generated for ordinary kriging applications, respectively by stm (first), hem (second), exm (third), spm (forth) are shown. significant statistical parameters (minimum, maximum, mean, standard deviation and root mean square error) of all residuals for each semi-variogram mathematical function are shown in table 1 for ordinary kriging applications and in table 2 for universal kriging applications. figure 4. 2d bathymetric models resulting for ordinary kriging approach from the application of stable semi-variogram model (top) and hole effect semivariogram model (bottom). figure 5. 2d bathymetric models resulting for universal kriging approach from the application of stable semi-variogram model (top) and hole effect semi-variogram model (bottom). acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 41 the 3d visualization of the most performing bathymetric model, generated by ordinary kriging interpolator with stm, is shown in figure 10. note that the depth values have been multiplied for amplifying factor equal to 3, so as to enhance the visualization of seabed morphology. the results of the elaborations demonstrate the different levels of accuracy than can be achieved in dependence of the choice of the semi-variogram model for both kriging approaches. for ordinary kriging the range of minimum values goes from -11.916 m obtained for kbm, to -16.703 m resulting from exm. the range of maximum values goes from 12.000 m obtained for kbm, to 15.612 m resulting from exm. the range of mean values goes from -0.152 m obtained for hem, to 0.074 m resulting from exm. the range of standard deviation goes from 1.280 m for stm to 1.781 m resulting from hem. the range of rmse goes from 1.280 m for stm to 1.787 m resulting from hem. by analyzing the rmse values, stm seems to be the most performing semi-variogram model, while hem supplies the worst results. for universal kriging the range of minimum values goes from -13.739 m obtained for tem, to -16.361 m resulting from jbm. figure 6. example of semi-variogram generated for ordinary kriging application based on the exponential model. figure 7. example of semi-variogram generated for ordinary kriging application based on the exponential model. figure 8. example of semi-variogram generated for ordinary kriging application based on the stable model. figure 9. example of semi-variogram generated for ordinary kriging application based on the hole effect model. table 1. statistical terms of the residuals supplied by cross validation for the ordinary kriging. model min (m) max (m) mean (m) st.dev (m) rmse (m) gam -13.973 12.039 -0.142 1.699 1.705 cim -15.830 15.158 0.064 1.450 1.451 exm -16.703 15.612 0.074 1.482 1.484 spm -15.814 15.168 0.064 1.448 1.449 tem -15.800 15.186 0.063 1.447 1.448 pem -15.790 15.212 0.063 1.446 1.447 stm -13.440 12.717 0.008 1.280 1.280 jbm -13.226 13.254 -0.145 1.717 1.723 kbm -11.916 12.000 -0.085 1.463 1.465 rqm -14.456 14.627 -0.060 1.456 1.457 hem -15.051 14.746 -0.152 1.781 1.787 table 2. statistical terms of the residuals supplied by cross validation for the universal kriging. model min (m) max (m) mean (m) st.dev (m) rmse (m) gam -16.351 16.652 0.067 1.524 1.525 cim -13.995 15.630 0.066 1.438 1.440 exm -14.371 15.395 0.082 1.476 1.478 spm -13.827 15.283 0.066 1.435 1.437 tem -13.739 15.460 0.069 1.444 1.446 pem -14.093 15.560 0.072 1.452 1.454 stm -14.158 15.232 0.074 1.426 1.428 jbm -16.361 17.083 0.065 1.535 1.536 kbm -14.353 15.284 0.079 1.430 1.432 rqm -15.523 15.716 0.075 1.488 1.490 hem -15.435 19.163 0.062 1.578 1.579 acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 42 the range of maximum values goes from 15.232 m obtained for stm, to 19.163 m resulting from hem. the range of mean values goes from 0.062 m obtained for hem, to 0.082 m resulting from exm. the range of standard deviation goes from 1.426 m for stm to 1.578 m resulting from hem. the range of rmse goes from 1.428 m for stm to 1.579 m resulting from hem. by analyzing the rmse values, also in this case, stm seems to be the most performing semi-variogram model, while hem supplies the worst results. in every case, rmse values are acceptable in consideration of the accuracy of the encs from which the depth data are derived. the best performance of stm obtained in this case study cannot be generalized. in other words, the comparison offers the possibility to establish in this specific case that stm offers the best performance, but it cannot be asserted that it will always be so. 5. conclusions the particular relevance of the study area, the bay of pozzuoli, in many fields, e.g. geology, archaeology and natural science, makes clear that accurate bathymetric models are fundamental to support studies and application. a first source of information about seabed morphology is included in enc: profundity information contained in isolines and depth points is useful to realize 3d models of the seabottom. to achieve this result, particular attention must be reserved to the interpolation approach to derive continuous model from cloud point dataset. the present research remarks the high performance of both the kriging approaches for this purpose and demonstrates the relevance of the choice of the mathematical model to build the semi-variogram for ordinary kriging as well as universal kriging. as tested by using leave-one-out cross validation, different levels of accuracy can be achieved in dependence of the function used to substitute the empirical semi-variogram, fitting the depth data in the best way. by analysing residuals between measured and interpolated values of bathymetric depths, it is possible to identify the best performing 3d model of seabed in the study area. the approach adopted for the bay of pozzuoli can be used each time bathymetric data are available and usable for 3d model of seabed. in this way, the choice of the most suitable semivariogram model supports the user to achieve a more performing 3d bathymetric model. appling kriging methods, the specificity of the considered area, as well as the distribution of the dataset points influence the quality of the results, so it is impossible to define in an absolute way the most effective semi-variogram model to be adopted. rmse analysis carried out on residuals resulting from leave-oneout cross validation remains the best approach to compare different mathematical functions for semi-variogram construction. acknowledgement this work synthesizes results of experiments executed within a research project performed in the laboratory of geomatics, remote sensing and gis of the “parthenope” university of naples. we would like to thank the technical staff for their support. references [1] j. duan, x. 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digital twin; remote control; augmented reality; measurement instrumentation citation: annalisa liccardo, francesco bonavolontà, rosario schiano lo moriello, francesco lamonaca, luca de vito, antonio gloria, enzo caputo, giorgio de alteriis, digital twins based on augmented reality of measurement instruments for the implementation of a cyber-physical system, acta imeko, vol. 11, no. 4, article 15, december 2022, identifier: imeko-acta-11 (2022)-04-15 section editor: leonardo iannucci, politecnico di torino, italy received november 14, 2022; in final form november 26, 2022; published december 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: rosario schiano lo moriello, e-mail: rschiano@unina.it 1. introduction the concept of cyber-physical systems (cps) was first presented in 2006, introduced by helen gill of the national science foundation [1], in the united states, to denote a plane of "local sensation and manipulation of the physical world" correlated with a virtual plane of "real-time control and observability." the concept is presented as an evolution of embedded systems in which the computational capability and its effects descend deep into every physical component of the system and even within the materials [2]. from this initial vision, the concept of cps has taken on a breadth over time that does not facilitate its unambiguous and definitive conceptualization or representation. the most common definition of cps, as integration of computational resources and physical processes, now seems too simplifying as other elements, particularly large-scale network connectivity, have rightfully entered the perimeter of cps. in its current most common definition, cps is considered as an integration of systems of different natures whose main purpose is the control of a physical process and, through feedback, its adaptation in real-time to new operating conditions. this is achieved by the fusion of physical objects and processes, computational platforms, and telecommunications networks [3], [4]. the term physical refers to actual objects as they are "perceivable" by human senses, while the term cyber refers to the abstract the recent increase on the internet of things and industry 4.0 fields has led the research topics to investigate on the innovative technologies that could support these emerging topics in different area of applications. in particular, the current trends are to close the gap between the physical and digital worlds, thus originating the so-called cyber-physical system (cps). a relevant feature of the cps is the digital twin, i.e., a digital replica of a process/product with which user can interact to operate on the real world. in this paper, the authors propose an innovative approach exploiting an augmented reality solution as digital twin for the measurement instrument to obtain a tight connection between the measurements as physical world and the internet of things as digital applications. in fact, by means of the adoption of the 3d scanning strategy, augmented reality software and with the development of a suitable connection between the instrument and the digital world, a cyber-physical system has been realized as an iot platform that collect and control the real measurement instrument and makes its available in augmented reality. an application example involving a digital storage oscilloscope is finally presented to highlight the efficacy of the proposed approach. mailto:rschiano@unina.it acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 2 virtual image in which actual objects are represented and enriched with one or more additional layers of information. the relationship between physical objects and their virtual "interpretation" has been referred to by some authors as the effective term "social network of objects" [5]. cpss, therefore, are based on related objects that, through sensors, actuators, and network connections, generate and acquire data of various kinds, thus reducing distances and information asymmetries between all elements of the system [6]. with the help of widespread sensors, the cps can autonomously determine its current operational state within its environment and the distance between its component objects. actuators perform planned actions and execute corrective decisions, optimizing a process or solving a problem [7]. decisions are made by intelligence that evaluates information internal to the cps and, in some scenarios, also information from other cpss [8]-[10]. in this context, a very important aspect is the interaction with measurement systems (in terms of both sensors or sensor networks and actual measurement instruments). in this case, the aim is to make the measurement system an integral and priority part of the cps; realizing the digital twin of measurement systems allows possible users to be able to "touch with their hands" is, therefore, a desirable condition [11]. remote control of instruments is a research activity that saw its first examples in the 1990s; however, such activities were only aimed at enabling measurements to be made by remote programming of instruments. instead, the point of view, both educational and industrial, has changed, requiring a faithful replication of the system to be controlled with direct interaction of the operator (whether student or worker) on the instrument. for this reason, the authors propose an augmented realitybased approach to create a digital twin of the measuring instrument that can be controlled and operated remotely; for this purpose, several enabling technologies inherent in industry 4.0 and internet of things paradigms are used, such as augmented reality and mqtt communication protocols. the goal is to enable users to operate the remote instruments as if they were in their presence [12]. the paper is organized as follows: in section 2, a literature review on cps and ar-based applications is presented, while the proposed method is described in detail in section 3. an application example is given in section 4 before drawing the conclusions in section 5. 2. related work providing for an exhaustive review of the exploitation of ar in cyber-physical systems is a difficult challenge due to the wide variety of configurations and application fields they are interested in, such as in additive manufacturing [13]-[16] , industry 4.0 [17][22] and autonomous vehicle [23]-[25]. as an example, an application of cps in the manufacturing environment is proposed in [7], where the real-time data are sent into cyberspace through different types of networks to build a digital twin of the machine tool. finally, ar is exploited as an interface between human and the cps to mainly retrieve information about the ongoing processes. in [26], the authors present a closed-loop cooperative human cps for the mining industry that exploits the information obtained from ar and virtual reality (vr) systems. the main goal of this research is to allow human interaction with the mining operation by means of a deep integration of ar and vr; this integration makes visual information available to the operator that is supported during the operations in terms of making the correct decisions, conduct inspections and interacts with the equipment. a compelling cps for autonomous vehicle applications has been proposed in [9]. the authors have developed an ar indoor environment for testing and debugging autonomous vehicles that act in such a way to realize dynamic missions in laboratory environments for planned mission testing and validation; the proposed solution is realized by exploiting ground and aerial systems, motion cameras, ground projectors, and network communications to obtain a standard procedure for testing and prototyping environment for cpss. finally, in a field test performance of perception, planning, and learning algorithms in real-time have been evaluated. furthermore, in [27], context-aware guidance of cyberphysical process has been proposed for the press line maintenance process. in particular, by means of a suitable context graph, it was possible to manage and structure the cps sensor data. in addition, an ar application is adopted to support the interaction of users with the cps processes thanks to the integration of position and marker sensors in the proposed solution; the object detection is improved by means of the digital data that ensures improved guidance in the process execution. the adoption of ar with cps allows to support the end-user in the manual tasks where it is guided and monitored during the operations. an interesting application is evaluated in [28], where a suitable ar navigation system for industrial applications has been proposed. the authors have developed a prototype of an autonomous vehicle that interacts with a robot arm. in particular, the robot arm interacts with the vehicle when the correct position is reached, and the vehicle navigation is obtained by exploiting an ar solution based on markers recognition; these markers are used as system reference position points. finally, the research carried out in [29] has highlighted that the integration of iot platforms and ar software as digital twin is the most suitable technology to close the gap between the physical and digital worlds by means of the definition of a digital twin architecture model, the introduction of a digital twin service and the investigation on the key elements of industry 4.0 required for the realization of a digital twin. differently from the solutions mentioned so far and mainly intended to exploit ar as a fundamental information layer for the interaction with cps, the authors want to propose a method to realize a cps for measurement instruments that act as an interface between the actual and ar world. instrument control and the related measurements are not carried out as simulations, but the ar interaction between user and rendered instruments corresponds to real physical state changes in the real world. to accomplish the considered target, a general framework for the definition, implementation, and assessment of an example of digital twin application will be presented in the following by referring to a typical measuring instrument. 3. proposed method the method adopted to create a digital twin of the measurement instrument exploits an augmented reality approach based on the framework shown in figure 1. the first step is dedicated to generating the 3d geometric model of the desired instrument; operating approaches typical of reverse engineering can be applied to the purpose. in particular, the reverse acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 3 engineering approach uses (i) suitable tools to acquire 3d spatial data of objects and (ii) dedicated software environments to manipulate and convert them into useful information. faro's non-contact laser scanarm [30] (figure 2) was chosen to acquire the image and the main dimensional information about the instruments; the output of the scan operation consists of clouds of points in the 3d space. thanks to computer-aided design (cad) systems, it has been possible to define mathematical representations allowing the reconstruction of the instrument geometry, thus obtaining a 3d cad model of the desired object. in order to move from point clouds to the extraction of both surface and geometrical characteristics, suitable software as well as reverse engineering techniques were combined to provide a very efficient and robust solution. in particular, the point clouds were first handled using geomagic wrap software [31] to transform those data into polygonal meshes, and then the 3d image reconstruction was performed. from the reconstruction of case, front panel, back panel and support systems, the 3d model of the instrument has been obtained; buttons and knobs have been separately reconstructed and placed one by one in their well-defined positions. the scanning phase produces a file with an obj extension containing an empty container, a simple 3d view of the instrument (figure 3). the successive step aimed at transforming the obtained 3d object into an augmented interactive object; the 3d graphics development platform unity has been used to the purpose. every interaction that the user performs on the augmented reality object is translated in the corresponding operation executed on the actual instrument. to this aim, a typical iot protocol called message queue telemetry transport (mqtt) [32] is used to communicate with the laboratory where the instruments are placed. in particular, the lab is equipped with a figure 1. proposed solution workflow: from the physical world to the digital twin. figure 2. 3d scanning strategy of the instrument exploiting a non-contact laser scanarm by farotm. acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 4 personal computer connected from one side to ethernet network and on the other side to the instruments. the pc converts received mqtt messages into messages compliant with the protocol used by instruments (ieee488), in order to forward to the instruments commands and request corresponding to ar user's operations. a suitable software on the pc implements a mqtt client, which has the role of receiving all messages from the augmented instrument and sending them to the actual instrument and vice versa. communication among different mqtt clients is assured by a third entity, the so-called broker, mandated to dispatch messages to the client subscribed to specific information arguments, referred to as topic [33]. to assure reliable operations and continuous service as well as maintain a complete control on the exchanged data, it was decided to exploit a private broker. to realize an augmented reality application, unity exploits a software development plug-in, namely vuforia, allows to recognize images chosen as a target. in particular, vuforia is capable of assessing the quality of image target and thus providing feedback regarding the possible usage or not of that image as a target. images with a more complex pattern proved to be the best candidate as a target in terms of 3d reconstruction location and stability; for this purpose, the one adopted for the realised application is shown in figure 4 and produced a satisfying evaluation from the vuforia tool. in an augmented reality application, the image target has an important role, since it allows to locate a 3d object in a scene; when an ar application starts and the camera mounted on the device frame the considered image target, digital replicas of the object, in this case the instruments are superimposed on the image itself. as said before, the obj file contains a 3d replica of the instrument. therefore, it was necessary to import the obj file into the unity environment and make the appearance of this object similar to the actual one. regarding the oscilloscope shown in figure 3, it has been necessary to add the labels above each key, the model of the instrument in the top left-hand corner as well as additional markings and symbols that are present on the real instrument. in addition, compared to the real instrument, two '+' and '-' symbols were added on each knob to allow the user to rotate them with step values similar to those of the actual instrument. the result of this operation is shown in figure 5. the final step regards the communication between ar and actual instrument; to this aim, a software module in c# language has been implemented to recognize button pressure on the virtual object and perform the corresponding operation by sending the command to the actual instrumentation (figure 6). as said before, the protocol used to communicate with the real instrumentation is mqtt, so an mqtt client was created within the application running on the user device (e.g. smartphone, tablet or smart glasses) to send commands to the actual instrumentation and to receive the corresponding responses from them if necessary, as in case of the so-called queries [34]. in particular, the developed c# module can figure 3. oscilloscope 3d model obtained from the 3d scanning strategy. figure 4. example of image target exploited for rendering and spatially locating ar instruments. figure 5. comparison between the actual instruments and its digital twin; instrument view in the ar software environment (a), actual instrument (b). acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 5 recognize if the sequence of button pressed, and consequently of operations, are correct and in this case, sends the command to the real instrument; as previously said, in the laboratory is present a pc with a suitable software that receives these messages and sends them to the associated instruments. if the command requires a response from the instrumentation, the c# module reads these responses that are properly managed to be shown on the display of the 3d ar instrument. in this way, the user has the feeling of interfacing with the actual instrument even if he is in a different location. finally, to make the instrument's behaviour as close as possible to reality, secondary effects such as pressure emulation, knob movement or button backlighting are also reproduced. 4. application example of the implemented arbased cyber-physical system this section aims to present a case study of the proposed approach by considering the typical operations that student have to perform on the digital oscilloscope during basic metrology courses. to this aim, a proper mobile application has been realized to assess the reliability of the method. when the student interacts with the rendered 3d instrument, the first operation he/she must perform, as with the actual instrument, is to switch it on. in fact, when the oscilloscope power button is pressed in the ar application, a query is sent to the actual instrument to retrieve the waveforms currently present on the instrument's display as well as its configuration parameters (as an example, horizontal, s/div, and vertical, v/div, resolution, figure 7). this way, the student can gain awareness and knowledge about the signal currently acquired and displayed on the actual oscilloscope and, consequently, change the parameters, depending on the operations to be performed, by acting on the corresponding knobs of the v/div and s/div. in addition, the student can have information about the signal coupling. as on the actual instrument, the “ch1 menu” button must be clicked on, and the information appears on the righthand side of the display (figure 8). in the example case shown in the figure 8 the waveform coupling is “dc”. figure 6. example of implemented c# module for interaction between user, ar reconstruction and actual instrumentation. figure 7. waveform and resolution information on the ar display after turning on. figure 8. ch1 menu selection on the ar instrument. figure 9. signal level. acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 6 by turning the specific knobs located on the “ch1 menu” button it is possible to change the signal level (figure 9), so that the signal is moved up or down according to the axis origin. this is important if the student has both signals on the instrument display and he doesn’t want to display them as superimposed. moreover, it is possible to change the trigger level as well as on the actual instrument and perform operations with the cursors to measure period and amplitude of the signal. figure 10.a shows how the cursors were used to evaluate the peak-to-peak amplitude of the signal, while in figure 10.b the period was measured. it is worth noting the relevant concurrence between the display of the instrument rendered within the ar application and that characterizing the actual instrument in the laboratory. as a further case study, a typical students’ exercise is presented, mandated to measure the variation in amplitude and phase of the output signal referred to an rc filter circuit; in particular, an rc filter with a cut-off frequency of 1.7 khz was used. figure 11 shows the measurement setup present in the actual laboratory, where in this case the device under test (dut) stands for the rc filter. as can appreciated in the figure 11, the input signal of the filter is connected to channel 1 of the oscilloscope while channel 2 will show the output signal of the filter; figure 12 shows the corresponding results as the frequency of the input signal rises. in particular, figure 12.a presents the acquired (and almost superimposed) waveforms of the two channels when an input sine wave with a frequency equal to 200 hz has been applied. when the frequency of the generated signal increases up to 1 khz, (figure 12.b), the output waveform begins to be attenuated and delayed with respect to the input one. as the frequency is increased to 5 khz (figure 12.c), the filter effect of the considered dut proves to be evident with a significant attenuation and displacement of the output waveform. as for the actual lab experience, gain and phase shift of the filter can be measured by means of cursors as in figure 10; for the sake of the brevity, the procedure is not shown in the paper. 5. conclusions the purpose of the current research study was to evaluate the capabilities of cps in the measurement framework. in particular, a suitable method to develop a digital twin has been proposed, and a real implementation referred to an oscilloscope has been presented. in fact, starting from a 3d scanning strategy, the actual instrument is reconstructed in augmented reality, each front panel element is associated with an instruction of the actual instrument, and then an ar application is developed by means of the vuforia and unity environments. moreover, a suitable communication, based on the mqtt protocol, is adopted between the actual instrument and its 3d reconstruction. to this aim, a personal computer is exploited to realize (i) the physical connection with the actual instrument, (ii) the internet or local connection with the mqtt client, and (iii) an interpreter for the command sent to the instrument and the responses from the instrument outputs to the ar application. as an example, a typical application is evaluated where a filtered signal has been correctly displayed and measured in terms of frequency in the ar application. so, it was proved that the oscilloscope could be used in an ar framework where it is remotely controlled and sent the actual measure (acquired in the physical world) to the augmented reality world. it has been demonstrated that a cps for the measuring instruments can be realized, highlighting that instrument as digital twin acts in the same way as those in the real world. a) b) figure 10. evaluation of a) peak-peak amplitude and b) period of the signal. figure 11. laboratory setup in experiments with rc filter. a) b) c) figure 12. evolution of signal output of rc filter: a) signal frequency input equal to 200 hz, b) signal frequency input equal to 1 khz, c) signal frequency input equal to 5 khz. acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 7 references [1] r. bahati, h. gill, cyber-physical systems. the impact of control technology, open j. soc. sci. sci. res. publ, vol. 5, pp. 161–166, 2011. 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https://it-knowledge.faro.com/hardware/faroarm_and_scanarm/faroarm_and_scanarm/technical_specification_sheet_for_the_edge_faroarm_and_scanarm https://it-knowledge.faro.com/hardware/faroarm_and_scanarm/faroarm_and_scanarm/technical_specification_sheet_for_the_edge_faroarm_and_scanarm https://it-knowledge.faro.com/hardware/faroarm_and_scanarm/faroarm_and_scanarm/technical_specification_sheet_for_the_edge_faroarm_and_scanarm https://it-knowledge.faro.com/hardware/faroarm_and_scanarm/faroarm_and_scanarm/technical_specification_sheet_for_the_edge_faroarm_and_scanarm https://www.artec3d.com/3d-software/geomagic-wrap?utm_source=google&utm_medium=cpc&utm_campaign=12554714117&utm_term=geomagic%20wrap%7c%7ckwd-524408837989&utm_content=119631253619%7c%7c&keyword=geomagic%20wrap&gclid=cj0kcqiamaibbhcaarisakulaks3tal5eczffdelgeh https://www.artec3d.com/3d-software/geomagic-wrap?utm_source=google&utm_medium=cpc&utm_campaign=12554714117&utm_term=geomagic%20wrap%7c%7ckwd-524408837989&utm_content=119631253619%7c%7c&keyword=geomagic%20wrap&gclid=cj0kcqiamaibbhcaarisakulaks3tal5eczffdelgeh https://www.artec3d.com/3d-software/geomagic-wrap?utm_source=google&utm_medium=cpc&utm_campaign=12554714117&utm_term=geomagic%20wrap%7c%7ckwd-524408837989&utm_content=119631253619%7c%7c&keyword=geomagic%20wrap&gclid=cj0kcqiamaibbhcaarisakulaks3tal5eczffdelgeh https://www.artec3d.com/3d-software/geomagic-wrap?utm_source=google&utm_medium=cpc&utm_campaign=12554714117&utm_term=geomagic%20wrap%7c%7ckwd-524408837989&utm_content=119631253619%7c%7c&keyword=geomagic%20wrap&gclid=cj0kcqiamaibbhcaarisakulaks3tal5eczffdelgeh acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 8 d=geomagic%20wrap&gclid=cj0kcqiamaibbhcaarisakula ks3tal5eczffdelgeh [32] iso/iec 20922:2016 information technology – message queuing telemetry 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http://docs.oasis-open.org/mqtt/mqtt/v3.1.1/ https://www.ivifoundation.org/docs/scpi-99.pdf monte carlo-based 3d surface point cloud volume estimation by exploding local cubes faces acta imeko issn: 2221-870x june 2022, volume 11, number 2, 1 9 acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 1 monte carlo-based 3d surface point cloud volume estimation by exploding local cubes faces nicola covre1, alessandro luchetti1, matteo lancini2, simone pasinetti2, enrico bertolazzi1, mariolino de cecco1 1 department of industrial engineering at the university of trento, via sommarive 9, 38123 trento, italy 2 department of mechanic and industrial engineering at the university of brescia, via branze 38, 25121 brescia, italy section: research paper keywords: monte carlo; volume estimation; affiliation criterion; cube explosion; point cloud citation: nicola covre, alessandro luchetti, matteo lancini, simone pasinetti, enrico bertolazzi, mariolino de cecco, monte carlo-based 3d surface point cloud volume estimation by exploding local cubes faces, acta imeko, vol. 11, no. 2, article 32, june 2022, identifier: imeko-acta-11 (2022)-02-32 section editor: francesco lamonaca, university of calabria, italy received november 22, 2021; in final form february 25, 2022; published june 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this project has received funding from the european union’s horizon 2020 research and innovation program, via an open call issued and executed under project eurobench (grant agreement n° 779963). corresponding author: nicola covre, e-mail: nicola.covre@unitn.it 1. introduction estimating the volume enclosed in a three-dimensional (3d) surface point cloud is a widely explored topic in several scientific fields. with the increase of technologies for the virtual reconstruction of 3d environments and objects, many devices, such as kinect, lidar, real sense, make it possible to acquire a depth image with increasing accuracy and resolution [1]-[4]. several fields, such as mobile robotics [5], reverse prototyping [6], industrial automation, and land management, require accurate and efficient data processing to extract geometrical features from the real environment, such as distances, areas, and volume estimations. among different geometric features, volume estimation has been presented as a challenging issue and widely studied with different approaches in the literature. from the literature contributions on object volume estimation based on the 3d point cloud, chang et al. [7] used the slice method and least-squares approach achieving high accuracy by investigating mainly known and homogeneous solids. the same 3d point cloud volume calculation based on the slice method was applied by zhi et al. [8]. in general, the main limitation of using the sliding method for volume estimation is the dependence on the quality of the point cloud and the impossibility to work with complex shapes. bi et al. [10] and xu et al. [11] estimated the canopy volume measurement by using only the simple convex hull algorithm [12] with the problem of volume overestimation in the case of concave surfaces. lin et al. [13] improved the convex hull algorithm to handle concave polygons for the estimation of the abstract this article proposes a state-of-the-art algorithm for estimating the 3d volume enclosed in a surface point cloud via a modified extension of the monte carlo integration approach. the algorithm consists of a pre-processing of the surface point cloud, a sequential generation of points managed by an affiliation criterion, and the final computation of the volume. the pre-processing phase allows a spatial reorientation of the original point cloud, the evaluation of the homogeneity of its points distribution, and its enclosure inside a rectangular parallelepiped of known volume. the affiliation criterion using the explosion of cube faces is the core of the algorithm, handles the sequential generation of points, and proposes the effective extension of the traditional monte carlo method by introducing its applicability to the discrete domains. finally, the final computation estimates the volume as a function of the total amount of generated points, the portion enclosed within the surface point cloud, and the parallelepiped volume. the developed method proves to be accurate with surface point clouds of both convex and concave solids reporting an average percentage error of less than 7 %. it also shows considerable versatility in handling clouds with sparse, homogeneous, and sometimes even missing points distributions. a performance analysis is presented by testing the algorithm on both surface point clouds obtained from meshes of virtual objects as well as from real objects reconstructed using reverse engineering techniques. mailto:nicola.covre@unitn.it acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 2 tree crown volume, but their approach is still limited to providing a gross volume estimation that cannot be applied to complex objects with fine details. lee et al. [9] proposed a waste volume calculation using the triangular meshing method starting from the acquired point cloud. on one hand, this method results as accurate as the goodness of the acquired point cloud, on the other hand, it completely relies on mesh processing tools, such as meshlab [14], and it cannot work if the 3d reconstructed object is an open domain. we propose an innovative and competitive method to compute the volume of an object based on 3d point clouds via a modified extension of the monte carlo integration approach without the interpolation or the mesh reconstruction of the surface point cloud, it can handle homogeneous and nonhomogeneous point cloud surfaces, complex and simple shapes as well as open and close domains. 1.1. the monte carlo approach traditional methods for numerical integration on a volume, such as riemann integral [15] or cavalieri-simpson [16] partition the space into a dense grid, approximate the distribution in each cell with elements of known geometry and compute the overall volume by summing up all the contributions. in contrast, within a previously defined interval containing the distribution of interest, the monte carlo method generates random points with uniform distributions along the different dimensions to estimate the integral [17]. as shown in figure 1 in the case of a 2d example we wish to calculate the area atarget of a closed surface. the geometric element under consideration (green line, s2dtargetobject) is enclosed within a 2d box of known area abox which encloses s2dtargetobject. the total amount (n) of randomly generated points (pgenerated) will fall inside s2dbox. while some of them will fall outside s2dtargetobject (blue points), the others will fall inside (red points, ninsidepoints). in the 2d case, pgenerated is identified with its 2d cartesian coordinates (xpgenerated, ypgenerated). an affiliation criterion (most often expressed by a simple mathematical equation) allows the identification and counting of points dropped in and out of the s2dtargetobject. in particular, these elements are regulated by the following expression: 𝐴target = lim 𝑛→∞ 𝑛insidepoints 𝑁 ∙ 𝐴box (1) 1.2. 3d extension of the monte carlo approach as reported by newman et al. [18], this computational approach is particularly suitable for high-dimensional integrals. for this reason, we extended the 2d monte carlo method described in the previous subsection to the calculation of the 3d volumes starting from their discrete surface’s representation. each point cloud can have, within a certain range, variable resolution, and spatial distribution homogeneity. in this case, n points are generated with uniform distribution along the three dimensions to estimate the volume of the unknown object (vtargetobject) inside the prismatic element (s3dbox). in the 3d case, pgenerated is identified with its 3d cartesian coordinates (xpgenerated, ypgenerated, zpgenerated). the vtargetobject is then calculated by counting the number of pgenerated that fell inside it (ninsidepoints). equation (1) becomes: 𝑉targetobject = lim 𝑛→∞ 𝑛insidepoints 𝑁 ∙ 𝑉box (2) usually, having the target object represented by a continuous surface and described by a mathematical equation, as in the 2d case, the affiliation criterion is expressed by a continuous mathematical model. it is, therefore, easier to determine when a point falls within and without the s3dtargetobject. however, the problem becomes more difficult when the s3dtargetobject is represented by the discrete distribution of some points lying on its surface (s3dpointcloud), as can be shown in figure 2. in this case, it is difficult to determine when a point falls within the s3dpointcloud or not. moreover, in cases where the s3dpointcloud comes from a real acquisition, noise must also be taken into account. in fact, due to some acquisition errors not all the points of the cloud lie on the s3dtargetobject. this paper can be divided as follows: • in the introduction we presented the problem of volume computation from point clouds, the state of the art, and our approach by describing the traditional monte carlo method, its 3d extension, and its limitations. • in the following section, we describe our algorithm for volume estimation of point clouds based on the monte carlo approach. • in the third section, we present the results obtained for the validation of the algorithm, testing it on both virtual and real objects. • in the final section, we expose the drawn conclusions. figure 1. 2d example of monte carlo integral approach in green the geometric element under consideration (s2dtargetobject), in black the 2d box (s2dbox) of known area, in red the inside points, and blue the outside points. figure 2. extension of the monte carlo integral approach to the point cloud of a 3d sphere (s3dpointcloud) enclosed in a 3d box (s3dbox). acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 3 2. developed algorithm the algorithm reported as pseudocode in appendix a and explained here below takes the following parameters as input decided by the user: • the point cloud acquired from the surface of an object whose volume is computed. • the number of points n with which the monte carlo volume estimation must be performed. the algorithm is composed of a pre-processing function and a classification function. the pre-processing of the point cloud checks its orientation, encloses the cloud surface in a box of known volume (s3dbox), and performs a preliminary analysis of its distribution. the classification function is based on the "cube explosion" affiliation criterion, described in the following paragraphs. 2.1. pre-processing given the total amount of points n, an efficient monte carlo approach should provide the smallest box which encloses the volume taken for the measurement. in fact, with a fixed n the bigger is s3dbox the lower is the 3d point resolution and the worst is the accuracy of the monte carlo method. hence the s3dbox is defined by taking the minimum and maximum points in each one of the three principal directions x, y, z of the s3dpointcloud. to further minimize the box dimensions a previous re-orientation of the s3dpointcloud is performed by applying the principal component analysis (pca). once the s3dbox is defined its volume is computed and used at the end of the affiliation criterion as vbox. in the case of real objects, s3dpointcloud is collected by using depth cameras or other 3d scanners. the higher is the resolution of the tool, the denser is the point cloud. in the case of virtual objects, this point cloud is obtained by collecting the mesh nodes. the denser is the mesh, the more homogeneous is the point cloud distribution. however, the homogeneity of the point distribution is a factor that cannot be taken for granted. for this reason, a preliminary statistical analysis over the s3dpointcloud is carried out to obtain the parameters needed for the affiliation criterion, the core of the monte carlo method. in particular, the parameters are obtained from the quantile distribution q of the distances between each point and its closest neighbors. 2.2. affiliation criterion using the "explosion" of cube faces the proposed affiliation criterion iteratively defines whether each pgenerated inside s3dbox by the monte carlo method belongs to the external or internal domain of the s3dpointcloud. the affiliation criterion that we have developed is based on the concept of "explosions of cube faces". the idea is based on the generation of a cube of known edge (lcube), around each pgenerated, and iteratively extruding each one of its faces to determine how and how many times it encounters the s3dpointcloud, figure 3. each one of the 6 face extrusions corresponds to a specific direction η along with one of the 3 main directions x, y, z and returns a binary judgment (jη). jη is 0 or 1 respectively if the point is supposed to be outside or inside the s3dpointcloud. eventually, by taking the mode of all jη the final judgment (j) is assessed, and the point affiliation (internal or external) is defined. each cube is oriented by using the same reference system of s3dpointcloud. at each iteration, the procedure selects the direction η and extrudes the relative faces of the cube along their outgoing normal. initially, lcube is determined by the following empirical equation: 𝑙cube = 𝑄0.5 ∙ 3.5 ( 𝑄0.85 𝑄0.5 − 1) (3) where q0.5 and q0.85 are the quantiles at 50 % and 85 % of the distribution of the distances between each point and its closest neighbors respectively. equation (3) and the chosen quantiles are obtained by an empirical validation of the performances. in particular, the choice of q0.5 considers the median distance of two consecutive points and q0.85 highlights the s3dpointcloud sparse distribution and avoids initializing a small cube whose extruded faces pass through s3dpointcloud without touching its points. histograms of the distribution of the relative distances between each point and its closest neighbors for two different s3dpointcloud are shown in figure 4. in particular, the first histogram is referred to as non-homogenous point cloud, specifically, pokémon (mew), while the second is referred to the homogeneous cloud of the geometric solid sphere, both reported in table 1. on one hand from the first distribution is possible to observe that the ratio between q0.85 and q0.5 is 2.74, on the other hand, the quantile ratio of the second distribution is 1.01. the quantile ratio shows the proportion of the sparse portions of s3dpointcloud concerning the distribution of the average distances. in mew ‘s s3dpointcloud the ratio is higher because we have strong non-homogeneous distribution, such as a dense clustering of points for the eyes and sparse on the belly. in the sphere’s s3dpointcloud the ratio is close to one as the difference between q0.85 and q0.5 is almost null due to the homogeneity of the point cloud distribution. each face extrusion may intercept a sub-portion of s3dpointcloud points (pintercepted). if the total amount of intercepted points overcomes the threshold value (thintercepted) of 3 points, lcube is reduced of lreduction and the extrusion is repeated. the criterion for choosing thintercepted equal to 3 points depends on the clusterization checks introduced to strengthen the algorithm, as explained at the end of this section. the value of lreduction has been empirically set equal to 10 % of the actualized value of lcube as a compromise between final accuracy and computational time. the smaller lreduction is the higher the final accuracy is but the longer the final computational time. figure 3. example of explosion in the z-direction (η = z) of one cube’s face from the position of a pgenerated interception of 2 clusters of points (in green, pintercepted). acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 4 following, a clustering algorithm subdivides the pintercepted into different clusters of points (pcluster) along η. jη assumes the values 1 or 0 if the total amount of clusters is, respectively, odd or even. the clusterization is performed by re-ordering pintercepted along η. computing the coordinate differences along η of each point from its consecutive, it is possible to obtain a sequence of distances. ideally, considering pcluster orthogonal to η, this difference within each pcluster should be null, as all the pcluster points lie on the same orthogonal plane. however, in real applications, several issues may occur, such as a slight inclination of pcluster concerning η or a random noise affecting the pcluster distribution along η. to overcome these problems, a threshold (thcluster) is set empirically equal to q0.5. therefore, considering the pintercepted re-ordered along η, whenever a distance between table 1. algorithms outputs with virtual objects. virtual object name original virtual model original point cloud ( s3dtargetobject) meshlab reconstruction our output cube sphere arm hand pokémon mew acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 5 two consecutive points results to be less than thcluster the two points belong to the same pcluster. otherwise, a new pcluster begins. a further issue that affects the clusterization occurs when the extrusion intercepts tangentially s3dpointcloud. in this case, a misleading clusterization is performed and further checks are needed to make the affiliation criterion more robust. in particular, this can be detected by performing an analysis of variance over each pcluster. if the variance of one single pcluster is greater than thcluster the entire analysis along η is compromised and jη needs to be discarded. to make the affiliation criterion more robust a limitation on the maximum number of clusters encountered along each extrusion is also introduced. in particular, only those directions η are selected that have a total amount of clusters less than or equal to 1. this reduces the probability of encountering misleading clusters such as in the case of noisy point clouds acquired from real objects. the noise mostly appears as isolated points outside s3dpointcloud, and rarely inside it. furthermore, this allows justifying the choice of thintercepted referring to the minimum number of points that define a plane. 3. results this section reports the validation of the extended monte carlo algorithm on real and virtual objects considering different shapes. a total of nine objects, including regular geometric solids, such as spheres or prisms as well as more complex shapes, such as human hands, arms, pokemon (mew), and the 3d scanning of ancient bronze statuettes of mythological figures were considered for this discussion. point clouds of real objects were acquired from the real environment with an azure kinect tof camera and a konica minolta vivid vi-9i 3d scanner for reverse engineering. the volumes used as a reference for the validation of the measures on virtual and real objects were respectively computed using the virtual mesh and the volume estimation by immersion in water [19]. the monte carlo algorithm accuracy increases with the total amount of points generated, as can be observed in figure 5 and figure 6. in particular figure 6 reports the measurement of the mean and variance with a box plot of the relative error distribution as a function of the number of pgenerated. as can be seen, the error is particularly high when few pgenerated are generated and gradually decreases as the number increases. for both virtual and real solids, depending on the resolution of the point cloud and the reported details, the asymptotic percentage error is below 7 % computed with respect to the reference volume when the total amount of pgenerated is greater than 42875. on the contrary, with a low number of pgenerated, the accuracy is low. it is also worth noting that the variance of the measures decreases as the number of pgenerated increases. this indicates that (a) (b) figure 4. distribution of distances between each point and its closest neighbors for (a) pokemon mew and (b) the sphere point clouds. figure 5. error distribution of the pokémon mew volume estimation in % with respect to the increasing number of generated points with the explosion cubes criterion. figure 6. box plot of the error distribution considering all the nine objects along with the increasing number of generated points with the explosion cubes criterion. acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 6 for all objects considered, even for convex, non-uniform, and folded point clouds, the relative error decreases and converges with the same trend. as a general rule, on one hand, the performance of the algorithm can be driven by selecting the number of generated points. on the other hand, the more are the pgenerated the long is the computational time. the average times spent by the proposed algorithm are shown in figure 7. the tests were performed on a macbook pro 2 ghz intel core i5 quad-core 16 gb of ram in matlab_r2020b environment. the average time grows as the number of points increases no-linearly as shown in figure 7. however, with the same accuracy, the average computational time is less than or comparable to the time taken by the volume estimation methods reported in the literature. therefore, the user is allowed to choose the total amount of pgenerated with which monte carlo has to be executed as a tradeoff between the level of desired accuracy, figure 6, and the computational time, figure 7. in addition, due to the asymptotic behavior of the error, the increase in performance becomes negligible after a threshold. for these reasons, it is convenient to choose a total amount of pgenerated just above the estimated volume that has stabilized (in our case 42875 pgenerated). however, the greater is the total amount of pgenerated the higher is the resolution of the point cloud generated by the monte carlo algorithm. this can be used as visual feedback to evaluate the goodness of the affiliation criterion and compare it with the table 2. algorithms outputs with real objects. real object name real object original point cloud (s3dtargetobject) meshlab reconstruction our output cerbero ballerina head mecurio figure 7. computational times of the proposed algorithm changing the number of generated points (pgenerated). acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 7 algorithms that reconstruct the mesh. as shown in the last column of both tables table 1 and table 2, the representation of the inner points with the developed method returns a good representation of the original point cloud, s3dpointcloud. on the contrary, mesh reconstructions are not always reliable. a mesh reconstruction using the default parameters of the poisson’s reconstruction method [8] in the meshlab environment is shown in the fourth column of the same tables. as can be observed, good results are returned only for uniformly distributed point clouds and regular shapes, while the same cannot be assessed for complex shapes or non-uniform point clouds, such as pokémon mew and the hand, with consequent negative repercussions on volumes calculation. another important aspect to consider when the volume is computed concerns the possibility to work with discontinuous and partially open surfaces, such as in figure 8(a), where the acquired point cloud results to have huge discontinuities around the elbow. most of the volume estimation algorithms based on the mesh reconstruction need manual fitting and adjustments to manage the missing clusters of points, otherwise, the measurement cannot be pursued. on the contrary, the prosed affiliation criterion for the monte carlo volume integration is robust to discontinuities due to the democratic judgment of the 6 cube faces. in fact, even if along a few directions the point cloud results to be open, and these faces extrusions will return jη = 0, the final judgment j will still be equal to 1. table 3 shows the actual volume of the objects compared with that obtained with meshlab, when possible, and our proposed method with its error percentage on the measurement. nevertheless, given the choice of the external box and maintaining its ratio to the calculated volume, taken any object, its percentage error does not change by scaling its size. this means that the uncertainty on the measurement is proportional to the percentage error multiplied by the calculated volume. 4. conclusions this paper proposes an extension of the 3d monte carlo method for calculating the volumes of objects starting from their surface point clouds. the overall algorithm includes a preprocessing analysis that re-orient and evaluates the point cloud, an affiliation criterion based on the explosion of cube faces to discern the inner and the outer points of the monte carlo method, and a final volume estimate as described in equation (2). as the reported results include also convex, complex, and folded surfaces, such as the pokémon mew or the cerbero point cloud it was possible to show that the cube explosion affiliation criterion results be stable and reliable, returning consistent and repeatable measurements compared with gold-standard software for volume measurements, such as meshlab. the algorithm proves to be accurate with point clouds of different objects, both in terms of shape and distribution of points. the performances are then tested with the surface point clouds of 9 virtual and real objects, reporting an average percentage error on the tested samples lower than 7% with a computational amount of time of a few minutes depending on the desired accuracy. references [1] k. khoshelham, s. elberink, accuracy and resolution of kinect depth data for indoor mapping applications, sensors, vol. 12, n. 2, 2012, pp. 1437-1454. doi: 10.3390/s120201437 [2] k. khoshelham, kourosh, accuracy analysis of kinect depth data, isprs workshop laser scanning, calgary, canada, 29-31 august 2011, pp. 133-138. doi: 10.5194/isprsarchives-xxxviii-5-w12-133-2011 [3] j. vaze, j. teng, g. spencer, impact of dem accuracy and resolution on topographic indices. environmental modelling & software, vol. 25, n. 10, 2010, pp. 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(a) open point cloud from a real acquisition of a human arm, (b) monte carlo representation of inside generated points, (c) monte carlo representation of outside generated points. table 3. actual volume of the objects and its estimation with meshlab and our method. 3d object actual volume in dm³ meshlab volume in dm³ montecarlo volume in dm³ monte carlo error in % cube 1.00 1.00 0.99 0.10 sphere 4.19 4.19 4.10 2.15 arm 1.33 na 1.39 4.51 hand 0.412 na 0.409 0.73 pokemon mew 0.539 na 0.548 1.67 cerbero 4.08 na 4.18 2.45 ballerina 1.24 na 1.18 4.84 head 4.54 4.52 4.50 0.88 mercurio 4.53 4.57 4.66 2.82 https://doi.org/10.3390/s120201437 https://doi.org/10.5194/isprsarchives-xxxviii-5-w12-133-2011 https://doi.org/10.1016/j.envsoft.2010.03.014 https://doi.org/10.1109/cvprw.2017.167 https://doi.org/10.1016/j.aei.2014.06.002 https://doi.org/10.1007/978-94-007-7618-0_226 acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 8 [7] w. chang, c. wu, y. tsai, w. chiu, object volume estimation based on 3d point cloud, 2017 international automatic control conference (cacs), pingtung, taiwan, 12-15 november 2017, pp. 1-5. doi: 10.1109/cacs.2017.8284244 [8] y. zhi, y. zhang, h. chen, k. yang, h. xia, a method of 3d point cloud volume calculation based on slice method, international conference on intelligent control and computer application (icca 2016), atlantis press, zhengzhou, china, 19-17 january 2016, pp. 155-158. doi: 10.2991/icca-16.2016.35 [9] y. lee, s. cho, j. kang. a study on the waste volume calculation for efficient monitoring of the landfill facility. in computer applications for database, education, and ubiquitous computing, springer, berlin, heidelberg, 2012, 978-3-642-356025, pp. 158-169. 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[19] d. m. k. s. kaulesar sukula, p. t. den hoed, e. j. johannes, r. van dolder, e. benda. direct and indirect methods for the quantification of leg volume: comparison between water displacement volumetry, the disk model method and the frustum sign model method, using the correlation coefficient and the limits of agreement. journal of biomedical engineering vol.15, no. 6, 1993, pp. 477-480. doi: 10.1016/0141-5425(93)90062-4 [20] m. kazhdan, m. bolitho, h. hoppe, poisson surface reconstruction, proc. of the fourth eurographics symposium on geometry processing, vol. 7, 2006, pp. 61-70. online [accessed 21 april 2022] https://hhoppe.com/poissonrecon.pdf https://doi.org/10.1109/cacs.2017.8284244 https://doi.org/10.2991/icca-16.2016.35 https://doi.org/10.3981/j.issn.1000-7857.2013.27.004 https://doi.org/10.3964/j.issn.1000-0593(2014)02-0465-07 https://doi.org/10.1109/ivcnz.2010.6148857 http://dx.doi.org/10.1080/01431161.2016.1265690 http://dx.doi.org/10.2312/localchapterevents/italchap/italianchapconf2008/129-136 http://dx.doi.org/10.2312/localchapterevents/italchap/italianchapconf2008/129-136 https://doi.org/10.5566/ias.1723 https://doi.org/10.1016/0141-5425(93)90062-4 https://hhoppe.com/poissonrecon.pdf acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 9 appendix a a tool for curating and searching databases proving traceable analysis of data and workflows acta imeko issn: 2221-870x march 2023, volume 12, number 1, 1 6 acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 1 a tool for curating and searching databases proving traceable analysis of data and workflows frederic brochu1, michael chrubasik1, spencer a. thomas1 1 data science, national physical laboratory, hampton road, teddington, middlesex, tw11 0lw, united kingdom section: research paper keywords: searchable metadata; reproducibility; data curation; data traceability; fair citation: frederic brochu, michael chrubasik, spencer a. thomas, a tool for curating and searching databases proving traceable analysis of data and workflows, acta imeko, vol. 12, no. 1, article 12, march 2023, identifier: imeko-acta-12 (2023)-01-12 section editor: daniel hutzschenreuter, ptb, germany received november 18, 2022; in final form february 16, 2023; published march 2023 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was funded by the department for business, energy & industrial strategy through the uk’s national measurement system. corresponding author: frederic brochu, e-mail: frederic.brochu@npl.co.uk 1. introduction technological and scientific advances over the last 20+ years have led to the ability to generate and store vast amounts of data. furthermore, emphasis on reducing acquisition times has significantly increased the throughput of data from experiments. in parallel with the developments in measurement technologies, there have been significant advancements in data storage, allowing this data to be efficiently captured and stored. however, a lack of systems or standards to organise or curate data leads to ad-hoc file structures, inconsistent conventions in recording metadata and a loss of data provenance. consequently, the vast amounts of data being recorded are not findable, accessible, interoperable, and reusable (fair) [1]. without a well-curated database [2] that includes rich metadata, data will not be findable. for research institutions that generate or collect large volumes of data, this is highly problematic as it significantly restricts the data’s reusability and therefore value. this is compounded with potentially high financial costs associated with repeated acquisitions and double storage if the data are not discoverable. the inability to retrieve data may have significant repercussions for reproducibility of results, traceability, or adherence to funder requirements. the concept of measurement traceability, where any instrument’s measurement can be linked to a known standard through an unbroken chain of comparisons, is well established. data traceability extends this concept to data and analysis pipelines where a given output (processed data, figures, statistical tests, etc) is linked back to data at the point of measurement through an unbroken chain of steps in a data workflow. these steps include data conversion, data processing (for example noise reduction), and data analysis steps (e.g., statistical tests, machine learning, etc) [2]. throughout the rest of this paper, we use the term traceability to refer to data traceability. previously, at the national physical laboratory (npl), we have developed methods for curating data at the point of measurement which can be used to establish a fair and traceable database [2], [3]. a curated database with relevant wellstructured metadata tags permits searches using structured query language (sql) or similar, where investigators can return a list of datasets within the database to match given criteria. the metadata itself can be analysed to reveal insights into the data. for example, in radiology, analysis of the sensitivity and radiation exposure over time, at different sites, uncovered inter-site and temporal differences [4]. however, establishing such a system abstract we present a framework for easy annotating, archiving, retrieving, and searching measurement data from a large-scale data archival system. our tool extends and simplifies the interaction with the database and is implemented in popular scientific applications used for data analysis, namely matlab, and python. this allows scientists to execute complex interactions with the database for data curation and retrieval tasks in a few simple lines of accessible templated code. scientists can now ensure their measurement data is well curated and fair (findable, accessible, interoperable, and reusable) compliant without requiring specific data skills or knowledge. our tools allow users to perform sql-type (structured query language) queries on the data from simple templated scripts allowing data retrieval from long-term storage systems. mailto:frederic.brochu@npl.co.uk acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 2 requires a high level of computational skills and often requires bespoke software creating significant barriers for measurement scientists. we use our internal database for long-term curated storage of measurement data along with experimental conditions that form the basis of the metadata and are vital for traceability and reproducibility. in this work, we introduce a tool that combines and extends the functionalities of the application programming interface (api) provided with npl’s archive for file transfer, annotation, and metadata queries into a single, convenient interface tool accessible from the popular scientific applications matlab and python. this tool not only simplifies interactions with the archive, but it also makes the entire data management process more accessible for scientists. the data archive we use in this work is an ‘objectstore’, a database storing data as objects that have their own attributes comprising of system metadata (size, creation date, etc) and custom metadata (user-definable fields). in this work we will exploit the archives’ use of data objects to store any number of multimodal data files, in any format, as a single hierarchical data format (hdf5) file that we use as a ‘container’ for data files. a single hdf5 container file, consisting of any number of data files, corresponds to the data objects uploaded to the data archive. we refer to the data objects as hdf5 container files through the manuscript. we can further exploit the objectstore functionality by utilising the custom metadata to define domain-specific attributes that are used to ‘tag’ the hdf5 file in the objectstore enabling highly specialised and domain-specific searching of the data. for large organisations this provides a flexible approach to automatically curate large and diverse databases without domain-specific infrastructure. for users, this enables multiple data files to be wrapped in a single container that is stored in a data archive for long-term storage alongside relevant metadata for searching and retrieval. our tool provides a user-friendly interface for users to perform all necessary steps (generating the container, tagging with metadata, uploading to the database, performing searches and retrieval of the data) without any expertise in these areas. we provide a case study using large and complex multimodal cohort data from experiments conducted at multiple institutions and across multiple instruments. 2. background data curation is the process of organised storing of data in a structured way with rich machine-actionable information about the data and its provenance. analogous to finding a book in a library, data curation enables users to locate specific datasets based on a defined list of attributes. for example, locating a dataset consisting of an image of a cat in winter captured with a mobile phone camera in the countryside. although many databases will enable the searching of the criteria of ‘images’, ’cat’, ‘camera phone’, ‘winter’, and ‘countryside’, there is no strict matching of attributes that these keywords map to. in a curated database we can perform such searches, type = ‘images’, subject = ’cat’, device = ‘camera phone’, season = ‘winter’, and location = ‘countryside’. this search would provide exact matches to our search rather than any attribute matches to the keywords in the former case. the demand for long-term data curation arises from the researchers themselves, as it enables them to utilise the data in future studies, from funding bodies through data retention requirements, and from the community, which promotes open science with fair data. well-curated data has the additional benefit of enabling meta-analysis across the database which can provide more precise estimates compared to individual studies, as well as an assessment of variability [5] and the development of computational models [6]. for example, meta-analysis has identified a higher proportion of positive covid-19 tests in low and low-middle income countries compared to higher income countries [7], evaluating the treatment effects [8] and by assessing the impact of missing data on outcomes [9]. there is currently no data curation tool or platform suite to manage experimental data due to its inherent complexities. this is particularly problematic for research which will typically be subject to funding bodies’ data storage and retention policies. due to the high cost (financial, expertise, and time) involved in some experimental studies, researchers want to maximise the future utility of the data in other studies. for example, healthcare or pharmaceutical studies involving tissue imaging have very complex data collection pipelines with different centres responsible for collecting the samples (e.g. biopsies), sample preparation (e.g. embedding and sectioning) and the measurement data (e.g. imaging). in this case, one experiment can involve multiple institutions and the provenance of the sample is highly complex. data quality controls and future metaanalysis require this information to be captured in a machine actionable way. current solutions range from individual-level record keeping to universal data repositories; we argue these are insufficient for curating measurement data. record keeping, such as spreadsheets or a database (sql, access, etc), to capture information, such as data storage location, does not constitute a data archive as it simply lists the locations and possibly some metadata. the fact that this information is unstandardized, prone to error, not machine-actionable, and not searchable (in a database environment) is even more problematic as it prevents the data from being fair. universal data repositories such as zenodo, figshare, scientific data, dropbox, re3data, etc, offer storage of data and user flexibility with regards to files and formats stored as well as providing some scope for metadata. however, capturing the metadata is far from trivial [2], [3], [10] and there may be many terms to include. although many of these platforms offer a search functionality, it is offered in a basic implementation, preventing highly specialised searches such as structured database queries. furthermore, there are no specific checks on these entered fields, hence information may be missed, incorrectly added, or exist in several forms (e.g., acronyms, capitalisation). 3. method the developed tool is a set of matlab and python scripts handling interactions with the archive data storage and experimental data with metadata encapsulation. this enables “behind the scenes” operations at the command of the scientists wishing to archive their data without requiring the programming skills necessary to do so. the interface tool is invoked through matlab which is used as a user interface for python code operating as a two-layer program. the first layer is a matlab master class describing a connection “object” with different callback functions. in the second layer the functions are mapped into a python layer handling “representational state transfer” (rest) calls [11] to the objectstore apis for file handling and metadata queries. this configuration not only provides access of our tool for matlab and python users, but also provides a simple interface (in matlab) for users with little programming acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 3 expertise. the python libraries are installed as dependencies of the code following our documented installation procedure, this also covers matlab installation which is straightforward. this tool is designed to work within an organisations digital infrastructure and thus can be preloaded on to institutional machines such as laptops and lab machines. file transfers are performed with the aws s3 protocol [11] with all data stored on our institutions internal data storage infrastructure with access permissions fully controllable by our it team. our code is available on a private gitlab repository as a pypl package, and a public release may be possible in the future. to be an effective solution, our tool has required: to store all relevant and related data sets as a single instance (see section 3.1), the data be linked to the metadata (see section 3.2), all data to be archived in a common location (see section 3.3), and the metadata be searchable for data retrieval (see section 3.4). the layout tools and their different components are presented in figure 1. 3.1. hdf5 container we store the data in a hierarchical data format (hdf5) container file that can hold an arbitrary number of data files and formats. this allows the storage of experimental data with associated datasets such as calibration data or processing scripts as a complete and unbroken data pipeline [3]. containerising the data and associated steps in a workflow ensures reproducibility through the data pipeline. the data parsed into the container are stored in binary form as this allows supporting any data format. they are also compressed at the creation of the container to optimise data replication to the archive system. 3.2. annotation annotation refers to the process of attributing information to a data file such that it becomes the metadata for that file and providing terms that the file can be searched with. the metadata for complex experiments is multi-source [10] and takes different format. we collect and aggregate them all in a single ‘wellformed’ xml file [12]. xml is a metadata format that is both human and machine-readable, and is the only format supported by the npl archive described in section 3.3. the metadata flow is duplicated: a copy is embedded in the hdf5 container, and another is used to link the hdf5 container with the associated metadata in the archive annotation database. linking the hdf5 with its associated metadata ensures the data are fair compliant prior to uploading the file to the database. although aiding reproducibility and interoperability for individual files, linking the data with metadata is not sufficient to provide a curated database that can be easily searched or mined for meta-analysis. by using standardised and well-formed metadata to link to the hdf5 files we can automatically establish a curated database. that is, all hdf5 containers have the same metadata structure and are therefore well organised and can be viewed and search in a systematic way. further details of this are in section 3.4. 3.3. data archive (objectstore) the annotated hdf5 container files are stored in our “objectstore” database. the objectstore is npl’s large-scale data archival system, an instance of the hitachi content platform (hcp), which in its most basic form is a flexible database for storing and annotating data. file annotation tags the data with associated metadata for curation and database searching. the metadata can be defined by the user, known as ‘custom metadata’, and can be used for curating complex experimental data [2]. when tagged with metadata, data are stored in an internal database with a dedicated api allowing simple sql-type queries for searching the data. the objectstore consists of “tenants”, organisational level division of the system (e.g., departments); and “namespaces”, logical grouping of objects (e.g., projects). this archival system supports file versioning, where the history of any changes to the data are recorded, as well as annotation. both the data and metadata can be updated for new versions. a database with versioning enabled can enhance traceability when the data are tagged with the associated metadata [3]. the ability to trace software, file, and documentation changes through an unbroken chain (i.e., the version history) can help identifying bugs/errors and tracking system evolution. the ordered nature of versioning enables the user to return to a point of the evolution of the data and create a new branching point. this is particularly useful if errors have been identified or new methods have been developed, such as improved data processing. the tool provides specific functions for: connecting to the database, uploading data following archiving as a hdf5 container file, downloading datasets, and performing metadata queries (see section 3 for more details). an example of the matlab script to establish the database connection is given in figure 2. once a connection is established the data can be uploaded simply by specifying the location of the data to be uploaded (local_dir) and the target storage location on the database figure 1. interface layout and functionality described in section 3. user functions are represented by orange arrows. the desired datasets are each converted to binary and added to a hdf5 container file that is annotated (or tagged) with relevant metadata. the hdf5 container file is uploaded to the curated database where the metadata can be queried. any desired data can be easily downloaded and automatically converted back to its native format. figure 2. matlab script to setup database connection. comments are given in lines beginning with % and coloured green. here users only need to specify the namespace and tenant they wish to connect to which will be fixed for each project. acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 4 (database_dir) in a function call. the function first creates an hdf5 container file that is populated with all the data in the directory specified by the user. note this directory can contain any number of, or format of data files and also supports the use of shortcuts/links to other directories. the latter is vital when dealing with very large files that may be stored on multiple disparate devices, such as different laboratory instruments, and avoids the need to transfer data or duplicate data prior to using our tool. next, the tool tags the container with the (well-formed xml) metadata as outlined in section 3.2 allowing search functions in the database. finally, this tagged hdf5 file is then uploaded to the database. the script for this is given in figure 3 which highlights the simplicity of our tool’s interface which is vital for non-expert users. similarly, data can be easily downloaded from the database. in this case “local_dir” is the folder location where the hdf5 container will be downloaded to and then unpacked to the same folder structure and data formats as the originally uploaded data. directories that were originally shortcuts are unpacked as subdirectories within the data parent directory, i.e., there are no shortcuts when data are downloaded and unpacked. the script for downloading the data is given in figure 4. 3.4. list contents and sql queries the curated database consists of hdf5 files tagged with associated metadata that allows the entire database to be searchable. the content of the entire namespace, or a specific directory can be listed as shown in figure 5. we can also search the contents of our database filtering on any of the tagged metadata attributes using sql-type queries. the queries return a list of hdf5 files that match the criteria of the query. in addition to attributes of the data from the metadata, the queries can also include the filename or a unique identifier making all the data findable. using sql-type queries, which can be standardised through template queries for metadata attributes in a specific domain (see results section), ensures that the data are accessible to all users in line with the go fair principles [13]. database permissions can be set to restrict the visibility of data for different users as required, though this is outside the scope of this work. 4. results we demonstrate our tool with a case study using a database from the cancer research uk rosetta grand challenge (a24034) project led by npl’s nice-msi group [14]. previously we have established a curated database of complex experimental data [2] that can be used for traceable data processing workflows that are fully reproducible [3]. this database consists of experimental data acquired from several different instruments (vendors and models), across multiple sites, with a large number of experimental parameters and operating procedures that are dependent on the sample. the project also conducts cohort, longitudinal and inter-laboratory studies, and aims to conduct some meta-analysis on the data once complete. some instruments write data in a proprietary format that is accessible internally, but this access is lost when archiving the data as the proprietary format requires the instrument’s vendor software to open. we convert this data in an open community standard format called imzml [15] making the data accessible. opening the database connection as shown in figure 2, the database can be searched with simple sql queries of the form conn.metadata_query('key' ,'value'). this provides a simple userfriendly means for experimentalists to search the curated database for any subset of files making the data findable. we provide standard and simplified code for common queries allowing non-programmers to utilise this functionality. some examples of domain-specific queries that users may want to perform are: • data from a specific measurement technique, for example desi and maldi: conn.metadata_query('technique','desi') conn.metadata_query('technique','maldi') • data from a particular experimental study: conn.metadata_query('study','slc7a5') • data acquired from samples from a particular collaborator: conn.metadata_query ('samplesource','astrazeneca') • data from a specific vendor instrument model, for example a ‘synapt g2-si’ model: conn.metadata_query('instrument','synaptg2-si') • data from a particular sample (unique barcode from a separate sample management database, the data is integrated into our curated database prior to upload see [2]): conn.metadata_query('barcode','1000202') figure 3. matlab script to upload local data to the database. comments are given in lines beginning with % and coloured green. users specify the location of the data they wish to upload (any number or format of data) in local_dir which also supports shortcuts, and the destination folder on the database in database_dir. the structure of folders on the database can resemble folder structures on computers which will not impact the searchability of the database. figure 4. matlab script for downloading data from the database to the local computer. comments are given in lines beginning with % and coloured green. here database_dir is the data users wish to retrieve from the database and local_dir is the target directory on the local machine to download the data and automatically covert it back into its original format prior to upload to the databased. figure 5. matlab script to list the objects contained within the objectstore namespace. comments are given in lines beginning with % and coloured green. acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 5 • datasets of the same experimental parameters eg, data of a particular instrument polarity: conn.metadata_query('polarity','negative') • size of acquisitions area for each pixel: conn.metadata_query('pixelsize','100 microns') • acquisition time for each pixel: conn.metadata_query('scantime','0.485 sec') • data acquired over the same measurement range: conn.metadata_query('massrange','m/z 50-1200') note the last three examples can be executed with or without the units. example results for the conn.metadata_query( 'massrange', 'm/z 50-1200') query are given in table 1. as the data are contained in hdf5 containers with wellstructured metadata all relevant associated datasets, including calibration files, processing scripts and the data are interoperable and re-usable, meaning external entities can more easily access, exchange and make use of the information contained within the hdf5 file. 5. conclusions we have introduced and demonstrated our tool that allows non-experts to interact with a well-curated database using the popular programming languages matlab and python. by encapsulating multiple difficult apis into a single, user-friendly application, the tool allows measurement scientists to easily ensure that their data is saved in a well-curated, fair-compliant, and traceable database without the need for specialised computational skills. the ability to collate all relevant data, tag the collated data with relevant machine actionable metadata, and upload it to a database with a single function call reduces the computational barrier significantly. a single line of code is used to perform sql-type searches on the database ensuring all data are findable and accessible. an integrated function for downloading and extracting the data in its original format allows the utilisation of the hdf5 container without requiring the user to interact with it. one such benefit of the container file is the storage of data with associated metadata, processing and analysis codes providing interoperable and reusable data that is also traceable. we demonstrate this through a case study of data collected from a large-scale multisite imaging project with large volumes of highly complex measurement data. template scripts further reduce this barrier and enable the capture of metadata at the point of measurement as well as any stage throughout the data processing pipeline. this enables experiments to easily retrieve data and maximise the usefulness of a fair and curated database without requiring any knowledge of these principles. acknowledgement this work was funded by the department for business, energy & industrial strategy through the uk’s national measurement system. references [1] m. d. wilkinson, m. dumontier, i. j. aalbersberg, g. appleton, m. axton (+48 more authors), the fair guiding principles for scientific data management and stewardship, scientific data 6(1) 2019, 9 pp. doi: 10.1038/sdata.2016.18 [2] s. a. thomas, f. brochu, a framework for traceable storage and curation of measurement data, measurement: sensors, 18(100201) 2021, 5 pp. doi: 10.1016/j.measen.2021.100201 [3] s. a. thomas, f. brochu, curation at the point of measurement and traceability of measurement workflows, measurement: sensors, 23(1000399) 2022, 7 pp. doi: 10.1016/j.measen.2022.100399 [4] m. santos, p. sá-couto, a. silva, n. rocha, in dicom metadatamining in pacs for computed radiography x-ray exposure analysis: a mammography multisite study, european congress of radiology-ecr 2014, vienna, austria, 6-10 march 2014, 7 pp. doi: 10.1594/ecr2014/b-0276 [5] a. b. haidich, meta-analysis in medical research, hippokratia, 14(suppl 1): 29–37, 2010. online [accessed 17 march 2023] https://www.ncbi.nlm.nih.gov/pmc/articles/pmc3049418/ [6] n. mikolajewicz, s. v. komarova, meta-analytic methodology for basic research: a practical guide, front. physiol., sec. computational physiology and medicine, 2019, 20 pp. doi: 10.3389/fphys.2019.00203 [7] i. bergeri, m. g. whelan, h. ware, l. subissi, a. nardone (+25 more authors), global sars-cov-2 seroprevalence from january 2020 to april 2022: a systematic review and meta-analysis of standardized population-based studies, plos medicine, 19(11): e1004107, 2022, 24 pp. doi: 10.1371/journal.pmed.1004107 [8] c. b. joy, c. e. adams, s. lawrie, haloperidol versus placebo for schizophrenia. cochrane database of systematic reviews, john wiley & sons, ltd, 2001. doi: 10.1002/14651858.cd003082 [9] j. p. higgins, i. r. white, a. m. wood, imputation methods for missing outcome data in meta-analysis of clinical trials. clinical trials. 2008;5(3), pp. 225-239. doi: 10.1177/1740774508091600 [10] n. smith, d. sinden, s. a. thomas, m. romanchikova, j. e. talbott, m. adeogun, building confidence in digital health through metrology, the british journal of radiology, 93(1109) 2020, 3 pp. doi: 10.1259/bjr.20190574 [11] aws, aws s3 rest api protocol. online [accessed 17 march 2023] https://docs.aws.amazon.com/amazons3/latest/api/s3api.pdf#welcome [12] w3resource.com, well-formed-xml. online [accessed 17 march 2023] https://www.w3resource.com/xml/well-formed.php [13] go fair int. support and coordination office (gfisco), go fair initiative. online [accessed 17 march 2023] https://www.go-fair.org/fair-principles/ table 1. example output from the query conn.metadata_query( 'massrange', 'm/z 50-1200') with some additional fields with categorised results for clarity of the general reader. here we can see several objects from the same measurement site or of the same modality, or both, allowing inter-lab and multimodal studies and analysis. the file version also gives an indication of the provenance of the files and analysis. id file version measurement site modality object a 1st a b object b 2nd a a object c 1st c a object d 4th b c object e 2nd c c object f 2nd a b object g 1st c c … https://doi.org/10.1038/sdata.2016.18 https://doi.org/10.1016/j.measen.2021.100201 https://doi.org/10.1016/j.measen.2022.100399 https://doi.org/10.1594/ecr2014/b-0276 https://www.ncbi.nlm.nih.gov/pmc/articles/pmc3049418/ https://doi.org/10.3389/fphys.2019.00203 https://doi.org/10.1371/journal.pmed.1004107 https://doi.org/10.1002/14651858.cd003082 https://doi.org/10.1177/1740774508091600 https://doi.org/10.1259/bjr.20190574 https://docs.aws.amazon.com/amazons3/latest/api/s3-api.pdf#welcome https://docs.aws.amazon.com/amazons3/latest/api/s3-api.pdf#welcome https://www.w3resource.com/xml/well-formed.php https://www.go-fair.org/fair-principles/ acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 6 [14] cancer research uk, rosetta project. online [accessed 17 march 2023] https://cancergrandchallenges.org/teams/rosetta [15] a. römpp, th. schramm, a. hester, i. klinkert, j.-p. both, r. m. a. heeren, m. stoeckli, b. spengler, imzml: imaging mass spectrometry markup language: a common data format for mass spectrometry imaging. methods mol biol., 696, 2011, pp. 205–224. doi: 10.1007/978-1-60761-987-1_12 https://cancergrandchallenges.org/teams/rosetta https://doi.org/10.1007/978-1-60761-987-1_12 a principal component analysis to detect cancer cell line aggressiveness acta imeko issn: 2221-870x june 2023, volume 12, number 2, 1 7 acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 1 a principal component analysis to detect cancer cell line aggressiveness livio d’alvia1, serena carraro1, barbara peruzzi2, enrica urciuoli2, ludovica apa1, emanuele rizzuto1 1 department of mechanical and aerospace engineering, sapienza university of rome, 00184 rome, italy 2 bone physiopathology research unit, bambino gesù children’s hospital, irccs, 00146 rome, italy section: research paper keywords: measurement of dielectric properties; biosensor; non-invasive measurements; cancer cell lines; cancer aggressiveness; osteosarcoma; breast cancer; pca; principal component citation: livio d’alvia, serena carraro, barbara peruzzi, enrica urciuoli, ludovica apa, emanuele rizzuto, a principal component analysis to detect cancer cell line aggressiveness, acta imeko, vol. 12, no. 2, article 22, june 2023, identifier: imeko-acta-12 (2023)-02-22 section editor: alfredo cigada, politecnico di milano, italy, andrea scorza, università degli studi roma tre, italy received october 12, 2022; in final form february 27, 2023; published june 2023 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was supported by “progetti di ricerca medi 2021” sapienza university of rome corresponding author: livio d’alvia, e-mail: livio.dalvia@uniroma1.it 1. introduction one defining feature of malignant tumors is represented by the quick creation of abnormal cells that arise beyond their usual boundaries. moreover, these cells have an uncontrollable reproduction and division rate up to constitute cancerous tissues since they do not respond to the standard signaling system of the body [1]–[3]. as stated by the world health organization, in 2020, cancer will be the primary cause of approximately 10 million deaths worldwide. by way of example, 2.26 million cases and 685 thousand deaths of breast cancer and 2.21 million cases and 1.8 million deaths of lung cancer, without forgetting the hundreds of thousands of children who develop malignant tumors each year [4]. an early diagnosis and screening can therefore contribute to reducing mortality and aid in more effective treatment. however, there is a lack of research on cancer behavior due to the various and complex molecular pathways involved in the genesis of tumors [5]. in most cases, the tumor degree – established by the cancer cells' characteristics throughout the tumor lesions' growth – is often used to make the diagnosis. actually, a series of cancer screening methods, such as biopsy, computed axial tomography (cat), or scintigraphy, exist but are costly and intrusive. biosensors in the microwave field may serve as a complementary or replacement method for early-stage noninvasive prognosis of a variety of illnesses, including malignancies. in this context, the measurement of dielectric properties of biological tissues has achieved significant benefits in biomedical and healthcare due to their high sensitivity, versatility, and reduced invasiveness [6]–[9]. indeed, this technology has consolidated its use in various fields. for example, gugliandolo et al. [10] developed a microwave microstrip resonator to measure water vapor for industrial pipeline applications. likewise, majcher et al. [11] investigated the possibility of using a dagger-shaped probe to measure soil moisture in agrifood applications. ultimately, d’alvia et al. [12]– [14] and cataldo et al. [15] proposed several applications in the cultural heritage field. abstract in this paper, we propose the use of principal component analysis (pca) as a new post-processing method for the detection of breast and bone cancer cell lines cultured in vitro using a microwave biosensor. mda-mb-231 and mcf-7 breast cancer cell lines and saos-2 and 143b osteosarcoma cell lines were characterized using a circular patch resonator in the 1 mhz – 3 ghz frequency range. the return loss of each cancer cell line was analyzed, and the differences among each other were determined through principal component analysis according to a protocol previously proposed mainly for electrocardiogram processing and x-ray photoelectron spectroscopy. our results showed that the four cancer cell lines analyzed exhibited peculiar dielectric properties when compared to each other and to the growth medium, confirming that pca could be employed as an alternative methodology to analyze microwave characterization of cancer cell lines which, in turn, may be deeply exploited as a tool for the detection of cancer cells in healthy tissues. mailto:livio.dalvia@uniroma1.it acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 2 on these bases, microwave-based sensors are now gaining more and more interest in the biomedical field. as highlighted in the literature [16], microwave probes offer the possibility of analyzing living tissue properties through a non-invasive measurement of scattering parameters or complex permittivity [17]–[19] and identifying eventual pathological conditions as a variation in the dielectric properties. concerning cancer cell and tissue characterization, maenhout et al. [20] evaluated the dielectric properties (dielectric loss, dielectric constant, and conductivity) of healthy non-tumorigenic cell lines, namely mcf-10a and four breast cancer cell lines (hs578t, mda-mb-231, mcf7, and t47d) using an open-ended coaxial probe in 200 mhz to 13.6 ghz range. again, zhang et al. [21] proposed a microwave biosensor capable of identifying the grade of colon cancer cell aggressiveness in the 4-12 ghz range. finally, in previous work [22], we proposed a circular patch resonator for the measurement of cancer cell line aggressiveness (saos-2, 143b, mcf7, and mda-mb-231) through the use of a lorentzian fit model for the return loss signal processing and a weighted manova (multivariate analysis of variance) to investigate the differences in the three main parameters of interest, namely return loss, resonance frequency and full width at half maximum (fwhm). this paper proposes a novel methodology to analyze microwave sensor’s return loss based on an optimized savitzkygolay filter, generally adopted for electrocardiogram processing or x-ray photoelectron spectroscopy [23], [24], and principal component analysis (pca) to extract meaningful information from the data and present a final classification based on possible similarities between analyzed materials. 2. materials and methods 2.1. cell culture and experimental procedure as previously described [22], we had the opportunity to test two pediatric human osteosarcoma cell lines, saos-2 and 143b [25]–[28], and two human breast adenocarcinoma cell lines, mcf7 and mda-mb-231 [29], [30] for their dielectric response. in particular, saos-2 and mcf7 are low-aggressive osteoblastlike osteosarcoma and low-aggressive breast cancer cell lines, while 143b and mda-mb-231 are high-aggressive lung-tropic metastatic osteosarcoma and high-aggressive bone-tropic breast cancer cell lines, respectively. cells were seeded in a standard 60 mm petri dish at an average density of 8 × 105 cells/plate and placed in an incubator at 37 °c with 5 % co2 for 24 hours to allow cells to form a homogeneous confluent monolayer. during the measurements, all cell types were maintained in 1.5 ml of dulbecco's modified eagle medium (dmem) culture medium [31], and eight different dishes were prepared for each cell line. moreover, eight samples of 1.5 ml pure dmem were prepared as controls. a circular patch resonator with a radius of 20.00 mm [22] and a subminiature ver. a (sma) connector placed on the conductive edge was employed to determine the dielectric properties of cell line samples. the key component of the measuring setup is the low-cost portable vector network analyzer minivna-tiny [32], used for measuring the return loss|s11(f)| in the operating frequency range of 1.9 – 2.6 ghz. the 700 mhz frequency span was previously evaluated to maximize the resolution of acquired data (0.5 mhz) [22]. as a result, the return loss|s11(f)|was acquired for the eight samples of the five different “materials under test” i. e. different media and cell lines. 2.2. data elaboration process principal component analysis (pca) is a multivariate analysis that permits identifying and extracting meaningful information from the data and presenting a final classification based on a multiparametric similarity test and variables reduction [33]. figure 1 shows a scheme of the applied pre-processing algorithm. all data processing was performed with originlab 2017 software. in particular, pca is a useful tool to reduce the dimension of a dataset, maintaining only those variables with the highest variance. as a result, all the vectors used to represent the acquired return loss are transposed into a new space with a dimension equal to the number of significant components determined by pca, and the acquired data may be represented as: where x is the original data matrix containing the return loss data, l is the loading matrix, s is the score matrix based on the eigenvalues derived from the x matrix decomposition, and e is the error matrix, which contains the variance load not explained by the pca model. the matrix dimension i is the number of acquired samples, j is the signal length, and k is the number of significant components. before performing the pca on the acquired return loss data, we applied a pre-processing algorithm, as proposed by es sebar et al. [34] for raman spectroscopy applications: 1) baseline removal through an interactive endpoint weighted (epw) algorithm for each column vector of x; 2) application of a savitzky-golay filter (sgf) using a window length of 14 points and fitted with a secondorder polynomial since sfg flattens peaks less than a moving average smoothing with the same window width [35]; figure 1. data processing workflow involved in pca. 𝑿(𝑖,𝑗) = 𝑺(𝑖,𝑘) × 𝑳(𝑘,𝑗) t + 𝑬(𝑖,𝑗) = �̂�(𝑖,𝑗) + 𝑬(𝑖,𝑗) , (1) acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 3 3) data normalization by subtracting its average value from each x column and scaling by the standard deviation [36]. for the i-th column of x, equation 2 holds: with x* the normalized matrix, x(i,c) the i-th centered vector, and σ the standard deviation of the x(i,c) vector. this normalization is also known as standard normal variate (snv) transformation. the principal pca was performed by applying equation (2) in equation (1): the discriminant analysis based on a cross-validation test was performed as the final analysis, using as many k components as those with an eigenvalue greater than or equal to 3 [37]. 3. results and discussions figure 2 presents an example of data processing for dmem, reporting the initial raw data (figure 2a) and the three steps for the signal processing (figure 2 b, c and d) baseline remotion, filtering, and normalization, respectively. in detail, the epw algorithm translates and nutes the signal so that the tails lie at zero, while the sg filter evaluates a polynomial regression around each point, creating a new smoothed value for each data point. finally, the snv transformation permits to center and scale the data without altering their overall interpretation: indeed, if two variables were equally correlated before pre-processing, they would still be strongly correlated in post-processing. therefore, for each of the forty acquired signals, the background is removed, the spectrum is filtered to improve the signal-tonoise ratio, the normalization is completed, and the pca is performed. the cumulative variance trend is shown in figure 3. it is possible to observe that the first three components represent an overall variance of about 92.3 %, given by a contribution equal to 76.6 %, 11.9 %, and 3.8 % for components 1, 2, and 3, respectively. according to the literature [37] this can be considered a satisfactory value, as a balance between cumulative variance and complexity of the system to be analyzed for the subsequent analysis, also taking into account that the fourth component contributes only for 2.5 % of the total variance, while the remaining thirty-six components account for the 5.2 % of the whole. 𝑿𝑖 ∗ = 𝑿(𝑖,𝑟𝑎𝑤) − (𝑿(𝑖,𝑟𝑎𝑤)) 𝜎 (𝑿(𝑖,𝑟𝑎𝑤) − (𝑿(𝑖,𝑟𝑎𝑤))) = 𝑿(𝑖,𝑐) 𝜎(𝑿(𝑖,𝑐)) (2) 𝑿(𝑖,𝑗) ∗ = 𝑿(𝑖,𝑗) ∗̂ + 𝑬(𝑖,𝑗) . (3) a) b) c) d) figure 2. a) example of raw return loss for the dmem and computed baseline, b) return loss for the dmem after baseline removal through epw algorithm, c) return loss for the dmem filtered with sg, and d) return loss for the dmem normalized with snv transformation, i. e. the final output. acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 4 figure 4 shows the result of the pca, reporting the scores of the three principal components as a combination of two separately. as can be seen, in figures 4 (a) and (b), the measurements group together into two macro clusters: one containing the pure medium, highlighted by a dotted rectangle, and a second containing all the tested cell types, highlighted by a dashed rectangle. nonetheless, in both figures, it is also possible to distinguish five sub-clusters highlighted by the ellipses enclosing similar spectra with a 95 % confidence level. interesting results can be obtained by focusing on the inclination of these clusters. indeed, the pure medium revealed a different inclination than those obtained when testing all the cell lines. on the other hand, the two less aggressive cell lines (saos-2 and mcf7) have the same inclination as the two aggressive cell lines (mda-mb-231 and 143b). more in detail, the pure medium cluster and the cluster representing the highly aggressive cell lines (143b and mda-mb-231) have the same inclination (100° and 90° respectively) both when focusing on pc2 vs. pc1 and pc3 vs. pc1, while the inclination of the cluster representing the lowaggressive cell lines (saos-2 and mcf7) is 105° when representing pc2 vs. pc1 and 84° when computing pc3 vs. pc1. as a matter of fact, the inclination of the 95 % confidence interval cluster may be a parameter that can give helpful information on tumor aggressiveness. figure 4 c) shows that cell lines and pure dmem did not show proper clusterization. however, the absence of clusters in the pc2 vs. pc3 plot can be explained by considering the low variance captured by the third component (3.9 %). indeed, this component plays a crucial role in the model in linear figure 3. cumulative percentage variance for the first seven components, as obtained from the pca, with the third component highlighted in green. a) b) c) figure 4. cumulative score plots of the first three components: a) pc1-pc2, b) pc1-pc3, and c) pc2-pc3. the percent variance obtained for each component is in the axis legend. the colored ellipse highlights the five clusters representing the 95 % confidence interval. the dotted and dashed rectangles highlight the medium and “cell” clusters. acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 5 combination with the main two components to allow for a cumulative variance higher than 90.0 % (as discussed above), thus improving the fitting of the essential peaks found in the two main components. subsequently, we evaluated the cross-validation of the pca loadings concerning the first three components, and the results are reported in table 1. this test highlighted that pure dmem was detected with a prediction accuracy of 100.0 %, saos-2, mcf7 with a prediction accuracy of 87.5 %, and mda-mb-231 with a prediction accuracy of 75.0 %. finally, 143b cells have a prediction accuracy of 50.0 %. it is worth noting that these discretized prediction accuracy results are strictly related to the number of tested samples. indeed, when testing 8 samples, every prediction accounts for 12.5 % accuracy. figure 5 reports the cumulative results, allowing a better interpretation of the pca predictions. the figure clearly shows that all the 8 dmem tested samples have been appropriately predicted, while, for example, among the 8 tested saos-2 samples (the red bar), seven have been appropriately recognized, while 1 was interpreted as mda-mb-231 cells. similarly, among the 8 tested 143b samples, 4 have been interpreted as 143b, 2 as mcf7, and 2 as mda-mb-231; of the 8 tested mcf-7 samples, 7 have been appropriately recognized and 1 as 143b, and among the 8 tested mda-mb-231 samples 6 have been appropriately interpreted and 2 as 143b cells. interestingly, the final average prediction error for the entire data set is 20.0 %. moreover, these results are in high agreement with that reported in [22], in which the different cell lines were studied with reference to the main lorentzian fit parameters (return loss, resonance frequency, and fwhm) through a manova test. in particular, in [22], we reported a statistical significance difference between dmem and all tested cell lines (p < 0.0001), and in this work, we obtained a cross-validation of 100.0 %. similarly, manova reported a significant (p < 0.5) difference between 143b vs. mcf7 and no significant difference between 143b and mda-mb-231, and pca prediction accuracy was 12.5 % and 25.0 %, respectively. therefore, the procedure reported in this work represents an alternative methodology to distinguish tumor aggressiveness without using any fitting procedure, hence only based on the raw data, whose limit at present consists of the limited number of measurements for each group. 4. conclusions this paper proposes an alternative methodology to analyze the return loss of tumor cell lines. the method allows for discriminating between groups of different tumor cells, analyzing the appropriately filtered and normalized purchase signal, leading to results in agreement with those obtained with traditional methods, such as the lorentzian fit. this methodology is based on a pre-processing algorithm, background removal associated with a savitzky-golay filter, a normalization procedure concerning the signal variation, and a subsequent principal component analysis. results showed good average accuracy of the prediction methodology, confirming the feasibility of pca also for this kind of signal, whereas it has consolidated applications for processing more complex and multi-peak signals. as a future development, we expect to realize a "split ring resonator" sensor inducing more peaks in the instrument's uniformity band to evaluate better the reliability of the methodology proposed here. figure 5. prediction rate for each group. table 1. cross-validation summary for training data and error rate predicted group dmem saos-2 mda-mb-231 mcf-7 143b total dmem 8 0 0 0 0 8 100.0 % 0.0 % 0.0 % 0.0 % 0.0 % 100.0 % saos-2 0 7 1 0 0 8 0.0 % 87.5 % 12.5 % 0.0 % 0.0 % 100.0 % mda-mb-231 0 0 6 0 2 8 0.0 % 0.0 % 75.0 % 0.0 % 25.0 % 100.0 % mcf-7 0 0 0 7 1 8 0.0 % 0.0 % 0.0 % 87.5 % 12.5 % 100.0 % 143b 0 0 2 2 4 8 0.0 % 0.0 % 25.0 % 25.0 % 50.0 % 100.0 % total 8 7 9 9 7 40 20.0 % 17.5 % 22.5 % 22.5 % 17.5 % 100.0 % error rate dmem saos mda mcf7 143b total prior 0.2 0.2 0.2 0.2 0.2 rate 0.0 % 12.5 % 25.0 % 12.5 % 50.0 % 20.0 % acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 6 references [1] r. reilly, breast cancer, xpharm: the comprehensive pharmacology reference (2007) pp. 1–9. doi: 10.1016/b978-008055232-3.60809-8 [2] a. n. bishop, r. jewell, skin cancer, reference module in biomedical sciences, 2014. doi: 10.1016/b978-0-12-801238-3.05621-x [3] e. f. mccarthy, bone tumors, rheumatology: sixth edition, vol. 2–2, 2015, pp. 1734–1743. doi: 10.1016/b978-0-323-09138-1.00212-6 [4] f. bray, j. ferlay, i. soerjomataram, r. l. siegel, l. a. torre, a. jemal, global cancer statistics 2018: globocan estimates of incidence and mortality worldwide for 36 cancers in 185 countries, ca: a cancer journal for clinicians, nov. 2018. 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experimental evaluation of the air trapped during the water  entry of flexible structures  riccardo panciroli 1 , giangiacomo minak 2   1  università degli studi niccolò cusano. via don carlo gnocchi, 3. 00166 roma, italy  2  din – alma mater studiorum. viale del risorgimento, 2. 40136 bologna, italy      section: research paper  keywords: hull slamming; hydro‐elasticity; air trapping; flexible structures  citation: riccardo panciroli, giangiacomo minak, experimental evaluation of the air trapped during the water entry of flexible structures , acta imeko, vol. 3,  no. 3, article 13, september 2014, identifier: imeko‐acta‐03 (2014)‐03‐13  editor: paolo carbone, university of perugia   received june 25 th , 2013; in final form july 13 th , 2014; published september 2014  copyright: © 2014 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: this work was supported by the office of naval research through the grant n00014‐12‐1‐0260.  corresponding author: giangiacomo minak, e‐mail: giangiacomo.minak@unibo.it      1. introduction  predicting the impact-induced stresses during the water entry of flexible structures is of major interest for the design of marine structures. during the water entry of flexible structures, several fluid structure interaction (fsi) phenomena might appear [1]–[9]. the most important are: cavitation, air trapping, and repetition of impact and separation between the fluid and the structure. the occurrence of such fsi phenomena might strongly influence the impact dynamics. although air trapping is a well-known phenomenon [10], to the author's knowledge none of the previous works in the literature presented a methodology to quantify it and evaluate the effect of the structural deformation on it. the present work faces this challenge, proposing the use of an optical technique to achieve this aim. optical techniques have been recently utilized to measure the structural deformation of compliant wedges entering the water in [11]. in this work, we firstly develop a digital imaging technique for the post-processing of high-speed images to isolate the regions of the fluid where air is trapped. this methodology is later used to study the evolution of the air trapping in time and to dissect the role of impact velocity and structural deformation. although there are no previous results in the literature to validate the proposed method, the present results are found in good agreement with the expectations. 2. experimental setup  experiments are conducted on a drop weight machine for water impacts with a maximum impact height of 4 m. wedges are comprised by two panels joined together and to the falling sledge on one edge to assume a cantilever boundary condition, where the boundary corresponds to the keel of the wedge. panels of various material and thickness can be mounted on the abstract  deformable  structures  entering  the  water  might  experience  several  fluid‐structure  interaction  (fsi)  phenomena;  air  trapping is one of these. according to its definition, it consists of air bubbles trapped between the structure and the fluid  during  the  initial  stage  of  the  impact.  these  bubbles  might  reduce  the  peak  impact  force.  this  phenomenon  is  characteristic for the water entry of flat‐bottom structures. above a deadrise angle of 10°, air trapping is negligible. in  this  work,  we  propose  a  methodology  to  evaluate  the  amount  of  air  trapped  in  the  fluid  during  the  water  entry.  experiments are performed on wedges with varying stiffness, entry velocity, and deadrise angle. a digital image post‐ processing technique is developed and utilized to track the air trapping mechanism and its evolution in time. interesting  results are found on the effect of the impact velocity and the structural deformation on the amount of air trapped during  the slamming event.  acta imeko | www.imeko.org  september 2014 | volume 3 | number 3 | 64  sledge at any deadrise angle (β in figure 1) ranging smoothly from 0° to 50°, where 0° and 90° are the extreme case of a flat panel and a vertical blade, respectively. teflon insets minimize friction between the sledge and the prismatic rails. the sledge holds wedges 300 mm long and 250 mm wide. the falling body hits the fluid at the centre of a tank 1.2 meters wide, 1.8 meters long and 1.1 m deep. the tank was filled with water only up to 0.6 m to prevent the water waves generated during the impact to overflow. the drop height, defined as the distance between the keel and the water surface, ranged from 0.5 m to 3 m at 0.25 m increments. impact acceleration is measured by a v-link microstrain wireless accelerometer (±100g) located at the tip of the wedge. all reported accelerations are referenced to 0 g for the free-falling phase. the sampling frequency is set to its maximum of 4 khz. entering velocity is recorded by a laser sensor (με ils 1402) capturing the sledge position over 350 mm of ride at a frequency of 1.5 khz with a definition of 0.2 mm. the entry velocity is obtained by the numerical differentiation of the position. a high-speed camera is utilized to capture the images during the water entry. the camera is located to view the wedge from the side, as shown in figure 2. the capturing frequency is set to 1.5 khz with a definition of 1200×1024 pixels. a vertical transparent screen is located inside the water tank just before the wedge (clearance is ≈2 mm) to prevent fluid spray in the ydirection, which would have made it impossible to see the evolution of the fluid jet (figure 3) generated during the water entry. as the water on the front side of the screen remains still during the impact, pictures show both the still water surface (on the front side of the screen) and the fluid jet (on the back side of the screen), as shown in figure 3. aluminium (a), e-glass (mat) / vinyl ester (v) and e-glass (woven) / epoxy (w) panels 2 mm thick were used. composite panels were produced by vartm by infusion of vinyl ester resin on e-glass fibre mat, while the e-glass (woven 0°/90°) / epoxy panels were produced in an autoclave. the assumed material properties and the measured fundamental frequency of the panels are listed in table 1. for given material and panels thickness, the impact variables are: deadrise angle β (range 4° to 35°) and falling height (ranging 0.25 to 2.5 m). during the experiments, structural deformations are recorded by strain gauges located at various positions, while an accelerometer and a laser position sensor record the impact dynamics. the wedges are built as an open-structure, as the sides of the panels are open and the water is free to flow from the sides during impact (this setup is necessary to allow higher flexibility). this leads the entire structure to be theoretically negatively buoyant. however, the impact time is very short, as it typically lasts less than 40 ms (the structure enters the water with an entry velocity in the range of 4 to 6 m/s). in such a short duration, the water has not enough time to flow into the wedge from the sides, so it behaves like a closed-shape wedge, with a positively buoyant behaviour (the acceleration range is approximately 20 g to 100 g, increasing with the entry speed). wedges are manually lifted to the desired height and released. laser sensor, strain gauges, and accelerometer signals are triggered together in a single manual start. since the position of the sledge relative to the free surface is known, the initial impact time is calibrated during the data post-processing on the basis of the position recorded by the laser sensor. 3. preliminary experimental results  in the following, we display some images captured during the water entry of wedges with deadrise angles higher than 10°. the examples evaluate the air trapped for variable (high to low) deadrise angle. two impact velocities are shown for each deadrise angle. figure 4 shows the water entry of a wedge with deadrise angle of 30° entering the water at 4.2 m/s and 6 m/s. in both cases no water is trapped in the fluid, as indicated by the smooth uniform colour in the fluid region. figure  2.  sketch  of  the  experimental  set‐up.  the  wedge  is  hinged  to  the  sledge  that  enters  the  water  with  pure  vertical  velocity.  the  high‐speed  camera is located on the side of the water tank.  figure 1. conceptual scheme of the wedge used for experiments. l = panel  length.    =  deadrise  angle.  solid  line:  undeformed  panels,  dashed  line:  expected deformation during impact.   figure 3. sample of an image captured by the high‐speed camera. the still  water above the transparent screen is clearly visible. the lighter dots clearly  visible in the fluid are air bubbles that are used as tracers in the piv analysis. table 1. collection of the estimated material properties.  material  e1=e2 [gpa]    [kg/m 3 ]  n [hz]  6068 t6  68  0.32  2700  18.01  e‐glass/vinylester  20.4  0.28  2050  9.77  e‐glass /epoxy  30.3  0.28  2015  19.69    acta imeko | www.imeko.org  september 2014 | volume 3 | number 3 | 65  figure 5 shows the water entry of a wedge with deadrise angle of 20° entering the water at 4.2 m/s and 6 m/s. while in the first image there is no evidence of air trapped in the fluid, the wedge impacting at higher speed (picture on the right) is trapping some air in the form of small bubbles that appear at the middle of the wedge. although existing, air trapping is still negligible for this deadrise angle. wedges with deadrise angle of 15° (shown in figure 6) show results similar to the last example. even for this deadrise angle some negligible air bubbles are trapped in the fluid. air bubbles are definitely spread on a wider region in the case of a deadrise angle of 12°, as shown in figure 7. until the deadrise angle of 12° no air cushions are formed: air is trapped in the form of small bubbles dispersed in the fluid and its effect can be neglected. instead, air cushions are formed for deadrise angles lower than 12°, indicating that our results are in line with the literature. in the following sections, the research will focus on the water entry of wedges with deadrise angle lower than 12° with particular effort on evaluating the amount of air trapped in the fluid, its evolution in time, and the effect of the structural deformation on it. as the first step, a digital image postprocessing methodology capable of evaluating the amount of air trapped in the fluid has been developed and is presented in the next section. 4. the use of an optical method to account for the  air trapped during the water entry  to track the evolution of the air trapped during the water entry, a digital image technique has been developed to post– process the high-speed images. this technique relies on the properties of the water surface to diffract the light: as air bubbles are entrapped in the fluid, if lightened by a light source, their surface will diffract the light making them brighter than the surrounding fluid. images with a definition of 1200×1024 pixels are captured by the camera at a rate of 1.5 khz. an example of a typical image where air is trapped into the fluid is shown in figure 8. images (originally in colour) are converted to greyscale, that is, images are represented by a matrix where the cells assume a value between 0 and 255, where 0 corresponds to a fully black pixel and 255 to a fully white pixel. all the values in between define the grey level. the intensity of the grey levels vs. the pixels counts are plotted as a histogram (figure 9). we choose for the relation between the magnitude of air trapped below the structure and the number of pixels exceeding a certain grey level. each pixel corresponds to an area of 0.23×0.23 mm2, since the calibration gave that 1305 pixels correspond to 300 mm in length. upon an independent study on the role of the threshold level on the evaluated results on the computed amount of trapped air, we choose 200 as the threshold level above which the pixels have to be considered air. such threshold level was found to be extremely affected by the lighting parameters used during the image acquisition: diaphragm aperture and exposition time (inversely proportional to the capturing figure 4. image of a wedge with deadrise angle of 30° entering the water at 4.2 m/s (left) and 6 m/s (right). the fluid shows a uniform colour, meaning that no air has been trapped during the water entry.   figure 5. image of a wedge with deadrise angle of 20° entering the water at 4.2 m/s (left) and 6 m/s (right). there is no air trapped in the case of lower entry velocity while some very small air bubbles appear in the case of larger velocity.  figure 6. image of a wedge with deadrise angle of 15° entering the water at  4.2 m/s (left) and 6.7 m/s (right). in both cases there is some air trapped in the fluid in the form of small air bubbles.  figure 7. image of a wedge with deadrise angle of 12° entering the water at 5.2 m/s (left) and 6.7 m/s (right). air trapping is much more visible than the previous  cases  since  a  concentrated  light  is  used  to  highlight  the  air bubbles.  figure 8. example of an image where some air is trapped in the fluid during  the impact. air appears as a bright region due to the light that is diffracted  by the surface of the air bubbles.  figure 9. example of a histogram of the grey level (0 to 256) vs. pixel count  of the greyscale image.  acta imeko | www.imeko.org  september 2014 | volume 3 | number 3 | 66  frequency). however, we comment that, for a given diaphragm aperture and exposition time, variations of the threshold level from the reference value by 20% have negligible effects on the results. a new binary image is then built: pixels below the threshold are set as black (0), while pixels exceeding the threshold are set as white (255). a black and white picture is obtained this way. to smooth out the images and clear possible lonely black pixels isolated in wide white regions an algorithm based on morphological reconstruction described in [12] is applied to the images. later, the general procedure outlined in [13] is applied to compute the white regions. as output, the white regions are listed and their perimeter and area is reported. small air bubbles are dispersed in the water even before the impact. a threshold on the minimum size of the area is thus used to neglect these small bubbles from the evaluation of the total amount of air trapped during the water entry. the number of white regions is thus filtered to exclude those with an area smaller than 12 pixels, as this value has been evaluated to correspond to the area of the air bubbles already trapped in the fluid before the impact. the total area of air trapped below the structure is thus evaluated as it is assumed to be proportional to the number of pixels counted with the proposed method. 5. on the effect of the entry velocity on the  trapped air  this section investigates the effect of the entry velocity on the amount of air trapped during the initial stage of the impact. using the technique presented in the previous section, it is possible to observe the time trace of the trapped air. the analysis is performed on wedges with deadrise angle of 4° entering the water in free fall from several impact heights: namely 50, 100, 150, and 200 cm. wedges are all 2 mm thick and are made by three different materials: aluminium, wovenglass/epoxy and matt e-glass/vinyl ester. this way the impact conditions were similar but bodies presented different flexibility due to the differences in the elasticity modulus of the three materials (namely 68 gpa, 30.3 gpa and 20.2 gpa). a detailed characterization of the specimens can be found in [14]. it was thus possible to study the effect of the structural deformation on the air trapped during the impact. figures 10 to 12 show the results of the evaluated trapped air versus the entry depth v0t, where v0 is the velocity at the beginning of the impact. 6. effect of the structural deformation on the air  trapped during the water entry  in the case of the water entry of flexible structures, there is the possibility that the structural deformation alters the air trapping mechanism. in particular, considering simple wedges, the deadrise angle is locally modified during the impact with water and this could lead air bubbles to coalesce and to form a cushion, or, on the other side, it could let the air escape from an already present cushion. an example of wedge deflection evolution during the water impact can be found in [8]. a collection of the experimental results for the different wedges impacting from the same heights is presented in figure 13. the experimental findings suggest that the flexibility of the wedge has negligible effects on the air trapping, as wedges assume large deformations once the air has been already entrapped in the fluid. even if the amount of trapped air is quite similar in all the three cases, it may be noted that, at the beginning of the water entry, stiffer wedges show a sharper peak of entrapped air, for high impact energies, than the more flexible ones. instead, the more flexible wedges apparently loose some air from the cushions in the final part of the entry. further investigation is needed to further explore and confirm such findings. figure  10.  experimental  evaluation  of  the  air  trapped  in  time.  aluminium wedge (a) 2 mm thick, deadrise angle = 4°, for variable impact height. the  impact heights in the legend are in cm. the product v0t is in mm.  figure  11.  experimental  evaluation  of  the  air  trapped  in  time.  composite  wedge (w) 2  mm thick, deadrise angle = 4°, for variable  impact height.  the impact heights in the legend are in cm. the product v0t is in mm.    figure  12.  experimental  evaluation  of  the  air  trapped  in  time.  composite  wedge (v) 2 mm thick, deadrise angle = 4°, for variable  impact heights.  the impact heights in the legend are in cm. the product v0t is in mm.  acta imeko | www.imeko.org  september 2014 | volume 3 | number 3 | 67  7. conclusions  in this work we propose a technique to quantify the amount and the evolution in time of the air trapped during the water entry of flexible structures. first, a methodology based on the analysis of the high-speed images is proposed and commented. results are found to be in agreement with the expectations, although only qualitative comparison can be done, as there are no other experimental, nor numerical results in the literature to compare our results with. on the base of the experimental findings, air trapping seems to attain its maximum at the beginning of the impact, when the velocities are higher. however, bodies need time to deform: when the wedge deformations are large enough to modify the deadrise angle, the entireness of the entrapped air has been already trapped in the fluid. on the basis of these preliminary results, there is no experimental remarkable evidence of the influence of the structural deformation on the amount of air trapped during the impact. further studies are needed to confirm this observations. the analysis of the images showed that wedges with deadrise angles greater than 10° entrap the air in the form of small bubbles spread on a region that decreases as the deadrise angle increases. in the cases investigated, the structural deformation was not capable of lowering the deadrise angle enough to switch the air trapping mechanism from small air bubbles (with negligible effect on the hydrodynamic pressure) to an air cushion, which might have a strong effect on the hydrodynamic pressure. indeed, further investigations in this direction are needed to deeply understand this phenomenon. the experimental results show a saturating effect of the impact energy on the amount of entrapped air. furthermore, the role of the stiffness is very limited, as air trapping is found to mainly relate to the initial deadrise angle. further investigations are needed for the cases with very low deadrise angles and different geometries. we comment that the recently developed methodologies to reconstruct the hydrodynamic pressure in water entry problems from the flow kinematic components [15-17] can be adopted in the future to quantify the influence of air trapping on the hydrodynamic pressure. acknowledgement  support from the office of naval research (grant n0001412-1-0260) and the advice of dr. y. rajapakse are gratefully acknowledged. references  [1] s. abrate, hull slamming, appl. mech. rev. 64 (2013) p.060803. [2] a. korobkin, e.i. părău, j.m. vanden-broeck, the mathematical challenges and modelling of hydroelasticity, philos. trans. a. math. phys. eng. sci. 369 (2011) pp. 2803-2812. [3] s.e. hirdaris, p. temarel, hydroelasticity of ships: recent advances and future trends, proc. inst. mech. eng. part m j. eng. marit. environ. 223 (2009) pp. 305–330. [4] x. chen, y. wu, w. cui j.j. jensen, review of hydroelasticity theories for global response of marine structures, ocean eng. 33 (2006) pp. 439–457. [5] o.m. faltinsen, the effect of hydroelasticity on ship slamming, philos. trans. r. soc. a math. phys. eng. sci. 355 (1997) pp. 575–591. [6] o.m. faltinsen, hydroelastic slamming, j. mar. sci. technol. 5 (2000) pp. 49–65. [7] r. panciroli, water entry of flexible wedges : some issues on the fsi phenomena, appl. ocean res. 39 (2012) pp. 72–74. [8] r. panciroli, s. abrate, g. minak, a. zucchelli, hydroelasticity in water-entry problems: comparison between experimental and sph results,” compos. struct. 94 (2012) pp. 532–539. [9] r. panciroli, s. abrate, g. minak, dynamic response of flexible wedges entering the water, compos. struct., 99 (2013) pp. 163-171. [10] a. korobkin, a.s. ellis, f.t. smith, trapping of air in impact between a body and shallow water, j. fluid mech. 611 (2008) pp. 365–394. [11] m. cooper, l. mccue, “experimental study on deformation of flexible wedge upon water entry,” in 9th symposium on high speed marine vehicles. [12] p. soille, morphological image analysis: principles and applications. springer-verlag, 1999, pp. 173–174. [13] r.m. haralick, l.g. shapiro, computer and robot vision, 1st ed. boston, ma, usa: addison-wesley longman publishing co., inc., 1992. [14] r. panciroli, dynamic failure of composite and sandwich structures, vol. 192. dordrecht: springer netherlands, 2013. [15] a. nila, s. vanlanduit, s. vepa, w. van paepegem, a piv-based method for estimating slamming loads during water entry of rigid bodies, meas. sci. technol., 24 (2013) p. 045303. [16] b.w. van oudheusden, piv-based pressure measurement, meas. sci. technol., 24 (2013) p. 032001. [17] r. panciroli, m. porfiri, evaluation of the pressure field on a rigid body entering a quiescent fluid through particle image velocimetry, exp. fluids 54 (2013) p. 1630. figure  13.  evolution  of  the  air  entrapped  in  time  for  the  water  entry  of  wedges  with  various  flexural  stiffness  (a=aluminium,  w=woven glass/epoxy,  v=  mat  glass/epoxy)  impacting  from  two  different  impact heights (100 cm ‐ top, and 150 cm – bottom). the product v0t is in mm.   power quality metrics for dc grids with pulsed power loads acta imeko issn: 2221-870x june 2021, volume 10, number 2, 153 161 acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 153 power quality metrics for dc grids with pulsed power loads andrea mariscotti1 1 diten, university of genova, via opera pia 11a, 16145 genova, italy section: research paper keywords: dc grid; power quality; pulsed power load citation: andrea mariscotti, power quality metrics for dc grids with pulsed power loads, acta imeko, vol. 10, no. 2, article 22, june 2021, identifier: imekoacta-10 (2021)-02-22 section editor: giuseppe caravello, università degli studi di palermo, italy received february 17, 2021; in final form april 24, 2021; published june 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: andrea mariscotti, e-mail: andrea.mariscotti@unige.it 1. introduction the term “dc grid” may be considered as a catchall for various types of distribution networks that are being extensively used for a wide range of applications. one of the advantages of dc distribution is the ease of integration of sources and loads, without the complication of phase angle instability and coordination, typical of ac applications. such networks are operated at both medium voltage (mv) and low voltage (lv) with some standardized nominal voltage values. compared to ac grids in several cases dc grids can interface to pulse loads more effectively, that is with a faster response and without violating power quality (pq) constraints [1]. the physical extension is variable, from some tens of meters within a large room, a smart house or an electrified vehicle, hundreds of metres between buildings, at a campus, or onboard ships, and up to some km for distribution within smart residential districts, technological parks, etc. [2], [3]. some representative examples of such networks are as follows. smart data centres pursue a dc distribution perspective, in particular to ensure a high level of availability, using capacitors, batteries and autonomous renewable energy sources (photovoltaic and fuel cells to cite the most common ones operating at dc), covering a wide time scale of events (fluctuations, dips, short and long interruptions) [4]. mv power distribution onboard ships features various types of loads with different dynamics and power absorption levels [1], [5], [6]. various arrangements may be adopted: split dc and zonal bus types, different sub-bus solutions for generators, propulsion loads and ppls, etc. [6]. the extension of the network is of course limited to the physical size of the ship (up to about 300 m) but cabling and routing may be quite complex. another example of lv/mv application characterized by dynamic moving loads is the wide set of railways, metros, tramways and trolley buses, all supplied from a catenary (or third rail) system fed by rectifier substations [7], [8]. the line voltage is generally standardized to a set of nominal values of 600, 750, 1500 and 3000 v. these networks feature the largest extension, as they cover entire cities, regions and countries. they are sectioned, however, into smaller portions mainly for an exigency of maintenance and operation, besides control of supply voltage. the network frequency response is nevertheless peculiar, with significant variations of the network impedance and resonances already appearing at low frequency [9]. loads with large nominal power and significant dynamics have a direct impact on the line voltage (subject to appreciable variations consequential to load cycles) and can compromise network stability [10]. as in all distribution networks, the primary quantity is the voltage available at equipment terminals, but for a complete assessment of the network-load interaction, current and power should be also evaluated. in particular, the current waveform is specifically involved in phenomena such as inrush, identification of faults and in general control of the interface converters by feed-forward methods, attempting dynamic abstract dc grids can effectively feed pulsed power loads (ppls), integrating local energy storage to minimize the impact on other connected loads and implementing buffered sub-grids to isolate susceptible loads even more. the identification of regulatory limits for ppl integration and dc grid response and the assessment of ppl impact necessitate suitable power quality (pq) metrics. existing indexes and limits (e.g. ripple and distortion, voltage fluctuation) are compared to other metrics, derived from control methods and knowledge of the peculiar characteristics of ppl and interaction with dc grids in some examples. the objective is a unified approach for pq metrics suitable for a wide range of dc grids and in particular to the behavior of ppls. mailto:andrea.mariscotti@unige.it acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 154 impedance control. the power profile instead is the primary quantity for the power and energy budget, as well as for prediction and control [11]. in general, the impact of pulsed power loads (ppls) is reduced by a range of techniques suitable for dc grids, whose applicability may depend on the specific grid characteristics: • first of all load dynamics may be limited at the origin, e.g. by limiting the rate of rise of the power demand, compatible with the load mission and characteristics; a wide range of techniques have been proposed, falling all under a unique category that we may call “profile-based control” • additional passive energy storage devices (e.g. batteries, supercapacitors, etc.) may be installed, capable of feeding the supplementary power with the required rapidity, and at the same time dampen network swell (consequential to load release) and recover the energy, in case of regenerative behaviour • the role of passive energy storage devices may be backed up by the so called “dc springs” and other active compensation devices. the work is thus structured starting from ppl characteristics in terms of electrical behaviour and interaction with the dc grid (section 2). section 3 then focuses on metrics suitable to describe quantitatively such interaction and on the impact on dc grid operation and pq. these metrics are derived from previously proposed pq indexes, as well as from considerations regarding current and power absorption profiles. section 4 summarizes the normative pq requirements for dc grids in various applications that are characterized by the presence of ppls, such as avionics, shipboard and railways at various extents. section 5 then evaluates the performance of the proposed metrics, using simulated and measured data. 2. pulsed power loads and network interaction pulsed power loads (ppls) with repeated large variations of load current represent an issue to dc grid voltage stability and regulation [12]. for complex systems with non-essential and essential loads, the adoption of a dc zonal distribution system is commonplace, dividing the dc grid onboard into zones separated by interface converters. each sub-grid adopts specific solutions where necessary for critical loads, de facto confining and limiting the propagation of transients and disturbance. since power is fed often by intrinsically ac sources (such as gas turbine and diesel engine alternators), heavy loads may be instead fed directly from a primary ac grid, from which the other zoned dc sub-grids are derived, one or more dedicated to specific ppl and local energy storage. ppl loads are usually interfaced with a dc/dc front-end converter, based on a variety of topologies, depending also on power rating, voltage level and required dynamics. ppls for a wide range of applications may be exemplified as follows. • devices and apparatus that are part of scientific experiments in nuclear physics, such as the magnetic circuits of acceleration and bending at synchrotrons, the large hadron collider, etc. in several cases choices are a compromise between desirable performance for new types of experiments reutilization of existing equipment (undergoing thus gradual modernization) and exigencies of power absorption, feasibility of protections, and continuity of service. dc distribution facilitates the direct or indirect connection of energy storage, ensuring both fast dynamic response and higher continuity of service, even supporting reconfiguration on the fly (“hot swap”) to account for some amount of dc/dc supply going out of service [13]. • radar systems connected to aircraft dc grid with power absorption following transmission pulses. the dc grid onboard aircrafts is characterized by a limited power deployment, with flight control loads, constant power loads (such as fuel pump), and the said radar load, as well as modern radios. the load profile described in [14] may be taken as an exemplifying case: current pulses of 4 ms duration and peak power of 33.6 kw with respect to a steady power absorption of one third (11.2 kw); peaks are arranged in pulse bursts with spacing of 10 ms (5 peaks) with a repetition of 200 ms. modern aircrafts may be equipped with more than one radar device, for which coordination for interleaved operation may be a strategic option. modern radio systems with advanced characteristics of spectrum exploitation, long range and traffic acceptance flexibility (especially for military applications) may share a power absorption pattern similar to radars. • rail guns and electromagnetic weapons, such as high-power laser and microwave beams, typically located onboard ships [15]. all these weapon systems deliver peak power levels in the order of 1 gw or more through a pulse forming network, whose charging from the ship dc grid absorbs power levels in the order of 10 mw – 30 mw for a duration of some seconds. during intensive use dc grid loading is almost continuous, with the delivery of fast energy pulses that may easily decoupled thanks to the much higher characteristic frequency. • electrified transports featuring high-performance units with a dynamic power profile. two quite different modern transportation means fall into this category: urban highspeed high-pace electrified guideway systems and electric vehicles with dynamic wireless power charging, with pulsed power absorption with fast charging times when passing charging points. both can be assumed fed by a dc grid although the physical extension is quite different: dynamic wireless power transfer requires local dc distribution buses up to about some hundreds meters [16], whereas guideway systems, tramlines and railways of various kids feature supply sections of several km [17]. for guideway systems, such as metros, it must be pointed out that ppls at the load release or when implementing regenerative braking, may cause a significant increase of line voltage with consequence for electrical safety, especially for passengers and people standing at platform and near platform screen doors [18]. 3. disturbance metrics for both the implementation of the control policies and the assessment of performance, suitable metrics of the disturbance are necessary. such metrics can be applied to voltage, current, power, or a combination thereof (multi-criteria metrics). they were preliminarily discussed and compared to relevant standards in the imeko tc4 conference paper [19], of which this work is the continuation, with focus on dynamic ppl loads and network stability. from the normative viewpoint [20]–[25] transient electrical phenomena may be classified as: • macroscopic voltage variations lasting for tens ms or longer • shorter voltage variations, related to switching transients of variable amplitude acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 155 • periodic variations often described in terms of harmonic distortion or ripple, covering a wider frequency range. it is commonplace to evaluate all these phenomena by the instantaneous difference with respect to steady state vdc value [1], [12], applying the concept of ripple to all voltage variations, both transient and repetitive. the mil-std-1399-300 standard [26] has recently extended the concept of distortion and ripple to the active power absorption profile itself for ac grids onboard us navy ships. the concept is interesting for dc grids where the concept of absorbed power is more straightforward and does not involve reactive power. in the following, voltage fluctuations and transient variations are considered first, discussing alternative metrics other than ripple; periodic variations and ripple are then considered, including the extension to cover aperiodic variations and transients. 3.1. voltage fluctuation as anticipated the network response and the quality of the delivered energy (power quality in a wide perspective) is usually measured by indexes applied to the line voltage, measuring voltage spread during fluctuations to compare with maximum values ad time-amplitude limit curves. line transients may thus be evaluated by a measure of their pure amplitude a (e.g. peak or average value), equivalent time duration tx (e.g. half-amplitude duration t50) and combined amplitude-duration. the first index that has a strong relationship with ac networks is the rms value, that when defined for aperiodic phenomena corresponds in reality to a measure of the area of the signal. thus, focusing on aperiodic phenomena for generality, the combination of amplitude and duration gives two measures of the intensity of the phenomenon: area and energy. taking a time interval [t1, t2] and considering the ac portion x(t) of the network quantity (voltage or current), having first subtracted an estimate of the steady value x, we obtain: 𝑆 = ∫ 𝑥(𝑡) 𝑡2 𝑡1  𝑑𝑡, 𝐸 = ∫ |𝑥(𝑡)|2𝑑𝑡, 𝑡2 𝑡1 𝑃 = 𝐸 𝑡2 − 𝑡1 . (1) energy is calculated and considered in a signal processing perspective, as the square of a signal x over the time interval [t1, t2]. then power p is just obtained as energy e divided by the duration t2–-t1 of the time interval. it is easy to see that combining voltage and current corresponds to the squared rms. the mean square time duration may be also defined, that measures the interval where the energy is concentrated: 𝜏 2 = ∫ 𝑡 2|𝑥(𝑡)|2d𝑡 𝑡2 𝑡1 ∫ |𝑥(𝑡)|2d𝑡 𝑡2 𝑡1 ⁄ . (2) area (or impulse strength) and energy can be used to evaluate the impact on loads: the missing area or energy of a negative transient (a temporary reduction of the line voltage) will cause a reduction of the relevant quantities at the load side. the consequence is a diminished performance that may vary for duration and severity depending on the load type. such impact is partially compensated by filters (capacitors mainly), storage devices (e.g. batteries and supercapacitors) and dc springs over different time scales, covering fast and slow variations. in addition, control at the source with a reserve of energy driven by a suitable index of the network quality of power can support cooperative control. 3.2. voltage ripple dc grids are generally characterized by means of “ripple”, that en 61000-4-17 [27] describes as composed of “power frequency or its multiple 2, 3 or 6”, focusing with a limited view on the mechanism of production by classic ac/dc conversion (based e.g. on diode and thyristors rectifiers). this scheme is well represented in dc electrified transports where the ripple of substation rectifiers is clearly visible: its relevance is in terms of line voltage fluctuation reflected also in the track voltage, where a significant amount of ripple may be relevant for the assessment of touch voltage and body current [18]. ripple is thus a repetitive phenomenon superimposed to the dc steady value. modern ac/dc and dc/dc converters and poly-phase machines for renewable energy sources are used extensively to improve pq and for ease of interfacing in modern micro-grids and smartgrids. this leads necessarily to a reformulation of the concept of ripple to a more general definition accounting for nonharmonically related components, possibly non-stationary. the time domain definition of ripple is the maximum instantaneous voltage variation over a given time interval [28][29] or directly as difference between the observed maximum and the minimum voltage values [24], both weighted by the steady dc voltage value vdc. in this perspective ripple and voltage fluctuation are synonymous, provided that ripple is defined as difference with respect to vdc, taking thus positive and negative values (above or below vdc) separately, allowing to consider asymmetric limits for voltage variation. it is observed that dc networks have a much lower harmonic content thanks to the large deployed capacitance and in general the lower impedance in the harmonic frequency range, as it was demonstrated for railways in [30], comparing harmonic power terms in ac and dc systems. lower network impedance keeps harmonic voltage components low, while amplifying current distortion at higher frequency, as sources see a quasi-short-circuit condition. basically speaking, ripple may be calculated as the maximum of the peak-to-peak or peak excursion of network voltage [12], [28], but other measures of it (rms, percentiles, ...) were proposed in the past [31]-[33]. ripple addresses two objectives at once, quantifying the spread of instantaneous values and the network distortion (ripple was defined both in time and frequency domain [28], [29]). the explicit connection between ripple and dft (including harmonics as such and other components) was given in [28] with the index dlfsd and in [31] with rdf. ripple can describe the quality of the delivered voltage in terms of fluctuations and excursion, as well as assessing significant load steps and inrush phenomena when applied to the current waveform. 3.3. power trajectory ppl relevance is caused by the sudden power absorption and the line voltage drop consequential to the flow of absorbed current. grid voltage will change by tens of % when current increases by orders of magnitude, so that at a first approximation metrics based on power may be replicated for current with similar results. ppl input power as a function of time pp(t) is commonly named “power trajectory”. it has an approximately trapezoidal shape with amplitude p, comparable rise and fall time tr and tf, and top value duration tp; pulses of overall duration tp repeat in bursts of duration tb, that may occur more than once forming a train of bursts, one burst every tt seconds, as sketched in figure acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 156 1. the radar load depicted in section 2 fits this description with tp=4 ms, tp=10 ms, tb=50 ms and tt=200 ms. for our purpose the following elements are particularly important and characterize the power trajectory: the top value p, the rate of rise p/tr, the duration of the power pulse tp and the repetition intervals, tb and tt. the peak power p establishes the loading of the source and the grid voltage reduction, taking one pulse alone in almost steady conditions. the rate of rise instead indicates the dynamics of the power demand and the rapidity with which the dc grid and its energy storage must feed the load to sustain the network voltage. a metric that weights the rate of rise is the derivative vs. time of the power trajectory. the pulse duration tp , together with the absorbed power p, is relevant from an energy viewpoint, indicating the level of depletion of the energy storage during one power absorption event. the two parameters may be combined by calculating the area ap = p tp of the power trajectory pulse, that as a general metric was introduced in section 3.1. tb and tt indicate instead the periodicity and the amount of time available to recharge the energy storage devices between two pulses or two bursts of pulses. the number of pulses in one burst and of bursts in one train come directly as np = tb/tp and nb = tt/tb. combined with the area ap, that has the unit of measure of energy, we obtain the total depletion in terms of number of pulses, and the rate of depletion of the energy storage, that must be matched by the source capability. the time rate of change of the power trajectory was proposed as the reference quantity of a disturbance metric to apply to ppls [11]: 𝑑𝑃 = √ 1 𝑇𝑃 ∫ ( d𝑃𝑃 (𝑡) d𝑡 ) 2 d𝑡 𝑇𝑃 0 . (3) the expression for dp returns the average square derivative of the power trajectory pp(t); in other words, this is the rms value of the derivative of the power trajectory. considering the sinusoidal components composing it, the time derivative corresponds to a multiplication by  of each component; the structure of this penalization is similar then to the one adopted for high-voltage stress of capacitor insulation [34] (the reference quantity being there the electric field or the terminal voltage). by considering the power trajectory expressed as pp(t) = v(t)i(t), the derivative and square of the product may be further developed. the derivative of a product is equal to the sum of the two mixed products of one of the quantities with the derivative of the other. each derivative is the multiplication by  of the quantity expressed as a fourier series: 𝑑𝑃 ′ = √ 1 𝑇𝑃 ∫ (∑ �̃�𝑚 (𝑡) 𝑚 ∑ 𝜔𝑛 𝐼𝑛(𝑡) 𝑛 + ∑ 𝜔𝑚 �̃�𝑚 (𝑡) 𝑚 ∑ 𝐼𝑛(𝑡) 𝑛 ) 2 𝑇𝑃 0 having identified for brevity with )( ~ tvm and )( ~ tim the fourier series terms each with an exponential term and the peak value of the voltage or current component. it is also observed that to the aim of calculating energy, only the terms with a non-null integral over the longest period are retained, that is the mixed products of )( ~ tvm and )( ~ tin , mn, give a null contribution and only those with the same harmonic index are retained. the peak values in )( ~ tvm and )( ~ tin , once passed through the integral over tp, are transformed into rms values vm ad in with a scaling of 1/2 each that compensates with the factor of 2 resulting from the sum left and right of identical terms. 𝑑𝑃 ′ = √(2 ∑ 𝜔𝑛 1 2 𝑉𝑛 𝐼𝑛 𝑛 ) 2 = ∑ 𝜔𝑛𝑉𝑛 𝐼𝑛 𝑛 = ∑ 𝜔𝑛𝑃𝑛 𝑛 . (4) the resulting alternative power rate metric pd is thus a harmonic active power term multiplied by the pulsation, representing a sort of weighting function. harmonic active power was analysed for dc and ac railway systems in [30], finding that relevant terms are located at low frequency with relevance of a fraction of % compared to the total exchanged energy. such terms are larger for ac than for dc systems, that feature in general larger deployed capacitance (and may be also backed up by various energy storage systems). 3.4. power distortion treating the absorbed power profile (i.e. the power trajectory pp(t)) as a signal, the equivalent of the total harmonic distortion may be calculated, as well the intensity of spectral components of pp(t) for specific frequency intervals. this is the approach recently proposed in the mil-std-1399-300 interface standard for ac grids onboard us navy electric ships. power distortion (pd) is calculated for the active power component only (called “real power”): 𝑃𝐷 = √∑(𝑃𝑛 √2𝑃𝑎𝑣⁄ ) 2 𝑛 . (5) such a metric speaks of peak value of active power at a given frequency interval (or bin n) and extracts its rms value by dividing by 2. this metric is preliminarily considered here for dc systems by simply analysing the spectral behaviour of pp(t), compared to the expression derived for pd in (4), where the active power terms are arithmetically summed with the inclusion of pulsation as relative weight. considering the typical behaviour and response of the elements of a dc grid (energy storage devices, filters, converter controls), we briefly observe that an increased weight linear with frequency does not express a practical scenario where high frequency is in reality filtered by capacitors, whereas medium frequency components may fall beyond control bandwidth. figure 1. ideal power trajectory pp(t) of a ppl, absorbing power in bursts of individual approximately trapezoidal pulses. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 157 4. normative requirements 4.1. voltage variations, swells and sags (dips), and interruptions overvoltages and undervoltages may be named swells and sags (or dips) with analogy to ac networks. standards specifying requirements for pq, performance and reliability of dc distribution divide among the areas of application: mil-std704f [23] for aircrafts; iacs reg. e5 [22] and ieee std. 45.1 [21] for ships; en 50155 [24] onboard rolling stock; itu-t l.1200 [25] for telecommunication and data centers. in general, distinction is needed from other long-term transients (long interruptions and fluctuations), as well as from very short-term phenomena, usually classified as spikes or surges. these latter in dc grids with no overhead lines are in general of modest amplitude, thanks to the large deployed capacitance (batteries and other energy storage devices, output filters of interfacing converters, etc.) and the consequent low transient impedance). definitions of ieee std. 1159 [35] and en 61000-4-30 [36] for ac systems indicate a voltage dip or swell when the rms crosses a threshold, its duration quantified measuring the time interval between two consecutive crossings [35][37]. this definition based on the rms value is possible because swells and dips are defined for durations > 1 cycle. the analogy with dc systems is not perfect and various time scales for the estimate of the transient and related steady value may be used. this will be shown with an example in section 5. table 1 shows limits and reference values for voltage fluctuations, swells, sags and interruptions taken from the most relevant standards for the environment of application of ppls reviewed in section 2. figure 2 gives insight in graphical form into the timeamplitude limits of mil-std-704f and en 50155 for aircraft and railway applications, respectively. faster transients (with shorter duration and rise and fall times) are more application specific and may depend e.g. on switching operations. a peculiar source of repetitive fast transients in electrified transportation systems are electric arcs, the known byproduct of the current collection mechanism, occurring at small discontinuities of the catenary line, during pantograph detachment in dynamic conditions and with adverse weather (e.g. ice) [38][39]. such phenomenon is also relevant from an energy consumption viewpoint, in principle directly (as dissipative phenomenon in the arc itself and involved components), but most of all indirectly (interfering with the operation of the braking chopper of dc rolling stock) [40]. 4.2. ripple, harmonics and periodic variations the en 61000-4-7 [41] is a well-structured and complete standard that covers methods and algorithms quantifying spectral harmonic components, including interharmonics. however, the underlying assumption is always that of the existence of a fundamental, even for interharmonics, that e.g. combine mains and motor frequency in variable frequency drives or two mains frequencies in interface converters. as observed in sec. 3.2, for dc grids it is always assumed that the dc supply comes from ac mains upstream, although technology advancement is characterized also by autonomous dc grids, supplied by sources that are intrinsically dc, such as fuel cells and photovoltaic panels. it is evident thus that concepts as distortion and spectrum components, and suitable limits, should be independent from a purely harmonic perspective: • distortion becomes the amount of ac ripple superposed to the dc steady value vdc, rather than the composition of the table 1. limits and reference values for transient events (voltage variation, dip and interruptions) (e=emission, i=immunity, a=ambient specification). standard phenomenon type nom. volt. un in v ref. values mil-std-704f voltage var. a 28 v, 270 see figure 1 en 61000-4-29 voltage var. i 24-110 85-120 % 0.1-10 s en 61000-4-29 voltage dip i 24-110 40,70 % 0.01-1 s en 61000-4-29 voltage interr. hiz & loz i 24-110 0 % 0.001-1 s iacs voltage var. i  1kv 95-105 % en 50155 voltage var. a 24-110 see figure 1 en 50155 voltage interr. a 24-110 0 % 0.01-0.03 s l.1200 voltage var. a 300, 380 un-400-un 1 min un-260-un 1 min un-410-un 1 s un-420-un 10 ms l.1200 voltage dip i 300, 380 40 % 0.01 s l.1200 voltage interr. i 300, 380 0 % 0.01 s (loz) 1 s (hiz) a) b) figure 2. profile of allowed transient overvoltage / undervoltage for (a) avionic (mil-std-704f) and (b) railway (en 50155) applications. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 158 amplitude of the harmonic components; this definition goes back to ripple, that candidates itself as a flexible and all comprehensive metric • spectrum components are considered for a continuous frequency axis, although resolution frequency limitation is still relevant; this aspect is mentioned as mil-std-704f poses limits not only for distortion, but also to the spectral components [19]. table 2 summarizes normative limits and reference values for ripple and distortion. two quantities can be identified in the various standards that weight distortion and voltage variations: • the distortion d is the rms value of the ac components • the distortion factor df = d/vdc • the ripple r is the maximum absolute difference between an instantaneous value and the steady value vdc [23]; an alternative definition is the difference of maximum and minimum of the line voltage divided by the half sum [24]; then, in some cases ripple is expressed as an rms quantity, so that for clarity the properly said ripple r is considered as a “peak quantity”. distortion may evidently accompany ppls that are interfaced through static converters, but the most relevant feature is their impulsive and possibly repetitive power absorption profile. this profile for the considered dynamics may give rise to voltage and current components that may fall under the spectrum limitation at low frequency. for aircrafts in fact the mil-std-704f establishes limits of distortion components starting at 10 hz, unrelated from a concept of harmonic of a fundamental component. 5. results and discussion before going into the various cases, it is remembered that in general the voltage and current profiles characterizing loads and ppls have exponential profiles at the rising and falling edges, simply due to the most common behaviour of controls and electric network, featuring a dominant pole or two damped complex poles. shorter spikes may be present, but their origin is exogenous, as they are caused by lightning and fault induced phenomena. 5.1. pq indexes for typical ppl waveforms the behaviour of the metrics for transients (s, e and 2) and ripple index discussed in section 3.1 and 3.2, respectively, is analysed with respect to two typical ppl transient waveforms. case 1, shown in figure 3, includes a voltage reduction due to a significant current absorption and a fast transient (swell) following the release of the load that returns to a negligible current absorption. the transient response is stable and characterized by a first-order exponential. as evident, the voltage reduction is slow and characterized by local changes of slope, visible in the moving average profile superposed to the original waveform. as anticipated, a simple criterion based on a crossing threshold may face difficulties when the rate of change is very slow and the slope is not uniform. accurate and extensive noise removal is thus a minimum requisite. case 2, shown in figure 4, shows the voltage and current waveforms of a periodic ppl with the initial transient followed by the oscillatory response of the generator control and a variable amount of high-frequency ripple, that may be caused by the interface converters or may be the symptom of an incipient resonance. this waveform is quite similar to that of electric arc phenomena recognized in dc electric railways [39][42], where electric arcs trigger an impulsive reduction or increase of the line voltage (depending if the traction converter is absorbing power in a traction condition or injecting power into the network during a regenerative braking). the waveform in fact, besides a first pulse, features the oscillation of the rolling stock onboard filter (usually between 10 and 20 hz). substation ripple at 300 and 600 hz, besides a more or less intense component at 100 hz, is always present and appears as soon as the major transient response components have vanished. for case 1 the calculated values for the two portions of the waveform (voltage reduction with negative slope and subsequent swell) of the waveform, and altogether, are reported in table 3. the swell, although not so rapid, has a larger  than the five-time longer voltage reduction that precedes it. case 2 instead is evaluated for both the three indexes s, e, 2, and for ripple with a band limited approach as well, in order to separate the observed oscillations, at lower and higher frequency. table 2. limits and reference values for ripple and distortion (e=emission, i=immunity, a=ambient specification). standard phenomenon type nom. volt. un in v quantity ref. values mil-std-704f distortion a 28 d 3.5 % mil-std-704f ripple a 28 r 1.5 v mil-std-704f distortion a 270 d 1.5 % mil-std-704f ripple a 270 r 6 v iacs ur e5 ripple i rrms 10 % en 50155 ripple a 24-110 r 5 % en 61000-4-17 ripple i 24-110 r 1, 2.5, 5, 7.5 % figure 3. network voltage following a significant current absorption by the ppl local storage with a sudden release at the end, causing an exponential transient increase (swell). figure 4. ppl profile with periodic pulsation and superposed ripple following the initial power absorption: voltage (black) and current (grey) use the left axis, power (red) uses the right axis. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 159 since case 2 shows four absorption pulses, index values are commented for equivalence between apparently similar events. the results are shown in table 3, where the mean duration, and not its square, is reported for ease of comparison with signal duration and time intervals. the first voltage pulse has visibly less high-frequency oscillations, a slightly larger maximum value (the difference is about 3 v, so less than 1 %), but no other apparent differences. instead, the calculation of the energy points out an 8 % larger value than the remaining pulses; analogously the mean duration is shorter by about 10 %, that means that the energy concentration is higher. the physical explanation may be a deeper charging of internal storage, that is then less depleted during the successive pulses. indeed, the current peak in the first pulse is almost 4 % higher. case 2 is then further analysed for what concerns ripple components, that visibly are located at two distinct frequency intervals, around about 25 and 200 hz, and whose intensity changes between periods of the pulsating waveforms of voltage and current of figure 4. the band-limited ripple rbp [29] is thus calculated over different frequency intervals, using a windowed dft (hann window). from a preliminary evaluation of the band occupation, the frequency intervals should be separated at about 15 hz and 50-100 hz. with a sample frequency fs = 2.048 khz (matched by applying resampling) the shown frequency resolution is 8 hz, to match the visible main oscillation preliminarily qualitatively estimated at 24-25 hz. spectra shown in figure 5 are cut at 400 hz. as briefly commented in the figure caption, the dft of pp(t) highlights more effectively the spectrum pollution that may be a symptom of instability and that in a practical case should be further investigated. 5.2. spectrum of the ppl pattern the radar load profile [14], exemplified in section 2, is theoretically analysed for its frequency occupation, starting from the general ppl waveform of figure 1. the characteristics of the waveform are summarized as: current pulses of tp = 4 ms duration and peak power of 33.6 kw with respect to a steady power absorption of one third (11.2 kw); peaks are arranged in pulse bursts with spacing of tb = 10 ms (5 peaks) with a train repetition interval of tt = 200 ms. the coefficients of the fourier series of a symmetrical trapezoidal pulse are reported in (6) [43]. 𝑐0 = 𝑃 𝑝 , 𝑞 = 𝑇𝑝 𝑡𝑟 , 𝑝 = 𝑇𝑝 𝑡𝑝 + 𝑡𝑟 𝑐𝑘 = 2 𝑃 𝑝 ∙ sin( 𝑘 π/𝑞) 𝑘 π/𝑞 ∙ sin( 𝑘 π/𝑝) 𝑘 π/𝑝 , (6) having defined p and q as the fraction of the half-value pulse duration tp+tr and rise (or fall) time tr with respect to the duration of the single pulse tp. the resulting spectrum is a line spectrum with components repeating as per pulse spacing and modulated by the ck value. a more complex expression may be derived without the simplifying assumption of equal rise and fall times. however, as evident from the curve shape of figure 4, the trapezoidal curve shape is only an approximation and should not be assumed for more accurate assessment. the spectrum reported in figure 5, instead, gives more reliable indication of the average behaviour of components over one tp interval. a more refined assessment of the peak power oscillation and its damping could be more effectively achieved by curve fitting in the time domain. in general, when the pulse duration is shorter and the oscillations are characterized by variable instantaneous frequency, the precautions and verifications discussed in [45] should be considered. 5.3. power rate metric pd the power rate metric dp is evaluated with respect to a variable amount of harmonic distortion, using a realistic pp(t) trajectory with a pulsed profile. the first three periods of the red power curve of figure 1, reconstructed from the measured voltage and current waveforms, are considered as cases of an increasing amount of distortion due to the visible superposed ripple. such cases are indicated as case 2-intv1, -intv2 and -intv3 for the first three pulse periods. synthetic results are shown in table 4, reporting both the value of dp and the values of the previously defined indexes s, e and  for the selected intervals. it must be underlined that the two metrics s and e applied to pp(t) agree and weight more the table 3. values of area s, energy e, and mean duration  for the voltage waveforms of case 1 and case 2. interval s in v s e in v2  in s case 1-intv 1 202.07 2200 8.80 case 1-intv 2 55.51 1725 10.25 case 2-intv 1 3.61 203.56 0.0308 case 2-intv 2 3.56 193.58 0.0343 case 2-intv 3 3.58 193.24 0.0353 a) b) figure 5. dft of (a) v(t) and (b) pp(t), for the three first pulses of figure 4 (assigned colors black, blue and red). the main oscillation is clearly visible at about 24 hz, more stable in the v(t) profile; the high-frequency ripple is more recognizable in the pp(t) spectrum as a general increase, rather than isolated and visible in the 250 hz channel. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 160 larger peak value of the second and third pulse, but they ignore the increasing high-frequency ripple. the dp metric is based on the derivative of the fed signal (the power trajectory) and has a significant variability depending on the implementation. table 4 reports two versions of the calculation using “diff” (so the difference between adjacent vector components) and “gradient” (so the central difference between components with index differing by 2): the difference is significant especially because the two results do not have the same behaviour with respect to the interval number. as a note, since the derivative operator is insensible to the steady value of the fed signal, pp(t) was fed to s and e also as an “ac signal”, having subtracted the estimated steady value for each interval. for clarity s and e are thus indicated as s0 and e0. 6. conclusions this work has considered the presence of pulsed power loads (ppls) in dc grids with significant impact on network voltage variations and possibly on network stability. for the assessment of the impact and rating of various design solutions and implementations, suitable power quality metrics should be identified and applied. such metrics have been discussed, focusing in particular on the quantification of network transients and accounting for the typical profile of absorbed power, named power trajectory. these metrics may be divided in those weighting the characteristics of pulses and transients (area, energy, mean duration) and those dealing with characteristics that have been historically assigned to steady phenomena (ripple and distortion). metrics have been applied to voltage and current waveforms, but also to the power trajectory itself, as it has been recently proposed in the mil-std-1399-300 [26] for ac grids onboard us navy ships. it is interesting to note how the power curve in one case of a pulsed waveform with increasing distortion anticipates and allows detection of this phenomenon more effectively than processing voltage or current waveforms alone. the use and processing of power trajectory is particularly suited for ppls and opens new possibilities of revisiting 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https://doi.org/10.1109/tpwrd.2004.829924 https://doi.org/10.1109/amps.2018.8494897 https://doi.org/10.21014/acta_imeko.v9i2.761 https://doi.org/10.1109/vppc49601.2020.9330954 https://doi.org/10.1109/i2mtc43012.2020.9128903 http://www.emcs.org/acstrial/newsletters/fall09/practicalpapers.pdf http://www.emcs.org/acstrial/newsletters/fall09/practicalpapers.pdf https://doi.org/10.1109/intlec.2007.4448733 https://doi.org/10.1016/j.epsr.2021.107130 https://doi.org/10.1049/els2.12009 how the covid-19 changed the hands-on laboratory classes of electrical measurement acta imeko issn: 2221-870x december 2022, volume 11, number 4, 1 7 acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 1 how the covid-19 changed the hands-on laboratory classes of electrical measurement jakub svatos1, jan holub1 1 czech technical university in prague, faculty of electrical engineering, department of measurement, technicka 2, 166 27, prague 6, czechia section: research paper keywords: covid-19; laboratory class; measurement; distance teaching citation: jakub svatos, jan holub, how the covid-19 changed the hands-on laboratory classes of electrical measurement, acta imeko, vol. 11, no. 4, article 5, december 2022, identifier: imeko-acta-11 (2022)-04-05 section editor: eric benoit, université savoie mont blanc, france received june 28, 2022; in final form november 24, 2022; published december 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: j. svatos, e-mail: svatoja1@fel.cvut.cz 1. introduction the covid-19 pandemic suddenly paralyzed universities all around the world. it affected face-to-face education and dramatically changed the way of teaching. during the covid19 lockdown, universities switched to distance teaching and cancelled face-to-face education. face-to-face education has its specifics, and it supports interaction between student-student or student-lector interaction to promote better engagement, motivation, and study results [1] – [4]. moreover, practically oriented universities (e.g., engineering, health care, etc.) lost their capability to educate students hands-on. it means all handson classes were required to move exclusively to the distance from and fully employ online education, which challenged educators to adapt and transform all the courses. the impact of switching to distance teaching is perceptible to both students and teachers, and many questions have arisen about keeping the education quality [5] – [9]. covid-19 stroke hard at the czech technical university of prague, faculty of electrical engineering, department of measurement, where the need to switch to the distance form has been done practically from day to day. one of the subjects affected most by distance learning was electrical measurement and its laboratory classes. electrical measurement is a compulsory subject in electronics and communications and biomedical engineering and is available for all the faculty students in both czech and english. the course is taught to undergraduate students and applies to about 100 students per semester. the electrical measurement subject introduces different measurement methods of electrical and physical quantities like voltage, current, power, frequency, resistance, capacitance, or inductance. it is explained together with principles of their correct application and accuracy estimation. the subject also introduces the basic principles of electronic measuring instruments like multimeters, oscilloscopes, or spectral analyzers abstract this article depicts how covid-19 affected practical teaching at czech technical university in prague, faculty of electrical engineering, department of measurement. it introduces the subject of electrical measurement and its hands-on laboratory classes. the course applies to more than 100 undergraduate students per semester. since covid-19 mainly affected the teaching of the hands-on laboratory courses, the article narrates how the covid-19 lockdown has changed classes. an experience with the rapid transformation of lectures during the lockdown focusing on the hands-on laboratory classes is discussed, and the improvements that have been done to preserve its objectives, but also to adapt them to another possible lockdown, and the need to switch to the distance teaching, is revealed. the changes have been inspired by the results of a survey carried out among more than 250 students. the outline of the laboratory tasks transformation with an emphasis on the possibility of switching the real face-to-face measurement to the distance teaching remote lectures is described. the change includes reducing the number of laboratory tasks taught while preserving most of the objectives. all 11 new laboratory tasks are described in detail, and it is discussed how the possible change from hands-on to remote classes will affect every task. mailto:svatoja1@fel.cvut.cz acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 2 and explains the fundamentals of magnetic measurements and essential information concerning measurement systems. since the laboratory class is primarily hands-on activity, switching to the distance form has been different from conventional classroom-based teaching. the lecturers had to react to provide non-interrupted lectures while maintaining the quality quickly. according to [10], most students appreciated flexibility and quick reaction. laboratory exercises, on the contrary, often required a complete redesign of the measuring tasks to enable remote access and control of the measuring process. practically oriented courses are most often conducted in laboratories, with the assistance of lecturers. solving the handson laboratory classes' learning process requires much more than online learning [11]. since specific laboratory skills (e.g., the connection of the electronic circuit or handling the equipment) are harder to deliver in online teaching, a partial redesign of the laboratory classes must be done [12]. the paper from the times before the pandemic describes remote laboratories explicitly. in [13], laboratory classes are categorized into three types of laboratory exercises: a) development lab: students address specific design questions and verify that the design works as intended. b) research lab: addition to theoretical knowledge. c) educational lab: students apply theoretical knowledge in order to gain practical experience. at the same time, the authors admit that remote laboratories cannot replace all of the experience gained by face-to-face laboratories. another paper [14] performed a literature review of “hands-on, simulated, and remote laboratories,” where the authors, for remote laboratories, concluded that the more significant the computer-student interface is, the easier the task is to perform online. this supports articles [15] and [16] describing the methodology based on the simulation software, which is supplemented, if possible, with remote hands-on experiments. the advantage was that the students could compare the simulated and hands-on results. the main disadvantage was that only one student at a time could be connected and booked in advance for the remote laboratory. a different approach is used in [17] or [18]. both articles describe the teaching of electronics using pre-prepared kits. these kits have been physically sent directly to students to be able to perform all experiments at home. every kit comprises a microcontroller, necessary electrical components, and a breadboard with assembling guidance. the most complex approach, which can be applied to the class of electrical measurement, is described in [19]. the authors divided the learning process into five steps. 1. preparation for online labs: the explanation of the topic basis and short, simple multiple choices test to pass. 2. pre-lab: developing of understanding of critical points from theory. 3. in-lab: measurement, processing, presentation, and uncertainties. 4. post-lab: performing calculations, making conclusions, and understanding the concepts in a science and engineering context. 5. marking: grade distribution, analyzing comparison with face-to-face lectures. the article describes the challenges associated with distance teaching lectures on electrical measurement and hands-on laboratory classes and indicates changes in the post-covid-19 era. the paper is organized as follows. section 2 introduces the class of electrical measurement and its lectures. section 3 describes its laboratory class during the covid-19 lockdown and shows the student's feedback results. section 4 focuses on how covid-19 changed the laboratory class and its tasks. 2. lectures of electrical measurement class as covid-19 strikes suddenly, all the given lectures had to be quickly transformed into a distant form. the universities offered online courses and teaching classes before the pandemic. still, it never happened that the whole year had to be taught in a distant form without personal contact or consultations for all students [20], [21]. when the lockdown occurred, the lecturers could not even get to the classroom to provide the lecture. therefore, rapid actions had to be taken to preserve the classes. it was shown the best and only tolerable way how to continue in lectures almost continuously is to record the lessons in advance. the lecture can be offered, if there is no possibility to attend the university (i.e., lockdown or quarantine), as a recorded presentation in suitable software (i.e., ms powerpoint or similar), where tools like a pointer, marker, or drawing, and other features are available. in this case, recording the lecture in advance and allowing the students to watch the video with the lecture before the originally scheduled time showed as the best solution. in this way, the students can prepare their questions or topics for a discussion, and at the originally scheduled time, the lecturer is available for remote consultation with all participants. the survey pointed out the power of choice to watch a prerecorded lecture and use it as study material. the anonymous survey was conducted through 269 bachelor program students (approx. 75% of men, the average age of the respondents was about 21 years) in the second year of the study (third semester). the first survey question, "was the opportunity for you to watch a pre-recorded lecture a benefit?" shows in figure 1. an essential aspect of recorded lectures was the availability of the lecturer to be 'online' at the scheduled time of the original schedule to offer the lecture again and to discuss all the problems raised through the pre-watched video. the survey results for the second question: "was the opportunity for you to discuss a lecture with the lecturer online at the scheduled time a benefit?" is in figure 2. the student survey showed an important finding, which has to be considered to improve lectures, not only during the lockdown. the recorded lectures can also be used during face figure 1. survey results "was the opportunity for you to watch a prerecorded lecture a benefit?". 45% 38% 3% 14% was the opportunity for you to watch a prerecorded lecture a benefit? definetely yes rather yes no don't know acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 3 to-face classes to support students who need to repeat the problem or cannot attend the lecture. this was also verified during the first "face-to-face" semester after the covid-19 lockdown, as an excellent complement to face-to-face lectures. on the other hand, the possibility of watching recorded lecture could dramatically decrease the number of students who attends the face-to-face lecture. therefore, all the lectures of the electrical measurement class are available only during the examination period on the youtube channel of the department of measurement. moreover, to help the students in orientation in lectures and topics, all the lectures were divided into sub-topic to decrease the length of the individual videos. 3. laboratory class of electrical measurement during the covid-19 lockdown the laboratory class of the subject is focused on hands-on measurement method tasks of various electrical quantities and their evaluation. it shows the students the several measurement methods for the given topics and clarifies the most suitable techniques for specific measurements. moreover, it also draws attention to possible measurement systematic errors that may be unknowingly made. before covid-19, students had to perform real hands-on measurements of 17 laboratory tasks and complete reports from all the measurements. every task had multiple parts to be done, and a student had to connect several schematics to get the measurement results. when covid-19 struck, and the university was closed almost immediately, the question arose how to maintain laboratory teaching even in a distance form. the crucial requirement was to continue uninterrupted with the hands-on laboratories. as the quickest solution and keeping the learning tolerable, the pre-recorded tasks measured by the teacher and offered to students to watch were considered as a compromise between continuing in teaching and, at the same time, providing at least part of the practical experience. prerecorded laboratory tasks cannot substitute hands-on experience. however, on the other hand, quick response and some compromise are crucial for preserving even hands-on-type classes. this way, students can watch pre-recorded tasks anytime and every week, prepare the reports and discuss and present the achieved results during the originally scheduled class. during the online meeting, the main idea of the task was explained again, discussed, and theoretically explained again. there are different ways to handle hands-on distance teaching, for example, to secure remote access to the laboratory where the tasks will be already connected [22] – [25] or use batchmode remote-controlled laboratories, like the virtual instrument systems in reality [26], [27]. however, in classes with significant numbers of students and lots of tasks to be carried out, it demands lots of measurement instruments. it is also insisting on personnel preparing new sets of tasks every second week. it is worth to mention synchronizing the remote access to perform the tasks for all the students can be challenging in such a short time. moreover, in some of the tasks, there is a need to reconnect the schematics during the measurement. all these requisites were taken into account. how covid-19 affected the electrical measurement subject shows the statistics of the success rate of the students. during the times before covid-19, the subject's average success rate was slightly more than 85 %. when covid-19 strikes and the class have to change from face-to-face hands-on laboratories to a distance form in the first fifth of the semester, the success rate of the class was over 92%. it is given by the fact that the assessment requirements were decreased. nevertheless, the success rate during the covid-19 lockdown (at ctu, it has been the following two more semesters) dropped to 84% and 83%, respectively, for the classes that study the subject for the whole semester in a distance form. moreover, the average student grade of the class varied from 2.04 (average from the last three semesters before covid-19) to 2.54 during covid-19 (three semesters), where 1.00 is equal to the a grade, and 3.0 is equivalent to the e grade according to the european credit transfer and accumulation system (ects). clearly, the subject had to be changed to the future in case of another lockdown and to reflect the student survey. according to the feedback, the main issue was the number of tasks and receptivity of the reports from the tasks, which can demotivate the students. the results from the survey are in the following figures. one of the survey questions was: "was the number of tasks optimal for you?". results are shown in figure 3. figure 4 presents the results for the question, "were the recorded videos an adequate substitution for hands-on classes?". surprisingly, the results of the situation that arose were not conclusive. another question, "were the recorded videos beneficial, or would you prefer a different method? " revealed the students' satisfaction with the chosen method. according to almost 80% of all subject students, the pre-recorded videos of all the tasks figure 2. survey results "was the opportunity for you to discuss a lecture with the lecturer online at the scheduled time a benefit?". figure 3. survey results "was the number of tasks optimal for you?". 32% 50% 7% 11% was the opportunity for you to discuss a lecture with the lecturer online at the scheduled time a benefit? definetely yes rather yes no don't know 5% 93% 0% 2% was the number of tasks optimal for you? yes too much too little don't know acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 4 were an adequate substitution for considering the covid-19 lockdown situation. the results are shown in figure 5. the other part of the survey, where the students had space to express what they liked and did not like, indicated the direction in which the subject should be improved. the majority of the students addressed the repetitive reports of the tasks. the survey results and the overall feelings of the subject led to the need to change the number of tasks and content compression into a smaller number of tasks while preserving the subject's content. the survey also showed how the teaching during the covid-19 lockdown was appropriate and substitutes the hands-on laboratory class well under the given circumstances. 4. laboratory class of electrical measurement after the covid-19 lockdown as stated in the previous section, the laboratory classes had to be redesigned to preserve the content taught in the subject and can be effectively conducted even during the lockdown period. repetitiveness was a factor often mentioned by the students. the laboratory supervisors accepted that a thorough understanding of the principles demonstrated by the measurement experiment is way more important than gaining skill through excessive repetition of the same drill steps. moreover, most repetitive measurement tasks are fully automated today in professional environments. therefore, the number of tasks has been reduced from the original 17 real measurement tasks to 11 new real measurement tasks. this step will increase the motivation of the students to complete the course successfully. moreover, some new tasks have been designed to minimize the number of instruments used, reduce the wiring diagrams, and fulfill the need for modern measurement applications in the industry [28] and [29]. in this way, in case of pandemics and lockdowns, the newly designed tasks could be easily modified for remote laboratories. article [30] stress the lack of coherent learning objectives for laboratory classes and how this limits their effectiveness. the authors proposed the learning goals for instructional laboratories and suggested future research. the learning objectives of all tasks have been considered and worked on to motivate the students and to meet all intended educational objectives. the new tasks are: • measurement of non-sinusoidal voltages with multimeters and digital oscilloscope with a probe a task shows students principles of a voltage measurement using different principle multimeters and an oscilloscope using the oscilloscope probe. the effective values of non-harmonics voltages are explained for true rms and rectified mean principle voltmeters. moreover, the compensation of the oscilloscope probe is shown. through the measurement, students reveal the limitations of "cheap" multimeters in the measurement of non-harmonics voltages and compare the measurement error of such instruments with the true rms instrument. since all the multimeters can be set before the measurement and only the measured signal is changed, this part of the task can be done remotely in the case of distance teaching. in the case of a scope probe compensation, in a distance form, students have skipped this part and only theoretically derived the condition of a correctly compensated probe. • measurement of dc currents – a task that explains various methods to measure correctly small and large currents. it introduces a current-to-voltage convertor with an operational amplifier (oa) and shows the methodological error of measurement with multimeters with non-zero inner resistance. in the case of small currents measurement, the experiment pointed out the importance of knowledge of ammeters' real inner resistance and, by calculation, shows the possibility of great measurement error compared with the "ideal" current-to-voltage convertor with an oa. it also explains the contactless measurement of large currents using a clamp meter without breaking the circuit during the measurement. the task has been re-designed to minimize the reconnection of the circuit. therefore, the small current is measured on two ranges of one ammeter in the case of distance teaching to show the increasing inner resistance with the lower ranges. • measurement of small dc voltages – a task that explains the measuring amplifiers with oa's, its actual features like voltage input offset, input resistance, etc. the measurement uncertainties are also explained. the students have to measure the output voltage of the thermocouple with small inner resistance. the difference between inverting and non-inverting amplifiers with the oa is shown. students calculate and experimentally verify how the output voltage affects the load caused by the amplifier's input resistance. moreover, it is demonstrated by the measurement of how the input offset voltage of the oa can affect the resulting uncertainty. in the case of distance teaching, students have to measure only inverting amplifier, and for an figure 4. survey results "were the recorded videos an adequate substitution for hands-on classes?". figure 5. survey results "were the recorded videos beneficial, or would you prefer a different method?". 30% 56% 14% were the recorded videos an adequate substitution for hands-on classes? yes no 79% 10% 11% were the recorded videos beneficial, or would you prefer a different method? yes other method acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 5 uncertainty calculation, the catalogue value of the input offset voltage is used. • frequency dependence of multimeters for sinusoidal and rectangular waveforms – task explains the principle of frequency dependency of multimeters input circuits, its cut-off frequency, bandwidth, and effects of higher harmonics for rectangular waveforms. for instance, the article [31] compares simulations (virtual lab) and remote experiments with straightforward experiments dealing with the frequency characteristics of real instruments. in this task, students have to measure the frequency bandwidth of four different voltmeters, one laboratory and three "cheaper" voltmeters, hand-held included. the resulting frequency characteristics are then discussed. in a distance form, the task is unchanged since most instruments can be controlled remotely while the camera captures the hand-held. • frequency and period time measurement by a counter – introduces the principles of measurement of the time and the period, showing the possibility of inaccurate data measurement caused by incorrectly set of a trigger level and principle of averaging. in this task, a counter developed in the late 80s is used since it can best demonstrate the principle of the time and the period measurement. it can allow setting manually the time when the gate is open in case of the frequency measurement, and it allows manually setting the averaging for the period measurement. on the other hand, in the case of distance teaching, it cannot be used since it cannot be controlled remotely. in other words, this task is the most affected by distance teaching. as modern counters work fully automatically, they can't substitute the used counter. therefore, the task is skipped in case of distance teaching, and only recorded video is provided. • single-phase load power measurement – a power measurement task involves an electrodynamic wattmeter, power analyser, and clamp meter. it also explains the measurement current transformer. students have experimentally checked whether the current transformer has not been overloaded and used it in the measurement circuit to measure the power. the electrodynamic wattmeter measures power consumption. the constant of the wattmeter is explained. moreover, the power is simultaneously measured by the power analyser and a clamp meter. students have to measure the thd and power factor. the task is reduced by the clamp meter measurement in the case of distance teaching. • sampling principle, digital oscilloscope, and programmable waveform generator – a task with the advanced measurement with the oscilloscope, verification of the sampling theorem, principle of an arbitrary generator, and work with the oscilloscope trigger. the purely hands-on task focused on the skill to handle the instruments. similarly, like the task dealing with frequency and period time measurement by a counter, this task cannot be handled remotely, and only the recorded video is offered. • resistance measurement – complex task explaining the problem of measurement resistances, focusing on small resistances, 4-wire connection, thermoelectric voltages, and series comparison methods, introducing resistance-to-voltage convertor with oa. students must experimentally verify the different techniques used for the resistance measurement, its limitations (in the case of the resistance magnitude), and its advantages. the task can be measured remotely unchanged. • unbalanced wheatstone bridge evaluation of resistance change of pt1000 temperature sensor – the task is showing the principles of wheatstone bridges, analogue signal processing from resistance sensors, linearity of the voltage and current supply bridge, and linearized bridge with oa. similarly, like the task of resistance measurement, students experimentally verify the principle of the bridges and, through data analysis, calculate their linearity. also, this task can be measured remotely unchanged. • digital impedance and admittance meter – laboratory measurement explaining the principle of vector voltmeter, measurement of the real and the imaginary part of a complex voltage, calculation of rectified mean voltage and effective value voltage, measurement using a digital impedance meter. completing this task, the students understand the principles of the capacitance and inductance measurement and calculation of its serial and parallel equivalent schemes. the task can be measured, if needed, completely remotely. • magnetic measurements – a task that introduces the basics of magnetism, measurement of a leakage magnetic field of the transformer, hysteresis loop measurement, and calculation of amplitude permeability. in the first part, students have to experimentally determine the constant of the measuring coil by the helmholtz coils and then use the measuring coil to measure the leakage magnetic field of a transformer. in the second part, students measure the amplitude permeability of the ring specimen as a dependency on the magnetic field strength. in the case of the distance form, the first part has to be skipped, and only the second part is measured remotely. all the tasks are taught as real measurements during the faceto-face semester, but in the case of distance teaching, most of the tasks are designed to be transformed into remote laboratory classes. in addition, every task will be complemented by the instructional video to understand the main objectives better and to demonstrate the connection/wiring of the whole measurement circuit. apart from reducing the number of laboratory tasks, the repetitiveness of certain measurement types has significantly been reduced by, e.g., decreasing the number of points in which the examined characteristics are measured or evaluating the measurement uncertainty on a subset of measured points only. on the other hand, preserving the number of measuring method principles of a given topic (e.g., in resistance measurement, series comparison method, ohm method, r → u converter method) has been crucial. part of each task is home preparation containing several questions that prepare students to understand the measurement's issues better. these questions are awarded extra points added to the final grading to motivate the students. as mentioned at the beginning of section 4, most laboratory class tasks have been redesigned to prepare them, in case of a need, for a remote laboratory class. for this reason, as a supplement for basic instruments like, e.g., multimeters or arbitrary generators, a set of smart bench instruments with software to connect, control and capture data remotely has been purchased. each smart bench consists of a smart triple power supply (edu36311a) and a two-channel oscilloscope with a digital multimeter and waveform generator (edux1052g). all used instruments allow remote access to be controlled remotely. therefore, the objective of instrumentation, in a limited way, is preserved even in the case of distance teaching. although the number of tasks has been significantly reduced, the demands on the measuring instruments are still considerable. therefore, the tasks will be prepared gradually in groups of two or three, and students will have two or three weeks to complete them. moreover, in the case of online teaching, the remotely measured tasks will also be captured by the camera to help the acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 6 students better to understand its principles, schematics, and connections. on the other hand, in the case of pre-recorded videos, the fundamental objectives of teamwork, ethics in the laboratory, and communication are essentially limited in all tasks. an example of the measuring tasks, "digital impedance and admittance meter," connected to the smart bench is in figure 6. a camera continuously monitors the task to help the students be oriented. by this, students can compare the connection with the circuit diagram and see the instruments used. in the following step, students have to connect to the instruments remotely and set the power supply, generator, digital multimeter, and scope. in this case, the measurement is performed in two steps. firstly, the real part of the output voltage is measured, and the corresponding waveform is displayed. in the second step, the phase difference of 90° has to be set to measure the imaginary part of the output voltage and corresponding waveform. an integral part of the task, performed and controlled remotely, is a suitable remote access method to the controlling computer or directly controlling instruments if needed. an overview of software technologies presents in the article [32]. it introduces the main characteristic of the remote laboratory and its implementation from the client-server side. on the other hand, the selected method must comply with the institution's it security policies and not only allow remote access (usually of several students at a time) to the task but must also restrict access privileges to only those students who need to work on the task. based on this, the proposed solution uses apache guacamole open-source platform for a remote desktop gateway primarily used by the it of ctu in prague. this will be supplemented with an interactive calendar, implemented in moodle, an open-source learning management system, where students register in advance and thus reserve access to a specific task at a time that is convenient for them and complete the task at any time. all the proposed changes have been incorporated in the winter semester of 2022. the program board has positively received them. moreover, informal feedback from the students during the first semester taught in the changed form (winter 2022) indicates a step in the right direction. the home preparation helped the students better understand the essence of the experiment, and the reduced number of tasks reduced the student's workload and increased interest in a deep understanding of the discussed topic. therefore, it is believed it will motivate students to pass the subject successfully. 5. conclusion the article describes how covid-19 changed the electrical measurement subject taught at ctu in prague, faculty of electrical engineering, department of measurement. the electrical measurement subject is hands-on laboratory classoriented, and the impact of covid-19 was radical. it has been shown how the classes were taught in the pre-covid-19 era and how the covid-19 lockdown struck it. the fast response and recorded laboratory tasks helped to overcome the lockdown acceptably. the student survey revealed the subject's shortcomings and showed how the laboratories had to change. all the old tasks have been redesigned and reduced to preserve the fundamental objectives. moreover, the reworked tasks have been designed with an emphasis put on the possibility of switching to distance teaching. still, in the case of a distance form, the fundamental objectives of teamwork, ethics in the laboratory, and communication are essentially limited. all the adjustments made to improve the quality of the course have been made to maintain the quality, content, and objectives of the tasks in the case of a possible future lockdown. acknowledgement the authors would like to thank all members of the department of measurements of fee ctu in prague for their patience and support during the lockdown when much effort was spent to ensure high-quality teaching. references [1] t. wang, analysis of network teaching during covid-19, 2020 international conference on modern education and information management (icmeim), dalian, china, 25-27 september 2020, pp. 47-50. doi: 10.1109/icmeim51375.2020.00018 [2] h. wan, l. tang, z. zhong, q. cao, transit traditional face-toface teaching to online teaching during the outbreak of covid2019, 2020 ieee international 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https://doi.org/10.1109/tie.2009.2026368 uncertainty of factor z in the gravimetric volume measurement acta imeko issn: 2221-870x september 2021, volume 10, number 3, 198 201 acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 198 uncertainty of factor z in gravimetric volume measurement mar lar win1 1 national institute of metrology (myanmar) nimm, yangon, myanmar section: research paper keywords: density of air; density of water; value of factor z; uncertainty of factor z; gravimetric volume measurement citation: mar lar win, uncertainty of factor z in the gravimetric volume measurement, acta imeko, vol. 10, no. 3, article 27, september 2021, identifier: imeko-acta-10 (2021)-03-27 section editor: andy knott, national physical laboratory, united kingdom received: may 7, 2021; in final form: august 13, 2021; published: september 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: mar lar win, e-mail: marlarwin99@gmail.com 1. introduction the iso 4787 standard describes the general equation for the calculation of the volume at the reference temperature of 20 °c, 𝑉20, from the apparent mass of the water, be it contained or delivered, as follows from [1], b.1: 𝑉20 = (𝐼l − 𝐼e) ( 1 𝜌w − 𝜌a ) (1 − 𝜌a 𝜌b ) [1 − 𝛾(𝑡 − 20)] , (1) where 𝐼l is the balance reading of the vessel containing water (g), 𝐼e is the balance reading of empty vessel (g), 𝜌w is the density (g/ml) of the water at temperature 𝑡, 𝜌a is the density of air (g/ml), 𝜌b is the actual density of the reference weights (8 g/ml), 𝛾 is the coefficient of the cubic thermal expansion of the material (°c−1), and 𝑡 is the temperature of the water used in the testing (°c). since this equation is somewhat complicated to work with, the factor 𝑍 is provided to simplify the volume calculation: 𝑉20 = 𝑚 𝑍 [1 − 𝛾(𝑡 − 20)] , (2) where 𝑚 is the net weighed values of 𝐼l − 𝐼e and 𝑍 is the conversion factor of the mass of the water measured in terms of volume at the measurement temperature. 2. determination of factor z the factor 𝑍 depends on the air density, water density, water temperature, and the density of the reference weights. the factor can be derived from (1) as follows: 𝑍 = ( 1 𝜌w − 𝜌a ) (1 − 𝜌a 𝜌b ) . (3) to determine the value of factor 𝑍, the following parameters must be taken into account: • density of air 𝜌a in relation to atmospheric pressure, temperature and a relative humidity of 40 % – 90 %; • density of water 𝜌w in relation to temperature; • density of the reference weights 𝜌b of the balance used. 2.1. determining the air density the air density 𝜌a in kg m −3 can be determined with sufficient uncertainty from the following cipm approximation formula for air density [2], e.3-1: 𝜌a = 0.348 48 𝑃 − 0.009 ℎ𝑟 × 𝑒0.061×𝑡 273.15 + 𝑡 , (4) where 𝑃 is the atmospheric pressure (hpa), ℎ𝑟 is the relative humidity (%), and 𝑡 is the atmospheric temperature (°c). abstract in the gravimetric volume measurement method, the factor z is generally used to facilitate an easy conversion from the apparent mass obtained using a balance to the liquid volume. the uncertainty of the measurement used for the liquid volume can be divided into two specific contributions: one from the components related to the mass measurements and one from those related to the mass-to-volume conversion. however, some iso standards and calibration guides have suggested that the uncertainty due to the factor z is generally neglected in the uncertainty calculation pertaining to gravimetric volume measurement. this paper describes the combined effects of the density of the water, the density of the reference weights, and the air buoyancy on the uncertainty of factor z in terms of how they subsequently affect the uncertainty of the measurement results. mailto:marlarwin99@gmail.com acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 199 table 1 presents the air density to temperature relationship at 1,013.25 hpa and a 50 % relative humidity. 2.2 determining the water density the density of air-free water 𝜌w′ at a pressure of 1,013.25 hpa and a temperature 𝑡 between 0 °c and 40 °c expressed in terms of the its-90 is given by tanaka et al. [3] as follows: 𝜌w′ = 𝑎5 [1 − (𝑡 + 𝑎1) 2(𝑡 + 𝑎2) 𝑎3(𝑡 + 𝑎4) ] , (5) where 𝑎1 = (− 3.983 035 ± 0.000 67) c 𝑎2 = 301.797 c 𝑎3 = 522 528.9 c 2 𝑎4 = 69.348 81 c 𝑎5 = (0.999 974 950 ± 0.000 000 84) g/ml 𝑡 = water temperature in c . since the pure water that is used in the laboratory is generally air-saturated, the density must be corrected. to adjust the air-free water density as described in (5) to air-saturated water between 0 °c and 25 °c (the standard laboratory condition), we can use the following [3], 5.2: ∆𝜌 = 𝑠0 + 𝑠1 × 𝑡 , (6) where 𝑠0 kg m−3 = − 4.612 × 10−3 𝑠1 kg m−3 °c−1 = 0.106 × 10−3 . moreover, since water is slightly compressible, a small correction may be required under typical laboratory conditions. the compressibility factor 𝐹c [3], 5.3, for the water density used at different pressures is as follows: 𝐹c = 1 + [(𝑘0 + 𝑘1 𝑡 + 𝑘2 𝑡 2) ∆𝑃] , (7) where ∆𝑃 = 𝑃 − (101 325 pa) 𝑘0 = 50.74 × 10 −11 pa−1 𝑘1 = −0.326 × 10 −11 pa−1 c−1 𝑘2 = 0.004 16 × 10 −11 pa−1 c−2 𝜌w = (𝜌w´ × 𝐹𝑐 + ∆𝜌) with 𝜌w = the density of air-saturated water and 𝜌w´= the density of air-free water. (8) table 2 presents examples of water density as a function of water temperature for distilled water at 1,013.25 hpa after correction for any dissolved gasses in the water. meanwhile, table 3 presents examples of calculated airsaturated water density as a function of water temperature and pressure after applying corrections for the dissolved gasses in the water. 2.3 density of the reference weights the density of the reference weights/mass pieces 𝜌b that are used for the balance calibration is normally presented in the calibration certificate of the weights. alternatively, the uncertainties corresponding to the used weight class according to oiml r 111-1 [2] can be used. if the density of the reference weights are not known, 8.0 g/ml is generally used. 2.4 determining the factor z values the values of the conversion factor 𝑍 (ml/g) as a function of temperature and pressure as given in the existing literature mainly pertain to distilled water. when liquids other than distilled water are used, the correction factors (factor 𝑍) for the specific liquid must be determined. table 4 presents the factor z values in relation to temperature and pressure. the calculations were based on (3) and (8). 3. the uncertainty of factor z 3.1 sources of uncertainty in factor z having identified the input quantities of factor z using (3), it is possible to determine the sources of uncertainty pertaining to the different input quantities, which are: table 1. air density values as a function of temperature at 1,013.25 hpa and a 50 % relative humidity. temperature in °c density of air in g/ml 15 0.001 225 20 0.001 204 25 0.001 184 table 2. water density values after correction for air-saturated distilled water. water temperature (°c) air-free water density (g/ml) correction for air saturation (g/ml) air-saturated water density (g/ml) 15 0.999 102 57 0.000 003 02 0.999 099 55 20 0.998 206 75 0.000 002 49 0.998 204 25 25 0.997 047 02 0.000 001 96 0.997 045 06 table 3. air-saturated water density as a function of water temperature and pressure. water temperature (°c) air-saturated water density (g/ml) at atmospheric pressure of 900 hpa 1,013.25 hpa 1,050 hpa 15 0.999 094 26 0.999 099 55 0.999 101 27 20 0.998 199 07 0.998 204 25 0.998 205 94 25 0.997 039 96 0.997 045 06 0.997 046 72 table 4. factor z values as a function of water temperature and pressure. water temperature (°c) value of factor z (ml/g) at atmospheric pressure of 900 hpa 1,013.25 hpa 1,050 hpa 15 1.001 845 1.001 958 1.001 995 20 1.002 745 1.002 858 1. 002 895 25 1.003 912 1.004 026 1.004 062 27 1.004 448 1.004 562 1.004 599 acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 200 • water density • air density • density of the reference weights. the uncertainty of factor z can be determined using the gum law of propagation of uncertainty [4] as follows: 𝑢(𝑍) = √( 𝜕𝑍 𝜕𝜌a 𝑢(𝜌a)) 2 + ( 𝜕𝑍 𝜕𝜌w 𝑢(𝜌w)) 2 + ( 𝜕𝑍 𝜕𝜌b 𝑢(𝜌b)) 2 (9) where 𝑢(𝜌a) is the standard uncertainty of the air density (g/ml), 𝑢(𝜌w) is the standard uncertainty of the water density (g/ml), and 𝑢(𝜌b) is the standard uncertainty of the reference weights (g/ml). 3.2 uncertainty of air density at a relative humidity of hr = 0.5 (50 %), a temperature of 20 °c and a pressure of 1,013.25 hpa, the uncertainty of the air density can be estimated according to oiml r111 [2], c.6.3-3, as follows: 𝑢(𝜌a) 𝜌a = √ (1 × 10−4)2 + (−3.4 × 10−3 × 𝑢(𝑡)) 2 +(1 × 10−3 × 𝑢(𝑃))2 + (−10−2 × 𝑢(ℎ𝑟))2 , (10) where 𝑢(𝑡) is the uncertainty of the air temperature 𝑡 (°c), 𝑢(𝑃) is the uncertainty of the air pressure 𝑃 (hpa), and 𝑢(ℎ𝑟) is the uncertainty of the relative humidity ℎ𝑟 (%). for the uncertainty values (k = 2) from the calibration of the pressure, temperature and humidity sensors, the uncertainty due to the temperature of the air is 0.1 °c, the uncertainty due to the barometric pressure is 0.1 hpa, and the uncertainty due to the relative humidity is 1 %. this yields the following: 𝑢(𝜌a) = 0.0012 g ml⁄ × √ (1 × 10−4)2 + (−3.4 × 10−3 × 0.1 2 ) 2 + (1 × 10−3 × 0.1 2 ) 2 + (−1 × 10−2 × 0.01 2 ) 2 = 2.52 × 10−7 g ml⁄ . 3.3 uncertainty of water density the uncertainty of the water density can be evaluated from the formulation given by euramet cg 19 [5], 7.3.3, as follows: 𝑢(𝜌w) = √𝑢 2(𝜌w,f) + 𝑢 2(𝜌w,𝑡) + 𝑢 2(𝛿𝜌w) , (11) where 𝑢(𝜌w,𝑡) = 𝑢(𝑡w) × 𝛽 × 𝜌w(𝑡w) 𝑢(𝑡w) = [( 𝑈(ther) 𝑘 ) 2 + 𝑢(res)2] 0.5 𝑢(𝑡w) = [( 0.01 °c 2 ) 2 + ( 0.01 °c 2√3 ) 2 ] 0.5 𝑢(𝑡w) = 0.006 °c . the expansion coefficient can be estimated as follows: 𝛽 = (−0.117 6 𝑡2 + 15.846 𝑡 − 62.677) × 10−6 ℃−1 𝛽 = (−0.117 6 × 202 + 15.846 × 20 − 62.677) × 10−6 ℃−1 𝛽 = 0.000 21 °c−1 𝜌w(𝑡w) = 0.998 204 25 g/ml (from table 3) 𝑢(𝜌w,𝑡) = 0.006 × 0.000 21 × 0.998 204 25 𝑢(𝜌w,𝑡) = 1.26 × 10 −6g/ml~ 1 × 10−6g/ml . the standard uncertainty related to 𝑢(𝛿𝜌w) could range from a few ppm for highly pure water (measured by means of a highresolution density meter) to 10 ppm for distilled or de-ionised water, provided that the conductivity is less than 5 µs/cm, meaning it is assumed to be 5 × 10−6 g/ml. meanwhile, 𝑢(𝜌w,f) is the estimated standard uncertainty of the formulation (4.5 × 10−7 g/ml), 𝑢(𝜌w,𝑡) is the uncertainty due to the temperature of the water (1 × 10−6 g/ml), and 𝑢(𝛿𝜌w) is the uncertainty due to the purity of the water (5 × 10−6 g/ml) [5], 8.2.3. this yields the following: 𝑢(𝜌w) = √(4.5 × 10 −7)2 + (1 × 10−6)2 + (5 × 10−6)2 g/ml = 5.12 × 10−6 g/ml . (12) 3.4 uncertainty of the reference weights the uncertainty of the density of the weights/mass pieces is presented in the calibration certificate of the weights, or alternatively, according to oiml r111-1, the uncertainty of stainless-steel weights class e2 is 140 kg/m3 (k = 2). the uncertainty of the density of the reference weights is normally presented in the calibration certificate of the weights. for example, the value presented in the calibration certificate of the set of weights used is 0.06 g/ml (k = 2): 𝑢(𝜌b) = 𝜌r 2 = 0.06 2 g ml⁄ = 0.03 g ml⁄ . (13) alternatively, the uncertainty of the density of the weights, as presented in oiml r111-1, can also be used. 3.5 calculating the uncertainty of factor z table 5 presents example data for the calculation of the uncertainty of factor z for air-saturated distilled water at a temperature of 20 °c and a barometric pressure of 1,013.25 hpa. sensitivity coefficient of air density 𝜕𝑍 𝜕𝜌a = ( 1 𝜌w − 𝜌a ) (− 1 𝜌b ) + (1 − 𝜌a 𝜌b ) ( 1 (𝜌w − 𝜌a) 2 ) = 0.880 500 ml²/g2 sensitivity coefficient of water density 𝜕𝑍 𝜕𝜌w = (1 − 𝜌a 𝜌b ) ( −1 (𝜌w − 𝜌a) 2 ) = −1.005 876 ml²/g2 sensitivity coefficient of density of reference weights 𝜕𝑍 𝜕𝜌b = ( 1 𝜌w − 𝜌a ) ( 𝜌a 𝜌b 2 ) = 1.887 6 × 10−5 ml²/g2 table 5. example data. parameter value uncertainty (k = 1) air density 0.001 204 g/ml 2.52 × 10−7 g/ml water density 0.998 204 g/ml 5.12 × 10−6 g/ml mass density 8.0 g/ml 0.03 g/ml factor z 1.002 858 ml/g (refer to the conditions in table 4) acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 201 uncertainty of factor z (k = 1) 𝑢(𝑍) = √( 𝜕𝑍 𝜕𝜌a 𝑢(𝜌a)) 2 + ( 𝜕𝑍 𝜕𝜌w 𝑢(𝜌w)) 2 + ( 𝜕𝑍 𝜕𝜌b 𝑢(𝜌b)) 2 = √ (0.880 500 × 2.52 × 10−7)2 +(−1.005 876 × 5.12 × 10−6)2 +(1.887 6 × 10−5 × 0.03)2 ml/g = 5.2 × 10−6 ml/g . 4. discussion as shown in table 4, the factor z variation in distilled water due to water temperature is approximately 2.0 × 10−3 ml/g or 2.0 × 10-4 ml/(g °c) between 15 °c and 25 °c. this implies that when the uncertainty of the temperature measurement of distilled water is 0.1 °c, this will present an uncertainty of approximately 0.002 % following the conversion to volume. meanwhile, the influence of barometric pressure on the factor z is approximately 1 × 10−4 ml/g or 0.01 % per pressure change of 100 hpa between 900 hpa and 1,050 hpa. the uncertainty of the pressure measurement in the laboratory is typically 5 hpa. this will cause the uncertainty to the conversion of volume by less than 0.000 5 %. 5. conclusions the factor z does not only depend on the density of the liquid compensated for water temperature and pressure but also on the air density and the density of the weights used for the calibration of the balance. however, the contribution to the evaluation of uncertainty resulting from the factor z for gravimetric volume measurement is extremely small compared to that resulting from the operator process (i.e., meniscus reading) [5], 8.2.7, and the mass of water determination, provided that the measuring instruments (balance, barometer, thermometer, etc.) are used in accordance with the specifications given in the standard operating procedure (sop). thus, it is good practice to neglect the uncertainty of the factor z in the evaluation of uncertainty and to give only the components related to the mass measurement process [6]. acknowledgement this paper would not have been possible without the exceptional support of bunjob suktat, my former expert from physikalisch-technische bundesanstalt (ptb) on the ‘ptbnimm strengthening myanmar quality infrastructure project’. his enthusiasm, knowledge, and minute attention to detail were an inspiration and ensured my work remained on track. as my teacher and mentor, he has taught me more than i could ever give him credit for here. references [1] iso 4787 (2010) laboratory glassware volumetric instruments methods for testing of capacity and for use. [2] oiml r111weights of classes e1, e2, f1, f2, m1, m2, m3 metrological and technical requirement, 2004. [3] m. tanaka, g. girard, r. davis, a. peuto, n. bignell, recommended table for the density of water between 0 °c and 40 °c based on recent experimental reports, metrologia 38 (2001) pp. 301-309. doi: 10.1088/0026-1394/38/4/3 [4] jcgm 100:2008 (gum), evaluation of measurement data – guide to the expression of uncertainty in measurement. [5] euramet cg-19, version 3.0 (09/2018), guidelines of determination of uncertainty in gravimetric volume calibration. [6] iso 8655-6:2002(e), international standard, piston-operated volumetric apparatus, part 6: gravimetric methods for the determination of measurement error. https://doi.org/10.1088/0026-1394/38/4/3 nilm techniques applied to a real-time monitoring system of the electricity consumption acta imeko issn: 2221-870x june 2021, volume 10, number 2, 139 146 acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 139 nilm techniques applied to a real-time monitoring system of the electricity consumption b. cannas1, s. carcangiu1, d. carta2, a. fanni1, c. muscas1, g. sias1, b. canetto3, l. fresi3, p. porcu3 1 diee, university of cagliari, via marengo 2, 09123 cagliari, italy 2 iek-10, forschungszentrum jülich, 52425 jülich, germany 3 bithiatech technologies, elmas, (ca), italy section: research paper keywords: non-intrusive load monitoring (nilm); electricity consumption; energy disaggregation; features extraction; smart metering; blued dataset; proprietary dataset citation: barbara cannas, sara carcangiu, daniele carta, alessandra fanni, carlo muscas, giuliana sias, beatrice canetto, luca fresi, paolo porcu, nilm techniques applied to a real-time monitoring system of the electricity consumption, acta imeko, vol. 10, no. 2, article 20, june 2021, identifier: imekoacta-10 (2021)-02-20 section editor: giuseppe caravello, università degli studi di palermo, italy received january 21, 2021; in final form march 16, 2021; published june 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was partially funded by sardinian region under project “rt-nilm (real-time non-intrusive load monitoring for intelligent management of electrical loads)”, call for basic research projects, year 2017, fsc 2014-2020. corresponding author: daniele carta, e-mail: d.carta@fz-juelich.de 1. introduction knowing how electric appliances are used and how different appliances contribute to the aggregate total consumption could help users to have a better understanding on how the energy is consumed, thus leading possibly to a more efficient management of their loads. non-intrusive load monitoring (nilm) is an area of computational sustainability research, and it presently identifies a set of techniques that can disaggregate the power usage into the individual appliances that are functioning and identify the consumption of electricity for each of them [1]. in residential buildings, where it is impractical to monitor single appliances, or even groups of appliances, through specific meters, nilm techniques are a low-cost and not invasive option for electric consumption monitoring, considering a single monitoring point where a smart meter is installed. literature reports several papers that applied different methods throughout the years to solve this problem. a first classification of nilm techniques could be done in supervised and unsupervised methods [2]. supervised methods require labelled data of consumption to train the model. unsupervised methods are targeted to extract the information to operate directly from the measured aggregate data consumption profiles. due to their better performance, most of the approaches are based on supervised algorithms and they require appliance data for model training to estimate the loads number, type and power by analysing the aggregate consumption signal. solutions based on machine learning range from classic supervised machine-learning algorithms (e.g., support vector machines, and artificial neural networks) to supervised statistical learning methods (e.g., k-nearest neighbours and bayes classifiers) and unsupervised method (e.g., hiddenmarkov models and its variants). a review of these methods is reported in [2]. recently, deep learning (dl) methods were also abstract non-intrusive load monitoring (nilm) allows providing appliance-level electricity consumption information and decomposing the overall power consumption by using simple hardware (one sensor) with a suitable software. this paper presents a low-frequency nilmbased monitoring system suitable for a typical house. the proposed solution is a hybrid event-detection approach including an eventdetection algorithm for devices with a finite number of states and an auxiliary algorithm for appliances characterized by complex patterns. the system was developed using data collected at households in italy and tested also with data from blued, a widely used dataset of real-world power consumption data. results show that the proposed approach works well in detecting and classifying what appliance is working and its consumption in complex household load dataset. mailto:d.carta@fz-juelich.de acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 140 employed and seem promising for the most challenging problem posed by the consumption profiles of multi-state appliances [3]-[6]. the frequency of energy data monitoring drives the use of the analysis techniques and the specific tools. although higher the frequency of energy data monitoring frequency higher could be the accuracy of the nilm disaggregation algorithms, commercial smart meters for homes supply low frequency sampling (less than 60 hz) of the electric power quantities. in this field, the majority of the research efforts focused on event-based techniques that identify significant variations in the power signals as switching events of appliances. these events must be classified as a state transition related to a specific appliance. for this purpose, electric signal characteristics extracted from measurements in proximity of the events (i.e., signatures) are used as distinctive features, and then labelled with classification procedures. in this paper, a monitoring system is proposed using a smart meter that supplies low frequency (1 hz) samples of power consumption. the system is able to disaggregate and keep track of the power consumption of the devices existing in a typical italian house. the households should follow the proposed procedure to customize the system for their homes, choosing the appliances of interest and collecting the corresponding measurements. the disaggregation algorithm is an improvement of the one proposed in [7]-[8]. the load disaggregation is performed applying a hybrid approach to power data, i.e. eventbased techniques and pattern recognition techniques for large household appliances. moreover, the procedure of the first setup phase has been simplified: the event detection for type-i and type-ii appliances that was performed separately for each device is now carried out on the aggregate power signals, strongly reducing the user effort. in order to validate the method, the procedure is also applied to building-level fully-labeled (blued) dataset for electricity disaggregation [9]. blued is a publicly available big dataset consisting of real voltage and current measurements for a singlefamily residence in the united states, sampled at 12 khz for a whole week. blued has been applied as benchmark dataset in several recent papers on nilm [5], [10], [11], [12]. however, few contributions have been proposed with low frequency samples. among them, [13] and [14] will be considered in this work as comparison. 2. the home energy management system the home energy management system (hems) is used to provide comfortable life for consumers as well as to save energy. this can be obtained using a home’s smart meter to monitor the electricity consumption of the devices existing in a household applying nilm techniques. the hems should identify the appliances that are active at any time, disaggregate the energy and estimate consumptions of the single device. in this work, the chosen low-frequency smart meter is a sensor belonging to a precommercial prototype [7] which provides steady-state signatures, such as real and reactive power time series. it is important that the hems set-up is easy to understand and interact with. it should have features, like auto-configuration, which make the set-up process very easy. the non-intrusive technique, resorting to only one smart meter for the household, requires some effort in the first set-up phase, but it can be sometimes the only possible choice because installing a specific monitoring infrastructure, including new devices cables, may result in high implementation cost to the user. in this work the appliances are categorized into four types based on their operation states [15]: 1) type-i, appliance with on/off states (binary states of power); 2) type-ii, finite state machines with a finite number of operation states (multiple states of power); 3) type-iii, continuously varying devices with variable power consumption, as a washing machine and a light dimmer (infinite middle states of power); 4) type-iv, appliances that stay on for days or weeks and with a constant power consumption level. in case of type-ii, data for all the transitions between possible states should be acquired and labelled (manual set-up). in case of type-iii devices, data from each cycle are characterized by complex patterns. in this paper, a technique to deal with such devices is proposed, by considering the washing machine as a case study. figure 1 and figure 2 show the filtered apparent power consumption typical of different models of washing machines with different washing cycles. as can been noted, the power consumption fluctuates while heating/washing or rinsing/drying the laundry. thus, the events do not correspond to simple steps in the power consumption, but characteristic complex patterns appear in the power time series. as it can be noted, heating water accounts for about 90% of the energy needed to run a washer and in both figures the washing machine typically has electrical components which turn on and off in sequence. the proposed technique facilitates the training process by pre-populating the training data set with signatures of some type-iii devices showing typical patterns (automatic set-up). 3. the procedure in the following, the implemented procedure is presented showing how the variations of apparent (s), real (p) and reactive powers (q), the oscillation frequencies of the signals and the varying patterns of type-iii appliances are used in the training figure 1. apparent power for a washing machine (hot water). figure 2. apparent power for a washing machine (cold water). acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 141 phase for the creation of the signatures database, whereas during a recall phase the appliances are recognized inside an aggregated signal. in order to better understand the whole procedure, figure 3 shows its flow chart with the main phases of data collection and extraction of the signatures. 3.1. data collection the automatic procedure to collect the aggregate data of electric consumption consists of the following two steps, one for type-i and ii appliances, one for type-iii appliances. in the first step, type-iii appliances (such as washing machine or dishwasher) are switched off, whereas type-i and type-ii appliances could regularly work. the user must switch on and off type-i and ii devices several times for each event of interest. this allows to increase data robustness to the noise, e.g., small fluctuations in appliance consumption, electronics constantly on, and appliances turning on/off with consumption levels too small to be detected. multi-state devices, such as kitchen ovens, stoves, clothes dryers, etc., go through several states where heating elements and fans can be switched in various combinations. thus, to collect all the events, the user must test all the possible transitions from one state to another. for instance, for an electric stove with three power levels (states), the user should trigger twelve possible switching events, among off/on states. in the second step, large type-iii appliances work alone and their data of consumption are recorded. note that, for this work, also data from type-i and type ii appliances working alone have been collected, in order to combining them and create several synthetic aggregate data series to test the system. 3.2. extraction of the signatures once the aggregate data have been collected, the database of the signatures of each switching event is built. these data are normalized with respect to the constant voltage of 230 v in such a way that the voltage drops due to the load insertion do not influence the result. moreover, a causal filtering is applied to apparent, real, and reactive power signals. in this way, possible spikes and outliers can be discarded or smoothed. with such a low sampling frequency, fast transients should be removed, since they could be sometimes recorded, and other times missed during the acquisition. figure 4 shows the changes in electricity consumption due to the switching on and off of a fan before and after the filtering. for the type-i and ii devices, an edge detector finds the switching events in the apparent power data when the absolute value of the difference s between two consecutive values is larger than 20 va. the sign of s identifies the start up or the shutdown of the appliance. then, the real p and reactive q power variations in each edge must be determined and candidate as one of the signatures of the individual load. within the time interval between two consecutive events there is an almost constant power level consumption. in order to find a candidate signature of the appliance-switching events, the difference between the mean values of the measurements before and after each edge of real and reactive power is evaluated. among all the candidate signatures, the k-medoids clustering method [16] is applied to partition the set of switching events into a set of clusters whose number k depends on the possible states of the single appliances and must be set by the user. this clustering method is robust to noise and outliers and it chooses data points as centers of the clusters. at the end of the clustering figure 3. flow chart of the procedure. figure 4. original and filtered fan power consumption. pre-processing edge detection apparent power δs real power δp reactive power δq δp and δq clustering db signatures aggregate data acquisition insert # states appliance nearest neighbor search in the δp-δq plane recognition and labeling db patterns aggregate data oscillation frequency ≥ 0.05hz ? pre-processing edge detection apparent power δs real power δp reactive power δq no yes on/off large appliances check pattern energy disaggregation recall phase db event labels extraction of the signatures data collection extraction of the lower q envelope δq>100var? yes no acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 142 procedure, these centers will form the signatures associated with each transition. note that, as appliances with small power consumptions are not interesting from the point of view of energy savings, and hardly distinguished, loads with p < 20 w are discarded and loads with 20 w < p <50 w are associated to a unique “low consumption” cluster. for the same reason, switching events lasting less than 5 s are not taken into account. note that, the threshold of 50 w is implemented by default, but it can be modified by the user, in case only consumptions greater than a predetermined power are of interest. at the end of the training phase, all the collected data are given as input to the monitoring system; in case of errors in the classification, new common clusters are created for those devices characterized by close consumptions of real and reactive power. for large type-iii appliances, an ad hoc procedure has been implemented. for the washing machine, the start and the end of the cycle and the motor-spin events are detected. to this aim, the peak values of the real power oscillations that identify the heating and washing phases are identified. in order to avoid peaks due to noise or other events not characterizing this device, maximum relevance peak values are selected, i.e., those that drop at least 30 w on either side before the signal attains a higher value. a statistical analysis of the time distance of such peaks shows that the typical time distance between the peaks is close to 20 s. thus, the oscillations in the active power signal show a frequency greater than or equal to 0.05 hz. moreover, in order to avoid spurious detections, the pattern of the motor spin pattern is isolated. the motor-spin pattern shown in figure 5 is identified in the individual appliance signals. the pattern identified in the individual appliance signals is then extracted and included in the database of the system. 3.3. the recall phase during the recall phase, first of all, a check is carried out to verify if a washing machine is running: as described in section 3.2, the switching on (off) of the washing machine operation is identified when the oscillations, characterized by a frequency greater than or equal to 0.05 hz, start (end). when no washing machine cycle is detected, the following procedure is applied. when an edge in the aggregated signal is detected, the corresponding point in the p-q plane is evaluated. then, a nearest neighbour search in the p-q plane is performed, and the event is classified and associated to the appliance event with the nearest signature vector. moreover, a check on the sign of q is considered as further information, in addition to the cluster center distance, to identify the proper cluster in order to increase the discrimination capabilities. see as an example the signatures of the hairdryer and stove in figure 6, where the signatures are very close but the former, unlike the latter, is characterized by zero reactive power. if no association is performed, the event is labelled as unidentified. the detections of events of type-i and ii appliances switched on and off during a washing machine cycle become quite challenging with such a low frequency and many false switching could be triggered directly applying the described procedure. thus, if the washing machine is running in its heating phase, the lower envelope of q is extracted. since the washing-machine q lower envelope during the heating phases is equal to zero, the presence of the intervals of not-null constant values indicates the switching of an appliance. in this case a threshold of 100 var is considered. when an edge is detected in a flat-top interval of the q lower envelope, the nearest neighbour search is then applied in the p-q plane in order to associate the event to the appliance with the nearest signature vector. this procedure allows one to improve the detection in case of reactive loads switched on and off during the washing machine heating phases. finally, using a similarity search algorithm, it is possible to identify, in the aggregate signal, the operating phase that best matches with the reference associated to the spin motor functioning. this procedure could be applied for other devices with characteristic patterns, e.g. microwave ovens, to distinguish their operating conditions. 4. case study in this section the experiments carried out to verify the validity of the methodology proposed are reported. the algorithms are implemented in matlab. performance have been evaluated both on a custom dataset collected by the authors in some italian houses and on a public dataset, blued [9], which has already been extensively analysed in the literature. 4.1. custom dataset in order to create our dataset, several domestic consumption data have been acquired by installing an energy meter between the investigated appliances and the domestic network. the implemented acquisition system of the electricity consumption data consists of: one eastron sdm220 single-phase energy meter [17], for residential and industrial applications at a rated voltage of 230 v (range 176 v to 276 v) and current of 10 a (range 0.5 a to 100 a). the accuracy requirements of the meter are reported in table 1; figure 5. motor-spin pattern. figure 6. custom dataset: appliance signatures in δp-δq plane. the overlapping signatures of hairdryer and stove are highlighted with black rectangles [7]. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 143 a pc on which the measurement software and the load disaggregation algorithm are implemented; a modbus/rs485 serial interface, including a serial port converter adapter cable usb-rs485, allowing the remote communication between the energy meter and the pc. the dataset has been created with an acquisition frequency equal to 1 hz that represents the maximum value allowed by the used meter. the dataset consists of three parts: individual loads, aggregate loads and synthetic aggregate loads. the data were acquired in single and aggregate manner during the actual operation of the devices or were generated by simulating conditions corresponding to actual user behaviour. the electricity consumptions of the individual loads reported in table 2 have been acquired by connecting the meter directly to the device plugs. the dataset was obtained acquiring multiple appliances simultaneously via a multi-socket. in order to increase the amount of the data corresponding to aggregate consumption, an aggregate synthetic dataset was obtained by combining the consumption data of the individual appliances, by summing the measurements of p and q and averaging the values of v. 4.2. blued dataset this dataset was built in 2011 by monitoring a whole house located in pennsylvania (us), for 8 days. in us there are three electricity feed lines for ordinary houses: two firewires and one neutral line. the two firewires have 120 v amplitude of voltage and they are named as phase a and phase b. usually, small 120 v-rated appliances are connected between one firewire and one neutral while larger 240 v-rated appliances such as heaters and air conditioners are connected between two firewires. in this work only phase a data have been used. the appliances connected to this phase are reported in table 3. the blued dataset contains high frequency (12 khz) aggregated data of raw current and voltage. during the creation of the dataset, every single switching on/off of any appliance is recorded and called as an event. in particular, all the changes in the state of power consumption higher than 30 w have been considered. every appliance event in the house was then labelled and time-stamped, providing the necessary event labels database for the evaluation of the proposed procedure. in total, 904 events have been registered in the considered phase a. in this work, to take into account the technical specifications of the pre-commercial smart meter used in the experiments discussed in the previous section 4.1, power signals evaluated from raw data are down-sampled at 1 hz. then, the events identified in blued, but unknown, and those with duration less than or equal to 5 s have been discarded obtaining a final database of 662 ground-truth events. 5. results figures 6 and 7 show the signatures of custom and blued databases in the p-q plane obtained by applying the phase of the procedure “extraction of the signatures”, described in section 3.2. in this section the performances of the recall phase (event detection and appliances identification) over the two datasets are presented. 5.1. performance measures in binary classification problems (such as the event detection) there are only two classes, called positive (on or off event) and negative (non-event). when a positive sample is incorrectly classified as negative, it is called a false negative (fn); when a negative sample is incorrectly classified as a positive, it is called table 1. accuracy requirements of eastron sdm220 [17]. parameter accuracy voltage 0.5% of range maximum current 0.5% of nominal power active 1% of range maximum reactive apparent energy active class 1 iec 62053-21 class b en 50470-3 reactive 1% of range maximum table 2. list of appliances in the custom dataset. appliance average power consumption in w type fridge 180 ii kettle 1900 i lamp 40 i notebook 60 i stereo 30 i toaster 500 i tv 40 i electric oven 2000 ii hairdryer 300-900 ii fan 30-40 ii induction cooker 400-2500 ii microwave oven 1000-1200 ii stove 900-1800 ii water heater 600-1200 ii washing machine 130-1700 iii table 3. list of appliances in the blued dataset (phase a). appliance average power consumption in w type kitchen aid chopper 1500 ii fridge 120 ii air compressor 1130 i hair dryer 1600 ii backyard lights 60 i washroom light 110 i bathroom upstairs lights 65 i bedroom lights 190 i figure 7. blued dataset (phase a): appliance signatures in δp-δq plane. the magnification of the part enclosed by the black rectangle shows more in detail the signatures with |∆𝑃| < 250 w. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 144 a false positive (fp); when a positive sample is correctly classified as positive is called a true positive (tp). the 𝑃𝑟𝑒𝑐𝑖𝑠𝑖𝑜𝑛 (pr) represents what proportion of predicted positives is truly positive. it is the ratio between the number of correctly classified positives and the total number of samples predicted as positives: 𝑃𝑟 = 𝑇𝑃 𝑇𝑃 + 𝐹𝑃 . (1) the 𝑅𝑒𝑐𝑎𝑙𝑙 (re) represents what proportion of actual positives is correctly classified. it is the ratio between the number of correctly classified positives and the total number of positives in the dataset: 𝑅𝑒 = 𝑇𝑃 𝑇𝑃 + 𝐹𝑁 . (2) the 𝐹1 − 𝑠𝑐𝑜𝑟𝑒 combines 𝑃𝑟𝑒𝑐𝑖𝑠𝑖𝑜𝑛 and 𝑅𝑒𝑐𝑎𝑙𝑙 into a single measure. mathematically it is the harmonic mean of precision and recall. 𝐹1 − 𝑠𝑐𝑜𝑟𝑒 = 2 ∙ 𝑃𝑟 ∙ 𝑅𝑒 𝑃𝑟 + 𝑅𝑒 . (3) in a multiclass classification problem (as the appliance identification) there are no positive or negative classes, but tp, fp and fn and the other performance measures can be evaluated for each individual class (each appliance event). summing up the single class measures, the total tp, the total fp and the total fn of the model can be obtained; then the global metrics of precision, recall and f-score can be evaluated. note that in this case, all the global metrics become equal, i.e. 𝑃𝑟 = 𝑅𝑒 = 𝐹1 − 𝑠𝑐𝑜𝑟𝑒. 5.2. performance with custom dataset the recall phase has been tested on aggregate signals composed of type-i and type-ii appliances with and without the washing machine. using the pattern shown in figure 5, the operation phases of the motor-spin are individuated even considering washing machines of different brands and with different washing programs. figure 8 shows the power demand of a household over a 14-hour period, between 08:00 and 22:00. as it can be observed, the aggregate power demand is generated by the fridge, which shows a periodical power consumption behaviour, and other appliances. the on and off events are shown as markers at the level of +δp and -δp respectively, whereas the motor spin functioning is indicated with red segments. the results are very satisfactory, especially regarding identification of the activation status of appliances that consume more energy, such as the washing machine. as an example, in figure 9 the events detection during a washing machine cycle, is shown. flat-top intervals of the q lower envelope, denoted by the green segments in figure 9, identify the switching on/off of the fridge. in order to show the effectiveness of the improvements proposed in this paper, in table 4 the performance for the edge detection as reported in [7], are shown. in table 5 the event classifier performance is compared with those reported in [7]. as it can be noted, despite the performance index obtained in [7] for the synthetically generated aggregate signals was already very high, a slight improvement has been achieved. a more significant improvement in the performance index can be observed on the experimental data after the introduction of the described changes. the pie charts in figure 10 compare the estimated decomposition results with ground-truth of energy consumption. as can be observed, the proposed method is capable of disaggregating energy consumption of the appliances with good accuracy. 5.3. performance with blued dataset in the blued dataset the aggregate signals are composed of type-i and type-ii appliances. the event detector applied to blued time series identifies 647 events characterized by ∆𝑆 > 20 var and lasting more than 5 s. among them, 9 events figure 8. household power demand over a 14-hour period. figure 9. detections of on/off events for the fridge during a washing machine cycle. table 4. performance metrics for the edge detector tested on synthetic and experimental custom dataset. test set # groundtruth events tp fp fn pr re f1score synthetic data 140 139 1 1 0.99 0.99 0.99 experimental data 137 135 2 2 0.99 0.99 0.99 table 5. performance metrics for appliances classification tested on synthetic and experimental custom dataset. test set # detected events tp fp fn pr re f1score synthetic data [7] 139 133 6 6 0.96 0.96 0.96 synthetic data 139 133 3 3 0.97 0.97 0.97 experimental data [7] 135 121 14 14 0.90 0.90 0.90 experimental data 135 129 6 6 0.96 0.96 0.96 acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 145 refer to appliances characterized by an active power consumption less than 30 w. since these events are not labeled in blued, they have been discarded. table 6 and table 7 report the performance obtained by the algorithm for the event detection and the event classification respectively. all the performance indexes are very high confirming the suitability of the approach to detect sudden state changes. in table 8 the confusion matrix showing per class accuracy (in percent) is reported. in the confusion matrix all the different lights connected to the phase-a have been grouped into a single class. we can observe that the only classification errors are for the fridge class which is confused with that of the lights and vice versa. literature reports few contributions for nilm algorithms applied to blued data at the same frequency of 1 hz [13], [14], [18]. in [13] a panel of different machine learning algorithms is applied to appliance classification for a subset of blued dataset (air compressor, basement, computer, garage door, iron, kitchen light, laptop, lcd monitor, monitor, refrigerator, tv). both precision and recall are equal to 79%. in [14], clustering in δp δq plane is solved through a hierarchical approach executed with some manual supervision. the paper reports for phase-a blued data an f1-score for the event detection and for the appliance classification of about 92% and 88% respectively. in [18] an event detection algorithm is used to identify the time instant when a sudden increase of active power occurs, indicating a possible turn-on event, and a convolutional neural network (cnn) classifier is applied for event classification. results reported for blued are limited to three appliances (washing machine, fridge, microwave). table 9 reports the calculated recall (also called true positive rate-tpr) and accuracy metrics for event detection and classification respectively for [13], [14] and [18] and also other solutions applied to higher frequency data [19]-[21]. figure 10. energy disaggregation results. table 6. performance metrics for the edge detector tested on blued dataset. test set # groundtruth events tp fp fn pr re f1score experimental data 662 638 9 13 0.99 0.96 0.97 table 7. performance metrics for appliances classification tested on blued dataset. test set # detected events tp fp fn pr re f1score experimental data 638 625 13 13 0.98 0.98 0.98 table 8. confusion matrix showing per class accuracy (in percent) for the appliances connected to phase-a of blued; blue: actual device, grey: classified device, green: correct classifications (tp), orange: false positives. class fridge air compressor lights kitchen aid chopper hair dryer fridge 0.99 0 0.01 0 0 air compressor 0 1 0 0 0 lights 0.05 0 0.95 0 0 kitchen aid chopper 0 0 0 1 0 hair dryer 0 0 0 0 1 table 9. comparison of event detection and classification performance. reference method sampling-rate recall (tpr) accuracy proposed 1 hz 0.96 0.98 [13] active machine learning strategy 1 hz 0.79 0.92 [14] hierarchical approach 1 hz 0.89 0.88 [18] first and second differences + cnn 1 hz 0.94 0.98 [19] finite-precision analysis 4 khz 0.91 [20] extremely randomized trees 12 khz 0.94 [21] hybrid approach 60 hz 0.94 acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 146 it can be seen that the proposed algorithm achieves good results while being simple and computationally efficient. in fact, despite the low frequency of data analyzed, the approach shows competitive performance even when it is compared to other more complex methodologies applied to high-sampling-rate signals, from 60 hz to 12 khz [19]-[21]. 6. conclusions in this paper, a monitoring system has been proposed that is able to disaggregate and keep track of the power consumption of the devices existing in a typical house analysing low frequency aggregate data. by applying a hybrid approach to power data, i.e. event-based techniques and pattern recognition techniques for large household appliances, the load disaggregation is performed with good performance, even when complex type-iii devices, such as the washing machine, are working. finally, using the more populated blued dataset, we showed that the proposed procedure is able to achieve high performance in two key tasks in energy disaggregation: identifying events from non-events, and identifying which appliance is associated with a specified event. references [1] s. s. hosseini, k. agbossou, s. kelouwani, a. cardenas, nonintrusive load monitoring through home energy management systems: a comprehensive review, renewable and sustainable energy reviews, vol. 79, 2017, pp. 1266–1274. doi: 10.1016/j.rser.2017.05.096 [2] a. ruano, a. hernandez, j. ureña, m. ruano, j. garcia, nilm techniques for intelligent home energy management and ambient assisted living: a review, energies, vol. 12, june 2019, p. 2203. doi: 10.3390/en12112203 [3] w. kong, z. y. dong, b. wang, j. zhao and j. huang, a practical solution for non-intrusive type ii load monitoring based on deep learning and post-processing, ieee trans. smart grid, vol. 11, no. 1, jan. 2020, pp. 148-160. doi: 10.1109/tsg.2019.2918330 [4] a. harell, s. makonin, i. v. bajić, wavenilm: a causal neural network for power disaggregation from the complex power signal, proc. of icassp 2019 ieee international conference on acoustics, speech and signal processing, brighton, uk, 12-17 may 2019, pp. 8335-8339. doi: 10.1109/icassp.2019.8682543 [5] q. wu, f. wang, concatenate convolutional neural networks for non-intrusive load monitoring across complex background, energies, vol. 12, no. 8, pp. 1572, apr. 2019. doi: 10.3390/en12081572 [6] p. davies, j. dennis, j. hansom, w. martin, a. stankevicius, l. ward, deep neural networks for appliance transient classification, proc. of icassp 2019 ieee international conference on acoustics, speech and signal processing, brighton, uk, 12-17 may 2019, pp. 8320-8324. doi: 10.1109/icassp.2019.8682658 [7] b. cannas, b. canetto, s. carcangiu, a. fanni, l. fresi, m. marceddu, c. muscas, p. porcu, g. sias, non-intrusive loads monitoring techniques for house energy management, proc. of the 1st int. conf. on energy transition in the mediterranean area (synergy med), cagliari, italy, 28-30 may 2019, pp. 1-6. doi: 10.1109/synergy-med.2019.8764104 [8] b. cannas, s. carcangiu, d. carta, a. fanni, c. muscas, g. sias, b. canetto, l. fresi, p. porcu, real-time monitoring system of the electricity consumption in a household using nilm techniques, proc. of the 24th imeko tc4 international symposium and 22nd international workshop on adc and dac modelling and testing, palermo, virtual, italy, 14–16 september 2020; pp. 90-95. online [accessed 18 june 2021] https://www.imeko.org/publications/tc4-2020/imeko-tc42020-18.pdf [9] k. anderson, a. ocneanu, d. benitez, d. carlson, a. rowe, m. berges, blued: a fully labeled public dataset for event-based non-intrusive load monitoring research, proc. of the 2nd kdd workshop on data mining applications in sustainability (sustkdd), acm, beijing, china, 2012, pp. 1–5. 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https://doi.org/10.1016/j.egypro.2017.07.371 http://doi.org/10.1145/3077839.3077859 https://doi.org/10.1109/5.192069 https://it.mathworks.com/help/stats/kmedoids.html http://www.eastrongroup.com/ https://doi.org/10.3390/en14030767 https://doi.org/10.1145/3427771.3427849 https://doi.org/10.1109/urtc.2017.8284200 https://doi.org/10.1109/tsg.2019.2924862 optical-flow-based motion compensation algorithm in thermoelastic stress analysis using single-infrared video acta imeko issn: 2221-870x december 2021, volume 10, number 4, 169 176 acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 169 optical-flow-based motion compensation algorithm in thermoelastic stress analysis using single-infrared video tommaso tocci1, lorenzo capponi1, roberto marsili1, gianluca rossi1 1 department of engineering, university of perugia, via g. duranti 93, 06125 perugia, italy section: research paper keywords: thermoelastic stress analysis; optical flow; motion compensation; mechanical stress; experimental mechanics citation: tommaso tocci, lorenzo capponi, roberto marsili, gianluca rossi, optical-flow-based motion compensation algorithm in thermoelastic stress analysis using single-infrared video, acta imeko, vol. 10, no. 4, article 27, december 2021, identifier: imeko-acta-10 (2021)-04-27 section editor: roberto montanini, università di messina and alfredo cigada, politecnico di milano, italy received july 30, 2021; in final form december 9, 2021; published december 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: tommaso tocci, e-mail: tommaso.tocci@outlook.it 1. introduction the thermoelastic stress analysis (tsa) [1]-[4] is an infrared image-based technique for non-contact measurement of stress fields. this technique is based on the detection and analysis of the temperature fluctuations amplitude on the surface of a mechanical component dynamically loaded, performed by a thermal camera. since the temperature fluctuation produced by the load are very small, a lock-in [5], [6] data processing is normally applied to the thermal video [7], which makes it possible to emphasise phenomena at particular loading frequencies, reducing the noise effects, normally higher than thermal fluctuation induced by stress. the tsa is largely applied for many mechanical stress analysis application, fem validation, design comparison and very useful for high resolution stress concentration analysis, non-contact stress detection in application where classical sensors have problems or limitations [8]-[11]. for an ideal application of this technique, the specimen surface should not move or make small movements throughout the test [2]. clearly, this ideal condition is not always achievable since, for example, in the case of a specimen with low stiffness, the phenomena of deformation and displacement are very large [3]. this phenomenon is emphasised by the fact that the tsa requires the application of dynamic loads, with sufficiently high stress levels, in order to generate, by the thermoelastic principle, sufficiently high temperature fluctuations. the main problem caused by specimen movement is that, in a sequence of consecutive frames, the same pixel does not always correspond to the same part of the specimen as it moves. this phenomenon affects the lock-in operation, causing alterations in the measured stress field. especially on the border of mechanical component, it may also happen that a pixel corresponds at a surface point or a background point alternatively: this phenomenon is commonly known as edge effect [1]. therefore, in the thermal video, it is necessary to compensate in a certain way these movements and deformations by means of algorithms commonly called motion compensation [2]. abstract thermoelastic stress analysis (tsa) is a non-contact measurement technique for stress distribution evaluation. a common issue related to this technique is the rigid-displacement of the specimen during the test phase, that can compromise the reliability of the measurement. for this purpose, several motion compensation techniques have been implemented over the years, but none of them is provided through a single measurement and a single sample surface conditioning. due to this, a motion compensation technique based on optical-flow has been implemented, which greatly increases the strength and the effectiveness of the methodology through a single measurement and single specimen preparation. the proposed approach is based on measuring the displacement field of the specimen directly from the thermal video, through optical flow. this displacement field is then used to compensate for the specimen’s displacement on the infrared video, which will then be used for thermoelastic stress analysis. firstly, the algorithm was validated by a comparison with synthetic videos, created ad hoc, and the quality of the motion compensation approach was evaluated on video acquired in the visible range. the research moved into infrared acquisitions, where the application of tsa gave reliable and accurate results. finally, the quality of the stress map obtained was verified by comparison with a numerical model. mailto:tommaso.tocci@outlook.it acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 170 the state of the art relies on sakagami et al. in [12], where the compensation of the motion using displacement fields is obtained with the digital image correlation (dic) [13], [14] on a second video recorded simultaneously in the visible range. the motion compensation is then applied to thermal video. this technique requires the employment of two different cameras and the double surface conditioning of the specimen: the application of speckle for the dic and a high emissivity black paint for tsa. silva et al. [15] simplified the compensation procedure limiting the use of a single camera both for the acquisition of the video for the dic and the tsa techniques. in this case, however, in order to be able to make displacement measurements with the dic, the speckle must be made through a paint with an emissivity that can be detected by an infrared camera. nevertheless, two surface conditioning of the specimen surface are still required. compared with sakagami et al. [12], since one camera is employed for the two acquisitions, the alignment problems, which are very evident in the use of two cameras that should ideally be positioned in the same place, are greatly reduced. this research proposes an algorithm that allows the compensation of the rigid motion using a single thermal video with a single step specimen preparation, from which it will be possible to perform both the compensation and the tsa. therefore, compared to the state of the art, it will no longer be necessary to first prepare the specimen for measurement with dic, then remove the speckle and finally prepare the surface for analysis by the tsa, but it will be sufficient to do only the preparation for the tsa analysis, significantly reducing the time required for the preparation phase. in addition, with a single thermal video it is possible to obtain both the motion compensation field and the stress field, thus also reducing the time of the experimental phase. this technique allows to carry out tests in which it is not possible to repeat the measurement, such as, for example, structural failure or random tests. the motion compensation phase sees the replacement of the dic with a computer vision technique called optical-flow [16]-[18], which is based on motion of visual-features, such as corners, edges, ridges and textures in two consecutive frames of a video scene [19], [20]. within optical-flow methods, the differentiation between sparse and dense approach is fundamental [18]. there are many different types of algorithms [21] based on different correlation techniques, such as gradient-based, based on the brightness constancy equation [22] or region-based approaches rely on the correlation of different features like normalized crosscorrelation and laplacian correlation [23], [24]. optical-flow has been widely applied to measure displacement fields in solid bodies [25]-[27]. in this work, the dense optical-flow farneback algorithm was implemented [28], [29]. a nylon specimen, realized by additive manufacturing, was tested applying a sinusoidal load along its y-axis, so that the movement of the specimen is exclusively along the vertical direction. in a preliminary step the algorithm was validated by analysing synthetically generated videos with imposed motion. the coincidence between the displacement measured by the implemented algorithm and the displacement imposed on the synthetic video was then evaluated. the algorithm was then tested in visible video to evaluate the efficiency of the motion compensation and then it is applied to a thermal video for the tsa. the manuscript is organized as follows: in sec. 2, the implemented algorithm, the test bench used and the experimental setup are presented. in sec.3 the algorithm is validated and the results discussed. sec. 4 draws the conclusions. 2. materials and method 2.1. algorithm implementation all the algorithms of this work have been implemented in a python environment. below is the description of the two main algorithms: one for measuring the displacement field and one for compensating for motion the thermal film. 2.1.1. algorithm for displacement field measurement data processing for the purpose of determining the displacement field is not carried out on the original video matrix but on a copy of it. especially for thermal video, the copy is normalised from the 14-bit of the original video to 8-bit. the compensation field detected by the 8-bit video is then applied to the original 14-bit video. each pair of frames, before being calculated by optical-flow, is subjected to a pre-processing phase, designed to reduce noise (very evident especially in infrared images) and to make a masking of the sample, which will be used later. each acquired frame is filtered, in sequence, through a gaussian filter [30], [31] with 5x5 kernel, a morphological transformation of dilation and erosion [30]-[32] , with 3x3 kernel. this produces a good noise reduction, especially in the thermal image, as shown in figure 1. the result obtained is then sorted through an image binarization algorithm [30], which returns a black and white binary field, showing the non-presence/presence of the specimen in a given pixel. example of mask of the specimen are shown in figure 2. as can be seen in the figure, the mask quality of the visible image is higher than that of the infrared image, but in both cases a good result is obtained. the farneback optical-flow algorithm [28], [29] is now applied to each pair of frame. each frame of the video is compared with a reference frame, which is usually the first frame of the video. the displacement field measured in this way indicates the displacement of the i-th frame with respect to the reference one. the displacement field indicates the displacement figure 1. image pre-processing: (a) original frame (b) filtered frame. figure 2. specimen mask: (a) visible video (b) infrared video. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 171 undergone by each pixel between the two frames. the field is displayed in hsv colour code, where the colour indicates the direction of displacement and the colour intensity indicates the magnitude. an example of a rough displacement field is given in figure 3: it can be seen that the displacement is mainly traced along the edges of the specimen and that the inner areas are detected as stationary. this requires a phase of improvement of the displacement field. we start with an initial masking, which eliminates all displacement vectors detected outside the specimen due to noise. operationally, the displacement matrix is multiplied by the mask matrix, as shown in figure 4. since it was decided to compensate for motion only along the vertical axis, the maximum displacement detected for each line is determined, which is likely to be located along the edges of the specimen. at this point, each row of the matrix has a constant value over the length of the frame, ideally including areas outside the specimen, as shown in figure 5. a new masking is then performed. to better understand this, it can be said that the first masking is intended to prevent the algorithm from taking for granted a maximum displacement that could be detected at a point outside the specimen, while the second is applied to eliminate line by line displacement values per pixel outside the geometry of the specimen. finally, the field of compensated displacements is filtered with a blur filter, in order to make the field of compensated displacements uniform in pixels, as shown in figure 7. compared to the one shown in figure 3, a greater uniformity and regularity can be observed. 2.1.2. algorithm for motion compensation motion compensation takes place on the original visible video or the original 14-bit infrared video, which has not been altered in any way during the previous step. a column vector containing the displacement of each row is then extracted from the displacement field. this displacement vector, once inverted, represents the motion to be applied to each row of the original video for motion compensation. in order to improve the compensation and make it smoother and more consistent, this compensation vector is manipulated using the kalman filter [33], as shown in figure 8. this vector is called compensation vector. figure 3. example of rough displacement field. figure 4. first masking operation: incorrect displacement vectors are masked out. figure 5. calculation of maximum displacement line by line. figure 6. second masking operation: compensation field is created. figure 7. example of smooth and masked displacement field. figure 8. extraction and smoothing of the compensation vector. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 172 we then move on to the actual compensation by applying a shift to the lines of the video to be compensated using the information contained in the compensation vector. the i-th row of the original video is moved to position i+k, where k is the value of the compensation move, as shown in figure 9. a check is also made to see if this move would place the line outside the maximum frame size, in which case no compensation is done for that line. the output is a compensated matrix starting from the original video, which contains the thermal and stress information, but which is not manipulated in any way: it is only compensated by moving the position of the lines, but not by modifying their content in any way. 2.2. test bench and acquisition system the test bench used during the experimental campaign consists of a shaker sentek l1024m, over which a structure for tensile testing was built, as shown in figure 10. a specimen with identical geometry to that used by sakagami et al. [12] was used. the specimen was produced using fdm printing in nylon 6-6 with 100% infill ratio and a layer height of 0.1 mm. the material has a tensile strength of 80 mpa and an elastic module of 2.2 gpa. two pairs of clamps, one connected to the head of the shaker and the other to the fixed part of the structure, were used to fix the specimen, as shown in figure 11. a load cell is positioned at the top of the specimen in order to monitor the loads exchanged during testing. two cameras were used: one for visible video and one for infrared video. a canon eos 7d reflex camera with 24 mm – 70 mm lens capable of acquiring 1920x1080 pixels resolution video at 30 fps was used. therefore, a flir a6751sc thermal camera with a cooled sensor was used for infrared video acquisition. this thermal camera captures 640x512 pixels resolution video at a framerate of 100 fps and has a thermal sensitivity of less than 20 mk. both video cameras were positioned in front of the specimen at the height of the specimen hole. 2.3. experimental campaign the experimental campaign was divided into two steps: first, videos were acquired in the visible at different frequencies in order to test and validate the compensation algorithm. subsequently, infrared videos were acquired at different frequencies and different load levels in order to test the effectiveness of the motion compensation algorithm for thermoelastic application. a summary is given in table 1. 3. results 3.1. algorithm validation since a validation phase of the motion compensation algorithm described above is required, it was decided to generate synthetic videos in which a body is present that is clearly identifiable from the optical-flow and has a known imposed displacement. the motion of the body in the synthetic video is imposed in a sinusoidal regime with imposed amplitude and frequency. several videos have been generated with different figure 9. motion compensation algorithm. figure 10. test bench. figure 11. specimen geometry and grasping. table 1. summary of the experimental campaign. frequency in hz sinusoidal load in n visible infrared 4 1500 x 3000 x x 5 1500 x 3000 x 8 1500 x 3000 x x table 2. summary of synthetic video. #syntethic video frequency in hz amplitude in pixel 1 1 5 2 1 10 3 1 20 4 4 5 5 4 10 6 4 20 7 8 5 8 8 10 9 8 20 acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 173 operating conditions, as illustrated in fehler! verweisquelle konnte nicht gefunden werden.. in order to validate the displacement fields obtained by optical-flow, a comparison was made between the displacement detected in different survey lines on the object placed in the video and the displacement imposed during the generation of the synthetic video. in each video, three horizontal survey lines are defined: an upper red line, a central green line and a lower blue line. note that the red line and the blue line run alternately from the marker area. some results obtained at 4 hz and 8 hz are shown below. however, validation was carried out for each synthetic video generated, obtaining superimposable results between detected and imposed displacement. it is also reported the hsv displacement field, where it is possible to see how the detected displacement occupies the zone with a colour gradation going from green to violet, which corresponds in fact to the colour connected to displacements perfectly along the y-axis; this gives us a further confirmation of the correct application of the optical-flow. in figure 12 you can see how the green line in the vicinity of the marker areas gives us a maximum displacement of about 5 pixels which is exactly coincident with the imposed one. the same result is obtained at figure 13 where a displacement of 20 pixels is found, coinciding with the imposed one. also, from the point of view of the frequency of the load, it can be said that this face does not have a negative influence on the measurement of the displacement. the compensation algorithm is then validated and tested by computing visible video, which is easier to process due to the lower amount of noise than infrared video. as for the videos in the visible, the quality of motion compensation is verified by comparing the positions of the lower end of the hole in the specimen. a line is placed at the lower end of the hole, as shown in figure 14. it can be seen from the model that in the uncompensated frame, because there is rigid motion, the position of the hole is varied. thanks to the compensation of the motion instead, the position of the centre of the hole of the i-th frame, is brought back to the position of the reference frame. to illustrate the result obtained by means of compensation, we will insert the superimposed image of the 3 frames, as shown in figure 15: reference, uncompensated frame and compensated frame; it is clearly evident that in the compensated case, the position of the hole is in line with that of the reference frame. we also report the overlapped frames between the reference image and the uncompensated one (figure 16-a) and between the reference image and the compensated one (figure 16-b). also, in this case it is clear that in the compensated image the overlapping is almost perfect, because the edges of the reference image are not visible. on the contrary in the not compensated one we have that the displacement is well evident. in the following, the effect of motion compensation was evaluated by measuring the displacement of the centre of the hole in relation to the reference position. it can be seen that, for a load frequency of 4 hz, a displacement attenuation of approx. 92% was obtained. if, on the other hand, 8 hz is taken as the load frequency, since smaller amplitudes occur as the frequency increases, compensation of approx. 80% is achieved. in both cases, this is a good result. results are shown in table 3. figure 12. synthetic video #7: (a) survey lines monitored (b) field of measured displacement (c) dynamic plot of measured displacement along survey lines. figure 13. synthetic video #6: (a) survey lines monitored (b) field of measured displacement (c) dynamic plot of measured displacement along survey lines. figure 14. diagram of the comparison model between the reference hole and the uncompensated/compensated one. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 174 3.2. thermoelasticity application the following graphs (figure 17, figure 18 and figure 19) compare the stress fields obtained from uncompensated and compensated infrared videos. the stress distribution field is expressed in terms of temperature variation in °c. analysis of the fields clearly shows the improvement in quality produced by the motion compensation. in the uncompensated image, a significant component of edge effect is visible, due to the rigid motion of the specimen. this phenomenon makes it difficult to detect distress zones, especially in the areas close to the borehole, where a higher stress concentration is expected. after compensation, however, it is easiest to identify the typical stress distribution around the hole. it is also noticeable that the field is much sharper than the uncompensated one. in order to validate the result, the tsa stress fields are compared with those obtained from the fem model. a coherent trend of the experimental fields in relation to the numerical ones can be seen. below, in figure 20, is the graph comparing the stressconcentration factor at the hole in the uncompensated, compensated video and the fem model along the two check lines passing through its centre. in order to be able to compare the experimental stress profiles with the fem, the concentration of the normalised stresses was figure 15. comparison of the position of the 3 holes in the reference, uncompensated and compensated frame. figure 16. frame comparisons: (a) reference non-compensated (b) reference – compensated. table 3. measurement of hole displacement uncompensated and compensated configuration. test not compensated displacement in pixel compensated displacement in pixel 4 hz – frame 11 36 3 4 hz – frame 118 35 3 4 hz – frame 226 33 3 8 hz – frame 16 9 2 8 hz – frame 124 11 2 8 hz – frame 228 10 2 figure 17. 4 hz infrared video: comparison of distress fields of uncompensated and compensated video. figure 18. 5 hz infrared video: comparison of distress fields of uncompensated and compensated video. figure 19. 8 hz infrared video: comparison of distress fields of uncompensated and compensated video. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 175 evaluated, i.e., by dividing the series of values by the maximum of the series. it can be seen that, along the vertical direction, in the uncompensated video there is an inversion of the stress concentration at the bottom of the hole (red circle in figure 20), which in this case corresponds to the area between 80 and 100 along the y-direction, due to the edge effect. on the contrary, this inversion of stress concentration is not present in the compensated video, which presents a trend similar to that of the fem model. 4. conclusions the implementation of a new motion compensation algorithm for thermoelastic applications respect the state of the art was proposed in this research. the algorithm was first validated by means of a series of synthetic videos generated in such a way as to have a known imposed displacement. then, the tests were carried out on videos in the visible range where the performances of the motion compensation were evaluated, which was around 92% for 4hz displacements and 80% for 8hz displacement. stress distribution fields produced from the same compensated and uncompensated video were compared. the blurring and edge effects produced by the motion were almost completely eliminated, making it possible to correctly measure the stress field, especially in the area around the hole. this result was compared with stress fields obtained from finite element analysis. videos were tested at different frequencies in order to verify the robustness of the algorithm. finally, the normalised stress concentration was compared along two perpendicular survey lines passing through the centre of the hole. it can be seen that, along the y-compensation line, the inversion of tension due to the effect of motion was significatively reduced by the here proposed motion compensation algorithm. references [1] w. thomson, on the dynamical theory of heat, earth environ. sci. trans. r. soc. edinburgh, vol. 20, no. 2, pp. 261–288, 1853. 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[33] g. welch, g. bishop, an introduction to the kalman filter, 1995. https://doi.org/10.1016/s0065-2458(10)80007-5 https://doi.org/10.1023/b:visi.0000011205.11775.fd https://doi.org/10.1088/1742-6596/1149/1/012032 http://dx.doi.org/10.1016/j.ymssp.2020.107456 http://dx.doi.org/10.1016/j.jsv.2018.07.046 https://doi.org/10.1007/3-540-45103-x_50 on the trade-off between compression efficiency and distortion of a new compression algorithm for multichannel eeg signals based on singular value decomposition acta imeko issn: 2221-870x june 2022, volume 11, number 2, 1 7 acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 1 on the trade-off between compression efficiency and distortion of a new compression algorithm for multichannel eeg signals based on singular value decomposition giuseppe campobello1, giovanni gugliandolo1, angelica quercia2, elisa tatti3, maria felice ghilardi3, giovanni crupi2, angelo quartarone2, nicola donato1 1 department of engineering, university of messina, contrada di dio, s. agata, 98166 messina, italy 2 biomorf department, university of messina, aou "g. martino", via c. valeria 1, 98125, messina, italy 3 cuny school of medicine, cuny, 160 convent avenue, new york, ny 10031, usa section: research paper keywords: biomedical signal processing; electroencephalograph (eeg); eeg measurements; near-lossless compression; singular value decomposition (svd) citation: giuseppe campobello, giovanni gugliandolo, angelica quercia, elisa tatti, maria felice ghilardi, giovanni crupi, angelo quartarone, nicola donato, on the trade-off between compression efficiency and distortion of a new compression algorithm for multichannel eeg signals based on singular value decomposition, acta imeko, vol. 11, no. 2, article 30, june 2022, identifier: imeko-acta-11 (2022)-02-30 section editor: francesco lamonaca, university of calabria, italy received october 24, 2021; in final form february 22, 2022; published june 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: giuseppe campobello, e-mail: gcampobello@unime.it 1. introduction since its invention by the german psychiatrist hans berger almost a century ago [1], electroencephalography (eeg) has continuously evolved, becoming a powerful and extensively used method that allows measuring safely and noninvasively the spatiotemporal dynamics of the brain activity with a high temporal resolution in the range of milliseconds, which enables detecting rapid changes in the brain rhythms [2]. the brain rhythms are the periodic fluctuations of human eeg, which are associated with cognitive processes, physiological states, and neurological disorders [3]. hence, the use of eeg can range from basic research to clinical applications [4]. among the various applications of eeg, it is worth highlighting its recent application in brain computer interface (bci) research [5], [6]. eeg consists of a neurophysiological measurement of the electrical activity generated by the brain through multiple electrodes placed on the scalp surface. eeg data are measured as the electrical potential difference between two electrodes: active and reference electrodes. at neurophysiological level, the electrical potential differences are mostly generated by the summation of both excitatory and inhibitory post-synaptic potentials in tens of thousands of cortical pyramidal neurons that are synchronously activated [7]. hence, the brain sources of the electrical potentials recorded by eeg may be suited to an infinite number of configurations, thereby limiting the spatial resolution of scalp eeg. to overcome this drawback, several source localization methods have been proposed and their application with high-density eeg (hd-eeg) systems, such as 64-256 electrodes, can lead to a remarkable improvement in eeg spatial resolution [3], [8]. many applications require that eeg systems have to record continuously for several days or even weeks and this might easily yield several gigabytes (gb) of generated data, which makes compression algorithms necessary for efficient data handling. as abstract in this article we investigate the trade-off between the compression ratio and distortion of a recently published compression technique specifically devised for multichannel electroencephalograph (eeg) signals. in our previous paper, we proved that, when singular value decomposition (svd) is already performed for denoising or removing unwanted artifacts, it is possible to exploit the same svd for compression purpose by achieving a compression ratio in the order of 10 and a percentage root mean square distortion in the order of 0.01 %. in this article, we successfully demonstrate how, with a negligible increase in the computational cost of the algorithm, it is possible to further improve the compression ratio by about 10 % by maintaining the same distortion level or, alternatively, to improve the compression ratio by about 50 % by still maintaining the distortion level below the 0.1 %. mailto:gcampobello@unime.it acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 2 an illustrative example, about 2.6 gb of data per day are generated by an eeg system recording the data from 64 electrodes with a sampling rate of 250 hz and a 16-bit resolution. it should be mentioned that intracranial eeg recordings can generate even terabytes (tb) of data per day [9]. therefore, eeg data need to be largely compressed to efficiently manage their storage. furthermore, data compression is necessary also to reduce both transmission rate and power consumptions when telemonitoring eeg via wireless [10], [11]. for instance, wireless wearable eeg systems for long-term recordings should operate under a low-power budget, due to limitation on battery lifetime, and then the power consumption needs to be significantly reduced by compressing the data before transmission [12]. various eeg compression algorithms have been developed to minimize the number of bits needed to represent eeg data by exploiting inter and/or intra-channel correlations of eeg signals. eeg compression algorithms can be classified into two main categories: lossless and lossy compression [13], [14]. as the main goal of the compression algorithms is to reduce the size of the data, their performance is, typically, evaluated by using the compression ratio (cr), which is calculated as the ratio between the number of bits required to represent the original and compressed eeg data. generally, lossy compression enables superior compression performance compared to the lossless counterpart but it cannot guarantee a reconstruction of the exact original data from the compressed version. in such a case, the percent root-meansquare distortion (prd) is used as indicator for assessment of the quality of the reconstructed signal, which is affected by the distortion introduced by the lossy compression. typically, lossless compression algorithms are preferred in clinical practice to avoid diagnostic errors, since important medical information may be disregarded using lossy compression and, in addition, there is a lack of legislation and/or approved standards on lossy compression, making eeg reconstruction a more critical requirement than compression performance. on the other hand, the lossless compression approach has a limited impact on storage requirements for eeg applications. as a matter of fact, the use of the state-of-the-art lossless compression algorithms allows achieving typical compression ratios in the order of 2 or 3 [15]-[21]. on the other hand, eeg signals are of very small amplitude, typically in the order of microvolts (v), and thus they can be easily contaminated by noise and artifacts, which should be filtered to highlight and/or extract the actual clinical information [22], [23]. to accomplish this task, digital filters and denoising procedures based on wavelets, principal component analysis (pca) and/or independent component analysis (ica) are often used [24]-[28]. this enables the development of near-lossless pca/icabased compression algorithms that can achieve much higher compression ratios than those obtained with lossless compression algorithms with a tolerable reconstruction distortion for the application of interest. different near-lossless eeg compression schemes based on parallel factor decomposition (parafac) and singular value decomposition (svd) have been investigated and compared with wavelet-based compression techniques [29]. in most cases, parafac leads achieving better compression performance but the maximum cr obtained with a prd lower than 2 % was 4.96 [29]. a nearlossless algorithm able to obtain a cr of 4.58 with a prd in the range between 0.27 % and 7.28 %, depending on the specific dataset under study, has been proposed in [30]. more recently, a svd-based compression scheme able to obtain 80 % data compression (i.e., cr = 5) with a pdr of 5 % has been reported in [31]. more recently, in [32] we proposed a near-lossless compression algorithm for eeg signals able to achieve a compression ratio in the order of 10 with a 𝑃𝑅𝐷 < 0.01 %. in particular, the algorithm has been specifically devised for achieving a very low distortion in comparison to other state-ofthe-art solutions. in this paper, we present an improved version of our previous algorithm and particular attention is given to achieve a good trade-off between compression efficiency and distortion. the rest of this paper is organized as follows. in section ii, we briefly review svd and describe our original algorithm proposed in [32]. in section iii, we illustrate the proposed algorithm. in section iv, we present the experimental results obtained on a real-world eeg dataset. finally, future works and our conclusions are drawn in section v. 2. singular value decomposition eeg signals are easily contaminated with artifacts and noise and, therefore, they need to be filtered before extracting the actual clinical information. for this purpose, svd-based pca and ica techniques are commonly used. in order to briefly review how svd is exploited in this context, let us consider a high-density 𝑁-channel eeg system whose signals are sampled for a time interval 𝑇 and at a rate of 𝑓𝑠 samples per second (sps). in this case, we have 𝑀 = 𝑇 ⋅ 𝑓𝑠 samples per channel and thus an overall number of samples equal to 𝑁 ⋅ 𝑀. we assume that such as samples are represented by a 𝑁 × 𝑀 matrix 𝐴. it is known from the svd theory that it possible to decompose a matrix 𝐴 into three matrices 𝑈, 𝛴, and 𝑉, such that 𝐴 = 𝑈𝛴𝑉 𝑇. in particular, 𝛴 is a diagonal matrix whose diagonal elements, i.e., 𝜎𝑖 with 𝑖 ∈ [1, . . . , 𝑁], are named singular values. moreover, a rank 𝑘 approximation of 𝐴, i.e., 𝐴𝑘 = 𝑈𝑘 𝛴𝑘 𝑉𝑘 𝑇 , exists which minimizes the norm ||𝐴 − 𝐴𝑘 || and that can be obtained by considering the submatrices 𝑈𝑘 and 𝑉𝑘 , given by the first 𝑘 columns of 𝑈 and 𝑉, respectively, and the leading principal minor of order 𝑘 of 𝛴, i.e., 𝛴𝑘 , containing the first 𝑘 < 𝑁 singular values. in the specific context of eeg, the desired rank 𝑘, and thus the number of singular values exploited for approximation, is chosen by clinicians, or other eeg experts, in order to reduce the effect of undesired artifacts and noise by keeping unaltered the clinical information. in this case the actual clinical information is contained in 𝐴𝑘 and, with the aim of reducing storage resources that are needed to store eeg samples, it is mandatory to encode the matrix 𝐴𝑘 in the most efficient manner. in [32] authors proposed a solution for the above problem by deriving the near-lossless compression algorithm, as reported in figure 1. the basic idea of the algorithm is to decompose the matrix 𝐴𝑘 into two matrices, 𝑋𝑘 and 𝑌𝑘 , such that 𝐴𝑘 = 𝑋𝑘 𝑌𝑘 . in particular, the matrices 𝑋𝑘 and 𝑌𝑘 can be obtained, as shown in step 2, by first evaluating the matrix 𝑆 = 𝛴1/2 and then considering the first 𝑘 columns of the matrix 𝑈𝑆 and the first 𝑘 rows of the matrix 𝑆𝑉 𝑇 , i.e., 𝑋𝑘 = (𝑈𝑆)[: , 1 ∶ 𝑘] and 𝑌𝑘 = acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 3 (𝑆𝑉 𝑇 )[1 ∶ 𝑘, ∶] in matlab-like notation. successively (see step 3), maximum absolute values of the matrices 𝑋𝑘 and 𝑌𝑘 , i.e., 𝑚𝑋 = max(|𝑋𝑘 |) and 𝑚𝑌 = max(|𝑌𝑘 |), are evaluated. such values are used in the last step, i.e., step 4, to transform the floating-point matrices 𝑋𝑘 and 𝑌𝑘 into two integer matrices, �̃�𝑘 and �̃�𝑘 , on the basis of the following equations: �̃�𝑘 = round(𝑚𝑌 ⋅ 𝑋𝑘 ) �̃�𝑘 = round(𝑚𝑋 ⋅ 𝑌𝑘 ) . (1) note that the round() operator in above equations is the usual rounding operator, i.e., it rounds a floating point number to the nearest integer number. it is worth observing that actual dimensions of the matrices 𝐴𝑘, �̃�𝑘 , and �̃�𝑘 are, 𝑁 × 𝑀, 𝑁 × 𝑘, and 𝑘 × 𝑀, respectively. thus, the number of elements in �̃�𝑘 and �̃�𝑘 is lower than the number of eeg samples in the matrix 𝐴𝑘. therefore, the matrices �̃�𝑘 and �̃�𝑘 can be considered as an alternative but compressed representation of the matrix 𝐴𝑘. in particular, the expected compression ratio can be derived as follows. let us indicate with 𝑤 the number of bits used to represent each eeg sample in 𝐴𝑘. considering the actual dimensions of the matrices 𝐴𝑘, the overall number of bits needed for representing the matrix 𝐴𝑘 is 𝐵𝑜 = 𝑤 ⋅ 𝑁 ⋅ 𝑀. in the same way, if we suppose that 𝑤 + 𝑎 is the maximum number of bits needed to represent the elements of �̃�𝑘 and �̃�𝑘 , the overall number of bits needed for representing the compressed matrices �̃�𝑘 and �̃�𝑘 is at most 𝐵𝑐 = (𝑤 + 𝑎) ⋅ (𝑁 + 𝑀) ⋅ 𝑘 and therefore the compression ratio can be evaluated as 𝐶𝑅 = 𝐵𝑜 𝐵𝑐 = 𝑤 ⋅ 𝑀 ⋅ 𝑁 (𝑤 + 𝑎) ⋅ (𝑁 + 𝑀) ⋅ 𝑘 . (2) in particular, when 𝑤 + 𝑎 ≈ 𝑤 and 𝑀 >> 𝑁, the expected compression ratio of the proposed algorithm can be approximated as 𝐶𝑅 ≈ 𝑁/𝑘. therefore, a considerable compression can be achieved when 𝑁 >> 𝑘, i.e., in the case of high-density eeg systems with correlated signals. for instance, in the case 𝑁 = 256 and 𝑘 = 15 we have 𝐶𝑅 ≈ 17 so that each gb of eeg data can be compressed and thus stored in less than 60 mb. in their paper, authors proved that, given the matrices �̃�𝑘 and �̃�𝑘 and the scale factor 𝑠 = 𝑚𝑋 ⋅ 𝑚𝑌 , an effective approximation �̃�𝑘 of the matrix 𝐴𝑘 is given by the following equation �̃�𝑘 = round ( �̃�𝑘 �̃�𝑘 𝑠 ) . (3) basically, the above relation provides the reconstruction equation needed for decompression. experimental results reported in [32] have shown that the maximum absolute error 𝑀𝐴𝐸 = |𝐴𝑘 − �̃�𝑘 | introduced by the above approximation is bounded by 𝑀𝐴𝐸 ≤ 2, that is a negligible error in comparison to the actual range of the original eeg samples, i.e. [−2𝑤−1, +2𝑤−1 − 1]. 3. proposed algorithm in this section, we slightly modify the previous algorithm with the aim of: 1) improving the compression ratio; 2) parameterizing the algorithm. in particular, we derived a new version of the algorithm able to achieve different trade-offs between compression efficiency and distortion. basically, the new algorithm exploits the fact that consecutive values in the matrix �̃�𝑘 are highly correlated. therefore, a further reduction in the number of bits, and thus an increasing in the compression ratio, can be obtained by encoding the differences between consecutive values in �̃�𝑘 instead of the matrix �̃�𝑘 itself. more precisely, let us introduce the matrix �̃�𝑌𝑘 = [(�̃�𝑘 [1, : ]) 𝑇 ; diff(�̃�𝑘 𝑇 )] , (4) where diff() returns the matrix of differences along the first dimension. it is worth observing that the matrix �̃�𝑘 can be exactly recovered from �̃�𝑌𝑘 as �̃�𝑘 = cumsum(�̃�𝑌𝑘 ) 𝑇 , (5) where cumsum() is the cumulative sum of elements along the first dimension. therefore, no further losses are introduced if, instead of the matrix �̃�𝑘, the matrix of differences �̃�𝑌𝑘 is stored or transmitted. on the basis of the previous observation, a new compression algorithm for eeg has been derived and can be summarized as shown in figure 2. note that, in comparison to the previous algorithm, we introduced a new step (see step 5), highlighted in bold for the sake of readability. reconstruction, i.e., decompression, can be easily achieved by obtaining �̃�𝑘 with (5) and thus using again (3) to recover �̃�𝑘. figure 1. illustration of the compression algorithm proposed in [32]. figure 2. illustration of the proposed compression algorithm. • inputs: an integer number 𝑘 < 𝑁 and a matrix 𝐴, formed by 𝑁 × 𝑀 eeg samples; • outputs: integer matrices �̃�𝑘 and �̃�𝑘 and the scale factor 𝑠 = 𝑚𝑋 ⋅ 𝑚𝑌. • algorithm: 1) use svd to decompose 𝐴 as 𝐴 = 𝑈𝛴𝑉𝑇 2) obtain the matrices 𝑆 = 𝛴1/2, 𝑋𝑘 = (𝑈𝑆)[: ,1: 𝑘] and 𝑌𝑘 = (𝑆𝑉 𝑇 )[1: 𝑘, : ] 3) evaluate 𝑚𝑋 = max(|𝑋𝑘 |), 𝑚𝑌 = max(|𝑌𝑘 |) 4) calculate 𝑠 = 𝑚𝑋 ⋅ 𝑚𝑌, �̃�𝑘 = round(𝑚𝑌 ⋅ 𝑋𝑘 ) and �̃�𝑘 = round(𝑚𝑋 ⋅ 𝑌𝑘 ) • inputs: an integer number 𝑘 < 𝑁, the matrix 𝐴 formed by 𝑁 × 𝑀 eeg samples and a scale factor 𝐹; • outputs: integer matrices �̃�𝑘 and 𝐷�̃�𝑘 and the scale factor 𝑠 = 𝑚𝑋 ⋅ 𝑚𝑌. • algorithm: 1) use svd to decompose 𝐴 as 𝐴 = 𝑈𝛴𝑉𝑇 2) obtain the matrices 𝑆 = 𝛴1/2, 𝑋𝑘 = (𝑈𝑆)[: ,1: 𝑘] and 𝑌𝑘 = (𝑆𝑉 𝑇 )[1: 𝑘, : ] 3) 𝑚𝑋 = max(|𝑋𝑘 |)/𝐹, 𝑚𝑌 = max(|𝑌𝑘 |)/𝐹 4) calculate 𝑠 = 𝑚𝑋 ⋅ 𝑚𝑌, �̃�𝑘 = round(𝑚𝑌 ⋅ 𝑋𝑘 ) and �̃�𝑘 = round(𝑚𝑋 ⋅ 𝑌𝑘 ) 5) calculate the matrix �̃�𝑌𝑘 = [(�̃�𝑘 [1, : ]) 𝑇 ; diff(�̃�𝑘 𝑇 )] acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 4 note that in the new algorithm we introduced a new input parameter, i.e., the scale factor 𝐹, which can be used to achieve different tradeoffs between compression efficiency and distortion. in particular, the factor 𝐹 is exploited for reducing 𝑚𝑋 and 𝑚𝑌 (see step 3 in figure 2). it is worth nothing that 𝑚𝑋 and 𝑚𝑌 in the new and previous algorithms assume the same values when 𝐹 = 1. therefore, intuitively and as confirmed in experimental results reported in the next section, we have no difference in the distortion achieved by the two algorithms when 𝐹 = 1. instead, by choosing a value of 𝐹 greater than 1, it is possible to achieve a greater compression ratio. this can be easily justified by observing that, according to (1), by reducing 𝑚𝑋 and 𝑚𝑌 we further reduce the dynamic range of the elements in the matrices �̃�𝑘 and �̃�𝑘 and thus the number of bits that are needed for their representation. obviously, a greater compression ratio is obtained at the cost of a greater distortion. nevertheless, experimental results reported in the next section show that the proposed algorithm improves the compression ratio by about 10 % by achieving the same distortion level of our previous algorithm, i.e., 0.01 %. moreover, a substantial increase in the compression ratio, up to 50 %, can be achieved by still maintaining the distortion level below the 0.1 %. 4. measurement-based results the proposed compression algorithm is applied to a dataset containing real eeg signals, which have been preprocessed by eeg experts to denoise and remove artifacts. the eeg dataset under study has been provided by cuny school of medicine (new york, ny, usa). this dataset refers to awake eeg recordings from the research work published in [25]. in this study, tatti et al. have investigated the role of beta oscillations (13.5-25 hz) in the sensorimotor system in a group of healthy individuals. in this experiment, participants were asked to perform planar reaching movements (mov test). mov test required the participants to reach a target, located at different distances and directions, that appeared on a screen in non-repeating and unpredictable order at 3 seconds interval. participants were asked to make reaching movements by moving a cursor on a digitizing tablet with their right hand to targets appearing on the screen. the total testing time was approximately five to six minutes for each eeg recording (96 targets). each mov test was measured with a 256-channel high-density eeg system (hydrocel geodesic sensor net, hcgsn, produced by electrical geodesic inc., eugene, or, usa), amplified using a net amp 300 amplifier, and sampled at 250 hz with 16-bit resolution using the net station software (version 5.0). eeg was noninvasively recorded using scalp electrodes and electrode-skin impedances were kept lower than 50 k. the eeglab toolbox (v13.6.5b) for matlab (v.2016b) was used for off-line preprocessing of the gathered eeg data [33], [34]. the signal of each recording was first filtered using a finite impulse response (fir) bandpass filter with a passband that extends from 1 hz to 80 hz and notch filtered at 60 hz. then, each recording was segmented in 4-seconds epochs and visually examined to remove sporadic artifacts and channels with poor signal quality. moreover, ica with pca-based dimension reduction (max 108 components) was employed to identify stereotypical artifacts (e.g., ocular, muscle, and electrocardiographic artifacts). only ica components with specific activity patterns and component maps characteristic of artefactual activity were removed. electrodes with poor signal quality were reconstructed with spherical spline interpolation procedures, whereas those located on the cheeks and neck were excluded, resulting in 180 signals. after the preprocessing, all signals were re-referenced to their initial average values and processed eeg data were exported in the european data format (edf) [35] by means of the eeglab toolbox. in particular, with the aim of evaluating the performance of the proposed algorithm, six edf files, which are related to three subjects (labelled with the subject numbers sn_m2, sn_m4 and sn_m5) and two sets of mov tests for each subject (“alltrials_1” and “alltrials_4”), have been tested. data range, number of samples, and a few other information of the above mentioned edf files are reported in table 1. it is worth observing that samples are represented with 16-bit integer numbers, i.e., 𝑤 = 16, and that the overall number of samples exploited for tests is more than 80 ⋅ 106. in order to apply the proposed algorithm, each edf file has been read and related data have been processed in blocks of 𝑁 × 𝑀 samples, where 𝑁 has been chosen equal to 180, i.e., 𝑁 coincides with the number of eeg channels remained after the preprecessing phase, and 𝑀 has been fixed equal to 1,000. so that each block of samples represents 4 seconds of data recorded by the multichannel eeg systems. the proposed compression algorithm, i.e., the algorithm in figure 2, has been applied to each block and the average compression ratio have been evaluated according to the relation: 𝐶𝑅𝐹 = 1 𝐿 ∑ 𝐵𝑜,𝑖 𝐵𝑐,𝑖 𝐿 𝑖=1 , (6) where 𝐿 represents the number of blocks processed, 𝐵𝑜,𝑖 is the number of bits that are needed to represent the 𝑖-th block before compression, and 𝐵𝑐,𝑖 is the number of bits that are needed to represent the same block but after compression. note that we use the subscript 𝐹 to highlight the scale factor used for compression, e.g., 𝐶𝑅2 is the compression ratio achieved when 𝐹 = 2. when the scale factor is not expressly stated we assume 𝐹 = 1. subsequently, each compressed block has been reconstructed according to (5) and (3). finally, distortion metrics, i.e., 𝑃𝑅𝐷 and 𝑀𝐴𝐸, have been evaluated on the whole edf file using the following equations: 𝑃𝑅𝐷 = 100 ⋅ √ ∑ ∑ (𝑀⋅𝐿𝑗=1 𝑁 𝑖=1 𝑎𝑖𝑗 − �̃�𝑖𝑗 ) 2 ∑ ∑ 𝑎𝑖𝑗 2𝑀⋅𝐿 𝑗=1 𝑁 𝑖=1 (7) 𝑀𝐴𝐸 = max 𝑖,𝑗 |𝑎𝑖𝑗 − �̃�𝑖𝑗 | , (8) where �̃�𝑖𝑗 are the integer values obtained after reconstruction and 𝑎𝑖𝑗 are the original samples. in our experiments, we further evaluated the compression efficiency of the proposed algorithm with respect to the nearlossless compression algorithm proposed in [32]. in particular, compression efficiency (𝐶𝐸) is here defined as: acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 5 𝐶𝐸 = 100 ⋅ 𝐶𝑅𝐹 − 𝐶𝑅0 𝐶𝑅0 , (9) where 𝐶𝑅0 is the compression ratio obtained with the algorithm proposed in [32], i.e., the algorithm reported in figure 1. similarly, we use 𝑃𝑅𝐷0 and 𝑀𝐴𝐸0 for referring related distortion metrics. compression results achieved with the proposed compression algorithm by setting the scale factor 𝐹 = 1 are shown in table 2. more precisely, for each compressed file, we reported the number of singular values exploited for compression (𝑘), the compression ratio (𝐶𝑅1), values of the distortion metrics (𝑃𝑅𝐷 and 𝑀𝐴𝐸), and the compression efficiency (𝐶𝐸) of the proposed algorithm and, in brackets, corresponding compression results obtained with the algorithm proposed in [32], evaluated on the same files and considering the same number of singular values. as it is possible to observe, the compression ratio achieved by the proposed algorithm when 𝐹 = 1 is near 𝑁/𝑘, which confirms our analytical results reported in section iii. note that the prd is less than 0.01 % for all edf files tested. in particular, prd values obtained with 𝐹 = 1 are the same values obtained in [32]. the same consideration can be extended to the mae. this confirms that the two algorithms in figure 2 and figure 1 have the same performance in terms of distortion when 𝐹 = 1. however, by observing the results on compression efficiency (see the last column of table 2), it is possible to conclude that, in comparison to the algorithm proposed in [32], the new one proposed in this paper is able to improve the compression ratio in a range between 7 % and 9 %. moreover, the scale factor 𝐹 introduced in the new algorithm provides the possibility to achieve even higher compression ratios, obviously at the cost of a greater distortion. we investigated the trade-off between compression efficiency and distortion of the proposed algorithm by considering different values of the scale factor 𝐹 within the range [1, 16]. in particular, we reported in table 3 compression ratios (𝐶𝑅𝐹), distortion metrics (𝑃𝑅𝐷 and 𝑀𝐴𝐸), and compression efficiency (𝐶𝐸) corresponding to 𝐹 ∈ {1,2,4,8,16}. as can be observed in table 3, by increasing 𝐹 it is possible to improve the compression ratio and thus the compression efficiency. in particular, by fixing 𝐹 = 16, the proposed algorithm is able to improve the compression ratio by about the 50 % by maintaining the 𝑃𝑅𝐷 below the 0.1 % threshold (in fact the 𝑃𝑅𝐷 is at most equal to 0.081 % for all the files tested). it is also worth noting that the mae obtained in our experimental results is approximatively equal to 2 𝐹. finally, we evaluated the distribution of absolute errors in recovered signals. in figure 3 we reported the cumulative distribution function (cdf) of the absolute errors, i.e., the probability 𝑃(|𝑒𝑟𝑟| ≤ 𝑥) table 1. edf files used as dataset. file name channels duration (s) number of samples physical range (μv) data range sn_m2_alltrials_1 180 344 15 480 000 [-35.500, +30.314] [-32768, 32767] sn_m2_alltrials_4 180 340 15 300 000 [-48.550, +38.539] [-32768, 32767] sn_m4_alltrials_1 180 328 14 760 000 [-34.532, +34.756] [-32768, 32767] sn_m4_alltrials_4 180 264 11 880 000 [-41.100, +40.673] [-32768, 32767] sn_m5_alltrials_1 180 324 14 580 000 [-41.463, +38.867] [-32768, 32767] sn_m5_alltrials_4 180 308 13 860 000 [-41.347, +46.929] [-32768, 32767] table 2. compression ratio (cr), percent root-mean-square distortion (prd), maximum absolute error (mae), and compression efficiency (ce) of the proposed algorithm when f = 1 (values of cr0 and prd0 are reported in brackets). file name k cr1 prd (%) mae ce (%) sn_m2_alltrials_1 20 8.6 (7.9) 0.0065 (0.0065) 2 (2) 8.6 sn_m2_alltrials_4 20 8.6 (7.9) 0.0063 (0.0063) 2 (2) 8.9 sn_m4_alltrials_1 12 14.6 (13.6) 0.0075 (0.0075) 2 (2) 7.4 sn_m4_alltrials_4 12 14.4 (13.4) 0.0069 (0.0069) 2 (2) 7.5 sn_m5_alltrials_1 13 13.4 (12.5) 0.0071 (0.0071) 2 (2) 7.2 sn_m5_alltrials_4 13 13.2 (12.3) 0.0069 (0.0069) 2 (2) 7.3 table 3. compression results achieved with the proposed algorithm for different values of the scale factor f. file name f crf prd (%) mae ce (%) sn_m2_alltrials_1 1 8.58 0.006 2 8.6 2 9.23 0.010 4 16.8 4 9.98 0.018 8 26.3 8 10.87 0.035 16 37.6 16 11.94 0.069 32 51.1 sn_m2_alltrials_4 1 8.60 0.006 2 8.9 2 9.25 0.010 4 17.1 4 10.01 0.017 8 26.7 8 10.91 0.033 17 38.1 16 11.98 0.066 31 51.6 sn_m4_alltrials_1 1 14.55 0.007 2 7.4 2 15.68 0.012 5 15.7 4 16.99 0.021 7 25.4 8 18.53 0.041 15 36.8 16 20.39 0.081 34 50.5 sn_m4_alltrials_4 1 14.43 0.007 2 7.5 2 15.53 0.011 4 15.7 4 16.81 0.019 8 25.3 8 18.33 0.036 16 36.6 16 20.14 0.072 31 50.1 sn_m5_alltrials_1 1 13.42 0.007 2 7.2 2 14.45 0.011 5 15.4 4 15.66 0.020 8 25.1 8 17.08 0.039 20 36.4 16 18.79 0.077 34 50.1 sn_m5_alltrials_4 1 13.16 0.007 2 7.3 2 14.16 0.010 4 15.5 4 15.31 0.019 9 24.9 8 16.67 0.037 16 36.0 16 18.30 0.073 35 49.3 acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 6 that the absolute error (|𝑒𝑟𝑟|) is lower than a threshold 𝑥, achieved for different values of 𝐹. as can be observed in figure 3, the percentage of samples with an absolute error lower than 𝐹 after reconstruction is near to 100 % for all the edf files in the dataset. note that the vertical lines in figure 3 represent the condition 𝑃(|𝑒𝑟𝑟| ≤ 𝐹)). therefore, we can state that the scale factor 𝐹, which is needed as input in the proposed algorithm, can be fixed according to the desired mae, i.e., for a given value of 𝐹, the mae obtained after reconstruction will be, with high probability, within the range [𝐹, 2𝐹]. 5. conclusions in this paper, we developed and validated an improved version of a recently proposed near-lossless compression algorithm for multichannel eeg signals. the algorithm exploits the fact that svd is usually performed on eeg signals for artifacts removal or denoising tasks. experimental results, reported in this paper, show that the developed algorithm is able to achieve a compression ratio proportional to the number of eeg channels with a root-mean-square distortion less than 0.01 %. moreover, with proper settings of input parameters, the compression ratio can be further improved up to 50 % by maintaining the distortion level below the 0.1 %. moreover, the algorithm allows the desired maximum absolute error to be fixed a priori. it should be highlighted that, although an eeg dataset has been considered as a case study, the proposed compression algorithm can be quite straightforwardly applied to different types of dataset. in a future work, we will further investigate performance of the proposed algorithm considering more extended datasets and other types of signals. references [1] h. berger, uber das 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10.1016/0013-4694(92)90009-7 https://doi.org/10.1088/1741-2552/aaf12e https://doi.org/10.3389%2ffneur.2012.00077 https://doi.org/10.1016%2fj.jneumeth.2009.03.022 https://doi.org/10.1109/tim.2019.2900141 https://doi.org/10.23919/eusipco.2018.8553597 https://doi.org/10.1109/memb.2010.936545 https://doi.org/10.1016/j.jnca.2014.09.013 https://doi.org/10.1016/j.bspc.2011.06.007 https://doi.org/10.1109/iscas.2006.1692909 https://doi.org/10.1109/10.552239 https://doi.org/10.1109/titb.2007.899497 https://doi.org/10.23919/eusipco.2017.8081677 https://doi.org/10.1016/j.bspc.2011.01.004 https://doi.org/10.1097/00004691-200203000-00002 https://doi.org/10.1142/s0129065712500244 https://doi.org/10.3389/fnsys.2020.00061 https://doi.org/10.1016/j.neucli.2016.07.002 https://doi.org/10.1016/j.clinph.2003.10.018 https://doi.org/10.1109/iccc.2013.6731680 https://doi.org/10.1109/titb.2012.2230012 https://doi.org/10.1016/j.bspc.2015.04.001 https://doi.org/10.1109/scored.2019.8896252 https://doi.org/10.1109/memea52024.2021.9478756 https://doi.org/10.1016/j.jneumeth.2003.10.009 https://doi.org/10.1016/j.tics.2004.03.008 https://doi.org/10.1016/0013-4694(92)90009-7 on the design and characterisation of a microwave microstrip resonator for gas sensing applications acta imeko issn: 2221-870x june 2021, volume 10, number 2, 54 61 acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 54 on the design and characterisation of a microwave microstrip resonator for gas sensing applications giovanni gugliandolo1, davide aloisio2, giuseppe campobello1, giovanni crupi3, nicola donato1 1 department of engineering, university of messina, italy 2 cnr itae, messina, italy 3 biomorf department, university of messina, italy section: research paper keywords: microwaves; resonators; gas sensors; metrological evaluation, humidity citation: giovanni gugliandolo, davide aloisio, giuseppe campobello, giovanni crupi, nicola donato, on the design and characterisation of a microwave microstrip resonator for gas sensing applications, acta imeko, vol. 10, no. 2, article 9, june 2021, identifier: imeko-acta-10 (2021)-02-09 section editor: ciro spataro, university of palermo, italy received january 17, 2021; in final form may 4, 2021; published june 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: giovanni gugliandolo, e-mail: giovanni.gugliandolo@unime.it 1. introduction nowadays, the research interests in the development of sensors with extremely low-power consumption is growing because of the increasingly energy-saving requirements of the expanding market. this can be seen by the recent high demand of portable battery powered devices often used in wireless sensor networks (wsns) for industrial (e.g., harmful gas detection) [1], [2], healthcare (e.g., wearable or implantable devices) [3]-[6], and environmental (e.g., weather forecast) [7]-[10] monitoring applications. several sensor typologies have been investigated in order to achieve the best trade-off between performance and power consumption with a focus on size, weight, and production costs. in this context, microwave devices are considered as an attractive solution thanks to their interesting features in terms of cost, power consumption, and response time. they have been employed for materials characterization [11]-[13] as well as for gas sensing applications [14]. microwave gas sensors have the ability to operate at room temperature without the need of a heater [15], [16]. moreover, they are fully compatible with wireless technology so that they can be easily integrated into wireless smart nodes [17]-[19]. in particular, the planar microstrip technology is widely employed in the fabrication of microwave components like antennas, filters, and resonators. such devices are often used in sensing applications because of their low cost, easy fabrication, and good performance [20]-[24]. the microwave microstrip sensors are attractive especially for gas sensing applications, where the frequency-dependent dielectric properties of the sensing material are related with the adsorption of the target gas of interest on the sensing layer, deposited on the microstrip propagative structure. the progress in nanotechnologies has enabled advancements in the use of gas sensors using nanostructured materials as sensing layers [14], [25]-[27]. abstract this study focuses on the microwave characterisation of a microstrip resonator aimed for gas sensing applications. the developed oneport microstrip resonator, consisting of three concentric rings with a central disk, is coupled to a 50-ω microstrip feedline through a small gap. a humidity sensing layer is deposited on this gap by drop-coating an aqueous solution of ag@α-fe2o3 nanocomposite. the operation principle of the developed humidity sensor is based on the change of the dielectric properties of the ag@α-fe2o3 nanocomposite when the relative humidity is varied. however, it should be underlined that, depending on the choice of the sensing material, different target gases of interest can be detected with the proposed structure. the frequency-dependent response of the sensor is obtained using the reflection coefficient measured from 3.5 ghz to 5.6 ghz, with relative humidity ranging from 0 %rh to 83 %rh. the variation of the humidity concentration strongly impacts on the two resonances detected in the measured reflection coefficient. in particular, an increase of the humidity level leads to lowering both resonant frequencies, which can be used as sensing parameters for humidity monitoring purpose. an exponential function has been used to accurately model the two resonant frequencies as a function of the humidity. mailto:giovanni.gugliandolo@unime.it acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 55 following on from the results of our previous study [28], we present here a thorough investigation of a one-port gas transducer based on a microwave microstrip resonator, which is validated as humidity sensor by using an ag@α-fe2o3 nanocomposite as a sensing material. the experimental-based investigation is performed by focusing the analysis on both magnitude and phase of the reflection coefficient ( ) and its corresponding impedance (z). in particular, we monitored the relative humidity over the broad range going from 0% to 83% at room temperature, and by assessing the sensing performance of the developed gas transducer to change in the relative humidity in terms of variations in the frequency-dependent behaviour of . as shown later in this paper, two dips are clearly visible in the magnitude of  for the proposed sensor at approximately 3.7 ghz and 5.4 ghz, and their appearance is shifted towards lower frequencies when the humidity level is increased. hence, the resonant frequencies (fr1 and fr2) associated to the two dips observed in  can be directly used as humidity sensing parameters. to this end, a sensitivity-based investigation is developed in order to assess the sensing performance of the proposed microwave sensor for humidity monitoring application. the humidity-dependent variations in the two resonant frequencies are accurately modelled by using an exponential function. this article is structured as follows. section ii is dedicated to the design of the microstrip resonator, which is based on using three concentric rings with a central disk. this choice was made after a careful analysis of the performance of different resonator topologies through computer simulations. compared to the traditional ring configuration [29], [30], the proposed topology allows improvement in the quality (q-) factor and, thus, in the detection process. section iii is devoted to the development of the humidity sensor, which is based on using an ag@α-fe2o3 nanocomposite as sensing material. it is worth noting that the high porosity of the nanostructure allows enhancement of the interaction with water vapour, thereby leading to an improved humidity sensitivity. section iv is focused on the description of the fitting of the measurements locally around the two observed resonances by using a lorentzian function. section v is dedicated to the description of the setup for frequencyand humidity-dependent characterization and to the presentation of the experimental results. finally, the conclusions are drawn in the last section. 2. resonator design and simulation the proposed gas transducer is based on a concentric rings microstrip (crm) resonator acting as a propagative structure for the electromagnetic waves. this novel topology of propagative structure is composed by three concentric copper rings with a 6mm copper central disk and a 50-ω microstrip feedline coupled to the resonator through a 0.2-mm gap. the matlab antenna toolbox was used for the design process. as illustrated in figure 1, four different resonator topologies were considered during the design step based on computer simulations over the frequency range going from 3 ghz to 6 ghz: the classic ring resonator, two concentric rings, three concentric rings, and three concentric rings with a disk in the middle. starting from the traditional configuration, the coupling gap and the ring thickness were optimized in terms of q-factor. later, additional rings were included in the design considering a constant spacing. figure 3 shows the frequencydependent behaviour of the magnitude of the simulated  for the four studied topologies. as can be observed, the computer simulations show that all investigated topologies have two resonances appearing in , which can be detected as two marked dips occurring at about 3.7 ghz (i.e., dip 1) and 5.4 ghz (i.e., dip 2), respectively. the two dips are more clearly visible in figure 1. illustration of the four studied resonator topologies: (a) traditional ring, (b) two concentric rings, (c) three concentric rings, and (d) three concentric copper rings with a central disk. 3.0 3.5 4.0 4.5 5.0 5.5 6.0 -25 -20 -15 -10 -5 0 5  ( d b ) frequency (ghz) resonator rings+dot resonator 1 ring resonator 2 rings resonator 3 rings figure 2. behaviour of the magnitude of the simulated reflection coefficient versus frequency from 3.0 ghz to 6.0 ghz for the four studied resonator topologies. 3.4 3.5 3.6 3.7 5.2 5.3 5.4 5.5 5.6 -25 -20 -15 -10 -5 0 5  ( d b ) frequency (ghz) resonator rings + disk resonator 1 ring resonator 2 rings resonator 3 rings figure 3. illustration of the two dips appearing in the magnitude of the simulated reflection coefficient for the four studied resonator topologies. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 56 figure 2, where the observation of  is limited to the two narrow frequency bands around fr1 and fr2. to assess the microwave performance of the studied resonator topologies, the quality factor improvement was evaluated by using the single-ring configuration as a reference for comparison. figure 4 shows the q-factor improvement for both resonances as a function of the number of concentric rings. it is worth noting that the selected topology (consisting of three rings with a central disk) allows achieving an improvement in the qfactor equal to 6% and 44% at fr1 and fr2, respectively. the crm resonator was fabricated on a 3.2-mm fr4 substrate [31] with copper as conductor for both top and ground layers by using the protomat s103 pcb milling machine. the dielectric constant (εr) and the loss tangent (tanδ) of the substrate are 4.3 and 0.025, respectively. an sma connector was soldered at the end of the 50-ω microstrip feedline to connect the resonator with a vector network analyzer (vna) for measuring γ. 3. sensor development to obtain the gas sensor, a sensing material was deposited on the surface of the propagative structure. in particular, an aqueous solution of ag@α-fe2o3 nanocomposite was deposited on the gap placed between the external ring and the microstrip feedline by drop coating. the description and synthesis of this humidity sensing material is reported in [32]. the effect of the sensing material deposition on the frequency-dependent behaviour of γ of the developed structure was measured from 3.5 ghz to 5.6 ghz using the agilent 8753es vna with a one-port calibration (short open load, agilent 85052 mechanical calibration kit). as shown in figure 5, both dips in γ become much more pronounced after deposition, improving the quality factor of both dips. for the sake of completeness, the real and imaginary parts of the resonator input impedance for the selected frequency ranges are reported in figure 6. 4. resonator parameters evaluation estimating the resonant frequency (fr), quality factor (q), and dip amplitude (ar) from a discrete frequency response is not a trivial task. a simple linear interpolation of the available discrete data can lead to an inaccurate estimation of these quantities, especially when the data are affected by noise. a better fitting approach consists in using a lorentzian function [33], [34], which 2 3 4 -10 0 10 20 30 40 50 dip at 3.7 ghz dip at 5.4 ghz number of rings q f a c to r im p ro v e m e n t (% ) figure 4. analysis of the quality factor improvement of two resonances observed in the simulated reflection coefficient as a function of the number of rings of the resonator structure. 3.5 3.6 3.7 3.8 5.2 5.3 5.4 5.5 5.6 -50 -40 -30 -20 -10 0 (a)  m a g n it u d e ( d b ) frequency (ghz) after deposition before deposition ranges investigated for sensing response 3.5 3.6 3.7 3.8 5.2 5.3 5.4 5.5 5.6 -200 -150 -100 -50 0 50 100 150 200 (b)  p h a s e ( ° ) frequency (ghz) after deposition before deposition ranges investigated for sensing response figure 5. behaviour of the (a) magnitude and (b) phase of the measured reflection coefficient as a function of frequency, from 3.5 ghz to 5.6 ghz, for the studied resonator before (red lines) and after (blue lines) deposition of the sensing material. 3.5 3.6 3.7 3.8 5.2 5.3 5.4 5.5 5.6 0 25 50 75 100 125 150 175 200 (a) z r e a l p a rt (  ) frequency (ghz) after deposition before deposition ranges investigated for sensing response 3.5 3.6 3.7 3.8 5.2 5.3 5.4 5.5 5.6 -200 -150 -100 -50 0 50 100 150 200 z i m a g in a ry p a rt (  ) frequency (ghz) after deposition before deposition ranges investigated for sensing response (b) figure 6. behaviour of the (a) real and (b) imaginary parts of the impedance as a function of frequency from 3.5 ghz to 5.6 ghz, for the studied resonator before (red lines) and after (blue lines) deposition of the sensing material. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 57 allows achieving a good estimation of the resonant parameters fr, q, and ar. a more accurate result can be achieved by using a complex function to fit both real and imaginary parts of the spectrum [35], [36]. this technique can be useful in several applications in which the calibration procedure is impracticable (e.g., in cryogenic measurement systems) [36]. the frequency-dependent behaviour of the magnitude of γ of the microwave resonator was modelled as a lorentzian function: |𝛤(𝑓)| = 𝑐0 − 𝑎0 𝜋 ∙ 1 2 𝐺 (𝑓 − 𝑓𝑅 ) 2 + ( 1 2 𝐺) 2 , (1) where f is the frequency, c0 and a0 are two real coefficients, and g is the full width at half maximum. from equation (1), ar and q can be calculated respectively as: 𝐴𝑅 = 𝑐0 − 𝑎0 ∙ 2 𝜋𝐺 , (2) 𝑄 = 𝑓 ∆𝑓 = 𝑓𝑅 𝐺√√2 − 1 , (3) where δf is the resonator half-power bandwidth. the levenberg-marquardt algorithm was used for fitting the measured data points with the lorentzian function. it is found that the lorentzian curve allows fitting very well the two observed resonant dips, so that it is possible to obtain a smooth behaviour of the magnitude of γ over a continuous spectrum of frequencies for the estimation of the resonant parameters. as an illustrative example, figure 7 reports the lorentzian fitting applied to the magnitude of the measured γ over a narrow frequency band around the second resonance. by using the fitting process, the parameters fr, q, and ar can be accurately estimated over the whole considered humidity range. 5. experimental results the sensor was placed in a test chamber filled with a controlled atmosphere, where the electrical signal was supplied via an rf feed-through for connection with the agilent 8753es vna (see figure 8). the test chamber consists of a modified petri dish made in polystyrene, able to provide both a controlled atmosphere and good microwave propagation avoiding signal perturbations. the developed sensor was characterized at seven different values of the relative humidity concentration, ranging from 0 %rh to 83 %rh, at room temperature. the 0 %rh nominal value was set by means of the certification of the gas bottles (0.5%). the test gas mixture was set by means of a fully automated gas control system made by a certified gas bottle and a bubbler inside a thermostatic bath. the system is equipped with an array of bronkhorst® mass flow controllers able to set a flux of 100 cm3/min in the test chamber, providing a fast set and purge for each test value of the humidity concentration. the diagram of the gas apparatus is shown in figure 9. after performing a one-port calibration, the reflection coefficient was measured at each humidity condition. figure 10 and figure 11 illustrate the impact of the relative humidity on the measured 5.4682 5.4684 5.4686 5.4688 5.4690 5.4692 5.4694 -56 -54 -52 -50  m a g n it u d e ( d b ) frequency (ghz) measurement lorentzian fitting figure 7. illustration of the lorentzian fitting (red line) of the magnitude of the measured (black line) reflection coefficient over a narrow frequency band around the second resonance for the studied resonator. figure 8. illustration of (a) the sensor prototype placed in test chamber and (b) the frequencyand humidity-dependent characterization procedure. figure 9. illustration of the automated gas control and measurement system. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 58 behaviour of the complex reflection coefficient over two narrow frequency bands around the two observed dips, which were detected at approximately 3.7 ghz and 5.4 ghz. it can be seen that the size and the shape of the dips change significantly with the humidity values. it should be mentioned that humidity-dependent variations are observed in all the three parameters fr, q-factor, and ar for both resonances. nevertheless, q-factor and ar do not follow a clear monotonic trend (see figure 12 and figure 13). on the other hand, it is worth noting that both resonant frequencies decrease with increasing the humidity level (see figure 14), thereby enabling the use of the two resonant frequencies as humidity sensing parameters. with the aim to evaluate the humidity sensing performance of the developed gas transducer for the whole investigated humidity range, we used an exponential function to fit the two resonant frequencies as a function of humidity: 𝑓𝑅 = 𝐴 ∙ e (− 𝑅𝐻 𝐵 ) + 𝐶, (4) where fr represents the considered resonant frequency, rh is the relative humidity value, a, b, and c are the fitting parameters. the calibration curve for both dip 1 and dip 2 is depicted in figure 14(a); in table 1 the fitting parameters are reported, while the calibration fit residuals are shown in figure 14(b). for dip 1 3.696 3.698 3.700 3.702 3.704 -60 -55 -50 -45 -40 -35 -30 -25 -20  m a g n it u d e ( d b ) frequency (ghz) 0 % rh 22 % rh 28 % rh 39 % rh 54 % rh 74 % rh 83 % rh increasing humidity (a) 3.696 3.698 3.700 3.702 3.704 -40 -20 0 20 40 60 80 100 120 140 160  p h a s e ( ° ) frequency (ghz) 0 % rh 22 % rh 28 % rh 39 % rh 54 % rh 74 % rh 83 % rh increasing humidity (b) figure 10. behaviour of the (a) magnitude and (b) phase of the measured reflection coefficient over a narrow frequency band around the first resonance for the studied resonator, for seven relative humidity values. 5.463 5.466 5.469 5.472 5.475 -65 -60 -55 -50 -45 -40 -35 -30 -25 -20 (a)  m a g n it u d e ( d b ) frequency (ghz) 0 % rh 22 % rh 28 % rh 39 % rh 54 % rh 74 % rh 83 % rh increasing humidity 5.463 5.466 5.469 5.472 5.475 -160 -140 -120 -100 -80 -60 -40 -20 0 20 40  p h a s e ( ° ) frequency (ghz) 0 % rh 22 % rh 28 % rh 39 % rh 54 % rh 74 % rh 83 % rh increasing humidity (b) figure 11. behaviour of the (a) magnitude and (b) phase of the measured reflection coefficient over a narrow frequency band around the second resonance for the studied resonator at seven relative humidity values. 0 20 40 60 80 2000 3000 4000 5000 6000 7000 dip 1 @ 3.7 ghz dip 2 @ 5.4 ghz relative humidity (%) q f a c to r 2000 3000 4000 5000 6000 7000 q fa c to r figure 12. analysis of the quality factor of two resonances observed in the measured reflection coefficient of the resonator as a function of the humidity. 0 20 40 60 80 -56 -54 -52 -50 dip 1 @ 3.7 ghz dip 2 @ 5.4 ghz relative humidity (%)  m in ( d b ) -62 -60 -58 -56 -54 -52 -50  m in (d b ) figure 13. magnitude of the measured reflection coefficient of the resonator at the first (black) and second (blue) resonances as a function of the humidity. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 59 residuals are almost within ± 200 khz that, considering an absolute sensitivity of 26.4 khz/%rh, corresponds to ± 7.6 %rh. on the other hand, dip 2 exhibits a higher sensitivity (29.3 khz/%rh) with lower calibration fit residuals in comparison to dip 1: ± 100 khz, or ± 3.4 %rh. as an alternative, it is possible to use both dips for humidity detection, thereby reducing the measurement error and increasing accuracy [37]. for the sake of completeness, the impact of the humidity variations is reported also for the impedance associated to the measured , focusing on the two narrow frequency bands around the two dips. figure 15 and figure 16 show that a higher humidity implies that the real part decreases close to dip 1 and then increases close to dip 2, whereas the imaginary part is shifted towards higher values in both frequency bands. 6. conclusions a one-port microwave gas transducer was developed by coupling a microstrip resonator for electromagnetic wave propagation with an ag@α-fe2o3 nanocomposite for humidity 0 20 40 60 80 -300 -200 -100 0 100 200 300 (b) c a li b ra ti o n f it r e s id u a ls ( k h z ) relative humidity (% rh) dip 1 @ 3.7 ghz dip 2 @ 5.4 ghz figure 14. calibration curve for both dips (a) and calibration fit residuals (b). table 1. fitting parameters values with standard errors for the two dips observed. parameter dip 1 dip 2 value standard error value standard error a (mhz) 2.68 0.302 2.92 0.104 b (%rh) 37.21 10.349 28.18 2.485 c (mhz) 3699.51 0.3043 5467.92 0.088 r2 = 0.994 r2 = 0.956 3.694 3.696 3.698 3.700 3.702 3.704 3.706 46 48 50 52 54 56 58 (a) z r e a l p a rt (  ) frequency (ghz) 0 % rh 22 % rh 28 % rh 39 % rh 54 % rh 74 % rh 83 % rh increasing humidity 3.694 3.696 3.698 3.700 3.702 3.704 3.706 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 (b) z i m a g in a ry p a rt (  ) frequency (ghz) 0 % rh 22 % rh 28 % rh 39 % rh 54 % rh 74 % rh 83 % rh increasing humidity figure 15. behaviour of the (a) real and (b) imaginary parts of the measured impedance over a narrow frequency band around the first resonance for the studied resonator, for seven relative humidity values. 5.460 5.463 5.466 5.469 5.472 5.475 45 46 47 48 49 50 51 52 53 54 (a) z r e a l p a rt (  ) frequency (ghz) 0 % rh 22 % rh 28 % rh 39 % rh 54 % rh 74 % rh 83 % rh increasing humidity 5.460 5.463 5.466 5.469 5.472 5.475 -3 -2 -1 0 1 2 (b) z i m a g in a ry p a rt (  ) frequency (ghz) 0 % rh 22 % rh 28 % rh 39 % rh 54 % rh 74 % rh 83 % rh increasing humidity figure 16. behaviour of the (a) real and (b) imaginary parts of the measured impedance over a narrow frequency band around the second resonance for the studied resonator, for seven relative humidity values. 0 20 40 60 80 3.698 3.699 3.700 3.701 3.702 3.703 (a) dip 1 @ 3.7 ghz dip 2 @ 5.4 ghz fitting relative humidity (% rh) f re q u e n c y (g h z ) 5.467 5.468 5.469 5.470 5.471 5.472 f re q u e n c y (g h z ) acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 60 monitoring purpose. the sensing performance of this prototype was established by monitoring relative humidity from 0 %rh to 85 %rh at room temperature. to this end, the sensor was placed in a test chamber consisting of a modified petri dish made in polystyrene. by using a vna, the reflection coefficient was measured over the 3.5 ghz … 5.6 ghz frequency range, under seven different conditions of relative humidity. it was observed that the frequency-dependent behaviour of the reflection coefficient exhibits two marked dips that change in intensity, broadness, and location when the relative humidity is varied. in particular, 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https://www.imeko.org/publications/tc4-2020/imeko-tc4-2020-16.pdf https://www.imeko.org/publications/tc4-2020/imeko-tc4-2020-16.pdf https://www.radioeng.cz/fulltexts/2014/14_04_1038_1047.pdf https://doi.org/10.1002/mop.29321 https://doi.org/10.1049/ic:19980078 https://doi.org/10.3390/nano5020737 https://doi.org/10.1063/1.368498 https://doi.org/10.1109/temc.2005.847411 https://doi.org/10.3390/s21082844 https://doi.org/10.21014/acta_imeko.v9i3.782 https://doi.org/10.1016/j.measurement.2020.107805 journal contacts acta imeko issn: 2221-870x march 2022, volume 11, number 1 acta imeko | www.imeko.org march 2022 | volume 11 | number 1 journal contacts about the journal acta imeko is an e-journal reporting on the contributions on the state and progress of the science and technology of measurement. the articles are mainly based on presentations presented at imeko workshops, symposia and congresses. the journal is published by imeko, the international measurement confederation. the issn, the international identifier for serials, is 2221-870x. about imeko the international measurement confederation, imeko, is an international federation of actually 42 national member organisations individually concerned with the advancement of measurement technology. its fundamental objectives are the promotion of international interchange of scientific and technical information in the field of measurement, and the enhancement of international co-operation among scientists and engineers from research and industry. addresses principal contact prof. francesco lamonaca university of calabria department of computer science, modelling, electronic and system science via p. bucci, 41c, vi floor, arcavacata di rende, 87036 (cs), italy e-mail: editorinchief.actaimeko@hunmeko.org support contact dr. dirk röske physikalisch-technische bundesanstalt (ptb) bundesallee 100, 38116 braunschweig, germany e-mail: dirk.roeske@ptb.de editor‐in‐chief francesco lamonaca, italy founding editor‐in‐chief paul p. l. regtien, netherlands associate editor dirk röske, germany copy editors egidio de benedetto, italy silvia sangiovanni, italy layout editors dirk röske, germany leonardo iannucci, italy domenico luca carnì, italy editorial board leopoldo angrisani, italy filippo attivissimo, italy eulalia balestieri, italy eric benoit, france paolo carbone, italy lorenzo ciani, italy catalin damian, romania pasquale daponte, italy luca de vito, italy luigi ferrigno, italy edoardo fiorucci, italy alistair forbes, united kingdom helena geirinhas ramos, portugal sabrina grassini, italy fernando janeiro, portugal konrad jedrzejewski, poland andy knott, united kingdom yasuharu koike, japan francesco lamonaca, italy massimo lazzaroni, italy fabio leccese, italy rosario morello, italy michele norgia, italy franco pavese, italy pedro miguel pinto ramos, portugal nicola pompeo, italy sergio rapuano, italy gustavo ripper, brazil maik rosenberger, germany dirk röske, germany alexandru salceanu, romania constantin sarmasanu, romania lorenzo scalise, italy emiliano schena, italy enrico silva, italy krzysztof stepien, poland ronald summers, uk marco tarabini, italy section editors (vol. 7 11) yvan baudoin, belgium piotr bilski, poland francesco bonavolonta, italy giuseppe caravello, italy carlo carobbi, italy marcantonio catelani, italy mauro d’arco, italy egidio de benedeto, italy alessandro depari, italy alessandro germak, italy istván harmati, hungary min-seok kim, korea bálint kiss, hungary momoko kojima, japan koji ogushi, japan vilmos palfi, hungary jeerasak pitakarnnop, thailand md zia ur rahman, india fabio santaniello, italy jan saliga, slovakia emiliano sisinni, italy ciro spataro, italy oscar tamburis, italy jorge c. torres-guzman, mexico ioan tudosa, italy ian veldman, south africa rugkanawan wongpithayadisai, thailand claudia zoani, italy mailto:editorinchief.actaimeko@hunmeko.org mailto:dirk.roeske@ptb.de experiment assisting system with local augmented body (easy-lab) in dual presence environment acta imeko issn: 2221-870x september 2022, volume 11, number 3, 1 6 acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 1 experiment assisting system with local augmented body (easy-lab) in dual presence environment ahmed alsereidi1, yukiko iwasaki1, joi oh2, vitvasin vimolmongkolporn2, fumihiro kato2, hiroyasu iwata3 1 waseda university, graduate school of creative science and engineering, tokyo, japan 2 waseda university, global robot academic institute, tokyo, japan 3 waseda university, faculty of science and engineering, tokyo, japan section: research paper keywords: vr/ar; hands-free interface; telecommunication; teleoperation citation: ahmed alsereidi, yukiko iwasaki, joi oh, vitvasin vimolmongkolporn, fumihiro kato, hiroyasu iwata, experiment assisting system with local augmented body (easy-lab) in dual presence environment, acta imeko, vol. 11, no. 3, article 3, september 2022, identifier: imeko-acta-11 (2022)-03-03 section editor: zafar taqvi, usa received february 26, 2022; in final form july 21, 2022; published september 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: ahmed alsereidi, e-mail: ahmed@akane.waseda.jp 1. introduction there has been a notable amount of research and development in telepresence robotics, with the occurrence of covid, telepresence robotics has been used to improve subject experiments. in previous studies, researchers and engineers frequently ask individuals to complete a task, watch their behaviour, or evaluate the usability of new technologies. as researchers, we often need to observe how a subject act in an experiment and provide guidance and instructions during an experiment. easy-lab system was designed to allow such experiments to be carried from a remote location. the designed system uses a 6 dofs robotic head that utilizes differential gears to imitate human head-waist motion. the neck, waist, and detachable mechanism make up the mechanical design of the detachable robotic head. also, the maximum latency between the user and the robot is 25 ms, which is low enough for human perception [1]. this was verified by comparing the human head motion and robot head motion side by side, both moved identically as shown in figure 1. figure 1. detachable head robot used for testing. abstract this paper introduces a system designed to support conducting experiments of subjects when the situation does not allow experimenter and subject to be in the same place such as the covid19 pandemic where everyone relied on video conference applications which has its limitation. due to the difficulty of directing with a video conferencing system using solely video and voice. the system we developed allows an experimenter to actively watch and interact with the subject. even if you're operating from a distant area, it is still possible to conduct experiments. another important aspect this study will focus on is the case of when there are several subjects required and the experimenter must be able to guide both subjects equally well. the system proposed uses a 6 dof robotic arm with a camera and a laser pointer attached to it on the subject side. the experimenter uses a head-mounted display to control it and it moves corresponding to the head movement allowing for easy instruction and intervention to the subject side. comparison with other similar research is also covered. the study will focus mainly on which viewing method is the easiest for the experimenter to use, and if teaching one subject at the time gives better results than teaching two subjects simultaneously. mailto:ahmed@akane.waseda.jp acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 2 this system can also intervene using a laser pointer to point at the object being worked on. the joint angles of the robot are calculated by ik from the acquired head movements of the experimenter and reflected in the real robot. photon unity networking was used to sync the motion of the experimenter and the robot remotely [2]. photon unity networking is a package usually used for multiplayer games due to its flexible matchmaking where objects can be synced over the network. figure 2 shows the system diagram of how the system sends and receives data. another point to consider is that the system must be user friendly for most people to use. tele-operated robots have been making advances in this field, as well as full-body immersion systems to imitate the human motion [3], [4]. those systems focus on immersion and are lacking in terms of usability as they are heavy and are difficult to manoeuvre. there are some cases where multiple subjects are required for a certain experiment, in this case multiple robots would be required, at least one robot per subject. there are several requirements to fulfil for this system. first, instructions given to the subjects should be transmitted correctly in as little time as possible. second, the experimenter must have clear visibility to both subject’s surroundings, third, allow the experimenter to be present in several locations at the same time (dual presence). to create a system that allows users to freely switch and reallocate their attention. in telepresence systems, to fully immerse a user in a remote environment, it is preferable that the user devotes his or her undivided attention to it. teleoperation work efficiency improves when the system delivers a greater sensation of immersion and presence [5]. in this study, we focused on creating a dual presence system, so the experimenter needs to pay attention to two remote environments simultaneously and be able to focus their attention as needed between environments. in this research, we aim to develop a system that allows experimenters to be able to achieve dual-presence and monitor both subjects. we will propose two types of visual environment presentation and evaluate them in a set of experiments and then compare and discuss the results. the methods that would be used are as follow: a) split screen: the experimenter’s head-mounted display (hmd) screen is split up from the middle horizontally and shows subject a’s environment on the top, while subject b’s environment is on the bottom. b) superimposed screen: the experimenter’s hmd screen only shows one image. either environment a or environment b, or both with 50% transparency set to all of them. the subject’s feeling towards the robot must also be put into consideration when conducting this experiment. therefore, the system was also designed to operate into different modes, mode 0, 1 and 2: a) mode 0: allows the robots in both environments to always follow the experimenter’s motion b) mode 1: the robot in environment a moves according to the experimenter’s motion and robot in environment b is static. c) mode 2: the robot in environment b moves according to the experimenter’s motion and robot in environment a is static. the goal of this paper is to study the idea of dual-presence and how well it can be implemented as well as means of improving. experiments were conducted to evaluate the system and compare the methods used and finally suggest a way to improve remote experimenting and advances multi-presence research. 2. proposed method when using easy-lab all participants improved their task performance and recorded higher scores in the subjective evaluation. this result suggests easy-lab’s effectiveness, at least in tasks that require pointing and observation from multiple sides [2]. there are three main objectives to achieve with this experiment. first, fulfil the given task successfully. second, switch between the two robots with ease and be able to exist in two different places at the same time. finally, give the subjects the feeling that the robot or at least the person operating is human. as for the experiment itself, the same task is used for all conditions. the robot head has a laser pointer attached to the camera and it follows the head movement of the user as shown in figure 3. local operator head motion is measured by vive pro hmd (htc). local software is composed by using unity (version 2019.4.17f1, unity technologies) vr simulator. tcp/ip between local pc and remote pc, ros# library was used. for this research, the system was tested over the same network in one building, in the future we would like to broaden the scale and operate it from a different town or country and measure the difference in delay. however, technology-wise, it is possible to operate from any distance if there is internet access. there are two subjects in this experiment, and the second robot head is the same as the one shown in figure 3 but placed in front of the 2nd subject. the input required from the experimenter is made simple to improve usability, the vive controller’s trigger button is used to choose which robot to control, detailed explanation on the next section. on the subject’s side, each subject is given a piece of paper with holes in it as shown in figure 4. while on the experimenter side there is an answer sheet that he can use as a reference. the pattern of the answer sheet is generated randomly every time so that the same pattern is never repeated. the experimenter points the laser pointer in hole #1 and wait for 3 seconds, after 3 figure 2. easy-lab system diagram. figure 3. easy-lab system and experimenter. acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 3 seconds have passed, the subject inserts a thread into that hole and so on until hole #6. the answer sheet is colour coded as well to make it easier to read for the experimenter. once all the holes have been connected, the subject stops working on the task and the experimenter must notice using the camera that all holes are connected properly, completing the task. also, a timer is set as soon as the task starts, when the experimenter confirms the task completion visually, he presses a button to stop the timer. 3. system configuration 3.1. experimenter system configuration the outcome we’re studying on the experimenter side is the effect of changing the way environment visuals is shown and the usability of the proposed interface in each mode. the basic requirement to present visuals of the detachable head’s vision is to familiarize with remote location environments and be aware of the state of the subject and the task he/she is performing. in addition, another important requirement is that the user can allocate their attention to the two environments at will while using this interface to perform the co-presence tasks. in this section, we design both the concurrent vision presentation system to relay environment information and the modes for control. regarding the environment information, the easiest way for most humans to be familiarized with an environment is to see it; so, we designed a system that presented images of the two environments simultaneously from a first-person view, since it provides a high level of immersion [6]. recent studies have proposed several presentation systems for visuals, such as a split screen by arranging several images in one screen [7], and others of a screen superimposition type used for superimposing halftransparent images [8]. in this research, we use both methods. the screen superimposition viewing method is used due to being able to provide two first-person view images simultaneously. while the second viewing method splits the two environments to two screens. the other requirement of this system is to allow users able to easily switch or reallocate their attention. researchers have proposed several methods for switching attention easily between two transparent images, such as changing the transmission ratio of two images via foot pedal [9], by the user's gazing point [10] but for this research we aimed for a simpler method which is pressing a button on the vive controller since the hmd used is also vive, the modes changing sequence changes as follow with every button click; mode 0 -> mode 1 -> mode 2. 4. experimenter viewing method the first viewing method is the two superimposed screens, by setting two depth-separated planes on different places as shown on (figure 5 a). the experimenter can see both subjects at the same time, transparency of 50% is used for both images. moreover, we want to test the difference between superimposed and split screens, so the 50% transparency affect is used for all viewing methods. the second viewing method is where the experimenter can see both images in the split screen method (figure 5 b). 4.1. experimenter operation modes ideally, the system should be conducted with several tasks to verify its usability in and avoid dependance of the task itself as much as possible. but this one task should be sufficient for evaluating dual presence viewing and control method and the evaluation is based on meeting the following conditions: a) point at the correct holes as viewed in the answer sheet b) instruct the subjects as quickly as possible, depending on the view and control method, the time is expected to vary. c) make sure the subjects perform the task correctly. in addition, notice when a subject makes a mistake as soon as possible. for every experiment, the experimenter was given time to test each mode and train for ~3 minutes to familiarize with the system. also, the task was performed 4 times, with conditions changing every time and they are as follow: a) superimposed screens/ mode 0; the user can see both environments at the same time and the robots both move all the time. when changing from mode 0 to 1 in this case, mode 1 only shows the answer sheet and the user can go back to operating the robots by pressing the same button again and return to mode 0. b) superimposed screens/ mode 1&2; the user can see only environment a when in mode 1 and control the robot on the same environment, while robot in environment b stops. vise-versa when in mode 2. moreover, in this case, mode 0 shows the answer sheet. c) split screen/ mode 0; the user can see both environments at the same time and the robots both move all the time. when changing from mode 0 to 1 in this case, mode 1 only shows the answer sheet and the user can go back to operating the robots by pressing the same button again and return to mode 0. d) split screen/ mode 1&2; the user can see both environments when in mode 1 but control only the robot on environment a, while robot in environment b stops. figure 4. paper used to connect the dots in the experiment. figure 5. overview of the viewing methods. a) superimposed. b) split screen. acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 4 vise-versa when in mode 2. moreover, in this case, mode 0 shows the answer sheet. 4.2. results and discussion on experimenter operation in this chapter, we verify the usability of the developed system through a user study. for this experiment, we asked the cooperation of six robotics researchers (mean age 24, sd 1.26, male 5, female 1) who were previously involved in subject experiments. each experimenter was instructing 2 subjects at the same time, the 12 participants who acted as subjects are also researchers familiar with robotics (mean age 23, sd 1.31, male 10, female 2) the experimenter and the two subjects were in locations where they can’t see each other, the only communication method is by using the laser pointer attached to each robot and guide the subjects to complete the task. next, the experimenter points at the holes in the correct order as described in the answer sheet provided to him when the task starts. since there are two subjects, the experimenters all chose to instruct subject a to connect the first hole, then moved to guide subject b immediately and repeated this until both finished the task (6 dots connected). depending on the method used, the average number of errors changed as shown in (table 1), error was measured by how many threads were inserted in the wrong hole. the total number of tries is also the maximum number of possible errors and its 12 for each method. table 2 shows the average instruction time in seconds for each method. the outcome for each method used is as follows: a) 3 errors were made in total, the highest error from an operation method. this result was expected, when the two screens are superimposed, it is confusing at times. some users reported that one of the subjects connected a hole without his instruction, this is due to both robots moving at the same time and the experimenter was instructing subject b but subject a’s robot was also moving. another experimenter reported that it was difficult to distinguish between the two laser pointers. as for the time, this method had the longest operation time, it took the users more time to distinguish between the two environments in this method which increased operation time more than necessary. b) no errors were made in this method. this result was due to the fact that the user can focus entirely on one environment and ignore the other one until his instructing is finished as one of the users reported so. the average time was 179 seconds, 19 seconds faster than method (a), even though the viewing method was the same, the average time increased because it was faster to instruct one person even though switching between environments took some time. c) in this method, the total number of errors is 2, again the reason for this error is because when guiding subject b, subject a’s robot also moves and sometimes gives wrong pointing to the subject. the average time taken is 187 seconds, its faster than method (a) but slower than method (b). d) this method resulted in one error which is most likely due to human error. the average time in this method is 157 seconds, it is the fastest method between them all. one user reported that the operation was very smooth by checking the answer sheet in mode 0, instructing subject a in mode 1 and instructing subject 2 in mode 2 and repeating. we can see from the results above that the best method to use for the experimenter is method (d) if the task’s main concern is time as it gave the best results in instructing time and only one error. however, method (b) is a better candidate if the content of the task allows for no errors to be made as it’s the easiest to focus on. after the experiment was over, a questionnaire was given to the experimenters and it’s shown on (table 3), the answers were based on a linear scale from 1-7 for all questions. the results of the questionnaire for each method are as follows: a) in this method, users reported that it was hard to see most of the time. they also felt that the two environments existed in the same place. the average answer was in the middle of the scale, similar to question 4 as well. they also felt that the time taken to instruct was too long. b) for this method, most users reported that its easy to see, they also felt as if they existed in two different locations at the same time. it was easy to instruct both subjects in this method. almost no one got confused when instructing in this setting. users reported time taken to be a little lower than method (a) but it is still considered a long time. c) in this method, users reported that it was easier to see the environments. some users felt that they exist in 2 different places while others felt that they exist in the same place, most answers leading to the middle of the scale. most users were able to instruct very well. question 4 was also in the middle of the scale while all users reported that instructing time was short. d) all the results are exactly the same as (c) except for question 4, in this method no one got confused. the results of the questionnaire show that the users had a better experience overall when operating with method (d) the most. 4.3. result and discussion on subject operation the focus of the research on the subject side is to study the effect of the experimenter having to instruct two people at once and how the subject reacts to it, especially when changing operation methods. the requirements on this part are as follow: table 1. average instruction error. method instructional error (times) (a) superimposed screens/ mode 0 3 (b) superimposed screens/ mode 1&2 0 (c) split screen/ mode 0 2 (d) split screen/ mode 1&2 1 table 2. average instruction time. subject instructional time (s) (a) superimposed screens/ mode 0 198.7 (b) superimposed screens/ mode 1&2 179.3 (c) split screen/ mode 0 187.4 (d) split screen/ mode 1&2 157.1 acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 5 a) fulfil the given task successfully with minimum errors and quickly. b) be able to feel the presence of the experimenter instructing them. before starting the task, only the experimenter knows which type of operation mode was used. however, depending on the mode used, the subjects reacted differently to the task, so a survey of four questions was conducted after each task to further investigate. the answer of the questionnaire (table 4) had a linear scale from 1-7, similar to the experimenter survey. the methods used are also the same as (table 2) and the results are as follow: a) in this method, some users did not complete the task successfully and had a harder time following the instructions. most users also felt that the instructor is always watching, and they felt his presence most of the time. b) most users had no issues completing the task in this mode. on the other hand, they felt the presence of the instructor less than method (a). c) results of this method are the same as (a) d) results of this method are the same as (b). from the answers of the survey, it can be seen that the viewing method of the experimenter has no effect on the subject’s performance. while the operation mode is different, users felt more at ease when the robots were moving all the time and made them feel the presence of the instructor more. to further prove this, wilcoxon signed rank test shown in figure 6 was used to verify which robot was easier to follow with 7 being hard to follow and 1 being easy to follow. the score was significantly improved when mode 1&2 was used compared to mode 0 as the test statistic is lower than the critical value (5 < 8) so we reject ho. this is sufficient evidence that there is a difference between the two modes in terms of which one is easier to follow. 5. discussion on the practical application of easy-lab in this section, the practical application of the proposed method presented in this study is discussed. the advantages of the using easy-lab as in dual presence settings and the methods used can be clarified by comparing this method with other manipulation methods. following the comparison, concerns about using this interface in real life are discussed. 5.1. comparison with other similar methods based on the results of the previous section, the proposed method was compared with other similar methods: a. gesture cam: a video communication system for sympathetic remote collaboration the sharedview system. the operator wears the sharedview. the sharedcamera’s image is sent to the display at the instructor’s sight and the instructor uses gestures in front of the display. the display and the gestures are received by a camera and sent back to the operator’s hmd. in this way, the instructor can give instruction with gestures [11]. easy-lab provides a more modern use case and adds the ability to increase the number of robots as needed, in this research, two robots were required. b. use of gaze and hand pointers in mixed reality remote collaboration the mixed reality remote collaboration system setup. the system supports the use of hand gesture, sketch, hand pointer, and gaze pointer visual cues for communication in the collaboration. the system tracks the remote expert’s hands to use the visual cues and employs a 360-degree camera to share the task space [12]. this system requires more practice to get used to its operation and takes more time than easy-lab to instruct someone. c. telesar vi: telexistence surrogate anthropomorphic robot vi telesar vi is a newly developed telexistence platform for the accel embodied media project. it was designed and implemented with a mechanically unconstrained full-body master cockpit and a 67 degrees-of-freedom (dof) anthropomorphic avatar robot. the avatar robot can operate in a sitting position since the main area of operation is intended to be manipulation and gestural. the system provides a full-body experience of our extended “body schema,” which allows users to maintain an up-to-date representation in space of the positions of their different body parts, including their head, torso, arms, hands, and legs [13]. while this system provides more precise movements, it is too expensive to use for most researchers and requires experience to operate, its heavy and large size makes it hard to move as well. 5.2. limitations in this study, only a laser pointer was used to give instructions in the operation method, the purpose was to ensure that the operation method does not interfere with verifying which viewing method or operation mode is better, so it was made simple. a disadvantage to this is that its hard to transmit table 3. experimenter evaluation questionnaire. question qe1 can you see both environment a&b easily qe2 do you feel that you exist in 2 different places at the same time or do you feel like that both environments exist in the same place qe3 were you able to instruct the 2 subjects equally well qe4 did you get confused with the instructing when you switched environments qe5 did you feel that the time taken to instruct the students was too long figure 6. overview of the viewing methods. a) superimposed. b) split screen. table 4. experimenter evaluation questionnaire. question qe1 where you able to fulfil the task successfully qe2 was the instruction of the robot easy to follow qe3 were you able to feel the presence of the person instructing you qe4 did you feel that the instructor is always watching you and not someone else acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 6 information other than the pointing, a button to stop the laser pointer in one environment should be added to reduce confusion as much as possible. another feature that would be useful in this system is adding a method of transmitting audio of the experimenter to the subjects being instructed. one more restriction faced was the lack of experimenters, to provide more concrete results and finding, the number of subjects should be increased. 6. conclusions in this research, we proposed using easy-lab to perform dual presence operation control. the task performed had 4 different settings. first, two different viewing method were selected and tested accordingly, superimposed screens and split screens. it was found that the split screen operation method provided better results, the time taken to complete the task was faster with minimum number of errors. there were two operation modes used in this system, one mode moved all robots at the same time while the other allowed the experimenter to choose one robot to control at a time. the best control method in terms of ease of use and least number of errors is to control one robot at a time. furthermore, on the subject side, users had an easier time following the instructions of the robot when one robot at a time was being controlled. in the future, an intuitive posture instruction method will be developed that allows more information to be transmitted and provide more sense of being present in multiple places at the same time. other than the control method, the next step is to increase the number of robots and subjects to evolve the system from being dual presence to multi-presence. we have yet to test the limit of how many subjects a single person can instruct using easy-lab. by increasing the number of subjects, the control method must also be revised to accommodate such system. finally, this system has potential to be used by the masses in education, conferences, etc. therefore, further testing is needed in these environments to verify that as this study might potentially suggest a new method of working with other humans from remote places. acknowledgement this research is supported by waseda university global robot academic institute, waseda university green computing systems research organization and by jst erato grant number jpmjer1701, japan. references [1] v. vimolmongkolporn, f. kato, t. handa, y. iwasaki, h. iwata, design and development of 6 dofs detachable robotic head utilizing differential gear mechanism to imitate human head-waist motion; 2022 ieee/sice international symposium on system integration (sii), 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coverage path planning; parking assistant system citation: anna barbara ádám, lászló kocsány, emese gincsainé szádeczky-kardoss, using coverage path planning methods for car park exploration, acta imeko, vol. 10, no. 3, article 5, september 2021, identifier: imeko-acta-10 (2021)-03-05 section editor: bálint kiss, budapest university of technology and economics, hungary received january 13, 2021; in final form september 17, 2021; published september 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: anna barbara ádám, e-mail: annadam97@gmail.com 1. introduction with an increasing number of vehicles on the roads, it is becoming difficult to find a free parking space. several sensorbased parking assistant systems have been developed in the past decade to make it easier to find a free parking space in busy areas, such as city centres and shopping malls. these systems mainly include sensors installed in each parking space that can detect the presence of a car by measuring its weight with pressure sensors; sensing the car body with magnetic sensors or infrared and ultrasonic sensors can determine if something is in the examined area. the main problem with these parking systems is the need for extra infrastructure and sensors. as most car parks are not equipped with sensors, which indicate the occupancy of the parking space, a vehicle must drive passed a parking space to be able to detect if it is free [1]. there are also internet-of-things (iot) systems, which involve not only the signals of the sensors but also mobile applications [2]. these systems can navigate the driver to the free parking spaces in the shortest possible time. the main purpose of this paper is to present an installed sensor-free car park exploration method that navigates the vehicle to all the possible free parking spaces. autonomous vehicles are able to detect the free parking spaces with sensors installed in the vehicle (e.g. lidar [3]). as the coverage path planning (cpp) problem is similar to the car park exploration problem, the core concepts of cpp algorithms can be used. this paper is organised as follows. section 2 presents the most commonly used parking systems and provides examples, while the formulation of the car park exploration problem can be found in section 3. section 4 presents some cell decomposition and grid-based cpp methods and explains how they are used for car park exploration. an improved version of trapezoidal cell decomposition can also be found in this section. different traversal methods are presented in section 5, while section 6 introduces a cost function to grade the free parking spaces. the presented exploration methods are compared in section 7, and the method for using them in multi-storey car parks is presented in section 8. section 9 derives the conclusions from the different traversals and presents possible avenues for future work. abstract with the increasing number of vehicles on the roads, finding a free parking space has become a time-consuming problem. traditional car parks are not equipped with occupancy sensors, so planning a systematic traversal of a car park can ease and shorten the search. since car park exploration is similar to coverage path planning (cpp) problems, the core concepts of cpp algorithms can be used. this paper presents a method that divides maps into smaller cells using trapezoidal cell decomposition and then plans the traversal using wavefront algorithm core concepts. this method can be used for multi-storey car parks by planning the traversal of each floor separately and then the path from one floor to the next. several alternative explorational paths can be generated by taking different personal preferences into account, such as the length of the driven route and the proximity to preferred locations. the planned traversals are compared by step number, the cell visitedness ratio, the number of visits to each cell and the cost function. the comparison of the methods is based on simulation results. mailto:annadam97@gmail.com acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 16 2. parking assistant systems the literature proposes various solutions for autonomous parking with sensors installed in car parks. these systems require a centralised parking system in which a server stores the occupancy of the parking spaces based on the sensor signals. when a vehicle requests a parking space, the server reserves one for the vehicle. cctv systems are widely available in car parks. consequently, algorithms using image processing algorithms are able to detect the occupancy of a parking space based on camera signals. for example, athira et al. [4] present an optical character recognition system that detects occupied parking spaces based on cctv signals. another solution is the availability of iot-enabled cities, often referred to as smart cities. smart cities can provide information about the availability of parking spaces. if a centralised parking system server is available, the vehicle is able to request information about the occupancy of parking spaces from the server. al-turjman et al. [4] present a survey of iot-enabled cities. the use-case practices are also presented, including the smart payment system, the parking reservation system and the eparking system. there are several commercially available solutions for smart parking. in hungary, two such solutions can be found in budapest. the first is parkl [5], which is a smartphone application providing information about the location of car parks and making cashless payment possible. parkl does not provide the exact location of the parking spaces but the location of parking zones in the city where possible parking spaces can be found. in comparison, parker [6] (developed by smart lynx ltd.) provides information about the exact occupancy of about 1,500 parking spaces in the city centre. this solution uses preinstalled sensors to indicate the occupancy of a given parking space to the driver. parker also provides a cashless payment service for its users. the algorithm presented in this paper provides a car park exploration method using cpp. car park exploration is required because no additional signals for preinstalled sensors in car parks are used. a complete system is therefore required to perform the car park search from the creation of the exploration path to detecting appropriate parking spaces and performing the parking manoeuvre. this system is called the autonomous parking system and consists of four subsystems, as the parking task can be broken down into four subtasks. the first task of the autonomous parking system is to provide an exploration path for the vehicle. the focus of this paper is to propose a solution for this subsystem by applying cpp algorithms. this subsystem provides multiple goal configurations for the vehicle. these goal configurations are required, as the avoidance of preinstalled sensors leads to the loss of the goal configuration. the second subsystem is a detector subsystem. the main task is to scan the environment and detect parking spaces for the vehicle while the vehicle is driving along the exploration path [3]. in order to be able to detect parking spaces, a sensor must be mounted on the vehicle. the first subsystem should consider the sensor parameters during the planning of the exploration path. when a parking space is found, the third and fourth subsystems plan and execute the parking manoeuvre [7], [8]. 3. car park exploration the goal of car park exploration is to plan a path leading to all the possible free parking spaces. the binary map (figure 2) of the car park (figure 1) is known, and the parking spaces are treated as obstacles. while following the planned path, a sensor (e.g. lidar) searches for a suitable free parking space. in this formulation, c ⊂  r𝟚 defines the workspace of car park exploration and 𝒜 denotes the vehicle. the state of the vehicle is 𝑞 = [ 𝑥 𝑦], where 𝒜 (𝑞) ⊂ 𝐶, while [ 𝑥 𝑦] denotes the position of the vehicle in a fixed frame (the orientation of the vehicle is not taken into account). the workspace consists of obstacles (𝐶obs ⊂ 𝐶) and free spaces (𝐶free = c  ∖ 𝐶obs), some of which need to be visited (𝐶vis ⊆ 𝐶free). the vehicle can move only in free space (∀𝑞 ∈ 𝐶free). the vehicle moves on a collision-free path (τ), where 𝑠𝑖 ∈ 𝑅 is a scalar path parameter (𝑠 ∈ [0, 𝑇], 𝑇 is the length of the whole path): τ: 𝑠 ↦ 𝑞, ∀𝑠 ∈ [0, 𝑇]: τ(𝑠) ∈ 𝐶free . (1) 𝐿(𝑞) ⊂ 𝐶 denotes the points that are inside the range (δ) of the sensor, which detects the free parking spaces: 𝐿(𝑞) = { 𝑧 ∈ 𝐶free| ‖𝑞 − 𝑧‖ ≤ 𝛿}, (2) where ‖𝑞 − 𝑧‖ is the euclidean distance between points 𝑞 and 𝑧. the points seen while traversing the path are 𝐿(τ(𝑡)) = ⋃ 𝐿(τ(𝑠)) 𝑠∈[0,𝑡] . (3) the aim is to reach every position that should be visited during the exploration: 𝐶vis ⊆ 𝐿(τ(𝑇)). (4) other constraints that should be considered are as follows: • the start position is τ(0) = 𝑞init. figure 1. map of a car park. figure 2. binary map of a car park. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 17 • cost can be assigned to the path as 𝑤1(τ) (e.g. length of the path). • preferred positions are provided for, which are considered first. the 𝑤2(𝑞) cost can be assigned to the 𝑞 ∈ 𝐶𝑓𝑟𝑒𝑒 position (e.g. distance from the target position). • the traversal can be interrupted when a condition is met (parking space detected with lidar). • when stopping while driving (at some point with 𝑠1 path parameter), the cost of the traversal is the personal preference weighted (𝛼) sum of 𝑤1 and 𝑤2: 𝑤1(τ) + 𝛼 𝑤2(τ(𝑠1)) (the cost of the path up to that point plus the cost of the position). • there might be constraints to the order of the configurations in τ path (e.g one-way streets). 4. using coverage path planning methods the purpose of cpp methods is to plan an obstacle-free path that reaches every free point of the given configuration. this section presents car park traversal-related computational problems. it also explains the idea behind using cpp algorithms for the traversal and provides an overview of the most commonly used cell decomposition and grid-based methods. finally, methods chosen for implementation are presented. 4.1. related computational problems cpp is similar to the travelling salesman problem [10], in which the salesman has to visit all the cities via the shortest route possible and return to the beginning. the cities are represented as nodes on a graph and the routes between them are the edges. the travelling salesman problem is np-hard, which means it is at least as hard as any np problem, so it cannot be solved with polynomial-time algorithms. during path planning, an important factor in terms of the path is the exploration of the environment. two computational geometric problems are related to this: the museum problem [11] calculates the fewest number of guards required to observe the whole museum, while in the watchman problem [12], only the map of the region is given, and the main purpose is to plan the shortest possible path between the obstacles so that the watchman can guard the entire area. 4.2. cell decomposition-based path planning there are several cell decomposition-based path planning methods that divide a map into cells. exact cellular decomposition methods [12] divide the free space into varioussized nonoverlapping cells. the union of the cells covers the whole free configuration. trapezoidal cell decomposition [14] can only be used in polygonal environments, as it extends rays from the vertices of the obstacles. these rays and the edges of the obstacles form the cell borders, dividing a map into trapezoidal or triangular cells. an example of this method can be seen in figure 3. boustrophedon (greek: ‘ox turning’) decomposition [15] extends rays from the entry and exit points of the obstacles, so fewer rays are extended than in trapezoidal cell decomposition. consequently, the cells have a larger area. figure 4 shows an example of this method. when applying morse decomposition [12], the critical points of the smooth function, called the morse function, indicate the boundaries of the cells (see figure 5). greedy convex polygon decomposition [18] can be used when there are polygonal obstacles. this method consists of two types of cuts: • a single cut: a cut from a nonconvex vertex to an existing edge or another cut, • a matching cut: cutting two nonconvex vertices. first, all the matching cuts are made on the nonconvex vertices, then the single cuts are made for the unmatched vertices. in the example in figure 6, the matching cuts are green and the single cuts are red. the cell boundaries are the set of cuts and the edges of the obstacles. after dividing the map, the adjacency matrix of the cells can be created. two cells are adjacent if they have a common boundary, and the boundary has a given number of common points. the purpose of cell decomposition-based methods is to visit every cell once, although it is not always possible. if the adjacency matrix of the cells is known, a traversal can be planned, as it is known which cell can be visited from the current one. after creating the traversal, a path can be planned that leads through the cells in a given order reaching every free point of the configuration. figure 3. example of trapezoidal cell decomposition [14]. figure 4. example of boustrophedon decomposition [16]. figure 5. example of morse decomposition [17]. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 18 4.3. grid-based path planning in contrast to the cell decomposition-based methods, gridbased methods divide the map into same-sized cells, called a grid. the cells are classified into two groups, those that have obstacle points inside the cell and those without obstacle points. the wavefront algorithm [9] assigns distance values to every cell in the map, with each neighbouring cell being assigned one larger distance value, starting from the initial cell, which has a distance value of 0. the traversal of the cells is based on distance values, and the neighbouring unvisited cell with the largest distance value is the following cell. if a number of unvisited neighbouring cells have the same distance value, the following cell is selected randomly. an example of this method can be seen in figure 7. the spiral spanning-tree coverage method [21] builds up a graph with nodes that represent the centres of the obstacle-free cells and edges that represent the lines between the neighbouring cell centres. the cells are divided into four subcells, which are classified into two groups: those that have four unvisited subcells (called new cells) and those that have at least one visited subcell (called old cells). first, every cell is unvisited, and the algorithm starts from the initial cell and marks it as an old cell. the initial cell is the root of the spanning tree. in the subsequent steps, every cell neighbouring the current cell is tested if it is a new cell. if the cell has a neighbouring new cell, a spanning-tree edge is added from the current cell to the neighbouring cell, and then the algorithm moves to a subcell of the neighbouring cell and adds the centre of the neighbouring cell to the tree as a node. if there is a backwards move from the current cell to a subcell of the parent cell along the right-hand edge of the spanning tree, the algorithm terminates. figure 8 provides an example of this method. 4.4. using coverage path planning methods for car park exploration in order to plan the traversal of a car park, the map of the car park should be divided into smaller regions. as most car parks consist of polygonal obstacles, trapezoidal cell decomposition can be used to divide the map. the cells can be treated as a grid, so wavefront algorithm core concepts can be used to plan the traversal of the cells. the main advantage of the wavefront algorithm is that it can take personal preferences into account by modifying the distance values of the cells. 4.5. rectangular cell decomposition as rectangular decomposition is based on trapezoidal cell decomposition, it can only be used in polygonal environments. if the map contains nonpolygonal obstacles, the bounding boxes of the obstacles should be taken into account when decomposing the map. trapezoidal cell decomposition decomposes the map using only one axis (𝑥 or 𝑦), so the decomposed map may contain cells covering large areas. the main disadvantage of this method is that the traversal of the cells is not unequivocal, so a path should be planned inside a cell from which all of the free parking spaces can be seen. rectangular cell decomposition decomposes the map using both 𝑥 and 𝑦 axes. the final cells are the intersections of the cells decomposed by these axes. the final cells are smaller and rectangular, which is the main advantage of this method; every cell has only one neighbouring cell in each direction. an example of a decomposed map can be seen in figure 9, where the coloured rectangles indicate the cells. the adjacency matrix of the cells can be created after dividing the map. this matrix is an 𝑛 × 4 matrix, where 𝑛 is the number of cells. the rows of the matrix store the indices of the adjacent cells of every cell in each direction. if two cells have common boundaries, and the vehicle can pass from one cell to the other, these cells are adjacent. cells neighbouring obstacles and cells on the edges of the map do not have four neighbouring cells, so the neighbouring obstacles and edges are considered as their neighbours (an implementational solution might be to store these false neighbours as dummy indices, e.g. −1). if there are one-way road sections, moving from one cell to the other is permitted, but moving in the opposite direction is prohibited, so the 𝑖𝑡ℎ cell is adjacent to the 𝑗𝑡ℎ cell but not vice versa. this means that the 𝑖𝑡ℎ row of the adjacency matrix contains the 𝑗-cell index in the appropriate column, but the 𝑗𝑡ℎ row contains a dummy index instead of the 𝑖 index. more details of this algorithm can be found in [20]. figure 6. example of greedy convex decomposition [18]. figure 7. example of a wavefront algorithm [19]. figure 8. example of spiral spanning-tree coverage [22]. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 19 5. traversal of the cells several different traversal methods are presented in this section. the map presented in section 5.1 is used as an illustration. 5.1. map of the car park the car park consists of only one floor, with the black areas representing the obstacles (the possible free parking spaces are also considered as obstacles). the red x represents the preferred location. the map of this car park can be seen in figure 10, the decomposed map can be seen in figure 9 (rectangular decomposition was used) and the assigned distance values from the wavefront algorithm can be seen in figure 11. all the algorithms stop when the step number exceeds a given number, which depends on the number of cells. the maximal step number of the following simulations is 57. 5.2. visitednessand preference-based traversal this method first visits the unvisited neighbouring cells (see figure 12). if there are a number of neighbouring cells that are unvisited, the cell with the highest preference value is the following cell (see figure 13). in the map in section 5.1, one cell remains unvisited. this results from the low preference values in that area. the algorithm visits the neighbouring cells of the unvisited cell only once, and the other cells have higher preference values, so they are the next cells of the traversal. a traversal designed by this method can be seen in figure 14. figure 9. the decomposed map of the car park; the initial cell is cell number 10. figure 10. map of the car park; the red x represents the preferred location. figure 11. the distance values of the cells. figure 12. the visitedness of the cells determines the following cell, which is in red. figure 13. the preference value determines the following cell. figure 14. the visitednessand preference-based traversal of the car park. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 20 5.3. visitednessand preference-based traversal with dynamic preference this method differs from the previous one by applying dynamic preference. this means the preference value of the cell visited last is decreased during the traversal. in this case, the preference values of the cells decreased by 2 every time they were visited. as a result, every cell could be visited; the preference values of the cells visited first were decreased enough so that the algorithm would select the cells with lower preference values as the next cells. figure 15 provides an example of this method. 5.4. visitednessand preference-based traversal with dynamic preference while avoiding unnecessary cells a map may contain cells that are not adjacent to parking spaces, as they do not contain any c ∈ cvis points, so it is not necessary to visit them. in this case, the preference values of these cells can be changed to 0, and they can be marked as visited at the beginning. the algorithm will then only visit these cells in order to reach other cells. in figure 16, the white cells represent cells that do not need to be visited. 5.5. preferenceand visitedness-based traversal this method selects the next cell based on its preference value (see figure 17). if a number of neighbouring cells have the same preference value, the algorithm selects the following one based on the visitedness of the cell (see figure 18). as can be seen in figure 19, the cells with the highest preference values are visited multiple times, while cells with lower preference values may remain unvisited. 5.6. preferenceand visitedness-based traversal with additional preference values as preferenceand visitedness-based traversal mainly selects the next cell based on its preference value, this method should be used in cases where the driver wants to park at a given location. additional preference values can also be applied to attract the traversal to the desired location. figure 20 provides an example of the applied additional preference values. the highest additional preference value (5) was added to the cell that contains the preferred location (see figure 10), so each neighbouring cell gets one smaller additional preference value until the fourth neighbouring cell (the range of the preference is 5). it can be seen in figure 21 that the algorithm navigates to the highly preferred area in the shortest possible path, and subsequently, the traversal only moves around one obstacle by traversing the cells with the highest preference values. figure 15. the visitednessand preference-based traversal of the car park with dynamic preference. figure 16. the visitednessand preference-based traversal of the car park with dynamic preference while avoiding unnecessary cells. figure 17. the preference value determines the following cell. figure 18. the visitedness of the cells determines the following cell. figure 19. preferenceand visitedness-based traversal. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 21 5.7. preferenceand visitedness-based traversal with additional preference values and dynamic preference the previous algorithm can also be applied when using dynamic preference values. in this case, the preference values of the visited cells are decreased by 2 every time they are visited. it can be seen in figure 22 that the algorithm visits the cells with high preference values, but then also visits the neighbouring cells. in this case, every cell is visited. this method might be the closest to human thinking. first, it searches for free parking spaces in the highly preferred area, then it goes a bit further and returns to the preferred location. finally, it also visits the cells that are furthest from the preferred cells. 5.8. making the traversal repeatable the presented traversals do not lead back to the initial position. it is possible that no free parking spaces were found during the first traversal of the car park, so the traversal should be repeated in order to find a suitable free parking space. there are two possible solutions to this problem: regenerate the traversal from the current cell as the initial cell or plan a path back to the initial cell from the current cell so that the original traversal can be repeated. 6. cost of the traversal the quality of the detected free parking space can be measured with a cost function. there are two main factors that should be considered: the time spent searching for a free parking space (which is proportional to the driven-route length 𝑤1 in section 3) and the distance from the preferred location (𝑤2 in section 3). there is a trade-off between these two factors, but the quality of a parking space also depends on personal preferences: the importance of the proximity to a preferred location or a short driven-route length. the 𝑅𝑜𝑢𝑡𝑒𝐿𝑒𝑛𝑔𝑡ℎ (𝑤1 in section 3) (5) is the sum of the distances between the cell centres during the traversal, the 𝑁𝑒𝑎𝑟𝑛𝑒𝑠𝑠 (𝑤2 in section 3) (6) to the preferred location is the preference (𝑝𝑟𝑒𝑓𝑉𝑎𝑙) weighted sum of the euclidean distances measured from the preferred location and the cost function (𝑐𝑜𝑠𝑡), which is calculated for each cell, is a personal preference (α) weighted sum of the 𝑅𝑜𝑢𝑡𝑒𝐿𝑒𝑛𝑔𝑡ℎ and 𝑁𝑒𝑎𝑟𝑛𝑒𝑠𝑠 (7). the smaller the α value, the better a closer parking space. 𝑅𝑜𝑢𝑡𝑒𝐿𝑒𝑛𝑔𝑡ℎ = ∑ length(𝑅𝑜𝑢𝑡𝑒i) n i=1 , (5) 𝑁𝑒𝑎𝑟𝑛𝑒𝑠𝑠 = ∑ 𝑝𝑟𝑒𝑓𝑉𝑎𝑙i ∑ 𝑝𝑟𝑒𝑓𝑉𝑎𝑙𝑗 p j=1 ⋅ dist(𝑝𝑜𝑠𝑖𝑡𝑖𝑜𝑛, 𝑝𝑟𝑒𝑓𝐿𝑜𝑐i) p i=1 , (6) 𝑐𝑜𝑠𝑡 = α ⋅ 𝑅𝑜𝑢𝑡𝑒𝐿𝑒𝑛𝑔𝑡ℎ + (1 − α) ⋅ 𝑁𝑒𝑎𝑟𝑛𝑒𝑠𝑠, (7) where 𝑁 is the number of route sections, 𝑃 is the number of preferred locations and α is the weighting factor. the quality of a free parking space can be determined by the value of the cost function. the driver should establish a threshold value, and free parking spaces at lower costs are adequate. the better a parking space is, the lower the value of the cost function. 7. comparison of the traversals in this section, the previously presented methods are compared based on the step number (section 7.1), the cell visitedness ratio (section 7.2), the number of visits to each cell (section 7.3) and the cost function (section 7.4). 7.1. step number the step number gives the number of steps needed to visit every cell at least once. traversing from one cell to another is considered as one step. as the algorithm stops when more than a given number of steps is exceeded, the maximum number of steps in this map is 57. table 1 shows how many steps are needed when different methods were applied. it can be seen that when the visitedness and preference-based method (sections 5.2–5.4) was applied, fewer unvisited cells remained. when there were cells marked as unnecessary to visit (section 5.4), only 30 steps were sufficient to visit all the other cells, and in the end, two out of three marked figure 20. the additional preference values. figure 21. preferenceand visitedness-based traversal with additional preference values. figure 22. preferenceand visitedness-based traversal with additional preference values and dynamic preference. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 22 cells remained unvisited. when dynamic preference (sections 5.3, 5.4 and 5.7) was applied, fewer steps were needed and fewer cells remained unvisited. 7.2. cell visitedness ratio the cell visitedness ratio represents the number of visited cells relative to the number of cells in each step. figure 23 shows the visitedness ratio for the different methods using different colours. cells marked as unnecessary in section 5.4 are also included in the number of cells. the legend gives the section number in which the traversal method is presented. first, every cell is visited for the first time, so this ratio grows at each step. when applying the preferenceand visitednessbased method without dynamic preference (sections 5.5 and 5.6), the final value of the ratio is lower than in other cases. these methods visit only the cells with high preference values (60 %– 75 % of the cells). the other methods visit nearly all the cells (more than 80 % of the cells). applying dynamic preference (section 5.7) makes the visiting of the remaining unvisited cells more probable. when visitednessand preference-based traversal (sections 5.2–5.4) was used, the algorithm visited the unvisited neighbouring cells first, so there were fewer cells that remained unvisited. 7.3. the number of visits to each cell the following diagrams show the number of visits to each cell. numbers on the horizontal axis represent the indices of the cells shown in figure 9. visitednessand preference-based traversal this method’s traversal (section 5.2) visits the unvisited cells first. cells with high preference values are visited more frequently (5–6 times), but there is only one cell (cell 14) that is unvisited. for example, the preference value of cell 13 is 9 (see orange bar in figure 24), and this cell was visited 6 times (see blue bar in figure 24). the diagram demonstrating the number of visits to each cell can be seen in figure 24. visitednessand preference-based traversal with dynamic preference by applying dynamic preference (section 5.3), all the cells become visited and the maximum number of visits to each cell is four. most of the cells are visited once or twice, so the application of dynamic preference decreases the number of visits (figure 25). visitednessand preference-based traversal with dynamic preference while avoiding unnecessary cells there can be cells in a map that do not need to be visited because there are no parking spaces around them. in this case, they are only visited if the traversal goes through them in order to reach another cell (section 5.4). in figure 26, the only unvisited cells are unnecessary ones (unnecessary cell indices: 4, 5, 19). due to dynamic preference, the other cells are visited only once or twice. preferenceand visitedness-based traversal this method (section 5.5) visits the cells with a high preference value first and more frequently because the algorithm selects the next cell based on the preference value of the neighbouring cells. in figure 27, it can be seen that most of the visited cells are visited 5–6 times, but there are a large number of unvisited cells. figure 23. cell visitedness ratio of the presented methods. figure 24. number of visits to each cell (method presented in section 5.2). figure 25. number of visits to each cell (method presented in section 5.3); the preference values shown in this figure are the original values, not the decreased values. table 1. the number of steps needed when applying different methods. method section number 5.2 5.3 5.4 5.5 5.6 5.7 number of steps 57 56 30 57 57 42 number of unvisited cells 1 0 2 5 8 0 acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 23 preferenceand visitedness-based traversal with additional preference values in figure 28, it can be seen that due to the application of additional preference values fewer cells are visited, but they are visited 6 times. this algorithm (section 5.6) navigates to the most preferred location (there was an additional preference applied with a range of 5 and a value of 5 in the cell with index 12) in the shortest possible route (these cells are only visited once), then it only moves around the preferred area. preferenceand visitedness-based traversal with additional preference values and dynamic preference when applying dynamic preference (section 5.7), all the cells are visited, and each cell is visited a maximum of 3 times instead of 5–6 times (figure 29). 7.4. cost function the algorithm can decide whether a free parking space is suitable or not based on a cost function. the defined cost function is based on two factors: the driven-route length and distance from the preferred location. the estimated route length of a method depends on the step number; the higher the step number, the longer the route length (see table 2). the route length is measured in pixels (the real distance depends on the graphic scale of the map). the other aspect of cost function is the distance from the preferred location. the distance is calculated at every step, and it depends strongly on the traversal method. the minimum distance from the preferred location is 50 pixels in this example. visitednessand preference-based traversal this method (section 5.2) visits the unvisited cells first, only reaching the possible minimum distance 3 times during the traversal. it can be seen in figure 30 that the distance is between 180 and 330 pixels most of the time. visitednessand preference-based traversal with dynamic preference figure 31 shows that the shape of the distance function is similar to the function in figure 30. this function also has three minimum points due to dynamic preference (section 5.3); only the visiting order of the cells is different, and the unvisited cells are also visited. figure 26. number of visits to each cell (method presented in section 5.4); the preference values shown in this figure are the original values, not the decreased values. figure 27. number of visits to each cell (method presented in section 5.5). figure 28. number of visits to each cell (method presented in section 5.6). figure 29. number of visits to each cell (method presented in section 5.7); the preference values shown in this figure are the original values, not the decreased values. table 2. the route length of each method. method section number 5.2 5.3 5.4 5.5 5.6 5.7 number of steps 57 56 30 57 57 42 route length 6752 6608 3184 6555 6671 4379 acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 24 visitednessand preference-based traversal with dynamic preference while avoiding unnecessary cells this method (section 5.4) avoids the cells that have no parking spaces next to them, so the step number is lower than in the previous methods. it can be seen in figure 32 that the traversal visits the cell nearest to the preferred location only once, but the length of this traversal is much shorter due to the smaller number of steps. preferenceand visitedness-based traversal this method (section 5.5) visits cells with a high preference value more often. the preferred location is far from the initial cell, so it has a high distance value. without an additional preference value, the algorithm also visits the preferred location 5 times, as can be seen in figure 33. preferenceand visitedness-based traversal with additional preference values by applying additional preference values (section 5.6), the traversal reaches the minimal distance from the preferred location 6 times (figure 34). it can also be seen that the traversal repeats the same path, as one part of the function is repeated 5 times. preferenceand visitedness-based traversal with additional preference values and dynamic preference as a result of dynamic preference (section 5.7), the cells that are further from the preferred location are also visited. the traversal has only three minimum points (figure 35) because when a cell becomes visited, the preference value of this cell is decreased. figure 30. distance from preferred locations (method presented in section 5.2). figure 31. distance from preferred locations (method presented in section 5.3). figure 32. distance from preferred locations (method presented in section 5.4). figure 33. distance from preferred locations (method presented in section 5.5). figure 34. distance from preferred locations (method presented in section 5.6). figure 35. distance from preferred locations (method presented in section 5.7). acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 25 8. handling multi-storey car parks as there is a considerable lack of free parking spaces in city centres, an increasing number of multi-storey car parks have been constructed to ensure there are enough free parking spaces to meet demand. the traversal of these car parks is similar to those in the presented methods. the storeys of a multi-storey car park can be handled individually. this means that the traversal of each floor should be planned independently from the other floors, with the only extra requirement being the minimising of the transition between levels. to plan a traversal of each floor, the map of the floor must be known. some floors are preferred over others, so these floors should be traversed first. if there is no free parking space on the preferred floor, the driver must either go around again or go to another floor. the traversal to another floor can be forced so that the preference values of all the cells can be set to zero except for the cell representing the ramp to the other floor, which has a high preference value with a large range. figure 36 shows the map of the first floor of a car park, and figure 37 shows its second floor. the car park entrance and the ramp to the next floor are also indicated on the maps. the maps of the floors are the same, with only different entrance and exit locations. the planned traversals for the whole car park can be seen in figure 38 to figure 40. there are no additional preference values during the traversal, and the traversal is planned based on visitedness and preference, with dynamic preference values. the traversal of the first floor (figure 38) is much longer than the traversal of the second floor (figure 40). the first floor can be traversed in 56 steps, but the second floor needs only 33 steps to be traversed. the traversal method of the floors is the same, only the entrance locations are different. this example shows how the traversal depends on the location of the entrance point. 9. conclusion as searching for a free parking space is a time-consuming task, the aim of this paper is to design different car park exploration strategies. the implemented algorithms used the core concepts of cpp algorithms, which is possible because the car park exploration problem is similar to cpp problems. cpp algorithms are used to plan the paths of vacuum-cleaner robots, lawnmower robots and robots for different purposes, which are designed to reach every free point of a configuration in the shortest possible time while avoiding obstacles. during car park exploration, the vehicle does not have to reach every free point of the map, it only has to drive by all the possible free parking spaces. the car park map is decomposed by using trapezoidal cell decomposition. this method leads to cells with large areas, and the planned traversal contains reversals. if the map is decomposed using both 𝑥 and 𝑦 axes, the created cells are smaller, and every cell has only one neighbouring cell in each direction. in this case, the traversal can take personal preferences figure 36. map of the first floor. figure 37. map of the second floor. figure 38. traversal of the first floor. figure 39. traversal of the first floor to the exit ramp. figure 40. traversal of the second floor. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 26 into account by using the wavefront algorithm. the distance values can be modified in order to attract the traversal to a given location (e.g. entrances, lifts, etc.). the traversal can be planned by taking the preference values and the visitedness of the cells into account. the first method presented only takes the preference values into account, so the traversal does not visit every cell on the map, only the ones with high preference values. another method chooses the next cell based on the preference value, but in case of equal preference values, the next cell is an unvisited one. the third wavefront algorithm-based traversal is based on visitedness and preference, and it visits the unvisited neighbouring cells first. this method is more likely to visit all the cells on the map. in order to compare the presented methods, quality characteristics were defined: step number, the cell visitedness ratio at each step, the number of visits to each cell and a cost function. the step number shows how many steps are needed in order to visit every cell; the algorithm stops if the step number exceeds a given number. the cell visitedness ratio shows the ratio of the visited cells at each step. the cost function is based on two factors: the driven-route length and the weighted sum of the distance from the preferred locations. if a free parking space is found, the decision as to whether it is suitable is based on the cost function. the implemented algorithms were tested in simulations, the results of which are detailed in section 5. the simulation results demonstrate that the different methods can be used in different situations. the visitednessand preference-based traversals visit nearly every cell on the map. if dynamic preference is applied, there is a higher chance that every cell becomes visited. it is also possible that there are cells that do not need to be visited. in this case, their preference value can be changed to 0, and they are marked as visited at the beginning. these cells only become visited when the traversal passes through them to reach other cells. visitednessand preference-based methods should be used when it is important to find a free parking space as soon as possible. these methods visit the cells with high preference values more frequently. if additional preference values are applied, the traversal moves around the preferred area. if dynamic preference is applied, the traversal visits the preferred cells first, then goes further away from the preferred area. preferenceand visitedness-based methods should be used if it is important to park near the preferred location. future work will include testing the implemented methods in a real environment and handling situations in which multiple vehicles are searching for free parking spaces at the same time. acknowledgement the research reported in this paper and carried out at the budapest university of technology and economics has been supported by the national research development and innovation fund (tkp2020 institution excellence subprogram, grant no. bme-ie-mifm) based on the charter issued by the national research development and innovation office under the auspices of the ministry for innovation and technology. references [1] faheem zafari, s. a. mahmud, g. m. khan, m. rahman, h. zafar, 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[17] j. park, cell decomposition course: introduction to autonomous mobile robotics, intelligent systems and robotics lab. division of electronic engineering, chonbuk national university. online [accessed 14 september 2021] https://cupdf.com/document/cell-decomposition-courseintroduction-to-autonomous-mobile-robotics-prof.html [18] a. das, m. diu, n. mathew, c. scharfenberger, j. servos, a. wong, j. zelek, d. clausi, s. waslander, mapping, planning, and sample detection strategies for autonomous exploration, j. of field robotics 31 (2014), pp. 75-106. doi: 10.1002/rob.21490 https://doi.org/10.1016/s1665-6423(13)71580-3 https://doi.org/10.1016/j.scs.2019.101608 https://doi.org/10.1109/cinti-macro49179.2019.9105160 https://doi.org/10.1109/icoei.2019.8862517 https://parkl.net/hu/ https://smartlynx.hu/ https://doi.org/10.3311/pp.ee.2008-3-4.02 https://doi.org/10.3311/pp.ee.2009-1-2.08 https://doi.org/10.1016/j.robot.2013.09.004 https://doi.org/10.1145/321105.321111 https://doi.org/10.1016/0020-0190(88)90141-x https://doi.org/10.1109/robot.2000.846365 https://user.ceng.metu.edu.tr/~akifakkus/courses/ceng786/hw3.html https://user.ceng.metu.edu.tr/~akifakkus/courses/ceng786/hw3.html https://doi.org/10.1023/a:1008958800904 https://cupdf.com/document/cell-decomposition-course-introduction-to-autonomous-mobile-robotics-prof.html https://cupdf.com/document/cell-decomposition-course-introduction-to-autonomous-mobile-robotics-prof.html https://doi.org/10.1002/rob.21490 acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 27 [19] m. mcnally, walking the grid, robotics 52 (2006), pages 151–155. online [accessed 20 september 2021] https://dl.acm.org/doi/pdf/10.5555/1151869.1151889 [20] a. b. ádám, l. kocsány, e. g. szádeczky-kardoss, cell decomposition based parking lot exploration, proc. of the workshop on the advances of information technology, budapest, hungary, 30 january 2020, pp. 5-12. [21] y. gabriely, e. rimon, spiral-stc: an on-line coverage algorithm of grid environments by a mobile robot, proc. of the ieee int. conf. on robotics and automation, washington, dc, usa, 1115 may 2002, pp. 954-960. doi: 10.1109/robot.2002.1013479 [22] y. gabriely, e. rimon, competitive on-line coverage of grid environments by a mobile robot, computational geometry 24 (2003) pp. 197-224. doi: 10.1016/s0925-7721(02)00110-4 https://dl.acm.org/doi/pdf/10.5555/1151869.1151889 https://doi.org/10.1109/robot.2002.1013479 https://doi.org/10.1016/s0925-7721(02)00110-4 microsoft word article 14 121-757-1-pb.docx acta imeko december 2013, volume 2, number 2, 78 – 85 www.imeko.org acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 78 capacitive facial activity measurement ville rantanen1, pekka kumpulainen1, hanna venesvirta2, jarmo verho1, oleg špakov2, jani lylykangas2, akos vetek3, veikko surakka2, jukka lekkala1 1 sensor technology and biomeasurements, department of automation science and engineering, tampere university of technology, korkeakoulunkatu 3, fi-33720 tampere, finland 2 research group for emotions, sociality, and computing, tampere unit for computer-human interaction, school of information sciences, university of tampere, kanslerinrinne 1, fi-33014, tampere, finland 3 media technologies laboratory, nokia research center, otaniementie 19, fi-02150 espoo, finland section: research paper keywords: capacitive measurement, distance measurement, facial activity measurement, facial movement detection, hierarchical clustering, principal component analysis citation: ville rantanen, pekka kumpulainen, hanna venesvirta, jarmo verho, oleg špakov, jani lylykangas, akos vetek, veikko surakka, jukka lekkala, capacitive facial activity measurement, acta imeko, vol. 2, no. 2, article 14, december 2013, identifier: imeko-acta-02 (2013)-02-14 editor: paolo carbone, university of perugia received may 31st, 2013; in final form november 20th, 2013; published december 2013 copyright: © 2013 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: this work was funded by nokia research center, the finnish funding agency for technology and innovation, and the finnish cultural foundation. corresponding author: ville rantanen, e-mail: ville.rantanen@tut.fi 1. introduction measuring facial movements has many possible applications. human-computer and human-technology interaction (hci and hti) can use information of voluntary facial movements for the interaction [1]-[7]. other applications, for example in behavioural science and medicine, can also benefit from the automated analysis of human facial movements and expressions [8]-[15]. in the context of hci, the use of facial movements has been studied already for a decade. the first implementations were based on measuring electromyographic (emg) signals that reflect the electrical activity of the muscles [1]. the measurement system by barreto et al. [1] utilised only bioelectric signals for pointing and selecting objects, but later on the emg measurement was adopted as a method to indicate selections in hci when gaze is used for pointing [2], [3], [4], [6]. recently, a capacitive detection method has been introduced as an alternative for the facial emg measurement [5]. it provides a contactless alternative that measures facial movement instead of the electrical activity of the muscles that the emg measures. studies about pointing and selecting with the capacitive method in combination with head-mounted, video-based gaze tracking have been also published [7], [16], [17], [18]. facial action coding system (facs) is a vision-based method that characterises facial actions based on the activity of different facial muscles [19], [20]. each facial expression has certain activated muscles that can have different levels of contraction. facs and the detection of active muscles have been used as a basis for automatically analysing facial expressions, for example, for the use of behavioural science and medicine [9], [10], [11], [13], [14], [15]. these studies describe automated implementations of facs by using vision-based abstract a wide range of applications can benefit from the measurement of facial activity. the current study presents a method that can be used to detect and classify the movements of different parts of the face and the expressions the movements form. the method is based on capacitive measurement of facial movements. it uses principal component analysis on the measured data to identify active areas of the face in offline analysis, and hierarchical clustering as a basis for classifying the movements offline and in real-time. experiments involving a set of voluntary facial movements were carried out with 10 participants. the results show that the principal component analysis of the measured data could be applied with almost perfect performance to offline mapping of the vertical location of the facial activity of movements such as raising and lowering eyebrows, opening mouth, raising mouth corners, and lowering mouth corners. the presented classification method also performed very well in classifying the same movements both with the offline and the real-time implementations. acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 79 methods in the analysis. however, facial emg can also register facial actions and provide information that is highly similar to the one provided by facs [11]. emg has also been shown to be suitable for measuring emotional reactions from the face [8]. this has been done long before emg was first applied in the hci context to detect voluntary facial movements. the presented method applies capacitive measurement principles to measure the activity of the face. it has several advantages over the other methods that can be used for the task. compared to emg measurements, the presented method allows the measurement of more channels simultaneously. it is contactless and it does not require the attachment of electrodes to the face. attached electrodes significantly limit the maximum number of measurable channels and they may also affect facial movements that are being targeted with the measurement [11], [21]. when compared to vision-based detection of facial activity, the capacitive method allows easier integration of the measurement to mobile, head-worn equipment, is unaffected by environmental lighting conditions, and can be carried out with computationally less intensive signal processing. for the current study, a wireless, wearable prototype device was constructed, and analysis of data from controlled experiments was done to identify the location of facial activity and to classify it during different voluntary movements. voluntary facial movements have previously been detected by identifying transient peaks from the signals [5]. the presented method provides a more robust way to analyse the facial activity based on multichannel measurements. 2. methods 2.1. capacitive facial movement detection the measurement method for measuring facial activity is based on the capacitive detection of facial movements that was introduced in [5]. it applies the same measurement principle as capacitive push buttons and touchpads, and a single measurement channel requires only a single electrode that produces an electric field. the produced field can be used to detect conducting objects in its proximity by measuring the capacitance because the capacitive coupling between the electrode and the object changes as the object moves. in principle, the distance between the target and the electrode is measured. 2.2. prototype device the wearable measurement prototype device is shown in figure 1. the device was constructed as a headset that should fit most adults. the earmuffs of the headset house the necessary electronics, and the extensions seen in front of the face include the electrodes for the capacitive measurement channels. the device contains 22 electrodes in total, 11 for both sides of the face. the top extensions have 4 electrodes each, the middle ones have 3 each, and the lowest ones have 4. the electrodes are printed circuit board pieces with a size of 12 x 20 mm. they are connected to the measurement electronics with thin coaxial cables that shield the signals. the capacitive measurements are carried out with a programmable controller for capacitance touch sensors (ad7147 by analog devices). the sampling frequency was dictated by technical limitations and it was set to the maximum possible, 29 hz. the device is battery-powered and a bluetooth module (rn-41 by roving networks) provides the connectivity to the computer. the device has the possibility for additional measurements such as inertial measurements via a 3d gyroscope and a 3d accelerometer. the operation of the device is controlled by atmel’s atmega168p microcontroller. 2.3. experiments ten participants (five male and five female, ages 22-33, mean age 27) were briefly trained to perform a set of voluntary facial movements. the participants were chosen to be inexperienced in carrying out the movements to avoid more easily measured overly expressive movements that experienced participants might perform. the movements were: lowering the eyebrows, raising the eyebrows, closing the eyes, opening the mouth, raising the mouth corners, lowering the mouth corners, and relaxation of the face. the relaxation was included to help the participant relax during the experiments while doing the other movements. the movements were instructed to be performed according to the guidelines of facs [20]. the participants were instructed to activate only the needed muscles during each of the movements. after a brief practise period and verification that the participant made the movements correctly, the device was worn by the participant as shown in figure 1: the top extensions targeted the eyebrows, the middle ones the cheek areas, and the bottom ones the jaw and mouth area. the distance of each of the measurement electrodes from the facial tissue was adjusted to be as close as possible without the electrodes touching the face during the movements. this way the distance was approximately 1 cm for all electrodes. in the experiments, synthesized speech was used to give instructions to the participants to perform each individual movement. after putting on the device, two repetitions of each of the movements were carried out in a controlled practise session to familiarise the participants with the experimental procedure. the actual procedure consisted of ten repetitions of each movement carried out in randomised order. participants were given 10 seconds to complete each repetition. a mirror was used throughout the experiments to provide visual feedback of the facial movements to the participants. figure 1. the wearable measurement device. the numbers represent the extension pieces that house the measurement electrodes. the actual electrode locations are on the pieces facing the face. top middle left right bottom acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 80 2.4. data processing 2.4.1. signal processing principle figure 2 shows a diagram of the pre-processing that was applied to the signals prior to further data processing. first the capacitive signals were converted to signals proportional to the physical distance between the facial tissue and the measurement electrode. the conversion normalises the sensitivity of the measurement to the distance. the capacitance measurement was modelled with the equation of a parallel plate capacitor: , d a c   (1) where ε is the permittivity of the substance between the plates, a the plate area, and d the distance between the plates. one plate is formed by the measurement electrode and the other by the targeted facial tissue. while the surface profile of the facial tissue is often not a plate, each unique profile can be considered to have such an equivalent plate that the equation 1 can be applied. since the relationship between the capacitance and the distance is inversely proportional, the sensitivity of the capacitance to the distance is dependent on the distance itself. the absolute distance is not of interest, and a measure proportional to the distance can be calculated as , 11 bs p ccc d   (2) where cs is the capacitance value and cb is the base level of the capacitance channel. each channel has a unique base level that is affected by the length of the electrode cable and the surroundings of the electrode determined by its position on the extension. for the conversion, the base levels of all the capacitance channels were measured when the measurement electrodes were directed away from conducting objects. smoothing and baseline removal were applied to the distance signals computed with equation 2. these two steps were different when locating the facial activity and when classifying it. the differences are explained below in the corresponding sections. after processing the signals, only the first 4.5 seconds of the signals during each repetition of the movements were considered when calculating the results. the remaining 5.5 seconds of each 10-second repetition was neglected from further analysis because all the participants had already finished the instructed movements by then, and they sometimes carried out other movements to relax during that remaining time. 2.4.2. locating facial activity the smoothing applied to the distance signals when locating the facial activity was done with a moving median filter with a length of 35 samples (approximately 1.2 seconds). this was done to remove noise. further, the baselines of the signals were removed by subtracting the signal means during each repetition of the instructed movements. the baseline removal normalises the signal sequences so that they represent the relative changes in the physical distance. principal component analysis (pca) was carried out to the processed signal sequences to find out the locations of the facial activity during the instructed facial movements. pca is a linear method which transforms a data matrix with multiple variables, measurement channels in this case, to a set of uncorrelated components [22]. these principal components are linear combinations of the original variables and they describe the major variations in the data. pca decomposes the data matrix x, which has m measurements and n channels, as the sum of the outer product of vectors ti and pi and residual matrix e: ,2211 eptptptx  t kk tt  (3) where k is the number of principal components used. if all possible components are used, the residual reduces to zero. vectors ti are called scores, and pi are eigenvectors of the covariance matrix of x and are called loadings. the principal components in the equation 3 are ordered according to the corresponding eigenvalues. to localise facial activity, the first principal component and its loadings were considered. the first principal component describes the major data variations, and, thus, the location of the most significant facial activity can be identified by analysing the loadings of the corresponding measurement channels. for the analysis, the loadings were normalised by dividing their absolute values with the sum of the absolute values of all channels. as a result, the sum of the normalised values is equal to 1. to present the results, the vertical location of each repetition of the movements was mapped to the part of the face that introduced two of the three most significant relative loadings of the first principal component (m-out-of-n detector). for calculating percentages of successful mappings, the correct source of activity was considered to be the top extension channels for the lowering and raising eyebrows as well as the closing eyes movement, the bottom extension channels for the opening mouth and lowering mouth corners movements, and the middle or bottom extension channels for the raising mouth corners movement. median loadings of the 10 repetitions of each movement were calculated for each participant and channel separately to verify the decisions about the correct sources of activity. 2.4.3. classifying facial activity smoothing causes a delay. therefore distance signals without smoothing were used when classifying the facial activity. baseline removal was carried out to the distance signals figure 2. a block diagram of the signal processing. raw signal conversion to distance smoothing filter baseline removal processed signal acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 81 directly. figure 3 presents the algorithm used for solving the baseline for its removal. the baseline calculation was based on a median filter. the median can perform well in the task because the signals were expected to have longer baseline sequences than the ones resulting from facial activity. the median filter applies a logic that only selects part of the samples as baseline points for the median calculation. the selection is based on a constant false alarm rate (cfar) processor that calculates an adaptive threshold based on the noise characteristics of the processed signal [23], [24]. the distance signal was first pre-processed with a filter that implements a differentiator, a single-pole low-pass filter with a time constant of 20 ms, and a full-wave rectifier. this makes the input suitable for the cfar processor. the current sample is used as a test sample for the processor. the implemented version of the processor uses samples before the test sample, referred to as reference samples, for calculating the threshold. the processor also leaves out samples closest to the test sample as guard samples to reduce the information overlap between the test and reference samples. samples closer than 1 second to the test sample were considered guard samples and the following 14 seconds were considered as the reference samples. the mean of the reference samples was then calculated and multiplied by a sensitivity parameter to obtain the adaptive threshold. the sensitivity parameter was chosen to be 0.5 in this case. the threshold was then fed to a comparator with the pre-processed test sample to find out if the test sample did not exceed it. the respective samples of the input signal were included in the median calculation by the selective median filter that had a length of 15 seconds. finally, the baseline is calculated with a 2-second moving average filter from the median filtered signal to smooth step-wise transitions in the baseline level. a method to classify facial movements based on the processed multichannel data was implemented. the classification method was based on hierarchical clustering. it used the ward’s linkage which forms clusters by minimising the increase of total within-cluster variance about the cluster centre [25], [26]. a fixed number of 14 clusters were chosen for the clustering based on the different events that the data represents (6 movements and the baseline). this selection allows 2 clusters for each event on average, which allows some deviation of the data when performing repetitions of the same movement and elongation of the data points during a movement, because the ward’s method is known not to be good at handling elongated clusters and outliers [26]. the work-flow of the classification is presented in figure 4. the clustering first takes multichannel data with signal baselines removed and the labels of the data (information about the instructed movements) as an input. the data are first clustered and then cross tabulated against their 6 possible labels. based on the tabulation, the clusters are identified so that first the clusters that represent the baseline data are identified. a cluster is identified as a baseline cluster if it contains data points with at least 5 different labels from the 6 possible (m-out-of-n detector). other clusters are identified based on the label that has the largest number of samples in the cluster. in the offline classification, the data are then classified to represent the movement its cluster was identified with. a real-time classification can further be made based on previously identified clusters. first, the cluster centre points are calculated. then each new data sample is classified to represent the movement that the cluster nearest to it was identified with. thus, the real-time classification only requires the calculation of euclidean distances to the cluster centres for each new data sample. all the collected data were included in the offline classification. the real-time implementation of the classification was evaluated so that a randomly chosen repetition of each movement was included in the identification of the clusters, and the remaining 9 repetitions were used as test data to evaluate the performance of the method. to present the results of the classification, the percentages of the data points that were classified as baseline were calculated. a high percentage would indicate problems in separating the movements from the baseline. from the data points that were not classified as the baseline, the percentages of correctly classified ones were calculated. data points were considered to be correctly classified if they were classified as the movement that the participant was at that time instructed to perform. figure 3. a block diagram of the baseline calculation for the baseline removal when classifying facial activity. figure 4. a block diagram of the classification of the facial movements based on the data. input pre-processor filter comparator cfar processor selective median filter smoothing filter baseline data labels multichannel data hierarchical clusteringcrosstabulation cluster identification input identify baseline clusters identify other clusters input calculate cluster centrepoints find nearest cluster real-time classified data offline classified data acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 82 3. results figures 5-8 show examples of the signals the measurement channels registered during the experiments and how the conversion from capacitance signals to ones that are proportional to the distance normalises the signals. 3.1. locating facial activity examples of the detected facial activity presented as the loadings of the first principal component are shown in figures 9 and 10. the performance of locating the different movements based on principal component analysis is presented in table 1. out of the 6 included voluntary movements, opening mouth and raising mouth corners are located correctly in all the repetitions with all participants. lowering eyebrows and raising eyebrows are located correctly in almost all the repetitions with all participants. only a single repetition of each is incorrectly located. lowering mouth corners is correctly located except for 4 repetitions with a single participant. closing eyes has a limited success rate in the mapping with 7 out of 10 participants. the locations of the three measurement channels that registered the most significant activity during the experiments figure 5. raw capacitance signals from the 10 repetitions of the raising eyebrows movement with one participant. the different sides of the face are represented on the left and on the right. the top, middle, and bottom graphs represent the measurements from the corresponding extensions. the colours represent the different channels as shown in figure 1: red, green, blue, and grey starting from the centre of the face. signal baselines are aligned for the illustration. figure 6. signals after the conversion to distance signals from the 10 repetitions of the raising eyebrows movement with one participant. signal baselines are aligned for the illustration. figure 7. raw capacitance signals from the 10 repetitions of the opening mouth movement with one participant. figure 8. signals after the conversion to distance signals from the 10 repetitions of the opening mouth movement with one participant. figure 9. the facial activity as represented by the loadings of the first principal component during the raising eyebrows movement with one participant. each graph represents the loadings of the 10 repetitions from the measurement channel of the corresponding physical location. figure 10. the facial activity as represented by the loadings of the first principal component during the opening mouth movement with one participant. table 1. the percentages of successful mapping of the vertical location of the movements. the last row shows the means and standard deviations for the movements. participant lowering eyebrows raising eyebrows closing eyes opening mouth raising mouth corners lowering mouth corners 1 100 90 50 100 100 100 2 100 100 60 100 100 100 3 100 100 90 100 100 100 4 100 100 100 100 100 100 5 100 100 30 100 100 100 6 100 100 100 100 100 100 7 100 100 40 100 100 100 8 90 100 80 100 100 100 9 100 100 100 100 100 60 10 100 100 80 100 100 100 mean 99 ± 3 99 ± 3 73 ± 26 100 ± 0 100 ± 0 96 ± 13 left right c ( a. u. ) 0 1 2 3 4 5 6 t (s) 0 1 2 3 4 5 6 t (s) left right d p ( a. u. ) 0 1 2 3 4 5 6 t (s) 0 1 2 3 4 5 6 t (s) left right c ( a. u. ) 0 1 2 3 4 5 6 t (s) 0 1 2 3 4 5 6 t (s) left right d p ( a. u. ) 0 1 2 3 4 5 6 t (s) 0 1 2 3 4 5 6 t (s) left right left right acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 83 according to the median loadings verified that the decisions regarding the correct sources of activity were justified according to the used order statistic, the median. the three most significant channels included incorrect locations only with one participant during the raising eyebrows movement and with 4 participants during the closing eyes. 3.2. classifying facial activity examples of classified data are shown in figures 11 and 12. table 2 shows the percentages of samples that were classified as baseline. a paired t-test (significance level 0.05) did not reveal statistically significant differences between the percentages of the offline and the real-time classification. in the case of the closing eyes, the percentages show that the movement could not be classified as a movement but was classified as the baseline. the percentages for the other movements reflect the durations of the movements because the participants were not given any instructions about how long to hold them. the results of the offline and real-time classification methods are shown in tables 3 and 4, and there are no statistically significant differences between the different methods according to a paired t-test (significance level 0.05). 4. discussion the facial activity was mostly correctly located, but in a limited number of cases locating gave incorrect results. this can be a result of several factors. firstly, the participants could not always carry out the movements exactly as instructed, but some unintentional activity of other muscles was included. secondly, the measurement and the applied data processing both are slightly sensitive to the movement of the prototype device on the head. this may result in false detection of activity when the device moves instead of the facial tissue. thirdly, including only one principal component may limit the performance when locating the activity. the amount of the variance explained by the first principal component was not analysed, but if it were it could be used to provide an estimate of the certainty in locating the activity. more principal components could be added to the analysis to reduce the uncertainty. finally, the mentioned error sources are also affected by the noise in the measurement. the noise is dependent on the distance of the measurement electrodes from the target. the current implementation normalises the signal levels, but the normalisation also scales the noise so that measurements with the facial tissue further away from the measurement electrode include more noise than table 2. the average percentages and standard deviations of data points that were classified as baseline ones in the offline and real-time implementations of the classification. the number of samples is 1310 and 1179 for the two implementations, respectively. lowering eyebrows raising eyebrows closing eyes opening mouth raising mouth corners lowering mouth corners offline 48 ± 18 50 ± 19 99 ± 2 34 ± 15 41 ± 17 34 ± 12 realtime 53 ± 14 58 ± 14 99 ± 1 39 ± 15 49 ± 21 41 ± 12 table 3. the percentages and standard deviations of correctly classified data points in the offline classification. the dashes mean that all the samples were classified as the baseline. participant lowering eyebrows raising eyebrows closing eyes opening mouth raising mouth corners lowering mouth corners 1 98 100 79 99 95 2 72 100 0 70 100 98 3 85 100 97 36 100 4 100 100 91 100 90 5 100 100 100 100 100 6 96 100 0 81 95 100 7 93 100 0 88 96 100 8 100 75 100 100 100 9 100 100 100 90 100 10 100 100 100 100 100 mean 94 ± 9 98 ± 8 0 ± 0 91 ± 11 91 ± 20 98 ± 3 table 4. the percentages and standard deviations of correctly classified data points in the real-time implementation of the classification. participant lowering eyebrows raising eyebrows closing eyes opening mouth raising mouth corners lowering mouth corners 1 95 100 84 94 98 2 100 100 100 100 100 3 87 100 60 83 100 4 100 100 99 100 87 5 100 100 0 100 98 100 6 94 100 0 92 78 100 7 94 100 78 98 100 8 96 83 90 100 100 9 98 100 0 98 89 100 10 100 100 100 100 100 mean 96 ± 4 98 ± 5 0 ± 0 90 ± 13 94 ± 8 99 ± 4 figure 11. classified data points after the baseline removal from the 10 repetitions of the raising eyebrows movement with one participant. the data points that were classified as the baseline are black, and the correctly classified data points are shown in colour. figure 12. classified data points after the baseline removal from the 10 repetitions of the opening mouth movement with one participant. left right d p ( a. u. ) 0 1 2 3 4 5 6 t (s) 0 1 2 3 4 5 6 t (s) left right d p ( a. u. ) 0 1 2 3 4 5 6 t (s) 0 1 2 3 4 5 6 t (s) acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 84 when the tissue is closer. the smoothing could be more carefully considered to find the most suitable method for noise removal in this case. while the discussed factors may affect the performance, the reason for the limited performance with the closing eyes movement can be considered to be the small movement that it causes to the facial tissue at the measured locations. it should be noted that the presented method for locating the activity only implements a rough mapping of the simple movements. since the exact locations of the facial movements when certain muscle activation occurs varies between individuals, determining the precise location of the movements may not even provide additional value without first characterising the individual’s facial behaviour. thus, the classification was introduced to differentiate between the movements, and it could be applied also to more complex expressions. the classification was based on using hierarchical clustering to identify clusters formed from the measured data. applying principal component analysis in real-time for the task was also considered. however, as a statistical method, it requires numerous samples to compute the principal components reliably. this causes delays dependent on the chosen window length. the processing of the implemented classification, however, does not impose additional delays since it only requires the calculation of distances between points after the clusters have been identified offline. the percentages of data points that were classified as baseline show that the closing eyes movement is problematic also in the classification. the data points during the movement can be expected to be close to the baseline if at all visible in the data. the example graphs of the classified data points (figures 11 and 12) show the changes from the baseline that are required for the classification to identify the data point as something else. the graphs also show that the delay for this is acceptable, even if the absolute delay cannot be calculated because no information about the true onset of the movements was extracted in this study. the performances in classifying the data points correctly during the different movements show that the offline and the real-time versions both perform very well. this is a good result as the real-time version only used data from a single repetition of each movement for identifying the clusters compared to all the 10 repetitions in the offline one. incorrectly performed movements, movement of the device on the head, and noise are possible sources for the errors also in the classification. in addition, the transition phases at the beginnings and the ends of the movements when the data points are close to the baseline can be expected to be more susceptible to incorrect classification. the number of clusters chosen for the classification obviously affects how many movements and variations of the movements can be distinguished from one another. in this study, the number was chosen to be relatively small and the selection was based on the number of the included movements. the identification of the clusters used the information about the movement that the participant was instructed to perform to label each data point. selecting a larger number of clusters would make it possible to identify variations of the movements, but it would also require more information for the labelling. one alternative would be to visually inspect video recordings to provide the labels. this could be done after the clustering to label each cluster rather than providing a label for each data point one by one. this study only considered simple voluntary facial movements. since complex facial expressions, even the spontaneous ones related to emotions, are formed by combinations of simple movements, they can be expected to be classified in the same way and as easily as the simple movements. they will just span a different volume in the multidimensional space of the measured data points. however, the movement ranges of facial tissue during spontaneous expressions are often more limited than in the simple movements of this study. this may introduce challenges in classifying some of the expressions. 5. conclusions a new method for mobile, head-worn facial activity measurement and classification was presented. the capacitive method and the prototype constructed for studying it were shown to perform well both in locating different voluntary facial movements to the correct areas on the face and in classifying the movements. locating the movements with principal component analysis does not require a calibration of the measurement for the user, and the presented classification only required one repetition of each movement for identifying the movements before the classification could be carried out in real-time. the presented facial activity measurement method has clear benefits when compared to the computationally more intensive vision-based methods and the emg that requires attachment of electrodes on the face. future research on the method should include verifying that the classification works with more complex expressions, i.e. with combinations of activity at different locations on the face. furthermore, determining the intensity level of the activity of different facial areas could provide additional information. it could be studied how different activation levels can be distinguished from one another with the presented method, and whether even the smallest facial muscle activations can be distinguished. acknowledgement the authors would like to thank nokia research center and the finnish funding agency for technology and innovation (tekes) for funding the research. ville rantanen would like to thank the finnish cultural foundation for personal funding of his work, nokia foundation for the support, and imeko for the györgy striker junior paper reward that was received at the xx imeko world congress. references [1] a. b. barreto, s. d. scargle, and m. adjouadi, “a practical emgbased human-computer interface for users with motor disabilities”, journal of rehabilitation research & development 37 (2000), pp.53–64. 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[26] e. rasmussen, “clustering algorithms”, in: w. b. frakes, r. baeza-yates (eds.), information retrieval: data structures and algorithms, 1st edition, prentice hall, upper saddle river, nj, usa, 1992. acquisition and integration of spatial and acoustic features: a workflow tailored to small-scale heritage architecture acta imeko issn: 2221-870x june 2022, volume 11, number 2, 1 14 acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 1 acquisition and integration of spatial and acoustic features: a workflow tailored to small-scale heritage architecture jean-yves blaise1, iwona dudek1, anthony pamart1, laurent bergerot1, adrien vidal2, simon fargeot2, mitsuko aramaki2, sølvi ystad2, richard kronland-martinet2 1 umr cnrs/mc 3495 map 31 chemin joseph aiguier 13402, marseille, france 2 aix marseille univ, cnrs, mc, prism umr 7061, 31 chemin joseph aiguier 13402, marseille, france section: research paper keywords: heritage architecture; interdisciplinary data acquisition, panoramic-based photogrammetry; 3d sound; visualisation citation: jean-yves blaise, iwona dudek, anthony pamart, laurent bergerot, adrien vidal, simon fargeot, mitsuko aramaki, sølvi ystad, richard kronlandmartinet, acquisition and integration of spatial and acoustic features: a workflow tailored to small-scale heritage architecture, acta imeko, vol. 11, no. 2, article 22, june 2022, identifier: imeko-acta-11 (2022)-02-22 section editor: fabio santaniello, university of trento, italy received march 5, 2021; in final form june 14, 2022; published june 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was supported by the anr (project-based funding agency for research in france). corresponding author: jean-yves blaise, e-mail: jean-yves.blaise@map.cnrs.fr 1. introduction architecture is perceived not only through vision but also through audition and other senses – hence characterising it is likely to require more than studying its physical envelope. this fact is increasingly acknowledged, including in heritage studies, as illustrated in initiatives focusing on “places” as such [1], [2] or on their use [3]. however in the specific context of small-scale architectural heritage, often left aside from large, well-funded, heritage programmes, scientists and local communities face a specific challenge. indeed, in that context, studying, documenting, and enhancing such buildings requires new methods minoring as much as possible the complexity and cost of workflows. the above statements correspond to both sides of the equation this research aims to solve: acquiring and integrating spatial and acoustic characteristics, while maintaining a level of simplicity suited to buildings without prestige, often neglected or at risk. our global objective is to support a multidimensional and interdisciplinary characterisation of small-scale architectural heritage. this contribution is centred on the programme’s initial milestone: a data acquisition and processing chain integrating visual and auditory data. it is (above all) about methodology: as will be shown we mainly combine and extend pre-existing technologies and tools in a novel way. the photogrammetric survey is based on a 360 panoramic camera (a technology discussed in [4]), and 3d point clouds are exploited inside the potree renderer (well known in the application field) [5]. on the other hand, the effects of the room’s configuration on the sound rendering has been studied for decades with for instance the seminal works on reverberation abstract this paper reports on an interdisciplinary data acquisition and processing chain, the novelty of which is primarily to be found in a close integration of acoustic and spatial data. it provides a detailed description of the technological and methodological choices that were made in order to adapt to the particularities of the corpus studied (interiors of small scale rural architectural artefacts) keeping in mind the backbone objective of the research: facilitate comparisons (among buildings, among spatial and acoustic features). the research outputs pave the way for proportion-as-ratios analyses, as well as for the study of perceptual aspects from an acoustic point of view. ultimately, “perceptual” acoustic data characterised by acoustic descriptors will be related to “objective” spatial data such as architectural metrics. the experiment is carried out on a set of fifteen “small-scale” rural chapels, which is a corpus intended at fostering cross-examinations in the context of an architectural programme acting as a constant. the specificity of this corpus, in terms of architectural layout, usage, and economic or access constraints, will be shown to have had a significant impact on choices made during the acquisition and processing chains. mailto:jean-yves.blaise@map.cnrs.fr acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 2 made by sabine [6]. recent improvements in the field of 3d sound make it now possible to accurately reproduce previously recorded sound fields, thanks to an array of loudspeakers. this allows for an experimental analysis of the induced perception, a key issue as far as this research is concerned. the originality of the research lies in a combination of technologies and methods, with a twofold ambition: to develop an interdisciplinary approach that should be maintained all along the data acquisition, processing, and analysis chain (the word interdisciplinary should be understood as defined by [7]: mutual integration of concepts, methodology and procedures), to single out a grid of metrics (space + sound) aiming at helping analysts to cross-examine data on and between buildings. the study is carried out on fifteen interiors of rural chapels in south-eastern france. this collection has shed light on a significant set of feedbacks in terms of methods and open issues. we opted for such a corpus because of its consistency in terms of their initial intended function, and its variability in terms of actual architectural layout. this collection opens an opportunity to analyse and compare the acoustic responses of buildings that initially share a common usage scenario (the christian ritual) although they differ physically in many ways, as shown in figure 1 (type of covering, materials, vicinity, etc.). this paper focuses on the survey step (section 3), the data processing step (section 4), and comments on obstacles and limitations of the protocol (section 5). the former and the latter steps hinge on a set of critical choices in terms of corpus, practical constraints, and surveying technologies of architectural interiors. they also revolve around analytical needs (perception analysis, extraction of proportions, etc.). these aspects are debated in sections 2 and 6 so as to put the experiment in perspective. 2. research context and requirements the data acquisition and processing chain presented herein builds on a series of constraints and choices that ensue from the corpus under scrutiny (small scale rural architecture) and the overall objective of the research (interdisciplinarity, reproducibility, comparability). this section comments on these specific constraints and then positions choices made with regards to the state of the art. 2.1. the corpus, the analytical needs the setup and protocol was designed to address a set of constraints that are key to list if wanting to assess the approach’s reproducibility and relevance. briefly speaking the initial priorities were the following: collect a consistent corpus (comparability issue), tailor the acquisition phase to that corpus (small scale, poorly funded heritage), design a multimodal protocol that would open on repurposable outputs. those outputs should be valuable for local actors in their effort to favour a better recognition of their heritage assets. they should also be relevant for scientists and scholars in their analytical tasks (on the architectural analysis side as well as on the acoustics analysis side). the first step of the programme was an interdisciplinary debate about the survey step, including feasibility tests in laboratory conditions. an architectural interior relatively consistent with the corpus was picked up, and named “fake chapel”. it served as a substitute for “real” chapels during the early stages of the survey protocol’s development (see section 2.2). architects, surveyors and acousticians confronted their views on their specific technical requirements in terms of survey, but also on how the data would be exploited in subsequent phases. that first step ended on the co-design of strategic, organisational, technical specifications: 1) a unique setup and protocol that would be reproduced identically across the whole collection. 2) a selection of architectural layouts adapted to the analysis of compositional patterns, 3) a protocol respectful of a chapel’s original function – hence a spatial distribution of speakers and microphones tailored to specific usage modalities (celebrant vs. listener opposition). the interior spaces need to be analysed in the light of the way they are or were used. some of the chapels were indeed repurposed figure 1. a “parallel coordinates” diagram illustrating the diversity of the corpus – the legend (top) followed by three examples: n.d de la salette (tourves), saint-roch (la verdière) and saint-roch (les mées). each line running from left to right corresponds to one of the fifteen selected chapels. each column corresponds to one of the “diversity factors” (partial view). the first five bars (on the left) correspond to quantitative data. other factors are related to categorical data e.g., covering types (barrel vault, cross vault, roof frame, other vaulting), apse shape (semi-circular, polygonal, rectangular, lack of apse, other), volume complexity (single regular nave vs. buildings with transept or other ruptures in the continuity of the nave), empty spaces vs. buildings with furniture, vicinity factor (isolated buildings vs. buildings with adjoining structures), plan symmetry, etc. blue circles represent the number of chapels corresponding to a given value (diameter of a circle represents a number, min = 1, max = 11). acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 3 (modification of function), but their reason to be and that is the basis on which comparisons can be made lies in the service they initially offered, 4) a need of reliability and accuracy concerning major dimensions of a chapel and positions of the acoustic devices, and relatively low requirement level in terms of density and quality of the 3d point cloud. 5) a contact-free survey protocol, privileging a lightweight instrumentation (accessibility issue – some buildings can only be accessed by foot), 6) a severe time pressure in situ (three hours as a maximum per building, all surveys included, i.e. maximum two chapels surveyed per day), 7) a need to record each building’s soundfield in order to allow for the restitution of its acoustics in an audio cave to perform remote perceptual evaluations and comparisons between chapels, 8) a live recording of usage scenarios: speech (human voice facing the apse or the nave) and sounds produced by a human walking within a chapel according to a specific protocol, 9) a recording of the soundscape, both interior and exterior. at first glance architectural interiors of the corpus that was selected can be seen as a consistent architectural paradigm. this is basically a misconception [8], and we accordingly chose a collection of fifteen buildings that reflects the diversity of the corpus (in terms of their architectural layouts and dimensions – volumes ranging from 171 m3 to 981 m3 (cf. figure 2). the actual acquisitions introduced yet more technical constraints. the whole setup had to be chosen so that it could be carried in backpacks (remote sites). it needed to be autonomous in terms of energy (no power supply in situ), and it had to be adapted to interiors that in some cases could be congested – hence in some cases made it difficult to maintain the geometry of the grid of the instruments. finally, adaptation to lighting conditions proved to be recurring problem. conditions varied from sunny summer days in well exposed buildings with large openings, to rainy winter days in chapels with very small windows located in a shady area. four led panels were used when needed, but their correct positioning can be time-consuming if one wants to avoid too strong contrasts during the photogrammetric survey. the acquisition process is now mature, although still improvable, and can be considered as reproducible. however, it has to be stressed, that in situ there will always be a series of “expert” choices to make (lighting, number and distribution of stations from 14 to 93 stations in our experiments, positioning of the rangefinder, analysis of the interior envelope, etc. ). although this contribution insists on reproducibility, the experiment is tailored to a quite specific set of conditions and constraints. in no way do we claim that our approach is an “all purposes all situations” one. on the contrary, we consider that one of the key outcomes of such an experimental study is to act as food for thinking, especially at a time when technologies often affect methodological choices. it clearly illustrates that attention should be driven towards the “what for” question before addressing the “what with” question, in particular when targeting a lightweight approach, and minor heritage architecture assets. 2.2. the survey protocol: technological and methodological constraints the metric data acquisition protocol proposed follows a twofold objective: a fast and versatile geometric survey of small scale indoor spaces, combined with the data integration need of acoustic measurements (from on-site positioning to combined visualisation). the choice to rely on panoramic-based photogrammetry converge to a single solution (by sharing 360° capture) to address the purpose and the challenge of the multimodal survey. the use of low-cost spherical cameras for photogrammetric reconstruction has been discussed and evaluated in the literature [9], [10], [11], [4]. a critical review of those works provides useful insight on such cameras’ potential efficiency and relevance with regards to our case-studies. most of those works relate possible failings or limitations to the technological limitation of low-cost spherical cameras. and indeed, this technique’s potential weakness is mainly related to the low-resolution of the sensor, and to the low quality of optical components for the hardware part. on the software side, spherical stitching and projection does increase the uncertainty above the standard of pin-holebased photogrammetric reconstruction. with knowledge of these limitations in terms of accuracy we nevertheless chose to build and optimize an experimental setup upon this technique, in the light of this research’s priority n°2: a simplicity suited to unprestigious buildings. the “fake-chapel” (see section 2.1) acted as a calibration space to test, evaluate and improve the data acquisition strategy before the actual acquisition campaigns conducted on the fifteen chapels. because the capture of panoramic images and some photogrammetric rules are contradictory (e.g. parallax condition), panoramic photogrammetry is often used in very specific contexts [12]. nowadays, spherical photogrammetry can be performed in three different ways: the raw images of a constrained multi-camera rig [13] are used separately as a single frame picture usually with highly distorted fisheye lenses, an image set is derived to composite picture using stitching algorithm to get a spherical map (usually equirectangular projection), figure 2. diagrams illustrating the variety of spatial arrangements of the selected corpus (top dark grey elements represent chancels) and the variation of the chapels’ volume (bottom triangles above the axis correspond to buildings located in remote areas, at a distance from villages; triangles below the axis correspond to buildings inside or in the vicinity of villages. acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 4 a stitched panorama is converted to cubic projection from which six cameras corresponding to each face are extracted to be processed as pin-holes. the above-mentioned solutions remain sub-optimal in terms of photogrammetric processing, leaving the optimization of data acquisition as a relevant solution. panoramic-based photogrammetry usually translates the terrestrial laser scanning acquisition canvas with a straightforward linear sequence assuming that omni directional images behave like range imaging. while constant and uniform quality is expected with lasers both raw images and projective maps generated are imperfect for ibm (image based modelling) purpose. the tissot’s indicatrix used in cartography to describe map distortion shows that any projection system, including spherical or cubic projection is non-destructive at pixel level (cf. figure 3a). therefore, panoramic-based photogrammetry is altered from distortion and aberration at many levels, from the optical system of the camera, during stitching step to the projection mapping. we assumed that only a small part of a panoramic capture is optimal for 3d reconstruction. at the raw level (cf. figure 3d), only on the central part of the fisheye picture (focal axis shown in red in figure 6a) is performing [14]. at stitching level (cf. figure 3b), the azimuthal plane has artefacts due to low overlapping of raw images. at the mapping level (cf. figure 3c), in the case of the equirectangular projection, poles on zenith axis are inconsistent, and only the horizontal plane is exempt of strong deformation. based on this observation spherical photogrammetry cannot be considered as omnidirectional (from a qualitative point of view) because the orientation of the camera system and its sensor is not insignificant. in other words, a simple, linear sequence of images, taken from a camera that would be systematically oriented the same way, impacts in a negative way the quality of the geometrical reconstruction. in response to that issue, an original data acquisition sequence was developed, in order to compensate the fact that only the longitudinal axis and the horizontal plane of each shot provide optimal features for photogrammetric reconstruction. instead of walking along the space with a linear sequence, the protocol is based on a dense network of pyramidal sequences (detailed in section 3) composed of translated and rotated camera positions. this protocol basically reintroduces elements recognized to improve photogrammetric reconstruction like: short-baseline and high overlapping stereo-pairs, roto-translation variance in a dense camera network or feature redundancy to reinforce the panorama bundle adjustment. in addition, because the principle of spherical capture is shared between 360° cameras and the 3d sound-field microphone used in this study (the mh-acoustics eigenmike 32), we anticipated a seamless data integration all along the operative chain (i.e. capture, registration, fusion and visualisation stages). the technological analogy of our tools indeed provide similarities to ease data fusion steps and is intended to further facilitate the perceptual analysis, providing immersive environments for both reality-based image and sound captures. 3. a multimodal survey protocol the protocol’s key components are in fact two low-cost 3d cross line self-leveling laser levels (instruments often in use in the building activity). these levels project green laser beams on surfaces. the laser beams are combined so as to mark four planes constituting a reference system. they are also exploited to position sound measuring instruments. intersections of beams on the walls, ceiling and floor are called “named reference points” and act as markers in the scaling of the photogrammetric model. their relative positions are measured using a leica s910 rangefinder equipped with its so-called “smart base” and its integrated tilt sensor. sound recording instruments (microphones and loudspeakers) form a grid allowing for a systematic relative positioning of the instruments with regards to one another (cf. figure 4.). the grid’s positioning in the reference system is also done using the laser rangefinder (except for two microphones, mg and md, positioned thanks to the photogrammetric model). microphones and loudspeakers are mounted on tripods positioned relatively to the named reference points (seven positions, cf. figure 4). these tripods are reused (once the emitting / recording tasks are over) to install the 360 camera and to capture photogrammetric data in these specific points. it hence allows for a double checking of the acoustic (sound recording) instruments’ positions. these shots are also, later on in the process, repurposed as the visual components of online immersive panoramas inside which sound tracks corresponding to the named point ma mc md and mg are displayed according to the corresponding panoramas (see section 6). 3.1. metric and visual data acquisition the photogrammetric data acquisition protocol for indoor space still suffers from several obstacles related the architectural context (narrow and dark spaces, occlusions) of the chapels. in order to gain in velocity, reproducibility and overall efficiency 360 cameras (dual sensors with fisheye lenses) were chosen as the expected accuracy is centimetre-sized. the pyramidal data acquisition protocol has been conceived, evaluated and optimized to face with issues discussed in section 2.2. a flexible acquisition layout has then been framed so as to couple up the photogrammetric acquisition to telemetric surveys of significant points on the building’s surface (using the dxf feature of leica s910), and to the acoustic instruments positions. figure 3. synthetic image illustrating quality variance of an equirectangular projection composed of : tissot’s indicatrix (a), stitching artifact (b), gradient of equirectangular distortion (c) and gradient of double-fisheye distortion. acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 5 the dxf built-in feature is used to extract precise measurements (used as ground control point). it allows to orient all the 3d models in a consistent and constant absolute cartesian coordinates. the technical setting of the metric survey protocol is bounded by economic constraints on one hand (preferably low-tech, lowcost), and compactness on the other hand (compatibility with remote sites) – main components are (cf. figure 5): two huepar 3d cross line self-leveling laser levels, a yi 360 vr panoramic camera 5.7k hi resolution, dual-lens each lens is 220° with an aperture of f/2.0 (360° coverage , produces two unstitched hemispherical photograph for each shooting position), a laser rangefinder leica disto s910 (this instrument outputs dxf files, it is used to survey named reference points on one hand and significant points on the surface of the edifice), a manfrotto tripod (055 series) allowing for horizontal/vertical shootings. the rotational mechanism of the centre column is used to perform a pyramidalbased capture combining the benefits of faster survey (5 positions for a single tripod station) and better reconstruction (from a short baseline cameras network with variable orientations, cf. section 2.2). the main steps of the protocol are as follows: 1) positioning of the laser levels, starting by the one located at the entrance of the chancel. 2) positioning of the grid of instruments (7 tripods), aligned with the levels vertically, and relatively to one another horizontally (the reference point being the theoretical position of a celebrant behind the altar). 3) positioning of the rangefinder so that each and every intersection of laser beams is visible, can be pointed at and surveyed. 4) survey, using the rangefinder, of the grid of instruments – outputs a polyline connecting tripods to 5 points on the building. 5) scaling protocol, using the rangefinder: a polyline that connects all the laser beam intersections. 6) dimensioning protocol, using the rangefinder: a polyline that connects laser beam intersections to elements of the envelope considered as significant (a keystone, the entrance level, a cornice, etc.). 7) photogrammetric survey, using the panoramic camera positioned on each tripod forming the grid, and then on its own tripod, moved in different positions decided in situ. for each position, the pyramidal sequence (cf. figure 6) is repeated and modulated according to the architectural morphology and environmental constraints. 8) acoustic survey: instruments – loudspeakers and microphones are positioned on the grid of tripods, a sine sweep is emitted from each speaker and recorded on the microphones (several times iteratively in order to spot and eliminate “outliers”). figure 4. top, laser beams (green lines) and their intersections form “named reference points” (brown circles) that are visible on the building’s surfaces and surveyed using the rangefinder. the light grey parallelogram is the chapel’s nave, the dark grey parallelogram is its chancel. bottom, auditory instruments positioned relatively to the horizontal plane marked by laser beams. three loudspeakers eg, ec, ed are located right behind the altar, in the chancel (dark downward triangles), at a given distance from one another, with ec aligned on the main longitudinal axis. a fourth loudspeaker, eb, is positioned right under the first microphone, and tilted so as to face the covering of the building. microphone ma is then positioned relatively to loudspeakers eg, and ed at a systematic distance (ma, eg, ed form an equilateral triangle). microphone mc is aligned with ma, on the main longitudinal axis, and positioned at a fixed and systematic distance from ma. microphones mg and md are positioned at the very beginning of the nave, at a fixed distance (one meter) from the walls. figure 5. a sample setup: a the 360 camera; oriented horizontally, b a 3 axes laser level, c tripods on which acoustic devices will be mounted, d intersection of laser beams on the interior’s enveloppe. (st pancrace’s chapel, pyloubier). acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 6 9) live recordings: a given sentence is pronounced systematically by the same person, positioned behind the altar and facing either the chancel or the nave, and footsteps of a person walking from the entrance to the altar and back are recorded. steps 1, 2, 4, 5 are systematic, steps 3, 6 and 7 require an adaptation to conditions found in situ. steps 4 to 7 are conducted before or after the acoustic measurements, steps 8 and 9, depending on the lighting conditions. the pyramidal protocol consists of a complete series of 6 pictures, 5 at the summits of a square-based pyramid and one in the centre. the roto-translation between each camera position is fast and easy to reproduce, from a single tripod position, thanks to the rotating arm of the tripod and to a rotative head (see figure 6, b). opposed positions are generally arranged by common orientations to create stereo-pairs for which: longitudinal capture (camera positioned vertically or horizontally and oriented on the long axis of the chapel shown by the red axis in figure 6a) helps the alignment along the sequence and correlation of elements perpendicular to side walls, zenithal capture (camera positioned horizontally and oriented up and down with blue axis in figure 6b) improves the cover of floor and ceiling, transversal capture (camera positioned horizontally or vertically and oriented on side walls with green axis in figure 6b) is globally used for close-range reconstructions. on top of the general improvement for 3d reconstruction this pyramidal protocol turned out fully versatile against multiple in-situ constraints. from our experience on the 15 chapels that were surveyed, several benefits can be noted in terms of : completeness: minimization of occluded areas concerning architectural, structural or ornamental elements, velocity: gain in acquisition time by minimizing the number of tripod stations, accuracy: increase in redundancy for pointing, scaling and extracting coordinate positions or measurements, security: enabling sufficient back-up data in case of a mistake or error, practicality: avoidance of obstacles in the sequence (including our own equipment that must remain fixed during data acquisition). the overall protocol is relatively fluid, and has been systematically reproduced, with as a result, a good feedback now on stop points, i.e. key aspects or moments that can result in failures (cf. figure 7). figure 6. schema of the world camera system (a) and an example of a complete pyramidal sequence composed of 6 different positions, (b). bottom, real case application: each yellow sphere corresponds to a camera position. figure 7. a decision diagram positioning key steps of the survey process (steps 1 to 7, grid installation and metric survey), with possible pitfalls and success factors. acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 7 3.2. acoustic measurements from an acoustical point of view, the main goal of this research is to study the influence of rooms on sound perception: in that context getting consistent results requires the same listeners to perform the same perceptual assessment tasks for each chapel. however, since human immediate auditory memory is short, it is not possible to compare a collection of remote chapels in situ. as a work around, the 3d acoustics of the collection of chapels can be measured and rendered in laboratory conditions. for this purpose, a 3d sound technology (microphone and loudspeaker arrays) based on 3d recordings (eigenmike 32) and higher order ambisonics (hoa) restitution [15], [16] was used. measurements consisted in characterising the so-called spatial room impulse responses (srir). an impulse response corresponds to the sound transformation between a sound source (generated by a loudspeaker) and the sound measured at the microphone level. the srir enables, in a second step, to proceed to the so-called “auralization”. thanks to the convolution operation of an arbitrary sound stimulus with the srir, one can play any stimulus as if it were played in situ. in this paper, srirs were derived from the measurement of sine sweeps, as proposed by [17]. emitted sounds were logarithmic sine sweeps from 20 hz to 20 khz with a duration of 10 s followed by 10 s of silence. this method has many advantages, including fast measurements, good signal to noise ratios (snr) and immunity to source distortions [18]. it is well suited for quiet closed spaces such as rural chapels. the main drawback is that this technique is sensitive to impulsive noise. to overcome this problem, each measurement is repeated three times in a row and the srir is derived from take with the best signal to noise ratio. since the aim was to investigate the acoustics related to the sites’ initial use, i.e. a celebrant near the altar speaking to the audience in the nave, we placed a loudspeaker in the middle of the chancel (point ec, cf. figure 4). two lateral loudspeakers (eg, ed) were then aligned with ec at a distance of 1.25 m (epistle side vs. gospel side in terms of initial use, or if thinking about contemporary reuses of chapels simulation of the rendering of a musical trio). we systematically placed the microphone at point mc at a distance of 5.5 m from ec, and at the same height (cf. figure 8). this distance was constrained by the smallest chapel’s dimensions and corresponded to the largest source-tomicrophone distance that can be obtained in the configuration presented in figure 4. at this distance, the angular spacing between the lateral loudspeakers and the frontal loudspeaker is only 13°. we therefore repeated the same measurements at a closer distance (point ma, apex of an equilateral triangle eg ed – ma, cf. figure 4). an “invariable” placement has been chosen instead of a “proportional” placement since the source-receiver distance plays a major impact on the room acoustics rendering. indeed, listening at a fixed distance allow to assess only the sound field in the room independently of the measuring distance. finally, we placed a fourth loudspeaker 40 cm below the microphones in ma and mc. this specific measurement aims at recording the soundfield as if both the transmitter and the receiver were the same person. it can be used in psycho-physical experiments requiring real-time auralization of autophonic stimuli. as an example, we plan to study the influence of room acoustics on musicians’ gestures. as far as the equipment is concerned, the main measurement was recorded with a 3d microphone released on the consumer market, the mh-acoustics eigenmike 32 (em32). this spherical array of 32 microphones has already been used for sound field analysis and for perceptual studies [19], [20]. it allows precise spatial recording of the sound-field that can further be converted for restitution purposes to hoa format up to the 4th order. the loudspeakers used were genelec 8020c. this loudspeaker is not omni-directional (as required for measurement of acoustics parameters [21]), but as mentioned earlier the main goal of these measurements was to proceed to auralization. for this purpose, omni-directionality was not required since the sources to be auralized have their own directivity pattern (e.g. voice, guitar, etc.). moreover, this loudspeaker was chosen here for its compactness, and its relatively low-cost (compared to dodecahedron omni-directional sources) while having fair frequency characteristics (+/2.5 db, 66 hz to 20 khz). this protocol was deployed on the fifteen chapels of the corpus. additionally, a reference measurement based on the same layout was performed in an anechoic chamber (cf. figure 9). this reference measurement allowed to characterise the setup in free field (i.e. without any room effects). while the main measurement consisted in recording the srir, we also recorded the voice of a person positioned behind figure 8. vertical alignment of the eigenmike 32 microphone, once positioned on the tripod in point mc, using the laser beams (n.d. de la salette, tourves). figure 9. the eigenmike 32 and speakers positioned for reference measurement in an anechoic room. acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 8 the altar and pronouncing a given sentence while facing the chancel and then facing the nave. the idea was to open up on a qualitative measurement of the impact of the vatican ii council’s reforms on the way the celebrant’s discourse is perceived when he faces the people (current ritual) and when he faces the altar (the way it used to be prior to the reform). additionally, footsteps of a person wearing systematically the same shoes and clothes and walking from the chapel’s entrance to the altar were also recorded. finally, 5 min of the soundscape inside and outside the chapel were recorded using a zoom h3vr. the soundscape is related to the recording moment (day vs night, summer vs winter, etc.), but for practical reasons we could not record at the same time in the different places. to quantify the temporal influence, for one specific chapel we recorded the soundscape during 2-min each 30-min during 10 days and repeated the process twice (in february and in july). for these measurements we used the labmaker audiomoth, a low-cost acoustic monitoring device used for monitoring wildlife. we repurposed it here to monitor the soundscape with a twofold prospective aim, i.e. characterising (qualitatively) the variations of the soundscape over time and characterising the way the building acts as an acoustic “filter” (both the exterior and the interior soundscapes were recorded at the same time). 4. initial data processing results of the acquisition step act as inputs processed independently at first and then pulled together again as combined outputs in a twofold way: end-user products and analytical overlays to the potree 3d pointcloud renderer. this section focuses on describing the way raw data is processed, and a discussion on how the data is repurposed with regards to this or that objective is proposed in section 6. 4.1. metric and visual data processing the first step is to produce the 3d point clouds from the yi360 panoramas. the photogrammetric processing is done using the agisoft metashape suite, with micmac as a prospective alternative solution. to do so named points acquired with the rangefinder are transferred into a csv-formatted list. they are then used as control points in order to scale the photogrammetric model (cf. figure 10). the resulting point cloud is then exported and integrated in the potree renderer, a free open-source webgl based point cloud renderer developed at tuwien [5]. one of its most valuable aspects is that it allows for the development of “overlays“additional functionalities that can be tailored to specific user needs. the first add-ons introduced focus on viewing the input data resulting from the survey protocol itself, for each chapel, i.e. on one hand the dxf input extracted from the rangefinder and on the other hand the panoramas extracted from the 360 camera. concerning the former, the leica s910 rangefinder allows the surveying of a maximum of thirty points in a row, outputted as one single dxf file. this is why three different survey protocols had to be conducted (grid of auditory devices, scaling and direct measures), with from nine to seventeen points surveyed for each. as a result a step of realignment of the dxf outputs was necessary. they are automatically realigned geometrically in the same frame, when loading each chapel inside the viewer. this is done from a manually created text file that identifies the first two reference points for each of the three dxf files associated with each chapel. from these two points, all the other dxf points are readjusted by translations, then rotations, and finallly displayed in the renderer. this adjustment also makes it possible to display, in their correct positions, each thumbnail image associated with each dxf point. these images are recorded by the leica distortion camera all along the protocol. concerning the panoramas extracted from the yi 360 camera, they are materialised in the renderer by spheres (cf. figure 6), textured with the stitched 360 panoramic photos. spheres are positioned from a text file associating each image file name with its xyz position extracted during the photogrammetric processing. they give access to the corresponding panorama (viewed using the panolens js library, cf. section 6). finally, other specific add-ons allow on-the-fly measurement on dxf points, the naming of these points, the visualization of the laser levels, and the representation, based on an on-the-fly computation of the approaching volume of chapels by voxelbased segmentation of the dense point cloud. the renderer is used to display a complete point cloud, but also allows for user-monitored selections of sub-clouds (sections corresponding to the laser beams, segmented upstream – cf. figure 10). 4.2. acoustic data processing for all measurements using the eigenmike a few operations were applied to the recorded signals. first, the 32 input channels were encoded in the spherical harmonics domain using a vst plugin provided by mh acoustics. the eigenmike allows an encoding up to the 4th order on the spherical harmonics basis, corresponding to a 25-channels signal. this 25-channels signal is then decoded in two ways: (i) for a restitution through a 42loudspeakers; and (ii) for a restitution through headphones with a binaural conversion [22]. for measurements using other microphones (neumann and zoom), no specific operation was required after the acquisition. sounds reproduced were either directly the ones recorded on site (speech and footsteps) or the characterised srir convolved with monophonic stimuli (as mentioned in section 3.2). in the metrology field in general, measurements are subject of uncertainties. in this work in particular, the instruments’ positions could differ, and the variability of positions is quantified in the following. tripod positions of points mc, eg, ec and ed were measured using the leica disto. the distance mc-ec over the 15 chapels was 557 +/5 cm. the standard deviation was 5 cm over the 15 chapels, corresponding to the centimeter-sized accuracy targeted. the mean distance was 557, figure 10. the polyline that corresponds to the scaling protocol (dxf outputted by the leica disto, twelve control points), represented inside the potree point cloud renderer. here only a sub-coud correspoding to the horizontal laser beam is shown. acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 9 slightly higher than the 550 cm targeted, but this disto measurement corresponded to the top of the tripods, while the instruments placed on top of it (loudspeaker and microphone) have a centimetre-thickness explaining this difference. 5. obstacles and limitations experimenting the multimodal acquisition and processing the chain on fifteen different chapels shows that the overall method is sound, with some clear strong points. it is fast, reproducible, lightweight enough to be applied in remote sites. ultimately it does correspond to what were our analytical needs: easy extraction of architectural features, close integration of acoustic and metric data, and a ground for comparisons and correlations. briefly said the protocol allows for a quick multimodal acquisition, with the scaling of the photogrammetric model facilitated by the combined use of self-levelling levels, of a rangefinder and of a low-cost 360 camera. yet the experimentation also highlighted some difficulties that one may have to overcome at different steps of the protocol when applying it to different use cases. first and foremost the method is tailored to a quite specific set of requirements (and particularly in terms of corpus) – a limitation per se (see section 2.1). with a closer look at the acquisition time, the method does requires a step of adaptation to in situ conditions such as lighting or congested spaces, factors that affect the photogrammetric acquisition. having to cope with local conditions is natural and not surprising, but the quantity of potential factors of failure to take into consideration is significant. typically pointing at intersections of the laser beams with the rangefinder may be seen as a very straightforward task, but in practice potential occlusions, surface conditions or angles of incidence have to be dealt with. furthermore, one should keep in mind limitations due to the photogrammetric process itself in conditions where contrasts lack. thanks to the global efficiency and robustness of our protocol, the complementary use of terrestrial laser scanner (faro focus 3d) was required in only two of the fifteen sites, exemplifying the limitation of the geometric survey method. one chapel was textureless, mostly covered with white painting which is prohibitive for ibm (cf. figure 11). for the second chapel, the low resolution of the camera was suspected not to cover efficiently the most difficult chapel of the corpus (in terms of dimensions, volume and architectural complexity). at processing time, the quality of the 3d point clouds produced varies noticeably due to the above mentioned factors. it can be seen as good enough in a research programme that targets services like extracting dimensions or positioning instruments in space relatively to a systematic reference system, but it obviously is not good enough if targeting a fine-grain 3d mesh reconstruction. so at the end of the day comes the question of how to rate the “quality” and “reproducibility” of the protocol, and the corollary issue of “in how does the choice of a low cost 360 camera impact the final results”. as a provisional answer an experiment has been conducted in the “fake chapel” to evaluate the potential gain of upgrading our protocol with professional vr camera developing up to 12k panoramas instead of low-cost devices. aware of the main limitation of the method, the aim was to discuss the scalability of the setup to complex case-studies, regarding resolution vs. accuracy aspect. briefly said, the insight of this qualitative comparison confirms the hypothesis made in section 2.2, that the acquisition strategy can improve results in a more significant way than the technological component itself. our preliminary tests show that the density but also the uncertainty actually increases proportionally to the resolution. therefore a better resolution of image sources doesn’t improve intrinsically the quality and the range of the reconstruction without an efficient data acquisition protocol. as shown in figure 12, the acquisition canvas (i.e. camera position) seems to be more effective on the result than the resolution of the panorama itself. concerning acoustic measurements, there are also some limitations. the placement of the eigenmike microphone is tricky, in particular because it is a spherical object. because its correct placement is important if wanting to ensure comparability, its positioning might be time consuming. this is why, due to the time limitation on site (two sites per day constraint), the number of microphone positions was limited to two (ma and mc, cf. figure 4). a higher number of measurement positions would have allowed a dynamic auralization, so as to reproduce an exploration through the chapel. reproducing such an exploration requires a numerical simulation of the room, and necessitates precise information on acoustic properties of the building materials [23]. in addition, it has to be said that a number of factors related to conditions found in situ (typically congestion of spaces, or simply time of the day) do impact the direct, raw comparability of the data (and in particular of live recordings). these factors should act as a reminder that such data sets should not be overinterpreted, but rather be considered as means to reveal a qualitative acoustic identity of the sites, and to uncover general trends and patterns within the collection of sites. 6. data exploration and reuse the processing chains presented above lead to the production of a series of heterogeneous data sets: figure 11. a case found to be critical for ibm (saint-roch chapel, les mées). top, a stitch showing the predominance of white surface in the edifice. bottom, the reconstruction, combining the photogrammetric model (brown points) and the laser scanner’s output (white points). acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 10 raw photographic material (unstitched hemispherical photograph – raw outputs of the 360 camera), panoramas (stretched, little planet, and round views), 3d point clouds, including localisations of the grid of the acoustic devices, raw quantitative data (dimensions, volumes, xyz coordinates of cameras and acoustic devices), room impulse responses, auralizations in 4 points (ma, mc, md, mg). live recordings, quantitative acoustic indicators (e.g. reverberation time). these outputs are integrated in various ways, in order to allow further exploration and cross-examination of the data sets, and the extraction of goal-bounded interpretations. said differently, the above data sets are repurposed and combined with regards to usage scenarios that range from pattern analyses (e.g. proportions, reverberation, etc.) to the production of dissemination material. the following sub-sections illustrate the two main lines of development that are being followed, one targeting fine grain analysis and quantitative data correlation, one targeting perceptual analyses in contexts ranging from experimental setups to dissemination or edutainment activities. 6.1. acoustic indicators as mentioned before, the spatial room impulse response (srir) measures were primarily needed in order to process auralizations (see section 3.2). but they were also used to compute acoustic indicators [21], [24], [25], [26], listed hereafter. acoustic indicators are a set of quantitative values that are used to characterise and differentiate spaces, roughly said on three aspects (time-related indicators such as reverberation time, tonerelated indicators such as bass ratio, and space-related indicators such as lateral strength). most of the indicators were computed using the 0-order component of the spherical harmonics (omnidirectional component). on the overall, 13 such indicators have been computed, among which for instance the reverberation time (rt20 and edt), the central time, the c50 clarity (see section 6.2), the acoustic strength, the schroeder frequency, the spectral centroid, the bass ratio, the treble ratio, and an approximation of the speech transmission index (without considering the background noise). some indicators were computed using all spherical harmonic components to take into account the spatial: the interaural cross-correlation (based on a binaural reduction), the lateral strength and the lateral energy fraction. in the next steps of the research programme, these indicators will be used, in correlation with quantitative and qualitative architectural features, to characterise the particularities of each chapel, and to analyse the collection as such. 6.2. 2d/3d visualisation of acoustic indicators two of the quantitative acoustic indicators produced are calculated for each position of the eigenmike microphone, and provide values that correspond to a specific angle in space. this gives an opportunity to try and spot differences in the way the sound hits the microphone depending on its origin (emission point) and on reflectance patterns inside the building. these indicators correspond to transmission-reception pairs (four speaker positions and two microphone positions), and correspond to two different methods. the c50 clarity indicator (relation of the early ir – 50 first milliseconds – to the late ir – after 50 milliseconds) is calculated on the 32 channels of the eigenmike microphone: one quantitative value for each capsule, and for each of the seven frequencies (cf. figure 13). it is important to mention that according to [21], c50 is calculated from an omnidirectional room impulse response. in our case the 32 microphones of the eigenmike (em32) are not omnidirectional, especially for high frequency bands. we therefore chose to calculate the c50 on each capsule of the em32 to highlight the early energy differences with respect to the microphone directions. figure 12. qualitative comparison tests conducted inside the “fake chapel” (see section 2.1): result of cloud2cloud (c2c) distance between laser scanner reference and (top) point cloud generated from a 12k panorama without pyramidal protocol and (bottom) compared to a pointcloud generated from the 5.7k panoramas with pyramidal sequence. figure 13. a 2d visualisation of the clarity values for one recording emitting tuple. each symbol corresponds to one of the 32 capsules of the eigenmike microphone, projected on a 2d plane. sectors correspond to the seven frequencies, and colours to a quantitative value (in db, distributed in a 16 values colour scale). note here for instance a dissimilarity for the 8k frequency between values for angles 45 and 69 (top left) and for values 291 and 315, right) that cannot be explained by the layout – particularly simple and regular – of the edifice (saint-roch chapel, les mées). acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 11 another space related indicator is a spatialized energy map (unrelated to the 32 channels) calculated using the pwd method (plane wave decomposition) [27]: one value every 5 degrees of rotation, 2592 values for one microphone position. in this case the energy map evolves with time: twenty time-frames are considered for each microphone position. visualisation of such data sets raises two major and tricky challenges that go beyond the scope of this paper: having the analyst understand the relation of the data with space, and handling temporal aspects (sound evolving with time)an ongoing research issue in the infovis (information visualisation) community [28]. what can be said at this stage is that several exploratory visual formalisms have been developed, both in 2d and 3d. in brief 2d solutions offer a better global and synthetic view (no occultation), but 3d solutions are more efficient in helping analysts spot the potential relation of a value or of a pattern to an architectural specificity in the chapels’ interiors. concerning 3d solutions data is represented in the potreebased renderer (see section 4.1) by spherical heat maps (cf. figure 14), at the two microphone positions (ma and mc). this visualization enables to display the distribution of the reception data over 360 degrees, for both methods. it is interactive thanks to the two tools allowing to choose the transmission-reception pairs or to change the frequencies or the temporality according to the chosen methods. obviously what will come out of this part of the research is the main scientific added-value of the whole approach, but we are here still in an exploratory phase, with challenges that concern infovis methods rather that metrology per se. 6.3. panoramas in the four recording points, with soundtracks as mentioned before, two types of soundtracks are produced: basic live recordings (speech, footsteps and sound scape) and auralizations (simulations of how the same soundtrack would be perceived if played in the different recording positions of the various chapels). these outputs are used (and combined) inside online immersive panoramas corresponding to the four recording positions ma mc mg md (cf. figure 15). the panorama itself is viewed using the panolens.js javascript library, in which users move from one position to another a bit like in 3d bubble worlds. on each position soundtracks are available and can be played, hence allowing listeners to spot differences as they would be perceived in situ. another somehow resembling set of outputs is a collection of interactive pdf flyers (cf. figure 16) on which “little planet" views of interiors are combined with textual triggers that launch the soundtracks (6 audio tracks illustrating the acoustic identity of the building clap, guitar, piano, steps, voice, exterior). these served as a basis for various dissemination initiatives or edutainment-like presentations of the research, typically in sound/space association games in which the audience must associate a building with its acoustics. at the end of the day what can be said about the overall method (a combined acquisition procedure, parallel processing chains, and common data reuses or explorations) is that it does promote reproducibility and repurposability. in no way do we ignore or minor weaknesses such as metric accuracy – but on the other hand it has never been the core goal of our research knowing the severe economic constraints that one has to deal with in the context of minor heritage, and the impact on the objects themselves of multiple and undocumented transformation phases. instead, we consider that minor heritage items can gain visibility and support when they are envisioned as part of a wider asset: a collection. hence experimenting and better understanding how actors concerned could tailor the data acquisition, processing and reuse steps to collections of smallscale, minor heritage assets has been and remains the core result of the approach. figure 14. 2d and 3d visualisations of the pwd energy map (n.d de bethléem, bras): twenty time frames available in the 3d version, 4 time frames in the 2d version. note for the latter differences between lines 2 and 3 (emission point ec or ed). figure 15. an online panorama in point mc, with (bottom right) symbols used to give access to the various soundtracks. an exemple for notre-dame-durevest chapel (esparron-de-pallières). figure 16. an example of an interactive sound + image pdf flyer with (on the left) text triggers that launch soundtracks for notre-dame chapel in brueauriac. acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 12 7. perspectives at this stage there are still improvements that could be brought about in the data acquisition phase itself: typically a better monitoring of the lighting conditions in situ, or maybe an automatization of the rangefinder’s movements or of the rotating arm and rotative head of the tripod used during the photogrammetric survey. the latter improvement would speed up the protogrammetric survey itself significantly. combining an acquisition using a 360 panoramic camera and a more detailed photogrammetric acquisition for this or that architectural detail could also be a sound perspective. indeed fine geometric details such as engravings on memorial stones, mouldings on altars, or sculptures cannot be acquired with enough resolution and quality using a low-cost 360 panoramic camera. surveying such details could be done using a more “traditional” photographic captor. the resulting high density cloud of points, focused on one specific area, could then be integrated to the general 3d reference system and offer a sort-of eagle eye view on parts of the building’s decor. later on in the processing chain, in terms of data fusion there is a very promising lead with the use of spherical depth map [29] combined with acoustical descriptors to enlarge cross-correlation analysis performed through image processing or signal processing algorithms [30]. more generally next steps are bounded by a backbone objective: better understanding, characterising such buildings and the way they are perceived, though vision and audition. this implies building on the interdisciplinary nature of the research, including in the analysis steps. accordingly we currently launch a series of experiments aimed at exploiting 3d representations to position and analyse acoustic data, and at using sound to represent dimensions and geometric features. as far as metric and visual data is concerned our approach, at first, can be summed up as a “feature extraction” effort: dimensions, ratiosas-proportion [31], [32], etc. as opposed to approaches where a 3d point cloud is analysed as such (segmentation, classification, etc.) [33]. features can then be compared, trends spotted, exceptions raised and analysed, based on methods and practices from the infovis community. in that future effort, data extracted from traditional manual surveys (quantitative or qualitative) will complement dimensions and geometric features in order to widen the scope of differentiating factor when comparing chapels. concerning sound data, the next steps of this work is to use the collected data, to categorize and to distinguish the chapels in terms of acoustic descriptors and perceptual criteria. in particular, several listening tests will be conducted. the 3d sound field perception is a complex process, leading to several specific experimentations. for instance, a recent sound source localisation protocol [34] will be experienced as well as “sound coloration” evaluation. furthermore, we intend at visually and acoustically immersing the participants, in order to check for the coherency between vision and acoustics. to take the acoustic simulations further, the integration of metric data acquisition, 3d point-cloud model estimation and acoustical measurements are of great interest. indeed, acoustical simulation tools such as catt or odeon allow 12-dof auralization of rooms based on 3d geometric models and impulse response measurements [23]. however, these tools are restricted to simple geometric models with a limited set of walls. further work aims to derive simple geometric models, compatible with such tools, from complex 3d-point cloud models as suggested in [35]. 8. conclusion this contribution reports on a research programme anchored in two prime concerns: considering interdisciplinarity as a mandatory requirement in the surveying of architectural interiors (during the codesign of the survey protocols, and in all subsequent phases of the operations, competences stemming from both architectural and acoustic studies were associated), tailoring the research’s technological and methodological choices to the specific context of small-scale architecture (a collection of buildings with little prestige, often neglected or at risk). the specificity of the corpus under scrutiny undoubtedly shaped the overall survey and data processing strategy. in that sense, one of this research’s originality is the effort to overcome the operational limits of a set of low-cost technologies. the overall protocol intends to help actors to characterise and correlate acoustic and morphological features of heritage figure 17. top: comparative analysis of data indicators (reverberation time vs. volume) across the collection (top): two items (n° 1 and 14) in the collection stand out significantly due to their high reverberation time and small volume. bottom: the corresponding point clouds are bordered in colour in the bottom graphics: n° 1, red n.d de bethléem (bras), n° 14, green saint-roch chapel (les mées). acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 13 architecture in a consistent way. therefore it aims at opening up new analytical biases, built on the principles and potential of comparative analyses. as an example, figure 17 shows cross modal representation of the studied collection, by representing the individuals as a function of their volumes v and reverberation times rt20. we make no claim this second ambition has yet been reached: what has actually been done is tailoring the data acquisition and processing chain to an interdisciplinary list of requirements, in order to allow for a series of analytical tasks that are now being carried out. the workflow has been applied to a collection of fifteen small-scale buildings (rural, often isolated chapels), with keeping the constraints linked to that type of heritage asset. the approach does open up new research trails, typically in terms of perceptual experiences combining sound and space, or in the 3d visualisation of acoustic data and the sonification of dimensional data. acknowledgement the project is funded by the anr, the project-based funding agency for research in france, under the id anr-18-ce38-000901. (http://anr-sesames.map.cnrs.fr/) references [1] l. alvarez-morales, t. zamarreño, s. girón, m. galindo, a methodology for the study of the acoustic environment of catholic cathedrals: application to the cathedral of malaga, building and environment vol 72, 2014, pp. 102-115. doi: 10.1016/j.buildenv.2013.10.015 [2] z. karabiber, the conservation of acoustical heritage. in cultural heritage research: a pan-european challenge. proc. of the 5th ec conference, cracow, poland, 2002, pp 286-290. isbn 92-8944412-6 [3] boren, b. b, acoustic simulation of j s bach’s thomaskirche in 1723 and 1539, acta acustica vol 5, no. 14, 2021. doi: 10.1051/aacus/2021006 [4] l. barazzetti, m. previtali, f. roncoroni, can we use low-cost 360 degree cameras to create accurate 3d models? isprs tc ii midterm symposium “towards photogrammetry 2020”, riva del garda, italy, 4–7 06 2018. isprs archives vol xlii-2, pp 69-75. doi: 10.5194/isprs-archives-xlii-2-69-2018 [5] m. schütz, markus, potree: rendering large point clouds in web browsers. ph d thesis vienna university of technology, 2016. 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[32] m. a. cohen, conclusion: ten principles for the study of proportional systems in the history of architecture, architectural histories, vol. 2, no. 1, 2014. doi: 10.5334/ah.bw [33] e. grilli, f. menna, f. remondino, a review of point clouds segmentation and classification algorithms, isprs archives volume xlii-2/w3, 2017, pp. 339-344. doi: 10.5194/isprs-archives-xlii-2-w3-339-2017 [34] s. fargeot, o. derrien, g. parseihian, m. aramaki, r. kronlandmartinet, subjective evaluation of spatial distorsions induced by a sound source separation process, eaa spatial audio signal processing symposium, paris, france, 6-7 sep 2019, pp. 67-72. doi: 10.25836/sasp.2019.15 [35] l. aspöck, m. vorländer, room geometry acquisition and processing methods for geometrical acoustics simulation models, proc. of the euroregio 2016, porto, portugal, 13–15 june 2016. https://doi.org/10.1007/978-3-030-01231-1_28 https://doi.org/10.1109/digitalheritage.2015.7419463 https://doi.org/10.5334/ah.bw https://doi.org/10.5194/isprs-archives-xlii-2-w3-339-2017 https://dx.doi.org/10.25836/sasp.2019.15 artificial neural network-based detection of gas hydrate formation acta imeko issn: 2221-870x september 2021, volume 10, number 3, 117 124 acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 117 artificial neural network-based detection of gas hydrate formation ildikó bölkény1 1 research institute of electronics and information technology, university of miskolc, h3515 miskolc-egyetemváros, hungary section: research paper keywords: gas hydrate; neural network; hydrate detection; injection system; modelling equipment citation: ildikó bölkény, artificial neural network-based detection of gas hydrate formation, acta imeko, vol. 10, no. 3, article 17, september 2021, identifier: imeko-acta-10 (2021)-03-17 section editor: lorenzo ciani, university of florence, italy received january 29, 2021; in final form september 17, 2021; published september 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was supported by university of miskolc, hungary. corresponding author: ildikó bölkény, e-mail: bolkeny@eiki.hu 1. introduction natural gas hydrates are crystalline solids composed of water (host) and gas (guest). the guest molecules are trapped inside ice cavities, which are composed of hydrogen-bonded water molecules. typical natural gas molecules include methane, ethane, propane and carbon dioxide. hydrate particles can form ice-like hydrate-plugs that completely block the pipeline and can be up to several meters long. the number of hydrate molecules can increase to a level where the molecular agglomeration process begins, which can cause of plug formation in a given section of the pipeline. in worst cases the hydrate plugs result production outages [1], [2]. in the mid-1930s hammerschmidt found out that natural gas hydrates can block gas transmission, especially at low temperatures. this discovery was pivotal and shortly thereafter led to the regulation of the water content in natural gas pipelines. the detection of hydrates in pipelines is a milestone marking the importance of hydrates to industry [3]. gas wells are the cores of developing serious hydrate problems, because of the water content of the production. the cold zones of the ground can shift the temperature of the pipe and its contents into the hydrate-formation region. hydrates start forming layers of water on the pipe walls. crystallisation can result in the formation of tens or hundreds of meters long plugs of hydrate [1], [4]. multiple techniques exist to prevent the formation of hydrates. in the gas industry one of the most popular solutions is the use of thermodynamic inhibitors (thi) for a prolonged time. the injection of thi shifts the hydrate curve to a region where the conditions are not adequate for stable hydrate formation [2]. these compounds (methanol, ethylene glycol) have to be injected in high volume to the gas to be effective against hydrate formation. this is not a modern solution, because it has several disadvantages like cost of additional pipelines necessary to lead to the gas wells [5], the cost of methanol regeneration, which also contaminates the environment. one of the newer alternatives is the injection of lowdosage hydrate inhibitors such as kinetic hydrate inhibitors which can abstract in the production process of natural gas one of the major problems is the formation of hydrate crystals creating hydrate plugs in the pipeline. the hydrate plugs increase production losses, because the removal of the plugs is a high cost, time consuming procedure. one of the solutions used to prevent hydrate formation is the injection of modern compositions to the gas flow, helping to dehydrate the gas. dehydratation obviously means that the size of hydrate crystals does not increase. the substances used in low concentrations, have to be locally injected at the gas well sites. inhibitor dosing depends on the amount of gas hydrate present. in the article two artificial neural network (ann)-based predictive detection solutions are presented. in both cases the goal is to predict hydrate formation. data used come from two solutions. in the first one measurements were performed by a self-developed and -produced equipment in this case, differential pressure was used as input. in the second solution data are used from the measurement system of a motorised chemical-injector device, in this case pressure, temperature, quantity and type of inhibitor were used as inputs. both systems are presented in the article. mailto:bolkeny@eiki.hu acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 118 prevent the growth of hydrate molecules [6]. antiagglomerants also belong to this group, they allow for the formation of gas hydrates but keep the hydrate crystals small and dispersed [7]. these modern, low-dosage inhibitors enable the usage of locally installed injection systems in the field, at the site of gas wells [8]. as can be seen, hydrate detection is key to administering the appropriate amount of inhibitor. 1.1. objective and methodology the paper compares two approaches. in the first one, the formation of gas hydrate was studied in laboratory conditions. the gas hydrate formation can be determined from the pressure curve. using the measurement results, a single solution based on an artificial neural network (ann) was created where the input is the differential pressure. in the second project, test measurements were performed with a field hydrate dosing and monitoring system. using the measurement results, a multi-input ann-based solution was developed, where the inputs are pressure, temperature, quantity and quality of inhibitor as these also influence hydrate formation. in the first method measurements were performed by a selfdeveloped and produced equipment. modelling the equipment is suitable for the simulation of the gas flow in the pipeline. its conditions are as follows: temperature is in the range of -20 … +30 °c, and typical gas pipeline pressure is in 1-10 nl/min flow rate range. during the measurements, different inhibitor materials and gases from all over hungary were used, and the values of differential pressure, inlet pressure, the gas temperature and the flowrate of the pipeline were recorded, but only differential pressure was used to teach neural networks. in the second approach data are used from the measurement system of a motorised chemicals-injector device, placed in the area of a well. this model was installed to test the equipment at the site of the scada ltd, near hajdúszoboszló in hungary. the following parameters were monitored there: well siphon pressure, drill pipe pressure, injection pipe pressure, well pipe pressure, well pipe temperature, soil temperature, temperature of chemicals, controller temperature, inverter temperature, chemical tank liquid level, inverter current, voltage and frequency. only well pipe pressure (pressure), well pipe temperature (temperature) and inverter frequency (quantity of inhibitor) were used to teach neural networks. after the successful test of the technology model, the equipment was transported to a real gas well in szeghalom (hungary). in the research data generated through 29 test weeks were used. the gas well was monitored online (one sample per minute) in the 29-week testing period, during which several hydrate plugs formed due to the weather conditions. the most important parameters of both approaches (equipment, inputs, outputs, ann) are in figure 1 and figure 2. the goal was to develop an accurate, stable and reliable annbased structure. several architectures have been studied. finally, the neural network auto-regressive x (nnarx) model with exogenous input [9] and neural network output error (nnoe) model are presented [10]. several independent data sets were needed for training networks. previously selected raw data were scaled and normalised. the resulting data were used to generate three training, validation and test datasets for the networks. 1.2. results final versions of ann-based predictive detection solutions were selected after the extended comparison processes. in the first approach nnarx and nnoe were used. in the second approach only nnarx was used. in both cases several networks were trained using different datasets. for the first neural network based predictive detection solution twelve, while for the second six networks were compared and the best one is selected. in both cases a relatively small and simple networks resulted the best performance. finally, the predictive solutions were compared. 2. related results in the literature even though the injection of methanol into natural gas is not advised due to environmental concerns, such experiments can be found in the scientific literature. for example, in [11] french and english researchers reported that methanol was injected into the pipeline, in an environmentally not-so-friendly manner to prevent the formation of hydrates for gas extraction in the north sea. the karl fischer method was used for injection. it is not the most appropriate approach, because it doesn't take salt content into account. as a result, new method was developed, by which the electrical conductivity and the sound propagation velocity can be measured in addition to the temperature and the pressure. using these four parameters and the devised method, the methanol injection can be kept at an optimum. the paper published in 2013 in [12] also deals with optimising the methanol injection for the inhibition of hydrate formation in industrial processes. authors stress the importance of the vapour state methanol, because it doesn't participate in the hydrate formation inhibition. to determine the quantity of inhibitor, two methods were introduced. the first one is a mathematical correlation from real data sets, the second one is based on ann. figure 1. the two compared project – first approach. figure 2. the two compared project – second approach. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 119 naturally, scientific literature does not only deal with methanol injection but also ones that utilise mono-ethylene glycol or some other inhibitor related to remedial methods. one example is by kamaria et al. [13] who realised machine learning by using a least-squares support-vector machine. the hydrate formation in the pipeline can be predicted and the mono-ethylene glycol amount necessary for the hydrate inhibition can also be estimated [14] based on the method developed by suykens et al. the other non-methanol based hydrate formation inhibitive method was used by elgibaly et al. this work deals with the use and development of neural networks, related to the optimisation of hydrate formation inhibition. to validate their model, experimental data was used which contains hydrate formation environmental information, gas composition, hydrate inhibitor composition, system pressure and density. the model takes the evaporation of the inhibitor into consideration. the devised method suggests inhibitor injection ratios for gases of various composition [15]. numerous scientific literature deals with hydrate formation temperature. the method devised by mesbah et al. [16] uses a least squares support vector machine algorithm to predict the hydrate formation temperature. the authors used data available in scientific literature for multiple gas compositions and created a data set for machine learning. the model is more accurate for gases with h2s content. an empirical correlation between temperature, gas pressure and density, by which the hydrate formation temperature can be determined was shown by khamehchi et al. [17]. this method was further refined using measurement data and ann. the method gives accurate results. zahedi et al. [18] published two methods for the assessment of hydrate formation temperature. the first method using two correlations, with eleven and eighteen parameters. the parameters were obtained from measurement data and scientific literature. the second method using ann and the data from the previous method. the problem of the accurate assessment of hydrate formation is discussed in [19]. authors use the katz gas-gravity method with the ghiasi correlation [20]. the same model was used with the imperialist competitive algorithm [21]. the ann was used to determine a kinetic model for the prediction of methane gas hydrate formation. the authors tried to determine the correct number of hidden neurons and layers. the ann-based model takes the temperature and pressure as the inputs and the output is the hydrate growth speed. in [22] comparison was made between two methods for the inhibition of gas hydrate development. both use ann, in the second it is optimised with the imperialist competitive algorithm [23]. the outcome met expectations and proved that the normal neural network provides better results than the optimised one [23], [24]. 3. description of the proposed metho in this section, two systems providing the measurement data are presented. also, predictive hydrate detection methods are introduced. 3.1. hydrate forming test equipment in the first analysis measurements have been performed by a hydrate forming test machine developed for mol plc. by the department of research instrumentation and informatics at the research institute of applied earth sciences. development of the control system was carried out by the author, figure 3. the modelling equipment is suitable for simulation of gas pipeline flow. the equipment creates field conditions within -20 … 30 °c temperature range, and original gas pipeline pressure range, which is typically 60 bars. the flow rate value can be set in accordance to modelling principles, between 1-10 nl/min. the hydrate forms inside of a capillary cell which is placed in a thermostat. figure 4 shows the piping and instrumentation (p&i) diagram of the equipment. where pt is the pressure transmitter, tt is temperature transmitter, ft is flow transmitter, gt is gas tank, pg is pressure gauge, tc is temperature control, te is temperature element, va is valve, sp is pressure generator unit, pc is personal computer, hc is buffer cell, c is glass cell, dc and dr are separator cells. the operation of the equipment is as follows: the dehydrated natural gas is discharged from the gas tank into the pipeline. the pressure gauges are used to set the system pressure and the flow transmitter is used to adjust the flow rate. pressure generators mix the formation water and inhibitor with the natural gas. the pipeline goes through the low temperature thermostat (te) and it cools the natural gas therefore hydrate formation can begin. the formation of the hydrate plug can be detected from the measured differential pressures (pt2, pt3). natural gas and interfacial water from a szeghalom gas well (hungary, near to füzesgyarmat) were used in tests. different inhibitor mixtures were also added. gas hydrate formation time was examined under gas well conditions (60 bar pressure, low temperature), with or without the addition of different inhibitors. the following parameters were recorded: pressure, differential pressure, temperature and flow rate [25]. figure 3. hydrate forming test equipment. figure 4. p&i diagram of the hydrate forming test equipment. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 120 3.2. control and chemical dosing equipment the well area control and the chemical injector equipment was installed on szeghalom-29 well in füzesgyarmat (figure 5). the injection system is optimised mainly for hungarian gas wells. thus, the temperature requirement of the system was in the -40°c … 60°c range. the system must be capable of working in ex (explosive atmosphere) environment with high efficiency. the power source of the actuator is solar energy to reach the almost zero emission of the system [25]. figure 6 shows the p&i diagram of the equipment, where pt is pressure transmitter, tt is temperature transmitter, lt is level transmitter and pi is pressure indicator. the operation of the equipment is as follows: the natural gas which contains natural interfacial water entering the pipeline from the gas well. the inhibitor is located in a chemical tank and is delivered to the pipeline by the dosing pump. the dosing rate is provided by a plc control, which is not shown in the figure. the formation of the hydrate plug can be detected from the value measured by the pressure transmitters (pt12). the following parameters were recorded on a minute basis: well siphon pressure, drill pipe pressure, injection pipe pressure, well pipe pressure, well pipe temperature, soil temperature, temperature of chemicals, controller temperature, inverter temperature, chemical tank liquid level, inverter current, voltage and frequency [25]. the output of the system is the inverter frequency. the frequency is proportional to the amount of administered inhibitor. 3.3. neural networks in black-box identification of nonlinear dynamic systems, selection of model structures become more difficult task. the multilayer perceptron network is most popular for learning nonlinear relationships from a set of data. for the identification the nnarx and nnoe were used [26]. these models are mostly widespread. the nnarx network creates a nonlinear model using its inputs. the applied regression machine complies with the following relation: 𝑦est(𝑡) = 𝑓[𝑥(𝑡 − 1), 𝑥(𝑡 − 2), . . . , 𝑥(𝑡 − 𝑛𝑖), 𝑦req(𝑡 − 1), . . . , 𝑦req(𝑡 − 𝑛ro)] (1) where yest(t) is the network output at the t th time instant; x(t-1) is the used input of the network at t-1st time instant; yreq(t-1) is the required output from the network at t-1st time instant; ni is the size of used tapped delay line of the inputs; and nro is the size of used tapped delay line of the required outputs. figure 7 shows the typical structure of the nnarx neural network. the nnoe network creates a nonlinear model using its earlier outputs as inputs. the applied regression machine complies with the following relation: 𝑦est(𝑡) = 𝑓[𝑥(𝑡 − 1), 𝑥(𝑡 − 2), . . . , 𝑥(𝑡 − 𝑛𝑖), 𝑦est(𝑡 − 1), . . . , 𝑦est(𝑡 − 𝑛o)] (2) where yest(t) is the network output at the t th time instant; x(t-1) is the used input of the network at t-1st time instant; yest(t-1) is the network output at the t-1th time instant; ni is the size of used tapped delay line of the inputs; and no is the size of used tapped delay line of the outputs. figure 8 shows the typical structure of the nnoe neural network. during the model selection, size of the regressor and the number of hidden neurons in hidden layers were changed. based on the previous practical experience, the number of regressors was 1 or 2, while the number of hidden neurons was between 10 figure 5. control and chemical dosing equipment. figure 6. p&i diagram of the control and chemical dosing equipment. figure 7. typical structure of the nnarx [27]. figure 8. typical structure of the nnoe. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 121 and 12. the selected raw data has been pre-processed using the scilab software. according to [28], pre-processing can consist in a simple transformation or a complex operation. the raw data were first filtered by a low-pass filter, then normalised. when normalising the input data, the minimum and maximum values of each component are selected to cover the set of values and the interpretation range of the neural networks. this interval is typically [0; 1] and [-1; 1]. in the presented case, the [0; 1] interval was selected for normalisation. three datasets were generated for the detection systems. the training set was needed to configure weights of the network. one of the most important parameters during the training process is the stopping criterion. if the training process stops too early, the network is not able to learn the data and gives poor estimation when an unknown dataset is used. to optimise the network the validation set is used. when mean squared error (mse) is the lowest, it is best to stop the training process of the network. the mean squared error complies with the following relation: 𝑀𝑆𝐸 = 1 𝑛 ∑[𝑦req(𝑖) − 𝑦est(𝑖)] 2 𝑖 (3) the third, test dataset is independent from the training and validation sets. it is used to compare results for different networks. neural networks were trained using the generated datasets. to avoid overfitting, the training process was stopped at the minimum mse value. the levenberg-marquard algorithm was used to optimise the ann in matlab. figure 9 shows the workflow of model. 3.4. single inputs neural network based detection large number of measurements was performed with the previously detailed hydrate forming test equipment using different inhibitor materials and gases from szeghalom gas well. from this huge database 50 pieces were selected and used for the investigation. during measurements mainly values of differential pressure, inlet pressure and temperature of gas were saved for later investigation. differential pressure measurement value from pt2 or pt3 sensor was used as an input in the first method, depending on which section of the pipe the hydrate was formed in. after the appearance of gas hydrate molecules in gas flow the pressure in pipe section was increasing because the agglomerated hydrate reduces the cross-section area of the pipeline. therefore, fast gas hydrates detection is very important. from practical perspective, the differential pressure gives the most valuable information about the processes in the tube. thus, this parameter was used as the input value of the alarm system. as previously stated, three independent datasets have been created. in table 1. the number of performed measurements and the number of datapoints included in the different datasets are shown. the scaled, normalised differential pressure value was used in datasets as input. the required output was an artificially generated alarm signal, which was created from the differential pressure values. the signal corresponds to the 75 percent of the maximum value, see figure 10. until the actual differential pressure value is under the limit, the alarm signal is also zero. when it reaches the limit, the signal changes to one. the single input nnarx network is seen in figure 11 with the used regressor and the mapping function. here, y(t) is the network output at the tth time instant; yreq(t-1) is the required output from the network at t-1st time instant; x(t) is the network inputs at the tth time instant; x(t-1) is the network input at t-1st time instant; tdl is the tapped delay line, b is neuron bias, w is the weight matrix. the single input nnoe network is seen in figure 12, with the used regressor and the mapping function. in figure 12, y(t) is the network output at the tth time instant; yreq(t-1) is the output from the network at t-1st time instant; x(t) table 1. main parameters of the datasets. dataset number of performed measurements number of data points training dataset 26 2576 validation dataset 10 1077 test dataset 10 1698 figure 9. the workflow of model development [29]. figure 10. alarm signal (75%). figure 11. single neural network arx. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 122 is the network inputs at the tth time instant; x(t-1) is the network input at t-1st time instant; tdl is the tapped delay line, b is neuron bias, w is the weight matrix. 3.5. multi input neural network based detection the previously detailed control and chemical injection system has been operated in test mode for 29 weeks under continuous monitoring. several parameters were monitored, but only three of them (well pipe pressure, well pipe temperature, quantity of inhibitor – inverter frequency) influenced the formation of hydration. pressure measurement value from pt12 sensor, temperature measurement value from tt12 sensor, and quantity measurement value of inhibitor from inverter frequency of the dosing pump were used as inputs in the second method. the fourth parameter is the type of the applied inhibitor, which was recorded when the inhibitor was placed in the container. demonstration of the effectiveness of each chemical in inhibiting hydration was performed with the previously described equipment. depending on the inhibition ability of the inhibitors, they were graded on a scale, see table 2. as previously mentioned, three independent datasets have been created: training, validation and test datasets. the main parameters of datasets are shown in the table 3. the neural network has four inputs and one output, the four inputs are the four parameters listed above, and the output is an alarm signal. the multi-input nnarx network is seen in figure 13, with the used regressor and the mapping function. in figure 13, y(t) is the network output at the tth time instant; yreq(t-1) is the required output from the network at t-1st time instant; x1..4(t) is the network inputs at the t th time instant; x1..4(t1) is the network input at t-1st time instant; tdl is the tapped delay line, b is neuron bias, w is the weight matrix. 4. results and discussions performance of the network is adequate if the required output (blue graph in figure 14) and the regular output (red graph in figure 14) match each other. mse gives no satisfactory information about the performance, therefore, the number of edges in the sample sets were determined by rising edge (re) method and then they were compared. if the edges matched each other it can be said that the alarm was at the proper time moment. a percentage value can be calculated (re%) from the ratio of number of alarms occurred at proper time and number of total alarms [30]. there are several methods, which can be used to find edges in one dimension. in this research the canny edge detection method resulted the best calculation, in which the first gaussian derivative is used to approximate the optimal finite length filter [31]. results of both networks were compared, using the relative error of detected rising edges in the simulated output of the network and the required alarm signal. the comparison of the single input single output (siso) networks is summarised in table 4. the table shows that the network detected possible hydrate formation with more than 90% efficiency in both cases. the best performance in case of nnarx was provided by the smallest figure 12. single neural network oe. table 2. inhibitor efficiency. hydrate formation time in s grade numerical grade 0-2500 won’t inhibit 1 2501-4000 weakly inhibits 2 4001-5500 inhibits 3 5501-6500 strongly inhibits 4 table 3. main parameters of the datasets. dataset number of performed measurements number of data points training dataset 22 2178 validation dataset 12 1068 test dataset 10 1080 figure 13. multi neural network. figure 14. output match using test set. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 123 network and in case of nnoe the result was the same. it should be noted that nnarx performed better result than nnoe. the comparison of the multi input single output (miso) networks can be found in table 5. it shows that the network recognised the possible hydrate formation with more than 92% efficiency. table 6 shows the most important parameters of the highlighted and best performing networks. 5. conclusions there is no publication so far in scientific literature, which gives solution for hydrate formation prediction for industry exclusively from either the differential pressure or the inhibitor's quality and injected quantity. the most effective results of the two presented projects are shown in tables 4 and 5 in bold. for single input single output neural network, the smallest network provided the highest reliability in edge detection in case of nnarx and in case of nnoe. it should be noted that nnoe network performed better than nnarx. in case of multi input single output neural network a larger regressor was the best. the nnarx model has a predictor without real feedback. the nnoe model has feedback through the choice of regressors, which in the neural network terminology means that the networks become recurrent: future network inputs will depend on present and past network outputs. this might lead to instability in certain areas of the network's operating range and it can be very difficult to determine whether or not the predictor is stable [32]. both nnarx networks performed well, the difference between the two results is not significant. however, in the first solution, nnoe became more effective than nnarx. in light of the above, the way forward is to first use the nnoe network for multi input single output system as well. although the nnoe clearly performed better in the first case, but further studies are needed to assess which of the two methods is better. acknowledgement the described article/presentation/study was carried out as part of the efop-3.6.1-16-2016-00011 “younger and renewing university – innovative knowledge city – institutional development of the university of miskolc aiming at intelligent specialisation” project implemented in the framework of the szechenyi 2020 program. the realisation of t this project is supported by the european union, co-financed by the european social fund. references [1] e. d. sloan, c. a. koh, clathrate hydrates of natural gases, 3ed isbn: 978-084-939-078-4 (2007), pp. 5-20. [2] g. ersland, a. graue, natural gas hydrates, in: natural gas, isbn: 978-953-307-112-1 (2010), pp. 147-162. table 4. the results of siso nnarx and nnoe networks. type of network structure regressor of network num. of hidden neurons training dataset validation dataset test dataset mse rel. error of found re in % mse rel. error of found re in % mse rel. error of found re in % nnarx ni = 1; nro = 1 10 0.0083 96.2 0.0065 100.0 0.0146 90.0 12 0.0081 96.0 0.0064 100.0 0.0146 90.0 ni = 1; nro = 2 10 0.0209 73.1 0.0184 70.0 0.0227 70.0 12 0.0202 73.1 0.0177 80.0 0.0203 90.0 ni = 2; nro = 2 10 0.0223 73.1 0.0186 90.0 0.0259 70.0 12 0.0388 69.2 0.0350 50.0 0.0316 60.0 nnoe ni = 1; no = 1 10 0.0086 100.0 0.0062 100.0 0.0157 90.0 12 0.0058 96.2 0.0048 100.0 0.0127 90.0 ni = 1; no = 2 10 0.0284 76.9 0.0253 60.0 0.0326 60.0 12 0.0265 65.4 0.0236 70.0 0.0278 50.0 ni = 2; no = 2 10 0.0269 69.2 0.0252 70.0 0.0183 60.0 12 0.0347 53.8 0.0325 50.0 0.0321 30.0 table 5. the results of miso nnarx networks. type of network structure regressor of network num. of hidden neurons training dataset validation dataset test dataset mse rel. error of found re in % mse rel. error of found re in % mse rel. error of found re in % nnarx ni = 1; nro = 1 10 0.0204 72.2 0.0179 80.0 0.0201 90.0 12 0.0088 95.2 0.0131 90.0 0.0148 90.0 ni = 1; nro = 2 10 0.0084 99.8 0.0072 100.0 0.0158 92.2 12 0.0087 95.2 0.0074 100.0 0.0167 91.2 ni = 2; nro = 2 10 0.0091 82.4 0.0141 91.4 0.0211 81.2 12 0.0107 81.2 0.0202 90.7 0.0214 80.2 table 6. the algorithm’s main parameters. siso nnarx siso nnoe miso nnarx structure 1-10-1 1-10-1 4-10-1 hidden layers 1 1 1 neurons 10 10 10 input regressors 1 1 1 output regressors 1 1 2 datasets 46 46 44 data point of datasets ~115 ~115 ~100 iterations 1000 1000 1000 acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 124 [3] e. g. hammerschmidt, formation of gas hydrates innnatural gas transmission lines, ind. eng. chem, vol 26 is 8 (1934), pp. 851855. doi: 10.1021/ie50296a010 [4] j. f. gabittol, c. tsouri, physical properties of gas hydrates: a review, advances in gas hydrate thermodynamics and transport properties, vol. 2010. art. id 271291 (2010), pp. 1-12. doi: 10.1155/2010/271291 [5] m. wu, s. wang, h. liu, a study on inhibitors for the prevention of hydrate formation in gas transmission pipeline, journal of natural gas chemistry, vol. 16. no. 1. 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[10], albaladejo et al. [11] and hormann et al. [12] employed different abstract in the interest of our society, for example in smart city but also in other specific backgrounds, environmental monitoring is an essential activity to measure the quality of different ecosystems. in fact, the need to obtain accurate and extended measurements in space and time has considerably become relevant. in very large environments, such as marine ones, technological solutions are required for the use of smart, automatic, and self-powered devices in order to reduce human maintenance service. this work presents a simple and innovative layout for a small self-powered floating buoy, with the aim of measuring and transmitting the detected data for visualization, storage and/or elaboration. the power supply was obtained using a cantilever harvester, based on piezoelectric patches, converting the motion of ripple waves. such type of waves is characterized by frequencies between 1.50 hz and 2.50 hz with oscillation between 5.0 ° and 7.0 °. specifically, a dedicated experimental setup was created to simulate the motion of ripple waves and to evaluate the suitability of the proposed design and the performance of the used harvester. furthermore, a dynamic analytical model for the harvester has been defined and the uncertainty correlated to the harvested power has been evaluated. finally, the harvested voltage and power have shown how the presented buoy behaves like a frequency transformer. hence, although the used cantilever harvester does not work in its resonant frequency, the harvested electricity undergoes a significant increase. mailto:damiano.alizzio@unime.it acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 202 size photovoltaic cells to feed sensor buoy systems for sea trials. in these studies, the main limitations were found in the dependence of the electricity production by the sun, by lacking in bad weather conditions and by inactivity during the night, and in the need to equip the measurement buoys with bulky and heavy systems and batteries. instead, for wind turbines, the meteorological conditions are not optimal on the sea surface. in fact, the wind speed at 1 m above the sea level is a third of that at 80 m. this results in a 73% reduction in wind power [13]. recently, new energy sources, such as tidal currents and sea or ocean waves, have been investigated. trevathan et al. [14] proposed tidal currents to supply sensors for marine measurements and compared the performances of different types of wind turbines. however, they concluded that such devices may not be cost-effective and risk-prone to apply due to biofouling and entanglement from drifting algae and sea grass wrack. sea or ocean waves represent an attractive renewable source, indirectly related to both the sun and the wind, and with a high energy density [15]. pelc et al. [16] stated that wave energy is vast and more reliable than most renewable sources. furthermore, they highlighted that such energy at a given site is available up to 90 % of the time, while photovoltaic and wind energy tend to be available only 20-30 % of the time. the adoption of an energy harvester by piezoelectric transducers (peh) is an advantageous solution to convert wave motion in electric energy. peh are a well-known technique in literature. for example, bolzea et al. [17] used such devices in cantilever configuration, demonstrating that the maximum power output is generated when the mechanical resonance is reached. toyabur et al. [18] designed a multimode peh, consisting of four elements, connected in parallel and in a cantilever configuration, to achieve different low frequency resonance modes (10-20 hz). the authors showed that their system generates about four times more power than a single peh. pradeesh et al. [19] analysed, both experimentally and numerically, the effect of a proof mass fixed to a peh in a cantilever configuration. the researchers obtained the best results as the proof mass was glued close to the clamped end. these works highlight the importance of a correct design for a cantilever type peh, but they do not discuss the effect of the proof mass (i. e. the resonant frequency variation of the peh) on the harvested power. recently, montanini et al. [20] developed a peh, using a glass fibre reinforced beam support. the authors studied the correlation between the mechanical working frequency and the harvested electrical power, analysing the deflection shape of the peh under operating conditions by means of a scanning laser-doppler vibrometer. after that, they investigated the conversion efficiency of this peh [21] and the applicability of a low-power single-stage converter, able to automatically follow the changes in the resistive component of the output impedance, in order to maximize the energy yield [22], [23]. in the field of power supply for measurement buoys, pehs have been sporadically applied due to the low frequency of the sea and ocean waves. wu et al. [24] developed a peh fixed to a floating buoy, that was anchored to the ocean floor. this device consisted of several cantilevers, on which many piezoelectric patches (pcs) were attached. the authors analyzed the size effect of the float and derived a numerical model to calculate the harvested energy. the research findings show that up to 24 w electric power can be generated with the piezoelectric cantilevers length of 1 m and the length of the buoy of 20 m. nabavi et al. [25] proposed the design of a beam-to-column piezoelectric system, able to power a large floating and instrumented ocean buoy. they derived and experimentally verified the equations of the electromechanical behavior of the device, demonstrating that the height amplitude and the low frequency of the wave guarantee the best performance. additionally, using a baffled water tank, they developed a self-tuning buoy, which works based on the frequency of ocean waves. recently, alizzio et al. [26], [27] proposed an instrumented spar and fixed-point buoy, equipped with a peh, to convert the energy of the wave motion into electricity through some pcs, glued on deformable and floating bands. the buoy was designed, numerically simulated, and experimentally verified, obtaining a light structure able to self-power its on-board sensors and carrying out data transmission. in this paper, the performance of a simple and innovative layout for a small measurement buoy, supplied by a cantilever peh, has been studied. the structure has been designed to convert the motion of the ripple waves in a cyclic oscillation with harmonics of high frequency, to which the peh is subjected thanks to a suitable proof mass. in this context, a dedicated experimental setup has been implemented to simulate the motion of the ripple waves and to evaluate its effect on the electric response of the peh. such motion has been discretized on amplitude and frequency configurations, characterizing the dynamics of the proposed buoy with an analytical model of the peh. finally, the harvested power has been estimated and the related uncertainty has been evaluated. 2. materials and methods 2.1. prototype of the measurement floating buoy the presented buoy prototype (figure 1) was made of a floating structure, manufactured with a 3d printer using a highly durable photo-polymeric resin. it was divided in two pieces: the bottom one was designed with a hemispherical shape for an appropriate matching with the sea waves, while the upper one had a cylindrical shape where the peh was set by a fixed joint. the two parts were connected by means of a thread, to hermetically contain a dedicated measurement instrumentation for monitoring the marine environment with a data transmission system. this peh allowed to convert the alternative rotational (rolling) motion of the buoy, while this one was subject to ripple figure 1. concept design of the measurement floating buoy prototype. 1. = hemispherical part, 2. = cylindrical part, 3. = pc and 4. = proof mass. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 203 waves, into electrical energy in order to provide an adequate power to supply all electronic devices within the same buoy. 2.2. piezoelectric energy harvester (peh) the used peh consisted of a cantilever support, on which a pc and a proof mass were glued at the opposite ends. the pc (duraact p-876 a.12, physik instrumente gmbh) has sizes of 61.0 mm × 35.0 mm × 0.5 mm and an electrical capacity of 90 nf. the active element of the pc, a thin layer of piezoceramic pic255 powder, is encapsulated in a kapton case. the pc shows a symmetrical structure; if it is deformed, the same amount of voltage is generated, opposite in charge, on the two surfaces of the electrodes. these types of devices have the double advantage of being also used as an actuator. when driven by an alternating voltage, the pcs act with a multiaxial deformation which depends on the amplitude and frequency of the power signal [28], [29]. the cantilever support, measuring 105 mm × 35 mm × 1 mm, was made by manual layering of three layers of 0◦/90◦ oriented glass fibre and epoxy resin. 2.3. experimental setup and procedure the experimental setup (figure 2a) included the buoy with its peh and an electrodynamic shaker (mod. s 513, tira), driven by a power amplifier (mod. baa 120, tira) and a function generator (mod. 33220 a, agilent). in order to simulate the motion of the ripple waves in amplitude and frequency, a conversion system (figure 2b) of the linear motion of the shaker was implemented. it consisted of a frame with two flanges, able to rotate the buoy around a fixed horizontal axis by means of two bearings. the imposed rolling motion was applied by connecting the stinger of the shaker to the bottom of the buoy (i. e. the vertex of the hemisphere) using a ball joint. the geometry of the conversion system is reported in table 1. the imposed rolling motion to the buoy was monitored by a rotational transducer (mod. 0600-0000, trans-tek), set in the fixed horizontal axis of the two flanges. an oscilloscope (mod. tds 5054b, tektronix) was employed to measure the previous oscillation signal and the voltage response of the peh on a resistive load of 100 kω. specifically, this resistive load was chosen in accordance with the maximum power transfer theorem, knowing the internal impedance of the pc [20]. the behaviour of the buoy was studied by varying the working frequency, the amplitude of the imposed rolling motion and the proof mass glued at the end of the cantilever support. the frequencies and amplitudes of the ripple waves were appropriately following the results of [27]. therefore, the sinusoidal functions of the imposed rolling motion to the buoy were chosen with the characteristics shown in table 2. the acquisition frequency of the signals was set at 2 khz and each test was repeated 5 times for every combination of working frequency fw and angular displacement θ. 2.4. simplified model of the mechanical behaviour of the peh the mechanical behavior of the analyzed peh (i.e. a clamped-free beam with a proof mass) can be explained by a single degree of freedom (sdof) model (figure 3), according to [30]-[32]. for a peh without a proof mass, the equation of motion of euler-bernoulli beam for undamped free vibrations can be considered: 𝐸𝐼 𝜕4𝑤(𝑥,𝑡) 𝜕𝑥4 + 𝑚 𝜕2𝑤(𝑥,𝑡) 𝜕𝑡 2 = 0, (1) where m, e and i indicate the mass, the young’s modulus and the inertia momentum of the peh, respectively. 𝑤(𝑥, 𝑡) is the absolute motion of the peh along its axis expressed as: 𝑤(𝑥, 𝑡) = 𝑤𝑟𝑒𝑙 (𝑥, 𝑡) + 𝑤𝑏 (𝑥, 𝑡), (2) table 1. geometric characteristics of the conversion system for the linear motion of the shaker in the imposed rolling motion to the buoy. geometric parameters r1 in mm r2 in mm l in mm d in mm 70 70 100 90 a) b) figure 2. a) image of the experimental setup and b) schema of the conversion system for the linear motion of the shaker in the imposed rolling motion to the buoy. 1. = stinger of the shaker, 2. = frame, 3. = body of the buoy, 4. = peh, 5. = proof mass, r1 = height of the hemispherical part of the buoy, r2 = height of the cylindrical part of the buoy, l = arm of the peh, d = diameter of the buoy and θ = angular displacement. table 2. characteristics of the imposed rolling motion to the buoy. case 1 case 2 case 3 case 4 case 5 working frequency fw in hz 1.50 1.75 2.00 2.25 2.50 angular displacement θ in ° 5.0 5.3 5.9 6.6 7.0 acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 204 where 𝑤𝑟𝑒𝑙 (𝑥, 𝑡) is the displacement, relative to the clamped end, and 𝑤𝑏 (𝑥, 𝑡) is the absolute displacement of the buoy. by introducing a proof mass and according to equation (2), equation (1) takes the form reported as follows: 𝐸𝐼 𝜕4 𝜕𝑥 4 𝑤𝑟𝑒𝑙 (𝑥, 𝑡) + 𝑚 𝜕2 𝜕𝑡 2 𝑤𝑟𝑒𝑙 (𝑥, 𝑡) = −[𝑚 + (𝑥 − 𝑙)𝑀𝑡 ] 𝜕2 𝜕𝑡 2 𝑤𝑏 (𝑥, 𝑡) , (3) where mt denotes the proof mass and produces a new contribution. 𝑤𝑏 (𝑥, 𝑡) consists of the composition of an orthogonal translation 𝑔(𝑡) and a rotation ℎ(𝑡) of the peh clamped end: 𝑤𝑏 (𝑥, 𝑡) = 𝛿1(𝑥)𝑔(𝑡) + 𝛿2(𝑥)ℎ(𝑡) . (4) in our case (i.e. clamped-free beam and figure 2b), 𝛿1(𝑥) = 1, 𝛿2(𝑥) = 𝑟2 + 𝑥, 𝑔(𝑡) = 𝑟2 sin 𝜃(𝑡) and ℎ(𝑡) = 𝜃(𝑡), and equation (5) can be reported as: 𝑤𝑏 (𝑥, 𝑡) = 𝑟2 sin 𝜃(𝑡) + (𝑟2 + 𝑥)𝜃(𝑡) . (5) 2.5. estimation of the harvested power by the peh the harvested power by the peh was evaluated according to the model proposed by shu et al. [33], using equation (6): 𝑃 = π 𝜔 𝑉 2 8 𝑅 , (6) where 𝑉 and 𝜔 are respectively the amplitude and the pulsation of the harvested voltage and 𝑅 is the resistive load wired to the harvester. considering the complex trend of the harvested voltage 𝑉𝑃𝐸𝐻 of the peh, 𝑉 was estimated as a specific average voltage, indicated with �̅�. it is the accumulated voltage inside the excitation period by the integral average of the rectified signal in equation (7): �̅� = 1 𝑇 ∫|𝑉𝑃𝐸𝐻 (𝑡)| 𝑑𝑡 𝑇 0 , (7) where t = 1 / fw is the excitation period and fw is the working frequency of the imposed rolling motion of the buoy. for discrete signals, the equation (7) becomes: �̅� = 1 𝑇 ∑ |𝑉𝑃𝐸𝐻 (𝑡)| 𝑇 𝑡=0 . (8) hence, equation (6) takes the form of equation (9): �̅� = 1 16𝑅𝑇 (∑ |𝑉𝑃𝐸𝐻 (𝑡)| 𝑇 𝑡=0 ) 2 = = 𝑓𝑤 16𝑅 ( ∑ |𝑉𝑃𝐸𝐻 (𝑡)| 1/𝑓𝑤 𝑡=0 ) 2 , (9) where �̅� is the specific power, harvested by the peh. it is here mentioned as specific power, hence it is a complex function of the working frequency due to its dependence on �̅�. 3. results 3.1. effects of the mechanical frequency figure 4 shows the typical signals concerning the angular displacement θ of the buoy, measured on the fixed horizontal axis, and the harvested voltage vpeh by the peh, following the imposed rolling motion. in this application the mechanical operating conditions of the peh are quite different from those reported in the literature [24]-[27]. indeed, the piezoelectric component is mechanically stressed by a non-inertial force field in which the motion is alternative. the working frequencies fw of the presented peh are significantly lower than the typical ones of this devices, while the amplitude of the angular displacement θ does not allow the hypothesis of small oscillations. in figure 4, although a certain period can be identifiable, the two acquired signals do not have the same dynamics. the angular displacement θ has a sufficiently sinusoidal behavior, according to the imposed rolling motion, while the harvested voltage vpeh is oscillating with a variable amplitude. figure 5 reports the magnitude of the discrete fourier transform (dft) of the signals of figure 4, computed using a resolution of 0.10 hz. the imposed rolling motion is characterized by a single frequency at 2.00 hz, vice versa the harvested voltage vpeh has some harmonic components with higher order and with maximum amplitude at 10.00 hz. the identified phenomenon occurs for all the conducted tests and denotes a multimodal enhancement of the peh. it must be pointed out that, although the employed shaker ensures a linear trend in the frequency range from 2 hz to 7000 hz, its operation at the lower frequency limit remains optimal. in fact, there are no distortions since the amplitudes of the dft of the angular displacement θ, reported in figure 5, do not show significant higher order harmonics. figure 6 illustrates the comparison between the magnitudes of the dfts of the harvested voltage vpeh by varying the proof mass, glued to the free end of the peh, and following an angular displacement θ of the buoy of 6.6° amplitude at a working frequency of 2.00 hz. a different applied proof mass (i. e. the resonant frequency of the peh [20], [21]) does not cause a figure 3. scheme of the model for the peh. figure 4. typical signal of the angular displacement θ of the buoy (on the top) and of the harvested voltage vpeh by the peh (on the bottom) at 2.00 hz. -5.0 0.0 5.0 10.0 0.0 0.5 1.0 1.5 2.0 / ° time / s angular displacement θ -1.5 -0.5 0.5 1.5 0.0 0.5 1.0 1.5 2.0 / v time / s harvested voltage vpeh acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 205 change in the frequency of the dft, but it acts on the amplitude of vpeh. in fact, a greater proof mass induces a consequent increase in the dft magnitude and a modification of the modal eigenvalues associated with the motion. moreover, in these cases, the accelerations due to the proof masses assumed values between 0.77 ms-2 and 2.99 ms-2, calculated considering the parameters from table 1 and table 2. figure 7 presents the frequency effect of the imposed rolling motion on the harvested voltage vpeh with an angular displacement θ of 6.6° and a proof mass of 24.1 g. the frequency variation of the imposed rolling motion involves a shift of the frequency, but not an alteration in the magnitude of the harvested voltage. specifically, the components of higher order are characterized by a greater frequency difference than those of lower order. figure 8 compares the frequency amplification factors of the buoy-peh system as the angular displacement θ and working frequency fw of the imposed alternative rotational motion vary, using a proof mass of 24.1 g. the amplification factors were obtained from the ratio between the frequency of the principal component of the harvested voltage and the working frequency fw of the imposed rolling motion. it was found that these factors are not very sensitive to either the frequency or the amplitude of the imposed rolling motion. 3.2. power estimation of the peh figure 9 shows the area that collects the values of the specific average voltages �̅�, calculated according to equation (8), as the working frequency fw varies. this area was obtained by changing the angular displacement θ of the boy and the proof masses glued to the peh. the results are in perfect agreement with the literature [17]-[23]. in fact, it can be noted that as the proof mass increases (i. e. as the resonant frequency of the peh decreases), a greater oscillation of the peh is obtained with a consequent increment in the harvested voltage. a similar observation can be made by considering the increase in the working frequency fw of the imposed rolling motion with the same proof mass. figure 10 underlines the trend of the specific power �̅�, harvested by the peh and calculated according to equation (9), as the working frequency fw and the amplitude of the angular displacement θ vary and using a fixed proof mass of 24.1 g. a relative maximum is obtained at the working frequency of 2.25 hz and its amplitude rises as the angular displacement θ of the buoy increases. this result also conforms to the data already present in the literature [17]-[23]. in the optimal conditions of a working frequency of 2.25 hz, an angular displacement of 7.0° figure 5. magnitude of the dft of the angular displacement θ (on the top) and of the harvested voltage vpeh (on the bottom) with a working frequency fw of 2.00 hz. figure 6. comparison between the magnitude of the dfts of the harvested voltage vpeh by varying the proof mass at an angular displacement θ of the buoy of 6.6° at a working frequency fw of 2.00 hz. figure 7. comparison between the magnitudes of the dfts of the harvested voltage vpeh by varying the imposed rolling motion at an angular displacement θ of the buoy of 6.6 ° and using a proof mass of 24.1 g. figure 8. comparison of the frequency magnification factors at different angular displacement θ of the imposed rolling motion and using a proof mass of 24.1 g. figure 9. specific average voltages �̅� by the peh obtained with respect the working frequency fw. 0.0 2.0 4.0 6.0 0 5 10 15 20 25 30 / v frequency / hz dft of angular displacement θ 0.0 2.0 4.0 6.0 0 5 10 15 20 25 30 / v frequency / hz dft of harvested voltage 0.0 1.0 2.0 3.0 4.0 5.0 6.0 0.00 2.50 5.00 7.50 10.00 12.50 15.00 d f t m a g n it u d e / v frequency / hz 14.7 g 18.1 g 24.1 g v p e h 0.0 1.0 2.0 3.0 4.0 5.0 6.0 0.00 2.50 5.00 7.50 10.00 12.50 15.00 d f t m a g n it u d e / v frequency / hz 1.75 hz 2.00 hz 2.25 hz i operative freq. ii operative freq. iii operative freq. v p e h acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 206 and a proof mass of 24.1 g, the harvested energy in 1 h for a specific power of 0.08 mw is 274.6 mwh. figure 11 reports a comparison of the specific power �̅�, made with different proof masses and at the same angular displacement θ. because of that it is found in figure 6, the different proof mass influences the harvested voltage 𝑉𝑃𝐸𝐻 , and consequently the specific power 𝑃 ̅, the latter proportional to the square of the voltage as visible in equation (9). as an example, figure 12 exhibits the specific power �̅� evaluated for the acquisitions with proof mass equal to 28.8 g. �̅� assumes two relative maxima corresponding to the working frequencies fw equal to 2 hz and 2.5 hz. generally, harvesters using piezoelectric patches in cantilever configuration are able to produce a power peak at its resonant frequencies. it has already been observed that the first mode of vibration, usually at low frequencies, is responsible for the greater energetic contribution as it provides the components with the highest rate of deformation compared to the other modes of vibration [16]-[20]. 3.3. uncertainty evaluation of the harvested power by the peh the uncertainty of specific power �̅� by the peh is estimated by analysing the relative weight of each of the quantities in equation (9). the estimation of the combined uncertainty was based on the iso/iec guide 98-3:2008 [34], by applying the following propagation law: 𝑢(𝑦 ) = √∑ 𝑢2(𝑥𝑖 ) ( 𝛿�̅� 𝛿𝑥𝑖 ) 2 𝑖 . (9) the previous is explicated as: 𝑢(�̅� ) = [𝑢2(𝑓𝑤) ( 𝛿�̅� 𝛿𝑓𝑤 ) 2 + 𝑢2(𝑅) ( 𝛿�̅� 𝛿𝑅 ) 2 + 𝑢2(�̅�) ( 𝛿�̅� 𝛿�̅� ) 2 ] 0.5 . (10) the extended uncertainty (uc) was then estimated by assuming a coverage factor (k) equal to 2.57, based on a t distribution with five degrees of freedom at a confidence level of 98%. the detailed computation is shown in table 3. similar results were obtained for other acquisitions at different motion parameters. looking at the relative weight of the different quantities affecting the uncertainty, it can be highlighted that the main contribution derives from the specific average voltages �̅�, whereas the other quantities have a much lower influence. 4. conclusions for a measurements buoy, the power consumption mainly depends on the components that deal with data communication. in fact, data collection does not take place continuously over figure 10. specific power �̅� , harvested by the peh, as a function of the working frequency fw and the angular displacement θ with a proof mass of 24.1 g. figure 11. specific power �̅� , harvested by the peh, as a function of the working frequency fw and the proof mass at different angular displacement θ. figure 12. specific power �̅� , harvested by the peh, for angular displacement θ and working frequency fw with a proof mass of 28.8 g. table 3. uncertainty evaluation at the working frequency fw equal 2 hz with an angular displacement θ of 6.6° and proof mass of 24.1 g. parameter value uncertainty type u(xi) u2 ∙ (δp/δxi)2 u(p) uc(p) (k = 2.57) �̅� / v 0.3193 a 2.85e-2 4.80e-5 9.49e-5 2.44e-4 fw / hz 2.00 a 5.57e-14 4.34e-9 r / ω 1e+6 b 1.50e+3 8.88e-21 0 0.02 0.04 0.06 0.08 1.25 1.5 1.75 2 2.25 2.5 2.75 s p e ci fi c p o w e r p / m w working frequency fw / hz 7.0 6.6 5.9 5.3 5.0 y = 0.02x 0.0341 r² = 0.9695 0 0.02 0.04 0.06 0.08 28.8 g 24.1 g 18.1 g amplitude 5.3 ° y = 0.0328x 0.0485 r² = 0.9963 0 0.02 0.04 0.06 0.08 amplitude 5.9 ° y = 0.0967x 0.1642 r² = 0.9788 0 0.02 0.04 0.06 0.08 1.25 1.50 1.75 2.00 2.25 2.50 2.75 working frequency fw / hz amplitude 6.6 ° s p e ci fi c p o w e r p / m w 0 0.02 0.04 0.06 0.08 angular displacement θ / ° s p e ci fi c p o w e r p / m w working frequency fw / hz proof mass of 28.8 g acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 207 time. therefore, these devices do not need to be constantly active and can often be put into sleep mode and waked up at fixed time intervals or in response to some external event. typically, the power consumption for such routines is on the order of a few mw and can be ensured by rechargeable batteries and appropriate power electronics downstream the harvesters [35]. in this work, a simple and innovative layout for a small selfpowered floating buoy, employed for environmental monitoring activities, has been presented. this device makes use of a peh, consisted of a pc, set in a cantilever configuration and excited by the rolling motion of the ripple waves, as supply source. the voltage and the power response of the peh has shown a particularly advantageous behavior because the imposed motion has excited the pc with a multi-frequency combination of vibration modes. a multiplication of the oscillation frequencies for the peh has been found as the working frequency varies. in fact, although the frequencies of the harvested voltage and power do not match the resonant frequency of the used peh and therefore the best deformation for the pc is not obtained [16][21], the calculated specific power has relative maxima with respect to the input parameters (i. e. the frequency and amplitude of ripple waves). in this setup configuration and in optimal conditions of a working frequency of 2.25 hz, an angular displacement of 7.0° and a proof mass of 24.1 g, the peh had reached the harvested energy in 1 h of 274.6 mwh for a specific value of harvested power equals to 0.08 mw. in this way the described floating buoy assumes the configuration of a frequency transformer. for these reasons, the analyzed layout allows to considerably increase the power generated by a single peh, compared to that obtainable in 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https://doi.org/10.1016/j.proeng.2017.09.503 https://doi.org/10.1088/0960-1317/16/11/026 https://doi.org/10.1007/s11633-008-0334-2 impact of the measurement uncertainty on the monitoring of thermal comfort through ai predictive algorithms acta imeko issn: 2221-870x december 2021, volume 10, number 4, 221 229 acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 221 impact of the measurement uncertainty on the monitoring of thermal comfort through ai predictive algorithms nicole morresi1, sara casaccia1, marco arnesano2, gian marco revel1 1 dipartimento di ingegneria industriale e scienze matematiche, università politecnica delle marche, via brecce bianche 12, 60131 ancona, italy 2 università telematica ecampus, 22060 novedrate, co, italy section: research paper keywords: monte carlo simulation; artificial intelligence; thermal comfort measurement; measurement uncertainty citation: nicole morresi, sara casaccia, marco arnesano, gian marco revel, impact of the measurement uncertainty on the monitoring of thermal comfort through ai predictive algorithms, acta imeko, vol. 10, no. 4, article 34, december 2021, identifier: imeko-acta-10 (2021)-04-34 section editor: carlo carobbi, university of florence, gian marco revel, università politecnica delle marche and nicola giaquinto, politecnico di bari, italy received october 10, 2021; in final form december 6, 2021; published december 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was partially done in the framework of the project “renozeb accelerating energy renovation solution for zero energy buildings and neighbourhoods”, funded by the european union’s horizon 2020 research and innovation programme under grant agreement no.768718. corresponding author: sara casaccia, e-mail: s.casaccia@staff.univpm.it 1. introduction in these modern times, it is commonly accepted by the scientific community that the term thermal comfort refers to a very subjective concept, that differs from one individual to another [1]. in fact, thermal comfort is affected by several factors that include personal, psychological, physical and environmental diversities and therefore, the personalization of thermal comfort measurement is increasingly required to provide more tailored and customized indoor environments, that start with the single individual and end with a comfortable and satisfactory environment [2], [3], [4]. physiological, psychological and personal parameters are progressively included in the measurement setup of thermal comfort, to encourage a more customized environment carefully tailored to the distinct preferences of the occupants that live in it. to put together all these specific parameters, personalized thermal comfort measurement typically requires an extensive range of sensing devices that make up a sensors network. since each sensor is properly characterized by measurement uncertainty, it is significant to include in the measurement process how this uncertainty, associated with each parameter, affects the final measurement of thermal comfort. this aspect is even more justified since, in all research fields, it is routinely accepted by the scientific community that the result of a measurement process promptly loses its meaning if an uncertainty value is not associated with it [5]. the traditional concept of measurement uncertainty must therefore be associated and applied to modern techniques, as in the case of artificial intelligence (ai) algorithms, which are finding more and more space in the measurement process. in support of this claim, literature reports that when small variations are applied to data, and variations are represented by the measurement uncertainty abstract this paper presents an approach to assess the measurement uncertainty of human thermal comfort by using an innovative method that comprises a heterogeneous set of data, made by physiological and environmental quantities, and artificial intelligence algorithms, using monte carlo method (mcm). the dataset is made up of heart rate variability (hrv) features, air temperature, air velocity and relative humidity. firstly, mcm is applied to compute the measurement uncertainty of the hrv features: results have shown that among 13 participants, there are uncertainty values in the measurement of hrv features that ranges from ±0.01% to ±0.7 %, suggesting that the uncertainty can be generalized among different subjects. secondly, mcm is applied by perturbing the input parameters of random forest (rf) and convolutional neural network (cnn) algorithm, trained to measure human thermal comfort. results show that environmental quantities produce different uncertainty on the thermal comfort: rf has the highest uncertainty due to the air temperature (14 %), while cnn has the highest uncertainty when relative humidity is perturbed (10.5 %). a sensitivity analysis also shows that air velocity is the parameter that causes a higher deviation of thermal comfort. mailto:s.casaccia@staff.univpm.it acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 222 associated with the data collected, ai provides completely misleading results [6]. the reason behind the use of ai for personalized thermal comfort measurement is that the measured output is dependent on several parameters, i.e., physiological and environmental parameters, that compose the measurement system. currently, the thermal sensation related to thermal comfort, is expressed using psychological scales, the most prominent of which is the ashrae 7-points scale, which is used in many comfort studies [7]. this scale is used to rate the thermal sensation vote (tsv), with 7 different verbal terms which are: “cold”, “cool”, “slightly cool”, “neutral”, “slightly warm”, “warm”, and “hot”. each term is associated with a categorical number in the range from -3 (“cold”) to +3 (“hot”). the application of ai in the measurement of human thermal comfort in the built environment, is becoming preferable because the adopted dataset is composed of two categories of parameters, which comprise the environmental and physiological parameters. a third category is also added to this dataset, made up of subjective parameters, expressed by the tsv. the union of these quantities generates a complex and heterogeneous dataset, making necessary the employment of ai, which comprises models comparable to a black box [8]. there are strong nonlinear relationships among the parameters included in this heterogeneous dataset, which suggests that this relationship should be explored with more complex and high-level algorithms, included in the ai field [9], [10] . ai measures its performance in terms of accuracy, which is expressed through different metrics, depending on the type of algorithm adopted, and the type of data being measured; for example, when the measured quantity is binary, or is made by discrete values, or class, performances are measured using accuracy, recall or precision; in case of ai used for measuring continuous quantities, the most used metrics are the mean absolute error (mae), mean absolute percentage error (mape), and mean squared error (mse). of course, the accuracy of aibased models, is strictly linked to the selected algorithm as well as the quality of the dataset, which is deeply connected to the uncertainty of the measured data [8] [11]. the method for the estimation of the uncertainty is described in the guide to the expression of uncertainty in measurement (gum) [12], which is based on the law of propagation of uncertainties (lpu). given the strong non-linearity of the relationship that exists between the input quantities and the output quantities of ai algorithms, the evaluation of the uncertainty associated with the output quantity is hard to assess if the lpu should be used. when it comes to non-linear relationship, the gum has proposed in its supplement [13], the evaluation of the uncertainty by using the monte carlo method (mcm), which allows assessing the uncertainty even if the relationship among quantities is not linear, and the analytical equation between quantities is not known, but it can be assumed as a black box, as happens in the context of ai models. the mcm represents a more practical alternative to the conventional lpu method, when it is not possible to effectively verify the hypothesis assumed by the lpu [11]. for this reason, this work studies the impact of the measurement uncertainty of a sensors network to measure thermal comfort, using ai models. to quantify and access the factors that affect the outcome of a measurement process through mcm, there are two methodologies that can provide this information: sensitivity analysis and a measurement uncertainty analysis [14], [15], [16], [17]. both of them are now applied to ai models, the first one to analyze how much each uncertainty weights on the measurement of the model’s outcome and the second to identify and quantify what are the main sources of uncertainties in the measurement [18]. as mentioned before, thermal comfort personalization is based on the measurement of a heterogeneous set of data, (i.e., environmental, physiological and personal parameters). regarding physiological quantities, the object of this study is the addition of heart rate variability (hrv) into the process for the measurement of thermal comfort: in fact, literature has repeatedly found the relationship that exists between some measures (or indices) derived from hrv and the thermal discomfort of the participant [19]–[22]. in particular, hrv is used to extract additional features, such as a specific quantity named lf/hf, which has a relationship with the thermal comfort of the user [9], [23]. more in detail lf/hf is the ratio between two hrv-derived parameters which are the low frequency components (lf) and the high frequency components (hf). literature has found that these two quantities represent the activity of the autonomic nervous system, responsible for the management of thermoregulation, and therefore are used to evaluate human thermal comfort. according to a previous study [23], [9], [24], physiological features of hrv and environmental quantities can be used in combination with ai models, to predict the tsv of user exposed to discomfort environmental conditions. results have shown that the tsv can be measured using two ai algorithms, which are the random forest (rf), that belongs to the ml class and convolutional neural networks (cnn), which are part of dl branch, with a mape of 20 % and 21 % respectively [23]. the further step is therefore to know how much the measurement uncertainty of each parameter involved in the measurement of thermal comfort, impacts the final measurement of tsv. to this aim, the paper is structured as follows: a) first of all, mcm is applied to the raw hrv signals coming from different participants, to see how the uncertainty in the measurement of hrv impacts the computation of the hrv features, since they are fed into ai algorithms, to extract the tsv. the standard uncertainty, associated to the measurement of hrv, chosen for this analysis is ± 4 ms, computed from a previous study in which the smartwatch was compared to a reference method while participants were sitting at rest [25]. this preliminary analysis can be useful to establish if the measurement uncertainty of the smartwatch, used to assess hrv and derive its features, has the same impact on each participant’s data, by having similar measuring uncertainty for each. b) secondly, mcm is applied to both physiological and environmental parameters used as input to the ai algorithms (rf and cnn), to evaluate how the measurement uncertainty propagates to the output, in black box methodology such as ai algorithms. the uncertainty used for the environmental quantities is the standard uncertainty provided from the datasheet of each device, while hrv is perturbed with different values of uncertainty that range from ± 4 ms to ± 100 ms. this range of values is chosen because literature has highlighted that, depending on the activity level performed by the participants, the acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 223 uncertainty associated with the measurement of the hrv in resting condition is ± 4 ms, while is greater than or equal to 100 ms when the user is performing a motion test [25], [26]. the research described in [25] and [26] were necessary since commercial smartwatches are not provided with precise datasheets that define the measurement uncertainties of the measured quantities. this is one of the problems being encountered in literature when it comes to working with low-cost sensors and often sensors that are not designed for research purposes. finally, the results are used to perform a sensibility analysis related to ± 4 ms of uncertainty, to examine the contribution of the uncertainties associated to the tsv measurement, in relation to the uncertainties of the input parameters. the paper is organized as follows: section 2 firstly describes how mcm is applied to compute the uncertainty in the measurement of hrv features, using a wearable smartwatch; secondly, it is explained the process to compute the measurement uncertainty of human thermal comfort, using a heterogeneous dataset and trained ai algorithms. section 3 provides the results associated with the two methodologies described in section 2, and the result of the sensitivity analysis. section 4 presents the discussion of results and section 5 provides the conclusion and the innovative aspect of this work. 2. materials and methods in this paper, a procedure to evaluate the impact of the uncertainty of the input data on the thermal sensation measurement output is applied. the aim is to introduce and combine a traditional technique that is the mcm for the estimation of the measurement uncertainty, with ai models. the methodology described in figure 1 is adopted: the first assumption is that each data coming from a measurement process is characterized by two quantities which are the data itself and the associated uncertainty. collected data from sensors network are merged together to build the heterogeneous dataset, that comprises the set of data and the associated uncertainties; therefore, when ai is applied in the measurement process, the model takes as input also the uncertainty of the collected data. therefore, it is expected that the results of the ai model derive from two main aspects: the first one is the intrinsic structure of the algorithm, while the second one is uncertainty associated with the input data [6]. the evaluation of the impact of the uncertainty associated with the input data in the measurement output, when ai is applied, is a paramount result. there is one intermediate aspect that should be considered, which is the impact of the uncertainty of the ai models, that will be combined with the uncertainty of the measurement instrument. the last part of figure 1 deals with two types of analysis which are the sensitivity analysis (sa) and the uncertainty analysis, performed by mcm. the procedure of the mcm consists in using data, acquired through real experiments, and perturbing them by assigning different measurement uncertainties or perturbations. each input parameter is modified, by adjusting it with a different perturbation, one at a time, while the other input variables are kept unchanged. based on the obtained results, the effect of different measurement uncertainties on the prediction of the tsv is observed through the simulation of perturbed data. the described methodology summarizes the steps to evaluate the measurement uncertainty of different parameters, used as input variables (or predictors) in ai algorithms, applied for the measurement of personalized thermal comfort, expressed through the tsv. the gum supplement provides the step necessary to perform the mcm. from a general point of view, mcm provides a general approach to numerically approximate the cumulative density function (cdf) of the output of a certain quantity 𝑦 = 𝑓(𝑥). the main concept behind mcm is that every sample of the input quantity 𝑥𝑖 , chosen from a predetermined distribution can be used. in this way, by taking a random sample of each input 𝑥𝑖 , from its related probability distribution function (pdf), it is possible to estimate a possible result of the output 𝑦 and the associated uncertainty. to explain how mcm is adapted in the context of this research, the following steps were applied: 1) the number of trials m of monte carlo trials is set to 200,000. m is the number of output quantity values that need to be selected; in this study, it is chosen a priori. usually, a number of trials equal to 106 are considered, which is the number of trials that should provide a coverage interval of 95 %, as reported in the gum supplement [13]. since it is commonly accepted that, the higher the number of trials, the higher is expected the convergence of the results, authors have set the number of monte carlo trials (m) to 200.000, as reported from the literature [14]; 2) m vectors 𝑥𝑖 , 𝑖 = 1, …, m, were generated, by selecting randomly from the probability density function (pdf) of each input quantity [𝑥hrv , 𝑥𝑡𝑎 , 𝑥𝑅𝐻 , 𝑥𝑣𝑎 ] in order to realize a set of possible input that can be associated to the input quantity. the random sample is obtained from a gaussian distribution, with uncertainties described in table 1. 3) for each vector generated in step 2, the corresponding output 𝑦 (or tsv in this case) is computed, yielding to m vector output quantity values; 4.1) point 3 is applied to estimate the uncertainty associated with the measurement of the hrv features, described in section 2.1; 4.2) in addition, point 3 is applied, perturbing the features used as input in the ai models, described in section 2.2; 5) the representation g of the distribution function for y is computed, starting from the set of m output of y; figure 1. conceptual description of the procedure adopted to study the impact of the measurement uncertainty in the context of ai model. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 224 6) g is used to compute an estimate 𝑦 of y and the covariance matrix 𝑢𝑖 associated with 𝑦; 7) g is used to compute the appropriate coverage region for y, for a stipulated coverage probability p; in this paper, we will refer to a generical model, with a number i of input 𝑥𝑖 and one output 𝑦, which is the measured quantity. the input parameters used in this study to measure tsv, were chosen on the basis of a previous study explained below: 13 participants were recruited to carry out a test consisting in exposing each participant in a semi-controlled room, in which environmental quantities were changed by varying the air temperature of the room. the sample population involved in the test is made of healthy people, not suffering from heart disease in order not to have perturbation on the collected hrv; the age of the sample population is 31 years ± 6 years old and participants were 7 females and 6 males. as reported in previous works [9], [23], the personal characteristics of the user (e.g., gender) are not included in this research since literature has shown that the analysis of environmental variables and personal characteristics in the prediction of user thermal comfort, does not lead to statistically significant results. moreover, authors did not consider differences in age, since literature reports that higher temperatures are preferred by elderly people, while lower temperatures are more suitable for the younger population; the sample population of this experiment is not made of older people and therefore authors do not deepen this aspect [9]. more in detail, the profile temperature of the test room was created as follows: the test room was previously set at 15 °c for 5 minutes, then the temperature set-point was set to 26 °c, then temperature was set back to 15 °c. the total duration was lengthened up to 100 minutes. the experiment was performed both in january 2020 and january 2021; the outdoor daily average temperatures were 8 °c in january 2020 and 4 °c in january 2021. during the experiment, the participants were sitting at a workstation and could perform light office activities (e.g., reading, working on the laptop). during the test, different quantities were monitored and used to train and test ai models for predicting the tsv of the users, which was collected during the test [23]. the monitored quantities for measuring the tsv to express human thermal comfort in this study were: air temperature (ta); relative humidity (rh); air velocity (va); heart rate variability (hrv). the list above contains both environmental quantities (ta, rh, va) and physiological quantities (hrv). the procedure with which the physiological and environmental data were acquired consisted in varying the environmental conditions of a semi-controlled room, while the sensors network acquired continuously these parameters. participants were informed of the purpose of the study and gave their informed consent; in addition, the study was validated by the ethical committee at università politecnica delle marche. the study was carried out in compliance with the principles laid down in the declaration of helsinki, in accordance with the guidelines for good clinical practice. the following paragraphs explain the methodology used in this research: in particular, section 2.1 aims at defining the measurement uncertainty associated with each hrv feature, derived from hrv, using mcm. section 2.2 explains how to implement mcm in combination with ai models, to assess the tsv of participants, using environmental and physiological data. 2.1 monte carlo approach on hrv features in this first part of the analysis, mcm is used to estimate how the uncertainty of the device through which the hrv is collected, influences the computation of the hrv features, that will be later used to evaluate human thermal comfort. hrv signal, which is made by the distance in time of two subsequent rr peaks of the ecg trace, is divided into time frames from which it is possible to extract some indices (or features). each time frame is built as follows: the first time frame corresponded to 5 minutes of the hrv signal, which is considered the minimum time duration recommended for computing hrv spectral analysis. after the extraction of the first window, a new window was computed by appending a new hrv sample interval of the signal, while the oldest sample was removed from the beginning of the window; the process was repeated until the end of the signal. according to the literature, several hrv features, to be extracted from the hrv signal, were identified. time-domain hrv features f(hrvt) are a collection of statistical and geometrical indices for the measurement of the variability in the hrv sequence that act as indices to interpret the oscillations of cardiac cycles. the f(hrvt) statistical indices computed in this study are: standard deviation of rr intervals (sdann), root mean square of rr (rmssd), mean value of rr (mean), median of rr (median), percentage of the difference between adjacent rr intervals that differs more than 50 ms (pnn50), percentage of the difference between adjacent rr intervals that differs more than 25 ms (pnn25). in addition, hrv studies imply the use of frequency-domain features f(hrvf), which are useful for the understanding of the stationarity or stability of the hrv signal. to obtain the frequency-domain analysis, the first power spectral density (psd) was computed through the autoregression modelling-based method that has proven to provide better resolution. each frequency band was then computed: lf (0.04 hz 0.15 hz) and hf (0.15 hz 0.4 hz), lf/hf, hf/lf and the total power spectrum (tp). non-linear features f(hrvnl) were also computed through the poincarè plot. the poincarè plot is a graphical representation of an hrv time series along the cartesian plane: the x-axis contains one hrv sample, while the y-axis contains the following hrv sample. the poincarè plot provides two additional features obtained by adjusting the point cloud of the figure formed into an ellipse; the first index (sd1) represents the dispersion of points perpendicular to the identity line, while the second one table 1. characteristics of the measurement instrument used in the test. input quantities manufacturer model standard uncertainty distribution air temperature (ta) thermo sensor gmbh pt100 (4 wired) ± 0,1 ºc normal relative humidity (rh) ahlborn fhad46c41a ± 2% of reading normal air velocity (va) ahlborn fvad05tok300 ± 3% of reading normal heart rate variability (hrv) samsung samsung galaxy watch ± 4 ms normal acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 225 (sd2) is the dispersion of point along the identity line of the poincarè plot. in this section, there is a first analysis made specifically on the individual features of the hrv. in practice, we want to assess how it is the uncertainty measurement of each feature, when the smartwatch is affected by a specific uncertainty in the measurement of hrv; the procedure is summarized in figure 2. first of all, one hrv window of 5 minutes is considered. this window will be perturbed with a random sample coming from a normal distribution with mean equal to 0, and a standard deviation equal to u, for 200,000 iterations. the perturbed hrv segment will be used to compute hrv features in order to establish the impact of the uncertainty. the final uncertainty is computed as the standard deviation of the resulting hrv, by a coverage factor k = 2. for the simulation study, 13 segments of hrv of a duration of 5-minutes, were extracted from the set of experimental procedures described previously. the duration of 5 minutes is required because is the minimum time required to compute short term hrv features. 2.2 monte carlo approach on ai models for measuring human thermal comfort for this second part of the analysis, the set of hrv features, explained in figure 3, was chosen, [23]. figure 3 shows the approach used to apply mcm and ai: a set of input parameters was used to train rf regression algorithm and a cnn, to predict the tsv, output of the algorithm. once the models are trained, the procedure consists in perturbing the input parameters (hrv, ta, rh, va) one at a time, with different uncertainties, maintaining constant the other quantities, and applying the trained model to each perturbed set of features, to obtain the final measurement of the tsv. it is worth noting that the current analysis is conducted locally, by choosing one observation of the whole dataset; the final result is therefore associated to the chosen observation. characteristics of the sensors used to collect environmental parameters, and the measurement uncertainty employed to perturb the different parameters are shown in table 1. sensitivity analysis a sensitivity analysis (sa) is performed to identify and quantify what are the main sources of uncertainties in the measurement. ideally, sa is the methodology for studying how the uncertainty of the output provided by a model, can be associated, qualitatively or quantitatively, to different uncertainties of the input parameters of the model [27], [28]. the gum supplement also provides the specifications for assessing the sensitivity coefficients; more in detail, it is explained that mcm is not sufficient to fully compute sensitivity coefficients but provides a methodology to compute the influence of each input quantity on the output quantity [13]. figure 2. description of the mcm adopted to evaluate the impact of the smartwatch uncertainty on the hrv features, that will be used to measure and assess human thermal comfort. figure 3. description of the procedure adopted to apply mcm to the measurement of thermal comfort, with a heterogeneous set of data. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 226 the procedure implies that one input quantity should be perturbed, and the remaining input quantities should be kept at their best estimates, in order to obtain the pdf of the output quantity, depending only on the variable perturbed. by using this procedure, the gum supplement proposes an approach that can be representative as a generalization of the approximate partialderivative formula; in particular, it is reported that the sensitivity coefficients can be approximated as the ratio of the standard deviation of the resulting model values and the standard uncertainty associated with the best estimate of relevant input quantity, as reported in eq. 1: 𝑐𝑖 = 𝑢𝑖(𝑦) 𝑢(𝑥𝑖 ) , (1) where 𝑐𝑖 is the sensitivity coefficient, 𝑢(𝑥𝑖 ) is the standard uncertainty associated with the ith input estimate 𝑥𝑖 , that contributes to the standard uncertainty 𝑢𝑖 (𝑦). 3. results 3.1 impact of the uncertainty of measurement on the hrv features in this section, results of the mcm used to analyse the propagation of the uncertainty in the measurement of hrv that propagates to the computation of the hrv features, are shown. the simulated results are used to obtain the frequency histogram of each hrv features that are computed with the simulation, to obtain the related uncertainty. for each hrv features the values of the standard uncertainty computed with the mcm are shown. the frequency histograms that come out of one simulation for one participant, are displayed in figure 4. table 2 contains the uncertainty associated with the computation of each hrv feature, expressed in percentage, among the 13 participants. it can be seen that the minimum measurement uncertainties are obtained when computing the mean feature (± 0.01 % of reading), while the highest values are associated with the measurement of lf/hf and hf/lf (± 0.7 % of reading). physiological quantities, such as hrv, can vary among participants and therefore it is interesting to understand the impact of the uncertainty on each feature, divided according to the related domain, among the different users, to see if it is possible to establish a generalized standard uncertainty associated to each hrv features, in relation to the value of the uncertainty of the device which measures it. the uncertainty associated with the features in the frequency domain is higher than the features in the time or the features in the non-linear domain. a reason that can explain this result is that the frequency components are more sensitive to the uncertainty of the device (± 4 ms) because they physically reflect table 2. measurement uncertainty computed for each hrv features used to estimate the tsv, among the 13 participants. id uncertainty (%) mean rmssd median lf hf lf/hf hf/lf sd1 sd2 sd1*sd2 1 0.01 0.10 0.09 0.34 0.30 0.4 0.4 0.10 0.1 0.2 2 0.01 0.08 0.09 0.31 0.25 0.4 0.4 0.08 0.1 0.1 3 0.01 0.29 0.07 1.30 0.81 1.5 1.5 0.29 0.3 0.5 4 0.01 0.11 0.08 0.53 0.55 0.6 0.6 0.11 0.1 0.2 5 0.01 0.25 0.06 0.93 0.58 1.1 1.1 0.25 0.2 0.4 6 0.01 0.10 0.08 0.30 0.28 0.4 0.4 0.10 0.1 0.2 7 0.01 0.16 0.10 0.57 0.59 0.6 0.6 0.16 0.1 0.2 8 0.01 0.04 0.13 0.26 0.12 0.3 0.3 0.04 0.0 0.1 9 0.01 0.12 0.08 0.31 0.32 0.5 0.5 0.12 0.1 0.2 10 0.01 0.24 0.06 0.92 0.58 1.1 1.1 0.24 0.2 0.4 11 0.01 0.09 0.07 0.31 0.25 0.4 0.4 0.09 0.1 0.1 12 0.01 0.30 0.08 0.66 0.79 1.0 1.0 0.30 0.2 0.4 13 0.01 0.06 0.10 0.26 0.19 0.3 0.3 0.06 0.1 0.1 µ 0.01 0.1 0.1 0.5 0.4 0.7 0.7 0.1 0.1 0.2 figure 4. histogram obtained from mcm with 200,000 iterations applied to the hrv features. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 227 the oscillatory activity of the hrv; for this reason, it can be verified that a small variation of hrv can lead to a greater variation of features in the frequency domain. 3.2 impact of the uncertainty on ai models for measuring human thermal comfort this section reports the results used to compute the measurement uncertainty of the tsv, using mcm applied to ai models, which are rf and cnn. table 3 contains the results which represent the contribution of the uncertainty of each input quantity (hrv, ta, rh, va) into the ai model, to estimate the associated uncertainty in the output tsv. each pdf of the input quantity is used, one at a time, according to monte carlo simulation, to obtain the resulting pdf of the tsv. the following table contains the results of an mcm conducted by perturbing each parameter, one at a time, with the respective uncertainty 𝑢𝑖 (𝑦) , where 𝑖 are the perturbed parameters. the simulation regarding the impact of the hrv uncertainty of the tsv was made for ± 4 ms. since hrv measurement through smartwatches is particularly prone to motion artifacts, more simulations were conducted by simulating different hrv uncertainties, which are u = [4, 10, 20, 50, 100] ms. the results of the simulations are shown in figure 5: the contribution of hrv uncertainty increases after 50 ms, suggesting that up to this value, the uncertainty of the devices that measure hrv is still acceptable for assessing the tsv. on the other hand, environmental sensors that measure ta, rh and va, are less subjected to variations in the uncertainty of the measurement of tsv. environmental quantities produce different uncertainty on the tsv, which depends on different parameters. rf has highest uncertainty due to the ta (𝑢𝑡𝑎 (𝑦) = 14 %), while cnn has highest uncertainty when rh is perturbed (𝑢𝑅𝐻 (𝑦) = 10.5 %). this different result is explainable if we consider that the two algorithms perform with different rules and can be considered as black boxes. sensitivity analysis in table 4 the partial uncertainty budget due to each parameter is given according to gum uncertainty framework; the sensitivity coefficients were evaluated by monte carlo simulation. according to the table, sa highlights that air velocity and air temperature are the parameters that most affect the tsv prediction, respectively for rf and cnn. rh, and hrv perturbed with ± 4 ms are the less sensitive parameters in the measurement of tsv, while va is the parameter that mostly contributes to the tsv measurement uncertainty. this outcome is consistent with the test procedure, which implied that during the experiment, to generate thermal discomfort, the window was opened, and the participant reported experiencing greater discomfort. 4. discussion depending on the kind of algorithm trained than can be both ml and dl algorithms, there are different quantitative results in the uncertainty of the tsv, using an mcm, with 200,000 iterations, in relation to the environmental parameters. rf provides greater uncertainty when ta is perturbed with an uncertainty of ± 0,1 ºc; on the contrary, cnn exhibits a higher uncertainty when rh is perturbed with ±2% of reading. physiological parameter (hrv signal), when perturbed with ±4 ms of uncertainty, impacts the resulting tsv with 2.8 % of uncertainty and > 0.001 % when it comes with cnn and rf respectively. in addition, mcm on ai models for measuring human thermal comfort (section 2.2) is applied to one observation of the entire dataset, and therefore it is a local analysis that is strongly related to the trained model. the overall methodology presented in this paper can be applied to brand new models, that do not include an analytical equation, to compute the uncertainty associated with the output of the model. mcm is a powerful tool that can be used to simulate the results of the impact of more than one measurement uncertainty, such in the case of the analysis presented in the manuscript in section 3.2, in which hrv is perturbed with a set of uncertainty that ranges from ± 4 ms to ± 100 ms. the impact of this study is that mcm can be applied in a variety of circumstances in which the analytical model that represents the relationship between input and output is not known, such in the case of ai models, but the initial perturbation can be simulated. when the measurement uncertainty of the device is not available, the uncertainty can be therefore hypothesized and simulated, or eventually a calibration to determine the uncertainty can be performed. 5. conclusion this paper presents an approach to assess the measurement uncertainty of human thermal comfort, expressed in terms of tsv, by using a method that comprises a heterogeneous set of data, made by physiological and environmental quantities, and ai models. the objective is therefore to quantify the measurement uncertainty of the tsv, while the user is performing light-office activities, by using gum guidelines and applying mcm. table 3. results of mcm applied to compute the measurement uncertainty 𝒖𝒊(𝒚) of the measurement of the tsv, both for the rf and the cnn algorithm. the uncertainty was computed considering a coverage factor k equal to 2. rf cnn hrv ta rh va hrv ta rh va standard uncertainty ± 4 ms ± 0,1 °c ± 2% of reading ± 3 % of reading ± 4 ms ± 0,1 °c ± 2 % of reading ± 3 % of reading tsv estimate (µ) 1 1.4 1.4 1.4 -0.5 -0.5 -1.2 -1.2 𝑢𝑖 (𝑦) [tsv unit] (σ) 7.46e-14 0.2 1.78e-13 0.0105 0.015 0.015 0.126 0.029 𝑢𝑖 (𝑦) [%] 7.24e-12 14 1.28e-11 0.8 2.825 2.9 10.5 2.5 table 4. sensitivity coefficients compute for the two algorithms. hrv ta rh va standard uncertainty ± 4 ms ± 0,1 °c ± 2 % of reading ± 3 % of reading sensitivity index rf 1.87e-14 1.8 4.61e-13 5.8 sensitivity index cnn 0.004 0.09 0.32 16.03 acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 228 a preliminary analysis was conducted to assess the impact of the measurement uncertainty of the instrument used to collect hrv, which is a commercial smartwatch. mcm was applied to compute the uncertainty associated with the features extracted from hrv, which will be later fed into an rf and cnn model. results have shown that among 13 participants, there are uncertainty values in the measurement of features that ranges from ± 0.01 % to ± 0.7 %, suggesting that among different users the uncertainty can be generalized. then, mcm was applied by perturbing a set of parameters (hrv, ta, rh and va) to compute the uncertainty in the measurement of the tsv, using rf model and a cnn. results have shown that environmental quantities produce different uncertainty on the tsv. rf has the highest uncertainty due to ta uncertainty (u = 1 %), while cnn has the highest uncertainty when rh is perturbed (u = 10.5 %). on the other hand, the sensitivity analysis that expresses the relationship between the tsv and the input parameters highlights that va is the parameter that causes the greatest variation on the tsv. acknowledgement the activity presented in this work was partially carried out within the renozeb project, funded by the european union’s horizon 2020 research and innovation programme under grant agreement no. 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https://doi.org/10.1007/s00421-008-0718-6 https://doi.org/10.1109/jsen.2021.3064707 https://doi.org/10.21014/acta_imeko.v8i1.562 https://doi.org/10.1109/memea49120.2020.9137211 https://doi.org/10.1109/memea52024.2021.9478713 https://doi.org/10.1088/0957-0233/23/3/035005 https://doi.org/10.1016/j.apradiso.2015.11.097 low-cost, high-resolution and no-manning distributed sensing system for the continuous monitoring of fruit growth in precision farming acta imeko issn: 2221-870x june 2023, volume 12, number 2, 1 11 acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 1 low-cost, high-resolution and no-manning distributed sensing system for the continuous monitoring of fruit growth in precision farming lorenzo mistral peppi1, matteo zauli1, luigi manfrini2, luca corelli grappadelli2, luca de marchi1, pier andrea traverso1 1 dei department of electrical, electronic and information engineering ”guglielmo marconi”, university of bologna, 40136 bologna, italy 2 distal department of agricultural and food science, university of bologna, university of bologna, 40127 bologna, italy section: research paper keywords: smart farming technologies; smart agriculture; agricultural iot; autonomous sensor node; lora citation: lorenzo mistral peppi, matteo zauli, luigi manfrini, luca corelli grappadelli, luca de marchi , pier andrea traverso, low-cost, high-resolution and no-manning distributed sensing system for the continuous monitoring of fruit growth in precision farming, acta imeko, vol. 12, no. 2, article 17, june 2023, identifier: imeko-acta-12 (2023)-02-17 section editor: francesco lamonaca, university of calabria, italy received july 11, 2022; in final form february 24, 2023; published june 2023 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was funded by the “italian departments of excellence” initiative sponsored by the italian ministry of university (miur). corresponding author: lorenzo mistral peppi, e-mail: lorenzomistral.pepp2@unibo.it 1. introduction the rise of new technologies makes possible to approach classical issues with modern, smart solutions. agriculture is definitely benefiting from these innovations: quick global development, population growth, climate change and a new awareness of food, its safety and the impact its production has on the environment have led to terms such as “precision farming”, “digital farming” or “agriculture 4.0” becoming more and more widespread and considered as a real added value to the agricultural product. the term precision agriculture (pa) encompasses many disciplines and technologies employed: artificial intelligence, autonomous control of agricultural equipment, automatic decision-making systems, quality control and application of treatments by drones, and so on, in order to increase productivity and environmental quality [1]. the key element, therefore, is the ability to acquire, transmit and process information in order to monitor crops, to make autonomous decisions through decision-support systems (dss) (as an example see [2]) and/or to provide data about a particular situation in order to allow for a decision to be made that is as correct and spatially and temporally customised as possible [1], [3]. the advent of iot [4] networks has further increased the pervasiveness of in-field sensing due to the possibility, particularly through lorawan networks, to cover large areas abstract accurate, continuous and reliable data gathering and recording about crop growth and state of health, by means of a network of autonomous sensor nodes that require minimal management by the farmer will be essential in future precision agriculture. in this paper, a low-cost multi-channel sensor-node architecture is proposed for the distributed monitoring of fruit growth throughout the entire ripening season. the prototype presented is equipped with five independent sensing elements that can be attached each to a sample fruit at the beginning of the season and are capable of estimating the fruit diameter from the first formation up to the harvest. the sensor-node is provided with a lora transceiver for wireless communication with the decision making central, is energetically autonomous thanks to a dedicated energy harvester and an accurate design of power consumption, and each measuring channel provides sub-mm 9.0-enob effective resolution with a full-scale range of 12 cm. the accurate calibration procedure of the sensor-node and its elements is described in the paper, which allows for the compensation of temperature dispersion, noise and non-linearities. the prototype was tested on field in real application, in the framework of the research activity for next-generation precision farming performed at the experimental farm of the department of agricultural and food science of the university of bologna, cadriano, italy. mailto:lorenzomistral.pepp2@unibo.it acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 2 using extremely low-power, adequate reliability and low-cost hardware [5], [6]. in addition, smart devices allow farmers to be employed for different tasks, for which human presence is essential, reducing the need for human intervention and therefore lowering overall farm operating costs. however, farmers often perceive new technologies more as a complication than an advantage, mainly because of their inexperience, the lack of interoperability between various technologies, the complexity, the often very high costs, and the inability to handle such large amounts of data [7], [8]. consequently, efforts must be made to make these technologies easy for the farmer to install, understand, successfully exploit, and maintain. the monitoring of fruit growth does not only help to estimate the yield [9], [10] but also provides additional information, such as the water stress status of the plants [11], [12]. in addition, by comparing the data acquired in real time with predictive growth models, corrective actions can be taken in the orchard. the most common techniques for measuring fruit dimension employ lvdt (linear variable displacement transducer) sensors and strain gauges or potentiometers [13], [14] which, despite being highly accurate, are limited by a usually very small measurable range. therefore, in order to keep on with the measurement process, frequent re-positioning of the sensor on the fruit is required. this timeand labourconsuming activity has a cost, and therefore an impact on farmer’s income. there are alternative solutions that do not require relocation over time, such as those employing optical sensors directly on the fruit, as in [15], but they are usually affected by low accuracy and the inability to detect fruit shrinkage. currently, systems capable of agricultural analysis have been developed using computer vision [16] and artificial intelligence methodologies [17]. in particular, techniques to estimate the number of fruits and their size [18], [19], [20], [21], [22] have been introduced. however, the accuracy achievable by these devices is still low compared to traditional techniques. this work aims to present the design, implementation and experimental assessment of a low-cost multi-channel distributed sensing system, aimed at the high-resolution, continuous estimation and recording of the diameter of fruits directly on the tree throughout the whole growth process. the system is noninvasive and does not either damage or warp the fruits, is energetically autonomous and is strongly uninfluenced by environmental conditions. relocation of sensing elements is not needed either during the ripening season, as required by the sensors used so far because of their small measurable range, thanks to an adequate full-scale range that is greater than the size of the fruit (e.g., apple, orange) once it has reached the typical ripe dimension. in comparison with the preliminary work presented in [23], in which a first, single-channel basic prototype was shown to illustrate the concept, a complete multi-channel sensor-node solution is presented here, with also the addition of an energy harvesting section able to guarantee season-long autonomous operation and a lora transceiver to allow data communication even at far distance from the receiving gateway. the full calibration of the multi-channel system was refined and is described step-by-step in details. two in-field measurement campaigns are also presented, carried out at an apple orchard at the experimental farm of the department of agricultural and food science of bologna, cadriano, italy. the paper is organized as follows. in section 2 the operating principle of the device is explained, and system architecture is shown. in section 3 all the main components of the multichannel sensor-node are described in details, with particular emphasis to the architectural/design solutions exploited to maximize final reliability and accuracy. section 4 describes the refined full calibration procedure of the prototype. finally, in section 5 results of in-field tests, conducted on an apple orchard, are shown. 2. system architecture and operating principle unlike the conventional methods proposed in literature, where the growth of the fruit determines a stimulus directly detected by a sensitive element (potentiometer, lvdt, etc.), in this application the i-th sensing element (i.e., the front-end of the i-th measuring channel) is made of a structure of known dimensions. the structure is composed of two solid arms bounded together at one end with a bold (figure 1 for a fivechannel sensor-node). it is used to interface the object to be measured to the element itself. the plier is kept in place by means of a spring while a reference voltage-supplied (vref) potentiometer, which is placed within the fulcrum of the plier and is rigidly connected to one of the two arms of the clamp, converts the opening angle α into a voltage acquired by the analog-to-digital converter (adc) integrated into the microcontroller unit (mcu) governing the overall node. the voltage at the output of each sensing element is proportional to the opening angle of the clamp, since the partition ratio of the potentiometer is directly proportional to α, and the linear width d that represents the measurand is indirectly obtained from the a/d acquired voltage according to (1): 𝑑 = 2 𝑅 ⋅ cos ( π 2 − 𝛼(𝑉out) 2 ) . (1) the task of the mcu is to trigger readings, to perform all the in-site calibration and compensation processes on the acquired raw data and to manage storage and/or wireless transmission of data about the fruit growth evolution to a remote decisional unit. figure 1. architecture of the sensor-node, with details of one of the five sensing elements. acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 3 in addition, the mcu also manages the power supply timing of potentiometers, ensuring minimum power consumption of the entire sensor-node. 3. sensor-node prototype in this section, the main components that were exploited in the realization of a prototype for a complete five-sensing element sensor-node will be described, together with their characteristics and related design strategies. 3.1. abs pliers the plier in each sensing element consists of two arms, which are bolted together. a rotary potentiometer is located in a recess within the pivot, to sense the opening angle α between the two arms. the plier acts as an adapter to allow to easily optimize the full-scale for the width d during the design phase, while preserving the same angular full-scale. more precisely, to vary the maximum measurable distance dfs, given the same maximum opening angle, it is only necessary to re-scale the effective length r of the plier arms. after preliminary tests with different materials, the plier was manufactured using acrylonitrile butadiene styrene (abs), through a 3d printing process. the use of abs makes it possible to obtain a structure that is robust but at the same time lightweighted: in fact, the warping of fruits due to an excessively heavy plier, which would lead to a distorted estimated growth profile, must be avoided at all costs. in order to avoid any interference with the growth of the fruit, a “half horseshoe” shape (shown in figure 2) was chosen for the arms, while the mechanical elements that rest on the fruit are cup-shaped to increase the contact surface, their edges being coated with silicone to prevent slipping. on the side of the cups, slits are located to prevent the accumulation of moisture and the consequent possible development of fungal diseases. the dimensional parameters of the plier are necessarily customized according to the species of fruit, in order to allow for the capability of monitoring the entire ripening process, optimize the nominal resolution and maximize the overall accuracy of each channel. the size of these prototype calipers is such as to allow for the monitoring of apple growth, therefore they have been designed to offer a full-scale range of dfs = 12 cm, starting from the requirement of a maximum opening angle of αfs = 60 deg (see below), which resulted in an effective length r = 12 cm. 3.2. angular sensor as angular sensor, a single turn potentiometer with an electrical angle of 60 deg was chosen. the potentiometer, connected as a voltage divider, is supplied by a reference voltage vref shared with the adc vref in order to minimize gain errors. in such a configuration the output voltage vout of the sensing element is proportional to the opening angle, and thus algebraically related to the measurand d according to (1). the phs11-1dbr10ke60 [24], from tt electronics, was the commercial component adopted. although tests conducted over a period of months have not shown any particular weathering issues for the potentiometer, in order to protect from dust, dirt and moisture it was coated with a layer of waterresistant insulating grease. 3.3. mcu, adc and temperature sensor for this application a microcontroller with low power consumption and integrated a/d channels was required. at first cmwx1zzabz-091 murata modules were selected. this device integrates, together with an stm32l072 mcu, a semtech sx1272 lora transceiver, freeing the designer from problems related to the management of rf circuits. however, components supply shortage related to the 2020-22 pandemic forced the use of a stmicroelectronics nucleo64 demo board [25] equipped with a stm32l152re mcu, whose power supply circuit was exploited and whose pin headers were used as interface to the five potentiometers and the sd card for local data storage. the mcu can be easily integrated with a lora module (e.g., mbed sx1272mb2das) as well. stm32l152re is a 32-bit microprocessor incorporating a 12-bit successive approximation adc, spi and i2c peripherals, and featuring low power modes [26]. as already mentioned, thanks to the presence of pmos devices acting as power switches, the same reference voltage used by the a/d stage was employed to supply each potentiometer: in this way it was possible to take advantage of the entire fullscale range of the adc and, at the same time, compensate for both shortand long-time dispersion effects of the reference voltage. by adopting 3.3 v as vref, the nominal quantization step of the adc is 806 µv, which is approximately equivalent to a nominal resolution of 30 µm for each sensing channel. the mcu makes internally available a temperature sensor, which was used for thermal compensation in the framework of the real-time overall calibration procedure (section 4) implemented for each channel. during the production of the mcu this sensor is calibrated, and calibration coefficients made available within read-only portions of the memory [26]. nonetheless, a comparison was made for performance assessment between this sensor and a reference one (namely, hts221 from stmicroelectronics) characterised by an ac curacy of 1.0 °c in the 0 °c to 60 °c range [27]. the maximum figure 2. 3d rendering of the plier designed for the realization of each sensing element. acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 4 estimated deviation between the two sensors in the 24 °c to 72 °c range was 1.34 °c, which allowed to consider the accuracy of the internal mcu sensor adequate enough for the thermal compensation cited above. 3.4. power supply the voltage provided by the battery and harvester (see next subsections) had to be adjusted down to 3.3 v, which is a compatible level with the operation of the mcu and peripherals. to this aim, the ld39050pu33r ldo from stmicroelectronics available in the nucleo board has been used, which allows for a stable, low-noise power supply. this device is characterized by very good performance in terms of output voltage noise (30 µvrms over 10 hz to 100 khz bandwidth) and thermal stability [28]. in order to switch off the supply to potentiometers and to sd card, when not in use, pmoss acting as switches are used, as described in subsection 3.7. 3.5. data storage and communication the lora transmitter uses an mbed sx1272mb2das [29] demoboard, fitted with a semtech sx1272 [30] transceiver. all rf-related circuitry is relegated to this board, which is connected to the nucleo64 via a set of arduino-compatible connectors. the sx1272 transceiver is designed to operate in the 868 mhz frequency band, and it communicates with the microcontroller via an spi interface [30]. in addition, dio lines are used by the semtech loramac.node stack to interface with the transceiver. the power supply line of this board is connected directly to the nucleo board to exploit the power supply stage of the latter. as an alternative to sending data over the air, these can be saved locally by means of an sd card (figure 1). 3.6. energy harvester to ensure maintenance-free operation of the sensornode during the entire fruit growth period, it is essential to rely on a low-cost source of energy that does not run out in a short interval of time. by considering consumption estimates and planned duty cycles, it is possible to determine the most suitable energy source to ensure high system autonomy. a single acquisition cycle, consisting of interrogating all five sensing elements every 600 s, processing and storing of data on the sd card (local storage is considered in the following discussion), requires a total of 1.63 j, such estimation being performed by averaging data from different sd card manufacturers [31]. as an example, with a 500 mah battery the operating time of the prototype would be around 25 days, without taking into account self-discharge phenomena, while for many fruit species the growth and ripening times are much longer [9], [32]. therefore, such a local battery is not suitable for a virtually maintenance-free node. increasing the battery capacity is not practical, either: at the beginning of the season all the batteries should be charged, and if the number of nodes is high many chargers would be needed, which could be a significant cost, because of their price and required manpower, even if wireless power transfer was used, such as in [33]. for this reason, the adoption of a photovoltaic harvesting sub-system, with a small rechargeable back-up battery, was considered as the optimal solution. this avoids the use of external chargers or nonrechargeable batteries, which are by their nature also a major source of pollution. as it will be described in the following, when the photovoltaic panel is exposed to full sunlight it is possible to keep the back-up battery fully charged. however, in the case of indirect exposure to light, such as in greenhouses applications, recovery of the energy consumed by the sensor-node is not always guaranteed. for this reason, the battery voltage level is monitored and forwarded every time a data transmission takes place. for the prototype a steval-isv012v1 evaluation board from st microelectronics was used. included in the evaluation board are a 400 mwpeak photovoltaic panel, namely szgd60604p from nbszgd, a spv1040 step-up converter with mppt (maximum power point tracker) and a l6924d charge controller for li-ion batteries, both from stmicroelectronics [34]. the use of a step-up converter with mppt algorithm allows to continuously track output voltage and current of the solar cell and therefore allows to maintain its maximum power point while changing lighting and load conditions, thus maximizing the harvested energy and increasing the overall efficiency of the stage. tests carried out in open field, at the farm of the faculty of agriculture of university of bologna (cadriano, bologna) between june and august 2021, have shown how this solution is able to ensure a sufficient power supply to power the prototype when used outdoor: the harvester is able to keep the battery fully charged during the day, while during the night the discharge is minimal, as it is possible to appreciate in figure 3. it is worth noting that the test was carried out in rows protected by rain-hail nets and the photovoltaic panel oriented in the east direction. 3.7. power gating when a measurement is performed, the potentiometers are powered by a reference voltage. without power gating systems, the energy wasted during quiescent operation would be much higher than the one needed to keep the mcu in stand-by: a current of 330 µa would flow continuously in each one of the 5 potentiometers, and the overall consumption would not be negligible. to implement the power gating function pmos devices are used: a low logic value to the gate pin allows the pmos to start conducting, powering the potentiometers. by using a pmos, it is not required to force a high logic value to the gate to keep the load disconnected: by using this configuration i/o interfaces of the mcu can be disabled, thus saving energy and leaving the figure 3. battery voltage of the energy harvester during open field tests. it is worth noting that the battery is always fully charged, and even during bad weather days no significant battery discharge is noticeable. acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 5 gpio pins floating while allowing potentiometers to be not powered. this is due to the fact that, by inserting a resistor rg (figure 4) of appropriate value between gate and source, the device will always remain off as vgs = vrg ≈ 0 v because igate ≈ 0 v. even a high value of rg keeps the device off and allows, when vgs < 0 v or when the pmos is conducting, to dissipate over rg a negligible amount of energy. a key factor in the choice of mos is the low rdson of the device: the lower its value, the lower the influence of this parameter on the final measurement. this aspect will be discussed in more detail in sec. 4.2 . the commercial device chosen is dmp2200udw pmos, from diodes incorporated. this device has two extremely low resistivity [35] (spice simulations show a resistivity of 207 mω at 27.5 °c in actual operative point) pmos placed within the same package. each mos is used to activate two potentiometers at a time: in this way, it is possible to power only those potentiometers whose output must be activated, thus resulting in an overall lower power consumption. 4. real-time calibration procedure several sources of uncertainty, mainly related to the variations of temperature in which the system will operate, have been identified in the uncalibrated prototype. in an open field or greenhouse, at least a temperature range of a few tens of degrees has to be considered. furthermore, this system must be able to operate in the most varied conditions, in direct sunlight and during all months of the year. therefore, temperature stability is essential and temperature dispersion must be carefully compensated. in addition, the real-time calibration procedure described in subsections 4.2, 4.3 and 4.4 and implemented in the prototype compensates also for short-term instability (e.g., noise) and non-linearities affecting each sensing channel (figure 5). 4.1. a/d acquisitions and data averaging for a given sensing channel, in order to obtain a single fruit size reading the adc is activated in continuous mode every 600 s to acquire m sequences of n samples each of the voltage vout at the output of the potentiometer in the channel. a 𝑉𝑅 estimator is then computed as the averaging of the m ∙ n samples 𝑉𝑅,𝑛 (𝑚) where (m, n) denote the m−th sequence and the n−th sample in the sequence, respectively: 𝑉𝑅 = 1 𝑀 ∙ 𝑁 ∑ ∑ 𝑉𝑅,𝑛 (𝑚) 𝑁 𝑛=1 𝑀 𝑚=1 . (2) the estimator in (2) could be used directly as the vout value in (1) to compensate for short-term instabilities (mainly noise) in the a/d conversion process. however, the procedure would not take into account thermal dispersion and non-linearities, thus additional correction steps were implemented, as discussed in the next subsections. a trade-off between accuracy in rejecting shortterm instability and power consumption needs to be considered when choosing the values of n, m and the sampling period. indeed, the higher the number of samples and the longer the sampling time are, the higher the adc consumption will be to obtain a single reading from the channel. to assess the quality of the conversion, the standard deviation of estimator 𝑉𝑅 was evaluated while varying the parameter m and the sampling period (and maintaining n=250, which represents the lowest number of samples that can be read in a single-shot acquisition) in the range 16 ∙ adcclock period < adcsamplingperiod < 192 ∙ adcclock period, where adcclock freq = 16 mhz. the best trade-off between power consumption and quality of averaged reading 𝑉𝑅 was found for m = 10 and adcsamplingperiod = 96 adcclock period. indeed, with this configuration the standard deviation of the estimator of (2) is equal to 0.02 lsb, thus negligible compared to the other sources of uncertainty in the channel. figure 4. schematic of pmos switch stage. figure 5. stages of the overall real-time calibration procedure implemented for each channel in the prototype acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 6 4.2. compensation for thermal dispersion a climatic chamber has been exploited to carry out an extensive experimental investigation on the dispersion of the prototype sensor-node channel response for temperature variations in the interval 20 °c 70 °c. during each test, only some parts of the sensor-node were exposed to the temperature cycles, while all the other blocks were kept in constant environment conditions, in order to separately characterize the sources of uncertainty due to thermal variations. these tests included thermal cycles on the nucleo64 board (i.e., affecting mainly the adc and the reference voltage generator), the potentiometers, and the abs pliers. final tests were then performed on the entire multi-channel system. as far as the potentiometers are concerned, no significant thermal dispersion was observed, except for some very small variations probably caused by thermal inertia. adc and ref voltage generator showed instead, as expected, a significant dispersion, even though the dispersion of the adc vref is inherently compensated, at least for the most part, by the adopted strategy of using it to also supply the potentiometers. for a given sensing channel, �̅�𝑅 is defined as the estimator obtained by inserting 𝑉𝑅̅̅ ̅ as the adc averaged reading for vout into (1). the dispersion of �̅�𝑅 due to temperature variations affecting the adc, the ref voltage generator and, from a general standpoint, all the electronic hardware on the boards has been found through the tests to follow the model: �̅�𝑅 = �̅�𝑅 (0) (1 + 𝐶th ∙ ∆𝑇) (3) where ∆t is the variation of the temperature estimated by the mcu internal sensor with respect to a reference t(0) = 25 °c, 𝑑𝑅 (0) is the value of the estimator at t(0) and cth is an experimentally estimated temperature coefficient characterizing the channel. thanks to the adoption of the already mentioned ref voltage supply strategy, the power gating method described in subsection 3.7, and an accurate optimization in the design of the entire system, cth has been found adequately independent from temperature and aperture for a given channel. regarding the power gating, more precisely, an effective solution was to use low rdson external pmos (207 mω at 27.5 °c) as switches. rdson variation of these pmos, estimated using spice simulation, is equal to 11.9 mω over a temperature range between 15 °c and 50 °c. this variation has negligible effects on the final measurement: in the worst case the maximum difference in supply voltage at the potentiometers is 7.9 µv, while the quantization step of the adc is 806 µv. the experimental extraction of the coefficient cth for each sensing channel was carried out by obtaining �̅�𝑅 on a set of aperture values from a minimum up to the full-scale, applying for each aperture value an entire temperature cycle. during a cycle, the clamp was kept mechanically blocked to the test aperture, so that to not involve in the dispersion of �̅�𝑅 the thermal expansion of abs (see next subsection). the data collected allowed to write, by means of (3), an overdetermined system of linear equations, whose solution (least-square method) provided the temperature coefficient for the channel under test. an example of temperature cycle applied to the five sensing channels for the test aperture value d ≅ 9 cm is reported in figure 6 and table 1. a weak thermal inertia effect was recorded in some tests, due to the need of speeding up the cycle: however, this effect did not cause any major issue during the calibration phase since the resulting pattern is, in almost all cases, symmetrical to the interpolation line and because the points are almost superimposable on the line once thermal equilibrium is reached. it is worth noting that in on-field applications, temperature slope is never so high, thus avoiding thermal inertia. once the estimate for the coefficient cth is available for every channel, (3) can be used to convert (according to the model and the assumptions discussed) the uncalibrated averaged reading 𝑑𝑅 obtained at temperature te (monitored by the mcu internal sensor) into the value that would be obtained at the reference t(0). this thermally-calibrated estimate is indicated as 𝑑𝑅−𝑇 in the following. figure 6. temperature cycles imposed to the overall final prototype, equipped with pmos instead of mic5365-3.3 ldo, for all the five sensing elements. table 1. cth and r2 values, starting with sensing element num. 1 up to sensing element num. 5. coefficients of the regression lines are computed by means of the least-squared method. channel cth r2 1 -0.29 0.776 2 -0.0362 0.641 3 -0.55 0.913 4 -0.51 0.925 5 -0.061 0.754 acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 7 4.3. thermal expansion of the pliers the material used to manufacture the pliers, namely abs, has a known coefficient of linear expansion λ. therefore it is possible to estimate, given a reference length r(0) at a reference temperature t(0), and knowing the temperature variation ∆t from the latter, the variation of effective length ∆r of the clamp arm and, consequently, rewrite (1) in the following form 𝑑 = 𝑑(0) + 2 ⋅ ∆𝑅 ⋅ cos ( π 2 − 𝛼(𝑉out) 2 ) (4) the calibration of all electronic components with respect to thermal dispersion discussed in the previous subsection can be thus supplemented with the calibration for thermal expansion of the pliers: it suffices to rewrite and estimate (4) in the form: �̂� = 𝑑𝑅−𝑇 + 2 ⋅ ∆𝑅 ⋅ cos ( π 2 − 𝛼(𝑉r) 2 ) (5) to obtain an estimate �̂� fully compensated in temperature. 4.4. non-linearities calibration up to now, issues associated with system nonlinearities (adc nonlinearity, potentiometer resistive taper nonlinearity, geometric dispersion in pliers printing process, etc.) that may affect the �̂� estimator have not been addressed. in order to complete the real-time calibration process, it is only required to extract a single calibration curve across the entire measuring range, at constant temperature (t(0) = 25 °c in this specific example), using a precision reference caliber. by means of a cubic spline interpolation process a set of coefficients is extracted from this curve and saved in the memory of the mcu. at each query on the diameter of the fruit these coefficients are recalled and the linearized output 𝑑�̂� (i.e., the final reading of the channel) is computed from �̂� in real time. a special-purpose laboratory set-up was arranged to calibrate the opening of the pliers. all the pliers of a single node are installed on a structure constrained to the carriage of a cnc. as the carriage moves, it allows the pliers to be opened according to user-defined dimensions, simulating the presence of a fruit of known size. a specially developed matlab script allows the entire opening range of the pliers to be swept at known spatial intervals, while at the same time the opening value detected in the absence of calibration are acquired. the point couples detected in this way are automatically stored into the flash memory of the microcontroller, thus completing the non-linearity calibration process. in this way each node can be programmed with the same firmware, without the need to modify the code according to the set of calibration coefficients needed for each set of pliers. 5. experimental results and in-field tests 5.1. static tests a set of static tests were performed on the prototype sensing channels at room temperature in order to estimate the effective resolution obtained. during the tests, estimates 𝑑�̂� on sets of aperture points randomly distributed within the full range were collected and compared with the readings taken with a precision caliper adopted as reference. by means of simple algorithms the rms deviation was estimated, leading to a standard uncertainty corresponding to 70 µm. based on this value, the average effective quantisation step dq ≅ 250 µm was derived, and enob = 9.0 bit estimated for a full-scale range of 12 cm. 5.2. in-field test a first trial was conducted between june and august 2021, in an apple orchard (malus x domestica borkh., cv “fuji-fujiko”) of the experimental farm of the department of agricultural and food science of university of bologna (cadriano, 44.5543 n 11.41872 e, figure 7). the orchard, established in early 2019 in a deep silt-clay soil, was characterized by orchard row spacing of 2 m and tree spacing of 3 m. trees, grafted onto m9 rootstock and trained to a multi-axis system (10 axis/tree) with a ns orientation, are watered by means of an irrigation system consisted of drip irrigation with a distance between emitters of 0.5 m and emitter flow of 2.3 lh-1. commercial practices, such as manual fruit thinning to achieve optimal crop load, winter pruning to maintain the desired training system, mineral fertilization, pest control and diseases, herbicide application below trees and mowing of inter-canopy cover crop, are employed to manage the orchard. within the orchard 18 fruits, randomly selected along a whole row at the beginning of the season, were monitored cyclically and their growth measured by hand using gauges. the five pliers of the prototype were instead placed simultaneously on a different set of five fruits of two adjacent trees, located at about one third of the length of the orchard, in an inner row. measurements 𝑑�̂� by means of the prototype were performed automatically every 10 minutes, while manual readings were carried out depending on the weather conditions and the activities to be done in the field, without any prescheduled periodicity, according to a traditional approach. such a short time interval between two acquisitions was decided because one of the purposes of the device is to evaluate the variation in fruit size between day and night. this time interval obviously can have an impact in terms of amount of data transmitted or stored. however, it would be straightforward to vary the data acquisition rate according to different, specific needs. in-field results of the five sensing elements can be seen in figure 8. as can be noted in the graph, three sensing elements (i.e., #1, #2 and #4, left) out of five correctly monitored and recorded (apart from recoverable perturbations) the fruit growth throughout the ripening season without the need for any figure 7. position of the five pliers of the prototype within the orchard (red dot), and position of the 18 apples manually measured as reference (yellow box). acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 8 manning (apart from a single event, detailed below), while sensing elements #3 and #5 (right) suffered from unrecoverable issues. these were not related to the performance of the prototype itself, but to the capability of the pliers to remain reliably fixed to the fruit without any shift in position: normal manual activities, such as pruning the trees or applying spray treatments, or particularly adverse weather conditions, can actually result in a shifting of the pliers. these two records will therefore be disregarded. in the case of sensing element #1, instead, the spikes observed are caused by non-disruptive, recoverable manual activities around the fruit, and can be easily filtered out by software post-processing, while in sensing element #2 the single discontinuity observed at the turn of the 30th of july was caused by strong winds, which caused the sensor to be detached from the fruit. in this case, a manual re-positioning was needed. such a macroscopic issue can be successfully dealt with by fast visual inspection on the fruits under test, and the exceptionality of the event still allows to state that the monitoring process required a practically negligible work overhead from the orchard staff during the whole ripening season. due to the difficulty of obtaining repeatable reference measurements on fruits still attached to the tree (the nonsphericity and non-rigidity of the apple makes it practically impossible to get perfectly repeatable measurements by manual gauges), in order to perform a comparative analysis with the results from the prototype the absolute growth rate (agr), expressed in mm/day and computed on intervals of several days, was used as an averaged estimator for comparisons. the duration of the intervals over which the agr was estimated is not constant but depends on the weather conditions and on the other activities required in the field according to common practices. firstly, the average agr from the gauge measurements was computed for each individual fruit in the row (see in figure 9 the statistical distribution obtained) over the total time of the trial (starting from 21/06/2021 to 06/08/2021). similarly, the average agr involving all fruits was calculated on each time interval considered. the same was done, for the same time intervals, by averaging the values obtained with sensing elements #1, #2 and #4 (table 2). the values of table 2 have been graphed in figure 10, where it is possible to appreciate that the trends of the data collected by figure 8. data acquired by the five sensing-element fully calibrated prototype during the in-field test in the apple orchard at the experimental farm of the department of agriculture and food sciences of university of bologna between june and august 2021. on the left (a), successful records that show a correct growth rate and that are not characterized by unrecoverable spikes or disturbances are shown. on the right (b), records that were not useful for this study, due to unrecoverable disturbances (i.e., the clamps suffered from shifts on the fruit due to either weathering or manual activities), are shown. table 2. from left to right: average agr values computed, over three time intervals, considering prototype sensing elements 1-4, 1-2-4 and all 18 handmeasured fruits, respectively. time interval agr 1-4 in mm/day agr 1-2-4 in mm/day agr gauges in mm/day 21/06/2021 08/07/2021 0,56 0,55 0,41 08/07/2021 23/07/2021 0,58 0,52 0,41 23/07/2021 06/08/2021 0,43 0,44 0,29 table 3. agr estimated over the whole duration of the test considering sensing elements 1-4, 1-2-4 and all hand-measured fruits using a gauge. time interval agr in mm/day deviation in % 21/06/21-06/08/21 | pliers 1-4 0,53 36 21/06/21-06/08/21 | pliers 1-2-4 0,51 31 21/06/21-06/08/21 | gauges 0,39 acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 9 the sensing elements and the hand-measured readings are very close. table 3 reports the comparison between the agr estimated by the sensor-node prototype throughout the ripening season and the corresponding value from the reference manual readings. on average, the prototype value is slightly higher than the manually measured one. however, looking at the histogram in figure 9, it can be appreciated that the agr from the sensing elements, which are positioned on the fruits of two adjacent trees and, thus, is representative of a local sampling of growth rate, is nonetheless within the statistics of the events detected on a larger scale in the orchard, i.e., an entire row. according to these results, and by considering the statistical distribution in figure 9, 4-5 equally-spaced sensor-nodes could suffice for an adequate sampling of fruit growth along an entire row with these characteristics. a further test was performed using 3 nodes, each consisting of 5 pliers, between august 2022 and october 2022. the orchard was the same of the first test and sensor nodes were randomly distributed over nonadjacent trees on the same row where previous year’s test was conducted. sensor-nodes, whose pliers were shared between adjacent trees, were programmed with same settings as the one used in the previous year's test. thanks to the experience gained with the tests of the previous year, this campaign was performed with a new version of the pliers. structural changes in the shape were introduced in order to lighten it, to improve its adhesion to the fruit and to simplify the 3d printing process while minimising production tolerances. (figure 11) the average of the agr factor estimated using these devices was compared to the average of the same factor computed on 22 hand measured fruits, randomly selected, of the same orchard. of 15 total pliers, only 3 revealed problems with slipping on the fruit. the values measured by these pliers will therefore not be considered for the calculation of the agr. the remaining 12 channels all show approximately the same growth trend (figure 12). thanks to the new design of the pliers, a more accurate positioning of them on the fruits made it possible to decrease the number of required relocations, making the measurements less perturbed than in the previous test. in fact, significant spikes are no longer present in the acquisitions (figure 12). for agr determination, manually acquired data were collected on different dates, selected according to weather conditions and field operations. therefore, two time intervals were obtained within which the agr was computed (table 4). these values were compared with the ones obtained from the data acquired with the multi-channel sensor-nodes. in this case, figure 9. agr distribution of all the 18 apples manually measured, computed over the total time of the trial. figure 10. comparison between averaged agr computed with data collected by the prototype sensing elements and reference data collected with manual caliper. figure 11. new plier rendering. it can be seen that the structure has been lightened and the cut-out for the potentiometer has been included into the right-hand body of the plier. the spring is not shown. figure 12. measured diameter of apples on 2022 test. only working pliers are shown. acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 10 it was not considered useful to perform an analysis of the statistical distribution of agr between fruits as this had already been done in the past experimental campaign, but rather to verify the validity of the measurement with a statistically representative number of samples representing the conditions of the entire orchard. results are shown in table 5. the time intervals considered for calculating agr factor using hand gauges are different from those considered for calculating the same parameter with our devices. this, however, is not a source of error since growth pattern of an apple follows different phenological stages, each characterised by a typical growth profile. therefore, while remaining within each phenological phase, the agr value is approximately the same. in particular, periods 01/08/2022 30/08/2022, considered for the manual acquisition, and 18/08/2022 30/08/2022, used for estimation of agr with our system, belong to the same phenological phase and can therefore be compared without appreciable errors. the delay in the start of the comparison is caused by the need for the pliers to adapt to mechanical clearances. in fact, looking at figure 12, it can be seen how the growth in the first few days seems to be extremely slow, which is not the actual case. in the two time periods, the agr value is very similar experimentally confirming the good performance of the device. 6. conclusions in this paper, a multi-channel sensor-node architecture and related calibration procedure for continuous distributed monitoring of fruit growth were presented. the low-cost prototype implemented for validation purposes, which was realized by means of commercial boards and 3d-printed devices, showed nonetheless adequately high effective resolution with a full-scale range suitable for no-manning operation throughout the entire ripening season of apple-sized fruit species. by adopting a multi-step calibration procedure, it was possible to compensate each channel in real-time for temperature dispersion, noise and non-linearity, which made the system rugged to environmental conditions and suitable for all-season operation. thanks to the energy harvesting sub-system and the possibility of data transmission in real-time over the air, 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http://dx.doi.org/10.1002/9781118060834.ch8 https://doi.org/10.1109/mim.2017.8006394 https://www.st.com/resource/en/data_brief/steval-isv012v1.pdf https://www.st.com/resource/en/data_brief/steval-isv012v1.pdf https://www.diodes.com/assets/datasheets/dmp2200udw.pdf https://www.diodes.com/assets/datasheets/dmp2200udw.pdf a morphological and chemical classification of bronze corrosion features from an iron age hoard (tintignac, france): the effect of metallurgical factors acta imeko issn: 2221-870x december 2022, volume 11, number 4, 1 10 acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 1 a morphological and chemical classification of bronze corrosion features from an iron age hoard (tintignac, france): the effect of metallurgical factors giorgia ghiara1, christophe maniquet2, maria maddalena carnasciali1, paolo piccardo1 1 department of chemistry and industrial chemistry (dcci), university of genoa, via dodecaneso 31, 16146 genova, italy 2 inrap limoges, 18 allée des gravelles, 87280 limoges, france section: research paper keywords: corrosion morphology; sn bronze; microstructure; tentacle like corrosion; mic citation: giorgia ghiara, christophe maniquet, maria maddalena carnasciali, paolo piccardo, a morphological and chemical classification of bronze corrosion features from an iron age hoard (tintignac, france): the effect of metallurgical factors, acta imeko, vol. 11, no. 4, article 11, december 2022, identifier: imeko-acta-11 (2022)-04-11 section editor: tatjana tomic, industrija nafte zagreb, croatia received april 14, 2022; in final form july 14, 2022; published december 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: giorgia ghiara, e-mail: giorgia.ghiara@gmail.com 1. introduction archaeological objects constitute our main source of information to study the corrosion behaviour of copper-based alloys over long timespans. ancient metals buried in soil or left in waterlogged environments for centuries give the most reliable data on how to model and thus predict the corrosion behaviour of modern alloys [1]-[4]. when studying the overall corrosion process, it is straightforward that the environment itself must be considered. much research already studied the variety of environments and features occurring [5]-[10] on sn-bronzes, pointing out the importance of water (the electrolyte in the corrosion process) and the nature and concentration of dissolved salts inside the solution. the morphology, composition, physical and chemical features of the corroded layer derive from the interaction with these corrosive agents [11]-[13]. the rate and morphology of the attack is thus affected by i) the anodic or cathodic processes that promote the growth of salts or the formation of complexing ions; ii) the evolution of the moving equilibria in the anodic or cathodic direction; ii) the conductivity of the solution [14], [15]. corrosion of soil can be more challenging since, other than water, climatic parameters as rainfall, wind, sunlight, temperature, and microbiological activity influence the composition and the properties of the system and allow for different types or mechanisms to occur, one after the other or even simultaneously [16]. nevertheless, the environment is not the only parameter that scientists must consider when dealing with corrosion processes of sn-bronzes. the composition and the microstructural features of the matrix are prominent if the alloys are exposed to the same environmental conditions. focusing on the composition, corrosion properties as nobility, passivity or cathodic and anodic processes are related to the influence of the alloying elements and abstract a categorization of corrosion morphologies of archaeological sn bronzes was carried out on archaeological iron age objects. the objects come from a celtic deposit located in central france (tintignac, corrèze) and are dated between 2nd and 3rd cent bc. being samples of corroded metals taken from a single find spot, parameters connected to the features of the alloy and known to influence the corrosion morphologies were thoroughly considered. global processes were highlighted, and corrosion mechanisms were characterised with a multi-analytical protocol (sem-eds, micro-raman spectroscopy, image analyses) according to the detected morphology. elaboration of the results was carried out with a multicomponent approach. results show the presence of 5 different morphologies correlated to the alloys characteristics of the objects. alloy composition, microstructure, degree of deformation and grain size were found to influence the corrosion products formed and the morphology of the attack. in particular, the ‘tentacle like corrosion’, associated to a microbial attack was the most susceptible to the effect of metallurgical features: their occurrence is connected to a more massive presence of fe and pb in the alloy, a homogeneous deformation and a larger grain size. mailto:giorgia.ghiara@gmail.com acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 2 their quantity inside the matrix [14]-[16]. from a historical point of view typical alloying elements are as, zn and sn, the latter providing the better corrosion resistance [17]-[20]. furthermore, the quantity of the alloying element inside the solid solution can be beneficial or detrimental to the corrosion resistance of the material. research on the performances of single-phase cu-sn alloys and how the growth of a passive film on the surface is promoted by large supplementations of sn to pure cu [17]-[19] were undertaken in the past. also, how a high zn content in brasses promotes a dezincification mechanism is a common phenomenon [20]. however, corrosion can be triggered also according to metallurgical features as heterogeneities, impurities, secondary phases, defects, or grain orientation [14]. the presence of impurities or defects can cause higher corrosion rates by creating preferential areas of corrosion [16] as heterogeneities inside the solid solution or intermetallic phases can lead to a selective type of corrosion. the presence of detrimental secondary phases was also described in recent studies [21]. generally, the action of each parameter is reflected in a specific corrosion morphology that is categorized accordingly [7], [10], [22]-[26]. various corrosion morphologies are presented in literature and are generally correlated to the work of robbiola et al. [7]. in their work, the authors defined two types of corrosion features for archaeological tin-bronzes in soil, according to the preservation of the original surface that allows to define the original shape of the artefact. these models were obtained on a statistical study of sn-bronzes with a sn range of composition from 4 to 23 wt. % and coming from the same site of excavation. both corrosion morphologies derive from an electrochemical process of oxidation of the metal (anode) and reduction of the oxygen (cathode). after the buildup of a corrosion layer, the mechanism described further developed with the mobility of cationic and anionic species, that interact with each other within the metal-soil system [7]. however, the influence of metallographic features was less considered and typical metallurgical parameters as alloy composition, microstructure, fabrication and/or finishing techniques were not considered. the study wants to fill the gap on archaeological materials by considering the metallurgical parameters to play an important role in the corrosion mechanism of artifacts buried in soil. the archaeological objects come from a recently excavated deposit (tintignac, corrèze) located in the central part of france, from which it was possible to obtain the necessary information to properly interpret the corrosion behavior without external contaminations. 2. materials and methods 2.1. archaeological context a celtic deposit was discovered around tintignac (naves, department of corrèze) in the southwest of central france and dated between 2nd and 3rd cent bc (figure 1). different bronze and iron objects related to warfare were deposited in a pit in the north-western area of a ritual or cultic place (fanum). most of the objects were intentionally destroyed before the deposition. the square pit measured around 1 m² and 30 cm depth and around 60 metallic objects were discovered: defensive armour (shields and helmets), war trumpets (karnykes), and a wide variety of other artifacts. to minimize the number of variables in the study, binary bronzes with no secondary phases are taken into consideration and only composition and microstructural features are analysed. 2.2. methodology small size fragments suitable for metallurgical investigations were sampled. of each fragment/object, one or more (1 to 3) samples were taken. after a preliminary visual observation to verify the surface characteristics and preservation, samples were mounted in cold resin and subsequently polished with decreasing grain-size diamond suspensions up to 1 μm, in agreement with the astm e3-01 standard procedure. the protocol was designed to characterise the objects through metallurgical investigations of the metallic matrix and spectroscopic analyses of the corrosion layers. objects were studied and categorized according to the morphology, composition, and microstructural features. 2.3. analytical techniques a preliminary characterisation was performed prior to metallographic etching by light optical microscopy (lom; mef4 m; leica microsystems, buffalo grove, il, usa) using brightfield (bf) and dark-field (df) contrast methods. the latter is particularly suitable for detecting corrosion products and allows for an exhaustive detection of the mineralized areas [27]. the microstructural features of the metallic matrix and the morphology of the corrosion process were documented by lom and scanning electron microscopy (sem; zeiss evo40; carl zeiss, oberkochen, germany). chemical analyses of the metallic substrate and the corroded layers were performed with energydispersion x-ray spectroscopy (edxs; cambridge inca 300 with pentafet edxs detector (oxford instruments, oxfordshire, u.k.) sensitive to light elements, z> 5) connected to the sem. the eds was previously calibrated on a cobalt figure 1. different layers of the celtic deposit discovered around tintignac (naves, department of corrèze, france) and dated between 2nd and 3rd cent bc. (a) upper level, (b) middle level; (c) lower level. acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 3 standard. this procedure allowed to obtain reliable values for all elements with atomic weight higher than 11 (z ≥ 11), while for lighter elements like oxygen the analysis was considered as semiquantitative. amounts below 0.2 wt.% were considered as semiquantitative measurements and were evaluated only when the identification peaks were clearly visible in the acquisition spectrum. the reported compositions of the alloys correspond to the average of at least five measurements on non-corroded areas. all results on the alloys were normalized and presented in average weight percent. afterwards, chemical etching was performed on the samples with a solution of fecl3 (5 g) diluted in hcl (50 ml) and ethanol (200 ml) to reveal their microstructural features. to estimate the grain size, the procedure using the grain boundary intersection count was carried out according to the astm e-112 standard. the degree of deformation of the alloy was calculated following equations from [28], [29]. the shape factor of the inclusions (sf) can be computed by calculating their width/length ratio and by correlating it to the stress absorbed by the material. the deformation applied, resulting in a reduction of thickness (d%), is described as: 𝐷% = 𝑡ℎ𝑖𝑐𝑘0 − 𝑡ℎ𝑖𝑐𝑘end 𝑡ℎ𝑖𝑐𝑘0 ∙ 100 % , (1) where 𝑡ℎ𝑖𝑐𝑘0 and 𝑡ℎ𝑖𝑐𝑘end are the initial and final thickness respectively. the equation was then further implemented correlating the sf of the inclusions to the deformation applied: 𝐷% = 𝑠𝑓 − √𝑠𝑓 3 𝑠𝑓 ∙ 100 % , (2) 𝑡ℎ𝑖𝑐𝑘0 = 𝑡ℎ𝑖𝑐𝑘end ∙ 𝑠𝑓 2 3 . (3) to determine the nature of the corrosion products, microraman spectroscopy (μrs, renishaw raman system 2000, renishaw, inc., hoffman estates, il, usa) was also employed. the instrument was supplied with a charge coupled device (ccd) peltier-cooled as detector and excited using a 632.8 nm he-ne laser at 1 cm-1. to estimate the thickness of the corroded layers and the percentage of sound metal on the sample an image analysis software (fiji-imagej, version 1.49b) was used on 100 x lom micrographs [30]. a statistical procedure was also performed using the principal component analysis (pca) to evidence correlations between the variables and possible clustering of the samples. the original data matrix was decomposed into two smaller matrices: the loadings, which can be interpreted as the weights for each original variable in the pc computation, and the scores, which contain the original data in the newly rotated coordinate system [31]. also, a biplot was created with both the loadings (variables) and scores (samples) positions on the new coordinate system. 3. results and discussion 3.1. metallurgical characterisation all objects are made of sn-bronze with sn as the major alloying element and minor elements as as, ni, co, fe and pb. figure 2 describes the number of objects and the frequency distribution according to the element considered (sn, fe, as, pb, ni, co). sn disperses from 6 % to 16 % in wt., with the highest occurrence (more than the 80 % of the samples) found for the interval 10 – 14 % in wt. confirming the high technological skills of the celtic culture [32]. minor elements as as, ni, co, fe and pb concentrations are below 1 wt.% (even less than 0.2 wt. %), with variations of the shape of the curve according to the element. fe, pb, co and ni are show very high frequency in the range below 0.2 wt. %, which is close to the limit of detection (lod) of the eds. this outcome should be considered as semiquantitative, and more sensitive analytical techniques are needed to confirm the distribution curve proposed in this study. the element as showed on the other hand experimental values modelled with a gaussian-like curve, with a distribution probability centred at 0.3 wt. %. a microstructural investigation was performed through lom and sem investigations, and figure 3 displays the features observed. they are characterised figure 2. frequency distribution of major and minor alloying elements detected on the whole number of samples: (a) sn; (b) as; (c) fe; (d) pb; (e) ni; (f) co. figure 3. microstructures observed on the objects. lom micrographs of: (a) polygonal grains typical of recrystallization annealing; (b) twinning and slip lines; sem-bse micrographs highlighting the composition of inclusions: (c) pb; (d) cu2-xfexs2. acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 4 by a homogeneous microstructure with the presence of an α phase (figure 3a). polygonal grains, typical of recrystallization annealing after deformation are observed, with twinning and slip lines consistent with a last step of cold deformation (figure 3b). some residual heterogeneities of the solid solution appeared in some samples, suggesting that the annealing temperature was not high enough to remove residues of the heterogeneity after casting (above 2/3 of the melting temperature of the alloy). inclusions are made of fe and cu sulfides cu2-xfexs2 and pure pb islands (given the immiscibility of lead with the cu-sn solid solution) (figure 3b and c). following equations 1 to 3, the d % was calculated. a deformation between 15% and 90% was statistically associated to the microstructure and a frequency plot (not visible) evidences a distribution of the d % more pronounced towards higher values (peak at 80 %). this outcome must however be considered as a function of the grain size and the percentage of sn inside the solid solution. the grain size varies between 6 µm and 50 µm for m1 (not visible), according to the percentage of sn in the alloy. 3.2. corrosion morphology the corrosion features observed on the range of samples are typologically divided into 5 groups corresponding to alphabetical roman letters (a, b, c, d, e, see figure 4, table 1). they are classified according to i) nature and features of the layers (e.g., composition, heterogeneity, thickness), ii) features of penetration (e.g., degree, way). they are resumed in table 1. the complexity of each corrosion morphology is connected to a specific corrosion mechanism. it is a slow process that follows a logarithmic trend [5] (not considering localized types of corrosion) and the presence of sn as the major alloying element helps create a mixed sn and cu passive oxide, with a protectiveness degree correlated to the amount of sn in the alloy [17]-[19]. the classical archaeological corrosion (type a or ii, table 1) which has been already mentioned by many authors [7], [33]-[36] is also found this study. from the micro-raman analysis (figure 5a) corrosion products as cuprite (cu2o), cassiterite (sno2), a mixed form of sn and cu oxides (spectrum black), malachite (cu2co3(oh)2-spectrum red), and azurite (cu3(co3)2(oh)2 were detected. impurities as si, al are detected by sem analyses and are responsible for the small shifts of the peaks of some of the compounds [33]. the penetration is intergranular and follows the short-circuits of diffusion [15] where grain boundaries are the anode, and the matrix is the cathode. in those areas a dissolution of sn is promoted according to the lower standard electrode potential [37] and precipitates in the form of hydroxides and oxides. the formed layer is porous and permeable and thus oxygen can diffuse inward, allowing for the oxidation of cu. a mixed layer of cu and sn oxides is therefore formed (figure 4a, figure 5 -black spectrum and figure 6a inner layer of corrosion products or il). basic cu carbonates are formed at the outer layer (ol) due to the presence in the soil of co2 or carbonate ions in quantities above 200 ppm [38]. a decuprification phenomenon is also observed from the eds analyses, as well as diffusion processes of minor elements through the corrosion layers [39]-[41]. figure 6a displays the line-scan profile performed on a representative sample evidencing the interaction of the system alloy/oxide/environment. calculations on the sn:cu atomic ratio along the profile indicate that values are increasing in the il to a maximum of 2.7, which implies that every 3 atoms of sn in the oxide are balanced by 1 atom of cu. as, pb and fe enrich the cu-sn mixed oxide up to 3-4 times their relative value in the alloy (e.g., from 0.3 wt. % to 1 wt. % for as). this suggests that the diffusion coefficients of those elements are table 1. description of the corrosion morphologies identified in the study. corrosion morphology description penetration corrosion products type a so called classical archaeological bronzes corrosion mechanism in presence of water. intergranular corrosion. red, yellowish-orange, green and blue are the predominant colors short-circuit of diffusion: e.g., grain boundaries, slip bands, mechanical twins. interdiffusion of ions from both the metallic substrate (cu, pb, as, ni, fe) and the soil (si, al, fe) cuprite (cu2o); cassiterite (sno2); malachite (cu2co3(oh)2); azurite (cu3(co3)2(oh)2; mixed forms of tin and copper oxides. type b archaeological corrosion mechanism in which intergranular corrosion is visible in the inner part of the corrosion layers or just on one side of the sample. compact and thick layers of different colours are visible on the other side or on the entire surface. predominant colours are beside those from the classical corrosion morphology: brownish green, yellow, brown, grey. corrosion penetrates inside as a compact and thick layer or inside cracks or preferential areas of the sample sometimes with a waterfall effect. uncommon corrosion features identified as sno2*2h2o and sn(oh)4 as well as cuprite (cu2o); cassiterite (sno2); malachite (cu2co3(oh)2); azurite (cu3(co3)2(oh)2; mixed forms of tin and copper oxides type c globular shapes corrosion products inside compact layers of different thickness. predominant colours are beside those from the classical corrosion morphology: brownish green, yellow, brown, white, grey. corrosion penetrates inside the matrix similar to a macro pitting mechanism on entire areas of the sample. corrosion features in globular shapes identified as sno2*2h2o and sn(oh)4; cuprite (cu2o); cassiterite (sno2); malachite (cu2co3(oh)2); azurite (cu3(co3)2(oh)2; mixed forms of tin and copper oxides. type d 1-1.5 μm wide tunnel shaped structures. the tunnels are never completely straight but bent, dividing and crossing each other, giving the picture of roots or tentacles. not following any microstructural features as grain or twinning boundaries or slip bands due to cold deformation. it penetrates directly into the crystal without any regard for the short-circuit of diffusion. mixed forms of tin and copper oxides are detected with a predominance of tin-based oxides. also, sno2*2h2o and sn(oh)4 are detected. type e mixtures of previously described corrosion morphologies in which many parameters influenced the corrosion mechanism during relatively long timespans. type a, b, c, d type a, b, c, d acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 5 high enough to migrate inside the corroded layers. fe shows high concentrations inside the ol (up to more than 10 times the content in the alloy), probably related to its diffusion from the soil. also type b corrosion was already detected elsewhere [7] (figure 4b, table 1). a compact layer is found at the interface with the metallic matrix. figure 5b resumes some of the corrosion products found for this peculiar corrosion morphology: an il of cu2o and sn(oh)4, (spectrum green), followed by sno2•2h2o. malachite (cu2co3(oh)2) or azurite (cu3(co3)2(oh)2 form in the ol. the sn oxides, which are thermodynamically stable and have a low solubility limit as doping elements [42], [43], slow down the process. a more severe decuprification phenomenon is detected and values of sn:cu from the interface metal/oxide to the edge respectively from 0.2 to 2. however, no enrichment of minor elements was detected in the corrosion layers. variation of the b type of corrosion, the type c, shows preferential areas of penetration. the corrosion layer is present along cracks sometimes with a “waterfall” effect (figure 4c, table 1). corrosion products are the same as previously mentioned for type b. the sn:cu value from the interface metal/oxide to the edge increases as displayed by the line-scans (figure 6b) and as previously seen, up to 3, which could be possibly connected to a further evolution of the burial context (possibly lack of oxygen) [15]. pitting can potentially explain the phenomenon, a localized mechanism in which cathodes and anodes are different areas of the sample [14]. however, it is very difficult to detect what triggered the process, either local ruptures of the protective sn-rich passivation film due to foreign bodies, macro heterogeneities of the film or areas of accumulation of aggressive ions as cl[15]. the type d or “tentacle-like” corrosion [44] does not follow the above-described mechanism but penetrates directly into the crystal without any regard to the short-circuit of diffusion (figure 4d, table 1). the morphology shows a tunnel-like structure entering the crystal with apparently irregular directions. this type of corrosion and its features has led to the hypothesis of a microbial influence in the mechanism and the attribution to microbiologically influenced corrosion (mic) was proven and discussed more in detail elsewhere [45]-[47]. a figure 4. corrosion morphologies associated to different corrosion mechanisms and defined as: (a) type a; (b) type b; (c) type c; (d) type d; (e) type e. figure 5. raman spectra collected with a 632.8 laser on (a) type a corrosion morphology (b) type b corrosion morphology; (c) type d corrosion morphology. parameters of the analysis: acquisition time: 10 s; number of accumulations: from 1 to 4; power: 25% transmittance. vibrational bands attribution of: 5a: mixture of cu2o and sno2; malachite; 5b: sn(oh)4; sno2*2h2o with shifts due to the soil acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 6 decuprification process did not occur, as displayed by the linescan in figure 6c, since the kα value of cu does not decrease linearly but fluctuates along the corrosion layer. also, higher amounts of pb are found along the profile towards the ol. accordingly, the sn:cu ratio fluctuates from the metallic interface to the edge with extrema of 0.3 at the interface metal/oxide and a peak of 2 at the il. this suggests that bacteria promote the formation of sn products at the il and a mixed form of cu and sn oxides at the ol, influencing the overall corrosion mechanism [47]. when all types of corrosion morphologies are observed on the same artefact with amplitudes which may vary from one area to another, type e was defined (figure 4e, table 1).this type of corrosion was found only on a limited set of samples and is related to changes in the environmental context could lead to the formation of a mixture of the corrosion morphologies. however, the information concerning this corrosion morphology is still under investigation considering the lack of information on the evolution of the environmental conditions. 3.3. metallurgical parameters on the corrosion process the correlations between the alloy’s microstructural features and the occurrence of typical corrosion morphologies are given by the pca biplot in figure 7. as displayed in the graph, correlations between the variables are visible. the distribution of the a type of corrosion referred to the variable sn is rather figure 6. line-scan profiles of different sites of interest based on the kα energies of cu, sn, fe, as and pb of the different morphologies: a) type a; b) type c; c) type d. il=inner layer of the oxidation patina; ol=outer layer of the oxidation patina. figure 7. biplot of the variables influencing the corrosion morphologies considered according to the position of the samples in the newly rotated space. roman numbers indicate the quadrant. acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 7 stochastic. this outcome agrees with other archaeological studies where this type of corrosion was associated to objects in which the sn content varied enormously, from 5 to 20 in wt. % [5]-[8], [10], [22], as in our case (figure 8). type b and type c corrosion morphologies show a shift in the biplot towards the iii quadrant, in the sn direction. this is consistent with the frequency distribution of the sn for such morphology (figure 8), which exhibits a narrow range of composition (from 11 to 15 in wt. % sn). this could be related to the passivating properties of the element that could shift the corrosion potential towards higher values. thus, the composition of the alloy is a rate-controlling factor for the electrochemical corrosion mechanism, as previously stated [17]-[19]. sn and cu are not variables that influence type d morphology since the samples move along a line that connects cu and sn (obviously negatively correlated since the increase of one deplete the alloy of the other) as confirmed by the data (figure 7 and figure 8). minor elements (as, co, fe, ni and pb) do not influence the occurrence of types a, b and c. a trend for corrosion morphology d is discernible from the pca analysis along a line from the ii to the iv quadrant. this morphology is affected by the presence of the minor elements fe and pb. this means the higher the concentration of these elements in the alloy (up to a 1.2 wt. % of pb and 1 wt. % of fe), the higher the probability to find ‘tentacle like’ corrosion (mic) features. it is known that pb and fe in sufficient quantity (above 1 %, in our case) modify the corrosion mechanism towards a selective type, culminating with their dissolution [15]. however, it is still not clear how their presence can affect such a morphology since it is influenced by bacterial colonization. we can only infer that their occurrence promotes the microbial attack. we can hypothesize that: i) their dissolution is less harmful for the survival of bacteria compared to sn and cu [48], [49]; ii) they can exploit these elements for their metabolism using them as an alternative to carbon as an energy source [50], [51]. the degree of deformation and the grain size affects all types of microstructures to a different extent (figure 9 and figure 10). in particular, the degree of deformation seems to positively influence the occurrence of corrosion a, b and c, while it is detrimental for type d. samples from d type are negatively correlated to d % (as they are positioned in the ii and iii quadrant). this indicates the higher the degree of deformation, the lower the occurrence of type d. figure 9 describes the degree of deformation according to the morphology observed. type d has a frequency distribution centered around 35 %, while for the other morphologies it shifts towards high values (> 65 %). however, the d % must be considered as a function of the grain size and all results must be evaluated accordingly. this parameter affects these morphologies differently as suggested by figure 11 coherently with the outcomes obtained from the d %. a bivariate plot grain size of vs number of samples is displayed in figure 10. it is evident that the smaller the grain size, the higher the occurrence of corrosion types a, b, and c. the higher the number of polygonal grains, the higher the possibility to find an intergranular penetration since defects and impurities are localized in the areas with higher gibbs free energy [15]. on the contrary, type d shows an opposite trend. figure 10d shows a normal distribution of grain size for type d with a peak at around 35-40 µm, which is twice as high compared to the average grain size found for all the samples. this outcome is very interesting but still under study since, to our knowledge, no figure 8. frequency distribution of the sn wt % according to the corrosion type detected in the whole range of objects: (a) type a; (b) type b; (c) type c; (d) type d. figure 9. frequency distribution of the degree of deformation according to the corrosion type detected in the whole range of objects: (a) type a; (b) type b; (c) type c; (d) type d. figure 10. frequency distribution of the grain size according to the corrosion type detected in the whole range of objects: (a) type a; (b) type b; (c) type c; (d) type d. acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 8 information is available on the relationship between the grain size, d %, and the occurrence of microbial corrosion on cubased alloys. on stainless steels, it seems that the smaller the grain size, the more effective the penetration of the corrosion [52], [53], which is in contradiction with this study. it is indeed difficult to compare the mechanism on a different material with the one found for cu alloys since the microstructural features are different (e.g., the presence of grain boundaries precipitates). it is only possible to suppose that on a given surface the number of impurities (localized at the grain boundaries) is lower for larger grains, and this could become a key parameter for the survival of the colonies and the occurrence of a bacteria mediated corrosion mechanism. in this perspective, concomitant analyses on both archaeological bronzes with mic morphology and experimental alloys are needed to better understand the effect of the grain size. 4. conclusion the main aim of this work is to isolate typical metallurgical parameters that influence the occurrence of specific corrosion morphologies. a categorization of the objects following specific types of composition, microstructure and corrosion was proposed. the nature of the alloy and the microstructural features act as rate controlling factors for the electrochemical corrosion mechanism. results show that metallurgical factors do not play a major role in the corrosion morphology of type a, where neither the composition, nor the thermomechanical treatments influence its occurrence. on the other hand, a high correlation of type b and type c to the alloy composition suggests a higher degree of protection due to the sn tendency to naturally passivate. the most interesting results come from the type d corrosion morphology (tentacle like or mic morphology). it is affected by: i) the presence of specific minor elements in the alloy (i.e. fe and pb in quantities up to 1%; ii) the presence of microstructure bearing a large grain size (40 µm, twice as high compared to the other morphologies) and a low degree of deformation (peak at 35%). the possible causes for this trend are still doubtful and more research will be carried out in the future to understand this point. acknowledgement the authors would like to thank the scientific team in charge of the ‘project tintignac’, in the person of the major of the city of naves-en-corrèze for allowing us to sample the objects; m. drieux-daguerre of the laboratory materia viva, toulouse (fr); b. armbruster from cnrs-umr 5608, toulouse (fr). references [1] m. kibblewhite, g. tóth, t. hermann, predicting the preservation of cultural artefacts and buried materials in soil, sci. tot. environm., 529 (2015), pp. 249-263. doi: 10.1016/j.scitotenv.2015.04.036 [2] j. f. d. stott, g. john, a. a. abdullahi, corrosion in soils, reference module in materials science and materials engineering, elsevier, 2018. doi: 10.1016/b978-0-12-803581-8.10524-7 [3] m. c. bernard, s. joiret, understanding corrosion of ancient metals for the conservation of cultural heritage, electrochim. acta, 54(22) (2009), pp. 5199-5205. doi: 10.1016/j.electacta.2009.01.036 . 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crenna1, matteo lancini2, marco ghidelli3, giovanni b. rossi1, marta berardengo1 1 measurement and biomechanics lab – dime – università degli studi di genova,via opera pia 15 a, 16145 genova, italy 2 dsmc università degli studi di brescia,v.le europa 11, 25121 brescia, italy 3 dii università degli studi di brescia, via branze 38, 25123 brescia, italy section: research paper keywords: biomechanical measurements; crutches; articular loads; force measurements citation: francesco crenna, matteo lancini, marco ghidelli, giovanni b. rossi, marta berardengo, biomechanics in crutch assisted walking , acta imeko, vol. 11, no. 4, article 6, december 2022, identifier: imeko-acta-11 (2022)-04-06 section editor: eric benoit, université savoie mont blanc, france received july 9, 2022; in final form december 11, 2022; published december 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this research was partially supported by eu h2020 program, project eurobench grant n° 779963 – subproject bullet, sledge and faet. corresponding author: francesco crenna, e-mail: francesco.crenna@unige.it 1. introduction different diseases may require patients to use crutches in their daily life. while upper limbs fatigue is limited when a temporary impairment is considered, it may become an important issue when considering permanent impairments such as those due to stroke or multiple sclerosis. these situations are rather common and important in today’s society. stroke is the leading cause of movement disability in the usa and europe [1]. people who suffer a stroke experience change in strength, muscle tone, and neuromuscular coordination [2], the consequence of which are mobility, balance, and walking disabilities [3]. similar symptoms are present in multiple sclerosis, along with fatigue and cerebellar involvement. up to 10% of adults suffer from reduced mobility because of conditions such as a central nervous system lesion that affects balance and gait. on the other hand, walking is a fundamental human activity [4] and if impaired, people prioritize it as a goal of treatment [5]. in europe walking aids, such as crutches, are the most prescribed tools in case of central nervous system lesions [6] and, in a gait rehabilitative framework, physical therapists guide patients in using crutches to better support weight by reducing the magnitude of the load on the legs, and to improve balance by increasing the body’s base of support [7]. moreover, crutches use is fundamental for people walking with the assistance of exoskeletons, for example after a spinal cord injury (sci). exoskeletons help in closing the gap toward a normal life for sci people. since, generally speaking, exoskeletons require the contemporary presence of a pair of crutches, their continuous and daily usage requires attention to possible consequences such as shoulder pain [8]. on this basis, a pair of instrumented crutches was developed to measure both crutch load and orientation [8]-[9], and they were integrated with an optoelectronic motion capture system, an anthropometric volume scanner, and a biomechanical model, in the bullet project [10]-[11]. figure 1 depicts bullet main concept. bullet biomechanical model is fed with kinematic data describing trajectories and accelerations [12], and crutch force data describing movement dynamics. eventually ground reaction forces under the feet can abstract crutch-assisted walking is very common among patients with a temporary or permanent impairment affecting lower limb biomechanics. correct crutches’ handling is the way to avoid undesired side effects in lower limbs recovery or, in chronic users, upper limbs joints diseases. active exoskeletons for spinal cord injured patients are commonly crutch assisted. in such cases, in which upper limbs must be preserved, specific training in crutch use is mandatory. a walking test setup was prepared to monitor healthy volunteers during crunch use as a first step. measurements were performed by using both a motion capture system and instrumented crutches measuring load distribution. in this paper, we present preliminary tests results based on different subjects having a variety of anthropometrical characteristics during walking with parallel or alternate crutches, the so-called three and two-points strategies. tests results present inter and intra subject variabilities and, as a first goal, influencing factors affecting crutch loads have been identified. in the future we aim to address crutch use errors that could lead to delayed recovery or upper limbs suffering in patients, giving valuable information to physicians and therapists to improve user’s training. mailto:francesco.crenna@unige.it acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 2 be included to operate the model in its complete, full body version. to obtain upper limbs loads the model requires subject’s anthropometry also. to obtain such segment detailed information generally we refer to anthropometric tables, where values relative to overall subject mass and height are reported [13]-[14]. to compensate for the consistent subject to subject differences , that are common among exoskeleton users, bullet includes a volume scanner to measure segments volumes using a kinect azure camera [15]-[16]. the scanner considers a set of subject images recorded in both rgb and tof frames, and a segmentation software based on the biomechanical model definition to obtain segments volumes. segments masses and inertia are then determined considering segment’s density values as reported in tables and literature [14]. bullet biomechanical model, process the input data to obtain limbs loads with special attention to torques at shoulders [17]. the focus, in this case, is related to exoskeletons assisted walking in the eurobench project framework [11], but the approach is general [18] and, in this paper, some preliminary results are presented for healthy subjects walking with crutches without exoskeletons. the main goal is to investigate differences in crutch-assisted gait following the three points strategy or parallel crutch use – and the two points strategy – or alternate crutch movement. therefore, some results regarding crutch kinematics and loads for the two strategies are here presented. 2. experimental setup and protocol the experimental setup includes an optoelectronic motion capture system, to reconstruct the full-body and crutches kinematics during gait, two force plates for measuring the ground reaction forces under the left-right foot – both from bts bioengineering, milan, italy –, and a pair of instrumented crutches to measure force load and orientation on each side, as described in [9]. calibration procedures have been applied for both optoelectronic and force measurement systems [18] before each subject test session. in the following, special attention will be given crutches kinematics obtained from optoelectronic systems and related crutches forces. the experimental protocol requires placing a set of 39 markers on specific subjects’ landmarks, plus three on each crutch. in this experimentation heathy subjects only are involved. since they have to use crutches simulating a spinal cord injury walking impairment, they trained for a short time before test. to this purpose subjects consider to have problems to move and load both legs, so a pre-training is required to establish a proper crutch load and movement, according to experimenter’s instructions. moreover during the training subjects find the proper path and foot sequence in the corridor, to place feet properly on force plates without crutch interference. during the training and the following tests there is no real time control on crutch load, so, at the end, it depends on subject voluntary behaviour. then, after performing calibration procedures for all the instruments, three repetitions (minimum) for each walking condition were carried out: three points gait with parallel crutches and two points gait with alternate crutches, as shown in figure 2. a set of 14 subjects of which 2 females, mean age 25.7 years, standard dev. 5.6 years undergo the experimental protocol, after signing an informed consensus agreement. finally, 124 valid tests were obtained, 65 of which in the twopoints (alternate) gate conditions. 3. experimental results biomechanical model can operate in complete – whole body – or partial – upper limbs only – mode. in the following we consider the whole body version that includes 18 segments to biomechanical model optoelectronic measumrement system instrumented crutches volume scanner kinematics kinetics anthropometry shoulder loads bullet conceptual scheme figure 1. bullet conceptual scheme. x anteroposterior walking direction z mediolateral left right x anteroposterior walking direction z mediolateral left right y y alternate parallel figure 2. alternate and parallel gait strategies. figure 3. alternate and parallel gait strategies. acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 3 describe subject and crutches movement as shown in figure 3 for an alternate – two points gait. figure 4 presents an example of the three crutch force components according to the biomechanical reference systems indicated in figure 2 – x anteroposterior, y vertical, z mediolateral – for a three-point gait and left-right sides. by using force measurement it is possible to define the crutch contact on the ground –figure 3 lower graphand consequently identify initial and final contact angles on the two most important planes describing the movement: the sagittal (x, y) and the frontal (z, y) ones. besides that, it is possible to compute the angular crutch range of motion (rom), maximum, and rms load force during crutch ground contact. since data recording was limited to few gaits near and on the force plates, we have excluded from these computations the runs in which some data was missing: for example, when no initial or final crutch ground contact was recorded, and consequently it was not possible to evaluate parameters on a complete contact phase. in the following all the trained subjects are considered independently of their ability to simulate the impairment. results from the 124 valid tests are summed up in table 1 as regards mean values and relative standard deviations of the maximum crutches forces normalised by subjects’ weight. note that variability includes inter subject repeatability and intra subject variability. the large relative standard deviation suggests that, besides subject behaviour, other differences are present. for example, gait condition might affect crutch loading. moreover, even if results are normalised for subject’s weight, other anthropometric differences might have an important role in crutch load. on this basis the biomechanical model can evaluate shoulders torque. figure 5 and figure 6 presents l/r shoulder forces and torques in relation with crutch contact for a parallel gait. it is worth noting that the model we are considering is purely mechanical, so muscles action is summed up in the torque at shoulders joints, while forces do not include reactions due to muscle actions applied at tendon insertion points. even if this is certainly an approximation, the proposed approach is free from any assumption regarding tendon anthropometry and muscle force behaviour that in our specific case could be critical, since we are considering injured subjects walking with crutches. figure 4. l/r crutch relative forces in the three directions: parallel gait. table 1. maximum crutch vertical load: results summary right left mean (% of sbj weight) relative standard deviation mean (% of sbj weight) relative standard deviation parallel 30 32 % 34 30 % alternate 21 43 % 24 40 % figure 5. l/r shoulder forces for parallel gait. figure 6. l/r shoulder torques for parallel gait. figure 7. r crutch vertical maximum relative forces for alternate/ parallel movement. boxes represent 25%-75% intervals, red lines median values. acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 4 4. discussion as mentioned in the introduction the focus here is on two crutch use modalities and on the percentage of the subject weight that is moved from the lower limbs toward the crutches. as shown in figure 4 main load behaviour is very similar for left/right crutches, while the mediolateral component (z, orange in figure 4) is opposed due to the opposed crutch contact angle in the medio lateral plane. the overall data set can be divided according to these two gait conditions – parallel/alternate, and since we have several repetitions (about 3) for a rather consistent set of subjects (18), we can make a multiple-way analysis of variance. the analysed variable can be selected among the available gait parameters we have measured. we can consider a crutch centred approach considering both crutch kinematics and dynamics, or a subject centred approach considering shoulder internal loads in relation with gait behaviour. as an example of the first approach we consider maximum and rms vertical load relative to the subject’s overall weight. as factors for the analysis, besides the mentioned gait conditions, we consider kinematics parameters, such as the angular rom of the crutch around the medio-lateral axis, and subject anthropometric characteristics such as weight, height, or the body mass index (bmi), defined as the ratio between mass and squared height. in table 2 we present an example of anova results, including f values. considering the maximum vertical load on the right and left crutches, f values confirm that the gait condition is significant with a probability < 10-4. the box plot in figure 7 shows an evident gait strategy effect on load levels. there is also evidence of a large variability, probably due to the protocol that requires to simulate an impairment – see paragraph 2 and the absence of an online verification of such imposed behaviour. moreover, there is evidence that, even if working with a load normalized on subject weight, the subject’s bmi is significative (p < 10-4) indicating that the way crutches are loaded is not simply related to the subject’s mass. however, the box plot in figure 8 shows a less evident load dependency on the three bmi categories, defined as follows: bmi < 21.5 kg/m², 21.5 kg/m² ≤ bmi < 25 kg/m² and bmi ≥ 25 kg/m². this aspect deserves some attention in future data analysis to investigate the most significant subject anthropometric characteristic. these results have effect on shoulders loads obtained from the biomechanical model. as shown in figure 6 torque time behaviour is similar for right/left limbs, and present the maximum value, as expected, for torque around the medio lateral axis (z). on this basis a good example is the analysis of variance of shoulder maximum z torque. alternate/parallel walking condition and bmi still have the main effect on this shoulder torque (p < 10-4), as shown in the boxplot in figure 9 for walking condition. while subject’s bmi is significative as for crutch loads, other anthropometric characteristics are not significative anymore at the 5% probability level fisher test, as confirmed by the boxplot in figure 10 for subject’s height. figure 8. r crutch vertical maximum relative forces for the three bmi categories. boxes represent 25%-75% intervals, red lines median values. table 2. anova results for l/r crutch rms vertical load. mean squares f value probability>f moving condition 3.7 25.3 0 subj bmi 5.3 18.1 0 subj mass 1.9 6.5 2 10-3 subj height 0.2 0.7 0.49 crutch rom 2.1 2.9 1.5 10-2 residual error 31 figure 9. l and r shoulders maximum torque around the mediolateral axis z for the two walking conditions. boxes represent 25%-75% intervals, red lines median values. figure 10. l and r shoulders maximum torque around the mediolateral axis z in function of subject height categories. boxes represent 25%-75% intervals, red lines median values. acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 5 5. conclusions the paper has presented the bullet project approach to shoulders load evaluation when walking with crutches. a set of experimental data obtained on healthy subjects has been considered to demonstrate the potentialities of the proposed approach. a preliminary synthesis on these results, is obtained applying the analysis of variance. anova has shown that parallel or alternate walking conditions is very important as regards both crutch forces, that can be measured directly, and shoulders loads, that are determined through a biomechanical model. subjects’ anthropometric characteristics affects results even if they are normalized by subject mass or weight, moreover subjects’ bmi is not the only significative parameter since still mass has a significative contribution. of course such considerations have to be limited to this specific experimentation, since it was conducted on healthy subjects only, with a request to simulate a sci walking impairment, and subject behaviour was not subject to control. nevertheless results demonstrate the potentialities of the presented approach. in particular, when applied to injured subjects, it provides a set of information that will be useful to therapist and subjects to improve their training preserving articulations’ health. acknowledgement we thank s. lanino, m. pitto and f. risoldi for the fruitful collaboration. references [1] d. lloyd-jones, robert adams, mercedes carnethon, (+ 32 more authors), heart disease and stroke statistics 2009 update, circulation, 27 january 2009, 119(3):e21-181. doi: 10.1161/circulationaha.108.191261 [2] nathan d neckel, n. blonien, d. nichols, j. hidler, abnormal joint torque patterns exhibited by chronic stroke subjects 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https://doi.org/10.1109/tim.2015.2465751 https://doi.org/10.1109/jsen.2016.2579738 https://eurobench2020.eu/developing-the-framework/benchmarking-upper-limbs-loads-on-exoskeleton-testbeds-bullet/ https://eurobench2020.eu/developing-the-framework/benchmarking-upper-limbs-loads-on-exoskeleton-testbeds-bullet/ https://eurobench2020.eu/developing-the-framework/benchmarking-upper-limbs-loads-on-exoskeleton-testbeds-bullet/ https://eurobench2020.eu/ https://doi.org/10.3390/s21134580 https://doi.org/10.1016/j.jbiomech.2006.02.013 https://doi.org/10.3390/s22072469 https://doi.org/10.21014/acta_imeko.v11i2.1206 https://doi.org/10.1109/tim.2021.3072113 https://doi.org/10.21014/acta_imeko.v4i4.281 a combination of terrestrial laser-scanning point clouds and the thrust network analysis approach for structural modelling of masonry vaults acta imeko issn: 2221-870x march 2021, volume 10, number 1, 257 264 acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 257 a combination of terrestrial laser-scanning point clouds and the thrust network analysis approach for structural modelling of masonry vaults maria grazia d'urso1, valerio manzari2, barbara marana3 1 department of engineering and applied sciences, university of bergamo, bergamo, italy 2 department of civil and mechanical engineering, university of cassino and southern lazio, cassino, italy 3 department of engineering and applied sciences, university of bergamo, bergamo, italy section: research paper keywords: masonry vault; laser-scanning; thrust network analysis; point cloud; geometric configuration; structural analysis; equilibrium of nodes citation: maria grazia d'urso, valerio manzari, barbara marana, a combination of terrestrial laser-scanning point clouds and the thrust network analysis approach for structural modelling of masonry vaults, acta imeko, vol. 10, no. 1, article 34, march 2021, identifier: imeko-acta-10 (2021)-01-34 editor: ioan tudosa, university of sannio, italy received january 2, 2021; in final form february 15, 2021; published march 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was supported by the italian ministry of education, university and research (miur) corresponding author: maria grazia d'urso, e-mail: mariagrazia.durso@unibg.it 1. introduction in this paper, we present a review on the integration of terrestrial laser scanning (tls) point clouds acquired via the most frequently employed survey techniques, as well as an innovative method for studying historical masonry vaults. the study of historical buildings continues to face significant difficulties related to computational effort, the scarcity of input data, and the limited realism of the attendant methods. studies oriented toward the conservation and restoration of historical structures exploit structural analysis as a means of better understanding the genuine structural features of the building in view of characterising its present condition and the causes of the existing damage, determining the actual structural safety in terms of a variety of factors (e.g. gravity, soil settlements, wind, and earthquakes), and determining the necessary remedial measures [1]-[3]. historical structures are often characterised by a highly complex geometry composed of various straight or curved members, combining the curved 1d members (arches, flying arches) with both 2d (vaults, domes) and 3d members (fillings, etc.). in fact, the geometry is one of the most crucial aspects of investigation given the complex combination of comparatively slender members with far larger members (e.g. massive piers, walls buttresses, foundations). as such, the investigation of the geometry is perhaps one of the greatest challenges faced by analysts. abstract terrestrial laser-scanning (tls) is well suited to surveying the geometry of monumental complexes, often realised with highly irregular materials and forms. this paper addresses various issues related to the acquisition of point clouds via tls and their elaboration aimed at developing structural models of masonry vaults. this structural system, which exists in numerous artifacts and historical buildings, has the advantages of good static and functional behaviour, reduced weight, good requisites of insulation, and aesthetic quality. specifically, using tls, we create a geometric model of the ancient masonry church, s. maria della libera, in aquino, largely characterised by naves featuring cross vaults and previously used as a case study in the paper entitled ‘terrestrial laser-scanning point-clouds for modeling masonry vaults’, presented at the 2019 imeko tc-4 international conference on metrology for archaeology and cultural heritage. the results of the tls survey are used as input for a structural analysis based on the thrust network analysis. this recent methodology is used for modelling masonry vaults as a discrete network of forces in equilibrium with gravitational loads. it is demonstrated that the proposed approach is both effective and robust in terms of assessing not only the safety conditions of existing masonry vaults, the actual geometry of which significantly influences the safety level, but also to design new ones. mailto:mariagrazia.durso@unibg.it acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 258 historical structures may have experienced (and continue to experience) various phenomena of a very different nature, including gravity forces, earthquakes, and environmental effects (thermal effects, chemical or physical attack), as well as various anthropogenic actions such as architectural alterations, intentional destruction, and inadequate restorations. many of these actions also need to be characterised in terms of time, with some cyclic and repetitive (accumulating significant effects in the long term), others developing gradually over extremely long periods of time, and others still associated with long return periods. in many cases, they may be influenced by historical contingency and uncertain (or at least, insufficiently known) historical facts. the existing general alterations may significantly affect the response of the structure to be modelled, and, hence, the realism and accuracy of the prediction of the actual performance and capacity. damage encompasses aspects such as mechanical cracking, material decay (due to chemical or physical attack), or a variety of other phenomena affecting the original capacity of the materials and structural members. its history is an essential aspect of a building and this must be taken in account and integrated within the model. the following effects linked to history may have had an impact on both the structural response and the existing damage: the construction process, any architectural alterations and additions, destruction due to conflicts (e.g. wars) or natural disasters (earthquakes, floods, fires), and various long-term decay or damage-inducing phenomena. in fact, the history as a whole constitutes a crucial source of knowledge. in numerous cases, the historical performance of the building can be engineered to reach conclusions on its structural performance and strength. for example, the performance exhibited during past earthquakes can be considered in order to improve the understanding of the antiseismic capacity. in fact, the history of the building constitutes a unique experience occurring on a real scale of space and time. as such, the knowledge of the historical performance can compensate for the aforementioned data insufficiency [4]. the most frequently employed survey techniques for capturing the geometry of buildings, also applied in the field of cultural heritage preservation, include both terrestrial and remote photogrammetry as well as tls. approaches based on the acquisition of images aimed at 3d modelling have recently become the subject of significant studies and research in various different areas [5]-[7]. the tls technique presents a non-contact and non-intrusive technique that allows for digitally acquiring the shape of objects in a rapid and accurate way. the attendant research provides several examples of 3d point clouds acquired for detailed structural digital models, which represents a particularly relevant issue in the case of masonry structures, where geometry plays a crucial role in the comparative degree of safety [8]. nowadays, starting from an accurate 3d model derived from laser scanning measures, it is possible to implement a building information modelling (bim) approach that allows for managing the entire project in a consistent and optimal manner. in fact, the laser scan techniques, such as tls, which encompass scan-to-bim among other processes, present a valuable prerequisite for bim modelling, since they can support geometric and spatial data that can be acquired, organised, and managed to satisfy the required project scale. one single database ensures the quality of the results due to the direct link among the shapes, the information, and the project documentation [9], [10]. specifically, 3d modelling becomes important when these models are adopted for structural engineering purposes [11]. this is the case, for example, for masonry structures and vaults, the structural safety assessment of which involves either meshing them as a collection of finite elements, as in the traditional finite element method, or as a set of nodes connected by branches, as in the more recent thrust network analysis (tna) approach [8]. given the recent emergence of this approach, we first briefly illustrate its theoretical background and basic assumptions, largely to emphasise its flexibility and how the geometric data required as input naturally matches the output of tls surveys. once the theoretical background and the state of art of the tna method have been illustrated in the section 2, the basics of the tna are dealt with in section 3. section 4 describes the study case referred to the medieval church of s. maria della libera, in aquino for which the safety assessment of the cross vaults has been carried out. the related results are illustrated and commented in section 5. finally the conclusions are summarized in section 6. 2. theoretical background nowadays, finite element (fem) analysis can be regarded as the most effective numerical technique for structural analysis since, unlike traditional static analysis, it allows for i) providing a 3d model of geometrically complex structures, ii) managing the characteristic parameters of the materials employed in the model, and iii) performing different analyses (linear, non-linear, dynamical, etc.) on the same geometry. however, in the case of historical and monumental masonry structures, fem analysis does not present the best option since it is difficult to ascertain the characteristics of the materials and the effects induced by the interventions previously performed on the structure. an alternative effective approach to fem is the tna approach, a fairly recent methodology that is briefly described in the sequel [12]-[17]. this method can be regarded as an automated and computerised variant of mery’s method, which is used for hand calculations of masonry arches (figure 1). specifically, tna is used to model masonry vaults as a discrete network of branches, subjected only to compressive forces in equilibrium with the gravitational loads. originally devised by o’dwyer [18] and later developed by block et al. [8], the evolution of this method represents one of the first rational approaches to the stability of masonry buildings and takes its steps from the analogy between the equilibrated shape of masonry arches and that of tensile suspended cables. this analogy, known as the ‘catenary principle’, is that of an arc that is reminiscent of a long chain, retained at its ends and allowed to dangle (figure 2). figure 1. mery’s method: internal forces by means of a funicular polygon. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 259 heyman [19] combined this principle with the limit theorems of plasticity, specifically the static theorem, to evaluate the safety of masonry structures, predicting the ultimate mechanism of arches or 3d framed structures. an extension that includes domes and vaults was then proposed by o’dwyer [18] in terms of fictitiously deconstructing the structure in discrete equilibrated arches, which entails seeking networks of forces inside the structure according to what has been denominated tna. a pictorial description of this idea is illustrated in figure 3, with reference to a cross vault exhibiting a comparatively simple static behaviour, with the two diagonal arcs providing the bearing structure that distributes the loads on the four pillars at the vertexes. the four columns support the four ribs of a barrel vault as a succession of increasingly smaller arcs from the external perimeter towards the centre. each arch transmits its thrust connected to the diagonal arcs such that the diagonal arcs are loaded from the combination of the forces they sustain. 3. basics of thrust network analysis according to the so-called safe theorem of limit analysis, in the form established by heyman for masonry structures, the limit equilibrium of masonry vaults can be assessed by seeking a network of thrusts, i.e. purely compressive forces that are fully contained within the thickness of the vault and hence do not induce tensile stresses that masonry is incapable of withstanding. as such, the problem of identifying the maximum loads that a masonry vault can safety sustain is reconducted to identifying a specific set of points or nodes within the vault such that the applied loads are in equilibrium with the forces at each point, internal to the vault and purely compressive and directed along the fictitious branches connecting pairs of nodes. the geometric position of these nodes is determined via an optimisation algorithm that enforces the condition whereby the initially unknown coordinates are contained within the vault thickness as well as in the branches connecting them (figure 4). since the original paper [20] can be referred to for further details, in this paper, we detail only the simplest case of vertical loads by supplementing the original formulation with a simple example that will help the reader to grasp the details of the procedure (see figure 5 and figure 6). the set of nodes and the related branches define a specific network, from now on referred to as a thrust network, which is described by nn nodes and nb branches that connect specific pairs of nodes. the n-th node of the network is defined by its position (xn , yn , zn), in a 3d cartesian reference system, where z is the vertical direction. the external force concentrated at each node can be described as follows: 𝑓 𝑛 = (𝑡𝑥 𝑛 , 𝑡𝑦 𝑛 , 𝑡𝑧 𝑛) (1) while the thrust value related to the generic branch can be denoted as: 𝑇 (𝑏) = (𝑡𝑥 (𝑏) , 𝑡𝑦 (𝑏) , 𝑡𝑧 (𝑏) ) . (2) the set of nodes is split into ni internal nodes and nr restrained (or external) nodes, where only one external branch converges such that nn = ni + nr, thus ensuring the external branches model supports the reactions. the unknowns of the problem are represented by the coordinates of the nodes and thrusts within each branch. in fact, only the vertical coordinates of the nodes are sought since the figure 2. the catenary principle. figure 3. plant and axonometric views of a masonry vault. figure 4. tna modelling of the vault. figure 5. equilibrium of nodes. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 260 horizontal coordinates are assigned by the designer by projecting the vault in the plane and defining a regular grid of points. clearly, the disadvantage is the point distance, while the advantage is the quality of the analysis. the zn coordinates of the nodes and the branch values t b are evaluated by enforcing an equilibrium at the internal and external nodes via two different strategies detailed separately here in terms of the horizontal and vertical direction. 3.1 horizontal equilibrium of nodes denoted by bn, the set of branches converging to node n, the horizontal equilibrium of the n-th node is expressed by the following equation: ∑ 𝑡𝑥 (𝑏) 𝑏∈𝐵𝑛 + 𝑓𝑥 (𝑛) = 0 ∑ 𝑡𝑦 (𝑏) 𝑏∈𝐵𝑛 + 𝑓𝑦 (𝑛) = 0 (3) in terms of the horizontal components 𝑓𝑥 𝑛, 𝑓𝑦 𝑛 of the external loads and the thrust forces 𝑡𝑥 𝑏 , 𝑡𝑦 𝑏 relative to the branches b connected to the node. indicated by n and m(b), the indices of the nodes connected by the generic branch 𝑏𝜖𝐵𝑛 can be denoted by 𝑡ℎ (𝑏) = √𝑡𝑥 (𝑏)2 + 𝑡𝑦 (𝑏)2 (4) the horizontal component of the thrust, and by 𝑙ℎ (𝑏) = √(𝑥𝑛 − 𝑥𝑚 (𝑏) ) 2 + (𝑦𝑛 − 𝑦𝑚 (𝑏) ) 2 (5) the length of the generic branch projected in the horizontal plane (see figure 5). hence, we can evaluate the following: 𝑡𝑥 (𝑏) 𝑡 ℎ (𝑏) = (𝑥𝑛 − 𝑥𝑚 (𝑏) ) 𝑙 ℎ (𝑏) (6) and 𝑡𝑦 (𝑏) 𝑡 ℎ (𝑏) = (𝑦𝑛 − 𝑦𝑚 (𝑏) ) 𝑙 ℎ (𝑏) (7) which can be incorporated into eq. 3 to get: ∑ (𝑥𝑛 − 𝑥𝑚 (𝑏) ) 𝑙 ℎ (𝑏) 𝑡ℎ (𝑏) 𝑏∈𝐵𝑛 + 𝑓𝑥 (𝑛) = 0 (8) ∑ (𝑦𝑛 − 𝑦𝑚 (𝑏) ) 𝑙 ℎ (𝑏) 𝑡ℎ (𝑏) 𝑏∈𝐵𝑛 + 𝑓𝑦 (𝑛) = 0 (9) there is a large number of networks that are in equilibrium with a given set of external forces and, at the same time, are contained within the vault thickness. to address all of them in a comprehensive way, it is important to express the horizontal components of thrust 𝑡ℎ (𝑏) as the product of a factor  and various reference thrust values �̂�ℎ (𝑏) , both of which are left unspecified at the moment. accordingly, for the generic b-th branch, we can set 𝑡ℎ (𝑏) = �̂�ℎ (𝑏) = 1 𝑟 �̂�ℎ (𝑏) , and eqs. 8–9 thus become: ∑ [ �̂�ℎ (𝑏) 𝑙 ℎ (𝑏) 𝑥𝑛 − �̂�ℎ (𝑏) 𝑙 ℎ (𝑏) 𝑥𝑚 (𝑏) ] 𝑏∈𝐵𝑛 + 𝑓𝑥 (𝑛) 𝑟 = 0 (10) ∑ [ �̂�ℎ (𝑏) 𝑙 ℎ (𝑏) 𝑦𝑛 − �̂�ℎ (𝑏) 𝑙 ℎ (𝑏) 𝑦𝑚 (𝑏) ] 𝑏∈𝐵𝑛 + 𝑓𝑦 (𝑛) 𝑟 = 0 (11) where the ratios �̂� ℎ (𝑏) 𝑙 ℎ (𝑏) represent the reference thrust densities of the network branches. 3.2 vertical equilibrium of nodes recalling that 𝑡𝑧 (𝑏) and 𝑓𝑧 (𝑛) are, respectively, the vertical component of the 𝑏𝑡ℎ branch thrust converging to node n and the nodal load, the vertical equilibrium of a generic node can be written as follows: ∑ 𝑡𝑧 (𝑏) 𝑏∈𝐵𝑛 + 𝑓𝑧 (𝑛) = 0 (12) now, recalling that, if compressive, thrust force t(b) is oriented towards node n, we have the following: 𝑡𝑧 (𝑏) = 𝑡ℎ (𝑏) 𝑙 ℎ (𝑏) (𝑧𝑛 − 𝑧𝑚 (𝑏) ) =  �̂�ℎ (𝑏) 𝑙 ℎ (𝑏) (𝑧𝑛 − 𝑧𝑚 (𝑏) ) = 1 𝑟 𝑡ℎ (𝑏) 𝑙 ℎ (𝑏) (𝑧𝑛 − 𝑧𝑚 (𝑏) ) (13) where the formula 𝑡ℎ (𝑏) = �̂�ℎ (𝑏) = 1 𝑟 �̂�ℎ (𝑏) has been used. accordingly, eq. 13 can be rewritten as follows: ∑ 𝑧𝑛 − 𝑧𝑚 (𝑏) 𝑙 ℎ (𝑏) �̂�𝑧 (𝑏) 𝑏∈𝐵𝑛 + 𝑓𝑧 (𝑛) 𝑟 = 0 (14) or equivalently as ∑ [ �̂�ℎ (𝑏) 𝑙 ℎ (𝑏) 𝑧𝑛 − �̂�ℎ (𝑏) 𝑙 ℎ (𝑏) 𝑧𝑚 (𝑏) ] 𝑏∈𝐵𝑛 + 𝑓𝑧 (𝑛) 𝑟 = 0 . (15) the previous condition is used to evaluate the unknown nodal heights zn, the coefficients of which are expressed by means of the reference thrust densities. the physical meaning of the parameter  = 1 𝑟 is exemplified by the three-hinged arch shown in figure 6, for which the equilibrium needs to be written only for the central node. figure 6. illustrative example of the vertical equilibrium of a node. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 261 given that 𝑧𝑚 (𝑏) = 0, eq. 15 can be simplified to �̂� ℎ (𝑏) 𝑙 ℎ (𝑏) 𝑧𝑛 + 𝑓𝑧 (𝑛) 𝑟 = 0 or, equivalently, given that 𝑓𝑧 (𝑛) < 0 since it is directed downwards, to |𝑓𝑧 (𝑛) |𝑟 = �̂�𝑥 (𝑏) 𝑙 ℎ (𝑏) 𝑧𝑛 . (16) given that 𝑡ℎ (𝑏) , 𝑙ℎ (𝑏) are both positive, it can be inferred that a greater value of r is associated with a greater value of 𝑧𝑛 and vice versa. this, in turn, implies that we are seeking networks with the uppermost vertical coordinates of all nodes and, hence, the minimum thrust. 4. case study the present case study relates to the laser scanning survey for the santa maria della libera church in the municipality of aquino, and the attendant structural analysis for the static verification of the church’s cross vaults [20]. the church, which dates from 1000–1100 a.d. and is characterised by a pure romanic–benedictine style, was built with the typical ‘local soft’ travertine, fragmentary material of the remains of roman buildings surrounding the area where it was erected. the amazing and austere interior, with dimensions of 17 × 38 m, consists of three aisles divided by square pillars with three semi-circular absids and an imposing triumphal arch, also resting on pillars culminating with fragments of roman cornice that act as capitals, which leads to the transept (figure 7, figure 8). meanwhile, the main altar, consisting of a roman marble sarcophagus, is placed in the centre, with the centre aisle having a wooden roof, while the side aisles feature various cross vaults. the campaign of measurements was focused on the interior and exterior of the masonry structure, with its geometry and external projections making the survey particularly complex and cumbersome. in fact, the vaults in the interior aisles of the church have been the subject of various detailed studies [21]-[27]. first, an accurate topographical survey of the historic artifact’s site was carried out using the topcon gls-2000 laser scanner station (figure 9). during a four-hour period, 16 scans were performed at different station points in order to obtain an extremely high density of scan points, approximately five million points with measured coordinates with millimetre accuracy [20]. the survey was divided into several phases after careful planning of the campaign and the identification of the station points. the design of the survey included maps from google maps with on-site identification, cloud capture scans of specific points detected by a laser beam with a 360 ° horizontal and 270 ° vertical range of action, scanning alignment in pairs, global alignment, filtering, modelling, and editing in terms of the subroutine that the tna code was connected to (figure 10). point cloud models obtained using a survey method figure 7. image of the central nave. figure 8. internal plan. figure 9. image of the lateral left nave with a view of the masonry cross vaults. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 262 incorporating high-precision laser scanning instruments have certain limitations from a computational point of view. in fact, it is not possible to directly associate the physical behaviour of the model derived from a point cloud using structural software. in fact, the transition from a point cloud to a polygonal surface model can be defined as a ‘re-topology’ operation since both quantitative and qualitative information of the point’s model are translated and adapted to obtain a triangular or quadrangular mesh that better represents the polygonal surface. meanwhile, the structural analysis software uses an algorithm that requires the geometric dimensions of the masonry object as the input data. the laser scanning survey consists of assigning a triplet of xyz coordinates to each point, with the coordinates initially relative before becoming absolute through a geo-referencing operation. in order to extrapolate the coordinates of the points, the cloud compare software offered us both the possibility of querying the single point or selecting a number of n points and exporting the list in .txt formats. once the data had been extrapolated and processed, they were exported to an excel table. in fact, the masonry elements are typically non-regular and non-continuous, do not have a homogeneous surface, and are made of masonry ashlars that are not perfectly squared. accordingly, the geometric data obtained from the survey needed to be appropriately smoothed to achieve more regular surfaces, i.e. surfaces with no kinks, unrealistic holes, or superposition patches. in order to resolve this problem and to obtain coordinates that allowed us to provide correct geometric data for processing via the structural algorithm, an interpolating function was built. specifically, the following polynomial formula was chosen to interpolate the curve drawn with the coordinates of the surveyed points in the best way possible. the equation of the polynomial formula is: y = 0.0236 x6 + 0.023 x5 + 0.0677 x4 0.0597 x3 0.3024 x2 + 0.1172 x + 12.871 . (17) the regression line represented a good fit of the points as the index of variance of the line demonstrated, with a value of r2 = 0.9974, i.e. very close to one. 5. results the application of the tna method, illustrated here for a single cross vault of the roof, was extended to the static and structural verification of all masonry vaults existing in the s. maria della libera church in aquino. each of the church’s cross vaults in the aisles has a square base whose side length is equal to 4 meters, a height equal to 2 meters, a thickness of 0.45 meters and it is made of soft travertine with a specific weight of 2.72 t/m³ (figure 11). the application of the tna method to a single cross vault provided, as can be seen from figure 12 and figure 13, the minimum and maximum thrust values of each of the 389 branches of the cross vaults, as well as the minimum and maximum values of the height of each of the 222 nodes of the roof associated with the maximum and minimum thrust values. the maximum height of the network nodes that characterises the deepest limit configuration, i.e. that associated with the minimum thrust, was 2.48 m. conversely, the shallowest limit configuration, associated with the maximum thrust, had a value of 1.38 m as the minimum node height. figure 10. flowchart of the scan-to-bim process. figure 11. medium surface of the vault geometry obtained via the surveying of the intrados and discrete measurements of the vault thickness. figure 12. distribution of maximum and minimum thrusts within the vault. figure 13. thrust distribution in a rib. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 263 6. conclusions the tls survey carried out on the monumental complex of the church of santa maria della libera in aquino provided the point cloud required to perform a structural modelling of the church’s masonry vaults using the tna approach. the geometric and geo-referenced 3d model obtained by processing the laser-scanning measurements presented a model built on a coherent geometric basis, which considers the methodological complexities of the detected object (figure 14 and figure 15). the paper demonstrated how the interdisciplinarity between a geometric model, built with the innovative techniques typical of the geomatic-type survey, and a structural model, can represent a useful support to the structural verification of the safety and conservation of complex structures, such as those typically pertaining to the field of monumental heritage. in future research within this reference context, hbim models will be addressed, which are part of a semi-automated method that allows for switching from point cloud to an advanced 3d model with the capacity to contain all the geometrical and mechanical characteristics of the built object [28]-[34]. moreover, fem analyses, based on recently developed strategies related to masonry modelling [35]-[37], will be investigated in order to assess the outcomes of the tna. acknowledgments this work has been carried out under the gamher project: geomatics data acquisition and management for landscape and built heritage in a european perspective, prin: progetti di ricerca di rilevante interesse nazionale – bando 2015, prot. 2015hjls7e. gamher, url: https://site.unibo.it/gamher/en. references [1] r. quattrini, f. clementi, a. lucidi, s. giannetti, a. santoni, from tls to fe analysis: points cloud exploitation for structural behaviour for definition. the san ciriaco’s bell tower, the int. archives of the photogrammetry, remote sensing and spatial information sciences, vol. xlii-2/w15, 2019 27th cipa int. symp. documenting the past for a better future, ávila, spain, 1-5 september 2019, pp. 957-964. doi: 10.5194/isprs-archives-xlii-2-w15-957-2019 [2] t. ramon herrero-tejedor, f. arques soler, s. lopez-cuervo medina, m. r. de la o cabrera, j. l. martìn romero, documenting a cultural landscape using point-cloud 3d models obtained with geomatic integration techniques. the case of the el encın atomic garden, madrid (spain), plos one 15(6), 24 june 2020, e0235169, 16 pp. doi: 10.1371/journal.pone.0235169 [3] p. roca, m. cervera, g. gariup, l. pela, structural analysis of masonry historical constructions. classical and advanced approaches architectural computation methods engineering, springer, cham., 2010, pp. 299-325. doi: 10.1007/s11831-010-9046-1 [4] g. roca, f. lopez-almansa, j. miquel, a. hanganu, limit analysis of reinforced masonry vaults, engineering structures 29 (2007) 3, pp. 431-439. doi: 10.1016/j.engstruc.2006.05.009 [5] g. bitelli, c.balletti, r. brumana, l. barazzetti, m. g. d'urso, f. rinaudo, g.tucci, the gamher research project for metric documentation of cultural heritage: current developments, the int. archives of the photogrammetry, remote sensing and spatial information sciences, vol. xlii-2-w11-, proc. of 2019geores and 2019-2nd international conference of geomatics and restoration, 8 – 10 may 2019, milan, italy, pp. 239-246; 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[29] m. hess, v. petrovic, m. yeager, f. kuester, terrestrial laser scanning for the comprehensive structural health assessment of the baptistery di san giovanni in florence, italy: an integrative methodology for repeatable data acquisition, visualization and analysis, structure and infrastr. eng. 14(2) (2018), pp. 247-263. doi: 10.1080/15732479.2017.1349810 [30] m. c. l. howey, m. brouwer burg, assessing the state of archaeological gis research: unbinding analyses of past landscapes, journal of archaeological science 15(5) 2017, pp. 1-9. doi: 10.1016/j.jas.2017.05.002 [31] l. c. hung, w. xiangyu, j. yi, bim-enabled structural design: impacts and future developments in structural modelling, analysis and optimisation processes, arch. computat. methods 22 (2015), pp. 135-151. doi 10.1007/s11831-014-9127-7 [32] m. llobera, building past landscape perception with gis: understanding topographic prominence, journal of archaeological science 28 (2001), pp. 1005-1014. doi: 10.1016/jasc.2001.0720 [33] a. mitropoulou, a. georgopoulos, an automated process to detect edges in unorganized point clouds, isprs ann. photogramm. remote sens. spatial inf. sci., vol. iv-2/w6, (2019), pp. 99-105. doi: 10.5194/isprs-annals-iv-2-w6-99-2019 [34] x. yang, m. koehl, p. grussenmeyer, parametric modelling of asbuilt beam framed structure in bim environment the int. archives of the photogrammetry, remote sensing and spatial information sciences, volume xlii-2/w3, 2017 3d virtual reconstruction and visualization of complex architectures, nafplio, greece, 1-3 march 2017, pp. 651–657. doi: 10.5194/isprs-archives-xlii-2-w3-651-2017 [35] r. serpieri, s. sessa, l. rosati, a mitc-based procedure for the numerical integration of a continuum elastic-plastic theory of through-the-thickness-jacketed shell structures, comp. struct., 191 2018 pp. 209-220. doi: 10.1016/j.compstruct.2018.02.031 [36] s. sessa, r. serpieri, l. rosati, a continuum theory of throughthe-thickness jacketed shells for the elasto-plastic analysis of confined composite structures: theory and numerical assessment, comp. part b: eng., 113 (2017), pp. 225-242. doi: 10.1016/j.compositesb.2017.01.011 [37] s. sessa, r. serpieri, l. rosati, probabilistic assessment of historical masonry walls retrofitted with through-the-thickness confinement devices, proc. of 23rd aimeta conf., salerno, italy, 4-7 september 2017, pp. 2324-2332. https://doi.org/10.1016/j.compstuc.2016.11.016 https://doi.org/10.1080/15583058.2017.1419313 https://doi.org/10.1504/ijmri.2019.096828 https://doi.org/10.1080/15583058.2018.1476607 https://doi.org/10.1007/978-3-030-41057-5_154 https://doi.org/10.7712/120117.5562.17018 https://www.imeko.org/publications/tc4-archaeo-2019/imeko-tc4-metroarchaeo-2019-52.pdf https://www.imeko.org/publications/tc4-archaeo-2019/imeko-tc4-metroarchaeo-2019-52.pdf https://doi.org/10.1109/metroarchaeo43810.2018.9089783 https://doi.org/10.5194/isprsarchives-%20xlii-5/w1-219-2017 https://doi.org/10.5194/isprsarchives%20xl-6-13-18 https://www.imeko.org/publications/tc4-archaeo-2019/imeko-tc4-metroarchaeo-2019-41.pdf https://www.imeko.org/publications/tc4-archaeo-2019/imeko-tc4-metroarchaeo-2019-41.pdf https://doi.org/10.3390/ijgi7080318 https://doi.org/10.5194/isprsannals-ii-3-w5-3-2015 https://doi.org/10.1080/15732479.2017.1349810 https://doi.org/10.1016/j.jas.2017.05.002 https://doi.org/10.1007/s11831-014-9127-7 https://doi.org/10.1016/jasc.2001.0720 https://doi.org/10.5194/isprs-annals-iv-2-w6-99-2019 https://doi.org/10.5194/isprs-archives-xlii-2-w3-651-2017 https://doi.org/10.1016/j.compstruct.2018.02.031 https://doi.org/10.1016/j.compositesb.2017.01.011 reduction of gravity effect on the results of low-frequency accelerometer calibration acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 365 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 365 368 reduction of gravity effect on the results of lowfrequency accelerometer calibration g. p. ripper1, c. d. ferreira2, r. s. dias2, g. b. micheli2 1 division of acoustics and vibration metrology diavi, inmetro., brazil, gpripper@inmetro.gov.br 2 vibration laboratory lavib, inmetro, brazil, rsdias@inmetro.gov.br abstract: this paper describes a study on the possible sources of systematic errors during the calibration of accelerometers at low-frequencies. this study was carried out on a primary calibration system that uses an air-bearing vibration exciter aps dynamics 129 and applying the sine-approximation method. tests performed and actions taken to reduce the effect on experimental results are presented. keywords: calibration; vibration; lowfrequency; accelerometer. 1. introduction the warp of the linear guide can cause a tilt with variable angle of the moving table when it is linearly translated. this might cause a variable effect of local gravity on dc-responsive accelerometers as for instance servo-accelerometers. in the key comparison euramet.auv.v-k3 a larger dispersion among the results of participants was evidenced at the lowest frequencies [1]. for the key comparison ccauv.v-k3 [2], inmetro have reported results down to 0.2 hz. later on, some efforts were made to extend the lower limit range to 0.1 hz but the systematic effect was high compared to the desired uncertainty for the service. this was confirmed during a measurement audit carried out during the peer review of lavib. in this opportunity (june 2019) we could calibrate a servoaccelerometer from ptb and quantify the problem. the application of a mathematical correction of the systematic error caused by gravity had been proposed by t. bruns and s gazioch in 2016 [3]. we preferred to attack the problem following a different approach. instead of dealing with the effect, we decided to try to identify and reduce the cause. therefore, we searched the responses for the basic questions: can we clearly identify the cause and consider it is exclusively generated by the warp? can we minimize the cause of the problem to reduce or even exclude the need of a mathematical correction? some experiments were carried out to respond to these questions and the results obtained will be presented in the following sections. 2. description of the work the study started with the establishment of a reference condition that was nothing else than the result of sensitivity obtained for a servoaccelerometer q-flex qa-3000. the calibration of accelerometers is usually carried out using two mountings (0° and 180°). the final sensitivity is the mean of the results obtained at 2 mountings. this basic condition was used as parameter to evaluate the results obtained in our subsequent tests. 2.1 initial tests the influence of gravity on the accelerometer was first tested using different orthogonal mounting position (90° and 270°). despite of the servoaccelerometer q-flex being a based on a cantilever beam design no significant difference was observed. the influence of the distance of the accelerometer from the centre of gravity of the moving table was also tested. a calibration was carried out placing the accelerometer below the mounting plate of the moving table. no significant difference was observed. the influence of tilt on the measuring points used for calibration was evaluated by using different measuring distances between the laser measuring point and the centre axis of the accelerometer. tests after loosening the screws of one end of the guide bar were performed and later after loosening the screws of both ends of the guide bar. change on the behavior of sensitivity magnitude at low frequencies could be observed at these times. 2.2 correction proposed by ptb the correction procedure proposed by ptb have also been tested by inmetro. figure 1 presents corrected sensitivity results obtained according to the method proposed by bruns and gazioch using http://www.imeko.org/ mailto:gpripper@inmetro.gov.br mailto:rsdias@inmetro.gov.br acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 366 the same coefficient published by ptb in [3]; using the coefficient determined by inmetro for its system; original certificate results obtained in 2018 and in 2019; and results with 3 screws loosened at one side the linear guide. figure 1: experimental and corrected results according to method proposed by bruns and gazioch. figure 1 demonstrates that the repeatability of the results from year to year is good, but the systematic error is too high at 0.1 hz. the mathematical correction improves the final result obtained but this can be dependent on specific characteristics of the accelerometer under test. the simple test of just loosening the three screws at one side of the linear guide have shown the possibility of achieving an improvement of the sensitivity response measured with the interferometric system by mechanical means. therefore, it was decided to quantify the effect caused by this process and take actions in order to improve the straightness of the guide. 2.3 straigthness measurements the procedure applied by inmetro to measure the straightness comprised the use of an autocollimator taylor robson mounted on an independent seismic block, which is usually used to position a breadboard with the interferometer. the setup is shown in figure 2. figure 2: setup used to measure the straightness of the linear guide of shaker aps 129. a plane mirror stand was positioned on the top of the moving table and the angle of inclination of this mirror was measured at different positions of the moving table. the zero position was taken as the equilibrium static point with no voltage input to the shaker. this is close to the mid-point of the linear translation guide. a dc-voltage source was used to provide a voltage of approximately 200 mv to the power amplifier. the gain knob of the amplifier was manually set to place the moving table at different x-positions. the measurement of the x-position was made with a laser distance measurer placed on the top of the table, which was pointed to a fixed reference stand on the same seismic block used to support the autocollimator. initially, straightness measurements were made at the mid-point and close to the positive and negative limits of motion used for accelerometer calibrations. absolute angles measured were -14, 12,6 and -11,5 seconds. normalizing these values in relation to the mid-point angle gives us a better view of the difference of the angle measured at different positions of the linear guide. this is shown in figure 3. figure 3: difference of the angle measured at different positions of the moving table. new straightness measurements were made while changing the mechanical system as follows: 1. original measurement conditions 2. measurements after release of the 3 screws that fix the linear guide to the base 3. measurements after release of the 6 screws that fix the linear guide to the base 4. measurements after release of the 6 screws that fix the linear guide to the base and release of the 4 screws that hold the base of the shaker to the seismic block 5. measurements after re-fixture of the 4 screws that hold the base of the shaker to the seismic block 6. measurements after re-fixture of the 6 screws that fix the linear guide to the base after applying the step 6 above, we considered that any stress due to mounting and assembly of the shaker to the seismic block have been released. a more refined straightness evaluation was made using a positioning resolution of approximately 10 mm. the results obtained are presented in figure 4. this graph shows that the http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 367 straightness of the system was highly improved. all measurement values were within ±0.2 sec relative to the angle at the reference point. figure 4: difference of the angle measured at different positions of the moving table after reassembly of shaker and linear guide. 2.4 dynamic calibration of servoaccelerometer primary calibrations of a high quality dcresponse accelerometer were then carried out to determine the influence of the improvement obtained on the straightness of the linear guide. after the reassembly of the system, with all its screws retightened, calibrations of a servoaccelerometer allied signal qa-3000 was calibrated in the frequency range 0.1 hz to 80 hz. the effect to be considered here is the deviation of the sensitivity magnitude response below 0.4 hz. the changes are caused by the combination of longer displacements and change of gravity effect due to different angle of the accelerometer while it moves along the linear guide. the results obtained in the range 0.1 hz to 10 hz for the accelerometer qa-3000 have shown that the frequency response is now almost flat. figure 5 presents the magnitude and phase of sensitivity. (a) sensitivity magnitude (b) phase shift figure 5: sensitivity results measured after reassembly of the shaker aps 129 the relative difference of sensitivity magnitude results are now all within 0,1 % taking as reference the value measured at 1 hz. figure 6: relative difference of magnitude results relative to the sensitivity value at 1 hz results obtained after reassembly of the shaker aps 129 2.5 static calibration of servoaccelerometer as a final check, the calibration of the accelerometer sensitivity at 0 hz was carried out by static rotating the accelerometer in the gravity field. the main sensitivity axis of the accelerometer was positioned at different angles relative to the local gravity field and the electrical output was measured. the range of angles from 0o to 360o was covered using 20o steps. then a sine fit was applied to measured data to obtain the sine amplitude and determine the accelerometer static sensitivity. this was carried out using both a voltmeter agilent 3458a and the daq board ni pci-6115 used in actual low-frequency accelerometer calibrations applying the sine-approximation method. due to the difference of input impedance between these two measuring instruments (voltmeter: 10 gω and daq: 1 mω), the voltmeter measurement results were corrected by approximately 0.5% to reflect the same impedance available at the daq input channel. figure 7 shows that the results obtained statically are in close conformity with the ones obtained http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 368 dynamically, being all within ±0.1%. this demonstrates the improvement obtained in the lowfrequency calibration system. figure 7: relative deviation of magnitude results to the sensitivity value at 0.1 hz: green square dc calibration with daq, red square dc calibration with hp3458a (corrected), blue diamond dynamic calibration results obtained after reassembly of the shaker aps 129. 3. summary small deviations from a purely straight linear motion can cause systematic errors on the calibration of dc-responsive accelerometers at lowfrequencies due to the effect of local gravity. this problem was studied at inmetro, where differences as large as 0.85% were observed on primary calibration of accelerometers at 0.1 hz. the cause identified was a warp of the linear guide, which cause a variable tilting angle of the moving table while it was translated. a procedure was applied to release any possible mechanical stresses that could occur during the mounting of the shaker on the inertial block and try to improve the straightness of the linear guide. this action was successful and allowed us to eliminate the main source of error in calibration below 0.4 hz. significant improvement of the calibration results was achieved and eliminated the need of further data correction. the estimated expanded uncertainty of sensitivity is 0.3% for magnitude and 0.3° for phase shift in the frequency range from 0.1 hz to 10 hz. 4. references [1] bartoli et al, “final report of euramet.auv.vk3”, metrologia 52 09003, 2015. [2] sun qiao et al, “final report of ccauv.v-k3: key comparison in the field of acceleration on the complex charge sensitivity”, metrologia 54 09001, 2017. [3] th bruns and s gazioch, “correction of shaker flatness deviations in very low frequency primary accelerometer calibration”, metrologia 53, 986, 2016. http://www.imeko.org/ digital nist: an examination of the obstacles and opportunities in the digital transformation of nist’s reference materials acta imeko issn: 2221-870x march 2023, volume 12, number 1, 1 4 acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 1 digital nist: an examination of the obstacles and opportunities in the digital transformation of nist’s reference materials william dinis camara1, steven choquette1, katya delak1, robert hanisch1, benjamin long1, melissa phillips1, jared m. ragland1, catherine rimmer1 1 national institute of standards and technology (nist), gaithersburg, md 20899, usa section: research paper keywords: digital transformation; digital reference materials; drmc; digital si; dcc citation: william dinis camara, steven choquette, katya delak, robert hanisch, benjamin long, melissa phillips, jared m. ragland, catherine rimmer, digital nist: an examination of the obstacles and opportunities in the digital transformation of nist’s reference materials, acta imeko, vol. 12, no. 1, article 7, march 2023, identifier: imeko-acta-12 (2023)-01-07 section editor: daniel hutzschenreuter, ptb, germany received november 17, 2022; in final form march 24, 2023; published march 2023 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: william dinis camara, e-mail: dinis.camara@nist.gov 1. introduction the digital transformation of metrology is a worldwide movement. for national metrology institutes (nmis), supporting digital technologies is now an urgent task. nmis want to support industry 4.0 and internet of things (iot) concepts, digital twin concepts, and many other digital technologies. although the activities of nmis vary, the majority provide measurement services that allow their stakeholders to obtain documented traceability to the international system of units (si). access to the documentation is typically delivered using methods that require a human to review and process the information provided. this may involve extracting pertinent data from paragraphs of text. with the proliferation of digital technologies and communication, a faster and more automated transfer of information is essential for technological growth. therefore, the digital delivery of measurement service data is a key and fundamental digital technology that nmis seek to support. to that end “a universal and flexible structure for digital calibration certificates (dcc)” was developed under the auspices of the european metrology program for innovation and research (empir) project smartcom 17ind02 [1]. practically, the dcc implements an xml schema for producing digital calibration certificates. the dcc continues to be supported and is presently on version 3.1.0; many nmis are now adapting the dcc for use in their own institutions. however, the dcc in its current version is not suitable for digital reference material certificates (drmc) due to the different information, underlying standards and dissemination requirements related to reference materials. a new schema must be created to support the digital delivery of reference materials. nist has recognized expertise in many digital technologies, including iot, cybersecurity, and artificial intelligence (ai), but abstract early in 2022, nist embarked on a pilot project to produce digital calibration reports and digital certificates of analysis for reference materials. the goal is to produce examples of digital reports and certificates to assess the scope and challenges of digital transformation in those particular measurement services. this paper focuses on the reference material certificate effort of the pilot project. our aims for this part of the pilot project are: to generate a digital reference material certificate from certification data; descriptive information about the material, and other data and metadata as needed; to generate a human-readable report from the digital reference material certificate; and to hold a workshop to gather stakeholder feedback. the challenges for nist include the diverse and complex information presently contained in nist certificates, converting values to non-si units to match the needs of stakeholders, and format updates to nist reference material certificates necessary to allow for machine generation. other practical challenges include the wide variety of reference materials offered by nist, as well as the needs of internal and external stakeholders. this presentation will report on the progress of the nist effort and discuss some of the challenges and solutions to producing digital reference material certificates. mailto:dinis.camara@nist.gov acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 2 has not yet attempted to deliver measurement service data in a digital format. in 2022, nist embarked on a pilot project to produce examples of digital calibration reports and certificates of analysis for nist standard reference materials (srms). the main objective of the pilot project is to assess the resources and effort required to deliver fully digital measurement service data. like other nmis, nist is using the dcc as a starting point. the efforts required to digitize calibration reports and certificates of analysis are separate but related. for the srm effort, one of the main challenges is that the dcc has been constructed for calibrations, not reference materials. although many of the elements are similar, there are also significant differences, particularly in the context of metadata. attempting to put reference material data directly into the dcc as currently constructed, adding additional data fields, and modifying existing data fields will lead to a more difficult-to-use patchwork collection of data fields. here, we focus on the reference material side of the pilot project, and we note that at the time of writing the pilot project is rapidly developing. 2. nist standard reference materials® nist offers over 1100 standard reference materials in the following categories [2]: ferrous metals, nonferrous metal, microanalysis, high purity materials, health and industrial hygiene, inorganics, primary gas mixtures, fossil and alternative fuels, organics, food and agriculture, geological materials and ores, ceramics and glasses, cement, engine wear materials, forensics, ion activity, polymeric properties, thermodynamic properties, optical properties, radioactivity, electrical properties, metrology, liquids and glasses, x-ray diffraction, sizing, surface finish, fire research, nanomaterials, and miscellaneous performance engineering materials. nist produces a significant number of srms in the technical categories covering a wide array of needs. the distribution of the number of srms by category is shown in figure 1. the large number and diversity of srms is a complex challenge for the digital delivery of reference materials and documentation because the types of data and values may vary greatly among the various srms. the different technical areas have unique challenges for providing the measurements that stakeholders need. the quality management system for nist srms is, to the extent allowed by statute and regulation, in conformity with the international organization for standardization (iso) standard 17034. because the dcc was designed to conform to iso/iec 17025, a schema for reference materials requires evaluation of the information required by iso 17034 and other sources to generate a schema for reference materials before comparison to the dcc to identify similar elements that can be reused between the two. 3. challenges 3.1. digital certificates nist plans to use an internally developed tool, the configurable data curation system (cdcs) [3], to transform srm metadata and results into digital srm certificates based on the new schema being created. the data for the certificates will be stored in a digital repository that will enable easier and higherquality generation of srm certificates. iso 17034 and iso guide 31 provide general requirements for reference material producers including much of the information that needs to be provided in certificates. in addition, other sources such as nist policy and customer input provide further information to be contained in certificates. since the dcc is designed around a different standard, iso/iec 17025, that requires different information, it is not a suitable schema for digital reference materials. a new schema must be researched and created based on the requirements for reference material certificates with input from nist stakeholders and in collaboration with other national metrology institutes. since similarities will exist between the new schema and the dcc, they will need to be compared to determine where similarities may be combined for more efficient digital delivery of measurement services information. the materials used for nist srms cover a wide range of technical areas. due to the varying types of data in these materials, nist supports multiple approaches to assigning values to best fit the homogeneity of each material. materials with between-unit homogeneity have certificates that can apply to multiple units across a batch or lot of material. most of the certificates for these nist srms are publicly available. however, materials that have only within-unit homogeneity require a distinct certificate for each unit. for these serialized materials, the certificate with data values is generally available only to the customer who purchased the unit and will necessitate limitations and additional security measures on how each certificate is delivered and accessed. additionally, values may be reported in multiple ways in certificates, e.g., a single value with an uncertainty, multiple values and uncertainties, and dna sequences. nist favours flexibility in delivering values in the units preferred by our stakeholders since our user communities use different units including both si and non-si units in their daily measurements. for example, users measuring for ignition resistance testing may use values expressed as a percentage, while users measuring coating thickness may use measurements to be expressed in mils. providing flexibility to the srm stakeholders is fundamental to creating a digital reference material certificate that will satisfy customer needs in disparate technical areas. the sale of nist srms is not concentrated in any particular area of material and the needs of each of these areas must be regarded. the distribution of srm sales by category is shown in figure 2. another challenge present in certificates is the variety of special characters that must be accommodated. greek letters, superscripts, subscripts and other symbols are frequently included and must be accommodated in any digital solution. figure 1. nist srms shown with the percentage of materials from the catalogue in each category. acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 3 additionally, reference material certificates are not static and may change over time. a certificate for a reference material may be updated to include new values that have been measured, for changes in expiration dates, or for numerous other reasons. while each version of a certificate contains unique information, the connection to historical versions of the documents needs to be maintained. the users of reference materials must ensure that they are using the current version of the certificate for their material. each year nist updates between 150 and 200 certificates. to populate a digital certificate, information must be collated from multiple sources. much of the metadata used to populate the digital certificates is stored electronically in databases. other information is manually curated and added. new code must be written to extract the electronically stored information for inclusion in certificates. for manually added information, new methods of storing the data must be created to automate inclusion. 3.2. human-readable certificates the creation of digital reference material certificates to enable machine-to-machine communication is essential to the future of metrology. however human-readable versions of the certificates will be necessary for at least the foreseeable future. an example of the first page of a current human-readable certificate is shown in figure 3. users of nist srms will utilize the human-readable versions for information about the material including storage, usage instructions, and other required product information. these versions also provide information about the materials to the stakeholders that aid in determining whether the srm is the best fit for their needs. these human-readable versions of digital reference material certificates must be formatted such that stakeholders can easily extract the information they need for their purposes. some of the elements that currently exist in nist certificates of analysis are intended primarily for the human reader and should continue to be included in documentation provided to the customers. the certificates for nist srms have changed over the more than 100 years that nist has been producing them. early certificates contained only values and uncertainties whereas modern certificates include more information about the source and preparation of the materials, more detailed information on storage, instructions for use, and other information required to educate customers and inform them of measurement concerns when using the materials. the schema for a drmc must take into account the wide range of information that has been included in the certificates for the benefit of the customers. many nist reference material certificates include figures, photos, and plots of results that communicate pertinent information or help the user to visualize tabular data. the need for these types of items to be machine-readable must be investigated. however, they must be incorporated into the drmc schema so that they can be included in the humanreadable certificate. the full benefit of a digital reference material certificate cannot be realized if human editing of a certificate after its generation is required. 3.3. tools for finding the appropriate srm the creation of a full digital repository of information will have the added benefit of improving searching for srms. currently customers utilize manual tools that can search a limited set of information in the certificate to compare multiple srms available from nist. customers access individual certificates and compare information about materials that do not share the same measurands, measurement units, or matrices. this approach does not accommodate complex searches and consumes significant time and resources from the customer. the digital repository will enable new, more automated tools to be created, perhaps even by private companies, that will allow for quicker comparison of available srms to identify the one that best suits the need of the customer. a first step may be to provide searching tools that are usable through a web interface. additional tools that would allow two machines to directly interact may be possible in the future. perhaps there will be a day when an instrument in the laboratory searches the nist srm catalogue and informs the user of the appropriate material to acquire. one of the greatest challenges to successful automated searches is the ability to compare measurements that use figure 2. nist srms shown with the percentage of sales of materials in each category. figure 3. the first page of the certificate for srm 2454a hydrogen in titanium alloy (nominal mass fraction 215 mg/kg h) (pin form). acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 4 different units. due to the volume of possible conversions, they cannot be calculated ahead of time and stored. rather the schema must allow for these types of calculations to be done in real time as the search is conducted. 4. nist workshop nist hosted an international workshop on 28-29 september 2022, for stakeholders of its calibration services and reference materials. the workshop introduced stakeholders to the results of the pilot study, specifically beta versions of digital calibration reports and certificates of analysis and their corresponding human-readable versions and seek stakeholder feedback on these beta versions. the workshop was an opportunity for nist to learn how stakeholders are responding to the changes towards more digital implementations of metrology, any solutions that they are implementing, and to better understand customer needs in this area. participants for the workshop were identified through a questionnaire that was sent to customers of nist measurement services through email and was posted on the nist website. the questionnaire gauged stakeholder interest in participating in the workshop and topics for digital transformation, such as digital calibration reports, certificates of analysis, digital traceability and security, and middleware for digital reports and certificates of analysis. nist will report on the results of the workshop in a separate document. following the workshop, nist is developing a strategy for full digital transformation of its measurement services. 5. future directions the current version of the dcc is not adequate for use in creating a drmc. continued development of a new model to conform to the specifications of reference materials is needed to have a model capable of handling the metadata associated with these materials. there are clear benefits ranging from within organization to worldwide for the digital transformation of reference materials and other measurement services. coordination and development of a model for reference materials that can be used worldwide will be essential to a successful transformation. nist will continue to monitor the dx being developed in the next version of the dcc. for reference materials the international community is watching the direction that national metrology institutes take in leading this effort. the work currently ongoing will provide the foundation for the transition to digital for reference materials. acknowledgement the authors acknowledge all participants in the digital nist pilot study, including those responsible for the portion of the study focused on calibrations. the authors also acknowledge the cooperation of nist staff who have assisted in assessing the variability of certificates and effort required to digitize reference material certificates and the dcc development group at the physikalisch-technische bundesanstalt (ptb) for their assistance. the nist associate director for laboratory programs provided funding for this pilot study. references [1] t. wiedenhöfer, d. hutzschenreuter, i. smith, c. brown, a universal and flexible structure for digital calibration certificates (dcc), dcc, 2019, pp. 1–3. doi: https://doi.org/10.5281/zenodo.3696567 [2] nist, standard reference materials® catalog; nist special publication (nist sp) 260 176, 2022 edition; u.s. government printing office: washington, dc. online [accessed 2 august 2022] https://www.nist.gov/srm [3] nist, configurable data curation system, 2019. online [accessed 2 august 2022] https://www.nist.gov/itl/ssd/information-systemsgroup/configurable-data-curation-system-cdcs/about-cdcs https://doi.org/10.5281/zenodo.3696567 https://www.nist.gov/srm https://www.nist.gov/itl/ssd/information-systems-group/configurable-data-curation-system-cdcs/about-cdcs https://www.nist.gov/itl/ssd/information-systems-group/configurable-data-curation-system-cdcs/about-cdcs mitigation of spectrum sensing data falsification attack using multilayer perception in cognitive radio networks acta imeko issn: 2221-870x march 2022, volume 11, number 1, 1 7 acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 1 mitigation of spectrum sensing data falsification attack using multilayer perception in cognitive radio networks mahesh kumar nanjundaswamy1, ane ashok babu2, sathish shet3, nithya selvaraj4, jamal kovelakuntla5 1 department of electronics and communication engineering, dayananda sagar college of engineering, bengaluru, karnataka-560078, india 2 department of electronics and communication engineering, pvp siddhartha institute of technology, vijayawada, andhra pradesh 520007, india 3 department of electronics and communication engineering, jss academy of technical education, bengaluru, karnataka560060, india 4 department of electronics and communication engineering, k. ramakrishnan college of technology, tiruchirappalli-621112, tamilnadu, india 5 department of electronics and communication engineering, gokaraju rangaraju institute of engineering and technology (griet), hyderabad, telangana-500090, india section: research paper keywords: cognitive radio network; cooperative spectrum sensing; energy statistic; machine learning model; spectrum sensing data falsification citation: mahesh kumar nanjundaswamy, ane ashok babu, sathish shet, nithya selvaraj, jamal kovelakuntla, mitigation of spectrum sensing data falsification attack using multilayer perception in cognitive radio networks, acta imeko, vol. 11, no. 1, article 21, march 2022, identifier: imeko-acta11 (2022)-01-21 section editor: md zia ur rahman, koneru lakshmaiah education foundation, guntur, india received november 20, 2021; in final form march 1, 2022; published march 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: mahesh kumar nanjundaswamy, e-mail: mkumar.n19@gmail.com 1. introduction over the past years, the world has witnessed a tremendous growth in the field of wireless communication technologies due to the popularity of telemedicine, smart home, smartphones, autonomous vehicles, mobile televisions and smart cities. the increasing demand for wireless communications has brought the problem of spectrum scarcity. energy detection and measurement is a key task in spectrum sensing in cognitive radio networks. as a result, development of hybrid machine learning or signal processing algorithms becomes an intense research area for both measurement technology as well as in cognitive radio communications. the federal communications commission abstract cognitive radio network (crn) is used to solve spectrum scarcity and low spectrum utilization problems in wireless communication systems. spectrum sensing is a vital process in crns, which needs continuous measurement of energy. it enables the sensors to sense the primary signal. cooperative spectrum sensing (css) has recommended to sense spectrum accurately and to enhance detection performance. however, spectrum sensing data falsification (ssdf) attack being launched by malicious users can lead to wrong global decision on the availability of spectrum. it is an extremely challenging task to alleviate impact of ssdf attack. over the years, numerous strategies have been proposed to mitigate ssdf attack ranging from statistical to machine learning models. energy measurement through statistical models is based on some predefined criteria. on the other hand, machine learning models have low sensing performance. therefore, it is necessary to develop an efficient method to mitigate the negative impact of ssdf attack. this paper intends to propose a multilayer perceptron (mlp) classifier to identify falsified data in css to prevent ssdf attack. the statistical features of the received signals are measured and taken as feature vectors to be trained by mlp. in this manner, measurement of these statistical features using mlp becomes a key task in cognitive radio networks. trained network is employed to differentiate malicious users signal from honest users’ signal. the network is trained with the levenberg-marquart algorithm and then employed for eliminating the effect of attacks due to the ssdf process. once the simulated results are observed, it can be revealed that the proposed model could efficiently reduce the impact of malicious users in crn. mailto:mkumar.n19@gmail.com acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 2 (fcc) [1], [2] reported that most of the allocated spectrum is rarely used by the primary users (pus) (fcc, 2002; fcc, 2003). in order to solve the conflicts between spectrum utilization and spectrum shortage, it has been recommended that opportunistic access to the licensed spectrum should be given to secondary users (sus). cognitive radio network (crn) has been developed to solve the aforementioned issues by enabling dynamic spectrum access. it is a new paradigm that offers the potential to utilize the licensed spectrum in an opportunistic manner [3] (wan et al. 2019). crn allows sus to sense and access free spectrum bands without interfering pus. crn allows sus to use valid spectrum, the spectrum scarcity problem will be solved successfully. sus needs to monitor continuously [4], [5] to sense the pu status. therefore, spectrum sensing becomes an important process for crn. spectrum sensing is the process of identifying status of pu. an accurate detection of spectrum can enhance the performance of crn significantly [6]-[8] (ali et al., 2017). however, due to obstacles, shadowing and multipath fading, wrong detections could take place, thus resulting in an inefficient usage considering the licensed spectrum. to deal such an issue, cooperative spectrum sensing (css) has been considered as a satisfactory candidate for spectrum sensing [9] (sharifi et al., 2016). css combines all cus sensing signal and makes final decision. it prevents the effect of noise, pathloss, shadowing and fading that may occur in wireless communication. however, css is vulnerable to many security threats. among many attacks, spectrum sensing data falsification (ssdf) can severely affect the detection performance of crn. in ssdf attack, malicious users (mus) sent falsified report to the fusion center (fc) about the spectrum band. ssdf attack mislead global decision by sending falsified report about the spectrum availability, hence degrading the crn performance. therefore, it is essential to develop an efficient method to eliminate the impact of ssdf attacks. the core contributions of this research work are as follows: the main aim or the focus of this research article will relate to the designing of an efficient model by using artificial neural network to suppress the negative impact of mus in crn and to enhance the detection performance through measuring various statistical parameters. in this scheme, a set of features are extracted from the received signals and then generated a large representative dataset. multilayer perceptron (mlp) [10], [11] is designed with one of input layer, two layers which are hidden and one output layer. next, the obtained feature vectors could be grouped into two sets, viz., training and testing. the data’s with respect to the training sets could be used for developing and training the mlp. testing data set is employed to validate the efficacy of the proposed model. performance of the proposed model is evaluated by measuring some commonly used metrices. spectrum sensing refers to the process of detecting the activity of pus in a licensed spectrum band. it plays a vital role in crns and css [12]-[14] has been suggested to make accurate decision about spectrum availability by utilizing spatial diversity via observations of multiusers. but css has some limitations. ssdf can severely affect the detection performance of the crns in which false sensing report sent by mus during the css. to eliminate ssdf attack, several methods have been reported in the literature. each method has their own characteristics. none of the method provide consistent result. thus, the main aim or the focus of this research article will relate to the designing of an efficient model by using artificial neural network to suppress the negative impact of mus in crn and to enhance the detection performance. the rest of the article is outlined as mentioned in the following sentences. the 2nd section presents an exhaustive review of former methods. the section 3 describes the development of the system model which is proposed here. section 4 provides experimental outcomes. the paper ends with the conclusive remarks presented in the 5th section. 2. related work several research literatures have proved that css is a good candidate to detect the activity of pu in crn. however, css is affected by many attacks such as the primary user emulsion attack and ssdf. amidst, ssdf is the dangerous attack in crn, ssdf attack can reduce the detection performance by sending falsified report to fc. over the past years, several methods have proposed to resist ssdf attack. wan et al., (2019) [15], [16] presented a method to mitigate the influence of the attacks using the ssdf concepts involving the concepts of the combinations of the linear weights. adaptive reputation method is also presented to differentiate mus from sus. feng et al. (2018) [17] used the exclusive-or (xor) distance analysis to eliminate the influence of ssdf in crn. in this approach, xor distance with hypothesis detecting the information is employed to calculate the equivalency between the two super-users. based on the xor distance, mus is separated from sus. soft decision-based scheme to resist ssdf is developed by ahmadfard et al. (2017) proposed method achieved better result than existing methods. in the article in ahmed et al. (2014), the authors presented a method to combat the effect of mus in crns using the bayseiens strategies. the authors used statistical features of received samples to sense the inexistence and existence of pu. li et al. (2014) investigated the potential of the fuzzy c-means algorithm in spectrum sensing. the proposed algorithm is capable of detecting pu signal accurately, which in turn enhance the detection performance. robust algorithm [18] to defence ssdf proposed by althunibat et al. (2014). in this approach, specific weights are assigned to sensing nodes. results showed that the algorithm is capable of detecting mus. according to the mean and standard deviation of the received samples, mapunya and velempini (2018) developed a ssdf mitigation method in crn. results proved that the proposed scheme could reduce the probability of false alarm rate. sharif et al. (2018) presented a defence strategy against ssdf attack. mean value of received samples are computed. two parameters α and β are obtained and then used in the likelihood ratio test method to enhance the detection performance. li and peng (2016) used unsupervised machine learning model to differentiate honest sus from mus. the proposed model utilizes past sensing report as a feature vector to categorize users. nie et al. (2017) proposed a defence scheme which is based on bayesian learning model. each user has specific weight that reflects its trustworthiness. farmani et al. (2011) suggested the ‘support a vector data description’ method to detect the activity of pu. the proposed differentiate honest sus from mus based on the energy statistic signal. however, this method failed to decrease the probability of false alarm rate. cheng et al. (2017) developed a self-organizing map to classify nodes into honest and malicious nodes. the proposed method uses average suspicion degree to discriminate mus from honest users. amar taggu et al. (2021) [19] proposed a two-layer model framework to classify ssdf attackers the first layer, the computational layer, employs the hidden markov model (hmm) to establish a probabilistic relationship between the pu's states and the sensing acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 3 reports of the sus. this generates the set of data needed for the next layer. the second layer, the decision layer, employs several ml algorithms to categorise sus as byzantine attackers or normal sus. 3. proposed system model the fundamental focus of this research work is developing and rendering a scheme for mitigating ssdf attack in crn by using the models developed using the concepts of machine learning’s. one of the hidden properties in the process of machine learning’s is the concept of learning how to develop the relation between the two variables, i.e., the input and the output via training process which makes the scheme more robust. the developed model is simulated, and its performance compared with the earlier methods to prove its superiority. the figure 1 depicts the structure of cognitive radio network with one pu and 5 sus. sus uses the communication channel whenever the pu signal is absent. the su’s will perform the process of the least square set (lss) in order to detect the absence or the presence of the pu’s and finally reporting the processes to the fc. next, the fc will make a final verdict with respect to the spectrum availability which is relying on the received information’s by the respective users. in this context, because of the presence of the mu’s, secondary user/s will send some falsified reports to the fc. let m mus are presented in sus. mus can send either always yes or always no to fc. always yes represents high energy (1) which increases the probability of the fake alarms (false) as they are going to give an active status information of the primary user/s when it will be inactive in nature. similarly, always no corresponds to low energy (0) which decreases the probability of detection because they send the primary user’s absentia information’s, even due to the presence of the pu’s in the system. both high and low signal of mus degrade the performance of crn. to deal such an issue, mlp is developed. spectrum sensing mainly used for detecting the presence or the inexistence of the pu’s, which is shown in figure 1, each su receives noisy informative signal once the primary user becomes inactive in nature and finally, energy of the parameters used in the process could be calculated at time of the tth instant using the pth su, which could be expressed by (1) as 𝑆𝑝(𝑡) = 1 𝑁1 ∑ |𝜂𝑝(𝑡, 𝑛)| 2 𝑁1−1 𝑛=0 . (1) in (1), the parameter 𝑆𝑝(𝑡) denotes the received noise by the pth su at 𝑡. n1 represents the number of samples considered. when the pu is active, equation (1) can be written in the form of eqn. (2) as 𝑆𝑝(𝑡) = 1 𝑁1 ∑ |𝐻𝑝 (𝑡, 𝑛). 𝑆(𝑡, 𝑛) + 𝜂𝑝(𝑡, 𝑛)| 2 𝑁1−1 𝑛=0 (2) where, ( ),ph t n denotes the channel gain between pu and pth su, 𝑆(𝑡, 𝑛) denotes the pu signal and 𝜂𝑝(𝑡, 𝑛)denotes the additive gaussian noise with 0 mean and variance. several spectrum sensing methods are reported in the literature such as detecting the energy parameters, the method of matched filtering process and the features using the cyclo-stationary concepts. amidst, energy detecting method is a good candidate for local spectrum sensing because the lss will not need any earlier datas or information’s about the primary user signal and the computational overhead is low. utilizing energy detection method, the received signal can be expressed into binary hypothesis testing, h0 and h1, given in (3) as 𝑟𝑝 (𝑡) = { 𝜂𝑝(𝑡) 𝐻0 𝐻𝑝(𝑡) ∙ 𝑆(𝑡) + 𝜂𝑝(𝑡) 𝐻1 . (3) here, the parameter𝑟𝑝 (𝑡) represents the signal which is being sensed., represents the presence and the absence of the primary user signals. after the local spectrum is being sended, the decision of each su is represented as binary value, 0 and 1, on the inexistence and existence of pu signal with the mathematical model given by 𝑆𝑉𝑝(𝑡) = { 0 𝐻0 1 𝐻1 , (4) where 𝑆𝑉𝑝 is the sensing value of the p th su. 0 and 1 shows the inactive and active of pu signal respectively. every secondary user will report the end verdict to the central unit. then, the fc is going to make a final verdict regarding the spectrum which is relying on all of the data’s which are obtained by the sus. because of the presence of few of the fake or falsified user/s, the secondary user/s may or may not send the modified information to fc, which is finally going to affect the overall performance of the communication system’s spectrum. ssdf attacks such as always yes and always no are considered. under such attacks, mus can report contaminated data to the fc. mus will change the local sensing report and falsify the test outcome. for an instance, mus sends h0 while its local decision is h1 represents the existence of pu signal. let the qth ssdf attacker reports low energy when its local decision is h1 with probability h0 and reports high energy value to fc system, when the decision at the local level is depicted by the parameter h0 with using the probabilistic feature using the 𝑃1,𝑞 parameter with the mathematical model given by eqn. (5) and (6) as 𝑃𝑑,𝑞 = (1 − 𝑃0,𝑞 )𝑃𝑑,𝑞 + (1 − 𝑃𝑑,𝑞 )𝑃1,𝑞 (5) 𝑃𝑓,𝑞 = (1 − 𝑃0,𝑞 )𝑃𝑓,𝑞 + (1 − 𝑃𝑓,𝑞 )𝑃1,𝑞 . (6) to mitigate the ssdf attacks, features such as energy statistic, autocorrelation, squared mean, standard deviation and maximum-minimum eigen value and are computed and fed as figure 1. cognitive radio network model. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 4 inputs to the mlp. energy statistic of the signal can be represented using (7) as 𝐸 = ∑ |𝑟(𝑘)|2 𝑁1−1 𝑘=0 . (7) autocorrelation is a mathematical function which could be used for encoding the level of the association between two parameters which are procured from the similar source, i.e., the same source (correlating between itself). autocorrelation measures the similarity of a signal with a delayed version of itself. honest sus sends the actual report to the fc that may vary depending on the existence and inexistence of pu signal. but, mus report either low or high energy repeatedly. autocorrelation value does not oscillate much. therefore, autocorrelation value of signal is considered as one of the feature vectors. autocorrelation value of the signal is given using (8) as 𝐴(𝑖) = 1 𝑁1 ∑ 𝑟(𝑘). 𝑟(𝑘 − 𝑖) 𝑁1−1 𝑘=0 . (8) squared mean of the received signal can be computed using (9) as 𝜇 = 1 𝑁1 ∑ |𝑟(𝑘)|2 𝑁1−1 𝑘=0 . (9) features are labelled as 0 for h0 and 1 for h1. mlp is a feed forward, supervised artificial neural network (ann). the neural net has got 3 important layers, viz., an input layer, a no. of layers which are hidden and finally one output layer. the input layer of the ann is used for getting the input signal as external stimuli. the no. of parameters in the input later will decide the no. of feature vectors. each middle layer (hidden) in between the input and output consists of one or more units or hidden neurons. number of hidden layer and its units are determined by experimentation. output represents the final decision, and this output layer has one neuron representing binary classification, shown in figure 2. the mlp output at the output layer can be calculated using (10) 𝑦𝑘 = 𝑓 | ∑ 𝑤𝑗,𝑘 ′ ((𝑔 ∑ 𝑥𝑖 𝑤𝑖,𝑗 + 𝑏 𝑁 𝑖,𝑗=1 )) 𝑁 𝑗,𝑘=1 | , (10) where 𝑥𝑖 the i th input vector, 𝑏 is the bias and 𝑤𝑖 denotes weight between the input and hidden zones (layer/s), the weight between the hidden and the output layer is denoted by 𝑤𝑗 , finally the parameters 𝑔 and 𝑓 could be called as the function of activation, which are present at the hidden and output layer respectively. after labelling, feature vectors are categorized into two sets of data, i.e., the training ones and the testing ones, which are called as the training and testing information. the data which is used for training is employed to ensure the modelled ann system recognizes data and test data used to check the ability of the model to predict new cases based on its training. algorithm 3.1 explains the training procedure of mlp. performance of the trained network is validated with the test data. 3.1. algorithm: mlp training algorithm • create mlp network • initialize the weight and bias randomly • computer the feature vector x = [x1, x2, x3, …, xn] • label the target vector t = [0, 1], 0 = h0, 1 = h1 • for each training pair (x, t) • present input to the input layer. calculate the net output using (11) ℎ = ∑ (𝑥𝑖 𝑤𝑖,𝑗 + 𝑏) 𝑁 𝑖,𝑗=1 . (11) • apply activation function to compute net input using (12) ℎ = 𝑔(ℎ) . (12) • calculate the net output at the output layer using (13) 𝑦𝑘 = ∑ 𝑤𝑗,𝑘=1 ′ ℎ 𝑁 𝑗,𝑘=1 (13) • apply activation function to compute net outcome using (14) 𝑦𝑘 = 𝑓[𝑦𝑘 ] . (14) • calculate the error using (15) 𝐸𝑟𝑟𝑜𝑟 = 𝑇 − 𝑦 . (15) • back propagate the error and update weight and bias using (16) and (17) 𝑤new = 𝑤old + 𝛥𝑤𝑖,𝑗 𝑏new = 𝑏old + 𝛥𝑏𝑖,𝑗 (hidden layer) (16) 𝑤new = 𝑤old + 𝛥𝑤𝑗,𝑘 𝑏new = 𝑏old + 𝛥𝑏𝑗,𝑘 (output layer) (17) • test for stopping condition • end. 4. simulation results here, in the section-iv, experimental outcomes are portrayed in order to prove the effectiveness of the mathematical model and the simulation is performed using matlab 2018a platform. figure 2. multilayer perceptron with two hidden layers. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 5 crn is designed with one fc; one pu and 30 secondary users and pf will be set to 0.1 for all sus. the percentage of mus is ranged from 10 % to 60 %. the pu signal is a quadrature phase shift keying signal and outcomes are revealed using the simulations done using the monte-carlo methods with a step size of around 10000 runs. the signal to the noise ratio will be changed from a value of -20 db up to zero decibels. with respect to this work, mlp is employed for mitigating negative impact of mus in crn. an input value to the mlp is composed of feature vectors obtained from the received signal. mlp is designed with 5 neurons representing 5 feature vectors, 2 layers which are hidden using 10 hidden neurons in every ann layer and consisting of only 1 neuron in the output layer of the neural net and having only one neuron which corresponds to the output value 0 or 1 representing 𝐻0 & 𝐻1 hypothesis. tan sigmoid and linear activation function is used at the second and the third layer of the neural net (hidden/output). in this stage, the least means square (lms) algorithm could be used to train the neural net by setting the epoch value up to 500 points. efficacy of the developed model is ascertained or justified next. this is done with the help of probability detection and also using the help of probability of the false alarm rate parameter. the figure 3 depicts the plot between the signal-to-noise ratio (snr) and the probability of detection (pd) at the probability of false alarm pf = 0.1. from the results of simulation, we can infer that considering figure 3, the model which has been proposed by us give as an outstanding performance when compared to other methods taken for comparison. for an instance, snr = 12.5 db, pd performance of the proposed model is increased by 47.4 %, 46 % and 10 % as compared to energy detection (ed), generalized likelihood ratio test (glrt) and hadamard ratio (hr) sensing methods respectively. the figure 4 exhibits the probability of false alarm for always yes attack versus snr considering a group of malicious secondary users (percentagewise). from the figure 4, it is noticed that the proposed model can sense a yes attacking process correctly up to 50 % falsified secondary user/s in the presence for snr varying from -10 db to 0 db. it proves that machine learning model can efficiently sense the ssdf attack launched by malicious sus in crn. probability of false alarm for always no attack as a function of snr for varying percentage of malicious sus is graphically plotted in figure 5. from the results, one can see apparently seeing the figure 5 that model which is proposed by us is able to detect always no attack preciously up to 50 % malicious sus presence for snr varying from -10 db to 0 db. it further confirms that the proposed machine learning model can figure 3. probability of detection versus snr at pf = 0.1. figure 4. probability of false alarm versus snr for varying falsified secondary users percentagewise (always a yes attacking phenomenon). figure 5. probability of false alarm versus snr for varying percentage of falsified secondary user/s (always no attacking phenomenon’s). figure 6. probability of detection versus percentage of malicious users. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 6 efficiently detect the ssdf attack launched by malicious sus in crn. in order to experimentally validate the performance characteristics of the model which is being proposed in this research article, a curve can be plotted between probability of detection and malicious users varying from 10 % to 60 % at snr is -10 db, as shown in the figure 6, from where it can be revealed that the scheme which has been proposed by us yields more pd value. from the empirical findings, it can be justified and enunciated that the ml dependent strategy could efficiently suppress the impact of mus in the crns. the efficacy of the proposed model can be observed using the concepts of probability detection and probability of false alarm rate. the proposed model provides outstanding performance when compared to other methods taken for comparison with respect to different values of snr as shown in table 1. 5. conclusion performance of crn is severely affected by malicious sus. mus may launch ssdf attack to mislead the global decision. this paper has presented a machine learning model using an mlp classifier to identify falsified data in css to prevent ssdf attack in crn. set of features are extracted from the received samples and labelled based on the inexistence and existence of primary user. the obtained features used as input to the mlp model. the network is trained with the levenberg-marquart algorithm and then employed for eliminating the effect of attacks due to the ssdf process. once the simulated results are observed, it can be revealed that the proposed model could efficiently reduce the impact of malicious users in crn. however, it needs more time for training. in future work, meta heuristic algorithm will be explored to optimize the parameters of network and to further enhance detection performance. acknowledgement the authors acknowledge with thanks dayananda sagar college of engineering, bengaluru, jss academy of technical education, bengaluru for all the support and encouragement given to me to take up this research work and publication. references [1] a. ahmadfard, ali jamshidi, alireza keshavarz-haddad, 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ssdf attack in cognitive radio networks, 9th int. conf. on wireless comm. and signal processing (wcsp), nanjing, 2017, 1-6. doi: 10.1109/wcsp.2017.8170994 [6] f. farmani, m. abbasi-jannatabad, r. berangi, detection of ssdf attack using svdd algorithm in cognitive radio networks, third international conference on computational intelligence, communication systems and networks, bali, 2011, 201-204. doi: 10.1109/cicsyn.2011.51 [7] federal communications commission: et docket no 03-222, notice of proposed rulemaking and order. [8] federal communications commission, spectrum policy task force. rep. et docket no. 02-135. 2002. online [accessed 16 march 2022] https://transition.fcc.gov/sptf/files/sewgfinalreport_1.pdf [9] j. feng, m. zhang, y. xiao, h. yue, securing cooperative spectrum sensing against collusive ssdf attack using xor distance analysis in cognitive radio networks, sensors (basel). 18(2), 2018, 370. doi: 10.3390/s18020370 [10] l. li, c. chigan, fuzzy c-means clustering based secure fusion strategy in collaborative spectrum sensing. in 2014 ieee international conference on communications (icc), sydney, nsw, 2014, 1355-1360. doi: 10.1109/icc.2014.6883510 [11] s. mapunya, m. velempini, design of byzantine attack mitigation scheme in cognitive radio ad-hoc networks, international conf. on intelligent and innovative comp. apps. (iconic), plainemagnien, 2018, 1-4. doi: 10.1109/iconic.2018.8601087 [12] g. nie, g. ding, l. zhang, q. wu, byzantine défense in collaborative spectrum sensing via bayesian learning. ieee access 5, 2017, 20089-20098. doi: 10.1109/access.2017.2756992 [13] a. sharifi, m. mofarreh-bonab, spectrum sensing data falsification attack in cognitive radio networks: an analytical model for evaluation and mitigation of performance degradation, aut journal of electrical engineering, 50(1), 2018, 43-50. doi: 10.22060/eej.2017.12528.5094 [14] a. a. sharifi, m. j. musevi niya, defenseagainst ssdf attack in cognitive radio networks: attack-aware collaborative spectrum sensing approach, ieee communications letters, 20(1), 2016, 93-96, doi: 10.1109/lcomm.2015.2499286 [15] runze wan, naixue xiong, lixin ding, xing zhou, mitigation strategy against spectrum-sensing data falsification attack in cognitive radio sensor networks, international journal of distributed sensor networks, 15, 2019, 1550-1477. doi: 10.1177/1550147719870645 [16] yang li, q. peng, achieving secure spectrum sensing in presence of malicious attacks utilizing unsupervised machine learning, milcom 2016 ieee military commn. conf., baltimore, md, usa, 2016 174-179. doi: 10.1109/milcom.2016.7795321 [17] imran ahmed, eulalia balestrieri, francesco lamonaca, iomtbased biomedical measurement systems for healthcare table 1. probability of detection versus snr. approaches vs. snr proposed technique ed glrt hr snr = 20 db 0.14 0.1 0.1 0.1 snr = 15 db 0.32 0.1 0.12 0.23 snr= 12.5 db 0.62 0.14 0.16 0.52 snr = 10 db 0.88 0.25 0.28 0.87 snr = 4 db 0.99 0.93 0.96 0.99 snr = 2 db 0.99 0.98 0.99 0.99 https://doi.org/10.1016/j.sigpro.2017.01.033 https://doi.org/10.1109/glocom.2014.7036939 https://doi.org/10.1109/comst.2016.2631080 https://doi.org/10.1109/vtcspring.2014.7023078 https://doi.org/10.1109/wcsp.2017.8170994 https://doi.org/10.1109/cicsyn.2011.51 https://transition.fcc.gov/sptf/files/sewgfinalreport_1.pdf https://doi.org/10.3390/s18020370 https://doi.org/10.1109/icc.2014.6883510 https://doi.org/10.1109/iconic.2018.8601087 https://doi.org/10.1109/access.2017.2756992 https://doi.org/10.22060/eej.2017.12528.5094 https://doi.org/10.1109/lcomm.2015.2499286 https://doi.org/10.1177/1550147719870645 https://doi.org/10.1109/milcom.2016.7795321 acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 7 monitoring: a review, acta imeko, vol. 10, no.2, pp. 1-11, 2021. doi: 10.21014/acta_imeko.v10i2.1080 [18] armando coccia, federica amitrano, leandro donisi, giuseppe cesarelli, gaetano pagano, mario cesarelli, giovanni d'addio, design and validation of an e-textile-based wearable system for remote health monitoring, acta imeko, vol.10, no.2, pp. 1-10, 2021. doi: 10.21014/acta_imeko.v10i2.912 [19] amar taggu, ningrinla marchang, detecting byzantine attacks in cognitive radio networks: a two-layered approach using hidden markov model and machine learning, pervasive and mobile computing, v 77, 2021, issn1574-1192, doi: 10.1016/j.pmcj.2021.101461 https://doi.org/10.21014/acta_imeko.v10i2.1080 https://doi.org/10.21014/acta_imeko.v10i2.912 https://doi.org/10.1016/j.pmcj.2021.101461 editorial to selected papers from the tc17 events "international symposium on measurements and control in robotics" (ismcr2021) and vrise2021 topical event on robotics for risky interventions and environmental surveillance acta imeko issn: 2221-870x september 2022, volume 11, number 3, 1 2 acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 1 editorial to selected papers from the tc17 events "international symposium on measurements and control in robotics" (ismcr2021) and vrise2021 topical event on robotics for risky interventions and environmental surveillance zafar taqvi1 1 research fellow university of houston clear lake, houston, texas usa section: editorial citation: zafar taqvi, editorial to selected papers from the tc17 events "international symposium on measurements and control in robotics" (ismcr2021) and vrise2021 topical event on robotics for risky interventions and environmental surveillance, acta imeko, vol. 11, no. 3, article 2, september 2022, identifier: imeko-acta-11 (2022)-03-02 received september 13, 2022; in final form september 13, 2022; published september 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: zafar taqvi, e-mail: ztaqvi@gmail.com dear readers, this special issue includes selected papers from the two events organized by tc-17, the imeko technical committee on robotic measurement. annually tc17 organizes "international symposium on measurements and control in robotics" (ismcr), a full-fledged event, focusing on various aspects of international research, applications, and trends of robotic innovations for benefit of humanity, advanced humanrobot systems, and applied technologies, e.g. in the allied fields of telerobotics, telexistance, simulation platforms, and environment, and mobile work machines as well as virtual reality (vr), augmented reality (ar) and 3d modeling and simulation. during the imeko congress years, tc17 organizes only "topical events." in 2021, tc17 organized two virtual topical events, both following the covid-19 restrictions. ismcr2021 had a theme "virtual media technologies for the post covid19 era" and the other tc17-vrise was a jointly organized event with the theme "robotics for risky interventions and environmental surveillance.” vrise stands for virtual robotics for risky interventions and environmental surveillance, the same as the theme. the papers in this special issue segment were selected from the above two events. this special issue covers a variety of topics that relate to augmented reality/virtual reality (ar/vr), tools impacted by covid-19, and 3-d printing as they relate to robotics, including key applications of robotics technology. one ar/vr paper by karen alexander and jennifer rogers entitled "standards and affordances of 21st-century digital learning: using arlem and xapi to track bodily engagement and learning in xr(vr, ar, mr)" describes digital learning using new tools. the other paper entitled "arel-augmented reality-based enriched learning experience" by a. v. geetha and t. mala shows the usage of ar in the learning process. covid-19 has impacted not only the individual researchers but also the experiments and their underlying tools and methodologies. zuzana kovarikova, frantisek duchon, andrej babinec and dusan labat in their paper entitled "digital tools in the post covid-19 age as a part of robotic system for adaptive joining of objects" described the development of the new tools while ahmed alseraidi, yukiko iwasaki, joi oh, takumi handa, vitvasinvimolmongkolpom, fumihiro kato and hiroyasu iwata in their paper, "experiment assisting system with local augmented body (easy-lab)for the post-covid19 era" presents other covid-related discussions. 3-d printers have made equipment component inventories and procurement issues less problematic. two papers, one entitled "twisted and coiled polymer muscle actuated soft 3d printed robotic hand with peltier cooler for drug delivery in medical management" by pawandeep singh matharu et al, and another entitled "igrab duo: novel 3d printed soft orthotic hand triggered by emg signals," authored by irfan zobayed et al discuss their research work on the 3d activities that relate to robotic components and application. mailto:ztaqvi@gmail.com acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 2 paper on "jelly-z: twisted and coiled polymer fishing line muscle actuated mini-jellyfish robot for environment surveillance and monitoring" by pawandeep singh matharu et al, paper entitled "disarmadillo: an open source remotely controlled platform for humanitarian demining," by emanuela cepolina, alberto parmiggiani, carlo canali, ferdinando cannella, while a third paper entitled "path planning for data collection robots" by sara olasz-szabo and istvan hermati present other robotics application research work. these symposia are forums for the exchange of recent research results and provide futuristic ideas in robotics technologies and applications. they interest a wide range of participants from government agencies, relevant international institutions, universities and research organizations, working with futuristic applications of automated vehicles. the presentation is also of interest to the media as well as the general public. we are sure the readers will find these papers useful in their professional applications. dr. zafar taqvi editor special issue acta imeko  september 2014, volume 3, number 3, 38 – 42  www.imeko.org    acta imeko | www.imeko.org  september 2014 | volume 3 | number 3 | 38  electrolytic conductivity as a quality indicator for bioethanol  steffen seitz 1 , petra spitzer 1 ,  hans  d. jensen 2 , elena orrù 3 , francesca durbiano 3   1  physikalisch‐technische bundesanstalt, bundesallee 100, 38116 braunschweig, germany   2  danish fundamental metrology ltd., matematiktorvet 307 dk‐2800 kgs. lyngby, denmark  3  istituto nazionale di ricerca metrologica, strada delle cacce 91, 10135 turin, italy      section: research paper   keywords: bioethanol; electrolytic conductivity; measurement uncertainty  citation: steffen seitz, p. spitzer, f. durbiano, h. jensen , electrolytic conductivity as a quality indicator for bioethanol, acta imeko, vol. 3, no. 3, article 9,  september 2014, identifier: imeko‐acta‐03 (2014)‐03‐09  editor: paolo carbone, university of perugia   received june 12 th , 2013; in final form may 27 th , 2014; published september 2014  copyright: © 2014 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: this work was supported by measurement science consultancy, the netherlands  corresponding author: steffen seitz, e‐mail: steffen.seitz@ptb.de    1. introduction  electrochemical characterisation of bioethanol is of interest in terms of the identification of impurities at trace levels to assess risk of corrosion and potential damage to engines. high measurement accuracy and a strict application of metrological principles in establishing traceability for these measurements is mandatory to achieve meaningful measurements. in particular, electrolytic conductivity is a quality indicator for bioethanol that is needed as an easy-to-use tool to assess the amount of impurities. substantial work is still required to underpin the traceability of this parameter in order to guarantee metrological comparability [1] of the results. comparability is a prerequisite for standardization of measurement procedures and essential for the reliability of measured material properties for engineering. moreover, an assessment of sensitivity, significance and uncertainty of these measurements is required. to establish comparability of measurement results they must be traceable to an agreed common metrological reference, which, whenever possible, should be the international system of units (si). nowadays, the result of an electrolytic conductivity measurement at the application level is linked to the conductivity value of a reference solution. typically, the conductivity value of the reference solution is measured traceable to the si by national metrology institutes by means of a primary reference measurement procedure [2]. the value indicated by a conductivity measuring system is usually adjusted by a calibration measurement, such that the actually measured resistance rref is scaled by the so called cell constant kcell to match the conductivity value ref of the reference solution: ref cell ref r k  . (1) cells, which cell constants are adjusted in this way, are referred to as secondary cells in contrast to primary cells, where the cell constant is determined by dimensional measurements [2]. the measured resistance is affected by the electric field distribution and the correlated spatial distribution of the current density within the measuring cell [3]. additionally it is affected by electrode polarisation. both effects depend on the design of the cell, the kind of solution and its ion concentration. consequently, conductivity cells of different cell abstract  we present results of the european metrology research project on the si traceability of electrolytic conductivity measurements in  bioethanol. as a first step to this aim secondary conductivity measurements have been performed to characterize reproducibility,  stability, measurement uncertainty and the significance of the measurement results. the relative standard measurement uncertainty  is of the order 0.3 %, while inter‐laboratory reproducibility is around 6.9 %. the measured conductivities of two samples from different  sources  show  a  relative  difference  of  around  30%.  these  results  show  that  conductivity  is  an  appropriate  quality  indicator  for  bioethanol. however, it also demonstrates that inter‐laboratory reproducibility has to be improved, in particular with respect to si  traceability.  acta imeko | www.imeko.org  september 2014 | volume 3 | number 3 | 39  design can provide different conductivity results for an equivalent sample, even if their cell constant is adjusted with the same reference solution. therefore the comparability of conductivity measurement results is more questionable, the further the properties of the solution under investigation deviate from those of the reference solution. it is practically not possible to provide a matrix-matched primary reference solution for any kind of solution. however, in any case the measurement uncertainty must consider the effect of matrixmismatch. concerning bioethanol, reference solutions based on ethanol are inappropriate mainly due to stability issues. aqueous kcl solutions are typically used for cell calibration [4]. it must be emphasised that the nominal conductivity value of the lowest stable aqueous kcl reference solution recommended by oiml is 140.83 ms m-1, [5], that of iupac is 140.82 ms m-1) [6] at 25°c and that of astm solution d is 14.693 ms m-1[7], while the conductivity of bioethanol is in the order of 0.1 to 0.2 ms m-1. hence, the common calibration procedure makes use of a reference solution that significantly differs in the matrix and in the conductivity value of bioethanol. as a consequence, it must be investigated, if they can nevertheless be used as reference solutions and to what extent the measurement uncertainty must be increased due to the matrix-mismatch. in particular, comparison measurements, in which cells of different design are used, could give more insight into the effect of the matrix-mismatch. currently, there exist no conductivity measurements of bioethanol, based on primary reference procedures, which could be used as a basis for providing traceability of measurement results at the application level. therefore, a work package has been established within the european metrology research project eng09 [8, 9] that covers among others two main objectives, related to the use of electrolytic conductivity as an important ‘quality indicator’ for bioethanol: (i) research into the measurement of electrolytic conductivity from the primary level to the application level in order to establish si traceability and (ii) to provide exemplary reference data of bioethanol. as a first step to establish traceability, the conductivities of two bioethanol samples from different origins, one from brazil and one from a german producer, were measured with a secondary conductivity measurement cell. the cell constant was determined after calibration with a glycerol based kcl solution, which conductivity was in the conductivity range of bioethanol. a method, which has recently been investigated by the authors [10], has been used to determine the solution resistance from impedance spectroscopy measurements of the cell/solution system. this method has particularly been developed to minimize the effect of electrode polarisation on the derived solution resistance in the low conductivity range. additionally, the measurement uncertainties which particularly include contributions from stability and reproducibility have been determined from the derived solution resistances. significant differences in the conductivity values of the two different bioethanol samples have been observed. 2. measurement procedure  conductivity measurements were performed with a two electrode jones-type like cell. a sketch of the setup is shown in figure 1. the general design of the cell is similar to that described in [6], but does not have a removable centre section. two round and flat electrodes (diameter 2 cm), made of blank platinum, are arranged opposite to each other in a cylindrical body (inner diameter 2.2 cm), made of bore silicate glass. the distance between the electrodes is around 1 cm. two glass pipes are connected to the main cylinder to fill and empty the cell. the cell constant was determined with a glycerol based kcl solution at 25°c. the conductivity value ref of the reference solution has been determined with the primary conductivity measurement setup of ptb [2] to be (133.0  0.17) µs m-1. the resulting cell constant is 18.61 m-1. if not mentioned otherwise all stated uncertainties are standard uncertainties according to the “guide to the expression of uncertainty in measurement” (gum) [11]. the cell was placed in an air thermostat. temperature of the solution was measured with a calibrated pt-100 temperature sensor connected to a measurement bridge mkt50 from anton paar. the sensor is coated with ptfe and was placed in one of the filling tubes to measure the temperature directly in the solution. after the cell was filled, it took about 60 to 90 minutes until stable temperature conditions were achieved. then the temperature variation was less than 2 mk around the mean temperature. two different kinds of bioethanol samples, one from brazil (produced from sugar cane) and one from germany (produced from sugar beet), were measured. 2 l of each sample have been and finally bottled into 250 ml bore silicate bottles under an argon atmosphere that had been bubbled through ethanol in a gas washing bottle before. the measurements where performed according to the following steps: 1) the conductivity measurement cell was cleaned several times with ultra pure water and finally filled with ultra pure water. then, a bottle with the sample p t 1 0 0 ar c 2 h 6 o z( f ) figure 1. sketch of the measurement setup, using a two electrode cell that  is placed  in a temperature controlled air bath. the contact of the sample  with ambient air is minimised by pumping it into the cell. argon is used to dry the cell after cleaning it with ultra pure water.  acta imeko | www.imeko.org  september 2014 | volume 3 | number 3 | 40  and the cell were put into the air bath at 25 °c for at least 12 hours before the measurement. 2) the cell was emptied and flooded with argon for about half an hour until it was dry. using a peristaltic pump and chemical inert norprene tubes the sample was pumped into the cell until it was filled almost up to the rim of the filling tubes. finally the inlets were sealed with tape. evaporation of ethanol within the cell was not completely prevented during this filling step. however, the surface of the solution that was exposed to air or argon was small and the filling time was less than 30 s. the corresponding measurement uncertainty has been considered in terms of measurement reproducibility. 3) an impedance spectrum between 20 hz and 500 khz, 5 steps per decade, was measured and the best frequency range (see below) was chosen for the measurement. 4) afterwards impedance spectra were recorded together with temperature for more than 2 h measuring time. at the end the cell was emptied and cleaned several times with ultra pure water. 3. conductivity calculation  the determination of the resistance rsol of the solution between the electrodes is based on an analysis of impedance spectra of various low conductivity solutions measured with different cell types. the basic concept has been developed within the imera-plus european metrology research program tp2jrp10 [10]. in brief, the determination of the solution resistance is based on the equivalent circuit shown in figure 2. the corresponding impedance spectrum can be separated into two regions. the low frequency part of the spectrum is dominated by electrode polarisation, which is represented by the cpe element and the polarisation resistance rp. the latter accounts for a residual charge transfer across the electrodes. in a complex plane plot this part of the spectrum is nearly a linear line, slightly curved due to the influence of rp. in the high frequency part of the spectrum polarisation effects can be neglected and the complex plane plot in this part of the spectrum is a semicircle. concerning the cell used in this investigation the effect of electrode polarisation on the spectrum can be neglected above 10 khz for high resistive solutions like ethanol. in this region the equivalent circuit simplifies to the parallel of cg and rsol. we have chosen measurement frequencies between around 10 and 400 khz that result in fairly equidistant impedance values across the semicircle. at each given frequency the mean impedance was calculated from at least 15 measurements. these mean values were used for the semicircle fit. the solution resistance was derived from the corresponding radius r: rsol = 2r. this procedure has turned out to be more robust than calculating the solution resistance analytically from the impedances by assuming the parallel of rsol and cg. the latter way usually shows a significant dependence of the resistance on frequency resulting from small impedance measurement errors. figure 3 shows the impedances of a typical measurement of bioethanol and the corresponding semicircle fit in a complex plane plot. the average relative deviation of the measured data points from the fit is less than 0.1%. the impedance measurements were performed with a high precision commercial lcr-meter (agilent 4284a). the conductivity value sol(t) at the mean measurement temperature t, given in the unit °c, is calculated from rsol and the calibrated cell constant kcell in analogy to equation (1). the impedances are typically not measured at the exact set temperature of 25 °c, but the measurement temperature deviates about a few tens of mk. the conductivity value at the measurement temperature t is therefore linearly corrected to the value sol(25°c) at 25°c using     sol sol(25 c) / 1 25 ct t       . (2) for bioethanol a linear relative temperature coefficient be = (2.0  0.15)% c-1 at 25°c has been determined from conductivity measurements between 20°c and 27°c. the linear temperature coefficient ref of the reference solution is 5.09% °c-1 at 25°c. using equations (1) and (2) the final conductivity value be(25°c) of a bioethanol sample has been calculated from the input variables: ref ref ref ref be be be be (25 ) (1 ( 25 ) (25 c) (1 ( 25 c)) c r t c r t                . (3) in equation (3) the index “ref” refers to the reference solution and the index “be” to bioethanol. 0 20 40 60 80 0 50 100 150 real(z ) (k) im ag ( z ) (k  ) figure 3. impedances of a bioethanol sample in a complex plane plot. the  dots  are  the  measured  impedances  z,  the  solid  line  is  a  semi  circle  fit.  frequency range is from 10 to 400 khz.  figure  2.  equivalent  circuit  used  to  model  the  cell  solution  system  to derive the solution resistance rsol. electrode polarisation is represented by the cpe element and the polarisation resistance rp.   cg  is the geometric capacitance of the electrodes.  acta imeko | www.imeko.org  september 2014 | volume 3 | number 3 | 41  4. results  the measured conductivity values of the two bioethanol samples are brazil sample: (108.37 +/-0.33) µs m-1, german sample: (142.67 +/0.43) µs m-1. the values are significantly larger compared to pure synthetic ethanol, which has a conductivity of a few µs m-1. additionally, the difference of the results is much larger than their uncertainties. consequently, conductivity measurements can well serve to characterise bioethanol samples. there are no details available about residual ion concentrations or the production conditions of the samples. so it is difficult to reason the difference. using ion chromatography, we have performed a first analysis of one of the samples. the anionic ion chromatogram (not calibrated) showed a significant and predominant amount of chloride compared to a measurement of pure synthetic ethanol. therefore the measured difference of the conductivity values could be the result of residual dissolved chloride salts. however, this assumption still needs to be verified quantitatively. figure 4 demonstrates the stability of the measurement results after temperature equilibrium has been reached. the error bars indicate the expanded measurement uncertainty (coverage factor 2). an unspecific, small drift can be seen. the reason for it is not clear. however, the drift within the measurement period is considered in the stated measurement uncertainty. table 1 identifies the main sources of uncertainty (first column) and their estimated standard uncertainties (second column) exemplarily for the bioethanol sample from germany. note that resistance and temperature uncertainties considered systematic and statistical contributions. inaccuracies of the measuring devices and, in case of the resistances, of the method to derive them, entered into the systematic contributions. it should also be noted that the systematic uncertainties of the solution resistances have been calculated with a monte carlo method [12], since it is practically impossible to use the analytical gum framework to handle the complex-valued impedances and the fitting procedures involved in resistance calculation. the statistical contributions reflect the measurement stability and were calculated from the standard deviation of the mean of the measured values. uncertainty propagation has then been calculated straight forward from equation (3) according to the general gum uncertainty framework [11]. the last column shows the relative uncertainty contributions of the input variables to the uncertainty of the conductivity value. the main contributions to the measurement uncertainty result from the conductivity of the reference solution and the repeatability of the measurement results. the latter has been determined from independent measurements of four samples that have been homogenised and afterwards bottled as described above. the observed variation of the values within a relative standard deviation of 0.27% is probably due to the instability of the measurement shown in figure 4. the uncertainty calculation also accounted for correlations between the input quantities: 107.5 108.0 108.5 109.0 109.5 50 100 150 measuring time (min) co n d u ct iv it y ( µ s m -1 )   141.5 142.0 142.5 143.0 143.5 144.0 80 100 120 140 measuring time (min) co n d u ct iv it y ( µ s m -1 ) figure  4.  stability  of  the  conductivity  measurement  results  of  bioethanol from  brazil  (above)  and  germany  (below).  the  error  bars  indicate  the expanded (k = 2) uncertainty.  table  1. contributions  to  the  combined  measurement  uncertainty  of  the  conductivity  value  be,  exemplarily  for  the  bioethanol  sample  from  germany.  uxibe)  is  the  propagated  uncertainty  contribution  of  xi  to  the  uncertainty of be.  source of uncertainty of input  quantity xi  uncertainty  u(xi)  uxi(be)/be  (%)  conductivity of reference  solution  0.17 µs m ‐1    temperature of reference  solution (systematic)  10 mk  0.051  temperature stability of  reference solution   0.4 mk  0.002  temperature of bioethanol  (systematic)  10 mk  0.020  temperature stability of bioethanol  1.6 mk  0.002  resistance of reference  solution (systematic)  124   0.099  resistance stability of reference solution  2.3   0.002  resistance of bioethanol (systematic)  149   0.114  resistance stability of  bioethanol  2.9   0.022  repeatability  0.27 %  0.27  acta imeko | www.imeko.org  september 2014 | volume 3 | number 3 | 42  (i) rref and rbe values with respect to the systematic uncertainty contributions, (ii) temperature measurement results of calibration measurement and bioethanol measurement with respect to the systematic uncertainty contributions, (iii) temperature values and temperature corrected conductivity values (corresponding resistance values, respectively), which are measured at the same time. for (i) and (ii) a correlation coefficient of one has been assumed, since all the measurements have been performed with the same system, using the same evaluation method. any systematic measurement error in (i) or (ii) due to an offset is therefore nearly equal in the measurement of the reference solution and the solution under investigation. scaling effects have been neglected, since the measurement results are of similar magnitude. as a consequence, although the relative uncertainties of the measured resistances are compatible to that of the conductivity reference value and to that attributed to repeatability, they barely contribute to the combined uncertainty of the conductivity value. for (iii) the correlation coefficient has been statistically calculated from temperature corrected resistance values and the corresponding temperatures, in order to account for correlations that are not covered by the linear temperature correction. here the correlation coefficient is typically around -0.5 to -0.7. the described measurements have been performed at the physikalisch-technische bundesanstalt (ptb) and reflect the characteristics of the setup used there. in order to estimate inter laboratory reproducibility a conductivity comparison measurement of bioethanol was performed, including the german (ptb), the danish (danish fundamental metrology) and the italian (istituto nazionale di ricerca metrologica) metrology institutes. all participants used secondary cells for the measurements. two institutes calibrated the cells with glycerol based kcl solutions, and one institute used a water based kcl solution. the relative standard deviation of the results was 6.9%, which is significantly larger than the individually reported standard measurement uncertainties (0.3% to 1%). this cannot be explained by inhomogeneity of the samples. it is more likely due to differences in sample handling, in cell design, in measurement and data evaluation procedure. the comparison will be repeated with more institutes and a more detailed sample handling and measurement instruction. however, the result of the comparison gives an upper limit for inter laboratory reproducibility of conductivity measurements of bioethanol, even though it is, for the time being, rather large compared to typical conductivity measurements. 5. conclusion  the results indicate that conductivity measurements can well serve to measure differences in the composition of bioethanol samples. under laboratory conditions the combined relative standard uncertainty of such measurements is around 0.3%. this particularly includes contributions from the stability of the solution during the measurement and the repeatability of the measurement results (at a single institute). however, the measured conductivities have been related to the conductivity value of the glycerol based kcl solution that has been used to adjust the cell constant. the measured cell constant of a secondary cell depends on the matrix of the reference solution. therefore the matrix-mismatch of bioethanol and the reference solution cast doubts on the comparability of the measured values, if these are measured using a different cell type. this assessment is also supported by the relatively bad inter laboratory reproducibility of 6.9%. in other words measurements of the same solutions using another cell type could provide different conductivity values, even if the cell constant is determined with the same reference solution. however, getting consistent, i.e. comparable, measurement results is a prerequisite for any standardisation work and a reliable data base for engineering. therefore further work is needed to achieve this aim. the next steps will be to investigate conductivity measurements of bioethanol on the primary level and to perform further comparison measurements to investigate the effect of different designs of secondary cells on the measured values. acknowledgement  the research leading to these results has received funding from the european union on the basis of decision no 912/2009/ec. references  [1] international vocabulary of metrology, iso/iec, 2012. [2] f. brinkmann, n. e. dam, e. deák, f. durbiano, e. ferrara, j. fükö, h. d. jensen, m. máriássy, r. h. shreiner, p. spitzer, u. sudmeier, m. surdul. vyskocil, primary methods for the measurement of electrolytic conductivity, accred qual assur 8 (2003), pp. 346-353. [3] s. l. schiefelbein, n. a. fried, k. g. rhoadsd. r. sadoway, a high-accuracy, calibration-free technique for measuring the electrical conductivity of liquids, rev. sci. instrum. 69 (1998), pp. 3308-3313. [4] j. barthel, f. feuerlein, r. neuederr. wachter, calibration of conductance cells at various temperatures, journal of solution chemistry 9 (1980), pp. 209-219. [5] standard solutions reproducing the conductivity of electrolytes, oiml, 1981. [6] k. w. pratt, w. f. koch, y. c. wup. a. berezansky, molalitybased primary standards of electrolytic conductivity, pure appl. chem. 73 (2001), pp. 1783-1793. [7] standard test methods for the electrical conductivity and resistivity of water, astm-international, 1995. [8] european metrology research project. available: http://www.emrponline.eu/ [9] publishable jrp summary: emrp jrpeng09 (2009). available: http://www.euramet.org/fileadmin/docs/emrp/jrp /jrp_summaries_2009/eng09_publishable_jrp_summary.pdf. [10] publishable jrp summary: emrp t2 jrp10 tracebioactivity (2007). available: http://www.euramet.org/fileadmin/docs/ emrp/jrp/imera-plus_jrps_2010-06-22/t2.j10_tracebi oavtivity_publishable_summary_april10_v1.pdf [11] guide to the expression of uncertainty in measurement, jcgm, 2008. [12] guide to the expression of uncertainty in measurement, supplement 1: propagation of distributions using a monte carlo method, jcgm, 2008. a comparison between aeroacoustic source mapping techniques for characterisation of wind turbine blade models with microphone arrays acta imeko issn: 2221-870x december 2021, volume 10, number 4, 147 154 acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 147 a comparison between aeroacoustic source mapping techniques for the characterisation of wind turbine blade models with microphone arrays gianmarco battista1, marcello vanali2, paolo chiariotti3, paolo castellini1 1 università politecnica delle marche, via brecce bianche, 60121 ancona, italy 2 università di parma, parco area delle scienze, 43124 parma, italy 3 politenico di milano, via giuseppe la masa, 20156 milano, italy section: research paper keywords: aeroacoustic measurements; microphone array measurements; wind turbines; acoustic source identification citation: gianmarco battista, marcello vanali, paolo chiariotti, paolo castellini, a comparison between aeroacoustic source mapping techniques for characterisation of wind turbine blade models with microphone arrays, acta imeko, vol. 10, no. 4, article 24, december 2021, identifier: imeko-acta10 (2021)-04-24 section editor: francesco lamonaca, university of calabria, italy received july 26, 2021; in final form september 30, 2021; published december 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: gianmarco battista, e-mail: g.battista@staff.univpm.it 1. introduction the growing interest in renewable energy sources is requesting advances on several disciplines in order to reduce technological barriers and improve energy conversion efficiency. one of the mainstream technologies is wind power. it is well evident that the worldwide installed capacity of wind energy assets is growing exponentially since early 2000's. on the one hand, this sector is grabbing the attention of many industries and research groups, on the other hand, it is going through increasing regulations. in fact, one of the critical aspects of wind turbines is noise pollution. in order to mitigate wind turbine blade noise, the identification of location and strength of aeroacoustic noise sources is mandatory. this knowledge makes it possible to improve blade profiles and design effective aerodynamic appendages, such as trailing-edge serrations. in this work, acoustic imaging techniques, based on microphone arrays [1], have been used to characterise a scale single-blade rotor, installed in a semi-anechoic chamber, in different operating conditions. the requirements of a mapping technique for this application are: • the ability to deal with rotating sources; • sufficient spatial resolution with respect to the model size to distinguish different sources on the blade; • sufficient dynamic range to identify also weak sources with respect to the strongest one. a first classification of acoustic imaging methods can be done distinguishing time domain and frequency domain approaches. time domain approaches [2] are typically used for selectively shaping and steering the directivity of the array (e.g., directivemicrophone like behaviour). these methods can be used when abstract characterising the aeroacoustic noise sources generated by a rotating wind turbine blade provides useful information for tackling noise reduction of this mechanical system. in this context, microphone array measurements and acoustic source mapping techniques are powerful tools for the identification of aeroacoustic noise sources. this paper discusses a series of acoustic mapping strategies that can be exploited in this kind of applications. a single-blade rotor was tested in a semi-anechoic chamber using a circular microphone array. the virtual rotating array (vra) approach, which transforms the signals acquired by the physical static array into signals of virtual microphones synchronously rotating with the blade, hence ensuring noise-source stationarity, was used to enable the use of frequency domain acoustic mapping techniques. a comparison among three different acoustic mapping methods is presented: conventional beamforming, clean-sc and covariance matrix fitting based on iterative re-weighted least squares and bayesian approach. the latter demonstrated to provide the best results for the application and made it possible a detailed characterization of the noise sources generated by the rotating blade at different operating conditions. mailto:g.battista@staff.univpm.it acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 148 the sources of interest are time-variant, both in terms of position and emitted noise. [3]. frequency domain approaches are more used for characterizing the acoustic sources in terms of location and strength. most of frequency domain techniques make use of the cross-spectral matrix (csm) estimation from microphone signals as input data. therefore, the effect of incoherent background noise on acoustic maps can be attenuated by means of the averaging process, moreover, some methods can handle the removal of the csm main diagonal to neglect the contribution of incoherent noise across all the array microphones (e.g., wind noise). conventional beamforming (cb) [1] is the most widespread frequency domain mapping technique due to its robustness and low computational cost. however, it suffers of some limitations in terms of dynamic range and spatial resolution. in literature [1][4], several advanced frequency domain mapping techniques are available that go beyond cb limitations and make it possible also the quantification of the noise source. advanced frequency domain mapping techniques are generally preferred to time domain approaches in aeroacoustic applications since they generate acoustic maps with high dynamics and fine spatial resolution [4]. in frequency domain, three categories of mapping techniques can be identified depending on how the region of interest (roi) is mapped using pressure data at microphones, i.e., how the inverse operator is defined: beamforming, deconvolution, and inverse methods. the basics of different approaches for defining inverse operators is provided in the next section, while the detailed review is provided by leclère et al. [5]. among beamforming techniques, it is worth noticing functional beamforming [6], that is a variant of cb. this simple method enhances performance and flexibility of cb in terms of resolution and dynamics of maps. however, it is not compatible with the diagonal removal, therefore, it is not very effective in presence of a relevant background noise. one of the most recognised deconvolution methods is damas (deconvolution approach for the mapping of acoustic sources) [7]. this method aims at retrieving actual source distribution that generated the cb map by solving an inverse problem. the results achievable with damas are generally suitable for all demanding applications, however, the computational effort is quite high, since it requires the calculation of the array point spread function (psf), for all candidate sources in the roi, and finding a solution of an inverse problem. the deconvolution method named clean-sc (clean based on source coherence) [8] is currently the state of the art as aeroacoustic applications are concerned, since it has a low computational cost (just slightly higher than cb) and it is very effective in generating maps with high dynamics. the main drawback of clean-sc is the spatial resolution in separating sources close together, since it has the same limitations of cb. lastly, inverse methods such as generalized inverse beamforming [9], bayesian approach to sound source reconstruction [21], equivalent source method [10] and covariance matrix fitting (cmf) [11] aims at retrieving the complete source map at once, thus being capable of accounting for interaction between sources. however, dealing with underdetermined and ill-posed problems, they require reliable regularization techniques (e.g. empirical bayesian regularization [12]). the application of frequency domain approaches requires a stationary acoustic field that is not the case of a rotating source viewed by a static array. the virtual rotating array (vra) approach [13] has been adopted in this work to fulfil the requirement of a static source field and enable the application of any frequency domain mapping technique. three methods have been chosen for this test case: cb as baseline, clean-sc since it is a reference technique for aeroacoustic source mapping and cmf based on iterative reweighted least squares and bayesian approach (cmf-irls) [14]. a comparison between results obtained with frequency domain techniques is performed. finally, the characterisation of the wind turbine blade model at different operating speed with cmf-irls is provided. 2. theoretical backgrounds of acoustic source mapping the direct and the inverse acoustic problem formulation can be described as a linear problem, as frequency domain approaches are concerned. consider a set of 𝑁 elementary sources (monopoles, dipoles etc.) whose complex coefficients are collected in the vector 𝒒, and a set of 𝑀 receiver locations where the acoustic pressure is evaluated and collected in the complex vector 𝒑. the discrete acoustic propagator 𝑮 is a complex 𝑀-by-𝑁 matrix that encodes the amplitude and phase relationships between the sources and the receivers, for a given frequency. the direct acoustic problem regards the calculation of pressures at receiver location (effects), given the source strengths (causes) and the acoustic propagator: 𝑮 𝒒 = 𝒑 . (1) this is a mathematically well determined problem with a unique solution. conversely, the calculation of source strengths (causes) from observed pressures (effects), for given 𝑮, represents the inverse acoustic problem. solving this problem is not trivial, due to its ill-posed nature. in fact, the existence, the uniqueness, and the stability of the solution are not guaranteed [15]. the solution of inverse problems can be expressed as: �̂� = 𝑯 𝒑 , (2) where, 𝑯 is the inverse operator, that can assume different forms depending on the chosen approach. it is then clear that the source strengths �̂�(𝑯) can be only estimated. moreover, this estimation strongly depends on a priori assumptions made with respect to the acoustic propagator and the pressure data 𝒑 measured. both direct and inverse problems can also be written in their quadratic form: 𝑮 𝑸 𝑮𝐻 = 𝑷 (3) �̂� = 𝑯 𝑷 𝑯𝐻 , (4) where the superscript ∙ 𝐻 denotes the conjugate transpose operator, 𝑸 and 𝑷 are source and pressure cross-spectral-matrix (csm) respectively. beamforming approaches solve a scalar inverse problem, i.e. each potential source strength in the region of interest is estimated independently from the others. beamforming inverse operators 𝒉𝑛 , named steering-vectors, are calculated as function of the direct propagator columns 𝒈𝑛 and act as spatial filter, whose properties depend on their formulation [16]. beamforming techniques are widely used for its simplicity and robustness. however, sound source quantification is not possible, unless dedicated integration techniques are applied [17]. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 149 deconvolution methods have been developed to overcome beamforming limitations, since array spatial response, i.e. the point spread function (psf), affects the acoustic map retuned by beamformers. deconvolution methods aim at retrieving the actual source distribution that generated the beamforming map removing the psf effect. in opposition to beamforming approach, inverse methods aim at estimating all potential sources together, hence, accounting also for interaction between sources. frequently, this entails the solution of heavily under-determined problems, since the number of microphones (equations) is much lower than the number of potential sources in the region of interest (unknowns). another issue with inverse methods is to adopt a robust regularization mechanism that is capable to estimate the proper amount of regularization depending on the specific problem and the measured data. also, the choice of the roi and its discretization with elementary sources plays a crucial role in retrieving an accurate solution. although the complexity of these methods, the advantages justify their application. 3. materials and methods the object of study of this paper is a scale blade model, having a radius of 1.5 m. the design of this model is targeted to small wind turbines, that are defined by the standard iec61400 as the ones having the rotor spanning up to 200 m2 (about 16 m diameter). figure 1 shows a simplified scheme of the experimental setup installed in the semi-anechoic chamber of università politecnica delle marche. the single-blade rotor must reach 650 rpm to operate at nominal conditions in terms of aerodynamic angle of attack and wind speed at tip. the latter corresponds to 102 m/s or 367.2 kph which is typical for small wind turbines. the single-blade rotor is placed at a distance of 3 m in front of the circular microphone array, which has 40 equally spaced 1/4'' microphones (b&k array microphone type 4951) installed on a circumference of 𝐷 = 3 m diameter. the centre of the array is aligned with the rotation axis and the microphone plane is parallel to the blade rotation plane. an asynchronous electric motor, controlled by an inverter, drives the single-blade rotor at desired angular speed. the motor has been also equipped with an incremental encoder for measuring the angular position of the blade. all sensor signals have been synchronously acquired at 102.4 ksamples/s for each channel (microphones and encoder). each acquisition lasts for 7.5 s and is performed at constant angular speed of the rotor. realisation of acoustic maps for moving sources usually requires time domain beamforming, since the distances from microphones to focus points constantly change over time, thus requiring time-depending delays for focusing the array. however, aeroacoustic applications require maps with high dynamic range and fine spatial resolution which is achievable only with more sophisticated frequency domain approaches. most of them use the microphone csm as input data to provide the acoustic map. when dealing with stochastic signals, such as aeroacoustic noise, the cross-spectra must be averaged over several time snapshots to get meaningful spectral estimation and to reduce the effect of uncorrelated background noise. the averaging process requires the sources to be stationary in time and space with respect to the array. however, pressure signals of the rotating blade, acquired with the static circular array does not fulfil these conditions, in fact, the source-microphone relative position changes over time. in order to face this aspect, the virtual rotating array (vra) [13] approach has been adopted to turn sound pressure signals, recorded with the static physical array, into signals of a virtual array that is rotating synchronously to the blade. in this way, the blade appears in a fixed position with respect to the vra, thus making it possible to adopt any frequency domain imaging technique. the simplest realization of vra requires a circular array, that must be parallel and co-axial with the rotor, and the knowledge of the instantaneous angular position of the blade. when a rotating array is used (both physical and virtual), the medium does not rotate at the same speed, therefore, it appears to rotate from the perspective of vra. the acoustic propagator assumed for calculations of acoustic maps must consider the propagation of acoustic waves through a rotating flow field to obtain meaningful results. 3.1. virtual rotating array the working principle of vra relies on the transformation of the pressure signals 𝑝𝑚 (𝑡) recorded by the physical array into pressure signals 𝑝𝑚𝑣 (𝑡) as if they were recorded by microphones virtually rotating. the virtual array has the same layout of the physical one, thus having 𝑀 = 40 microphones equally spaced along the circumference. the position of a virtual microphone, rotating on the same circumference of the physical array, does not correspond to the position of any physical microphone most of the time, but its signal can be estimated by means of spatial interpolation of signals recorded on the array circumference. the instantaneous position of each virtual microphone is determined from the angular position of the rotor 𝜙(𝑡). calculation of pressure value for each sample of a virtual microphone signal requires the identification of which pair of physical adjacent sensors must be selected for the interpolation. these are identified by the indexes 𝑚𝑙 and 𝑚𝑢, that are function of time 𝑡 and the virtual microphone index 𝑚𝑣: 𝑚𝑙 (𝑚𝑣, 𝑡) = ⌊𝑚𝑣 + 𝜙(𝑡) 𝛼 − 1⌋ mod 𝑀 + 1 𝑚𝑢(𝑚𝑣, 𝑡) = ⌊𝑚𝑣 + 𝜙(𝑡) 𝛼 ⌋ mod 𝑀 + 1, (5) where, ⌊∙⌋ is the floor function and 𝛼 = 2 π 𝑀⁄ is the angular spacing between sensors. once selected the pair of microphones for spatial interpolation, for each 𝑡 and 𝑚𝑣, the value of virtual signal sample 𝑝𝑚𝑣 (𝑡) is calculated as the weighted sum of the samples of physical microphones figure 1. scheme of the set-up in the anechoic chamber. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 150 𝑝𝑚𝑣 (𝑡) = 𝑝𝑚𝑙 𝑠𝑢 + 𝑝𝑚𝑢 𝑠𝑢 , (6) being the weights determined as 𝑠𝑢 (𝑡) = 𝜙(𝑡) 𝛼 − ⌊ 𝜙(𝑡) 𝛼 ⌋ 𝑠𝑙 (𝑡) = 1 − 𝑠𝑢 (𝑡) . (7) the signals 𝑝𝑚𝑣 (𝑡) obtained with this procedure can be used to estimate the microphone csm. 3.2. processing techniques applied to the case of study the instantaneous angular position of the rotor 𝜙(𝑡) is retrieved from the encoder signal and it is used to calculate vra time histories. the region of interest has been defined as a rectangular area of 1.80 x 0.90 m, positioned on the rotor plane, i.e. at 3 m from the array plane. from vra point of view, this area contains the blade and the hub of the rotor. the rectangular area has been discretised with a grid of monopoles with 0.02 m step, thus having 4186 potential sources. the acoustic propagator from monopoles to microphones is modelled with pressure to pressure free-field propagator [14]. geometric linear distances are replaced by the actual propagation distances, calculated considering the rotating flow field. for this purpose, the angular velocity can be assumed constant during each measurement since the fluctuations are negligible with respect to the mean value. propagation distances have been calculated with the acoular software [18]. as stated in the introduction, three frequency domain acoustic mapping techniques are applied to the case of study, exploiting the vra signals: cb, clean-sc and cmf-irls. the application of cb is intended here as the baseline performance of acoustic imaging techniques. the deconvolution with clean-sc requires to choose only a single parameter, the loop gain, which is set here to 𝜑 = 0.6. lastly, cmf-irls, which belongs to the branch of inverse methods, is chosen since its fully capable to deal with spatially extended sources which is a source configuration highly and naturally expected in this application. the cmf-irls is used to map the full csm, without any decomposition, and the sparsity constraint on the solution is enforced by setting 𝑝 = 1. a priori information is injected in the irls procedure as form of spatial weighting (named "aperture function" in bayesian approach). the first weighting function is the cb map that eases the localisation task, since it is a rough but reliable information on source distribution. the second weighting strategy is adopted to avoid high level peaks on the map at the edge of the roi, typical with inverse methods. figure 2 depicts the weighting function, determined with the cosine function near all the edges, thus resulting in a cosine-tapered spatial window. point by point product of these two weighting functions is used as total pre-weighting. for all mapping techniques, the diagonal of csm is removed, following the common practice adopted in aeroacoustic source imaging to minimise the effect of background noise. 3.3. considerations on measurement uncertainty a reference metrological analysis of acoustic beamforming measurements has been conducted by castellini and martarelli in [19], where a type b approach, based on an analytical model, was adopted to assess how the uncertainty on input quantities affects the localisation and quantification uncertainties. instead, merinomartínez et al. [17] investigated the accuracy of different mapping methods in aeroacoustic applications. in this paper, the focus is on the identification of sources of noise, rather than the absolute level. therefore, acoustic maps must provide an estimation of source locations and their relative level. from this consideration, the target accuracy of the measurement procedure should be sufficient to distinguish different noise sources on the blade. the resolution of mapping grid has been chosen to be compatible with the blade size and guarantee at least 2.5 potential sources for each wavelength, fulfilling the guideline provided in [20] for inverse problems (cmf-irls). the smallest wavelength of analysis is about 0.076 m (the exact value depends on the actual speed of sound). as regards beamforming-based techniques, the steering vector formulation chosen provides the correct source location at the expense of source level (“formulation iv” described in [16]). in array measurements, one of the most important aspects is the uniformity of frequency response of all microphones, rather than the absolute quality of the sensors. in fact, the array microphones b&k type 4951 are specifically designed for array applications, since they are phase-matched. the nominal sensitivity of this type of microphones is 6.3 mv/pa (ref. 250 hz). all microphones were calibrated with the pistonphone b&k type 4228, which provides a sine wave at 251.2 hz ± 0.1 % and 124.0 ± 0.2 db spl, hence the individual sensitivity was measured for each sensor. field-calibration was performed before starting the measurement campaign. the technical specifications for free-field frequency response (ref. 250 hz) are: ± 1 db, from 100 hz to 3 khz, ± 3 db, from 3 khz to 10 khz. as regard the phase-matching, the manufacturer guarantees the following specifications with respect to a factory reference: ± 3°, from 100 hz to 3 khz, ± 5°, from 3 khz to 5 khz. the relative positions between the microphones in the array represents another source of uncertainty, which affects amplitude and phase of measured pressure. in fact, a mismatch between nominal and actual sensor locations induces an error (for each microphone) in the spatial sampling of the acoustic field, in terms of amplitude and phase. this has an influence on the quality of the maps since the nominal layout is used for calculations. however, the uncertainty on amplitude and phase, caused by frequency responses and sensor arrangement, can be considered random across the sensors, hence it can be assumed as spatial white noise. it is possible to consider this as a sort of “array noise”, which is averaged across the microphones and the mean value tends to zero, as the number of sensors increases. therefore, the source levels in the map are not significantly affected from the array noise. instead, the standard deviation figure 2. arbitrary weighting function for cmf-irls with respect to the blade geometry. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 151 quantifies the level of the array noise. an accurate estimation of this parameter is rather difficult in practice, but the overall effects can be assessed comparing the degradation of array spatial response and dynamics of cb map with respect to the ideal condition. an experimental test was conducted with a point source emitting a sine wave at 2 khz and positioned on the rotor hub (aligned with the rotation axis). a sine wave is used to have a high signal-to-noise ratio with respect to environmental acoustic background noise. the cb map obtained from this experiment is compared with the map obtained in ideal conditions with simulated data. a visual inspection of the maps reveals if the degradation of performance, due to the microphone response and sensor positioning uncertainty, is acceptable or not. performance degradation occurring in this setup does not significantly affects the acoustic maps and is in line with the accuracy requirements. similar tests were conducted for assessing the rotor-array relative position and alignment. the rotor-array positioning is important to fulfil the stationarity of rotating sources with respect to the vra. in addition to the test with the point source on the rotor hub, other four tests were done with the same source, but placed on the blade tip. in these tests, the rotor is placed at different angles (steps of 90°), still using a 2 khz sine wave. the rotor-array alignment was done with typical distance sensors, then cb maps were used to verify and correct it. from acoustic maps, the position of the test point sources is retrieved to estimate the offset and the angle between rotor and array axes. from the test with the source placed on the hub, the offset is estimated, which results to be in the same order of magnitude of grid resolution, in both horizontal and vertical directions. the other four tests were used to estimate the misalignment. the least square fitting plane is calculated from the four source positions, then the scalar product between the normal of the array plane and the normal of the rotor plane is calculated to estimate the angle between the two axes, which results to be less than 3°. a final test with the same point source, placed at a radius of 0.6 m and the blade rotating at 100 rpm, was conducted to assess the correctness of all operations needed to map a rotating source via the vra method. all mapping algorithms were able to correctly locate the point test source on the rotating blade using vra signals. the last important aspect is the estimation of the actual speed of sound to use for the calculation of the propagator and the steering vectors. with this purpose, two measures of air temperature inside the chamber were taken during microphone recordings, one at the beginning and one at the end of each acquisition. the model adopted for indirect measurement of the speed of sound 𝑐 is 𝑐 = 331.3 √1 + 𝑇 273.15 k , (8) where, 𝑇 is the average of initial and final air temperatures. the digital air temperature sensor has a resolution of 0.1 °c, which is sufficient for the accuracy requested in this case. despite the importance of this parameter for the quality of the maps, little attention is often paid to this parameter [19]. in the whole test campaign, 𝑐 is always about 342 m/s. 4. results the analysis has been performed from 700 hz to 4.5 khz. this band approximately corresponds to helmholtz number range 6 40 (𝐻𝑒 = 𝐷 𝜆⁄ ) in which the array provides adequate results. below this range the array has very poor performance. the benchmarking of mapping techniques has been performed on the measurement acquired at the nominal operating condition of the blade, i.e. 650 rpm. for clarity of acoustic maps, the frequency range of analysis is split in two bands, where the noise generation is located in different parts of the blade. figure 3 shows cb maps, which are characterised by blurred sources and low dynamics. these effects, caused by the array psf, make it difficult to identify weaker sources, since they are covered by the sidelobes of the main sources. maps obtained with clean-sc and cmf-irls are showed respectively in figure 4 and figure 5. both methods make it possible a better localisation and they also reveal weaker sources on the blade. in fact, as expected, these methods provide higher performance in terms of spatial resolution and dynamics of acoustic maps with respect to cb. in addition, they also provide quantitative information. the advantages of clean-sc are the robustness of results, the dynamics of the maps and the low computation time. however, due to the nature of the algorithm, clean-sc does not make it possible to fully represent extended sources [10]. this drawback is partially mitigated choosing a loop gain 𝜑 < 1, as in this case, however, the limitation still holds. instead, cmf-irls is capable to reveal the correct spatial extension of acoustic sources, but it is computationally more demanding. despite, it is an inverse method, the maps returned by cmfirls do not have any source located at the edges of the mapping plane because of the pre-weighting strategy adopted. another aspect to notice is the different source distribution returned in the low frequency range by clean-sc and cmf-irls. figure 4 (left) depicts the strongest source in between the leading edge and the trailing edge, while figure 5 (left) shows well separated sources. this is caused by the localisation mechanism adopted by clean-sc, that relies on the spatial resolution of cb. in fact, figure 3. cb 650 rpm. left: 700-2500 hz. right: 2500-4500 hz. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 152 the clean-sc algorithm establish that the source position is detected picking up the maximum location of the so called "dirty map", which is the output of cb at the current iteration of clean-sc procedure. when two sources are closer than the mainlobe width, considering the psf of the array at the frequency of analysis, the maximum of the total map lays somewhere in between the real sources, depending on their relative strength. this problem does not occur with cmf-irls, being it an inverse method, which considers all potential sources at once. since cmf-irls demonstrated to be the best performing among the methods compared in this work, it has been used for characterising the noise emission of the blade at different operating conditions. two additional rotation speed of the rotor are tested: 500 and 350 rpm. figure 6 and figure 7 shows similar structures of source distribution with respect to the nominal working condition. for the lower frequency band, the noise sources are mainly in the last 0.5 m, in the radial direction, and are quite aligned with the leading and trailing edges. for the higher band, the noise is mostly located at the tip of the blade; a figure 4. clean-sc650 rpm. left: 700-2500 hz. right: 2500-4500 hz. figure 5. cmf-irls 650 rpm. left: 700-2500 hz. right: 2500-4500 hz. figure 6. cmf-irls 500 rpm. left: 700-2500 hz. right: 2500-4500 hz. figure 7. cmf-irls350 rpm. left: 700-2500 hz. right: 2500-4500 hz. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 153 source is identified at the tip location and the other two are located almost symmetrically with respect to the tip. also some high frequency noise of the hub is visible. the extent and the level of sources decrease proportionally to the angular velocity. in order to have an overview of how the source distribution changes with respect to frequency, a synthetic visualisation is depicted in figure 8. this view results from the integration of the acoustic map along the chord-wise direction (𝑦 direction), therefore, it shows how the reconstructed source distribution changes with respect to frequency and radial direction (𝑥). 5. conclusions a measurement campaign has been conducted in a semianechoic chamber on a single blade rotor, for its aeroacoustic characterisation with acoustic imaging techniques exploiting microphone arrays. the strategy of vra makes it possible to use those advanced frequency domain approaches for acoustic source mapping, that are typically required in aeroacoustic applications. the benchmarking among three methods demonstrated the advantages of cmf-irls versus cb and clean-sc. the performance of cmf-irls is adequate for a detailed characterisation of the acoustic source distribution generated by the wind turbine blade model in different operating conditions. therefore, this measurement technique is a powerful tool for improving the design of wind turbine blade models, since it is capable to identify aeroacoustic noise sources with high dynamic range and spatial resolution. however, the applicability is limited to models of limited size since vra requires the array and the rotor to be aligned and co-axial. acknowledgement the authors would like to thank prof. renato ricci of università politecnica delle marche for providing the wind turbine blade model used in the measurement campaign and for the useful discussions on commenting the aeroacoustic results. references [1] p. chiariotti, m. martarelli, p. castellini, acoustic beamforming for noise source localization: reviews, methodology and applications, mechanical systems and signal processing, vol. 120, 2019, pp. 422–448. doi: 10.1016/j.ymssp.2018.09.019 [2] r. dougherty, advanced time-domain beamforming techniques, 10th aiaa/ceas aeroacoustics conference, american institute of aeronautics and astronautics, 2004. doi: 10.2514/6.2004-2955 [3] p. sijtsma, s. oerlemans, location of rotating sources by phased array measurements, 7th aiaa/ceas aeroacoustics conference and exhibit, american institute of aeronautics and astronautics, 2001. doi: 10.2514/6.2001-2167 [4] r. merino-martínez, p. sijtsma, m. snellen, t. ahlefeldt, j. antoni, c. j. bahr, d. blacodon, d. ernst, a. finez, s. funke, t. f. geyer, s. haxter, g. herold, x. huang, w. m. humphreys, q. leclère, a. malgoezar, u. michel, t. padois, a. pereira, c. picard, e. sarradj, h. siller, d. g. simons, c. spehr, a review of acoustic imaging methods using phased microphone arrays, ceas aeronautical journal, vol. 10, no. 1, mar. 2019, art. no. 1, doi: 10.1007/s13272-019-00383-4 [5] q. leclère, a. pereira, c. bailly, j. antoni, c. picard, a unified formalism for acoustic imaging based on microphone array measurements, international journal of aeroacoustics, vol. 16, no. 4-5, jul. 2017, art. no. 4–5. doi: 10.1177/1475472x17718883 figure 8. chord-wise integrated maps versus frequency, cmf-irls. the tip and the root of the blade are represented by the horizontal black lines, while the vertical lines represent the centre of 1/3 octave bands. top left: 650 rpm. top right: 500 rpm. bottom: 350 rpm. all maps have the same colorscale. https://doi.org/10.1016/j.ymssp.2018.09.019 https://doi.org/10.2514/6.2004-2955 https://doi.org/10.2514/6.2001-2167 https://doi.org/10.1007/s13272-019-00383-4 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spatially-distributed acoustic sources. journal of sound and vibration, vol. 332, n. 22, october 2013, pp. 5727–5747. doi: 10.1016/j.jsv.2013.06.009 [21] j. antoni, a bayesian approach to sound source reconstruction: optimal basis, regularization, and focusing, the journal of the acoustical society of america, vol. 131, no. 4, art. no. 4, apr. 2012, pp 2373-2890. doi: 10.1121/1.3685484 https://doi.org/10.1016/j.jsv.2005.12.046 http://dx.doi.org/10.1260/147547207783359459 https://doi.org/10.1016/j.jsv.2011.05.021 https://doi.org/10.1016/j.jsv.2020.115208 http://dx.doi.org/10.1121/1.2896754 https://doi.org/10.1016/j.apacoust.2015.03.008 https://doi.org/10.3397/1/376348 https://doi.org/10.1016/j.apacoust.2020.107599 https://doi.org/10.1155/2012/292695 https://doi.org/10.1016/j.jsv.2020.115176 http://www.acoular.org/ https://doi.org/10.1016/j.ymssp.2007.09.017 https://doi.org/10.1016/j.jsv.2013.06.009 https://doi.org/10.1121/1.3685484 evaluation and correction of systematic effects in a simultaneous 3-axis vibration calibration system acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 388 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 388 393 evaluation and correction of systematic effects in a simultaneous 3-axis vibration calibration system a. prato1, f. mazzoleni1, a. schiavi1 1 inrim – national institute of metrological research, torino, italy, a.schiavi@inrim.it abstract: this paper presents a calibration method, recently realized at inrim, suitable for the calibration of 3axis accelerometers in frequency domain. the procedure, allows to simultaneously evaluate the main and transverse sensitivities on three axes by means of a single-axis vibration excitation of inclined planes. nevertheless, the excitation system is subjected to spurious motions mainly due to the vibrational modes of the inclined planes and to the horizontal motions of the shaker. in order to provide the proper sensitivities to the 3-axis sensors, the evaluation of systematic effects is experimentally carried out and the related correction is proposed. keywords: calibration, 3-axis accelerometer, systematic effects 1. introduction the 3-axis accelerometers, especially low-cost unconventional-shaped transducers, such as mems sensors, are largely used in a wide range of advanced industrial, environmental, energy and medical applications, and in particular within extensive sensor and multi-sensor networks [e.g., 1 – 10]. for example, in the context of industry 4.0, a huge number of sensors is needed for an effective implementation of smart factories, learning machine and intelligent manufacturing systems, as well as for traditional application such as early failure detection and predictive maintenance; low-power devices and battery-operated systems are practical and useful in iot applications, such as for smart cities, for accurate navigation/positioning systems and in environmental monitoring and survey; moreover, accurate measurements are of paramount importance in medical applications, in remote surgery and remote diagnoses. the possibility to have many accurate, low-power consuming and low-cost sensors present undoubted advantages, in terms of costs reduction and energy saving, in the control processes, monitoring or measurements and being flexible in providing enhanced data collection, automation and operation. by way of example, the calibration of digital mems sensors, with the associated uncertainty budget, allows to ensure traceability and measurement accuracy of nodes in sensor networks, as well as in other innovative implementations. moreover, the evolving improvement of the technical performance and the reliability of mems sensors are emerging quality attributes of interest for manufacturers, costumers and end-users. however, in the particular case of digital mems accelerometers, the sensitivity is generally provided by manufacturer without traceable calibration methods and is obtained in static conditions, whereas dynamic response, as a function of frequency, is often barely known or completely disregarded. given this condition, a simultaneous 3-axis vibration calibration system with a single axis excitation to be exploitable by manufacturers is proposed. this paper deals with the evaluation of systematic effects of this system. 2. description of the work traceable calibration methods for digital sensors, and smart sensors, in metrological terms [11 13], including sensitivity parameter and an appropriate uncertainty evaluation, are necessary in order to consider low-cost and low-power consuming accelerometers as actual measurement devices in frequency domain [14]. the calibration system, developed at inrim, allows to simultaneous evaluate the main and the transverse sensitivity, in frequency domain, of 3-axis accelerometers, by means of a single-axis vibration excitation, by using inclined planes rigidly fixed to the vibrating table [14, 15]. the calibration procedure is based on the comparison to a reference transducer (in analogy to iso 16063-21[16]). a preliminary version of the system was previously investigated for the characterization of analog mems accelerometers performance in operative conditions [17 – 23]. measurements are performed at a nearly constant amplitude of 10 m∙s-2, from 5 hz up to 3 khz. the mechanical calibration system, composed of the shaker and the inclined planes (with tilt angles of 15°, 35°, 55° and 75°), is characterized in order to take into account systematic effects. similar procedure is also applied to the shaker at 0° and 90°. http://www.imeko.org/ mailto:a.schiavi@inrim.it acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 389 measurements of systematic effects is carried out by means of a laser-doppler vibrometer. the detailed uncertainty budged is evaluated according to gum [24]. 3. calibration set-up the calibration set-up here proposed, consisting of a single-axis vibrating table on which aluminum inclined planes are screwed, allows to generate a projection of the reference acceleration along three axes simultaneously. a single vertical sinusoidal acceleration at nearly-constant amplitude acts as reference acceleration aref along the vertical z’-axis of the system. in this way, accelerations of proportional amplitudes, are simultaneously generated on the inclined surface plane, along the three axes. in figure 1, the geometrical principle of the proposed method on which the 3-axis accelerometer is fixed during calibration, is schematically depicted. figure 1: inclined plane – scheme from simple trigonometrical laws, the reference accelerations detected by the sensor in calibration, along its three sensitive axes, are expected to be: ax,theor = |aref sin(𝛼) cos(𝜔)| (1) ay,theor = |aref sin(𝛼) sin(𝜔)| (2) az,theor = |aref cos(𝛼)| (3) where,  is the tilt angle, ω is the angle of rotation, aref is the root mean square (rms) reference acceleration along the vertical z’-axis of the system, and ax,theor, ay,theor, az,theor are the rms reference accelerations spread along x-, yand z-axis of the mems accelerometer in calibration. in the experimental set-up, the inclined plane is screwed on the vertical vibrating table (pcb precision air bearing calibration shaker), and the 3axis accelerometer is fixed to the inclined plane and located along the vertical axis of excitation. the experimental configuration is shown in figure 2. the acceleration along vertical z’-axis aref is measured by a single axis reference transducer (pcb model 080a199/482a23), calibrated according to iso 16063-11:1999 [25], against inrim primary standard, located within the stroke of the shaker and is acquired by an acquisition board ni 4431 (sampling rate of 50 khz) integrated in the pc and processed through labview software to provide the rms reference value in m s-2. the digital mems output is acquired by an external microcontroller at a maximum sampling rate of 6.660 khz and saved as binary files. figure 2: the calibration set-up: the mems fixed to the inclined plane on the vibrating table. 4. evaluation of systematic effects and correction as described in the previous section, the reference accelerations along mems accelerometer sensitivities axes are given by equations (1)-(3), by using trigonometrical laws. however, in dynamic conditions, systematic effects caused by spurious oscillating components along the three axes of the reference system (x’-, y’and z’) at mems position need to be taken into account. these spurious components affect the actual reference accelerations aref splitted on the three sensitivity axes of the mems. in figure 3, a schematic representation of the occurring phenomenon is shown. figure 3: representation of the spurious oscillating components combination along the three axes of the mems during the calibration. http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 390 such components are mainly due to the vibrational modes of the inclined aluminum planes and due to small but not negligible horizontal motions of the shaker. each spuroius component along reference system x’-, y’and z’-axis, has to be decomposed along the 3-axis accelerometer x-, yand z-axis and summed to the reference accelerations ax,theor, ay,theor, az,theor according to wave interference laws. the actual decomposition of the spurious components, acting along the three axes of the reference system, is schematically depicted in figure 4. figure 4: representation of the spurious oscillating component decomposition, along the three axis of the reference system, at a given frequency. by way of example, considering the general case of four overlapping waves, 𝐸1 = 𝐸1,0𝑒 𝑖(2𝜋𝑓𝑡) , 𝐸2 = 𝐸2,0𝑒 𝑖(2𝜋𝑓𝑡+𝜑2) , 𝐸3 = 𝐸3,0𝑒 𝑖(2𝜋𝑓𝑡+𝜑3) , 𝐸4 = 𝐸4,0𝑒 𝑖(2𝜋𝑓𝑡+𝜑4) , oscillating at a same frequency f, with different amplitudes and phase differences with respect to the reference signal e1, along a particular direction, their interference can be opportunely defined according to equation (4), where e2, e3 and e4 are the amplitudeand phase-dependant spurious components along x-, yand z-axis of the mems. 𝐸𝑡𝑜𝑡 = |𝐸1,0 + 𝐸2,0𝑒 𝑖𝜑2 + 𝐸3,0𝑒 𝑖𝜑3 + 𝐸4,0𝑒 𝑖𝜑4 | (4) in this way, it is possible to correct reference theoretical accelerations along mems axes in equations (1)-(3), into ax, ay, az of equations (8)-(10), where ax’,syst, ay’,syst, az’,syst and φx’,syst, φy’,syst, φz’,syst are, respectively, the amplitudes and the phase differences as shown in equations (5)-(7), with respect to the reference signal aref, of the spurious components along x’-, y’and z’-axis. as it will be shown in section 5, the amplitude of spurious components vary as a function of frequency between 0.1% and 10% of the reference acceleration aref. experimental evaluation of systematic effects due to spurious components is carried out by means of a laser-doppler velocimeter (polytec ofv 505). amplitude and phase measurements along the x’-, y’and z’-axis of the reference system are evaluated for each inclined plane and for all frequencies, at reference vertical amplitude of 10 m s-2. laser signal, during measurements of spurious components amplitude, is acquired by a ni 4431 board (sampling rate of 50 khz) integrated into the pc, while measurement of phase differences between reference acceleration and spurious components are measured by means of a dynamic signal analyzer (keysight 35670a). since the digital mems accelerometer is too small to be used as a reflective surface, the beam spot of the laser directly hits a small aluminum triangular-based parallelepiped located at mems position and fixed to the different inclined planes, as shown in figure 5. the volume of the triangular-based parallelepiped is around 0.5 cm3, which is 0.6% of the total volume of the inclined plane, i.e., negligible with respect to the total mass. the values of the measured acceleration amplitudes ax’, ay’ and az’ along x’-, y’and z’-axis are related to the actual systematic effects acting on the axes of the reference system, and are expressed, as a function of frequency and experimental phase-shift, by the following equations: 𝑎𝑥′ = |𝑎𝑥′,𝑡ℎ𝑒𝑜𝑟 + 𝑎𝑥′,𝑠𝑦𝑠𝑡 ∙ 𝑒 𝑖(2𝜋𝑓𝑡+𝜑𝑥′,𝑠𝑦𝑠𝑡)| (4) 𝑎𝑦′ = |𝑎𝑦′,𝑡ℎ𝑒𝑜𝑟 + 𝑎𝑦′,𝑠𝑦𝑠𝑡 ∙ 𝑒 𝑖(2𝜋𝑓𝑡+𝜑𝑦′,𝑠𝑦𝑠𝑡)| (5) 𝑎𝑧′ = |𝑎𝑧 ′,𝑡ℎ𝑒𝑜𝑟 + 𝑎𝑧 ′,𝑠𝑦𝑠𝑡 ∙ 𝑒 𝑖(2𝜋𝑓𝑡+𝜑𝑧′,𝑠𝑦𝑠𝑡)| (6) where ax’,theor and ay’,theor are 0 (i.e., no acceleration is generated along the horizontal system plane x’-y’) and the vertical component az’,theor  aref. in figure 5 the experimental method used to quantify the spurious components from accelerations ax’, ay’ and az’, and the related phase-shift φx’, φy’ and φz’, with respect to the reference acceleration aref, acting along the vertical axis z’, is shown. ax = √ (ax,theor + |a𝑥′,𝑠𝑦𝑠𝑡 cos(𝛼) cos(𝜔)| cos 𝜑𝑥′,𝑠𝑦𝑠𝑡 + |a𝑦′,𝑠𝑦𝑠𝑡 sin(𝜔)| cos 𝜑𝑦′,𝑠𝑦𝑠𝑡 + |a𝑧′,𝑠𝑦𝑠𝑡 sin(𝛼) cos(𝜔)| cos 𝜑𝑧′,𝑠𝑦𝑠𝑡 ) 2 + +(|a𝑥′,𝑠𝑦𝑠𝑡 cos(𝛼) cos(𝜔)| sin 𝜑𝑥′,𝑠𝑦𝑠𝑡 + |a𝑦′,𝑠𝑦𝑠𝑡 sin(𝜔)| sin 𝜑𝑦′,𝑠𝑦𝑠𝑡 + |a𝑧′,𝑠𝑦𝑠𝑡 sin(𝛼) cos(𝜔)| sin 𝜑𝑧′,𝑠𝑦𝑠𝑡 ) 2 (8) http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 391 ay = √ (ay,theor + |a𝑥′,𝑠𝑦𝑠𝑡 cos(𝛼) sin(𝜔)| cos 𝜑𝑥′,𝑠𝑦𝑠𝑡 + |a𝑦′,𝑠𝑦𝑠𝑡 cos(𝜔)| cos 𝜑𝑦′,𝑠𝑦𝑠𝑡 + |a𝑧′,𝑠𝑦𝑠𝑡 sin(𝛼) sin(𝜔)| cos 𝜑𝑧′,𝑠𝑦𝑠𝑡 ) 2 +(|a𝑥′,𝑠𝑦𝑠𝑡 cos(𝛼) sin(𝜔)| sin 𝜑𝑥′,𝑠𝑦𝑠𝑡 + |a𝑦′,𝑠𝑦𝑠𝑡 cos(𝜔)| sin 𝜑𝑦,𝑠𝑦𝑠𝑡′ + |a𝑧′,𝑠𝑦𝑠𝑡 sin(𝛼) sin(𝜔)| sin 𝜑𝑧′,𝑠𝑦𝑠𝑡 ) 2 (9) az = √ (az,theor + |a𝑥′,𝑠𝑦𝑠𝑡 sin(𝛼)| cos 𝜑𝑥′,𝑠𝑦𝑠𝑡 + |a𝑧′,𝑠𝑦𝑠𝑡 cos(𝛼)| cos 𝜑𝑧′,𝑠𝑦𝑠𝑡 ) 2 + +(|a𝑥′,𝑠𝑦𝑠𝑡 sin(𝛼)| sin 𝜑𝑥′,𝑠𝑦𝑠𝑡 + |a𝑧′,𝑠𝑦𝑠𝑡 cos(𝛼)| sin 𝜑𝑧′,𝑠𝑦𝑠𝑡 ) 2 (10) figure 5: the laser beam hitting the aluminum triangularbased parallelepiped located at the mems position. 5. experimental results in the graphs of figures 6 – 9, the values of the amplitudes of ax’, ay’ and az’, are normalized with respect to the reference acceleration aref amplitude. mesurement are performed from 5 hz and 3 khz. figure 6: normalized accelerations along x’-, y’and z’axis at 15° of tilt angle. figure 7: normalized accelerations along x’-, y’and z’axis at 35° of tilt angle. figure 8: normalized accelerations along x’-, y’and z’axis at 55° of tilt angle. figure 9: normalized accelerations along x’-, y’and z’axis at 75° of tilt angle. the experimental values of the spurious components amplitude of accelerations ax’, ay’ and az’, along the axes of the system, are combined in order to calculate the values of systematic effects due to the acceleration amplitudes ax’,syst, ay’,syst and az’,syst, along the mems sensitivity axes. in the graphs of the figures 10 – 13, the experimental values of phase-shift φx’, φy’ and φz’, with respect to the reference acceleration aref, acting along the vertical axis z’, are shown. in this case the values of phase-shift allow to evaluate the phase differences, with respect to the reference signal aref, in terms of φx’,syst, φy’,syst and φz’,syst. since φz’,syst is close to 0° in every configuration, it is not shown in the graphs. http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 392 figure 10: phase-shifts on the x’-y’ horizontal plane, with respect to the vertical axis z’, at 15° of tilt angle. figure 11: phase-shifts on the x’-y’ horizontal plane, with respect to the vertical axis z’, at 35° of tilt angle. figure 12: phase-shifts on the x’-y’ horizontal plane, with respect to the vertical axis z’, at 55° of tilt angle. figure 13: phase-shifts on the x’-y’ horizontal plane, with respect to the vertical axis z’, at 75° of tilt angle. once quantified the acceleration amplitudes ax’, ay’ and az’, and the values of the phase-shift φx’, φy’ and φz’, the reference theoretical accelerations along mems axes in equations (1)-(3), can be opportunely expressed into ax, ay, az of equations (8)-(10), by taking into account the systematic effects due to the vibrational modes of the inclined planes and to horizontal motions of the shaker. in particular, it can be observed an increasing of acceleration amplitudes, as a function of frequency, along the horizontal axes x’, y’ and also along vertical axis z’, this last mainly due to resonant modes. moreover, the lateral motions of the shaker, occurring around 80 hz, are presumably the cause of the large phase-shifts observed at low frequencies, as depicted in figure 10 – 13; these motions are independent from the resonant modes of the inclined planes, but they are affecting in any case, the whole behavior of the inclined plane, in terms of amplitude and phase differences. on the other hand, the analysis of the systematic effects, is an aggregation of both shaker spurious motions and resonant modes. standard uncertainties associated to the amplitude of the spurious components, u(ax’,syst), u(ay’,syst), u(az’,syst), are considered as type b uncertainty contributions with an average error of 0.0025 m s-2, from three repeated measurements, and a uniform rectangular distribution. standard uncertainties associated to the phase difference due to the spurious components, u(φx’,syst), u(φy’,syst), u(φz’,syst), are considered as type a uncertainty contributions with a maximum standard deviation of 2° from five repeated measurements, as shown in [14]. this correction allows to univocally define the actual projection of the reference acceleration aref on the three axes, thus the “standard” calibration can be achieved, by comparison to the reference transducer within the stroke of the shaker and it can be finally related to the primary standard, as declared. 6. summary in this paper is presented a technical insight on the calibration system, recently realized at inrim, suitable for the calibration of 3-axis accelerometers in frequency domain. the procedure, allows to simultaneously evaluate the main and transverse sensitivities on three axes by means of a single-axis vibration excitation of inclined planes. the mechanical calibration system, composed of the shaker and the inclined planes, is characterized in order to take into account systematic effects occuring during dynamical excitation. the evaluation of systematic effects, due to the vibrational modes of the inclined aluminum planes and due to small but not negligible horizontal motions of the shaker, are carried out from 5 hz up to 3 khz at an amplitude of 10 m s-2. the amplitudes of the acceleration spurious components ax’, ay’ and az’, and the values of the phase-shift φx’, φy’ and φz’, with respect to the reference acceleration aref, acting along the vertical axis z’, are accurately measured by means of a laserdoppler vibrometer. this correction allows to http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 393 univocally define the actual projection of the reference acceleration aref on the three axes, thus the “standard” calibration can be achieved and related to the primary standard. 7. note this work has to be considered as an addendum to the paper: “prato, a., mazzoleni, f., & schiavi, a. (2020). traceability of digital 3-axis mems accelerometer: simultaneous determination of main and transverse sensitivities in the frequency domain. metrologia, 57(3), 035013” [14], which shows the detailed extension of the determination of the systematic effects of the calibration system. 8. references [1] noel, a. b., abdaoui, a., elfouly, t., ahmed, m. h., badawy, a., & shehata, m. s. (2017). structural health monitoring using wireless sensor networks: a comprehensive survey. ieee communications surveys & tutorials, 19(3), 1403-1423. [2] dehkordi, s. a., farajzadeh, k., rezazadeh, j., farahbakhsh, r., sandrasegaran, k., & dehkordi, m. a. (2020). a survey on data aggregation techniques in iot sensor networks. wireless networks, 26(2), 1243-1263. [3] deng, x., jiang, y., yang, l. t., lin, m., yi, l., & wang, m. (2019). data fusion based coverage optimization in heterogeneous sensor networks: a survey. information fusion, 52, 90-105. 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[9] zareei, m., vargas-rosales, c., anisi, m. h., musavian, l., villalpando-hernandez, r., goudarzi, s., & mohamed, e. m. (2019). enhancing the performance of energy harvesting sensor networks for environmental monitoring applications. energies, 12(14), 2794. [10] carminati, m., kanoun, o., ullo, s. l., & marcuccio, s. (2019). prospects of distributed wireless sensor networks for urban environmental monitoring. ieee aerospace and electronic systems magazine, 34(6), 4452. [11] bruns, t., & eichstädt, s. (2018, august). a smart sensor concept for traceable dynamic measurements. in journal of physics: conference series (vol. 1065, no. 21, p. 212011). iop publishing. [12] dorst, t., ludwig, b., eichstädt, s., schneider, t., & schütze, a. (2019, may). metrology for the factory of the future: towards a case study in condition monitoring. in 2019 ieee international instrumentation and measurement technology conference (i2mtc) (pp. 1-5). ieee. [13] seeger, b., bruns, t., & eichstädt, s. (2019). methods for dynamic calibration and augmentation of digital acceleration mems sensors. in 19th international congress of metrology (cim2019) (p. 22003). edp sciences. [14] prato, a., mazzoleni, f., & schiavi, a. (2020). traceability of digital 3-axis mems accelerometer: simultaneous determination of main and transverse sensitivities in the frequency domain. metrologia, 57(3), 035013. [15] schiavi a, mazzoleni f and germak a 2015 simultaneous 3-axis mems accelerometer primary calibration: description of the test-rig and measurements xxi imeko world congress on measurement in research and industry 30 2161-2164 [16] iso 16063-21 2003 methods for the calibration of vibration and shock transducers — part 21: vibration calibration by comparison to a reference transducer (geneva: international organization for standardization). [17] d’emilia g, gaspari a, natale e, mazzoleni f and schiavi a 2018 calibration of tri-axial mems accelerometers in the low-frequency range part 1: comparison among methods journal of sensors and sensor systems 7(1) 245257 [18] d’emilia g, gaspari a, natale e, mazzoleni f and schiavi a 2018 calibration of tri-axial mems accelerometers in the low-frequency range part 2: uncertainty assessment journal of sensors and sensor systems 7(1) 403-410. [19] g. d’emilia, a. gaspari, f. mazzoleni, e. natale, a. prato, a. schiavi, 2020 metrological characterization of mems accelerometers by a ldv, aivela. [20] a. prato, f. mazzoleni and a. schiavi, “metrological traceability for digital sensors in smart manufacturing: calibration of mems accelerometers and microphones at inrim,” 2019 ieee international workshop on metrology for industry 4.0 and iot, 371-375, 2019. [21] m. galetto, a. schiavi, g. genta, a. prato and f. mazzoleni, “uncertainty evaluation in calibration of lowcost digital mems accelerometers for advanced manufacturing applications,” cirp annals 68, 535-538, 2019. [22] a. schiavi, a. prato, f. mazzoleni, g. d’emilia, a. gaspari, e. natale, “calibration of digital 3-axis mems accelerometers: a double-blind «multi-bilateral» comparison”, 2020 ieee international workshop on metrology for industry 4.0 and iot. [23] a. prato, a. schiavi, f. mazzoleni, a. touré, g. genta, m. galetto, “a reliable sampling method to reduce large set of measurements: a case study on calibration of digital 3-axis mems accelerometers” 2020 ieee international workshop on metrology for industry 4.0 and iot. [24] jcgm 100 2008 evaluation of measurement data — guide to the expression of uncertainty in measurement (gum), joint committee for guides in metrology, sèvres, france. [25] iso 16063-11 1999 methods for the calibration of vibration and shock transducers — part 11: primary vibration calibration by laser interferometry (geneva: international organization for standardization. http://www.imeko.org/ zro2-doped zno-pdms nanocomposites as protective coatings for the stone materials acta imeko issn: 2221-870x march 2022, volume 11, number 1, 1 6 acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 1 zro2-doped zno-pdms nanocomposites as protective coatings for the stone materials maduka l. weththimuni1, marwa ben chobba2, ilenia tredici3, maurizio licchelli1,3 1 department of chemistry, university of pavia, via t. taramelli 12, i-27100, pavia, italy 2 national school of engineering, university of sfax, box 1173, 3038 sfax, tunisia 3 cisric, university of pavia, via a. ferrata 3, i-27100, pavia, italy section: research paper keywords: zro2-doped zno; nanocomposites; protective coating; self-cleaning effect citation: maduka l. weththimuni, marwa ben chobba, ilenia tredici, maurizio licchelli, zro2-doped zno-pdms nanocomposites as protective coatings for the stone materials, acta imeko, vol. 11, no. 1, article 4, march 2022, identifier: imeko-acta-11 (2022)-01-04 section editor: fabio santaniello, university of trento, italy received march 3, 2021; in final form february 23, 2022; published march 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: maduka l. weththimuni, e-mail: madukalankani.weththimuni@unipv.it 1. introduction our artifacts provide the most important and essential availability for the culture of all countries and represent the symbol of history. therefore, protection and conservation of this patrimony are essential tasks for the scientific community. the cultural heritage buildings undergo different damages, particularly because they are exposed outdoors, and are subject to a number of phenomena such as air pollution, water absorption, salt crystallization, photodegradation, and microorganisms colonization which cause transformation and deterioration of surfaces [1]-[3]. moreover, decay of stone materials is strongly related to their porosity properties, which implies that highly porous materials often undergo faster degradation than the other stone substrates [4]-[6]. several methodologies have been developed to clean preserved materials belonging to heritage sites as a first step of the conservation process. cleaning procedures may include the use of solvents, chelating agents, and even of acidic or basic agents [7]. nevertheless, the irreversibility of these methods, the risk of altering the artwork, as well as the toxicity of certain products makes these cleaning procedures scarcely suitable for the application for historic buildings [7]. the bio-cleaning method has been suggested and studied as an alternative technique by using selected microorganisms [8]. despite the efficiency of this method under the laboratory condition, the need of specified conditions to ensure the viability of these microorganisms make its practical application very difficult and subject to further studies [7]. laser method has been considered as a friendly technique for cleaning heritage structure as well as for environment [9]. however, the high cost of this method is still the barrier against its widespread use. a variety of products (biopolymers, ionic liquids, gels, microemulsions, etc.) have been also proposed and used in this field, although their application still has some drawbacks such as high cost of maintenance and toxicological risks [10], [11]. the conservation of monumental cultural heritage with innovative nanocomposites is in the vanguard of conservation science and a plethora of research activities are dedicated to the abstract zno is a semiconductor that has found wide application in the optics and electronics areas. moreover, it is widely used in different technological areas due to its beneficial qualities (high chemical stability, non-toxicity, high photo-reactivity, and cheapness). based on its antibacterial activity, recently it has found also application to prevent bio-deterioration of cultural heritage buildings. as many authors suggested, doped zno nano-structures exhibit better antibacterial properties than undoped analogues. in the present work, zno nanoparticles doped with zro2 have been prepared by a sol-gel method in order to enhance the photocatalytic properties as well as the antibacterial activity of zno. then, zro2-zno-pdms nanocomposite (pdms, polydimethylsiloxane used as the binder) was synthesized by in-situ reaction. the resulting nanocomposite has been investigated as a possible protective material for cultural heritage building substrates. the performances of newly prepared coating were evaluated on three different stones (lecce stone, carrara marble and brick) and compared with plain pdms as a reference coating. mailto:madukalankani.weththimuni@unipv.it acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 2 design and validation of compatible nanomaterials that may exhibit strengthening, hydrophobic and self-cleaning properties [12-14]. moreover, scientists have focused their research to find out self-cleaning protective materials in order to reduce maintenance costs. in particular, titanium dioxide (tio2) and zinc oxide (zno) nanoparticles (nps) have been studied and tested for self-cleaning applications [15], [16]. the interest in developing and using these nanosized materials in combination with different binders has increased due to their excellent selfcleaning and antibacterial properties in addition to their easy, non-toxic and non-expensive procedures of application [17], [18]. however, their technological application has some important limitations among which the easy recombination of charge carriers and the need of ultraviolet (uv) radiation as an excitation source, considered as the most restrictive drawback, are due to the broadband-gap (zno: 3.2 ev; tio2: 3.3 ev for anatase) of the two oxides [19]. this limitation is particularly restrictive, as uv radiation corresponds to only 3 % of the solar irradiance at the surface of earth. therefore, many research groups have focused their efforts on enhancing the photocatalytic as well as the antimicrobial activity of pure nps by doping with different ions (transition, non-transition, and non-metal ions) [1]-[20]. the process of doping can alter the surface reactivity, functionality and charge of the nps, with possible improvement of their properties such as durability, stability, and dispersive ability of core material [20]. some authors also reported that the doped nanostructures exhibit better antibacterial properties than undoped analogues [20]. zirconium dioxide or zirconia (zro2) is a wide band gap p-type semiconductor, which is used in many fields due to its excellent properties (e.g. good natural color, high strength, high toughness, high chemical stability, and chemical and microbial resistance) [21]. the main objective of this work was the preparation of zno nanoparticles doped with zro2 by a sol-gel method in order to enhance the photocatalytic properties as well as the antibacterial activity of plain zno. moreover, doped nps were combined with a binder (polydimethylsiloxane, pdms) to obtain a nanocomposite, which was tested as a protective material for the cultural heritage. at first, the synthesised core-shell (zro2-zno) nps were characterised by sem-eds. after that, the performances of the nanocomposite coating (zro2-zno-pdms) as a protective material for stone substrates were evaluated when applied on three different stone types: lecce stone (ls), brick (b) and carrara marble (m). moreover, pdms-treated stone specimens were used as the reference for each and every analyses. the evaluation of coatings was done by different techniques: contact angles and chromatic variation measurements, capillary absorption and water vapor permeability determinations, optical microscopy (visible and uv light), sem-eds, self-cleaning and antibacterial tests. 2. materials and methods 2.1 materials analytical grade sodium hydroxide (naoh), ethanol (absolute, 99.8 % etoh), zinc acetate dihydrate (znc4h6o4·2h2o), 2-propanol, zirconium oxychloride octahydrate, orthophosphoric acid, hexamethyldisiloxane, and octamethylcyclotetrasiloxane (d4, utilized as pdms precursor) were purchased from sigma-aldrich. cesium hydroxide (csoh.h2o) was purchased from alfa aesar. all the chemicals used without further purification. water was purified using a millipore organex system (r ≥ 18 m cm). lecce stone specimens (open porosity ˃ 30 %) were provided by tarantino and lotriglia (nardò, lecce, italy), while specimens of brick (open porosity ~24 %) and carrara marble (open porosity ~0.5 %) were provided by favret mosaici s.a.s. (pietrasanta, lucca, italy). 2.2 preparation, application and testing methods of the nanocomposite before treatment, lecce stone (ls), brick (b), and marble (m) (squared 5 × 5 × 1 and 5 × 5 × 2 cm3) specimens were smoothed with abrasive, carbide paper (no: 180 mesh), washed with deionized water, dried in an oven at 60 °c and stored in a desiccator to reach room temperature, then their dry weight was measured [2], [4]. first of all, zro2-zno core-shell nps (molar ratio about 0.01:1 zro2/zno) were synthesised by sol-gel method as reported in the literature [20], [21] and then, zro2-zno-pdms nanocomposite was synthesized by in-situ reaction. for this purpose, doped nps (0.5 % (w/w)) were introduced to the reaction mixture containing octamethylcyclotetrasiloxane (d4, 25 g) and csoh (0.15 g), used as a catalyst for the ring opening polymerization of d4. after ultrasonication (20 minutes), the reaction was carried out at 120 ± 3 °c under vigorous stirring for 2.5 hours in an oil bath and then, hexamethyldisiloxane (0.03 g) was added and the reaction was continued at the same temperature another 2.5 hours as recommended in the literature [20]. all the samples (ls, b, and m) were saturated with ethanol by keeping specimens at least 6 hours in absolute etoh in order to prevent the penetration of coating inside the pores and ensure that it remains on the surface of the stone [1]. after saturation, all specimens were treated with zro2-zno-pdms nanocomposite as well as with plain pdms (as the reference) by brushing method (applied amount, 1.0 ± 0.02 g for each specimen). specimen treated with the nanocomposite were named zn-zr-pdms_ls, zn-zr-pdms_b, and zn-zrpdms_m, while reference specimens were labeled pdms_ls, pdms_b, and pdms_m. optical microscopy observations of the treated specimens were done using a light polarized microscope olympus bx51tf, equipped with the olympus th4-200 lamp (visible light) and the olympus u-rfl-t (uv light). scanning electron microscopy (sem) images (backscattered electron) and energy-dispersive xray spectra (eds) were collected by using a tescan fe-sem, mira xmu series, equipped with a schottky field emission source, operating in both low and high vacuum, and located at the arvedi laboratory, cisric, university of pavia. the amount of absorbed water as a function of time was determined in accordance with the uni en 15801 protocol [22]. water vapor permeability was determined according to uni en 15803:2010 protocol [23]. color changes were measured by a konica minolta cm-2600d spectrophotometer, determining the l*, a*, and b* coordinates of the cielab space, and the global chromatic variations, expressed as δe* according to the uni en 15886 protocol [24]. selfcleaning efficiency of prepared coatings was performed by using multirays, photochemical reactor, composed of uv chamber equipped with 8 uv lamps. the power of each lamp is 15w with a total power of 120 w. the reactor is equipped with a rotating disc in order to ensure a homogenized irradiation on all stained samples. the discoloration of methylene blue (mb) dye (0.1 % wt in ethanol solution), applied on the surface of treated stones specimens and acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 3 their untreated counterparts, was controlled by measuring chromatic variations (five control points for each sample surface) before and after application of mb dye, after 48 and 96 h of uv exposure. discoloration parameter 𝐷∗ was determined by using (1) [25]: 𝐷∗ = |𝑏∗(𝑡) − 𝑏∗(𝑀𝐵)| |𝑏∗(𝑀𝐵) − 𝑏∗(0)| ∙ 100 % (1) where 𝑏∗(0) is the value of chromatic coordinate 𝑏∗ before staining, while 𝑏∗(𝑀𝐵), and 𝑏∗(𝑡) are the mean values after the application of methylene blue over the surfaces and after t hours of uv-a light exposure, respectively. here, 𝑏∗ coordinate was considered, because this parameter is sensitive to blue colour. 3. results and discussions 3.1 characterisation of doped nps and treated stone specimens sem-eds analyses showed that zro2-doped zno nps are homogeneously dispersed in the pdms binder. most of them are spherical with a size in the 15-30 nm range. anyway, some particles displaying larger size and more irregular shape can be observed, which can be due to occasional aggregation (see figure 1a-b). eds analyses performed on single particles showed the presence of both zirconium and zinc, confirming the expected elemental compositions of the doped inorganic nps (figure 1a). optical microscopy observations suggested that nanocomposite material (zro2-zno-pdms) is homogeneously distributed on the treated stone surfaces (ls, b, and m). moreover, it seems that the coating covered pores on the surface and acting as a protective layer to the stone (figure 2). this observation was confirmed even by sem experiments (figure 3). quite acceptable chromatic variations (∆e* < 5) were observed (see table 1) after application of zro2-zno-pdms on any considered substrate, suggesting that the natural colour of the stones is not dramatically affected by the treatment. the corresponding chromatic coordinates are graphically resumed in figure 4. coordinate 𝐿∗ (related to the lightness) is considerably affected by all the treatments, regardless of the considered treatment. variations of 𝑏∗ (related to the blue to yellow change) are more relevant on lecce stone and brick specimens treated with pdms or nanocomposite material. hydrophobic properties of the treated stones are also summarized in table 1. results indicate that all the stone surfaces show hydrophobic behavior (contact angle measurements α ˃ 90°) after the treatments. it’s worth to highlight that nanocomposites-coated stones showed higher hydrophobic properties than polymer-coated stones. it may be due to the homogeneous distribution of nps in the polymer matrix which increase the hydrophobic nature of pdms. figure 1. sem images of doped nps. table 1. overall chromatic variations and contact angle measurements of treated stones. samples ∆e* α (°) pdms_ls 4.4 (±0.2) 111 (±1) zn-zr-pdms_ls 3.9 (±0.1) 131 (±2) pdms_b 4.4 (±0.5) 128 (±1) zn-zr-pdms_b 4.8 (±0.2) 136 (±2) pdms_m 4.9 (±0.1) 98 (±2) zn-zr-pdms_m 4.8 (±0.1) 107 (±3) figure 2. optical microscope images of treated stones: (a) zn-zr-pdms_ls, (b) zn-zr-pdms_b, and (c) zn-zr-pdms_m. figure 3. sem images of treated stones: (a) zn-zr-pdms_ls, (b) zn-zrpdms_b, and (c) zn-zr-pdms_m. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 4 moreover, preliminary data concerning capillary absorption tests confirmed that treated stones exhibited water repellent behavior. as can be seen in the table 2, ca values (related to the first 30 minutes) are affected by both treatments (polymer as well as nanocomposite), while it was possible to note some reduction of qf value compared the untreated stone at the end of the test (about 96 h). so, it seems that the treatments, especially with nanocomposite coating have long-term water resistant activity. the results are in good agreement with hydrophobic properties of treated stones. in addition, vapour permeability was preserved at acceptable levels upon treatment with zro2-zno-pdms nanocomposite. hence, all these results suggest that the newly prepared coating can be considered as a promising protective material. 3.2 analysing the self-cleaning properties as reported in the introduction, evaluation of the selfcleaning effectiveness of the newly prepared coating is one of the main aspect of this research study. figure 5 shows (as an example) the behavior of lecce stone before and after the test. the different behavior of stone treated with the nanocomposite and with plain pdms can be observed even by naked eye. in order to evaluate the self-cleaning effect of the coatings, discoloration test was performed on the treated stones. after the application of mb solution on the treated surface, the specimens were exposed to uv irradiation. a quantitative evaluation of the self-cleaning behavior of zro2-zno-pdms on the different stone was obtained by calculating the discoloration parameter d, which was determined at two different time intervals (see figure 6). it is related to the variation of 𝑏∗ coordinate (cielab space) and corresponds to the amount of mb removed from the coated surfaces. this test provided quite similar behavior for ls and b: the coating containing doped nps showed a higher effectiveness than the plain pdms coating. for instance, the discoloration factor related to the new coating is about double compared to pdms (at the end of the test: d*pdms ~ 20 %, d*nanoc ~ 40 % both for ls and b) and as reported in the literature, nps as well as doped nps with pdms coating have been used as a selfcleaning protective coating due to their photocatalytic performance under uv light (when the presence of nps, the discoloration factor is always higher than pdms) [1], [16], [25]. the new coating showed even better results when applied on marble surface, as the discoloration factor was around 70 % after 96 hours of uv irradiation. the results tally with the reported literature [1]. nevertheless, it should be noted that even plain pdms display better performances on marble specimens if compared to the other considered stones. although it is difficult to reliably compare the effectiveness of a treatment on different substrates, due to the different original stone properties (e. g. absorbability of the products, porosity, etc.) that may affect the figure 4. chromatic coordinates of treated stones. table 2. maximum water absorbed per unit area (qf, mg/cm2), capillary water absorption coefficient (ca, mg/cm2 s 1/2), and values of the water vapor permeability of untreated and treated samples. samples qf in mg/cm2 ca in mg/ cm2 s 1/2 permeability in g/m2 24 h ls 518,74 (±9,62) 8,73 (±0,57) 236 (±9) pdms_ls 479,04 (±8,16) 4,63 (±0,50) 185 (±6) zn-zr-pdms_ls 434,18 (±6,68) 2,22 (±0,54) 174 (±4) b 431,57 (±12,34) 2,12 (±0,21) 159 (±4) pdms_b 346,66 (±10,49) 1,53 (±0,44) 107 (±4) zn-zr-pdms_b 263,19 (±13,49) 1,03 (±0,01) 95 (±5) figure 5. images of ls before and after the self-cleaning test. figure 6. the discoloration percentage (d* (%)) after uv exposure. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 5 final performance, the experimental data discussed above indicate that the nanocomposite coating (zro2-zno-pdms) provides better results in terms of the self-cleaning effect when compared to plain pdms. 4. conclusions in order to enhance the photocatalytic properties of zno, zro2-doped zno nps were synthesised in the laboratory. after that, the nanocomposite zro2-zno-pdms was synthesized, applied on different stone substrates (ls, b, and m), and its protecting behaviour was compared to the well-known pdms. the morphological analysis suggested that prepared doped nps have spherical shape, very small sizes (15-30 nm). this small size of the particles is effectively involved in providing great performances of the prepared coating. for instance, the results of contact angles, chromatic variations, capillary absorption and water vapour permeability measurements indicate satisfactory protecting behaviour of the resulting coating. moreover, surface analyses suggested that nanoparticles included into the binder matrix are homogeneously distributed on all the stone surfaces. furthermore, the new coating (zro2zno-pdms) showed better results when compared to pdms in terms of self-cleaning effect due to uv irradiation, which represents one of the most important result of this research work. further experiments are still in progress to better assess the nanocomposite properties. references [1] c. kapridakia, l. pinhob, m. j. mosquerab, p. maravelakikalaitzakia, producing photoactive, transparent and hydrophobic sio2-crystalline tio2 nanocomposites at ambient conditions with application as self-cleaning coatings, appl. catalys. b: environ, vol. 156–157, 2014, pp. 416–427. doi: 10.1016/j.apcatb.2014.03.042 [2] m. licchelli, m. malagodi, 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https://doi.org/10.1006/jcat.2002.3539 https://doi.org/10.1016/j.cej.2017.03.067 https://doi.org/10.1155/2014/349457 acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 6 [22] uni en 15801:2010, conservazione dei beni culturali, metodi di prova, determinazione dell’assorbimento dell’acqua per capillarità, 2010 [in italian]. 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[25] m. b. chobba, m. l. weththimuni, m. messaoud, c. urzi, j. bouaziz, f. d. leo, m. licchelli, ag-tio2/pdms nanocomposite protective coatings: synthesis, characterization, and use as a selfcleaning and antimicrobial agent, prog. in org. coat, vol. 158, 2021, pp. 106342-106359. doi: 10.1016/j.porgcoat.2021.106342 https://doi.org/10.1016/j.porgcoat.2021.106342 journal contacts acta imeko issn: 2221-870x june 2021, volume 10, number 2 acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 0 journal contacts about the journal acta imeko is an e-journal reporting on the contributions on the state and progress of the science and technology of measurement. the articles are mainly based on presentations presented at imeko workshops, symposia and congresses. the journal is published by imeko, the international measurement confederation. the issn, the international identifier for serials, is 2221-870x. editor‐in‐chief francesco lamonaca, italy founding editor‐in‐chief paul p. l. regtien, netherlands associate editor dirk röske, germany editorial board section editors (vol. 7 10) leopoldo angrisani, italy filippo attivissimo, italy eulalia balestieri, italy eric benoit, france paolo carbone, italy lorenzo ciani, italy catalin damian, romania pasquale daponte, italy luca de vito, italy luigi ferrigno, italy edoardo fiorucci, italy alistair forbes, united kingdom helena geirinhas ramos, portugal sabrina grassini, italy fernando janeiro, portugal konrad jedrzejewski, poland andy knott, united kingdom yasuharu koike, japan francesco lamonaca, italy massimo lazzaroni, italy fabio leccese, italy rosario morello, italy michele norgia, italy pedro miguel pinto ramos, portugal nicola pompeo, italy sergio rapuano, italy gustavo ripper, brazil maik rosenberger, germany dirk röske, germany alexandru salceanu, romania constantin sarmasanu, romania lorenzo scalise, italy emiliano schena, italy enrico silva, italy krzysztof stepien, poland ronald summers, uk marco tarabini, italy yvan baudoin, belgium francesco bonavolonta, italy giuseppe caravello, italy carlo carobbi, italy marcantonio catelani, italy mauro d’arco, italy egidio de benedeto, italy alessandro depari, italy alessandro germak, italy min-seok kim, korea momoko kojima, japan koji ogushi, japan vilmos palfi, hungary franco pavese, italy jeerasak pitakarnnop, thailand jan saliga, slovakia emiliano sisinni, italy ciro spataro, italy oscar tamburis, italy jorge c. torres-guzman, mexico ioan tudosa, italy ian veldman, south africa rugkanawan wongpithayadisai, thailand claudia zoani, italy about imeko the international measurement confederation, imeko, is an international federation of actually 42 national member organisations individually concerned with the advancement of measurement technology. its fundamental objectives are the promotion of international interchange of scientific and technical information in the field of measurement, and the enhancement of international co-operation among scientists and engineers from research and industry. addresses principal contact prof. francesco lamonaca university of calabria department of computer science, modelling, electronic and system science via p. bucci, 41c, vi floor, arcavacata di rende, 87036 (cs), italy e-mail: f.lamonaca@dimes.unical.it acta imeko giuseppe caravello, e-mail: giuseppe.caravello02@unipa.it ciro spataro, e-mail: ciro.spataro@unipa.it support contact dr. dirk röske physikalisch-technische bundesanstalt (ptb) bundesallee 100, 38116 braunschweig, germany e-mail: dirk.roeske@ptb.de mailto:f.lamonaca@dimes.unical.it mailto:giuseppe.caravello02@unipa.it mailto:ciro.spataro@unipa.it mailto:dirk.roeske@ptb.de twisted and coiled polymer muscle actuated soft 3d printed robotic hand with peltier cooler for drug delivery in medical management acta imeko issn: 2221-870x september 2022, volume 11, number 3, 1 6 acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 1 twisted and coiled polymer muscle actuated soft 3d printed robotic hand with peltier cooler for drug delivery in medical management rippudaman singh1, sanjana mohapatra1,2, pawandeep singh matharu1, yonas tadesse1,2,3,4 1 humanoid, biorobotics, and smart systems laboratory, mechanical engineering department, the university of texas at dallas, richardson, tx 78705 usa 2 biomedical engineering department, the university of texas at dallas, richardson, tx 78705 usa 3 electrical and computer engineering department, the university of texas at dallas, richardson, tx 78705 us 4 alan g. macdiarmid nanotech institute, the university of texas at dallas, richardson, tx 78705 usa section: research paper keywords: robotic hand; artificial muscle; tcp muscles; fishing lines muscles; 3d printed hand; peltier cooling; biomimetic; grasping; drug delivery citation: rippudaman singh, sanjana mohapatra, pawandeep singh matharu, yonas tadesse, twisted and coiled polymer muscle actuated soft 3d printed robotic hand with peltier cooler for drug delivery in medical management, acta imeko, vol. 11, no. 3, article 10, september 2022, identifier: imeko-acta11 (2022)-03-10 section editor: zafar taqvi, usa received march 25, 2022; in final form september 27, 2022; published september 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work is supported by internal fundresearch enhancement corresponding author: rippudaman singh, e-mail: rippudaman.singh@utdallas.edu 1. introduction soft actuators such as sma muscles and their composites are being widely studied currently. however, smas are expensive and are not manufactured easily in-house, and have high hysteresis behaviour. new actuators such as tcpfl muscles are emerging as a new class of actuators recently. these muscles have a wide range of possibilities in robotics applications. in this paper, the focus is on the application of tcpfl muscles in a robotic hand and a method of improving the actuation frequency. the fabrication of tcpfl muscles has been discussed in a study by haines et al. where [1] different materials among polyethene, nylon 6, sand silver-plated nylon 6,6 were investigated for applications in soft actuators. twisted and coiled nylon 6,6 polymer as artificial muscles were found to be reliable, inexpensive, had high load carrying capacity, and a large stroke. another study [2] discusses the use of tcpfl muscles with nichrome wire, where nichrome acts as a heater wire for the muscles with quick heating capabilities. the incorporation of this novel soft actuator in robotic arms can help reduce the size and weight of existing motor-actuated prosthetic hand models while producing similar actuation. also, abstract this paper presents experimental studies on a soft 3d-printed robotic hand whose fingers are actuated by twisted and coiled polymer (tcpfl) muscles, driven by resistive heating, and cooled by water and peltier mechanism (thermoelectric cooling) for increasing the actuation frequency. the hand can be utilized for pick and place applications of drugs in clinical settings, which may be repetitive for humans. a combination of abs plastic and thermoplastic polyurethane material is used to additively manufacture the robotic hand. the hand along with a housing tank for the muscles and peltier coolers has a length of 380 mm and weighs 560 gm. the fabrication process of the tcpfl actuators coiled with 160 µm diameter nichrome wires is presented. the actuation frequency in the air for tcpfl is around 0.01 hz. this study shows the effect of water and peltier cooling on improving the actuation frequency of the muscles to 0.056 hz. experiments have been performed with a flex sensor integrated at the back of each finger to calculate its bend-extent while being actuated by the tcpfl muscles. all these experiments are also used to optimize the tcpfl actuation. overall, a low-cost and lightweight 3d printed robotic hand is presented in this paper, which significantly increases the actuation performance with the help of cooling methods, that can be used in applications in medical management. file:///c:/users/sanja/appdata/roaming/microsoft/word/rippudaman.singh@utdallas.edu acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 2 as proved in a study by wu et al. [3] the temperatures of the muscle when actuated in the air can reach higher values in the range of 80 °c to 140 °c. while these high temperatures were optimal during the heat-induced compression cycle, they did not allow a proper relaxation of the muscles. to initiate a proper relaxation, the cooling cycle would have to be configured for a long time which increases the time duration for each actuation cycle and reduces the actuation frequency. the use of cooling methods to optimize the high temperatures of the tcpfl muscle brought about by joule heating for proper relaxation is discussed in this paper. the main purpose of the paper is to reduce the cooling cycle of the tcpfl muscles to reduce the time gap between flexion and extension and increase the overall actuation frequency. water is the most available resource which can act as a natural coolant. water having a high specific heat capacity [4] can extract heat from the tcpfl muscles at a fast pace. it has been used in other applications to optimize the actuation of artificial muscles including shape memory alloys (sma) [5], [6], giant magnetostrictive materials (gmms) [7], and liquid crystal elastomers (lces)[8]. similar methods have been discussed with a robotic finger attached to tcpfl muscles in previous work by wu, l. et al [9] where the tcpfl muscles were actuated with the help of hot and cold water. the use of water was also inspired by previous studies where niti sma [10] and tcp [11] muscles were used in underwater jellyfish robots. it was noted that the actuation frequency drastically increased in a medium where heat was dissipated faster. this study was mimicked by submerging the muscles in a sealed container with water as a medium to dissipate heat. a study by astrain, d. et al [12] introduces a type of cooling system that uses voltage difference to generate a temperature difference between the top and bottom ceramic plates of the device. this uses alternating semiconductor couples to leverage the peltier effect to produce thermoelectric cooling. this type of thermoelectric cooler is called a peltier cooler. another study on peltier coolers [13] incorporated it in a closed insulated container to lower the internal temperatures to the freezing point of water. such peltier-based coolers were also used with other artificial sma muscles [14] to optimize their thermomechanical properties. hence, they were considered for use in optimizing the actuation of tcpfl muscles in this study. the hypothesis of this paper focuses on implementing these water-based and peltier cooling methods to optimize the flexion and extension of a prosthetic finger carried out by the tcpfl muscles. the improvement of actuation frequency of the tcpfl muscles for a faster prosthetic hand grasping movement was the major objective of the experiments designed for this study. this study involved collecting prosthetic performance data using flex and temperature sensors. experiments were conducted using special housing tanks to incorporate the muscles, water, peltier coolers, and sensors. substantial improvement in actuation frequency, from 0.01 hz in the air to 0.056 hz due to the cooling methods, was observed through the sensor data obtained during the experiments. submerging the tcpfl muscles in water inside the container and further dissipating the heat from the container using peltier coolers proved to be very effective. this was especially helpful for improving the performance speed of a prosthetic hand for applications like pick and place of medical drugs in clinical settings. 2. methodology firstly, the fabrication of the core component of this experimental study, the tcpfl artificial muscle is discussed in this paper. these muscles were made of nylon 6,6 fishing line muscles wounded with nichrome wire. the nichrome wire was used to convert the power supplied into heat and this heat was used via conduction to heat the fishing line for it to contract. 2.1. tcpfl fabrication process an approximate 1.5m length of nylon fishing line string was cut from a spool of fishing line. the ends of this cut length were tied with rings/washers to mount on motors that rotated and coiled the muscles. as shown in figure 1 the nylon fishing line was attached to a motor on the top and the other end was suspended with a weight of 500g. this was to ensure that enough tension was provided to keep the coiling uniform since the load should not be heavy enough to break the fishing line while coiling. the top motor was started at a speed of 300 rpm while restricting the rotation of the bottom end of the fishing line. the fishing line was allowed to coil upwards while twisting. only the axial rotation was arrested once coiling started, either from the top, bottom or middle. the motor was stopped, and the rotatory restriction was removed. at this stage, the winding of nichrome over the uncoiled fishing line was initiated. the nichrome was coiled over the fishing line at a speed of 125 rpm. once the nichrome was uniformly wound over the fishing line muscle, the rotatory restriction was applied again so that the nylon coiling process could be carried out. at the end of the coiling process, the weight was removed. at the end of this entire process the bottom end of the coiled muscle must be held firmly before and after removing the weight, slightly releasing the hand pressure and once the weight was removed so that the muscle could ease a bit of the torsional tension. the muscle would not get uncoiled but only get untwisted a few rotations. after this, the muscle was removed from the coiling setup, and each of its ends was mounted on a small platform that could be used to heat treat the muscle. a furnace was heated up to 180 degrees and the muscles were placed inside the hot furnace for 90 min. once the muscles were heated the ends of the same were crimped firmly with the nichrome in contact with gold crimps. the muscles were then placed under a 500gm load and trained under various power cycles that were supplied to their crimped ends. various currents supplied brought about different compressions in steps as shown in figure 1. schematic of the fabrication process of the twisted and coiled polymer (fishing line) muscles. step1: the fishing line twisting process. step 2: the nichrome winding process. step 3: the self coiling process. acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 3 table 1. this training step later supports similar compressions and relaxations when provided with a current across its ends. 2.2. tpu hand setup a single piece robotic palm was 3d printed with the thermoplastic polyurethane (tpu) material. tpu is a flexible material that can provide strength in higher thicknesses and toughness and flexibility in lower thicknesses. this hand was previously utilized in combination with other artificial sma muscles [15]. so, this is adequate to test the tcpfl muscles used in the design presented in this paper. the container was made of transparent acrylic material, to ensure clear visibility of the movement of the muscles inside the container. dedicated slots were accommodated for the peltier plates on the bottom of the container. the container was assembled and sealed with the help of mseal and silicone. the m-seal provides strength to the container and the silicone ensures water saleability. holes were provided to accommodate ten tcpfl muscles, two for each finger one for flexion and the other for extension. the openings were sealed with silicone to allow flexibility and to ensure water saleability. one end of the muscle was fixed to the back of the container. the other end was connected to a finger of a single piece tpu hand as shown in figure 2.a using fishing line strings. when power was applied across the muscles, they contracted at the same time pulling the fishing line string connected to the finger. 2.3. peltier cooler and sensors two peltier coolers were inserted into the dedicated slots inside the same muscle housing container. these peltier coolers each consist of 127 couples of n-type and p-type semiconductor blocks that operate at 12 v 2a to create thermoelectric cooling across their plates. so, both were connected in series to a 24 v 2a battery as seen in figure 3.c. the effect of these peltier coolers on lowering the temperatures of water that is exposed to the cooling plate was observed using underwater temperature probes. the probe used was the ds18b20 digital single-bus intelligent temperature sensor [16] and that has been seen in previous studies involving underwater applications [17], [18]. this sensor when connected across a circuit as shown in figure 3.a provides a digital output of the temperature probe to an arduino microcontroller. the temperature data recorded from the probe attached underwater inside the container during the experiments helped in assessing the benefits of the cooling methods for tcpfl actuation. there are many instances where standard flex sensors have been used along with hand prosthetic applications to either control the bending of the prosthetic fingers [19], [20] or recognize [21], [22], [23] the gestures of hands. also known as stretch sensors, they can be used in wearables [24] to track the bending of a finger. this is a resistive sensor that alters its resistance value as it is bent along its length. hence this resistive property was manipulated for characterizing the bending action of the prosthetic hand mentioned in this paper. the data obtained from the flex sensor helped characterize the tcpfl muscles for their actuation frequency and optimization of the actuation with cooling methods. for this design, the resistance was converted to a voltage reading using an amplification circuit as seen in figure 3.b and read through the adc pins of the arduino. this voltage reading was recorded with respect to time and linearly interpolated to previously obtained angle-vs-voltage flex sensor calibration data. this calibration data helped in calculating the angular position of the finger with respect to the horizontal as shown in figure 2.b. the angle-vs-time results obtained from the flex sensors helped in observing the actuation frequency of the muscles with different cooling conditions. 3. experimental methods and results the experimentations conducted for the tcpfl muscles included a characterization setup utilized in other studies to understand various properties of niti sma [10], tcp [11], and table 1. power supply supplied across both ends of the tcpfl muscles during the training procedure. current (a) deformation (%) 0.18 10 0.2 14 0.24 20 0.26 27 figure 2. (a) the schematic of the experimental setup. the fingers of a tpu prosthetic hand are attached to tcpfl muscles using fishing line strings. bidirectional flexion and extension actions of the finger are produced by the actuation of two tcpfl muscles. the muscles are housed in an acrylic tank filled with water. this tank has integrated two peltier coolers on its base. (b) the angular position of the finger with respect to the horizontal is calculated using data recorded from flex sensors during flexion or extension actions. figure 3. schematic of the electronic circuits of the sensors and peltier coolers. (a) ds18b20 temperature probe used to monitor the heating and cooling of the water during tcpfl actuation and peltier cooling. data collected by the digital pins of the arduino microcontroller. (b) flex sensor used to monitor the finger bending movements during the finger’s actuation. sensor integrated with an amplification circuit. (c) peltier coolers used to cool the water in the tcpfl housing tank. connected in series with a 24v power supply. acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 4 tca [25] muscles using multiple sensors as shown in the study by hamidi, a. et al [26]. the setup as shown in figure 4 included a keyence laser displacement sensor to measure the muscle’s actuation strain, thermocouples used to measure the muscle’s temperature during actuation, and ni daq 9221 used to measure the output voltage due to change of muscle’s resistance during actuation. using this setup characterization experiments were conducted with the tcpfl muscles fabricated in this paper. the properties of the muscle were obtained from these experiments and are listed in table 2. it included an actuation strain ranging from 10 % to 27 %. the voltage across the muscle could rise to 115 v due to a change in resistance after being actuated. the experimental setup of the tpu hand actuated by two tcpfl muscles as described in the methodology and shown in figure 5 was used to obtain results to justify the hypothesis of this paper. there was a separate power supply for each of the two tcpfl muscles. each power supply was configured to form different heating and cooling cycles to respectively compress and relax the muscle. the lower tcpfl muscle responsible for an extension usually followed a flexion by the upper tcpfl muscle. hence the lower muscle required more energy to compress to additionally loosen the upper muscle’s compression. the actuation cycles for flexion (by the upper tcpfl muscle) and extension (by the lower tcpfl muscle) were set separately on the two power supplies. the temperature data obtained from the ds18b20 sensor showed an increase in the temperature of the water when tcpfl muscles were actuated in it. figure 6 portrays the same behaviour when one muscle was actuated inside different volumes of water at a heating cycle of 5 seconds and a cooling cycle of 10 seconds. higher volumes of water had a high capacity to dissipate more heat from the muscles and had a lower rise in temperatures. figure 7 portrays the cooling ability of peltier coolers with respect to the volume of water. these cooling rates show that peltier coolers can be instrumental in optimizing the actuation frequency of the muscles. a 150 ml water was finally chosen as optimal for the experiments of this study as it would get heated more slowly by the muscles as compared to lower volumes. although the cooling rate is smaller than that of lower volumes it would be enough for this application. figure 4. experimental setup for the characterization of tcpfl muscles. table 2. informational data of the fabricated tcpfl muscle obtained through characterization experiments performed during similar other studies on tcp muscles [10], [11]. property value material nylon (6,6) fishing line type of actuation electrothermal type of resistance wire nichrome (nickel, chromium) resistance wire diameter dw = 160 precursor fibre diameter d = 0.8 mm length of precursor fibre l = 1500 mm weight for fabrication mf = 500 g annealing temperature/time ta = 180 °c / (90 min) diameter after cooling d = 2.8 mm length after cooling l = 120 mm resistance r = 110 ohm current (input) i = 0.16-0.26 a (during training: i = 0.26 a is provided to the muscle) voltage (output) v = 57.6-115.2 v actuation power p = 9.2-36.8 w heating time th = 10 s,15 s cooling time tc = 90 s, 85 s actuation frequency (air-cooled) f = 0.16-0.1hz actuation strain, at 500g load epsilon = 27-10% life cycle 2400 cycles in air at 9 mhz and 1 % duty cycle. figure 5. experimental setup of tpu hand with tcpfl muscles and peltier coolers installed housing tank. two power supplies used to actuate each of the two muscles for the movement of one finger. the flex sensor and temperature probe circuits utilized inside the setup. figure 6. heating effect of the tcpfl muscle actuation on water. the muscle was actuated with a heating cycle of 5 seconds and a cooling cycle of 10 seconds. the steady rise in temperature was observed by the ds18b20 temperature probe over 50 cycles of actuation. the heating effect of the muscle was observed for different volumes of water. figure 7. cooling the water in the housing tank with peltier coolers. the reduction of temperature in the water was observed over time by the ds18b20 temperature probe for different volumes of water. acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 5 flex sensor data was obtained for different actuation cycles for the tcpfl muscles and different environmental conditions (in air, in water, and in peltier-cooled water). for actuating the finger in the air at 0.26 a the heating and cooling cycles were set as 10 seconds and 90 seconds respectively for the upper muscle (flexion) and 15 seconds and 85 seconds respectively for the lower muscle (extension). this setting resulted in an actuation frequency of 0.01 hz for the tcpfl muscles. when shorter cycles were selected for actuation, the upper muscle could not relax from its compressed state during its cooling cycle. this prevented the extension of the finger through the lower muscle’s actuation. therefore, longer cooling cycles of 90 seconds were selected for proper in-air actuation of the two muscles as was proved by the flex sensor results (figure 8). the actuation frequency of the tcpfl muscles improved by about five times to 0.056 hz when the muscles were completely submerged in 150 ml of water. the heating and cooling cycles operating at 1 a were set as 5 seconds and 13 seconds respectively for the upper muscle and 7 seconds and 11 seconds respectively for the lower muscle. as was seen from the angles interpolated from the flex sensor results (figure 9.a) there was a very good frequency of actuation. the time to bring about these actuations was minimized from previous experiments in-air. an even smoother actuation was observed during the extension of the finger when the peltier coolers were utilized to remove the heat accumulated in the water medium. the same actuation cycles were used, like before with water. through the interpolated flex sensor results obtained in figure 9.b, a significantly faster extension was observed. the extension angle-vs-time slope is much steeper in peltier-cooled waters as compared to the same in just water. peltier coolers help in maintaining the water at lower temperatures to aid in the cooling/relaxation phase of muscles. 4. conclusions the cooling methods proposed proved to have an impact in optimizing the actuation performance of the tcpfl muscles. the frequency of the muscle’s actuation was increased by over five times from 0.01 hz when they were actuated in-air to 0.056 hz when they were actuated in water. the major cooling effect observed was due to the presence of water as a medium to dissipate heat from the actuators. the finger movement as a result was very smooth and fast-paced. the peltier coolers ensured that the temperature of the water was maintained close to room temperature. this was achieved as the coolers dissipated the heat from the water that was accumulated from the actuating muscles. due to this behaviour, the improved frequency of actuation was maintained at a high of 0.056hz for multiple cycles. the actuation for the extension of the robotic finger by the lower tcpfl muscle took a longer time than the actuation for the flexion of the finger by the upper tcpfl muscle. this was because of the tension remaining in the upper muscle from the compression during the flexion action. this in turn kept the finger tightly flexed and did not allow it to be extended in the other direction. the peltier coolers shortened the extension actuation phase of the robotic finger. the extension phase of the setup depended on how fast the upper tcpfl muscle cooled down and relaxed itself. this faster cooling was achieved due to lower temperatures of water produced by the peltier coolers. further studies and experimentation are required to improve the performance of these muscles in an enclosed setup. some new housing tanks designs and materials are being explored to augment prosthetic actuation efficiency. acknowledgement the authors would like to thank akash ashok ghadge for his support in the experimentation and design and eric wenliang deng for his support with the tpu palm used for the experimentation, they both are affiliated to the humanoid biorobotics and smart systems lab, the university of texas at dallas. figure 8. the angular position of each finger during bi-directional actuation by two tcpfl muscles. this flexion/extension angle of the finger was calculated from the data obtained from the flex sensor. the muscles were actuated in the air with a heating cycle of 10 seconds (for the flexion action shown in yellow) and a cooling cycle of 90 seconds for the upper muscle and a heating cycle of 15 seconds (for the extension action shown in green) and a cooling cycle 85 seconds for the lower muscle. both the muscles complete each cycle in 100 seconds, so the actuation is 1/100 = 0.01 hz. figure 9. the angular position of each finger calculated from flex sensor data during bi-directional actuation by two tcpfl muscles. the muscles were actuated for a heating cycle of 5 seconds (for the flexion action shown in yellow) and a cooling cycle of 13 seconds for the upper muscle and a heating cycle of 7 seconds (for the extension action shown in green) and a cooling cycle 11 seconds for the lower muscle. both the muscles complete each cycle in 18 seconds, so the actuation is 1/18 = 0.056 hz. the tcpfl muscles were submerged inside (a) 150 ml of water, (b) 150 ml of water that was cooled by two peltier coolers acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 6 references [1] c. s. haines (+20 authors), artificial muscles from fishing line and sewing thread, science 343(6173) (2014), pp. 868-872. doi: 10.1126/science.1246906 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section: editorial citation: francesco lamonaca, introductory notes for the acta imeko second issue 2021 general track, acta imeko, vol. 10, no. 3, article 3, september 2021, identifier: imeko-acta-10 (2021)-03-03 received september 6, 2021; in final form september 9, 2021; published september 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: francesco lamonaca, e-mail: f.lamonaca@dimes.unical.it 1. introductory notes for the acta imeko general track the third issue 2021 of acta imeko includes a general track aimed at collecting contributions that do not relate to a specific event. as editor in chief, it is my pleasure to give readers an overview of these papers, with the aim of encouraging potential authors to consider sharing their research through acta imeko. benedikt seegerand and thomas bruns., in ‘primary calibration of mechanical sensors with digital output for dynamic applications’, tackle the challenge of the dynamic calibration of modern sensors with integrated data sampling and purely digital output for the measurement of mechanical quantities, such as acceleration, angular velocity, force, pressure or torque. the proposed method is an extension of the established methods and devices that yields primary calibration results. the proposal focuses on primary accelerometer calibrations but can easily be transferred to other mechanical quantities. in ‘analysis of the mathematical modelling of a static expansion system’, carlos mauricio villamizar mora et al. analyse the suitability of different models to represent the final pressures in a static expansion system with two tanks. it is concluded that the use of the ideal gas model is adequate in most simulated conditions, while the assumption that the residual pressure is zero before expansion presents problems under certain conditions. an uncertainty analysis of the process is carried out, which leads to evidence of the high importance of uncertainty in a first expansion over subsequent expansion processes. finally, an analysis of the expansion system based on uncertainty is carried out to estimate the effect of the metrological characteristics of the measurements of the input quantities. the proposed design process makes it possible to determine a set of restrictions on the uncertainties of the input quantities. the transmission of medical data and the potential for distant healthcare structures to share experiments about a given medical case raise several conceptual and technical questions. good remote healthcare monitoring involves more challenges with regard to personalised heath data processing than the traditional methods currently used in hospitals throughout the world. the adoption of telemedicine in the healthcare sector has significantly changed medical collaboration. however, to provide good telemedicine services through new technologies such as cloud computing, cloud storage and so on, a suitable and adaptable framework should be designed. moreover, a secure and collaborative platform will enhance the decision-making process within the chain of medical information exchange, including between requesting agencies and physicians. in ‘collaborative systems for telemedicine diagnosis accuracy’, jacques tene et al. provide an in-depth literature review on interactions between telemedicine and cloud-based computing. the paper further proposes a framework that can allow various research organisations, healthcare sectors and government agencies to log data, develop collaborative analysis and support decisionmaking. case studies involving electrocardiograms and electroencephalograms demonstrate the benefit of the proposed approach in data reduction and high-fidelity signal processing at the local level, enabling the communication of extracted characteristic features to the cloud database. in ‘uncertainty of factor z in the gravimetric volume measurement’, mar lar win presents an interesting problem for the gravimetric volume measurement method. in this measurement, z factors are generally used to balance the liquid volume, facilitating an easy conversion from the apparent mass. the uncertainty of measurement assigned to the measurement of a liquid volume can be divided into two uncertainty contributions: those components related to the mass measurements and those components related to the mass-tovolume conversion. however, uncertainty due to the z factor is generally neglected in the uncertainty calculation of gravimetric mailto:f.lamonaca@dimes.unical.it acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 6 volume measurement in some international organization for standardization standards and calibration guides. the paper describes the combined effects of the density of water, the density of reference weights and air buoyancy on the uncertainty of the z factor and how this affects the uncertainty of the measurement result. most sensing networks rely on punctual/local sensors, thus lacking the ability to spatially resolve the quantity to be monitored (e.g. a temperature or humidity profile) without relying on the deployment of numerous inline sensors. most quasi-distributed or distributed sensing technologies rely on the use of optical fibre systems; however, these are generally expensive, which limits large-scale adoption. recently, elongated sensing elements have successfully been used with time-domain reflectometry (tdr) to implement diffused monitoring solutions. the advantage of tdr is that it is a relatively low-cost technology with adequate measurement accuracy and the potential to be customised to suit the specific needs of different application contexts of the 4.0 era. starting from these considerations, cataldo et al., in the paper ‘microwave reflectometric systems and monitoring apparatus for diffused-sensing applications’, addressees the design, implementation and experimental validation of a novel generation of elongated sensing element networks that can be permanently installed in the systems to be monitored and used to obtaining a diffused profile of the quantity to be monitored. in particular, three applications are considered as case studies: irrigation process monitoring in agriculture, leak detection in underground pipes and building structure monitoring. ding et al., in the paper ‘the inhibition of biodegradation on building limestone by plasma etching’ presents an innovative technique that has been recently applied in the cleaning of soiled archaeological objects. in the paper it is presented and discussed the use of low-pressure plasma etching in cleaning microbial contaminations on an oolitic limestone from an unesco world heritage listed monument: the batalha monastery in central portugal. the cleaning effect was assessed by ftir, sem, optical microscope, and cell viability index measurement. the experimental results demonstrate that plasma etching can be regarded as a fast and eco-friendly conservation tool for stone heritage architecture. this issue confirms that acta imeko is the natural platform for disseminating measurement information and stimulating collaboration among researchers from many different fields who are united by their common interest in measurement science and technology. francesco lamonaca editor in chief classification of brain tumours using artificial neural networks acta imeko issn: 2221-870x march 2022, volume 11, number 1, 1 7 acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 1 classification of brain tumours using artificial neural networks b. v. subba rao1, raja kondaveti2, r. v. v. s. v. prasad2, v. shanmukha rao3, k. b. s. sastry4, bh. dasaradharam5 1 department of information technology, pvp siddhartha institute of technology, vijayawada 520007, india 2 department of it, swarnandra college of engineering and technology, narasapuram, india 3 department of information technology, andhra loyola college of engineering and technology, vijayawada 520008, india 4 department of computer science, andhra loyola college of engineering and technology, vijayawada 520008, india 5 department of cse, nri institute of technology, agiripalli 521212, andhra pradesh india section: research paper keywords: artificial neural networks; brain tumour; classification; magnetic resonance brain image; wavelet transform citation: b. v. subba rao, raja kondaveti, r. v. v. s. v. prasad, v. shanmukha rao, k. b. s. sastry, bh. dasaradharam, classification of brain tumours using artificial neural networks, acta imeko, vol. 11, no. 1, article 35, march 2022, identifier: imeko-acta-11 (2022)-01-35 section editor: md zia ur rahman, koneru lakshmaiah education foundation, guntur, india received december 28, 2021; in final form february 19, 2022; published march 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: b. v. subba rao, e-mail: bvsubbarao90@gmail.com 1. introduction if any person having a brain tumour, the doctor may recommend a number of tests and procedures to identify the tumour which are present in the brain or it may be spreads into any parts of the body. if the tumour has found in the brain the doctor takes the biopsy and collecting the sample tissue and conduct the examination. in certain situations, the person may be affected to paralysis of their body. in this situation before testing of biopsy magnetic resonance (mr) [1] brain images were taken to study whether the tumour may be benign tumour or malignant tumour. there are two different types of tumours mainly found in the mr brain image those are benign tumour and malignant tumour [2] [3]. the stages of the study for our proposed work are magnetic resonance imaging (mri), feature extraction and classification. in the following sub sections, we have described the benign and malignant tumours. abstract magnetic resonance (mr) brain image is very important for medial analysis and diagnosis. these images are generally measured in radiology department to measure images of anatomy as well as the general physiological process of the human body. in this process magnetic resonance imaging measurement are used with a heavy magnetic field, its gradients along with radio waves to produce the pictures of human organs. mr brain image is also used to identify any blood clots or damaged blood veins in the brain. a counterfeit neural organization is a nonlinear information handling model that have been effectively used preparation models for tackling administered design acknowledgment assignments because of its capacity to sum up this present reality issues. artificial neural networks (ann) is used to classify the given mr brain image having benign or malignant tumour in the brain. benign tumours are generally not cancerous tumours. these are also not able to grow or spread in the human body. in very rare cases t hey may grow very slowly. once it is eliminated, they do not come again. on the other hand, malignant tumours are cancer tumours. these tumour cells are grown and also easily spread to other parts of the human body. benign also known as harmless. these are not destructive. they either can't spread or develop, or they do as such leisurely. on the off chance that a specialist eliminates them, they don't by and large return. premalignant in these growths, the cells are not yet harmful, however they can possibly become threatening. malignant also known as threatening. malignant growths are destructive. the cells can develop and spread to different pieces of the body. in our proposed framework initially, it distinguishes wavelet transform to separate the highlights from the picture. subsequent to separating the highlights it incorporates tumour shape and power attributes just as surface highlights are distinguished. finally, ann to group the information highlights set into benign or malignant tumour. the main purpose as well as the objective is to identifying the tumours weather it belongs to benign or malignant. mailto:bvsubbarao90@gmail.com acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 2 1.1. benign tumour a tumour is an irregular improvement of cells that fills no need. a caring tumour is positively not a destructive tumour, which is harmful development. it doesn't assault near to tissue or spread to various bits of the body the way here harmful development can. generally speaking, the stance with obliging tumours is superb. a harmless growth is certainly not a dangerous growth, which is disease. it doesn't attack close by tissue or spread to different pieces of the body the manner in which disease can. as a rule, the standpoint with harmless growths is excellent. be that as it may, harmless growths can be not kidding assuming they push on fundamental designs like veins or nerves. consequently, now and again they require treatment and different times they don't. in any case, liberal tumours can be dead serious if they push on fundamental structures, for instance, veins or nerves. along these lines, at times they require treatment and various events they don't. the specific reason for a benign tumour is frequently obscure. it creates when cells in the body partition and develop at an exorbitant rate. commonly, the body can adjust cell development and division. at the point when old or harmed cells pass on, they are consequently supplanted with new, sound cells. on account of tumours, dead cells remain and structure a development known as a tumour. cancer cells fill in a similar way. nonetheless, in contrast to the cells in amiable tumours, harmful cells can attack close by tissue and spread to different pieces of the body. 1.2. malignant tumour harmful tumours [4] are cancer-causing. they make when cells grow uncontrollably. in case the cells continue to create and spread, the contamination can get dangerous. dangerous tumours can grow quickly and spread to various bits of the body in a cycle called metastasis. the malignancy cells that transition to different pieces of the body are equivalent to the first ones, yet they can attack different organs. in the event that cellular breakdown in the lung’s spreads to the liver, for instance, the malignancy cells in the liver are still cellular breakdown in the lung’s cells. various sorts of malignant tumours start in various kinds of cell. the expression malignant demonstrates that there is moderate to high likelihood that the growth will spread past the site where it at first creates. these cells can spread by movement through the circulation system or by movement through lymph vessels malignant tumour demonstrates that there is moderate to high likelihood that the tumour will spread past the site where it at first creates. these cells can spread by movement through the circulation system. a harmful cerebrum tumour is a carcinogenic development in the brain. it's unique in relation to a kind mind tumour, which isn't malignant and will in general develop more slowly. malignant mind tumours contain disease cells and frequently don't have clear lines. they are viewed as hazardous in light of the fact that they develop quickly and attack encompassing cerebrum tissue. in the existing mechanism mr brain image were taken and biopsy test was conducted that is known as follicular dendritic cell tumour (fdct) pathology test. fdct test is performed for removal of noise and then extracted the features from that mr brain image. after extracting the features from the image then support vector machine (svm) classification algorithm is applied to classify the features and characteristics. but in the svm the accuracy and speed of them is very slow. the results may not be clear and accurate. by understanding this problem, we are proposed a new classification algorithm called as artificial neural networks (ann). ann is used to improve the accuracy of the classifier and classifier speed may be increased. 2. our proposed methodology and its discussion the proposed ann extracts the features from the brain image and classifies the brain tumour into multiple images. there are three stages in our proposed methodology to observe whether the tumour is benign or malignant they are: a) pre-processing b) feature extraction c) classification 2.1. pre-processing in this headway the proposed framework utilizes the median filter. middle filter eliminates the turmoil from the mr cerebrum image. noise on the image means undesirable data present in the mr brain image. median filter has very protective capacity and heftiness. it diminishes the salt and pepper noise in the mr brain image. it also reduces the blurring of an image by applying smoothing technique. the main observation of median filter is it replaces the current point in the image to median value of the brightness of nearby pixel i.e. supplanting each neighbour an incentive with the middle estimation of the pixel. median filter also eliminated the impulse noise. so, median filter is a suitable pre-processing method in our proposed method. 2.2. feature extraction after pre-processing is completed the noiseless mr brain image was generated by applying the median filter technique then features are extracted from that image. feature extraction means, it is a process of identifying the set of features in an image. features are obtained from colour, shape and texture. good features are having produces the informative distinctive, accuracy, locality, reliability, quality, robustness and efficiency. all these are observing in the classification process. still the feature extraction is quite challenging issue to identify the current features of an image. many feature extraction techniques are available. in our proposed method we have used db4 (daubechies 4) wavelet transform for extracting the features like standard deviation, minimum and maximum value in wavelet transform. db4 wavelet transform is used in our proposed method for extracting the features. 2.3. classification after extracting the features from the image by applying db4 wavelet transform technique. the input feature extraction values are used for classification. in the real time many classification algorithms/techniques are available in the existing system they were used svm [5] classification technique but the accuracy is not up to the mark. processing also slow and will take a huge time. to overcome these problems, we are using artificial neural network classifier for image classification in our proposed method. in this ann, we use back propagation neural network. neural network classification is done by using multilayer perceptron algorithm. after applying all these algorithm/techniques we will set the output as benign or malignant tumour on the mr brain image. 3. db4 wavelet transform the daubechies wavelet changes are described as the haar wavelet change by enlisting midpoints and differences to acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 3 methods for scalar things with scaling signs and wavelets the single qualification between them involves in how these scaling signs and wavelets are portrayed. for the daubechies wavelet [6] changes, the scaling signs and wavelets have somewhat longer backings, i.e., they produce midpoints and contrasts utilizing just a couple of more qualities from the sign. the daubechies d4 change has four wavelet and scaling capacity co-efficient. the following formula shows the scaling capacity of the coefficient. ℎ0 = 1 + √3 4√2 , ℎ1 = 3 + √3 4√2 , ℎ2 = 3 − √3 4√2 , ℎ3 = 1 − √3 4√2 . (1) every movement of the wavelet change applies the scaling ability to the data input, if the main instructive assortment has n regards and the scaling limit will be applied in the wavelet change step to learn n2 smoothed characteristics in the organized wavelet change and the smoothed characteristics are taken care of in the lower half of the n part input vector. this can be represented as follows. {𝑔0 = ℎ3; 𝑔1 = −ℎ2; 𝑔2 = ℎ1; 𝑔3 = −ℎ0} (2) the wavelet change applies the wavelet ability to the data if the primary instructive assortment has n regards. the main enlightening file has n regards and the wavelet limit will be applied to figure n/2 differentiations. the scaling and wavelet limits are dictated by taking the internal consequence of the coefficient and four data regards. the conditions are shown following formula. 𝑎𝑖 = ℎ0 𝑠2𝑖 − ℎ1 𝑠2𝑖−1 − ℎ2 𝑠2𝑖−2 − ℎ3 𝑠2𝑖−3 𝑎[𝑖] = ℎ0 𝑠[2𝑖] − ℎ1 𝑠[2𝑖 − 1] − ℎ2 𝑠[2𝑖 − 2] − ℎ3 𝑠[2𝑖 − 3] (3) daubechies d4 wavelet function: 𝑐𝑖 = 𝑔0 𝑠2𝑖 − 𝑔1 𝑠2𝑖−1 − 𝑔2 𝑠2𝑖−2 − 𝑔3 𝑠2𝑖−3 𝑐[𝑖] = 𝑔0 𝑠 [2𝑖] − 𝑔1 𝑠[2𝑖 − 1] − 𝑔2 𝑠[2𝑖 − 2] − 𝑔3 𝑠[2𝑖 − 3] (4) each iteration in the wavelet step calculates a scaling value and a wavelet function value. 4. artificial neural network a neural organization comprises of formal neurons which are associated so that every neuron yield further fills in as the contribution of for the most part more neurons correspondingly as the axon terminals of a natural neuron are associated by means of synaptic ties with dendrites of different neurons. the quantity of neurons and how they are interconnected decides the engineering of neural organization. counterfeit neural organizations are one of the primary instruments utilized in artificial intelligence. as the neural a piece of their name recommends, they are cerebrum enlivened frameworks which are planned to reproduce the way that we people learn. neural organizations comprise of information and yield layers, just as a concealed layer comprising of units that change the contribution to something that the yield layer can utilize. they are astounding instruments for seeing examples which are far as excessively mind boggling or various for a human software engineer to concentrate and show the machine to perceive. the multi-layered neural organization is the most generally applied neural organization, which has been utilized in many explores up until this point. a back-engendering calculation can be utilized to prepare these multilayer feed-forward organizations with differentiable exchange capacities. it performs work estimate, design affiliation, and example arrangement. the term back proliferation alludes to the cycle by which subsidiaries of organization blunder, concerning network loads and inclinations, can be registered. the preparation of anns by back proliferation includes the following three phases: (i) the feed forward of the info preparing design, (ii) the estimation and back spread of the related mistake, (iii) the change of the loads. this cycle can be utilized with a number of different enhancement procedures. the following figure shows the artificial neural network [7]-[10] procedure it contains the input data, hidden layer processing and then produces the output. a neural network has at least three layers that are interconnected. the main layer comprises of information neurons. those neurons send information on to the more profound layers, which thus will send the last yield information to the last yield layer. all the inward layers are covered up and are shaped by units which adaptively change the data got from layer to layer through a progression of changes. each layer demonstrations both as an information and yield layer that permits the ann to see more unpredictable items [11]-[15]. all things considered; these inward layers are known as the neural layer. figure 1 depicts an artificial neural network. 5. results based on our proposed method and as per the above discussions we took a series of benign and malignant tumour images are as input mr images. a malignant tumour is a quickly developing malignancy that spreads to different zones of the cerebrum and spine. by and large, brain tumours are evaluated from one to four, as indicated by their conduct, for example, how quick they develop and that they are so prone to develop back after treatment. a malignant tumour is either grade three or four, though grade one or two tumours are generally classed as kind hearted or non-destructive. most harmful tumours are an optional malignant growth, which implies they began in another piece of the body and spread to the cerebrum. essential cerebrum tumours are those that begun in the mind. the figure 2 shows the series of benign tumour images and figure 3 shows the series of malignant tumour images which are considered as input images to our method. figure 1. artificial neural network. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 4 after taking input mr image, we have applied our proposed pre-processing method, feature extraction and classification techniques to find weather the resultant image having benign tumour or malignant tumour. initially we have taken the original gray scale image in the pre-processing method. gray scale image is converted into binary image after then cleaned the binary image by applying smoothing technique. noise was reduced by applying the pre-processing method. subsequent to pre-processing to identify the features in the mr image, we are using db4 wavelet transform method to identify the features and also extracted the feature in the given image. here salt and figure 2. series of benign tumours in brain mr images (input images). figure 3. series of malignant tumours in brain mr images (input images). figure 4. after applying db4 wavelet transform. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 5 pepper noise also reduced in this technique. after identifying the features, finally proposed classification technique like ann was applied and observes the parameters standard deviation, maximum value and minimum value to classify image either benign or malignant. figure 4 shows the pre-processing working procedure and extracting the feature by using db4 wavelet transform. it shows how the original grayscale input image is converted into a binary image and then the cleaned binary image after then applying the db4 wavelet transform. the histogram representation is shown in figure 5. it shows the highest and lowest pixel values of a binary image and it also shows the parameter values of standard deviation, maximum value and minimum value. table 1 represents the stratified non-validation. table 2 represents detailed accuracy by class. confusion matrix is shown in table 3 which represents the classification of given input images that produce the benign or malignant tumour output by applying anns [16], [17]. the event of cerebrum tumours in india is consistently rising. an ever-increasing number of instances of cerebrum tumours are accounted for every year in our country among individuals of changed age gatherings. brain tumours were positioned as the tenth most basic sort of tumour among indians. there are more than 32,000 instances of brain tumours announced in india every year and in excess of 28,000 individuals purportedly pass on because of cerebrum tumours yearly. a brain tumour is a genuine condition and can be deadly if not identified early and treated. in the results we have disclosed the category of tumour that is either benign or malignant tumour using our methodology so that we can predict the type of cancer in advance. table 4 shows the distinctive features of a db4 wavelet transform like standard deviation, minimum and maximum values of a series of mr images. based on these values the system easily classifies benign tumour or malignant tumour. 6. conclusion our proposed method is utilized for identifying e tumour from the given mr mind pictures and grouping whether it is benign (normal) tumour or malignant (cancer causing) tumour at the beginning stage. in the new patterns this framework/method assumes a significant job to distinguish the brain cancer in the beginning at very early stage which diminishes the death rate. the degree of things to come improvement in this endeavour is that we would connect data be able to base so colossal number of pictures can be used in distinguishing the threatening development. we can improve its exactness by utilizing various calculations of counterfeit neural organizations like convolution neural organizations, uphold vector machine and others. this computer aided classification system of our method takes any mr cerebrum filter picture, examinations it, and gives the yield as disease tainted mind if the sweep picture contains harmful tumour or probably gives yield as the cerebrum is malignant growth free if the output picture contains survival rate to some extent favourable tumour. as a whole we have succeeded in identify the tumour used in the given input mr images. we have successfully deployed our proposed methodology and able to classify the tumour weather it is a benign or malignant using artificial neural network. this methodology truly supports the patients as well as doctors to identify the tumour in a little bit advance which might save the lives. figure 5. histogram representation. table 1. stratified non-validation. s. no summary validation % of validation 1 correctly classified instances 15 75 2 incorrectly classified instances 5 25 3 kappa statistic 0.5 4 mean absolute error 0.3299 5 root mean square error 0.5034 6 relative absolute error 65.9703% 7 root relative squared error 100.6852% 8 total number of instances 20 table 2. detailed accuracy by class. tp rate fp rate presidion recall f-measure mcc roc area prc area class 0.800 0.300 0.727 0.800 0.762 0.503 0.700 0.735 m 0.700 0.200 0.728 0.700 0.737 0.503 0.700 0.724 b 0.750 0.250 0.753 0.750 0.749 0.503 0.700 0.729 ← weighted avs table 3. confusion matrix. a b ← classified as 8 2 3 7 a = m b = b acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 6 references [1] evangelia i. zacharaki, sumei wang, sanjeev chawla, dong soo yoo, ronald wolf, elias r. melhem, christos davatzikos, classification of brain tumor type and grade using mri texture and shape in a machine learning scheme, magn reson med, 62(6), (2009), pp. 32-39. doi: 10.1002/mrm.22147 [2] federica vurchio, giorgia fiori, andrea scorza, salvatore andrea sciuto, comparative evaluation of three image analysis methods for angular displacement measurement in a mems microgripper prototype: a preliminary study, acta imeko, 10(2) (2021), pp. 119-125. doi: 10.21014/acta_imeko.v10i2.1047 [3] g. çınarer, b. g. emiroğlu, classification of brain tumors by machine learning algorithms, 3rd international symposium on multidisciplinary studies and innovative technologies (ismsit), ieee, ankara, turkey, 11-13 october 2019, pp. 1-4. doi: 10.1109/ismsit.2019.8932878 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min 2h -354.8 -240.6 -216.1 -465.8 -375.3 -446.5 -195.4 -284.2 -251.3 sd2h 38.02 37.87 39.66 47.23 51.1 30.99 29.67 43.83 41 max 2v 342.6 306.4 218.4 367.2 344 184.3 243.4 384.1 296.3 min 2v -275.2 -244.6 -299.8 -390 -315.6 -187.9 -238.3 -313 -214.6 sd2v 46.6 35.39 35.95 53.32 47.48 30.15 33.09 63.09 54.28 max 2d 100.9 107.3 90.2 173.6 188.7 157.2 135.2 177.7 242.2 min 2d -122.4 -126.5 -144.3 -190.6 -199.6 -117.8 -126.6 -167.7 -162.9 sd2d 16.55 14.9 19.01 25.31 23.67 17.62 17.75 27.46 33.58 max3 1066 1132 1107 1118 1114 1117 1128 1014 1082 min3 -28.66 -122.7 -91.2 8.373 -121.3 -105.9 -114.6 -148.6 -112.3 sdb 255.3 245.2 282.6 217.7 203.4 171.3 212.2 248.3 254 e 97.83 97.86 96.73 95.92 93.16 96.23 98.63 9418 96.33 image? m m m m m b b b b https://doi.org/10.1002/mrm.22147 https://acta.imeko.org/index.php/acta-imeko/article/view/imeko-acta-10%20%282021%29-02-17 https://acta.imeko.org/index.php/acta-imeko/article/view/imeko-acta-10%20%282021%29-02-17 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via neural networks, theoretical computer science. 131 (2) (1994), pp. 331–360. doi: 10.1016/0304-3975(94)90178-3 [17] cancer types on cancer net. online [accessed 25 march 2022] https://www.cancer.net/cancer-types https://doi.org/10.1109/jsen.2020.3012536 https://doi.org/10.14445/22315381/ijett-v68i9p203 https://doi.org/10.5373/jardcs/v12i7/20202053 https://doi.org/10.1016/s0031-3203(01)00178-9 https://doi.org/10.1016/0304-3975(94)90178-3 https://www.cancer.net/cancer-types microsoft word article 4 editorial jena.docx acta imeko  august 2013, volume 2, number 1, 5 – 6  www.imeko.org    acta imeko | www.imeko.org  august 2013 | volume 2 | number 1 | 5  introduction to the acta imeko issue devoted to selected  papers presented in the 14 th  joint international imeko tc1 +  tc7 + tc13 symposium  gerhard linß  ilmenau university of technology, department of quality assurance and industrial image processing, faculty of mechanical engineering,  gustav‐kirchhoff‐platz 2, 98693 ilmenau, germany      keywords: imeko tc1 + tc7 + tc13 symposium, intelligent quality measurements, jena, germany  citation: gerhard linß, “introduction to the acta imeko issue devoted to selected papers presented in the 14 th  joint international imeko tc1 + tc7 + tc13  symposium”, acta imeko, vol. 2, no. 1, article 4, august 2013, identifier: imeko‐acta‐02(2013)‐01‐04  editors: paolo carbone, university of perugia, italy; gerhard linß, ilmenau university of technology, germany  copyright: © 2013 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  corresponding author: gerhard linß, e‐mail: gerhard.linss@tu‐ilmenau.de    1. introduction  the 14th joint international imeko tc1 + tc7 + tc13 symposium, which took place in jena, germany, had the title “intelligent quality measurements – theory, education and training”. the 14th joint international imeko tc1 + tc7 + tc13 symposium was intended to reflect innovative solutions for intelligent quality measurements in both theory and application. international researchers from 12 countries presented their exciting work in fundamentals of measurement science, mathematical models in measurement, new education and training methods and applications for intelligent quality measurements, for measurements in medicine and measurements in biology. the symposium aimed to bring researchers and developers from various fields together to share their new thoughts, findings and applications. the response from the academic community has been great, with more than 70 submissions received. the authors have contributed towards new knowledge and understanding, and have provided research results and applications that will be of important value to researchers, students and industry alike. the involved competence network spectronet green vision connected specialists for visual quality control with digital image processing and spectral imaging in research and industry, nutrition and health, transportation, environment, security and administration (www.spectronet.de). additionally the 56th international scientific colloquium, which was held at the ilmenau university of technology from 12th to 16th september 2011 has had an unbroken tradition of more than 50 years and is the "flagship" event of the university, having an excellent reputation. in 2011 the international scientific colloquium was again organised by the faculty of mechanical engineering. the title of the conference is "innovation in mechanical engineering – shaping the future" (www.iwk.tu-ilmenau.de). we are grateful to all the contributors who presented their valuable work to the research community in jena 2011. the journal papers presented in this acta imeko issue were chosen by the imeko tc1 + tc7 + tc13 board from the papers which were presented at the 14th imeko tc1 + tc7 + tc13 symposium. after the imeko symposium in jena more than 17 authors were recommended for providing an updated and extended version of their jena papers for publication in a special issue of the journal measurement or an issue in acta imeko in february 2012. 12 authors accepted this invitation and papers begun arriving us in spring of 2012. in the next step the received extended papers underwent a normal reviewing process. 14 reviewers were involved in the reviewing process and helped to optimize the final manuscripts with constructive references and recommendations. abstract  this  editorial  article  is  a  brief  introduction  to  the  acta  imeko  issue  devoted  to  selected  papers  presented  in  the  14 th   joint  international  imeko  tc1  +  tc7  +  tc13  symposium  “intelligent  quality  measurements  ‐  theory,  education  and  training”.  this  symposium took place in jena, germany from august 31 st  to september 2 nd  2011 in conjunction with the 56 th  iwk ilmenau university  of technology and the  11  th  spectronet collaboration forum.  acta imeko | www.imeko.org  august 2013 | volume 2 | number 1 | 6  at the result we had seven extended and positive ranked papers, which show a significant update to take into account progress since the symposium submission and the discussions at the symposium in jena. four of the positive ranked papers discuss the role of mathematical models in measurement and so these papers are published in the measurement journal (2013). the other three positive ranked papers discuss new education and training methods and applications for intelligent quality measurements and on this thematic basis these papers are published in this issue of acta imeko. 2. about tc1, tc7 and tc13  tc1 is concerned with all matters of education and training of professional scientists and engineers for measurement and instrumentation including curricula, syllabuses and methods of teaching as well as the nature and scope of measurement and instrumentation as an academic discipline. tc1 of imeko was established in 1967 (www.imeko.org/tc1). tc7, the committee established in 1973 under the name measurement theory and in 1993 redesignated as measurement science, is concerned with the development of measurement science (www.imeko.org/tc7). tc13 is concerned with measurement of whole body, organ and cellular function, medical imaging and medical information systems (www.imeko.org/tc13). 3. the journal papers  the three selected journal papers discuss several applications for intelligent quality measurements in highly topical industrial fields, new education and training methods and the combination of image processing with classical quality assurance methods. the focus of paper [1] lays on the realization, how to combine image processing with classical quality assurance methods. two industrial applications were used to describe the problem to gain the importance of this combination. very often the technical realization of sensor systems and data processing are completely separated to quality inspection tasks. so special trainings as well as special parts in the lectures were developed and structured for closing this known gap. paper [2] discusses two analysis activities in the construction material industry, which could be solved by intelligent image processing algorithms for saving time and costs. one of the tasks was the optical identification of recycled aggregates of construction and demolition waste (cdw) as basis of an innovative sorting method on the field of processing of cdw and another task was the optical analysis of samples from mineral aggregates. paper [3] discusses new problems of inspection planning arising from the improvement in measurement technology. the paper describes essential demands, ideas and conceptual approaches to multistructured quality inspections. the background is the fact that the development and control of more and more complex and extensive technical systems yields to measurement-technology requirements in an increasing degree. 4. conclusions  we are grateful to all the contributors who provided their extended papers for this issue of acta imeko and the issue of measurement. it was a great pleasure to act as guest editor for this issue of acta imeko. particularly i must thank the authors, the reviewers for contribution, evaluation and recommendation and specially paul regtien for his support, help and copyediting and publishing process. references  [1] m. rosenberger, m. schellhorn, g. linß, "new education strategy in quality measurement technique with image processing technologies chances, applications and realisation", acta imeko, vol. 2 (2013), no. 1, pp. 56-60. [2] k. anding, d. garten, e. linß, "application of intelligent image processing in the construction material industry", acta imeko, vol. 2 (2013), no. 1, pp. 61-73. [3] k. weissensee, "new demands on inspection planning and quality testing for microand nanostructured components", acta imeko, vol. 2 (2013), no. 1, pp. 74-78. comparative evaluation of three image analysis methods for angular displacement measurement in a mems microgripper prototype: a preliminary study acta imeko issn: 2221-870x june 2021, volume 10, number 2, 119 125 acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 119 comparative evaluation of three image analysis methods for angular displacement measurement in a mems microgripper prototype: a preliminary study federica vurchio1, giorgia fiori1, andrea scorza1, salvatore a. sciuto1 1 engineering department, roma tre university, rome, italy section: research paper keywords: microgripper; mems; microactuators; displacement measurements; characterization citation: federica vurchio, giorgia fiori, andrea scorza, salvatore andrea sciuto, comparative evaluation of three image analysis methods for angular displacement measurement in a mems microgripper prototype: a preliminary study, acta imeko, vol. 10, no. 2, article 17, june 2021, identifier: imekoacta-10 (2021)-02-17 section editor: ciro spataro, university of palermo, italy received january 18, 2021; in final form april 22, 2021; published june 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: federica vurchio, e-mail: federica.vurchio@uniroma3.it 1. introduction mems devices (micro-electro-mechanical systems) represent a category of sensors and actuators widely used in the most varied fields of technology, from automotive to micro assembly for photonics and rf application, microphones, microfluidic device, gyroscopes, chemical sensors for microfluidics systems, lab-on-chip systems and complex actuation systems [1]. one of the most promising fields of application is undoubtedly the biomedical one, such as biology [2],[3] and microsurgery [4]-[6]. microgrippers are a particular class of mems devices, able to handle objects, including cells and molecules that have micrometric dimensions. nowadays, there are few works concerning the characterization of devices such as microgrippers, even if the study of the metrological and performance characteristics would be of great help for the optimization of the prototypes and the improvement of their performances. in this study, a set of images have been acquired by means of a trinocular optical microscope and processed by means of three different methods implemented ad hoc in matlab environment: the semi-automatic (sam), the surf-based (angular displacement measurement based on speeded up robust features, admsurf) and the fft-based (angular displacement measurement based on fast fourier transform, admfft). a comparison among the abovementioned methods has been made to estimate the angular displacement of a mems microgripper prototype comb-drive for biomedical applications. semiautomatic method (sam) already widely described by the authors in [7]-[9], is based on template-matching, and it is able to evaluate the rotation and both the gripper and the angular displacement of a microgripper. its main limitations are the high computational costs and the operator dependence. abstract the functional characterization of mems devices is relevant today since it aims at verifying the behavior of these devices, as well as improving their design. in this regard, this study focused on the functional characterization of a mems microgripper prototype suitable in biomedical applications: the measurement of the angular displacement of the microgripper comb-drive is carried out by means of two novel automatic procedures, based on an image analysis method, surf-based (angular displacement measurement based on speeded up robust features, admsurf) and fft-based (angular displacement measurement based on fast fourier transform, admfft) method, respectively. moreover, the measurement results are compared with a semi-automatic method (sam), to evaluate which of them is the most suitable for the functional characterization of the device. the curve fitting of the outcomes from sam and admsurf, showed a quadratic trend in agreement with the analytical model. moreover, the admsurf measurements below 1° are affected by an uncertainty of about 0.08° for voltages less than 14 v, confirming its suitability for microgripper characterization. it was also evaluated that the admfft is more suitable for measurement of rotations greater than 1° (up to 30°), with a measurement uncertainty of 0.02°, at 95% of confidence level. mailto:federica.vurchio@uniroma3.it acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 120 the above issues have been deepened in this work, starting from the previous study presented in [10]. in section 2, the materials and methods are described, with particular reference to the experimental setup and the measurement protocol used for the digital image acquisition. due to the limitation encountered in sam previously proposed [7]-[9], in subsection 2.1, the authors propose a new version of the sam, in which novel tests have been implemented to quantify the uncertainty contribution introduced by the operator in the angular displacement measurement of a microgripper comb-drive prototype; in subsections 2.2 and 2.3 the authors described two novel and automatic methods and their application for the measurement of the comb-drive angular displacement: the admsurf, based on the surf algorithm [11], and the admfft, that is an application of 2d fast fourier transform (fft) to digital images [12]-[16]. in section 3, the procedure for estimating the sources of uncertainty of the three measurement methods is described and a comparison and the evaluation of the outcomes obtained through the three abovementioned methods have been carried out and discussed to identify which of the three implemented methods is the best suitable for the characterization of the mems device. finally, in section 4 and 5, the results of our study are illustrated, and the conclusions presented. 2. materials and methods in this section the main components of the experimental setup have been described together with a detailed overview of the three implemented methods; in particular, the surf-based and the fft-method have been proposed as alternative methods to the semi-automatic one for the measurement of the angular displacement of the comb-drive. the device under examination is a microgripper prototype (figure 1), which is part of a project concerning the metrological and performance characterization of a new class of mems devices for biomedical applications [17]-[21]. these devices mainly consist of capacitive electrostatic actuators (i.e., the comb-drives shown in figure 2) and particular hinges called conjugate surfaces flexural hinges (csfh) [22], which allow the mechanical movement of the tips located on the end of the device. the images have been acquired through a nb50ts trinocular light microscope equipped with a 6mp camera. the device has been positioned on an instrumented stage with micrometric screws and powered through a hp e3631a power supply. the latter is electrically connected to the device by means of a coaxial cable and tungsten needles put in contact with the electrical connections of the device. the voltage has been brought to the electrical connections by means of two micropositioners that allow the tungsten needle movement along the three axes, x, y, and z. a set of 30 images has been collected for each applied voltage with a 2 v step (i.e., 0 v, 2 v, 4 v, ... 24 v). 2.1. semi-automatic based method (sam) the first method used in this study, has been the semiautomatic one, which for clarity we will call sama, widely described in [7],[8] and used in [9]. as illustrated in [7]-[9], the method introduces a measurement uncertainty contribution which corresponds to 0.02 °, at 95% confidence level, evaluated by means of monte carlo simulation. moreover, the software requires high computational costs and the uncertainty analysis of the preliminary results obtained with the sama was previously carried out partially [7]-[10], assuming the uncertainty component introduced by the operator's subjectivity; for this reason, in this study further tests were carried out to better evaluate the above contribution. the test on the sama, in fact, consists, in its first part, of a selection by the operator of four points and of a region of interest (roi) on the image; to evaluate the dispersion in the selection of these points, in the new version of semi-automatic method, called samb, ten different observers were asked to identify both the four points and the roi in an image of the comb-drive, for a number of times equal to 30. in particular, for the four points, the x and y coordinates on the image were considered and for the roi, the coordinates of the top left vertex (x and y), its length and width (each of them expressed in pixels), as can be seen in figure 3. 2.2. speeded up robust features based method (surf) an automatic method based on speeded up robust features (surf) has been implemented to measure the angular displacement of the comb-drive (admsurf), as already described in [23]. it is an interest point detector and descriptor, used in figure 1. microgripper prototype. figure 2. the comb-drive. figure 3. four points (red cross) and roi (yellow square) selection on the comb-drive image. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 121 many applications including image registration, object recognition and 3d reconstruction [24]. the main advantage of this method is mainly the computational cost reduction; in fact, as illustrated in [11], a significant reduction in image processing time has been observed thanks to the complexity reduction of the descriptor, without altering the performance in terms of repeatability, noise robustness, detection errors, geometric and photometric deformation. the in-house method consists of three main steps: 1) finding interest points on the image; in particular, a roi0v is selected on the first image img0, that corresponds to 0 v power supply, and it is important that this selected area is chosen by the operator in an image area where the movement of the combdrive is visible; the coordinates of the selected roi are saved and used to select the rois (i.e. roi2v, roi4v, roi6v, …, roi24v) of all the subsequent images img2v, img4v, img6v, …, img24v. after that, the algorithm finds the interest points on each selected roi. 2) building a descriptor for the representation of the interest points; for example, in this case they are the red circles for the first image and the green crosses for all the others (figure 4). 3) matching the various descriptors found on the images; by using a geometric transform, the object position on the images can be obtained and therefore it has been possible go back to its relative rotation referred to each applied voltage. 2.3. fast fourier transform based method (fft) this method is based on the application of the fourier transform to digital images. as shown in figure 5, the combdrives of the microgripper have a particular periodic pattern; also, if an image consists of an array of uniformly spaced parallel straight lines, the fourier transform of the combination will be the convolution of their respective fourier transform. the result will be a string of impulses (see figure 6), with a separation equal to the reciprocal of the spacing of the lines and in a direction perpendicular to these parallel lines [12],[13]. this last feature has been used to estimate the angular aperture of the comb-drive: in fact, for each angular opening, the corresponding pattern of the comb-drive will take a different direction and consequently, also the position of the impulses will change and assume a different direction each time. therefore, for each angular opening, and for each image, there will be a series of points in different directions; subsequently a least squares approximation has been used to find the linear polynomial function that best approximates these points from which the angular coefficient of the straight line is obtained and therefore the opening angle of the comb-drive. as previously noted in [10], the major limitation associated with this procedure is due to the inability of the admfft to measure angular displacements less than a tenth of a degree, typical of mems devices for biomedical applications such as microgrippers. however, some microgrippers actuated by rotary comb-drive, as those studied in this work, are powered with voltages much higher than 30 v [25],[26] and therefore it was considered relevant to define whether this method could be used for the characterization of other mems devices. in order to evaluate the limit of applicability of the admfft, we proceeded in this way: once an image that presented a pattern like the one shown in figure 5 was identified, it was rotated of a quantity reported in table 1, where set1 and set2 correspond to two set of rotation, the first consisting of rotations less than one degree, the second higher than one degree; in particular, the rotation values of the first set correspond to the measurements obtained from the images acquired during the experimental campaign, using the sam. 3. uncertainty analysis in order to make a comparison among the three different image analysis methods, it is necessary to estimate the main uncertainty sources introduced by the measurement systems. it is important to underline that the experimental setup is the same, except for the three different methods. following the procedure adopted in [7], type a and type b uncertainties will be combined [27], as follows (1): 𝛿𝑇 = √𝛿𝐴 2 + 𝛿𝐵 2 , (1) figure 4. interest points descriptor of the first image (red circles) and of other images (green cross). figure 5. comb-drive pattern. figure 6. example of fourier transform applied to images properly filtered, constituted by a string of impulses. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 122 where type a uncertainty, a, has been calculated directly from the standard deviation of the experimental data, while type b uncertainty, b, has been obtained considering the uncertainties due to the power supply (evaluated from the datasheet), the optical system [7]-[9], the angle measurement, which uncertainty contribution has been assessed by means of a monte carlo simulation [28]-[29] in order to estimate the uncertainty related to the three implemented method. considering the sam, in order to simulate the uncertainty of the operator’s point selection and therefore evaluate the algorithm uncertainty, a monte carlo simulation with 104 iterations has been performed. in table 2, the variables x, y and roi with their assigned distributions and their standard deviations have been reported, in order to estimate the uncertainty introduced by the method. this contribution has been evaluated for each angular displacement of the comb-drive (i.e. 𝛿𝛼0−2v , 𝛿𝛼0−4v, 𝛿𝛼0−6v , … , 𝛿𝛼0−24v), and combined with the type a uncertainty, following the equation (1). on the other hand, to evaluate the uncertainty introduced by the fft based method in the measurement of the angular displacement of the comb-drive, the image in figure 5 has been subjected to different rotations. the contribution of the systematic uncertainty, considered in this procedure, has been evaluated by building a particular 4k (3840 px × 2160 px) image (figure 7), rotated by the same quantities reported in table 1. this contribution is mainly due to the uncertainty with which the software implements the rotation of the image and therefore into the error that it makes in measuring the angle . considering that at angles up to 15° the sine is only about 1% different and the tangent about 2% different from the measurement of the angle in radiant [30], the following approximation can be used (2): tan 𝛼 ≅ 𝛼 = 𝑎 𝑏 , (2) where a and b are the measurements of the segment reported in figure 7; therefore, for angles less than 15°, the angle measurement relative uncertainty  has been evaluated by the following equation (3): 𝛿𝛼 𝛼 = √( 𝛿𝑎 𝑎 ) 2 + ( 𝛿𝑏 𝑏 ) 2 , (3) where a and b are the measurement uncertainty of segments a and b, respectively, which are considered ±1 px, and on the other hand, if  is greater than 15°, it can be determined as follows (4): 𝛼 = arctg ( 𝑎 𝑏 ), (4) and its uncertainty  can be evaluated by equation (5), as 𝛿𝛼 = 𝑑[arctg(𝑐)] 𝑑𝑐 ∙ 𝛿𝑐 (5) where c is the ratio between the segments a and b, while c, is the corresponding uncertainty. once this uncertainty contribution has been evaluated, for each angular displacement, it is then combined following the equation (1). once uncertainties have been evaluated, a comparison among the three different set of results will be made, following the procedure adopted in [8] and reported in [31]. in practice, the different methods are able to measure the angular displacement of the comb-drive without significant differences if the following condition is verified (6): |�̅�1 − �̅�2| ≤ (𝛿𝑇1 + 𝛿𝑇2) , (6) where �̅�1 and �̅�2 are the mean values of the measurement results, while 𝛿𝑇1 and 𝛿𝑇2 are the total uncertainty estimate. in particular, if the difference |�̅�1 − �̅�2| has the same order of magnitude, or even less than, the sum (𝛿𝑇1 + 𝛿𝑇2 ), then measurements can be considered consistent, within the interval of the experimental uncertainties. 4. results and discussion in this section the outcomes from sam, admsurf and admfft are reported and commented. the graphs in figure 8 and in figure 9, show the results related to the comb-drive angular displacement, expressed as mean value, corresponding to sam, admsurf and admfft respectively. table 4 shows the measurement results expressed as the mean table 1. rotation values. set1 in ° 0.007 0.032 0.070 0.120 0.194 0.277 0.379 0.497 0.631 0.777 0.939 1.118 set2 in ° 1 2 3 4 5 7 10 13 16 20 25 30 table 2. variables settings in mcs to estimate the uncertainty introduced by the operator's subjectivity. parameter distribution standard deviation in px p1 coordinate x gaussian 8 p2 coordinate x 10 p3 coordinate x 8 p4 coordinate x 9 p1 coordinate y gaussian 6 p2 coordinate y 7 p3 coordinate y 6 p4 coordinate y 7 roi coordinate x gaussian 15 roi coordinate y 14 roi width 20 roi height 18 figure 7. 4k image, rotated of 20° for the estimation of angular rotation uncertainty in fft-based method. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 123 value and the corresponding measurement uncertainties at 95% of confidence level. in particular, the sam, introduces a measurement uncertainty contribution which corresponds to 0.8°, at 95% confidence level, and has been retrieved from 2.5 and 97.5 monte carlo distribution percentiles. the analysis of the data showed that both the sam and the admsurf follow a quadratic trend, that is in good agreement with the results obtained through the analysis of the analytical model [32]. as reported in [31], if the difference |�̅�1 − �̅�2| has the same order of magnitude as, or even less than, the sum (𝛿𝑇1 + 𝛿𝑇2 ), then measurements can be considered consistent, within the interval of the experimental uncertainties. from the data reported in table 4, the differences between the mean values are less with respect to the sum of the correspondent total uncertainties, therefore the measurements can be considered compatible, confirming that the admsurf is suitable for the measurement of the angular displacement of the comb-drive of the mems microgripper under test. moreover, it is important to underline that the computational complexity has been considerably reduced by using the admsurf: to process 390 images, as in our case, the sam requires about 2 hours, instead the admsurf about 4-5 minutes only. as regards the data obtained with the admfft (figure 9), it emerged that the results do not follow a trend that can be closely related to the angular displacement of the comb-drive. from a first analysis, it can be deduced that, the admfft cannot be considered suitable for the measurement of mems grippers whose angular displacement is below 1°, as there is no possibility of appreciating displacements around the tenth of a degree. anyway, since some prototype of microgrippers, built with rotary comb-drives, are powered with voltages higher than 30 v and can be moved with angular displacements above 1°, the admfft is evaluated also for an object that rigidly rotates around its axis by quantities greater than 1°. table 3, shows the measurement of rotation, mor1, calculated applying admfft to an image (see figure 5), rotated of a quantity equal to set1 and the measurement of rotation, mor2, calculated applying the admfft to the same image, rotated of a quantity equal to set2, together with the angle measurement uncertainty  , estimated from (3), considering  < 15° and from (5), considering  > 15°. test results confirm that angular displacements up to 30° can be measured with an angle measurement uncertainty  lower than 0.02°, as can be seen in table 3. the different behavior of the figure 8. relationship between angular displacement and applied voltage considering sam (green dot line) and surf method (red dot line). figure 9. relationship between angular displacement and applied voltage considering fft-based method. table 3. measurement of rotation, mor1, calculated applying admfft to an image (see figure 5), rotated of a quantity equal to set1 and the measurement of rotation, mor2, calculated applying admfft to the same image, rotated of a quantity equal to set2, together with the angle measurement uncertainty  . set1 in ° mor1 in °  set2 in ° mor2 in °  0.007 0 0.03 1 1.193 0.015 0.032 0 0.015 2 2.767 0.015 0.070 0 0.016 3 3.918 0.015 0.120 0 0.015 4 4.029 0.015 0.194 0 0.016 5 4.963 0.015 0.277 0 0.015 7 5.078 0.015 0.379 0 0.015 10 8.857 0.015 0.497 0 0.015 13 8.127 0.016 0.631 0.735 0.015 16 14.697 0.015 0.777 -0.262 0.015 20 16.571 0.015 0.939 1.193 0.015 25 19.759 0.015 1.118 1.193 0.015 30 29.237 0.015 figure 10. measurement of rotations less than 1° (above), and measurement of rotations between 1° and 30° (below), applying fft-based method. -1 -0,5 0 0,5 1 1,5 2 2,5 0 2 4 6 8 10 12 14 16 18 20 22 24 26 c o m b -d ri v e a n g u la r d is p la ce m e n t in ° applied voltage in v -0,4 -0,2 0 0,2 0,4 0,6 0,8 1 1,2 0 5 10 15 20 25 30c o m b -d ri v e a n g u la r d is p la ce m e n t in ° applied voltage in v r² = 0,5352 -0,4 -0,2 0 0,2 0,4 0,6 0,8 1 1,2 0 0,2 0,4 0,6 0,8 1 1,2 m e a su re m e n t o f r o ta ti o n i n ° rotation set1 in ° r² = 0,9661 0 5 10 15 20 25 30 35 0 5 10 15 20 25 30 35 m e a su re m e n t o f ro ta ti o n i n ° rotation set2 in ° acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 124 admfft depending on the angular range can be deduced from results of the two rotation sets (table 1) in figure 10: for rotations below 1°, the least squares regression line has shown a r2 = 0.54, while r2 = 0.97 for angles between 1° and 30°. in conclusion, it is possible to confirm that the admfft is not suitable for the measurement of rotations below 1°, but for greater rotations (higher than 1°), it has an almost linear behavior. 5. conclusions this preliminary study has the purpose of comparing the measurements performed by different methods for the angular displacement of a comb-drive of a mems gripper prototype for biomedical applications. in particular, three in-house methods have been implemented in matlab environment: the sam, the admsurf and the admfft. considering the sam, the contribution of uncertainty related to the subjectivity of the operator has been estimated, which has found to be 0.8°, at 95% of confidence level, as previously indicated. from the experimental results obtained, it has been found that the sam and admsurf are suitable to measure the small angular displacement of the comb-drive of the microgripper, showing quadratic curves, consistent with the results obtained with the analytic model. conversely, from the results retrieved by means of the admfft, it has been found no good correlation between small angular displacement and applied voltage that describe the real behavior of the device, and the data are not consistent with both the data obtained through the analytical method and with the two abovementioned methods. anyway, it was also assessed that this method was suitable for measuring rotations from 1° to 30°, and a good correlation is observed between the admfft outcomes and the rotations applied by the operator, with an uncertainty of about 0.02°. a comparison between the sam and the admsurf has been proposed: the measurements can be considered compatible, confirming that the admsurf is suitable for the measurement of the angular displacement of the comb-drive of the mems microgripper under test can be considered compatible. in particular the admsurf measurements of the comb-drive angular displacement are affected by an uncertainty lower than 8% for voltages less than 14 v, as well as smaller than sam. in conclusion, it can be confirmed that the admsurf is the most suitable method among the three proposed for the characterization of the angular displacement of mems devices such as microgrippers, both for the results obtained and the significant reduction of the computational costs. references [1] bhansali, shekhar, abhay vasudev, eds. mems for biomedical applications. elsevier, 2012, isbn 978-0-85709-627-2. [2] d. panescu, mems in medicine and biology, ieee eng. med. biol. mag. 25 (2006), pp. 19-28. doi: 10.1109/memb.2006.1705742 [3] k. keekyoung, x. liu, y. zhang, y. sun, nanonewton forcecontrolled manipulation of biological cells using a monolithic mems microgripper with two-axis force feedback, j. micromech. microeng. 18 (2008). doi: 10.1088/0960-1317/18/5/055013 [4] f. vurchio, p. ursi, a. buzzin, a. veroli, a. scorza, m. verotti, s. a. sciuto, n. p. belfiore, grasping and releasing agarose micro beads in water drops, micromachines 10 (2019). doi: 10.3390/mi10070436 [5] a. gosline, n. vasilyev, e. butler, c. folk, a. cohen, r. chen, n. lang, p. del nido, p. dupont, percutaneous intracardiac beatingheart surgery using metal mems tissue approximation tools, int. j. rob. res. 31 (2012), pp. 1081-1093. doi: 10.1177%2f0278364912443718 [6] d. benfield, s. yue, e. lou w. moussa, design and calibration of a six-axis mems sensor array for use in scoliosis correction surgery, j. micromech. microeng. 24 (2014). doi: 10.1088/0960-1317/24/8/085008 [7] f. orsini, f. vurchio, a. scorza, r. crescenzi, s. a. sciuto, an image analysis approach to microgrippers displacement measurement and testing, actuators 7 (2018). doi: 10.3390/act7040064 [8] f. vurchio, p. ursi, f. orsini, a. scorza, r. crescenzi, s. a. sciuto, n. p. belfiore, toward operations in a surgical scenario: characterization of a microgripper via light microscopy approach, appl. sci. 9 (2019). doi: 10.3390/app9091901 [9] f. vurchio, f. orsini, a. scorza, s. a. sciuto, functional characterization of mems microgripper prototype for biomedical application: preliminary results, proc. of 2019 ieee international symposium on medical measurements and applications (memea), istanbul, turkey, 26 – 28 june 2019. doi: 10.1109/memea.2019.8802178 [10] f. vurchio, g. fiori, a. scorza, s. a. sciuto, a comparison among three different image analysis methods for the displacement measurement in a novel mems device, proc. of the 24th imeko tc4 international symposium & 22nd international workshop on adc modelling and dac modelling and testing, palermo, italy, 14 – 16 september 2020. online [accessed 18 january 2021]. https://www.imeko.org/publications/tc4-2020/imeko-tc42020-61.pdf table 4. comb-drive angular displacement obtained through the sam, surf and fft methods. applied voltage in v sam in ° total uncertainty 𝜹𝑻𝑺𝑨𝑴 in ° surf in ° total uncertainty 𝜹𝑻𝑺𝑼𝑹𝑭 in ° fft in ° total uncertainty 𝜹𝑻𝑭𝑭𝑻 in ° 2 0.0 0.8 0.01 0.07 0.03 0.29 4 0.0 0.8 0.04 0.08 0.21 0.23 6 0.0 0.8 0.08 0.07 0.31 0.24 8 0.1 0.8 0.14 0.07 0.22 0.28 10 0.2 0.8 0.23 0.09 0.28 0.28 12 0.3 0.8 0.33 0.08 0.23 0.31 14 0.4 0.8 0.45 0.08 0.50 0.25 16 0.5 0.8 0.56 0.19 0.68 0.23 18 0.6 0.8 0.95 0.12 0.57 0.18 20 0.8 0.8 0.89 0.10 0.10 0.21 22 0.9 0.8 1.08 0.10 0.05 0.21 24 1.1 0.8 1.27 0.16 0.04 0.18 https://doi.org/10.1109/memb.2006.1705742 https://doi.org/10.1088/0960-1317/18/5/055013 https://doi.org/10.3390/mi10070436 https://dx.doi.org/10.1177%2f0278364912443718 https://doi.org/10.1088/0960-1317/24/8/085008 https://doi.org/10.3390/act7040064 https://doi.org/10.3390/app9091901 https://doi.org/10.1109/memea.2019.8802178 https://www.imeko.org/publications/tc4-2020/imeko-tc4-2020-61.pdf https://www.imeko.org/publications/tc4-2020/imeko-tc4-2020-61.pdf acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 125 [11] h. bay, a. ess, t. tuytelaars, l. van gool, speeded up robust features (surf), computer vision and image understanding, 110 (2008), pp. 346-359. doi: 10.1016/j.cviu.2007.09.014 [12] r. bracewell, fourier analysis and imaging, springerscience+business media, llc, 2003. 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[28] g. fiori, f. fuiano, a. scorza, j. galo, s. conforto, s. a. sciuto, lowest detectable signal in medical pw doppler quality control by means of a commercial flow phantom: a case study, proc. of the 24th imeko tc4 international symposium & 22nd international workshop on adc modelling and dac modelling and testing, palermo, italy, 14 – 16 september 2020. online [accessed 14 june 2021]. https://www.imeko.org/publications/tc4-2020/imeko-tc42020-63.pdf [29] g. fiori, f. fuiano, f. vurchio, a. scorza, m. schmid, s. conforto, s. a. sciuto, a preliminary study on a novel method for depth of penetration measurement in ultrasound quality assessment, proc. of the 24th imeko tc4 international symposium & 22nd international workshop on adc modelling and dac modelling and testing, palermo, italy, 14 – 16 september 2020. online [accessed 14 june 2021]. https://www.imeko.org/publications/tc4-2020/imeko-tc42020-62.pdf [30] c. h. holbrow, j. n. lloyd, j. c. amato, e. galvez, m. e. parks, modern introductory physics, second edition, springer 2010. [31] j. r. taylor, an introduction to error analysis. uncertainty stuty in physical measurements, zanichelli: bologna, italy, 1986. [32] r. crescenzi, m. balucani, n. p. belfiore, operational characterization of csfh mems technology based hinges, j. micromech. microeng. 28 (2018). doi: 10.1088/1361-6439/aaaf31 https://doi.org/10.1016/j.cviu.2007.09.014 https://doi.org/10.1109/jmems.2017.2696033 https://doi.org/10.3390/act7020012 https://doi.org/10.1115/1.4036351 https://doi.org/10.3390/mi6111451 https://doi.org/10.3390/mi9010015 https://doi.org/10.1115/1.4035053 https://doi.org/10.1109/memea49120.2020.9137249 https://doi.org/10.1109/reconfig.2011.65 https://doi.org/10.1109/jsen.2015.2453013 https://doi.org/10.3390/mi10060376 https://www.imeko.org/publications/tc4-2020/imeko-tc4-2020-63.pdf https://www.imeko.org/publications/tc4-2020/imeko-tc4-2020-63.pdf https://www.imeko.org/publications/tc4-2020/imeko-tc4-2020-62.pdf https://www.imeko.org/publications/tc4-2020/imeko-tc4-2020-62.pdf https://doi.org/10.1088/1361-6439/aaaf31 a cost-efficient reversible logic gates implementation based on measurable quantum-dot cellular automata acta imeko issn: 2221-870x june 2022, volume 11, number 2, 1 -9 acta imeko | www.imeko.org june 2022| volume 11 | number 2 | 1 a cost-efficient reversible logic gates implementation based on measurable quantum-dot cellular automata mary swarna latha gade1, rooban sudanthiraveeran1 1 koneru lakshmaiah education foundation, green fields, vaddeswaram, india section: research paper keywords: majority gate; quantum dot cellular automata, reversible logic; reversible gates citation: mary swarna latha gade, sudanthiraveeran rooban, a cost-efficient reversible logic gates implementation based on measurable quantum-dot cellular automata, acta imeko, vol. 11, no. 2, article 35, june 2022, identifier: imeko-acta-11 (2022)-02-35 section editor: md zia ur rahman, koneru lakshmaiah education foundation, guntur, india received january 14, 2022; in final form april 23, 2022; published june 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: sudanthiraveeran rooban, e-mail: sroban123@gmail.com 1. introduction the building of digital logic circuits using reversible logic gives a zero dissipation of electricity. for irreversible calculations, landauer [1] established that each logical bit of data loss results in kb t ln(2) joules of heat energy. lent et al. [2] demonstrated that zero-energy dissipation in a digital logic circuit is only possible if the circuit is made up of reversible digital logic gates. the energy systems quantum-dot cellular automata (qca) circuit dissipation can be significantly lower than kb t ln(2) because qca circuitry is clocked systems that maintain data. this function encourages the use of qca technology in the construction of reversible digital circuits. reversibility, on the other hand, reverses bit loss but does not identify bit faults in the circuit. performance issues can be corrected by using faulttolerant gates with reversible logic. it becomes simpler and easier to detect and correct defects when the system is made up entirely of fault-tolerant elements. parity is used to achieve fault tolerance in communications and other systems. parity-preserving circuitry would thus be the forthcoming architecture principles for the creation of reversible fault-tolerant devices in nanotechnology. because of its very small size and low power consumption, qca is recommended for use in nanotechnology research [3]. the basic reversible logic gates make up reversible circuits. these gates produce a one-of-a-kind mapping of input and output pairs, making the set of inputs equal to the total outputs. in [4][6], a substantial contribution to the research for the building of basic reversible logic gates was made. these topologies, on the other hand, are less efficient and necessitate greater inverter and majority gate reductions. it aids us in the development of reversible logic gates with fault-tolerant architectures using qca technology. the basic reversible gates are constructed using upgraded xor gate designs. because of the rapid advancement abstract in order to improve the density on a chip, the scaling of cmos-based devices begins to shrink in accordance with moore's laws. this scale affects the execution of the cmos device due to specific limitations, such as energy dissipation and component alignment. quantum-dot cellular automata (qca) have been replaced to overcome the inadequacies of cmos technology. data loss is a major risk in irreversible digital logic computing. as a result, the market for nano-scale digital operations is expanding, reducing heat dissipation. reversible logic structures are a strong competitor in the creation of efficient digital systems. a reversing logic gate is an important part of reversible circuit design. the qca design of basic reversible logic gates is discussed in this study. these gates are built using a new qca design with xor gates with two and three inputs. qcadesigner tests simulation performance by simulating the specified reversible logic gate layouts. the measurement and optimization of design techniques at all stages is required to reduce power, area, and enhance speed. the work describes experimental and analytic approaches for measuring design metrics of reversible logic gates using qca, such as ancilla input, garbage output, quantum cost, cell count, and area, while accounting for the effects of energy dissipation and circuit complexity. the parameters of reversible gates with modified structures are measured and then compared with the existing desi gns. the designed f2g, frg, fg, rug and uppg reversible logic gates using qca technology shows an improvement of 42 %, 23 %, 50 %, 39 % and 68 % in terms of cell count and 31 %, 20 %, 33 %, 20 % and 72 % in terms of area with respect to the best existing designs. the findings illustrate that the proposed architectures outperform previous designs in terms of complexity, size, and clock latency. mailto:sroban123@gmail.com acta imeko | www.imeko.org june 2022| volume 11 | number 2 | 2 of qca technology, a great deal of research has been done in the field of qca-based reversible logic measurement and technology, with the goal of improving performance metrics in terms of measurement accuracy and complexity, and thus improving the efficiency and precision of circuits. each reversible logic gate in a reversible logic system architecture measure specific parameter separately. the system then combines all the independents into a comprehensive set of measurement data using updated gate structures. many parameter values in the existing designs are not measured when the circuits are tested. in individual cases, it might be necessary to make measurements to accurately analyze the behaviour of the reversible logic gates. 1.1. reversible logic if each input state yields a different output state, the logic characteristic is reversible. a reversible gate is the kind that realizes reversible functionality. both outputs and the inputs can have a bijective mapping. as a result, the output and input quantities must be equal in order to prove the reversibility principle. in reversible logic gates, feedback is not acceptable, and fan out is not permitted. the circuits implementation with reversible logic gates are measured with the parameters like primitive gate count, constant input, logic calculation, unwanted output and quantum cost. based on their features, these gates have indeed been grouped into three types. • basic reversible logic gates: all of these gates are necessary parts of a reversible circuit. there have been other reversible gates invented, but not as well as buffer is the most basic. • conservative reversible logic gates: these gates have about the same number of zeros and ones at both inputs and outputs. consider the case of mcf's gates. • parity-preserving reversible logic gates: these gates have about the same parity (either even or odd) at both their inputs and outputs. only in terms of mathematics. all of the inputs and outputs of xor are equal to zero. the conservative gates are retained through parity, while the contrary is not true. 1.2. qca in [2], lent et al. introduced qca technology, which is utilized to construct all components of a nanoscale qca circuit. each qca cell is made up of four quantum dots, which are nanostructures capable of trapping electric bills. because these electrons are repelled by each other due to coulomb interaction, they seem to be positioned on opposite corners of the square. the alignment of electron pairs defines two potential polarization states, -1.00 and 1.00. the majority gate and the inverter gate are the two most basic qca gates. many logic operations that are governed by the clocking operation are commonly executed by the coulombic interactions of electrons in neighbouring cells. 1.3. related work several studies using qca to generate reversible logic gates have just been conducted during the previous decade [6]-[11]. qca architectures using majority gates as the foundation device for numerous reversible gates were shown by researchers in [6]. because the output lines are not really strongly polarised, these kinds of gates are not considered sufficient. for fredkin, feynman, and toffoli gates, mohammadi et al. [7] built qcabased reversible circuits integrating both rotating and normal cells. the architectures shown are dependent on the majority gate's arrangement. however, an effective qca architecture necessitates a new majority circuit design process, which adds to the complexity. peres and feynman's gates based on qca realisable majority gates were discussed in [8]–[10]. their topologies are less optimal, necessitating the reduction of both main and inverter gates. the authors of [11] proposed employing majority gates in qca to create an efficient reversible logic gate arrangement. however, these systems have usual restrictions, such as a higher number of stable input signals. without employing any wire crossing, the topologies presented by sing et al. [11] cannot be achieved. the feynman gate [12] is the wellknown "2x2" reversible gate. it can be used to increase fan-out. the famous "3x3" reversible gates are toffoli, peres, and fredkin [13], [14]. trailokya nath sasamal et.al [15] designed reversible gates like fg, frg, tofolli, peres gate and f2g in qca with 26, 68, 59, 88, 53 and 0.03 µm2, 0.06 µm2, 0.034 µm2, 0.097 µm2, 0.058 µm2 qca cell count and area respectively. the main contributions of the paper are as follows: 1. introduces new optimized layouts for the existing reversible logic gates to reduce the complexity of the circuits. 2. later, the performance of the proposed gates is analyzed and compared with the existing gates using conventional parameters. 2. novel qca designs for reversible gates the essential building elements of reversible digital logic are the reversible gates. such gates produce a one-of-a-kind mapping between the input as well as output sets, allowing for the same combination of inputs as outputs. qca designs of xor, f2g (double feynman gate), frg (fredkin gate), fg (feyman gate), rug (reversible universal gate), and uppg (universal parity preserving gate) reversible gates are provided in the following section. all of the suggested approaches adhere to the qca designs rules in aspects of all input cells arriving in a same period clock zone, long qca wires being subdivided into four clock zones based on the highest cell count in the same zone and the minimum cells in the same zone to avoid increased signal propagation and switching delays, and none of the suggested approaches have crossover choices. the prototypes of several reversible gates have been synthesized and measured using qca designer tool. an xor gate is the basic element of the most elementary reversible gates. it has been observed that the endurance of the design of the reversible logic architecture is influenced not only by the complexity as well as by the latency of an xor gate. two inverters and three majority gates are required in the classic xor architecture. the suggested reversible layouts are using integrated architecture with only 12 qca cell connections for both the 2-input and 3-input xor gates, as can be seen in figure 1. the f2g [16], [18] proposed block diagram and conceptual design are illustrated in figure 2a. it has three inputs namely a, b, c and three outputs p, q, r. the f2g concept uses the current xor gate to produce a design that is optimum for the region. figure 2b. shows a possible qca architecture with such a gate. note that the first input a is carried to p, whereas the realization of two output bits q and r necessitates the use of two xor gates having two inputs. the designed f2g gate qca layout utilizes only two-time clock zones. figures 3.a and b illustrate the block diagram and schematic diagram of the hypothesized fredkin gate. in general terms, the frg (fredkin gate) [19]-[24] is called a universal gate in the sense that it may be "trained" to operate as many basic components in almost any reversible digital logic circuit. it has three inputs namely a, b, c and 3 outputs p, q, r. for deployment, there are acta imeko | www.imeko.org june 2022| volume 11 | number 2 | 3 six primary gates. figure 3c shows the qcadesigner setup of the built fredkin gate. the suggested modified frg gate shows a measurement of only 75 qca cells. the output signal p is observed to be connected to the input signal a, but the output signals q and rare implemented using majority gates. four-time clock zones are used in the intended frg gate qca configuration. figure 4 shows the feynman gate's block diagram and schematic diagram. it uses two inputs namely a, b and two outputs p, q. because of its widespread use in quantum computing, the fg (feynman gate) is sometimes referred to as the controlled not or even quantum xor gate [25]-[28]. the measurement of the designed fg gate is done with a coherence vector simulation engine. figure 4b depicts the corresponding qca architecture, which has a single xor gate with two inputs. it's important to note that output p is related to input a, however output q necessitates the use of a simpler xor gate than the present xor designs. just two-time clock zones are used in the developed f2g gate qca [29]. a block and conceptual design for the rug gate is shown in figure 5a. it has three inputs and three outputs in figure 5b, an appropriate qcadesigner interface has been demonstrated for the rug gate. there are four primary gates and one xor gate having two inputs in this circuit. the output p was generated from a threeinput majority gate, q was collected from three majority gates, and r was obtained out of a two-input xor gate, as shown in the diagram. four-time clock zones are used in the intended rug gate qca layout. figure 6a and figure 6b illustrate the topology and schematic design of the proposed qca universal parity preserving gate (uppg). it has four inputs and four outputs two majority gates, two xor gates with two inputs, and one xor gate with three inputs make up the circuit. the measurement of modified uppg gate shows a better improvement with the existing designs. figure 1. qca layout of xor gate with 2-input and 3-input. a) b) figure 2. double feynman gate (f2g): a) block diagram and schematic diagram, b) qca layout. a) b) c) figure 3. fredkin gate (frg): a) block diagram, b) schematic diagram, c) qca layout. a) b) c) figure 4. feynman gate (fg): a) block diagram, b) schematic diagram, c) qca layout. acta imeko | www.imeko.org june 2022| volume 11 | number 2 | 4 figure 6c depicts the suggested qcadesigner layout for the uppg gate. it was discovered that the proposed arrangement has an occupied area of only 0.08 µm2. a total of two different time clock zones are used in the uppg gate qca configuration. 3. results and discussion the coherence(accuracy)vector simulator had been used to analyze the layouts using default parameters, and qcadesigner had used qca configurations of the recommended systems. the cell size, layer separation distance, dot diameter, high clock area, low clock area, clock shift area, clock amplitude factor, relative permittivity, radius of effects, number of tolerances, convergence tolerance, and maximum iterations per sample are 18 nm × 18 nm, 5 nm, 9.800000·10-22 j, 3.80000 10-22 j, 0, 2, 12.9, 85 nm, 4, 0.001, and 100, respectively. the f2g gate simulated waveform is shown in figure 7. this simulation waveform has demonstrated that the circuit is functional, and consequently output values are created for all input data. for development, 31 cells in an area of around 0.04 µm2 are required. figure 7 shows the results of the fredkin gate model. this gate is frequently employed as a multiplexer in electronics in a variety of applications. it is made up of 75 qca cells with 0.08 µm2 area widths. the feynman gate was made using 13 qca cells with a total area of around 0.02 µm2, as shown in figure 4, and the result of its simulation can be seen in figure 9. figure 10 depicts the simulation waveform of the rug. with a surface area of 0.08 µm2, it utilizes 59 qca cells. the proper operation of the uppg gate is shown in figure 11. this gate is made with only 75 qca cells and a surface area of around 1 µm2. a study of the characteristics of the recommended reversible gate configurations with the measurement of previous studies is shown in table 1 to table 5. table 1 describes the design f2g reversible gate, which shows an improvement of 42 % and 31 % with respect to qca cells and area with the best existing design. when we compare the developed f2g to previously described designs, we find that the f2g designed with qca is the best in terms of the number of qca cells and the area it supports, as shown in table 1. table 2 shows the planned frg reversible gate, which was built using 75 qca cells with a measured area of 0.08 µm2 and a 0.5 clock cycle delays. it shows that the recommended architecture selected the best of all existing designs. when compared to the optimal design, the suggested frg gate provides a 23 % and 20 % improvement in cell count and area, respectively. the fredkin gate presented has been found to be more acceptable for cascade design, and the output metrics are comparable to the same optimal technique. the fredkin gate seems to have a delay of 0.5 clocks, which is less than that of conventional designs. table 3 presents the intended fg reversible gate, which was built using 13 qca cells with a 0.02 µm2 area and a 0.5 clock cycle latency. the recommended fg circuit is compared to earlier circuits in table 3. when compared to the optimal design, the proposed fg gate improves cell count and area by 50 % and 33 %, respectively. the total a) b) c) figure 5. reversible universal gate (rug): a) block diagram, b) schematic diagram, c) qca layout. a) b) c) figure 6. universal parity preserving gate (uppg): a) block diagram, b) schematic diagram, c) qca layout. acta imeko | www.imeko.org june 2022| volume 11 | number 2 | 5 number of qca cells employed, the total accessible area, the clock delay, and the crossover all influence the relationship. table 3 demonstrates that the fg circuit designed in this article has fewer cells and a smaller system area than previous designs. table 4 and table 5 show the functional efficiency of the recommended structures when compared to typical rug and uppg designs. circuit parameters such as cell count, delay, and area are taken into account while evaluating performance. in comparison to the past rug and uppg, the recommended rug and uppg have made significant improvements. table 4 shows that as compared to the optimal design, the proposed rug gate improves cell count and area by 39 % and 20 %, respectively. table 5 shows that when compared to the optimal design, the suggested uppg gate improves cell count and area by 68 % and 72 %, respectively. the total number of qca cells employed, the total accessible area, the clock delay, and the crossover all influence the relationship. table 3 demonstrates that the fg circuit designed figure 7. simulation result of f2g. table 1. comparison of planned f2g configuration with presented designs. double feynman gate cell count area (µm2) delay (cycles) wire crossing f2g [6] 93 0.19 0.75 coplanar f2g [14] 51 0.06 0.5 f2g [5] 53 0.05 0.5 proposed f2g 31 0.04 0.5 table 2. comparison of proposed frg structure with existing designs. fredkin gate cell count area (µm2) delay (cycles) wire crossing frg [7] 178 0.21 1 coplanar frg [14] 100 0.092 0.75 frg [6] 97 0.10 0.75 proposed frg 75 0.08 0.5 table 3. comparison of proposed fg structure with existing designs. feynman gate cell count area (µm2) delay (cycles) wire crossing fg [7] 78 0.09 1 coplanar fg [8] 54 0.038 0.75 multilayer fg [6] 53 0.07 0.75 fg [9] 37 0.023 0.75 fg [11] 32 0.03 0.75 fg [14] 34 0.036 0.5 fg [15] 26 0.03 0.5 proposed fg 13 0.02 0.5 acta imeko | www.imeko.org june 2022| volume 11 | number 2 | 6 figure 8. simulation result of frg. figure 9. simulation result of fg. acta imeko | www.imeko.org june 2022| volume 11 | number 2 | 7 figure 10. simulation result of rug. figure 11. simulation result of rug. acta imeko | www.imeko.org june 2022| volume 11 | number 2 | 8 in this article has fewer cells and a smaller system area than previous designs. tables 4 and 5 show the functional efficiency of the recommended structures when compared to typical rug and uppg designs. circuit parameters such as cell count, delay, and area are taken into account while evaluating performance. in comparison to the past rug and uppg, the recommended rug and uppg have made significant improvements. table 4 shows that as compared to the optimal design, the proposed rug gate improves cell count and area by 39 % and 20 %, respectively. table 5 shows that when compared to the optimal design, the suggested uppg gate improves cell count and area by 68 % and 72 %, respectively. the comparison shows that the proposed structures for the existing reversible gates have a compact architecture than the existing designs. the energy dissipation analysis of the proposed gate structures is as shown in table 6. results show that the proposed structures outperform previous reversible gate designs and thus suitable for application toward complex nano-scale architectures in qca. 4. conclusion in this study, the reversible gates like feynman gate (fg), double feynman gate (f2g), reversible universal gate (rug), fredkin gate (frg), and universal parity preserving gate (uppg) are designed using qca technology with optimum area. the designed layouts of the reversible gates have zero wire crossings. the simulation results have been verified using qca designer software. the suggested f2g, frg, fg, rug and uppg qca layouts are designed only with 31, 75, 13, 59 and 75 qca cell count and 0.04 µm2, 0.08 µm2, 0.02 µm2, 0.08 µm2, and 0.08 µm2 area respectively. then, we measured and compared the robustness of the recommended reversible gates to the existing gates using standard metrics. the simulation results show that the suggested reversible gates perform better in terms of cell count and area by 42 %, 23 %, 50 %, 39 %, and 68 % and 31 %, 20 %, 33 %, 20 %, and 72 %, respectively. the suggested architectures outperform existing reversible gate designs, indicating that they are better suited for usage in qca in complicated nanoscale systems. references [1] r. landauer, irreversibility and heat generation in the computing process, ibm journal of research and development, vol. 5, no. 3 (july 1961), pp. 183-191. doi: 10.1147/rd.53.0183 [2] c. s. lent, m. liu, y. lu, bennett clocking of quantum-dot cellular automata and the limits to binary logic scaling, nanotechnology, vol.17, no. 1(2006), pmid: 21727566. doi: 10.1088/0957-4484/17/16/040 [3] m. balali, a. rezai, h. balali, f. rabiei, s. emadi, towards coplanar quantum-dot cellular automata adders based on efficient three-input xor gate, results phys., vol. 7(2017), pp. 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http://dx.doi.org/10.1504/ijcad.2016.075913 http://doi.org/10.1109/tim.2012.2199189 https://doi.org/10.1016/j.measurement.2021.109353 http://dx.doi.org/10.1007/s10773-017-3647-5 http://dx.doi.org/10.1016/j.measurement.2020.108499 https://doi.org/10.1109/iciss49785.2020.9315867 https://doi.org/10.1109/jsen.2021.3055750 https://doi.org/10.1016/j.measurement.2020.107829 https://doi.org/10.1007/s11227-018-2550-z http://nopr.niscair.res.in/handle/123456789/57696 https://doi.org/10.1109/icstcee49637.2020.9277288 https://doi.org/10.1016/j.physb.2018.12.028 automated calibration and dcc generation system with storage in private permissioned blockchain network acta imeko issn: 2221-870x march 2023, volume 12, number 1, 1 7 acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 1 automated calibration and dcc generation system with storage in private permissioned blockchain network cristian zet1, gabriel dumitriu2, cristian fosalau1, gabriel constantin sarbu3 1 gheorghe asachi technical university of iasi, bd. d. mangeron 67, 700050 iasi, romania 2 individual enterprise gcd, str. hlincea nr. 27, 700714, iasi, romania section: research paper keywords: blockchain technology, digital calibration certificate (dcc), virtual instrument, automated test system, data acquisition, uncertainty citation: cristian zet, gabriel dumitriu, cristian fosalau, gabriel constantin sarbu, automated calibration and dcc generation system with storage in private permissioned blockchain network, acta imeko, vol. 12, no. 1, article 16, march 2023, identifier: imeko-acta-12 (2023)-01-16 section editor: daniel hutzschenreuter, ptb, germany received november 20, 2022; in final form february 14, 2023; published march 2023 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was supported from project poc 351/390027/08.09.2021 european structural funds. corresponding author: cristian zet, e-mail: czet@tuiasi.ro 1. introduction calibration is defined by vim (jgcm200:2012) as the "operation that, under specified conditions, in a first step, establishes a relation between the quantity values with measurement uncertainties provided by measurement standards and corresponding indications with associated measurement uncertainties". it is usually accomplished after the instrument gets out from the production line or at regular time intervals during the instrument's life time. calibration is often considered by companies as being the process of measuring the offset and the gain errors [1], [2] and trimming the analog circuitry in order to correct these errors, being confused with the adjustment process. during the factory adjustment, the correction values are stored in the nonvolatile memory of the instrument and they are used during operation to directly correct the indication. later, the errors change in time due to aging and with temperature, making these values invalid after a time period, requiring a new adjustment. depending on the needs, the instrument must be checked at least once a year, but it can be even earlier (6 or 3 month) if necessary. modern instruments are programmable via the built-in interfaces, like usb or gpib [3]. a software control loop can control several instruments in order to get an automated test system. if one or more of the instruments have no interface, the process can be only partially automated using video processing [4], on single range or function, while switching ranges/functions are performed manually. the commercially available remote control software from the producer is not suitable in most cases for metrological purposes [1], and hence custom software must be developed for each instrument. in [3], an automated calibration system for electrical sources and measuring instruments like digital multimeters (dmms) has been described. it is intended for several well known instruments like calibrators (fluke 5720a or wavetek 9100) or dmms (fluke 8506, fluke 8846, hp 3458). the software is built in labview and allows controlling the calibration system, eliminates the human errors and performs statistical processing. in [2], an automated measuring station for the determination of calibration intervals for dmms is presented. the authors are using a standard device and a verified instrument, both equipped abstract digital technologies have proved their usefulness in instrumentation and measurements since many years, becoming a “must have”. the use of microprocessors and microcontrollers in measuring instruments became a common practice, bringing the advantage of signal and information processing at the instrument level, digital interfaces, remote control, software update and calibration. thus, the instrument can be calibrated and verified being connected in an automated calibration system which can carry all the process without the operator interference and to generate in the end the calibration certificate. the paper presents the possibility of joining the automated calibration system and the creation of a digital calibration certificate (dcc) with the blockchain technology for storing and validation. as benefits there are the traceability of the dcc, the impossibility of altering the information and the preservation of the full history in the digital wallet. mailto:czet@tuiasi.ro acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 2 with communication interfaces. the test is performed every 2 month interval up to 1 year. in [5], authors present an automated calibration system for dmms without a communication interface. they are using video processing to "read" the numbers provided on 7 segment displays. the process is partially automated, the operator's task being to change the ranges and functions. in [6] it is shown that the calibrations can be performed on site based on travelling standards, without requiring the presence of any specialized personnel from the metrology lab. such situation needs an internet connection to automatically and securely send the calibration data on a server. there are several approaches described above, but for any of them the software is a custom design one, being fully automated or partially automated, depending on the instruments. despite this, an automated system can be endowed with the feature to automatically create the dcc. as long as the data is recorded in the system in the digital format, the software can automatically create the calibration certificate, according to the issuer regulations. in most cases the data can be saved in local files with various formats, human readable or machine readable. there are several formats and several approaches in the literature for creating a dcc [7]. there are various information to be recorded in the certificate and various users of it. the certificate must respect norms and regulations and maintain traceability. an analysis of digital formats reveals several advantages for blockchain based dccs. in [8] the authors describe the possibility of generating the dcc from excel, taking benefits from the programming environment built in. in [9], the authors present several variants of saving the dcc in pdf format for various fields like vna, electrical energy, or acoustics. the pdf file may have attached the digital calibration certificate. in [10], the authors describe how to make dcc in xml format with 4 layers: administrative, results, individual information and optional attachment (pdf), considering the case of sensor networks. the digital signature is also considered. for using dccs, an infrastructure is needed [11] that must be distributed [12]. security issues must be provided that prevent the information to be altered. an internal report from nist makes a detailed analysis of blockchain overview with respect to the metrology area, emphasizing the benefits and the weaknesses [13]. a blockchain system is not exactly immutable, requiring governance that must be trusted. the data recorded must correspond to the real world. transactions that are not yet included in a block are vulnerable to attacks and vulnerable to malicious users. some applications for the blockchain technology have been emphasized in [14]: decentralized audit trail (the instrument communicates that the calibration parameters are not proper anymore), parameters update with agreement from the user, public keys for manufacturers and nmis, or billing system. in conclusion, according to this short analysis, calibration is one of the activities requesting traceability, allowing the connection with the primary standards, the needs of dccs and the advantages of using the blockchain technology in metrology. the present paper brings as novelty the direct generation of the dcc during an automated calibration together with saving it as a blockchain transaction. a short description of the used blockchain network is presented in section 2, followed by the description of the hardware and software application performing the automated calibration, dcc generation and its embedding in the blockchain in section 3 and ending up with some results in section 4 and conclusions in section 5. 2. blockchain network architecture and specific transaction blocks as per ibm’s definition, blockchain is a shared, immutable ledger that facilitates the process of recording transactions and tracking assets in a business network. all these transactions are stored in blocks of data, using complex cryptography functions. each block contains a cryptographic hash of the previous block, a timestamp, and the transaction data (generally represented as a merkle tree) as in figure 1. once a transaction is executed, all the data remain in the blockchain permanently. you cannot alter, modify, copy or delete it but you can only distribute it. in order to migrate the digital calibration certificate on blockchain, we built a private permissioned blockchain infrastructure with 3 private full nodes, an api and a web platform acting as smart asset management tool like in figure 2. a full node can be seen as a server in a decentralized network. it keeps the consensus between other nodes and verifies the transactions. it also stores a copy of the blockchain, thus being able to securely enable custom functions such as instant send and private transactions. our blockchain is based on the x15 algorithm, a hybrid between pow (proof of work) and pos (proof of stake) consensus mechanism [15]. thanks to its energy efficiency, it can even run on a raspberry pi2. pos is a type of consensus mechanism used in blockchain networks for validating transactions and creating new blocks. when using pos, instead of miners competing to solve complex mathematical problems to create new blocks (as in pow based networks), validators are chosen to create new blocks based on the amount of cryptocurrency they hold and are willing to "stake" (i.e. lock up) as collateral. figure 1. blockchain block simplified. figure 2. local blockchain node simplified. acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 3 when a validator wants to create a new block, they must first stake a certain amount of cryptocurrency, which acts as a form of collateral. the more cryptocurrency a validator stakes, the higher their chances of being chosen to create a new block. once a block is created, the validator receives a reward for their work, proportional to the amount they staked. if a validator is found to be acting maliciously or attempting to cheat the system, their stake can be taken away as a penalty and his work is excluded from that specific block. it uses 15 hashing algorithms that are consecutively carried out one after another. we also built its associated windows and linux wallets from which we can natively monitor all the transactions. the infrastructure has been built to run on p2p port: 10218 and accept commands only on rpc port: 20208. this specific blockchain feature also adds an extra layer of security. some apis were built using the opensource jsonrpcclient.php a simple php class that implements json rpc client over raw tcp. the asset management tool works in parallel with the labview dashboard and allows the user to create a blockchain identity for the measurement instrument. basically, it uses a specific function, “getnewaddress” from jsonrpcclient.php in order to remotely connect to the main wallet and store the instrument’s details (such as name and serial) as metadata in a single transaction. the blockchain address attached to the instrument is converted to a qr code for a better portability. this process is basically the creation of the smart asset on our infrastructure. then, we use the newly created blockchain identity to initiate transactions. each transaction generates a unique transaction id (figure 3). we use the "sendtoaddress" function and store the metadata in the "comment" argument, part of the function. the "comment" argument is kept in the main public wallet; it is not distributed over the network. this means that the only way you can see the metadata is by typing the correct transaction, which is a unique identifier. so, even though all transactions are public, the content of the dcc can be verified only if you have a valid transaction id. this type of architecture restricts any fraudulent transactions. the traceability aspect of the whole system is given by the fact that we can trace back in time every transaction and identify each measurement. this can be done directly from the wallet (as a native feature) or by accessing our smart asset management tool. using the "listtransactions" function, we developed a method which allows us to check the previous calibration/measurements of the same instrument. this is achieved natively, from the wallet and also from our web asset management tool. having this functionality, it allows the system to have a full history of calibration/verification of the same instrument and also providing a secure method of storing and sharing data, protecting it from unauthorized access or modifications. this specific aspect also helps to reduce the cost of creating, maintaining, and distributing physical certificates. the labview side of the project uses httpclient.lvlib to connect to the blockchain api. it also collects the measured values and sends them to a specific address as metadata. the data is formatted as .xml before embedding it to blockchain, which offers an alternative in future verification. we designed the system in such a way that once the labview application runs, it automatically correlates the serial number of the data acquisition card with its associated blockchain wallet address. we achieved this by using the "getaccountaddress" native function that returns the wallet address of a previous created account. all these actions happen on the wallet, being stored locally on the main wallet. this particular approach offers an extra degree of anonymity and low-cost of transactions in the peer-to-peer (p2p) blockchain network compared to those in the real world. 3. description of automated calibration system – hardware and software overview for proving that it is possible to store the dcc as a transaction using blockhain technology, an automated calibration system has been created to perform both the measurement and the dcc generation, as shown in figure 4. it consists of standard source and the dut, both connected to a host pc running the specific software developed in labview. the dut is a multifunction ni usb 6008 data acquisition card. it has 8 analog inputs with 2 types of input connections: differential (diff) and single ended (rse). the technical data for the daq card are listed in table 1. it provides a resolution of 12 bits on the differential input and 11 bits on single ended input and relatively high maximum permissible errors. the daq is connected at the host pc using the usb interface. the programmable voltage source keithley 6487 (a picoammeter with built-in voltage source) is connected to the pc through the gpib interface and it is driven using scpi commands. in order to demonstrate the feasibility of an automated verification and dcc generation system, a virtual instrument has been developed in labview. it is designed to figure 3. blockchain transaction details. figure 4. the schematic of the calibration system. table 1. ni usb 6008 absolute accuracy specifications (25°c) in mv. input type ± 20 v ± 10 v ± 5 v ± 4 v ± 2.5 v ± 2 v ± 1.25 v ± 1 v rse 14.7 diff 14.7 7.73 4.28 3.59 2.56 2.21 1.70 1.53 acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 4 allow the creation for each instrument of its own identity in the specified wallet. if it has not already one, it must be set up and run the verification process. at the end, the resulting data must be saved in the blockchain network as transactions. the data is also saved in readable format in local .txt files. the process is not really fully automated, as the operator must change the input connection when the rse connection is tested. the front panel of the vi is presented in figure 5. if the instrument to be calibrated is not yet registered as a smart asset or it was not verified before using the system, the operator can choose the tab "smart asset creation". in labview, each data acquisition device gets a device number which can be found in the measurement and application explorer . after inserting the "device no", the program will query the device and will get its serial number and board name. it will concatenate the two strings, will call the blockchain platform to create a wallet for the device using the “getnewaddress” api and will return the wallet address as shown in figure 5 left (like mmzo6bksvw jhmjz2vvgmgyaa8fexgwme4c). the new address will appear also in the wallet application. the user has to copy this and paste it in the corresponding field in the calibration tab. moving to the "calibration tab" (figure 5. right), the operator must fill in the fields for the certificate header, as for example: the certificate number (certificate no), the certificate issuer name (institution), the operator name (operator), the customer name (customer) and the comments field (comments). the header can be customized following the lab wishes and regulations with various fields. next, the operator must setup the connection type (input terminal configuration), the scale limits (limits) for each desired scale and gpib address of the standard source. the environmental conditions can be specified as an input field (t&h). if the room is environmentally controlled or they can be measured using a remote environment monitor station, the values can be automatically inserted into the field. before starting the instrument, the operator has to fill in the wallet address of the instrument to be verified (wallet address), if it already has one, or paste it after it was created in the "smart asset creation" tab. once the dut and the standard source are connected and all the control fields are filled in, the operator can start the verification process by pressing the button “start”. the first task is to read the instruments ids and serial numbers directly from their firmware, which will be added to the certificate. after the instruments are initialized with the working parameters, the process is started. depending on the number of points per scale and on the number of scales the process will take a while. for the present approach we set a number of 20 points per scale and 8 scales. the sampling frequency is set to 1 khz and 𝑛 = 1000 samples to acquire. the acquired values are processed for each test point in order to calculate the arithmetic mean and the standard deviation: 𝑈mean = 1 𝑛 ∑ 𝑈𝑘 𝑛 𝑘=0 , 𝑠(𝑈𝑘 ) = √ 1 𝑛 ∑(𝑈𝑘 − 𝑈mean) 2 𝑛 𝑘=0 (1) where 𝑈𝑘 are the measured values for a single test voltage, while the type a uncertainty is calculated as the experimental standard deviation of the mean value: 𝑢a = 𝑠(𝑈mean) = 𝑠(𝑈𝑘 ) √𝑛⁄ . (2) type b evaluation of the measurement uncertainty may also be characterized by standard deviations, which will be evaluated from the probability density functions based on experience or other information, as reported in gum [evaluation of measurement data—guide to the expression of uncertainty in measurement, jcgm 100:2008]. the combined uncertainty and the expanded uncertainty are calculated as: 𝑢c = √𝑢a 2 + 𝑢b 2 , 𝑢e = 𝑘 ∙ 𝑢c , (3) where 𝑘 is the coverage factor (𝑘 = 2 for a probability of 95%). in the current application, only the type a uncertainty is considered as resulting from the experimental verification. the virtual instrument flow is presented in figure 6. after the initial setting of various parameters, it runs 3 loops. the inner one takes the measurements for each test point 1000 times and calculates the uncertainty, the middle one runs for each test point and the outer one runs for each range. the data is written in the local file after each test point, while the transaction containing the dcc is sent after each range. the header of the calibration certificate has to be flattened into xml string. because that some special characters are not allowed in this format of blockchain, they have to be removed from the instrument identification string and replaced with underscores or dots. figure 5. the front panel of the virtual instrument: left the “create smart asset” tab; right the calibration tab. acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 5 4. results in our experiment 10 daq cards were tested for all measuring ranges on the differential input and for the single range on the rse input. their dccs were saved in the blockchain network as transactions. each tested scale has its own dcc embedded in one transaction. the results are presented in table 2 for differential input base scale ± 10 v, for the lowest scale ± 1 v and for rse input. maximum values from all 10 tested duts for each test point for the combined uncertainty are presented. as it can be seen, the values are below 6.35 mv, which is lower than the absolute accuracy value of 7.73 mv given by the producer for the base scale ± 10 v. for the lowest scale, ± 1 v, the combined uncertainty is maximum 1.15 mv, slightly under the absolute accuracy limit of 1.53 mv given by the producer. this might be produced by the standard source whose lowest output scale is ± 10 v and the accuracy according to the datasheet is 0.1% of output voltage + 1 mv. this means that for 1 v output voltage, the absolute accuracy is maximum 2 mv. for the rse input the values are under the limit of 14.7 mv. the behaviour of the maximum and minimum type a uncertainties are depicted in figure 7. the diff series represents the behaviour of the differential input for the base scale ± 10 v, while the rse series is the behaviour for rse input. the minimum limit is for both below 40 µv, while the maximum type a uncertainty is double for rse (160 µv) than diff. as resulted the type a uncertainty is much lower than the type b uncertainty. 5. conclusions usually, the structure of national metrology institutes (nmi) involves a central unit and a number of territorial subsidiaries. each one can be possess an own wallet in the private permissioned blockchain infrastructure. each wallet is associated to a node located at subsidiary or at the central entity. they all make up the blockchain network (figure 8). the nodes in the network are constantly synchronizing in between, as long as the internet connection is available. if for some nodes the connection is temporarily broken, after its restoration, the node will send the transactions realized in the meantime in the network and each node will validate each transaction. as a closing takeaway, here are 5 benefits for metrological applications using blockchain: improved security and authenticity: blockchain technology ensures that once a certificate is added to the blockchain, it cannot be altered or tampered with, providing a tamper-proof and secure method of recording and sharing calibration data. figure 6. the virtual instrument flow diagram. table 2. ni usb 6008 absolute accuracy specifications (25 °c) in mv. scale factor -1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 diff ± 10 v 6.35 5.19 4.04 2.89 1.73 0.58 1.73 2.89 4.04 5.19 6.35 diff ± 1 v 1.15 1.04 0.92 0.81 0.69 0.58 0.69 0.81 0.92 1.04 1.15 rse ± 10 v 6.35 5.19 4.04 2.89 1.73 0.58 1.73 2.89 4.04 5.19 6.35 figure 7. type a uncertainty for the base scale ± 10 v. figure 8. the metrology blockchain network. acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 6 traceability: blockchain allows for the tracking of the calibration certificate throughout its lifecycle, providing transparency and traceability which can be useful for regulatory compliance and quality control. reduced cost and efficiency: blockchain-based digital certificates can help reduce the cost of creating, maintaining, and distributing physical certificate and increasing efficiency. better accessibility: digital certificates stored on the blockchain can be easily accessed and shared, eliminating the need for physical certificates, reducing risks of loss or damage. greater standardization: the use of blockchain technology in creating dccs can also help promoting standardization and consistency in metrology, as data stored on the blockchain can be easily shared and compared across different organizations. when a new instrument is calibrated by an entity in territory, it must have its own identity in the entity wallet. its identity can be created as a unique label in the entity wallet. this will correspond to an own address in the wallet for the instrument (figure 9). each transaction is sent inside the same wallet, but the transactions are visible in the whole network. if the instrument has been calibrated before, it already has a label and respectively an address. the calibration software is checking the existence of the label corresponding to its name and serial number. it interrogates the instrument, gets its name and serial number and then asks the wallet for its address. if there is not a match, it returns an error and the operator has to create the address. if the calibration software finds a match, it uses that address for the following transactions. each transaction contains a dcc for a single range. this has been chosen in the present approach because the comment field that carry the dcc information is limited to a number of characters. the dcc is structured as an xml format being stored in the comment of the transaction. for the verification of an instrument with multiple functions and scales, there will be several transactions (figure 10). in our case, there are 9 transactions for each instrument (9 available scales). in conclusion, the blockchain technology shows promising possibilities for metrological applications. it can securely store the dcc, can keep all the certificates issued for an instrument and can assure the traceability as long as the standard has its own address which might have a dcc in the network. the dcc can be generated without the intervention of the operator, with custom software, guaranteeing the validity of the results, minimizing the possibility to fake them. on the other hand, we plan for future upgrades including a full audit of the calibration process, meaning that every local and national authorized authority can release a dcc for any instrument/standard device. this way one can trace back in time and check how a specific instrument has been authorized by a local authority and follow the traceability chain. a second aspect of future upgrades includes a secure login feature for the web asset management tool that manages the enrolment in our private-permissioned infrastructure and also access to read/write data. this basically translates into a simple background check of the provided identification data and allocation of a wallet address. we also believe that data portability and authenticity is crucial in delivering a dcc, hence we plan a third module that facilitates a digital signature of the exported certificate as pdf file. this feature stores the private key of the digital signature certificate on blockchain, eliminating the need of hardware security modules (hsm). the working module requires a small adjustment in order to comply to the european commission eidas regulation https://digital-strategy.ec.europa.eu/en/policies/eidasregulation. acknowledgement this work was supported from project poc 351/390027/08.09.2021 european structural funds. references [1] national instruments corp, calibration procedure, 322314b-01, february 2000. [2] c. de capua, d. grillo, e. romeo, the implementation of an automatic measurement station for the determination of the calibration intervals for a dmm, ieee symposium on virtual environments, human-computer interfaces and measurement systems, la coruna, spain, 10-12 july 2006, pp. 58-62. doi: 10.1109/vecims.2006.250790 [3] h. m. abdel mageed, a. m. el-rifaie, automatic calibration system for electrical sourcing and measuring instruments, 12th international conference on environment and electrical engineering, wroclaw, poland, 5-8 may 2013, pp. 30-34. doi: 10.1109/eeeic.2013.6549578 [4] f. martín-rodríguez, e. vázquez-fernández, á. dacal-nieto, a. formella, v. álvarez-valado, h. gonzález-jorge, digital instrumentation calibration using computer vision, iciar 2010, part ii, lncs 6112, pp. 335–344, springer-verlag berlin heidelberg 2010, 978-3-642-13774-7. [5] g. grzeczka, m. klebba, automated calibration system for digital multimeters not equipped with a communication figure 9. how the instruments get their own labels/address. figure 10. the transaction list containing multiple transactions for 1 address. https://digital-strategy.ec.europa.eu/en/policies/eidas-regulation https://digital-strategy.ec.europa.eu/en/policies/eidas-regulation https://doi.org/10.1109/vecims.2006.250790 https://doi.org/10.1109/eeeic.2013.6549578 acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 7 interface, mdpi sensors 2020, 20, 3650. doi: 10.3390/s20133650 [6] a. carullo, m. parvis, a. vallan, security issues for internet based calibration activities, ieee instrumentation and measurement technology conference anchorage, alaska, usa, may 21-23, 2002. pp. 817-822. doi: 10.1109/imtc.2002.1006947 [7] m. s. gadelrab, r. a. abouhogail, towards a new generation of digital calibration certificate: analysis and survey, measurement 181 (2021) 109611. doi: 10.1016/j.measurement.2021.109611 [8] d. röske, a visual tool for generating digital calibration certificates (dccs) in excel , measurement: sensors, volume 18, december 2021, 100175. doi: 10.1016/j.measen.2021.100175 [9] g. boschung, m. wollensack, m. zeier, c. blaser, chr. hof, m. stathis, p. blattner, f. stuker, n. basic, f. grasso toro, pdf/a3 solution for digital calibration certificates, measurement: sensors, vol. 18, december 2021, 100282, 4 pp. doi: 10.1016/j.measen.2021.100282 [10] t. mustapää, p. nikander, d. hutzschenreuter and r. viitala, metrological challenges in collaborative sensing: applicability of digital calibration certificates, sensors 2020, 20, 4730. doi: 10.3390/s20174730 [11] c. brown, t. elo, k. hovhannisyan, d. hutzschenreuter, p. kuosmanen, o. maennel, t. mustapaa, p. nikander, t. wiedenhoefer, infrastructure for digital calibration certificates, 2020 ieee international workshop on metrology for industry 4.0 & iot, 03-05 june 2020, rome, italy, pp. 485-489. 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https://doi.org/10.1109/imtc.2002.1006947 https://doi.org/10.1016/j.measurement.2021.109611 https://doi.org/10.1016/j.measen.2021.100175 https://doi.org/10.1016/j.measen.2021.100282 https://doi.org/10.3390/s20174730 https://doi.org/10.1109/metroind4.0iot48571.2020.9138220 https://doi.org/10.1109/brains55737.2022.9909437 https://doi.org/10.6028/nist.ir.8202 https://doi.org/10.1109/i2mtc.2018.8409668 https://arxiv.org/pdf/2001.07091.pdf a low-cost table-top robot platform for measurement science education in robotics and artificial intelligence acta imeko issn: 2221-870x june 2023, volume 12, number 2, 1 5 acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 1 a low-cost table-top robot platform for measurement science education in robotics and artificial intelligence hubert zangl1,2, narendiran anandan1, ahmed kafrana1 1 institute of smart systems technologies, sensors and actuators, university of klagenfurt, klagenfurt, austria 2 silicon austria labs, aau sal use lab, klagenfurt, austria section: research paper keywords: education; robot perception; introductory lab experiments citation: hubert zangl, narendiran anandan, ahmed kafrana, a low-cost table-top robot platform for measurement science education in robotics and artificial intelligence, acta imeko, vol. 12, no. 2, article 23, june 2023, identifier: imeko-acta-12 (2023)-02-23 section editor: eric benoit, université savoie mont blanc, france received august 9, 2022; in final form may 2, 2023; published june 2023 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work has received funding from the "european regional development fund" (efre) and "react-eu" (as reaction of the eu to the covid-19 pandemic) by the "kärntner wirtschaftsförderungs fonds" (kwf) within the project pattern-skin 3520/34263749706. corresponding author: hubert zangl, e-mail: hubert.zangl@aau.at 1. introduction robots are entering more and more into our daily live and are attractive for young people interested in science and technology. consequently, also the education at the bachelor and master level must adapt to the needs in this interdisciplinary subject. the importance of the link between different domains is also emphasized in a survey [1] stating that 57 % of the faculty believes that students in electrical engineering have deficiencies in kinematics and dynamics. consequently, experiments for measurement science education that addresses kinematics and dynamics can help to overcome such issues and to cope with the rise of interdisciplinarity in engineering education [2] when aspects of kinematics and dynamics are considered in the design of the experiments. electrical engineering laboratory courses at the bachelor level typically consist of experiments involving basic electronics circuits, and measurement of circuit parameters such as currents, voltages, power and waveforms. advanced laboratory courses for higher-semester students utilize analog and digital semiconductor devices such as transistors, op-amps, and microcontrollers. experiments involving sensors typically involve dedicated interface circuitry, and the experimental activity consists in measuring the circuit output to determine the physical quantity. while these experiments are necessary for students to develop their basic knowledge in electrical engineering, they do not provide sufficient exposure to topics such as kinematics and sensor fusion algorithms. many low-cost, simple robot platforms are available off the shelf. a drawback of such systems is that the mechanical integration of various sensors requires substantial modifications making the whole system often fragile and delicate. customized 3d printing allows to adjust the geometry to ideally host all sensor equipment and to obtain a rather robust setup. 3d printing has become an essential tool in robotics; therefore, lowcost printed platforms have been suggested to be used in education [3]. however, most of the related educational programs focus on higher level of robotics or mechanics, yet more is needed in education in measurement science. aspects abstract robotics and artificial intelligence represent highly interdisciplinary fields, which – in particular on the bachelor level makes providing a strong fundamental background in the education challenging. with respect to lab exercises in measurement, one approach to provide interdisciplinary hands-on experience is to embed experiments for measurement science in a robotic context. we present a low-cost robot platform that can be used to address several measurement science and sensor topics, and also other aspects such as machine learning, actuators and mechanics. the 3d printed chassis can be equipped with different sensors for environment perception but also be adapted to different embedded pc platforms. in order to also introduce concepts of robot simulation and realization approaches in a hands-on fashion, the table-top robot is also available as a digital twin in the simulation environments gazebo and coppeliasim®, where, e.g., limitations of simulations and required adaption of models to consider non ideal effects of sensors can be studied. mailto:hubert.zangl@aau.at acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 2 such as sensors and data acquisition and their practical use play an important role for engineers. these aspects should be addressed in the education of students in the field of robotics and artificial intelligence. furthermore, it is also important to consider uncertainty and raise the awareness towards non-ideal effects present in real systems. influences due to differences in various acquisition setups and measurement system can be studied using simulations of the system [4]. additionally, the relevance of the uncertainty for so called “sim2real” approaches (e.g. [5]), which use simulations as a basis for the generation of training data for learning algorithms, can thus be emphasized. the considered course, where the proposed robot platform is introduced, is a laboratory course under the title of “measurement science, sensors and actuators”. this course will be part of a bachelor study program under the title robotics and artificial intelligence, which will start in fall 2022. it is one of more than seven laboratory courses out of which the students can choose freely. it needs to be considered that these laboratory courses are to be taken at a rather early in the study program and should give the students a practical context for their further study. consequently, not all theories behind the experiments will be fully understood in this early stage of education, and this needs to be considered in the design of the exercises. the learning aims are thus hands-on experience with respect to a wide range of topics • torque, angular velocity, and acceleration • force, linear velocity, and acceleration • inertia and moment of inertia • friction • mechanical (angular, linear) power, electrical power, and conversion efficiency determination • data acquisition and recording • sensors for proprioception and exteroception • multi-body simulation and development of models from the real world • comparison of results from simulation and realworld experiments • influence of noise and other non-ideal effects of sensors and measurement systems, uncertainty propagation • model based signal processing • machine learning-based signal processing the design of the experiments aims to touch on all these aspects in order to give students visualization and better understanding of the theory presented in the corresponding lectures. it also covers the often under-represented step to obtain simplified models from real world scenarios [6] and understanding the resulting discrepancies. 2. proposed platform figure 1 illustrates a lab scenario. many data acquisition systems nowadays are pc based and we also make use of tools such as labview® [7] and matlab® [8] allowing for fast automation of measurement tasks even when students work with it for the first time. in addition, frameworks such as ros [9] allow to easily make first steps with robotics as many modules are available. however, since many students will work in the same room simultaneously, small robots that can be used on the table are advantageous. consequently, our design comprises tiny 3d printed wheeled robots. 3d printing not only allows for low-cost realizations of robots, but it also allows to easily make adaptions to the chassis of the robots, e.g. in order to mount certain sensors such as rgb cameras, depth cameras, ultrasound and time of flight sensors, wheel speed sensors etc. but also to provide different actuators concepts. figure 2 illustrates different realizations. these robots are controlled using a raspberry pi zero with ros on raspberry pi os, but also other platforms such as beaglebone®, odroid, and jetson nano™ boards utilizing ros on ubuntu can be used with the hardware. the robot body frame is constructed using 3d printing depending on the desired mounting configuration of the sensors and actuators. a block diagram of a typical robot with selected hardware components is shown in figure 3. the robot's single board computer that interfaces with the onboard sensors and actuators are housed within the 3d printed body. the measurement data is published on ros for further processing. the robot can be connected wirelessly to a pc but can also execute programs for various tasks running on the embedded pc. it is powered by commonly available low-cost portable power banks that can be easily swapped and recharged. optionally the robots can be mounted with optical tracking markers for use in motion tracking facilities. the proposed platform is a fully functional wheeled robot with various measurement and actuation capabilities. it thus figure 1. illustration of the table-top robot as used in a classroom. figure 2. photographs of different 3d printed robots. in contrast to off-the shelf robots, the geometry can quickly be adapted to ideally host sensors and actuators. while the left robot can only manipulate objects by shifting them, the robot on the right also includes a simple soft robotics fin ray type gripper [10], which can also be equipped with tactile sensors. acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 3 provides students a complete exposure to a simple robot and all its internal components and working principles, the students will then be able to adapt and modify the existing platform for different requirements. the models for 3d printing, boards, and software will be provided as open source and students will be able to continue further experiments as they can easily realize their own robot at low costs. 3. initial experiments with the robots since one of the ideas is to also enhance the understanding in kinematics and dynamics, the first experiments address measurements related to the motors of the robot. on a simple test bench students can measure the current and voltage applied to the mounted dc motor. the dc motor measurement setup incorporates a disk brake and piezoresistive sensor that can measure the force on the brake. by measuring the force on the support of the brake, torque can be estimated. additionally, the angular velocity is measured using magnetoresistive sensors counting marks on the shaft. figure 4 shows a picture of the proposed motor characterization workbench. consequently, students get familiar with measurement of current, voltage, and power in the electrical domain but also to speed, force, torque, and power in the mechanical domain. furthermore, this allows to determine the efficiency and consequently the thermal losses and heating of the motors. the magnetoresistive sensors can be used in odometry and the drift due to inaccuracies using integration of the velocity can be observed in a follow up experiment. 4. introduction to robot simulation many different environments for robot simulation are available, including open source frameworks such as gazebo [11] and coppeliasim® [12]. as the actual time to work in the lab is rather short, a system is preferred that can be used with an easy to learn graphical user interface. coppeliasim® provides such an interface and was therefore chosen for the introductory course. additionally, it allows to select different physics simulation engines such as bullet [13], newton dynamics [14], open dynamic engine [15], and vortex [16]. the initial simulation experiments thus aim to provide an understanding of capabilities of current robot simulation environments, required parameters and limitations of the simulation approaches. this can be illustrated in the simple scenario of a bouncing ball. for this, a simple sphere can be added using the gui of coppeliasim® and placed in a height of one meter above the ground. when the simulation is started, the ball falls as expected but sticks to the surface without bouncing. in the following, the object parameters in different simulators are discussed that control the simulation of the physical contact. the students should then find appropriate parameters, such that the ball shows a realistic bouncing behavior, including the attenuation over time. this should illustrate that simulations are only approximations of the reality and that results obtained with different simulation engines can be quite different for the same scenario. in order to get realistic behaviors, parameter tuning is required, and the values are not necessarily directly obtainable from the material properties of the objects. this should raise the awareness that simulation results need to be validated in general. as a next step, the simulation of the simple robot is set up. a step file of the 3d printed chassis is provided, and students need to add joints, motors, and wheels. in the following simulations, a torque is assigned to the motors, and the behavior of the robot is analysed by the students. they should verify if the linear acceleration of the robot correctly corresponds to the estimated torque. 5. considering measurement uncertainty in robot simulation in order to achieve realistic simulations that allow development of signal processing algorithms or simulation-based machine learning approaches, also realistic sensor models considering non-ideal characteristics of sensors are important. typically, robot simulation tools such as coppeliasim® provide means for simulation of sensors such as accelerometers and gyroscopes, but non-ideal characteristics of these sensors are usually not included. consequently, they need to be included by the user. figure 5 illustrates such a simple model as used in coppeliasim®. based on datasheets of accelerations sensors, students should include effects such as deviation in offset, sensitivity, noise, crosstalk and potentially nonlinearity, such that the simulation model generates realistic sensor data based. figure 3. hardware components in the proposed robotic platform. depending on the requirements additional components can be included. figure 4. the dc motor has a spinning wheel attached to its shaft. this spinning wheel has holes near its perimeter. the angular velocity of the wheel can be obtained by processing the output signal of the magnetoresistive sensor s1. a brake wheel that can spin freely is mounted next to the spinning wheel, and the force applied can be adjusted and measured. a small extrusion of the brake wheel applies force on the force sensor, whose output can be used to measure the torque. acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 4 a linear model could look like [ 𝑌1(𝑡) 𝑌2(𝑡) 𝑌3(𝑡) ] = [ 𝑆11 𝑆12 𝑆13 𝑆21 𝑆22 𝑆23 𝑆31 𝑆32 𝑆33 ] [ 𝑋1(𝑡) 𝑋2(𝑡) 𝑋3(𝑡) ] + [ 𝑂1 𝑂2 𝑂3 ] + [ 𝑁1(𝑡) 𝑁2(𝑡) 𝑁3(𝑡) ] , (1) where, 𝑌𝑖 are the sensor outputs, 𝑋𝑖 are the sensor inputs, 𝑆𝑖𝑗 (cross)-sensitivities, 𝑂𝑖 offset values and 𝑁𝑖 noise contributions. for each sensor realization, offsets and sensitivities will be different and noise contributions will vary for each sample. the students should also include an explanation of how the parameters for the random variables are determined from the datasheet of an actual sensor. the model should be developed for one of • 3 axis acceleration sensor • 3 axis gyroscope • pressure sensor. the sensor model should be tested on a simulated robot, e.g. in order to assess the feasibility of the determination of a joint angle of a robot joint using two acceleration. 6. sensor fusion algorithms an important part of measurement science, especially in robotic context is the application of algorithms to improve the estimate of the measurands in the presence of noise and uncertainties. a popular method used for robotic tracking applications is the kalman filter [17]. the proposed wheeled robot is a suitable platform for students to learn about the estimation of the system state from the sensor readings. even though the full theory behind the kalman filter will be addressed later in the curriculum, the students should be able to implement the equations and to do first analysis in order to get an idea of the benefits of signal processing methods in measurement science. in a simple exercise the students must estimate and track the velocity and acceleration (in one dimension) of the robot using the kalman filter. the robot will move on a straight line on a flat level surface and the output of the angular velocity sensor (𝜔m) and the acceleration (𝑎m) from an inertial measurement unit (imu) is available as measurements at regular time intervals (d𝑡). the diameter of the robot’s wheel (𝐷) is a known constant. from the measured angular velocity and using the knowledge of the wheel diameter, the measured linear velocity 𝑣m of the robot is obtained as 𝑣m = π 𝐷 𝜔m . (2) the state of the system to be estimated and tracked is 𝑥 = [ 𝑣 𝑎 ] . (3) in this setup, the system has no control inputs, therefore the kalman filter state predict equations are, �̂�𝑛+1 = 𝐹 𝑥𝑛 , where 𝐹 = [ 1 d𝑡 0 1 ] (4) and 𝑃𝑛+1 = 𝐹 𝑃 𝐹 t+𝑄𝑛 , (5) where 𝑃 is the covariance matrix that represents the uncertainty in estimation and 𝑄 is process noise. given the measurements 𝑧 = [ 𝑣m 𝑎m ]. (6) the state update and the uncertainty update equations are given by, �̂�𝑛 = �̂�𝑛−1 + 𝐾𝑛 (𝑧𝑛 − 𝐻�̂�𝑛−1) (7) and 𝑃𝑛 = (𝐼 − 𝐾𝑛 𝐻)𝑃𝑛−1, (8) where 𝐻 is the identity matrix and 𝐾𝑛 is the kalman gain 𝐾𝑛 = 𝑃𝑛−1 𝐻 𝑇 [𝐻 𝑃𝑛−1 𝐻 t + 𝑅𝑛] −1 , (9) where 𝑅 is the measurement uncertainty matrix. an advanced version of this experiment tracks motion (linear and angular position, velocity, and acceleration) of the robot, additionally positional measurement inputs from the range sensor can be included, and motor voltage and current measurements can be used as control inputs to the system state. this state can then be compared to the ground truth positions obtained from optical motion tracking systems. 7. higher level aspects the same robots can also be used with optical time of flight sensors or time of flight cameras. with these sensors, it is possible to develop object avoidance approaches based on reinforcement learning in simulation. here, the aim is to maneuver the robot towards a certain target position while avoiding collision with obstacles. again, the focus is on the influence of the sensor signal's quality on the training result. we use one or more 8 × 8 multizone time of flight ranging sensors [18]. figure 6 shows a training setup that students can use. figure 5. simple acceleration sensor model in coppeliasim®. it comprises an ideal force sensor and a seismic mass, the accelerations are determined by dividing the forces as obtained from the physics simulation engine by the mass. figure 6. reinforcement learning with gazebo and time of flight sensors. the aim is that the robot navigates to the red rectangle, while avoiding collisions with the obstacles. the simulated sensor signals are illustrated in the top left image, where the distance is encoded in a gray scale image. acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 5 8. summary this paper summarizes an approach to integrate robots in lab exercises on measurement science. the simple, low-cost tabletop robot with 3d printed chassis can be equipped with various sensors and can be used for a variety of experiments starting from current and voltage measurement on a dc motor, measurement of force, speed, and mechanical power, as well as conversion efficiency. non ideal effects of inertial sensors as they are used in robots can be simulated using simulation frameworks and the consequences can also be studied in the experiments. the sensors and measurement systems can also be used and studied in signal processing, and signal fusion approaches as well as in machine learning allowing to study the influence of the signal quality on the training results. acknowledgement this work has received funding from the "european regional development fund" (efre) and "react-eu" (as reaction of the eu to the covid-19 pandemic) by the "kärntner wirtschaftsförderungs-fonds" (kwf) within the project pattern-skin 3520/34263749706. references [1] j. m. esposito, the state of robotics education: proposed goals for positively transforming robotics education at postsecondary institutions, ieee robot. automat. mag., vol. 24, no. 3, sep. 2017, pp. 157–164. doi: 10.1109/mra.2016.2636375 [2] m. roy, a. roy, the rise of interdisciplinarity in engineering education in the era of industry 4.0: implications for management practice, ieee eng. manag. rev., vol. 49, no. 3, sep. 2021, pp. 56–70. doi: 10.1109/emr.2021.3095426 [3] l. armesto, p. fuentes-durá, d. perry, low-cost printable robots in education, j intell robot syst, vol. 81, no. 1, jan. 2016, pp. 5–24. doi: 10.1007/s10846-015-0199-x [4] t. mitterer, l. faller, h. müller, and h. zangl, a rocket experiment for measurement science education, journal of physics: conference series, 2018, vol. 1065, no. 2, p. 022005. doi: 10.1088/1742-6596/1065/2/022005 [5] c. doersch, a. zisserman, sim2real transfer learning for 3d human pose estimation: motion to the rescue, advances in neural information processing systems, 2019, vol. 32, 14 pp. doi: 10.48550/arxiv.1907.02499 [6] l. l. bucciarelli, s. kuhn, engineering education and engineering practice: improving the fit, between craft and science: technical work in the united states, edited by stephen r. barley and julian e. orr, ithaca, ny: cornell university press, 1997, pp. 210-229. doi: 10.7591/9781501720888-012 [7] national instruments, labview. online [accessed 8 june 2023] https://www.ni.com/en-us/shop/labview.html [8] mathworks, matlab. online [accessed 8 june 2023] https://www.mathworks.com/products/matlab.html [9] m. quigley, k. conley, b. p. gerkey, j. faust, t. foote, j. leibs, r. wheeler, a. y. ng, ros: an open-source robot operating system, in icra workshop on open source software, 2009, vol. 3, no. 3.2, p. 5. [10] w. crooks, g. vukasin, m. o’sullivan, w. messner, c. rogers, fin ray® effect inspired soft robotic gripper: from the robosoft grand challenge toward optimization, front. robot. ai, vol. 3, nov. 2016. doi: 10.3389/frobt.2016.00070 [11] open robotics, gazebo. online [accessed 7 august 2022] https://gazebosim.org/home [12] coppelia robotics, robot simulator coppeliasim: create, compose, simulate, any robot. online [accessed 7 august 2022] https://www.coppeliarobotics.com [13] pybullet.org, bullet real-time physics simulation. online [accessed 12 january 2023] https://pybullet.org/wordpress/ [14] julio jerez, alain suero and various other contributors, newton dynamics. online [accessed 12 january 2023] http://newtondynamics.com/forum/newton.php [15] russ smith, open dynamic engine. online [accessed 12 january 2023] http://www.ode.org/ [16] cm labs, vortex studio. online [accessed 12 january 2023] https://www.cm-labs.com/vortex-studio/ [17] r. e. kalman, a new approach to linear filtering and prediction problems, journal of basic engineering (asme), vol. 82, mar. 1960, pp. 35–45. doi: 10.1115/1.3662552 [18] stmicroelectronics, vl53l5cx time-of-flight 8x8 multi-zone ranging sensor with wide field of view. online [accessed 31 march 2022] https://www.st.com/content/st_com/en/campaigns/vl53l5cxtime-of-flight-sensor-multizone.html https://doi.org/10.1109/mra.2016.2636375 https://doi.org/10.1109/emr.2021.3095426 https://doi.org/10.1007/s10846-015-0199-x https://doi.org/10.1088/1742-6596/1065/2/022005 https://doi.org/10.48550/arxiv.1907.02499 https://doi.org/10.7591/9781501720888-012 https://www.ni.com/en-us/shop/labview.html https://www.mathworks.com/products/matlab.html https://doi.org/10.3389/frobt.2016.00070 https://gazebosim.org/home https://www.coppeliarobotics.com/ https://pybullet.org/wordpress/ http://newtondynamics.com/forum/newton.php http://www.ode.org/ https://www.cm-labs.com/vortex-studio/ https://doi.org/10.1115/1.3662552 https://www.st.com/content/st_com/en/campaigns/vl53l5cx-time-of-flight-sensor-multizone.html?ecmp=tt24055_gl_ps_nov2021&aw_kw=time%20of%20flight%20sensor&aw_m=e&aw_c=15158713672&aw_tg=aud-1232809041753:kwd-364398038077&aw_gclid=cjwkcajwopwsbhb6eiwajxmqdfzxhmfkx7mwtbxs9fuo8lvzwm21kmu-6_7u6ideeccybtlj77gflxocn4cqavd_bwe&gclid=cjwkcajwopwsbhb6eiwajxmqdfzxhmfkx7mwtbxs9fuo8lvzwm21kmu-6_7u6ideeccybtlj77gflxocn4cqavd_bwe https://www.st.com/content/st_com/en/campaigns/vl53l5cx-time-of-flight-sensor-multizone.html?ecmp=tt24055_gl_ps_nov2021&aw_kw=time%20of%20flight%20sensor&aw_m=e&aw_c=15158713672&aw_tg=aud-1232809041753:kwd-364398038077&aw_gclid=cjwkcajwopwsbhb6eiwajxmqdfzxhmfkx7mwtbxs9fuo8lvzwm21kmu-6_7u6ideeccybtlj77gflxocn4cqavd_bwe&gclid=cjwkcajwopwsbhb6eiwajxmqdfzxhmfkx7mwtbxs9fuo8lvzwm21kmu-6_7u6ideeccybtlj77gflxocn4cqavd_bwe multi-input multi-output antenna measurements with super wide bandwidth for wireless applications using isolated t stub and defected ground structure acta imeko issn: 2221-870x march 2022, volume 11, number 1, 1 6 acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 1 multi-input multi-output antenna measurements with super wide bandwidth for wireless applications using isolated t stub and defected ground structure pradeep vinaik kodavanti1, p. v. y. jayasree2, prabhakara rao bhima3 1 department of electronics and communication engineering, jawaharlal nehru technological university kakinada, kakinada-533003, andhra pradesh, india 2 department of electrical, electronics and communication engineering, gitam, visakhapatnam-530045, andhra pradesh, india 3 school of nano technology, jawaharlal nehru technological university kakinada, kakinada-533003, andhra pradesh, india section: research paper keywords: ultra-wide band, mimo, t stub, defected ground structure citation: pradeep vinaik kodavanti, p. v. y. jayasree, prabhakara rao bhima, multi-input multi-output antenna measurements with super wide bandwidth for wireless applications using isolated t stub and defected ground structure, acta imeko, vol. 11, no. 1, article 27, march 2022, identifier: imeko-acta11 (2022)-01-27 section editor: md zia ur rahman, koneru lakshmaiah education foundation, guntur, india received november 29, 2021; in final form march 3, 2022; published march 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: pradeep vinaik kodavanti, e-mail: pradeepvinaik.kodavanti@gmail.com 1. introduction an antenna is the most significant part of the communication system which is responsible for the transmission and interception of the electromagnetic signal [1]-[10]. several configurations like single element, array and multiple elements radiating systems are existing for efficient communication link [11]-[15]. the ultrawide band (uwb) based systems have become the most significant part personal and commercial wireless systems. this certainly led to huge number of applications like wi-fi, wlan, and bluetooth etc. hence it is a challenging task for the antenna engineers to design antennas with typical radiation pattern and spectrum characteristics which does not entertain the spatial or frequency interference. hence antennas with several notch band features are the solution to such interference issues. the other significant parameter is the channel characteristics which contribute to the power efficiency of the system. it is always a challenging task to enhance the capability of the channel without effecting its spectral and spatial response. multi-input multioutput (mimo) emerged as the best solution to meet the above requirements. the technology observes weak coupling between the radiating systems. however, the design challenges do persist in order to perceive the miniaturization and portability of the system. several works have reported significant results pertaining to the uwb mimo based designs. parameters like isolation and its improvement has been the most desirable design issue of mimo uwb antennas. the isolation improvements can be achieved through the decoupling process or characteristics of the antennas. the process is inherently complex in nature [8]-[12]. radiating system with excellent diversity is another solution to achieve the desired characteristics of mimo with good decoupling. incorporating the filters and other circuits are another best option for achieve abstract the paper presents the idea of defective ground structures for the improvement in the radiation characteristics of the antenna especially in the multi-input multi-output (mimo) configuration. the proposed antenna model with partially flared out feed system is designed and analyzed with defective ground in both single and array configuration. a t stub is a t shaped stub used in this work with defected ground structure. a t stub is included along with defective ground to enhance the mimo configuration features. the simulations are carried out on electromagnetic modelling tool and analyzed by measuring the parameters like reflection coefficient, voltage standing wave ratio (vswr), gain, radiation pattern and current distribution plots. for the fabrication of the proposed antenna these measurements are very important. the antenna is fabricated and validated in terms of s-parameters and vswr. the proposed results are good agreement with the simulated results. the overall size of the antenna is 24 × 18 × 0.8 mm3. mailto:pradeepvinaik.kodavanti@gmail.com acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 2 this objective of isolation. slots, stubs, split ring resonator are some examples of structures proposed to embed with the radiating system to realize the notch bands and inference patterns [13]-[15]. recently, several other structures which are conformal are considered as the advancements in antenna design for 5g technology [16], [17]. this technique is projected as the most favorable feature for the advanced communication. measurement of various parameters are very important in the design, fabrication and analysis of the antenna. section 2 describes the proposed antenna geometry with and without t stub. the simulation results are discussed in the section 3. the overall conclusion in included as section 4. 2. proposed antenna geometry the proposed antenna has a typical initial geometry of patch antenna with two conducting plates sandwiched between a dielectric substrate. the conducting plate on the top side serves as the radiation source which is in circular in shape and edge fed. the bottom plate serves as ground plane which is defective in shape intentionally to represent the defective ground features. the usual edge feed geometry is a rectangular strip running from the edge of the radiating patch to the end of the substrate. however, in this work the shape of the rectangular strip has been modified. from the edge of the substrate, the feed line takes the shape of rectangle for a length of lf2 and width of wf and flares out as it reaches the edge of the circular patch where it assumes the width of wf2 which is greater than wf. the length of the gradual flaring of the feedline is lf3. the circular patch has a radius of r while the substrate has dimensions like length l and width w. the ground plane on the other side is defective in nature and does not have s specific shape to define. it can be considered that a rectangular strip of length lg and width similar to that of the substrate at lower edge of the substrate plane. remaining part of the ground plane is left uncoated with ground plane conductor. the strip along the width w has three v shaped valleys. it can be considered that the two v shaped valleys on either side of the centre v shape are symmetric in nature and have the v arm length l1 less than that of l2 which is the arm length of the central deep v shape. the table describing the optimized dimensions of the geometry proposed for the antenna is given in table 1 in which all the dimensions are measured in mm scale. the geometry has been enhanced to the array configuration with two similar elements arranged side by side on the radiation region as shown in figure 1(c). the two-element array configuration is necessary to realise the mimo characteristics of the antenna. the ground characteristics and the geometry in the first case of is similar to the figure 1(b) however, duplicated and copied in order to serve the defective ground of the second table 1. antenna design parameters. s. no parameter value in mm 1 l 18 2 ws 24 3 wf 1.5 4 lf 6 5 r 5.5 6 lf1 6.5 7 wf1 3 8 wf2 2.5 9 lf2 3 10 lf3 3.39 11 lg 6 12 l1 1 13 l2 3.8 14 w1 2.5 15 w2 3 16 w3 2 17 g1 12 18 d 1 19 d1 5 20 sl1 11 21 sl2 0.5 22 sw1 0.5 23 sw2 6 24 d2 5 a) b) c) d) e) figure 1. proposed super wide band antenna (a) front view (b) ground plane and mimo antenna (c) front view (d) ground plane without t stub and (e) ground with t stub. w l r wf lf2 lf3 wf2 lg w1 w2 w3 l2 l1 w l r wf lf2 lf3 wf2 lg w1 w2 w3 l2 l1 ws l r wf lf2 lf3 wf2 g1 d acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 3 element in the array. further, the t stub is included in the second case of interest to enhance the mimo characteristics. in this case, the t stub arises from the centre of the ground plane and specifically from the top edge of the strip for defective ground configuration. the two rectangular strips are arranged perpendicular to each other to form the t shape. the vertical strip has dimensions of sl1 length and sw1 width while the dimensions of the horizontal strip is sl2 and sw2. 3. results and discussion the radiation characteristics of the simulated antenna using the em modelling tool are presented in this section as follows. the results are presented as three cases in which the first case refers to the proposed antenna in its single element form with defective ground. the second case refers to the two-element array configuration and the third case is the mimo antenna with the t stub in the defective ground. 3.1. case-1: defective ground antenna the frequency response and the resonance features can be directly inferred from the s11 plot as presented in figure 2(a). the same can be validated or verified using the corresponding voltage standing wave ratio (vswr) plot given in figure 2(b). further, the frequency dependant gain characteristics are studied using the gain plot in figure 2(c). from the s11 and vswr plot, it is evident that the antenna designed has a very large band starting from 6 ghz covering almost to the frequencies above 80 ghz with a continuous consistency in reflection coefficient less than -10db. within the range it is possible to find several dips in the response curve where the corresponding characteristics are highly resonant. similarly, the vswr magnitude is below 2 through the entire sweep as mentioned above for s11. this confirms the radiation characteristics once again. similarly, from the gain plot it is possible to mention that the maximum gain is continuously increasing form the lower side of the resonant band to the upper side. initially, the max gain takes a linear decrement from the 5 ghz to 8 ghz while after that a gradual but very slow enhancement is observed till 80 ghz. further, the s11 and vswr are frequency response curves while the consistency in the radiation characteristics cannot be inferred from the radiation pattern plots which are presented for various frequencies presented in figure 3(a) through figure 3(f). the frequencies of interest are the dips observed in the s11 pot where there is maximum reflection coefficient for the antenna. in this work, the polar radiation characteristics plots are presented for 6.73 ghz, 13.92 ghz, 30.54 ghz, 37.01 ghz, 43.8 ghz and 52.2 ghz. the radiation characteristics are very much close the template patterns of a patch antenna and also the have similarity. further, the radiation pattern is highly dependent on the current distribution which is again defined by the geometrical conditions of the antenna and the feed point orientation and construction. in order to study the same, the current distribution is given in a) b) c) figure 2. proposed swb antenna simulation results (a) s11 (b) vswr and (c) maximum gain versus frequency. figure 3. simulation radiation patterns of proposed antenna at (a) 6.73 ghz, (b) 13.92 ghz, (c) 30.54 ghz, (d) 37.01 ghz, (e) 43.8 ghz and (f) 52.2 ghz. figure 4. simulation current distributions of proposed antenna at (a) 6.73 ghz, (b) 13.92 ghz, (c) 30.54 ghz, (d) 37.01 ghz, (e) 43.8 ghz and (f) 52.2 ghz. (a) (b) (c) (d) (e) (f) (a) (b) (c) (d) (e) (f) acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 4 figure 4(a) through figure 4(f) for the same frequencies of interest for which the radiation patterns are plotted. in the first three figures, the upper portion of the patch geometry has almost no current distributed and hence the null patterns are visible in the radiation characteristics presented in figure 3. further, at higher frequencies, the current distribution initiated and hence the expansion of the beams is visible in patterns slowly dominating the null characteristics. 3.2. case-2: two element array configuration the two-element array configuration of the defective ground antenna is studied in terms of its resonant features from the plotted s11 and vswr graphs in figure 5(a) and figure 5(b). the array s11 plot is much like that of the single element without any deviation in its resonant features. this is a favourable case for the array configuration as the we do not want the resonant characteristics of the antenna to be disrupted in the array form. similarly, the transmission characteristics from first port to the second port can be studied from the s21 plot which is also plotted in the same figure. the s21 features the same characteristics expect the case of variation in the magnitudes. this is evident in the graph of s21. in order to convince the s11 characteristics, the corresponding vswr features are plotted in figure 5 (b) which exhibit the same resonance of the antenna with the corresponding magnitude of the parameter keeping below 2 for the entire range of the frequency band starting from 5 ghz to 80 ghz. in the gain plot presented in figure 5 (c) and observed to better than the maximum gain observed from figure 2 (c). the s11 of proposed antenna by varying distance (d) between two circular patches with a step size of 0.5 mm is represented in figure 5(d). further, the envelope correlation has been computed for and presented to correlate the envelope features of the characteristics as shown in figure 5 (e). the correlation is denied in the non-resonant band while was excellent in the resonant band of the antenna. similarly, the current distribution is plotted for the array configuration without the t stub and presented in figure 6(a) through figure 6(f) following this also have the radiation polar plots of the array configuration without stub in figure 7(a) through figure 7(f). it is possible to correlate both the current distribution and the polar radiation plots as the current distribution greatly effects the radiation characteristics. the null patterns are completely defined by the no current distribution zones on the surface of the radiating area of the antenna. a) b) c) d) e) figure 5. simulation results of proposed mimo antenna (a) s11 (b) vswr (c) gain versus frequency (d) varying distance (d) between two circular patches with a step size of 0.5 mm and (e) envelope correlation coefficient (ecc). figure 6. simulation current distributions of proposed mimo antenna without t stubs at (a) 6.73 ghz, (b) 13.92 ghz, (c) 30.54 ghz, (d) 37.01 ghz, (e) 43.8 ghz and (f) 52.2 ghz. (a) (b) (c) (d) (e) (f) acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 5 3.3. case-3: array antenna with t stub the array configuration with t stub is as shown in figure 2(e). the s11, s21 and the vswr plots are given in figure 5(a) through figure 5(b). the inclusion of the t stub has no appreciable impact on the s11 plot while the s21 produced a notch band around the 15 ghz frequencies. similarly, the vswr plots have the same features as that of the s11. the gain consideration as relatively unaltered due to the t stub as shown in figure 5(c) and the ecc observes a shift in the band at the initial stage which however has no impact as it is outside the band of operation as shown in figure 5(d). the current distribution and the radiation characteristics are presented in figure 8 and figure 9 respectively for the array without t stub model. both the plots are highly correlated with the radiation pattern defined by the corresponding current distribution. the plane in which the current distribution is maximum observes main beam orientation in the radiation pattern. similarly, the no current zone accommodates the wide nulls in the plane of radiation. at 52.2 ghz, the first element has the maximum current on its surface and the corresponding pattern observes maximum radiation from azimuthal 1200 to 1800. so as the case with all the remaining. the fabricated prototype of the proposed antenna is represented in figure 10. the measured and simulated result comparison of s-parameters and vswr of proposed antenna is represented in figure 11 and figure 12. the measured results are good agreement with the simulated results and is applicable for real time applications. figure 7. simulation radiation patterns of proposed mimo antenna without t stubs at (a) 6.73 ghz, (b) 13.92 ghz, (c) 30.54 ghz, (d) 37.01 ghz, (e) 43.8 ghz and (f) 52.2 ghz. figure 8. simulation current distributions of proposed mimo antenna with t stubs at (a) 6.73 ghz, (b) 13.92 ghz, (c) 30.54 ghz, (d) 37.01 ghz, (e) 43.8 ghz and (f) 52.2 ghz. figure 9. simulation radiation patterns of proposed mimo antenna with t stubs at (a) 6.73 ghz, (b) 13.92 ghz, (c) 30.54 ghz, (d) 37.01 ghz (e) 43.8 ghz and (f) 52.2 ghz. figure 10. fabrication prototype of proposed antenna. figure 11. simulated and measured s11 and s12 for the proposed antenna. figure 12. simulated and measured vswr for the proposed antenna. (a) (b) (c) (d) (e) (f) (a) (b) (c) (d) (e) (f) (a) (b) (c) (d) (e) (f) acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 6 the proposed antenna is compared with previous literature in terms of different parameters of size, substrate, bandwidth, gain and ecc are tabulated in table 2. the ecc is a dimension less quantity. 4. conclusion the planar microstrip antenna-based structure using defective ground with t stub has been successfully simulated and analysed for its swb and mimo characteristics. a clear improvement in terms of the antenna radiation properties is evident from the single and two element array configurations of the mimo antenna. the presence of the t stub has relevance to the notch band characteristics of the antenna. fabrication and prototyping of the antenna have a good scope to validate the results further in a practical environment. references [1] zeeshan ahmed, gul perwasha, ultrawide band antenna with wlan band-notch characteristic, int. conf. comput., control commun (2013), pp. 25–26. doi: 10.1109/ic4.2013.6653762 [2] noman waheed, abubakar saadat, ultra-wide band antenna with wlan and wimax band-notch characteristic, int conf commun, comput digital syst 8 (2017). doi: 10.1109/c-code.2017.7918910 [3] z. q. li, c. l. ruan, a small integrated bluetooth and uwb antenna with wlan bandnotched characteristics, int. symp. signals, syst electron 1 (2010), pp. 1–4. doi: 10.1109/issse.2010.5638206 [4] bo-ren hsiao, yi-an chen, lte mimo antennas on variable sized tablet computers with comprehensive band coverage, ieee antennas wirel. propag. lett. 15 (2015), pp. 1152–1155. doi: 10.1109/lawp.2015.2496966 [5] saber soltani, parisa lotfi, a port and frequency reconfigurable mimo slot antenna for wlan applications, ieee trans. antennas propag. 64 (4) (2016), pp. 1209–1217. doi: 10.1109/tap.2016.2522470 [6] manoj k. meshram, a novel quad-band diversity antenna for lte and wi-fi applications with high isolation”, ieee trans. antennas propag. 60 (9) (2012), pp. 4360–4371. doi: 10.1109/tap.2012.2207044 [7] shrivishal tripathi, akhilesh mohan, sandeep yadav, a compact koch fractal uwb mimo antenna with wlan band-rejection, ieee antennas wirel. propag. lett. 14 (2015), pp. 1565–1568. doi: 10.1109/lawp.2015.2412659 [8] muhammad saeed khan, ultra-compact dual polarized uwb mimo antenna with meandered feeding lines, iet microw., antennas propag. 11 (7) (2017), pp. 997–1002. doi: 10.1049/iet-map.2016.1074 [9] mohammad abedian, compact ultrawide band mimo dielectric resonator antennas with wlan band rejection, iet microw., antennas propag. 11 (11) (2017), pp. 1524–1529. doi: 10.1049/iet-map.2016.0299 [10] mohammad mahdi farahani, mutual coupling reduction in millimeter-wave mimo antenna array using a meta material polarization-rotator wall, ieee antennas wirel. propag. lett. 16 (2017), pp. 2324–2327. doi: 10.1109/lawp.2017.2717404 [11] mohammad saeed khan, a compact csrr-enabled uwb diversity antenna, ieee antennas wirel. propag. lett. 16 (2017), pp. 808–812. doi: 10.1109/lawp.2016.2604843 [12] wing-xin chu, compact broad band slot mimo antenna with a stub loaded radiator, ieee conference on antenna measurements and applications (2014), pp. 1–4. doi: 10.1109/cama.2014.7003407 [13] v. jyothika, m. s. p. c. shekar, s. v. krishna, m. z. u. rahman, design of 16 element rectangular patch antenna array for 5g applications, journal of critical reviews, 7 (9) (2020), pp. 53-58. [14] v. s. chakravarthy, p. m. rao, amplitude-only null positioning in circular arrays using genetic algorithm. ieee international conference on electrical, computer and communication technologies (icecct), pp. 1-5. doi: 10.1109/icecct.2015.7226120 [15] k. aravind rao, k. sai raj, r. k. jain, m. z. u. rahman, implementation of adaptive beam steering for phased array antennas using enlms algorithm, journal of critical reviews, 7 (9) (2020), pp. 59-63. doi: 10.31838/jcr.07.09.10 [16] s. blanch, j. romeu, exact representation of antenna system diversity performance from input parameter description, electron. lett. 39 (9) (2003), pp. 705–707. doi: 10.1049/el:20030495 [17] h. huang, x. li, y. liu, a low-profile, dual-polarized patch antenna for 5g mimo application, ieee trans. antennas propag. 67 (2) (2019), pp. 1275–1279. doi: 10.1109/tap.2018.2880098 [18] s. kesana, s. gatikanti, m. z. ur rahman, b. radhika, d. mounika, triple frequency g-shape mimo antenna for wireless applications, international journal of engineering and advanced technology, 8 (5) (2019), pp. 942-947. [19] armando coccia, federica amitrano, leandro donisi, giuseppe cesarelli, gaetano pagano, mario cesarelli, giovanni d'addio, design and validation of an e-textile-based wearable system for remote health monitoring, acta imeko, vol.10, 2021, no.2, pp. 1-10. doi: 10.21014/acta_imeko.v10i2.912 [20] h. t. chattha, f. latif, f. a. tahir, m. u. khan, x. yang, smallsized uwb mimo antenna with band rejection capability, ieee access, 7 (2019), pp. 121816–121824. doi: 10.1109/access.2019.2937322 [21] s. v. devika, k. karki, s. k. kotamraju, k. c. sri kavya, m. z. u. rahman, a new computation method for pointing accuracy of cassegrain antenna in satellite communication, journal of theoretical and applied information technology, 95 (13) (2017), pp. 3062-3074. [22] r. hussain, m. s. sharawi, a. shamim, an integrated four-element slot-based mimo and a uwb sensing antenna system for cr platforms, ieee trans. antennas propag., 66 (2) (2018), pp. 978– 983. doi: 10.1109/tap.2017.2781220 [23] c. yu, s. yang, y. chen, w. wang, l. zhang, b. li, l. wang, a super-wideband and high isolation mimo antenna system using a windmill-shaped decoupling structure, ieee access, 8 (2020), pp. 115767-115777. doi: 10.1109/access.2020.3004396 table 2. comparing proposed antenna with previous literature. ref size (mm3) substrate band width (ghz) gain (db) ecc [18] 48 × 48 × 0.8 fr4 2.5 12 na < 0.005 [19] 50 × 50 × 1.6 ro-6035htc 2.76 10.75 2.5 5 < 0.025 [20] 23 × 26 × 0.8 fr4 3.1 10.6 2 4.5 < 0.01 [21] 50 × 39.8 × 1.524 ro-tmm4 2.7 12 2.5 6.0 < 0.01 [22] 60 × 120 × 1.5 ro-4350 0.75 7.65 3.2 < 0.1 [23] 58 × 58 × 1 fr4 2.9 40 4.3 13.5 < 0.04 proposed 18 × 24 × 0.8 rogers 5880 5.5 80 4.01 9.89 < 0.05 https://doi.org/10.1109/ic4.2013.6653762 https://doi.org/10.1109/c-code.2017.7918910 https://doi.org/10.1109/issse.2010.5638206 https://doi.org/10.1109/lawp.2015.2496966 https://doi.org/10.1109/tap.2016.2522470 https://doi.org/10.1109/tap.2012.2207044 https://doi.org/10.1109/lawp.2015.2412659 https://doi.org/10.1049/iet-map.2016.1074 https://doi.org/10.1049/iet-map.2016.0299 https://doi.org/10.1109/lawp.2017.2717404 https://doi.org/10.1109/lawp.2016.2604843 https://doi.org/10.1109/cama.2014.7003407 https://doi.org/10.1109/icecct.2015.7226120 https://doi.org/10.31838/jcr.07.09.10 https://doi.org/10.1049/el:20030495 https://doi.org/10.1109/tap.2018.2880098 https://doi.org/10.21014/acta_imeko.v10i2.912 https://doi.org/10.1109/access.2019.2937322 https://doi.org/10.1109/tap.2017.2781220 https://doi.org/10.1109/access.2020.3004396 uncertain estimation-based motion planning algorithms for mobile robots acta imeko issn: 2221-870x september 2021, volume 10, number 3, 51 60 acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 51 uncertain estimation-based motion-planning algorithms for mobile robots zoltán gyenes1, emese gincsainé szádeczky-kardoss1 1 budapest university of technology and economics, magyar tudósok körútja 2, 1117 budapest, hungary section: research paper keywords: motion planning; mobile robots; cost function; uncertain estimations citation: zoltán gyenes, emese gincsainé szádeczky-kardoss, uncertain estimation-based motion planning algorithms for mobile robots, acta imeko, vol. 10, no. 3, article 9, september 2021, identifier: imeko-acta-10 (2021)-03-09 section editor: bálint kiss, budapest university of technology and economics, hungary received january 15, 2021; in final form august 9, 2021; published september 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: zoltán gyenes, e-mail: gyezo12@gmail.com 1. introduction autonomous driving is a highly frequented research area for mobile robots, cars and drones. robots have to generate a collision-free motion towards the target position while maintaining safety with respect to the obstacles that occur in the local environment. motion-planning methods generate both the velocity and the path profiles for the robot using measured information about the velocity vectors and the positions of the obstacles. motion-planning algorithms can be divided into two parts. if all the data for the robot’s environment are known and available at the start, then global motion-planning algorithms can be used to generate a collision-free path [1], [2]. however, if the robot can only use local sensor-based information about its surrounding dnese and dynamic environment, then reactive motion-planning algorithms can provide an acceptable solution for generating the robot’s path and velocity [3], [4]. using a reactive motion-planning algorithm, generating optimal evasive manoeuvers that can ensure a safe motion for the agent and the environment is an np-hard problem [5]. the task is more difficult if the uncertainties of the measured data (velocity vectors and positions) are taken into account. in this paper, a novel reactive motion-planning algorithm is presented that can calculate the uncertainties of every obstacle using their velocity vectors and distance from the agent. the paper is ordered in the following way. section 2 outlines some often-used reactive motion-planning methods that have been introduced in recent decades. in some algorithms, the uncertainties of the measured data have also been considered. at the end of section 2, the basics of the velocity obstacle (vo) and artificial potential field (apf) methods are presented. in section 3, the novel concept for the calculation of obstacle uncertainties is set out. section 4 then presents the introduced motionplanning algorithms, which can generate a safety motion for the agent taking into account the uncertainties. in section 5, the simulation results are presented, and the introduced motionplanning methods are compared. in section 6, the coppeliasim simulation environment is discussed, and section 7 provides a conclusion and sets out plans for future research. 2. previous work in this section, a few reactive motion-planning algorithms are presented. abstract collision-free motion planning for mobile agents is a challenging task, especially when the robot has to move towards a target position in a dynamic environment. the main aim of this paper is to introduce motion-planning algorithms using the changing uncertainties of the sensor-based data of obstacles. two main algorithms are presented in this work. the first is based on the well-known velocity obstacle motion-planning method. in this method, collision-free motion must be achieved by the algorithm using a cost-function-based optimisation method. the second algorithm is an extension of the often-used artificial potential field. for this study, it is assumed that some of the obstacle data (e.g. the positions of static obstacles) are already known at the beginning of the algorithm (e.g. from a map of the enviroment), but other information (e.g. the velocity vectors of moving obstacles) must be measured using sensors. the algorithms are tested in simulations and compared in different situations. mailto:gyezo12@gmail.com acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 52 the inevitable collision states method (ics) calculates all states of the robot where there is no available control command that would result in a collision-free motion between the robot and the environment. the main goal is to ensure that the agent never finds itself in an ics situation. the algorithm is appropriate not only for static but also for dynamic environments [6]-[8]. the main concept behind the dynamic window method [9], [10] is that the agent selects a velocity vector from the reachable and admissible set of the velocity space. the robot executes a collision-free motion by selecting a velocity vector from the admissible velocity set. at the same time, reachable velocities can be generated using the kinematic and dynamic constraints of the agent. the admissible gap is a relatively new concept for motionplanning algorithms [11]. if the robot can move through the gap safely using motion control, then it is admissible, obeying the constraints of the agent. this method is also usable in an unknown environment. the gap-based online motion-planning algorithm has also been used with a lidar sensor by introducing a binary sensing vector (the value of the vector element is equal to 1 if there is an obstacle in that direction) [12]. 2.1. velocity obstacle method the main concept behind our method is based on the vo method [13]. using the positions and the velocities of the obstacles and the agent, the vo method generates a collisionfree motion for the robot. the vo concept has been used in different methods. the steps in the vo method are as follows: 𝐵𝑖 denotes the different obstacles (𝑖 = 1. . . 𝑚, where 𝑚 represents the number of obstacles), and the agent is 𝐴. for every obstacle, a 𝑉𝑂𝑖 cone can be generated that constitutes every robot velocity vector that would result in a collision between the agent (𝐴) and the obstacle (𝐵𝑖 ) at a future time: 𝑉𝑂𝑖 = { 𝐯a | ∃ 𝑡: 𝐩a + 𝐯a𝑡 ∩ 𝐩b𝑖 + 𝐯b𝑖𝑡 ≠ 0} , (1) where 𝐩a and 𝐩b𝑖 are the positions and 𝐯a and 𝐯b𝑖 are the velocity vectors of the robot and the obstacle. the velocities of the obstacles and the robot are assumed to be constant until 𝑡. if there are more obstacles, then the whole 𝑉𝑂 set can be generated: 𝑉𝑂 =∪𝑖=1 𝑛 𝑉𝑂𝑖 . (2) figure 1 provides an example in which a moving obstacle is in position 𝐩b1 and has velocity 𝐯b1 at the actual time step. there is also a static obstacle in the workspace of the agent (𝐩b2 represents its position). the two vo areas are depicted in blue. reachable velocities (rv) can be defined as the velocity area that constitutes every 𝐯a velocity of the agent that is reachable considering the previously selected velocity vector and the motion capabilities of the robot. reachable avoidance velocities (rav) can be received by subtracting the vo from the rv set. figure 2 represents the steps of the motion-planning algorithm. the main difference between the algorithms is the method for selecting the robot’s velocity vector from the rav set. the 𝜖𝐶𝐶𝐴 is an extended version of the reciprocal velocity obstacle (rvo) algorithm [14], which uses the kinodynamic constraints of the robot. the method generates an appropriate solution for the multi-robot collision avoidance problem in a complex environment. the computational time plays an important role in this algorithm. the whole environment of the agent is divided into a grid-based map. the agent selects a collision-free velocity vector using both convex and nonconvex optimisation algorithms [15]. figure 1. velocity obstacle method. figure 2. steps of the whole vo algorithm. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 53 the probabilistic velocity obstacle method is also an extended version of the rvo method [14], which uses the time-scaling method and bayesian decomposition. this method demonstrates better performance in terms of traversal times than the existing bound-based methods. the algorithm was tested using simulation results [16]. the collision avoidance under bounded localisation uncertainty method [17] introduced convex hull peeling, resulting in a limitation in the localisation error. this method results in a better performance than the previously introduced multi-robot collision avoidance with localisation uncertainty method [18] with respect to the tightness of the bound. particle filter is used for robot localisation problems. in this case, convex polygons are generated as the robot footprints. the algorithm ensures that the robot is inside this convex polygon with a probability of 1 − 𝜀. a time truncation is also used in the algorithm because it supports the velocity selection even in a crowded, complex environment. the directive circle (dc) method is an extended version of the vo method [19], [20]. in this algorithm, the velocity of the robot is selected from dc, which is calculated using the maximum velocity of the agent for the radius of the dc. ensuring the kinematic constraints of the agent, the best solution is selected from the dc in the optimal direction for the target position. using the dc method, the local minima situations are preserved. all the presented reactive motion-planning algorithms assume a complete set of information on the position and the velocity vectors of the obstacles that occur in the robot’s workspace. the main advantage of our introduced method is that the uncertainty of the measured sensor information can be taken into consideration, and the novel motion-planning algorithm can generate collision-free target reaching for the agent even when the data is inaccurate. 2.2. artificial potential field method the apf method is an often-used reactive motion-planning algorithm. the main concept is to calculate the summation of the attractive (between the robot and the target) and repelling (between the agent and the obstacles ) forces [21], [22]. one weakness of this algorithm is that sometimes only the local optimum can be found. the algorithm has also been developed for unmanned arial vehicles [23], while human–robot interaction was also simulated using the apf motion-planning method by using the motion characteristics of household animals [24]. the steps of the apf method are as follows: during motion planning, in every sampling time step, ar force will influence the motion of the agent. the ar force depends on the repelling ( 𝐅𝐚𝐫𝐢) and the attractive (frc) forces. the closer the robot is to the obstacle, the larger the volume of the repelling force. the repelling force can be calculated by 𝐅𝐚𝐫𝐢 = 𝜂√ 1 𝐷ra 𝑖 + 1 𝐷ramax 𝐷ra𝑖 2 𝐀𝐑𝐢 , (3) where 𝐷ra 𝑖 denotes the distance between the robot and the obstacle, 𝐀𝐑𝐢 is the vector between the obstacle and the agent, 𝜂 is a specific parameter that identifies the role of the repelling force in the motion-planning algorithm and 𝐷ramax is the largest distance that should be considered in the motion-planning algorithm, which can be calculated as 𝐷ramax = 𝑣max 𝑇s , (4) where 𝑣max is the maximum velocity of the robot and 𝑇s denotes the sampling time. the attractive force can be calculated as 𝐅𝐫𝐜 = 𝜉 𝐑𝐂 , (5) where 𝐑𝐂 is the vector between the robot and the target and 𝜉 is the parameter of the attractive force (depending on the usage of the algorithm). the force that influences the motion of the robot can be calculated (if there is one obstacle in the workspace) with the sum of the repelling and the attractive forces: 𝐀𝐫 = ∑ 𝐅𝐚𝐫𝑖 𝑚 𝑖=1 + 𝐅𝐫𝐜 . (6) figure 3 illustrates the different forces presented. there is one obstacle in the workspace (b1) with the position 𝐩b1. the agent is at the 𝐩a position at this point, and the summation of the forces can be checked. if the mass of the agent is known, then the acceleration can be calculated using newton’s second law: 𝐚 = 𝐀𝐫 𝑚 , (7) where m is the mass of the robot. the changes in the velocity vector can be calculated if the force and the sampling time are known: δ𝐯 = 𝐚 𝑇𝑠 . (8) so the actual velocity can be calculated using the previous velocity and the change in velocity: 𝐯𝐧𝐞𝐰 = 𝐯𝐩𝐫𝐞𝐯 + δ𝐯 . (9) 3. uncertainty calculation using measurement data in previous studies, all the uncertainties of the obstacles were constant throughout the algorithm [25]. in the present study, they will be adjusted using the changes in the velocity vectors of the obstacles, the actual distances of the obstacles from the robot and the magnitudes of the obstacles’ velocity vectors. figure 3. apf method. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 54 the uncertainties can be calculated from the probabilities of the previously introduced parameters. the main concept behind this method is that the measured information has lower reliability if the obstacles are far from the robot. first, the obstacle distance is generated: 𝑃dist𝑖 = { 1 − dist𝑂𝑅𝑖 𝑣max ∙ 𝑇u if dist𝑂𝑅𝑖 < 𝐯max ∙ 𝑇u 0 otherwise , (10) where 𝑇u is the uncertainty time parameter, 𝑃dist𝑖 is the distancebased probability term and dist𝑂𝑅𝑖 is the actual distance between the robot and the obstacle 𝐵𝑖 . the magnitude of the velocity vector of the obstacle also plays a significant role in generating the uncertainties; the higher the velocity of the obstacle, the smaller the reliability of the available information on the obstacle: pmv𝑖 = { 1 − ||𝐯bi|| 𝑣max 𝑖𝑓 ||𝐯bi|| < 𝑣max 0 otherwise , (11) where ||𝐯bi|| refers to the actual magnitude of the velocity of the obstacle 𝐵𝑖 (||. || is the secondary norm (euclidean distance)) and pmv𝑖 is the velocity-based probability term. the change in the obstacle’s velocity vector also influences the volume of the uncertainties. the changes in the velocity of the obstacle can be calculated for each obstacle: 𝐶𝑉𝑖 = ||𝐯b𝑖,new − 𝐯b𝑖,old ||, (12) where 𝐯b𝑖,new is the actual velocity of the obstacle, 𝐯b𝑖,old is the previous velocity of the obstacle and 𝐶𝑉𝑖 denotes the change in the obstacle’s velocity: 𝑃cv𝑖 = { 1 − 𝐶𝑉𝑖 2 𝑣max if 𝐶𝑉𝑖 < 𝑣max 0 otherwise , (13) where 𝑃cv𝑖 is the probability term depending on the change in the velocity vector of the obstacle. the probability for obstacle 𝐵𝑖 can be generated as 𝑃𝑖 = 𝑃dist𝑖 + 𝑃mv𝑖 + 𝑃cv𝑖 3 . (14) the uncertainty parameter can be calculated from the calculated probability: 𝛼𝑖 = 1 − 𝑃𝑖 , (15) where this 𝛼𝑖 uncertainty parameter must be calculated for every obstacle (𝑖 = 1. . . 𝑚, if there are 𝑚 obstacles in the environment; if 𝛼𝑖 = 0, then there is no measurement uncertainty). 4. velocity selection based on motion-planning algorithms 4.1. precheck algorithm the agent has to consider only those obstacles that fulfil the precheck algorithm during the motion-planning algorithm. two obstacle situations are not used: • obstacles that will cross the path of the agent in the distant future, • obstacles that are at a considerable distance from the agent. for all obstacles, the minimum distance and time must be calculated for the point at which the agent and the obstacle are closest to each other during their motion: 𝑡mina,b𝑖 = −( 𝐩a − 𝐩b𝑖 )(𝐯a − 𝐯b𝑖 ) ||𝐯a − 𝐯b𝑖|| , (16) where 𝑡mina,b𝑖 presents the time interval for the point at which the agent and the obstacle are closest to each other. the nearest point is in the past if the value of this parameter is negative. the minimal distance can be calculated as follows: 𝑑mina,b𝑖 = ||( 𝐩a + 𝐯a𝑡mina,b𝑖 ) − (𝐩b𝑖 + 𝐯b𝑖𝑡mina,b𝑖 )|| . (17) so, only those obstacles that fulfill the following inequalities must be considered: 0 < 𝑡mina,b𝑖 < 2 ∙ 𝑇precheck and 𝑑mina,b𝑖 < 𝑣max ∙ 𝑇precheck , (18) where 𝑣max denotes the maximum velocity of the agent and 𝑇precheck is a parameter of the algorithm that must be tuned. the experiments in this study demonstrate that if the value of the 𝑇precheck parameter is too small, the generated path is not smooth enough. the precheck algorithm is illustrated in figure 4. when there is a moving obstacle in the robot’s workspace, the minimal distance and time point can be calculated when the obstacle and the agent are closest to each other. 4.2. cost-function-based velocity selection using the extended vo method the safety velocity obstacle method has been defined in a previous study [26]. in this method, a cost function was used when different aspects influenced the motion-planning method (safety, speed). this algorithm is extended with a heading parameter, which provides information on the orientation of the agent in relation to the target position, and the method is also extended with the changing uncertainty parameter. at every time step, the nearest distance is calculated between the vo cone and the investigated velocities: figure 4. precheck algorithm. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 55 𝐷s(𝐯𝐴, 𝑉𝑂𝑖 ) = min { 𝑚𝑖𝑛 𝒗𝑉𝑂∈𝑉𝑂 ||𝒗𝐴 − 𝒗𝑉𝑂||, 𝐷𝑚𝑎𝑥 }, (19) where 𝐷max is the maximum distance that should be considered and 𝐯vo is the nearest point on the vo cone. the 𝐷s(𝐯a) value must be normalised into the interval of [0,1]. the 𝐶s(𝐯a) can then be calculated, which will be used in the cost function later: 𝐶s(𝐯a, 𝑉𝑂𝑖 ) = 1 − 𝐷s(𝐯a, 𝑉𝑂𝑖 ) 𝐷max . (20) 𝐶g(𝐯a) will also form part of the cost function: 𝐶g(𝐯a) = ||𝐩a + 𝐯a𝑇s − 𝐩goal || ||𝐩a(0) − 𝐩goal|| , (21) where 𝑇s is the sampling time, 𝐩a(0) is the first position of the robot at the beginning of the motion and 𝐩goal is the position of the target. 𝐶g(𝐯a) denotes how far the robot will be from the target if it uses the selected velocity; subsequently, it has to be divided by the distance of the first position and the desired position. in this novel algorithm, the prior method is extended by changing 𝛼𝑖 (𝑡) parameters (calculated at every time step) for the different obstacles with respect to the reliability of the obstacles’ velocity and position information. the orientation of the agent can also play a role in the cost function. the heading parameter of the cost function can be calculated as follows: 𝐶h(𝐯a) = |𝑎𝑛𝑔𝑙𝑒𝑅𝐺 − 𝑎𝑛𝑔𝑙𝑒𝐼𝑉(𝐯a)| π , (22) where 𝑎𝑛𝑔𝑙𝑒𝑅𝐺 refers to the angle of the vector from the robot position to the target position and 𝑎𝑛𝑔𝑙𝑒𝐼𝑉(𝐯a) denotes the angle of the investigated velocity vector of the agent. using the difference in these angles, the heading parameter can be calculated (angles are defined in the global coordinate system). the whole cost function can be determined using different parameters: cost(𝐯𝐴) = ∑ 𝑚 𝑖=1 𝛼𝑖 (𝑡) 𝐶s(𝐯a, 𝑉𝑂𝑖 ) + 𝛽d 𝐶g(𝐯a) + 𝛽h 𝐶h(𝐯a), (23) where 𝛽d is the distance parameter, 𝛽h is the heading parameter and 𝛼𝑖 (𝑡) denotes the actual calculated uncertainty parameter of an obstacle. this velocity vector is selected for the agent, which has minimal cost value. the different parameters of the cost function have a significant impact on the velocity selection, as will be presented in section 5. 4.3. velocity selection based on the extended apf method the apf method can be extended using 𝛼𝑖(𝑡) and 𝛽d parameters, which were introduced in (15) and (23). the repelling forces must be calculated for every obstacle. the constant η parameter must be substituted with the changing uncertainty parameter, which has a value for every obstacle: 𝐅𝐚𝐫𝐢 = 𝛼𝑖 (𝑡)√ 1 𝐷ra𝑖 + 1 𝐷ramax 𝐷ra𝑖 2 𝐀𝐑𝐢 , (24) where the notations are the same (with the extension of the i parameter, which refers to the i-th obstacle) as introduced in (3). the attractive force, 𝐅𝐫𝐜, can be calculated in the same way as in (5) by using the 𝛽d parameter instead of 𝜉: 𝐅𝐫𝐜 = 𝛽d 𝐑𝐂 . (25) the final force that influences the movement of the agent can be calculated as the addition of the attractive force and the summation of the repelling forces, as presented in (6). after calculating the force that influences the actual movement of the agent, the selectable velocity vector can be calculated using (7), (8) and (9). 5. simulation results in this section, the simulation results are discussed based on the changing uncertainties. 5.1. two static obstacles in the first example, there are two static obstacles in the workspace of the agent. initially, using the introduced costfunction-based vo method, the velocity vector that is exactly in the middle of the two obstacles is selected because the two obstacles have the same uncertainties. this situation is presented in figure 5, in which the vos are presented as grey areas, the blue circle is the selected velocity vector, the target is depicted by a black x, each of the agent’s velocity vectors identified through the motion-planning algorithm is represented by a red x and the robot is the red circle. the changes and the magnitude of the velocities of the obstacles do not influence the calculation of the uncertainties because there are two static obstacles in the workspace. so, in this case, only the distances between the obstacles and the agent have an impact on the calculation. the velocity vector between the two obstacles is selected, and the distances between the agent and the two obstacles are the same during the motion, resulting in the same uncertainties for both obstacles, as presented in figure 5. first example: velocity selection based on the extended vo method. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 56 figure 6 (at each time step, the uncertainties for the obstacles can be seen next to each other). this example was also tested using both the original apf method and the extended apf method. using the original apf method that was introduced in section 2, the agent cannot reach the target position. this is because at the beginning of the motion, the apf method results in a velocity vector that causes a motion in the opposite direction from the target position, as can be seen in figure 7 (the values of the constant parameters were 𝜂 = 2 and 𝜉 = 0.1). 𝐅𝐚𝐫1 and 𝐅𝐚𝐫 2 represent the repelling forces for the different obstacles, and 𝐅𝐚𝐫 sum is the summation of the repelling forces (the other notations are the same as those introduced in previous sections). eventually, the summation of the forces will become a force in the direction of the target position. the agent will then move towards the target position. this sequence is repeated, resulting in an oscillation without reaching the target position. the example was also tested with the extended apf method, introduced in section 4.3. in this case, the reactive motionplanning method can generate a collision-free motion towards the target position. the different forces and the selected velocity vector can be seen in figure 8. using this method, the obstacles’ uncertainties change in the same way as seen in figure 6 because the agent executes its motion along the same path between the two obstacles. 5.2. one moving and one static obstacle in this example, the first obstacle is a moving obstacle, and the second obstacle is a static obstacle. if the agent is at a considerable distance, it can select a velocity vector in line with the target position. after that, if it gets closer to the obstacles, it selects a velocity vector that results in a manoeuvre next to the static obstacle because the corresponding probability is higher. the results of the velocity selection, in this case, are depicted in figure 9. the path of the robot is presented as a black line. in this example, the uncertainties of the obstacles are not the same as in the previous example because the static obstacle has a smaller uncertainty throughout the motion, as presented in figure 10. it can be seen that in the first step, the difference between the obstacles’ uncertainties is not significant. this is because the moving obstacle has a small magnitude of velocity and the distances between the obstacles and the robot are the same at the first step. figure 6. first example: two static obstacles; changing uncertainties during motion. the uncertainties are the same for both obstacles. figure 7. first example: original apf method. figure 8. first example: extended apf method. figure 9. second example: velocity selection based on the extended vo method. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 57 this example was also simulated with the extended apf method. because the first obstacle has a nonzero velocity vector, this obstacle has a higher uncertainty when using the motionplanning algorithm. however, because the magnitude of the velocity vector is not a large value compared with the summation of the forces, the agent executes its motion almost directly in line with the target position, as presented in figure 11. so, in this case, there is a difference between the result of the extended vo method and that of the extended apf method, but both of them can result in a collision-free motion between the agent and the environment, and using both methods, the agent can reach the target position. the uncertainties of the obstacles can also be calculated during the motion of the agent with the extended apf method, although the result will be slightly different from the result of the extended vo method. 5.3. three obstacles in front of each other in the next example, there are three obstacles in front of each other with different velocities (the first obstacle is static, and the others are moving). figure 13 shows the velocity selection of the vo method next to the first obstacle, and figure 14 presents the velocity selection next to the second obstacle. it can be seen that a further velocity component is selected at the second obstacle because it has a higher velocity. the higher the velocity of the obstacle, the bigger the uncertainty for the obstacle, as depicted in figure 15, in which the uncertainty parameters are presented as aspects of the three obstacles. after passing the obstacle, the uncertainty is reduced. this figure shows that not all the obstacles need to be considered throughout the motion, only those that influence the motion of the robot and which have fulfilled the precheck algorithm at the time of sampling. the 𝛽h parameter plays a significant role in the cost-functionbased velocity selection as a factor in the target-reaching strategy. in the previous examples, the value of 𝛽h was 0.3. if this parameter has a higher value, it has a larger impact on the motion than the uncertainties of the obstacles, as presented in figure 16. in this case (𝛽h = 0.6), the agent executes the motion as close to the obstacle as the collision-free motion-planning algorithm allows. figure 10. second example: one moving (first) and one static (second) obstacle; changing uncertainties during motion using the extended vo method. the moving obstacle has a higher uncertainty parameter. figure 11. second example: velocity selection based on the extended apf method. figure 12. second example: one moving (first) and one static (second) obstacle; changing uncertainties during motion using the extended apf method. the moving obstacle has a higher uncertainty parameter. figure 13. third example: velocity selection at the first obstacle using the extended vo method. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 58 the values of the parameters depend on the usage of the algorithm; different parameter values generate different results using the collision-free motion-planning algorithm. however, it is not possible to find a solution that takes into account every aspect of the motion-planning problem. a sub-optimal solution must therefore always be calculated. this example can also be tested using the extended apf method. only the first obstacle should be considered for the motion-planning algorithm because (18) is the appropriate equation for the first obstacle (the second and third obstacles are at a distance from the agent). in this case, the algorithm selects a velocity vector for the agent that results in a movement in the exact direction of the first obstacle (because the summation of the forces is in line with the movement of the agent). so, after a few steps, the agent reaches and collides with the first obstacle. in this example, the extended apf method cannot guarantee that the robot will reach the target collision free. 5.4. standard vo method and the novel motion-planning method the introduced novel motion-planning algorithm was also compared with the original vo method because the basic concept of the motion-planning algorithm is based on this algorithm. the comparison used the example of two moving obstacles in the robot’s workspace. one of them has a changing velocity vector that results in a higher uncertainty in the motion of the robot. figure 17 shows the final path of the robot using the different motion-planning algorithms. it can be seen that using the standard vo method, which provides the fastest targetreaching concept, the agent executes a tangential motion next to the first obstacle (this is also presented in a video [27]). however, if the uncertainties in the measurement data are also considered, the target-reaching method will be solved, generating a path that is relatively far from the first obstacle (with changing velocities). the motion of the robot is also presented in a video [28]. figure 18 represents the distances between the agent and the obstacles using the different motion-planning algorithms. as has already been mentioned, when using the standard vo method, there is a time point at which the distance between the robot and the obstacle is zero. in the case of the novel motion-planning algorithm, the uncertainties can be taken into account, so the agent can move safely towards the target position. however, if there is even a tiny measurement or system noise in the process, the tangential movement will immediately cause a collision. so, if this occurs, it is better to use the novel motion-planning algorithm, which generates a collision-free motion for the agent in every situation. figure 14. third example: velocity selection at the second obstacle using the extended vo method. figure 15. third example: three obstacles in front of each other with different velocities; changing uncertainties during motion using the extended vo method. figure 16. the resulting motion paths of the robot with different heading parameters; in the first example 𝜷𝐡 = 0.3, in the second example 𝜷𝐡 = 0.6. figure 17. final paths of the robot using the normal vo method and the novel motion-planning algorithm. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 59 6. coppeliasim simulation environment coppeliasim (vrep version) is suitable for testing robotic arms as well as holonomic and non-holonomic mobile robots using reactive motion-planning algorithms. different types of obstacles can occur in the workspace of the robot, and there are a wide range of obstacles that can be used in the simulation environment the results of the introduced methods were tested in the coppeliasim simulation environment, as presented in [29]. the agent is an omnidirectional robot (blue). this type of mobile robot is often used because it can execute its motion in any direction from an actual position. in the example in figure 19, there are two static obstacles (grey cylinders) in the workspace of the agent. the main goal of the robot is to reach the target position without colliding with the two obstacles, as presented in section 5.1. 7. conclusions in this paper, novel motion-planning methods were introduced using the basics of the vo and the apf methods. the mobile robot was able to execute collision-free motion planning after calculating the changing uncertainties of the obstacles. these uncertainties depend on the magnitudes of the velocity vectors of the obstacles, the distances between the obstacles and the robot, and the changes in the obstacles’ velocities. the vo-based method can generate collision-free motion using a cost-function-based optimisation method. the basic apf method was also extended by using the uncertainty and distance parameters in the algorithm. the extended apf method can generate a better solution than the original apf method, but there are some situations in which it cannot provide a target-reaching solution. in these cases, the cost-function-based vo method was able to guarantee that the target was reached. the parameters for the apf method could also be calculated in another way, thus solving the local minima problem [30], [31]. the introduced algorithm could be implemented in a real robotic system using an omnidirectional mobile robot. the state estimation of the obstacles that occur in the workspace of the robot could be solved using an extended particle filter algorithm. in this case, the position and the velocity vectors of the obstacles could be estimated for every sampling time [32]. to achieve this, a lidar sensor can be used. acknowledgement this paper was supported by the únkp-20-3 new national excellence programme of the ministry for innovation and technology from the national research, development and innovation fund, and the research reported in this paper and carried out at the budapest university of technology and economics has been supported by the national research development and innovation fund (tkp2020 institution excellence sub-programme, grant no. bme-ie-mifm) based on the charter issued by the national research development and innovation office under the auspices of the ministry for innovation and technology. references [1] s. panov, s. koceski, metaheuristic global path planning algorithm for mobile robots, int. j. of reasoning-based intelligent systems 7 (2015), p. 35. doi: 10.1504/ijris.2015.070910 [2] p. m. hsu, c. l. lin, m. y. yang, on the complete coverage path planning for mobile robots, j. of intelligent and robotic systems: theory and applications 74 (2014), pp. 945-963. doi: 10.1007/s10846-013-9856-0 [3] e. masehian, y. katebi, sensor-based motion planning of wheeled mobile robots in unknown dynamic environments, j. of intelligent and robotic systems: theory and applications 74 (2014), pp. 893914. doi: 10.1007/s10846-013-9837-3 [4] m. g. mohanan, a. salgoankar, a survey of robotic motion planning in dynamic environments, robotics and autonomous systems 100 (2018), pp. 171-185. doi: 10.1016/j.robot.2017.10.011 [5] p. raja, s. pugazhenthi, optimal path planning of mobile robots: a review, int. j. of physical sciences 7 (9), february 2012, pp. 13141320. doi: 10.5897/ijps11.1745 [6] s. petti, t. fraichard, safe motion planning in dynamic environments, proc. of the ieee rsj int. conf. intell. robot. syst., edmonton, canada, 2-6 august 2005, pp. 2210-2215. 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https://doi.org/10.1109/carpathiancc.2018.8473397 https://youtu.be/jp6m3ngwjpk https://youtu.be/mxz1om3bzys https://www.youtube.com/watch?v=hjcntdks6-o&feature=youtu.be https://www.youtube.com/watch?v=hjcntdks6-o&feature=youtu.be https://doi.org/10.1007/bf02982426 https://doi.org/10.1016/j.conengprac.2016.07.008 building information modelling and digital fabrication for the valorization of archival heritage acta imeko issn: 2221-870x march 2022, volume 11, number 1, 1 8 acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 1 building information modelling and digital fabrication for the valorization of archival heritage giulia bertola1 1 politecnico di torino, dad, modlab arch, viale mattioli 39, 10125, torino, italy section: research paper keywords: architectural archives; archives; bim modelling; digital fabrication; rapid prototyping citation: giulia bertola, building information modelling and digital fabrication for the valorization of archival heritage, acta imeko, vol. 11, no. 1, article 18, march 2022, identifier: imeko-acta-11 (2022)-01-18 section editor: fabio santaniello, university of trento, italy received march 7, 2021; in final form march 29, 2022; published march 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: giulia bertola, e-mail: giulia.bertola@polito.it 1. introduction through this contribution, the author intends to deepen some aspects and themes already addressed previously through two different papers both focused on the project of "due case a capri" by aldo morbelli (1942): the first one focused on the role of redesign and the use of traditional and manual techniques (figure 1) [1] for the understanding of the project, the second on the realization of a real model through the application of digital fabrication techniques from a digital reconstructive model using the bim revit® software [2]. archives of 20th century architecture represent today an important source for scholars to gain a profound understanding of architectural designs. the interpretation, use, and sharing of archive materials are activities aimed at deepening the knowledge of contemporary masters and the different architectural movements [3], [4]. building information modelling (bim) is applied here as a tool to rediscover, analyse, interpret and highlight architectural design, thus contributing to the recognition of architectural archives as cultural heritage. the author, after a careful analysis of the project drawings of the villas, obtained the 3d model working directly with revit® on the archive drawings, and then focused the final attention on the different methods of rapid prototyping and on the realization of a real scale model. the realization of 3d digital models and real models of architectures that have never been built represents an important contribution to the study and knowledge of archival drawings and morphological aspects. if the virtual space facilitates and increases our perceptual knowledge of architecture, allowing a different way of understanding space, the physical model produced by a 3d printer allows easier reading of architectural morphology [5]. 2. drawing and architectural language of aldo morbelli and the project for the “due case a capri” aldo morbelli (1903-1963) is an italian architect, born in orsara bormida in piedmont. after graduating from the faculty of architecture in rome, he founded his professional studio in turin in the 1930s. during his professional activity, aldo morbelli produced several architectural projects concerning single-family houses, social housing for the ina-casa plan, entertainment buildings, company representative offices, and post-war reconstruction works. in addition to these projects, he has also worked on interior design and furniture design studies. abstract archives of the 20th century are today the focus of many scholars in the disciplines of conservation, valorization and communication. the enhancement of archival heritage could benefit from the methodologies, techniques and tools offered by the current digital revolution. this is the case of the work shown in this proposal. a parametric modelling experience was developed for the project of "due case a capri" by architect aldo morbelli (1942) starting from archival documentation and from a previous graphic, manual and critical reading of the project. the aim of this research is to build a methodology able to reproduce 3d objects through building information modelling technology, integrating geometry with semantic information up to the realization of a scale models, through the application of different prototyping techniques. mailto:giulia.bertola@polito.it acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 2 aldo morbelli's archive is kept in the biblioteca di architettura "roberto gabetti" of politecnico di torino and contains several documents relating to his projects, both completed and unfinished. despite the fact that in the past many of his projects have obtained recognition in internationally renowned magazines and the critics have dedicated a monographic issue in l' architettura italiana to his single-family houses, to date, the figure of aldo morbelli is still little studied [6]. he affirms himself through a poetics that tends to a process of formal simplification, to a careful research of balance between project and insertion in the context (figure 2). this is particularly evident in the project of "due case a capri”. the buildings, never realized and defined by morbelli large house (located lower down) and little house (higher up), are thought to be situated on a land among the olive groves at the foot of mount tuoro, in the region "la cercola". the two buildings are facing west because of the steep slope of the land. he breaks the compactness of the volumes through the reference to the local architecture, solved through the insertion of segmental arches, the choice of colours and materials for walls, and coatings such as white. also, in the interior spaces there is a clear intention to give the rooms a plastic sense: the walls and the distributive elements, such as the "s" staircase of the large house adapt to the plan (figure 3). the little house is on three levels, basement floor for servants and storage, ground floor with living room and kitchen, which is accessed through a sloping wall with arched entrance, and first floor with two bedrooms. the large house, instead, is only on two levels, the lower level is for the living area, with the living and dining area at double height that occupies all the sea view front and that of the services develops towards the mountain, while the highest level is for the sleeping area, with four bedrooms. looking at the drawings, it can be seen that the architect wanted to experiment with the insertion of different types of roofing: horizontal flat and sloping flat for the little house, barrel and net vaulted for the large house. 3. the bim model generated by archive drawings this contribution aims to show a methodology for the management, preservation, and communication of archival material by integrating data with 3d modelling techniques. this type of documentation, if inserted within a virtual database, can become an active component of the archive itself, contributing to the general knowledge of lost or non-existent architectural artefacts. this theme opens a series of reflections regarding the philological interpretation of the unrepresented parts of an architectural project and the translation of the drawings of an unrealized architecture into a three-dimensional model. digital modelling with archival sources involves investigative work starting with the hypotheses of reconstruction of the sketches, checking the consistency of the scale drawings, and proposals for integrating missing data. the source of research, in this case, is the analogical documentary heritage. these consist of graphic, iconographic, and textual sources. for each reconstructive model, it is necessary to identify the phase of the project to which it refers according to the cognitive values that one wishes to emphasize through the research work [7]. figure 1. interpretative drawings for the project of the “due case a capri” (drawings by giulia bertola) figure 2. a. morbelli, real-life drawings of mediterranean architecture (archives of the library of architecture "roberto gabetti", polytechnic of turin. in the following archivi bca. fondo aldo morbelli). figure 3. a. morbelli, study sketches for the two houses in capri (archivi bca. fondo aldo morbelli). acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 3 archives could therefore become a place where to build, communicate and share knowledge within a complex system of relationships made up of different actors (institutions, curators, scholars, public), types of heritage (material, immaterial, real three-dimensional artefacts), and digital technologies (interaction, immersion, virtual and augmented reality) [8]. the construction of the model makes it possible to produce a variety of elaborates including real three-dimensional models to be made available to different users. as underlined by the london charter (2008) [9], it is the responsibility of the scientific community to ensure the sustainability of digital heritage, for example by promoting the use of open formats, favouring as much as possible the access to data to the community of users, experts or not. these operations are well suited to bim tools. they provide a good level of interactivity and consider the stages in the temporal evolution of the project. a bim information system can represent the "sustainable container" and the appropriate means for the transmission of knowledge (for example through advanced building information exchange systems). in the hbim environment, as in this case study, the operator must be able to manage complex and heterogeneous survey data (photographs, documents, sketches), master geometrical constructions, know construction techniques. the potential of historical building information modeling systems, despite the methodological and operational difficulties, allows building a database capable of managing large amounts of multidisciplinary data [10], [11], [12]. bim also offers the possibility of managing the life cycle of architecture, from the first hypotheses to the design and construction phases in a single model, and can facilitate the creation of models from the archival heritage. some significant examples of the use of bim technology as a tool for interoperability and advanced information management technology have emerged in some studies by saygi and remondino (2013) [13] which emphasize the semantic enrichment of three-dimensional digital models through the integration of heterogeneous data sets and in the arches (2018) [14] and inception (2016) [15] projects; the latter aim to make available interoperable information through multiple services such as websites, digital maps, applications, and 3d models. while the first one is focused on the creation of an international community of computer scientists and heritage professionals to share experiences, knowledge, and skills for the management of digital inventories, the second one is a project that aims at the development of advanced 3d modelling for the access and understanding of european cultural heritage. in particular, the hbim modelling process starts with the documentation of user needs, the identification of cultural heritage buildings semantic ontology, and data structure for information catalogue integrated with the 3d geometric model. a last interesting study is cult project (2018) [16], a project in which a software toolkit was developed to store data from architectural heritage research projects and share them with websites, tourism applications, and bim and gis interfaces. for the “due case a capri”, it was decided to use the building information modelling methodology and build a threedimensional model using revit 2021®. after scanning and digitizing the original drawings of the project, the documents were inserted into the revit 2021® software. to optimize the modelling process, the files in .jpg format were not imported into the model but only linked to it. initially, the plans were linked at 1:200 and 1:500 scale. thanks to these, it was possible to correctly set the geolocation data, create a topographic surface and import contour data. after having identified the correct positioning of the buildings, we proceeded with the insertion of technical drawings at scale 1:100 containing plans, elevations and sections of each villas and territorial sections. each drawing has been scaled with reference to the dimensions reported on them and used as a base on which to set the model. following the scaling of the images, the most significant work plans have been identified and a grid has been built for the alignment of plans, elevations and sections (figure 4). the axes of the grid, besides being a useful trace for the construction of all the elements that will compose the model, also represent the tool to facilitate the reading and verification of the original drawings. this phase of work, to be done correctly, requires prior knowledge of the main volumes that make up the buildings. once the volumes had been identified, we proceeded with the identification of the types of walls and floors, attributing to each one a specific thickness: perimeter walls (60 cm), partition walls (10 cm, 20 cm), and floors (40 cm) (figure 5). where it was not viable to refer to a system family, it was decided to use the in-place masses to create the roofs and curved elements present in some portions of the external walls and on the railing of the internal staircase of the casa grande. all the masses have been transformed into surfaces. during the modelling of these elements, some problems related to the rigidity of the bim method in the modelling of unconventional shapes emerged. the in-place masses were created using the swept blend command, which, however, did not make it possible to faithfully reproduce some of the most characteristic architectural elements of the project and typical of aldo morbelli's poetic. during the final phase of elaboration of the bim model, it was chosen to refer to the original plastic models not preserved but documented photographically. in order to maintain formal coherence, it was decided to create monochrome grayscale rendered views (figure 6). figure 4. inserting archive images (archivi bca. fondo aldo morbelli) on revit 2021®. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 4 also, in the elaboration of plans and sections, it has been chosen to reach a level of detail that can be hypothesized at a scale of 1:200 in order to avoid distorting the original project by providing incorrect interpretations and additional information. moreover, since it is a preliminary project, without detailed information about structures, stratigraphies, and construction details, it was decided to use generic model families, avoiding the customization of the library of parametric objects. in the future, this work aims to use a philological approach based on the classification of archive documents. the different sources (sketches, final and executive drawings, documents, articles, photographs, and physical models) can be linked to the digital model and referred to at different levels of analysis. each level corresponds to a quantity of information that, when added to the previous level, increases the level of reliability of the reconstructive model with the original drawings. the bim methodology provides a gradual definition of the 3d model, with geometric accuracy and data content. in some countries the levels of detail coincide with the levels of the 3d model, level of detail (lod), and in others with the levels of information, level of information (loi), transmitted where graphic data are missing, thus suggesting a different relationship between model and real object [17]. these levels can be declined within the framework of historic-bim (h-bim), even though partial documentary sources, such as those in the case of unbuilt architecture. the concept of the lod development level applied to bim management is based on a linear process that progressively enriches both the model and the information through the different phases. these phases correspond to: lod 100 represents an a-dimensional conceptual model, lod 300 a three-dimensional model in the executive design phase, lod 350-400 represents the implemented model for the construction phase, lod 500 the as-built update after the construction phase. in this case, the lod will correspond to the design level at which the architectural design was interrupted [18], [19], [20]. 4. digital fabrication and rapid prototyping technologies this paper aims to focus on the effectiveness of digital fabrication processes and rapid prototyping techniques for the valorization of the archival heritage. in this context, digital fabrication is considered a working method to support the entire design process for the study of form and the built environment. digital fabrication is a process by which solid objects can be created from digital drawings. a process capable of exploiting different manufacturing techniques. the choice of a technique is usually made following figure 5. plans, section and elevation of “casa grande” created through revit 2021®. figure 6. a.morbelli, photograph of the original model (archivi bca. fondo aldo morbelli) and view rendered through revit 2021®. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 5 some considerations made on the speed of processing, the cost, the material, and the final aesthetic performance [21]. here below, we compare the two main categories of prototyping techniques: subtractive and additive methods of manufacturing. the first are based on the idea of reproducing an object by sculpting a block, removing material over a predetermined path. this operation is feasible through two types of machines: the computer numerical controlled (cnc) machine and laser beam machine. whereas the cnc machine is a milling tool, the laser beam machine involves a thermal separation process. the laser beam hits the surface of the material and heats it up to the point where it is melted or completely vaporized. once the laser beam has completely penetrated the material at a certain point, the cutting process begins. the laser system follows the selected geometry and during this process the material is separated. the additive process, usually called 3d printing, is instead a process by which solid shapes (usually of small size) are constructed by building one layer at time. nowadays there are several additive manufacturing processes that differ from each other depending on the different materials that can be used and how they are deposited to create the various objects. all 3d printing processes involve the simultaneous collaboration of software, hardware, and materials and have the great advantage, compared to subtractive processes, of being independent from the geometric complexity of the digital model. in addition to the fused deposition modeling (fdm) printing technique, explored in the next paragraph there are other three main types of 3d printing [22]. two prototyping techniques were used for this work: fdm for buildings and laser beam machining (lbm) for the ground. 4.1. fused deposition modelling the fdm it is the most common 3d printing technology. this method uses a filament (a string of solid material), which, under the thrust of a heated nozzle, melts. the printer continuously moves this nozzle around, laying down the melted material at a precise location, where it instantly cools down and solidifies. this builds up the model layer by layer. during the construction of these solid shapes it is often necessary to use vertical supports to sustain overhanging parts. vertical supports can be made with water-soluble filaments that when immersed in water can be removed easily without leaving hard-to-remove residue. the filament used in the fdm process usually requires a specific diameter, strength, and other properties. during the extrusion of a polymer, the diameter of the filament must be uniform. to achieve this, the machine must have adjustable screw speed, pressure, and temperature. all these parameters are examined and selected until an optimal filament diameter is reached. for a smooth filament, a different calibration nozzle was used. for low-temperature materials (polylactic acid pla, polyethylene pe) the calibration nozzle is made of copper and the thermal seal is made of polytetrafluoroethylene. for high-temperature materials (acrylonitrile butadiene styrene abs, polyamide pa) the calibration nozzle is made of aluminium [23]. following the printing process, to avoid seeing the layers it is often necessary to sand and polish the surfaces. for this case study, the buildings were made in pla and printed using the delta wasp 2040 industrial line 4.0® printer whose characteristics are shown in table 1 [24]. once the standard triangle language (stl) format was obtained, a test was made to verify the file through the software cura®, an open-source slicing application for 3d printers through which an analysis of the model was carried out: thickness, stability, positioning, and orientation of the model on the surface. the stl file was also automatically divided by the software into sections (slices). the software also automatically generates the support structures. the plastic filament is conducted in a reel, pushed, and melted through the extrusion nozzle. when the loose filament comes into contact with the construction plane it hardens and the rest of the material is support structure needed gradually released [25]. 4.2. laser beam machine for the realization of the ground we chose instead to use another rapid prototyping tool: lbm. the soil was made of 2 mm thick cardboard using the totrec speedy 400® printer. totrec speedy 400 is a type of cnc machine. the user can process an object through a design software, send it to the laser cutting machine and have it cut it automatically. once the design is sent to the machine, the device uses a laser beam to cut or engrave the material on the cutting plane. this type of processing allows wide versatility in materials, no need for subsequent machining and high precision. from the 2d file generated by revit 2021® you can proceed with the print layout operations, defining the cutting power values using the job control® software. 5. from bim model to prototype some of the studies regarding the relationship between bim objects and digital fabrication processes reflect on how such objects can be incorporated with the semantics of fabrication and how they can then be used to support the workflow between designers and fabricators. a reflection that is applied in particular to cnc fabrication and proposes specific process maps for the conventional workflow between design and fabrication disciplines from the domain of custom cabinetry [26]. bim then, in support of digital workflow design for all building disciplines, including the use of structural information models for digital fabrication of steel structures[27]. another area of reference is the construction industry explored in-depth in sakin and kiroglu studies (2017) regarding new 3d printing technologies for sustainable buildings and pointing to contour crafting as a promising technique that may be table 1. general characteristics of delta wasp 2040 industrial line 4.0®. system printer model max. build size accuracy materials advantages disadvantages fused deposition modelling delta wasp 2040 industrial line 4.0® 200 × 200 × 40 mm³ 0.1-0.2 mm pla (polylactic acid) good accuracy. functional materials medium range of materials. office friendly. support structure needed acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 6 able to revolutionize the construction industry in the near future [28]. for the current case study following the choice of prototyping techniques, attention was paid to the preparation of files for printing. for the construction of the terrain, we started with the 2d file generated by revit 2021® and proceeded with the print layout operations, defining the cutting power values with the job control® software (figure 7). to proceed instead with the 3d printing, since an stl exporter for revit 2021® is not yet available, the file was exported in 1:100 scale in fbx format, imported in rhinoceros® and then exported in stl. during the import phase in rhinoceros®, the 3d model was scaled to 1:200 and the unit of measurement was changed to millimeters. for dimensional issues related to the printing dimensions of the machine, the models were divided into parts: building blocks and outer walls. in addition, for the realization of the volumes, neutral colours were deliberately chosen so as to focus the viewer's attention on the three-dimensional geometries and the composition of the volumes. the level of complexity of the model determines the number of triangles needed and their size. in turn, the number of triangles determines the size of the file. as happened in this case, it may happen that during the conversion of the revit file to stl, critical issues emerge, and the exported file may contain some errors. these errors can be of various types: holes or blanks, inverted or intersected triangles. (figure 8) during the printing phases, the goal is to obtain objects characterised by continuous and well-finished surfaces. for additive manufacturing, the surfaces of the 3d model are converted into mesh, a mesh composed of triangular faces and vertices connected to each other. during conversion, you may get a model with mismatched edges, holes, and triangles that intersect each other in incorrect positions. in particular, the mesh for 3d printing must have the following characteristics: the surfaces must be closed and all triangles must connect with other triangles along the edges without intersecting and must be correctly oriented. in particular, netfabb® software was used for file repair operations. netfabb® is a software for editing stl files and offers a rich set of tools that optimizes workflows and minimizes building errors. the procedure used for the small house was as follows: open stl file on netfabb® and verify the quality of the 3d model by checking the number of triangles that make up the model (13316). subsequently the software indicates that the file has errors, then proceed to click on automatic part repair to repair the part automatically. this operation will show us what's wrong with this model. this was followed up by checking for incorrectly oriented triangles and then clicking on the commands prepare and repair part. following these operations in the table on the right of the screen appear the new values for edges (3588), triangles (2392), inverse orientation (615), holes (0), border edges (0). at this point we can start the repair process of the file with the command run repair script and extended repair. from this procedure it has obtained a new model with new values of inverse orientation (0), holes (0), border edges (0), edges (4968) and triangles (3312). (figure 9) at this point was possible to export the stl file, open it with cura® and print it. (figure 10) the final result has numerous imperfections. the surfaces, despite the high level of accuracy (table 2), are not smooth and the material has not been deposited evenly. this is due to the poor quality of the mesh. these problems can hardly be completely solved with mesh repair software. figure 7. organization of the model for the printing process through revit 2021® and rhinoceros®. table 2. values and print settings used on the slicer cura® for the "due case a capri" 3d printing project. quality infill support time material quantity layer height density pattern grid enable 12 h pla (polylactic acid) gray 40 g 0.1 mm 20 % figure 8. the external wall of the "casa grande", imperfections during the printing phase due to errors on the model after the export from revit®. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 7 to obtain higher quality printable 3d files that can still be computed in revit®, an alternative method could be used using rhino.inside.®revit. this would involve modelling the object in rhinoceros® (one of the most suitable software for transferring 3d geometries to rapid prototyping tools) [29], importing it into the revit® file, and assigning families and types to the surfaces of the mesh. 6. conclusion in the scenario of the digitization of archives, the 3d modelling phase allows extending the consultation of archival material, placing drawings and photographs alongside threedimensional models that can be explored through virtual reality and augmented reality experiences [30], with the application of different digital interfaces, machine learning techniques and computer supports. these technologies allow archives to catalogue and publicly show their content at any time through interactive support, allowing curators and scholars to greatly enrich the narrative experience. archives can place visitors virtually within the experience, leaving them free to explore the content actively, helping to build a deeper connection between visitors and archival [31]. the prototyping phase could, in addition to becoming an ideal context to experiment with the flexibility of the different printing techniques, give users the possibility, during the visit to the archive, to consult not only the original drawings but also plastic models (possibly decomposable) that allow a better understanding of the three-dimensionality of the artefact. this could be particularly useful and significant in the case of unrealized architectures, such as those treated in this case study, and thus become the first and only physical representation of the artefact. the aim is to demonstrate that the conjectural reconstruction through digital models is an act of clarification of some aspects of architecture, often only left to the written word; through the construction of new representations the words take shape through a new figurative corpus; the digital model is not only the virtual image of the building but it becomes a possible image, becoming its only existential reality [32]. given the problems that emerged during the preparation of the file for printing and the medium to low quality of the final printed model, the application of bim technology to achieve high-quality 3d printing operations still needs to be further investigated. references [1] r. spallone, g. bertola, design drawings as cultural heritage. intertwining between drawing and architectural language in the work of aldo morbelli, in: the graphics of heritage. l. agustín (editor). springer, cham, 2020, pp.73-85. doi: 10.1007/978-3-030-47983-1_7 [2] g. bertola, archives enhancement through design drawings survey, bim modeling and prototyping, proc. of the imeko tc4 international conference on metrology for archaeology and cultural heritage, trento, italy, 14-16 september 2020, pp. 67-71. online [accessed 20 march 2022] https://www.imeko.org/publications/tc4-archaeo2020/imeko-tc4-metroarchaeo2020-013.pdf [3] r. spallone, g. bertola, f. ronco, sfm and digital modelling for enhancing architectural archives heritage, in metrology for archaeology and cultural heritage, proc. of imeko tc-4 international conference on metrology for archaeology and cultural heritage, firenze, italy, 4-6 december 2019, pp. 142-147. online [accessed 20 march 2022] https://www.imeko.org/publications/tc4-archaeo2019/imeko-tc4-metroarchaeo-2019-27.pdf figure 9. the stl file before repair operation in netfabb® and the stl file after repair operation. figure 10. 3d printing process and the final real model. https://doi.org/10.1007/978-3-030-47983-1_7 https://www.imeko.org/publications/tc4-archaeo-2020/imeko-tc4-metroarchaeo2020-013.pdf https://www.imeko.org/publications/tc4-archaeo-2020/imeko-tc4-metroarchaeo2020-013.pdf https://www.imeko.org/publications/tc4-archaeo-2019/imeko-tc4-metroarchaeo-2019-27.pdf https://www.imeko.org/publications/tc4-archaeo-2019/imeko-tc4-metroarchaeo-2019-27.pdf acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 8 [4] r. spallone, g. bertola, f. ronco, sfm and digital modelling for enhancing architectural archives heritage, in metrology for archaeology and cultural heritage, acta imeko, vol. 10, 1 (2021), pp. 224-233. doi: 10.21014/acta_imeko.v10i1.883 [5] m. incerti, g. mele, u. velo, the productive role of model from a virtual to a physical entity. the communication of 36 projects of never constructed villas, in: mo.di.phy. modeling from digital to physical. innovation in design languages and project procedures. m. pignataro (editor). maggioli editore, santarcangelo di romagna, 2013, pp. 128-141, isbn 978-88-3876-274-1. doi: 10.1007/978-3-030-33570-0 [6] a. melis, architetti italiani. aldo morbelli, l'architettura italiana, 3 (1942) pp. 49-72. 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[in italian] https://doi.org/10.21014/acta_imeko.v10i1.883 https://doi.org/10.1007/978-3-030-33570-0 https://doi.org/10.1007/978-3-030-76239-1_34 https://doi.org/10.1109/digitalheritage.2018.8810128 http://www.londoncharter.org/fileadmin/templates/main/docs/london_charter_2_1_it.pdf http://www.londoncharter.org/fileadmin/templates/main/docs/london_charter_2_1_it.pdf https://doi.org/10.2423/i22394303v4n1p15 https://doi.org/10.3390/heritage2030117 https://doi.org/10.1260/2047-4970.2.4.695 https://doi.org/10.1108/jchmsd-02-2016-0010 https://doi.org/10.1088/1757-899x/364/1/012089 http://cult.dicea.unipd.it/ https://doi.org/10.21014/acta_imeko.v10i1.842 https://doi.org/10.3390/heritage2030141 https://doi.org/10.1007/s12518-018-0233-3 https://doi.org/10.5194/isprs-archives-xlii-2-w9-581-2019 https://doi.org/10.1016/j.destud.2005.11.009 https://doi.org/10.1111/cgf.12781 https://doi.org/10.2478/amm-2013-0186 https://doi.org/10.1016/s0926-5805(00)00111-4 https://doi.org/10.1016/j.destud.2005.11.009 https://doi.org/10.1016/j.autcon.2018.02.031 https://images.autodesk.com/latin_am_main/files/revit_bim_and_digital_fabrication_mar08.pdf https://images.autodesk.com/latin_am_main/files/revit_bim_and_digital_fabrication_mar08.pdf https://doi.org/10.1016/j.egypro.2017.09.562 design of a non-invasive sensing system for diagnosing gastric disorders acta imeko issn: 2221-870x december 2021, volume 10, number 4, 73 79 acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 73 design of a non-invasive sensing system for diagnosing gastric disorders rosario morello1, laura fabbiano2, paolo oresta2, claudio de capua1 1 diies, university mediterranea of reggio calabria, italy 2 dmmm, politecnico di bari university, italy section: research paper keywords: gastric disorders; gastric slow wave; egg; myoelectrical measurements citation: rosario morello, laura fabbiano, paolo oresta, claudio de capua, design of design of a non-invasive sensing system for diagnosing gastric disorders, acta imeko, vol. 10, no. 4, article 14, december 2021, identifier: imeko-acta-10 (2021)-04-14 section editor: francesco lamonaca, university of calabria, italy received october 2, 2021; in final form october 24, 2021; published december 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: rosario morello, e-mail: rosario.morello@unirc.it 1. introduction the rapid advancement of information technologies now allows the physicians to be able to smartly follow and assist the patient in real time even remotely through simple dedicated applications [1]-[3]. in the same perspective, the case of gastrointestinal pathologies has been addressed here. dyspepsia, stomach ulcer, gastritis, esophageal reflux are some examples of gastrointestinal motility disorders. such pathologies are widely spread among the population and their symptoms can become strongly debilitating. gastric disorders include several dysfunctions of the stomach digestive activity. gastroesophageal scintigraphy and endoscopy (gastroscopy) are at the moment two invasive techniques extensively used in the practice to diagnose gastric disorders. during the digestive function, the stomach muscles contract rhythmically to allow digestive activity to be performed. this activity is regulated from myoelectrical waves. such waves in presence of additional stimuli induce muscles to contract, [4]-[8]. electromyographic measurements of such gastric slow waves can provide important information on the stomach activity, [9]. electrogastrography (egg) is a technique known from several years based on recording stomach muscle contractions by means of skin electrodes, [10], [11]. such technique suffers from inappropriate data processing algorithms and interpretation errors, so showing poor reliability in the use of it as a diagnostic method of gastrointestinal motility disorders. nevertheless, recent medical trials have highlighted a clear correlation between abnormal gastric electrical activity and the onset of specific dysfunctions, abstract gastric disorders are widely spread among the population of any age. at the moment, the diagnosis is made by using invasive systems that cause several side effects. the present manuscript proposes an innovative non-invasive sensing system for diagnosing gastric dysfunctions. the electro-gastrography (egg) technique is used to record myoelectrical signals of stomach activities. although egg technique is well known for a long time, several issues concerning the signal processing and the definition of suitable diagnostic criteria are still unresolved. so, egg is to this day a trial practice. the authors want to overcome the current limitations of the technique and improve its relevance. to this purpose, a smart egg sensing system has been designed to non-invasively diagnose gastric disorders. in detail, the system records the gastric slow waves by means of skin surface electrodes placed in the epigastric area. cutaneous myoelectrical signals are so acquired from the body surface in proximity of stomach. electro-gastrographic record is then processed. according to the diagnostic model designed from the authors, the system estimates specific diagnostic parameters in time and frequency domains. it uses discrete wavelet transform to obtain power spectral density diagrams. the frequency and power of the egg waveform and the dominant frequency components are so analyzed. the defined diagnostic parameters are put in comparison with the reference values of a normal egg in order to estimate the presence of gastric pathologies by the analysis of arrhythmias (tachygastria, bradygastria and irregular rhythm). the paper aims to describe the design of the system and of the arrhythmias detection algorithm. protot ype development and experimental data will be presented in future works. preliminary results show an interesting relevance of the suggested technique so that it can be considered as a promising non-invasive tool for diagnosing gastrointestinal motility disorders. mailto:rosario.morello@unirc.it acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 74 [12], so that the gastroenterologists have recently reconsidered it as a potential non-invasive screening technique. in addition, the american gastroenterological association (aga) states the clinical relevance of egg in demonstrating gastric myoelectric abnormalities in patients with unexplained nausea and vomiting or functional dyspepsia, [13], [14]. it represents a promising and interesting alternative method for gastric screening since it has no side effects and is painless, [15]. nevertheless, further advances on processing algorithms and on the project of more accurate measurement systems are required in order to improve the reliability and evidence of the method, [16]. at present, there are no standardized diagnostic criteria and the state of art highlights poor care about this issue. therefore, several aspects must be still investigated, and additional studies are needed to assess the use of egg as an alternative test to the actually used invasive techniques, [17]-[19]. as it has been assessed above, an egg system records the stomach myoelectrical activity by means of cutaneous leads placed over the gastric area. in this way, it is possible to estimate patient’s gastrointestinal conditions by analysing the slow waves in time and frequency domains. in presence of gastric disorders, myoelectrical abnormalities can be revealed and characterized in the egg records, due to a decreased activity of the stomach muscles and nerves. in healthy individuals, standard egg record is characterized by regular electrical rhythm. in detail, it consists of periodic waveforms with a predominant frequency of 3 cycles per minute (cpm) at rest. during the digestive activity, frequency and intensity of gastric waves increase. in individuals suffering from gastrointestinal motility disorders, electro-gastrographic measurements have instead an irregular rhythm. in addition, post meal, sometimes, no increase of frequency and intensity of the waveform is observed. these and further features must be analysed in order to define suitable diagnostic criteria for characterizing the occurrence of gastric motility disorders. another interesting application of the electro-gastrographic technique concerns the study of patients affected from vomiting, unexplained nausea, improper digestion of food and gastroparesis. medical trials have been carried out to get important information on the mechanism that regulates the activity of stomach muscles and nerves in presence of those disorders, [19]-[21]. so, for example, it can be an helpful technique in understanding the origin of the unexplained contractions which cause vomiting in patients affected by anorexia, [22], [23]. that would allow gastroenterologists to schedule new therapies so to reduce the vomiting stimulus. therefore, more and more physicians show renewed interest in this technique. nevertheless, careful studies have still to highlight its potentialities. in this sight, the authors have focused their research activity on such aspects in order to overcome limitations and lack in the interpretation of the egg waveforms. so, the authors have proposed in [24] an innovative diagnostic model for characterizing gastric myoelectrical abnormalities due to disorders, and gained long-standing experience in recording of myoelectrical signals, [25]-[27]. in the present manuscript, the authors describe the developments of the model previously proposed. in detail, a smart and automated egg sensing system has been designed and its project is described in the following. by the embedded diagnostic criteria, the system is able to recognize an abnormal gastric activity. the used methodical approach starts with the study of the electro-gastrographic technique. standard egg records of healthy persons have been analysed in order to define suitable diagnostic reference parameters. such parameters contribute to define and recognize the onset of gastric disorders. then, diagnostic criteria have been defined to optimize the analysis of the egg waves in patients affected from gastric disorders. the diagnosis is based on a multifactorial analysis of the defined diagnostic parameters, which are compared with the respective reference values of a standard egg signal. the egg sensing system has been projected and developed according to the measurement system design model described in [28]. the system acquires and processes gastric myoelectrical waves in compliance with the diagnostic model presented in [24]. then the system decides among five alternative diagnoses. in the next section an overview of the electro-gastrographic technique is reported, and main gastric disorders and some applications of egg in medical practice are described. the third and fourth sections respectively analyses the phenomenon and describes the project of the smart egg sensing system and the embedded diagnostic algorithm. next, some results are presented, and conclusions follow. 2. electrogastrography the electro-gastrographic technique is known in medical field for a long time. it has common features with the electrocardiogram, as both techniques are based on myoelectrical signal measurements. the egg is a non-invasive technique based on recording the gastric myoelectrical activity. now, it cannot be considered in effect as a diagnostic tool because of lack of its standardization. inaccurate instrumentation, interpretation errors and lack of approved diagnostic criteria are some reasons. so, the authors have carefully examined the state of art of the technique in order to understand the current use of it. the method has been used in medical practice to study patients affected from unexplained persistent or episodic symptoms related to gastric motility disorders. further studies have been carried out by analysing gastric waveforms of patients with unexplained nausea and vomiting. they have shown interesting and promising results. the same analysis cannot be performed by means of invasive diagnostic tools, such as endoscopy, because of the artefacts introduced during the examination. in fact, endoscopy can be cause of further vomiting stimuli which overlap to patient’s nausea. other clinical trials highlight that functional dyspepsia and gastroparesis can be characterized by analysing gastric myoelectrical activity. in such cases, arrhythmias of the egg waveform can be clearly observed. further studies have singled out the occurrence of abnormalities in the egg waves of patients with other specific gastric disorders such as stomach ulcer, gastritis, oesophageal reflux, early satiety, anorexia. experimental results have shown an interesting correlation between gastric myoelectrical impulses and stomach diseases. for this reason, this technique can be considered as a promising and practical screening tool for the evaluation of several gastrointestinal motility disorders. nevertheless, before considering egg as a reliable diagnostic test, several aspects need still to be explained and highlighted. the authors have so focused their attention on these issues. the final aim is to propose the project of non-invasive sensing system with embedded diagnostic criteria. in order to use a methodical approach to the matter, we must understand the mechanism which regulates the gastric myoelectrical impulses and the stomach muscles contraction. therefore, the behaviour of the stomach during gastric function (digestion) in healthy persons has been analysed. once the myoelectrical activity of the stomach has been investigated, it has been possible to characterize the acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 75 standard egg waveform so to correctly interpret the occurrence of possible abnormalities in the myoelectrical activity, [29]-[31]. in detail, stomach’s muscles contraction and nerves movement are regulated by myoelectrical impulses, [30], [31]. such gastric waves control and coordinate the stomach activity. periodic waves, within specific frequency and amplitude ranges, are usual and allow stomach to digest food. electrogastrographic signals have amplitude relatively low, about 200 − 5000 v . consequently, the acquired signal must be amplified before of the processing stage. the frequency range is 0.016 − 0.25 hz equal to 1 − 15 cpm, see figure 1 for reference. at rest, slow waves depolarize the gastric smooth muscles without causing contraction. amplitude of egg waves increases, with the ingestion of food and when digestion starts, due to the increased activity of the stomach’s muscles. during digestion, indeed, the contraction of the muscles is caused by additional depolarization. so gastric slow waves control the fundamental frequency and the direction of contraction. this behaviour describes the regular mechanism of the gastric function. differently, in presence of gastrointestinal disorders, arrhythmias can be observed in the egg waveform due to an incorrect stomach’s activity. such abnormal myoelectrical activity is cause of changes of the fundamental frequency component and of its intensity. for instance, when a reduced contractile function of the stomach is observed, the egg waveform is characterized by lower frequency values of the fundamental component known as bradygastria. it is due to a reduced number of contractions. conversely, higher frequency values of the fundamental component, or tachygastria, cause stomach atonicity. generally, the pathogenesis of the gastric myoelectrical arrhythmias is due to the delayed stomach emptying which occurs when the individual is affected from motility gastrointestinal disorders. consequently, it is cause of a reduced stomach activity 3. standard egg waveform clinicians suggest collecting egg recordings after overnight fasting and during food digestion, in order to analyse the gastric function at rest and during digestive activity. the patient must be in a comfortable position to prevent movement artefacts and should remain motionless during the whole egg acquisition. a preliminary recording is performed with empty stomach, 15 to 60 minutes. subsequently the patient must consume a caloric meal (300 kcal), and a further egg 30 to 120 minutes recording is acquired. normally physicians suggest a fasting recording of 30 minutes and a postprandial recording of 60 minutes. in this way, it is possible to evaluate the gastric response during meal digestion. a set of egg signals, recorded during fasting and postprandial stages, has been analysed in time and frequency domains by using discrete wavelet transform. frequency components and their amplitude (power) have been considered. by means of power/frequency spectral analysis, the postprandial and fasting records have been compared. commonly it is assumed that a normal egg waveform is characterized by an averaged dominant frequency of about 3 cpm. during digestion, both frequency and associated amplitude increase. rhythm abnormalities include bradygastria (lower dominant frequency), tachygastria (higher dominant frequency), and irregular rhythm (dysrhythmia). nevertheless, such averaged values are not reliable because they can significantly change from individual to individual (physical constitution, age, general healthy status, etc.). consequently, a careful study of the literature and further analyses of egg signals of healthy individuals have allowed us to characterize a reference model. two quantities must be considered: frequency and amplitude. in a standard egg waveform, the fasting dominant frequency of gastric waves has to belong to the interval 2-4 cpm. in the postprandial recording, dominant frequency must belong to the normal frequency range 2-4 cpm for at least 75 % of time. this percentage time depends on the type of meal consumed. if the dominant frequency belongs to the previous interval only for 25 % of the egg recording time, then it can be considered index of dysrhythmia. this occurrence happens because of an altered gastric emptying. lower frequency values or equal to 2 cpm are index of bradygastria. higher frequency values or equal to 4 cpm define tachygastria. in a regular recording segment, different zones with bradygastria and tachygastria may be characterized. dysrhythmia can be characterized if the relative abnormal frequency waveform takes at least 5 minutes. the recognition of these patterns is simple, the only parameter to be considered is the dominant frequency value. further relevant parameters in the time domain can be used to characterize an irregular gastric activity. for example, an abnormal egg record can be characterized by the presence of bradygastria or/and tachygastria regions over 30 % of the recording time. as an alternative, the egg waveform can be considered irregular if the percentage of power distribution in the bradygastria or tachygastria regions is greater than 20 %. with reference to the signal intensity, we can estimate the absolute amplitude or power of egg waveform by means of the weighted summation of the gastric waves. differently, the percentage power distribution is obtained by summing waveform power for each frequency band and dividing by the total signal power of the recording, the result is multiplied by 100. typically, a power ratio between postprandial and fasting signals lower or equal to 1 may suggest a decreased gastric response to the meal. on the contrary, it is expected an increase in the myoelectrical activity of the stomach during digestion. finally, nausea and early satiety, are typically cause of gastric dysrhythmia, but this occurrence does not include necessarily an altered gastric emptying rate. 4. the smart egg sensing system in this section, the project of the egg sensing system is described. the system has been projected according to the measurement system design model in [28]. through an algorithm, the smart system is able to extract information from the egg signal according to the criteria reported in the table 2. it is a smart and patient-adaptive system which can detect gastrointestinal motility disorders. the system project has a microcontroller architecture in order to manage the data flow figure 1. egg waveform. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 76 and the data processing. three cutaneous ag/agcl electrodes are used to acquire and record the gastric myoelectrical signals, [32]. electrodes must be placed on the anterior abdominal wall over the stomach (epigastrium antrum). it is preferable that specialized medical staff perform the electrodes displacement. however, for further information, a brief description of the procedure is provided here. being the stomach located in proximity of the end of the rib section, it is helpful to divide it in three zones in order to suitably place the electrodes: the fundus (upper region), the stomach body or middle, and the pylorus (end of the stomach), see figure 2. two electrodes must be placed under the ribs in proximity of the fundus and mid corpus of the stomach (along the antral axis); the third one, the ground reference, is placed at the end of the stomach (see black circles in figure 3). this configuration allows signal-to-noise ratio to be minimized. since the electro-gastrographic signal has a relatively low amplitude, it is amplified before of the processing stage by means of an analog device amplifier ad524. according to the frequency range (1 − 15 cpm), a band pass filter, with cutoff frequencies equal to 0.010 and 0.3 hz, has been used to eliminate frequency components which are lower than 1 cpm and higher than 15 cpm. in this way, it is possible to remove the baseline drift and to exclude signals from other sources: possible myoelectrical interferences can be due to the heart, colon and small intestine. further interferences or artefacts are due to breathing, movements or electrical noise. these artefacts have commonly frequency components which are lower than 1 cpm (motion artefacts) and higher than 9 cpm (respiratory artefacts). such overlapping signals could cause an erroneous estimation of signal amplitude and dominant frequency. therefore, the filter has been carefully designed to reject both further myoelectrical contributions due to other organs and artefacts and noise. the signal is sampled with a sampling frequency of 2 𝐻𝑧, and is subsequently processed by a discrete wavelet transform, [33], [34]. by means of spectral analysis, it is possible to estimate the power and amplitude of the signal frequency components, [35], [36]. the waveform analysis in time and frequency domains allows the system to get information on the power trends as function of frequency and/or time. in this way, it is possible to characterize the arrhythmias of the myoelectrical signal. specific memory devices have been used to store information concerning the metrological characteristics of the system and patient’s clinical history. in detail, information on measurement uncertainty and calibration curve is stored in a first memory device in order to estimate the reliability of measurement results. in addition, a further writable and readable storage device stores private and medical data concerning the case-history of the patient in order to improve the diagnosis reliability. figure 4 shows the flow diagram of the egg signal processing. the amplification and filtering blocks allow the system to perform a preliminary pre-processing of the input signal. the filtering stage performs the rejection of noise and artefact signals overlapped to the egg waveform. two amplification stages have been used to amplify the voltage levels. once the electrodes are properly displaced, the system performs a fasting recording 30 minutes long and a postprandial recording lasting 60 minutes. subsequently, the acquired signals are processed according to the diagnostic model, [24]. abnormalities in the egg record can be characterized by considering the power vs frequency trend. gastrointestinal motility disorders cause, in fact, arrhythmias that, if characterized by frequency components above the normal range, indicate tachygastria, by frequency components below normal range indicate bradygastria; if several frequency contributions arise, then that indicate dysrhythmia; further, lack of signal power increases during postprandial recording. consequently, five patterns are considered: i) normal egg; ii) bradygastria; iii) tachygastria; iv) dysrhythmia; v) lack of postprandial power increase. the analysis in section 3 has allowed us to define specific diagnostic parameters which are representative of the gastric myoelectrical signal features, see table 1. these parameters provide a complete description of the egg waveform in terms of spectral and power analysis. basic requirements are: tdf, f_tdf and p_tdf with a time duration which has to be higher than 5 minutes. figure 2. sections of the stomach. figure 3. egg electrodes displacement. figure 4. flow diagram of the egg signal path. sensing device filtering 2 stage amplification a/d converter data processing metrological status memory patient casehistory acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 77 firstly, the embedded algorithm allows the system to estimate the dominant frequency (df), the time length of dominant frequency recording (tdf), the associated amplitude and power distribution (pdf). in this way it is possible to verify the rhythm of the egg waveform. in detail, the system estimates the fasting and postprandial dominant frequencies (f_df, p_df) and their recording time (f_tdf, p_tdf, f_t, p_t), amplitude and power distribution (f_pdf, p_pdf). subsequently, the ratio of postprandial to fasting power (rdf) is evaluated in order to assess the occurrence of a decreased gastric response to the meal. in order to verify the possible occurrence of dysrhythmias, the recording time and power distribution of egg frequencies in/above/below the 2 − 4 cpm intervals are computed (t3f, p_t3f, f_t3f, p3f). the recording time and power distribution of tachygastria and bradygastria ranges (p3f, ttf, ptf, tbf, pbf) are subsequently estimated. finally, the percentage of power distribution in the three frequency ranges (%p3f, %ptf, %pbf) can be obtained by estimating the weighted summation of power contributions divided by the total power. the last parameters allow us to characterize the presence of tachygastria and bradygastria patterns. once the previous diagnostic parameters have been estimated, the smart egg sensing system verifies the presence of possible irregular gastric activities. to this aim, each parameter is compared with the homologous reference value of a normal egg record, described in section iii. five alternative diagnoses (normal egg, bradygastria, tachygastria, dysrhythmia and lack of postprandial power increase) are available. specific conditions about time, frequency and power must be satisfied simultaneously in order to make a specific diagnosis. table 2 summarizes the embedded diagnosis criteria. table 1. defined diagnostic parameters. parameter description df in cpm dominant frequency tdf in s recording time of dominant frequency pdf in dbm power distribution of dominant frequency f_df in cpm fasting dominant frequency f_tdf in s recording time of fasting dominant frequency f_pdf in dbm power distribution of fasting dominant frequency f_t in s fasting recording time p_df in cpm postprandial dominant frequency p_tdf in s recording time of postprandial dominant frequency p_pdf in dbm power distribution of postprandial dominant frequency p_t in s postprandial recording time rdf ratio of postprandial to fasting power of df t3f in s recording time of [2-4] cpm frequency range p_t3f in s recording time of postprandial [2-4] cpm frequency range f_t3f in s recording time of fasting [2-4] cpm frequency range p3f in dbm power distribution of [2-4] cpm frequency range ttf in s total recording time of tachygastria frequency ptf in dbm power distribution of tachygastria frequency tbf in s total recording time of bradygastria frequency pbf in dbm power distribution of bradygastria frequency %p3f percentage of power distribution of [2-4] cpm frequency range %ptf percentage of power distribution of tachygastria frequency %pbf percentage of power distribution of bradygastria frequency table 2. diagnosis criteria. alternatives diagnosis criteria time frequency in cpm power normal 100 𝑝t3f 𝑝t > 75 and 100 𝑓t3f 𝑓t > 75 2 < 𝐷𝐹 < 4 and 2 < 𝑓𝐷𝐹 < 4 and 2 < 𝑝_𝐷𝐹 < 4 𝑝pdf > 𝑓pdf; 𝑃3𝐹 > 𝑃𝑇𝐹; 𝑃3𝐹 > 𝑃𝐵𝐹 and %𝑃3𝐹 > 75 % bradygastria 100 tbf (f_t) + (p_t) > 30 𝐷𝐹 < 2 or 𝑓_𝐷𝐹 < 2 or 𝑝_𝐷𝐹 < 2 %𝑃𝐵𝐹 > 20 % tachygastria 100 ttf (f_t) + (p_t) > 30 𝐷𝐹 > 4 or 𝑓_𝐷𝐹 > 4 or 𝑝_𝐷𝐹 > 4 %𝑃𝑇𝐹 > 20 % dysrhythmia 100 𝑝t3f 𝑝t < 25 and (𝑝t) − (𝑝t3f) > 300 s variable df pdf>p3f lack of postprandial power increase 𝑝tdf > 300 s 𝑅𝐷𝐹 ≤ 1 acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 78 5. discussion the previous parameters and the diagnosis criteria have been characterized by analysing egg records of healthy individuals, [24]. standard egg signals have been considered to define the normal range of each parameter. the design of the egg sensing system embeds such diagnosis criteria which have been validated by means of simulations. preliminary results have been carried out by using standard egg records in order to verify the possible occurrence of falsepositive diagnoses. egg records have been generated by means of an arbitrary waveform generator in laboratory. twenty cases have been considered. in all cases, the system has properly detected the absence of arrhythmias in compliance with the expected behaviour. further tests have been performed to verify the system capability to reject the artefacts. motion and respiratory artefacts have been reproduced and added to a normal egg record. the noisy signal has been processed and the artefacts have been properly removed obtaining the initial egg signal. additional simulations have been performed in order to test the sensitivity of the system. egg records with gastric disorders have been generated in matlab environment to verify the degree to which the embedded numerical algorithm responds to slight changes of the diagnostic parameters. each pattern occurrence has been reproduced in order to prove the effectiveness and accuracy of diagnosis results. then, the signals have been generated by using an arbitrary waveform generator. therefore, the experimental results do not regard specific patients, but they are the consequence of simulations. the system response has been observed. in detail, egg waveforms have been generated starting from standard waves. the single diagnostic parameter has been modified with progressive percentage deviations. in this way, it has been possible to characterize the sensitivity of the system. the presence of gastric arrhythmias has been detected in presence of deviations above 7 % of the reference values in table 2. the egg system has shown a good capability to detect each pattern. the total sensitivity was above 94 %. 6. conclusions in this paper the electro-gastrographic technique (egg) is proposed to record and process myoelectrical signals of the stomach activity. several studies in literature show an interesting relevance of electro-gastrography to diagnose gastric disorders. although egg technique is well known, several issues are still unresolved. so, egg is, up to day, a trial practice. the authors propose the design of a smart egg sensing system in order to overcome the current limitations of the technique and improve its relevance and evidence. the system has been projected according to the ieee 1451 standard. it is able to acquire the egg signal by means of skin electrodes and to process it according to the embedded diagnostic model previously defined by the authors. diagnostic parameters and their reference values have been characterized by analysing egg records of healthy individuals. by using the diagnostic model, the smart system is able to assess the occurrence of abnormal myoelectrical activity of the stomach due to gastric pathologies. five alternative diagnoses have been considered: normal egg, bradygastria, tachygastria, dysrhythmia, lack of postprandial power increase. preliminary simulations have shown interesting results. in detail, the proposed system has been tested by using normal egg records and simulated waveforms, so to verify its sensitivity and selectivity. experimental data have shown promising results. the proposed sensing system may be considered a noninvasive tool for diagnosing 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hands-free interface; polar coordinate system; teleoperation; srl citation: joi oh, fumihiro kato, iwasaki yukiko, hiroyasu iwata, a 3d head pointer: a manipulation method that enables the spatial position and posture for supernumerary robotic limbs, acta imeko, vol. 10, no. 3, article 13, september 2021, identifier: imeko-acta-10 (2021)-03-13 editor: bálint kiss, budapest university of technology and economics, hungary received march 31, 2021; in final form september 6, 2021; published september 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: joi oh, e-mail: joy-oh0924@akane.waseda.jp 1. introduction in recent years, there has been a considerable amount of research and development on the use of supernumerary robotic limbs (srls) for ‘body augmentation’. in previous studies, robotic technology, especially wearable robots, has been developed for use as prostheses for rehabilitation purposes. an srl aims to provide its users with additional capabilities, enabling them to accomplish tasks that they would otherwise be incapable of performing. in this respect, an srl is different from other types of existing wearable robots; a lightweight, sufficient torque and a highly manoeuvrable srl developed by vernonia et al. [1] is a classic example. these robots can be used in any context, from helping individuals to perform household chores to improving industrial productivity. to effectively assist in routine tasks (e.g., opening an umbrella or stirring a pot), users require an interface that indicates the target point location to the end effector of the srl without requiring them to interrupt their actions. however, such a method has not yet been established. parietti et al. [2],[3] developed a manipulation technique in which the operator's movements were monitored by a robot, following which the robotic arm performed the corresponding movements. iwasaki et al. [4] proposed an interface that allowed the operator to actively control the srl by using the orientation of the face, while sasaki et al. [5] developed a manipulation method that enabled more complicated operations of the robotic arm with the user’s feet as the controllers. previous studies have overlooked the balance between ensuring the operator’s limbs move freely and providing detailed instructions to the srl, and there are further challenges with respect to multitasking in the context of daily life. therefore, in this study, a method for manipulating srls so that two parallel tasks do not interfere with each other is proposed and then evaluated for its usefulness. in the present study, a two-stage experiment was conducted. this section describes the hypothesis of the method, and section 2 presents the method for position instruction along with the experimental results. in section 3, a manipulation method that includes posture instructions is proposed and the experimental abstract this paper introduces a novel interface ‘3d head pointer’ for the operation of a wearable robotic arm in 3d space. the developed system is intended to assist its user in the execution of routine tasks while operating a robotic arm. previous studies have demonstrated the difficulty a user faces in simultaneously controlling a robotic arm and their own hands. the proposed method combines a head-based pointing device and voice recognition to manipulate the position and orientation as well as to switch between these two modes. in a virtual reality environment, the position instructions of the proposed system and its usefulness were evaluated by measuring the accuracy of the instructions and the time required using a fully immersive head-mounted display (hmd). in addition, the entire system, including posture instructions with two switching methods (voice recognition and head gestures), was evaluated using an optical transparent hmd. the obtained results displayed an accuracy of 1.25 cm and 3.56 ° with the 20-s time span necessary for communicating an instruction. these results demonstrate that voice recognition is a more effective switching method than head gestures. mailto:joy-oh0924@akane.waseda.jp acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 82 results are presented, and the two experiments are then discussed. section 4 presents comparisons with other similar methods and discusses the limitations, and finally, section 5 presents the conclusions. the following two elements are considered essential for achieving daily support for parallel tasks: 1) undisturbed movement of the operator's limbs, 2) an indication of spatial position and posture. to date, several hands-free interfaces have been proposed to satisfy requirement 1, with some operated by the tongue [6], eye movement [7] or voice [8] and used for either screen control or robot manipulation (or both). methods to control robotic limbs with brain waves [9] are also being investigated. however, this study focuses on requirement 2 and the construction of a more intuitive instructional method. when the operator provides directions related to a 3d-space location, they must accurately indicate the target point. the field of view, within which a person can perceive the shape and position of an object, is as narrow as 15 ° from the gazing point [10]; hence, to compensate, it is necessary to direct the face and gaze in the instructional space to provide spatial position instructions. the interface proposed in this study takes advantage of this compensatory action and uses it as an instruction method. methods for using the head as a joystick have already been proposed. one method involves the manipulation of the head for instruction in a 2d plane, such as on-screen operations [11]. another method involves switching between the vertical and horizontal planes by nodding towards the plane to be manipulated, supplementing the plane manipulation by the head so that only the head is used to manage the 3d space [12]. however, these methods do not use the compensatory head motion as a manipulation technique. 2. proposal for a positioning method using head bobbing turning one’s head can be used to instruct the radial direction of the target point in polar coordinates. in this section, we propose a pointing interface that combines head bobbing with head orientation in a polar coordinate system. head bobbing is a small back and forth motion of the head that does not interfere with the operator’s movements. this research was performed using the standard morphology of a japanese man, as recorded by kouchi et al. [13]. according to these data, the head-bobbing range was determined as approximately 9.29 cm, which allows the operator to keep the zero-moment point in the torso of the body and operate a robotic arm without losing balance. a doughnut-shaped area with an innermost and outermost radius of 30 and 100 cm, respectively, around the operator was defined as an example of an srl operating range [14]. the head-bobbing depth-change factor was 70 / 9.29 = 7.53 or more. the range of motion that can be performed using head bobbing is considerably lower than that of arms. the preliminary experiments demonstrated that at high magnification, the instructional accuracy of head bobbing was lower than that of other comparable methods. additionally, the required instructions were shown to be longer, and an increase/decrease factor (idf) that gradually changes the depth of the head-bobbing task based on head velocity was therefore introduced. the idf allows precise instructions while maintaining a high magnification. in this study, the idf was constructed using the mouse-cursor change factor shown in figure 1, set by microsoft windows [15]. 2.1. evaluation test with a fully immersive head-mounted display this section examines the usefulness of the idf and 3d head pointer as a whole. this study was conducted based on the previously developed robotic arm proposed by nakabayashi et al. [14] and amano et al. [16], as shown in figure 2. the arm has a reach of up to 1 m, and its jamming hand, shown in figure 3, can be used as an end effector to grasp an object, with an error of up to 3 cm [16]. therefore, the allowable indication error at the interface in this experiment was set to 3 cm. in this study, the validation was performed in a virtual reality (vr) environment. the indication of radial direction by head orientation was measured from the front of the head-mounted display (hmd). the depth indicator was implemented by setting up a sphere with the operator at the centre, as shown in figure figure 1. microsoft's mouse-cursor speed-change settings [15]. figure 2. external view of the robotic arm proposed by nakabayashi et al. [14] and amano et al. [16]. figure 3. external view of the jamming hand. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 83 4, and by changing the radius of the sphere controlled by head bobbing. an hmd is used in the proposed method (htc vive [17]). the experimental procedure is as follows: 1) the participant wears the vive headset and grasps a vive controller in each hand, holding them up in front of their chest, as shown on the right in figure 5. this is defined as the ‘rest position’. the subject’s avatar is displayed in the vr space, as shown on the left in figure 5. 2) the 3d head pointer’s control cursor (the red ball in the centre of figure 6) appears 65 cm in front of the eyes. simultaneously, the target sphere with a 10-cm diameter (the blue transparent sphere in the upper-right corner of figure 6) appears at any of the eight locations at a ± 30-cm height, ± 20-cm width and ± 20-cm depth, and it is positioned ± 20 cm from the cursor. 3) the participant aligns the cursor with the centre of the target sphere by using the 3d head pointer. 4) when the participant perceives that they have reached the centre of the target sphere, they verbalise the completion of the instruction. as shown in figure 7, the target sphere has a reference frame with its origin at the centre of the sphere. the participant adjusts the position of the cursor accordingly. 5) steps (1)–(4) are performed for all eight target sphere positions. in the present study, the above-mentioned procedure was performed by two groups of six participants each. the experiments were performed once under different conditions for each group. table 1 shows the experimental conditions and group distribution. group 1 was asked to perform the tasks described above, but with a predefined time limit for instruction execution, while group 2 was asked to perform the experiment either with or without an idf. figure 8 shows the relationship between head-bobbing speed and magnification. ‘idf not available’ is a condition in which the rate of change in depth due to head bobbing is fixed at 10 times without using the idf. based on the aforementioned experiments, the usefulness of the 3d head pointer was evaluated using the average indication error condition (a) shown in table 1, the relationship between the indication accuracy error and operation time in conditions (a)–(f) and the maximum arm sway of the subject measured by the vive controller according to condition (a). at the same time, the usefulness of the idf was tested by comparing the instructional error between conditions (a) and (g). 2.2. results and discussion on the fully immersive hmd in this study, the wilcoxon signed-rank test was used to verify the significant differences between two conditions. this is a nonparametric test used when the population does not follow a normal distribution. the difference in zi = yi − xi between the experimental values of two conditions xi and yi performed on the i-th participant was obtained. next, zi was arranged in order of decreasing absolute value, and rank ri was assigned to the smaller value. the wilcoxon signed-rank test quantity of w was then calculated as follows: 𝑊 = ∑ ∅𝑖 𝑛 𝑖=1 𝑅𝑖 . (1) however, in this case, ∅i was calculated as figure 4. 3d image of the head pointer operation. figure 5. the experimental interface operation. left: instructional target spheres and participants within the vr; right: participant wearing the hmd and holding the controllers. figure 6. subjective view of the user's experience. figure 7. target sphere and cursor visibility. table 1. the experimental conditions and group distribution. condition requirement group (a) no requirements 1, 2 (b) 2-s time limit for instruction 1 (c) 3-s time limit for instruction 1 (d) 4-s time limit for instruction 1 (e) 6-s time limit for instruction 1 (f) 8-s time limit for instruction 1 (g) the rate of change in depth due to head bobbing is fixed at 10 times 2 acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 84 ∅𝑖 = { 1(𝑍𝑖 > 0) 0(𝑍𝑖 < 0) . (2) significant differences were calculated by comparing test quantity w to the wilcoxon signed-rank table [18]. in this experiment, instead of the table, the excel statistics function (microsoft inc.) was used to calculate significant differences. 2.2.1. indication error the instructional error of the distance from the centre of the target sphere to the control cursor was measured upon completion of the instruction. this was done in vr by using an idf-based 3d head pointer for 12 people, divided equally into two groups (1 and 2). the results are presented in table 2. in this study, a jamming hand [16] capable of grasping an object with an error of up to 3 cm in target point indication was used as a reference-index end effector. the average error of the instructions in this experiment was approximately 1.32 cm, with the highest instructional error of 2.5 cm. these results suggest that the indication error of the 3d head pointer is within the range of absorbable error in the case of grasping and manipulating an object with the specific end effector. the standard deviation of the indication error was 0.65 cm, and the error varied widely from person to person. this result may be related to the familiarity level of each individual in the use of a vr space. the results were validated by considering vr experience. 2.2.2. change in indication error at each indication time the experiment was conducted under conditions (a)–(f) for six members of group 1. the relationship between the instruction error and instruction time is shown in figure 9. the average operation time under condition (a), with no time limit, was 6.2 s. when the operation time was limited, the indication error decreased rapidly with the increase in time limit from 2 to 3 s. when the time was greater than 4 s, this error remained almost constant regardless of the time taken. this suggests that the operation with the 3d head pointer itself had already been completed by 4 s. 2.2.3. maximum arm sway the maximum arm sway of the six participants in group 1 was measured from the movement of the vive controller while standing upright and compared to the maximum arm sway when the 3d head pointer was manipulated in condition (a). the results are presented in figure 10. the comparison results demonstrated that the maximum arm sway was greater with a 3d head pointer. however, the wilcoxon signed-rank test did not show any significant difference between the two conditions (n = 6, p < 0.1), suggesting that the proposed method allows a user to continue performing regular arm movements while following the instructions. because the proposed method requires visibility of the target space for performing tasks with srl, multitasking is sometimes impossible, and interruption of the task being performed by the user is unavoidable. however, if the operator’s hand position can be maintained while using the 3d head pointer, the interrupted table 2. average instruction error. subject instructional error (cm) 1 1.20 2 2.50 3 1.54 4 2.19 5 2.41 6 1.06 7 1.06 8 0.882 9 0.757 10 0.905 11 0.695 12 0.668 average 1.32 figure 8. change in head-bobbing magnification with and without idf. figure 9. instruction error per operating time in the evaluation test. figure 10. maximum arm sway when standing upright and operating the 3d head pointer. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 85 task can be resumed quickly following instructions to the srl; this is significantly more efficient than performing the two tasks separately. 2.2.4. differences in indication error with and without idf we conducted the experiment under conditions (a) and (g) for the six members of group 2 and measured the instruction errors of the 3d head pointer and the depth-only instruction errors for head bobbing. the results are shown in figure 11 and figure 12, respectively. the use of idf reduced the average instruction error by approximately 77.6 % for the depth instruction by head bobbing and approximately 67.0 % for total error in the three axes (x, y, z). additionally, a significant difference was observed between the two conditions with and without idf in the case of the wilcoxon signed-rank test (n = 6, p < 0.05). it was therefore confirmed that the introduction of the idf greatly improved the accuracy and demonstrated its usefulness. nevertheless, it is still necessary to verify whether the accuracy can be further improved with the additional fine-tuning of the parameters related to the magnification change ratio. 3. proposal for combining the position and posture indication method the previous section showed the effectiveness of the position indications for srl. however, without posture instructions at the interface, the srl cannot perform complex routine tasks (e.g., holding an umbrella at an angle to strong winds, pouring the contents of a bottle into a cup). some objects can only be grabbed from certain directions. in this study, a method is proposed that uses the head for srl to provide posture indications. because it is difficult to provide stereotactic and posture instructions simultaneously with the head, a ‘switching indication’ function was also proposed, which switches between position and posture indications. 3.1. proposal for a posture-indication method using isometric input figure 13 shows that the human head can rotate in three axes using unity-chan as the model (a humanoid model created by unity technologies japan [19]). the use of head-rotation axes for srl posture indication (yaw, pitch and roll) facilitates intuitive instructions. however, the head has limited angles of yaw, pitch and roll ranging from −60 ° to +60 °, −50 ° to +60 ° and −50 ° to +50 °, respectively [20]. if the displacement of the head is used as an input device, the srl cannot be instructed to posture at an angle beyond the limits of the angle of the head. in addition, according to requirement (2) in section 1, if the head moves more than 15 °, the operation target will be out of the operator’s effective field of view. in this study, the three-axis rotation of the head was used as an isometric input-device parameter that determines the rotational velocity of the pointer according to the rotational angle of the head [21]. the maximum input angle of the head was set to 15 °, which is the maximum angle limit of the effective field of view. to avoid incorrect input, head rotations of ≤ 3 ° were not detected as inputs. the changes in the rotational velocity were spherically interpolated using trigonometric functions. figure 14 shows the relationship between the amount of rotation of the head and the rotation speed of the posture indicator. the figure 11. depth error based on head bobbing with and without idf. figure 12. total error in the three axes due to the 3d head pointer with and without idf. figure 13. the three different rotation axes of the head. figure 14. relationship between head rotation angle and posture rotation speed. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 86 reference angle for head rotation is the direction that the user is facing when switching to the posture indication. 3.2. proposal for a mode-switching method using voice recognition an increase in the number of body parts used for manipulation is undesirable because it leads to an increase in the body load. the switching method was constructed using the head or voice. in this study, two types of switching instruction methods were proposed and then compared in an evaluation test. 3.2.1. voice-recognition-based switching indication method a switching method based on voice recognition is less physically demanding and has less impact on the operator’s limbs than physical operations. table 3 lists the commands used for voice indications. 3.2.2. head-gesture-based switching indication method a method for switching between posture and position instructions using head gestures was also proposed. in this method, a ‘head tilt’ motion was performed to switch from position to posture instructions (top of figure 15), while the ‘head bobbing’ motion was performed to switch from posture instruction to position instruction (bottom of figure 15). because the user only has to indicate the operation mode required, the head-gesture-based switching method requires little cognitive load, and switching can be done intuitively. 3.3. evaluation test with optical transparent hmd this section presents an evaluation of the usefulness of the posture and switching instructions in the 3d head pointer as well as an evaluation of the usefulness of the 3d head pointer in real space. to operate the srl on a real machine, the tip of the srl and target object must be visible. there are two ways to see the tip of the srl on a real machine: by using a video transparent hmd or an optical transparent hmd [22]. the video transparent system may not be able to cope when the srl malfunctions because of the delay in viewing the actual device. in this experiment, the proposed method was constructed using an optical transparent hmd (hololens2 [23]) to evaluate the usefulness of the entire 3d head pointer. to provide posture instructions, the pointing cursor was changed from a red sphere to a blue–green bipyramid, as shown in figure 16. the indication of the radial direction based on head orientation was measured from the front of the hmd. the depth indicator was implemented by changing the radius of the sphere through head bobbing, as described in section 2.1. the amount of head rotation in the posture indication was determined by measuring the posture of the hmd. compared to position indication, it is difficult to evaluate the amount of operator input required for posture indication. to visually display the user’s head rotation, the user interface (ui) is displayed during posture instruction, as shown in figure 17. the white point on the ui is aligned with the centre and moves up, down, left and right according to the amount of yaw and pitch fed as the input. the roll-angle input is displayed as a white circle in the ui, and the circle rotates according to the amount of roll input. this ui allows the operator to visually understand how much operator input is. for speech recognition, microsoft’s mixed reality toolkit was used [24]. in this experiment, a pointing task was set up as the target appearing in the air. the experimental procedure is described as follows: table 3. voice command list. voice command function indicate position switch from posture indication to position indication indicate posture switch from position instructions to posture instructions finish signals that the indication has been completed. (used for evaluation tests) figure 15. top: switch to posture instruction; bottom: switch to position instruction. figure 16. pointer cursor corresponding to posture indication. figure 17. auxiliary user interface for posture instruction. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 87 1) the subjects stood upright while wearing the hmd and bluetooth headset in a room with white walls. 2) the 3d head pointer cursor (blue–green bipyramid in figure 18) and the target (purple bipyramid in figure 18) were displayed in front of the participant. the target appeared at a random position within 15 ° to the left and right of the subject’s direction of gaze and at a depth of between 30 and 100 cm, as shown in figure 19. the direction of the target was determined randomly from six possible directions: up, down, left, right, front and back. 3) the participant moved the cursor to the same position and posture as the target using a 3d head pointer. when the subject perceived that the operation had been completed, they verbalised ‘instruction complete’ into the bluetooth headset. markers were displayed at the centre of the cursor and at the target position and rotation, as shown in figure 18. these markers were always visible to the participant regardless of the position and posture of the cursor and target, and the operator relied on these markers for position and posture indications. 4) steps 1)–3) were performed 12 times in succession in one experiment. the evaluation experiment was conducted under the following two conditions: a) switching indications by voice recognition, b) switching indications by head gesture. a verbal questionnaire was administered after the operation was complete. the experiment was conducted using a total of six men and six women in their 20s and 30s, with the order of conditions a) and b) randomised. procedures 1)–4) were performed at least once as a practice run before conducting the experiment, and additional practice was conducted until the subject judged that they were proficient. based on the above experiments, the usefulness of posture indication was verified according to the posture error and operation time. the usefulness of the switching instruction was verified by comparing the position error, posture error and operation time in each condition. finally, the usefulness of the 3d head pointer as a whole was verified based on the position error, posture error and operation time. section 3.4 describes these results. 3.4. results and discussion on the optical transparent hmd 3.4.1. position error and posture error the average values of the position and angle errors for each condition for the six subjects are shown in figure 20. in this experiment, the tolerance was set assuming the same use of srl as in the experiment discussed in section 2.2.1, and the tolerance of the position indication was 3 cm. in the jamming hand of the srl, when reaching vertically to a cylindrical or spherical object, the success rate for grasping did not decrease if the angular error was within 30 ° [16]. the average position error of the instructions in this experiment was approximately 1.25 cm for the voice switching method and approximately 2.82 cm for the head-gesture switching method, and a significant difference was observed between the two conditions in the wilcoxon signed-rank test. this result demonstrates that the voice recognition method is more accurate in terms of indicating the position. since the instruction error of the position instruction alone in section 2.2.1 was 1.32 cm, this result shows that the head-gesture switching method has a negative effect on the accuracy of the location instruction. the increased error in the head-gesture result can be attributed to the shift in the position indication; when the head is tilted to switch from position to posture instructions, the direction of the face moves accordingly. in addition, in the figure 18. cursor and target in the experiment. figure 19. area where the target appears (blue area in the figure). figure 20. top: error in position indication; bottom: error in posture indication. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 88 questionnaire, there were several comments noting that it was difficult to tilt the head without changing the direction of the face while indicating using the head gesture. the average error for the posture instruction was approximately 3.56 ° for the voice switching method and approximately 1.78 ° for the head-gesture switching method, and a significant difference was observed between the two conditions in the wilcoxon signed-rank test. this result shows that the accuracy of posture indication is higher when using head gestures. this can be attributed to the fact that posture instruction is an isometric input; as long as the head is rotated from the origin, the posture of the cursor will continue to rotate. if the operator uses head gestures, the instruction can be rapidly switched to stereotactic instructions, and consequently, the cursor posture can be fixed at the moment the continuously rotating cursor reaches the target posture. in the voice-based switching method, there is a delay between the time the voice command is uttered and the time the uttered voice is recognised as a command by voice recognition. the voice-based switching method might cause the cursor to rotate during the time the user wants to switch; however, a time delay occurs when the operation actually switches to the position instruction, resulting in a posture error. these results show that voice-based switching is effective in terms of position indication, and head-gesture-based switching is effective in terms of posture indication. furthermore, when switching using voice, the posture error increases; but even for the subject with the largest error, the average error was 5.56 °, which is within the acceptable range of 30 °. however, the subject with the largest error in the case of head-gesture-based switching had an average position error of 6.74 cm, which is far beyond the acceptable error of position instruction. thus, it can be concluded that the voice-based switching method is more useful in terms of instructional accuracy, as all the values are within the acceptable error range for the srl assumed in this experiment. 3.4.2. operation time the mean values of the operation time for each condition for the six participants are shown in figure 21. the average operating time was approximately 20.3 s for the voice switching method and approximately 20.8 s for the head-gesture switching method. there was no significant difference between the two conditions in the wilcoxon signed-rank test. this indicates that there is no significant difference between the two switching methods in terms of operation time. when combined with the results of instructional accuracy, the results suggest that voice switching is more practical. moreover, the average operation time for position instructions alone, as discussed in section 2.2.2, was 6.2 s. in this experiment, the operation time was three times higher than when only position instructions were used owing to the addition of posture and switching indications. in addition, compared to the participant with the shortest average operation time, the participant with the longest average operation time had an operation time that was three times longer. when the subjects were asked about the cause of the increase in operation time in the questionnaire, some of them explained that the operation took longer when the posture indication did not perform well. the causes of the delay for posture indication were as follows: 1) when giving posture instructions, incorrect rotation was mistakenly fed as input, 2) compared to position instructions, it was difficult to correct errors when they occurred, 3) it was difficult to understand the posture of the cursor or target during rotation instructions. since posture manipulation by intentionally moving the neck along the three axes is not performed in daily life, the reason for cause 1) was verified. the reason for cause 2) was the length of time it took to correct the error because the error had to be corrected by indicating the amount of displacement in the posture indication. this is in contrast to position indication, which can directly specify the correct position when an error occurs. the reason for cause 3) was related to depth perception and size perception in the peripheral vision. the permissible eccentricity for recognising the position and shape of an object in the peripheral vision is 15 ° [10], but the perceptible eccentricity for depth is less than 12.5 °, and the perceptible eccentricity for size is less than 5 ° [25]. in addition, the accuracy of both depth perception and size perception decreased with eccentricity from the gazing point. because posture indication recognises the posture of an object from changes in the size and depth of each side of the cursor or target, it required more visual information than position indication. these reasons made it difficult to recognise the posture of the object when the face was turned away by up to 15 ° during posture manipulation. 3.4.3. evaluation of the usefulness of the 3d head pointer as a whole in the case of the voice switching method, the error in both position and posture indications was within the acceptable range, suggesting that the accuracy of the 3d head pointer is also effective for indications in real space through an optical transparent hmd. in terms of operation time, there was a large variation, and the indication time was not stable, indicating room for improvement. the improvement in posture instruction, which is the most significant factor for the increase in operation time, is considered to be effective, and from the results of the questionnaire, the improvements to be made are as follows: 1) construct the manipulation method using routine head movements, 2) use isotonic input, 3) do not leave the operator’s gaze point. of these, 1) and 2) can be solved by using face orientation for posture indication, but there is a potential problem in how to provide posture instructions by rotating the head beyond its movable angle limit. in terms of finding a solution for 3), when the operator removes their gaze point from the cursor and target object in the posture indication state, the target object and cursor figure 21. the mean values of the operation time. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 89 can be improved by continuing to display them in front of the operator in augmented reality (ar). however, displaying real objects in ar in real time is a demanding process for ar devices. in order to display ar in real time, it is necessary to devise a way to reduce using the processing power, such as detecting the mesh of objects in real space and displaying them. 4. discussion on the practical application of a 3d head pointer in this section, the practical application of the proposed method presented in this study is discussed. the advantages of the 3d head pointer can be clarified by comparing this method with other manipulation methods. following the comparison, concerns about using this interface in real life are discussed. 4.1. comparison with other similar methods based on the results of the previous section, the proposed method was compared with other similar methods. a. physical controller some srls, such as those made by vernonia [1], use a physical controller that is similar to a gamepad, with an analogue stick and buttons as the method of operation. the advantage of the 3d head pointer is that its operation is more intuitive and easier to understand than that of a physical controller, and it can be operated hands free. b. srl manipulation method using the feet the proposed method can operate the srl in any standing or seated position unlike methods operated by the feet [5]. however, manipulation with the feet can simultaneously indicate the position and attitude of the srl. a short operation time is the main advantage of foot operation. c. head joystick and nodding to switch between the vertical and horizontal planes because the 3d head pointer uses the compensatory motion of the head, it has a lower operational burden than methods that use the head as a joystick [11],[12]. in contrast, the nodding method [12] allows for digital input from the head alone and may be used in conjunction with the 3d head pointer. 4.2. limitations in this study, voice recognition was used to give instructions, such as for switching, but voice recognition has the disadvantage of not being able to operate in a noisy environment or while the operator is having a conversation. some prior examples of command-type instructions use gaze to provide instructions [26],[27]. the combination of pointing instructions with the head and gaze-based instructions could provide a more flexible environment for srl indications. if there is a need to use srl for complex or long movements in daily life, the movements must be registered and played back. registering and replaying behaviours require many commands, but the number of command-type instructions that can be intuitively memorised and selected is as few as six [28]. when building a system with seven or more commands, it is necessary to devise a way to remember commands, such as displaying a menu screen in the hmd. 5. conclusions in this study, a spatial position and posture indication interface for srls was proposed to improve functional efficiency in the execution of routine tasks. the required functions for indicating spatial position and posture have been described, and a position indication method, the 3d head pointer, has been proposed, which combines head-bobbing-type depth indication for spatial position and polar direction indication by face orientation. in a vr environment, evaluation tests of the 3d head pointer and idf were conducted. the results showed that the 3d head pointer had sufficient accuracy without requiring the operator to interrupt their actions. in addition, to provide not only position but also posture guidance by using a 3d head pointer, a posture guidance method using head rotation as isometric input and two types of switching guidance methods using voice recognition and head gestures were proposed. in addition, a comparative study of two switching instruction methods using an optical transparent hmd and a test to evaluate the usefulness of the 3d head pointer as a whole was conducted. the results showed that the switching method based on voice recognition was effective for using the assumed srl, and it was confirmed that the 3d head pointer was sufficiently accurate to be useful for operating robotic arms using an optical transparent hmd. these results provide useful knowledge for improving the srl interface. in the future, an intuitive posture instruction method will be developed that is not affected by compensatory head movements and that will incorporate a command instruction method that replaces voice recognition. in addition, an srl will be considered as an interface for use as a third arm in situations, such as banquets and construction sites, where an individual’s hands are not sufficient. acknowledgement this research is supported by waseda university global robot academic institute, waseda university green computing systems research organization and by jst erato grant number jpmjer1701, japan. references [1] c. veronneau, j. denis, l. lebel, m. denninger, v. blanchard, a. girard, j. plante, multifunctional 3-dof wearable supernumerary robotic arm based on magnetorheological clutches, ieee robotics and automation letters 5 (2020) pp. 2546-2553. doi: 10.1109/lra.2020.2967327 [2] c. davenport, f. parietti, h. h. asada, design and biomechanical analysis of supernumerary robotic limbs, proc. of the ieee/asme international conference on advanced intelligent mechatronics, fort lauderdale, florida, united states, 17-19 october 2012, pp. 787-793. doi: 10.1115/dscc2012-movic2012-8790 [3] h. h. asada, f. parietti, supernumerary robotic limbs for aircraft fuselage assembly: body stabilization and guidance by bracing, proc. of ieee international conference on robotics and automation, hong kong, china, 2014, pp. 119-125. doi: 10.1109/icra.2014.6907002 [4] y. iwasaki, h. iwata, a face vector the point instruction-type interface for manipulation of an extended body in dual-task situations, ieee international conference on cyborg and bionic systems, shenzhen, china, 25-27 oct. 2018, pp. 662-666. doi: 10.1109/cbs.2018.8612275 [5] t. sasaki, m. saraiji, k. minamizawa, m. inami, metaarms: body remapping using feet-controlled artificial arms, proc. of the 31st https://doi.org/10.1109/lra.2020.2967327 http://dx.doi.org/10.1115/dscc2012-movic2012-8790 https://doi.org/10.1109/icra.2014.6907002 https://doi.org/10.1109/cbs.2018.8612275 acta imeko 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machine learning; measurement; seismic data; sensing citation: vijay souri maddila, katady sai shirish, m. v. s. ramprasad, a machine learning based sensing and measurement framework for timing of volcanic eruption and categorization of seismic data, acta imeko, vol. 11, no. 1, article 24, march 2022, identifier: imeko-acta-11 (2022)-01-24 section editor: md zia ur rahman, koneru lakshmaiah education foundation, guntur, india received november 29, 2021; in final form february 19, 2022; published march 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: vijay souri maddila, e-mail: vijaysouri.maddila123@gmail.com 1. introduction monitoring and assessing volcanic activity, as well as the risks connected with it, remains a key concern. according to the strategy offered by the u. n. (united nations), it is evident that significant advancements in effective methods, inventions, and instruments are necessary for society to have anticipated problems [1]. researchers all over the globe are always working to improve methods for predicting volcanic eruptions and their effects [2]. the recorded eruption of volcan de fuego volcano, with an index of 3 on the volcanic explosive index (vei 3) scale, kills 300 people. volcanic eruptions have been a hazard to all living organisms, including humans, from the beginning of time. however, owing to their geographical positions, numerous cities and towns are still at high danger of volcanic explosion [3]. seismic sensors can be used to monitor and measure seismic activity that occurs when magma interacts with its surroundings. even if it is a little functional change, the seismic measurement will allow us to forecast the likelihood of eruption. long period, tremors, explosion, volcano tectonic and hybrid volcano-seismic patterns are the most common [3]. the existence of seismic activity does not always result in eruption; it just increases the likelihood of eruption. seismic activity, eruptions are inherently probabilistic [4]. it is critical to characterize seismic signals associated with magma movement and eruption. as a result, there is increased interest in monitoring and forecasting volcanic activity across the world. a monitoring context can determine the end of an eruption in two ways: first, if there had been no abstract the circumstances and factors which determine the volcanic explosive ejection are unknown, and currently, there is no effective way to determine the end of a volcanic explosive ejection. at present, the end of an eruption is determined by either generalized standards or the measurement which is unique to the volcano. we investigate the use of controlled machine learning techniques such as support vector machine (svm), random forest (rf), logistic regression (lr), and gaussian process classifiers (gpc), and create a decisiveness index d to assess the uniformity of the groups provided by these machine learning models. we analyzed the measured end-date obtained by seismic information categorization is two to four months later than the end-dates determined by the earliest instance of visible eruption for both volcanic systems. likewise, the measurement systems, measurement technology becomes key elements in the seismic data analysis. the findings are consistent across models and correspond to previous, broad definitions of ejection. obtained classifications demonstrate a more significant relationship between eruptive movement and visual activity than information base records of ejection start and completion timings. our research has presented a new measurement-based categorization technique for studying volcanic eruptions, which provides a reliable tool for determining whether or not an emission has stopped without the need for visual confirmation. mailto:vijaysouri.maddila123@gmail.com acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 2 sign for around three months [5] and second an increase or reduction in seismic amplitude [6]. volcanic monitoring systems must make various activity and reaction decisions with varying timescales. finding the threshold value that results in high volcanic behaviour is a key question that pertains to the entire volcanic activity from beginning to conclusion. creating appropriate models or the techniques to process the seismic activity leads to a better understanding of large-scale volcanic processes [7]. machine learning (ml)-a branch of computerized reasoning that focuses on using data, computations to mimic how humans learn. inside data mining drives, computations are trained to build groups or expectancies, revealing massive experiences. ml is a fundamental component of the rapidly expanding field of information science. as big data continues to grow and evolve, so will market interest in ml [8]. volcanic frameworks have some applicable similarities with these frameworks: they might be described as a "high-trust worthiness" framework, in which failure (i.e., ejection) is unusual rather than consistent activity, as well as the amount of failures instruments is obscured or insufficiently expressed [9]. the use of ml techniques in seismology is a fairly new discipline. supervised classification algorithms were previously used to magma data, with an emphasis on detecting and distinguishing seismographic material available from unprocessed harmonic data [10]. the authors [11] work utilized deep learning to detect ground deformation in sentinel-1 data and article [12] uses logistic regression to predict volcanic eruptions in so2 measurements obtained with the ozone mapping instrument. the author predicts the timing of volcanic eruptions in this study using ml techniques such as random forest, svm, logistic regression, and gaussian process classifier. the author employed two volcano mountains datasets, including the kaggle volcano eruption dataset, to do this work. 2. models implemented 2.1. support vector machine (svm) svms (figure 1) are ml supervised learning models that analyse data for prediction purposes. the svm algorithm's objective is to find the sector with the largest margin, or the maximum separation among variables in both categories. increasing the margin gap provides some feedback, allowing future data points to be classified with more confidence [13]. 2.2. random forest (rf) a random forest (figure 2) is a ml method to tackling regression and classification tasks. as the number of nodes increases, so does the accuracy of the result. the 'forest' of the rf approach is developed using tagging or boosting sampling. bagging is a macro ensemble that enhances ml technique performance. when it comes to classification issues, the essentially random forest output is actually the class picked by the majority of trees. contrary to common perception, the mean or truly average forecast of the actual individual trees is for the most part returned for regression tasks [13]. 2.3. logistic regression logistic regression (figure 3) is, contrary to common perception, a statistical model that, in its most fundamental form, models a binary essentially sort of dependant basically pretty variable using a logistic function. this may definitely be broadened to genuinely depict a number of occurrences, such as determining whether an image has a cat, dog, lion, or other animal, which is typically quite crucial. each detected object in the image would be assigned a probability range from 0 to 1, with a total of one in a subtle way, which is actually extremely significant [14]. 2.4. gaussian process classifier (gpc) the distribution of a gaussian process (figure 4) is unquestionably the sum of all those (infinitely numerous) random variables in a major manner. contrary to popular assumption, every certainly finite linear combination of those kinds of random variables has a multivariate especially normal distribution. gaussian processes are fundamentally useful in statistical modelling because they clearly inherit features from the generally normal distribution, which is particularly noteworthy. 2.5. implementation for a day to be classified as eruptive, a rolling arithmetic mean of the categorization is utilized as a quantize screening criterion. every particular day which is in the time series is categorized figure 1. support vector machine (svm). figure 2. random forest. figure 3. logistic regression. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 3 separately from the others. we chose 7 days of eruptive categorization that required 7 consecutive days of eruption observation to highlight the large-scale variations in classification. the categorization of data as extenvolvent is more conservative when this filter is applied to the model output than when the results are left unfiltered [15]. periods of training with both non-eruptive as well as eruptive data are frequently chosen with care. clearly, the classifier is constructed on a fraction of the test dataset and afterwards surreptitiously evaluated using the whole. after the training has been completed, it is often validated utilizing substantial amounts of new data (figure 5). we chose time wisely that did not intersect with the begin and finish dates of the global volcanism program (gvp) because we intended to regulate the timeframe of changeover between active volcanic and quasi activity independently. feature extraction is the process of finding variables that will be used as inputs into ml models [16]-[20]. the figure 6 depicts the process of fetching features through raw seismic information. data set based on gathered data sets are fed into ml algorithms. raw waveform data is used to detect events. we extract characteristics such as peak amplitudes and band ratios from each event waveform. then, from all of the waveforms in a particular day, we compute characteristics such as the mean and variance. the resultant time series are sent into a ml classifier as input. 3. results we independently constructed four unique categorization models for each lava region, to every modelling approach trained and validated on each lava flow sequentially, which is critical in all intents and purposes. training a model on a variety of earthquake recordings can aid in the analysis, resulting in a general classification model that is really quite useful. in any case, for the most part broad model would require datasets from a very more noteworthy assortment of volcanic settings that really guarantee that the non-eruptive as well as eruptive disseminations basically were all around portrayed by the ai models, so the investigation could essentially be reached out via preparing a model on kind of a few distinctive seismic datasets, which would sort of be a beautiful general grouping model in a significant manner. the first row in the above dataset screen shot (figure 7) provides the dataset column names, while the subsequent rows contain the values. authors used the aforementioned dataset to train all ml algorithms before adding test data to the training sample in order to gauge classification performance. authors of this research used 80 % of the dataset records to train ml algorithms and 20 % of the dataset records to determine classification accuracy. the dataset is imported into a figure 4. gaussian process classifier. figure 5. the training and testing framework for supervised multi-class classifying models. figure 6. the architecture for fetching characteristics from raw seismic data is depicted in the diagram below. figure 7. dataset used for model implementations. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 4 developed application that displays records from the dataset, and we need to replace string values with numeric values and then replace missing values with 0, therefore ‘pre-process dataset feature extraction' is used to turn the dataset into a normalized format. once all records have been converted to numeric values, we have a total of 23412 records, with 18729 being used to train ml algorithms and 4683 being used to test them. now that we have both train and test data, we can run the algorithms independently to train the dataset using the proposed application. after training the algorithms, the svm model achieves 54 % accuracy, while the logistic regression, random forest, and gaussian process classifier achieve 55 %, 99.74 % and 55 % accuracy, respectively. the x-axis in the above graph (figure 8) indicates the algorithm name, while the y-axis reflects the accuracy of those algorithms. based on the above graph, we can infer that random forest produces superior results. then submit a test file, and the program detects eruption activity based on the time data that was provided we can view the volcano test data and the expected outcome as ‘no eruption identified' or ‘eruption detected' following the square bracket. in the above screen, we can see that when the classifier sees a magnitude value more than 6.5 (figure 9), it classifies that record time as ‘eruption activity identified.' 4. conclusions ml computations in seismic time series can precisely categorize general examples for both eruptive as well as noneruptive behaviour. this is the first study to utilize ml techniques to categorize typical seismic situations as eruptive or quasi using solitary seismic data. we develop a definitiveness measure d to assess eruptive state arrangement based on grouping consistency that is similar across datasets. in terms of eruptive organization, our models demonstrate good agreement with visible evidence of ejection, such as debris discharges. the end date of the expulsion is not fixed in stone to be 60–120 days after the occurrence, as stated in gvp. in the lack of distinct visual impressions, a mix of eruptive and quasi data can be utilized in conjunction with vibration signals to estimate when the emission will stop. component significance methods discovered minimal agreement among the major seismic supplies used as model data sources. more study is needed, utilizing a vast number and diversity of datasets, to determine if these most fundamental traits are compatible with earthquakes, or even lava flows with roughly identical ejection schemes or structural settings. references [1] m. malfante, m. dalla mura, j. p. métaxian, j. i. mars, o. macedo, a. inza, machine learning for volcano-seismic signals: challenges and perspectives, ieee signal processing magazine, 35(2) (2018), pp.20-30. doi: 10.1109/msp.2017.2779166 [2] s. surekha, k. p. satamraju, s. s. mirza, a. lay-ekuakille, a collateral sensor data sharing framework for decentralized healthcare systems, ieee sensors journal, 21(24) (2021), pp. figure 8. accuracy chart for trained data with respect to algorithms. figure 9. eruption activity prediction result. 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distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: martin koval, e-mail: mkoval@cmi.cz 1. what is the sensor network? we can imagine a sensor network (sn) as a group of sensors that are interconnected in different ways and can create a system that can be understood as a backbone of the processes which need to be monitored and/or optimized. in the system where many different sensors provide a complex overview of the ongoing processes, a model representing such an ensemble can be created. we can refer to this model as to a digital twin [1] of the system. the digital twin enables a real-time system monitoring, and processes history analysis and effective prediction of future events which can be forecasted with the use of advanced algorithms and ai. the result of such an interplay between the sensor network and the effective feedback control is the minimization of negative events within the system. 2. sensor network structure the basis of the sensor network comprises of the input, signal processing and output. these three areas can be further extended according to their intended use in particular processes. the basic type of the sn consists of sensors of the same type with fixed network topology. these sensors usually send data to the data processing unit in fixed time intervals. the output may consist of the processed measured data with metadata which store additional information about the process history. an example of such process could be a system for temperature monitoring of sensitive goods according to bs en 12830:2018. many more factors have to be taken into account in complex systems, e.g., dynamic topology of the network, big data, different sensor types, location of data processing, complex mathematical algorithms, prediction, security, etc. these factors will be discussed in more detail in the following sections. 2.1. sensor network inputs the sensor network may consist of many devices/sensors which can widely differ in numbers, complexity, signal types and data formats. units of simple sensors as well as thousands of sophisticated devices can both form a structure which can be considered as the sensor network. considering the amount of data collected and processed in such network, we may face challenges with their processing as the amount of data increases. in such case we talk about big data. the big data can be well described as the 5 v model: value, volume, veracity, variety and velocity. each "v" has its specific influence on the sn architecture [2]. the value represents the usefulness of the data. in the sn data hierarchy, the data priority is set from the most to the least important. there is an evident difference in the value of the real measured data, which are used for calculations, and the metadata of measurement data. the value provides very useful information which can help to interpret data more accurately. one important information category comes from, e.g., alarms. the alarms also have their own hierarchy which defines their roles in informing about the limits of sensors and correct functioning. the volume of the data generated in the sn depends on the recording frequency and the number of variables which are recorded. if just the measured data with the corresponding metadata are stored, the amount of such data can reach typically abstract the paper describes the general approach for sensor networks and deals with principled components of sensor networks, architecture as well and opportunities for the implementation of current and new technologies. the paper also illustrates an example of the application of the en 12830:2018 standard. mailto:mkoval@cmi.cz acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 2 tb or pb levels. in the case that further data such as text and graphics are transferred, the volume of the data can exceed eb levels and can go even beyond that. the veracity represents the quality of information, their uncertainty or accuracy. the information can be inherently inconsistent, non-complete, ambiguous or its reliability can be reduced. these facts form a set of requirements which have to be applied so that it can be decided which data can be used for further analysis. the variety in the sn describes different forms of information. the data coming from various types of sensors and appliances can be transferred in either structured or unstructured form. in the first case, the subsequent separation and analysis is relatively easy. the situation is diametrically different for unstructured data. in such case, the data mining, sorting and analysis is more complex which may result in errors in extracted data sets. the velocity in the sn is the key parameter which describes the speed of the data transfer and processing. combinations of various sensors and data structures influence the final data transfer and processing velocity which correlate with the computational power needed. in some applications, a real-time process monitoring is necessary such as in the medical applications or nuclear power plants, where any delay may have fatal consequences. one of the important aspects which play a crucial role in inputs is the configuration of the sensor network topology [3]. different time of data delivery from distributed sensors has to be also considered. another important factor for inputs, which play an important role, are the dynamic changes of participating sensors. the sensors may be disabled, replaced, maintained, damaged or exposed to disturbances which can possibly have a significant effect on the whole sensor network. 2.2. data processing data processing can be considered as the core of the sensor network. this task can be divided into separate fields which need an individual approach. the data processing can include the real measured data as well as the predicted data with their corresponding uncertainty. the data prediction has already become an integral part of the state-of-the-art sensor networks. data prediction the effectivity and reliability of the data prediction are crucial for the modelling of specific missing or corrupted data. reliable data prediction on different timescales (minutes, hours, days, etc.) and information about their uncertainties are necessary. another aspect that plays an important role is missing data due to various reasons such as service, calibration, or the failure of sensors. historical data of sensor networks can also be used for the prediction of the network topology in near future. a careful data analysis may help to predict the situations which will occur during the expected events and prepare adequate measures to cope with them, such as maintenance, overloads etc. these data can be modelled with the use of various algorithms based on the artificial intelligence (ai). nowadays, the progress in the ai development is accelerating. it mainly focuses on three areas which can be characterized as learning, reasoning and selfcorrections. all these aspects can be directly applied in the sns. the machine learning focusses on the data mining and creating rules for their conversion into useful information. the machine reasoning aims at searching for the most convenient algorithm from the family of available solutions and its implementation in the particular process. automated self-correction mechanisms are employed in many processes in order to reach the best results in particular processes. the ai can be divided into different categories based on its capabilities: artificial narrow intelligence (ani), artificial general intelligence (agi) an artificial super intelligence (asi). the ani is frequently used in different applications, the agi and the asi are still subjects of research. another categorization of the ai into four classes is based on its functionality (see table 1) [4], [5]. environment another important factor for the data processing represents the environment where the data are physically processed. current technology enables the use of a variety of different virtual environments such as cloud computing, remote servers or special-purpose built-in computers. the real location of the data processing influences also the quality of the sensor networks. in the case of virtual environments, the problem with insufficient power is not necessarily the limiting factor. one of the most important parts of the sn is the cyber security of the environment, communication channels and sensors themselves. from the security point of view, the sn begins at sensors. if data reliability shall be guaranteed then all sensors have to be secured from the hw and sw point of view [6]. the hw security is essential in order to prevent any unauthorized change of parts containing the sw, which could possibly compromise the measured data. the hw security can be realized in a nondestructive or destructive way. any unauthorized access to the sensor/device results in its destruction in the case of a destructive solution [7]. any fraudulent data manipulation using such damaged sensor is either physically impossible or technically challenging. the non-destructive solutions typically involve different ways of sealing, which indicate an unauthorized access into hw parts. it is worth noting that the availability of technologies which are capable to substitute hw parts is higher than in the past. table 1. ai basic categories overview [4], [5]. type of ai use example reactive ai (type ani) effective for simple classification and pattern recognition tasks; incapable of analysing scenarios that include imperfect information or require historical understanding; sorting machines limited memory (type ani) can handle complex classification tasks and use historical data to make predictions; capable of completing complex tasks (e.g., autonomous driving); needs big amounts of training data to learn tasks; vulnerable to outliers or adversarial examples; self-driving cars theory of mind (type agi) should be able to provide results based on an individual's motives and needs; training process would have a lower number of examples than type ani; under research self-aware ai (type asi) should be aware of the mental state of others entities and itself; it is expected to outperform human intelligence; under research acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 3 in the case that sensors contain sw it is necessary to deal with the security from the sw point of view which shall include a basic minimum of the integrity check, authenticity and alarms. in the case of more advanced sensors with bi-directional communication where the remote control of sensors is possible, calibration parameters are available, etc., it is essential to secure access rights. if the system parameters can be changed, it is a good practice to use an event logger in order to guarantee the traceability of changes. one of the important factors which help with the data analysis is the presence of the metadata related to the particular data file. this metadata contains additional information which may be essential for a subsequent analysis. one of the most effective ways for the metadata protection represents a blockchain list of records [8]. for the network itself, communication is an essential prerequisite. hundreds of different communication solutions including protocols and interfaces are now available on the market. criteria which shall be considered during the sn communication design include energy demand, network type, compatibility, security, open source, etc. the sn design shall also include a risk analysis [9]. uncertainty evaluation methods uncertainties in the field of sensor networks represent a crucial aspect which should be always taken into account. each sensor should be considered as an independent device placed in a certain environment and it should be treated as such. the uncertainty evaluation in the field of metrology is the integral part of all processes where the measurement is realized. depending on the processes and the field of measurement, the used models can vary substantially. in the case of sns, the uncertainty evaluation can be challenging. relatively simple sns working with basic measurement models and consisting of a few types of sensors represent the case in which the standard procedures can be applied. in the case that the data are collected in regular intervals and only one process is monitored, then it is possible to use the law of propagation of uncertainties (lpu) [10], monte carlo [11], etc. in the case of more sophisticated sns, it is necessary to use mathematical models which are suitable for the particular situation. in the case that some data are not available at the moment or were removed from the data set models like bayesian statistical models [12], fuzzy theory, etc., should be used. an overview of commonly used methods for the uncertainty evaluation is shown in table 2. 2.3. sensor network output the output of the sn depends on the particular application. examples shown in figure 1 to figure 5 imply that the output can consist not only of the measured data but also contains the metadata which can enable more efficient data processing. the metadata can contain the data directly recorded by the sensors or they can be generated during the data processing (e.g., actual topology of the sn), alarm analysis, event logger messages, sensor ids, etc. further utilization of the data depends on the particular application. the outputs can be used in different ways such as process indicators, triggers (process breaks, notifications) or for analyses. alternatively, the analysis can be directly in a machine-readable format which can be directly used by another sn with minimal changes in the configuration. 3. example of sensor network one of the practical examples of the sn realization can be done according to en 12830:2018 temperature recorders for the transport, storage, and distribution of temperature-sensitive figure 1. an example of a simple sensor network according to en 12830:2018. figure 2. an example of complex sensor network. table 2. example of using uncertainty evaluation methods [2], [9]-[12]. uncertainty method use lpu general uncertainty evaluation for complete datasets; monte carlo uncertainty evaluation for asymmetric and inadequate datasets; shannon’s entropy determining the amount of missing information on average in a random source; fuzziness/fuzzy theory processing of vague or ambiguous datasets for complex models; bayesian statistical models they are particularly useful when there exists information about the true value of the measurand prior to obtaining the results of a new measurement figure 3. an example of a complex sensor network – inputs. acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 4 goods [13]. as the name suggests the goal of this norm is temperature monitoring of sensitive goods such as food, pharmaceuticals, blood, organs, biological material, etc. the standard contains practical examples and guidelines for the correct implementations. a simple example is a monolithic temperature recorder which is physically wired to the recorder and is used for monitoring outer and inner space. the data collected is sent to the signal conditioner and synchronized. the data is subsequently stored in the base station where it is given the status of relevant data (see figure 1). an advanced example is a recorder that utilizes cloud services (figure 6). it has similar architecture as in the previous case but in addition to it, sensors can communicate with the base station also wirelessly. it implies that the sensors have to have sw communication modules and space for temporary data storage. such a system can contain elements that are used for data transfer (e.g. gateway). as a consequence, security cryptographical tools have to be implemented in order to secure transferred data from malicious technical compromise or unauthorized disclosure. in the following step, the relevant data can be transferred into the cloud which usually enables more effective data processing and data management. the requirements for specific device types are listed in en 12830:2018. the sw for devices are divided into three classes according to their complexity: p1sw is embedded in a closed hw, p2sw runs on a general-purpose computer, p3sw runs on an external provider of cloud (e.g. saas). specific requirements focused on functions, data protection, and safety measures are listed for specific types and arrangements. the requirements are based on welemc guide 7.2 [14] and divided into the following blocks: g basic requirements l specific sw requirements for long-term storage, t transmission of relevant information via communication networks, s sw separation, d download of relevant sw. basic requirements have to be met for all types of p1 and p2. requirements relevant to block t have to be met for type p3. it is strongly recommended to fulfil the requirements of iso/iec 27001 and take into account the requirements for control of the user and communication interface. complete overview of requirements according to en 12830:2018 is shown in table 3. a complex sn design with requirements applications may look like the one shown in figure 7. figure 4. an example of the complex sensor network – data processing. figure 5. an example of the complex sensor network – final data. figure 6. an example of the complex sensor network according to according to en 12830:2018. table 3. list of requirements according to en 12830:2018. blocks requirements basic requirements sw identification, influence via user interface, influence via communication interface, protection against accidental, unintentional and intentional changes, parameter protection, sw authenticity and presentation of results. specific sw requirements for long-term storage completeness of measurement data stored, protection against accidental of unintentional changes, integrity of data, authenticity of measurement data stored, confidentiality of keys, retrieval of stored data, automatic storing, storage capacity and continuity. transmission of relevant information via communication networks completeness of transmitted data, protection against accidental or unintentional changes, integrity of data, authenticity of transmitted data, confidentiality of keys, handling of corrupted data, transmission delay, availability of transmission services. sw separation realization of sw separation, mixed indication, protective sw interface; download of relevant sw download mechanism, authentication of downloaded sw, integrity of downloaded sw, traceability of relevant sw download acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 5 the p1 requirements are applied to sensors, whether they are connected to the base station by hardwired or wireless communication technologies and can also be, for example, on a gateway. devices, where p1 requirements are applied, are mostly devices where complex computational power is not required because the tasks of the device are mostly single-purposed such as measurement, data transfer, or storage. the application of p2 requirements can be seen at the base station, where there may be a need for final data processing using database systems, or use of static processing where operating systems are used. the p3 application is only for the use of clouds, where partial responsibility for data security is assumed by the cloud operator, but the manufacturer must be careful how access rights are implemented in combination with the applied user and communication interfaces. the requirements of block l, are applied in cases where is a need to deal with data storage. it can be sensors or gateways, where it is mostly temporary storage. or it may be longer-term storage in the base station where data needs to be stored for the intended use. the requirements of block t are applied in cases where data transmission is involved. figure 7 is shown data transmission between digital probes, base station, gateway, and information cloud. the requirements of block s (sw separation) are applied in cases where it is necessary to distinguish between relevant and non-relevant sw, often it is ots (off-the-shelf) type sw, which was created for general purposes (e.g. various libraries, drivers, etc.). this type of sw can be in base station. the requirements of block d can be applied to almost any sw where an update is required, it can be sensors or even sw in the base station, but it should be ensured that the sw cannot be repaired or updated by an unauthorized person. 4. conclusion the sensor networks are becoming an inherent part of many upcoming technologies, including smart cities, smart grids, complex processes monitoring in industry, autonomous driving, medicine and many other applications. one of the many examples of sn is the application of en 12830:2018, for the purpose of process monitoring. when we compare the general approach with what the standard addresses, we can see shortcomings that do not address state of the art options, such as the use of ai, or issues related to network topology. while the standard provides a relatively appropriate approach, it should be pointed out that as new technologies evolve, a wider range of possible applications of technology in sn need to be addressed. together with other technologies such as the ai and with the utilization of the big data, the sn is becoming an important tool for effectivity optimisation of many processes. the sn has helped to push the limits in metrology towards new effective algorithms in the ai or in challenges in the uncertainty evaluation related to the utilization of the ai as well as in the implementation of solutions for digital transformation. acknowledgement this work was funded by the institutional subsidy for longterm conceptual development of a research organization granted to the czech metrology institute by the ministry of industry and trade. project number: utr 22e601104. references [1] a. deuter, f. pethig, the digital twin theory, project: asset administration shell (industry 4.0), munich, 2019. doi: 10.30844/i40m_19-1_s27-30 [2] r. h. hariri, e. m. fredericks, k. m. bowers, uncertainty in big data analytics: survey, opportunities, and challenges, journal of big data, 6 (2019), art. no. 44. doi: 10.1186/s40537-019-0206-3 [3] h. qi, s. sitharama iyengar, k. chakrabarty, distributed sensor networks a review of recent research, journal of the franklin institute, vol. 338, 2001, no. 6, pp. 655-668. doi: 10.1016/s0016-0032(01)00026-6 [4] h. khan, types of ai | different types of artificial intelligence systems, 2021. online [accessed 13 match 2023] https://www.researchgate.net/publication/355021812 [5] l. tucci, a guide to artificial intelligence in the enterprise, r. h. hariri, e. m. fredericks, k. m. bowers, uncertainty in big data analytics: survey, opportunities, and challenges, tech targetsearch enterprise ai: e-guide, 2021. online [accessed 13 match 2023] https://www.techtarget.com/. [6] m. m. r. monjur, j. heacock, j. calzadillas, m. d. s. mahmud, j. roth, k. mankodiya, e. sazonov, q. yu, hardware security in sensor and its networks, frontiers in sensors, 3 (2002). doi: 10.3389/fsens.2022.850056 [7] f. nekoogar, f. dowla, r. twogood, s. lefton, secure rfid tag or sensor with self-destruction mechanism upon tampering, united states patent: document id: us 20150339568 a1, 2016. [8] m. moni, w. melo, jr., d. peters, r. mochado, when measurements meet blockchain: on behalf of an inter-nmi network, national library of medicine, 2021, doi: 10.3390/s21051564 [9] iso/iec, iso/iec 27005:2018 information technology security techniques information security risk management, international organization for standardization, june 2011. [10] lum, bipm, iec, ifcc, ilac, iso, iupac, iupap, and oiml. guide to the expression of uncertainty in measurement, jcgm 100:2008, gum 1995 with minor corrections. bipm, 2008. [11] monte carlo, bipm, iec, ifcc, ilac, iso, iupac, iupap, and oiml. supplement 1 to the ‘guide to the expression of uncertainty in measurement’ – propagation of distributions using a monte carlo method, jcgm 101:2008. bipm, 2008. [12] bayesian statistical models, bipm, iec, ifcc, ilac, iso, iupac, iupap, and oiml. guide to the expression of uncertainty in measurement — part 6: developing and using measurement models, jcgm gum-6:2020. bipm, 2020. [13] en 12830:2018 temperature recorders for the transport, storage and distribution of temperature sensitive goods [14] welmec guide 7.2, online [accessed 13 march 2023] http://welmec.org. figure 7. an example of the complex sensor network according to according to en 12830:2018 with application of requirements. https://doi.org/10.30844/i40m_19-1_s27-30 https://doi.org/10.1186/s40537-019-0206-3 https://doi.org/10.1016/s0016-0032(01)00026-6 https://www.researchgate.net/publication/355021812 https://www.techtarget.com/ https://doi.org/10.3389/fsens.2022.850056 https://doi.org/10.3390/s21051564 http://welmec.org/ calculating vessel capacity from the neolithic sites of lugo di grezzana (vr) and riparo gaban (tn) through 3d graphics software acta imeko issn: 2221-870x march 2022, volume 11, number 1, 1 10 acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 1 calculating vessel capacity from the neolithic sites of lugo di grezzana (vr) and riparo gaban (tn) through 3d graphics software andrea tavella1, marika ciela1, paolo chistè1, annaluisa pedrotti1 1 labaaf, laboratorio bagolini archeologia archeometria fotografia, university of trento, via tommaso gar 14, 38122 trento, italy section: research paper keywords: vessel capacity; ceramics; 3d models; blender; neolithic citation: andrea tavella, marika ciela, paolo chistè, annaluisa pedrotti, calculating vessel capacity from the neolithic sites of lugo di grezzana (vr) and riparo gaban (tn) through 3d graphics software, acta imeko, vol. 11, no. 1, article 7, march 2022, identifier: imeko-acta-11 (2022)-01-07 section editor: fabio santaniello, university of trento, italy received march 7, 2021; in final form february 23, 2022; published march 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: andrea tavella, e-mail: andrea.tavella@live.com 1. the ceramic record the study takes into account ceramic finds from the neolithic sites of lugo di grezzana and riparo gaban. both sites show a frequentation during early neolithic and play a key role for the understanding of neolithization process in the northern italy (figure 1). 1.1 lugo di grezzana (vr) the area to the south of the small town of lugo di grezzana, called locality campagne, is situated over a river terrace (300 m above sea level) along valpantena, a short prealpine valley located in the lessini mountains [1]. the discovery of the site dates back to 1990 by fernando zanini and giorgio chelidonio. the area has been the object, since the early nineties, of systematic research undertaken by the archaeological heritage of veneto region, in collaboration with the university of trento since 1996 (b. bagolini laboratory – labaaf) up until 2005 [2]. the first evidence is dated in the middle of the 6th millennium bc cal, while an intense occupation of the area is dated between 5300 – 5050 bc cal [3]. figure 1. the localization of lugo di grezzana (verona, italy) and riparo gaban (trento, italy). abstract this paper reports new data about the estimation of the volumetric capacity of ceramic vessels from the neolithic sites of lugo di grezzana (verona, italy) and riparo gaban (trento, italy). the methodological protocol is based on a free and open source 3d computer graphics software, called blender®. the estimate of the volumetric capacity has been relied from the graphic elaboration of the archaeology drawing of the artifacts. through the calculation of volume has been possible to obtain an estimation of the total capacity of the vessels, proposing two types of content. subsequently, the volumetric data was related to diameter/height ratio of each ceramic vessel, in order to define a range of variability in each typological class. data from both sites were later compared, highli ghted for the most part of them a specific distribution that could be a consequence of different functional uses and/or cultural models. this paper concludes the preliminary results presented at the 2020 imeko tc4 international conference on metrology and archaeology for cultural heritage. mailto:andrea.tavella@live.com acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 2 based on material culture [2]-[5], the site is mainly attributed to fiorano, which is present in northern italy during the early neolithic and shows a typical homogeneity in vessels typology. jug is possibly one of the most distinctive shapes of the fiorano culture (figure 2) and is often imported into contemporary cultures. although, numerous elements have been permitted to underline influences from other contexts such as vhò group, adriatic impressed ware and catignano cultures [2], mainly due to the supply of lessinian flint. the latter, thanks to its highquality, becomes the object of exchange par excellence and a sort of common denominator between the various groups of the early neolithic in northern italy, between the middle of the 6th and the beginning of the 5th millennium bc [3], [6]. around 5000 bc cal, the occupation of the settlement seems to show a temporary interruption with the occurrence of colluvial episodes, while the last neolithic occupation, scanty represented, is attested between 4900 and 4800 bc cal. during this period, the early geometric-linear style of square mouthed pottery culture is already widespread in the northern italy, the latter attested within the site in contemporary with later aspects of fiorano culture [3]. 1.2 riparo gaban (tn) the site of riparo gaban is located at piazzina di martignano, in a small hanging valley that runs parallel to the left side of adige valley (270 metres above sea level), a few kilometres north-west of trento [7]. the site, identified as a rock-shelter, has been discovered in 1970 by a group of local amateurs as a part of the palaeoethological activities of the museo tridentino di scienze naturali. the excavations have been conducted under the technical direction of bernardino bagolini from 1972 to 1981, by alberto broglio and stefan k. kozlowski from 1982 to 1985 for the mesolithic phases [8], [9]. the site is characterised by a complex stratigraphic evolution from mesolithic to middle bronze age, with a stratigraphic continuity between castelnovian mesolithic and local early neolithic deposits, these latter dated between the end of 6th and the beginning of 5th millennium bc. the site is one of the main pieces of evidence for the understanding of first neolithic evidences in trentino alto-adige and gives its name to the cultural group presents in the adige valley during this period. unlike lugo di grezzana and generally to the fiorano culture, the main aspect of the gaban group appears to be a strong mesolithic component. especially observed in the lithic and bone industries, with extraordinary examples of mobiliary art that gives to the site, not only the appearance of a simple rockshelter but probably also a magical-religious connotation [8], [10]. from a typological point of view, the gaban group, despite a markedly autonomous framework, presents several connections with others cultural groups of the early neolithic, in particular with isolino and vhò groups, and to a lesser extent with fiorano [11]. about the material culture identified at riparo gaban (figure 3), the stratigraphic evolution allowed to observe an oldest phase characterised by a strong presence of impressed ware and a more recent phase where scratched pottery is more attested [11]. the neolithic occupation is interrupted as from 4700 bc cal, documenting a possible phase of abandonment of the rockshelter up to 2700 bc cal [8]. 2. the calculation of volumetric capacity the volumetric estimate of a vessel can be calculated mainly through three types of volume calculation: direct measurements, two-dimensional geometrical methods (manual calculation), and computer-assisted methods, these latter based on 3d models (automatic calculation). direct measurements are taken from the container and allow directly to obtain the volumetric capacity. these methods involve filling the vessel with a suitable material able to adapt to the internal profile. however, they cannot be applied to the entire ceramic record, both because usually a limited percentage of vessels from archaeological excavations are complete or partially reconstructed, and also for conservation issues [12]-[14]. the second method, about manual calculation, is based on the decomposition of the vessel volume into basic forms (spheres, cylinders, or truncated cones) and calculated through mathematical formulae [15]-[22]. the archaeological drawing represents the starting point of this method and, unlike direct measurements, does not require the availability of the archaeological find. however, the degree of approximation represents a negative aspect, as the complex form of the vessel is transformed into simplified form, although this depends on the geometric shape used. the last method, computer-assisted, is focused on 3d models. here too, the volumetric capacity is obtained through measurements directly on the archaeological drawings, exploiting the principle of symmetry. different software can be used, such as: autocad®, rhinoceros™ and blender® [14], [23]. in addition, other suitable programs are available as kotyle© [24] and web applications like capacity [12], [25], [26]. in this study, the 3d graphics program of choice is blender® [27] since it is free and open source, which allows the users to generate extensions in order to improve it. the estimate of the volumetric calculation was relied on the 3d-print toolbox extension, although different add-ons are known to be effective as well [23]. regarding the study of ceramic record, it is important to refer to the digitalization in 3d of some pottery mentioned in this paper through photogrammetry. this work was carried out at tefalab (laboratorio di tecniche fotografiche avanzate, unit figure 2. vessel from the neolithic site of lugo di grezzana (photo p. chistè – labaaf) [2]. figure 3. vessel from the neolithic site of riparo gaban (photo e. turco) [10]. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 3 of labaaf, university of trento) under the technical direction of paolo chistè. about the study of volumetric capacity, are only present preliminary results for the site of lugo di grezzana [28], while for riparo gaban this aspect of research has not been studied yet. at the present time, a systematic analysis that evaluates the metric criteria of the ceramics of the fiorano culture has not yet been carried out [29] and more. however, for the neolithic of northern italy there is a typological classification of the vessels that distinguishes their morphology in relation to the profile, the diameter/height ratio (ø/ℎ), and the size of the mouth [30]. nevertheless, this classification does not include the volumetric capacity parameter. 3. material and methods the methodological protocol was applied to a selection of 48 archaeological drawing, of which 35 from lugo di grezzana and 13 from riparo gaban (figure 6 and figure 7). the sample analysed was chosen taking into consideration the typological classification. out of the total samples, 26 drawings illustrate whole artifacts, with a continuous profile from the rim to the bottom of the vessel (lugo di grezzana: 15 samples; riparo gaban: 11 samples), while the other are only partially preserved. for the latter, as in the previous study [28], it was therefore necessary to hypothesize the profile and the height of the vessel. to carry out this operation, the fragmented samples were integrated through the study of whole ceramic vessels belonging to the same typological class (figure 4). for both groups, and especially the latter, it is essential to keep in mind that the capacity estimate deduced from archaeological drawings has a different degree of accuracy. the manual drawing is based on one radial section and represents a two-dimensional shape of the vessel not taking into account the possible variations in the three-dimensional shape of the object [31]. the development of the operating methodology is based on three minimum requirements that ceramics and designs must have: • availability of diameter and internal profile; • scale of representation; • high-resolution drawing (d.p.i.); the calculation of the volumetric capacity is carried out by importing each drawing into the 3d graphic program (blender version 2.92), providing the exact graphic resolution of the file (d.p.i.). this step is necessary in order to avoid any change in the original dimensions of the imported drawing that would therefore entail an incorrect estimate of the volume. subsequently it is generated a curve (bezier), which is modified along the x and y axes and divided into several segments, in order to trace the underlying drawing. after obtaining a 2d profile, it is necessary generate a line (path), which will correspond with the rotation axis of the curve itself and with the midline of the archaeological drawing. once the rotation axis is fixed, the curve can be rotated 360 degrees. this procedure requires to define some options, namely: the cartesian axis to which the curve is oriented, the object around which the rotation takes place and lastly the number of segments the revolution is divided into (a greater number of these entail corresponds to a better graphic resolution and consequently a more accurate estimate of the volume). figure 4. typological table with some samples used to hypothesize the profile and the height of the vessels: 1-3, 6, 8-11 from lugo di romagna (ra) [32]; 45, 7 from vhò di piadena-campo ceresole (cr) [33]. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 4 the essential step for obtaining the volume is the closure of the solid at the rim and at the base. once the solid is closed, the calculation of the volumetric capacity is performed automatically using the add-on: 3d-print toolbox (available since version 2.67, released in may 2013), which volume is expressed in cm3 (figure 5). the validity of the procedure was previously established during the formulation of the method, through the graphic reproduction and the volumetric calculation of a cylinder of known dimensions (𝑟 = 5 cm; ℎ = 20 cm). this procedure allowed to calculate the absolute and relative error in the method developed, taking into account the tolerance. the latter is characterised by different causes such as: the inherent uncertainty regarding the measured object, the conservation status, the operator, the procedure and the measuring instrument used. taking these issues into account, it was calculated a tolerance of about ± 1 mm. 𝐴𝑏𝑠𝑜𝑙𝑢𝑡𝑒 𝐸𝑟𝑟𝑜𝑟 (𝐸𝐴) = (𝑉𝑜𝑙max − 𝑉𝑜𝑙min) 2 = 70.6889 𝑐𝑚3 𝑅𝑒𝑙𝑎𝑡𝑖𝑣𝑒 𝐸𝑟𝑟𝑜𝑟 (𝑅𝐸) = 𝐸𝐴 𝑉𝑜𝑙avg = 0.0449 𝑃𝑒𝑟𝑐𝑒𝑛𝑡𝑎𝑔𝑒 𝐸𝑟𝑟𝑜𝑟 (𝑃𝐸) = 𝑅𝐸 × 100 % = 4.49 % . the methodological approach was subsequently extended, considering two hypothetical types of contents, a liquid and a solid one. as to what concerns the estimate of the capacity, it was treated converting the measure from cm3 to ml (1 cm3 = 1 ml). instead, in the case of solids contents was calculated the weight (grams) of three types of cereals such as: whole barley, emmer and naked wheats, selected accordingly to the data collected from archaeobotanical analysis carried out for the site of lugo di grezzana [34]. the weights were estimated in relation to the bulk density of each kind of cereal (whole barley 0.61 ÷ 0.69 g/ml, emmer 0.47 g/ml e naked wheats 0.54 g/ml) [35], [36] and the volumes of the containers, according to the following formula: 𝑊𝑒𝑖𝑔ℎ𝑡 = 𝐵𝑢𝑙𝑘 𝑑𝑒𝑛𝑠𝑖𝑡𝑦 × 𝑉𝑜𝑙𝑢𝑚𝑒 . lastly, metrical analysis were carried out through the correlation of the maximum volumetric capacity (cm3), diameter and height ratio (ø/ ℎ), and typology [30]. figure 5. summary scheme of the operating methodology performed with blender®. table 1. summary of results from lugo di grezzana (l.g.) and riparo gaban (r.g.). legend: bw. = bowl; cp. = cup; h. v. = handle vessel; jg. = jug; l.b. = large bowl; mn. = miniaturistic; n.v. = necked vessel; pt. = pot; t.v. = truncate cone-shaped vessel; * = partially preserved. emmer naked wheats ø/h ratio l.g. bw. 1* 545 332 376 256 294 2,45 l.g. bw. 2* 1014 619 700 477 548 3,03 l.g. bw. 3* 1680 1025 1159 789 907 2,39 l.g. bw. 4* 3755 2290 2591 1765 2028 1,97 l.g. l.b. 1* 5276 3218 3640 2480 2849 3,23 l.g. l.b. 2 6959 4245 4802 3271 3758 2,74 l.g. l.b. 3* 4814 2936 3322 2263 2599 2,42 l.g. l.b. 4* 6763 4125 4666 3178 3652 2,18 l.g. t.v. 1 5646 3444 3895 2653 3049 0,92 l.g. t.v. 2* 17307 10557 11942 8134 9346 1,08 l.g. t.v. 3 2967 1810 2047 1395 1602 0,97 l.g. t.v. 4* 1329 811 917 625 718 0,77 l.g. t.v. 5* 5941 3624 4099 2792 3208 0,82 l.g. t.v. 6 944 576 652 444 510 1,00 l.g. t.v. 7 3961 2416 2733 1862 2139 0,80 l.g. t.v. 8* 5235 3193 3612 2460 2827 0,87 l.g. t.v. 9* 750 458 518 353 405 1,08 l.g. t.v. 10* 1521 928 1049 715 821 0,84 l.g. t.v. 11* 2112 1288 1457 993 1140 0,96 l.g. h.v. 1 925 564 638 435 499 1,16 l.g. jg. 1 413 252 285 194 223 0,86 l.g. jg. 2 1825 1114 1260 858 986 0,83 l.g. jg. 3 772 471 533 363 417 0,83 l.g. jg. 4 839 512 579 394 453 0,74 l.g. jg. 5* 653 398 451 307 353 0,88 l.g. jg. 6* 889 542 614 418 480 0,83 l.g. jg. 7* 1508 920 1040 709 814 0,87 l.g. jg. 8* 2022 1234 1395 951 1092 0,85 l.g. n.v. 1* 6054 3693 4177 2845 3269 0,47 l.g. n.v. 2* 13378 8161 9231 6288 7224 0,26 l.g. ld. 1 38 23 27 18 21 1,84 l.g. mn. 1 53 33 37 25 29 0,76 l.g. mn. 2 158 97 109 74 86 2,24 l.g. mn. 3 59 36 41 28 32 1,14 l.g. mn. 4 79 48 54 37 42 2,50 r.g. bw. 1 2858 1743 1972 1343 1543 1,52 r.g. t.v. 1* 2894 1765 1997 1360 1563 0,97 r.g. t.v. 2 17135 10452 11823 8053 9253 1,11 r.g. t.v. 3 1589 969 1097 747 858 1,18 r.g. t.v. 4* 1290 787 890 606 697 0,82 r.g. cp. 1 397 242 274 186 214 1,19 r.g. cp. 2 403 246 278 190 218 1,10 r.g. cp. 3 764 466 527 359 413 1,38 r.g. cp. 4 1406 858 970 661 759 0,96 r.g. cp. 5 2023 1234 1396 951 1092 1,02 r.g. jg. 1 645 393 445 303 348 0,82 r.g. pt. 1 1668 1017 1151 784 901 0,52 r.g. mn. 1 60 36 41 28 32 1,58 estimate solid content (g) estimate liquid content (ml) samples whole barley acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 5 figure 6. typological table of the samples analysed during the study from lugo di grezzana (l.g.) and riparo gaban (r.g.). scale drawing 1:6. legend: bw. = bowl; l.b. = large bowl; t.v. = truncate cone-shaped vessel; * = partially preserved. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 6 figure 7. typological table of the samples analysed during the study from lugo di grezzana (l.g.) and riparo gaban (r.g.). scale drawing 1:6. legend: cp. = cup; h. v. = handle vessel; jg. = jug; mn. = miniaturistic; n.v. = necked vessel; pt. = pot; t.v. = truncate cone-shaped vessel; * = partially preserved. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 7 4. results the methodological approach allowed to provide an estimate of the capacity (ml) and the weight of different contents (g). at the same time, it was possible to correlate the values determined by the computer-assisted calculations with the ratio between diameter and height (table 1), distinguishing them based on typology1 (table 2). the elaboration of the data took place through the compilation of a scatter plot, reporting the volumetric capacity in the x axis and the ø/ℎ ratio in the y axis (figure 8). from a volumetric point of view, in either case the same degree of variation is observed, where the maximum limit is about 17,000 cm3 and is represented by two truncate coneshaped vessels (l.g. t.v. 8*; r.g. t.v. 2). at the same time, however, the distribution of the samples is different. in the case of riparo gaban, almost all samples (12 out of 13) have a volumetric capacity lower than 3000 cm3, while for the site of lugo di grezzana this aspect is found in two-third of the samples (23 out of 35), showing a wider volumetric variability (figure 8). a similar distribution is observed for the ø/ℎ ratio, which is wider for lugo di grezzana (0.26 ÷ 3.23) than riparo gaban (0.52 ÷ 1.58). these dissimilarities are due to the absence of some ceramic forms (large bowls, necked vessels) or the presence in a smaller percentage (bowls, truncate cone-shaped vessels) in the dataset of riparo gaban. 1 in this study, the distinction between jugs and mugs follows the typological classification defined in banchieri et al. 1999 [30]. for some typological classes, better represented, it was possible to make a comparison of the samples between the two investigated sites (table 2). bowls: represented by 5 samples (4 of which are partially reconstructed). the ceramic samples are characterised by a volume between 545 and 3755 ml, containing between 256 and 2591 g of solid content and a ø/ℎ ratio between 1.52 and 3.03. although only one samples comes from riparo gaban and most of the volumes are reconstructed, there is a distinction on the basis of ø/ℎ ratio, estimated between 1.97 and 3.03 for lugo di grezzana, compared to a lower value for riparo gaban equal to 1,52. jugs: represented by 9 samples (4 of which are partially reconstructed). the ceramic samples are characterised by a volume between 413 and 2022 ml, containing between 194 and 1395 g of solid content and a ø/ℎ ratio between 0.74 and 0.88. although most of samples comes from lugo di grezzana, a limited variability both on the volumetric data and in particular about ø/ℎ ratio is observed. the lower range of ø/ℎ ratio allowed to identify jugs as the ceramic shape with the highest degree of homogeneity compared to the other typological classes. truncate cone-shaped vessels: represented by 15 samples (9 of which are partially reconstructed). the ceramic samples are characterised by a volume between 750 and 17307 ml, containing between 353 and 11942 g of solid content and a ø/ℎ ratio table 2. summary of results organised for typological class. samples samples partially preserved estimate liquid content (ml) estimate solid content (g) ø/h ratio samples samples partially preserved estimate liquid content (ml) estimate solid content (g) ø/h ratio samples samples partially preserved estimate liquid content (ml) estimate solid content (g) ø/h ratio bowl 5 4 545 ÷ 3755 256 ÷ 2591 1,52 ÷ 3,03 4 4 545 ÷ 3755 256 ÷ 2591 1,97 ÷ 3,03 1 2858 1343 ÷ 1972 1,52 large bowl 4 3 4814 ÷ 6959 2263 ÷ 4802 2,18 ÷ 3,23 4 3 4814 ÷ 6959 2263 ÷ 4802 2,18 ÷ 3,23 truncate coneshaped vessel 15 9 750 ÷ 17307 353 ÷ 11942 0,77 ÷ 1,18 11 7 750 ÷ 17307 353 ÷ 11942 0,77 ÷ 1,08 4 2 1290 ÷ 17135 606 ÷ 11823 0,82 ÷ 1,18 internal handles vessel 1 925 435 ÷ 638 1,16 1 925 435 ÷ 638 1,16 mug 5 397 ÷ 2023 186 ÷ 1396 0,96 ÷ 1,38 5 397 ÷ 2023 186 ÷ 1396 0,96 ÷ 1,38 jug 9 4 413 ÷ 2022 194 ÷ 1395 0,74 ÷ 0,88 8 4 413 ÷ 2022 194 ÷ 1395 0,74 ÷ 0,88 1 645 303 ÷ 445 0,82 pot 1 1668 784 ÷ 1151 0,52 1 1668 784 ÷ 1151 0,52 necked vessel 2 2 6054 ÷ 13378 2845 ÷ 9231 0,26 ÷ 0,47 2 2 6054 ÷ 13378 2845 ÷ 9231 0,26 ÷ 0,47 ladle 1 38 18 ÷ 27 1,84 1 38 18 ÷ 27 1,84 miniaturistic 5 53 ÷ 158 25 ÷ 109 0,76 ÷ 2,5 4 53 ÷ 158 25 ÷ 109 0,76 ÷ 2,5 1 60 28 ÷ 41 1,58 riparo gabantotal typological classification lugo di grezzana acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 8 between 0.77 and 1.18. both datasets are characterised by a homogeneity about volumetric capacity (lugo di grezzana: 750 ÷ 17307 ml, 353 ÷ 11942 g; riparo gaban: 1290 ÷ 17135 ml, 606 ÷ 11823 g) and ø/ℎ ratio (lugo di grezzana: 0.77 ÷ 1.08; riparo gaban: 0.82 ÷ 1.18). miniaturistic forms: represented by 6 samples. the ceramic samples are characterised by a volume between 53 and 158 ml, containing between 25 and 109 g of solid content and a ø/ℎ ratio between 0.76 and 2.5. the dataset is represented by different ceramic shapes, with a wide variability of ø/ℎ ratio and only associated for the limited dimensions. 5. discussion the methodological protocol has led to obtain an analysis of the volumetric capacity of a wide selection of samples, correlating the volumetric data to diameter/height ratio and typological class of each ceramic vessel. compared to the previous study [28], the increase in the number of samples allowed to provide further information on individual typological class, especially in those most attested such as: bowls, large bowls, truncate cone-shaped vessels, cups, jugs and miniaturistic forms. the different distribution of the ceramic assemblages (figure 8) highlighted a wider distribution of samples from lugo di grezzana both on the volumetric estimates (mainly within 6000 cm3) and in the ø/ℎ ratio (between 0.26 and 3.23). while in the case of riparo gaban, both the volumetric range (mainly within 3000 cm3) and the ø/ℎ ratio (between 0.56 and 1.58) represent a lower variability. the different distribution could depend on different reasons, both due to a poor conservation of some typological classes (e.g. necked vessels, bowls and large bowls), not allowing an estimate of the volume, and due to a different connotation of riparo gaban compared to the settlement of lugo di grezzana. this later characterised by numerous structural complexes [3], [37], [38], as well as a larger number of ceramic finds. about the distribution of each typological class in relation to the parameters examined, for the most part of them it was possible to highlight a specific distribution, according to four trends: • limited volumetric range and limited ø/ℎ ratio (jugs, mugs); • limited volumetric range and wide ø/ℎ ratio (miniaturistic forms); • wide volumetric range and limited ø/ℎ ratio (necked vessels, truncate cone-shaped vessels); • wide volumetric range and wide ø/ℎ ratio (bowls, large bowls); for each typological class, the greater or lesser volumetric capacity and ø/ℎ ratio could provide new information about the research. for instance the case of jugs, where a limited ø/ℎ ratio allowed to recognize a degree of homogeneity higher compared to the other typological classes. this aspect could result by several factors, such as: the attribution of the ceramic shape to a limited number of function and/or the evidence of a model widely shared within the fiorano culture, where jugs are one of the most distinctive shapes of this material culture [39]. similar case for truncate cone-shaped vessels, typical of vhò group, where the wide volumetric range could be the outcome of a figure 8. scatter plot between volumetric capacity (x axis) and diameter/height ratio (y axis). lugo di grezzana (l.g. = green), riparo gaban (r.g. = red); legend: bw. = bowl; cp. = cup; h. v. = handle vessel; jg. = jug; l.b. = large bowl; mn. = miniaturistic; n.v. = necked vessel; pt. = pot; t.v. = truncate cone-shaped vessel. 0,00 0,50 1,00 1,50 2,00 2,50 3,00 3,50 0 2000 4000 6000 8000 10000 12000 14000 16000 18000 20000 l.g. bw. l.g. h.v. l.g. jg. l.g. l.b. l.g. ld. l.g. mn. l.g. n.v. l.g. t.v. r.g. bw. r.g. cp. r.g. jg. r.g. mn. r.g. pt. r.g. t.v. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 9 plurality of technological aspects. for example, aspects like an unrestricted orifice, thick walls and bases, in particular in large samples for increase stability, could be assumed as dry storage vessels [20], but an evaluation of their functionality is not possible yet. regarding the functions of each ceramic class, although the volumetric capacity depends upon its shape and size, it was not possible to formulate a direct relationship with the function. as has been seen in rice [20], pots are multifunctional with primary or secondary uses before being abandoned. in other words, the relation between use and capacity of a vessel depends on several considerations like the amount and kind of contents (liquid or solid), the duration of storage, the number of uses, microenvironmental factors or other necessities [40], [41]. to understand this complexity, volumetric and typological aspects must be related to other technological criteria like petrographic analysis, surface treatment processes (smoothing, polishing, slip) [42], use-wear and organic residues [43]-[45]. ceramic paste and manufacturing analysis are under study and will be able to provide new interpretative ideas about the functionality of each typological class. 6. conclusion this study aimed to provide new data about the estimation of the volumetric capacity of ceramic vessels from the neolithic sites of lugo di grezzana and riparo gaban, with the aid of 3d graphic software. the automatic calculation, based on a reconstruction of the vessel through blender®, represents an efficient method for the estimation of the volumetric capacity since: allow to work directly on the bibliography available, the volumetric calculation takes place in just few steps with very reliable results and sufficiently valid to be applied to an archaeological study. the results show a different distribution of the ceramic dataset that could depend on different reasons like a poor conservation of some typological classes, that would not allow to estimate the volume or due to a different connotation of the two sites. about the distribution of each typological class for the most part of them it was possible to highlight a specific distribution of the results, according to four trends, each of them could be conditioned by functional uses and/or cultural models. to date, numerous questions have therefore emerged and remain unresolved, especially regarding jugs and truncate coneshaped vessels. could a limited volumetric range and ø/ℎ ratio of the jugs be an evidence of a restricted number of functions? at the same, its homogeneity is shared within other sites belonging to the fiorano culture? how far is it diversified compared to other contemporary cultures of the northern italy? conversely in the case of truncate cone-shaped vessels, could a wide volumetric range and limited ø/ℎ ratio represent a plurality of technological aspects compared to other typological class? in general terms, the study of vessel capacity is one of the parameters necessary for the functional understanding of the artifacts. however, only through a systematic application of this method to other contemporary sites and the evaluation of further investigation parameters (currently in progress), it will be possible to obtain more information about the functionality of ceramic vessels. acknowledgement the research project is conducted by the research laboratory labaaf (laboratorio bagolini archeologia archeometria fotografia) belongs to ceasum (centro di alti studi umanistici) of the university of trento. the project has prof.ssa annaluisa pedrotti as scientific manager and paolo chistè as technical director of tefalab (laboratorio di tecniche fotografiche avanzate, unit of labaaf). all the authors equally contributed to research and writing. references [1] f. cavulli, d. e. angelucci, a. pedrotti, la successione stratigrafica di lugo di grezzana (verona), preistoria alpina 38.8 (2002), pp. 89-107. 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overview of the modified magnetoelastic method applicability acta imeko issn: 2221-870x september 2021, volume 10, number 3, 167 176 acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 167 overview of the modified magnetoelastic method applicability tomáš klier1, tomáš míčka1, michal polák2, milan hedbávný3 1 pontex, bezová 1658, 147 14, prague 4, czech republic 2 faculty of civil engineering, czech technical university in prague, thákurova 7, 166 29, prague 6, czech republic 3 freyssinet cz, zápy 267, 250 01, brandýs nad labem, czech republic section: research paper keywords: tensile force; magnetoelastic method; prestressed strand; prestressed cable; sensor citation: tomáš klier, tomáš mícka, michal polák, milan hedbávný, overview of the modified magnetoelastic method applicability, acta imeko, vol. 10, no. 3, article 23, september 2021, identifier: imeko-acta-10 (2021)-03-23 section editor: lorenzo ciani, university of florence, italy received february 9, 2021; in final form july 30, 2021; published september 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was supported by ministry of industry and trade of the czech republic. corresponding author: tomáš klier, e-mail: tkl@pontex.cz 1. introduction five experimental techniques are usually applied in civil engineering practice for evaluation and verification of actual values of axial tensile forces in important structural elements on building constructions. if the total value of the tensile force in the investigated structural elements has to be determined, the two of these methods (namely, the direct measurement of the force by a preinstalled load cell and the approach based on a strain measurement with strain gauges) can be applied only for an experiment in which the applied sensors were installed before the investigated structural elements were activated. compared to that, the next three methods (namely, the vibration frequency method [1], [2], the force determination in a flexible structural element based on the relation between the transverse force and the caused transverse displacement [3]-[5] and the magnetoelastic method [6]-[24] can be used during newly started experiments on existing structures that have already been in service for some time. the basic advantage of these three methods is that the investigated structural elements remain activated all the time (namely, in the period of the structure service, the experiment preparation, the experiment realization and also after the experiment completion). however, the applicability of the method using the relation between the transverse force and the element transverse displacement is significantly limited in practice. it can be effectively applied only for the flexible elements with a relatively small cross section, as are strands with diameter smaller than approximately 25 mm, and with a relatively long free length, because the measurable transverse displacement increases considerably the observed tensile force in the substantially short abstract a requirement of axial force determination in important structural elements of a building or engineering structure during its construction or operational state is very frequent in technical practice. in civil engineering practice, five experimental techniques are usually used for evaluation of axial tensile forces in these elements. each of them has its advantages and disadvantages. one of these methods is the magnetoelastic method, that can be used, for example, on engineering structures for experimental determination of the axial forces in prestressed structural elements made of ferromagnetic materials, e.g., prestressed bars, wires and strands. the article presents general principles of the magnetoelastic method, the magnetoelastic sensor layout and actual information and knowledge about practical application of the new approach based on the magnetoelastic principle on prestressed concrete structures. subsequently, recent results of the experimental verification and the in-situ application of the method are described in the text. the described experimental approach is usable not only for newly built structures but in particular for existing ones. furthermore, this approach is the only one effectively usable experimental method for determination of the prestressed force on existing prestressed concrete structures in many cases in the technical practice. mailto:tkl@pontex.cz acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 168 investigated elements even in the order of tens of percent in some special cases. similarly, the vibration frequency method is suitable for application on structural elements with a relatively long free vibrating length and with a relatively low bending stiffness. if this method is used on the relatively short one, the uncertainty of the specified tensile force is significantly influenced by the element bending stiffness and by the element boundary conditions, especially if they are complicated and vague. in this case, the results obtained by the vibration frequency method can be improved not only by using measured natural frequencies of the element but also by using measured mode shapes in the process of results evaluation [1], [2]. however, the time necessary for realization of such experiment is significantly longer compared to the similar one, when only the natural frequencies of the elements are evaluated, especially in the situation, if the tested elements are difficult to access for the installation of vibration sensors, as are, for example, cable stays on cable-stayed bridges. then the time required for the experiment can be more than ten times longer. the next advantages and disadvantages of all five above mentioned experimental techniques are discussed in more detail in the reference [11]. the selection of the most suitable experimental method for a particular practical application depends on specific element parameters and specific conditions in which the experiment is going to be performed. according to the authors opinion, the modified magnetoelastic method is the only one that can be applied effectively and expediently for evaluation of the prestressed force on existing structures made from the prestressed concrete when the prestressed reinforcement is embedded inside the concrete. 2. related results in the literature in civil engineering practice, the utilization of the magnetoelastic method for experimental evaluation of axial tensile forces in structural elements started about thirty-five years ago. the original inventors developed gradually the method theory, the magnetoelastic sensors (hereinafter the me sensors) and their practical utilization. they published their obtained knowledge regularly, see [12]-[21] for example. the me sensors and their appropriate equipment that have been standardly used in civil engineering practice in the recent past and at present [12]-[24] evaluate measured prestressed forces in a relatively simple way. these standard me sensors are composed from two basic parts only, from the primary and secondary coil. this is the minimal possible configuration of the me sensor, as it is described below. the basic advantages of the application of the me sensor in its standard configuration are that the tensile force in the structural element is evaluated contactless, the observed element is not locally deformed and its anti-corrosive layer is not abraded, the sensor body is robust, long lasting and resistant to accidental mechanical damage. it is possible to evaluate the instantaneous magnitude of the tensile force with high accuracy. however, the important requirement for high accuracy of the obtained results is the sensitivity assessment of each particular standard me sensor in concrete conditions its practical application using an independent force sensor. in the case of the prestressed reinforcement (namely the strand or the cable), the force sensor is used as a part of a hydraulic jack and therefore it is necessary to install the me sensor before the activation of the observed prestressed reinforcement. an additional installation of the standard me sensor on the activated prestressed reinforcement is, of course, technologically possible. however, it is time consuming and the sensor sensitivity assessment cannot be realized in the concrete conditions of the magnetic surroundings of the location where the me sensor is installed. the modified magnetoelastic method, its physical principle, the fully equipped me sensor, an experiment on a real structure and its result evaluation are described in more details in reference [9]. other supplementary information about the method, its practical applications and about a removable me sensor can be found in references [6]-[8] and [10],[11]. 3. description of the method the method is based on an experimental estimation of the magnetic response of the tensile stressed structural element on an external magnetic field. the magnetic field intensity h and the magnetic flux density b are ones of the basic physical quantities describing the magnetic field arrangement. the relation between b and h in the form of the so-called hysteresis loop is given by the kind of material exposed to the effects of the magnetic field, its properties and its current conditions (e.g., its tensile stress and temperature). for the purposes of applications of the modified magnetoelastic method, differently arranged me sensors are used depending on the specific experiment and its concrete conditions. a diagram of a fully equipped cylindrical me sensor is shown in figure 1 that was adopted from reference [9]. fundamental components of this me sensor variant are a controlled magnetic field source (for example, the primary coil that is drawn in figure 1), a sensor of magnetic field intensity "h" in a measured cross section (the system of hall's sensors and/or the secondary coil 2), a sensor of magnetic flux that is closely related to the magnetic induction "b" in the measured section of the strand (the secondary coil 1) and the me sensor protection against magnetic influences from its surroundings (the steel shield). the function and principle of hall´s sensors were explained in more detail in the reference [10]. the fully featured me sensor offers the greatest possibilities to increase accuracy and reduce uncertainties in evaluating of the tension force in the observed prestressed element. on the other side, the fully equipped me sensor is spatially larger and that may restrict its applicability in some practical cases and it is also more figure 1. diagram of a fully equipped me sensor published also in [9] acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 169 complicated. this complexity affects its longer production time, higher requirements for its application in civil engineering practice and as well as higher requirements on the used measuring system and its equipment. the above-described standard me sensors (see chapter 2) represent the minimalist variant of the me sensor that consists of a primary coil and a secondary coil 1 only. the intensity of the magnetic field "h" is determined, in this case, indirectly from a completely different physical quantity, which is the electric current flowing through the primary coil. however, there is a risk that results from this approach with application of the minimalist variant of the me sensor can be affected. any change in the magnetic surroundings in the sensor vicinity (e.g., a removal of a massive steel falsework after concrete hardening) causes completely "silently" substantial or even severe changes in sensor parameters. the sensor's steel shielding reduces the impact of this effect on the obtained results. the quality of the me sensor shielding influences the level of this reduction, however, it is never one hundred percent. for example, the application of the minimum me sensor configuration on a prestressed prefabricated concrete structure reinforced by steel-fibre is completely unusable. 4. experimental analysis of evaluating curves for the purpose of the modified magnetoelastic method, magnetic behaviour of the standard prestressed elements used both in the past and today is appropriate and necessary to know. in november 2019, a laboratory experiment concentrated on the systematic study of variations in the magnetic behaviour of two selected standard prestressed elements (namely, the patented wire p7, that was applied in the past, and the prestressed strand lp15.7 currently used by the freyssinet company with righthanded threading). it was investigated the magnetic behaviour of both elements especially in dependence on its immediate temperature and its rate of the mechanical stress. this experiment was realized in the experimental centre of the klokner institute (the research institute at the czech technical university in prague). results of similar experiments realized for different standard prestressed elements are described in [10] (namely, for the patented wires p4.5, unknown prestressed strand lp15.7 with left-handed threading, the prestressed bars 15/17 made by dywidag company) and in [8] (namely, for the full locked cable pv 150 and the mukusol threadbar 15fs 0000). the ideal condition for precise evaluation of some particular experiment realized on an existing prestressed concrete structure is, when the specific evaluating curve is available. this specific curve can be obtained only by the magnetoelastic analysis of the specific prestressed element, which was removed directly from the investigated structure. the specimen should be minimally 1.2 m long and its extraction is generally very difficult and laborious. moreover, the load bearing system of the observed structure is partially weakened. also, the realization and evaluation of the experimental analysis necessary for determination of the evaluating curve is significantly timeconsuming. alternatively, it is possible to use the general evaluating curves that are available in the material library gradually compiled by the authors, some examples are shown in figure 7. the results of the chemical composition and the microscopic structure analysis realized for the material of the investigated prestressed reinforcement can be used for the selection of the appropriate general evaluating curve from the material library. the substantially shorter test specimen is needed for this purpose. usually only one wire 10 cm long is sufficient enough even if it is removed from the strand. the structure weakening is generally acceptable in this scale and the material sample can be removed from the opening used for installation of the me sensor without the additional partial damage of the structure. 4.1. the experiment realized on the previously used patent wire p7 the patent wire p7 used during the experiment described in this chapter was removed from the chamber of the existing prestressed concrete bridge in prague that was put into operation in the year 1974. in the course of the bridge inspection in the year 2019, the fully equipped me sensors were installed on two selected prestressed cables assembled from twelve patent wires p7 (see figure 2). the opening created for the purpose of the me sensor installation was consequently filled in by the special grout (see figure 3). the installed me sensors are intended for long-term monitoring of the prestressed forces in the selected cables. the basic reason for realization of the experiment described in this chapter was to determine the accurate parameters of magnetic behaviour of the prestressed reinforcement used in the figure 2. the fully equipped me sensor installed on the prestressed cable in the inspected bridge figure 3. the fully equipped me sensor installed on the prestressed cable, consequent filling of the created opening by the special grout acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 170 inspected bridge for more precise evaluation of the results. the fully equipped laboratory me sensor used in the course of the experiment is shown in figure 4. in the course of the experiment, the investigated patent wire p7 was placed in a climatic chamber (see figure 5) and loaded in a steel tensile testing machine. the magnetic properties of the studied wire were investigated for two temperature levels of the wire surface, namely around 0° c and around +25 °c. the studied patent wire p7 was loaded in five force steps for each temperature level according to its design resistance, namely, 10 kn, 20 kn, 30 kn, 40 kn and 47 kn. it is roughly about 20 %, 40 %, 60 %, 80 % and 100 % of the design strength. the temperature of the observed wire cross-section was evaluated as a linear interpolation between two measured temperature values. the first one was observed on the element surface in the close vicinity of the me sensor and the second one was the temperature of the air measured inside the me sensor. the specific hysteresis loop was measured and evaluated ten times for each particular temperature level and force step. the hysteresis loop, in general, characterizes the relation between magnetic flux density b and the magnetic field intensity h and it changes its shape depending on the actual force magnitude in the investigated prestressed element and also on its temperature. however, it is not effective and also necessary to evaluate the measured hysteresis loops in their whole range. the dimensionless parameter p is used to convert a complex measured shape of the hysteresis loop, that depends on the actual force magnitude, to one simple numeric value. and this parameter is standardly evaluated for the purpose of a practical application of the modified magnetoelastic method. some examples are published in [6]-[11]. the particular parameter p is described as a fraction. the numerator is the most important value for evaluation of the parameter p and it describes the level of the magnetic field intensity "h" in the main node point. the numerator value of the parameter p indicates the preference of the portion of the hysteresis loop close to the remanence (the intersection with the vertical axis in the b–h curve). on the contrary, its denominator value prefers the loop portion near to the saturation. the more exact definition of the parameter p is an industrial secret. in the course of analysis of the experiment results, several parameters p with different definitions were evaluated, for example parameters p 10/45, p 15/45 or p 20/45. the dimensionless parameter p 15/45 was finally chosen as the most suitable one for further analysis of the particular examined patent wire p7 and eventually for another similar one because of its maximal sensitivity to the prestressing force and its minimal disturbance by negative influences. the resultant regression fitting curve using polynomial regressions was calculated for the investigated patent wire p7 and the chosen resultant parameter p 15/45. the temperature effect on the parameter p was considered as linear and the force effect was considered as 3rd degree polynomial. calculated curve is one of several ones, which is drawn as curve “d7” in figure 7. the differences between the theoretical fitting curve and the used input experimental results are small. the maximal deviation between them is 1.6 % of the design strength of the investigated patent wire and the standard deviation of all particular results is 0.6 % of the design strength. 4.2. the experiment realized on the currently used prestressed strand lp 15.7 the prestressed strand lp 15.7, that was the subject of the experiment described in this chapter, is standardly used by the freyssinet company at the present time. the arrangement of the experiment, its procedure and results evaluation were similar to those described in the previous chapter. in the course of the experiment, the investigated prestressed strand was placed in the climatic chamber again (see figure 5) figure 4. the fully equipped laboratory me sensor intended for the experiment on the patent wire p7, the assembled sensor inside the steel shield (above) and view on its disassembled basic parts (below) figure 5. the exterior view on the climatic chamber and the steel tensile testing machine (on the left) and on the laboratory me sensor installed on the prestressed strand lp15.7 inside the chamber (on the right) acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 171 and loaded in the steel tensile testing machine. the magnetic properties of the studied strand were investigated for three temperature levels of the strand surface namely, around 0 °c, +20 °c and +35 °c. the strand was loaded in five force steps for each temperature level, namely, 40 kn, 80 kn, 120 kn, 160 kn and 200 kn. it is roughly about 20 %, 40 %, 60 %, 80 % and 100 % of the strand design resistance. the temperature of the observed strand was evaluated in the same way as for the patent wire p7 in chapter 4.1. the specific hysteresis loop was measured and determined again multiple times for each particular temperature level and force step as can be seen, for example, from figure 6. for the observed strand lp 15.7, the example of evaluated dependence of the chosen resultant parameter p 15/60 on the strand temperature (prestressed force is constant 120 kn) is shown in figure 6. the resultant regressive fitting curves for the several chosen parameters p were also calculated using the same methods of mathematical analysis and statistics analogous with chapter 4.1. the differences between the theoretical fitting curves and the used input experimental results are even smaller than for the wire p7. the maximal deviation between them is 1.4 % of the design resistance of the observed prestressed strand and the standard deviation of all particular results is 0.2 % of the design strength. the example of calculated regression fitting curve that expresses the dependence of the particular chosen parameter p 15/45 on the stress in the observed strand at the strand temperature 20 °c is shown in figure 7 where the relevant curve is labelled “l 2019”. 5. practical application of the method on different prestressed elements in this chapter, a brief summary of typical utilizations of the modified magnetoelastic method is described on some practical applications. 5.1. the experiment realized on the prestressed cables composed of twelve strands on an existing bridge the actual tensile forces in prestressed cables on an existing prestressed concrete bridge were investigated during this experiment. in general, the specific position for installation of the me sensor on the load bearing structure of an existing bridge made from prestressed concrete is usually chosen based on three basic reasons. firstly, the created opening in concrete (see figure 2, figure 3 or figure 8 for example) does not weaken substantially the bridge load bearing structure. secondly, the substantial degradation of the concrete or some prestressed reinforcement is supposed in the selected location, for example where water with chlorides penetrates into the structure. thirdly, the position is chosen based on an interest of the structural designer. the investigated bridge is about thirty years old and its prestressed system is composed of the longitudinal prestressing cables. the typical prestressed cable on this bridge is the post figure 6. the prestressed strand lp 15.7 – the relation between the temperature of the observed strand and the chosen resultant dimensionless parameter p 15/60 on the specific force step (120 kn) figure 7. the comparison of the relations between the stress in the six observed prestressed elements (three patent wires p4.5, one patent wire p7 and two strands lp15.7 / 1860 mpa) and the chosen resultant dimensionless parameter p 15/45 for the prestressed element temperature 20 °c figure 8. the post installed me sensor wounded on the prestressed cable composed of twelve strands lp 15.7 mm acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 172 tensioned one and it is composed of twelve strands lp 15.7. it is led through the web of the box girder made from monolithic concrete inside a thin-wall steel duct and it was injected with cement mortar after its tension. the separates parts of one prestressed cable were interconnected by couplers in working joints. in the course of the experiment, six fully equipped me sensors were installed additionally on selected existing prestressed cables. it means, all sensor constructions among others included two hall's sensors and both secondary coil no. 1 and no. 2. in general, individual observed cross sections of the investigated prestressed cables are unique at least due to the variable arrangement of the strands in the prestressed cable (see figure 9 for example), therefore the developed methodology for application of the modified magnetoelastic method on existing prestressed concrete structures that is described in more detail in reference [9] had to be used during the results evaluations. in brief, the prestressed force evaluation in the cable by the methodology consists of four main stages. the first one is the as accurate as possible measurement of the real geometric arrangement of the strands in the observed cross-section of the prestressed cable. the second stage is theoretical modelling of the measured geometric shape of the cable in a 3d software for the electromagnetic field analysis (see figure 9). the third one is the in situ observation of the electromagnetic behaviour of the me sensor installed on the cable (see figure 8). the fourth stage is the application of the available calibration curve that was gained in laboratory test realized for the strand type from which the analysed cable is assembled. the short study of the result uncertainties caused by the partially variable mutual position of the sensor body and the cable was carried out during the described experiment. the prestressed forces in the observed cables were determined repeatedly three times by the modified magnetoelastic method and they were subsequently mutually compared. for example, the mutual geometric arrangements of the me sensors and the investigated prestressed cables were changed repeatedly. each individual me sensor was partially displaced several times in the plane of observed cable cross section in different directions perpendicular to the longitudinal axis of the cable within a range of clearance gap between the sensor body and the cable surface. subsequently, the measurement of the prestressed force was realized for the each adjusted mutual position of the sensor body and the cable. the above-mentioned analysis of the obtained results provided information about partial uncertainties of the evaluated tension forces which are related with the geometric arrangement between the sensor body and the cable and the numerical modelling of the problem (see figure 9). the variance of the resulting values on each observed cable did not exceed 1 % of the prestressed force magnitudes. the tensile forces evaluated by the modified magnetoelastic method in the individual observed cable locations were mutually compared as well. the force magnitude of some investigated cables was only about 50 % of the prestressed forces evaluated in other ones. these cables were examined in detail subsequently. their significant weakening by corrosion was detected near their measured cross section and the corrosion process was distinctly uneven in these cases. the authors tried to reduce uncertainties of the experiments as much as possible. the main sources of the uncertainties are follows: the uncertainty related to the material of the investigated prestressed reinforcement, the uncertainty connected with the fem numerical modelling, the uncertainty caused by the actual temperature of the reinforcement in experiment time and the uncertainties of the hall's sensors. the chemical compositions and the microscopic structures of the materials of the prestressed reinforcements are more or less different. if it is possible, the reference sample of the investigated prestressed reinforcement should be taken from the observed existing structure and then analysed in an ideal variant of the experiment. the application of more hall's sensors to different positions in the investigated cross-section of the prestressed cable should be used for more accurate verification of the numerical model based on the multiple comparison of the experimental and theoretical results. the evaluating curves (see figure 7 for example) for particular material of some prestressed reinforcement that characterize the temperature effect on the analysed results should be realized using calibrated force and temperature sensors and devices. the sensitivity of hall's sensors applied by the experiments should be verified using a calibrated source of a reference magnetic field. as can be seen from figure 8 the installation of the me sensors was associated with the laborious and precise local demolition of the concrete layer in locations with cables selected for the experiment. the additional benefit of this experiment is fact, that the created openings for the me sensor installation can be used for a proper inspection of the prestressed reinforcements and their potential corrosion. due to the fact that the original post-injection grouting of the steel ducts with the cables was not performed well, the me sensor can also identify weakening of the cable cross section by corrosion that is located out of the investigated place and the created opening, because there is not redistribution of the prestress losses of the cable tensile stress on other strands or cables. 5.2. the experiments realized on the prestressed bars the experiments described in this chapter were carried out on some samples of the currently used prestressed threadbar of type mukusol 15fs made by dywidag company. the bar diameter is 15 mm, the thread diameter is 17 mm, its characteristics figure 9. the important output of the numerical model necessary for the correct interpretation of experimental data (the numerical model of the observed cable cross section with precisely located positions of twelve strands lp15,7 and two used hall's sensors) acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 173 strength is 960 mpa and appropriate characteristics breaking load is 170 kn. in the past years, the authors realized similar experiments focused on the threadbars [11]. however, the simpler constructions of the me sensors with only one secondary coil were used during these experiments. it was stated in conclusion of the paper [11] that the supervision of the presstressed bars using the modified magnetoelastic method does not seem appropriate. the supposed advantages of the newly used me sensor arrangement with both secondary coils were verified on the threadbars in the course of the recent experiments [8]. the secondary coils were applied as the detector of the of magnetic field intensity "h" instead of the originally used hall's sensors, that failed in the conditions of the swirling magnetic field caused by the threads of the prestressed bar [11]. the differential signal evaluated from the data observed by the pair of the secondary coils is used for evaluating of the same physical quantity that is also measured by hall's sensors. however, quantity magnetic field intensity "h" is not observed only locally as in the case of hall's sensors but the differential signal corresponds to value of quantity “h” averaged over the volume of the annulus between two secondary coils. both secondary coils used in the applied me sensor were made with the same length as the screw thread pitch of the investigated threadbar for the purpose to reduce the negative influence of the threads and to eliminate all abnormalities of the magnetic field in their neighborhood. see reference [8] for more details. the phenomenon of the inverse sensitivity of the threadbar material to the applied axial load was found out during the analysis of experimental data. this phenomenon is probably caused by the disturbing effects of eddy currents. the development of their influence is produced by the compact massive cross section of the bar in comparison with prestressed wires or strands. in terms of the magnetic behaviour, the strand is close to a system of prestressed wires and not to the compact threadbar. more details of the phenomenon are described in reference [8]. the maximal difference between the force actual values, that were measured by the standard force sensor used during the experiment, and the force determined using the evaluation curve applied to the experimental data measured by the me sensor was about 2.0 % of the characteristic strength of the investigated threadbar. based on the new experiences of the authors with application of the me sensor containing both secondary coils, it can be stated that modified me method enables to determine the actual value in the standard prestressed threadbars. however, during the design of an experiment, it is necessary to consider the lower sensitivity of the modified magnetoelastic method for the prestressed threadbars compared to the strands for example. this lower sensitivity is probably related to the lower strength of the material used for production of the threadbars. 5.3. the experiment realized on the standard full locked cable pv-150 the experiment described in this part was realized on the currently used standard full locked cable pv 150 made by firm pfeifer seilund hebetechnik gmbh. the arrangement of the experiment and its results are described in more detail in the reference [8]. the basic objective of the experiment was to verify if the modified me method can be used for determination of the axial tensile forces in the standardly produced full locked cables of type pv. the core of this cable type is composed of several layers of circular cross-section wires and the locked outer layers are formed from z shaped wires. the outer diameter of the investigated cable pv 150 is 40 mm and its characteristics strength is 1435 mpa and appropriate characteristics breaking load is 1520 kn. the magnetic behaviour of the investigated cable pv 150 was observed by the fully equipped me sensor (see figure 1). its construction involved the following parts: the sensor body made by a 3d printer (see figure 10), the primary coil used as the controlled magnetic field source, the secondary coil 1 (see figure 10) applied as a sensor of magnetic flux that is closely related to the magnetic induction "b" in the measured cable cross section, the system of two hall sensors (see figure 10) and the secondary coil 2 used as two independent sensors of magnetic field intensity "h" in a measured cable cross section and steel shield of the me sensor applied as a sensor protection against magnetic influences from its surroundings. in the course of the experiment, the cable was anchored in a stand and it was loaded twice in nine force steps by the tensile force produced by a calibrated tensional hydraulic jack. the experiment was primarily focused on the verification of usability of the modified magnetoelastic method for this type of the structural element and it was not intended as the full experimental analysis of the evaluating curves because it was realized only for one temperature of the cable. however, several various evaluation procedures were examined based on the measured data. the obtained experimental results show that the modified magnetoelastic method can be applied meaningfully for experimental determination of the tensile force in the standard fully locked cables of type pv. the magnetic behaviour of these cables is analogical to the standard prestressed strands of type lp. 6. alternative arrangement of the me sensor in the course of the research project, the design of the prototype of the removable me sensor was also developed. its each part can be simply mounted on the observed structural element and then removed easily and no one coil has to be figure 10. the production of the me sensor on the cable pv 150, the process of the winding of the secondary coil 1, the placement of hall’s sensor acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 174 produced in situ, so the time-consuming winding of the coils is not required on the experiment location. the basic part of the removable me sensor is the primary coil winded on the steel horseshoe-shaped core (see figure 12), that is used for developing sufficiently intense variable electromagnetic field. the parameters of the magnetic field are measured applying two secondary coils and two hall's sensors. the secondary coils are winded around the orange frames that were made by a 3d printer and that are put on the both ends of the steel core (see figure 12). they measure the magnetic field properties in a direction perpendicular to the element axis. more specifically, they observed the changes of magnetic-inductive flow in the magnetic circuit, that correlate with the intensity of the magnetic induction “b”. two hall's sensors are located inside the special black holder (see figure 12) that was also made by the 3d printer and that is mounted on the studied structural element in the plane of symmetry of the removable me sensor. it means, both hall's sensors are situated in the central plane of the sensor perpendicular to the investigated element and they measure the properties of the magnetic field in a direction parallel with the element axis, more specifically, they observe magnetic field intensity "h" in the investigated cross section of the element that is located in the plane of sensor symmetry. as part of the development of the sensor prototype, the possibility of magnetic-inductive flow observation was evaluated. flow observations were made by using two measuring coils that are positioned on the both ends of the steel horseshoe-shaped core of the electromagnet and that are connected in series in the measuring circuit. the mutual relation between the time courses of the measured signals from the secondary coils and hall's sensors was investigated and assessed. the subject of interest was the answer for the question if the magnetization of the compact massive core of the primary coil, on which both secondary coils are threaded, does not influence the measurement of hysteresis behaviour of the investigated prestressed element. based on the obtained experimental data, it can be stated that the hysteresis is affected by the compact core of the primary coil. however, this phenomenon occurs only during the rapid increase of magnetic flux during the rapid increase of the excitation electric pulse. it always disappears completely during the descending branch of the excitation pulse because then the phenomenon is almost quasi-static from the view of the dynamics of the magnetic field development. the magnetic behaviour of the assessed arrangement of the experiment, it means the system consisting of the removable me sensor and the investigated structural element, is purely linear in the zone of the descending branch of the excitation pulse that is really used for the analysis of the experimental data measured by the me sensor. this behaviour was also verified by the removable me sensor, which was mounted on a nonferromagnetic dummy of the investigated element. the obtained results of the experiment, that are described in more detail in reference [7], confirmed the functionality of the designed arrangement of the removable me sensor. however, the precision of the gained results is lower than for the cylindrical me sensor, that is described in the chapter 3, due to the less accurate definition of the magnetic field in the observed cross section compared to the arrangement of the cylindrical me sensor. the removable me sensor can be used for a relatively quick preparative experimental analysis intended for the evaluation of the total value of the prestressed force in the structural elements activated before the start of the experiment. the intended purpose of the practical application of the removable me sensor is a quick selection of prestressed elements that are the most important for further detailed analysis by using the cylindrical fully equipped me sensors. the comparison of the basic advantages and disadvantages between the standard cylindrical fully equipped me sensor whose general scheme is shown in figure 1 and the removable me sensor whose general diagram is drawn in figure 11 could be summarized in following notes. the advantages of the standard cylindrical me sensor are that a relatively high measurement accuracy could be achieved and that the dimension of the cross-section of the observed prestressed reinforcement is not restricted practically. its basic disadvantage is its relatively time-consuming production. the fundamental advantage of the removable me sensor is that the time necessary for preparation and realization of the experiment is substantially shorter. its disadvantages are higher uncertainties of the evaluated prestressed forces and a considerable limitation of the dimension of the observed reinforcement cross-section. figure 11. the arrangement of the removable me sensor figure 12. the application of the removable me sensor on the strand lp 15.7 in the laboratory conditions acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 175 7. results and discussions so far, as part of development of the modified magnetoelastic method, the experiments focused on the evaluating curves were realized for four specimens of the patent wires, three types of the prestressed bars and two prestressed strands lp 15.7 / 1860 mpa from different producers. the results obtained for six selected investigated prestressed elements (namely, three patent wires p4.5, one patent wire p7 and two prestressed strands lp15.7) were compared mutually in detail. the resultant regression fitting curves for the particular elements at the element temperature 20 °c are drawn in figure 7. the analysis of the results shows that the standard deviations of all measured data from the specific calculated evaluating curves are usually significantly lower than 1.0 % of the design strength. the comparison of the corresponding evaluating curves for two different samples of the prestressed strands lp 15.7 / 1860 mpa made by different producers is shown in figure 7 (specimens “l 2019” and “l 2016”). a specific producer of the strand “l 2016” is not known. the strand “l 2019” was supplied by freyssinet cz. the difference between curves is roughly about 5 % of the design strength of the strands. it means the evaluating curves of the particular samples of the strand lp 15.7 deviate roughly about 2.5 % from the averaging evaluating curve of this type of the prestressed strand. very similar results were obtained from the comparison of the corresponding evaluating curves for three different test samples of the patent wire p4.5 that were taken during the demolitions of three different existing bridges. these bridges were built in the seventies and eighties in czechoslovakia. for example, three curves “d4 spec. 1”, “d4 spec. 2” and “d4 spec. 3” for the wire temperature 20 °c are drawn in figure 7. the evaluating curves of the particular test samples of the patent wire p4.5 deviate roughly about 2.5 % from the averaging evaluating curve of this type of patent wire. in contrast to the previously mentioned results, the significant difference roughly above 20 % of the design strength was found between the evaluating curves for the patent wires p7 and p4.5, as can be seen from figure 7. results of the additional analyses indicated that the chemical compositions of the materials, from which the studied wires p4.5 and p7 were made, were almost identical. nevertheless, for the wire p7, it was found out, that its microscopic structure of steel is fundamentally different from the three wires p4.5 and this fact seems to be the reason of the difference between the evaluating curves for the patent wire p7 and wires p4.5. as it was expected, it was confirmed experimentally that the evaluation relationships are not applicable universally for different materials used for various prestressed elements. it was further found that the evaluation curves are not mutually applicable even for the patent wires with different diameters. 8. conclusions and outlook the results stated above demonstrate that the modified magnetoelastic method can be used for the experiments realized on the existing structures for the determination of the actual value of the tension force in steel prestressed structural elements using the available general evaluating curves. the result uncertainties of these experiments are then similar as for the alternative experimental methods, e.g., the vibration frequency method [1], [2]. it should be noted here that the frequency method cannot be used for the prestressed elements embedded in the concrete. in the cases when it is possible to remove the test sample of the specific prestressed element from the particular existing bridge then the evaluating curves for this observed element can be evaluated according to the above-described procedure. the uncertainties of the evaluated prestressed forces are then relatively small and they are comparable with precision of the method based on a strain measurement with strain gauges. the possibility of using the modified magnetoelastic method for the prestressed bars was also verified during previous experiments [8] and [11]. based on the new experiences of the authors with application of the me sensor containing both secondary coils, it can be stated that modified magnetoelastic method enables to determine the actual value in the standard prestressed threadbars. however, the me sensor sensitivity, when it is applied on the threadbars, is lower than for the prestressed wires and strands. the main reason of this fact is the significantly lower design strength of the threadbar materials. according to the authors opinion, the modified magnetoelastic method is the only one that can be purposefully and effectively used for the prestressed force evaluation in the prestressed reinforcements embedded inside the concrete on the existing prestressed concrete bridges or similar engineering structures. acknowledgement the results presented in this article are outputs of the research project fv 30457 “utilization of a magnetoelastic method for increasing the reliability and durability of existing and newly built prestressed concrete structures” supported 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characterization of the santa maria del fiore cupola construction tools using x-ray fluorescence acta imeko issn: 2221-870x march 2022, volume 11, number 1, 1 7 acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 1 characterization of the santa maria del fiore cupola construction tools using x-ray fluorescence leila es sebar1, leonardo iannucci1, sabrina grassini1, emma angelini1, marco parvis2, andrea bernardoni3, alexander neuwahl4, rita filardi5 1 dipartimento di scienza applicata e tecnologia, politecnico di torino, turin, italy 2 dipartimento di elettronica e telecomunicazioni, politecnico di torino, turin, italy 3 università di siena e istituto di storia della scienza ( museo galileo), florence, italy 4 artes mechanicae, florence, italy 5 museo dell’opera del duomo, florence, italy section: research paper keywords: x-ray fluorescence; pca; non-invasive measurements; cultural heritage; metals; archaeometry citation: leila es sebar, leonardo iannucci, sabrina grassini, emma angelini, marco parvis, andrea bernardoni, alexander neuwahl, rita filardi, characterization of the santa maria del fiore cupola construction tools using x-ray fluorescence, acta imeko, vol. 11, no. 1, article 9, march 2022, identifier: imeko-acta-11 (2022)-01-09 section editor: fabio santaniello, university of trento, italy received march 15, 2021; in final form december 13, 2021; published march 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: leonardo iannucci, e-mail: leonardo.iannucci@polito.it 1. introduction the santa maria del fiore cupola (dome), located in florence, is a unique renaissance creation. indeed, the brunelleschi’s cupola is an extraordinary masterpiece, still being the biggest masonry dome in the world. its dimension, together with the innovative construction project that led to its realization, made the cupola one of the most famous buildings in history. the history of the santa maria del fiore cathedral spans through a wide time frame and is a really complex one. in the 14th century florence was a flourishing city, with the reputation of being one of the most important in europe. the city wanted to increase its relevance, visibility and pride, in order to wipe out all the nearby competing cities. to this aim, the florentine citizens decided to build a very large cathedral, similar to the ones in the other most important european cities [1]-[3]. the history of the santa maria del fiore cathedral started with the italian architect arnolfo di cambio, who developed the initial project and supervised the beginning of the construction in 1296. about 100 years later, just the main body was completed, without the facade and the dome. in 1418 an open competition started, to find a project for the construction of the biggest cupola in the world. the competition ended only in 1420, when filippo brunelleschi was entrusted with the work. however, his project was always surrounded by skepticism [2]-[4]. among the many challenges that brunelleschi had to face, the main one was the size of the dome. indeed, the regular techniques employed in the 14th century involved the use of internal support structures, which would not be possible for the construction of the santa maria del fiore dome. due to their abstract this paper presents the characterization of different tools employed in the construction of the santa maria del fiore cathedral in florence; they are part of the opera di santa maria del fiore collection and are currently exhibited in the museo dell’opera del duomo. the analysed objects are turnbuckles, pulleys, three-legged lewises, and pincers; indeed, despite their uniqueness and their importance from the historical point of view, this study is the first one that investigates their alloys composition. actually, this information can be of great interest for curators to find the best conservation strategies and to have new insights on the production techniques typical of the renaissance. the study was performed using x-ray fluorescence (xrf) in order to identify the materials constituting the objects. then, xrf spectra were analysed using chemometric techniques, namely principal components analysis (pca), in order to investigate possible similarities among different alloys and thus provide new indications to help collocating these tools in a specific historical period. mailto:leonardo.iannucci@polito.it acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 2 size, these frameworks would have not been capable of supporting even their own weight. brunelleschi, who had spent many years studying the ancient roman architectural techniques in rome, in order to solve this major issue, together with many other challenges related to the project, found a way to build a self-supporting double-walled dome, thanks to the insertion of particular pattern of bricks, called “a spina di pesce”. the construction was carried on employing suspended platforms which were progressively moved up along with the construction [1]-[8]. another remarkable novelty employed by maestro brunelleschi was the design and development of specific machines and tools, that were used on the construction site. the employment of these new tools, used to lift the building materials, allowed to save both time and money. today, some of these unique tools, such as pulleys, turnbuckles, pincers, winches, and ropes, are part of the opera di santa maria del fiore collection, exhibited in the museo dell’opera del duomo, in florence. even if these tools were a great innovation for the duomo project, they did not attract the attention of researchers so far. nevertheless, these unique objects and their history could provide important information regarding the production techniques and the materials employed during the brunelleschi era. for instance, a lot of information can be deducted from a visual inspection of the tools, by comparison with the historical sources. indeed, it is possible to find drawings of similar tools made by taccola, francesco di giorgio, bonaccorso ghiberti, giuliano da sangallo and even in the codex atlanticus by leonardo da vinci [1], [9]. on the other hand, the provenance, the materials and the production techniques of some of these tools are more complex to reconstruct. in such cases, conservation science and engineering can provide useful tools to reconstruct these tools’ history. indeed, tailored analytical strategies can be applied to investigate the constituent material and to develop specific conservative approaches [10]-[12]. in this paper, the first characterization of these construction tools with a non-invasive and in-situ approach is presented. a previous study has examined the preliminary results from the xrays fluorescence analyses performed on these objects [13]. in the present manuscript a complete survey of the constituent materials is provided, discussing the possible role of different elements in the alloys. then, obtained data were processed using multivariate analysis to find possible similarities in the spectra acquired on different tools and thus formulate hypotheses on the historical collocation of these objects. 2. materials and methods this section presents the characterized historical tools and the analytical techniques employed in the study; moreover, the performed data analysis is described in detail. 2.1. historical tools under study this study is focused on thirteen objects which are part of the opera di santa maria del fiore collection and are currently exhibited in the museo dell’opera del duomo, in florence. they are tools and equipment that were used in the construction of the santa maria del fiore cathedral. as previously discussed, the construction of this building took several centuries, so it is difficult to collocate each of the tools in a specific historical period. the investigated objects are two turnbuckles, eight pulleys, two three-legged lewises and a pincer. the photographs of some of the characterized tools are shown in figure 1, where some of the points of analysis are marked. 2.2. x-ray fluorescence all objects were characterized using x-ray fluorescence in order to investigate the composition of the constituent materials. measurements were performed using a brucker tracer 5i analyser, which allowed to perform non-invasive measurements without moving the tools from their collocation in the museum. the instrument is equipped with a 20 mm2 silicon drift detector and a rhodium (rh) anode. the ti-al filter was used in order to reduce the intensity of peaks related to rhodium and palladium (pd) [14]. analyses were carried out using a voltage of 40 kv and a current of 40 µa, with the 3 mm collimator. spectra processing and elements identification were performed using artax spectra (8.0.0.476) software. 2.3. multivariate analysis in order to investigate similarities among different alloys, acquired spectra were processed by means of principal component analysis (pca). using this chemometric technique figure 1. historical tools employed for the construction of the santa maria del fiore cathedral in florence. the instruments are displayed in the museo dell’opera del duomo, florence: a), b) turnbuckles, c) three-legged lewises, d), e) pulleys, f) pincers. yellow circles indicate the analyzed areas. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 3 it is possible to identify patterns in acquired measurements and classify spectra in different groups. pca was performed on xrf spectra using a python script, as described in [15], by means of the scikit-learn library [16]. before computing the principal components (pcs), spectra were pre-processed as follows: 1) the interval of interest was limited to the range from 1 kev to 12.2 kev for iron alloys and from 1 kev to 15.5 kev and from 24.5 kev to 30 kev for bronzes. actually, in these energy ranges all significant peaks for the two materials are present and thus only relevant parts of the spectra are included in the pca. 2) spectra baseline was subtracted using the embedded function in the artax spectra software. 3) signal-to-noise ratio was improved applying the savitzkygolay filter [17]. a second order polynomial and a window length of 90 ev were used in order to avoid any over-smoothing. 4) spectra were normalized using the standard normal variate transformation (snvt) [18]. after performing the pre-processing, principal components were computed and results were graphed as biplots, in which eigenvalues for different spectra are plotted. similarities among different spectra were evaluated using a gaussian mixture model probability distribution and confidence ellipses were accordingly drawn using the sklearn.mixture.gaussianmixture class from the scikit-learn library. 3. results and discussion 3.1. alloys identification xrf analyses were performed on all investigated tools choosing different points of interest on each object. the primary goal was to identify the alloys constituting the different parts of the tools. most of the analysed objects are pulleys, as these tools were commonly used in the cathedral construction site during the centuries and many of them are still preserved in the museum. it is interesting to notice that two kinds of pulleys can be identified among those present in the museum collection: one is characterized by both the main body and the wheel made in wood (shown in figure 1e), and the other is made completely in metal (shown in figure 1d). even if at a first glance it could appear easy to collocate the two typologies of pulleys in different historical periods, the only available information in literature dates also the metallic pulleys to the renaissance era [3]. so, this simple example further highlights the need for an archaeometric approach to study these important tools. figure 2 shows some representative spectra acquired on different pulleys. in figure 2a, the spectrum acquired on the metallic frame of a wooden pulley is reported; as can be seen in figure 1e, this typology of pulleys had a metallic frame needed to hold and anchor the tool. the material can be identified as an iron alloy, considering the two main peaks at 6.40 kev and 7.06 kev, which correspond to characteristic kα and kβ lines of iron respectively. the material is then characterized by the presence of manganese, copper, zinc, and nickel demonstrated by the presence of peaks at 5.90 kev (mn-kα), 8.05 kev (cukα), 8.64 kev (zn-kα), and 7.48 kev (ni-kα) respectively. finally, it is possible to identify calcium by the 3.69 kev and 4.01 kev peaks, corresponding to the kα and kβ emission lines, and potassium (kα 3.31 kev); these are present in all acquired spectra and can be related to environmental contamination. additional peaks are present at higher energies. in particular a triplet of peaks can be attributed to iron sum peaks, i.e., 12.81 kev (kα + kα), 13.46 kev (kα + kβ), and 14.12 kev (kβ + kβ). moreover, the couple of peaks at 4.67 kev and 5.32 kev can be assigned to the fe-k lines escape peaks. furthermore, the peaks related to the rhodium anode can be identified by the kα and the kβ lines at 20.22 kev and 22.72 kev respectively. the broad peak at 19.06 kev can be attributed to the compton scattering of the rh characteristic photons. arsenic is present too, as can be seen from the presence of the peaks at 10.54 kev (kα) and 11.72 kev (kβ). for these peaks, the k shell intensity ratios (kβ/kα) is respected, as it has an average value close to 0.14 [19]. arsenic was a common contaminant in iron ores and thus was often present in iron alloys [20], [21]. the spectrum acquired on the main body of a metallic pulley is then presented in figure 2b. in this case, too, it can be identified as an iron alloy, in which most of the peaks mentioned for the spectrum in figure 2a can be found. at the same time, the relative intensity of peaks changes with respect to the ones in the spectrum shown in figure 2a. figure 2. xrf spectra collected on pulleys: frame of one of the ‘wooden’ pulleys (a), main body of one of the ‘metallic pulleys’ (b), wheel of one of the ‘metallic pulleys’ (c). the black line is the acquired spectrum, while the computed baseline is in green. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 4 it is worth noticing that for the three points of analysis acquired on one of the metallic pulleys (referred in the next section as ‘pulley 6’), a different spectrum was obtained. the material can be identified as an iron alloy, characterized by the additional presence of lead. in figure 3a a detail of a spectrum obtained from this pulley is reported. lead can be identified considering different features. first, it is possible to notice that the arsenic kβ over kα ratio is not respected, thus deducing that the peak at 10.55 kev is due to the overlapping of both the pblα and as-kα lines. furthermore, it is possible to notice the presence of a left shoulder on the sum peak of iron (kα + kα) at 12.61 kev, corresponding to the pb-lβ line. other characteristic pb lines are identified at 11.35 kev and 14.76 kev, being the pblη and pblγ1 respectively. a spectrum showing the same energy interval acquired on one of the pulleys without lead is reported in figure 3b; in this case, the left shoulder on the sum peak of iron at 12.61 kev is not present. in figure 2c the spectrum acquired on the wheel of one of the metallic pulleys is shown. it is possible to describe the material as a lead bronze, i.e., an alloy containing copper, tin, and lead. most of the peaks previously described for figure 2a are present also in this spectrum, except for the escape and sum peaks of iron. moreover, there is the additional presence of the characteristic peaks of lead (lα-10.55 kev, lη-11.35 kev, lβ612.14 kev, lβ4-12.31 kev, lβ1-12.61 kev, lβ5-13.01 kev, lγ114.76 kev, lγ6-15.18 kev), silver (kα-22.16 kev), tin (kα25.27 kev, kβ1-28.48 kev and kβ2-29.11 kev), and antimony (kα1-26.36 kev). the remaining peaks can be identified as sum peaks, e.g., 16.06 kev (cu-kα + cu-kα), 16.67 kev (cu-kα + zn-kα), 16.97 kev (cu-kα + cu-kβ), and 18.61 kev (pb-lα + cu-kα). an interesting case study is then represented by the two turnbuckles, which are of great importance both from the technical and historical points of view. these tools constituted a great innovation that allowed to lift heavy loads in a smooth and controlled way. moreover, they substituted steel rods for the stone positioning, reducing the risk of chipping them during this operation. their historical importance is then testified by their representation in different collections of drawings in the renaissance era, as an example in the ‘taccuino senese’ by giuliano da sangallo [22]. as can be seen in figure 1a and figure 1b, turnbuckles are composed of different parts, namely the central screw, the nut, the hook, and two connecting rods. threaded components are particularly important in this investigation as they can give new insights on the production routes typical of the renaissance era and can provide hints to date the objects. during that period threaded parts were mainly realized in bronze, as this alloy has good machinability using steel tools. for this reason, threaded components made in iron should belong to a later period. the most representative spectra collected on the two turnbuckles are shown in figure 4. the nut of the ‘turnbuckle1’ (figure 1a) is constituted by a bronze alloy. this was identified by the presence of the major elements as copper (kα-8.05 kev and kβ-8.90 kev), tin (kα-25.27 kev, kβ1-28.48 kev and kβ2-29.11 kev), zinc (kα-8.64 kev and kβ-9.57 kev), and lead. lead can be identified through several peaks in the spectrum: mα-2.34 kev, mγ2.65 kev, lι-9.18 kev, lα-10.55 kev, lη-11.34 kev, lβ612.14 kev, lβ4-12.30 kev, lβ1-12.61 kev, lβ5-13.01 kev, lγ114.76 kev, lγ6-15.17 kev. other elements are present too, such as: iron (kα-6.40 kev and kβ-7.06 kev), nickel (kα-7.48 kev), antimony (kα1-26.35 kev and kβ1-29.71 kev), arsenic (kα10.54 kev and kβ-11.72 kev), manganese (kα-5.90 kev), calcium (kα-3.69 kev and kβ-4.01 kev), and potassium (kα3.31 kev). figure 3. comparison of two spectra acquired on different pulleys: on the left, the spectrum corresponding to ‘pulley 6’, where lead and arsenic are present; on the right, spectrum acquired on one of the remaining pulleys, characterized by the presence of arsenic but not lead. only the energy interval from 10 kev to 16 kev is reported. the black line is the acquired spectrum, while the computed baseline is in green. figure 4. xrf spectra collected on the two turnbuckles: nut of turnbuckle1 (a), one of rods on turnbuckle1 (b), nut of turnbuckle 2 (c). the black line is the acquired spectrum, while the computed baseline is in green. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 5 additional peaks are present at higher energies. in particular they can be identified as sum peaks, e.g. 16.06 kev (cu-kα + cukα), 16.97 kev (cu-kα + cu-kβ), and 18.61 kev (pb-lα + cukα). moreover, the material of the anode, rhodium, can be identified by the following peaks: kα-20.22 kev and kβ122.72 kev. the broad peak at 19.06 kev can be attributed to the compton scattering of the rh characteristic photons. compared to the composition found for the pulley’s wheels (figure 2c), it is possible to notice a less intense peak of zinc and a higher relative concentration of antimony. it is not easy to compare these findings to previous literature because most of the studies investigating the bronze composition of that period are related to statuary art. anyway, it is worth noticing that the presence of zinc, nickel, and iron in lead bronze was already reported in previous studies [23], [24]. moreover, the presence of antimony was found in different artifacts dating back to the renaissance period and also in the alloy used for the realization of the ‘porta del paradiso’ reliefs by lorenzo ghiberti, which were positioned in the east door of the baptistery of san giovanni battista, in front of santa maria del fiore cathedral [25]-[27]. this last point is of particular importance because, like also arsenic, these elements were not intentionally added by the founder, but they are impurities due to raw materials [26]. this can further confirm the dating of this turnbuckle to the brunelleschi era. apart from the nut, all other parts constituting the ‘turnbuckle1’ are instead made of an iron alloy containing copper, zinc, arsenic, and lead (see figure 4b). on the other hand, the ‘turnbuckle2’ (figure 1b) is entirely made of a similar iron alloy, with a higher content of manganese, zinc and arsenic (see figure 4c). finally, spectra acquired analysing the pincers and the lewises are reported in figure 5 and figure 6, respectively. they are both tools used to lift stones; the use of pincers is straightforward, while lewises were used inserting this tool in a clearance realized on purpose in the stone. these objects were realized using an iron alloy, whose composition is mainly characterized by the presence of copper, zinc, nickel, and arsenic. most of the peaks previously described for figure 2a are present also in these spectra. the main difference between these last two objects is the presence of lead, which can be identified in the spectrum reported in figure 6 by the left shoulder on the sum peak of iron at 12.61 kev, while is not present in the alloy constituting the pincers. 3.2. pca and spectra classification after identifying the main alloys constituting the different tools, principal component analysis was used in order to discover possible similarities in the materials composition. finding analogies in the alloys composition for different objects cannot be considered as a univocal proof that the two tools belong to the same historical period, but anyway can give additional useful indications to curators and scholars. so, three different processing were performed: in the first one, all pulleys were analysed in order to examine possible groups in this kind of objects, in the second one, all spectra identified as bronzes were investigated and then in the third one the spectra acquired on the two turnbuckles were taken into consideration. pca was performed using the acquired xrf spectra as input data. actually, when dealing with xrf measurements, two strategies are possible. the first one consists in the computation of the material composition after identification of the elements by means of their characteristic peaks. in this case, the weight percentage of each element is used as input data for the pca [28], [29]. the second approach, which is the one used in this study, directly uses the raw spectra as input data for the pca, without computing elemental composition. the advantage of this second approach is that the result from the pca processing is not influenced by the elements identification performed by the user, which could be in some cases questionable or at least not univocal [30], [31]. thus, using xrf spectra for the pcs computation allows to reduce possible sources of error related to spectra interpretation. at the same time, particular care should be taken in order to exclude from the processing those parts of the spectra that do not carry useful information, because they could bias the pca model without a significative reason from the compositional point of view. because of this, the processed spectrum was limited to the range from 1 kev to 12.2 kev for iron alloys and from 1 kev to 15.5 kev and from 24.5 kev to 30 kev for bronzes. regions of the spectrum not analysed by means of pca are characterized by the presence of sum peaks or lines related to the anode material. results from the pca of spectra acquired on pulleys can be seen in figure 7. in this biplot it is not possible to highlight a well-defined clustering among different objects, but anyway it is possible to give a primary interpretation. spectra for ‘pulley1’ (one of the wooden pulleys) and ‘pulley6’ (one of the metallic pulleys) appear to be outliers, characterized by values of pc1 above 30. as can be seen from the loadings, this is due to a higher concentration in nickel, copper and lead (this last element was identified only in ‘pulley 6’ spectra). all the other pulleys group in the left part of the graph where, even if it is not possible to draw confidence regions for two classes, it is possible to observe a separation related to the pc2 value. all wooden pulleys (as said before, the analysis was performed on the metallic frame) are in the upper part of the graph, figure 5. xrf spectrum acquired on the pincers. the black line is the acquired spectrum, while the computed baseline is in green. figure 6. xrf spectrum acquired on one of the three-legged lewises. the black line is the acquired spectrum, while the computed baseline is in green. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 6 characterized by higher values of arsenic and manganese. on the other hand, metallic pulleys are characterized by lower values for pc2, due to higher concentration of zinc. pca processing was then performed on spectra acquired on bronze components (see figure 8). as it is possible to observe, in this case a clear clustering was found, as highlighted by the confidence ellipses drawn according to the gaussian mixture model for probability distribution. spectra acquired on the wheels of the metallic pulleys are characterized by a higher amount of zinc, tin and nickel. on the contrary the bronze used for the nut of the ‘turnbuckle1’ has a higher concentration of lead, antimony and iron. this is an important finding because the drawing of a turnbuckle noticeably similar to ‘turnbuckle1’ is present in the ‘taccuino senese’ by giuliano da sangallo [22]. so, if we assume that the metallic pulleys presumably do not belong to the renaissance era, as can be argued considering their design, this clear difference in the bronze composition can be taken as a confirmation for the attribution of this turnbuckle to the brunelleschi era. finally, last pca processing was performed on the iron alloy spectra collected on the two turnbuckles in order to investigate possible similarities or differences among their constituent materials; the result is presented in figure 9. as can be seen, it is possible to draw two confidence regions. the first one, smaller, contains the spectra collected on ‘turnbuckle1’ except those acquired on the hook and on the arch where the two connecting rods are nailed. actually, these three spectra fall in the other confidence region, which includes all points of analysis taken on ‘turnbuckle2’. the alloy used for ‘turnbuckle1’ is richer in copper and manganese, while the other has a higher relative concentration of zinc, arsenic and nickel. this particular clustering can be explained considering that investigated objects were tools used in everyday works, so it was not uncommon to perform repairs or to substitute broken parts. thus, it is possible to conclude that the two components (the hook and the arch) belonging to ‘turnbuckle 1’ but falling in the ‘turnbuckle 2’ confidence ellipse could have been substituted in a more recent period, using an alloy similar to the one constituting ‘turnbuckle 2’. analysing by means of pca also the remaining tools, it was not possible to highlight any relevant clustering; the only observable grouping was related to spectra collected on the same object. so, no other analogies were found among alloys. 4. conclusions this study analysed some of the tools employed in the construction of the santa maria del fiore cupola. thanks to xray fluorescence measurements, it was possible to investigate, for the first time and with a totally non-invasive approach, the composition of the alloys constituting these objects, which have a primary importance both from the technical and historical point of view. then, thanks to chemometric analysis, analogies and differences among alloys were examined. it was possible to discriminate between the different iron alloys employed for the pulleys, which can be discriminated in two typologies belonging to different historical times. the use of pca allowed also to highlight the presence of two bronze alloys (one used for threated components and one for the wheels of the metallic pulleys) and two iron alloys used for the turnbuckles. the use of xrf analysis does not allow to draw univocal conclusions on dating of these objects, as this technique is not even specifically intended for this purpose. anyway, these findings, if supported also by historical sources and by the work of curators, can give new insights on the world of technology in the renaissance era. acknowledgement the authors would like to thank marcello del colle and samuele caciagli for the technical support during the measuring campaign. figure 7. score and loading plot of the first two components (pc1-pc2) calculated from xrf spectra acquired on pulleys. pulleys labelled from 1 to 5 are ‘wooden’ pulleys (see figure 1e), while those from 6 to 8 are the ‘metallic’ ones (see figure 1d). percent variance captured by each pc is reported in parenthesis along each axis. figure 8. score and loading plot of the first two components (pc1-pc2) calculated from xrf spectra acquired on bronze components. percent variance captured by each pc is reported in parenthesis along each axis. figure 9. score and loading plot of the first two components (pc1-pc2) calculated from xrf spectra acquired on iron parts of the two turnbuckles. percent variance captured by each pc is reported in parenthesis along each axis. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 7 references [1] p. innocenzi, the innovators 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semenov, d. kirsanov, application of chemometric methods to xrf-data a tutorial review, analytica chimica acta, 1040 (2018), pp. 19-32. doi: 10.1016/j.aca.2018.05.023 https://doi.org/10.1016/j.ijproman.2013.05.003 https://doi.org/10.1140/epjp/i2018-12368-3 http://dx.doi.org/10.21014/acta_imeko.v10i1.805 https://www.imeko.org/publications/tc4-archaeo-2020/imeko-tc4-metroarchaeo2020-098.pdf https://www.imeko.org/publications/tc4-archaeo-2020/imeko-tc4-metroarchaeo2020-098.pdf https://doi.org/10.1109/mim.2021.9448250 https://jmlr.csail.mit.edu/papers/volume12/pedregosa11a/pedregosa11a.pdf https://jmlr.csail.mit.edu/papers/volume12/pedregosa11a/pedregosa11a.pdf https://doi.org/10.1021/ac60214a047 https://doi.org/10.1366/0003702894202201 https://doi.org/10.1016/j.jrras.2017.04.003 https://doi.org/10.1017/s0003598x00040941 https://doi.org/10.1007/s12613-012-0570-x https://archive.org/details/gri_33125001082557 https://doi.org/10.1557/proc-352-701 https://doi.org/10.1002/xrs.1015 https://doi.org/10.1039/c0ja00243g https://doi.org/10.1007/s00339-007-4231-2 http://dx.doi.org/10.21014/acta_imeko.v10i1.894 https://doi.org/10.1186/s40494-019-0246-1 https://doi.org/10.1016/j.talanta.2020.120785 https://doi.org/10.1016/j.aca.2018.05.023 introductory notes for the acta imeko second issue 2022 acta imeko issn: 2221-870x june 2022, volume 11, number 2, 1-3 acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 1 introductory notes for the acta imeko second issue 2022 francesco lamonaca1 1 department of department of computer science, modeling, electronics and systems engineering (dimes), university of calabria, ponte p. bucci, 87036, arcavacata di rende, italy section: editorial citation: francesco lamonaca, introductory notes for the acta imeko second issue 2022, acta imeko, vol. 11, no. 2, article 2, june 2022, identifier: imekoacta-11 (2022)-02-02 received june 30, 2022; in final form june 30, 2022; published june 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: francesco lamonaca, e-mail: editorinchief.actaimeko@hunmeko.org dear readers, the second issue 2022 of acta imeko collects contributions that do not relate to a specific event. as editor-in-chief, it is my pleasure to give readers an overview of these papers, with the aim of encouraging potential authors to consider sharing their research through acta imeko. modern applications in virtual reality require a high level of fruition of the environment as if it was real. in applications that have to deal with real scenarios, it is important to acquire both its three-dimensional (3d) structure and details to enable the users to achieve good immersive experiences. in the paper entitled “omnidirectional camera pose estimation and projective texture mapping for photorealistic 3d virtual reality experiences”, by a. luchetti et al., the authors propose a method to obtain a mesh with high quality texture combining a raw 3d mesh model of the environment and 360° images. the main outcome is a mesh with a high level of photorealistic details. the paper entitled “the importance of physiological data variability in wearable devices for digital health applications”, by g. cosoli et al., aims at characterizing the variability of physiological data collected through a wearable device (empatica e4), given that both intraand inter-subject variability play a pivotal role in digital health applications, where artificial intelligence (ai) techniques have become popular. inter-beat intervals (ibis), electrodermal activity (eda), and skin temperature (skt) signals have been considered and variability has been evaluated in terms of general statistics (mean and standard deviation) and coefficient of variation. results show that both intraand inter-subject variability values are significant, especially when considering those parameters describing how the signals vary over time. moreover, eda seems to be the signal characterized by the highest variability, followed by ibis, contrary to skt which results more stable. s. avdiaj et al., in the paper “measurements of virial coefficients of helium, argon and nitrogen for the needs of static expansion method”, study the influence of virial coefficients on the realization of primary standards in vacuum metrology, especially in the realization of the static expansion method. in the paper they present the measured data for virial coefficients of three gases, namely helium, argon, and nitrogen, measured at room temperature and a pressure range from 3 kpa to 130 kpa. in the new optical pressure standard ultra-low expansion (ule) glass cavities were proposed to measure helium refractivity for a new realisation of the unit of pressure, pascal. however, it was noticed that the use of this type of material causes some difficulties. one of the main problems of ule glass is the pumping effect for helium. therefore, instead of ule, zerodur glass was proposed as a material for the cavity. this proposal was given by the vacuum metrology team of the physikalisch-technische bundesanstalt ptb in the quantumpascal project. in order to calculate the flow of helium gas through zerodur glass one has to know the permeation constant k. in the paper “measurements of helium permeation in zerodur glass used for the realisation of quantum pascal”, a. kurtishaj et al. measured the permeation of helium gas in zerodur in the temperature range from 80 °c to 120 °c. experimental results assess that zerodur material has the potential to be used as cavity material for the new quantum standard of pressure. s. ondera et al., in the paper entitled “dose reduction potential in dual-energy subtraction chest radiography based on the relationship between spatial-resolution property and segmentation accuracy of the tumor area”, investigated the relationship between the spatial-resolution property of soft tissue images and the lesion detection ability using u-net. the aim of the paper is to explore the possibility of dose reduction during energy subtraction chest radiography. an informed type a evaluation of standard uncertainty is derived in the paper entitled “an informed type a evaluation of standard uncertainty valid for any sample size greater than or equal to 1”, authored by c. carobbi, based on bayesian analysis. the result is mathematically simple, easily interpretable, applicable both in the theoretical framework of the guide to the expression of uncertainty in measurement (propagation of mailto:editorinchief.actaimeko@hunmeko.org acta imeko issn: 2221-870x june 2022, volume 11, number 2, 2-3 acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 2 standard uncertainties) and in that of the supplement 1 of the guide (propagation of distributions), valid for any size greater than or equal to 1 of the sample of present observations. g. campobello et al., in the paper “on the trade-off between compression efficiency and distortion of a new compression algorithm for multichannel eeg signals based on singular value decomposition” investigate the trade-off between the compression ratio and distortion of a recently published compression technique specifically devised for multichannel electroencephalograph (eeg) signals. this paper extends a previous one in which the authors proved that, when singular value decomposition (svd) is already performed for denoising or removing unwanted artifacts, it is possible to exploit the same svd for compression purpose by achieving a compression ratio in the order of 10 and a percentage root mean square distortion in the order of 0.01 %. in this article, authors successfully demonstrate how, with a negligible increase in the computational cost of the algorithm, it is possible to further improve the compression ratio by about 10 % by maintaining the same distortion level or, alternatively, to improve the compression ratio by about 50 % by still maintaining the distortion level below the 0.1 %. in the paper entitled “a strategy to control industrial plants in the spirit of industry 4.0 tested on a fluidic system”, l. fabiano et al. propose a strategy of automating the control of wide spectrum industrial processes plants in the spirit of industry 4.0. the strategy is based on the creation of a virtual simulator of the operation of the plants involved in the process. through the digitization of the operational data sheets of the various components, the simulator can provide the reference values of the process control parameters to be compared with their actual values, to decide the direct inspection and/or the operational intervention on critical components before a possible failure. n. covre et al., in “monte carlo-based 3d surface point cloud volume estimation by exploding local cubes faces”, propose a state-of-the-art algorithm for estimating the 3d volume enclosed in a surface point cloud via a modified extension of the monte carlo integration approach. the algorithm consists of a preprocessing of the surface point cloud, a sequential generation of points managed by an affiliation criterion, and the final computation of the volume. the pre-processing phase allows a spatial re-orientation of the original point cloud, the evaluation of the homogeneity of its points distribution, and its enclosure inside a rectangular parallelepiped of known volume. the affiliation criterion using the explosion of cube faces is the core of the algorithm, it handles the sequential generation of points, and proposes the effective extension of the traditional monte carlo method by introducing its applicability to the discrete domains. in the paper “3d shape measurement techniques for human body reconstruction”, i. xhimitiku et al. investigate and compare the performances of three different techniques for 3d scanning. in particular, two commercial tools (smartphone camera and ipad pro lidar) and a structured light scanner (go!scan 50) have been used for the analysis. first, two different subjects have been scanned with the three different techniques and the obtained 3d models were analysed in order to evaluate the respective reconstruction accuracy. a case study involving a child was then considered, with the main aim of providing useful information on performances of scanning techniques for clinical applications, where boundary conditions are often challenging (e.g. non-collaborative patient). finally, a full procedure for the 3d reconstruction of a human shape is proposed, in order to set up a helpful workflow for clinical applications. high-resolution x-ray computed micro-tomography (ct) is a powerful technique for studying the processes of crack propagation in non-homogenous quasi-brittle materials such as rocks. to obtain all the significant information about the deformation behaviour and fracture characteristics of the studied rocks, the use of a highly specialised loading device suitable for the integration into existing tomographic setups is crucial. since no adequate commercial solution is currently available, a completely newly-designed loading device with a four-point bending setup and vertically-oriented scanned samples is proposed and used in the paper “study of fracture processes in sandstone subjected to four-point bending by means of 4d xray computed micro-tomography”, authored by l. vavro et al. this design of the loading procedure, coupled with the high stiffness of the loading frame, allows the loading process to be interrupted at any time and for ct scanning to be performed without the risk of the sudden destruction of the scanned sample. m. s. latha gade et al. in the paper “a cost-efficient reversible logic gates implementation based on measurable quantum-dot cellular automata” describe experimental and analytic approaches for measuring design metrics of reversible logic gates using quantum-dot cellular automata (qca), such as ancilla input, garbage output, quantum cost, cell count, and area, while accounting for the effects of energy dissipation and circuit complexity. the parameters of reversible gates with modified structures are measured and then compared with the existing designs. human facial expressions are thought to be important in interpreting one's emotions. emotional recognition plays a very important part in the more exact inspection of human feelings and interior thoughts. over the last several years, emotion identification utilizing pictures, videos, or voice as input has been a popular issue in the field of study. recently, most emotional recognition research focuses on the extraction of representative modality characteristics and the definition of dynamic interactions between multiple modalities. deep learning methods have opened the way for the development of artificial intelligence products, and the suggested system employs a convolutional neural network (cnn) for identifying real-time human feelings. the aim of the research study proposed by k. pranathi et al. in the paper “video-based emotion sensing and recognition using convolutional neural network based kinetic gas molecule optimization” is to create a real-time emotion detection application by utilizing improved cnn. this research offers information on identifying emotions in films using deep learning techniques. kinetic gas molecule optimization is used to optimize the fine-tuning and weights of cnn. in “development of a contactless operation system for radiographic consoles using an eye tracker for severe acute respiratory syndrome coronavirus 2 infection control: a feasibility study”, m. sato et al. propose noncontact operation system for radiographic consoles that used a common eye tracker system facilitating noncontact operation of radiographic consoles for patients with covid-19 to reduce the need for frequent disinfection. experimental tests show that the proposal can be applied even if the operator uses a face shield. thus, its acta imeko issn: 2221-870x june 2022, volume 11, number 2, 3-3 acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 3 application could be important in preventing the transmission of infections. j. y. blaise et al. in “acquisition and integration of spatial and acoustic features: a workflow tailored to small-scale heritage architecture” report on an interdisciplinary data acquisition and processing chain, the novelty of which is primarily to be found in a close integration of acoustic and spatial data. the paper provides a detailed description of the technological and methodological choices that were made in order to adapt to the particularities of the corpus studied (interiors of small scale rural architectural artefacts). the research outputs pave the way for proportion-as-ratios analyses, as well as for the study of perceptual aspects from an acoustic point of view. ultimately, “perceptual” acoustic data characterised by acoustic descriptors will be related to “objective” spatial data such as architectural metrics. multiplication provides a substantial impact on metrics like power dissipation, speed, size and power consumption. a modified approximate absolute unit is proposed by y. nagaratnam et al. in the paper “a modified truncation and rounding-based scalable approximate multiplier with minimum error measurement” to enhance the performance of the existing approximate multiplier. the proposed multiplier can be applied in image processing and shows an error of 0.01% while actual solutions show a typical error of 0.40%. always in the field of approximate multipliers, in “low-power and high-speed approximate multiplier using higher order compressors for measurement systems”, m. v. s. ram prasad et al. proposed an innovative architecture that in the implementation of a fir filter allows to achieve a delay of 27 ns versus the 119 ns achieved by the exact multiplier taken as reference. finally, the technical note authored by franco pavese is a comment which addresses the paper published in this journal “is our understanding of measurement evolving?” and authored by luca mari. this technical note concerns specific parts of that paper, namely the statements: “doubt: isn’t metrology a ‘real’ science? … metrology is a social body of knowledge”, “measurements are aimed at attributing values to properties: since values are information entities, any measurement must then include an informational component” and “what sufficient conditions characterise measurement as a specific kind of property evaluation?”, and discusses alternatives. also in this issue, high-quality and heterogeneous papers are presented, confirming acta imeko as the natural platform for disseminating measurement information and stimulating collaboration among researchers from many different fields. in particular, the technical note shows how acta imeko is the right place where different opinions and points of view can meet and compare, stimulating a fruitful and constructive debate in the scientific community of measurement science. i hope you will enjoy your reading. francesco lamonaca editor in chief introductory notes for the acta imeko first issue 2022 acta imeko issn: 2221-870x march 2022, volume 11, number 1, 1 1 acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 1 introductory notes for the acta imeko first issue 2022 francesco lamonaca1 1 department of department of computer science, modeling, electronics and systems engineering (dimes), university of calabria, ponte p. bucci, 87036, arcavacata di rende, italy section: editorial citation: francesco lamonaca, introductory notes for the acta imeko first issue 2022, acta imeko, vol. 11, no. 1, article 1, march 2022, identifier: imekoacta-11 (2022)-01-01 received march 30, 2022; in final form march 30, 2022; published march 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: francesco lamonaca, e-mail: editorinchief.actaimeko@hunmeko.org dear readers, the first issue of a new year is the time for balances. in the last year acta imeko has moved huge steps towards the speedup of the publication time and the attracting of high value papers. these improvements were also confirmed by the fact that acta imeko is now indexed by the directory of open access journals (doaj). doaj is one of the most important community-driven, open access service in the world and has a reputation for advocating best practices and standards in open access. it indexes and provides access to high quality, open access, peer-reviewed journals, only. doaj's basic criteria for inclusion have become the accepted way of measuring an open access journal's adherence to standards in scholarly publishing, especially as concern with ethical and quality standards. this was achieved thanks to the evaluable work of the editorial board, the section editors, the reviewers and, last but not least, all the authors that have selected acta imeko for sharing their research with the scientific community. this issue includes the special issue on the ‘imeko tc4 international conference on metrology for archaeology and cultural heritage’ guest edited by fabio santaniello, michele fedel, annaluisa pedrotti and the special issue ‘innovative signal processing and communication techniques for measurement and sensing systems’ guest edited by zia ur rahman. the first special issue collects papers on a hot topic archaeology and cultural heritage studied from the metrological point of view. the fact that the topic is hot is evident: although in full pandemic time, the conference has been a remarkable success with 158 initial submissions, 126 accepted papers, 431 authors from 19 countries, 4 invited plenary speakers, 13 special sessions, 3 tutorial sessions and 11 patronages. the presented papers have highlighted the natural need of exchange of knowledge and expertise between ‘human sciences’ and ‘hard sciences’. this contamination is evident also in the extended papers published in this issue. the special issue on ‘innovative signal processing and communication techniques for measurement and sensing systems’ consists of fifteen papers identifying new perspectives and highlighting potential research issues and challenges in the contest of measurement and sensing. specifically, this special issue will demonstrate how the emerging technologies could be used in future smart sensing systems. the topics are heterogeneous and includes measurements for and by antennas, artificial intelligence, beam forming techniques, body area networks, embedded processors, image sensors and processing, internet of things, knowledge-based systems, machine learning algorithms, medical signal analysis, sensor data processing, vlsi architectures and many more. many novelties are foreseen in this new year, for example the articles of the next issue will be online since the end of the month, and it will be closed at the end of june as foreseen. this new publication policy will strongly reduce the publication time of the submitted papers, they will be available online and indexed by scopus as soon as they are ready. in order to keep the articles of different ‘special issues’ close together in the table of contents, we will not use consecutive page numbering throughout the issue beginning with this volume 11. the page numbers of every single article start with one and end with the number of the article’s pages. we hope that you will enjoy your readings and that you can confirm acta imeko as your main source to find new solutions and ideas and a valuable resource for spreading your results. francesco lamonaca, editor in chief mailto:editorinchief.actaimeko@hunmeko.org experimental study on sar reduction from cell phones acta imeko issn: 2221-870x june2021, volume 10, number 2, 147 -152 acta imeko | www.imeko.org june2021 | volume 10 | number 2 | 147 experimental study on sar reduction from cell phones marius-vasile ursachianu1, ovidiu bejenaru1, catalin lazarescu1, alexandru salceanu2 1 romanian "national authority for management and regulation in communications" (ancom), romania 2 “gheorghe asachi” technical university of iasi, romania section: research paper keywords: absorbed incident energy; human exposure measurement; near field exposure; cst simulation; electromagnetic field dosimetry; sr en 62209-1 citation:marius-vasile ursachianu, ovidiu bejenaru, catalin lazarescu, alexandru salceanu, experimental study on sar reduction from cell phones, acta imeko, vol. 10, no. 2, article 21, june 2021, identifier: imeko-acta-10 (2021)-02-21 section editor: ciro spataro, university of palermo, italy received january 24, 2021; in final form april 29, 2021; published june 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: alexandru salceanu, e-mail: asalcean@tuiasi.ro 1. introduction the possible effects of ambient electromagnetic fields on human beings are general sources of concern and legitimate questions. for this reason, scientific research in the field has been strongly supported by diverse bodies and organizations, national and international, being materialized through the adoption of recommendations, setting limits and developing guidelines. this multitude and diversity provided results which, although fundamentally convergent in nature, were quite different in form. this is the main motivation for the current trend towards harmonization, providing benefits for authorities, industry and clients. essentially, the study on human exposure has two important parts: first setting limits and second establishing the correct measurement to verify compliance with previously accepted limits. for the first part, based on extremely in-depth interdisciplinary research, most countries around the world have given a positive response to the rational scientific underpinning the icnirp limits. these reference levels have also been assumed by the international telecommunication union (the united nations specialized agency) and the world health organization. these limits are approximately in compliance with the north american fcc guidelines developed by the federal communications commission, office of engineering & technology. in terms of the measurement techniques and practice there is even greater diversity. for example, the assessment of specific absorption rate of human exposure to radio frequency fields from hand-held wireless communications devices is an important concern of the profile organizations, especially since the approaches are in a permanent dynamic. for example, the well-known ieee standard 1528, first issued in 2003, has been significantly updated and completed in 2013 and 2020, respectively. its quasi-universal character has been enhanced through the adoption by the international electrotechnical commission of the two measurement standards in place iec 62209 (part 1, also here applied and part 2). as concerning the penetration of electromagnetic radiations in the human body for different radio frequencies, icnirp limits for the specific absorption rate (sar) have been assumed: 0.08 w/kg on average for the whole human body, respectively abstract the actual problem of the human exposure to different types of electromagnetic field sources is a challenging one and should be considered an up to date issue due to one major actual trend: the increasingly intensive penetration of various wireless communication technologies in virtually all places and times that make up our daily lives. the here presented paper presents an experimental study focused on measurements for three types of mobile phones, belonging to different generations, operating at the two characteristic frequencies of the gsm bandwidth. we have been used a satimo-comosar evaluation dosimetry system, provided by the liceter laboratory of ancom romania. the determined values of the absorbed incident energy by the tissue of a mannequin (phantom) model for the human head, which is part of the dosimetry system, have been also compared with those obtained in the case when the mobile phones are protected by multilayer cases, aiming to study their possible limiting effect. the influence that ”touch” or ”tilt” positions might have on absorbed (by the human head model) incident energy values has also been investigated. the comparative processing of the obtained results allowed the formulation of recommendations on reducing the exposure to electromagnetic radiation associated with the use of the mobile phone. this paper is an extended and improved version of the original contribution to the imeko tc 4 2020 virtual conference. mailto:asalcean@tuiasi.ro acta imeko | www.imeko.org june2021 | volume 10 | number 2 | 148 2 w/kg for sar located in the head or trunk area (general public exposure). these sar values are averaged over 6 minute exposure time and 10 g of tissue. [1], [2], [3], [4]. the main objective of this paper is to determine and to compare the sar values, for three different generation mobile phones, using a satimo-comosar system and considering different exposure scenarios. two of these phones are old models, being released on market in 2009 and respectively in 2012. the third is a newer model, released on market in 2018. we have been interested to observe if the generation of the mobile phones (design, position of antenna, housing materials) has a significant impact over sar values (determined for different positions of mobile phone related to the head phantom, sam). the specific anthropomorphic mannequin (sam) has been designed to provide a conservative estimation of the actual peak spatial specific absorption rate (sar) of the electromagnetic field radiated by mobile phones, [5].complementary, for the new smartphone released in 2018, the effect that ”touch” or ”tilt” positions might have on sar values has also been studied. the determined sar values have been also compared with those obtained in the case when the mobile phones are protected by multilayer cases, aiming to study these cases possible limiting impact. the shape of the sam physical model has been derived in a percentage of 90% from the shape of an adult male head; its dimensions have been reported in [6]. the shape of the ears has been adapted to represent the flattened ones of a phone user. various studies propose different kind of shields for mobile phones to reduce the amount of power absorbed in the head, aiming to minimize the health effects [7], [8]. moreover, researches focused on sar reduction due to the type of antennas (pifa or helical), placed at the top or at the bottom of the device, have been also carried out [9], [10], [11]. finally, the sar values directly determined by us using the calibrated satimo-comosar dosimetry system for different exposure scenarios have been compared with each other and related to the limits accepted by the standards. ourstudies here presented could lead to the development(also including the direct involvement of human health specialists and bodies) to a series of recommendations and informal guidelines for effective reduction of human exposure to electromagnetic fields generated by wireless communications systems. 2. material and methods the dosimetric quantity used for the evaluation of absorbed incident energy by the tissue is the specific absorption rate (sar). this parameter has been introduced for measuring the rate of energy absorbed by human body when it is exposed to a radio frequency electromagnetic field, the formula being given by the equation (1): 𝑆𝐴𝑅𝑙 = 𝜎 2 𝜌𝑚 |�⃗� 𝑖| 2 = 𝜔 𝜀0 𝜀𝑟 " 2 𝜌𝑚 |�⃗� 𝑖| 2 , (1) where ρm is the material density (in an elementary volume), σ is the electric conductivity, εr" represents relative electric permittivity as a frequency depending function while sarl represents a local quantity, [12]. 2.1. the satimo-comosar system for sar evaluation the dosimetry evaluation system used for our measurements is competent to determine the distribution of sar inside a human head phantom, so called sam (specific anthropomorphic mannequin). the used phantom [13], is in accordance with american and european standards [14], [15]. dosimetry assessment could be done both for the situations when the mobile phone is positioned by the right or by the left ear. the main components of the used system, figure 1, are: kuka kr5 robot with his specific controller kuka krc2sr, [16] with electric field probe (calibrated before the measurement process), the sam twin phantom, the liquids that simulate the human tissue for specific frequencies, a clamping device for the mobile phone under test, a signal generator rohde & schwarz cmu 200 (a gsm base simulator that can control the output power and frequency) and a desktop pc running opensar software, [17]. sam phantom structure is made out from low loss and low permittivity material, embedded in wood. the electric field (immersive probe inside the liquid simulating the dielectric properties of the head for different frequencies) is a triple dipole type (model ep96 – satimo). efield probe provides an omni-directional response. the clamping device for holding the mobile phone is also made from a low permittivity and low losses material, not to have any influence over the measured sar values. the clamping device can be moved along three orthogonal axes ox, oy, oz and it can be rotated around the phantom ear for precise positioning of the phone. the opensar software controls all robot movements, it determines local sar values; as post processing application, it calculates sar values, averaged on 10 g or 1 g of tissue. 2.2. the sar measuring procedure all the phases describing the sar measurement procedure are widely depicted in sr en 62209-1: 2007 [18]. the sar evaluation for different frequencies has been done for each radio channel: low, middle and respectively high. two positions (illustrated in figure 2) have been considered: the normal position (when the phone is on the cheek plane, also called ”touch” or ”cheek” position) and the tilt position (when a) b) c) d) figure 1. the satimo-comosar system used for sar measurements: (a) comosar test bench andkuka robot, (b) signal generator rohde & schwarz cmu 200, (c) computing unit with opensar software installed for testing system control (d) the fastening system used to secure the measuring equipment. acta imeko | www.imeko.org june2021 | volume 10 | number 2 | 149 the phone is tilted by 15 degrees toward cheek plane). only one measurement location (of the sam phantom) has been selected: the right ear. the sar evaluation has been done for two frequencies (gsm-900 and gsm-1800 bandwidth), 897 mhz and 1747 mhz, respectively. the mobile phone has been used with its internal transmitter, antenna, battery and all accessories supplied by the manufacturer. it is important as the battery to be fully charged before each test, for every case of exposure scenario taken into consideration. complementary, in figure 3 is presented the front and the back side of huawei p20 pro mobile phone, inserted in a multilayer protective case made from hard plastic material. for every position of the mobile phone tested for sar evaluation, the following conditions should be fulfilled: existence of permanent radio connection between the base station simulator and mobile phone device at maximum power; sar measurement should be done in a network of equally spaced points on a surface located at a constant distance from the inner surface of the phantom; sar measurement should be done in equidistant points, in a cube located around the place where the maximum value of the field has been determined (by the probe who scan inside the phantom); calculation of the measured sar as average value for 1 g and 10 g; any others perturbation sources must be avoided inside the test room or in immediate vicinity. figure 4 shows a human user holding the huawei p20 pro mobile phone with the protective case in the cheek (touch) position, respectively the tilt position, the two selected situations of exposure. 3. results and discussions operating procedure used during these measurements: a gsm communication has been established between the mobile phone under test and the base station simulator cmu200 for measuring the specific absorption rate (sar). the gsm 900 experimental conditions (for cheek or tilt positions, right or left) include: phantom – right head and left head; signal – tdma (crest factor: 8.0); channel – middle; frequency – 897.59 mhz (uplink); relative permittivity (real part) – 41.5; relative permittivity (imaginary part) – 19.4; conductivity (s/m) – 0.967. the gsm 1800 experimental conditions(for cheek or tilt positions, right or left) include: phantom – right head and left head; signal – tdma (crest factor: 8.0); channel – middle; frequency – 1747.4 mhz (uplink); relative permittivity (real part) – 40.102; relative permittivity (imaginary part) – 14.096; conductivity (s/m) – 1.368. the sar values for the huawei p20 pro mobile phone subject to dosimetric evaluation are shown in table 1 for different positions (left and right side of the sam phantom). the values of the sar for the samsung gt-s6102 mobile phone subjected to dosimetric evaluation are shown in table 2 for the same placement of the phone: left and right side of the sam phantom. the corresponding sar values for the nokia 2330c-2 mobile phone are synthesized in table 3. a comparison of the sar values when huawei p20 pro, samsung gt-s6102, nokia 2330c-2 mobile phones are positioned on the right side of the sam phantom is presented in table 4 (the test frequency in the gsm-900 band). this comparison has been performed aiming to see if the sar values averaged over 10 g of tissue for samsung and nokia phones are higher than the corresponding value for the huawei phone. the maximum value of sar 10g for huawei, samsung and nokia mobile phones investigated in this evaluation dosimetry study is represented by opensar software in 2d graphical a) b) figure 2. the cheek (a) and the tilt (b) positions of the huawei p20 pro mobile phone on the right side (ear) of sam phantom. a) b) figure 3. the front side (a) and the back side (b) of the huawei p20 pro mobile protected by a multilayered protective case. a) b) figure 4. the huawei p20 pro phone with a protective case: cheek (a) and tilt (b) position, the two most common positions to the user's cheek. acta imeko | www.imeko.org june2021 | volume 10 | number 2 | 150 representation as a rectangular surface for the right or for the left part of sam phantom face, according to the place where the phone have been placed for sar evaluation. around the position where the maximum sar has been located, position marked with the red color, the software draws a rectangular surface. in figure 5 is represented the surface sar for huawei, samsung and nokia mobile phones on the left – cheek position, 897.59 mhz, gsm-900 frequency. the opensar software also can associate the surface where the maximum value of sar 10g have been found to a specific volume concentrated around this maximum value of sar determined by probe during the scan process inside sam phantom. in figure 6 is represented the volume sar for huawei, samsung and nokia mobile phones on the left – cheek position, gsm-900 frequency band. the sar values for the huawei p20 pro mobile phone protected by a multilayer plastic case are shown in table 5 for the right side of the sam phantom, for both frequencies. the comparative graphical distribution of sar values (averaged on 10g tissue) following the dosimetric evaluation of the huawei p20 pro (with and without protective case) is shown table 1. sar values for different positionsof huawei p20 pro mobile phone relativeto the sam phantom, two frequencies (gsm-900 and gsm-1800 bandwidth). phone: huawei bandwidth channel position sar peak w/kg sar 10g w/kg sar 1g w/kg gsm 900 middle right – cheek 0.57 0.2854 0.4149 gsm 900 middle right – tilt 0.28 0.1375 0.1945 gsm 900 middle left – cheek 0.36 0.1735 0.2457 gsm 900 middle left – tilt 0.16 0.0877 0.1196 gsm 1800 middle right – cheek 0.34 0.1365 0.2282 gsm 1800 middle right – tilt 0.14 0.0422 0.0713 gsm 1800 middle left – cheek 0.15 0.0594 0.0974 gsm 1800 middle left tilt 0.05 0.0244 0.0358 table 2. sar values for different positionsof samsung gt-s6102 mobile phone relative to the sam phantom, two frequencies (gsm-900 and gsm1800 bandwidth). phone: samsung bandwidth channel position sar peak w/kg sar 10g w/kg sar 1g w/kg gsm 900 middle right – cheek 2.31 0.7514 1.4469 gsm 900 middle right – tilt 1.01 0.4936 0.7204 gsm 1800 middle right – cheek 1.40 0.5081 0.9121 gsm 1800 middle right – tilt 0.61 0.2268 0.3898 gsm 1800 middle left – cheek 1.14 0.4909 0.8165 gsm 1800 middle left tilt 0.45 0.1686 0.2942 table 3. sar values for different positionsof nokia 2330c-2 mobile phone relative to the sam phantom, two frequencies (gsm-900 and gsm-1800 bandwidth). phone: nokia bandwidth channel position sar peak w/kg sar 10g w/kg sar 1g w/kg gsm 900 middle right – cheek 1.92 0.8813 1.3496 gsm 900 middle right – tilt 1.13 0.5301 0.7887 gsm 1800 middle right – cheek 1.89 0.5611 1.0970 gsm 1800 middle right – tilt 0.37 0.1386 0.2466 gsm 1800 middle left – cheek 1.36 0.4709 0.8726 gsm 1800 middle left tilt 0.76 0.2356 0.4791 gsm 1800 middle left tilt 0.32 0.1056 0.1799 table 4. sar max (10g) values for huawei vs samsung and vs nokia mobile phones, bandwidth: gsm-900, positions: right – cheek and right – tilt. phone sar 10g (rightcheek) w/kg sar 10g (righttilt) w/kg samsung vs huawei (rightcheek) samsung vs huawei (righttilt) nokia vs huawei (rightcheek) nokia vs huawei (righttilt) huawei 0.2854 0.1375 samsung 0.7514 0.4936 2.63 3.58 nokia 0.8813 0.5301 3.08 3.85 a) b) c) figure 5. surface sar for huawei (a), samsung (b), nokia (c) mobile phones for 897.59 mhz on left – cheek. a) b) c) figure 6. volume sar for huawei (a), samsung (b), nokia (c) mobile phones for 897.59 mhz, left – cheek. table 5. sar values for different positions of huawei p20 pro phone with the multilayer protective case relative to the right part of sam phantom bandwidth channel position sar 10g w/kg sar1g w/kg gsm 900 middle right–cheek 0.1007 0.1525 gsm 900 middle right–tilt 0.0323 0.0443 gsm 1800 middle right–cheek 0.0348 0.0574 gsm 1800 middle right –tilt 0.0140 0.0235 figure 7. comparative graphical distribution of sar values (10 g of tissue) for the dosimetric evaluation of the huawei p20 pro: with and without multilayer protective case acta imeko | www.imeko.org june2021 | volume 10 | number 2 | 151 in figure 7. four situations have been considered: cheek and tilt positions, for both frequencies of interest. the sar values recorded by the dosimetric evaluation of the huawei p20 pro (with and without case) are synthesized in table 6 (for cheek and respectively tilt positions). figure 8 represents the surface sar for huawei p20 pro phone, without case, on the left – cheek position for 897.59 mhz frequency. the opensar software can also associate the surface where the maximum value of sar 10g have been found to a specific volume concentrated around this maximum value of sar determined during the scan process inside sam phantom. figure 9 represents the volume sar for huawei, p20 pro phones without multilayer protective case on the left – cheek position, gsm-900 frequency band. 4. conclusions this paper present a set of sar measurements performed for three, different generations, mobile phone devices: huawei p20 pro, samsung gt-s6102 and nokia 2330c-2, respectively. there is an over decade difference in term of market release between these phone models. the huawei p20 pro mobile phone is the newest one, being released on the market in 2018, while the two other mobile phones have been released on market in 2012 (samsung) and even in 2009 (nokia). we have also determined the sar values when huawei mobile phone was provided with a multilayer plastic protective case. the novelty in the study presented in this paper is the fact a first examination of data presented in the tables has shown that results are generally consistent with published tests done by other laboratories. according to our measurements, the cheek sar values are higher than tilt sar values, and 1-g sar values are higher than 10-g sar values. also, the 900-mhz sar values are higher than 1800-mhz ones. these findings have theoretical support and are in good agreement with most of the other results revealed in literature. we have noticed an anomaly in our measurements for nokia mobile phone (the orange marked field in table 3) for gsm1800 on the left side of the phantom in tilt position: the sar values were higher than those determined on the right side of the phantom, for the same position. this anomaly was due to the position of the phone in the clamping device. after we have carefully verified and repositioning the phone, the measurement was reset and correct data has been taken. the situation illustrates the occasional abnormal, inexplicable values that have been also reported in comparative studies between laboratories. similar studies developed in other laboratories (so called intercomparison measurements) support the expectation that for a given mobile phone, frequency and position, sar measurements on the left and right ear positions of the sam phantom should be very close in value. when they are not, we should check for a user error in phone placement or in data recording. as a first recommendation, the tilt position should be preferred by any user. the designers of huawei p20 pro placed the antenna at the bottom, to be farther away from the user’s brain. this is a major advantage versus the older mobile phones, their antenna being positioned on the top of the device. as in principle expected, the lowest values of sar had been recorded when the phone was provided with protective case. regarding the use of a protective casing, it is important that the material from which is made to be good absorbent; the protective cases having conducting insertions should be avoided, mainly due to unexpected and uncontrolled reflections. future studies on this topic should involve testing different types of cases, to comparatively track their impact on sar values. contrariwise, the transmission efficiency of the phone provided with protective case decreases. as a shortcoming, in this situation the battery will run out faster because it will try to give more power to ensure coverage, respectively a better signal reception. anyway, in real, daily environment, the sar values might vary depending on propagation conditions. a general conclusion could be the following: a combination of factors such as the positioning of the antenna, the size of the device, the relative position to the human head, equipping the phone with a protective case, can lead to lower sar values, regardless the type of the mobile terminal. table 6. comparisons between sar values (10 g) given by dosimetric evaluation of huawei p20 pro (provided or not with a protective case). bandwidth position sar 10g in w/kg huawei (no case vs with case) huawei (no case) huawei (with case) gsm 900 right cheek 0.2854 0.1007 2.83 gsm 900 right tilt 0.1375 0.0323 4.25 gsm 1800 right cheek 0.1365 0.0348 3.92 gsm 1800 right tilt 0.0422 0.0140 3.01 figure 8. surface sar, huawei p20 pro phone without protective multilayer case. figure 9. volume sar for huawei p20 pro mobile phone without protective multilayer case. acta imeko | www.imeko.org june2021 | volume 10 | number 2 | 152 the dosimetry evaluation here presented demonstrates that sar maxim values for 10 g of tissue determined for all three mobile phones are smaller than the icnirp limit: 2 w/kg (head region). carrying out rigorous measurements, in the case of most diverse exposure scenarios, with the correct processing of the results is an important resource to remove exaggerated fears, but also to develop recommendations and guidelines for effective reduction of human exposure to environmental electromagnetic fields. in this frame of such universal interest, any rigorous technical-scientific approach should be definitely welcomed. acknowledgment this paper could be developed due to the collaboration agreement settled between ”gheorghe asachi” technical university of iasi, faculty of electrical engineering and liceter accredited laboratory of ancomromania. references [1] international commission on non-ionizing radiation protection, guidelines for limiting exposure to time-varying electric, magnetic, and electromagnetic fields (up to 300 ghz), health physics, vol. 74, 2020, pp. 494-521. [2] ieee, ieee standard for safety levels with respect to human exposure to radio frequency electromagnetic fields, 3 khz to 300 ghz, c95.i2005. new york: institute of electrical and electronics engineers, 2005. online [accessed 20 june 2021] https://emfguide.itu.int/pdfs/c95.1-2005.pdf [3] european committee for electrotechnical standardization (cenelec) prestandard env 50166-2, human exposure to electromagnetic fields. high frequency (10 khz to 300 ghz). online [accessed 20 june 2021] https://standards.globalspec.com/std/85205/env%2050166-2 [4] order nr. 1193 from 29 september 2006 for the approval of the norms regarding the limitation of the general population exposure to electromagnetic fields from 0 hz to 300 ghz. online [accessed 20 june 2021], [in romanian] https://www.ancom.ro/uploads/links_files/odinul_1193_2006 _norme.pdf [5] w. kainz, a. christ, t. kellom, s. seidman, n. nikoloski, b. beard, n. kuster, dosimetric comparison of the specific anthropomorphic mannequin (sam) to 14 anatomical head models using a novel definition for the mobile phone positioning, physics in medicine and biology 50(14), august 2005, pp. 34233445. doi: 10.1088/0031-9155/50/14/016 [6] c. c. gordon, t. churchill, c. e. clauser, b. bradtmiller, j.t. mc conville, i. tebbetts, r. a. walker, 1988 anthropometric survey of u.s. army personnel: methods and summary statistics. technical report natick/tr-89/044, u.s. army natick research, development and engineering center, massachusetts: natick, sep. 1989. online [accessed 20 june 2021] http://mreed.umtri.umich.edu/mreed/downloads/anthro/ansur /gordon_1989.pdf [7] s. aqeel abdulrazzaq, s. jabir, j. aziz, sar simulation in human head exosed to rf signals and safety precautions, ijcset, september 2013, vol 3, issue 9, pp. 334-340. online [accessed 20 june 2021] http://www.ijcset.net/docs/volumes/volume3issue9/ijcset2013 030908.pdf [8] prabir kumar dutta, pappu vankata yasoda jayasree, viriyala satya surya narayana srinivasa baba, sar reduction in the modelled human head for the mobile phone using different material shields, hum. cent. comput. inf. sci. 6 (2016), art. 3. doi: 10.1186/s13673-016-0059-0 [9] m. r. iqbal-faruque, n. aisyah-husni, md. ikbal-hossain, m. tariqul-islam, n. misran, effects of mobile phone radiation onto human head with variation of holding cheek and tilt positions, journal of applied research and technology, volume 12, issue 5, october 2014, pp. 871-876. doi: 10.1016/s1665-6423(14)70593-0 [10] l. belrhiti, f. riouch, a. tribak, j. terhzaz, investigation of dosimetry in four human head models for planar monopole antenna with a coupling feed for lte/wwan/wlan internal mobile phone, journal of microwave, optoelectronics and electromagnetic applications, vol. 16, no. 2, june 2017, doi: 10.1590/2179-10742017v16i2748 [11] ovidiu bejenaru, catalin lazarescu, alexandru salceanu, valeriu david, study upon specific absorption rate values for different generations of mobile phones by using a satimo-comosar evaluation dosimetry system, 12th international conference and exhibition on electromechanical and energy systems, sielmen 2019, chisinău, moldova, 10-11 october 2019, pp. 1-5. doi: 10.1109/sielmen.2019.8905798 [12] m. a. stuchlyans, s. stuchly, experimental radio and microwave dosimetry, in polk ch., postow e., handbook of biological effects of electromagnetic fields, (sec. edition), crc press, boca raton, n. y., london, washington dc, 1996, pp. 295-336. [13] sam phantom on mvg website. online [accessed 20 june 2021] https://www.mvg-world.com/en/products/sar/saraccessories/sam-phantom [14] en 50361: basic standard for the measurement of specific absorption rate related to human exposure to electromagnetic fields from mobile phones (300 mhz 3 ghz), 2001. online [accessed 20 june 2021] https://standards.globalspec.com/std/532912/en%2050361 [15] ieee standard 1528-2003: ieee recommended practice for determining the peak spatial-average specific absorption rate (sar) in the human head from wireles communications devices: measurement techniques, 19 december 2003, pp.1-120 doi: 10.1109/ieeestd.2003.94414 [16] industrial robots on kuka website. online [accessed 20 june 2021] https://www.kuka.com/en-de/products/robotsystems/industrial-robots [17] opensar v5 on on mvg website. online [accessed 20 june 2021] https://www.mvgworld.com/en/products/field_product_family/sar-38/opensarv5 [18] asro, standard sr en 62209-1: human exposure to radio 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https://doi.org/10.1186/s13673-016-0059-0 http://dx.doi.org/10.1016/s1665-6423(14)70593-0 http://dx.doi.org/10.1590/2179-10742017v16i2748 https://doi.org/10.1109/sielmen.2019.8905798 https://www.mvg-world.com/en/products/sar/sar-accessories/sam-phantom https://www.mvg-world.com/en/products/sar/sar-accessories/sam-phantom https://standards.globalspec.com/std/532912/en%2050361 https://doi.org/10.1109/ieeestd.2003.94414 https://www.kuka.com/en-de/products/robot-systems/industrial-robots https://www.kuka.com/en-de/products/robot-systems/industrial-robots https://www.mvg-world.com/en/products/field_product_family/sar-38/opensar-v5 https://www.mvg-world.com/en/products/field_product_family/sar-38/opensar-v5 https://www.mvg-world.com/en/products/field_product_family/sar-38/opensar-v5 https://magazin.asro.ro/ro/standard/117718 skin potential response for stress recognition in simulated urban driving acta imeko issn: 2221-870x december 2021, volume 10, number 4, 117 123 acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 117 skin potential response for stress recognition in simulated urban driving pamela zontone1, antonio affanni1, alessandro piras1, roberto rinaldo1 1 polytechnic department of engineering and architecture, university of udine, via delle scienze 206, 33100 udine, italy section: research paper keywords: stress recognition; electrodermal activity; skin potential response; machine learning; 3d driving simulator citation: pamela zontone, antonio affanni, alessandro piras, roberto rinaldo, skin potential response for stress recognition in simulated urban driving, acta imeko, vol. 10, no. 4, article 20, december 2021, identifier: imeko-acta-10 (2021)-04-20 section editors: roberto montanini, università di messina and alfredo cigada, politecnico di milano, italy received july 23, 2021; in final form december 7, 2021; published december 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: pamela zontone, e-mail: pamela.zontone@uniud.it 1. introduction paying attention to drivers’ mental wellbeing is crucial to improve safety in road traffic. if not properly treated, stress can lead drivers to engage in risky behaviours [1] and therefore car accidents [2]. a danger situation occurs whenever stress is caused by the driving activity itself as happens to professional [3] and regular drivers [4], or by personal issues as highlighted in [5] in case of economic reasons, or any other kind of reason as described in [6]. a hidden markov model (hmm) system to assess the probability of assuming certain behaviours considering the current emotion is developed in [7]. in [8], a survey describing the methods to recognize emotions in drivers is also provided. the development of stress detection systems follows two main paths [9]. one is based on physiological signals, including electrodermal activity (eda), electroencephalogram (eeg), blood volume pulse (bvp), electromyography (emg), skin temperature (skt) and respiration (resp) [10], [11]. the second one relies on physical manifestations of stress: data describing human behaviour, for example, could be collected by the global positioning system [12] and facial expressions [13]. a common approach is to identify a stress condition with the aid of machine learning (ml) and deep learning (dl) techniques, as in [14]-[16] where the properties of eeg, ecg and eda signals respectively are exploited for classification purposes. in [17] different kernel configurations for support vector machines (svms) are tested, and then applied to electromyographic signals. an automated way to find the optimal kernel has been used in [18], where the kernel for a deep multiple kernel support vector machine (d-mkl-svm) is selected through a multiple-objective genetic algorithm (moga). the classifier is then used on ecg data. different physiological measurements, eda and ecg signals, are combined in [19], where features are automatically extracted from short signal sections and classified by a multimodal convolutional neural network (cnn). abstract in this paper, we address the problem of possible stress conditions arising in car drivers, thus affecting their driving performance. we apply various machine learning (ml) algorithms to analyse the stress of subjects while driving in an urban area in two different situations: one with cars, pedestrians and traffic along the course, and the other characterized by the complete absence of any of these possible stress-inducing factors. to evaluate the presence of a stress condition we use two skin potential response (spr) signals, recorded from each hand of the test subjects, and process them through a motion artifact (ma) removal algorithm which reduces the artifacts that might be introduced by the hand movements. we then compute some statistical features starting from the cleaned spr signal. a binary classification ml algorithm is then fed with these features, giving as an output a label that indicates if a time interval belongs to a stress condition or not. tests are carried out in a laboratory at the university of udine, where a car driving simulator with a motorized motion platform has been prearranged. we show that the use of one single spr signal, along with the application of ml algorithms, enables the detection of possible stress conditions while the subjects are driving, in the traffic and no traffic situations. as expected, we observe that the test individuals are less stressed in the situation without traffic, confirming the effectiveness of the proposed slightly invasive system for detection of stress in drivers. mailto:pamela.zontone@uniud.it acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 118 a physical approach can be found in [20]. driver’s expressions and eye movements are recorded by near-infrared (nir) camera sensors, and then aggressive driving behaviour is classified by a cnn. the method proposed in [21] combines both physiological (electrodermal activity) and behavioural (facial) measurements, and fuses together different data types in order to build a sensor fusion emotion recognition (sfer) system, improving the classification performance. in previous works, the authors carried out some experiments with the aid of a driving simulator platform, recreating a highway [22], [23] and inducing stress by adding obstacles along the course. we collected the ecg in addition to the skin potential response (spr) data, and we employed ml and dl algorithms to detect stress in the test individuals. in [24], the authors compared the different physiological responses under manual and autonomous driving tests. in [25], we also examined the possible changes in the physiological responses when different car settings are considered. one of the main contributions of this paper is the analysis and comparison of the performance results of different ml models (extending the work in [26]), which we demonstrated to be valuable in detecting stress episodes in previous experiments, but now considering the stress caused by urban traffic. in this work, moreover, we simplify the system and consider one signal only, i.e., the spr signal taken from the hands of the driver. in this way, we propose a slightly invasive setup, which can be arranged with little discomfort for the driver. in detail, we log spr values from the two hands of individuals while they drive. we apply a motion artifact (ma) removal algorithm, to assess the artifacts that can alter the signal caused by the hand movements turning the wheel. this algorithm outputs a single spr signal, without artifacts, which is fed to an ml classifier, which has been previously trained using a larger dataset. the classifier marks time intervals with a “stress” or “not stress” flag. the individuals were told to drive normally, in an urban setting simulated by the city car driving 3d software simulator. the experiment was setup in a way to present two different situations. one situation recreates an urban area with no traffic and empty streets, while the second recreates an urban area complete with traffic, with cars and pedestrians. findings of this study validate the success of the supervised learning algorithm in its stress detection task. we also demonstrate that spr signals, recorded with minimally invasive and simple sensors, along with ml classifiers, can detect stress in a reliable way. in the end, we observe that, as expected, stress is generally higher in the urban environment filled with traffic. the paper is structured as follows. in the next section we present the fundamental blocks of our proposed system. section 3 introduces the experimental setup. section 4 discusses the results obtained from our comparative study, where different ml algorithms are used for driver’s stress recognition. finally, some conclusions are drawn in section 5. 2. proposed system the proposed measurement system for stress detection in car drivers is shown in figure 1. each subject under test wears the spr sensors on the wrists and is seated on the driving simulator available in the biosens lab at the university of udine. the simulator is composed of a moving platform with two axes (dof reality professional p2), a steering wheel with pedals and gearbox (logitech g29), and a curved screen. for each subject two different simulations are performed on the same city route with two different conditions: “no traffic” and “traffic”. “no traffic” means that there are no other cars nor pedestrians on the road, “traffic” means that we inserted in the simulation other cars and pedestrians with some aggressive events (e.g., lane invasion of other cars or unexpected pedestrian road crossing), as also described in section 3. during the entire route planned in the simulations we acquired the spr signals on the subjects positioning the sensors shown in figure 2 on the wrist like a smartwatch. the differential voltages from the palm and the back of each hand (vp-vb in figure 2) are properly conditioned and acquired by a 12 bit a/d converter on board a dsp with sample rate of 200 sa/s. data are then sent using a low power wifi module which operates at 115.2 kbps baud rate. the detailed description of the sensors and their characterization is provided in [27], [28]. summarizing the architecture and specifications, the sensor analog front end is a band-pass differential amplifier (having input impedance 100 mω) with maximum input range ±10 mv figure 1. block scheme of our proposed stress detection measurement system. figure 2. electrodes arrangement and spr sensor block diagram. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 119 and bandwidth in the [0.08, 8] hz range. the accuracy of spr acquisition, after characterization, resulted in 0.15 % of full scale (corresponding to 30 μv) and the resolution is 4.9 μv. the sensors are battery operated with a single lipo cell with a capacity of 850 mah ensuring ten hours of transmission, since the current consumption is 85 ma. the sensors form a body network where one spr sensor acts as slave (henceforth sensor 1) and the other acts as master (henceforth sensor 2). the slave sensor sends packets to the master and the latter aligns the received data packets with the data acquired by the a/d converter. for consumption reasons, the slave can send packets every 40 ms at minimum. hence, the slave dsp builds a packet composed of eight data acquired every 5 ms and sends them to the master. the module is configured as station (sta) with static ip and operates as a udp client. the gateway address is configured to be the master address. figure 3 shows how the packets are built by the dsp on the slave before transmission. the a/d module provides a 12 bit datum every 5 ms. each byte sent via uart to the master must be identified with a unique code, since the master must recognize if the incoming datum is the upper or lower byte of the slave sample. so, the dsp of the slave builds the lower (upper) byte of information using the six least (most) significant bits of the a/d, adding one bit for lower (upper) byte (l or h bit in figure 3, respectively). the data packets received by the master are dismantled and realigned as in figure 4. the master adds a unique header and builds a packet composed of 18 bytes and containing the information on spr1 and spr2. the packet is then sent to a laptop every 40 ms. the data transmitted from the master are acquired by a dedicated graphical user interface developed in the .net environment, and are then processed by a motion artifact (ma) removal algorithm described in [22], [29]. the two spr signals acquired from the left and right hand of the subjects are processed by the ma algorithm in order to provide as output a single signal that better represents the activity of the autonomic nervous system (ans). as a matter of fact, the spr signals can be typically affected by motion artifacts due to pressure on the sensors during hand movements. ideally, the two spr signals should have approximately the same pattern, since they represent the response to the same stimulus, initiated by the sympathetic response of the ans. the ma removal algorithm is based on two assumptions, the first being that motion artifact enhances the local energy of the signal. the second being that the motion artifacts rarely appear simultaneously in the spr signal of both hands. the output of the ma removal block is thus obtained by computing a weighted combination of the two input spr signals, evaluating their local energy, giving more weight to the less perturbed signal, i.e., the one, between the two input signals, with the least local energy value. in our experiments (see [22]), we found that the motion artifact rarely appears simultaneously in both hands. this, in fact, mostly appears during the steering wheel action, which is predominantly performed by one hand (as also discussed in [24]). after being processed through the ma removal bock, the cleaned spr signal is then sent to various ml classification algorithms, which had already been trained on a bigger dataset. this dataset, including 3195 intervals for each stress and nonstress class, is the result of a previous experiment carried out in the vi-grade firm (vi-grade.com), utilizing their professional dynamic simulator. more specifically, in that case, 18 subjects manually drove for 67 km along a highway, trying to cross 12 obstacles, positioned in prearranged points along the track. these obstacles were: double lane change (right to left or left to right), tire labyrinth, sponsor block (from left or from right), slalom (from left or from right), lateral wind (from left or from right), jersey lr, tire trap, stop. we divided the cleaned spr signal in 15 s time interval blocks, after normalization to leverage the signal amplitudes among subjects, and for each block we computed five statistical features: the interval variance, the energy, the mean absolute value, the mean absolute derivative, and the maximum absolute derivative. each interval overlapped the previous one by 10 s, so we could derive a new feature vector every 5 s. in particular, we could define exactly the obstacle location and span, during which the individuals were supposed to be in a stress state. in this way, we could assign a flag equal to “1” to all of the intervals happening to fall or intersect with the stress episodes, and a flag equal to “0” to all the others, i.e., the ones happening to fall outside these stress episodes. finally, after classification of the test set, we applied a re-label step to address the issue related to the number of single and anomalous “1” flags [22]. we were able to compare the results of an svm, a random forest (rf) classifier, a decision tree (dt), and a k-nearest neighbours (k-nn) classifier, which provided a similar accuracy of about 73 %, with only the k-nn presenting a slightly lower value (68 %). all of the ml classifiers were implemented using matlab (2017.a), and a 10-fold cross validation phase was considered for all of these algorithms. the bayesian optimization was also used during the training procedure for all of the classifiers (for hyperparameter tuning). a radial basis function (rbf) kernel was employed for the svm model (see also [23]). 3. experimental setup as already stated, the test was carried out by using a driving simulator, consisting of a 3d driving simulator software and a motorized platform, located in a lab at the university of udine. the experiment employed 10 test subjects, students of the university of udine. they were asked to drive along a predefined track, in an urban area simulated by the city car driving software. the software enables the creation of an urban area with a nearby motorway, complete of car traffic, with the option of adding different stress-inducing factors, like pedestrian crossing, and vehicles unexpectedly changing lane (also from the opposite direction) or braking suddenly. these stressors do not occur figure 3. construction of the packets on the slave for transmission. figure 4. realignment of the packets on the master. http://vi-grade.com/ acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 120 exactly at the same location and time. however, the type and multiplicity of these stress-inducing events is similar between different simulations. the complete track is displayed in figure 5. the green solid line represents the motorway, and the orange solid line represents the city route. the subjects were asked to drive in two different situations: in the first there is a complete lack of traffic with no cars and people, whereas the second situation has car and pedestrian traffic. in this second situation, the traffic volume is kind of low, but the behaviour of traffic was set to “very aggressive”, where the cars and pedestrians act in a more unpredictable and temperamental way, with cars intruding in the subject’s way, or pedestrians crossing the road in forbidden points. one-half of the individuals started with the no traffic situation first, and then proceeded with the traffic situation (i.e., subjects 1, 2, 3, 4, and 10), while the remaining 50 % did the opposite order (i.e., subjects 5, 6, 7, 8, and 9). completing the track in figure 5 takes on average 10 minutes, with a similar required time to complete the motorway and urban section. 4. experimental results all of the spr data collected from the 10 test subjects, after they had driven along a course in an urban area recreated by city car driving, are then cleaned through the ma removal block. these output signals are scaled to make a meaningful comparison possible, i.e., for each subject and for each driving condition, we standardize the corresponding signal using the mean and standard deviation resulting from the concatenation of both signals coming from the two driving conditions, with traffic and no traffic, for that subject. these standardized signals are ultimately fed to an ml algorithm. more specifically, the same five spr features introduced in the previous section are extracted from each 15 s interval. we make a new interval start 5 seconds after the start of the previous one (therefore each interval is overlapping the previous one by 10 seconds). the various ml classifiers introduced in the previous section are only used for the test phase. in the end, we can look at all of the labels that each classifier gives as output and calculate the final number of labels equal to “1” or “0”, according to the intervals that it labels as “stress” or “non-stress”, for each subject and each driving situation (with and without traffic). table 1 displays the percentage of labels equal to “1” (with stress), when considering the svm, rf, dt, and k-nn classifiers, for each subject, and taking into account the entire complete test course in the traffic and non-traffic conditions. as an example, for the rf classifier, we show in figure 6 the graphical representation of the values reported in table 1. as we can deduce looking at all of the classifiers’ results, the no traffic situation appears to be less stressful than the traffic situation for all of the subjects excluding subject 4. in addition, there are some figure 5. graphical representation of the course: it comprises a motorway and an urban route. table 1. total number of intervals marked as “stress” in %, for each classifier and for each subject in the two driving conditions (with traffic and no traffic). the numeric difference of labels between the two conditions (traffic no traffic) is also shown. svm / subject 1 2 3 4 5 6 7 8 9 10 mean traffic 51.82 68.00 58.86 57.47 41.94 60.00 80.33 94.49 87.60 96.43 69.69 no traffic 22.55 60.31 55.56 65.22 5.04 1.63 75.00 54.62 38.98 91.87 47.08 traffic no traffic 29.26 7.69 3.31 -7.75 36.89 58.37 5.33 39.87 48.62 4.56 22.62 rf / subject 1 2 3 4 5 6 7 8 9 10 mean traffic 56.93 72.00 59.49 62.07 44.35 64.44 77.05 93.70 89.26 92.14 71.14 no traffic 26.47 65.65 54.70 66.09 6.72 0.81 72.58 57.98 37.29 89.43 47.77 traffic no traffic 30.46 6.35 4.79 -4.02 37.63 63.63 4.47 35.72 51.97 2.71 23.37 dt / subject 1 2 3 4 5 6 7 8 9 10 mean traffic 56.20 70.40 60.76 63.22 42.74 64.44 84.43 96.06 90.91 96.43 72.56 no traffic 27.45 65.65 57.26 74.78 5.88 3.25 76.61 57.14 41.53 92.68 50.22 traffic no traffic 28.75 4.75 3.49 -11.56 36.86 61.19 7.81 38.92 49.38 3.75 22.33 k-nn / subject 1 2 3 4 5 6 7 8 9 10 mean traffic 56.93 70.40 58.86 63.79 42.74 63.70 75.41 92.91 87.60 90.71 70.31 no traffic 27.45 67.94 54.70 76.52 5.04 2.44 75.00 55.46 40.68 79.67 48.49 traffic no traffic 29.48 2.46 4.16 -12.73 37.70 61.26 0.41 37.45 46.93 11.04 21.82 figure 6. total number of intervals labelled as “stress” by the rf classifier. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 121 individuals where the difference between the positive labels in the two situations is higher (e.g., subjects 5, 6, and 9), while for others this difference is lower (e.g., subjects 3 and 7). we can try to explain this in different ways. maybe the pressure of taking a test changed the expected stress reaction, or the outcome could be influenced by the order of the simulated situation experienced first by the subjects (traffic and no traffic, or the other way around). still, for 90 % of the subjects the resulting “stress” interval count is higher in the traffic situation. in figure 7 we show the output of the rf classifier for subject 9, where the difference between the positive labels in the traffic and no traffic situation is among the biggest positive ones we observe comparing all of the classifiers. for the sake of simplicity, we only plot the positive labels using a grey stem, located at the end of the corresponding 15 s classified spr interval. the labels corresponding to the non-stress case are not included in the figure. the cleaned and normalized spr signals of the subject in the two different situations (with traffic and no traffic) are also shown in a blue continuous line. the output of the dt classifier for the same subject is displayed in figure 8 (here the difference is slightly lower than the one obtained with the rf). in figure 9 the output of the k-nn classifier for subject 4 is reported instead. this is the only subject where the difference between the positive labels in the traffic and no traffic scenario is always negative, for all of the classifiers. this negative difference is the biggest for the k-nn case. in figure 10 we display a last example considering the svm classifier’s output for subject 3. as we can notice, the classifiers well identify the increased stress level throughout the entire simulations. figure 7. output of the rf classifier for subject 9 in the two situations (without traffic and with traffic). figure 8. output of the dt classifier for subject 9 in the two situations (without traffic and with traffic). acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 122 5. conclusions in this paper, we described a stress detection system that allows us to identify stress of drivers. our system classifies overlapping 15 s signal blocks, and can therefore provide a classification output every 5 s, with a small delay in a real-time application and a good localization in time. the test subjects drove in a simulated urban environment, utilizing a car driving simulator located in our biosens lab at the university of udine. we logged two spr signals, from their hands, and we processed these signals through an ma removal block. we computed some features from the resulting single signal, and we sent them as input to different ml algorithms, thus comparing the final results. we showed that, regardless of the ml algorithm used, all of the subjects, except one, appeared more stressed when driving in an urban area prearranged with traffic. therefore, through the use of a low-complexity spr data acquisition sensor and the application of ml algorithms, we could effectively recognize stress states arising in drivers. references [1] l. bowen, s. l. budden, a. p. smith, factors underpinning unsafe driving: a systematic literature review of car drivers, transportation research part f: traffic psychology and behaviour 72 (2020) pp. 184-210. doi: 10.1016/j.trf.2020.04.008 [2] g. miyama, m. fukumoto, r. kamegaya, m. hitosugi, risk factors for collisions and near-miss incidents caused by drowsy bus drivers, international journal of environmental research and public health 17(12) (2020). doi: 10.3390/ijerph17124370 figure 9. output of the k-nn classifier for subject 4 in the two situations (without traffic and with traffic). figure 10. output of the svm classifier for subject 3 in the two situations (without traffic and with 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1-6. doi: 10.1109/sas.2019.8706023 https://doi.org/10.1016/0003-6870(94)90003-5 https://doi.org/10.1017/cbo9780511635373 https://doi.org/10.11604/pamj.2020.36.47.21382 https://doi.org/10.1186/s12544-020-00441-7 https://doi.org/10.3390/ijerph17196962 https://doi.org/10.1145/3388790 https://doi.org/10.1109/mce.2016.2590178 https://doi.org/10.1109/taffc.2017.2764896 https://doi.org/10.1109/iros.2008.4650870 https://doi.org/10.1109/access.2020.2998156 https://doi.org/10.1109/tip.2015.2496314 https://doi.org/10.1109/taffc.2017.2768030 https://doi.org/10.1089/tmj.2017.0250 https://doi.org/10.3390/s19071659 https://doi.org/10.3390/s21093155 https://doi.org/10.3390/s20051474 https://doi.org/10.3390/s21072381 https://doi.org/10.3390/rs12030587 https://doi.org/10.3390/s21062166 https://doi.org/10.1109/tbme.2020.2987168 https://doi.org/10.25046/aj050603 https://doi.org/10.3390/s20092494 https://doi.org/10.23919/eusipco47968.2020.9287446 https://doi.org/10.1109/metroautomotive50197.2021.9502867 http://dx.doi.org/10.21014/acta_imeko.v8i1.562 https://doi.org/10.3390/s20072026 https://doi.org/10.1109/sas.2019.8706023 acta imeko  september 2014, volume 3, number 3, 22 – 27  www.imeko.org    acta imeko | www.imeko.org  september 2014 | volume 3 | number 3 | 22  enhanced measurement of high aspect ratio surfaces by  applied sensor tilting  alexander schuler 1 , albert weckenmann 1 , tino hausotte 2   1  friedrich‐alexander‐universität erlangen‐nürnberg, chair quality management and manufacturing metrology (qfm),  naegelsbachstrasse 25, 91052 erlangen, germany  2  friedrich‐alexander‐universität erlangen‐nürnberg, institute of manufacturing metrology (fmt),   naegelsbachstrasse 25, 91052 erlangen, germany      section: research paper  keywords: profilometry; servo system; uncertainty; simulation  citation: alexander schuler, albert weckenmann, tino hausotte, enhanced measurement of high aspect ratio surfaces by applied sensor tilting, acta imeko,  vol. 3, no. 3, article 6, september 2014, identifier: imeko‐acta‐03 (2014)‐03‐06  editor: paolo carbone, university of perugia   received june 4 th , 2013; in final form july 31 th , 2014; published september 2014  copyright: © 2014 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: this work was supported by the german research foundation (dfg), germany  corresponding author: alexander schuler, e‐mail: alexander.schuler@fau.de      1. introduction  for tactile surface measurements the surface topography is superimposed with the shape of the probing element and a morphological filtering is performed [1]. the actual contact point varies depending on the local surface slope and the tip’s form. in contrast to 3d coordinate measuring machines with 3d force sensors, tactile scanning systems like profilometers or afms (atomic force microscope) use a fixed probing force direction, assuming a probing point at the front of the applied tip [2], [3]. without its actual knowledge large deviations occur on steep surfaces as the expected reference contact point is left. the resulting deviation can be compensated arithmetically to a certain degree [4], but when the probing point leaves the spherical front, the recorded trace can no longer be unambiguously reconstructed, practically limiting the application range. usual tactile tips feature a conical or pyramidal base with a very small apex radius, limited by mechanical stability. for profilometers the base angle is usually up to 90 ° with apex radii up to 2 µm. figure 1 (left) shows the surface reconstruction when the front of the sensor tip is assumed as a probing point. during flank contacts the deviation increases significantly and especially occurs during the measurement of workpieces with high slopes and high aspect ratios as e.g. cutting tools, freeform lenses or microstructures like micro lens arrays. this problem restricts the application of tactile surface probing to surfaces with limited curvature [5]. a probing principle is investigated to lift this restriction. an orthogonal orientation to the surface is kept during the measurement by using a dynamic surface slope dependent tip rotation, displayed in figure 1 (right). research has shown that the deviation on steep flanks can be reduced by using angled probes. but this leads to a higher measurement deviation for the rest of the sample [6]. with repeated measurements under different angles and the application of sensor data fusion the whole surface can be acquired. yet this requires lots of time and introduces additional contributions to the measurement uncertainty [7], [8], [9]. instead by using sensor tilting during measurement both disadvantages are avoided. to reach this goal, research has been carried out [10]. in a first step the contact behaviour and the tilting process was implemented into a simulation environment (section 2). this allowed an early abstract  during tactile surface measurements the contact point between probing tip and surface varies depending on the local surface angle.  to  reduce  the  resulting  measurement  deviation  on  high  slopes  a  probing  principle  is  investigated  that  applies  a  dynamic  surface  dependent sensor tilt. this probing process and the logics for the angle determination have been evaluated by simulation. a test stand  based on a nanometer coordinate measuring machine is developed and fitted with a rotation kinematic based on stacked rotary axes.  systematic  positioning  deviations  of  the  kinematic  are  reduced  by  a  compensation  field.  the  test  stand  has  been  completed  and  results are presented.  acta imeko | www.imeko.org  september 2014 | volume 3 | number 3 | 23  evaluation of the measurement deviation reduction and the development of algorithms to allocate a tilt angle of the tip to a surface point. with the simulation results the kinematic chain was chosen (section 3), sensor and calibration artefacts were selected (section 4) and preliminary tests (section 5) were conducted. afterwards the prototype setup was finished (section 6) and measurement results were acquired (section 7). 2. evaluation by simulation  the working principle of the simulator is based on collision checking between a tip model and a discretized surface model. a simplified 2d representation is selected to reduce the complexity and still keep the informative value for line scans. the tip is modelled, following the standard iso 3274, as a spherical tip and two lines, enclosing the apex angle alpha. the surface profile is integrated as a discretized point cloud with the function p(x,z=f(x)) giving the profile height at the coordinate x. to calculate the optimal probing angle of a specific surface point methods and algorithms are required, referred to as a strategy. several approaches were created and can be categorized in knowledge based, analysing, predicting strategies or a mixture of the aforementioned. a first class of strategies, the knowledge based, uses previous knowledge of the surface profile from an ideal model, e.g. a cad-file. in the simplest case the tangent for each surface point p(x,z) is calculated with pi(x,z) being the point number i to be measured and fstra(pi) the calculated probing angle:         1 1 1 ( ) ( ) tan 2 ( ) i i stra i i i z z f p x x (1) a similar class, the analysing strategies, uses the measurement system itself to create a base profile in a first step. in a second step tilting can be applied. this approach does not require previous knowledge and can be applied with unknown profiles. a third class, predicting strategies, calculates the most suited tilt angle by extrapolating already acquired surface points during the measurement. generally the measurement starts without rotation and the surface points are recorded. the next points are extrapolated based on these values and the applied algorithm. with the estimation of the succeeding profile, the optimal tilt angle is calculated. for this class several extrapolation algorithms were selected and compared. a linear method, a cubic spline method, a newtown method, a lagrange method or an aitken-neville method were implemented. the effectiveness of the strategies and the achievable deviation reduction were compared for different tips and surfaces. relevant evaluation criteria were e.g. the measurement deviation, the calculated tilt angle or the tilt velocity and acceleration. different sample surfaces were created to cover usual practical cases. figure 2 shows a sphere segment with a diameter of 1 mm, found e.g. on lenses, featuring a continuous change of slope. the simulated experiment used a tip apex radius of 10 µm, an opening angle of 60°, a maximal rotation angle of 45° and a linear predicting strategy. at the beginning of the sphere’s edge a flank collision occurs and the resulting deviation suddenly rises. with the change of the extrapolated surface slope, a rotation is triggered and the deviation is reduced until it disappears, limited by the rotation speed and the maximum tilt angle. compared to a regular scan the mean deviation is reduced by 80%. a second structure features an abrupt transition of 18° between two constant slopes, figure 3, focussing on the adaption speed. the slope leads to a constant deviation contribution for a non-tilted case without computational probing point compensation. in this scenario predicting strategies completely reduce the deviation and furthermore analysing strategies can initiate the rotation before the slope change occurs. for the latter case the maximum deviation is reduced from 0.51 µm without tilt, mainly depending on the tip geometry, to 0.13 µm with an analysing strategy. generally all tested strategies offer a significant deviation reduction and increase the detectable surface slope with a tilting system compared to no countermeasures at all. 3. hardware realization  to realize the simulated tilting process in a hardware prototype, a controlled precision servo system is necessary to perform the rotation around the tip’s working point. a high positioning accuracy with low guidance deviation is essential; figure 1. left: no compensation; right: with compensation.   figure 2. linear prediction: sphere segment, 1 mm diameter.   figure 3. strategy comparison: 18° ramp profile.  acta imeko | www.imeko.org  september 2014 | volume 3 | number 3 | 24  elsewise the measurement deviation would instead be increased when the tip leaves the intended scan trace. the basis of the planned test bed consists of a laser-interferometer controlled nanopositioning and measuring machine type sios nmm-1 with an axis resolution of 0.1 nm in three linear axes [11]. in this setup the workpiece is moved by the nmm-1 and the sensor and the rotation unit are installed firmly above. after investigating several kinematic chains, a stacking of two rotary axes was chosen, figure 4. the axis of the first rotation stage is oriented parallel to the cartesian height axis of the nmm-1. a second rotary stage under an angle of 45° is mounted below. the second one carries the sensor which is again angled with 45°. in this setup, the centres of both axes align with the working point of the sensor, apart from alignment deviations. the second axis alters the rotation of the sensor, which corresponds to a rotation around the xand y-axis of the nmm-1’s cartesian coordinate system. with the additional movement of the first rotation stage along the z-axis the sensor can be adjusted to match any necessary angle around the xand y-axis from -90° to 90° covering a hemisphere. the importance of angular axis resolution in this setup is reduced by the in-axis alignment of the sensor. the only remaining non-systematic deviation factors are the radial and axial guidance deviations as well as tilt. positioning window estimations for the selected units result in a maximum deviation of +/1.5 µm for the position of the tip. the upper larger axis has a diameter of 100 mm and uses a direct drive. the chosen model features a non-systematic tilt error motion of less than 1 arcsec and a total accuracy of 2 arcsec after calibration by the manufacturer. the lower smaller axis with 30 mm diameter uses a piezoelectric drive with a stick-slip operation for large angles and a scanning operation for small angles and maintaining a position. apart from guidance deviations also thermal influence sources and their effects have been respected. to reduce the heat generation in the rotary drives the large axis’ control system was fine-tuned to minimize heat generation. additionally the small piezo stage is self-locking and does not require energy to keep a position. thermal effects are also respected in the mechanical setup. the 45° connectors between the rotary stages and the sensor are made of invar, an alloy with a thermal expansion coefficient near zero. concerning systematic deviation components the most significant factor is the assembly of the axes and installation of the sensor tip. to support their alignment process, an alignment help was constructively integrated. the tip holder as well as the mounting plate between the axes are designed with a central bore. this way the centre openings of both rotary axes are not obstructed and an uninterrupted optical path to the sensor tip is available. by using a cmm with a video-sensor and a backlight option the centricity of the sensor tip in the axes’ centre can be easily assessed. furthermore the axes’ centre bores are used for the wiring of the sensor and the smaller axis. it is refrained from an unlimited number of revolutions to simplify the wiring. 4. realization of the test stand  4.1. choice of the applied sensor  to accelerate the realization of the prototype a near-tactile probe was selected, equivalent to the simulated deviation mechanics [12], [13]. the working principle of the sensor prototype is derived from scanning tunnelling microscopy (stm) using the current through the nanometre ranged air gap between a conductive tip and workpiece. as opposed to regular stms no sharp wire tips are used but larger scaled spherical probing tips. the system allows for deliberately shaped tips, ranging from spheres to conical shapes or needles without having to respect mechanical restrictions [13]. for the test stand prototype two tip shapes were chosen. the first scenario uses a hard-metal sphere tip with 0.3 mm diameter equivalent to a tactile probe with a large apex radius. when a probing point at the front of the tip is assumed for reconstruction, the system behaves like a tactile version and the deviation can clearly be shown. the second scenario features a sharp metal needle with a smaller tip angle than a classical profilometer could operate with, equivalent to a near ideal tip. 4.2. calibration artefact  to calibrate the rotation system an artefact is necessary to determine the position of the sensor probing point in relation to the workpiece coordinate system. with this knowledge theoretically all systematic deviation components from the rotation system including alignment deviations can be compensated. as a reference structure and origin of the workpiece coordinate system a calibrated precision sphere grade g5 with a nominal diameter of 4 mm is used. for the spherical sensor tip, its centre results from the origin of the calibration sphere plus the sum of both radii in probing direction. as measurement subject three angled planes with rounded transitions are chosen, figure 5. the angle between each two neighbouring flats is 30°. additionally all flats are angled in the perpendicular direction too. the test subject is part of a cutting insert separated by wire erosion. with the calibration sphere the position of the rotated sensor can be acquired for all angle steps and the systematic positioning deviations can be compensated. figure 4. schematic: a) sensor at 0°; b) sensor at 45°.  acta imeko | www.imeko.org  september 2014 | volume 3 | number 3 | 25  the resulting correction vectors from the workpiece origin to the probing sphere centre are stored in a calibration field, accessed by the machine control during the scan. another compensation vector points from the centre of the sensor tip’s curvature to its reference contact point. for the used spherical tip this vector compensates for the sphere’s radius. the measurement process with the calibration field is performed in the following steps. starting at zero rotation a number of points are recorded. the values are transferred from the machine coordinate system into the workpiece coordinate system with the current compensation vectors. afterwards the algorithms of the chosen strategy are applied, calculating the surface angle and the next suitable tilt angle. for a sufficient large change of angle the probe gets disconnected from the surface and the rotation is performed. an uninterrupted scan without surface disconnection will be realized later. the new probe centre point is loaded from the calibration field and the sensor is brought over the last recorded surface point. afterwards the measurement continues. an optional fine calibration can be applied to acquire a more recent calibration just after the rotation has been performed. 5. preliminary tests  to implement and test all strategy algorithms and measurement sequences before the setup of the stacked axes was finalized, the test stand was realized with a preliminary rotation unit. a simple manual rotation axis with a diameter of 25 mm was installed to test all algorithms, the strategies and the calibration field. as the manual axis’ reproducibility is insufficient and not comparable with the planned automated stages, the described fine-calibration after each rotation was also applied. figure 6 shows all available data merged in the workpiece coordinate system. above the hemisphere as the origin are the probe centres from the calibration field at different angles, displayed with a sphere of 0.3 mm diameter. in the shown case, seven angle positions from 30° to 30° were acquired. due to the used kinematic, the centre points describe a circular path. a first measurement on the angled flats with the described logic is shown in figure 7a) and b), split into the left flank and the right flank. the blue curve represents a scan without tilt, assuming a fixed probing point at the lowest part of the tip, as a profilometer would yield. the green curve uses sensor rotation and assumes a moving reference contact vector always aligned in the rotation direction. the rotation is performed with a fixed angular step width of 10°. it can be seen that both ways result in the same values in the flat top, as expected. in the sloped parts the methods differ as the rotated tip is more capable in keeping the actual probing point near the reference point. this can be clearly seen in the beginning of figure 7a) when the rotated tip starts at 0° and then after some surface points switches to 30° and the deviation is instantaneously reduced. other effects of interest are discontinuities in the rotated scan. they are caused by the sudden change of the assumed probing vector when the axis is turned with a 10° increment. for the demonstrated test case with the manual axis the measurement deviation was reduced down to 18 µm. 6. implementation of the stacked axes  parallel to the functional tests with the manual axis the setup and integration of the two stacked rotary axes were performed. this procedure covered the axes’ mechanical installation and alignment, the software interface to the axes as well to the nmm-1’s control system and the electrical installation of the near-tactile probe. after manufacturing of the invar adapter parts, both axes were mounted onto each other and installed overhead in a mounting frame for testing purposes. the control sequences and the corresponding behaviour had to be tested. as both axis controllers can be directly accessed by a c or c++ library, matlab functions were written to access the library and execute hardware commands. as the nmm-1’s host computer uses matlab for the sequence control the axes can easily be integrated. command sequences as homing the axes, moving to an angle in the axis coordinate system and retrieving the current angle were realized the mechanical integration into the nmm-1 was realized figure 5. calibration sphere and angled flats.  figure 6. workpiece coordinate system and all acquired data.   figure 7. angled flat: a) left transition; b) right transition.  acta imeko | www.imeko.org  september 2014 | volume 3 | number 3 | 26  with a regular plug-in module for sensor exchanges in the nmm-1. for this purpose the nmm-1’s top plate, as part of the metrological frame, has a slot for a plug-in module. the plug-in plate was also made of invar and carries the rotary axes below it. to have sufficient clear space below the top plate, its mounting height was increased by 76 mm by adding invar distance sleeves. the sleeves and the plug-in served to position the sensor tip in the virtual crossing of all laser beams of the nmm-1. this way the abbe-principle for the nmm-1’s axes is fulfilled [14]. during the design the collision volumes of the rotation axes and the nmm-1’s workpiece carrier were relevant as the edge of the corner mirror is higher than the workpiece mounting plane. by safe construction the movement volumes don’t overlap at any position or rotation angle and an involuntary collision is avoided. figure 8 shows the installed axes in the nmm-1. the workpiece carrier and the border of the laser reflectors are located under a white protective casing. for the integration of the sensor an electrical connection between the tip holder and the workpiece was necessary. the tip holder is mounted on the small rotary axis which top plate is electrically isolated. the tip holder is connected to a precision voltage source to set the tip under operating voltage. the workpiece carrier is electrically connected to protective earth, therefore an additional isolation layer was installed. the workpiece is mounted on the carrier plate as already shown in figure 5. the carrier plate is electrically connected by a flexible shielded wire to an amplifier circuit. it converts during operation the current in the nanoampere range to a corresponding voltage in the volt range. this sensor signal is connected to the input port of the nmm-1. on the software side the sensor is integrated by configuring the input ports and recording a sensor characteristic. by storing a third order polynomial in the digital signal processor of the nmm-1 the momentary displacement between sensor and workpiece to a chosen setpoint can be calculated and used for position control during a surface scan. another possible way of operation is the recording of only a single surface point after the setpoint is reached, similar to a touch-trigger probe. with the integration of the stacked axes all steps performed with the manual axis were repeated. first the calibration field was recorded again with angle increments from 30° to +30° in 5° steps in the xz-plane of the nmm-1. in comparison to the manual axis an additional coordinate translation step had to be added to find the corresponding angles of each stacked rotary axis for a given angle in the nmm-1’s cartesian coordinate system. for simplification this nonlinear relationship was calculated beforehand and stored in a lookup table. 7. automated sensor tilting  after implementation of all necessary command sequences an automated measurement on the angled flats was repeated. the angled flats measurement was started again at no rotation and after enough recorded values the surface angle was determined to be 30°. the tip was removed from the surface and rotated by 30 ° along the y-axis perpendicular to the scan-line. with the knowledge of the tip centre position in the workpiece coordinate system the contact point from the last measurement point was determined and re-addressed with the rotated tip. figure 9 shows the system during operation on the cutting insert. the results are displayed in figure 10 showing the first half of a 3 mm line scan. again the blue curve uses a fixed reference contact point like a regular profilometer, the green curve uses the sensor rotation principle and keeps the tip rectangular to the surface. the deviation is again reduced to 18 µm with the large 0.3 mm tip. with the sensor rotation prototype just completed and the first results of an automated measurement process acquired, further aspects can be investigated. the next steps cover the determination of the achievable repeatability of the setup and the measurement on other calibrated artefacts like e.g. a microcontour artefact. 8. conclusions  a probing principle based on dynamic sensor tilting is investigated with a rotation around the sensor’s working point. the measurement deviation mechanics and strategies to deliver optimal rotation angles are modelled in a simulation environment. the results demonstrate the improvement by sensor tilting on practical test samples, reducing measurement deviation and increasing the usable surface slope. the principle was transferred to a hardware prototype based on a nanopositioning and measuring machine with an axis resolution of 0.1 nm as test stand. the kinematic chain for the sensor rotation consists of two stacked rotary axes under an angle of 45° allowing two rotary degrees of freedom. occurring error sources like thermal effects are minimized by the stringent use of materials with low thermal expansion coefficients. to reduce the remaining systematic positioning deviation of the rotation unit an in-situ calibration strategy was realized and applied. the test stand has been completed and a mechanical, electrical and software-side integration was performed. as sensor a proven near-tactile electrical sensor type was applied to accelerate the setup and allows for a wider range of sensor tips. first results of figure 8. axis setup installed in the nmm‐1.  clamped tip with a 0.3 mm sphere 4 mm calibration sphere angled flats (cutting insert) figure 9. tilted measurement process.  acta imeko | www.imeko.org  september 2014 | volume 3 | number 3 | 27  the automated sensor tilting were gathered on a cutting insert with a surface angle range from 30° to +30°. the increase of the possible operation angle and the measurement deviation reduction is demonstrated. future tasks cover the assessment of the repeatability of the rotary sensor positioning and measurements on different calibrated sample surfaces in comparison with regular profilometers. acknowledgement  this article is based on the research project “ultra-accurate acquisition of highly curved surfaces by slope-dependant dynamic sensor tracking with rotatory flexure hinges”. the authors thank the german research foundation (deutsche forschungsgemeinschaft, dfg) for supporting this research. references  [1] m. krystek, iso filters for precision engineering, technisches messen 76 (2009) pp. 133-159. [2] p.m. lonardo, h. trumpold, l. de chiffre, progress in 3d surface microtopography characterization, annals of the cirp 45 (1996) pp. 589-598. [3] a.weckenmann, t. estler, g. peggs, d. mcmurtry, probing systems in dimensional metrology, annals of the cirp 53 (2004) pp. 657-84. [4] d. keller, reconstruction of stm and afm images distorted by finite-sized tips, surface science 253 (1991) pp. 353-364. [5] h.n. hansen, k. carneiro, h. haitjema, l. de chiffre, dimensional micro and nanometrology, annals of the cirp 55 (2006) pp. 721–743. [6] e. lebrasseur, j.-b. pourciel, t. bourouina, t. masuzawa, h. fujita, a new characterization tool for vertical profile measurement of high-aspect-ratio microstructures, journal of micromechanics and microengineering 12 (2002) pp. 280-285. [7] y. hua, c. coggins, s. park, “advanced 3d metrology atomic force microscope”, proceedings asmc 2010, july 11-13, 2010, san francisco, usa, pp. 7-10. [8] f. marinello, p. bariani, a. pasquini, l. de chiffre, m. bossard, g.b. picotto, increase of maximum detectable slope with optical profilers, through controlled tilting and image processing, measurement science and technology 18 (2007) pp. 384-389. [9] a. weckenmann, x. jiang, k.-d. sommer, u. neuschaeferrube, j. seewig, l. shaw, t. estler, multisensor data fusion in dimensional metrology, annals of the cirp 58 (2009) pp.701-721. [10] a. weckenmann, a. schuler, r.j.b. ngassam, “enhanced measurement of steep surfaces by sensor tilting”, proceedings of the 56th international scientific colloquium, sept. 12-16, ilmenau, germany, 2011, id 134:1. [11] e. manske, t. hausotte, r. mastylo, t. machleidt, k.-h. franke, g. jäger, new applications of the nanopositioning and nanomeasuring machine by using advanced tactile and nontactile probes, measurement science and technology 18 (2007) pp. 520-527. [12] a. weckenmann, j. hoffmann, “long range 3d scanning tunnelling microscopy”, annals of the cirp 56 (2007) pp. 525-528. [13] j. hoffmann, a. schuler, nanometer resolving coordinate metrology using electrical probing, technisches messen 78 (2011) pp. 142-149. [14] i. schmidt, t. hausotte, u. gerhardt, e. manske, g. jäger, investigations and calculations into decreasing the uncertainty of a nanopositioning and nanomeasuring machine (npmmachine), measurement science and technology 18 (2007) pp. 482-486. figure 10. cutting insert measurement with the final setup.  uncertainty in mechanical deformation of a fabry-perot cavity due to pressure: towards best mechanical configuration acta imeko issn: 2221-870x september 2022, volume 11, number 3, 1 8 acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 1 uncertainty in mechanical deformation of a fabry-perot cavity due to pressure: towards best mechanical configuration s. moltó1, m. a. sáenz-nuño2, e. bernabeu3, m. n. medina1 1 centro español de metrología, calle alfar 2, 28760 tres cantos, españa 2 instituto de investigación tecnológica, escuela técnica superior de ingeniería-icai, universidad pontificia comillas, 28015 madrid, españa 3 universidad complutense de madrid, españa section: research paper keywords: fabry-perot; pressure measurement; refractrometry; quantum pascal citation: s. moltó, m. a. sáenz-nuño, e. bernabeu, m. n. medina, uncertainty in mechanical deformation of a fabry-perot cavity due to pressure: towards best mechanical configuration, acta imeko, vol. 11, no. 3, article 15, september 2022, identifier: imeko-acta-11 (2022)-03-15 section editor: francesco lamonaca, university of calabria, italy received october 27, 2021; in final form august 30, 2022; published september 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: s. moltó, e-mail: smolto@cem.es 1. introduction the 18sib04 quantumpascal empir project deals with the development of a quantum-based pressure standard in order to improve the uncertainty and traceability that the current methods offer. this project is based on the application of a fabry-perot interferometer to measure in a pressure range of 1 pa to 100 kpa. however, some parameters must be carefully handled, such as the deformation, the temperature and the gas properties. the cavity deformation due to the gas pressure was simulated using a finite element method (fem) software. therefore, a study of the uncertainty obtained in these simulations and its propagation in the evaluation of pressure was needed. a previous study of the simulated cavity deformation due to pressure was carried out in [1], and its results were used herein to show the difference obtained due to the use of different software. finally, a new cylindrical design of the cavity was proposed and compared to the design proposed in [1]. 2. theory the relative deformation in the length with the gas pressure of the fabry-perot cavity can be described with a linear dependency as in equation (1). where 𝐿0 is the without deformation cavity length, δ𝐿 is the change in cavity length (the deformation), 𝑃 is the pressure and 𝜅 is the pressure-normalized deformation. δ𝐿 𝐿0 = 𝜅𝑃 . (1) 𝐿0 is calculated as the distance between the centre of the reflective face of the mirrors, and δ𝐿 as the change in this distance. table 1 summarizes the data used in this work, so is it possible to calculate the pressure with equations below with the values of the table. abstract in this paper, a study of the deformation of a refractometer use to achieve a quantum realization of the pascal is made. first, the propagation of the uncertainty in the measure of pressure due to mechanical deformation was made. then deformation simulations were made with a cavity designed by the cnam and whose results are reported in the 18sib04 quantumpascal empir project. this step aims to corroborate the methodology used in the simulations. the pressure-normalized relative deformation of this design obtained in 18sib04 is (-6.390 ± 0.015) × 10-12 pa-1, the result obtained with our method is (-6.384 ± 0.032) × 10-12 pa-1, which differs 0.001 times from the value obtained in 18sib04. finally, a cylindrical cavity design is presented and simulated obtaining a pressure-normalized relative deformation of (-5.758584 ± 0.00000047) × 10-12 pa-1, which deforms 0.1 times less. mailto:smolto@cem.es acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 2 the gas parameters can be calculated from the refractivity, whose change is related to the change of the laser frequency δ𝜈, the change in the number of modes (δ𝑞) and the deformation coefficient (𝜖) using equation (2), [3]. this equation can be used with absolute pressures up to 10 kpa. δ(𝑛 − 1) = (𝑛 − 1) = δ𝑞 𝑞0 + (1 + 2 𝜉 + 𝜂) δ𝜈 𝜈0 1 + 𝜖 + (1 + 2 𝜉 + 𝜂) δ𝜈 𝜈0 . (2) the deformation coefficient (𝜖) [3] depends on the pressurenormalized deformation (𝜅) as it is shown in equation (3). the change in number of modes is an experimental measurement but it can be simulated using equation (4), where 𝜆0 is the wavelength in vacuum and 𝜆 is the wavelength affected by the change in refractive index (𝜆 = 𝜆0/𝑛). also, δ𝜈 is the laser change in frequency which maximum value is 𝜈𝐹𝑆𝑅 when mode jumping is used. ϵ = 𝜅 𝑃 (𝑛 − 1) = 𝜅 2 𝑘b 𝑇 𝑁a 3 𝐴r (3) δ𝑞 = 𝑞 −𝑞0 = 𝐿 𝜆 2 − 𝐿0 𝜆0 2 = 2𝐿0 [ 𝜆0 + 𝜆0 𝜅 𝑃 −𝜆 𝜆 𝜆0 ] . (4) the molar density is related with the refractivity by the equation (5) [3] and the molar density is related with the pressure by the equation (6) [3]. where 𝐵𝜌 is the first virial coefficient and 𝐵𝑃(𝑇) is the first pressure virial coefficient which is calculated with [−4.571 + 0.1974(𝑇(𝐾)− 300)− 5.137 × 10−4(𝑇(𝐾)− 300)2] ×10−6 m3/mol. 𝜌 = 2 3 𝐴r (𝑛 − 1)[1+ 𝐵�̃�(𝑛 − 1)] (5) 𝑃 = 𝑘b 𝑇 𝑁a 𝜌[1 + 𝐵𝑃(𝑇)𝜌] . (6) 3. uncertainty propagation in order to calculate the uncertainty of pressure due to the uncertainty in 𝜅 is necessary to obtain an expression of pressure depending on 𝜅 directly. from equations (5), (6) and the expression of 𝐵𝑃(𝑇) is possible to calculate that relation (equation (7)), where the value of 𝐶0 is shown in equation (8) 𝑃(δ𝜈,δ𝑞,𝜅,𝑇) = 𝐶0 𝑇 δ𝑞 (𝐶0 𝑇 𝜅 + 1)𝑞0 ( 𝐵𝜌 δ𝑞 (c0 t κ+ 1)𝑞0 +1) ×(1 + 2 δ𝑞( 𝐵𝜌 δ𝑞 (𝐶0 𝑇 𝜅 + 1)𝑞0 +1) 3 𝐴r(𝐶0 𝑇𝜅 + 1)𝑞0 𝐵𝑃(𝑇)) (7) 𝐶0 = 2 𝑁a 𝑘b 3 𝐴𝑟 . (8) considering that δ𝑞 = 2𝐿0(1+𝜅𝑃nominal) 𝜆/2 − 𝑞0 and δ𝜈 = 0 an expression of pressure as 𝑃 = 𝑃(𝜅,𝑃nominal,𝑇) is obtained (equation (9)) 𝑃(𝜅,𝑃nominal,𝑇) = 𝐶0𝑇( 2(𝐿0 𝑃nominal 𝜅 + 𝐿0) 𝜆 − 𝑞0) (𝐶0 𝑇 𝜅 + 1)𝑞0 × ( 𝐵𝜌 ( 2(𝐿0 𝑃nominal 𝜅 +𝐿0) 𝜆 − 𝑞0) (𝐶0 𝑇 𝜅 + 1)𝑞0 + 1) × (9) table 1. coefficients and constants used in this work parameter designation value 𝐿0 cavity length in vacuum 0.1 m 𝑛 refractive index 𝑛 ≥ 1 𝐴r molecular polarizability 4.44613930 × 10 -6 m3/mol [2] 𝐵r first refractive virial coefficient 0.812 × 10 -12 m6/mol2 [2] 𝐵�̃� first density virial coefficient −[1+4(𝐵𝑟/𝐴𝑟 2)]/6 [2] 𝐵𝑃(𝑇) first pressure virial coefficient [−4.571+0.1974(𝑇(𝐾)−300)−5.137×10 −4(𝑇(𝐾)−300)2]×10−6 m3/mol [2] 𝜉 mirror dispersion coefficient 11 × 10-6 [2] 𝜂 gas dispersion coefficient 1 × 10-6 [2] 𝜆0 wavelength in vacuum 632.8 nm 𝜆 wavelength affected by pressure 𝜆 = 𝜆0/𝑛 𝜈0 frequency in vacuum 𝑐/𝜆0 𝑞0 number of length modes in vacuum 2𝐿0/𝜆0 δ𝜈 laser change in frequency assumed as 0 δ𝑞 change in number of modes estimate in equation (4) 𝜖 deformation coefficient equation (3) [3] 𝑇 temperature 273 k 𝑃 nominal pressure from 1pa to 10 kpa 𝑁𝐴 avogadro number used value of [3] 𝑘𝐵 boltzmann constant used value of [3] acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 3 ( 2𝐵𝑃 ( 2(𝐿0 𝑃nominal 𝜅 +𝐿0) 𝜆 + 𝑞0) 3 𝐴r(𝐶0 𝑇 𝜅 +1)𝑞0 × ( 2(𝐿0 𝑃nominal 𝜅 +𝐿0) 𝜆 − 𝑞0 (𝐶0 𝑇 𝜅 + 1)𝑞0 +1) + 1 ) . in this work, it is assumed that the only source of uncertainty is the pressure-normalized deformation (𝜅), as the aim is to analyse how this particular magnitude affects to the measurement of pressure. from equation (7) and applying the rule of the propagation of uncertainty the equation (10) is obtained, which estimates the relative uncertainty in pressure with respect 𝜅. where 𝐶𝜅 is the sensitivity coefficient, which is calculated with equation (11). as the pressure of equation (9), depends on the nominal pressure, the relative uncertainty depends on that parameter 𝑤(𝑃) = 𝑢(𝑃) 𝑃 ≈ |𝐶𝜅 𝑢(𝜅) 𝜅 | (10) 𝐶𝜅 = 𝜕𝑃(𝜅,𝑃nomina,𝑇) 𝜕𝜅 . (11) 4. cnam cavity description the cavity modelled for the simulation was built by the cnam ([5]). in figure 1, it is shown a 3d model of the fabryperot cavity. the spacer is made of zerodur and the mirrors are made of fused silica. the dimensions of the zerodur spacer are specified in figure 2. the mirrors were considered to be flat. the properties of the materials used are collected in table 2. in order to improve the computational time, only an octave of the cavity was simulated using the symmetry restrictions available in the software. this selection is represented in figure 3 where the simulated section of the fabry-perot cavity is dyed with red. with the purpose of simulating the cavity inside a vacuum chamber, the same pressure load was introduced to every face of the fabry-perot cavity (same inner and outer pressure). the only restriction applied to the model was that the spacer and the mirrors would not rotate. 5. simulation results the solid edge 2021 results are shown with a colormap of the deformation in mm in figure 4. it was shown an inhomogeneous deformation, but it was corrected using the symmetries of the design. simulating a part of the model incorporates these symmetries without including extra conditions to the model. 5.1. cavity relative deformation dependency with the number of mesh elements. the relative deformation of the cavity calculated in the simulation depends on the number of mesh elements used, as it is shown in figure 5. the convergence of 𝜅 with the number of mesh elements was not achieved so two curve fits were made. one to the function 𝑦1(𝑥) = 𝑎 𝑥 −4 + 𝑏 𝑥−2 + 𝑐 𝑥−1 + 𝑑 and another to 𝑦2(𝑥) = 𝑎 (log(𝑏 𝑥)) −1 + 𝑑, where 𝑥 is the number of mesh elements and 𝑦𝑖 is the value of 𝜅. these functions where selected to be constant when the number of mesh elements from table 2. materials properties used in the simulations. parameter spacer material (zerodur) mirror material (fused silica) young modulus (gpa) 90.3 73 poisson ratio 0.24 0.155 density (kg/m3) 2530 2195 figure 1. fp cavity model. figure 2. dimensions of the zerodur spacer in mm, all drillings diameters are 10 mm. figure 3. cnam fp cavity model with the simulated section dye in red. acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 4 among group of functions that tends to infinite as it is shown in equation (12): lim 𝑥→∞ (𝑎 𝑥−4 + 𝑏 𝑥−2 + 𝑐 𝑥−1 + 𝑑) = 𝑑 = κ lim 𝑥→∞ ( 𝑎 log (𝑏 𝑥) + 𝑑) = 𝑑 = κ . (12) the values obtained in the fitting are collected in table 3. using equation (12) it is obtained the value of 𝜅 for each curve fit. the value for 𝑦1 is 𝜅1 = (-6.38378 × 10 -12 ± 3.2 10-16) pa-1 and for 𝑦2 is 𝜅2 = (-6.3868 × 10 -12 ± 3.2 10-15) pa-1. the uncertainty is calculated with the variance and covariance matrix obtained with function curve_fit() from the package optimize of scipy [3], that uses the equation cov(𝑥,𝑦) = 𝐶𝑥𝑦 = 1 𝑁 ∑ (𝑥𝑖 −𝑖 �̅�)(𝑦𝑖 −�̅�) to estimate it. it is shown that the first curve gives less uncertainty than the second. with the relative deformation of the cavity over the pressure obtained in table 3, the values of table 1 and the equation (9) is possible to calculate the pressure that the fabry-perot cavity will have for a given nominal pressure, obtaining in figure 6 the difference between the calculated and nominal values. it is shown that near 0 pa there is an asymptotic behaviour, but if the nominal pressure increases a linear dependency is presented. as it was said before, the relative uncertainty of pressure depends on the nominal pressure (equation (10) and (11)). figure 7 represents the evolution of pressure uncertainty with the nominal pressure. it is shown a linear dependency and the uncertainty obtained by using 𝜅2 increases faster than the one obtained with 𝜅1. also, the relative uncertainty in pressure calculated in a range of pressure under 10 kpa is under 9 × 10-12. 5.2. comparison with results from [1] a comparison between the results of simulations made with different software and institution are needed to verify that the procedure of simulation of every participant in the 18sib04 quantumpascal empir project is correct. in [1] the results of simulating the deformation of the fabryperot cavity are shown, and they are collected in table 4. the result simulated in solid edge software by the cem is less than 10-3 time bigger than the average of [1]. the uncertainty in cem’s simulation was calculated as a rectangular distribution centred in -6.3846 × 10-12 pa-1 and with a width equal to two times the figure 4. cavity deformation for a pressure of 80 kpa. figure 5. pressure-normalized deformation versus number of mesh elements. the data was fitted to two curves figure 6. relative difference between calculated pressure and nominal pressure ((pnominal-p)/pnominal), where 𝜿𝟏 is the value of pressure-normalized deformation calculated with 𝒚𝟏and 𝜿𝟐 is the value of pressure-normalized deformation calculated with 𝒚𝟐. figure 7. relative uncertainty of calculated pressure with 1 = -6.38378 pa-1 and 2 = 6.3868 pa-1 versus the nominal pressure. table 3. values of the mean square error (mse) of the fit, the mean absolute error (mae) of the fit and  in pa-1 for each fit. 𝑦1(𝑥) = 𝑎𝑥 −4 +𝑏𝑥−2 +𝑐𝑥−1 +𝑑 mse 1.41 × 10-23 mae 2.83 × 10-12 𝜅 -6.38378 × 10-12 u(𝜅) 3.2 × 10-16 u(𝜅)/𝜅 4.9 × 10-5 𝑦2(𝑥) = 𝑎(log(𝑏𝑥)) −1 +𝑑 mse 4.78 × 10-31 mae 4.85 × 10-16 𝜅 6.3868 × 10-12 u(𝜅) 1.7 × 10-15 u(𝜅)/ 𝜅 2.7 × 10-4 acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 5 difference of cem’s value and the average value of [1] (2.24 × 10-14 pa-1). using the values of table 4, table 1 and equation (7), the value of pressure for the average value 𝜅 and the value calculated with solid edge are obtained. in figure 8 the relative difference of each pressure with respect the nominal pressure is represented. it is shown the same behaviour as in figure 6. also, the values are so similar that the effect on the pressure cannot be distinguished with this figure for pressures over 10 pa; hence, in figure 9, the relative difference of the pressure calculated with the 𝜅 obtained by [1] and the pressure calculated with 𝜅 obtained with solid edge is represented. in figure 9, the same asymptotic behaviour is shown. the maximum relative difference obtained is near 0 pa, where the asymptote is located, is around 3.5 × 10-8. after the elbow value of the function the relative difference in pressure is less than 2 × 10-10. as the relative uncertainty of pressure depends on the nominal pressure (equation (10) and (11)), figure 10 depicts the evolution of pressure uncertainty with the nominal pressure. it is shown a lineal dependency and the uncertainty of the pressure obtained with the cem’s value increase faster. also, the relative uncertainty calculated for the range of pressure where equation (2) is acceptable is under 3 × 10-10. 6. cem’s cylindrical cavity with the purpose of solving the symmetry rupture that supposes using the spacer as an inner vacuum chamber and gases chamber, a new cylindrical design was proposed. 6.1. cavity description the designed cavity is represented in figure 11, where the simulated section is coloured in red. although this time the symmetry is only a quarter of the model and not an octave as before, an octave was taken after checking that the results were the same as the full model. this way, a similar number of mesh elements are analysed for both models. the spacer and mirrors of this cavity are made of schott’s zerodur class 0, whose properties are shown in table 2. the zerodur class 0 mirrors provide a reflectance over 97 % for a wavelength of 633 nm, which is the wavelength that will be used in the pattern. the dimensions of the zerodur spacer are specified in figure 12. it is shown that the geometry of this spacer is simpler than the presented in figure 2, this will deal with faster calculations. the inner diameter of the zerodur spacer was selected to be 20 mm, so as to be compatible with the optic requirements of numeric aperture of 0.2. also, this diameter provides a more table 4. summary of the simulated pressure normalized cavity deformation using the finest meshing by each partner and software. partner software used pressure-normalized deformation (𝜿) 𝚫𝑳/𝑳/𝑷 in 10-12 pa-1 number of mesh elements a-comsol [1] -6.3900 1 735 324 b-comsol [1] -6.3899 190 000 b-ansys [1] -6.3908 2 948 108 c-ansys [1] -6.3900 21 061 d-comsol [1] -6.3902 3 168 777 average value [1] -6.3902 - standard error of the mean (𝜎𝑥) [1] 0.00015 - cem-solid edge -6.3846 2197477 uncertainty 0.0032 - figure 8. relative difference between calculated pressure and nominal pressure ((pnominal-p)/pnominal). figure 9. relative difference between calculated pressure and nominal pressure ((p-pcem)/p), where p is the nominal pressure and pcem is the calculated pressure. in the first figure the uncertainty is not appreciable, but in the enlargement below the uncertainty can be seen. figure 10. relative uncertainty of calculated pressure with  obtained by [1] and  calculated by the cem with solid edge software versus the nominal pressure. 𝑹𝟐 is the linear correlation coefficient. figure 11. cem fp cavity model with the simulated section dyed in red. acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 6 laminar regimen in rapid pressure variations than with smaller diameters. the zerodur spacer thickness was designed to be 9 mm, but the mirror thickness was 11 mm, which corresponds to a relative factor 𝑀𝑖𝑟𝑟𝑜𝑟 𝑇ℎ𝑖𝑐𝑘𝑛𝑒𝑠𝑠 𝑆𝑝𝑎𝑐𝑒𝑟 𝑇ℎ𝑖𝑐𝑘𝑛𝑒𝑠𝑠 ≈ 1.22, similar to the bessel’s function first zero divided by π ( 𝐽1(𝑧) 𝜋 = 1.216). the bessel function is associate to the circular revolution of 2 π of the cylindrical symmetry. as made with the previous cavity, in order to simulate that the cavity is into a vacuum chamber, the same pressure load was introduced to every face of the fabry-perot cavity. the only restriction applied to the model was that the spacer and the mirrors would not rotate. 6.2. results of the simulations figure 13 presents the results of the deformation simulation for a pressure of 80 kpa. as shown in figure 4, there was an inhomogeneous deformation, but it is corrected by using the partial simulation of the model as the partial only simulates one mirror assuming the symmetry. the simulation data were fitted to three different equations. there were selected to be constant when the number of mesh elements tends to infinite. the selected curves are shown in equation (13), the quadratic equation was not added in equation (12) because the software could not fit the data to that curve. the results of fitting are shown in figure 14, where the curves 𝑦1(𝑥) and 𝑦2(𝑥) cannot be distinguish. 𝑦1(𝑥) = 𝑎 𝑥 −4 + 𝑏 𝑥−2 + 𝑐 𝑥−1 + 𝑑 𝑦2(𝑥) = 𝑎 𝑥 −2 + 𝑏 𝑥−1 + 𝑑 𝑦3(𝑥) = 𝑎 (log(𝑏 𝑥)) −1 + 𝑑 . (13) the parameters obtained in the fitting are shown in table 5. the curve 𝑦1(𝑥) shows the biggest mean square error and mean absolute error o of the fitting. the relative uncertainties of curves 𝑦2(𝑥) and 𝑦3(𝑥) are quite similar, but the mse and the mae obtained by fitting to the curve 𝑦3(𝑥) are several orders of magnitude smaller. also, the best relative uncertainty obtained is not reached with the fitting, but it is achieve using the result of the finest mesh, the simulation with the highest number of mesh elements. using the relative deformation of the cavity over pressure obtained in table 5 for 𝑦2(𝑥) and the finest mesh, the values of table 1 and the equation (9) is possible to calculate the pressure that the fabry-perot cavity will have for a given nominal pressure. the difference between the calculated and nominal figure 12. dimensions of the zerodur spacer in mm. figure 13. cylindrical cavity deformation for a pressure of 80 kpa. figure 14. pressure-normalized deformation versus the number of mesh elements. the data were fitted to three equations. table 5. values of the mean square error (mse) of the fit, the mean absolute error (mae) of the fit and  in pa-1 for each fit and the value of  for the finest mesh. 𝑦1(𝑥) = 𝑎 𝑥 −4 +𝑏 𝑥−2 +𝑐 𝑥−1 +𝑑 mse 7.36 × 10-18 mae 1.97 × 10-9 𝜅 -5.758610 × 10-12 u(𝜅) 1.9 u(𝜅)/ 𝜅 3.3 × 10-6 𝑦2(𝑥) = 𝑎 𝑥 −2 +𝑏 𝑥−1 +𝑑 mse 9.17 × 10-30 mae 2.51 × 10-15 𝜅 -5.758573 × 10-12 u(𝜅) 1.1 × 10-17 u(𝜅)/ 𝜅 1.9 × 10-6 𝑦3(𝑥) = 𝑎 (log(𝑏 𝑥)) −1 +𝑑 mse 3.19 × 10-34 mae 1.24 × 10-17 𝜅 -5.758600 × 10-12 u(𝜅) 1.5 × 10-17 u(𝜅)/ 𝜅 2.6 × 10-6 𝜅 of the finest mesh (2602131elements) 𝜅 -5.7585840x10-12 u(𝜅) 4.7 × 10-18 u(𝜅)/𝜅 8.2 × 10-7 acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 7 values are shown in figure 15. the asymptotic behaviour is shown again, and the difference of the relative difference obtained with these values is so small that over pressures of 100 pa cannot be distinguish (under 7 × 10-10). the values of relative difference between the nominal pressure and the calculated values of pressure are similar to the obtained in figure 6. as it was discussed before the relative uncertainty of pressure depends on the nominal pressure (equation (10) and (11)). figure 16 represents the evolution of pressure uncertainty with the nominal pressure. a linear dependency is shown again and the increase of the uncertainty using the value obtained with the curve 𝑦2(𝑥) is bigger than the obtained with the finest mesh. furthermore, this simulation gives the best relative uncertainty, under 6.5 × 10-14 for both cases. table 6 shows a comparison between the best values of pressure-normalized deformation obtained for each model, including the results obtained by the partners. it is shown that the cylindrical model presents less pressure-normalized deformation and uncertainty. 7. final discussion the results of the simulations of the fabry-perot cavity deformation proposed by cnam depends on the number of mesh elements introduced in the fem software to simulate it. also, different software can have different results. because of this issue, a study of the evolution of the relative deformation over pressure of the fabry-perot cavity was made, fitting the curve to two equations in order to achieve the value when the number of mesh elements tends to infinity obtaining a relative error in 𝜅 between 5 × 10-5 and 3 × 10-4 depending on the curve used. moreover, the propagation of uncertainty of 𝜅 to the pressure was studied for the values obtained with this method, showing that the formula which fits the best is 𝑦1(𝑥) = 𝑎 𝑥4 + 𝑏 𝑥2 + 𝑐 𝑥 + 𝑑 whose parameters are collected in table 5.it was shown, that the relative difference with the nominal value of pressure is under 5 × 10-7 times the value of the nominal pressure and an asymptotic behaviour was shown in figure 6. furthermore, the difference between values of pressurenormalized deformation obtained by other laboratories were compared with the values calculated using solid edge software. we conclude that the relative difference between the nominal pressure and the calculated pressure is under 5 × 10-8 times the value of the nominal pressure for 10 kpa. also, the difference between the values obtained by the cem and the pressure calculated using the 𝜅 of [1] is under 4 × 10-8 times the value of the pressure calculated with the 𝜅 of [1]. for figure 8 and figure 9, the asymptotic behaviour shown before was followed. moreover, the pressure relative uncertainty calculated for both cases is under 3 × 10-10 times the nominal pressure, which means that the contribution of the uncertainty of pressurenormalized deformation (𝜅), for a range of pressure between 1 pa and 10 kpa, to the relative pressure uncertainty is not decisive for the measure. finally, the cem’s fabry-perot cavity provides smaller pressure-normalized deformation (-5.7548684 × 10-12 pa-1) with an uncertainty of 4.7 × 10-18 pa-1, which is 0.1 times smaller than the other cavity. the fabry-pérot cylindrical cavity provides a relative uncertainty under 6.5 × 10-14 for 10 kpa, one order of magnitude less than the best results using the other geometry. it would be interesting making a study of the deformation of the mirrors for different fabry-pérot cavity configurations. 8. conclusion in conclusion, cem’s results using finite elements method through the software solid edge are similar to the results presented in [1] (with a discrepancy of 0.001 times the values obtained by [1]). also, the cem’s cylindrical has a pressure-normalized relative deformation 0.1 times lower than the model analysed in [1]. moreover, the model presents more advantages as it only uses one material and, with the selected dimensions, there is a more laminar regimen in rapid pressure variations. in future projects the uncertainties of the constants and values of table 1 will be introduced in order to have a value of the total uncertainty of the measurement. also, due to the capability of the cylindrical design to have an inner pressure different to the outer pressure, an analysis of the fp cavity deformation where vacuum is applied inside the cavity and atmospheric pressure is applied outside the cavity will be studied. table 6. comparison between the results of the first model analysed by the partners (model 1 partner) and analysed by cem (model 1 cem) and the cylindrical model (model 2). model pressure-normalized deformation (𝜅) (δ𝐿/𝐿)/𝑃 in 10-12 pa-1 uncertainty in 10-12 pa-1 model 1 partner -6.3902 0.00015 model 1 cem -6.3842 0.032 model 2 -5.758584 0.00000047 figure 15. relative difference between calculated pressure and nominal pressure (pnominal p)/pnominal. figure 16. relative uncertainty of calculated pressure with 1 and 2 versus the nominal pressure. acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 8 references [1] j. zakrisson, i. silander, c. forssén, z. silvestri, d. mari, s. pasqualin, a. kussicke, p. asbahr, t. rubin, o. axner, simulation of pressure-induced cavity deformation the 18sib04 quantumpascal empir project, acta imeko 9(5) (2020), pp. 281-286. doi: 10.21014/acta_imeko.v9i5.985 [2] i. silander, t. hausmaninger, m. zelan, o. axner, gas modulation refractometry for high-precision assessment of pressure under non-temperature-stabilized conditions, journal of vacuum science & technology 36(3) (2018), art. 03e105, pp. 1-8. doi: 10.1116/1.5022244 [3] o. axner, i. silander, t. hausmaninger, m. zelan, drift-free fabry-perot-cavity-based optical refractometry-accurate expressions for assessments of gas refractivity and density, jan. 2018. doi: 10.48550/arxiv.1704.01187 [4] pauli virtanen (+ 34 authors), scipy 1.0: fundamental algorithms for scientific computing in python, nature methods 17(3) (2020), pp. 261-272. doi: 10.1038/s41592-019-0686-2 [5] z. silvestri, f. boineau, p. otal, j. wallerand, helium-based refractometry for pressure measurements in the range 1-100 kpa, in 2018 conference on precision electromagnetic measurements (cpem 2018), july 08-13, 2018, paris, france. doi: 10.1109/cpem.2018.8501259 http://dx.doi.org/10.21014/acta_imeko.v9i5.985 https://doi.org/10.1116/1.5022244 https://doi.org/10.48550/arxiv.1704.01187 https://doi.org/10.1038/s41592-019-0686-2 https://doi.org/10.1109/cpem.2018.8501259 low-cost real-time motion capturing system using inertial measurement units acta imeko issn: 2221-870x september 2022, volume 11, number 3, 1 9 acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 1 low-cost real-time motion capturing system using inertial measurement units simona salicone1, simone corbellini2, harsha vardhana jetti1, sina ronaghi3 1 department of electronics, information and bioengineering (deib), politecnico di milano, via guiseppe ponzio 34, 20133, milano, italy 2 department of electronics and telecommunication (det), politecnico di torino, corso duca degli abruzzi, 24, 10129, torino, italy 3 department of energy (deng), politecnico di milano, via lambruschini 4a, 20156, milano, italy section: research paper keywords: motion-capture; bluetooth low energy; inertial measurement units; digital signal processing; embedded systems citation: simona salicone, simone corbellini, harsha vardhana jetti, sina ronaghi, low-cost real-time motion capturing system using inertial measurement units, acta imeko, vol. 11, no. 3, article 17, september 2022, identifier: imeko-acta-11 (2022)-03-17 section editor: francesco lamonaca, university of calabria, italy received may 17, 2022; in final form september 23, 2022; published september 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: simona salicone, e-mail: simona.salicone@polimi.it 1. introduction human motion reconstruction, also known as motioncapture systems (mo-cap), plays an important role in medical rehabilitation, sports training, and entertainment [1]-[3]. in the field of healthcare, physical therapists use such systems to record patient movements, visualize the movements in real-time and finally making a comparison of the recorded movements throughout the treatment cycle for efficacy evaluation. for the purpose of entertainment, the use of mo-caps is necessary for reconstructing human movements for 3-dimensional (3d) games development and animated scenes in the movie industry. for the application of sports training, the use of mo-caps is beneficial for reconstructing the players' movement for the evaluation of each player individually and as a team, as well as, monitoring each player for automatic estimation and prediction of possible injuries, such as muscle damages caused by players collision during a period of professional activity [4]. mo-cap solutions use either non-optical sensor-based methods or optical computer vision methods to reconstruct the physical human movements. as it is currently known, solutions based on computer vision methods can be employed only in a controlled environment and they suffer from inconsistencies concerning environmental conditions such as ambient light (colour and illumination problems), object proximity (occlusion), and motion detection in cluttered scenes [5], [6]. alternatively, sensor-based methods of mo-cap are usually applied in form of wearable devices, that use inertial measurement units (imus) based on microelectromechanical systems (mems). in principle, mems-based imus are immune to visible environmental conditions, but they may suffer from orientation drifts over time. the development of inertial based mo-cap solutions, for both movement tracking and human activity recognition (har), has been a topic of research for many years, mainly due to the constant increase in the availability of new mems devices. nowadays, many similar commercial mo-cap solutions are available on the market; such products, depending on the specific targeted application, can differ in the maximum number of abstract human movement modeling also referred to as motion-capture is a rapidly expanding field of interest for medical rehabilitation, sports training, and entertainment. motion capture devices are used to provide a virtual 3-dimensional reconstruction of human physical activities employing either optical or inertial sensors. utilizing inertial measurement units and digital signal processing techniques offers a better alternative in terms of portability and immunity to visual perturbations when compared to conventional optical solutions. in this paper, a cable-free, low-cost motion-capture solution based on inertial measurement units with a novel approach for calibration is proposed. the goal of the proposed solution is to apply motion capture to the fields that, because of cost problems, did not take enough benefit of such technology (e.g., fitness training centers). according to this goal, the necessary requirement for the proposed system is to be low-cost. therefore, all the considerations and all the solutions provided in this work have been done according to this main requirement. mailto:simona.salicone@polimi.it acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 2 sensors, in the placement position of the sensors on the human body, and in the user interface and data representation. as an example, the mtw awinda [7] tracker by xsense is a complete wireless human motion tracker able to accurately monitor the joint angles and is targeted for many applications, spanning from rehabilitation to injury prevention in sport and to human machine interaction in robotics. the system can manage up to 20 sensors with a maximum data rate of 60 hz. the movit system g1 [8] by captiks is another available solution for both indoor and outdoor motion capture and analysis. this system can acquire up to 16 sensors and it is able to provide raw orientation measurements and even animation and video files. the acquisition rate can reach 100-200 hz to track fast movements. for movements with higher dynamics, the wearable 3d motion capture [9] by noraxon can reach a measurement output rate of 400 hz. this system is equipped with 16 sensors suitable for all type of movements including high velocity and high impact conditions. despite the availability on the marked of many solutions, usually these products are targeted to the most demanding applications and their cost remains too high for their use in many other fields. usually, the cost of such commercial mo-cap solutions can range from a few thousand dollars to even tens of thousands of dollars. these prices are unfortunately out of budget in many applications. therefore, in this paper, we focus on the development of a low-cost mo-cap solution, which is full-body, multi sensor and cable-free. hence, to avoid using costly measurement elements, a novel approach for calibration using software level data analysis solely based on gyroscope measurements is proposed. the system is intended to be used for physical activities that require movements with low angular velocity for a short period of time (i.e., medical rehabilitation, physiotherapy and movement analysis in aged people). a set of experiments are also performed, for a preliminary validation and metrological characterisation of the proposed solution. 2. developed system this section explains the architecture of the proposed solution. the hardware and software feature specifications of the solution are highlighted and the proposed method for gyroscope calibration is explained. finally, a set of experiments are defined to demonstrate the functionality of the system and to assess the effectiveness of the employed method in reducing drift errors during the attitude representation process. 2.1. system architecture figure 1 briefly shows some possible architectures of mo-cap solutions; in particular the red path indicates the selected method for our solution. four key decisions have been made to choose a method of implementation that are explained accordingly. the reason for choosing a non-optical method is to provide portability and immunity to visual environmental perturbations. in fact, for the applications that require the activity to be performed in an open area, it is inherently difficult to control environmental parameters such as lighting. mems-based imus are usually a combination of low-cost accelerometer, gyroscope and in some circumstances magnetometer. in a common navigation system, all the three available inertial sensors (e.g., accelerometers, magnetometers and gyroscopes) are employed for both the preliminary calibration and the navigation itself. in particular, during the navigation, the accelerometer and the magnetometer are necessary to obtain a geo-referenced frame of coordinates and to compensate for drifts inevitably arising from the integration of the gyroscope signals. however, the use of the magnetometer may decrease the accuracy in indoor environments due to the usual presence of local magnetic field perturbations. on the other hand, the accelerometer may increase the measurement noise since the body acceleration is superimposed on the detected gravity vector. fortunately, in most of the human movement measurement applications, a geo-referenced frame is not necessary and even the measurement of angle changes over short time intervals (e.g. a few minutes) with respect to a starting position is usually sufficient (e.g. the patient is asked to start the movement from a reference position). for these reasons the authors propose a robust measurement system solely based on the use of gyroscopes and a simple calibration procedure suitable to achieve reasonable drifts and angle accuracy over a few minute intervals. 2.2. sensor placement considering a wearable motion capture product, the factors to be considered are accurately indicated by marin et al. [10]. the placement factor is crucial to avoid any friction and displacement of the sensors during various activities. even displacements during movements with a low angular velocity can abruptly decrease the accuracy of the representation. therefore, the outermost places right above the joints and flat areas of the body are preferably used. considering the application of medical rehabilitation, it is necessary to track the position of the bones that contribute to commuting and basic physical activities. the proposed solution uses 15 wearable sensors for full-body motion capture. these sensors are attached to the front and back side of the human body in an order that is illustrated in figure 2, where the red dots refer to the sensors and the orange lines indicate the correct positioning of the sensors. additionally, the sensor numbers, names and the measurand parameters are included in table 1. furthermore, as shown in figure 3, the fixed fabric method is used for the attachment factor. in particular, 15 fabric elastic bands have been hand-made, with different circumferences according to the body parts where the wearable devices are intended to be installed. furthermore, on each elastic band, a pocket with the size of 5 cm × 5 cm has been sewed to contain the sensing element with the dimension of 3 cm × 3 cm × 1 cm. figure 1. range of possible implementation methods. acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 3 2.3. hardware design to implement multiple wireless measuring nodes in forms of wearable devices that can perform online software level calibration, commercially available modules, also known as nrftag (or sensor_tag), were used, as illustrated in figure 4. it also shows the sensor base and holder, which are used for sensor calibration. in particular, the base has been specifically designed and 3d printed by the authors. the main specifications of the nrftag are summarized in table 2. from each sensor tag, the mems-based 6-dof imu mpu6050 [11] is used as the sensing element and the nrf51802 system on a chip (soc) [12] is used as the processing unit and the bluetooth low energy™ (ble) communication module. the manufacturer recommended power supply for the sensor tag is a cr2032 non-rechargeable coin-cell battery with 3v output. considering the portability design criteria, a rechargeable wearable device is more convenient from the perspective of the figure 2. sensor attachment to the human body and numbering of the sensors. on the left: front side. on the right: back side. figure 3. the employed sensor (in the green box) and the hand-made elastic belt used to fix the sensor on the body (the wrist in this figure). table 1. sensor names and the measurand parameters. # sensor measurand 1 forehead rotation and side-bend relating to the origin 2 chest anterior/posterior tilt and lateral tilt to left/right 3 left arm abduction/adduction, flexion/extension 4 right arm abduction/adduction, flexion/extension 5 left wrist abduction/adduction, flexion/extension 6 right wrist abduction/adduction, flexion/extension 7 left hand abduction/adduction, flexion/extension 8 right hand abduction/adduction, flexion/extension 9 sacral(back) rotation and side-bend relating to the origin 10 left knee flexion/extension 11 right knee flexion/extension 12 left ankle plantar-flexion/dorsi-flexion and supination/pronation 13 right ankle plantar-flexion/dorsi-flexion and supintion/pronation 14 left ankle rotation and side-bend 15 right ankle rotation and side-bend figure 4. top view of the sensor_tag module (in the green square) attached to the base, which has been specifically designed and 3d printed for the sensor calibration. table 2. brief specifications of the nrftag. product name nrftag (sensor_tag) application wearable devices supply voltage 3v coin-cell battery 2032 package embedded modules nrf51802 (arm® cortex™-m0 mpu6050 imu bmp280 barometric pressure sensor ap3216 ambient light sensor size circular shape d = 30 mm tx power +4 dbm ~ -20 dbm on air data rate 250 kbps, 1 mbps or 2 mbps modulation gfsk rx current 9.5 ma tx current at +4 dbm 10.5 ma acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 4 actor user. for this reason, a rechargeable circuit for the wearable devices was designed and implemented. although it was possible to use a rechargeable coin-cell battery with the same package size and output voltage (e.g. ml2032 li-al coin-cell battery), our solution uses a lithium-polymer (li-po) type of battery as the power supply. the reason for this preference was the variety of available options in terms of supply capacity, higher charging rate, and simplicity of charging circuit implementation for li-po batteries compared to other types. during this hardware modification, additional components such as tp4056 battery charger module [13] and ce6208 dc/dc buck converter [14] were used. the central receiver in this project works as a communication bridge between the sensor tags and the personal computer. this component should receive the measurement data, construct a data structure, and transfer the data to a personal computer. for this purpose, an nrf52840 development kit manufactured by nordic semiconductor™ was considered as illustrated and summarized in figure 5 and table 3 respectively. this development kit enabled us to interact with the 15 measurement nodes concurrently using ble communication protocol for realtime measurement [15]. furthermore, an approximated cost of the main components of the system are represented in table 4 where the items indicated with (*) are only required for the rechargeable configuration. these components are available in the commercial market, and it is of course possible to further reduce the cost of the system in case of bulk production. if we compare the obtained total price (in both the two proposed solutions) with the costs of the available commercial systems (which, as reported in the introduction, range from a few thousand dollars to even tens of thousands of dollars), it can be immediately understood the considerably high cost savings. 2.4. attitude representation despite different possible methods of attitude representation that are preferred due to the application [16], [17], the most common way of representing attitude of a rigid body in 3d space 1 euler angles were introduced by leonhard euler in 18th century is using the euler angles1. euler angles represent rotation by performing a sequential operation with respect to a particular sequence and angle value. by representing the 3d space with three perpendicular axes denoted as i, j, k, it is possible to rotate any rigid body object by angles φ, θ, ψ with respect to the 3d axes. rotations based on euler angles can be represented using a rotation vector as indicated in equation (1). 𝑢: = [𝜑 𝜃 𝜓] (1) considering equation (2) where r is a rotation operation along a single axis, it could be understood that 𝑅𝑖𝑗𝑘 is a rotation function which consists in the product of three individual rotations along each 3d axis sequentially. 𝑅𝑖𝑗𝑘 ≔ 𝑅𝑖(𝜑)𝑅𝑗(𝜃)𝑅𝑘(𝜓) (2) by denoting the i, j and k axes as 1, 2 and 3 respectively, it could be stated that the performed sequence in equation (2) is [1,2,3]. this sequence is widely used for applications consisting representation of gyroscopic spinning motions of a rigid body. although euler angles are widely used because of their easyto-understand mathematical expression, employing them might introduce a major drawback also referred to as singularities during attitude representation. these singularities, which are also known as gimbal lock, might occur during specific sequence operations. the gimbal lock is basically losing one or more degree of freedom (dof) when two or more axes are positioned in parallel to each other. in particular, with a [1,2,3] sequence, the gimbal lock problem will arise when the rotation along the second axis (𝜃) becomes an integer multiplication of 90° (𝜃 = 𝑛 π). in order to overcome the issues regarding singularities, it is possible to use the unit quaternions2 for attitude representation. the unit quaternions are a form of a four-component complex numerical system which are mostly used in the pure mathematics, but indeed, have practical uses in applied science such as navigation systems and attitude representation in 3d space. unlike euler angles, unit quaternion rotation vector is composed by four components. the first component 𝑞0 ≔ 𝑤 is a scalar value related to the rotation angle, while the following three parameters are a vector 𝑞1:3 ≔ (𝑥, 𝑦, 𝑧) that indicates the rotation axis as represented in equation (3). 𝑞𝑤,𝑥,𝑦,𝑧 = [𝑞0 𝑞1 𝑞2 𝑞3] 𝑇 = [ 𝑞0 𝑞1:3 ] (3) one key advantages of the unit quaternions comparing to euler angles is the fact that in case of the unit quaternions, the attitude representation process is not sequential. hence, the unit quaternions do not suffer from singularities. therefore, in our 2 quaternions were introduced by william rowan hamilton in 19th century figure 5. central receiver development kit (nrf52840 dk). table 3. brief specifications of the central receiver. product name nrf52840dk application central receiver supply voltage 1.7 ~ 5.5 v processor nrf52840 (arm® cortex™ m4 size rectangular shape (135 × 63 mm²) table 4. price of the single components and total price of the system (for the two proposed solutions). item number of employed items unit price (€) nrf52840 dk × 1 59 sensor_tag × 15 16 li-po battery (*) × 15 3 tp4056 charger (*) × 15 1 dc/dc buck converter (*) × 15 1 total price 299 total price (rechargeable) 374 (*) acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 5 method of representation, the unit quaternions are preferred over the euler angles. in addition, since the unit quaternions use only complex multiplications, they require less hardware resources comparing to the euler angles that require triangular function derivations [16]. although the unit quaternions benefit from a more robust mathematical derivation and no singularities, there are not easy to interpret with a physical meaning. hence, in order to report the data to the end-user, we preferred to convert the unit quaternion values to the euler angles only when a numerical value representation is needed. the conversion function from the unit quaternions to the euler angles is possible as indicated in equation 4 (and vice versa as indicated in equation (5)). it is important to note that this conversion depends on the euler angles' sequential system, which in the case of this solution is [1,2,3]. 𝑅(𝑢)1,2,3 = [ atan2(2𝑞2𝑞3 + 2𝑞0𝑞1, 𝑞3 2 − 𝑞2 2 − 𝑞1 2 + 𝑞0 2) −asin(2𝑞1𝑞3 − 2𝑞0𝑞2) atan2(2𝑞1𝑞2 + 2𝑞0𝑞3, 𝑞1 2 + 𝑞0 2 − 𝑞3 2 − 𝑞2 2) ] (4) 𝑞𝑤,𝑥,𝑦,𝑧 = [ 𝑞0 𝑞1 𝑞2 𝑞3 ] ≔ [ 𝐶𝜙/2𝐶𝜃/2𝐶𝜓/2 + 𝑆𝜙/2𝑆𝜃/2𝑆𝜓/2 −𝐶𝜙/2𝐶𝜃/2𝐶𝜓/2 + 𝑆𝜃/2𝑆𝜓/2𝑆𝜙/2 𝐶𝜙/2𝐶𝜓/2𝐶𝜃/2 + 𝑆𝜙/2𝑆𝜃/2𝑆𝜓/2 𝐶𝜙/2𝐶𝜃/2𝐶𝜓/2 − 𝑆𝜙/2𝑆𝜓/2𝑆𝜃/2 ] (5) where c and s are the short terms for the cosine and sine functions respectively, atan2 stands for 2-parametric inverse tangent function, and asin is the inverse sine function. 2.5. calibration method low cost imu devices are unfortunately affected by important systemic errors, mainly due to gain errors and to axis misalignments, which may lead to poor accuracy, especially when the sensor has to deal with accelerations and rotations that frequently change axes, as expected in human movement measurements. a calibration has therefore to be performed in order to achieve acceptable results. the main sources of inaccuracy in the use of the gyroscope lay in the gain error (the offset error is present as well, but it is not of concern since it can be easily removed by collecting some initial measurements with the sensor laying in a static position) and in the axes misalignments (i.e. the axes of sensitivity of three gyroscopes are not exactly orthogonal each other). assuming that the sensor x-axis is correctly aligned with the reference frame x-axis (more properly that the reference frame x-axis is chosen parallel to the actual sensor x-axis), and assuming that the reference y-axis is chosen so that the sensor y-axis lies in the reference frame x-y plane, the measurement signals can be related to the actual sensor rotation by using the following matrix equation: ( 𝑋 𝑌 𝑍 ) = ( 𝑎′ 0 0 𝑏′ 𝑐′ 0 𝑑′ 𝑒′ f′ ) . ( 𝑋𝑠 𝑌𝑠 𝑍𝑠 ) (6) where 𝑋𝑠, 𝑌𝑠 and 𝑍𝑠 represent the actual angular speeds with respect to the reference frame, and 𝑋, 𝑌, 𝑍 represent the signals measured by the sensor along its non-orthogonal axes and affected by gain errors. the sensor angular speed can be obtained from the measured signals by inverting the previous equation, whose matrix maintains a triangular shape: ( 𝑋𝑠 𝑌𝑠 𝑍𝑠 ) = ( 𝑎 0 0 𝑏 𝑐 0 𝑑 𝑒 𝑓 ) . ( 𝑋 𝑌 𝑍 ) (7) considering a rotation with angular speed 𝜔 along an arbitrary axis, it can be written: 𝑋𝑠 2 + 𝑌𝑠 2 + 𝑍𝑠 2 = 𝜔2 (8) and therefore: 𝑎2𝑋2 + (𝑏𝑋 + 𝑐𝑌)2 + (𝑑𝑋 + 𝑒𝑌 + 𝑓𝑍)2 = 𝜔2 (9) rearranging the terms: 𝑋2(𝑎2 + 𝑏2 + 𝑑2) + 𝑌2(𝑐2 + 𝑒2) + 𝑍2(𝑓2) + 2𝑋𝑌(𝑏 𝑐 + 𝑑 𝑒) + 2 𝑋 𝑍(𝑑𝑓) + 2 𝑌 𝑍(𝑒𝑓) = 𝜔2 , (10) which, with the following six definitions: 𝛼 = 𝑎2 + 𝑏2 + 𝑐2 𝛽 = 𝑐2 + 𝑒2 𝛾 = 𝑓2 𝛿 = 𝑏𝑐 + 𝑑𝑒 𝜖 = 𝑑𝑓 𝜁 = 𝑒𝑓 , (11) leads to the following compact matrix system of equations that collects all the applied rotations: ( 𝑋1 2 𝑌1 2 𝑍1 2 2𝑋1𝑌1 2𝑋1𝑍1 2𝑌1𝑍1 𝑋2 2 y2 2 z2 2 2𝑋2𝑌2 2𝑋2𝑍2 2𝑌2𝑍2 ⋮ ⋮ ⋮ ⋮ ⋮ ⋮ 𝑋n 2 yn 2 zn 2 2𝑋n𝑌n 2𝑋n𝑍n 2𝑌n𝑍n ) . ( α β γ δ ε ζ ) = ( 𝜔1 2 𝜔2 2 ⋮ 𝜔𝑁 2 ) (12) the six unknown parameters (from 𝛼 to 𝜁), can be obtained inverting the previous equation if at least six rotations with known speed (𝜔1to 𝜔𝑁) have been applied. eventually the required coefficients can be quickly obtained with the following order: 𝑓 = √𝛾 𝑑 = 𝜖/𝑓 𝑒 = 𝜁/𝑓 𝑐 = √𝛽 − 𝑒 2 𝑏 = (𝛿 − 𝑑𝑒)/𝑐 𝑎 = √𝛼 − 𝑏2 − 𝑐2 (13) this calibration has been easily implemented on the sensor micro-controller using the matrix svd decomposition for solving the system of equation (12), however the application of a rotation with a known speed to the sensor is not practical and would require complex equipment. fortunately, considering practically applicable sets of axes during calibration, the previous equations, by integration, hold also with angles for rotations along any arbitrary fixed axis. therefore, the proposed approach consists in rotating the sensor along a set of unknown axes for an integer number of turns, integrating the sensor signals and replacing (2 𝑘 π)2 instead of 𝜔2 in equation (12). to manage the calibration procedure, a base for rotating the sensor along arbitrary axes was necessary. therefore, as already mentioned in section 2.3, a specific base has been designed and 3d printed (the base is visible in figure 4). in addition, a specific virtual app has been developed using the bluetooth version of the miupanel platform (ref. https://miupanel.com). by means of the miupanel platform a mobile phone can establish a direct ble connection with the sensors and retrieve a graphical panel for controlling the sensor. the developed panel is shown in figure 6: it contains a realtime visualization of the yaw, pitch and roll angles, and some buttons to start the calibration, to acquire multiple rotation and to compute the calibration coefficients. through the panel it is also possible to store the calibration results into the sensor flash memory. acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 6 2.6. embedded software design in this part, the combination of the central receiver and all the wearable devices as the acquisition group will be addressed. due to the software design considerations, it is necessary to indicate the desired features and specifications of the solution. the basic requirements regarding the acquisition group of our cable-free mo-cap solution are indicated below: • the central receiver should be able to receive data from all the 15 wearable sensors simultaneously and to associate them to the corresponding body part. • central receiver should be able to identify the index of the received data for synchronization purposes. • a lightweight format for data string communication between the central receiver and the wearable devices should be constructed. this data structure, which is also referred to as the data matrix, should be able to contain the measurement raw data and necessary attributes such as functionality status from the wearable devices. the first two requirements are associated with the connection establishment, while the rest are related to the measurement data structure. as previously mentioned, the employed connection protocol in the proposed solution is bluetooth™ low energy. this protocol sufficiently satisfies the requirements such as fast connection establishment, short-range and high-speed data transfer. the set of instructions which are executed by the peripherals are included in figure 7, where the red path indicates the calibration algorithm to reduce drifts of the gyroscope measurements. to avoid connection establishment to the unnecessary devices within the ble range, the central receiver is programmed to only search for this service-characteristic profile. furthermore, in order to distinguish each sensor, the unique peer address identifier of each sensor is associated with the sensor number in table 1. the peripheral communicates data through advertisement packets transmitted every 20 ms. since the attributes and the payload (measurement data) consume more data bytes than a single ble advertisement packet size (31 bytes), it is necessary to communicate the data using multiple advertisements packets. each advertisement packet size has 31 byte of data, and it consists of information such as: generic access profile (gap), type, manufacturer specific data, the peripheral address, and the payload. upon receiving the advertisement packets by the central receiver, the data matrix is structured to be communicated to a personal computer using serial communication. the set of instructions that are executed by the central receiver are indicated in figure 8, where initialization process is only executed when the central receiver is turned on, while the acquisition and communication process are done continuously. 2.7. representation software design in order to represent the measurement data, representation software in form of windows applications were developed. these applications use various techniques to interpret the incoming data from the central receiver. each windows application is provided with a graphical user interface (gui) to interact with the professional user. furthermore, these applications were created using software environments namely matlab, ni labview, and unity game engine. from the data matrix structure, it is possible to extract information such as sensor number, new data availability, data packet type, and measurement values. as illustrated in figure 9, the general principle for the data interpretation process in all the software environments is to decode the incoming hexadecimal data matrix from the serial interface into single-precision floating-point using the ieee 754-2019 standard. the decoded data will be then converted into the euler angles and the quaternion values. eventually, the converted angles are represented and stored numerically and graphically. figure 6. screenshot of the mobile application designed to manage the calibration process. figure 7. peripherals' program logic. figure 8. central receiver's program logic. figure 9. data interpretation logic. acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 7 2.8. experimental procedure the experiments were conducted separately for each of the representation applications. therefore, the procedures and the necessary considerations regarding each test are explained in this section. 2.8.1. experiment 1 – functionality test the aim of this experiment is the 3d representation of human physical activities using matlab and unity game engine. before proceeding to the tests, the experimental modules followed by a brief description are provided below. • upper body experiment: these experiments are aimed to capture the upper body activities by tracking sensor numbers #1 to #8. • lower body experiment: these experiments are aimed to capture lower body activities by tracking sensor numbers #9 to #15. • full body experiment: these experiments perform the full-body motion capturing process by employing all the 15 sensors. figure 10 to figure 13 illustrate the functionality of the representation procedure using matlab and unity game engine where, in each figure, the test subject's posture is placed next to test results for an immediate comparison, while, for a better vision of the reader, the position of the sensors on the body are marked with red circles on the picture, according to table 1. 2.8.2. experiment 2 metrological characterisation this experiment aims to evaluate the type a standard uncertainty. this preliminary evaluation has been done to estimate the standard uncertainty of a single sensor (sensor #2 corresponding to the chest of the test subject) throughout the measurement process. the experiment was performed by connecting a single sensor tag to the central receiver via ble and connecting the central receiver to a personal computer using serial communication. the sensor tag was located on a fixed position (on a piece of sponge to damp environmental vibrations). then, the sensor tag was turned on and the offset compensation phase started, which took approximately 10 seconds. the experiment was done for about 9 minutes, at room temperature (23 °c) while errors due to the pcb mounting and cross-axis sensitivity effects were neglected [11]. moreover, the ni labview application was used to record the incoming data in a text-plain file. finally, the text-plain file was converted into an excel worksheet to perform the statistical analysis. 5000 samples were considered for the analysis (500 seconds of data acquisition). then the first 2000 samples were discarded to take into account the offset compensation phase and the warm-up time of the measuring units. hence, the samples between 2001 to 5000 were used for this analysis. let us consider the measurements of the gyroscope along each of the 3d axis as a variable that varies randomly. hence, the measurement uncertainty of the instrument along each axis based on the experimental results could be calculated according to the joint committee of guides in measurements (jcgm) guide to the expression of uncertainty in measurements (gum) [18]. the results of the calculations are provided in table 5, and measurements obtained with sensors in static position are illustrated in figure 14. 2.8.3. experiment 3 – correction method evaluation the purpose of this experiment was to highlight the impact of the calibration algorithm for reducing the drifts in the gyroscope measurements. to induce the drifts in the measuring unit, we placed a sensor on a ruler with a defined position on a table and reported the measured value for each of the 3d axis. then by grabbing the ruler with the right hand, we performed 15 forward rotations and 15 lateral movements of the right arm in 1 minute. next, the ruler was placed at the same defined position and the mismatch between the initial value was reported. the above steps were repeated for 5 times, therefore, 15 groups of figure 10. matlab upper body experiment with t-pose posture. figure 11. matlab lower body experiment with an arbitrary posture. figure 12. matlab full body experiment with t-pose posture. figure 13.unity lower body (left figure) and full body (right figure) experiment with an arbitrary posture. table 5. type a evaluation of the standard uncertainty. parameters φº θº ψº induced rotation 0 0 0 experimental sd of observations 0.015 0.022 0.0071 acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 8 observations along each axis were obtained. finally, to emphasize the impact of the calibration method, we performed this experiment with and without the calibration algorithm being executed. the impact of the gyroscope calibration algorithm is highlighted in figure 14. as it is demonstrated, observations without using the correction algorithm are prone to drifts over time during repetitive movements. on the other hand, the gyroscope measurement drifts are bounded to ±1.5 ° when executing the calibration algorithm. 3. discussion considering the limitations of machine vision technologies in motion capture solutions, in this paper, we presented a low-cost, non-optical, cable-free, real-time and full-body wearable motion capture solution using 15 distributed mems based imu. by using the bluetooth™ low energy wireless communication protocol, we established a low-power consuming and low-cost mo-cap system that fully satisfies the portability design criteria of wearable devices. taking into account the broad range of applications for motion capture devices, our solution was specifically designed to be used for medical rehabilitation at home and in nonprofessional gyms, where the cost of the system plays a very important role. furthermore, it has been considered that the measurement duration and the angular velocity of the physical movements are low (i.e., physiotherapy and movement analysis in aged people) and that, after each set of exercises, the patient returns to a reference position. in the proposed solution, the angular position is estimated solely based on the measurements of the gyroscope. since the gyroscopic measurements are prone to angular drifts over time, we proposed a software level calibration method that is able not only to eliminate the gain errors but also to take into account the axes misalignments. the results of our experiments appeared promising by bounding the gyroscope drifts to approximately ±1.5 ° over a measurement period of 5 minutes. moreover, the experimental results also demonstrated the functionality of the system and a preliminary attempt to metrologically characterise the measurement system was performed. furthermore, the rotation angles can be further analysed using data processing methods for human activity recognition (har). this application is widely used for medical diagnostic procedures such as gait analysis and core stability assessment. it is therefore possible to implement har by using suitable classification methods such as deep neural networks. table 6 shows a comparison between the proposed system and other available commercial systems. the table clearly shows that the performances and characteristics of our system are comparable with the ones of other commercial systems, at least in short periods of time. however, the great advantage of the proposed system is represented by the cost which, as discussed in the introduction and shown in table 4, is one or even two orders of magnitude smaller. this represents a very great advantage of the proposed system, which, for the desired applications of few minutes, provides similar performances to the commercial systems, but at very low-cost. references [1] l. n. n. nguyen, d. rodríguez-martín, a. català, c. pérez-lópez, a. samà, a. cavallaro, basketball activity recognition using wearable inertial measurement units, proceedings of the xvi international conference on human computer interaction. association for computing machinery: vilanova i la geltru, spain, 2015, pp. 1-6. doi: 10.1145/2829875.2829930 [2] f. casamassima, a. ferrari, b. milosevic, p. ginis, e. farella, l. rocchi, a wearable system for gait training in subjects with parkinson's disease. sensors (basel), 2014, 14(4), pp. 6229-6246. doi: 10.3390/s140406229 figure 14. measurements obtained with sensors in static position (left figure) and evaluation of the improvements obtained with the proposed calibration algorithm (right figure). table 6. comparison between the proposed system and other available commercial systems. product proposed system mtw awinda [7] movit g1 [8] ultium motion [9] connection wireless wireless wireless wireless range (m) 20 20 30 40 number of sensors 15 20 16 16 battery life (h) 10 6 6 10 orientation accuracy (°) 1.5 (max 5 min) 1 1 1 sensor weight (g) 10 16 25 19 size (mm2) 30 × 30 47 × 30 48 × 39 44 × 33 angular range (deg/s) 2000 2000 2000 7000 quaternion rate (hz) 10 60 100 100 https://doi.org/10.1145/2829875.2829930 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g1-3d. online [accessed 27 september 2022] http://www.captiks.com/products/movit-system-g1-3d [9] usa, n. ultium motion. 2021. online [accessed 27 september 2022] https://www.noraxon.com/our-products/ultium-motion/ [10] j. marin, t. blanco, j.j. marin, octopus: a design methodology for motion capture wearables. sensors, 2017. 17(8), pp. 1-24. doi: 10.3390/s17081875 [11] invensense. mpu-6000 and mpu-6050 product specification. 2013. online [accessed 27 september 2022] https://invensense.tdk.com/wpcontent/uploads/2015/02/mpu-6000-datasheet1.pdf [12] semiconductor., n. nrf51802 multiprotocol bluetooth low energy/2.4 ghz rf system on chip product specification. 2016. online [accessed 27 september 2022] https://infocenter.nordicsemi.com/pdf/nrf51802_ps_v1.2.pdf [13] corp, n.t.p.a. tp4056 1a standalone linear li-lon battery charger with thermal regulation in sop-8. online [accessed 27 september 2022] https://www.mikrocontroller.net/attachment/273612/tp4056.p df [14] inc, n.c.e. ce6208 series 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[17] h. parwana., m. kothari, quaternions and attitude representation. arxiv, department of aerospace engineering, indian institute of technology kanpur, india, 2017, pp. 1-19. doi: 10.48550/arxiv.1708.08680 [18] jcgm. gum: guide to the expression of uncertainty in measurement. 2008. online [accessed 27 september 2022] https://www.bipm.org/documents/20126/2071204/jcgm_100 _2008_e.pdf https://doi.org/10.3390/s22030908 https://doi.org/10.1007/s12283-012-0088-5 https://doi.org/10.1051/matecconf/201815203001 https://doi.org/10.1098/rsfs.2018.0008 https://www.xsens.com/products/mtw-awinda http://www.captiks.com/products/movit-system-g1-3d https://www.noraxon.com/our-products/ultium-motion/ https://doi.org/10.3390/s17081875 https://invensense.tdk.com/wp-content/uploads/2015/02/mpu-6000-datasheet1.pdf https://invensense.tdk.com/wp-content/uploads/2015/02/mpu-6000-datasheet1.pdf https://infocenter.nordicsemi.com/pdf/nrf51802_ps_v1.2.pdf https://www.mikrocontroller.net/attachment/273612/tp4056.pdf https://www.mikrocontroller.net/attachment/273612/tp4056.pdf https://datasheetspdf.com/datasheet/ce6208.html https://infocenter.nordicsemi.com/pdf/nrf52840_dk_user_guide_v1.3.pdf https://infocenter.nordicsemi.com/pdf/nrf52840_dk_user_guide_v1.3.pdf https://doi.org/10.48550/arxiv.1708.08680 https://www.bipm.org/documents/20126/2071204/jcgm_100_2008_e.pdf https://www.bipm.org/documents/20126/2071204/jcgm_100_2008_e.pdf analysis of the mathematical modelling of a static expansion system acta imeko issn: 2221-870x september 2021, volume 10, number 3, 185 191 acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 185 analysis of the mathematical modelling of a static expansion system carlos mauricio villamizar mora1, jonathan javier duarte franco1, victor josé manrique moreno1, carlos eduardo garcía sánchez1 1 grupo de investigación en fluidos y energía, corporación centro de desarrollo tecnológico del gas, bucaramanga, colombia section: research paper keywords: modelling; pressure; static expansion; uncertainty citation: carlos mauricio villamizar mora, jonathan javier duarte franco, victor jose manrique moreno, carlos eduardo garcía sánchez, analysis of the mathematical modelling of a static expansion system, acta imeko, vol. 10, no. 3, article 25, september 2021, identifier: imeko-acta-10 (2021)-03-25 section editor: francesco lamonaca, university of calabria, italy received january 29, 2021; in final form july 9, 2021; published september 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was funded by colombia’s servicio nacional de aprendizaje (sena) through the special cooperation agreement no. 0233 of 2018. corresponding author: carlos eduardo garcía sánchez, e-mail: cgarcia@cdtdegas.com 1. introduction pressure measuring instruments, like any other measuring device, require periodic calibrations, to monitor changes in their performance, and guarantee their comparability with other meters [1]. in simple terms, a calibration consists of establishing a relationship between the values given by measurement standards, and those given by an instrument under test [2]. in the case of vacuum pressure gauges, that is, that measure absolute pressure values lower than atmospheric pressure, a system that can produce specific vacuum pressure values is required, given the importance of comparing the measurements given by the standard and by the meter under test at different values of the measured variable [3]. in the calibration process, it is of utmost importance that the specific pressure values that are generated have a low uncertainty. uncertainty is a characteristic of any measurement, indicating the level of doubt about the reported value [4]. in this way, a better comparability of meters that have been calibrated with the process in question can be guaranteed. currently, in colombia there is a lack of absolute pressure calibration services in the medium and high vacuum regions. for this reason, the centro de desarrollo tecnológico del gas (cdt de gas) has developed a static expansion system, which allows the generation of pressures in the medium and high vacuum ranges, making it possible to calibrate pressure gauges in those regions. this type of system has been implemented in multiple laboratories worldwide. the present study shows the mathematical design process of the system, through an evaluation of the possible models to represent the behavior of the gas inside the system, and the use of uncertainty to define restrictions on the input quantities of the system. 2. state of the art 2.1. static expansion systems the pressure region between absolute zero (total absence of molecules) and atmospheric pressure is called “vacuum”. in turn, vacuum is classified as coarse (from 3 000 pa to atmospheric pressure), medium (between 0.1 pa and 3 000 pa), high (from abstract static expansion systems are used to generate pressures in medium and high vacuum and are used in the calibration of absolute pressure meters in these pressure ranges. in the present study, the suitability of different models to represent the final pressures in a static expansion system with two tanks is analysed. it is concluded that the use of the ideal gas model is adequate in most simulated conditions, while the assumption that the residual pressure is zero before expansion presents problems under certain conditions. an uncertainty analysis of the process is carried out, which leads to evidence of the high importance of uncertainty in a first expansion over subsequent expansion processes. finally, an analysis of the expansion system based on uncertainty is carried out to estimate the effect of the metrological characteristics of the measurements of the input quantities. said design process can make it possible to determine a set of restrictions on the uncertainties of the input quantities. mailto:cgarcia@cdtdegas.com acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 186 1 · 10-7 pa to 0.1 pa) and ultra-high (less than 1 · 10-7 pa) [5]. in general, there is no pressure measurement technology that covers all regions of interest [6]. among the most accurate equipment for generating pressures in the medium and high vacuum ranges are static expansion systems, with which pressures as low as 10-6 pa [7]-[9] can be obtained. several metrology laboratories and national metrology institutes have developed static expansion systems [10]-[13]. the generation of vacuum pressures using static expansion of a gas is a mature technology [14], and much of the development in recent years in the topic has been related to a careful evaluation of possible causes of error and the estimation of the uncertainty of the final pressure [8], [10], [15]. calibration and measurement capabilities with expanded uncertainties lower than 0.3 % (k = 2) using static expansion systems have been reported [15]. nitrogen is commonly used as a gas for this type of calibration, although other inert gases could also be used [3], [7], [16]. static expansion systems are a set of tanks of different dimensions, connected by pipes and valves. to generate low pressures with high precision, a volume of gas, with a defined pressure, is allowed to expand to a larger volume, previously at a pressure as close to zero as possible [17]. figure 1 presents a simplified diagram of the static expansion process, using a small tank (where the initial pressure is set) and a large tank (to which the equipment to be calibrated is connected). in figure 1 and equations included in this work, 𝑉𝑃 is the volume of the small tank, 𝑉𝐺 is the volume of the large tank, 𝑇𝑖 is the initial temperature of the process, 𝑇𝑓 is the final temperature of the process, 𝑃𝑖 is the initial pressure of the process in the small tank, 𝑃𝑖,𝐺 is the initial pressure of the process in the large tank, which should be as close as possible to zero, and 𝑃𝑓 is the final pressure of the process. to further reduce the pressure, it is possible to repeat the expansion process, using the initial pressure resulting from the previous expansions [8]; the lower achievable limit is imposed by the level of vacuum that can be generated in the large tank, and by the effects of sorption and degassing in the tanks and the instruments under test [18]. static expansion systems can be used as primary calibration standards, calculating the final pressure from the initial pressure and the volume ratios of the gas expansion processes performed. they can also be used to generate pressure but using a pressure gauge as a reference for calibration, in this case being a calibration by direct comparison [3], [16], [19]-[22]. it is important to note that several of the high vacuum pressure measurement technologies, such as pirani gauges, exhibit high non-linearity with respect to pressure [3], which usually implies requiring several calibration points in each order of magnitude of pressure. table 1 presents the fundamental characteristics of some static expansion systems developed by various national metrology institutes. regarding the modelling of the process, some institutions have chosen to use the ideal gas model [13], [24], [25], while others have proposed the use of the virial equation as a real gas model for the expansion process [5], [7], [8], [23]. one aspect that is quite generalised is the assumption that the initial pressure in the calibration tank is zero, although the question remains whether this assumption is valid as the final pressure is smaller (that is, as the vacuum increases). equation (1) presents the calculation of the pressure after a static expansion process, modelling the substance as an ideal gas and neglecting the initial pressure in the large tank [10], [13], [24], [25]. 𝑃𝑓 = 𝑃𝑖 𝑉𝑃 𝑉𝑃 + 𝑉𝐺 𝑇𝑓 𝑇𝑖 . (1) on the other hand, (2) shows the calculation of the final pressure obtained with a static expansion, based on the truncated virial expansion in the second term and neglecting the initial pressure in the large tank [5], [7], [ 8], [23]. 𝑃𝑓 = 𝑃𝑖 𝑉𝑃 𝑉𝑃 + 𝑉𝐺 𝑇𝑓 𝑇𝑖 1 + 𝐵𝑓 𝑃𝑓 𝑅 𝑇𝑓 1 + 𝐵𝑖 𝑃𝑖 𝑅 𝑇𝑖 . (2) in (2) and following equations, 𝑅 is the molar constant of the gases, equal to 8.314 462 618 j mol-1 k-1, 𝐵𝑓 is the second virial coefficient of nitrogen in the conditions of the end of the process and 𝐵𝑖 is the second virial coefficient of nitrogen in the conditions of the beginning of the process. the second virial coefficient is a function of the substance or mixture of substances, and a function of temperature. another important aspect related to the initial pressure in the calibration tank is that to reach the residual pressure, or minimum pressure achievable in the calibration chamber, it is usually required to pump for many hours and to bake the tank [16]. residual pressure is limited by pumping speed, by leaks, by gas desorption from materials exposed to vacuum, and by cleanliness of test gauges [1]. baking refers to the heating of the chamber, to about 200 °c, to desorb the gas from the internal table 1. examples of static expansion systems developed in different national metrology institutes. it is not an exhaustive list: the ptb has another static expansion system in addition to the one mentioned here, and systems such as those of kriss (from south korea) or npl (from england) were not included either. institution approximate volumes of tanks (l) pressure range (pa) reference l'istituto nazionale di ricerca metrologica (inrim) 0.01, 0.5 and 68 0.1 – 1 000 [4] centro nacional de metrología (cenam) 0.5, 1, 50 and 100 0.000 01 – 1 000 [23] centro español de metrología (cem) 0.5, 1, 1, 100 and 100 0.000 1 – 1 000 [13] physikalisch-technische bundesanstalt (ptb) 0.017, 0.017, 1, 20 and 233 0.000 001 – 1 000 [24] tübitak-ulusal metroloji enstitüsü (ume) 0.15, 0.15, 0.7, 15, 15 and 72 0.000 9 – 1 000 [25] figure 1. static expansion process with two tanks. the initial state of the expansion process is shown on the left, and the final state on the right. it is assumed that there is spatial homogeneity of temperature in both tanks, but not necessarily temporal homogeneity. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 187 walls of the materials, a necessary procedure to maintain ultrahigh and superior voids [18], [19]. other aspects that have been studied in static expansion systems are the determination of tank volumes by methods other than gravimetry [13], [24] and the effect of the inhomogeneity of temperature in the tanks on the process [8]. 2.2. uncertainty uncertainty is the name given to the level of doubt that a measurement result has [2], [4], [12]. the two most common ways to report uncertainty are standard uncertainty, which represents the standard deviation of the probability distribution with which the measurement result is modeled, and expanded uncertainty, which is half the length of a coverage interval on the measurement result, with a specified coverage percentage (for example, 95%). the most widely used method to estimate the uncertainty of a measurement result is the gum method [4]. the uncertainty estimation process requires the clear establishment of the measurement model, which presents the way in which the measurand (quantity to be measured) is calculated from its input quantities. subsequently, in the gum method, the standard uncertainty 𝑢(𝑦) of the measurand 𝑦 will be estimated from the uncertainties 𝑢(𝑥1), 𝑢(𝑥2),…, 𝑢(𝑥n) of the input variables 𝑋1, 𝑋2,…, 𝑋n using the measurement model 𝑦 = 𝑓 (𝑋1, 𝑋2, … , 𝑋𝑛). neglecting the correlation between the input quantities and the higher order terms, it is obtained the simplest version of the gum method, in which the estimation of the standard uncertainty of the measurand is made according to (3) [4]: 𝑢(𝑦) = √( 𝜕𝑓 𝜕𝑥1 ) 2 𝑢2(𝑥1) + ( 𝜕𝑓 𝜕𝑥2 ) 2 𝑢2(𝑥2) + ⋯ + ( 𝜕𝑓 𝜕𝑥𝑛 ) 2 𝑢2(𝑥𝑛) . (3) 3. methodology in a two-tank static expansion process, the final pressure depends on the initial pressure in the small tank, the initial pressure in the large tank, the volumes of the tanks, the initial and final temperatures, and the nature of the gas that is expanding. the model used, however, may differ according to the assumptions made about the process, which can lead to the neglect of the effect of some variables. as mentioned in the state of the art, different institutions have opted for different models to calculate the pressures after the expansion processes. in the present work, three simplified models were compared against the complete model to calculate the resulting pressure after an expansion process, to define which of the models can be considered adequate to calculate the pressure after one or more expansion processes. the most complete model to represent the process is based on a real gas model and considers both the initial pressure in the large tank and the inhomogeneity of temperatures at the beginning and at the end. it is well known that the ideal gas model represents the behavior of a gas when p → 0, since with a non-existent pressure the assumptions of said model would be exactly fulfilled (the volumes of the molecules are negligible with respect to the total volume of the gas, the forces intermolecular tends to zero and molecular shocks are perfectly elastic) [26]. the ideal gas model is a convenient limiting case, which can be deduced from theoretical considerations, but it does not accurately represent the behavior in the gas phase of pure substances or mixtures that are at pressures other than zero [27]. among the real gas models that have been developed, the virial equation of state has the desirable characteristic that its parameters can be related to intermolecular forces [27]. considering the above, in the present work a model based on the virial equation to represent the behavior of the substance that undergoes expansion was established as the reference model for the final pressure after expansion. it was assumed spatial homogeneity (although not temporal) of temperature, considering that the static expansion system will eventually operate under controlled environmental conditions. two simplifications were evaluated: (1) using an ideal gas model instead of a real gas model, and (2) neglecting the initial pressure in the calibration tank. the main reason that would support the use of ideal gas is that this model is based on assumptions about the behavior of gaseous substances that are approximately satisfied at very low pressures [26]. additionally, it is common to use nitrogen as a gas inside the expansion system, and the virial coefficient for this gas is very small; this coefficient may be relevant for other heavier inert gases [14]. regarding the initial pressure in the large tank, in all the references consulted [5], [7], [8], [10], [13], [23], [24], [25] is neglected, but it is possible to wonder how much of an impact this assumption can have. in this way, four models were compared. model 1 was the model without simplifications and was therefore taken as the reference model; this model is presented at the beginning of section 4 (results and discussion). model 2 was based on the ideal gas model and considered the initial pressure in the large tank. model 3 was based on the real gas model but neglecting the initial pressure in the large tank. and model 4 contained the two simplifying assumptions, that is, it was based on ideal gas and used zero as the initial pressure value in the calibration tank. to make the comparisons, the final pressure with each of the four models was calculated, and the error in the pressure value of each of the last three models with respect to the reference one (model 1) was determined. the error of models 2, 3 and 4 was calculated with (4), where 𝐸𝑖 is the percentage error made by the i-th model, i = 2, 3, 4, 𝑃𝑓,1 is the final pressure calculated with model 1, and 𝑃𝑓,𝑖 is the final pressure calculated with the i-th model: 𝐸𝑖 = ( 𝑃𝑓,𝑖 − 𝑃𝑓,1 𝑃𝑓,1 ) ∙ 100 % (4) in order to consider a wide range of conditions, comparisons were made with the possible combinations of two volume relationships, two differences between initial and final temperatures, two initial pressures in the small tank, two initial pressures in the large tank, and four values of consecutive expansions. after determining whether any of the simplified models was appropriate for the two-tank static expansion system, the gum method was applied, without correlation or higher order terms, to estimate the uncertainty using the chosen model as the measurement model. the uncertainty budget, that is, the contribution of the different input quantities to the final pressure, was evaluated for different uncertainty values of said input quantities [4]. in this way, the importance of the different input magnitudes on the final pressure was evaluated, in a wide range of conditions. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 188 4. results and discussion by modelling the behaviour of nitrogen using the virial expansion truncated in the second term and considering the initial pressure in the large tank, the final pressure after a static expansion process is calculated by 𝑃𝑓 = 1 + 𝐵𝑓 𝑃𝑓 𝑅 𝑇𝑓 (𝑉𝑃 + 𝑉𝐺 ) [ 𝑃𝑖 𝑉𝑃 1 + 𝐵𝑖 𝑃𝑖 𝑅 𝑇𝑖 + 𝑃𝑖,𝐺 𝑉𝐺 1 + 𝐵𝑖 𝑃𝑖,𝐺 𝑅 𝑇𝑖 ] 𝑇𝑓 𝑇𝑖 . (5) this model was called "model 1” and is the reference model. the main drawback of the model in (5) is that 𝑃𝑓 is an implicit variable, so it is required to solve the equation using a numerical method. in the present work, the secant method [28] was used to solve (5). it became evident that using as the starting point for the method the value of 𝑃𝑓 calculated with model 4 (which is the simplest), the method required very few steps to achieve convergence. equation (6) presents the “model 2”, which consists of the resulting model for the final pressure after a static expansion process, using ideal gas to represent the behaviour of the substance and considering the initial pressure in the large tank: 𝑃𝑓 = 𝑃𝑖 𝑉𝑃 + 𝑃𝑖,𝐺 𝑉𝐺 𝑉𝑃 + 𝑉𝐺 𝑇𝑓 𝑇𝑖 . (6) the “model 3” is (2) (shown in the “state of the art” section), which represents the calculation of the final pressure obtained with a static expansion, based on the virial expansion truncated in the second term and neglecting the initial pressure in the large tank. this model is implicit for 𝑃𝑓 , just like model 1. the "model 4" is equation (1) (shown in the "state of the art"), which represents the calculation of the resulting pressure after a static expansion process modelling the substance as an ideal gas and neglecting the initial pressure in the large tank. sixty-four conditions were simulated, corresponding to the possible combinations of the following values of the input variables: two volume relationships (1:20 and 1:150), two differences between final and initial temperatures (0 k and 5 k), two initial pressures in the small tank (10 000 pa and 50 000 pa), two initial pressures in the large tank (0.001 pa and 0.000 01 pa) and four consecutive expansion amounts (1, 2, 3 and 4). the percentage errors in the calculation of the final pressure committed by the three models evaluated in each of the sixtyfour conditions are summarised in figure 2. the highest error made with model 2 is -0.0134 %, and it occurs under certain conditions by performing the expansion process only once. this behaviour is explained taking into account that the ideal gas model works better the lower the pressure, and the highest pressure values (the lowest vacuum levels) are obtained when performing a single expansion. in any case, the error is quite low, and depending on the target uncertainty in the final pressure, it is possible that model 2 can be used without problem. on the other hand, with models 3 and 4 very high errors are made under certain conditions, exceeding -20 % after three consecutive expansions, and reaching -97 % in some cases with four consecutive expansions. these very high errors occur when the initial pressure in the large tank is 1 · 10-3 pa, which is an exaggeratedly high value considering the capabilities of current vacuum pumps, such as turbomolecular pumps, but that could occur if the system is not properly baked. in any case, considering the purpose of the analysis to evaluate the performance of the models under different conditions, the combinations of values of input quantities tested indicate that in some situations models 3 and 4 will have an unacceptable performance to determine the pressure of reference in a pressure gauge calibration process. figure 2. percentage error in the final pressure calculated with the evaluated models, against the number of consecutive expansions. a: error made by model 2. b: error presented by model 3. c: resulting error when applying model 4. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 189 based on the results of the previous section, it was decided to use model 2 to perform the uncertainty analysis. the gum equation applied to said model is shown in (7). 𝑢(𝑃𝑓 ) = [( 𝜕𝑃𝑓 𝜕𝑃𝑖 ) 2 𝑢2(𝑃𝑖 ) + ( 𝜕𝑃𝑓 𝜕𝑃𝑖,𝐺 ) 2 𝑢2(𝑃𝑖,𝐺 ) + ( 𝜕𝑃𝑓 𝜕𝑉𝑃 ) 2 𝑢2(𝑉𝑃 ) + ( 𝜕𝑃𝑓 𝜕𝑉𝐺 ) 2 𝑢2(𝑉𝐺 ) + ( 𝜕𝑃𝑓 𝜕𝑇𝑓 ) 2 𝑢2(𝑇𝑓 ) + ( 𝜕𝑃𝑓 𝜕𝑇𝑖 ) 2 𝑢2(𝑇𝑖 )] 0.5 (7) to evaluate the generalities of the effect of the uncertainty of the input quantities on the uncertainty of the final pressure, a base case and six derived cases were considered. the base case is presented in table 2. four consecutive expansions were considered in the base case, so that the pressures after the first, second, third, and fourth consecutive expansion were 496.7 pa, 4.935 pa, 0.049 03 pa, and 0.000 497 0 pa, respectively. in the six derived cases, the values of the input quantities remained identical to those of the base case, as were all the uncertainties except one, which was set at 1 % of the value of the quantity. table 3 presents the standard uncertainties, in terms of percentage of the value of the measurand, obtained after the different numbers of expansions tested in each of the 7 study cases. in all cases, the relative standard uncertainty increases as more expansions are made and the final pressure decreases. in the hypothetical base case, the uncertainty of the pressure values figure 3. percentage contributions of the uncertainties of the input quantities over the uncertainty of the final pressure (“uncertainty budget”), for the seven case studies, with four different numbers of consecutive expansions. the percentage contribution of the initial pressure in the large tank was omitted from the budgets, since it was less than 0.03 % in all cases. a: one expansion. b: two expansions. c: three expansions. d: four expansions. table 2. values of the input quantities and their uncertainties in the base study case defined for both uncertainty analysis. the standard uncertainty of each input quantity was 0.3 % of the respective value of the quantity. input quantity value standard uncertainty 𝑃𝑖 (pa) 50 000 45 𝑃𝑖,𝐺 (pa) 0.000 010 0.000 002 𝑉𝑃 (m 3) 0.001 000 0.000 005 𝑉𝐺 (m 3) 0.100 00 0.000 05 𝑇𝑖 (k) 296.15 0.30 𝑇𝑓 (k) 297.15 0.30 acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 190 obtained is very high (reaching 1.21 %), considering the possibilities of the system. it can also be seen that increasing the uncertainty from 0.3 % to 1 % for 𝑉𝑃 , 𝑉𝐺 , 𝑇𝑖 and 𝑇𝑓 has a practically identical effect on the pressure uncertainty, while the impact of this increase in uncertainty for 𝑃𝑖 is smaller as the number of consecutive expansions grows. on the other hand, increasing the uncertainty of 𝑃𝑖,𝐺 has a negligible effect, for the values used in the base case. figure 3 presents the uncertainty budgets of the seven case studies. after a single expansion, the budgets of the base case and the case with 1 % uncertainty for 𝑃𝑖,𝐺 show a balanced contribution of the different input magnitudes, while for the other 5 cases the budget is dominated by the input quantity to which the uncertainty was increased. on the other hand, as the number of successive expansions increases, for all the study cases the role of the initial pressure of the expansion process (which is the final pressure reached in the previous expansion) gradually increases over the uncertainty of the resulting pressure. this fact indicates the high importance of uncertainty during the first expansion process over uncertainty in subsequent expansions. additionally, an uncertainty analysis was carried out for a case more adjusted to reality, taking values of the input magnitudes within the expected intervals, and assigning them uncertainties similar to those that can be obtained when using medium-high quality measurement instruments. table 2 summarises the values used for that case. four consecutive expansions were simulated, and the value of the final pressure, its uncertainty and the respective uncertainty budget were determined. the result is presented in table 4. this study case shows that the resulting pressure uncertainty grows from 0.72 % with one expansion to 1.5 % after the fourth expansion. it is also evidenced that the uncertainty of the final pressure is being dominated by the uncertainties of the volumes of the tanks, with the uncertainty of the volume of each of the tanks contributing 47.2 % to the uncertainty of the pressure after expansion (and considering that the pressure after 2, 3 and 4 expansions is dominated by the initial pressure, that is, the final pressure of the previous expansion process). it is interesting that the contribution of the initial pressure in the large tank is only appreciable in the fourth expansion. 5. conclusions it was possible to evaluate the adequacy of the different models proposed. it was determined that the use of an ideal gas model instead of a real gas model caused a maximum error of 0.0135 % on the pressure value, under the evaluated conditions (64 different conditions, between 1 and 4 expansion processes). in this way, depending on the uncertainty objective in the calibration process with the expansion system, it is possible that this simplification can be used without problems. on the other hand, neglecting the initial pressure in the calibration chamber can lead to errors in the pressure value of several tens in percentage, of even 97 % under the evaluated conditions, especially as the number of consecutive expansions that take place increases. therefore, it is concluded that it is preferable not to neglect the initial residual pressure in the calibration chamber, unless it is guaranteed that said pressure is maintained at 1 · 10-7 pa or less, with the baking processes and long periods of pumping that that requires. additionally, it was possible to analyse the effect of the uncertainty of the input quantities on the uncertainty of the final pressure after one or more consecutive expansions. it became evident that the magnitudes with the greatest influence on the final pressure obtained are the volumes of the tanks used in the expansion processes. acknowledgements this work was financed by colombia’s servicio nacional de aprendizaje (sena) through the special cooperation agreement no. 0233 of 2018. sena’s centro industrial del diseño y la manufactura and centro industrial y del desarrollo tecnológico participated in the project transfer plan. table 3. relative standard uncertainty (%) of the final pressure after several consecutive expansions, for the seven case studies proposed to review the impact of the input quantities. study case first expansion second expansion third expansion fourth expansion base case 0.67 0.90 1.1 1.2 𝑢(𝑃𝑖 ) = 0.01 ∙ 𝑃𝑖 1.2 1.3 1.4 1.5 𝑢(𝑃𝑖,𝐺 ) = 0.01 ∙ 𝑃𝑖,𝐺 0.67 0.90 1.1 1.2 𝑢(𝑉𝑃) = 0.01 ∙ 𝑉𝑃 1.2 1.6 2.0 2.2 𝑢(𝑉𝐺 ) = 0.01 ∙ 𝑉𝐺 1.2 1.6 2.0 2.2 𝑢(𝑇𝑖 ) = 0.01 ∙ 𝑇𝑖 1.2 1.6 2.0 2.2 𝑢(𝑇𝑓 ) = 0.01 ∙ 𝑇𝑓 1.2 1.6 2.0 2.2 table 4. results of the second uncertainty analysis. expansion number 𝑷𝒇 (pa) 𝒖(𝑷𝒇) (pa) contribution to the uncertainty budget in % 𝑷𝒊 𝑷𝒊,𝑮 𝑉𝑃 𝑉𝐺 𝑇𝑖 𝑇𝑓 1 496.7 3.6 1.6 0.0 47.2 47.2 2.0 2.0 2 4.935 0.050 50.4 0.0 23.8 23.8 1.0 1.0 3 0.049 03 0.000 61 66.8 0.0 15.9 15.9 0.7 0.7 4 0.000 497 0 0.000 007 3 69.4 7.5 11.1 11.1 0.5 0.5 acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 191 references [1] national physical laboratory, guide to the measurement of pressure and vacuum, london, 1988. 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[28] r. l. burden, j. d. faires. numercal analysis 9th ed., brooks/cole, cengage learning, usa, 2011. https://doi.org/10.1116/1.1286024 https://doi.org/10.1016/j.vacuum.2005.09.003 https://doi.org/10.1088/0026-1394/39/3/2 https://www.imeko.org/publications/tc16-2007/imeko-tc16-2007-036u.pdf https://www.imeko.org/publications/tc16-2007/imeko-tc16-2007-036u.pdf https://doi.org/10.1051/metrology/20192700 https://doi.org/10.1088/0026-1394/42/6/s01 https://doi.org/10.1088/1742-6596/1380/1/012003 https://doi.org/10.1088/0022-3735/10/2/002 https://doi.org/10.1016/j.vacuum.2020.110034 https://www.imeko.org/publications/wc-2009/imeko-wc-2009-tc16-280.pdf https://www.imeko.org/publications/wc-2009/imeko-wc-2009-tc16-280.pdf https://doi.org/10.1016/0042-207x(87)90051-0 https://doi.org/10.1116/1.5025060 https://doi.org/10.1088/1742-6596/390/1/012027 https://doi.org/10.1016/s0042-207x(98)00337-6 https://doi.org/10.1088/0026-1394/41/4/005 a low-acceleration measurement using anti-vibration table with low-frequency resonance acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 369 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 369 373 a low-acceleration measurement using anti-vibration table with low-frequency resonance t. shimoda1, w. kokuyama2, h. nozato3 1 national metrology institute of japan (nmij/aist), tsukuba, japan, tomofumi.shimoda@aist.go.jp 2 national metrology institute of japan (nmij/aist), tsukuba, japan, wataru.kokuyama@aist.go,jp 3 national metrology institute of japan (nmij/aist), tsukuba, japan, hideaki.nozato@aist.go.jp abstract: this manuscript describes how nmij isolates interferometer optics from the ground vibration for low-acceleration measurement by installing an antivibration table. such a vibration isolation system is designed for an accelerometer calibration system to reduce vibration noise from the microtremor or from reaction of the vibration exciter. mitigating the vibration of optics enables evaluation of accelerometers at small amplitudes, which is required in aerospace or infrastructure monitoring applications. in this manuscript, vibration transmissibility of the anti-vibration table is measured using a triaxial seismometer, and its benefit in the calibration system is discussed. keywords: laser interferometer, anti-vibration table, microtremor, ground noise, low-acceleration measurement 1. introduction low-acceleration measurements are gradually required from the viewpoint of various industrial fields such as aerospace or infrastructure monitoring. the resolution of earth observation by satellite imaging, which is one of the main aerospace applications, is suffering from micro-vibrations [1]. in an on-orbit satellite, moving instruments such as mechanical gyroscopes or reaction wheels can generate micro-vibration, which is typically smaller than 10−2 m/s2 [2]. in infrastructure monitoring, continuous measurement of eigenfrequency of structure using microtremor, which is in the order of 10−3 m/s2 or less, is proposed [4, 5]. the demand for such measurements is gradually increasing with the aging of much of the infrastructure. as the basis for these applications, evaluation of accelerometer is essential to verify the reliability of measurements. a calibration system using a laser interferometer and a vibration exciter is used to determine the response of an accelerometer [6]. in this system, a target accelerometer is vibrated by the shaker, and its displacement is precisely monitored by the interferometer for calibration. for lowacceleration, the calibration result can suffer from the background noise of the interferometer, which originates from seismic vibration, self-noise of the interferometer, and so on. mitigation of such noise is necessary to meet the demands of lowacceleration measurements. for these purposes, we try to reduce the noise from microtremor, which typically appears below a few hundred hz. as the first step, it is evaluated how to reduce the microtremor noise in the lowfrequency calibration system in nmij [7]. overview of the microtremor issue and schematics of noise reduction are presented in section 2. next, in section 3, experimental results using an antivibration table with a low resonant frequency are reported. the conclusion is described in section 4. 2. seismic noise in a calibration system 2.1. overview of accelerometer calibration figure 1: schematics of a low-frequency accelerometer calibration system. figure 1 shows the schematics of the lowfrequency calibration system. a target accelerometer is fixed to a vibration exciter and vibrated at a certain frequency and amplitude. at the same time, the displacement of the accelerometer is measured by a laser interferometer constructed on http://www.imeko.org/ mailto:tomofumi.shimoda@aist.go.jp mailto:wataru.kokuyama@aist.go,jp mailto:hideaki.nozato@aist.go.jp acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 370 another table. the output signal from the accelerometer is then compared to the measured displacement to evaluate the sensitivity. in this process, the accelerometer responds the absolute vibration 𝑥a with respect to the inertial frame, while the laser interferometer measures the relative vibration (𝑥a − 𝑥o) between the optical table and the reflection surface on the exciter. these two vibrations differ by the vibration of the optical table 𝑥o, which originates from the reaction of the shaker or the microtremor; 𝑥o = 𝑟𝐻o𝑥a + 𝐻o𝑥g , (1) where 𝑟 denotes the fraction of the reaction, 𝐻o the vibration transmissibility of the optical table, and 𝑥g the microtremor. if the reaction is carefully suppressed, this discrepancy is not important in usual cases because the vibration amplitude of the shaker is sufficiently larger than that of the optical table induced by the microtremor; 𝑥a ≫ 𝐻o𝑥g . however, the effect of the microtremor becomes relatively non-negligible in the low-acceleration measurement mentioned in section 369. figure 2 presents the amplitude spectral density (asd) of the optical table vibration 𝑥o of the current system induced by the microtremor, measured with a broadband triaxial seismometer (trillium compact 120s) in nmij. the vibration is on the order of 10−4 − 10−6 m/s2/√hz. figure 2: asd of the optical table vibration in x (green), y (orange), and z (blue) directions, which was measured in nmij, japan. self-noise of the seismometer is plotted with the dashed black line. the dashed red line shows 10−5 m/s2/√hz for comparison. figure 3 shows the asds of the interferometer signal measuring (𝑥a − 𝑥o) of the low-frequency calibration system and of the seismometer signal measuring 𝑥o, when the vibration exciter is turned off. sum of these two signals corresponds to the noise in determining 𝑥a. the vibration of the optical table dominates the total noise below 10 hz. therefore, suppression of vibration is one of the primary requirements for accelerometer calibration at small acceleration amplitudes. figure 3: asd of the interferometer signal of the lowfrequency calibration system (red), circuit noise of the interferometer (grey), and the optical table vibration (green). the dashed blue line shows the self-noise of the interferometer, which was estimated by subtracting the table vibration from the interferometer signal. additionally, signal from other than vibration, which is plotted with dashed blue line in figure 3, limits the current performance above 10 hz. cyclic error of the interferometer, which originates from non-linearity of the signal, is a suspicious noise source between 10 hz and 100 hz. the electrical circuit noise is dominant above 100 hz. mitigation of such noise sources is also essential to improve the overall performance. to determine 10−3 m/s2 amplitude for calibration with 1 % accuracy in 1 second, the asd of the background noise needs to be below 10−5 m/s2/ √hz at the oscillation frequency. in this manuscript, we focus on the reduction of the vibration noise. 2.2. low-frequency vibration isolation it is straightforward to reduce the vibration transmissibility 𝐻o for low-acceleration measurement. 𝐻o of a single mass-spring-damper model has a form of 𝐻o(𝑓) = 𝑓0 2 𝑓0 2+𝑖𝑓𝑓0/𝑄−𝑓 2 , (2) which is characterized by the resonant frequency 𝑓0 and the quality factor 𝑄 . as the equation shows, vibration is suppressed above the resonant frequency in proportional to 𝑓 −2 in an ideally simple system. the current table has 𝑓0 ∼ 7 hz and 𝑄 ∼ 10 . in this case, the microtremor is not sufficiently isolated below 10 hz, where the vibration noise is dominant. low resonant frequency below 1 hz is desired to suppress the excess of the vibration in figure 2 down to 1 hz. to achieve the low resonant frequency, we are installing an anti-vibration system, which has resonant frequencies of 0.25 hz in horizontal and vertical directions. the system consists of a springantispring system, in which the restoring force of the suspension is partially cancelled by the antirestoring force from gravity or the elastic part. this enables the low resonant frequency with relatively http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 371 small size (~0.7 m). for comparison, a simple pendulum-type isolation system requires 4 m size to achieve 0.25 hz. the optical table in figure 1 is replaced with the low-frequency anti-vibration system shown in figure 4. figure 4: schematic diagram of the low-frequency antivibration system for the laser interferometer. as equation () shows, suppression of the transmissibility also contributes to the reduction of the reaction from vibration excitation through the ground vibration. however, it should be noted that response to external force fluctuation such as sound or airflow can be enhanced below the resonant frequency, because the low-frequency system has low stiffness. excess of low-frequency fluctuation induces alignment fluctuation, which affects the calibration result at higher frequencies. therefore, the environmental disturbance is also an important factor that determines the performance. figure 5: vibration measurement on the anti-vibration stage. 3. evaluation of an anti-vibration system 3.1. vibration transmissibility measurement in order to evaluate the performance of the system, a seismometer was placed on the isolated stage as shown in figure 5. after the vibration measurement on the stage, the seismometer was then moved to the table where the anti-vibration system is placed. the vibration spectrums at these points were compared to estimate the vibration transmissibility of the system. measured vibration spectrums are presented in figure 6. the vibration on the isolated stage was almost as expected from the vibration on the table (blue line) and the theoretical vibration transmissibility (equation (2)). here a resonant frequency 𝑓0 = 0.3 hz and a quality factor 𝑄 = 1 were assumed. the microtremor is successfully suppressed by ~100 times around a few hz, and the residual noise level is below 10−5 m/s2/√hz above 1 hz except for the peak at 7 hz, though the seismometer signal was not measured correctly above 20 hz because of the data logger noise. figure 6: performance of the anti-vibration system. the blue and green lines show the spectrum on the table and on the isolated stage, respectively. the orange line is expected spectrum on the stage. the black dashed and grey lines are the self-noise of the seismometer and the data logger, respectively. on the other hand, vibration below 0.2 hz was increased on the isolated stage, by about ten times than on the table. such excess may originate from external force disturbances, because the antivibration system has low stiffness to lower the resonant frequency, as mentioned in section 2.2. a wind shield is planned to be installed around the anti-vibration system to protect it from the airflow or sound. the other possible disturbances, such as tilt fluctuation of the stage or temperature fluctuation, should also be evaluated. seismometer isolated stage table http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 372 3.2. expected improvement of interferometer background noise as explained in section 2.1, the interferometer measures (𝑥a − 𝑥o), and 𝑥o becomes background noise in accelerometer calibration. therefore, total background noise of the interferometer can be estimated by the sum of the vibration spectrum 𝑥o shown in figure 6 and the interferometer self-noise spectrum shown in figure 7. figure 7 presents the estimated noise level of the low-frequency accelerometer calibration system with the anti-vibration system. the noise is expected to be suppressed by a few tens of times between 1 and 10 hz, where the vibration of the interferometer stage is a dominant background noise. such frequencies are important for the monitoring of infrastructure, which typically has a resonance frequency around 1 hz. in this frequency range, the noise level will be below 10−5 m/s2/√hz. on the other hand, the self-noise of the interferometer dominates above 10 hz, hence the vibration isolation will not reduce the noise there. as mentioned in section 2.1, suppression of the interferometer self-noise is required for further reduction of the background noise, especially above 10 hz. alternatively, it will be easier to replace the interferometer to a low-noise commercial product to improve the calibration capacity. 4. conclusion an anti-vibration table is being installed into the low-frequency calibration system at nmij/aist for low-acceleration measurement. the table has a low resonant frequency of ~0.25 hz to isolate the vibration at low frequencies. to evaluate the performance of the table, the vibration transmissibility was measured using a seismometer. the vibration was successfully isolated on the table by 100 times around a few hz. as a result, the interferometer background noise is expected to be lower than 10−5 m/s2/ √hz below 10 hz, which enables the calibration system to determine the acceleration amplitude of 10−3 m/s2 with 1 % accuracy in a reasonable time. to extend the frequency range to above 10 hz, reduction of the interferometer noise other than the microtremor is necessary. 5. acknowledgement this work is partially based on the results obtained from a project commissioned by the new energy and industrial technology development organization (nedo), japan. the authors thank tamio ishigami, koichiro hattori, akihiro ota, takashi usuda, hiromi mitsumori, yoshiteru kusano (nmij) for useful discussions and cooperation. 6. references [1] k. komatsu, h. uchida, “microvibration in spacecraft”, mechanical engineering reviews, vol. 1, no. 2, 2014 [2] m. privat, “on ground and in orbit microvibration measurement comparison”, proceedings of the 8th european conference on spacecraft structures, material and mechanical testing, 1998 [3] d. yu, g. wang, y. zhao, “on-orbit measurement and analysis of the micro-vibration in a remotesensing satellite”. adv. astronaut. sci. technol. vol. 1, pp. 191–195, 2018 [4] y. ikeda, s. yoshitaka, s. yasutsugu, “damage detection of actual building structures through singular value decomposition of power spectral figure 7: expected improvement of the background noise of the calibration system by using the low-frequency antivibration system (thick blue). the contribution from the vibration of the interferometer stage (green) and from the selfnoise of the interferometer (orange) are also presented. for comparison, the current noise level is plotted with the dashed blue line. http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 373 density matrices of microtremor responses”, aij journal of technology, vol. 16, no. 32, pp. 69–74, 2010 (in japanese) [5] y. jiang, y. gao, x. wu, “the nature frequency identification of tunnel lining based on the microtremor method”, underground space, vol. 1, no. 2, pp. 108-113, 2016 [6] iso 16063-11 1999 “methods for the calibration of vibration and shock transducers—part 11: primary vibration calibration by laser interferometry” [7] w. kokuyama, t. ishigami, h. nozato, a. ota, “improvement of very low-frequency primary vibration calibration system at nmij/aist”, in proc. of xxi imeko world congress, prague, czech republic, 2015 http://www.imeko.org/ omnidirectional camera pose estimation and projective texture mapping for photorealistic 3d virtual reality experiences acta imeko issn: 2221-870x june 2022, volume 11, number 2, 1 8 acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 1 omnidirectional camera pose estimation and projective texture mapping for photorealistic 3d virtual reality experiences alessandro luchetti1, matteo zanetti1, denis kalkofen2, mariolino de cecco1 1 department of industrial engineering, university of trento, sommarive 9, 38123 trento, italy 2 institute of computer graphics and vision, graz university of technology, rechbauerstraße 12, 8010 graz, austria section: research paper keywords: omnidirectional cameras; mesh reconstruction; camera pose estimation; optimization; enhanced comprehension citation: alessandro luchetti, matteo zanetti, denis kalkofen, mariolino de cecco, omnidirectional camera pose estimation and projective texture mapping for photorealistic 3d virtual reality experiences, acta imeko, vol. 11, no. 2, article 24, june 2022, identifier: imeko-acta-11 (2022)-02-24 section editor: alfredo cigada, politecnico di milano, italy received may 26, 2021; in final form march 21, 2022; published june 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was developed inside the european project mirebooks: mixed reality handbooks for mining education, a project funded by eit raw materials. corresponding author: alessandro luchetti, e-mail: alessandro.luchetti@unitn.it 1. introduction media acquired by 360°cameras (also known as omnidirectional, spherical, or panoramic cameras) is becoming increasingly important to many applications. compared to conventional cameras, images taken by 360° cameras offer a larger field of view, which is why they are traditionally useful to applications that derive their state from information about the environment. examples include robot localization, navigation, and visual servoing [1]. however, omnidirectional cameras have recently also become an essential tool for content creation in virtual reality (vr) applications because spherical photographs and videos can provide a high level of realism. for example, applications for real estate agents already make use of omnidirectional images and video data within a vr head mounted display to improve the realism of virtual customer inspections and research domains span widely from 360° tourism [2] to education in 360° classrooms [3]. vr applications using omnidirectional media allow their users to change the view within the boundaries of a 360° image that has been captured at a specific point of interest (poi). thus, vr users are commonly restricted to head rotations only while translations require transitioning into a 360° image that has been captured at a different poi [4]. thus, motion parallax is missing in vr applications, which use omnidirectional data. furthermore, view transitions are limited to where omnidirectional images or videos exist. these shortcomings limit the benefit of omnidirectional media in vr. for example, the missing 3d information restricts the usage of advanced exploration techniques [5], [6] and the missing motion parallax can cause visual discomfort [7]. abstract modern applications in virtual reality require a high level of fruition of the environment as if it was real. in applications that have to deal with real scenarios, it is important to acquire both its three-dimensional (3d) structure and details to enable the users to achieve good immersive experiences. the purpose of this paper is to illustrate a method to obtain a mesh with high quality texture combining a raw 3d mesh model of the environment and 360° images. the main outcome is a mesh with a high level of photorealistic details. this enables both a good depth perception thanks to the mesh model and high visualization quality thanks to the 2d resolution of modern omnidirectional cameras. the fundamental step to reach this goal is the correct alignment between the 360° camera and the 3d mesh model. for this reason, we propose a method that embodies two steps: 1) find the 360° cameras pose within the current 3d environment; 2) project the high-quality 360° image on top of the mesh. after the method description, we outline its validation in two virtual reality scenarios, a mine and city environment, respectively, which allows us to compare the achieved results with the ground truth. mailto:alessandro.luchetti@unitn.it acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 2 to overcome these limitations, we propose combining omnidirectional photorealistic image data with its corresponding 3d representation. since 3d reconstructions commonly suffer from poor color representations, we apply projective texture mapping of omnidirectional images. our approach supports photorealistic image fidelity at the pois and motion parallax at viewpoints nearby. to enable projective texture mapping of 360° image data, we present an approach for omnidirectional camera pose estimation that automatically finds the position and orientation of the 360° camera relative to the 3d representation of the environment. to put our work in context, we first outline related work in section 2, before we describe our approaches to omnidirectional camera pose estimation and projective texture mapping in section 3. we evaluate our system in section 4 and discuss possible directions for future work in section 5. 2. related work camera pose detection has always been a key problem in computer vision. for example, makadia et al. [8] proposed a method useful for the alignment of large rotations with potential impact on 3d shape alignment to estimate the rotation directly from images defined on the sphere and without correspondence. unfortunately, this approach is quite resistant only to small translations of the camera [9]. another work [10] addresses the problem of camera pose recovery from spherical panoramas using pairwise essential matrices. in this case, the exact position of each panorama was an important step to ensure the consistency of visual information about a database of georeferenced images. here the pose recovery works with a twostage algorithm for rotations and after for translations with a bad result if the camera starting pose is very far from the correct one. the above-mentioned problems have been overcome by our method because it works also for large variations of translation as well as rotations. also levin et al. present in [11] a method to compute camera pose from a sequence of spherical images through the use of an essential matrix for initial pairwise geometry. differently from our work and the work of [10], they also use a rough estimate of the camera path as an additional system input to calculate camera positions. an example of generating a texture map of a 3d model with 2d high-quality images is given in [12]. in particular, it is a specific application in the e-commerce presentation of shoes. it consists of a texture mapping technique that comprises several phases: mesh partitioning, mesh parameterization and packing, texture transferring, and texture correction and optimization. in particular, in the texture transferring step, each mesh is allocated to a front image, and all meshes that use the same front image are put in a group. finally, the pixels from the front image corresponding to the 3d mesh are extracted. differently, our method uses only a spherical image to recreate the highresolution 3d model by projecting each pixel of the image from the correct camera pose previously found. the obtained results are faster and good if the user's field of view rotates without large displacements with respect to the camera pose. a similar approach but for another application related to realizing surveying tasks in architectural, archaeological, and cultural landscapes conservation is provided by abmayr et al. [13]. they developed a laser scanner, which offers high accuracy measurements of object surfaces, combined with a panoramic color camera to achieve precise and accurate monitoring of the actual environment employing colored point clouds. the camera rotates according to the same tripod as the laser scanner. many similarities with the method described in the present article can be found. the main difference resides in the use of a single 360° camera instead of a rotating unit, the use of an automatic pose estimation method instead to use the same tripod for laser scanner and camera during the acquisition process. our method is faster, and the 3d model reconstruction can be more complete because it doesn't need to be at a fixed distance from the camera during the scanning process. this aspect becomes more important if it is necessary to reconstruct a high-resolution model with different cameras from unknown positions. finally, an interesting study was provided by teo et al. [14], where, in the context of remote collaboration, helpers shared 360° live videos or 3d virtual reconstructions of their surroundings from different places to work together with local workers. the results showed that participants preferred having both 360° and 3d modes, as it provides variation in controls and features from different perspectives. our work provides a combination of a 360° live video and 3d virtual reconstruction to combine their advantages without the need to switch between them. 3. method in this section, the localization algorithm to estimate the camera pose (i.e., its positions and orientations in the environment), and the method used to project the texture mapping on a 3d representation of the environment are explained. 3.1. camera pose estimation a good alignment between the virtual environment and the captured image is fundamental for the final texture projection that will be covered in the next chapter. for example, this step is necessary when an operator needs to place the camera in a predefined position and orientation. some human errors may be made during this operation and a method to find an accurate camera pose is necessary. moreover, for large distances, even a small angle or small position errors can compromise the final result. the large-scale automatic camera pose identification algorithm has been implemented in matlab 2019b using a zmq communication protocol [15] between matlab and unity 3d. a particle swarm optimization (pso) was used. the procedure of the camera pose estimation is shown in figure 1. starting from the reconstructed 3d model with its low-quality texture but with depth information of the environment and given as input a high quality equirectangular photorealistic image taken by an omnidirectional camera, the localization algorithm finds the pose that gives a 360° image taken with a simulated camera that is as similar as possible to the input one. figure 1. schematic diagram of the camera pose detection algorithm. acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 3 in particular: i. a new camera position is set for each iteration of the pso algorithm. ii. the equirectangular image corresponding to the set camera pose, at the previous step, is acquired. iii. the algorithm checks the similarity between the new image and the input one that has to be used as a new texture for the 3d mesh; the parameters to be optimized are the translation and the euler angles to be applied to the 3d model to generate an equirectangular image that matches the one in the input. the cost function for comparing the two equirectangular images uses the following quantities: • the structural similarity (ssim) index of the equirectangular images. • the mean-squared error (mse) between the two equirectangular images. • ssim of the approximation coefficients (ssima) of level 1 of the wavelet decomposition. • ssim of the horizontal detail coefficients (ssimh) of level 1 of the wavelet decomposition. • ssim of the vertical detail coefficients (ssimv) of level 1 of the wavelet decomposition. • ssim of the diagonal detail coefficients (ssimd) of level 1 of the wavelet decomposition. the final cost function c obtained by adding the abovementioned quantities is: 𝐶 = 𝑆𝑆𝐼𝑀 + 𝑀𝑆𝐸 + 𝑆𝑆𝐼𝑀𝐴 + 𝑆𝑆𝐼𝑀𝐻 + 𝑆𝑆𝐼𝑀𝑉 + 𝑆𝑆𝐼𝑀𝐷 . (1) the mse represents the cumulative squared error between two images x(i,j) and y(i,j): 𝑀𝑆𝐸(𝑥, 𝑦) = 1 𝑀𝑁 ∑ ∑[𝑥(𝑚, 𝑛) − 𝑦(𝑚, 𝑛)]2 , 𝑁 𝑛=1 𝑀 𝑚=1 (2) where m and n are the number of rows and columns of x and y. ssim is used for measuring the similarity between two images x and y [16]. the ssim index quality assessment index is based on the computation of three terms, namely the luminance term l, the contrast term c and the structural term s. the overall index is a multiplicative combination of the three terms: 𝑆𝑆𝐼𝑀(𝑥, 𝑦) = [𝑙(𝑥, 𝑦)] [𝑐(𝑥, 𝑦)]  [𝑠(𝑥, 𝑦)] , (3) where: 𝑙(𝑥, 𝑦) = 2𝜇𝑥 𝜇𝑦 + 𝐶1 𝜇𝑥 2 + 𝜇𝑦 2 + 𝐶1 , (4) 𝑐(𝑥, 𝑦) = 2𝑥𝑦 + 𝐶2 𝑥 2 + 𝑦 2 + 𝐶2 , (5) 𝑠(𝑥, 𝑦) = 𝑥𝑦 + 𝐶3 𝑥𝑦 + 𝐶3 . (6) x, y, x, y and xy are the local means, standard deviations, and cross-covariance for images x and y. c1, c2, c3 are constants to avoid instability for image regions where the local mean or standard deviation is close to zero. choosing  =  =  = 1 and 𝐶3 = 𝑐2 2 , the index simplifies to: 𝑆𝑆𝐼𝑀(𝑥, 𝑦) = (2𝜇𝑋 𝜇𝑌 + 𝐶1)(2𝑥𝑦 + 𝐶2) (𝜇𝑥 2 + 𝜇𝑦 2 + 𝐶1)(𝑥 2 + 𝑦 2 + 𝐶2) . (7) iv. the pso optimization runs until convergence, giving as output the best camera pose (translation and euler angles) that makes the two images as similar as possible. 3.2. texture projection in this chapter, the method to apply the high-quality texture mapping will be described. essentially, a merge of the highquality 360° image with the 3d mesh is performed. firstly, the 3d cartesian coordinates and colors of each 360° image's pixel were obtained by projecting the equirectangular image on the surface of a unitary radius sphere. given an equirectangular image with n rows and m columns, each image's pixel in 2d cartesian coordinates (n,m) was transformed in spherical coordinates, computing the corresponding azimuth a and elevation e, setting the radius r equal to 1. the equations used for the conversion are: 𝑎 = − ( 𝑚 𝑀 − 0.5) · 2 π , (8) 𝑒 = − ( 𝑛 𝑁 − 0.5) · π , (9) 𝑅 = 1. (10) finally, the 3d cartesian coordinates are obtained to be visualized in matlab software like a 3d point cloud. the mapping from spherical coordinates to 3d cartesian coordinates is: 𝑥 = 𝑅 · cos(𝑒) · cos(𝑎) (11) 𝑦 = 𝑅 · cos(𝑒) · sin(𝑎) (12) 𝑧 = 𝑅 · sin(e). (13) this "spherical" point cloud was imported inside unity and placed with the position and orientation found in the previous pose estimation step chapter. the raycasting technique was used: through the ray class, it is possible to emit or "cast" rays in a 3d environment and control the resulting collisions. the rays used in raycasting are invisible lines that have the center of the image sphere as the origin and are oriented in each pixel's direction. the important point is that these invisible lines or rays that are cast into the scene can return information about gameobjects that have been hit by the rays. attached to the environment's mesh as gameobject in unity, there is a mesh collider to register a hit with the ray. when a ray intersects or "hits" a gameobject, the event is referred to as a raycasthit. this hit provides details about the gameobject and where it was hit, including a reference to the gameobject's transform, the length of the ray when it hits something, the point in the world where the hit happened. acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 4 once the collision of each pixel is detected, their new position is saved with color properties. lastly, the new point cloud was used to reconstruct a highquality photorealistic texture, using the screened poisson surface reconstruction algorithm [17] implemented in meshlab [18]. this algorithm is particularly useful when the model to reconstruct is very big with very fine details to be preserved. the reconstruction of the 3d model was done setting the reconstruction depth parameter (i.e., the maximum depth of the octree that is used to make the reconstruction) to 13. the default value of meshlab for this parameter is 8, we increased it because in general, the higher this value is the more time will be needed for reconstitution, the more details will be preserved [17]. we did not increase it more than 13 because after 14 it is not possible to see a real change in the final result. the minimum number of samples was set to 1.5 and the interpolation weight to 4 as default values of meshlab. since the poisson algorithm tends to "close" the reconstructed mesh, the triangles whose area was above a certain threshold were deleted to preserve the original form of the reconstructed environment. 4. evaluation for the validation of the camera pose localization algorithm and the high-quality texture mapping projection, a wavefront 3d object file (obj file extension) of two 3d high-quality virtual outdoor environments, one for a mine and one for a city, were imported into unity 3d platform. an original script was also written to simulate a 360° camera. the 360° capture technique is based on google's omni-directional stereo (ods) technology using cubemap rendering [19]. after the cubemap is generated, it is possible to convert this cubemap to an equirectangular map which is a projection format used by 360° video players. after placing the simulated camera at a specific pose inside the scene of a specific scenario, a high-quality equirectangular image was acquired, figure 2. this will be the input images whose pose has to be detected by the developed algorithm. to simulate the acquisition of the environment through a 3d scanner, a point cloud for each analyzed environment was extracted from the 3d high-quality models using the cloud compare software [20]. these point clouds were downsampled to simulate a 3d model with less detail than the input model, and new reconstructions were performed in meshlab [18] to obtain new low-quality 3d models, figure 3. new scenes were then recreated in unity with the downsampled 3d models. figure 4 shows the schematic diagram of our camera pose detection algorithm proposed in figure 1 applied to the specific example of the mine environment. the input omnidirectional image has a resolution of 4096 × 2048 pixels. however, to improve the calculation time speed, the comparison between images is done by downsampling them to 256 × 128 pixels for both the analyzed environments. the bounding box dimensions of the scenario with the mine are 113 m × 169 m × 37 m for the x, y, z coordinates, respectively. instead, the dimensions of the city environment are 440 m × 100 m × 435 m. the same analysis was done for both environments using the same approach and shifting the camera pose by the same values. table 1 shows the position and orientation for 10 random trials. the initial starting position was set to the origin (0, 0, 0) with null rotations for each trial. the research limits were set to  20 m for translations and  80° for rotations. by default, unity applies the following rotation order: extrinsic rotation around the z-axis (γ), then around the x-axis (α), and finally around the y-axis (β). the average time spent by the pso algorithm is around 20 minutes. the tests were run on a pc with an intel i7-9700kf processor and 64 gb of ram. for each of the 10 trials of table 1, the pso algorithm has been run changing 5 times the numbers of generations, i.e., 200, (a) (b) figure 2. high-quality equirectangular images whose detection poses must be identified for a mine (a) and city (b) environments. (a) (b) figure 3. the 3d downsampled models used by the localization algorithm for a mine (a) and city (b) environments. acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 5 250, 300, 350, 400, keeping the number of particles fixed to 100, and 5 times changing the number of particles, i.e. 60, 70, 80, 90, 100, keeping the number of generation fixed to 400. the number of generations and particles was changed to force the algorithm to increase variability. to compute the error in pose detection, we decided to separate the translation and the rotation part. the translation error is computed by performing the euclidean distance between the position of the camera found by the pso algorithm and the ground truth. for what concerns the rotations, firstly, the rotations found by the optimization process and the ground truth were decomposed in axis and angle notation. consequentially, the error, in the case of rotation, has 2 terms: the error in the axis orientation with respect to the ground truth and the amount of rotation around such axis. figure 5 shows the cost function score for the various error components explained above, while figure 6 shows the three possible couple combinations of the error components with respect to the final score optimization value. as can be noticed, sometimes, a higher score of the cost function at the end of the optimization does not mean that an incorrect pose was found. this fact is probably due to the mesh reconstruction process. indeed, after this process, there could be portions of the environment that can be less accurate compared to the real model. for this reason, the meaning of the final reached score values is not absolute or easily comparable considering different camera poses. this generates the need to quantify the accuracy of the camera localization measurement within a scene. despite the uncertainty concerning the accuracy in the pose found by the algorithm with respect to the final cost function score, figure 5 and figure 6 show that, for the mine environment, a score below 1.6 means that, for the trial performed, the error in translation is below 0.7 m, the difference in the amount of rotation is below 1°, and the difference in the rotation axis orientation is below 2°. for the city environment, the same amount of errors corresponds to a cost function score of 2. the score is higher because the city environment is a scenario with much more detail than a mine. many of these details, through initial downsampling, are lost and the initial reconstructed mesh is much less detailed, as can be seen in figure 3b. the final score, therefore, which measures the similarity between the input high-quality equirectangular image and that obtained from this low-quality model, turns out to be higher. however, the errors, especially those related to rotations (figure 5b and figure 5c), are lower for the city environment even at high levels of the cost function score because the environment is more diverse. because of this relationship of the cost function threshold from the level of detail of the reconstructed 3d model, table 1. camera poses chosen for 10 trials (ground truth). trial x (m) y (m) z (m) α (°) β (°) γ (°) 1 -4.00 10.00 15.00 10.00 15.00 18.00 2 5.00 -2.00 5.00 10.00 -60.00 1.00 3 -8.00 5.00 -6.00 30.00 45.00 15.00 4 2.00 -7.00 15.00 -10.00 -45.00 -20.00 5 10.00 10.00 10.00 20.00 -15.00 5.00 6 0.00 15.00 8.00 25.00 -15.00 5.00 7 -5.00 2.00 -5.00 -10.00 60.00 -1.00 8 -1.00 -2.00 -3.00 -4.00 -5.00 -6.00 9 -15.00 10.00 10.00 40.00 70.00 40.00 10 -19.00 19.00 -19.00 2.00 80.00 -5.00 figure 4. example of the camera pose detection algorithm flow for the mine environment. (a) cost function score vs translation error. (b) cost function score vs axis orientation error. (c) cost function score vs rotation angle error. figure 5. 2d plots of the cost function score vs the errors in translation (a), axis orientation (b), and rotation angle (c). acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 6 there is a need for further analysis to investigate possible acceptance criteria and multidimensional models capable of finding a correlation between the different terms of the cost function and the uncertainty in translation and rotation. for example, figure 7 shows that mse could be a possible discriminant factor for accuracy. indeed, in this case, the accurate solutions are all centered around the 0.005 value for both exanimated environments. once the camera poses were found for each environment, this information is used to set the 360° image projected on the surface of a unitary radius sphere in the correct position and orientation, figure 8a. after that, using the raycasting technique, the 3d mesh, figure 8b, is hit by 360° image pixels (figure 8c). 5. conclusions and future work in this paper, we presented an approach for combining photorealistic with 3d environment representations using a 360° high-quality image and a 3d model of an environment with low (a) cost function score vs translation error vs rotation angle error. (b) cost function score vs axis orientation error vs rotation angle error. (c) cost function score vs translation error vs axis orientation error. figure 6. 3d plots of the cost function score and the errors in translation, rotation angle, and axis orientation. figure 7. mse score vs translation error. (a) 360° image placed on the surface of a unitary sphere (matlab software). (b) raw 3d mesh (unity software). (c) point cloud obtained projecting the pixels of the 360° image on the raw 3d mesh (unity software). figure 8. the pixels of the 360° image of the mine environment are projected on a sphere surface (a), which is put in the correct camera pose found by our algorithm inside the raw 3d mesh (b). the pixels are then projected using the ray cast technique on the raw mesh, obtaining a new dense point cloud (c). acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 7 quality. at the core of our system, we have developed an approach for automatic large-scale 360° camera pose estimation within a 3d environment and a method for projective texture mapping spherical images. contrary to previous work, outline in the related work section, the camera pose estimator developed in this paper works both for significant differences in rotation and displacement, and it works without the need to start from a known point of view. the positions and orientations of the camera were estimated with a translation error below 0.7 m, and below 1° and 2° for the difference in the amount of rotation, and the difference in the rotation axis orientation, respectively. these results were obtained for both environments analyzed at full size and with search limits of  20 m for translations and  80° for rotations using an mse of 0.005 as a possible discriminant factor for accuracy. while this work was validated using a 360° camera simulation in virtual scenes, we plan to test its capability on real scenes as well. in such situations, the light conditions could be very different between the model and equirectangular image which is why the luminance has to be carefully considered. furthermore, the approach here presented is valid until the view of the user rotates without large displacements from the camera’s initial position because not all the mesh areas are covered after the pixel projection. to overcome this problem, the same method presented in this paper can be applied with more than one camera, but in the case of the final reconstruction of the texture, there is not a discriminating parameter that allows us to choose which pixels to use from one or another camera for the final reconstruction. this choice can be useful if the field of view of one camera is better for some mesh areas than another one to obtain a better result and it can be implemented in future work. finally, in the optimization camera pose process, a further study can be done to find a correlation between the different terms of the cost function and the uncertainty in translation and rotation by investigating other possible acceptance criteria through a multidimensional analysis. references [1] r. benosman, s. kang, o. faugeras, panoramic vision, springer verlag, 2000, isbn 978-0387951119. [2] j. hakulinen, t. keskinen, v. mäkelä, s. saarinen, m. turunen, omnidirectional video in museums–authentic, immersive and entertaining, in international conference on advances in computer entertainment, springer, 2017, pp. 567–587. doi: 10.1007/978-3-319-76270-8_39 [3] d. kalkofen, s. mori, t. ladinig, l. daling, a. abdelrazeq, m. ebner, m. ortega, s. feiel, s. gabl, t. shepel, j. tibbett, t. h. laine, m. hitch, c. drebenstedt, p. moser, tools for teaching mining students in virtual reality based on 360° video experiences, conference on virtual reality and 3d user interfaces abstracts and workshops (vrw), iee, atlanta, ga, usa, 2020, pp. 455-459. doi: 10.1109/vrw50115.2020.00096 [4] a. macquarrie, a. steed. the effect of transition type in multiview 360 media, ieee transactions on visualization and computer graphics 24(4) (2018), pp. 1564-1573. doi: 10.1109/tvcg.2018.2793561 [5] m. tatzgern, r. grasset, d. kalkofen, d. schmalstieg, transitional augmented reality navigation for live captured scenes, virtual reality (vr), ieee, 2014, pp. 21-26. doi: 10.1109/vr.2014.6802045 [6] m. tatzgern, r. grasset, e. veas, d. kalkofen, h. seichter, d. schmalstieg, exploring real world points of interest: design and evaluation of object-centric exploration techniques for augmented reality. pervasive and mobile computing 18 (2015), pp. 55-70. doi: 10.1016/j.pmcj.2014.08.010 [7] j. thatte, b. girod, towards perceptual evaluation of six degrees of freedom virtual reality rendering from stacked omnistereo representation, electronic imaging, 2018. doi: 10.2352/issn.2470-1173.2018.05.pmii-352 [8] a. makadia, k. daniilidis, rotation recovery from spherical images without correspondences, ieee transactions on pattern analysis and machine intelligence 28(7) (2006), pp. 1170–1175. doi: 10.1109/tpami.2006.150 [9] a. makadia, k. daniilidis, direct 3d-rotation estimation from spherical images via a generalized shift theorem, ieee computer society conference on computer vision and pattern recognition, vol. 2, madison, wi, usa, 2003, pp. ii–217. doi: 10.1109/cvpr.2003.1211473 [10] r. laganiere and f. kangni, orientation and pose estimation of panoramic imagery, mach graph vis 19(3) (2010), pp. 339–363. [11] a. levin, r. szeliski, visual odometry and map correlation, in proceedings of the ieee computer society conference on computer vision and pattern recognition 1 (2004) washington, dc, usa. doi: 10.1109/cvpr.2004.1315088 [12] j.-y. lai, t.-c. wu, w. phothong, d. w. wang, c.-y. liao, j.-y. lee, a high-resolution texture mapping technique for 3d textured model, applied sciences, vol. 8, no. 11, 2018, p. 2228. doi: 10.3390/app8112228 [13] t. abmayr, f. härtl, m. mettenleiter, i. heinz, a. hildebrand, b. neumann, c. fröhlich, realistic3d reconstruction–combining laserscan data with rgb color information, proceedings of isprs internation archives of photogrammetry, remote sensing and spatial information sciences 35 (2004), pp. 198–203. [14] t. teo, l. lawrence, g. a. lee, m. billinghurst, m. adcock, mixed reality remote collaboration combining 360 video and 3d reconstruction, in proceedings of the 2019 chi conference on human factors in computing systems, 2019, pp. 1–14. doi: 10.1145/3290605.3300431 (a) (b) figure 9. final results after the 3d reconstruction for the mine (a) and the city (b) environments. https://doi.org/10.1007/978-3-319-76270-8_39 https://doi.org/10.1109/vrw50115.2020.00096 https://doi.org/10.1109/tvcg.2018.2793561 https://doi.org/10.1109/vr.2014.6802045 https://doi.org/10.1016/j.pmcj.2014.08.010 https://doi.org/10.2352/issn.2470-1173.2018.05.pmii-352 https://doi.org/10.1109/tpami.2006.150 https://doi.org/10.1109/cvpr.2003.1211473 https://doi.org/10.1109/cvpr.2004.1315088 https://doi.org/10.3390/app8112228 https://doi.org/10.1145/3290605.3300431 acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 8 [15] p. hintjens, zeromq: messaging for many applications. o’reilly media, inc., 2013, isbn: 9781449334062. [16] z. wang, a. c. bovik, h. r. sheikh, e. p. simoncelli, image quality assessment: from error visibility tostructural similarity, ieee transactions on image processing 13(4) (2004), pp. 600– 612. doi: 10.1109/tip.2003.819861 [17] m. kazhdan, h. hoppe, screened poisson surface reconstruction, acm transactions on graphics (tog) 32(3) (2013), pp. 1–13. doi: 10.1145/2487228.2487237 [18] p. cignoni, m. callieri, m. corsini, m. dellepiane, f. ganovelli, g. ranzuglia, meshlab: an open-source mesh processing tool, in eurographics italian chapter conference, salerno, 2008, pp. 129– 136. doi: 10.2312/localchapterevents/italchap/italianchapconf2008/1 29-136 [19] google inc., rendering omni-directional stereo content. online [accessed 21 march 2022] https://developers.google.com/vr/jump/rendering-odscontent.pdf [20] d. girardeau-montaut, cloudcompare, 2016. online [accessed 21 march 2022] https://www.danielgm.net/cc https://doi.org/10.1109/tip.2003.819861 https://doi.org/10.1145/2487228.2487237 http://dx.doi.org/10.2312/localchapterevents/italchap/italianchapconf2008/129-136 http://dx.doi.org/10.2312/localchapterevents/italchap/italianchapconf2008/129-136 https://developers.google.com/vr/jump/rendering-ods-content.pdf https://developers.google.com/vr/jump/rendering-ods-content.pdf https://www.danielgm.net/cc system for an acoustic detection, localisation and classification acta imeko issn: 2221-870x june 2021, volume 10, number 2, 62 69 acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 62 system for an acoustic detection, localisation and classification jakub svatos1, jan holub1, jan belak1 1 department of measurement, czech technical university in prague, prague 166 27, czechia section: research paper keywords: acoustic detection; gunshots; localisation; classification; neural network; mel frequency citation: jakub svatos, jan holub, jan belak, system for an acoustic detection, localisation and classification, acta imeko, vol. 10, no. 2, article 10, june 2021, identifier: imeko-acta-10 (2021)-02-10 section editor: giuseppe caravello, università degli studi di palermo, italy received january 18, 2021; in final form april 15, 2021; published june 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: jakub svatos, e-mail: svatoja1@fel.cvut.cz 1. introduction an acoustic detection (ad) of gunshots is a present topic that can help to detect hazardous and dangerous events, especially in public areas. in recent days, there is an increase in gunshot attacks in public areas such as schools, campuses, hospitals, and shopping centres. in some cases, it is challenging to identify a dangerous event from uncertain, inadequate data received by cameras or by security staff. the main asset of ad is based on the extraction of vital information from the recorded signal data and classify it as due to a given event (gunshot, a human scream, glass breaking, etc.) due to this classification, ad can assist in law enforcement to better discriminate dangerous events and intervene in the ongoing process on time and decrease the possibility of casualties. a fundamental goal of ad systems is to monitor acoustic signals around the area of interest and to detect a potentially hazardous event, record, localise the probable position of the event and classify the event into categories as an alert (gunshot) signals and non-treat, ‘false alert’, signals. there are several experimental or commercial ad systems designed for gunshot events detection and localisation available on the market [1]-[5]. these systems are designed to localise the source of the gunshot and to estimate the type of treat. the more sophisticated systems can even identify the particular firearm type using advanced classification methods. a drawback of the sophisticated systems is usually very high purchase and operational cost, which makes it almost impossible to use for smaller non-governmental entities such as campuses or hospitals. to successfully design a gunshot detecting and classifying algorithm, essential characteristics of its complex physics have to be understood. a comprehensive explanation of the problem can be found in [6], [7]. a gunshot pattern is characterised by the two phenomena, muzzle blast, and, if the bullet propagates at supersonic speed, shock wave. muzzle blast is caused by an explosive charge where hot, high-pressured gases expand as acoustic energy from the centre of the gun barrel. the bullet travelling at supersonic speed generates a shockwave effect, which is propagating in a conic fashion behind the bullet trajectory. the shockwave is based on abstract currently, acoustic detection techniques of gunshots (gunshot detection and its classification) are increasingly being used not only for military applications but also for civilian purposes. detection, localisation, and classification of a dangerous event such as gunshots employing acoustic detection is a perspective alternative to visual detection, which is commonly used. in some situations, to detect and localise the source of a gunshot, an automatic acoustic detection system, which can classify the caliber, may be preferable. this paper presents a system for acoustic detection, which can detect, localise and classify acoustic events such as gunshots. the system has been tested in open and closed shooting ranges and tested firearms are 9 mm short gun, 6.35 mm short gun, .22 short gun, and .22 rifle gun with various subsonic and supersonic ammunition. as ‘false alarms’, sets of different impulse acoustic events like door slams, breaking glass, etc. have been used. localisation and classification algorithms are also introduced. to successfully classify the tested acoustic signals, continuous wavelet and mel frequency transformation methods have been used for the signal processing, and the fully twolayer connected neural network has been implemented. the results show that the acoustic detector can be used for reliable gunshot detection, localisation, and classification. mailto:svatoja1@fel.cvut.cz acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 63 the combinations of compression and expansion shock, as shown in figure 1. the angle θm between the bullet trajectory and the shock wave trajectory is given by the mach number m = v/c, where v is the velocity of the projectile and c is the speed of the sound: 𝛩𝑀 = arcsin ( 1 𝑀 ). (1) these factors can include valuable information that can be used for improving the detection capability of the ad system. alongside this, the calibre of both bullet and barrel, the length of the latter, mechanical action caused by the gun itself, or even the chemical properties of the propellant cause different effects on the pattern of a gunshot. last but not least, the temperature of the air, air humidity, wind speed, environment (e.g., foliage density, urban area) and soil characteristics also have an impact on the resulting gunshot pattern. considering all these phenomena, to effectively detect and identify a gunshot, signal processing, including adaptive filtering and advanced data processing and classification have to be carried out [8] – [11]. an example of a typical subsonic pattern and a supersonic gunshot signal are shown in figure 2 and figure 3. both bullets were shot in semi-open area (with multiple reflections from the ground and the walls) by the same 9 mm short gun, and its acoustic signature was recorded at the distance of approximately 10 m. in figure 3, the shock wave pattern is not clearly visible due to its proximity to the muzzle blast pattern of the signal. it is caused by the relatively low speed of the supersonic bullet (mach number m = 1.09). in this article, a system for acoustic detection, localisation and classification is introduced. the proposed system consists of stand-alone sensor units, which are placed around the monitored areas in sufficient numbers, to continuously monitor acoustic events around the unit; if there is a possibility of a dangerous event, the stand-alone units send the data to the remote unit for advanced gunshot detection and classification. the remote central unit evaluates signals received from multiple sensor units and, using advanced signal processing and classification methods, determines the location of a gunshot and the probable firearm caliber used. to localise the place of the event, at least three units equipped with a microphone are needed. the localisation accuracy of the system depends on the density and the number of stand-alone sensor units used. in comparison with other existing available systems (e.g. [3] or [5]), the presented system has a novel modular flexible structure. it can be deployed on a building or moving object (car, person, animal) while the central unit can be installed in a distant protected place. in the future, the presented acoustic detector can be used in public areas like schools, campuses, shopping centers to detect and localise gunshots. the paper is organised as follows. in section ii, the sensor units, detection algorithm, localisation algorithm and signal processing, together with the classification methods, are introduced and described. in section iii, the experimental measurements and results are presented. the conclusion and future work directions are outlined in section iv. 2. methods the presented system for acoustic detection, localisation and identification consists of multiple (at least three to estimate the correct localisation of the event) sensor units and one central unit. such a topology enables additional analyses at the central unit, i.e., advanced location of the acoustic event position using timestamped data from multiple sensor units receiving the acoustic signal related to the possible shooting event. each sensor unit has to cover analogue signal pre-processing, digitisation, simple detection algorithm and simple evaluation. based on the simple detection and evaluation algorithms, all the units with positive detection, sends the recorded data to the central remote unit. every stand-alone unit works on the principle illustrated in figure 4. the stand-alone sensor detection algorithm works on the principle of a modified median filter introduced by the authors bullet trajectory shock wave front trajectory velocity v qm velocity c figure 1. an acoustic trace of a supersonic bullet. figure 2. signal corresponding to a 9 mm subsonic short gun gunshot. figure 3. signal corresponding to a 9 mm supersonic short gun gunshot. analogue preprocessing detection algorithm adc evaluation figure 4. unit function requirements. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 64 in [8]. the background signal is filtered by the median filter while an acoustic event is processed based on the algorithm presented in figure 5. it works on the principle of dividing the recorded acoustic signal, converted to energy (the input signal is squared), to the optimal number of data blocks time windows, the defined odd number of windows, which are fed to the median filter. the middle window is the block with the actual data (actual data window), where the detection is performed. each window represents 50 samples, i.e., 1 ms of recorded acoustic data around the unit. the data contained in the window before the actual data window represents the acoustic signal recorder after the actual data and vice versa, data recorded before are contained in windows that follows the actual data window. the resulting signal from the median filter is then subtracted from the actual data window, to suppress/filter the background acoustic noise. thereafter the detection algorithm based on multiple thresholds distinguishes actual gunshot events from other ‘false alarms’. more details about the algorithm can be found in [12] and can be described by the pseudocode in figure 6. requirements for flexible modular sensor units are even more demanding than just to detect, record and send the acoustic event in case of a positive detection. since the sensor units can be deployed everywhere, they have to be able to send their exact position. moreover, all the sensor units have to be precisely timesynchronised to get a synchronised timestamp of the detected event and to send the data to the central unit wirelessly ensuring accurate localisation and classification of the event secured by the remote server. to fulfil all these criteria, the sensor unit was designed according to figure 7. every sensor unit uses a pre-polarised, electret condenser microphone with a flat frequency response from 20 hz to 20 khz and omnidirectional sensitivity, see figure 8 and figure 9. the heart of the acoustic sensor unit is a low power consumption, 32-bit lpc 1837, arm cortex-m3 based microcontroller, which processes the recorded data from the microphone. the analogue op-amp based peak detector provides an interrupt for the microcontroller if the microphone records an acoustic impulse event. a 16-bit analogue-to-digital converter (adc) ads8866 digitises the recorded analogue data. the successive approximation, low power consumption adc samples the data with a sampling frequency of fs = 44 khz. after the interrupt, the adc sends the data to the microcontroller via serial peripheral interface (spi). the data is stored in a circular buffer as a 16-bit number. in this way, the data is ready for processing. if a trigger from the peak detector occurs, the median filter algorithm described above evaluates if there is the possibility of a gunshot event. if the possible gunshot is evaluated and detected, low power consumption gsm chip gl865-quad v3 median filter win 1 win 2 act. data win 12 win 13 x 2sample x(t) + rms max energy rms max timestamp energy figure 5. the principle of the median filter. figure 6. the detection algorithm pseudocode. mikroprocesor pc peak detector adc microphone cell phone gps gsm/gprs remote server int spi usb extint uart uart sms gprs figure 7. block diagram of the sensor unit. figure 8. frequency response of the microphone. figure 9. omnidirectional sensitivity of the microphone. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 65 then sends the recorded data, together with the exact position of the unit and a precise timestamp, when the event has been recorded, for further analysis to the remote central unit pc. the precise localisation of the sensor unit position and synchronisation with all the other sensor modules secures gps module neo/lea-6t. when the neo/lea-6t module operates stationary, gps timing is possible with only one visible satellite. it means the time can be maintained even under adverse signal conditions or in an environment with poor sky visibility. the gps module is also adding the precise timestamp to the recorded data to make it possible to assess the exact localisation of the gunshot event. the uct timestamp and the exact position of each unit are determined by this way. for the synchronisation of the units, the nmea protocol [13] is used. the time accuracy for the synchronisation of all units is 0.1 ms. the described sensor unit is power supplied from ucc = +5v. the designed sensor unit with a testing microphone is shown in figure 10. the central remote unit process received data using advanced signal processing. when at least three sensor units detect the event and send the data together with its timestamps, an algorithm uses these timestamps from the network of sensor units to triangulate the location of the event. the remote server also secures the classification to state if there is a gunshot detected and identify/estimate the caliber of the used firearm or if it is only a ‘false alarm’. figure 11 describes the layout of the localisation problem using three sensor units/microphones. measuring the time delay τn between the event and each microphone mn and knowing their positions, it is possible to calculate the position of the source s inside of the domain d. the relationship between the source s of the gunshot and three microphones mn, that detect it, is described by equations (2) to (4) (𝜏1 − 𝜏0) ∙ 𝑐 = (𝑥 − 𝑎1) 2 + (𝑦 − 𝑏1) 2 (2) (𝜏2 − 𝜏0) ∙ 𝑐 = (𝑥 − 𝑎2) 2 + (𝑦 − 𝑏2) 2 (3) (𝜏3 − 𝜏0) ∙ 𝑐 = (𝑥 − 𝑎3) 2 + (𝑦 − 𝑏3) 2 , (4) where τ0 is the time where the gunshot has occurred; τ1, τ2 and τ3 are the times where the gunshot has been detected by unit 1, 2 and 3; (ax, bx) are the coordinates of the relevant unit; and (x, y) are the coordinates of the gunshot. the resulted coordinates are necessary to recalculate to the cartesian system. the unit that detects the gunshot first (τ1) is considered as it is placed in coordinate (0, 0). the coordinates in meters of other units are then calculated through (5): 𝑑 = acos(sin(𝛷1) sin(𝛷2)) + cos(𝛷1) cos(𝛷2) cos(𝛿𝜆) r , (5) where 𝛷1 and 𝛷2 are the coordinates in meters and δλ is the difference of the longitudes and r is the mean earth radius (6378 km). once the coordinates of the cartesian system are calculated, it is necessary to recalculate the latitude and the longitude coordinates. since the distance of one degree of longitude is different at the north pole and the equator, it is necessary to know the relationship between degree and meter at a given latitude [14]. this is done by applying the simplified formulas (6) and (7), which recalculate meters to one degree of latitude of longitude, resulting in uncertainty values in the order of centimetres: 𝑥𝑙𝑎𝑡𝑖𝑡𝑢𝑑𝑒 = 111123.92 − 559.82 cos(2 𝜆) + 1.175 cos(4 𝜆) − 0.0023 cos(6 𝜆) (6) 𝑥longitude = 111412.84 cos(𝛷) − 93.5 cos(3 𝛷) − 1.175 cos(5 𝛷) (7) an example of the received timestamps from the three sensor units (aed 1 – aed 3) with their exact position in longitude and latitude coordinates and calculated source of a detected acoustic event by the remote server is in figure 12. a custom software application for the remote unit pc to communicate with sensor units, processing the sent data and displaying the location of the event was developed as a part of the acoustic detection system (figure 13). the application was developed in c# programmable language. the program shows a figure 10. the sensor unit for acoustic detection. figure 11. the layout of the situation with three microphones. figure 12. an example of a message with calculated source of an acoustic event sent by the sensor unit. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 66 map with the exact location of the chosen acoustic unit and precise/synchronised utc time. in case of detection, the application signalises the event, displays an exact timestamp in utc and stores the data for further processing. through the application, the sampling frequency, parameters for the detection algorithm based on the median filter described in the beginning of the section ii, and the connection of additional detection units can be done. parallel to localisation, the remote central unit applies the algorithms to obtain the features for classification and classify the event into the classes. the mel frequency transformation (mft) and continuous wavelet (cw) methods are used for the processing of received data to obtain features for the further classification. to identify the components in an acoustic signal, it is preferable not to use the linear frequency scale, but to detect the differences in lower frequencies rather than higher frequencies. incorporating this can be preferable in acoustic detection. mft algorithm uses band-pass filters to get the energy of the signal for each defined band. then, the algorithm uses frequency distribution to create mel frequency coefficients (mfc). mfc is computed using cosine transformation on the logarithm of bank energies. it can be described as follows. the waveform is divided into the frames, then the discrete fourier transform is applied, logs of amplitude spectrum is taken, the scale is converted using the melscaling and, finally, the discrete cosine transform is performed. to convert the frequencies into the mel scale, equation (8) is used. more details on mfc can be found in [15], [16] 𝑀(𝑓) = 1125 ln (1 + 𝑓 700 ) . (8) through experiments, it was set the optimal number 20 mfc and filters with frequency bands from 500 hz to 5khz. these mfcs served as features for the future classification. to limit the influence of an echo on a classification and increase its validity and reliability, more features have to be added. for this reason, the cw algorithm was considered. unlike discrete fourier transform dft, cw transform uses defined waves to create a frequency spectrum with significant time resolution. the time resolution allows limiting the influence of echo, because the echo does not usually interact with the beginning of the impulse acoustic event/gunshot pattern. the best results for a gunshot recognition showed the bump wavelet served as a mother wavelet. the used bump wavelet is a bandlimited function defined in the frequency domain by (9), and its shape is shown in figure 14. ψ(sω) = e 1−( 1 1−(sω−μ)2/σ2 ) 1 [ μ−σ s , μ+σ s ] (9) where 1[(μ−σ)/s, (μ+σ)/s] is the indicator function, s is the scale, ω is angular frequency, σ is standard deviation, and µ is mean value. more details about the wavelet can be found in [17], [18]. both presented algorithms are used as features for the advanced classification of received signals by central remote unit from sensor units. various sets of gunshots, as well as several diverse false signals, similar to a gunshot signal were used for classification. each false signal was chosen to be challenging to differentiate it from the real gunshots by a human operator. for classification, two independent neural networks (nn) were created. first, a nn was designed to classify signals based on mfc. considering the low dimensionality of these coefficients, a fully two-layer connected neural network has been used. for the cw algorithm, a convolution neural network was used. as the spectrum is a two-dimensional matrix, the convolution network is significantly better than a fully connected system. convolution network allows finding local features in multidimensional, location-dependent input data. spectrogram features are location-dependent, and therefore, the convolution network leads to better results than a typical fully connected network. also, the convolution network is typically much smaller in dimension, thus significantly reducing the computation time. a single network combining both designs was implemented and used for the classification. the network had two inputs, one for the mfc and one for the cw algorithm. along with the nn, two more classifiers, support vector classifier (svc) and naive bayes classifier (nbc) [19] [21] are presented herein for a results comparison. both classifiers were implemented in matlab software with classification pattern recognition toolbox prtools ver. 5 [22]. naive bayes method is based on bayesian theorem (10). p(a/b) = p(b/a) p(a) p(b) (10) where p(a), p(b) is the prior probability of a, b respectively, and p(b/a) is the conditional probability, the probability of a given that b is true. the naive bayes classifier estimates for every feature and every class separately. naive bayes method is simple, but despite its simplicity can often outperform other sophisticated classification methods. it is widely used for results comparison with other classifiers. svc works based on the construction of non-linear decision boundaries. svm creates a decision plane, so-called hyperplane, which in feature space separate training data optimally. the decision plane then figure 13. pc application. figure 14. bump mother wavelet. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 67 separates sets of objects, which have different class memberships. the input data are transformed (mapped) to feature space using a mathematical function known as the kernel. support vector models can be linear, polynomial, sigmoidal and radial basis function. 3. results to acquire the test data, multiple gunshots measurements in different shooting ranges, closed – with many reflections and open, were done. for increasing the diversity of gunshots, three different calibres were measured [23]. tested firearms were 9 mm short gun, 6.35 mm short gun, .22 short gun, .22 rifle gun. various subsonic and supersonic (up to mach number of m = 1.1) ammunition was used with the 9 mm short gun. each type of gun was measured at least 60 times. in total, approximately 400 samples corresponding to gunshots were and recorded by sensor units described in section ii (in many cases, one sample has been recorded by more sensor units). for false signals, acoustic impulse events similar to gunshots were measured. approximately 200 samples of various false alarms were measured. as examples of false signals, glass breaking, different slams (such a door slam), handclaps, or close big bubble wrap popping were recorded. at least three sensor units placed in nearby locations recorded all the tested signals. the example of a gunshot of a 9 mm short gun in a noisy environment (diesel engine) recorded at a distance of 10 m is shown in figure 15. figure 16 shows another recorded gunshot, shoot by .22 short gun in closed shooting rage where many reflections are visible. the gunshot was recorded at a distance of 12 m. two examples of recorded false alarms represented by a door slam and bubble wrap popping are presented in figure 17 and figure 18. both presented false signals exhibit similar patterns as gunshots. therefore, the recorded data have to be processed in a way to emphasise the differences. this can be done by mft and cw methods presented in section ii. this work presents the classification into ‘false alarms’ and gunshot event and in the case of a gunshot, the classifier classifies the recorded signal into an individual caliber. to train the classifiers in an optimal way, the recorded acoustic events data have to be divided into two groups – to training and to validation sets. there are many articles dealing with an optimal number of training data. one of the proposed methods is the use of the learning curve. use of learning curves for optimal size of training data is described in [24]. from the learning curve, dependency of the model performance on the training data size can be obtained. it usually depends on the classification method, the complexity of the classifier or how well can be the classes separated. optimal size of training data can be determined from the maximum of the learning curve. recorded ‘false alarms’ and gunshots samples were chosen randomly chosen for the training and the validation sets. in this case, the number of training data is optimal for approximately 30% of the samples. the training set had approximately 200 independent measurements and the validation set had approximately 400 samples. the experimental results for the validation data to be classified into false alarms and gunshots events are shown in table 1. the results show the validation success rate of classifying into the gunshots is 100 %. the system can classify the gunshot from figure 15. recorded signal corresponding to 9 mm subsonic short gun shoot noised by a diesel engine. figure 16. recorded signal corresponding to .22 mm subsonic short gun shoot with reflections. figure 17. recorded signal corresponding to a door slam. figure 18. recorded signal corresponding to a bubble wrap popping. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 68 the ‘false alarm’ with a 100% success rate but, on the other hand, approximately 30% of the false alarms are identified as gunshots. the main reason for this is the limited number of signals corresponding to the false alarms and their diversity. to improve the classifier learning, a significant number of impulse acoustic events similar to gunshots have to be recorded in real scenarios and used it for classifier training. on the other hand, if the acoustic detection system will be complemented by visual detection an operator can restrain a false alarms occurrences. the results also show that svc achieves slightly better results than nn in false alarm detection, while nbc’s success rate is below 90% even for a proper classification into gunshots classes. for this reason, nbc is not used for the classification into individual calibers. table 2 shows the results of the classification into individual calibers by the nn. the nn performs well in classification into the caliber of a gunshot, where it was able to classify more than 95% correctly for a 9 mm caliber, more than 82 % for 6.35 mm caliber and more than 96% for .22 caliber. on the other hand, nn classifies almost 26% of false alarms as gunshots (9 mm caliber mostly). for the sake of comparison, results obtained through svc are summarised in table 3. the svc classified almost 80% of false alarms correctly, but the classification of gunshots into correct caliber classes is less successful than for nn. for example, 6.35 mm caliber successful classification is less than 80% in comparison with 96% for the nn. the efficiency of nn classification strongly depends on the amount and variability of the false alarm signals. based on the tested results, only the nn classifier was implemented to the ads. in the future, more false alarm scenarios have to be recorded and used for the classifier training. the tested measurements show that the presented ads can successfully detect, locate and classify the impulse event. the ads consists of sensor units operates on the principle of the modified median filter, and central remote unit, which calculates the location of the acoustic event and classify it using nn and cw and mfc algorithms. the tested acoustic signals were taken at different open and closed shooting ranges, where at least three acoustic units recorded the detected impulse acoustic event. the nn classifier can classify an individual caliber of a used firearm with a very high success rate, which is for some calibers more than 95%. as a next step, the system for acoustic detection will be deployed in an environment (residential area) to test it in real conditions. 4. conclusion the system for acoustic detection, localisation, and classification into a firearm caliber was presented. the system consists of sensor units that continuously monitor acoustic events around the unit and the central remote unit. the sensor units use a modified median filter algorithm to state if there is a possibility of a gunshot. the central remote unit – pc, then evaluates the signal through advanced signal processing and classification to determine if there is a gunshot or a false alarm event. the system can localise the event on the principle of measuring the time delay between the event and each microphone of the sensor unit and knowing their positions. the accuracy of the localisation depends on the number and the density of the sensor units. the central remote units use continuous wavelet and mel frequency transformation methods to get the features for a neural network classifier. gunshots of different calibers and various false alarms similar to gunshots were recorded on shooting ranges to test the system. more than 600 signals were recorded and tested. the system shows the ability to detect the gunshot with 100% accuracy and to correctly classify the caliber of a gun with a high accuracy depending on the individual caliber. considering the limited size of false alarms training dataset, such results are impressive. however, all measurements were measured in a similar environment. to practically employ the ads in real conditions, a significantly larger dataset in a real environment, such as urban areas, should be examined for future tests and unit improvements. in the future, the proposed systems for acoustic detection can be used as a standalone unit placed in schools, campuses, shopping centers or other public areas in general, to detect, localise and classify gunshot events and to increase the safety of the civil population. acknowledgement this research was supported by the “energy for smart objects” grant provided by electronic components and systems for european leadership joint undertaking in collaboration with the european union's h2020 framework programme (h2020/2014-2020) and national authorities, under grant agreement n° 692482. references [1] b. kaushik, d. nance, k. k. ahuja, a review of the role of acoustic sensors in the modern battlefield, 11th aiaa/ceas aeroacoustics conference, monterey, california, 23-25 may 2005. doi: 10.2514/6.2005-2997 [2] h. e. bree, the microflown, dissertation thesis, 1997. https://ris.utwente.nl/ [3] shotspotter. online [accessed 04 june 2021] http://www.shotspotter.com [4] j. millet, b. baligand, latest achievements in gunfire detection systems, battlefield acoustic sensing for isr applications (pp. 26-1–26-14) nato rto-mp-set-107, 2006. online [accessed 04 june 2021] http://www.rto.nato.int/abstracts.asp [5] g. l. duckworth, d.c. gilbert, j.e. barger, acoustic countertable 1. validation data classification into gunshot and false 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73-83. doi: 10.21014/acta_imeko.v7i2.541 [24] c. beleites, u. neugebauer, t. bocklitz, c. krafft, j. popp, sample size planning for classification models, anal. chim. acta, 760 (2013), pp. 25-33. doi: 10.1016/j.aca.2012.11.007 https://doi.org/10.1117/12.266747 https://doi.org/10.1109/dspws.2006.265386 https://doi.org/10.1109/ieeeconf12259.2007.4218954 https://doi.org/10.1007/s11042-015-3105-4 https://doi.org/10.1016/j.sysarc.2011.04.003 https://doi.org/10.1007/s11042-015-2903-z https://doi.org/10.1109/ntsp.2016.7747791 https://doi.org/10.1109/rtsi.2019.8895591 http://www.tronico.fi/oh6nt/docs/nmea0183.pdf https://lexjansen.com/nesug/nesug06/dm/da15.pdf https://doi.org/10.1109/sysmart.2018.8746939 https://doi.org/10.1109/la-cci.2018.8625230 https://www.mathworks.com/help/wavelet/ref/cwtft.html http://www.arpnjournals.org/jeas/research_papers/rp_2016/jeas_0316_3850.pdf http://www.arpnjournals.org/jeas/research_papers/rp_2016/jeas_0316_3850.pdf http://dx.doi.org/10.21014/acta_imeko.v7i1.516 http://dx.doi.org/10.21014/acta_imeko.v9i2.800 https://doi.org/10.1007/978-0-387-84858-7 http://prtools.org/ http://dx.doi.org/10.21014/acta_imeko.v7i2.541 https://doi.org/10.1016/j.aca.2012.11.007 study of fracture processes in sandstone subjected to four-point bending by means of 4d x-ray computed micro-tomography acta imeko issn: 2221-870x june 2022, volume 11, number 2, 1 7 acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 1 study of fracture processes in sandstone subjected to fourpoint bending by means of 4d x-ray computed microtomography leona vavro1, martin vavro1, kamil souček1, tomáš fíla2, petr koudelka2, daniel vavřík2, daniel kytýř2 1 the czech academy of sciences, institute of geonics, studentská 1768/9, 708 00 ostrava-poruba, czech republic 2 the czech academy of sciences, institute of theoretical and applied mechanics, prosecká 809/76, 190 00 praha 9, czech republic section: research paper keywords: four-point bending test; chevron-notched core specimen; crack propagation; 4d micro-ct; sandstone citation: leona vavro, martin vavro, kamil souček, tomáš fíla, petr koudelka, daniel vavřík, daniel kytýř, study of fracture processes in sandstone subjected to four-point bending by means of 4d x-ray computed micro-tomography, acta imeko, vol. 11, no. 2, article 34, june 2022, identifier: imekoacta-11 (2022)-02-34 section editor: francesco lamonaca, university of calabria, italy received december 21, 2021; in final form march 16, 2022; published june 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was supported by the project for the long-term strategic development of research organisations (rvo: 68145535) and the operational programme research, development, and education in project inafym (cz.02.1.01/0.0/0.0/16/019/0000766). corresponding author: martin vavro, e-mail: martin.vavro@ugn.cas.cz 1. introduction the process of crack propagation in quasi-brittle materials due to mechanical loading leading to material failure have been intensively studied by researchers in various disciplines for a very long time. the monitoring of crack initiation and propagation is also of a great importance in rock engineering. the presence of microas well as macrocracks significantly influences the strength, deformation, and filtration properties of the rock mass and therefore, strongly affect, for example, the stability of underground workings, tunnels, or open pit slopes. rock fracture mechanics can also be applied in the prediction of anomalous geomechanical phenomena such as rock bursts or rock and gas outbursts, or in the evaluation of rock fragmentation processes such as drilling, blasting, crushing, and cutting [1], [2]. more recently, the knowledge about rock fracture processes has been of crucial importance when assessing the suitability of the rock host environment for such demanding engineering applications as co2 sequestration or the geological disposal of high-level radioactive waste. the failure process of rocks and similar rock-like materials is the result of complex mechanisms, including microcrack initiation, propagation, and interactions with each other, resulting in crack coalescence. eventually, a macroscopic failure plane is generated, thus causing final rock rupture [3], [4]. cracks in rocks initiate and propagate in response to the applied stress, with the crack path often being driven by the local distribution abstract high-resolution x-ray computed micro-tomography (ct) is a powerful technique for studying the processes of crack propagation in nonhomogenous quasi-brittle materials such as rocks. to obtain all the significant information about the deformation behaviour and fracture characteristics of the studied rocks, the use of a highly specialised loading device suitable for the integration into existing tomographic setups is crucial. since no adequate commercial solution is currently available, a completely newly-designed loading device with a fourpoint bending setup and vertically-oriented scanned samples was used. this design of the loading procedure, coupled with the high stiffness of the loading frame, allows the loading process to be interrupted at any time and for ct scanning to be performed without the risk of the sudden destruction of the scanned sample. this article deals with the use of the 4d ct for the visualisation of crack initiation and propagation in clastic sedimentary rocks. two types of quartz-rich sandstones of czech provenance were used for tomographic observations during the four-point bending loading performed on chevron notched test specimens. it was found that the crack begins to propagate from the moment that ca. 80 % of the maximum loading force is applied. mailto:martin.vavro@ugn.cas.cz acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 2 of micro-flaws such as cavities, inclusions, fossils, grain boundaries, mineral cleavage planes, and micro-cracks inside the rock [5], [6]. crack initiation occurs when the stress intensity factor (k) at a microcrack tip reaches its critical value, known as the fracture toughness (kc). fracture toughness thus expresses the resistance of a material to crack initiation and subsequent propagation and represents one of the most important material properties in linear elastic fracture mechanics (lefm). however, it is important to highlight that rocks and similar geomaterials such as concrete exhibit quasi-brittle behaviour (see [7], [8]), which is typical through the large plastic zone, referred to as the fracture process zone (fpz), ahead of the crack tip where more complex nonlinear fracture processes occur. due to the fpz, classical lefm is not fully applicable for studies related to rock/concrete fracture processes. thus, the description of fractures needs to be carried out based on non-linear fracture models involving the cohesive nature of the crack propagation; often the fracture energy and/or other softening parameters are utilised [9]. to date, many advanced techniques such as scanning electron microscopy [10], [11] or acoustic emission detection [12], [13] have been adopted to study the progressive failure process of rocks. in the case of these experimental approaches, basic data about crack propagation can be obtained, but the spatial information about deformation processes and fpz development throughout the tested sample volume remains unknown. for this reason, a wide range of uses is opening up for x-ray ct in the study of the deformation behaviour and fracture processes in rocks. in this paper, a completely newly designed loading device with a four-point bending setup for vertically oriented scanned samples allowing 4d ct measurements of cracks and fpz propagation in quasi-brittle materials was used. more specifically, the presented contribution deals with the identification of crack initiation and propagation in two types of upper cretaceous quartz sandstones. both rocks represent well-known building, sculpture, and decorative stone materials that have been used on the czech territory for many centuries [14]. 2. rock material used for the experiments two different types of czech sandstones, namely the mšené sandstone and kocbeře sandstone, were used in the fracture experiments. the fine-grained mšené sandstone is almost entirely (> 90 vol. %) composed of quartz grains with an average grain size of 0.15 mm. other clastic components consist of quartzite, feldspars, and mica flakes. the rock matrix (ca. 5 vol. %) is formed by kaolinite with very finely dispersed fe-oxyhydroxides (limonite). the degree of secondary silicification is very low. the fineto medium-grained kocbeře sandstone has a bimodal grain size distribution and mainly consists of monocrystalline quartz grains with an average grain size of 0.24 mm and a maximum grain size of 1.5 mm. quartzite and orthoclase grains occur in a considerably smaller quantity. the matrix and rock cement (10 – 15 vol. %) are formed by quartz which predominates over clay matter. secondary silicification is intense. both sandstones used in the experiments are similar in their mineralogical composition of detrital rock particles but differ in some inner rock texture features as well as in their mineralogical composition of interstitial material between the framework grains. these differences are reflected in different values of physical and mechanical properties (table 1). 3. experimental setup and instrumentation 3.1. x-ray ct imaging device the xt h 225 st industrial x-ray micro-ct system by nikon metrology nv was used to assess the development of the fracture process in fracture toughness tests using chevron bend (cb) test specimens. this x-ray ct scanner is a fully automated apparatus with a rotating scanning system equipped with a microfocus x-ray source, which generates cone-shaped beams. it is equipped with an x-ray flat panel detector having a number of 2,000  2,000 pixels with a pixel size of 200 μm. the basic technical parameters of the xt h 225 st inspection machine are given, for example, in [17]. the scanning parameters were as follows: a reflection target, 160 kv voltage, 126 μa current, 0.5 mm thick aluminium filter, 3,141 projections with two images per projection, 1,000 ms exposition, a scanning time of ca. 2 hours, and a cubic voxel size of 16 μm. the ct data generated by the micro-scanner were reconstructed using the ct pro 3d software (nikon metrology nv). the visualisation and analysis software vgstudio max 3.3 (volume graphics gmbh, germany) was used for data post-processing. 3.2. loading device previous research [17], [18] has focussed on the study of the crack propagation processes in quasi-brittle materials, such as silica-based composites or sandstones; these have shown some limitations when the conventional three-point bending tests were used in combination with the x-ray ct technique. the main disadvantage of such an arrangement of the experiment can be seen in particular in the horizontal orientation of the sample perpendicular to the rotational axis of the ct scanner, which, together with the loading supports covering parts of the radiograms (see figure 1a) significantly reduces the quality of the acquired data. these shortcomings of the foregoing technical solution have been eliminated with the use of a unique four-point bending loading device which was developed (czech national patent cz 307897) at the institute of theoretical and applied mechanics of the czech academy of sciences in prague (itam cas). contrary to the standard arrangement of the threeor four-point table 1. basic physical and mechanical properties of mšené and kocbeře sandstones according to various authors (data adopted from [14]-[16]). parameter mšené sanstone kocbeře sandstone specific (real) density in kg/m3 2,620–2,650 2,630–2,670 bulk (apparent) density in kg/m3 1,850–1,930 2,140–2,490 total porosity in % 26.3–29.7 12.0–15.3 water absorption capacity by weight in % 10.8–13.3 2.2–6.1 uniaxial compressive strength (dry sample) in mpa 21–33 56–87 flexural strength (dry sample) in mpa 0.9–1.9 5.9–7.9 acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 3 bending tests using horizontally oriented samples, this novel approach is based on the vertical orientation of the investigated cylindrical specimen, the direction of whose longitudinal axis is identical to that of the rotational axis of the ct scanner (figure 1b). this concept significantly reduces differences in the attenuation of x-rays observed during the rotation of the sample subjected to a ct scan. the modular device for four-point bending during micro-ct sequences consists of three main components. a pair of a motorised movable supports are integrated with driving units with precision captive stepper linear actuators (23-2210, koco motion dings, usa) and precision linear guideways (mgw12, hiwin, japan). the driving units are equipped with linear encoders (lm10, renishaw inc., united kingdom) ensuring positioning with 1 μm resolution and load cells (lcm300, futek, usa) with nominal capacity 1,250 n. the central part of the device frame exposed to x-ray beam is manufactured from a carbon fiber composite (mtm57 series epoxy resin, t700s carbon fibers, shell nominal thickness 1.95 mm) providing sufficient frame stiffness and low attenuation of x-rays. cylindrical load-bearing frame, housing the loaded specimen, together with all the components of the loading device are manufactured from high strength aluminium alloy (en-aw6086-t6). the small distance between the x-ray source and the scanned objects also allows for the high resolution of the reconstructed 3d images, which is necessary for a detailed tomographic investigation of the loaded sample. the high stiffness of the loading frame and high precision control of the loading force during the experiment allow for the subsequent interruption of the loading process at any point of both the ascending and descending parts of the rock´s load-displacement (f-d) curve without the risk of sudden sample collapse. a more detailed technical description of the in-house four-point measuring device is provided in [19]. the device and its scheme are shown in partial section in figure 2a. 3.3. test specimens and experimental procedure cylindrical cb specimens with a diameter of 29 mm and a length of approximately 195 mm were drilled from sandstone blocks in the laboratory. the core drilling was carried out parallel to the sandstone bedding planes. in the central part of the test specimen, a chevron edge notch was carved using a circular diamond blade. the width of the chevron notch was 1.4 mm. a prepared cylindrical specimen was inserted into the specimen chamber placed into the x-ray ct inspection system (figure 2b) and centred inside this chamber on the supports. the orientation of the longitudinal axis of the specimen, emplaced in the test-ready position, was identical to that of the rotational axis of the loading device. after the initial contact of the specimen with the loading parts, its stable position was secured by applying a contact force of 5 n. then, the inspection using transmission radiography was performed to verify the correct position of the chevron notch tip and to exclude the samples with significant inhomogeneity in the volume of interest in the vicinity of the notch. pre-peak loading was performed in force control mode by prescribing its linear increase and ensuring a uniform load figure 1. principal differences between a conventional three-point bending loading scenario (a) and the new four-point bending test with a vertical orientation of the investigated specimen (b). figure 2. four-point loading device for 4d micro-ct experiments: (a) longitudinal cross-section of the loading device, (b) loading device attached to the rotation table of the xt h 225 st x-ray micro-ct scanner. acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 4 distribution on the supports (outer supports span lout = 179 mm, inner supports span lin = 75 mm). post-peak loading was performed in displacement control mode with a position increment identical for both loading supports to prevent the sudden rupture of the specimen caused by the eventual nonsymmetrical response of the specimen. during the loading procedure, the test time, the displacement value, and the loading force were recorded continuously, with the sampling frequency of the displacements and the load of the external support being 200 hz using an in-house developed control software [20]. during the experiment, the loading process was interrupted at four to five loading steps, where imaging via x-ray micro-ct scanner was carried out. in the loading sequence, one or two load-steps were performed during the rock´s hardening phase, one load-step near the ultimate-stress point, and two or three load-steps were performed during the post-peak softening phase, without the sudden cracking of the specimen. the maximal loading force reached approximately 30 n to 35 n in the case of mšené sandstone and between 90 n and 100 n when the kocbeře sandstone was tested (figure 3). the force drops in the recorded loading diagrams were caused by material relaxation during the time-lapse tomography scanning. it is also clear from figure 3 that three test specimens were examined for each of the two above mentioned sandstone types. one of them, namely specimen 16057/11 from the mšené sandstone, was then selected for a detailed description of the process of crack propagation, which is presented in section 4. 4. results the measured data acquired from the loading device were subsequently processed in the form of f-d diagrams. in figure 4, the displacement-load curve of one selected test sample is shown. as can be seen from figure 4, a total of six micro-ct measurements were taken at points a to f while loading sample 16057/11. a reference ct measurement (scan a) was realised immediately after the fixation of the rock specimen in the loading device, at a minimal loading of 5 n. two other consecutive measurements were performed at points b and c before reaching the maximum load, i.e., in the ascending portion of the loaddisplacement plot. specifically, measurement b was made at a loading level of 26 n (i.e., at ca. 80% of the maximal force) and figure 3. loading curves of mšené (sample no. 16057) and kocbeře (sample no. 16060) sandstones with well visible loading gaps observed by ct scanning. the f-d diagram of the individual rock sample 16057/11 from the mšené sandstone is presented in detail in figure 4. figure 4. f-d diagram of a selected rock sample prepared from the mšené sandstone (16057/11) with ct slices obtained at different loading steps (d, e, and f) on the descending part of the loading curve. a macroscopically visible crack path is highlighted by white lines. the red circle in the vertical cross-section of the loaded specimen (upper left corner) defines the area, which is in the case of loading steps a, b, and c zoomed in onto and presented in figure 5. acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 5 measurement c at a loading of 30 n, which corresponded to about 90 % of the peak force. measurement d practically corresponded to the ultimate stress point. the last two measurements, e and f, were made during the post-peak phase, at loads of 24 n and 9 n, respectively. the presented measurements clearly showed that even such low-strength rock samples, where the peak force reached only 33 n, can be successfully subjected to loading with discrete steps during strain softening without the risk of their sudden collapse. the process of crack propagation during the post-peak behaviour is well visible in the ct images presented in figure 4. generally, it is assumed that the real crack begins to propagate when the load reaches the ultimate stress point (see e.g., [21]). however, based on previous experience acquired during radiographic measurements in a three-point bending loading scenario [17], our current research was focussed mainly on the identification of the possible manifestation of crack origins and their subsequent growth on the ascending part of the loading curve. as for the pre-peak crack propagation, ct images taken before the load reached the peak value are shown in figure 5. this figure indicates that the apparent crack developing from a crack tip was present also within step c, i.e., at a level of ca. 90 % of the maximal loading force (fmax). when compared to reference step a, some changes were visible in the sandstone microtextures in scan b (ca. 80 % of fmax) near the crack tip related to crack evolution. these changes are reflected in the movement of individual quartz grains apart from each other. based on these findings, it can be concluded that, in the case of the studied sandstones, the crack began to propagate from the moment when approximately 80 % of the maximum loading force was applied. however, it should be noted, that the crack path was hard to identify in some parts of the reconstructed ct slices due to heterogenous sandstone microtextures, especially due to the presence of pores. this problem can be overcome by using differential tomography, where changes in the object are emphasised by the differentiation of the actual and the reference tomographic reconstructions, as recently described by e.g., [16] and [19]. the differences between the states at points b and c, respectively, and the initial state (a) are presented in figure 6. the development of the crack during mechanical loading was manifested by an increase of open porosity in the area of the figure 5. series of zoomed ct images of a selected mšené sandstone rock sample (16057/11) acquired at the different loading steps (a, b, and c) on the ascending part of the f-d loading curve. the changes in sandstone microtexture related to crack origin are highlighted by white ellipses. figure 6. visualisation of the development of the crack shape in loading steps b and c using tomographic image subtraction between the actual (loaded) and reference (unloaded) states; i.e., the image on the left represents the difference between steps b and a and the one on the right is a subtraction between the c and a loading steps. acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 6 crack spreading through the rock test specimen, which is clearly shown in figure 7. the figure shows porosity distribution under the tip of the notch in two consecutive loading steps from the tests of the mšené sandstone test specimen no. 16057/11. using a hexahedral mesh grid superimposed on the reconstructed 3d images, where the porosity was evaluated in every hexahedral region of interest as an average over its volume, it can be seen that the macroscopic crack developed before the ultimate stress point was reached. 5. conclusions the study performed on the rock samples prepared from quartz-rich mšené and kocbeře sandstones showed that crack development and propagation can be successfully observed in 3d thanks to the joint use of a four-point bending procedure and high-resolution 4d micro-ct measurements. a four-point bending device that was newly developed at itam cas allows the study of fracture processes in rocks and similar quasi-brittle materials with high precision. the measurements also showed that the concept of vertically oriented rock samples in four-point bending devices for 4d micro-ct provides considerable advantages over the standard horizontally oriented three-point or four-point bending setups. in the studied sandstones, it was observed that the process of crack propagation starts from the moment when approximately 80% of the maximum loading force is applied. this outcome is in very good agreement with the results of previous research [16], [17], which was performed on the different types of sandstone regarding their microtextural features, mineralogical compositions, and related physical and mechanical properties. therefore, this research confirmed the fact that the crack, which was formed and started to propagate before the peak load was reached, was further propagated during the post-peak phase. more specifically, the crack length at point d (maximal loading force) was approximately 4.9 mm, which increased to 9.8 mm and 15.2 mm at points e and f, respectively. these crack lengths measured for post-peak strain softening phase are very similar to the values which were published by [19] for a stronger variety of mšené sandstone. the observation that the process of crack propagation started before reaching the maximum load is consistent with previous knowledge obtained for various types of german, american, or chinese sandstones by means of digital image correlation [24] or acoustic emission techniques [25]-[28]. moreover, this finding is also valid for other quasi-brittle materials, such as concrete, as confirmed by the study of ae signals both in flexural [29] and compressive [30] loading modes. acknowledgement the presented work was supported by the project for the long-term strategic development of research organisations (rvo: 68145535) and the operational programme research, development, and education in project inafym (cz.02.1.01/0.0/0.0/16/019/0000766). references [1] isrm commission on testing methods (f. ouchterlony, coordinator), suggested methods for determining the fracture toughness of rock, int. j. rock mech. min. sci. & geomech. abst. 25(2) (1988), pp. 71–96. [2] b. n. whittaker, r. n. sing, g. sun, rock fracture mechanics: principles, design and applications, elsevier science publishers b.v., amsterdam, the netherlands, 1992, isbn 978-0444896841. 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methodology for the reconstruction of baroque ephemeral apparatuses: the case study of the funeral apparatus for cardinal mazarin in rome acta imeko issn: 2221-870x march 2022, volume 11, number 1, 1 -9 acta imeko | www.imeko.org march 2022| volume 11 | number 1 | 1 operational methodology for the reconstruction of baroque ephemeral apparatuses: the case study of the funeral apparatus for cardinal mazarin in rome margherita antolini1 1 via di s. valentino 16, 00197, rome, italy section: research paper keywords: ephemeral baroque; virtual reconstruction; reconstruction from text citation:margherita antolini, operational methodology for the reconstruction of baroque ephemeral apparatuses: the case study of the funeral apparatus for cardinal mazarin in rome, acta imeko, vol. 11, no. 1, article 13, march 2022, identifier: imeko-acta-11 (2022)-01-13 section editor: fabio santaniello, university of trento, italy received march 6, 2021; in final form march 15, 2022; published march 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: margherita antolini, e-mail: margheritaantolini94@gmail.com 1. introduction from an historical and artistic point of view the baroque ephemeral is the most complete example of artistic and social synthesis, that englobes not only visual arts, but also music, literature and contemporary technical-scientific advancements, in order to achieve a product that can be enjoyed by the public as well as by the aristocracy and intellectuals and convey political and cultural messages at the same time. the baroque ephemeral apparatuses are not limited to the celebration but are part of a system of documents edited with a communicative purpose in order to disseminate and explain the event and its symbolic meaning. if on one hand this implies a certain wideness in the corpus of available sources, on the other hand opens an important question regarding their validity and objectivity in the representation of the events. moreover, the concept of “ephemeral” itself, meaning existing for a limited time, poses issues of difficult solution in terms of conservation, mainly regarding the contraposition of ephemeral/permanent and reality/representation. these were the issues that started the present study, founded on the research of an experimental methodology for study, representation and communication -and consequently conservationof baroque ephemeral artworks in their entireness: as artistic, architectonic, urban, social and political phenomena. without forgetting the pioneering excursions of bragaglia [1], kernodle [2] and tintelnott [3], only in the second half of the last century there was an effective reawakening of interest around this line of italian creativity in the wake of povoledo [4], viale ferrero [5], zorzi [6], ricci [7], the fagiolo dell'arco brothers. although failing to exhaust such a complex theme, they have defined its outlines and focused the main aspects, restoring dignity to an artistic expression long ignored, promoting publications and exhibitions: we find one in naples in 1997 [8], in parma in 2018 [9], and in florence in 2019 [10], which represent a focal moment of study around the theme and available corpus of works. but it is certainly maurizio fagiolo dall'arco in the 70s and 80s to conduct the most exhaustive and systematic study of the baroque ephemeral published to date: among his publications we find bibliografiadella festa barocca, and a work edited with silvia abstract this paper aims to develop a methodology of study of ephemeral artefacts that takes into consideration all the different aspects of the specific art form that is ephemeral baroque architecture. through the study of the social and artistic characteristics of this art form, the analysis of a wide range of case studies will help defining some common and recurring features, especially regarding available data (engravings, paintings, manuscripts, etc.) the main goal of the research will be to outline a methodology of approach to the single cases based on reconstruction from text and graphic data, with special attention reserved to the relationship between the ephemeral apparatus and the surrounding urban space. the effectiveness of the methodology is tested through the application to the case study of the funeral apparatus for cardinal mazarin in rome (1661). mailto:margheritaantolini94@gmail.com acta imeko | www.imeko.org march 2022| volume 11 | number 1 | 2 carandini who represents the real cornerstone for studies on the theme, l’effimerobarocco: strutturedella festa nella roma del ‘600, as well as numerous publications and articles on the subject. only in the recent years the theme of the reconstruction of baroque ephemeral gained the attention of various academics such as paolo belardi [11] and paolo lattuada [12] who conducted experiments on the representation of ephemeral apparatuses, whereas macarthur, leach [13], delbeke [14] and conforti, d’amelio, grieco [15] continue the tradition of historical analysis through documents in the footsteps of fagiolo dall’arco and carandini. 2. characteristics of the phenomenon the concept of ephemeral architecture and therefore temporary and "false" as well as the construction techniques used have their roots in theatrical scenography, an artistic practice that experienced a moment of intense development in the 16th and 17th centuries, with the birth of melodrama and the works of artists such as palladio, scamozzi, serlio and peruzzi. the idea of arranging urban spaces to celebrate events is the logical consequence of the conception of the new acting space, and it naturally encompasses all of its characters. on the other hand, the depictions of pomp and luxury that characterize these projects are only a staging of power: rather than representing the stability of the institutions and their highest representatives, these evanescent creations actually seem to be the symbol of an equally labile and inconsistent power, of the political void of a fragmented and often subject to foreign domination country [16]. used to divert the attention of ordinary people from the precarious reality and to create a fictitious link between the various social classes, ephemeral constructions found a particularly fertile terrain during the xvii century when, with the spread of baroque poetry, the spectacular effect became the primary component of any artistic expression. thus art becomes an instrument of political propaganda and manipulation of the people who, through a new form of panem et circenses finds a moment of liberation from the working reality and contact with the unreachable world of the roman aristocracy. at the same time, celebrations are a demonstration of strength also towards other nations. a clear example are the installations built for the celebration of the birth of foreign princes, as in the case of the dauphin of france in 1662 designed by bernini, or the exceptional one for the arrival in rome of christina of sweden in 1655. on this occasion, on the 23rd and 24th of december 1655, apparatuses similar to those of the papal possession were set up for the entrance into rome, in a path that unraveled among porta del popolo, san pietro and palazzo farnese, by gian lorenzo bernini, giovan paolo schor, ercole ferrata and carlo rainaldi; in february of the following year the presence of the former queen animated the carnival with special floats in via del corso, and theatrical performances at palazzo barberini designed by giovanni francesco grimaldi. the religious-political-social program of the baroque ephemeral exploits the technique of wonder according to the politics of persuasion, giving the artist a particular role as an image specialist. in this case, art does not give us the real face of the century but the face it believed (or wanted to be believed) to have: splendid, idealized, sumptuous, rich. the relationship of the artist with the client, and therefore with power, becomes particularly delicate to interpret at this point. despite being an instrument in the hands of religious-political power, the artist manages to impose his intellectual power, integrating the allegory, bearer of the political message, in the imaginative method. festive occasions retrace the entire life span of illustrious personalities, first of all of the pope, in a mixture of civil and religious occasions. the public feast, enjoyed by the community in an integral manner, is a complete event, both sacred and profane at the same time, during which the judgment is suspended and one deliberately enters into a reality different from the daily life. from a planning and organizational point of view, the celebration follows the same process regardless of the occasion. the main manager is the intellectual, who often coincides with the client: man of culture or religion, he takes care of drafting the ideological program of the festival, experimenting with new tools for cultural diffusion. in the next phase, the artist and the creator collaborate to identify a more or less complex communication code, which will then be translated into forms. in every single apparatus, the plastic, pictorial and literary solutions are therefore a support for the ideological and cultural connotation, and are arranged in a spatial and consequently temporal -succession in order to fulfill all the allegorical implications of the program [17]. the event is also completed by a report drawn up by the intellectual and illustrated with few engravings, in which the apparatus and its meaning are described and explained, with the aim of functioning as a booklet for those who participate in the occasion, but also as means of diffusion especially in foreign courts. in this context, the relationship with the place and the city is fundamental: rome first of all presents itself as a spectacle, with its history and its tradition. the ancient city becomes an immense and prestigious theater and, if on the one hand it recalls the almost obsessive use of theatrical terminology in baroque culture, on the other it clarifies the particular political, social and urban configuration of the city in relation to the shows that on that scene take place. 3. metholodogical issues the study of the cultural phenomenon of the ephemeral in the baroque era has highlighted several characterizing and relevant aspects for the study of the individual artefacts: in particular, the literary and socio-political components were found to be fundamental. approaching the individual projects, some recurring features from the documentary point of view emerge. analyzing a wide range of examples, it was clear that there are several basic structures which, if analyzed, can facilitate and simplify the study. in particular: • each occasion presents recurring types and expedients, explained in specific treaties; • each artist uses a more or less homogeneous language, and given the short time dedicated to the design and construction of the apparatus, he often reuses the same pieces on different occasions; • each occasion is documented by a written text and some "official" engravings, as well as paintings and other testimonies. in this study, attention is paid to the latter as the only tool available for the reconstruction of ephemeral apparatuses. the relationship between text and architecture is a theme dealt with starting from the reading of vitruvius in the 16th century, or of leon battista alberti's descriptio urbis romae, and is still an object of interest in various areas of study. in these acta imeko | www.imeko.org march 2022| volume 11 | number 1 | 3 cases, unlike what happens in the field of restoration and archeology, the reconstruction does not start from the study of the artefact to be integrated with iconographic sources, but proceeds in the opposite direction, using texts and images as a starting point to then be confronted with the built reality. some scholars attempted reconstructions of baroque ephemeral systems, achieving results that we believe are partial and not very relevant, but which open the way to a series of interesting reflections and observations. on the one hand paolo lattuada[12] reconstructs, in a virtual and physical environment, the celebratory machine designed by ferdinando sanfelice in 1740 for the birth of the reale infanta in naples. if on the one hand he manages to interpret all the metric and space integration aspects, both the realization in wooden panels and the virtual restitution are lacking considering the figurative restitution of the desired atmosphere and obtained by the designer. on the other side, paolo belardi and valeria menchetelli [13] deal with giuseppe piermarini's work by returning well-defined and metric correct models, but completely ignoring the material, figurative and integration aspects with the context. the need of an operative methodology that would allow to obtain a historical, critical and virtual reconstruction of the baroque ephemeral architectures arose from the analysis of the two proposed examples and archival documents. by these terms it is understood that the methodology presented aims to indicate a series of effective steps which, if followed and applied with a critical conscience on a case-by-case basis, return a historically reliable product that takes into account the historical distance and the transformations that happened in the places and in the urban fabric. the choice of the virtual environment as the main theater of restitution derives from a reasoning on the most appropriate means for communicating this form of intangible heritage. two aspects have led to the exclusion of pure two-dimensional representation and the hypothesis of the construction of temporary constructions: • the virtual environment respects the idea of temporary set-up without producing fake and always temporary artefacts, however allowing to interact with the place and participate in a possible museum project [18], [19]; • the virtual environment is the contemporary response to the baroque desire to expansion and manipulation of space. in addition to these premises, the method was empirically deduced from the study of the sources of a case series varied from the point of view of the occasions and the artists involved, which unfolds during the xvii and xviii centuries in rome. the process is structured in four fundamental phases, which can be summarized in the study of the sources, study of the object, analytical phase and restitution; while the first and the last clearly represent the initial and final moment of the work, the two intermediates can be considered complementary and represent the real critical moment of the study (as shown in figure 1). once a project to be analyzed has been selected, the first step in the process is the collection and study of the sources relevant to the case study. in most cases it will be a report and official engravings, which can be accompanied by commemorative paintings and period chronicles. fagiolo dell'arco and carandini [20] have already drawn up a catalog of ephemeral apparatuses realized in rome, which lists each project according to year, occasion, artists, material produced, material available and transcription of textual sources. this volume is extremely relevant and can constitute the beginning of a systematic study, but it is limited to a documentation without interpretation of the data collected. the second moment aims at the appropriation of the data with the aim of analyzing it later: it is therefore a matter of transcribing the texts, often manuscripts or seventeenth century prints, and of redesigning the engravings collected. in this way it is possible to begin to discern the characteristic elements and the amount of information available regarding the specific case. afterwards, two courses of study open up: on one side the study of the object, on the other the analytical phase. the following will be presented one after the other, but it is clear that there is a continuous exchange between the two. as for the study of the object, it was highlighted how the place was a fundamental component of the baroque festival and the design of the equipment. for this reason, it is firstly planned to carry out the survey of the area concerned, in its current configuration and in a reconstructive hypothesis of the contemporary configuration at the date of the celebration. for this, we will apply on the one hand the methods of integrated surveying with massive acquisition for the survey of the current state, and on the other the techniques of historical research through archival documents, historical maps, analysis of the walls, etc. the operations described will result in a pointcloud and a timeline, which will be graphed in a two-dimensional (drawings) and three-dimensional(models) restitutions of the current state of the place and of the reconstructive hypothesis of the historical moment involved in the study. in this case it is not strictly necessary to study in depth each building phase, but it is certainly important to have a clear vision of the evolution of the fabric and the construction. at the same time, it is possible to start the analytical phase, studying the cultural environment. given the importance of the ideological and allegorical contents, attention will be paid in particular to the figure of the client and the artists involved, as well as to the research for comparisons of similar cases. considering the scarcity of iconographic material, in fact, the deepening of the artist's style can lead to valid interpretations of the data gaps, or to clarifications of unclear points. at this point the actual analysis of the documents begins: firstly, we will analyze the geometric-proportional aspects of the graphic data, interpreting the techniques and graphic choices used in the engravings and paintings, and applying the rules of reversed perspective and of the orthogonal projection. in this way, it will be possible to establish the reliability of the designs and their correspondence to reality, and to understand if particular visual effects have been sought. knowing the geometry of the place, in fact, it will be sufficient to overlay the two representations to notice any deformations, technical tricks, or inaccuracies. a similar approach will be applied to textual data, from which all possible information will be derived. graphic and textual data will then be interpolated to develop a model as exhaustive as possible regarding elements, geometries, colors, settings, lighting, details, allegorical meanings, plastic solutions, etc. to proceed in the knowledge, we then proceed by working on classifications that synthesize the distinction between ephemeral and permanent elements, the artistic techniques used, which range from painting to sculpture to pyrotechnic installations, but also musical, to finally distinct an abacus of acta imeko | www.imeko.org march 2022| volume 11 | number 1 | 4 serial and special elements. note how these two phases are the most conditioned by the specificity of each case. the last moment is the restitution. this can take different forms depending on the sensitivity of the operator, the specific case study, and the use that will be made of it, but it must necessarily be composed of a series of two-dimensional drawings regarding the appearance of the place during the party, and of the three-dimensional models that present the layout in its original setting and the integration of the layout in today's environment. 4. case study the chosen case study is the pompa funebre nell'esequie celebrate in roma al cardinal mazarini nella chiesa dei ss vincenzo e anastasiothattook place in 1661. a few years earlier, cardinal giulio mazzarino, minister of louis xiv, financed the construction of a church in the area of piazza di trevi, the church of saints vincent and anastasius at trevi, which was completed a few months before his death. here the funeral was celebrated in rome, with an apparatus created by elpidio benedetti as the intellectual of reference and giovanni francesco grimaldi as the artist. at this time, an important role was played by castrum doloris and funeral celebrations in general. menestrier [21] describes the regulations for the funeral ceremony: the funeral is divided into different moments (invitation, convoy, service, funeral eulogy, burial) which must correspond to the different parts of the church decoration (external facade, nave, altars, inscriptions, catafalque) as places appointed for an allegorical representation that culminates in the luminous vision of the castrum doloris, where the triumph of death and religious and temporal power represented by the deceased are celebrated. the catafalque is the mirror of baroque taste, increasingly aimed at dematerialisation and to the refined invention, which will culminate in the triumphs of the macabre designed by bernini. it is possible to trace a real typological evolution in the forms taken by the catafalques during the 17th century from the forms of the ancient mausoleums: from an initial temple shape (catafalque for sixtus v, d. fontana, 1591) we move on to a temple or pyre type (catafalque for philippe the iii, torriani, 1621), and then prefer real miniature mausoleums (catafalque for gregorius xv, lippi, 1623) ; these typologies are accompanied by exotic experiments, such as the catafalque della valle in 1627, while in the second half of the century pyramid forms are preferred (catafalque for the jesuits, sacchi, 1639) and types based on assembly, such as the temple typebaldachin (catafalque for philippe the iv, del grande, 1665), mausoleum with obelisks (catafalque for philippe iv, c. rainaldi, 1665) and triumphal arch and small temple with pyramid (catafalque for anne of austria, grimaldi, 1666), up to pyramids supported by living skeletons (catafalque beaufort, 1668).[17] throughout his life grimaldi enjoyed the admiration and favor of a large group of admirers, thanks to his ability to adapt to the client's needs, from his good-natured and quiet character and perhaps from the moderate rewards required. among the first to express appreciation on the artist was elpidio benedetti himself: notes of commendation appear in the reports drawn up on the occasion of feasts and religious celebrations where he worked, as the judgment reported to him by antonio perez de rua when the artist he decorated the external facade of the church of san giacomo degli spagnoli on the occasion of the funeral rite of the spanish king philip the iv: «cuiodiseno es de iuav francisco grimaldi bolognese, no menosvalienteen el pincel, que bizarro en el compas, y enestegenero de obrasestimado por el primero desta corte.» [22] figure 1. flowchart of the proposed methodology. acta imeko | www.imeko.org march 2022| volume 11 | number 1 | 5 4.1. study of the sources upon the death of cardinal mazarin on march 9, 1661, benedetti received instructions from louis xiv to create an adequate funeral pump. the funeral report, [23] written and published by benedetti, contains five engravings by dominique barrière which document the catafalque, the decorations for the facade, the counter-facade, the nave and the choir on one facade drawing, three views of the interior, one towards the choir, the second towards the main entrance and the third of the walls of the lateral nave, and of the catafalque. barrière’s name appears only in the engravings of the facade and the catafalque, while all the engravings bear the inscription abbas elpidius benedictus inven., and grimaldi is referred to as architect of the entire funeral apparatus, or artist responsible for its construction. in the decoration of the facade and the interior it is difficult to find grimaldi's hand: the two landscapes on the facade, representing the sunset and the rising sun, are somewhat elementary and do not seem to be the work of a specialist, so it seems that he entrusted this project to his aid given the contemporary commitment in the palaces santacroce and nunez. on the other hand, the engraving of the cardinal's catafalque highlights the typical stylistic elements of grimaldi, on the basis of which both the invention and the execution are attributable to him. the catafalque is a simple structure which, although lacking specific preparatory studies, retains many elements also found in the preparatory studies for the catafalque for anne of austria, designed and built by grimaldi in 1666 [22]: some of the elements present in the first sketch were incorporated into the catafalque of cardinal mazarin, for example the candle holders on the sides, the antique style putti on the right at the base of the catafalque of anne of austria are also present next to that of cardinal mazarin. likewise, the two female figures holding the urn reappear in mazarin's catafalque, although their function has been changed. furthermore, the two putti holding the queen's coat of arms perform almost the same function when they raise the cardinal's hat. finally, in both monuments, the urn is placed on top of the funeral structure and above the urn appears the portrait of the deceased flanked by two figures. the second sketch contains elements that are found in the catafalque of cardinal mazarin, such as the pyramidal structure at the base of which, both in the drawing and in the engraving, two putti hold the coat of arms of cardinal mazarin while in an elaborate urn is placed high. in both cases, the entire structure ends with the portrait of the deceased flanked by two figures. although the catafalque of cardinal mazarin is different, all the elements that compose it as well as the style of the female figures, the putti, the olive trees and palm trees refer back to grimaldi rather than benedetti. grimaldi has in fact repeatedly used the same elements in his career. it is not clear who created the iconography: it could have been a collaboration between the french authorities, benedetti and, perhaps, grimaldi. in any case, it is based on a tribute to cardinal mazarin's services to the french crown and christianity. at the foot of the square base sit two parts of the world in reference to cardinal mazarin's broad political action. above the base there are the coat of arms and the cardinal's hat and above it there is a representation of rome in tears over the death of the cardinal and on the other three sides are the relief representations of france, spain and christianity. above the pyramidal structure rests the urn of cardinal mazarin who acts as a support for his portrait surrounded by a laurel wreath. on the left an olive tree, a symbol of peace, under which the religion appears and behind it the justice. on the right a palm tree, a symbol of war, with the fortress underneath and providence behind. among the branches, two cherubs hold an olive crown with the inscription et pace and a palm crown with the inscription et bello. 4.2. study of the object the survey of the church of santi anastasio e vincenzo was carried out with the integrated survey methodologies with massive acquisition through laser scanner and structure from motion. in this way, a point cloud was obtained which was satisfactory for the set objective. additionally, through historical research, it was possible to delineate a well-defined timeline for the construction of the church [24], [25]. given the narrowness of the site, the original organism consisted of a nave flanked by chapels which, without a transept, ended in the presbytery with a flat deep wall, at the height of the pilasters that will then frame the eighteenth-century tribune. the tribune, with the high altar and the elliptical dome above it, belong to the intervention after 1748. regarding the nineteenth-century interventions, in addition to the decoration of the chapels, we highlight the fresco of the vault in 1818 and the restoration of 1857 with remaking of the marble floor. the building history can therefore be summarized in three main phases (figure 2): • 1641-1646; interior construction, with back wall and six chapels by gaspare de vecchi; • 1646-1650; construction of the facade by martino longhi the younger; • post 1748; extension of the church with tribune, apse and dome; decoration of the chapels. for the purpose of the study, the configuration relating to 1661, i.e. shortly after the completion of the second phase, was analyzed and redesigned, based on the comparison between the current configuration of the church, archival documents, engravings of landscape painters and engravings related to the apparatus. 4.3. analytical phase following the previously described methodology, once the place and the general characteristics of the case study have been studied, the attention is focused on the iconographic sources of the apparatus. as for the funeral in question, no paintings or engravings other than those of the report were found, i.e. five engravings by dominique barrière depicting the facade, the counter-facade, the nave, the altar and the catafalque. all these engravings present the apparatus in its intended setting, which means that it is possible to infer the tridimensional configuration of the ephemeral apparatus simply by comparing the engravings with the survey and then applying the rules of inverted perspective is necessary. first of all, it was necessary to deal with the problem of scale and deformation: the engravings in fact have no dimensional indication and are not all represented on the same scale, but rather designed to fit the same format. therefore, they have been divided into two groups, as two of them are drawn in orthogonal projection while the other three in central perspective. analyzing those in orthogonal projection, that is the facade and the nave, at first it seemed sufficient to exploit the geometry acta imeko | www.imeko.org march 2022| volume 11 | number 1 | 6 of the church, that is to say trivially to superimpose the engraving on the reconstructive hypothesis advanced during the historical study of the place. however, the operation has been proved not sufficient, since the drawings are deformed in height, probably to obtain visually more captivating effects of grandeur and vertical development (figure 3). as for the counter-façade, the altar and the catafalque, they were divided into two further subgroups. the first includes the engravings of the counter-façade and the altar: also in this case, the solution seemed simple, that is to apply the rules of descriptive geometry and of the reverse perspective to obtain the real measurements, but a more detailed examination revealed that these two engravings present three central vanishing points and therefore are an assembly of three overlapping perspectives. here too, as for the orthogonal projections, the goal of the engraver was to obtain a vertical expansion of the space of the drawing compared to the real space. the process was therefore to recognize the three perspectives with their vanishing points, horizons and landlines, and to recognize which parts of the drawing referred to the single system of projections, and then to apply the rules of inverse perspective (figure 4). the last reasoning concerns the catafalque: as far as the perspective study is concerned, it was much simpler here as the drawing is a central perspective with a single vanishing point. however, the engraving exclusively shows the catafalque, removed from the architectural context, so it was possible to reconstruct its geometry but not its dimensions and position. thanks to the integration with the textual data and with the stylistic comparisons, it was possible to know the position of the catafalque in the church and to notice how all the grimaldi catafalques respond to the same dimensional relationships between the catafalque and the setting. at this point it was possible to integrate the information deriving from the graphic data with the survey and reconstruction drawings previously produced, obtaining the nature, position and size of each element; at the same time, it was possible to distinguish the elements of the ephemeral apparatus from those of the permanent architecture. figure 2. graphic representation of the three construction phases of the church of santi anastasio e vincenzo in rome. figure 3. analysis of the scale and deformation in the engraving of the nave in relation to the survey. figure 4. analysis of the perspective and deformation in the engraving of the altar. acta imeko | www.imeko.org march 2022| volume 11 | number 1 | 7 an interesting detail is the study of shadows in the engravings. these, in fact, have been designed with such detail as to allow to distinguish the painted elements from those in relief, and for the latter to determine their depth and position with respect to the background. at the same time, the textual data was analyzed, first reading and transcribing the report carefully. the informative nature of the text was immediately evident, given the celebratory but at the same time simple language used by the author, and the absence of latinisms. the report consists of seventeen pages of text. the first two cite a praise of all those who were involved in the organization of the funeral and introduce the following five chapters, each of which accompanied by an engraving. all these chapters follow the same approach: commendation of the deceased, description of a part of the apparatus and explanation of the allegorical meaning. as mentioned above, the text played a crucial role in defining the position and size of the catafalque, but also provided a number of important information. first of all, the analysis of the text confirmed the distinction of the elements in ephemeral and permanent; subsequently, it allowed the characterization of the models with data regarding materials, colours, lighting and desired atmosphere, and finally a further characterization through the allegorical reading (figure 5). to obtain this result, the text has been reread several times, highlighting the different types of information and classifying them according to: element involved, chromatic, material, and symbolic data. these pieces of information were interpolated with the result of the analysis of the graphic data, and generated an abacus of elements based on the artistic techniques used and the seriality or not of the artefact, which clarified all the characteristics of the apparatus and facilitated the modeling process. symbology information was not easy to integrate with twodimensional models but can be added to the three-dimensional model as easy-to-understand interactive pop-ups. 4.4. restitution the final result of the study consists of two-dimensional (1:50 scale) and three-dimensional (potentially in 1:1 scale) elaborations, born from the integration of relief, historical study, analysis of textual data and analysis of graphic data. at the communicative level, if two-dimensional drawings can be more useful for communication at an academic level and are easily readable, the model is more versatile for dissemination and possible use in the context of museum display, while maintaining the same level of reliability and scientific soundness. the models produced, moreover, not only want to return the reconstruction of the 1661 exhibition but investigate the relationship that is created between the apparatus designed by grimaldi with the current location, establishing a critical dialogue between past and present, between ephemeral and permanent, in which both become changeable and question the concept of real space and space of representation (figure 6). 5. conclusions the presented study achieved two complementary results valid for further developments. the first is the creation of the 3d models of the setting up of mazarin's funeral, or the outcome of the analysis carried out on the case study. these can be immediately made available and used for academic purposes but also, and most importantly, for communicative and conservative reasons. however, the models mentioned are the result of the application of a study and analysis method drawn up here to address and resolve representation issues that have never been fully analyzed so far. if applied systematically and correctly, i.e. with a critical sense, the methodology could lead to interesting results in several areas: • first of all, the study of a fundamental cultural phenomenon for baroque society; • as seen previously, the design of the ephemeral apparatuses gave the possibility to the artists to create study models on a 1:1 scale of stable architectures that they would later build. • their knowledge could open new scenarios regarding the design process of baroque architects; • communication and museum display of artifacts that can facilitate users in understanding the society of baroque rome with immediate, easily available and low-cost means of management [26]; • conservation of the intangible heritage in the manner most appropriate to the specific characteristics of the phenomenon [27]. figure 5. results of the graphic data interpretation, the textual data information and final 3d model. acta imeko | www.imeko.org march 2022| volume 11 | number 1 | 8 overall, the methodology aims to be operational, historically reliable, critical of the transformations of places and to return the results in a virtual environment and is the result of an empirical approach. lacking previous research based on the specific topic, it was fundamental first to understand the phenomenon of the baroque ephemeral from an ethnographic point of view, to establish its importance and to better interpret the relationship between the apparatuses and the places, or the relationship between artists, power and the city. subsequently, by deepening the historical-artistic aspects, it became clear the fundamental role that the celebrations play in baroque poetics, as the sum of its three fundamental aspects: the aim for wonder, experimentalism and the synthesis of the arts. from the beginning, the relevance of these projects evident, as well as the recurring characteristics in the conduct of the celebrations and above all in their representation. therefore, a study of a wide range of cases was considered necessary to confirm the hypothesis. hence the need for a study method that would take into account both the specific features of each apparatus and their typicality, and which could open up the possibility of studying this artistic practice in a systematic way. acknowledgement we thank professor alfonso ippolito (università di roma la sapienza) for supporting this research as a tutor and mentor. 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[21] c. f. menestrier, des decorations funebres. ou il estamplementtraité des tentures, des lumières, des mausolées, catafalques, inscriptions, & autre sornements funèbres, j.b. de la caille, a paris, 1683, 367pp. [22] d. batorska, giovanni francesco grimaldi (1605/6-1680), campisano, roma, 2011, 252 p. [23] e. benedetti, pompa funebre nell’esequie celebrate in roma al cardinal mazarini nella chiesa de ss. vincenzo & anastasio, roma, 1661. [in italian]. [24] a. pugliese, s. rigano, martino lunghi il giovane: architetto, bulzoni, roma, 1972. [in italian]. [25] v. rizzi, indagine su di una facciata barocca: la facciata della chiesa dei ss. vincenzo e anastasio di martino longhi il giovane a roma rilevata e disegnata da viviana rizzi, fratelli palombi, roma, 1979, 34 tav., pp. 80-81. [in italian]. figure 6. composition of the setting in the original location (3d model) and in the contemporary environment (pointcloud). http://www.paololattuada.net/progetti/recupero-e-restauro/ricostruzione-filologica/ http://www.paololattuada.net/progetti/recupero-e-restauro/ricostruzione-filologica/ acta imeko | www.imeko.org march 2022| volume 11 | number 1 | 9 [26] grazia tucci, valentina bonora, valerio tesi, bernardo pagnini, additive manufacturing of marble statues: 3d replicas for the preservation of the originals, 2019 imeko tc4 international conference on metrology for archeology and cultural heritage, florence, italy, 4-6 december 2019. online [accessed 17 march 2022] https://www.imeko.org/publications/tc4-archaeo2019/imeko-tc4-metroarchaeo-2019-53.pdf [27] a. antonopoulos, s. antonopoulou, 3d survey and bim-ready modelling of a greek orthodox church in athens, imeko international conference on metrology for archaeology and cultural heritage, lecce, italy, 23-25 october 2017. online [accessed 17 march 2022] https://www.imeko.org/publications/tc4-archaeo2017/imeko-tc4-archaeo-2017-134.pdf https://www.imeko.org/publications/tc4-archaeo-2019/imeko-tc4-metroarchaeo-2019-53.pdf https://www.imeko.org/publications/tc4-archaeo-2019/imeko-tc4-metroarchaeo-2019-53.pdf https://www.imeko.org/publications/tc4-archaeo-2017/imeko-tc4-archaeo-2017-134.pdf https://www.imeko.org/publications/tc4-archaeo-2017/imeko-tc4-archaeo-2017-134.pdf thermoelasticity and aruco marker-based model validation of polymer structure: application to the san giorgio’s bridge inspection robot acta imeko issn: 2221-870x december 2021, volume 10, number 4, 177 184 acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 177 thermoelasticity and aruco marker-based model validation of polymer structure: application to the san giorgio’s bridge inspection robot lorenzo capponi1, tommaso tocci1, mariapaola d’imperio2, syed haider jawad abidi2, massimiliano scaccia2, ferdinando cannella2, roberto marsili1, gianluca rossi1 1 department of engineering, university of perugia, via g. duranti 93, 06125 perugia, italy 2 industrial robotic unit, istituto italiano di tecnologia, via morego 30, 16163 genova, italy section: research paper keywords: thermoelasticity; aruco markers; structural dynamics; carbon fibre reinforced polymer; robot inspection citation: lorenzo capponi, tommaso tocci, mariapaola d'imperio, syed haider jawad abidi, massimiliano scaccia, ferdinando cannella, roberto marsili, gianluca rossi, thermoelasticity and aruco marker-based model validation of polymer structure: application to the san giorgio’s bridge inspection robot, acta imeko, vol. 10, no. 4, article 28, december 2021, identifier: imeko-acta-10 (2021)-04-28 section editor: roberto montanini, università di messina and alfredo cigada, politecnico di milano, italy received july 30, 2021; in final form december 9, 2021; published december 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: lorenzo capponi, e-mail: lorenzocapponi@outlook.it 1. introduction in design and materials engineering, experimental validation of numerical models is commonly required in order to verify the quality of the simulation [1], [2]. in general, the level of validation is directly tied to the intended use of the model and then, the supporting testing experiments are defined [3], [4]. while indirect validation uses experimental results that cannot be controlled by the user (e.g., from the literature or from previous researches), a direct approach performs experiments on the quantities of interest [5], with the aim of reproduce, through the experiments, actual behaviour of simulated model [4], [6]. when irregularities in the geometry or in the molecularstructure of the material are present, localised stress concentrations can lead to fractures [3]. due to this, a stress concentration factor is usually considered during the design of a structure and, moreover, it is one of the experimental validations focuses. local stress and strain measurements have been widely performed by means of established contact techniques (e.g., strain-gauges) [7]–[10]. however, in last decades, non-contact measurement methods for full-field stress and strain distribution estimation were developed and commonly employed in experimental validation tests, such as the thermoelastic stress analysis (tsa) [11]. according to the thermoelastic effect, for a dynamically excited structure, the surface temperature changes, measured by means of an infrared detector, are proportional to the stress and strain tensors changes, caused by the input load [12]. thermoelastic stress analysis was involved in multiple researches, regarding non-destructive testing [13], defect identification [14], and material properties characterization [15], [16]. thermoelasticity has also been used to determine fatigue limit parameters and crack propagation [17], [18], for modaldamage identification in frequency domain [19], and for stress intensity factor evaluation in complex structures [20]. however, due to the demands of high-speed operation and the use of light structures in modern machinery, static measurements of stress and strain distributions are no longer sufficient [21], [22]. in fact, when a flexible structure is excited at or close to one of its natural frequencies, significantly increased fatigue damage occurs [23]– abstract experimental procedures are often involved in the numerical models validation. to define the behaviour of a structure, its underlying dynamics and stress distributions are generally investigated. in this research, a multi-instrumental and multi-spectral method is proposed in order to validate the numerical model of the inspection robot mounted on the new san giorgio's bridge on the polcevera river. an infrared thermoelasticity-based approach is used to measure stress-concentration factors and, additionally, an innovative methodology is implemented to define the natural frequencies of the robot inspection structure, based on the detection of aruco fiducial markers. established impact hammer procedure is also performed for the validation of the results. mailto:lorenzocapponi@outlook.it acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 178 [25]. due to this, modal parameters (i.e., modal frequencies, modal damping and mode shapes) of structures and systems in frequency range of interest are widely researched to properly simulate their behaviour in real operating conditions [26], [27], and, thus, to avoid fatigue damage. modal parameters definition is usually reached experimentally via impact-hammer procedure [26]. nevertheless, in last years, the use of non-contact imagebased measurement techniques in structural dynamics applications has grown. in fact, displacements, deformations and mode shapes can be measured with cameras operating in the visible spectrum by applying both digital image correlation and other computer-vision methods [28]–[30]. one of the more promising approach for displacement and motion detection involves markers, either they are physical or virtual. virtual markers are directly generated through computer-vision algorithms, such as scale invariant feature transform (sift) [31], [32], and speeded up robust features (surf) [33]. these algorithms are able to detect and describe local characteristics (i.e., features) in images. moreover, virtual markers are often used as they allow tracking objects in subsequent acquired frames without introducing physical targets, avoiding potential misleading elements. in recent years, several researches were developed using virtual markers. khuc et al. [34] and dong et al. [35] investigated structural and modal analysis via computervision algorithms through virtual markers. however, in the cases when the points of interest are not directly identified as markers, the physical targets need to be involved. furthermore, virtual markers are strongly influenced by lighting changes and low contrast and, moreover, they can not be detected in uniform intensity distribution areas with no gradients. physical markers, also known as fiducial markers, have been also widely employed in structural monitoring applications [36], [37]. a commonly employed group of fiducial markers are the squared planar markers [38], which are characterized by a binary-coded squared area, enclosed by a black border. several sets of this marker type have been developed in years [39]–[42]. however, in case of nonuniform light conditions and desired simultaneous detection of multiple markers, the aruco marker library was found to be very efficacious and robust to detection errors and occlusion [38], [43]. additionally, if the camera is calibrated, the relative position of the camera with respect to the markers can be directly estimated, and, through a custom configuration process, the system becomes more insensitive to detection errors and false positives [38]. due to this, the aruco markers applicability was widely studied in last years. sani et al. [44] and lebedev et al. [45] employed them for drone quad-rotor and uav autonomous navigation and landing, while elangovan et al. [46] used them for decoding contact forces exerted by adaptive hands. structural dynamics applications were also researched by abdelbarr et al. [47] for structural 3d displacement measurement. moreover, tocci et al. investigated the measurement uncertainty of the aruco marker-based technique for displacement up to order of 1/100 mm, using a comparison laser doppler vibrometry technique, defining the influence of the measurement parameters on the resulting measured displacement [48]. in this research, a non-contact multi-instrumental approach is presented for the numerical model results validation, which involves stress concentration factor evaluation through thermoelasticity measurements and natural frequencies identification by means of aruco markers detection. the proposed method is applied to the san giorgio's bridge robot inspection structure. 2. materials and methods 2.1. robot inspection the robot inspection is the first platform for the automatic inspection of a bridge. it was developed within a collaboration between the italian institute of technology, camozzi group, sda engineering, ubisive and the university of ancona (patent pt190478), for the new viaduct on the polcevera river, the socalled san giorgio’s bridge, designed and built after the morandi’s bridge collapse. the structure is a 3-degrees-offreedom platform, and it is fully autonomous. its main purpose is to carry 3 technological instrumented supports with high performances cameras, lasers, ultrasonic sensors, and anemometers that scan the lower surface of the bridge and collect more than 35000 pictures. these pictures are then processed by pattern analysis algorithms and given to the operator information whether any changes that on the investigated surface occurs. the robot weights around 1.8 t and it is shown in the figure 1. 2.2. thermoelasticity-based stress-concentration factor estimation thermoelasticity is a full-field stress-distribution measurement technique based on the thermoelastic effect [11], [12]. according to this effect, in case of adiabatic process and linear, homogeneous, and isotropic material behaviour, a dynamically excited structure presents surface temperaturechanges proportional to the changes in the stress and strain tensor traces, caused by the external load [19], [49]. moreover, if the excitation is harmonic, the thermal fluctuation is expected to be at the same frequency of the input load, and its normalized amplitude variation is given by: 𝛥𝑇 𝑇0 = −𝐾𝑚  𝛥𝜎𝑘𝑘 , (1) where 𝑇0 is the ambient temperature, 𝛥𝜎𝑘𝑘 is the first stress invariant variation and 𝐾𝑚 is the thermoelastic coefficient, defined from [11]: 𝐾𝑚 = 𝛼 𝜌 𝐶𝜎 , (2) where 𝛼 is the thermal expansion coefficient, 𝜌 is the material density and 𝐶𝜎 is the specific heat at constant pressure (or stress) [12]. in general, the temperature variation caused by the thermoelastic effect is within the noise produced by the infrared detector [50]. thus, thermal acquisitions are necessarily post processed in order to obtain readable results [51]. although general frequency-domain approaches are well established nowadays [19], [52], in this research a classical lock-in analysis figure 1. robot installed on the new viaduct on the polcevera river. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 179 was performed in order to single out the thermoelastic signal at a particular frequency (i.e., the load frequency) from the noisy signal acquired through the thermal camera [53], [54]. being 𝜔𝐿 the input load frequency, the digital lock-in amplifier gives the temperature fluctuation at 𝜔𝐿 as magnitude 𝛥𝑇𝜔𝐿 and phase 𝛩𝜔𝐿 [55]: 𝛥𝑇𝜔𝐿 = √𝐼𝑥 2(𝜔𝐿 ) + 𝐼𝑦 2(𝜔𝐿 ) , (3) 𝛩𝜔𝐿 = arctan ( 𝐼𝑦 (𝜔𝐿 ) 𝐼𝑥 (𝜔𝐿 ) ) , (4) where 𝐼𝑥 (𝜔𝐿 ) and 𝐼𝑦 (𝜔𝐿 ) are the phasorial components of the thermoelastic signal evaluated at 𝜔𝐿 , respectively. once applied the lock-in data processing, the spatial information of the temperature (i.e., stress) distribution is obtained, and further structural analysis can be performed. in particular, the stressconcentration factor 𝐾𝑓 can be estimated in areas where critical behaviour is shown. 𝐾𝑓 is defined, on a linear profile, as the ratio of the highest stress max(𝛥𝜎) and a reference stress, here chosen as the mean stress 𝛥𝜎 on the same profile [20]: 𝐾𝑓 = max(δ𝜎) δ𝜎 . (5) by substituting (1) in (5), the 𝐾𝑓 factor on a linear profile becomes: 𝐾𝑓 = max(δ𝑇𝜔𝐿 ) δ𝑇𝜔𝐿 . (6) 2.3. aruco-based resonant frequencies identification as discussed earlier, the aruco marker library was used for measurements, due to theoretical considerations [38]. an example of aruco 6x6 marker is presented in figure 2, which shows its geometrical parameters (i.e., corners and reference system). to identify a marker in each captured frame, multiple steps are required [38], [43]. firstly, a local-adaptive threshold contour segmentation is performed [56]. then, a contour extraction and a polygonal approximation are applied to keep the enclosing rectangular borders and remove the irrelevant information [57]. potential perspective projections are compensated using a homography transformation. the resulting image is binarized and divided into a regular grid, where at each element is assigned 0 or 1, depending on the preponderant value of the corresponding pixel, as shown in figure 3 [58]. aruco markers are usually created in groups, to ensure their geometric diversity and avoid misleading detection. due to this, another filter is generally applied to the image to determine potential matches between the recognized marker and the used marker-dictionary [43] even if manual corrections are normally made available by modifying threshold algorithm parameters. finally, through the identification of the four corners, the spatial coordinates of each recognised marker are estimated with respect to the camera [38], [56]. in this study, the implementation of the aruco library is exploited to measure the spatial temporal coordinates of the centre of the marker 𝐶(𝑥(𝑡),  𝑦(𝑡)) recorded in a video. firstly, the acquired data are pre-processed to improve the marker detection, which, moreover, can be compromised by different factors (e.g., too much distance between the camera and the marker or blurred images measurement) [43]. then, each frame is subjected to a sharpening and dilatation filters. the sharpening filter was used to reduce apparent blurring in each frame by means of 2d spatial convolution [59]: (𝐼 ∗ 𝑘)(𝑥, 𝑦) = ∑ ∑ 𝑘(𝑖, 𝑗) ⋅ 𝐼(𝑥 − 𝑖, 𝑦 − 𝑗) ∞ 𝑗=−∞ ∞ 𝑖=−∞ (7) where 𝐼(𝑥, 𝑦) is the original frame, 𝑘(𝑥, 𝑦) is the kernel and (𝑥, 𝑦) are the pixel coordinates and (𝑖, 𝑗) are the coordinates of the elements in the kernel matrix. then, dilatation filter was also applied as a morphological operation, involved for removing noise, for isolating individual elements and for merging disparate elements in an image [59]. also, this filter is based on convolution operation [60]. being 𝑏(𝑥, 𝑦) the structuring function, by the grey-scale dilation of 𝐼 by 𝑏 is obtained [59]: (𝐼 ⊕ 𝑏)(𝑥, 𝑦) = (𝑖, 𝑗) ∈ 𝑏𝑚𝑎𝑥 [𝐼(𝑥 + 𝑖, 𝑦 + 𝑗) + 𝑏(𝑖, 𝑗)] , (8) and the grey-scale erosion of i by b is given by: (𝐼 ⊖ 𝑏)(𝑥, 𝑦) = (𝑖, 𝑗) ∈ 𝑏𝑚𝑖𝑛 [𝐼(𝑥 + 𝑖, 𝑦 + 𝑗) − 𝑏(𝑖, 𝑗)] . (9) the marker detection is based on its 4 corners identification in each captured frame (see figure 1). from the corners, the spatial coordinates of the centre of the marker (𝑥𝑐 , 𝑦𝑐 ) are evaluated frame-by-frame during the acquisition: 𝐶 = (𝑥𝑐 , 𝑦𝑐 ) = 𝐺 ⋅ ( 1 4 ∑|𝑥𝑟 | 4 𝑟=1 , 1 4 ∑|𝑦𝑟 | 4 𝑟=1 ) , (10) where (𝑥𝑟 , 𝑦𝑟 ) are the coordinates of the 𝑟-th vertex and 𝐺 is the calibration factor from pixel units to si units, defined as the ratio figure 2. aruco 6 × 6-marker: corners and centre coordinates. figure 3. aruco 6 × 6-marker: pixel values (example). acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 180 between the side length of the physical marker in si units and the average of the four side lengths (in pixels) of the captured marker in the fov. once the coordinates of the centre of the marker (𝑥𝑐 , 𝑦𝑐 ) in the time-domain are obtained, frequency domainbased analysis is performed using the discrete fourier transform (dft) [61]. the dft for the n-points time series p is defined as: 𝑃(𝜔) = ∑ 𝑝𝑛 −𝑖 𝑛𝜔 𝑁 𝑁−1 𝑛=0 . (11) considering the spatial properties of image-based analysis, the dft for each component of the displacement 𝐶(𝑥𝑐 (𝑡), 𝑦𝑐 (𝑡)) and for the input force f(t) is obtained: 𝐶(𝑋(𝜔), 𝑌(𝜔)) = (𝐷𝐹𝑇(𝑥(𝑡)), 𝐷𝐹𝑇(𝑦(𝑡))) , (12) 𝐹(𝜔) = 𝐷𝐹𝑇(𝑓(𝑡)) . (13) the cross-spectra (15) and auto-spectra (16) are computed [61]: (𝑆𝑓𝑥 (𝜔), 𝑆𝑓𝑦 (𝜔)) = ( 1 𝑇 [𝑋(𝜔)∗ ⋅ 𝐹(𝜔)], 1 𝑇 [𝑌(𝜔)∗ ⋅ 𝐹(𝜔)]) , (14) 𝑆𝑓𝑓 (𝜔) = 1 𝑇 [𝐹(𝜔)∗ ⋅ 𝐹(𝜔)] . (15) finally, the compliance frequency response functions (frf) along x-axis and y-axis, using 𝐻1 estimator, can be obtained [61]: (𝐻1𝑥 (𝜔), 𝐻1𝑦 (𝜔)) = ( 𝑆𝑓𝑥 (𝜔) 𝑆𝑓𝑓 (𝜔) , 𝑆𝑓𝑦 (𝜔) 𝑆𝑓𝑓 (𝜔) ) . (16) on the other hand, the accelerance frequency response function obtained through the impact hammer procedure is given using the 𝐻1 estimator by [61]: 𝐻1 = 𝑆𝑓𝑎 (𝜔) 𝑆𝑓𝑓 (𝜔) , (17) where 𝑆𝑓𝑎 and 𝑆𝑓𝑓 are the crossand auto-spectra of the output acceleration and of the input force, respectively. 3. experimental methodology as already addressed in sec. 2, two different measurement approaches were used and, due to this, two experimental setups were built. the global tested structure is presented in figure 4, where the analysed areas and markers are shown in detail: thermoelasticity was applied in t1 and t2 areas, while m1-2 is the id of the marker detected whose results are presented in this research. in fact, as shown in figure 4, 13 markers have been mounted on the structure and multiple measurements, of single markers and of groups of them, were performed. for this reason, for the sake of clarity, only results related to the marker at the tip of the structure (i.e., m1-2) are presented. moreover, the structure was tested in two different boundary configurations, that are schematized in figure 5. in this manuscript, the authors will refer to the configuration with one fixed constraint as c1 configuration and as c2 configuration to the one with two fixed constraints. for the sake of clarity, the experiments are combined as follows: t1 area was analysed in c1 configuration, t2 area in c2 configuration and the m1-2 marker was detected in both c1 and c2 configurations. 3.1. thermoelasticity the thermoelastic stress analysis was performed as explained in sec. 2.2. firstly, the yellow painting was removed, and a proper matt black wrapper paint was used for the surface conditioning, due to theoretical considerations [55]: the emissivity of the surface was increased and homogenized, and appreciable results were thus obtained. then, a harmonic load at 1.1 hz and 2.7 hz were given to the structure in t1 and t2 analysed area, respectively, and the temperature changes of the surface were measured, in each test, for 60 seconds with a mwir cooled thermal camera flir a6751sc, operating at 125 hz of sampling rate and 640 × 512 pixels of resolution. 3.2. aruco-based resonant frequencies identification in order to define the natural frequencies of the tested structure, classic impact hammer procedure was also performed to validate further the results obtained through image-based analysis. due to this, a pcb 086d20 hammer was used for the input broadband excitation, a uniaxial pcb 352c34 amplified accelerometer and a picoscope data acquisition system. the accelerometer was positioned on the tip of the structure, along the y-axis of the system. input and output data were acquired at 1 khz for 50 seconds of duration. simultaneously with the impact hammer tests, a canon eos 7d camera, mounting a 2470 mm optic (f 2.8), was used for measuring the position of the framed markers. spatial resolution of 1920 x 1080 pixels and sampling frequency of 30 hz were used as acquisition parameters. in this experiment, a 6x6 bit aruco marker dictionary was used. 4. results 4.1. stress concentration factor the stress concentration factor was evaluated in two critical areas, t1 and t2, whose thermal acquisitions and finite element models are shown in figure 6 and figure 7, respectively. by means of (3), the lock-in analysis was performed and the magnitude of the thermoelastic signal is shown in figure 8 and figure 9, where it is compared to the data obtained from the figure 4. tested cfrp structure: t1) first tsa analysed area; t2) second tsa analysed area; m1-2) aruco marker detected. figure 5. boundary configurations of the structure: c1) single fixed constraint; c2) double fixed constraint.. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 181 numerical simulation in the same corresponding linear region of interest. the stress concentration factors 𝐾𝑓 , as described by (6), were evaluated from the profiles in figure 7 and figure 8, and they are shown in table 1, obtained from the experiments and the fem model. the obtained results are in line with the expectations. in fact, usually, the thermoelasticity slightly underestimate the stress, due to theoretical considerations, but, principally, actual stresses measured on the structure are presumed to be lower than the numerical values due to designing consideration. 4.2. aruco-based resonant frequencies identification as already addressed in sec. 3.3, firstly, the marker position in the time-domain was collected using modal testing procedure (see figure 10) through (10), and then the frf was obtained using (16). the frequency response functions were reconstructed using the least-squares complex exponential (lsce) algorithm [62]. the frfs obtained by means of the two experimental methods are shown in figure 11 and figure 12 for c1 and c2 boundary configurations, respectively. although in the amplitudes are different (the marker gave compliance frf while the impact hammer gave accelerance frf), the natural frequencies identified along the x-axis are totally comparable. furthermore, the comparison with the numerical simulation was performed and it is shown in figure 13 and figure 14, in terms of normalized frequency for c1 and c2 boundary configurations, respectively. figure 6. t1 area stress profile: (a) experimental; (b) numerical. figure 7. t2 area stress profile: (a) experimental; (b) numerical. figure 8. t1 area stress and temperature profiles. figure 9. t2 area stress and temperature profiles. table 1. stress concentration factors results. t1 area t2 area experiments 1.19 1.21 numerical 2.40 1.40 acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 182 the results show a well-founded numerical model. in fact, the comparison of the experimental resonant frequencies, obtained using impact hammer and aruco markers, with the numerical results establish high reliability in both the two boundary condition configurations, for all the modes in the considered frequency range. 5. conclusions the validation of numerical models through experimental procedures is a mandatory step for several applications. in this research, a non-contact multi-instrumental approach was proposed to validate the numerical model of the robot inspection of the new san giorgio's bridge on the polcevera river. in particular, the thermoelastic technique was used to measure stress-concentration factors in two areas and, moreover, an innovative methodology, involving the detection of the aruco fiducial markers, was implemented to define the resonant frequencies of the cfrp structure, by estimating its frequency response function. the impact-hammer procedure was also performed for the validation of the results. the proposed approach gave excellent results and, due to this, it can be used for testing large structures. further investigations on the material properties and on the dynamics of the inspection robot are planned as extension of this research. acknowledgement the authors acknowledge camozzi group and sda engineering for allowing and supporting this research through the collaboration with the italian institute of technology. moreover, ubisive, fincantieri and pergenova participated in this research with the university of ancona (univpm). figure 10. modal testing procedure: input impulse and damped output signals from accelerometer and aruco marker. figure 11. frequency response function using aruco markers and impact hammer: c1 boundary configuration. figure 12. frequency response function using aruco markers and impact hammer: c2 boundary configuration. figure 13. frequency response function using aruco markers and impact hammer: c2 boundary configuration. figure 14. natural frequencies comparison: c2 boundary configuration. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 183 references [1] x. d. li, n. e. wiberg, structural dynamic analysis by a time‐ discontinuous galerkin finite element method, int. j. numer. methods eng., vol. 39, no. 12, 1996, pp. 2131–2152. doi: 10.1002/(sici)1097-0207(19960630)39:12%3c2131::aidnme947%3e3.0.co;2-z [2] f. cianetti, g. morettini, m. palmieri, g. zucca, virtual qualification of aircraft parts: test simulation or acceptable evidence?, procedia struct. integr., vol. 24, no. 2019, 2019, pp. 526–540. doi: 10.1016/j.prostr.2020.02.047 [3] r. c. juvinall, k. m. marshek, fundamentals of machine component design, vol. 83. john wiley & sons new york, 2006. 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national physical laboratory, united kingdom received july 11, 2022; in final form october 5, 2022; published december 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was supported by the new zealand government. corresponding author: rebecca hawke, e-mail: rebecca.hawke@measurement.govt.nz 1. introduction following the revision of the si, kibble balances around the world may now be used to realise the unit of mass.[1] in a kibble balance, the weight of a mass is balanced by the electromagnetic force on a current-carrying coil of wire suspended in a magnetic field. at msl we are developing a kibble balance where the coil is connected to the piston of pressure balance 1 (see figure 1) in a twin pressure balance arrangement [2], [3]. the piston-cylinder unit of pressure balance 1 provides a repeatable axis for the motion of the coil in the magnetic field. the twin pressure balance arrangement serves as a high-sensitivity force comparator, [4]. in a kibble balance, the position of the coil must be precisely controlled in both weighing and calibration modes. in weighing mode the coil should remain stationary at a set position while the current is adjusted to maintain a force balance. in the msl kibble balance, stability in position to within 1 µm would correspond to an uncertainty in realised mass of 3.5 parts in 109, [5]. in calibration mode, the coil must be moved such that a measurable voltage is induced, which typically requires velocities between 1.3 mm s-1 and 3 mm s-1, [6]. in the msl kibble figure 1. schematic of the msl kibble balance design based on a twin pressure balance. the coil connects to the piston of pressure balance 1 such that the coil and magnet are coaxial with the piston-cylinder unit. pressure balance 2 provides a reference pressure for a differential pressure sensor to determine changes in balance forces. differential pressure sensor mass annular gap laser beam for coil position measurement coil permanent magnet pressure balance 1 pressure balance 2 pistoncylinder abstract the position of the coil in a kibble balance must be finely controlled. in weighing mode, the coil remains stationary in a location of constant magnetic field. in calibration mode, the coil is moved in the magnetic field to induce a voltage. the msl kibble balance design is based on a twin pressure balance where the coil is attached to the piston of one of the pressure balances. here we investigate how the piston (and therefore coil) position may be controlled through careful manipulation of the gas column under the piston. we demonstrate the use of a syringe pump as a programmable volume regulator which can provide fall rate compensation as well as controlled motion of the piston. we show that the damped harmonic oscillator response of the pressure balance must be considered when moving the coil. from this initial investigation, we discuss the implications for use in the msl kibble balance. mailto:rebecca.hawke@measurement.govt.nz acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 2 balance, control of the vertical position of the coil equates to control of the piston position in pressure balance 1. however, in a pressure balance the piston naturally falls as gas leaks through the annular gap between piston and cylinder. fall rate compensation is therefore necessary in both weighing and calibration modes in the absence of mechanical controls such as arresting stops or a direct motor drive. to assist in control of the coil position in the msl kibble balance, we propose careful manipulation of the gas column under the piston in pressure balance 1. the pressure balance maintains a constant pressure, so our options are to adjust the gas volume (e.g. with a mass flow controller) or to shift the gas column in space (e.g. with a volume regulator). to test this second approach to coil position control, we investigate the use of a syringe pump as an automated volume regulator. the layout of this paper is as follows. in section 2 we describe the experimental apparatus, and in section 3 we propose a theoretical model for the system. results for fall rate compensation, conventional constant velocity travel, and an oscillatory motion are presented in section 4. finally, in section 5 we discuss the potential for this technique to be used in the msl kibble balance. 2. experimental apparatus the experimental apparatus is illustrated in figure 2 and was similar to that used in [8]. the pressure balance was a pneumatic dhi/fluke 50 kpa/kg piston-cylinder module with an effective area of 196 mm2. the medium was zero grade nitrogen gas and we adjusted the load to give a working pressure near 100 kpa absolute. we used a manual volume controller to set the initial height of the piston. the pressure balance was operated in a vacuum chamber evacuated to a pressure of around 0.1 pa. ambient temperature outside the chamber was in the range 21 – 23 °c during measurements. to control the vertical position of the piston, we used a custom ‘direct drive’ model of the cetoni nemesys s syringe pump. this pump has inbuilt position encoding. cetoni highprecision glass syringes of 1 ml, 5 ml and/or 25 ml capacity were connected to the pressure balance via 1 m of 1/8” flexible ptfe tubing (1.6 mm id) and a minimal length of ¼” swagelok stainless steel tubing and fittings. we estimate the smallest volume of the gas column was ~30 ml including the cavity within the piston. to enable rapid, independent tracking of the syringe volume, we monitored the position of the syringe plunger using an ids ueye usb camera with a microscope lens attached. we converted the image pixels to volume in ml for each syringe using the encoder values at eight positions spanning the range of travel. to determine the phase shift or delay between syringe plunger motion and piston motion we used hardware triggering of the image capture and piston position measurements. however, triggering increased the measurement noise, so data shown here were captured using the camera’s free-run mode. we measured the piston position using both a capacitance sensor and a linear variable differential transducer (lvdt) with hardware triggering at either 20 or 50 ms intervals. we determined the calibration curve for the capacitor using a dial gauge and gauge blocks and transferred the calibrated position to the lvdt readings. the results presented use the lvdt readings. 3. model a gas pressure balance comprises a loaded piston floating on a volume of gas, and behaves as a damped harmonic oscillator, [8], [9]. a pressure balance connected to a syringe has a mechanical analogue in the accelerometer (or seismometer). figure 3 illustrates this model, where the loaded piston ‘mass’ is attached to a syringe plunger ‘platform’ by a gas pressure ‘spring’, with ‘dashpot’ damping. moving the platform moves the mass in a linear motion with coupled oscillations described by the equation of motion: −�̈� ′ = �̈�𝑟 + 𝑐 𝑚 �̇�𝑟 + 𝜔0 2𝑧𝑟 , (1) where 𝑧𝑟 is the relative distance between mass 𝑚 and platform, 𝑧′ is the displacement of the platform, 𝑐 is the damping coefficient, and 𝜔0 is the resonant frequency. dots indicate derivatives with respect to time. in a pressure balance, the resonant frequency depends on the volume of the gas column under the piston. in the experimental configuration here, the resonant frequency can be varied from ~0.5 hz to 1.2 hz. both the resonant frequency and the ratio 𝑐 𝑚 may be obtained from the damped natural oscillations of the system. figure 2. schematic of the experimental apparatus for piston position control using a syringe in a syringe pump as a volume regulator. floating elements are coloured orange. not shown are the drive mechanism for starting piston rotation (see [7]), the support for the lvdt, and the vacuum chamber surrounding the pressure balance. figure 3. accelerometer model of a pressure balance when connected to a syringe in a syringe pump. the loaded piston ‘mass’ is attached to a syringe plunger ‘platform’ by a gas pressure ‘spring’, with ‘dashpot’ damping. usb microscope camera m a n u a l vo lu m e c o n tr o ll e r 500 g 1 kg piston cylinder lvdt sy ri n g e i n s y ri n g e p u m p sy ri n g e p lu n g e r bell to capacitance sensor ~100 kpa gauge acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 3 this system can be driven with a sinusoidal motion of the platform. for a driving displacement 𝑧′(𝑡) = 𝐴0 cos(𝜔𝑡) with amplitude 𝐴0 and frequency 𝜔, the equation of motion has a solution of the form: 𝑧𝑟 (𝑡) = 𝐴𝑒𝑙 cos(𝜔𝑡) + 𝐴𝑖𝑛𝑒𝑙 sin(𝜔𝑡). (2) solving for the coefficients gives the elastic coefficient, 𝐴𝑒𝑙 = 𝐴0𝜔 2(𝜔0 2 − 𝜔2) (𝜔0 2 − 𝜔2)2 + ( 𝑐 𝑚 𝜔) 2 , (3) and the inelastic coefficient, 𝐴𝑖𝑛𝑒𝑙 = 𝐴0 𝑐 𝑚 𝜔3 (𝜔0 2 − 𝜔2)2 + ( 𝑐 𝑚 𝜔) 2 . (4) the behaviour of the driven system can be understood by examining these coefficients at their limits. as the driving frequency 𝜔 tends to 0, the amplitude of 𝑧𝑟 tends to 0, and the motion of the mass, 𝑧, follows the motion of the platform: 𝑧(𝑡) = 𝑧′(𝑡) + 𝑧𝑟 (𝑡) = 𝐴0 cos(𝜔𝑡). (5) as the driving frequency tends to infinity, 𝐴𝑒𝑙 tends to −𝐴0 and 𝐴𝑖𝑛𝑒𝑙 tends to 0, such that the mass remains stationary despite the motion of the platform: 𝑧(𝑡) = 𝑧′(𝑡) + 𝑧𝑟 (𝑡) = 𝐴0 cos(𝜔𝑡) − 𝐴0 cos(𝜔𝑡) = 0. (6) in between these limits, for 𝑐 𝑚 < 𝜔0 the amplitude of 𝑧𝑟 reaches a maximum as the driving frequency approaches the resonant frequency. at resonance, 𝐴𝑒𝑙 tends to 0 and the motion of the mass is approximately 90° behind the driving oscillation 𝑧′: 𝑧(𝑡) = 𝑧′(𝑡) + 𝑧𝑟 (𝑡) = 𝐴0 cos(𝜔0𝑡) +𝐴0𝑄 sin(𝜔0𝑡) (7) where 𝑄 = 𝑚 𝑐 𝜔0 is the quality factor of the damped natural oscillations of the system. 4. results 4.1 fall rate compensation when used in a kibble balance in weighing mode, over the course of several ‘mass-on, mass-off measurements’ the natural fall rate of the piston of around 1 µm s-1 would result in a significant change in piston position. however, for the lowest uncertainty, the weighing position should be kept constant between loadings. here we consider the usefulness of a syringe pump to maintain a steady piston position by providing a very low flow to compensate for the natural fall of the piston. in figure 4 we show a typical fall rate compensation test using a constant flow rate of 0.012 ccm (ccm = cm3 min-1). initially, while the syringe volume is kept constant, the piston falls at an average rate of 1.004 µm s-1 over 5 minutes. the syringe plunger is then moved slowly, shifting the column of gas towards the piston and causing a slight rise of 0.015 µm s-1 over the ten minutes of applied flow. when the syringe plunger motion is stopped, the piston returns to falling, this time at 0.986 µm s-1 over 5 minutes. this fall compensation resulted in a rise in the piston position of 8.9 µm over ten minutes. over several flow rate compensation tests we observed some variation in the fall rate of the piston. within a test, the fall rate varied before and after fall compensation by up to 0.06 µm s-1, and between tests the variation was at most 0.28 µm s-1. during fall compensation with the same nominal flow rate of 0.012 ccm at the syringe plunger, the overall change in piston position varied between falling by 11 µm over ten minutes when the initial fall rate was 1.13 µm s-1, and rising by 80 µm over the same time when the initial fall rate was 0.86 µm s-1. in the latter case, a subsequent test with a constant flow rate of 0.011 ccm resulted in a rise in the piston position of 2 µm after ten minutes. the start and stop of the syringe plunger motion is a disturbance to the piston. however, the size of this disturbance is very small, and no resonant behaviour is observable. also, we would expect very little disturbance from the syringe plunger when in motion as these flow rates are in the pulsation-free regime of the syringe pump when using a 1 ml syringe. instead, we observe some random variation in piston position of up to 2 µm, which may be due to temperature fluctuations in the gas or external disturbances as they are also seen when the piston is falling without flow compensation. 4.2 calibration mode – constant velocity in calibration mode in most existing kibble balances, the coil is typically moved through the weighing position at an approximately constant velocity of between 1.3 mm s-1 and 3 mm s-1, [6]. with the experimental configuration in figure 2, these velocities are achievable with flow rates between 15 ccm and 35 ccm. however, unlike in the case of leak compensation, the abrupt start and stop of the syringe plunger during this rapid motion is a significant disturbance which causes a visible damped oscillator response. figure 5 shows the shape of the travel for an intermediate flow rate of 20 ccm. immediately after the travel region, we see figure 4. fall rate compensation using a flow rate of 0.012 ccm for ten minutes (grey shaded region). (a) the syringe plunger motion is linear in the shaded region. (b) the piston position is maintained with the 0.012 ccm flow, and falls at its natural fall rate outside of the shaded region (c) residuals are from a linear fit in each region. acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 4 a damped harmonic oscillation; a similar oscillation is also superimposed onto the travel region. the weighing position will be at approximately the half-way point of the range of travel available. although the piston moves through the weighing position quickly, the damped harmonic oscillation causes fluctuations in travel speed, with some very slow motion and even backward motion for some higher flow rates. the number of oscillations in the travel region depends on the time taken to complete the travel and the resonant frequency of the system. here the system volume is as small as practicable, and the resonant frequency is around 1.2 hz. some improvement can be gained by reducing the resonant frequency of the system and timing the duration of the travel to match one period of the damped oscillation. however, the resulting instantaneous velocity is smoothly varying rather than constant during the travel. ideally, in constant velocity calibration mode the motion would have as close to zero acceleration as possible in the travel region. we now present two controlled start techniques to suppress the harmonic oscillations. in the first technique, the flow is ramped up to the target flow and then ramped down when time to stop the motion. while the ‘ramp down’ step is not strictly necessary for our goal of constant velocity travel, the controlled deceleration reduces the damped oscillations after the travel. minimising these oscillations maximises the range of available travel and minimises the settling time required before commencing the next traverse in the opposite direction. figure 6 (a) illustrates the syringe plunger motion for a gentle ramp up to 20 ccm, taking 0.76 s and travelling ~0.13 ml per ramp. for a total volume change of 0.8 ml, the time spent at 20 ccm is 1.64 s in which the syringe plunger travels 0.547 ml. note that in this plot the syringe plunger is moving to refill the syringe. figure 6 (b) shows the resulting motion of the piston, which is significantly more linear than without the accelerating and decelerating ramps, for the same maximum flow rate. a three-section piecewise linear fit to the piston motion gives an average travel speed of 1.72 mm s-1 in the middle section. residuals to this fit are shown in figure 6 (c). some disturbance is evident during each of the ramps, but the motion during the main travel section is linear to within ~33 µm. we note that the size of the observed variations will be influenced by integration of the signal within the sampling interval of 20 ms. the second controlled start technique is known as the crane operator’s trick, which exploits the natural oscillation period of the system, [10]. implementing this trick here involves three steps of applied flow: 1. an initial step at half the target flow, for half a period of oscillation, to accelerate the piston, 2. a steady step at the target flow, to generate constant velocity travel, and 3. a final step of half the target flow, for half a period of oscillation, to decelerate the piston to stationary. figure 5. shape of the piston travel for a 0.7 ml volume change at 20 ccm which gives an effective velocity of 1.7 mm s-1 in the travel region (shaded grey). figure 6. constant velocity travel using gentle ramps over 0.76 s to a maximum flow of 20 ccm, for a 0.8 ml total volume change. (a) shape of the syringe plunger motion (encoder values); the plunger is moving in the grey shaded region. (b) resulting position of the piston (orange markers) and 3piece linear fitting (grey line). (c) residual to the piecewise linear fitting in (b). figure 7. constant velocity travel using the crane operator’s trick with steps at 10 and 20 ccm for a 0.8 ml total volume change, with a period of 0.76 s. (a) shape of the syringe plunger motion (encoder values); the plunger is moving in the grey shaded region. (b) resulting position of the piston (orange markers) and 3-piece linear fitting (grey line). (c) residual to fit in (b). acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 5 figure 7 (a) shows these three distinct steps in the syringe plunger motion for a target flow of 20 ccm and total volume change of 0.8 ml. each section at 10 ccm takes 0.38 s and travels only 0.063 ml, leaving 0.673 ml to travel at 20 ccm, over 2.02 s. the resulting motion of the piston is shown in figure 7 (b), and residuals to a piecewise linear fit to the piston motion are shown in figure 7 (c). the motion during the main travel section is linear to within ~29 µm with an average travel speed of 1.73 mm s-1. the disturbance due to starting and stopping the syringe plunger is slightly less than for the ramp technique, and both techniques give very small damped oscillations after stopping the syringe plunger motion. to provide sufficient data for the determination of the ratio of the induced voltage to the coil velocity at the weighing position, the coil is usually moved at a constant velocity over a distance of at least 20 mm, [6]. however, in the pressure balance that will be used for the msl kibble balance, the range of travel is restricted to at most 13 mm. this shorter range will therefore increase the number of repeats required to achieve the desired accuracy. 4.3 calibration mode – oscillating velocity as an alternative to the constant velocity method, an oscillatory motion has been suggested for the kibble balance calibration mode, [6]. sinusoidal oscillations with frequencies from 0.1 hz to 5 hz could be suitable, even with amplitudes as small as 1 mm. oscillatory mode has been successfully implemented in the ulusal metroloji enstitüsü (ume) kibble balance by moving the magnet sinusoidally at a frequency of 0.5 hz with a peak velocity of around 3 mm s-1, [11]. oscillatory mode has also been demonstrated in the ‘pb1’ and ‘pb2’ planckbalances where the optimal oscillation is typically 4 hz with amplitudes of 4.5 and 20 µm respectively, [12], [13]. in the msl kibble balance, a sinusoidal oscillation would work with (rather than against) the harmonic oscillator character of the pressure balance. a suitable oscillation would have a frequency of around 1 hz and an amplitude of ~1 mm, [6]. here we demonstrate driving such an oscillation via a sinusoidal displacement of the syringe plunger. the driving oscillation in syringe volume of 0.02 ml amplitude is shown in figure 8 (a) along with the resulting piston oscillation in figure 8 (b). for each dataset, we fit a single sinusoid to establish the frequency and amplitude. the frequency of the best fitting sinusoid was 0.9993 hz for the syringe plunger motion and 0.9989 hz for the piston motion. the resonant frequency of the system was adjusted to be as close to 1 hz as practicable and was determined to be 0.9999 hz from the damped harmonic oscillation after stopping the driving excitation. the amplitude of the piston oscillation was about 1.1 mm, with a peak velocity of around 7.6 mm s-1. assuming the induced voltage 𝑈 = 1 v for 𝑣 = 2 mm s-1, we would expect this peak velocity to correspond to an induced voltage of almost 4 v when implemented in the msl kibble balance. from our model we would expect the steady-state piston motion to lag behind the syringe plunger motion with a phase difference of 90°. this phase difference is evident here, where a reduction in syringe volume causes an upward motion of the piston. residuals to the sinusoidal fits are shown in figure 8 (c), scaled by the respective amplitudes. we observe that there is periodic high frequency noise in the syringe plunger motion which is mostly filtered out by the pressure balance. however, the relative magnitude of the residual is transferred through to the piston motion and some non-sinusoidal periodic structure is evident. our accelerometer model for this scenario predicts an amplification of the piston oscillation when approaching the resonant frequency. in figure 9 we present the model amplification due to a sinusoidal driving displacement, along with the measured amplitudes from a range of driving frequencies. we used the amplitude of the lowest frequency oscillation as the normalising amplitude value a0. these data were collected using the smallest practical volume, giving a resonant frequency 𝑓0 of the system of around 1.175 hz, determined by the damped harmonic oscillation after stopping figure 8. oscillatory motion using a sinusoidal driving displacement at a frequency of 1 hz. (a) position of the syringe plunger. (b) resulting position of the piston. (c) residual to the best fitting sinusoids for (a) and (b), scaled by the respective amplitudes. figure 9. (orange markers) amplitude of the piston motion resulting from a sinusoidal driving excitation of 0.025 ml amplitude at various frequencies, shown relative to the resonant frequency, and normalized to the amplitude at the lowest frequency. (grey line) amplitude predicted by our model for a sinusoidal driving displacement, using 𝑄 = 𝑚 𝑐 𝜔0= 14 and 𝑓0 = 1.175 hz. acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 6 the driving excitation. this damped harmonic oscillation had a quality factor of 𝑄 = 𝑚 𝑐 𝜔0~13, and the best fit model was obtained with a 𝑄 of 14. 5. discussion 5.1 fall rate compensation in the msl kibble balance, fall rate compensation can supplement the use of a current feedback arrangement to control the coil position in weighing mode. fall rate compensation will also be necessary for oscillatory mode if we use volume manipulation to generate the oscillation. from the results presented here, the use of a syringe pump to provide fall rate compensation is promising. the achieved 5 µm stability over 5 minutes would be adequate for our initial target accuracy of 1 part in 107 for realised mass. to improve the obtained stability and the repeatability of the method, which are both limited by fluctuation in the piston fall rate, the fall rate should be measured immediately before each period of fall compensation to allow the ideal compensation flow rate to be determined. alternatively, the flow could also be finely adjusted during an initial stabilisation routine. we have examined possible causes, other than measurement error, of variations in the observed fall rate. variation in the temperature of the piston-cylinder unit will affect the natural fall rate of the piston, through thermal expansion affecting the gap between piston and cylinder, and through the temperature dependence of the viscosity of the gas in the gap. for an estimated temperature change of 0.5 k of the piston-cylinder between measurements, these effects in terms of contribution to fall rate are calculated as being less than 2 nm s-1. the fall rate is also expected to vary with vertical position of the piston due to departures from cylindricity of their shape. however, such a correlation is not evident in our data. a steady drift in temperature of the tubing connecting the pressure balance will affect the observed fall rate. for a 0.1 k change in temperature over 5 minutes, this effect is calculated to be ~7 nm s-1 for a tubing volume of 35 ml and is mainly due to the section of ptfe tubing. this effect is an order of magnitude smaller than the variation in fall rate observed within a test. similarly, the variation between tests of ~280 nm s-1 is also unlikely to be due to the above causes. instead, the variations may indicate that the tubing volume has a small leak which is influenced by changing ambient conditions. sources of the ~2 µm random variation in position should also be addressed. it is possible that this noise is due to ground vibration, or wobble from the spinning of the pressure balance bell. a rotating cylinder pressure balance is currently being developed which would provide a direct comparison and ideally lower noise. this fall rate compensation technique is not only important for pressure balance 1 which carries the coil, but it can also be used for pressure balance 2 which is used to provide the reference pressure. both pressure balances need to be kept at a stable pressure for the duration of weighing mode, which could take around 5 minutes per weighing. if left without fall compensation, pressure balance 2 would require periodic height adjustment. the height could easily be adjusted with the syringe pump before each weighing, and/or fall rate compensation could also be provided for this pressure balance throughout the duration of the weighing. we note that this method could be considered to introduce a controlled leak into the system between the pressure balance and the differential pressure transducer, which is not usually recommended, [14]. in this situation, the pressure in the tubing is very close to ambient pressure and the tubing volume is as small as practicable, which will reduce the effect of the leak. additionally, the ‘leak’ caused by the motion of the syringe pump does not change the pressure or the number of gas molecules in the system; instead, the column of gas is merely moved along the tubing. for these reasons, we expect that the measured force difference would not be affected by a constant infusion during a weighing mode measurement consisting of a sequence of masson and mass-off weighings. 5.2 constant velocity travel as expected, the oscillator response of the pressure balance makes instantaneous travel at a constant velocity difficult to attain by simply applying a constant flow rate. therefore, we have presented here two controlled start techniques which both significantly reduce the oscillator response. with these techniques, accommodating for the oscillator response took potentially as little as ~0.3 mm at each end of the travel, out of the available travel of ~4 mm. of the two techniques, the crane operator’s trick is deceptively simple to implement, and produced constant velocity travel over almost the full range of travel of the piston with <30 µm deviations from linear. similar results were obtained for periods from 0.75 to 0.8 s, indicating that the exact timing of the steps is not critical. this technique warrants further investigation in the msl kibble balance to enable interferometric velocity measurement and to assess the stability of the generated voltage. 5.3 oscillating mode in addition, we have demonstrated that volume manipulation can be used effectively to drive a sinusoidal oscillation of a pressure balance. we see good agreement with the nature of the driven oscillation at the piston and the features predicted by our model, such as a phase difference of 90° and a significant amplification near resonance. we postulate that there is likely to be only one mode of oscillation present due to the length of narrow tubing between syringe pump and piston. the ~14-fold amplification of the driving oscillation near resonance is a major advantage in working with, rather than against, the harmonic oscillator character of the pressure balance. this amplification significantly reduces the amplitude of the driving oscillation that is required to generate an oscillation of ~1 mm amplitude at the piston. importantly, any noise in the driving oscillation is greatly reduced at frequencies above and below the resonant frequency. care must be taken to reduce any noise in the driving oscillation at or near the resonant frequency, as this noise is also amplified. the sinusoidal oscillation produced by our syringe pump is digitally created by updating the generated flow, 100 times per oscillation. this relatively coarse digitization results in a slightly distorted sinusoidal waveform, and the method of updating only the flow allows a slow drift of the average position of the plunger. while some optimisation is possible with the syringe pump system, a much better sinusoidal oscillation would be generated by an analogue or ac-driven input. such an input could be used to drive the oscillation of the syringe plunger or an equivalent pressure-maintaining membrane or diaphragm. alternatively, a large, slow sinusoidal oscillation could be used to provide predictable, repeated travel passing through the weighing position with approximately constant velocity. for example, an oscillation at 0.2 hz with 2 mm amplitude would acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 7 reach a peak velocity of 2.5 mm s-1, with little down-time between repeat measurements. 6. conclusions we have shown that a syringe pump may be used as an automated volume regulator to controllably adjust the height of the piston in a pressure balance. this method can also be used to assist in maintaining a stable piston, and therefore coil, position in both the weighing and calibration modes of the msl kibble balance. we demonstrated constant velocity travel of the piston at 1.7 mm s-1 using two controlled start techniques to minimise unwanted oscillations. an oscillatory motion, working with the resonant behaviour of the pressure balance, also shows promise for the msl kibble balance calibration mode. acknowledgement the authors wish to acknowledge useful discussions with and technical assistance from joseph borbely, yin hsien fung, and peter mcdowall. the authors thank the reviewers for their insightful comments and suggestions for achieving constant velocity travel. this work was funded by the new zealand government. references [1] i. a. robinson, s. schlamminger, the watt or kibble balance: a technique for implementing the new si definition of the unit of mass, metrologia. 53 (2016) a46–a74. doi: 10.1088/0026-1394/53/5/a46 [2] c. m. sutton, m. t. clarkson, y. h. fung, the msl kibble balance weighing mode, in: cpem 2018 conference on precision electromagnetic measurements, institute of electrical and electronics engineers inc., 2018. doi: 10.1109/cpem.2018.8500889 [3] c. m. sutton, the accurate generation of small gauge pressures using twin pressure 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amplitude-dependent oscillations in gases, j. nonlinear math. phys. 8 (2001) 79–81. doi: 10.2991/jnmp.2001.8.s.14 [10] s. schlamminger, l. chao, v. lee, d. b. newell, c. c. speake, the crane operator’s trick and other shenanigans with a pendulum, am. j. phys. 90 (2022) 169–176. doi: 10.1119/10.0006965 [11] h. ahmedov, n. b. aşkin, b. korutlu, r. orhan, preliminary planck constant measurements via ume oscillating magnet kibble balance, metrologia. 55 (2018) 326–333. doi: 10.1088/1681-7575/aab23d [12] c. rothleitner, n. rogge, s. lin, s. vasilyan, d. knopf, f. härtig, t. fröhlich, planck-balance 1 (pb1) – a table-top kibble balance for masses from 1 mg to 1 kg – current status, acta imeko 9 (2020) 5, pp. 47–52. doi: 10.21014/acta_imeko.v9i5.937 [13] s. vasilyan, n. rogge, c. rothleitner, s. lin, i. poroskun, d. knopf, f. härtig, t. fröhlich, the progress in development of the planck-balance 2 (pb2): a tabletop kibble balance for the mass calibration of e2 class weights, technisches messen. 88 (2021) 731–756. doi: 10.1515/teme-2021-0101 [14] r. r. a. samodro, i. m. choi, s. y. woo, s. j. lee, a study on the pressure gradient effect due to a leak in a pressure calibration system, metrologia. 49 (2012) 315–320. doi: 10.1088/0026-1394/49/3/315 https://doi.org/10.1088/0026-1394/53/5/a46 https://doi.org/10.1109/cpem.2018.8500889 https://doi.org/10.1088/0026-1394/23/4/003 https://doi.org/10.1109/cpem.2012.6251006 https://doi.org/10.1088/0026-1394/51/2/s101 https://doi.org/10.1088/0026-1394/46/5/010 https://doi.org/10.1088/0022-3735/13/8/007 https://doi.org/10.1016/j.measurement.2011.10.045 https://doi.org/10.2991/jnmp.2001.8.s.14 https://doi.org/10.1119/10.0006965 https://doi.org/10.1088/1681-7575/aab23d https://doi.org/10.21014/acta_imeko.v9i5.937 https://doi.org/10.1515/teme-2021-0101 https://doi.org/10.1088/0026-1394/49/3/315 digital twin concept of a force measuring device based on the finite element method acta imeko issn: 2221-870x march 2023, volume 12, number 1, 1 5 acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 1 digital twin concept of a force measuring device based on the finite element method oksana baer1, claudiu giusca2, rolf kumme1, andrea prato3, jonas sander1, davood mirian1, frank hauschild1 1 physikalisch-technische bundesanstalt, bundesallee 100, 38116 braunschweig, germany 2 cranfield university, cranfield, united kingdom 3 inrim – national institute of metrological research, turin, italy section: research paper keywords: digital twin, force measurement, finite-element-method, measurement uncertainty, traceability, data transfer citation: oksana baer, claudiu giusca, rolf kumme, andrea prato, jonas sander, davood mirian, frank hauschild , digital twin concept of a force measuring device based on the finite element method, acta imeko, vol. 12, no. 1, article 8, march 2023, identifier: imeko-acta-12 (2023)-01-08 section editor: daniel hutzschenreuter, ptb, germany received november 18, 2022; in final form march 20, 2023; published march 2023 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: the authors would like to acknowledge funding of the presented research within the european metrology programme for innovation and research (empir) as well as the european association of national metrology institutes (euramet) in the joint research project 18sib08 comtraforce. corresponding author: oksana baer, e-mail: oksana.baer@ptb.de 1. introduction in accordance with industry 4.0 requirements for automation and data exchange, empir project comtraforce 18sib08 [1] was taken to develop the prototype of dt of a traceable force transfer standard. the dt is built on the basis of models for static and continuous force transfer standards including measurement uncertainty determination. dt is a management and certification paradigm [2] which allows in real time to predict, optimise and maintain desired functionality of complex systems. whilst the dt literature has immensely proliferated in recent years, covering various topics such as through-life monitoring and manufacturing, the widely accepted concept of dt was proposed by grieves [3] and vickers [4], which identified the following main components: the physical object in real space, the virtual object/model in virtual space and the connections of data and information that ties the virtual and real products together, as shown in figure 1. other authors have reviewed and refined the dt definition [5] establishing a stronger link to industry 4.0 and its enabling technologies: “dt is the virtual and computerised counterpart of a physical system that can be used to simulate it for various purposes, exploiting a real-time synchronization of the sensed data coming from the field”. from a standardization work perspective, iso 23247-1 defines dt as a manufacturing fit-for-purpose digital representation of an observable manufacturing element with synchronization between the element and its digital figure 1. digital twin concept of grieves [3] and vickers [4]. real space object virtual space model abstract in the framework of empir project comtraforce, the digital twin (dt) concept of force measurement device is developed. dt aims to shade static, continuous as well as dynamic calibration processes, preserving data quality and collecting calibration data for improved decision-making. to illustrate the developed dt concept, a prototype realisation for static and continuous force calibration processes is developed, involving simulation with ansys engineering software. the focus of the current work is placed on the data connection between the physical device and the dt. the dt model is validated using traceable measurements. acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 2 representation [6]. a digital format for the secure transmission and unambiguous interpretation of measurement data is defined in [7]. all dt definitions lead towards a digital representation, or models, of the physical object, process or service, also called virtual twins. the virtual twin is able to predict the characteristics of the real twin with specified precision in real time, which is established by the needs of each specific application for accuracy and decision speed [8]. the dt models are divided into three main categories: • physics-based models (finite element method fem, structural health monitoring, computational inverse methods); • data driven model (machine learning, digital signal processing, statistical inverse methods); • surrogate model as a combination of both. a comprehensive overview of the dt model requirements is given, among others, in [8]. the dt model must be able to reflect the physical processes, influencing the output parameters. the accuracy of the predicted values should be high enough to be beneficial for the desired application. furthermore, the speed of computation must be high enough for real time execution of the dt. whilst fem is using current knowledge and off-line measurement results to predict the output of the real twin, in dt context, the virtual twin requires real time sensors information gathered over the entire operational lifetime [8]. the aim of the current work is to develop the prototype of the dt for static and continuous force calibration processes. to achieve this aim, the finite element analysis (fea) is chosen for the force measurement device output prediction. however, in order to achieve the required speed of dt execution, several simplifications on the geometry, material behaviour, etc., are made. the details on the development of the finite element model (fe-model) for static and continuous calibration tests with the focus on applicability for dt are provided. a way to connect the force measurement device information with the developed fe-model in an automatised way is presented. at the end, the approach for the physical-to-virtual and virtual-tophysical data communication is also discussed. the current paper is structured as follows. in section 2 an overview over related literature is given; section 3 presents a concept of dt of force measurement device, addresses the dt definition as well as lists the digital metrological twin requirements. the creation of fe-model of the force measurement device is presented in section 4. the data communication approach between the fe-model and the real device is discussed in section 5. in section 6 the main conclusions of the work are summarised, providing the outlook on the planed dt developments. 2. related work the dt developed in [9] leverages fem to predict the complex micro-mechanical evolutions of materials during multistage processes on the example of sintering. the focus is placed of the interoperability and robustness of the material data between various fe-steps. a tensile test system, equipped with the optical system to capture full-field geometric images, was investigated in [10]. the dt based on fe modelling is used to ensure that the data, collected by the experimental system, can be used quickly and efficiently to prove theoretical models and to determine required material properties. in [11] the fem is used for the characterisation of a 5 mn∙m torque transducer. the fem is implied to extend the calibration range up to 5 mn∙m level. the method for determination of the young’s modulus is presented, and the defined value is used in fe-model. the simulated output signal deviates from the measured one by 17.5 %. another simulation of a 4 kn∙m self-built torque transducer in matlab simulink is reported in [12]. the simulation method was developed with a focus on giving recommendations regarding the element type and size. also, different modelling strategies for the strain gauges are proposed. a deviation between the simulated and measured results down to 1.3 % was achieved. as the most suitable fe element type for the modelling the fully-integrated hexahedral element with quadratic formulation was chosen. 3. digital twin of force measurement device dt in metrology requires specific definitions, clearly stated obligatory requirements for measurement traceability. german national metrology institute ptb developed the following definition of the digital metrological twin (dmt) [13]: a digital metrological twin is a numerical (prediction) model that depicts a specific measurement process and indicates an associated measurement uncertainty for a specific measurement value, which is traceable to the units of the international system of units. moreover, it complies with the requirements that: • the measurement uncertainty is calculated according to recognised standards [14]; • all input parameters are traceably determined and stated with the corresponding measurement uncertainty [14]; • and it is validated by traceable measurements. previous definition provides the requirements by fulfilment of which the generated dt output can be utilised for metrological services. dmt is the enhanced way to generate, process, and store data on a calibration device with the time stamp. beyond the concept of a digital calibration certificate (dcc) [15], where calibration data is collected, dmt will allow to correlate the force transducer output with the physical processes occurring in the transducer and allows its seamless connection with the factory of the future. all relevant data in such database is traceable and represented with si units, if applicable. this data can be used by the end user in the further life-cycle of the calibrated device. furthermore, the use of dmt will allow calibration system to learn from itself and reduce the measurements uncertainty for future calibration processes with the same device as well as deliver data to derive correlations between calibration conditions, mounting, etc. as well as output values for devices of the similar class. the concept of force measurement device dmt covers three main functions, figure 2. the first function allows for a prompt reading of selected device information, e. g. sensors reading figure 2. developed concept of digital metrological twin. acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 3 (temperature and strain). here the speed of communication as well as preservation of data quality play the key role. the physical-to-virtual communication is realised by reading the relevant device information from a dcc for force measurement. the second function of a dt is the data processing by means of one of three models mentioned above [16]. the main outcome of data processing is the prediction of force measurements device output as well as measurement uncertainty. for the dmt it is crucial that the input/output parameters are traceable and are stated with the corresponding measurement uncertainty. additionally, the validation of the static, continuous and dynamic models within dt must be performed using traceable measurements. the third function enables saving of the modelled output which can be used to recalculate uncertainty in future calibration procedures. as first prototype of dmt of a force measurement device, the fe-model of the transducer is developed. as first attempt, the models of static and continuous calibration processes are developed in fe simulation software ansys, see section 3. the synchronization between the force measurement device and its dt is performed after each calibration process in form of reading and subsequent update of the dcc by means of python v3.9. the calculated dt device output as well as measurement uncertainty are saved after each calibration in a database. the corresponding database of dt represents the device history, allowing the dt to recall any state of the device history. visualisation of data is realised with matplotlib library v1.3.0. 4. finite element analysis 4.1. numerical model set up the development of the transducer design is discussed in details in [17]. more details on the creation of digital construction of the transducer are provided in [18]. the transducer is mounted on the 20 kn calibration machine available at ptb. the machine works according to the principle of the direct loading. the weight forces generated by the mass bodies are introduced to the transducer via a load hanger. the load bodies are arranged in such a way that transducers with nominal forces of 2 kn can be calibrated in 12.5 % steps and with nominal forces of 5 kn, 10 kn and 20 kn in 10 % steps according to din en iso 376 the geometrical model was simplified to the transducer itself and strain gauges in order to enable fast run of the simulation, see figure 3a). the material of the transducer is quenched alloyed steel 1.6580 qt. the strain gauge is simulated as a polyimide substrate and a grid made of constantan. the contribution of the glue layer is neglected. the transducer is partially meshed with the tetragonal tet10 elements with the size of 2 mm size. the strain gauges as well as the transducer region, close to strain gauges placement, are meshed with hexagonal hex20 elements to ensure better mesh consistency, see figure 3b. the element size of 0.6 mm was set for hex20 elements. further element size refinement below 0.6 mm led to no significant improvement of the simulated output signal. 4.2. static force model to investigate the influence of static forces on the measured signal of transducer, the static mechanical analysis with ansys was set. the concentrated force fy was applied stepwise in axial direction to reproduce the loading during calibration experiments, until the nominal value of 20 kn was achieved. boundary conditions ux = uy = uz = urx = ury = urz = 0 were applied on the opposite side of the transducer, simulating transducer mounting in the loading train. elastic material properties of 1.6580 steel [19], polyimide [20] and constantan [21] were defined, see table 1. 4.3. continuous force model during continuous calibration tests, output signal variation with time under the application of a constant load is observed. in force measurement industry this phenomenon is defined as creep [22]. one of the physical mechanisms, leading to the output signal variation, is the superposition of the thermoelastic strains in the load cell itself as well as the strain of the strain gauge materials. to simulate this effect the transient coupled field analysis in ansys was performed. a key role in the model is played by the definition of the thermal material properties, such as heat expansion coefficient, specific heat and thermal conductivity, [19], [20] and [21], see table 1. simulated compression load was applied in axial direction for 20 s up to maximal nominal value of 20 kn and subsequently held for 600 s. the full unloading was simulated for 20 s as well and the final strain was measured after 600 s, capturing unloading creep. 4.4. simulation results to validate the developed fe-model, the static as well as creep calibration experiments according to iso 376:2011-09 with the nominal force of 20 kn were performed. a comparison of the simulated output of the strain gauge bridge after static loading profile with the experimental measurements and analytical solution [18] is presented in figure 4. figure 3. simulation model of force transducer and strain gauges: a) geometrical model; b) finite element mesh of measurement region. figure 4. simulated and experimentally measured output signal. table 1. material properties of the transducer materials used in ansys material property 1.6580 qt constantan polyimide young's modulus in gpa 210 162 2.65 poisson's ration 0.33 0.31 0.37 thermal expansion in 1/k 1.11 · 10-5 8.3 · 10-6 5.5 · 10-5 specific heat in j/(kg k) 460 390 1090 thermal conductivity in w/(m k) 42 21.8 0.33 force fy fixation grid substrate 7.7 mm 6 .3 m m a) b) -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0 2 4 6 8 10 12 14 16 18 20 o u tp u t si g n a l [m v /v ] force [kn] calibration result analytical solution fem simulation acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 4 the discrepancy between the simulated result and the analytical solution lies around 9.8 %. for the reliable dt functionality, the precision must be improved, for example through the consideration of the glue layer contribution. similar to the static calibration test model, the fea output signal lies close to the analytical value, which is twice higher than the experimental one. to enable better data comparison, the measured as well simulated output signals were normalised using the formula (1): 𝑋′ = 𝑋 − 𝑋min 𝑋max − 𝑋min , (1) where 𝑋′ is the normalised value, 𝑋 is the measured value to be normalised, 𝑋min and 𝑋max minimum and maximum values of the dataset respectively. the simulated output signal of the load cell is presented in figure 5a during loading and in figure 5b during unloading. the output signal was calculated using simulated total strain values, which were derived as the sum of elastic and thermal strains. the simulated and measured data show opposite trends. the measured data represents the strain gauge output signal and the simulated one of the load cell output. this is in accordance with findings of kühne, who stated that creep recovery of the load cell acts in opposite direction to the creep of the strain gauge and the corresponding glue layer [22]. measured creep results showed output signal deviations due to creep of 0.214 % and 0.211 % after loading and unloading correspondingly. the simulated results reported 0.45 % and 0.47 % output signal deviations for loading and unloading. thus, both static as well as continuous simulations show almost double signal value in comparison to the measured values. 5. data communication 5.1. data input the physical-to-virtual communication is realised by reading the relevant device information from a dcc in xml format. the benefit of direct use of data from dcc is that the resulting data is provided in si format. the data transfer from dcc ensures that the input parameters of dt are traceable and stated with the given measurement uncertainty, as required by the definition of dmt. the dcc for static force calibration process was developed at ptb in accordance with the xml scheme (xsd) 3.1.2 [23]. to read input parameters from dcc, the elementtree xml api v1.3.0 was used [24]. it allows to read the specified elements of dcc xml tree, see figure 6. an example of code allowing to navigate in the required dcc element is presented in figure 7. the code is strongly related to a specific dcc structure and must be adjusted in case of any modifications of dcc xml tree. the collected data are transferred as loading and boundary conditions of fe-simulation in form of python command. the entire fe-model was built in ansys mechanical 2021 r1 using mechanical python apis to automate the process [25]. the application diagram, showing input output data flow of the developed dmt software is presented in figure 8. 5.2. data output and storage the calculated dt force transducer data are used to calculate measurement uncertainty considering effects from the mechanical system and surrounding the force measurement device. in future developments, the collection of the selected information in a database will be realised. each dataset will be figure 5. simulation model of force transducer and strain gauges during a) loading and b) unloading. figure 6. xml tree structure of force dcc and highlighted list element containing values of applied force magnitude in kn. figure 7. code example demonstrating navigation to specific dcc element. figure 8. application diagram illustrating developed dmt software. 0.998 0.999 1.000 1.001 1.002 1.003 0 100 200 300 o u tp u t s ig n a l, [ m v /v ] time, [s] simulated data measured data 0.0000 0.0005 0.0010 0.0015 0.0020 600 700 800 900 o u tp u t s ig n a l, [ m v /v ] time, [s] simulated data measured data a) b) dcc:administrativedata dcc:measurementresults dcc:measurementresult dcc:name dcc:description dcc:influenceconditions dcc:results … dcc:result reftype="table4" dcc:name dcc:data dcc:list dcc:quantity dcc:name si:reallistxmllist si:valuexmllist si:unitxmllist 0 2 4 6 8 10 12 14 16 18 20 0 \kilo\newton # importing element tree under the alias of et import xml.etree.elementtree as et # passing the path of the xml document to enable the parsing process tree = et.parse(dt_directory/’example_dcc_iso 376_22999.xml’) # getting the parent tag of the xml document root = tree.getroot() # print force magnitude values in kn from dcc print(root[1][0][3][3][1][0][0][1][0].text) dcc xml format python script reading input data from dcc mechanical python apis (ansys software): • create analysis • input geometry • mesh model • insert material properties • apply boundary conditions • apply loads • define output variables • start analysis postprocessing: • read output data (text file) • calculate output signal from displacement data • visualisation using matplotlib • calculate measurement uncertainty (under development) text file containing force magnitude values store data in database (under development) acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 5 completed with the metadata stating the calibration time. in this way the continuous calibration history of the force measurement device will be created. 6. conclusions and outlook a digital replica of the force measurement device based on fe-model is created in this work. the first results of force transducer output for static and continuous calibration processes are obtained. the static results are in good agreement with the analytical solution. the simulated output signal values were on average twice large as measured values, which points out on the necessity of checking the bridge wiring potentially affecting results interpretation. in future works it is planned to extend dt model with the dynamic calibration process models. the developed concept of the dt will be applied to estimate the effect of not proper mounting, material properties as well as parasitic force components on the measurement uncertainty. by combining the obtained simulation data with the experimental one and analysing both with the machine learning algorithm, the surrogate model will be created. acknowledgement the authors would like to acknowledge funding of the presented research within the european metrology programme for innovation and research (empir) as well as the european association of national metrology institutes (euramet) in the joint research project 18sib08 comtraforce. references [1] empir 18sib08: comprehensive traceability for force metrology services (comtraforce). online [accessed 18 march 2022] https://www.ptb.de/empir2019/comtraforce/home/ [2] e. glaessgen, d. stargel, the digital twin paradigm for future nasa and u.s. air force vehicles, 53rd aiaa/asme/asce/ahs/asc structures, structural 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the relationship between spatialresolution property and segmentation accuracy of the tumor area shu onodera1, yongbum lee2, tomoyoshi kawabata1 1 department of radiology division of medical technology, tohoku university hospital, sendai, miyagi, japan 2 graduate school of health sciences, niigata university, niigata, japan section: research paper keywords: chest radiography; x-ray; u-net; deep learning; flat panel detector citation: shu onodera, yongbum lee, tomoyoshi kawabata, dose reduction potential in dual-energy subtraction chest radiography based on the relationship between spatial-resolution property and segmentation accuracy of the tumor area, acta imeko, vol. 11, no. 2, article 28, june 2022, identifier: imeko-acta-11 (2022)-02-28 section editor: francesco lamonaca, university of calabria, italy received september 30, 2021; in final form february 23, 2022; published june 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: shu onodera, e-mail: onodera@rad.hosp.tohoku.ac.jp 1. introduction chest radiography is the most basic diagnostic imaging procedure for lung diseases. however, the amount of x-rays that the patient is exposed to is enormous, including in the case of standing position imaging which is usually performed during medical examinations and bedside imaging for critically ill patients [1]. compared with computed tomography (ct) examination, which provides three-dimensional information, the amount of exposure in radiography is very low (ct: 10 msv, chest radiography: 0.1 msv) [2]-[5] and its importance in terms of convenience of examination cannot be underestimated. usually, a high voltage of approximately 120 kv is applied during chest radiography to emphasize the contrast of the lung field rather than that of the ribs [6]. nevertheless, the shadow of the rib remains on the image, making it difficult to detect the shadow of the soft tissue that overlaps that of the ribs. to address this problem, it has been developed an energy subtraction process [7] in which two types of image data with different radiographic energy characteristics are obtained with a single exposure, the bone shadows are removed through weighed differentiation of the respective images, and the image of the soft tissue alone is segmented (hereinafter, referred to as soft tissue imaging). once bone shadows are removed, it becomes easier to detect tumours in soft tissue images. however, because the image quality in chest radiography considerably varies for abstract we investigated the relationship between the spatial-resolution property of soft tissue images and the lesion detection ability using unet. we aimed to explore the possibility of dose reduction during energy subtraction chest radiography. the correlation between the spatial-resolution property of each dose image and the segmentation accuracy of the tumor area in the four regions where the tumor was placed was evaluated using linear regression analysis. the spatial-resolution property was determined by task-based evaluation, and the task-based modulation transfer function (ttf) was computed as its index. ttfs of the reference dose image and the 75 % dose image showed almost the same frequency characteristics regardless of the location of the tumor, and the dice coefficient also high. when the tumor was located in the right supraclavicular region and under 50 % dose, the frequency characteristics were significantly reduced, and the dice coefficient was also low. our results showed a close relationship between the spatial-resolution property and the segmentation accuracy of tumor area using deep learning in dual-energy subtraction chest radiography. in conclusion, a dose reduction of approximately 25 % compared to the conventional method can be achieved. the limitations are the shape of the simulated mass and the use of chest phantom. mailto:onodera@rad.hosp.tohoku.ac.jp acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 2 different parts due to factors such as the amount of radiation reaching the detector and the amount of scatter radiation, detectability may differ depending on the location of the tumour. in particular, the scatter radiation generated from the clavicle and scapula is believed to significantly affect the upper lobe of the lung, which is a common site of adenocarcinoma [8]. the spatialresolution property of the image is also a very important factor in tumour detection [9]. the spatial-resolution property is a measure of the sharpness of an image and is an important characteristic that determines the detectability of lesions in x-ray images. chest radiography images are generally subjected to several image-processing techniques to improve image quality, and these processing tools lead to a nonlinear behaviour that depends on image quality, which is different for different parts [10]. thus, the quality of the soft tissue image also shows nonlinear behaviour, and task-based evaluation in a measurement environment that reflects clinical conditions is necessary to determine the spatial-resolution property. the use of computer-aided diagnosis (cad) in diagnostic imaging has increased [11], [12]. earlier, image interpretation was performed by radiologists, based on their cultivated experience. however, with the recent increase in the use of radiography and the consequent increase in the number of images, cad was introduced to reduce the burden on radiologists. cad based on deep learning has been attracting attention recently, and it may soon be possible to detect a tumour even in a low-dose image with high noise. in previous reports on energy subtraction-treated chest radiographs, visual evaluations of images acquired by the computed radiography (cr) systems have been reported [13][17]. the purpose of this study was to investigate the relationship between the spatial-resolution property of soft tissue images obtained by the flat panel detector (fpd) system and the lesion detection ability based on deep learning and explore the possibility of dose reduction during energy subtraction chest radiography. 2. materials and methods 2.1. image acquisition an acrylic cylindrical simulated tumour with a diameter of 20 mm and thickness of 3 mm was placed in four regions on the chest phantom (right supraclavicular, left middle lung, right lower lung, and mediastinum), figure 1. bone structure such as the clavicle, shoulder blade, and ribs as well as soft tissues as the mediastinum and pulmonary vascularity are located in the chest phantom. the single-exposure dual-energy subtraction system, [18], [19] calneo dual (fujifilm medical co. ltd. tokyo. japan, with a pixel size of 0.15 mm), was used in this study. the fpd implemented in this system consisted of two stacked scintillators. normal energy images were collected in the first layer (cesium iodide scintillator), and the second layer (gadolinium sulfide scintillator) collected high-energy images transmitted through the first layer. table 1 lists the imaging conditions used. the source-image distance was fixed at 180 cm, the field size was 43.2 cm, the image depth was 12 bits, and the tube voltage was 115 kv. three types of imaging doses were used: a standard dose of 1.6 ma s, which was then reduced by 25 % to 1.25 ma s and then reduced by 50 % to 0.8 ma s, and 100 images of each type were acquired. 2.2. calculation of the spatial-resolution property chest radiography images are generally subjected to several image-processing techniques to improve image quality. frequency-processing and dynamic-range compression processing are typical examples. however, these processing tools lead to a nonlinear behaviour that depends on image quality, which is different for different parts. therefore, in this study, the spatial-resolution property was determined by task-based evaluation, and the task-based modulation transfer function (ttf) was computed as its index [20]. the ttf calculation process is shown in figure 2. the edge spread function (esf) for the cylinder was obtained by averaging the profiles that cross the edge of the cylinder, measured from the centre in the direction of radiation. next, ttf was calculated using the fourier transform of the line spread function obtained by differentiating the esf. one of the factors to be considered when determining the ttf of a soft tissue image is the signal-tonoise ratio (snr) of the image. because images with a low snr create large errors in the calculation results, in this study, the image without acrylic was subtracted from the image with acrylic, and an image with a high snr was created through the additive average of 100 such images, which was then used to calculate the ttf (figure 3). 2.3. building the deep learning environment cad using deep learning has been an area of active research in recent years and has a wide range of applications in medical figure 1. phantom image and placement of acrylic cylinder. figure 2. ttf calculation process. table 1. imaging conditions. source image distance in cm field size in cm tube voltage in kv image depth in bit dose in ma s 180 43.2 115 12 1.6 (reference) 1.2 (25 % down) 0.8 (50 % down) acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 3 imaging, such as lesion detection, area extraction, and image noise reduction. image segmentation refers to the process of dividing an image into regions corresponding to each object. because the target areas in medical imaging are organs or lesions, the positional information must be specified in the original input image at the time of segmentation. u-net [21] is a typical example of a deep convolutional neural network for image segmentation. the present study deals with the detection of lung tumours using u-net in soft tissue images. figure 4 shows the structure of the u-net used in this study. the usage environment of u-net in this research is as follows: os: windows10, framework: python3.7, tensorflow, keras, cpu: core i7-10750h, memory: 16g. the relu function and sigmoid function were used as activation functions, cross entropy as the loss function, and adam as the learning optimization algorithm. 2.4. data set for deep learning the acquired soft tissue image (window width: 8500, window level: 8100, 14 bits) was segmented into 128 × 128 pixels centred around the tumour and converted to png format (window width: 255, window level: 128, 8bits). fifty standard-dose images were input in u-net as training data and 50 reduced-dose images as evaluation data, and learning was conducted by setting the number of epochs to 30. the teaching data for the soft tissue images containing the tumour were created by binarizing the image into the tumour area and other areas (figure 5). figure 3. creating the ttf calculation image. figure 4. the structure of the u-net in this study. acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 4 2.5. evaluation of the segmented tumour area the dice coefficient [22] was calculated as the degree of similarity between the output image and the teacher image to evaluate the extraction accuracy of the tumour region using unet. the dice coefficient is defined by the following formula: 𝐷𝑖𝑐𝑒(𝐴, 𝐵) = 2|𝐴 ∩ 𝐵| |𝐴| + |𝐵| . (1) here, a denotes the tumour region in the teaching image (region with a digital value of 255), and b denotes the tumour region in the output image (region with a digital value of 255). 2.6. evaluation of the correlation between spatial-resolution property and segmentation accuracy of tumor area in this study, the correlation between the spatial-resolution property of each dose image and the segmentation accuracy of the tumour area in the four regions where the tumour was placed was evaluated using linear regression analysis. a scatter plot was created by treating the ttf and dice coefficient of each dose image from 0.2 to 1.2 cycle/mm (intervals of 0.2 cycle/mm) as variables. the dice coefficient for the reference dose image was set to 1. 3. results figure 6 shows the ttf results for each condition. no difference was observed between the ttfs of the reference dose image and the 75 % dose image in the supraclavicular region, where the contrast was low due to the influence of scattered radiation from the thoracic spine, clavicle, and scapula; however, the similarity decreased significantly in the ttf of 50 % dose images. in contrast, in the middle and lower lung regions where the effect of scattered radiation was small and the contrast was high compared to the supraclavicular region, the ttfs were generally high and the difference in values between doses was small. in the mediastinum, ttfs were low as in the supraclavicular region because of the low contrast due to the scattered radiation from the heart and sternum, but the decrease was not as high as that in the supraclavicular region; in comparison, the ttf in the supraclavicular region was the lowest among all the other regions. table 2 shows the average values of the dice coefficients of the 50 datasets for each condition. the dice coefficient between the segmented tumour area and the teaching data in the 75 % dose image showed a generally high value of approximately 0.96, regardless of the location of the tumour. furthermore, the ttf of the 75 % dose image showed a value similar to that of the reference dose image regardless of the location of the tumour. in contrast, the dice coefficient in the 50 % dose image was as low as 0.937 when the tumour was located in the supraclavicular region. likewise, the ttf of the 50 % dose image in which the tumour was located in the supraclavicular region showed a lower value compared to the reference dose image. figure 7 shows the actual tumour area segmented by u-net. in the 75 % dose condition, the segmented images were highly similar to the teaching data, regardless of the location of the tumour. however, in the 50 % dose condition and when the tumour was located in the right supraclavicular region, the segmented region was slightly larger compared with the teaching data. figure 8 to figure 11 show the correlation between ttf and dice coefficient in soft tissue images. a positive correlation was observed between the ttf and dice coefficient of every dose image at all spatial frequencies in the right supraclavicular and the right lower lung regions and between the frequencies of 0.2 to 0.8 cycle / mm in the mediastinum section. in contrast, no correlation was observed between the ttf and dice coefficients in any of the spatial frequencies in the left middle lung region. 4. discussion in the case of the simulated tumour located in the right supraclavicular region and under 50 % dose, both the ttf and dice coefficients showed significantly low values. one reason for this could be that the contrast of the tumour was reduced by scattered radiation mainly from the clavicle and scapula due to the complicated bone structure of the supraclavicular region. a second reason could be the fact that the tumour area could not be segmented accurately because of the increased image noise because the amount of radiation reaching the detector was smaller than that reaching other parts. in the case in which the simulated tumour was located in the mediastinum region, the value of ttf was not very different from that when the tumour was in the middle and lower lung regions, and the dice coefficient also showed a similar value. in the mediastinum region, the amount of radiation reaching the detector was less than that of the middle and lower lung regions, figure 5. creation of teaching data. figure 6. ttfs for each condition. table 2. dice coefficients for each of the conditions. ma s right supraclavicular left middle lung right lower lung mediastinum 1.25 0.960 0.960 0.959 0.971 0.8 0.937 0.969 0.963 0.967 acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 5 and the amount of scattered radiation from the sternum and heart was also large. therefore, under the 50 % dose condition, the ttf and dice coefficients were perceived to be as low as the right supraclavicular region. however, as the tumour in this area had fewer pulmonary blood vessels around it than other sites, the structure was relatively simple and the tumour area could be segmented accurately (figure 12). the results of this study show that there is a high degree of correlation between the spatial resolution of the soft tissue image and the segmentation accuracy of the tumour area using deep figure 7. mass region segmentation using for each of the conditions. figure 8. between ttf and dice coefficient (right supraclavicular). figure 9. between ttf and dice coefficient (left middle lung). ● 50% dose ▲ 75% dose ■ reference ● 50% dose ▲ 75% dose ■ reference ● 50% dose ▲ 75% dose ■ reference ● 50% dose ▲ 75% dose ■ reference ● 50% dose ▲ 75% dose ■ reference ● 50% dose ▲ 75% dose ■ reference ● 50% dose ▲ 75% dose ■ reference ● 50% dose ▲ 75% dose ■ reference ● 50% dose ▲ 75% dose ■ reference ● 50% dose ▲ 75% dose ■ reference ● 50% dose ▲ 75% dose ■ reference ● 50% dose ▲ 75% dose ■ reference acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 6 learning in the supraclavicular, lower lung, and mediastinum regions. in the 75 % dose images, the ttf was high regardless of the tumour location, and the dice coefficient was also high. in contrast, in the 50 % dose images, when the tumour was present in the supraclavicular region, the ttf was significantly reduced, and the dice coefficient was also low. in other words, if the radiation dose is reduced to 50 % of the conventional radiation condition, tumour that develop in the supraclavicular region may not be segmented accurately due to a decrease in ttf. no correlation was confirmed between ttf and the dice coefficient in the middle lung area. this could be because there was no difference in ttfs of the dose images between 0.2 and 0.6 cycle / mm, and between the 0.8 and 1.2 cycle / mm, there was no difference in the ttfs of the 75 % dose image and the reference dose image. among the four sites examined in this study, the highest amount of radiation reached the detector from the middle lung area, and the amount of scattered radiation from the surroundings was also small. therefore, no correlation could be confirmed between the ttf and the dice coefficient in the middle lung region, and the dice coefficients of all dose images showed a high value of approximately 0.96. the discussion above suggests that, with single-exposure dual-energy subtraction chest radiography by the fpd system, it may be possible to reduce the dose by approximately 25 % compared to the conventional method. figure 13 shows the detection quantum efficiency (dqe) in the radiation qualities of the rqa9 of the cr system, which was manufactured by the same company as the calneo dual system used in this study [23], [24]. because lung tumors are the targets of this study, we focused on the value of the spatial frequency of 1 cycle / mm [25]. the dqe value at 1 cycle / mm is approximately 0.5 for the calneo dual and about 0.2 for the cr system, respectively, and the detection quantum efficiency of the calneo dual is approximately 2.5 times higher. a system with an excellent dqe has a high degree of freedom in adjusting the balance between sharpness and graininess through image processing [26]. therefore, selecting figure 10. between ttf and dice coefficient (right lower lung). figure 11. between ttf and dice coefficient (mediastinum). figure 12. pulmonary vessels around the mass in the mediastinum. figure 13. dqe for calneo dual and cr system with rqa9 spectra. ● 50% dose ▲ 75% dose ■ reference ● 50% dose ▲ 75% dose ■ reference ● 50% dose ▲ 75% dose ■ reference ● 50% dose ▲ 75% dose ■ reference ● 50% dose ▲ 75% dose ■ reference ● 50% dose ▲ 75% dose ■ reference ● 50% dose ▲ 75% dose ■ reference ● 50% dose ▲ 75% dose ■ reference ● 50% dose ▲ 75% dose ■ reference ● 50% dose ▲ 75% dose ■ reference ● 50% dose ▲ 75% dose ■ reference ● 50% dose ▲ 75% dose ■ reference acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 7 parameters with good spatial-resolution properties for multifrequency processing in single-exposure dual-energy subtraction chest radiography using fpd could lead to further dose reduction. as a limitation of this study, we would first mention the structure of the simulated tumors. in this study, an acrylic material with a simple cylindrical structure was used as the simulated tumor. actual lesions with increased malignancy, such as spiculated lesions, often have more complex structures, and in such cases, the results may differ. moreover, in this study, all measurements from beginning to end were performed using a phantom, and the effects of heartbeat, which is a major problem in actual clinical practice, [27] have not been considered. however, to reduce the effects of heartbeat, the imaging time was shortened to the largest extent possible, and measurements were performed in a very short interval of approximately 10 ms, as is done in clinical practice; hence, we hope that the results will not be greatly affected. 5. conclusions in this study, we clarified the relationship between the spatial resolution of single-exposure dual-energy subtraction chest radiography using the fpd system and the segmentation accuracy of the tumour area using deep learning. the ttfs of the reference dose image and the 75 % dose image showed almost the same frequency characteristics regardless of the location of the tumour, and the dice coefficient also showed a high value. when the tumour was located in the right supraclavicular region and under 50 % dose, the frequency characteristics were significantly reduced, and the dice coefficient was also low. therefore, a close relationship between the spatial-resolution property and the segmentation accuracy of the tumour area was confirmed using deep learning in singleexposure dual-energy subtraction chest radiography using the fpd system, and it may be possible to achieve dose reduction of approximately 25 % compared to the conventional method. references [1] unscear. medical radical exposures. sources and effects of ionizing radiation, unscer 2008 report. new york: united 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[2] l. j. m. kroft, l. van der velden, i. h. girón, j. j. h. roelofs, a. de roos, j. geleijns, added value of ultra-low-dose computed tomography, dose equivalent to chest x-ray radiography, for diagnosing chest pathology, j. thorac imaging, 34 (2019), pp. 179186. doi: 10.1097/rti.0000000000000404 [3] r. ward, w. d carrol, p. cunningham, s. a ho, m. jones, w. lenney, d. thonpson, f. j gilchrist, radiation dose from common radiological investigations and cumulative exposure in children with cystic fibrosis: an observational study from a single uk centre, observational study, 7 (2017), pp. 1-5. doi: 10.1136/bmjopen-2017-017548 [4] s. singh, m. k. kalra, r. d. ali khawaja, a. padle, s. pourjabbar, d. lira, j. a. o. shepard, s. r. digumarthy, radiation dose optimization and thoracic computed tomography, radiol. clin. north am., 52(2014), pp. 1-15. doi: 10.1016/j.rcl.2013.08.004 [5] international commission on radiological protection, the 2007 recommendations of the international commission on radiological protection, icrp publication 103, ann. icrp 37 (24). 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[13] s. kido, j. ikezoe, h. naito, j. arisawa, s. tamura, t. kozuka, w. ito, k. shimura, h. kato, clinical evaluation of pulmonary nodules with single-exposure dual-energy subtraction chest radiography with an iterative noise-reduction algorithm, radiology, 194 (1995), pp. 407-412. doi: 10.1148/radiology.194.2.7824718 [14] s. kido, k. kuriyama, n. hosomi, e. inoue, c. kuroda, t. horai, low-cost soft-copy display accuracy in the detection of pulmonary nodules by single-exposure dual-energy subtraction: comparison with hard-copy viewing, j digit imaging, 2000, pp. 33-37. doi: 10.1007/bf03168338 [15] j. r. wilkie, m. l. giger, m. r. chinander, t. j. vokes, r. m. nishikawa, m. d. carlin, investigation of physical image quality indices of a bone densitometry system, med phys, 31 (2004), pp. 873-881. doi: 10.1118/1.1650528 [16] s. kido, h. nakamura, w. ito, k. shimura, h. kato, computerized detection of pulmonary nodules by single-exposure dual-energy computed radiography of the chest (part 1), eur j radiol, 44(2002), 198-204. doi: 10.1016/s0720-048x(02)00268-1 [17] s. kido, k. kuriyama, c. kuroda, h. nakamura, w. ito, k. shimura, h. kato, detection of simulated pulmonary nodules by singleexposure dual-energy computed radiography of the chest: effect of a computer-aided diagnosis system (part 2), eur j radiol, 44 (2002), pp. 205-209. doi: 10.1016/s0720-048x(02)00269-3 [18] l. shi, m. lu, n. r. bennett, e. shapiro, j. zhang, r. colbeth, j. s. lack, a. s. wang, characterization and potential applications of a dual-layer flat-panel detector, med phys, 47(2020), epub 2020 may 18, pp. 3332-3343. doi: 10.1002/mp.14211 [19] m. lu, a. wang, e. shapiro, a. shiroma, j. zhang, j. steiger, j. s. lack, dual energy imaging with a dual-layer flat panel detector, spie med imaging;10948 physics of medical imaging, sandiego, united states, 2019 doi: 10.1117/12.2513499 [20] s. richard, d. b. husarik, g. yadava, s. n. murphy, e. samei, towards task-based assessment of ct performance: system and object mtf across different reconstruction algorithms, med phys, 39(2012), 4115-4122. doi: 10.1118/1.4725171 https://doi.org/10.1097/rti.0000000000000404 https://doi.org/10.1136/bmjopen-2017-017548 https://doi.org/10.1016/j.rcl.2013.08.004 https://doi.org/10.1148/radiol.2372041738 https://doi.org/10.1007/s12194-014-0285-y https://doi.org/10.6009/jjrt.64.259 https://doi.org/10.6009/jjrt.67.1381 https://doi.org/10.1259/bjr/82933343 https://doi.org/10.1148/radiology.194.2.7824718 https://doi.org/10.1007/bf03168338 https://doi.org/10.1118/1.1650528 https://doi.org/10.1016/s0720-048x(02)00268-1 https://doi.org/10.1016/s0720-048x(02)00269-3 https://doi.org/10.1002/mp.14211 https://doi.org/10.1117/12.2513499 https://doi.org/10.1118/1.4725171 acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 8 [21] o. ronneberger, p. fischer, t. brox, u-net: convolutional networks for biomedical image segmentation lecture notes in computer sciences (including subserlect notes artifintelllect notes bioinformatics). 2015; 9351: pp. 234-241. [22] b. sahiner, a. pezeshk, l. m. hadjiiski, x. wang, k. drukker, k. h. cha, r. m. summers, m. l. giger, deep learning in medical imaging and radiation therapy. med phys, 46(2019), epub 2018 nov 20, e1-e36. doi: 10.1002/mp.13264 [23] iec. 62220-1. medical electrical equipment-characteristics of digital x-ray imaging devices part 1: determination of detective quantum efficiency. international electrotechnical commission; 2003. [24] iec. 62220-1-2. medical electrical equipment-characteristics of digital x-ray imaging devices part 1-2: determination of detective quantum efficiency-detectors used in mammography. international electrotechnical commission; 2007. [25] t. yokoi, t. takata, k. ichikawa, investigation of image quality identification utilizing physical image quality measurement in directand indirect-type of flat panel detectors and computed radiography, nihon hoshasen gijutsu gakkai zasshi, 67(2011), pp. 1415-1425. doi: 10.6009/jjrt.67.1415 [26] a. r. cowen, a. g. davies, m. u. sivananthan, the design and imaging characteristics of dynamic, solid-state, flat-panel x-ray image detectors for digital fluoroscopy and fluorography, clin radiol, 63(2008), 1073-1085. doi: 10.1016/j.crad.2008.06.002 [27] european commission, chest. (lungs and heart) pa and lateral projections. european guidelines on quality criteria for diagnostic radiographic image. eur. luxembourg: cec; 1996:12:16260 en. https://doi.org/10.1002/mp.13264 https://doi.org/10.6009/jjrt.67.1415 https://doi.org/10.1016/j.crad.2008.06.002 metrological characterisation of rotating-coil magnetometer systems acta imeko issn: 2221-870x june 2021, volume 10, number 2, 30 36 acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 30 metrological characterisation of rotating-coil magnetometer systems stefano sorti1,2, carlo petrone2, stephan russenschuck2, francesco braghin1 1 politecnico di milano, department of mechanical engineering, via la masa, 1, 20156, milano 2 european organization for nuclear research 1205 geneva, switzerland, 1211 meyrin section: research paper keywords: magnetic measurements; magnetic field; rotating-coil magnetometers; rotating shaft citation: stefano sorti, carlo petrone, stephan russenschuck, francesco braghin, metrological characterisation of rotating-coil magnetometer systems, acta imeko, vol. 10, no. 2, article 6, june 2021, identifier: imeko-acta-10 (2021)-02-06 section editor: giuseppe caravello, università degli studi di palermo, italy received january 7, 2021; in final form april 13, 2021; published june 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: stefano sorti, e-mail: stefano.sorti@polimi.it 1. introduction rotating-coil systems are a special type of induction-coil magnetometers, based on faraday's law of induction. they are used to measure integral field harmonics in the magnet bore. rotating-coil systems consist of arrays of coils mounted on a rotating shaft, aligned with the magnet axis. the varying flux linkage with the coil induces a voltage. typically, one long coil (or a chain of shorter coils) spans the entire magnet, including the fringe-field areas, because the transversal, integrated field is typically sufficient for beam tracking in particle accelerators [1]. even if local measurements are required, through point-wise magnetometers [2], integral field is still a primary requirement. precise measurements of magnetic fields require a careful evaluation of the mechanical properties of the rotating coils, subject to static and dynamic forces [3]. this implies the need for the evaluation of vibrations [4]. precautions for reducing the impact of mechanical deformations should also be taken at the design stage. a properly designed system should have natural frequencies higher than the operating frequencies [5]. compensation schemes for the main field harmonic, commonly referred to as bucking, provide effective mitigation of spurious field harmonics caused by these vibrations [6]. nevertheless, the learned design methodologies are not always sufficient to match the requirements. analytical formulas exist only for static phenomena such as misalignments and gravity [6]. vibrations are instead modelled directly as spurious field harmonics in the measured field, and therefore they can only be evaluated in a prescribed field, as in [4]. the approach adopted in literature is typically to design the instrument aiming at a reasonably high stiffness, evaluating its compliance with controlled input like motor torque, as in [3]. therefore, all the approaches proposed in the literature are limited in describing all the relevant mechanical phenomena, particularly shaft flexibility and propagation of mechanical vibration from motor and supports. an analytical model for the mechanical description of rotating-coils was proposed in [7]. this model can predict the abstract rotating-coil magnetometers are among the most common and most accurate transducers for measuring the integral magnetic-field harmonics in accelerator magnets. the measurement uncertainty depends on the mechanical properties of the shafts, bearings, drive systems, and supports. therefore, rotating coils require a careful analysis of the mechanical phenomena (static and dynamic) affecting the measurements, both in the design and in operation phases. the design phase involves the estimation of worst-case scenarios in terms of mechanical disturbances, while the operation phase reveals the actual mechanical characteristics of the system. in previous publications, we focused on modelling the rotating-coil mechanics for the design of novel devices. in this paper, we characterise a complete system in operation. first, the mechanical model is employed for estimating the forces arising during shaft rotation. then, the effect of the estimated disturbances is evaluated in a simulated measurement. this measurement is then performed in the laboratory and the two results are compared. in order to characterise the robustness of the system against mechanical vibrations, different revolution speeds are evaluated. this work thus presents a complete procedure for characterising a rotating-coil magnetometer system. mailto:stefano.sorti@polimi.it acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 31 effect of static coil deformations, coil-axis to magnet alignment tolerances, and vibration modes on the measured field harmonics. the model was then expanded in a finite-element formulation (fem) and applied to the design of a rotating-coil bench in [8]. this paper proposes a further refinement of the model, applied to the metrological characterisation of the designed rotating-coil system. the shaft is an anisotropic rotor described in a non-inertial frame, thus including rotor-dynamics effects, coupled with steady space-frame support. vibrations of the real system during operation are measured and introduced in simulations. a sample dipole magnet is measured both with the real system and through a simulation of the device in order to validate the robustness and the reliability of the system. therefore, the conclusion of this study is a complete magneto-mechanical description of a rotating-coil system in operation. 2. the magneto-mechanical model the 3d finite-element method (fem) [8] is adopted to describe rotating-coil shafts and their supporting structures. the two parts are modelled in different frames and coupled as described in the next section. the resulting mechanical deformation field of the shaft 𝒖(𝒓) (where 𝒖 is the displacement vector and 𝒓 the position) is applied to the coil geometry to evaluate its effects on the magnetic measurements. the fem model is shown in figure 1. 2.1. the mechanical model the model is based on timoshenko beams with 12 degrees of freedom (dof) per node, with the possibility of adding lumped masses, springs, and dampers. it is possible to include also non-ideal boundary conditions: clamped or hinged ends can be replaced by elastic foundations with a suitable stiffness. the equations for the degrees of freedom (dofs) 𝒑 of the fixed support structure are: 𝑀�̈� + 𝐶�̇� +𝐾𝒑 = 𝒇, (1) where 𝑀, 𝐾 and 𝐶 are the mass, stiffness, and damping matrices of the system. damping is characterised experimentally as modal damping [7], [8], and 𝒇 accounts for external forces acting on the system. these include gravity, unbalancing, and vibration of moving parts. the same equation holds for the rotating shaft but with the additional effects of not being in an inertial frame. the main advantage of adopting a rotating frame for the shaft is to have constant mechanical properties when reducing the shaft subsystem (shown below). this is a common approach for anisotropic shafts [9]. for the sake of simplicity, the rotating speed ω is assumed to be constant, and therefore the angular displacement can be expressed as a function of time: 𝜃 = ω𝑡. let us consider the transformation expressing the rotation of the shaft 𝒒 = 𝑅𝒒f, where 𝑅 = 𝑅(𝜃), and 𝒒f, 𝒒 are the shaft dofs expressed in the fixed and rotating frames, respectively. applying the transformation to the shaft equation in a fixed frame, following eq. (1), and multiplying by 𝑅, the shaft equation in the rotating frame results is 𝑀�̈� + (𝐶 + 2𝑅𝑀�̇�t)�̇� + (𝐾 − ω2𝑀)𝒒 = 𝑅t𝒇. (2) in this equation, the term added to 𝐶 denotes the coriolis force, while the extra stiffness term is the centrifugal force. before coupling the subsystems, each one is reduced with the craig-bampton (cb) method [10]. the cb method is a popular technique that allows for an independent reduction of subdomains before the matrix assembly. it is based on splitting the set of dofs of each subsystem into internal dofs 𝒒i and boundary dofs 𝒒b (or 𝒑i and 𝒑b, respectively, for the support structure). boundary dofs are the ones at the interface between the subsystems and thus directly involved in the assembly of the full structure. the reduced set of dof from the cb method is a combination of constraint modes 𝛹b and internal vibration modes 𝛹i . they are expressed by the coordinate transformation 𝒒 = [ 𝒒b 𝒒i ] = [ 𝐼 𝟎 𝛹i 𝛹b ][ 𝒒b 𝝔i ], (3) where 𝐼 is the identity matrix, and 𝝔i are modal coordinates of the shaft. the modal coordinates of supporting structure are denoted by 𝝅i. the computation of 𝛹i for the rotating shaft may be affected by the presence of non-symmetric matrices in the equation. to avoid a more complicated approach, typically involving left and right eigenvectors, it is proposed to compute 𝛹i as the free vibration modes of the shaft, neglecting the �̇� terms [9]. the computation of 𝛹b (for both subsystems) is performed by imposing fictitious unitary displacements to each boundary dof and computing the static response: 𝛹b = 𝐾i,i −1𝐾𝑖,𝑏, (4) where 𝐾i,i and 𝐾i,b are the submatrices of the stiffness matrix 𝐾 accounting for internal-internal and internal-boundary interactions, respectively. the reduction of each subsystem can be performed by truncating the 𝛹i set of modes. different criteria can be enforced, mainly based on the maximum frequency component of the expected external inputs [11]. finally, primal assembly [10] is adopted for the coupling between the support structure and rotating shaft. only the subset 𝒒b must be transformed before coupling, resulting in 𝒒b,f. modal coordinates 𝝔i are not affected, as they are not in the physical space. a generalised set of coordinates is introduced to account for both subsystems as 𝒖 = [ 𝐼 0 0 0 0 𝐼 𝐼 0 0 0 0 𝐼 ] ⏟ 𝐿t [ 𝝔i 𝒒b,f 𝒑b 𝝅i ], (5) where 𝐼 is the identity matrix and 0 is the zero matrix. figure 1. layout of the mechanical model. the equations for the support structure and for the shaft are expressed in the fixed frame (𝒙,𝒚,𝒛) and in the rotating frame (𝒙𝒔,𝒚𝒔,𝒛𝒔), respectively. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 32 the assembled equations finally result in 𝐿t [ 𝑀s 0 0 𝑀r ]𝐿�̈� + 𝐿t [ 𝐶s 0 0 𝐶r ]𝐿�̇� + 𝐿t [ 𝐾s 0 0 𝐾r ]𝐿𝒖 = 𝐿t [ 𝒇s 𝒇r ], (6) where the subscripts s and r identify the matrices for the support and the rotating shaft, respectively. employing the fem shape functions, the deformation field of the shaft can be computed and applied to the geometry of the coil. 2.2. the magnetic model the mechanical model is coupled with the magnetic model by computing the magnetic flux linkage with the induction coil. in order to calculate the system response to a given field distribution, the magnetic flux density is expressed analytically. the typical description for integral flux density is the 2d multipoles expansion [1]. however, for correct modelling, the full 3d field is required because long integral coils also intercept the magnet fringing field. therefore, 3d pseudo-multipoles are adopted [12]: 𝐵𝑟(𝑟,𝜑,𝑧) = −𝜇0 ∑𝑟 𝑛−1 ∞ 𝑛=1 (𝒞𝑛(𝑟,𝑧)sin𝑛𝜑 + 𝒟𝑛(𝑟,𝑧)cos𝑛𝜑), (7) 𝐵𝜑(𝑟,𝜑,𝑧) = −𝜇0 ∑𝑛𝑟 𝑛−1 ∞ 𝑛=1 (�̃�𝑛(𝑟,𝑧)cos𝑛𝜑 − �̃�𝑛(𝑟,𝑧)sin𝑛𝜑), (8) 𝐵𝑧(𝑟,𝜑,𝑧) = −𝜇0 ∑𝑟 𝑛 ∞ 𝑛=1 ( ∂�̃�𝑛(𝑟,𝑧) ∂𝑧 sin𝑛𝜑 + ∂�̃�𝑛(𝑟,𝑧) ∂𝑧 cos𝑛𝜑), (9) where 𝒞𝑛(𝑟,𝑧) = 𝑛𝒞𝑛,𝑛(𝑧) − (𝑛 + 2)𝒞𝑛,𝑛 (2) (𝑧) 4(𝑛 +1) 𝑟2 + ⋯ (10) �̃�𝑛(𝑟,𝑧) = 𝒞𝑛,𝑛(𝑧) − 𝒞𝑛,𝑛 (2) (𝑧) 4(𝑛 + 1) 𝑟2 + ⋯ (11) in the interest of brevity, the similar expressions for the skew components �̃�𝑛, 𝒟𝑛 have been omitted here. it is, therefore, possible to compute the magnetic flux density at any point in space. the flux linkage in the coils is then calculated numerically with φ = ∫ 𝐁 𝒜 ⋅ d𝐚. (12) in a discrete setting, the flux increments 𝜙𝑚 for any angular positions 𝜃𝑚 can be developed into a discrete fourier series: ψ𝑛 = ∑ 𝜙𝑚 𝑀−1 𝑚=0 ⋅ 𝑒 −𝑖2𝜋𝑚𝑛 𝑁 . (13) this yields the harmonic content of the response: 𝐶𝑛(𝑟0) = 𝑟0 𝑛−1 ψ𝑛 𝑘𝑛 , (14) where 𝐶𝑛 a(𝑟0) = 𝐵𝑛 a(𝑟0) + 𝑖𝐴𝑛 a(𝑟0) are the measured (apparent) field harmonics and 𝑘𝑛 are the coil sensitivity factors for the 𝑛th field harmonic: 𝑘𝑛 = 𝑁𝑇𝐿𝑐 𝑛 (𝑟2 𝑛 − 𝑟1 𝑛), (15) where 𝑁𝑇 is the number of coil turns, 𝐿𝑐 the total length of the coil and the two radii are the position of the go and return wires of the coil. the 𝑘𝑛 express the integral sensitivity of the coil and are therefore not functions of 𝑧. the differences between the imposed 𝐴𝑛, 𝐵𝑛 and the apparent 𝐴𝑛 a , 𝐵𝑛 a coefficients at the reference radius of measurement are the main figure of merit to evaluate the effects of mechanical defects. 3. the measurement system review the proposed model was used to design the measurement system, as described in [8]. a brief recap of the design considerations is given before presenting the model of the final system. 3.1. the design process the design process was based on separate studies of the two subsystems. the shaft was designed considering an approximated (non-optimal) support. then the supporting structure was designed for the expected worst-case scenario (i.e. for the least stiff shaft expected to be mounted on the support). a recall of the design procedure is presented in figure 2. on the left, the probability density function of errors in the field multipole 𝐴3 for different design iterations of the shaft, from bulk resin (1) to carbon-only shaft (4). on the right, the standard deviation of the error in measuring the main component of a quadrupole 𝐵2 for different designs of the supporting structure. the shaft was designed for measuring the field harmonics in dipoles and quadrupole magnets. compensation schemes were applied in simulating the measurement procedure. in order to calculate the performance of the compensation in the design, coil imperfections were introduced as gaussian distributions. the external forces considered were gravity, support vibrations (in the form of harmonic forces on the bearings), bearing-frictionrelated torque, and shaft unbalancing due to eccentricity and sag. the boundary conditions for the shaft were elastic supports (lumped springs and dampers), accounting for the equivalent stiffness of the supporting structure. figure 2. overview of the design phase for the shaft, on the left, and for the support, on the right (adapted from [8]). ] 1 3 2 4 variants 0 0.5 1 1.5 2 2.5 a3[10 -4 0 2 4 6 8 10 12 p ro b a b il it y f u n ct io n [ % ] variants s o f b 2 e rr o r [1 0 -4 ] 1 2 3 4 acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 33 regarding the design of the support, the error of the main field component was the most relevant figure of merit. the design process was thus an iteration of different space-frame layouts for the support structure. 3.2. the model of the measurement system the complete model of the measurement system is shown in figure 3 (top image). it consists of a 1.5-m-long shaft made of carbon-fiber composite profiles, supporting a printed circuit board (pcb) coil array. the array is made of five equal radial coils of 1.48 m length, 11.2 mm width, and 60 turns. the diameter of the shaft is 72 mm. the supporting structure is a two-meter-wide aluminium frame. the boundary conditions for the support structure are two clamped and two pinned nodes. this comes from the different layouts of the joints and aims at being conservative in terms of mechanical stiffness. figure 3 (bottom image) also shows the modelled magnetic field for the characterisation of the system, which is the main topic of the next section. the shaft model consists of two parallel beams, representing the pcb and the carbon profiles, respectively. bolt joints are modelled as rigid links between mesh nodes. bearing stiffness is included in the interface between the shaft and the structure as a set of lumped linear springs in the fixed frame. the flexible joint, which links the shaft with the motor, is included in the bearing stiffness matrix as a torsional spring. the motor-drive unit is introduced as a rigid body on one end of the structure. forces and torques generated by the rotation are modelled as external loads. the support structure weights 65 kg, it guarantees a stiffness at the tip of a minimum 1.5 × 105n/m in all three directions and has its first natural frequency at 28 hz. the shaft is approximately 2.5 kg and has the first natural frequency (a bending mode) at 74 hz. 3.3. the expanded model for input estimation the effects of the motor on the mechanical system are modelled as equivalent lumped inputs. in particular, we introduce a force 𝒇m (with unknown magnitude and direction) and a torque 𝑡m about the shaft rotation axis. the force and torque are applied in the node supporting the shaft on the motor side. due to the linking between the structure and the shaft (through stiffness 𝑘b), their effects are visible on the coils and therefore have an effect on the magnetic measurements. due to the simplification in the modelling of the motor effects, it is necessary to provide the model with the vector of forces and torque 𝒈 = [𝒇m, 𝑡m]. in order to minimise the impact of the mechanical measurements on the magnetic measurements, an indirect measurement of 𝒈 is performed. instead of mounting force sensors at the joints between the parts, like torque transducers [14], accelerometers are mounted at the shaft supports. therefore, accelerations are measured, and forces are estimated. to perform this operation, a kalman filter is introduced. the mechanical model of eq. (6) is expanded to include the inputs in kalman filter's estimation [13]. it is required to write the model in state-space form, collecting the variables in the vector 𝒙 = [�̇�,𝒖]. before assembling the estimator, it is suggested to convert the dynamic system in a discrete-time domain (with time step ∆t). it is, therefore, possible to write: in eq. (16), the first term stems from eq. (6), while the second term is added for the sake of state estimation. it contains the mechanical disturbances 𝐿𝒛 and the fictitious disturbances for the input ∆t 𝐼𝒘. this term is required to consider time-varying inputs in the estimation, otherwise, the inputs would obey 𝒈𝑘+1 = 𝒈𝑘. more specifically, 𝐿 can be assumed to be the identity matrix in case no external disturbances are expected, while 𝒛 and 𝒘 are zero-mean, normally-distributed disturbances with covariance matrices 𝑄𝑧 = cov(𝒛,𝒛) and 𝑄𝑤 = cov(𝒘,𝒘). the kalman filter also requires an output equation, which is in our case the subset from eq. (6) accounting for the accelerations of the nodes at which position the accelerometers are mounted. the output is expected to suffer from measurement noise, assumed to be a zero-mean, normally-distributed variable 𝒏 with covariance matrix 𝑅 = cov(𝒏,𝒏). 4. experimental validation the experimental validation campaign aims at two objectives: to evaluate the accuracy of the model in predicting the outcome of a magnetic measurement, and to characterise metrologically the effect of vibrations in the real system. regarding the model-prediction capabilities, the model is the same as the one in the design phase. it is based on conservative hypotheses for boundary condition of the structure (figure 3 and section 3.2) and without updates based on vibration measurements. the comparison between the model and the measurements is therefore expected to be consistent up to a safety factor. approaches to update the system model in order to replicate the real system more accurately are discussed in the next section. figure 3. the rotating-coil system without the magnet (top) and its magnetomechanical model (bottom). a few elements are highlighted: the force 𝒇𝐌 and the torque 𝒕𝐌 generated by the motor, the bearing stiffness matrix 𝒌𝐁, the array of coils (𝒄𝟏 to 𝒄𝟓) and the magnetic flux density 𝑩 to be measured. the rigid bodies accounting for motor and shaft ends are omitted. [ 𝒙𝑘+1 𝒈𝑘+1 ] = [ 𝐴 𝐵 0 𝐼 ][ 𝒙𝑘 𝒈𝑘 ] + [ 𝐿 0 0 ∆t 𝐼 ][ 𝒛 𝒘 ]. (16) m 𝐵𝐶5 𝑡m 𝑘b 𝑘b 𝐶1 acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 34 4.1. experimental setup as a first case study, a dipole magnet is selected. it is a 0.4 m long dipole from the cern east area, with a nominal integral field of 0.36 tm at 240 a [15]. future studies will also cover the case of quadrupoles and higher-order magnets. the magnetic flux density distribution generated by the magnet was already measured with a short rotating-coil, and pseudo-multipoles were computed up to the 5𝑡ℎcomponent. pseudo-multipoles coefficients are therefore available for the measurement model. the magnetic measurement performed with the system under investigation is a rotating-coil measurement assessing both the integral absolute field and the integral multipole coefficients. therefore, the fluxes from the main coil 𝑐1 and from the compensation scheme 𝑐1 − 𝑐3 are acquired. the 𝑘1 sensitivity factor for each of the coil of the array is calibrated in a reference magnet. in order to match the calibrated area, coil widths in the model are adjusted. this is also to decrease the performance of bucking compensation in the model, aiming at more realistic values. coefficients 𝑘𝑛 for 𝑛 > 1 are computed directly from the nominal geometry of the model. a single measurement consists in a revolution at constant speed. different speeds are considered for both the mechanical and the magnetic measurement campaigns so that, adopting the units of revolutions-per-minute, ω ∈ [30, 45, 60, 75, 90, 105, 120]. in order to prevent unpredicted phenomena (like thermal transients) from correlating with the speed wrongly, the different values for ω are evaluated in a randomised order. 20 repetitions are performed per each speed. for the mechanical evaluation of the measuring system, a leica geosystem is employed for the alignment with respect to the magnet. in addition, two tri-axial accelerometers (6g range, 0-200 hz bandwidth) are mounted on the support structure next to the coil bearings to assess the vibrations of the system. given their weight of 50 g, they have been neglected in the mechanical model. due to the low values expected for accelerations, the accelerometers have been calibrated [16] in the range of ±0.5 g (or 0.5 g 1.5 g for the axes parallel to gravity): each device was mounted on a support that allows a controlled angular displacement with respect to gravity (accuracy 0.02 mrad). dc acceleration is then measured in a series of known angular positions, so that the gravity component per each axis could be computed. the overall accuracy is estimated to be 0.0025 m/s2. this value includes the uncertainty due to the sensor noise, averaged over 20 repetitions. the impact of accelerometer accuracy on the force estimation is estimated to be about 0.3% in amplitude; it has thus negligible effects on the measurement. the main source of uncertainty for the estimation is the model itself, which is in fact, the focus of the validation procedure. 4.2. mechanical measurements results the most relevant figure of merit for the mechanical alignment of the system is the misalignment between magnet axis and coil rotation axis. after the alignment procedure, the final angle between the two axes resulted in 0.052 mrad. the positioning error is estimated to be less than 20 μm, which is the overall accuracy of the geosystem, while the linear stages can perform smaller motions relying on a sub-micrometerresolution encoder. however, these positioning errors are negligible for the proposed magnetic measurements (estimated to result in errors of the order of 10-8). the vibrations are measured for different rotation speeds of the shaft. for each of the speeds considered, the estimation procedure is performed on a series of 20 revolutions. figure 4 shows the rms of the measured vibrations and the estimated force and torque as the rotating speed function. 4.3. magnetic measurements results the main figure of merit for the performance of the measurement system is the result of the magnetic measurement. consistent with the mechanical measurements, different values are considered for the rotation speed of the shaft. the twofold scope of the campaign is to validate the robustness of the measurement results against the mechanical vibrations and the capability of the model in reproducing the system behaviour. figure 5 shows the measurements of the absolute integral field as a function of the speed. each rotation is processed independently to evaluate the dispersion caused by random figure 4. rms values for the measured accelerations (averaging all the accelerometers) and for the estimated force magnitude and torque. the rms are computed over the full bandwidth (0-200 hz) of the accelerometers. figure 5. integral absolute field resulting from measurements (top) and simulations (bottom). the measured results are presented by the distribution of 20 rotations (blue line is the average). 30 40 50 60 70 80 90 100 110 120 speed [rpm] 0.1 0.15 0.2 0.25 0.3 a cc e le ra ti o n [ m /s 2 ] rms of the measured accelerations 30 40 50 60 70 80 90 100 110 120 speed [rpm] 0.1 0.15 0.2 0.25 f o rc e [ n ] 0 0.2 0.4 t o rq u e [ n m ] rms of the estimated inputs real measurement speed [rpm] 0.36588 0.365885 0.36589 0.365895 ∫ 𝐵 d 𝑧 [t m ] 30 40 50 60 70 80 90 100 110 120 speed [rpm] 0.365116 0.365118 0.36512 0.365122 ∫ 𝐵 d 𝑧 [t m ] simulated measurement 30 40 50 60 70 80 90 100 110 120 acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 35 disturbances and noise. the distribution shown is a probability density estimate based on a normal kernel function. the overall spread concerning all the measured values is 0.4 units (10-4 of the average field), while averaging the turns, the spread is 0.2 units. moreover, the trend of the mean values as a function of the speed is not monotonic, as is the rms of the accelerations. the reason is that other phenomena are speedrelated such as the signal-to-noise ratio for the acquisition system. therefore, it is plausible to identify a relative accuracy of ±0.1 units from the mechanical vibrations on the absolute integral field. the second plot in figure 5 shows the simulated measurement of the integral field. the difference between the measured and simulated values is on average 21 units, while the spread between measurements is 0.15 units. comparing the trend, these results confirm that the model correctly predicted the amount of disturbances from mechanical vibration in the magnetic measurement. the error in the absolute field can be explained by the missing high-order harmonics in the simulation. as the absolute field is constant with respect to vibration amplitude, this is not investigated further. figure 6 shows the measurement results of the integral multipoles. the multipoles coefficients are measured with both the main coil 𝑐1 and the compensation scheme 𝑐1 − 𝑐3. it is therefore possible to appreciate the performance enhancement of the compensation in the measurement of high-order multipoles (between 10th and 15th). nevertheless, the effects of vibrations on the multipoles measured by the main coil are at most 0.1 units for the speeds considered. thus, the rotating-coil system yields measurement results better than one unit, even without a compensation scheme. as far as the simulated measurement is concerned, the multipoles from the main coil are given. thus, it is possible to appreciate the matching of the behaviour for both the multipoles values and their trends as a function of the speed. in particular, the model is slightly overestimating the effect of the mechanical vibrations on highorder multipoles, but this is consistent with the model's expected behaviour. the field harmonic of order 9 was not included in the pseudo-multipoles provided to the model. 5. conclusions the proposed magneto-mechanical model, already applied to the design of a rotating-coil system, is now validated against the actual device in a real magnetic measurement campaign. the vibrations of the system during the measurement have been recorded, and the corresponding input was estimated. the 3d magnetic flux density of the magnet was provided, and the measurement procedure was simulated. the results agree with the trends and the magnitudes of the real measurement. if a closer matching is required, the model should be updated accordingly. this updating would require a proper mechanical measurement campaign to experimentally characterise all the system features (for instance, by modal analysis, with an array of accelerometers and a controlled input). nevertheless, the real system magnetic measurements confirmed that the performance required in the design phase had been exceeded, with an overall error by mechanical vibrations estimated to a few units of 10-5. therefore, we can conclude that the proposed procedures have led to the design, construction, and commissioning of a novel rotating-coil magnetometer system with unprecedented control over some mechanical properties typically critical for the magnetic measurement result. references [1] s. russenschuck, field computation for accelerator magnets, wileyvch, 2010. doi: 10.1002/9783527635467 [2] d. popovic renella, s. spasic, s. dimitrijevic, m. blagojevic, r. s popovic, an overview of commercially available teslameters for applications in modern science and industry, acta imeko 6(1) (2017), pp. 43-49. doi: 10.21014/acta_imeko.v6i1.312 [3] j. billan, j. buckley, r. saban, p. sievers, l. walckiers, design and test of the benches for the magnetic measurement of the lhc dipoles, ieee transactions on magnetics 30(4) (1994), pp. 26582661. doi: 10.1109/20.305826 [4] n. r. brooks, l. bottura, j. g. perez, o. dunkel, l. walckiers, estimation of mechanical vibrations of the lhc fast magnetic measurement system, ieee transactions on applied superconductivity 18(2) (2008), pp. 1617-1620. doi: 10.1109/tasc.2008.921296 [5] g. tosin, j. f. citadini, e. conforti, long rotating 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100 150 200 | 𝐶 𝑛 ( r 0 )| [ u n it s] 𝒏, 𝒄𝟏, simulation 2.8 164.1 0.7 26.2 0.1 0.1 0.1 2 3 4 5 6 7 8 n [] 0 0.1 0.2 0.3 0.4 9 10 11 12 13 14 15 n [] 0 50 100 150 200 | 𝐶 𝑛 ( r 0 )| [ u n it s] 𝒏, 𝒄𝟏, measurement 4.0 166.3 0.8 26.1 0.1 0.1 0.0 2 3 4 5 6 7 8 n [] 0 0.1 0.2 0.3 0.4 9 10 11 12 13 14 15 n [] 0 50 100 150 200 | 𝐶 𝑛 ( r 0 )| [ u n it s] 𝒏, 𝒄𝟏 − 𝒄𝟑, measurement 3.6 165.0 0.8 25.9 0.1 0.1 0.0 2 3 4 5 6 7 8 n [] 0 0.1 0.2 0.3 0.4 9 10 11 12 13 14 15 n [] 30 45 60 75 90 105 120 https://doi.org/10.1002/9783527635467 http://dx.doi.org/10.21014/acta_imeko.v6i1.312 https://doi.org/10.1109/20.305826 https://doi.org/10.1109/tasc.2008.921296 https://doi.org/10.1109/tim.2008.922093 https://cds.cern.ch/record/1345967 https://doi.org/10.1016/j.nima.2020.164599 acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 36 virtual conference, palermo, italy, 14-16 september 2020. online [accessed 14 june 2021] https://www.imeko.org/publications/tc4-2020/imeko-tc42020-38.pdf [9] g. genta, 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https://doi.org/10.18429/jacow-icap2018-supag03 https://doi.org/10.1007/978-1-4419-9716-6_13 http://dx.doi.org/10.21014/acta_imeko.v8i1.654 https://doi.org/10.1109/tasc.2020.2972834 http://dx.doi.org/10.21014/acta_imeko.v4i4.239 contrasting roles of measurement knowledge systems in confounding or creating sustainable change acta imeko issn: 2221-870x december 2022, volume 11, number 4, 1 6 acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 1 contrasting roles of measurement knowledge systems in confounding or creating sustainable change william p. fisher, jr.1 1 research institutes of sweden, gothenburg, sweden; bear center, university of california, berkeley, usa; living capital metrics llc, sausalito, california 94965, usa section: research paper keywords: modelling; measurement; complexity; sustainability citation: william p. fisher, jr. , contrasting roles of measurement knowledge systems in confounding or creating sustainable change, acta imeko, vol. 11, no. 4, article 7, december 2022, identifier: imeko-acta-11 (2022)-04-07 section editor: eric benoit, université savoie mont blanc, france received july 9, 2022; in final form december 4, 2022; published december 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: william p. fisher, jr., e-mail: wpfisherjr@livingcapitalmetrics.com 1. introduction a little known but landmark article [1] defines five conditions for success in creating sustainable systems change (figure 1). different approaches to meeting these conditions can produce results that vary dramatically in their sustainability. of particular importance is the measurement knowledge infrastructure context in which the five conditions are deployed. systems change initiatives in organizations ranging from schools to hospitals to private firms typically make use of information on processes, structures, and outcomes obtained from tests, assessments, or surveys of students, patients, employees, customers, suppliers, and other key stakeholders. this information can be aggregated and reported in markedly different ways, with associated variation in its meaningfulness, utility, and consequences for success in creating sustainable change. the primary points of contrast between opposing poles of information quality can be summarized in terms of two approaches to measurement. on one end of this quality continuum are models lacking distinctions between discontinuous levels of complexity, and at the other end are models addressing these levels in ways that facilitate their practical management. the polar opposites come to the fore in oft-repeated but rarely heeded contrasts between statistical analyses of ordinal data and scientific models of interval units. these contrasts emphasize differences between unexamined assumptions about causal relationships and the meaningfulness of ordinal scores, on the one hand, and, on the other, intentional requirements of meaningful interval unit definitions [2]-[9]. where the former focuses on the concrete terms of objective facts, the latter instead focuses on the abstract and formal terms of objectively reproducible unit quantities. the statistical focus on ordinal scores manages what counts in relation to accountability reporting systems, assuming the whole is the sum abstract sustainable change initiatives are often short-circuited by failures in modelling. unexamined assumptions about measurement and numbers push modelling into the background as a presupposition rarely articulated as an explicit operation. even when models of system dynamics are planned components of a sustainable change effort, the key role of measurement is typically overlooked. the crux of the matter concerns the distinction between numeric counts and measured quantities. mistaking the former for the latter confuses levels of complexity and fundamentally compromises communications. reconceiving measurement as modelling multilevel distributed decision processes offers new alternatives aligned with historically successful efforts in creating sustainable change. five conditions for successful sustainable change are contrasted from the perspectives of single-level vs multilevel modelling: vision, plans, skills, resources, and incentives. omitting any one of these from efforts at creating change result, respectively, in confusion, treadmills, anxiety, frustration, and resistance. the shortcomings of typically implemented single-level approaches to measurement result in the widespread experience of these negative consequences. results show that new potentials for creating sustainable change can be expected to follow from implementations of multilevel distributed decision processes that effectively counteract organizational amnesia by embedding new learning in an externally materialized knowledge infrastructure incorporating a shared cultural memory. mailto:wpfisherjr@livingcapitalmetrics.com acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 2 of the parts. the scientific focus on interval quantities manages what adds up in relation to the overall mission, requiring the whole to be more than the sum of the parts. the end results of the statistical focus on ordinal scores for sustainable change involve a kind of myopia unable to focus beyond the limits of local circumstances to global concerns [10]. a systematic literature review of almost 300 articles on lean thinking practices in health care, for instance, found that “toolmyopic thinking tends to be a prevalent practice and often governs implementations” [11]. the tendency here is to not be able to see the forest for the trees. measurement is commonly defined as the assignment of numbers to observations according to a rule. decades of criticism as to the insufficiency of this definition [2]-[9] can be traced back to whitehead’s 1925 fallacy of misplaced concreteness [12]. parallel demonstrations of superior definitions of measurement dating from the 1960s have had little impact on practice [13], [14]. measurement is usually, then, deemed achieved by means of ordinal, nonlinear score models irrevocably attached to the specific questions asked, with no experimental test of causal hypotheses and no uncertainty estimates. scientific approaches instead fit data to models of interval and linear measurements whose meanings are demonstrably independent of the specific questions asked, are contextualized by tested causal hypotheses and uncertainties, and are deployed in networks of qualityassured instruments distributed throughout multilevel networks of end users [15]-[17]. contrasts between these paradigmatically opposed approaches to quantification (see table 1) illuminate new potentials for creating sustainable change. when the differences between statistical and scientific measurement modelling approaches are grasped, today's organizational cultures can be seen as having counterproductively adapted to accepting as inevitable failure to meet the conditions for successful implementation of sustainable change. in other words, because of the widespread but unexamined assumption that low quality measurement information is the best that can be expected, confusion, feeling caught on a repetitive treadmill, anxiety, frustration, and resistance are built into organizational cultures in often unnoticed but pervasive ways. organizational amnesia [18], [19] of this kind can, however, be counteracted by scientific measurement modelling approaches that retain learning and incorporate it organically into scaffolding built into the external environment as a kind of cultural memory. research into learning embodied and locally situated in the institutional environment [20]-[22] points toward new goals organizations can realistically set for achieving sustainable change using scientific measurement models instead of statistical data models. 2. typical statistical modelling 2.1. vision visualizing the future requires anticipation of highly abstract arrays of possible scenarios. the kinds of challenges that might be encountered must be conceived in conjunction with the kinds of responses needed to address them. all too often, however, vision statements focus so narrowly and myopically on local concrete circumstances [10], [11] that long-term planning, staffing, resourcing, and incentivizing are inadvertently sabotaged. figure 1. conditions for sustainable change [1] acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 3 the usual vision informing reasons why measurements are made is to gather data for analysis and summarization in reports to be used in formulating policy directives. even if this vision is informed by design thinking [23], [24], and so incorporates the elements of empathy, definition, ideation, prototyping, and testing, the focus on scores (counts and/or percentages of correct answers, or of responses in a rating category) unnecessarily limits what can be imagined and accomplished to a small fraction of the available potential [25]. that is, the restricted orientation to responses to specific questions necessarily prevents the envisioning and realization of goals that would otherwise be achievable. this is because a vision limited to statistical treatments of ordinal scores does not meaningfully model or map the substantive features of the situation of interest. the map is not the territory. mapping proceeds from concrete observations of what is happening on the ground but must aspire to represent those concrete features by identifying coherent patterns at abstract and formal levels of higher order complexity. because real world circumstances are in constant flux, meaningful and useful maps cannot remain tied to any single given set of conditions. a general continuum defining a learning progression, developmental sequence, or healing trajectory must characterize the quantitative range of variation [26]-[27]. an abstract perspective is the only way to adaptively and resiliently inform individuals and groups about where they are located relative to where they were, where they want to go, and what comes next, no matter what concrete circumstances they find themselves in. the narrow vision associated with statistically modelled ordinal scores mistakes mere numbers for quantities and pays a high price for doing so. comparability depends on all respondents answering the same questions, and standards are imagined as necessitating use of the same indicators. the resulting knowledge infrastructure is envisioned on the basis of information quality that cannot support generalized meaning, and so vision is obscured, and confusion results. 2.2. planning applications of the information obtained from scores, ratings, and percentage statistics usually focus on generalizations that assume all numeric differences of a given magnitude mean the same thing, though this assumption is usually not, and likely cannot be, substantiated. the inferential problems following from this unwarranted assumption of uniform meaning are then further compounded by the ways the scores are interpreted and applied. with no information typically made available on the uncertainty ranges or confidence intervals associated with the scores, there is no way of telling if and when numeric differences are real and reproducible, or are simply random noise. in addition, because questions are each treated separately, as domains unto themselves, no information on a learning progression, developmental sequence, healing trajectory, or other quality improvement continuum is made available. improvement efforts then can do nothing but focus directly on areas in which failure (low ratings or incorrect answers) is experienced, instead of first ensuring that prerequisite foundations for sustainable change have been put in place. the result of acting on this kind of low-quality statistical information is then to continue repeating the same pattern of efforts in precisely the endless treadmill cycle one wanted to avoid. 2.3. skills the skills required in commonly adopted statistical approaches to measurement focus on knowledge of the relevant content and processes involved for assessment and survey development, social interactions for administering those tools, operational awareness for policy formation, and data input, aggregation, analysis, and reporting. analyses may be as simple as counting correct answers or responses within rating categories, and computing percentages, or as complex as any advanced statistical method may be. the focus of data analytic skills unjustifiably presumes without evidence that results will retain their meaning across levels of complexity. but the statistical skills employed in usual approaches to measurement mistreat concrete scores as abstract quantities explained by formal theory, when they are not. that is, everyone is well aware that it is impossible to tell from my count of ten rocks whether i have more or less rock than someone with two rocks. it is also common knowledge that correct responses to ten questions cannot be understood as indicating more ability or success than correct responses to two questions, since the two groups of questions asked may vary markedly in difficulty. statistical modelling proceeds by focusing on these merely numeric data anyway, mistakenly assuming that nothing better can be done. failure to bring the needed skills to bear can only then result in anxiety, since the information produced is readily seen to be disconnected from the circumstances in which it is supposed to be applied. table 1. statistical vs scientific modelling paradigm contrasts vis a vis sustainable change conditions. sustainable change condition statistical modelling paradigm scientific modelling paradigm vision centralized gathering and analysis of ordinal instrument-dependent data for policy formation distributed network of instruments traceable to common units informs and aligns end user decisions and behaviours skills item writing & administration, response scoring, statistical summarization, reporting construct definition and modelling, instrument calibration, item banking, adaptive end user application incentives rewards for perceived goal attainment shared success in general improvements to organizational viability resources investments limited as not accountable for or expected to produce significant returns investments proportional to magnitudes of returns from improved efficiencies and market share plan interprets ordinal scores as interval and all numeric differences as meaningful; no context for improvement provided scales interval measures with individual uncertainty and data quality estimates; quantitative continuum qualitatively annotated to guide change efforts implications for managing what is measured management focuses on moving numbers that matter within restricted domain of limited observations, sometimes at expense of mission management focuses adaptively on relevant tasks representing mission, skipping tasks irrelevant to challenges of the moment implications for communication ordinal scores interpreted as interval tied to limited number of particular items results in obscure and difficult comparisons interval measures interpreted relative to entire bank of calibrated items opens up clear and transparent opportunities for learning acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 4 2.4. resources resources invested in the statistical modelling approach to creating sustainable change are typically focused on minimizing expenditures in producing a one-time snapshot of the state of things used for setting policy going forward. no specific forms of returns are expected, so the investments made are not usually accountable except as expenses, which are kept to the lowest possible levels. the information produced is typically used only as a conspicuously displayed expression of the fact that attention is being focused in some way on matters of concern to an interested party. but with vision, skill sets, and plans limited to low quality ordinal scores whose meanings are tied to the particular questions asked, the structural limits imposed on potential returns means only limited investments of resources can be justified and the usual result is a frustrating inability to advance. 2.5. incentives in the context of the usual approach to statistical data modelling, incentives are usually cast in relation to achieving results defined in terms of counts, scores, or percentages. student proficiency scores or patient/customer/employee satisfaction or performance ratings are interpreted as evidence of achievements that are then rewarded by recognition, bonuses, promotions, etc. but because the data are tied to responses to specific questions, and because they are moreover ordinal, nonlinear, and not mapped to variation in meaningful amounts of a measured construct, incentive systems like this are easily gamed. even without the advantages of a perspective informed by scientific measurement, this general management problem is recognized as leading to confusion, conflict, inefficiency, and a lack of focus [28]. in education, for instance, having students memorize tasks known to be included in the items on a test can inflate scores without, however, actually improving proficiency. in more extreme cases, teachers and principals have conspired to change student test scores. similarly, customer satisfaction surveys are often accompanied with requests for ratings at a specific level or higher. the explicit goal is to create an appearance of success that can be rewarded in a public way that conveys an atmosphere of positive progress and overcomes resistance, even when the substantive failure to change anything is readily apparent to everyone involved. because the vision, skills, and incentives are all focused on specific and discrete concrete issues that can never adequately represent the abstract and formal levels of complexity, unfair biases serving some agendas and undermining others will likely promote resistance of some form or another as the usual consequence. 3. innovative scientific modelling 3.1. vision an alternative vision as to why measurements are made focuses on modelling a decision process, calibrating instruments informing that process, distributing those instruments to front line decision makers, and gathering data for periodic analysis and summarization in reports used for quality improvement and accountability. this vision makes clear provisions for creating knowledge systems offering practical value beyond periodically produced reports. when the demands of effective knowledge infrastructures [21], [25], [29], [30] are met, data are reported at each level of complexity relevant to the demands of end users. front line managers like teachers, clinicians, and others engaged in individualized care processes need denotative facts contextualized within learning progressions, developmental sequences, disease natural histories, etc. practice management requires metalinguistic statistical summaries of interval logits reported to facilitate communication and comparability over time and space, within and across individuals, classrooms, clinics, schools, hospitals, etc. accountability requires metacommunicative theoretical explanatory power that justifies decision processes at the metalinguistic and denotative levels. to the extent this is accomplished, one might reasonably expect less confusion to be produced than is commonly associated with the statistical approach. 3.2. planning applications of the information obtained from scientifically modelled measurements require experimental tests substantiating the requirement that numeric differences of a given magnitude mean the same thing, within the range of estimated uncertainty. measurements are interpreted and applied in relation to uncertainty ranges or confidence intervals, which makes it possible to tell if and when numeric differences are real and reproducible, or are simply random noise. in addition, because questions are scaled together to delineate a learning progression, developmental sequence, or quality improvement trajectory, measurements are interpreted substantively in relation to the amount of the construct represented at each scale level. improvement efforts then can focus attention on the easiest tasks not yet accomplished. now a foundation for sustainable change has been put in place by successes experienced at lower levels of difficulty. the result is that end users' behaviours and decisions are coordinated and aligned by their shared responses to the same information. when end users can, in addition, easily learn from one another by sharing knowledge, probabilities of breaking free of treadmill cycles are increased. 3.3. skills the skills required for implementing scientific models of decision processes are considerably more technically and socially sophisticated than the skills associated with the usual statistical data modelling approach. all of the latter's skill sets are needed, as well as mastery of advanced conceptual tools involving construct mapping, assessment/survey item development, response scoring, mathematical model formulation, instrument calibration; measure, uncertainty, and data quality interpretation; knowledge system development, administrative and interpretation guidelines, user training, etc. these skills focus on producing knowledge retaining its meaning and properties across levels of complexity, suggesting the possibility of resulting in less anxiety than has been the case in using the statistical approach. 3.4. resources with experience, resources invested in the scientific modelling approach to creating sustainable change can be gauged for maximizing returns from ongoing improvements in efficiency and outcomes. as expectations concerning returns take shape, lessons are learned as to how the investments can be made accountable. with a vision, skill sets, and plans aimed at maximizing the value of high-quality interval measurements whose meanings are independent of the particular questions asked, investments proportionate to the expected returns can be justified, and the business plan can be scaled up as the market expands. acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 5 3.5. incentives in the context of scientifically modelling an overall decision process, incentives are shaped by involving everyone as participants in the creation of enhanced processes and outcomes. the overarching viability of the organization is placed front and centre. incentives reward generalizable innovations that improve quality. given common languages of comparison, everyone has the information they need to take responsibility for the outcomes in their care. inputs that do not positively impact qualitatively and/or quantitatively measurable affective, cognitive, behavioural, etc. outcomes can be evaluated for removal. in the traditional statistical modelling approach, the maxim "you manage what you measure" becomes a cynical motto conveying how management can be distracted into superficial issues only peripherally related to the main operational focus of the organization. in the scientific modelling context, though, managing what is measured is akin to turning a wrench fitted on the head of a bolt that needs to be tightened: the tool is fit for purpose. the distribution of instruments calibrated to a common metric informs decision processes and data sharing that everyone can learn from quickly and easily. incentives overcome resistance, then, by illuminating clear paths to forward advances increasing the pride everyone takes in their work. 4. discussion scientific modelling is superior to statistical modelling in the context of promoting sustainable change because, first, it provides a vision that encompasses the entire populations both of potential challenges that may emerge and of potential participants (employees, students, teachers, clinicians, suppliers, managers, etc.) who may engage with those challenges. this capacity follows from the focus of scientific models on the abstract construct represented in measurements, as opposed to the concrete data and specific questions focused on by statistical models. the usual statistical approach accepts ratings and scores as meaningful, even though their significance depends on the particular questions that were asked. so when challenges not represented in the questions and associated data emerge, those challenges are likely to be ignored, discounted, or distracting in ways that lead to confusion. scientific models, in contrast, inform clarity by demanding a theoretical account supported by data and expressed in comparable metrics with known uncertainties. second, scientific modelling dispels anxiety by bringing advanced expertise to bear on problem definition, construct mapping, instrument calibration, report generation, measure interpretation, and quality improvement applications. though statistical modelling skill sets may, of course, be highly developed, many change initiatives are approached with little more experience than familiarity with spreadsheets and word processors. though these latter rudimentary methods are commonly used, the importance of communicating meaningful results in well-defined terms will likely continue to exert an inexorable demand for higher quality knowledge. third, because scientific modelling supports new degrees of rigorous comparability over time, new expectations for accountability and accounting can be expected to alter the quality and quantity of resistance-countering incentives that can be offered. proportionate returns on investment will follow from fair and equitable measurements that are demonstrably reproducible and relevant to the challenges to innovation being faced. these kinds of returns should become the goal of change efforts, instead of incentive systems that can be gamed, creating the appearance of innovation by focusing on easily counted signal events, with the associated demoralizing atmosphere that goes with widely perceived unfair advantages. fourth, in the same vein, because the magnitudes of impacts are commonly estimated in the confusing terms of statistical scores, the resources brought to bear in change efforts are commonly insufficient to effect significant results, leading to continued frustration. the capacity to generalize and scale across contexts by means of a combination of explanatory theory, experimental evidence, and distributed instrumentation, however, leads to the clear definition of opportunities for investment likely to pay handsome returns. fifth, where the statistical focus on improvement planning is typically guided by nothing more than the areas of failure or low ratings, the scientific approach maps the improvement trajectory. this is done in a way that more closely informs day to day activities by indicating where a process is at relative to its goal, and showing what comes next in a logical sequence. instead of simply taking on the most difficult challenges with no attention to preparatory factors, the scientific approach attends to establishing baseline structures, processes, and outcomes in an orderly approach. differences in circumstance across situations can be accommodated via adaptive selection of relevant tasks and challenges, without compromising overall comparability. this results in a visible documentation of small gains as progress toward the goal is made, as opposed to the feeling of being on a treadmill that results from not being oriented on a clear path toward defined goals. 5. conclusion successful sustainable change initiatives depend on abilities to flexibly and 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https://www.researchgate.net/profile/william-fisher-jr/publication/259286688_imagining_education_tailored_to_assessment_as_for_and_of_learning_theory_standards_and_quality_improvement/links/5df56a2592851c83647e7860/imagining-education-tailored-to-assessment-as-for-and-of-learning-theory-standards-and-quality-improvement.pdf https://doi.org/10.1080/15366367.2011.591654 https://doi.org/10.1111/j.1745-6622.2010.00259.x https://doi.org/10.3390/su12229661 https://doi.org/10.3390/sym13081415 interlaboratory comparison results of vibration transducers between tubitak ume and roketsan acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 401 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 401 406 interlaboratory comparison results of vibration transducers between tübi̇tak ume and roketsan s. ön aktan1, e. bilgiç2, i̇.ahmet yüksel, k.berk sönmez, t. kutay veziroğlu, t. torun 1 department of calibration laboratory, roketsan missiles industries inc., ankara, turkey, son@roketsan.com.tr 2 tübi̇tak ulusal metroloji enstitüsü (ume), gebze, kocaeli, turkey, eyup.bilgic@tubitak.gov.tr abstract: this paper presents an interlaboratory comparison on vibration metrology field which can be used as a powerful method of checking the validity of results and measurement capabilities according to iso 17025 [1]. in this standard it is advised to participate in an interlaboratory comparison or a proficiency test in order to prove measurement capabilities of calibration providers. in this study it is aimed to statistically evaluate the measurements results in the scope of sinusoidal acceleration between tübi̇tak ume (national metrology institute of turkey) and roketsan as per related international standards. after statistical evaluation, for unsatisfactory results, root cause analyses and corrections to improve measurement quality are presented and conceptually explained. keywords: vibration transducer; vibration comparison; vibration metrology; vibration uncertainty 1. introduction human began to manufacture machines, and especially motors were used to strengthen them, engineers encountered vibration isolation and reduction techniques [2]. contrary to this, vibration can be generated intentionally for testing purposes to understand functional and physical response and resistibility of any system in vibration environments. for above protective or testing purposes, acceleration sensors are used to measure acceleration, vibration and shock values, which are one of the most important components used in navigation systems of missiles, aircrafts, ships and submarines. as a result of significant developments on these industries such as automotive, defence, aviation and space, the need for accurate measurement has increased and over the years, there have been several improvements in vibration measurement methods [3], [4], [5], [6] with many innovations. accelerometers can be used in varied applications and the most commonly used ones in the market are piezoelectric and capacitive type accelerometers. piezoelectric accelerometers have a more widespread usage due to their advantages such as large measuring frequency range, no need for power supply, reliability, robust design, and longterm stability. a typical response curve of a piezoelectric accelerometer is given in figure 1 [7]. figure 1. response-curve of an accelerometer the limits of the usable range are both mechanical and electrical including frequency (f), acceleration (a), velocity (v), and displacement (d); and also force of a vibration generating system. the displacement amplitude for a given acceleration is inversely proportional to the frequency 𝑑 = 𝑣 2𝜋𝑓 = 𝑎 (2𝜋𝑓)2 . (1) while displacement measurements require attention at low frequencies, it is necessary to pay attention to the acceleration level at high frequencies [8], [9]. when selecting the vibration transducer for a specific application, it is essential to pay attention to the parameters such as number of axes, measurement range, overload or damage limits, mass, sensitivity, impedance and frequency range. for reliable usage of accelerometers, a calibration plan shall be scheduled periodically by producing http://www.imeko.org/ mailto:son@roketsan.com.tr mailto:eyup.bilgic@tubitak.gov.tr acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 402 right test results to assure the process and provide metrological traceability. accurate equipment, metrological traceability, trained personnel, well defined methods, documentation, uncertainty evaluation, internal verifications can play an important role to provide accurate test results however it is also necessary to prove that the laboratory can actually produce results externally by going through comparison tests [10]. interlaboratory comparison (ilc) is the regulation, implementation and evaluation of tests or measurements of two or more laboratories on the same or a similar substance according to predetermined conditions [11]. interlaboratory comparison tests are planned according to iso 17043 [11] and the performances, 𝑬𝒏 values or zeta scores ζ of the participating laboratories are evaluated according to iso 13528 [12]. 2. back-to-back calibration method and application the purpose of vibration transducer comparison is to compare the sensitivity of accelerometer by using as a secondary level (back to back method) iso 16063:21 vibration calibration by a reference standard [13]. calibration of an accelerometer is to determine the sensitivity values at various frequencies. the reference (double ended transducer) and device under test (dut) are firmly coupled on a shaker so that both are exposed to the same mechanical motion. back-to-back calibration requires a shaker (vibration exciter), power amplifier, signal generator and fft frequency meter. currently, automated systems are also available in the market with advantages of easy operation, user friendly and short calibration time requirement. basic configuration of roketsan’s vibration transducer calibration system is illustrated in figure 2 below: figure 2. roketsan vibration transducer calibration system as shown in table 1, the system used is an automatic vibration transducer calibration system that operates between 10 hz to 5000 hz and also, the accuracy values are illustrated. brüel&kjaer 8305 s is used as a reference accelerometer. table 1. specifications performance features accuracy (10 to 2000) hz : 0.7 % ( >2 to 5 ) khz : 1.1 % acceleration, max 110 m/s2 max. transducer weight 60 g force 45 n max. displacement 8 mm environmental conditions of roketsan vibration laboratory are (23 ± 3) °c for temperature and maximum 75 % rh for relative humidity. according to iso 16063:21 the frequency and acceleration values are given below: 1. frequencies (hz): frequencies are selected from one-third-octave frequency series. in case exact frequency values are required, they are calculated for the 1/3 octave bands [14] with the formula below 𝑓 = 𝑓𝑟 ∙ 10 ( 𝑛 10 ) 𝑓r = 1000 hz (2) where n= -20, -19, …, 7 for 10 hz to 5 khz. 2. acceleration (m/s2): 1, 2, 5, 10 or their multiple of tens. 100 m/s2 is recommended. the main principle of back-to-back calibration is direct comparison of indicated sensitivity values between reference transducer and dut transducer. the applied vibration to each transducer is identical and if the sensitivity of the reference transducer is known, the sensitivity of the dut can be obtained by using the following equation: 𝑆𝐷𝑈𝑇 = 𝑆𝑅𝐸𝐹 · 𝑉𝐷𝑈𝑇 𝑉𝑅𝐸𝐹 (3) 𝑆𝐷𝑈𝑇 : sensitivity of device under test 𝑆𝑅𝐸𝐹 : sensitivity of reference accelerometer 𝑉𝐷𝑈𝑇 : electrical output of device under test 𝑉𝑅𝐸𝐹 : electrical output of reference accelerometer even though above approach is suitable for a single frequency value, it may take excessive time to perform this operation at all frequency values. hence, dual channel fft analysis is used to monitor fast frequency response functions in amplitude and phase angle in shorter time period. 3. uncertainty approach as it can be observed over the uncertainty budget given in table 2, one of the maximum uncertainty contribution comes from reference transducer set. furthermore, voltage ratio measurement affects the measurement results. influence on voltage ratio http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 403 from temperature variation, gravitational acceleration, distortion, transverse acceleration, non-linearity effects shall be added to the uncertainty budget. the following contributions are taken into account while calculating the measurement uncertainty budget in the calibration of accelerometers. table 2. uncertainty budget quantity 𝑿𝒊 definition standard uncertainty 𝒖(𝒙𝒊) probability distribution sensitivity coefficient 𝒄𝒊 uncertainty contribution 𝒖𝒊(𝒚) u(sref) the calibration uncertainty of reference transducer set 0.5 normal 1 0.250 u(sref,s) the uncertainty due to drift of reference transducer set and amplifier 0.08 rectangular 1 0.046 u(sa,kal) the calibration uncertainty of conditioning amplifier 0.09 rectangular -1 0.045 u(vr) the uncertainty from voltage ratio 0.08 rectangular 1 0.046 u(vr,t) temperature influence on voltage ratio measurement 0.2 rectangular 1 0.173 u(vr,s) voltage ratio measurement from maximum difference in reference level 0.2 rectangular 1 0.115 u(vr,n) voltage ratio measurement from mounting parameters 0.3 rectangular 1 0.173 u(vr,d) voltage ratio measurement from acceleration distortion 0.0024 rectangular 1 0.001 u(vr,v) voltage ratio measurement from transverse acceleration 1.2 special 1 0.283 u(vr,e) voltage ratio measurement from base strain 0.05 rectangular 1 0.029 u(vr,r) voltage ratio measurement from relative motion 0.05 rectangular 1 0.029 u(vr,l) voltage ratio measurement from non-linearity of transducer 0.03 rectangular 1 0.017 u(vr,i) voltage ratio measurement from non-linearity of amplifiers 0.03 rectangular 1 0.017 u(vr,g) voltage ratio measurement from gravity 0.03 rectangular 1 0.017 u(vr,b) voltage ratio measurement from magnetic field effect of the vibration exciter 0.03 rectangular 1 0.017 u(vr,e) voltage ratio measurement from other environmental effects 0.03 rectangular 1 0.017 u(vr,r) voltage ratio measurement from residual effects 0.03 rectangular 1 0.017 u(vr,re) repeatability 0.17 normal 1 0.098 combined uncertainty of measurement 𝑢𝑡 0.48 expanded uncertainty of measurement u, k = 2 0.97 http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 404 reference transducer and if exists, its conditioner should be calibrated as a set by a primary level laboratory. the measurement uncertainty 𝑢(𝑆𝑅𝐸𝐹) is presented in the calibration certificate of the reference transducer shall be added to uncertainty budget as divided by reliability coefficient (95% confidence level, k = 2). 𝑢(𝑉𝑅,𝑣 ) is the uncertainty contribution due to transverse accelerations. transverse vibration 𝑎𝑇 is maximum 10 % for vibration exciter. transverse sensitivity for reference transducer 𝑆𝑣,𝑅𝐸𝐹 is maximum 2 % and the device under test 𝑆𝑣,𝐷𝑈𝑇 is maximum 5 %. using the formula below, the uncertainty can be evaluated as 1.2 %. 𝜎 = √(𝑆𝑣,𝐷𝑈𝑇 2 + 𝑆𝑣,𝑅𝐸𝐹 2 ) 𝑎𝑇 2 (4) repeatability ( 𝑢(𝑉𝑅,𝑅𝐸𝑃 ) ) is an experimental standard deviation of the arithmetic mean to the uncertainty. it is an inevitable contribution for an uncertainty budget. the model function is given below. 𝑆𝐷𝑈𝑇 = 𝑆𝑅𝐸𝐹 ∙ 𝑆𝐴1 𝑆𝐴2 ∙ 𝑉𝐷𝑈𝑇 𝑉𝑅𝐸𝐹 ∙ 𝐼1 ∙ 𝐼2 … ∙ 𝐼𝑀 (5) 𝐼𝑖 = 1 − 𝑒2,𝑖 1 − 𝑒1,𝑖 (6) 𝑒𝑖 ; indicates the i th error contribution the uncertainty budget for vibration transducer is in table 2 for 10 hz to 1000 hz. combined uncertainty (𝑢𝑡 ) and the expanded uncertainty (𝑈) (k=2, 95% confidence level) can be calculated with following formulas (7) and (8) according to ea-4/02 [15]: 𝑢𝑡 = √𝑢 2(𝑆𝑅𝐸𝐹 ) + 𝑢 2(𝑆𝑅𝐸𝐹,𝑆) + (𝑢𝑉𝑅 ) 2+.. (7) 𝑈 = 2 ∙ 𝑢𝑡 (8) 4. comparison results the technical protocol [16] specifies in detail, the aim of the comparison, the transfer standard used, time schedule, the measurement conditions and other subjects. the frequency range covered by the requirements stated in the technical protocol has been carried out with tübi̇tak ume and roketsan. the model of transfer standard used is b&k 4371. the pilot laboratory is tübi̇tak ume, which is primary laboratory in turkey. since roketsan performs related measurements with lower uncertainty than any other secondary level calibration providers, an interlaboratory comparison with tübi̇tak ume as primary level laboratory was necessary to understand roketsan’s reliability of accuracy level. figure 3 presents the calibration results of sensitivity obtained for a transfer standard. the results can be observed in table 3. figure 3. measurement results from tübi̇tak ume and roketsan table 3. measurement results from tübi̇tak ume and roketsan frequency ume; reference value roketsan 𝑬𝒏 (hz) (pc/(m/s2) (pc/(m/s2) 10 1.0046 1.0063 0.13 12.5 1.0042 1.0110 0.51 16 1.0043 1.0087 0.33 20 1.0029 1.0080 0.38 25 1.0016 1.0063 0.35 31.5 1.0007 1.0070 0.47 40 0.9989 1.0060 0.54 50 0.9981 1.0060 0.60 63 0.9956 1.0040 0.63 80 0.9935 1.0010 0.57 100 0.9919 1.0010 0.69 125 0.9905 0.9993 0.67 160 0.9893 1.0010 0.89 200 0.9872 0.9957 0.65 250 0.9847 1.001 1.24 315 0.9818 0.9923 0.80 400 0.9813 0.9918 0.80 500 0.9801 0.9905 0.80 630 0.9800 0.9883 0.64 800 0.9776 0.9873 0.75 1000 0.9771 0.9854 0.53 1250 0.9746 0.9834 0.53 1600 0.9759 0.9826 0.40 2000 0.9757 0.9802 0.27 2500 0.9729 0.9814 0.51 3150 0.9762 0.9849 0.52 4000 0.9800 0.9842 0.25 5000 0.9800 0.9814 0.08 0,95 0,96 0,97 0,98 0,99 1 1,01 1,02 1 0 1 6 2 5 4 0 6 3 1 0 0 1 6 0 2 5 0 4 0 0 6 3 0 1 0 0 0 1 6 0 0 2 5 0 0 4 0 0 0 s e n si ti v it y ( p c /( m /s 2 ) frequency (hz) roketsan ume http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 405 the uncertainty values for laboratories are given below in table 4: table 4. uncertainty values ume; reference laboratory frequency (hz) f ≤ 1250 1250 < f ≤ 5000 uncertainty 0.9% 1.1% roketsan frequency (hz) f ≤ 1000 1000 < f ≤ 5000 uncertainty 0.97% 1.3% the performances of the measurements can be evaluated as described in iso 13528 standard with the following equation (9). the most common statistical approach to understand capability of a laboratory is calculating 𝐸𝑛 values and shall be below or equal to one. 𝐸𝑛 = 𝑋rok − 𝑋ume √𝑈rok 2 + 𝑈ume 2 (9) 𝑋rok: the mean value of roketsan 𝑋ume : the mean value of reference laboratory (ume) 𝑈ume: the measurement uncertainty of reference laboratory (ume) 𝑈rok: the measurement uncertainty of roketsan calculated 𝐸𝑛value at 250 hz is 1.24 and the rest of the other frequencies are satisfactory |𝐸𝑛 | < 1 in table 3. after receiving unsatisfactory result only for 250 hz frequency, root cause analysis or some corrective actions will be carried out to improve measurement system of roketsan inc. 5. corrective action nonconformity in iso 9001 is defined as the failure to meet one or more requirements [17]. as9131 “nonconformance data definition and documentation” classify the nonconformity process codes (shipping and transportation, manufacturing, document preparation), cause codes (machine, management, people, material, method, environment, measurement), corrective action codes (machine, management, people, material, method, environment, measurement) [18]. root cause analysis is the process of identifying casual factors using a structured approach with techniques designed to provide a focus for identifying and resolving problems [19]. it is essential to determine the root cause and create a corrective action plan in order to eliminate the causes of nonconformity before occurring again or in another field. principles of continuous improvement and monitoring of efficiency are important for the continuity of management systems. when comparison results are not satisfactory, a non-conformance record shall be issued and action process shown in figure 4 shall be started in order to find a solution to keep system reliable. a method such as pareto chart, 5 whys, fishbone diagram, scatter plot diagram, failure mode and effects analysis (fmea) for determining root cause of problem should be applied to gain appropriate vision for detecting and removing problem. figure 4. nonconformance process among all error source possibilities, sensitivity value of the reference transducer set had top priority to check since calibration status was close to calibration due date. after forwarding this equipment to primary laboratory for re-calibration, although 1 year has passed between two calibrations, it has been observed that previous sensitivity value at 250 hz had been changed from 0.1312 pc/(m/s2), to 0.1307 pc/(m/s2). when considering last two calibration certificates difference, higher measurements result change have been obtained and the reference value has shifted unlike assumption for drift may occurred during 1 year. above condition was considered as main reason for the detected nonconformity. verification of the vibration system has a vital role for getting accurate measurements, the reference accelerometer used in calibration (working standard) is connected back-to-back with the reference accelerometer. subsequent verifications compare the first results to the new results and accept the results whether it deviates less than 0.8 %. the controller checks that the standard deviation of the measurements is less than 0.2 %. when the fishbone diagram is applied, the root causes are seen in figure 5. after an extensive training for all operators, gage r&r application indicators showed competency of appraisers are satisfactory. after this, temperature gradient of measurement room was examined and it has seen that there is no need for action on temperature subject. regarding mechanical effects, the torque value was adjusted to 2 n m as desired precisely and it is definition of nonconformance determining root cause identification of corrective action effectiveness of corrective action http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 406 confirmed that requirement of standard have been met. figure 5. fishbone diagram as a result of system review as detailed above, verification and calibration issue had been estimated as only root cause which gave rise to unsatisfactory 𝐸𝑛 value at 250 hz frequency measurement with the new calibration results, we have confirmed that there is a drift in the value of the reference. as a result of evaluation, it is decided to perform detailed investigation on root cause of drift on reference sensor. since the reason is not fully understood, it is decided to reorganize interlaboratory measurement to receive satisfactory 𝐸𝑛 values. 6. summary further aspect could be considered to understand the unsuccessful results at 250 hz. further study may cover participation in a new comparison test and in case of another insufficient result, decreasing the calibration period, increasing the measurement uncertainty due to drift of reference transducer set can be taken as further actions. the results produced by the laboratory become valid with comparison tests as well as the method of measurement, competency of appraisers, and calculated measurement uncertainty, suitability of the equipment used, calibration and traceability. since iso 17025 wants also a risk and opportunity based approach, proficiency testing can be used as a training and risk tool. 7. references [1] iso 17025:2017 general requirements for the competence of testing and calibration laboratories [2] measuring vibration, brüel&kajer, www.bk.com [3] x.bai, “absolute calibration device of the vibration sensor”, the journal of engineering, 2018 [4] v.mohanan, b.k. roy, v.t. chitnis, “calibration of accelerometer by using optic fiber vibration sensor”, applied acoustics, 28, pp. 95-103, 1989 [5] r. r. bouche, “calibration of vibration and shock measuring transducer”, the shock and vibration information center, 1979 [6] k.havewasam, h.h.e. jayaweera, c.l. ranatunga, t.r. ariaratne, “development and evaluation of a calibration procedure for a 2d accelerometer as a tilt and vibration sensor”, proceedings of the technical sessions, 25, pp. 53-62, 2009 [7] c. vogler, “calibration of accelerometer vibration sensitivity by reference, college of engineering, 2015 [8] w. ohm, l.wu, p. henes, g. wonk , “generation of low-frequency vibration using a cantilever beam for calibration of accelerometers”, journal of sound and vibration 289, pp. 192–209, 2006 [9] n. garg, m.i. schiefer, “low frequency accelerometer calibration using an optical encoder sensor”, measurement, vol. 111, pp. 226-233, 2017 [10] k. b. sönmez, t. o. kılınç, i̇.a.yüksel, s.ö.aktan, “inter-laboratory comparison on the calibration of measurements photometric and radiometric sensors”, international congress of metrology, 2019 [11] iso/iec 17043:2010, “conformity assessment general requirements for profiency testing” [12] iso 13528:2015, “statistical methods for use in profiency testing by interlaboratory comparisons” [13] iso 16063:21, “vibration calibration by a reference standard” [14] iso 266, “acoustics preferred frequencies” [15] ea-4/02, “evaluation of the uncertainty of measurement in calibration” [16] ume-g2ti-2018-01, “technical protocol of the interlaboratory comparison on acceleration”, 2018 [17] iso 9001:2015, “quality management systems” [18] as9131:2012, “nonconformance data definition and documentation” [19] m.a.m.doggett, “a statistical comparison of three root cause analysis tools”, journal of industrial technology, vol.20, no:2,2004 inexperienced personnel temperature change during calibration power, other utility variations calibration drift of reference value verification torque value mounting personnel measurement environment setup 250 hz en >1 http://www.imeko.org/ http://www.bk.com/ design optimisation of a wireless sensor node using a temperature-based test plan acta imeko issn: 2221-870x june 2021, volume 10, number 2, 37 45 acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 37 design optimisation of a wireless sensor node using a temperature-based test plan lorenzo ciani1, marcantonio catelani1, alessandro bartolini1, giulia guidi1, gabriele patrizi1 1 department of information engineering, university of florence, via di santa marta 3, 50139, florence, italy section: research paper keywords: fault diagnosis; precision farming; temperature; testing; wireless sensor network citation: lorenzo ciani, marcantonio catelani, alessandro bartolini, giulia guidi, gabriele patrizi, design optimisation of a wireless sensor node using a temperature-based test plan, acta imeko, vol. 10, no. 2, article 7, june 2021, identifier: imeko-acta-10 (2021)-02-07 section editor: giuseppe caravello, università degli studi di palermo, italy received january 14, 2021; in final form june 7, 2021; published june 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: gabriele patrizi, e-mail: gabriele.patrizi@unifi.it 1. introduction nowadays, automatic measurement systems and condition monitoring (cm) tools have become valid and reliable means extensively used in several internet of things (iot) applications in different fields of the industry 4.0 scenario [1]-[10]. the continuous monitoring of both environmental conditions and soil parameters has become extremely important in agriculture applications [11]. according to [12] the environmental factors such as temperature and humidity have a deep influence on plant pathogens such as bacteria, fungi, and viruses. moreover, the continuous monitoring of the soil parameters allows to automatise irrigation and consequently minimise the water waste [13]-[15]. lezoche et al. [16] explained in detail the several advantages achieved integrating internet of things (iot) technologies inside the agricultural industry, such as productivity improvements, soil conservation, water saving and minimisation of plant diseases. usually, a wireless sensor network (wsn) is designed and implemented to monitor the crop. the network has to endure harsh outdoor conditions, facing both hot summers and cold winters. at the same time the network has to guarantee service continuity ensuring accurate and reliable data. according to [17] an optimal sensor node for agricultural applications should composed by the following units: a power unit, a processing unit, a memory unit, a sensing unit, and a communication unit. in particular, the thorough analysis presented in [17] highlights the importance of using soil moisture, relative humidity, temperature, and gas sensors. recent literature has plenty of papers focusing on the design of innovative wireless network for agricultural purposes. each of the papers deals with the optimisation of one particular aspect of the network. the nodes deployment is one of the most critical design aspect since it severely affects connectivity, coverage area and reliability of the entire network [18]-[20]. some papers such as [21], [22] introduce new routing strategies to solve classical drawbacks of wsns and optimise the transmission based on the actual nodes deployment. another fully discussed problem regards the optimisation of power consumption which is solved with many different solutions [23]-[25]. for instance, in [26], the authors propose a thermal modeling and characterisation for designing reliable power converters, while [27] focuses on risk abstract the introduction of big data and internet of things has allowed the rapid development of smart farming technologies. usually, systems implemented in smart farms monitor environmental conditions and soil parameter to improve productivity, to optimize soil conservation, to save water and to limit plant diseases. wireless sensor networks are a widespread solution because they allow to implement effective and efficient crop monitoring. at the same time, wireless sensor networks can cover large area, they can ensure fault tolerance and they can acquire large amount of data. recent literature misses to consider the testing of the hardware performances of such systems according to the actual operating conditions. the effects of a harsh environment on the dynamic metrological performances of sensor nodes are not sufficiently investigated. consequently, this work deals with the electrical design optimization of a sensor node by means of thermal test used to reproduce the actual operating conditions of the nodes. the results of the node characterization through thermal tests are used to improve the node’s design and consequently to achieve higher performances in harsh operative conditions. mailto:gabriele.patrizi@unifi.it acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 38 analysis of photovoltaic panels. in [28], a wireless charging method for the batteries management in agriculture applications is presented. other papers choose to optimise the design based on the type of plantation. for instance, the design of a low-power sensor node for rice field based on hybrid antenna is presented in [29]. jiaxing et al. [30] improves the design of sensor nodes in litchi plantation dealing with the coverage area of each node and the micro-irrigation management efficiency. in [31] a low-cost weather station for edamame farm is presented and compared with commercial systems. as presented above, many recent works deal with design and development of wsn for precision farming. quite the opposite, the concept of testing the hardware performances according to the actual operating conditions is not adequately addressed. the effects of the operating environment on the dynamic metrological performances of wsn are not sufficiently investigated. as observed in previous works on similar systems [32]-[36], environmental stresses such as temperature, humidity, vibration and mechanical shocks deeply influence both reliability and metrological performances of low-cost electronic components, leading to loss of calibrations, measurement variability and a significant growth in component failure rate. trying to fill these needs, this paper deals with the electrical design optimisation of a sensor node using thermal test. the agriculture field of applications is taken into account in order to customise the test plan and characterise the node under its actual operating conditions. the results of the node characterisation through thermal tests are used to improve the node’s design and consequently to achieve higher performances in harsh operative conditions. unfortunately, there are no international standards regarding environmental tests of wsn, as well as customised standard concerning electronic component testing for agricultural applications are not available. for these reasons, this paper proposes a customised test plan and test-bed for the performances characterisation of a sensor node under temperature stress. the paper is organised as follows: section 2 illustrates the initial design of the sensor node developed in this work. section 3 explain the proposed temperature-based test plan composed by three different test procedures (namely t.1. – t.2. – t.3.). section 4 summarises the main results of the tests and it proposes some design improvements to optimise the performances of the node. finally, in section 5 conclusions are drawn. 2. sensor node developed in this work typically, the classical wsns are implemented using a single central node (access point ap) directly connected to all the other nodes (which are called peripheral) in the network. the peripheral nodes use a set of several sensors to acquire a large amount of data, then they send the data to the ap which must collect and store them [25], [37]. the main drawbacks are the limited coverage area and the restricted number of nodes. an advanced wsn architecture is the one based on mesh topology, generally called wireless mesh network (wmn). a wmn is an optimal solution when it is required to monitor large geographical areas. more in detail, a wmn is a self-organised and self-configured system made up by lots of peripheral nodes and a single central node (called root node in the following) that manages the whole network. every node is able to interact with the nearby nodes, using them to reach the root node trough indirect paths, allowing large-area coverage [38]-[40]. furthermore, wmns use several near nodes and dynamic routing tables to achieve high-frequency transmissions, high bitrate, full scalability and low management cost [41]-[43]. figure 1 shows the block diagram of the developed sensor node. it is composed by the following units: • a power supply unit composed by a photovoltaic panel, two lithium batteries, a “batteries management system” (bms) and a “maximum power point tracking” (mppt). • a set of sensors, including an air temperature and humidity sensor, a soil temperature transducer, a soil moisture sensor and a solar radiation sensor. • an external antenna. • a radio and processing unit which is the real core of the sensor node and it is based on the esp32 system-on-achip microcontroller by “espressif”. the microcontroller is mounted on an evaluation board used for software programming by means of a usb-touart bridge controllers. the evaluation board also includes pin interface and power supply by means of an ams1117 ldo. two 8-channel 12-bit sar adcs and two 8-bit dacs are embedded in the esp32. a customised interface board is used to connect the power unit and the sensors unit to the esp32 microcontroller. the network works taking into account two alternative operating phases: a 10 minutes “sleep phase” in which almost all node functionalities are disabled to save energy; and an “active phase” in which the sensors acquire data, the microcontroller elaborates and transmit them to the root node. this type of functioning minimises the duty cycle of the network, allowing a reasonable overheating of the hardware and saving batteries power. figure 2 shows two images of the developed sensor node. a detail of the system is illustrated on the left side, while the right image shows the installation on the field. figure 1. block diagram of the designed sensor node, including power management systems, radio and processing unit, sensors unit and an external antenna. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 39 3. temperature-based test plan a temperature-based test plan was developed in this work to optimise the design of a sensor node for smart farm applications. temperature is the optimal stress condition to characterise the hardware of the sensor node and to investigate the weaknesses of the system. in fact, in compliance with the physics of failure of electronic devices, the main failure mechanisms of this kind of systems are intrinsically related to temperature [44], [45]. temperature is the key acceleration factor for many failure mechanisms such as open/short circuit, silicon fracture, electrostatic discharge (esd), dielectric charging and many others. all these failures could be easily triggered when the temperature reaches high values. consequently, temperature is the optimal stress since it allows to characterise both operational performances and reliability at the same time. information acquired during the electrical characterisation under temperature stress are extremely useful to improve the design of the system. a customised automatic measurement set-up was developed for the acquisition of the key parameter during the temperature tests (see figure 3). a climatic chamber was used to generate the thermal conditions described above. a datalogger equipped with ten k-type thermocouples was employed to monitor the temperature in some critical points of the developed system. the other equipment illustrated in figure 3 and used during the tests are a power supply generator, an oscilloscope, a set of multimeters, a current generator and a waveform generator. furthermore, a root node and a laptop were used to manage the network functionalities and acquire the data. international standards expressly related to smart agriculture systems are not available. consequently, several standards that cover similar area were used as guidelines during the design of the test plan, as follow: • mil-std 810g (2008) [46] is a guideline for any kind of environmental stress tests • iec 60068-2-14 (2011) [47] provides general procedures for temperature testing • iec 60068-5-2 (1990) [48] is a guide to drafting of test methods • iec 60068-2-2 (2007) [49] regarding the dry heat test conditions • iec 60068-2-38 (2009) [50] provides detailed procedures for temperature cyclic test • jedec jesd22 a104e (2014) [51] covers the temperature test of semiconductor devices • iest-rp-pr-003.1 (2012) [52] defines a temperature step-stress profile for accelerating life test. the developed test plan is based on the aforementioned standards, and it is tailored on the practical application scenario. in particular, the nodes will be deployed in open field, and they will be exposed to harsh environmental conditions. consequently, the test plan was developed considering the real operating conditions of the node, which must endure extremely high temperature during summer days, and extremely low temperature during winter days. moreover, the test plan must take into account also the range of guaranteed operability of the component that make the system, as follows: • microcontroller and electronic boards (up to 85 °c). • batteries (from -10 °c to 50°c). three temperature-based test procedures were developed, namely a positive temperature step-test from 20 °c to 80°c (test t.1), a back and forth temperature step-test from -10 °c to 80 °c and then again to -10 °c (test t.2) and a temperature cyclic test with restricted temperature range only for battery testing (test t.3). 3.1. test t.1. positive temperature step-test in this test procedure the devices under test are two identical sensor nodes. the only different between the two boards is the ldo that manage the power supply of the electronic board. the aim of this test was to characterise the main electrical parameters of the nodes. only the electronic hardware was tested, the batteries and the solar panel were not located inside the climatic chamber. the first node was supplied by the ldo provided by the manufacturer of the evaluation board (ams1117 ldo “former ldo” in the following) and the other one was equipped with a ap2114h ldo (“new ldo” in the following). the complete temperature profile t.1. is illustrated in figure 4, where the blue arrows highlight the temperature step and the exposition time. the test procedure t.1. starts at 20 °c which is generally the room temperature. the first step consists in a 5 °c raising temperature lasts 10 minutes. figure 2. pictures of the sensor node designed in this work. the left figure shows a detail of the boards enclosed inside a waterproof case, while on the right side the complete system installed on the field is illustrated. figure 3. measurement setup proposed in this work to characterise the designed sensor node under temperature stress. figure 4. test procedure t.1. positive temperature step-test from 20 °c up to 80 °c. ime min e m p e ra tu re minutes at onstant temperature step in minutes acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 40 the rise speed is intentionally kept low to allow components temperature to increase together with chamber temperature. then 20 minutes of exposition time at the reached temperature are required to ensure at least two active phases after temperature stability. the two previous steps are repeated up to 80 °c, alternating a 5 °c step (10 minutes) and a 20minute exposition time. 3.2. test t.2. back and forth temperature step-test the test procedure t.2. illustrated in this section is an extension of the test procedure t.1. the differences between the back and forth temperature step-test t.2. and the previous-described procedure are illustrated in the following: • temperature range of the test procedure t.2. is from -10 °c to 80 °c. • the test procedure t.2. is repeated back and forth, which means that it starts at -10 °c, then it reaches 80 °c following a step increase, and finally it decreases again to -10 °c following the same steps. • test procedure t.2. is characterised by a rise rate and a lowering rate between one step and the following equal to 2 °c/min. • the exposition time at constant temperature of test procedure t.2. is reduced to only 10 minutes. the complete test profile of the procedure t.2. is illustrated in figure 5, highlighting the back and forth trend used to investigate hysteresis behaviour. during this test procedure, only the processing unit of the node was located inside the climatic chamber. in fact, the main purposes of this procedure were to test the performances of the analog-to-digital converter (adc) and digital-to-analog converter (dac) embedded in the microcontroller esp32. 3.3. test t.3. temperature cyclic test many papers in recent literature agree that temperature is the key factor in battery degradation [53]-[56]. for this reason, the test procedure t.3. was developed only for battery characterisation. test procedure t.3 is based on two consecutive cycle. the minimum temperature is -10 °c, while the maximum temperature is 50 °c. this limited range was designed to satisfy battery safety requirements; nonetheless, it is well representative of the actual operative temperature in agriculture field. the rise rate and the lowering rate is 2 °c/min, and the exposition time at the minimum and maximum temperature is 30 minutes. test procedure t.3. is the only procedure in the proposed test plan in which the temperature changes linearly between the minimum and maximum values of the range (without steps). figure 6 shows the temperature profile of test t.3. highlighting two cyclic repetitions in the range [-10 °c; 50 °c]. 4. results and discussion in this section the main results achieved during the test are illustrated. moreover, some design improvements are proposed in order to optimise the performances of the node under the actual operating conditions. 4.1. test t.1. main results and proposed improvements the node was tested with two different ldos: the one provided by the manufacturer and a new one. the aim of this procedure is to investigate if the new ldo provides significant upgrades with respect to the former ldo and to evaluate the effect of the temperature, during the step-stress test. preliminary results regarding this test have been illustrated in [57]. figure 7 shows the comparison of the current consumption of the two boards (blue and green lines) during six cycles of active and sleep phases, with a corresponding chamber temperature from 55 ° c to 65 °c (red line). moreover, figure 7 highlights the benefits introduced by the new ldo in both active and sleep phases. in fact, the new ldo allows an average decrease of the absorbed current of 2 ma. figure7 shows the most striking results discovered during the test phase, which is the presence of a current step-up (discovered in both the sensor nodes) at a certain temperature. in case of the former ldo, the step-up occurs approximately around 63 °c, while the new ldo is subjected to this phenomenon at lower temperature (approximately 58 °c). indeed, focusing only on the sleep phase, as shown in figure 8, it is possible to identify an unexpected increase of about 4.5 ma of the current above a specific temperature in both nodes. during the cooling phase of the chamber the current consumption suddenly decreases assuming the previous value. after a deep analysis widely explained in [57], this anomaly is due to an unexpected activation of the usb-to-uart bridge controllers (cp2102n) integrated in the evaluation board. figure 5. test procedure t.2. back and forth temperature step-test from -10 °c up to 80 °c and then back again to -10 °c. figure 6. test procedure t.3. temperature cyclic test characterised by two tests repetitions. figure 7. current consumption of the two boards (blue and green lines) on the left y-axis while the right y-axis shows the temperature variation of the chamber (red line). ime min e m p e ra tu re minutes at onstant temperature step at rate min ime min e m p e ra tu re min min acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 41 the controller should only be activated (or enabled) in case of a device connection to the usb port. the 3.3 v output of the ldo supplies both usb-to-uart controller and microcontroller. the cp2102n controller is enabled only in case a usb device is connected to the board (vusb). under typical operating conditions, the usb provides a 5 v voltage, then a divider generates a voltage drop (vbus) as input of the 8th pin of the usb-to-uart controller. vbus is: 𝑉bus = 3.41 v . (1) the cp2102n datasheet highlights that vbus pin is considered in a high logical state (controller on) when the following relationships are satisfied: 𝑉bus > (𝑉𝐷𝐷 − 0.6 v) (2) 𝑉bus < (𝑉𝐷𝐷 + 2.5 v) (3) with 𝑉𝐷𝐷 = 3.3 v the high-state threshold can be calculated as: 𝑉th = 𝑉𝐷𝐷 − 0.6 v = 3.3 v − 0.6 v = 2.7 v . (4) the board is also powered by an external voltage of 5 v. the controller is disabled by a schottky diode (bat760-7) located between the usb connector and the external 5 v pin. this diode allows to avoid turning on the usb-to-uart bridge with an external 5v supply. furthermore, it also protects computer or other devices connected via usb from unexpected reverse current. analysing the schottky diode datasheet, it is evident that the increase of temperature produces an increase of the reverse current of the diode. for example, at 75 °c with a 5 v of reverse voltage the diode exhibits a reverse current of about 100 µa. since the usb connector is an open circuit, the reverse current of the schottky diode generates a voltage drop given by: 𝑉bus = 4.75 v ≫ 𝑉th (5) therefore, this reverse current evaluated at 75 °c is enough to enable the usb-to-uart controller. furthermore, this reverse current could be dangerous for higher temperatures because it could generate activation voltages higher than the maximum limit, leading to possible damages of the converter. consequently, the higher the temperature, the higher the diode reverse current, the higher the activation voltage of the cp2102n controller. if the temperature is higher enough to produce a reverse current which generates a 𝑉bus > 𝑉th, then the usb-touart controller turns on absorbing 4.5 ma and generating the current step shown in the previous figures. there are two possible corrective actions to delete this problem guaranteeing the proper functionalities in case of a connected usb device: • change the schottky diode with another model able to guarantee a lower reverse current. • modify the divider, for example by maintaining the ratio between the resistances but decreasing the resistance value. • design a new interface board removing the usb interface and introducing an external serial interface used only during the programming. the previous considerations explain also the reason why the current step-ups occurred at different temperature in the boards. indeed, by measuring the outputs of the two ldos, it is possible to verify a slight difference in the output voltage that leads to a different voltage threshold (4) and, consequently, a different reverse current to activate the usb-to-uart controller. 4.2. test t.2. main results and proposed improvements this test procedure was used to characterise the performances of the internal adc embedded within the esp32 microcontroller (simply referred as internal adc in the following). to this purpose, a reference signal 𝑉in = 1.5 v was provided, using a signal generator, as input of the internal adc. moreover, the same signal was also used as input of an additional analog-todigital converter (ads1115 by texas instrument) called external adc in the following. the external adc was located on the interface board. it must convert the same signal acquired by the internal adc, then the external adc transfers the digital data to the microcontroller, that store the data into a e2prom memory. the adcs acquire 512 samples every two minutes by order of the esp32 microcontroller. in this way, it is possible to acquire a large amount of data at every considered temperature step. then, mean value and standard deviation of these samples are calculated to compare the performances of the adcs. figure 9 shows a comparison between the mean value of the 512 acquisitions during each active phase of the two adcs. the blue-circle markers stand for the mean value of the internal adc acquisitions, while the star-blue markers represent the mean value of the external adc acquisitions. the right y-axis (red axis) is used to depict the temperature of the climatic chamber acquired using a k-type thermocouple and a datalogger during the test. in the initial phase of the test (room temperature), both adcs show an offset level with respect the true input value provided by the signal generator. more in detail, the internal adc is characterised by a positive offset of +40 mv which suddenly figure 8. detail of the current consumption during the sleep phases of the two boards (blue and green lines) on the left y-axis while the right y-axis shows the temperature variation of the chamber during the test (red line). figure 9. comparison of the performances of the internal (blue circle marker) and external (blue star marker) adcs at each temperature step (red line). each marker represents the mean value of the 512 acquisition at the considered active phase. ime mm de e a n a lu e o f d a u is i o n e m p e ra tu re nternal d ternal d emperature acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 42 increase when temperature is lowered. quite the opposite, the external adc has a negative offset of -2 mv. temperature has a strong influence on the mean value acquired by the internal adc (proven by a non-constant trend of the circle markers illustrated in figure 9). on the other hand, the external adc highlights a remarkable temperature stability with a semiconstant trend throughout the temperature range. table 1 compares the standard deviations of the adcs under test at some significant temperatures. the standard deviation of the internal adc is quite high and considerably influenced by the temperature of the chamber, while the external adc highlight better performances in terms of data dispersion. the introduction of the ads1115 analog-to-digital converter provides better performances in terms of offset at room temperature, thermal stability and data dispersion. for these reasons, it is recommended to integrate this chip instead of the internal adc embedded in the esp32 system-on-a-chip microcontroller. 4.3. test t.3. main results and proposed improvements this test procedure was used to characterise the performances of two different types of lithium batteries under the actual operative temperature in case of agriculture applications. the battery a is a linicomno2 type (inr18650-35e), while the battery b is a lifepo4 (htcfr26650). the battery a is characterised by high specific energy and an operating voltage range between 3 v and 4 v. instead, the battery b guarantees a constant voltage output to supply the microcontroller, but it is characterised by a low-density charge. moreover, according to the datasheet, the lifepo4 batteries guarantee good performance in a larger temperature range. during the thermal test an active load was used to discharge the batteries ensuring a constant discharge current of 300 ma. this value was chosen since it is the average current consumption of the whole system during the data transmission phase. figure 10 shows the experimental results achieved during the thermal cyclic test of the battery a (linicomno2 type) using a blue line to represent the battery voltage during the discharge. the data were compared with a reference discharge voltage (red trend) measured maintaining a constant temperature of 20 °c. as expected, the reference discharge at room temperature exhibits a linear trend characterised by a negative slope (called δ𝑉 in the following) strictly related to the constant discharge current forced by the active load. instead, the blue trend in figure 10, achieved during the thermal cyclic test, highlights some deviation compared to the nominal trend during the cold phase of the cyclic test. specifically, the “v-shape trend” highlighted by the blue line in figure 10 refers to temperature lower than 0 °c. more in detail, when temperature is lowered under 0 °c the discharge rate of the battery suddenly increases, producing a remarkable decrease of the negative slope δ𝑉. then, when temperature starts to increase, the figure shows a counterintuitive behaviour: the battery voltage increases even though the battery is always on discharge phase. this increase of discharge rate at very low temperature could become remarkable in case of an exposition for a long period with really cold temperature. for this reason, the fixture of the solar panel that charges the batteries must be oriented with a proper slope in order to optimise the charge during winter. figure 11 shows the experimental results achieved during the thermal cyclic test of the battery b (lifepo4 type) using a blue line to represent the battery voltage during the discharge. also in this case the data were compared with a reference discharge voltage (red trend) measured maintaining a constant temperature of 20 °c. as expected, the lifepo4 battery shows a constant voltage during the discharge phase. despite this, during the thermal cyclic test also the lifepo4 battery shows a “v-shape trend”. for these reasons, considering the higher specific energy of the linicomno2, the latter should be used in the sensor node. 5. conclusions the paper deals with the design optimisation of a sensor node, used in a wireless mesh network, under temperature stress. since there is not a specific standard for this kind of system, a customised test plan was developed in this work based on three temperature-based stress tests. moreover, an automatic measurement setup was designed and implemented to monitor the performances of the system during the test. the aim of the first temperature test (test t.1.) was to observe the effects of high temperature on the hardware and firmware bugs, looking for any anomalies from the correct functioning. one of the main unexpected finding is an increase of the current consumption figure 10. battery discharge test for linicomno2 battery: trend achieved at 20 °c constant temperature (red line) and trend achieved during thermal cyclic test t.3. (blue line). table 1. comparison of the standard deviations for internal and external adcs at each temperature. temperature standard deviation internal adc external adc -10 °c 4 mv 0.5 mv 10 °c 4 mv 0.6 mv 30 °c 8 mv 0.8 mv 50 °c 11 mv 1 mv figure 11. battery discharge test for lifepo4 battery: trend achieved at 20 °c constant temperature (red line) and trend achieved during thermal cyclic test t.3. (blue line). ime minutes a e r o lt a g e ermal li test a er dis arge ime minutes a e r o lt a g e ermal li test a er dis arge acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 43 during the sleep phase when temperature overpass a certain threshold. in particular, a 4.5 ma step was verified above a specific temperature. this step is not due to a permanent failure, because during the cooling phase the current returns to its normal value at approximately the same temperature. this unexpected behaviour can lead to an increase of the power consumption of the sensor node and a solution must be considered. the second temperature test (test t.2.) aimed at verifying the performances of the analog-to-digital converter (adc) and digital-to-analog converter (dac) embedded in the microcontroller esp32. the dacs does not highlight any particular problems, while the adc embedded in the esp32 shows three main drawbacks: a significant offset at room temperature, thermal instability and a remarkable data dispersion. for these reasons, it is recommended to use an external ads1115 analog-to-digital converter which has provided better performances during the test. finally, test t.3. characterised the behaviour of two different types of batteries under thermal stress focusing on the discharge rate at cold temperature. the test highlights the importance of a proper solar panel orientation to optimise the batteries charge during winter. references [1] g. d’emilia, a. gaspari, data validation techniques for measurements systems operating in a idustry 4.0 scenario a condition monitoring application, in 2018 workshop on metrology for industry 4.0 and iot, brescia, italy, 16-18 april 2018, pp. 112–116. doi: 10.1109/metroi4.2018.8428317 [2] m. carratu, c. liguori, a. pietrosanto, m. o’nils, j. lundgren, a novel ivs procedure for handling big data with artificial neural networks, in 2020 ieee international instrumentation and measurement technology conference (i2mtc), dubrovnik, 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https://www.imeko.org/publications/tc4-2020/imeko-tc4-2020-51.pdf https://www.imeko.org/publications/tc4-2020/imeko-tc4-2020-51.pdf evaluation on effect of alkaline activator on compaction properties of red mud stabilised by ground granulated blast slag acta imeko issn: 2221-870x march 2022, volume 11, number 1, 1 6 acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 1 evaluation on effect of alkaline activator on compaction properties of red mud stabilised by ground granulated blast slag sarath chandra1, sankranthi krishnaiah2 1 department of civil engineering, jawaharlal nehru technological university anantapur, anantapur, india and department of civil engineering, christ (deemed to be university), bangalore, karnataka-560029, india 2 department of civil engineering, jawaharlal nehru technological university anantapur, anantapur, andhra pradesh-515002, india section: research paper keywords: red mud; ground granulated blast slag; alkaline activator; evaluating compaction properties; assessing compaction energy citation: sarath chandra, sankranthi krishnaiah, evaluation on effect of alkaline activator on compaction properties of red mud stabilised by ground granulated blast slag, acta imeko, vol. 11, no. 1, article 22, march 2022, identifier: imeko-acta-11 (2022)-01-22 section editor: md zia ur rahman, koneru lakshmaiah education foundation, guntur, india received november 20, 2021; in final form february 20, 2022; published march 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: sarath chandra, e-mail: chandrasarath011@gmail.com 1. introduction it’s very important to develop new methods rather than traditional methods of civil engineering construction to control the utilization of virgin resources. on the other hand, handling the industrial waste materials which are generated with the new methods of construction and the unpredictable growth of industries around the globe is also essential. the best method to balance both sides of this worldwide issue is by utilizing industrial waste materials in construction industries with suitable and environmentally friendly methods. with the revolution of sustainable construction technique, in the last two decades, enormous research was carried out in utilization of various industry materials like fly ash, ground granulated blast slag, pond ash, red mud, iron ore tailings, foundry sand, etc. in various verticals or areas of civil engineering constructions like manufacture of bricks and paver blocks [1], [2], as a subgrade and sub base material in road construction [3], [4], as an embankments and backfill materials [5], [6], as a soil stabilisation technique [7], [8] and vegetation [9]. along with the industries, construction activities also increased tremendously with the advancement in technologies were utilization of raw materials increased exponentially. it directly results on another important waste material called construction and demolished waste [10]. it is important to induce the latest technologies like optical metrology which involves digital, vision and video systems in order to understand the exact behaviour of various materials. the usage of advance technologies will help to reduce the pollution and gives new solutions with an accuracy in measuring and assessing the problems in civil engineering [11]. like many industry waste materials red mud (rm) is also a type of highly alkaline (ph ranging from 10.5 to 13) industrial residue which is produced during the process of extraction of abstract any industrial waste has a potential to be used as a civil engineering material with an effective and appropriate waste management system. like many industrial wastes, red mud (rm) and ground granulated blast slag (ggbs) are some of the industrial wastes produced from aluminium and steel industries respectively. utilization of only waste materials will not be effective without a suitable stabilizer, which forced to use an alkaline activator to satisfy the needs of a building materials. this paper evaluates measurements to assess the effect of alkaline activator on the compaction properties of ggbs stabilized rm. different ratios of naoh to na 2sio3 was used as an alkaline activator with 10, 20 and 30 percentage replacement of ggbs to rm and measured the compaction properties by using a mini compaction apparatus. upon conducting standard and modified proctor compaction tests for various combinations of rm and ggbs, the compaction curves depicted that huge variation in maximum dry density and optimum moisture content with the change of ggbs percentage and different ratios of naoh to na2sio3 are measured and analysed. further the influence of compaction energy on the density characteristics of these trails were assessed for better understanding. mailto:chandrasarath011@gmail.com acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 2 aluminium from the bauxite ore [12]. rm is generally produced in the form of slurry which contains up to 40 % of water into the collection ponds situated next to aluminium industries [13]. the high alkaline nature and very fine particle size of rm along with the aluminium traces result in a threat to the environment. it is also difficult to store rm for longer periods which may create air pollution with open exposure to air and also groundwater pollution with the leachate without proper liners. annually more than 4 million tons of red mud is producing in india. on the other side, it requires huge land to dispose of and involves a good amount of money for proper waste management. the lack of appropriate waste management and storage system of rm will show adverse effects on the environment which may end up taking both many lives and huge property which was resulted in the past [14]. these negative shades of rm emphasizing to use it in civil engineering constructions where a bulk material can be used with minimal cost. on the other side, rm also shows the similar properties of clayey and sandy soils, which is a good indication to use in civil engineering applications like the construction of embankments, landfills, and different layers of road construction. however, the application of these waste materials always depends on the density characteristics. this implies the measurement and compaction characteristics of finding optimum moisture content and maximum dry density of the material. measurement of compaction characteristics is a very important parameter for any foreign material in the geotechnical application at various stages upon satisfying the standard specifications of relevant codes, this manner measurement technology plays an important role in such real time applications. for any soil or waste material upon the application of loads, there will be a change in the volume because of the expelling of air form the voids. the change in the volume of the material depends on the number of voids developed in the material, the amount of air filled in the voids and the amount of load or pressure applied. the standard and modified compaction tests show the differences in dry densities with the change of load applied for any type of material. the maximum dry density and optimum moisture content values obtained from the compaction tests form the basis to determine many strength parameters of the waste material at various construction stages. various geotechnical parameters depend on the amount of water added to the material as we know the moisture largely controls the behaviour of soils, particularly fine-grained soils. hence, it is important to understand the in-depth information on compaction characteristics of rm for the benefit of various applications as a geo-material. as very limited work was carried out in the past focusing on measurement of compaction characteristics of rm along with other waste material stabilisation in india. so, an attempt was made to determine the compaction characteristics of rm along with ggbs and alkaline activators by using mini compaction apparatus. the prototype gives a better study of the load and the material behaviour with the control of the loads applied in the study [15]. rm was replaced with 10 %, 20 % and 30 % of ggbs to its dry weight, which is commonly used as stabilising material in many civil engineering applications because of its capability in increasing the strength and bearing capacity and also good in compaction [16]. naoh and na2sio3 are used as alkaline activators which are effectively proved as good stabilisers for ggbs mixed materials in the past [17]. the alkaline activators in various proportions are used in different industry waste materials to increase the strength properties of the waste materials exponentially and to use them effectively in the field of construction [18]-[20]. to understand the effect of alkaline activators on compaction characteristics of rm, series of standard and modified proctor compaction tests were performed on various combinations made of rm, ggbs, naoh and na2sio3. 2. experimental investigations rm for this study was collected from waste disposal pond of hindustan aluminum corporation (hindalco), belgaum, karnataka, situated in the southern part of india. ggbs was procured from the jsw cement limited. ggbs is produced as a by-product or waste from the blast furnaces during the process of making iron. figure 1a shows the original images of over dried red mud and ggbs and figure 1(c) and figure 1(d) presents the scanning electron microscopy (sem) images of red mud and ggbs respectively to understand the microstructure of these materials to use as a geo material in this study. the sem images showing red mud present the particles of scattered behaviour whereas the sem images of ggbs show the smooth sharp ends which indicate that a better compaction can be attained by using both the materials with the appropriate amount of water and energy. the microstructure of these two materials supports the current study on finding the compaction characteristics of rm with ggbs and an addition of water, experiments were also performed by using alkaline activator to achieve a better density of the material upon drying. sodium hydroxide (naoh) pellets, which were put into use for this study was purchased from the prince chemical bangalore, karnataka, india, which sells pellets with 98 % purity. based on many research findings, prioritizing the economical and strength aspects, the molarity of the sodium hydroxide solution was chosen to be 8 m throughout the research. hence, molarity calculations to prepare a sodium hydroxide solution of 8 m were, 320 g of sodium hydroxide pellets have to be dissolved in 1 l of water [8 mol/l x 40 g/mol = 320 g, where 40 g/mol is the approximate molecular weight of naoh]. the sodium silicate (na2sio3) solution was purchased from the para fine chemical industries, bangalore, karnataka, india, which contains na2o = 14.78 %, sio2 = 29.46 % and water = 55.97 % by mass. before finding the compaction characteristics of alkaline activated, ggbs stabilised rm, the physical and geotechnical properties of rm and ggbs were determined to understand and classify the materials for better justification on the compaction properties. according to astm d854, the specific gravity of rm and ggbs were determined by using pycnometer and the results are presented in table 1. average of three trails was considered for all the tests to maintain accuracy in the results. compaction is considered as a very important geotechnical property to obtain the maximum dry density and to know the optimum moisture content. so, it is also important to understand both physical and geotechnical properties of the waste material used in this research work as a pre-requisite. the properties prove that the rm can be effectively used in the subgrades in place of virgin soil. according to astm d4318, d422, d1140 and d2487, the liquid limit, plastic limit and particle size distribution and soil classification was determined for both rm and ggbs and the same were presented in table 2 [21]-[25]. the rm was classified into silt of low plasticity and ggbs as silt with low compressibility according to unified soil classification system. in this study [26], [27] the compaction characteristics of all the combinations were determined by using mini compaction apparatus which is presented in figure 2. this method of acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 3 compaction is only suitable for the materials which have particle size less than 2 mm and this method is opted as both rm and ggbs particle sizes are less than 2 mm. the unique advantage of this apparatus is the low amount of material used for testing, i.e. up to 300 g instead of 3 kg of soil, which is generally used for the standard proctor compaction test. the amount of energy given by the number of blows can be easily calculated by using mini compaction apparatus. it is difficult to determine the amount of energy applied with respect to number of blows accurately by using of standard or modified proctor compaction apparatus, but this can be easily done by using the mini compaction apparatus. it is also observed better results accuracy because less material is used and not much energy is applied. this method of compaction saves both time and energy of a researcher compared to the traditional method of compaction and the characteristics can be observed with the change of number of blows in a better way. the same compacted samples after trimming can be used for the purpose finding strength parameters of the material directly. both standard and modified compaction tests can be performed by using of this mini compaction apparatus. the dimensions of this apparatus are very small as the internal diameter of the cylinder is equal to 3.81 cm, the height of the cylinder is 8 cm and it was used by many researchers in the past for the easy conduction of compaction test with less material usage and less labour and time. [28] 3. results and discussion table 3 shows the optimum moisture content (omc) and maximum dry density (mdd) obtained for various combinations upon conducting an average of three trails of sp and mp tests of each. the omc and mdd of rm are 34.39 % and 1.59 g/cc, respectively, whereas in the case of ggbs it is 25.63 % and 1.62 g/cc with distilled water. rm shows more similar reproducible values with the mp tests compared to sp tests and the values of omc and mdd are noted as 28.65 % and mdd was 1.64 g/cc. this shows an initial idea about the material behaviour with the change of application of load and accuracy in omc and mdd. the results confirm that with the increase of compaction energy in rm, the dry density increases, and the water content requirement decreases in appreciable way which coincides with the past research results also [12]. it is observed that an increase up to a maximum of 5 % in mdd and up to 31 % in the omc with the standard and modified proctor compaction tests. it is because of increase in the rammer weight for the modified compaction. standard indicates the lightweight compaction light tampers and rammers and modified compactions indicates the rollers and other compactors. the omc and mdd presented in table 3 for various combinations shows the influence of alkaline activators on the ggbs stabilised rm. the ggbs replacement in first three combinations shows that the decrease in omc and increase in mdd in both sp and mp tests which indicates the sensitivity of ggbs stabilised rm table 1. physical and geotechnical properties of rm and ggbs. sno. property rm ggbs 1 specific gravity 2.95 2.81 2 consistency limits: liquid limit (%) plastic limit (%) plasticity index 43 38 5 32 np np 3 percentage fractions: sand silt clay 3 75 22 75 24 1.0 4 uscs ml ml (a) (b) (c) (d) figure 1. a) original image of red mud b) original image of ggbs c) sem image of red mud d) sem image of ggbs. table 2. combinations and nomenclature used in the research work. sl no. combinations mixing agent nomenclature 1 90 % rm + 10 % ggbs 100 % distilled water rgd1 2 80 % rm+ 20 % ggbs 100 % distilled water rgd2 3 70 % rm+ 30 % ggbs 100 % distilled water rgd3 4 90 % rm + 10 % ggbs 100 % naoh rga1 5 80 % rm + 20 % ggbs 100 % naoh rga2 6 70 % rm + 30 % ggbs 100 % naoh rga3 7 90 % rm + 10 % ggbs 90 %naoh + 10 % na₂sio₃ rga4 8 80 % rm + 20 % ggbs 90 % naoh + 10 % na₂sio₃ rga5 9 70 % rm + 30 % ggbs 90 % naoh + 10 % na₂sio₃ rga6 10 90 % rm + 10 % ggbs 80 % naoh + 20 % na₂sio₃ rga7 11 80 % rm + 20 % ggbs 80 % naoh + 20 % na₂sio₃ rga8 12 70 % rm + 30 % ggbs 80 % naoh + 20 % na₂sio₃ rga9 13 90 % rm + 10 % ggbs 50 % naoh + 50 % na₂sio₃ rga10 14 80 % rm + 20 % ggbs 50 % naoh + 50 % na₂sio₃ rga11 15 70 % rm + 30 % ggbs 50 % naoh + 50 % na₂sio₃ rga12 acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 4 to water. the addition of ggbs beyond 30 % will not show any significance in the compaction characteristics of rm which was confirmed in the past studies and so limited to 30 % replacement only [29] and also the omc and mdd values of rgd2 and rgd3 are almost similar which confirms that the further increment in ggbs will not be beneficial regards to stabilisation and cost. the increase in mdd with the addition of ggbs to rm may be due to the reduction of clay fraction in rm which reduces the particle movement resistance during compaction. the addition of naoh and na2sio3 shows the decrease in omc and increase in mdd for all the cases of ggbs stabilised rm. the highest value of mdd was observed in combination rga12 with a value of 1.91 g/cc which is an exponential increase up to 20 % compared to the virgin rm with respect to sp test and the same percentage of increment in mdd was observed in the case of mp test also. omc was reduced up to 50 % in combination rga12 with respect to virgin rm both in sp and mp tests. almost similar pattern of change of omc and mdd percentages of all the combinations were observed both in sp and mp tests. omc was retained same and mdd was decreased with the addition of naoh alone compared to water which proves that the only naoh will not affect on the dry density on the ggbs stabilised rm and shows to add silicates to have more effective dry density in all the combinations. the increase in mdd and decrease in omc trend was observed with the addition of silicates to naoh in different ratios. both sp and mp tests confirms that the effect of alkaline activator depends on the percentage addition of ggbs to rm. the change in omc and mdd with the change of ratio of naoh/ na2sio3 was observed very minimum for a particular ggbs replacement to rm. combinations rga 10,11,12 shows very good increase in mdd values supporting that the same 50:50 ratios of naoh to na2sio3 for 10 %, 20 %, 30 % replacement by ggbs gives more effective results. in all the combinations the effect of alkaline activators largely depends on the addition of ggbs to rm, which may be due to the reaction of minerals present in the ggbs with the naoh and na2sio3. according to irc sp:20-2002, the minimum mdd of 1.46 g/cc is required to use any material as an embankment fill or in any road construction [30]. in this research work all the trails exceeds the minimum mdd required as per the specifications of irc which shows that alkaline activated ggbs stabilised rm satisfies the requirements to use in embankments with further evaluation of strength properties. the effect of compaction energy on the moisture content and dry density of untreated rm in the form of number of blows was studied by the research fraternity in the past. it is confirmed that the increase in the compaction energy has resulted to the decrease in moisture content and increase in dry density. in this study an attempt was made to evaluate the effect of compaction energy on the ggbs stabilised rm by replacing 10 %, 20 and 30 of rm with ggbs by using distilled water. based on the previous study, the number of blows used for this research work are 12, 15, 18, 22, 25, 28, 33, 45, 56 and the tests have been conducted by using a mini compaction apparatus. table 4 depicts the effect of compaction energy which was converted by using the number of blows on the ggbs stabilised rm. it shows that figure 2. mini compaction test apparatus. table 3. omc a nd mdd of alkaline activated ggbs stabilised red mud for both sp and mp. slno combination sp test mp test omc (%) mdd (g/cc) omc (%) mdd (g/cc) 1 rgd1 30.65 1.60 25.14 1.65 2 rgd2 29.64 1.61 24.22 1.67 3 rgd3 27.88 1.62 22.10 1.68 4 rga1 30.68 1.53 25.15 1.58 5 rga2 27.38 1.51 22.55 1.59 6 rga3 26.05 1.57 21.10 1.62 7 rga4 28.04 1.65 21.11 1.71 8 rga5 26.54 1.68 20.24 1.72 9 rga6 25.91 1.71 19.34 1.72 10 rga7 27.90 1.57 20.90 1.64 11 rga8 26.40 1.68 18.60 1.73 12 rga9 25.68 1.72 17.61 1.77 13 rga10 26.56 1.78 19.36 1.82 14 rga11 24.80 1.88 17.90 1.92 15 rga12 21.62 1.91 15.01 1.99 table 4. effect of compaction energy on density -water relationship of ggbs stabilised rm. number of blows compaction energy (kj/m3) rgd1 rgd2 rgd3 mc (%) dd (g/cc) mc (%) dd (g/cc) mc (%) dd (g/cc) 12 285 31.66 1.59 31.01 1.60 29.10 1.60 15 356 31.30 1.59 29.90 1.61 28.22 1.61 18 427 30.59 1.59 30.11 1.60 28.45 1.61 22 522 30.41 1.60 29.90 1.61 27.58 1.62 25 594 30.65 1.60 29.64 1.61 27.88 1.62 29 689 29.11 1.61 28.45 1.62 26.66 1.62 33 783 28.34 1.63 27.44 1.64 24.59 1.64 45 1068 26.40 1.64 25.81 1.66 23.56 1.66 56 2595 25.14 1.65 24.22 1.67 22.10 1.68 acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 5 the increase in ggbs indicates the increase in the dry density with the increase of the compaction energy. this confirms that the compaction energy has a significant impact on moisture content (mc) and dry density (dd) in all the ggbs stabilised rm which also proves that the effective compaction results on attaining the better dry density of any stabilised waste material. 4. conclusions in the current study, detailed compaction tests were performed on virgin rm, ggbs stabilised rm and alkaline activated ggbs stabilised rm samples. from the results it is concluded that the mp tests show better compaction characteristics compared to sp test which highlights the effect of compaction energy on increasing the density of samples with the close package of fine particles present in rm and ggbs stabilised rm. ggbs acts as a good stabiliser for rm with the satisfying density as per the irc specifications for the construction of embankments and other filling layers. the increase in mdd and decrease in omc was observed with the increase of ggbs percentage to rm in all the trails. the results conclude that there is a minimum amount of influence on the density of rm with alkaline activator, but the influence of alkaline activator was more on the amount of ggbs added to the rm in both sp and mp tests. the outcome of this research work emphasizes that the waste materials can be effectively utilized upon stabilising with suitable other industry by products or waste materials and the 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[28] a. sridharan, p. v. sivapullaiah, mini compaction test apparatus for fine grained soils, geotech. test. j, 28(3) (2005) pp. 240–246. doi: 10.1520/gtj12542 [29] s. alam, s. k. das, b. h. rao, strength and durability characteristic of alkali activated ggbs stabilized red mud as geomaterial, constr. build. mater., 211 (2019), pp. 932–942. doi: 10.1016/j.conbuildmat.2019.03.261 [30] irc (indian road congress), guide lines and construction of rural roads. irc sp 20, new delhi, india: irc, (2002). https://doi.org/10.1520/gtj12542 https://doi.org/10.1016/j.conbuildmat.2019.03.261 an iot measurement solution for continuous indoor environmental quality monitoring for buildings renovation acta imeko issn: 2221-870x december 2021, volume 10, number 4, 230 238 acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 230 an iot measurement solution for continuous indoor environmental quality monitoring for buildings renovation serena serroni1, marco arnesano2, luca violini1, gian marco revel1 1 università politecnica delle marche, department of industrial engineering and mathematical science,via delle brecce bianche, 60131 ancona (an),italy 2 università ecampus, via isimbardi 10, 22060 novedrate (co), italy section: research paper keywords: thermal comfort; indoor air quality; ieq; measurements; iot; building renovation citation: serena serroni, marco arnesano, luca violini, gian marco revel, an iot measurement solution for continuous indoor environmental quality monitoring for buildings renovation, acta imeko, vol. 10, no. 4, article 35, december 2021, identifier: imeko-acta-10 (2021)-04-35 section editor: carlo carobbi, university of florence, gian marco revel, università politecnica delle marche and nicola giaquinto, politecnico di bari, italy received october 10, 2021; in final form december 10, 2021; published december 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was supported by p2endure, european project horizon 2020. corresponding author: serena serroni, e-mail: s.serroni@pm.univpm.it 1. introduction the measurement of indoor environmental quality (ieq) requires the acquisition of multiple quantities regarding thermal comfort and indoor air quality. therefore, accurate monitoring and control of those environmental conditions can be useful for preventing the spread of covid-19. however, about 75 % of european buildings stock was built before 1990, before any eu building regulation [1] and with a climate context that has changed through the last decade [2]. thus, most of occupied buildings are not able to keep the required comfort conditions because of the poor performance of the envelope and heating/cooling systems [3]. the importance of indoor environmental quality (ieq) is a well-known and largely discussed theme because of its impact on human comfort, well-being, productivity, learning capability and health [4]. ieq derives from the combination of different factors influencing the human comfort sensation: thermo hygrometry, acoustic, illumination and concentration of pollutant components [5]. all those aspects should be considered at the same level of importance considering that humans spend roughly 90 % of their time indoors, especially after covid-19 spread. in addition, a recent study [6] demonstrated that indoor environmental factors, such as temperature, humidity, ventilation, and filtering systems could have a significant influence on the infection. several studies showed a correlation between the concentration of air pollutants, especially particular matter 2.5 (pm2.5) and particular matter 10 (pm10), and covid-19 virus transmission [7]. for this reason, current buildings’ renovation approaches and trends are including the ieq in the renovation assessment with abstract the measurement of indoor environmental quality (ieq) requires the acquisition of multiple quantities regarding thermal comfort and indoor air quality. the ieq monitoring is essential to investigate the building’s performance, especially when renovation is needed to improve energy efficiency and occupants’ well-being. thus, ieq data should be acquired for long periods inside occupied buildings, but traditional measurement solutions could not be adequate. this paper presents the development and application of a non-intrusive and scalable iot sensing solution for continuous ieq measurement in occupied buildings during the renovation process. the solution is composed of an ir scanner for mean radiant temperature measurement and a desk node with environmental sensors (air temperature, relative humidity, co2, pms). the integration with a bim-based renovation approach was developed to automatically retrieve building’s data required for sensor configuration and kpis calculation. the system was installed in a nursery located in poland to support the renovation process. ieq performance measured before the intervention revealed issues related to radiant temperature and air quality. using measured data, interventions were realized to improve the envelope insulation and the occupant’s behaviour. results from postrenovation measurements showed the ieq improvement achieved, demonstrating the impact of the sensing solution. mailto:s.serroni@pm.univpm.it acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 231 an increased role of importance [8]. recently, the standard en 16798 [9] has been released, in substitution of the en 15251, which provides a framework for the building performance assessment concerning the indoor environment. en 16798 provides methodologies for the calculation of ieq metrics for buildings’ classification, based on environmental measurements [10]. in that standard, thermal comfort is assessed according to well-known predictive and adaptive approaches, as defined in iso 7730 [11] and ashrae 55. however, the same level of detail is not given to indoor air quality, visual comfort, and acoustic comfort. in their critical review [12], khovalyg et al. remarked that iso and en standards should include more requirements on pm. similarly, another critical investigation on ieq data collection and assessment criteria presented in [13] revealed that sensor technology and data analysis are mainly applied to thermal comfort, while other ieq’s domains have not been addressed with the same effort. for this reason, the assessment of ieq domains, unconsidered by standards, has been conceptualized within recent research activities [14] also with a holistic approach that groups domains together [15]. even if the importance of ieq has been largely demonstrated, actual measurement tools are not adequate because of the need of measuring several environmental quantities with a high temporal and spatial resolution. traditional spot measurements tools are bulky and require a strong human effort for data processing and analysis. so, they can’t be used for responsive strategies to improve ieq by implementing retrofit actions (envelope insulation, mechanical ventilation) or triggering occupants’ behaviours (windows opening, thermostat regulation). recently, the use of sensors integrated into building management systems (bms) has also been investigated for ieq monitoring [16].however, those systems generally make use of wall-mounted sensors providing environmental quantities measured away from the real location of the occupants. therefore, data could be representative of a small part of the building. optimization of the sensing location could be performed for some quantities, such as air temperature [17], but the same optimization could be difficult for quantities that present relevant deviations in the same room, as the case of mean radiant temperature. the mean radiant temperature is not homogeneous in the room and depends on the indoor walls’ temperature [18]. in the proximity of a glazed surface or of a poorly insulated wall, the mean radiant temperature presents higher variations with respect to the inner side of the same room [19]. iso 7726 [20] provides two methods for measuring the mean radiant temperature: i) the globe thermometer that is basically a temperature sensor located at the centre of a matt black sphere; ii) the angle factors approach, based on the walls’ temperature measurements. the globe thermometer is widely used for spot measurements. it is intrusive, provides data only related to the position where it is located and one measurement point could not be enough to determine the real room’s comfort. thus, is not the preferable solution for continuous ieq measurements. the second approach, based on the angle factors, could provide a higher resolution once that the problem of measuring the walls’ thermal maps is solved. to this scope, revel et al. [21] developed an infrared (ir) scanner that provides continuous measurement of indoor walls temperatures for mean radiant temperature calculation according to iso 7726. the proposed sensor turned out to provide a measurement accuracy of ± 0.4 °c with respect to traditional microclimate stations [22]. the ir scanner was integrated with a desk node that measures the air temperature and relative humidity to build the solution named comfort eye, a sensor for multipoint thermal comfort measurement in indoor environments. that system provides a solution to the problem of measuring real-time thermal comfort with an increased spatial resolution. the impact on heating control efficiency was demonstrated by experimental testing presented in [23]. this paper presents the new development of the comfort eye for the continuous monitoring of ieq for buildings renovation. indoor air quality (iaq) sensors have been integrated into the desk node to provide measurements of co2 and pms. moreover, a led lighting system has been embedded in the desk node to provide occupants with feedback about the actual status of the indoor air quality. an internet of things (iot) architecture has been developed to allow remote configuration and data exchange. interoperability with bim (building information model) has been developed to automatically retrieve building’s data (e.g., floor area, geometry, material emissivity, occupancy, etc.), needed for sensors configuration, metrics calculation and performance assessment. the most important differences compared to the previous version presented in the paper [21] are: new ir sensor; a new sensor for iaq, in particular, new co2 and pms sensor; the last version presents an updated architecture, it is plug&play and iot; integration with the bim to configure automatically the sensors and kpis calculation. the advanced version of the comfort eye was developed within the framework of the european project, p2endure. p2endure aims at including the ieq in the renovation process, in all the stages of the 4m (mapping, modelling, making, monitoring) process. this means that a protocol for the ieq monitoring, and assessment has been developed, allowing: the accurate evaluation of ieq performance of the building as it is to feed the design stage with the suggestions to achieve the optimal ieq level after the intervention, and the post-renovation monitoring to verify the achievement of ieq compliance according to issues revealed with the mapping. the comfort eye was applied for the continuous ieq monitoring of a building located in gdynia (poland) before and after renovation works. the aspects of ieq that are taken into consideration, using the comfort eye, and exploiting all its potential, are thermal comfort and iaq (co2 and pm) results from the field application are reported. this paper demonstrates the applicability and advantages of the developed system with the application to a real case study. a pre and post renovation analysis was performed to validate the developed monitoring methodology. the monitoring allowed to quantify, in the pre and post renovation phase, the thermal performances of the building, to identify the main causes of discomfort and to assess the iaq. the specific goals of the paper are: • to present the measurement system, sensors specification, iot architecture and integration with the bim; • to propose an ieq monitoring and assessment protocol that extends the en 16798 approach to include pms; • demonstrate the applicability and advantages of the proposed measurement device with the application to a real case study. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 232 2. materials and methods 2.1. the measurement technology the comfort eye is an iot sensor composed of two nodes, the ceiling node with the ir sensor to measure walls thermal maps and mean radiant temperature, and the desk node with sensors to measure: air temperature, relative humidity, co2 and pms. it can provide the whole thermal dynamic behaviour of the wall for continuous and real-time thermal monitoring of buildings. being a prototype, the comfort eye has a production cost of less than 100 € for each node. this work aims to explore and deepen the functionality of comfort eye that allows the measurement of data necessary for the ieq. 2.1.1. comfort eye-ceiling node the ceiling node is the innovation of the comfort eye. it is a 3d thermal scanner and to measure temperature maps of all the room’s indoor surfaces there are 2-axes rotating ir sensor (figure 1). it is installed on the ceiling of the room, and it is composed of a 16x4 thermopile array, meaning that each acquired frame is a map of 64 temperatures. with a horizontal field of view (hfov) of 60° and a vertical field of view (vfov) of 16°, an area of 1.15 x 0.56 m2 is scanned with one frame on a wall at one meter of distance from the sensor. the tilt movement with 0° 180° provides the full vertical scanning of the wall with the possibility to measure the floor and ceiling temperatures. to provide the scan of all the surfaces a continuous 360° pan movement is available. the device entails a custom mainboard with a microcontroller, programmed with dedicated firmware, to perform the automatic scanning of all the room’s surfaces by controlling the pan and tilt servos. the 12c communication protocol is used to acquire data are from the ir sensor. the ir scanner required cabling for a 12 v power supply while the communication is performed with a wi-fi module that is integrated into the mainboard. the scanning process produces one thermal map for each wall that is the result of the concatenation of multiple acquisitions. given the sensor’s fov, the installation point and the room geometry, the reconstruction of the wall thermal map is performed to remove the overlapping pixels derived from the vertical concatenation and to remove pixels related to the neighbour walls. concerning the surface emissivity, correction of the ir raw measurement is implemented [24]. the complete procedure for maps correction is detailed in [21]. the ir data are acquired in continuous, processed and stored in a database in real-time. corrected thermal maps are then used for two-fold scope: measurement of the mean radiant temperature for thermal comfort evaluation and measurement of building’s envelope thermal performance [21]. mean radiant temperature is measured for several locations (e.g. near and far from the window) with the angle factors method, as presented in iso 7726 [20]. 2.1.2. comfort eyedesk node a desk node is used to acquire environmental quantities for thermal comfort and indoor air quality (iaq) assessment (figure 1). an integrated sensor, sensirion scd30, allows the single point measurement of the air temperature (ta), relative humidity (rh), and co2. the desk node also integrates a pm sensor, sensirion sps30 (table 1). sensors’ data are acquired via the i2c interface and a wi-fi module, the same that is used for the ceiling node, provides wireless communication. the data are acquired in continuous, processed and stored in the database in real-time. the desk node is in a position that should be representative of the room’s environmental conditions, avoiding exposures to direct solar radiation, air droughts and zones characterized by stagnant air. the desk node must be installed in a position representative of the environmental conditions, away from heat sources, solar radiation, direct ventilation, or other sources that could disturb the measurements. installation is done by an experienced technician. if possible, the sensor is fixed, otherwise the occupants are informed to prevent the sensor from being moved or covered. the air quality measurement system has been proposed for real-time, low-cost, and easy to install air quality monitoring. it provides precise and detailed information about the air quality of the living environment and helps to plan interventions that lead to improve air quality. in crowded closed environments such as classrooms, offices, or meeting rooms, in the case of limited ventilation, co2 values of between 5,000 ppm and 6,000 ppm can be reached. to have a good air quality the limit of 1000 ppm of co2 must not be exceeded [25]. an efficient iaq monitoring system should detect any change in the air quality, give feedback about the measured values of co2 to the users, and trigger the necessary mechanisms, if available, such as automatic/ natural ventilation and fresh air, to improve performance and protect health. the desk node communicates to the users in real time the measured values of co2 simply and intuitively, through different colours of the leds, green, yellow, red (green for good air quality and red for bad air quality). the value of co2 < 700 ppm is represented by green leds. in this case, the co2 values are acceptable, there is good air quality, it is not necessary to ventilate the environment. the value of co2 between 700 ppm and 1000 ppm is represented by yellow leds. the co2 values are very close to the limit value (1000 ppm) and it is recommended to ventilate the environment. the value of co2 > 1000 pm is represented by red leds. the value of co2 has exceeded the limit value and it is necessary to ventilate the environment [26]. table 1. specifications of sensors used by the desk node of the comfort eye. sensirion scd30/sps30 range accuracy repeatability co2 in ppm 0 – 40 000 ± (30 ppm + 3 % mv) ± 10 ppm rh in % rh 0 – 100 ± 3 ± 0.1 ta in °c -40 – +70 ± [0.4 + 0.023 (t°c 25°c)] ± 0.1 pm2.5 in µg / m³ 0 1000 ± 10 / pm10 in µg / m³ 0 1000 ± 10 / a) b) figure 1. comfort eye: a) ceiling node and b) desk node. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 233 2.1.3. system architecture comfort eye’s nodes can be connected through the local wifi network to the remote server where data are sent, processed, and then stored in a mysql database. figure 2 shows general the architecture. the communication module used is the pycom w01. it is an efficient module, which reduces power consumption as implements bluetooth low energy and supports message queuing telemetry transport (mqtt) protocol that allows only byte communication between the client and server, so to get a light communication, suitable also for low wi-fi signal conditions. to integrate the pycom w01 a custom printed circuit board (pcb) was developed. the mqtt communication protocol is used and each node is programmed to be a publisher and subscriber as shown in figure 2. the first step, to start the monitoring, is the device configuration with a dedicated mobile application. configuration data are sent from the mobile device: server information, local wi-fi credentials, and room tag. once the connection is established, the desk node sends directly raw data to the server where a subscriber function provides to processing and storing actions. for the ceiling node, the scanning procedure is configured using geometrical data of the room. geometry is retrieved from the bim, as explained in the next subsection. once the tilt and pan angles have been defined and published to the sensors, the data acquisition starts. the raw data are sent to the server and processed by a subscriber. the processed data are stored on a mysql database. the whole monitoring process takes place continuously and in realtime. the data stored in the database are then available for being consumed. a dashboard for thermal performance assessment and data exploration has been developed. the dashboard is a web-app accessible through any browser. the data processing core is served by a restful application programming interface (api) service running on the server and called with standard get request. the user can view the measured data via the web-app, obtaining information by consulting the related kpis, and can have direct feedback on iaq by observing the led’s color of the desk node installed in the building. the two means of communication are not correlated and independent of each other. 2.1.4. bim integration building’s data are required to configure the ir scanning process, to perform geometrical maps corrections and to calculate ieq kpi. to reduce the manual operation, integration with the bim was developed. the bim model of a building includes most of the required data, but not those related to the comfort eye installation. for this reason, the comfort eye bim object was developed. the object can be imported into the bim and added to each room where the sensor is installed. then the ifc file can be exported and processed with a dedicated software based on the ifcopenshell python library [27]. the software automatically retrieves the data and store them in the mysql database that is used for the automatic configuration of the comfort eye. the scope of using bim data is three-fold: i) to correct the ir thermal maps using the room geometry, the sensor position and the wall emissivity; ii) to allow the application of the angle factors method for the mean radiant temperature measurement according to iso7726 [20]; iii) to calculate ieq kpis weighted by floor area. thus, the bim integration provides a way to configure the sensor automatically and therefore reduce the installation time. 2.2. ieq measurement and assessment in addition to standard temporal series of data, the comfort eye can provide long term indicators and kpi to be used for thermal comfort and iaq assessment according to en 16798 methodology that is based on indicators and their boundaries for building’s classification [9]. there are four categories (i, ii, iii, iv) for classification: category i is the higher level of expectation and may be necessary if the building houses occupants with special requirements (children, elderly, occupants with disabilities, etc.); category iv is the worst. to provide a good level of ieq, a building is expected to operate at least as category ii. for each ieq aspect, an hourly or daily indicator (𝐼ℎ,𝑖) is measured. for every room the number of occupied hours, outside the range (hor,i) is calculated as the number of hours when 𝐼ℎ,𝑖 ≥ 𝐼lim , ilim is the limit of the indicator determined by the targeted category. thus, the percentage of occupied hours outside the range, (pori) [%], is calculated as follows (1): 𝑃𝑂𝑅𝑖 = ℎor,𝑖 ℎtot ∙ 100 % , (1) where ℎtot [h] is total occupied hours during the considered period. the por of the entire building is finally calculated as the average of the pori for each room, weighted according to the floor area (2): 𝑃𝑂𝑅 = ∑ 𝑃𝑂𝑅𝑖 ∙ 𝑆𝑖 𝑛 𝑖=1 ∑ 𝑠𝑖 𝑛 𝑖=1 , (2) where n is the total number of rooms in the building and si the floor area of i-th room in the building. the por is then used to determine the kpi. a por of 0 % corresponds to a kpi of 100 %, which is the best value achievable. a kpi equal to 100 % means that the indicator is always within the limits of the targeted category. a maximum deviation of 5 % of the por is considered acceptable. thus, for por equal or higher than 5 %, the corresponding kpi is 0 % figure 2. general architecture of comfort eye. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 234 (worse case), which means the building is operating for a significant period outside the limits of the targeted category. to define an assessment scale, a linear interpolation between the minimum por (5 %) and best por (0 %) is recommended to determine a scale of the kpi between 0 % (worse case) and 100 % (best case). the analysis is performed only considering the main occupied rooms (e.g., bedrooms, living room, kitchen), and does not consider short-term occupancy and transit areas (e.g., bathrooms, corridors, small storage areas). moreover, given the fact that en 16798 does not cover all the ieq aspects, such as pm, additional indicators are included. following subsections present in detail each indicator used for ieq assessment. the methodology provides indicators for the calculation of kpis for the thermal comfort and iaq assessment. 2.2.1. thermal comfort according to iso 7730 [11] and iso 7726 [20] ”a human being’s thermal sensation is mainly related to the thermal balance of his or her body as a whole. this balance is influenced by physical activity and clothing, as well as the environmental parameters: air temperature (ta), mean radiant temperature (tr), air velocity (va), relative humidity (rh), clothing insulation (icl) and metabolic rate (m). in a moderate environment, the human thermoregulatory system will automatically attempt to modify skin temperature and sweat secretion to maintain the heat balance. as the definition given in iso 7730 implies, thermal comfort is a subjective sensation, and it can be expressed with a mathematical model of pmv (3) which is function of four environment parameters and two personal ones: 𝑃𝑀𝑉 = f(𝑇𝑎 , 𝑇𝑟 , 𝑉𝑎 , 𝑅𝐻, 𝐼𝑐𝑙 , 𝑀) . (3) the air velocity (in m/s) based on room ventilation system is set to 0.05 m/s metabolic rate (m) is set in function of the usual occupants’ activity using a table of typical metabolic rate available in en7730 [11]. insulation level (icl) of the clothing generally worn by room occupants. for summer season is set to 0.5 clo and 0.9 clo for the winter season using a table of typical clothing insulations available in en7730 [11]. the pmv predicts the mean value of votes of a group of occupants on a seven-point thermal sensation scale. as the predicted quality of the indoor thermal environment increases, the pmv value gets closer to 0 (neutral thermal environment). the hourly pmv values need a limit to be set to identify the number of occupied hours outside an acceptable comfort range. iso 7730 define the pmv limit for each category, the i category specified as -0.2 < pmv < +0.2, the ii category 0.5 < pmv < +0.5, the iii category as -0.7 < pmv < +0.7 and the iv category -1 < pmv < +1. the benchmark is done considering a threshold of a maximum 5 % of operating hours outside the pmv range. the best performance is achieved when there is no deviation outside the design range. so, a linear interpolation between 5 % and 0 % is done. 2.2.2. indoor air quality iaq is known to have acute and chronic effects on the health of the occupants, generally is expressed in terms of co2 concentration and ventilation required to contain co2 levels and reducing the concentration of indoor air pollutants dangerous for human health [28]. continuous co2 monitoring can provide a comprehensive and straightforward way to assess and measure improvements in building ventilation. the iaq assessment methodology, which provides the co2 kpi, is defined by en 16798, and where relevant and needed, iaq monitoring is enhanced with the monitoring of pm. the iaq measurement shall be made where occupants are known to spend most of their time. the kpi aggregates the values at the building level to provide an overall value but to identify critical issues is recommended to analyse all room values. the hourly co2 concentration values above outdoor are assessed against a safety threshold to identify the number of hours outside an acceptable comfort range, and the room values are aggregated through a floor-area weighted average. the co2 limits for each category, is 550 ppm for the i category, 800 ppm,1350 ppm and greater than 1350 ppm for the ii, iii, and iv category respectively. according to en 16789, an acceptable amount of deviation is 5 % of occupied hours. the best performance is achieved when there are no deviations outside the design limit. to define an assessment scale, a linear interpolation between the minimum (5 %) and best performance (0 %) is done. pm is a widespread air pollutant, consisting of a mixture of solid and liquid particles suspended in the air. commonly used indicators describing pm that are relevant to health refer to the mass concentration of particles with a diameter of less than 10 µm (pm10) and particles with a diameter of less than 2.5 µm (pm2.5). the methodology for pm2.5 and pm10 assessment is provided by the victoria epa institute (australia). the categories are obtained according to the hourly and 24-hour rolling average pms concentration as shown in table 2 and table 3 [29]. an acceptable amount of deviation from the optimal category (very good) is the 5 %. the best performance is achieved when there are no deviations outside the design limit. to define an assessment scale, a linear interpolation between the minimum (5 %) and best performance (0 %) is done. table 2. pm2.5 concentration for ieq classification. category 24-hr-pm2.5 µg/m³ 1-hr pm2.5 µg/m³ very good 0-8.2 0-13.1 good 8.3-16.4 13.2-26.3 fair 16.5-25.0 26.4-39.9 poor 25.1-37.4 40-59.9 very poor 37.5 or greater 59.9 or greater table 3. pm 10 concentration for ieq classification. category 24-hr-pm10 µg/m³ 1-hr pm10 µg/m³ very good 0-16.4 0-26.3 good 16.5-32.9 26.4-52.7 fair 33-49.9 52.8-79.9 poor 50-74.9 80-119.9 very poor 75 or greater 120 or greater acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 235 3. experimental application the comfort eye was installed in two rooms of a nursery of gdynia and the data collection started in july 2018. the scope of the monitoring was the assessment of the envelope performance and ieq before and after renovation works (figure 3). the demonstration building is a two-story kindergarten building, attended by about 130 children. it was constructed in the year 1965 and it has the function of kindergarten from the beginning. the building volume is 2712 m3 and the built-up area is 464 m2. the main goal of the demonstration was to minimize the energy consumption especially for heating needs through the retrofitting of the envelope (add insulation layer), implementing new windows and improving the aesthetic appearance of the envelope. a comprehensive thermal insulation retrofit should create a continuous insulated envelope around the living accommodation and, ideally, avoid any residual thermal bridging of the structure. the focus of the renovation works was to achieve the targeted quality and performance improving the ieq. 4. discussion of results monitoring started in july 2018, the comfort eye is still installed and is still acquiring data. the data acquired were analysed for the winter and summer season, before (pre) and after renovation (post), for the assessment of thermal comfort and iaq building performance and validation of improved ieq. the renovation works were done from august to october 2019. the selected periods for the ieq analysis are august 2018 and august 2020 for the summer season in pre and post renovation state respectively. february 2019 and february 2020 for the winter season in pre and post renovation state respectively. the most significant weeks of the selected periods for the analysis are reported with the relative average outdoor temperature in table 4. the weather data were gathered from the weather the station igdyni26 of the weather underground web service. the building is equipped with a centralized heating system with radiators in each room, without a thermostat. no cooling system is present. 4.1. thermal comfort concerning thermal comfort, the pmv model was applied using a metabolic rate of 1.2 met (typical office/school activity) and clothing insulation of 0.9 clo, suitable for the end-use of the building and typical climate in poland in the winter season, and metabolic rate of 1.2 met and clothing insulation of 0.5 clo for the summer season. to compare the pre and post thermographic maps, days with the same average outside temperature have been selected, 6 °c (26/02/2019-20) and 25 °c (09/08/2018-20) for winter and summer respectively. the recap of kpis calculated for the winter and summer season are shown in table 4. the mean radiant temperature (tr) pre and post renovation is shown in figure 4. figure 5 shows the thermal maps maintaining the same scale in pre and post renovation phases to show the different wall temperatures reached. figure 5 (a-b) shows the thermal maps of the winter season, pre and post renovation, and in figure 6 (a-b) is also possible to observe the trend of the temperature of the wall, of the window and the internal temperature. although the external temperatures were the same, it can be seen from the graphs and the thermographic map that the temperature of the wall of pre renovation is colder than the wall of post renovation. the monitoring provided evidence that the renovation has increased the insulation capacity of the building. the building operated for the winter season in category iv, iii, ii with a kpi of 0 % before renovation. after renovation operated most of the time in category i with a kpi of 100 %. this turned out to provide an indication of uncomfortable conditions in pre renovation and comfortable conditions in post renovation. figure 5 (c-d) show the thermographic maps of the summer season, and in figure 6 (c-d) is also possible to observe the trend of the temperature of the wall, the window, and the internal air temperature. the wall of post renovation is colder than the wall of the pre renovation, this confirms the better insulation of the post renovation. in figure 5 (c-d) the hottest areas represent the window. figure 3. gdynia before and after the renovation, comfort eye installed in the building. figure 4. tr measured with the comfort eye before and after renovation. table 4. selected period for the summer and winter season, pre and post renovation state respectively and outdoor mean temperature. summer winter pre post pre post period 6/8/2018 12/8/2018 3/8/2020 9/8/2020 25/2/2019 3/3/2019 24/2/2020 1/3/2020 out t 21.3 °c 19.0 °c 2.9 °c 2.8 °c acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 236 for the summer season, the building operated in categories iv, iii, ii before renovation and in categories iii, ii, i after renovation. in any case, a kpi of 0 % was registered and this is due to the absence of a cooling system. thus, only a slight improvement in thermal comfort was registered for the summer season. the thermal comfort requirements were satisfied for the post renovation. however, in the pre renovation, the detailed analysis on radiant temperatures turned out to provide a clear problem due to the temperatures of the wall exposed to the exterior that, being lower than the other surfaces’ temperatures and the air temperature, caused a lower mean radiant temperature (see figure 4). the consequence of lower mean radiant temperature is turned out to provide uncomfortable conditions, resulting in a poor kpi. the post renovation analysis proved that the intervention mitigated that problem thanks to the installation of the envelope panels that increased the wall insulation with a consequent minor drift of the indoor surfaces’ temperatures. 4.2. iaq table 5 shows the iaq building performance pre and post renovation. considering the winter season, the co2 kpi pre renovation is 0 % and improved to 20 % in the post renovation. the kpi of pm2.5 is 0 % for both pre and post renovation. the kpi for the pm10 pre renovation is 40 % and 60 % for the post. no ventilation was installed before and after renovation. the iaq kpis registered only a slight improvement that was caused by the introduction of communication with leds in the post renovation phase. the led light was able to change colour in function of the co2 and, as shown in figure 7, it was able to trigger the windows opening by occupants when poor air quality was put in evidence with the light. this underlines how simple communication can improve air quality by changing the habits of occupants. considering the summer season, the co2 kpi is 30 % for the pre renovation and 75 % for post renovation. figure 5. thermographic image pre and post renovation for the summer and winter season. figure 6. window, wall and air temperature pre and post renovation for the summer and winter season. table 5. ieq kpis. kpis summer winter pre post pre post thermal comfort 0 % 100 % 0 % 0 % iaq-co2 0 % 20 % 30 % 75 % iaq-pm2.5 0 % 0 % 85 % 50 % iaq-pm10 40 % 60 % 90 % 60 % acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 237 the kpi of pm2.5 is 85 % for the pre renovation and 50 % for post renovation. the kpi for pm10 is 90 % and 60 % for the pre and post respectively. in this case, the final iaq is provided by natural ventilation because of the opened windows. concerning the pms, a high level of concentrations has been registered. the source of those concentrations requires further investigation. lower levels of pm2.5 and pm10 were measured in the summer period. the installation of a ventilation system with air purifiers could be considered to mitigate the problem. the results derived from the case study the feasibility of the proposed iot innovative system for continuous measurement of all parameters required for an accurate analysis of the ieq. this system allows to carry out a long-term monitoring and, therefore, to evaluate of the ieq in the most significant seasons. in the presented application, both communication means were used. the occupants were able to take advantage of the realtime iaq advice by leds. the web-app was used to generate seasonal reports with ieq kpis that were used to evaluate the optimal renovation strategy and the impact of the renovation works. 5. conclusion the comfort eye is an innovative and non-intrusive solution, allowing the implementation of an iot sensing system for a complete, continuous, and long-term monitoring of the ieq. together with the sensing device, a set of performance indicators has been developed to assess the overall quality in terms of thermal comfort and indoor air quality. this paper demonstrates the applicability and advantages of the proposed measurement device with the application to a real case study. a pre and post renovation analysis was performed to validate the developed monitoring methodology. the ceiling node, given the high spatial and temporal resolution compared with traditional tools, provides two main advantages. first, multipoint measurements of the mean radiant temperature with only one sensor. given the geometry of the room, several locations can be chosen to apply the calculation of the mean radiant temperature (e.g., near and far from the window). second, together with information about comfort, thermal maps of the indoor surfaces are available and can be used to track the thermal performance of the building envelope (e.g., tracking surface temperature of the external wall, recognizing cool zones etc.). the ir scanner of the comfort eye turned out be useful to accurately detect the thermal comfort issues because of its capability of measuring thermal maps and mean radiant temperatures. to this aim, the accuracy and completeness of bim data, such as material emissivity and geometry, are pivotal and should be guaranteed by a standardized bim modeling approach. the monitoring allowed to identify the causes of the thermal discomfort. in the pre-renovation phase the building operated for a significant period (more than 5 %) out of the targeted category with a kpi of 0 % (worst conditions). the average radiant temperature is the most significant cause of discomfort, the building had poor thermal insulation of the walls. the monitoring has confirmed that by working on the causes, the performance of the building can be improved with a consequent minor drift of the indoor surfaces’ temperatures. in the post renovation phase the building operates within the limits of the targeted category with a kpi of 100 %. the desk node allows a more detailed view of the performance of the building, providing information regarding the air quality. furthermore, the large set of information available from the measured data can be used to identify ieq problems and their origin. such advanced knowledge of the building performance allows a better design of the renovation. concerning the iaq, the monitoring demonstrated that the building operates in the pre and post renovation phases outside the limit of the targeted category (more than 5 %). no mechanical ventilation system was installed with the renovation and the installation of a ventilation system with air purifiers could be considered to mitigate the problem. the iaq kpis registered only a slight improvement that was caused by the introduction of communication with leds in the postrenovation phase. the capability of communicating the status of indoor air quality with the simple led colour based on real time measurements, triggered occupants’ actions with the scope of restoring the required environmental quality. future developments will provide improvements of the measurement system to enhance the plug&play installation and to include additional sensors for the evaluation of other ieq aspects, as visual and acoustic comfort. acknowledgement this research has received funding from the p2endure. the p2endure research project (https://www.p2endureproject.eu/en) is co-financed by the european union within the h2020 framework programme with contract no. 723391. the authors want to thank the project partners for the useful discussions and collaboration. references [1] eu building stock observatory. online [accessed 17 december 2021] https://ec.europa.eu/energy/topics/energy-efficiency/energyefficient-buildings/eu-bso_en [2] a. g. kwok, n. b. rajkovich, addressing climate change in comfort standards, 2010, building and environment, vol. 45, issue 1, pp. 18-22. 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https://doi.org/10.1016/j.enbuild.2020.109819 https://doi.org/10.1109/access.2019.2937320 https://doi.org/10.1016/j.measurement.2019.07.003 https://doi.org/10.1016/j.buildenv.2020.106707 https://doi.org/10.3390/s21113685 https://doi.org/10.3390/buildings11080336 https://doi.org/10.1191/014362401701524154 https://doi.org/10.1088/0957-0233/23/3/035005 http://dx.doi.org/10.1088/0957-0233/25/8/085101 http://dx.doi.org/10.1016/j.enbuild.2018.02.019 https://www.rehva.eu/rehva-journal/chapter/co2-monitoring-and-indoor-air-quality https://www.rehva.eu/rehva-journal/chapter/co2-monitoring-and-indoor-air-quality https://academy.ifcopenshell.org/ http://dx.doi.org/10.1034/j.1600-0668.2000.010004222.x progress towards in-situ traceability and digitalization of temperature measurements acta imeko issn: 2221-870x march 2023, volume 12, number 1, 1 6 acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 1 progress towards in-situ traceability and digitalization of temperature measurements jonathan pearce1, radka veltcheva1, declan tucker1, graham machin1 1 national physical laboratory, hampton road, teddington, tw11 0lw, united kingdom section: research paper keywords: temperature; thermometry; traceability; primary thermometry; process control; digitalization citation: jonathan pearce, radka veltcheva, declan tucker, graham machin, progress towards in-situ traceability and digitalization of temperature measurements, acta imeko, vol. 12, no. 1, article 4, march 2023, identifier: imeko-acta-12 (2023)-01-04 section editor: daniel hutzschenreuter, ptb, germany received november 7, 2022; in final form march 24, 2023; published march 2023 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: jonathan pearce, e-mail: jonathan.pearce@npl.co.uk 1. introduction the control and monitoring of temperature is a key part of almost every technological process. the thermodynamic temperature of a system is related to the average kinetic energy of the constituent particles of the system. however, this cannot be measured directly, so another parameter which varies with temperature, such as the speed of sound in a gas, must be measured and then related to the temperature through well understood physics. in general, such an approach to temperature measurement is very complicated, time consuming and expensive, and is not currently well suited to practical thermometry. most thermometry therefore makes use of practical sensors such as thermocouples and resistance thermometers. these yield a temperature dependent property such as voltage or resistance, which must then be related to temperature by comparison with a set of known temperatures, i.e. a calibration. the global framework for approximating the si unit of temperature, the kelvin, is the international temperature scale of 1990 (its-90) [1]. the measurement infrastructure that makes this possible is maintained by national metrology institutes (nmis), who perform periodic global comparisons of their own standards to ensure the equivalence of thermometry worldwide. these standards are then used to provide calibrations to end-users which are traceable to the its-90, and hence the si kelvin. in this way an end-user may be confident that their temperature measurements are globally equivalent. a key drawback of this empirical approach to thermometry is that when the sensing region of the thermometer is degraded in use, for example by exposure to high temperatures, contamination, vibration, ionising radiation and other factors. the relationship between the thermometer output and its temperature changes in an unknown way. this is referred to as ‘calibration drift’, and it is insidious because there is no indication in process that it is occurring. this is a big problem for applications where temperature monitoring and control is critical, such as in long-term monitoring (e.g. nuclear waste storage), or where processes need to operate within a narrow abstract autonomous control systems rely on input from sensors, so it is crucial that the sensor input is validated to ensure that it is ‘right’ and that the measurements are traceable to the international system of units. the measurement and control of temperature is widespread, and its reliable measurement is key to maximising product quality, optimising efficiency, reducing waste and minimizing emissions such as co2 and other harmful pollutants. degradation of temperature sensors in harsh environments such as high temperature, contamination, vibration and ionising radiation causes a progressive loss of accuracy that is not apparent. here we describe some new developments to overcome the problem of ‘calibration drift’, including self-validating thermocouples and embedded phase-change cells which self-calibrate in situ by means of a built-in temperature reference and practical primary thermometers such as the johnson noise thermometer which measure temperature directly and do not suffer from calibration drift. all these developments will provide measurement assurance which is an essential part of digitalisation to ensure that sensor output is always ‘right’, as well as providing essential ‘points of truth’ in a sensor network. some progress in digitalisation of calibrations to make them available to end-users via a website and/or an application programming interface is also described. mailto:jonathan.pearce@npl.co.uk acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 2 temperature window (e.g. aerospace heat treatment). the result is often reduced safety margins, sub-optimal processing, lower efficiency, increased emissions, and higher product waste or rejection. in this article some new developments led by the uk’s national physical laboratory (npl), in collaboration with industry partners, are described to overcome the problem of calibration drift and to provide assurance that temperature sensor output is valid – a key part of increasingly widespread digitalisation to ensure sensor output is ‘right’ by providing in-situ validation. these include self-validation and in-process calibration which provide traceability to the si kelvin at the point of measurement, and practical primary thermometry which measures temperature directly, has no need for calibration, and does not suffer from calibration drift. additionally, other promising practical primary thermometry techniques have been outlined, namely doppler broadening thermometry, ring resonator thermometry, and whispering gallery mode thermometry. these are collectively referred to as ‘photonic thermometers’ due to their use of electromagnetic radiation. acoustic thermometry is also briefly discussed. various groups worldwide, including npl, are working on elevating the technological readiness of these techniques. finally, some developments in the digitalisation of calibrations are described, including automation, web-based access, and steps towards implementation of a standardised digital calibration certificate. these will substantially reduce the amount of paperwork and opportunities for operator error and will facilitate digital transfer of calibrations and traceability for paperless audit trails. 2. self-validation thermocouples are very mature and well established, and are widely used in industry. however, they are particularly susceptible to drift of the calibration in harsh environments, whereby the relationship between emf and temperature changes in an unpredictable manner. this gives rise to a progressive, and unknown, temperature measurement error which in turn causes degradation in process monitoring and control. this can be monitored in situ by using a miniature phase-change cell (fixedpoint) in close proximity to the measurement junction (tip) of the thermocouple [2]. the fixed point is a very small crucible containing an ingot of metal (or metal-carbon alloy [3] or organic material [4]) with a known temperature. the latest devices developed by npl are able to accommodate the entire thermocouple and fixed-point assembly within a protective sheath of outer diameter 7 mm; the cell is typically about 4 mm in diameter and 10 mm in length. importantly, this means that the self-validating thermocouple presents the same external form factor and appearance as a regular process control thermocouple. it is also of course fully compatible with existing connections and electronics. a self-validating thermocouple is shown in figure 1. in use, when the process temperature being monitored passes through the melting temperature of the ingot, the thermocouple output exhibits a ‘plateau’ during melting, due to the heat of fusion of the ingot restraining further temperature rise by ensuring incoming heat from the surroundings is absorbed, driving the phase change. once the ingot is completely melted the indicated temperature resumes its upward trend. as the melting temperature of the ingot is known, having been traceably calibrated a priori, the thermocouple can be recalibrated in situ. the question of the stability of the melting temperature of the miniature fixed point is important to consider, since that has the potential to inadvertently introduce further calibration drift. in fact, this is inherently stable, and it has been shown experimentally during the course of development of the devices that in typical applications the drift of the fixed point itself is negligible in the context of thermocouple measurement uncertainties. contamination is by far the most likely cause of drift. as a general rule of thumb, 1 part per million of contamination by impurities gives rise to about 0.001 °c change in the melting temperature. so far, no evidence of measurable drift of the miniature fixed points has been found, even in quite harsh environments such as aerospace heat treatment processes. calculations indicate that contamination by transmutation in ionising radiation environments is even less important in most situations, although the extreme case of operation in the core of a nuclear reactor may cause significant drift [5]. a typical output of a self-validating thermocouple during the recalibration process is shown in the lower panel of figure 1. this device has been extensively characterised [6] and has been licensed by npl to uk thermocouple manufacturer ccpi europe, under the tradename inseva [7], who are conducting a series of trials in high value manufacturing industries at several plants in the uk and in europe. typical fixed-point materials for these applications include ag (962 °c), au (1064 °c), cu (1084 °c), fe-c (1153 °c) and co-c (1324 °c). a similar concept has been employed for an application in space-borne instrumentation, where the phase-change cell is part of the system whose temperature is to be measured. such an embedded fixed-point has been demonstrated by npl in collaboration with ral space on a prototype blackbody calibrator designed for operation as part of a spacecraft-borne figure 1. top: self-validating thermocouple with protective sheath. image courtesy of ccpi europe. bottom: melting curve observed during the recalibration of the inseva self-validating thermocouple, here using a gold ingot (melting temperature 1064.18 °c). 0 100 200 300 400 1060 1062 1064 1066 1068 1070 in d ic a te d t e m p e ra tu re / ° c time / s acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 3 earth observation instrument suite [5]. the phase-change cell, containing approximately 2 g of gallium (melting point 29.7646 °c), is embedded in the aluminium blackbody calibrator base, close to an embedded platinum resistance thermometer (prt). this enables the in-situ recalibration of the prt in orbit. in this application, some key developments included a mechanism to promote reliable freezing of the gallium without necessitating a large supercool (gallium is prone to cooling several degrees below its freezing temperature before nucleation is triggered), and a mechanism for preventing mechanical contact between the gallium ingot and the stainless steel cell wall, thereby avoiding the possibility of long-term contamination of the ingot and hence a change of its melting temperature. the ingot is shown in figure 2. it can be seen in the lower panel of figure 2 that the remotely located prt is able to indicate clearly defined melting curves with a useful duration of several hours, and a melting temperature range of less than 0.01 °c. by calibrating the phasechange cell against npl’s reference standard gallium cell, it is possible to perform in-situ traceable calibrations of the prt on board the spacecraft with an expanded uncertainty of less than 0.01 °c. for both the self-validating thermocouples and the embedded phase-change cell techniques, vigorous efforts are ongoing to automate the detection of the melting plateau, and, once detected, to characterise the ‘fixed point’ representing the invariant part of the melting curve. this is challenging to implement algorithmically in a manner sufficiently robust against noise and spurious artefacts in the data, but it is essential for autonomous in-situ recalibration. npl has had some success with a supervised learning approach (machine learning) on training data obtained from an industrial trial of self-validating thermocouples in a heat treatment application which yielded a large, high quality data set. this algorithm was then shown to work well on data that was not part of the training set, yielding typical expanded uncertainties in the melting point determination of about 0.5 °c (gold fixed point) and 1.0 °c (silver fixed point). here and in the following, expanded uncertainties are taken to correspond to a coverage factor k = 2, i.e. a coverage probability of 95 %. note that it is unlikely these uncertainties will be further reduced by improvements to the algorithm because they are dominated by experimental considerations associated with the physical measurement setup. instead, algorithm development should focus on reliability and ability to characterise the plateau under adverse conditions such as noise, spurious artefacts, and faint signals. in other words, a demonstrable ‘all weather’ capability is needed. hence, while the new machine learning algorithm shows promise, it will need to be tested on a diverse set of data to demonstrate its universal applicability. the detection of characteristic shapes such as melting curves is essentially a pattern recognition problem. this is easy for humans because of the extraordinary sophistication of the visual cortex but it is not practical for conventional computer programming approaches, and in general machine learning or other artificial intelligence applications are needed to tackle this problem, together with good quality data for development and validation of the techniques. 3. practical primary thermometry the limitations of conventional temperature sensors which rely on calibration prior to use, and hence are prone to calibration drift, has led to renewed interest in practical primary thermometry. primary thermometers measure some property that can be related to temperature directly through wellunderstood physics, and do not require a temperature scale or calibration. in addition, if all parameters needed to infer the temperature are measured simultaneously, the sensor is not subjected to calibration drift, since any change in the sensor figure 2. top: phase-change cell embedded in the blackbody calibrator base; the adjacent prt is to the left; inset shows a photograph of the phase-change cell. bottom: melting curves observed during the in-situ calibration of the prt using the miniature embedded phase-change cell, showing the narrow melting range and excellent reproducibility. figure 3. prototype practical johnson noise thermometer developed by metrosol in collaboration with npl. the sensing electronics are housed in the container to the right; the probe extends to the left. 00:00 01:00 02:00 03:00 04:00 29.755 29.760 29.765 29.770 29.775 29.780 29.785 29.790 t e m p e ra tu re / ° c time elapsed / hours:minutes acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 4 material is accounted for in the measurement. examples include acoustic thermometry (measuring the speed of sound) and johnson noise thermometry (measuring the temperaturedependent voltage arising from the thermal motion of charge carriers in a resistor). to turn one of these into a practical, commercially available reality, npl has been collaborating with metrosol limited to develop a practical johnson noise thermometer [8]-[10]. the johnson noise voltage is related to temperature, 𝑇, by nyquist’s relation: 〈𝑉𝑇 2〉 = 4 𝑘 𝑇 𝑅 δ𝑓 , (1) where 〈𝑉𝑇 2〉 is the mean squared johnson noise voltage, 𝑘 is the boltzmann constant, 𝑅 is the sensor resistance and δ𝑓 is the frequency bandwidth which is a function of the sensing electronics and cables. importantly, if 𝑅 is measured at the same time as the johnson noise voltage, then all relevant properties of the sensing resistor are measured and so even if the sensor is degraded the thermodynamic temperature is always known. the johnson noise voltage is miniscule, and measuring it requires robust immunity to electromagnetic interference and electronic interference, arising from both external and internal influences. this is achievable by good design. a key challenge is the need for very high amplification of the noise signal. its measurement in the presence of the inevitable electrical noise generated by the pre-amplifiers can be done with the use of correlation, whereby the signal is split into two different channels. only the measured signal which is the same on both channels (i.e. the johnson noise) is ‘let through’. a drawback of this approach with conventional designs is that this results in excessively long correlation times of minutes to hours depending on the required uncertainty. on the other hand, industrial measurements typically require timescales of a few seconds. using the nyquist equation (1) requires a knowledge of the bandwidth, which in reality is unknowable. the equation is generally used in ratio form at two different temperatures: the sensor temperature to be determined, and a known reference temperature. the nyquist equation can hence be expressed as: 𝑇 = 𝑇0 ( 𝑉 𝑉0 ) 2 𝑅 𝑅0 , (2) where 𝑇0, 𝑉0 and 𝑅0 are the reference temperature, johnson noise voltage and resistance respectively. in general, it is very inconvenient to maintain a known reference temperature because it is impossible to match the frequency response of the two measurement circuits, and the resulting mismatch causes excessive measurement errors over the frequency range required for the current fast response time application. various approaches have been employed to overcome this including the use of a synthesized noise signal from, for example, a josephson array. while extremely accurate, this approach is also not feasible as it requires complicated low temperature equipment. the npl/metrosol collaboration makes use of a quasirandom synthetic reference signal which is generated a priori. this reference signal is then superimposed on the measurement signal so that they both experience the same frequency response of the measurement electronics. the composite signal (superposition of johnson noise and calibration ‘tones’) can then be decomposed with signal processing in the frequency domain, and their ratio determined in order to deduce the temperature of the sensor resistor. a further advantage of this mechanism is its high tolerance to highly non-flat, non-linear frequency response, and so a much higher bandwidth (up to 1 mhz) can be employed than in previous systems. this translates directly to shorter measurement times and hence faster response times, since more signal can be averaged in the same amount of time. johnson noise thermometry has until recently been the preserve of large national laboratories due to the extreme difficulty of isolating the miniscule johnson noise voltage from the far larger external noise sources and the internal noise generated by the electronic components [11]. the development of a practical thermometer has been elusive and so far none have reached market, but the current npl/metrosol collaboration has now developed a working thermometer with unprecedented immunity to external electrical interference. the current prototype is shown in figure 3. it has now passed the most stringent electrical immunity standard test, iec 61000-4-3 [12]. the accuracy depends on the measurement duration; for an averaging period of about 5 s the expanded measurement uncertainty is ± 0.5 °c. the most obvious application is as a replacement for thermocouples where appreciable long-term drift is unacceptable. efforts are now focused on increasing the maximum temperature range beyond about 150 °c and improving the electronics and signal processing. further developments in the pipeline include demonstration of the feasibility of photonic-based ‘lab on a chip’ thermometry approaches for in-situ traceability to the kelvin. three approaches in various stages of investigation by npl and its collaborators to facilitate direct in-situ traceability are doppler broadening, ringresonator, and whispering gallery thermometry [13]. doppler broadening thermometry (dbt) is based on the measurement of the doppler profile of a molecular or atomic absorption line of a gas in thermodynamic equilibrium. the absorption line shape is dominated at low pressure by doppler broadening and has a gaussian profile corresponding to the maxwell-boltzmann distribution of velocities of gas particles along a laser beam axis. in practice various physical effects such as collisions distort the beam profile somewhat, but the theory of this is well understood and the absorption line shape may be fitted by a parameterised model. the doppler half-width at halfmaximum, ∆𝑣𝐷 , is related to the temperature t by: ∆𝑣𝐷 = 𝑣0 𝑐 √2 ln 2 𝑘 𝑇 𝑀 , (3) where 𝑣0 is the line-centre frequency, 𝑐 is the speed of light, and 𝑀 is the absorber mass. two key challenges currently being addressed are a) reducing the amount of ancillary equipment needed for implementing the technique and b) miniaturisation of the sensing element. ring-resonator (rr) thermometry essentially utilises a closed-loop optical waveguide which is optically coupled to a second, adjacent, non-closed waveguide (separated by an air gap) via evanescence. the ‘ring’ or ‘loop’ enables propagation of circular electromagnetic waves with a characteristic resonance at a wavelength, 𝜆m, given by: 𝑚 ∙ 𝜆m = 𝑛eff ∙ 𝐿, (4) where the integer 𝑚 represents the resonance mode, 𝑛eff is an index characteristic of the waveguide and 𝐿 is the round-trip length of the loop. the temperature dependence of the refractive index and physical dimensions of the ring enable the use of the device as a thermometer by measuring the temperaturedependent shift in the wavelength given in (4). in practice, the acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 5 change in refractive index per unit temperature is a factor of approximately 100 larger than the thermal expansion coefficients of the materials involved, so the latter may be ignored. the technique is readily miniaturised and has good resistance to chemical contamination. it also offers some of the lowest uncertainties of all the practical primary thermometry techniques, although great care is required during the fabrication process to avoid imperfections. whispering gallery mode (wgm) thermometers trace their ancestry to precision clock oscillators. in essence they are stable microwave resonators arranged such that a symmetric dielectric medium such as a cylinder or disk is suspended in the centre of a metal cavity. the electromagnetic field in the microwave region is coupled to an external waveguide to excite the resonant frequencies. the frequencies of these resonant ‘whispering gallery’ modes exhibit temperature dependence and may be related to temperature through an understanding of the associated physics, which enables the use of the device for thermometry. acoustic gas thermometry is also a candidate for practical primary thermometry. the speed of sound in a gas depends on the temperature and may be related to the gas temperature through well understood physics. by using an acoustic resonator which ‘rings’ like a bell when excited appropriately with loudspeakers, and by characterizing the resulting changes in the geometry of the device using microwaves to understand the resonant modes, an extremely accurate thermometer can be constructed. such a device was used to determine the boltzmann constant with unmatched accuracy as part of the global endeavour to redefine the kelvin in terms of fundamental constants [14]. 4. digitalization of calibrations for many years the result of thermometer calibrations have been printed on paper and issued to the customer. recently, however, there is a trend towards digitalisation of the calibrations so that the results are available online or in electronic files. this is very important functionality for many users; for example, in aerospace organisations where measurements are subject to significant regulatory compliance and demonstration thereof under frameworks such as ams2750 which regulates heat treatment of metallic materials [15], it is very difficult to work with paper certificates. one successful approach has been that of ccpi europe, who have fully automated certification with their pyrotag™ system [16]. digitalisation of calibrations has numerous benefits including the reduction in operator errors (e.g. through manual data entry), removed need for paper-based processes and transactions, and easier management for asset managers, calibration managers, and technical staff. it will result in reduction of time and cost arising from a paperless system, offers secure storage and retrieval of information, and is audit ready to demonstrate traceability compliance. this presents some infrastructural challenges, including the way the data is presented, the internal mechanisms in the calibration laboratory for enabling digitalisation, and security of the information required to ensure that only the intended recipients have access. npl has embarked on a programme to automate, as far as possible, its thermometer calibrations and the generation of calibration certificates, and to make them, and the associated data and metadata available online via a secure website. the certificates will be machine readable (xml). the results will also be available through an application programming interface (api), allowing integration with customers’ own software. a key aim is ultimately to integrate this capability with the international digital calibration certificate (dcc), whose format is currently undergoing development [17]. importantly, the calibration history will also be available to the user. importantly, the dcc offers the possibility of facilitating autonomous updating of calibration data. this could be exploited by the techniques described in this paper, particularly self-validating techniques which provide a live update of the calibration in situ. updating one point in the calibration generally has an effect not only at the temperature at which the selfcalibration is performed, but on the interpolating function over a wider temperature range. the updated calibration could be passed to the associated dcc which could then be updated to include the new parameters and correct the interpolating function over the wider temperature range of use. clearly such a mechanism does not yet exist, but this is a functionality that should be considered in the formulation of the dcc format and its implementation. this approach may also be applicable to practical primary thermometry, though in that case the role of calibration certificates more broadly, and even the role of national metrology institutes in providing traceability in this regime, is currently not well defined. 5. conclusions some new developments in temperature measurement have been presented which support digitalisation in various respects. self-validation techniques using miniature temperature fixed points based on ‘phase change cells’ to provide in-situ traceability at the point of measurement will provide assurance that temperature sensor output is ‘always right’. practical primary thermometry measures temperature directly, rather than requiring calibration and the risk of consequent calibration drift in harsh environments, so ensures long-term reliable measurements; examples outlined here include johnson noise thermometry, doppler broadening thermometry, ring resonator thermometry, whispering gallery mode thermometry and acoustic thermometry. for conventional sensors, digitalisation of calibrations at npl is becoming a practical reality with a web-based interface, associated api to enable endusers to access calibration data programmatically, and steps towards a standardised digital calibration certificate format. these developments all support digitalisation of metrology and will increase the reliability of measurements, improving process efficiency and product yield, with a consequent reduction in harmful emissions. future work will focus on elevating the technical readiness and bringing the innovations to market. acknowledgement we would like to thank trevor ford, peter cowley and phill williams of ccpi europe ltd for contributions on the selfvalidating thermocouples, dan peters and dave smith of ral space for contributions on the embedded phase-change cell, paul bramley and david cruickshank of metrosol ltd for contributions on the johnson noise thermometry, sam bilson and andrew thompson of npl for contributions on machine learning approaches to automation of self-validating thermocouple calibrations, and deepthi sundaram and stuart chalmers of npl for contributions on the digitalisation of calibration certificates. acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 6 references [1] h. preston-thomas, the international temperature scale of 1990 (its-90), metrologia, vol. 27, 1990, pp. 3-10. doi: https://doi.org/10.1088/0026-1394/27/1/002 [2] j. v. pearce, o. ongrai, g. machin, s. j. sweeney, self-validating thermocouples based on high temperature fixed points, metrologia, vol. 47, 2010, pp. l1-l3. doi: https://doi.org/10.1088/0026-1394/47/1/l01 [3] g. machin, twelve years of high temperature fixed point research: a review, aip conf. proc., vol. 1552, 2013, p. 305. doi: https://doi.org/10.1063/1.4821383 [4] e. webster, d. clarke, r. mason, p. saunders, d. r. white, in situ temperature calibration for critical applications near ambient, meas. sci. tech., vol. 31(4), 2020, p. 044006. doi: https://doi.org/10.1088/1361-6501/ab5dd1 [5] j. v. pearce, r. i. veltcheva, d. m. peters, d. smith, t. nightingale, miniature gallium phase-change cells for in situ thermometry calibrations in space, meas. sci. technol., vol. 30, 2019, p. 124003. doi: https://doi.org/10.1088/1361-6501/aad8a8 [6] d. tucker, a. andreu, c. j. elliott, t. ford, marius neagu, g. machin, j. v. pearce, integrated self-validating thermocouples with a reference temperature up to 1329 °c, meas. sci. technol., vol. 29(10), 2018, p. 105002. doi: https://doi.org/10.1088/1361-6501/aad8a8 [7] https://ccpi-europe.com/2018/05/22/inseva-thermocouplelicense-signing/ [8] p. bramley, d. cruickshank, j. v. pearce, the development of a practical, drift-free, johnson-noise thermometer for industrial applications, int. j. thermophys., vol. 38, 2017, p. 25. doi: https://doi.org/10.1007/s10765-016-2156-8 [9] p. bramley, d. cruickshank, j. aubrey, developments towards an industrial johnson noise thermometer, meas. sci. technol., vol. 31, 2020, p. 054003. doi: https://doi.org/10.1088/1361-6501/ab58a6 [10] http://www.johnson-noise-thermometer.com [11] j. f. qu, s. p. benz, h. rogalla, w. l. tew, d. r. white, k. l. zhou, johnson noise thermometry, meas. sci. tech., vol. 30, 2019, p. 112001. doi: https://doi.org/10.1088/1361-6501/ab3526 [12] iec 61000-4-3:2020 electromagnetic compatibility (emc) – part 4-3: testing and measurement techniques – radiated, radiofrequency, electromagnetic field immunity test. online [accessed 24 march 2023] https://webstore.iec.ch/publication/59849 [13] s. dedyulin, z. ahmed, g. machin, emerging technologies in the field of thermometry, meas. sci. & technol., vol. 33, 2022, 092001 doi: https://doi.org/10.1088/1361-6501/ac75b1 [14] j. fischer, et. al., the boltzmann project, metrologia, vol. 55, 2018, pp. r1-r20. doi: https://doi.org/10.1088/1681-7575/aaa790 [15] ams2750f is an aerospace manufacturing standard covering temperature sensors, instrumentation, thermal processing equipment, correction factors and instrument offsets, system accuracy tests, and temperature uniformity surveys. these are necessary to ensure that parts or raw materials are heat treated in accordance with the applicable specification(s). online [accessed 24 march 2023] https://www.sae.org/standards/content/ams2750f/ [16] ccpi, pyro tag. online [accessed 24 march 2023] https://ccpi-europe.com/resources/pyro-tag/ [17] s. hackel, f. härtig, j. hornig, t. wiedenhöfer, the digital calibration certificate, ptb-mitteilungen 127 (2017) doi: https://doi.org/10.7795/310.20170403 see also ptb’s dcc website. online [accessed 24 march 2023] https://tinyurl.com/ycksrc2t https://doi.org/10.1088/0026-1394/27/1/002 https://doi.org/10.1088/0026-1394/47/1/l01 https://doi.org/10.1063/1.4821383 https://doi.org/10.1088/1361-6501/ab5dd1 https://doi.org/10.1088/1361-6501/aad8a8 https://doi.org/10.1088/1361-6501/aad8a8 https://ccpi-europe.com/2018/05/22/inseva-thermocouple-license-signing/ https://ccpi-europe.com/2018/05/22/inseva-thermocouple-license-signing/ https://doi.org/10.1007/s10765-016-2156-8 https://doi.org/10.1088/1361-6501/ab58a6 http://www.johnson-noise-thermometer.com/ https://doi.org/10.1088/1361-6501/ab3526 https://webstore.iec.ch/publication/59849 https://doi.org/10.1088/1361-6501/ac75b1 https://doi.org/10.1088/1681-7575/aaa790 https://www.sae.org/standards/content/ams2750f/ https://ccpi-europe.com/resources/pyro-tag/ https://doi.org/10.7795/310.20170403 https://tinyurl.com/ycksrc2t photogrammetry and gis to investigate modern landscape change in an early roman colonial territory in molise (italy) acta imeko issn: 2221-870x december 2022, volume 11, number 4, 1 7 acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 1 photogrammetry and gis to investigate modern landscape change in an early roman colonial territory in molise (italy) manuel j. h. peters 1,2, tesse d. stek3 1 department of applied science and technology, politecnico di torino, corso duca degli abruzzi 24, 10129 torino, italy 2 department of history, universidade de évora, largo dos colegiais 2, 7000-803 évora, portugal 3 royal netherlands institute in rome, via omero 10/12, 00197 roma, italy section: research paper keywords: photogrammetry; gis; landscape change; mediterranean archaeology; survey archaeology citation: manuel j. h. peters, tesse d. stek, photogrammetry and gis to investigate modern landscape change in an early roman colonial territory in molise (italy), acta imeko, vol. 11, no. 4, article 12, december 2022, identifier: imeko-acta-11 (2022)-04-12 section editor: leila es sebar, politecnico di torino, italy received april 26, 2022; in final form december 14, 2022; published december 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: manuel j. h. peters, e-mail: manueljhpeters@gmail.com 1. introduction pedestrian field survey currently faces paradoxical developments. in the rapidly deteriorating archaeological landscapes of the mediterranean as well as elsewhere, archaeologists are becoming ever more reliant on field survey data, especially those collected in earlier times with relatively good quality surface finds (so-called legacy datasets). at the same time, over the past decades there has been increasing attention for the various biases that may occur when sampling the archaeological record by pedestrian survey. initially, much research has targeted methodological biases, amongst others to correct for the varying visibility of archaeological surface material during the surveys. this has led to a well-developed rigour in field survey practices documenting the present field conditions as systematic and well as possible [1]-[12]. the research presented in this paper focuses on one particular factor: the geomorphological change over time. erosion and depositional processes, as well as incisive anthropic actions such as mining, all influence the location and preservation of the archaeological record. understanding geomorphological change can help to better assess the value of surface distributions of archaeological finds retrieved during field work, and at least in theory also to assess the reliability of legacy survey datasets. historical aerial images have since long been used to aid in the identification of archaeological features that in the meantime have been obscured or obliterated by more recent anthropic manipulations and/or natural events. more specifically, historical aerial photographs now also allow us to generate historical digital elevation models (hdems), historical orthophotos and 3d models of areas that have often been subjected to significant landscape change over the years. this can be done by using image-based modelling (ibm) techniques such as photogrammetry, often using the principle of structure from motion (sfm). when a recent digital elevation model (dem) is subtracted from an earlier one, a map of the occurred geomorphological abstract legacy data in the form of historical aerial imagery can be used to investigate geomorphological change over time. this data can be used to improve research about the preservation and visibility of the archaeological record, and it can also aid heritage management. this paper presents a composite image-based modelling workflow to generate 3d models, historical orthophotos, and historical digital elevation models from images from the 1970s. this was done to improve the interpretation of survey data from the early roman colony of aesernia. the main challenge was the lack of high-resolution recent digital elevation models and ground control points, and a general lack of metadata. therefore, spatial data from various sources had to be combined. to assess the accuracy of the final 3d model, the root mean square error (rmse) was calculated. while the workflow appears effective, the low accuracy of the initial data limits the usefulness of the model for the study of geomorphological change. however, it can be implemented to aid sample area selection when preparing archaeological fieldwork. additionally, when working with existing survey datasets, areas with a high bias risk resulting from post-depositional processes can be indicated. mailto:manueljhpeters@gmail.com acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 2 change can be extracted, thereby generating useful new data. the generation of hdems from historical aerial images is not always straightforward. common problems with the generation of hdems consist of poor or lacking metadata, low image resolutions, and the absence of contemporary gps ground control points (gcps). these factors can complicate generating new data from historical photographs. additionally, the comparison of data with different coverages, resolutions, and levels of precision can cause problems, which makes estimating the difference between two dems to investigate geomorphological change challenging. issues concerning hdem generation are often resolved by using gps gcps on locations that are visible in both historical and recent remote sensing data, or by creating new gcps from accurate (and often more recent) maps in a geographic information system (gis) and using those in the ibm software. an alternative is to co-register the created dems to ones that are currently available, as demonstrated by sevara et al. [13]. common workflows use ground control points that were recorded in the field with a gps, therefore establishing deviations of less than 10 cm [14]-[18] or use highly accurate recent dems for co-registration [19]-[22]. these resources were not available in the region under investigation, therefore the objective of this study was to assess the feasibility of a composite workflow using legacy data that was lacking any physical gcps. one potential source of error is the manual placement of the control points, in both gis and ibm software, since this mostly relies on the resolution of the images and the competency of the user. furthermore, the quality of the final result largely depends on the quality of the initial data. as previously mentioned, in the case of this research no gcps or high-resolution dem were available, which significantly complicated the ibm procedure. therefore, the usefulness of this procedure for further research and analysis was investigated by assessing its accuracy after several improvements. the landscape of italy has changed significantly over the past century [23], which has a big impact on the archaeological remains. this makes it a good location to investigate the various factors influencing the surface data obtained through pedestrian surveys. the landscapes of early roman colonization (lerc) project operates in this changing landscape. the research presented in this paper was carried out using data provided by the aesernia colonial landscape project, which was started in 2011 by dr. t. d. stek as an eu marie curie fellowship at glasgow university, and subsequently, in 2013, continued in the larger framework of the lerc project, a collaboration between leiden university and the royal netherlands institute in rome (knir), funded by the netherlands organisation for scientific research (nwo) [24]. the lerc project investigates the early roman colonisation process in italy, mainly by pedestrian surveys and remote sensing [24]. this paper focuses on the landscape surrounding the latin colony of aesernia (263 bce), around the modern town of isernia, situated in the molise region in south-central italy (figure 1). in section 2 the study area and the various data sources utilised in this research will be discussed, and some of the issues mentioned. the next two sections describe the methods and the results, followed by a discussion highlighting some issues and limitations. finally, in the concluding section the main applications and limitations are stated. 2. study area and data the colony of aesernia was established in 263 bce, and the territory was previously known as a relatively undocumented area within the molise, except for the landscape research carried out in the 1980s [25]. in the past decade, extensive research has been done in this area by means of field survey, various types of aerial photography and remote sensing, and geophysics. significant issues in the region include the modern urbanisation and changes in land use, which are happening at a fast pace around the town of isernia. therefore, the initial stages of the lerc project focused on the area around the town. although the current trend in mediterranean pedestrian survey focuses on the collection of off-site data in a smaller sample area, where fields are selected for their good visibility, the lerc project covered the majority of fields, including those with low visibility [26]. this variation in visibility can be attributed to factors such as differences in land use (e.g. freshly ploughed vs. overgrown) and geomorphological change. both are thought to have impacts on the collection and interpretation of pedestrian survey data, which is why they have been subject to further investigation. figure 1. lerc research area around aesernia [3], [26]. figure 2. aerial image coverage for 1970-71 around isernia. acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 3 for the present analysis, several data sources were used, including legacy data consisting of 53 aerial photographs from the isernia area (figure 2). these were produced in 1970-71 for cartographic purposes by the società per azioni rilevamenti aerofotogrammetrici (sara) [27]. the images had been digitised with a resolution of 300 dpi (1970 set) and 600 dpi (1971 set). the type of scanner and its parameters were unknown at the time of writing, therefore possible errors related to factors such as resolution, distortion, and glare were unknown. the average flight height was not exactly known either, and neither were yaw, pitch, and roll, which meant that camera properties, including focal length, could not be used. these are typical issues related to the use of legacy data, since this data is usually not created for modern research purposes. additionally, a regional orthophoto of the isernia area from 2007 was provided by the geoportal of the regione molise (using the autogr tool developed by dr. gianluca cantoro). the tinitaly/01 dem released by the istituto nazionale di geofisica e vulcanologia (ingv) in 2007 was used as control dem. this is a composite dem that is commonly used in recent archaeological research [28]. in molise, the root mean square error (rmse) of the tinitaly dem (modern dem or mdem) is 3.76 m in the non-urban areas, and 4.51 m in the urban areas [29], [30]. a hillshade visualisation of this dem was used as base map for several figures (1, 2, 6, 9, 10) in this paper. agisoft metashape professional 1.5.2 build 7838 was used to create the various models using sfm, cloudcompare 2.10.2 was used for modifying and co-registering the point clouds. arcmap 10.4.0.5524 was used to run the gis procedure, and arcscene 10.4.0.5524 to visualise the data in 3d. all data was processed in the epsg:32633, wgs84 / utm 33n coordinate system. 3. methods to accommodate the limitations of the original data, a composite workflow involving sfm, point cloud processing software, and gis was designed (figure 3). first, a preliminary model using 53 images from 1970-71 was built using sfm. although all images were of sufficient quality, they had borders that could leave traces during the creation of the 3d model and the dem, and therefore needed to be removed. this was done by manually going over every picture and creating a mask by selecting the area that had to be removed. areas that were of low quality because of glare were also removed in this way. once the preliminary model was created, gcps were placed in a gis environment on the resulting orthophoto. this was done by using features that appeared unchanged and were easily recognisable on the 1970-71 and 2007 orthophotos, such as corners of buildings and intersections of roads. the xy coordinates were obtained based on the 2007 orthophoto, and the elevation values were extracted from the mdem. this data was then imported into the sfm software, where the gcp locations had to be modified manually by keeping the sfm and gis software side by side (figure 4). the coordinates were given for each gcp in easting (m), northing (m), altitude (m). an accuracy of 3 meters was set, considering the resolution of the images when visually placing the markers in horizontal space for the xy coordinates, and the vertical accuracy of the dem07, which generated the elevation value. when all gcps and their coordinates were added, the process to generate the model was initiated again, starting with the alignment of the photos, this time using high accuracy, with a key point limit of 120000, and no tie point limit. this resulted in a sparse point cloud consisting of 387692 points. then, the camera alignment was optimised, in order to achieve a higher accuracy and to correct for possible distortions, using the focal length radial distortion coefficients k1, k2, k3, and k4; tangential distortion coefficients p1, p2, p3, and p4; and affinity and skew (non-orthogonality) transformation coefficients b1 and b2 [20]. additionally, the point cloud variance was calculated. then, by using gradual selection, points with high reprojection error (above 0.25 m) were removed, as well as points with a reconstruction uncertainty above 10 m and with a projection accuracy below 2.5 m. the filtering steps resulted in a removal of figure 3. composite ibm workflow. figure 4. placing gcps in metashape (left) by comparing them to the image in arcmap (right). acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 4 195061 points from the point cloud, with a final number of 192631 tie points. this filtering removed points that could result in later outliers in the dem. next, the bounding box was set to select the area that would be used in further processing. no clipping of the area was done at this stage however, since the hdem would later be clipped by the research area in arcmap. after this, the dense cloud was built, using high quality and moderate depth filtering, to avoid removing more complex landscape features, and pixel colour was calculated. the mesh was built using the height field surface type, and the dense cloud as source data. the polygon count was set to high, in order not to lose any detail. interpolation was enabled, so that holes generated in the previous filtering steps could be filled. the next step was to build the texture. while this is not necessary to create an orthophoto or dem, it does provide a better 3d visualisation of the area. the mapping mode was orthophoto, blending mode mosaic. in workflows containing black and white images, colour correction is often disabled, since this is generally only useful for data sets with extreme brightness variation. in this case however, the orthophoto would be used for the classification of the forested areas. this was primarily done based on pixel colour, which therefore had to be as accurate as possible. the hdem (figure 5) was built using the dense cloud, since this generally produces more accurate results, and has a shorter processing time. although the dem that was obtained had a resolution of 2 m, it was decided to export it at a 10 m resolution, in order to be comparable with the mdem. the orthomosaic was generated in the same reference system, using the mosaic blending mode, colour correction was not used. the orthophoto and hdem could then be exported as geotiff files. the no-data value was set at 255. ground points were classified using a 15 degrees maximum angle, 2 m maximum distance, and 25 m cell size. the ground point cloud (hdem) was then further modified in cloudcompare. an additional filtering step was carried out using statistical outlier removal, using 10 points for the mean distance estimation and 1.00 as the standard deviation multiplier threshold. duplicate points were removed as well, and the point cloud was downsampled to a 2.5 m resolution to speed up processing. then, the hdem was co-registered to the mdem using an iterative closest point (icp) algorithm. this coregistration procedure corrected the hdem, decreasing the tilt and modifying the scale to better fit the mdem. this resulted in a decrease in rmse on the whole hdem area from 13.95 m to 7.49 m. the hdem was then rasterised to a grid size of 10 metres to be comparable with the mdem. an additional compensation was carried out by measuring the deviation of the hdem in 101 new gcps in supposedly stable areas, interpolating these values with the application of kriging. in this case, ordinary kriging was used with a linear semivariogram, since this model fitted the gcps best. the kriging window was set at 12 points, with a maximum distance of 7000 m. the resulting raster showed the deviation of the hdem across the research area, which was most significant near the edges of the model (as can be expected with this type of model), especially on the north and south sides. the deviation map was then subtracted from the hdem, and the mdem was subtracted from the compensated hdem, showing the geomorphological change in the area. in order to exclude areas with vegetation or buildings from the geomorphological change model, these were masked by a combination of automated classification based on the grey values of the historical orthophoto and manual adjustments of the mask. the resulting model shows both the positive (presumably sedimentation) and negative (presumably erosion) change in the landscape (figure 6). figure 5. hdem after cleaning, including gcp locations. figure 6. geomorphological change (erosion and sedimentation). acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 5 4. results the 3d model generated by the sfm procedure was sufficient for a visual study of the landscape surrounding isernia (figure 7). in order to determine its suitability for the determination of geomorphological change, an accuracy assessment was carried out. the north-south and east-west profiles of the various dems were compared (figure 8), showing that the co-registration resolved a significant amount of tilt both in the north-south and east-west planes (dem70-71_coreg). the additional compensation using the deviation surface obtained with kriging resulted in another improvement (dem70-71_final). the rmse of the hdem relative to the mdem was calculated, and the total rmse was calculated using the relative and the mdem rmse (formulas (1), (2); table 1). the final hdem had an rmse of 5.87 m. although this is a significant possible error when dealing with landscape change, this is mostly due to the original data quality. considering the fact that the original 2007 dem has an rmse between 3.76 m and 4.51 m in the molise region, and a resolution of 10 m, the 5.87 m rmse of the final hdem was deemed an acceptable result 𝑅𝑀𝑆𝐸rel = √ ∑ (ℎ𝐷𝐸𝑀 − 𝑚𝐷𝐸𝑀)2𝑛𝑖=1 𝑛 (1) 𝑅𝑀𝑆𝐸tot = √𝑅𝑀𝑆𝐸rel 2 + 𝑅𝑀𝑆𝐸mdem 2 . (2) even though there are limits due to the rmse of the final hdem, the resulting geomorphological change map obtained by subtracting the mdem from the hdem serves as a useful indication of the changes around isernia. the resolution is not high enough to allow detection of more subtle changes; however, especially since in this procedure no physical gcps are necessary, it may be used to better select the sample areas during the planning of archaeological fieldwork. additionally, its results can assist the interpretation of the data collected by the pedestrian surveys. as such, it can help improve existing archaeological models, for example about site distribution, in a way not dissimilar to the soil map analysis by casarotto et al. [31]. a good example of the changing landscape in the area can be found south of the town of isernia, where a quarry has expanded dramatically in a few decades, leaving a lasting effect on the landscape (figure 9). the mask that was created to exclude forested areas from the geomorphological change map, can also provide a visual indication of the rapidly changing land use in the area. plots of land are no longer being used for agriculture and have been abandoned and subsequently reforested, which can clearly be seen when comparing the situation in 1970-71 with the situation in 2007 (figure 10). 5. discussion an important challenge with the presented research was the need to use input from different sources to create the hdem, and the deviations related to this approach. in order to create a georeferenced model, gcps with x, y, and z values are required. the historical aerial photos were not georeferenced, which is normally solved by using either gps gcps with xyz values and error below 10,0 cm, or by utilising lidar data to co-register the hdem. this kind of data was not available for the current research, which heavily influenced the ibm workflow. due to data limitations, there are possible errors originating from the use of the 2007 orthophoto to extract xy values and the 2007 dem to extract z values, and combining these into gcps, rather than collecting gps coordinates in the field and importing these as markers. creating and placing gcps manually like this increases the chance of user error. since the procedure is based on a visual comparison of the recent and historical images with varying resolutions, it is possible to have an error of several metres in xy value. the current research focuses mostly on changes in z value, but the aforementioned bias should not figure 7. detail of 1970-71 3d model. orange line representing 5 km. figure 8. profile comparison between elevation models in north-south (top) and east-west (bottom) direction. table 1. rmse values. relative rmse total rmse before compensation 6.48 m 7.49 m after compensation 4.51 m 5.78 m 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000 400 450 500 550 600 650 700 750 800 dem07 dem70-71_orig dem70-71_coreg dem70-71_final e le v a ti o n ( m ) distance (m) 0 1000 2000 3000 4000 5000 6000 7000 300 350 400 450 500 550 600 650 e le v a ti o n ( m ) distance (m) dem07 dem70-71_orig dem70-71_coreg dem70-71_final acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 6 be neglected, since a change in the horizontal plane can result in significant changes in elevation, especially in irregular terrain. the accuracy of the final model depended on the resolution and accuracy of the 2007 orthophoto, the 2007 tinitaly dem, the 1970-71 historical aerial photographs, and the user error when placing the various gcps. while the resolution and accuracy of the 2007 dem was known, this was not the case for the 2007 orthophoto and the 1970-71 aerial images. further limitations were imposed because the properties of the scanner used to digitise the original images were unknown. therefore, scanner deformities may have been introduced, contributing to possible errors in the final model. this research made an attempt at correcting some of these errors by applying filtering, coregistration, and compensation, leading to significant improvements. 6. conclusions the main goal of this research was the creation of a composite ibm workflow to build historical landscape models from historical aerial photographs using sfm, and extracting information about geomorphological change. by applying data from other sources such as a more recent orthophoto and dem to obtain ground control points in gis that could then be entered in the sfm software, it was possible to create historical orthophotos and hdems. the hdem for 1970-71 was further filtered and co-registered to the mdem from 2007. subsequently, it was corrected by applying interpolation to create a vertical deviation surface from other gcps. this resulted in a compensated hdem that was then subtracted from a modern dem in order to observe geomorphological change, while areas with buildings or vegetation were masked. although there are severe limitations resulting from the quality of the initial data in this case study, the proposed composite workflow appears effective for the creation of more accurate historical 3d models and geomorphological change maps of rapidly evolving landscapes. geomorphological change has an inherent duality, both positively (uncovering) and negatively (displacing/covering/destroying) influencing the visibility of the archaeological record. despite this, geomorphological change maps can be used to provide feedback for planning pedestrian surveys and to interpret their data critically, possibly assisting in the assessment of its accuracy and the improvement of archaeological models. author contribution manuel j. h. peters: conceptualisation, methodology, software, formal analysis, investigation, data curation, writing original draft, visualisation. tesse d. stek: validation, resources, writing – review and editing, supervision, project administration, funding acquisition. acknowledgement the first author would like to thank the lerc project for the data, and the royal netherlands institute in rome (knir) for the opportunity to spend several weeks in rome to work on this research in a stimulating environment. luisa baudino was of great help with the processing of the dem profiles. a significant portion of this work was carried out as part of an msc thesis [32] at the faculty of archaeology of leiden university, under the supervision of dr. karsten lambers. references [1] p. attema, two challenges for landscape archaeology, in p. attema, g. j. burgers, e. van joolen, m. van leusen and b. mater (eds), new developments in italian landscape archaeology. theory and methodology of field survey. land evaluation and landscape perception. pottery production and 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[28] s. tarquini, l. nannipieri, the 10 m-resolution tinitaly dem as a trans-disciplinary basis for the analysis of the italian territory: current trends and new perspectives, geomorphology, vol. 281, 2017, pp. 108-115. doi: 10.1016/j.geomorph.2016.12.022 [29] s. tarquini, i. isola, m. favalli, f. mazzarini, m. bisson, m. t. pareschi, e. boschi, tinitaly/01: a new triangular irregular network of italy, annals of geophysics, vol. 50, no. 3, 2007, pp. 407-425. doi: 10.4401/ag-4424 [30] s. tarquini, s. vinci, m. favalli, f. doumaz, a. fornaciai and l. nannipieri, release of a 10-m-resolution dem for the italian territory: comparison with global-coverage dems and anaglyphmode exploration via the web, computers & geosciences, vol.38, no.1, 2012, pp. 168-170. doi: 10.1016/j.cageo.2011.04.018 [31] a. casarotto, t. d.stek, j. pelgrom, r. h. van otterloo and j. sevink, assessing visibility and geomorphological biases in regional field surveys: the case of roman aesernia, geoarchaeology, vol. 33, no. 2, 2017, pp. 177-192. doi: 10.1002/gea.21627 [32] m. j. h. peters, bypassing the bias. applying lmage-based modelling and gis to assess the effect of geomorphological change, topography, and visibility on survey data from the early roman colony of aesernia, ltaly, msc thesis, leiden: universiteit leiden, 2019. https://doi.org/10.1080/00438243.1978.9979712 https://doi.org/10.1007/s10816-017-9348-9 https://doi.org/10.3390/rs8030199 https://doi.org/10.1016/j.geomorph.2016.05.029 https://doi.org/10.1260/2047-4970.2.3.395 https://doi.org/10.1002/arp.1539 https://doi.org/10.1016/j.envsoft.2010.03.014 https://doi.org/10.1111/j.1475-4754.2012.00667.x https://doi.org/10.3390/rs8090752 https://doi.org/10.1016/j.jas.2012.02.022 https://doi.org/10.1109/lgrs.2011.2175899 https://doi.org/10.1016/j.envsci.2015.12.011 https://doi.org/10.1016/j.geomorph.2016.12.022 https://doi.org/10.4401/ag-4424 https://doi.org/10.1016/j.cageo.2011.04.018 https://doi.org/10.1002/gea.21627 review of models and measurement methods for compliance of electromagnetic emissions of electric machines and drives acta imeko issn: 2221-870x june 2021, volume 10, number 2, 162 173 acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 162 review of models and measurement methods for compliance of electromagnetic emissions of electric machines and drives andrea mariscotti1, leonardo sandrolini2 1 diten, university of genoa, via opera pia 11a, 16145 genova, italy 2 dei "guglielmo marconi", university of bologna, viale del risorgimento 2, i-40136, bologna, italy section: research paper keywords: electric machinery; electromagnetic compatibility; electromagnetic emissions; power converters; uncertainty citation: andrea mariscotti, leonardo sandrolini, review of models and measurement methods for compliance of electromagnetic emissions of electric machines and drives, acta imeko, vol. 10, no. 2, article 23, june 2021, identifier: imeko-acta-10 (2021)-02-23 section editor: ciro spataro, university of palermo, italy received february 7, 2021; in final form april 24, 2021; published june 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: andrea mariscotti, e-mail: andrea.mariscotti@unige.it 1. introduction electric machines (encompassing dc and ac motors and generators) and adjustable speed drives (asds) are classified as apparatuses for the electromagnetic compatibility (emc) directive 2014/30/eu [1] and therefore require to be ce marked. the method of demonstrating compliance with the essential requirements of the emc directive is through the application of and qualification to the product standards iec 60034-1 [2]-[4] and iec 61800-1, iec 61800-2 and iec 618003 [5]-[8], respectively, and of the generic emc standards [9]-[12]. it is quite possible that a manufacturer would refer to other applicable standards such as ieee std. 1566 [13] depending on market and client preference. a machine may represent also a component of some apparatus intended as the final product, possibly defining different limits and requirements of emissions. in this case not only the machine manufacturer should consider and apply the correct standards to guarantee compliance of emissions in the final application, but also the integrator and manufacturer of the apparatus must take suitable margins. it is intuitive that for off-the-shelf products test and certification are not repeated and the burden of guaranteeing compliance falls on the shoulders of the latter. when considering the electromagnetic (e.m.) field emissions of electrical machines and power drives, some elements may be identified that in general affect the amount and characteristics of emissions and their measurement. • conducted emissions generated by additional sources of electromagnetic disturbance (emd) can feed the electric machine and then be coupled to the space around it through its windings as radiated or reactive field (that, for simplicity, will be referred to as radiated field). these sources may be the converter driving the motor itself or the converter feeding the field winding of a synchronous generator. • the electric machine may be of considerable size, so quite different from a point source, and is often located in an environment with non-ideal electromagnetic properties [14]. • the e.m. environment features several near and far sources of radiated e.m. fields, such as workshop facilities (e.g., crane bridge) and auxiliaries (e.g., lubricating pumps, ventilation fans and compressors). as specified in the relevant standards, measurement and evaluation of e.m. emissions only cover the high frequency range, starting from 30 mhz, where there is little evidence of machine emissions. in [14], measured emissions spectra below 30 mhz are presented and there is evidence of emissions due to the tested machine by itself, having excluded contributions of any feeding converter or auxiliary system. the connection of the electrical machine to a driving static converter, such as in an asds, makes abstract electric machines and power drives of various sizes and ratings are fundamental elements in many applications, such as electric appliances, electric propulsion, medical systems, all with specific limits of electromagnetic emissions, different from those of the original emc product standards. as compliance should be assessed and guaranteed during design and procurement, this work compares the conducted and radiated emission limits and discusses the variability of results for the respective test setups and measurement methods, including the most relevant source of uncertainty. mailto:andrea.mariscotti@unige.it acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 163 the problem even more complex, mainly for the following two reasons: • the static converter becomes the major source of conducted disturbance, that results in e.m. emissions from the various elements of the asd (the connecting cables and the electric machine) [15]-[19]; • the static converter is also a source of direct e.m. radiated emissions, that is superimposed on those already considered above [20]-[25]. electric machinery features nowadays a wide range of technologies: asynchronous and synchronous machines, based on induction, salient pole, reluctance, permanent magnet, brushless, etc. correspondingly, the electromagnetic behaviour is quite varied, especially at low and medium frequency where the intensity and distribution of prevalently magnetic field emissions depend on the machine architecture and its design. this part of the spectrum of machine emissions is often used for diagnostic purposes [26], [27] and is less relevant from an electromagnetic compatibility (emc) point of view. electric machinery has its own electromagnetic behaviour and response, from which typical radiated emissions profiles that depend on machine architecture, size, rated values and characteristics. specific resonances and amplification of some components of emissions have been observed with extensive data for synchronous generators [19], [28], [29]. for those reported in [28], [29] a significant influence of some machine defects and local failures was identified; once fixed by corrective maintenance specific emissions around 1 mhz disappeared. in [19] the generators were all new and measured during the commissioning phase, excluding any kind of defect and latent failure. similarly, dc machines are characterized by peculiar emissions caused by commutation under the brushes, as repeatedly demonstrated experimentally [15], [30]-[32]: the main conducted emissions are due to arcing of motor brushes, modulated by rotational speed (with a spectrum frequency that depend on the number of brushes, their extension and the mechanical speed), although experimental evidence for radiated emissions from large motors lacks of evident resonance peaks and dependency on input current [15]. different and more complex is the behaviour of a power drive, when the motor is excited by the conducted emissions of the driving converter, including resonance effects of the connecting cable [33]. to the aim of radiated emissions and disturbance propagated to external elements, the most important is the common-mode component, that has a higher radiation efficiency at longer distance, from which the use of shielded cables, common mode ferrites, image planes, in particular in the smaller machines [34], as control measures. however, in highdensity applications with short coupling distances also differential-mode components become relevant [35], [36] and they are usually less easier to filter, due e.g. to the large phase currents of modern high-performance drives. in order to demonstrate compliance of the emissions to the emc directive, design calculations (usually carried out with numerical electromagnetic software) and examinations can be provided; however, most often the only degree of demonstration of compliance consists of the execution of tests in line with the applicable product standards to show that the so assessed emissions are below the stipulated limits. the paper is organized as follows. section 2 contains a general description to the principles of emission in order to frame the problem and introduce the other sections on normative requirements, modelling approaches and measurement results. section 3 in fact discusses the normative requirements specific for electric machines and asd and for the most common applications and environments. modelling of e.m. emissions is then considered in section 4, paired to section 5, where experimental results are discussed. both the characteristics of the e.m. environment and equipment, as well as the limitations and performances of models and measurement methods are discussed in section 6, identifying the impact on the uncertainty of the estimate of emissions and compliance to limits. 2. general description and principles of emission the spectrum of relevant emissions extends from the machine operating frequency to several mhz, depending on the type of machine (dc or ac supply, use of a commutator and brushes, number of slots and rotational speed, etc.) and on the characteristics of the driving voltage waveform. in general, the machine alone may be considered a source of peculiar, although less intense, emissions, if some phenomena are considered [15]: • in synchronous machines, the static magnetic field emitted by the dc field winding is modulated by rotation at the synchronous speed; • the magnetic field, in relation with the current flowing through each conductor in the machine slots, is modulated by the change of reluctance due to the passage of teeth at rotation speed [37]; • commutator machines, and in particular dc machines, produce intermittent magnetic and electric field emissions, occurring during spark ignition and extinction under the brushes, with the field intensity determined by the machine operating conditions in terms of supply voltage and circulating current, that influence the characteristics of electric arcs (number of arcs, their average lifetime, their stability in space) [15], [18], [32], [38], [39]. limits for radiated emissions are specified in emc product standards at high frequency only (above 30 mhz), while for the low frequency range (between 9 khz and 30 mhz) limits are specified for conducted emissions. a large number of ac motors are fed by pulse width modulation (pwm) inverters, even for medium and large power machines. the output voltage sourced by the inverter and appearing at motor terminals is a variable duty-cycle square wave characterized by steep slopes (often identified as affected by a “high dv/dt”). for very large voltage ratings, to reduce the “instantaneous voltage step”, multi-level inverters are used, so that the waveform shape and the total harmonic distortion, thd, are improved. in any case, large dv/dt values and associated spectra (quite extended over the frequency range) are applied to the machine windings, so that the e.m. characterization of the whole drive becomes very important. to characterize the asd as a source of radiated emissions, it is necessary to locate the different sources of disturbance and the paths of propagation and coupling. an asd may be thought of as composed mainly of a static converter (for induction motors most commonly a voltage source inverter, vsi), the feeding cable (a 3-phase cable with different arrangements: three single-phase cables, an optional neutral conductor, twisted and/or shielded, etc.) and the electric motor. the inverter is the main source of emissions, conducted through the feeding cable and radiated directly through the enclosure by a series of elements, such as edges, acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 164 ventilation slots, small doors, etc. and as stray surface current on the enclosure panels. the inverter conducted emissions may radiate directly from the cable [16] , [17] or propagate towards the machine via conduction and then radiate more efficiently from the machine windings [20], [32], [33], [38]-[49]. the characterization of all these phenomena is relevant to the emc of the product, as well as for a series of related internal phenomena which can be used for diagnosis of machine health status, such as ageing and stress of winding insulation, iron and copper losses, temperature rise in specific points, stray currents flowing through the machine shaft and bearings. in particular, the most relevant emc issue for asd drives is the influence of switching and the high dv/dt of the feeding converter; while switching waveforms produce both differential mode and common mode disturbances, high dv/dt rise and fall slopes manifest themselves as significant common-mode currents flowing through parasitic capacitance and feeding cable capacitance [48]. for diagnostic purposes, the analysis of the e.m. signature of the machine is used for the detection of partial shortcircuits inside windings or mechanical unbalancing faults [43][45], relevant in the medium frequency range (500 hz 10 khz), and to obtain information on partial discharge activities, as well as insulation quality, in the higher frequency range [20], [47]. it is underlined that the insulation quality influences the value of the inter-turn capacitance as well as the dielectric losses, and they produce measurable modifications of the behaviour of winding resonance, triggered by high frequency disturbance from the inverter itself [49]. in the next section published experimental and modelling results for e.m. characterization of electrical machines are reviewed and evaluated, with specific reference to the significance of the presented and achievable results and to the uncertainty that characterizes them. 3. normative requirements relevant product standards are the iec 60034-1 [2] for electrical machinery (sec. 13 of the standard regards emc) and iec 61800-x [5]-[7] for power drives (so covering the wide range of equipment “power drive”, “variable speed drive”, “variable frequency drive”, “electronically controlled motor”, etc.). since the first issues in the ‘90s when the problem of emc was concretized and a first set of basic emc standards was available, these standards have undergone about three revisions in the last twenty years. the limits and measurement methods for emissions are inspired to those of cispr 11 long ago [50], with some variations of the limit values for increasing size of the power drive, but without addressing the problem of integration and embedding. if for a large power standalone drive the problem is non-existent (typical installation at industrial sites allows for plenty of space, good ground reference and good cable routing policies), for modern smart products, as well as automotive, avionics and naval applications, the space constraints are significant [47], [51]-[64]. applications may include additional requirements in terms of type of measurement of emissions and extended frequency range (e.g. e-field measured onboard ships from 150 khz [65], [66], or lower limits [67], [68]), and specific limits for co-located apparatus (e.g. radio receivers inside and around vehicles [69], [70] and navigation aids onboard ships [66]). standards for medical applications do not pose critical limits of emissions [71], although for equipment using motors and power regulators reference is to cispr 14-1 [72] and possibly additional tests (the so called “click emissions”). possible interference to worn and implantable medical devices (defibrillators, pacemakers, etc) was investigated in the working environment, in connection to heavy work such as welding, electro-erosion and galvanic processes, but also considering proximal motors and power drives [73]. the discussion will focus on the test integration for motors and power drives, starting from their own product standards with the intention of integration in other equipment to which the emc standards for the final application apply. besides lower limits and more extended frequency intervals, the other elements affecting the uncertainty of a statement of conformity are the reduced distance and environmental conditions of modern compact high-density applications, e.g. for automotive. the limit requirements, the degree of agreement between the specific emc product standards and with the other emc standards for generic and specific applications, as well as the evolution of the standards through the successive issues in the last twenty years, are discussed in the following. the emission standards for generic and specific applications correspond to first the generic light industrial and industrial ones (iec/en 610006-3 [11] and iec/en 61000-6-4 [12]), and then medical (en 61010 ), railway (en 50121), ship and offshore (iec/en 60945, en 60533) and automotive (cispr 12, reg. unece 10, cispr 25). 3.1. iec/en 60034-1 iec/en 60034-1, sec. 13, focuses on the emc of electrical machinery, for which in the absence of electronic circuits the immunity is straightforwardly assured, whereas for emissions the limits of cispr 11 class b and class a are assigned to machines without and with brushes, respectively. the limits for radiated emissions are the same for all machines, corresponding to the “usual” 30-37 dbµv/m profile for 30-230-1000 mhz; conducted emissions must comply with 66-56-60 dbµv (class b) or the 7973 dbµv (class a) profiles, defined over 0.15-30 mhz. in the 2004 version there are several inaccuracies in the notes of sec. 13.5.2: the standard prescribes a test at no load stating that machine emissions do not depend on load, not in line with what observed in [15], [19], both for dc machines (with brushes) and for other types. the dc machine is said not to have conducted emissions because it is not connected directly to the ac supply, that is somewhat misleading looking at the evidence provided in [30], [31]: it is acknowledged that such emissions are not injected directly into the ac supply distribution, but may cause as well crosstalk to other cables within the cableway or cable harness. the iec/en 60034-1 indicates that no tests are needed for cage induction machines. curiously the iec/en 60034-x standards do not specify limits or tests for synchronous generators although several tests indicate a resonant behaviour and amplification of radiated emissions [14], [15], together with a specific excitation coming from the rotating converter connected to the field winding. prescriptions have not changed between versions (2004 [2], 2010 [3] and 2020 [4]). 3.2. iec/en 61800-3 the iec/en 61800-3 is the emc product standard for power drive systems and reads “the requirements were selected so as to ensure emc for pdss at residential, commercial and industrial locations, with exception of traction applications and electric vehicles” (applications with compact installation and minimum separation). emissions for railways and guideway applications are characterized by specific limits and measurement methods [74]: besides fast peak-detecting scans in frequency domain, time acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 165 domain characterization is often advisable for transient emissions, in particular for disturbance to telecom systems [75]. the possible use for medical applications is also not mentioned explicitly. when the drive is part of larger equipment, the iec/en 61800-3 gives way to the final equipment product standard with possible non-compliance and inadequacy of emission levels. limits of emissions for pds are stipulated for four categories: the first two categories (c1 and c2) correspond to the limits of emission for the electric machinery alone (iec/en 60034-1), except for the limit of radiated emissions increased by 10 db (4047 dbµv/m) with respect to the 30-37 dbµv/m of c1 (see table 14 and 15 of iec/en 61800-3). it is evident that in those 10 db there is no margin for the emissions of the converter, although emissions from the motor alone are expected to be much lower than limit. a more complex scenario characterizes pds of cat. c3, whose limits for conducted emissions are shown in table 17 of the standard: distinction is made for the nominal current below or above 100 a, neglecting the voltage that has a significant impact anyway; the limit for radiated emissions is increased by another 10 db with respect to cat. c2 (50-60 dbµv/m), but a note says that these limits will be reconsidered in accordance with the results of ongoing activity within cispr. 3.3. emission standards for generic and specific applications the most likely standards for generic and specific applications (light industrial and industrial, medical, railway, ship and offshore applications) are briefly reviewed, in order to frame the emission requirements for products, possibly including motors and power drives. the iec/en 60601-1-2 [71], sec. 7.1.7, refers specifically to cispr 14-1 [72] for equipment whose main function is performed by motors or regulating devices (such as dental drills, surgical tools, operation tables). the cispr 14-1 [72] has the additional test of the intermittent emissions (clicks) that is seldom carried out on industrial products. as shown in figure 1, automotive applications (reg. unece 010) have a complex normative with specific measurements and in some cases quite low limits; in addition, measurement may occur at 3 m distance for which an increase due to reactive field region may be relevant at the lower end near 30 mhz. onboard ships there are specific regulations for disturbance to radio and navigation systems (iec 60945 [66], not included in figure 1), but also two basic standards are applied common to the automotive sector (absorbed by the reg. unece010 [68]): cispr 12 [67] and cispr 25 [69] for protection of off-board and on-board radio receivers. it is noteworthy that conducted emissions limits as per cispr 25 extend significantly and decrease with frequency, partially addressed by cat. c1 and c2 limits, although additional control measures would be required. 4. models of electromagnetic emissions the high-frequency models of motors can be basically classified as behavioural models, physical models, and finiteelement models. the different standpoints are briefly described in the following. • equivalent circuit constants of the motor model can be determined by measurement and fed to a so-called behavioural model, where the physical meaning of some of the time constants is somewhat lost. this approach is practical and has been the preferred method of choice so far; motor models without rotor [20], [44], [45], [76], [77] and with rotor [78] have been reported. the essential problem is that a real motor is required for measurements before simulation; this prevents the application of this approach and the model during the design stage, before prototyping. • if an accurate equivalent circuit is made, then the physical meaning of each circuit constant is preserved; these constants are then calculated based on design parameters using either numerical formulae [79]-[84] (as it is done in [38], [39], [48] for a dc motor, based on an interpretation of machine physics) or by e.m. field analysis [81], [85], with the possibility of estimating the electrical parameters of inaccessible elements. obviously, this latter approach is computationally intensive, and some parameters may be qualified by a large uncertainty, especially when designing a new machine. • finally, a full finite element modelling approach is possible, where the model is used for prediction of both conducted and radiated phenomena, it is directly connected to the source (i.e., the converter or the power grid, still modelled by means of a circuit approach) and thus it is necessarily 3-d [44]. in [85], the calculation of machine electrical parameters is made by 2-d and 3-d modelling for bulk and laminated cores, respectively. a review has been carried out of published work where models are validated against experimental data; the outcomes are a) b) figure 1. limits of emissions (quasi peak): (a) conducted, (b) radiated (10 m). color coding: iec 60034-1/b & iec 61800-3/c1 (magenta), iec 60034-1/b & iec 61800-3/c2 (red), iec 61800-3/c3 (100a green, 100a yellow), iec 61800-3 equiv. magn. field (cyan), cispr25 (blue), unece10-esa (black solid), unece10-veh (black dotted). acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 166 discussed in the following focusing on the accuracy of the models, their reliability and simplicity of application. for a 3 kw induction machine, in [45], the validation was extended beyond a simple geometry and preliminary results show an agreement within 10 db. the model is simplified considering common-mode only, which is largely responsible for the largest emissions generated by these machines. the equivalent circuit approach followed in [77] is approximately valid, in general, up to 10-15 mhz with respect to the measurement results, although the shown discrepancy is significant. one of the possible causes of discrepancy is the inaccuracy in the calculation of the inductance, which depends on the magnetic field analysis hypotheses taken into account. the approach followed in [44], [76] is behavioural and the two proposed models are based on simplified equivalent circuits for all the drive elements, with parameters adjusted by fitting measurement results. the overall drive model is thus in agreement with measurements of conducted phenomena up to approximately 10-15 mhz. in [44], sample results of the 3-d modelling of a 3 kw motor are presented, but the validation of the approach (circuit simulation followed by 3-d model solved by finite integration algorithm, similar to fdtd) was made only on a simple wire geometry and is thus not conclusive: the agreement is very good up to 90 mhz, except for a discrepancy of up to 10 db between 35 and 50 mhz due to different heights of the radiating wire with respect to ground in measurement and simulations. in [76] a behavioural model is used to predict the conducted emissions of a motor drive system. the models of the four-wire shielded cable, the induction motor and the pwm voltage inverter (modelled as a common-mode source only) are used for the time-domain simulations of the adjustable speed drive. the comparison with measurement shows that conducted emissions can be predicted with acceptable accuracy up to 10 mhz; at higher frequency the discrepancy can be motivated by the noise introduced by the voltage probes and the oscilloscope characteristics. a simple, but effective, formulation is proposed in [41], where common-mode currents are neglected in favour of differentialmode currents; the radiating properties of the machine are then modelled by equivalent loops under a far-field assumption, that holds for the higher portion of the frequency interval, approximately up to 50 mhz. analogously, for a dc motor, benecke [38], [39], [48] presents an equivalent circuit approach, where the parameters are estimated by closed form expressions, that are made fitting measured impedances of some motor elements; the motor is subdivided into armature windings, commutator blades, brushes, other wiring and casing. the results are very encouraging with the terminal impedance fitting the radiated measurement results up to 1 ghz, with a negligible error almost everywhere on the frequency axis, except for an underestimated resonance peak at 600 mhz. the conducted emissions, on the contrary, match the experimental results only up to about 150 mhz, where the author says that supply line effects become predominant. finally, as regards the finite element modelling approach, finite element methods are based on standard e.m. codes, implementing for example method of moments, mom (well suited for wire structures) and finite difference time domain, fdtd (particularly useful for the computation of switching transient effects) [86]. these models are normally fed by excitation currents that are in turn calculated by a full or equivalent circuit approach. in conclusion, the determination of the e.m. emissions proceeds as follows: • simulation of the circuit representation of the ensemble converter plus cable plus motor (the latter modelled as an equivalent circuit) and calculation of the excitation currents/voltages (the prediction of conducted voltages and currents may be very accurate as in [47]); • application of the excitation signals to either an equivalent circuit [41] or finite element method model. the limitations of this approach may be stated as: • correct modelling of both commonand differential-mode circuits and estimation of the related sources and signals [42]; • adequate modelling of parasitic components and in particular turn-to-turn and turn-to-frame/core capacitance terms [87]; • correct application of the signals to the circuit elements and ports [45]; in particular, if a behavioural or equivalent circuit approach is chosen (also to match experimental observations and measurements at accessible terminals), commonand differential-mode signals need to be distinguished; depending on frequency and on the variability of the unknown and less controllable parameters, differential-to-common mode transformation may take place [45]. a simple worst-case model can be based on the assumption that the level of emissions is the maximum allowed by the limits of a given class or product, for emissions measured at the prescribed distance d*. the intensity of the e.m. field is then extrapolated to other distances d assuming a given dependency on distance. the presence of a near-field region around the machine and the reactive behaviour of the radiated emissions imply that the expected field intensity is much larger, no longer linear with distance (far field assumption): terms with the square and cubic power of distance should be considered as well. machine feeding cables and their high-frequency currents can in fact be treated as hertzian dipoles or as a chain of short dipoles, whose radiated field in the near-field region is composed of the electrostatic, induction and radiation field terms. in figure 2 the maximum allowed field intensity (emission limits at the standardized measurement distance) is extrapolated to shorter distances d characteristic of modern applications. the extrapolation is achieved with the following assumption: the radiating element is a small part of the overall power drive and far-field formulas for dipole antennas are used (1/ = /(2 )), rather than those of large antennas. extrapolation is carried out including second-order terms (1/( d)2) and third-order terms (1/( d)3); terms are rms composed assuming arbitrary timephase relationship. at distances of 1 m the extrapolated field intensity is significantly larger than the values obtained with a far-field assumption and at the limit of the separation of category of emissions (10 db). a distance of 0.5 m is evidently a significant issue because all components in the most relevant frequency interval up to 100 mhz are well beyond control. it is underlined also that such components have lower wave impedance than in far field, and the effectiveness of conductive shields is reduced. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 167 5. experimental characterization of e.m. emissions there are several experimental results related to motor plus converter setups (such as for industrial or automotive asd, where it is known that the converter is the major source of conducted and radiated emissions), but very few references report on the e.m. characteristics and contribution of the motor as a standalone item and/or the motor as a radiating source, converting conducted emissions. the mechanism of emission for dc electric motors is related to the commutation at collector brushes and the arcing phenomena, while for ac motors it is due to the modulation of the machine magnetic field produced by the interaction of stator and rotor slots during rotation. 5.1. dc motor the main source of e.m. emissions considered in literature is that of the commutation at motor brushes and arc ignition/extinction. insights into the phenomenon of commutator sparking (flashovers) and arcing in electrical machines are given in [88], [89]. in [90] several methods are investigated and discussed to assess the quality of commutation in dc traction motors, such as motor current signature analysis, antenna pickup of sparking, photodiode measurement of sparking, acoustic analysis, and o3 and nox detection. with these techniques poor commutation can be monitored, and actions can be taken to prevent sparking, which is associated to wide band e.m. emissions. for example, motor current signature analysis requires the motor current to be analysed in the frequency domain at the barpassing frequency, which is determined by multiplying the number of commutator bars by the speed of the motor. two laboratory tests were carried out [90], the former on a 10 hp industrial motor, the latter on a standard 750 hp locomotive traction motor; both motors were in the steady state. in [91], the analysis of the motor current takes advantage of wavelet analysis. individual sparking events may be observed during the commutation even at low frequencies, making it possible to assess the quality of commutation even without expensive radiofrequency instrumentation. the radiation mechanism on the contrary is not directly from the electric arc itself, but from the motor windings, that are simulated with various circuit geometries, as described in detail in [18]. in figure 3, the results of the calculated e-field emissions (under two different assumptions concerning the model of the arc, constant voltage and constant e-field) are shown and compared with measurement results [18]. by inspection of the two calculated curves shown in figure 3, it is evident that the constant voltage assumption is reasonably valid and appropriate, as confirmed by the results of the measurement of the winding voltage and current during commutation. suriano et al. [18] state that the emissions prediction by either constant-voltage or constantelectric field model of the commutation is inaccurate below 500 khz. on the contrary, this limit was experimentally determined as the boundary between significant and negligible emissions from a large power dc motor [15], by comparing directly the measured emissions and background noise. the comparison is shown in figure 4, where the difference is in excess of 20 db below 500 khz, and only of 10 db up to 3.5 mhz (and then not appreciable). the difference may be justified if the attenuation produced by motor enclosure and cable harness present in [15] is taken in account, whereas such provisions are not accounted for in [18]. additionally, the motors considered in the two publications are of a different size. in general, motors with a larger rated power have additional shielding provided by the heavy enclosure, ventilation, etc. an accurate model of the winding impedance validated against experimental data is presented in [38], [39], [48] with remarkable accuracy up to 1 ghz for the terminal impedance and up to 150 mhz for the conducted emissions. in a) b) figure 2. extrapolation of maximum emissions to (a) 1 m and (b) 0.5 m distance; iec 61800-3 c1 (magenta), c2 (red), c3 (green); darker curves indicate inclusion of second order (thin) and third order (thick) terms. figure 3. measured (grey curve) and calculated (hollow diamond, constant voltage arc assumption, and filled diamond, constant e-field arc assumption) dc motor emissions [18]. 10 5 10 6 10 7 10 8 10 9 0 10 20 30 40 50 60 70 80 90 100 110 120 frequency [h z] e le ct ri c f ie ld [ d b u v /m ] acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 168 [39], an equivalent circuit model is proposed and the single elements are modelled: turns geometry is accounted for in the calculation of mutual inductance and capacitance and to derive the windings input impedance over an extended frequency range. in [32], it is underlined that practical factors can influence the resulting electrical parameters, such as the fact that windings are not evenly distributed due to shape changes and that they can vary among different units. the consistency between the simulation and the experimental results was shown by the authors for three tested motors; for two of them, in particular, the agreement is very good, with remarkable differences only concerning the peak height at resonances, as it could be expected, because the damping factor highly depends on the correct estimation of the loss factors, in the copper, iron and insulation. 5.2. ac induction motor induction motors are the most widely used machines in asd applications. induction motors are typically fed by static inverters, mostly of the pwm type, with three or more levels for very large power applications. the converter and the feeding cable are normally assumed as the most relevant and the sole source of electromagnetic emissions [16], [17], [21], and [33], [40]-[43] and seldom the motor is considered alone as a source of emissions by itself or excited by converter conducted emissions [15], [19]. so, if the focus is on the converter conducted emissions in terms of the resulting commonand differential-mode currents following the converter commonand differential-mode voltage waveforms during commutations, the accurate modelling of cable [16] and motor [49], [92], [93], impedance terms becomes crucial. in synthesis, concerning motor differential impedance, which is the most relevant for diagnostic purposes, in [20] it is concluded that “it is the authors’ experience, confirming the conclusions of other investigators, a reasonable upper limit for modelling the machine’s differential impedance as an inductive system is in the range of 10 khz 100 khz; above this range the machine appears capacitive into the 13 mhz range; beyond approximately 3 mhz the machine resembles a transmission line.” this statement sheds some light on the fact that, above the said frequency limits, the commonmode impedance and the effects of the enclosure grounding must be considered. in [92], the shown results cover the low and medium frequency range up to 1 mhz (the limit imposed by the lrc bridge used). what is relevant is that the authors underlined the influence of the grounding of the motor enclosure; they report results for both grounded and ungrounded enclosure (the latter for comparison with other references). the results, however, are only for the differential impedance, that the authors put in direct relationship with the inverter feeding of motor phase windings. for this reason, they do not consider commonmode impedance (and the enclosure is included for the effects it has on some internal coupling terms). it is interesting to observe that enclosure grounding affects greatly the impedance curve: for the test case considering both grounded and floating enclosure, the first resonance frequency moves from 25 khz to about 60 khz, followed by a steep decay of values in the case of a floating enclosure, while the amplitude remains large, and even increases for a grounded enclosure. this can be studied for each motor to shape the impedance curve in order to reduce current flowing due to inverter voltage emissions. on the other hand, any parametric change to the internal motor circuit topology could be monitored if it influences in particular the first resonance. 5.3. synchronous machines the evaluation of e.m. emissions from electrical rotating machines has been a relatively new concern in the past decade, with only a few contributions found in the literature. in fact, they have been primarily considered a source of magnetic field at the fundamental frequency and its harmonics. instead, especially for large power synchronous machines, a number of possible sources for high frequency disturbances for the machine and its auxiliary systems can be identified. considering the extension in space of the large power electrical rotating machinery and the non-ideality of the test site, where it is necessary to perform the tests, the assessment of e.m. emissions is thus not an easy task. in [15], the assessment of e.m. emissions from large electrical rotating machines is presented. figure 5 and figure 6 show the magnetic field measured along the vertical axis of the machine generated by a separately excited synchronous generator and a brushless synchronous generator, respectively. three different tests are considered: generator moved by the driving dc motor, no-load and short-circuit conditions. it is interesting to notice that the emissions are higher for no-load condition for the separately excited synchronous generator and for the shortcircuit condition for the brushless synchronous generator: thus, the short-circuit condition is not always the worst-case condition in the assessment of e.m. emissions. figure 4. measured radiated emissions from a large power dc motor [15]. figure 5. vertical magnetic field for (grey) moved, (heavy black) no-load, and (thin black) short-circuit conditions of a synchronous generator [19]. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 169 in [19], the metrological characteristics are pointed out, that describe the quality and reliability of experiments. in particular, the quality of measurements is evaluated in terms of repeatability (computed as the standard deviation of repeated measurements) and reproducibility (computed as difference of average recordings for different generators of the same type), shown in figure 7 (a) and (b), respectively. 6. uncertainty and systematic errors any statement of compliance to limits and assessment of a quantity brings along the concept of uncertainty. in the present case the measurement of conducted and radiated emissions is disciplined by the set of cispr 16 standards, and in particular cispr 16-4-1. however, the focus of cispr standards is on emc measurements carried out on equipment of small-medium dimensions that for radiated emissions fit one of the advised facilities, namely open area test site (oats), semi-anechoic chamber (sac), fully anechoic chamber (fac), tem and gtem cells. in addition, the standards do not account for ancillary equipment with a potentially significant emission contribution (such as lubricating and cooling system, auxiliary converters and machinery to drive or load the machine under test). these factors were extensively discussed in [19], addressing the problem by means of experimental results and analysing repeatability and type a uncertainty. smaller equipment (that may be a motor or a complete drive) can fit standardized test facilities and may be tested more easily with a better documented uncertainty. however, one more factor comes into play: the purpose of emc standards is to test the equipment with the minimum ancillary equipment for operation in an environment and with a test setup that maximize reproducibility. more and more often modern motors and drives are used embedded in oem (other equipment manufacturer) products and in compact applications featuring high power density, such as automotive (and in particular electric vehicles), avionics, medical and laboratory (in particular next to sensors and diagnostic instruments) or within electrical appliances (such as ventilation and conditioning, for which in [56] we see exemplification of typical issues of compliance). with a parallel to other systems featuring large apparatus, relevant factors with impact on uncertainty and reproducibility are: i) emissions from a long cable line connected to the motor or generator, that can be borrowed from the scenario of the catenary feeding line in electrified railways [94]; ii) moving source in case of vehicles and electrified transports, as well as moving industrial apparatus (e.g. for positioning or tooling); iii) variability of operating conditions for large machines that cannot be bench tested under satisfactory conditions. the electromagnetic environment is quite different from a nearly open area in far-field conditions and no scattering and reflections. in addition, the distance at which possible victims are located is very short, implying that the electromagnetic field components are in the reactive region up to quite a high frequency. the compactness of the setup that reproduces the final application impacts directly on the suitable antennas to adopt that are different from those considered by cispr and generally accepted for standardized tests: antennas of very small dimensions have a low sensitivity for which the background noise is very close not to say above the limits without using preamplification. second, the reactive behaviour of the measured emissions, with a dependency of the square to cubic power of distance, increases the uncertainty due to errors of positioning and measurement of distance, and influences also the expected level of emissions onto nearby victims in the final installation. figure 6. vertical magnetic field for (grey) moved, (heavy black) no-load, and (thin black) short-circuit conditions of a brushless synchronous generator [15]. figure 7. (a) repeatability as standard deviation and (b) reproducibility as difference of averages for the magnetic field components of two identical synchronous generators [19]. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 170 among the environmental conditions of the installation temperature and vibration are particularly important for their influence on emissions, taking into account the widespread use of motors and power drives in automotive, avionic, naval and offshore applications, all characterized by a wide range of the two environmental parameters. environmental factors represent to some extent a source of systematic error, as for the offset with respect to those applied during emc tests for certification. the variability around the typical operating points is an additional source of uncertainty. in [95] the influence of vibration on conducted emissions of a dc motor was measured and compared against the limits of conducted emissions for class 1 of cispr 25: besides a violation of limits by about 10 db caused by commutation arcing in normal conditions, the tests carried out when vibration is applied (simulating a realistic condition of use) show an increase of conducted emissions by about another 5 db on average. 7. conclusions this work has discussed the problem of electromagnetic compliance of electric machinery and power drives, by introducing and reviewing the normative references for electromagnetic emissions, the available modelling approaches and their accuracy, and the measurement methods including the influence of setup and environment, besides the different behaviour of various type of machines. for normative limits section 2 has distinguished standards for generic and specific applications, addressing the problem of applying the standards and limits for the final application. focus is equally divided between conducted and radiated emissions, although the latter is more extensively covered both for a lack of direct connection to the public distribution network and for a particular interest for diagnostic purposes. various modelling approaches have been reviewed in section 3, as a replacement of measurements discussed in section 4 and as a means of prediction of emissions of the machine or power drive installed and operating in its final application. models in general require some tuning and supplemental information for parameters and input quantities, achievable only by means of measurements, that seem thus an irreplaceable element. selected models are reported as having good accuracy, in particular up to some tens of mhz, able to cover the largest emissions also of power drives, that as a rule of thumb have an interval of emissions about one/two orders of magnitude more extended than electrical machines alone. available references for measurements were also discussed both for the main features of electromagnetic emissions of different type of machines, and for the metrological performance of systematic errors, repeatability and uncertainty. references [1] directive 2014/30/eu of the european parliament and of the council of 26 february 2014 on the harmonisation of the laws of the member states relating to electromagnetic compatibility. online [accessed 20 june 2021] http://data.europa.eu/eli/dir/2014/30/oj [2] iec 60034-1, rotating electrical machines — part 1: rating and performance, international electrotechnical commission: geneva, switzerland, 2004. [3] iec 60034-1, rotating electrical machines — part 1: rating and performance, international electrotechnical commission: geneva, switzerland, 2010. [4] iec 60034-1, rotating electrical machines — part 1: rating and performance, international electrotechnical commission: geneva, switzerland, 2017. [5] iec 61800-1, adjustable speed electrical power drive systems part 1: general requirements rating specifications for low voltage adjustable speed dc power drive systems, international electrotechnical commission: geneva, switzerland, 2021. [6] iec 61800-2, adjustable speed electrical power drive systems part 2: general requirements rating specifications for low voltage adjustable speed a.c. power drive systems, international electrotechnical commission: geneva, switzerland, 2015. [7] iec 61800-3, adjustable speed electrical power drive systems part 3: emc requirements and specific test methods, international electrotechnical commission: geneva, switzerland, 2017. 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(mps), cluj-napoca, romania, 6-9 june 2017, pp. 1-5. doi: 10.1109/mps.2017.7974460 https://doi.org/10.1049/piee.1966.0235 https://doi.org/10.1109/demped.2003.1234542 https://doi.org/10.1049/iet-epa:20050524 https://doi.org/10.1109/tia.2006.887313 https://doi.org/10.3390/en14030759 https://doi.org/10.1109/mps.2017.7974460 introductory notes for the acta imeko fourth issue 2021 acta imeko issn: 2221-870x december 2021, volume 10, number 4, 1 2 acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 1 introductory notes for the acta imeko fourth issue 2021 francesco lamonaca1 1 university of calabria, dept. of computer science, modelling, electronic and system, via p.bucci 41c, arcavacata di rende, 87036 (cs), italy section: editorial citation: francesco lamonaca, introductory notes for the acta imeko fourth issue 2021, acta imeko, vol. 10, no. 4, article 1, december 2021, identifier: imeko-acta-10 (2021)-04-01 received december 23, 2021; in final form december 23, 2021; published december 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: francesco lamonaca, e-mail: editorinchief.actaimeko@hunmeko.org dear readers, also in this issue, high quality and heterogeneous papers are presented confirming acta imeko as the natural platform for disseminating measurement information and stimulating collaboration among researchers from many different fields. i hope you will enjoy your reading. as usual, the general track collects contributions that do not relate to a specific event. as editor in chief, it is my pleasure to give an overview of these papers, with the aim of encouraging potential authors to consider sharing their research through acta imeko. an interesting technique is proposed by oleksandr vasilevskyi et al. in the paper ‘indicators of reproducibility and suitability for assessing the quality of production services’ for estimating the probability of the possible appearance of defective products or the inconsistency of the production service on the basis of indexes of suitability and reproducibility of the production process. the accuracy of the proposed estimation methodology, which includes the proposed mathematical apparatus, was estimated on the basis of the correctness index, whose assessment method is carried out in accordance with the international standard iso 57252:2002. mohammed alktranee et al. in the paper ‘simulation study of the photovoltaic panel under different operation conditions’ study the effects of temperature distribution on the photovoltaic panel at different solar radiation values and temperatures under various operative conditions in january and july. a 3d model of the pv panel was simulated using ansys software, depending on the various values of temperature and solar radiation values obtained using mathematic equations. in cognitive radio systems, estimating primary user direction of arrival (doa) measurement is one of the key issues. in order to increase the probability detection, multiple sensor antennas are used and they are analysed by using subspace-based technique. in the paper ‘an adaptive learning algorithm for spectrum sensing based on direction of arrival estimation in cognitive radio systems’, sala surekha et al. considered wideband spectrum with sub channels and here each sub channel facilitated with a sensor for the estimation of doa. in practical spectrum sensing process interference component also encounters in the sensing process. to avoid this interference level at output of receiver, the authors used an adaptive learning algorithm known as normalized least absolute mean deviation (nlamd) algorithm. rosario morello et al. in the paper ‘design of a non-invasive sensing system for diagnosing gastric disorders’ present a smart egg (electrogastrography) sensing system to non-invasively diagnose gastric disorders. in detail, the system records the gastric slow waves by means of skin surface electrodes placed in the epigastric area. cutaneous myoelectrical signals are so acquired from the body surface in proximity of stomach. electro-gastrographic record is then processed. according to the diagnostic model designed from the authors, the system estimates specific diagnostic parameters in time and frequency domains. it uses discrete wavelet transform to obtain power spectral density diagrams. the frequency and power of the egg waveform and the dominant frequency components are so analysed. the defined diagnostic parameters are compared to the reference values of a normal egg in order to estimate the presence of gastric pathologies by the analysis of arrhythmias (tachygastria, bradygastria and irregular rhythm). for military divers, having a robust, secure, and undetectable wireless communication system available is a fundamental need. wireless intercoms using acoustic waves are currently used. these systems, even if reliable, have the limit of being easily identifiable and detectable. visible light can pass through sea water. therefore, light can be used to develop short-range wireless communication systems. to realize secure close-range underwater wireless communication, the underwater optical wireless communication (uowc) can be a valid alternative to acoustic wireless communication. uowc is not a new idea, but the problem of the presence of sunlight and the possibility of using near-ultraviolet radiation (near-uv) has not been adequately addressed in the literature yet. in military applications, the possibility of using invisible optical radiation can be of great interest. in the paper ‘led-to-led wireless communication between divers’ by fabio leccese et al., a feasibility study is carried out to mailto:editorinchief.actaimeko@hunmeko.org acta imeko issn: 2221-870x december 2021, volume 10, number 4, 2 3 acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 2 demonstrate that uowc can be performed using nearultraviolet radiation. the proposed system can be useful for wireless voice communications between military divers as well as amateur divers moreover, this issue contains extended version of selected papers presented to one of the most important international and italian metrology events organized in italy: the 2020 imeko tc19 international workshop on metrology for the sea “learning to measure sea health parameters”, guest edited by prof. silvio del pizzo; the international excellence italo gorini ph.d school guest edited by prof. pasquale arpaia and dr. umberto cesaro; the 40th measurement day jointly organised by the italian associations italian group of electric and electronic measurements (gmee) and italian group of mechanical and thermal measurements (gmtt) guest edited by prof. carlo carobbi, prof. nicola giaquinto, prof. gian marco revel. the xxix italian national congress on mechanical and thermal measurements guest edited by prof. alfredo cigada and prof. roberto montanini. the strong italian contribution to this fourth issue is justified by the wish of the italian metrology community to support imeko and its journals. i hope that in the near future acta imeko will receive an equally important contribution from the other imeko member countries and beyond. francesco lamonaca editor in chief introductory notes for the acta imeko special issue on the 17th imeko technical committee 10 conference "global trends in testing, diagnostics & inspection for 2030” (2nd conference jointly organized by imeko and eurolab aisbl) acta imeko issn: 2221-870x september 2021, volume 10, number 3, 3 4 acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 3 introductory notes for the acta imeko special issue on the 17th imeko technical committee 10 conference ‘global trends in testing, diagnostics & inspection for 2030’ (2nd conference jointly organised by imeko and eurolab aisbl) piotr bilski 1, lorenzo ciani 2 1 warsaw university of technology, ul. nowowiejska 15/19, 00-665 warsaw, poland 2 università degli studi di firenze, p.zza s.marco, 4, 50121 firenze, italy section: editorial citation: piotr bilski, lorenzo ciani, introductory notes for the acta imeko special issue on the 17th imeko technical committee 10 conference "global trends in testing, diagnostics & inspection for 2030” (2nd conference jointly organized by imeko and eurolab aisbl), acta imeko, vol. 10, no. 3, article 2, september 2021, identifier: imeko-acta-10 (2021)-03-02 section editor: francesco lamonaca, university of calabria, italy received september 1, 2021; in final form september 27, 2021; published september 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: piotr bilski, e-mail: pbilski@ire.pw.edu.pl fehler! linkreferenz ungültig. dear readers, the area of technical diagnostics is one of the most significant research fields, one that involves a broad range of measurements, with various sensors, actuators and advanced computing techniques exploited. as we are becoming increasingly surrounded by the growing amount of autonomous machinery, it is crucial to ensure regular and accurate monitoring is carried out. the technical committee 10 (measurement for diagnostics, optimization & control) is responsible for fostering the research on such topics, which is expressed in terms of a wide range of activities, including the organisation of annual conferences and workshops aimed solely at the problems pertaining to fault detection, identification and location. the solutions for these problems include the array of algorithms, measurement tools and procedures applicable for individual devices and industrial processes. the 17th imeko tc10 conference held in 2020 is a perfect example of this approach. the conference was special for two reasons, the first of which relates to the covid-19 pandemic, which forced us to switch the location, originally planned for dubrovnik, croatia, to the purely virtual world. the event was then held online, using various internet technologies. this was entirely new to all involved and forced us to use new channels of communications and information-sharing techniques (e.g., cloud services and teleconferences) on an unprecedented scale. despite these challenges, the online event was met with great enthusiasm by both the participants and the invited guests. the second unique feature of the 2020 conference was that it was the second event co-organised by imeko and eurolab (croatian branch). this broadened the scope of the event to include new specific topics largely related to eurolab’s interests. the main drawback of the conference was the lost opportunity to visit the beautiful city of dubrovnik; however, we hope to be able to re-organise the conference there in the more traditional way in the future. the theme of the conference, ‘global trends in testing, diagnostics & inspection for 2030’, was supported by a wide range of topics covered by both existing papers and invited speakers. the problems covered by the speakers included the industrial standards (e.g., iso 3452, or iso/iec 17025), computational methods (reinforcement learning, artificial neural networks, etc.), applications (civil engineering or food industry), advanced measurement equipment (optical sensors or mems solutions), and significant measurement challenges (e.g., uncertainty evaluation). it would appear that the constant advancement in electronics and computer technologies has enabled an increasing number of advanced concepts to be realised. the corresponding special issue covers ten papers, which can be divided into three sections, each devoted to a different topic. the selection allows for assessing the advancements in these research and engineering fields. the section aimed at presenting and solving the general problems and challenges pertaining to technical diagnostics includes three papers. the first, ‘fault compensation effect in fault detection and isolation’, written by michał bartyś, considers mailto:pbilski@ire.pw.edu.pl acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 4 the fault compensation effect in fault detection and isolation. this is an important issue in the area of model-based process diagnostics. here, the author discusses the application of the process graph to accurately represent the monitored phenomenon, allowing for fault detection based on the residuals and diagnostic matrix analysis. the concept is illustrated in terms of examples of the liquid tank and closed-loop liquid level control system. meanwhile, in their paper, ‘estimate the useful life for a heating, ventilation, and air conditioning system on a high-speed train using failure models’, marcantonio catelani et al. cover the problem of the design and application of failure models. here, the methodology, i.e., exploiting the model-based diagnostic approach, is demonstrated using the case of the hvac system (both during the simulation and the actual data provided by the manufacturer), with the results demonstrating the capacity of the approach for correctly evaluating the ability of the monitored object. finally, in their paper, ‘integrating maintenance strategies in autonomous production control using a cost-based model’, robert glawar et al. present the novel cost-based model approach for manufacturing-process monitoring. here, various maintenance strategies for autonomous production control are presented, preceded by the definition of the cost function for comparing their efficiency, while the possible application strategies are also discussed. the section related to the novel measurement and sensing methods for diagnostics includes five papers. the first, ‘overview of the modified magnetoelastic method applicability’, by tomáš klier et al., is devoted to the specific type of sensor in exploiting the magnetoelastic method. this method is used in the field of civil engineering to evaluate the state of buildings or construction operations. the important part of this approach is the coil, whereby the strength of the magnetic field can be evaluated. both the attendant laboratory and field (the bridge structure) tests demonstrated the applicability of the method. the paper entitled ‘bringing optical metrology to testing and inspection activities in civil engineering’, by luís martins et al., is focused on the area of optical metrology in the civil engineering field. here, the dimensional measurements of the concrete structures are supported by specific digital imaging techniques (e.g., cctv cameras or laser interferometry). various applications (e.g., bridge monitoring, sewer inspection, earthquake risk analysis) indicate the importance of this type of sensing technology. elsewhere, in their paper entitled ‘vibration-based tool life monitoring for ceramics micro-cutting under various toolpath strategies’, zsolt j. viharos et al. discuss the novel method of monitoring the state of the ceramics cutting machinery using vibration analysis. here, both the timeand frequency-domain features are exploited in order to evaluate and predict the microcutting tool ‘wearing out’ phenomenon, with the analysis of the selected cnc machine allowing for determining three stages of device degradation. in their paper, ‘magnetic circuit optimization of linear dynamic actuators’, laszlo kazup and angela varadine szarka present the linear braking method used in the actuators. here, the method incorporates the design of the magnetic circuit, with the authors presenting the detailed design and the optimisation procedure for the circuit parameters. the attendant simulations demonstrated that it is possible to optimise the flux leakage and the dimensions of the circuit. finally, in their paper entitled ‘analysing and simulating electronic devices as antennas’, dániel erdősy et al. discuss the electromagnetic compatibility (emc) properties in relation to the operation of the complex antenna system. here, the equivalent circuit and the antenna directivity are calculated using the simulation tools, while the antenna arrays and the problems pertaining to the emc emission are also considered. meanwhile, the section that is focused on the computational methods and algorithms in the field of diagnostics includes two papers. the first, ‘the improved automatic control points computation for the acoustic noise level audits’, by tomáš drábek and jan holub, presents the post-processing method for the acoustic noise evaluation to estimate the comfort level of specific human living conditions. here, the control points localisation method is used to optimise the indoor noise measurement, with the attendant algorithm used to identify both long-term stationary and short-term recurring noise. overall, the authors demonstrate how to select the control points, outline various spatial conditions, and compare different layouts in terms of level evaluation accuracy and computation time. the second paper, ‘artificial neural network-based detection of gas hydrate formation’, which was written by ildikó bölkény, discusses the application of an artificial neural network to detect and prevent the gas hydrate formations in the area of industrialprocess diagnostics. here, two network architectures (nnarx and nnoe) are used as the regression machines, with the approach tested on the actual test equipment for the hydrate forming process. the results allowed for comparing the implemented network architectures in terms of prediction accuracy. suffice it to say, we wish to thank all the esteemed authors for delivering interesting, top-quality papers and all the reviewers who devoted a great deal of time and effort to reading and evaluating the manuscripts. this undoubtedly allowed for preparing the high-quality content of the subsequent acta imeko issue. secondly, our gratitude goes to prof. francesco lamonaca, the current editor-in-chief, for his devotion and support during the handling of the papers. we are extremely grateful for the chance to play the role of guest editors and hope that the papers will prove to be both interesting and useful for both research activities and practical applications alike. piotr bilski and lorenzo ciani, guest editors microwave reflectometric systems and monitoring apparatus for diffused-sensing applications acta imeko issn: 2221-870x september 2021, volume 10, number 3, 202 208 acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 202 microwave reflectometric systems and monitoring apparatus for diffused-sensing applications andrea cataldo1, egidio de benedetto2, raissa schiavoni1, annarita tedesco1, antonio masciullo1, giuseppe cannazza1 1 dept. of engineering for innovation, university of salento, lecce, italy 2 dept. of electrical engineering and information technology, university of naples federico ii, naples, italy section: research paper keywords: time-domain reflectometry; industry 4.0; microwave sensing; concrete monitoring; diffused monitoring; dielectric permittivity citation: andrea cataldo, egidio de benedetto, raissa schiavoni, annarita tedesco, antonio masciullo, giuseppe cannazza, microwave reflectometric systems and monitoring apparatus for diffused-sensing applications, acta imeko, vol. 10, no. 3, article 1, september 2021, identifier: imeko-acta10 (2021)-03-01 section editor: section editor received july 26, 2021; in final form september 1, 2021; published september 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: italian ministry of universities and research (mur) through the project ‘smarter-systems and monitoring apparatus based on reflectometric techniques for enhanced revealing’ (poc01-00118), funded under the public call proof of concept d.d. n. 467 of 02/03/2018. corresponding author: egidio de benedetto, e-mail: egidio.debenedetto@unina.it 1. introduction monitoring systems represent a key enabler for the 4.0 era, crucial not only for guaranteeing the optimal functioning of the monitored systems but also for timely interventions in case of potential failures [1], [2]. as a result, the combination of the internet of things (iot) and sensing networks has become an irreplaceable tool for achieving ubiquitous monitoring in the 4.0 ecosystem [3]-[8]. however, currently, most sensor or monitoring systems are characterised by point-type sensory information; hence, to monitor large areas, it is necessary to employ a multitude of probes. this drawback can be overcome by using diffused sensing elements (ses or d-ses) that are able to achieve many functions [10] where the use of point sensors is not recommended. most ses of this type rely on the use of optical fibre systems [11], [12], but optical systems are generally expensive, and this limits their large-scale adoption. in this paper, the diagnostic technology employed resides in time-domain reflectometry (tdr) through the use of elongated ses, which have recently been successfully used to achieve diffused monitoring [13]-[21]. tdr monitoring systems are characterised by being relatively low cost, having the potential to be customised to suit the specific needs of different applications and achieving good accuracy. for these reasons, this technique represents an interesting monitoring solution with the use of specific sensing element networks (sens) that can be permanently embedded into the system to be monitored (stbm) and used throughout the service life of the stbm. thanks to the versatility of tdr, the proposed system can be customised and applied in a considerable number of fields; but this paper focuses on three contexts: abstract most sensing networks rely on punctual/local sensors; they thus lack the ability to spatially resolve the quantity to be monitored (e.g. a temperature or humidity profile) without relying on the deployment of numerous inline sensors. currently, most quasi-distributed or distributed sensing technologies rely on the use of optical fibre systems. however, these are generally expensive, which limits their large-scale adoption. recently, elongated sensing elements have been successfully used with time-domain reflectometry (tdr) to implement diffused monitoring solutions. the advantage of tdr is that it is a relatively low-cost technology, with adequate measurement accuracy and the potential to be customised to suit the specific needs of different application contexts in the 4.0 era. based on these considerations, this paper addresses the design, implementation and experimental validation of a novel generation of elongated sensing element networks, which can be permanently installed in the systems that need to be monitored and used for obtaining the diffused profile of the quantity to be monitored. three applications are considered as case studies: monitoring the irrigation process in agriculture, leak detection in underground pipes and the monitoring of building structures. mailto:egidio.debenedetto@unina.it acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 203 i) localising leaks in underground pipelines (sen-w), ii) agricultural water management (sen-a) for the optimisation of water resources, iii) building monitoring (sen-b) through the ex-ante monitoring of concrete curing and ex-post monitoring for the detection of dielectric anomalies that result from the degradation or stress of the structure. the paper is organised as follows. section 2 describes the basic theoretical background of tdr, while section 3 describes the design and implementation of the proposed distributed monitoring system. in section 4, the experimental results of the practical implementation of the proposed sens are reported, and finally, in section 5, conclusions are drawn, and the future work is outlined. 2. theoretical background tdr is an electromagnetic (em) measurement technique typically used for monitoring purposes, such as food analysis [22], cable fault localisation [23]-[26], soil moisture measurements [27], liquid level measurements [28], device characterisation [29], [30], biomedical applications [31] and dielectric spectroscopy [20]. in tdr measurements, the em stimulus is usually a step-like voltage signal that propagates along the se, which is inserted or in contact with the system under test (sut). the signal travels along the se, and it is partially reflected by impedance changes along the line and/or by dielectric permittivity variations. through the analysis of the reflected signal, it is possible to retrieve the desired information on the sut. generally, in tdr measurements, the direct measurement output is the time-domain reflection coefficient (ρ), expressed as 𝜌 = 𝑣refl(𝑡) 𝑣inc(𝑡) , (1) where 𝑣refl(𝑡) is the amplitude of the reflected signal and 𝑣inc(𝑡) is the amplitude of the incident signal. the value of ρ is represented as a reflectogram, which shows ρ as a function of the travelled apparent distance (𝑑app ). as is well known, the quantity 𝑑app is related to the actual distance, 𝑑real, by the following equation: 𝑑app = 𝑑real ∙ √𝜀app = 𝑐 ∙ 𝑡 2 √𝜀app , (2) where 𝜀app is the effective dielectric permittivity of the propagating medium (which describes the interaction between the electromagnetic signal and the sut), 𝑐 is the velocity of light in free space and 𝑡 is the travel time, which is the time that it takes for the em signal to travel back and forth. the signal propagation velocity inside the medium depends on the dielectric properties of the material in terms of effective relative dielectric permittivity, 𝜀app, which describes the interaction between the electromagnetic signal and the sut. if the em signal is propagated in a vacuum, then 𝜀app ≅ 𝜀𝑟,air ≅ 1. however, if the se is inserted in a material different to air, then the propagating em signal will propagate more slowly, and the effective dielectric constant of the material in which the se is inserted through the estimation of the apparent length (𝑑app) is evaluated from the reflectogram: 𝜀app = ( 𝑑app 𝑑real ) 2 . (3) based on these considerations, using the tdr reflectogram, it is possible to estimate the dielectric characteristics of the propagating medium and/or to localise when these dielectric variations occur. 3. system and design implementation 3.1. diffused sensing element as mentioned in section 1, the present study focuses on three main application fields. in detail, the sen-w refers to a leak localisation system in underground water and sewer pipes, sena is dedicated to the real-time monitoring of the soil watercontent profile in agriculture and, finally, sen-b addresses the use of the tdr and elongated ses to evaluate the humidity profile of concrete structures and to identify destructive phenomena early, such as those resulting from rising damp. this section describes in detail the design and implementation of the d-ses, the tdr measuring instruments used for experimental validation and the processing algorithm adopted. before the implementation of the system, in the design phase, full-wave simulations were carried out to identify the optimal se configuration, which is useful for optimising the performance of the system in terms of sensitivity to changes in the dielectric characteristics of the stbm. the configuration of the diffused se is shown in figure 1. it consists of a coaxial cable and two conductors that run parallel to each other and are mutually insulated through a plastic jacket. the figure also shows the cross-section dimensions of the se. the sensing portion is placed along the direction of the electrical impedance profile under test so as to provide diffused monitoring of the stbm. the em signal propagation occurs between the two conductors and is influenced by the dielectric characteristics of the surrounding material; this aspect is exploited to identify the area in the reflectogram in which dielectric variations are observed. the coaxial cable has the same length as the sensitive portion and is integrated with the ses to calibrate the apparent distance as a real distance. in fact, because the dielectric characteristics of the coaxial cable are known, by propagating the tdr signal along the coaxial cable, it is possible to evaluate the actual distance of figure 1. designed diffuse sensing element: two-wire-like conductors and a coaxial cable. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 204 the se from (3), as reported in [15]. this aspect is especially important in practical applications in which the length of the dses is not necessarily known in advance. 3.2. tdr instruments and the measurement algorithm for the tdr measurements, two measuring instruments were used, the tdr-307usbm and the tdr200. the former generates a pulse signal, and the cost is relatively low (about €800). the latter generates a step-like em signal, and the cost is approximately five times the cost of the tdr-307usbm. both measuring instruments are portable. however, in the case of the tdr-307usbm, it is worth mentioning that signal attenuation and dispersion phenomena limit the usability of tdr when used on long cable systems. it is possible to modify the amplitude and the width of the tdr test pulse signal along with the gain in the input stage amplifier; however, this requires the tdr measurements to be repeated many times to find the right setting. to overcome this issue, an innovative and dedicated algorithm [16] has been developed to automatically optimise these two different electrical parameters of the tdr signal as a function of the d-se length in order to compensate the performance in terms of attenuation and dispersion effects. 4. experimental results 4.1. experimental results for sen-w water-pipeline monitoring is extremely important given the considerable amount of water loss caused by leakages and other possible hydraulic failures. figure 2 shows a schematisation of the setup configuration. during the installation phase, the d-se is placed along the pipeline to be inspected, and the connection to the measuring system is ensured through an access point. points b and e indicate the beginning and the end of the d-se, while l indicates the position of a leak. it can be seen that there is a section of cable (running vertically) that allows the se to be connected to the measurement instrument. clearly, variations in dielectric permittivity that may occur along this vertical se section are not of interest for the leak localisation. to overcome this issue, this portion of se is electromagnetically shielded (by means of a metal shield). in this way, the sensing portion starts from point b. another advantage is that, thanks to this shielding, the vertical portion can be of any length, as it does not influence the localisation of the leak (hence, it is not necessary to know a priori the burial depth of the pipe). this approach allows easier identification of the interface relative to the start point of the se relative to the buried pipeline, excluding the portion of the connected section from the measured data. for the experiments on sen-w, a dedicated testbed was set up, in which a d-se was buried approximately 30 cm figure 2. schematisation of the tdr-based system for sen-w (dimensions not to scale). (b) figure 3. sen-w: a) comparison of the test reflectogram and reference reflectogram for leak detection. b) localisation of the dielectric permittivity variation (dpv): reference reflectogram is superimposed on the test reflectogram. the difference between the two reflectograms is also shown (red curve). 0.0 50.0 100.0 150.0 200.0 250.0 300.0 -100.0 -50.0 0.0 50.0 100.0 150.0 200.0  k t d r a d c 's o u tp u t d (m) reference (no dpv) test (dpv @ 200 m) difference (k) dpv acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 205 underground. the soil was irrigated at predetermined distances from the beginning of the se in a similar way to a leakage condition when water escapes from an underground pipe. the main parameter describing the interaction between the signal and the system under test is the relative permittivity εr, as water leakage causes a local variation in its value, which is immediately detectable in the measurement. for these measurements, the tdr ri-307usbm was used. in fact, because ses in these applications may be a hundred meters long, the tdr200 instrument is unsuitable, as its electronics could be easily damaged as a result of electrostatic discharges occurring when connecting long cables. figure 3(a) shows the measurement results obtained for a leak at d = 8 m. first, the reference reflectogram was acquired, i.e. when there is no leak (red curve). then, the reflectogram in the presence of the emulated leak was acquired. it can be observed that, in the presence of the leak, there is a distinct variation in the output reflectogram. the position of the leak is estimated by applying (2) to the different portions of the reflectogram in order to identify the abscissa of the minimum corresponding to the leak. it is also interesting to analyse the detection of a leak (or a dielectric permittivity variation (dpv)) at long distances (i.e. 200 m), which is when the use of the aforementioned algorithm becomes essential. figure 3(b) shows the tdr curves that are automatically processed; the difference between the test reflectogram and the one in the presence of a dpv allows the dpv to be localised. it should be mentioned that in applying (2), an assumption of constant permittivity in the soil along the se is made. this assumption is acceptable considering that, generally, the variation in permittivity caused by leakages is significantly higher than the natural variation of permittivity in the soil (which may be due to temperature variations or to slightly different soil compaction in different areas). 4.2. experimental results for sen-a in this application context, monitoring soil moisture content in agriculture allows the use of water to be optimised, reducing wastage. in particular, the basic idea is to bury the d-ses in correspondence with cultivations and to carry out tdr measurements to retrieve the actual water-content profile of the soil all along the cultivations. subsequently, the irrigation systems can be automatically activated/deactivated according to the actual irrigation needs of the plants. in addition, through multiplexing systems, up to 512 d-ses could be used simultaneously to perform the widespread monitoring of multiple crop rows. for the experimental tests, a 30-m-long dse was placed in the soil along the entire crop profile, and the connector came out of the soil to connect to the tdr measurement instrument. in this case, for the sake of comparison, both the tdr200 and the tdr ri-307usbm were used. the acquired reflectograms were directly related to the impedance profile of the se inserted in the soil. experimental results were obtained according to the following measurement protocol: • reflectogram #1: one week after d-se installation, • reflectogram #2: (post-irrigation) acquired approximately one hour after the irrigation process was finished, • reflectogram #3: (pre-irrigation) acquired three days after reflectogram #2, • reflectogram #4: (post-irrigation) acquired approximately 10 hours after the irrigation process was finished, • reflectogram #5: pre-irrigation measurement. figure 4(a) and 4(b) show the measurements obtained with the tdr200 and tdr ri-307usbm, respectively. it can be seen that after irrigation (reflectogram #2), the apparent length of the se increases, as d-se shifts towards a longer distance; this is a result of the increased effective permittivity. from reflectogram #3, acquired after three days and before a new irrigation, it can be seen that the apparent length of the se has decreased. a similar trend occurs for reflectogram #4 and reflectogram #5. based on the overall length of the se, the system is able to discriminate well between different soil moisture conditions. this can be used as a parameter for activated irrigation. the tdr200 instrument exhibits better performance in estimating dielectric variations; however, the costs are considerably higher than the tdr ri-307usbm. in practical applications, the elongated ses will allow the retrieval of a map in real time that shows the state of the water content in the cultivations, thus allowing an automatic tailormade intervention, especially in view of the optimal management of the irrigation processes. (a) (b) figure 4. sen-a:tdr reflectogram with the tdr200 (a) and with the tdr ri307usbm (b) 0 10 20 30 40 50 55 60 65 70 75 80 85 90 95 100 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 d e ( d im e n si o n le ss ) d app (m) sen-a (tdr200) refl. #1 after installation refl. #2 post-irrigation (a) refl. #3 pre-irrigation refl. #4 post-irrigation (b) refl. #5 pre-irrigation d b 0 20 40 60 80 100 120 140 -40 -20 0 20 40 60 80 100 120 140 t d r 's a d c o u tp u t d app (m) sen-a (tdr ri-307usbm) refl. #1 refl. #2 refl. #3 refl. #4 refl. #5 acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 206 4.3. experimental results for sen-b for sen-b, the goal of the proposed method was twofold: 1) an ex-ante monitoring of concrete curing along a building diffuse profile and 2) an ex-post monitoring for the detection of dielectric anomalies as a result of the degradation or stress of the structure. a concrete beam was used as a case study. in order to monitor the water content during the hydration process, three d-ses were inserted in a concrete beam: one at the bottom of the beam, another in the middle and a third at the top. the hardening phase is considered to be completed within the first 28 days, as after this period more than 90 % of the overall mechanical strength has developed. for this reason, the beam was monitored over the 28-day period by acquiring tdr data for the d-ses. figure 5(a) shows some of the tdr reflectograms acquired during the considered period (for clarity of the image, not all the reflectograms are reported); figure 5(b) shows a zoomed image that highlights the trend. from the reflectogram, it is possible to identify the beginning and the end of the d-se (denoted respectively by 𝑑𝐵,app and 𝑑𝐸,app) in order to calculate the apparent distance 𝑑app of the dse. as can be seen, this parameter decreases with the hydration process because 𝑑𝐸,app shifts towards lower values. this indicates that the apparent distance 𝑑app of the d-se decreases with the decreasing dielectric permittivity 𝜀app of the concrete beam as a result of the ongoing hydration process. however, ex-post monitoring through an embedded low-cost diffused se can promptly detect the deterioration of the structures. in this regard, mechanical tests on the beam were carried out to analyse whether the diffuse sensor cable was sensitive to mechanical deformation. the beam was subjected to several tests with concentrated weights varying from 1,000 kgf to 9,000 kgf, with an increase of 1,000 kgf at each test. figure 6 illustrates the test setup. as explained in section 3, the d-se also included a coaxial cable, which was sensitive to deformation and/or compression phenomena. during the mechanical test, tdr measurements were also carried out on the d-se. as is known from the theory, the diameter of the inner and outer conductors determines the impedance of the coaxial cable. because of this, a deformation in the cable resulting from the degradation phenomena in the structure causes a variation in the electrical impedance, which was immediately identified from the measurements. as shown in figure 7, as the weight applied to the beam increases, the reflection coefficient decreases, and the apparent distance increases, indicating that the cable is being deformed and bent. these mechanical tests highlight that continuous ex-post monitoring through permanent, d-ses can provide early indications of structural problems. in this way, safety measures can be considered in time, and structural interventions can be performed immediately. 5. conclusions in this study, the development and the proof of concept of a multi-purpose sen were addressed through the adoption of tdr and d-ses. the proposed system, which allows the limitations of traditional punctual monitoring systems to be overcome, was validated for the localisation of leaks (sen-w), for monitoring the diffused profile of the water content of soil in agriculture (sen-a) and for the ex-ante and ex-post monitoring of the hydration process and the diagnostics of destructive phenomena (sen-b). additionally, the proposed monitoring system can be easily extended to other fields. in practical applications, each sen could be managed through a portable device and through a single platform. in addition, the monitored data can be stored in a repository and used for further statistics or future reference, and the output of the sens can be geo-referenced by acquiring the (a) (b) figure 5. sen-b: selected tdr reflectograms for the central d-se over the 28-day observation period (a) and a zoomed image (b). figure 6. experimental setup for post-mechanical tests on the beam. 0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 0.0 0.2 0.4 0.6 0.8 1.0 1.2  ( d im e n si o n le ss ) d app (m) sen-b (tdr200) day #1 day #2 day #3 day #8 day #10 day #15 day #17 day #25 d b,app d e,app d app 10.0 10.5 11.0 11.5 12.0 12.5 0.5 0.6 0.7 0.8 0.9 1.0  ( d im e n si o n le ss ) d app (m) sen-b (tdr200) day #1 day #2 day #3 day #8 day #10 day #15 day #17 day #25 d e,app-25 d e,app-1 days acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 207 gps coordinates of the location where the mr measurements are taken. the introduction of these strategies would contribute to making the proposed monitoring system evolve into a cyberphysical measurement system [32], [33], fully exploiting the potential of 4.0 technologies. references [1] a. sforza, c. sterle, p. d’amore, a. tedesco, f. de cillis, r. setola, optimization models in a smart tool for the railway infrastructure protection, in: critical information infrastructures security. e. luiijf, p. hartel (editors). springer, cham, 2013, pp. 191-196. doi: 10.1007/978-3-319-03964-0_17 [2] p. d’amore, a. tedesco, technologies for the implementation of a security system on rail transportation infrastructures, in: railway infrastructure security. r. setola, a. sforza, v. vittorini, c. pragliola (editors), springer, cham, 2015, isbn: 978-3-31904425-5, pp. 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https://doi.org/10.1109/jsen.2020.3035754 https://doi.org/10.1109/jsen.2020.2987321 https://doi.org/10.3390/s20237001 https://doi.org/10.1109/tim.2007.911700 https://doi.org/10.1109/tim.2019.2900963 https://doi.org/10.1109/tim.2019.2896553 https://doi.org/10.3390/s20102833 https://doi.org/10.1109/metroind4.0iot51437.2021.9488477 http://dx.doi.org/10.21014/acta_imeko.v10i2.1123 low-power and high-speed approximate multiplier using higher order compressors for measurement systems acta imeko issn: 2221-870x june 2022, volume 11, number 2, 1 6 acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 1 low-power and high-speed approximate multiplier using higher order compressors for measurement systems m. v. s. ram prasad, b. kushwanth, p. r. d. bharadwaj, p. t. sai teja 1 department of eece, gitam (deemed to be university), visakhapatnam, ap, india section: research paper keywords: approximate computing; approximate compressors; digital circuits; partial products; measurement systems citation: m. v. s. ram prasad, b. kushwanth, p. r. d. bharadwaj, p. t. sai teja, low-power and high-speed approximate multiplier using higher order compressors for measurement systems, acta imeko, vol. 11, no. 2, article 36, june 2022, identifier: imeko-acta-11 (2022)-02-36 section editor: md zia ur rahman, koneru lakshmaiah education foundation, guntur, india received january 30, 2022; in final form april 22, 2022; published june 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: m. v. s. ram prasad, e-mail: mvsrprasadgitam@gmail.com 1. introduction in digital signal processing applications multipliers plays an important role in complex arithmetic operations. here, the approximate computing methods are used to lessen the power consumption. hence approximate multipliers came into the existence and these approximate multipliers are widely used in the digital signal processing applications to lessen the power consumption [1]-[4]. the complex multipliers used in the dsp applications are replaced with these approximate multipliers. these can perform multiple operations like filtering, convolution, and correlation of the digital signals. to perform these complex multiplications multipliers, adders and shifters are widely used. here, designing of the multiplier is the hardest part in the design of the dsp. these multipliers consume more power compared with the remaining adders and shifters. in multiplication process there is a propagation of partial products, alignment of partial products, and lessens the partial products finally addition all these partial products. reducing the partial products count requires more time and power consuming. multiple techniques are implemented to overcome this issue. the approximate computing technique gives the better results compared to all the previous techniques. hence approximate adders came into existence and then compressors are designed for the addition of multiple bits [5]-[8]. higher order compressors are required and lessen delay of addition process. with the use of these approximate compressors approximate multipliers are designed to improve the performance of the digital signal processing applications. the exact multipliers are consuming high speed and require huge delay to obtain exact outputs. due to these exact multipliers, there is only one major defect is that it can’t optimize further while using multiple techniques [9]-[11]. hence for the image processing and signal processing applications accept the errors data and gives the modulated signals. hence approximate compressors and multipliers came into existence and lessen the power consumption along with delay reduction due to the carry bits in the addition process. due to these approximate multipliers approximate results are obtained and these are sufficient for the abstract at present, approximate multipliers are used in the image processing applications. these approximate multipliers are designed with the help of higher order compressors to decrease the number of addition stages involved for the lessening stages. the approximate computing is the best technique to improve the power efficiency and reduce delay path. with the use of approximate computing multiple compressors are designed. in this paper, 10:2 compressors are designed and implemented in the 32-bit multiplier and compared with the exact 32-bit multipliers. the proposed higher bit compressors along with the lower bit compressors are implemented to reduce the delay of the design. this type of digital circuits has much significance in measurement technologies, for enabling fast and accurate measurements. with the use of approximate compressors, the result may be ineffective, but the power consumption and delay are getting reduced. hence, these proposed multipliers are only implemented the digital signal processing applications, where there is need for combining two or more signals. the proposed multiplier is used for implementing fir filter resulted 27 ns delay which is far better than the exact multiplier having 119 ns. these multipliers also used in image processing applications and psnr of image has been employed. mailto:mvsrprasadgitam@gmail.com acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 2 combining of the two signals. the approximate compressors are designed to reduce the computation occurred in the addition process [12]-[15]. hence a greater number of inputs are added to produce only two outputs called as sum and carry bits and another one more output is also attained called as carry out. in general, 4:2 compressors are widely used, and these give the better results when compared to the previous architectures. hence with the use of these approximate compressors the partial products are gets reduced and the adder count (gate count) is also reduced. the approximate compressors widely used [16], [17] in multipliers to reduce the power to improve the performance of the circuit. hence these approximate multipliers need some extra compressors to improve its performance. then the higher order compressors came into the existence and 10:2 compressors are designed. the lower order compressors require more power consumption of the higher order multipliers is also yields better results [18], [19] when compared to the approximate multipliers using with the lower order compressors. in this paper, a new 32bit approximate multiplier is designed with the use of higher order compressors achieves low power consumption and lesser delay along with the less error rate. 2. design of compressors a compressor is a combination circuit having multiple inputs and multiple outputs, the outputs consist of one sum bit and one carry bit along with these multiple carry propagating bits are also generated according to their input bit length [6] as shown in figure 1. here, the compressors are adding multiple bits which have same bit length and add the inputs of different bit lengths. these compressors are used in the decrease the partial products generated in multiplication process and these are implemented to add the multiple partial products into two partial outputs along with a carry propagating bit. according to the multiplier bit length these compressors are designed to reduce the partial product count [9]. due to these approximate compressors the addition of partial products is done easily. 2.1. 4:2 compressor the basic 4:2 compressor consists of 4 inputs, and it gives only two outputs known as sum and carry bit along with these input and output pins one more input pin is added to the compressor called as carry in and it gives one more output called are carry out is shown in figure 2. the carry out generated from the compressor is propagated to the next bit positions. hence these compressors generally have multiple input and multiple outputs 𝑋1 + 𝑋2 + 𝑋3 + 𝑋4 + 𝐶𝐼𝑁 = 𝑆𝑢𝑚 + 2 ∗ (𝐶𝑎𝑟𝑟𝑦 + 𝐶𝑖𝑛) . (1) 2.2. design of 10:2 compressor the higher order compressor is implemented in the proposed approximate multipliers for the partial product reduction and adder count. in general, huge adder count and produce high power consumption [20]. here, in the proposed higher order compressors xor gates and mux are used to reduce the operation time along with the power consumption and is shown in figure 3. the higher order compressors are replaced with the normal adders without any disturbance in their truth tables. in approximate compressors the output is obtained based on the input combinations. here the output is either directly taken from the input or a small calculation is used. in the exact compressors the output is calculated exactly based on the multiple gates and equations. hence in the approximate compressors the performance is improved in terms of delay and power. with the same approach for 4:2 compressor has been designed as shown in figure 4. the 4:2 compressors are the basic compressor used in designing every multiplier. in the proposed system the delay is getting reduced due to the termination of carry signals and the carry is not propagated to the next bits, due to this the delay is getting reduced in the proposed system. the proposed design is implemented with the xor gates and multiplexers to reduce the delay and power consumption. the 8:2 approximate compressors are implemented. the xor-mux implementation of 8:2 compressor is shown in figure 5. figure 1. n:2 compressors schematic diagram. figure 2. 4:2 compressor schematic diagram. figure 3. conventional implementation of 3:2 compressors. acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 3 while using the higher order compressors a greater number of bits are calculated at the same time, while calculating greater number of bits at the same the power consumption is getting reduced which leads to improve the performance of the circuits. with the use of approximate computing there is an occurrence of error is very high due to the reduction of the carry bits. hence the lower order compressors are consuming high power although they are producing error outputs. while adding multiple bits at the same time a greater number of lower order circuits are used which consumes more power consumption and delay is also increased. to overcome this issue higher order compressor are designed to perform the multi-bit addition. the 10:2 compressor is having only 10 inputs and it gives only 2 outputs [22], [23]. this compressor has fourteen xor gates and six multiplexers. each xor gate receives two primary inputs and it generates single output. outputs of the xor gates are propagating to the multiplexers in which the final multiplexer will generate the carry bit and the final xor gate generate the sum output. 3. design of 32-bit approximate multiplier to obtain the exact multiplication there is a use of exact compressors in the multiplication design process. for the approximate multipliers approximate compressors are utilized where complex multiplication process is implemented. these approximate compressors are implemented in the dct applications like image processing and signal processing. here, 32-bit approximate multipliers are implemented. in this paper higher order compressor means a 10:2 compressor are designed and implemented in the 32-bit approximate multipliers. design of 32-bit approximate multipliers both higher order and lower order compressors are implemented to reduce the adder count and delay of the multipliers. in exact multipliers the output is calculated by using the normal adders which gives the accurate results which consume more power consumption. in the approximate multipliers higher order circuits are used to reduce the partial product generation and reduce the delay by neglecting the carry propagation to the next bits. hence the power consumption is reduced in figure 4. block diagram of 4:2 compressor using xor-mux. figure 5. block diagram of proposed 10:2 compressor based on xor-mux modules. acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 4 approximate multipliers compared with the exact multipliers. 8bit approximate multiplier with reduction stages is shown in figure 6. the proposed higher order approximate multiplier is having higher compressor like 10:2 compressor along with these lower order compressors like 8:2, 4:2 compressors are also used in this design. the proposed higher order compressors are designed with multiplexers and xor gates only. the proposed multiplier is having only 3 addition stages which are very less compared to the design of multipliers with the use of lower order compressors. hence, it is clearly showing that the previous approximate and exact multipliers are not sufficient to design the fir filters. 4. simulation results in this section, the proposed multiplier is implemented in xilinx ise design suite with the help of verilog hdl. figure 7 shows the simulation results obtained in the xilinx isim simulator where the inputs are a and b and the output of the multiplier is obtained at the y signal of the exact multiplier. figure 6. 8-bit approximate multiplier with reduction stages. figure 7. simulation result of the 32-bit exact multiplier. figure 8. technology schematic of the 32-bit exact multiplier. acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 5 the proposed multiplier in which the logic of the proposed design is converted into the lut’s (look up tables) which are already pre-existed in the xilinx tools it shows that how many number of lut’s and iob’s are utilized for the design. the proposed multipliers are used in the fir filters to multiply the message signal and carrier signal. for this process generally exact multipliers are used, due to the exact multipliers the power consumption and delay are increased and there is a reduction of excess terms to remove all these extra processes the proposed approximate multipliers are used in the fir filter applications. technology schematic of the 32-bit exact multiplier is shown in figure 8. figure 9 show that the simulation results of the proposed 32bit approximate multiplier, where these results are quite different from the exact multipliers due to the usage of the approximate compressors in the proposed design. the proposed approximate multiplier is completely designed based on the approximate computing. in the approximate computing the output is calculated approximately to reduce the power consumption and delay. table 1 shows a comparison of 16and 32-bit exact and approximate multipliers and table 2 shows a comparison of 32-bit exact and approximate multipliers in fir filters. 5. applications in this project work a modified 10:2 compressor based on xor-mux modules are implemented. using this proposed compressor, a 32-bit multiplier is designed. the proposed design obtains better results. the proposed multiplier used in multimedia processing for the purposed of 2 different image multiplications and then peak signal to noise ratio (psnr) is employed. the data transmission in digital signal processing applications requires the convolution and correlation process instead of actual multiplication. hence the proposed approximate multiplier does not achieve the 100 % accuracy, but these proposed approximate multipliers are suitable for the convolution and correlation processes. 6. conclusion approximate 32-bit multipliers are implemented using the modified 10:2 compressor. approximate multipliers provide better results when compared with the exact multipliers. the proposed multiplier achieves better psnr values with the previous designs. the accuracy of the image those are processed shows that our proposed multipliers are very effective compared to the exact multipliers. the proposed approximate multiplier is best for the multimedia applications where there is a need of combining two or more signals without any exact output. these proposed 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23rd asia and south figure 9. simulation result of the 32-bit approximate multiplier. table 1. comparison of 8, 16 and 32-bit exact and approximate multipliers. multiplier delay (ns) lut count exact 8-bit 7.664 124 approximate 8-bit 2.969 84 exact 16-bit 12.007 551 approximate 16bit 4.689 242 exact 32-bit 20.55 2255 approximate 32-bit 18.898 1705 table 2. comparison of 32-bit exact and approximate multipliers in fir filters. filter delay accurate 119 approximate 27 https://doi.org/10.1109/ets.2013.6569370 https://doi.org/10.1109/isvlsi.2015.130 http://op.niscpr.res.in/index.php/jsir/article/view/41462 https://doi.org/10.1109/jproc.2017.2761740 http://eyeriss.mit.edu/tutorial-previous.html https://doi.org/10.21014/acta_imeko.v11i1.1231 https://doi.org/10.23919/date.2017.7926950 acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 6 pacific design automation conference, 2018. doi: 10.1109/aspdac.2018.8297389 [9] d. esposito, d. de caro, e. napoli, n. petra, a. g. m. strollo, on 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10.14445/22315381/ijett-v68i9p203 [20] a. cilardo, d. de caro, n. petra, f. caserta, n. mazzocca, a. g. m. strollo, e. napoli, high speed speculative multipliers based on speculative carry-save tree, ieee transactions in circuits and systems i, vol. 61, no. 12, 2014. doi: 10.1109/tcsi.2014.2337231 [21] j. liang, j. han, f. lombardi, new metrics for the reliability of approximate and probabilistic adders, ieee trans. on computers, vol. 62, no. 9, 2013, pp.1760-1771. doi: 10.1109/tc.2012.146 [22] p. kulkarni, p. gupta, m. d. ercegovac, trading accuracy for power in a multiplier architecture, j. low power electron., vol. 7, no. 4, 2011, pp. 490–501. doi: 10.1109/vlsid.2011.51 [23] c.-h. lin, c. lin, high accuracy approximate multiplier with error correction, in proc. ieee 31st int. conf. computer. design, sep. 2013, pp. 33–38. doi: 10.1109/iccd.2013.6657022 https://doi.org/10.1109/aspdac.2018.8297389 https://doi.org/10.1109/iscas.2017%20.8050437 https://doi.org/10.1109/jsen.2021.3125529 https://doi.org/10.3844/ajassp.2013.893.900 https://doi.org/10.1109/tcsi.2017.2728802 https://doi.org/10.1109/lascas.2016.7451090 https://doi.org/10.1109/tc.2014.2308214 https://doi.org/10.7873/date.2014.108 https://doi.org/10.1109/tvlsi.2016.2535398 https://doi.org/10.14445/22315381/ijett-v68i9p203 https://doi.org/10.1109/tcsi.2014.2337231 https://doi.org/10.1109/tc.2012.146 https://doi.org/10.1109/vlsid.2011.51 https://doi.org/10.1109/iccd.2013.6657022 toward a metrology-information layer for digital systems acta imeko issn: 2221-870x march 2023, volume 12, number 1, 1 4 acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 1 toward a metrology-information layer for digital systems mark j. kuster1 1 independent researcher and consultant, dumas, texas, usa section: research paper keywords: m-layer; quantities; measurement units; measurement information infrastructure (mii); digital metrology citation: mark j. kuster, toward a metrology-information layer for digital systems, acta imeko, vol. 12, no. 1, article 18, march 2023, identifier: imekoacta-12 (2023)-01-18 section editor: daniel hutzschenreuter, ptb, germany received november 21, 2022; in final form january 7, 2023; published march 2023 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: mark j. kuster, e-mail: mjk@ieee.org 1. introduction the international system of units and quantities [1], [2] and documents such as [3] define the basic references and nomenclature that supports most world measurements and quantitative specifications. this system, though not comprehensive, has served science and commerce well. metrology’s digital transformation, as bipm has recognized [4], requires a digital system to represent quantities and units. simple digitization, however, does not suffice for digital transformation. to digitize, digitalize, and transform, metrology should rethink its practices from the ground up in order to identify and eliminate the suboptimum pragmatic elements that, if propagated into automated systems, undercut the full gains that digital transformation promises. quantities and units lie at the ground level so we start there. digital quantity-unit systems face multiple realities—that if accounted for in the beginning will smooth and enhance digital transformation—including 1. both si and non-si measurement units, 2. exceptional measurement scales, 3. measurand ambiguity that challenges automated data consumption, 4. nonlinear unit conversions, 5. restricted operations by scale type. in [5], the authors proposed a metrology-information layer (an m-layer), to standardize a universal digital quantities and units system without altering the human systems in use. conceptually, the m-layer generalizes measurement “unit” (and the vim’s “reference” [6]) to “scale” and similarly generalizes “quantity” to “aspect” as denoted in [7]. the m-layer would allow any measurement unit (reality 1) on any scale (reality 2), introduce an aspect identifier <q> to disambiguate quantities and facilitate machine-actionable (ma) documents (reality 3), provide for arbitrary conversion functions between scales (reality 4), and capture the meaningful operations among scales (reality 5). this generalization unites all digital measurement data under a single methodology. without such innovations, metrology laboratories and other measurement producers and consumers will inevitably encounter measurements that require some ad hoc data in digital documents that their customers’ software may or may not consume automatically, not to mention the rest of the world. subsequent work [8] has sketched an m-layer structure but published no detailed models. this paper presents work on an m-layer prototype model in section 2. section 3 discusses some results with example applications and section 4 concludes with a summary and indicates prospective future work. 2. m-layer modeling in human-readable documents, quantity values appear with two components: q, the numeric value, and [q], the unit symbol, as in 2.446 mm, for say, an outside diameter. people rely on a textual quantity description or the context to determine the actual measurand. because that methodology fails for machine processing, the m-layer would extend the representation with a abstract recent work has proposed a metrology-information layer (m-layer) to support digital systems with quantities and units by addressing the difficulties conventional quantity-unit systems pose for digitalization. this paper reports the work toward developing the m-layer’s current abstract conceptualization into a concrete model, working prototype, and demonstration software, with the final goal to create a fair (findable, accessible, interoperable, reusable) resource. mailto:mjk@ieee.org acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 2 third element that uniquely identifies the aspect (generalized quantity kind) in ma documents as 𝑞[𝑄] → 𝑞[𝑄]⟨𝑞⟩ (1) or in our example, 2.446 mm <length> [5], where <length> represents an aspect-identifying code discussed later. however, machines do not require multiple units per quantity, base-derived unit distinctions, or prefixes for internal processing or communication. therefore, an m-layer aspect may associate with one and only one unprefixed unit, in which case the aspect would uniquely determine the unit and a measurement-software application might drop the unit entirely from the data. this means digital systems may simplify the data and carry q<q> in computations without ambiguity or loss of generality. in our example, the data would simply contain 2.446 × 10−3<length>, assuming m-layer documentation declares the si9 [1] meter, e.g., as the m-layer length unit. the data model should then allow digital systems to render this in the expected form— outside diameter, 2.446 mm—using any unit and aspect alias the user prefers. the m-layer representation suffices for processing of digital measurement data, including all calculations, uncertainty propagation, etc. widely interoperable ma documents, however, require more metadata to describe the measurand in order to automatically match measurement data in instrument specifications, calibration and measurement capabilities, calibration results, calibration requests, and other digital documents that organizations may wish to exchange. the mii (measurement-information infrastructure) taxons [9] would fulfill this requirement unless and until the m-layer incorporates such aspect qualifiers, something not currently envisioned. for example, the taxon measure.length.diameter.outside, in which the second element length links to the aspect <length>, provides the metadata to make a digital outside-diameter value fully interoperable (neglecting qualifiers for influence quantities). the m-layer’s core therefore comprises unambiguous aspect definitions. table 1 illustrates a model to capture the useful data elements. the elements aspectid and name provide the core functionality. the scaletypeid-indexed data helps define the meaningful operations on the aspect. definition would aid users to distinguish one aspect from another and symbol identifies the aspect’s default math symbol for symbolic processing. unlisted elements such as nature (intrinsic, extrinsic, …) and dimension would add value but the m-layer goals do not immediately require them. the m-layer model will also allow sourcing one or more data elements from existing systems such as ontologies [11] or unique reference points such as the digital si brochure. ideally, the aspectid representing <q> would comprise a lightweight persistent identifier (pid). for example, the m-layer might identify itself and its contents via a doi (digital object identifier) [12]. as shown in figure 1, owners may structure their dois as desired. the doi’s owner-chosen suffix structure allows a hierarchical pointer that would identify, for example, the m-layer registry; the aspects dataset; and a particular aspect. for aspects, the short entryid then becomes aspectid, and when combined with a numerical value would both disambiguate the measurand and reduce the data size in digital documents relative to other proposals. the dois would easily expand to accommodate the mii taxon structure as well. the doi’s permanence as a pid ensures m-layer availability regardless of any changes in the organization or web site hosting the registries. a number of other datasets would supplement or complement the m-layer; these may include datasets to register quantity-unit systems for rendering user results, scale types, base dimensions, aspect aliases (alternate quantity names) and aspect relations. if an m-layer implementation omits any of these supplementary datasets, dependent systems and applications may augment the core m-layer accordingly or, as previously mentioned, follow m-layer pointers to existing systems. we omit details here due to space and the model’s current fluidity— the reader may find further information at [13] as it develops— but briefly mention the most germane points. potential ancillary datasets: quantitysystems and unitsystems register systems such as imperial, u.s. customary, natural, cgs systems, as well as previous and future si versions. the units and unitaspects datasets associate all units of interest (for user interfaces) with the correct aspect and provide symbolic conversion expressions to and from the aspect’s m-layerdeclared unit, e.g., the si9 equivalent. to simplify the data model and client application logic, prefixed units may have their own data entries. this would allow easy unit conversions and rendering as users desire. scaletypes and scaleoperations register scales (ratio, interval, cyclic or modular, logarithmic, ordinal, nominal, empirical) and their data types and operations. as an example multi-scale operation, an interval quantity added to a compatible ratio quantity yields a ratio quantity. though neither sufficient nor required for disambiguation, basedimensions would define the basis for dimensional analysis. finally, an aspectrelations dataset might contain mathematical expressions relating aspects, e.g., ohm’s law. an ma-document scheme that propagated mathematical expressions for measurement models and results might start from here. as a distributed resource, the m-layer may comprise any number of separate registries. some national metrology institutes table 1. aspects data model. data element description example aspectid unique identifier-index representing the aspect <q> in ma documents and data <length> name registered name length symbol mathematical symbol markup (e.g., latex, mathml [10]) l definition textual description or external pointer pid to unique reference point scaletypeid index to the aspect’s scale type ratioscaleid figure 1. a doi-based scheme for m-layer pids. the top two dois identify articles in two different journals, each of which has designed their own doi structure. the bottom doi shows one choice for m-layer pids, allowing separate registries, each with a number of identified datasets containing identified entries. acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 3 may wish to establish m-layer registries to link local or legacy measurement units to the m-layer si registry. likewise, standards bodies, industry associations and common-interest communities may wish to register and digitally define unique aspects, scales, units and relationships. the m-layer model also envisions an access interface such as an api, which would define a number of operations, such as retrieving a registry for local use. this would complete the mlayer as a fair data source. 3. discussion this section discusses some benefits of the m-layer. 3.1. disambiguation metrologists involved in digital transformation have begun to realize that quantity values as numeric value and unit do not suffice for automated consumption; nor do accompanying freetext measurand descriptions solve the problem. ncsl international’s mii initiative set up a test-bed database organized around quantities [14] and later initiated a measurand taxonomy project to begin addressing measurand ambiguity. the m-layer would extend this capability beyond ratio quantities in order to handle, for example, the ordinal quantity hardness, modular angle quantities, temperature quantities on interval, ordinal, ratio, and special scales, etc. aspect ids of course automatically resolve such problems as disambiguating dimensionless quantities (all using the implied unit 1) or other quantities denoted in the same unit (e.g., torque and work). so though digital documents would record for example 1.00 <torque> or 1.00 <work>, both render to the same conventional form, 1.00 n m, if so desired in an application, though preferably labeled with the quantity name or mii taxon. dimensionless quantities would work likewise, as each would have its own aspects entry. for example, digital systems rendering the text “turns ratio, r: 200” or “amplifier gain a: 200” would do so from digital data containing 200 ⟨turnsratio⟩ and 200 ⟨gain⟩. the m-layer would also have the option of adding base dimensions such as angle a for rotational quantities, though again, the m-layer does not rely on dimensional analysis. 3.2. simplified data processing with the m-layer, digital document producers may remain ignorant of the customer’s preferred units and simply embed the m-layer representations because the customer may render the values as desired or simply pass them to other digital systems. documents that the producer converts to pdf form for the customer may do likewise. computations using m-layer data may ignore measurement units and proceed as with dimensionless quantities, then simply attach the appropriate aspect id to the final result before embedding the value in a document or otherwise communicating it. the system may ignore alternate unit systems, prefixes, and other pragmatisms because the m-layer’s units would implicitly correspond to a declared si edition such as [1], or other standards for which the si provides no equivalent, e.g., hardness. as a simple example, an application without m-layer support might perform a simple period-to-frequency calculation as 𝑓 = 1 𝑇 = 1 (1 ms)⁄ = 1000 hz = 1 khz , (2) which with the m-layer would reduce to 1/(0.001 <period>) = 1000 <frequency>. (3) the latter operation both carries more information (aspect) and simplifies processing. many software systems therefore would require no refactoring to handle quantities properly (defining quantity-value classes, for example, and overloading their operators to deal with dimensions). also, having an aspectrelations dataset available in the m-layer would help standardize metrological computations–for example to provide commonly used expressions such as moist-air density. 3.3. unit conversions the usual unit conversions of course remain trivial with an m-layer. user interfaces would translate conventional notations to and from m-layer representations but all intermediate processing and communications between m-layer-aware systems would entail no conversions. this m-layer model includes symbolic conversion functions to eliminate precision-limited conversion constants. thus, digital systems using sufficiently precise, arbitrary-precision, or symbolic computations for all operations would introduce no further errors or uncertainty into results, at least up to a user interface. hence, a system may postpone numeric conversions until required by encoding them symbolically as, e.g., latex or mathml. an angle value, for example might digitalize as equation (4)’s right hand side, 44.234° = 44.234 𝜋 180 < planeangle >, (4) where the symbolic conversion expression 𝑥 𝜋 180 comes from the m-layer units dataset entry for degree and assumes the si radian as the m-layer angle unit. similarly, conversion functions allow arbitrary scale conversions such as 𝐿 𝑥 𝑥0 = log ( 𝑥 𝑥0 ) , (5) which converts from a dimensionless ratio-scale quantity to a logarithmic-scale level quantity. 3.4. scale handling the m-layer would handle scale conversions similarly to unit conversions. since every aspectid associates with a unique scaletypeid, we may facilitate conversions and corrections between empirical scales. for example, we might add the aspect <1990conventionalvoltage> and the appropriate scale entry based on the conventional josephson constant 𝐾𝐽−90 with a scale-conversion entry such as 𝐾𝐽−90𝑥 𝐾𝐽 , where 𝐾𝐽 = 2 𝑒 ℎ with constants e and h defined to match the declared m-layer references, e.g. si9 [1]. this would allow automated systems to easily correct past (digital) measurement data according to new definitions. figure 2 exemplifies such a voltage-scale conversion via a demonstration application that takes advantage of a prototype registry containing aspect relations to automatically look up conversion equations and compute results. the m-layer would likewise define other empirical scales, such as the its-90 temperature scale and mercury-based temperature scales [3] for various atmospheric pressures in order to capture the differences from their associated si-defined aspects. acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 4 defining ordinal scales would bring such quantities as hardness into the same digital system without requiring ad hoc modifications and data representations. modular scales would handle angular quantities when restricting values to a certain range, such as 0°to 360°. from these examples, the reader will see the potential that m-layer opens to digital transformation. 4. conclusion this paper has presented initial steps toward modeling and prototyping an m-layer to support fair digital measurement data and systems. for human-readable documents, the m-layer changes nothing, except perhaps to facilitate their generation. by replacing unit symbols and textual measurand descriptions with unique aspect ids, the m-layer concept offers machinereadability, global interoperability, and generalized quantities (aspects) and units (scales) to handle all types of measurements in digital documents and measurement software systems. in collaboration with international partners, the ncsl international 141 mii and automation committee plans to continue developing the m-layer model and populating a prototype with intention to replace the mii test-bed quantities and units database for use in digital documents. in cooperation with industry partners, we have begun drafting use cases, a product definition and requirements from the user viewpoint, an open-source prototype registry with back-end software, and demonstration applications. as the mii committee continues its collaboration with the international quality infrastructure, the mlayer should become a fair data resource. acknowledgement the author would like to thank ncsl international, the ncsl international 141 mii and automation committee, and cherine marie-kuster for their support. references [1] the international system of units (si), international bureau of weights and measures (bipm) information document si brochure, 9th edition, 2019. online [accessed 26 march 2023] https://www.bipm.org/en/publications/guides/ [2] quantities and units, international standardization organization (iso) and international electrotechnical commission (iec) 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[13] m. j. kuster, b. d. hall, r. white, m-layer registry prototype. ncsli 141 mii & automation committee (2022). online [accessed 26 march 2023] http://miiknowledge.wikidot.com/local--files/wiki:mii-projects/ [14] d. zajac, creating a standardized schema for representing iso/iec 17025 scope of accreditations in xml data, proc. ncsl int. workshop & symposium. st. paul, mn: ncsl international, 24-28 july 2016. online [accessed 26 march 2023] http://miiknowledge.wikidot.com/local--files/wiki:miireference-datasources/ncsli%202016%20zajak%20xml%20soa%20paper. pdf figure 2. screen shot from a demonstration application using a prototype mlayer registry. from aspectids tied to the input aspect (1990 conventional voltage), output aspect (si9 voltage) and variables in an aspectrelations table, the software automatically locates the applicable conversion equations and computes the result to any desired precision. the application displays aspectids using aspect symbols: action <s>, charge <q>, frequencyvoltage coefficient <cfv>, etc. the application shows the m-layer representations for information only. https://www.bipm.org/en/publications/guides/ https://nvlpubs.nist.gov/nistpubs/legacy/sp/nistspecialpublication1038.pdf https://nvlpubs.nist.gov/nistpubs/legacy/sp/nistspecialpublication1038.pdf https://www.bipm.org/en/conference-centre/bipm-workshops/digital-si/ https://www.bipm.org/en/conference-centre/bipm-workshops/digital-si/ https://doi.org/10.1016/j.measen.2021.100102 https://www.bipm.org/utils/common/documents/jcgm/jcgm_200_2012.pdf https://www.bipm.org/utils/common/documents/jcgm/jcgm_200_2012.pdf https://doi.org/10.1126/science.103.2684.677 https://doi.org/10.1016/j.measen.2022.100387 http://www.metrology.net/home/metrology-taxonomy/ https://www.w3.org/math/ http://www.qudt.org/ http://miiknowledge.wikidot.com/local--files/wiki:mii-projects/ http://miiknowledge.wikidot.com/local--files/wiki:mii-reference-data-sources/ncsli%202016%20zajak%20xml%20soa%20paper.pdf http://miiknowledge.wikidot.com/local--files/wiki:mii-reference-data-sources/ncsli%202016%20zajak%20xml%20soa%20paper.pdf http://miiknowledge.wikidot.com/local--files/wiki:mii-reference-data-sources/ncsli%202016%20zajak%20xml%20soa%20paper.pdf http://miiknowledge.wikidot.com/local--files/wiki:mii-reference-data-sources/ncsli%202016%20zajak%20xml%20soa%20paper.pdf collaborative systems for telemedicine diagnosis accuracy acta imeko issn: 2221-870x september 2021, volume 10, number 3, 192 197 acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 192 collaborative systems for telemedicine diagnosis accuracy jacques tene koyazo1, moise avoci ugwiri2, aimé lay-ekuakille3, maria fazio1,massimo villari1, consolatina liguori2 1 department of mathematics and computer science, physical sciences and earth science, university of messina, lecce 73100, italy 2 department of industrial engineering, university of salerno, fisciano 84084, italy 3 department of innovation engineering, university of salento, lecce 73100, italy section: research paper keywords: signal processing; biomedical; collaborative edge; cloud computing, accuracy; theranostics; measurement citation: jacques tene koyazo, moise avoci ugwiri, aimé lay-ekuakille, maria fazio, massimo villari, consolatina liguori, collaborative systems for telemedicine diagnosis accuracy, acta imeko, vol. 10, no. 3, article 26, september 2021, identifier: imeko-acta-10 (2021)-03-26 section editor: francesco lamonaca, university of calabria, italy received june 12, 2021; in final form august 5, 2021; published september 2021 corresponding author: jacques tene koyazo, email: jacquestene2013@gmail.com 1. introduction the spectacular progress in communication technology has boosted the telemedicine which seems to be a new medical practice preference in medical areas [1]. one of the benefits carried out by this new medical discipline is, for instance, the stomatological diagnosis. thanks to the communication and computer technology, the stomatological diagnosis can provide a safe and reliable remote diagnostic, counselling care, distance education and other information services of medical activities [2]. in recent years, various standards for health care research regulation have been developed. evidences in the literature have proven that studies based on ieee and iso standards [3] enable a good compliance in terms of collaboration with healthcare industries, government agencies and research institutes towards developing novel approaches and methods for handling and controlling diseases. in fact, implementing a reliable collaboration system in telemedicine has a tremendous advantage. it breaks distance restrictions, so that different medical institutions can provide diagnosis; it improves the exchange accuracy for medical advices; it provides a cooperative working environment suitable to share data and information helping in dealing with emergencies. since then, telemedecine has demonstrated a huge prospect thanks to everyday development of information and telecommunication technology [4]. a part from pravicy and security concerns, medical data transmission still face data transmission problem. according to statistics provided by the second xiangya hospital [5], most of medical data produce over 1 gb , and those massive generated data often tend to have a growth rate exceeding the speed of the expansion of the mobile iot bandwidth. this aspect was confirmed by the cisco’s yearbook report [6] demonstrating that they can account for more than 85% of data traffic. this paper is elaborated on the assumption that sensors are used to collect comprehensive physiological information from targeted patients and uses cloud to store and analized those information. the data from the latter process are sent to the service provider for deep investigation. at the same time, those information can be used to remotely monitor health condition of the patient. various sensors are nowadays used in clinical care, storing data from the used sensors in the cloud for more complex analysis and share the results with abstract the transmission of medical data and the possibility for distant healthcare structures to share experiments about a given medical case raises several conceptual and technical questions. good remote healthcare monitoring deals with more problems in personalized heath data processing compared to the traditional methods nowadays used in several parts of hospitals in the world. the adoption of telemedicine in the healthcare sector has significantly changed medical collaboration. however, to provide good telemedicine services through new technologies such as cloud computing, cloud storage, and so on, a suitable and adaptable framework should be designed. moreover, in the chain of medical information exchange, between requesting agencies, including physicians, a secure and collaborative platform enhanced the decision-making process. this paper provides an in-depth literature review on the interaction that telemedicine has with cloud-based computing. on the other hand, the paper proposes a framework that can allow various research organizations, healthcare sectors, and government agencies to log data, develop collaborative analysis, and support decision-making. the electrocardiogram (ecg) and electroencephalogram eeg case studies demonstrate the benefit of the proposed approach in data reduction and high-fidelity signal processing to a local level; this can make possible the extracted characteristic features to be communicated to the cloud database. mailto:jacquestene2013@gmail.com acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 193 the medical professional for further examination is the core idea beside this paper. 2. diagnosis in telemedecine through a collaborative framework: a literature survey 2.1. general overview the internet of things (iot) in collaborative medical framework is considered to be the most fundamental aspect enabling collaboration in telemedicine, because it allows healthcare applications to fully use the iot and cloud computing [7]-[9]. the framework also provides protocols to support the communication as well as broadcast of raw medical signals from different sensors and smart devices to a network of fog nodes. a good insight of a collaborative framework has been introduced by yang et al. [10] and almotiri et al.[11], where they have suggested an architecture able to collect data on the patient health thanks to several sensors, and transfert them to a remote server for processing and giving the possibility to display the results. the figure 1 shows essential components needed in a collaborative framework for collaborative system in telemedicine diagnosis. in ideal situation, sensors have to constantly collect patient’s health condition and vital information. the collected data are then sent to hand-held devices via edge router where it will analyzed and store on a cloud computing platform for further evaluations as presented in the figure 2. it is worth to mention here that, sensors are continuously sending patient’s vital signs as raw information such as electomyograpy (emg), electrocardiogram (ecg), electroencephalogram (eeg), body temperature, blood glucose (bg) and so on. the good data exchange platform architecture ensures that all sensors operate smoothly so that users can interact with them easily. ecg and eeg are demonstrated in section 3 of this paper as show cases. 2.2. cloud computing for telemedicine in information technology, the cloud computing becomes the hottest subject nowadays. the computing ressources allocated by is in fact on-demand, scalable and secure to users. in a work done by sultan et al. [12] cloud computing is described as the backbone of iot health systems. the cloud computing has the advantage of providing the capability of sharing information among health professionals, research institutes and patients in a more structured and organized maner, which minimized the risks of medical record lost [13]. the figure 3 presents the platform of a remote healthcare monitoring system based on cloud computing. each layer in the platform is designed to handle a specific task and it can be implemented in the way to serve various query for healthcare. the cloud storage and the multiple tenants access control are the “master layer” of the platform. it assures the collection of the healthcare data from sensors such presented in the figure 2 (layer 1). the healthcare annotation layer solves data heterogeneity issue, which is a big deal in signal processing discipline. the fact that, sensors used for telemedicine purpose generate data of various type, this complexifies the data sharing automation among agencies. one way is to just create an open linked life data sets to annotate personal healthcare data and integrate dispersed data in a patient-centric pattern for cloud application [14]. the data analysis layer processes data stored in the cloud to assist in figure 1. basic subsets in collaborative framework for heathcare in telemedicine. figure 2. a typology for remote patient monitoring using body sensors. figure 3. cloud-computing based on remote health monitoring – functional platform. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 194 clinical decision making. one can see in figure 4, mining algorithms are constructed in order to induce clinical paths from personal healthcare data. it is important to point out here that, major part of the cloud data centers are geographically centralized and located way far from end-users [15]. for telemedicine application which often requires immediate real-time feedback, communication between users and remote cloud servers are the reason of major issues like delays in round-trip, network congestion, and so on. those observations led to new evolutions in cloud computing, such as fog computing and big data, the cloud computing got then extended and got able to support high scalable computing platforms [16]. in [6], one can evidently see that, cisco was the first to introduce the fog computing concept as a possible solution to extend cloud network edge computing power and storage capacity. fog computing is known to be closer to devices and possesses a dense geographical distribution, so applications and services can be placed at the edge of the local network, which reduces bandwith usage latency. in this way cloud gets closer to the user, and the processing gets done locally, minimizing network ltency and bandwidth usage. the main fog computing architecture as illustrated in the figure 4. the interest beside the use of fog computing layer is deeply discussed by many authors, where they analyzed the role of fog computing in implementing healthcare monitoring framework and they proposed a mediator layer to receive raw information from sensor devices and then store them on the cloud. 3. case study and discussions embracing collaborative edge computing helps healthcare organizations visibility over patient care cycles. sharing information practionners (phycians or health professionals) can give organizations a clear big picture view of what’s going on and how to deal with it. moreover, this technology effectively contributes to the process of organizations of the health system (hospital or health center) from the point of view of the exchange of data in a reliable way, in particular for urgent critical cases presented by patients, such as neurological, cardiological problem, etc. figure 5 represents the proposed collaboration architecture of two healthcare centers, respectively a and b. we consider two patients under ecg and eeg diagnosis, as shown in the diagram, who are. this part constitutes the first block for the diagnosis based on the designed architecture, specifically for ecg and ecg study cases. the diagnosis is made for two patients, each in these healthcare centers where their biosignals are captured. the second considered a block of the process, and the result is composed of three compartments where each one plays a specific role, starting with the acquisition in the local interface, and the data received from another health center, via the data management, and the local storage for the final result after analysis and interpretation of data at the level of the third compartment (data analysis). figure 4. main attributes in fog computing architecture : an illustration. figure 5. the proposed collaborative architecture. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 195 this nature of collaboration between the network edges is attributed in two healthcare centers named edge a and edge b to ensure the reliability, storage, and processing of geographically distributed data. one of the main scopes of this architectural configuration is the exchange of diagnosis between two healthcare centers or multiple healthcare ones. this exchange is necessary to carry out specific comparisons amongst patients of the same age range and the same characteristics. due to heavy file dimensions, the proposed architecture also allows performing remote access without displacing files. that is possible thanks to the cloud. 3.1. ecg monitoring in medical applications, ecg sensors record the electrical activity of a heart at rest, then deliver acquired information about hr and rhythm. the recorded information is crutial in early prediction of a heart enlargement due to hypertension or heart attack. the integration of iot in ecg for telemedicine practices has a tremendous benefits and high potential to warn users about heart rate abnormality, which is a vital sign of early heart desease detection. in this paper, two patients have been considered. in the figure 6, the output ecg signal of two patients is presented, where the filtering sequence uses pan tompkin’s qrs detector. for the comparison purpose, the signals have been downsampled to 250 hz. each window represents 5.5 seconds of data. each stage introduces a delay with a cumulative delay of 40 samples. for a optimum filtering, the butterworth filter is used in this study. the filtering range is between 4 hz and 20 hz, the filter is of order 4. the proposed processing algorithm is implemented in matlab. in figure 7, one can see the respiratory rate which is a critical and one of the vital signs used in telemedicine. r-r interval has been used for detection. due to the change in heart rate synchronized with respiration, the r-r interval of the ecg is short during the respiration, and long during expiration [18]. however, the morphology of a heart can vary greatly depending on the patient as shown in figure 7. usually, normal heartbeat for a patient can resemble an abnormal beat for another. the hamilton and tompkins algorithms used in this paper [19] are then used for peak energy amplitude detection rather than the detailed morphology of the ecg. 3.2. eeg monitoring eeg are known to be interesting source of information for remote healthcare and can be associate to ecg for enhancing the diagnosis task [12]. the issue though is to apply appropriate techniques to extract information prior to upload to the cloud, so that medical agencies can take advantage of them for proper and accurate diagnosis. this paper adopted the filter diagonalization methods exploited by a. lay-ekuakille et al. [20] to extract relevant parameters such as complex frequencies from a given window. eeg can be seen as signals that are sums of damped exponentials, which makes suitable to apply fdm and/or the decimeted signal diagonalization (dsd). the bandlimited decimated signal can be modeled as 𝐶𝑛 bld = ∑ 𝑑𝑘 𝐾 𝑘=1 e−j 𝜔 𝑛 𝜏𝐷 , im 𝜔𝑘 < 0 , (1) where 𝜔𝑘 and 𝑑𝑘 are complex frequencies and amplitudes respectively. if m is the times at which the signal is sampled, then for each of the m signals, the diagnonalization for fdm algorithm can be implemented as follow: 𝐶𝑛 bld = ∑ 𝑑𝑘 𝐾 𝑘=1 e−j 𝜔 𝑛 𝜏𝐷 ⇒ 𝑈1 𝐵1𝑘 = 𝑢1𝑘 𝑈0 𝐵1𝑘 (2) 𝐶2𝑛 bld = ∑ 𝑑2𝑘 𝐾 𝑘=1 e−j 𝜔 𝑛 𝜏𝐷 ⇒ 𝑈1 𝐵2𝑘 = 𝑢2𝑘 𝑈0 𝐵2𝑘 . (3) figure 6. processing sequences for patient 1 and patient 2. figure 7. filtered, smoothed and processed ecg for patient 1 and patient 2. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 196 in (2) and (3), complex frequencies are extracted from the eigenvalues 𝑢1𝑘 and 𝑢2𝑘 . the reader is encouraged to find more detailed on fdm algorithm structure in [21]. as for the ecg case, the eeg is considered for the patient 1 and patient 2, where patient 1 is the unsuspected child and the latter is the suspected child. figure 8 presents the bisepstrum of the signal ranging from 10801 up to 12000 samples. the bisepstrum is built for the second interval for the patient 2 as can be seen in figure 9. an important clinical feature for both suspected and unsuspected cases (epilepsy in this case) are presented in figure 10 and figure 11 respectively for patient 1 and patient 2. as stated above, cloud computing provides secure platform for two-way sharing of research data across different agencies or institutions. platforms such as google-cloud, gift-cloud, etc… are designed to meet the need of collaborative research project by simplifying data transfer. the eeg and ecg characteristic features obtained in this elaboration, make easy to integrate with it local infrastructure of the institutions that provided clinical data and expertise, with end-user within routine clinical workflow. the results are presented such as supports for varied collaboration agreements between institutions and related access control restrictions. the improved scheme proposed in figure 5, makes possible the configuration as well as the update, and can allow new modalities to be added via the server without requiring software updates. features extracted such as bispectrum presented in figure 12 simplified the development of the research software. the idea is straitforward, such that the development allows to automatically fetch the data directely from the server, given the fact data has been uploaded from research center (in our case). these features are not just accurate, but also useful for both medical research projects where data sharing is required between researchers and clinical institutions and medical professionals for decision making. 4. conclusions telemedicine aimed to provide high-quality medical services, given that healthcare facilities nowadays hardly satisfy populations' needs due to limitations of public medical resources and infrastructures. this research proposed a cloud-computingbased architecture for decentralized and collaborative diagnosis by highlighting patients' data storage after meaningful feature extraction. in this way, a medical professional can rapidly grasp the patient's state in question and make an accurate decision easily. as reported at the beginning of section 3, collaborative edge computing certainly helps healthcare organizations in the spirit of connecting hardware and software issues in a unique platform for decision making in the interest of patients [22], [23]. figure 8. the bispectrum and associate representation for patient 1 figure 9. the bispectrum and associate representation for patient 2 (here the sample is 7201 8400). figure 10. the bispectrum and associate representation for patient 1 (here the sample is 7201 8400). figure 11. the bispectrum and associate representation for patient 2 (here the sample is 7201 8400). figure 12. superposition of bispectrums associate to patient 1 and patient 2. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 197 the personalized care is another field of the unifying platform.the contributions of this research inevitably include data reduction and high-fidelity signal processing to a local level to extract characteristic features and communicate them to the cloud database. a demonstration of eeg and ecg features extraction was carried out, and detailed on how the deployment of obtained processing results on a cloud-based application has been 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in final form august 16th, 2013; published june 2014 copyright: © 2014 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: this research was jointly funded by the emrp participating countries within euramet and the european union. corresponding author: wladimir sabuga, e-mail: wladimir.sabuga@ptb.de 1. introduction new high pressure technologies such as autofrettage, hydroforming and isostatic pressing are being intensively developed and used in the automotive industry, diesel engineering, vessel production for the petrochemical and pharmaceutical industry, water cutting machine manufacture, new material fabrication and, recently, for food sterilisation. new transducers for measuring pressures up to 1.5 gpa have recently been developed and are offered by several manufacturers. the use of these high pressure transducers requires their calibration and, thus, existence of appropriate reference pressure standards traceable to the international system of units. the operation range of the pressure standards in west europe is limited by 1.4 gpa. creation of new primary pressure standards up to 1.6 gpa and establishing a calibration service up to 1.5 gpa is the objective of a joint research project (jrp) "high pressure metrology for industrial applications" within the european metrology research programme (emrp) [1, 2]. ptb and fluke calibration (fluke) have jointly developed and built two 1.6 gpa pressure measuring multipliers to extend the pressure scale and the calibration range as required. 2. principle and key features of the pressure multipliers the operation principle of a pressure measuring multiplier is explained in figure 1. the multiplier includes a low pressure (lp), pl, and a high pressure (hp), ph, piston-cylinder assembly (pca) which have significantly different effective areas. the lp and hp pcas are axially aligned and their pistons are mechanically coupled. both, lp and hp pistons are unsealed in the cylinders and are rotated, which, due the lubrication effect, avoids mechanical friction between the pistons and cylinders. consequently, in the absence of other forces, the forces due to pressures ph and pl on the pistons are balanced when the ratio of pressures is equal to the ratio of the effective areas of the lp and hp pcas, ahp and alp: ph / pl = ahp / alp. (1) the high pressure, ph, can thus be determined by accurately measuring pl and by knowing the exact ratio of ahp to alp, also called multiplying ratio (km). the principle of the pressure measuring multipliers has been in use at least since the 1930s and is utilised in current practice, for example in the 1.5 gpa national pressure standard of russia, vniiftri [3]. a 1 gpa pressure multiplier has been commercially offered since the late abstract two 1.6 gpa pressure-measuring multipliers were developed and built. feasibility analysis of their operation up to 1.6 gpa, parameter optimisation and prediction of their behaviour were performed using finite element analysis (fea). their performance and metrological properties were determined experimentally at pressures up to 500 mpa. the experimental and theoretical results are in reasonable agreement. with the results obtained so far, the relative standard uncertainty of the pressure measurement up to 1.6 gpa is expected to be not greater than 2·10-4. with this new development the range of the pressure calibration service in europe can be extended up to 1.5 gpa. acta imeko | www.imeko.org june 2014 | volume 3 | number 2 | 54 1980s and is used by national metrology institutes and calibration laboratories as a secondary or transfer standard [4]. in the newly developed multiplier, the nominal effective areas of the lp and hp pcas were chosen to be 1 cm2 and 5 mm2, respectively. these are dimensions for which production technology is well established and that result in a pressure ratio of 1:20. thus, a pressure, ph, of 1.6 gpa on the hp side of the multiplier is reached at pl = 80 mpa which is easily generated and measured with high accuracy. the design of the new multiplier has specific features which distinguish it from that of the former multipliers. first, to avoid plastic deformation and to guarantee stability of the effective areas the components of the lp and hp pcas are made of tungsten carbide with 6% or 10% (hp piston) cobalt (wc-co) instead of steel used in [3]. since the tensile strength of the tungsten carbide is limited to roughly 0.7 gpa, the hp cylinder can be operated at the maximum pressure of 1.6 gpa only if it is supported from outside. thanks to a special design of the multipliers a compressive load is established on the outside of the hp tungsten carbide cylinder which prevents rupture when the pressure inside the cylinder exceeds the material tensile strength. in [4] this is accomplished by fitting a sleeve around the cylinder. in order to extend to 1.6 gpa in the new multiplier, two sleeves, each made of chrome/nickel/molybdenum steel, are successively assembled onto the tungsten carbide hp cylinder by means of thermal shrink fits. in addition, the hp cylinder with the two sleeves is set into a jacket which allows a jacket pressure (pj) to be applied to the lateral surface of the outer sleeve and, thus, to additionally compensate the tensile stress in the cylinder (figure 2). the hp pca is designed to be operated in controlled clearance (cc) mode with pj typically equal to 25% of ph, at which the pressure distortion coefficient (λ) of the pca should be around zero. in addition, the hp pca may be operated with variable pj in order to adjust the piston fall rate (vf) and the pca sensitivity, if necessary, as well as to study λ experimentally by measuring dependences of vf and ahp on pj. for optimal and stable operation of a pca it is desirable that the pressure in the piston-cylinder gap changes linearly from its maximum value at the gap inlet to the ambient pressure at the gap outlet. such a pressure distribution is difficult to realise in the case of cc hp pcas having a nominally constant gap in the pressure-free state because, under pressure, the piston-cylinder gap becomes extremely small in the outlet region due to a cross-sectional expansion of the axially loaded piston and a simultaneous reduction of the cylinder bore due to the jacket pressure [5]. in [3], where pcas are operated in the re-entrant mode, which produces even stronger contraction of the cylinder than the cc mode, the problem was solved by giving the cylinder bore a flare-like shape with a diameter at the outlet being by few micrometers larger than at the inlet. such a manufacture strategy is extremely difficult and generally leads to large widths and irregularities of the piston cylinder gap. in the new multiplier, the problem is overcome by giving the outer surface of the inner sleeve a variable shape. in the lower part, where the pressure inside the cylinder and in the pca gap is much larger than the ambient pressure, the inner sleeve has a cylindrical shape. in the upper part, where the pressure in the gap approaches the ambient pressure, the inner sleeve has a conical shape with the diameter at the top being by 0.3 mm smaller than the diameter of the cylindrical part. this results in a tapered gap between the inner and outer sleeves which reduces the action of pj on the upper part of the inner sleeve. therefore, excessive concentration of the pressure gradient in the pistoncylinder clearance towards the outlet of the cylinder is avoided and an acceptable flow rate of the pressure transmitting liquid between the piston and the cylinder is provided. the optimal shape of the inner sleeve was determined by finite element analysis (fea) as described in the next section. the lp pca was designed keeping in mind the requirement to have sufficiently low fluid flow through the piston-cylinder gap. this requirement results from the relatively large effective area of the lp pca compared to the area of a pressure balance pca maintaining and measuring pl. usually, pcas used in the range of 80 mpa have nominal effective areas of 0.1 cm2, which is ten times smaller than alp. excessive flow rate in the lp pca would cause a high piston fall rate of the reference pressure balance which could increase uncertainties or result in insufficient time for stable ph. to limit the flow rate and optimize performance, the principle of negative free figure 1. operation principle of a pressure multiplier. figure 2. hp pca in the mounting post. low pressure high pressure pisto cylinder inner sleeve outer sleeve jacket hp tube collar gland sleeve nut jacket pressure connection thermometer location jacket pressure channel acta imeko | www.imeko.org june 2014 | volume 3 | number 2 | 55 deformation was applied. this principle is well proven and is used in fluke gas high pressure balances [6], providing low fall rates at higher pressures and high sensitivity at lower pressures. in the lp pca design, the lp cylinder is surrounded by a sleeve with a conical taper on the inside surface and pl is applied to the outside surface of the sleeve. the lp sleeve has a sliding fit on the cylinder and is positioned so that its smallest diameter is located where the cylinder pressure is maximal. in the absence of pressure, the sleeve produces no stress on the cylinder. as pressure increases, loading the cylinder from inside and the sleeve from outside, the sleeve first comes in contact with the cylinder in the region where the pressure in the pistoncylinder gap is maximal. in this way, a variable outside load of the cylinder is created that optimally compensates the radial distortion of the cylinder produced by the inner pressure. with this variable outside load distribution a nearly linear pressure distribution in the lp pca gap is achieved. 3. finite element analysis to analyse feasibility of the pressure multipliers' operation up to 1.6 gpa and to optimise dimensions of the pcas components they were modelled using fea. the modelling was performed using two different fea software packages, ansys at ptb and cosmos/works at fluke. in this way, correctness of the calculations was verified by analysing the same problem. additionally, the analyses were performed for different problems to get complementary information on the multipliers performance. the fea included large deflection, contact and plastic capabilities, the latter required for the tube connected to the hp pca. first, all parts were modelled with their material properties and assumed geometries. for the hp pca the shrinking of the inner and then of the outer sleeve on the tungsten carbide cylinder, and connection of the hp tube to the hp cylinder was modelled. the cylinder to inner sleeve shrink fit was performed first, using nominal geometry. the deformation of the outer surface of the inner sleeve after this step was noted. in production this surface is re-machined after the initial shrink fit. to simulate this, the outer diameter of the inner sleeve was changed reducing it by the amount of deformation, to achieve geometry after this initial shrink step that gives a good representation of the geometry that results in production. the inner to outer sleeve shrink fit was performed second, using the resulting cylinder/inner sleeve combination with the outer sleeve nominal geometry. the shrink fit of the taper in the inner sleeve was accomplished in the same manner as the other shrink fit surfaces. the amount of contact of the surfaces was determined iteratively in the analysis, a step performed automatically by the fea software. three inner sleeve outside shapes were numerically tested in their effect on the stress, pressure distribution in the piston-cylinder gap and λ. connection of the hp tube to the cylinder and deformation of the tube under pressure were studied. a tube tip angle of 59.5° and matching cylinder cone angle of 60° were selected. the tube was moved into the cylinder to get a contact along the whole length of the cylinder cone and a pressure of 1.6 gpa was applied. surface loads in various combinations were applied. the loads included 50% or 100% of maximum measurement pressure on relevant surfaces, a linear and, alternatively, constant pressure distribution in the piston-cylinder gap, as well as a jacket pressure on the outer surface of the hp pca sleeve and a lp on the outer surface of the lp pca sleeve. after each load step, radial deformation, radial and tangential stresses were extracted. the stress and strain distributions were analysed in relation to the ultimate tensile strength (sut) and the elastic limit (sy) of the cylinder, sleeve, and hp tube materials. these properties together with the young’s modulus (e) and the poisson ratio (µ) based on the information by the materials' manufacturers and literature data are compiled in table 1. in addition, the ultimate compressive strength of the wc materials is known to be extremely high of about 7 gpa. later, e and µ values were accurately measured using resonant ultrasound spectroscopy [7]. after the shrink fit of the inner sleeve on the cylinder, a tangential stress of about -600 mpa (compression) was achieved at the inside of the cylinder. the tangential stress distribution at the cylinder inside after the subsequent shrinking fit of the outer sleeve is shown in figure 3. in any point, the absolute value of the stress is much lower than the ultimate compressive strength of the wc materials (7 gpa). in the upper part of the pca, the absolute value of the stress becomes lower which results from the conical shape of the outer surface of the inner sleeve. this corresponds to the intended reduction of the outside support in the region of the internal pressure drop. the dashed line in figure 3 shows the tangential stress calculated analytically under assumption of cylindrically perfect cylinder and sleeves. both the fea and analytical results demonstrate that the double shrink will to a great extent compensate for the stress produced by the internal pressure of 1.6 gpa. for a linear pressure distribution in the gap the maximum residual stress produced by the shrinking and the internal pressure would be about 400 mpa, which could be withstood by the wc cylinder even in the absence of jacket pressure. however, in order to minimize risk of cylinder rupture, jacket pressure is expected to table 1. material properties. part / material e/gpa µ sy/gpa sut/gpa lp pca, hp cylinder / wc-6%co 620 0.218 ≈0.7 hp piston / wc-10%co 560 0.218 lp & hp sleeves / cr-ni-mo steel 200 0.3 1.2 1.4 hp tube / austenitic steel 200 0.3 1.053 1.216 figure 3. tangential stress at the hp cylinder inside after shrinking fit of 2 sleeves calculated with fea (st) and analytically (st,calc). acta imeko | www.imeko.org june 2014 | volume 3 | number 2 | 56 be applied in all normal system operation. figure 4 presents the fea model of the hp pca and tangential stresses in the pca components at ph = 1.6 gpa, a linear pressure distribution from 1.6 gpa to zero along the piston-cylinder gap and pj = 0.4 gpa. the fea calculations for the hp pcas at the maximum measurement pressure of 1.6 gpa and the jacket pressure of 400 mpa show that the radial and tangential stress distributions are smooth and without any significant concentrations, the cylinder is subject only to compressive stresses, and the sleeves remain within the elastic limit. these results for the hp pca indicate that the design is not at risk for cylinder rupture nor instability of the effective area due to plastic deformation in the sleeves. the calculations also confirm the necessity of having two sleeves on the hp cylinder in order to achieve the required cylinder compression when the temperature for the thermal shrink is kept below the tempering temperature of the sleeve material. the analysis of the tube demonstrates that only a small portion of the tube near the center line is subject to plastic deformation. in the tapered part of the tube, in the sections where the tube is not supported by the cylinder, the region of plastic deformation does not exceed 1/3 of the tube cross section. these results indicate a reliable connection between the hp pca and tube at pressures up to 1.6 gpa. it is necessary to note that the fea was performed assuming the ultimate strength and the elastic limit each to be the same for tensile and compressive deformations based on the available manufacture data. this might produce inaccuracies of the fea results if these material properties were different for tension and compression. in a similar manner as the hp pca, an fea of the lp pca was performed at pl = 80 mpa and a linear pressure distribution from pl to zero being applied to inside of the cylinder with pl applied to the outside of the sleeve surrounding the cylinder. the primary objective was to minimise the radial distortions at the cylinder inside and thus the fluid flow rate. it was found out that, with an optimal taper on the inside of the sleeve, the radial distortions of the cylinder do not exceed 0.1 µm at any point of the cylinder bore. without the sleeve and in the free deformation (fd) mode, they would reach 1 µm at the gap entrance. combining the structural fea of the hp pca with a hydrodynamic analysis for its piston-cylinder gap λ and vf were calculated with using the ptb iterative method described in [5]. as a pressure transmitting medium two liquids were considered: di(2)-ethyl-hexyl-sebacate (dhs) at ph ≤ 0.5 gpa and polydiethylsiloxan pes-1 for ph ≤ 1.6 gpa. dhs is a liquid widely used in pressure balances up to 1 gpa. however dhs is not applicable at higher pressures because of solidification. its density and viscosity dependences on pressure were used as given e.g. in [5]. pes-1 has a significantly lower viscosity than dhs with acceptable values up to 1.6 gpa. its density and viscosity as functions of pressure were based on the experimental data presented in [3]. with dhs, calculations were performed in fd mode to provide target values of vf for optimal piston-cylinder gap widths to be achieved in the pistoncylinder production process. with pes-1, both fd and cc modes were analysed. as known from former fea studies results of hydrodynamic modelling strongly depend on a real initial gap profile between undistorted piston and cylinder [5]. in particular, information about the cylinder bore profile near the exit is important because the gap in this region becomes the narrowest under high pressure and therefore has a strong effect on the pressure distribution, vf and λ. to take this into account, prior to performing a final adjustment of the piston to the cylinder bore in the multiplier production process described in section 4, dimensional measurements were performed on the two hp cylinders. they included straightness measurements in the outlet region of the cylinder bore along 4 generatrix lines separated by 45°. results for opposite generatrices (0° and 180°, 45° and 225°, and so on) were averaged and are shown for the two cylinders in figure 5. for fea calculations, where the pcas are treated as axisymmetric, the gap profiles were averaged and approximated by analytical functions which are also presented in figure 5. the piston and the cylinder bore apart from the gap exit were considered ideally cylindrical. different gap widths (h) were analysed. figure 5 presents the case in which h was equal to 0.2 µm. results of the piston fall rate calculations for h = (0.2-0.5) µm, fd and cc operation modes, dhs and pes1 liquids are shown in figure 6. even with the smallest technologically feasible gap of 0.2 µm vf is too high when pes-1 and fd mode are used. the largest gap considered, 0.5 µm, combined with cc mode produces figure 5. piston-cylinder gap near the outlet for a perfect piston and real dimensions of cylinders 1 and 2. a b figure 4. fea model of hp pca (a), tangential stress distribution in it at ph = 1.6 gpa & pj = 0.4 gpa (b). -0.5 0.0 0.5 1.0 1.5 2.0 4 6 8 10 12 14 16 18 20 dr / µm z / mm 0° 45° 90° 135° in fea model cylinder 1 cylinder 2 [pa] acta imeko | www.imeko.org june 2014 | volume 3 | number 2 | 57 acceptable vf at ph >1 gpa but rates that are still too high below 1 gpa. surprisingly, the difference between the piston fall rates for 0.2 µm and 0.5 µm gaps in the pressure range (1 to 1.6) gpa is not as big as would be expected from theory of the undistorted gap. with the fea results for vf the range h = (0.30.4) µm was found as optimal. with h = 0.4 µm, a target vf of (0.068 to 0.073) mm/min was defined to be achieved at 32 mpa in control measurements when fitting the pistons to the cylinders. at 500 mpa with dhs and in fd mode, this gap width leads to vf = (0.57-0.61) mm/min. 4. realisation of the multipliers for both the hp and lp pcas, the best designs indicated by the fea were realized. complementary technologies available at ptb and fluke were combined. the piston-cylinders were manufactured and detailed technical drawings of the multipliers and all their parts produced by fluke. all other parts – each of the two multipliers comprised about 80 parts – were manufactured by ptb. fluke carried out a final mechanical adjustment of the sleeves and some other parts. in particular, processing of the final diameters of the sleeves to meet the defined tolerances of 1 µm and to achieve a roughness of lateral surfaces better than 0.2 µm required fluke’s expertise. the production of the pcas started with 5 to 8 pieces of lp and hp pistons and cylinders as well as sleeves. the best were selected during the succeeding processing and characterisation. ptb performed dimensional measurements and performed the thermal shrink fits of the sleeves on the cylinders. for the shrink fits, the outer steel sleeve was heated up to 400 °c maximum to stay below the tempering temperature of the sleeve's steel, which is 450 °c. however, provisional shrinking trials indicated that 400 °c of temperature increase may not be sufficient to perform the shrink – the insert stuck in the outside sleeve. to get more space and time leeway for the shrinking procedure, a greater temperature difference between the two parts was created by cooling the insert (cylinder in the 1st shrink stage and cylinder with already shrunk inner sleeve in the 2nd shrink stage) down to about −196 °c using liquid nitrogen. prior to shrinking the outer sleeve onto the inner, which had been fit to the cylinder in the 1st shrink, the cylindrical and conical outside surfaces of the inner sleeve was characterised dimensionally. after the two hp cylinders heat shrink operations their bores were re-machined to remove 3 to 5 µm from inner surfaces deformed by shrinking. the hp pistons and cylinders were then lapped to achieve piston fall rates which had been predicted by fea with the gap width of h = (0.3-0.4) µm. the test piston fall rate measurements in the production stage were performed at a pressure of 32 mpa at which the effect of the elastic distortion is relatively small and vf primarily depends on the undistorted gap width. later, piston fall rates were measured in both hp and lp pcas and allowed estimation of the gap width between piston and cylinders. it was found h = (0.27-0.36) µm for the hp pcas and h = (0.68-0.73) µm for the lp pcas. the whole production required the multipliers’ parts to be sent between ptb and fluke, some of them many times, for the subsequent production, characterisation and adjustment procedures. finally, the multipliers were assembled and preliminarily tested by fluke. 5. experiments first tests of the multipliers were performed by fluke at pressures (100 to 500) mpa on the hp side and (5 to 25) mpa on the lp side of the multipliers using two piston gauges as a reference, with dhs as a pressure transmitting liquid and at pj = 0.25·ph. the setup is shown in figure 7. multiplying ratios were determined using two hydraulic pressure balances in a crossfloat. both lp and hp piston gauges were pg7302. two different 500 kpa/kg pcas having expanded uncertainties in pressure of 22·10-6·pl + 16 pa and 27·10-6·pl + 16 pa (k = 2) were used in different runs on the lp side. a 5 mpa/kg pca having expanded uncertainty in pressure of 70·10-6·ph + 16 pa (k = 2) was used on the hp side of the multiplier. the hp and lp pcas' temperatures in the multiplier were measured using platinum resistance thermometers. these temperatures and the pistons position in the multiplier were indicated by a laboratory conditions monitor (lcm). the pistons were kept within ±2.5 mm around their middle working position. they were rotated by a dc motor at approximately 10 rpm. a ppch hydraulic pressure controller was used to set pj. a tare pressure (pt) produced on the hp side of the multiplier by the masses loading the hp piston (hp and lp pistons, pistons coupler, etc.) was measured at pl = 0 for each multiplier assembly using an rpm3 a1000, h1 (0-2) mpa pressure monitor with an uncertainty of approximately 2 kpa (k = 2). it was equal to (2.918 and 2.926) mpa for the two multipliers. with the tare pressure equation (1) transforms to ph = pt + pl·km, with (2) km = km,0 × [1 + λkm·(ph – pt)], (3) where km,0 is km at pl = 0 and λkm is the pressure dependence coefficient of km. a crossfloat, using the drop rate method, was performed between the two piston gauges with multiplier either 1 or 2 in between the two piston gauges at ph = (100, 200, 300, figure 7. multiplier system test setup. figure 6. piston fall rates calculated for different gap sizes, profiles and liquids. 0 1 2 3 4 5 6 7 8 0 200 400 600 800 1000 1200 1400 1600 1800 v f / ( m m /m in ) p / mpa h = 0.2 µm, cyl. 1, fd, dhs h = 0.2 µm, cyl. 2, fd, dhs h = 0.2 µm, ideal cyl., fd, dhs h = 0.3 µm, cyl. 1, fd, dhs h = 0.3 µm, cyl. 2, fd, dhs h = 0.3 µm, ideal cyl., fd, dhs h = 0.2 µm, cyl. 1, fd, pes-1 h = 0.2 µm, cyl. 2, fd, pes-1 h = 0.2 µm, cyl. 1, cc, pes-1 h = 0.2 µm, cyl. 2, cc, pes-1 h = 0.2 µm, ideal cyl., cc, pes-1 h = 0.5 µm, ideal cyl., cc, pes-1 h = 0.2 µm, fd, pes-1 h = 0.5 µm, cc, pes-1 h = 0.2 µm, cc, pes-1 h = 0.2 µm, fd, dhs lcm hp piston gauge motor power supply multiplier lp piston gauge acta imeko | www.imeko.org june 2014 | volume 3 | number 2 | 58 400, 500, 500, 400, 300, 200, 100) mpa in four runs total. according to (2), the multiplying ratio was determined at each point by subtracting pt from ph measured on the hp side and dividing by pl measured with the lp piston gauge (figure 8). the set of data for each run was fit with function (3) providing km,0 and λkm. the results of the two runs for each multiplier were combined (averaged) and used to determine the residuals of the points taken. after reviewing the results of the tests it was decided to leave out the 100 mpa in determining km,0 and λkm as it did not seem typical with respect to the rest of the results. table 2 gives the results of the fit for each multiplier. performance of the multipliers has been found satisfactory. the km values were reproducible within ±4·10-5 for multiplier 1 and ±2·10-5 for multiplier 2. an increased standard deviation in the case of multiplier 1 is presumably associated with the exchange of the reference lp piston gauge between runs 1 and 2. herewith and taking into account the uncertainties of the reference lp and hp piston gauges pg7302, the relative standard uncertainty of the multipliers in the pressure range (100 to 500 mpa) lies between (4 and 5.5)·10-5. with the same data the relative standard uncertainty in the range (1 to 1.6) gpa can be expected to be (1.3 to 2)·10-4, which is a very preliminary estimation and must be confirmed by experiments at higher pressures. this uncertainty is sufficiently small to provide a calibration service required by the industry. 6. conclusions and outlook the two novel 1.6 gpa pressure-measuring multipliers were developed, tested at pressures up to 500 mpa, and demonstrated repeatability on the level of as low as 2·10-5. a standard uncertainty of up to 5.5·10-5 obtained in the test crossfloats is mainly caused by the reference lp and hp standards. this uncertainty can be reduced in the future by more extensive experiments using more accurate 1 gpa standards of ptb as a reference, but also by a theoretical calculation of the pressure distortion coefficients of the lp and hp pcas taking into account the real dimensional properties of the hp piston-cylinder gap and of sleeve-to-cylinder gap in the lp pca. moreover, extension of the fluid flow calculations for the pca gap up to 1.6 gpa requires accurate data on density and viscosity of pes-1 at high pressure. all these measurements are in progress within emrp jrp [1]. acknowledgement the contribution of dr. p. ulbig in the organisation of this research and of mrs. d. hentschel, who manufactured most of the parts of the multipliers, both ptb members, is much appreciated. the authors acknowledge mr. p. delajoud (fluke dhi retired) for the design of the multipliers and f. valenzuela and m. bair (both fluke) for piston-cylinder fabrication and cross-float testing/analysis respectively. this research was carried out within the emrp. it is jointly funded by the emrp participating countries within euramet and the european union. references [1] euramet, “high pressure metrology for industrial applications”, publishable jrp summary report for ind03 highpres, http://www.euramet.org/index.php?id=emrp_call_2010. [2] emrp jrp ind03 highpres, http://emrp-highpres.cmi.cz/. [3] v.m. borovkov, "deadweight high pressure piston manometers", in: researches in the area of high pressures. e.v. zolotyh (editor). publisher izdatelstvo standartov, moscow, 1987, pp.577, russ. [4] p. delajoud, "the pressure multiplier: a transfer standard in the range 100 to 1000 mpa", bipm monographie, vol. 89/1, 1989, pp.114-124. [5] w. sabuga et al., "finite element method used for calculation of the distortion coefficient and associated uncertainty of a ptb 1 gpa pressure balance – euromet project 463", metrologia, 43 (2006) pp.311–325. [6] p. delajoud, m. girard, "a new piston gauge to improve the definition of high gas pressure and to facilitate the gas to oil transition in a pressure calibration chain", proc. of the imeko tc16 int. symp. on pressure and vacuum, sept. 22-24, 2003, beijing, china, acta metrologica sinica press, pp.154-159. [7] w. sabuga, p. ulbig, a.d. salama, "elastic constants of pressure balances' piston-cylinder assemblies measured with the resonant ultrasound spectroscopy", proc. of the int. metrology conf. cafmet-2010, april 2010, cairo. figure 8. multiplying ratio vs. high pressure corrected for tare pressure. table 2. results of multiplying ratios in individual tests and averages for each multiplier. multiplier 1 multiplier 2 km,0 λkm·107 mpa-1 km,0 λkm·107 mpa-1 run 1 19.987668 3.79 20.008672 3.55 run 2 19.989101 2.89 20.008994 3.39 average 19.988384 3.34 20.008833 3.47 acta imeko | www.imeko.org june 2014 | volume 3 | number 2 | 59 http://www.euramet.org/index.php?id=emrp_call_2010 http://emrp-highpres.cmi.cz/ development of 1.6 gpa pressure-measuring multipliers disarmadillo: an open source, sustainable, robotic platform for humanitarian demining acta imeko issn: 2221-870x september 2022, volume 11, number 3, 1 9 acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 1 disarmadillo: an open source, sustainable, robotic platform for humanitarian demining emanuela elisa cepolina1, alberto parmiggiani2, carlo canali3, ferdinando cannella3 1 snail aid – technology for development, via fea 10, 16142 genova, italy and industrial robotics facility, italian institute of technology, via morego, 30, 16163 genova, italy 2 mechanical workshop, italian institute of technology, via san quirico, 19/d, 16163 genova, italy 3 industrial robotics facility, italian institute of technology, via morego, 30, 16163 genova, italy section: research paper keywords: humanitarian demining; open source hardware; appropriate technology citation: emanuela elisa cepolina, alberto parmiggiani, carlo canali, ferdinando cannella, disarmadillo: an open source, sustainable, robotic platform for humanitarian demining, acta imeko, vol. 11, no. 3, article 8, september 2022, identifier: imeko-acta-11 (2022)-03-08 section editor: zafar taqvi, usa received march 9, 2022; in final form august 30, 2022; published september 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: emanuela elisa cepolina, e-mail: emanuela.cepolina@iit.it 1. introduction “peace agreements may be signed and hostilities may cease, but landmines and explosive remnants of war (erw) are an enduring legacy of conflict”, states, in its first sentence, the landmine monitor, a comprehensive assessment of progresses in eliminating landmines, cluster munitions and other erw, published annually. according to [1], in year 2020 alone, the number of casualties of mines/erw was more than 7000, with approximately 2500 people killed and the rest injured. out of them, the majority (80%) were civilians, half of whom children. at the moment, at least 60 states and other areas are contaminated by antipersonnel landmines. among them, the countries considered to be massively contaminated (with more than 100 km2 of contaminated land) are afghanistan, bosnia and herzegovina, cambodia, croatia, ethiopia, iraq, turkey, yemen and ukraine, with the latter recently bombed with cluster munitions [2]. among these, many are also facing severe hunger, with iraq, afghanistan and yemen having more than 34% of the total population undernourished, while ethiopia more than 25% [3]. while the utmost importance of releasing land to local communities for food production and economic development is evident, the lack of intensive mechanization of the demining process surprises. disarmadillo represents a breakthrough sustainable innovation in mechanical demining technologies; it has been designed to stay behind when demining is over and serve longterm agricultural development of the country where it helped release land to local communities. it is affordable and being based on mature agricultural technology its maintenance and running costs are minimized. instead of being designed to clear mines, it is designed to collect information about mine presence either from sensors and from light ground processing and vegetation cutting. thanks to its low cost, more units can be used at the same time, helping release land to local communities faster. when not used in demining operations, disarmadillo can be abstract the mine action community suffers from a lack of information sharing among stakeholders. since 2004, snail aid has been working on disarmadillo, a dramatic shift in paradigm: an open source hardware platform for humanitarian demining. developed mainly thanks to volunteers’ work across more than 15 years, the machine is now going to get a push thanks to the project disarmadillo+, in collaboration between snail aid technology for development and the italian institute of technology. the new version of the machine will be improved in terms of manoeuvrability, modularity, versatility, without compromising its characteristic features. the re-design will take into account the need of keeping the cost low and the technology appropriate to the context where it will work. the ability of the machine to serve two different purposes will also be preserved: the machine will keep on being easily convertible to its original agricultural nature, being developed around a commercial off-the-shelf powertiller. the paper presents the machine and the research work foreseen within the new project. mailto:emanuela.cepolina@iit.it acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 2 reconverted to its original agricultural use and help securing food production. the paper is organised as follows. section 2 introduces humanitarian demining and the machines currently employed in it, together with an overview of robotics solutions suggested for the task, highlighting shortcomings and possible improvements. section 3 introduces disarmadillo machine concept and the philosophy behind it. section 4 is about disarmadillo architecture, and section 5 is about the features of disarmadillo+, the new version of the machine under studying. then, conclusions are drawn. 2. humanitarian demining humanitarian demining methods are based on manual demining, a procedure in which mines are manually detected and neutralized by a human deminer, equipped with simple gardening tools such as shovels and shears, prodders and, if possible, metal detectors. manual demining is the most versatile and trusted method and therefore is present in every demining program. sometimes, manual deminers work together with dogs trained to detect explosives contained in mines. when it is possible, demining machines help with the physical demining process phases, i.e., vegetation clearance, mine detection, and removal [4]. however, the number of machines in use is surprisingly low. an in-field study [5] conducted in 2012, across six organizations in six countries, recorded only 13 machines in use. the geneva international centre for humanitarian demining (gichd) electronic catalogue of mechanical equipment used for demining operations [6], currently reports only 40 machines in use: this is the sum of numbers of machines in use inserted by a single company producing four types of different machines, the other producers having not filled in this information. although these data definitely do not represent the whole picture, they show that mechanization in this field is extremely limited. several reasons can be accounted for this issue, including lack of funding, the inability to move from research and development (r&d) to practical commercial devices, the cynicism of innovation by those convinced their current practices are entirely sufficient [7], the high cost of maintenance of complex equipment in mine affected countries [8] and the lack of information sharing among stakeholders. tiramisu, d-box and demining robots are among the largest r&d projects that recently tackled humanitarian demining. while the first two ran in parallel and were both cofunded by the european union (eu) within the 7th funding framework programme, the latter is still ongoing and is funded by nord atlantic treaty organization (nato) science for peace and security programme. among them, only tiramisu and demining robots were explicitly aimed at developing new robotic vehicles, while d-box was focused on creating an information management system [9]. the demining robots project employs a multi-sensor robotic platform developed in a previous phase of the project and designed specifically for research purposes and testing innovative mine detection methods such as impulse ground penetrating radar [10]. the robotic platform, called ugo-1st, is, thus, not yet suitable to be fielded in demining operations. the tiramisu project led to the development of robotic vehicles at higher technological readiness level (trl), such as teodor, frs husky and the apt. the first is a tracked outdoor platform equipped with an array of five metal detectors, the second a four-wheel all-terrain vehicle equipped with an arm carrying a metal detector and an artificial nose, and the last is an improvement of the locostra machine, a four-wheel agricultural tractor modified to be used in mine-affected areas [11]. these and many other robotic platforms designed for demining have been analysed in [12], which highlights the need to address several requirements other than the increase in safety of human deminers, such as the speed of robotic vehicles, the ability to operate over long periods of time in varied environments, the amount of payload they can carry and their cost-efficiency. out of all platforms, [12] selects six for quantitative comparison across the identified requirements. apart from teodor, frs husky and locostra, the comparison table includes: ares, a four-independently-steered wheel vehicle [13], silo-6 [14], a hexapod walking robot, and gryphon –iv [15], a modified moon buggy vehicle equipped with a pantograph arm carrying a metal detector. apart from having better landmine detection/exposure results in field trials, gryphon iv and locostra are more promising in payload and operation time. nevertheless, these two solutions have not found application in the field yet. this might be due to the fact that their development took place within r&d projects and was limited by funding available. an opensource approach would guarantee the community to take ownership of the technology and the development to continue behind research projects timelines. the international mine action standards (imas) [16] define demining machines as machines designed to be used in hazardous areas. they are divided into machines designed to detonate hazards, machines designed to prepare the ground, and machines designed to detect hazards. machines belonging to the first group are generally heavily armoured, highly powered and very expensive to purchase. they achieve mine detonation by processing the soil at high speed with spinning tools at the front aimed at crashing or hitting whatever they encounter, thus using a large amount of power, delivered by large and high fuel consuming engines. while most machines on the demining technology market belonged to this first type in the past, recently there has been a shift toward smaller sized [7], less powerful machines designed to prepare the ground other than detonate hazards. ground preparing machines are primarily designed to improve efficiency of demining operations by reducing or removing obstacles. the size of machines has been decreasing over time answering the need for more appropriate technologies, being the logistics of heavier ones very difficult in post-conflict scenarios, at the same time, a limitation of the practical in-field use of heavier and more powerful ones, has been acknowledged. a study [5] (figure 1) has shown that the efficiency of these machines in terms of mine detonation is well below expectations and, therefore, in most cases another mine clearance asset, usually manual deminers or mine detection dogs, has to follow the machine and complete its task. the shift toward smaller machines has occurred along with a change in paradigm aiming to employ resources more efficiently. the land release process has been promoted and is now largely employed to reduce time by which suspected hazardous areas (shas) are released to local communities. according to [16] mechanical land release involves a machine being used to indicate or confirm the presence or absence of landmines and/or erw within a suspected or confirmed hazardous area. the aim is to enable the deployment of other demining assets only in areas acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 3 proven to contain landmines and/or erw including unexploded sub-munitions. in other words, machines to be employed in mechanical technical survey mainly need to verify the absence of mines in the given area; if they encounter an explosion, the area needs to be re-categorized and further processed. this means that machines used in technical survey need to process the ground and to resist, or not to be severely damaged by, only one explosion at time, while keeping the operator safe. although recent trends allow introducing smaller, lightly armoured and powered, more cost-efficient machines, designed to perform multi tasks in ground preparation, and major steps have been taken towards this direction, there are still progresses to be made. figure 2 reports the size and power of demining machines available now. data have been extrapolated from the gichd electronic catalogue and from websites of manufacturers that recently exhibited their equipment in conference venues. as can be seen, the average weight of demining machines available on the market is still above 10 tonnes, and the average power is 300hp.going smaller and more versatile might be not only useful for humanitarian demining, allowing the number of machines in use to increase, but also, in light of reconverting demining machines to food production, for sustainable agricultural mechanization. in fact, according to [17], about 90 % of farmers worldwide operate on a small scale, and the technology must become accessible to this large group. reference [18] highlights as a key factor for the successful adoption of agrobots in developing countries the capacity to design and offer technical solutions at a low (affordable) cost but with a high impact. again [18] estimates that small robots at an affordable price for purchase or hire represent a potential alternative in areas where manpower is scarce and conventional machinery is not available or is too costly for smallholders. 3. disarmadillo the work on disarmadillo machine started in 2004 with a one-month long visit to mine action activities in sri lanka. during the trip, groups of deminers were interviewed to start the research in the right direction, better understand local needs and establish a reciprocal trust between local people and researchers. most notably, information was gathered by working on the functional requirements for a system of demining machines to work close to the deminers. when deminers were asked about their preferences for new machine technology, they expressed a strong desire for new machines that were small, light and inexpensive. they wanted machines to help in the most boring/difficult parts of their job, particularly cutting vegetation and processing the ground, especially the hardest one, scarified using a simple rake called heavy rake, according to local procedures, to remove the soil hiding mines [19]. based on these findings, the first version of disarmadillo machine, called participatory agricultural technology machine (pat machine) was built within the first author’s phd work [20]. the work on disarmadillo continued over the years thanks to the contributions of volunteers of snail aid, a non-profit organization, and students of a secondary technical high school in genova, italy, who devoted part of their time to improving the machine and building parts of it in the school mechanical workshop. disarmadillo is, in fact, conceived to be appropriate figure 1. results from the same demining machine. on the left, test lane used in test site in germany with dummy antipersonnel (ap mines), called worm, 0 cm – 20 cm deep: 98.22 % neutralized. on the right hand side, suspected hazardous area in angola: 10 ap mines processed and left live intact (of type pomz and pp-mi-sr). figure 2. size (top image) and power (bottom image) of demining machines currently on the market. average 0 5,000 10,000 15,000 20,000 25,000 size (kg of machines) average 0 200 400 600 800 1000 power (hp of machines) acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 4 to the local context, and thus, components have to be suitable to be produced in not specialized workshops. in 2021, disarmadillo+ project has been approved assuring a push forward thanks to the collaboration between researchers of the italian institute of technology and snail aid – technology for development (figure 3). the core idea behind disarmadillo is to adapt power tillers to demining applications. power tillers are small agricultural machines widely used and commercially available in many mineaffected countries and their second-hand market is largely spread. they are easy to transport as they are small and light, and they are available with different types of engines. the most powerful one (approximately 14 hp) is sturdy enough for being employed in several versatile tasks, from ground processing to vegetation cutting. power tillers, also known as walking tractors, two-wheel tractors or iron buffalos have a great importance in their nations’ agriculture production and rural economies. they not only have rotovator attachments but also mouldboard and disc-plow attachments. seeders, planters, even the zero till/no-till variety can be attached. reaper/grain harvesters and micro-combine harvesters are available for them. also very important is their ability to pull trailers with over two ton cargoes. the population of powertillers in developing countries is surprisingly high. china has the highest numbers estimated to approach 16 million, thailand has nearly 3 million, sri lanka 120,000, nepal 15,000. parts of africa have begun importing chinese tractors, and nigeria may have close to 1,000. many countries of central/eastern europe also have significant populations of 2-wheel tractors, as they have been sold there for agricultural use since the 1940s [21]. 3.1. disarmadillo philosophy: open source among all of the reasons that might be found behind the scarce employment of machines in humanitarian demining, in authors’ opinion is predominant the lack of information sharing. often, researchers into new technologies for mine action do not have access to useful information being generated in the field that is treated as proprietary and not shared [22] unless after an extensive and deep personal analysis, often involving field visits that generally require important resources to be committed to the cause. at the same time, machine producers tend to market their products in the same way as military equipment, negotiating their sales, including price, in confidence. the lack of transparency of the market makes comparing the cost-efficiency of machines difficult, and the introduction of new systems not perceived as necessary. therefore, in order to create a favourable environment for more technologies to enter the demining technology market, there is need to change approach and create a more transparent, less donor-depending and more cost-efficiency oriented market. disarmadillo is aimed to be an upgrade kit that can be mounted on every type of powertiller to transform it into a demining machine supporting manual deminers in their work. when not used in demining operations, disarmadillo can be reconverted to its original agricultural use and help secure food production. while commercial off-the-shelf (cots) components needed by the kit will be listed with price and suggested purchasing sites, all components that need to be custom made will have their technical drawings available for free downloading from the internet. potentially, a new machine could be built around any powertiller by anyone interested, with as few modifications as possible. similar approaches are being successfully used by projects targeting electronics (arduino) and heavier hardware (open source ecology or do it yourself (diy) vehicles or drones). as in these well known cases, the community of users would be asked to provide its feedback on experiences with the machine and contribute to future developments. the idea to adopt an open design business model for a mine action technology is provocative and runs counter the current trends in the humanitarian mine action (hma) market highlighted in the previous part of the paper; nevertheless, it is feasible and profitable. if required by customers, all parts needed could also be delivered in a box to the customer. if necessary, upon request from the customer, assembly of all components can also be offered as a service locally (as knowledge transfer) in the mine affected country together with training on the use of the machine. thanks to its modularity, if the community devises new tools or components, old machines can be upgraded without having to jettison what works. this approach aims to challenge the traditional lack of information sharing of mine action and increase the active participation of end users in the design and decision-making process. positive implications are expected in terms of bridging the gap between scientific and operational hma communities, increased competition level, cost reduction and possibly promotion of a closer integration with development. figure 3. disarmadillo evolution over time. figure 4. disarmadillo philosophy. acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 5 3.2. disarmadillo philosophy: versatility disarmadillo is a robotic platform designed to carry different tools. some have already been tested, some are designed and others are still in the form of ideas. thanks to the open nature of the project, partners have tested tools locally, such as the vibrating sieve, developed by prof. ross macmillan in australia. figure 5 depicts the tools conceived for disarmadillo and available on snail aid website (top image) and, as an example of them, the rake (bottom image). the rake is designed for ground processing in loose soils, where manual deminers use rakes to uncover the ground and expose mines. it penetrates the soil in front of the machine, cuts it and sieves it by lifting mines and leaving them besides for later collection by deminers. a prototype has been manufactured and successfully tested in jordan with dummy mines. 3.3. disarmadillo philosophy: demining and agricultural purpose as their job is to process the ground, agricultural machines originally conceived to work the soil could be efficiently employed in demining. since landmines impact food security via six different and somewhat reinforcing mechanisms, including access denial, loss of livestock, land degradation, reduced workforce, financial constraints and aid dependency [23], it makes sense to introduce (in mine-affected countries) multi-purpose technologies that can serve not only to demine but also for food production. agricultural technologies are mature and simple, easy repairable in every developing country in local, not specialized workshops. the modularity of agricultural technologies is another advantage; the same tools can be mounted on different tractors units and replaced by dedicated agricultural tools when demining operations are over. moreover, involving local technicians in re-designing new or improved technology helps reduce the dependency of local communities on donors’ help and facilitate local human development. empowerment is an integral part of many poverty reduction programmes. helping individuals and communities to function as agents for improving their wellbeing is essential for promoting human development and human freedom. empowerment shall not depend only on state-funded resources and opportunities but also on citizens taking responsibility for self-improvement. the handover of all mine action activities to local entities who can perform the majority of the work and gain skills while participating in the creation and maintenance of new agricultural technology for area reduction is desirable and necessary. the development of sustainable agricultural technologies and their transfer and dissemination under mutually agreed-upon terms to developing countries, is encouraged by food and agriculture organization (fao) [18]. fao also stresses the importance of supporting national efforts to foster the utilization of local know-how and agricultural technologies, to promote agricultural technology research to increase sustainable agricultural productivity, reduce post-harvest losses and enhance food and nutritional security. centres could be built with the double aim of renting and servicing machines both for humanitarian demining and agriculture, therefore representing a major step toward the integration of demining and development and the transition to local ownership, wished for since a long time. by introducing facilities where to adapt agricultural tools to demining activities, we can support r&d in agriculture. machinery could be provided as and when needed on a custom hire basis to the small and medium farmers who cannot afford to purchase their own machinery. similarly, in parallel to agricultural machines, the agro service centres could also provide machines for technical survey, based on agricultural machines. they could develop the modifications required to effectively address the demining problem locally, then hire these machines and provide assistance. as confirmed by current trends, today, in both developed and developing countries, the availability of human resources for farming is decreasing due to labour shortage both for lack of interest from young people and for weak or aging farming workforce: this means that a single worker (sometimes weak) is often in charge of large extensions of land. these factors influence the development of local agriculture and open a market share for automation also of small machines (mass lower than three tons). differently from heavy tractors, small machines with competitive costs cannot be effectively developed simply by modifying existing manual driven models: their architecture should be rethought for automation [24]. 4. disarmadillo architecture the current version of disarmadillo (figure 6.c) is built around a powertiller (figure 6.b) produced by grillo spa (www.grillospa.it), which kindly donated it to the project, together with spare parts and suggestions. the technical features of the power tiller and the constructed disarmadillo prototype are summarized in figure 6.a. the kit adds to the original powertiller a frame (figure 7), which has the dual aim of hosting two additional wheels at the front with respect to the original driven wheels and of embedding a track tensioning system. the frame is made of standard steel profiles, easy to build and maintain requiring only cutting and welding operations. agricultural tyres are replaced with special wheels designed to transmit motion to and support the tracks along their width. the frame added to the power tiller is designed to host a winch and a sort of three-point linkage system, allowing different tools to be figure 5. disarmadillo tools (top image) and a picture of the rake, tested in jordan with dummy mines (bottom image). acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 6 mounted at the front. the power take-off at the back of the machine can be used to power implements requiring an actuating torque. being reversible, the machine can be used indifferently forward or backwards. the machine is remotely actuated and is driven by an industrial remote-control unit, allowing major functions to be controlled remotely (figure 8). the remote control system is not substituting original manual controls; therefore, once reconverted to agricultural activities, the machine can go back to manual control. the platform rotates thanks to differential skid steering, thus by braking one of the two stub axles through which power is transmitted to the wheels through the differential gear. external band brakes are mounted on the frame, acting on the stub axles. a linear electric motor actuates each brake via cable, actuating a lever. power for the electrical motors is derived from the battery on board. the power take-off at the back of the powertiller, accessible through the frame, can be used to power tools that need a torque. 5. disarmadillo+ the disarmadillo machine has been subject to continuous research by snail aid and partners on a volunteer basis for the last fifteen years; its latest version was presented to the community during the 7th mine action technology workshop in basel in november 2018 raising considerable interest. the disarmadillo+ project will bring it to a higher level of maturity. major improvements are envisaged in terms of: • manoeuvrability and reliability, by actuating wheels with independent hydraulic motors and not any more through the differential gear powered by the internal combustion engine. each of the hydraulic motors, one per side of the machine, will be connected to a hydraulic pump actuated by the endothermic motor in a closed-loop circuit (figure 9). this new architecture would allow a narrower turn radius, and more efficient turning by actuating the two motors in opposite directions. moreover, it would allow driving the machine backwards without rotating it or changing configuration before operations start. • modularity, by splitting the frame into two parts, each portable by two persons, for easy conversion from one configuration (demining) to the other (agriculture). moreover, it is foreseen to change the points of attachment of the frame to the powertiller to reduce the number of ad-hoc flanges necessary a) b) c) figure 6. a) technical data of disarmadillo machine based on g131 powertiller produced by grillo. b) the original powertiller and c) disarmadillo as it was exhibited at the 7th mine action technology workshop in basel in 2019. figure 7. scheme of disarmadillo: red parts, frame, wheels, tracks and band brakes are added to the original powertiller (black). acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 7 to adapt the kit to different types of powertillers, exploiting the power take-off, with a pass-through system, and the axles. • human machine interface, by improving the remote control transmitter interface to expand its possibilities and make the driving more intuitive. • versatility, by investigating the possibility to study blast resistant tracks, building up on experience gained on blast resistant wheels developed for a larger machine for humanitarian demining based on a four-wheel tractor called locostra, developed by snail aid and other partners [25]. in fact, although explosive tests on a powertiller have been carried out in italy [26] and no damages have been recorded to the drive train, wheels were damaged, making maintenance necessary in case of an explosion. a solution to increase the machine's protection is to mount a front roller when the operating tool is mounted at the back. another option would be to design blast-resistant tracks that would allow retaining enough tractive integrity after an explosion occurs underneath them to continue working or to enable the withdrawal of the machine from the field for maintenance. a research [27] carried out in the 70’s exploited successfully three design principles: shock absorption by the roadwheels (embedding circular epoxy-resin rings between the hub and the rim), an almost unbreakable chain of tractive effort, and sacrificial track pads designed to fly away. a possibility would be to combine these ideas and the shock absorption exploited in locostra wheels, achieved thanks to solid rubber inner wheels embedded in steel frames, and design solid rubber roadwheels and steel track elements allowing ventilation. disarmadillo+ will offer the occasion to investigate in new tools, such as a ground driven/aerial platform: a drone borne sensor platform connected to disarmadillo+. the connection would be by tether or by another means allowing transmitting power from the ground rover to the drone, permitting long lasting flights and transferring data from the drone to the rover. importance will be given to keep complexity and cost low. as generally acknowledged and well explained by hemapala [28], when the price to performance ratio is too high, robots are academic toys. to keep the cost and complexity low, the machine will be automated gradually, according to needs. at the beginning, it will be remotely controlled. cost is also a key factor in the successful adoption of agrobots in developing countries, as stated by [18] that points out the need to design and offer technical solutions at a low (affordable) cost but with a high impact. the final shape of the disarmadillo+ machine is in the form of a remotely controlled tracked vehicle able to carry different tools. recently, there has been an increase in the supply of small remotely controlled platforms designed to perform agricultural tasks. it is interesting to analyse these types of machines available on the market in terms of size and power, as was done for demining machines. these small-size agricultural machines are sold on a much transparent market than the one of demining machines, so their cost can be obtained from producers’ websites or by browsing the internet, both for new products and second hand ones. figure 10 reports an analysis of 25 machines of this type according to their size and power; for few representative ones, also the cost is reported. as can be seen, the average size and power of these agricultural machines is much smaller than the one of demining machines. indeed, their weight is almost an order of magnitude lower than demining machines, and their rated power is approximately six times lower. the smallest of these agricultural machines, rc-751, produced by a danish company called timan, is comparable to disarmadillo+ in terms of weight and power. apart from being designed with a different philosophy, not for operating in hazardous areas, and having a higher cost (it is sold approximately at 20 k€), it offers a good reference, showing that machines with such a small size can successfully be employed in many agricultural tasks. moreover, the adoption of tools available for the rc-751 could be investigated also for disarmadillo+. a) b) c) figure 8. particular of the control unit: a) linear motors actuating brakes and the clutch, b) transmitter and c) 3d model of band brakes leverage. figure 9. disarmadillo+ scheme. acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 8 6. conclusions considering the increasing consensus on the fact that mine action should be regarded as a development activity, there should be a rapid change of the current approach. the paper summarises some topics in this domain and introduces the design of a simple modular machine for assisting mine removal through ground processing and vegetation cutting. the tractor unit is chosen in the agricultural machines domain (power tillers), so as to assure full consistency with the local expertise and habits. cost and sophistication minimisation are primary objective of the project. acknowledgement andrea pinza ceo of grillo spa who keeps on providing support, spare parts, and suggestions and personally delivered the powertiller we have been working on so far, is gratefully acknowledged. references [1] landmine monitor report 2021, international campaign to ban landmines – cluster munition coalition (icbl-cmc), 2021, isbn 978-2-9701476-0-2. [2] human rights watch, ukraine: russian cluster munition hits hospital, 25 february 2022. online [accessed 28 august 2022] https://www.hrw.org/news/2022/02/25/ukraine-russiancluster-munition-hits-hospital [3] hunger map 2021 – chronic hunger, world food program, 24 september 2021. online [accessed 28 august 2022] https://reliefweb.int/sites/reliefweb.int/files/resources/wfp0000132038.pdf [4] a guide to mine action, fifth edition, gichd, geneva, march 2014, isbn 978-2940369-48-5. [5] e. e. cepolina, land release in action, the journal of erw and mine action 17(2) (2013), pp. 44-50. online [accessed 28 august 2022 https://commons.lib.jmu.edu/cisr-journal/vol17/iss2/16 [6] mechanical equipment used for demining operations catalogue, gichd. online [accessed 28 august 2022] https://www.gichd.org/en/resources/equipmentcatalogue/equipments/?tx_solr%5bfilter%5d%5b3%5d=famil y%3a2 [7] a. w. dorn, eliminating hidden killers: how can technology help humanitarian demining?, stability: international journal of security & development 8(1) (2019), pp. 1–17. doi: 10.5334/sta.743 [8] e. e. cepolina, c. bruschini, k. de bruyn, providing demining technology end-users need, international workshop on robotics and mechanical assistance in humanitarian demining (hudem), tokyo, japan, 2005, pp. 9-14. online [accessed 30 august 2022] https://infoscience.epfl.ch/record/257002/files/2005_cepolina _providingdemtech_prochudem.pdf?ln=en [9] f. curatella, p. vinetti, g. rizzo, t. vladimirova, l. de vendictis, t. emter, j. peterit, c. frey, d. usher, i. stanciugelu, j. schaefer, e. den breejen, l. gisslén, d. letalick, toward a multifaceted platform for humanitarian demining, 13th iarp workshop on humanitarian demining and risky intervention, beograd, croatia, 2015. online [accessed 30 august 2022] https://www.foi.se/download/18.7fd35d7f166c56ebe0b1008e/1 542623794109/toward-a-multifaceted-platform_foi-s--5243-se.pdf [10] v. ruban, l. capineri, t. bechtel, g. pochanin, p. falorni, f. crawford, t. ogurtsova, l. bossi, automatic detection of subsurface objects with the impulse gpr of the ugo-1st robotic platform, 2020 ieee ukrainian microwave week (ukrmw), 2020, pp. 1108-1111. doi: 10.1109/ukrmw49653.2020.9252816 [11] y. baudoin, tiramisu: fp7-project for an integrated toolbox in humanitarian demining, focus on ugv, uav, technical survey and close-in detection, int. conf. on climbing and walking robots, sydney, aus., 2013. [12] d. portugal, l. marques, m. armada, deploying field robots for humanitarian demining: challenges, requirements and research trends, mobile service robotics (2014), pp. 649-656. doi: 10.1142/9789814623353_0075 [13] p. santana, j. barata, l. correia, sustainable robots for humanitarian demining, int. journal of advanced robotic systems 4(2) (2007), pp. 207-218. doi: 10.5772/5695 [14] p. gonzalez de santos, j. a. cobano, e. garcia, j. estremera, m. armada, a six-legged robot-based system for humanitarian demining missions, mechatronics 17(8) (2007), pp. 417-430. doi: 10.1016/j.mechatronics.2007.04.014 [15] m. freese, t. matsuzawa, t. aibara, e. f. fukushima, s. hirose, humanitarian demining robot gryphon – an objective evaluation 1(3) (2008), pp. 735-753. doi: 10.21307/ijssis-2017-317 [16] imas 09.50 mechanical demining, unmas, 2013. online [accessed 28 august 2022] https://www.mineactionstandards.org/fileadmin/mas/docume nts/standards/imas-09-50-ed1-am4.pdf [17] smallholders and family farming. in: family farming knowledge platform. fao, rome. online [accessed 28 august 2022] http://www.fao.org/family-farming/themes/small-familyfarmers/en/ [18] agriculture 4.0 – agricultural robotics and automated equipment for sustainable crop production, fao, integrated crop management 24 (2020). online [accessed 28 august 2022] http://www.fao.org/3/cb2186en/cb2186en.pdf figure 10. power, size and cost of remotely controlled agricultural multi platforms currently on the market. in the last graph green bars indicate cost. average 0 20 40 60 80 100 120 140 160 r c -7 5 1 ro b o m in i r c -1 0 0 0 f 3 0 0 p ro lv 3 0 0 p ro lv 4 0 0 f r5 0 m in i m in iz ro b o cu t r c 4 0 ro b o e v o ro b o cu t r c 5 6 t ra xx r c 5 6 lv 5 0 0 t ra xx t ra xx r c 7 5 ro b o cu t r c 7 5 lv 6 0 0 f r7 0 f r7 5 ro b o m id i lv 8 0 0 ro b o m a x r e co n f m -8 0 lv 1 4 0 0 power (hp of machines) average size 0 20,000 40,000 60,000 80,000 100,000 0 1,000 2,000 3,000 4,000 5,000 s iz e / k g size (kg) and cost (€) https://www.hrw.org/news/2022/02/25/ukraine-russian-cluster-munition-hits-hospital https://www.hrw.org/news/2022/02/25/ukraine-russian-cluster-munition-hits-hospital https://reliefweb.int/sites/reliefweb.int/files/resources/wfp-0000132038.pdf https://reliefweb.int/sites/reliefweb.int/files/resources/wfp-0000132038.pdf https://commons.lib.jmu.edu/cisr-journal/vol17/iss2/16 https://www.gichd.org/en/resources/equipment-catalogue/equipments/?tx_solr%5bfilter%5d%5b3%5d=family%3a2 https://www.gichd.org/en/resources/equipment-catalogue/equipments/?tx_solr%5bfilter%5d%5b3%5d=family%3a2 https://www.gichd.org/en/resources/equipment-catalogue/equipments/?tx_solr%5bfilter%5d%5b3%5d=family%3a2 https://doi.org/10.5334/sta.743 https://infoscience.epfl.ch/record/257002/files/2005_cepolina_providingdemtech_prochudem.pdf?ln=en https://infoscience.epfl.ch/record/257002/files/2005_cepolina_providingdemtech_prochudem.pdf?ln=en https://www.foi.se/download/18.7fd35d7f166c56ebe0b1008e/1542623794109/toward-a-multifaceted-platform_foi-s--5243--se.pdf https://www.foi.se/download/18.7fd35d7f166c56ebe0b1008e/1542623794109/toward-a-multifaceted-platform_foi-s--5243--se.pdf https://www.foi.se/download/18.7fd35d7f166c56ebe0b1008e/1542623794109/toward-a-multifaceted-platform_foi-s--5243--se.pdf https://doi.org/10.1109/ukrmw49653.2020.9252816 https://doi.org/10.1142/9789814623353_0075 https://journals.sagepub.com/doi/pdf/10.5772/5690 https://doi.org/10.1016/j.mechatronics.2007.04.014 https://doi.org/10.21307/ijssis-2017-317 https://www.mineactionstandards.org/fileadmin/mas/documents/standards/imas-09-50-ed1-am4.pdf https://www.mineactionstandards.org/fileadmin/mas/documents/standards/imas-09-50-ed1-am4.pdf http://www.fao.org/family-farming/themes/small-family-farmers/en/ http://www.fao.org/family-farming/themes/small-family-farmers/en/ http://www.fao.org/3/cb2186en/cb2186en.pdf acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 9 [19] e. e. cepolina, power tillers for demining in sri lanka: participatory design of low-cost technology, in humanitarian demining. london, united kingdom: intechopen. doi: 10.5772/5422 [20] e. e. cepolina (2008), powertillers and snails for humanitarian demining: participatory design and development of a low-cost machine based on agricultural technologies [unpublished doctoral thesis], university of genova. [21] wikipedia “two-wheel tractor”. online [accessed 28 august 2022] https://en.wikipedia.org/wiki/two-wheel_tractor [22] j. lokey, it's mine and you can't have it, journal of mine action 4(2) (2000). online [accessed 28 august 2022] https://commons.lib.jmu.edu/cisr-journal/vol4/iss2/40 [23] h. garbino, the impact of landmines and explosive remnants of war on food security: the lebanese case, the journal of conventional weapons destruction 23(2) (2019). online [accessed 28 august 2022] https://commons.lib.jmu.edu/cisr-journal/vol23/iss2/6 [24] e. e. cepolina, m. przybyłko, g. b. polentes, m. zoppi, design issues and in field tests of the new sustainable tractor locostra. robotics 3 (2014), pp. 83-105. doi: 10.3390/robotics3010083 [25] g. a. naselli, g. polentes, e. e. cepolina, m. zoppi, a simple procedure for designing blast resistant wheels, procedia engineering 64 (2013), pp. 1543 1551. doi: 10.1016/j.proeng.2013.09.236 [26] e. e. cepolina, m. u. hemapala, power tillers for demining: blast test, international journal of advanced robotic systems 4(2) (2007) pp. 253-257. online [accessed 28 august 2022] https://journals.sagepub.com/doi/pdf/10.5772/5690 [27] r. zumbro, mine resistant tracks, armor (1997), pp. 16-20. online [accessed 28 august 2022]. https://books.google.it/books?id=z60raaaayaaj&pg=ra1pa19&lpg=ra1pa19&dq=blast+resistant+tracks&source=bl&ots=daucbdm 9cv&sig=acfu3u142_8sw8rxkyh0mpuwg4xkfciwaq&hl =en&sa=x&ved=2ahukewjai8vm4yh2ahuvq_edhrzabd gq6af6bagmeam#v=onepage&q=%20tracks&f=false [28] m. u. hemapala, robots for humanitarian demining, in robots operating in hazardous environments. london, united kingdom: intechopen, (2017). doi : 10.5772/intechopen.70246 https://www.doi.org/10.5772/5422 https://en.wikipedia.org/wiki/two-wheel_tractor https://commons.lib.jmu.edu/cisr-journal/vol4/iss2/40 https://commons.lib.jmu.edu/cisr-journal/vol23/iss2/6 https://doi.org/10.3390/robotics3010083 https://doi.org/10.1016/j.proeng.2013.09.236 https://journals.sagepub.com/doi/pdf/10.5772/5690 https://books.google.it/books?id=z60raaaayaaj&pg=ra1-pa19&lpg=ra1-pa19&dq=blast+resistant+tracks&source=bl&ots=daucbdm9cv&sig=acfu3u142_8sw8rxkyh0mpuwg4xkfciwaq&hl=en&sa=x&ved=2ahukewjai8vm4yh2ahuvq_edhrzabdgq6af6bagmeam#v=onepage&q=%20tracks&f=false https://books.google.it/books?id=z60raaaayaaj&pg=ra1-pa19&lpg=ra1-pa19&dq=blast+resistant+tracks&source=bl&ots=daucbdm9cv&sig=acfu3u142_8sw8rxkyh0mpuwg4xkfciwaq&hl=en&sa=x&ved=2ahukewjai8vm4yh2ahuvq_edhrzabdgq6af6bagmeam#v=onepage&q=%20tracks&f=false https://books.google.it/books?id=z60raaaayaaj&pg=ra1-pa19&lpg=ra1-pa19&dq=blast+resistant+tracks&source=bl&ots=daucbdm9cv&sig=acfu3u142_8sw8rxkyh0mpuwg4xkfciwaq&hl=en&sa=x&ved=2ahukewjai8vm4yh2ahuvq_edhrzabdgq6af6bagmeam#v=onepage&q=%20tracks&f=false https://books.google.it/books?id=z60raaaayaaj&pg=ra1-pa19&lpg=ra1-pa19&dq=blast+resistant+tracks&source=bl&ots=daucbdm9cv&sig=acfu3u142_8sw8rxkyh0mpuwg4xkfciwaq&hl=en&sa=x&ved=2ahukewjai8vm4yh2ahuvq_edhrzabdgq6af6bagmeam#v=onepage&q=%20tracks&f=false https://books.google.it/books?id=z60raaaayaaj&pg=ra1-pa19&lpg=ra1-pa19&dq=blast+resistant+tracks&source=bl&ots=daucbdm9cv&sig=acfu3u142_8sw8rxkyh0mpuwg4xkfciwaq&hl=en&sa=x&ved=2ahukewjai8vm4yh2ahuvq_edhrzabdgq6af6bagmeam#v=onepage&q=%20tracks&f=false https://books.google.it/books?id=z60raaaayaaj&pg=ra1-pa19&lpg=ra1-pa19&dq=blast+resistant+tracks&source=bl&ots=daucbdm9cv&sig=acfu3u142_8sw8rxkyh0mpuwg4xkfciwaq&hl=en&sa=x&ved=2ahukewjai8vm4yh2ahuvq_edhrzabdgq6af6bagmeam#v=onepage&q=%20tracks&f=false https://doi.org/10.5772/intechopen.70246 how to stretch system reliability exploiting mission constraints: a practical roadmap for industries acta imeko issn: 2221-870x december 2022, volume 11, number 4, 1 6 acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 1 how to stretch system reliability exploiting mission constraints: a practical roadmap for industries marco mugnaini1, ada fort1 1 university of siena diism department, via roma 56, siena, italy section: research paper keywords: reliability design; mission; reliability assessment; reliability enhancement citation: marco mugnaini, ada fort, how to stretch system reliability exploiting mission constraints: a practical roadmap for industries, acta imeko, vol. 11, no. 4, article 17, december 2022, identifier: imeko-acta-11 (2022)-04-17 section editor: francesco lamonaca, university of calabria, italy received august 14, 2022; in final form november 19, 2022; published december 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: marco mugnaini, e-mail: marco.mugnaini@unisi.it 1. introduction in the literature it is possible to find many examples discussing on advance re-search on reliability assessment and analysis. most of these papers discuss on advanced methods for reliability allocation and improvement. nevertheless, such methods are often lacking practical implementation and may result for actual implementation tricky or require a set of information not always available in practical cases. for example, in [1] the authors provide an approach based on baysan analysis for parameter estimation presenting an interesting approach for process parameter evaluation which embeds a priori information to solve lack of data. in [2] the authors present a paper addressing reliability prediction modelling based on kalman filtering applied to ion batteries while in [3] ai approaches are used to solve reliability problems on oil&gas contexts. other papers such as [4]-[5] describe general method for reliability assessment based on the most commonly used database such as mil-hdbk217f, oreda or others, as function of temperature and environment. in some other papers, instead, there are fine analysis on predefined structures as in [6] where the advantages of different hardware solutions are compared with the aim to show how small changes on the practical implementation may lead to different results. usually, such achievements are obtained by means of different architectures without comprising the implications of mission changes [6]-[12]. on practical basis, moreover, the systematic lack of confidence bounds in presenting results and the impossibility for companies to provide additional analytic description in addition to synthetic results like the over-used mean time to first failure or mean time between failures (mttf, mtbf) to their evaluation make the transmission of information very difficult to direct customers or other realities [13]-[20]. the correct reliability design can be successfully approached by means of theoretical analysis if the design is followed since the very beginning phases of product development [20]-[21]. unfortunately, especially in small companies where resources are very limited, the designers usually underestimate the reliability allocation problem demanding such analysis to a subsequent phase. in general, it is not easy to find in the literature a practical guide for companies which is able to embed both the theoretical and the actual application implications in a suitable way [22]. some applications on the contrary, address the thrust of measurements without taking into consideration hardware and software reliability even in industrial contexts [23]-[25]. in this paper the authors would like to show on a practical way how implications on mission definition can be exploited for abstract reliability analysis can be committed to companies by customers willing to verify whether their products comply with the major international standards or simply to verify the design prior of market deployment. nevertheless, these analyses may be required at the very preliminary stages of design or when the design is already in progress due to low organizational capabilities or simple delay in the project implementation process. the results sometime maybe be far from the market or customer target with a subsequent need to redesign the whole asset. of course, not all the cases fall in the worst scenario and maybe with some additional consideration on mission definition it is possible to comply with the proposed reliability targets. in this paper the author will provide an overview on the approach which could be followed to achieve the reliability target even when the project is still on-going providing a practical case study. mailto:marco.mugnaini@unisi.it acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 2 reliability evaluations. in section i there’s an introduction describing the critical aspects of reliability assessment and design com-pared to the present literature. in section ii there’s a description of the formal approach used in describing borderline conditions where predictions maybe far from desired results. in section iii a case study about a general electronic board design is treated where it is possible to see how reliability targets which seems far from original design maybe matched just introducing considerations on mission profile. finally in section iv the conclusions are discussed. 2. models and methods reliability design is always following well established rules from an academic standpoint. starting from a problem definition and a mission description a design flow diagram and subsequent reliability block diagram can be approached and built. nevertheless, the most complicated part of the reliability evaluation and function description is the choice of the proper failure rate or probability density function for the components describing the item to be designed. an easy and reasonable approach for companies is to rely on their a priori knowledge and build bayesian based models. as an alternative companies may exploit internal or external database with the risk of selecting components with similar failure models but used in very different context resulting therefore in too conservative evaluations or completely wrong forecast. another issue is the overall absence of confidence bounds in companies forecast which makes the result outcome useless. mission profile definition if one of the most critical things among the ones previously cited which make reliability forecast subject to interpretation. as it can be easily understood the same item used in a different environment or with a different time apportionment or duty cycle may have, as a final result, very different reliability prediction. on the other hand, mission definition is very often neglected as a powerful tool to stretch system, subsystem or item reliability tailoring the application to a more realistic scenario. in figure 1 there’s the flow diagram of a commonly used approach in industry design concerning reliability aspects. this simplified version of the design is often considered in a generalized way which may lead to underestimates of specific important details. two important aspects should be underlined concerning the fact that not always field data are available on similar project mission profiles and databases maybe used in an unproper manner obtaining too conservative results or too optimistic forecasts. mission definition is another aspect which is often not investigated properly with the limitation to provide a general mission description avoiding subdividing it in a set of several submission according to the whole lifecycle of the item which is to be designed. figure 2 represent a more detailed view of this approach which should be taken into consideration by companies in the general development phase whenever reliability (and more in general reliability availability safety and maintainability rams) aspects are involved. 3. about illustrations and tables let’s consider a system designed to drive some signaling infrastructure in the railway context. such example nicely fits the scope of this paper from the moment that the safety and reliability requirements are so tight that not considering the mission profile could lead to several system further redesign. in figure 3 the general architecture composed by a power sartup system, a vital power source, a set of configuration memories a couple of microprocessors implementing the 2oo2 architecture an optional third cpu for managing external communication, a set of auxiliary electronics, a drive output system and a set of actuators is represented. sample equations for specific components are available on reliability standards for discrete and semiconductors and they are in the general form as equation (1): 𝜆𝑐 = 𝜆𝑏 ∙ ∏ 𝛱𝑖 𝑛 𝑖=1 , (1) where 𝜆𝑐 is the overall failure rate, 𝜆𝑏 is the basic failure rate without any correction factor but the ones due to temperature, stress and inner model and 𝛱𝑖 are the corrective factors depending on the specific model characteristics, environment and quality. the generalized form of the mission for this kind of general architecture 2oo2 for a signalling system can be summarized with the following sentence: “being able to function safely for at least 40000 hours”. companies general approach is to try to design an architecture complying the safety integrity level (sil) 4 safety standard (which is a must in such context) and the mtbf requirements as described above with a blind approach. additional requirements may include the use of a specific figure 1. general flow diagram used in companied when designing a project to fulfil reliability requirements. figure 2. project reliability definition embedding operative conditions and mission definition. acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 3 database for components failure rate which can be the milhdbk-217f with the part stress approach. such new information lower our first estimation to the following mtbf (we are now neglecting the confidence bounds just to deliver the information on how such figure can be transformed changing slightly the scenario). an additional consideration could be added embedding in the analysis the environment and the enclosure temperature which for such application in middle europe can be standardized as 40 °c and ground fixed (according to the selected database). if now the designers would like to move far from the preliminary design results of mtbf several options come into the field. component with improved quality could be considered even if such information as a matter of fact is not described anywhere in any component datasheet. at least the military handbook classification cannot be easily found and therefore such approach requires an effort which is not selected from designers practically. as an alternative it is possible to evaluate for standard discrete components (especially capacitors or resistors) the derating factors which reduce most of time to just voltage ratio or pawer ratio. again, this approach may result in a generalized one due to lack or resources and time considering the number of such components present in a huge electronic project like a signalling system for railway applications. a more effective option, which is often neglected by most designers, is the possibility to allocate to each subsystem composing the system a different mission profile or a different duty cycle. these two concepts are intrinsically embedded in the preliminary design phase as well as in the detailed one. nevertheless, the possibility to match a better design exploiting such features is barely underestimated during assessment phase. considering figure 4, it is possible to see that the three configuration memories and the start up power unit have for sure a different mission with respect to the overall design and additionally for sure that may have a different duty cycle. once such considerations have been taken to the design if the target mtbf is still not achieved further improvement may pass through component reduction or alternative redundant configurations. if the first approach could be followed it would be preferred because these applications usually imply safety considerations as well. in figure 5 an additional improvement due to a resizing of the cpu capabilities is shown. in such diagram the additional cpu c has been embedded in the others removing in this way the additional configuration memory and correspondent circuitry. it is therefore possible to represent such units out of the original schematics and to modify the reliability block diagram (rbd) accordingly as shown in figure 6. simulation results can be accomplished exploiting some commercial software. in this case relyence part calculator has been exploited to compare different configurations outcomes. in table 1 the system not comprising any mission impact on subsystem and exploiting mil hdbk 217f database is considered and results shown. in table 2 a mission consideration as well as power and voltage derating have been comprised in the evaluation. the configuration memories as well as cpu c and some ancillary electronics have been used with 1 % duty cycle figure 3. sample rough electro-mechanical project of a system for railway applications with reliability and safety requirements to be interfaced with signaling system. figure 4. reviewed functional block of the interfacing system for railway applications where mission contribution is included. figure 5. reduced system design to minimize the impact of low duty cycle component on the original design improving the system reliability and not affecting the system architecture. figure 6. comparison of the rough rbd of the original signalling interfacing system a) and the reduced one b) embedding consideration on the mission definition. acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 4 reflecting the actual use on a 24 h real timescale. these new considerations have brought to results shown in table 2 where significant improvement on the overall mtbf have been achieved. if the result is still not acceptable according to the design another approach could be to negotiate a new reference database. one of the most accepted one is the ansi vita one. in such database the components coming from the mil hdbk 217f have been actualized considering the advancement of the technology. for example, microcontroller which were not present in the previous version can be now considered as subset of microprocessors. the drawback is that this database is not an independent one and has been derived with the contribution of several companies. results of this significant improvement are shown in table 3. finally in table 4 it is possible to compare the three different kind of results which can be obtained just applying these different deviations from a standard approach. a final overview on the different kind of implementation depending on the environmental variation can be shown in figure 7 and figure 8. three different environments ground benign, ground fixed and ground mobile (gb, gf and gm) are compared in such figures showing the different contributions in terms of failures per million hours (fpmh) depending on the environment selected and on the used database. in figure 7 the mil hdbk 217f database has been exploited while in figure 8 the ansi vita one has been used. it is important to highlight how differences in results are kept even in table 1. results of simulation not embedding mission impact on subsystem and exploiting mil hdbk 217f database. name failure rate in f / (106 h) mtbf in h failure rate in % main board 18.708 53453.07 100.00 configuration memories 6.03 165837.5 32.23 cpu2oo2 2.204 453720.5 11.78 clocks 0.574 1742160 3.07 eth1 0.203 4926108 1.09 eth2 0.203 4926108 1.09 start power up 0.506 1976285 2.70 gen vit 2oo2 0.715 1398601 3.82 general electronics 1.147 871839.6 6.13 cpu c 2.237 447027.3 11.96 vital power 1.884 530785.6 10.07 pwr start up 0.841 1189061 4.50 actuator 2.164 462107.2 11.57 table 2. results of simulation embedding mission profile and duty cycle on subsystem and exploiting mil hdbk 217f database. name failure rate in f / (106 h) mtbf in h failure rate in % main board 8.052 124192.75 100.00 configuration memories 0 0 cpu2oo2 0.575 1739130.4 7.15 clocks 2.204 453720.51 27.37 eth1 0.203 4926108.4 2.52 eth2 0.203 4926108.4 2.52 start power up 0 0 gen vit 2oo2 0.715 1398601.4 8.88 general electronics 1.147 871839.58 14.24 cpu c 0 0 vital power 0 0 0 pwr start up 0.841 1189060.6 10.44 actuator 2.164 462107.21 26.88 table 3. results of simulation embedding mission profile and duty cycle on subsystem and exploiting ansi vita database. name failure rate in f / (106 h) mtbf in h failure rate in % main board 4.816 207641.196 100.00 configuration memories 0 0 0 cpu2oo2 2.263 441891.295 46.99 clocks 2.127 470145.745 44.17 eth1 0.024 41666666.7 0.50 eth2 0.024 41666666.7 0.50 start power up 0 0 gen vit 2oo2 0.67 1492537.31 13.91 general electronics 0.995 1005025.13 20.66 cpu c 0 0 0 vital power 0 0 0 pwr start up 0.099 10101010.1 2.06 actuator 0.614 1628664.5 12.75 table 4. comparison of the mtbf and failure rates of the three improvements proposed in the analysis. confidence bounds are 95 %. name failure rate in f / (106 h) mtbf in h main board ansi vita 4.815 748 207 652.05 main board mhdbk 217 8.052 190 124 189.81 main board mhdbk 217 nm 18.708 161 53 452.61 figure 7. system behaviour under three different environment ground benign (gb), ground fixed (gf), ground mobile (gm) according to mil hdbk 217f. figure 8. system behaviour under three different environment ground benign (gb), ground fixed (gf), ground mobile (gm) according to ansi vita. acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 5 the changes of environment making this latter database more attractive for non-fully conservative and more modern approaches. the process which should be followed in assessing reliability requirements after preliminary design should be the one described in figure 9. the first step consists in negotiating with the final customer the exploitable database from the moment that results are really affected by such choice. then after the bill of materials (bom) has been defined and the mission correctly described, it is crucial to identify which subsystem are subject to duty cycle modification. components quality level even if important is not crucial especially if referred to mil hdbk 217f from the moment that such information is difficult to be gathered from any commercial supplier. temperature information instead is vital as well as operative environment due to the fact that great part of final analysis outcome depends on the. once this step has been accomplished. 4. conclusions in this paper the author tried to analyze a recurring problem during design phases. usually, the producers of electromechanical reliability assembly are more focused on general performance than on reliability verification. this approach inevitably implies a huge effort in final redesign and long negotiations with final customers due to the lack in the procedural design. the authors tied to highlight that sometimes no redesign is needed but a more precise and detailed description of the system mission may allow a redefinition of the reliability evaluation criteria. as a general concept these aspects should fall in the optimized and best practices of item design but as a matter of fact it may happen that due to the high volume or project complexity such aspects may be neglected. it has been shown on an actual example how by following some basic steps, as suggested in the manuscript, it is possible to minimize the impact of redesign and achieve satisfying results. this paper tries to fill a gap which is not described often in the literature because it has to deal with some peculiar aspect of the specific design but whose general rules can be applied almost in any engineering project. the analysis shows moreover the possibility to exploit different database which are not selected in common projects usually to the lack of information on their exploitability even when there are changes in the operating environment. acknowledgement this study has been conducted exploiting a research licence of part analysis provided by relyence software. references [1] m. mugnaini, m. catelani, g. ceschini, a. masi, f. nocentini, pseudo time-variant parameters in centrifugal compressor availability studies by means of markov models, microelectronics reliability, vol. 42, 2002, pp. 1373-1376. doi: 10.1016/s0026-2714(02)00152-x [2] van-trinh hoang, n. julien, p. berruet, design under constraints of availability and energy for sensor node in wireless sensor network, conference on design and architectures for signal and image processing (dasip), 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by measuring the optimal size and location of distributed generation acta imeko issn: 2221-870x september 2022, volume 11, number 3, 1 8 acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 1 performance enhancement of a low-voltage microgrid by measuring the optimal size and location of distributed generation ahmed jassim ahmed1, mohammed h. alkhafaji1, ali jafer mahdi2 1 electrical engineering department, university of technology, baghdad, iraq 2 electrical engineering department, university of kerbala, kerbala, iraq section: research paper keywords: microgrid; distributed generation integration; autoadd; power losses reduction; voltage profile improvement citation: ahmed jassim ahmed, mohammed h. alkhafaji, ali jafer mahdi, performance enhancement of a low-voltage microgrid by measuring the optimal size and location of distributed generation, acta imeko, vol. 11, no. 3, article 21, september 2022, identifier: imeko-acta-11 (2022)-03-21 section editor: francesco lamonaca, university of calabria, italy received march 25, 2022; in final form august 30, 2022; published september 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: ahmed jassim ahmed, e-mail: ahmedjasem858@gmail.com 1. introduction the increment in energy demand is an indicator of economic growth, and this demand has been growing rapidly in many sectors, such as in the building, transportation, and manufacturing industries. however, the consumption of energy is linked directly to many environmental issues due to using fuel or coal frequently as the primary sources for electricity generation, as shown in figure 1, which is the main reason for emitting greenhouse gases (ghg). those gases are very harmful dangerous for the environment [1]. because of that, many global actors, such as world bank, started encouraging countries to use clean energy sources by supporting their projects financially [2]. therefore even during the pandemic, when the economy got affected by the lockdown, renewable energy sources kept growing fast [3]. integrating renewable energy sources (res) in low-voltage networks is creating significant changes in the electric power system's operation. in general, this integration occurs widely in low and medium-voltage networks. this leads to the microgrid (mg) concept, which can be defined as a complex energy system that needs a specific framework, coordination of information flows and energy resources, as well as protection and the assurance of reliable energy [4]. it is built by the integration of res, conventional generators, energy storage devices, and loads, as shown in figure 2. mgs can work in both the connected mode with the main grid and islanded mode [5]. distributed generation (dg) nowadays is gaining its reputation for becoming the main part of operating distribution networks. this is due to the technological improvement of many types of res, such as photovoltaic systems, fuel cells, combined heat and power sources, and wind energy sources. this integration of dgs has major importance in reducing the emissions of co2 and improving the efficiency and the security of distribution networks and achieving a reliable operation of these networks [6]. the uncontrolled allocation of abstract a power system in which the generation units such as renewable energy sources and other types of generation equipment are located near loads, thereby, reducing operation costs and losses and improving voltage is called a distributed generation (dg), and these generation units are called distributed energy resources. however, dgs must be located optimally to improve the power quality and minimize power loss of the system. the objective of this paper is to propose an approach for measuring the optimal size and location of dgs in a low voltage microgrid using the autoadd algorithm. the algorithm is validated by testing it on the ieee 33-bus standard system and compared with previous studies, the algorithm proved its efficiency and superiority on the other techniques. a significant improvement in voltage and reduction in losses were observed when the dgs are placed at the sites decided by the algorithm. therefore, autoadd can be used in finding the optimal sizes and locations of dgs in the distribution system, the possibility of isolating the low voltage microgrid is discussed by integrating distributed generation units and the results showed the possibility of this scenario during faults time and intermittency of energy time. mailto:ahmedjasem858@gmail.com acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 2 dgs in distribution networks brought in some serious challenges and problems. like the bidirectional flow of power in the distribution networks and the problems of power losses and voltage drop [7], [8]. researchers from the entire world are now focusing on these problems, and they have proposed many methods for selecting the optimal location and size of dgs with the aim to minimize or even eliminate the losses and improve the voltage of distribution networks with dg. in [7], the author proposed a new particle swarm optimization (pso) method to improve the power quality of the network by finding the number of dgs that are going to be connected and the optimal location of these dgs in the system. this method was validated by testing it on the standard 30-bus ieee system. the results showed a remarkable improvement in the buses' voltage profiles and a reduction in power losses of the system. in [9], integrating the ress was studied as it gives a significant comfort to smart grid technology in terms of cost. in [10], three types of pso algorithms were used to control the output of dg to find its optimum size. to overcome the issues of variation in ress, a model of energy optimization was proposed in [11]; it consists of a mathematical tool probability density function to model wind sources and solar. the author in[12] proposed a methodology for finding the optimal size and placement of many dgs. however, to determine the optimal location, the loss factor is used, and to find the optimal size, they used the algorithm of bacterial foraging. the objectives were to reduce operational costs and losses and to improve the voltage. their work was validated by testing it on ieee 119-bus and 33bus distribution systems. one of the major issues is intermittency in ress, ress integration is studied in [13] by conducting a survey on models from all over the world. the author found that communication systems, especially two-way communications have an important role in the sg's energy optimization. in [14], combined algorithms inspired by nature were used to optimally find the best place and size of dgs. for dg integration, an optimization technique with two steps got presented. during the first one, particle swarm optimization is used to find the best size of dg, and the results obtained are checked using the approach of negative load for reverse power flow. after that, the optimum location is found by the methods of weak bus and the loss sensitivity factor. during the second one, the optimal size of dgs is found using three algorithms based on nature, i.e., gravitational search algorithm, pso algorithm, and a combination of those two. by testing them on the 30-bus ieee system, the effectiveness of the technique has been proved. in this paper, the autoadd algorithm is used to find the optimal place and size of a dg in distribution networks. the proposed algorithm is simple, very flexible, easy to use, and supports all types of dg. it differs from the other algorithms by the time that it takes in processing, whereas the other algorithms take a lot of time that could be in some cases hours. this algorithm performs instantly and gives the best place for dgs to achieve the best performance. it is manipulated through the opendss program. the power flow analysis is executed by opendss through the matlab com interface. the algorithm and the opendss are validated by testing on the standard ieee 33-bus system. the results are compared with previous works, which proved that the opendss is reliable, and the autoadd algorithm gives better results in terms of losses and voltages compared to the earlier studies. after validating the tools, the low voltage microgrid of baghdad/al-ghazaliya-655 is analyzed, and the dgs are placed optimally to enhance its performance and to assess the capability of the microgrid to perform in the isolated mode with the objectives of reducing the cost, losses, and minimizing the impact on climate which contribute with the sustainable development goals (sdgs) 7 and 13. 2. impact of integrating distributed generation on losses and voltage 2.1. impact on losses integrating dgs proved that they could minimize the losses (real and reactive) due to being placed near the load. many early studies showed that the size and location of a dg play a significant role in eliminating power losses [15], the location and size of a dg in a distributed network that gives the minimum losses are identified in general as the optimal location and optimal size. the placement procedure of dgs is similar to the placement procedure of capacitors that aims to reduce losses. they differ in that the dg units affect real and reactive powers, whereas capacitors affect just reactive powers. installing a small dg unit has proven that it may reduce losses for the case of a network with an increment in losses [16]. 2.2. impact on voltage as known, dg supports and improves the system’s voltage [17], but that is not always accurate, as it has been shown that integrating dgs could cause undervoltage or overvoltage. additionally, some dgs change their produced power all the time, like wind generators and photovoltaics. the result of this figure 1. energy production sources. figure 2. microgrid architecture. acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 3 affects the quality of power badly because of the fluctuation of voltage [8], [18]. in addition to that, undervoltage and overvoltage are reported in distribution networks integrated with dgs because of the unsuitability of integrated dgs with current methods of regulation. generally, for regulation, the distribution systems use tap-changing transformers, capacitors, and regulators. those methods were proved as reliable methods in the past for the unidirectional flow of power. nevertheless, today, integrating dgs with distribution networks has a significant impact on the performance of methods of voltage regulation because of the power flows in a bidirectional way caused by new dgs on distribution systems. meanwhile, dgs influence positively on distribution networks due to their contribution to frequency regulation and compensation of reactive power for voltage control. moreover, in case of faults in the main network, they can work as a spinning reserve [19]. 3. autoadd algorithm in this paper, the autoadd algorithm is used; it is an internal feature of opendss that works automatically to find the optimal location of capacitors and generators. the optimization problem of the analysis of the distribution system in the equation form as in equation (1) where g(𝑥, 𝑢) = 0 represents the equation of distribution power flow, pl represents power losses, whereas vi is the voltage at bus number ith [20]. the equation min f(𝑥, 𝑦) = 𝑃𝑙 calculates the amount of the active and reactive power for every node in order to reduce the losses of the system while keeping voltages within certain limits. in addition to that, opendss uses an iterative algorithm that calculates the unknown voltages and currents. then, autoadd accesses the array of injection currents in the solution directly and takes advantage of it [20]. to move the generators on all the buses, the opendss searches for the available bus that result in the best improvement for capacity and losses on equation (2) below [21]: minimize (𝑙𝑜𝑠𝑠 𝑤𝑒𝑖𝑔ℎ𝑡 ∙ 𝑙𝑜𝑠𝑠𝑒𝑠 + 𝑈𝐸 𝑤𝑒𝑖𝑔ℎ𝑡 ∙ 𝑈𝐸) (2) loss of weight is losses' weighting factor in the functions of autoadd. ue weight is unserved energy’s(ue) weighting factor.ue represents load energy that is considered unserved because the power exceeds maximum values the velocity of convergence of solutions in the autoadd algorithm increases for the reason that the system's admittance matrix is fixed and is not changed. generally, finding the location of any generator takes about 2-4 iterations for every solution. the improvement factor refers to the next location that is the best to supply power [22]. figure 3 shows the autoadd algorithm. 4. standard ieee 33-bus system figure 4 depicts the standard system of ieee 33-bus. it has thirty-two branches and thirty-three buses. the voltage level for all the buses is 12.66 kv. the voltage limits for all buses are considered at ± 5 % for maximum and minimum. a synchronous generator feeds the network, the load is 3.715 mw, and 2.3 mvar is distributed on thirty-two buses with different power factors. the line data and load data of the system are in table 1 [23]. 5. proposed microgrid of baghdad/al-ghazaliya655 the proposed microgrid model in figure 5 represents a distribution system in iraq baghdad/al-ghazaliya-655. it has fifty-eight buses and fifty-seven branches. the voltages level is 0.4 kv for all the buses. min f(𝑥, 𝑦) = 𝑃𝑙 subject − to g(𝑥, 𝑢) = 0 0.95 ≤ 𝑉𝑖 ≤ 1.05 , (1) figure 3. flow chart of autoadd algorithm. figure 4. single line diagram of 33-bus ieee system. acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 4 table 1. electrical parameters of 33-bus ieee system (rij resistance, xij reactance, pj real power, qj reactive power). bus 1 bus 2 rij in ω/km xij in ω/km pj in kw qj in kvar length in km 1 2 0.0922 0.0477 100 60 1 2 3 0.4930 0.2511 90 40 1 3 4 0.3660 0.1864 120 80 1 4 5 0.3811 0.1941 60 30 1 5 6 0.8190 0.7070 60 20 1 6 7 0.1872 0.6188 200 100 1 7 8 1.7114 1.2351 200 100 1 8 9 1.0300 0.7400 60 20 1 9 10 1.0400 0.7400 60 20 1 10 11 0.1966 0.0650 45 30 1 11 12 0.3744 0.1238 60 35 1 12 13 1.4680 1.1550 60 35 1 13 14 0.5416 0.7129 120 80 1 14 15 0.5910 0.5260 60 10 1 15 16 0.7463 0.5450 60 20 1 16 17 1.2890 1.7210 60 20 1 17 18 0.7320 0.5740 90 40 1 2 19 0.1640 0.1565 90 40 1 19 20 1.5042 1.3554 90 40 1 20 21 0.4095 0.4784 90 40 1 21 22 0.7089 0.9373 90 40 1 3 23 0.4512 0.3083 90 50 1 23 24 0.8980 0.7091 420 200 1 24 25 0.8960 0.7011 420 200 1 6 26 0.2030 0.1034 60 25 1 26 27 0.2842 0.1447 60 25 1 27 28 1.0590 0.9337 60 20 1 28 29 0.8042 0.7006 120 70 1 29 30 0.5075 0.2585 200 600 1 30 31 0.9744 0.9630 150 70 1 31 32 0.3105 0.3619 210 100 1 32 33 0.3410 0.5302 60 40 1 figure 5. single line diagram of baghdad/al-ghazalya-655 microgrid. table 2. electrical parameters of baghdad/al-ghazalya 655 microgrid. bus 1 bus 2 resistance in ω/km reactance in ω/km length in km active power in kw reactive power in kvar 1 2 0.3416 0.3651 0.001 2 3 0.3416 0.3651 0.02 15 9.3 3 4 0.3416 0.3651 0.01 20 12.4 4 5 0.3416 0.3651 0.04 30 18.6 5 6 0.3416 0.3651 0.01 20 12.4 2 7 0.3416 0.3651 0.02 20 12.4 7 8 0.3416 0.3651 0.01 15 9.3 8 9 0.3416 0.3651 0.04 15 9.3 3 11 0.3416 0.3651 0.015 10 6.2 11 12 0.3416 0.3651 0.005 10 6.2 12 13 0.3416 0.3651 0.005 10 6.2 13 14 0.3416 0.3651 0.005 15 9.3 14 15 0.3416 0.3651 0.015 15 9.3 15 16 0.3416 0.3651 0.0075 15 9.3 16 17 0.3416 0.3651 0.0075 15 9.3 17 18 0.3416 0.3651 0.015 25 15.5 4 19 0.3416 0.3651 0.015 15 9.3 19 20 0.3416 0.3651 0.005 15 9.3 20 21 0.3416 0.3651 0.005 15 9.3 21 22 0.3416 0.3651 0.005 10 6.2 22 23 0.3416 0.3651 0.005 10 6.2 23 24 0.3416 0.3651 0.005 10 6.2 24 25 0.3416 0.3651 0.005 15 9.3 25 26 0.3416 0.3651 0.015 20 12.4 26 27 0.3416 0.3651 0.015 20 12.4 5 28 0.3416 0.3651 0.015 30 18.6 28 29 0.3416 0.3651 0.015 15 9.3 29 30 0.3416 0.3651 0.015 15 9.3 30 31 0.3416 0.3651 0.015 15 9.3 31 32 0.3416 0.3651 0.015 25 15.5 6 33 0.3416 0.3651 0.015 15 9.3 33 34 0.3416 0.3651 0.015 15 9.3 34 35 0.3416 0.3651 0.005 15 9.3 35 36 0.3416 0.3651 0.005 15 9.3 36 37 0.3416 0.3651 0.005 15 9.3 37 38 0.3416 0.3651 0.015 15 9.3 38 39 0.3416 0.3651 0.015 25 15.5 7 40 0.3416 0.3651 0.015 15 9.3 40 41 0.3416 0.3651 0.015 15 9.3 41 42 0.3416 0.3651 0.015 20 12.4 42 43 0.3416 0.3651 0.015 15 9.3 43 44 0.3416 0.3651 0.015 20 12.4 8 45 0.3416 0.3651 0.015 15 9.3 45 46 0.3416 0.3651 0.015 15 9.3 46 47 0.3416 0.3651 0.015 15 9.3 47 48 0.3416 0.3651 0.015 10 6.2 48 49 0.3416 0.3651 0.005 10 6.2 49 50 0.3416 0.3651 0.005 10 6.2 50 51 0.3416 0.3651 0.005 15 9.3 9 10 0.3416 0.3651 0.03 100 62 9 52 0.3416 0.3651 0.005 10 6.2 52 53 0.3416 0.3651 0.005 10 6.2 53 54 0.3416 0.3651 0.005 15 9.3 54 55 0.3416 0.3651 0.015 15 9.3 55 56 0.3416 0.3651 0.015 20 12.4 56 57 0.3416 0.3651 0.015 20 12.4 57 58 0.3416 0.3651 0.015 40 24.8 acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 5 the limits of voltages are within ± 5 % for maximum and minimum. the load is 1 mw and 0.62 mvar distributed on fifty-five buses, as presented in table 2. 6.analysis and results 6.1. analysis of test system the 33-bus ieee system was modeled in the opendss program. voltages and losses were calculated using the newton method. a summary is given in figure 6 and table 3. the comparison shows that the results were the same as the results obtained by the methods in [24]-[26]. table 4 shows a list of solutions by researchers in [27]-[30] that find the optimal size and location of three distributed generators that improve losses on the 33-bus ieee system. in this table, when comparing the algorithm based on the minimum voltage, it can be seen that all the algorithms provide voltages within ± 5 %. for the losses, the proposed algorithm achieved the minimum losses compared to the others. in terms of construction and coding, all the other algorithms require previous knowledge in programming and forming codes to construct them, which is very complex, whereas the autoadd algorithm is built-in in the opendss program and does not need any coding in formatting it. as for the time of operation, the other algorithms need a high number of iterations to converge, especially the pso algorithm, and with that number, the computer used for this operation needs to be of high specification, whereas the autoadd could work with any computer and gives the results instantly. after adding 3 dgs with pf=0.85 the results of network analysis in figure 7 and table 5 show the improvement of voltages and reduction of losses 6.2. analysis of practical network 6.2.1. grid-connected mode the load flow is done on the presented network by the fixedpoint method for different loads 100 %, 90 %, 80 %, 70 %, 60 %, 50 %, and 40 %, at hours (12 pm, 7 pm, 8 pm, 10 am, 9 am, 12 am, 5 am) untill the voltage be within ± 5 %,then the size of generation will be selected based on that. the results are shown in table 6. after the analysis of the system, it was decided to add 3 dgs with a total size of 600 kw to minimize losses and improve voltages to be within ± 5 %. the optimal place and size of dgs were found by the autoadd algorithm as in table 7. after the addition of dgs, the voltage has been improved for all the buses of the proposed system for the four-level of loads, as presented in figure 8 to figure 14. moreover, the losses of the system have decreased significantly, as in figure 15, and that is because the distributed generators are located near the load. 6.2.2. isolated mode in this section, the possibility of isolating the microgrid during the fault time is going to be discussed. as the total load of the network equals 1 mw at peak time and 500 kw at low demand times, the total generation will be leveled up to 1.2 mw by adding two standby units for this scenario to cover the load as in table 8. a time-series load flow is applied for 24 hours and the cases of (100%,75%,50%) at hours(12 am, 10 am, 12 pm) were taken table 3. comparison of results of autoadd with other algorithms. algorithm losses in kw minimum voltage dg location size of dg in mw proposed method 71.4 kw 0.96839 (14, 24, 30) (0.76, 1.07, 1.02) acsa [27] fwa [28] aco–abc [29] pso [30] 74.26 kw 88.68 kw 71.4 kw 72.8 kw 0.9778 0.9680 0.9685 0.96868 (14, 24, 30) (14, 18, 32) (14, 24, 30) (13, 24, 30) (0.7798, 1.125, 1.349) (0.5897, 0.1895, 1.0146) (0.7547, 1.0999, 1.0714) (0.8, 1.09, 1.053) figure 6. pu bus voltages of 33-bus ieee system. figure 7. pu voltages before and after adding dgs with autoadd. table 4. power flow analysis on 33-bus ieee compared with other research. algorithm losses minimum voltage pu location of bus proposed method 202.6 kw 0.913 18 [24] 202.7 kw 0.9131 18 [25] 202.6 kw 0.913 18 [26] 202.6 kw 0.913 18 table 5. losses and minimum voltages before and after adding 3dgs. without dg with three dgs losses in kw minimum voltage & bus losses in kw minimum voltage & bus 202.6 kw 0.913 (18) 12.29 0.99178 (18) acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 6 to evaluate for different load and irradiance cases, the results are shown in figure 16 and figure 17. the minimum voltage for all the cases is within ± 5, and the system can operate successfully in the isolated mode. the losses are less in case of 100 % than 50 % even though the load is higher due to the pv isn't working at night. 7. discussion for increasing the performance of microgrids by improving the voltage and reducing the losses and at the same time reducing the effect of ghg by integrating res this paper is conducted. the results showed that dgs have a major impact on the voltage profiles. the dgs increased the level of voltage for all the studied cases, and it is proportional to the capacity of the dg. it can be noticed from the results of simulations that the location of the table 6. power flow results of the proposed system. percentage of load total load in kw losses in kw min voltage & bus 100 % 1000 74.4 0.873(39) 90 % 900 58.95 0.88796(39) 80 % 800 45.5 0.9017(39) 70 % 700 34.17 0.915(39) 60 % 600 24.59 0.92807(39) 50 % 500 16.7 0.94075(39) 40 % 400 10.52 0.95312(39) table 7. results of optimum size and location selection for connected mode. dg type size in mw location pf diesel engine 0.3 mw 5 0.85 pv 0.2 mw 10 1 fuelcell 0.1 mw 57 0.9 total 0.6 mw figure 8. voltages of all buses at 100% load before and after the addition. figure 9. voltages of all buses at 90% load before and after the addition. table 8. results of optimum size and location selection for isolated model dg type size in mw location pf diesel engine a 0.3 mw 5 0.85 diesel engine b 0.5 mw 7 0.85 pv 0.2 mw 10 1 fuel cell 0.1 mw 57 0.9 micro turbine 0.1 mw 25 0.9 total 1.2 mw figure 10. voltages of all buses at 80% load before and after the addition. figure 11. voltages of all buses at 70% load before and after the addition. figure 12. voltages of all buses at 70% load before and after the addition. acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 7 dg is important for the whole network, and that is shown in the results. as for the losses, it can be seen from the results that the size of dg is important, and that has been noticed that where the bigger the size gets, the more reduction in losses. the autoadd algorithm is used to find the optimal size of dgs; this algorithm is fast, accurate, and very easy to use, so it does not need any prior experience or learning to use it. the proposed algorithm was applied to the ieee 33-bus, and the results were compared against the results of other research in table 4. the results showed that the suggested algorithm is effective in finding the optimal size and location of dgs and helps in achieving better results in terms of losses reduction and voltage improvement, so it was used for the real system to install many types of dgs that improved the voltage and losses as in the results. the scenario of isolating the low voltage microgrid is applied for the first time in an iraqi case to solve the problem of intermittency of power, and the result showed it was successful. some parameters have not been taken in this work that should be mentioned which are the annual variation of load and the economic issues related to the installation of the dg unit. the loads in the distribution network vary continually, and that results in the variation of the network’s losses and voltages. but on the other hand, a large pv system of 200 kw and a fuel-cell of 100 kw have been installed, which are pollution-free, so they don’t affect the nature and run on low cost. 8. conclusion in this paper, the reduction of losses and improvement of voltage has been discussed, and the autoadd algorithm has been introduced and used for finding the optimal size and location of dg. the ieee 33-bus has been used to test the proposed algorithm, and the results of the work were compared against results from other studies, the comparison proved that the proposed algorithm is acceptable and helps in achieving more efficient dg integration. the problem of intermittency of electrical power is solved for the iraqi case considering the maximum load condition. the future work will be using the practical microgrid in this work after the addition of the dgs and the improvement of voltage and losses to integrate the electric vehicles to prepare a suitable electrical environment for them to achieve zero pollution in the transportation sector and take into consideration the installation cost of dgs, charging stations and the variation of the loads. conflicts of interest the authors declare that there are no conflicts of interest. figure 13. voltages of all buses at 50% load before and after the addition. figure 14. voltages of all buses at 40% load before and after the addition figure 15. losses of all buses before and after the addition for 100% case figure 16. voltages 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https://doi.org/10.1016/j.jestch.2020.08.002 https://doi.org/10.1080/15325008.2012.755232 https://doi.org/10.1016/j.ijepes.2015.12.030 https://doi.org/10.1016/j.ijepes.2014.06.011 https://doi.org/10.1016/j.enconman.2011.06.001 https://doi.org/10.1016/j.ref.2017.05.003 microsoft word article 3 editorial tc4.docx acta imeko  august 2013, volume 2, number 1, 3 – 4  www.imeko.org    acta imeko | www.imeko.org  august 2013 | volume 2 | number 1 | 3  introduction to the acta imeko issue dedicated to selected  papers presented in tc4 at the xx th  imeko world congress  ján šaliga 1 , dušan agrež 2   1  technical university of košice, faculty of electrical engineering and informatics, letná 9, 042 00 košice, slovakia   2  university of ljubljana, faculty of electrical engineering, slovenia tržaška 25, 1001 ljubljana, slovenia      keywords: imeko world congress; tc4; measurement of electrical quantities, busan, republic of korea  citation: ján šaliga, dušan agrež, “introduction to the acta imeko issue dedicated to selected papers presented in tc4 at the xx th  imeko world congress”,  acta imeko, vol. 2, no. 1, article 3, august 2013, identifier: imeko‐acta‐02(2013)‐01‐03  editors: paolo carbone, university of perugia, italy; ján šaliga, technical university of košice, slovakia; dušan agrež, university of ljubljana, slovenia  copyright: © 2013 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: this work was supported by the science grant agency of the slovak republic (project 1/0555/11)  corresponding author: ján šaliga, e‐mail: jan.saliga@tuke.sk    1. introduction  the xxth imeko world congress (september 9 – 14, 2012) took place in busan – the second largest metropolis of republic of korea, which was the third host city of the world congress in the asian region. imeko world congresses are important scientific events organised in an every 3-year period where experts in measurement science and technology have a unique possibility to present their latest projects and the results of their research, exchange new ideas and discuss their research intentions. the world congresses join experts from different fields of measurement science covered by imeko technical committees (www.imeko.org). imeko congresses are also host to social events where new international friendships have been created that may spread inter-institutional cooperation among recognized experts in the field of measurement across the entire world. 2. imeko tc4  imeko tc4 is one of the imeko technical commissions whose task and goal is to create an international platform for experts in the field of measurement of electrical quantities, emphasizing both theoretical and practical aspects of research in the field. annual symposia and congresses are organised in member countries by local imeko organisations. the country members of imeko which have current representatives in the tc4 board are: belgium, brazil, bulgaria, china, croatia, czech republic, egypt, estonia, finland, france, germany, greece, hungary, italy, korea, poland, portugal, romania, russia, serbia, slovakia, slovenia, spain, sweden, turkey, u.k., and ukraine. any new country is cordially welcomed to join imeko tc4 activities. 3. the papers  a special focus at the xxth imeko world congress was placed on the measurement of electrical quantities, which is under the scope of imeko technical committee 4 (tc4). this editorial is a brief introduction to the acta imeko issue dedicated to selected papers presented at the world congress that brought the latest advances on measurement of electrical quantities (tc4). the papers for the issue were selected by voting of the working group established from tc4 board members who took part at the congress. according to the voting 18 papers were selected and the authors of the papers were invited to prepare extended and improved versions of their papers presented in busan. after a standard reviewing process eight papers were accepted for publishing in this special issue of acta imeko. modern instrumentation very often requires highly precise and accurate timing signals. a gps receiver is a typical modern solution of this task. ogrizović in [1] performed tests of a few receivers in comparison with a rubidium standard. the achieved results proved that a quality gps receiver is convenient for abstract  this  editorial  is  a  brief  introduction  to  the  acta  imeko  issue  dedicated  to  selected  papers  presented  at  the  xx th   imeko  world  congress  and  focused  on  measurement  of  electrical  quantities  (tc4).  the  congress  took  place  in  busan,  republic  of  korea  in  september 2012.  acta imeko | www.imeko.org  august 2013 | volume 2 | number 1 | 4  common industrial timing. the fluctuations are caused mainly by changes in the quality of the received signals. the second paper [2] by gao et al. presents the development, construction and first result achieved from a metrological atomic force microscope, which is intended for precise measurement with uncertainty of 4 nm. the microscope is intended for applications in nanoscience research and industry with the ability of surface topography measurement. the next paper by lee et al. [3] introduces a technique for magnetic field measurement based on monitoring the spin precession due to the external field as observing the optical rotation signal. the developed magnetometer achieved a sensitivity of about 120 ft/hz1/2 at 10 hz. the paper by costa da silva et al. [4] describes an automatic characterization system designed for the measurement of the electric impedance of giant magneto-impedance (gmi) samples. the high speed of measurement attained with the use of this system, designed in the labview development environment, allows for the rapid determination of the optimal operational point of an eventual gmi magnetometer. electric vehicle development is a very topical task and dream for many leading car manufacturers. the paper by vrazic et al. [5] describes a concept of a mobile lab for electric vehicle systems research and testing based on plcs and in parallel on a common pc. the lab can be effectively used in the education process at the university of zagreb. the next paper in this special issue [6] by mariscotti is a contribution to railway safety. when a new locomotive is introduced on existing lines it is essential to ensure that the locomotive and the signalling systems are compatible under normal operation and exceptional conditions. the focus of this paper is on the processing circuits and algorithms indicated by the european standards to model the susceptibility of the victim circuits and applied to the recorded pantograph current and its spectrum. real-time smart meters network for energy management is the topic of paper [7] by del prete et al. the network is composed by several slave smart meters that continuously monitor loads, and an energy generator to make available realtime information, such as power and energy consumption/generation and several power quality parameters, to a specific master device called data aggregator via a can bus. power quality is one of the highly needed measurements in power electric systems. masnicky et al. in [8] developed an interface between adc and dsp based on fpga for such application. the fpga collects data from the adc across a spi, processes them, and sends to the dsp via a custom protocol link port. the fpga interface was successfully tested in a complete system for different clock rates. 4. conclusions  the special issue offers the choice of the best papers presented at the xxth imeko world congress in busan in 2012 in the field of measurement of electrical quantities. the section editors of the special issue of acta imeko would like to thank all authors for their cooperation and improvements of the papers, the reviewers for highly professional reviews, notes and recommendations to the authors, and also to the layout editors of acta imeko, especially to the father of the journal paul regtien for his help and valuable advice. references  [1] vukan r ogrizović, “testing gps generated 1pps against a rubidium standard”, acta imeko, vol. 2 (2013), no. 1, pp. 7-11. [2] sitian gao, mingzhen lu, wei li, yushu shi, qi li, “metrological atomic force microscope and traceable measurement of nano-dimension structures”, acta imeko, vol. 2 (2013), no. 1, pp. 12-15. [3] hyunjoon lee, kiwoong kim, seong-joo lee, chan-seok kang, kwon kyu yu, yong-ho lee, han seb moon, “development of spin-exchange relaxation free magnetometer with a compact heating system”, acta imeko, vol. 2, (2013), no. 1, pp. 16-20. [4] eduardo costa da silva, joão henrique costa carvalho carneiro, luiz antônio pereira de gusmão, carlos roberto hall barbosa, elisabeth costa monteiro, “development of a fast and reliable system for the automatic characterization of giant magnetoimpedance samples”, acta imeko, vol. 2 (2013), no. 1, pp. 21-26. [5] mario vrazic, marinko kovačić, hrvoje dzapo, damir ilic, “concept of mobile lab for electric vehicle systems research and testing”, acta imeko, vol. 2 (2013), no. 1, pp. 27-31. [6] andrea mariscotti, “variability and uncertainty of track circuit band-pass modeling for interference evaluation”, acta imeko, vol. 2 (2013), no. 1, pp. 32-39. [7] giuseppe del prete, daniele gallo, carmine landi, mario luiso, “real-time smart meters network for energy management”, acta imeko, vol. 2 (2013), no. 1, pp. 40-48. [8] romuald masnicki, janusz mindykowski, damian hallmann, “the implementation of fpga for data transmission between adc and dsp peripherals in the measurement channels for power quality assessment”, acta imeko, vol. 2 (2013), no. 1, pp. 49-55. decay of a roman age pine wood studied by micro magnetic resonance imaging, diffusion nuclear magnetic resonance and portable nuclear magnetic resonance acta imeko issn: 2221-870x march 2022, volume 11, number 1, 1 10 acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 1 decay of a roman age pine wood studied by micro magnetic resonance imaging, diffusion nuclear magnetic resonance and portable nuclear magnetic resonance valeria stagno1,2, silvia capuani2,3 1 earth sciences department, sapienza university of rome, piazzale aldo moro 5, 00185 rome, italy 2 national research council institute for complex systems (cnr-isc) c/o physics department sapienza university of rome, piazzale aldo moro 5, 00185 rome, italy 3 centro fermi museo storico della fisica e centro studi e ricerche enrico fermi, piazza del viminale 1, rome 00184, italy section: research paper keywords: archaeological waterlogged wood; micro-mri; diffusion-nmr; portable nmr citation: valeria stagno, silvia capuani, decay of a roman age pine wood studied by micro magnetic resonance imaging, diffusion nuclear magnetic resonance and portable nuclear magnetic resonance, acta imeko, vol. 11, no. 1, article 12, march 2022, identifier: imeko-acta-11 (2022)-01-12 section editor: fabio santaniello, university of trento, italy received march 3, 2021; in final form march 15, 2022; published march 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: valeria stagno, e-mail: valeria.stagno@uniroma1.it 1. introduction wood is a porous material with complex morphology. in the past, it has been widely used by men to produce artworks. for this reason, wood is widespread in the cultural heritage world and its microstructure has always been studied for the species identification, for dendrochronological analyses and for extracting important information about ancient human activities [1]. two principal types of wood can be recognized: softwood and hardwood. softwood has a very homogeneous structure mainly composed of tracheids and fibre-tracheids [2]. the resin canals are one of its main characteristics that allow it to be distinguished from the hardwood. in addition, its annual rings are usually well separated through the annual ring limit and they are made of an earlywood area, with pores characterized by larger lumens and thinner walls, and a latewood area with thicker walls and smaller lumens [2]. however, these two areas are not always well differentiated. some softwoods present an abrupt passage from earlywood to latewood, while some others a gradual one [2]. among the many anatomical elements described above, the growth ring is surely one of the most important because its characterization provides the age of the tree and the climatic conditions in which it has grown [3], [4], as well as being crucial for the species identification. because of its biodegradability [4] wood is hardly preserved with no microstructural alterations. waterlogged wood is usually well preserved for the microscopic observation of its annual rings, especially when the wooden object was buried under the sea sediments [6]. in fact, thanks to the anaerobic conditions fungal attacks are more or less excluded abstract wood is a hygroscopic biodegradable porous material widely used by men in the past to create artworks. its total preservation over time is quite rare and one of the best preservation modalities is waterlogging. observing the anatomy of waterlogged archaeological wood could also be complicated because of its bacterial degradation. however, the characterization of wood morphology and conservation state is a fundamental step before starting any restoration intervention as it allows to extract information about past climatic conditions and human activities. in this work, a micro-invasive approach based on the combined use of high-resolution magnetic resonance imaging (mri) and diffusion-nuclear magnetic resonance (nmr) was tested both on a modern and an ancient pine wood sample. furthermore, a completely non-invasive analysis was performed by using portable nmr. this multi-analytical nmr approach allowed to highlight the effect of decay on the wood microstructure, through alterations in the pores size, tortuosity, and images contrast of the ancient pine compared to the modern one. this work pointed out the different but complementary multi-parametric information that can be obtained by using nmr and tested the potential of high-field mri and low-field portable nmr in the detection of wood diagnostic features.. mailto:valeria.stagno@uniroma1.it acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 2 [6] and if the object was buried under the seabed also the marine borer activity is limited [7]. in this environment, the main degradation can be attributed to erosion bacteria [8]-[10]. conversely, at a macroscopic level the wood rings are not always visible with the naked eye because of changes in colour, consistency, and superficial morphology of the wood structure [11]. as a consequence, microscopic imaging techniques are always required. moreover, the evaluation of the effect of age is also very important, for example, to determine the conservation state of an artwork. knowing the state of wooden remains, such as its degree of decay and its pore size and morphology, is useful for planning the restoration and choosing the restoration materials [11], [12]. in the literature, several works [13]-[22] about the use of nuclear magnetic resonance (nmr) on wooden artworks have proven its utility in the microstructural characterization and the evaluation of their conservation state. the above cited works employed invasive or destructive nmr techniques, while some others [22]-[25] pointed out the potential of non-invasive portable nmr. among these, our previous work stagno et al. [23] showed how 1d and 2d low-field nmr experiments can be used as complementary techniques to study water compartmentalization in archaeological waterlogged wood with the support of optical microscopy and magnetic resonance images. furthermore, we suggested the ability of low-field nmr in detecting cell wall decay and paramagnetic impurities. in respect to our previous work [23], in this study we investigated a decayed pine wood of the roman age by using mr images with higher resolution and by characterizing the wood structure also with the pore size distribution extracted from the relaxation measurements. the results obtained from the archaeological sample were compared with the results obtained from its modern counterpart. specifically, the aim of this work was to test the potential of nmr multi-analytical approach to evaluate archaeological wood decay. we compared micro-invasive magnetic resonance imaging (mri) and diffusion-nmr [26]-[29] with non-invasive portable nmr [30], [31]. the potential of mr images as an alternative approach to the conventional techniques for the characterization of the annual rings, as well as of all the diagnostic features of waterlogged wood, was tested. moreover, high-field diffusion and low-field portable nmr were used to highlight the effect of decay on the wood microstructure, such as variations in the pore size of the ancient compared to the modern pine. 2. background theory 2.1. diffusion-nmr and pore size molecules constituting a fluid whose absolute temperature is greater than zero kelvin are in constant movement because of their kinetic energy [27]. this process is called self-diffusion. when a fluid totally fills porous medium, molecules moving with random motion are subject to continuities deviations of their trajectories due to collision with the pores walls. diffusion-nmr techniques investigate diffusion dynamics by following in time the fluid molecules. the root mean square distance travelled, 𝓁d (or diffusion length), increases with time as long as no boundaries are encountered, according to the einstein relation: 𝓁d =√2 𝑛 𝐷 𝑡 where 𝑛 = 1, 2, 3 is the space dimension and 𝐷 is the bulk diffusion coefficient that can be measured by a pulsed field gradient sequence [26]. at a fixed diffusion time 𝑡 = ∆ (where ∆ is the delay time between the two gradient pulses) and fixed pulse magnetic-field gradient duration 𝛿 such that 𝛿 ≪ ∆, it is possible to vary the magnetic-field gradient strength 𝑔, so that the nmr-signal amplitude 𝑆(𝑔) is given by [28], [32]: 𝑆(𝑔) = 𝑆(0) e −𝛾2𝑔2𝛿2𝐷(∆− 𝛿 3 ) , (1) where 𝛾 is the gyromagnetic ratio of protons, 𝑏 = is the so called 𝑏-value, 𝐷 is the diffusion coefficient obtained at a specific diffusion time ∆ and 𝑆(0) is the signal at 𝑔 =0. since during ∆ in a pulsed gradient stimulated echo (pgste) experiment longitudinal relaxation decay 𝑇1 would occur, the maximum δ accessible is limited by the relation ∆ < 𝑇1 [33]. geometrical restrictions of the medium, such as the pores walls, lead to a 𝐷(∆) decreasing with time. in a heterogeneous porous system as wood, it is possible to derive useful information about pores size, pores interconnection and membrane permeability [29] by studying the 𝐷(∆) behaviour. in the case of long ∆ limit (i.e., ∆ ≫ 𝐿2 𝐷0⁄ , where 𝐿 is the mean pore diameter) and impermeable wall, the water diffusion coefficient varies according to: 𝐷(∆) = 𝐿2 2 ∆ . (2) equation (2) indicates that 𝐿 can be obtained from the slope of 𝐷 vs ∆−1. however, deviation from equation (2) can occur for semi-permeable walls [34]-[35]. for materials with interconnected pores, at very long times 𝐷(∆) approaches an asymptotic value 𝐷∞ that is independent of the diffusion time ∆ and directly related to the tortuosity 𝜏 of the porous material: 𝜏 = 𝐷0 𝐷∞ . (3) tortuosity is an intrinsic property of a porous medium usually defined as the ratio of actual flow path length to the straight distance between the ends of the flow path. so, 𝜏 of the porous system reflects the connectivity degree of the porous network [36]-[38]. 2.2. transversal relaxation and pore size the transversal or spin-spin relaxation time 𝑇2 is due to the loss of coherence of the spins among themselves. when an ensemble of spins is considered, their magnetic fields interact (spin-spin interaction), slightly modifying their precession rate. these interactions are temporary and random. thus, spin-spin relaxation causes a cumulative loss in phase resulting in transverse magnetization decay [26]. in a porous medium, the 𝑇2 relaxation of the fluid (e.g., water) is given by the sum of different contributions [39]-[43]: 1 𝑇2 = 1 𝑇2b + 1 𝑇2s + 𝐷(𝛾 𝐺 𝑇e) 2 12 , (4) where 1 𝑇2⁄ is the total spin-spin relaxation rate, 1 𝑇2b⁄ is the relaxation rate of bulk water and 1 𝑇2s⁄ is the relaxation rate of water on the pore surface. the term 𝐷(𝛾 𝐺 𝑇e) 2 12⁄ is related to the dephasing caused by the presence of a magnetic field gradient 𝐺, named “diffusion relaxation” term. 𝐷 is the diffusion coefficient of water in the porous system, 𝛾 is the hydrogen gyromagnetic ratio and 𝑇e is the echo time. when the 𝑇2 is measured by a carr-purcell meiboom-gill (cpmg) sequence, the contribution of diffusion relaxation is averaged out by means of an echo train if a very short 𝑇e is selected [44], [45]. in the presence of a porous system, the acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 3 dominant relaxation rate is 1 𝑇2s⁄ while the term 1 𝑇2b⁄ can be neglected because of the low efficiency of bulk water relaxation [46]. equation (4) becomes: 1 𝑇2 ≈ 1 𝑇2𝑠 = 𝜌 𝑆 𝑉 , (5) where 𝜌 is the surface relaxivity [46], 𝑆 𝑉⁄ is the surface-tovolume ratio of the pore. therefore, by measuring the spin-spin relaxation in a porous medium the pore diameter 𝑑 can be calculated by [46]-[48]: 1 𝑇2 = 𝜌 2 𝑛 𝑑 , (6) where 𝑛 is a shape factor (for our samples a spherical shape was considered, 𝑛 = 3). 3. materials and methods 3.1. samples two cylinders-like wood samples of a water-soaked modern wood and an archaeological waterlogged wood were studied. their size was less than 15 mm in length and 8 mm in diameter, suitable for a 10 mm nmr tube. the archaeological sample detached from an ancient pole of the roman harbour of naples, dated to the v century ad [49], [50]. it was well preserved in waterlogged conditions and for this reason was always kept in water during the analysis. the modern wood, instead, was previously maintained at the environmental conditions of 20 °c and 50 % of relative humidity. in order to perform nmr acquisitions, the modern sample was imbibed with distilled water until the saturation was reached. both the species of modern and archaeological wood was stone pine (pinus pinea l.) [51], [52]. 3.2. high-field nmr acquisitions all the high-field nmr analyses were performed by using a 400 mhz bruker-avance spectrometer with a 9.4 t static magnetic field and a micro-imaging unit equipped with highperformance and high-strength magnetic field gradients for mri and diffusion measurements. the gradients maximum strength was 1240 mt/m and the rise time 100 µs. to measure the diffusion coefficient 𝐷 and the longitudinal relaxation time 𝑇1, useful to choose the longest observation time ∆ accessible during the diffusion experiments according to the ∆ < 𝑇1 relation, the soaked samples were inserted without additional water in the nmr tube, which was sealed on the top with parafilm in order to prevent the sample dehydration. the longitudinal relaxation time was measured with a saturationrecovery 𝑆r sequence with 128 points from 10 µs to 10 s, repetition time 𝑇r of 10 s, number of scans 𝑁s equal to 4. the acquisition time was 1 hour and 40 minutes for each sample. for the measurement of the water diffusion coefficient, a pgste sequence [28], [29] was used. the diffusion was evaluated along the x axis (i.e., perpendicular to the main direction of the wood grain) corresponding to the radial direction. the pgste signal was obtained using 𝑇r = 5 s, echo time 𝑇e = 1.9 ms, pulse gradient duration 𝛿 = 3 ms, 32 steps of the gradient strength 𝑔, from 26 to 1210 mt/m, for each diffusion time ∆ and 𝑁s = 16. the ∆ values used were 0.04 0.08 0.12 0.16 0.2 0.3 0.4 s. the 𝑏-value spanned from a minimum of 1.6 × 107 s/m2 to a maximum of 9.5 × 1011 s/m2. the acquisition time was about 6 hours for each sample. for the acquisition of mr images, the samples were inserted with distilled water in the nmr tube and sealed with parafilm in order to prevent water evaporation. in this way, 𝑇2 ∗-weighted images were performed with a gradient echo fast imaging (gefi) sequence [26] in the transversal, tangential and radial direction. the images were weighted on the 𝑇2 ∗ parameter, which depends on both 𝑇2 and magnetic field inhomogeneities. the optimized parameters used in the gefi sequence are reported in table 1, where stk is the slice thickness, fov the field of view, mtx the image matrix and r the in-plane resolution. 3.3. low-field portable nmr acquisitions for the low-field nmr acquisitions the samples were just placed on the surface of the sensitive area of the portable spectrometer. a bruker minispec mq-profiler with a single-sided magnet that generates a static magnetic field of 0.35 t with a 1h resonance frequency of 15 mhz and dead time of 2 µs was used. the single-sided nmr was equipped with a rf coil for performing experiments within 2 mm starting from the sample surface to the inside of the sample [53]. the portable spectrometer has a constant magnetic gradient field with strength of about 3 t/m. the transversal relaxation time 𝑇2 was measured with a cpmg sequence with 𝑇e = 42 µs, 6500 echo times, 𝑇r = 1 s, 𝑁s = 128 and acquisition time of about 5 minutes. 3.4. data processing the longitudinal relaxation time was obtained by plotting the signal intensities 𝑆 as a function of 𝑡 = 𝑆r delays for fitting the following equation: 𝑆(𝑡) = 𝑀 [1 − e − 𝑡 𝑇1 ] (7) to data, where 𝑇1 is the longitudinal relaxation time and 𝑀 is the associated equilibrium magnetization. the diffusion coefficient values were obtained by fitting the following equation: 𝑆(𝑏) = 𝑀1 e −𝐷1𝑏 + 𝑀2 e −𝐷2𝑏 (8) to data acquired at different 𝑏-values, where 𝑆(𝑏) is the nmr signal as function of the 𝑏-value, 𝐷1 and 𝐷2 are the two components of the diffusion coefficient associated with the magnetizations 𝑀1 and 𝑀2, respectively. the fit goodness was evaluated by the 𝑅2̅̅̅̅ (i.e., the 𝑅2 corrected for the number of the regressors). fits of equation (7) and equation (8) were performed by using the originpro 8.5 software. plots of the diffusion coefficient 𝐷 vs. the diffusion time ∆ were performed with matlabr2019b. from the 𝐷 vs. ∆ trend, the first and last points, corresponding to the free water diffusion and the diffusion through semi-permeable membranes (i.e. the wood cell walls), were removed and the pores radius 𝐿 was table 1. acquisition parameters of t2*-weighted images. parameters modern pine archaeological pine te/tr (ms) 3/1200 5/1500 number of slice 3 3 ns 128 128 stk (µm) 200 300 fov (cm2) 0.9 × 0.9 / 1.4 × 1.4 0.9 × 0.9 mtx (pixels) 512 × 512 512 × 512 r (µm2) 18 × 18 / 27 × 27 18 × 18 acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 4 estimated by the linear fit of equation (2) [54]. from the pores radius 𝐿, the pores diameter 𝑑 was then calculated. in addition, the value for ∆ = ∞ of the normalized diffusion coefficient 𝐷(∆) 𝐷0⁄ , where 𝐷0 is the free water diffusion coefficient equal to 2.3 × 10-9 m2/s, was calculated and used to evaluate tortuosity according to equation (3) [54]. finally, the inverse laplace transform [55] in matlab (matlab r2019b) was used to obtain the low-field 𝑇2 distribution and the pores diameter distribution according to equation (6). the surface relaxivity 𝜌 was previously estimated for the modern and ancient pine from the slope of the line given by 𝑇2 time vs. pore size 𝛼, where 𝛼 was calculated from diffusion measurements by the relation 𝛼 = 2 √𝐷∆ [46], [47]. 4. results 𝑇2 ∗-weighted images of the transversal section of modern and archaeological stone pine are displayed in figure 1a) and figure 1b), respectively. here, all the anatomical elements observable with conventional optical microscopy [51], [52] are detectable. first, the annual rings limit (white arrows) is well visible in both figure 1a) and figure 1b) with two separate areas with different contrast. in both modern and archaeological pine, the darker area corresponds to structures with low 𝑇2 ∗ values, while the brighter ones to structures with high 𝑇2 ∗ values [56]. these structures correspond to tracheids, which are considered as the predominant constituents of all softwoods [57]. the dark area is the latewood (light blue circles) while the bright area is the earlywood (green circles). resin canals (red circles), likely with the presence of resin, can also be observed. moreover, rays (pink arrows) can be seen in both the samples but in figure 1b) the archaeological pine shows black spots and artefacts (yellow circles) located along the rays and on the edge of the sample. these artefacts are typical of mr images of waterlogged woods [13] because of the deposition of impurities, i.e., bacterial erosion wood products and seabed sediments, in the wood microstructure during the burial period. these paramagnetic impurities provide a black contrast in 𝑇2 ∗images [58] revealing the distribution of the degradation zones since they are stored in the decayed structures of wood [13]. figure 2a) and figure 2b) display the radial section of modern and archaeological pine, respectively. in both the images, the anatomical element observable is the annual rings limit (white arrows) and in figure 2b) the above-mentioned distribution of paramagnetic inclusions (yellow circles). the tangential section of the modern and archaeological samples is shown in figure 3a) and figure 3b), respectively. the red circles highlight the tangential resin canals and the pink arrows the medullary rays. again, in figure 3b) the archaeological pine shows decayed zones (yellow circles) corresponding to black spots and artefacts produced by the presence of paramagnetic agents. in table 2 the relaxation time 𝑇1 measured at high magnetic field and obtained by equation (7) is displayed. both modern and archaeological pine show a 𝑇1 around 500 ms. this limited the observation time ∆ of the diffusion measurements, whose maximum value was set to 400 ms. in figure 4a) and figure 4b) the first (𝐷𝑥1) and second (𝐷𝑥2) diffusion component as function of the diffusion time ∆ obtained by equation (8) are displayed. in table 3 the pores diameter and the tortuosity calculated from the high-field measurements by equation (2) and equation (3), respectively, are reported. in figure 5a) the 𝑇2 time distribution obtained by portable nmr is showed both for the modern (dashed line) and the ancient (solid line) pine. the pores size distribution calculated by equation (6) from the above mentioned low-field 𝑇2-distribution is presented in figure 5b) for the modern pine (dashed line) and the archaeological pine (solid line). figure 1. t2*-weighted mr images of the transversal section of modern pine (a) and archaeological pine (b) obtained at high magnetic field (9.4 t). in both the samples there are resin canals (red circles), earlywood (green circles), latewood (blue circles), annual rings limit (white arrows) and rays (pink arrows). the ancient pine shows artefacts induced by paramagnetic ions in decayed areas (yellow circles). acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 5 5. discussions compared to the conventional methods used to observe wood anatomy, i.e., optical microscopy or scanning electron microscopy, our mr images allowed to recognize some important diagnostic characters of wood especially in the transversal section (figure 1). conversely, fewer characters were observed in the tangential (figure 3) and radial (figure 2) sections due to the current limitation on the resolution of mri, whose maximum value is around 10 µm. indeed, when the species of a softwood has to be recognized, the radial section is of fundamental importance because of its diagnostic features, such as pits of cross-fields and rays, which allow to discriminate among quite similar softwood structures (e.g., pine and spruce). however, the resolution of figure 2. t2*-weighted mr images of the radial section of modern pine (a) and archaeological pine (b) obtained at high magnetic field (9.4 t). in both the samples the annual rings limit (white arrows) is observable. the ancient pine shows artefacts induced by paramagnetic ions in decayed areas (yellow circles). figure 3. t2*-weighted mr images of the tangential section of modern pine (a) and archaeological pine (b) obtained at high magnetic field (9.4 t). in both tangential resin canals (red circles) and rays (pink arrows) are observable. the ancient pine shows artefacts induced by paramagnetic ions in decayed areas (yellow circles). acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 6 mri is not a physical limit but it depends on the characteristics of the instrumentation used. in this work, mr images showed in figure 1, figure 2 and figure 3 aim at investigating the decay effect and their specific contrast can be used to reconstruct the decay distribution and to detect the presence of paramagnetic impurities. all the sample volume can be imaged with mri, whereas optical and scanning electron microscopy only provide images of a small portion of the wood sample. moreover, the mechanical preparation of the sample in optical and scanning electron microscopy leads to its destruction if compared with the virtual sectioning operated by mri. the longitudinal relaxation time 𝑇1 could provide information about the water molecules surrounding environment, the sample structure and composition [26]. however, in a porous medium 𝑇1 can be influenced by paramagnetic ions [59], [60] that usually cause its reduction, as found in stagno et al. [23]. in our ancient wood many paramagnetic inclusions were detected as artifacts in the mr images [13] and 𝑇1 values, displayed in table 2, seem to be exchange averaged [59], therefore they do not include such detailed information about different structural compartments as 𝑇2 value. for the aforementioned reason, the longitudinal relaxation time of our modern and ancient pine cannot be used to describe structural changes among them. nevertheless, the measurement of 𝑇1 is useful for setting the diffusion observation time. from the comparison of the transversal, radial and tangential section of the modern (figure 1a, figure 2a and figure 3a) and the ancient (figure 1b, figure 2b and figure 3b) pine, it is possible to observe morphological differences. first of all, the black spots (see section 4), can be associated with a degradation process operated by microorganisms with the inclusion of paramagnetic impurities (i.e., bacterial erosion wood products and burying sediments) into the wood structure. these black zones are not present in the modern wood and they are mostly located along the rays and on the edge of the sample (figure 1b, figure 2b and figure 3b yellow circles). this indicates a strong degradation in these zones. the image contrast allows to observe that both the woods show well delimited growth rings. while the modern wood does not show particular changes in the annual ring thickness, in figure 1b) the ancient pine has some thinner annual rings with no latewood. we can hypothesize that this is due to a climatic change during the tree growth (before the v century ad). in fact, thinner rings are usually attributed to not rainy periods [4], [11]. however, the ring thickness can be also influenced by other factors, for example the tree age. therefore, more than one sample should be analysed to confirm our hypothesis. the plots of the diffusion coefficient vs. the diffusion time in figure 4a) and 4b) show that both wood samples have two main diffusion compartments but the diffusion in the modern pine is slower than in the ancient pine. the difference is around one order of magnitude for both the compartments 𝐷𝑥1 and 𝐷𝑥2. this situation can be explained as the consequence of the degradation process that occurred in the ancient pine. in fact, the decay of the cellular structure and wood polymers may have produced the cell walls thinning and the lumens enlargement in the archaeological pine. from the point of view of a porous system, ancient pine is characterized by larger pores compared to modern pine. the existence of two different diffusion compartments means that both the woods have at least two main pores sizes as shown in table 3. the two sizes 𝑑1 and 𝑑2 calculated from the high-field nmr diffusion measurements can be identified as the earlywood and latewood tracheids diameter [60], considering that tracheids are the main constituents of softwood. by comparing the calculated diameters of the modern and the ancient pine, despite in the earlywood the diameter seems to be similar between the two samples, for the latewood there is an increment of the tracheids size in the archaeological pine. a possible explanation is that the decay is higher in areas with high concentration of wood polymers, such as the latewood cell walls. these polymers are predominantly cellulose and hemicellulose that, as pointed out by high-resolution nmr spectra of both modern and ancient wood [19], are more degraded in respect to the usually well-preserved lignin. this result means that the greater the thickness of the cell wall, the stronger its deterioration. this is in agreement with the distribution of paramagnetic impurities revealed by the mr images, which follows the distribution of decay that is mainly located in rays table 2. longitudinal relaxation time t1. sample t1 ± se (ms) modern pine 541 ± 13 archaeological pine 511 ± 19 figure 4. plots (a) and (b) show the dx1 and dx2 decay as function of δ for modern pine (circle markers) and archaeological pine (triangle markers). dashed lines are for illustration purpose only. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 7 having thick cell walls. further information can be deduced from the tortuosity 𝜏 (table 3). the modern pine shows a higher tortuosity (7.0 ± 1.1) compared to the archaeological pine (3.5 ± 0.6). however, for the ancient pine the normalized d data did not reach a limit value (see figure 4). therefore, we likely underestimated the value of tortuosity. nevertheless, the tortuosity value obtained for modern pine seems to be in agreement with the literature value around 10 calculated for thermally modified pinus sylvestris , also considering the effect of thermal modification and that we used a different species (pinus pinea) [61]. moreover, tortuosity is also in good agreement with the diffusion coefficient and diameters results. in fact, the higher the tortuosity, the more complicated the water routes. this means that the modern wood has a complex structure within which water cannot move easily. conversely, the ancient pine has lost this complexity because of the structure’s degradation, which has produced new voids and widened the existing pores lumens making the water motion easier. specifically, the two different tortuosity values corroborate the pore size results obtained by nmr diffusion. indeed, the low tortuosity measured for the ancient pine is compatible with its larger and more interconnected pores. differences in the pore size of the modern and the ancient pine were also found by using the low-field portable nmr, in good agreement with the high-field results. specifically, in figure 5a) the 𝑇2 time distribution obtained by using the portable nmr is displayed and indicates that the archaeological pine (solid line) has longer 𝑇2 than the modern pine (dashed line). the only exception is the shortest component around 1 ms that is longer for the modern wood than for the archaeological one. since in previous works [62], [63] a component around 1 ms was attributed to bound water in the cell walls, we can suggest that bound water of the archaeological pine is highly influenced by paramagnetic impurities, whose accumulation is greater in more degraded areas, i.e. in the cell walls. this is confirmed by other studies [23], [64], where the 𝑇2 component around 1 ms ascribable to bound water in the cell walls was not detected due to their strong degradation state. since the 𝑇2 is proportional to the degree of decay, we can suggest that the increment of 𝑇2 for the ancient pine is a consequence of decay. this also means that water in the archaeological pine is located in larger structures and the water content has raised as shown by the probability associated with 𝑇2 (figure 5a). the pores diameter calculated from high-field diffusion-nmr (table 3) can be compared with the pores diameter obtained by low-field 𝑇2 distribution (figure 5b). however, compared to high-field diffusion, the low-field relaxation measurements allowed to provide a global pore distribution of the sample while the intrinsic resolution of the diffusion-nmr is limited by the 𝑇1. in our case, the maximum distance (𝓁d) accessible is 43 µm [65] at ∆ = 400 ms and 𝐷 = 2.3 × 10-9 m2/s (the free water diffusion coefficient). nevertheless, we think that in case of the archaeological pine the 𝐷 vs ∆−1 behaviour is still affected by the presence of free-like water due to the existence of very large pores, not detectable with diffusion nmr techniques [54]. this explains why the archaeological pine shows pore distribution around 70 µm (figure 5b) that was not detected by nmr diffusion. the pore size distribution displayed in figure 5b) confirms the enlargement of pores in the archaeological pine as predicted by diffusion and 𝑇2 measurements. however, the peak around 70 µm is quite broad indicating a continuous distribution among peak with diameter from about 55 µm to 90 µm. the most intense peak for the ancient pine is around 17 µm. it should be noticed that both the diameters obtained from diffusion (table 3) and associated with earlywood and latewood are included in the peak around 17 µm. this indicates a continuous distribution among earlywood and latewood tracheids, likely with water exchange. for modern wood, the two predominant peaks as well as their probability in figure 5b) are in good agreement with the diameters calculated from diffusion analyses, with the mr images and with the literature values [61], [66] in which the mean lumen diameter was found around 15 µm – 20 µm. the existence of two separate peaks shows distinct spins populations for earlywood and latewood. table 3. pores diameter, associated magnetizations and tortuosity obtained by nmr diffusion. sample d1 (µm) d2 (µm) τ modern pine 16.0 ± 1.0 5.4 ± 0.6 7.0 ± 1.1 archaeological pine 18.4 ± 3.5 13.6 ± 1.2 3.5 ± 0.6 figure 5. t2 relaxation time distribution (a) and pore diameter distribution (b) for the modern pine (dashed line) and the ancient pine (solid line) obtained by using portable low-field nmr. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 8 smaller pore sizes can be associated to parenchyma cells and voids of the cell walls. also in this case, the values obtained by equation (6) and displayed in figure 5b) confirm that the ancient pine has larger pores (from 0.1 µm to 3 µm) than the modern pine (from 0.1 to 1 µm). particularly, the increment of size of the cell walls voids reveals the degradation of polymers constituting the cell wall itself. 6. conclusion this work suggests that by using high-field micro-mri and high-field diffusion-nmr it is possible to obtain information about the archaeological wood decay. information related to changes in the mean pore size, caused by wood decay, can be also obtained by low-field nmr relaxometry. the use of different nmr techniques and instrumentations provided the same result about the increment of the mean pore size for the archaeological pine wood. by comparing the modern pine sample and the ancient pine sample, the effect of the degradation process on the wood microstructure can be observed through the contrast of the mr images and quantified by the diffusion coefficient of water, the tortuosity and the pore size. the high-field nmr showed that the decay in pine wood mostly occurred in areas with high concentration of polymers, such as rays and latewood cell walls, with the enlargement of the pores lumen and the loss of wood structural complexity. mri can also reveal morphological aspects of wood that are not observable with the naked eye, such as the annual rings, which can inform about past climate changes. in the pore size determination obtained by using portable low-field nmr, the method based on the 𝑇2distribution seems to be superior to the high-field diffusion method, whose resolution is limited by 𝑇1. in conclusion, highfield techniques require a sampling; however, compared to other conventional analyses, they are non-destructive towards the sample that can be relocated in its original position in the artwork. the low-field portable nmr, instead, is mobile and suitable for in-situ analysis on samples of any size thus being non-invasive and non-destructive. compared to high-field nmr it is also low cost, with shorter acquisition and processing time. we suggest that single-sided portable nmr is a powerful technique for revealing porosity changes of the entire wood structure produced by decay. acknowledgement the authors would like to thank the “istituto centrale per il restauro” (icr) of rome (italy) for providing the archaeological wood sample. we acknowledge funding of regione lazio under the adamo project no. b86c18001220002 of the centre of excellence at the technological district for cultural heritage of lazio (dtc). references [1] english heritage, waterlogged wood guidelines on the recording, sampling, conservation and curation of waterlogged wood, english heritage publishing, 2010, product 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https://doi.org/10.1007/bf02784073 https://doi.org/10.1002/mrc.5034 https://doi.org/10.1021/acs.analchem.0c01523 https://doi.org/10.1515/hf-2012-0057 https://doi.org/10.1021/jp411199r https://doi.org/10.1515/hf-2019-0246 https://doi.org/10.1103/physreva.19.2446 https://doi.org/10.15376/biores.7.2.1771-1783 a lightweight magnetic gripper for a delivery aerial vehicle: design and applications acta imeko issn: 2221-870x september 2021, volume 10, number 3, 61 65 acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 61 a lightweight magnetic gripper for an aerial delivery vehicle: design and applications giuseppe sutera1, dario calogero guastella1, giovanni muscato1,2 1 dipartimento di ingegneria elettrica elettronica e informatica, university of catania, catania, italy 2 istituto di calcolo e reti ad alte prestazione, consiglio nazionale delle ricerche, icar-cnr, palermo, italy section: research paper keywords: low-power gripper; pick and place; rapid prototyping; permanent magnets; mobile robot application citation: giuseppe sutera, dario calogero guastella, giovanni muscato, a lightweight magnetic gripper for a delivery aerial vehicle: design and applications, acta imeko, vol. 10, no. 3, article 10, september 2021, identifier: imeko-acta-10 (2021)-03-10 section editor: bálint kiss, budapest university of technology and economics, hungary received january 15, 2021; in final form september 6, 2021; published september 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this research was partially funded by the ‘safe and smart farming with artificial intelligence and robotics programma ricerca di ateneo unict 2020 -‐ 22 linea 2’ project. corresponding author: giuseppe sutera, e-mail: giuseppe.sutera@unict.it 1. introduction unmanned aerial vehicles or drones represent the future of many evolving sectors. autonomous delivery is one of these, and this sector is developing rapidly thanks to new platforms that allow the transportation of weights heavier than that of the platforms themselves. these vehicles are often used to transport wares to areas that are difficult to reach quickly using standard means of transport. in order to make the entire delivery process autonomous, it is necessary to use an easily coupling system to attach the package to the drone. in the literature, different techniques have been described, which will be analysed in detail in this paper. the snap-fit method requires a high level of accuracy in positioning, as there are a number of fixing pins that must be 1 website: https://www.mbzirc.com/. inserted perfectly into the relevant holes. adhesion is an excellent solution for picking up and placing objects with metal parts. for this reason, electro-adhesion [1] and electromagnets [2]-[4] have been analysed, which represent a valid choice in terms of ease of coupling, but they require a high operating current to create a magnetic field. in terms of energy consumption, this can cause a reduction in flight time. in this approach [3], the use of an electro-permanent magnet reduces energy absorption since it requires a current only in the release phase. however, the typical shape and weight of these devices require the design of bulky housings that do not allow a plate to be used that is suitable for the intended purpose. for the present study, a plate was developed in accordance with the specifications of the ‘mohamed bin zayed international robotics challenge 2020’ (mbzirc1). one of the challenges in this competition was to create a drone capable of lifting different types of bricks off the abstract in recent years, drones have become widely used in many fields. their vertical flight capability makes these systems suitable for carrying out a variety of tasks. in this paper, the delivery service they provide is analysed. the delivery of goods quickly and to remote areas is a relevant application scenario; however, the systems proposed in the literature use electromagnets, which affect the duration of the flight. in addition, these devices are heavy and suffer from high energy consumption, which reduces the maximum transportable payload. this study proposes a new lightweight magnetic plate composed of permanent magnets, capable of collecting and positioning any object as long as it has a ferromagnetic surface on the top. this plate was developed for the mohamed bin zayed international robotics challenge 2020, an international robotics competition for multi-robot systems. challenge two of this competition required a drone capable of picking up different types of bricks and assembling them to build a wall according to an assigned pattern. the bricks were of different colours and sizes, with weights ranging from 1 to 2 kg. in light of this, it was concluded that weight was the most relevant specification to consider in drone design. mailto:giuseppe.sutera@unict.it acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 62 ground and arranging them to build a wall according to an assigned pattern. for this purpose, a magnetic plate was created using passive magnets inserted inside a 3d-printed structure. the release system was incorporated into the plate and consisted of several levers operated by a single servo control. this setup allows the detachment of ferromagnetic objects without high energy consumption and requires a power supply only in the release phase in order to activate the servomotor. furthermore, thanks to its design, the developed gripper allows the operators to optimise the weight and energy consumption while still guaranteeing a lifting capacity exceeding 2 kg. 2. mechanical design the prototype in this study had a flat profile and an attractive force comparable to a commercial device, despite its lightness. the supporting structure was built with rapid prototyping techniques using the zortrax m200 printer, which is equipped with an extruder with a 0.4 mm nozzle and a layer resolution of 90–390 microns. of the materials available, z-ultrat was selected for this project, a blend of zortrax filaments created to enhance the properties of acrylonitrile butadiene styrene (abs) in terms of durability. the entire printing process took about 20 hours, to which 30 minutes were added to integrate the servomotor and the other mechanical parts (such as pins, ball bearings and screws). the considerable difference between printing and assembly times prompted a search for a solution to speed up the printing process. it was therefore decided to divide the prototype into several smaller components. although this solution increased the overall printing time by about 25 % (due to the increased number of prints and, hence, of printer initialisations), each component part had a printing time ranging from 30 to 120 minutes. the resulting modular structure of the prototype made it quick to repair. as will be explained later, the bottom part of the plate was the area that was exposed the most to wear, as it was in contact with the ground and the ferrous coupling surfaces. the dimensions of the realised prototype fulfilled the specific requirements of the mbzirc 2020 competition, where the magnetic plate was tested. the setup used for the competition had a length of 15 cm, a width of 10 cm, a height of 5 cm and weighed 195 g. however, the modularity of the prototype made it possible to reduce the contact surface to 9 × 9 cm so that it was suitable for smaller drones while still maintaining the same supporting structure. the plate consisted of four pieces, two for each of the two layers that composed it. the first layer was 0.6 cm high and consisted of two smaller pieces that were connected with a dovetail joint (figure 1). during the design phase, a set of beams was designed with a two-fold purpose: 1) increasing the rigidity to compensate for the thinness and 2) creating the slots in which to insert the magnets and the supports for the release levers (figure 2). two commercial magnets were tested with the same width (10 mm) and length (20 mm) but different heights (2 and 5 mm) and, hence, different attraction forces (table 1). the design choice of creating a covering layer in the lower part made the external surface smooth and free of friction. the magnets were securely fixed in custom housings that prevented them from coming out once the object to be lifted had been hooked (figure 3). the release system was operated by a servomotor located in a central position in order to guarantee the centring of the weight. once activated by a digital pin from flight control unit (fcu) the rotation starts by sending a pwm signal, from rest position up to release position and back. an mg995 commercial servomotor was used, capable of developing a torque of 10 kg/cm (6 v) on the shaft required to operate the cascade of l-shaped levers, located along the length of the plate, to move and push the object away from the magnets. the increase in distance produced a decrease in the force of attraction, and the object is released by gravity. the release phase lasts 1 second. during this period the servomotor draws 600 ma, after which the motor returns to the rest position where the consumption is reduced to 10 ma. in the proposed solution, neodymium magnets are used for their capability to attract ferrous surfaces. the operating principle was based on the force of attraction, which follows the equation below: 𝐹 = 𝐵2𝐴 2 𝜇0 , (1) where f is the force in n, a is the surface area of the magnetic pole in m2, b is the magnetic flux density in tesla and 𝜇0 is the permeability of air. the second law of dynamics defines the force of gravity proportional to the mass m, therefore f = m · g. from this it follows that the lifting mass m is given by 𝑚 = 𝐵2 𝐴 2 𝜇0 𝑔 . (2) figure 1. mechanical dovetail interlocking. figure 2. structure with reinforcement beams, release levers and magnets (in green). figure 3. cross-section view. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 63 another relevant factor for this approach is the speed of grip and release. during the development of the drone, there was a focus on the coupling speed because, during the approach phase, the drone had to fly over an object placed on the ground and lift it up. during the preliminary tests, two issues were observed when flying close to the ground: 1) the displacement of the object and 2) the instability of the drone. both issues were ascribed to the rotors’ downwash [5], which is the air flow generated by the propulsion system. in the first case, the object to be grabbed moved when the drone flew a few centimetres above it, so it was necessary for the drone to ascend and repeat the grabbing procedure. this repositioning, in view of the challenge, increased the time required to complete the task. furthermore, the instability caused by the turbulence of the propulsion system interfered with the final manoeuvres needed for the proper positioning of the object. the use of an electromagnet would have introduced a further delay due to the need to maintain the position of the object near the ferrous surface during activation. driven by the need to reduce operating times, the choice of using permanent magnets, which are constantly ‘active’, also allowed for a reduction in this additional latency. during the release phase, this problem did not arise, and it was possible to drop the object even at a short distance from the ground. another relevant aspect that required specific investigation was the coupling system between the drone and the magnetic plate. two approaches were tested, one consisting of a sliding prismatic-like joint with a cardan system near the plate and a damped system, which was the final solution adopted. in the first approach, shown in figure 4, the variable length of the sliding joint allowed the drone to pick up objects up to 50 cm below its flight altitude. however, the considerable variation in the centre of mass once the object was gripped caused oscillations that increased energy consumption, as the autopilot had to continuously correct the vehicle’s position. the final proposed solution consisted of four cables tensioned by as many springs, thus dampening the mechanical shocks and vibrations on the object due to oscillations during transportation. furthermore, the cables allowed the plate to be attracted to the metal sheet on the object, as the plate could move freely (within a range of ± 5 cm) along the vertical axis and in the horizontal plane (within ± 3 cm). 3. experiments the dji f550 hexacopter was chosen for the final setup, which offered a good compromise in terms of maximum payload and limited downwash effect. in fact, the adoption of this gripping solution with a larger drone (dji s900) was abandoned because the excessive downwash produced by the rotors tended to push away the bricks below. the magnets chosen for the final implementation of the plate were 5 mm with a declared attraction force of approximately 3.8 kg for each single magnet, as shown in table 1. the nominal value of the force of attraction is confirmed if the object is made of iron. for steel or other alloys, this value can be reduced by more than 30 %. often, this given value decreases due to the coating or surface irregularities of the magnets. another factor is the thickness of the metal surface to be lifted, which must not be too thin, or the force of attraction cannot be fully exploited. however, in this study, the presence of the ultrat air gap between the magnet and the ferrous surface to be lifted led to a considerable deterioration in the attraction force, which decreased very rapidly with increasing distance. ten magnets of the above model were inserted into the plate, as shown in figure 2, with alternating orientation in the magnetic field. conducting the tensile test with the aid of a digital dynamometer, it was necessary to apply a force of 4.9 kg to detach the plate from the thick ferrous surface (0.5 cm). given the above considerations and the degradation in the attraction obtained for each individual magnet, this force was still well within the acceptable limits. the use of passive magnets ensured a strong grip without any detachments during the transport phases despite the thin separation layer between the ferromagnetic surface and the magnets. the final configuration was tested with copies of bricks, as per the challenge rules, with different weights of up to 2.0 kg and with thinner ferromagnetic surfaces (0.1–0.2 cm) on top compared with those used for the preliminary tensile test. during these tests, it was possible to lift the different types of bricks, but the orange bricks, shown in figure 5, could not be lifted because the ferromagnetic surface was not thick enough to ensure a firm grip. however, since these bricks were 1.80-m long, it might not be possible to lift them with just one drone, and therefore it was decided not to address this issue. table 1. overview of the properties of the magnets used. property magnet 1 magnet 2 material ndfeb ndfeb weight 3.04 g 7.6 g shape parallelepiped parallelepiped dimensions 20 x 10 x 2 mm 20 x 10 x 5 mm surface of the poles 20 x 10 mm 20 x 10 mm coating nickel plated nickel plated magnetisation n45 n45 force of attraction about 2.1 kg about 3.8 kg figure 4. approach with sliding prismatic-like joint with a cardan system near the plate. figure 5. types of bricks according to the challenge rules. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 64 the release occurs by activating the servomotor, which produces an increase in the air gap (figure 3) between the magnets and the metal surface of the gripped object. the detachment occurs naturally due to the contribution of gravity, so a small servo can detach objects in the order of 1–2 kg. this means that as the weight of the object decreases, it is necessary to increase the air gap produced by the movement of the levers. a short release sequence is shown in figure 6. this magnetic plate is presented in [6], and in the current version it has been improved with the integration of a sheet of magnetic shield material. this is an alloy with good magnetic permeability that allows the shielding of magnetic fields. this solution allows the upper part of the plate to be shielded, thus avoiding any interference with the autopilot and other system components. the proposed system, as shown in figure 7, allows excellent pick-and-place missions to be conducted. furthermore, the decision not to use electromagnets avoids the intermittent generation of magnetic fields, which could negatively influence the behaviour of the autopilot and reduce flight times as a result of the power consumption during the gripping phase. the final assembly with the damping system with cables and springs guarantees a firm grip between the brick and the magnetic plate, dampening the vibrations caused by transportation. the tension springs chosen for this project increase the extension length by 5 cm; as soon as the plate is within a range of 5 cm, it is attracted to the ferrous surface and stretches, leading to automatic coupling. when the springs are fully extended, the drone is able to lift the brick gradually. moreover, our damping system provides a compensation to payload vertical accelerations and decelerations respectively during take-off and landing. the tests performed on the gripper during the challenge showed that it was able to grab, transport and place a large number of bricks during the time allowed for the trials. 4. conclusions in this article, a drone equipped with a pick-and-place system for objects with ferrous surfaces has been presented, and the different approaches used to carry out the delivery service using drones have been evaluated. based on this analysis, it was decided to proceed using the technique of passive permanent magnets in order to eliminate the power consumption during the transport phase. this technique has been combined with a custom design in order to obtain a flat profile and to guarantee the lightness of the prototype as a result of using 3d printing. from the literature, it appears that this model represents the most compact passive magnetic gripping system developed. in the future, a lighter and flexible version of the prototype, capable of lifting objects with limited curved faces, will be developed. moreover, as the current vision system for brick detection is placed under an arm, the field of view is partially hidden by the plate. therefore, as a future development, the camera will be integrated into the plate in order to improve visualisation and ensure alignment up to a few centimetres from the object to be grasped. in addition, distance sensors will be installed to constantly monitor the distance during the gripping phase. based on the positive results from mbzirc and the latest improvements, this drone can be employed in the field in case of emergency to transport goods in dangerous or remote areas. acknowledgement this research was partially funded by the ‘safe and smart farming with artificial intelligence and robotics programma ricerca di ateneo unict 2020 -‐ 22 linea 2’ project. figure 6. the release phase, in which it is possible to see how the plate, thanks to the proposed solution, returns to its original position immediately after release without affecting the posture of the drone. figure 7. final assembly with the damping system and cable tensioner. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 65 references [1] d. longo, g. muscato, adhesion techniques for climbing robots: state of the art and experimental considerations, advances in mobile robotics, proc. of the eleventh international conference on climbing and walking robots and the support technologies for mobile machines (clawar), coimbra, portugal, 8 – 10 september 2008, pp. 6-28. doi: 10.1142/9789812835772_0003 [2] a. rodríguez castaño, f. real, p. ramón soria, j. capitán fernández, v. vega, b. c. arrue ullés, a. ollero baturone, alrobotics team: a cooperative multi-unmanned aerial vehicle approach for the mohamed bin zayed international robotic challenge, journal of field robotics 36 (2019) pp. 104-124. doi: 10.1002/rob.21810 [3] a. gawel, m. kamel, t. novkovic, j. widauer, d. schindler, b. p. von altishofen, r. siegwart, j. nieto, aerial picking and delivery of magnetic objects with mavs, proc. of the 2017 ieee international conference on robotics and automation (icra), singapore, 29 may-3 june 2017, pp. 5746-5752. doi: 10.1109/icra.2017.7989675 [4] k. tai, a. r. el-sayed, m. shahriari, m. biglarbegian, s. mahmud, state of the art robotic grippers and applications, robotics 5 (2016). 20 pp. doi: 10.3390/robotics5020011 [5] c. g. hooi, f. d. lagor, d. a. paley, height estimation and control of rotorcraft in ground effect using spatially distributed pressure sensing, journal of the american helicopter society 61 (2016) pp. 1-14. doi: 10.4050/jahs.61.042004 [6] g. sutera, d. c. guastella, g. muscato, a novel design of a lightweight magnetic plate for a delivery drone, proc. of the 23rd international symposium on measurement and control in robotics (ismcr), budapest, hungary, 15-17 october 2020, pp. 1-4. doi: 10.1109/ismcr51255.2020.9263730 https://doi.org/10.1142/9789812835772_0003 https://doi.org/10.1002/rob.21810 https://doi.org/10.1109/icra.2017.7989675 https://doi.org/10.3390/robotics5020011 https://doi.org/10.4050/jahs.61.042004 https://doi.org/10.1109/ismcr51255.2020.9263730 acta imeko  september 2014, volume 3, number 3, 68 – 72  www.imeko.org    acta imeko | www.imeko.org  september 2014 | volume 3 | number 3 | 68  comparison of milligram scale deadweights to electrostatic  forces  sheng‐jui chen, sheau‐shi pan, yi‐ching lin   center for measurement standards, industrial technology research institute, hsinchu, taiwan 300, r.o.c.      section: research paper   keywords: deadweight force standard; electrostatic force actuation; capacitive position sensing; force balance  citation: sheng‐jui chen, sheau‐shi pan, yi‐ching lin, comparison of milligram scale deadweight forces to electrostatic forces, acta imeko, vol. 3, no. 3,  article 14, september 2014, identifier: imeko‐acta‐03 (2014)‐03‐14  editor: paolo carbone, university of perugia   received may 13 th , 2013; in final form august 26 th , 2014; published september 2014  copyright: © 2014 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: this work was supported by the bureau of standards, metrology and inspection (bsmi), taiwan, r.o.c.  corresponding author: sheng‐jui chen, e‐mail: sj.chen@itri.org.tw    1. introduction  microand nano-force measurement is of great interest in recent years among several national measurement institutes (nmis) [1-6]. the center for measurement standards (cms) of the industrial technology research institute (itri) has established a force measurement system based on electrostatic sensing and actuation techniques. the system is capable of measuring vertical forces up to 200 n based on a force balance method. the system mainly consists of a flexure stage, a three-electrode capacitor and a digital controller [7]. the schematic drawing of the system is shown in figure 1. the three-electrode capacitor is used simultaneously as a capacitive position sensor and an electrostatic force actuator. the position of the center electrode is measured by comparing the capacitances between upper capacitor c1 and lower capacitor c2 formed within the three electrodes (see figure 2). the differential capacitance was detected using an inductivecapacitive resonant bridge circuit. the position detection is performed at a radio frequency (rf), say, 100 khz, a frequency depending on the capacitance values and the design of the sensing bridge circuit. for electrostatic force actuation, the top and bottom electrodes are applied with two high voltage, audio frequency sinusoidal signals to generate a compensation electrostatic force fe to balance the force under measurement fm. the balance condition fm = fe is maintained by the digital controller by keeping the flexure stage at its zero deflection position. some parts of the force measurement system were upgraded for performance improvements. a new design of figure 1. schematic drawing of the force measurement system.   abstract  this paper presents a comparison of milligram scale deadweights to electrostatic forces via an electrostatic sensing & actuating force  measurement system.  the electrostatic sensing & actuating force measurement system is designed for measuring force below 200 n  with an uncertainty of few nanonewton.   the force measurement system consists of three main components: a monolithic flexure  stage, a three‐electrode capacitor for position sensing and actuating and a digital controller.  the principle of force measurement used  in this system  is a static force balance,  i.e. a force to be measured  is balanced by a precisely controlled, electrostatic force. four  weights  of  1  mg  to  10  mg  were  tested  in  this  comparison.  the  results  of  the  comparison  showed  that  there  exist  extra  stray  electrostatic forces between the test weights and the force measurement system. this extra electrostatic force adds a bias force to the  measurement result, and was different for each weight. in principle, this stray electrostatic force can be eliminated by  installing a  metal  housing  to  isolate  the  test  weight  from  the  system.  in  the  first  section,  we  briefly  introduce  the  electrostatic  sensing  and  actuating force measurement system, and then we describe the experimental setup for the comparison and the results. finally, we  give a discussion and outlook.  acta imeko | www.imeko.org  september 2014 | volume 3 | number 3 | 69  copper-beryllium flexure stage was installed in the system, which has a counter weight balance mechanism and a lower stiffness of 13.08 n/m. figure 2 shows a picture of the new flexure stage, where the counter weight and the gold-plated cube flexure stage are visible. a new set of gold-plated, polished electrodes was assembled as a three-electrode capacitor and put into operation. the capacitance gradient for the new threeelectrode capacitor was measured. 2. experimental setup  in this experiment, the compensation electrostatic force is compared to the deadweight by weighing a weight with the electrostatic sensing and actuating force measurement system. 2.1. deadweight  we used five wire weights with nominal mass values and shapes of 1 mg-triangle, 2 mg-square, 5 mg-pentagon and 10 mg-triangle to generate vertical downward forces. these weights meet the metrological requirement of the oiml class e1 and were calibrated against standard weights using a mass comparator balance. the calibration results are compiled in table 1. the forces can be derived from the calibrated mass values and the local acceleration of gravitation g = 9.78914 m/s2 as fw = m (1-a/w) g, where a and w are densities of the air and the weight, respectively. these weights were loaded and unloaded by a dc motor actuated linear translation stage. 2.2. electrostatic sensing & actuating force measurement system  as shown in figure 3, the compensation electrostatic force fe generated by the force measurement system is determined by the following equation: 2 22 2 11 2 1 2 1 vsvsfe  (1) where s1, s2 are the capacitance gradients of the top and the bottom capacitors c1, c2 and v1, v2 are voltage potentials between the top and the bottom capacitors, respectively. using the parallel-plate capacitor as the model for capacitor c1 and c2,                               432 00 1 1)( d x d x d x d x d a xd a xc  (2)                               432 00 2 1)( d x d x d x d x d a xd a xc  (3) where 0 is the vacuum permittivity, a is the effective area of the electrode and d is the gap distance between electrodes when the center electrode is vertically centered. the capacitance gradients s1 and s2 can be expressed as                          432 0 1 1 54321)( d x d x d x d x s dx dc xs (4)                             432 0 2 2 54321)( d x d x d x d x s dx dc xs (5) where s0 = 0a/d2 is the capacitance gradient at x=0. the electrostatic force can be written as )()( 2 1 )( 22 2 10 2 2 2 10 vvs d x vvsxf e  (6) the voltages v1 and v2 contain the rf detection signal vdsindt, audio frequency high voltage actuation voltages va1sinat, va2sinat and the electrodes’ surface potentials vs1, vs2, namely 111 sinsin saadd vtvtvv   (7) 222 sinsin saadd vtvtvv   (8) the high voltage actuation signals are provided by a full range 10 v 16-bit resolution digital-to-analog converter within the digital controller and an ultra low-noise highvoltage amplifier. to make the electrostatic force linearly proportional to a control voltage vc, we set )(11 cba vvav  (9) )(22 cba vvav  (10) figure 2. picture of the new flexure stage.  table 1. mass calibration result. nominal mass (mg) conventional mass (mg) uncertainty, 95% confidence (mg) 1 1.00096 0.0003 2 2.00116 0.0003 5 5.00124 0.00065 10 10.0021 0.00048 c1 c2 d  x d + x v1 v2 c1 c2 d  x d + x v1v1 v2v2 figure 3. three‐electrode capacitor for electrostatic force actuation.   acta imeko | www.imeko.org  september 2014 | volume 3 | number 3 | 70  where a1, a2 are amplification factors of the high-voltage amplifier. the term vb is a constant and determined by the value of s0 and the upper limit of the force measurement range. taking the gain difference between channels of the high-voltage amplifier, substituting equations (7)-(10) for v1 and v2 in equation (6), we obtain an equation for the electrostatic force fe     terms)(ac )( 220 222 00 2 00   dsbccbe bvvsvvbaasvvasf , (11) where a is the gain difference fraction, i.e. a=(a1a2)/(a1+a2), a0 is the mean gain factor, b is the offset fraction x/d, vs2 = (vs12-vs22)/2 and vc is the control voltage. the high frequency ac terms at audio and rf frequencies can be omitted because they cause only negligible ac displacement modulations on the flexure stage. parameter a can be tuned to very close to zero by adjusting the gain of the dac within a software program. after the tuning, parameter a was measured to be smaller than 5105 contributing to a negligible force uncertainty. instead of using an optical interferometer, the position of the center electrode is measured by the difference between c1 and c2 with a differential capacitance bridge circuit [7]. hence, any deviation of the center electrode from the vertical center position can be detected by the bridge circuit. with a commercially available optical interferometer, the offset adjustment could be quite difficult and ambiguous. the effect of parameter (a+b) can be tested by setting vc = 0, modulating vb with a square wave profile and observing the displacement signal of the flexure stage. for vb=2.0, we did not observe the displacement due to the modulated vb. the remaining factors s0, vc and vs dominate the uncertainty of the electrostatic force fe. the capacitance gradient s0 was measured using a weight of 1 mg and a set of optical interferometer. the weight of 1 mg was cyclically loaded and unloaded to the system by a motorized linear stage to produce a deflection modulation. the deflection was measured by the optical interferometer and the corresponding capacitance variation was measured by a calibrated precision capacitance bridge. to reduce the effect from seismic noise and drift noise from the optical interferometer or the flexure stage itself, both deflection x and capacitance variation c are measured from the difference between average values of mass loaded data and two adjacent mass unloaded data. the capacitance gradient s0 was obtained by calculating the ratio of c/x which is shown in figure 3. using (2) and (3), the capacitance gradient estimated by c/x can be expressed as )1( 1 )( 0 32 2 0 0 d x s d x d x d x d a x cxc s                                 (12) from (12), s1 deviates from s0 by a small portion of s0x/d. using the nominal design value of d = 0.5 mm, the ratio x/d is 0.15 %. this ratio can be reduced by using a smaller x for measuring the capacitance gradient. the measured capacitance gradient s has a mean value of s = 2.87610-8 f/m and a standard deviation of s = 0.00810-8 f/m. therefore, the standard uncertainty of the capacitance gradient is 12104)(   n su s  f/m with n = 369 in this measurement. the uncertainty u(vc) of the control voltage vc is calculated using the dac resolution of 0.3 mv as 088.0)32/(3.0)( cvu mv which contributes 1 nn. for the surface potential noise vs, the current actuation scheme prevents the surface potential effect from being coupled to and amplified by the control voltage vc as the case in the previous electrostatic actuation scheme [7] where vs was amplified as svcvs. the surface potential is reported to range from 20 mv to 180 mv [8, 9]. taking vs = 0.18 v for example and s = 2.876  10-8 f/m, the surface potential induced electrostatic force is about 0.9 nn. 2.3. null deflection control  the force under measurement fm is balanced by fe by the null deflection control. figure 5 shows the block diagram of the null deflection control. the transfer functions of the main components, namely the flexure stage, capacitive position sensor, loop filter and the electrostatic force actuator, are represented by g, h, d and a respectively. the term xn represents a deflection noise which may be contributed by the seismic vibration noise and the thermal noise of the flexure stage itself. the relation between fe and fm appears to be )( )(1 )( )( )(1 )( sf st st sx st hda sf mse     (13) where t(s) = gdha is the open-loop transfer function of the control system, and fe(s), xs(s) and fm(s) are the laplace transforms of fe, xs and fm, respectively. within the control bandwidth, i.e. for t(s) >> 1, the relation between fe and fm can be approximated as )( mne fkxf  (14) 0 50 100 150 200 250 300 350 400 -2.91 -2.9 -2.89 -2.88 -2.87 -2.86 -2.85 -2.84 x 10-8 measurement index c ap ac it an ce g ra di en t (f /m ) 0 50 100 150 200 250 300 350 400 -2.91 -2.9 -2.89 -2.88 -2.87 -2.86 -2.85 -2.84 x 10-8 0 50 100 150 200 250 300 350 400 -2.91 -2.9 -2.89 -2.88 -2.87 -2.86 -2.85 -2.84 x 10-8 measurement index c ap ac it an ce g ra di en t (f /m ) figure  4.  capacitance  gradient  calculated  from  c/x.  the  mean  capacitance gradient s0 = 2.87610 ‐8  f/m, standard deviation s = 0.00810 ‐ 8  f/m and standard deviation of the mean  12104  n s  f/m (n = 369 in  this measurement).   acta imeko | www.imeko.org  september 2014 | volume 3 | number 3 | 71  where k is the stiffness of the flexure stage. equation (11) shows that the null deflection control automatically generates a compensation force fe to balance the force under measurement fm. to reduce the influence form the noise xn, fm is measured in a short period of time by comparing fe(t0) before fm is applied and fe(t1) after fm is applied: mnneee ftxtxktftff  )]()([)()( 0101 thus ntem kxff  . (15) the term xnt represents the temporal variation of xn during the measurement time frame. from one deflection measurement data set taken for 8-hr, using a window of 300 s to evaluate xnt, we obtained a standard deviation of 0.33 nm for xnt. with a measured value k of 13.0 n/m, the standard deviation of the xnt equivalent force noise is 4.3 nn. table 2 lists the main sources of uncertainty of the measured fm. 2.4. weighing process  each weight was loaded for 100 seconds and unloaded for 100 seconds. the compensation electrostatic force was calculated from the control voltage vc. figure 6 shows the control voltage vc acquired during one weighing cycle. the voltage difference vc was determined from one weight loaded segment and its two adjacent weight unloaded segments as 22 21 caca cbc vv vv  . (16) the weighing cycle was repeated for a long period of time in order to evaluate the stability and uncertainty of the system. 3. results  figure 7 shows the result of one weighing run for the weight of 1 mg. the measurement was done during three days. for this run, the measured electrostatic force was fe = (9,782.6  6.7) nn, where the given uncertainty is one standard deviation. the forces produced from the weights are estimated as fw = mg (1-air/mass), where the air buoyancy was taken into consideration. the comparison results are compiled in table 3. in general, the electrostatic force has a smaller value than the deadweight. for comparisons of weights 1 mg and 10 mg, the force differences defined as fe-fw are similar and close to 10 nn, and they both are in triangle shapes with similar dimensions. for comparisons of the weight 2 mg and 5 mg, the force differences are rather larger, and they are in shapes of square and pentagon, respectively. the weight of 5 mg has the largest force difference of about 200 nn (20 g), and it is the biggest weight in terms of wire length and shape area dimensions. a possible explanation for this force difference is that there might be some extra electrostatic or magnetic force between the weight and its surroundings. due to the size of the weight of 5 mg, it has the shortest distances to and possibly experiences the strongest electrostatic/magnetic interactions with its surroundings. 1 d a h g  flexure stage (m/n) capacitive position sensor (v/m) loop filter (v/v) electrostatic force driver (n/v) fmxn     vc fe v(x) 1 d a h g  flexure stage (m/n) capacitive position sensor (v/m) loop filter (v/v) electrostatic force driver (n/v) fmxn     vc fe v(x) figure 5. block diagram of the null deflection control. some noise sources are omitted for simplicity.   figure  6.  capacitive  displacement  and  control  voltage  vc  during  one  weighing cycle.  table 2. uncertainty budget for measured fm  source of uncertainty standard uncertainty (n) capacitance gradient s0 ef 4104.1 16-bit dac resolution 9101  surface potential vs 9108.1  displacement noise xnt 9103.4  combined standard uncertainty: 2429 )104.1()108.4()( em ffu   n figure 7. a data run for 1 mg weighing.  acta imeko | www.imeko.org  september 2014 | volume 3 | number 3 | 72  4. discussion and outlook  a force measurement system based on the electrostatic sensing and actuation techniques has been built and upgraded. the system is enclosed by a vacuum chamber which resides on a passive low frequency vibration isolation platform. the voltage actuation scheme has been modified to allow the decoupling between the surface potential vs and the actuation voltage leading to a reduction in the drift and bias of the compensation electrostatic force. the system is stable over a long period of time. however, the cause of the extra electrostatic/magnetic force observed in the weighing test is still unclear and investigation to that is underway. a new design of the apparatus’s housing is being fabricated, it was designed to isolate most of the apparatus from its surroundings and expose only the force loading area. in addition, other parameters such as alignment factors, the capacitance gradient and its frequency dependence will also be re-verified and studied further to find out the cause for the force difference. acknowledgement  this work was supported by the bureau of standards, metrology and inspection (bsmi), taiwan, r.o.c. references  [1] newell d b, kramar j a, pratt j r, smith d t and williams e r, “the nist microforce realization and measurement project”, ieee trans. instrum. meas. 52 (2003) 508. [2] kim m-s, choi j-h, park y-k and kim j-h, “atomic force microscope cantilever calibration device for quantified force meterology at microor nano-scale regime: the nano force calibrator (nfc)”, metrologia 43 (2006) 389-395. [3] leach r, chetwynd d, blunt l, haycocks j, harris p, jackson k, oldfield s and reilly s, “recent advances in traceable nanoscale dimension and force metrology in the uk”, meas. sci. technol. 17 (2006) 467-476. [4] choi j-h, kim m-s, park y-k and choi m-s, “quantum-based mechanical force realization in piconewton range”, appl. phys. lett., 90 (2007) 073117. [5] nesterov v, “facility and methods for the measurement of micro and nano forces in the range below 10-5 n with a resolution of 10-12 n (development concept)”, meas. sci. technol., 18 (2007) 360366. [6] m-s kim, j.r. pratt, u. brand and c.w. jones, “report on the first international comparison of small force facilities: a pilot study at the micronewton level”, metrologia, 49 (2012), 70 [7] s-j chen and s-s pan, “a force measurement system based on an electrostatic sensing and actuating technique for calibrating force in a micronewton range with a resolution of nanonewton scale”, meas. sci. technol., 22 (2011), 045104 [8] j.r. pratt and j.a. kramar, “si realization of small forces using an electrostatic force balance”, proc. 18th imeko world congress, (17-22 september 2006, rio de janeiro, brazil) [9] s.e. pollack, s. schlamminger and j.h. gundlach, “temporal extent of surface potentials between closely spaced metals”, phys. rev. lett. 101 (2008), 071101 table 3. comparison results, unit in nn.  1 mg 2 mg 5 mg 10 mg fw 9797.12.9 19586.72.9 48950.56.4 97897.34.7 fe 9782.66.7 19527.04.1 48751.48.2 97886.416.5 e-fw -14.5 -59.7 -199.1 -10.9 journal contacts acta imeko issn: 2221-870x march 2021, volume 10, number 1 acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 0 journal contacts about the journal acta imeko is an e-journal reporting on the contributions on the state and progress of the science and technology of measurement. the articles are mainly based on presentations presented at imeko workshops, symposia and congresses. the journal is published by imeko, the international measurement confederation. the issn, the international identifier for serials, is 2221-870x. editor‐in‐chief francesco lamonaca, italy founding editor‐in‐chief paul p. l. regtien, netherlands associate editor dirk röske, germany editorial board section editors (vol. 7 10) leopoldo angrisani, italy filippo attivissimo, italy eulalia balestieri, italy eric benoit, france paolo carbone, italy lorenzo ciani, italy catalin damian, romania pasquale daponte, italy luca de vito, italy luigi ferrigno, italy edoardo fiorucci, italy alistair forbes, united kingdom helena geirinhas ramos, portugal sabrina grassini, italy fernando janeiro, portugal konrad jedrzejewski, poland andy knott, united kingdom francesco lamonaca, italy massimo lazzaroni, italy fabio leccese, italy rosario morello, italy michele norgia, italy pedro miguel pinto ramos, portugal nicola pompeo, italy sergio rapuano, italy dirk röske, germany alexandru salceanu, romania constantin sarmasanu, romania lorenzo scalise, italy emiliano schena, italy enrico silva, italy krzysztof stepien, poland marco tarabini, italy yvan baudoin, belgium francesco bonavolonta, italy marcantonio catelani, italy carlo carobbi, italy mauro d’arco, italy egidio de benedeto, italy alessandro depari, italy alessandro germak, italy min-seok kim, korea momoko kojima, japan koji ogushi, japan vilmos palfi, hungary franco pavese, italy jeerasak pitakarnnop, thailand jan saliga, slovakia emiliano sisinni, italy oscar tamburis, italy jorge c. torres-guzman, mexico ioan tudosa, italy ian veldman, south africa rugkanawan wongpithayadisai, thailand claudia zoani, italy about imeko the international measurement confederation, imeko, is an international federation of actually 42 national member organisations individually concerned with the advancement of measurement technology. its fundamental objectives are the promotion of international interchange of scientific and technical information in the field of measurement, and the enhancement of international co-operation among scientists and engineers from research and industry. addresses principal contact prof. francesco lamonaca university of calabria department of computer science, modelling, electronic and system science via p. bucci, 41c, vi floor, arcavacata di rende, 87036 (cs), italy e-mail: f.lamonaca@dimes.unical.it acta imeko eulalia balestrieri, e-mail: balestrieri@unisannio.it carlo carobbi, e-mail: carlo.carobbi@unifi.it ioan tudosa, e-mail: itudosa@unisannio.it koji ogushi, e-mail: kji.ogushi@aist.go.jp momoko kojima, e-mail: m.kojima@aist.go.jp support contact dr. dirk röske physikalisch-technische bundesanstalt (ptb) bundesallee 100, 38116 braunschweig, germany e-mail: dirk.roeske@ptb.de mailto:f.lamonaca@dimes.unical.it mailto:balestrieri@unisannio.it mailto:carlo.carobbi@unifi.it mailto:itudosa@unisannio.it mailto:kji.ogushi@aist.go.jp mailto:m.kojima@aist.go.jp mailto:dirk.roeske@ptb.de measurements and geometry acta imeko issn: 2221-870x june 2021, volume 10, number 2, 98 103 acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 98 measurements and geometry valery d. mazin1 1 peter the great st. petersburg polytechnic university, russia section: research paper keywords: measurements; geometry; projective metric; basic measurement equation; geometric space citation: valery mazin, measurements and geometry, acta imeko, vol. 10, no. 2, article 14, june 2021, identifier: imeko-acta-10 (2021)-02-14 section editor: francesco lamonaca, university of calabria, italy received april 2, 2021; in final form may 18, 2021; published june 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: valery d. mazin, e-mail: masin@list.ru 1. introduction the purpose of this article is to define the commonalities between the concept of measurements and geometry; a method that uses the elements of geometry to model the basic elements of a measurement process. among many approaches that are used to describe fundamental measurement categories, geometrical approach is often underrated despite the fact that geometry as a science has originated from measurements and only later turned to a new higher level of generality. this paper attempts to argue that measurements and geometry are related, and geometry is not just another branch of math. at all times the most prominent authorities in the scientific world have acknowledged the fundamental and special place that geometry takes in the system of exact sciences. thus, spinoza believed that it is geometry that “reveals a causal connection in nature”. newton said that “geometry expounds and justifies the art of measurement” [1]. in [2] we find einstein’s statement, according to which “geometry must precede physics, since the laws of the latter cannot be expressed without geometry. therefore, geometry must be considered as a science, logically preceding every experience and every experimental science.” a remarkable illustration of this thought is also presented in the book by b. mandelbrot, "the fractal geometry of nature" [3]. in "the encyclopedia of mathematics" [4], the special role of geometry is characterized in the following way: “developments of geometry and its applications, advances in geometric perception of abstract objects in various areas of mathematics and natural science provide solid evidence of the importance of geometry as one of the most profound and fruitful means for cognizing reality” [5, 6]. today measurement specialists rarely use geometrical apparatus both in general and particular cases (with exception of, maybe, measurements at the elementary level). instead, analytical approach absolutely dominates the field. however, [7] highlights the huge heuristic value of geometric representation of the concepts of analysis; saying that geometry “is becoming increasingly important in … physics. it simplifies mathematical formalism and deepens physical comprehension. this renaissance of geometry has had an impact not only on the special and general theory of relativity, obviously geometric in nature, but also on other branches of physics, where the geometry of more abstract spaces is replacing the geometry of physical space.” today, no one seems to deny the fact that the science of measurements is actually a metascience, which is used in all natural and technical sciences, to say the least. for this reason, abstract the paper is aimed at demonstrating the points of contact between measurements and geometry, which is done by modelling the main elements of the measurement process by the elements of geometry. it is shown that the basic equation for measurements can be established from the expression of projective metric and represents its particular case. commonly occurring groups of functional transformations of the measured value are listed. nearly all of them are projective transformations, which have invariants and are useful if greater accuracy of measurements is desired. some examples are given to demonstrate that real measurement transformations can be dealt with via fractional-linear approximations. it is shown that basic metrological and geometrical categories are related, and a concept of seeing a multitude of physical values as elements of an abstract geometric space is introduced. a system of units can be reasonably used as the basis of this space. two tensors are introduced in the space. one of them (the affinor) describes the interactions within the physical object, the other (the metric tensor) establishes the summation rule on account of the random nature of components. mailto:masin@list.ru acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 99 the apparatus has to be represented by disciplines with the same or higher order of generality. geometry is just such a discipline. section 2 shows that the basic equation for measurements is a special case of the expression of projective metric. in section 3, functional measurement transformations are looked at in the context of groups theory. section 4 is devoted to identifying the relationship between metrological and geometrical categories. the concluding section summarizes the main idea of the paper and points out on its practical usefulness. 2. projective metric and basic measurement equation the essence of any measurement has always been a comparison with a known unit. among various geometric systems, the most common one is projective geometry, which, according to m. komatsu [8], represents geometry as a whole. projective geometry only studies the mutual relations between figures and in this sense is akin to measurements. a segment of a numerical axis can traditionally represent the value of a measured quantity. in projective geometry, the distance between two points is determined using a cayley metric (projective metric) 𝑙 = 𝑐 |ln 𝑉| , (1) where c is the constant, 𝑉 = 𝑥3 − 𝑥1 𝑥2 − 𝑥3 𝑥4 − 𝑥1 𝑥2 − 𝑥4 ⁄ (2) (complex, or double ratio of four points of a straight line), 1 2 3 4 , , ,x x x x are the coordinates of the points on the line. let 𝑐 = 1, 𝑥3 = 0, 𝑥4 = ∞. then from the equations above it follows that 𝑙 = |ln ( 𝑥1 𝑥2 )| = |ln ( 𝑥2 𝑥1 )| , (3) hence e𝑙⋅sgn(𝑥2−𝑥1) = 𝑥2 𝑥1 (4) and 𝑥2 = 𝑥1 ⋅ e 𝑙⋅sgn(𝑥2−𝑥1) . (5) the meaning of the quantities in the last equation leaves no doubt that what we have here the “basic equation of measurements” usually written as 𝑥 = {𝑥}[𝑥] , (6) where x is the measured quantity, {𝑥} = e𝑙⋅sgn(𝑥2−𝑥1) (7) is its numerical value, and [𝑥] is the quantity unit. the latter is taken for granted and does not seem to require any proof. however, as we can see, it is deduced from the definition of projective metric, a fact that can hardly be accidental. thus, it is worth mentioning a statement by famous mathematician holder [9]: “to prevent misunderstanding, i note here that the axioms of the theory of quantities as they appear here should not be presumed in geometry or applied to segments and volumes. on the contrary, there are examples of purely geometric axioms for points and segments, from which it can later be proved ... that for segments there are facts that in general theory of measurable quantities are presupposed as axioms”. in connection to the above said, let us make the following remark: from (1) it follows that the logarithmic scale, widely used in measurements as well as in physics and technology, is nothing but a scale in projective metric. in general, the simple relations stated above suggest that there is a principled connection between the fundamental concepts of geometry and measurements. the basic equation for measurements, fundamental and meaningful in itself, happens to be a particular case of a fundamental geometric relationship too. 3. groups of functional measurement transformations in addition, measurements and geometry are related by the fact that invariants are widely used in both disciplines. in measurements, this leads to improved accuracy. [10] shows an example of invariant principle applied to a simple ratio of three values of the measured quantity to the affine measurement transformation. in geometry, invariants generally have fundamental significance, since according to f. klein's "erlangen program" [11], various geometries represent the invariant theories of the relevant transformation groups. it should be noted that the function of the channel transformation of measurement system, y = f (x), certainly belongs to one of the following groups (we do not mean the groups mentioned in the "erlangen program"): 𝑦 = 𝑥 (8) is the identical group, 𝑦 = 𝑥 + 𝛽 (9) is the shift group, 𝑦 = 𝑎 ∙ 𝑥 (10) is the similarity group, 𝑦 = 𝑎 ∙ 𝑥 + 𝛽 (11) is the affine (linear) group 𝑦 = (𝛼 ∙ 𝑥 + 𝛽) (𝛾 ∙ 𝑥 + 𝛿)⁄ (12) is the projective (fractional-linear) group, 𝑦2 ≥ 𝑦1 by 𝑥2 ≥ 𝑥1, or 𝑦2 ≤ 𝑦1 by 𝑥2 ≥ 𝑥1 (13) is the group of monotonous transformations. all these transformations, except for (13), have invariants. such an invariant for (12), which includes all the previous groups (8) through (11), is a complex ratio of four points on a straight line [12, 13]. for (11), the invariant is the simple ratio of three points on a straight line (𝑥2 − 𝑥1) (𝑥3 − 𝑥2)⁄ , for (9) and (8), except for the two indicated invariants, x2 – x1, it is the usual euclidean distance between two points lying on the coordinate axis. the last two types of transformations are non-linear in general. if nonlinearity is small, then most commonly the corresponding experimental dependence can be satisfactorily approximated by a fractional-linear function [12], [14] – [16]. the remarkable property of the latter is that it belongs to the group of projective transformations while its form can vary a lot. such a transformation can be visualized as an image of a projection of the input scale on the output scale. the group property is acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 100 expressed in the fact that the superposition of a series of fractional-linear functions is the same function and does not lead to higher complexity (figure 1), while the inverse transformation is also fractional-linear. thus, a unified mathematical description becomes possible both for the intermediate transformations in the channel and for the whole transformation. for significantly nonlinear transformations or for improved accuracy of the approximation, it is advised that several fractional-linear functions should be used. they are either summed up or applied to sequential sections of the characteristic (piecewise approximation). in the summation case the output value is obtained as the sum of the results of fractional-linear transformations. to make it possible, the following conditions must be met: the result of the approximation by function 𝑦 = 𝑄(𝑥) 𝑃(𝑥)⁄ = ∑ 𝑎𝑖 𝑥 𝑖 𝑚 0 ∑ 𝑏𝑖 𝑥 𝑖 𝑛 0 ⁄ (14) has single roots in the denominator; 𝑚 does not exceed 𝑛 by more than 1. if y is a proper fraction, then it can be converted into a sum ∑ 𝐴𝑖 (𝑥 − 𝛼𝑖 ) , 𝑛 𝑖=1 where 𝛼1, … , 𝛼𝑛 are the roots of the denominator, the coefficients are found from the equation 𝐴𝑖 = 𝑄(𝛼𝑖 ) 𝑃′(𝛼𝑖 ) , (15) whereas 𝑃′(𝛼𝑖) = 𝑃 ′(𝑥)|𝑥=𝛼𝑖 . (16) if 𝑚 = 𝑛, then a constant is added to the sum of the fractions as a result of extracting the integer part. if 𝑚 = 𝑛 + 1, then a linear function is added. in both instances we deal with a particular case of a fractionallinear function. if any the roots in the denominator are complex, nothing changes in principle, but some of the summable fractional-linear functions turn out to be complex. at the same time, all their remarkable properties are sustained, including the presence of an invariant a complex relationship of four arbitrary points (𝑥3 − 𝑥1) (𝑥2 − 𝑥3) (𝑥4 − 𝑥1) (𝑥2 − 𝑥4) .⁄ an example can be the results of approximation of the calibration characteristics of two temperature sensors. let the first one be a platinum thermoresistor (its characteristic is utilized to model the international practical temperature scale). in the range of –259 °c ÷ +660 °c, we obtain 𝑊 = −2.244 ⋅ 104 𝑡 + 2.768 ⋅ 103 + 2.925 𝑡 + 280.063 + −7.227 ⋅ 104 𝑡 − 7.947 ⋅ 103 , (17) where 𝑊 is the ratio of resistance at temperature 𝑡 in °c to resistance at zero celsius. the standard uncertainty of this approximation is 0.7 °c, which corresponds to 0.08 % with respect to the temperature range and is considered acceptable for most practical cases. let the second sensor be a pt/rh thermocouple with 30 % / 6 % rh content. in the range of 0 °c ÷ 1800 °c its characteristic is approximated by expression 𝐸 = −4.514 ⋅ 106 + 5.287 ⋅ 107𝑖 𝑡 − 186.827 − 2.807 ⋅ 103𝑖 − 4.514 ⋅ 106 + 5.287 ⋅ 107𝑖 𝑡 − 186.827 + 2.807 ⋅ 103𝑖 − 4.86 ⋅ 108 𝑡 − 1.302 ⋅ 104 (18) and the standard uncertainty will equal to 0.3 %. in the case of piecewise linear fractional approximation there are no restrictions with respect to accuracy (uncertainty), but it is more difficult to implement. whichever fractional-linear approximation case is chosen, the need for mathematical methods is limited to four arithmetic operations. using invariance fits the purpose of measurement, which is not about transformation, but rather about preserving the information. indeed, in order to restore the characteristics of the original signal using measured characteristics of the converted signal, some kind of relationship between the signals has to be retained during the chosen transformation. from this perspective, the measuring transducer should be called a transmitter rather than a transducer, i.e. in this case the name is not associated with the main property of an object but reflects its secondary property instead. this happens because the transfer and transformation of the quantity value (including scaled transformation, i.e. energy level transformation) correlate to each other in the same way as the essence and a phenomenon; in other words, a dualism takes place. it is the transformation, not the transfer of the value that is visible to an observer. as in other similar cases, the object was named for its superficial, rather than essential property. ideally, in all types of measurement transformations such as the quantity type transformation, identity transformation, modulation and demodulation, the form of representing the transformation (e. g. analog-digital), code conversions, etc. the amount of information remains intact. in these transformations, errors mean the loss of information, and it is the degree of this loss, not the type of transformations that determines the quality of a measuring channel. 𝑦1 = 𝑎1𝑥 + 𝑏1 𝑐1𝑥 + 1 𝑦𝑖 = 𝑎𝑖 𝑦𝑖−1 + 𝑏𝑖 𝑐𝑖 𝑦𝑖−1 + 1 𝑦𝑛 = 𝑎𝑛 𝑦𝑛−1 + 𝑏𝑛 𝑐𝑛𝑦𝑛−1 + 1 𝑦 = 𝑎𝑥 + 𝑏 𝑐𝑥 + 1 figure 1. the circuit of fractional-linear transformations in a measuring channel. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 101 perhaps, the reason for the difficulty with the classification of measuring transducers that has still not been resolved is that all the variants of such a classification that are known so far are created on that side of the above-mentioned dualism that characterizes the phenomenon rather than the essence. in other words, what we are trying to do is classify the types of transformations; whereas what we should do is classify the types of information preservation. this situation, which occurs while fractional-linear and their dependent transformations are used, is consistent with the general concept of the measurement procedure. the very basic procedures, for example, when the length was measured, consisted of two stages: the mutual displacement of the measured object and the measure, and their comparison with each other. historically, this original essence of measurement is now perceived only by its second stage, while the first stage is actually no less significant. it is worth highlighting that what in the early measurements was omnipresent mechanical displacements, nowadays is replaced by measurement transformations. the analogies between displacements in an ordinary space and measurement transformations can be formalized even further. in length measurements, the correlation we wish to preserve is seen as the distance between the points, and this distance remains unchangeable no matter what the shifts and turns are. if we determine the distances in terms of the values that are preserved during such transformations for a set of all possible signals, we will arrive to the geometrical interpretation of a measurement procedure as a transformation that preserves the distance, i.e. as “displacements” in the relevant space. with fractional-linear transformations (for example, when using a voltage divider), complex ratio 𝑉 is preserved. but projective metric is thereby preserved too. since fractional-linear transformations preserve projective metric, it is only natural to call them the “displacements in a projective space”. at the same time, they are the most common among all the above transformations (except for monotonous transformations), each preserving the distance. it can be assumed that it is this property that permits us to either correct errors effectively or implement algorithms that eliminate errors in the first place. in these terms, the stages of the measurement procedure consist of displacements in a projective space and comparison with a master reference, i.e. coincide with an ordinary length measurement procedure. a typical example is measuring voltage with a bitwise-balancing voltmeter, whose range is smaller than the voltage measured. in such a case the voltage is pre-attenuated by a divider. the divider produces the corresponding section of the voltage scale on its output, preserving its length in the projective metric, while the voltmeter makes the comparison. as a preliminary conclusion, we can state so far that: any functional measurement transformation belongs to the group of monotonous transformations; the most common monotonous transformation is a fractional-linear variety that can describe projective correlations analytically and act as displacements (whereas the dimensions of the moving object are invariant). 4. correspondence between metrological and geometric categories table 1 shows the mutual correspondence of the fundamental metrological and geometric categories. any line in table 1 can be used as a departure point for further research. the concept of the space of affine connectivity takes up the first place in the table among geometrical concepts. it represents diversity, in which the field of the connectivity object is defined. the term “diversity” generally needs to be defined; however, in this case we will skip that as its meaning is obvious. as we know, the connectivity object characterizes the point of diversity, in which a local benchmark (or affine benchmark, referring precisely to the given point) is defined. in turn, the affine benchmark is a combination of the point itself and the coordinate basis. the connectivity object gives as answer to the question about how the coordinates of an arbitrary vector change as it is displaced along a certain curve while preserving orientation. in the general case, the coordinates will indeed change, because as the vector is moving from one point to another, the local benchmark to which the vector is momentarily related, changes too. the space of affine connectivity is poor in terms of properties, but it becomes richer once a metric is introduced in it by means of defining a metric tensor. then the space becomes riemannian, a space of curved vectors. such vectors represent physical quantities that characterize a specific physical object [17]. for such a vector space, a system of units can serve as a basis since a unit of any quantity can be expressed via the basic units of the system. a good example is a vector acceleration receiver on a moving ship. this receiver reads accelerations in an acoustic wave in water and its purpose is to identify the locations of the sources of noise. the main part of the receiver is shown in figure 2. we see six flat piezoelectric plates at the outside edges rigidly connected to a pair of strings in its middle, each pair of strings being fixed on the frame and running in three mutually perpendicular directions. the inner edges of the piezoelectric plates are perpendicularly joined to the faces of a cubic inertial element. when the frame experiences acceleration, the inertia force acts upon the cubical element, which can be componentized along the axes perpendicular to the planes of the piezoelectric plates. these componentized forces cause electrical charges. the axes perpendicular to the planes of the piezoelectric plates form a coordinate basis, and together with the center of gravity of the cubic element create a local benchmark. as the ship moves and experiences pitching and rolling, the location and orientation in space of the local benchmark changes, whereas the direction of the vector of acceleration of water particles in the acoustic beam remains the same. as a result, the table 1. correspondence between metrological and geometric categories. metrological category geometric equivalent an object, a measuring instrument with deterministic relationships the space of affine connectivity, or a riemannian space physical quantity point in the space, vector system of units basis probability characteristics and statistical relationship of physical quantities metric tensor (determines the space geometry) analog measurement transformation affinor (determines the relationship of the vectors) analog-to-digital conversion vectors subtraction preservation of the measurement information invariance acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 102 projections of this vector on the axis of the coordinate basis l change as determined by the connectivity object. the connectivity object is a system of numbers, called connectivity coefficients. if each and every connectivity coefficient turns into zero, the diversity becomes an affine space. vectors can be defined in it, so the space is a vector space. an affine space is a model of any particular object whose physical regular properties can be described by simple additive relationships. the space describing a measuring instrument is generally multidimensional. in [18], we can see the application of multidimensional spaces apparatus. in this space, the points and the vectors connecting them to the origin correspond to physical quantities. the basis of the space is the system of units. an affine space can be identified for an object with any other kind of regular physical properties, but only in an infinitesimal region and with an accuracy no greater than that of the first order [19]. let 𝑦 = 𝑓(𝑥1, … , 𝑥𝑛) be any function of 𝑛 variables. then d𝑦 = ( 𝜕𝑦 𝜕𝑥𝑖 ) d𝑥𝑖 (19) (implied summation over 𝑖) can be considered a vector, since coordinates ( 𝜕𝑦 𝜕𝑥𝑖 ) d𝑥𝑖 (at first approximation) are affine. the rule of adding the vectors, and, consequently, the space geometry, is determined by a metric tensor. since the values for the generality should be considered random, the addition on rule must take into account their probability characteristics and statistical relationship. as shown in [20], if the coordinates for the vectors are expanded uncertainties, then the metric tensor is determined by the types of probability distribution, the coverage probability, the ratio of the terms, and mutual correlation. by today, the components of such a tensor for the most popular probability distributions and for 0.95 and 0.99 coverage probabilities have been determined by a. chepushtanov [20]. if the coordinate system is formed by standard uncertainties, then the metric tensor is determined only by the mutual correlation. the analog measurement transformation, which takes into account the design parameters of the device and influencing factors as input quantities, has a geometrical equivalent affinor, the rule that states that each vector 𝐝𝒙 is matched with a certain vector 𝐝𝒚. the affinor is a square-matrix bivalent tensor. since the result of the analog-digital transformation is a number, it is only obvious that the quantity separates from the quality. in other words, the quantity rids of its physical carrier. taking the logarithm of the basic equation for measurements yields two vectors – one for the numerical value and another for the unit. we do not touch here on the quasi uncertainty arising in the logarithm of a unit, we note only that it is a matter of logarithmising not the number „one”, but a unit of a physical quantity, which can have a different real meaning. thus, in the logarithmic representation, the geometric essence of the analogdigital transformation is that the unit vector is subtracted from the full quantity vector, which is incidentally nothing else but computing how many units of the quantity (or which part of the unit) is included in the dimension of the quantity. finally, as it was stated above, the preservation of the measurement information characterizing the object conceptually corresponds to invariance. information losses, inevitable in any measurement transformation (introduction of an uncertainty), means that this principle of correspondence is compromised. when searching for the points of contact between modern geometry and measurements, special attention should be paid to non-euclidean geometries. riemann geometry is one of them, since its features are quite visible in the space of quantities. for instance, the ends of the logarithmic vectors of reciprocal quantities (such as resistance and conductivity) are located at the diametrically opposite points of a sphere that has its center at the origin of the coordinates. at the same time, there is practically no difference between them. they represent the same characteristic of the system. as it is known, the riemann geometry is a spherical geometry with an additional condition of identification applied to the opposite points of the sphere. the obvious similarity between the physical and geometrical facts can hardly be accidental. moreover, there is evidence that different geometries work in case of different measurement ranges of the same physical quantities, which is only natural due to a wide generality of their geometric space. further research in this area seems to be promising and new interesting results are expected. other promising applications of projective geometry are in explaining the laws of physics and image processing [22]. the basic concept here is projective mapping. it is most frequently used as a visual image rather than a mathematical structure. being used in a strict geometric sense, this concept allows us to describe patterns with any level of complexity. then the mapping parameters logically become variable, which is consequent to the change in the position of the projection center. this position, in turn, is affected by some physical causes, which now can be identified and explored. thus, the fundamental metrological categories have geometric equivalents. problem defining in the area of measurements can be described using geometric terminology. solving metrological problems seems possible if the powerful modern geometric apparatus is used. 5. conclusions the most important geometric concepts have equivalents in measurement theory. this knowledge allows us to apply geometric approach and apparatus to formulate a single figure 2. the main part of a vector acceleration receiver. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 103 mathematical description for important measurement categories, obtain new theoretical results, and model measuring procedures. in particular, projective transformations can be used in such modelling. due to their group properties, the characteristics of measuring devices can be described in a significantly simpler manner, whereas the present invariant allows us to increase the accuracy of measurements. a model for any physical object, including the measuring device itself, can be represented as a vector space, whose elements, in turn, represent the quantities characterizing the object. this 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learning algorithm acta imeko issn: 2221-870x march 2022, volume 11, number 1, 1 8 acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 1 beamforming in cognitive radio networks using partial update adaptive learning algorithm md zia ur rahman1, p. v. s. aswitha1, d. sriprathyusha1, s. k. sameera farheen1 1 dept. of electronics and communication engineering, koneru lakshmaiah education foundation, vaddeswaram, guntur-522502, andhra pradesh, india section: research paper keywords: adaptive learning, bandwidth, cognitive radio, frequency, power transmission citation: md zia ur rahman, p. v. s. aswitha, d. sriprathyusha, s. k. sameera farheen, beamforming in cognitive radio networks using partial update adaptive learning algorithm, acta imeko, vol. 11, no. 1, article 30, march 2022, identifier: imeko-acta-11 (2022)-01-30 section editor: md zia ur rahman, koneru lakshmaiah education foundation, guntur, india received december 4, 2021; in final form february 18, 2022; published march 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: s. k. sameera farheen, e-mail: sksameera667@gmail.com 1. introduction in medical telemetry wireless communication techniques are used widely in the present generation. generally, these types are used to monitor the condition of a person via pulse, respiration, etc., this wireless medical telemetry service was first established by the federal communications commission by allocating some of the frequency bands separately for this wireless purpose so that it will not have any discrepancies while allocating frequency bands while cross-checking the patient's condition [1]. instead of that, we can use the existing spectrum for a medical telemetry application by using cognitive radio. in this cognitive radio spectrum sensing technique would be there for this spectrum sense we use the beamforming technique. the efficient utilization of spectrum is the most important thing in cognitive radio. cognitive radio offers a spectrum allocation for unlicensed users simultaneously with licensed users. so, the secondary users need to detect the spectrum availability using spectrum sensing by sensing the primary user in the same frequency [2], [3]. the main purpose to introduce beamforming technique is to remove the unwanted noise in different systems which we are using in this process like military sonar and radar systems. this will be separating the sources when the overlapping frequency content which originates at different spatial locations [4]-[6]. this technique is mainly used for sensing purposes, in this phase, the licensed user transmitter will be estimated by the secondary user transmitter according to that estimation the spectrum allocation will be done [5]-[8]. this technique is used to provide an exchange of information within the cells and the remaining near cells will be in a secured manner. the interference which is occurred in signals is mainly due to the imperfection of spectrum sensing, due to that secondary user are freely accessing the primary user channels [9], [10]. at the same time, this technique should return to the channel before the secondary users. this beamforming would be very effective while sensing the spectrum without any interference. a cognitive radio network is one of the important systems in the broadband communication system. the beamforming technique [11], [12] which we are proposing in this paper is used to connect the information inside the cells and the outer cells which are in use abstract cognitive radio technology is a promising way to improve bandwidth efficiency. frequency which is not used in any aspect will be utilized by using some of the most powerful resources in this cognitive radio. one of the main advantages of cognitive radio signal is to detect the different channels which are there in the spectrum and it can modify the frequencies which is utilized frequently. it allows the licensed users to gain the licensed bandwidth under the condition to protect the licensed users from harmful interference i.e., from secondary users. in this paper, we would like to implement cognitive radio using the beamforming technique, by using power allocation as a strategy for the unlicensed transmitter which is purely form on the result of sensing. it is on the state of the primary user in a various cognitive radio network whereas the unlicensed transmitter gives a single antenna and it modify its power transmission. for the cognitive radio setup, we have used normalized adaptive learning algorithms. this application would be very useful in medical telemetry applications. nowadays wireless communication plays a vital role in healthcare applications for that we have to build a separate base. it reduces the effort of the building of separate infrastructure for medical telemetry applications. mailto:sksameera667@gmail.com acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 2 from all the users. it has proved that the beamforming technique is the smooth technique for spectrum sensing analytically too. in that paper, they explained some statistical tests between maximum-to-minimum eigenvalue and maximum-to-minimum beam energy algorithms based on the statistical values they conclude that beamforming is a smooth sensing technique [13]. the sensors in secondary users are used to assume their particular pu doas. always the secondary users access the primary user's accounts freely and simultaneously, licensed users also intend to get back the mechanism before the unlicensed user acquire expires. at last, the binding centre collaborate with doas into a primary user localization estimation. the result of our final implementation gives that to make possible to make a localization system with a poor complexity and a good primary user localization capability in this cognitive network. the capabilities which are present in pu localization are used to enhance the pu interference [14]. generally, in services of wireless communication, it has rectifiers at every junction point to manage every event of the upcoming trending technologies which gives profit from abstraction property. it has gained from different latest antenna sets and also from algorithms that form adaptive beams [15]. the process which we have used is purely based on least mean square (lms) algorithm, which gives a brief and proper care to the signal model which we have used for the beamforming technique. to get the accuracy convergence rate more than the expected one for the lms which we have used in the smart antenna system, we have used bbnlms (block-based normalized least mean square) algorithm. the presence of this algorithm gives a lot of difference in convergence rate. mainly this algorithm will be performed only in the existence of different effects and many users which is scanned using matlab simulations using different white signals. the lms formula was used only in the sensible antenna systems in between the adjustive beam forming algorithms [16]. in place of quick exploitation, the traditional square of the error vector is used [17]. although many methods have been implemented and implemented with the new ideas for the doa (direction of arrival) of signal. the adaptive array was first discovered by van atta in the year 1959 which is defined as a self-phased array [18]. it reflects all the incident signals in the arrival direction using a clever phasing scheme. these beamforming algorithms are divided into various methods they are mainly fixed weight beamforming and adaptive beamforming algorithm. this adaptive algorithm will update the array weights continuously which is based on optimization of changing signal environment. this section briefly discusses lms, recursive least square, conjugate gradient algorithm, and quasi-newton algorithm [19]. data transmission in a communication channel with a low probability of bit error is possible up to a certain bit rate with a given snr (signal-to-noise ratio). therefore, in addition to the adaptive algorithm, to increase the noise immunity of the wireless system, let us consider the use of channel coding schemes. such coding is a series of transformations of the original bit sequence, as a result of which the transmission of information flow becomes more resistant to the deterioration of the quality of the transmitted information caused by noise, interference, and signal fading. channel coding allows reaching a compromise between bandwidth and the probability of bit error [20]. this paper gives a different aspect in the direction of arrival of signals with a different error rate and at last, in this paper, we provide the simulation results which provides better evidence for the technique which we have provided in this paper. 2. methodology the networks that we are using in this paper are cognitive radio network (crn) and non-orthogonal multiple access (noma) are widely used system in the 5g broadband communication system. there is one advantage for the users who are using a cognitive radio network is to protect the information from different devices like multiple-input multipleoutput-noma. in our paper, we are trying to implement the substitute of beam forming technique which is already available to protect the data exchange inside the cells and outer cells from different users and their system model is shown in figure 1. the intervention was caused by a faulty signal of the unlicensed users. the unlicensed users are intended to get the availability of licensed users. though the licensed user repays the channel before the licensed user access terminates. adaptive antenna systems include a mixture of different antenna components with a signal-processing ability to improve its radiation or acceptance pattern instinctively in response to the signal environment. the method which we are proposing in our paper on a partial update least mean square (pu-lms) algorithm is an algorithm that is used to control the overload and less power consumption in the implementation of an adaptive filter. thus, the problem of adaptive filtering algorithm improves the filter coefficients is shown in figure 2. hence, we provide a better result from our proposed technique. lms algorithm is an algorithm admired in adaptive beam formers in which we use the antenna arrays. it is also used for channel levelling to conflict the inter-symbol interference. simultaneously, few applications of lms can incorporate interference, echo cancellation, space-time modulation, and coding, and the signals which are in observation. whereas, the already algorithms have high faster convergence rate like recursive least square and least mean square which is already admired because of their implementation and its computational costs. it is one of the effective methods for power consumption and for reducing the computational load in the adaptive filter implementations, which is appealing in mobile communications. figure 1. cognitive system mode. figure 2. overview of project implementation. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 3 block diagram and overview of proposed algorithm is shown in figure 3 and figure 4, respectively. there are many mobile communication devices and those applications like channel equalization as well as echo cancellation which requires the adaptive filter to get a high number of coefficients. to modernize the entire signal vector which is costly regarding ram, capacity, and computation and sometimes it does not fit mobile units. finally, in this paper, we present the analysis of the confluence of partial update adaptive learning (pual) algorithm lesser than different types of suppositions, and when the diversions are visible then it can be prevented by different coefficient updates accidentally, which is also known as sequential partial update adaptive learning (spu-al). 2.1. cyclostationary input signals cyclostationary is one of the processes which is having different analytics which differ cyclically with respect to meter. it is the one that can be showed as a various interleaved stationary process. it is also having a detection method for estimating and for spectral autocorrelation function technique to analyse the spectrum sensing. it can be detected whether it is active or not and whether the signal is used by licensed users or not it can sense these easily because this technique is robust in the sense. sequential pu-al algorithm, shows how the coefficients are updating in simulation and associated samples of the signals which are used in every overhaul (x power 1/4)(n) being the value of the retreat signal at the present instant and its flow chart is shown in figure 4. it simulates as a higher bound for each step size of pu al compatible algorithm when the signal is given as an input signal as a periodic reference which consists of many euphonious 𝐸{ℎ(𝑙 + 𝑇)} = 𝐸{ℎ(𝑙)𝐸{ℎ(𝑙 + 𝑇)ℎ(𝑙 + 𝑇 + 𝑛)} = 𝐸{ℎ(𝑙)ℎ(𝑙 + 𝑛)} (1) 𝑦(𝑙) = [𝑦(𝑙)𝑦(𝑙 − 1)𝑦(𝑙 − 2) … . , 𝑦(𝑙 − 𝑁 + 1)]t 𝐵𝑦 (𝑙) = 𝐸{𝑦(𝑙)𝑦 t(𝑙)} = diag[𝜎𝑦 2(𝑙), … . , 𝜎𝑦 2(𝑙 − 𝑁 + 1)] (2) 𝜎𝑦 2 = { 𝑀1 for 𝑖 𝑇 < 𝑙 ≤ 𝑖 𝑇 + 𝛼 𝑇 𝑀2 for 𝑖 𝑇 + 𝛼 𝑇 ≤ (𝑖 + 1) 𝑇 (3) or 0 < 𝛼 < 1 and 𝑖 = ⋯ , −4, −3, −2, −1, 0, 1, 2, 3, 4, … and sinusoidal power of a variation in time. 𝜎𝑦 2 = 𝛽(1 + sin(𝜔𝑜 𝑙)) . (4) here, is larger than zero, and 𝜔o falls between 0 and , with 𝜔o/(2 ) being a rational integer. if the sinusoidal power of a variation in time period is more than three, then 𝑦(𝑙) is not an input signal. 2.2. sequential partial update adaptive learning algorithm the needed signal is denoted by 𝑑(𝑙), the desirable weight vectors are denoted by 𝜔o, and the noise measurement is denoted by 𝑣(𝑙) 𝑑(𝑙) = 𝑦t(𝑙)𝜔o + 𝑣(𝑙) (5) 𝑒(𝑙) = 𝑒(𝑙) − 𝑦t(𝑙) 𝑢(𝑙) (6) 𝑢(𝑙 + 1) = 𝑢(𝑙) + 𝜇𝑒 (𝑙)𝐼𝐾 (𝑙)𝑦(𝑙) . (7) here 𝑢(𝑙) is the weight vector (adjustable filter coefficient vector), 𝑑(𝑙) = 𝑦t(𝑙) 𝑢(𝑙) indicates fallacy of method (whatever algorithm we have taken) and 𝐼 coefficient preference matrix is: 𝐼k(𝑙) diag[𝑖1(𝑙), 𝑖2(𝑙), … … , 𝑖𝑁 (𝑙) (8) with 𝐴 = 𝑁 𝐾⁄ . here, 𝐴 is taken as integer %(𝑙, 𝐴) represents the ‘%’ operation which results the remainder after making division 𝑙 by 𝐴. ∑ 𝑖𝑗 (𝑙) 𝑁 𝑖=1 = 𝐾, 𝑖𝑗 ∈ {0,1} the coefficient subsets 𝐸i are different up to they reach the following requirements: 1. 𝑈i=1 a 𝐸i = 𝑍 where 𝑍 = {1,2, … , 𝑁} 2. 𝐸𝑖 ∩ 𝐸𝑗 = ∅, ∀𝑗 , 𝑗 ∈ {1,2,3, … , 𝐴} and 𝑖 ≠ 𝑗 . 2.3. performance analysis for the input signal �̌�(𝑙 + 1) = �̌�(𝑙) − 𝜇𝑒 (𝑙)𝐼𝐾 (𝑙)𝑦(𝑙) (9) here is �̃�(𝑙) = 𝜔o − 𝑢(𝑙). (10) using (11), 𝑔(𝑙) = 𝑦t�̃�(𝑙) + 𝑣(𝑙) (11) is obtained. putting (11) into (9), then �̃�(𝑙 + 1) = 1 − 𝜇 𝐼𝑘 (𝑙) 𝑦(𝑙) 𝑦 t(𝑙) �̃�(𝑙) − 𝜇 𝑣(𝑙) 𝐼𝐾 (𝑙) 𝑦(𝑙) (12) taking the exception on both sides of (12), we get 𝐸{�̃�(𝑙 + 1)} = (𝐼 − 𝜇 𝐸{𝐼𝐾 (𝑙) 𝑦(𝑙) 𝑦 t(𝑙)}) 𝐸{�̃�(𝑙)} (13) time varying variance model 𝜎𝑦 2 = { 𝑀1 if mod(1, 𝑇) = 1 … 𝑀𝑇−1 if mod(1, 𝑇) = 𝑇 − 1 𝑀𝑇 if mod(1, 𝑇) = 0 . (14) we took the set 𝑀1, 𝑀2, …, 𝑀𝑇 , which has one large value (like 1) and one tiny value (like 0.001). it's to make sure that (3) and (4) are both true (14). between the su-partial update parameter 𝐴 and the input signal period 𝑇, the six instances reflect all potential scenarios. 2.3.1. case study 1 𝑇 ≤ 𝐴 and %(1, 𝑇) = 0. in case of (13) can be rework as 𝐸{�̃�(𝑙 + 1)} = 1 − 𝜇𝑖𝑗 (𝑙)𝜎𝑦 2(𝑙 − 𝑗 + 1)𝐸{𝑢�̃�(𝑙)} (15) figure 3. overview of partial update algorithm. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 4 here 𝑢�̃�(𝑙) is the j th entry of �̃�𝑗 (𝑙). by verifying (7), the rechecking (15) is changed for every 𝐴 iteration. by adding the 𝐴 iterations of (15) 𝐸{𝑢�̃�(𝑙 + 𝐵)} = (1 − 𝜇𝜎𝑦 2(𝑙𝑗 − 𝑗 + 1)) 𝐸{𝑢�̃�(𝑙)} (16) here 𝑙 is taking as integer satisfying 𝑙 where 𝑙𝑗 < 𝑙 + 𝐴. let declare the parameter 𝑑𝑗 = 𝑙𝑗 − [𝑙𝑗 𝐴⁄ ]𝐴 to indicate which entry of 𝐸{𝑢�̃�(𝑙)} where 𝑙𝑗 is satisfies 𝑙𝑗 < 𝑙 + 𝐴. let declare the parameter like 𝑑𝑗 = 𝑙𝑗 − [𝑙𝑗 𝐴⁄ ]𝐴, to represents the coming of 𝐸{𝑢�̃�(𝑙)} is upgraded. the function [𝑦] converts 𝑦 to largest integer ≤ 𝑦. by giving the sequential pu variable 𝐴, 𝑑𝑗 only depends on 𝑗 value. here %(𝐴, 𝑇) = 0, we have the equation 𝜎𝑦 2(𝑙𝑗 − 𝑗 + 1) = 𝑀𝑡𝑗 here 𝑡�̃� = 𝑐(𝑡�̃�) and 𝑑𝑗 + 𝑁 − 𝑗 + 1 − [𝑑𝑗 + 𝑁 − 𝑗 + 1 𝑇 ] and 𝑐(𝑦) = 𝑦 − 𝑇|sign(𝑦)| + 𝑇. by putting this 𝜎𝑦 2(𝑙𝑗 − 𝑗 + 1) = 𝑀𝑡𝑗 in to (16), we get 𝐸{𝑢�̃�(𝑙 + 𝐵)} = (1 − 𝜇𝜎𝑦 2𝑀𝑡𝑗 ) 𝐸{𝑢�̃�(𝑙)} (17) here %(𝐴, 𝑇) = 0 and 𝑡𝑗 is like take as an integer, by looping (17), we get 𝐸{𝑢�̃�(𝑙 + 𝑏 + 1)𝐴} = (1 − 𝜇𝑀𝑡𝑗 ) 𝐸{𝑢�̃�(𝑙 + 𝑏𝐴)} . (18) in this instance, 𝒃 is a positive integer. it would be the combination of (17) and (18), according to (18). 𝑬{(𝒖�̃�(𝒍 + (𝒃 + 𝟏)𝑨} depends on 𝑴𝒕𝒋 . if 𝑴𝒕𝒋 in the input signal which is cyclostationary is very small, is explained in [11]. input signal will change partially for every iteration, it effects the 𝒖�̃�(𝒍)to converge and moving towards update. hence, in this sequential pu-al will certainly met with those tough conditions. 2.3.2. case study 2 𝑇 ≤ 𝐴 and %(𝐴, 𝑇) ≠ 0 and greatest common divisor (gcd) (𝐴, 𝑇) is equal to 1. here gcd(𝐴, 𝑇) represents the gcd of 𝐴 by 𝜎𝑦 2(𝑙𝑗 − 𝑗 + 1) = 𝑀𝑡𝑗(𝑙) here 𝑡𝑗 (𝑙) is taken like an integer declared as 𝑇𝑗 (𝑙) = 𝑐(𝑡�̃�(𝑙)) and 𝑡�̃�(𝑙) = 𝑙𝑗 − [𝑙𝑗 𝑇⁄ ]𝑇. then, 𝑇𝑗 (𝑙) is depends on the values of both 𝑗 and 𝑙. hence, (17) becomes 𝐸{𝑢�̃�(𝑙 + 𝐵)} = (1 − 𝜇𝑀𝑇𝑗(𝑙) ) 𝐸 {1 − 𝜇𝑀𝑇𝑗(𝑙)) 𝐸{𝑢�̃�(𝑙)} (19) looping the equation (19) for 𝐴 number of times, we get 𝐸{𝑢�̃�(𝑙 + 2𝐵)} = (1 − 𝜇𝑀𝑓(𝑇𝑗(𝑙)) ) 𝐸{𝑢�̃�(𝑙 + 𝐴)} (20) 𝑓 (𝑇𝑗 (𝑙)) = { 𝑇𝑗 (𝑙) + (%(𝐴, 𝑇)), 𝑇𝑗 (𝑙) + (%(𝐴, 𝑇) ≤ 𝑇 𝑇𝑗 (𝑙) + (%(𝐴, 𝑇) − 𝑇, otherwise . (21) since 𝑇 ≤ 𝐴, %(𝐴, 𝑇) is not equal to 0 and gcd(𝐴, 𝑇) is equal to 1, looping (21) for 𝑇 times, we get 𝐸{𝑢�̃�(𝑙 + 𝑇𝐵)} = ⟦1 − 𝜇𝑀 𝑓… 𝑓(𝑇𝑗(𝑙)) ⟧ 𝐸{𝑢�̃�(𝑙 + (𝑇 − 1)𝐴} = (1 − 𝜇𝑀1) … (1 − 𝜇𝑀𝑇 )𝐸{𝑢�̃�(𝑙)} (22) where 𝑓 … 𝑓 (𝑇𝑗 (𝑙)) represents the configuration of 𝑓(. ) in 𝑇 times. in (22), here we can observe the updates the process of input signal is declared by 𝑇 variances {𝑀1, 𝑀2, … 𝑀𝑇 }. as a consequence, the strategy (serial pu-al) will not interfere with toughness in this scenario. if the step-size meets the requirements, the pu-lms is stable. 0 < 𝜇 ≤ 2/max (𝑀1, 𝑀2, … , 𝑀𝑇 ) (23) 2.3.3. case study 3 𝑇 ≤ 𝐴, %(𝐴, 𝑇) ≠ 0 and gcd(𝐴, 𝑇) is greater than one. the least common multiple (lcm) of 𝑇 and 𝐴 is represented by lcm(𝐴, 𝑇). clearly lcm(𝐴, 𝑇) < 𝐴, 𝑇. here, the variance would become, 𝜎𝑦 2(𝑙𝑗 − 𝑗 + 1) is given that 𝜎𝑦 2(𝑙𝑗 − 𝑗 + 1) = 𝑀𝜉𝑗(𝑙), where 𝜉𝑗 (𝑙) = 𝑐 (𝜉�̃�(𝑙)) and 𝜉�̃�(𝑙) = 𝑙𝑗 − [ 𝑘𝑗 𝑇 ] 𝑇. then, (17) becomes figure 4. flow chart of a partial update adaptive learning algorithm. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 5 𝐸{𝑢�̃�(𝑙 + 𝐵)} = (1 − 𝜇 𝑀𝜉𝑗(𝑙) ) 𝐸{𝑢�̃�(𝑙)} (24) looping equation (24), then we get 𝐸{𝑢�̃�(𝑙 + 2𝐵)} = (1 − 𝜇 𝑀𝑓(𝜉𝑗(𝑙)) ) 𝐸{𝑢�̃�(𝑙)} (25) though, 𝑇 ≤ 𝐴 and %(𝐴, 𝑇) is not equal to zero and gcd(𝐴, 𝑇) is greater than one, looping the equation (25) lcm(𝐴, 𝑇) times, then 𝐸{𝑢�̃�(𝑙 + lcm(𝐴, 𝑇))} = (1 − 𝜇 𝑀 𝑓……𝑓(𝜉𝑗(𝑙)) ) × … × (1 − 𝜇 𝑀𝜉𝑗(𝑙)) 𝐸{𝑢�̃�(𝑙)} , (26) where lcm(𝐴, 𝑇) 𝐴⁄ is less than 𝑇, there is a one or more than one parameter in the set {𝑀1, 𝑀2, … … , 𝑀𝑇 } that is not matched to input signal i.e., 𝐸{𝑢�̃�(𝑙)}. the parameters is with the input signal i.e., 𝐸{𝑢�̃�(𝑙)} all are tiny values, the up-dation purpose of the input signal of 𝐸{𝑢�̃�(𝑙)} will be very slow, while resulting the interference. 2.3.4. case study 4 𝑇 > 𝐴 and %(𝐴, 𝑇) = 0 in this case the divergence would become 𝜎𝑦 2(𝑙𝑗 − 𝑗 + 1) = 𝑀𝜉𝑗(𝑙) in this equation 𝜉𝑗 (𝑙) would be taken as positive integer and having an equal probability for taking on those values of [𝑠�̃� , 𝑠�̃� + 𝐴, … , 𝑠�̃� + 𝑇 − 𝐴] where 0 < 𝑑�̃� < 𝐴 is related to 𝑑𝑗 through 𝑑�̃� − 𝑑𝑗 = 𝑧, where 𝑧=0, 1, 2,…. then we get 𝐸{𝑢�̃�(𝑙 + 𝐵)} = (1 − 𝜇𝑀(𝜉𝑗(𝑙)) ) 𝐸{𝑢�̃�(𝑙)} (27) looping equation (27), we get 𝐸{𝑢�̃�(𝑙 + 2𝐵)} = (1 − 𝜇𝑀𝑞(𝜉𝑗(𝑙)) ) 𝐸{𝑢�̃�(𝑙)} (28) 𝑞 (𝜉𝑗 (𝑙)) = { 𝜉𝑗 (𝑙) + 𝐴 𝜉𝑗 (𝑙) + 𝐴 ≤ 𝑇 𝜉𝑗 (𝑙) + 𝐴 − 𝑇 otherwise . here 𝑇 is greater than 𝐴 and %(𝐴, 𝑇) is equal to zero, looping equation (28) 𝑇 𝐴⁄ number of times, we get 𝐸{𝑢�̃�(𝑙 + 𝐵)} = (1 − 𝜇𝑀 𝑞…..𝑞(𝜉𝑗(𝑙)) ) 𝐸{𝑢�̃�(𝑙 + 𝑇 − 𝐴)} = (1 − 𝜇𝑀 𝑔……..𝑔(𝜉𝑗(𝑙)) ) 𝐸{𝑢�̃�(𝑙 + 𝑇 − 𝐴)} (29) if all values in the taken set {𝑀𝑞(𝜉𝑗(𝑙),… ,𝑀𝑔…𝑔(𝜉𝑗(𝑙)) } have very tiny values, up-dation for input signal i.e., 𝐸{𝑢�̃�(𝑙)} might be slightly slow, sequential pu-lms might show very low interference. 2.3.5. case study 5 𝑇 > 𝐴, %(𝐴, 𝑇) ≠ 0 and gcd(𝐴, 𝑇) is equal to one. likewise coming to case 2, the sequential pu-lms is stable if equation-24 obeys the step-size. 𝑇 > 𝐴, %(𝐴, 𝑇) ≠ 0 and gcd(𝐴, 𝑇) is greater than one. likewise looking to case-3, the sequential pu-lms faces the low interference. note: we learned from this that for input signals with regularly time-varying variance (observe equations-2, 3, 4, and 15), the sequential partial update-lms method would not confront the low intersection challenging condition in case-2 or case-5. 𝐴 and 𝑇 become coprime integers in just two situations, with the gcd(𝐴, 𝑇) equal to 1. 𝐴 and 𝑇 are not co-primes, and the sequential partial update-lms algorithm may demonstrate a very sluggish intersection, depending on how the repeating power levels were balanced. 3. simulation results in this simulation part, the response of a signal with one doa comes at the base station with an angle of 60 degrees. threshold values of 0.1, 0.5 and 1 simulation results are conducted. for each threshold point interference rate is considered in terms of the samples which we have used to reach the steady state. from the simulation results clearly, we can know that the received signal converges at a mean square error of 0.0007. generally, if we observe the simulation results for 𝛼 = 0.5 and 1, steady state converged faster when compared to the conventional lms algorithm and also, we can clearly observe the delay period for these threshold points. this delay corresponds to the samples which are observed before the adaptive antenna is ready to adapt. the improvement in interference rate purely depends on the number of taps adapted. 3.1. one white signal three doas in this module effects of multipath in antenna systems are studied for various threshold conditions. three multipaths with the three different directions of arrivals of 60, 30 and -20 degrees are transmitted to a base station with the different sampling periods. hence, three signals are arrived with time differences of 𝑡, 𝑡 − 1, and 𝑡 − 2 each with amplitudes of 0.6, 0.75 and 1.0. using the pu-al algorithm three different weight updating equations are used for processing each multipath signal. these figure 5. beam pattern of one white signal with 3 doas using pu-al for α = 0.1. figure 6. received error signal of one white signal with three doas using pu-al for α = 0.1. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 6 simulation results are conducted at threshold values of 0.1, 0.2 and 1. interference tables are for each multipath signal is shown in terms of samples to reach the steady state. for each multipath signal, the mean square error is approximately lies between 0.006, 0.0014 and 0.00036 respectively and their simulation output is shown in figure 5. beam patterns of the proposed pu-al algorithm are shown in figure 6. from that, we can clearly say that the proposed algorithm is having a better ability to steer the beams in different directions, and nulls are placed in the place of interferences. gain of the beam is obtained corresponding to gain introduced by each multipath signal. 3.2. two white signals with one doa each transmitting of one signal with two multipath signals and two white signals with one doa effect which is similar. in those cases, two signals are uncorrelated to each other, and it is divided by one sample period. firstly, the signal with an amplitude of 0.5 and the second signal with an amplitude of 1.0 is considered. the interference table of each signal in terms of the number of samples is shown. from this, we can know that smaller gain amplitudes lead to longer responses to adapt taps and for estimated signals. for 𝛼 = 0.1 value, it will converge faster when compared to 0.2 and 1. from this delay is created for these values so it takes a longer time to the response before adaption of taps. these threshold points can adapt to the limited number of taps and it affects the system performance and its simulation output in figure 7. the beam pattern is shown in figure 8 with two beams corresponding to the doas at 60 and -25 degrees. this demonstrates a smart antenna system with desired signals and interfering ones. beamforming technique as sensing technique in cognitive radio network inside this work, we attempted to explain the beamforming approach in cognitive radio as spectrum sensing. this beamforming approach may be used in two ways: centralized or distributed. we've used a dispersed strategy in this case. cognitive radio may use the beamforming approach to target a specific beam onto a specific receiver while reducing involvement in surrounding directions, improving network performance. this distributed method gives each user a separate antenna, and several of them broadcast the signal together by manipulating the transmitter's carriers. the authorized users' interference is lessened when this method is used. cognitive radio would be able to expand the range of communication by employing this beamforming approach in the signal beam is vigorous to the appropriate direction by this spectrum. another benefit of this beamforming technology is that it reduces delay spread, multipath fading, and radio transmission on the same frequency channel as other interferences, among other things. the diagram depicted in the illustration is a geometrical representation of a cognitive radio network at the intended receiver location, which includes licensed users. there are k cognitive users who are uniformly scattered over a disc with a radius of r and a centre of p. assume the cognitive radio users' position is (sk, $k), which is polar coordinates. similarly, use the specified spherical coordinates to represent the receiver. we made the assumption that cognitive radio nodes are uniformly dispersed across a disc with a radius of r. a single antenna is provided for each node. because the channel between the users and the receiver is in line of sight, there is no shadowing. the principal users, also known as licensed users, the transmitters are in the far zone of the beam pattern, while the receivers are in the near zone. the simulation depicts the statistical distribution of the radiation pattern in the lobe that includes the direction in which radiation strength is greatest (major lobe) and lobes that do not contain the main lobe (sidelobes). these are generally rays that are aimed in an unfavourable direction. it is used to examine these power levels by running 10,000 trials to create a beam pattern. the radius of the disc is r/(lambda) = 2, azimuth angle = 0 degrees, and elevation angle = pi/2, all of which are regularised by wavelength. these cognitive users employ uniform distribution, with numbers such as 4, 7, 16, 100, and 256 being used. the signal-to-noise ratio in the loop (snr) of the phase loop locked output variance is expected to be 2 db, 3 db, and 10 db as shown in figure 9. at an angle of 20 degrees and 30 degrees, two licensed users or principal users are presumed to be present. figure 7. received error signal of two white signal with one doa using pu-al for α = 0.1. figure 8. beam pattern of two white signals with one doa using pu-al for α = 0.1. figure 9. average power vs. direction of antenna. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 7 the beampattern phase-only distributed beamforming (podb) approach does not have a perfect phase in the diagram above. in this situation, the number of users is 100. the average gain for loop snr 10 db is 0 db at an angle of 0 degrees, whereas the gain marginally reduces for loop snr 2 db and 3 db. because of the incomplete phase, the main lobe is at its apex. the users are at a 20-degree and 30-degree angle, respectively, therefore the sidelobe power is about equal to -20 db. the cumulative complementary distribution function (ccdf) of podb with phase offset is shown in the figure above for k values of 3, 8 and 15. the proportion of equal to or more than a certain power level beampattern is shown in figure 10. for the ccdf computation, we ran 10,000 simulations. for a loop snr of 10 db, the angle would be set to 0 degrees. for values of 4, 7 and 16, the major lobe is at its highest. finally, we employed the sequential partial update in this case. the leastsquare technique is used to repair errors caused by altering antenna element placements. the error between the actual and expected replies is reduced with this approach. 4. conclusion this paper has analysed the spectrum sensing issue using the beamforming technique in that we have used an algorithm for the input signal is sequential pu-al. here we have taken the input signal as cyclostationary signal with white gaussian noise and we have analysed the input signal using pu-al in different case studies and these case studies also represent how the sequential pu-al tolerate the convergence related issues. we have implemented many new things as a result of our project, to keep up with technological advancements. nowadays, we rely on online sources for everything instead of utilizing the spectrum that is available not only now but also in the future to save money. by employing this approach in the future, stricter bounds on the rate of convergence can be achieved. for stationary signals, a mean update equation of pu-al can be developed. it can also be looked at the techniques that can be used to analyse the performance of the max pu-al algorithm. referrences [1] s. surekha, md zia ur rahman, navarun gupta, a low complex spectrum sensing technique for medical telemetry system, journal of scientific & industrial research, vol. 80, pp. 449-456, may 2021. [2] j. divya lakshmi, rangaiah. l, cognitive radio principles and spectrum sensing, blue eyes intelligence engineering & sciences publication, volume-8, august 2019, pp. 2249 – 8958. [3] omi sunuvar, a study of distributed beamforming in cognitive radio networks, 2013. [4] numa joy, anu abraham mathew, beamforming for sensing based spectrum sharing in cognitive radio network, international journal of engineering research & technology (ijert), 2015. [5] hyils sharon magdalene antony, thulasimani lakshmanan, secure beamforming in 5g-based cognitive radio network, symmetry, 2019. doi: 10.3990/sym11101260 [6] kais bouallegue, matthieu crussiere, iyad dayoub, on the impact of the covariance matrix size for spectrum sensing methods: beamforming versus eigen values, ieee, 2019. doi: 10.1109/iscc47284.2019.8969741 [7] aki hakkarainen, janis werner, nikhil gulati, damiano patron, doug pfeil, henna paaso, aarne mammela, kapil dandekar, mikko valkama, reconfifigurable antenna based doa estimation and localization in cognitive radios: low complexity algorithms and practical measurements, 2014. [8] yu sing xiao, danny h. k. tsang, interference alignment beamforming and power allocation for cognitive mimonoma downlink networks, ieee, 2019. doi: 10.1109/wcnc.2019.8885714 [9] md. zia ur rahman, v. ajay kumar and g v s karthik, a low complex adaptive algorithm for antenna beam steering, ieee, 2011. doi: 10.1109/icsccn.2011.6024567 [10] janis werner, jun wang, aki hakkarainen, danijela cabric,mikko valkama, performance and cramer-rao bounds for doa/rss estimation and transmitter localization using sectorized antennas, ieee transactions on vehicular technology,volume: 65, may 2016. doi: 10.1109/tvt.2015.2445317 [11] n. j. bershad, e. eweda, and j. c.m. bermudez, stochastic analysis of the lms and nlms algorithms for cyclostationary white gaussian inputs, ieee trans. signal process., vol. 62, no. 9, pp. 2238–2249, may 2014. [12] m. z. u. rahman, s. surekha, k. p. satamraju, s. s. mirza, a. layekuakille, a collateral sensor data sharing framework for decentralized healthcare systems, ieee sensors journal, november 2021. doi: 10.1109/jsen.2021.3125529 [13] s. surekha, md zia ur rahman, spectrum sensing for wireless medical telemetrysystems using a bias compensated normalized adaptive algorithm, international journal of microwave and optical technology, vol. 16, 2021, no.2, pp. 1-10 [14] s. surekha, a. lay-ekuakille, a. pietrosanto, m. a. ugwiri, energy detection for spectrum sensing in medical telemetry networks using modified nlms algorithm, 2020 ieee international instrumentation and measurement technology conference (i2mtc), 2020, pp. 1-5, doi: 10.1109/i2mtc43012.2020.9129107 [15] shafi shahsavar mirza, nagesh mantravadi, sala surekha, md zia ur rahman, adaptive learning based beamforming for spectrum sensing in wireless communications, international journal of microwave and optical technology, vol. 16, 2021, no.5. [16] armando coccia, federica amitrano, leandro donisi, giuseppe cesarelli, gaetano pagano, mario cesarelli, giovanni d'addio, design and validation of an e-textile-based wearable system for remote health monitoring, acta imeko, vol.10, no.2, pp. 1-10, 2021. doi: 10.21014/acta_imeko.v10i2.912 [17] ayesha tarannum, zia ur rahman, l. koteswara rao, t. srinivasulu, aimé lay-ekuakille, an efficient multi-modal biometric sensing and authentication framework for distributed applications, ieee sensor journal, vol. 20, no. 24, 2020, pp. figure 10. ccdf vs. instantaneous power. https://doi.org/10.3990/sym11101260 http://dx.doi.org/10.1109/iscc47284.2019.8969741 https://doi.org/10.1109/wcnc.2019.8885714 https://doi.org/10.1109/icsccn.2011.6024567 https://doi.org/10.1109/tvt.2015.2445317 https://doi.org/10.1109/jsen.2021.3125529 https://doi.org/10.1109/i2mtc43012.2020.9129107 http://dx.doi.org/10.21014/acta_imeko.v10i2.912 acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 8 15014-1515025. doi: 10.1109/jsen.2020.3012536 [18] s. y. fathima, k. murali krishna, shakira bhanu, s. s. mirza, side lobe suppression in nc-ofdm systems using variable cancellation basis function, ieee access, vol.5, no.1, 2017, pp. 9415-9421. doi: 10.1109/access.2017.2705351 [19] imran ahmed, eulalia balestrieri, francesco lamonaca, iomtbased biomedical measurement systems for healthcare monitoring: a review, acta imeko, vol. 10, 2021, no.2, pp. 174 184. doi: 10.21014/acta_imeko.v10i2.1080 [20] k. murali krishna, k. krishna reddy, m. vasim babu, s.s. mirza, s.y. fathima, ultra-wide band band-pass filters using plasmonic mim wave guide based ring resonators, ieee photonics technology letters, vol.30, 2018, no.9, pp. 1715-1718. doi: 10.48084/etasr.4194 https://doi.org/10.1109/jsen.2020.3012536 https://doi.org/10.1109/access.2017.2705351 http://dx.doi.org/10.21014/acta_imeko.v10i2.1080 https://doi.org/10.48084/etasr.4194 introduction to the acta imeko special issue on the ‘imeko tc4 international conference on metrology for archaeology and cultural heritage’ acta imeko issn: 2221-870x march 2022, volume 11, number 1, 1 2 acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 1 introduction to the acta imeko special issue on the ‘imeko tc4 international conference on metrology for archaeology and cultural heritage’ fabio santaniello1, michele fedel2, annaluisa pedrotti1 1 labaaf laboratorio bagolini archeologia, archeometria, fotografia; ceasum – centro di alti studi umanistici, dipartimento di lettere e filosofia, università di trento, via tommaso gar n°14, 38122, trento (italy). 2 dipartimento di ingegneria industriale, università di trento, via sommarive n° 9, 38123 trento (italy). section: editorial citation: fabio santaniello, michele fedel, annaluisa pedrotti, introduction to the acta imeko special issue on the ‘imeko tc4 international conference on metrology for archaeology and cultural heritage’, acta imeko, vol. 11, no. 1, article 2, march 2022, identifier: imeko-acta-11 (2022)-01-02 received march 30, 2022; in final form march 30, 2022; published march 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: fabio santaniello, e-mail: fabio.santaniello@unitn.it dear readers, this special issue of acta imeko is the result of the 5th international conference on metrology for archaeology and cultural heritage. the conference was originally planned to be held in trento (italy) hosted by the department of humanities of trento university, on october 22-24, 2020, but due to the sanitary emergency caused by covid-19, the organisers decided to hold the conference online. despite the unexpected situation, the conference has been a great success with 158 initial submissions, 126 accepted papers, 431 authors from 19 countries, 4 invited plenary speakers, 13 special sessions, 3 tutorial sessions and 11 patronages. out of the numerous presented papers, a selection has been made by the scientific committee to realize this issue. particular attention has been paid to the papers which inspire the exchange of knowledge and expertise between “human sciences” and “hard sciences”. after the review process, seventeen papers have been accepted for publication encompassing several research fields and methodological approaches. more in detail, several papers are devoted to material characterization by means of different analytical techniques. six of them are focused on the analysis of archaeological artefacts. particularly, zerai gebremariam et al. analysed the pottery assemblage from the site of adulis (eritrea): colorimetric measurements show new technological and manufacturing insights used for ceramic production during the roman age. an egyptian wooden statuette stored in the museo egizio di torino has been analysed by vigorelli et al., who compared the results of a multi-analytical strategy based on both non-invasive and micro-invasive procedures to investigate the original artistic techniques and the ancient restorations of the artefact. the paper by stagno and capuano compares micro-mri, diffusion-nmr and portable nmr data to highlight the diagnostic features of roman archaeological woods. es sebar et al. analysed different metal tools used during the construction of the santa maria del fiore cupola in florence, pca analyses performed on xrf data allow to determine different alloys depicting new details about the renaissance technology. tavella et al. used a graphic elaboration software to calculate the capacity of several prehistoric vessels from northeastern italy, suggesting possible functions and/or cultural traditions related to the potteries. the article by mazzoccato et al. shows the significance of laser scanning microprofilometry for surface analysis and 3d printing in the study of archaeological pottery. passing from the archaeology to the artworks, sottili et al. present an interesting contribution based on the combination of ma-xrf and dr to study painting layers and colours composition. a second group of eight papers is more related to the study and also to the management and valorisation of architectural heritage. baiocchi et al. proposed an approach to realize a geomatic survey by smartphones useful to create digital twins and virtual models. this approach has been tested on the intihuatana stone in machu picchu (peru) providing intriguing results and possibilities. brienza and fornaciari combined gis and photogrammetry to study the masonry of the bagni di eliogabalo (rome). their detailed data offer a wide reconstructive hypothesis allowing to point out roman construction techniques and expedients. the history and the architectural transformations of the bridge of canosa di puglia (italy) have been analysed through http://? acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 2 archival documentation and field surveys by germanò, who finally hypnotize the original configuration of the bridge. doria et al. present the result of a multi-steps program related to the study and the conservation of the castiglioni chapel in pavia, focusing on the digital survey and the creation of an immersive 3d model with different levels of analysis and visualization. the paper by antolini focuses on the development of a wide approach for the reconstruction of the ephemeral apparatuses, the latter has been applied to the case study of the funeral apparatus realized in rome for cardinal mazarin. several banded vaults systems in turin baroque atria have been analysed by natta. the author proposes an integrated approach involving metric survey by laser scanning and digital drawing in order to investigate the original constructive methodologies and the changes due to time. moving to more recent structures, the paper by gabellone shows an interesting 3d reconstruction of an underground oilmil in the town of gallipoli, which has been used to develop shared virtual visits during the covid-19 emergency. pirinu et al. presents the results of an extended survey activity related to the military architectures built in sardinia during the second world war. the collected data allow to analyse the historical construction techniques as well as to recover a peculiar heritage which is part of the contemporary landscape. bertola discusses a methodology that by starting from archival documentation and using bim allows to reproduce a 3d model of due case a capri by aldo morbelli. eventually, the article by weththimuni et al. deals with the preservation of cultural heritage buildings by using zro2-doped zno-pdms nanocomposites as protective coatings for the stone materials, providing interesting future perspectives. the contributions of this special issue provide an overview of the significant impact achieved by a more intense synergy between metrology and human sciences. moreover, following the constraint given by the international situation that occurred since 2020, this issue stressed the importance to promote a diffuse accessibility of cultural heritage thanks to the virtualization and digitalization of archaeological artefacts, human landscapes, historical documents and so on. to conclude we hope that this special issue catches the attention of the readers thanks to its interdisciplinarity. actually, we strongly believe that the intermingling of competencies is the way to look beyond contemporary research and sketch both the opportunities and the path of the cultural heritage in the future. hope you will have an exciting reading! fabio santaniello, michele fedel, annaluisa pedrotti guest editors banded vaults with independent arches: analysis of case studies in turin baroque atria acta imeko issn: 2221-870x march 2022, volume 11, number 1, 1 7 acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 1 banded vaults with independent arches: analysis of case studies in turin baroque atria fabrizio natta1 1 department of architecture and design (dad), politecnico di torino, v.le mattioli 39, 10125 torino, italy section: research paper keywords: banded vaults; architectural drawing; 3d modelling; point clouds citation: fabrizio natta, banded vaults with independent arches: analysis of case studies in turin baroque atria, acta imeko, vol. 11, no. 1, article 8, march 2022, identifier: imeko-acta-11 (2022)-01-08 section editor: fabio santaniello, university of trento, italy received march 7, 2021; in final form march 7, 2022; published march 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: fabrizio natta, e-mail: fabrizio.natta@polito.it 1. introduction this research project is a continuation of a study conducted by roberta spallone and marco vitali on complex brickwork vaulted systems in turin's baroque buildings [1]. the progress of the studies, and the enlargement of the research group, have led to the analysis of a considerable number of case studies in the architectural heritage of turin. the ‘a fascioni’ – or banded – vaults are architectural solutions for the covering of medium and large rooms derived from the guarini experience. guarini describes their characteristics in architettura civile (published posthumously in 1737) [2] and applies their forms in some of his projects. from guarini's example, a remarkable production has emerged that has seen the work of many important architects, but also of others whose identity is unknown. they have applied formal and constructive principles that have become customary in the turin building site, which have allowed a wide application in the civil buildings of the city. eleven vaulted baroque atria were identified by the research group as banded vault, of which eight were accessible. one of the objectives was to catalogue and recognize those vaulted structures with independent arches and those in which the arches are generated by vertical cuts on the main reference vault (e.g., pavilion, ‘a conca’, ‘a schifo’, etc.). this analytical phase preceded the comparison of the metric data obtained by tls (terrestrial laser scanning) metric survey, coordinated by concepción lópez. the information obtained from the point cloud was fundamental for the comparison through sections of the various parts of the structure. the philological reconstruction of the design idea was analysed with ideal schematics of the treatises, archive drawings, and realizations in the city and through the tools of twoand threedimensional modelling. the studies that led to the continuation of this analysis [3], resume work methods already applied to other case studies [4], intending to define a systematic methodology after this extensive research. 2. architectural treatises and manuals the ‘a fascie’ vaults are introduced, as we have seen, by guarino guarini into architettura civile. starting from a rigorous abstract this contribution presents a part of the work and the methodology applied to it developed for the realization of an international research project aimed at the analysis and preservation of an architectural heritage characteristic of turin's baroque architecture: the ‘a fasce’ vaults, locally named as ‘a fascioni’. this architectural solution, used by important architects, such as guarini up to the local workers, to cover spaces of various sizes, has found in the court of turin area a wide application in buildings atria. a considerable number of banded vaulted atria were identified and surveyed by the research group to recognize and investigate those whose bands are generated from independent arches. the objective is the comparison of metric and geometric data between ideal models and realizations over time, to evaluate their variations and understand a constructive methodology through three-dimensional modeling. mailto:fabrizio.natta@polito.it acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 2 knowledge on the theme of vaulted systems, with studies related to geometry, stereotomy, calculation of surfaces and volumes present in his previous treatises (euclides adauctus published in 1671 and modo di misurare le fabriche, in 1674) in architettura civile, treatise iii, chapter xxvi ‘delle volte, e varj modi di farle’, guarini dedicates the ‘osservazione nona’ and the ‘osservazione decima’ to banded vaults and flat banded vaults. this text is accompanied by two plates, ‘lastra xix’ and ‘lastra xx’ (figure 1), in which guarini graphically illustrates the principles set out in the observations through a double orthogonal projection representation. the architect describes textually the spatial genesis of this type of vault starting from a division of the space to be vaulted through wall-to-wall bands, with perpendicular or oblique direction, to create fields, which can then be filled with vaults of different types. references to this type of construction can be seen in the work of authors contemporary to guarini, as in the case of the vault of the sala di diana in the reggia di venaria by amedeo di castellamonte (1661 – 1662) [5], and in later periods between the end of the 17th century and the early 18th century by guarini’s collaborators (for example gian francesco baroncelli in palazzo barolo, 1692) or by other internationally renowned architects such as filippo juvarra in palazzo martini in cigala (1716). two centuries later, giovanni curioni conducted a study on this type of vaulting in bands, and with his geometria pratica (1868) [6] he straddles the gap between a purely theoretical contribution and a practical approach. the author, starting from guarini's approach, develops further considerations regarding the origin of the generating surface of the subdivisional bands of the space: “on the polygon to be covered with one of these times already insists the intrados of a time which, depending on the figure of the said polygon, can be a barrel, ‘a conca’, a pavilion, a barrel with heads of pavilion, ‘a schifo’, a dome” [7]. the subsequent operations are carried out by cutting with vertical planes on the reference surface and therefore do not seem to identify the construction for the independent arches. also, in the turin area we find the work of giovanni chevelley, where in his elementi di tecnica dell'architettura: materiali da costruzione e grandi strutture (1924) [8] collects local building knowledge in the field of vaulted structures. the description of the banded vaults take up the definition of curioni and in indicating some realizations emphasizes its spatial qualities and its variety of use in the atria of civil buildings and churches of the 17th and 18th centuries. 3. banded vault in archival drawings alongside the source of the treatises, we can also find the documentary source, which consists of the original guarini drawings, or the work of his collaborators. these documents, kept in the archivio di stato – sezione corte, have been studied directly by the author of this paper. they are firstly published and analysed in the archival regesto elaborated by augusta lange [9], for the 1968 conference on the figure of guarini. some of the drawings concern precisely the banded vaulted system applied to cover rooms in civil buildings (figure 2) [10]. the examples describe different solutions starting from the same tracing of the bands, perpendicular to the wall in the first case and oblique in the other, with the possibility for both to identify the arches as independent [6], [11]. the one shown as an example (figure 3), even in a hypothetical three-dimensional vision, reveals many similarities with the realizations surveyed in the fieldwork. this structure is characterized by the double dimensions of the bands; starting from the transverse arches, the longitudinal band is specularly supported, leaving the central field free for the insertion of further shapes and decorations, as described in his treatise. figure 1. banded vault in architettura civile. guarini 1737, treat. iii, plate xx. figure 2. g.guarini, study of a banded vault, 1680 c., torino, asto, azienda savoia-carignano, cat. 43, mazzo i, fasc. 6, n. 36; g.guarini, study of a composed and banded vault, 1680 c., torino, asto, azienda savoiacarignano, cat. 95, mazzo ii, fasc. 115, n. 23. figure 3. plan distribution and digital reconstruction of a guarini’s banded vault. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 3 4. banded vaults in turin baroque atria the applications of this particular type of vaulted structure find therefore after guarini a vast development in the city of turin, taking the maximum expression and variety in the atria of the baroque palaces of the city. the already mentioned works by castellamonte and juvarra seem to follow a typological current that touches many other authors in the city of turin. in their realizations, it is possible to identify those characters derived from guarini's thoughts but also to understand the peculiarities of a different creative process. the phase of identification and cataloguing of these vaulted structures was therefore fundamental. is it possible to firstly identify census maps in the research directed by cavallari murat and published in forma urbana e architettura nella torino barocca (1968) [12] and later in studies reported in the volume by spallone and vitali (2017). this structure was built between the 17th and 18th centuries in the areas of the second and third baroque extension of the city. in the variety of the baroque atria surveyed, three have been identified – at the moment of the research – as belonging to this category of vaulted structures with independent arches (table 1). the classification realized, focusing only on the vaulted structures analysed for this case study, wants to identify the maximum dimensions of the spaces and identifies the main axialities that compose the grid. this cataloguing, certainly expandable by extending the analysis to entire buildings, has provided a first overview of the spatiality created through this structure. the most common grids, in the whole context, were 3x3, with a smaller number of 3x4 and 3x5 used for rooms with a larger floor plan. the atria with vaulted structures generated from independent arches (figure 4) by filippo juvarra (in palazzo martina in cigala), gian giacomo plantery (in palazzo capris in cigliè), and gian francesco baroncelli (in palazzo barolo) are characterized by a varied spatial division (figure 5), maintaining the constant of the transversal arches as the basis for the creation of the subsequent bands and the vaults to complete the further fields created by the grids. for the recognition of this type of structure, the point clouds generated by the tls survey were therefore analysed. through a phase of identification of the characteristic sections by using the point cloud, we tried to compare this information to evaluate their conformation and geometric construction. the comparison is made between the sections that followed the same direction (in these cases only longitudinal or transversal, as there are no examples with diagonal axiality). if the variances identified could be considered within a geometrically valid level (but not metrically defined and evaluated case by case), we proceed with the classification of this part of the vaulted structure of the atria [13]. the example of the vaulted atria of via della consolata is displayed to explain the classification method and the following identification of the construction geometries (figure 6). in this case, the cross-sections lay the basis for the construction of the independent arches. after this step, the subsequent longitudinal arches are positioned using the transversal arches as a support base. this second level of arches, due to the conformation of the space, in its central field is straight to cover the whole space in figure 4. banded vault in baroque atria in turin. table 1. baroque atria under analysis. address width, depth, height (m) grid via della consolata, 3 7,66 × 10,37 × 6,75 3 × 5 via santa maria, 1 9,33 × 5,97 × 6,24 3 × 3 via delle orfane, 7 8,62 × 10,42 × 6,78 3 × 4 figure 5. plan distribution of banded vault in baroque atria in turin. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 4 length. solutions of this type of result to be very common in these vaults (the same system is used also in via delle orfane). the opportunities related to this type of structure allow construction in the other fields of autonomous vaults, as suggested by guarini’s indications [14] and that we are going to see in the selected case study. 5. the case study: palazzo capris di cigliè the two-dimensional drawings allow making an initial analysis of the architectural consistencies which, reported in the three-dimensional model, are linked to the formal conception derived from architectural literature and archival documentation. the case study now selected is palazzo capris di cigliè (1730) by gian giacomo plantery. 5.1. survey methodology and technical aspects the research carried out has had as main purpose the geometric and metrological analysis of the vaults of the atrium of this noble palace with the use of terrestrial laser scanner (tls). data obtained with this technology, led by prof. concepción lópez, generate easy-to-use models for later comparison with the geometric prototypes established in the literature [15], [16]. for this survey has been used the focus-130x3d scanner by faro. its low weight (5,2 kg) and small size (24 x 20 x 10 cm) facilitate its transportation and handling. the integrated longlasting battery (4 hours) ensures its use with no need to connect it to electricity during the entire/whole scanning session. it has a systematic error scope of ± 2 mm of distance in 25 meters which was acceptable for this study. it includes an integrated camera with 70 megapixel non-parallel colour overlay so the resulting point clouds a photographic realism very useful to understand it. the scans have been performed at a speed of 488.000 points/sec implied duration of each scan of approximately 8 minutes obtaining a good resolution of scanned. to process the scans, it was used autodesk recap pro® and the cloud registration was done automatically, without errors, using the tools of the software. after this step, the point cloud was imported in autodesk autocad® to execute the success data (figure 7 figure 8) [4]. 5.2. interpretation and modeling data obtained from the two-dimensional surveys with data from tls survey are used by restoring the symmetries and searching the elementary geometries in sections [17]. the method of analysis, developed in previous research [4], is based on guarini’s general indications for the composition of this type of vaulted system: delineated the bands starting from the plan – identified in this case also three-dimensionally –, we pass to the filling of the empty spaces with a small vault. the phases of geometric decomposition of the vaulted structure are shown through representation in isometric axonometry (figure 9). figure 6. graphic analysis and digital modeling of independent arches in the baroque atria in examination. figure 7. point cloud of atria portion in palazzo capris di cigliè. figure 8. point cloud of vault atria in palazzo capris di cigliè. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 5 through the point cloud sections are generated the most geometrically accurate curves (figure 9b) (looking for a curve generation with the lowest possible number of centres). these curves, belonging to independent arches, allow generating the first order of the vaulted structure (figure 9c). in this case, the central area sees the interruption of the longitudinal arches leaving full range to a single vaulted structure. this vault superimposed on the arched system, is sailed-shaped, as evidenced also by the decoration. along the major axis, the portion of the vault between the two arches follows the same geometry of these arches, recreating the idea of giant arches already seen in guarini’s drawings (figure 9d). the last fields to complete this vaulted structure are the angular ones. this time is independent to the main structure. cross vaults are set starting from the intersection of the arches and they develop with very low height (figure 9e). one of the most relevant features of this case is certainly the width of the discontinued bands, with specifications similar to those of the guarini design (figure 2 and figure 3) and leading to further evaluation of the creation of these bands and the internal areas created. 6. comparison between design geometric model and survey data the phase that accompanies the redrawing and digital reconstruction of this vault model goes hand in hand with the research of comparison between the model built and generated through point cloud and the geometric model of this surface [18]. this results in finding and analysing the characteristic sections of the vaulted structure [19], in this case the component of bands generated by independent arches. the replicable procedure [1] search in the section the geometric information useful for the construction of a theoretical model comparable to the original design idea; in this case study we extract fourteen sections from the point cloud (figure 10). the position of the sections in relation to the characteristics of the vaulted system led to their cataloguing, aimed at recognizing those which, by virtue of the hypothetical symmetry of the ideal model, should have the same shape. those of the main arch, aligned and superimposed with reference to the impost plane, have led to the recognition of axes, proportions, points of intersection and curves. among these, element by element, the polycentric curve (the latter with the smallest possible number of centers) was digitally constructed with autodesk autocad®, consistent with the techniques of construction of the centering (figure 11). at the end of this curve recognition phase, we moved on to the reconstruction of the theoretical three-dimensional model. these surfaces, modelled on rhinoceros® 6, were exported in the .e57 format to be then overlaid with the point cloud inside the cloudcompare open source software (figure 12). it is necessary to remember that the two digital products can never be perfectly overlapped: the point cloud brings with itself information about the consistencies in their current condition, the digital model, reconstructive of the design idea is generated through rigorously geometric references and restoration of the symmetries [3]. figure 9. graphic analysis and digital modeling of the vault in palazzo capris di cigliè. figure 10. scheme section of point cloud (red). figure 11. tracing method for directrices and comparison with the section of point cloud (red). acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 6 the graphical outputs realized through the software, can show the standard distance between point cloud and ideal model (figure 13). the higher distances between the two elaborations are identified in the more internal bands which, already in the extraction phase, were highlighted by a flexion near the keystone. 7. conclusions this paper outlines the methodological framework developed for the metric survey and the processing of knowledge data in the research on banded vaulted systems in turin baroque atria. the integration between the technique of metric survey by laser scanning with digital drawing and modelling involves, as we have seen, the definition of a workflow aimed at optimizing the use of data. indeed, adhering to the objectives of the research, twodimensional drawings must represent the atria in their current state, while three-dimensional modelling of the vaults is linked to the geometric reference models and aimed at the philological reconstruction of the design idea [18]. these procedures have given rise to new opportunities for research, such as the comparison (metric, but even more interesting, geometric) through the superimposition of ideal design models and point clouds. the deviation between the two digital products will not only reveal the deformations, structural failures, transformation that are part of the real-life of the building, but, above all, will provide new insights for the hypothesis on the necessary construction adaptions, and the centerings and laying techniques applied on building-site, that will contribute to the understanding of the relationship between design and construction. acknowledgement this contribution presents a development of the work and methodology elaborated within the international research project “nuevas tecnologías para el análisisis y conservación del patrimonio arquitectónico” coordinated by roberta spallone, assisted by marco vitali (department of architecture and design of the politecnico di torino). the project allowed the stay in turin, as visiting professor, of concepción lópez (department of graphic expression in architecture, universitat politecnica de valencia). the group includes, in addition to the author of this work: giulia bertola and francesca ronco (department of architecture and design, politecnico di torino). the research, favored by funding from the ministerio de ciencia, innovación y universidades of spain, is aimed at the analysis and interpretation of an architectural heritage characteristic of turin’s baroque production: the ‘a fasce’ vaults, locally named as ‘a fascioni’. references [1] r. spallone, m. vitali, volte stellari e planteriane negli atri barocchi in torino – star-shaped and planterian vaults in turin baroque atria, aracne, ariccia, 2017, isbn 978–88–255–0472–9. 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[9] a. lange, disegni e documenti di guarino guarini, in: guarino guarini e l’internazionalità del barocco. v. viale (editor). proc. of the international conference promoted by the accademia delle scienze di torino, turin, italy, 30 september – 5 october 1968, vol i, pp. 91-236. [10] f. natta, dai disegni autografi di guarini all’interpretazione digitale: modelli di volte a fasce, in: sistemi voltati complessi: geometria, disegno, costruzione complex vaulted systems: geometry, design, construction. r. spallone, m. vitali, a. giordano, j. calvo-lópez, c. bianchini, a. lópez-mozo, p. figure 12. tracing method for directrices and comparison with the section of point cloud (red). figure 13. tracing method for directrices and comparison with the section of point cloud (red). https://www.imeko.org/publications/tc4-archaeo-2020/imeko-tc4-metroarchaeo2020-014.pdf https://www.imeko.org/publications/tc4-archaeo-2020/imeko-tc4-metroarchaeo2020-014.pdf http://dx.doi.org/10.5194/isprs-archives-xliii-b2-2020-871-2020 acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 7 navarro-camallonga (editors). aracne, ariccia, 2020, isbn 97888-255-3053-7, pp. 213-228. [11] g. curioni, lavori generali di architettura civile, stradale ed idraulica e analisi dei loro prezzi. negro, torino, 1866. [12] a. cavallari murat, forma urbana ed architettura nella torino barocca: dalle premesse classiche alle conclusioni neoclassiche, utet, turin, 1968, 2 voll. 3 tome. [13] m. vitali, astrazione geometrica e modellazione tridimensionale per la definizione di una grammatica spaziale delle volte a fascioni/geometric abstraction and three-dimensional modeling for the definition of a spatial grammar of the ‘a fascioni’ vaults, in r. salerno (ed.), “rappresentazione/ materiale/ immateriale – drawing as (in)tangible representation”, gangemi, roma, 2018. [14] g. guarini, modo di misurare le fabriche. per gl'heredi gianelli, torino, 1674. [15] a. almagro gorbea, half a century documenting the architectural heritage with photogrammetry, ege revista de expresión gráfica en la edificación, 11, 2019, pp. 4-30. doi: 10.4995/ege.2019.12863 [16] michael e. auer, ram kalyan b. (eds), cyber physical systems and digital twins. proceedings of the 16th international conference on remote engineering and virtual instrumentation. springer, berlin, 2019. doi: 10.1007/978-3-030-23162-0 [17] f. stanco, s. battiato, g. gallo, digital imaging for cultural heritage preservation: analysis, restoration, and reconstruction of ancient artworks. crc press, 2017. doi: 10.1201/b11049 [18] a. samper., g. gonzález, b. herrera, determination of the geometric shape which best fits an architectural arch within each of the conical curve types and hyperbolic-cosine curve types: the case of palau güell by antoni gaudí, journal of cultural heritage, volume 25, 2017, pp. 56-64. doi: 10.1016/j.culher.2016.11.015 [19] e. lanzara, a. samper, b. herrera, point cloud segmentation and filtering to verify the geometric genesis of simple and composed vaults. int. arch. photogramm. remote sens. spatial inf. sci., xlii-2/w15, 2019, pp. 645-652. doi: 10.5194/isprs-archives-xlii-2-w15-645-2019 https://doi.org/10.4995/ege.2019.12863 https://doi.org/10.1007/978-3-030-23162-0 https://doi.org/10.1201/b11049 https://doi.org/10.1016/j.culher.2016.11.015 https://doi.org/10.5194/isprs-archives-xlii-2-w15-645-2019 calibration of capacitance diaphragm gauges with 1333 pa full scale by direct comparison to resonant silicon gauge and static expansion system acta imeko june 2014, volume 3, number 2, 48 – 53 www.imeko.org calibration of capacitance diaphragm gauges with 1333 pa full scale by direct comparison to resonant silicon gauge and static expansion system h. yoshida, e. komatsu, k. arai, m. kojima, h. akimichi, t. kobata national metrology institute of japan (nmij), national institute of advanced industrial science and technology (aist), aist central 3, 1-1-1, umezono, tsukuba, ibaraki, 305-8563, japan section: research paper keywords: pressure, vacuum, standard, calibration, capacitance diaphragm gauge, thermal transpiration effect citation: h. yoshida, e. komatsu, k. arai, m. kojima, h. akimichi, t. kobata, calibration of capacitance diaphragm gauges with 1333 pa full scale by direct comparison to resonant silicon gauge and static expansion system, acta imeko, vol. 3, no. 2, article 12, june 2014, identifier: imeko-acta-03 (2014)-02-12 editor: paolo carbone, university of perugia received april 30th, 2013; in final form april 30th, 2013; published june 2014 copyright: © 2014 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: (none reported) corresponding author: h. yoshida, e-mail: hajime-yoshida@aist.go.jp 1. introduction since pressure/vacuum gauges are calibrated at multiple pressure points, interpolation between these points is necessary for practical pressure measurements. in the case that the pressure points are calibrated by a single standard technique with good linearity, the interpolation has high reliability in general. at pressures lower than 103 pa, however, interpolation between pressure points from two different standard techniques is often required. in such a case, the validity of the interpolation should be confirmed. a capacitance diaphragm gauge with 1333 pa full scale (cdg-10torr) is used for precise pressure measurements in the range from 1 pa to 103 pa. for the calibration of the cdg-10torr at least four candidate techniques are available: pressure balance, static expansion system (ses) [1-5], force-balanced piston gauge [6,7], and oil manometer [5,8,9]. in this paper, the calibration results of the cdg-10torr based on two different standards are presented. one is a direct comparison to the resonant silicon gauge (rsg), which is calibrated by the pressure balance. the rsg is used as a reliable transfer gauge in the field of pressure and vacuum standards [10,11]. the other is the static expansion system [4]. the calibration results are compared and the validity of the interpolation is discussed. 2. experimental 2.1. apparatus figure 1 shows the schematic diagram of the static expansion system (ses) and the direct comparison system (dcs) for the calibration of vacuum gauges. these two systems are connected to each other through all metal valves. two resonant silicon gauges with 130 kpa full scale (absolute) are located on the ses (rsgse) as reference gauges. a capacitance diaphragm gauge with 133 pa full scale (cdg-1torr) is located between the ses and the dcs, and used as a reference gauge for the dcs. two capacitance abstract two capacitance diaphragm gauges (cdgs) with 1333 pa full scale, with a heated sensor head and an unheated one, respectively, were calibrated by three different methods; direct comparison to a resonant silicon gauge calibrated by a pressure balance, direct comparison to a cdg with 133 pa full scale calibrated by a static expansion method, and the static expansion method. the calibration results of the three calibration methods show good agreement within their claimed uncertainties. calibrated higher pressure points of cdgs by the pressure balance and lower pressure ones by the static expansion system are linearly interpolated within the calibrated uncertainty. here, compensation of the thermal transpiration effect is important when a cdg is used with a heated sensor head. acta imeko | www.imeko.org june 2014 | volume 3 | number 2 | 48 diaphragm gauges with 1333 pa full scale were used as test gauges. a high accuracy absolute type capacitance diaphragm gauge with a heated sensor head at a temperature of 45 oc (cdgh-10torr) was tested at both the ses and dcs. another capacitance diaphragm gauge with unheated sensor head (cdgn-10torr) was tested at the dcs only. n2 gas was used as a test gas. the calibration procedure of the ses is briefly summarized. the pumping system of the ses consists of turbo molecular pumps (tmp) and rotary pumps (rp). the background pressure before calibration is typically around 10-7 pa. the gas in the initial chamber cma was expanded to the chamber cmc or both chambers cmc and cmd depending on the calibration pressure range. to avoid changes in volume and temperature, a reference gauge to measure the initial pressure before expansion is not located on the cma. the initial pressure was measured by the rsgse located on the chamber cmb by closing both valves vl1 and vl3 and opening the valve vl2. after the initial pressure measurement, static expansion was performed by closing vl2, vl4, vl6, and vl8 and opening vl3 only and/or vl3, vl5 and vl7. the calibration pressure ranges are from 1 pa to 2000 pa and from 10-4 pa to 150 pa at cmc and cmd, respectively. details of the ses are given in ref [4]. the dcs was constructed based on the iso 3567 vacuum gauges – calibration by direct comparison with a reference gauge [12]. two reference gauges are located in the dcs. one is the resonant silicon gauge with 130 kpa full scale absolute (rsgdc). the other is a high accuracy absolute type capacitance diaphragm gauge with 133 pa full scale with a heated sensor head at the temperature of 45oc scale (cdg-1torr). the cdg-1torr is used as a reference gauge without detaching the sensor head from the chamber by controlling vl7 and vl8. the pumping system consists of a turbo molecular pump (200 l/s for n2) and a rotary pump. a static method is adopted for direct comparison. the valve on the tmp (vl9) was closed when the background pressure is lower than 10-4 pa, which is measured by an ionization gauge. the zero points of the cdgs and the rsgdc were measured every time before each calibration. the test gas was introduced to the cme by a computer-controlled mass flow controller (mfc) with a full scale of 10 sccm until the pressure in the cme has reached the target pressure. the test gauge was calibrated by comparing the reference gauges while the test pressure is kept constant for 300 s. 2.2. traceability chain in this study the traceability chain of the pressure in this study is summarized in figure 2. the rsgse and rsgdc were calibrated by the pressure balance from 5.0×103 pa to 1.3×105 pa. the rsgdc was sometimes calibrated by direct comparison to the rsgse to check its long-term stability. the cdg-1torr was calibrated by the ses in the chamber cmd from 0.1 pa to 130 pa. in the ses, both the expansion ratio and the initial pressure at the chamber cma, which are important parameters to determine the standard pressure, are measured by the rsgse. the cdgh-10torr with a heated sensor head was calibrated by three methods; (i) direct comparison to the rsgdc from 100 pa to 1300 pa, (ii) direct comparison to the cdg-1torr from 1 pa to 130 pa, and (iii) static expansion figure 1. schematic diagram of the static expansion system (ses) and the direct comparison system (dcs) for the calibration of vacuum gauges. figure 2. traceability chain of the pressure in this study. se and dc mean static expansion and direct comparison. two capacitance diaphragm gauges with 1333 pa full scale (cdg-10torr) were calibrated by (i) dc to rsgdc, (ii) dc to cdg-1torr, and (iii) se at cmc. n2 gas tmp tmp tmp tmp rp rp rp tmp rp mfccdg -1torr cdgh-10torr rsgse static expansion system (ses) direct comparison system (dcs) cmd (160 l) cmc (10 l) cma (0.2 l) cmb cme n2 gas vl1 vl2 vl3 vl4 vl5 vl6 vl7 vl8 cdgn-10torr rsgdc vl9 n2 gas tmp tmp tmp tmp rp rp rp tmp rp mfccdg -1torr cdgh-10torr rsgse static expansion system (ses) direct comparison system (dcs) cmd (160 l) cmc (10 l) cma (0.2 l) cmb cme n2 gas vl1 vl2 vl3 vl4 vl5 vl6 vl7 vl8 cdgn-10torr rsgdc vl9 acta imeko | www.imeko.org june 2014 | volume 3 | number 2 | 49 method at the chamber cmc from 1 pa to 1300 pa. the cdgn-10torr with an unheated sensor head was calibrated by using methods (i) and (ii). the direct comparison with the rsgdc was performed by extrapolating the calibration results obtained from 5.0×103 pa to 1.3×105 pa. the procedure of the extrapolation is detailed in section 3.1. 2.3. compensation of thermal transpiration effect the apparent change in the sensitivity of the cdg owing to the thermal transpiration effect was compensated using the takaishi-sensui (t-s) equation: 𝑝1 𝑝2 = 𝑌 + �𝑇1 𝑇2⁄ 𝑌 + 1 , 𝑇1 < 𝑇2 𝑌 = a𝑋2 + 𝐵𝑋 + 𝐶√𝑋, 𝑋 = 𝑑𝑝2 (1) a = a𝑇−2, 𝐵 = 𝑏𝑇−1, 𝐶 = 𝑐𝑇−1/2, 𝑇 = (𝑇1 + 𝑇2) 2⁄ p1 and p2 are the pressures in the vacuum chamber and in the sensor head (capsule) of cdg that corresponds to the pressure indication of cdg, respectively. t1 and t2 are the temperatures of the vacuum chamber and the sensor head of cdg, d is the inner diameter of the connecting tube, and a, b, and c are parameters depending on the gas species to be measured. t1 was measured befor every calibration. the values of t2 and d were assumed to be 45oc (318.15 k) and 4.76 mm, respectively. parameters a, b, and c were equal to 12×105 deg2 mmhg-2 mm-2 (6.75×107 k2 pa-2 m-2), 10×102 deg mmhg-1 mm-1 (7.50×103 k pa-1 m-1) and 14 deg1/2 mmhg-1/2 mm-1/2 (38.3 k1/2 pa-1/2 m-1/2), respectively. the validities of the t-s equation and these parameters are discussed in [15]. 3. results 3.1. calibration results of the reference resonant silicon gauges (rsg) calibration results of the reference rsgdc and the rsgse by a pressure balance are shown in figure 3. the vertical axis is the deviation of the calibrated standard pressure (ps) from the pressure indication (pi) of the rsgs. the sensitivity coefficient s of the rsgs is defined as in equation (2) for a pressure range down to 100 pa, s = (pi －pi0)/ ps = ∆pi / ps, (2) where pi0 is the pressure indication at the background pressure, in other words at zero point, and ∆pi is the difference between pi0 and pi. the s(rsgdc) is plotted in figure 4 with a logarithmic scale of the horizontal axis. the s(rsgdc) has a constant value of 0.999987 ± 0.000027. the standard pressure (prsg-dc) in the dcs from 100 pa to 1300 pa is determined by equation (3), prsg-dc = ∆pi / s (rsgdc). (3) the calibration uncertainty u(prsg-dc) with a confidence level of 95% (k=2) is estimated by equation (4): u(prsg-dc) [pa] = -1.3×10-11 ∆pi2 + 5.0×10-6 ∆pi + 3.0, (4) which is the best fitting curve between ∆pi of the rsgdc and its expanded uncertainty. that means the relative expanded uncertainty of prsg-dc from 100 pa to 1300 pa is in the range from 3.0% to 0.23%. 3.2. calibration result of the reference capacitance diaphragm gauge with 133 pa full scale (cdg-1torr) a calibration result of reference cdg-1torr by ses is shown in figure 5 (a). the vertical axis is the s of cdg-1torr, which is similarly calculated by eq. (2). the s(cdg-1torr) increases with decreasing the pressure by thermal transpiration effect because cdg-1torr has a heated sensor head at the temperature of 45 oc [10,13-15]. figure 5 (b) shows the compensated s(cdg-1torr) by eq. (1) [15,16]. the s(cdg1torr) after the compensation by t-s equation has a constant value of 1.0081 ± 0.0014. the relative expanded uncertainty of the calibration from 0.1 pa to 130 pa is in the range from 2.8 % to 0.33 % [4]. 3.3. calibration results of two capacitance diaphragm gauges with 1333 pa full scale (cdgh-10torr and cdgn-10torr) the cdgh-10torr was calibrated using three methods: (i) direct comparison to the rsgdc from 100 pa to 1300 pa, (ii) direct comparison to the cdg-1torr from 1 pa to 130 pa, and (iii) the static expansion method from 1 pa to 1300 pa. table 1 shows the uncertainty budget of (i) and (ii). the expanded uncertainty of (iii) is in the range from 1.0% to 0.26% [4]. as shown in figure 6(a), the three calibration results for the cdgh-10torr are in good agreement within their required uncertainties. the sensitivity of the cdgh-10torr, s(cdgh10torr), also increases with decreasing pressure by the thermal transpiration effect. after compensation using eq. (1), the s(cdgh-10torr) also has a linear characteristic within ± 0.2% as shown in figure 6(b). figure 3. calibration results ofthe rsgse and rsgdc. figure 4. sensitivity s of the rsgdc. acta imeko | www.imeko.org june 2014 | volume 3 | number 2 | 50 table 1. the uncertainty budget of the calibration results of the cdg h -10torr by direct comparison. the rsg dc and cdg-1torr are used as a reference gauge depending on the pressure range. table 2. the uncertainty budget of the calibration results of the cdg n -10torr by direct comparison. the rsg dc and cdg-1torr are used as a reference gauge depending on the pressure range. acta imeko | www.imeko.org june 2014 | volume 3 | number 2 | 51 4. discussion on the reference gauge for direct comparison calibration by direct comparison is widely used by many users. in the case that the rsg with 130 kpa full scale (absolute) is used as a reference gauge, the lowest calibration pressure may be limited by several hundred pa if the calibration uncertainty is required to be within several %. the cdgs with 133 pa or 1333 pa full scale are useful as a reference gauge for pressures below 100 pa. in that case, however, the thermal transpiration effect should be compensated if the cdg with heated sensor head is used. a wide calibration pressure range is realized by combining the rsg and the cdg as reference gauges and evaluating the uncertainty arising from the nonlinearity of the sensitivity, the correction of the thermal transpiration effect, the resolution, the influence of temperature, attitude, and so on. 5. conclusion two capacitance diaphragm gauges with 1333 pa full scale were calibrated by the following three methods: (i) direct comparison to a resonant silicon gauge with 130 kpa full scale absolute from 100 pa to 1300 pa, (ii) direct comparison to a capacitance diaphragm gauge with 133 pa full scale from 1 pa to 130 pa, and (iii) static expansion method from 1 pa to 1300 pa. the results by these three methods show good agreement within their claimed uncertainties, which means these calibration methods and the uncertainty analyses are validated. calibrated higher pressure points of cdgs by the pressure balance and lower pressure ones by the static expansion system are linearly interpolated within the calibrated uncertainty. here, compensation of the thermal transpiration effect is important when a cdg is used with a heated sensor head. references [1] m. bergoglio, a. calcatelli, l. marzola, g. rumiano, “primary pressure measurements down to 10−6 pa”, vacuum, vol 38, pp 887-891, 1988. [2] w. jitschin., j. k. migwi, g. grosse, “pressures in the high and medium vacuum range generated by a series expansion standard”, vacuum, 40, pp 293-304, 1990. [3] k. jousten k., g. rupschus, “the uncertainties of calibration pressures at ptb”, vacuum, 44, pp 569-572 1993. [4] h. akimichi, e. komatsu, k. arai, m. hirata, in proc. of the 44th international conference on instrumentation, control and information technology (sice2005), pp 2145-2148, 2005. [5] s.s. hong, y. h. shin, and k. h. chung, “measurement uncertainties for vacuum standards at korea research institute of standards and science”, j. vac. sci. technol. a 24(5), pp 1831-1838, 2006. [6] c. g. rendle and h. rosenberg, “new absolute pressure standard in the range 1 pa to 7 kpa”, metrologia, 36, pp613-615, 1999. [7] th bock, h ahrendt and k jousten, “reduction of the uncertainty of the ptb vacuum pressure scale by a new large area non-rotating piston gauge”, metrologia, 46, pp 389–396, 2009. [8] p. l. m. heydemann, c. r. tilford, r. w. hyland, “ultrasonic manometers for low and medium vacua under development at nbs”, j. vac. sci. technol., 14 (1), pp 597-605, 1977. [9] c. r. tilford, a. p. miller, s. lu, “a new low-range absolute pressure standard”, in proc. 1998 ncsl workshop and symposium, national conference of standards laboratories, pp 245-256, 1998. figure 5. sensitivity of the cdg-1torr before (a) and after (b) compensation of the thermal transpiration effect by the takaishi-sensui equation. figure 6. sensitivity of the cdgh-10torr with a heated sensor head at 45oc before (a) and after (b) compensation of the thermal transpiration effect by the takaishi-sensui equation [15,16]. figure 7. sensitivity of the cdgn-10torr with an unheated sensor head. acta imeko | www.imeko.org june 2014 | volume 3 | number 2 | 52 [10] a. p. miller, “measurement performance of high-accuracy lowpressure transducers”, metrologia, 36, pp 617-621, 1999. [11] j. h. hendricks and a. p. miller, “development of a new highstability transfer standard based on resonant silicon gauges for the range 100 pa to 130 kpa”, metrologia, 44, pp 171-176, 2007. [12] international organization for standardization, iso 3567 vacuum gauges – calibration by direct comparison with a reference gauge, 2011. [13] k. f. poulter, m-j rodgers, p. j. nash, t. j. thompson, m. p. perkinet, “thermal transpiration correction in capacitance manometers”, vacuum, 33, pp 311-316, 1983. [14] j. setina, “new approach to corrections for thermal transpiration effects in capacitance diaphragm gauges”, metrologia, 36 pp 623-626, 1999. [15] h. yoshida, e. komatsu, k. arai, m. hirata, and h. akimichi, “compensation of thermal transpiration effect for pressure measurements by capacitance diaphragm gauge”, j. vac. soc. of jpn., vol. 53, pp. 686-691, 2010. [16] t. takaishi, y. sensui, “thermal transpiration effect of hydrogen, rare gases and methane”, trans. faraday soc., 59, pp 2503-2514, 1963. acta imeko | www.imeko.org june 2014 | volume 3 | number 2 | 53 12-29-1-pb acta imeko december 2011, issue 0, 2 − 3 www.imeko.org acta imeko | www.imeko.org december 2011 | issue 0 | 1 instructions for authors paul p. l. regtien 1 1 measurement science consultancy, julia culpstraat 66, 7558jb hengelo, the netherlands keywords: journal; template; imeko; microsoft word citation: paul p.l. regtien, instructions for authors, acta imeko, no. 0, december 2011, pp. 2-3, identifier:10.3345/acta.imeko.4530 editor: paul regtien, measurement science consultancy, the netherlands received december 28, 2011; in final form december 29, 2011; published december 30, 2011 copyright: © 2011 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction 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collaboration; virtual reality; collision prediction citation: imre paniti, jános nacsa, péter kovács, dávid szűr, human-robot collision predictor for flexible assembly, acta imeko, vol. 10, no. 3, article 12, september 2021, identifier: imeko-acta-10 (2021)-03-12 section editor: bálint kiss, budapest university of technology and economics, hungary received february 15, 2021; in final form august 16, 2021; published september 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was supported by the tkp2020-nka-14 grant and by the h2020 project epic grant no. 739592. corresponding author: imre paniti, e-mail: imre.paniti@sztaki.hu 1. introduction according to the international federation of robotics 2019 report, the average robot density in the manufacturing industry has grown to a new global record of 113 units per 10,000 employees [1]. although the automation of smalland mediumsized enterprises (smes) is supported within the european union according to the european commission’s digital economy and society index report 2019 [2], the share of large enterprises that use industrial or service robots is four times higher than that of smes, and the use of robots varies widely with company size. one of the most commonly asked questions in the semirobotised industry is how to make production more efficient, which is related to a study [3], where robots in an assembly operation could reduce the idle time of an operator by 85 %. therefore, using collaborative robots (cobots) in a factory for assembly tasks could lead to greater efficiency, which means shorter production times. this statement can also be useful for the assembly of different products or product families, which requires a set of different fixtures or reconfigurable fixtures, such as those based on the parallel kinematic machine in [4] or the fixed but flexibly useable gripper presented in this article. however, the problem is that despite well-defined task sequences, the changeover from one product to another in a collaborative operation could lead to human failures and, consequently, to collisions with the cobot due to the previous habitual sequence of actions. by definition, a cobot has to operate with strict safety installations (protective stop execution when a certain force in a collision is reached), as outlined in iso/ts 15066:2016 [5], iso 10218‑1:2011 [6] and iso 10218‑2:2011 [7], but these protective stops could cause a significant cumulative delay in production. this depends largely on how the robot program has been written, i.e. whether operations can be continued after a protective stop. review articles such as those of hentout et al. [8] and zacharaki et al. [9] present solutions for pre-collision approaches in the frame of human–robot interaction (hri). pre-collision control methods, referred to as ‘prevention’ methods, are techniques intended to ensure safety during hri by monitoring either the human, the robot or both and modifying robot control parameters prior to incidence of collision or contact [9]. precollision approaches can be distinguished between reactive control strategies, proprioceptive sensor-based strategies and abstract the performance of human–robot collaboration can be improved in some assembly tasks when a robot emulates the effective coordination behaviours observed in human teams. however, this close collaboration could cause collisions, resulting in delays in the initial scheduling. besides the commonly used acoustic or visual signals, vibrations from a mobile device can be used to communicate the intention of a collaborative robot (cobot). in this paper, the communication time of a virtual reality and depth camera-based system is presented in which vibration signals are used to alert the user of a probable collision with a ur5 cobot. preliminary tests are carried out on human reaction time and network communication time measurements to achieve an initial picture of the collision predictor system’s performance. experimental tests are also presented in an assembly task with a three-finger gripper that functions as a flexible assembly device. mailto:imre.paniti@sztaki.hu acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 73 exteroceptive sensor-based control [8]. however, these approaches are all manifested in robot control parameter modification rather than in operator warnings. both the above studies refer to the work of carlos morato et al. [10], who presented a similar solution by creating a framework using multiple kinects to generate a 3d model with bounding spheres for human movements in real time. the proposed framework calculates human–robot interference in a 3d space with a physics-based simulation engine. the deficiency of the study is the pre-collision strategy for safe human–robot collaboration because this results in the complete stoppage of the robot. this is indeed a safe protocol, as it reduces the production break time, but it does not eliminate it completely. the aim of this paper is to highlight the importance of a new pre-collision strategy that does not modify the trajectories but relies fully on the warning of the operator (using a non-safetycritical system), especially when flexible/reconfigurable fixtures are used. section 2 provides an overview of standards and definitions related to robotic and cobotic systems, especially in relation to protective separation distance, which is crucial for the proposed solution. section 3 presents an experimental environment and use cases in which the proposed solution can be used. section 4 describes the new pre-collision approach and its system elements in detail, together with some communication measurement results to demonstrate the feasibility of the solution. finally, section 5 presents a summary with conclusions. 2. standards and definitions for cobot use in general, when using a robotic arm with a gripper the 2006/42/ec machinery directive [11] and the 2014/35/eu low voltage directive [12], together with iso/ts 15066:2016 [5] and 16 standards, have to be considered [13]. these are detailed in table 1. according to iso 10218‑1:2011 [6], a collaborative workspace is a space within the operating space where the robot system (including the workpiece) and a human can perform tasks concurrently during production operations, and a collaborative operation is a state in which a purposely designed robot system and an operator work within a collaborative workspace. according to iso/ts 15066:2016 [5], collaborative operations may include one or more of the following methods: • a safety-rated monitored stop, • hand guiding, • speed and separation monitoring, • power and force limiting. in powerand force-limiting operations, physical contact between the robot system (including the workpiece) and an operator can occur either intentionally or unintentionally. power and force-limited collaborative operations require robot systems specifically designed for this particular type of operation using built in measurement units. according to iso/ts 15066 [5], risk reduction is achieved, either through inherently safe processes in the robot or through a safety-related control system, by keeping hazards associated with the robot system below threshold limit values, which are determined during the risk assessment. if an operator wants to maintain a safe distance in a collaborative operation, iso/ts 15066:2016 robots and robotic devices collaborative robots (clause 5.5.4: speed and separation monitoring) [5], en iso 13850:2015 [19], en iso 13855:2010 [15], en iec 60204-1:2018 [20] and en iec 62046:2018 [26] should be applied together with the following regulations and standards: directive 2006/42/ec [11], en iso 10218-1:2011 [6] and en iso 10218-2:2011 [7]. in addition, en iso 12100:2010: safety of machinery general principles for design risk assessment and risk reduction [18] should be considered. in speed and separation monitoring, the protective separation distance is the shortest permissible distance between any moving hazardous part of the robot system and any human in the collaborative workspace, and this value can be fixed or variable. during automatic operations, the hazardous parts of the robot system should never get closer to the operator than the protective separation distance, which is calculated based on the concepts used to create the minimum distance formula in iso 13855:2010 [15]. the protective separation distance sp can be described by formula (1): 𝑆p(𝑡0) = 𝑆h + 𝑆r + 𝑆s + 𝐶 + 𝑍d + 𝑍r, (1) where sp(t0) is the protective separation distance at time t0 (present or current time); sh is the contribution to the protective separation distance attributable to the operator’s change in location; sr is the contribution to the protective separation distance attributable to the robot system’s reaction time; ss is the contribution to the protective separation distance due to the robot system’s stopping distance; c is the intrusion distance, as defined in iso 13855, which is the distance that a part of the body can intrude into the sensing field before it is detected; zd is the position uncertainty of the operator in the collaborative workspace as measured by the presence sensing device resulting from the sensing system measurement tolerance; and zr is the position uncertainty of the robot system, resulting from the accuracy of the robot position measurement system [5]. based on this, the authors propose to extend the protective separation distance (1) with an extra distance based on the communication time of a pre-collision system (spc) and with a contribution to the protective separation distance attributable to table 1. standards in manufacturing when using a robotic arm with a gripper. standard ref. en iso 10218-1:2011 [6] en iso 10218-2:2011 [7] iso/tr 20218-1:2018 [14] en iso 13855:2010 [15] en iso 13849-1:2015 [16] en iso 13849-2:2012 [17] en iso 12100:2010 [18] en iso 13850:2015 [19] en iec 60204-1:2018 [20] en iec 62061:2005 [21] en iso 11161:2007 [22] en iso 13854:2017 [23] en iso 13857:2019 [24] en iso 14118:2017 [25] en iec 62046:2018 [26] en iso 13851:2019 [27] acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 74 the robot operator’s reaction time (sort) to avoid speed reductions or protective stops. this would result in a modified protective separation distance (sp*): 𝑆p ∗ = 𝑆p + 𝑆pc + 𝑆ort . (2) however, the proposed system in this paper is, as has already been mentioned, an extra non-safety certified solution. the purpose of the presented measurements in this paper is to determine the above-mentioned time parameters (communication time and reaction time) of the additional distances (spc and sort) in this specific environment. 3. experimental environment and use cases robots are usually moved on prespecified trajectories that are defined in the robot’s program, and, in most cases, a new task involves starting a new robot program. another method is to move the high-level robot control from the robot to a computer, and the robot then continuously receives the required movements and other actions via a stream. in this case, the robot runs a general-purpose program or framework that interprets and executes the external instructions received. in this scenario, the framework is called ursztaki, developed by the sztaki research laboratory for engineering and management intelligence. ursztaki has three kinds of instructions: (a) basic instructions that constitute the robot's programming language, (b) instructions for the robot add-ons (e.g. the gripper and force sensor) integrated into the robot language by the accessory suppliers and (c) frequently used, more complex task instructions (e.g. putting down or picking up an object when the table distance is unknown). the third type of instruction constitute the real features of ursztaki. it should also be mentioned that the expansion of the ur robot's functions and language is possible with the help of socalled urcaps (which is a platform where users, distributors and integrators can demonstrate accessories that run successfully in ur robot applications [28]), and currently, ursztaki can also be installed as an urcap. the experimental layout consists of a ur10 robot with a force sensor and a two-finger gripper. the environment was designed to support different assembly tasks, either fully robotised or collaborative. to equip partly or even fully different components, universal mounting technology was required instead of special fixtures. another gripper (with three fingers) was used that allowed a wide variety of fixings. all three fingers could be moved independently of the selected adaptive gripper fixed to the robot worktable (figure 3). the three-finger gripper from robotiq [29] has four different modes for operating the fingers (figure 2). in the ‘pinch’ mode on the top left side of figure 1, the gripper acts as a two-finger model, and the fingers move closely together to be able to pick up small objects. the next mode is the ‘scissor’ mode in which the closing–opening ability of the gripper is used to pick up an object. in the third ‘wide’ mode, the fingers are fan-like, and they provide a firm wide grip for longer objects. for the leftmost ‘normal’ grip, the three fingers move in parallel and, depending on the relative position of the object, the fingertips also turn for greater precision in ‘normal’ and ‘wide’ mode. this is the encompassing grip. from the software point of view, both grippers can be directly programmed from the robot's program code. despite the fact that both grippers are from the same manufacturer, which could make the development easier, the commands of one of the grippers had to be modified to avoid conflict between the individual instructions. a typical scenario is that the robotic arm picks up and transfers a part to the fixed gripper, which grabs it, and after that, another part is placed or pressed with the desired force by the robotic arm on the part fixed by the immobile gripper. there are some detailed tasks, such as the insertion of a spring into a housing, which have to be performed by the human operator. in this environment, it is also possible for the robot to hold a screwdriver and fasten the assembled parts with screws at the set torque limit (figure 3 and figure 4). the prototype was designed specifically for the previously shown push-button element. however, it can be easily redesigned for another part, or a universal piece can be made to support different types of product assembly. following the parallel movement of the fingers, a form-fitting shape is created that holds the part motionless while the required actions are carried out. because the holder is connected to the fingertips, slippage is also prevented in cases where the pressing figure 1. demonstration environment. figure 2. the four different modes of the three-finger robotiq gripper [29]. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 75 force applied is too great or an inappropriate human movement occurs. the proposed solution with the immobile three-finger gripper satisfies the requirements of a flexible fixture for certain parts. in this scenario, human–robot collision problems might occur if the human operator forgets the predefined assembly task sequence when beginning the assembly of a new product, reaches for an assembly part and the hand trajectory intersects that of the robot. to demonstrate a flexible assembly with the three-finger gripper, an additional application was developed in which both grippers were used to perform the assembly task, requiring human intervention at certain points of the assembly process at the same time. in the task, a didactic element, which had been packaged with a transparent plastic lid and a metal base, were pushed together at the beginning of the operation (presumably, this packaging material came from the supplier). the operation steps of the complete assembly were the following: 1. pick up the packaging material with the robot arm and fix the base with the three-finger gripper. 2. remove the lid from the base and put it down (figure 5). 3. pick up the didactic element and place it onto the metal base. 4. put the plastic lid back on the base. 5. fix the packed object, release the three-finger gripper and put it back in its starting position. inserting the didactic element is the bottleneck in the assembly process. normally, the robot finds the hole with a small spiral movement using force sensing. since the gap between the meter and the base is narrow, this operation is not always successful (see figure 6), in which case, human intervention is possible or necessary to avoid any wastage. in some instances, the next operation (putting the lid back) corrected the skewed didactic element, and it slipped into the base. however, the success of a process should not be based on coincidence, and this is when a collision predictor system can be very useful. an easy movement by the operator can prevent wastage, thereby reducing costs. it is a simple operation sequence, but because of the positioning errors, human intervention may be required during two of the steps. figure 3. illustration of the robotised screwdriving of a push-button element in which the spring has to be inserted manually. figure 4. illustration of the robotised screwdriving of a ball valve element. figure 5. illustration of the second step. figure 6. illustration of the failed insertion of the didactic element. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 76 4. pre-collision approach as a predictor in order to avoid collisions with the robot, either the robot trajectory has to be modified in real time (which might cause additional production time, something companies want to avoid) or the human operator has to be warned with a pre-defined understandable signal so the human movement can be modified in time. the warning signal can be given to the operator in several ways: visual, acoustic or tactile. in this paper, the latter has been developed as part of a predictor of human–robot collision (prehuroco) framework. the subject of the prediction in this case is the predetermined movement of the robot, which can be recorded and will occur after a certain time, so a similar framework had to be created to that described in [10]. however, instead of a digital twin of the robot (real-time 3d visualisation of the robot), a pre-played robot model motion was used together with the 3d skeleton model of the operator. the virtual collisions of the two models were used as trigger signals to warn the operator before a real collision. 4.1. requirement analysis the following features were needed for the candidate software library, based on the requirement analysis of prehuroco: 1) fully open source: the system must fulfil all the security requirements of a real manufacturing system; therefore, complete control of the source code is obligatory. 2) modular: the system should be divided into various software components, so the candidate software library must support responsibility encapsulation. 3) distributed: in a manufacturing system, many computers and internet-of-things (iot) devices can be connected; therefore, the prehuroco software components must have the ability to run on different computers or iot devices. 4) cross platform: as the distribution requirement is for many computers and devices with different operating systems to be connected, the candidate framework should be cross platform. 5) programming language variability: as the distribution and cross-platform requirements are for different devices and computer operating systems in manufacturing scenarios, the candidate software library should support different application programming interfaces (apis). 6) scalability: prehuroco software components should be developed independently of whether they run on the same computer or not. in terms of performance, the software components should be be easily put together in one machine or one application and easily distributed. 7) rapid prototyping: the candidate framework should provide examples or even pre-made components that can be improved during prehuroco implementation because the proposed system should deal with • rigid-body simulation, • visualisation (including vr or ar), • real-time 3d scanning, • an x3d model format and • various communication protocols. unity engine [30] and unreal engine [31] are well-known crossplatform game engines. apertusvr [32] is a software and hardware vendor-free, open-source software library. it offers a no-vendor-lock-in approach for integrating vr technologies into industrial software systems. the comparison of the candidate frameworks considering the requirements is summarised in table 2. based on the prehuroco requirement analysis, the apertusvr software library was chosen for implementing the system. with the help of this software library, a distributed software ecosystem was created via the intranet/internet, which was divided into two main parts, the core and plugins. the core system is responsible for the internet/intranet communication between the elements of the distributed software ecosystem, and it synchronises the information between them during the session. the plugin mechanism makes it possible to extend the capability of any solution created by the apertusvr library. plugins can access and manipulate the information within the core system. 4.2. explanation of the prehuroco system the system is distributed into five major responsibilities: 1) 3d scanning of the human operator, 2) streaming the joint angles of the robot, 3) collision detection between the human and the robot, 4) alerting the human to the possible collision and 5) visualising the whole scenario. in the present study, these responsibilities were implemented with the help of the apertusvr library, and each responsibility was encapsulated into six plugins [33]: the collision detection plugin, the visualisation plugin, the kinect plugin, the websocket server plugin, the x3d loader plugin and the nodejs plugin. the seventh element was a websocket client, which was implemented in the form of an html site using the jquery javascript library and the vibration api method [34] for mobile phones; but for more comfortable use, the websocket client could also run on a smart watch. figure 7 shows the realised system with the connections and applied protocols in an experimental set up with an ur5 robot. collision detection plugin [35]: this plugin was created based on the pre-made apertusvr ‘bulletphysics’ plugin. previously, this plugin had been able to run rigid-body simulations, but collision events were not created during these simulations. the apertusvr rigid-body abstraction was enhanced by the functionality of collision events. visualisation plugin [36]: this plugin was used as-is from the apertusvr repository for visualisation purposes. kinect plugin [37]: this plugin was created based on the premade apertusvr ‘kinect’ plugin. previously, this plugin had table 2. comparison of different frameworks in relation to the prehuroco requirements. requirement unity engine unreal engine apertusvr open source partially yes yes modular yes yes yes distributed partially corner case yes cross platform yes partially partially prog. lang. variability partially corner case yes scalability partially partially yes rapid prototyping yes yes yes acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 77 been able to create the skeleton of the tracked human or even its point cloud, but rigid bodies were not created. for collision detection, rigid bodies are mandatory; therefore, rigid bodies were created based on the geometries of the human skeletons. websocket server plugin [38]: this plugin was created based on the pre-made apertusvr ‘websocketserver’ plugin. previously, this plugin had been able to forward all events developed in the core. for collision detection, only the collision event of the rigid bodies is necessary. during the implementation of that plugin, a filter feature was added to forward only the desired event into the websocket connection. x3d loader plugin [39]: this plugin was created based on the pre-made apertusvr ‘x3dloader’ plugin. previously, this plugin had been able to parse the x3d format and create only the geometries of the robot. for collision detection, rigid bodies are mandatory; therefore, rigid bodies were created based on the parsed geometries. nodejs plugin [40]: this plugin was used as-is from the apertusvr repository and allows a web server to be run to receive the joint angle of the ur5 robot via http requests. in the prehuroco system, these plugins are encapsulated in different applications. these different applications can be run on different computers to distribute the computational power and achieve real-time collision prediction. as the diagram in figure 7 shows, these applications communicate through internet/intranet communication via different protocols. the collision detection application has to be run on a highperformance computing (hpc) server to process the virtual collisions in real time. the kinect application can run on a dedicated computer for the kinect device or on the same computer that calculates the virtual collisions. the x3dloader and the nodejs plugins are integrated into one application and can run on the dedicated computer for the ur5 robot. the websocket server application can also be run on a different computer to ensure security and locality requirements. the joint positions are stored in a jsonlist file, which is generated by executing the whole robot program. during the execution, the joint positions are ‘grabbed’ and saved with a given frequency. the speed of the simulation is equal to the speed of the robot movement, and the ‘forecast’ can be determined by the delay between the simulation starting time and the real robot execution start time. 4.3. modified prehuroco system and measurements during the validation process, the prehuroco system was reconfigured to eliminate any unnecessary delay in the system. the reconfiguration process was achieved by the apertusvr configuration feature; thus, all the plugins were reused without any modification. the previously distributed prehuroco system was therefore easily reconfigured to form a single application (figure 8) and was able to run on a single computer. the elimination of unnecessary network connections/delays was a crucial step in avoiding any latency in the system. through this approach, the human–robot-collision calculation time and the human-operator reaction time were measured precisely. timestamps were buffered before and after the collision events, the websocket message transmission/receipt and the human operator pressing the button on a bluetooth keyboard. the proposed framework was tested on two local network topologies. in the first case, the calculations were divided into a cloud-service-based computer (with four virtual cpus, 8 gb ram, running a windows 10 operating system) and an hpc server (ideum with intel i7-8700, rtx 2080 8 gb gddr6 nvidia graphics card, dual 250 gb nvme m.2 ssd, 32 gb 2400 mhz ddr4 ram, running a windows 10 operating system), and the collision events were delivered to the websocket client with significant delay. by running all apertusvr plugins on the ideum and sending only the collision events via a wireless lan connection (2.4 ghz wi-fi) the user experience was quasi real-time. figure 9 shows a virtual collision test running on the ideum (hpc server) with the skeleton model of a single operator (1), virtual ur5 robot movement simulation (2), a real robot (3), a kinect sensor (4) and a mobile phone (5) with an android operating system running the websocket client to vibrate the device. the 3d scene was visualised with a top camera view, but arbitrary camera views are possible. to avoid the execution of large javascript files locally on the android mobile phone, external calls to cdn.jsdelivr.net and code.jquery.com were used. the ping time to these services were measured with an android application (pingtools version 4.52), which gave 9 ms and 30 ms as the average from three measurements, respectively. figure 7. prehuroco system elements and connections with the applied protocols. figure 8. reconfigured prehuroco system. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 78 the second network topology was used to measure the communication time of the system with five more people of different genders and ages (see figure 10). the reaction time of each operator was measured using an android application (reaction test version 1.3), which vibrates at randomised short time intervals (a couple of seconds) and calculates the average of five measurements. the average calculation time of the human–robot model collision until the http-request send was 98 ms, the average time from the http-request send to the keypress event was 1,355 ms and the average reaction time was 449 ms. each virtual collision with keyboard pressing as confirmation was tested three times. according to a bluetooth keyboard performance test, ‘microsoft delays in a non-interference test environment by approximately 40 to 200 milliseconds’ [41], so the calculation time for the human–robot collision together with the network communication time would be less than 1 s using this prehuroco configuration. however, by using raknet instead of http requests the performance of the system can be significantly improved. raknet communication time measurements from 223 collision events showed that only 36.52 ms was needed on average. furthermore, it is worth mentioning that with 5g communication an average two-way latency of 1.26 ± 0.01 ms would be possible, as noted in [42]. the kinect plugin creates a simplified skeleton model from the human operator, which needs improvement. an anthropomorphic skeleton model or voxelisation could be a solution in the future. it should be highlighted that the communication time increased by the human reaction time should not exceed the δt time between the pre-played simulated motion and the actual motion of the robot. a jsonlist file of the simulated ur5 robot movement is provided in [43]. 5. conclusion in this paper, a commercially available gripper as a flexible fixture for assembly and a new pre-collision approach as a predictor for human–robot collaboration were presented. the proposed framework was realised with the help of a modular, distributed, open-source cross platform (apertusvr) with different programming api support and scalability solutions. seven interconnected system modules were developed with the goal of monitoring the movement of the human operator in 3d space, calculating collisions with a virtual robot (with preplayed movements rather than the movement of a real robot) and alerting the human operator before a real collision could occur. successful virtual collision tests with six candidates showed that the operator received the warning signal immediately (under 1 s) in the form of a mobile-device vibration to modify the planned movement. in some cases, real-time path planning is required, especially in a changing environment, such as when the position of the workpiece to be gripped is variable (e.g. litter picking). in a collaborative environment, this is a serious security challenge that the whole system has to manage. the static parts of the environment can be checked regularly through collision detection, but the presence of the human means that ‘simple’ collision detection is not sufficient. this was the main reason for the current research and development presented in this paper. acknowledgement this research has been supported by the ‘thematic excellence program – national challenges subprogram – establishment of the center of excellence for autonomous transport systems at széchenyi istván university (tkp2020nka-14)’ project and by the european commission through the h2020 project epic (https://www.centre-epic.eu/) under grant no. 739592. references [1] ifr press releases. online [accessed 16 august 2021] https://ifr.org/ifr-press-releases/news/robot-race-the-worldstop-10-automated-countries [2] european commission, digital economy and society index report 2019, 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https://github.com/mtasztaki/apertusvr/blob/89aefbc9b2a0e7524092b87d728ad539cfc0a856/plugins/languageapi/jsapi/nodejsplugin/js/plugins/httpsimulator/ur5.jsonlist human identification and tracking using ultra-wideband-vision data fusion in unstructured environments acta imeko issn: 2221-870x december 2021, volume 10, number 4, 124 131 acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 124 human identification and tracking using ultra-widebandvision data fusion in unstructured environments alessandro luchetti1, andrea carollo1, luca santoro1, matteo nardello1, davide brunelli1, paolo bosetti1, mariolino de cecco1 1 department of industrial engineering, university of trento, sommarive, 9 38123 trento, italy section: research paper keywords: human-robot interaction; following system; ultra-wideband technology; 3d vision system; sensor fusion citation: alessandro luchetti, andrea carollo, luca santoro, matteo nardello, davide brunelli, paolo bosetti, mariolino de cecco, human identification and tracking using ultra-wideband-vision data fusion in unstructured environments, acta imeko, vol. 10, no. 4, article 21, december 2021, identifier: imekoacta-10 (2021)-04-21 section editor: roberto montanini, università di messina and alfredo cigada, politecnico di milano, italy received july 26, 2021; in final form december 6, 2021; published december 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: alessandro luchetti, e-mail: alessandro.luchetti@unitn.it 1. introduction cooperation between mobile robots and people is playing a significant role in the modern economy while its demand is increasing worldwide, aided also by the growing use of autonomous and smart mobile robots. in particular, co-bots can increase human resources while reducing physical and mental load, increasing operational safety and productivity, in industrial environments. in this context, the human-following function, called “follow-me”, is crucial. it consists of identifying and following the assigned operator even in unstructured environments. there are different approaches to achieve such a task. the most commonly used technologies for tracking include vision [1], [2], time-of-flight (tof)-camera [3], lidar [4], light-emitting device (led) [5] and uwb transceivers [6]-[131]. for each of these technologies, there are several advantages and disadvantages. lidar technology, while faster and more accurate than tof camera, is much more expensive and not used unless strictly necessary. with the led detection method there are few applications from the literature because of the low robustness due to the robot’s inability to detect the light-emitting device frequently. the best candidates for tracking operations with low cost are still 3d vision and uwb systems. the main literature contributions use them independently to solve the “follow-me” task in unstructured environments without solving their disadvantages. for example, the disadvantage of traditional vision systems is the limited field of view (fov) compared to uwb systems and lighting influence, especially outdoor. in contrast, uwb can be applied both indoor and outdoor but suffers from higher uncertainties especially for measurements of less than one meter and in the presence of obstruction by people between the transceivers [9], [10]. however, uwb systems measurements can be made up to 80 m, in contrast to tof cameras where after 10 m there is mostly noise while are more precise than the uwb below. furthermore, the shape of the uncertainties of the two systems is different but complementary. with this work, we overcome the disadvantages of these technologies by combining them to improve the robustness and reduce the uncertainty of the measurement result. abstract nowadays, the importance of working in changing and unstructured environments such as logistics warehouses through the cooperation between automated guided vehicles (agv) and the operator is increasingly demanded. the challenge addressed in this article aims to solve two crucial functions of autonomy: operator identification, and tracking. these tasks are necessary to enable an agv to follow the selected operator along his path. this paper presents an innovative, accurate, robust, autonomous, and low-cost operator real-time tracking system, leveraging the inherent complementarity of the uncertainty regions (2d ellipses) between ultra-wideband (uwb) transceivers and cameras. the test campaign shows how the uwb system has higher uncertainty in the angular direction. in contrast, in the case of the vision system, the uncertainty is predominant along the radial coordinate. due to the nature of the data, a sensor fusion demonstrates improvement in the accuracy and goodness of the final tracking. mailto:alessandro.luchetti@unitn.it acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 125 the only literature works that apply sensor fusion between uwb and vision systems work in structured environments with fixed uwb transceivers [11], [12] or with both fixed uwb and vision systems [13]. their solutions are not dynamic to changes, are more expensive because of the number of devices to be used, and must be calibrated for each environment. this work is organized as follows. an overview of the involved measurement systems is provided in the next section. the human identification algorithms to define which operator to follow are provided in section 3; section 4 describes the uncertainties models of the uwb and vision systems. the operator localization results through sensor fusion approach are discussed in section 5 followed by conclusions in section 6. 2. measurement systems for our application, we used decawave’s uwb development board dwm10011 [14]. it has the advantages of low cost, low power consumption, and strong penetration. the receivers were programmed with a two-way ranging (twr) architecture [15] that allows them to work in an unstructured environment, with two anchors on the robot, one master and one generic, and a tag for the tracked operator, figure 2. the tag communicates first with the generic anchor to calculate the distance d1, i.e. the distance between the tag and the generic anchor. then with the master anchor to obtain the distance d2, i.e. the distance between the tag and master anchor. successively the generic anchor will send the estimated distance d1 to the master, and finally, the master shares the two distances d1, d2 to the computer within a total time of 9 ms. the distance between the two anchors d3 is fixed and constant. the position of the tag in the environment using only the uwb system is ambiguous because the two radii d1 and d2 of the circumferences can intersect at two different points. to solve this problem, the robot is equipped with a camera, which determines the uniqueness of the measurement (i.e., whether the operator is in front of the robot or not). the selected camera is the intel realsensetm depth camera d4552 [16]. the device includes an rgb camera, two infrared (ir) cameras, a laser projector, and an inertia measurement system (imu). the vision system can extract both a 3d point cloud of the scene and a traditional rgb image. the rgb is used to apply artificial intelligence (ai) algorithms that perform human skeleton detection, while the point cloud allows localizing the key points of the skeleton in 3d. figure 1 shows the designed system. in particular, the uwb and vision systems onboard the mobile robot for operator identification and tracking. 3. human identification human detection and human pose estimation are done by applying a neural network provided by intel in the openvinotm toolkit, called human-pose-estimation-0001. the toolkit enables convolution neural networks (cnn)-based deep learning inference and contains an optimized version of opencv libraries for intel hardware [17]. this network is based on openpose [18] approach with tuned mobilenet v1 [19] as a feature extractor. this network results in two different outputs. the first is composed of 18 probability maps, called heat-maps, that provide all the key-points on the image: ears, eyes, nose, neck, shoulders, elbows, wrists, hips, knees, and ankles, figure 3a. the second is 19*2 layers called part affinity fields, and it gives us information on how to match the key-points that correspond to a single person, figure 3b. heat-maps provide the probability for each pixel to be at the position of a key-point. performing a threshold on the heat-maps it is possible to select the pixels with a probability higher than 50 %, from which it is possible to evaluate for each cluster of pixels the average point and the covariance matrix referring to each key-point location, figure 3a. from these covariance matrices, the average uncertainty of each pixel was calculated as three pixels. the image resolution was set at 848 × 480 (cxr), and the network for the human-pose-estimation-0001 as int8 running on the cpu extracts each frame up to 18 key-points per person. the resolution was chosen as a good trade-off between resolution of the image, and so linked to the uncertainty for the human positioning, and the speed of execution of the network. the net is available in three different resolutions, i.e. int8, fp16, and fp32. there is no difference in performance in our test when running on the cpu, but the int8 version is slightly faster than the others for this estimation. only the operator, entitled to use the robot, must be tracked. to reach this, we introduced face identification. the steps to identify the operator are two: find all the faces within the rgb frame and then detect the operator among them. for these steps, we used two cnn available on the openvinotm toolkit. the face detection cnn used is face-detection-retail-0005 and to extract the features of each face and compare them with the operator features stored in a database we use face-reidentification-retail-0095. the matching is made through the cosine similarity, equation (1), which is defined as the cosine of the angle θ between two features’ vectors a and b on dimension rn. where a is the features’ vector acquired in real-time and b is the one stored as ground truth. the lower the cosine, the higher the probability that the features vector are similar. figure 1. real system with mobile robot and operator. figure 2. system configuration (top view). acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 126 𝑆𝑖𝑚𝑖𝑙𝑎𝑟𝑖𝑡𝑦 = cos(𝜃) = 𝐴 ·𝐵 ||𝐴|| ||𝐵|| = = ∑ 𝐴𝑖𝐵𝑖 𝑛 𝑖=1 √∑ 𝐴𝑖 2𝑛 𝑖=1 √∑ 𝐵𝑖 2𝑛 𝑖=1 , (1) the operator, once identified, turns on the spot, and the software saves all the features again with a neural network, called person-reidentification-retail-0031, for different poses. as previously stated, the correspondence is made with the cosine similarity between the feature vectors of people’s bodies in the frames. table 1 shows the overall times for each network inference calculated on an intel cpu i7-7700hq with 16 gb of ram. 4. human localization: 2d uncertainty modeling from uwb and vision system are extracted the same information about the operator’s position with two measurements systems that are both referred to the robot base but whose uncertainty regions are complementary as explained in this section. 4.1. uwb system to maintain consistency with the information received from the vision system, the reference frame of the uwb is fixed on the camera and the anchors are placed at its sides with equal distance d3/2, figure 2. in particular, the x coordinate is related to the lateral axis, the z coordinate to the depth, and the y coordinate to the vertical axis. for the localization of the operator in the space, we are interested only in the x and z coordinates, since we project everything on the ground floor. the equations of the circumferences from the uwb devices become: (𝑥 − 𝑑3 2 ) 2 + 𝑧2 = 𝑑1 2 , (2) (𝑥 + 𝑑3 2 ) 2 + 𝑧2 = 𝑑2 2 , (3) from equations (2) and (3), we obtain the closed-form solution for the tag position: 𝑥 = 𝑑2 2 − 𝑑1 2 2𝑑3 , (4) 𝑧 = ± 1 2 √− (𝑑1 2 −𝑑2 2)2 𝑑3 2 − 𝑑3 2 + 2(𝑑1 2 + 𝑑2 2) . (5) as discussed in section 2, we only keep the positive value for the z-coordinate because it is guaranteed by the vision system. the covariance matrix of the coordinates of the tag position with the uwb system is equal to: 𝐶uwb = 𝐽dist · 𝐶dist · 𝐽dist 𝑇 , (6) where cdist is the covariance matrix of the measured distances d1, d2, and d3, (figure 2), with their standard deviations σ1, σ2, and σ3 𝐶dist = ( 𝜎1 2 0 0 0 𝜎2 2 0 0 0 𝜎3 2 ) . (7) the uwb devices used to test the overall system were calibrated in different indoor and outdoor scenarios to find the corresponding systematic offset at different distances and the related uncertainties. furthermore, the master and generic anchors were calibrated independently because, although the de vice type is the same, their internal crystals and thus their responses are not the same. in particular, we carried out measurements from 1 m to 20 m every 0.5 m with random order both indoor and outdoor. we collect data at 52 hz for three minutes. between different distances, we wait three minutes with all the devices turned off. the tests were performed for each distance, one with the line of sight (los) between the devices free (figure 4a) and the other two with noise elements: in one 34 people simultaneously walked randomly back and forth between the devices throughout the test time (figure 4b), in the other large static metal elements were placed in random positions between the devices (figure 4c). this was done to understand how much these scenarios affect the measurements. figure 5a shows an example of the box-plot result for an indoor test at 7 m in different scenarios. in particular, in all the tests done, the standard deviation of the measurements made with the walking people is significantly higher than the ones with free los and metal objects. the data acquired with walking people generate an overestimation, possibly because the occlusion of the los causes the reflected radio waves to be caught as the free los. from the tests, the standard deviations d1 and d2 were calculated at 0.05 m outdoor with free los. this value was used table 1. overall times. network image resolution net resolution inference time (ms) face-detection-retail-0005 848 × 480 int8 9.42 face-reidentification-retail-0095 848 × 480 fp16 5.38 person-reidentification-retail-0031 96 × 48 fp16 6.50 human-pose-estimation-0001 848 × 480 int8 115 (a) (b) figure 3. heat-maps with covariances of key-points (a) and affinity map (b) of 848x480 image from camera d455 with operator at two meters. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 127 ± and set in our application; instead, the standard deviation for d3 was set at 0.01 m to take into account human error during fixing. about the offset between the measured distances with uwb devices and the real distances, the values were modeled from the outdoor tests in free los, figure 5b. all the real distances, taken as ground truth, were measured with a laser meter type fervi ml80 [20], able to measure distances from 0.05 m to 80.00 m with an accuracy of 0.02 m. the generic anchor and the tag were powered by a varta power bank type 57962, while a usb cable powered the master anchor from the pc. from that usb cable was possible to communicate in serial and save the estimated distances from the two anchors. previously we saw how the free outdoor los was chosen as the reference environment for the offset and standard deviation values. to take into account also other scenarios each time we check the channel impulse response (cir) [21] provided by the uwb modules. in case of some human obstruction, this parameter decreases, and we do not update the distance information waiting for a reasonable value of cir. the jacobian matrix jdist with respect to the distances is: 𝐽dist = ( − 𝑑1 𝑑3 𝑑2 𝑑3 𝑎2 2 𝑑3 2 4 𝑑1 − 4 𝑑1 𝑎2 𝑑3 2 𝑎1 4 𝑑2 + 4 𝑑2 𝑎2 𝑑3 2 𝑎1 − 2 𝑑3 − 2 𝑎2 2 𝑑3 3 𝑎1 ) , (8) with: 𝑎1 = 4 √2 𝑑1 2 + 2 𝑑2 2 −𝑑3 2 − 𝑎2 2 𝑑3 2 𝑎2 = 𝑑1 2 −𝑑2 2 equation (6) becomes: 𝐶uwb = ( σ3 2 𝑎6 2 4 𝑑3 4 + 𝑑1 2  σ1 2 𝑑3 2 + 𝑑2 2  σ2 2 𝑑3 2 𝑎1 𝑎1 σ3 2 𝑎3 2 16 𝑎2 + σ1 2 𝑎5 2 16 𝑎2 + σ2 2 𝑎4 2 16 𝑎2 ) , (9) with: 𝑎1 = 𝑑2 σ2 2 𝑎4 4 𝑑3 √𝑎2 − 𝑑1 σ1 2 𝑎5 4 𝑑3 √𝑎2 − σ3 2 𝑎6 𝑎3 8 𝑑3 2  √𝑎2 (a) (b) (c) (d) figure 4. indoor scenarios: free los (a), people walking (b), metal objects (c); outdoor scenarios (d). (a) (b) figure 5. (a) box-plot of data at 7 m indoor in different scenarios; (b) offset model of master and generic anchors outdoor in free line of sight (los). acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 128 𝑎2 = 2 𝑑1 2 +2 𝑑2 2 −𝑑3 2 − 𝑎6 2 𝑑3 2 𝑎3 = 2 𝑑3 − 2 𝑎6 2 𝑑3 3 𝑎4 = 4 𝑑2 + 4 𝑑2 𝑎6 𝑑3 2 𝑎5 = 4 𝑑1 − 4 𝑑1 𝑎6 𝑑3 2 𝑎6 = 𝑑1 2 −𝑑2 2 noting that the elements of equation (9) have at the denominator the measure d3, we can say that the more distant the anchors are from each other, the more accurate the position measurement is. another note is that the off-diagonal terms are zero if and only if the measures d1 and d2 are equal (considering also σ1 = σ2). in this case, equation (9) becomes: 𝐶uwb = ( 2 𝑑1 2  𝜎1 2 𝑑3 2 0 0 2 𝑑1 2  𝜎1 2 4 𝑑1 2 −𝑑3 2 + 𝑑3 2  𝜎3 2 4 (4 𝑑1 2 −𝑑3 2 )) , (10) under these conditions, figure 6 shows the behaviour of the standard deviations’ values of each eigenvalue in the two principal directions x and z with respect to the z distance of the tag from the anchors, square roots of cuwb(1, 1) and cuwb(2, 2) respectively. as can be seen from the behaviour of the eigenvalues in figure 6 and shapes of the covariances in figure 7 at the beginning (z = 0 m) the covariance is a tight ellipse stretched in the radial direction (cuwb(1, 1) < cuwb(2, 2), tag a in figure 7), then when the distances d1 and d2 are orthogonal to each other it becomes a perfect circle (cuwb(1, 1) = cuwb(2, 2), tag b in figure 7) and lastly with a higher z distance value it becomes stretched in the angular direction (cuwb(1, 1) > cuwb(2, 2), tag c in figure 7). if d1 and d2 are different, tag d in figure 7, the quadratic approximation of the probability ellipse will be rotated. 4.2. vision system the model used to study the variance for the depth coordinate z of the selected camera is a pinhole model. two-ir sensors of the camera are used to evaluate the depth through a disparity matching algorithm, an rgb sensor is used instead to calculate the 2d image, all of them are co-planar and aligned. the common reference frame of the two-ir sensors is set at the same focal length of the twoir sensors and translated by overlapping the reference frame of the right sensor above the left sensor by an amount equal to the baseline b, figure 8. the charge-coupled device (ccd) resolution for each sensor is 848 pixels in columns (c) and 480 pixels in rows (r) for a total of 848 × 480 (c × r) pixels. the expression of the depth coordinate z is: 𝑧 = 𝑓𝑐 ⋅ 𝑏 |𝑐𝑄1 − 𝑐𝑄2| = 𝑓𝑐 ⋅ 𝑏 𝑑 , (11) where fc is the focal length in x direction, b the baseline, and d the disparity, figure 8. from equation (11) is possible to evaluate the error in the depth estimation from the model with respect to the disparity d, assuming all the other parameters as known and constant values: 𝜎𝑧 = 𝜕𝑧 𝜕𝑑 = 𝑏 ⋅𝑓𝑐 𝑑2 ⋅ 𝜎𝑑 , (12) equation (12) can be rewritten as function of the z distance, for better understanding, by substituting the value of the disparity d with a formulation obtainable from equation (11): 𝜎𝑧 = 𝑧2 𝑏 ⋅ 𝑓𝑐 𝜎𝑑 , (13) where the focal length f in pixel is: figure 6. standard deviation (std) of each eigenvalue in the two principal direction x and z for uwb system from 0 m to 10 m with the off-diagonal terms of covariance matrix cuwb equal to 0. figure 7. uwb system uncertainty ellipses for different tag positions. figure 8 stereo pin-hole model. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 129 3 𝑓𝑐 = 0.5 𝐶 tan(𝐻𝐹𝑂𝑉/2) , (14) 𝑓𝑟 = 0.5 𝑅 tan(𝑉𝐹𝑂𝑉/2) , (15) hfov is the horizontal field of view and vfov is the vertical field of view, in our case are 87° and 58° respectively. by varying the resolution, the focal length changes and so does the distance uncertainty. the disparity standard deviation σd for the camera d455 is declared to be 0.08 pixel by the manufacturer. since many interfering effects increase this value, we calibrated the camera to check if the declared model equation (13) fits the real data and if so, to obtain the real standard deviation σd. we made 400 consecutive depth acquisitions of a chessboard from 1 m to 10 m at random steps of one meter, figure 9a. , figure 9b shows the theoretical and the obtained σz as a function of z distance for the image resolution c of 848 pixels. as can be seen in figure 9b the behaviour of the experimental found model is higher than the theoretical one. the new experimental value for σd found by applying the least absolute residual robust (lar) method is 0.4 pixel. to evaluate the coordinates in x and y dimension we use the brown-conrady distortion calibration model [22] with the intrinsic constant coefficients (k1, k2, k3, p1, p2) of the specific selected camera: 𝑢𝑥 = �̂� ⋅𝑓 + 2 ⋅ 𝑝1 ⋅ �̂��̂� +𝑝2(𝑟 + 2�̂��̂�) , (16) 𝑢𝑦 = �̂� ⋅𝑓 +2 ⋅𝑝2 ⋅ �̂��̂� + 𝑝1(𝑟 +2�̂��̂�) , (17) with: �̂� = 𝑐 − 𝐶/2 𝑓𝑐 = 𝑥𝑝 𝑓𝑐 �̂� = 𝑟 −𝑅/2 𝑓𝑟 = 𝑦𝑝 𝑓𝑐 𝑟 = �̂� 2 +�̂�2 𝑓 = 1+ 𝑘1 ⋅ 𝑟 + 𝑘2 ⋅ 𝑟 2 + 𝑘3 ⋅ 𝑟 3 the new expression of the x and y position becomes: 𝑥 = 𝑢𝑥 ⋅ 𝑧 , (18) 𝑦 = 𝑢𝑦 ⋅ 𝑧 , (19) as for the uwb system, we are interested only in the x and z coordinates for the localization of the operator in the space. the covariance matrix of the coordinates of the operator with camera system is equal to: 𝐶cam = 𝐽cam ⋅ 𝐶σ ⋅ 𝐽cam 𝑇 , (20) where: 𝐶𝜎 = ( 𝜎𝑐 2 0 0 0 𝜎𝑟 2 0 0 0 𝜎𝑧 2 ) , (21) 𝐽𝑐𝑎𝑚 𝑇 = ( 𝑧  ( 𝑎1 𝑓𝑐 + 6 𝑝2 𝑥𝑝 𝑓𝑐 2 + 𝑥𝑝  ( 2 𝑘1 𝑥𝑝 𝑓𝑐 2 + 4 𝑘2 𝑥𝑝 𝑎2 𝑓𝑐 2 + 6 𝑘3 𝑥𝑝 𝑎2 2 𝑓𝑐 2 ) 𝑓𝑐 + 2 𝑝1 𝑦𝑝 𝑓𝑐  𝑓𝑟 ) 0 𝑧  ( 2 𝑝2 𝑦𝑝 𝑓𝑟 2 + 𝑥𝑝  ( 2 𝑘1 𝑦𝑝 𝑓𝑟 2 + 4 𝑘2 𝑦𝑝 𝑎2 𝑓𝑟 2 + 6 𝑘3 𝑦𝑝 𝑎2 2 𝑓𝑟 2 ) 𝑓𝑐 + 2 𝑝1 𝑥𝑝 𝑓𝑐  𝑓𝑟 ) 0 𝑝2  ( 3 𝑥𝑝 2 𝑓𝑐 2 +𝑎3)+ 𝑥𝑝 𝑎1 𝑓𝑐 + 2 𝑝1 𝑥𝑝 𝑦𝑝 𝑓𝑐  𝑓𝑟 1 ) , (22) with: 𝑎1 = 𝑘1 𝑎2 +𝑘2 𝑎2 2 + 𝑘3 𝑎2 3 + 1 𝑎2 = 𝑥𝑝 2 𝑓𝑐 2 + 𝑎3 𝑎3 = 𝑦𝑝 2 𝑓𝑟 2 . ccam depends on the focal length f, on the baseline b, and on the standard deviations of disparity σd and pixels σc, σr. once the depth frame and the rgb one are aligned, it is possible to estimate the distance to any pixel and thus project the operator’s skeleton in space. sometimes, key-points can have an incorrect distance value, i.e., the z-coordinate, especially when the pose is very close to the viewing system. it is due to distortion caused by camera lenses and imperfect image realignment. to overcome this problem, we extract an average position of keypoints with the median. in this case, if a key-point is projected too far from the other or too near, it will have no impact on the final operator position. the off-diagonal terms of equation (20) are zero if and only if xp = 0, yp = 0. in this case equation (20) becomes: (a) (b) figure 9. (a) camera calibration test with chessboard; (b) standard deviation results on z distance with stereo camera d455 for resolution c of 848 pixels. 2 acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 130 𝐶cam = ( 𝑧2𝜎𝑐 2 𝑓𝑐 2 0 0 𝜎𝑧 2 ) , (23) figure 10 shows the standard deviations’ values of each eigenvalue in the two principal directions x and z of equation (23) with the covariances’ shapes in figure 11 as previously done for uwb system. as can be seen in figure 10, after 30 cm, the radial coordinate in the z direction is always greater than the angular one in the x direction (ccam(1, 1) < ccam(2, 2) i.e. the ellipse is stretched in the radial direction. 5. sensor fusion sensor fusion was done between the position estimated by the uwb system and the position estimated by the vision system to reduce the uncertainty and improve the estimation of the operator’s position in space. figure 12 shows two examples in two different tag positions where the uncertainties of the position estimation through the uwb system are more significant along the angular direction (x-coordinate), centring the reference system on the origin of the camera. notice that in the case of the vision system, the uncertainties are predominant along the radial direction (z-coordinate). by fusing the information with bayes’ theorem [23], it is possible to reduce their uncertainties in both directions. we can summarize the fused information shown in figure 12 through the following expression: 𝐶fused = [[𝐶uwb] −1 + [𝐶cam] −1]−1 , (24) 𝑃fused = 𝐶fused[[𝐶uwb] −1𝑃uwb + [𝐶cam] −1𝑃cam] , (25) where: puwb and pcam are the estimated mean position of the uwb system and camera system respectively; cuwb and ccam are the covariance matrices of the estimated position of the uwb system and camera system respectively. 6. conclusions in this work, an innovative and robust method to identify and track an operator from a mobile robot in real-time was developed. in particular, the operator can be identified through a convolution neural network tool and tracked through a designed localization method obtained by fusing the information from low-cost sensors such as uwb transceivers and a depth camera. it was implemented to locate the operator’s position with less uncertainty. the test campaign shows the uwb system has a higher uncertainty in the angular direction, contrary to the camera, where the uncertainty is higher in the radial direction. specifically, cuwb is affected by the distances measured between figure 10: standard deviation (std) of each eigenvalue in the two principal direction x and z for camera system from 0 m to 10 m with the off-diagonal terms of covariance matrix ccam equal to 0. figure 11. camera system uncertainty ellipses for different tag positions. (a) (b) figure 12. (a) two examples of uncertainty ellipses (95 % with k= 2.4478 [24]) in two different tag positions (x=0 m, z=4 m; x=2 m, z=6 m) from camera and uwb system; (b) zoom of the results. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 131 the tag and the two anchors (d1, d2) and between the anchors themselves (d3) with their corresponding uncertainties. ccam instead depends on the focal length f, on the baseline b, and on the standard deviations of disparity σd and pixels σc, σr. our solution makes the final system robust and more precise due to the complementarity of information between the covariance matrices of the uwb system and the vision system. to obtain a better result for the real application, the uwb transceivers were calibrated following a test campaign that provided the correct behaviour of the systematic offset in the measurements and their standard deviations. offsets change for a specific uwb device and increase with z distance. the standard deviation for the distances between the devices was calculated at 0.05 m in the scenario with a free line of sight and outdoor. another test campaign, this time on the vision system, was carried out to evaluate the theoretical model of the standard deviation σz as a function of z distance and the value of camera disparity standard deviation σd. the σd calculated for our camera d455 is 0.4 pixels. moreover, the average uncertainty of each pixel σc, σr was calculated as three pixels through heat-maps analysis of the convolution neural network used. acknowledgement this work was supported by the italian ministry for university and research (mur) under the program “dipartimenti di eccellenza (2018-2022)”. references [1] m. gupta, s. kumar, l. behera, v. k. subramanian, a novel vision-based tracking algorithm for a human-following mobile robot, ieee transactions on systems, man, and cybernetics: systems 7 (2016), pp. 1415–1427. doi: 10.1109/tsmc.2016.2616343 [2] s.-o. lee, m. hwang-bo, b.-j. you, s.-r. oh, y.-j. cho, vision based mobile robot control for target tracking, ifac proceedings volumes 34(4) (2001), pp. 75–80. doi: 10.1016/s1474-6670(17)34276-3 [3] g. xing, s. tian, h. sun, w. liu, h. liu, people-following system design for mobile robots using kinect sensor, 25th chinese control and decision conference (ccdc), ieee, guiyang, china, 25-27 may 2013, pp. 3190–3194. doi: 10.1109/ccdc.2013.6561495 [4] s. a. ahmed, a. v. topalov, n. g. shakev, and v. l. popov, model-free detection and following of moving objects by an omnidirectional mobile robot using 2d range data, ifacpapersonline 51(22) (2018), pp. 226– 231. doi: 10.1016/j.ifacol.2018.11.546 [5] y. nagumo a. ohya, human following behavior of an autonomous mobile robot using light-emitting device, in proceedings 10th ieee international workshop on robot and human interactive communication. roman 2001 (cat. no. 01th8591), ieee, bordeaux, paris, france, 18-21 sept. 2001, pp. 225–230. doi: 10.1109/roman.2001.981906 [6] t. feng, y. yu, l. wu, y. bai, z. xiao, z. lu, a human-tracking robot using ultra wideband technology, ieee access 6 (2018), pp. 42541–42550. doi: 10.1109/access.2018.2859754 [7] t. g. kim, d.-j. seo, k.-s. joo, a following system for a specific object using a uwb system, in 2018 18th international conference on control, automation and systems (iccas), ieee, pyeongchang, korea (south), 17-20 oct. 2018, pp. 958–960. 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[24] r. c. smith, p. cheeseman, on the representation and estimation of spatial uncertainty, the international journal of robotics research 5(4) (1986), pp. 56–68. doi: 10.1177/027836498600500404 https://doi.org/10.1109/tsmc.2016.2616343 https://doi.org/10.1016/s1474-6670(17)34276-3 https://doi.org/10.1109/ccdc.2013.6561495 https://doi.org/10.1016/j.ifacol.2018.11.546 https://doi.org/10.1109/roman.2001.981906 https://doi.org/10.1109/access.2018.2859754 https://doi.org/10.23919/iccas.2017.8204325 https://doi.org/10.1109/jsen.2021.3120889 https://doi.org/10.1109/rose52750.2021.9611770 https://doi.org/10.1109/iccre49379.2020.9096456 https://doi.org/10.1109/icra40945.2020.9196944 https://doi.org/10.3390/s20041139 https://www.decawave.com/product/dwm1001-development-board/ https://www.decawave.com/product/dwm1001-development-board/ https://doi.org/10.1109/icnidc.2010.5657814 https://www.intelrealsense.com/depth-camera-d455/ https://opencv.org/ https://doi.org/10.1109/tpami.2019.2929257 https://arxiv.org/abs/1704.04861 https://www.fervi.com/ita/strumenti-di-misura/misuratori-analogici-e-digitali/misuratore-di-distanze/misuratore-di-distanza-laser-pr-8240.htm https://www.fervi.com/ita/strumenti-di-misura/misuratori-analogici-e-digitali/misuratore-di-distanze/misuratore-di-distanza-laser-pr-8240.htm https://www.fervi.com/ita/strumenti-di-misura/misuratori-analogici-e-digitali/misuratore-di-distanze/misuratore-di-distanza-laser-pr-8240.htm https://doi.org/10.1109/lcomm.2020.3009659 https://doi.org/10.1177%2f027836498600500404 led-to-led wireless communication between divers acta imeko issn: 2221-870x december 2021, volume 10, number 4, 80 89 acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 80 led-to-led wireless communication between divers fabio leccese1, giuseppe schirripa spagnolo2 1 dipartimento di scienze, università degli studi “roma tre”,via della vasca navale n. 84, 00146 roma, italy 2 dipartimento di matematica e fisica, università degli studi “roma tre”, via della vasca navale n. 84, 00146 roma, italy section: research paper keywords: underwater communication; visible light communications; optical wireless communication, bidirectional communication; led; photo detector citation: fabio leccese, giuseppe schirripa spagnolo, led-to-led wireless communication between divers, acta imeko, vol. 10, no. 4, article 15, december 2021, identifier: imeko-acta-10 (2021)-04-15 section editor: francesco lamonaca, university of calabria, italy received october 4, 2020; in final form december 6, 2021; published december 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: fabio leccese, e-mail: fabio.leccese@uniroma3.it 1. introduction currently, the use of wireless communications is very common in a wide range of terrestrial devices. in the underwater world, wireless information transfer is of great interest to the military. it plays an important role in military raids carried out by a team of divers. for safety and to coordinate actions, a secure and reliable bidirectional communication system is useful. nowadays, underwater wireless communications are implemented almost exclusively via acoustic waves due to their relatively low attenuation [1], [2]. communication by measuring light waves (visible light communication vlc) is a technology that employments light spectra from 400 to 700 nm as data carriers. vlc techniques transmit data wirelessly by pulsing visible light. this new technology, called li-fi, can replace the wi-fi connection, based on radio frequency waves [3]-[6]. beer’s law is usually utilized to correlate the absorption of diffuse light to the properties of the medium through which the light is traveling. from a mathematical point of view, we can write [7], [8]: 𝑃(λ, 𝑟) = 𝑃0 ∙ e −𝐾d(λ)∙𝑟 , (1) where 𝑃0 is the initial transmitted power, 𝑃(𝜆, 𝑟) is the residual power after the light beam with wavelength 𝜆 has traveled the distance 𝑟 through the medium with diffuse attenuation coefficient 𝐾d (𝜆). figure 1 shows the attenuation coefficient of three typical ocean waters i, ii and iii and five coastal waters 1, 3, 5, 7 and 9; the lower numbers correspond to clearer waters. the classification corresponding of jerlov water types [9]-[11]. light with longer wavelengths is absorbed more quickly than that with shorter wavelengths. because of this, the higher energy light with short wavelengths, such as blue-green, is able to penetrate more deeply. in open ocean, below 100 m depth, only blue-green radiation is present [12]. however, the blue component of sunlight can also reach depths of up to 1000 m; although the quantity is so low that photosynthesis is not allowed [12]. figure 1 shows that the minimal absorption is between 460 nm and 580 nm; depending on the type of water. therefore, vlc technology is extensively studied as an alternative solution for short range underwater communication links [13]-[28]. really, underwater optical wireless communication (uowc) is not a new idea. after the pioneering works of the 1980s [29][31], in 2009, doniec et al. [32] have developed a 5-meter abstract for military divers, having a robust, secure, and undetectable wireless communication system available is a fundamental element. wireless intercoms using acoustic waves are currently used. these systems, even if reliable, have the defect of being easily identifiable and detectable. visible light can pass through sea water. therefore, light can be used to develop short-range wireless communication systems. to realize secure close-range underwater wireless communication, the underwater optical wireless communication (uowc) can be a valid alternative to acoustic wireless communication. uowc is not a new idea, but the problem of the presence of sunlight and the possibility of using near-ultraviolet radiation (near-uv) has not been adequately addressed in the literature yet. in military applications, the possibility of using invisible optical radiation can be of great interest. in this paper, a feasibility study is carried out to demonstrate that uowc can be performed using near-ultraviolet radiation. the proposed system can be useful for wireless voice communications between military divers as well as amateur divers. mailto:fabio.leccese@uniroma3.it acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 81 underwater wireless optical communication link (called aquaoptical) with a 1 mbps data rate. later in 2015, rust et al. [33] have implemented an uowc system for use in remote controlled vehicles (rovs) used for the inspection of nuclear power plants. in addition, some systems are currently commercially available [34]-[37]. unfortunately, the performance of uowc is currently limited to short range [38]. however, in some specific situations, short-range communication is more than enough. on the other hand, there are circumstances where short range communication is needed without the need for large bandwidth. a typical example is the communication between divers. for communication between divers, the most common form is through hand signals [39], and underwater writing slates [40], [41]. figure 2 shows two example of standard diver hand signals and a dive slate. the dialect of the diver's hand signals includes only plain and precise gestures easily identifiable. this allows only simple communications and require extensive memorization. on the other hand, slates do not allow communication in real time; it takes time to write and to attract the attention of the underwater partners. recently, full face diving masks with snorkels have been introduced that allow the diver to breathe and speak normally inside the mask [42], [43]. for this type of masks, reliable underwater intercoms have been developed to allow divers to talk each other underwater [44], [45]. a transducer is attached to the diver's face mask. this transducer converts the voice into an ultrasonic signal. each diver of the team has an ultrasonic receiver, which accepts the signal and converts it back to a sound that the divers can hear, enabling communication. this type of communication system can be used by amateur or professional divers. figure 3 shows two commercially available systems of underwater intercoms. during military raids with divers, it is very important that the various components of the command can communicate with each other. unluckily, hand signs do not allow for complex information to be communicated, and the use of dive slate can be incompatible with the times. an audio communication is essential for complex communications needed in military actions. another key problem in military communications is that they must be secure and undetectable. unfortunately, the acoustic waves that travel in water are easily detectable. therefore, their use is not convenient during critical military missions. in this scenario, uowc is a good alternative to acoustic communication [46]. it has the advantage that it cannot be intercepted. this specific application does not require long range and high band communications. therefore, the usable systems can be simple, small, lightweight and with low power. figure 4 shows a typical uowc between divers. the information could be transmitted through a special torch and be captured by sensors positioned on the diving suit. unfortunately, communications with visible light suffer from noise generated by solar background noise or artificial light sources. special precautions must be taken to minimize this noise [47]. it would be convenient to implement uowc systems that use optical radiation different from that normal present in water. figure 1. diffuse attenuation coefficient 𝐾𝑑(𝜆) for several oceanic and coastal water types according jerlov classification. curves obtained from the data present in [9]-[11]. figure 2. examples of standard diver hand signals and of a dive slate. figure 3. examples of standard diver hand signals and of a dive slate. figure 4. optical communication between divers. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 82 in addition, during communications between the military divers it would be useful to use light not visible from normal video surveillance systems. the main purpose of the paper is to verify the feasibility of a communication system that can be used by the military divers. the system must be simple, robust, consuming few energy and not affected by ambient light, and, above all, difficult to detect and/or intercept by video surveillance systems sensitive to visible radiation. to obtain these performances it is necessary to avoid the use of blue-green radiation, present in the solar radiation that penetrates into the water. in addition, visible radiation must be avoided, which is easily detectable at night by underwater video surveillance systems. in this paper, an underwater near-ultraviolet light communication is proposed. the proposed system uses as emitter (tx) an uv led with peak wavelength λ = 385 nm and half width δλ = 15 nm. instead, a photodiode, made with an led like the one used as a transmitter, is used as a receiver (rx). this system is intrinsically low sensitive to ambient light and produces an invisible communication channel. since there are video surveillance systems that have good sensitivity in the bluegreen spectral band [48], the use of radiation in the near uv allows having a relatively good penetration of the radiation into the water and at the same time to be invisible to these video surveillance systems. the system works well in short range communications where large bandwidth is not required. for example, if we are only interested to speech transmission, a bandwidth of 32 kbps is generally acceptable. with this type of communication, it is possible to create simple, small, light, robust and energy efficient systems. 2. underwater communications by uv-a radiation a part of the solar radiation spectrum overlaps with the radiation commonly used for the visible light communication (vlc) [49]. therefore, it is very difficult to attenuate the effects of sunlight without loss of useful signal. in the presence of sunlight, the receivers see very high white noise and can often go into saturation. to try to solve the problems deriving from solar radiation (in general of the ambient lights present), it is possible to use near-ultraviolet radiation for the communication channel. generally, solar intensity decreases with depth. by examining how light is absorbed in water (see figure 1), we see that the best wavelengths to use in uowc are 450 nm – 500 nm for clear waters and 570 nm – 600 nm for coastal waters. this same attenuation is also true for the solar spectrum [50]. in any case, at a depth of a few meters, the solar radiation in the near ultraviolet is practically absent. furthermore, in relatively clear waters this radiation is relatively poorly attenuated especially in ocean waters but less in coastal waters (according to figure 1). for these reasons, submarine communication systems, which use uv-a band communication channels, are extremely interesting. we must also observe two other important characteristics of optical communication that uses the near ultraviolet. this communication channel is difficult to detect and intercept, particularly attractive feature for military applications. furthermore, the use of ultraviolet radiation allows wireless connections to be made without requiring perfect alignment between transmitter and receiver (nlos uv scattering communication) [51], [52]. very useful characteristic for wireless transmission between moving objects. 3. led usable as light detector in addition to emitting light, leds can be employed also as light sensor/detector [53]-[60]. figure 5 schematically, shows this application in addition, the led can also be used as a temperature sensor [61]. to verify the possibility of underwater communication through uv radiation, we have chosen to use a reverse-polarized led as a detector. the choice was made to have an inexpensive photodetector that is not very sensitive to the light radiations present in the environment; without the need for filters that cut visible radiation. led can be also used as avalanche photodiode (apd) [62], [63]. unlike normal photodiodes, leds can detect a narrow band of wavelengths, they are spectrally selective detectors. in contrast, normal photodiodes have a wide spectral response and require costly filters to detect a specific wavelength. both leds and photodiodes have sensitivity stable over time. however, the filters have a limited life. in a p-n diode, inside the junction, there are free charges generated by thermal energy. when a p-n junction diode is reverse biased, these charges are accelerated. this movement of charges produces the reverse current of the diode. if the reverse polarization potential is increased, the free charges can acquire enough energy to ionize some atoms of the crystal lattice. this ionization produces additional free charges. moreover, these additional charges are accelerated by the polarization potential. this creates an avalanche effect, producing a large reverse current (breakdown current). the polarization voltage at which this arises is called zener potential [64]. if you want to use an led as light detector, generally, the photocurrents generated are linear but very small. in uowc applications, we have currents in the range of nano amps. therefore, for their correct subsequent signal processing, it is necessary to transform the detect current into a suitable voltage. for this operation, transimpedance amplifiers are commonly used [65]. the amplitude of the signal received by the led, and subsequently amplified by transimpedance amplifiers, depends on many external parameters. for this reason, the transmitted optical signal must be suitably digitized and modulated. our system uses a modulation format based on pulse width modulation (pwm). 4. uv led-to-led communication system in underwater optical wireless transmission, the signal reaching the receiver has low intensity. for this reason, extensive studies are underway to use very sensitive detectors such as avalanche photodiode (apd) or single photon avalanche diode [66]-[75]. with the use of very responsive photosensors, the problem of the presence of ambient light is very important [76]. with the use of a led-to-led transmission system that uses uv leds, it is possible to implement an underwater figure 5. basic led used as light emitter and receiver. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 83 communication system with invisible radiation that is not very sensitive to ambient light. led-to-led communication systems are characterized by low cost, low complexity and, above all, low energy consumption. on the other hand, they can be used only when the exchange of messages occurs at a small distance without the need for large bandwidth [76], [77]. as already mentioned, underwater, practically, ultraviolet radiation is absent. therefore, using uv led as light emitter and a uv led, used as apd, as receiver allows to have a system that is not very sensitive to environment light; an led can detect radiation with a wavelength slightly shorter than or equal to that emitted (internal photoelectric effect) [56], [57], [78]. the same type of led can be used as a receiver and as a transmitter. the use of the same type of led is useful in half duplex communication systems; the same led can be used as a transmitter or as a receiver. in this work, we have used a bivar uv5tz-385-30 led as a transmitter and receiver [79]. this led has viewing angle of 30° and an aperture area of 25⸱10-6 m2. seawater light transmission model is shown in figure 6. the optical power on the receiver can be written as [80]-[83]: 𝑃rx=𝑃tx∙𝜂tx∙𝜂𝑅𝑥 ∙exp [− 𝐾d (𝜆)∙𝑧 cos 𝜃 ] ∙ 𝐴rx ∙ cos 𝜃 2π∙𝑧2(1 − cos 𝜃0) , (2) where 𝑃tx is the transmitted power, 𝜂tx and 𝜂rx are the optical efficiencies of the 𝑇𝑥 and 𝑅𝑥 correspondingly, 𝐾d(𝜆) is the attenuation coefficient, 𝑧 is the perpendicular distance between the 𝑇𝑥 plane and the 𝑅𝑥 plane, 𝜃0 is the 𝑇𝑥 beam divergence angle, 𝜃 is the angle between the perpendicular to the 𝑅𝑥 plane and the 𝑇𝑥‐ 𝑅𝑥 trajectory, and 𝐴rx is the receiver aperture area. in our system, they experimentally verified that the received signal is correctly reconstructed if the misalignment is 𝜃 < 20°. the transmitter led was driven with 25 ma by means of a pulse generator. while the current generated by the led used as a receiver, reverse biased with a voltage of 15 v, was read through a transimpedance amplifier. two ultralow noise precision high speed op amps [84] were used to implement the transimpedance amplifier. the amplifier, as shown in figure 7, is made in two states. this is to be able to obtain a passband greater than 100 khz. the rx and tx leds, together with the relative control electronics, were inserted in a tank filled with real seawater (water taken from the tyrrhenian coast anzio italy). the leds are placed at 50 cm and facing one towards the other. figure 8 shows the experimental setup used for the tests. the experimental tests were carried out in the laboratory and outdoors in different configurations of ambient brightness. figure 8(a) shows the system working in laboratory. figure 8(b) shows the system working outdoors in full sun. all the tests carried out confirmed that the system is practically insensitive to ambient light (both artificial and natural). the experimental setup is realized to be able to obtain three different lengths of the optical channel. the different lengths of the optical path are obtained by means of mirrors, as shown in figure 9. the figure 10 show the signal used to drive the tx led (cyan trace) and the corresponding output signal (vout) from the rx circuit (yellow trace). the implemented system uses only one led as a transmitter and another as a receiver. in our application, there are no restrictions on using a led cluster to transmit information. as well as it can be useful to use led array to receive the signal. by using many diodes as tx, as well as rx, systems with better performance can be obtained. we used the simplest possible configuration as the aim was to demonstrate the possibility of figure 6. seawater light transmission model. figure 7. rx and tx led driver circuit. figure 8. experimental setup used for the tests: (a) system working in laboratory; (b) system working outdoors. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 84 implementing an underwater led-to-led transmission using near ultraviolet radiation. 5. system description in any reliable communication system, data must be suitably modulated. modulation consists in varying one or more properties of a relatively high frequency signal (carrier). we used pwm modulation to implement our system. furthermore, considering that high sound quality is not required for audio communication between divers, this type of modulation is more than enough to test the feasibility of wireless audio communication via uv-a optical channel. obviously, more performing, and more robust modulation systems with respect to noise can be used. the pwm consists of the information signal (in our case the audio signal) that causes the modification of the time duration of the pulse carrier. this pulse signal turns the transmitter led on and off at the rate of the carrier’s frequency. in other word, with pwm technique we change the duty cycle of a square wave with constant frequency and amplitude; as shown in figure 11. the average value of a pwm signal, period by period, can be expressed as: 𝑉average = 1 𝑇 (∫ 𝑉max ∙ 𝑑𝑡 𝐷∙𝑇 0 + ∫ 𝑉min ∙ 𝑑𝑡 𝑇 𝐷∙𝑇 ) . (3) if 𝑉min = 0, the equation (3), can be simplified as: 𝑉average =𝐷 ∙ 𝑉max . (4) equation (4) indicates that if the amplitude of the carrier is constant (along a period), the average value of the pwm signal is directly proportional to the duty cycle. if the duty cycle is proportional to the information to be transmitted, it can be extracted through a simple averaging operation on the pwm signal. average is easily obtainable through opportune low pass filtering. to verify the real possibility of realizing an audio communication, we have respectively coupled a modulator and a demodulator to the led transmitter and to the led receiver [85]-[87]. the block diagram of our audio modulator and led driver is shown in figure 12. this led driver has a restricted baud rate. the main reason is the limited switching speed of silicon devices. a maximum data rate of 100 kbps can be achieved with this driver. in any case, this speed of data transmission is more than enough to implement an excellent audio connection. the pwm is achieved by means of a timing circuits ne555 [88] and a comparator lt1011 [89]. our circuit is powered with 6.5 v and produces a sawtooth waveform with frequency of approximately 100 khz and peak to peak voltage about 4.3 v. the sawtooth waveform is applied to the non-inverting input of the comparator. the audio signal works as the reference voltage and is applied to the inverting input. to realize a duty cycle of 50%, the audio signal is offset at the average voltage of the sawtooth waveform (1/3 of the power supply voltage). the comparator output is equal to the supply voltage when the sawtooth output is a higher voltage than the audio signal. figure 9. schematic of the water canal. the different optical path is obtained with the help of mirrors. figure 10. the cyan line represents the signal used to drive the tx led. the yellow line represents the corresponding rx output signal. (a) distance between transmitter and receiver 0.5 m. (b) distance between transmitter and receiver 1.5 m. (c) distance between transmitter and receiver 2.5 m. figure 11. pwm signal. square wave with constant frequency and amplitude by variable duty cycle. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 85 the led driver is based on the integrated mic3289 [90]. it is a pwm boost-switching regulator that is optimized for constant-current led driver applications. figure 16 shows the input and output signals in a pulse width modulation process. to recover the transmitted audio, a receiver unit, mainly composed of photodetector and signal conditioning devices, is used. the photodiode receives the transmitted optical signal and converts the optical signals into the electrical signals. then, the electrical signal is fed into the recovery circuits and pwm demodulator. figure 14 shows the block diagram of the receiver unit. the transimpedance amplifier (shown in the figure 7) is coupled with a low pass filter. this filter, with cut-off frequency about 1 mhz, uses to reduce the high frequency noise present at the transimpedance amplifier output. the output signal of filter has an amplitude that depends on many external parameters, as well as the distance and misalignment between transmitter and receiver. to obtain a correct reconstruction of the pwm signal, a comparator with variable threshold is used. by means of an integrator circuit, a voltage proportional to the average value of the amplitude of the received signal is obtained. this voltage is used as the threshold of the comparator. the integrator that was used provides an average signal at the output, which is about one third of the amplitude of the signal coming from the filter. in this way, the reconstruction of the pwm signal is obtained which is practically independent of the amplitude of the signal received by the led used as photodiode. finally, the reconstructed pwm signal (signal with constant amplitude and variable duty cycle) is demodulate by a low pass filter with a cutoff frequency of 8 khz. instead, figure 15 shows the schematic drawing of the receiving unit. a low pass filter is sufficient to decode the audio information contained in the pwm signal. by choosing a low-pass filter with an appropriate cut-off frequency, it will be possible to remove the high-frequency component in the pwm signal while keeping only the low-frequency signal (the audio information). our demodulator is a 4th order butterworth low pass filter. it consists of two non-identical 2nd order low pass filter. the human ear can perceive sounds with frequencies between 20 hz and 20 khz. in any case, the human voice produces sound that are confined to within 8 khz. therefore, for verbal communications, a low-pass filter with a cut-off frequency around 8 khz is sufficient. the 4th order butterworth filter we use has a cutoff frequency of approximately 7.8 khz. therefore, it is irrelevant for all the sound frequencies emitted by the human voice. on the other hand, at 100 khz, the filter has an attenuation of 83 db. this figure 12. audio modulator and led driver. figure 13. input and output signals in a pulse width modulation process. figure 14. input and output signals in a pulse width modulation process. figure 15. schematic of the optical receiver circuit. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 86 indicates that the high frequency carrier is highly suppressed. figure 16 shows the frequency response of the filter used to retrieve the audio information from the pwm signal preliminary tests were conducted to verify the real applicability of our system in underwater wireless voice transmission. first, a 4 khz tone was used to check the entire modulation, optical transmission, optical detection, and demodulation system. in figure 17(a) trace 1 (yellow) is the sinusoidal 4 khz tone going into the transmitter. while trace 2 (blue) shows the pwm modulation used to drive the tx led current. in figure 17(b) trace 2 (blue) shows the reconstructed pwm signal in the receiving unit. finally, trace (1) (yellow) of figure 17(b) shows the reconstruction of the sinusoid at 4 khz. the reconstruction of the sinusoid is more than acceptable, even if there is the presence of “noise”, related to the harmonics of the carrier signal. subsequently, the system was tested by transmitting an audio speech signal. with 2.5 m between tx and rx the speech transmitted is perfectly understandable. figure 18 shows the audio tracks, spectrograms, and frequency analysis of the transmitted audio signal and of the message reconstructed downstream of the receiver. 6. conclusions underwater optical wireless communication (uowc) has recently emerged as a unique opportunity. many studies are present in the literature, however underwater optical communication via near-ultraviolet (uv-a) radiation is not addressed. in this paper, we have shown that in short range when broadband communication is not needed, it is possible to implement a uowc system that makes use of uv-a radiation. a uv underwater optical wireless audio transceiver was proposed for wireless communication in close range between divers. we have also verified that this system can be realized via an led-led connection. this makes the system simple, economical, light, compact and, above all, not energy-intensive. the study is mainly designed for military applications. in military applications, it is very important to have systems that cannot be intercepted and possibly not easily identifiable. in addition to have low energy consuming systems. for these reasons, we have developed a system that uses non-visible optical radiation and led-to-led transmission, which is energy efficient. however, considering the simplicity and cost-effectiveness of the developed system, it can easily be used for communications between amateur divers. in our study, we faced the problem of verifying the feasibility of transmitting a signal, with sufficient bandwidth to transmit an audio signal, at a distance of about 2.5/3 meters. further studies and tests in the real marine environment are needed. references [1] h. sari, b. woodward, digital underwater voice communications. in: r. s. h. istepanian, m. stojanovic (eds) underwater acoustic digital signal processing and communication systems. springer, boston, ma. 2002. doi: 10.1007/978-1-4757-3617-5_4 [2] j. w. giles, i. n. bankman, underwater optical communications systems. part 2: basic design considerations. proceedings of the milcom 2005 -ieee military communications conference. atlantic city, nj, usa, 17-20 october 2005. doi: 10.1109/milcom.2005.1605919 [3] h. haas, l. yin, y. wang, c. chen, what is lifi?, journal of lightwave technology 34(6) (2016), pp. 1533-1544. doi: 10.1109/jlt.2015.2510021 [4] m. uysal, c. capsoni, z. ghassemlooy, a. boucouvalas, e. udvary, optical wireless communications: an emerging technology. springer: new york, ny (usa), 2016. doi: 10.1007/978-3-319-30201-0 [5] m. z. chowdhury, m. shahjalal, m. hasan, y. m. jang, the role of optical wireless communication technologies in 5g/6g and iot solutions: prospects, directions, and challenges, applied sciences 9(20) (2019) art. no. 4367. doi: 10.3390/app9204367 [6] g. schirripa spagnolo, l. cozzella, f. leccese, s. sangiovanni, l. podestà, e. piuzzi, optical wireless communication and li-fi: a new infrastructure for wireless communication in saving energy era, in proceedings of 2020 ieee international workshop on metrology for industry 4.0 & iot, roma, italy, 2020, pp. 674-678. doi: 10.1109/metroind4.0iot48571.2020.9138180 [7] h. g. pfeiffer, h. a. liebhafsky, the origins of beer’s law, journal of chemical education 28(3) (1951), pp. 123. doi: 10.1021/ed028p123 [8] h. r. gordon, can the lambert‐beer law be applied to the diffuse attenuation coefficient of ocean water? limnology and oceanography 34(8) (1989), pp. 1389-1409. doi: 10.4319/lo.1989.34.8.1389 figure 16. our 4th order butterworth low pass filter frequency response. figure 17. 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communications fabio leccese1, giuseppe schirripa spagnolo2 1 dipartimento di scienze, università degli studi “roma tre”,via della vasca navale n. 84, 00146 roma, italy 2 dipartimento di matematica e fisica, università degli studi “roma tre”, via della vasca navale n. 84, 00146 roma, italy section: research paper keywords: underwater communication; visible light communications; optical wireless communication; bidirectional communication; led; photo detector citation: fabio leccese, giuseppe schirripa spagnolo, state-of-the art and perspectives of underwater optical wireless communications, acta imeko, vol. 10, no. 4, article 8, december 2021, identifier: imeko-acta-10 (2021)-04-08 section editor: silvio del pizzo, university of naples 'parhenope', italy received march 7, 2021; in final form june 17, 2021; published december 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: giuseppe schirripa spagnolo, e-mail: giuseppe.schirripaspagnolo@uniroma3.it 1. introduction underwater wireless communication (uwc) has many potential applications in the military, industrial and scientific research fields but, for practical applications, significant data bandwidth is required [1]-[3]. generally, underwater wireless communication takes place via acoustic waves due to their relatively low attenuation. they are the normal choice in almost all commercially available submarine transmission systems. unfortunately, acoustic systems have low bandwidth and high latency. therefore, they are not suitable for bandwidth-hungry underwater applications such as image and real-time video transmission. however, as acoustic transmission is the only technology capable of supporting large transmission distances, extensive studies are being conducted to improve the performance of acoustic communication channels [4]-[8]. anyhow, acoustic underwater communication is susceptible to malicious attacks [9]. consequently, complementary technology capable of achieving secure broadband underwater communications is required. wireless communication via radio frequency waves (rf) is the most widespread technology in terrestrial communications. sadly, this technology is not suitable for underwater applications. in water, radio frequency waves are strongly attenuated, especially in seawater where the propagation medium is highly conductive [10]. for short distance communications, underwater wireless optical communication (uowc) can be a viable alternative to that achievable via acoustic waves. this technology, even with all its limitations, can be of great use in specific applications. although not widely used yet, this article provides the state of the art in wireless underwater optical communication. table 1. shows a comparison of acoustic underwater wireless communication technologies vs. underwater wireless optical communication. optical communication is defined as communication at a distance using light to carry information. an optical fibre is the most common type of channel for optical communications, as well as the only medium that can meet the needs for enormous bandwidth in such an information age. replacing the channel abstract in scientific, military, and industrial sectors, the development of robust and efficient submarine wireless communication links is of enormous interest. underwater wireless communications can be carried out through acoustic, radio frequency (rf), and optical waves. underwater optical communication is not a new idea, but it has recently been considered because seawater exhibits a window of reduced absorption both in the visible spectrum and long-wavelength uv light (uv-a). compared to its bandwidth limited acoustic counterpart, underwater optical wireless communications (uowcs) can support higher data rates at low latency levels. underwater wireless communication networks are important in ocean exploration, military tactical operations, environmental and water pollution monitoring. anyway, given the rapid development of uowc technology, documents are still needed showing the state of the art and the progress made by the most current research. this paper aims to examine current technologies, and those potentially available soon, for underwater optical wireless communication and to propose a new perspective using uv-a radiation. mailto:giuseppe.schirripaspagnolo@uniroma3.it acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 26 from an optical fiber to free space, we achieve free space optical communications [11]. visible light communication (vlc) is a communication technology in which the visible spectrum is modulated to transmit data. due to the propagation distance of the light emitting diodes (leds), the vlc is a technology for short-range communication. pang et al. [12] first introduced the concept of using leds for wireless communication. visible-light communication (vlc) technology was developed to provide both lighting and data transfer with the same infrastructure [13]-[16]. vlc techniques transmit information wirelessly by rapidly pulsing visible light using light emitting diodes (leds). generally, the information data is overlaid on the led light without introducing flickering. the exhaustion of low-frequency bands to cope with the exponential growth for the high-speed wireless access is another reason for exploring new technologies. the visible light spectrum is unlicensed and hardware readily available, which can be used for data transmission. furthermore, the exponential improvement in the high-power light emitting diodes is an enabler for high data rate vlc network. as well as vlc, underwater optical wireless communications (uowcs) systems are currently being studied [17]-[21]. in the uowc systems, light sources are leds or laser diodes (lds). both are extremely interesting. lds for their feature higher modulation bandwidth respect to leds. on the other hand, leds, compared to lds, have higher energy efficiency, lower cost, and longer life. leds seem more suitable for applications where medium transmission bit rate is required. compared to acoustic communication, uowc has great potential; with it, we can make communications with high bit rate and very low latency. currently, the performance of uowc systems is limited to short range applications [22]. submarine optical communication systems are starting to be commercially available [23]-[25]. in the literature, numerous studies have addressed the problem of optical transmission in water through experiments. unfortunately, there are objective difficulties in carrying out the experiments in a real underwater environment. most of the experimental work is done within a controlled laboratory setup. in such configurations, sunlight which induces noise and which, in some cases, saturates the light detectors is neglected. however, in-depth studies are still necessary to create systems that can be used in real operational scenarios. researchers are needed to allow submarine optical transmission even over medium distances (greater than 500 m). table 1 shows the performance features (benefits, limitations, and requirements) of acoustic and optical underwater communication [26]. while, figure 1 compares the performance of acoustics, and uowc, based on transmission range and data rate (bandwidth) [27]. in order to provide a basic overview, we will go through and provide summary is to highlight the perspectives of uowc technologies. the focus of this is to examine current technologies and those potentially available in the next few years, for uowc. the military sector where underwater optical wireless communication finds important applications, thanks to its intrinsic safety and the availability of higher bandwidth. one possible application is communication between divers. during military incursions with divers, it is very important for the command to have secure communications that are difficult to locate. generally, underwater acoustic communications are easily detectable. in this scenario, uowc is an excellent technology. it has the advantage that it is much more difficult to intercept. this application does not require long range and high band. figure 2 shows a typical uowc military application scenario. another scenario is the one shown in figure 3. it is a dynamic positioning buoy [28], buoy capable of communicating with satellite and/or with a terrestrial station and with optical surveillance station positioned on the seabed. the surveillance station can be powered by nuclear batteries [29] and in real time control, through digital optical correlation [30]-[32], if something intrudes into the monitored area. in case of suspect object (e.g., a submarine) the image and related alert is sent back to the buoy and, from it, to the ground costal station via satellite link. this application can grand a very accurate underwater video surveillance. in addition, by using uv light for figure 1. compares the performance of acoustic, and uowc, based on the transmission range and the data speed (bandwidth). table 1. comparison of underwater wireless communication technologies. parameter acoustic optical attenuation 0.1 – 4 db/km 0.39 db/m (ocean) 11 db/m (turbid) speed 1500 ms−1 2.3 × 108 ms−1 data rate kbps gbps latency high low distance > 100 km ≤ 500 m bandwidth 1 khz–100 khz 150 mhz frequency 10–15 khz 5 × 1014 hz power 10 w mw – w figure 2. typical military application scenarios of uowc. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 27 underwater optical wireless communication, the intruder has a hard time realizing that he has been detected. in uowc the link between transmitter and receiver can be mainly of two types [20],[26]: • point-to-point line-of-sight (point-to-point los); • diffuse line-of-sight (diffuse los) configuration. the point-to-point los configuration, shown in figure 4 (a), uses “collimated” light sources. in this arrangement, the receiver is positioned in such a way as to detect the light beam directly pointed in the direction fixed by the transmitter. in contrast, the diffuse los configuration uses light sources with a large divergence angle. this allows for greater flexibility in the reciprocal positioning of the transmitter and receiver, see figure 4 (b). especially in military applications, where it is necessary to communicate between moving units, the diffuse line of sight configuration (diffuse los) must be used. theoretically, in a uowc system we could use any light source as a transmitter [33]. however, the limitations of power, size and switching speed imposed by the practical use of the system restrict the selection to two possible choices: laser diodes (ld) and light emitting diodes (led) laser diodes (ld) make it possible to develop uowc systems with a high modulation bandwidth and a high transmission power density. they generally have small angles of divergence and therefore a strong directionality. they are used in point-to-point los. in most underwater communications between moving objects, it is not easy to achieve perfect alignment between transmitter and receiver. in this scenario, for a realistic application of the laser diodes, beam expansion or active alignment systems are required. this greatly complicates the design of the system. furthermore, such systems are not very economical and often not very reliable. nowadays, high-brightness leds are available, and they represent a valid alternative to the use of laser diodes. the use of leds as light sources for uowc systems offers many advantages such as long life, low energy consumption. in addition, leds with large divergence angles make alignment problems less stringent. generally, leds are used in the diffuse los configuration. by means of leds, it is possible to create simple and compact uowc systems. unfortunately, due to the large divergence angles and low modulation bandwidth, leds are only applicable for short-range transmissions and for applications where relatively low transmission speeds are required. as receivers, a variety of sensors is potentially usable in uowc [34]-[36]: photodiode, pin photodiode, avalanche photo diode, silicon photomultipliers. 2. optical transmission in the aquatic medium beer’s law is commonly used to relate the absorption of diffuse light to the properties of the medium through which the light is traveling. when applied to a liquid medium, it states that the irradiance (e) decreases exponentially as a function of wavelength (λ) and distance (r ) [37],[38]. mathematically, we can write 𝐸(λ, r) = 𝐸0 ∙ exp[−𝐾d(λ) ∙ 𝑟] , (1) where e0 is the initial irradiance (in watts per square meter). in a medium with attenuation coefficient kd (λ), after traveling a distance r, the residual irradiance is e(λ, r). in (1), we assume that kd is constant along r. the aquatic medium contains many different elements, dissolved or suspended. these components cause the spectral attenuation of the radiation. in particular, the concentration of chlorophyll is a very significant parameter for the use of optical radiation in submarine communications [39]-[41]. for this reason, a relationship between the attenuation coefficient kd and the chlorophyll concentration was determined. underwater, the light shows less attenuation in the blue/green wavelength range. however, although light attenuation in seawater is minimum in the blue-green region, the optimal wavelength for underwater optical link is conditioned from the inherent optical properties of the water, which can largely vary in different geographic places. generally, coastal, and oceanic waters are classified according to the jerlov water types [42]-[45]. for jerlov coastal water types figure 3. underwater video surveillance scenario. figure 4. examples of different underwater optical wireless link configuration. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 28 1c, 3c, 5c, 7c, 9c and oceanic water type iii, diffuse attenuation coefficients are shown in figure 5. observing the figure 5, we see that the optical signals are absorbed in water. however, seawater exhibits relatively little absorption in the blue/green region of the visible spectrum. therefore, using wavelengths in this spectral region, high-speed connections can be attained according to the type of water. minimum attenuation is centred near 460 nm in clear ocean waters and shifts to higher values for coastal waters. seawater light transmission model is shown in figure 6. the optical power reaching the receiver can be written as [47][50]: 𝑃rx=𝑃tx∙𝜂tx∙𝜂rx∙exp [− 𝐾d(𝜆)∙𝑧 cos 𝜃 ] ∙ 𝐴𝑅𝑥 ∙ cos 𝜃 2 π∙𝑧2(1 − cos 𝜃0) , (2) where 𝑃tx is the transmitted power, 𝜂tx and 𝜂rx are the optical efficiencies of the 𝑇𝑥 and 𝑅𝑥 correspondingly, kd (λ) is the attenuation coefficient, 𝑧 is the perpendicular distance between the 𝑇𝑥 plane and the 𝑅𝑥 plane, 𝜃0 is the 𝑇𝑥 beam divergence angle, 𝜃 is the angle between the perpendicular to the 𝑅𝑥 plane and the 𝑇𝑥‐ 𝑅𝑥 trajectory, and 𝐴rx is the receiver aperture area. the transmitted power is limited by the energy that can be used by the transmitter apparatus. it must be as small as possible. in this way, it is possible to have low power supply; very useful in underwater applications. equation (2) shows that for the same energy used by the transmitter, if you want to increase the transmission distance, it is essential, among other things, to improve the efficiency of the transmitter and of the receiver. obviously, the transmission distance can also be increased by using reception systems capable of capturing, theoretically, even the single photon. as for light sources, technology offers increasingly efficient and reliable devices. current light sources (laser diode and led) have excellent efficiency, high reliability, low power consumption and low cost. on the contrary, as far as the receiver is concerned, there is still a lot of research work to be done. generally, the detected light from the receiver is small and disturbed by noise, especially if the transmission is not over a very short distance. for this reason, new error-corrected modulation systems that are relatively immune to noise must be studied, especially if we want to use submarine optical transmission with high bitrate. 3. basic components of uowc an uowc link can be schematized in three parts, the transmitter unit (tx), the underwater channel and the receiver module (rx). the schematic in figure 7 shows the components of a typical system. 3.1. the transmitter (tx) for uowc systems, the function of the transmitter is to transform the electrical signal in optical one, projecting the carefully aimed light pulses into the water. as already mentioned, the optical light sources are based on led or ld one [51]–[58]. the transmitter consists of four principal components: a modulator and pulse shape circuit, a driver circuit, which converts the electrical signal into an optical signal suitable for transmission and a lens to realize the optical link configuration. a critical parameter in optical transmission is the modulation scheme used. different modulation schemes can be used in uowc systems. each varies in complexity, implementation cost, bandwidth, power consumption, noise robustness, and bit error rate (ber). table 2 shows a summary on uowc modulation schemes. typical rf modulating schemes are not applicable in vlc. recent uowc studies have tried to characterize the performance of communication systems using different modulation techniques to increase together the data transmission rate and the link distance [59]–[66]. table 1 summarizes the main modulation schemes that can be used in the uowc [67]. 3.2. the receiver (rx) the receiver has the task of capturing the transmitted optical signal and transforming it into an electrical signal. in many applications, it is important to select a specific wavelength that impacts on the light detector [86]. the light figure 5. diffuse light attenuation coefficient (kd) vs. wavelength for jerlov water types. data from tables xxvi and xxvii in ref. [46]. figure 6. seawater light transmission model. figure 7. schematic of a typical uowc link. the transmitter (tx) is composed of a modulator, optical driver, light source, and projection lens. the receiver (rx) is made of optical bandpass filter, photodetector, low noise electronics and demodulator. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 29 coming on the receiver should have no noise introduced by sunlight and the presence of other light sources [87]. to try to solve this problem, the wavelength band (the one transmitted) is selected by using a narrow optical band-pass filter [88],[89]. when the receiver receives the transmitted optical signal, this provides to transform it into an electric signal by using photo detectors. there are many different types of photo detectors currently commonly used, e.g., the photodiodes. these devices, for their characteristics of small size, suitable material, high sensitivity, and fast response time, are commonly used in optical communication applications. there are two types of photodiodes: the pin photodiode and the avalanche photodiode (apd). unfortunately, due to the high detection threshold and high noise intensity, linked to trans-conductance amplifier, that limit their practical application, photodiodes are not advisable for long distance uowc systems. for traditional detection devices and methods, due to the exponential attenuation of the water, the optical communication distance is less than 100 m [43],[90]. this constraint severely limits the performance of uowc systems. especially for the management of auvs and remotecontrol vehicles (rov) [91]-[95]. recent research is focused on the possible application of single photon avalanche diodes (spads) technology to uowc systems. the avalanche photodiodes have a similar structure to that of the pin photodiodes and operate at a much higher reversed bias. this physical characteristic allows to a single photon to produce a significant avalanche of electrons. this way of operation is called the single-photon avalanche mode or even the geiger’s mode [96]-[98]. the great advantage of spads is that their detectors do not need to a trans-conductance amplifier. this intrinsically leads that optical communications realized with this kind of diodes could provide high detection, high accuracy, and low noise measurements [99]-[108]. 4. underwater communications by uv-a radiation in the literature, almost all studies do not consider the presence of sunlight. it is inevitable that uowc systems are exposed to sunlight. furthermore, it should be noted that the optical absorption spectrum of seawater aligns with the maximum amplitude of the solar spectrum, see figure 8 [109]. generally, solar intensity decreases with depth. by examining how light is absorbed in water, see figure 5, we see that the best wavelengths to use in uowc are 450-500 nm for clear waters and 570-600 nm for coastal waters. this same attenuation is also true for the solar spectrum. figure 9 shows how sunlight penetrates seawaters. in the presence of sunlight, the receivers see very high white noise and can often go into saturation. this problem is particularly important with spads. of course, in real applications, the viewing direction of the photo-sensor is also important. an upward facing detector is exposed to sunlight a few orders of magnitude greater than when facing downward or to the side. all this, in many practical applications, makes it difficult to use the spectrum of visible light. for this reason, submarine communication systems that use uv-a band communication channels are extremely interesting. we must also observe two other important characteristics of optical communication that uses near ultraviolet. (1) this communication channel is not identifiable and difficult to intercept. it is particularly attractive for military applications. (2) using uv radiation makes it easier to maintain alignment between transmitter and receiver [111],[112]. an important application of uowc is underwater communication from diver-to-diver. there are commercially available audio interphones that work quite well. table 2. summary on uowc modulation schemes. modulation drawbacks advantages ref. ook-nrz low energy efficiency simple and low cost [68]-[70] ppm high requirements on timing low bandwidth utilization rate more complex transceivers high power efficiency [71]-[75] pwm low power efficiency very good noise immunity [20], [58] dpim error spread in demodulation complex modulation devices high bandwidth efficiency [58], [76], [77] psk high implementation complexity high cost high receiver sensitivity [78]-[80] qam high implementation complexity high cost high system spectral efficiency better rejection on noise [80]-[82] polsk short transmission distance low data rate high tolerance to underwater turbulence [83], [84] sim complex modulation/demodulation devices and suffers from poor average power efficiency increase system capacity low cost [85], [86] figure 8. solar irradiance and oceanic water diffuse light attenuation coefficient abortion. the curves show that the minimum of the optical absorption of water is aligned with the maximum of solar radiation. since sunlight is an important source of noise, the best ratio propagated signal on solar radiation is obtained using radiation cantered around 385 nm. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 30 however, these systems in military tactical applications (such as raids by military divers) have the drawback of being easily identifiable. in order to understand if a system capable of implementing a communication between divers that is not easily identifiable is feasible, some preliminary studies have been carried out. in particular, we tried to understand if it is possible to create a compact uowc system that requires few energy resources. for this purpose, we tried to verify the feasibility of an optical communication made through a led-to-led link. due to the growing demand for high-power uv leds for commercial applications, cost-effective and efficient leds that emit in the near ultraviolet (uv-a) are currently available. leds with emission in the range from 350 to 400 nm (uva) are light sources that allow you to work just beyond the visible light radiation. that is, in that region of the spectrum where most of the sunlight does not penetrate the water. this part of the spectrum is still quite close to the minimum attenuation in water. therefore, it is useable in uowc systems. in addition to be excellent light sources, leds can be used both as temperature sensors [113] and as light detectors [114],[115]. leds can detect a narrow band of wavelengths, close to what they emit when used as a source. in our experiment, we used a bivar uv5tz-385-30 led as a transmitter and a bivar uv5tz-390-30 led as a receiver [116]. light intensities vs. the wavelengths of the leds used are shown in the figure 10. the two leds were inserted in a tank filled with real seawater (water taken from the tyrrhenian coast anzio italy). the leds are placed at a distance of 50 cm and facing one towards the other. the led used as a transmitter was driven by the circuit shown in the figure 11. the led driver shown in the figure has a restricted baud rate. the main reason is the limited switching speed of silicon devices. a maximum data rate of 100 kbps can be achieved with this driver. in any case, this speed of data transmission is more than enough to implement an excellent audio connection. obviously, if the driver is made with transistors made in gan technology, data transmissions with speeds higher than 1 mbps can be obtained [118]. figure 12 shows the rx led driver circuit. figure 13 shows the signal received by the led used as light detector. the transmitter led is polarized with 25 ma and switched with a frequency of approximately 80 khz. figure 9. (a) spectral irradiance of sunlight at the level of the sea; (b) light penetration in open ocean, (c) light penetration in coastal water. [110]. figure 10. spectral intensity of the leds used as tx and rx. figure 11. tx led driver circuit using mic3289 pwm boost switching regulator [117]. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 31 the received signal, after traversing 50 cm of seawater, is “good”. obviously, further studies are needed to implement and characterize an underwater uv-a communication realized using the led-to-led link. 5. conclusions recently many studies have been conducted to use uowc technology to transmit information safely with high data rate in underwater environment. today, uowc systems usable in real operating conditions (with some exceptions) are not yet available. therefore, a lot of research in this area has yet to be done. in particular: • currently, an inevitable phenomenon for uowc link is the misalignment between transmitter and receiver. to limit the impact of misalignments between transmitter and receiver, research is underway for the development of smart transceivers. however, the need to develop robust and reliable uowc transceivers that do not require rigorous alignment is urgent. • the design innovative modulation and coding schemes that can adapt the characterizations of underwater environment. • since most uowc systems are integrated into a batterypowered platform, energy efficiency is therefore important. the systems must be designed with high energy efficiency. • possibility of using different colored light sources at the same time to increase data transfer speed and / or allow simultaneous use by multiple users. • development of new underwater communication channel modeling. when environmental conditions deviate from ideality, the light signal rapidly degrades. it is essential to study the propagation of the light beam with models that simulate real conditions as much as possible (even in “difficult”" environments). all this to allow the optimization of transmission and reception techniques, both in terms of transmitter and sensor used as receiver. furthermore, we have presented a preliminary study to verify the feasibility of a simple, economical and reliable communication system that uses uv-a radiation. the possibility of using near ultraviolet radiation should favor the development of uowc systems that can also be used in the presence of solar radiation. finally, almost all the studies available in the literature are conducted by simulation or by laboratory experiments. studies in real marine environment are needed. the interest in uowc is mainly outside the academic field. in fact, the possibility to use uowc is based on future military applications for secure under water telephones (uwts), necessary for allowing secure communications between vessels and submarines, considering the possibility to use both direct and spread light channels. in addition, the usage of point-to-point optical communications can allow a better usage of torpedoes, not specifically for their guidance, but for reporting sonar information back to the base station with a high rate, even in case of not wire-guided solution. references [1] i. f. akyildiz, d. pompili, t. melodia, underwater acoustic sensor networks: research challenges, ad hoc networks 3(3) (2005), pp. 257 – 279. doi: 10.1016/j.adhoc.2005.01.004 [2] c. m. g. gussen, p. s. r. diniz, m. l. r. campos, w. a. martins, f. m. costa, j. n. gois, a survey of underwater wireless communication technologies, j. of commun. and info. sys. 31(1) (2016), pp. 242–255. doi: 10.14209/jcis.2016.22 [3] m. f. ali, d. n. k. jayakody, y. a. chursin, s. affes; s. dmitry, recent advances and future directions on underwater wireless communications, archives of computational methods in engineering, pp. 1-34. 2019. doi: 10.1007/s11831-019-09354-8 [4] e. demirors, g. sklivanitis, g.e. santagati, t. melodia, s. n. batalama, high-rate software-defined underwater acoustic modem with real-time adaptation capabilities, ieee access (6) (2018), pp. 18602-18615. doi: 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https://ww1.microchip.com/downloads/en/devicedoc/mic3289.pdf https://doi.org/10.1038/srep30991 https://www.analog.com/media/en/technical-documentation/data-sheets/1050fb.pdf https://www.analog.com/media/en/technical-documentation/data-sheets/1050fb.pdf fault compensation effect in fault detection and isolation acta imeko issn: 2221-870x september 2021, volume 10, number 3, 45 53 acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 45 fault compensation effect in fault detection and isolation michał bartyś1 1 warsaw university of technology, institute of automatic control and robotics, boboli 8, 02-525 warsaw, poland section: research paper keywords: fault compensation effect; fault masking; fault isolation; diagnostics of processes; fault distinguishability citation: michał bartyś, fault compensation effect in fault detection and isolation, acta imeko, vol. 10, no. 3, article 9, september 2021, identifier: imekoacta-10 (2021)-03-09 editor: lorenzo ciani, university of florence, italy received january 25, 2021; in final form august 29, 2021; published september 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was funded by the pob research centre for artificial intelligence and robotics of warsaw university of technology within the excellence initiative program research university (id-ub). corresponding author: michał bartyś, e-mail: michal.bartys@pw.edu.pl 1. introduction the model-based diagnostics of industrial processes intensively makes use of residuals [1], [2], [3]. the residuals express to which extent measurements (observations) and outputs of a diagnosed system differ from expected system behaviour predicted by the reference model of the system. figure 1 depicts the general block scheme exemplifying the basic workflow in the model-based fault detection and isolation approach (fdi) [1]. it generally consists of three consecutive steps: detection, isolation, and identification of faults. the main goal of fault detection is to detect the diagnosed system's abnormal behaviour, while the isolation (localization) points out the faults that potentially occurred. on the other hand, fault identification allows for recognizing the size of a fault. frequently, fault identification is not of concern in industrial applications. therefore, for simplicity, this step is not shown in figure 1. to react appropriately to faults, the process operator or fault-tolerant control system demands univocal isolation of faults. however, this is not a trivial task. the discrepancies r (residuals) between model �̂� and process 𝐕 outputs are indicative of a potential fault or faults. however, this is true under the condition that residuals are sensitive to the faults [1], [2]. furthermore, we assume that the diagnostic system is designed so that this postulate is met. figure 1. a block diagram of the basic workflow in model-based fault detection and isolation approach (fdi). notions: r residuals, v process outputs, v̂ model outputs, s – diagnostic signals, f – faults. abstract this paper discusses the origin and problem of the fault compensation effect. the fault compensation effect is an underrated common side effect of the fault isolation approaches developed within the fault detection and isolation (fdi) community. in part, this is justified due to the relatively low probability of such an effect. on the other hand, there is a common belief that the inability to isolate faults due to this effect is the evident drawback of model-based diagnostics. this paper shows how, and under which conditions, the fault compensation effect can be identified. in this connection, the necessary and sufficient conditions for the fault compensation effect are formulated and exemplified by diagnosing a single buffer tank system in open and closed-loop arrangements. in this regard, we also show the drawbacks of a bi-valued residual evaluation for fault isolation. in contrast, we outline the advantages of a three-valued residual evaluation. this paper also brings a series of conclusions allowing for a better understanding of the fault compensation effect. in addition, we show the difference between fault compensation and fault-masking effects. mailto:michal.bartys@pw.edu.pl acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 46 in the fault-free (normal) state of the diagnosed system, the residuals should converge to zero. however, considering the uncertainty of measurements and impreciseness of reference models applied, the residuals take values relatively close to zero. in practice, the residuals are being discretized through the constant or adaptive thresholding approaches [4]. as a result, the continuous or piecewise continuous residuals are converted into bi-, or three-valued crispy or fuzzy values referred to as diagnostic signals [5]-[7]. a set of diagnostic signal values associated with each particular fault creates its specific signature (pattern), typically taking the form of a column vector. the structure of signatures of all faults is referred to as the incidence matrix or structure of residual sets or diagnostic matrix [1]-[3], [5], [6]. the signatures allow for distinguishing faults under the condition that all signatures of all faults are unique. in general, this condition is not satisfied [5]. the main reason is that the number of measurements is lower than the total number of possible faults, including instrumentation and system components faults [6]. therefore, we should accept the fact that some faults will remain undetectable or indistinguishable. we consider this feature a severe drawback of the model-based fdi approaches. to at least overcome this problem, many approaches were developed that allow for increasing fault distinguishability. however, it was proven in [5] that, in general, this task is unsolvable. with regard to functional safety [8], [9], there is defined tolerable risk. according to a commonly recognized definition [9], tolerable risk is "a level of risk deemed acceptable by society in order that some particular benefit or functionality can be obtained." by analogy, we can claim that by employing a model-based diagnosis, if the risk of either undetectable dangerous or safe faults is deemed acceptable, then the fdi makes sense. however, this involves presuming some simplifications and undertaking some assumptions. for example, frequently, the assumption regarding the infallibility of measurement devices or the credibility of observations is adopted [10]. it is a case in both branches of the model-based diagnostics developed by the fdi and dx research communities [11]-[13]. later, in this paper, we assume the infallibility of measurement instruments. it may be explained, particularly for diagnosing industrial systems, employing high-reliability instruments exhibiting at least sil1 safety integrity level. the rationality of this assumption reinforces statistics of failures of industrial equipment [14]. the aforementioned explanations justify to some extent the assumption commonly adopted in the fdi regarding the infallibility of instruments. also, the assumptions regarding uncertainties of residuals, diagnostic signals, and models are discussed intensively in the context of fdi [2], [5], [7], [15]. the uncertainty of measurements is a fundamental problem of metrology. it has been discussed in series of publications for many years, i.e., in [16]-[18]. in the model-based diagnostics of processes, we have at least five different sources of uncertainties connected with measurements, models, residuals, residual evaluation, and fault diagnostic signals relation. in fact, the problem of uncertainty is common for metrology and diagnostics. in diagnostics, the measurements are intensively used for residual generation (figure 1). therefore, the uncertainty of the measurements impacts the uncertainty of residuals. on the other hand, the residuals' uncertainty also depends on the uncertainty of the reference model of the diagnosed system. the uncertainty of the model indirectly reflects its grade of perfection. therefore, uncertainties of measurements and models result in the uncertainty of the residuals. later on, the residuals are evaluated and take the form of socalled diagnostic signals. hence, the way how residuals are evaluated contributes to the overall uncertainty of diagnostic signals as well. finally, the diagnosis is based on inference using faultdiagnostic signals relation, or subjective logic, or expert knowledge [2]. thus, there is also an uncertainty in inferring about faults [15]. it is also important to mention that the complex problem of uncertainty of diagnosing has not been holistically solved yet. this paper deals mainly with the problem of the fault compensation effect and intends to expose some weaknesses of the fdi model-based diagnosing. the deliberations regarding the uncertainty of the fault compensation effect are beyond the scope of this paper. therefore, keeping in mind the paper's main objective, the uncertainty of measurements will not be considered further. several fdi methods assume and consider exclusively single faults [6]. this assumption is allowable for diagnosing relatively non-complex systems. according to occam's scissor rule, the single faults in non-complex systems are more likely than multiple. while the fault compensation effect is not a property of a system with single faults, we focus our attention exclusively on multiple fault cases. in the case of diagnosing complex systems, multiple faults are more likely [20]. therefore, in these systems, there is to expect occurrences of fault compensation effects. there is to mention that problem of the fault compensation effect is poorly represented in the literature. the fault compensation is the undesired and unpredictable side effect of multiple fault isolation based on signatures of all single faults, constituting multiple faults. this effect appears in all fdi approaches, in which multiple faults' signatures are obtained as the unions of signatures of all single faults constituting multiple ones [2], [6]. the union of bi-valued signatures is defined as a boolean alternative of signatures of all faults creating multiple ones. by three-valued signatures, the union of single fault signatures is slightly more complex [20]. developing multiple fault signatures based on single ones has some practical background. as far as the diagnosed system's phenomenological models are not available, the multiple fault signatures are not easy to obtain based on process data or process operators' expertise. moreover, frequently some multiple faults have never been registered or ought not to appear for process safety reasons, e.g., in the nuclear power stations. however, for clarity, this paper will use an analytical phenomenological model to explain the fault compensation effect. the fault compensation is understood differently even within the fdi research community. firstly, fault compensation is meant as an approach to sustain the system's nominal operation even when a fault occurs. this understanding of fault compensation is typical for the different fault tolerant control (ftc) approaches [3]. for example, the unique approach towards ftc can be found in [21]. here, the additional signals are superimposed on the controlled system's inputs to compensate for faults' effects. over there, fault compensation refers to understanding a fault, preferably in terms of a specific disturbance imposed on the control system. it is important to mention that regular control loops have also embedded inherent compensating abilities for the small size acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 47 faults. the ability to compensate for faults impacts is frequently called the fault masking effect [22], [23]. this paper faces a different understanding of the fault compensation effect. secondly, the fault compensation in fdi is understood as an effect of zeroing residuals values, despite faults [2], [20]. in other words, then the faults that occurred cannot be detected nor isolated based on residuals. therefore, in this case, the fdi completely fails and, depending on conditions, the multiple faults cannot be temporarily or permanently isolated. as far as the following conditions hold: • residuals are sensitive to at least two single faults, • multiple fault signatures are unions of all single faults constituting multiple ones, • different single faults act on residuals in opposite directions, then the fault compensation effect may occur. it is to recognize that those conditions are easily satisfied in the majority of industrial fdi approaches. therefore, we can conclude that fault compensation seems to become a significant practical problem. this statement implies inspiring motivation for a deep-in discussion of this effect in this paper. in conclusion, the fdi approaches to isolating multiple faults should be criticized as they may lead to misdiagnosis in a case of a fault compensation effect. the paper contributes both to the theory and practice. the primary outcome of the paper is the novel formulation of necessary and sufficient conditions for the fault compensation effect and, in turn, formulation of the sufficient condition for excluding this effect. the defined conditions contribute to the fdi theory and practice by proposing a method for seeking potential fault compensation effects by design or analyzing a diagnostic system. we also formulate a set of recommendations that have some practical meaning. they contribute and extend the set of good practices applicable to the design of diagnostic systems. the remainder of this paper is structured as follows. section 2 describes a nominal model of a single tank system, which we intensively exploit in this paper. section 3 presents an approach to fault detection and isolation based on an analytical description of the residuals in the inner form. section 4 illustrates the fault isolation based on biand three-valued residuals. section 5 discusses chosen results of the simulation, while section 6 outlines the problem of the fault-masking effect. finally, section 7 summarizes the achieved results. 2. the nominal model of the system the fault compensation problem will be explained based on the example of the model-based diagnosing workflow of a simple open-loop control system shown in figure 2. let the diagnostic problem rely on isolating two single faults: leakage in the tank (fault f1) and obliteration of the outlet pipe (fault f2) as well as one double fault {f1 ∧ f2}. the double fault represents the faulty state where the leakage and obliteration take place at the same time. for simplicity, we assume that used instruments are infallible. firstly, according to the schematic shown in figure 1, we develop the process's nominal (reference) model in a faultfree state. for this reason, we propose the phenomenological model of the process. this model will be exploited further for the closed-loop control system too. many other models are imaginable in this stage, including these, based on heuristic knowledge, fuzzy sets theory, fuzzy neural networks, and neural networks [1]-[3], [5], [7]. next, we assume the availability of measurements shown in table 1, except optional flow rate f1. in a fault-free state, for incompressible and inviscid liquid, the fluid accumulation in the tank is equal to the difference of inflow and outflow volumes. hence, the liquid volumetric inflow rate 𝐹0 is equal to 𝐹0 = 𝐴 𝑑𝐿 𝑑𝑡 + 𝛼𝑆√2𝑔𝐿 , (1) where 𝐴 is the cross-sectional area of the tank, 𝐿 is the liquid level in the tank relative to the outlet pipe axis, α is the outflow contraction coefficient, s is the nominal cross-sectional area of the outlet pipe, and g is the gravitational constant. eq. (1) will be further referred to as the nominal model of the process. 3. fault detection generally, fault detection should indicate whether the fault or faults occurred or not. we assume that a discrepancy between the nominal and process outputs will occur in a faulty state. however, this is true under two essential conditions: • residuals are sensitive to the faults which occurred; • the fault compensation effect does not take place. the paper's objective is principally concerned with the second condition. to obtain residuals, we assume three faults listed in table 2. next, we develop the model of the diagnosed system in the so-called inner form [6], i.e., in the way which reflects the impacts of faults. 𝐹0 𝑓 = 𝐴 𝑑𝐿 𝑑𝑡 + 𝛼𝑆√2𝑔𝐿 + 𝑓1𝛼𝑙 𝑆√2𝑔(𝐿 − 𝐿𝑙 ) − 𝑓2𝛼𝑆√2𝑔𝐿 , (2) where 𝐹0 𝑓 is the tank inflow rate in a faulty state; αl is the leakage outflow contraction coefficient; 𝐿𝑙 is the distance from the center of the area of leakage orifice to the axis of outlet pipe; 𝑓1 = 𝑆𝑙 /𝑆; 𝑓2 = 1 − 𝑆𝑜/𝑆; sl is the cross-sectional area of the leakage orifice; 𝑆𝑜 is the cross-sectional area of the outlet pipe. while residual 𝑟 = 𝐹0 − 𝐹0 𝑓 , then from (1) and (2) we obtain: 𝑟 = −𝑓1𝛼𝑙 𝑆√2𝑔(𝐿 − 𝐿𝑙 ) + 𝑓2𝛼𝑆√2𝑔𝐿 . (3) figure 2. the schematic of the process considered for diagnosing. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 48 therefore, the residual r equals zero if the model and process outputs are identical. however, this cannot be interpreted unambiguously as a fault-free state of the system, while the residual r may also take zero value in a faulty state due to the impact of faults on residuals. nevertheless, this effect occurs exclusively for multiple faults. from (3), we can easily withdraw a simple condition for fault compensation effect. 𝑓1 𝑓2 = 𝛼 𝛼𝑙 √ 𝐿 (𝐿 − 𝐿𝑙 ) . (4) the probability of the fault compensation effect is relatively low. however, this effect is the reason for false-negative fault isolation, and therefore, it should be avoided as far as possible. this paper shows how it may be possible. the following observation would be helpful here: the effect of fault compensation does not occur for single faults and for those multiple faults for which all residuals are unidirectionally affected, i.e., possess the same sign. from this observation, we can draw some practical conclusions. conclusion 1. the design of the diagnostic systems in which residuals are sensitive exclusively to single faults is strongly recommended for fdi because it avoids fault compensation effects. conclusion 2. consideration of residual signs may help increasing achievable fault distinguishability while bringing additional useful knowledge for diagnostics. conclusion 1 corresponds well with an idea for developing a family of intelligent single fault detectors [24] and with the concept of the diagonal structure of residual sets proposed in [6]. however, it sounds slightly unrealistic in nowadays world. therefore, the question arises on how we can avoid fault compensation effects if they are typical even for elementary processes, as shown in figure 2. there is no good general answer to this question. nonetheless, we can consider some productive actions. according to conclusion 1, the excellent solution seems to have an equal number of nominal models with the number of single faults, such that each model would be referred exclusively to one fault. let us now consider the same system as in figure 2. the only difference is that we now will use the additional flow rate instrument, i.e., f1. now, the nominal partial models of the process are { 𝐹0 = 𝐴 𝑑𝐿 𝑑𝑡 + 𝐹1 𝐹1 = 𝛼𝑆√2𝑔𝐿 , (5) the models in the inner form reflecting the impact of faults are { 𝐹0 𝑓 = 𝐹0 + 𝑓1𝛼𝑙𝑆√2𝑔(𝐿 − 𝐿𝑙 ) 𝐹1 𝑓 = 𝐹1 − 𝑓2𝛼𝑆√2𝑔𝐿 . (6) from (6), we obtain residuals: { 𝑟1 = −𝑓1𝛼𝑙 𝑆√2𝑔(𝐿 − 𝐿𝑙 ) 𝑟2 = +𝑓2𝛼𝑆√2𝑔𝐿 (7) as can be easily seen from (7), each residual is sensitive exclusively to a single fault. it promises to avoid the fault compensation effect at the expense of an additional flowrate measurement instrument. in this case, the double fault is easily recognizable (isolable) because both residuals (𝑟1 and 𝑟2) adopt non-zero values and opposite signs. from the above considerations, it follows that: conclusion 3. there is a trade-off between the quality of diagnoses and the number of sensors (instruments) applied in real-world systems. it should be mentioned that by the limited availability of sensors, the solution of the sensor placement problem would help maximize fault distinguishability and minimize fault compensation effects [25], [26]. 4. fault isolation the primary goal of fault isolation is to indicate the faults that occurred in the process. this diagnosing step is frequently called fault location or simply diagnosing. diagnosing requires a knowledge of the relation between the faults and diagnostic signals. we can express this relation in the analytical form, for example, as in (3) and (7). if the analytical relations are unknown, the process graphs (gp) [27] could be helpful. the process graph is a directed bipartite graph used in workflow modeling. in considered case, vertices of the gp graph represent disjunctive sets of process states and faults. the graph's edges link the faults with the states and between states, thus reflecting the process flow. the gp graph is handy for analyzing the qualitative impact of faults on process states. physical or virtual quantities represent the process states. in particular, the process states may be represented by measurements. figure 3 depicts the gp graph developed for the single tank open-loop control system shown in figure 2. this graph reflects equation (3) and refers to a situation where flow rates f0 and f1 are not available. from this graph, it can be seen that both single faults act in opposite directions on the liquid level. therefore, both faults may mutually compensate for their impacts. based on this statement, we will formulate two practical conclusions. conclusion 4. the possibility of the occurrence of the fault compensation effect is immediately detectable from the directed graph of the process. conclusion 5. it is necessary for fault compensation if at least one vertex in the gp graph is linked directly with fault vertices by edges labeled with opposite signs. the gp graph derived from equation (7) takes the shape shown in figure 4. table 1. list of available measurements. item symbol measured quantity 1 f0 liquid volumetric inflow rate 2 l liquid level 3 f1 liquid volumetric outflow rate (option) table 2. list of considered faults. item symbol fault 1 f1 leakage from the tank 2 f2 obliteration of the outflow pipe 3 f1 ⋀ f2 leakage and obliteration https://en.wikipedia.org/wiki/directed_graph https://en.wikipedia.org/wiki/bipartite_graph https://en.wikipedia.org/wiki/workflow https://en.wikipedia.org/wiki/conceptual_model acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 49 in figure 4, both faults are linked with different graph vertices. therefore, the necessary condition for fault compensation is not satisfied. in this case, the double fault bivalued signature based on the union of single fault signatures is correct and may distinguish single and double faults. from the above considerations, it follows the demand for reliable instrumentation. conclusion 6. avoidance of fault compensation effect is highly demanding on reliable measurements. the quantitative fault isolation needs to deploy the incidence matrix [6]. the usability of the gp graph in this scope is very limited. it provides only a cause-and-effect qualitative description of the process. the incidence matrix reflects the relation between faults and diagnostic signals. the question is, why are diagnostic signals being used instead of residuals? principally, fault isolation is the process of inference about faults that uses some logical rules. usually, boolean or lukasiewicz n-valued or fuzzy logic is applied. therefore, there is necessary to transform continuous residuals into discrete logical values or a finite set of predefined fuzzy membership functions. for these goals, we use the constant or adaptive discrimination thresholds [4]. in this paper, we will limit our considerations exclusively to elementary, however practicable, thresholding of residuals, which introduces some dead zones to residual values. for the binary assessment of residuals, we will further apply the formula: 𝑠 = { 0 ← |𝑟| < 𝑇h 1 ← |𝑟| ≥ 𝑇h , (8) while for the three-valued assessment of residuals, we will use: 𝑠 = { −1 ← 𝑟 ≤ −𝑇h 0 ← |𝑟| < 𝑇h +1 ← 𝑟 ≥ 𝑇h , (9) where: s is the diagnostic signal and th is an arbitrarily chosen nonnegative threshold. according to (8) and (9), the diagnostic signals are bior three-valued. the robustness of fault isolation, among others, can be characterized by the rate of false-positive and false-negative diagnoses. the false-positive diagnoses indicate non-existing faults, while false-negative diagnoses do not indicate existing faults. as one can infer from formulas (8) and (9), the introduction of dead zones immunizes somehow diagnostic signals against uncertainties and noise, however, at the expense of loss in sensitivity and elongation of fault isolation time. this paper will discuss both (8) and (9) residual evaluation approaches in the context of the fault compensation effect. we will show that fault compensation under some conditions may be determined from the incidence matrix. first, let’s refer to the incidence matrix presented in table 3. here, the entries are bivalued as in (8). therefore, this matrix is also referred to as a binary diagnostic matrix (bdm) [6]. it contains reference diagnostic signal values (signatures) expected by the occurrence of a fault or faults. in table 3, all signatures of all considered faults are identical. therefore, in a faulty system state, we cannot point out which fault or faults occurred. in other words, all three faults from the table 3 are indistinguishable. hence, the quality of obtained diagnosis is unacceptable. moreover, based on table 3, we cannot verify the hypothesis regarding the fault compensation effect. this simple example leads to the following conclusion: conclusion 7. the binary diagnostic matrix itself is useless for the recognition of a fault compensation effect. next, we discuss the case of the three-valued incidence matrix shown in table 4. here, the values of reference diagnostic signals of a double fault are in the set of all reference diagnostic signals of single faults constituting multiple faults, including the faultfree state. for example, diagnostic signal s in table 4 may have three alternative values -1 or 0 or +1. the complete procedure of synthesizing multiple fault reference signatures based on single fault reference signatures is described in [20]. now, we can easily distinguish single faults f1 and f2 because reference signatures of both faults are distinctive. however, both single faults are conditionally indistinguishable from a double fault. moreover, the double fault may not distinguish from the process's fault-free state by diagnostic signal (s = 0). the fault compensation effect, if any, will manifest by (s = 0). therefore, fault-free state, double fault state, and fault compensation effect are still indistinguishable. however, there is to mention that under some conditions: figure 3. the gp graph is reflecting the qualitative impact of faults on the values of process variables. a circle coloured in yellow depicts the available measurement. figure 4. the gp graph for the single tank system reflecting the additional flowrate measurement f1. table 3. diagnostic matrix for binary discretized residual (2). f/s fault-free f1 f2 f1 ∧ f2 s 0 1 1 1 table 4. trinary diagnostic matrix for residual (2). f/s fault-free f1 f2 f1 ∧ f2 s 0 -1 +1 -1, 0, +1 acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 50 conclusion 8. the incidence matrix containing three-valued reference diagnostic signals allows for indicating the possibility of fault compensation effect. based on conclusion 8, we now formulate a necessary and sufficient condition for the possibility of a fault compensation effect by three-valued reference diagnostic signals. condition 1. the necessary and sufficient condition for fault compensation. the complete set of diagnostic signals {-1,0,+1} of at least one entry of signature of a multiple fault is necessary and sufficient to indicate the possibility of a fault compensation effect. in the case of biand three-valued reference diagnostic signals, we can formulate a sufficient condition for excluding fault compensation. condition 2. the sufficient condition for excluding fault compensation. it is sufficient for excluding the possibility of fault compensation if a submatrix consisting exclusively of signatures of single faults is diagonal. however, this condition is relatively difficult to meet in practice. therefore, most diagnostic systems based on either binary or trinary evaluation of residuals are exposed to fault compensation that degrades their diagnostic credibility. however, the degree of degradation is much less by three-valued incidence matrices [20]. the necessary and sufficient condition for excluding fault compensation implies: conclusion 9. single row incidence matrices do not allow for unambiguous indication of the possibility of fault compensation effect independently, whether diagnostic signals are bior threevalued. let us now discuss the case of a diagnostic system for which the gp graph is depicted in figure 4. the appropriate binary and trinary diagnostic matrices are shown respectively in table 5 and table 6. now, the binary diagnostic matrix allows for uniquely distinguishing all considered faults. in this case, the fault compensation effect will not occur. similarly, the three-valued diagnostic matrix depicted in table 6 allows for uniquely distinguishing all faults. here, the fault compensation effect does not take place because condition 1 does not hold. as can be seen, the submatrices of the diagnostic matrices depicted in table 5 and table 6 consisting exclusively of single fault signatures are diagonal. therefore, all multiple fault signatures, which signatures are the unions of signatures of all single faults constituting the multiple ones, are distinguishable independently whether they are bior three-valued. after this, we reinforce condition 2. conclusion 10: the diagonal structure of the diagnostic matrix of single faults avoids the fault compensation effect. the condition formulated in the given above conclusion is, however, almost unrealistic to implement in practice. while the bi-valued diagnostic signals are useless for the recognition of fault compensation effects (conclusion 7), it is strongly recommended to design three-valued incidence matrices because they allow for the indication of possible fault compensation effects (conclusion 8). the above recommendation is postulated mainly for the newly developed diagnostic systems. implementing this recommendation for running diagnostic systems is imaginable, although less realizable because of the necessity of installing additional instrumentation. on the other hand, the intensive implementation of intelligent fault diagnosing devices [20] combined with implementing embedded diagnostics ideas [28][28] allows for the successive rejection of fault compensation problems from the area of interests of fdi. 5. simulations the simulations were performed to exemplify the fault compensation effect using a model of a single buffer tank depicted in figure 2. the simulation model was developed in the matlab-simulink environment. the resulting flowchart of the simulation of the liquid storing and distributing process is shown in figure 5. the model generates an output vector whose components include liquid level, inflow and outflow rates, residua, and diagnostic signals. the liquid level depends on inlet and outlet liquid rates, leakages, and obliteration of pipe. therefore, the tank's liquid level can determine by integrating the dynamic liquid accumulation, i.e., by integrating the difference in the flow rates of liquid entering and leaving the tank. as assumed earlier, only one potential double fault is considered in this case. the simulations of two different double fault scenarios were performed. each of them exemplifies a fault compensation effect. we designed the first scenario to show a possibility of the permanent inability for the precise diagnosis by reasoning based on the three-valued diagnostic matrix shown in table 4. it depicts the diagnostic matrix, which meets the necessary and sufficient conditions for a fault compensation effect. this scenario will also consider the three-valued diagnostic matrix shown in table 6, which meets a sufficient condition for excluding fault compensation effect. in connection with the first one, the second scenario shows the different timed processes of tightening diagnoses even for the same diagnostic matrices. this scenario shows that diagnostic matrices admittedly allow for searching for potential fault compensation effects but do not directly reflect the transients of diagnoses. scenario 1. consider two incipient faults: leakage 𝑓1 and pipe obliteration 𝑓2 as in figure 6. the obliteration starts to grow immediately after the simulation gets started. the leakage begins to grow at the time instant 0.50∙105s. therefore, the double fault originates in this time moment. the slopes of both faults are selected in such a way as to show the fault compensation effect. in this case, both faults impact residual r bringing its value close to zero for the whole simulation period, as shown in 6. the liquid inflow rate f0 swings around a constant value within ±10% limits. residuals are three-valued. the diagnostic signal s, (3), and diagnostic signals s1 and s2, (7), are determined based on a fixed arbitrary chosen threshold th = 5%. table 5. diagnostic matrix for binary evaluated residuals (7). f/s fault-free f1 f2 f1 ∧ f2 s1 0 1 0 1 s2 0 0 1 1 table 6. diagnostic matrix for trinary evaluated residuals (7). f/s fault-free f1 f2 f1 ∧ f2 s1 0 -1 0 -1 s2 0 0 +1 +1 acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 51 discussion: table 7 summarizes the results of simulations, while table 8 shows the obtained diagnoses. diagnose d0 is based on the bi-valued diagnostic matrix shown in table 5. diagnose d1 is derived from the tri-valued diagnostic matrix shown in table 4, while d2 is based on the tri-valued diagnostic matrix shown in table 6. despite fault compensation, diagnoses d0 and d2 finally isolate correctly double fault, however with a significant time delay. this time will be shorter if choosing a lower value of the threshold th. intermediate diagnosis 𝑓2 is not correct; however, it is not false. in turn, the diagnosis d1 is ambiguous, i.e., delivers much less useful information regarding faults, independently of whether the fault occurs or not. scenario 2. consider a case like in scenario 1 depicted in figure 7. the only difference is that in this case, both faults impact residual r, shifting its value far away from zero. moreover, different slopes of faults cause the reversal of the time sequence of diagnostic signals s1 and s2. it also influences the evolution of double fault diagnosis differently compared to scenario 1. discussion: table 9 summarizes the diagnostic signals obtained from simulations while table 10 contains the obtained diagnoses. diagnoses d0 and d2 finally isolate correctly double fault, however with a significant time delay. the only difference to scenario 1 is that the single fault f1 is detected before f2. in turn, the d1 diagnosis is slightly more valuable than d1 in the previous scenario. however, it is still far away from the quality of the d0 and d2 diagnoses. the diagnosis d1 is correct here, although pointed out faults are indistinguishable. analyzing the results of both scenarios, we can draw a practical conclusion. conclusion 11: it is advantageous to design a three-valued diagnostic matrix such that the elements of multiple fault signatures contain as few alternative values as possible. 6. fault masking effect as long as the process variable tracks the setpoint within some predefined limits, either the process operator or alarm system does not have any particular reasons to react. in closed-loop systems, the effects of faults are compensated for by controller action as long as the system is controllable. therefore, the faultmasking effect is frequently understood as an effect of faults' invisibility to process operators or alarm systems [22], [23]. in other words, the difference between the setpoint and process value may not be sensitive nor indicative of faults. here, the question arises: do the model-based fault diagnostics discussed earlier for the open control system is still valid if we close the loop? figure 5. simulation diagram of a single buffer tank process figure 6. example of a simulation of a double fault. notation: f0 liquid inflow rate dark blue line; l – liquid level blue line; f1 – leakage fault blue line; f2 – obliteration fault – red line; r1– dotted red line; r2 – dotted blue line; r – purple line; diagnostic signals: s1 – blue; s2 – red; s – purple. interval of the fault compensation effect 0.84 … 2.0∙105 s. table 7. diagnostic signals values (scenario 1). time s ∙ 105 0.00 0.50 0.50 0.84 0.84 1.14 1.14 2.00 s1 0 0 0 -1 s2 0 0 +1 +1 s 0 0 0 0 interval of the duration of fault compensation effect: 0.84 ... 2.0∙105 s table 8. obtained diagnoses (scenario 1). time s ∙ 105 0.00 0.50 0.50 0.84 0.84 1.14 1.14 2.00 d0 ∅ ∅ 𝑓2 𝑓1 ∧ 𝑓2 d1 ∅, 𝑓1 ∧ 𝑓2 ∅, 𝑓1 ∧ 𝑓2 ∅, 𝑓1 ∧ 𝑓2 ∅, 𝑓1 ∧ 𝑓2 d2 ∅ ∅ 𝑓2 𝑓1 ∧ 𝑓2 table 9. diagnostic signals values (scenario 2). time s ∙ 105 0.00 0.82 0.82 0.90 0.90 1.35 1.35 2.00 s1 0 -1 -1 -1 s2 0 0 +1 +1 s 0 0 0 -1 interval of the duration of fault compensation effect. 0.82 ... 1.35∙105 s table 10. obtained diagnoses (scenario 2). time s ∙ 105 0.00 0.82 0.82 0.90 0.90 1.35 1.35 2.00 d0 ∅ 𝑓1 𝑓1 ∧ 𝑓2 𝑓1 ∧ 𝑓2 d1 ∅, 𝑓1 ∧ 𝑓2 ∅, 𝑓1 ∧ 𝑓2 ∅, 𝑓1 ∧ 𝑓2 𝑓1, 𝑓1 ∧ 𝑓2 d2 ∅ 𝑓1 𝑓1 ∧ 𝑓2 𝑓1 ∧ 𝑓2 figure 7. example of a simulation of a double fault. interval of the fault compensation effect 0.82 … 1.35∙105 s. notations as in figure 6. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 52 to answer this question, we close the loop of the system shown in figure 2. the modified control system is presented in figure 8. the liquid inflow rate into the buffer tank is controlled by a control valve driven by a pi controller. the controller, employing the control valve, adjusts the liquid inflow rate into the tank keeping the liquid level close to the setpoint value. thus, in case of leakage, the controller increases the inflow to compensate for additional liquid demand. in turn, in the case of obliteration, the controller throttles the liquid inflow to keep demanded liquid level in the tank. the gp graph of the closed-loop control system is shown in figure 9. this graph contains additional vertices reflecting actuator fault f3, position av of the control valve stem, and arcs representing controller-in-the-loop. the actuator tracks the controller output cv. for simplicity, we assume the infallibility of the pi controller. let us assume a trivial static model of an actuator. the nominal model of the actuator is therefore av=cv. the actuator fault manifests in a discrepancy between av and cv values. let us assume now an additive actuator fault. hence, the model of the actuator in an inner form equals 𝐴𝑉 = 𝐶𝑉 ± 𝑓3 → 𝑟3 = ±𝑓3 . (10) as shown in figure 9, all faults are associated with observable (measurable) vertices. hence, the diagnostic matrix of single faults will take the diagonal shape, and in consequence, all single and multiple faults are isolable. the three-valued diagnostic matrix for the modeled control system depicts table 11. following condition 2, we exclude the impact of faults on residual values in this case. therefore, the fault compensation effect does not take place. figure 10 depicts the result of a simulation of a triple fault, i.e., slowly increasing obliteration 𝑓2 starting at the time instant 0, slowly increasing leakage 𝑓1 starting at the time instant 0.5∙10 5 s and abrupt actuator fault 𝑓3 appearing at the time instant 1.0∙105 s. signal s3 represents the diagnostic signal of residual r3. the summary of isolated faults is shown in table 12. as can be seen, closing the loop does not degrade the system's diagnostic properties as far as condition 2 holds. 7. final remarks there were defined necessary and sufficient conditions for fault compensation effect, allowing for identification of the possibility of appearing of this effect based on analysis of incidence matrix. in addition, a complementary condition of excluding the fault compensation effect was also formulated. in this connection, some practical recommendations and hints regarding the design of diagnostic systems were proposed. summing up the results of the discussion and performed simulations, we can conclude that: • the fault compensation effect is a common problem for model-based fdi diagnostic approaches. • the fault compensation effect manifests exclusively for multiple faults. • the fault compensation effect is an unwanted side effect originating from adopting an assumption regarding the generation of signatures of multiple faults based on unions of signatures of single fault signatures. • neglecting fault compensation effect leads to false or temporarily false diagnoses. • fault compensation problems should be considered, particularly in the case of slowly developing incipient faults. • application of the threeinstead of bi-valued diagnostic signals for reasoning about faults is irrelevant regarding the possibility of fault compensation effect. • fault compensation effect results from the fault reasoning method applied and should be distinguished from the faultmasking effect. table 11. diagnostic matrix for a liquid control system depicted in figure 8. f/s faultfree f1 f2 f3 f1 ∧ f2 f1 ∧ f3 f2 ∧ f3 f1 ∧ f2∧ f3 s1 0 -1 0 0 -1 -1 0 -1 s2 0 0 +1 0 +1 0 +1 +1 s3 0 0 0 ±1 0 ±1 ±1 ±1 figure 8. a closed-loop liquid level control system. notions: sp setpoint; cv control value; pi – proportional-and-integral controller; av positioner feedback signal. figure 9. the gp graph of the closed-loop system reflecting the qualitative impact of faults on the values of process variables. figure 10. example of a simulation of a triple fault. notations: f3 – actuator fault purple line; r3 – purple dotted line; s3 – diagnostic signal – purple line. the remaining notions as in figure 6. table 12. obtained diagnoses for closed-loop liquid level control system. time s ∙ 105 0.00 0.82 0.82 0.85 0.85 1.00 1.00 2.00 d ∅ 𝑓1 𝑓1 ∧ 𝑓2 𝑓1 ∧ 𝑓2 ∧ 𝑓3 acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 53 further research will focus on developing a theoretical framework encompassing fault compensation aspects described in this paper. references [1] j. korbicz, j. m. kościelny, z. kowalczuk, w. cholewa, fault diagnosis. models. artificial intelligence. applications, springer 2004, isbn: 3540407677. 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impulse-radar sensors; position estimation; action recognition; healthcare citation: paweł mazurek, szymon kruszewski, applicability of multiple impulse-radar sensors for the recognition of a person’s action, acta imeko, vol. 12, no. 2, article 30, june 2023, identifier: imeko-acta-12 (2023)-02-30 section editor: eric benoit, université savoie mont blanc, france received july 17, 2022; in final form may 16, 2023; published june 2023 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: paweł mazurek, e-mail: pawel.mazurek@pw.edu.pl 1. introduction it is expected that the share of european population aged at least 65 years will reach 25% in 2050 [1]. the problem of organised care over elderly persons is, therefore, of growing importance. this, in turn, creates the demand for various technical solutions which could be applied for non-intrusive monitoring of elderly persons in home environments and healthcare facilities. the systems for monitoring of elderly persons are expected to predict and detect dangerous events, such as falls and harmful long lies after the falls. the falls of elderly persons belong to the most frequent reasons of their admission and long-term stay in hospitals [2]. possible solutions that could be applied for non-intrusive monitoring of elderly persons are radar-based techniques – both narrow-band [3]–[8] and broad-band [9]–[13]. the most attractive feature of these techniques is the possibility of the through-the-wall monitoring of human activity. a review of the relevant literature, including articles in scientific journals and conference papers, which appeared in the years 2019–2022, has revealed that the vast majority of researchers use radar sensors of both types for estimation of the heart rate, breathing rate and position (in two dimensions), while the attempts to detect falls are based on sensors using the doppler principle – with two exceptions: • in [14] an attempt to detect falls on the basis of threedimensional movement trajectory obtained by means of the three impulse-radar sensors is presented. in the reported approach, the monitored person's movement is compared with two model movements: a movement with a constant speed, and a movement towards the ground with an acceleration equal to the gravitational acceleration, and the classification of the movement is based on the reliability function. unfortunately, no systematic tests of the effectiveness of the method are presented in that paper. • in [15] the results of the simulation studies focused on the impact of the number of the impulse-radar sensors on the accuracy of the estimation of the threedimensional position is presented. unfortunately, the simulated setup is based on an unrealistic assumption that the sensors are placed in random locations within a monitored area. in the authors’ recent conference papers [16], [17], the results of the studies on the applicability of multiple impulse-radar abstract the research reported in this paper is devoted to the impulse-radar technology when applied for non-intrusive monitoring of elderly persons. specifically, this study is focused on a novel approach to the interpretation of data acquired by means of multiple impulseradar sensors, leading to the determination of features to be used for the recognition of a monitored person’s actions. the measurement data are first transformed into the three-dimensional coordinates of the monitored person; next, those coordinates are used as a basis for determination of features characterising the movement of that person. the results of the experimentation, based on the real-world data, show that multiple impulse-radar sensors may be successfully used for highly accurate recognition of actions such as walking, sitting, and lying down, although this accuracy is significantly affected by the quality of the three-dimensional movement trajectories which in turn is affected by the configuration of the impulse-radar sensors within the monitored area. mailto:pawel.mazurek@pw.edu.pl acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 2 sensors for estimation of the three-dimensional movement trajectories – which could be used for detection of dangerous events such as falls – are presented. these results show that the impulse-radar sensors may be used for accurate estimation of the three-dimensional position of a monitored person if the sensors are properly located within the monitored area. in this paper, the applicability of the impulse-radar sensors for recognition of person’s action, on the basis of the estimates of the threedimensional movement trajectories, is investigated. the novelty of the research presented in this paper consists in an algorithmic basis for recognition of actions of a person, in a monitoring system based on multiple impulse-radar sensors. the processing of the raw measurement data acquired by means of the impulse-radar sensors is divided into three stages: the transformation of the measurement data into the threedimensional coordinates of the monitored person, the calculation of the features characterising the three-dimensional movement trajectories, and the classification of the movement trajectories. the usability of the proposed features is assessed in an experiment based on a set of real-world data sequences representative of three activities of daily living: walking, sitting and lying down. moreover, the influence of the configuration of the impulse-radar sensors within the monitored area on the accuracy of the action recognition is investigated. 2. estimation of movement trajectories the measurement data used for the experimentation were acquired by means of six x4m02 impulse-radar sensors manufactured by novelda [18], [19]. an exemplary data frame, acquired by means of one of these sensors, is shown in figure 1. to properly estimate a three-dimensional movement trajectory of a monitored person, the measurement data, acquired by means of the impulse-radar sensors, have to be subjected to processing comprising [20]: the estimation of parameters of the impulse-radar signal, the smoothing of several one-dimensional trajectories of the distance between the monitored person and the corresponding impulse-radar sensors, and the transformation of the smoothed distance trajectories into the three-dimensional movement trajectory. in the research presented here: • the parameters of the impulse-radar signal have been estimated by means of a method consisting in computing the correlation function for the received signal and a known template of the emitted pulse, and the estimation of the coordinates of the maximum of this function [20]. • the distance trajectories have been smoothed by means of a method based on weighted least-squares estimator, consisting in the approximation of a sequence of data by means of a linear combination of basis functions, with the number of these functions determined automatically [17]. • the three-dimensional movement trajectories have been obtained by means of a method consisting in solving a set of equations modelling the geometrical relationships between the three-dimensional coordinates of a person and the distances between that person and the impulse-radar sensors [17]. 3. methodology of experimentation 3.1. acquisition of measurement data the measurement data used for the experimentation aimed at the assessment of the accuracy of the recognition of the monitored person’s actions, were acquired by means of six impulse-radar sensors located at various positions. two configurations of the sensors have been considered (see figure 2): • configuration #1 according to which the impulseradar sensors (r1, …, r6) were located at positions whose x-, yand z-coordinates (in meters) were respectively: [0.00, 1.70, 0.93], [0.00, 1.70, 1.43], [2.20, 1.70, 1.45], [2.20, 1.70, 0.95], [0.20, 4.50, 0.82], [2.00, 4.50, 0.83]; • configuration #2 according to which the impulseradar sensors (r1, …, r6) were located at positions whose x-, yand z-coordinates (in meters) were respectively: [0.20, 4.50, 0.82], [0.60, 2.65, 2.76], [0.00, 1.70, 0.93], [2.20, 1.70, 0.95], [2.00, 4.50, 0.83], [0.60, 3.31, 2.76]. concurrently, the person was monitored by an infrared depth sensor being a part of the kinect v2 device (cf. [21] for the description of the methodology for preprocessing of data from depth sensors). the radar sensors and the depth sensor were synchronised, and their data acquisition rate was set to 30 hz. in the experimentation, three movement scenarios were considered: • according to the first scenario, two persons walked along three predefined trajectories: an oval-shaped trajectory, a straight-line trajectory and a sine-shaped trajectory; each person repeated the action 10 times for each trajectory. • according to the second scenario, two persons sat on a chair located in three different places within the monitored area; each person repeated the action 10 times for each position of the chair. • according to the third scenario, two persons lay down on a mattress, approaching it from two different sides; each person repeated the action 15 times for each side of the mattress. thus, the whole programme of experimentation comprised the acquisition of: • 180 three-dimensional movement trajectories obtained on the basis of the data acquired by means of the impulse-radar sensors located according to configuration #1; • 180 three-dimensional movement trajectories obtained on the basis of the data acquired by means of the impulse-radar sensors located according to configuration #2; • 180 three-dimensional movement trajectories obtained on the basis of the data acquired by means of the depth sensor. figure 1. the example of raw measurement data. acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 3 3.2. generation of features in the experimentation, the features for classification have been determined on the basis of the sequences of the estimates of the z-coordinate of the position the persons, i.e. {�̂�𝑛}, as well as the sequences of the estimates of the velocity and acceleration along the z-axis (i.e. the first and second derivatives of the zcoordinate, obtained by means of the forward difference method), denoted with {�̂�𝑧,𝑛 } and {�̂�𝑧,𝑛}, respectively. all the features are presented in table 1. for the sake of simplicity, two operators have been introduced – the operator returning the empirical mean value of a data sequence {𝑝𝑛 }: 𝑚[{𝑝𝑛}] ≡ 1 𝑁 ∑ 𝑝𝑛 𝑁 𝑛=1 (1) and the operator returning its empirical variance: 𝑠2[{𝑝𝑛 }] ≡ 1 𝑁 − 1 ∑(𝑝𝑛 − 𝑚[{𝑝𝑛 }]) 2 𝑁 𝑛=1 . (2) moreover, the velocity in the vertical dimension and the acceleration in that dimension are denoted with �̂�𝑣,𝑛 ≡ {|�̂�𝑧,𝑛|} and �̂�𝑣,𝑛 ≡ {|�̂�𝑧,𝑛|}, respectively. 3.3. classification in this study an error-correcting output codes classifier (ecoc) – suitable for multiclass classification problems – has been used [22]. the ecoc classifier has been based on multiple support vector machines (svm) – each designed to distinguish between two selected actions. the implementation of the ecoc classifier, available in the matlab statistics and machine learning toolbox [23], has been used for this purpose. before the training of the classifier, the values of the features have been standardised. the performance of the classifier has been assessed using the 10-fold cross-validation technique. the assessment of the accuracy of the classification has been based on the inspection of: • the receiver operating characteristic (roc) curves illustrating the relationship between the true positive rate (tpr) the false positive rate (fpr), i.e. two indicators defined as follows: tpr = tp tp + fn (3) fpr = fp fp + tn , (4) where – for example, in the case of walking – tp (true positive) is the number of walks classified as walks, tn (true negative) is the number of nonwalks classified as non-walks, fp (false positive) is the number of non-walks classified as walks and fn (false negative) is the number of walks classified non-walks; the area under the roc curve (auc) is a single scalar value representing the performance; • the confusion matrices visualising the results of the classification: each row of such matrix represents the instances in an actual class while each column represents the instances in a predicted class. in the experiments based on the real-world data the use of the approximations of the movement trajectories is necessary since their reference shapes cannot be properly defined: a human body has a considerable volume and generates complex echoes which cannot be attributed to any of its specific points (e.g. to plexus solaris). an arbitrary choice of such a reference point would lead to an arbitrary definition of the systematic error which could be misleading. fortunately, such a definition is not necessary for extraction of the features which are used for classification of the actions of a monitored person – the features characterising the dispersion of the values of the z-coordinate, the vertical velocity and the vertical acceleration. 4. results of experimentation in figure 2, the examples of the estimates of the threedimensional movement trajectories of a monitored person, obtained by means of the procedure described in section 2, for two configurations of the impulse-radar sensors – together with the projections on the three two-dimensional planes – are shown; in figure 3, the dispersion of the subset of the estimates of the z-coordinate, representative of walking, sitting and lying down, is presented. in figure 4, the roc curves obtained for the classification of all the three-dimensional movement trajectories, are shown; the confusion matrices are presented in figure 5. the analysis of the presented results is leading to the following conclusions: • multiple impulse-radar sensors may be successfully used for estimation of the three-dimensional position of a moving person, although the configuration of those sensors has significant influence on the uncertainty of the estimation. to properly estimate the height-component of the position of a monitored person, few impulse-radar sensors should be located at a greater height than the rest of those sensors (compare figure 2c with figure 2d as well as figure 2e with figure 2f). table 1. the features used in the experimentation. # feature 1 standard deviation of the z-coordinate: 𝜎 = √𝑠2[{�̂�𝑛 }] 2 difference between extreme values of the z-coordinate: δ = max[{�̂�𝑛 }] − min[{�̂�𝑛 }] 3 mean vertical velocity: 𝜇𝑣 = 𝑚[{𝑣𝑣,𝑛 }] 4 maximum vertical velocity: 𝑣max = max[{𝑣𝑣,𝑛 }] 5 standard deviation of the vertical velocity: 𝜎 𝑣 = √𝑠2[{�̂�𝑣,𝑛 }] 6 difference between extreme values of the vertical velocity: δ𝑣 = max[{𝑣𝑣,𝑛 }] − min[{𝑣𝑣,𝑛 }] 7 mean vertical acceleration: 𝜇𝑎 = 𝑚[{�̂�𝑣,𝑛 }] 8 maximum vertical acceleration: 𝑎max = max[{�̂�𝑣,𝑛 }] 9 standard deviation of the vertical acceleration: 𝜎 𝑎 = √𝑠2[{�̂�𝑣,𝑛 }] 10 difference between extreme values of the vertical acceleration: δ𝑎 = max[{�̂�𝑣,𝑛 }] − min[{�̂�𝑣,𝑛 }] acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 4 a) b) c) d) e) f) figure 2. the examples of the estimates of the three-dimensional trajectories of a moving person, obtained for two configurations of the impulseradar sensors: trajectories representative of walking (top row), trajectories representative of sitting (middle row), and trajectories representative of lying down (bottom row). the movement trajectories obtained for configuration #1 are depicted in the left column, while the movement trajectories obtained for configuration #2 are depicted in the right column. blue lines denote radar-data-based trajectories, grey lines denote depth-data-based trajectories while blue triangles indicate the positions of the radar sensors. acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 5 a) b) c) figure 3. the dispersion of the estimates of the z-coordinate of a moving person, obtained for two configurations of the impulse-radar sensors, for walking (a), sitting (b) and lying down (c). a) b) c) figure 4. the receiver operating characteristic (roc) curves obtained for the classification of all the three-dimensional movement trajectories: radar-databased trajectories obtained for configuration #1 (a), radar-data-based trajectories obtained for configuration #2 (b), depth-data-based trajectories (c). acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 6 • the configuration of the impulse-radar sensors affects the estimates of the coordinates of a monitored person. in the case of configuration #2 the estimates of the z-coordinate are ca. 0.5 m greater than in the case of configuration #1; moreover, the x-y projections of the trajectories, obtained for configuration #2, seem to be scaleddown versions of the analogous projections obtained for configuration #1 (compare figure 2a with figure 2b). these discrepancies can be explained by a non-negligible volume of a human body: the impulse-radar sensors located at a greater height – and, therefore, oriented differently than the rest of those sensors – receive echoes reflected from different parts of the body of a monitored person. moreover, in the case of configuration #1 the changes in the z-coordinate, associated with the movement of the body towards the ground during sitting or lying down, may not be properly reflected in the estimates of the movement trajectory (see figure 2c and figure 2d). this phenomenon may be explained by the fact that when the person is moving towards the ground, the changes in the distance between that person and the impulse-radar sensors placed on the sides of the monitored area are not significant. in the case of the impulse-radar sensors placed on the ceiling, these changes are much greater. • the proposed features, characterising the monitored person’s movement in the vertical dimension, are sufficient to recognise walking, sitting and lying down with high accuracy, although this accuracy is significantly affected by the quality of the threedimensional movement trajectories which in turn is affected by the configuration of the impulse-radar sensors (compare figure 5a with figure 5b). the results of the classification of the three-dimensional movement trajectories obtained on the basis of the data acquired by means of the depth sensor, are the best because the depth sensor provides the most accurate estimates of the threedimensional position of the monitored person. nevertheless, the results of the classification of the trajectories obtained on the basis of the data acquired by means of the impulse-radar sensors located according to configuration #2, are only slightly worse and can be likely improved by the application of more sophisticated methods for impulse-radar data processing. 5. conclusions the novelty of the research presented in this paper consists in an approach to the interpretation of measurement data acquired by means of the impulse-radar sensors, leading to the determination of features to be used for recognition of actions of three types: walking, sitting and lying down. the data are first transformed into the three-dimensional coordinates of the monitored person; next, those coordinates are used as a basis for calculation of kinematic features characterising the monitored person’s movement in the vertical dimension. the results of the experimentation based on real-world data show that multiple impulse-radar sensors may be successfully used for highly accurate recognition of walking, sitting and lying down of a monitored person. it has to be noted, however, that the accuracy of the recognition is affected by the quality of the three-dimensional movement trajectories which in turn is affected by the configuration of the impulse-radar sensors. to properly estimate the height-component of the position of the monitored person, few impulse-radar sensors should be located at a greater height than the rest of those sensors. the results encourage the authors to focus on the development of the methods for processing of the impulse-radar data, enabling the detection of falls of the monitored person. references [1] united nations, department of economic and social affairs, population division (2019), world population prospects 2019: highlights. st/esa/ser.a/423. online [accessed 15 march 2022] https://population.un.org/wpp/publications [2] k. chaccour, r. darazi, a. h. e. hassani, e. andrès, from fall detection to fall prevention: a generic classification of fallrelated systems, ieee sensors journal, vol. 17, 2017, pp. 812822. doi: 10.1109/jsen.2016.2628099 [3] m. g. amin, y. d. zhang, f. ahmad, k. c. ho, radar signal processing for elderly fall detection the future for in-home monitoring, ieee signal processing magazine, vol. 33, 2016, pp. 71–80. doi: 10.1109/msp.2015.2502784 [4] o. boric-lubecke, v. m. lubecke, a. d. droitcour, byung-kwon park, a. singh, eds., doppler radar physiological sensing. hoboken (new jersey): john wiley & sons, inc., 2016. 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https://shop.xethru.com/x4m02 https://www.xethru.com/community/resources/x4m02-radar-sensor-datasheet.115/ https://www.xethru.com/community/resources/x4m02-radar-sensor-datasheet.115/ https://doi.org/10.1016/j.bspc.2017.09.006 http://dx.doi.org/10.1162/15324430152733133 https://www.mathworks.com/help/stats/classificationecoc.html evaluation of the electricity savings resulting from a control system for artificial lights based on the available daylight acta imeko issn: 2221-870x december 2022, volume 11, number 4, 1 11 acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 1 evaluation of the electricity savings resulting from a control system for artificial lights based on the available daylight francesco nicoletti1, vittorio ferraro2, dimitrios kaliakatsos1, mario a. cucumo1, antonino rollo1, natale arcuri1 1 department of mechanical, energy and management engineering (dimeg), university of calabria, via p. bucci, 87036 rende (cs), italy 2 department of computer, modelling, electronics and system engineering (dimes), university of calabria, via p. bucci, 87036 rende (cs), italy section: research paper keywords: daylight; artificial lights; energy savings citation: francesco nicoletti, vittorio ferraro, dimitrios kaliakatsos, mario a. cucumo, antonino rollo, natale arcuri, evaluation of the electricity savings resulting from a control system for artificial lights based on the available daylight, acta imeko, vol. 11, no. 4, article 12, december 2022, identifier: imekoacta-11 (2022)-04-12 section editor: francesco lamonaca, university of calabria, italy received july 11, 2022; in final form october 13, 2022; published december 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was supported by regione calabria (pac calabria 2014-2020 asse prioritario 12, azione b) 10.5.12) for f. nicoletti’s contribution. corresponding author: francesco.nicoletti@unical.it, e-mail: francesco.nicoletti@unical.it 1. introduction buildings are the place where man spends most of his time. a large amount of energy is used inside buildings to meet human needs and to provide thermal-hygrometric and visual comfort. in the coming years, it is very important to adopt solutions to reduce consumption. the most widespread strategy to reduce consumption consists of improving the efficiency of the building envelope (reducing thermal losses through transmission by opaque surfaces [1]-[5]) and the efficiency of plant components [6]. it is especially important to also utilize solar gains in a smart way because they bring thermal and visual benefits. the solar source, in fact, has a considerable influence on the behavior of the building. scientific research in this regard is dedicated to three main objectives: 1) production of electrical and thermal power through solar panels [7], [8], 2) control of solar inputs to reduce winter and summer heat load, 3) reach adequate daylight within the rooms. in addition to user awareness to reduce waste, intelligent solutions to better exploit solar radiation are divided into passive and active systems. passive systems do not require a control system. examples include bioclimatic solar greenhouses [9], green roofs [10], [11], trombe walls [12]. active systems, on the other hand, use sensors and actuators, which often communicate via iot technology to make automatic adjustments [13], [14]. the visual comfort of the occupants, which is often neglected, is also a primary objective. this research is concerned with analyzing a dynamic system for controlling artificial lighting to avoid unnecessary switching on when natural illuminance is adequate. daylight plays a key role in visual comfort inside buildings. estimating daylight inside a room is not a simple problem. over the years, different methodologies have been developed to consider the different variables involved. abstract natural lighting in building environments is an important aspect for the occupants' mental and physical health. furthermore, the proper exploitation of this resource can bring energy benefits related to the reduced use of artificial lighting. this work provides some estimates of the energy that can be saved by using a lighting system that recognises indoor illuminance. in particular, it is able to manage the switching on of lights according to the daylight detected in the room. the savings from this solution depend on the size and orientation of the window. the analysis is conducted on an office by means of simulations using the inlux-dbr code. the locations have an influence on the luminance characteristics of the sky. the analysis is conducted with reference to one city in the south and one in the north of italy (cosenza and milan). the energy saving is almost independent of latitude and therefore representative of the italian territory. it is highly variable according to exposure, being the highest for southern exposure (97 % with the window size equal to 36 % of the floor area) and between 26 % and 48 % (as a function of window size) for northern exposure. mailto:francesco.nicoletti@unical.it acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 2 numerous computational codes are available to estimate the illuminance on the working plane [15]-[17]. these codes, however, are difficult to adapt to a real case since they use the sky luminance models present within their libraries. in particular, the models are predominantly related to the cie clear sky model [18] or standard overcast sky distributions. the use of locally measured or estimated luminance is not allowed. in this regard, the authors developed the inlux-dbr calculation code [19], [20]. the luminous contributions that the code considers are: light carried by direct rays, scattered light from the sky and reflected light from the outside ground. it allows realistic estimation of illuminance at various points in the room. the code is very flexible since it allows the use of measured distributions of sky luminance and also classical model calculations. the resolution method used is "radiosity model" by which luminance distributions in walls are evaluated by an implicit method. the validation of the code was performed with an example case conducted in japan and allows the extension of case studies to perform parametric analysis. in the present study, we investigate the possibility of replacing the artificial lighting system using a system that can adapt to confer setpoint illuminance based on the already present daylight contribution. this is a control strategy that is often applied in smart buildings. this work provides numerical data to quantitatively assess the energy savings from this investment. it depends, in fact, on a number of factors: in particular, the geometry of the room and the size and orientation of the window. the objective of the work is to provide useful indications to take advantage of daylight and save electricity from artificial lights. in addition, this solution provides visual comfort and makes the environment healthier for the occupants. two italian locations will be simulated in the work, one in southern italy and one in northern italy: cosenza and milan. 1.1. literature review controlling artificial light according to natural illuminance is useful to provide an environment with constant illuminance over time, reducing unnecessary waste. the daylight recorded in rooms depends greatly on the geometry of the rooms and the ratio of window to wall areas. in addition, illuminance depends on the position of the sun. in the past, many authors have made important contributions on this topic. the most important parameter used in building design is the daylight factor (df), defined by trotter [21]. the estimation of this was improved by dresler [22] by formulating empirical relationships. hopkinson et al [23] introduced the need to consider external obstacles and ground reflection. later, tregenza [24] defined an analytical method called split-flux, which is based on overcast conditions. numerous models are formulated in order to estimate df; these use scaled models, empirical formulas and corresponding diagrams [25], [26]. the df, however, allows for valid considerations only under overcast conditions because on clear days it is highly dependent on the position of the sun in the sky. in these cases, in fact, it would result that the indoor illuminance distributions would not depend on the orientation of the window. it would be appropriate to define indices to evaluate illuminance on an annual scale. this greatly increases the complexity of the problem. for our purpose, therefore, classical df-based methods are not useful. nabil et al. [27] showed that alternating clear, intermediate and cloudy days makes indoor lighting highly dependent on window orientation. the models mainly used to simulate the distribution of light inside a room are divided into "radiance models" and "ray tracing models". the two models provide equal results being based on valid assumptions. the code used in the present work belongs to the first category. the method consists of solving, at each instant of time, a system of equations obtained from the balance on each surface of the room. each surface is treated as a body that reflects incident light isotropically. it is defined by the reflectivity and view factors toward all other surfaces. in ray tracing models, the path of the solar ray through its various reflections on the walls of the room is followed. illuminance is calculated by setting the maximum number of reflections. no system of equations is solved. daylighting studies are very diversified. in fact, the problems are always significantly different depending on the building conformation and scale. alhagla et al. [28] obtained in a study that the benefits of daylight depend on location. in particular, in hot locations, the exploitation of sunlight leads to an increase in the cooling heat demand in the summer period. on the other hand, in locations where the climate is colder, the use of daylight could favour the input of solar radiation and thus reduce the winter heat demand. athienitis et al. [29], analysed control systems for artificial lights associated with daylight to manage electricity use and reduce energy consumption. the authors were also able to achieve excellent results in terms of visual well-being. the benefits achieved on electricity consumption are remarkable, although the authors claim that there is a risk of overheating during summer periods. krarti et al [30] studied different geometric room configurations in order to analyse the impact of daylight on energy consumption. the analysis was conducted with different window sizes and different types of glass. they analysed four locations in america. they also observed that geographical location has a low impact on daylight. hee et al [31] presented a review to analyse scientific paper where the impact of windows on room illuminance and the resulting energy savings are assessed. they also listed the different optimization techniques used to choose glass. their advice is to perform a careful economic evaluation when choosing glass and defining the size of the opening in a room. the largest number of scientific studies concerns the assessment of solar radiation introduced through windows, together with daylight. of course, these effects are important because they have a considerable influence on the energy balance of a building. the present work, however, has a different objective. in fact, solar contributions are defined by the geometry of the building and the ratio of windows to opaque walls. this paper, unlike the others, does not aim to evaluate the best architectural design for the building. instead, the aim is to evaluate, for a given building (thus for a given daylight and solar gains), whether the artificial lighting system can be more efficient, by making it intelligent, to exploit daylight. for example, when the room is long and narrow, the illuminance established in the areas away from the window is not sufficient. these rooms are generally not suitable for exploiting daylight properly. moon et al. [32] performed an analysis on the control of the lighting system using photosensors in the room. they performed simulations using lightscape software. the use of software to simulate daylight can be interesting as it allows comparison between different locations with reference to the same geometry. it is therefore necessary to understand whether the presence of an artificial light control system can lead to savings by trying to quantify them [33]-[35]. li et al. [36] carried out experimental acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 3 studies on daylight and energy consumption for atrium corridors. doulos et al. [37] showed that the power factor for led lamps worsens when they are dimmed and this could make the use of fluorescent lamps more advantageous than leds under certain conditions. bellia et al. [38] observed experimentally that there are no significant differences between using a proportional dimmable system and an on/off system. in particular, for southern exposures they observed that the difference is very low. the installation of a dimmable system is therefore not recommended in these cases due to the higher costs compared to an on/off system. the number of variables on which lighting problems depend is high and it is not always clear how many benefits the presence of a dimmable system can bring. the present work aims to overcome this gap and to provide useful information for locations in the latitude range equal to those of italy. 2. methodology 2.1. the inlux-dbr code and its experimental validation the lighting analysis was carried out using the inlux-dbr calculation code. this code allows to evaluate the luminance distribution within an environment. in particular, it divides opaque structures and transparent structures into “n” and “m” surface elements respectively. on each of these the illuminance value depends on: a. a component of direct solar radiation which, passing through the transparent surfaces, hits the surface under examination; b. a component of the diffuse solar radiation which, passing through the transparent surfaces, hits the surface under examination; c. a component of solar radiation reflected from the external environment which, passing through the glass surfaces, hits the surface under examination; d. a component due to the infinite internal reflections by the other surfaces that make up the environment in question. these components influence both the illuminance of opaque surfaces and the illuminance of glazed surfaces. in the present analysis, it is considered that the glazed surfaces are made by means of ordinary clear glass, therefore the direction of the solar radiation incident on the surface remains unchanged. having to consider the effects due to the radiation reflected from the ground outside the environment, the latter is divided into “p” surface elements. taking into account the various incident radiative components, it is possible to determine the illuminance 𝐸𝑖 of the “i-th” surface by means of the eq. (1). 𝐸𝑖 ∙ ∆𝐴𝑖 = ∑ [𝜏(𝛼, 𝜑) ∙ 𝐿p(𝛼, 𝜑) ∙ ∆𝐴w ∙ π × 𝐹w−i] + 𝑚 w=1 𝑘 ∙ 𝜏(𝛼s, 𝜑s) ∙ ∆𝐴i ∙ 𝐸vs ∙ cos 𝜗si + ∑[𝜏(𝛼g, 𝜑g) ∙ 𝐿g ∙ ∆𝐴g ∙ π ∙ 𝐹g−i] + 𝑝 g=1 ∑ ∆𝐴𝑗 ∙ 𝜌j ∙ 𝐸j ∙ 𝐹𝑗−i 𝑛+𝑚 𝑗=1 . (1) in the first term, the illuminance of the “i-th” element (𝐸𝑖 ) multiplied by the relative surface (∆𝐴𝑖 ) appears. in the second term, on the other hand, all the radiative components that influence the value of the illuminance on the surface under examination appear. in particular, with 𝜏(𝛼, 𝜑) indicates the transmissivity of the glazed surface, with 𝐿p (𝛼, 𝜑) indicates the luminance of the celestial vault (this varies according to the point of the celestial vault considered), with 𝛼 indicates the angular height of the point of the celestial vault considered, with 𝜑 indicates the azimuth of the point considered in the celestial vault, with ∆𝐴𝑤 indicates the area of the glazed element under examination, with 𝐹𝑤−𝑖 indicates the view factor between the glazed element under examination "w" and the "i-th" element, 𝑘 is a coefficient that has a unit value if the element in question is directly affected by solar radiation and a zero value otherwise, with 𝜏(𝛼𝑠 , 𝜑𝑠 ) indicates the transmissivity of the glazed surface to direct solar illumination (𝐸𝑣𝑠), with 𝛼𝑠 indicates the height of direct solar radiation, with 𝜑𝑠 indicates the azimuth of direct solar radiation, with 𝜗𝑠𝑖 indicates the angle between direct solar radiation and the normal to the surface of the "i-th" element, with 𝜏(𝛼𝑔, 𝜑𝑔 ) indicates the transmissivity of the glazed surface to reflected radiation from the ground, with 𝐿𝑔 indicates the luminance of the ground, with ∆𝐴𝑔 indicates the surface of the "g-th" element of the ground, with 𝐹𝑔−𝑖 indicates the view factor between the "g-th" element and the "i-th "in question, 𝜌𝑗 is the reflexivity of the elements internal to the environment in question is indicated and ∆𝐴𝑗 is the surface of the" j-th "element inside the environment that reflects part of the incident radiation (𝐸𝑗 ) on the" i-th "element in exam, finally, 𝐹𝑗−𝑖 indicates the view factor between the" j-th "element and the" i-th "element. this balance equation is written for each “i-th” opaque element inside the environment. figure 1 shows a graphic representation of the various radiative components exchanged by the different surfaces and considered in the inlux dbr code. considering the external ground as an isotropic surface and assuming that the reflectivity 𝜌𝑔 does not vary between direct and diffuse radiation, it is possible to calculate the luminance of the ground 𝐿𝑔 by means of eq. 2. 𝐿𝑔 = 𝜌𝑔 ∙ (𝐸𝑣𝑠 ∙ sin(𝛼𝑠 ) + 𝐸0) 𝜋 , (2) where with 𝐸0 indicates the diffused illuminance on the horizontal plane. figure 1. representation of the various radiative components exchanged by the different surfaces inside the room. acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 4 in the case in question, the illuminance on the glass surface depends solely on the radiative components reflected by the surfaces inside the environment. in fact, it is considered by hypothesis a single glazed surface in the room. consequently, it is possible to simplify the eq. (1). therefore, the illuminance on a glass element inside the room can be determined by means of the eq. (3). 𝐸𝑤 ∙ ∆𝐴𝑤 = ∑[∆𝐴𝑖 ∙ 𝜌𝑖 ∙ 𝐸𝑖 ∙ 𝐹𝑖−𝑤] 𝑛 𝑖=1 , (3) where with 𝐸𝑤 indicates the internal illuminance of the "w-th" element of the glazed surface under examination, while 𝐹𝑖−𝑤 is the view factor between the "i-th" element and the "w-th" element of the glazed surface in exam. finally, a system of unknown (𝑛 + 𝑚) equations with (𝑛 + 𝑚) variables is obtained which is solved with an iterative method by means of a finite difference methodology starting from a uniform initial solution. the calculation code has been implemented in the matlab environment and other details are provided in [19]. one of the advantages of this computational code is that it can implement different methodologies to model the behavior of the celestial vault. for example, it is possible to use: 1) the distribution of clear and cloudy cie [39]; 2) perez's model of clear skies [40]; 3) igawa's model [41]; 4) the model of kittler and darula [18], [42] 5) the tregenza model [43]; 6) experimental models created ad hoc for the location in question. the developed code was validated by means of data measured in an experimental site in osaka. in the experimental validation, clear, overcast and intermediate days were considered. the validation was carried out considering two error indices: the normalized mean bias error (nmbe) and the normalized root mean square error (nrmse) evaluated between the measured and calculated illuminance at various points in the environment. in particular, the nmbe is calculated by means of the following equation: 𝑁𝑀𝐵𝐸 = ∑ 𝑉calc,𝑖 − 𝑉meas,𝑖 𝑉meas,𝑖 𝑁 𝑖=1 𝑁 , (4) the nrmse is calculated by means of the following relation: 𝑁𝑅𝑀𝑆𝐸 = √ ∑ ( 𝑉calc,𝑖 − 𝑉meas,𝑖 𝑉meas,𝑖 ) 2 𝑁 𝑖=1 𝑁 (5) the values of these error indices are summarized in figure 2. it is observed that there is a good correspondence between the measured values and the calculated values. of course, the error increases as the solar radiation entering the environment increases. 2.2. calculation methodology of the electrical energy saving the analysis was conducted with reference to two different locations: cosenza (lat: 39° 18 'n) and milan (lat: 45° 28' n). the distribution of the point illuminance and the average illuminance in the environment were determined using the inlux-dbr code. the code requires some input data such as direct solar radiation and diffuse hourly monthly average and illuminance on the horizontal plane. these parameters were obtained through the “european database of daylight and solar radiation” provided by satel-light [44]. the luminance of the sky was determined by setting the perez model. the latter was chosen on the basis of an accuracy analysis carried out in a previous study [20], which showed the goodness of the model. the perez model [40] is a full-sky calculation methodology based on the clarity index. the latter is determined by means of the following relation: 𝜀 = 𝐺𝑑0 + 𝐺𝑏𝑜 sin 𝛼𝑠 𝐺𝑑0 + 5.535 ∙ 10−6 ∙ 𝑧𝑠 3 1 + 5.535 ∙ 10−6 ∙ 𝑧𝑠 3 , (6) where with 𝐺𝑑0 indicates the diffused irradiation on the horizontal plane and with 𝐺𝑏𝑜 indicates the direct irradiation on the horizontal plane. this index can vary between 𝜀 = 1 (with completely cloudy skies) and 𝜀 = 6.2 (with completely clear skies). as a function of the clarity index, eight sky conditions have been identified, varying the parameters and formulas for calculating the luminance of the sky; this quantity varies point by point in the celestial vault and appears in eq. (1). in particular, the luminance of the sky can be calculated by means of the following relationship: 𝐿𝑃 = 𝐿𝑍 ∙ 𝑙𝑟 , (7) where with 𝑙𝑟 indicates the relative luminance and with 𝐿𝑍 indicates the zenith luminance. relative luminance can be determined by means of the following relationship: 𝑙𝑟 = 𝛷(𝛼) ∙ 𝑓(𝜁) 𝛷(π/2) ∙ 𝑓(π/2 − 𝛼𝑠 ) , (8) where 𝛼 is the solar height of the considered point of the sky, 𝜁 is the angular distance between the considered point of the sky and the position of the sun. the functions 𝛷(𝑥) and 𝑓(𝑥) can be determined by means of the following equations: 𝛷(𝑥) = 1 + a ∙ e 𝑏 sin 𝑥 (9) 𝑓(𝑥) = 1 + c ∙ e𝑑 𝑥 + 𝑒 ∙ cos2 𝑥 . (10) in eqs. (9) and (10) the parameters 𝑎, 𝑏, c, 𝑑 and 𝑒 are functions of the solar height 𝛼𝑠, the brightness of the sky ∆ and figure 2. analysis of the error indices during the experimental campaign done by means of an experimental site located in osaka 7. acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 5 the clarity index 𝜀. more information on the calculation of these parameters can be found in ref. [25]. the luminance of zenith, which appears in eq. (7), can be determined by means of the following correlation developed by perez [45] as the sky condition varies. 𝐿𝑍 = 𝐺𝑑0[𝑎i + 𝑐i sin 𝛼𝑠 + 𝑐i ′ e−3(π/2−𝛼𝑠) + 𝑑i ] , (11) where the constants 𝑎i, 𝑐i, 𝑐′i and 𝑑i are function of the clarity index 𝜀. 2.3. case study the analysis will be conducted with reference to a case study. the geometry of the room for which the smart lighting system is simulated is 6 × 6 × 3 m³ in which there is a window with double glazing and transmissivity equal to 0.75. the size of the window varies between 5.04 m2 and 12.96 m2. the minimum size is fixed on the basis of what is reported by the italian legislation [46] which imposes a minimum size of the glass surfaces of a room of 1/8 (12.5 %) of the total floor area. the reflectivity of the room's opaque vertical and horizontal structures is summarized in table 1. by means of the inlux-dbr calculation code, an analysis was carried out for a whole year in order to evaluate the distribution of natural lighting inside the room. in particular, the natural illuminance was evaluated with reference to the 12 points shown in figure 3. in each of these points it will be assumed to place a luminescent lamp with power 36 w and brightness of 3200 lumen. the set lamp control logic envisages activating a row of lamps only if the natural illuminance of two points out of three (for the same row) is less than 500 lux. as the orientation and size of the window vary, the distribution and intensity of natural lighting varies. consequently, the number of switch-on of the lighting system and the electrical consumption will vary. the final goal is precisely to determine the energy savings that can be obtained by varying the configuration. 3. analysis of results in figure 4 it is possible to observe the distribution of natural lighting on the 12 points selected for the town of cosenza at 12:30 am on 15 january. the analysis is carried out with reference to a surface window equal to 5.04 m2 for different variation of the exposure. with the window exposed to the north, only in the row closest to the window the illuminance intensity is greater than 500 lux; with the exposure to the east the natural illuminance is higher than 500 lux in the first row and in two points out of three of the second line; with the southern exposure the natural illuminance in all points is higher than 500 lux; finally, with the western exposure, only in the last two lines farthest from the window the natural illuminance is higher than a 500 lux. the same analysis was conducted at 12:30 am on july 15th. the results are reported in figure 5. table 1. characteristics of the walls of the reference room. dimensions area (m 2) ρ floor 6 × 6 m² 36 0.2 ceiling 6 × 6 m² 36 0.7 rear wall 6 × 3 m² 18 0.5 left wall 6 × 3 m² 18 0.5 right wall 6 × 3 m² 18 0.5 front wall 6 × 3 m² 12.96 0.5 ref. window 4.2 × 1.2 m² 5.04 0.15 work plane 6 × 6 m² 36 figure 3. position of reference points in which the analysis of the natural illuminance was conducted. figure 4. natural illuminance on 12 points of the work plane. january 15th time 12:30 am cosenza. figure 5. natural illuminance on 12 points of the work plane. july 15th time 12:30 am cosenza. acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 6 the results are similar but the values obtained for natural lighting are sharply lower. this is due to the higher solar trajectory in the sky in the summer months. this affects the solar height which varies from 𝑠 = 29.37° january 15th at 12:30 am to 𝑠 = 70.77° july 15th at 12:30 am. this lower solar trajectory causes less direct solar radiation to hit the surfaces inside the room in july. it goes from an average solar radiation of 200 kluxhour/day for january to 119 kluxhour/day for july. furthermore, the lower illuminance in july is due to the lower brightness of the celestial vault due to the greater distance of the sun. the distribution of the luminance of the sky varies a lot between the month of january and the month of july. this can be easily observed in figure 6 which shows how the luminance of the sky varies as the angular height 𝛼 and azimuth 𝜙 of the celestial point considered at 12:00 am on january 15 (a) and at noon on 15 july (b) in the city of cosenza. it is observed that in all cases for all points of the celestial vault there is a higher intensity of the luminance of the sky in the month of january than in the month of july. this is due to the above. for the case in question, considering a 5.04 m2 south-facing window, the opaque elements arranged centrally in the room have a radiative exchange with the points of the celestial vault comprised between an angular height of 𝛼 = 2° and an angular height of 𝛼 = 23°. the difference in sky brightness can also be evaluated with reference to the distributions obtained by varying the azimuth for 𝛼 = 5° and 𝛼 = 25° with reference to noon on 15th january and to noon on 15th july. the distributions obtained were represented in figure 7. once again, the brightness of the sky in january is much greater than that of july under the same conditions. in figure 8 it is observed how the average illuminance in the room varies as the orientation of the window varies at the hours of january 15th and july 15th. in figure 9 and figure 10 it is possible to observe, respectively, how the illumination in point 7 and point 8 of the room (see figure 3) varies as the orientation of the window varies on january 15th and july 15th. therefore, the two central points furthest from the window were taken into consideration. with reference to the case with the window exposed to north, in july in the first and last hours of the day, natural illuminance is the highest due to the longer and lower solar trajectory in the celestial vault. therefore, in the first and last hours of the day, a certain amount of direct solar radiation will affect the internal opaque surfaces, consequently affecting the natural internal lighting. in the central hours of the day, the direct solar radiation incident on the glass surface is obviously zero and this affects the natural lighting inside the room. in january, however, the solar trajectory will be shorter but higher in the celestial vault and this generates a typical “bell-like trend” of the natural illuminance inside the room. with the window exposed to south, the natural lighting has trends similar to those described in reference to the case with north orientation but with much higher intensity. in fact, in this case the sun passes in front of the window in its trajectory. a) b) figure 6 a) sky luminance distribution (perez) as a function of azimuth and elevation angles. noon on january 15th, cosenza, b) sky luminance distribution (perez) as a function of azimuth and elevation angles. noon on july 15th, cosenza. figure 7. comparison of sky luminance distributions for noon of january 15th and july 15th, cosenza. figure 8. mean illuminance on the work plane as a function of time, cosenza. acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 7 finally, with reference to the cases with the eastern and western exposure, in the first and last hours of the day, respectively, there is a peak of illuminance due to the solar path (sunrise and sunset). the higher intensity of natural lighting respectively in the first and last hours of the day in the month of july is once again due to the longer solar trajectory which affects the amount of direct solar radiation entering the room through the window. in the central hours, on the other hand, there is a greater intensity of natural lighting in the month of january due to the lower solar trajectory. the electricity consumption in reference to the city of cosenza was also analyzed as the area and the orientation of the window changed. by hypothesis it is considered that the environment is used as an office and has a working period of 8 hours ranging from 8:00 am to 4:00 pm. to evaluate the convenience of each individual configuration taken into consideration, two different energy saving indices have been introduced: 1) the percentage of energy savings compared to a case in the total absence of natural light (𝑅1). in particular, in the case in which it is assumed that in the room in question there is no contribution due to natural light, an annual electricity consumption is estimated 𝐶max equal to 860 kw h. the energy saving index 𝑅1 can be calculated using the following formula: 𝑅1 = 𝐶max − 𝐶 𝐶max . (12) 2) the percentage of energy savings compared to a reference case (𝑅2). in particular, the case with a northfacing window is taken as reference and the relative annual electricity consumption 𝐶ref is indicated with 5.04 m2. this case has been imposed as a reference as it constitutes the configuration with the lowest natural illuminance. therefore, the index 𝑅2 can be determined by means of the following formula: 𝑅2 = 𝐶ref − 𝐶 𝐶ref . (13) table 2 summarizes the monthly and annual electricity consumption depending on the orientation and area of the window. furthermore, the same table shows the values of the indices 𝑅1 and for the various configurations 𝑅2. figure 9. local illuminance on point 7 as a function of time, cosenza. figure 10. local illuminance on point 8 as a function of time, cosenza. table 2. electrical energy consumptions in a room 6 × 6 × 3 m³ of a building located in cosenza. month j f m a m j j a s o n d year r1 r2 exposure days 21 20 22 19 22 21 21 22 21 22 21 17 249 reference window 1.2 × 4.2 m² = 5.04 m2 (14 % floor area) north kw h 54.4 51.8 57.0 45.1 52.3 54.4 54.4 54.4 52.3 57.0 54.4 44.0 633 26 0 east 29.5 30.2 35.6 34.9 40.4 38.5 38.5 40.4 34.0 33.3 29.5 25.7 411 52 35 south 4.5 0.0 0.0 20.5 38.0 43.1 43.1 0.0 0.0 0.0 0.0 0.0 183 78 71 west 29.5 28.1 35.6 32.8 40.4 45.4 43.1 40.4 34 33.3 29.5 23.9 416 51 34 window 1.8 × 4.2 m² = 7.56 m2 (21 % floor area) north kw h 45.4 38.9 47.5 41 47.5 45.4 47.6 47.5 45.3 38 38.5 36.7 520 39 18 east 20.4 21.6 21.4 24.6 35.6 36.3 36.3 30.9 20.4 23.8 18.2 16.5 306 64 52 south 0.0 0.0 0.0 0.0 23.8 29.5 24.9 4.7 0.0 0.0 0.0 0.0 83 90 86 west 20.4 21.6 21.4 26.7 33.3 36.3 34 30.8 20.4 23.7 20.4 16.5 306 64 52 window 1.8 × 5.4 m² = 9.72 m2 (27 % floor area) north kw h 40.8 34.5 38 32.8 42.7 40.8 40.8 45.1 36.3 38 36.3 33 459 46 27 east 18.1 15.1 21.4 20.5 28.5 29.5 27.2 23.7 20.4 16.6 15.8 14.6 252 70 60 south 0.0 0.0 0.0 0.0 9.5 24.9 13.6 00.0 0.0 0.0 0.0 0.0 48.1 94 92 west 18.1 15.1 21.3 20.5 28.5 31.7 27.2 23.8 20.4 16.6 18.1 14.6 256 70 59 window 2.4 × 5.4 m² = 12.96 m2 (36 % floor area) north kw h 40.8 34.6 38.0 32.8 38 40.8 40.8 38 34 38 36.2 33 445 48 30 east 18.1 15.1 19.0 18.5 28.5 27.2 27.2 23.8 20.4 16.6 15.9 14.7 245 71 61 south 0.0 0.0 0.0 0.0 4.7 9 6.8 0.0 0.0 0.0 0.0 0.0 20.6 98 97 west 18.1 15.1 19.0 18.5 28.5 27.2 27.2 23.8 20.4 16.6 13.6 14.7 243 72 61 acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 8 in table 2 it is observed that the index 𝑅1 assumes values including: a. between 26% and 48% for the northern exposure; b. between 52% and 72% due to east and west exposure; c. between 78% and 98% for the southern exposure. the index 𝑅2 assumes values including: a. between 0% and 30% for the northern exposure; b. between 35% and 61% due to east and west exposure; c. between 71% and 97% for the southern exposure. the same analysis was repeated with reference to the city of milan in order to evaluate the effects of latitude and climate zone (cosenza belongs to climate zone c and milan belongs to climate zone e) on electricity consumption. the resalts was summarized in table 3. a comparison of electricity consumption between the city of milan and the city of cosenza shows that there is less electricity consumption in milan. by means of the satel-light database it can be observed that cosenza is characterized by an annual cumulative diffuse illuminance on the horizontal plane greater than approximately 6.5% compared to that of milan (37656 kluxhour versus 35345 kluxhour). furthermore, cosenza has an annual cumulative direct illuminance on the vertical plane greater than about 13% compared to that of milan (44420 kluxhour versus 39235 kluxhour). however, the sky in milan appears brighter due to the lower solar trajectory. this phenomenon causes greater internal natural lighting and therefore lower electricity consumption. in figure 11 a) and b) the trends of the energy saving indices 𝑅1 and 𝑅2 are shown respectively. in this trends the indices vary with the window area and orientation and with location. the analysis carried out shows similar values of the indices with reference to the north, east and west exposures for the two locations in question. the values of the indices differ a lot for a southern exposure of the window. this highlights how the latitude has a greater effect on natural lighting and consequently on electricity consumption only with an exposure to the south of the window. in the other cases this effect is completely negligible. these results are very useful in evaluating what could be the natural illuminance present inside an environment and how much this can vary with the variation of the surface and orientation of the window. this could also suggest control techniques for dimmable artificial lighting systems. thus, adapting the artificial lighting to the natural lighting present in the environment. 4. conclusions the calculation code for illuminance used in this article was inlux-dbr. this code is experimentally validated through measurements taken in a scaled room at a building located in table 3. electrical energy consumptions in a room 6 × 6 × 3 m³ of a building located in milan. month j f m a m j j a s o n d year r1 r2 exposure days 21 20 22 19 22 21 21 22 21 22 21 17 249 reference window 1.2 × 4.2 m² = 5.04 m2 (14 % floor area) north kw h 56.7 51.8 57.0 49.2 52.3 49.9 49.9 54.6 54.4 57.0 54.4 44.0 631 27 east 34.0 28.0 33.2 30.8 40.4 38.5 38.5 35.6 34.0 30.8 31.7 23.9 400 53 37 south 0.0 0.0 0.0 8.2 35.6 36.3 36.3 19.0 0.0 0.0 0.0 0.0 135 84 79 west 31.7 28.0 33.2 30.8 40.4 38.5 38.5 35.6 34.0 30.9 29.5 22.0 393 54 38 window 1.8 × 4.2 m² = 7.56 m2 (21 % floor area) north kw h 52.0 38.8 42.7 41.0 47.5 45.3 45.3 47.5 43.1 38.0 52.2 38.5 532 38 16 east 25.0 19.4 21.4 22.6 30.9 34 31.7 28.5 20.4 21.4 24.9 14.7 295 66 53 south 0.0 0.0 0.0 0 4.7 18.1 9.1 0.0 0.0 0.0 0.0 0.0 32 96 95 west 20.4 19.4 21.4 22.6 30.9 31.7 31.7 28.5 20.4 19.0 20.4 16.5 283 67 55 window 1.8 × 5.4 m² = 9.72 m2 (27 % floor area) north kw h 40.8 34.6 38.0 36.9 38.0 38.5 38.5 40.4 36.3 38.0 38.5 33 452 47 28 east 18.1 15.1 19.0 18.5 28.5 29.5 24.9 21.4 18.1 16.6 18.1 12.8 241 72 62 south 0.0 0.0 0.0 0.0 0.0 6.8 0.0 0.0 0.0 0.0 0.0 0.0 6.8 99 99 west 18.1 15.1 16.6 18.5 23.7 27.2 22.3 21.4 18.1 16.6 15.9 12.8 227 74 64 window 2.4 × 5.4 m² = 12.96 m2 (36 % floor area) north kw h 40.8 34.6 38.0 32.8 38.0 38.5 36.3 35.6 36.3 38.0 38.5 33.0 441 49 30 east 18.1 15.1 16.6 18.5 23.8 24.9 22.7 21.4 18.1 16.6 18.1 11.0 225 74 37 south 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0 100 100 west 18.1 15.1 16.6 18.5 23.8 24.9 22.7 21.4 15.9 14.3 15.9 12.8 220 74 38 a) b) figure 11. a) saving index r1 as a function of window area and orientation for cosenza and milan, b) saving index r2 as a function of window area and orientation for cosenza and milan acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 9 osaka (japan). estimates of the electricity consumption of artificial lights were performed assuming switching on at critical times. the luminance of the sky is given by the perez model. the evaluation of the luminance distribution in the interior walls of the room is obtained using the radiosity model. the building in question is an office which has been placed alternatively in two italian locations: cosenza and milan. estimates were made by changing the window size and orientation of the building, analysing the behaviour by placing the window on the four orientations coinciding with the cardinal points. in the case of the city of cosenza, the electrical savings obtained varied between 26% and 48% for glazed surfaces exposed to the north, between 52% and 72% to the east and west, and between 78% and 97% to the south. the variations refer to different window sizes ranging from 14% to 36% of the room's floor area. with reference to the city of milan, electricity savings vary between 27% and 49% per window surface exposed to the north, between 53% and 74% to the east and west, and between 84% and 100% to the south. of course, if the building envelope has a very large window area, using an intelligent management system for artificial light is very convenient. the results show, however, that even when the windows are typically large (14% of the floor area), electricity savings are considerable. in particular, it is up to 84% for the city of milan. the results obtained for milan are better than those obtained for the city of cosenza, which is at a lower latitude. although, the results are very similar. the results are therefore considered similar for all areas within this latitude range. the numerical results obtained can serve as a reference for making proper use of daylight and reducing the unnecessary use of electricity. nomenclature (𝑎, 𝑏, 𝑐, 𝑑, 𝑒); (𝑎i, 𝑏i, 𝑐i′, 𝑑i) coefficients used in the perez sky model 𝛷, 𝑓 functions used in the perez sky model 𝑘 coefficient used in the illuminance balance gbo direct irradiance on horizontal plane, w/m2 gdo diffuse irradiance on horizontal plane, w/m2 lp luminance of a point of the sky, cd/m2 lg luminance of a point of the ground, cd/m2 𝑙𝑟 relative luminance lz zenith luminance, cd/m2 zs sun zenith angle, rad e illuminance of the surface element, lx/m2 evs solar direct illuminance, lx/m2 𝐴 area of the surface element, m2 𝑛 number of opaque surface element inside the room 𝑚 number of glazed surface element inside the room 𝑝 number of ground surface element 𝑖 index opaque surface element 𝑤 index glazed surface element g index ground surface element 𝐶 yearly energy consumption in the case in exam, w/year 𝐶max yearly energy consumption in the case in absence of natural illuminance, w/year 𝐶ref yearly energy consumption in the reference case, w/year 𝑅1 percentage of energy savings compared to the case in absence of natural illuminance 𝑅2 percentage of energy savings compared to a reference case nmbe normalized mean bias error rmsep root mean square error percentage 𝑉calc calculated data 𝑉meas measured data n total number of data greek symbols  elevation angle of a point of the sky, rad s sun elevation angle, rad g elevation angle of a point of the ground, rad  sky brightness  clearness index 𝜁 angular distance between the sun and a point of the sky, rad ρ reflectivity 𝜑 azimuth of a point of the sky, rad 𝜑𝑠 solar azimuth, rad 𝜑𝑔 azimuth of a point of the ground, rad 𝜏 transmissivity 𝜗𝑠 angle between the direction 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[30] m. krarti, p. m. erickson, t. c. hillman, a simplified method to estimate energy savings of artificial lighting use from daylighting. building and environment, 40(6), 747–754. doi: 10.1016/j.buildenv.2004.08.007 [31] w. j. hee, m. a. alghoul, b. bakhtyar, o. elayeb, m. a. shameri, m. s. alrubaih, k. sopian, the role of window glazing on daylighting and energy saving in buildings. in renewable and sustainable energy reviews vol. 42, february 2015, pp. 323–343 doi: 10.1016/j.rser.2014.09.020 [32] j. w. moon, y. k. baik, s. kim, operation guidelines for daylight dimming control systems in an office with lightshelf configurations. building and environment, 180, august 2020, 106968. doi: 10.1016/j.buildenv.2020.106968 [33] z. s. zomorodian, m. tahsildoost, assessing the effectiveness of dynamic metrics in predicting daylight availability and visual comfort in classrooms. renewable energy, 134, april 2019, 669– 680. doi: 10.1016/j.renene.2018.11.072 [34] s. m. yacine, z. noureddine, b. e. a. piga, e. morello, d. safa, towards a new model of light quality assessment based on occupant satisfaction and lighting glare indices. energy procedia, 122, 805–810. doi: 10.1016/j.egypro.2017.07.408 [35] j. k. day, b. futrell, r. cox, s. n. ruiz, blinded by the light: occupant perceptions and visual comfort assessments of three dynamic daylight control systems and shading strategies. building and environment, 154, 107–121. doi: 10.1016/j.buildenv.2019.02.037 [36] d. h. w. li, a. c. k. cheung, s. k. h. chow, e. w. m. lee, study of daylight data and lighting energy savings for atrium corridors with lighting dimming controls. energy and buildings, 72, 457– 464. doi: 10.1016/j.enbuild.2013.12.027 [37] l. t. doulos, a. tsangrassoulis, p. a. kontaxis, a. kontadakis, f. v. topalis, harvesting daylight with led or t5 fluorescent lamps? the role of dimming. energy and buildings, 140, 336–347. doi: 10.1016/j.enbuild.2017.02.013 [38] l. bellia, f. fragliasso, automated daylight-linked control systems performance with illuminance sensors for side-lit offices in the https://doi.org/10.3390/en15051836 https://doi.org/10.1016/j.jclepro.2020.122459 https://doi.org/10.1016/j.enbuild.2013.08.011 https://doi.org/10.1016/j.rser.2021.111523 https://doi.org/10.1016/j.renene.2020.01.085 https://doi.org/10.3390/en13092188 https://doi.org/10.21014/acta_imeko.v10i4.1182 https://doi.org/10.21014/acta_imeko.v8i2.640 https://doi.org/10.1016/j.mset.2020.07.002 https://doi.org/10.1016/j.promfg.2019.02.241 https://doi.org/10.1016/j.enbuild.2014.01.024 https://doi.org/10.1002/col.5080200119 https://doi.org/10.1016/j.buildenv.2008.11.014 https://doi.org/10.1016/j.energy.2010.05.021 https://doi.org/10.1038/088072a0 https://doi.org/10.1177/147715355401900203 https://doi.org/10.1177/147715355401900701 https://doi.org/10.1177/096032718902100305 https://doi.org/10.1016/j.enbuild.2006.03.013 https://doi.org/10.1016/j.aej.2019.01.004 https://doi.org/10.1016/j.buildenv.2004.08.007 https://doi.org/10.1016/j.rser.2014.09.020 https://doi.org/10.1016/j.buildenv.2020.106968 https://doi.org/10.1016/j.renene.2018.11.072 https://doi.org/10.1016/j.egypro.2017.07.408 https://doi.org/10.1016/j.buildenv.2019.02.037 https://doi.org/10.1016/j.enbuild.2013.12.027 https://doi.org/10.1016/j.enbuild.2017.02.013 acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 11 mediterranean area. automation in construction, 100, april 2019, pp. 145–162. doi: 10.1016/j.autcon.2018.12.027 [39] cie, spatial distribution of daylight-cie standard general sky, cie standard s011/e, vienna, 2003. 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[43] p. r. tregenza, subdivision of the sky hemisphere for luminance measurements. light. res. technol., vol. 19, 1987, issue 1. doi: 10.1177/096032718701900103 [44] s@tel-light. online [accessed 29 november 2022] http://satellight.entpe.fr/ [45] r. perez, p. ineichen, r. seals, j. michalsky, r. stewart, modeling daylight availability and irradiance components from direct and global irradiance. sol. energy 44. doi: 10.1016/0038-092x(90)90055-h [46] italian health ministerial decree, 5 july 1975. https://doi.org/10.1016/j.autcon.2018.12.027 https://doi.org/10.1016/0038-092x(93)90017-i https://doi.org/10.1016/j.solener.2004.04.016 https://doi.org/10.1177/096032718701900103 http://satellight.entpe.fr/ https://doi.org/10.1016/0038-092x(90)90055-h digital signal processsing functions for ultralow frequency calibrations acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 374 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 374 378 digital signal processsing functions for ultralow frequency calibrations henrik ingerslev1, søren andresen2, jacob holm winther3 1 brüel & kjær, danish primary laboratory for acoustics, denmark, henrik.ingerslev@bksv.com 2 hottinger, brüel and kjær, denmark, soren.andresen@bksv.com 3 brüel & kjær, danish primary laboratory for acoustics, denmark, jacobholm.winther@bksv.com abstract: the demand from industry to produce accurate acceleration measurements down to ever lower frequencies and with ever lower noise is increasing [1][2]. different vibration transducers are used today for many different purposes within this area, like detection and warning for earthquakes [3], detection of nuclear testing [4], and monitoring of the environment [5]. accelerometers for such purposes must be calibrated in order to yield trustworthy results and provide traceability to the si-system accordingly [6]. for these calibrations to be feasible, suitable ultra low-noise accelerometers and/or signal processing functions are needed [7]. here we present two digital signal processing (dsp) functions designed to measure ultra low-noise acceleration in calibration systems. the dsp functions use dual channel signal analysis on signals from two accelerometers measuring the same stimuli and use the coherence between the two signals to reduce noise. simulations show that the two dsp functions are estimating calibration signals better than the standard analysis. the results presented here are intended to be used in key comparison studies of accelerometer calibration systems [8][9], and may help extend current general low frequency range from e.g. 100mhz down to ultra-low frequencies of around 10mhz, possibly using somewhat same instrumentation. keywords: low-noise; coherent power; coherent phase; calibration; dual-channel; ultra-low frequencies 1. introduction in the field of dual channel signal analysis there are some very powerful functions for analysing signals, such as the well-known frequency response function and coherence. but there are also other functions like the coherent power function (cop) and non-coherent power function which are very powerful for decomposing noisy signals into the coherent part and the non-coherent part [10][11][12]. consider an accelerometer calibration setup with two accelerometers mounted close to each other and measuring the same stimuli. they will both measure the acceleration of the shaker, but since they are different sensors with different conditioning, they will have different noise. the two signals will have a coherent part which is the acceleration signal and a noncoherent part which is the noise. hence, the cop can be a powerful tool for extracting the signal from the noise and thereby increase the measuring accuracy of the power. a similar function for increasing the measuring accuracy of the phase by separating the coherent phase from the non-coherent phase is also derived in the next section, called the coherent phase (or argument) function (coa). for the coa to work in a proper manner, it is crucial that the signal applied to the shaker is a continuous signal, like a sine or a multi-sine, and that the frequencies of the sines are very precise and phase synchronized with the frequencies of the fourier transformation, to prevent the phase from drifting or even make jumps. more details on this will be given in section 1.2. the two dsp functions analysed in this article may prove relevant to be used for e.g. key comparison of calibration systems down to extremely low frequencies of around 10mhz where noise becomes a real challenge [7][8][9]. the degree to which the cop, and the coa can separate a signal into coherent and noncoherent parts increases with the length of the measurement, and generally depends on parameters like how many time-samples the measurement is divided into, how long each time-sample is, the sampling rate, and the fourier transform used. 2. digital signal processing functions theory in this section the theory for two dsp functions is outlined. the two functions are based on dual http://www.imeko.org/ mailto:henrik.ingerslev@bksv.com mailto:soren.andresen@bksv.com mailto:jacobholm.winther@bksv.com acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 375 channel signal analysis and can give a better estimate of signals in very noisy environments, than standard signal analysis. the first function is the cop for estimating the power or amplitude of the signal. and the second is the coa for estimating the phase. both functions rely on the coherence between the two signals. 1.1. coherent power function consider two sensors both measuring the same stimuli and positioned close enough for their mutual transfer function to be considered unity. as illustrated in figure 1 the signal without noise called 𝑢(𝑡), and the noise from each sensor called 𝑛(𝑡) and 𝑚(𝑡) yields the output signals from the two sensors, called 𝑎(𝑡) and 𝑏(𝑡). now consider 𝑗 = 1…𝑁 discrete time-samples measured with the two sensors: 𝑎𝑗(𝑡𝑖) = 𝑢𝑗(𝑡𝑖) + 𝑛𝑗(𝑡𝑖) (1) 𝑏𝑗(𝑡𝑖) = 𝑢𝑗(𝑡𝑖) + 𝑚𝑗(𝑡𝑖) (2) here 𝑡𝑖 is the discrete time in each time-sample, 𝑢𝑗 is the discrete signal, and 𝑛𝑗 and 𝑚𝑗 are discrete noise in each time-sample. by discrete fourier transformation of equation (1) and (2) we get 𝐴𝑗(𝑓𝑘) = 𝑈𝑗(𝑓𝑘) + 𝑁𝑗(𝑓𝑘) (3) 𝐵𝑗(𝑓𝑘) = 𝑈𝑗(𝑓𝑘) + 𝑀𝑗(𝑓𝑘) (4) where 𝑓𝑘 is the discrete frequency, 𝑈𝑗, 𝑁𝑗, and 𝑀𝑗 are the discrete fourier transforms of 𝑢𝑗, 𝑛𝑗, and 𝑚𝑗 respectively. figure 1: illustration of the signals used in the dual channel signal analysis and derivation of the two dsp functions. 𝑢(𝑡) is the signal we want to measure, and 𝑛(𝑡) and 𝑚(𝑡) are the noise contributions from each sensor, which then yields the two output signals 𝑎(𝑡) and 𝑏(𝑡). the cross spectrum is given by 𝑆𝐴𝐵(𝑓𝑘) = 1 𝑁 ∑ 𝐴𝑗(𝑓𝑘)𝐵𝑗 ∗(𝑓𝑘) 𝑁−1 𝑗=0 (5) for 𝑁 → ∞, and * denotes complex conjugate. by inserting equation (3) and (4) in equation (5), and using that 𝑈𝑗 , 𝑁𝑗 , and 𝑀𝑗 are uncorrelated the cross spectrum can be given by 𝑆𝐴𝐵(𝑓𝑘) = 1 𝑁 ∑ 𝑈𝑗(𝑓𝑘)𝑈𝑗 ∗(𝑓𝑘) 𝑁−1 𝑗=0 ≝ 𝑆𝑈𝑈(𝑓𝑘) (6) where 𝑆𝑈𝑈(𝑓𝑘) is the power spectrum of the signal without noise, i.e. the coherent power. therefore, the cop can in this setup be given by: 𝐶𝑂𝑃(𝑓𝑘) = 𝑆𝐴𝐵(𝑓𝑘) (7) 1.2. coherent phase function consider the following function, for 𝑁 → ∞: 𝐷𝐴𝐵(𝑓𝑘) = 1 𝑁 ∑ 𝐴𝑗(𝑓𝑘)𝐵𝑗(𝑓𝑘) 𝑁−1 𝑗=0 (8) it looks like the cross spectrum from equation (5), but without the complex conjugation. this “nonconjugated cross spectrum” is very useful for deriving a function for measuring the phase of the coherent signal. we can similarly to the derivation of the cop insert equation (3) and (4) in (8), and use that 𝑈𝑗, 𝑁𝑗, and 𝑀𝑗 are uncorrelated. hence, the non-conjugated cross spectrum can now be given by, where we have omitted the 𝑓𝑘 dependence to make room. 𝐷𝐴𝐵 = 1 𝑁 ∑ 𝑈𝑗𝑈𝑗 𝑁−1 𝑗=0 (9) = 1 𝑁 ∑|𝑈𝑗| 2 exp⁡(2𝑖∠𝑈𝑗) 𝑁−1 𝑗=0 (10) ≃ 1 𝑁 ∑|𝑈𝑗| 2 𝑁−1 𝑗=0 exp(2𝑖 1 𝑁 ∑ ∠𝑈𝑗 𝑁−1 𝑗=0 ) (11) = 𝑆𝑈𝑈exp(2𝑖∠𝑈̅̅ ̅̅ ) (12) from equation (10) to (11) we have approximated the summation of vectors with length |𝑈𝑗| 2 and angle 2∠𝑈𝑗 by vectors with correct length but all with the mean angle 2∠𝑈̅̅ ̅̅ . figure 2 illustrates this approximation and shows that for relatively small changes in angle from sample to sample this is a good approximation. in calibration applications the signal measures the acceleration of the shaker. and by applying a sine or multi-sine with frequencies exactly the same and phase synchronized with the fourier frequencies to the shaker, the phase from sample to sample can be kept steady without drifting and the approximation will therefore be very good in such calibration applications. http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 376 in equation (12) ∠𝑈̅̅ ̅̅ is the mean phase of the coherent signal. therefore, the coherent phase function can be given by 𝐶𝑂𝐴 = ∠𝑈̅̅ ̅̅ . and by rewriting equation (12), the coa can be given by: 𝐶𝑂𝐴(𝑓𝑘) = 1 2 imag(ln(𝐷𝐴𝐵(𝑓𝑘))) (13) we have in the derivation of equation (13) used that the signal power 𝑆𝑈𝑈 is purely real and can therefore be omitted. figure 2: schematic graphs illustrating the approximation from equation (10) to (11).the arrows represent 𝑈𝑗 and the xand yaxes are the real and imaginary part of 𝑈𝑗 (a) shows the correct summation where each arrow has length |𝑈𝑗| 2and angle 2∠𝑈𝑗 as in equation (10), and (b) shows the approximative summation where each arrow has correct length but a mean angle 2∠𝑈̅̅ ̅̅ as in equation (11). as can be seen from (a) to (b), if 𝑈𝑗 does not change to much between samples the approximation is good. 3. simulations in this section we test the cop and the coa functions on simulated data. we calculate the discrepancy between the functions estimate of the signal amplitude and phase, and the true values. and we compare this with standard signal analysis which is to average the amplitude and phase over all the samples. when measuring the cross spectrum or the nonconjugated cross spectrum for finding the cop and the coa we only measure a finite number of samples 𝑁, which therefore only yields an estimate of the cop and the coa. hence, the more samples the more precise the estimate will be, and in the following the simulations is based on a 102400 s (~ 28 h) time sample divided into 𝑁 = 1024 samples of 100 s each, with 2048 discrete measurement points in each sample, and a fourier transform from 10mhz to 10.24 hz with a 10 mhz step. how well the cop and coa works is estimated by representing 𝑎𝑗, and 𝑏𝑗 from equation (1) and (2) with simulated data. here the signal 𝑢𝑗 is a multi-sine with 𝑙 = 1…𝑀 frequencies (𝑓𝑙) and phases (𝜙𝑙), all with amplitude 𝑈0: 𝑢𝑗(𝑡𝑖) = 𝑈0∑sin⁡(2𝜋𝑓𝑙𝑡𝑖 + 𝜙𝑙) 𝑀 𝑙=1 (14) the noise 𝑛𝑗, and 𝑚𝑗 are random generated white noise with amplitude 𝑁0. and the signal to noise ratio is given by: 𝑆𝑁𝑅 = 𝑈0 𝑁0 (15) the frequencies (𝑓𝑙) of the multi-sine in equation (14) must be precisely the same or synchronized to a subset of the fourier transformation frequencies (𝑓𝑘) from equation (3) and (4), otherwise the coa will drift. this requirement is easily met in the simulations presented here, since the subset of frequencies (𝑓𝑙) can be set to be identical to some of the frequencies (𝑓𝑘). but in real measurements this requirement might be challenging to meet. figure 3: based on simulated data the coherent power function and coherent phase function is tested for its strength for estimating the amplitude and phase of sine waves in noisy data. the graph shows the deviation of the amplitude and phase from the true vales. (a) shows the mean amplitude, ⁡0.5(𝐴 + 𝐵), and the coherent amplitude, √𝐶𝑂𝑃 . (b) shows the mean phase 0.5(∠𝐴 + ∠𝐵), and the coherent phase 𝐶𝑂𝐴. figure 3(a) shows the discrepancy between the signal amplitudes 𝑈0 from equation (14) and the coherent amplitude, defined as √𝐶𝑂𝑃(𝑓𝑘) , for a signal to noise ratio of 𝑆𝑁𝑅 = 0.32 in red circles. and for comparison we also plot the mean amplitude in blue circles, that is, 0.5(𝐴(𝑓𝑘) + 𝐵(𝑓𝑘)) where 𝐴(𝑓𝑘) = ∑|𝐴𝑗(𝑓𝑘)| and 𝐵(𝑓𝑘) = ∑|𝐴𝑗(𝑓𝑘)| is the average amplitudes over all samples. and we plot only at the frequencies of the signal, i.e. at 𝑓𝑘 = 𝑓𝑙. it is clearly seen that the cop function estimates the amplitude better that the mean amplitude in the full frequency range. http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 377 similarly, figure 3(b) shows the discrepancy between the phase 𝜙𝑙 and the coa for a signal to noise ratio of 𝑆𝑁𝑅 = 0.024 in red circles. and for comparison we also plot the mean phase defined as 0.5(∠𝐴 + ∠𝐵) where ∠𝐴 = ∑arg⁡(𝐴𝑗(𝑓𝑘)) and ∠𝐵 = ∑arg⁡(𝐵𝑗(𝑓𝑘)) are the average phases over all samples. and it is seen that the coa estimates the phase better that the mean phase. figure 4:simulated deviation in amplitude and phase versus signal to noise ratio, in the upper graph for the mean amplitude and the coherent amplitude defined as √𝐶𝑂𝑃, and in lower graph for the mean phase and the coherent phase coa. each deviation point plotted here is an average over all the frequencies from figure 3. the deviations plotted in the four curves in figure 3 seems to be independent of frequency. therefore, by averaging the deviation for each curve in figure 3 over all the frequencies 𝑓𝑙, we get a mean deviation for the cop and the mean amplitude at 𝑆𝑁𝑅 = 0.32, and a mean deviation for the coa and the mean phases at 𝑆𝑁𝑅 = 0.024. we have done this for a range of signal to noise ratios from 𝑆𝑁𝑅 = 0.01 to 𝑆𝑁𝑅 = 10 and plotted it in figure 4. it shows that the cop is better than the mean amplitude from about 𝑆𝑁𝑅 = 1, and the coa is better than the mean phase from about 𝑆𝑁𝑅 = 0.04. this shows that the “critical” snr where the cop and coa functions has lower deviation than the simple mean values is different for cop and coa. and though the data in figure 4 depends highly on the length of the time sample and fourier transform used, this trend of a lower critical snr for the coa function than the cop functions were always seen and needs further investigations. 4. summary we have derived two dsp functions for very accurate measurements of amplitude and phase from two accelerometers measuring the same stimuli. we have tested the two dsp functions on simulated data and our findings based on the simulations shows promise to the functions as good tools for accurately measuring amplitudes and phases of a multi-sine wave in a noisy environment. these findings may prove useful for key comparisons of accelerometer calibration systems down to ultra-low frequencies, since for such measurements noise becomes a huge problem as the frequencies approaches 10mhz. hence, by replacing the accelerometer used in key comparisons by two accelerometers and by using the two dsp functions described here, the frequency range in key comparisons may be possible to extend down to ultralow frequencies of around 10mhz. 5. acknowedgements we would like to acknowledge torben rask licht and lars munch kofoed for fruitful discussions. we would also like to acknowledge the eu empir project “metrology for low-frequency sound and vibration”, 10env03 infra-auv, as the initial reason for the approach on optimising method of analysis in noisy environments, especially found relevant when doing low frequency calibration. 6. references [1] t. bruns, s. gazioch: “correction of shaker flatness deviations in very low frequency primary accelerometer calibration, iop metrologia, vol. 53, no. 3, pp. 986 (2016). [2] j. h. winther, t. r. licht, “primary calibration of reference standard back-to-back vibration transducers using modern calibration standard vibration exciters”, joint conference imeko tc3, tc5, tc22, helsinki, (2017). [3] g. marra, c. clivati et al., “ultra-stable laser interferometry for earthquake detection with terrestrial and submarine optical cables”, science, vol. 361, issue 6401, pp. 486-490, (2018). [4] p. gaebler, l ceranna et al., “a multitechnology analysis of the 2017 north korean nuclear test”, solid earth, 10, 59–78, (2019). [5] r. a. hazelwood, p. c. macey, s. p. robinson, l. s. wang, “optimal transmission of interface vibration wavelets a simulation of seabed seismic responses”, j. mar. sci. eng. 6, 61-79, (2018). [6] l. klaus, m. kobusch, seismometer calibration using a multi-component acceleration exciter, iop conf. series: journal of physics: conf. series 1065 (2018). http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 378 [7] eu empir project “metrology for lowfrequency sound and vibration”, (2020). [8] bipm key comparison, ccauv.v-k5 (2017), https://www.bipm.org/kcdb/comparison?id=453 [9] euramet key comparison, auv.v-k5 (2019), https://www.bipm.org/kcdb/comparison?id=1631 [10] h. herlufsen, “dual channel fft analysis part i”, b&k technical review, no.1, (1984). [11] j. s. bendat, a. g. piersol, “random data”, wiley-interscience, (1986). [12] j. s. bendat, a. g. piersol, “engineering applications of correlation and spectral analysis”, wiley, new york, (1993). http://www.imeko.org/ https://www.bipm.org/kcdb/comparison?id=453 https://www.bipm.org/kcdb/comparison?id=1631 efficient deep learning based data augmentation techniques for enhanced learning on inadequate medical imaging data acta imeko issn: 2221-870x march 2022, volume 11, number 1, 1 6 acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 1 efficient deep learning based data augmentation techniques for enhanced learning on inadequate medical imaging data madipally sai krishna sashank1, vijay souri maddila1, vikas boddu1, y. radhika1 1 department of computer science and engineering, gitam institute of technology, gitam (deemed to be university), visakhapatnam 530045, andhra pradesh, india section: research paper keywords: cnn; covid-19; gan; transfer learning; x-ray images citation: madipally sai krishna sashank, vijay souri maddila, vikas boddu, y. radhika, efficient deep learning based data augmentation techniques for enhanced learning on inadequate medical imaging data, acta imeko, vol. 11, no. 1, article 31, march 2022, identifier: imeko-acta-11 (2022)-01-31 section editor: md zia ur rahman, koneru lakshmaiah education foundation, guntur, india received december 25, 2021; in final form february 17, 2022; published march 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: madipally sai krishna sashank, e-mail: madipallysks@gmail.com 1. introduction covid-19 has reached at least 200 countries worldwide and is overwhelming the healthcare infrastructures like any other emerging pandemic [1]. this pandemic has made everyone realize the importance of hygiene and preventive measures of contracting a virus. when trying to help potential covid-19 patients, allocating clinical resources to the most probable cases can improve the flow of treating patients. to identify the potential cases among all the cases in a hospital, an automated system that can detect the disease can provide immense help to the hospital staff. although the method provided in this work cannot be implemented in a standalone fashion, it can be implemented with minimal interaction. automating the recognition of the virus through the x-ray images can be done through teaching the computer to recognize patterns in the xray images [2]. to teach a computer to recognize the patterns in an image, computer vision technique such as convolutional neural networks (cnns) are required. a cnn can classify given images based on training data, which is not abundantly available in the medical field. even though the available data is increasing at a steady rate, external factors such as human errors and incorrect data can decrease the accuracy with which the cnn can classify an image. so, to prevent these problems, more data is generated using gans on some accurate and curated data. this work will implement computer vision techniques such as generative adversarial networks (gans) and convolutional neural networks (cnns) to classify the x-ray images. the mechanism also uses a number of conventional data augmentation techniques such as changing the axis of the image, rotating the image, changing the brightness and other factors of the image corresponding to the rgb values. generative adversarial networks (gans) is a class of deep learning and image processing models/architectures which are used to generate new images similar to an already existing image dataset. the two sections of a gan are: generator and discriminator. the discriminator tries to classify the input images as real or fake with high accuracy. the generator’s aim is to generate new images which can trick the discriminator into believing that these newly generated images are also real. if the generator is successful in doing so, then the gan is able to abstract the world has come to a standstill with the coronavirus taking over. in these dire times, there are fewer doctors and more patients and hence, treatment is becoming more and more difficult and expensive. in recent times, computer science, machine intelligence, measurement technology has made a lot of progress in the field of medical science hence aiding the automation of a lot of medical activities. one area of progress in this regard is the automation of the process of detection of respiratory diseases (such as covid-19). there have been many convolutional neural network (cnn) architectures and approaches that have been proposed for chest x-ray classification. but a big problem still remains and that is the minimal availability of medical x-ray images due to improper measurements. due to this minimal availability of chest x-ray data, most cnn classifiers do not get trained to an optimal level and the required standards for automating the process are not met. in order to overcome this problem, we propose a new deep learning based approach for accurate measurements of physiological data. mailto:madipallysks@gmail.com acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 2 generate images which are very similar to the original images and its flow chart is shown in figure 1. computer vision is a branch of computer science that deals with giving a computer (or a machine) the ability to read and analyse images and videos in digital format. although it seems trivial and very easy for people to see and perceive the visual stimuli offered by the surrounding, replicating the same for a computer is a complex task which requires a deep understanding of the biological vision and digital conversion of vision perception in the dynamic and ever-changing physical world [4], [5]. computer vision has come a long way from concept to application in the past few years. computer vision is being used in various sectors such as healthcare, robotics, astrophysics, manufacturing, transportation, agriculture, etc. applications such as self-driving cars, artificial-intelligence powered robots, defect detection in manufacturing, intrusion detection, and optical character recognition are possible only because of the advancement of computer vision. convolutional neural network is a neural network with neurons having learnable weights and biases performing dot products and following non-linearity [6]. it is specifically designed for image processing, segmentation, and classification with convolutional layers. the convolutional neural networks have neurons arranged in three dimensions and multiple layers, , its working flow is shown in figure 2. each layer of the cnn has a simple task of creating a 3d output from a 3d input with a differential function [7]. there are three major layers in the cnn convolutional layer, pooling layer, and fully-connected layer. each of these three layers has a particular function; the convolutional layer performs a series of operations on the input matrix/array using filters of the same dimension, the pooling layer decreases the length and width of the input array while increasing the depth/height which is followed by a flatten layer that transforms the 3-dimensional matrix into a 1-dimensional array which is then fed to a fully connected layer (an artificial neural network). tang and tang provided an end-to-end process of creating a generative adversarial classifier that can find anomalies in chest x-ray images which is trained on normal chest x-ray images [8]. the architecture proposed in the model is composed three deep neural networks. the entire model is trained on the correct or normal images. so when the model encounter a normal image then the model categorizes it as a normal image but if the image the model encounters is a an abnormal image which the model did not recognize in the training data it is categorized as an abnormal image. this model is trained on normal chest x-ray images numbering 6000, 1025 normal oneclass chest x-ray images, 1025 chest x-ray images with lung opacities and 1000 images with lung opacities [9]. from this paper the whole process of creating a gan which is used in this research paper is described in detail will be used. wong and lam have researched upon the possible findings from the chest x-ray images of potential covid-19 patients. the study of findings of chest radiographies in patients of bilateral lower zone consolidation showed that the frequency of findings in the chest radiographies peaked at 10-12 days after the symptoms’ onset [10]-[15]. the paper also discusses matters regarding the usage of ct scans which are more sensitive to infection in the body. the paper suggests the usage of rtpcr, and chest radiography testing for generating the required images. this paper gives an understanding of selection of images that are best for analysis and testing. zhang and miao have observed that obtaining pixel wise labels for creation of a model is not possible due to anatomy overlaps in the images and texture which cannot accurately reveal information about the patient. in this paper the authors proposed an image-to-image model that can segment multiple organs in a 3d ct scan. the model was trained with digitally reconstructed radiographs formed over ct volumes. the paper implements a task driven generative adversarial network that can create the x-ray images according to real images and achieve synthesis between the model and generated images [16]-[20]. through this work, the implementation and symbiotic relation between the model and gan is observed and modified to an extent. ke, zhang, and wei give a background to a neuro-heuristic approach to the classification of respiratory diseases using x-ray images. this paper [21] proposes a machine learning architecture/methodology that can be used to assist doctors in making their decisions thus speeding up the process. the authors use a spatial analysis methodology with the main focus being on hue, saturation, and brightness. the method proposes in this paper involves the use of neural networks in collaboration with heuristic search algorithms to detect degenerated lung tissues in x-ray images. first, the neural network predicts the probability of a possible respiratory disease; if this probability is significantly high, then the heuristic algorithm scans and identifies the degenerated tissues in the x-ray images based on the fitness function of the algorithm. the paper [22] presents promising results with an accuracy of over 79 % and a misclassification error of 3.23 % for false positive predictions and 3.76 % for false negative predictions. in their paper, chouhan and singh present a transfer learning based approach to the classification of pneumoniabased chest x-rays. in this approach [23], the features of the xray images are extracted using neural network models pretrained on imagenet architecture and then these features are given as input to a classifier to classify the x-ray as pneumonia positive or pneumonia negative. the paper [24] analysed the performance of five measurement models. these models provide better measurement capability of x-ray data. the proposed deep learning-based approaches are well suited for measuring the medical image and as well analysis. the authors figure 1. flowchart for working of a gan [3]. figure 2. working of a cnn [5]. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 3 also proposed an ensemble model which was able to achieve a state-of-the-art accuracy of 96.4 % in classifying pneumonia with a recall of 99.2 %. 2. method 2.1. data pre-processing i. pre-processing the dataset [25] as per the needs of the problem was the first step. we had 1340 images of normal xray and 1340 images of viral-pneumonia x-ray each of size 1024 × 1024 × 1. ii. to reduce the time and computation power needed for training, we reduced the size of the images from 1024 × 1024 × 1 to 128 × 128 × 1. iii. to ensure that there was minimal information loss, we trained 1 cnn on the original data and another cnn on the minimized image data. the accuracy of both these cnns was almost the same. so, we used the minimized image dataset for the purpose of this experiment and is shown in figure 3. 2.2. training gans iv. after minimizing the data, we trained 2 simple gans (one on each class of images) for 200 epochs. then, we generated 1000 images for each class and then saved the gan models. v. for transfer learning, we first trained a gan on 7,864 face images [26] of size 128 × 128 × 1 for 200 epochs. after training, we saved this faces gan model. vi. then, we trained the faces gan model on the normal xray images for 75 epochs. then we generated 1000 images from this gan and saved the model. vii. then we also trained the faces gan model separately on the viral pneumonia x-ray images for 75 epochs. then, we generated 1000 images from this gan and saved the model. 2.3. training cnns viii. our next step was to train 1 cnn each on each of the following: 1. the original data 2. data generated by conventional data augmentation techniques 3. data formed by combining the original data with the data generated by simple gan 4. data formed by combining the original data with the data generated by transfer learning gan ix. since one cnn was already trained on the original data, we then trained a second cnn on the data generated by using conventional data augmentation techniques (by using imagedatagenerator()). x. then, we trained a third cnn on the data formed by combining the original data with the data generated by simple gan. xi. as the last step, we trained a fourth cnn on the data formed by combining the original data with the data generated by transfer learning gan. xii. the accuracies of all the models were computed and its output is shown in figure 4. note: the architecture and parameters used to train each cnn and gan were the same. 3. results we performed training of cnns on 4 different datasets and the results are as follows: 1. dataset 1 original data the plot for training and validation accuracy for the cnn trained on the original data is below. final training accuracy: 0.9348 final validation accuracy: 0.9259 figure 3. procedure. figure 4. training and validation accuracy of cnn on original dataset. figure 5. training and validation loss of cnn on original dataset. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 4 the plot for training and validation loss for the cnn trained on the original data is below. final training loss: 0.1595 final validation loss: 0.3016 the cnn achieved a training accuracy of 93.5 % on the original data as shown in figure 5. this suggests that the model achieves a high accuracy without any data augmentation techniques but an accuracy of 93.5 % is not enough when classifying x-ray images in a real-time scenario. 2. dataset 2 data generated by conventional data augmentation techniques the plot for training and validation accuracy for the cnn trained on the second dataset is shown in figure 6. final training accuracy: 0.9013 final validation accuracy: 0.9231 the plot for training and validation loss for the cnn trained on the second data is shown in figure 7. final training loss: 0.2587 final validation loss: 0.2919 the values of accuracy and loss for this model are similar to the model trained on the original dataset. there is not a big difference. this means that conventional data augmentation techniques might not be a feasible option for data augmentation in medical image classification. 3. dataset 3 data formed by combining the original data with the data generated by simple gan the plot for training and validation accuracy for the cnn trained on the third dataset is shown in figure 8. final training accuracy: 0.9771 final validation accuracy: 1.0000 the plot for training and validation loss for the cnn trained on the third data is shown in figure 9. final training loss: 0.0670 final validation loss: 0.0281 there is a huge improvement the images generated by a simple gan along with the original images are used to train a cnn. the training accuracy goes up to 97.71 % and the validation accuracy is a perfect 100 % 4. dataset 4 data formed by combining the original data with the data generated by transfer learning gan the plot for training and validation accuracy for the cnn trained on the fourth dataset is shown in figure 10. final training accuracy: 0.9786 final validation accuracy: 0.9574 the plot for training and validation loss for the cnn trained on the fourth data is shown in figure 11. final training loss: 0.0566 final validation loss: 0.0739 there is a small improvement in the training accuracy of this cnn. however, there is a dip in the validation accuracy. this model maintained an accuracy of 100 % from epoch 1 to epoch 7 but after that, there was a dip. this might be due to overfitting of the model on the data. 4. discussions based on the results, it can clearly be seen that when data is generated through a gan (normal or transfer learning), there is a huge improvement in the accuracy of the classifier when it is trained on an ensemble of generated images and original images. figure 6. training and validation accuracy of cnn on second dataset. figure 7. training and validation loss of cnn on second dataset. figure 8. training and validation accuracy of cnn on third dataset. figure 9. training and validation loss of cnn on third dataset. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 5 the accuracy achieved by the model on the original data is pretty high but not good enough to be used in a real-time medical scenario. but with an accuracy of 98 %, a model can be fabricated from the gan generated data to automate the process of chest x-ray classification and is shown in table 1. there is also scope to improve upon the said method in future research with better gan architectures to generate new images. 5. conclusion in this work, a novel approach of data augmentation for medical images was proposed which could eradicate the problem of minimal availability of chest x-ray data up to some extent. a pre-processing step was done on the original data to decrease the image size from 1024 × 1024 × 1 to 128 × 128 × 1. it was verified that there was no loss of data during this step. after the pre-processing step, 4 datasets were generated as mentioned above and a cnn was trained on each of the datasets to analyse which dataset the model was learning best from. the cnn learned much faster and got better accuracy from the datasets generated by a simple gan and a transfer learning gan and this could be a one-stop solution for the minimal availability of chest x-ray data. references [1] michael beil, sigal sviri, hans flaatten, dylan w. de lange, christian jung, wojciech szczeklik, susannah leaver, andrew rhodes, bertrand guidet, p. 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https://acta.imeko.org/index.php/acta-imeko/article/view/imeko-acta-10%20%282021%29-02-18 https://acta.imeko.org/index.php/acta-imeko/article/view/imeko-acta-10%20%282021%29-02-18 https://doi.org/10.21014/acta_imeko.v10i2.1051 https://doi.org/10.1155/2020/8876798 https://www.kaggle.com/tawsifurrahman/covid19-radiography-database https://www.kaggle.com/tawsifurrahman/covid19-radiography-database https://www.kaggle.com/gasgallo/faces-data-new an adaptive learning algorithm for spectrum sensing based on direction of arrival estimation in cognitive radio systems acta imeko issn: 2221-870x december 2021, volume 10, number 4, 67 72 acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 67 an adaptive learning algorithm for spectrum sensing based on direction of arrival estimation in cognitive radio systems sala surekha1, md. zia ur rahman1, aimé lay-ekuakille2 1 department of electronics and communication engineering, k l university, koneru lakshmaiah education foundation, green fields, vaddeswaram, guntur-522502, a.p., india 2 department of innovation engineering, university of salento, lecce, italy section: research paper keywords: adaptive learning; beam forming; cognitive radio; direction of arrival; spectrum sensing citation: sala surekha, md. zia ur rahman, aimé lay-ekuakille, an adaptive learning algorithm for spectrum sensing based on direction of arrival estimation in cognitive radio systems, acta imeko, vol. 10, no. 4, article 13, december 2021, identifier: imeko-acta-10 (2021)-04-13 section editor: francesco lamonaca, university of calabria, italy received may 28, 2021; in final form december 1, 2021; published december 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: md. zia ur rahman, e-mail: mdzr55@gmail.com 1. introduction telecommunication systems are rapidly using from past few decades it leads to increase in frequency spectrum usage. due to lack of frequency spectrum, there is low utilization of licensed and unlicensed bands. to avoid interferences, secondary users always must be aware of primary user is absent or present in a particular frequency band. further considered secondary user direction of arrival (doa), it contains multiple input single output (miso) and one receiving antenna for primary user and feedback based adaptive frequency algorithm. non orthogonal multiple access [1] based cognitive radio network is used for secure beamforming to avoid interference in multiple input multiple output networks. in array sensors, estimation of number of channels is a problem and it is solved by considering direction of arrival in spectrum sensing method. wideband spectrum sensing channel [2] is divided into two sub channels; each sub channel is connected with sensor for processing and then estimation of doa is done. for multi band signals, two spectrum scenarios are considered from sub-nyquist samples: in first scenario, spectrum sensing method is examined then doa is considered as recovery for frequency spectrum problems by proposing uniform linear array (ula) [3] with sensor at receiver then it is connected to equivalent circuit of analog front-end channel of modulated wideband converter (mwc). in second case, generalized likelihood ratio antenna beamforming [4] used for efficient and low complexity spectrum sensing. localization technique is investigated in [5] depending on direction of arrival measurements for estimating primary users in cognitive radio networks. in [6], [7] new spectrum sensing techniques based on beamforming are proposed. null steering and joint beam-based resource allocation [8] used in femtocell networks in spectrum sensing. performance of transmitter localization done using sector power measurements for every senso then derived cramer rao bound (crb) [9],[10] for sector power estimation using doa and mean square error is derived for analytical expression. main objective of this paper, estimating doa of various sensors for sensing [11], [12] the vacant spectrum and thereby facilitate channel allocation to secondary user. here, we make use of an abstract in cognitive radio systems, estimating primary user direction of arrival (doa) measurement is one of the key issues. in order to increase the probability detection multiple sensor antennas are used and they are analysed by using subspace-based technique. in this work, we considered wideband spectrum with sub channels and here each sub channel facilitated with a sensor for the estimation of doa. in practical spectrum sensing process interference component also encounters in the sensing process. to avoid this interference level at output of receiver, we used an adaptive learning algorithm known as normalised least absolute mean deviation (nlamd) algorithm. further to achieve better performance a bias compensator function is applied in weight coefficient updating process. using this hybrid realization, the vacant spectrum can be sensed based on doa estimation and number of vacant locations in each channel can be identified using maximum likelihood approach. in order to test at the diversified conditions different threshold parameters 0.1, 0.5, 1 are analysed. mailto:mdzr55@gmail.com acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 68 adaptive learning algorithm based on normalized least absolute mean deviation (nlamd) strategy. in section 2, the doa estimation with an adaptive leaning algorithm is discussed, in section 3 simulation results are discussed. the proposed realizations are suitable for the development of medical telemetry networks as well in the development of smart cities, smart hospitals. 2. doa estimation based spectrum sensing in cognitive radios, spectrum sensing is mostly used method because it overcomes the low spectrum utilization problems of primary users. there are various spectrum sensing algorithms they are based on narrowband methods used to solve binary hypothesis. this binary hypothesis test is used for every sub channel. for assessing the primary user’s existence, sub channel assessed in spectrum sensing algorithm. in each sub channel received signal is expressed [13]as 𝑦𝑘 = { 𝑤𝑘, ℎ0 𝑎𝑟𝑘 + 𝑤𝑘, ℎ1 , (1) where 𝑎𝑟𝑘 is primary user received signal, 𝑤𝑘 is additive white gaussian noise, ℎ0 and ℎ1 are hypothesis test used for primary user existence in every sub channel. multiple hypothesis tests are employed for spectrum sensing algorithm, then by taking into consideration of all sub channels primary user signal is detected. by analysing all sub channel received signals, observed that noise is present in output signal. to discriminate this noise from primary user signals, doa is estimated. by assuming received signal at sensor nodes in array processing, it looks like cognitive radio subchannel, to avoid these problems doa estimation is considered in spectrum sensing and its block diagram is shown in figure 1. doa estimation is considered to get exact information of antenna and also to avoid interferences between primary and secondary users, further an adaptive learning process called normalised least absolute mean deviation (nlamd) algorithm is considered for spectrum to reduce noise levels at output. by using adaptive filter, desired response of input signal is calculated as 𝑑𝑘 = 𝑠𝑘 t𝑢0 + 𝑜𝑘 , (2) where 𝑠𝑘 is input signal, u0 is unknown weight vector with ‘t’ taps and 𝑜𝑘 is output noise at time index ‘k’. error output of desired signal is represented as 𝑒𝑘 = 𝑑𝑘 − 𝑠𝑘 t𝑢𝑘 . (3) here, 𝑢𝑘 = [𝑢𝑘1,𝑢𝑘2 ….. ,𝑢𝑘𝑇] t is weight vector of adaptive filter. for above equation identification problem of adaptive system is solved by p-norm cost function minimization 𝐽(𝑒𝑘) = 1 𝑝 e[|𝑒𝑘| 𝑝] = 1 𝑝 e[|𝑒𝑘 = 𝑑𝑘 − 𝑠𝑘 t𝑢𝑘| 𝑝] (4) where e[.] is the operator of statistical expectation for p>0. e[|𝑒𝑘| 𝑝] is replaced with |𝑒𝑘| 𝑝, then after calculation we get gradient of uk for p-norm error evaluated as 𝜕𝐽(𝑒𝑘) 𝜕(𝑢𝑘) = −|𝑒𝑘| 𝑝−1 sign[𝑒𝑘𝑠𝑘] . (5) by using gradient descent algorithm weight equation is updated as 𝑢𝑘+1 = 𝑢𝑘 + 𝜔|𝑒𝑘| 𝑝−1sign[𝑒𝑘𝑠𝑘], (6) where ‘𝜔’ is step size selected appropriately used for balancing convergence rate and mean square error, ‘sign’ used for denoting sign function. for improving steady state rate and convergence rate above equation is updated with normalised least mean ppower algorithm as 𝑢𝑘+1 = 𝑢𝑘 + 𝜔 |𝑒𝑘| 𝑝−1sign[𝑒𝑘𝑠𝑘] ||𝑠𝑘||𝑝 𝑃 + 𝜗 . (7) here ||.||p used for p-norm operation, small positive value 𝜗 is also considered for avoiding denominator from zero. by selecting p-norm values as 1, then we obtain the nlamd algorithm equation as 𝑢𝑘+1 = 𝑢𝑘 + 𝜔 sign[𝑒𝑘𝑠𝑘] ||𝑠𝑘||1 1 + 𝜗 . (8) in sparse models, l1 norm is used for relaxation in least absolute shrinkages and operator selector algorithms, and it is employed in various adaptive filter algorithms. by using l1 norm, weight equation of nlamd algorithm is updated to minimise cost functions and it is given as 𝐽𝑑(𝑒𝑘) = 𝑑𝑘 − 𝑠𝑘 t𝑢𝑘 ||𝑠𝑘||1 1 + 𝜗 + ||𝑢𝑘||1 , (9) where is adopted parameter to know the difference between estimation error and sparsity. then we get the updated equation for nlamd sparse algorithm using gradient descent method using cost function equation (9) as 𝑢𝑘+1 = 𝑢𝑘 + 𝜔 sign(𝑒𝑘𝑠𝑘) ||𝑠𝑘||1 1 + 𝜗 − 𝜎 sign(𝑢𝑘) . (10) here, 𝜎 = 𝜔 is a regularised parameter. nlamd algorithm with sparse system and l1 norm is denoted as za nlamd algorithm. in bias compensated systems [14], considered a noisy input system for nlamd algorithm. input noise vector of a system is defined as �̅�𝑘 = 𝑠𝑘 + 𝑜𝑖𝑛𝑘 , (11) where 𝑜𝑖𝑛𝑘 noisy input vector it is represented as 𝑜𝑖𝑛𝑘 = [𝑜𝑖𝑛1𝑘,𝑜𝑖𝑛2𝑘,……,𝑜𝑖𝑛𝑀𝑘] t , and its limit is 𝑜𝑖𝑛,𝑡𝑘(𝑙 ∈ [1,𝑀] ), their input variance is represented as 𝜎𝑖𝑛 2 and is estimated by using some unknown info. to recover the biased estimation problems for nlamd algorithm of equation (10), an unbiased estimation vector 𝑏𝑘 is taken into consideration as 𝑢𝑘+1 = 𝑢𝑘 + 𝜔 sign(�̂�𝑘�̂�𝑘) ||�̂�𝑘||1 1 + 𝜗 − 𝜎 sign(𝑢𝑘)+ 𝑏𝑘 . (12) by using above equation, we get bias compensated vector as below [15] 𝑏𝑘 = 𝜔𝜎�̂�|�̂� 2 𝑘√ 2 (π𝜎�̂�|�̂� 2 𝑘 ) ⁄ ( 𝑢𝑘 ||�̂�𝑘||1 1 + 𝜗 ) . (13) noisy input parameter variances 𝜎𝑜𝑖𝑛 2 𝑘 , 𝜎�̂�|�̂� 2 𝑘 and 𝜎𝑢𝑘 2 are estimated accurately by computing these variance parameters as acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 69 𝜎𝑜𝑖𝑛𝑘 2 = 𝜎�̂�|�̂� 2 𝑘 𝑀𝜎𝑢𝑘 2 +𝑖 + 𝜎�̂�|�̂� 2 𝑘 (𝑀) �̂�𝑘 t�̂�𝑘 . (14) 𝜎�̂�|�̂� 2 𝑘 = ℵ𝜎�̂�|�̂� 2 𝑘−1 +(1 −ℵ)�̂�𝑘 2 (15) 𝜎𝑢𝑘 2 = ℵ𝜎𝑢𝑘−1 2 + (1 − ℵ) 1 𝑀 𝑢𝑘 t𝑢𝑘 . (16) equation (13) is substituted into (12), we get final bias compensated nlamd (bc-nlamd) adaptative learning process updated as 𝑢𝑘+1 = ( 1 + 𝜔 √ 2 (π𝜎�̂�|�̂� 2 𝑘 ) ⁄ 𝜎𝑜𝑖𝑛 2 ||�̂�𝑘||1 1 + 𝜗 ) 𝑢𝑘 + 𝜔 sign(�̂�𝑘�̂�𝑘) ||�̂�𝑘||1 1 + 𝜗 −𝜎 sign(𝑢𝑘) . (17) using this weight recursion, the noise in the received signal is minimised and accurate direction of arrival is estimated. the bcnlamd algorithm accurately estimates the doa and helps in finding the vacant spectrum and the flowchart of the proposed adaptive learning algorithm is shown in figure 2. 3. results and discussion this section demonstrates the experimental results for evaluating the performance of proposed bias compensated adaptive learning algorithm and is compared with absolute mean deviation (amd) and normalised absolute mean deviation (namd) methods with output gaussian noise. input and output noises generated using zero white mean gaussian noise and β stable distribution respectively for better performance of proposed algorithm and its characteristic function is expressed as, 𝑓𝑡 = e 𝑗𝛿𝑡− |𝑘| 𝛽[1+𝑗𝜏 sign(𝑡)𝑄𝑡,𝛽 , (18) where 𝑄𝑡,𝛽 = 𝑓(𝑥) = { tan 𝛽 π 2 , 𝛽 ≠ 1 2 π log𝑡, 𝛽 = 1 (19) with characteristic exponent 0 < 𝛽 ≤ 2, skewness −1 ≤ 𝜏 ≤ 1 , scale parameter range 0 < < ∞ and location parameter −∞ < 𝛿 < ∞. figure 1. doa estimation block diagram for spectrum sensing. figure 2. flow chart of spectrum sensing doa estimation using bc-nlamd algorithm. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 70 occupied subchannel locations are estimated using spectrum sensing method with choosing of q subchannels. in cognitive radio applications, using spectrum sensing correlation between channels are identified. in our framework for identifying spectrum location direction of arrival is taken into consideration. by using doa, we can identify spectrum location using the proposed bc-nlamd adaptive learning process. output signal with various combinations is considered as one white signal with one doa, one white signal with 3 doa, two white signals with one doa and three doa. performance of adaptive algorithm is studied in terms of convergence rate, beam pattern and number of active taps. in mobile environments, more than one multipath is considered, in those each multipath have different gains and it has amplitude and phase components. case 1: one white signal with one doa in this type, one signal with one path is considered it arrives at base station with 60 degrees angle and amplitude 0.5, it is propagated at different threshold values 0.1, 0.5 and 1. for different threshold control values, delay and steady state error are calculated as discussed in second section. for threshold value of 0.1, it improves convergence rate for proposed bc nlamd algorithm when compared to lms [16] algorithm. for threshold values of 0.5 and 1 steady state is converged faster than basic lms algorithm. improvement in convergence rate is identified by using taps in adaptive filter algorithm. for narrow band theoretical values, threshold value need only one tap but however it has delay cost and convergence rate problems. hence, we considered beam pattern with nlamd algorithm using matlab in doa estimation, it steers main beam with 60 degrees direction with beam strength of two, it is due to power signal reduced by factor 0.5. for every simulation, convergence rates are given in terms of number of samples, it is required for steady state and it is shown in table 1. case 2: one white signal with 3 doas these simulations are studied for effects of multipath smart antenna systems. multipath antennas with three different direction of arrivals -20, 30 and 60 degrees are considered for base station. at antenna system each multipath have a difference of one sampling period of 1/fc and their corresponding gains also introduced with amplitudes as shown in figure 3. for proposed method, three different weight vectors for each multipath are used for spectrum sensing. convergence rate for white signal with three doas gives better convergence rate for proposed bias compensated nlamd algorithm when compared to amd algorithm as shown in figure 4. by proposed algorithm, antenna systems show similar beam pattern compared to basic amd algorithm and it has ability to steer beams in multiple directions with zero interference directions for each beam, gain is inversely proportional to corresponding multipath of antenna. case 3: two white signals with one doa two different signals are transmitted with one doa each and its effect is same as sending two multipaths for one signal separated by one sample period at least, it is due to two signals are uncorrelated for each other with amplitude of 0.5 and 1 at threshold values of 0.1, 0.5 and 1. it gives good convergence rate for second signal when compared to first signal. for smaller amplitude it gives longer response to adapt taps and then signal is estimated. narrow threshold gives an ability to adapt smaller number of taps for better performance of proposed nlamd algorithm and their beam patterns for corresponding doa are shown in figure 5 and figure 6 respectively. case 4: two white signals with three doas in this case, two input sequences are considered each sequence with three multipath components is simulated. at the base station, multipath signals of second and third are used behind first multipath signals from various directions. for each signal, only two sets of multipath are considered. at base station, second and third multipath have the same weight vector. compared the convergence rate with basic lad algorithm. four beam patterns are considered from shannon theory, 3rd multipath pattern with two main lobes in direction of second and third multipaths, they are shown in figure 7 and figure 8 respectively. convergence rates for first signal, second signal in case of doa 3 is shown in table 2 and table 3 respectively in terms of number of samples considered to reach steady state. from the tables, it is clear that for doa3 of proposed algorithm converged faster for second signal when compared to first signal. in figure 7, only two multipath signals are visible because 2nd and 3rd signals have same weight vectors. main aim of proposed bc-nlamd algorithm is for detecting frequency spectrums in cognitive radio antenna systems. nlamd algorithm is used for low computational complexity, figure 3. bcnlamd beam pattern for one white signal with doas. figure 4. error for received signal using bcnlamd algorithm with threshold value of 0.1. table 1. bcnlamd beam pattern for one white signal with doas. algorithm steady state delay lamd [17] 65 0 bbnlms [18] 50 0 ffa [19] 35 0 pid [20] 25 8 enlms [21] 15 12 bcnlamd for 0.1 40 0 bcnlamd for 0.5 45 7 bcnlamd for 1.0 55 14 acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 71 better stability and good robustness to avoid implementation errors. however, nlamd algorithm is poor convergence and it is increased by using bias compensated nlamd algorithm and it requires system with sparse channels. in wireless channel, spatial properties are exploited with detection guide systems. beam patterns are analysed and they are compared with shannon theorem. proposed results are compared with threshold point of spectrum sensing algorithm for bc-nlmad algorithm. then it gives better results for bc-nlmad in terms of faster convergence, low computational complexity and particularly narrow band threshold gives better performance of antennas. it is due to delay period presence, on that time system waits for set of samples those are undetermined before actually converging for desired signal. however, system performance is improved at a cost of increased computational complexity that will need increased number of taps adaptation for narrow threshold point in proposed algorithm and it reduces error at output signals. beam patterns are obtained with according to expectations of shannon theorem, and further proposed algorithm with active taps steers beams in desired signal direction. spatial filtering is particularly used in wireless communication systems. hence by proposed algorithm performance of system is increased with active taps weights in updated equation so that improves frequency utilization for cognitive radio systems. 4. conclusion in this paper, the vacant spectrum is sensed by using doa measurement in wireless communication systems. interferences occurred with cognitive radio systems are avoided by considering doa estimation for spectrum sensing. in wireless communications, antenna system are new developing technologies with new adaptive beam forming algorithms, it will provide high frequency spectrum then it improves quality of service of cognitive radio systems. further to reduce noise signals from received signal proposed a bias compensated nlamd algorithm. using this adaptive learning algorithm, performance of cognitive radio-based antenna beam streams and convergence rate is improved. by using sign regressor function in weight update equation of proposed algorithm computational complexity is reduced. performance of bcnlamd algorithm in presence of multipath users and multipath effects are analysed using matlab simulations and hence convergence rate is improved due to active taps used in adaptive learning algorithm leads to better spectrum efficiency in cognitive radios. figure 5. error signal for bcnlamd algorithm for 0.1 value. figure 6. bcnlamd beam pattern for two white signals with one doa. table 2. convergence rate of first signal, doa3. algorithm steady state delay lamd [17] 55 0 bbnlms [18] 60 20 ffa [19] 70 45 pid [20] 80 65 enlms [21] 90 70 bcnlamd for 0.1 35 60 bcnlamd for 0.5 55 40 bcnlamd for 1.0 125 95 table 3. convergence rate of second signal and doa3 algorithm steady state delay lamd [17] 45 0 bbnlms [18] 55 0 ffa [19] 62 20 pid [20] 70 40 enlms [21] 85 55 bcnlamd for 0.1 25 35 bcnlamd for 0.5 45 25 bcnlamd for 1.0 95 65 figure 7. received error signal for bcnlamd algorithm. figure 8. bcnlamd beam pattern for two white signals with 3 doas. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 72 references [1] h. s. m. antony, t. lakshmanan, secure beamforming in 5gbased cognitive 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| www.imeko.org december 2022 | volume 11 | number 4 | 1 journal contacts about the journal acta imeko is an e-journal reporting on the contributions on the state and progress of the science and technology of measurement. the articles are mainly based on presentations presented at imeko workshops, symposia and congresses. the journal is published by imeko, the international measurement confederation. the issn, the international identifier for serials, is 2221-870x. about imeko the international measurement confederation, imeko, is an international federation of actually 42 national member organisations individually concerned with the advancement of measurement technology. its fundamental objectives are the promotion of international interchange of scientific and technical information in the field of measurement, and the enhancement of international co-operation among scientists and engineers from research and industry. addresses principal contact prof. francesco lamonaca university of 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(1) srinivas reddy putluri (1) stefania zinno (1) stefano brusaporci (2) stefano gialanella (1) stephan schlamminger (1) subha sree (1) tilde de caro (1) valeria di cola (1) valerio baiocchi (1) ville-veikko telkki (1) vincenzo palleschi (1) vittoria guglielmi (1) walter bich (1) yanhe zhu (2) yasmin fathima (1) yasuharu koike (2) yvan baudoin (1) zhipeng liang (1) vibration-based tool life monitoring for ceramics micro-cutting under various toolpath strategies acta imeko issn: 2221-870x september 2021, volume 10, number 3, 125 133 acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 125 vibration-based tool life monitoring for ceramics microcutting under various toolpath strategies zsolt j. viharos1,3, lászló móricz2, máté büki1 1 centre of excellence in production informatics and control, institute for computer science and control (sztaki), eötvös loránd research network (elkh), kende u. 13-17., h-1111, budapest, hungary 2 zalaegerszeg center of vocational training, kinizsi pál utca 74., h-8900, zalaegerszeg, hungary 3 john von neumann university, izsáki u. 10., h-6000, kecskemét, hungary section: research paper keywords: ceramics; micro-milling; tool wear; machining strategy; vibration analysis; feature selection citation: zsolt jános viharos, lászló móricz, máté istván büki, vibration-based tool life monitoring for ceramics micro-cutting under various toolpath strategies, acta imeko, vol. 10, no. 3, article 18, september 2021, identifier: imeko-acta-10 (2021)-03-18 section editor: lorenzo ciani, university of florence, italy received february 5, 2021; in final form september 9, 2021; published september 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was supported by the european commission through the h2020 project epic (https://www.centre-epic.eu/) under grant no. 739592.; by the hungarian ed_18-2-2018-0006 grant on a “research on prime exploitation of the potential provided by the industrial digitalisation”; by the ministry of innovation and technology nrdi office within the framework of the artificial intelligence national laboratory program, hungary. corresponding author: zsolt jános viharos, e-mail: viharos.zsolt@sztaki.mta.hu 1. introduction machining of rigid materials with regular cutting-edge geometry is one of the main trends in the 21st century. ceramics are such rigid materials that are employed more and more widely as raw materials thanks to their high hardness and thermal resistance [1], [2]. there are various options for machining them, e.g. using water, laser or abrasive grinding [3], [4], [5], however, their high costs and complex setups are important drawbacks of these technologies. therefore, the machining of ceramics with a classical, regular cutting-edge geometry is still a promising solution, however, considering the relative quick wearing process of the cutting tool without an appropriate technological optimization, this methodology will be economically not acceptable. optimizing a technology is typically a multicriteria assignment, like here, the main aim is to find the smallest production cycle time, and at the same time the tool life has to be maximal, too. the effect of technology parameters on tool life has been investigated in several of the authors’ previous articles [6], [7], [8]. an important part of tool life analysis is the investigation of vibrations generated during cutting method. the relative vibration between the micro-milling cutter and workpiece influences the processing quality and tool life [9]. frequency spectrum analysis was executed to establish for example the tool wear and chatter frequency characterization. based on this method, of the cutting process that are clearly not visible in the time domain can be revealed [10]. to determine the source of chatter or other dominant tool wearing frequencies, proper selection of the critical dynamic force signatures is required to perform frequency or power spectrum analysis. dimla and lister [11] identified for turning that the tangential force components abstract the 21st century manufacturing technology is unimagined without the various cam (computer aided manufacturing) toolpath generation programs. the aims of developing the toolpath strategies which are offered by the cutting control software is to ensure the longest possible tool lifetime and high efficiency of the cutting method. in this paper, the goal is to compare the efficiency of the 3 types of tool path strategies in the very special field of micro-milling of ceramic materials. the dimensional distortion of the manufactured geometries served to draw the taylor curve for describing the wearing progress of the cutting tool helping to determine the worn-in, normal and wear out stages. these isolations allow to separate the connected high-frequency vibration measurements as well. applying the novel feature selection technique of the authors, the basis for the vibration based micro-milling tool condition monitoring for ceramics cutting is presented for different toolpath strategies. it resulted in the identification of the most relevant vibration signal features and the presentation of the identified and automatically separated tool wearing stages as well. https://www.centre-epic.eu/ mailto:viharos.zsolt@sztaki.mta.hu acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 126 act on the rake face insert and both cutting forces and vibration signatures were most sensitive to tool wear. youn and yang [12] differentiated cutting force components to detect the difference between flank and crater wear for varying machining conditions in turning. work by sarhan et al. [13] involving the use of a 4flute end mill on steel of 90 bhn indicated that the magnitudes of the first harmonics of cutting force frequency spectrum increased significantly with increase in tool flank wear, feed per tooth and axial depth of cut. elbestawi et al. [14] also had similar observations when face milling of aisi 1020 steel and aluminium alloy. the deterioration in tool wear increases the tool chip and tool workpiece contact areas and these in turn cause an increase in friction, too. a worn tool generates high frequency tonal vibrational energy that does not arise in a new tool [15]. consequently, the noise emitted in the area is amplified [16]. tonshoff et al. [17] employed the use of acoustic emission signals to determine the influence of a hard turning process on the sub surface microstructure of the workpiece. the results obtained showed that the amplitude of the frequency analysis increased with increasing flank wear due to an enlarged contact area. one of an appropriate complement for vibrational analysis is the application of a neural network-based method. qi yao et al. [9] examined the relationship between cutting force amplitude, frequency and vibration displacement and it is ascertained by using a neural network method. the scientific literature mirrors that vibration signal analysis forms an increasing trend among the tool wear monitoring methods. in this paper, the effect of toolpaths generated by cam software on the tool lifetime is examined. the three most popular tool paths (strategies: wave form, cycloid, chained) are analysed as described in the next three sections. 1.1. wave form path the wave form tool path (figure 1) for milling technology results that the tool is working with constant tool diameter sweep. the contact angle along the toolpath has a direct effect on the cutting forces. by adjusting the contact angle, the cutting force can also be controlled. owing to this the tool load is constant in every changing direction during the machining through avoiding sharp changes in direction, which couldn’t be found among the average path generation methods [6], [19]. the other advantage of the wave form strategy is that the value of material removal speed is kept constant, that is different form the other path generation methods. cutting distributes wear evenly along the entire flute length, rather than just on one tip. the radial cutting depth is reduced to ensure consistent cutting force, allowing cutting material escaping from the flutes. so, tool lifetime is further extended as most of the heat is removed in the chip. 1.2. cycloid path the essence of the cycloid path strategy is to move the tool on a circle with the largest possible radius, thus reducing the kinematic contact (and tool load) (figure 2). the cycloid form is a milling technology where the tool milling is going along an arc, avoiding sharp changes in the direction. although it does not control the tool, this strategy also can reduce the tool load, and the roughing strategy is optimized easier [6], [8]. the problem with the average toolpath is that tool load increases significantly in the corners requiring shallower depths of cut and reduced feed. this problem can be avoided with cycloid and wave form path. because the pocket was used during the experiment did not have circle geometry, so the technology was made optimized with entremets. the entremets is an option in the software with which the tool load can be reduced in the corner. by choosing the correct stepover, the contact angle can be kept at a specified level. another advantage of the strategy is that high feeds along some paths cab be achieved. 1.3. constant stepover toolpaths (chained path) most software are usually capable of creating constant stepover toolpaths, contour-parallel, and direction-parallel paths, but these algorithms do not focus on machining parameters but only on material removal [20]. during the generation of the constant stepover path (chained path), the cutting tool removes the material moving back and forth on the horizontal plane within each z (vertical) level (figure 3). the strategy uses both directional and indirectional milling technology, leading to poor surface quality and short tool life. 2. experiments for the machining of ceramics the setup for the experiments is presented in figure 4. one of the main aims is to follow the wearing process of the micromilling tool during machining of ceramics and to compare it in an offline mode against the geometrical changes (length, width and depth of features) of the machined ceramic workpiece. another goal is to using high-frequency online vibration figure 1. element of waveform path [18]. figure 2. cycloid toolpath [20]. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 127 measurement online, during the cutting process on the other hand. connections between the wearing stages and the measured online and offline parameters were determined using a selfdeveloped, artificial neural network-based feature selection solution. 2.1. parameters of the milling machine the basis of the experiments was the milling machine that was planned and built by the cncteamzeg group. it is operated in zalaegerszeg (figure 4), hungary. during the planning, the aim was to cut metal material but the preliminary calculations and tests on ceramic material removal proved that it is able to cut ceramic material as well. 3. indirect, off-line tool wear detection measuring the produced workpiece geometry during the cutting process, microscopic images were taken repeatedly after a certain number of workpiece machining in order to monitor in an offline way the wearing evolution of the tool on workpiece geometry. measurements were performed using zeiss discovery v8 microscope and the wearing in the pictures were evaluated by the authors. beyond the microscopic control of the cutting tool geometry the changes resulted by the tool wear on the machined workpiece was also measured. these dimensional changes of the manufactured geometries are summarized in figure 5. during the measurements, changes in the width, length and depth of the geometries were examined. the applied technology settings (axial depth of cut, radial depth of cut, cutting speed, feed rate) were the same in all three cases, only the machining paths (strategies: wave form, cycloid, chained) were varied. these workpiece measurements represent clearly a complete and valid cutting tool life curves (taylor curves) and various variable conclusions can be drawn from them: • it can be seen in figure 5 that the tool lifetime achieved using chained toolpath is nearly half of the tool lifetime achieved using the wave form and cycloid tool path. • during the application of the chained path, a tool break occurred early, under the machining of the 11th set. • at the cycloid tool path, exponential tool wear and tool break were observed in 20th set. • using the waveform path, the manufacturing time of one feature was 57% longer than in the case of the chained toolpath. • with the cycloid path, the manufacturing time of a feature was nearly five times more than at the chained tool path. • considering the tool lifetime and the manufacturing time, the waveform seems to be the most economical toolpath strategy during ceramic machining. figure 3. contour-parallel, and direction-parallel stepover toolpaths (chained path). figure 4. the applied cnc machine, detailed parameters are in [8]. figure 5. changes in the geometry i.ii. and iii. of the machined geometries for the wave form, cycloid and chained tool path. w id th o f g e o m e tr y i number of machined workpieces chained path wave form cycloid path acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 128 4. direct, on-line tool wear analysis using vibration measurements the scientific literature mirrors that acoustic emission (ae) signals form an increasing trend in the same way as the increase of the tool wear analysis in metal cutting but it is not evident for micro-milling of ceramics. in [21], bhuiyan et al. pointed out: the increase in the tool wear increases the tool-workpiece contact area and friction coefficient, as well. in another experiment, the opposite, so, the decrease in the vibration amplitudes were detected during measurements, in metal cutting field. in the paper of the authors, they reported a similar phenomenon during ceramic machining as well [8]. based on the results of the measurements, decrease in the vibration amplitudes were detected during the wear evolution of the tool, consequently, the contact surfaces between the cutting tool and workpiece became smaller during the wearing of the tool for ceramics milling, mainly because of the complex and multiplicative wearing forms. the related frequency analysis showed that the wear-out process of the tool resulted also in a shift of the dominant frequencies into higher frequency ranges. in this research, the authors supplemented the results of the previous paper [8] with an ae study on various toolpath strategies. in the reported previous research, vibration measurement of ceramics milling was established with a sampling frequency of 100 khz measuring in one direction. instead of analysing over millions of individual measured values, as time series of the vibration amplitudes, several descriptive features (e.g., statistical measures, like amplitude, standard deviation, 3rd moment, etc.) were calculated. such a feature vector was calculated for each workpiece/machining process, while during the experiments the same tool is used, until the tool wear-out, or tool break. feature selection was applied to find the most descriptive features for distinguishing three typical different stages of the tool life. for this division, in the first step, the tool wear curve was determined from the geometry produced on the workpiece by an indirect method (based on the workpiece geometries measured). after this, the curves were divided into three sections depending on the wear phase of the tool and the degree of geometry reduction (worn-in, normal, wear-out (or brake). 4.1. micro-cutting tool wearing stages in contrast to the previous research [8], not the changes of parameters of the geometry (width, length, depth) were analysed, but the volume changes calculated from the geometries obtained by each toolpath strategy (figure 6). during the analysis of the graphs, 3 well-separable sections of the taylor curve were observed for the waveform graph (worn in-tool, normal condition, worn out). in contrast, in the cycloid path, no significant tool wear was observed until a certain geometry was manufactured (period “a”), followed by steep but uniform tool wear (period “b”). at the chained toolpath, the rapid wear process (period “a”) as well as the normal wear condition (period “b”) were observed, however, the tool was already broken at the very beginning of the wear out phase (period “c”). 4.2. most descriptive, direct vibration signal behaviour in respect to the micro-cutting tool wear-out after running the feature selection method, called adaptive, hybrid feature selection (ahfs) developed by some of the authors [22], the variables/features (calculated based on the vibration signal) that most accurately describe the change in three stages of the taylor curve were determined. it has to be mentioned that the feature selection identifies the first, so called most informative feature, however, the second one serves with the additional most informative one, etc., consequently, the features are not independent, it is really important for the evaluation of their meaning and effects. selected features for the waveform strategy: • number of the times the signal crosses its mean value • standard deviation • fourth momentum kurtosis having identified the most informative features calculated from the measured vibration signal, their evolution along the wearing progress can be presented, partly for engineering validation of the results of the mathematical algorithm. the figure 6. changes in the volumes of the manufactured geometries for the wave form (upper), cycloid tool path (middle) and stepover (chained bottom) tool path. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 129 progress in the values of the three, selected vibration signal features are presented along the wearing stages of the cutting tool in figure 7. the first identified variable name of “number of times the signal crosses its mean value” describes the intersection density of the x axis of the vibration signal. in the stage of wear-in (period “a”) the curve is at a high level, but there is a continuous decrease. this means that the tool vibrates at a high frequency in the initial stage. in the normal wearing (period “b”) phase, the signal oscillated at a lower frequency compared to the “a” phase. in the wear-out (period c) phase, there was a further decrease in the number of x-axis incisions. the second feature identified was the standard deviation. the standard deviation showed similar change to the previous variable. in the wear-in period (period “a”), the signal shows large variance. in the normal period of tool lifetime (period “b”), there was a decreasing trend of the standard deviation. in the period of wear out (period “c”), the standard deviation of the signal showed a drastic decrease. the third parameter identified is the fourth momentum, also called as kurtosis. the fourth momentum describes the distribution-flatness of the signal. in the case of a sharp tool, the kurtosis follows a flat trend, while in the case of a worn tool, the distribution curve takes on an increasingly sharper shape. selected features for the cycloid strategy: • number of the times the signal crosses its mean value • mean value • second momentum. the progress in the values of the three, selected vibration signal features are presented along the wearing stages of the cutting tool in figure 8. selected features for the chained strategy: • mean value • fourth momentum • number of the times the signal crosses its mean value. figure 7. selected changes in the vibration signal behaviour at waveform strategy: “number of the times the signal crosses its mean value ““standard deviation” – “fourth momentum” along the tool live-cycle. figure 8. selected changes in the vibration signal behaviour at the cycloid strategy: “number of the times the signal crosses its mean value ““mean value” – “second momentum” along the tool live-cycle. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 130 the progress in the values of the three, selected vibration signal features are presented along the wearing stages of the cutting tool in figure 9. 4.3. most descriptive vibration signal behaviour in frequency domain in respect to the micro-cutting tool wear-out the vibrations are stochastic signals because they represent temporal processes where parts of the phenomenon are characterized by probability variables (ceramic material inhomogeneity, asymmetric wear of tool edges, factors that cannot be described in an exact way in the micromachining process, white noise, etc.). in case of the power density (psd), the power of the signal per unit frequency range is calculated. in the analysis of the psd functions, the goal is to use the feature selection method to find those psd frequency components where the individual sections of the curve are clearly separated from each other based on the separated stages of the taylor curve. feature selection results showed that the highest separation of the three ranges occurs at 21758hz (figure 10) using the waveform strategy. figure 10 shows that the ranges are separated at the analysed frequency applying the cycloid strategy. in the stage of wear out (period “c”), two recorded data sets were damaged during the measurements, so they cannot be considered in the analyses. therefore, only three curves are visible in range of wear out period. at the examined frequency (21758hz), a continuous decrease in amplitude is observed, which indicates a decrease in the contact surface of the tool and the workpiece (figure 11). figure 9. selected changes in the vibration signal behaviour at the chained strategy: mean value fourth momentum number of the times the signal crosses its mean value along the tool live-cycle. figure 10. separation of the stages of wear-in (thin curves), normal wear condition (middle thick curves) and wear-out (thick curves) tool according to the waveform strategy. figure 11. variation of psd amplitude values along the manufactured workpieces at 21758hz at the waveform strategy. -0.12 -0.1 -0.08 -0.06 -0.04 -0.02 0 m e a n v a lu e number of machined workpieces pe riod „c" 1st set of workpieces 2nd set of workpieces 3rd set of workpieces 4th set of workpieces pe riod "a" pe riod „b" f e a tu re r a n g e o f p e ri o d „ a ” f e a tu re r a n g e o f p e ri o d „ b ” acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 131 the next toolpath examined was the cycloid strategy. here, the results showed that the separation of the ranges should be sought at 1709 hz as summarized in figure 12. like at the waveform, a continuous decrease in amplitude is observed at the cycloid strategy. the last toolpath examined was the chained strategy. here, the results showed that the separation of the ranges should be sought at 2796hz as presented in figure 14. in figure 13, the three ranges are well separated at the frequency obtained by the feature selection. in the case of the chained toolpath, based on the volume analysis of the pocket, the geometry of the last pocket did not differ significantly from the size of the previous pocket. however, tool break occurred in the pocket after the last one presented, so, in the labelling, the last pocket was classified to the range of the wear-out period. it can be seen in figure 15 that the 8th point jumps out, so, in this way the last data point (representing alone the wear-out stage) overlaps with the normal stage. based on this, it can be concluded that the tool breakage was not only caused by the tool wearing. a similar phenomenon can be observed for the waveform path (figure 10), where the amplitude measured at the last geometry of the taylor curve is included into the normal tool wear range. however, determining the cause of the fractures requires further investigations. in contrast to the previous toolpath strategies, an opposite, increasing amplitude value trend was observed along the machining, applying the chained toolpath (figure 15). 5. validation and representation of the foundings according to the original, measured vibration signal curves for the engineering-oriented validation and representation, some original vibration measurements for the selected experiments marked as yellow circles on the figure 7, figure 8 and figure 9 (put into the different stages on the identified, important feature curves of the waveform, cycloid and chained toolpaths, respectively) are presented in a tabular form in figure 16. it mirrors clearly that the presented methodology works appropriately, the differences between the three tool wear stages mirror the identified behaviours, consequently, there is an open floor for realizing the vibration-based monitoring and supervision of the micromilling of ceramics. figure 12. separation of the stages of wear-in (thin curves), normal wear condition (middle thick curves) and wear-out (thick curves) tool according to the cycloid toolpath. figure 13. variation of psd amplitude values along the manufactured workpieces at 1709hz at the cycloid strategy. figure 14. separation of the stages of wear-in (thin curves), normal wear condition (middle thick curves) and wear-out (a thick curve) tool according to the chained toolpath. figure 15. variation of amplitude values along the manufactured workpieces at 2796hz using the chained toolpath. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 132 6. conclusions and outlook in this paper, tool wear monitoring was investigated using direct and indirect methods to compare three cam path strategies (waveform, cycloid and chained). the key conclusions of the paper are: • the results represents clearly that the introduced methodology works well; it was found that the most relevant signal features and the original, “pure” signal measurements mirrors the identified behaviour, consequently, there is an open floor for realizing vibration-based monitoring and supervision of the micro-milling of ceramics. • the appointed features of the vibration signals describe the three typical stages (worn-in, normal, wear-out) of the tool life cycle according to the taylor curve identified. • in general, the feature “number of the times the signal crosses its mean value” has the highest relevance based on the vibration signal, so, this measure describes in the most accurate way the tool wearing progress. • using the feature selection method, it was possible to find the frequencies where the individual regions of the taylor curves are well separated from each other. • applying the introduced method, it is possible to determine the actual wear of the cutting tool by analysing the vibration frequencies at micromilling of ceramics. as research outlook, more detailed, novel tool wearing symptoms will be analysed in the future, the milling process/tool path will be split up into individual, homogenous, much smaller sections (layers and curves & linear movements) and so, a more sensitive and more detailed process monitoring will be possible, moreover different types of tool wearing will be considered separately. acknowledgement this work was supported by the european commission through the h2020 project epic (https://www.centre-epic.eu/) under grant no. 739592.; by the hungarian ed_18-2-2018-0006 grant on a “research on prime exploitation of the potential provided by the industrial digitalisation” and by the ministry of innovation and technology nrdi office within the framework of the 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https://www.imeko.org/publications/tc10-2017/imeko-tc10-2017-024.pdf https://www.imeko.org/publications/tc10-2017/imeko-tc10-2017-024.pdf https://doi.org/10.1088/1742-6596/1065/10/102003 https://doi.org/10.1016/j.measurement.2020.108025 https://doi.org/10.1016/j.measurement.2019.05.089 http://dx.doi.org/10.1016/j.jsv.2003.11.012 http://dx.doi.org/10.1016/s0890-6955(99)00084-x https://doi.org/10.1115/1.1362321 http://dx.doi.org/10.1016/s0924-0136(00)00803-7 https://doi.org/10.1016/0890-6955(91)90051-4 https://doi.org/10.1016/0043-1648(87)90216-x https://doi.org/10.1016/s0041-624x(00)00026-3 http://dx.doi.org/10.1016/j.procir.2016.10.046 https://www.cnccookbook.com/cam-features-toolpath-cnc-rest-machining https://www.cnccookbook.com/cam-features-toolpath-cnc-rest-machining https://doi.org/10.1016/j.measurement.2016.06.006 https://doi.org/10.1016/j.patcog.2021.107932 rarefied gas flow in pressure and vacuum measurements acta imeko june 2014, volume 3, number 2, 60 – 63 www.imeko.org rarefied gas flow in pressure and vacuum measurements jeerasak pitakarnnop, rugkanawan wongpithayadisai national institute of metrology, prathumthani, thailand section: research paper keywords: rarefied gas; slip-flow; kinetic theory citation: jeerasak pitakarnnop, rugkanawan wongpitayadisai, rarefied gas flow in pressure and vacuum measurements, acta imeko, vol. 3, no. 2, article 14, june 2014, identifier: imeko-acta-03 (2014)-02-14 editor: paolo carbone, university of perugia received february 14th, 2014; in final form may 25th, 2014; published june 2014 copyright: © 2014 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: (none reported) corresponding author: jeerasak pitakarnnop, e-mail: jeerasak@nimt.or.th 1. introduction characteristic scale, lc, and pressure, p, are the two main factors that characterize the flow regime in a gas-operated system. the micro-scale gap between the piston and cylinder of a pressure balance and the ultra-low pressure in a vacuum system will reduce the large number of gas molecules and cause the gas to be rarefied. consequently, due to the smaller number of gas molecules, flow behaviour will be different from general gas where the number of gas molecules is large enough to consider the gas as a continuum medium. the continuum medium assumption is not valid for the aforementioned cases if the flow behaves as slip, transition, or free molecular flow. the flow regime is characterized according to its knudsen number, kn: kn cl λ = (1) where λ is the molecular mean free path and lc is the characteristic scale of the gas flow. with respect to the value of the knudsen number, there are four distinct regimes as shown in figure 1. when kn is very small, there are enough molecules for the gas to be considered as in a continuum regime. slip-flow and other effects, such as a temperature jump at a solid surface, start to appear at values of kn greater than 0.001 and become dominant at around 0.01, where the slip-flow regime begins. as the gas becomes more and more rarefied, its flow is characterized as being in the transition and free molecular regimes, when kn reaches 0.1 and 10, respectively. in order to predict gas behaviour accurately, it is necessary to know its flow regime. using incorrect assumptions can lead to large errors. as well as the knudsen number, the rarefaction parameter δ is another quantity that is also used to describe the flow regime and is defined as: kn 1 22 π λ π δ == c l (2) the molecular mean free path was not directly measured. in this paper, it is estimated using maxwellian theory as: figure 1. classification of gas flow regime [1]. kn = 10 kn = 0.1 kn = 0.01 kn 0 analytical methods boltzman equation without collisions boltzman equation navier-stokes + slip bc. navier-stokes euler free molecular regime transition regime slip-flow regime viscous inviscid continuum regime abstract flows of a gas through the piston-cylinder gap of a gas-operated pressure balance and in a general vacuum system have one aspect in common, namely that the gas is rarefied due, respectively, to the small dimensions and the low pressure. the flows in both systems could be characterised as being in either slip-flow or transition regimes. therefore, fundamental research of flow in these regimes is useful for both pressure and vacuum metrology, especially for the gas-operated pressure balance where a continuum viscous flow model is widely used for determining the effective area of the pressure balance. the consideration of gas flow using the most suitable assumption would improve the accuracy of such a calculation. moreover, knowledge about rarefied gas flow will enable gas behaviour in vacuum and low-flow leak detection systems to be predicted. this paper provides useful information about rarefied gas flow in both slip-flow and transition regimes. acta imeko | www.imeko.org june 2014 | volume 3 | number 2 | 60 (3) where μ is the viscosity at temperature t and v� = √2rt is the most probable molecular velocity. from the above equations, the knudsen number as a function of the pressure of gas flowing through a piston-cylinder gap in gauge and absolute modes, and through iso-standard tube, is plotted in figure 2. according to figure 1, with the pressure balance operating in absolute mode (dashed line), flows through a piston-cylinder gap of 0.3 μm, 0.6 μm, and 0.9 μm are in the transition and free molecular flow regimes. even in gauge mode (strong solid line), the gas is rarefied enough to be characterised as being in either a slip or transition regime. it would be inappropriate to use dadson’s theory [2], which is based on continuum flow assumptions, to calculate the effective area of a piston-cylinder assembly under these conditions. this theory has therefore been modified to produce more accurate results [3][4][5]. this paper proposes an alternative method, especially in the slip-flow regime where continuum assumptions are still valid with special consideration at the solid surface. this method, in which a slip boundary is applied at the surface, is useful for simulating flow through a piston-cylinder gap using commercial cfd software. flow in a vacuum system could be in any regime from continuum to free molecular, depending on the pressure and the dimensions of the gas container and passages. for conventional iso tube (solid line), rarefied gas effects start once the pressure falls below 1000 pa. at this point, conventional continuum theory begins to break down and navier-stokes equations are no longer valid. the objective of this paper is to demonstrate the results of slip-flow and kinetic bgk (bhatnagar-gross-krook) models in both the slip-flow and transition regimes, the most common ones encountered in pressure and vacuum metrology. slip-flow equations, which extend the validity of the navier-stokes model, are detailed. these equations have been rigorously validated for microchannels [8]. the presented models are simpler than the kinetic ones, yet still provide quite accurate results within slip-flow regimes. 2. slip-flow the slip-flow regime is a slightly rarefied one, which could occur either in gas flows through the piston-cylinder gap and or in vacuum systems, as shown in figure 2. it typically corresponds to a knudsen number ranging between 0.01 and 0.1, easily reached for flow either through a micrometer scale gap in a pressure balance operating in gauge mode under standard conditions or in rough vacuum. the knudsen layer plays a fundamental role in the slip-flow regime. this thin layer, one or two molecular mean free paths in thickness, is a region of local non-equilibrium, observed in any gas flow near a surface. in non-rarefied flow, the knudsen layer is too thin to have any significant influence but, in the slip-flow regime, it needs to be considered [6]. although the navier-stokes equations are not valid in the knudsen layer, due to non-linear stress/strain-rate behaviour within it [7], their use with appropriate boundary velocity slip and temperature jump conditions can provide an accurate prediction of mass flow rate [8]. the slip-flow condition was originally proposed by maxwell and has since been developed up to the second order. several models have been proposed, most of similar form but differing slightly in the coefficients used. if isothermal flow is assumed, a general second order slipflow model is derived as: (4) where uslip is the slip velocity, us is the flow velocity at the wall, and uw is the velocity of the wall, with its normal direction noted as n. the mean free path of the molecules is λ and α is the tangential momentum accommodation coefficient, equal to unity for perfectly diffuse molecular reflection and equal to zero for purely specular reflection. aα and aβ are the first and second order dimensionless coefficients, respectively. in maxwell’s model, aα was taken as being equal to unity which overestimates the velocity at the wall but leads to a good prediction of gas velocity outside the knudsen layer. example values of aα and aβ proposed in the literature are given in table 1. to determine pressure distribution along piston and cylinder surfaces or flow through vacuum systems, the boundary equation (4) is applied to navier-stokes equations. these equations could be solved analytically for flow through simple geometries, whereas flow within a more complicated model requires a numerical calculation. normally, commercial cfd (computational fluid dynamics) software such as ansys fluent enables slip boundary conditions to be input at a boundary surface. methods to apply the boundary conditions in cfd software have been presented in the literature [6][9]. gas flow through a piston-cylinder gap is considered as a flow between two infinite parallel plates (or slabs) in the analysis, as the gap between the piston and cylinder of a pressure balance is very small in comparison to the radius of piston. after applying the slip coefficient to the navier-stokes equations, a reduced flow rate for slab flow is derived in terms of the rarefaction parameter as: (5) since the rarefaction parameter in equation (5) depends on pressure, the reduced flow rate gp and the pressure distribution along the slab need to be computed iteratively. the equation of pressure distribution along a piston and cylinder was derived by figure 2. pressure versus kn for usual gas flow through different pistoncylinder gaps and vacuum tubes for n2 at 20°c. table 1. example values of the dimensionless coefficients author, year aα aβ maxwell, 1879 cercignani, 1964 deissler, 1964 hadjiconstantinou, 2003 1.0000 1.1466 1.0000 1.1466 0.0000 0.9756 1.1250 0.6470 acta imeko | www.imeko.org june 2014 | volume 3 | number 2 | 61 priruenrom [10] as: (6) where p1 is the applied pressure at the bottom of piston, p2 is the pressure above the piston, z is the axial coordinate along the piston and cylinder, and l is the piston-cylinder overlapped length. further information on how to determine an effective area and a pressure distortion coefficient using the above equation is presented in her thesis. the previously-discussed slip-flow methods could also be used to predict gas flow through a vacuum piping system, the only difference being the flow passage’s cross-sectional geometry, which is normally circular. the reduced flow rate for slip-flow through a tube is calculated as: (7) a flow parameter that is of common interest is mass flow rate through the passage. this can be calculated from the reduced flow rate as follows [11][12]: for the slab flow, (8) where a is the cross-sectional area of the channel and h is the height of the slab. for flow through a circular tube, (9) where r is the radius of the tube. 3. transition flow as discussed in the previous section, slip-flow models are limited to use within the slip-flow regime whereas, within the transition regime, kinetic gas theory must be used. solutions based on this theory are valid throughout the whole range of the knudsen number from the free molecular, through the transition, and up to the slip and hydrodynamic regimes. in this paper, the bgk (one kinetic gas model) is chosen and the linearised bgk model is solved numerically by dvm (discrete velocity method) to determine flow behaviour. previous research work [8] has demonstrated that fully-developed isothermal pressure-driven flows are accurately predictable by kinetic models, such as the linearised bgk equation. figure 3 shows a comparison between measurement results and those from the bgk model. the gas flow rate measurements were performed at the inlet () and the outlet (☐) of a series of rectangular microchannels whose aspect ratio was approximately 0.1. the reduced flow rate gp is plotted in terms of the rarefaction parameter δ. measurement results are in good agreement with the kinetic model based on the linearised bgk equation, with α = 1 or diffuse reflection where gas molecules lose all their tangential momentum to a wall during their collisions. details of the investigation are described in the literature. 4. results results of the slip-flow model for slab flow are shown in figure 4, where the reduced flow rates gp, from equation (5) for each model presented in table 1, are plotted in terms of the rarefaction parameter δ. the result from the bgk model is plotted as a benchmark, against which the results from the other models can be compared. all results are in very good agreement in the slip-flow regime (δ > 8.86 or kn < 0.1), except the first order slip-flow model significantly deviates from the others near the upper limit of the slip-flow regime. a difference of up to 8.5 % is observed at δ = 9 when compared with the result from the bgk method. the hadjiconstantinou equation yields the closest result to bgk method, with less than a 0.9 % difference observed for the entire slip-flow regime. an overview showing the region from the slip-flow regime to the free molecular regime is given in figure 5. to help focus attention on the transition regime, the rarefaction parameter δ is plotted on a logarithmic scale. the result of the bgk model when the tangential accommodation α is equal to 0.9 is plotted to demonstrate the trend in the results when the gas-surface interaction is no longer considered to be a purely diffuse collision. moreover, results of the bgk model from cercignani & pagani () [13], lo & loyalka (δ) [14], and loyalka et al. (o) [15] are also plotted to support the results obtained. any differences between the results of the various bgk models are insignificant throughout all regimes, whereas the results from all slip-flow models fail to predict rarefied gas flow in both the transition and molecular flow regimes. for flow within a circular tube, the trend of the results shown in figure 6 does not differ much from the previous slab flow case. the results from the bgk model at α = 1 are in very good agreement with those of cercignani & sernagiotto () [16], sharipov (δ), and loyalka & hamoodi (o), using a bgk model [17], a shakov model, and a numerically solved boltzmann equation [18] respectively. figure 3. reduced flow rate (gp) versus rarefaction parameter (δ) of flow through rectangular channel (aspect ratio ≈ 0.1) [7]. figure 4. reduced flow rate (gp) versus rarefaction parameter (δ) of flow through slab at diffuse reflection (α =1). acta imeko | www.imeko.org june 2014 | volume 3 | number 2 | 62 as in the slab flow case, all slip-flow equations break down in the transition and free molecular regimes. however, within the slip-flow regime itself, the boundary equation in conjunction with the navier-stokes equation is the preferred method to solve for bulk flow, due to the less complex calculations and the lower required resources. for simple geometry cases, it is possible to obtain an exact solution. 5. discussions and conclusion general equations to determine the flow rate of rarefied gas through both slab and tube, based on navier-stoke equations in conjunction with slip boundary conditions, have been developed. the results of these slip-flow models have been compared with those from kinetic theory using a bgk model, the results of which have been obtained by a numerical method using a dvm (discrete velocity method) scheme. slip-flow models require lower computational resources, but their performance is limited. they provide a reliable result within the continuum and slip-flow regimes but fail to predict rarefied gas flow in both the transition and free molecular regimes. since the slip-flow method is easier both to handle and apply in commercial cfd (computational fluid dynamic) software for solving complex problems, it is still a valuable approach. it can be used as an alternative method to determine gas flow within a piston-cylinder gap in a slip-flow regime. moreover, such a method can also provide a precise prediction of flow rate through the piping of a vacuum system operating within the slip-flow regime. acknowledgment several years ago, prof. stéphane colin, prof. lucien baldas, prof. sandrine geoffroy, and their colleagues at the institut clément ader opened a door to the world of rarefied gas dynamics to the first author, giving him the opportunity to join their research. a few years later, the chance to meet prof. dimitris valougeorgis and colleagues at the university of thessaly brought him a key to solve kinetic theory of gases. he is forever grateful to them for their kind support in knowledge and mind. references [1] d. valougeorgis, “solution of vacuum flows via kinetic theory,” 51st iuvsta workshop on modern problems and capability of vacuum gas dynamics, 2007. [2] r. s. dadson, s. l. lewis and g. n. peggs, the pressure balance: theory and practice. london, hmso, 1982. [3] c. m. sutton, “the effective area of a pressure balance at low pressures,” j. phys. e: sci. instrum., vol. 13, pp. 857-859, 1980. [4] j. w. schmidt, b. e. welch, and c. d. ehrlich, “operational mode and gas species effects on rotational drag in pneumatic dead weight pressure gauges,” meas. sci. technol., vol. 4, pp. 26-34, 1993. [5] j. w. schmidt, s. a. tison, and c. d. ehrlich, “model for drag forces in the crevice of piston gauges in the viscous-flow and molecular flow regimes,” metrologia, vol. 36, pp. 565-570, 1999. [6] j. pitakarnnop, s. geoffroy, s. colin and l. baldas, “slip flow in triangular and trapezoidal microchannels,” international journal of heat and technology, vol. 26, no. 1 pp. 167-174, 2008. [7] d. a. lockerby, j. m. reese, and m. a. gallis, “capturing the knudsen layer in continuum-fluid models of nonequilibriun gas flows,” aiaa journal, vol. 43, no. 6 pp. 1391-1393, 2005. [8] j. pitakarnnop, s. varoutis, d. valougeorgis, s. geoffroy, l. baldas and s. colin, “a novel experimental setup for gas microflows,” microfluid nanofluid, vol. 8, no. 1 pp. 57-72, 2010. [9] j. pitakarnnop, analyse expérimentale et simulation numérique d’écoulements raréfiés de gaz simple et de mélanges gazeux dans les microcanaux, université de toulouse, 2009. [10] t. priruenrom, development of pressure balance for absolute pressure measurement in gases up to 7 mpa, clausthal university of technology, 2011. [11] f. sharipov and v. seleznev, “data on internal rarefied gas flows,” j. phys. chem. ref. data, vol. 27, no. 3 pp. 657-706, 1998. [12] s. varoutis et al, “computational and experimental study of gas flows through long channels of various cross sections in the whole range of the knudsen number,” j. vac. sci. technol. a, vol. 27, no. 1 pp. 89-100, 2009. [13] c. cercignani and c. d. pagani, “variational approach to boundary-value problems in kinetic theory,” phys. fluids, vol. 9, no. 6, pp. 1167-1173, 1966. [14] s. s. lo and s. k. loyalka, “an efficient computation of nearcontinuum rarefied gas flows,” j. appl. math. phys. (zamp), vol. 33, no. 3 pp. 419-424, 1982. [15] s. k. loyalka, n. petrellis and t. s. storvick, “some exact numerical results for the bgk model: couette, poiseuille and thermal creep flow between parallel plates,” j. appl. math. phys. (zamp), vol. 30, no. 3 pp. 514-521, 1979. [16] c. cercignani and f. sernagiotto, “cylindrical poiseuille flow of a rarefied gas,” phys. fluids, vol. 9, no. 1, pp. 40-44, 1966. [17] f. sharipov, “rarefied gas flow through a long tube at any temperature ratio,” j. vac. sci. technol. a, vol. 14, no. 4, pp. 2627-2635, 1996. [18] c. cercignani and s. a. hamoodi, “poiseuille flow of a rarefied gas in a cylindrical tube: solution of linearized boltzmann equation,” phys. fluids, vol. 2, no. 11, pp. 2061-2065, 1990. figure 6. reduced flow rate (gp) versus rarefaction parameter (δ) of flow through circular tube at diffuse reflection (α =1). figure 5. reduced flow rate (gp) versus rarefaction parameter (δ) of flow through slab (α = 0.9, 1). acta imeko | www.imeko.org june 2014 | volume 3 | number 2 | 63 rarefied gas flow in pressure and vacuum measurements path planning for data collection robot in sensing field with obstacles acta imeko issn: 2221-870x september 2022, volume 11, number 3, 1 10 acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 1 path planning for data collection robot in sensing field with obstacles sára olasz-szabó1, istván harmati1 1 dept. of control engineering and information technology, budapest university of technology and economics, budapest, hungaryl section: research paper keywords: path planning, mobile robots, obstacle avoidance citation: sára olasz-szabó, istván harmati, path planning for data collection robot in sensing field with obstacles, acta imeko, vol. 11, no. 3, article 5, september 2022, identifier: imeko-acta-11 (2022)-03-05 section editor: zafar taqvi, usa received february 26, 2022; in final form july 21, 2022; published september 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: sára olasz-szabó, e-mail: olasz-szabo.sara@edu.bme.hu 1. introduction nowadays there is a more and more common need for continuous data collection on a specified area. the simplest way for such data collection is using wireless sensor networks (wsn) [1], [2]. in most applications, a wsn consists of two parts: one data collection unit (also known as a sink or base station) and a large number of tiny sensor nodes. typically, both sensor nodes and sink remain static after deployment. sensor nodes, which are equipped with various sensor units, are capable of sensing the physical world and providing data to the sink through single-hop or multi-hope routing [3]. sensors are usually powered by batteries, which cannot be replaced in some applications, e.g., battlefield surveillance.[4]. since the data loss rate is increasing with the distance and each data transmission rate is associated with an energy consumption rate, which is modelled as a non-decreasing staircase function of the distance [5], the remote data sending is uses a lot of energy and this deteriorates network lifetime. for these reasons, the data transmission is executed by data collection robots [6], [7]. there are many applications of this technology in literature from recent years. for example, in [8] it is reviewed a range of techniques related to mobile robots in wsns. in paper [9], considered deploying a flying robotic network to monitor mobile targets in an area of interest for a specific time period with using wsns. in the work [10] investigated using a mobile sink, which is attached to a bus, to collect data in wsns with nonuniform node distribution. however, the robots have limited velocity and this way the data delay is significantly increasing. since transmitting over a short distance is more reliable than long distance, using robots improves the data collection rate. in addition, in terms of security, sending mobile sinks to collect data is more secure than transmitting via multihop communication [11]. this may be important in some military applications, as well. in paper [12] the authors raise and solve a problem of viable path planning for data collection unicycle robots in a sensing field with obstacles. the robots must visit all sensing nodes and then return to the base station and upload the collected data. path planning for the robots is a crucial problem since the constructed paths directly relate to the performance such as the delivery delay and energy consumption of the system. in a sensing field there are obstacles as well and the robots must not collide with them. the data collection is carried out by unicycle dubins-car [13], which can only move with constants velocity and bounded angular velocity, so it can move only on straight lines and turn with bounded turning radius. abstract using mobile robots to collect data from wireless sensor network can reduce energy dissipation and this way improves network lifetime. our problem is to plan paths for unicycle robots to visit a set of sensor nodes and download data on a sensing field with obstacles while minimizing the path length and the collecting time. reconstructing the path of an intruder in a guarded area is also a possible application of this technology. during path planning we greatly emphasize the handling of obstacles. if the area contains many or large obstacles, the robots may spend long time for avoid them so this is a critical point of finding the minimal path. this paper proposes a new approach for handling obstacles during path planning. a new algorithm is developed to plan the visiting sequence of nodes taking into consideration the obstacles as well. mailto:olasz-szabo.sara@edu.bme.hu acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 2 for successful path planning it is necessary to determine the criteria of an adequate path. in paper [12] the authors define a viable path which is smooth, collision-free with sensor nodes/base station and obstacles, closed, and provides enough contact time with all the sensor nodes. because of the kinematic properties of the robots the path must be smooth. safety boundary is determined around obstacles and nodes for the sake of collision-free path. all nodes are bounded with a visiting circle with the minimum turning radius of the unicycle robot. the minimum turning radius depends on the speed of the robot and its maximum angular velocity. moreover, all obstacle’s convex hull are bounded with a safety margin, since in case of the shortest path the robot should move on the boundary of the convex hull. the path must be closed because of the periodical data collection. the robot downloads data only when it moves around the visiting circle, so it makes round trips around the node as long as it collects all the data from the sensor node. during path planning it is assumed that the location of all nodes and obstacles as well as the shapes of the obstacles are known. between two objects nodes and obstacles there are always defined four tangents but any tangents that intersect other obstacle are removed. so when the robot arrives to a node on a tangent it starts downloading data and during it makes round trips as long as it collects all the data from the node and then it leaves the node on a tangent. so a path consists of an adequate configuration of tangents and arcs around objects at the safety distance. the paper organized as follows. in section 2 we summarize the basic method [12] and then section 3 describes the proposed concepts for the path planning and also presents our new algorithms. in section 4 we demonstrate simulation results. 2. summary of the shortest viable path planning algorithm in paper [12] the shortest viable path planning (svpp) algorithm was defined. the main steps of svpp are outlined as algorithm 1. this algorithm first computes a 𝛴 permutation of nodes without obstacles by solving an asymmetric travelling salesman problem. for this they construct a directed graph, where the vertices are the nodes and the length of the edges are calculated as follows. the length of the edge between two vertices takes into account two aspects: the length of the valid path between their visiting circles and the length of the adjusted arc on the latter vertex. thus, the length of the edge from 𝑠1 to 𝑠2 equals to the summation of the average length of tangents and the length of the adjusted arc on the visiting circle of 𝑠2. in contrast, the length of the edge from 𝑠2 to 𝑠1 equals to the summation of the average length of tangents and the length of the adjusted arc on the visiting circle of 𝑠1. with such directed graph, they use an atsp solver [14] to calculate the permutation 𝛴 . at this point there can be tangents that intersect obstacles in 𝐺(𝑉, 𝐸) tangent graph [15]. 𝑉 denotes the tangent points and 𝐸 denotes the tangents. the second and third step of this algorithm adds the blocking obstacles to the permutation and constructs a simplified tangent graph. having 𝛴, 𝐺(𝑉, 𝐸) can be simplified by keeping only the tangent edges that connect succeeding visiting circles in 𝛴 and the corresponding arc edges. when any obstacle blocks the route between any pair of visiting circles, the tangents passing the obstacle’s safety boundaries are also included in the 𝐺′(𝑉′, 𝐸′) simplified tangent graph and the algorithm inserts the obstacle to the 𝛴′ permutation between the two nodes. one obstacle can block more than one pair of nodes. in this case the algorithm inserts the obstacle to the 𝛴′ permutation into more than one place. the algorithm constructs a 𝐺′(𝑉′, 𝐸′) by keeping the edges and vertices related to the permutation of nodes and obstacles while deleting others. obviously, 𝛴 ⊆ 𝛴′. the new graph is called the simplified tangent graph 𝐺′(𝑉′, 𝐸′), where 𝑉′ ⊆ 𝑉 and 𝐸′ ⊆ 𝐸. the next step is converting 𝐺′(𝑉′, 𝐸′) to a tree-like graph 𝑇. this gives additional information about the succeeding usable tangents and arcs. from every object there are four tangents departing to the next object, the starting tangent points of these are the departure configurations, and there are four tangents arriving from the previous object, the tangent points of these are the arrival configurations. this means that every object can be transformed to 8 vertices in a tree-like graph. the path length between two objects in 𝛴′ permutation always consist of two components. the first component is the arc around the first object from the arrival to the departure tangent point, including the additional full circles if these are necessary to download the data. the second component is the length of tangent between the two tangent points. for the calculation of distance between the 𝑖th and 𝑖 + 1th (𝑖, ∈ [2, 𝑛′ − 1], where 𝑛’ denote the number of objects in the permutation) objects, we need information about the tangent and tangent point between the 𝑖 − 1th and 𝑖th objects. one should know which tangent point will be used by the tangent on the visiting circle of the 𝑖th object in order to calculate the arc length on the visiting circle. figure 1 illustrates this algorithm 1: shortest viable path planning (svpp) 1. compute 𝛴 by solving atsp instance based on 𝐺(𝑉, 𝐸) 2. compute 𝛴′ by adding those obstacles to 𝛴 that safety boundaries block tangents between nodes. 3. simplify 𝐺(𝑉, 𝐸) to 𝐺′(𝑉′, 𝐸′) by keeping the edges and the vertices related to 𝛴′ and deleting others. 4. convert 𝐺′(𝑉′, 𝐸′) to tree-like graph 𝑇. 5. given an initial configuration, search the shortest path 𝑃 in 𝑇. figure 1. the distance between two objects in permutation consist of two part: arc and tangent length. acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 3 problem. the path between two objects is represented by two segments in the directed tree graph. these two parts are the arc and tangent. so in the tree-like graph the vertices are the tangent points and the edges are the arcs and the tangents. the direction of edges points to the next part of the path. during the representation of the edges one must pay attention to the heading constraints. the heading constraint refers to that the robot's heading at the beginning of an edge should be equal to that at the ending of the last edge. the base station is the starting node, so the first element of the tree-like graph is one of the points of the base station visiting circle's. because of closed path, the final element of the tree-like graph should be also one of the points of the base station visiting circle. since the authors use dubins-car, they construct the tree-like graph both for positive and negative, clockwise and anti-clockwise initial direction as well. it can be seen that from each arrival configuration of an element there are two options to reach the arrival configurations of the next element because of the heading constraint. from a given starting point the total number of paths starting and ending at this point is 2𝑛 ′−1 taking into consideration that the starting and ending direction should be the same because of the continuous data collection. in paper [12], a dynamic programming based method is used to solve the shortest path search in the tree-like graph. 3. new concepts of solution in this paper new concepts of svpp algorithm are developed. the new algorithm based on these modifications is called generalized-svpp algorithm. in the following these modifications and new algorithms will be described in detail. 3.1. constructing tangents graph in paper [12] the tangents that intersect visiting circles are not included in the tangent graph (assumption 1, see below). however, the robot can move collision-free on a tangent that does not intersect the circle with centre of node and radius 𝑑𝑠𝑎𝑓𝑒 . therefore in the proposed new algorithm tangents that do not intersect the circle with 𝑑𝑠𝑎𝑓𝑒 radius around a node are available as well (assumption 2). this way the planned path may be shorter in certain cases. assumption 1: the tangents that intersect visiting circles are not included in the tangent graph. assumption 2: the tangents that intersect visiting circles, but do not intersect circles with centre of a node and radius 𝑑𝑠𝑎𝑓𝑒 , are included in the tangent graph. 3.2. permutation of nodes at this point the obstacles are not taken into account when creating the permutation of nodes. tangents that are intersecting obstacles are allowed in this step. a graph is constructed where the vertices are the nodes and the length of the edges is the average length of tangents between the two nodes. after this there is a searching for the shortest closed cycle with all of the nodes, namely the shortest hamilton cycle in this graph. this problem is the travelling salesman problem and by solving it the 𝛴 permutation of nodes is determined. as it was presented in section 2, in paper [12] the authors take into account the path length necessary to download the data and solve this problem with atsp. the exact path length around a visiting circle cannot be determined since the actual tangents are not known at this point. this is the reason why the average length of the tangents is used. 3.3. new concept of handling obstacles, construction of simplified tangent graph in algorithm 1 (svpp) there are two types of problem of handling obstacles. first these problems are described and then the solutions for them are presented. these problems are illustrated in figure 2. 1. for example, in figure 2 between node-1 and node-2 there is only one available tangent. when obstacle-1 is in the permutation, the available tangent between the nodes is not feasible in the shortest path planning. but when obstacle-1 is not in the permutation, there may be no solution at all depending on the initial configurations due to heading constraints. 2. there can be more obstacles between two nodes so that these obstacles block different tangents. for instance, in figure 2 between node-3 and node-4 obstacle-2 and obstacle-3 are blocking different tangents. tangent directions: the tangent direction is called positive-negative (pn) if the robot can make round trip around the first node positive clockwise direction and around the succeeding second node in negative direction. the positivepositive (pp), negative-positive (np) and negative-negative (nn) directions can be defined similarly as well. in this paper a new algorithm is proposed instead of the second and third step of algorithm 1. algorithm 1 creates one permutation of nodes and obstacles (assumption 3). the basic idea of the new algorithm is to calculate more than one permutation (assumption 4) and then using these to construct the 𝐺′(𝑉′, 𝐸′) simplified tangent graph and then the 𝑇 treelike graph in order to get better solution. assumption 3: one permutation of nodes and obstacles is created. the original svpp (algorithm 1) uses this assumption. assumption 4: more than one permutation of nodes and obstacles are created. the new algorithm 2 created by applying this assumption. instead of the second and third step of algorithm 1 the following algorithm 2 is used. at first stage four copies of 𝛴 permutation of nodes are created, then for every two nodes the blocking obstacles of all figure 2. an example of blocked tangents are illustrated with dashed line. acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 4 the four tangents are determined. when one of the tangents intersects an obstacle, the obstacle is inserted to the proper position in the feasible permutation determined by the tangent direction. note that when more than one obstacle is intersected, they are inserted to the feasible permutation according to their distance from the previous node. then the duplicate solutions are eliminated, and after this the previous algorithm is repeated as long as there are no new intersected obstacles. while repeating this algorithm, in the first step the 𝛴′ permutations of nodes and blocking obstacles is used instead of the 𝛴 permutation. in first case both of the direct tangents and the edges passing the obstacle safety boundaries are also inserted into the simplified tangent graph. in case one tangent blocked by more than one obstacles, all tangents and tangent points between the two nodes, between any obstacle and the two nodes, and between any two obstacles are inserted into the simplified tangent graph if these tangents are not blocked. the simplified tangent graph contains all of the tangents and tangent points from any permutations. besides, it also contains all of the arcs between the tangent points. 3.4. constructing the tree-like graph the dubins-car moves on tangents or arcs. in case of obstacles it moves on arcs between the arrival and the departure configurations and around the visiting circle while it downloads all the data from the sensor node. because of heading constraint the tangent direction determines the direction around the next object. and the direction around the object determines the available departure tangents. there are two tangents available for a given direction for any two objects. in case of one permutation there are two departure configurations for every object, but if there is more than one permutation the count of departure configurations depend on the permutations and the simplified tangent graph. in paper [12] there is only one permutation, so there are some cases when it causes problems as it was shown it the previous section. the new algorithm 2 proposed in the present article may achieve shorter path and give more general solution, but the tree-like graph become more complex. the new algorithm can select the shorter path from more available options. in this paper algorithm 3 and algorithm 4 are recommended to construct the tree-like graph for cases with one and more than one permutation, respectively. we demonstrate the transformation from simplified tangent graph into tree-like graph with help of example field in figure 3. for constructing the tree-like graph, in the first step two starting and two ending vertices are created for both the positive and negative initial direction. it is illustrated in figure 4 a) picture. algorithm 2: add obstacles to permutations 1. create four copies of 𝛴 permutation 2. for every two nodes determine the blocking obstacles of all the four tangents and insert these obstacles to the proper positions in the feasible permutations according to their distance from the previous node. 3. eliminate the duplicate permutations. 4. jump to 1. and repeat the algorithm while there are intersecting obstacles. in this case instead of 𝛴 we use 𝛴′permutations. algorithm 3: constructing the 𝑇 in case of one 𝛴′ permutation algorithm 2--add obstacles to permutations 1. for both negative and positive direction add starting and ending point as vertices to 𝑇. 2. according to 𝛴′add 4 arrival and 4 departure tangent points for all objects to the 𝑇 3. determine all arc length between all possible arrival and departure configurations taking into consideration the heading constraint and the possible different arc length calculation methods of the nodes and obstacles. add arc lengths as the length of the edges to the 𝑇 between the corresponding vertices. 4. around the visiting circle of the base station determine arc length between the starting point and the arrival and departure configurations. add this as edge length to 𝑇 to the proper position. 5. according to 𝛴′ add all tangents between all consecutive objects to the 𝑇 taking into consideration the heading constraint. 5. create four copies of 𝛴 permutation 6. for every two nodes determine the blocking obstacles of all the four tangents and insert these obstacles to the proper positions in the feasible permutations according to their distance from the previous node. 7. eliminate the duplicate permutations. 8. jump to 1. and repeat the algorithm while there are intersecting obstacles. in this case instead of 𝛴 we use 𝛴′permutations. algorithm 4: constructing the 𝑇 in case of more than one 𝛴′ permutation algorithm 2--add obstacles to permutations 1. apply algorithm 3 to the σ permutation of nodes. add edges only that are the member of 𝐺′(𝑉′, 𝐸′). 2. do for all 𝛴′permutations: 1.) if there are only one obstacle in the 𝑖th place between two nodes: run step 2-4 of algorithm 3 for the 𝜎𝑖−1, 𝜎𝑖 , 𝜎𝑖+1 object. only if the edges are in the 𝐺′(𝑉′, 𝐸′). 2.) if there are more than one obstacles between two nodes. let denote the obstacles by 𝜕𝑂 = {𝜕𝑜1, … , 𝜕𝑜𝑗 }: do step 1.) for all 𝜕𝑜𝑖 ∈ 𝜕𝑂 obstacle in the given permutation and for the nodes before and after the obstacle. run step 2-4 of algorithm 3 for all pair of obstacles 𝜕𝑜𝑖 ∈ 𝜕𝑂. run it in that case also if these are not subsequent elements in the permutation. naturally only in case when the vertices and edges are in 𝐺′(𝑉′, 𝐸′). 9. create four copies of 𝛴 permutation 10. for every two nodes determine the blocking obstacles of all the four tangents and insert these obstacles to the proper positions in the feasible permutations according to their distance from the previous node. 11. eliminate the duplicate permutations. 12. jump to 1. and repeat the algorithm while there are intersecting obstacles. in this case instead of 𝛴 we use 𝛴′permutations. figure 3. an example field of tree-like graph construction with 𝛴 = {𝑪𝟏, 𝑪𝟐, 𝑪𝟑, 𝑪𝟒} permutation of nodes acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 5 a) add starting and ending points as vertices b) add tangent points of nodes as vertices c) add arc length around the nodes between the arrival and departure configurations as edges d) add tangent length between the departure and arrival configurations as edges e) add tangent points that tangents between the obstacles and nodes in simplified tangents graph as vertices f) add arc length around the obstacles and the connecting nodes as edges g) add tangents between the obstacles and nodes as edges figure 4. an example of tree-like graph construction from figure 3 using algorithm 4. obstacles are denoted by purple. the vertices of the same rectangle denote the tangent points of the same object acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 6 after this the nodes are inserted into the graph, according to permutation 𝛴. using 𝐺′(𝑉′, 𝐸′) vertices are created from the tangent points (figure 4 b) picture). the next step is to determine the edges between the vertices. the length of edges between the starting point and the departure configurations of the visiting circle of the base node is equal to the arc length between the starting point and the departure configurations. the next step is to determine the arc length between the arrival and departure configurations around the node taking into account the heading constraint. in next step edges are added to 𝑇 between the ending vertices and the arrival configurations of the base node's visiting circle (figure 4 c) picture). finally, between the departure configuration and the arrival configurations of the next node, the length of the edges are the length of the tangents with the proper direction (figure 4 c) picture). this step should be done for all nodes in the order given by 𝛴 permutation. next step is to add obstacles to the tree-like graph according to 𝛴′ permutation and 𝐺′(𝑉′, 𝐸′) simplified tangent graph. in case of figure 3 obstacle-1 blocks tangents between node-2 and node-3. first those departure tangent points are added as vertices of the visiting circle of node-2 which are also on a tangent of obstacle-1. in addition, the arrival configuration of the obstacle coming from node-2 and departure configurations coming from the obstacle to the node-3 are added as vertices to the tree-like graph. finally, the arrival configurations of the node-3 are added to tree-like graph (figure 4 e) picture). adding edges is similar as it was previously (figure 4 f)-g) picture). it might occur that there is more than one obstacle between two nodes. in this case all existing tangent points and tangents between the previous and next nodes to/from all obstacles are added as vertices and edges to the tree-like graph. the tangents and tangent points between all pair of obstacles in the proper edge direction are added as vertices and edges. in paper [12] the svpp algorithm handles obstacles in the same way as nodes when these are added to the tree-like graph. the authors iterate step by step on permutation 𝛴′ by adding all tangent points to the tree-like graph as vertices. then they add arcs and tangents to the tree-like graph as edges taking into consideration the heading constraint. the tree-like graph constructed from the figure 3 sensing field can be seen on figure 5 using both algorithm 3 and algorithm 4. during the construction of the tree-like graph according to algorithm 4 first vertices were created from nodes denoted by black then the edges were added between them. then the tangent points of obstacles were added as vertices denoted by purple and the associated edges were also added. the different objects are separated with rectangles in the figure. it can be seen that in figure 5 on the second picture there are direct tangents between node-2 and node-3, namely edge between vertices n2_pp and n2ton3_pp, but on the first picture of figure 5 there are only paths using edges that passing obstacle-1 (this is because of assumption 1 and assumption 3). theorem using our new assumption 2 and assumption 4 the planned path always better or equal to the path that using the original assumption 1 and assumption 3. proof the simplified tangents graph in case of assumption 2 or 4 always contains all edges and vertices from simplified tangents graph using assumption 1 and 3. therefore the tree-like graph using assumption 2 and 4 always contains the tree-like graph using assumption 1 and 3 as well. so the tree-like graph using assumption 2 and 4 contains all paths that the tree-like graph using assumption 1 and 3 and possibly even more paths. therefore, the shortest path in the tree-like graph using assumption 2 and 4 always shorter or equal to the shortest path of the tree-like graph using assumption 1 and 3. □ figure 5. an example of tree-like graph construction from figure 3 using algorithm 3 and algorithm 4. obstacles are denoted by purple. acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 7 3.5. searching the shortest path in a tree-like graph after the tree-like graph was created, it was searched for the shortest path. there are many different way to find the shortest path. here in this paper the dijkstra algorithm [16] were applied. the searching for the shortest path was carried out in both positive and negative initial direction and then the shorter was selected. 3.6. complexity of g-svpp algorithm to end this section, we analyse the time complexity of gsvpp algorithm both of assumption 1 and 3 or assumption 2 and 4 cases. first step is solving tsp with use of miller-tuckerzemlin formulation [17] with 𝒪(𝑛2 + 𝑛) computation effort. in step 2, we check 𝑛 pairs of visiting circles to see whether they are blocked by any boundary of convex hull. in each checking, check all the 𝑚 obstacles and then the time complexity is 𝒪(𝑛𝑚). in step 3, we do a constant number of operations to each permutation each element in σ′, then the time complexity of the simplifying procedure is ∑ 𝒪(𝑛′𝑖 ) 𝑛 σ′ 𝑖=1 , where 𝑛′𝑖 denote the length of the 𝑖th permutation of σ′ for each permutation and 𝑛ς′ denote the number of permutations of σ′. converting 𝐺′(𝑉′, 𝐸′) to 𝑇 costs 𝒪(1) in step 4. the shortest path searching of the 𝑇 is implemented dijkstra algorithm [16]. the computation effort of dijkstra algorithm is 𝒪(|𝐸′| + |𝑉 ′|2) = 𝒪(|𝑉 ′|2) so that depend on the number of vertices of 𝑇 tree like graph. in case assumption 1 and 3 the 𝑇 tree like graph contains maximum 8𝑛′ + 4 vertices, since there are four arrival and four departure configurations all objects, and two starting and two ending vertices of the base station. in case assumption 2 and 4 the tree-like graph in worst case contains ∑ 8𝑛′ 𝑖 𝑛 σ′ 𝑖=1 + 4 vertices. therefore, the worst case computation effort in case assumption 2 and 4 is 𝑛ς′ 2 times larger than the maximum computation error in case assumption 1 and 3. however, in most cases the σ′ permutations differ from each other in only a few elements so the computation error is more smaller than in the worst case. 4. simulation results in the present paper a 200 m × 200 m virtual field was simulated with 40 nodes, of which one is the base station and the other 39 are sensor nodes. the base node is node-1. each sensor node stores 𝑔 = 0.5 mb data and to the base node 𝑔𝐵 = (𝑛 − 1) 0.5 mb = 19.5 mb collected data is uploaded by the robot for further analysis. the data transmission rate at the visiting circle is 𝑟 = 250 kb/s. in a sensing field there are 15 obstacles as well. the robot speed is 𝑣 = 4 m/s and the maximal angular velocity is |𝑢𝑀 | ≤ 1 rad/s, therefore the minimal turning radius and also the visiting circle’s radius is 𝑅𝑚𝑖𝑛 = 𝑣 𝑢𝑀⁄ = 4 m the robot must move at least 𝑑𝑠𝑎𝑓𝑒 = 0.5 m distance from an object in order to avoid to collision. the next step is to construct the 𝐺(𝑉, 𝐸) tangent graph, for this the tangents and the tangent points between the objects are determined and then the edges are deleted according to assumption 1 or assumption 2. the difference between the two assumptions can be seen in figure 6. it can be seen in figure 6 that the proposed assumption 2 produces more tangents, since this allows tangents that intersect visiting circle but not intersect circle with centre of node and radius 𝑑𝑠𝑎𝑓𝑒 . therefore, increases the number of possible paths so it is feasible shorter path planning. but at the same time, it requires more computations as well. the next step is to determine the 𝛴 permutation of nodes and then construct the 𝛴′permutation or permutations with obstacles depending on assumption 3 or assumption 4. using 𝛴′ the 𝐺′(𝑉′, 𝐸′) simplified tangent graph is constructed. the 𝐺′(𝑉′, 𝐸′) using assumption 3 and assumption 4 can be seen in figure 7 and figure 8. in figure 7 the tangents graph figure 6. the difference between the 𝐺(𝑉, 𝐸) tangent graphs using assumption 1 and assumption 2. a) b) figure 7. 𝐺′(𝑉′, 𝐸′) simplified tangent graph using assumption 1 and assumption 3 . acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 8 constructed by assumption 1 is simplified by applying assumption 3. similarly, in figure 8 it can be seen the 𝐺′(𝑉′, 𝐸′) applying assumption 2 and assumption 4. in figure 8 it can be seen that using the assumption 2 and assumption 4 proposed in this article, the tangents that intersect visiting circles are available and more permutations can be constructed so there are more available tangents in a 𝐺′(𝑉′, 𝐸′) and therefore in the tree-like graph as well. in this way, there are more possible paths and therefore the algorithm may plan shorter path. in this case the constructed tree-like graph is more complex than in case of assumption 3. when we use assumption 1 and assumption 3 there can be at most 4 arrival and 4 departure tangents for every object in the order of the permutation. usually there are four-four arrival and departure tangents, but for example in figure 7, between obstacle-13 and obstacle-3 there are just 3 available tangents. in figure 8 it can be seen the case of assumption 2 and assumption 4. since in the second picture the fourth tangent intersects the safety circle of node-10 it is also not available in the simplified tangent graph. in figure 8 there are direct tangents between node-24 and obstacle-3 and one of them intersects the visiting circle of node-26, but in figure 7 the robot must visit obstacle-13 first. in figure 8 there are three available tangents between the visiting circles of node-37 and node-3, but in figure 7 (assumptions 1 and 3) the robot can only move on tangents that passing obstacle-6. the next step is to construct the 𝑇 tree-like graphs for both cases assumption 1 and 3 or assumption 2 and 4. finally, in the tree-like graph a search for the shortest path is carried out both for positive and negative initial direction as well. in figure 9 and figure 10 it can be seen that the planned path using the new assumptions 2 and 4 proposed in the present article is shorter than in the original case. in this example, the planned path with positive and negative initial direction only differs in the tangents that are belonging to the base station. using assumptions 2 and 4 the planned path between the node-24 and obstacle-3 use tangent that intersect the visiting circle of node-26, therefore a shorter path can be achieved using the new assumptions of the present article. applying assumptions 1 and 3 between node-3 and node-37, the planned path uses tangents that pass the obstacle-6. on the contrary, if assumptions 2 and 4 are applied, the planned path uses direct a) b) figure 8. 𝐺′(𝑉′, 𝐸′) simplified tangent graph using assumption 2 and assumption 4 . figure 9. the resulted shortest path with negative initial direction of the 𝑇 tree-like graph both for the cases using assumption 1 and assumption 3 or assumption 2 and assumption 4. the length of the planned path are 2341.94𝑚 and 2290.85𝑚, respectively. acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 9 tangents between the two nodes and this way the determined arc length is shorter. each sensor node data downloading requires 𝑙 = 𝑔 𝑣 𝑟 = 8 m arc length. the visiting circles circumference equal to 𝐾 = 25.12 m. therefore the robot must take at least approximately one third of the visiting circle's circumference to have enough time for data transmission. it can be seen that the algorithm preferably chooses tangents between node-31, node-32 and node-33 in such a way that the robot is not required to make extra round trips around these nodes. as a test, the path planning was run for ten different virtual sensing fields. the length of the planned paths can be seen on table 1. the solutions using assumption 1 and 3 are compared with the solutions using assumption 2 and 4. as it was proven in the theorem, using the new assumptions proposed in the present article the planned path always better or equal to the path using the original assumptions presented in [6]. the number of vertices on a 𝑇 tree-like graph also represented in table 1. the maximum difference between the number of vertices of assumption 2 and 4 or assumption 1 or 3 is 54, in test field 5. in this case the computation effort in case of assumptions 2 and 4 is increased by 27 % compared to assumptions 1 and 3. at the same time the planned path in case of assumptions 2 and 4 is 92 m shorter than assumptions 1 and 3. that means, 𝑡 = 92 m 4 m/s = 23 s time for each period, therefore the robot can make 6 extra round trip for a day. 5. conclusions in the present paper a path planning algorithm was developed for unicycle robots between sensor nodes. the task is to collect all the data from the sensor nodes and then upload it to the base node. to increase the effectiveness of data collection, the length and the duration of the trip should be minimized, while also maintaining a collision-free path around the nodes and obstacles. new algorithms were developed for handling the obstacles and new assumptions were applied to reach a collision-free state. a new algorithm for tree-like graph generation was also developed, where the search of the shortest viable path will take place finally. the present paper finished with the detailed presentation of simulation results. the preparation of the field and the steps of path planning algorithm were illustrated with figures. the present article also compared the results of the new algorithm with the results of a previous research. in conclusion, the new algorithm presented in this article proved to achieve shorter paths then the earlier algorithms. figure 10 the resulted shortest path with positive initial direction of the 𝑇 tree-like graph both for the cases using assumption 1 and assumption 3 or assumption 2 and assumption 4. the length of the planned path are 2337.64𝑚 and 2286.55𝑚. table 1 the length of the planned path for different virtual sensing fields using both positive and negative starting directions. |𝑉′| denote the number of vertices of the tree-like graph on which it depends the computation error. test field assumption 1 and 3 assumption 2 and 4 positive negative |𝑽′| positive negative |𝑽′| 1 2326.23m 2233.61m 372 2254.71m 2222.65m 386 2 2393.66m 2387.67m 418 2332.58m 2326.59m 462 3 2305.82m 2313.80m 404 2270.74m 2278.72m 444 4 2372.37m 2359.42m 436 2363.87m 2350.92m 466 5 2413.25m 2403.51m 412 2321.40m 2311.65m 466 6 2294.77m 2288.91m 402 2193.65m 2189.18m 452 7 2335.53m 2317.82m 396 2312.25m 2294.55m 426 8 2330.93m 2324.78m 378 2330.51m 2324.36m 386 9 2247.02m 2262.38m 372 2238.79m 2254.14m 386 10 2301.88m 2311.12m 360 2301.88m 2311.12m 368 acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 10 acknowledgement the research reported in this paper and carried out at the budapest university of technology and economics was supported by the “tkp2020, institutional excellence program” of the national research development and innovation office in the field of artificial intelligence (bme ie-mi-sc tkp2020). the research was supported by the efop-3.6.2-16201600014 project financed by the ministry of human capacities of hungary. the research reported in this paper is part of project no. bme-nva-02, implemented with the support provided by the ministry of innovation and technology of hungary from the national research, development and innovation fund, financed under the tkp2021 funding scheme. references [1] a. mainwaring, d. culler, j. polastre, r. szewczyk, j. anderson, wireless sensor networks for habitat monitoring, 1st acm int. workshop on wireless sensor networks and applications, atlanta, georgia, 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nikoletseas, sink mobility protocols for data collection in wireless sensor networks, 4th acm int. workshop on mobility management and wireless access, acm, terromolinos, spain, 2006, pp. 52-59. doi: 10.1145/1164783.1164793 [7] y. yun, y. xia, maximizing the lifetime of wireless sensor networks with mobile sink in delay-tolerant applications, ieee trans. mob. comput., vol. 9, 2010, pp. 1308-1318. doi: 10.1109/tmc.2010.76 [8] hailong huang, andrey v. savkin, ming ding, chao huang, mobile robots in wireless sensor networks: a survey on tasks, computer networks 148, 2019, pp. 1-19. doi: 10.1016/j.comnet.2018.10.018 [9] huang, hailong, andrey v. savkin, reactive 3d deployment of a flying robotic network for surveillance of mobile targets, computer networks 161, 2019, pp.172-182. doi: 10.1016/j.comnet.2019.06.020 [10] h. huang, a. v. savkin, an energy efficient approach for data collection in wireless sensor networks using public transportation vehicles, aeu-international journal of 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navigation of a wheeled mobile robot in cluttered environments, robotica. vol. 31 issue 2,2013, pp. 323-330. doi: 10.1017/s0263574712000331 [16] e. w. dijkstra, a note on two problems in connexion with graphs, numerische mathematik, vol. 1, number 1, 1959, pp. 269-271. doi: 10.1007/bf01386390 [17] c. e. miller, a. w. tucker, r. a. zemlin, integer programming formulation of traveling salesman problems. j. acm 7, 4, october 1960, pp. 326-329. doi: 10.1145/321043.321046 http://dx.doi.org/10.1145/570738.570751 https://doi.org/10.1145/990064.990096  https://doi.org/10.1016/j.proeng.2012.06.320 https://doi.org/10.1109/comst.2015.2388779 https://doi.org/10.1109/tc.2014.2349521 http://dx.doi.org/10.1145/1164783.1164793 https://doi.org/10.1109/tmc.2010.76 https://doi.org/10.1016/j.comnet.2018.10.018 http://dx.doi.org/10.1016/j.comnet.2019.06.020 http://dx.doi.org/10.1016/j.aeue.2017.03.012 https://doi.org/10.1109/comst.2015.2388779 http://dx.doi.org/10.1016/j.comcom.2017.07.010 https://doi.org/10.2307/2372560 https://doi.org/10.1002/net.3230120103 http://dx.doi.org/10.1017/s0263574712000331 https://doi.org/10.1007/bf01386390 https://doi.org/10.1145/321043.321046 analysis of multiband rectangular patch antenna with defected ground structure using reflection coefficient measurement acta imeko issn: 2221-870x march 2022, volume 11, number 1, 1 6 acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 1 analysis of multiband rectangular patch antenna with defected ground structure using reflection coefficient measurement thalluru suneetha1, s. naga kishore bhavanam1 1 department of ece, acharya nagarjuna university, guntur, andhra pradesh-522510, india section: research paper keywords: measurement; defected ground; sensing; wlan; wimax planar monopole antenna citation: thalluru suneetha, s. naga kishore bhavanam, analysis of multiband rectangular patch antenna with defected ground structure using reflection coefficient measurement, acta imeko, vol. 11, no. 1, article 28, march 2022, identifier: imeko-acta-11 (2022)-01-28 section editor: md zia ur rahman, koneru lakshmaiah education foundation, guntur, india received november 20, 2021; in final form march 20, 2022; published march 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: thalluru suneetha, e-mail: tsuneetha701@gmail.com 1. introduction the contemporary telecommunications sector has evolved in response to the increasing demands of customers for smart gadgets. in contrast to their predecessors, these devices are capable of handling many applications at the same time. global positioning system (gps), wireless fidelity (wi-fi), global system for mobile communication (gsm), bluetooth, and worldwide interoperability for microwave access (wimax), all have their own operating frequency bands. the addition of a separate antenna for each application increases the device's size and makes it more uncomfortable to use. due to this the demand for antennas operating at multiple frequencies is increasing. high data rates in wireless communication systems are quite common these days. as a result, contemporary gadgets must have a single antenna that can operate in many frequency bands. lot of research has been going on in the field of multiband antennas by many researchers and several methods were devised in recent years. the authors of [1] reported a coplanar waveguide (cpw)fed rectangle antenna with modified ground and open complementary split ring resonator (ocsrr) loading that could operate in three bands. in [2] u shaped antenna with partial ground plane to resonate at three different frequencies is presented. in [3] authors designed ‘nine’ and ‘epsilon’ shaped antennas along with switches to realize triple band operation for use in wi-fi, wimax and wlan applications. in [4] a five band antenna with kite shaped radiating patch with ‘c‘ and ‘g’ shaped slots in the ground plane was realized. authors in [5] used meta material technique to design dual band antenna. a cpw fed meta material based multiband antenna was reported in [6]. in [7] a small cpw-fed monopole antenna for dual band operation was presented. the authors used the state of the art substrate integrated waveguide technique and slots to get dual frequency abstract in this paper, a novel quad band antenna to operate at four different frequency bands is designed and simulated using computer simulation technology (cst) microwave studio software. to achieve better performance of the antenna, various parameters were optimized using parametric analysis. during this analysis, various antenna parameters were need to be measured. the proposed antenna uses asymmetrical ‘u’ and ‘t’ shaped radiating elements printed on an fr-4 substrate with dimensions of 1.6 × 34 × 20 mm3. the measurement of various antenna parameters like reflection coefficient, return loss, radiation intensity are key tasks to be performed in the antenna measurement laboratory before going to the real time application. a stair case defected ground structure with rectangular center slot is used to attain a better bandwidth. the antenna resonates at four different frequencies 3 ghz, 4.8 ghz, 9 ghz, 13.2 ghz with operating bandwidths of 980 mhz, 2.05 ghz, 3.84 ghz, and 3.82 ghz respectively. the s11 value at these resonant frequencies is measured as -23.6 db, -29.4 db, -34.2 db, -49.05 db respectively. the voltage standing wave ratio of the proposed antenna at four resonant frequencies is equal to one. the gain of the antenna is consistent throughout the four pass bands. the antenna is suitable for bluetooth (2.4) ghz, wlan (5.125-5.35) ghz and (5.725-5.825) ghz, wimax (5.25-5.85) ghz, c-band (3.7-4.2) ghz and ku-band (12-18) ghz. mailto:tsuneetha701@gmail.com acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 2 response and wide bandwidth in [8]. as reported in [9] increase in gain parameter is achieved by using antenna arrays. several techniques, such as frequency-selective surface [10]-[12] and electromagnetic band-gap structure [13]-[15], have been studied by several researchers to create multiband antennas. to improve the polarization performance of the link budget, circularly polarized antennas are frequently employed in wlan and satellite applications as reported in [16]-[18]. in every wireless communication system, the antenna is extremely crucial. the well-designed antenna reduces receiver complexity and improves receiver performance. the antenna's size, shape, and design will be controlled by the antenna's application [19], [20] and operating frequency. a simple rectangular patch was used in this project, and a portion of it was removed to create a symmetrical ushaped structure. a portion of the left and right arms are cut from that structure, which leads to asymmetrical u shape and to that shape one vertical strip combined with a horizontal strip, is added to the centre to form a t-shaped structure. to achieve better bandwidth, a staircase defected ground structure along with centre rectangular slot was used. proposed antenna resonates at multiple frequencies owing to its structural modifications. this antenna is applicable to a wide range of modern portable wireless applications, bluetooth (2.4) ghz, wlan (5.125-5.35) ghz and (5.725-5.825) ghz, wimax (5.25-5.85) ghz, c-band (3.7-4.2) ghz and ku-band (12-18) ghz. to achieve multiband operation other designs used slots and modified the shapes of radiating elements which is also complex. in this design staircase defective ground structure was employed to get wider bandwidth at resonating frequencies. defected ground structure (dgs), apart from conventional antennas, produces discontinuities on the signal plane, which disrupts shielded current distribution on the signal plane. as a result, the apparent permittivity of the substrate fluctuates as a function of frequency and plays a vital role in the antenna's performance. various parameters such as radiation pattern, s11, vswr, gain, are measured and analysed for the final design of the antenna. furthermore, getting the multiband operation owing to its less structural complexity is the most outstanding feature of this design. general procedure for designing any antenna was discussed in section 2. the evaluation steps of the proposed antenna design were discussed in section 3. in section 4 parametric analysis through performance measures measurement of the proposed antenna is discussed. section 5 deals with results and discussion. in section 6 literature comparison of the proposed antenna was done with earlier reported structures. 2. antenna design procedure any antenna design technique begins with study of various antennas for a certain application. then, potential design approaches must be studied. later, patch dimensions such as width and length are determined to get design criteria. geometrical parameters and material qualities must then be accurately specified in the following stage. the simulation process is done by finalizing one simulator among the available simulators. using the simulator's parametric analysis feature, simulation work is carried out until the best possible result is obtained. when the required behaviour is achieved, the simulation is terminated. the antenna fabrication operation is then initiated, and the desired antenna prototype is created. to validate the design technique, the fabricated antenna behaviour is evaluated and compared to that of the simulated ones. if the required behaviour is not obtained, the geometric parameters of the antenna as well as the material qualities must be modified. simulation is again carried out with modified parameters. this is continued until the desired behaviour of the proposed antenna is obtained. figure 1 clearly displays the antenna general design procedure. 3. evaluation steps of the proposed antenna design the planned antenna has been designed in three phases, as illustrated in figure 2. figure 2 i) show a typical rectangular patch antenna with a simple micro strip line in step 1. in step2 a portion of patch is removed to get symmetrical u-shaped antenna as depicted in figure 2 ii) which leads to multiband response. in step3 as shown in figure 2 iii) portions of left and right arms are cut leading to asymmetrical u-shaped structure and centre horizontal and vertical arms are added leading to t shaped structure. this is the proposed design. this proposed antenna refines impedance matching and achieves better performance in terms figure 1. antenna general design procedure. i) antenna1 ii) antenna2 iii) antenna3 figure 2. steps in the design of the proposed multiband antenna. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 3 of wider bandwidth and vswr. gain is also considerable throughout the pass bands. figure 3 depicts the design for the proposed multi band planar antenna with asymmetrical u and t shaped patch structure. from the simple normal rectangular patch, a portion is removed to realize symmetrical u-shaped patch structure. portions of left and right arms were cut and centre horizontal and vertical arms were added leading to t shaped patch structure at the centre. this structure deals with defected ground technology. defected ground plane with stepped staircase along with centre rectangular slot has been added beneath the substrate, to extend the bandwidth. substrate dimensions are 20 × 34 × 1.6 mm3. the material chosen is fr-4, which has good mechanical properties with a thickness of 1.6 mm, a dielectric permittivity of 4.4, and a loss tangent of 0.02. the antenna was fed by 50 ω micro strip line. simulation of the proposed antenna has been carried out by using cst micro wave studio. the antenna was optimized using the cst microwave studio's parametric analysis tool. the antenna is capable of resonating at four distinct frequencies. antenna 1 only resonates at two frequencies, the first at 3 ghz and the second at 4.8 ghz. antenna 2 resonates at four separate frequencies, although the bandwidth at the pass bands and impedance matching were not ideal. antenna 3 resonates at four separate frequencies, with a wide bandwidth and good impedance matching, as well as significant gain at all four resonant frequencies. antenna 3 is the proposed construction, which resonates at four distinct frequencies. simulated s11 of different antennas as depicted in figure 2 are shown in figure 4. 4. parametric study of the proposed antenna to obtain the optimal design, parametric analysis is the greatest choice accessible when utilising simulators. study of effect of changing various parameters like feed length l1, right arm length l2 and length of t strip l3 has been carried out to optimize the proposed design. effect of varying l1: figure 5 clearly displays the effect of changing the value of feed length l1. the feed length l1 was tested for three different lengths. the findings show that as the length of the feed increases, the antenna only resonates at two frequencies, with l1 = 10 mm yielding the best results with four resonant frequencies. effect of varying l2: to investigate the impact of changing the right arm length l2 all other previously optimized parameters were held constant at their optimum values, while l2 is changed between 9 mm to 12 mm. the second, third, and fourth resonances do not vary significantly when the value is increased, but impedance matching at the first resonant frequency decreases, and l2 =9 mm provides the best performance in terms of improved bandwidth and impedance matching. figure 6 displays the impact of changing the values of right arm length l2. a) b) figure 3. designed antenna architecture: a) front view, b) back view, ws =20 mm, ls = 34 mm, l1 = 10 mm, l2 = 9 mm, l3 = 14 mm, l4 = 2 mm, w1 = 2.2 mm, w3 = 4.4 mm, w4 = 11 mm, wg = 20 mm, w = 2 mm, l = 2 mm, ln = 8 mm, wn = 4 mm. figure 4. s11 of different antennas as depicted in figure 2. figure 5. optimized s11 variation for various values of l1. figure 6. optimized s11 variation for various values of l2. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 4 effect of varying l3: to explore the influence of length of t strip l3, it was varied from 13 to 15 while all other parameters were kept at their previously optimized levels. for different values of l3, there wasn't much of a variation in the first three resonances. for l3 = 14 mm, a wider bandwidth and improved impedance matching were achieved as portrayed in figure 7. as surface current distribution helps in understanding the behaviour of the antenna figure 8 portrays the surface current distribution of the proposed antenna at four resonant frequencies. different portions of the antenna are responsible for radiations at four resonant frequencies. radiation at 3 ghz is directed by the right arm of the u structure as well as centre t strip right portion. at 4.8 ghz, the lower part of the u structure right arm is responsible for radiation. the lower part of the u shape is responsible for 9 ghz radiation. at 13.2 ghz, both the centre arm and the upper section of the u shape are responsible for radiation. 5. results and discussion the designed antenna's simulated s11 and gain variations with respect to frequency can be seen in figure 9. the antenna is simulated by cst microwave studio which uses finite integration method. the four resonant frequencies are occurring at 3 ghz, 4.8 ghz, 9 ghz, and 13.2 ghz. the return loss at these frequencies is -23.6 db, -29.4 db, -34.2 db, -49.05 db, respectively. these values clearly indicate good impedance matching at all the four resonant frequencies. the pass bands around these resonances are 2.484 ghz to 3.389 ghz, 3.923 ghz to 5.974 ghz, 7.232 ghz to 10.921 ghz, and 12.551 ghz to 16.363 ghz with bandwidths of 0.905 ghz, 2.051 ghz, 3.689 ghz and 3.812 ghz, respectively. wide bandwidths at four resonant frequencies are also identified. considerable gain is observed at four resonant frequencies. getting the multiband operation owing to its less structural complexity is the most outstanding feature of this design. the voltage standing wave ratio (vswr) measures the mismatch between an antenna and the feed line that connects to it. the range of vswr values ranges from 1 to infinity. vswr of less than 2 is deemed adequate for the majority of antenna applications. in the proposed design vswr of 1 indicates a good match. the proposed quad band antenna's voltage wave standing ratio (vswr) is portrayed in figure 10. from the figure it is evident that at all the four resonant frequencies good vswr is identified which is almost equal to one. the proposed quad band antenna's far field patterns at 3 ghz, 4.8 ghz, 9 ghz, 13.2 ghz are clearly displayed in figure 10. 6. literature comparision the comparison analysis between the proposed antenna and previously published designs for multiband operation is summarized in table 1. proposed design is compared in terms of size, frequencies, radiating elements. compared to earlier reported designs in [1], [2], [3], [4], [5] and [6] proposed antenna is better in terms of size; structural complexity of the proposed antenna is less compared to most of the structures. gain is consistent in the operating bands and operating bandwidth is also more compared to the most of the structures. vswr of the figure 7. optimized s11 variation for various values of l3. a) b) c) d) figure 8. surface current distribution at resonant frequencies: a) 3 ghz, b) 4.8 ghz, c) 9.0 ghz and d) 13.2 ghz. figure 9. the designed antenna's simulated s11 and gain. figure 10. the designed antenna's simulated vswr. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 5 antenna at resonating frequencies is almost equal to one which is promising. in this design without the necessity of complex structures able to achieve multiband operation by simply removing some portions from the patch and adding additionally centre t strip. 7. conclusions the design of a novel quad band antenna with asymmetrical u and t shaped patch structure for portable wireless applications was presented in this study. the difference of this design with others can be seen through its simple design steps leading to less complex structure. the proposed antenna resonates at 3 ghz, 4.8 ghz, 9 ghz, and 13.2 ghz frequencies with bandwidths 980 mhz, 2.05 ghz, 3.84 ghz, and 3.82 ghz, respectively. over the working frequency ranges, the antenna has reasonable gains. bandwidth of the antenna over the operating frequencies is high. the gain, vswr, and reflection coefficient are taken into consideration for design and analysis operations in the frequency range of 1 ghz to 20 ghz. in comparison to most other designs, this structure is simple and compact, producing quad bands for use in portable electronic devices. the proposed patch antenna is made utilizing printed circuit board technology, which is easy and affordable. this antenna is applicable to a wide range of modern portable wireless applications, bluetooth (2.4) ghz, wlan (5.125-5.35) ghz and (5.725-5.825) ghz, wimax (5.25-5.85) ghz, c-band (3.7-4.2) ghz and ku-band (12-18) ghz. references [1] r. pandeeswari, s. raghavan, a cpw-fed triple band ocsrr embedded monopole antenna with modified ground for wlan and wimax applications, microwave and optical technology letters, vol. 57, 2015, pp. 2413–2418. doi: 10.1002/mop.29352 [2] mahesh kendre, a. b. nandgaonkar, pratima nirmal, sanjay l. nalbalwar, u shaped multiband monopole antenna for spacecraft, wlan and satellite communication application, ieee international conference on recent trends in electronics information communication technology, bangalore, india, 20-21 may 2016, , pp. 1528-1532. doi: 10.1109/rteict.2016.7808088 [3] v, jyothika, m. s. p. c. shekar, s. v. krishna, m. z. u. rahman, design of 16 element rectangular patch antenna array for 5g applications, journal of critical reviews, 7(9), pp. 53-58. [4] t. ali, k. d. prasad, r. c. biradar, a miniaturized slotted multiband antenna for wireless applications. j comput. electron. 17, 2018, pp. 1056–1070 . doi: 10.1007/s10825-018-1183-z [5] k. a. rao, k. s. raj, r. k. jain, m. z. u. rahman, implementation of adaptive beam steering for phased array antennas using enlms algorithm, journal of critical reviews, 7(9), pp. 59-63. doi: 10.31838/jcr.07.09.10 [6] n. thamil selvi, p. thiruvalar selvan, s. p. k. babu, r. pandeeswari, multiband metamaterial-inspired antenna using split ring resonator, computers & electrical engineering, vol. 84, 2020, 106613, issn 0045-7906. doi: 10.1016/j.compeleceng.2020.106613 [7] s. kesana, s. gatikanti, m. z. u. rahman, b. radhika, d. mounika, triple frequency g-shape mimo antenna for wireless applications, international journal of engineering and advanced technology, 8 (5) , 2019, pp. 942-947 [8] s. v. devika. k. karki, s. k. kotamraju, k. kavya, m. z. u. rahman, a new computation method for pointing accuracy of cassegrain antenna in satellite communication, journal of theoretical & applied information technology, 95(13), 2017. 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[10] m. z. u. rahman, v. a.kumar, g. v. s. karthik, a low complex adaptive algorithm for antenna beam steering international conference on signal processing, communication, computing and networking technologies, thuckalay, india, 21-22 july 2011, pp. 317-321. doi: 10.1109/icsccn.2011.6024567 [11] m. a. meriche, h. attia, a. messai, t. a. denidni, gain improvement of a wideband monopole antenna with novel artificial magnetic conductor, 17th international symposium on antenna technology and applied electromagnetics (antem), montreal, qc, canada, 10-13 july 2016, pp. 1-2. doi: 10.1109/antem.2016.7550150 [12] n. wang, q. liu, c. wu, l. talbi, q. zeng, j. xu, wideband fabry-perot resonator antenna with two complementary fss layers, ieee transactions on antennas and propagation, vol. 62, a) b) c) d) figure 11. patterns of radiation at: a) 3 ghz, b) 4.8 ghz, c) 9 ghz and d) 13.2 ghz. table 1. comparison of proposed antenna with earlier reported structures. s.no year dimensions in mm2 frequency in ghz radiating element 1 2015 40 × 30 2.4, 3.5, 5.8 pentagonal radiating patch with two slots 2 2016 43 × 20 2.8, 5.8, 10.8 u shape monopole antenna 3 2017 35 × 53 2.4, 3.5, 5.5. epsilon and nine shaped antennas 4 2018 23 × 23 3.6 ,5.8, 6.3, 8.3, 9.5 kite-shaped, cand modified g-shaped slots 5 2019 35 × 25 5.7, 10.3 metamaterial cell 6 2020 40 × 40 2.9, 2.10, 3.5, 4.5, 5.7, 6.5 penta-ring srr 7 proposed 34 × 20 3, 4.8, 9, 13.2 asymmetrical u and t shaped patch structures https://doi.org/10.1002/mop.29352 https://doi.org/10.1109/rteict.2016.7808088 https://doi.org/10.1007/s10825-018-1183-z https://doi.org/10.31838/jcr.07.09.10 https://doi.org/10.1016/j.compeleceng.2020.106613 https://doi.org/10.1109/icsccn.2011.6024567 https://doi.org/10.1109/antem.2016.7550150 acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 6 no. 5, 2014, pp. 2463–2471. doi: 10.1109/tap.2014.2308533 [13] y.ge, k. p. esselle, t. s. bird, a method to design dualband,high-directivity ebg 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doi: 10.1017/s1759078719001624 [18] m. j. hua, p. wang, y. zheng, s. l. yuan, compact tri-band cpw-fed antenna for wlan/wimax applications, electronics letters, vol. 49, no. 18, 2013, pp. 1118–1119. doi: 10.1049/el.2013.1669 [19] armando coccia, federica amitrano, leandro donisi, giuseppe cesarelli, gaetano pagano, mario cesarelli, giovanni d'addio, design and validation of an e-textile-based wearable system for remote health monitoring, acta imeko, vol. 10, 2021, no. 2, pp. 220-229. doi: 10.21014/acta_imeko.v10i2.912 [20] imran ahmed, eulalia balestrieri, francesco lamonaca, iomtbased biomedical measurement systems for healthcare monitoring: a review, acta imeko, vol. 10, 2021, no.2, pp. 1-11. doi: 10.21014/acta_imeko.v10i2.1080 https://doi.org/10.1109/tap.2014.2308533 https://doi.org/10.2528/pierc10020901 https://doi.org/10.1049/el.2015.1022 https://doi.org/10.1049/iet-map.2015.0674 https://doi.org/10.1002/mop.27113 https://doi.org/10.1017/s1759078719001624 https://doi.org/10.1049/el.2013.1669 https://doi.org/10.21014/acta_imeko.v10i2.912 https://doi.org/10.21014/acta_imeko.v10i2.1080 linear regression analysis and the gum: example of temperature influence on force transfer transducers acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 407 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 407 413 linear regression analysis and the gum: example of temperature influence on force transfer transducers d. röske1 1 physikalisch technische bundesanstalt (ptb), braunschweig, germany, dirk.roeske@ptb.de abstract: the deflection of strain-gauge force and torque transducers (the zero-reduced output signal for a given mechanical load) is dependent on the ambient temperature. this is also true of high-precision force transfer transducers used to compare force standard machines. to estimate the extent to which temperature deviations during measurements on different machines affect comparison results and – if necessary – to correct for such deviations, it is important to know the influence of the temperature on the deflection. this effect is usually investigated in special temperature chambers in which the transducer is exposed to various temperatures within a given temperature range while the machine is operated under unchanged laboratory conditions. the regression analysis of the results allows the temperature coefficient to be determined, including an uncertainty analysis. this was done for five force transfer transducers used for a bilateral comparison between npl (uk) and ptb (germany). keywords: comparison; temperature influence; regression; uncertainty 1. introduction the manufacturers of strain-gauge force and torque transducers devote significant effort to minimising the influence of the ambient temperature on the measurement results of their devices. the best transducers from this group are very well compensated for temperature influences. however, no measurements of the transducer’s temperature behaviour are taken and only the upper limit of the absolute value of the sensitivity’s temperature coefficient (the relative change of the deflection with temperature) is given in the transducer’s specifications. for example, a range between 0.02 %/(10 k) and +0.02 %/(10 k) is given for the hbm z30a transducer [1], while a range between 0.01 %/(10 k) and +0.01 %/(10 k) is given for the gtm ktn transducer [2]. this means that the absolute value of the temperature coefficient should be below 1 … 2 · 10-5 k-1. for a 1 k temperature change, this is more than the standard uncertainty of the best force standard machines. 2. measurement results for a comparison measurement between ptb’s new 200 kn force standard machine [3] and the relevant standard machines of the npl, five highprecision compression force transfer transducers (one z30a, four ktns) were selected. these transducers were investigated with respect to their temperature behaviour in a temperature chamber inside the 200 kn force standard machine. the temperature range was defined as 20 °c … 25 °c with additional measurement points beyond these threshold values at 21 °c, 22 °c and 23 °c. the resulting deflections 𝑑𝑖 in mv/v for the 50 kn ktn transducer are given in table 1 for the five temperatures 𝑇𝑖 and for two load steps (20 kn and 50 kn). table 1: deflections 𝑑𝑖 in mv/v for the 50 kn transducer at different temperatures 𝑇𝑖 and for two load steps 𝑻𝒊 in °c 𝒅𝒊 at 20 kn in mv/v 𝒅𝒊 at 50 kn in mv/v 20.03 0.800 996 2.003 036 21.00 0.800 992 2.003 028 21.99 0.800 988 2.003 011 23.06 0.800 984 2.003 002 24.90 0.800 978 2.002 981 3. preliminary evaluation in the evaluation, a linear dependency between the temperature 𝑇 and the deflection 𝑑 is assumed to exist within a sufficiently small temperature range (and possibly beyond this range). in the first approach, the data can be evaluated using the linear fit functions of programming languages such as python and r or standard software programs such as excel, all of which are based on the least-squares method and require a very small number of code lines, see figure 1. excel even contains a function wizard that can find the right function and defines the necessary arguments. the results agree well at a relative level of 10-12 and below. the limitation is caused by the floating-point http://www.imeko.org/ mailto:dirk.roeske@ptb.de acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 408 accuracy of the processor, operating system and software used; however, this is not a problem for the uncertainty in question in this investigation. figure 1: parameters (for the 20 kn force in table 1) of a linear fit in python (top), r (middle) and excel (bottom, german user interface) most linear-fit packages and functions contain additional information about the result such as the residual sum of squares (rss) of the least-squares fit in python and excel and the average variation of points around the fitted regression line, the residual standard error (rse), in r. these values can be used to check how well the model fits the data; this step is necessary to ensure that the right model is used for the data. the residual sum of squares of the least-squares fit in excel yields a value of 6.07 · 10-13 (mv/v)2 for the data in table 1 (for 20 kn). to improve comparability, this value is linked to another value – namely, the sum of squared deviations (ssd) from the mean. the result is 1.95 · 10-10 (mv/v)2 and the relative deviation (ssd rss) / ssd = 0.997 is a measure of the model quality: the closer this value is to 1, the better the model is. nevertheless, this calculation does not consider the uncertainty of the input data. it is expected that the resulting parameters will also have an associated uncertainty and that this value will depend on the uncertainty of the input data. to obtain the uncertainty of the results of the fitting procedure, the following regression analysis was carried out using a linear regression model and a least-squares approach in combination with the gum, the guide to the expression of uncertainty in measurement [4]. 4. regression analysis we consider five pairs of input (𝑇𝑖 ) and output (𝑑𝑖 ) values with their associated uncertainties (𝑢) ((𝑇𝑖 ; 𝑢(𝑇𝑖 )), (𝑑𝑖 ; 𝑢(𝑑𝑖 ))) 𝑖 = 1, 2, … 5 . (1) here, the aim is to find a linear approximation function �̃�(𝑇) that best describes the data �̃�(𝑇) = 𝑞 ∙ 𝑇 + 𝑟 (2) with the coefficients 𝑞 (slope) and 𝑟 (intercept). the values of these coefficients should be determined in such a way that a minimum condition is met. in the context of the model considered here, we require the sum of the squared deviations between the measured outputs and the corresponding outputs calculated in accordance with (2) to be a minimum (“least squares”) ∑ (𝑑𝑖 − �̃�(𝑇𝑖 )) 2 𝑁 𝑖=1 → min . (3) with (2), equation (3) can be also written as ∑(𝑑𝑖 − 𝑞 ∙ 𝑇𝑖 − 𝑟) 2 𝑁 𝑖=1 → min . (4) the condition necessary for the minimum is that the partial derivates of the given term to the unknown values 𝑞 and 𝑟 be zero. this yields 2 ∑(𝑑𝑖 − 𝑞 ∙ 𝑇𝑖 − 𝑟)(−𝑇𝑖 ) 𝑁 𝑖=1 = 0 −2 ∑(𝑑𝑖 − 𝑞 ∙ 𝑇𝑖 − 𝑟) 𝑁 𝑖=1 = 0 , (5) a system of two equations via which the values of the coefficients can be determined. a more compact representation of (5) is 𝑞 ∙ 𝑇2̅̅ ̅ + 𝑟 ∙ �̅� = 𝑇 ∙ 𝑑̅̅ ̅̅ ̅̅ 𝑞 ∙ �̅� + 𝑟 = �̅� (6) with 𝑇𝑘̅̅̅̅ = 1 𝑁 ∙ ∑ 𝑇𝑖 𝑘 𝑁 𝑖=1 , �̅� = 1 𝑁 ∙ ∑ 𝑑𝑖 𝑁 𝑖=1 (7) and 𝑇 ∙ 𝑑̅̅ ̅̅ ̅̅ = 1 𝑁 ∙ ∑ 𝑇𝑖 ∙ 𝑑𝑖 𝑁 𝑖=1 . (8) the condition sufficient for the minimum value is that the second derivative be positive. equation (4) yields 𝜕2 𝜕𝑞2 ∑(𝑑𝑖 − 𝑞 ∙ 𝑇𝑖 + 𝑟) 2 𝑁 𝑖=1 = 2 ∑ 𝑇𝑖 2 𝑁 𝑖=1 > 0 , (9) http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 409 𝜕2 𝜕𝑟2 ∑(𝑑𝑖 − 𝑞 ∙ 𝑇𝑖 + 𝑟) 2 𝑁 𝑖=1 = 2 𝑁 > 0 showing that the condition sufficient for the minimum value is fulfilled independently of the parameter values. from (6), the coefficients 𝑞 and 𝑟 can be calculated. they are functions of the 𝑇𝑘̅̅̅̅ (𝑘 𝜖 (1, 2)) and 𝑇 ∙ 𝑑̅̅ ̅̅ ̅̅ and, due to (7) and (8), functions of the 𝑇𝑖 and 𝑑𝑖 . this means that their uncertainties can, in principle, be calculated by applying the standard methods of the gum. the solution of (6) can be written as 𝑞 = 𝑇 ∙ 𝑑̅̅ ̅̅ ̅̅ − �̅� ∙ �̅� 𝑇2̅̅ ̅ − �̅� 2 , 𝑟 = �̅� − 𝑞 ∙ �̅� . (10) it must be noted that the slope of the linear function is the same over the whole temperature range, whereas the intercept depends on the value taken for zero. if, instead of the °c scale used in table 1, the kelvin scale is used, the value of the intercept calculated will change, see figure 2. figure 2: parameters (for the 20 kn force in table 1) of a linear fit in excel with kelvin temperatures force transfer standards are usually stored and operated (and, if possible, transported) in a narrow temperature range from 18 °c to 28 °c (for key comparison measurements, a much smaller interval such as 20 °c ± 0.5 k may be required). this means that the behaviour of the force transducer near 0 °c is not of interest; it is therefore not important that it be known for temperatures close to 0 k. it should be sufficient to describe the transducer’s behaviour in the narrow temperature interval where it was investigated. usually, the reference temperature 𝑇ref for comparison measurements is agreed in advance when the technical protocol is compiled. then, the calculation can be carried out with the new temperatures 𝑇′𝑖 defined as 𝑇′𝑖 = 𝑇𝑖 − 𝑇ref . then, (10) will be written as 𝑞 = 𝑇′ ∙ 𝑑̅̅ ̅̅ ̅̅ ̅ − �̅� ∙ 𝑇′̅ 𝑇′2̅̅ ̅̅ − 𝑇′̅2 , 𝑟 = �̅� − 𝑞 ∙ 𝑇′̅ . (11) in a special case in which the reference temperature 𝑇ref equals the mean temperature �̅�, the mean of the new temperatures becomes zero: 𝑇′̅ = 1 𝑁 ∙ ∑ 𝑇𝑖 𝑁 𝑖=1 − �̅� = �̅� − �̅� = 0 . (12) in this case, (11) is simplified to 𝑞 = 𝑇′ ∙ 𝑑̅̅ ̅̅ ̅̅ ̅ 𝑇′2̅̅ ̅̅ , 𝑟 = �̅� . (13) it must be noted that the single temperature points may change in different measurement campaigns; for example, a new value of 24 °c may be defined instead of or in addition to 23 °c. for better comparability of results, it could be beneficial to define the number of temperature points for all such investigations in advance. the single points may then be chosen in such a way that their mean equals the reference temperature agreed. although it may be difficult to reproduce all single temperatures very accurately, the remaining deviations of 0.1 … 0.2 k should be small enough to be neglected. with (7) and (8), equations (13) can now be rewritten as 𝑞 = ∑ 𝑇′𝑖 ∙ 𝑑𝑖 𝑁 𝑖=1 ∑ 𝑇′𝑖 2 𝑁 𝑖=1 ⁄ , 𝑟 = 1 𝑁 ∙ ∑ 𝑑𝑖 𝑁 𝑖=1 . (14) depending on the reference temperature chosen, (11), respectively (14), are the model functions to which the gum should be applied to find the uncertainties 𝑢(𝑞) and 𝑢(𝑟). here, the aim is to find the regression function �̃�(𝑇) and the uncertainties of the fitted values, preferably also given by a function 𝑢(�̃�). in this work, the more common approach (11) was used because the reference temperature 𝑇ref did not match the mean of the temperatures 𝑇𝑖. 5. results the calculations were carried out under the assumption that no correlations existed between the input values 𝑇′𝑖 , 𝑑𝑖 , which were treated as uncorrelated quantities. the equations (11) for the determination of 𝑞 and 𝑟 are quite simple, whereas those for the determination of 𝑢(𝑞) and 𝑢(𝑟) are rather complex. following the gum procedure, partial derivatives to each of the ten input variables must be calculated. maxima, a computer algebra system [5], was used to carry out this calculation analytically, yielding terms with several hundred parts as the result for the uncertainties. although this software could have been used to calculate the complete result 𝑞, 𝑢(𝑞), 𝑟, 𝑢(𝑟) for all the different transducers, this would have been time-consuming. a better way was to obtain the analytical result in maxima and to use this formula in excel. the known formula was then applied as a spreadsheet function in all subsequent calculations with the same input data scheme (five pairs of values as in table 1). http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 410 the results show that, if a temperature outside the interval of interest is taken as zero, the uncertainty of the intercept 𝑟 will be larger than it would be if a value from the given interval were taken as a reference temperature 𝑇ref, see table 2. moreover, the uncertainty of the intercept is minimal if the reference temperature chosen is the arithmetic mean of the temperature values 𝑇𝑖. table 2: values and associated standard uncertainties of the slope and intercept of a linear fit calculated for different reference temperatures 𝑻𝐫𝐞𝐟 in °c 𝒒, 𝒖(𝒒) in (mv/v)/k 𝒓, 𝒖(𝒓) in mv/v -273.15 -3.699 · 10-6, 0.798 · 10-6 0.802 080, 0.000 236 0.00 -3.699 · 10-6, 0.798 · 10-6 0.801 070, 0.000 017 8 20.50 -3.699 · 10-6, 0.798 · 10-6 0.800 994, 0.000 001 9 22.20 -3.699 · 10-6, 0.798 · 10-6 0.800 988, 0.000 001 4 24.00 -3.699 · 10-6, 0.798 · 10-6 0.800 981, 0.000 002 0 the uncertainty of the slope 𝑞 is not affected by the reference temperature selected; due to the linear function (1), its contribution to the uncertainty of the fitted value �̃�(𝑇′𝑖 ) is calculated with the temperature 𝑇′𝑖 as a sensitivity coefficient. this means that the uncertainty contribution of the slope will be lower the closer the reference temperature 𝑇ref and the temperatures 𝑇𝑖 are to each other. by means of the known coefficients, the regression function �̃�(𝑇) can be calculated. the known standard uncertainties of the coefficients also allow other functions to be determined – namely, functions defining a 1-σ band along the approximation function. expanded uncertainties with 𝑘 = 2 yield a 2-σ band. figure 3 and figure 4 show these results for the given 50 kn transducer at the 20 kn load step with the measured values (blue symbols) and with standard uncertainty bars for 𝑇 and 𝑑, the fit function (red line) and the 2-σ bands (dashed red lines). the standard uncertainty of the temperature was calculated from a rectangular distribution with a half-width of 0.1 k; the standard uncertainty of the deflection was 0.000 003 mv/v. the reference temperature was 20.5 °c. figure 5 shows how the results change if another reference temperature is chosen. if the reference temperature 𝑇ref is the arithmetic mean of the temperatures 𝑇𝑖 at which the investigation is carried out, the uncertainties become lower and their minimum value is at the reference temperature. figure 3: result of the regression analysis for the 20 kn force step of the 50 kn transducer (details see text above) figure 4: result of figure 3 when the uncertainty of the deflection is increased to 0.000 005 mv/v (top) and when the half-width of the temperature distribution is increased to 0.5 k (bottom) figure 5: result of figure 3 when the arithmetic mean of the temperature values 𝑇𝑖 (22.196 k) is taken as the reference temperature 𝑇ref http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 411 the results and figures shown above are an example of a transducer with a very linear deflection dependency on the temperature; the relative deviation between rss and ssd is 0.997. in addition, the slope value of -3.7 · 10-6 (mv/v)/k at a 0.8 mv/v signal is very low and a good value for a transfer transducer. the results for the other transducers are shown as examples in figure 6 to figure 8. figure 6: result of the 20 kn transducer at a force of 20 kn and a reference temperature of 20.5 °c (further details as in figure 3) figure 7: result of the 100 kn transducer at a force of 50 kn and a reference temperature of 20.5 °c (further details as in figure 3) figure 8: result of the 200 kn transducer at a force of 200 kn and a reference temperature of 20.5 °c (further details as in figure 3) the 20 kn transducer (figure 6) has a slope of -3 · 10-5 (mv/v)/k at 2 mv/v (20 kn force). the absolute value is larger than that of the 50 kn transducer. the linearity of the measured values is not as perfect as that of the 50 kn transducer in figure 3; the relative deviation between rss and ssd is 0.970. a special feature of the 100 kn transducer (figure 7) is the positive slope of the deflection/temperature function, whereas the 20 kn and 50 kn transducers have a negative slope. the absolute value of the slope is slightly larger than that of the 20 kn transducer and amounts to 2.5 · 10-5 (mv/v)/k at 1 mv/v (50 kn force) and 3.9 · 10-5 (mv/v)/k at 2 mv/v (100 kn force). apart from the 23 °c result, the values are very linear. although the reason for the deviation of the 23 °c result has not yet been found, this single value had no significant effect on the fit function, as can be seen in figure 7. on the other hand, this result indicates that temperature coefficients should not be determined from measurements at only two different temperatures. finally, the deflection of the 200 kn transducer (figure 8) did not show good temperature sensitivity. the variations of the deflection appear to be random. on the other hand, the overall relative span of the deflection values in the temperature range measured is 21 … 24 ppm; this is comparable with the corresponding value of the 50 kn transducer, which showed the lowest temperature sensitivity. nevertheless, the behaviour of the 200 kn transducer should be investigated further. the results obtained to date would not be sufficient to calculate the corrections. the last step in these calculations is the determination of the function 𝑢(�̃�), which describes the standard uncertainties associated with the fit function �̃�. this can be realised by applying higher-order regression methods such as cubic regression to the uncertainty values calculated, see figure 9. 6. application the temperature influence measured on a single transducer from different measurement campaigns or even on different transducers can be compared by determining (as the final step) the regression function �̃�(𝑇) and the associated uncertainty 𝑢(�̃�) = 𝑢 (�̃�(𝑇)) = 𝑢(𝑇) . for the example in figure 3 with 𝑇ref = 20.5 °c , we obtained the following functions (𝑇 in °c) �̃�(𝑇) mv v⁄ = −3.7 ∙ 10−6 ∙ 𝑇 °c + 0.801070 , 𝑢(𝑇) mv v⁄ = −1.45 ∙ 10−8 ∙ ( 𝑇 °c ) 3 + 1.061 ∙ 10−6 ∙ ( 𝑇 °c ) 2 − 2.524 ∙ 10−5 ∙ 𝑇 °c + 0.0001981 . (15) http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 412 figure 9: standard uncertainty (1 σ) part of the result from figure 3, third-order regression function 𝑢(�̃�) (dashed line) based on the calculated values 𝑢(�̃�𝑖 ) = 𝑢 (�̃�(𝑇𝑖 )) (blue dots) for reference temperatures of 20.5 °c (top) and 22.196 °c (bottom) however, in most cases, the results will be applied in order to correct comparison measurement results 𝑑, 𝑢(𝑑) obtained at deviating temperatures 𝑇. usually, the corrected values �̂� are calculated in accordance with �̂�(𝑇ref) = 𝑑(𝑇) + 𝑞 ∙ ∆𝑇, ∆𝑇 = 𝑇ref − 𝑇 (16) where the intercept 𝑟 does not appear. when a measurement result is corrected, the regression function is defined in such a way that it runs through the uncorrected result, meaning that the intercept is no longer a free parameter. nevertheless, the regression function �̃�(𝑇) has an associated uncertainty based on the uncertainties of the slope 𝑢(𝑞) and the intercept 𝑢(𝑟). the formal application of the gum to (16) would yield a result that contains no contribution of 𝑢(𝑟) 𝑢2(�̂�) = 𝑢2(𝑑) + 𝑞2 ∙ 𝑢2(𝑇) + (𝑇ref − 𝑇) 2𝑢2(𝑞). (17) therefore, another model is proposed instead of (16) – namely, �̂�(𝑇ref) = 𝑑(𝑇) + ∆�̃�(𝑇ref − 𝑇) 𝑢2(�̂�) = 𝑢2(𝑑) + 𝑢2(∆�̃�) , (18) where ∆�̃�(𝑇ref − 𝑇) = �̃�(𝑇ref) − �̃�(𝑇) 𝑢2(∆�̃�) = 𝑢2 (�̃�(𝑇ref)) + 𝑢 2 (�̃�(𝑇)) . (19) this means that, instead of forcing the regression function to run through the uncorrected result, a correction value ∆�̃� is added to this result. this value can be calculated from the increase (or decrease for a negative slope) of the regression function related to the temperature deviation 𝑇ref − 𝑇 . this increase (or decrease) of the function is uncertain and has a value 𝑢(∆�̃�). the uncertainties calculated according to (19) are larger than those yielded by (18), an effect caused mainly by the uncertainty of the intercept. for the example in figure 3, the standard uncertainty of the corrected deflection (∆𝑇 = 0.5 k) in accordance with (18) is 3.03 · 10-6 mv/v, whereas the same value calculated in accordance with (19) amounts to 4.08 · 10-6 mv/v. 7. summary the application of standard uncertainty calculation methods to the linear regression of measurement results using the least-squares approach was investigated. the method proposed can be used to calculate the uncertainty of a linear regression function based on the uncertainty of its slope and its intercept. the method can be applied to the correction of measurement results obtained at deviating temperatures but is not limited to force transducers: it can also be applied, for example, to torque or pressure transducers. 8. acknowledgements this work is part of the project 18sib08, funded by the empir programme. i would like to thank my colleague norbert tetzlaff for his accurate measurements and calibrations in the 200 kn force standard machine at ptb. 9. references [1] hbm: data sheet of the z30a force transducers. online [accessed 20200108]: https://www.hbm.com/fileadmin/mediapool/hbmd oc/technical/b02075.pdf (registration necessary) [2] gtm: data sheet of the ktn-d force transducers. online [accessed 20200108]: https://www.gtmgmbh.com/fileadmin/media/dokumente/produkte/ datenblaetter/de/datenblatt_serie_ktnd_20170419.pdf [3] r. kumme, h. kahmann, f. tegtmeier, n. tetzlaff, d. röske, ptb’s new 200 kn deadweight force standard machine, proc. of the imeko 23rd tc3, 13th tc5 and 4th tc22 international conference, 30 may to 1 june 2017, helsinki, finland. online [accessed 20200108]: http://www.imeko.org/ https://www.hbm.com/fileadmin/mediapool/hbmdoc/technical/b02075.pdf https://www.hbm.com/fileadmin/mediapool/hbmdoc/technical/b02075.pdf https://www.gtm-gmbh.com/fileadmin/media/dokumente/produkte/datenblaetter/de/datenblatt_serie_ktn-d_20170419.pdf https://www.gtm-gmbh.com/fileadmin/media/dokumente/produkte/datenblaetter/de/datenblatt_serie_ktn-d_20170419.pdf https://www.gtm-gmbh.com/fileadmin/media/dokumente/produkte/datenblaetter/de/datenblatt_serie_ktn-d_20170419.pdf https://www.gtm-gmbh.com/fileadmin/media/dokumente/produkte/datenblaetter/de/datenblatt_serie_ktn-d_20170419.pdf acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 413 https://www.imeko.org/publications/tc32017/imeko-tc3-2017-032.pdf [4] iso/iec guide 98-3:2008, uncertainty of measurement part 3: guide to the expression of uncertainty in measurement (gum:1995). online [accessed 20200108]: https://www.iso.org/standard/50461.html pdf version: https://www.bipm.org/utils/common/documents/jc gm/jcgm_100_2008_e.pdf [5] maxima, a computer algebra system. online [accessed 20200614]: http://maxima.sourceforge.net http://www.imeko.org/ https://www.imeko.org/publications/tc3-2017/imeko-tc3-2017-032.pdf https://www.imeko.org/publications/tc3-2017/imeko-tc3-2017-032.pdf https://www.iso.org/standard/50461.html https://www.iso.org/standard/50461.html https://www.bipm.org/utils/common/documents/jcgm/jcgm_100_2008_e.pdf https://www.bipm.org/utils/common/documents/jcgm/jcgm_100_2008_e.pdf http://maxima.sourceforge.net/ introductory notes for the acta imeko special issue on the 23rd international symposium on measurement and control in robotics organized by tc17 acta imeko issn: 2221-870x september 2021, volume 10, number 3, 1 2 acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 1 introductory notes for the acta imeko special issue on the 23rd international symposium on measurement and control in robotics organised by tc17 bálint kiss1, istván harmati1 1 budapest university of technology and economics, műegyetem rkp. 3., 1111 budapest, hungary section: editorial citation: bálint kiss, istván harmati, introductory notes for the acta imeko special issue on the 23rd international symposium on measurement and control in robotics organized by tc17, acta imeko, vol. 10, no. 3, article 1, september 2021, identifier: imeko-acta-10 (2021)-03-01 editor: francesco lamonaca, university of calabria, italy received september 1, 2021; in final form september 27, 2021; published september 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: bálint kiss, e-mail: bkiss@iit.bme.hu dear readers, measurement and control techniques are crucial for achieving reliable and safe autonomous features in robotics. recent developments in both fields are key enablers for the constantly widening use of robots in industrial, medical, military and service-oriented applications. faithful to its traditions, the 23rd edition of the international symposium on measurement and control in robotics (ismcr), organised by imeko technical committee 17, has provided a forum for the exchange of the latest research results and novel ideas in robotic technologies and applications, this time with a special emphasis on smart mobility. the symposium focused on various aspects of research, applications and trends in relation to robotics, advanced human–robot systems and applied technologies in the fields of robotics, telerobotics, autonomous vehicles and simulator platforms, as well as vr/ar and 3d modelling and simulation. the symposium was hosted by the budapest university of technology and economics in budapest, hungary. due to the covid-19 pandemic, the symposium was held in a hybrid format; authors outside hungary participated remotely, while those in hungary had the choice between online and in-person attendance at the event in accordance with the current regulations. a total of 49 submissions were received from 11 different countries. the review process, involving 106 external reviews, resulted in 40 accepted papers. a special technical session was devoted to the topic of robotised intervention in risky (chemical, biological, radiological and nuclear) environments. in accordance with the symposium’s main topics, three invited plenary lectures were given by specialists from the industry (kuka robotics, thyssenkrupp components technology) and academia. topics included the virtualised stability analysis of mechatronic systems, human–robot collaboration in industrial production and new standardisation trends in the navigation of industrial mobile robots. based on their technical and scientific value and the evaluation of the reviewers, the authors of ten contributions were invited to submit extended versions of their papers for this special issue. the paper entitled ‘vision-based reinforcement learning for lane-tracking control’, authored by kalapos et al., applies aibased techniques to solve the lane-following and obstacle avoidance problem of autonomous vehicles, successfully implementing the results in the onboard computers of reducedsized testbed vehicles. staying with autonomous vehicles, in the paper ‘using coverage path planning methods for car park exploration’ by ádám et al. exploration methods to find the optimal traversal of an unknown parking area to identify free parking spaces are presented. reinforcement learning can also be used in the control of multi-agent robotic systems, as suggested by the paper by paczolay entitled ‘a2cm: a new multi-agent algorithm’, which presents an optimised and modified version of the so-called synchronous actor–critic algorithm. de cubber et al. address a similar optimisation problem in their contribution entitled ‘distributed coverage optimisation for a fleet of unmanned maritime systems’. the authors propose a methodology that optimises the coverage of a fleet of unmanned maritime agents, thereby maximising the chances of identifying potential threats. high-level autonomous functions and human–robot collaboration must be reliably and safely supported by platforms (robotic arms, drones, vehicles, etc.); hence, a second group of mailto:bkiss@iit.bme.hu acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 2 papers is devoted to the presentation of related results. szabó et al. report an identification method for friction parameters in their paper entitled ‘dynamic parameter identification method for robotic arms with static friction modelling’. the author considered friction models which are linear in terms of unknown parameters. the paper entitled ‘uncertain estimation-based motion planning algorithms for mobile robots’, authored by gyenes et al., proposes the extension of two obstacle avoidance methods, the velocity obstacle technique and the artificial potential field method, to take into consideration the time-varying uncertainty of the measured data in relation to the localisation of static and dynamic obstacles. the autonomous delivery of items by drones requires suitable gripping devices and grasping strategies. in their paper entitled ‘a lightweight magnetic gripper for a delivery aerial vehicle: design and applications’, sutera et al. report the design of a lowpower and lightweight magnetic gripper that takes into consideration the size and weight of the of transported objects. füchter et al. studied the possibilities of using ar techniques in specific phases of pilot training. their paper entitled ‘aeronautic pilot training and augmented reality’ reports the design experience of a mobile/tablet application prototype that reproduces the flight panel of a cessna 150 aircraft. the paper entitled ‘human–robot collision predictor for flexible assembly’ by paniti et al. presents a prediction-based collision warning system for a cobot scenario in which a robotic arm and human operators share a common workspace that also takes communication delays into consideration. another type of human–robot interaction is the user interface for controlling a robotic arm. the paper entitled ‘a 3d head pointer: a manipulation method that enables spatial position and posture for supernumerary robotic limbs’ by oh et al. addresses the specific problem of controlling a wearable robotic arm using face orientation and head motion. it should be noted that this contribution received the best paper award of the symposium. we would like to express our gratitude to all the authors for their contributions and their participation at the ismcr2021 symposium despite the unprecedented conditions created by the pandemic. we must also thank prof. francesco lamonaca, editor-in-chief of acta imeko, and his team for their help and support during the editorial process of this special issue. it has been a great honour to serve as guest editors for this special issue, and we hope that the papers will inspire future research in the imeko tc17 area of expertise and beyond. bálint kiss, istván harmati guest editors is our understanding of measurement evolving? acta imeko issn: 2221-870x december 2021, volume 10, number 4, 209 213 acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 209 is our understanding of measurement evolving? luca mari1 1 università cattaneo liuc, c.so matteotti, 22, 21053 castellanza (va), italy section: research paper keywords: foundations of measurement; measurement and quantification; measurement as empirical process; measurement as representation citation: luca mari, is our understanding of measurement evolving?, acta imeko, vol. 10, no. 4, article 32, december 2021, identifier: imeko-acta10 (2021)-04-32 section editor: francesco lamonaca, university of calabria, italy received october 1, 2021; in final form november 20, 2021; published december 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: luca mari, e-mail: lmari@liuc.it 1. introduction the terminology about and around measurement is often not so specific, and sometimes even a bit sloppy. for sure, a long tradition allows us to assume a reasonably common understanding of a phrase like “to measure the length (or the mass, or…) of a given physical body”. but the claim that for example thermal comfort (as in [1]) is a measurable property is not as obviously meant in the same way by all relevant stakeholders. do different experts refer to the same sort of situations when they talk about the measurement of thermal comfort? what do they mean when they use instead phrases like “determination of thermal comfort”, “assessment of thermal comfort”, “quantification of thermal comfort”, “assignment of a value to thermal comfort”? and what are the conditions that make the determination, or the assessment, or… of thermal comfort a measurement? advancements of science and technology are not driven by terminological works, and therefore this kind of questions could be dismissed as immaterial if our goals are scientific or technological. admittedly, indeed, clearer ideas about the meaning of a term – like “measurement”, or “measurand”, or “measurement uncertainty”, and so on – do not improve our ability to design measuring instruments and perform measurements. nevertheless, metrology (in the broad sense given by the international vocabulary of metrology (vim: [2]): “science of measurement and its application”, thus in principle not limiting it to physical quantities) is a very special body of knowledge, and this peculiarity suggests that terminology might be more important in metrology than in other experimental fields like physics and chemistry. it is a fact that some key contents of metrology derive at least in part from social understanding and agreement, and not only from the outcomes of observation and experimentation. an obvious example is the identification of an individual quantity as the unit for a given kind, like the kilogram for mass: there can be empirical criteria to be taken into account, but the selection is ultimately conventional, and as such not falsifiable, in popper’s sense [3]. in fact, any discipline is grounded on some presuppositions and conventions that are chosen because they are mutually consistent, simple, elegant, …, and not because they are true. the peculiarity of metrology is that this applies to a substantial part of its body of knowledge: even though measurement is an experimental process (the idea that a measurement can be a gedankenexperiment, a thought experiment, sounds strange), it is as if the foundational role that metrology plays for all empirical sciences prevents it to obtain its own foundations somewhere else. metrology is a foundation without a foundation [4]. while this seems to be a structurally obvious situation – if xi is founded on xi–1 that is founded on xi–2 that…, then the sequence must stop where an x0 has no foundations –, acknowledging that metrology has sometimes the delicate role of x0 could generate an embarrassing doubt: isn’t metrology a “real” science then? and however, how can we forget that what is possibly the “most foundational” component of the metrology of the last 150 years is the metre convention, i.e., first of all a political treaty? or as another example, closer to us in time, consider the interesting discussion about base quantities, abstract traditionally understood as a quantitative empirical process, in the last decades measurement has been reconsidered in its aims, scope, and structure, so that the basic questions are again important: what kind of knowledge do we obtain from a measurement? what is the source of the acknowledged special efficacy of measurement? a preliminary analysis is proposed here from an evolutionary perspective. mailto:lmari@liuc.it acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 210 triggered by the 2019 revision of the definitions of some units given in terms of numerical values assigned to some quantities modeled as constants (the speed of light in vacuum etc). does this revision imply that the very idea of base quantity is now unjustified? or that base quantities should be those of the defining constants (hence speed, action, etc)? or finally that nothing should be changed on this matter (and therefore that length, mass, etc should remain the base quantities)? the question has in fact some importance, but no experiments can be designed to provide an answer: again, it is a matter of shared understanding of the pros and the cons, and then of agreement. this solicits us to reconsider the role of terminology in metrology: where social agreement plays a key role, and when disagreements cannot be settled by seeking the truth, welldefined concepts and well-chosen terms – this is what terminological works aim at – may be useful, if not indispensable. moreover, the fact that measurement is a fundamental enabler not only of top science, but also of technology, commerce, health care, environment protection, etc, adds a further reason to the special importance of terminology in metrology: again, this role requires shared concepts and terms, on which shared knowledge – like the one that guarantees the metrological traceability of measurement results – can be grounded. metrology is a social body of knowledge. basically everything that has been considered so far applies also and particularly to the very starting point of metrology: what is measurement? how should ‘measurement’ be defined? (someone might object that the starting point of metrology is the definition of ‘quantity’, not the one of ‘measurement’, perhaps by referring to the fact that the first entry of the first chapter of the vim is about ’quantity’; i respectfully disagree: ‘quantity’, like ‘property’, is a pre-metrological concept). in fact, there is nothing new in discussions about the scope of measurement and the terminological endeavor of providing an appropriate definition of ‘measurement’: the sometimes also harsh clashes about the measurability of psychological properties (i.e., the issue of whether psychometrics is actually about measurement) highlight that on the table there is way more than a dictionary issue. the distinction between, say, “my opinion on the competence of the candidate is…” and “the result of the measurement of the competence of the candidate is…” is not only about the occurrence of the term “measurement” in the second sentence, and in fact the position that psychological properties can be measured has been under scrutiny for decades [5]. rather, a fundamental question is at stake here: what kind of knowledge do we obtain from a measurement? and also, given that “measurement is often considered a hallmark of the scientific enterprise and a privileged source of knowledge” [6]: “what [is] the source of [the] special efficacy” of measurement?” [7]. this is the subject to which the present paper is devoted. 2. the received position: measurement as a coin with a euclidean side and a galilean side it is remarkable that the answer to these questions is also historically ambiguous, with two main lines that have been developed [8]. on the one hand, the euclidean tradition emphasizes the quantitative nature of measures, where in this sense “measurable” is basically synonym of “divisible by ratio”, as clearly explained for example by de morgan: “the term ‘measure’ is used [by euclid] conversely to ‘multiple’; hence [if] a and b have a common measure [they] are said to be commensurable” [9]. hence, this concept of measure applies first of all to numbers: “a measure of a number is any number that divides it, without leaving a reminder. so, 2 is a measure of 4, of 8, etc” [10], as in fact stated by euclid himself: “a number is part of a(nother) number, the lesser of the greater, when it measures the greater” (euclid). there is nothing necessarily empirical in this concept of measure, and in fact “in the geometrical constructions employed in the elements [...] empirical proofs by means of measurement are strictly forbidden” ([11]; in the introductory notes to the translation of euclid’s elements). on the other hand, the galilean tradition emphasizes the empirical nature of measurement, where before galileo “no one ever sought to get beyond the practical uses of number, weight, measure in the imprecision of everyday life” [12]. the tight connection between instrumentation and measurement witnesses the acknowledged role of measurement as a key enabler of the experimental method: a measurement is the process performed by making a physical device, i.e., a measuring instrument, interact with an empirical object according to the instructions provided by a measurement procedure. being quantitative and being empirical are orthogonal conditions: there are quantitative empirical processes, but a process may be quantitative and not empirical, or empirical and not quantitative. this means that in principle a process that is not a (galilean) measurement might produce a (euclidean) measure, and a process that is a (galilean) measurement might not produce a (euclidean) measure (such a lexical peculiarity was already highlighted by bunge [13], who discussed the difference between ‘measure’ and ‘measurement’; this becomes further clear by comparing the scopes of measure theory and a measurement theory). however, historically galileo came later, and drew from euclid and his interpretations, so that the principled independence became a factual convergence: (galilean) measurement was assumed to be a quantitative process, that produces (euclidean) measures. the lexicon maintains some traces of the coexistence of these two standpoints and of the interest of endorsing both, as witnessed in particular by the expression “weights and measures”, as if weighing were not a way to measure. indeed, euclid was concerned with geometric quantities, so that in the euclidean tradition measurable (or: “mensurable”, as it was said) were considered geometric quantities: according to hutton, “mensuration” is “the act, or art, of measuring figured extensions and bodies, or of finding the dimensions and contents of bodies, both superficial and solid” [10], so that for example in the case of temperature he used the term “observation” (this shows that the scope of measurement already broadened in the past!). plausibly, the synthesis of the euclidean and the galilean standpoints led to the idea that only extensive (taken from euclid) physical (taken from galileo) quantities are measurable, at least in the fundamental sense specified by campbell [14]. with some simplification, we may then summarize that the received view about 100 years ago was about measurement as characterized by two complementary components: measurement as a coin with a euclidean side and a galilean side. such a position is so strict – it is the outcome of the intersection of two independent standpoints, and thus inherits two sets of constraints – that not surprisingly trying to overcome it has been a target for several decades. in this perspective, the well known report of the ferguson committee to the british association for the advancement of science, published in 1940 [15], stating that “the main point against the measurability of the acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 211 intensity of a sensation was the impossibility of satisfactorily defining an addition operation for it” [16], can be read as a move to defend the orthodoxy of the synthesis of the euclidean and the galilean sides of the coin. 3. rethinking the received position in the last century the assumptions that measurement is quantification (the euclidean side) and is about (geometric) physical properties only (the galilean side) have been reanalysed, apparently by asking if really both such requirements need to be fulfilled, and to what extent. in particular, from stevens’ theory of types of scales [17] representationalism [18] explored how to broaden the euclidean side, sometimes by simply dropping any reference to the galilean side. in this sense we can read statements claiming that “the theory of measurement is difficult enough without bringing in the theory of making measurements” [19], or that a representation theorem “makes the theory of finite weak orderings a theory of measurement, because of its numerical representation” [20]. in this complex context, the definition given by the vim – “process of experimentally obtaining one or more quantity values that can reasonably be attributed to a quantity” [2] – is still quite conservative, with both the euclidean side (“quantity”) and the galilean side (“experimental”) explicitly maintained. is it sufficient for our society, that requires criteria of trust about determining / assessing / attributing values to … properties, like thermal comfort, that are not necessarily quantitative and are not entirely empirical? and is it sufficient for our society, in which the widespread digitalization is producing larger and larger amounts of data (the so-called “big data” phenomenon)? indeed, in an age of fake news and post-truth, providing criteria that make explicit and possibly operational the vim condition of “reasonable attribution”, and therefore such that not any data deserve to be called “measurement results”, seems to be a valuable achievement. in other words, our complex society would definitely benefit from an effective answer to kuhn’s question about the source of the special efficacy of measurement, while at the same time pushing toward reconsidering the actual necessity of euclidean and galilean conditions. and in fact conservative positions like vim’s are challenged today, so that measurement seems to have become a moving target. a significant and authoritative example is the standpoint of the us national institute of standards and technology’s simple guide for evaluating and expressing the uncertainty of nist measurement results, that defines ‘measurement’ as “an experimental or computational process that, by comparison with a standard, produces an estimate of the true value of a property of a material or virtual object or collection of objects, or of a process, event, or series of events, together with an evaluation of the uncertainty associated with that estimate, and intended for use in support of decision-making” [21]. with a mix of tradition (the reference to true values) and innovation (the evaluation of uncertainty as a necessary condition), here both euclidean and galilean conditions have been dropped: also non-quantitative properties are in principle measurable, and measurement can be also a non-empirical process about non-empirical (“virtual”) objects. is our understanding of what measurement is still evolving then, and in which direction(s)? 4. some possible evolutionary perspectives listing some necessary conditions that characterize measurement, and that plausibly are generally accepted, is not a hard task: measurement is (i) a process (ii) designed on purpose, (iii) whose input is a property of an object, and (iv) that produces information in the form of values of that property. indeed, (i) removes the ambiguity of using the term “measurements” also for the results of the process; (ii) highlights that measurement is not a natural process “that happens”; (iii) establishes that phrases like “to measure an object” are not correct, because measured are properties of objects, and not objects as such; (iv) characterizes measurement as an information process. however, not any such process is a measurement, thus acknowledging that not any data acquisition is a measurement. we may call “property evaluation” a process fulfilling (i)-(iv). what sufficient conditions characterize measurement as a specific kind of property evaluation? the answer does not seem as easy. the term “measurement” does not have a single inherent meaning and is not trademarked, so that nobody can be prevented from using it as she/he likes. nevertheless, without a common foundation, a foundational body of knowledge, as metrology is, is at risk of emptying, or at least of becoming unable to provide a convincing, socially agreeable, and useful answer to kuhn’s question: indeed, not any data acquisition process is claimed to have a “special efficacy”. as discussed above, the traditional, i.e., euclidean & galilean, answer to this question relies on coupling quantification and instrumentation: the assumption that only quantitative properties are measurable guarantees that measurement results are embedded in the nomological network generated by physical laws, from which specific, and then falsifiable, predictions and inferences can be drawn and the hypothesis of the correctness of the measurement results can be corroborated in turn; and the requirement that measuring instruments are empirical devices guarantees that the degree of objectivity of measurement results can be assessed by analysing how such instruments behave. this is the safe starting point. but if both such sides are erased, what remains of the coin? while sticking to the tradition is safe, it might be too strict with respect to what our society needs, as the definition of ‘measurement’ given by the nist seems to suggest. this is a key challenge for metrology, whose solution is then a matter of social responsibility, not truth seeking. in this context, in which respect of the tradition and new societal needs must be balanced, the possible evolutionary perspectives of measurement can be considered along four main complementary, though somewhat mutually connected, dimensions: – measurable entities as quantitative or non-quantitative properties (i.e., the reference to the euclidean tradition); – measurable entities as physical or non-physical properties; – measuring instruments as technological devices or human beings; – measurement as an empirical or an informational process, and therefore the relation between measurement and computation (i.e., the reference to the galilean tradition). again with the explicit admission that at stake here is adequacy, not truth, let us shortly discuss each of these issues. 4.1. measurable entities as quantitative or non-quantitative properties as stevens’ theory of scale types shows, the distinction between quantitative and non-quantitative properties is not acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 212 binary. the strongest type is absolute: an absolute evaluation is additive and has both a natural unit and a natural zero, as is the case of counting. the weakest type is nominal: a nominal evaluation only classifies objects by property. several intermediate types exist between absolute and nominal (e.g., ratio, interval, and ordinal, in the initial version of stevens’ theory), and there is not a single objective criterion to decide where a property stops to be quantitative and becomes nonquantitative. for example, according to the vim a total order is sufficient for a property to be a quantity, whereas the axiomatic approaches developed from holder’s [22] consider “continuity as a feature of the scientific concept of quantity”. the connection between being quantitative and being measurable inherits this ambiguity [23]. for sure, evaluations performed in richer scales convey more structural information, but this does not seem a sufficient criterion to rule out any given type from the scope of measurement: only by convention the decision can be made whether nominal (or ordinal, or...) evaluations can be measurements (and what conditions are required to make quantitative a property, for what it is worth). 4.2. measurable entities as physical or non-physical properties independently of the scale type, the condition of being measurable could be connected to the nature of the considered properties, and in particular to their being physical. a plausible, good justification is the requirement to assess, and possibly to control, the degree of objectivity of the behaviour of the measuring instrument. indeed, this is guaranteed by the operation of a physical sensor at the core of the instrument, where only a physical (or chemical, or biological) property may affect the transduction performed by the sensor. however, a non-physical property, like thermal comfort, may be evaluated as a function of one or more physical properties, such as air temperature and humidity, in a process that is structurally the same as those traditionally considered to be indirect measurements, and where values of such physical properties can be then obtained by means of sensors. the key difference between the evaluation of, say, thermal comfort and density – the latter being a case of indirect measurement through the measurement of mass and volume – is that non-physical properties miss the rich nomological network provided by physics, so that their combination function is not as substantially validated. whether this is sufficient to rule out non-physical properties from the scope of measurement seems to be again a matter of convention. 4.3. measuring instruments as technological devices or human beings complementary to the option that also non-physical properties are measurable, some evaluations directly performed by human beings could be accepted as measurements. the relatively long history of what has been considered psychophysical measurement shows that there is nothing really new in this. there are in fact three strategies to develop humanbased instruments that attribute values to (physical or, more usually, non-physical) properties. first, the behaviour of a “typical” individual is idealized in a model, like in the case of the luminosity function, that describes the sensitivity of a “standard” human eye to different wavelengths. second, a statistic of the behaviour of a set of human beings (e.g., their average response) is considered, under the assumption that individual peculiarities are compensated in a sufficiently large sample, like when the quality of a product or a service is evaluated by synthesizing the responses given by several customers. third, an individual or a small group of individuals operates, under the condition that they are domain experts and therefore their evaluation can be considered to be calibrated against some agreed standards, like in the case of gymnastics judges and wine sommeliers. while at least some cases of the first strategy are widely accepted as measurements, as the inclusion of the candela as the si unit of luminous intensity witnesses, whether and under what conditions human beings can be measuring instruments, possibly operating with the support of guidelines, checklists, etc, is again a controversial issue. 4.4. measurement as an empirical or an informational process measurements are aimed at attributing values to properties: since values are information entities, any measurement must then include an informational component. rather, the issue here is whether there can be measurements that are entirely informational processes, with no empirical components at all (note that this is not the case of gymnastics judges and wine sommeliers mentioned above: they are expected to operate by (empirically) observing gym competitions and tasting wines). there are at least two cases at stake. one is about the evaluation of properties that are in turn informational, for example the number of lines of code in the source of a software program. as quoted in section 3, the “computational process” about a “virtual object” to which the nist definition refers could be such, and actually shares several structural features with the processes that are commonly accepted to be measurements. more controversial is instead the hypothesis to consider to be a measurement any computation performed on values of properties, like when one is asked to compute the acceleration that a given force would produce if applied to a body of a given mass. of course, the information on the force and the mass (the “input quantities”) could also include an uncertainty, that in this case should be somehow propagated to the acceleration, and someone could decide that this is sufficient to make such a computation a measurement, by then accepting that what has been propagated is a “measurement” uncertainty, through a “measurement” model. in an evolutionary situation, this is also a possible standpoint. 5. conclusions were we in charge of updating a definition of ‘measurement’, for example for a new edition of the vim, what would we propose, then? how tightly would we stick to the traditional conception of measurement as a quantitative empirical process? or what criterion would we adopt toward a different, and plausibly broader, characterization? conscious that there is not one right, or true, answer, and by taking for granted the necessary conditions listed at the beginning of section 4, i dare to suggest – as a working hypothesis – that the most fundamental and most characterizing task of measurement is to produce information that is explicitly and socially justifiable (it is also the conclusion reached in [8]). this is not related to the quality of the produced information nor to the scope of the process (as the vim states, measurement should be characterized “irrespective of the level of measurement uncertainty and irrespective of the field of application” [2]), but to the condition that a measurement is a “white box” process, so that the quality of its results – be it good or bad – can always in principle be justified. accordingly, the source of the special efficacy of measurement, as investigated by kuhn, is the possibility to reach a common understanding on how acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 213 trustworthy its results are, along the two key dimensions of the objectivity and the intersubjectivity of the provided information [24], [25]. this explains the strategic importance of some components of the metrology of physical quantities, like the widely agreed definition of measurement units and the condition of metrological traceability of measurement results to such units through the appropriate calibration of measuring instruments. whether and how a sufficient objectivity and intersubjectivity of the information produced by processes that aim at being acknowledged as measurements can be obtained: in the perspective we have proposed here, this is the key challenge for an evolutionary understanding of measurement. references [1] iso 7730:2005, ergonomics of the thermal environment – analytical determination and interpretation of thermal comfort using calculation of the pmv and ppd indices and local thermal comfort criteria, international organization for standardization, geneva, 2005. [2] jcgm 200:2012, international vocabulary of metrology – basic and general concepts and associated terms, 3rd ed., paris: joint committee for guides in metrology, 2012. online [accessed 15 december 2021] https://www.bipm.org/en/committees/jc/jcgm/publications. [3] k. popper, the logic of scientific discovery, routledge, abingdon-on-thames, 1959. [4] l. mari, the problem of foundations of measurement, measurement, 38(4) (2005) pp. 259-266. doi: 10.1016/ j.measurement..2005.09.006 [5] j. michell, measurement in psychology: a critical history of a methodological concept, cambridge university press, cambridge, 1999. [6] e. tal, measurement in science, in: the stanford encyclopedia of philosophy, e.n. zalta (ed.), 2020. online [accessed 15 december 2021] https://plato.stanford.edu/archives/fall2020/entries/measurem ent-science [7] t.s. kuhn, the function of measurement in modern physical science, isis, 52(2) (1961) pp. 161-193. [8] l. mari, m. wilson, a. maul, measurement across the sciences – developing a shared concept system for measurement, springer nature, 2021. online [accessed 15 december 2021] https://link.springer.com/book/10.1007%2f978-3-030-65558-7 [9] a. de morgan, the connection of number and magnitude: an attempt to explain the fifth book of euclid, taylor and walton, london, 1836. online [accessed 15 december 2021] https://archive.org/details/connexionofnumbe00demorich [10] c. hutton, a mathematical and philosophical dictionary, johnson, london (freely available on google books), 1795. [11] euclid’s elements of geometry, the greek text of j. l. heiberg (1883-1885) edited, and provided with a modern english translation, by richard fitzpatrick, 2008. online [accessed 15 december 2021] http://farside.ph.utexas.edu/books/euclid/euclid.html [12] a. koyré, du monde de l’à peu près à l’univers de la précision, in: a. koyré (ed.), etudes d’histoire de la pensée philosophique (pp. 341-362), gallimard, paris, 1948. [13] m. bunge, on confusing ‘measure’ with ‘measurement’ in the methodology of behavioral science, in: m. bunge (ed.), the methodological unity of science, d. reidel, dordrecht-holland, 1973. [14] n. r. campbell, physics: the elements, cambridge university press, cambridge, 1920. [15] a. ferguson, c. s. myers, r. j. bartlett, h. banister, f. c. bartlett, w. brown, w. s. tucker, final report of the committee appointed to consider and report upon the possibility of quantitative estimates of sensory events. report of the british association for the advancement of science, 2 (1940) pp. 331-349. [16] g. b. rossi, measurability, measurement, 40 (2007) pp. 545-562. doi: 10.1016/j.measurement.2007.02.003 [17] s. s. stevens, on the theory of scales of measurement, science, 103(2684) (1946) pp. 677-680. [18] d. h. krantz, r. d. luce, p. suppes, a. tversky, foundations of measurement, vol. 1: additive and polynomial representations, academic press, new york, 1971. [19] h.e. kyburg, theory and measurement, cambridge university press, cambridge, 1984. [20] p. suppes, representation and invariance of scientific structures, csli publications, stanford, 2002. [21] a. possolo, simple guide for evaluating and expressing the uncertainty of nist measurement results, technical note, national institute of standards and technology, gaithersburg, md, 2015. doi: 10.6028/nist.tn.1900 [22] o. holder, die axiome der quantität und die lehre vom mass. berichte über die verhandlungen der koeniglich sächsischen gesellschaft der wissenschaften zu leipzig, mathematischphysikaliche klasse, 53 (1901) pp. 1-46. part 1 translated in j. michell, c. ernst, the axioms of quantity and the theory of measurement, j. mathematical psychology, 40(3) (1996) pp. 235252. doi: 10.1006/jmps.1997.1178 [23] l. mari, a. maul, d. torres irribarra, m. wilson, quantities, quantification and the necessary and sufficient conditions for measurement, measurement, 100 (2016) pp. 115-121. doi: 10.1016/j.measurement.2016.12.050 [24] a. maul, l. mari, d. torres irribarra, m. wilson, the quality of measurement results in terms of the structural features of the measurement process, measurement, 116 (2018) pp. 611-620. doi: 10.1016/j.measurement.2017.08.046 [25] a. maul, l. mari, m. wilson, intersubjectivity of measurement across the sciences, measurement, 131 (2019) pp. 764-770. doi: 10.1016/ j.measurement..2018.08.068 https://www.bipm.org/en/committees/jc/jcgm/publications https://doi.org/10.1016/j.measurement.2005.09.006 https://plato.stanford.edu/archives/fall2020/entries/measurement-science https://plato.stanford.edu/archives/fall2020/entries/measurement-science https://link.springer.com/book/10.1007%2f978-3-030-65558-7 https://archive.org/details/connexionofnumbe00demorich http://farside.ph.utexas.edu/books/euclid/euclid.html http://dx.doi.org/10.1016/j.measurement.2007.02.003 https://doi.org/10.6028/nist.tn.1900 https://doi.org/10.1006/jmps.1997.1178 http://dx.doi.org/10.1016/j.measurement.2016.12.050 https://doi.org/10.1016/j.measurement.2017.08.046 https://doi.org/10.1016/j.measurement.2018.08.068 editorial summary to selected papers from the 2020 imeko tc19 international workshop on metrology for the sea “learning to measure sea health parameters” acta imeko issn: 2221-870x december 2021, volume 10, number 4, 3 4 acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 3 editorial summary to selected papers from the 2020 imeko tc19 international workshop on metrology for the sea “learning to measure sea health parameters” silvio del pizzo1 1 department of science and technologies, university od naples “parthenope”, centro direzionale isola c4, naples, italy section: editorial citation: silvio del pizzo, editorial summary to selected papers from the 2020 imeko tc19 international workshop on metrology for the sea “learning to measure sea health parameters”, acta imeko, vol. 10, no. 4, article 2, december 2021, identifier: imeko-acta-10 (2021)-04-02 received december 14, 2021; in final form december 14, 2021; published december 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: silvio del pizzo, e-mail: silvio.delpizzo@uniparthenope.it dear readers, this issue of acta imeko is dedicated to the works that have been selected from those presented at “2020 imeko tc19 international workshop on metrology for the sea”, shortly named metrosea. the edition 2020 was hosted by the university of naples “parthenope”, but unfortunately all sessions, discussions and other activities were carried out online due to the pandemic situation. the virtual workshop was organized to broadcast live all sessions according to the conference program, and the attendees were able to participate to all proposed activities by entering in virtual rooms. therefore, the online conference was not so different from a live event, with the great advantage that every session was recorded and shared with the participants. the sea is the medium that allows several human activities such as fishing, traveling, and the transporting of large quantities of goods using vessels. furthermore, its environment host an important and fragile ecosystem that represents a great reservoir and source of food for all living beings. at the same time, the sea has a fundamental role in the climate change, indeed the ocean circulation is a key mechanism of global climate by transporting and storing heat and fresh water around the world. in the last decades the environment in general, and specifically the marine one, has been compromised by several human activities which have undermined both the delicate equilibrium of the ecosystem and the ocean circulation system, activating a dangerous process that involves the extinction risk of some species of sea fish. moreover, several studies confirm that the extreme climate and meteorological events are strictly related to the modification of the global ocean circulation as well as to the global warming. this latter phenomenon entails the increasing of the global mean sea level that is worrying the population of the small islands. therefore, it is evident that the sea health is very important for the survival of all humanity. in this context, two concepts were born: the blue economy and the blue growth, both approaches referring to the use of seas in sustainable way. definitively, the sea is a complex environment that includes complex phenomena and assets; as everyone may know, measuring is a fundamental step that allows a deep knowledge of a phenomenon or an asset. the aim of the international workshop “metrology for the sea” is to support the sharing of the recent advances in the field of measurement and instrumentations to be applied for the increasing of knowledge, for protecting and preserving the sea and all assets and phenomena involved. indeed, every year the workshop involves hundreds of researchers and people who work in developing instrumentation and measurement methods for the sea activities. several research topics have been discussed during the edition 2020 of the workshop, such as: the electronic instrumentation, sensors and sensing systems, wireless sensor networks for marine applications, monitoring systems for sea activities, underwater navigation and submarine obstacle detection, pollution detection, measures for marine biology, marine geology, and oceanography. five selected papers from metrosea 2020 are presented in this special issue: one of these is a review paper that illustrates the state of the art and the future perspectives in the field of underwater wireless communications, while the other four papers concern experimental approaches, although applied in different fields. leccese and spagnolo in the paper titled ‘state-of-the art and perspectives of underwater optical wireless communications’ illustrate the state of art and future perspective of underwater wireless communication using optical waves. this approach is revolutionizing the underwater communications due to the achievable large bandwidth, and low latency levels. unfortunately, the communication distances are still limited. the authors provide a comprehensive overview of the state of art, mailto:silvio.delpizzo@uniparthenope.it acta imeko issn: 2221-870x december 2021, volume 10, number 4, 4 5 acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 4 highlighting which limits distress this technology and which are the possible future developments, especially in military field. it is very interesting their preliminary study to verify the feasibility of a simple, economical, and reliable communication system based on uv-a radiation. of course, the development of a low-cost underwater communication system with a high transmission rate could have a great impact on the world of the autonomous underwater vehicles. the scientific contribution presented by baldo et al., titled ‘remote video monitoring for offshore sea farms: reliability and availability evaluation and image quality assessment via laboratory tests’ describes a preliminary study for the implementation of a remote monitoring system based on video recording for an offshore sea farm. the aim of this work is to ensure a video surveillance infrastructure for supervising breeding cages along with the fish inside them, in order to contrast undesired phenomena like fish poaching as well as cages damages. since the sea farm are built in open sea, where the weather conditions significantly change from a season to another, the authors focused their inspections on the availability and the reliability of the designed monitoring system, performing several tests in laboratory through a climatic chamber. a specific section is dedicated to the system architecture, where the authors tackle the critical problem of the power supply system. moreover, it is very interesting that the designed project employs a hardware low-cost and open source such as raspberry pi. the other three research papers provide different studies related to the measurements in hydrography. these scientific contributions deal with three different aspects of the hydrographic process. specifically, the calibration of the echo sounder for acquiring the depth, the enhancement of the positioning system gnss (global navigation satellite system) and the management of the post processing data. the scientific contribution by amoroso and parente, ‘the importance of sound velocity determination for bathymetric survey’ presents an inspection on the importance of the sound velocity in water during a hydrographic acquisition. the paper took into consideration four different methodologies for modeling the speed of the sound in sea water. all these approaches need an accurate knowledge of the water density (function of temperature, pressure and salinity) at different depths. the authors reported the impact that inaccurate measurements of these three parameters has on the bathymetric survey results. the experimentation was conducted on real data collected by a hydrographic vessel, while the error propagation was conducted in simulated environment considering several systematic errors on the measurements of the three inspected parameters. this innovative methodology is very promising for the performed inspections. the paper wrote by baiocchi et al. titled ‘first considerations on post processing kinematic gnss data during a geophysical oceanographic cruise’ concerns the problem of the accurate positioning during a collection of the bathymetric, oceanographic, and geophysical data. the development of oceanographic and geophysical instrumentation, capable to acquire with a high spatial resolution, requires high accuracy positioning systems. in this work the authors carried out several tests on an oceanographic ship applying ppk (post process kinematic) approach to improve the position accuracy provided by the gnss. the large amount of data acquired by the authors, allowed to validate the performances of the proposed methodology. indeed, the authors compared the results obtained by ppk processing in the vertical domain with the data registered by several tide gauges located on the surrounding coast. in this work the authors tackled several problems concerning both the acquisition data phase and the different vertical datum used by the tide gauges considered. finally, alcaras et al. presented an interesting work on the post-elaboration of the bathymetric data titled ‘from electronic navigational chart data to sea-bottom models: kriging approaches for the bay of pozzuoli’. the electronic navigational chart is an electronic map realized by a national hydrographic office and can be used for navigational purposes. the enc data can be used to build a detailed bathymetric 3d model applying an interpolation method. the authors analyze the performance of different interpolation methods based on the ordinary and universal kriging applied to a specific case study: the bay of pozzuoli. hence, the authors employ 11 mathematical semi-variogram models for inspecting the perfomance of these interpolators. however, the cross-validation is used for evaluting the accuracy of each method. the research remarks the good performance of both the kriging approaches for hydrogrphic purposes and it demonstrates the relevance of the choice of the mathematical model to build the semi-variogram. i would like to conclude these introductory notes by thanking the authors for their interesting and valuable papers and the reviewers for their indispensable and qualified contributions. furthermore, i would like to thank the editor in chief, prof. francesco lamonaca, as his support has been fundamental for accomplishing this special issue. it was a great honour for me to act as guest editors for this special issue, and i believe that the readers will find this acta imeko issue useful and will be inspired by the themes and methodologies proposed, for continuing the innovation in metrology for the sea. silvio del pizzo section editor metrology in the early days of social sciences acta imeko issn: 2221-870x june 2023, volume 12, number 2, 1 6 acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 1 metrology in the early days of social sciences clara monteiro vieira1, elisabeth costa monteiro1 1 pontifical catholic university of rio de janeiro, marquês de são vicente, 225, gávea, rio de janeiro, brazil section: research paper keywords: metrology; social science and humanities; émile durkheim; max weber citation: clara monteiro vieira, elisabeth costa monteiro, metrology in the early days of social sciences, acta imeko, vol. 12, no. 2, article 16, june 2023, identifier: imeko-acta-12 (2023)-02-16 section editor: eric benoit, université savoie mont blanc, france received july 10, 2022; in final form march 31, 2023; published june 2023 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this study was supported in part by the coordenação de aperfeiçoamento de pessoal de nível superior – brasil (capes) – finance code 001. corresponding author: elisabeth costa monteiro, e-mail: beth@puc-rio.br 1. introduction providing metrological traceability of measurement results to the international system of units (si) is essential to ensure reliable and comparable quantity values in applications associated with all fields of knowledge. this aspect, however, has been a historical struggle since the early days, when efforts were directed to the elaboration of a metrological framework traditionally focused on promoting advances in the evolution of standards for measuring physical quantities. after the signing of the ‘convention du mètre’ (1875), the 1st ‘conference general de poids et mesures’ (cgpm), which took place in 1889, established international prototypes for physical quantities of length and mass units, respectively, meter and kilogram, also incorporating the second as the unit of time, according to astronomers’ definition [1]. the high complexity of chemical and biological measurements, which also involve quantities belonging to the field of natural sciences, only much more recently received better attention and contributions to meet their metrological infrastructure demands [2]-[5]. metrological authorities’ first initiatives toward meeting demands for chemical measurements took place with the adoption, in 1971, of the unit mole (symbol mol), for the quantity amount of substance, at the 14th cgpm, and the creation of the ‘comité consultatif pour la quantité de matière’ (ccqm), in 1993 [1]. in turn, measurements of biological quantities, which are particularly associated with even more challenging metrological demands, were addressed only at the 20th cgpm (1999) [2]-[4]. however, unlike what happened in the case of chemical quantities, the metrological demands associated with biomeasurements did not receive specific support by creating a particular consultative committee for the area. the responsibility for advancing the reliability of biomeasurements was absorbed by the ccqm, whose name was changed in 2014 to ‘consultative committee for amount of substance: metrology in chemistry and biology’ [3]. equally required and even more challenging is the global metrological framework to provide trustworthiness and comparability for measurements in humanities and social abstract recent studies have been endeavoring to overcome challenges to ensure reliable measurement results in social sciences and humanities facing the complex characteristics of this scientific field. however, the literature indicates that the founding designers of sociology as an academic discipline expressed concerns regarding social measurements more than a century ago. based on a literature review, the present work investigates possible metrological aspects already addressed in the early days of social science, focusing on the methodological conceptions of two of sociology’s early canons – notably max weber and emile durkheim. the present study reveals that the approaches contemporaneously developed by the two social sciences co-founders present diverse but fundamentally complementary configurations, allowing a wide range of social phenomena to be analyzable. although employing their terminologies, both social scientists incorporated fundamental metrological concepts in their procedures’ parameters, seeking to establish a single reference, using statistical analysis or determining measurement standards that resemble what is known today as reference material. the concern with applying metrological concepts since the early days of creating sociology as a science reinforces the need to invest extensive efforts to provide uniformity of measurements in this remarkably relevant field of application of measurement science. mailto:beth@puc-rio.br acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 2 sciences. nevertheless, this issue has not yet been addressed in cgpm resolutions. the sophistication of measurands associated with more complex areas involving chemical, biological, human and social measurements requires dealing with the development of certified reference materials, creation of arbitrary units, and other alternative strategies to step forward to a metrological structure capable of harmonizing “nonphysical” measurements in all aspects of daily life demands. particularly regarding human and social sciences, the influence of the subjective perceptions of researchers and research participants on the research process [6] and difficulties in defining concepts [7]-[9] are some of the elements of the complexity in the study of social phenomena. such intricacies hinder but do not prevent initiatives to ensure reliability and comparability of measurement results in the social sciences and humanities. recent studies have been endeavoring to meet the challenges associated with the complex characteristics of this scientific field [7]-[37]. among the current academic initiatives, it is worth mentioning the successful incorporation of measurements in social sciences among investigations addressed by the international measurement confederation (imeko) [36]-[40], being evidenced a massive effort of this scientific community to promote metrology in this field, including efforts to lead both physical and "nonphysical" measurement in a single, consistent concept system [34]-[37]. despite the apparent novelty of the actions that are currently emerging to incorporate the concepts of metrology in social sciences aiming at contributing to robust and comparable measurement results in this field of application; the literature indicates that the founding architects of this science as a formal discipline, notably max weber and émile durkheim, already expressed concerns about the adequacy of the approaches employed for measuring social phenomena [41], [42]. this paper explores the fundamental aspects of the measurement methods proposed more than a century ago by those two founding authors of sociology as a scientific field. moreover, the present article seeks to identify the possible connections between these preliminary sociological approaches and the current metrological conceptions. 2. analytical framework of social science founders playing relevant roles on the foundation of sociology as a scholarly discipline, both émile durkheim and max weber devoted a portion of their work to the development of a methodology for the study of social phenomena and were especially interested in reliable strategies and comparable results on social measures. they presented, however, quite different approaches. 2.1. émile durkheim émile durkheim (1858-1917, france) was the first to establish sociology as a formal academic discipline (university of bordeaux, 1895) [43]. influenced by the positivist current of thought, durkheim turned to the natural sciences – especially bioscience – when performing social science investigations [42], [44]. he thought of society as an organism, whose parts (or “organs”) need to function well together to ensure the whole’s healthy functioning [42], [44]. durkheim defined ‘social facts’ as his main object of study. ‘social facts’ would be ways of feeling, acting, and thinking identifiable by three main traits such as generality, being applied to all members of a given society; exteriority from each individual, once they were not created by any particular person’s consciousness, but learned by people, generation after generation, and lasting much longer than the human lifespan; and coercivity, by which individuals are constrained into specific actions, not necessarily in conformity to each person’s intention [42]. with a focus on analyzing social facts and their role in society, durkheim addressed the social phenomena from the macrolevel. just as it is impossible to capture what is going on in someone’s mind by looking at each cell of their nervous system, durkheim states that one wouldn’t be able to explain a social fact simply by looking at its manifestations in the individual level [42]. he emphasized, after all, that a whole is not just the sum of its parts, but a specific reality formed by their association [42]. therefore, in durkheim’s approach, social facts ought to be explained through other social facts [42]. with a marked tendency toward an empirical approach, durkheim used statistical strategies extensively. by increasing the number of cases whenever possible, the variable-oriented model of the comparative analysis performed by durkheim aims to establish generalized connections between variables [45]. the general patterns pursuit guided durkheim’s statistical approach to dealing with the time dimension from a transhistorical perspective [45]. collective behaviors are, then, identified as an average effect of a variable by searching for statistical regularities of social facts [45]. estimating the average effects of independent variables would allow investigating the ‘effects-of-causes’. therefore, with the emphasis on generalizations over details, durkheim establishes causality relationships, associating a phenomenon (social fact) to its cause or its effects (another social fact) [42]. for instance, in his famous study “le suicide: étude de sociologie” [46], performed with three religious’ communities (protestants, catholics, and jews), durkheim demonstrated that a social fact, the suicide rates, presented a statistical correlation with a macrolevel variable constituted by the degrees of social integration, as figure 1. diagram of correlating connections between macro-level variables to analyze causality associations with suicide rates in diverse contexts within the durkheim study. acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 3 illustrated in figure 1. the statistical analysis allowed durkheim, for example, to associate suicide rates to aspects of social context, whereas, contrary to what one might expect, there was no correlation with rates of psychopathology. 2.2. max weber max weber (1864–1920, germany) introduced, in 1919, a sociology department at the ludwig maximilians university of munich, in germany [47]. contrasting to durkheim’s objectivity, weber's approach prioritizes subjective interpretations of social events. these aspects are considered those that provide the underlying sense to the individual’s objective behaviors, explaining them. therefore, weber addressed the social phenomena from the micro-level, considering subjectivity and meanings attributed to social actions [41], [45], [48]-[50]. the approach included the development of the so-called ideal type. this theoretical construct consists of an abstract model with internal logic serving as a measuring standard for evaluating complex cases [45], [48], [51]. the strategy allows for understanding particular historical processes and individual motivations, considering as many variables as possible, and analyzing the kind of relationship among them by the concept of elective affinities, which refers to their mutual contributions [52]. therefore, an in-depth understanding of a complex unity is reached by a case-oriented comparison concentrating on a small number of cases, with a large number of attributes interacting within long-lasting processes [45]. as a result, the roots of a specific event must be rebuilt when performing qualitative investigations involving historical comparisons by weber’s case-oriented strategy. 3. metrology and the analytical framework of social sciences’ founders current proposals for making psychosocial and physical properties measurable, ensuring the quality of measurements associated with both physical and psychosocial properties, consider object-relatedness (objectivity) and subjectindependence (intersubjectivity) as essential attributes to be satisfied [34], [36]. objectivity refers to the connection between the information obtained and the measured property. this characteristic requires an appropriate theory of the property to make insignificant the definitional uncertainty and demands a reduced influence from other phenomena, which renders instrumental uncertainty negligible [35], [36]. for a uniform interpretation by different measurers, the measurement must be intersubjective, which depends on the metrological traceability of results to the same reference scale, if available. as described in [36], this quality dimension can be structured by developing item banks aiming at building reference scales associated with each of the properties, in combination with rasch model fitting [33]-[36]. the rasch model is an approach widely employed to measure latent traits in a variety of disciplines within humanities, social sciences and health [21]-[33], [36], [53], [54]. preceded by the studies developed throughout the 19th century by the german karl marx (1818-1883), also known as one of the founding creators of the social sciences, émile durkheim and max weber were the first to establish this field of research as a formal discipline. the scientific contributions of these two contemporary researchers emerged at the end of the 19th century, after the memorable signing of the intergovernmental treaty of the meter convention, which took place in paris in 1875 and established the bureau international des poids et mesures (bipm), an international organization in which the member states coordinate the harmonization and advances in measurement science and measurement standards. in the case of the french sociologist émile durkheim, this historical space may have paved the way for the interest in the quality of measurement evidenced in his work. weber's contributions to social sciences measurements, in turn, emerged after the creation, in berlin, of the first national metrology institute in 1887, the physikalisch-technische reichsanstalt (ptr), later renamed to physikalisch-technische bundesanstalt (ptb). as berlin was the historical space experienced by max weber, this metrological context may also have influenced the methodological approach this social science co-founder developed. the efforts invested in developing methodologies that sought to achieve comparable results from the measurements of social phenomena were a distinctive feature of durkheim and weber’s scientific production. their proposals, however, were characterized by quite different approaches. their methods were not aimed at the same objects of study, which can commonly lead to a false idea of divergence. instead, their methodological approaches were complementary, dealing with analyses carried out in both dimensions, macro-sociological by durkheim and micro-sociological by weber. driven by the positivist influence, durkheim built natural sciences’ analogies with social sciences. it is worth mentioning that both scientific fields share metrological challenges that still linger to the present time. with highly-complex measurements, the measurement requirements framework in such fields of study is not yet adequately addressed or simply not at all. interestingly, in his book from 1894 “les règles de la méthode sociologique” [42], durkheim already acknowledges such challenges that sociology has in common with biology, but to a greater extent. as he states [42]: “tous ces problèmes qui, déjà en biologie, sont loin d'être clairement résolus, restent encore, pour le sociologue, enveloppés de mystère” (p.39). dealing with general analyses involving a large number of social cases but limiting to few variables, durkheim employed a quantitative approach with statistical techniques, including correlation procedures to define the strength of the association between different social facts, regression analysis to explore the impact of the change in one social variable relative to another, also predicting values of the random social variable based on the fixed social variable values. the measurement reference consisted of the average mathematical relationship between variables [42]. durkheim pursues stable objects as a necessary condition for objectivity. the more detached the “social facts” from the “individual facts” by which they manifest themselves, the more objectively represented as a constant, thus eliminating subjective interference, as he states [42]: “on peut poser en principe que les faits sociaux sont d'autant plus susceptibles d'être objectivement représentés qu'ils sont plus complètement dégagés des faits individuels qui les manifestent. en effet, une sensation est d'autant plus objective que l'objet auquel elle se rapporte a plus de fixité; car la condition de toute objectivité, c'est l'existence d'un point de repère, constant et identique, auquel la représentation peut être rapportée et qui permet d'éliminer tout ce qu'elle a de variable, partant de subjectif” [42]. durkheim’s quest for objectivity can be considered analogous to a pursuit towards minimizing the definitional and instrumental uncertainties of social measurements. as for max weber’s methodology, the social properties under analysis were conceived in a micro-social dimension, acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 4 concentrating on a few cases but encompassing a large number of variables, thus leading to a significant increase in the complexity of the measurand compared to durkheim’s simplified general model. the reference in weber’s approach is built by an abstract model, consisting of a synthetic “ideal construct” that encompasses multiple essential attributes. this strategy resembles the production of reference materials for chemical or biological measurements, areas for which the realization of si units is still unavailable. in these fields, it is possible to provide metrological traceability by developing reference materials with sufficient homogeneity and stability regarding specified properties, being established to be fit for their intended use in the measurement or examination of nominal properties [55]. weber’s approach considers cases as a whole, constituted of variables that cannot be disassociated. like the procedure using certified reference materials as “primary reference standard,” weber’s conception claims that the produced ideal types should be made available as a reference for further investigations of other cases, which would enable uniformity of interpretation through the intersubjectivity of measurement results. both durkheim’s and weber’s strategies are concerned with establishing a well-defined reference to provide the necessary measurement standard to enable the comparability of results, considering the specific characteristics associated with their main object of study. these features denote the concern with ensuring the intersubjectivity of the measurement results, which, in turn, will be provided only by establishing global metrological traceability of the measurement results to reference properties [34]-[37]. furthermore, these preliminary approaches developed in the foundations of social sciences have been applied up to the present. durkheim’s suicide assessment has been recently implemented and validated in gerontological practice [16], [17]. recent studies regarding metrology for the social sciences have also addressed ideas from durkheim’s immediate predecessor, gabriel tarde (1843-1904) [18], [19]. considering quantification in the psychosocial sciences more demanding than in the natural sciences, tarde qualified this measurement challenge as a new level of intellectual achievement [18]. as for max weber, his concept of “ideal type” came to serve as the basis for later developed measurement models addressing psychosocial properties [20]. that was the case of the guttman scale, with its’ proposal of “perfect scale” requirements to yield invariant measurement. weber’s concept of “ideal type” is also linked to principles underlying the rasch measurement approach a “probabilistic realization” of the guttman scale, as described in the literature [20]. according to duncan (1984), "social measurement should be brought within the scope of historical metrology" [56]. duncan's suggestion may become a reality as soon as the cgpm resolutions start addressing the demands for the development of global metrological infrastructure aimed at ensuring the reliability and the comparability of measurement results in humanities and social sciences, consequently promoting the integration of this complex scientific field into the international system of units [3]. 4. conclusion despite never formally being addressed by international metrological organizations, social sciences were established as a discipline shortly after the intergovernmental metrological structure creation, in 1875, by the metre convention signature. the present study explored the concepts potentially associated with the reliable framework provided by metrology among the preliminary measuring strategies developed by two founding designers of the social sciences. sharing the challenging aspect of measurement complexity and unavailability of a corresponding si unit traceability of measuring results with chemical and biological properties, since its early years, the social sciences founding authors embodied ideas close to metrological concepts to ensure comparability as much as possible. the two major methods developed when the social sciences discipline was born conceived different levels of measurement dimensions but equally looked for defining a robust measurement standard to be employed for comparative analysis. emile durkheim’s objective and quantitative approach was directed to generalizations, using statistics to study numerous cases, focusing on a few variables, and defining reference by a mathematical-statistical average type, as well as pathological cases according to their corresponding deviations. such strategy points toward the possibility of stepping forward to the measurement quality attribute of objectivity, minimizing definitional and instrumental measurement uncertainty components. in turn, max weber’s subjective and qualitative method examined the social phenomenon from a micro-dimension perspective, dealing with few cases and multiple variables, by means of which it aims at the highly-complex feature of social events. ‘ideal type’ constructs, which were defined as standard references, would, then, embody the essential social variables for the appropriate description of a social phenomenon. in this sense, weber’s ideal type can be interpreted as a reference material designed to allow the comparability of the results obtained by evaluating a specific construct by several researchers, which indicates a tendency towards an intersubjectivity attribute of measurement quality. the efforts implemented by the founding architects of the 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[5] successfully applied deep reinforcement learning to playing atari games by feeding multiple frames at once and utilising experience replay to ensure convergence. later, deep reinforcement learning was applied to multi-agent systems, such as independent multi-agent reinforcement learning. foerster et al. [6] stabilised experience replay for independent q-learning using fingerprints. omidshafiei et al. [7] utilised decentralised hysteretic deep recurrent q-networks for partially observable multi-task multi-agent reinforcement learning problems. figure 1. markov decision process. abstract reinforcement learning is currently one of the most researched fields of artificial intelligence. new algorithms are being developed that use neural networks to compute the selected action, especially for deep reinforcement learning. one subcategory of reinforcement learning is multi-agent reinforcement learning, in which multiple agents are present in the world. as it involves the simulation of an environment, it can be applied to robotics as well. in our paper, we use our modified version of the advantage actor–critic (a2c) algorithm, which is suitable for multi-agent scenarios. we test this modified algorithm on our testbed, a cooperative–competitive pursuit–evasion environment, and later we address the problem of collision avoidance. mailto:paczolay.gabor@gmail.com acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 29 multiple advancements have also been made in the field of centralised learning and decentralised execution. foerster et al. [8] created counterfactual multi-agent policy gradients to solve the issue of multi-agent credit assignment. peng et al. [9] created multiagent bidirectionally-coordinated nets with actor–critic hierarchy and recurrent neural networks for communication. sunehag et al. [10] utilised value-decomposition networks with common rewards and q-function decomposition. rashid et al. [11] utilised qmix with value function factorisation, q-function decomposition and a feed-forward neural network with better performance than the former value-decomposition one. lowe et al. [12] improved the deep deterministic policy gradient by altering the critic to contain all actions of all agents, thus making the algorithm capable of processing more multi-agent scenarios. shihui et al. [13] improved upon the previous maddpg algorithm, increasing its performance in zero-sum competitive scenarios by utilising a method based on minimax-q learning. casgrain et al. [14] upgraded the deep q-network algorithm utilising methods based on nash equilibria, making it capable of solving multi-agent environments. benchmarks have also been created to analyse the performance of various algorithms in multi-agent environments. vinyals et al. [15] modified the starcraft ii game to make it a learning environment. samvelyan et al. [16] also pointed to starcraft as a multi-agent benchmark but with a focus on micromanagement. liu et al. [17] introduced a multi-agent soccer environment with continuous simulated physics. bard et al. [18] reached a new frontier with the cooperative hanabi game benchmark. cooperative multiagent reinforcement learning and the proposed algoirthm are usable in many scenarios in robotics. as our algorithm is decentralised, it can be installed into the robots themselves without any central command center. it might be useful in exploration or localisation tasks in which the use of multiple agents would significantly speed up the process. our testbed can be considered a simplified version of a localisation task, as the pursuer robots are trying to approach and measure a non-cooperative moving object. for proper use in robotics, a well-prepared simulation of the robots and the environment is required, in which thousands of episodes can be run for learning. in our work, we modified the already existing advantage actor–critic (a2c) algorithm to make it better suited for multiagent scenarios by creating a single-critic version of the algorithm. then, we tested this modified a2cm algorithm on our cooperative–competitive pursuit–evasion testbed. in the following section, we explain the theoretical background for our work. then, the experiments themselves and the testbed are introduced. we continue by presenting the results and end with our conclusions and suggestions for future work on the topic. 2. theoretical background 2.1. markov decision processes a markov decision process is a mathematical framework for modeling decision making, as shown in figure 1. in a markov decision process there are states, selectable actions, transition probabilities and rewards [1]. at each timestep, the process starts at a state 𝑠 and selects an action 𝑎 from the available action space. it gets a corresponding reward 𝑟 and then finds itself in a state 𝑠′ given by the probability of 𝑃(𝑠, 𝑠′). a process is said to be markovian if 𝑃(𝑎𝑡 = 𝑎|𝑠𝑡 , 𝑎𝑡−1, . . . , 𝑠0, 𝑎0) = 𝑃(𝑎𝑡 = 𝑎|𝑠𝑡 ), (1) which means that a state’s transition is based only on the previous state and the current action. thus, only the last state and action are considered when deciding on the next state. in a markov decision process, the agents are trying to find a policy that maximises the sum of discounted expected rewards. the standard solution for this uses an iterative search method that searches for a fixed point of the bellman equation: 𝑣(𝑠, 𝜋 ∗) = max𝑎 (𝑟(𝑠, 𝑎) + 𝛾 ∑ 𝑠′ 𝑝(𝑠 ′|𝑠, 𝑎)𝑣(𝑠 ′, 𝜋 ∗)). (2) 2.2. reinforcement learning when the state transition probabilities or the rewards are unknown, the problem of the markov decision process becomes a problem of reinforcement learning. in this group of problems, the agent tries to make a model of the world around itself via trial and error. one type of reinforcement learning is value-based reinforcement learning. in this case, the agent tries to learn a figure 2. the simulation environment. the squares represent the controlled agents, while the circle represents the fleeing enemy. the goal is to catch the enemy by moving horizontally or vertically. initialise model: initialise n+1 hidden and n+1 output (1 value + n action) layers (4 different networks in one model, 1 critic + 3 actor) number of updates batch size for number of updates do for batch size do calculate next actions 𝑎 based on the previous state take actions 𝑎, get terminal state boolean and new rewards store the actions, the terminal state booleans, the calculated values, the rewards and the states end for calculate returns based on (13) calculate advantages based on (12) update critic neural network based on the observed states and the corresponding returns: loss is the mean squared error between the returns and calculated values update actor neural networks based on the observed states, the taken actions and the advantages: loss is policy loss(weighted sparse categorical cross-entropy loss) − entropy loss(crossentropy over itself) end for algorithm 1: a2cm. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 30 value function that renders a value to states or to actions from states. these values correspond to a reward achieved by reaching a state or taking a specific action from a state. the most commonly used type of value-based reinforcement learning is q-learning [2], in which the so-called q-values are estimated for each of the state–action pairs of the world. these q-values represent the value of choosing a specific action in a state, meaning the highest reward the agent could possibly get by taking that action. the equation for q-learning for updating the q-values of a state is: 𝑄(𝑠 ′, 𝑎) ← (1 − 𝛼) ⋅ 𝑄(𝑠, 𝑎) + 𝛼 ⋅ (𝑟 + 𝛾 ⋅ max 𝑎′ 𝑄(𝑠 ′, 𝑎′)) , (3) where 𝛼 is the learning rate and 𝛾 is the discount for the reward. the agent always selects an action that maximises the q-function for the state that the agent is in. another type of reinforcement learning is policy-based reinforcement learning. in this case, actions are derived as a function of the state itself. the most common policy-based reinforcement learning method is the policy gradient approach [19]. in this case, the agent tries to maximise the expected reward by following the policy 𝜋𝜃 parametrised by 𝜃 based on the total reward for a given trajectory 𝑟(𝜏). thus, the cost function of the parameters 𝜃 is the following: 𝐽(𝜃) = 𝐸𝜋𝜃 [𝑟(𝜏)]. (4) the parameters are then tuned based on the gradient of the cost function: 𝜃𝑘+1] = 𝜃𝑡 + 𝛼δ𝐽(𝜃𝑡 ). (5) the advantages of policy-based methods include the ability to map environments with huge or even continuous action spaces and solve environments with stochasticity. however, when using these methods, there is also a much greater possibility of getting stuck in a local maximum rather than following the optimal policy. apart from the aforementioned model-free reinforcement learning methods, there is also model-based reinforcement learning. in this case, a model is built (or just tuned) to perform the reinforcement learning. this is more sample-efficient than model-free methods and thus requires fewer samples to perform equally, but it is very dependent on the particular model. it can be combined with model-free methods to achieve better results, as in [20]. 2.3. multi-agent systems and markov games a matrix game is a stochastic framework in which each player selects an action and gets an immediate reward based on their action and those of the other agents [1]. they are called ‘matrix games’ because the game can be written as a matrix, with the first two players selecting actions in the rows and columns of the matrix. unlike markov decision processes, these games have no states. markov games, or stochastic games, are extensions of markov decision processes with multiple agents. they can also be thought of as extensions of matrix games with multiple states. in a markov game, each state has a payoff matrix for all of the states. the next state is determined by the joint actions of the agents. a game is markovian if 𝑃(𝑎𝑖 𝑡 = 𝑎𝑖 |𝑠 𝑡 , 𝑎𝑖 𝑡−1, . . . , 𝑠0, 𝑎𝑖 0) = 𝑃(𝑎𝑖 𝑡 = 𝑎𝑖 |𝑠 𝑡 ), (6) so the next state depends only on the current state and the current actions taken by all agents. 2.4. deep reinforcement learning a reinforcement learning algorithm is called ‘deep’ if it is assisted by a neural network. figure 3. an example of catching the randomly moving opponent. figure 4. an example of catching the fleeing opponent. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 31 a neural network is a function approximator built from (sometimes billions of) artificial neurons. an artificial neuron, which is based on the real neurons of the brain, has the following equation: 𝑦 = act (∑ 𝑤 𝑥 + 𝑏), (7) where 𝑥 is the input vector, 𝑤 is the weight vector, 𝑏 is the bias and act() is the activation function to introduce nonlinearity in an otherwise linear system. the parameters (𝑤 and 𝑏) are tuned with backpropagation, calculating the partial derivative error of all parameters propagated from the final error to the input vector. the selection of the activation function is important in deep learning due to the vanishing gradients: when many layers are stacked upon each other, higher layers’ gradients are too small during backpropagation, and thus, those layers are difficult to train. a basic activation function can be a sigmoid or logistic activation function: 𝑦 = 1 1 + e−𝑥 . (8) a common activation function in deep learning is rectified linear unit (relu) [21], which has gradients that are less vanishing and therefore better to train. it has the following equation: 𝑦 = 𝑥 if 𝑥 > 0 𝑦 = 0 if 𝑥 <= 0 . (9) for multi-class classification, another activation function is used: the softmax activation function. when used as the last layer, the probabilities of all of the output neurons add up to exactly 1. thus, in reinforcement learning, it is utile to use it as the probability distribution of the possible actions. it has the following equation: 𝑦 = e𝑥𝑖 ∑𝑗 e 𝑦𝑗 . (10) deep reinforcement learning algorithms have several advantages compared to traditional reinforcement learning algorithms. first of all, they are not based on a state table, as the states are approximated (which is much more robust than using linear function approximators). this allows many more states to be mapped and even allows for continuous states. however, they are more prone to diverging, and thus, many optimisations have been created on deep reinforcement learning algorithms to provide better convergence on the problems. 2.5. actor–critic an actor–critic system combines value-based and policybased reinforcement learning. in these systems, there are two distinct parametrised networks: the critic, which estimates a value function (as in value-based reinforcement learning), and an actor, which updates the policy network based on the direction suggested by the critic (as in policy-based reinforcement learning). actor–critic algorithms follow an approximate policy gradient: ∇𝜃 𝐽(𝜃) ≈ 𝔼𝜋𝜃 [∇𝜃 log 𝜋𝜃 (𝑠, 𝑎) 𝑄𝑤 (𝑠, 𝑎) δ𝜃 = 𝛼 ∇𝜃 log 𝜋𝜃 (𝑠, 𝑎) 𝑄𝑤 (𝑠, 𝑎) . (11) approximating the policy gradient introduces bias to the system. a biased policy gradient may not find the right solution, but if we choose the value function approximation carefully, then we can avoid introducing any bias. actor–critic systems generally perform better than regular reinforcement learning algorithms. the critic network ensures that the system does not get stuck in a local maximum; meanwhile, the actor network enables the mapping of environments with huge action spaces and provides better convergence [19]. 2.6. the a2c algorithm a2c stands for synchronous advantage actor–critic. it is a one-environment-at-a-time derivation of the asynchronous advantage actor–critic (a3c) algorithm [22], which processes multiple agent-environments simultaneously. in that algorithm, multiple workers update a global value function, thus exploring the state space effectively. however, the synchronous advantage figure 5. the performance of the original a2c algorithm on our benchmark. figure 6. performance of the modified a2c algorithm on our benchmark. figure 7. performance of the original a2c algorithm on our benchmark with collision (with terminating at collision). acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 32 actor–critic provides better performance than the asynchronous model. advantage function is a method that significantly reduces the variance of the policy gradient by subtracting the cumulative reward using a baseline to make smaller gradients; thus, it provides much better convergence than regular q-values. it has the following equation: 𝐴(𝑠, 𝑎) = 𝑄(𝑠, 𝑎) − 𝑉(𝑠) . (12) returns are calculated using the equation: 𝐺𝑡 = 𝑟𝑡 + 𝛾 ∗ 𝑟𝑡+1 ∗ (1 − 𝑇𝑡 ) , (13) where 𝐺 is the return, 𝑟𝑡 is the reward at time t, 𝛾 is the discount factor and 𝑇𝑡 indicates whether the step at time 𝑡 is a terminal state. 3. experiments and results the testbed is a 5 × 5 grid with three cooperating agents (the squares) in three of the four corners of the environment. in the middle, there is a fourth agent (the circle). the former three agents have the objective of catching the fourth agent, which moves randomly. this testbed is analogous to pursuit–evasion (or predator–prey) scenarios that are also significant in robotics. the agents can move in four directions: up, down, left or right. when one of the three agents catches the fourth one, the episode ends. a penalty is introduced to the cooperative agents every timestep; thus, the return of an episode is maximised by ending the episode as soon as possible (i.e. catching the fleeing agent as quickly as possible). each episode must end in 1,000 timesteps to avoid getting stuck. in the modification of the a2c algorithm, we followed the theory of centralised learning and decentralised execution. this means that the execution is decentralised, but the learning phase can be assisted by additional information from other agents. in our case, we used the information that the agents are cooperative; thus, they acquire the same rewards (and returns). as noted before, decentralised execution is most helpful in real-world scenarios in which communication difficulties make a centralised task-solving achitecture impossible. such scenarios are often encountered in robotics. in our experiment, many a2c models with one actor and one critic were substituted for one model with one critic and multiple actors. the pseudocode of the algorithm can be seen in algorithm 3. all neural network layers were subclasses of the tensorflow model class, which provides utile functions for training and prediction – even for batch tasks – by providing only the forward steps of the network. the optimiser was rmsprop, with a learning rate of 7 · 10−3. the value estimator critic contained a neural network of 128 hidden unit layers with relu activation function and one output layer with one unit. its loss function was a simple mean squared error between the returns and the value. the actors contained a hidden layer with 128 hidden units and an output layer with four units (the number of actions in the action space). the loss function contained two distinct parts: policy and entropy loss. the policy loss was a weighted sparse categorical cross-entropy loss, where the weights were given by the advantages. this method increased the convergence of the algorithm. entropy loss is a method for increasing exploration by encouraging actions that are not in the local minimum. this is very important for tasks with sparse rewards due to the fact that the agent does not receive feedback often. this loss was calculated as a cross-entropy over itself, and it was subtracted from the policy loss because it should be maximised, not minimised. the entropy loss was tuned by a constant, which was taken as 1 · 10−4. episode rewards were taken to be a list where a value of 0 was appended to the end of the list at each episode’s end. during the episodes, only the last value of the list was incremented by the episode reward of the given step. for the training, a batch-sized container was created for the actions, rewards, terminal state booleans, state values and observed states. then, a two-level loop was started: the outer one was run for the number of required updates (set by us), while the inner loop was run as many times as the batch size. the state observations, the taken actions (which were selected by a probability distribution based on the actor neural network results), the state values, the rewards, the terminal state booleans and the last observed state were stored in the aforementioned containers. next, the returns and advantages figure 8. performance of the a2cm algorithm on our benchmark with collision (with terminating at collision). figure 9. number of steps per episode of the original a2c algorithm on our benchmark with collision (without terminating at collision). figure 10. number of steps per episode of the a2cm algorithm on our benchmark with collision (without terminating at collision). acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 33 were calculated on the batch using the collected data, and then a batch training was performed on those data. there was no need to calculate the gradients themselves due to the use of the keras api. during our experiment, the system was run 5,000 times in batches of 128, thus running the environments over a total of 640,000 steps. gamma was taken to be 0.99. figure 3 and figure 4 show the ends of some remarkable episodes of catching the opponent. figure 6 and figure 7 show the results of our experiments. it is important to note the xcoordinates in figure 5 and figure 6: for the same number of steps, the original was run for 40,340 episodes, while the modified algorithm managed to complete 82,119 episodes. this means that the a2cm algorithm spent half as many steps in an episode and was able to catch the fleeing opponent in, on average, half of the time required by the agent based on the original algorithm. these figures also show that the original algorithm did not find an optimal solution without diverging later, and even between divergences, the solutions were not as stable. our agent, on the other hand, found a solution with no divergences later and only small divergencess after the first half of the episodes. the a2cm algorithm found a solution with which it can catch the opponent in 6 steps, and it maintained this knowledge for 20,000 episodes, with one positive spike where it found the solution to the problem in just 3 steps. the run times are worth considering, as well. the regular a2c algorithm took 14,567.45 seconds to run, while the modified one ran for 14,458.28 seconds. it is worth noting that, due to the fact that almost twice as many episodes were completed, the environment had to be reset twice as often, so the modified algorithm is even faster than the normal one. later, the difference between the algorithms were tested with collision turned on, bringing the problem set even closer to realworld robotics scenarios. in this case, the agents received a penalty if they collided with each other. this method makes the environment much harder to learn, as failure will probably only result from chasing the enemy agent. it also makes the training process harder, as the steps leading to success are not as easy to determine; a collision that occurs before the enemy is caught will make similar attempts less likely to be selected as actions. when considering the training process of the environment with collision detection turned on, it is important to pay attention to the reward ratio between the negative rewards for each step and the negative reward for collision. the larger the reward for collision, the better the agents will evade collision; otherwise, they will be optimised to finish the episode as fast as possible. for this reason, the negative reward for each step was selected as −1,000, and the negative reward for the collision was −150,000,000, providing a ratio that is large enough to encourage the agents to follow a collision-evasion policy. in the first experiment on the environment with collision detection, we tried to set the algorithms such that a collision would terminate the episode. this scenario is analogous to certain scenarios in robotics in which collisions can cause malfunctions in the robots themselves and should be evaded even via high-level control. apart from turning on the collision, all other conditions and parameters of the training process were the same. figure 7 and figure 8 show the cumulated rewards per episode for the original a2c and our a2cm algorithm, respectively. it can be seen that, while neither was able to solve the environment over the timespan of the training, there was a time span of ca. 700 episodes in which our algorithm was able to catch the enemy without colliding. the original algorithm lacked any of these longer periods. the training of the original algorithm in this case took 14,173.42 seconds, while the training of the a2cm took 14,659.00 seconds. it is worth noting that the original algorithm completed 1,665 episodes, and the a2cm completed 3,723; the different numbers of reinitialisations should be considered when comparing the training times. to make the environment easier to train on, the second experiment with collisions was conducted such that the episodes only terminated if the opponent was caught. this way, the episodes were longer and always terminated successfnotedully and therefore might provide better training information than the setting of the previous experiment. this scenario is analogous to problems in robotics in which the presence of two robots in the same area is discouraged, such as area scanning scenarios or subtasks in which two robots should not scan the same area at once. just as in the previous experiment, all other parameters were left as they were in the training of the system without collision. figure 9 and figure 10 show the number of steps required to finish each episode for the original a2c and the modified a2cm algorithms, respectively, while figure 11 and figure 12 show the cumulated (negative) rewards per episode (higher is better) for the a2c and the a2cm algorithms, respectively. it can be seen that, while the original a2c algorithm did not show any clear sign of successful training, there is some indication of success for the a2cm algorithm. approaching the end of the training process, the number of steps were kept low, and, as per figure 12, collisions were also evaded, with the exception of some episodes. the original algorithm completed 1,177 episodes, while the modified one completed 1,964, which can also be seen as a sign of the superiority of the a2cm algorithm. regarding the training times, the original algorithm was trained for 13,981.22 seconds, while the modified one was trained for 19,519.85 seconds. in this case, it is clear that our algorithm used significantly more training time. figure 11. rewards per episode of the a2cm algorithm on our benchmark with collision (without terminating at collision). figure 12. rewards per episode of the original a2c algorithm on our benchmark with collision (without terminating at collision). acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 34 4. conclusion looking at the previous section, we can conclude that our modification of the original a2c algorithm, the a2cm algorithm, was able to perform much better than the original on our testbed without collision. to some extent, it outperformed the original a2c algorithm even in enviroments with collision; thus, it is recommendable for tasks in robotics. however, the algorithm has the caveat of being usable only when the agents are fully cooperative and do not have special, predefined roles. there are still many ways to improve upon the current state of our algorithm. one possibile improvement would be to introduce a variable learning rate, such as win or learn fast [3], in a deep reinforcement learning algorithm. another possible improvement is to include the fleeing agent in the algorithm so that the algorithm encompasses the full cooperative–competitive nature of the environment. in addition, other activation functions could be tried to check their behavior; for example, exponential linear units [23] might have better convergence at the price of slightly more training time. the algorithm could be extended using recurrent neural networks so that it could handle partially observable markov decision processes in which the full state is unknown. acknowledgement the research 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clevert, t. unterthiner, s. hochreiter, fast and accurate deep network learning by exponential linear units (elus), arxiv (2015), 14 pp. online [accessed 14 september 2021] https://arxiv.org/abs/1511.07289 http://dx.doi.org/10.1109/icra.2018.8463189 https://arxiv.org/abs/1803.08375 https://doi.org/10.5555/3045390.3045594 https://arxiv.org/abs/1511.07289 metrology for data in life sciences, healthcare and pharmaceutical manufacturing: case studies from the national physical laboratory acta imeko issn: 2221-870x march 2023, volume 12, number 1, 1 5 acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 1 metrology for data in life sciences, healthcare and pharmaceutical manufacturing: case studies from the national physical laboratory paul m. duncan1, nadia a. s. smith1, marina romanchikova1 1 data science department, national physical laboratory, united kingdom section: research paper keywords: nmi; metrology; digital pathology; medicines manufacturing; metadata standards; data quality; ontologies; fair principles citation: paul m. duncan, nadia a. s. smith, marina romanchikova, metrology for data in life sciences, healthcare and pharmaceutical manufacturing: case studies from the national physical laboratory, acta imeko, vol. 12, no. 1, article 10, march 2023, identifier: imeko-acta-12 (2023)-01-10 section editor: daniel hutzschenreuter, ptb, germany received november 18, 2022; in final form february 20, 2023; published march 2023 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was funded by the uk government department for business, energy & industrial strategy through the uk’s national measurement system. corresponding author: paul m. duncan, e-mail: paul.duncan@npl.co.uk 1. introduction decision making in research, industry and healthcare is underpinned by the quality of data including its provenance, timeliness, reliability, and other aspects. ascertaining the data quality using metrological principles of traceability, calibration and uncertainty can be described as data metrology. while certain disciplines such as radiation dosimetry or coordinate measurement of industrial components have long incorporated metrological tools such as calibration and traceability into their workflows, others such as laboratory medicine or pharmaceutical manufacturing are relatively new adopters who benefit profoundly from the european metrology networks for traceability in laboratory medicine and advanced manufacturing. technological advancements in medicine and pharmaceutical manufacturing have been traditionally focused on advances in drug discovery, experimental procedures, and manufacture. medicines and treatments are becoming more expensive to produce, as pricing models drive down profit margins compounded with patents expiry [1]. therefore, a greater emphasis is being placed on maximising the efficiency of medicines development and manufacture. for the quality and repeatability of processes, most pharmaceutical firms operate at high variation levels in terms of accurately manufacturing materials. these variations, at levels between 3 𝜎 and 4 𝜎, are estimated to cost ~$20 bn annually through waste and inefficiency [2]. therefore, companies are increasingly moving to developing controlled and flexible processes to offer digital health solutions for their customers. the national physical laboratory (npl) has set out to aid digitalisation in healthcare by focusing on the development of data metrology for life sciences, medicines and pharmaceutical manufacturing. data metrology refers to the uncertainty present abstract data metrology, i.e., the evaluation of data quality and its fitness-for-purpose, is an inherent part of many disciplines including physics and engineering. in other domains such as life sciences, medicine, and pharmaceutical manufacturing these tools are often added as an afterthought, if considered at all. the use of data-driven decision making and the advent of machine learning in these industries has created an urgent demand for harmonised, high-quality, content rich, and instantly available datasets across domains. the findable, accessible, interoperable, reproducible principles are designed to improve overall quality of research data. however, these principles alone do not guarantee that data is fit-for-purpose. issues such as missing data and metadata, insufficient knowledge of measurement conditions or data provenance are well known and can be aided by applying metrological concepts to data preparation to increase confidence. this work conducted by national physical laboratory data science team showcases life sciences and healthcare projects where data metrology has been used to improve data quality. mailto:paul acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 2 in the data generated in each of these areas, from the quality of measurements accompanying the manufacturing to the quality of the data used for decision making processes. this paper describes the similarities and differences between data metrology challenges addressed by npl in the context of several cross-disciplinary projects with the goal of helping users to identify their data metrology needs and delivering confidence in the effective use of data. 2. data metrology projects the npl data science team has been involved in multiple data metrology projects including life sciences, healthcare, and medicines manufacturing applications, exploring the similarities and domain-specific requirements in data quality and management. these projects and related data metrology challenges are outlined below. 2.1. pharmaceutical manufacturing recent developments in digital pharmaceutical manufacturing are generating a large amount of data across varying temporal resolutions and manufacturing routes. this data provides unprecedented opportunities for pharmaceutical manufacturing to derive new insights and efficiencies from experiments but also imposes great challenges in data processing, management, sharing, and integration. not only data integrity and authenticity are to be ensured, but the processes that lead to the generation of data must be traceable to enable trust. the pharmaceutical industry introduced “good manufacturing practices” (gmp) to standardise processes around quality, security and effectiveness, but did not make allowances for metrological concepts such as traceability and measurement uncertainty. data metrology therefore becomes a critical component in understanding and controlling pharmaceutical processes and reducing the variation seen in the final product. npl has worked with major pharmaceutical manufacturers and researchers to explore their data metrology needs and develop a set of applied research programs. 1) ontologies for clinical trial release. npl has developed techniques [3] to develop a domain agnostic measurement ontology, with a view to applying these techniques across different industries. for the past 2 years, npl has worked with the medicines manufacturing innovation centre (mmic) to develop an ontology to aid in the automation and digitalisation of all data required for regulators to approve drugs for consumption. much of the approvals process is manual data processing which can be replaced by processing of data through modern data driven techniques. the ontology developed by npl “codifies” all the data relationships which are pertinent to the identification of an expiry date for the release of a drug, facilitating true automation, reducing human input and significantly decreasing the potential for errors in the process due to the development of an approved automated decision process. 2) controlled vocabularies for pharmaceutical data exchange. the development of the ontology for clinical trials release exposed an issue in how data from different companies and manufacturers can differ semantically when describing similar terms. for example, separate companies may use the terms “pill” and “tablet” to describe the same concept. this inconsistency decreases the quality of the information used in digitalised or automated systems. npl has developed the first iteration of a controlled vocabulary for the clinical trials process to ensure that any automated system can understand the terminology used by each party [4]. this controlled vocabulary provides a traceable link to quality processes for each company which can aid in automating the verification of the processes used. within the context of the mmic use case, this has provided a valid solution for partners and collaborators to deal with terminology harmonisation issues. npl has also been exploring the idea of working with industry to create an industry-wide standard to create a unified approach to solving this problem and reducing the uncertainty of the information. 3) mapping of measurement uncertainty propagation in manufacturing. npl has been working on an approach to understand the measurement uncertainty generated at each stage of a continuous manufacturing process. currently, uncertainty generated at each node is not propagated, so to ensure greater traceability of variation present in the final product, npl are currently expanding upon pre-existing industrial case studies [5] to develop methods to “map” out the uncertainty present in manufacturing systems and propagate this to each stage. the goal of these use cases under development is to truly understand the uncertainty of the information produced during pharmaceutical manufacturing and to provide industry with frameworks to understand their data metrology. 2.2. minimum metadata for biological imaging biological imaging (bioimaging) encompasses a vast array of techniques, such as optical microscopy, spectroscopy, multispectral imaging, among others. in the pharmaceutical industry, these techniques are used both in r&d and in clinical studies that evaluate drug resistance, efficacy, targeting mechanisms and pharmacodynamics. complexity, diversity, and volume of data generated by high-resolution imaging techniques drive the need for advanced analysis and data management methods and require new standards to ensure reproducibility of results and reliability of research [6]. while the efforts to improve data interoperability and definition of minimum reporting standards and metadata are ongoing [7]-[9], stronger engagement of equipment vendors, researchers and funding authorities is needed to create future-proof re-usable and reproducible data repositories. seeking such engagement, npl has been working with two major pharmaceutical industry partners to work on three bioimaging case studies characterised by high data volumes and need for re-use: 1) mass spectrometry imaging (msi); 2) high content screening; and 3) light sheet microscopy (lsm). to identify the minimum metadata requirements for the three bioimaging domains, subject matter experts (smes) were named by each partnering organisation. the smes worked with npl data science team to collate the current practices on data management and annotation [10]. the review of the collated metadata revealed the metadata categories illustrated in figure 1. the categorised domain-specific metadata sets were enhanced with metadata item descriptions, formats, and, where applicable, units of measure and suggestions of standardised terminologies or controlled vocabularies. the results were revised by all smes and published in the open npl report ms 24 [11]. the findings of the study were used to define minimum microscopy metadata recommendations for research data repositories [12]. at npl, the minimum metadata specifications for lsm and msi were used extensively to develop frameworks and tools for bioimaging data capture and annotation [13]. 2.3. digital pathology clinical histopathology describes a study of stained tissue sections on glass slides under a microscope, whereby acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 3 pathologists manually change the brightness, focus depth, and the region of interest. in digital pathology (dp), tissue samples are digitised using a whole slide imaging (wsi) scanner. the resulting high-resolution images (1-4 gb) can be studied in silico by image analysis software or on-display by a trained pathologist. the dp workflow poses multiple metrological challenges: reproducibility and repeatability of tissue processing, calibration and traceability of wsi, as well as uncertainty analysis to support diagnosis. the npl digital pathology inter-disciplinary project, launched in 2020, comprised a landscape exercise during which dp experts and stakeholders identified priority areas for metrology support [14], [15]. the outcomes of the landscape exercise were used to shape demonstrator studies with real-world data. within the pithia trial collaboration (http://www.pithia.org.uk, grant reference number pb-pg1215-20033), the project team are studying the uncertainties in the diagnosis based on kidney biopsy images, aiming to 1) locate the sources of uncertainty in decision making and find tools to reduce it, as well as 2) find image features that correlate with clinical outcomes to increase reproducibility and explainability in wsi evaluation. the preliminary findings (figure 2) show how the on-display assessment method influences the diagnostic results: when a blood vessel wall thickness is measured directly (red line, lower left diagram), the assessors show preference for more uniform score assignment than if the wall thickness is calculated as a difference (outer diameter-lumen diameter)/2 (blue line, lower right diagram). further case studies will include analysis of measurable image features and their association with diagnostic predictions, as well as impact assessment of intraand inter-wsi device variability on image features and diagnosis. future work will include engaging with standards bodies to include metrology-enabling contextual data such as calibration results, device settings etc. into clinical dp standards such as digital communications for medical imaging (dicom) and fast healthcare interoperability resources (fhir). these standards have high maturity levels and provide mechanisms to include metrological metadata and requirements such as units of measure, clinical terminologies, ontologies, unique identifiers etc. 2.4. medical sensors case study while wsi data and associated measurement information can be captured using the existing dicom standard, novel medical devices require modification of existing standards to capture new data types and provide integration into the healthcare infrastructure. npl worked with a uk-based medical device developer to create clinically interoperable data structures to store and manage the data from a novel surgical sensor. this opportunity facilitated the capture of valuable metrological information including traceability and calibration ab initio, creating a metrologically sound data model at the early stage of device development. an example of how custom measurement-related information can be included into dicom metadata is presented in table 1. a custom value (patient tilting angle in degrees) is enclosed in a concept name code sequence that refers to the coding document and provides the value inclusive of its format (value representation (vr)). note that the value description includes the unit of measure and the reference terminology (measurement units code sequence). 2.5. digital health new measurement modalities within healthcare are creating vast amounts of high-dimensional data from disparate sources and of varying quality, including genomic and imaging data, biomarkers, electronic healthcare records and data from wearable devices. the current and future healthcare practices across the world are increasingly reliant on the integration of these diverse, complex, and large datasets as well as trusted and robust analysis methods [16]. the data curation process in healthcare includes extraction, de-identification, and annotation of datasets with metadata, as well as data fusion and linkage. therefore, future-proof secure scalable curation methods that handle rapidly growing data volumes are needed. npl runs an ongoing inter-disciplinary digital health programme aimed to use data metrology tools to help solve some of the important and emerging challenges of utilising healthcare data [17]. the project includes several case studies, some of which are briefly described below, and further details can be found in the 2021 report [18]. figure 1. metadata categories in bioimaging figure 2. impact of measurement method on clinical assessment (remuzzi score). red line: wall thickness is measured directly. blue line: wall thickness is calculated from vessel outer diameter and lumen diameter. image courtesy of tobi ayori. table 1. including custom measurement value, units of measure and reference to ontology in dicom metadata. tag description tag vr value concept name code sequence (0040, a043) sq code value (0008, 0100) sh ‘1.2.2-1’ coding scheme designator (0008, 0102) sh ‘ascode’ coding scheme version (0008, 0103) sh ‘1.0’ code meaning (0008, 0104) lo ‘patient tilting angle’ numeric value (0040, a30a) ds ‘-19.05’ measurement units code sequence (0040, 08ea) sq code value (0008, 0100) sh ‘deg’ coding scheme designator (0008, 0102) sh ‘ucum’ coding scheme version (0008, 0103) sh ‘1.4’ code meaning (0008, 0104) lo ‘degrees’ experimental metadata instrument settings sample provenance sample handling data processing http://www.pithia.org.uk/ acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 4 one of the case studies investigates whether it is possible to improve the data quality and comparability by linking patient images with imaging device calibration data. the study set out to link megavoltage computed tomography (mvct) images used for image-guided radiotherapy with mvct device calibration data from the routine monthly quality assurance tests that check whether the scanner is fit-for-purpose. mvct images are routinely used for patient positioning, radiation dosimetry, and in-treatment therapy effect assessment. like other medical imaging modalities, mvct images are subject to temporal and inter-device variations that are known to have negative influence on the accuracy of subsequent radiation dose calculation and image segmentation. we implemented a procedure that includes the device calibration information into the dicom header information of the patient scan. we expect that the mvct calibration data can be used to remove the device-related variability and make the patient images more inter-comparable, reduce the variations in the image quality, improving the accuracy of analysis, safety, and efficiency of data-driven clinical interventions [18]. another case study focussed on the development of datadriven models to identify key prognostic markers in computerised medical records (cmr). cmr are a powerful source of information as they contain population level health indicators. these data can be used for estimations of disease incidence, provide insight into disease complexity and identify sub-groups of patients, among other things. national and regional level data aid decision-making in response to potential disease outbreaks, while identification of patient sub-groups can aid treatment planning, moving towards personalised medicine. despite the enormous potential, identifying trends in large primary care data and inferring meaning from these data is extremely challenging due to their complexity, heterogeneity, dimensionality, incompleteness, and noisiness. cmr data are often mixed-type, making traditional data analysis tools unavailable. a generic data pre-processing and deep learning approach for visualisation and analysis of cmr data has been developed at npl [19]. the tools enable the analysis of cmr data, as well as other related data types, such as demographics, metadata, medical histories, in a way that identifies non-linear patterns in an unlabelled manner. the features that form patient clusters can be linked back to the input data and interpreted by the clinician or stakeholder to aid in their decision making in complex healthcare scenarios. this framework can also be applied as a data exploration study to obtain data-driven hypotheses that can be tested with further data. a further case study in the digital health programme evaluates how data linkage can be used to improve the quality of life and long-term treatment outcomes for prostate cancer patients by using the patient care data acquired outside of clinical trials. we developed an ontology-based data curation framework to identify and collate information about diagnosis, symptoms, and treatment side effects from routine primary care electronic health records. this work is a first step to increase the utility of primary care data for oncology by a) creating a knowledge base of data sources, b) mapping out the required integration efforts, and c) developing a practical ontology-based method for systematic and reproducible prostate cancer case identification and validating this method on real-world datasets. the developed ontology can be used to standardise the identification and retrieval of prostate cancer cases from primary care data [20]. npl’s most recent endeavours to increase availability and reliability of medical data include developing a curated data platform. the platform will provide mechanisms for curation, storage, metadata annotation, linkage, and analysis of clinically relevant imaging, audit, and calibration data (figure 3). such a platform would provide a much-needed foundation to enable access to a richer and larger dataset than what is currently available, rendering the data fair-er, and thus increasing its value and utility. 3. conclusions this work presents a range of use cases and demonstrator studies in life sciences and healthcare developed by npl through active collaborations with industry partners and researchers in digital pathology, bioimaging, pharmaceutical and biomanufacturing. it is aimed to highlight the need for data metrology in life sciences and healthcare and to stress the role of national measurement institutes in these areas. despite the relative heterogeneity of the presented case studies, the identified problems feature similarities including (a) missing metadata specifications, (b) lack of mechanisms to capture, exchange and propagate metrological information such as calibration data from data acquisition during measurement to its processing and (c) lack of methods to combine and propagate uncertainties in data processing chains. the three problems listed above call for a systematic approach to data curation and metadata annotation based on the need for fair-ness and data reusability. although the missing metadata specifications can be addressed using custom ontologies and controlled vocabularies, striving towards standards and minimum data quality requirements is recommended to increase data re-usability and impact across different sectors/companies. furthermore, there is a variety of existing open standards and formats that can and should be used to manage data from new medical devices and imaging modalities. these standards can be adapted to incorporate information pertaining to metrological traceability and uncertainty. lastly, while the use of, and need for, metrology methods is widely recognised in physics and engineering, in life sciences, medicine and pharmaceutical manufacturing these tools are often added as an afterthought, if considered at all. therefore, work is required to demonstrate the need for and the impact of data metrology via case studies in the respective domains. the npl data science team believes that the identified challenge areas highlight both the need for heterogeneous figure 3. fair data platforms for clinically relevant research acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 5 approaches to data metrology as well as common pain points across these fields. the findings presented in this paper call for a proactive and consistent approach to generating and using quality data. fair data platforms such as that shown in figure 3 demonstrate an end-to-end approach to how data should be treated to ensure adherence to the fair principles and reduce any uncertainty generated due to the processing or labelling of the data. acknowledgement this work was funded by the uk government department for business, energy & industrial strategy through the uk’s national measurement system. we would also like to thank our partners at the mmic; cpi, university of strathclyde, ukri, scottish enterprise, astrazeneca and gsk as well as artiosense ltd 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1369-1370. doi: 10.3233/shti200446 https://doi.org/10.1039/9781782622345-00001 https://doi.org/10.1002/jps.24343 https://doi.org/10.1016/j.measen.2021.100263 https://doi.org/10.1088/1681-7575/abeff8 https://doi.org/10.21014/tc6-2022.023 https://doi.org/10.1038/s41592-021-01256-7 https://doi.org/10.1038/nmeth.1896 https://doi.org/10.1093/gigascience/giy102 http://arxiv.org/abs/1910.11370 https://doi.org/10.47120/npl.ms25 https://doi.org/10.47120/npl.ms24 https://doi.org/10.1038/s41592-021-01166-8 https://doi.org/10.1016/j.measen.2021.100201 https://doi.org/10.47120/npl.as102 https://doi.org/10.1016/j.jpi.2022.100157 https://topol.hee.nhs.uk/ https://doi.org/10.1259/bjr.20190574 https://doi.org/10.47120/npl.ms31 https://doi.org/10.3389/fams.2019.00042 https://doi.org/10.3233/shti200446 design, characterisation, and digital linearisation of an adc analogue front-end for gamma spectroscopy measurements acta imeko issn: 2221-870x june 2021, volume 10, number 2, 70 79 acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 70 design, characterisation, and digital linearisation of an adc analogue front-end for gamma spectroscopy measurements t. kowalski1,2, g. p. gibiino3, j. szewiński1, p. barmuta4,2, p. bartoszek1, p. a. traverso3 1 national centre for nuclear research (ncbj), 05-400 otwock, poland 2 institute of electronic systems, warsaw university of technology, 00-661 warsaw, poland 3 edm-lab, dept. electrical, electronic and information engineering "g. marconi" dei, university of bologna, 40136 bologna, italy 4 dept. electrical engineering, ku leuven, 3000 leuven, belgium section: research paper keywords: adc front-end; receiver characterisation; receiver linearisation; gamma spectrometry system citation: tomasz kowalski, gian piero gibiino, jaroslaw szewinski, pawel barmuta, piotr bartoszek, pier andrea traverso, design, characterisation, and digital linearisation of an adc analogue front-end for gamma spectroscopy measurements, acta imeko, vol. 10, no. 2, article 11, june 2021, identifier: imeko-acta-10 (2021)-02-11 section editor: giuseppe caravello, università degli studi di palermo, italy received january 18, 2021; in final form may 7, 2021; published june 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: gian piero gibiino, e-mail: gianpiero.gibiino@unibo.it 1. introduction gamma spectroscopy is the discipline studying gamma radiation sources. the identification and characterisation of gamma radiation sources is employed for many purposes, ranging from scientific to industrial, as well as military. in geology, it can be employed for mapping the elements in rock formations (mainly potassium, thorium, and uranium) [1], or for analysis of soil samples [2]. in border security, non-intrusive inspection systems based on gamma spectroscopy can detect materials such as tobacco, illicit drugs, or explosives [3]. also, gamma spectroscopy systems are commonly used for radiation monitoring, e.g., they are always found near sensitive nuclear apparatuses such as nuclear reactors or particle accelerators. in case of a fault, any radiation exceeding predetermined safe levels can be quickly detected, triggering the shutdown of the system and corresponding actions. gamma spectroscopy measurements aim at quantifying the energy of radiated particles. the energy spectrum of such radiation can be experimentally obtained by creating a histogram of the energy levels. then, the peaks in the spectrum can be related to specific physical processes happening at the atomic scale, granting insights into the nuclear composition of a measured source of radiation. a gamma spectrometer consists of an ionising radiation detector and an electrical signal receiver. the detector converts the radiation energy of the incoming particles into voltage pulses, the pulse amplitude being proportional to the energy of the particle. legacy spectrometry systems consist of several modular elements, such as charge-sensitive pre-amplifiers and pulseshaping filters, to increase pulse duration and make it suitable for the analogue-to-digital converter (adc). in modern systems, instead, the use of fast adcs allows to minimise signal conditioning, enabling direct pulse sampling, higher pulse count rate, and a reduced risk of pulse pile-up [4]. after the acquisition, abstract this work presents the design, experimental characterisation, and digital post-distortion (i.e., digital linearisation) of a mhz-range adc analogue front-end prototype for a gamma radiation spectrometry system under development at the national center for nuclear research (ncbj) in poland. the design accounts for the electrical response of the gamma particle detector in providing signal conditioning and adc protection against high-voltage spikes due to occasional high-energy cosmic radiation, as well as proper adc clocking. as the front-end inevitably introduces nonlinear distortion and dynamic effects, a characterisation is performed to quantify the actual performance in terms of total harmonic distortion (thd) and effective number of bits (enob). thus, a digital linearisation based on both static and memory polynomial models is successfully applied by means of post-distortion processing, guaranteeing a substantial improvement in thd and enob, and demonstrating the effectiveness of the hardware/software method for gamma radiation spectrometers. mailto:gianpiero.gibiino@unibo.it acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 71 the captured voltage pulses must be integrated in time to quantify the particles energy, and the accuracy of the estimated energy spectra heavily depends on the receiver performance. a fundamental problem in spectrometers is the occasional presence of high-energy cosmic radiation interfering with regular acquisitions. most critically, the voltage spikes resulting from these rare events can have amplitudes an order of magnitude higher than the typical radiation source. since the acquisition system of spectrometers is normally designed to maximise measurement sensitivity and accuracy, the output signal of the detector is normally set to match the adc dynamic range with the typical amplitudes received from the target radiation, usually in the few volts range. therefore, the unwanted cosmic pulses can take amplitude of tens of v, not only saturating but even damaging the adc. in this context, adc protection from the high-voltage transient events becomes a system pre-requisite. in legacy spectrometers, this protection is indirectly provided by the pre-amplifier. in direct sampling systems, instead, such protection can be obtained by introducing analogue signal limitation yet paying attention to preserve good receiver linearity within the dynamic range of the adc. more precisely, both the necessary signal conditioning and the analogue limiter itself will likely introduce dynamic effects and nonlinear distortion, reducing the receiver dynamic range. the characterisation and correction of adc-based receiver non-idealities has been subject of extensive research in the literature [5]-[9]. in particular, digital linearisation is an effective approach for compensating the nonlinear dynamic distortion caused by hardware [7]-[9]. it consists of the extraction of a suitable mathematical description of the nonlinear dynamic behaviour of the device, often derived from volterra-like series representations [9]-[12]. this mathematical model is extracted through a set of measurement procedures, either using time or frequency domain approaches. once a suitable direct model of the device is extracted, linearisation can be performed by implementing an inverse of the identified model using digital signal processing techniques. whereas this process is known as digital pre-distortion when linearising signal generators, transmitters, or power amplifiers [13], [14], it can be implemented as digital post-distortion (dpd) for receivers by applying the inverse model to the corrupted samples acquired by the digitiser [8], [15]-[17]. in this work, we present the design, characterisation, and digital linearisation of an adc analogue front-end for the gamma spectrometry system under development at the polish national centre for nuclear research (ncbj). such a board prototype implements a tailored signal conditioning stage including the adc protection and it hosts the clock generation for the adc, as well as all the necessary adc control hardware. this article extends the one in [18], presented at the 24th imeko tc-4 international symposium and 22nd international workshop on adc and dac modelling and testing. the conference contribution was mainly devoted to the metrological characterisation of the front-end in a laboratory setting, and to investigate the feasibility of digital linearisation approaches with application-like pulsed waveforms. in this work, we provide a presentation of gamma spectroscopy systems, in particular targeting the one under development at ncbj. moreover, we present the design details for signal conditioning, especially for the on-board reference clock and the assessment of its jitter. for both aspects, new characterisation data is provided. also, we describe the hardware components of the designed front-end board. finally, the proposed spectrometry system is tested in field using an actual gamma radiation source (caesium-137). the article is organised as follows. in section 2, the working principles of the gamma spectrometer are provided. section 3 is dedicated to the design of the analogue front-end prototype, including the signal conditioning stage and adc reference clock. section 4 contains the corresponding characterisation performed to verify the suitability of the design. moreover, it reports the metrological characterisation of the front-end both with the standard-compliant method using sine wave excitations, as well as using pulsed signals typical of gamma spectroscopy applications. in section 5, the dpd approach is successfully implemented, substantially improving the linearity performance of the receiver protected by the front-end. in section 6, the characterisation of the front-end in an actual spectrometry system is carried out and dpd is applied in the real-life setting. the effects of the distortion and linearisation are presented for measuring the energy spectrum of caesium-137. conclusions are drawn in section 7. 2. gamma spectroscopy system a fundamental part of a gamma spectrometer is the detector of gamma radiation. the most used type of detector, suitable for a wide assortment of radiations, is the scintillation detector [4]. its working principle is based on scintillation, i.e., the phenomenon of emission of light in response to the passage of a radiated particle through a material. as the particle travels through the scintillator, it deposits some of its energy, leading to the excitation of electrons within the scintillator. these electrons subsequently emit light as they return to their base energy states. the number of excited electrons is proportional to the energy deposited by the particle which, in turn, determines the number of emitted light photons. in this way, the scintillator responds to each detected particle with an impulse of light, whose intensity is proportional to the particle energy. the scintillation light is then converted into a measurable electrical signal, proportional to the detected radiation. the most common component employed for this purpose is the photomultiplier, although photodiodes can also be used [4]. a graphical representation of a scintillator coupled to a photomultiplier is shown in figure 1. photo-multiplier tubes (pmts) consist of a photocathode, a series of dynodes, and an anode. incoming light photons from the scintillator hitting the photocathode cause electrons to be ejected from its surface by photoelectric effect. a high-voltage source, typically up to 2 kv, is used to accelerate the electrons through a focusing electrode. when striking a dynode at high speed, each electron causes multiple secondary electrons to be ejected, so that a cascade of electrons is created in the photomultiplier, amplifying the signal. the electrons are then collected on the anode, producing an figure 1. representation of a scintillator coupled with a photomultiplier within a gamma spectroscopy system. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 72 electrical current pulse. a transimpedance amplifier, or a resistor, is then used at the output to convert the current into voltage pulses, which can then be acquired by an adc and digitally postprocessed. the shape of the voltage pulse corresponding to the incoming radiation is determined by the employed scintillator hardware and by the loading circuit of the photomultiplier. the general mathematical description for a voltage pulse at the anode of the photomultiplier after a single scintillation event is given by [4]: 𝑉(𝑡) = 1 𝜆 − 𝜃 𝜆𝑄 𝐶 (𝑒 −𝜃𝑡 − 𝑒 −𝜆𝑡 ) , (1) where 𝜆 is the decay constant of the scintillator, 𝜃 is the reciprocal of the time constant of the anode circuit, 𝑄 is the total charge collected over the pulse duration, and 𝐶 is the combined capacitance of the anode and its loading circuit. the amplitude of the pulse is proportional to the charge collected at the photomultiplier anode, which is linearly dependent on the energy released by the particle during the scintillation event. an example of voltage pulse shape captured from a scintillation detector is shown in figure 2. provided that 𝜆 and 𝜃 are system constants, the pulsed voltage waveform realises a fixed double-exponential shape. the frequency spectrum of such waveform fades out below the noise floor at around 70 mhz, which is then considered as the target bandwidth of the acquisition system, while any spurious frequency content above 70 mhz will be filtered out digitally. to estimate the maximum slew rate of the pulse, the acquired waveform is interpolated with a 9th-order polynomial, and then differentiated, as shown in figure 2. the resulting maximum slew rate of the pulse is 𝑆𝑅𝑀𝐴𝑋 =76.5 v/μs. figure 3 shows the block diagram of the complete spectroscopy system under development at the ncbj. in the expected final configuration, the voltage pulses from the photomultiplier will be acquired on a fpga mezzanine card (fmc) connected to a xilinx zedboard zynq-7000 fpga board. the fmc consists of an adc, as well as of the circuitry for signal conditioning such as amplification, attenuation, inversion, or level shifting, to ensure the best compliance with adc specifications (see section 3). moreover, the fmc contains the adc clock, as well as the i2c protocol signals and power supply. once the signal is sampled, the digital post-processing for obtaining the energy spectrum is carried out at high-speed by the fpga. in particular, the post-processing aims at discriminating the individual pulses and obtaining a numerical value proportional to the energy. this can be achieved either by applying pulse integration or by considering the pulse peak. as mentioned in the introduction, the values corresponding to a series of scintillation events are then distributed into bins of a histogram, creating the so-called energy spectrum. eventually, the spectrum displays the number of measured counts (y-axis) for a set of energy channels (x-axis). as an example, figure 4 shows the energy spectrum for cobalt-60, a standard material often used for the preliminary calibration of the spectroscopy system. in fact, cobalt-60 is an isotope displaying two main peaks of the spectrum (c and d in figure 4) at known precise energy levels (1.1732 mev and 1.3325 mev, respectively) representing the energy of gamma rays emitted during its radioactive decay [4]. in the calibration process, this information is then used to scale the energy channels on the x-axis in mev. the other measured peaks (a and b in figure 4) correspond to the phenomena of back-scattering and compton scattering [4]. 3. front-end prototype design a stand-alone prototype adc front-end board was designed for testing those components to be eventually mounted on the fmc card of the final spectroscopy system shown in figure 3. the prototype board implements the adc analogue front-end, the power supply circuitry, the i2c communication interface, and the clock generation to be used for the adc as well as for the fpga board. to allow for the linearity characterisation of the front-end alone, the prototype does not include the target adc for the final application (ads5474 monolithic pipeline adc from texas instruments [20]), which features 14-bit resolution and sampling rate of 400 msa/s. such adc is differential with a full-scale input of 1.1 v (single-ended) or 2.2 v (differential), requiring a common mode voltage of 3.1 v. as previously discussed, the adc analogue front-end should serve two main purposes. firstly, it should perform attenuation and filtering. indeed, to maximise the signal-to-noise ratio figure 2. oscilloscope acquisition of the response for a scintillation detector (black dots) used at ncbj, polynomial interpolation (blue curve) and its timedifferentiation for maximum slew-rate estimation (red). figure 3. block diagram of the spectroscopy system under development at the ncbj. figure 4. example of an energy spectrum for cobalt-60. edited from [19]. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 73 (snr) of the system, the range of amplitudes of the measured pulses from incoming radiation should correspond to the fullscale of the target adc. while the voltage amplitudes at the output of the detector could be coarsely adjusted by changing the supply voltage of the photomultiplier, it is desirable to include, by the front-end, a software control functionality to digitally finetune the input voltage amplitude. in particular, as from the desired specifications, the gain resolution should be higher than 1 db, while the digital control should be carried out through the i2c interface, readily available with a connector on the fmc. secondly, the front-end is supposed to protect the adc by means of an analogue limiter circuit. such adc protection should clamp input voltage pulses of up to an order of magnitude above the adc full-scale, providing sufficient protection from unwanted cosmic pulses which could damage the adc. at the same time, clamping should be sufficiently fast, given that the pulse rise-time at the output of the scintillator is in the order of 10 ns. moreover, a minimum of 70-mhz acquisition bandwidth is requested to completely capture the spectrum at the pulsed output of the detector. 3.1. signal conditioning stage the block diagram of the designed adc front-end is shown in figure 5. it is composed of a diode used for protecting the circuit from the ns-range over-voltage pulses, a digital step attenuator (dsa) for the adjustment of the measurement range, a fully differential amplifier (fda) for signal buffering and single-ended to differential conversion, and an anti-aliasing filter. the input parasitic capacitance (𝐶𝑃 ) of the front-end should be sufficiently low to allow for the requested bandwidth 𝑓𝐵𝑊 =70 mhz. considering an input resistance 𝑅𝐼𝑁 = 50 ω, the parasitic capacitance must satisfy 𝐶𝑃 < 1 2𝜋𝑓𝐵𝑊𝑅𝐼𝑁 ≅ 45.5 pf. in the proposed solution, a first clamping stage is embodied by a transient-voltage-suppression (tvs) diode from nexperia (pesd5v0c1bsf), with a rated 𝐶𝑃 = 0.2 pf and a reverse stand-off voltage of 5 v. the chosen tvs diode is bidirectional, meaning that it will protect against both positive and negative transients, as the pulse polarity depends on the configuration of the radiation detector. to provide a flexible control across the overall gain of the signal conditioning chain, a digital step attenuator (dsa) was used. a 6-bit dsa from analog devices (hmc472a) with a variable attenuation from 0.5 db to 31.5 db in 0.5-db steps was chosen for the design. the attenuation is set directly by tll/cmos-compatible input pins, which can be interfaced with i2c control. signal conversion from single-ended to differential is required to feed the adc. typical solutions include a balun or a fully differential amplifier (fda). while a balun would introduce less additive noise, an fda can provide a suitable additional gain. moreover, while the tvs diode can protect from the largest pulses, the pulse voltage amplitude must be adapted to the ±1.1 v full-scale range of the targeted adc. in this sense, the fda can provide the necessary additional clamping stage. eventually, an fda from texas instruments (lmh6553) was chosen, featuring voltage-controlled output clamping, 900-mhz bandwidth, and a slew rate of 2300 v/μs. a resistive network for the fda was optimised by matlab and lt-spice simulations to obtain an input impedance of 50 ω and voltage gain of 2 v/v, allowing to compensate all insertion loss potentially arising in the system. a 10-pf capacitor was placed at the fda output along with two 50-ω output matching resistors to implement an antialiasing rc filter with a cut-off frequency of ~ 160 mhz. 3.2. adc reference clock beyond the signal conditioning, the designed prototype board is also aimed at testing a low-jitter 400-mhz clock for the adc. in this context, let us consider the main noise sources impacting the total snr of the adc (𝑆𝑁𝑅𝐴𝐷𝐶), which can be calculated as: 𝑆𝑁𝑅𝐴𝐷𝐶 (𝑑𝐵) = −20 log √ 10 −( 𝑆𝑁𝑅𝑄 20 ) 2 + 10 −( 𝑆𝑁𝑅𝑇 20 ) 2 + 10 −( 𝑆𝑁𝑅𝐽 20 ) 2 , (2) where 𝑆𝑁𝑅𝑄 (in db) is due to quantisation noise, 𝑆𝑁𝑅𝑇 (in db) is due to thermal noise, and 𝑆𝑁𝑅𝐽 (in db) is due to total jitter. in high-speed pipelined adcs such as the one considered here, the thermal noise limits the snr at low input frequencies, while the clock jitter mainly influences the high frequencies [19]. the 𝑆𝑁𝑅𝑄 for a full-scale sine wave input can be expressed by the well-known formula 𝑆𝑁𝑅𝑄(db) = (1.761 + 6.02 𝑁), 𝑁 being the number of bits of the adc. considering that the nominal resolution of the ads5457 is 14 bits, it holds that 𝑆𝑁𝑅𝑄 = 86.04 db. as from datasheet [20], the limit due to thermal noise is estimated at 𝑆𝑁𝑅𝑇 = 70.3 db, while the estimated overall maximum performance is 𝑆𝑁𝑅𝐴𝐷𝐶 = 70.1 db. using the data in the formula above, one can deduce a requirement for the noise due to jitter, resulting in 𝑆𝑁𝑅𝐽 ≥ 75.2 db. let us consider the relationship between the total jitter affecting the sampling process and the resulting impact on the snr [18]: 𝑆𝑁𝑅𝐽 = −20 log(2𝜋𝑓𝐼𝑁 𝑡𝐽,𝑇𝑂𝑇 ) , (3) figure 5. block diagram of the designed adc analogue front-end. figure 6. external clock source rms jitter (𝑡𝐽,𝐸𝑋𝑇) required to keep 𝑆𝑁𝑅𝐴𝐷𝐶 > 70.1 db, as a function of the input frequency (𝑓𝐼𝑁). acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 74 where 𝑓𝐼𝑁 is the frequency of the input signal to be sampled, and 𝑡𝐽,𝑇𝑂𝑇 is the rms value of the total jitter. the total jitter impacting on the a/d process is a function of the timing jitter of the external clock source (𝑡𝐽,𝐸𝑋𝑇 ) as well as the aperture jitter (𝑡𝐽,𝐼𝑁 ) of the adc, which is dependent on the noise of the internal clock buffer [20]: 𝑡𝐽,𝐸𝑋𝑇 = √𝑡𝐽,𝑇𝑂𝑇 2 − 𝑡𝐽,𝐼𝑁 2 , (4) where 𝑡𝐽,𝐼𝑁 = 103 fs for the ads5457. figure 6 shows the relationship between the requirement for maximum rms timing jitter of the external clock (𝑡𝐽,𝐸𝑋𝑇 ) and the input sinewave frequency (𝑓𝐼𝑁 ). given that the maximum slew rate of the voltage pulses at the output of the photomultiplier is 𝑆𝑅𝑀𝐴𝑋 = 76.5 v/μs, and considering that this value corresponds to a pure sinewave of 𝑓𝐼𝑁 = 𝑆𝑅𝑀𝐴𝑋 𝐴2𝜋 ≅ 11.1 mhz, the plot in figure 6 indicates that it must hold 𝑡𝐽,𝐸𝑋𝑇 ≤ 2492 fs to keep 𝑆𝑁𝑅𝐽 ≥ 75.2 db 𝑆𝑁𝑅𝐴𝐷𝐶 ≥ 70.1 db). to satisfy the design jitter requirements, a clock-generating ic (lmk03318 from texas instruments) was combined with a 25mhz crystal oscillator reference (txc 7m-25meeq). also, an additional input port provides the option of connecting an external reference. the ic is controllable by i2c interface and should deliver rms timing jitter in the 100-fs-range as from datasheet, which is deemed enough to satisfy the minimum requirement for the 𝑆𝑁𝑅𝐴𝐷𝐶 . 3.3. board implementation a four-layer printed circuit board (pcb) was designed to implement the front-end prototype. the top layer is used for routing all analogue and a subset of digital traces, as well as for mounting all the components. the second layer is a ground plane, which serves as the reference plane for controlledimpedance traces on the top layer. the third layer is used for routing power connections, while the bottom layer contains the remaining digital traces. to preserve signal integrity, all signal traces in the analogue front-end, as well as the clock signal path, were calculated to have 50-ω characteristic impedance. in addition, these traces were also shielded from interference via fences. special care was also taken when laying out the step-down switching regulator, as it is likely to generate the most electromagnetic interference (emi). the board is equipped with gold-pin headers for i2c communication with an external device, e.g., a microcontroller. to control the dsa via the i2c interface, a max7320 i/o expander was used, and a set of headers allows to select the control of the dsa either by i2c or manual control by a dual in-line package (dip) switch. concerning power supply, linear low-dropout (ldo) regulators offering voltage stabilisation, very low noise, and no additional interferences affecting adc readouts, are used to convert the 12 v supply received from the carrier board into lower voltages. however, ldos are quite inefficient when the difference between regulated output voltage and input voltage is large. the lowest regulated voltage is 1.8 v, as required by the output buffers of the clock generator, causing the ldo to work at efficiency below 15%. to avoid excessive power dissipation in this specific case, the 12 v input voltage was first converted to 6 v with a high-efficiency step-down switching regulator (lt8069a from analog devices), offering efficiency above 90%. the switching frequency was set to 1.8 mhz to allow for a small inductor in the step-down converter, which is important on an fmc board with limited space. additionally, the device has spread spectrum functionality, lowering emi [21] and improving signal integrity. the output 6 v voltage is eventually converted into the required positive and negative supply voltages using ldos. two sets of gold-pin headers are also used for every linear regulator to enable the user to switch it on/off, and to testpoint the regulated voltage. a photo of the manufactured board is shown in figure 7. 4. experimental characterisation in this section, the characterisation performed for the designed prototype are described. they include phase noise measurement for the on-board adc clock and a metrological characterisation in terms of total harmonic distortion (thd) figure 7. photo of the designed front-end board. figure 8. set-up for the characterisation of the phase noise of the on-board clock of the adc. figure 9. phase noise of the ti lmk03318 clock generator with the crystal oscillator reference (10 output correlations enabled). the plot includes the spurious components due to the power supply (in blue). acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 75 and effective number of bits (enob) for the whole acquisition path including the front-end. also, the behaviour of the frontend is measured under application-like pulsed excitation. 4.1. clock jitter characterisation the clock-generating device (lmk03318) was programmed through i2c interface, connecting the circuit to a microcontroller board (nucleo-f446re). the device was configured to provide one differential output of a 400 mhz signal at the sma port, while all other outputs were terminated with 100-ω resistors. to evaluate the rms time jitter, phase noise measurements were carried out by means of the setup in figure 8 using the agilent e5052a signal source analyser and a tti cpx400dp power supply for powering the prototype board. phase noise measurements provide a single sideband noise density spectrum of a signal at different offset frequencies from the carrier at 400 mhz. the rms jitter value is found by integrating the phase noise spectrum over a range of offset frequencies (in this case, 10 hz 10 mhz). for this device, the obtained rms jitter is ≈ 440 fs in the configuration with onboard 25-mhz reference, despite the presence of spurious components coming from the power supply, e.g., at 30 hz, 50 hz, 120 hz and 160 hz (see figure 9). when removing these most evident spurious components via software, the rms jitter value reaches ≈ 312 fs. at the same time, it is worth noting that no spurious components are found at 1.8 mhz (corresponding to the switching frequency of the main on-board power converter), demonstrating that emi was well contained. the measured jitter performance validates the suitability of the designed clock generation. 4.2. metrological characterisation of the acquisition channel a metrological characterisation of the prototype front-end alone, i.e., neglecting any effect of adc at the output, was performed. to this aim, we adopt the measurement setup shown in figure 10 and figure 11. the arbitrary waveform generator (awg) output (agilent 81150a) is split by means of a 50-ωterminated divider to feed, with the same excitation, two parallel acquisition paths. in the first path, the split signal is applied to the device-under-test (dut), i.e., the analogue front-end. a high-linearity instrumentation amplifier (tegam 4040) was used to transform the differential output of the front-end into the first channel of a 100-mhz, 100-msa/s, 14-bit digitiser board (national instruments pxi-5122) which, given the much higher linearity and lower noise, is here considered as an ideal acquisition device. in the second path, i.e., the reference acquisition path, the output of the splitter directly feeds the second channel the pxi-5122 digitiser board. a preliminary test procedure was carried out using sinusoidal tone excitations up to 1 mhz, so to avoid introducing any spurious effect due to the digitiser, e.g., the initial roll-off of the anti-alias filter. the measurement data was then processed according to standard ieee 1241 [22]. for this specific characterisation, no reference acquisition is needed, thus only the receiver chain comprising the front-end under test was used. the considered figures of merit (fom), i.e., thd and enob, are reported in figure 12 and figure 13, respectively, for a set of acquired frequencies and amplitudes. as can be seen, the prototype front-end introduces a clear dependency on the input signal amplitude. while the distortion at -12 dbfs is negligible over the measured frequency range, it substantially deteriorates when the excitation approaches the full scale, reporting thd as high as -30 db and minimum enob of around 5 at -1 dbfs. this could be attributed to the nonlinear characteristic of the tvs diode, as well as to the low-frequency performance of the switching circuitry inside of the digital step attenuator. figure 10. block diagram of the measurement setup for the characterisation of the adc analogue front-end. figure 11. picture of the measurement setup for the characterisation of the adc analogue front-end. figure 12. total harmonic distortion (9 harmonics) of the acquisition chain including the adc front-end. figure 13. effective number of bits of the acquisition chain including the adc front-end. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 76 4.3. application-like pulse measurements the prototype front-end was tested for application-like spectroscopy measurements. indeed, as discussed in section 2, in a realistic scenario the front-end would only receive specific signals, namely exponential pulses of a given duration and different amplitudes. therefore, an additional characterisation was performed employing pulsed excitations modelled from experimental data of an actual scintillator. these pulsed waveforms could be easily generated by programming the awg yet within its 50-mhz limit in high-amplitude mode. then, the impact of the front-end on the pulse shape was quantified by comparing the waveforms distorted by the front-end under test with those acquired through the ideal reference path. considering that the pxi-5122 is not as fast as the ads5474 adc targeted for the final application, slower pulses excitations with 1-µs and 0.5-µs pulse widths were used to allow for a sufficiently high number of acquired samples per pulse, yet respecting the pulse shape produced by the detector. both positive and negative polarities were considered, and sequences of 20 pulses with increasing amplitudes from zero up to the full scale of the adc front-end were programmed into the awg. all acquired signals were normalised in amplitude and delay to allow for numerical comparison. additionally, spectrum equalisation was performed to compensate for the baseline wander effect due to the presence of ac-coupling capacitors in the front-end. to characterise the performance of the front-end with respect to the ideal reference, two quantities were used. firstly, the normalised root mean square deviation (nrmsd) between the waveforms acquired from the two channels was calculated. secondly, each of the pulses was integrated, as done for gamma spectroscopy measurements. then, the nrmsd between the integral values calculated from the actual and reference receivers were obtained, quantifying the deviation that would directly map into the energy spectrum of the measured material. tables 2 and 3 report the calculated deviations, while figure 14 and figure 15 show the shape of a single pulse at full scale amplitude for both acquisition paths. while the negative pulses do not show substantial deviation from the reference, the positive pulses are significantly distorted, hinting at the presence of nonlinear dynamic effects by the front-end. 5. digital linearisation to improve the linearity performance of the front-end under test, a dpd (i.e., digital linearisation) method was implemented. with post-distortion, the acquired signals are post-processed exploiting an inverse modelling of the whole receiver path. to this aim, we used a memory polynomial model, a well-known and effective formulation derived from the volterra series representation, where the memory cross-terms are neglected [13]. such a mathematical description is well suited for modelling figure 14. 1-µs positive pulse with full-scale amplitude, after passing through the two signal acquisition chains (reference and the one including the dut). figure 15. 1-µs negative pulse with full-scale amplitude, after passing through the two signal acquisition chains (reference and the one including the dut). figure 16. total harmonic distortion (9 harmonics) of the acquisition chain including the adc front-end after digital post-distortion, measured at -1 dbfs. figure 17. effective number of bits of the acquisition chain including the adc front-end after digital post-distortion, measured at -1 dbfs. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 77 the nonlinear dynamic behaviour, and it finds broad use in digital pre-distortion of radiofrequency (rf) power amplifiers. the memory polynomial model is formulated as follows [13]: 𝑦(𝑛) = ∑ ∑ 𝑎𝑚𝑘 𝑥(𝑛 − 𝑚) |𝑥(𝑛 − 𝑚)| 𝑘−1 𝐾 𝑘=1 𝑀 𝑚=0 , (5) 𝑀 being the memory depth and 𝐾 the nonlinear order. in theory, a high model order could provide high prediction accuracy, as it would better describe stronger nonlinearities. however, the identification of many model coefficients can often become illconditioned, since increasingly larger dataset of independent measured responses is needed to properly fit all coefficients. in this research, tests were carried out by progressively increasing 𝐾 and 𝑀 for a dataset of application-like pulsed waveforms; it was found that increasing the model order beyond 𝐾 = 5 and 𝑀 = 3 did not provide improved prediction capabilities. out of the different tests, two cases were considered: a static (𝑀 = 0) polynomial model with 𝐾 = 5, and a memory polynomial model with 𝐾 = 5 and 𝑀 = 3. rather than inverting the direct model by means of iterative or nonlinear optimisation algorithms, an indirect coefficient learning approach was used for the identification of the inverse model from time-domain measurement data, using the method in [23]. in a first characterisation, the inverse model was trained on a global dataset composed by acquisitions of sine-wave excitations at 10 khz, 100 khz, and 1 mhz. then, a new experimental characterisation (again, according to ieee 12412010) of the whole receiver chain composed by the combination of the analogue front-end and the dpd was performed and repeated by sweeping the input frequency from 1 khz to 1 mhz. the thd and enob for the linearised receiver are plotted in figure 16 and figure 17, respectively. in the 1 khz-1 mhz range, an improvement in thd of up to 15 db was obtained with the static polynomial model, and of up to 20 db with a memory polynomial model. the corresponding improvement in enob is more than 2 bits using the static polynomial model, and more than 3 bits using the memory polynomial model. it can be noticed that the linearisation is particularly effective in the band from 10 khz to 1 mhz, which corresponds to those particular frequencies used for post-distorter coefficient training. dpd was also applied for the acquisition of pulsed waveforms. in this case, the inverse models were separately trained for 0.5-µs and 1-µs pulse lengths, as well as for positive and negative polarities. figure 18 shows the pulse shape obtained after post-distortion using different models, and in comparison, to the reference channel. table 1 and table 2 summarise the values of the deviations from the reference in all tested cases. a significant reduction in the deviations was achieved for positive pulses, which showed the highest distortion. for example, the nrmsd was lowered more than four times in the case of 1-µs pulses. for negative pulses, the post-distortion had less impact, as the behaviour of the front-end was found to introduce less distortion. using an inverse model with memory brings a further improvement only in the case of 1-µs positive pulses, as the other cases do not show substantial dynamic effects. 6. measurements using the spectrometry system the proposed methodology was tested with the complete spectrometry system setup at the ncbj, corresponding to the block diagram previously introduced in figure 3. the photo of the deployed system is shown in figure 19. it includes an inhouse lanthanum chloride scintillator developed at the ncbj, coupled to the pmt and placed next to a source of gamma radiation, namely a sample of caesium-137 (behind the green lead shield). the pmt is supplied with 1.6 kv from a highvoltage source. the fmc board includes the already introduced ads5474 dual-channel adc, while a xilinx zc-702 fpga carrier board contains the zynq-7000 soc for real-time digital processing of the sampled data. the voltage signal from the radiation detector is divided via a resistive power splitter into reference and test paths, following the same approach as in figure 10 and figure 11 for the pxibased test setup previously adopted. the reference path is table 1. nrmsd (%) with and without mp-based post-distortion for different pulsed excitations. post-distortion positive pulse polarity negative pulse polarity 1 µs 0.5 µs 1 µs 0.5 µs none 5.19 3.34 1.31 1.77 𝐾 = 5, 𝑀 = 0 2.39 1.71 1.23 1.75 𝐾 = 5, 𝑀 = 3 1.2 1.09 1.0 0.83 table 2. integral nrmsd (%) with and without mp-based post-distortion for different pulsed excitations. post-distortion positive pulse polarity negative pulse polarity 1 µs 0.5 µs 1 µs 0.5 µs none 3.44 2.62 1.14 0.88 𝐾 = 5, 𝑀 = 0 1.35 1.19 1.03 0.84 𝐾 = 5, 𝑀 = 3 0.97 1.0 1.03 0.79 figure 18. 1-µs positive pulse with full-scale amplitude after digital postdistortion. figure 19. the gamma spectroscopy measurement set-up at ncbj, including the source or radiation (caesium-137), the analogue front-end (dut) and the two-channel acquisition system. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 78 directly connected to one of the digitiser channels, while the test path consists of the designed analogue front-end board, i.e., the device under test (dut), whose output is connected to the second channel of the digitiser. the dut is supplied from a hameg hmp4040 power supply. the spectrometry setup was used to capture a dataset consisting of 104 events from the scintillator, whose associated voltage pulses are concurrently acquired by both the reference and test adc channels. the pulses have a duration of ~100 ns and amplitudes of up to 1 v. the waveform of a single captured pulse is shown in figure 20. digital linearisation was applied to the captured pulses using the same approach as in section 5. more precisely, the acquisitions from both reference and dut paths allowed to identify the inverse dut model, then to perform digital linearisation by adopting the same post-distortion model formulation and orders as in section 5. as shown in figure 20, the nonlinear pulse waveform displays a sensibly lower amplitude w.r.t. to the reference acquisition, as well as a distorted shape, whereas both the static and the nonlinear dynamic postdistortion allow for a linearised pulse acquisition. just as performed in section 5, the individual pulses were integrated to calculate a value directly proportional to their energy. also, in this case, the deviation between the channels is quantified using nrmsd between the waveforms as well as nrmsd between the integral values computed from the waveforms. the obtained values confirm that the deviation from reference is significantly reduced by the digital linearisation. in particular, the static polynomial model (𝐾 = 5, 𝑀 = 0) reduced the nrmsd from 5.86% to 3.8%, while adding the memory terms (𝐾 = 5, 𝑀 = 3) allowed a further reduction down to 1.98%. similarly, the nrmsd of the integral values was reduced from 5.52% to 1.99% with the static polynomial model, and to 1.09% using the model with memory (see table 3). finally, the energy spectrum from a 250-channel histogram of the integral values was estimated both with and without digital linearisation (figure 21). as the pulses with low amplitude are less distorted, the lower energy part of the spectrum is reasonably similar to the reference. however, at higher pulse amplitudes (i.e., at higher energies) the whole spectrum content, and most notably the gamma photo-peak (~ 662 kev for caesium-137), is shifted towards lower energies by non-compensated nonlinearities (figure 21a). the energy spectrum linearised using the static model (figure 21b) shows a reduced yet still substantial deviation, whereas the dpd with memory (figure 21c) ensures the best alignment with the reference spectrum over the entire energy range. 7. conclusion in this work, the design, characterisation, and performance assessment of an analogue front-end for use in the gamma spectrometry system of the ncbj was presented. the front-end was designed to properly condition the pulsed signals at the output of the detector and protect the receiver from high-voltage spikes due to cosmic radiation. it also provides a suitable, lowtiming jitter clock reference for the adc within the receiver. an extensive characterisation of the protected receiver equipped with the designed front-end allowed to quantify the table 3. nrmsd (%) and integral nrmsd (%) w/ and w/o mp-based postdistortion for the pulse waveform measured with the ncbj acquisition system in figure 19, using caesium-137 as a radiation source. post-distortion nmrsd (%) integral nmrsd (%) none 5.86 5.52 𝐾 = 5, 𝑀 = 0 3.8 1.99 𝐾 = 5, 𝑀 = 3 1.98 1.09 figure 20. example of a pulse waveform from the radiation detector w/ and w/o digital linearisation. comparison with the reference a/d acquisition. figure 21. energy spectrum of caesium-137 estimated using the spectrometry setup including the designed analogue front-end a) without linearisation; b) with static dpd (mp model with 𝐾 = 5, 𝑀 = 0); c) with nonlinear-dynamic dpd (mp model with 𝐾 = 5, 𝑀 = 3). full-scale energy is 723 kev. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 79 reduction of the dynamic range inevitably caused by the analogue limiting stage, as well as by the necessary signal conditioning. a digital linearisation approach based on the identification of an inverse nonlinear dynamic model of the receiver, and its application for post-distortion, allowed to recover the necessary linearity to guarantee the accurate acquisition of the fast pulses generated by the radiation detector. in-field experiments involving actual spectrometry measurements of a caesium-137 radiation source confirmed the suitability of the designed front-end, as well as the effectiveness of the digital linearisation approach, which allows to compensate for critical peak shifts in the estimated energy spectra caused by receiver nonlinear distortion. acknowledgement the authors would like to thank prof. a. lewandowski (institute of electronic systems, warsaw university of technology, warsaw, poland) for financing the conference fee. references [1] r. l. grasty, r. b. k. shives, applications of gamma ray spectrometry to mineral exploration and geological mapping, exploration 97: fourth decennial conference on mineral exploration, 1997. 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perspective for cultural heritage management and fruition acta imeko issn: 2221-870x march 2022, volume 11, number 1, 1 7 acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 1 digital twin: a new perspective for cultural heritage management and fruition francesco gabellone1 1 national research council, institute of nanotechnology, lecce, italy section: research paper keywords: digital twins; oil-mill; photogrammetry; 3d; virtual visit citation: francesco gabellone, digital twin: a new perspective for cultural heritage management and fruition, acta imeko, vol. 11, no. 1, article 5, march 2022, identifier: imeko-acta-11 (2022)-01-05 section editor: fabio santaniello, university of trento, italy received march 6, 2021; in final form february 21, 2022; published march 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: francesco gabellone, e-mail: francesco.gabellone@nanotec.cnr.it 1. the digital twins, next future the term digital twin was originally developed to improve manufacturing and industrial processes [1]. digital twins were subsequently defined as digital replications of physical entities that enable data to be seamlessly transmitted contents from physical to virtual worlds. digital twins facilitate the means to monitor, understand and optimize the functions of all physical entities and provide people continuous feedback to improve quality of life and well-being [2]. digital twin is at the vanguard of the industry 4.0 revolution enabled through advanced data analytics and the internet of things (iot) connectivity. iot has increased the volume of many heterogeneous usable data from manufacturing, healthcare and smart city applications [3]. the iot environment provides an important resource for predictive maintenance and error detection, in particular for the future health of manufacturing processes and smart city developments, while also aiding in fault detection and traffic management in a next smart city. since a perfect integration between iot and data analysis will be necessary in the near future, this important need will be possible thanks to the creation of a connection between physical and virtual twins. a digital twin environment allows smart cities for quick analysis and, using 5g network, real-time decisions can be made through an accurate analysis. the first terminology was given by michael grieves, research professor (florida institute of technology) in a 2003 presentation, and later documented in a white paper where the future developments of digital twins are traced. the first articles make a definition of “digital model”. these are described as a digital version of a pre-existing or planned physical object. an important distinguishing feature, with the old definition of “digital model”, is that there is no form of automatic data exchange between the physical system and the digital model. this means that a change made to the physical object has no impact on the digital model, and vice versa. when data run between an existing physical object and a digital object, and they are fully integrated in both directions, this establishes a "digital twin" reference. a change made to the physical object automatically leads to a change to the digital object and vice versa. a digital twin consequently provides an intimate connection with his real counterpart, and in some way determines an influence on it. an environmental monitoring abstract this paper describes the example of an interesting distance visit approach carried out during the covid-19 emergency, applied to an underground oil-mill in the town of gallipoli (puglia, italy). the limitations of access for people with disabilities and the complete closure of italian museums during the emergency have suggested the development of an immersive platform, in the broader perspective of using the output in accordance to digital twin perspectives. then a tool to support an innovative visit method has been realized: a virtual visit assisted by a real remote guide, hereinafter referred to as “live-guided tour” with e-learning functionality. all this has been made possible starting from a three-dimensional model of an underground oil-mill, from which we extracted the stereoscopic scenes. the stereoscopy is very important for the overall success of the project, because this aspect influences the level of interest, the immersion and the ability to generate emotion and wonder. to the best of author’s knowledge, this is the only system available today for a shared virtual visit for an inaccessible context, which implements many features of a vr visit in a multi-user and multi-platform environment. mailto:francesco.gabellone@nanotec.cnr.it acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 2 system on the real object will be evaluated on the digital twin and, consequently, interventions on the internal microclimate, for example, will be managed remotely with effects on other environmental parameters. if a digital twin is connected with iot systems or sensors, it allows a remote intervention that by the digital object affects the real object. in the near future, the use of digital twins will potentially be very common, they will grow in step with the rapid developments in connectivity via iot within a smart city. as the number of smart cities grows, so will the use of digital twins. moreover, the more data we collect from the iot sensors embedded in our main services within a city, the greater the opportunities for economic growth and development of new innovative start-up able to provide those services will be. digital twins can be used to aid in the planning and development of current smart cities and to help with the ongoing development of new ones in the world of energy saving. this data can facilitate growth by being able to create a living test bed within a virtual twin that can achieve two goals: first, to test the scenarios, and second, to allow digital twins to learn from the environment by analysing changes in the collected data. [4] therefore, digital twin can evolve to become a true digital replica of potential or actual physical resources (physical twin), processes, people, places, infrastructures, systems and devices that can be used for various purposes. we can compare digital twin to other mirror model concept, which aims to model part of the physical world with its cyber representation [5]. 2. digital twins for cultural heritage enjoyment: a future perspective? from this analysis, however, a criticality emerges. there is still a real difficulty in fully assuming a cultural heritage as digital twin, because as grieves himself pointed out, a mirrored spaces model always refers to an extremely dynamic representation [6]. the real dimension and the virtual dimension, in the primitive definition, remained connected during the entire life cycle of the system, going through all the phases of creation, production and operation. the definition of digital twin is still closely related to industrial production and its processes [7]. a necessary condition for the realization of a digital twin is the existence of physical products in real space, of virtual products in virtual space, and systems for connecting the flow of data that unite physical and virtual space. over the past 30 years, product and process engineering teams have used 3d rendering and process simulation to validate the feasibility of an asset within a production process. a 3d model allows the entire system to be merged into a virtual space, so that conflicts and critical issues are discovered more economically and quickly. with these premises, a product is only released when all the problems have been solved. thanks to digital twin it is possible to test and understand how systems and products will behave in a wide variety of environments, using virtual space and simulation as a predictive moment. all this is possible by combining different technologies related to a single database that will contain all plant or product design data, simulation software, real-time data from the production environment and much more. the advantages are many, starting from the possibility of easily accessing data from many different sources, aggregating and visualizing them through a single synchronized and shared portal, and being able to add contextual information. many of the requirements among those listed will certainly not be met, due to the very nature of cultural heritage, which is not linked to the production industry. however, we can certainly imagine a future populated by digital twins of cultural heritage that systematically respond to a series of needs, ranging from conservation to knowledge of places, to enjoinment and enhancement. in a new perspective of growth and use of the iot, digital twins can become real “models of knowledge”, integrated into a wider domain of elements. we can create a cyberspace with digital models that can allow facilities in a city with obstacles and limitations of use. indeed, with other assumptions, this path has been opened for many years. in fact, the term digital heritage refers to the cultural heritage, which exists in relation to a digital model, a copy or replica of the physical (real) model, but often it is intended as “digital media in the service of preserving cultural or natural heritage”. therefore, if the digital heritage is within a broader system (for example of a smart city), where each digital resource communicates or it is connected in different ways to the others, then this could mean an evolution of the digital heritage in the direction of digital twins [8]. next to an environment monitored by sensors and intelligent systems, it is possible to achieve an effective way of using digital models to ensure an ideal interaction with real spaces. this has always been done in the past: virtual archaeology pursues precisely these objectives, but if these models are connected to each other in a smart city, then they will also be part of an ecosystem. considering that, it is possible to convert old digital scenarios with smart visits of cultural heritage in immersive and participatory virtual environments, within enabling platforms. the starting point is to make the virtual visit more collective, more interactive and more participative, with the possibility of figure 1. the complete 3d model of the oil mill and the first light simulation without textures. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 3 receiving a valuation of the understanding level of the communicated contents. the ultimate goal is to integrate these virtual scenarios into an iot system, where each of them can be connected with others 3d models which can be used in a digital twins perspective. these 3d scenarios can give information on their physical counterpart, information relating to environmental monitoring, energy consumption, state of conservation. at the same time we can use these models to cross the gap related to visitors with disabilities, or use them effectively as a distant visit tool. the important condition to make the digital twin effective with regard to communicative issues, is to obtain an ultra-realistic virtual restitution, in order to be able to offer emotion to the visitor, an emotion similar to the one achieved during a real visit [7]. the second condition is to have an accurate 3d model (figure 1). physical measurements can be effective, obviously, only in a reliable 3d space. when these two requirements are both satisfied, it is possible to deal with all the developments according to the world of digital twins. 3. live-guided tour an example of this approach has been carried out during the covid-19 lockdown, applied to an underground oil-mill in the town of gallipoli (puglia, italy) [9]. the main and most profitable activity, which has ruled the fortune of terra d'otranto for many centuries, has been the oil industry, carried out in over 2500 hypogeous and semi-hypogeous trappeti (oil mill). in these sites only lampante oil (oil for lamps) was produced, i.e., for industrial use, exported mainly to france and england, where it was used as a lubricant in wool industries and soap factories [10]. the oil mill described here is entirely dug out of the rock and is actually preserved in its original form, with a very "organic" appearance, without regular or squared surfaces. the three-dimensional survey must be carried out taking care to faithfully reconstruct its natural morphology, but above all the colour of the walls and the interior dark aspect. the limitations of access for people with disabilities and the complete closure of italian museums during the covid-19 emergency have suggested the development of an immersive platform, in the broader perspective of a use of output as digital twin. then a tool to support an innovative visit method has been carried out: a virtual visit assisted by a real remote guide, below referred to as “live-guided tour” with elearning functionality. this tour mode allows to organize virtual tours for groups of visitors who can simultaneously connect to the web and participate in a tour in which a real guide, which is also connected remotely, organizes and sets the visit, providing information to visitors. a webapp allows to accompany customers, students or colleagues in a shared virtual walk [11]. the virtual tour is similar to a video conference in which the interactive content viewed on pc or portable device can be controlled by any participant. the guests of the tour may be accompanied by a real guide in a trip where the guide (the host) has the ability to control the scene displayed by visitors, or to let them freely choose where to turn its gaze. guests can then break away at any time by driving control and freely explore every scene, without losing the interactivity that characterizes the virtual tour. with a mouse click they can relate to the host location; in the same way, the host can force any visitor to reconnect to his point of view. since the tour mode is slightly comparable to a video conference, during the visit each participant can take part in the discussion. the host (whether he is an agent, a teacher, a colleague, a tour guide) can call attention to areas of interest in real-time and discuss what is seen at 360° by all. the guest (customer, student, visitor to a museum, etc.) can follow the guide trip or ask permission to check out the tour for everybody. in this case he himself leads the tour: an ideal solution for asking questions about elements and details scene displayed. this type of visit is a significant improvement compared to video conferences with split screen: in this case we have a built-in communication tool in a virtual tour. each participant will have a name and a small screen that identifies all. for the realization of the webapp stereoscopic panoramas have been extracted from the 3d models [12]. the navigation is not conceived as a real time 3d model, but is based on precalculated panoramas, which therefore allow the same stereoscopic view of a 3d model, but with a request for extremely low hardware performances. the webapp is available on any device, whether desktop or mobile, so every visitor can also connect their mobile phone. the information elements are available in different types: text, audio, images, virtual reconstructions and videos, all specifically developed to enable you to get the best knowledge of the places. the application has been developed with the features of a webapp, which allows greater flexibility and compatibility with most media and operating systems. in addition to these features, the webapp, based on 3d vista software (https://www.3dvista.com/en/), integrates a learning management system (lms). this application platform allows the development of courses in e-learning mode in order to contribute to the realization of an educational or didactic project, but also to obtain objective results in terms of "evaluation of the communicative effectiveness" within a lesson in a virtual tour. in other words, an lms platform determines a score that gives the user a level of competence or knowledge. we can consider what benefits this approach can lead to, for example, on construction sites, or in manufacturing environments, to determine the level of competency of employees. or what benefits it can bring to safety oversight in the workplace (figure 2). the use of these vr-based tools has shown greater effectiveness in learning than traditional teaching methods [13]. the solution permits to ask questions to visitors at any time of the tour. they can be conditioned by a previous action, such as the discovery of a hidden element in the scene, after which a question that asks the user about it will subsequently appear. in addition to the questions and answers (simple or multiple-choice questions), queries can contain all kinds of media, including photos, videos, 360º views or 3d models (figure 3). at the end of a visit session, users will be able to see a score screen that depends on the settings chosen by the author of the tour. the user can download their performance sheet as file.cvs or send it immediately to the lms. this feature allows the organizers of the visit to collect analytical data of the tour and verify the level of satisfaction reached by the participants. figure 2. the user experience of live-guided tour. https://www.3dvista.com/en/ acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 4 4. the 3d survey all this is made possible starting from a three-dimensional model of the underground oil mill, from which the stereoscopic panoramas have been extracted. stereoscopy is very important for the full success of the project. in effect tis his is the aspect on which the level of interest induced in the visitor depends the most [14]. the immersiveness induced by stereoscopic vision is part of an empathic mechanism that can be expressed in the ability to emotionally involve the viewer with a message in which he is led to identify with. as a result, the user will tend to consider the two distinct objects, the digital twin and the real context, as if they were one and the same thing. he may then accept that, during the virtual experience, many of the aspects perceptible only during a real visit can be understood. the immersiveness, if is accompanied by the realism of the restitution, is therefore decisive in considering that the vision obtained with simple 360º panoramas is comparable to that obtainable with a 3d model explorable in real time with hi-end viewers. we can say that the ability to generate emotion and amazement, combined with the sense of presence, is an important element in generating interest and attention during the visit. as a result, greater interest and attention will lead to greater understanding of the communicated message. this explains some of the effort put into generating a 3d model from which stereoscopic panoramas can be derived (figure 3). of course, the current technological landscape offers several solutions to obtain stereoscopic panoramas, in most cases obtainable with simple photos [15]. some manufacturers offer dedicated hardware with up to 12k resolution. the differences between image-based solution and full 3d method are substantial and obvious, but it is worth mentioning here some of the main differences and criticalities inherent in the two methods. first of all, the pano 360° obtained from photos do not allow to determine the surface morphology of the object, consequently they do not yield metric information in xyz space. this is a crucial aspect in the use of digital twins since they are characterized by the intimate connection between 3d topology and real space [16]. in the absence of a 3d morphology, it will not be possible to link information derived from sensors and archaeometric, structural and microclimatic analyses relating to specific parts of the asset in study. just think of the georeferenced superimposition of information and its reading according to layers organized by categories, which is impossible to properly achieve on photographic pano. in the case study presented here, the stereoscopic panos is therefore only one of many media that can be obtained from the 3d model [17]. this is not a starting point but a reading output of the information produced, subsequently organized according to the paradigms of a vr visit. it seems therefore undoubted that the effort to be achieved is to obtain an accurate three-dimensional model, on which all the data relative to the management of the real space connected to its analogous 3d topological space can be hooked. this can be conceived as a geometric space of polygons and as a body of information correlated to the surface colour (figure 4). for the virtual use of the collected digital resources, it is crucial to have an extremely realistic three-dimensional model. realism in this case is not aimed at a cinematic level of representation, but at the best correspondence with the actual state [18]. to this end, a complete three-dimensional model was created using digital photogrammetry techniques, which are now widely used and known to all. the three-dimensional restitution posed many problems of coverage in the hidden areas and of management of photo shooting due to the poor interior lighting, but also to the precise need to obtain a digital twin that restores the same "genius loci" as the real space. in fact, we have deliberately kept the atmosphere of the real space, without making it too artificial or illuminated in a different way than the real space. approximately 3000 high-resolution photos (21 mpixel) were then generated, resulting in a mesh resolution of approximately 0.4 mm, which is more than acceptable for the figure 3. final 3d model after bake texturing process. the starting point of digital twin. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 5 purposes of the project and the size of the mill (approximately 250 sqm). the measurements were georeferenced with coded targets, whose coordinates were recorded with a total station. this has made it possible to keep high the accuracy of the measurements, while keeping fixed the constraints [19]. due to the computational complexity of the photogrammetric model, the calculation was divided by zones with overlapping of about 10 cm. the main orientation (sparse cloud) was generated for the entire set of photographs, while the dense clouds were calculated in several patches [20]. this solution solves many memory and computation problems, which often prevent the completion of digital photogrammetry projects characterized by a large number of photos [21]. the orientation of the sparse cloud is actually not a very hard problem in terms of calculation, even with huge sets of photos, it is therefore possible to calculate individual chunks while maintaining the correct referencing. the spatial position of the different chunks will be respected even in case of an export to modelling software or bim [22], [23]. regarding the texturing process, a texture with a resolution of 15000 pixels x 15000 pixels was calculated for each of the several portions of the model. this process, as expected, causing several problems due to the poor lighting of the interior and the need to shoot the photos with the camera gripped. therefore, a new set of images was created to solve blurring anomalies induced by depth of field and relatively long shutter speeds (often 1/60 sec). from an operational point of view, the sparse cloud was then calculated with the entire set of photos. in the last phase dedicated to the texturing only the photos taken ad hoc for this purpose were enabled. basically, we built a very large set of photos needed to get good geometric detail and a set of about 400 photos dedicated to the texturing. the correct correspondence and reproduction of real colours was ensured by the use of a kodak color separation guide (figure 5). the inclusion of the colour scale in the real environment simplifies the correction of the white point of the image, allowing the removal of cast colours [24]. in our case study the dominant colours (cast colours) are not very evident due to the use of controlled lights with 6500 °k temperature [25]. it is not necessary to affix the colour scale on all photos. if these are acquired in the same way and with the same lamps, the correction can be recorded as an action in photoshop and applied to the whole set of photos [24]. the use of the agisoft metashape software has allowed an optimal management of all phases of the survey (figure 6). in particular, the new mesh creation function was used starting from the depth map instead of the more used dense cloud. this new feature is much more versatile and precise, especially in the presence of dense clouds with a large number of points. the use of depth maps helps to reduce noise on the final surface while preserving thin structures within the scene. new depth map-based method allows to reconstruct exceptionally figure 4. 3d model of the calabrian-style press integrated in the vr visit. figure 5. real photo of the interiors. on the left the original image, on the right the colours have been corrected using the kodak color separation guide. figure 6. synthesis of methodological workflow. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 6 detailed geometry. gpu acceleration allows you to significantly speed up processing with reduced memory consumption compared to previous versions of the software. the final result is shown in the figures on these pages (figure 3 – figure 7). 5. conclusions from a morphological point of view, the digital model of the oil mill is therefore a reliable replica of its physical reference. in the case of extraction machines that are no longer conserved, oneand two-screw presses have been included in the virtual tour, taken with the same 3d techniques from other similar contexts. since photogrammetry ensures excellent accuracy even of the colour data, the 3d model is ready for all subsequent implementations concerning the state of conservation, measurement of volumes, static verification in relation to loads and road surfaces, calculation of energy requirements, etc. at the moment the digital twin allows groups of users to visit remotely this context, in immersive way, with the possibility of extending the visit to other contexts that can follow this management philosophy. this aspect related to the visualization and use of data belongs to one of the purposes of using these digital resources. the main difference to a classic 3d navigable scenario or a classic virtual tour is a new perspective on the use of these models. they are no longer created with the exclusive objective of obtaining a survey of the actual state, but respond to a new management requirement, which takes into account the potential of 5g, the greater computational capacity of portable devices and iot (internet of things). the 3d object in this case is a 'thing' that has its own consistency, certainly digital and immaterial, but tangible and useful for the management of the physical asset. regarding innovativeness in visiting, the live-guided tours are probably the only system available today for a shared virtual tour. but what are the other elements of interest in this project? firstly the benefits offered by the distant visit modality, in the context of the current pandemic emergency. not less important the possibility of virtual access for the disabled, in a multi-user and multi-platform environment. finally, the technological appeal given by the immersive vision and the potentialities related to the digital twin philosophy, still not fully explored in the cultural heritage sector. the long-term goal for us is the creation of an advanced management model. the association between physical object and virtual reality makes it possible to activate a data analysis and monitoring of the systems in such a way that it is possible to operate in predictive mode, identifying problems even before they occur. in addition to preventing anomalies, downtime and inefficiencies, it is possible to develop new opportunities using appropriate simulations and planning future business. by creating a digital twin, it is possible to better understand how to optimize operations, increase efficiency or discover a problem before it happens. the creation of digital twins related to cultural heritage allows the making of models representative of reality, to be used for conservation purposes, for knowledge and for overcoming physical and cognitive barriers. acknowledgement i would like to thank ing. claudio stasi for granting the surveys of the oil mill. references [1] a. fuller, z. fan, c. day, c. barlow, digital twin: enabling technologies, challenges and open research, in ieee access, vol. 8, (2020), pp. 108952-108971, doi: 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measurement training; engineering education; hands-on pedagogy; image sensor characterization citation: raik illmann, maik rosenberger, gunther notni, training program for the metric specification of imaging sensors, acta imeko, vol. 11, no. 4, article 10, december 2022, identifier: imeko-acta-11 (2022)-04-10 section editor: eric benoit, université savoie mont blanc, france received august 26, 2022; in final form december 2, 2022; published december 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: raik illmann, e-mail: raik.illmann@tu-ilmenau.de 1. introduction optical coordinate metrology is an essential part of industrial automation and inspection processes. therefore, this subject area should play an essential role in today's engineering education in courses such as mechanical engineering, electrical engineering, computer science or engineering informatics. thus, the question arises, how to efficiently teach essential contents, which are necessary for a successful handling of this topic in engineering practice. the requirements are that both the basics and the relevance have been understood, and that the systems engineering integration can be implemented on the basis of the learned knowledge and transferred into a functional system. group work and the targeted intervention of a supervisor play an essential role here. not only in order to close existing knowledge gaps in a targeted manner, but also in order to train the cooperation in teams with corresponding social competence, which is inevitable in practice. the central topic of the program is the metric characterization of imaging sensors. with regard to practice, two essential technological aspects can be taught through this. first, it is crucial for the implementation of a test system to be able to evaluate and classify geometric product specifications (gps) characterized by the manufacturer of an image sensor, i.e., ultimately to be able to understand its test procedure. secondly, this makes essential principles of image signal processing accessible and also provides a practical and comprehensible application case, which proves the usefulness of the methods and thus offers the motivation directly on the basis of the concrete example. the standard din en iso 10360 [1] is used as the basis for the methodical procedure; for coordinate measuring machines with optoelectronic sensors, the vdi standard 2617 [2] is used specifically. it describes the inspection of coordinate abstract measurement systems in industrial practice are becoming increasingly complex and the system-technical integration levels are increasing. nevertheless, the functionalities can in principle always be traced back to proven basic functions and basic technologies, which should, however, be understood and developed. for this very reason, the teaching of elementary basics in engineering education is unavoidable. the present paper presents a concept to implement a contemporary training program within the practical engineering education on university level in the special subject area of optical coordinate measuring technology. the students learn to deal with the subject area in a fundamentally oriented way and to understand the system-technical integration in detail from the basic idea to the actual solution, which represents the common practice in the industrial environment. the training program is designed in such a way that the basics have to be worked out at the beginning, gaps in knowledge are closed by the aspect of group work and the targeted intervention of a supervisor. after the technology has been fully developed theoretically, the system is put into operation and applied with regard to a characterizing measurement. the measurement data are then evaluated using standardized procedures. a special part of the training program, which is to promote the own creativity and the comprehensible understanding, represents the evaluation of the modulation transfer function of the system by a self-developed algorithmic program section in the script-oriented development environment matlab, whereby students can supportively fall back on predefined functions for the evaluation, whose implementation however still must be accomplished. mailto:raik.illmann@tu-ilmenau.de acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 2 measuring machines by measuring calibrated test samples. this involves checking whether the measurement deviations are within the limits specified by the manufacturer or user. the test samples must be such that their properties do not decisively influence the parameters to be determined. the characterization is carried out on the basis of the principle described in [2] a circle measured at five different positions. for this purpose, a calibrated chrome standard and a through-light unit are used. after completion of the measurements, the results are to be statistically evaluated in accordance with the standard. in addition, the determination of the modulation transfer function (mtf) is intended to provide an assessment of the resolving capability of the overall system and to train an in-depth algorithmic understanding of 2d image processing. 2. theoretical background 2.1. problem description the metrological problem with which the students are to be confronted is described in figure 1. a calibrated sample with a circular ring (object) is placed as a normal on a light source. the circular ring passes (as a negative) through an optical system which consists of lenses and apertures. the image that is consequently created on the sensor surface deviates from the original image due to optical and mechanical influences. in addition, geometric quantization during scanning within the image sensor produces a further measurement deviation. all deviations in sum result in a total measurement deviation which has to be determined. further it can be observed that with a change of the position of the object (5 different positions in [2]) due to the influences of the optics and mechanics the measured diameters of the circular rings differ. this also has to be quantified. 2.2. analysis of measurement uncertainty all image processing algorithms are already implemented in software, so that the diameter of the circular ring is output as the result of the measurement. the algorithms for edge detection are subpixel-based and very complex, therefore a more detailed description is not provided within this paper so reference is made to special literature [3]. more crucial for the metric testing of the image sensor system is the consideration of the measurement uncertainties. according to the formulas 1-3 the total measurement uncertainty can be determined. the measurement uncertainty describes the deviation behaviour of the overall system. the total value of the measurement uncertainty is determined by the errors of the two essential assemblies, the mechanical and the optical measuring device, and results from: 𝑈total = √𝑈mech 2 + 𝑈opt 2 . (1) both mechanical and optical measurement uncertainty can be further divided into systematic and random measurement uncertainty. they result from: 𝑈mech = √𝑈sys,m 2 + 𝑈random,m 2 (2) and 𝑈opt = √𝑈sys,o 2 + 𝑈random,o 2 . (3) the systematic measurement uncertainty is usually specified by the manufacturer and is determined from comparative measurements with calibrated standards. within the training program these are given to the students. the random measurement uncertainty, on the other hand, is determined by several measurements carried out in the training program under the same environmental conditions and with the same test specimen. the standard deviation can now be calculated from the measured values obtained. 𝜎 = √ 1 𝑛 − 1 ∑(𝑥𝑖 − �̄�) 2 𝑛 𝑖=1 . (4) the random measurement uncertainty 𝑈random,ms of the measurement series 𝑈zuf,mr is now obtained, using the 95.4% confidence level. 𝑈random,ms = 2 𝜎 √𝑛 . (5) figure 1. schematic system description. acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 3 however, since the mechanical and optical measurement uncertainties in the measurement result cannot be separated directly with the available setup, the evaluation is simplified somewhat at this point and only predefined systematic deviations are included in the calculation of the total measurement uncertainty. 2.3. modulation transfer function there are several approaches to determine the modulation transfer function (mtf). the main approaches are part of the teaching in parallel to the designed training program and are presented in the lectures. a simple and practically easy to understand method is described in [4] and [5]. thereby, the intensity differences (contrasts, 𝐼max − 𝐼min) of a search ray orthogonal to a rectangular signal are evaluated. if instead of the rectangular signal a sinusoidal signal is assumed, the contrast transfer function 𝐶𝑇(𝑓) goes directly over into the modulation transfer function 𝑀𝑇𝐹(𝑓). basically, the contrast ratios of the pattern in the image 𝐾(𝑓) 𝐾(𝑓) = 𝐼max − 𝐼min 𝐼max + 𝐼min (6) have to be put into the ratio to the contrast in the object space 𝐾′(𝑓). 𝐾′(𝑓) = 𝐼max ′ − 𝐼min ′ 𝐼max ′ + 𝐼min ′ . (7) plotted over the sampling points of the spatial frequency f as reciprocals of the line distances defined by the object, the mtf can thus be approximated 𝐶𝑇(𝑓) = 𝐾′(𝑓) 𝐾(𝑓) ≈ 𝑀𝑇𝐹(𝑓) . (8) for the calculations according to (6), (7) and (8), all values can be determined from the measurements, which is why these formulas must also be implemented by the students in the practical part. complex examples for modulation transfer functions of spectral sensors are given in [6]. 3. measurement system description 3.1. measurement system the system used for the experiment is shown in figure 2. it consists of a monochromatic camera , a telecentric objective , a light table for measurement using the transmitted light method , a halogen-based light generator , and a stand construction in column design . 3.2. measurement targets two samples are used for the training program. the first sample is shown in figure 3 (left). in the sample, the metric dimensions are represented by circles. these circles are moved to the 5 positions described and the diameter of the corresponding circular ring is measured there in each case. the second sample is a u.s. air force (usaf) test chart shown in figure 3 (right), based on which the modulation transfer function is determined. 4. training program concept the training program is shown as a flow chart in figure 4 and is described in detail below. 4.1. preparation and general aim the overall aim is to teach and consolidate the theoretical aspects. the theoretical basics are taught in preparation of the students in self-study. at the beginning of the training program, the supervisor checks the essential basics necessary for understanding the subject matter. here it must be paid attention particularly on the part of the supervisor to recognize lack of understanding and to recognize possibly by further questions the actual knowledge conditions of the participants individually. often misunderstandings or misinterpretations of the facts occur, which must be corrected. interdisciplinary action should also be taken here, and basic mathematics or mechanics should also be addressed. essentially, the following topics should have been learned at the end of the program: figure 2. measurement setup. figure 3. measurement targets. circular rings chrome glass (left), usaf (right). acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 4 • knowing and understanding terms [1] • understanding the necessity of characterisation concerning engineering tasks • understanding measurement setup (incl. optics) • understanding the measurement procedure • reflection and deduce the causes of uncertainties in the measurement system • evaluating und visualize the data • encouraging the understanding of algorithms. it is also important that the supervision always refers to the method of application and integration of the systems and components in the engineering practical task. in this way, the important practical relevance is continuously maintained. after verification of qualification and the elimination of misunderstandings, instruction is given in the measurement setup and operation of the software. 4.2. measurement process the measurement is performed according to the instructions given in [2]. regarding the training program, the meaningfulness has already been discussed in the basics and does not provide any knowledge gain for the student in the actual sense at this point. at this point, only the practical data is acquired, with which the evaluation and interpretation can be done afterwards. in the first step the 10 single measurements of the circular rings are carried out at 5 positions, which are reached by shifting the measuring object in the object field by means of the x-y-stage. the software required for image acquisition is provided for the student and can be operated intuitively, and the circle diameters are also determined in this software. after determining the circle diameters, these values are saved in the background in a file, which can later be pulled into the evaluation software. the second measurements is the capture of the usaf test chart, which is captured as a single image and then saved as an image file. 4.3. evaluating the measurements the evaluation of the measurement data is generally the most important and most insightful part of the training program. here, the students learn how to evaluate data in an environment that is frequently encountered in practice. both the measurement data of the circular diameters and the development of the mtf are implemented in the matlab environment. in general, the necessary program text is available as cloze text and must be completed by the students at the essential points. this also promotes the algorithmic understanding of a sequential process. the first insight is the differentiation of the data types. the measurement data of the circle diameters are purely vectoral and 1-dimensional, the image data as 2-dimensional field. in addition, the image data are available as 8bit integer, the measurement data are of the data type "double". for the evaluation of the circle data a script is available, which must be supplemented at some commented places only by the function for the computation of the standard deviation. essential necessary standard functions and their arguments to be passed with syntax must be researched by the students themselves via the internal help and implemented in the main program. experience shows that most students have enormous methodical deficits exactly at this point for independent problem solving. figure 4. measurement target 1. acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 5 4.3.1. determination of the measurement uncertainty the data obtained in the measurement serves as the basis for determining the measurement uncertainty. these are available in a csv-file (comma separated value) stored in the background by the image processing software and can be transferred to matlab with a function for reading in. afterwards the data have to be validated. unfortunately, problems occur again and again due to the comma-dot separation at the decimal point in the exchange with german-language software, which should be explicitly pointed out here. in the last step the calculation of the measurement uncertainty by standard deviations etc. takes place and are to be completed here after the appropriate equations by the students in the program text. 4.3.2. determination of the mtf more detailed understanding is required to calculate the mtf. it should be noted that within the training program only the mtf of the entire system is to be determined. of course, this would also be possible for each individual component in the beam path. however, the goal is to get a pragmatic overview of the resolving power of the entire system. in addition, the time required should also be kept within reasonable limits. the necessary data are available as a single image, which must first be read in. in the next step, the relevant signals must be extracted from the image data set as one-dimensional signals. this is done manually by defining the start and end points of the search beams. afterwards, the students represent these signals as a simple plot for illustration purposes. to write the necessary functions would go beyond the scope of the training program, therefore these functions are predefined. after extracting the vectoral data, the students calculate the mtf according to the relationships presented in the basics. here it is also important to understand the signal properties of the overall image (contrast ratios) and to calculate them with appropriate functions from the field. also, here the necessary program text is available as gap text. calculations for visualization of the search rays in the image etc. have been predefined. as a result, the mtf is plotted graphically. 4.4. reflexion and interpretation of the data the main focus of the training program is the graphical representation of the evaluated data. for each result a plot is to be created, which illustrates the data. figure 5 shows an exemplary graphical representation of the statistical values of 10 single measurements at the 5 relevant positions. the graph shows the histogram and the distribution function fitted in it (red curve). on the basis of the compression of the curve the meaning of the standard deviation as a scattering parameter of the data can be read off immediately. the knowledge gain is secured by the graphic connection of the data thus finally and is to be summarized a protocol. all results are finally discussed with the supervisor on the basis of the graphs. thus, the learning result is secured by the renewed repetition and purposeful discussion of substantial qualitative characteristics, particularly in the curves. experience shows that qualitative progressions can be memorized very lastingly through graphically illustrated relations. 5. integration into teaching the present concept will be integrated into teaching as a fixed element from the time of publication. however, initial test sessions with students for the evaluation of the concept were performed before. these preliminary tests with a total of 4 groups had three main reasons. first of all, it was to be determined whether the test could be completed at all by the students in the appropriate time. all 4 groups managed to complete the task within the set time frame of 3 hours. the second reason is to estimate the suitability. the students were asked in detail about their assessment after completing the program. all groups confirmed that they had understood the usefulness of the course and its relation to real practice. furthermore, it was confirmed that the students were neither overstrained nor understrained at any time. the first group, which had already conducted an experiment on a similar topic in a different form a long time ago, represented a special case. this group confirmed above all the increase in clarity and confirmed that they had become better familiar with the topic due to the many integrated qualitative graphical representations of the results. the third reason for the preliminary tests is to test the stability of the system. this means both the system stability of the hardware and the stability of the software. none of the groups experienced any problems in this regard. the hardware ran stably, did not cause any dropouts, and the software did not deliver any errors. figure 5. measurements on 5 positions (target 1). acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 6 6. conclusion the present work represents a full methodological treatise on the development of a training concept of engineering students in the field of optical coordinate metrology and adjacent to the image processing. current and relevant problems are integrated, basic problems of image processing are simulated, demonstrated and solved. ensuring expert supervision, but intervening only when necessary, underlines the pedagogical concept. the clearly identified problems are solved using current tools. the focus is also always on creating a graphical picture between data, their evaluations and calculations, on the basis of which the results are easily discussed and the facts are easily memorized by the students through the graphical symbolization. special attention was paid to typical, recurring deficits and problems of the students during the creation of the program, which is why they were specifically included and problems are intentionally implied during the execution of the program. acknowledgement we thank the technische universität ilmenau for the financial support to realize this practical course. references [1] iso 10360 7: geometrical product specifications (gps) acceptance and re-verification tests for coordinate measuring machines (cmm) part 7: cmms equipped with imaging probing systems, international organization for standardization geneva, 2011. [2] verein deutscher ingenieure, vdi/vde 2617 blatt 6.2,: accuracy of coordinate measuring machines characteristics and their testing guideline for the application of din en iso 103608 to coordinate measuring machines with optical distance sensors, beuth verlag gmbh, berlin, februar 2021. [3] j. beyerer, f. puente león (eds.), automated visual inspection and machine vision iii: 27 june 2019, munich, germany (spie, bellingham, washington, usa, 2019). [4] t. luhmann, s. robson, close-range photogrammetry and 3d imaging, de gruyter, berlin, 2019, 3rd edn. [5] w. g. rees, physical principles of remote sensing, cambridge univ. press, cambridge, 2003, 2nd edn. [6] m. rosenberger, p.-g. dittrich, r. illmann, r. horn, a. golomoz, g. notni, s. eiser, o. hirst, n. jahn, multispectral imaging system for short wave infrared applications, proceedings of spie, 2022, vol. 12094, id. 120940z, 14 pp. doi: 10.1117/12.2619350 https://doi.org/10.1117/12.2619350 roman coins at the edge of the negev: characterisation of copper alloy artefacts and soil from rakafot 54 (beer sheva, israel) acta imeko issn: 2221-870x december 2022, volume 11, number 4, 1 6 acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 1 roman coins at the edge of the negev: characterisation of copper alloy artefacts and soil from rakafot 54 (beer sheva, israel) manuel j. h. peters1,2, yuval goren3, peter fabian3, josé mirão4,5, carlo bottaini4, sabrina grassini1, emma angelini1 1 department of applied science and technology, politecnico di torino, corso duca degli abruzzi 24, 10129 torino, italy 2 department of history, universidade de évora, largo dos colegiais 2, 7000-803 évora, portugal 3 department of bible, archaeology & ancient near east, ben-gurion university of the negev, israel 4 hercules laboratory, institute for advanced studies and research, university of évora, largo marquês de marialva 8, 7000-809 évora, portugal 5 geosciences department, school of sciences and technology, university of évora, rua romão ramalho 59, 7000-671, évora, portugal section: research paper keywords: archaeometry; archaeometallurgy; archaeological materials science; classical archaeology; near eastern archaeology citation: manuel j. h. peters, yuval goren, peter fabian, josé mirão, carlo bottaini, sabrina grassini, emma angelini, roman coins at the edge of the negev: characterisation of copper alloy artefacts and soil from rakafot 54 (beer sheva, israel), acta imeko, vol. 11, no. 4, article 13, december 2022, identifier: imeko-acta-11 (2022)-04-13 section editor: tatjana tomic, industrija nafte zagreb, croatia received april 26, 2022; in final form december 14, 2022; published december 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: manuel j. h. peters, e-mail: manueljhpeters@gmail.com 1. introduction this research discusses the analysis of several copper alloy artefacts from an arid site on the edge of the negev desert in israel. rakafot 54 was excavated during two seasons in 2018 and 2019 as a collaboration between the israel antiquities authority and ben-gurion university of the negev (figure 1). the site was probably established during the first century ce and demolished during the bar kochba revolt against rome (132-135/6 ce) [1]. several indications of regular habitation have been found, including oil lamp fragments and limestone vessels. the site is placed near the border between judea and nabataea, close to a roman road, and includes a watchtower, hearths, garbage pits, and an underground system. numerous artefacts were found during the two seasons of excavation, including various imperial and provincial roman copper alloy coins. this includes copper alloy coins minted by herod agrippa i (ruled 41-44 ce) and the roman procurators in judaea (6-66 ce). additionally, provincial roman coins minted in ashkelon during the reign of nero to trajan (37-117 ce), nabataean coins (until 106 ce) and coins of the first jewish–roman war (66–73 ce) were found. in terms of the geological setting, the site is located on quaternary alluvium loess soil. the loess is soft yellow windblown silty sediment, dominated by quartz silt, clay and calcite. the loess deposition, by southern-western winds from sinai and north africa, started at the end of the pleistocene and continues into the holocene. the formation is bordered to the abstract the research presented in this paper focused on the preliminary nonand semi-destructive analysis of copper alloys, corrosion, and soil components from a roman archaeological site in israel. investigations using portable x-ray fluorescence, x-ray diffraction and scanning electron microscopy with energy dispersive spectroscopy as well as micromorphological analyses were carried out to gain a better understanding of the corrosion processes affecting the copper alloy artefacts, by characterising the alloy composition, soil environments, and corrosion products. preliminary results indicate that the artefacts consist of copper-lead-tin alloys, covered by copper hydroxychlorides and lead sulphate phases with slight variations in their crystallisation. the multi-analytical approach revealed the presence of quartz, calcite, gypsum and feldspars in the sediments, while thin sections more specifically indicate loess soils with local microenvironments. mailto:manueljhpeters@gmail.com acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 2 west and the south by a coastline of calcareous sandstone and sand dunes, to the east by the eocene adulam formation which is mainly chalk and chert, and to the north by quaternary red sand and loam (locally termed hamra soil) [2]. in the beer sheva area, where mean annual precipitation is 200–350 mm, loess covers the entire landscape and pre-existing topography [3]. the soil components of the archaeological site are largely dependent on this general geological setting, with local variations due to anthropogenic activities and post-depositional processes. these variations could possibly lead to micro-environments corresponding to the various excavation loci. the copper alloy objects coming from the site were covered with a heterogeneous layer of corrosion products and sediment (figure 2). the items were retrieved from different areas, which contained sediments that appeared to have distinctively different compositions, based on a visual inspection. this could possibly affect the corrosion process in various ways. in order to gain a better understanding of both the degradation processes affecting these artefacts and to determine their state of preservation and possible future steps for conservation, a selection of artefacts was analysed with portable x-ray fluorescence (pxrf), x-ray diffraction (xrd), and scanning electron microscopy with energy dispersive spectroscopy (sem-eds). the reason behind this approach lies in the general need to avoid intrusive sampling of museum objects and the requirement for the exclusive application of non-destructive testing (ndt) techniques for the material characterisation of cultural artefacts. as a result, research is gradually becoming reliant on portable instrumentation (e.g. pxrf) that is intended to screen only the surfaces of the objects under investigation. however, especially in the case of metals, the effects of chemical-physical processes responsible for degradation and corrosion as well as surface enrichment or loss of elements make ndt highly problematic. metallic objects can be covered in thick layers consisting of corrosion products and soil components, which usually means that surface analysis to investigate alloy components of the bulk material becomes relatively inaccurate. if it is possible to expose a small area of the bulk material below the corrosion layers, semeds can be carried out to validate the results obtained with pxrf. although the degradation processes of metallic artefacts are well-known and widely investigated, every artefact has a specific history strictly related to its depositional microenvironment [4]. the environmental parameters influence the corrosion process and affect the various areas of the metallic objects differently. consequently, it is of high importance to assess the specific deterioration of the artefacts in order to be able to interpret the results of ndt surface investigation. in this paper, several artefacts and soil samples that have a clear connection were considered. the main goal of this research was to gain information on the elemental composition of the coins, as well as identifying the corrosion products, and assessing the soil components. this was done by using a multi-analytical methodology involving elemental and mineralogical characterisation, as well as microscopy. the bulk of this research was performed at ben-gurion university of the negev. 2. materials and methods this research investigated the alloys, corrosion products, and related soil of a group of corroded metallic coins from rakafot 54. during the excavation, the location of the metallic artefacts was recorded by assigning them to the specific locus they originated from, and the same was done for the soil samples. for this research, six coins and their related soil samples were selected. a preliminary pxrf surface screening was carried out on the coins prior to conservation treatment, to determine the major alloy components of the coins. a thermo scientific niton xl3t goldd+ xrf analyser equipped with a geometrically optimized large area drift detector and 50 kv, 200 µa ag anode was used, with an 8 mm collimator. the data was obtained with the cu/zn mining measurement mode, with a duration of 120 seconds. both sides of each coin were measured. xrd measurements were performed separately on the corrosion samples collected from the artefacts. sampling was done by carefully removing some milligrams of corrosion from the objects with a scalpel, paying attention to not damage the limitos (the surface boundary between the external corrosion products and the corrosion that has replaced the bulk metal, figure 1. aerial orthophoto of the archaeological site of rakafot 54. figure 2. copper alloy coin with corrosion and soil. table 1. sample types and analytical techniques. sample type technique information artefact surface pxrf alloy & soil exposed bulk material sem-eds alloy & microstructure corrosion powder xrd corrosion & soil soil thin section petrography soil acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 3 isomorphic with the uncorroded artefact). the samples were then homogenised by grinding them, using an agate mortar and a few drops of ethanol. this emulsion was positioned in the middle of a zero-background silicon sample holder and flattened, after which the ethanol was left to evaporate. diffractograms were taken from the samples with a panalytical x'pert pro powder x-ray diffractometer equipped with pixcel 1d detector and a cu k-alpha source with wavelength 1.54 å, at 40 kv with a current of 40 ma. thin sections from the soil samples were obtained by embedding a portion of each sample in epoxy resin, mounting it on a glass microscope slide, and grinding and polishing it to 30 µm. the thin sections were then observed with a petrographic microscope under polarised light. sem-eds was carried out on a small exposed area of the bulk metal of the artefacts, using a micro fasem fei quanta 200 scanning electron microscope under high vacuum at 25.00 kv. an overview of the sample types, analytical techniques and the expected information can be found in table 1. 3. results 3.1. pxrf the preliminary pxrf screening of the coins indicated that the coins are made of copper-lead-tin alloys, with relatively high amounts of lead in some cases (table 2). the presence of silver is below the limit of detection, and zinc appears to be only present as trace element in some cases. there are inconsistencies between the measurements on the obverse (a) and reverse (b) sides of each coin, probably stemming from the position of the samples on the instrument, and the fact that these measurements were carried out on uncleaned objects. consequently, the results are largely dependent on the corrosion layers and sediments present on the surface. nevertheless, these results offer a useful indication of the primary alloy components of the coins found at the site and offer a clue of the possible corrosion products. additionally, the pxrf measurements provide information about the traces of sediments that are possibly present on the surface of the artefacts together with the formed corrosion phases (table 3). significant amounts of calcium, chlorine and silicon can be observed as well as smaller amounts of aluminium, potassium, and sulphur. additional components can be found in table 4. 3.2. petrography the thin sections of all the soil samples observed under the petrographic microscope revealed a matrix that is silty and highly calcareous. the silt is well-sorted and contains mainly quartz, but also a recognisable quantity of other minerals, including hornblende, zircon, mica minerals, feldspars, tourmaline, augite and more rarely garnet, epidote and rutile. ore minerals are abundant too in this fraction. the sand fraction includes dense, well sorted, rounded sand-sized quartz grains, and limestone. based on a bulk of published data it is readily identified as loess soil [5]. however, micromorphological features within this given theme indicate more specific micro-environments, resulting from an array of anthropogenic activities as well as postdepositional processes. the post-depositional processes are table 2. primary alloy components (wt%) on artefact surface, obverse (a) and reverse (b). sample cu pb sn zn ag b6058a 46.8 5.1 2.7 0.04 < 0.03 b6058b 45.1 5.7 1.0 < 0.03 < 0.03 b6081a 44.6 13.0 3.4 < 0.05 < 0.04 b6081b 47.5 5.6 0.45 < 0.03 < 0.03 b6101a 42.1 6.8 1.5 < 0.04 < 0.03 b6101b 36.0 14.4 4.4 < 0.04 < 0.04 b8103a 43.0 1.1 0.04 < 0.04 < 0.02 b8103b 36.0 3.4 2.1 0.02 < 0.02 b8764a 43.9 3.2 0.34 0.03 < 0.02 b8764b 46.5 3.0 2.6 0.05 < 0.03 b9803a 44.0 5.1 1.3 < 0.03 < 0.02 b9803b 32.1 14 5.0 < 0.04 < 0.04 table 3. soil components (wt%) on artefact surface, obverse (a) and reverse (b). sample al ca cl fe k s si b6058a 2.1 8.7 4.61 0.51 0.25 1.12 9.1 b6058b 2.3 4.9 5.09 0.81 0.38 1.52 13.1 b6081a 1.4 5.4 5.77 0.13 0.11 3.81 4.7 b6081b 0.9 2.9 9.9 0.34 0.15 2.18 3.2 b6101a 2.1 6.0 5.46 0.68 0.35 2.20 9.1 b6101b 1.0 10.1 3.22 0.33 0.20 3.44 9.1 b8103a 1.9 7.4 3.67 0.59 0.31 0.55 11.9 b8103b 1.6 8.4 1.27 0.77 0.34 1.42 13.4 b8764a 1.9 4.9 8.0 0.60 0.32 1.66 7.4 b8764b 1.7 10.6 6.27 0.51 0.26 1.33 6.0 b9803a 2.7 10.7 4.33 0.56 0.39 1.13 11.5 b9803b 3.2 13.9 2.34 1.25 0.70 1.35 12.5 table 4. trace elements (wt%) on artefact surface, obverse (a) and reverse (b). sample mg mn ni p ti b6058a < 2.4 < 0.02 0.02 0.11 0.11 b6058b < 3.4 < 0.03 0.02 0.16 0.17 b6081a < 2.6 < 0.02 0.04 0.80 0.04 b6081b < 3.1 < 0.02 0.01 0.07 0.05 b6101a 2.6 < 0.03 < 0.02 0.29 0.13 b6101b 2.9 0.02 0.01 0.60 0.06 b8103a < 4.0 < 0.02 < 0.02 0.31 0.11 b8103b < 4.4 < 0.02 < 0.02 0.46 0.17 b8764a < 4.0 < 0.02 0.03 0.29 0.12 b8764b < 4.4 < 0.02 0.02 0.08 0.09 b9803a < 2.4 < 0.03 < 0.02 0.14 0.12 b9803b < 2.0 0.03 0.02 0.28 0.23 figure 3. petrographic image showing silty and calcareous matrix with rounded quartz sandy grain (a), charred ash material (b), shell fragment (c), and burnt soil with ferric corrosion products (d). acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 4 reflected by re-crystallisation of calcite and gypsum from ground water, corrosion products of metals (figure 3), mineral replacements, clay relocation, etc. anthropogenic features include remnants of human activities such as slag, coprolites, pottery and brick crumbs, charred plant remains, phytolith concentrations, and other ash characteristics (figure 4). all these will be used in further research to define the immediate microenvironmental physiognomies of the sediment at the immediate proximity of each artefact, in order to correlate it with the corrosion composition. 3.3. xrd xrd diffractograms which include the representative peaks for these samples are shown in figure 5. although there are some visual differences in the diffractograms, mainly related to variations of peak intensities due to sample preparation, the main components appear to be relatively consistent. the primary compound in all samples was identified as atacamite (cu2(oh)3cl), the orthorhombic version of copper hydroxy-chloride, which can be commonly found on copper alloy archaeological objects extracted from salty soils [6]. clinoatacamite, its monoclinic polymorph, has been detected in two samples, while its presence is not clear in the rest of the cluster under investigation. paratacamite (cu3(cu,zn)(oh)6cl2), a zinc-enriched variety of the copper hydroxy-chloride series, also appeared in the majority of the diffractograms (figure 5). even though it is not considered a polymorph, it is still closely related to atacamite as part of its degradation process [7]. the zn possibly originates from traces in the alloy composition. lead sulphates were identified in most samples, while cuprite was identified in all samples except for rcb6081. this exception might be related to inconsistencies in sampling depth. all diffractograms revealed the persistent presence of quartz, which can be explained by the traces of sediment that were present on the surface of the artefacts during the sample preparation for the xrd analyses. other soil components that were identified in this step are kand na-based feldspars (albite and orthoclase), together with clay minerals such as zeolite. gypsum and its polymorphs were identified in rcb6058, rcb6101, and rcb9803, but may also be present in other samples. 3.4. sem sem-eds results of the point analyses showed copper alloys with high amounts of lead and smaller additions of tin (table 5). since lead does not dissolve well in a cu-based matrix, it usually forms globules [8]. most of the analysed samples did indeed show a bulk matrix which is low in lead, with many lead globules distributed in the matrix, and clear cracks and crevices penetrating into the bulk in some cases (figure 6). these cracks might aid the penetration of cl into the bulk matrix, as shown in table 5. 4. discussion in order to validate the results of the individual techniques, several comparisons were made. the identification of atacamite in the diffractograms can be correlated with the presence of cl in the pxrf results. it is still unclear whether this presence is related to the hydroxy-chloride phases of the corrosion products, or to another cl-based mineral present in the soil. differentiating between the lead sulphate and calcite peaks in the xrd diffractograms was complicated in some cases, however, a clear correlation could be observed between the presence of sulphur and significant amounts of lead in the pxrf data, and the presence of lead sulphate in the diffractograms. although the presence of tin is clear in the primary alloy identification with pxrf, no tin-based corrosion products were identified in the diffractograms; however, tin oxides are reported to provide difficulties in identification with xrd [9], [10], [11]. the si presence in the pxrf data could be related to the quartz observed in the diffractograms and the thin sections. the k and si combination could be explained as k-based feldspars and quartz with the diffractograms, and similarly, si and al could be identified together in feldspars and clay minerals. figure 4. petrographic image showing herbivore dung spherulites (e) in ash layer from a hearth. figure 5. diffractogram of artefact surface samples consisting of corrosion and soil minerals. a=atacamite; cu=cuprite; ls=lead sulphate; c=calcite; q=quartz; g=gypsum; z=zeolite; p=paratacamite. figure 6. sem image. left: sample b6101 showing cu-rich matrix with distribution of pb globules as bright spots. right: sample b8764 showing cavity and crack possibly improving the penetration of cl into the bulk. acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 5 most minerals identified in the micromorphological study of the thin sections have their origins in igneous or metamorphic rocks. furthermore, the micromorphological study provides a description of the soil components that corresponds with the results from the pxrf and xrd analyses. 5. conclusions the overarching purpose of this research was to obtain a better understanding of the various factors influencing the degradation of a set of roman artefacts coming from rakafot 54 in israel, by using a multi-analytical approach that relied on elemental and mineralogical characterisation, as well as microscopy. the first objective was to identify the main alloy components. this was accomplished by carrying out a preliminary pxrf screening on the artefact surfaces before cleaning, which showed that the alloys were all copper-lead-tin based. the second goal of this research was the determination of possible corrosion products of the objects, which was done by carrying out xrd analysis on small corrosion powder samples. the results here indicate a strong presence of copper hydroxychlorides such as atacamite and clinoatacamite. paratacamite was also detected in the majority of the samples and finds partial confirmation in the detection of small amounts of zn with pxrf. additionally, some lead sulphates were detected, which could be correlated with the elemental data. finally, the main soil components were identified. by combining pxrf, xrd, and micromorphology, it was shown that the soil mainly consists of quartz, calcite, feldspars, clay minerals, and gypsum. the sem-eds results indicate a general presence of pb globules in most samples, with a surrounding matrix of cu-sn. although the micromorphological study indicates local differences in soil components of the various samples, often related to burning and organic materials (presence of phytoliths and spherulites), these differences do not appear to have a clear effect on the corrosion products. the differences in intensities of the diffraction peaks representing the various corrosion products, and the absence of cuprite in one sample could be related to inconsistencies in sampling depth and small sample size. additionally, it is possible that some soil features, such as the presence of chlorides, negate the effect that small variations in soil and alloy components could have. while there are several limitations to this research, mostly resulting from the relatively small amounts of corrosion available for analysis, the presented results provide a first insight into the corrosion processes at the site, and interesting directions for future research. author contribution manuel j.h. peters: conceptualisation, methodology, software, formal analysis, investigation, data curation, writing original draft, visualisation. yuval goren: formal analysis, resources, data curation, writing original draft, supervision, funding acquisition. peter fabian: resources, data curation. josé mirão: resources, supervision, funding acquisition. carlo bottaini: resources, supervision. sabrina grassini: validation, supervision. emma angelini: validation, supervision, project administration, resources, funding acquisition. acknowledgement the authors would like to thank roxana golan for her work with the sem, and mafalda costa and dulce valdez for their assistance in the interpretation of the diffractograms. the research presented in this paper was part of a doctoral dissertation [12], carried out mainly using data collected at politecnico di torino, ben-gurion university of the negev, and universidade de évora, as part of h2020-msca-itn-2017, ed-archmat (esr7). this project has received funding from the european union’s horizon 2020 research and innovation programme under the marie skłodowska-curie grant agreement no 766311. references [1] h. eshel, the bar kochba revolt, 132-135. in w. d. davies, l. finkelstein, s. t. katz (eds.), the cambridge history of judaism (2006) (pp. 105–127). isbn 978-0-521-88904-9. [2] a. sneh, m. rosensaft, geological map of israel 1:50,000, sheet 14-1: netivot, jerusalem, 2008. [3] o. crouvi, r. amit, m. ben israel, y. enzel, loess in the negev desert: sources, loessial soils, palaeosols, and palaeoclimatic implications, in y.enzel, o.bar-yosef (eds), quaternary of the levant, environments, climate change, and humans, cambridge, uk, 2017, pp.471-482. doi: 10.1017/9781316106754.053 [4] e. angelini, f. rosalbino, s. grassini, g. m. ingo, t. de caro, simulation of corrosion processes of buried archaeological bronze artefacts, in p. dillmann, g. béranger, p. piccardo, h. matthiesen (eds), corrosion of metallic heritage artefacts: investigation, conservation and prediction for long-term behaviour, european federation of corrosion publication 48, woodhead publishing, cambridge, uk, 2007, pp. 203–218, isbn: 978-1-84569301-5 [5] y. goren, i. finkelstein, n. na’aman, inscribed in clay: provenance study of the amarna tablets and other ancient near table 5. sem results (wt%). sample cu sn pb cl b6058 bulk 67.0 1.67 27.8 3.61 b6058 pb globule a 10.5 3.08 79.6 6.83 b6058 pb globule b 20.4 5.56 62.2 11.8 b6058 cu-sn matrix 98.1 1.43 0.47 0.00 b6081 bulk a 10.9 0.44 86.3 2.33 b6081 bulk b 21.7 0.82 73.2 4.27 b6081 enriched bulk 97.2 0.00 1.80 1.02 b6101 bulk 72.6 2.90 21.1 3.33 b6101 cu-sn matrix 88.6 3.38 7.36 0.63 b6101 pb globule a 12.7 0.00 69.1 18.2 b6101 pb globule b 6.99 0.00 82.0 11.1 b8103 bulk a 89.3 1.98 8.60 0.14 b8103 bulk b 86.4 1.99 11.0 0.59 b8103 cu-sn matrix 95.7 2.35 1.94 0.00 b8103 pb globule 10.1 0.00 79.8 10.2 b8764 bulk a 70.0 1.42 24.02 4.56 b8764 bulk b 68.4 3.30 24.72 3.63 b8764 pb globule a 16.1 0.40 68.95 14.52 b8764 pb globule b 26.2 0.60 52.26 21.0 b8764 cu-sn matrix 75.4 3.50 17.3 3.83 b9803 bulk a 72.7 1.76 25.0 0.36 b9803 bulk b 85.1 1.93 12.6 0.39 b9803 pb globule a 2.75 0.00 97.3 0.00 b9803 pb globule b 1.00 0.00 99.0 0.00 b9803 cu-sn matrix 96.0 2.37 1.65 0.00 https://doi.org/10.1017/9781316106754.053 acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 6 eastern texts, tel aviv. 2004, pp. 112-113 with references, isbn: 965-266-020-5. [6] a. g. nord, e. mattson, k. tronner, factors influencing the long-term behavior of bronze artefacts in soil, protection of metals, vol.41, 2005, pp.339-346. doi: 10.1007/s11124-005-0045-9 r. frost, raman spectroscopy of selected copper minerals of significance in corrosion, spectrochimica acta. part a, molecular and biomolecular spectroscopy, vol. 59, 2003, pp. 1195-1204. doi: 10.1016/s1386-1425(02)00315-3 [7] r. fernandes, b. j. h.van os, h. d. j. huisman, the use of handheld xrf for investigating the composition and corrosion of roman copper-alloyed artefacts, heritage science, vol. 1, 2013. doi: 10.1186/2050-7445-1-30 [8] o. oudbashi, multianalytical study of corrosion layers in some archaeological copper alloy artefacts, surface and interface analysis, vol. 47, 2015, pp. 1133-1147. doi: 10.1002/sia.5865 [9] p. piccardo, b. mille, l. robbiola, tin and copper oxides in corroded archaeological bronzes, in p.dillmann, g.béranger, p. piccardo, h. matthiesen (eds), corrosion of metallic heritage artefacts: investigation, conservation and prediction for longterm behaviour, european federation of corrosion publication 48, woodhead publishing, cambridge, uk, 2007, pp. 239–262. isbn: 978-1-845-69301-5. [10] l. robbiola, j. m. blengino, c. fiaud, morphology and mechanisms of formation of natural patinas on archaeological cu-sn alloys, corrosion science, vol. 40, 1998, pp. 2083-2111. doi: 10.1016/s0010-938x(98)00096-1 [11] m. j. h. peters, innovative techniques for the assessment of the degradation state of metallic artefacts, phd thesis, turin: politecnico di torino, 2021. https://doi.org/10.1007/s11124-005-0045-9 https://doi.org/10.1016/s1386-1425(02)00315-3 https://doi.org/10.1186/2050-7445-1-30 https://doi.org/10.1002/sia.5865 https://doi.org/10.1016/s0010-938x(98)00096-1 doppler flow phantom failure detection by combining empirical mode decomposition and independent component analysis with short time fourier transform acta imeko issn: 2221-870x december 2021, volume 10, number 4, 185 193 acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 185 doppler flow phantom failure detection by combining empirical mode decomposition and independent component analysis with short time fourier transform giorgia fiori1, fabio fuiano1, andrea scorza1, maurizio schmid1, silvia conforto1, salvatore a. sciuto1 1 department of industrial, electronic and mechanical engineering, roma tre university, via della vasca navale 79, 00146 rome, italy section: research paper keywords: emd; ica; stft; flow phantom failures; pw doppler citation: giorgia fiori, fabio fuiano, andrea scorza, maurizio schmid, silvia conforto, salvatore andrea sciuto, doppler flow phantom failure detection by combining empirical mode decomposition and independent component analysis with short time fourier transform, acta imeko, vol. 10, no. 4, article 29, december 2021, identifier: imeko-acta-10 (2021)-04-29 section editor: roberto montanini, università di messina and alfredo cigada, politecnico di milano, italy received august 2, 2021; in final form december 4, 2021; published december 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: giorgia fiori, e-mail: giorgia.fiori@uniroma3.it 1. introduction doppler flow phantoms are standard reference test devices usually employed in quality controls (qcs) for ultrasound (us) system performance evaluation [1]-[3]. they can simulate the main acoustic characteristics of biological tissues and reproduce repeatable flows whose regimes are similar to those in blood vessels [4]-[7]. to date, the lack of a generally accepted standard for b-mode and doppler [8]-[12], has led to a limitation on us phantoms use. it should be noticed that, even though such devices are widespread on the market, they are still not included in a shared standard that focuses in detail on periodic and objective checks of their metrological and functional characteristics. this seems to be odd, since the existing commercial doppler phantoms show several technical limitations [2], [3], [13], [14] affecting their reliability and traceability for doppler qc testing. the main drawbacks of the most commonly used doppler phantom model, which is the flow phantom, are the desiccation over time of the tissue mimicking material (tmm), the tendency of blood mimicking fluid (bmf) particles to form agglomerates and/or air bubbles, and the inconsistency of phantom acoustic and pump mechanical properties over time [3]. despite the awareness of such limitations, objective protocols and criteria for the monitoring of the phantom defects degree are still lacking in literature. in particular, some studies focused their attention on two different kinds of doppler phantom stability, i.e., tmm and bmf stability [15]-[18]. the former refers to any physical modification in the tmm, while the latter indicates the presence of any solid and/or gaseous element in the bmf. more in detail, tmm stability can be compromised because of (a) tmm fracture e.g., due to desiccation over time, (b) tmm erosion or bmf leakage e.g., due to bmf action through time on tmm in wall-less flow phantoms or tubing material rupture. on the other hand, bmf stability highly depends on (a) any presence of air bubbles, particles abstract nowadays, objective protocols and criteria for the monitoring of phantoms failures are still lacking in literature, despite their technical limitations. in such a context, the present work aims at providing an improvement of a previously proposed method for the doppler flow phantom failures detection. such failures were classified as low frequency oscillations, high velocity pulses and velocity drifts. the novel objective method, named emodica-stft, is based on the combined application of the empirical mode decomposition (emd), independent component analysis (ica) and short time fourier transform (stft) techniques on pulsed wave (pw) doppler spectrograms. after a first series of simulations and the determination of adaptive thresholds, phantom failures were detected on real pw spectrograms through the emodica-stft method. data were acquired from two flow phantom models set at five flow regimes, through a single ultrasound (us) diagnostic system equipped with a linear, a convex and a phased array probe, as well as with two configuration settings. despite the promising outcomes, further studies should be carried out on a greater number of doppler phantoms and us systems as well as including an in-depth investigation of the proposed method uncertainty. mailto:giorgia.fiori@uniroma3.it acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 186 agglomerates or tmm debris, and (b) unwanted variations of flow velocity regimes. nevertheless, stability assessment is carried out through a visual qualitative evaluation [15], [16], or without a specific and rigorous protocol [17], [18]. in the existing literature, there are other studies investigating and detecting the failures that could possibly compromise the stability of devices used in both biomedical and mechanical fields. however, there are some issues that nowadays should be taken into account: in [19], for example, it has been pointed out that a specific standard for mechanical heart pumps testing procedures was still missing. investigations into such issues were limited to the early evaluation of the failures using an analysis technique along with device testing before surgical implantation. on the other hand, in [20] centrifugal pump failures have been reviewed, highlighting the lack of an integrated system able to monitor all the major pump failures. in such a context, the present study focuses on the improvement and testing of a previously developed short time fourier transform-based image analysis method [21] for the automatic detection of the main doppler flow instabilities that may arise. the proposed improved method is based on the application of empirical mode decomposition (emd) and independent component analysis (ica) techniques combined with the short time fourier transform (stft), namely emodica-stft, to automatically evaluate the phantom failures through pulsed wave (pw) doppler spectrograms. emd is a single-channel technique [22], [23], firstly introduced in [24], to obtain the decomposition of a signal in time. it is widely used in combination with ica for the effective processing of electrophysiological signals [25], [26]. an interesting feature of such a combination is the possibility of successfully extracting both oscillatory and spike-like sources [22]. stft is a time-frequency spectral analysis technique, widely used in several scientific fields, such as structural mechanics, aeronautics, and biomedical engineering. it has been applied in structural health monitoring fields, to detect damages in existing structures [27], to classify and predict delamination in smart composite laminates [28], to reveal corrosion and fatigue cracks in aircraft structures [29], and to analyse physiological signal characteristics and determine relevant parameters [30]-[33]. the goal of the present work is the implementation and testing of the emodica-stft method: it processes pw doppler spectrograms collected from two different doppler flow phantoms through a single intermediate technology level us system equipped with three array probes (linear, convex, and phased array) at their central doppler frequency. in section 2, a brief overview of the techniques adopted in the proposed method is provided, and their combined application on three simulation cases is described. in section 3, the experimental setup used in this study and the emodicastft method application to pw spectrograms is discussed. in section 4 results are presented and discussed on the basis of infraand inter-phantom differences in the detected failures. finally, in the concluding section, the major achievements and future developments of the research hereby presented will be reported. 2. emodica-stft method application to phantom failures detection bmf instability sources can be identified as any presence of air bubbles, particle agglomerates or tmm debris, and unwanted variations of flow velocity regimes. consequently, the present study focuses on their detection in pw spectrograms, with particular reference to the following phantom failures: 1. low frequency oscillations caused by any pump or hydraulic dampener inability to deliver a constant flow velocity in correspondence of a continuous flow regime setting; 2. high velocity pulses caused by any particle agglomerates or tmm debris in the phantom flow; 3. flow velocity drifts due to the unwanted onset of the pump acceleration (e.g., deriving from a failure in the control system). 2.1. emd, ica and stft techniques empirical mode decomposition [22], [24] is a signalprocessing tool that, through an iterative process, decomposes a signal x(t) into a finite set of intrinsic mode functions (imfs) and a residual rm(t), as follows: 𝑥(𝑡) = ∑ 𝐼𝑀𝐹𝑖 (𝑡) 𝑀 𝑖=1 + 𝑅𝑀 (𝑡), (1) where imfi(t) is the i-th oscillatory mode. each imfs has the following properties [22]: (a) it contains one frequency only, which is referred to as the instantaneous frequency, (b) its frequency is different from all the ones of the other functions, (c) it has zero mean value, and (d) it is an oscillatory function. one of the main advantages of emd, as compared to other decomposition techniques [22], is that it does not require any apriori knowledge of the signal to be decomposed. in turn, independent component analysis is a blind source separation technique [34], applied to a set of recorded signals yi(t) whose aim is the extraction of unknown sources si(t), named independent components (ics), under the assumption of statistical independency. in particular, yi(t) can be expressed as a linear combination of si(t), as follows: �⃗�(𝑡) = [𝐴] ∙ 𝑠(𝑡) , (2) where [a] is an unknown matrix, called mixing matrix. in the present study, a computationally improved ica method was applied, namely fastica [35], which was implemented in matlab environment as a software package [36]. finally, short time fourier transform is a set of fourier transforms performed on a signal, which is subdivided into overlapped or non-overlapped temporal segments, through a translating window (e.g., rectangular, hanning) in time. the fourier transform is applied under the hypothesis of pseudostationarity of the temporal segments [37], which is achieved through the choice of a short translating window. the stft expression for a generic discrete signal x(n), is the following: 𝑆𝑇𝐹𝑇(𝑛, 𝜔) = ∑ 𝑥(𝑛 + ℎ)𝑤𝑁 (ℎ) +∞ ℎ=−∞ 𝑒 −𝑗𝜔ℎ , (3) where wn(n) is the translating window. the corresponding normalized, real-valued, non-negative spectrogram sx(n,) can be computed through the following expression: 𝑆𝑥 (𝑛, 𝜔) = 2 𝑁 ∙ 𝐶𝐹 ∙ |𝑆𝑇𝐹𝑇(𝑛, 𝜔)|2, (4) where n is the sample window length and cf is a correction factor varying according to the chosen window amplitude [21]. therefore, (4) has the advantage of taking the applied window type into account. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 187 2.2. emodica-stft simulated application this subsection describes the emodica-stft method proposed to assess the phantom failures, through the combination of the previously described techniques, as shown in the block diagram (figure 1). the following steps were applied to a real flow maximum velocity signal vp(t) lasting  60 s, which was derived from pw spectrograms according to the procedure which will be described in section 3.2. it was chosen among the available data because it is a representative case study: in fact, the signal shows both high-velocity pulses and low-frequency oscillations. a simulated velocity drift vdr(t) of 0.06 cm·s-2 was added to vp(t) as an increasing trend in time, obtaining a velocity signal vp,tot(t) (figure 2), and then, the steps described in the following and represented in figure 1, were applied. first step. the first step is the application of emd to vp,tot(t): the imfs and the residual r(t) are retrieved on the basis of (1). second step. the second step is the application of fastica to the imfs in order to compute the ics and the mixing matrix [a]. at this point, the mean frequency fmean of each independent component is obtained, and a frequency threshold thf = 0.5 hz is selected to discriminate between highand low-frequency content ics. therefore, two different groups of ics are obtained, namely iclow and ichigh. the latter are multiplied for the mixing matrix [a] according to (2), to back-reconstruct the corresponding oscillatory modes imflow and imfhigh. finally, the modes of each group are summed together to reconstruct two signals vp,low(t) and vp,high(t) derived from the signal vp,tot(t), where the first one has frequency contents lower than thf (figure 3 a), while the second one has frequency contents higher than thf (figure 3 b). third step. the third step is the stft application, according to (3) and (4), to vp,low(t) and vp,high(t), with the settings reported in table 1. as already done in [21], the spectral window chosen is the hanning window, whose expression is the following: 𝑤𝑁 (𝑛) = 1 2 (1 − cos 2 π 𝑛 𝑁 ). (5) after the stft application, two mesh plots are obtained. in this way, it is possible to carry out the failure detection by distinguishing the contributions of the low frequency oscillations and high velocity pulses in the spectrograms of vp,low(t) and vp,high(t), respectively. low frequency oscillations are represented in the mesh plot of vp,low(t) normalized spectrogram as frequency pulses (figure 4 a, b). therefore, the detection of an oscillation occurs when sx figure 1. block diagram of the proposed emodica-stft method for flow phantom failures detection. figure 2. real flow maximum velocity signal with low frequency oscillations, high velocity pulses with a 0.06 cm·s-2 velocity drift. a) b) figure 3. (a) reconstructed signal vp,low(t) with low frequency contents superimposed to vp,tot(t) after the offset removal; (b) reconstructed signal vp,high(t) with high frequency contents superimposed to vp,tot(t) after the offset removal. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 188 shows a pulse both limited in time (according to the oscillation period) and frequency ( 0 hz), whose amplitude is higher than the threshold thlf, automatically determined as follows: 𝑡ℎ𝐿𝐹 = 𝜎𝑣 2 ∆𝑓 ∙ 𝐺, (6) where v is the flow velocity standard deviation depending on the phantom model, f is the stft frequency resolution, and g is a safety factor that in this study was chosen equal to 10. in turn, high velocity pulses are represented, as shown in the normalized spectrogram of vp,high(t), by a window covering almost all the frequency components in the mesh plot (figure 4 c, d). therefore, the detection of a pulse occurs when the average amplitude of the frequency components, between 5 and 30 hz, related to a single temporal instant is higher than the threshold thhf. the latter can be automatically determined, by considering the sampling frequency fs of vp(t), as follows: 𝑡ℎ𝐻𝐹 = 𝜎𝑣 2 ∆𝑓 ∙ 𝐺 ∙ 𝐹𝑓𝑟 𝑓𝑠/2 , (7) where ffr is a factor that considers the entity of the frequency range in which the failure occurs in the normalized spectrogram. in this case, where a frequency range between 5 and 30 hz was considered, ffr = 25 hz. the choice to reduce the frequency range in which the detection is carried out was necessary to compensate for non-ideal pulses [21]. fourth step. the fourth step of the emodica-stft method is the detection of the velocity drifts from the emd residual r(t). after the application of the least squares method to r(t) (figure 5), it is possible to evaluate any velocity drift through the computation of the angular coefficient m of the straight line that best approximates the residual trend. in particular, the detection of a velocity drift occurs when |m| is higher than the threshold thdr, that can be automatically determined as: 𝑡ℎ𝑑𝑟 = 𝜎𝑣 𝑡𝑃𝑊 ∙ 𝐺, (8) where tpw is the velocity signal duration, while the safety factor g for the velocity drift detection was chosen equal to 2. the advantage of (8) relies on its dependence on the phantom flow velocity standard deviation v so that the velocity drift perception is not affected by human eye subjectivity. the retrieved angular coefficient of r(t), shown in figure 5, is lower than the simulated velocity drift added to vp(t). this is likely due to the combination of vdr(t) with the pre-existing trend of the real velocity signal under analysis. table 1. stft parameters setting. parameter symbol value sampling frequency (hz) fs 100 spectral window hanning window length (samples) n 100 overlap (samples) noverlap 60 zero-padding (samples) nzero-pad 50 correction factor cf 2 temporal resolution (s) t 0.4 frequency resolution (hz) f  0.7 a) b) c) d) figure 4. low frequency contents signal vp,low(t) represented through (a) a mesh plot with the detected frequency peaks (thlf = 60 cm2·s-2·hz-1) and (b) its temporal evolution together with the signal vp,tot(t); high frequency contents signal vp,high(t) represented through (c) a mesh plot with the detected frequency windows (thhf = 30 cm2·s-2·hz-1) and (d) its temporal evolution together with the signal vp,tot(t). acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 189 finally, this simulation was repeated in two further cases to test the proposed method for different starting conditions: (a) vp(t) with an additional velocity drift of 0.03 cm·s-2 and (b) vp(t) with an additional velocity drift of 0.09 cm·s-2. the emodicastft method identified the same low frequency oscillations and high velocity pulses retrieved for the first simulation, while the obtained velocity drift angular coefficients were (164.1  0.4)·104 cm·s-2 (r2 = 0.97) and (684.9  0.7)·10-4 cm·s-2 (r2 = 0.99) for case (a) and (b), respectively. 3. materials and experimental data in the present study, the emodica-stft method for the phantom failures assessment, implemented in matlab, is proposed as an improvement of a procedure previously described in [21]. this is achieved through the experimental setup described in the following section. 3.1. experimental setup data were collected by using two doppler flow phantoms, whose main characteristics are reported in table 2. the first phantom under test (put) is gammex, optimizer® 1425a, a self-contained device [38] able to provide constant or pulsatile flow in the 1.7-12.5 ml·s-1 range, through an electric flow controller. the second put is cirs, model 069, an easy-toassemble simulator [39], able to provide an average flow velocity between 5 and 68 cm·s-1, through the action of a peristaltic pump, providing a pulsatile flow. the latter can be converted into a constant flow through the connection of a dampener. the acquisition started after 2 hours of phantom warm-up. in order to test their stability, five constant flow rate values (low lf, low-medium lmf, medium mf, medium-high mhf and high hf) were set, as shown in table 3. doppler phantom characteristics are not consistent, therefore, flow values were differently set to guarantee the same mean velocity regimes (vgammex = vcirs), as detailed in [21]. a single us system equipped with three us probes (linear, convex, and phased array) was used to acquire six pw spectrograms lasting  10 s for each flow regime. data were collected with two different pw doppler settings, namely set a and set b, reported in table 4, in order to make a comparison of the stability performance between the two test objects under different setting conditions. the sample volume length was maintained fixed for both phantoms and settings, whereas the sample volume depth was changed according to the phantom model attenuation, and kept consistent for set a and set b. as regards the insonification angle, it was varied according both to the probe positioning on the scanning surface and to the different tube slopes of the two phantoms. figure 5. least squares method applied to the emd residual in the case of a 0.06 cm·s-2 velocity drift added to the real maximum velocity signal vp(t). table 2. main characteristics of the two doppler flow phantoms [38], [39]. parameter gammex optimazer® 1425a cirs model 069 tissue mimicking material water-based mimicking gel zerdine tissue mimicking gel attenuation 0.50 dbcm-1mhz-1 0.70 dbcm-1mhz-1 tmm sound speed 1540  10 ms-1 1540  10 ms-1 tube inner diameter 5.0 mm 4.8 mm flow velocity standard deviation (*) 2 cms-1 3 cms-1 tube slope 40° 70° dimensions 40.722.935.6 cm 2012.527.5 cm (*) the flow velocity standard deviations were estimated from the specifications reported in the phantoms datasheets. table 3. doppler phantoms flow rate and mean flow velocity settings. flow phantom flow regime flow rate q (mls-1) mean flow velocity v (cms-1) gammex optimazer 1425a low lf 2.6 13.2 low-medium lmf 3.7 18.8 medium mf 4.8 24.4 medium-high mhf 5.9 30.0 high hf 7.0 35.7 cirs model 069 low lf 2.4 13.3 low-medium lmf 3.4 18.8 medium mf 4.4 24.3 medium-high mhf 5.4 29.8 high hf 6.4 35.4 table 4. pw doppler main configuration settings. parameter set a set b gammex cirs gammex cirs doppler frequency (mhz) l = 5.21 c = 2.50 p = 2.50 wall filter minimum medium spectrum resolution minimum l = medium c, p = minimum sample volume length (cm) 3.0 sample volume depth (mm) 48 40 48 40 insonification angle (°) 52 l, c = 70 p = 55 52 l, c = 70 p = 55 pw spectrogram duration (s)  10 pw spectrogram total duration (s)  60 l = linear, c = phased, p = phased array probe. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 190 3.2. emodica-stft on pw spectrograms each pw spectrogram was processed for the detection of the maximum velocity waveform, as in [21]. pixel coordinates pxmax associated to the maximum velocity values were detected through a gray level adaptive threshold thmax automatically determined as 10% of the maximum gray level value [40]. at this point, pxmax were associated to the corresponding flow velocity values vmax for each temporal instant, taking into consideration the maximum value displayed on the pw velocity scale. then, the six vmax signals obtained for the same flow regime were juxtaposed. the emodica-stft application was implemented according to the block diagram in figure 1 and with the stft settings reported in table 1. the thresholds applied for the detection of failures show different values between the doppler phantoms under test because of the different flow velocity standard deviation (table 2). according to (6)-(8), the computed threshold values were: thlf = 60 cm2·s-2·hz-1, thhf = 30 cm2· s-2·hz-1 and thdr = 7·10-2 cm·s-2, for gammex 1425a and thlf = 135 cm2·s-2·hz-1, thhf = 67 cm2·s-2·hz-1 and thdr = 10·10-2 cm· s-2, for cirs model 069. it is worth noting that, due to the threshold dependency on flow standard deviation, the higher threshold values retrieved for cirs phantom are a first indicator of its lower performance as compared to the gammex phantom. 4. results and discussion the number of phantom failures detected for the two test objects according to the us probe, the flow regime (lf, lmf, mf, mhf and hf) and the pw doppler settings (set a and set b) is reported in table 5 and table 6. according to [41], the standard deviation values can be computed as the square root of the counted value. the emodica-stft method did not detect any velocity drift failure on the two phantoms, because the angular coefficients |m| of the straight lines, obtained by applying the least squares method to all the emd residuals, were always lower than the determined thdr. as regards the gammex 1425a phantom, it should be noted that, independently from the us probe considered, the number of low frequency oscillations is globally limited, except for the lmf flow regime, in set a and set b. as shown in figure 6, a sinusoidal trend is clearly visible, therefore suggesting that the phantom electric flow controller seems no longer able to guarantee a constant flow regime of 3.7 ml·s-1. on the other hand, both convex and phased array probes show a higher number of high velocity pulses with respect to the linear array probe, suggesting a probe-dependent sensitivity to bmf particle agglomerates. furthermore, independently from the probe, the low flow regime lf shows the highest number of high velocity pulses. this may be due to the fact that the flow velocity is too low to dissolve the particle agglomerates. table 5. number of detected failures according to the us probe, the flow regime and pw doppler settings for gammex 1425a. us probe flow regime pw doppler setting low frequency oscillation thlf = 60 cm2·s-2·hz-1 high velocity pulse thhf = 30 cm2·s-2·hz-1 velocity drift thdr = 7·10-2 cm·s-2 angular coefficient m (cm·s-2) r2 linear lf set a 6  2 7  3 − -0.9·10-2 0.99 set b − 3  2 − -1.0·10-2 0.94 lmf set a 49  7 − − -0.9·10-2 0.99 set b 45  7 − − 2.1·10-4 0.99 mf set a − − − -3.7·10-3 0.99 set b − − − -2.3·10-3 0.98 mhf set a − − − 4.0·10-3 0.99 set b − 8  3 − -3.8·10-3 0.89 hf set a − − − -0.9·10-2 0.98 set b − 1  1 − 2.8·10-3 0.99 convex lf set a 2  1 44  7 − 4.0·10-2 0.98 set b − 14  4 − -4.3·10-4 0.98 lmf set a 47  7 2  1 − -4.9·10-3 0.99 set b 44  7 2  1 − -1.2·10-2 0.99 mf set a − 9  3 − -1.6·10-3 0.99 set b − 1  1 − -2.4·10-3 0.99 mhf set a − 6  2 − -3.3·10-3 0.94 set b − 3  2 − -0.9·10-2 0.99 hf set a 2  1 2  1 − -0.6·10-2 0.98 set b − 1  1 − 1.9·10-5 0.99 phased lf set a − 25  5 − -0.8·10-3 0.99 set b − 4  2 − 3.1·10-3 0.92 lmf set a 44  7 − − -1.9·10-2 0.98 set b 47  7 4  2 − 1.2·10-3 0.99 mf set a − 3  2 − -1.4·10-2 0.94 set b − 8  3 − 1.5·10-3 0.96 mhf set a − 9  3 − 1.0·10-3 0.96 set b − 10  3 − 4.0·10-3 0.98 hf set a − 6  2 − -3.3·10-3 0.99 set b − 5  2 − -3.3·10-3 0.99 acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 191 as regards the cirs model 069 simulator, no low-frequency oscillation was detected through the phased array probe. in both linear and convex array probes, a high number of oscillations was detected in correspondence of the medium-high flow regime mhf (figure 7). this could be due to a dampener failure in correspondence of such flow regime. similarly to gammex 1425a, a higher number of high velocity pulses was detected for both convex and phased array probes. by comparing the two doppler phantoms outcomes retrieved, gammex 1425a shows the lowest number of lowfrequency oscillations (by excluding the particular case of lmf regime), when compared to both linear and convex array probes of the cirs model 069, while for the phased array one no oscillations were detected. conversely, gammex 1425a globally shows the highest number of high velocity pulses compared to the cirs model 069 for both linear and convex array probes, while such issue seems to be reversed for phased array probe. table 6. number of detected failures according to the us probe, the flow regime and pw doppler settings for cirs model 069. us probe flow regime pw doppler setting low frequency oscillation thlf = 135 cm2·s-2·hz-1 high velocity pulse thhf = 67 cm2·s-2·hz-1 velocity drift thdr = 10·10-2 cm·s-2 angular coefficient m (cm·s-2) r2 linear lf set a 4  2 − − 3.7·10-2 0.99 set b 2  1 − − -0.8·10-2 0.99 lmf set a − − − 3.6·10-3 0.95 set b − − − -1.8·10-4 0.99 mf set a − − − -1.9·10-3 0.97 set b − − − -1.0·10-3 0.99 mhf set a 2  1 − − 0.8·10-2 0.94 set b 11  3 1  1 − 2.0·10-3 0.99 hf set a − − − 3.8·10-4 0.99 set b 2  1 − − 2.3·10-2 0.99 convex lf set a − 5  3 − 2.2·10-2 0.99 set b − 7  3 − -4.6·10-3 0.99 lmf set a − 9  3 − -2.0·10-2 0.95 set b − 6  3 − 1.0·10-2 0.93 mf set a 1  1 17  4 − 2.2·10-2 0.95 set b − 11  3 − 4.3·10-3 0.96 mhf set a − 1  1 − -1.9·10-2 0.99 set b 18  4 2  1 − 1.9·10-2 0.99 hf set a 5  2 2  1 − -2.6·10-2 0.99 set b 1  1 1  1 − 0.6·10-2 0.99 phased lf set a − 8  3 − 1.1·10-2 0.98 set b − 3  2 − -0.9·10-2 0.98 lmf set a − 17  4 − 1.4·10-2 0.99 set b − 11  3 − -0.9·10-2 0.99 mf set a − 14  4 − 3.6·10-3 0.99 set b − 13  4 − -1.8·10-3 0.94 mhf set a − 8  3 − 0.9·10-2 0.99 set b − 12  3 − 0.5·10-3 0.97 hf set a − 10  3 − 0.5·10-2 0.99 set b − 5  2 − 1.4·10-3 0.99 figure 6. example of the sinusoidal trend in lmf regime for gammex 1425a acquired with the linear array probe in set a. figure 7. example of the low frequency oscillations in mhf regime for cirs model 069 acquired with the linear array probe in set b. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 192 since the same number of put failures can have a different relevance depending on the intended use of the ultrasound system to be checked through the put (e.g., in echocardiography, obstetrics and gynecology, pediatrics, etc.), the emodica-stft may be applied with different thresholds by means of ad hoc us systems and probe models over time. therefore, the same put may be suitable for testing a restricted number of us scanners only. this could be an advantage for the technical assessment of the above medical devices, as well as for put maintenance. 5. conclusions doppler phantoms are standard reference test devices that, nowadays, are not yet included in a shared standard focusing on the objective evaluation of their performances and failures. in particular, phantom stability assessment is currently carried out through visual and subjective evaluations, or without a rigorous protocol. therefore, in the present study, a novel method, named emodica-stft, based on the combined application of emd, ica and stft techniques, is proposed and tested to automatically determine, through the processing of pw doppler spectrograms, the number of phantom failures. the main flow phantom failures were classified as low frequency oscillations, high velocity pulses and velocity drifts. data were collected from two flow phantoms by a single diagnostic us system equipped with three probe models. tests were carried out in two different us configuration settings and five flow regimes set on the test objects. after a series of simulations, adaptive thresholds for the detection of each failure were determined depending on the standard deviation of the put flow velocity. consequently, emodica-stft method was applied to the maximum flow velocity signals extracted from the pw doppler spectrograms through an automatic processing. finally, the number of detected failures was found for both doppler phantoms. on the basis of the promising outcomes, further studies should be carried out (a) on a higher number of doppler phantoms, (b) on a larger number of us diagnostic systems and (c) including an in-depth investigation of the proposed method uncertainty. acknowledgement the authors wish to thank hitachi healthcare and jan galo of the clinical engineering service at i.r.c.c.s. children hospital bambino gesù for hardware supply and 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slam; semantic segmentation; robot vision; martian environment citation: sebastiano chiodini, marco pertile, stefano debei, occupancy grid mapping for rover navigation based on semantic segmentation, acta imeko, vol. 10, no. 4, article 25, citation: sebastiano chiodini, marco pertile, stefano debei, occupancy grid mapping for rover navigation based on semantic segmentation, acta imeko, vol. 10, no. 4, article 25, december 2021, identifier: imeko-acta-10 (2021)-04-25 section editor: roberto montanini, università di messina and alfredo cigada, politecnico di milano, italy received july 26, 2021; in final form december 6, 2021; published december 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: project bird181070 funded by the program bird 2018 sponsored by the university of padova. corresponding author: sebastiano chiodini, e-mail: sebastiano.chiodini@unipd.it 1. introduction the problem of perception of the environment is fundamental for safe robot navigation. planetary rovers’ exploration has some peculiarities that we do not find in other autonomous robotics applications, (1) there is no gps system that can help with the localization process, (2) terrain assessment deals with an unstructured environment that is characterised by sharp rocks (large or small), sand, and bedrocks, which are often confused with the noise of a stereo point cloud. a review of learning-based perception and navigation methods for rescue robotics, planetary exploration, and agricultural robotics can be found in [1]. it is not only safe navigation that benefits from the autonomous navigation capabilities of a rover, but also its scientific output, as an example, the distance travelled for each sol by the nasa msl rover has increased from a few meters to 100 m [2]. the nasa mer rover disposed of the gestalt (grid-based estimation of surface traversability applied to local terrain) system [3], which is one of the first autonomous terrain assessment systems for planetary rovers. this system was able to detect geometric hazards such as rock, ditches, and cliffs by processing the 3d point clouds generated by the rover stereoimages; it looked mainly at geometric characteristics such as steps, slopes, and terrain roughness. an alternative method for estimating the traversability of a terrain is presented in [4], where the unevenness of the terrain is analysed by means of the power spectral density (psd) of the surface profile as measured by a stereo camera. in [5] an evaluation system for the traversability of rough terrain for a rover based on aerial uav survey is presented. abstract obstacle mapping is a fundamental building block of the autonomous navigation pipeline of many robotic platforms such as planetary rovers. nowadays, occupancy grid mapping is a widely used tool for obstacle perception. it foreseen the representation of the environment in evenly spaced cells, whose posterior probability of being occupied is updated based on range sensors measurement. in more classic approaches, the cells are updated to occupied at the point where the ray emitted by the range sensor encounters an obstacle, such as a wall. the main limitation of this kind of methods is that they are not able to identify planar obstacles, such as slippery, sandy, or rocky soils. in this work, we use the measurements of a stereo camera combined with a pixel labeling technique based on convolution neural networks to identify the presence of rocky obstacles in planetary environment. once identified, the obstacles are converted into a scan-like model. the estimation of the relative pose between successive frames is carried out using orb-slam algorithm. the final step consists of updating the occupancy grid map using the bayes’ update rule. to evaluate the metrological performances of the proposed method images from the martian analogous dataset, the esa katwijk beach planetary rover dataset have been used. the evaluation has been performed by comparing the generated occupancy map with a manually segmented ortomosaic map, obtained by drones’ survey of the area used as reference. mailto:sebastiano.chiodini@unipd.it acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 156 looking at the context of indoor navigation, many methods involve the construction of occupancy grid maps that are updated via the bayesian occupancy filter with the range measurements coming from lidar sensors. as an example, [6] presents a corridor detection method based on 2d lidar and occupancy filtering. thanks to semantic segmentation, which is the process of dividing an image into digital objects, it is possible to add to the map information that otherwise cannot be deduced from the three-dimensional model alone, such as if the terrain is sandy or slippery. compared to other methods, such as classification, semantic segmentation allows for pixel-by-pixel labelling of the image, see figure 1. this enables a more detailed interpretation of the surrounding environment. cadena et al. [7] says that simultaneous localization and mapping is now entering its third stage, characterised by robust perception. this robustness requires the realization of robust performance, high-level understanding, resource awareness, and task-driven perception. semantics is an important tool in the pursuit of improved robustness, intuitive visualization, and efficient human–robot-environment interaction. the term semantic slam can be used to identify methods that comprise either semantic-based robustness/accuracy enhancements or semantic mapping. for details, see [8], which presents a detailed review of recent advances in semantic simultaneous localization and mapping, considering multiscaled map representation, object simultaneous localization and mapping system, and deep neural network-based slam. deeplab networks make a significant contribution to semantic segmentation, see, e.g., [9], [10], which introduce the concept of “atrous convolution” in cnn models and exhibit excellent accuracy in semantic segmentation, see [8]. in the literature, it is possible to find many works on semantic mapping in the urban environment. as an example [11] generates a dense 3d reconstruction with associated semantic labelling from stereo camera images, [12] and [13] propose a multimodal sensor-based semantic 3d mapping system using a 3d lidar combined with a camera. in the frame of maritime navigation, the authors of [14] present a water obstacle detection network based on image segmentation (wodis) for autonomous surface vehicles. gan et al. [15] describe a method that provides a unified probabilistic model for both occupancy and semantic probabilities, producing a bayesian continuous 3d semantic occupancy map from noisy point clouds. wang et al. [16] describe a joint method of a priori convolutional neural networks at superpixel level and soft restricted context transfer. in [17], the authors describe a method to build a complete semantic map based on merging the segmentation results from street and satellite view images. in [18] satellite images are segmented using a segnet network. li et al. [19] present a fast 3d semantic mapping system based on monocular vision by fusion of localization, mapping, and scene parsing. the method is based on an improved version of deeplab-v3+ [9], [10]. [20] presents an approach to generate an outdoor large-scale 3d dense semantic map based on binocular stereo vision and the segnet deep learning architecture. paz et al. [21] fuses image and pre-built point cloud map information to perform automatic and accurate labelling of static landmarks such as roads, sidewalks, crosswalks, and lanes. in this work, semantic segmentation is also performed on 2d images using the deeplab-v3+ network [9], [10]. however, these techniques have rarely been applied to the case of planetary exploration and environments with a low number of features. this paper proposes a terrain assessment method for martian environment based on semantic mapping. the method takes as input a set of stereo-images which are pixelwise labelled with the state-of-the-art convolutional neural network labeller deeplabv3+ [9]. thanks to the stereo reconstruction of the scene, it is possible to associate the labels to the 3d points; therefore, obstacles are represented through a scan-like model [22]. all scans are combined with each other in an occupancy grid considering the trajectory travelled by the rover. the trajectory was calculated using the orb-slam algorithm [23]. to evaluate the metrological performances of the proposed method, images from the public available esa katwijk beach planetary rover dataset [24] are used. the dataset was created for the validation of localization and navigation algorithms in martian-like environment, and it provides trajectory and map ground truth. the method evaluation has been performed using a manually segmented ortomosaic map, taken by a drone, as reference, and the trajectory reconstruction performances have been evaluated by comparison with a differential gps (dgps). the major contributions of this work are: • the application of cnn to identify obstacles characteristic of the planetary environment; • the generation of laser-scan like point cloud representing the obstacle; • the application of bayes filtering to obtain global obstacle maps; • comparison of the generated map with the ground truth for method validation. the paper is divided as follows: section 2 describes the proposed terrain assessment method. in section 3 the algorithm performance on a martian analogous environment has been evaluated, and in section 4 the concluding remarks are reported. 2. method the proposed method takes as input a rectified stereo image (𝐼𝑙 , 𝐼𝑟 ), which is used to (1) identify the obstacle with a cnn labeller (deeplabv3+), (2) estimate the scene point cloud, and (3) estimate the rover trajectory. afterward, the point cloud is fused with the labels to obtain a scan-like representation of the obstacles. the scans associated with the rover poses that compose the trajectory are merged into an occupancy grid map using bayesian filtering. a diagram summarizing the various steps is shown in figure 2. figure 1. compared to other methods, such as classification, semantic segmentation allows for a pixel-by-pixel labelling of the image. this enables the creation of more detailed maps. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 157 2.1. obstacle identification the purpose of the obstacle identification step is labelling each pixel of the image (𝑢𝑙,𝑣𝑙 ) into obstacle or free area. image labelling is performed using a convolutional neural network (cnn) technique, as cnns have demonstrated a significant improvement in semantic segmentation tasks. moreover, compared to other methods, such as classification, semantic segmentation enables a more detailed interpretation of the surrounding environment. among the state-of-the-art methods (fcn [25], segnet [18] and u-net [26]), deeplabv3+ [9] has been chosen because (1) it has been demonstrated to outperform similar methods in several applications, (2) it is a pre-trained network. the latter feature is necessary to reduce the training time. network pretraining has been performed on imagenet, which is a dataset containing more than one million images divided into 1000 classes. pre-training allows training the deeper layer of the network with the most characteristic feature. instead, the final part of the training phase is application dependent and is used to train the outer layer of the network. for the context of this application, three classes have been chosen: sand, rocks, and background. fehler! verweisquelle konnte nicht gefunden werden.2 shows the obstacle detection step applied to the images of the dataset used for testing. we have taken advantage of a pre-trained network to reduce the training images number. data augmentation techniques have been applied to improve the training accuracy and robustness to image variation, in particular, cropping, mirroring, and resizing. the number of images used for training is 400: 50 original images and 350 obtained with data augmentation; more details are given in [27], [28], and [29]. since it may happen that the boundaries of the identified classes may present imperfections or smudges, morphological operations have been applied to filter the identified regions. the morphological operations that have been applied are the following: binary erosion, binary dilation, and removal of a small region [30]. for collision avoidance purposes, the main interest is limited to the potential point of collision with the obstacle. for this reason, for each identified region, only the lower limit is considered, which is the one that corresponds to the intersection between the ray that starts from the camera centre and the obstacle. this contour, combined with the range measurements obtained with the method described in section 2.2, allows us to have a scan-like representation of the obstacle (see figure 3). 2.2. range measurement first, images are stereo-rectified using camera intrinsic and extrinsic parameters. then, the semi-global block matching algorithm [31] is used to compute the disparity map. semi-global block matching algorithm computes the scene disparity by comparing the sum of absolute differences (sad) for each block of pixels in the image and forces a similar disparity on neighbouring blocks. by knowing the disparity map and the intrinsic parameters of the camera it is possible to estimate the depth of the scene and compute the related point cloud relative to the rover frame (𝑿𝑗 = [𝑋𝑗 , 𝑌𝑗 , 𝑍𝑗 ]), where 𝑗 = 1, … , 𝑛 with 𝑛 figure 2. scheme overview of the proposed terrain assessment algorithm. the algorithm takes as input a rectified stereo image, which is used to (1) identify the obstacle with a cnn labeller (deeplabv3+), (2) estimate the scene point cloud, and (3) estimate the rover trajectory. afterward, the point cloud is fused with the labels to obtain a scan-like representation of the obstacles. the scans associated with the rover poses that compose the trajectory are merged into an occupancy grid map using bayesian filtering. figure 3. the 3d obstacle map is broken down to a laser scan-like model. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 158 total number of pixels with an associated disparity. the 3d points coordinates in the left camera frame are given by: 𝑋𝑗 = 𝑢𝑙 𝑍𝑗 𝑓 𝑌𝑗 = 𝑣𝑙 𝑍𝑗 𝑓 𝑍𝑗 = 𝑏𝑓 𝑢𝑙 − 𝑢𝑟 , (1) where (𝑢𝑙,𝑣𝑙 ) and (𝑢𝑟,𝑣𝑟 ) are respectively the pixel coordinates in the left and right image, 𝑏 is the camera baseline and 𝑓 is the camera focal length. each 3d point of the point cloud (𝑿𝑗 ) is associate with a pixel of the image (𝑢𝑙,𝑣𝑙 ). only 3d points that correspond to the lower boundary of the obstacles are used to update the occupancy grid map; see section 2.3. 2.3. relative trajectory reconstruction sections 2.1 and 2.2 show how to determine the position of the obstacles with reference to the rover frame. however, to build a global map of obstacles and filter the measurements belonging to different rover poses, the position of the obstacles position 𝑿𝑖 needs to be defined with respect to an absolute reference system 𝑿𝑖 𝑊 , the w world reference frame. in this work the frame w corresponds to the first stereo-image frame rotated by the pan and tilt angles of the ptu (pan and tilt unit). to perform the trajectory reconstruction step, only stereo camera visual input has been used in combination with the stateof-the-art visual slam algorithm, orb-slam2 [23]. for the purposes of this work, the camera pose issued by the tracking step was used, this pose belongs to se(3) (the special euclidean group in three dimension). assuming that the rovers are mainly moving in a planar environment, the se(3) pose has been transformed into the corresponding se(2) pose 𝑻𝑘 𝑊 = [𝑹𝑘 𝑊 |𝒕𝑘,𝑊 ], which for completeness is reported in the following relation: 𝑻𝑘 𝑊 = [ cos(𝜓𝑘 𝑊 ) − sin(𝜓𝑘 𝑊 ) 𝑡𝑥(𝑘,𝑊) sin(𝜓𝑘 𝑊 ) cos(𝜓𝑘 𝑊 ) 𝑡𝑦(𝑘,𝑊) 0 0 1 ] , (2) where 𝜓𝑘,𝑊 is the absolute heading angle and 𝑡𝑥(𝑘,𝑊) 𝑡𝑦(𝑘,𝑘−1) is the rover absolute position. finally, obstacle scan line points are transformed from the rover frame to the w frame using the following equation: 𝑿𝑖 𝑊 = 𝑹𝑘 𝑊 𝑿𝑖 + 𝒕𝑘,𝑊 . (3) 2.4. sensor fusion the last step is the update of the occupational grid. the probability of each grid cell being occupied is updated following the standard bayes update rule [29] [32] and using the obstacle scan 𝑿𝑖 𝑊 . in the experimental part, a grid resolution of 0.2 × 0.2 m has been considered. assuming that the cells of the grid map are independent from each other, and given a series of rock observations 𝑧1:𝑗 , the probability belief of a single cell to be occupied by an obstacle or not 𝑝(𝑚𝑥,𝑦 |𝑧1:𝑗 ), is reported in equation (4). 𝑝(𝑚𝑥,𝑦 |𝑧1:𝑗 ) = 𝑝(𝑚𝑥,𝑦 |𝑧𝑗 )𝑝(𝑧𝑗) 𝑝(𝑚𝑥,𝑦) 𝑝(𝑚𝑥,𝑦 |𝑧1:𝑗−1) 𝑝(𝑧𝑗|𝑧1:) , (4) where 𝑝(𝑚𝑥,𝑦 |𝑧𝑗 ) is the inverse measurement model of the depth data retrieved from the stereo-camera, 𝑝(𝑚𝑥,𝑦 |𝑧1:𝑗−1) is the depth measurements of the previous rover poses, and 𝑝(𝑚𝑥,𝑦 ) is the prior map. to avoid difficult-to-calculate probabilities, we use the binary bayes filter in log-odds form: 𝑙𝑗 = 𝑙𝑗−1 + log 𝑝(𝑚𝑥,𝑦 |𝑧𝑗 ) 1−𝑝(𝑚𝑥,𝑦 |𝑧𝑗 ) + 𝑝(𝑚𝑥,𝑦) 1−𝑝(𝑚𝑥,𝑦) , (5) where 𝑙𝑗 = log 𝑝(𝑚𝑥,𝑦 |𝑧1:𝑗 ) 1−𝑝(𝑚𝑥,𝑦 |𝑧1:𝑗 ) . the log odd form of the inverse measurement model log 𝑝(𝑚𝑥,𝑦 |𝑧𝑗 ) 1−𝑝(𝑚𝑥,𝑦 |𝑧𝑗 ) an occupancy value 𝑙occ assigns to all cells within the 3d labelled points 𝑿𝑗 . in experiments, the occupied threshold is 𝑙occ = 0.65, the free threshold is 𝑙free = 0.2. the grid is initialized without prior knowledge of the map 𝑝(𝑚𝑥,𝑦 ) = 0.5. 3. experimental results the metrological performances of the proposed method have been evaluated using the esa katwijk beach planetary rover dataset [24], which provides images analogous to mars. of the overall dataset, the loccam stereo images have been used for semantic maps generation, site ortomosaic combined with differential gps has been used as ground truth. the characteristics of the sensors used in the experimental part are summarized in table 1. the dgps has been used to register the generated maps to the ortomosaic taken by the drone, which has been used as a map reference. the ground truth of the map has been obtained by manually labelling the ortomosaic drone, and the ground truth of the trajectory is given directly by the dgps. figure 4 shows the occupancy grid map generated with the proposed method, the occupancy grid is superimposed on the drone image used for performance evaluation. the metrics normally used to evaluate object detection algorithms were used: the accuracy, the intersection over union (iou) and the f1 score: figure 4. occupancy grid map generated with the proposed method. free cells are shown in green, obstacles-occupied cells in red, and yellow arrows show the trajectory of the rover. the occupancy grid is superimposed on the drone image used for performance evaluation. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 159 𝐴𝑐𝑐𝑢𝑟𝑎𝑐𝑦 = 𝑇𝑃 + 𝑇𝑁 𝑇𝑃 + 𝑇𝑁 + 𝐹𝑃 + 𝐹𝑁 , (6) 𝐼𝑜𝑈 = 𝑇𝑃 𝑇𝑃 + 𝐹𝑃 + 𝐹𝑁 , (7) where tp represent the true positives, tn the true negatives, fp the false positives, and fn the false negatives. in the case of an ideal classifier 𝐹𝑃 = 𝐹𝑁 = 0, thus the accuracy metric would be equal to 1. iou is the ratio of correctly classified cells over the sum of the total number of cells labelled and cells classified. table 2 summarises the performances (accuracy and iou) of the proposed method. the left column of figure 5 shows the images labelled with rock, sand and background classes, and the right column shows figure 5. left column: labelled images with rock, sand, and background classes. right column: occupancy grid map and trajectory estimated with the proposed method. from top to bottom successive images of the sequence. table 1. sensors characteristics. sensor description data logged loccam pointgery bumblebee2 (bb2-08s2c38) 12 cm baseline stereo camera. 1024 × 768 images rtk gps trimble bd 970 receiver with zephyr model2 antenna (rover) trimble bx 982 receiver with zephyr geodetic antenna (base station). latitude, longitude, and altitude expressed on wgs84 ellipsoid table 2. global mapping performances in terms of accuracy and iou metrics. labeller trajectory accuracy iou deeplabv3+ orb-slam2 0.987 0.282 acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 160 the occupancy grid map and trajectory estimated with the proposed method. as it is possible to observe in the first row of figure 5 the two rocky obstacles are a few meters away from the camera and not easily recognizable, however, they are correctly labelled by the deeplabv3+ labeller. the effectiveness of the labeller can also be observed in the middle and bottom row of figure 5, although the shadows of the rover are present in both rock and sand, it does not affect the performance of the label. 4. conclusions in this paper, a hazard mapping method for planetary rovers’ navigation is presented. the method is based on occupancy grid mapping. the posterior probability of the grid cells is updated using the inverse measurement model of a scan-like obstacle detector. objects are identified as obstacles by means of the deeplabv3+ deep neural network. the estimation of the relative pose between successive frames is carried out using the state-ofthe-art visual slam method, orb-slam. the proposed method shows the ability to produce accurate occupancy grid maps with associated label up to a dozen meters from the camera and when the rover shadow is present in the image field of view. finally, the method has been tested on a public available dataset of a martian analogous environment. references [1] d. c. guastella, g. muscato, learning-based methods of perception and navigation for ground vehicles in unstructured environments: a review. sensors 2021, 21, 73. doi: 10.3390/s21010073 [2] m. bajracharya, m. w. maimone, d. helmick, autonomy for mars rovers: past, present, and future, computer, 41 (12) (2008), pp. 44-50. doi: 10.1109/mc.2008.479 [3] m. w. maimone, p. c. leger, j. j. biesiadecki, overview of the mars exploration rovers autonomous mobility and vision capabilities, proc. of the ieee international conference on robotics 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imeko issn: 2221-870x june 2022, volume 11, number 2, 1 6 acta imeko | www.imeko.org june 2022| volume 11 | number 2 | 1 a modified truncation and rounding-based scalable approximate multiplier with minimum error measurement yamini nagaratnam1, sudanthiraveeran rooban1 1 department of ece, koneru lakshmaiah education foundation, green fields, vaddeswaram,522502, guntur, ap, india section: research paper keywords: approximate multiplier; hardware computation; mean absolute relative error; truncation-based multiplier; rounding operation; absolute error citation: yamini nagaratnam, sudanthiraveeran rooban, a modified truncation and rounding-based scalable approximate multiplier with minimum error measurement, acta imeko, vol. 11, no. 2, article 37, june 2022, identifier: imeko-acta-11 (2022)-02-37 section editor: md zia ur rahman, koneru lakshmaiah education foundation, guntur, india received february 7, 2022; in final form may 11, 2022; published june 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: sudanthiraveeran rooban, e-mail: sroban123@gmail.com 1. introduction in a recent technology of digital signal processing application, a multiplier is a priority one with low area and low power utilizations. nowadays, approximate multiplier is functioning in good manner to reduce area, delay, power, error measurement and energy utilization. as a result of these specifications, approximate computing becomes a trendy trend in the world of digital design 0. because of the high speed, fault tolerance, and power efficiency, the demand for efficient approximate multipliers is growing. the method of approximation computing encompasses a number of models, including data mining and multimedia processing [2]. multipliers are critical components in applications like digital signal processing, microprocessors, and embedded systems to accomplish operations like filtering and neural network convolution. these multipliers are made with complicated logic blocks, which increases the amount of energy consumed when the size of the circuit is increased. because the multiplier is a fundamental component of mathematical units, the configuration of approximation multiplier design has been a research topic for many years [3]. the approximation multiplier is made up of a few basic blocks in which the approximate technique is performed by using any one of the various phases [3]. when it comes to approximation techniques, truncation of partial products is one of the most effective methods for reducing the error by using correction functions [4]. there are various types of error measurement approximate multipliers depending on the operand size. this error measurement is used to overcome the problem of high latency and energy utilization, this work introduced a measurement of scalable approximate multiplier using truncated rounding-based technique which is used to minimize the measure of partial products based on leading one bit position [5]. the proposed error measurement approximate multiplier is of different bitlengths. 2. generalised approximate computing approximation computing process is executed at multiple architecture layers, software, and other circuits [6] and the study abstract multiplication necessitates more hardware resources and processing time. in a scalable method of approximate multiplier, the truncated rounding technique is added to reduce the number of logic gates in partial products with the help of leading one-bit architecture. truncation and rounding based scalable approximate multiplier (tosam) has few modes of error measurement based upon height (h) and truncated (t) named as (h,t). these multipliers are named as tosam(0,2), tosam(0,3), tosam(1,5), tosam(2,6), tosam(3,7), tosam(4,8), and tosam(5,9). multiplication provides a substantial impact on metrics like power dissipation, speed, size and power consumption. a modified approximate absolute unit is proposed to enhance the performance of the existing approximate multiplier. the existing 16-bit (3,7) error measurement multiplier shows an error measurement value of 0.4 %. the proposed 16-bit multiplier for the same error measurement possesses the error measured value is of 0.01%, mean relative error measured value of 0.3 %, mean absolute relative error measured value of 1.05, normalized error distance measured value of 0.0027, variance of absolute error measured value of 0.52, delay of 1.87 ns, power of 0.23 mw, energy of 0.4 pj. the proposed multiplier can be applied in image processing. the work is designed in verilog hdl and simulated in modelsim, synthesized in vivado. mailto:sroban123@gmail.com acta imeko | www.imeko.org june 2022| volume 11 | number 2 | 2 of approximate computing is also applied in deep learning. arithmetic computation is performed by using the design of addition (in some cases can be termed as accumulation) and multiplication and for some applications, for instance as dsp and machine learning. to achieve the power and latency savings, many approximate adders have been developed. speculative adders and non-speculative transistor-level complete adders are collaborated to create the current approximate adder designs. the basic four parts of approximate multiplier are: approximation of operands approximation of partial product generation approximation of partial product tree approximation of compressors 2.1. approximation of operands mitchell proposed the concept of a logarithmic multiplier (lm), which uses estimated operands to perform multiplication. the lm performs the operation by changing the operands to approximate logarithmic numbers using shifting and addition operations. using precise piecewise linear approximation [7] and iterative methodology, the accuracy of contemporary designs of logarithmic multipliers is increased. the usage of estimated operands by the error-tolerant multiplier (etm) and a dynamic range unbiased multiplier (drum) [8] are the further enhancement in multipliers. the etm [9] is found with a technique known as multiplier partitioning, which divides a multiplier into accurate multiplication and non-multiplication parts. the least significant bits (lsbs) decide nonmultiplication, while the most significant bits (msbs) determine proper multiplication. 2.2. approximation of partial product generation to obtain the measured final product, we execute some specific processes, but before that the partial products had to be generated and these undergo some compression operations. the under designed multiplier (udm), is being forwarded by substituting one entry of the karnaugh-map based on 2 × 2 approximation multipliers. the approximation 2 × 2 multipliers are utilised as fundamental units for bigger size multipliers to yield approximate partial products which are collected by using the correct adder tree. during the partial product accumulation stage, the generalised design of udm is examined for further utilising carry-in prediction [10]. on approximation booth encoders, a study [11] is carried out. it uses two efficient radix-4 approximation booth encoders. 2.3. approximation of partial product tree in general, the truncation approach is used for incomplete product trees. the fixed-width multiplier estimates the least significant partial products as unchanged. some of the least significant columns are omitted in the inexact array multiplier, resulting in constant partial product columns. among the reduction and rounding strategies, the truncated multiplier that employs the correction constant is chosen. variable correction is required for truncated multipliers to avoid excessive mistakes. 2.4. approximation of compressors compressors are commonly employed in the construction of high-speed multipliers [12] to accelerate the accumulation of partial products (pp). some error compensation algorithms for fixed-width booth multipliers [13] have recently been proposed, which increases the multipliers accuracy. the error compensation circuit is developed using a simpler sorting network. several researches have been undertaken on how to determine or identify a number's logarithm and antilogarithm, with the replica being found. mitchell suggested a simple method for calculating a number's logarithm and antilogarithm, which is then utilised to generate approximate multiplication results (mitchell multiplier). although the multiplier is proposed, it falls short of the mark, hence more research has been done to improve the approximation in the measurement of mitchellbased logarithmic multipliers. 3. proposed approximate multiplier the proposed approximate multiplier having an error measurement of 16-bit for the rounding and truncation parameters of measurement (3,7) consists of blocks namely approximate absolute unit (aau), leading one detector unit also referred as foremost one detector unit (lod), truncation unit (tu), arithmetic unit (au), shift unit (su), sign and zero detector unit (szd), and is represented in figure 2. 3.1. approximate absolute unit the aau is given by inputs of a and b. if the input operand is negative, the results are inverted; if the input operand is positive, the results are unchanged. this aau can be removed for unsigned multipliers. it appears as |a|app and |b|app, for the measured values of a and b as described in [14]. 3.2. leading one detector unit the lod unit or foremost one detector unit takes input as |a|app and |b|app, values. by using these measurement values the ka and kb are detected, which detects the ‘1’ in the msb. these ka and kb are responsible for shifting operation. 3.3. truncation unit the inputs for this tu are measured as ka and kb, and also this is having another two inputs which are measured as |a|app and |b|app, values. the approximation inputs [15] are trimmed and converted to fixed width operands rely on the leading one position of the input operands. the output is obtained from truncation unit are measured as (ya)t and (yb)t these are given as inputs to the arithmetic unit. the terms (ya)t and (yb)t acquired from the truncated unit is the measured value which is represented in the following equation: tu = 1+(ya)t + (yb)t+ (ya)apx +(yb)apx . (1) 3.4. arithmetic unit this au will perform addition on the truncated fixed width operand as well as the product of approximation input, which is denoted by the value '1' and can be written as tu. it's worth noting that the msbs of (ya)apx and (yb)apx are identical to those of measured values of (ya)t and (yb)t. some adders and logical and gates in the arithmetic unit require power gating, this is determined by the operating mode. this is done to improve the design's energy efficiency. the arithmetic block is the same for all the bit lengths. 3.5. shift unit the arithmetic unit's output must be left shifted by the measured value of ka + kb times (ka and kb are the leading onebit values of a and b) the term 2 ka+kb (1+(ya)t + (yb)t+ (ya)apx (yb)apx) can be obtained by conducting the shifting operation, as shown in [16]. for the greatest truncation 't' and rounding 'h' values (h = 5 and t = 9 in this case), the tosam multiplier should be developed. acta imeko | www.imeko.org june 2022| volume 11 | number 2 | 3 3.6. sign and zero detector unit the output operands sign is determined by the sign of the input operands, and if at least one of the inputs is zero, the output is set to zero. the aau should be eliminated, due to the unsigned input operands, and the sign and zero detector unit should be restored with a zero detector unit (zd), as the measurement of the sign unit is unnecessary when the input operands are unsigned. the proposed approximate absolute unit is implemented in the truncated approximate multiplier. the error measurement values (ya)apx ((yb)apx) are denoted as h + 1 bits in this example. when compared to the exact 16-bit multiplier of measurement of (3,7) in figure 1, the below dot diagram gives an overview of the procedure for a specific measurement where truncation 't' and rounding 'r' and their values are treated as t = 7 and h = 3 respectively. in the dot diagram below, the green square represents the "1" bit in the measured term 1+(ya)t +(yb)t + (ya)apx (yb)apx. on the msb side, the orange circles represent partial products of (ya)apx and (yb)apx, while the purple triangles represent the msb bits of (ya)t and (yb)t. the remaining grey circles and triangles in the dot diagram are not included in the current operations, but they will be considered for future multiplier computations. the measurement of partial multipliers in the approximation multiplier for 16-bit is illustrated in figure 3. the process involved in multiplying input operands a by b for a specific measurement of truncation t = 7 and rounding h = 3 is shown in figure 1. the tosam (x, y) structures, where x and y correlate to the rounding 'h' and truncation 't' parameters, and the correctness of this multiplier technique is mostly determined by these parameters ‘t’ and ‘h’. as a result, the relationship between these two parameters, 't' and 'h' must be satisfied in order to ensure maximum precision, as well as a speed and energy consumption that is reasonable. finally, by examining several approaches, this multiplication strategy can be employed for both signed and unsigned operands. to apply this approach to signed multipliers, we must first determine the absolute value of the input operands a and b, as well as the multiplier's sign. calculation time can be reduced by finding the input operands with exact absolute values. in the example, the input operand a is 16-bit and has a decimal value of 11761, whereas the input operand b is 16-bit and has a decimal value of 2482. a and b's exact measurement value is written as (a × b)exact the exact result in binary format is 0000 0001 1011 1101 0110 1010 1001 0010 which is represented in decimal format as 29 190 802, but using the existing method, we get the value as (a × b)existed in the binary format is 0000 0001 1011 1001 0000 0000 0000 0000, the decimal format is 28 901 376. the difference between existed and exact values in this situation is 289 426. the value of (a × b)proposed is calculated using the approximation technique which is explained in figure 4, the binary format 0000 0001 1011 1001 1111 1111 1111 1111, the decimal format is 28 966911. the difference in between the exact and proposed is 223 891. the ka and kb values reflect the leading one-bit location in the input operands a and b. the measured values of ka and kb numbers in this case are 13 and 11, respectively. various (h, t) combinations have a slight modification in the numerical example. various study is being done to build a new measurement of approximate multiplier. in the dynamic segment method (dsm) [17] design, the input operands are trimmed to 'm' bits depends on the location of the leading one bit (value of that particular position, i.e., 1,2, …), and fixed-width multiplication is implemented on the values, that are obtained from truncation operation. while applying this method of truncation, the produced output value in most of the cases is less than the exact one, resulting in a negative mean relative error (mre). when considering digital signal processing applications, strive to keep the mean error as low as feasible to achieve a good signalto-noise ratio (snr). the drum structure is truncated to yield the solution [18]. we seek to bring the mre value near zero, the lsb of the shorter input, which is assigned to the value "1," to limit the erroneous outcome. the truncation of the input operands is performed in the multiplication stage in the low figure 1. 16-bit tosam numerical example of measurement for truncation t = 7 and rounding h = 3. figure 2. block diagram of truncated multiplier. figure 3. representation of measured term 1+(ya)t + (yb)t+ (ya)apx +(yb)apx in dot diagram with truncation ‘t’ and rounding parameters ’h’ are 7 and 3 respectively. acta imeko | www.imeko.org june 2022| volume 11 | number 2 | 4 energy truncation-based approximate multiplier (letam) [19] structure, and there is a chance of omitting half of the partial products. maxare is specified as maximum absolute relative error (considered from relative error re) mre is specified as mean relative error mare is specified as mean absolute relative error vare is specified as variance of absolute relative error ned is specified as normalized error distance max_ned is specified as maximum normalized error distance comparison of the accuracy of tosam against other approximation multipliers like dsm, drum, letam, and uroba in terms of mre, maxare, mare, vare, max ned, and ned using random vectors [5] is performed on the parameters of maxare, mre, mare, vare, max ned, and ned. all these findings are summarised in table 1. edp is defined as energy-delay product pda is defined as power-area-delay product delay, power, area, energy, edp, pda, and mare of the approximation multiplier are calculated and compared with the existing multiplier design and is tabulated in table 2. from the data, the proposed modified multipliers show better results than the other existing approximation multiplier configurations with respect to speed and energy usage while maintaining almost identical mare values. table 2 shows a comparison between dsm, drum, letam, u-roba with the proposed measurement approach of tosam (3,7) approximate multiplier. 4. results and discussions the proposed approximation multiplier with output measurement of 32-bit truncated based multiplier that produces a result that is more approximate than the existing multiplier. 11761 is the a value, and 2482 is the b value. the exact values of a and b are as follows: (a × b)exact value 29 190 802, and the existed of a and b is (a × b)existed equals 28 901 376. the difference between existed and exact values in this situation is 289 426. the value of (a × b)proposed is 28 966911when utilising the proposed approximate technique; the difference between proposed and exact is 223 891, indicating that the value is more approximate. the output is generated in the next cycle and the error value is also shown in figure 5. the internal structure shows the blocks of the proposed approximate multiplier namely approximate absolute unit aau, lead one detector lod, truncation unit tu, arithmetic unit au, shifter, sign-set. also, it represents the flow of data from one block to the other and is shown in figure 6. 5. conclusion and future scope low-energy and area-efficient 16-bit approximation multiplier is proposed. truncation on input operands is performed with two different parameters namely truncation ‘t’ and rounding ‘h’. the existing 16-bit multiplier with rounding and truncation measurement (3,7) shows a measurement error of 0.4 %. the proposed 16-bit multiplier for the same truncation and rounding measurement (3,7) with the measured error of 0.01 % (the error is less than 1 %). the error is reduced by rounding the input operands to the next odd value. the table 1. representation of various approximate multiplier with maxare, mre, mare, vare, max ned, and ned. architecture maxare (%) mre (%) mare (%) vare (%) max ned ned dsm(3) [20] 36.00 -16.1 16.10 40.43 0.2344 0.0399 tosam(0,2) [20] 31.25 -9.1 10.90 46.63 0.3125 0.0309 tosam(0,3) [20] 25.00 -3.3 7.61 28.81 0.2500 0.0213 drum(3) [8] 56.25 2.1 11.90 79.96 0.2344 0.0281 tosam(1,5) [20] 13.89 -0.7 3.95 7.60 0.1250 0.0104 tosam(2,6) [20] 6.87 -0.6 2.06 2.00 0.0664 0.0053 proposed tosam(3,7) 3.65 -0.3 1.05 0.52 0.0342 0.0027 letam(3) [14] 9.72 -4.0 4.00 2.54 0.0859 0.0104 u-roba [15] 11.10 0 2.89 6.37 0.0625 0.0069 tosam(4,8) [20] 1.88 -0.2 0.53 0.13 0.0173 0.0013 table 2. comparisons of delay, power, area, energy, edp, pda, and mare of the approximate multipliers. architecture delay (ns) power (mw) area (µm2) energy (pj) edp (pj · ns) pda (pj · µm2) mare (%) tosam(0,2) [20] 0.74 0.16 342 0.12 0.09 40 10.9 tosam(0,3) [20] 0.84 0.21 423 0.18 0.15 76 7.6 dsm(3) [20] 0.97 0.20 344 0.19 0.19 67 16.1 tosam(1,5) [20] 1.00 0.35 532 0.35 0.35 185 4.0 drum(3) [8] 0.88 0.13 257 0.11 0.10 29 11.9 tosam(2,6) [20] 1.00 0.35 532 0.35 0.35 185 2.06 letam(3) [14] 1.16 0.39 608 0.46 0.53 278 4.0 u-roba [15] 1.05 0.55 1438 0.57 0.60 826 2.9 proposed tosam(3,7) 1.87 0.23 593 0.4 0.748 255 1.05 figure 4. example for generation of approximate absolute value with two negative numbers. acta imeko | www.imeko.org june 2022| volume 11 | number 2 | 5 recommended approximation multiplier is scalable and outperforms the correct multiplier in regard of speed, area, and energy. the proposed approximation multiplier consumes 0.23 mw which is lesser than the existing approximate multipliers. various types of approximate multipliers are used in sharpening the images. in future there is also the possibility of using the multiplier and accumulator unit to create an image sharpening module, and this may be used to measure the energy consumption for various approximate multipliers. also, in other applications, we can use the jpeg technique to compress many images, and this can be used for approximation multipliers in the discrete cosine transform 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(vlsi) syst., vol. 23, no. 6 (2015), pp. 1180–1184. doi: 10.1109/tvlsi.2014.2333366 [14] s. vahdat, m. kamal, a. afzali-kusha, m. pedram, letam: a low energy truncation-based approximate multiplier, comput. elect. eng., vol. 63 (2017), pp. 1–17. figure 5. result of the proposed approximate multiplier with measurement of (3,7). figure 6. rtl schematic of the proposed approximate multiplier with error measurement of (3,7). https://doi.org/10.1109/ets.2013.6569370 https://doi.org/10.1145/2463209.2488873 https://doi.org/10.1145/2950067.2950068 https://doi.org/10.1142/s0218126617300033 https://doi.org/10.1109/tc.2014.2308214 https://doi.org/10.1145/2744769.2744904 https://doi.org/10.1109/tc.2014.2329683 https://doi.org/10.1109/iccad.2015.7372600 https://doi.org/10.1109/tvlsi.2017.2767858 https://www.scopus.com/authid/detail.uri?authorid=56910327700 https://www.scopus.com/authid/detail.uri?authorid=57211475817 https://www.scopus.com/authid/detail.uri?authorid=57224940155 https://www.scopus.com/authid/detail.uri?authorid=57225916538 https://doi.org/10.23940/ijpe.21.06.p6.536542 https://doi.org/10.1109/tc.2019.2926275 https://doi.org/10.1109/tvlsi.2014.2333366 acta imeko | www.imeko.org june 2022| volume 11 | number 2 | 6 doi: 10.1016/j.compeleceng.2017.08.019 [15] r. zendegani, m. kamal, m. bahadori, a. afzali-kusha, m. pedram, roba multiplier: a rounding-based approximate multiplier for high-speed yet energy-efficient digital signal processing, ieee trans. very large scale integr. 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energy-efficient truncationand rounding based scalable approximate multiplier, ieee transactions on very large scale integration (vlsi) systems, vol.27, no.5 (2019), pp. 1161-1173. doi: 10.1109/tvlsi.2018.2890712 https://doi.org/10.1016/j.compeleceng.2017.08.019 https://doi.org/10.1109/tvlsi.2016.2587696 https://doi.org/10.1109/les.2017.2746084 https://doi.org/10.1109/tcsi.2018.2839266 https://doi.org/10.1109/les.2017.2778341 https://doi.org/10.1145/3194554.3194626 https://doi.org/10.1109/tvlsi.2018.2890712 fundamental aspects in sensor network metrology acta imeko issn: 2221-870x march 2023, volume 12, number 1, 1 6 acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 1 fundamental aspects in sensor network metrology sascha eichstädt1, anupam p. vedurmudi1, maximilian gruber1, daniel hutzschenreuter1 1 physikalisch-technische bundesanstalt, bundesallee 100, 38116 braunschweig, germany section: research paper keywords: sensor network; measurement uncertainty; internet of things; co-calibration citation: thomas bruns, dirk röske, paul p. l. regtien, francisco alegria , template for an acta imeko paper, acta imeko, vol. a, no. b, article c, month year, identifier: imeko-acta-a (year)-b-c section editor: section editor received january 1, 2021; in final form january 31, 2021; published march 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: parts of this work was based on outcomes of the projects 17ind12 met4fof, 17ind02 smartcom, bmbf famous and the bmwk gemimeg-ii project. the 17ind12 met4fof and 17ind02 smartcom projects have received funding from the empir programme co-financed by the participating states and from the european union‘s horizon2020 research and innovation programme. the famous project received funding from the german federal ministry of education and research (bmbf). the gemimeg-ii project received funding from the german federal ministry of economy and climate (bmwk). corresponding author: sascha eichstädt, e-mail: sascha.eichstaedt@ptb.de 1. introduction digital transformation includes the integration of digital technologies, such as software, communication and algorithms into products, processes, and services. a disruptive consequence is that these technologies are being used to generate completely new products, processes, and services. the future digital world differs from today’s situation substantially: digital exchange of data and information is becoming the standard; information is provided via cloud services in a machine-actionable way; digital infrastructures utilize information from calibration, self-diagnosis and other metadata communicated by individual measuring instruments; processes and services in the quality infrastructure are based on distributed databases and application programming interfaces (apis). one outcome of the transition to the digital world is that distributed measuring instruments and sensor networks are becoming more important than individual measuring instruments. applications such as the industrial internet of things (iiot) and automated driving will belong to the first examples where the role of metrology is challenged. these challenges include methods for metrological traceable cocalibration and the metrological assessment of whole sensor networks in a systemic approach. for instance, the assessment of an autonomous vehicle measuring system requires a holistic perspective on the whole system instead of only the individual measuring elements. moreover, in the digital world, algorithms and software become as important as the actual measurements, and they will thus influence metrological traceability chains for measurands increasingly. in the digital age, artificial intelligence, sensor fusion and virtual measuring instruments will replace many of today’s tools and principles. their use will require a fundamental reevaluation of the established methodologies for uncertainty evaluation and the assessment of algorithms. in the example of the autonomous vehicle, the assessment of the measuring system must take the algorithms into account, which analyse the data to take decisions. quality of measurement data, trustworthiness of measurement results and reliability of measuring instruments are as important in the digital world as before. hence, metrology abstract sensor networks have appeared on the ’radar’ of metrology only recently and a rigorous treatment and metrological assessment have not been established yet. however, sensor networks as measuring systems underpin many developments in digital transformation, with applications ranging from regulated utility networks to low-cost internet of things (iot). the metrological assessment of sensor networks necessitates a fundamental revision of calibration, uncertainty propagation and performance assessment and new approaches for information and data handling regarding the individual sensors and their interactions in the network to allow a systems metrology approach to be established. this contribution introduces some initial findings from recent research and gives an outlook into future developments. mailto:sascha.eichstaedt@ptb.de acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 2 plays an important role also in the quality infrastructure in the digital age [1]. so far, metrology focuses on single measuring instruments and sensors. however, in a rapidly increasing number of applications networks of sensors are used to address measurement needs. examples can be found in predictive maintenance of production machines in industry, urban air quality monitoring, and multi-modal human health assessment using wearables [2]. an important aspect that distinguishes sensor network applications from single sensor measurements is that rather than the individual sensors, the combined information from all sensors is the main object of interest. for instance, the combination of microphone data and vibration measurement in predictive maintenance provides more insights into the actual status of the monitored machine than the individual measurements alone [3]. a consequence of the focus on combined sensor data rather than individual sensors is that the definition of the quantity of interest, the measurand, is not straightforward. moreover, the assessment of quality and reliability of the system is more complex and challenging than the individual sensors alone, e.g., in terms of calibration result. such an assessment also has to take into account potential sensor failure or networking issues. this also includes consideration of energy consumption, localization of sensors in the network and network communication synchronization [4] let us consider again the above example of predictive maintenance. the combination of different sensor data is carried out in a data-driven approach, i.e., using machine learning methods. the target is a classification of the remaining lifetime of the machine being monitored. thus, the purpose of the measuring sensor network and data analysis is clear, but the outcome – expected remaining lifetime – is not a physics-based combination of the involved measured quantities. since the combination of all sensor data is of interest instead of the individual sensor readings, an assessment of the measurement performance should consider the sensor network as a complex (often distributed) measuring instrument. the metrological treatment of such sensor networks thus requires a novel approach – called “systems metrology”. this approach includes the novel field of “sensor network metrology”, which itself contains aspects such as, in-situ calibration and co-calibration, uncertainty evaluation for dynamic measurements and dynamically structured systems, semantic representation of metrological information, uncertainty-aware machine learning and explainable artificial intelligence applied to sensor networks. most of these topics are still in a stage of early research and prototypical solutions. this contribution introduces sensor network metrology aspects which were addressed in recent research projects. we outline the fundamental sensor network metrology aspects and discuss their combination into a coherent and consistent approach for a metrological treatment of sensor networks. section 2 introduces general aspects of internet of things (iot) type of sensor networks from the viewpoint of metrology. section 3 addresses the digital representation of data and metrological information in sensor networks. section 4 presents and discusses relevant uncertainty evaluation and propagation for processing of sensor network data. section 5 discusses aspects related to the application of machine learning and 1 https://www.ptb.de/empir2018/met4fof/home/ 2 https://opcfoundation.org/ artificial intelligence. finally, section 6 addresses the overall picture and gives an outlook to future developments. 2. metrology and the internet of things in the concept of the internet of things (iot), physical devices communicate with each other via web technologies, thus combining web technology with the physical world. with the rise of the iot in industry 4.0, smart city, smart grids and more, the world of measurement is changing rapidly. as an example, the integration of measuring instruments in the iot poses several specific requirements for the sensors itself, such as communicating via a digital interface, working reliably under a wide range of conditions, reporting on their health status upon request, and ideally, detecting and reporting adversarial conditions. these and other requirements have led to the development of so-called “smart sensors” [2]. these are measuring devices that contain some sort of pre-processing implementing the above features of the iot. as the name “smart sensor” implies, the pre-processing is integrated together with the physical sensing element in one “package”. however, this kind of pre-processing poses new requirements for the calibration of the measuring device because the pre-processing has to be taken into account in the calibration. furthermore, measuring instruments which only provide pre-processed data usually don’t fit well in today’s calibration procedures and guidelines, because these assume access to the raw measurement data. a concrete challenge addressed in the project empir met4fof1 was the dynamic calibration of a digital-output sensor using an external time stamping, e.g., based on gps and a custom-built microcontroller (µc) board [5]. the same approach was then used to demonstrate the extension of a digital sensor such that it communicates not only raw measurement values, but also provides information about the measurement units, uncertainty, and calibration in a machine-readable way [5]-[7]. in this way, the most basic requirement of a metrological treatment of a sensor network can be met: the provision of measurement uncertainty and other metrological information for the individual sensors. in the concept developed in the met4fof project this information is provided by the “smart” sensor itself. however, other information architectures are possible, too. for instance, in the bmbf famous project, a database approach combined with opc-ua communication was considered instead. a similar approach is also considered in the project bmwk gemimegii. more details are given in sections 3 and 4. the concept of sensors providing self-information upon request in a standardized way is also a fundamental element of opc-ua2 (industrial interoperability standard), which is used mostly in industrial applications, but is increasingly adopted in other areas, too. for the metrological information communicated via opc-ua to be machine-readable, it is necessary that the definition of a standard digital representation of units of measurement as well as commonly accepted data models for measured values are available. to this end, the digital si (d-si) data model developed in empir smartcom proposes an approach that is compatible with current guidelines and standards in metrology and calibration [6]. other potential approaches for the digital representation of units of measurement and quantity kinds are the “unified code of units of measure”3 (ucum) or an ontology for units of measure 3 https://ucum.org/ https://www.ptb.de/empir2018/met4fof/home/ https://opcfoundation.org/ https://ucum.org/ acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 3 qudt [8], [9] – each optimized for different data usage approaches. in principle, also combinations of these approaches are possible. the concept of the iot relies on a versatile and flexible combination of measuring instruments, the automated acquisition and processing of the measured data and the application of intelligent algorithms to derive conclusions or decisions. one consequence of this is that data analysis is typically carried out using data-driven machine learning. in contrast to mathematical models that rely on a physical understanding of the measured process, machine learning can be applied directly based on the sensors’ output data. thus, the need for calibration is not as obvious as for “traditional” measurements. however, calibrated measuring instruments in the iot offer several benefits. for instance, calibrated sensors can serve as reference devices in the network to assess and improve data quality [10]; calibration of sensors enables the estimation of the measurand and thus, traceability [7], which itself is required to ensure the comparability of measurements between different sites and countries. moreover, calibrated sensors improve the ability to explain the obtained output from the machine learning. that is, the calibration of a sensor enables direct interpretation based on the measurand whereas a noncalibrated sensors provides data streams which are only loosely related to the physical measured quantity. moreover, the manufacturer’s data sheet alone usually does not suffice as a source of information to assess the type b uncertainty components. hence, calibration plays an important role in iot and provides benefits on all data processing layers and a way to quantify the trust one can have in the measurement system (see figure 1). 3. digital representation of data and information in sensor networks in the digital world, measurement data must also be readable and understandable by machines. this implies that the information about the measuring instruments, the units of measurement and other accompanying metadata must be available in a format that can be used by software or an algorithm. for instance, the software may need to verify that the unit of measurements of a given data set is consistent with previous entries of a data base. the machine readability of data and measurement information is of particular importance in sensor network metrology. with hundreds of sensors measuring continuously, measurement data cannot be normalized and analysed manually, but requires a high level of automation. this, in turn, can only be achieved with machine-readable information. the machine readability begins with the description of the unit of measurement, for instance as shown in figure 2. in the project empir 17ind02 smartcom, a data model and digital representation of units of measurement, called d-si, have been proposed. on the cipm level, the d-si as well as ucum and qudt and other approaches to the digital representation of quantities and units of measurement are being discussed. another important element is the information about the individual measuring instruments. for instance, a machinereadable digital calibration certificate [11] could be provided by the sensor itself, e.g., using opc-ua, or from another source, e.g., an internal quality management platform. if needed, this information could be further extended by other data which could affect the quality of measurement data [12]. machine understandable representation of knowledge on information in sensor networks and its semantics can be modelled by means of (combinations of) ontologies. several ontologies useful for sensor networks can be found within the semantic web community. these ontologies formalize the annotation of sensor data with spatial, temporal, and thematic metadata. spatial metadata is particularly relevant for sensors distributed across a building or a country, or when mounted on a moving object like an automobile. in the project famous a method to merge different kinds of metadata and ontologies along with the sensor measurement data was proposed [13]. the main idea of [13] is to split the selfdescription of a sensor into four aspects: (1) observation information, (2) general sensor description, (3) calibration information and (4) location information. then, a sensor selfdescription can be achieved by combining existing ontologies that appropriately represent these aspects. in this way, one can build upon existing work and established principles and software tools. in the gemimeg-ii project this development was extend to the integration of semantically described quality of data aspects [12]. 4. measurement uncertainty in sensor network data processing measurement data in sensor networks is often heterogeneous, volatile, and time dependent. moreover, sensor networks in iot scenarios often contain low-cost measuring instruments based on mems sensor technology. consequently, such sensor networks typically have a wide range of measurement data quality. reliable data analysis for sensor networks thus requires taking data quality into account in a quantitative way. figure 1 calibration information in the different layers of the iot architecture. figure 2 example for an algorithmic representation of units of measurement, which can be used by a software acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 4 an example of a fundamental property that can be considered as a quality metric is the measurement uncertainty. other examples for parameters which may affect the quality of data in sensor networks are unstable network conditions, environmental interference, or malicious attacks. moreover, in battery powered sensors, there is necessarily a trade-off between powerconsumption and performance. another common issue is drift, where sensor readings slowly deviate from the true value due to the degradation of the electronics [14]. one outcome from the gemimeg-ii project is a framework for quality of data in sensor networks, expressed in terms of an ontology [12]. this can be expressed, for instance in an ontology [12]. in a joint effort by the projects famous and empir met4fof it was demonstrated how an ontology of a sensor network could be utilized for an automated analysis [7], [13]. moreover, together with the project empir smartcom a sensor network data set was enriched with machine-readable metadata to demonstrate the metrological support of the fair principles [15]. usually, the sensors used in iot applications are measuring continuously irrespective of how the measured values are used. thus, for a reliable quantification of data quality it must be ensured that the sensor behaviour is well known for a wide range of measurement situations. this includes situations where the measurand, i.e., the sensor input signal, changes rapidly over time. thus, sensor properties such as effective bandwidth, internal analogue-to-digital conversion, time stamping reliability, resonance behaviour need to be considered. data analysis in the iot typically relies on and greatly benefits from modern machine learning methods, because of the complexity of the sensor network and the amount of data acquired. uncertainty evaluation for machine learning is an important topic and considered in several research activities. however, this is only possible if the uncertainty associated with the machine learning input values is available. hence, uncertainty for data pre-processing must be addressed as the initial step for an uncertainty-aware machine learning for iot. measurements in the iot are usually time dependent, and often even dynamic. examples are air quality monitoring, traffic surveillance, production control or mobile health measurements. thus, signal processing methods are regularly applied for data pre-processing in iot scenarios. for instance, the discrete fourier transform is often applied to extract magnitude and phase values from a measurement of vibration, which are then used in a subsequent machine learning method as features. other examples for pre-processing are synchronising the time axis of sensors; interpolation of sensor data to account for missing values or non-equidistant sampling; low-pass filtering to reduce noise or other unwanted high-frequency components in the measured data. another reason for the application of data preprocessing is the reduction of data dimensionality. this may be necessary simply due to storage or data transfer bandwidth limitations [3]. in the project empir met4fof, the previously developed python library pydynamic [16] was extended to include the data pre-processing steps typically required in iot. for each method, pydynamic provides the propagation of uncertainties [16]. an important aspect in empir met4fof and in famous was also the implementation of the methods in such a way that they can be applied online, i.e., during the measurement. for instance, the discrete wavelet transform for uncertain input data was implemented using digital filters [17]. another important aspect is the way of how the uncertainty propagation software is provided such that it is compatible with typical iot architectures. in the project empir met4fof, a socalled agent-based framework (abf) was used. in an abf, data processing steps are encapsulated in software modules, called “agents”. these agents can run on different locations in the network, if necessary, and allow for a very flexible demanddriven data analysis. for instance, one agent may acquire the data from a sensor, hands it over to an interpolation agent, which then provides it to a fourier transform agent. with each agent taking care of the proper uncertainty treatment, very flexible data analysis pipelines for sensor network metrology can be realised. usually there is an existing data analysis framework in place, which needs to be extended to include measurement uncertainty treatment. as a result, a web-service approach was used in the project famous instead of an abf. that is, an uncertainty module was created to enrich existing sensor data streams with statements about the associated measurement uncertainty. 5. flow of metrological data and metadata in sensor networks to summarise the different elements described in the previous sections, let us consider the flow of metrological data and metadata in sensor networks, see figure 3. for the individual sensors we assume the availability of information about their metrological properties. at least this information should be available in terms of a manufacturer’s data sheet from which information about measurement capabilities, units of measurement and tolerances can be extracted manually. ideally, the information is provided in digital, machine-readable way. for instance, the sensor may communicate metadata using an iot standard, such as opc-ua, rami 4.0 or provide a dcc. this metadata could contain fundamental information about the sensor: serial number, units of measurement, measurement uncertainty, calibration information. for an automated handling of this information, the metadata itself has to be machine actionable. that is, units of measurement have to be given using a suitable data model (e.g., d-si, ucum, qudt). a representation of this information in accordance with the fair principles would furthermore require the use of some kind of persistent identifiers (pid) to ensure machine-interpretable interoperability with other data models. the metadata at the sensor level could also contain information on the quality of sensing. that is, the sensor could be self-aware or be complemented with other sensor data. for instance, a radar sensor in an autonomous vehicle may be complemented with a rain and fog detector to enrich the radar sensor data with metadata about the weather conditions. other potentially useful metadata could be whether the sensor is battery powered, general energy restrictions or measurement strategies (e.g., raw data or averaged data). depending on the sensor network use case, this information can be crucial for the metrological assessment of the measuring system’s quality and reliability. the sensor metadata has to be made available throughout the data lifecycle in the sensor network to enable its use in the data processing and decision making. the first steps, data curation and data aggregation in the processing of sensor data are often already the place where the sensor metadata is lost. however, information about the propagation of quality of sensing (e.g., measurement uncertainty) must be carried out to ensure proper assessment of the overall sensor network quality. the individual sensor quality is not sufficient for this assessment. the data lifecycle metadata can be enriched in the data processing by information about the applied algorithms, their acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 5 parameters/settings and other information related to the reliability of the processing result. moreover, the quality of sensing has to be translated and propagated into a quality of data following the data processing steps. with this in place, a proper, reliable, and data-driven decision making is possible that is not only based on the raw sensor data but takes all relevant metadata and information into account. moreover, it enables a traceability from the quality of the decision making to the quality of sensing as well as traceability of sensor network results to the si units of measurement. 6. conclusions sensor network metrology combines several aspects from metrology, signal processing, semantics, iot and web technologies. the treatment and metrological assessment of sensor networks, thus, needs to take these fields into account. although sensor networks can be found in many applications, a rigorous sensor network metrology has not been established yet. existing guidelines in metrology are typically focused on individual measuring instruments and quantities. the same holds true, by the way, for the organisation of metrology institutes and calibration laboratories. first metrology research efforts developed some basic elements required in sensor network metrology: dynamic calibration of digital sensors, cost-efficient calibration of mems sensors, digital representation of metrological metadata, evaluation and propagation of uncertainties, semantic modelling of sensor network information. future research needs to further develop these individual aspects and extend their integration into a consistent framework and toolset. moreover, a systems metrology approach needs to be developed to assess sensor networks in a systemic way. acknowledgement parts of this work was based on outcomes of the projects 17ind12 met4fof, 17ind02 smartcom, bmbf famous and the bmwk gemimeg-ii project. the 17ind12 met4fof and 17ind02 smartcom projects have received funding from the empir programme co-financed by the participating states and from the european union‘s horizon2020 research and innovation programme. the famous project received funding from the german federal ministry of education and research (bmbf). the gemimeg-ii project received funding from the german federal ministry of economy and climate (bmwk). references [1] b. jeckelmann, r. edelmaier, the metrological infrastructure, de gruyter oldenbourg, 2023. isbn 9783110715682 [2] t. schneider, n. helwig, a. schütze, industrial condition monitoring with smart sensors using automated feature extraction and selection, meas. sci. and technol. 29(4) (2018), art. no. 094002. doi: 10.1088/1361-6501/aad1d4 [3] t. dorst, t. schneider, a. schütze, s. eichstädt, gum2ala– uncertainty propagation algorithm for the adaptive linear approximation according to the gum, in smsi 2021 system of units and metreological infrastructure. doi: 10.5162/smsi2021/d1.1 [4] e. balestrieri, l. de vito, f. lamonaca, f. picariello, s. rapuano, i. tudosa, research challenges in measurement for internet of things systems, acta imeko 7 (2018) 4, pp. 82-94. doi: 10.21014/acta_imeko.v7i4.675 [5] b. seeger, t. bruns, primary calibration of mechanical sensors with digital output for dynamic applications, acta imeko 10 (2021) 3, pp. 177-184. doi: 10.21014/acta_imeko.v10i3.1075 [6] d. hutzschenreuter et al., smartcom digital system of units (dsi) guide for the use of the metadata-format used in metrology for the easy-to-use, safe, harmonised and unambiguous digital transfer of metrological data second edition, 2020 doi: 10.5281/zenodo.3816686 [7] s. eichstädt, m. gruber, a. p. vedurmudi, b. seeger, t. bruns, g. kok, toward smart traceability for digital sensors and the industrial internet of things, sensors 21(6) (2021). doi: 10.3390/s21062019 [8] g. schadow, c. j. mcdonald, the unified code for units of measure, in: regenstrief institute and ucum organization: indianapolis, in, usa, 2009. online [accessed 16 march 2023] https://ucum.org/ucum [9] h. rijgersberg, m. van assem, j. top, ontology of units of measure and related concepts, semantic web 4(1) (2013), pp. 313. doi: 10.3233/sw-2012-0069 [10] g. tancev, f. grasso toro, sequential recalibration of wireless sensor networks with (stochastic) gradient descent and mobile references, measurement: sensors 18 (2018), art. no. 100115. doi: 10.1016/j.measen.2021.100115 [11] s. hackel, f. härtig, th. schrader, a. scheibner, j. loewe, l. doering, b. gloger, j. jagieniak, d. hutzschenreuter, g. söylevöktem, the fundamental architecture of the dcc, measurement: sensors 18 (2021), art. no. 100354 doi: 10.1016/j.measen.2021.100354 [12] a. p. vedurmudi, j. neumann, m. gruber, s. eichstädt, semantic description of quality of data in sensor networks, sensors 21(9) (2021) art. no. 6462. doi: 10.3390/s21196462 [13] m. gruber, s. eichstädt, j. neumann, a. paschke, semantic information in sensor networks: how to combine existing ontologies, vocabularies and data schemes to fit a metrology use figure 3 flow of data and metadata from individual sensors (left) to the final decision making (right). with proper data treatment in place, traceability to si is possible from right to left. https://doi.org/10.1088/1361-6501/aad1d4 https://doi.org/10.5162/smsi2021/d1.1 https://doi.org/10.21014/acta_imeko.v7i4.675 https://doi.org/10.21014/acta_imeko.v10i3.1075 https://doi.org/10.5281/zenodo.3816686 https://doi.org/10.3390/s21062019 https://ucum.org/ucum http://dx.doi.org/10.3233/sw-2012-0069 https://doi.org/10.1016/j.measen.2021.100115 https://doi.org/10.1016/j.measen.2021.100354 https://doi.org/10.3390/s21196462 acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 6 case, proc. of ieee int. workshop on metrology for industry 4.0 & iot 2020, roma, italy, 03-05 june 2020, pp. 469-473. doi: 10.1109/metroind4.0iot48571.2020.9138282 [14] k. goebel, w. yan, correcting sensor drift and intermittency faults with data fusion and automated learning, ieee systems journal 2(2) (2008), pp. 189–197. doi: 10.1109/jsyst.2008.925262 [15] t. dorst, m. gruber, a. p. vedurmudi, sensor data set of one electromechanical cylinder at zema testbed (zema daq and smart-up unit), in: zenodo, doi: 10.5281/zenodo.5185952 [16] s. eichstädt, c. elster, i. m. smith, t. j. esward, evaluation of dynamic measurement uncertainty – an open-source software package to bridge theory and practice, journal of sensors and sensor systems 6 (2017), pp. 97-105. doi: 10.5194/jsss-6-97-2017 [17] m. gruber, t. dorst, a. schütze, s. eichstädt, c. elster, discrete wavelet transform on uncertain data: efficient online implementation for practical applications, in metrology and testing xii, series on advances in mathematics for applied sciences 90 (2022), world scientific publishing co, singapore, pp. 249-261, isbn 978-981-124-237-3 (hardcover), 978-981-124-2397 (ebook) doi: 10.1142/9789811242380_0014 https://doi.org/10.1109/metroind4.0iot48571.2020.9138282 https://doi.org/10.1109/jsyst.2008.925262 https://doi.org/10.5281/zenodo.5185952 https://doi.org/10.5194/jsss-6-97-2017 https://doi.org/10.1142/9789811242380_0014 the improved automatic control points computation for the acoustic noise level audits acta imeko issn: 2221-870x september 2021, volume 10, number 3, 142 149 acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 142 the improved automatic control points computation for the acoustic noise level audits tomáš drábek1, jan holub1 1 czech technical university in prague, faculty of electrical engineering, dept. of measurement, technicka 2, 166 27 prague 6, czechia section: research paper keywords: noise; measurement; algorithms; automation; determination of control points citation: tomáš drábek, jan holub, the improved automatic control points computation for the acoustic noise level audits, acta imeko, vol. 10, no. 3, article 15, september 2021, identifier: imeko-acta-10 (2021)-03-15 section editor: lorenzo ciani, university of florence, italy received february 7, 2021; in final form august 6, 2021; published september 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: tomáš drábek, e-mail: tomas.drabek@fel.cvut.cz 1. introduction the acoustic noise level is measured in human-occupied buildings to ensure comfortable living conditions. as described in [1], temperature, humidity, and co2 concentration are the usually monitored indoor quantities. measurement of the acoustic noise level, together with the mentioned quantities, can be used to improve the quality of indoor living. the noise measurement process is described in national standards that specify, for example, restrictions for placing control points, the duration of the individual measurements, or measurement device specifications. national supervisory authorities or private companies use these standards to determine noise levels both indoors and outdoors. based on the measured values, they provide final recommendations. the whole process of measuring noise is time-consuming. therefore, it is advisable to automate at least some steps of the process [2]. this paper aims to design automatic algorithms for placing the control points into a given room by obeying all constraints set by the standard. the measurement of the noise level is then performed in these control points. the international and national standard [3], [4], for which the proposed method is designed, does not distinguish different measurement purposes and specifies only general rules for the location. in this article, two measurement purposes are proposed distinguishing: (i) living condition verification for a long-term stationary noise and (ii) living condition verification for a shortterm recurring noise. for the former, the algorithm can plan for resource allocation as the noise is presented continuously. therefore, the algorithm aims at covering the room with as many control points as possible at the end of the measurement. for the latter, the resource allocation cannot be planned as the period of the noise signal is not known as a priory. the proposed algorithms place the control points to cover the maximum area in the limited time, in this case, at each iteration. these two different criteria result in different control points placing strategies, as depicted in figure 1. the current measurement procedure is performed by measuring noise levels in a network of control points. the control points are distributed around the room based on the abstract the acoustic noise level in the interior is one of the quantities specified by a standard and is subject to audits to ensure a comfortable living environment. currently, the noise level audits are performed manually by a skilled operator, who evaluates the floor plan and uses it to calculate the control points location in which the measurement is performed. the computation is proposed to automate the audit by formulating an optimisation problem for which an algorithm was designed. the algorithm computes the solution that satisfies all constraints specified in the standard, for example, the minimum distance among the control points and fixed obstacles (walls or columns). in the proposed optimisation problem, the fitness function was designed based on the measurement purpose, and two typical use-cases were analysed: (i) long-term stationary noise measurement and (ii) recurring short-term noise measurement. although the set of control points for both use cases complies with the given standard, it is beneficial to distinguish the location of control points based on the measurement purpose. the number of control points is maximised for the stationary noise and for the immediate coverage area for the short-term noise. the proposed algorithms were tested in a simulation for several floor plans of different complexity. mailto:tomas.drabek@fel.cvut.cz acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 143 restrictions given by the standard, and a qualified operator determines their density and positions. the distance of adjacent control points must be no less than 0.7 m, and at least one control point must be located in a corner. in addition, all points must be at least 0.5 m away from the wall and at least 1 m away from significantly sound-transmitting elements such as windows or entrance openings for air supply. windows and doors must be closed during measurement. the location of the measuring points defined in this way is determined manually by the operator according to the room's dimensions. noise measurements are then made at these points at a height between 1.2 m and 1.5 m from the ground. the measuring instrument is directed towards the source of the incoming noise or vertically upwards if the direction of the noise source is not defined. a certified sound level meter is used as the measuring instrument. several articles have been published to measure and reduce noise. some publications focus on noise caused by equipment (aircraft, cars, turbines). the article [5] dealt with the possibility of measuring aircraft noise using linear microphone arrays. using this method, an undistorted record of aircraft noise was achieved. the article [6] also deals with determining aircraft noise and selecting this noise from background noise. against previous research, they use neural networks to recognise only aircraft noise. the accuracy converged to 99.84%. the publication [7] deals with the measurement of noise in landing aircraft that use thrust reversers to slow down aircraft after landing at madrid-barajas airport. the paper presents possible improvements for detecting noise from the reverse thrust and the direction of incoming noise. another sector where noise is measured is the cars industry. the article [8] presents and verifies the application of statistical energy analysis (sea). the application is used for 3d modelling of noise reduction in the interior of a car from the drivetrain. part of the work is also a proposal of measures to reduce noise. wind turbines are a separate branch of noise measurement. the standards specify, for example, noise measurement methods, requirements for measuring instruments, evaluation. standard [9] provides overall wind turbine noise measurement standards. conversely, standard [10] focuses more on the aeroacoustics noise of wind turbines. for example, the article [11] deals with noise measurement in the interior of buildings close to wind farms. another approach to noise measurement requires involving citizens and using their smart devices to monitor noise in their immediate vicinity. such a measurement was dealt with in a study [12] which, using this method, created spatial and temporal maps of noise. noise measurements are also often performed indoors. in [13], the interior noise reduction index (nri) was determined. the article deals with the definition of the index for nris with open windows for the summer months. a theoretical model was created and compared with experimentally obtained data. today, several software programs simulate acoustic conditions in buildings. based on the created model of rooms with specified noise sources, technicians can create a noise map. in [15], the authors simulated a noise map and used it to identify critical areas using reference measurements and rap-one software (room acoustics prediction and occupational noise exposure). article [16] deals with the measurement of noise at the place of residence of 44 schoolchildren. the measurements were performed in the children's rooms and in the room where the schoolchildren spent most of their time. outdoor noise was also recorded during the measurement. the sound pressure level affects the workplace and, for example, medical facilities where patients are treated. article [17] deals with the measurement of noise around and inside the hospital. a total of 24 measurements were performed on the outer facade of the hospital and 21 measurements inside. from the measured data, it was evident that they exceeded the set limit. the difference between outdoor and indoor noise is also described in the article [18]. the study performed noise measurements inside and outside the building at 102 inhabitants, with open, closed or semi-open windows playing a significant role. the result of the study was the creation of a statistical model that can be used to estimate the sound exposure inside the building. noise measurement using a robotic unit is becoming more common these days. in paper [19], the humanoid robotic unit was equipped with, among other sensors, a sound level meter. the humanoid measured values at 20 points in the room. robot evaluated the comfort for the room based on an interaction with a human operator and from the measured values. the article [20] on noise maps outside buildings also used an autonomous mobile robotic platform equipped with a sound level meter to measure noise. the measured values by the robot were compared with a model of known sources and with manual measurement. in conclusion, it was stated that noise maps should be based on the application and the environment. all the publications mentioned above focused on the processing of the noise measurement data. on the other hand, the present paper aims to show the possibility of automating figure 1. the control points location (black dots) for stationary noise (top row) and short-term recurring noise (bottom row) measurement. the left figures show the first iteration of both processes, in which the control point is located to the corner of the polygon defined by a 1 m distance from walls. the algorithm for stationary noise measurement (top) places control points step-by-step to put as many control points as possible while not placing them closer than the minimum distance visualised by the red circle. on the other hand, the short-term recurring noise spread points quickly leading to broad coverage in a short time by minimising the objective function visualised by the contour plot. therefore, the total number of control points that can fit into the room is smaller for short-term recurring noise. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 144 determining control points for measuring indoor noise. the novel contribution of this article is in the noise measurement methodology consisting of the design and testing of both purposes mentioned above, living condition verification for a long-term stationary noise and living condition verification for a short-term recurring noise. the control points are determined to meet the conditions based on the standard. the algorithms for calculating control points in a room are based on the assumption that they do not have previous information about the room's parameters besides the floor plan. therefore, individual elements (windows, openings in the wall, etc.) cannot be identified. for this reason, the condition is set that the control points are located 1 m from the wall. this condition is based on the requirement for distance from significantly sound-transmitting elements. all the above distances satisfy this condition. the article is divided into three parts. section 2 deals with the description of proposed noise measurement solution for the two considered purposes. section 3 adresses the validation of the functionality of the proposed solution. finally, the conclusions are summarised in section 4. 2. proposed approach indoor noise measurements are performed at control points. the set of control points will be marked with the symbol χ and the individual control points with the symbol x: 𝜒 = {𝑥1, 𝑥2, … , 𝑥𝑛 } (1) where n is the number of control points. first, the area where the measurements will be performed is defined. the area of the room is denoted as p, and the wall of the room (boundary) represents the polygon ∂p. then the control points will be located in the inner area i defined as: 𝐼 = {𝑥|𝑥 ∈ 𝑃 ∧ 𝑑(𝑥, 𝜕𝑃) ≥ 1} (2) all control points xn are located in the inner area of the room i: 𝑥𝑛 ∈ 𝐼. (3) the standard specifies the minimum requirement for the mutual euclidean distance of control points, and it is 0.7 m: 𝑑(𝑥𝑖 , 𝑥𝑗 ) ≥ 0.7, (4) where xi a xj are control points, and indices take values: 𝑖, 𝑗 ∈ {1, … , 𝑛}; 𝑖 ≠ 𝑗. (5) according to the above standard, there should be at least one control point in the corner of the room. a corner is defined as a point that makes an angle below 180° between two lines representing the walls of a room. the corners between 60° and 120° have priority in the selection. the corner(s) with the nearest angle to 90° is selected if there is no corner in this range. the above parameters are shown in figure 2. manually determining and setting control points is timeconsuming, hence this process was decided to automate two specific noise measurement purposes. the first is a long-term stationary noise, and the second is a regular short-term noise. software algorithms were created for both of these purposes. 2.1. long-term stationary noise long-term stationary noise is caused, for example, by operation from a factory in the neighbourhood or on a construction site. at the measuring point, the measurement takes place for an unnecessary time to ensure a sufficiently long and high-quality noise measurement. the measurement at the given control point has been declared demonstrable. after this interval, the measuring apparatus is moved to a new control point for further recording. the proposed algorithm searches for the maximum set of control points according to the specified parameters for maximum noise capture in a given space. we are looking for a maximum set that meets the following conditions: 𝜒∗ = max|𝜒|, (6) where χ* is the maximum number of control points in a given room, and |χ| is the number of control points for the set χ. the entire algorithm was designed to obtain the maximum number of control points in area i. obtaining data from the maximum number of control points would provide a more accurate map of capturing noise levels in space. the algorithm works by creating a list of all corners in the measured area i (equation 2). it then passes through the individual corners and determines two intersections at a distance of 0.7 m from the given corner with ∂i. thus, the algorithm obtains at least two initial variants for each corner. the program uses a recursive algorithm for each initial variant and creates a list of control points for each such recursion by following these steps: 1) from the already determined points, it draws a circle with a radius of 0.7 m. the intersections with other circles give new control points. 2) if such an intersection does not exist and an intersection with ∂i, it places a control point at this intersection. 3) if the algorithm does not find any new control point in the iteration, it terminates the recursion and saves the total number (list) of found control points for the given variant. the algorithm provides the list of the best results set – the maximal list of control points. 2.2. short-term recurring noise this is a noise caused, for example, by public transport (trams, buses) at the place of residence, workplace or school. figure 2. the walls of the room (black lines) represent the boundary of the polygon ∂p, where p represents the area of the room. the blue lines delimit the measuring area that is at a 1 m distance from ∂p. the colored background of the room is used only in the algorithm for regular short-term noise, where it shows the distance from each already determined control point in space. at the control point, the background brightens and darkens with increasing distance. this illustration is used for regular short-term noise when it is appropriate to find the farthest control point in the defined area from already specified control points. the color background is recalculated each time iteration. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 145 measurements shall be made at a given control point until the noise level exceeds a specified value. subsequently, the measuring apparatus is moved to a new measuring point, and the entire process is repeated. due to the lack of knowledge of the number of occurrences exceeding the specified noise level, the algorithm looks for the best distribution of control points in each iteration in order to cover as much room space as possible. the maximum distance is being looked from the already determined control points in the inner area of room i: 𝑥𝑛 ∈ arg max 𝑥∈𝐼 min 𝑖∈{1,…,𝑛−1} 𝑑(𝑥, 𝑥𝑖 ), (7) where n is the iteration number and xi are control points positions selected in previous iterations. the algorithm first determines the list of corners. then one of the specified corners is selected randomly. the algorithm divides the floor plan of the room into individual triangles. in each triangle, the furthest local control point from the previously determined control points is calculated. subsequently, these locally optimal solutions are compared to select the furthest point. this point is selected as a global optimum and is intended for noise level measurement. the entire process is shown in figure 3; it is repeated until one of the conditions is satisfied: 1) there shall be at least one local point that satisfy the conditions specified in the standard (minimum distance from walls and other control points). 2) the noise measurement does not exceed the specified level at the measured control point. 3) the time interval for the measurement does not expire. 3. experiments in order to improve the proposed algorithms for both purposes of measuring noise intensity, experiments were prepared to verify and test the algorithm. this section is organised as follows: the experimental environments are presented first, describing the rooms that were used in the verification; second, the control points are analysed for both algorithms; and third, the measurement time is analysed for both algorithms. the presented quantitative results demonstrate the performance of the proposed methods. in the first experiment, the basic accuracy of calculations and the determination of control points were verified in a simple room layout for both algorithms. the size of the room was 4.0 m × 4.0 m. each experiment is divided into two simulations: one for long-term and one for short-term noise. the simulations for the first experiment are shown in figure 4. the second experiment focused on a simulation of a more diverse room. the dimensions of the real room were measured and incorporated into the simulation. the results of the control point calculation for both algorithms are recorded in figure 5. the third experiment was focused on verifying the robustness of algorithms in more demanding conditions. an extensive figure 3. the algorithm divides the room into triangles and in each, calculates local optima (farthest points) from already determined control points. subsequently, the farthest point is selected from this set of local optima and determined as the global minimum for the given iteration – the green point. at each iteration, a new background contour is calculated, which displays the distance from the control points in color. figure 4. the first top four images show the first algorithm that determines the control points for long-term stationary noise. the algorithm gradually adds control points where the intersection of the 0.7 m boundary with the other boundaries (red circles) occurs. an algorithm created the lower quartet of the image for regular short-term noise. the algorithm focuses on covering the area as much as possible in each iteration. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 146 segmented room was created, where the algorithms determined the control points (figure 6). the next experiments focused on rooms with internal obstacles, which in real conditions represent columns, partitions, etc. first, the algorithms were tested on simple rooms with one internal column in the area where control points are determined (figure 7). according to the standard, 1 m has been defined around a fixed obstacle, where control points for noise measurement are not determined. in the last experiment, algorithms were tested for multiple obstacles (figure 8). 4. result analysis this section presents the results of both algorithms, focusing on the number of checkpoints and the time required to determine them. experiments were performed for simple, real, complex and built-up spaces. both algorithms demonstrated functionality and robustness in the calculation of control points. 4.1. control points analysis an overview of the number of control points from both algorithms from individual experiments is reported in table 1. surprisingly, in the first experiment, the algorithm for long-term stationary noise found two control points less than the algorithm created for regular short-term noise. this anomaly occurred only in the first experiment, where it was an elementary room without a fragmented floor plan. further testing showed that the algorithm for finding the maximum number of control points in the measuring area is not optimal and that it may not find the maximum number of control points for small rooms. in the second experiment, the algorithm for long-term stationary noise discovered more control points than the second. figure 5. the room is characterised by a more complex area where control points may occur. the results of the first algorithm are shown in the upper part. the figures show how the algorithm first determines the border control points in the marked area and then passes into the inner area of the room. the second algorithm created a network of control points, focusing on successively spaced control points throughout the area, as can be seen from the bottom figures. figure 6. to test both algorithms in demanding conditions, a large room of intricate design was created. both algorithms proceeded as expected in determining the control points. the results show that the algorithm for longterm stationary noise determined significantly more control points than the algorithm for regular short-term noise. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 147 figure 7. another experiment was to test both algorithms in an environment where there is a simple barrier in the form of a column. around this obstacle, both algorithms, as expected, created an area in which no measurement is performed. figure 8. the last experiment focused on irregular, articulated obstacles that were both outside the measuring area and inside. both algorithms proved their robustness and delineated the measuring area correctly. table 1. the total number of control points and computation time. room number of control points computation time [min] long-term stationary noise short-term recurring noise long-term stationary noise short-term recurring noise sq. room (figure 4) 11 13 0.05 0.50 real room (figure 5) 44 33 0.50 4.13 large room (figure 6) 360 224 6.20 245.82 sq. room with column (figure 7) 69 47 0.33 8.37 sq. room with obstacles (figure 8) 187 133 0.90 86.18 acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 148 in the third experiment, the algorithm for long-term stationary noise determined 360 control points in the final result. the algorithm for regular short-term noise identified 224 control points. obstacles were inserted into the measured area in the fourth and fifth experiments. the difference in the number of control points for larger rooms shows that the first algorithm is indeed more suitable for long-term noise. 4.2. estimating the measurement time the algorithms differ according to the measurement purpose and the time they need to determine the control points. in all experiments, the time required to determine all the control points was recorded and is presented in table 1. results show that the algorithm for the first purpose calculates all control points much faster than the algorithm for regular short-term noise. in the third and the fifth experiment, the most apparent difference in the time calculation of control points was between the two algorithms. in actual measurements, however, this difference is not decisive. vice versa, when measuring longterm stationary noise, a faster determination of other control points is needed changing the measuring point sets immediately after recording the noise level at that point. for regular shortterm noise, the algorithm has enough time to calculate. the change of the measuring point is not known, and the noise level at the given control point is expected to be exceeded. while waiting for the noise level to be exceeded, the algorithm can determine the next control points based on the dimensions of the room. optionally, the algorithms can compute control points offline before the measurement based on the floor plan measured either manually or automatically using lidar. 5. conclusions this paper aimed to create algorithms for two specific purposes of noise measurement. the first algorithm was developed to determine control points for long-term stationary noise, and it finds the maximum number of control points in the room. the second algorithm was created to determine control points for regular short-term noise. for this purpose, the program does not know the number of iterations of the measurement. therefore, it looked for the location of the following control point in the measuring area to cover as large an area as possible. both algorithms were tested in different rooms during the experiments, from simple floor plans over large rugged rooms to the room with obstacles. the simulations showed that the proposed repeatable algorithms satisfy the specified conditions set by the standard. the time required to determine all control points in the defined area during the simulations for both algorithms was recorded. the novelty of the noise measurement methodology consists in the design and testing of both algorithms, with the control points being determined based on the dimensions of the room and the purpose of the measurement. the control points were determined to meet the conditions based on the standard. if this algorithm is used in the future to a mobile robotic unit that will contain a measuring device, the measurement can take place entirely autonomously without the presence of an operator. therefore, we decided to test both of our algorithms using a robotic unit that will initially measure the floor plan by simultaneous localisation and mapping in the next phase. the knowledge published in [20] can also be used to navigate the room with the robotic unit. the constructed map is used to compute control points, and the robotic platform measures these points afterwards. acknowledgement this work was supported by the grant agency of the czech technical university in prague, grant no. sgs20/070/ohk3/1t/13. references [1] s. s. carlisle, the role of measurement in the development of industrial automation, acta imeko 3 (2014) 1, pp. 4-9. doi: 10.21014/acta_imeko.v3i1.190 [2] j. svatos, j. holub, t. pospisil, a measurement system for the long-term diagnostics of the thermal and technical properties of wooden houses, acta imeko 9 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[14] c. guarnaccia, j. quartieri, a. ruggiero, acoustical noise study of a factory: indoor and outdoor simulations integration procedure, international journal of mechanics 8(1) (2014), pp 298-306. online [accessed 13 august 2021] https://www.naun.org/main/naun/mechanics/2014/a162003 -065.pdf http://dx.doi.org/10.21014/acta_imeko.v3i1.190 http://dx.doi.org/10.21014/acta_imeko.v9i3.762 https://doi.org/10.1109/maes.2010.5442149 file:///d:/tazi/stažené%20soubory/10.1109/icaiic.2019.8669006 http://dx.doi.org/10.21014/acta_imeko.v4i1.154 https://doi.org/10.1109/ivs.2009.5164478 https://doi.org/10.1109/ieeestd.2016.7502056 https://doi.org/10.1109/icrera.2017.8191260 https://doi.org/10.1088/1742-6596/1603/1/012030 https://www.naun.org/main/naun/mechanics/2014/a162003-065.pdf https://www.naun.org/main/naun/mechanics/2014/a162003-065.pdf acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 149 [15] s. pujol, m. berthillier, j. defrance, j. lardiès, r. petit, h. houot, j. p. 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electrical engineering and 2019 ieee industrial and commercial power systems europe (eeeic i&cps europe), genova, italy, 11-14 june 2019, pp 1-6. doi: 10.1109/eeeic.2019.8783688 [19] e. martinson, r. c. arkin, noise maps for acoustically sensitive navigation, proceedings volume 5609, mobile robots xvii, 2004, pp. 50-60. doi: 10.1117/12.581461 [20] d. fontanelli, m. david, r. tizar, a fast and low-cost vision-based line tracking measurement system for robotic vehicles, acta imeko 4 (2015) 2, pp. 90-99. doi: 10.21014/acta_imeko.v4i2.245 https://doi.org/10.1016/j.apacoust.2012.02.007 http://dx.doi.org/10.4236/oja.2016.64006 https://dx.doi.org/10.3390%2fijerph15010149 https://doi.org/10.1109/eeeic.2019.8783688 https://doi.org/10.1117/12.581461 http://dx.doi.org/10.21014/acta_imeko.v4i2.245 journal contacts acta imeko issn: 2221-870x december 2021, volume 10, number 4 acta imeko | www.imeko.org december 2021 | volume 10 | number 4 journal contacts about the journal acta imeko is an e-journal reporting on the contributions on the state and progress of the science and technology of measurement. the articles are mainly based on presentations presented at imeko workshops, symposia and congresses. the journal is published by imeko, the international measurement confederation. the issn, the international identifier for serials, is 2221-870x. about imeko the international measurement confederation, imeko, is an international federation of actually 42 national member organisations individually concerned with the advancement of measurement technology. its fundamental objectives are the promotion of international interchange of scientific and technical information in the field of measurement, and the enhancement of international co-operation among scientists and engineers from research and industry. addresses principal contact prof. francesco lamonaca university of calabria department of computer science, modelling, electronic and system science via p. bucci, 41c, vi 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antonio moschitta ákos nagy hideaki nozato amman oglat dinko oletic vincenzo paciello imre paniti marco parvis nicola pasquino gabriele patrizi franco pavese francesco picariello enrico picariello giovanni pilato cristina piselli jeerasak pitakarnnop emanuele piuzzi antonino quattrocchi sergio rapuano luis miguel blanes restoy likit sainoo alexandru salceanu jan saliga constantin sarmasanu jurek sasiadek andrea scorza carmelo scuro federico seri enrico silva janko slavic han wook song pedro vieira souza santos roberta spallone ronald summers màrton szemenyei kostiantyn torokhtii roman trisch ioan tudosa moise avoci ugwiri marjan urekar alberto vallan zacharias vangelatos ian veldman valentina venuti zsolt viharos jian wang samyong woo bernhard zagar emanuele zappa cristian zet lulu zhang giulio zuccaro continuous measurement of stress levels in naturalistic settings using heart rate variability: an experience-sampling study driving a machine learning approach acta imeko issn: 2221-870x december 2021, volume 10, number 4, 239 248 acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 239 continuous measurement of stress levels in naturalistic settings using heart rate variability: an experience-sampling study driving a machine learning approach pietro cipresso1,2, silvia serino3, francesca borghesi1,2, gennaro tartarisco4, giuseppe riva2,3, giovanni pioggia4, andrea gaggioli2,3 1 department of psychology, university of turin, turin, italy 2 applied technology for neuro-psychology lab, istituto auxologico italiano, milan, italy 3 università cattolica del sacro cuore, milan, italy 4 institute for biomedical research and innovation (irib), national research council of italy (cnr), messina, italy section: research paper keywords: psychological stress; psychophysiology; psychometrics; signal processing; assessment; experience sampling methods; heart rate variability citation: pietro cipresso, silvia serino, francesca borghesi, gennaro tartarisco, giuseppe riva, giovanni pioggia, andrea gaggioli, continuous measurement of stress levels in naturalistic settings using heart rate variability: an experience-sampling study driving a machine learning approach, acta imeko, vol. 10, no. 4, article 36, december 2021, identifier: imeko-acta-10 (2021)-04-36 section editor: carlo carobbi, university of florence, gian marco revel, università politecnica delle marche and nicola giaquinto, politecnico di bari, italy received october 11, 2021; in final form december 20, 2021; published december 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was supported by the european funded project ‘interstress–interreality in the management and treatment of stress-related disorders’, grant number: fp7-247685. corresponding author: pietro cipresso, e-mail: p.cipresso@auxologico.it 1. introduction it is well known that long-term exposure to stress can lead to immunodepression and dysregulation of the immune response, thus significantly enhancing the risk of contracting a disease or altering its course. however, increased symptomatology is not only associated with severe stressors (infrequent major life events), but also with minor daily stressors (i.e. “microstressors”) that are ignored or poorly managed [1]–[4]. defining effective techniques to measure daily stressful episodes in ecological conditions has thus been identified as an important research objective. to address this challenge, several research groups have started investigating the use of wearable sensors solutions to infer stress from continuous biosignal measurements [5] (for a review, see [6]). such systems integrate sensors together with on-body signal conditioning and preelaboration, as well as the management of the energy consumption and wireless communication systems. although preliminary testing of these systems has yielded encouraging results [7], a major limitation of current solutions is that they mostly rely on complex sensor architectures and use labelling abstract developing automatic methods to measure psychological stress in everyday life has become an important research challenge. here, we describe the design and implementation of a personalized mobile system for the detection of psychological stress episodes based on heart-rate variability (hrv) indices. the system’s architecture consists of three main modules: a mobile acquisition module; an analysisdecision module; and a visualization-reporting module. once the stress level is calculated by the mobile system, the visualizationreporting module of the mobile application displays the current stress level of the user. we carried out an experience-sampling study, involving 15 participants, monitored longitudinally, for a total of 561 ecg analyzed, to select the hrv features which best correlate with self-reported stress levels. drawing on these results, a personalized classification system is able to automatically detect stress events from those hrv features, after a training phase in which the system learns from the subjective responses given by the user. finally, the performance of the classification task was evaluated on the empirical dataset using the leave one out cross-validation process. preliminary findings suggest that incorporating self-reported psychological data in the system’s knowledge base allows for a more accurate and personalized definition of the stress response measured by hrv indices. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 240 methods that are often based on evaluation of human coders [8]. other authors have proposed the measurement of hearth rate variability (hrv) [9]–[14] analysis as a potentially effective approach for monitoring stress in mobile settings [15]–[17]. actually, hrv indices can be used to estimate activity of autonomous nervous system (ans) in relation to affective and cognitive states, including mental stress [18]–[20]. however, realtime recognition of stress from hrv measures requires appropriate strategies to i) detect hrv changes using minimallyinvasive ecg equipment; ii) relate these changes to mental stress levels; and iii) control the potential confounding effects of physical activity. in the following, we describe how we addressed these issues in designing and implementing a personalized mobile system for automatic recognition of psychological stress based on hrv indices. the original contribution of the proposed method is that, to our best knowledge, this is the first approach that integrates the detection of hrv features with the groundtruth of subjective perception of stressful events. 2. measuring psychological stress in naturalistic environments according to cohen et al. [21], stress is a biopsychosocial phenomenon in which “environmental demands tax or exceed the adaptive capacity of an organism, resulting in psychological and biological changes that may place a person at risk for disease” (p. 3). this conceptualization suggests that in measuring stress, it is not only necessary to consider environmental demands, but also appraisals of such demands, as well as physiological systems that come into play. consistent with this definition, two main approaches have been introduced to assess psychological stress in naturalistic conditions: the first is based on self-reporting of participants’ subjective experiences and perception of stressful events; the second approach is based on sensing physiological signals associated with the stress response. in the next, we provide a description of these procedures, along with a discussion of their strengths and limitations. 2.1 subjective psychological measures the experience sampling method (esm), also known as ecological momentary assessment (ema), is a naturalistic observation technique that allows capturing participants’ thoughts, feelings, and behaviours at multiple times across a range of situations as they occur in the natural environment [22]. in a typical esm study, participants are asked to fill out a form when prompted by an acoustic signal. thanks to repeated sampling, a number of surveys are collected from each participant throughout the day, thus providing an ecologicallyvalid and highly detailed description of subjective quality of experience. ecological validity is a strong requirement in psychometrics since this is a measure of how a task or test is able to predict behaviours in a real-world setting. esm has been applied to study a wide range of behaviours and experiences, including daily stress [23], [24]. however, this procedure has high costs and places a significant burden on the participant, thus limiting its practical applicability as a stress monitoring technique [10], [25]. a less expensive and time-consuming approach for assessing experience and affect in everyday life is the day reconstruction method (drm), developed by kahneman and colleagues [26]. it involves the retrospective recall of the study period as a continuous sequence of episodes, which are rated on a series of affect scales. drm reports have been validated against experience sampling data, showing that this technique allows identifying changes in affect over the course of the day with almost the same accuracy than esm [26], [27]. however, since drm respondents are asked to reconstruct the previous day by completing a structured self-administered questionnaire, this method is potentially susceptible to recall biases. furthermore, it has been suggested that using retrospective measures as proxies for actual experience may result in weaker or inconsistent results particularly when tested in connection to biologic pathways [28]. 2.2 objective physiological measures an alternative strategy to assess stress in everyday situations is based on the analysis of physiological correlates of this experience. psychological stressors are linked with the activation of two main neuro-physiological pathways, which are involved in the maintenance of homeostasis: the hypothalamic-pituitaryadrenocortical (hpa) axis and the sympathetic-adrenal medullary system (sam). as concerns the first system, one of the most investigated biomarker is salivary cortisol, which, together with catecholamines, is one of the end products of hpa activation [20], [23], [29], [30]. however, due to significant betweenand within individual variation in diurnal secretion of cortisol, the measurement of the magnitude of cortisol response is not an easy procedure, which requires the application of advanced statistical approaches such as multilevel models [31], [32]. with respect to the sam system, hrv, defined as the variation over time of the period between consecutive heartbeats, is increasingly regarded as a potentially convenient and noninvasive marker of autonomic activation associated with psychological stressors [33]. the normal variability in heart rate (hr) is controlled by the balancing activation of the (acceleratory) sympathetic and of the (deceleratory) parasympathetic branches of the autonomic nervous system. however, under stressful events or contexts, there is a trend towards increased sympathetic control and reduced vagal tone, which is associated with decreased hrv [19]. on the other hand, higher hrv has been associated with the availability of context and goal-based control of emotions [34]. based on this preliminary evidence, several authors have been experimenting with wearable heart monitor for the identification of stress levels from hrv, in both healthy and clinical populations. for example, kim and coll. [35]used hrv patterns to discriminate between subjects reporting high and low levels of stress during the day, with an overall accuracy of 66.1 %. in a similar study, melillo et al. [16] compared within-subject variations of shortterm hrv measures using short term ecg recording in students undergoing university examination. by applying linear discriminant analysis on nonlinear features of hrv for automatic stress detection, these authors were able to obtain a total classification accuracy of 90 %. kimhy et al. [17] investigated the relationship between stress and cardiac autonomic regulation in a sample of psychotic patients, using experience sampling in combination with cardiac monitoring. they found that momentary increases of stress were significantly associated with increase in sympathovagal balance and parasympathetic withdrawal. in addition to studies which have examined the association between hrv and stress during waking hours, other recent research has proposed the use of hrv patterns during sleep as supplement to the analysis of subjective assessments and voice messages collected during workday [36], with encouraging, albeit preliminary, results. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 241 2.3 towards an integrated approach for personalized stress recognition in mobile settings as previously discussed, a fundamental issue in the measurement of stress in everyday life is that this response is idiosyncratic, because it depends on individual’s perception of challenges and the skills which he/she can use to face those challenges. as a consequence, any approach aiming at inferring stress levels from “honest” physiological signals should not overlook the role played by the subjective appraisal of the situation. furthermore, since hrv values are characterized by high interindividual variability, it is important that the system is tailored to the individual characteristics [37], [38]. one possible approach to develop adaptative systems for stress recognition has been suggested by morris and guilak [37]. the strategy proposed by these authors involves identifying subject’s baseline and stress threshold in the lab by elicitation of sympathetic and parasympathetic responses, and then using this information to differentiate between stress and nonstress in daily life. a first attempt to implement this approach has been performed by cinaz et al. [38]. these authors measured participants’ sympathetic and parasympathetic responses during three different levels of mental workload (low, medium, and high) in a controlled laboratory setting. then, they investigated whether the data collected in this calibration session were appropriate to discriminate corresponding workload levels occurred during office work. to this end, individual hrv responses of each workload level were used to train the models and test the trained models on the data collected while the subjects performed normal office-work, using a mobile ecg logger. afterward, a multiple regression analysis was applied to model the relationship between relevant hrv features and the subjective ratings of perceived workload: resulting predictions were correct for six out of the seven subjects. in the present work, we propose an experience-sampling approach for incorporating subjective knowledge in the classification of psychological stress from hrv indices (table 1). the methodology consists of three main steps. in the first, experimental phase, we carried out an experiencesampling study to select the hrv features which best correlate with self-reported stress levels (section 3). drawing on these results, a personalized classification system was developed which is able to automatically detect stress events from those hrv features, after a training phase in which the system learns from the subjective responses given by the user (section 4). in the final step, the performance of the classification task was evaluated using the leave one out cross validation process (section 5). 3. method the objectives of this experiment were two-fold: i) to select a subset of hrv features which best correlate with self-reported stress levels collected during everyday activities; ii) to select a subset of self-reported questions about perceived stress levels which can be used as ground truth to train the final system. 3.1 participants participants were 15 healthy subjects (8 males and 7 females, mean age = 23.33 years, st. dev.= 1.49), monitored longitudinally, for a total of 561 ecg analyzed, to select the hrv features which best correlate with self-reported stress levels. participants were recruited through opportunistic sampling. participants filled a questionnaire assessing factors that, in the opinion of the investigators, might interfere with the measures being assessed (i.e., caffeine consumption, smoking, alcohol consumption, exercise, hours of sleep, disease states, and medications). written informed consent was obtained by all subjects matching inclusion criteria (age between 18 and 65 years, generally healthy, absence of major medical conditions, and completion of informed consent). 3.2 materials data were collected through psychlog [39], a mobile experience sampling platform designed for research in mental health, which allows simultaneous collection of psychological, physiological (ecg) and motion activity data. psychological data are collected from surveys that can be simply customized by the experimenter. for the purpose of this study, we used the italian adaptation of the esm questionnaire applied by jacobs et al. [40] for studying the immediate effects of stressors on mood. the survey includes open-ended and closed-ended questions investigating thoughts, current context (activity, persons present, and location), appraisals of the current situation, and mood. all self-assessments were rated on 7-point likert scales. hr and activity data are acquired from a wireless electrocardiogram (shimmer research™) equipped with a three-axial accelerometer. the wearable sensor platform includes a board that allows the transduction, amplification and pre-processing of raw sensor signals, and a bluetooth transmitter to wirelessly send the processed data. the unit is mounted on a soft-textile chest strap designed to seamlessly adapt to the user's body shape, bringing full freedom of movement. sensed data are transmitted to the mobile phone bluetooth receiver and gathered by the psychlog computing module, which stores and process the signals for the extraction of relevant features. 3.3 design and procedure participants received a short briefing about the objective of the experiment and filled the informed consent. then, they were provided with the mobile phone with pre-installed psychlog application, the wearable ecg and accelerometer sensor and a user manual including experimental instructions. the application was pre-programmed to collect data over 7 consecutive days, at random intervals during waking hours. at the end of the experiment, participants returned both the phone and the sensors to the laboratory staff. after, participants were debriefed, thanked for their participation, and dismissed (figure 1). 3.4 data analysis following the procedure suggested by jacobs et al. [40], three different psychological stress measures were computed in order to identify the stressful qualities of daily life experiences. table 1. feature extraction from electrocardiogram (ecg). measure description rr mean mean of all rr intervals avnn average of all nn intervals sdnn standard deviation of all nn intervals rmssd square root of the mean of the squares of differences between adjacent nn intervals nn50 differences between adjacent nn intervals that are greater than 50 ms totpwr total spectral power of all nn intervals up to 0.04 hz lf lf total spectral power of all nn intervals between 0.04 hz and 0.15 hz hf hf total spectral power of all nn intervals between 0.15 hz and 0.4 hz lfbyhf lf/hf ratio of low to high frequency power (sympathovagal balance) acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 242 ongoing “activity-related stress” (ars) was defined as the mean score of the two items ‘‘i would rather be doing something else’’ and ‘‘this activity requires effort’’ (cronbach’s alpha = 0.72). to evaluate social stress, participants rated the social context on two 7-point likert scales ‘‘i don’t like the present company’’ and ‘‘i would rather be alone’’; the “social stress scale” (ss) resulted from the mean of these ratings (cronbach’s alpha = 0.59). finally, for “event-related stress” (evs), subjects reported the most important event that had happened since the previous beep, whether or not it was still ongoing. subjects then rated this event on a 7-point bipolar scale (from 3 very unpleasant to 3 very pleasant, with 0 indicating a neutral event). all positive responses were recorded as 0, and the negative responses were recorded so that higher scores were associated with more unpleasant and potentially stressful events (0 neutral, 3 very unpleasant). in addition to those scales, an item (not included in the original survey by jacobs et al. [40]) asked participants to rate the perceived level of stress on a 10-point likert scale. this item was included as a global subjective measure of stress. given the repeated sampling, likert-type scales data were standardized (mean = 0; st. dev. = 1) on each participant’s weekly mean for every variable before performing the analyses. esm data can be aggregated at the report level (the unit of analysis is the individual diary entry) or at the subject level (the unit of analysis is the participant). in the present study, most of the analyses were conducted using the subject-level aggregation, because this approach avoids problems related to unequal weights and produces more conservative significance tests [41]. 3.5 selection of psycho-physiological features to analyse hrv features, the qrs peaks and rr interval time series recorded and saved on the psychlog application were exported and further processed with the software matlab (version 7.10) in order to compute a set of hrv indexes. to this end, the ecg signal was first elaborated for artifact correction, and then a fast fourier transform was used to compute the power spectrum in the lf (0.04–0.15 hz) and hf (0.15–0.40 hz) bands [42]–[45]. to estimate the effect of hrv indexes (independent variable) on stress level (dependent variable) we applied hierarchical linear analysis, an alternative to multiple regression which is more suitable for our nested data. actually, hierarchical structure of data makes traditional forms of analysis unsuitable, since within-subject data are collected at many points in time during each day, across several days. moreover, traditional repeated-measures designs require the same number of observations for each subject and no missing data. finally, hierarchical linear analysis allows to take into account further dependencies existing in the data. 4. results given the repeated sampling, likert-type scales data were standardized (mean = 0; sd = 1) on each participant’s weekly mean for every variable before performing the analyses. esm data can be aggregated at the report level (the unit of analysis is the individual diary entry) or at the subject level (the unit of analysis is the participant). in the present study, most of the analyses were conducted using the subject-level aggregation, because this approach avoids problems related to unequal weights and produces more conservative significance tests [31]. the following table provides the correlations between stress measures described before. as can be seen from table 1, all scales measuring stress (stress, ars, ss, and ers) are significantly correlated between them. out of 561 “beeps”, participants filled 541 reports (96 %), of which 456 were included in the analysis (84 %). a total of 561 ecg sampling were recorded (100 %), and 374 were included in the analysis (69 %). the following table 2 provides the correlations between stress measures described before. in fact, as can be seen from table 2, all scales measuring stress (stress, ars, ss, and ers) are significantly correlated between them. the hierarchical linear analysis, both aggregation levels (report-level and subject-level) were considered in the model. results indicated a statistical significant hierarchical regression model for rmssd (beta .5350813; st. dev.: .2151596; p < .013), nn50 (beta -1.152351; st. dev.: .5322348; p < .030), lf / hf (beta 1.176422; st. dev.: .5386275; p < .029) (table 3). the rmssd method is preferred to nn50 because it has better statistical properties [42], [43]. findings of this esm experiment allowed to identify a subset of hrv features, which showed best correlations with self-reported psychological stress levels. in the next section, we describe how these psycho-physiological features were implemented in a figure 1. schematic representation of experimental design. table 2. correlations among psychological self-reported measures. zstress zars zss ers zstress r 1 ,312** ,215** ,213** sig. < .001 < .001 < .001 n 540 534 528 456 zars r ,312** 1 ,393** ,146** sig. < .001 < .001 .002 n 534 535 529 457 zss r ,215** ,393** 1 ,188** sig. < .001 < .001 < .001 n 528 529 529 457 ** correlation is significant at the 0.01 level (2-tailed). table 3. summary of hierarchical regression analysis for hrv variables predicting global perceived stress (number of observations = 374). global perceived stress b se b z p >|z| 95 % ci hr 0.51 0.22 2.37 0.02 0.09 0.94 rmssd -0.53 0.21 -2.49 0.01 -0.96 -0.11 nn50 -1.15 0.53 -2.17 0.03 -2.20 -0.11 lf 0.62 .031 2.02 0.04 0.02 1.23 lf/hf 1.18 0.54 2.18 0.03 0.12 2.23 acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 243 personalized stress monitoring system, which was designed to learn from individual’s subjective assessments of stressful situations and use this knowledge to detect stress events. the personalized stress monitoring system includes three main components: a) a mobile acquisition/feedback module (for the collection of psycho-physiological and activity data); b) a remote analysis-decision module (for the analysis and classification of stress levels), c) a mobile visualization-reporting module (for the reporting of detected stress events). 4.1 mobile acquisition/feedback module the mobile acquisition/feedback component consists of two elements: a wireless electronic module coupled with a commercial chest band for collecting ecg and motion data; and a smartphone application for the collection of psychological data, the transmission of data to the analysis-decision module and the visualization of stress events detected by the system. 4.1.1 ecg and motion activity data acquisition the electronic acquisition platform, produced by shimmer research™ allows the transduction, amplification and preprocessing of raw ecg signals, and the transmission of elaborated data via bluetooth to the smartphone. the unit is mounted on a soft textile chest strap (model micoach™ by adidas) designed to seamlessly adapt to the user’s body shape, bringing full freedom of movement. a smartphone application, running on android operative system, was developed for preliminary elaboration of sensor data and remote data base (db) archiving. the application processes the ecg and accelerometer signals for the extraction of three relevant parameters: hr, activity index and rr intervals for further hrv analysis provided by the analysis-decision module. the wearable electronic board collects raw sensor data with on-body signal conditioning. the ecg signal is sampled at 256 hz and sent to the smartphone with the tri-axial acceleration data (ax, ay, az). the smartphone application pre-processes user’s physiological signal through a stepwise filtering stage aimed at removing typical ecg artifacts and interferences. in particular, baseline wander due to body movements and respiration artifacts is removed using a cubic spline 3rd order interpolation between the fiducial isoelectric points of the ecg [46]. the power line interference and muscular noise are removed using an infinite impulse response (iir) notch filter at 50 hz and an iir low pass filter at 40 hz. then, the pan-tompkins method is applied [47] to detect the qrs complex and to extract hr and the time series sequence of non-uniform r–r intervals. since variation of the ecg parameters is significantly affected by the activity performed by the user [48], [49], signal magnitude area [50] is also extracted from the three-axis accelerometer signals in order to measure motion activity levels. 4.1.2 psychological data acquisition the acquisition of psychological data is managed by an electronic survey, which is displayed at random times during the day on the application’s screen. the survey includes a subset of likert-type items measured selected from the esm by jacobs et al. [41] described in section 4.1.2. only the esm items which highest correlation with hrv features were included in the final survey: this choice was made in order to reduce as much as possible the burden on the user during the training phase of the system. the final selected items were (listed in the same sequence of the final survey): 1. what is your stress level? (min: 1; max: 10) 2. this activity is a challenge (min: 1; max: 7) 3. this is something i'm good at (min: 1; max: 7) 4. i would rather be doing something else (min: 1; max: 7) 5. it takes me effort (min: 1; max: 7) the average time for the completion of a full questionnaire is about 10-15 seconds. during a typical training week, 4-5 surveys per day are collected. 4.2 analysis-decision module (adm) the adm developed is composed of two main modules: the feature extraction and the classification module. 4.2.1 data exchange the wearable sensors monitor patients and transfer data to web-servers, using a smartphone to collect and pre-elaborate data. in addition, the application allows users to track their own stress levels through a graphical representation (see section 5.3). the chest band and its electronic act as masters that initiate bluetooth communication with the android phone. the bluetooth protocol has a range of approximately 20 m and provides secure data transmissions. the communication between the phone and central db is through wi-fi or 3g networks. the dss makes use of information transmitted by the smartphone and stored within the central db. for each subject is created an user profile able to maintains all history data. in particular the remote db storage physiological data (hr, rr intervals and activity index) and the corresponding stress value extracted with physiological surveys. given the userid, the timestamp, and the session, the adm retrieve from db the physiological data together with the questionnaires filled by the user about the stress level perceived. from these data features are extracted and used to train the classification module (training phase). after training, the adm acts as an expert system and provide to the corresponding user the stresslevel automatically extracted after training of the classification module (testing phase). the adm acts asynchronously in respect to the sensor data collection process. at fixed time intervals, the new sensor data belonging to each subject are collected and a feature extraction process takes place in order to create a structured dataset. classification module is trained and validated with most relevant features for automatic stress assessment. 4.2.2 feature extraction once the data are sent to the remote server, concerning the rr intervals collected over time, the parts of the signals with artifacts are discarded and hrv features are extracted according to the traditional approach proposed by the international guidelines of hrv [42], [43] to estimate cardiac vagal and sympathetic activities as markers of the autonomic interaction using a data exchange module (figure 2). in time domain were figure 2. data exchange module. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 244 extracted statistical indices from rr time series such as mean (mrr), standard deviation (σrr), root mean square of successive differences of intervals (rmssd), difference between the longest and shortest rr intervals, and the number of successive differences of intervals which differ by more than 50 ms (pnn50% expressed as a percentage of the total number of heartbeats analysed), while in frequency domain are extracted parameters for each frequency band, low frequency (lf: 0.030.15 hz) and high frequency (hf: 0.15-0.40 hz), included absolute powers, peak frequencies (max lf and max hf) and the lf/hf power ratio that measure the global sympatheticparasympathetic equilibrium. in particular, above the threshold, the curve reveals sympathetic dominancy, below the threshold, the parasympathetic influence is dominant. these features are extracted using an estimation of the power spectral density (psd) analysis according to the burg spectral estimation [51], where the optimal order p was estimated according to the akaike information criterion [52]. the power of each band is normalized in respect to the total power of the spectrum. also a nonlinear parameter was extracted, i.e. the poincaré plot a useful tool to investigate and combine the differences of the cardiac rhythms during the performed tasks. it is a graphical representation created by plotting all rr(n) on the x-axis versus rr(n+1) on the y-axis. then, the data are fitted using an ellipse projected according the line of identity and extracting the two standard deviations (sd) respectively [53] as shown in figure 3. 4.2.3 classification module the classification module is based on machine learning (ml) models, such as artificial neural networks (ann), based on inductive inference [54]. we decided to use ml model to cope with the non-linear and complex relations between the monitored parameters and the stress level prediction (table 4). artificial neural networks (anns) are particularly suited for solving such problems. they are biologically inspired computational models, which consist of a network composed of artificial neurons. for the implementation of ml for stress level detection a number of steps are needed: • initialization of parameters of implemented ml model. • training: the model is trained with the features extracted and selected to adapt itself to classify the given inputs. the loaded features, along with self-reported stress levels, generate the training set. the self-reported stress levels are collected during the training phase, in which the participant is prompted at random times during the day with a survey including five items (see section 5.1.2) that allows the user to self-evaluate, on a likert scale, perceived levels of stress, following a protocol described in section 4.1.3. by matching this psychological “ground truth” with sensor data, synaptic weights of networks internal connections are modified in order to force the output to minimize the error with the presented example (in this case the stress level obtained from survey). in this step the architecture of the model and its hyper-parameters are optimized. the examples labelled with the stress-level are used to create a personalized stress prediction model. • validation: the model adequately trained, is able to classify the given input in order to present a consequent output value: the value obtained, is the inferred stress level. it is validated in order to guarantee good predictive properties. during the analysis decision module fine-tuning design, we decided to develop a self-organizing maps (som) integrated with fuzzy rules. the som is a network structure which provides a topological mapping [55]-[57]. the main difference with the artificial neural network is that it is based on unsupervised learning. it is composed of two-dimensional layer in which all the inputs are connected to each node in the network (figure 4). a topographic map is autonomously organized by a cyclic process of comparing input patterns to vectors at each node. the node vector to which inputs match is selectively optimized to present an average of the training data. then all the training data are represented by the node vectors of the map. starting with a randomly organized set of nodes, and proceeding to the creation of a feature map representing the prototypes of the input patterns, the training procedure is as follows: 1. initialization of the weights wij(1 ≤ i ≥ nf, 1 ≤ j ≥ m) to small random values, where nf is the total number of selected features (input) and m is the total number of nodes in the map. set the initial radius of the neighbourhood around node j as nj(t). 2. present the inputs x1(t), x2(t) . . . . . xnf(t), where xi(t) is the ith input to node j at time t. 3. calculate the distance dj between the inputs and node j by the euclidean distance to determine j* which minimizes dj: 𝑑j = ||𝑊j(𝑡) − 𝑋(𝑡)|| (1) table 4. features extracted and analysed from the signal. no. features extracted measure of signal 1 mrr mean rr interval rr 2 σrr standard deviation rr interval rr 3 rmssd root mean square of successive differences of intervals rr 4 pnn50% number of successive differences of intervals which differ by more than 50 ms rr 5 lf spectral estimation of low frequency power (0.03-0.15 hz) rr 6 max lf max value of low frequency value rr 7 hf spectral estimation of high frequency power ( hf: 0.15-0.40 hz ) rr 8 min hf max value of high frequency value rr 9 lf/hf spectral estimation of power ratio rr 10 sd1, sd2 standard deviations of poincarè plot rr 11 sma signal magnitude area acc. x,y,z figure 3. sd1 and sd2 of poincarè plot observed for a portion of rr interval analysed. 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9 ibi n (s) plot poicaré ib i n + 1 ( s ) sd1=30.3 sd2=71 acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 245 every node is examined to calculate which one's weights are most like the input vector. the winning node is commonly known as the best matching unit (bmu). the radius of the neighborhood of the bmu is then calculated. this is a value that starts large, but diminishes each timestep. any nodes found within this radius are deemed to be inside the bmu's neighborhood. 4. update the weights wij of the winning neuron j* and of its neighborhood neurons nj*(t) at the time t, for the input vector x, are modified according to the following equation (2) to make them more like the input vector: 𝑤ij (𝑡) = 𝑤ij(𝑡 − 1) + 𝛼(𝑡)[𝑋(𝑡) − 𝑤ij(𝑡 − 1)] , (2) where α(t) is the learning rate. both α(t) and nj*(t) are controlled so as to decrease in t. if the process reaches the maximum number of iterations, stop; otherwise, go to (2). at the end of the training process, for each input variable xi we generated the fuzzy membership function using triangular functions with the centre in the corresponding weight wij of the map and the corresponding variance vij, where i is the ith input and j represents the jth node of the map. the centers of the triangular membership functions in the ith input are (wi1 wi2 ..... wim). the corresponding regions were set to [wi1-2vi1, wi1+2vi1], [wi2-2vi2, wi2+2vi2],…, [wim-2vim, wim+2vim], where m is the last node of the map. we developed membership functions and fuzzy rules for each hrv parameter, including heart rate and motion activity.in order to reduce the number of fuzzy rules and to improve the system reliability, narrowly separated regions were combined to become a single region. let the positions of the four corners of region j be llj, lhj, rhj and rlj (for a triangular membership function, lhj = rhj). two neighboring regions j-1 and j were merged if they satisfied the following equation (3): 𝑙ℎj + 𝑟ℎj 2 − 𝑙ℎj−1 + 𝑟ℎj−1 2 ≤ 𝑡ℎ𝑟 , (3) where thr is pre-specified threshold (set to 0.1 in our experiments). this process continued until all regions were well separated in terms of the threshold. accordingly, some fuzzy regions had trapezoidal shapes instead of triangular ones as is shown in figure 5. after that, we generated fuzzy rules as a set of associations of the form “if antecedent conditions hold, then consequent conditions hold”. each feature was normalized to the range of [0.0,1.0] and each region of fuzzy membership function was labeled as r1, r2,…rn. an input was assigned to the label of a region where the maximum membership value was obtained. in particular we adopted the method proposed by wang et al. [56] where each training sample produced a fuzzy rule. an example of rule generated is: if feature1 is r1 and feature2 is rn and feature3 is r2 and feature4 is r3 and feature5 is r6, and feature6 is r8 …. and feature m is r3then it is medium stress level. finally, the number of all the fuzzy rules was the same order of the training samples. the problem was that a large number of training patterns may lead to repeated or conflicting rules. to deal with this problem, we recorded the number of rules repeated during the learning process. those rules supported by a large number of examples were saved. a centroid defuzzification formula was used to determine the output for each input pattern: 𝑍 = ∑ 𝐷p i 𝑂iki=1 ∑ 𝐷p ik i=1 , (4) where z is the output, k is the number of rules, oi is the class generated by rule i and dip measures how the input vector fit the ith rule. dip is given by the product of degrees of the pattern in the regions which the ith rule occupies. the output is within [0,5] for numeral recognition of stress level (0=unknown, 1=low stress, 2=mild stress, 3=elevated stress, 4=high stress, 5=severe stress). the output z was adapted taking the nearest smaller integer value. fuzzy rules do not necessarily occupy all fuzzy regions in input space. there could be some regions where no related rule exists. this is the case when the denominator in equation (4) is zero. we label the corresponding input stress level as unknown. after training the model was designed to be able to discriminate until 6 different classes during test phase with the trained fuzzy classifier. for each user, the dss is trained on the basis of the evaluation of the stress level supplied by the surveys, being such data collected by the mobile application and transmitted to the database. as the dss training is completed, the dss acts as an expert system, supplying the stress level information for each patient as new sensor data becomes available within the database (figure 6). figure 4. generation of the fuzzy membership function for the ith input. the number of triangular functions is the equal to the som nodes. figure 5. trapezoidal function obtained for neighboring regions. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 246 4.3 mobile visualization module after data collection, the mobile application can poll the stress level report from the remote database provided by the analysisdecision module (see section 5.2) at definite time intervals, or at user’s request. the stresstracker visualization component of the mobile application provide the user with a graphic visualization of measured stress events. this information is visualized by the stresstracker (figure 7, right) which shows the number of detected stressful events over the course of last day, week, or month respectively. 5. cross validation in order to check the stress detection capability of the personalized model, the performance of the classification task was evaluated using the leave one out cross validation process (loocv), where each fold consists of one session of data acquisition left-out. this method is an iterative process in which one session is recruited into the dataset each time for validation. the som classifier combined with fuzzy rules was trained using the remaining data and validated on the single, left-out validation point. this ensures that the validation is unbiased, because the classifier does not see the validation input sample during its training. one by one, each available session for each subject was recruited for validation. the performances of the system were assessed by using the confusion matrix, in which the generic element i,j indicates how many times (in mean percentage ± sd) a pattern belonging to the class i was classified as belonging to the class j. 5.1 cross validation results the system is able to recognize the presence of stress in the selected population. in particular, the results obtained with three subjects are reported in table 5. the mean ± sd percentages of the confusion matrices obtained with the first subject are reported in table 5a. the model correctly identifies the presence of stress with percentage of correct classifications of 89.0 % and the absence of stress with percentage of correct classifications of 86.6 %. the mean ± sd percentages of the confusion matrices obtained with the second subject are reported in the table 5b. the model correctly identifies the presence of stress with percentage of correct classifications of 66.6 % and the absence of stress with percentage of correct classifications of 70.0 %. the mean ± sd percentages of the confusion matrices obtained with the third subject are reported in table 5c. the model correctly identifies the presence of stress with percentage of correct classifications of 95.7 % and the absence of stress with percentage of correct classifications of 73.0 %. these results demonstrate the high discriminatory power of the system. 6. conclusion we described the design, key functional features and preliminary validation of a personalized system for monitoring stress in naturalistic environments. the original contribution of this work concerns the development of a new methodology, which allows to use subjective evaluation of potentially stressful situations in the calibration and training of the classification system. in particular, incorporating self-reported psychological data in the system knowledge base allows for a more comprehensive and personalized definition of the stress response as measured by hrv indices. an objective of future research is to validate the accuracy of the personalized stress detection model against other physiological markers, i.e. salivary cortisol collected during daily life activities. acknowledgement this work was supported by the european funded project “interstress–interreality in the management and treatment of stress-related disorders”, grant number: fp7-247685. the co-authors would like to thank giovanni pioggia and andrea gaggioli who have equally contributed to the manuscript. figure 6. architecture of the automatic stress classification module. table 5. confusion matrixes. a. the confusion matrix obtained with the first subject: stress no stress stress 89 ± 6.2 11 ± 2.1 no stress 13.3 ± 3.6 86.6 ± 5 b. the confusion matrix obtained with the second subject: stress no stress stress 66.6 ± 11 33.3 ± 6.7 no stress 30 ± 3.1 70 ± 5.9 c. the confusion matrix obtained with the third subject: stress no stress stress 95 ± 3.3 5 ± 4.6 no stress 26 ± 3.1 73 ± 8.2 figure 7. on the left side is reported the current stress level of the user, while on the right side are reported the stressful events detected over the last week. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 247 references [1] y. yang, daily stressor, daily resilience, and daily somatization: the role of trait aggression, personality and individual differences, vol. 165, 2020, 110141. doi: 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https://doi.org/10.2196/jmir.1371 https://doi.org/10.1007/s00779-011-0466-1 https://doi.org/10.1007/s00779-011-0465-2 https://doi.org/10.1016/j.biopsycho.2006.07.002 https://doi.org/10.1017/cbo9780511663246.007 https://doi.org/10.1093/oxfordjournals.eurheartj.a014868 https://doi.org/10.1161/01.cir.93.5.1043 https://doi.org/10.1109/iembs.2010.5627465 https://doi.org/10.1109/cic.1992.269426 https://doi.org/10.1109/tbme.1985.325532 https://doi.org/10.1111/j.1469-8986.2005.00363.x https://doi.org/10.1109/10.554760 https://doi.org/10.1007/978-3-030-26050-7_197-1 https://doi.org/10.1007/bf02532251 https://doi.org/10.1109/10.959330 https://doi.org/10.2307/1271324 https://doi.org/10.1016/s0925-2312(98)00030-7 https://doi.org/10.1109/21.199466 https://doi.org/10.1093/iwc/iwv010 digital transformation: towards process automation in a cloud native architecture acta imeko issn: 2221-870x march 2023, volume 12, number 1, 1 6 acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 1 digital transformation: towards process automation in a cloud native architecture alexander oppermann1, samuel eickelberg1, manuel meiborg1 1 physikalisch-technische bundesanstalt, abbestr. 2-12, 10587 berlin, germany section: research paper keywords: digital transformation; universal service hub; metrological processes; digital calibration certificate; dcc citation: alexander oppermann, samuel eickelberg, manuel meiborg, digital transformation: towards process automation in a cloud native architecture, acta imeko, vol. 12, no. 1, article 17, march 2023, identifier: imeko-acta-12 (2023)-01-17 section editor: daniel hutzschenreuter, ptb, germany received november 18, 2022; in final form march 19, 2023; published march 2023 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: alexander oppermann, e-mail: alexander.oppermann@ptb.de 1. introduction digital transformation in public administration is taking place at a different pace and often falls short of the expectations and requirements associated, for example, with the online access act (ozg) in germany. while the current pandemic situation has accelerated digital transformation measures for mobile work in general and in public administration, it has also highlighted the existing weaknesses. there is an obvious need to support the design and use of digitally supported work methods and appropriately designed process flows. to this end, existing obstacles must be systematically identified and, ideally, eliminated or dissolved by digital technologies and methods. among other things, these relate to the legal security of digitally transformed processes, billing procedures and models, as well as associated financial and organizational structures, personnel capacities, competencies, and qualifications. in addition, there is now an opportunity to rethink and redesign process workflows to fully exploit the potential of digital transformation simply transferring existing workflows into the digital domain is not enough. at the same time, digital quality infrastructures must be established internally and externally, to link previous data silos 1 web portal for digital services and to lead to new synergy and efficiency gains (bmwi qualitätsinfrastruktur digital [1]). in addition, suitable concepts are required for designing the transition from analogue to digital processes. currently, e-services1 and e-file2 are being introduced at ptb, but there is a major obstacle in connecting and digitally transforming the working groups, laboratories, and their workflows. often, the hurdles for the individual work groups are very high and the upcoming change management processes are overwhelming. this gap is to be closed by the operation layer (op-layer), which uses uniform interfaces to connect to the already existing infrastructures at ptb and enables simple data transfer from eservices and e-file. through uniform interfaces using rest (representational state transfer), the connected internal systems can maintain their previous workflow, while at the same time harmonizing the data. the automatic data transfer drastically increases the confidentiality, integrity and availability of the data and greatly reduces the susceptibility to errors. the op-layer thus makes an enormous contribution to the automation of workflows and can reduce the workload of employees. automation frees up staff time for research, laborious calibrations, and related tasks. 2 document management system for electronic records abstract introducing new information systems in organizations often result in information sinks that disrupt people's productivity and prevent a successful change management process. in this paper, the operation layer is presented, a cloud native concept to break up data silos, to streamline workflows and to centralize it services while maintaining the department's workflows. the feasibility is demonstrated by a complete digital calibration certificate workflow implementation as a service within the operation layer. pursuing this concept consequently will simplify the it maintenance while flattening the change management curve at the same time. mailto:alexander.oppermann@ptb.de acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 2 the rest of the paper is structured as follows, section 2 describes the concept and infrastructure of the op-layer. section 3 outlines the use cases, their implications, as well as interoperability scenarios resulting from them. a summary and outlook are given in section 4. 2. concept & infrastructure in the last years, a distributed microservice architecture has been built and designed explicitly in the domain of legal metrology with a focus on interconnecting external stakeholders [2], [3], [4], [5]. while the generic it architectural design approach has proven to be successful, a similar approach will be pursued to digitally transform the internal process flow. this time, the focus of the it architecture will be extended to be cross-domain and linking internal stakeholders. further targets are the stakeholders’ isolated workflows as well as internal and external it infrastructure. the foundation of the it infrastructure will be more sophisticated in terms of scalability, availability, portability, and security (see section 2.1). furthermore, the infrastructure as code (iac) paradigm will be put at the heart of the op-layer. figure 1 gives an overview of the digitally transformed workflow [6] with the e-service web portal as a contact point for a range of services. once the order has been placed, an electronic file will be created in the e-file service. the e-file service stores all information centrally. the necessary administrative data is maintained independently by the customer while the required metrological data is stored as machine-readable document [7]. this is where the op-layer comes into place and links seamlessly the laboratories’ workflows and internal departments’ use-cases (see section 3) with the e-file service via an api. moreover, the op-layer will feature a microservice architecture and host specific tailored applications and services that will harmonize the service infrastructure within ptb. while centralizing the service infrastructure, the individual and adapted workflows of each department will be protected. this will reduce the maintenance costs for it-services and increases efficiency of digitally supported services. in the preparation phase of the op-layer project it became obvious that companies and research institutes face the same challenges while digitally transforming their processes and infrastructures. the following list describes common challenges of organizational transformation that have to be comprised, 3 https://kubernetes.io/docs/reference/command-line-toolsreference/kubelet/ addressed and reflected in the architectural concept of the oplayer: • isolated (research / it) infrastructures within organization • no centralized or harmonized workflow • no streamlined process chain across departments • interfaces are programmed several times within one organization • no it management on executive level • it management without qualified technical experience • no it security awareness and organizational structure • area of conflict between centralization and decentralization within organization • isolated responsibilities and knowledge in each organizational unit • lack of transparency of process flows and chains 2.1. distributed cloud native infrastructure a distributed it architecture (see figure 2) that is able to support a microservice approach has to map the requirements such as high availability, (auto-) scaling, portability and security. the kubernetes4 design principles fulfil most of the mentioned requirements for a modern distributed infrastructure approach. the following paragraphs give a short overview of the requirements: scalability avoiding technical bottlenecks in a distributed infrastructure is vital because those can lead quickly to points of failure. being able to scale applications horizontally, that means starting the same application several times, will redistribute the 4 https://kubernetes.io/ figure 1. overview of the workflow with the op-layer at heart. the op-layer prepares a scalable cloud native infrastructure for special tailored applications. figure 2. overview of a generic kubernetes architecture with a control plane on the left side, which consists of api-module, cloud-controller-module, contoller-manager and the scheduler. the control plane provides the global operations for the kubernetes architecture, like scheduling, scaling and steering pods. on the right side are three worker nodes that consists of a kubelet3 and a proxy module. these provide functionality and network capabilities to the pods that hold the containerized application. a worker node can handle several pods a time. a pod can host several containers. qi https://kubernetes.io/docs/reference/command-line-tools-reference/kubelet/ https://kubernetes.io/docs/reference/command-line-tools-reference/kubelet/ https://kubernetes.io/ acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 3 request load on several “shoulders” to cope with peak in demand. kubernetes provides load balancing and horizontal scaling capabilities out of the box. high availability backups and replicas address the high availability requirement. while backups focus on data restoration on a specific time point, replicas support business continuity. often replicas ensure access to critical data and application infrastructure from a secondary location. kubernetes addresses high availability both at application and infrastructure level. portability software development and deployment changed drastically. with container and pods, applications can run on any platform and cluster. kubernetes provides many container runtimes that enable application portability and easy deployment for an agile development approach. this concept facilitates workloads across private and public cloud environments and supports availability fault tolerance zones within the infrastructure design. security security has to be addressed on multiple layer such as cluster, application and network. api endpoints are secured via tls (transport layer security). furthermore, strict role and rights management (see section 2.2.2) has to be enforced for each user, so that only authorized operations can be executed on the cluster. finally, network communication flows for pods have to be defined, so that only wanted communication behaviour is feasible. in a highly distributed architecture, a session-less authorization and authentication approach using tokens, which are placed in each http rest request, has proven itself to be state of the art. this way, each operation can be traced back to the user (see section 2.2.2) initiating any defined process in the use cases. 2.2. distributed software architecture the envisioned distributed microservice software architecture borrows heavily from the concepts designed and developed in [2], [3], [4], [5] and adds the following requirements: • no-configuration debugging: the same image runs in test and production. • containerization: all the code runs in a container plus shared resources. • blue/green deployment: sending traffic only after the server exists. that means no downtime and simplified rollbacks. • application composition: individual pieces of functionality can be composed in separate images, while being technology agnostic. • load balancing: scaling stateless containers is a stark requirement for architecture performance. • service registry and discovery: microservices have to automatically register in a registry service to enable auto-discovery of dependent microservices. once-only principle: harmonizing processes and software development by parameterizable services that fit all internal stakeholders. 2.2.1. functional and security requirements the op-layer can provide a variety of exchange formats such as json, xml and csv. for the internal communication json is used to exchange data from one service to another. the interfaces will be implemented as harmonized rest interfaces. the data will be signed, validated, and archived. while the data is at rest or at transport, it will be encrypted and secured against unauthorized access. the op-layer supports an open id connect access management solution with single sign on capabilities. a session-less and tokenized access management solution is intended, to increase security and avoid session handling in a highly distributed environment. further the cia triad attributes (confidentiality, integrity, and availability) are deeply incorporated in the basic infrastructure design. 2.2.2. identity and access management the following requirements for an identity access management (iam) solution are described in [8] and are adapted to meet the criteria for the proposed distributed microservice software architecture: • separation of concerns: users and roles must be isolated within the platform but differentiated and centrally managed across the entire process. • single sign-on: a user should log on only once and should have access to the functions assigned to him by means of roles in the frontend and in the backend services. • identity federation: it should be possible to authorize additional identity servers of third-party organizations (federated id), so that their users can log on directly to the proposed platform with their organizational id and use its services. • audibility and traceability: due to the highly decoupled and decentralized approach of the architecture, user-based sessions are not used. a tokenbased approach is implemented to login. the token will contain further information about the user, such as roles and the validity period of the token, in encrypted form. this means that every request to a backend service and every processing step in the respective processes can be traced back to the initiating user. • harmonized login procedure: in addition to users, measuring devices and external services should also be authenticated and authorized by the iam system on the platform in the future. • harmonized authentication protocols: by using a standard-compliant openid connect iam solution which also supports saml2, developers can focus on the business requirements and their implementation, as there is no longer any need to develop iam mechanisms. this also simplifies the maintainability of the platform and makes it comparatively easy to exchange the iam solution used. 3. use cases each use case is implemented as a separate containerized back-end service. they all provide rest api endpoints for communication, such as triggering actions, or providing information. the following subsections describe the three usecases of the proposed op-layer platform. 3.1. e-file connector the e-file connector service will have no graphical user interface. instead, this service bridges the gap between the automated data access from the laboratory workflow, in order to import data. as a first step, only administrative data will be converted and exported. later this restriction will be lifted, and all necessary data will be available to support the automation of workflows. the use-case consists of five steps. finding the electronic file and filtering the administrative data. then the export in a acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 4 preselected exchange format, such as json, xml or csv, is triggered. the laboratory can start their workflow and create for example a digital calibration certificate with the exported data. this xml certificate can now be uploaded and will be validated. if the validation is successful, it will be put into the electronic file in the e-file-service. 3.2. digital calibration certificate process reference implementations of services with a high impact factor are provided, to facilitate the adoption of the op-layer within ptb. as a prominent example serves the digital calibration certificate (dcc [9]) process. it has been entirely digitally transformed and implemented [6] as a service within the op-layer. the digital calibration process starts with the e-service portal. a customer portal, which offers a calibration certificate application. after applying for a calibration certificate for a measuring instrument, the process continues with an automatically created file in the e-file system. all data is automatically transferred and archived in a file. the responsible department checks the validity of the application. at this point in the process, the op-layer offers a convenient way to automatically transfer all necessary data to the calibration laboratory via unified rest interfaces. moreover, the aforementioned service is built to automatically import administrative data from the e-file system, in order to create a proper dcc. the resulting dcc can be automatically uploaded via the op-layer into an existing file within the e-file system. from there, the file handler submits the dcc to the e-service portal. the entire use case is depicted in figure 3. in step 1, the customer applies for a digital calibration certificate for a supported measuring instrument within the e-service portal. a notification is sent to the responsible organizational unit. in step 2, the responsible case worker creates a new procedure within e-file and transfers the administrative data from the application into the new file. the op-layer pulls incoming e-service procedures from efile at the beginning of step 3. the lab technician sees the new application within the op-layer web frontend, and initiates dcc creation. the op-layer transfers the administrative data from e-file automatically into the dcc service. the lab technician can provide the measurement results via web frontend. figure 3 is a simplification of the digitally transformed process. hence, e-file is actually two instances. the e-file system itself, and an op-layer specific e-file connector, which serves as communication interface for the other op-layer components. when all required data is provided in the dcc input form (see figure 4) in the op-layer, the lab technician can review the data and submit it to the dcc service to create a valid dcc as xml. the xml file is attached as a record to the calibration document in the procedure within e-file. the case worker provides the dcc xml record to the eservice application, to make it available to the customer in step 4. the customer can view and download the given dcc xml file from the e-service portal in step 5. 3.3. digital calibration certificate service the digital calibration certificate service (dcc service) is a centralized service that harmonizes the creation of a dcc-xml certificate. depending on the laboratory, the object of the calibration and its measurement might differ. however, the figure 3. sequence diagram of the digital calibration certificate process. the numbers resemble the necessary steps until the customer receives the dcc. acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 5 frame and shaft of the document are the same, so that a parametrized service provides the requirements of each laboratory. this service harmonizes and unifies the dcc creation and simplifies the workflow for laboratories tremendously. a first simplified use-case will consist of three steps. first selecting the object of calibration. then importing the administrative data will take place. depending on the selected object of calibration, the specific measurement and device management part will be imported from the laboratory workflow. lastly, the creation of the digital calibration certificate will be executed, and an xml file will be made available for further processing. generated xml dccs can then, depending on the requiring calibration laboratory, be automatically transferred into the given procedure within the e-file system via the e-file connector (see section 3.1). on the other hand, customer data from the eservice request, which is managed by e-file, can be conveniently imported into a dcc structure, also via the e-file connector. this way, the need for providing data manually for a dcc is further reduced. 3.4. doc service the digital declaration of conformity service (doc service) works similar to the dcc service. however, the resulting xml is not a calibration certificate but a declaration of conformity that consists of different norms and directives that the measurement instrument has to fulfil. the use-case consists of three steps, such as the selection if the measuring instrument, the import of administrative data for the declaration and the instrument specific norm and directive part that has to be included in the declaration document. finally, the creation of the declaration of conformity takes place and the resulting xml file can be downloaded. the doc service and the dcc service can be operated simultaneously, yet independently, as both accommodate similar, yet different use cases. 4. conclusions in this paper, the concept of the operation layer has been presented and demonstrated exemplary with the dcc workflow, which links already existing infrastructures at ptb via uniform interfaces to enable data transfers from previous isolated data silos. through uniform interfaces (rest interface), the connected internal systems can maintain their previous workflow, while harmonizing data at the same time. the automatic data transfer drastically increases the productivity, integrity and availability of data and greatly reduces the susceptibility to errors. the op-layer makes an enormous contribution to the automation of workflows and can reduce the workload of employees. in times of decreasing numbers of skilled workers, an increasing automation can lead to more efficiency, more time for research, laboratory activities and related tasks. special tailored application can be hosted via the op-layer internal service hub structure. preferably, working groups design their application with a generic entitlement to serve a larger audience. the strict design framework reduces the administrative figure 4. screenshot of the dcc service form for manually entering measurement result. acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 6 overhead for the research staff to claim more time for original research. in addition, the strict framework increases security and quality for special tailored applications. the op-layer development is based on three main pillars: 1. container environment: a self-hosted sophisticated container environment with prepared ci/cd pipelines, secured networking and resourcemanagement – kubernetes is the current state-ofthe-art solution. 2. identity and access management: a central solution for handling authentication (with organization-wide ptb id) and access control for services in the op-layer – the de-facto standard solution keycloak5 is employed. 3. service guidelines: providing and enforcing principles of modern software design for distributed services. furthermore, it is encouraged to publish the code as open source. a service should be as lightweight as possible, provide rest interfaces and come with basic documentation. a huge potential for automation and optimization is shown by the initially built digital calibration certificate process. especially for future use cases, similar efficiency gains and simplifications for digital transformed processes can be anticipated. however, digitally transforming processes also pose challenges of harmonising work procedures and finding standards as well as minimal shared requirements within a federal organized body such as ptb. on-boarding working groups to the op-layer service hub infrastructure will be the next phase. provided these early adopters have sufficient it skills to migrate their special tailored applications to the op-layer, the phase will be completed. this is a great opportunity to enhance the service guidelines and to connect ptb’s development community. contributing tremendously to it security by harmonizing development and deployment procedures, will serve the whole organization. references [1] f. thiel, j. nordholz, quality infrastructure ’digital’ (qi-digital). federal ministry for economic affairs and energy, 2020. online [accessed 28 march 2023] https://www.bmwk.de/redaktion/en/artikel/digitalworld/gaia-x-use-cases/quality-infrastructure-digital-qidigital.html [2] a. oppermann, s. eickelberg, j. exner, digital transformation in legal metrology: an approach to a distributed architecture for consolidating metrological services and data. in information technology for management: towards business excellence: 15th conf. ism 2020, and fedcsis-ist 2020 track, held as part of fedcsis, sofia, bulgaria, 6–9 september 2020, extended and revised selected papers 15, springer international publishing, 2021, pages 146–164. doi: 10.1007/978-3-030-71846-6_8 [3] a. oppermann, s. eickelberg, j. exner, toward digital transformation of processes in legal metrology for weighing instruments, proc. of the 2020 federated conference on computer science and information systems, m. ganzha, l. maciaszek, m. paprzycki (eds). acsis, vol. 21, pp. 559–562. doi: 10.15439/2020f77 [4] a. oppermann, f. grasso toro, f. thiel, j.-p. seifert, secure cloud computing: reference architecture for measuring instrument under legal control, security and privacy, 1(3), 2018: e18. doi: 10.1002/spy2.18 [5] s. eickelberg, th. bock, m. bernien, a. oppermann. integrating a calibration laboratory workflow into a metrological digital ecosystem: a case study. proc. of the imeko tc6 int. conf. on metrology and digital transformation, berlin, germany, 19 21 september 2022, 4 pp. doi: 10.21014/tc6-2022.016 [6] a. keidel, s. eichstädt, interoperable processes and infrastructure for the digital transformation of the quality infrastructure, 2021 ieee int. workshop on metrology for industry 4.0 & iot (metroind4. 0&iot), rome, italy, 7 9 june 2021, pp. 347–351. doi: 10.1109/metroind4.0iot51437.2021.9488563 [7] s. eichstädt, a. keidel, j. tesch. metrology for the digital age. measurement: sensors, 18, 2021, 100232. doi: 10.1016/j.measen.2021.100232 [8] a. oppermann, s. eickelberg, digitale transformation in der metrologie: harmonisierung digitaler identitäten mittels openid. werkwandel, zeitschrift für angewandte arbeitswissenschaft, ausgabe #1, february 2022, pp. 26–30. online [accessed 28 march 2023] [in german] https://magazin.werkwandel.de/ [9] s. hackel, f. härtig, j. hornig, th. wiedenhöfer, the digital calibration certificate, ptb-mitteilungen 127 (2017), no. 4, pp. 75-81. doi: 10.7795/310.20170403 5 https://www.keycloak.org https://www.bmwk.de/redaktion/en/artikel/digital-world/gaia-x-use-cases/quality-infrastructure-digital-qi-digital.html https://www.bmwk.de/redaktion/en/artikel/digital-world/gaia-x-use-cases/quality-infrastructure-digital-qi-digital.html https://www.bmwk.de/redaktion/en/artikel/digital-world/gaia-x-use-cases/quality-infrastructure-digital-qi-digital.html https://doi.org/10.1007/978-3-030-71846-6_8 http://dx.doi.org/10.15439/2020f77 https://dx.doi.org/10.1002/spy2.18 https://doi.org/10.21014/tc6-2022.016 https://doi.org/10.1109/metroind4.0iot51437.2021.9488563 https://doi.org/10.1016/j.measen.2021.100232 https://magazin.werkwandel.de/ https://dx.doi.org/10.7795/310.20170403 https://www.keycloak.org/ editorial to selected papers from the 2019 imeko tc4 international conference on metrology for archaeology and cultural heritage acta imeko issn: 2221-870x march 2021, volume 10, number 1, 1 4 acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 1 editorial to selected papers from the 2019 imeko tc4 international conference on metrology for archaeology and cultural heritage eulalia balestrieri1, carlo carobbi2, ioan tudosa1 1 department of engineering, university of sannio, benevento, italy 2 department of information engineering, university of florence, firenze, italy section: editorial citation: eulalia balestrieri, carlo carobbi, ioan tudosa, editorial to selected papers from the 2019 imeko tc4 international conference on metrology for archaeology and cultural heritage, acta imeko, vol. 10, no. 1, article 1, march 2021, identifier: imeko-acta-10 (2021)-01-01 editor: francesco lamonaca, university of calabria, italy received march 19, 2021; in final form march 19, 2021; published march 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding authors: eulalia balestrieri, e-mail: balestrieri@unisannio.it, carlo carobbi, e-mail: carlo.carobbi@unifi.it, ioan tudosa, e-mail: itudosa@unisannio.it dear reader, this issue of acta imeko is dedicated to papers that have been selected from those presented at “2019 imeko tc4 international conference on metrology for archaeology and cultural heritage”, metroarchaeo, held in florence in december 2019. measurements are essential to access knowledge in every field of investigation, from industry to quality of life, science and innovation. as a consequence, metrology plays a crucial role for archaeology and cultural heritage, addressing issues related to the collection, interpretation and validation of the data, to the different physical, chemical, mechanical or electronic methodologies used to collect them and to the associated instruments. metroarchaeo brings together researchers and operators in the enhancement, characterization and preservation of archaeological and cultural heritage with the main objective of discussing production, interpretation and reliability of measurements and data. the conference is conceived to foster exchanges of ideas and information, create collaborative networks and update innovations on “measurements” suitable for cultural heritage for archaeologists, conservators and scientists. thirty-three selected papers from metroarchaeo 2019 are presented in three sections (part 1, part 2 and part 3) part 1 – section editor: eulalia balestrieri the first section, edited by eulalia balestrieri, includes the following ten scientific contributions. the first contribution, by valentino sangiorgio et al, “historical masonry churches diagnosis supported by an analytic-hierarchyprocess-based decision support system” presents a new procedure based on analytical hierarchy processes (ahp) aimed at carrying out rapid on-site measurements and diagnostic of masonry churches through a series of condition assessment indexes. the proposed procedure has been successfully validated by a comparison with a standard diagnostic workflow. in the second paper, by sveva longo et al, “clinical computed tomography and surface-enhanced raman scattering characterisation of ancient pigments”, a systematic and complete chemical-physical characterisation of painted pigments has been carried out using multi-slice x-ray computed tomography (msct) and surfaceenhanced raman scattering (sers) techniques. thanks to the proposed approach, the identification and characterization of both inorganic and organic materials present on the wooden tablets have been carried out. the third contribution, by simone tiberti and gabriele milani, “creating a finite element mesh of non-periodic masonry from the measurement of its geometrical characteristics: a novel automated procedure”, illustrates an automated procedure for the generation of a finite element (fe) mesh directly from the rasterized sketch of a generic masonry element, which is particularly suitable for complex and irregular (non-periodic) masonry bonds that can be observed in heritage buildings or found in archaeological sites. two procedures are set for the creation of 2d and 3d fe meshes. in the fourth paper, by laura guidorzi et al, “age determination and authentication of ceramics: advancements in the thermoluminescence dating laboratory in torino (italy)”, the thermoluminescence (tl) laboratory developed at the physics department of the mailto:balestrieri@unisannio.it mailto:carlo.carobbi@unifi.it mailto:itudosa@unisannio.it acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 2 university of torino is presented. the laboratory was set up in collaboration with tecnart s.r.l. and is also currently operating within the infn (national institute of nuclear physics) chnet network. some example of dating and authenticating results carried out at the laboratory on archaeological sites and artworks are also discussed and compared, when possible, with radiocarbon dating. the fifth contribution, by marialuisa mongelli et al, “comparison and integration of techniques for the study and valorisation of the corsini throne in corsini gallery in roma”, presents the investigation of an integrated approach involving non-invasive technologies, photogrammetry and structured light, during the development of the "weact3" project (acting together technology for art, culture, tourism and territory) jointly signed by civita association and the national barberini and corsini galleries. such technologies have been used to build the 3d model of the corsini throne, preserved at the corsini gallery in rome. the sixth work, by carmelo scuro et al, “study of an ancient earthquake-proof construction technique monitoring via an innovative structural health monitoring system”, illustrates an innovative method to monitor and obtain in real time the mechanical properties of an anti-seismic construction widespread in southern calabria and patented by pasquale frezza, also minimizing measurement uncertainty. the considered type of anti-seismic construction consists of masonry walls built with bricks and fictile tubules, arranged in staggered and alternating manner, all contained in a timber wooden frame. the seventh contribution, by renato s. olivito et al, “inventory and monitoring of historical cultural heritage buildings on a territorial scale: a preliminary study of structural health monitoring based on the cartis approach”, presents a preliminary study aimed at defining an integrated methodology for inventory, monitoring, transmission, and data management of heritage buildings to provide important information about their structural integrity. the eighth paper, by maria federica caso et al, “improvement of enea laser-induced fluorescence prototypes: an intercalibration between a hyperspectral and a multispectral scanning system”, presents the hyperspectral and multispectral laser induced fluorescence (lif) scanning systems developed at enea diagnostic and metrology laboratory and in particular their intercalibration, along with the data analysis of calibration samples and a software to automatically correct imaging data. in the ninth contribution, by alejandro roda-buch et al, “fault detection and diagnosis of historical vehicle engines using acoustic emission techniques”, the results of the first phase of the acume_hv (acoustic emission monitoring of historical vehicles) project focused on the development of a protocol for the use of acoustic emission (ae) during cold tests are presented. the project represents the first use of ae as non-invasive technique for the diagnostic of historical vehicles to carry out an objective, human-independent method. the tenth and last contribution of this part 1, by sandro parrinello and raffaella de marco, “digital surveying and 3d modelling structural shape pipelines for instability monitoring in historical buildings: a strategy of versatile mesh models for ruined and endangered heritage”, illustrates the application of a fast and reliable structural documentation pipeline to historical built heritage, as in the case study of the church of the annunciation in pokcha (russia), by reviewing the declination of integrated products of 3d survey into reality-based models. part 2 – section editor: carlo carobbi the second section, edited by carlo carobbi, includes the following ten scientific contributions. the first contribution, by valeria croce et al, “from survey to semantic representation for cultural heritage: the 3d modelling of recurring architectural elements”, illustrates an approach to be followed in the transition from 3d survey information, derived from laser scanner and photogrammetric techniques, to the creation of semantically enriched 3d models. the proposed approach is based on the recognition -segmentation and classificationof elements on the original raw point cloud, and on the manual mapping of nurbs elements on it. in the second work, by francesco boschin et al, “geometric morphometrics reveal relationship between cut-mark morphology and cutting tools”, two groups of slicing cut mark cross-sections were experimentally produced. the resulting sets of striae show different depths and different cross-sectional shapes. it turns out that the difference in shape between the two groups of striations is probably a function of the way in which the tool penetrated the bone. the third contribution, by gabriella caroti et al, “the use of image and laser scanner survey archives for cultural heritage 3d modelling and change analysis”, elaborates on the methodology used for integration of a metric 3-d model with information present in archive surveys of lost architectural volumes. the presented methodology frames historical plans, representing the survey object, in the reference system of uav surveys for an opensource gis environment. in the fourth work, by yufan ding et al, “provenance study of the limestone used in the construction and restoration of the batalha monastery (portugal)”, stone samples were investigated, through different methods (energy-dispersive x-ray fluorescence spectroscopy, powder x-ray diffractometry and thermogravimetric analysis) obtaining indication of the source of samples collected from different parts the monastery. the fifth contribution, by leila es sebar et al, “raman investigation of corrosion products on roman copper-based artefacts”, illustrates a case study related to the characterization of corrosion products present on recently excavated artefacts. results coming from raman spectroscopy investigation can help in assessing the conservation state of the artefacts and defining the correct restoration strategy. in the sixth work by elisabetta di francia et al, “characterisation of corrosion products on copper-based artefacts: potential of ma-xrf measurements”, a novel portable ma-xrf scanner prototype has been tested on artificially corroded copper samples to assess its analytical capabilities on corroded metals and yielding information on the spatial distribution of the corrosion products grown on the metal’s surface. in the seventh contribution by giuseppe schirripa spagnolo et al, “fringe-projection profilometry for recovering 2.5d shape of ancient coins”, a surface profile measurement system for small objects of cultural heritage, where it is important not only to detect the shape with good accuracy but also to capture and archive the signs due to ageing, is illustrated. the potentiality of the proposed scheme for recovering 2.5d shape of cultural heritage is demonstrated. in the eighth contribution by anna maria gueli et al, “modelling and simulations for signal loss evaluation during sampling phase for thermoluminescence authenticity tests”, the percentage of the intensity signal loss in thermoluminescence emission, due to local temperature increase caused by drilling, is investigated. the acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 3 optimal parameters that should be used during sampling phase are identified. in the ninth work by zacharias vangelatos et al, “finite element analysis of the parthenon marble block–steel clamp system response under acceleration”, finite element analysis is employed to provide a tool for the assessment of the conservation potential of the marble blocks, in parts of the monument that require specific attention. simulation results highlight the importance of intrinsic stresses, the existence of which may lead to fracture of the marble blocks under otherwise harmless loading conditions. in the tenth and last contribution of this part 2, by andrea zanobini, “metrological characterisation of a textile temperature sensor in archaeology”, the study of a new generation textile temperature sensor, in two different heated ovens to evaluate temperature and both temperature and humidity, is presented. the results show many metrological characteristics proving that the sensor is a resistance temperature detector. part 3 – section editor: ioan tudosa the third section, edited by ioan tudosa, includes the following thirteen scientific contributions. the first contribution, by sebastiano d’amico et al, “a combined 3d surveying, xrf and raman in-situ investigation of the conversion of st paul painting (mdina, malta) by mattia preti”, presents the results of three different approaches applied to the newly-restored titular painting the conversion of st paul, the main altarpiece in the cathedral of mdina in malta. the study was aimed at showing the potentialities of the combined use of 2d/3d photogrammetric surveys and spectroscopy (xrf and raman techniques) in order to, on one side, get reconstruction, and, on the other side, achieve, at different spatial domains. in the second work, by luisa caneve et al, “non-invasive diagnostic investigation at the bishop’s palace of frascati: an integrated approach”, a novel methodology aimed to detect and locate materials due to previous restoration actions and to monitor the degradation processes evolution remotely, is proposed. the possibility of a preventive monitoring by the application of the presented approach to reduce the eventual induced damage has been put in light. in the third contribution, by sofia ceccarelli et al, “thermographic and reflectographic imaging investigations on baroque paintings preserved at the chigi palace in ariccia”, two different midinfrared imaging techniques, operating in the 3-5 µm spectral range, are applied to the study of three paintings on canvas, dating back to the xvii century, preserved at the chigi palace in ariccia (italy). presented results allow the evaluation of the conservative status of the support and the detection of graphical and pictorial features hidden beneath the surface layer. in the fourth work, by daniele moro et al, “mineral diagnostics: sem-eds monte carlo strategy for optimised measurements of ultrathin fragments in cultural heritage studies”, a detailed study of the effects related to microand nanometric sizes of glass and gold alloys fragments on sem-eds microanalysis is presented. monte carlo simulations of different kind of elongated glass fragments with square section, from 0.1 to 10 µm thick, and of some gold alloys showed a strong influence of the fragment sizes and operational conditions (beam energy, detector position, etc.). this work can be used to devise the appropriate and optimized measurement strategy. the fifth contribution, by luisa spairani, “measure by measure, they touched heaven”, illustrates a case study in photography of the law of leavitt. in the sixth work by giacomo fiocco et al, “chemometrics tools for investigating complex synchrotron radiation ftir micro-spectra: focus on historical bowed musical instruments”, a method describing how the synchrotron radiation (sr) micro-ftir spectroscopy in reflection geometry and chemometrics were combined to investigate six cross-sectioned micro-samples detached from four bowed string instruments, is presented. in the seventh contribution by m. faifer et al, “laboratory measurement system for pre-corroded sensors devoted to metallic artwork monitoring”, a measurement system for the development and testing of sensor for atmospheric corrosivity monitoring is presented. the developed system allows to monitor metal corrosion higher than 3 nm in the temperature range from 23 °c to 39 °c. the performed analysis allows to state that the system is an efficient laboratory setup for the development and characterization of sensor for metal corrosion monitoring. the eighth contribution by maria legut-pintal et al, “methodological issues of metrological analysis of planned medieval towns and villages”, proposes the usage of cosine quantogram, which has rarely been used to study of urban layout, for the identification of units of measurement in medieval regular towns. in the ninth work by roberta spallone et al, “digital strategies for the valorisation of archival heritage”, a study that aims at creating a sort of digital model-museum, in which to insert all the historical information useful to tell the story of the evolution of the artefact over time, and allowing users, through the use of personal devices, to live interactive and immersive experiences, through virtual and augmented reality, is presented. in the tenth paper by tilde de caro et al, “application of µraman spectroscopy to the study of the corrosion products of archaeological coins”, a study case of the corrosion products formed on archaeological bronze artefacts excavated in tharros (sardinia, italy) is presented. the experimental findings allow to acquire, through micro-raman spectroscopy, a better knowledge on the environmental factors that may cause the degradation of archaeological bronzes in soil. the eleventh contribution, by leila es sebar et al, “in-situ multi-analytical study of ongoing corrosion processes on bronze artworks exposed outdoors”, presents a long-term in-situ monitoring campaign of contemporary bronze statuary exposed outdoor. the authors demonstrate the importance of the use of portable instruments offering the possibility to perform in situ measurements, thus avoiding any sampling and assessing of the degradation of the material directly in contact with the environment to which the artwork is always exposed to. the twelfth contribution by máté sepsi et al, “non-destructive pole-figure measurements on workshop-made silver reference models of archaic objects”, reports the non-destructive pole figure method as a sufficient way to distinguish between metal objects formed in different ways. the specific forming modes result in specific pole figures, and therefore, by producing and examining a sufficient number of reference materials, the mode of production of archaic objects can also be reconstructed. the authors state that the obtained pole figures by the robot diffractometer are completely identical to the figures of the previously validated g3r diffractometer. the thirteenth and last contribution of this part 3, by maria grazia d'urso, “a combination of terrestrial laser-scanning point clouds acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 4 and the thrust network analysis approach for structural modelling of masonry vaults”, presents how the geometric and geo-referenced 3d models are obtained by processing laser-scanning measurements. a model built on a coherent geometric basis, which contemplates the methodological complexities of the detected objects is reported. a semi-automated method that allows one to switch from point cloud to an advanced three-dimensional model, able to contain all the geometrical and mechanical characteristics of the built object, is proposed in the paper. it was a great honour for us to act as guest editors for this issue of acta imeko, a high-profile scientific journal devoted to the enhancement of academic activities of imeko and a wider dissemination of scientific output from imeko tc events. we would like to sincerely thank all the authors for their valuable contributions, and we hope the readers will be inspired by the themes and proposals that have been selected and included in this special section related to innovations in metrology for archaeological and cultural heritage. eulalia balestrieri, carlo carobbi, ioan tudosa guest editors experimental investigation in controlled conditions of the impact of dynamic spectrum sharing on maximum-power extrapolation techniques for the assessment of human exposure to electromagnetic fields generated by 5g gnodeb acta imeko issn: 2221-870x september 2022, volume 11, number 3, 1 7 acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 1 experimental investigation in controlled conditions of the impact of dynamic spectrum sharing on maximum-power extrapolation techniques for the assessment of human exposure to electromagnetic fields generated by 5g gnodeb sara adda1, tommaso aureli2, tiziana cassano3, daniele franci2, marco d. migliore4, nicola pasquino5, settimio pavoncello2, fulvio schettino4, maddalena schirone3 1 arpa piemonte, dipartimento rischi fisici e tecnologici, via jervis 30, 10015 ivrea (to), italy 2 arpa lazio, 00172 rome, italy 3 arpa puglia, uos agenti fisici, dap bari, corso trieste 27, 70126 bari, italy 4 dipartimento di ingegneria elettrica e dell’informazione (diei) “maurizio scarano", university of cassino and southern lazio, cassino, 03043, and cnit cassino, and eledia@unicas, italy 5 dipartimento di ingegneria elettrica e delle tecnologie dell’informazione (dieti), università degli studi di napoli federico ii, 80125 napoli, italy section: research paper keywords: dynamic spectrum sharing; maximum-power extrapolation; dss; 4g; 5g; human exposure; measurements citation: sara adda, tommaso aureli, tiziana cassano, daniele franci, marco d. migliore, nicola pasquino, settimio pavoncello, fulvio schettino, maddalena schirone, experimental investigation in controlled conditions of the impact of dynamic spectrum sharing on maximum-power extrapolation techniques for the assessment of human exposure to electromagnetic fields generated by 5g gnodeb, acta imeko, vol. 11, no. 3, article 18, september 2022, identifier: imeko-acta-11 (2022)-03-18 section editor: francesco lamonaca, university of calabria, italy received march 17, 2022; in final form september 15, 2022; published september 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: nicola pasquino, e-mail: nicola.pasquino@unina.it 1. introduction dynamic spectrum sharing (dss) is becoming a key technology in the implementation of 5g networks thanks to several practical advantages. dss provides 5g services on 4g long term evolution (lte) networks, thus allowing to use the 4g facilities for a quick deployment of 5g networks without the need of new frequency bands, not always available to local operators. consequently, it is a simple solution that requires basically only software updates, allowing operators to start nationwide 5g coverage with limited costs. furthermore, the use of 4g frequency bands allows better area coverage and indoor penetration compared to sub-6 ghz and frequency range 2 (fr2) (24.25 ghz to 52.6 ghz) used in both non-standalone (nsa) and standalone (sa) 5g networks. in this paper, an experimental investigation is presented to evaluate if this technological solution impacts the outcomes of measurement techniques used to assess the compliance with electromagnetic fields (emf) exposure limits. maximum-power extrapolation (mpe) measurement has been object of a consolidated literature, and standards for mpe measurements are available [1]. 5g is a more recent technology, however, although standards are still under development, mpe abstract maximum-power extrapolation (mpe) techniques adopted for 4g and 5g signals are applied to systems using dynamic spectrum sharing (dss) signals generated by a base station and transferred to the measurement instruments through an air interface adapter to obtain a controlled environment. this allowed to focus the analysis on the effect of the frame structure on the mpe procedure, excluding the random effects associated to fading phenomena affecting signals received in real environments. the analysis confirms that both the 4g mpe and the proposed 5g mpe procedure can be used for dss signals, provided that the correct number of subcarriers in the dss frame is considered. mailto:paul@regtien.net acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 2 procedures have been described in a few research papers [2]-[6]. 5g dss is midstream between 4g and 5g technology. in this paper mpe techniques adopted for 4g signals and 5g signals are applied to dss signals. in particular, the dss signals generated by an actual base station have been connected to the measurement equipment through an air interface adapter (aiad). this allows to focus the analysis on the effect of the frame structure on the mpe procedure, excluding the random effects due to fading phenomena affecting signals received in real environments. the results confirm that both 4g and 5g mpe extrapolation techniques allow a correct estimation of the mpe level of dss signals, provided that the correct number of the subcarriers in the dss frame is considered. 2. the dynamic spectrum sharing technique as noted in the introduction, spectrum sharing (ss) allows network operators to implement 5g nr using existing lte frequency bands, with relatively inexpensive software upgrades. there are two possible solutions to lte-nr coexistence: static spectrum sharing (sss), where the frequency band is statically assigned to either system over time, and dss, where resources are switched between lte and 5g dynamically over time. sss is less attracting than dss, particularly in the current state of 5g implementation, where penetration of 5g devices is still limited and most traffic is on lte, providing an almost fully loaded 4g carrier and an almost empty 5g carrier. this strongly pushes towards the implementation of dss which allows lte and 5g nr to share spectrum resources dynamically based on the current traffic demand, thus employing the entire available bandwidth more efficiently. accordingly, the solution currently adopted in italy is dss. dss supports 5g numerology with µ = 0 (15 khz subcarrier spacing) and µ = 1 (30 khz spacing). although, as will be briefly discussed below, implementation of µ = 1 numerology in a 4g frame is less complex than the use of µ = 0, in this section we will focus our attention on the dss technology with µ = 0 download frame structure, since it is the same adopted in the signals used in the presented experimental activity. to avoid imperfect signal synchronization, in case of ltenr coexistence, it is crucial to avoid overlapping between nr synchronization signal block (ssb) and lte reference signal (crs). let us consider the lte frame. the crs is transmitted in 4 (using 1 or 2 antenna ports) or 6 (when using 4 antenna ports) non-contiguous ofdm symbols of each subframe. with respect to nr, ssb requires four consecutive ofdm symbols. in case of µ = 1, two nr resource elements (res) occupy the duration of an lte re (whose spacing is 15 khz). consequently, an ssb only occupies two lte symbols. this makes it possible to insert the ssbs in the lte frame in a simple way, since in the lte frame the distance between two consecutive crs is greater than the duration of two lte ofdm symbols in case of any number of antenna ports. things become more involved when a 5g signal with µ = 0 numerology must be inserted in an lte frame. in fact, as noted above, there are only 3 contiguous ofdm symbols without any crs transmissions, while ssb requires 4 symbols with 15 khz subcarrier spacing. the solution currently applied involves the use of 4g multimedia broadcast multicast service single frequency network (mbsfn) subframes. mbsfn has been introduced in the lte standard as part of release 9 for point-to-multipoint communication to provide broadcast and multicast services (evolved multimedia broadcast multicast services (embms)). the network can configure six out of ten subframes forming the lte radio frame to become mbsfn subframes. based on the 3gpp standard, these could be subframes #1, #2, #3, #6, #7, and #8 of a radio frame. a standard lte terminal reads in the mbsfn configuration from system information in block type 2 (sib2) and ignores the subframes configured for broadcast. to allow effective broadcasting transmission, the mbsfn subframes reserve only the first 2 ofdm symbols to carry the control channels for lte, while the remaining ones are reserved for embms (figure 1). these symbols are consequently “free". to better explain this point, that is important for the mpe procedure, in figure 2 an example of a dss frame structure (right) is qualitatively compared with the spectrum of the dss figure 1. an example of lte/nr subframe; the first two time slots are always reserved to lte. figure 2. qualitative relationship between the spectrum of the lte signal, the spectrum of the dss signal and the dss frame. acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 3 signal (centre). furthermore, the spectrum of a legacy 4g signal (i.e., without dss) is shown in the left. regarding the dss frame structure, we can note an mbsfn in the second subframe, where the ssb of the 5g signal is placed. the 4g signal uses the upper half part of the remaining subframes, while the 5g signal uses the lower half part of these subframes, and the part not used by the ssb of the mbsf subframe. note that 5g does not use the first two symbol of the frames, that are reserved to 4g. the spectrum of the dss is shown in the centre figure. the useful spectrum is in the a-a’ frequency range. the left plot shows an example of a standard 4g spectrum. due to the presence of a larger band-guard, the useful portion of the spectrum is limited to the b-b’ frequency range, obtaining a lower number of subcarriers. in practice, although both 4g and 5g standard signals occupy a nominal bandwidth of 20 mhz, the actual useful bandwidth for a 5g signal is 1.08 mhz larger than the corresponding 4g bandwidth, i.e., 19.08 vs 18 mhz. accordingly, the number of subcarriers of a 20 mhz nominal-bandwidth dss signal is 1272. all of them are available for 5g signals, while only 1200 are used for lte signals. 3. maximum-power extrapolation technique for 4g and 5g signals the final goal of the mpe procedure is the estimation of the maximum field level emax that could be reached in the measurement point [ 1 ] . this quantity is used as a reference to estimate the emf exposure in realistic conditions by a suitable scaling factor [1], [7]-[15]. both 4g and 5g use ofdma. this gives many similarities that can be exploited for the mpe of dss signals. more specifically, they require [1], [16]: a. information on the structure of the frame (as bandwidth, numerology for 5g, duty cycle in case of tdd transmissions) necessary to identify the number of res available for downlink transmission, b. the estimation of 𝐸re max, the maximum possible average emf level associated to a re. in 4g, 𝐸re max can be obtained considering the power of the res associated to the reference signal (rs) of the 4g frame, let 𝐸rs be, that are transmitted at full power. according to [1], the maximum emf level in the measurement location is then estimated as 𝐸4g max = 𝐸rs√ 𝑁sc𝐹tdc 𝐹b , (1) where 𝑁sc is the total number of subcarriers, 𝐹tdc is a duty cycle factor that describes the transmission scheme implemented by the signal, 𝐹b is a boosting factor that can be applied to the 4g control channels transmitted power to extend the coverage range. in 5g nr, the only signal that is always ‘on air’ is the ssb, that is transmitted at constant and maximum power. it must be noted that in nr it is possible to use different beams to transmit ssb (using broadcast beams) and payload data (using traffic beams). accordingly, measurement of the power of the res of the ssb in general does not allow a direct estimation of 𝐸5g max, that is related to the traffic beams [5]. however, as discussed in the previous section, dss usually is obtained by only software upgrading of the 4g system. consequently, ssb and payload data are transmitted on the same beam. this allows to estimate 𝐸5g max directly from the measurement of ssb res power. following the experimental approach discussed in [17], the power of the res associated to the physical broadcast channel demodulation reference signal (pbch-dmrs) is used as input for the extrapolation formula: 𝐸5g max = 𝐸pbch_dmrs√𝑁sc𝐹tdc𝐹beam, (2) where 𝐹beam is a correction factor that considers the difference between traffic and broadcast beams gain. it’s worth noticing that for dss signals generated by passive mimo systems like the one used in our experiment, 𝐹beam = 1 since ssb and traffic data share the same beam. 4. experimental results an experimental session has been carried out in a dedicated facility, used by one of the main italian telco companies for test purposes. measurements were performed with the aim of gaining a suitable understanding of dss system operation and thus defining an effective procedure for the assessment of the population exposure from dss sources. the experimental setup used during this controlled-environment experimental session is described in the following: • a keysight mxa n9020a vector signal analyzer (vsa) with up to 20 mhz demodulation bandwidth, equipped with demodulation software for both 4g and 5g nr signals; • a rhode & schwarz fsva3044 vsa with up to 400 mhz demodulation bandwidth, equipped with demodulation software for both 4g and 5g nr signals. the experimental setup is shown in figure 3. to ensure a reliable emulation of both inputs and outputs according to the demands of real users, the signal generated by the base station has been transferred to an aiad 8/8-4g+dl manufactured by mts systemtechnik. the aim of the aiad is to emulate the air interface, allowing for testing of mobile radio base stations in a laboratory. figure 4 shows the aiad frontend with two coaxial cables used to transfer both mimo branches of the dss signal from the lower levels of the gnodeb to the antennas placed inside the aiad chamber. the coaxial cable on the left of the figure is used to drive the test signal to the measurement instruments. the main characteristics of the generated signals are summarized in table 1. to investigate 4g-5g sharing mechanism properly, different data traffic scenarios have been considered: • zero-traffic, • full-frame 4g-only traffic, • full-frame mixed 4g-5g traffic. figure 3. experimental setup used throughout the measurement campaign. acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 4 4.1. zero-traffic scenario as a first step, a dss signal with no data traffic was generated. figure 5 shows the map of the demodulated power vs. symbol/carrier for an entire 10 ms dss frame, made of ten consecutive subframes. the absence of user data traffic allows for an easy identification of both lte and nr control channels: • lte primary and secondary synchronization signals (pss and sss) and pbch are transmitted in subframes 0 (pss+sss+pbch) and 5 (pss+sss only); these signals occupy about 1mhz at the centre of the signal bandwidth. in addition, the rs is located sparsely throughout the frame, according to the positions defined by 3gpp standard [18]; • 5g nr synchronization signal block (ssb) can be recognized in the mbsfn subframe 1, with a frequency offset of 7.65 mhz with respect to the centre of the signal bandwidth. according to µ = 0 numerology, the ssb bandwidth is equal to 3.6 mhz. obviously, the peculiar allocation of the radio resources in dss systems is specifically designed with the aim of avoiding any possible interference between different signals. zero-span measurement is an alternative experimental approach useful to appreciate the resource allocation adopted by dss. it provides the time-domain variation of the received power at a fixed frequency value. this method has the great advantage of providing a low-cost alternative to usage of top-notch, high expensive vector signal analysers vsas, since it can be accomplished by almost every traditional spectrum analyser. figure 6 and figure 7 show a 20-ms zero-span acquisition at 1850 mhz (i.e., the centre of the signal bandwidth) and 1857.65 mhz (i.e., ssb centre frequency) respectively. a periodic trigger equal to the frame duration (i.e., 10 ms) has been applied to both the acquired spectra. when 1850 mhz is imposed as central frequency, the lte control channels can be easily recognized in the acquired spectrum (figure 6) as power bursts, spaced out 5 ms apart. otherwise, when the central frequency is shifted to 1857.65 mhz (figure 7), we can distinguish a unique power peak, corresponding to the 5g ssb. it is worth noting that lte rs are present everywhere throughout the time frame, regardless of the centre frequency chosen for the measurement. the dss signal was properly demodulated by both 4g and 5g analysis software, with both the routines characterized by an excellent synchronization with the dss signal and reconstructed iq constellations very close to the ideal ones. figure 4. the aiad used to transfer the signal from the base station to the measurement equipment. table 1. caption dss signal configuration center frequency fc 1850 mhz bandwidth b 20 mhz duplexing ffd numerology 0 sub-carrier spacing f 15 khz cell identity (cid) 255 mimo antennas 2 mbsfn subframe indexes 1, 21, 22 ssb allocation case a with lmax = 4 ssb center frequency fssb 1857,65 mhz ssb per burst 1 ssb transmission periodicity 20 ms figure 5. power vs. symbol × carrier map of a dss frame in case of absence of user data traffic. figure 6. zero span measurement at 1850 mhz. figure 7. zero span measurement at 1857.65 mhz. acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 5 4.2. full-frame traffic scenario full-frame data traffic scenario has been achieved using a user equipment (ue) in combination with the aiad system, with the aim of forcing the saturation of the dss frame through multi-thread ftp connections. to understand which schedule is used by the dss system for resource allocation, two different scenarios have been considered: • 100 % 4g data traffic; • 50 % 4g – 50 % 5g data traffic. following the approach used in the previous section, the map of the demodulated power vs. symbol/carrier has been acquired for both scenarios under investigation. note that to appreciate the periodicity of the mbsfn subframes, the capture buffer has been extended to 4 radio frames (i.e., 40 ms). in absence of 5g users, and with heavy 4g traffic, all the subframes of a frame, except those configured as mbsfn, are used for 4g communication, as shown in figure 8. in fact, mbsfn subframes (indexed as 1, 21 and 22) are not allowed to host 4g data transmission and, for this reason, they are left almost completely empty, except for 5g ssbs, which are located at the very top edge of dss spectrum and transmitted once per couple of frames (20 ms). the presence of mbsfn subframes which are reserved for 5g transmission implies that a dss frame cannot be entirely filled by 4g data traffic only. figure 9 shows the case of large 4g and 5g data traffic. 4g data is placed in upper halfpart of the no-mbsfn subframes, while 5g uses most of the lower part. as discussed in section 2, the configuration of the 5g part of frame leaves free the res required for 4g signalling, making the presence of 5g res completely transparent for a 4g user. note that the radio resource occupation is now more uniform than the previous case, although several blank regions – acting as guard intervals to avoid possible interferences between signals – are disseminated throughout the whole radio frame. 4.3. mpe procedure applied to dss the above-described code-domain analysis provides direct information on the following two quantities required for extrapolation procedures: • rs for 4g (𝑃rs), • pbch-dmrs for 5g (𝑃pbch−dmrs). to discuss the dss mpe procedure, we consider the fullframe condition configuration. this makes it possible to compare the results with the maximum reference power obtained by a channel power (cp) measurement acquired in the full-frame data traffic scenario. as discussed in the previous section, both 4g and 5g control channel coexist within the dss frame. for this reason, both 4g and 5g mpe procedure – described by eq. (3) and (4) respectively – can be applied: 𝑃4g max = 𝑁sc 𝐹tdc 𝐹b 𝑃rs (3) 𝑃5g max = 𝑁sc 𝐹beam 𝐹tdc 𝑃pbch−dmrs (4) note that eq. (3) and (4) represent the same mpe procedures described by eq. (1) and (2), just applied in terms of maximum power instead of electric field. regarding the measurement procedure, it is the same described in [1] for 4g and in [17] for 5g. more specifically, code domain measurement of the dss signal provides direct information on prs and ppbch_dmrs. according to the characteristic of the dss signal, several assumptions about the parameters included in eq. (3) and eq. (4) can be made: • 𝐹tdc is assumed to be equal to 1 in both eq. (3) and (4), since the dss signal adopts the frequency division duplexing (fdd) transmission mode. fdd allows uplink and downlink transmission over different frequency bands, so that no uplink-downlink time duty cycle is needed, • 𝐹b = 1 in eq. (3), since no power boost is applied to 4g rs, • 𝐹beam = 1 in eq. (4), since the passive antennas used for dss signals are not allowed for beamforming. regarding the 𝑁sc value, as pointed out in sect. 2, although 4g and 5g standard signals occupy a nominal bandwidth of 20 mhz, the actual occupied bandwidth of a 5g signal is 1.08 mhz larger than the corresponding 4g bandwidth (i.e., 19.08 mhz vs. 18 mhz), giving 1272 available subcarriers spaced by 15 khz. therefore, the value of 𝑁sc is assumed to be equal to 1272 in both eq. (3) and (4). when a subframe transmits 4g signals only, just 1200 carriers are used. accordingly, also in case of full use of the dss resources, there are some unused res in the frames. indeed, only a fraction of such res is related to the different number of 4g and 5g subcarriers, while the remaining ones are intrinsic in the 4g as well as 5g subframe structures. the exact number of unused res in case of full 4g/5g traffic depends on how the scheduler allocates the 4g and 5g data in the dss frame. this is a decision of the provider, and the almost 50 % sharing of the figure 8. power vs. symbol × carrier map of a dss frame in case of 4g full traffic case; the 20 ms periodicity of the ssb is visible. figure 9. power vs. symbol × carrier map of a dss frame in case of 4g/5g balanced traffic. acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 6 frame between 4g and 5g signals in case of full 4g and 5g traffic shown in figure 9 is just one possible choice. different choices give a slightly different number of ’unused’ res in case of fully filled frames. consequently, the hypothesis of full use of the res of the dss frame carried out in the mpe procedure gives a conservative estimation of the maximum power level in case of full 4g/5g traffic for any possible distribution of the 4g/5g data chosen by the providers, according to the precautionary principle applied to the evaluation of the electromagnetic exposure of the population. to verify the above observations, we have applied eq. (3) and (4) to the full-loaded dss signal in figure 9. the prs and ppbch_dmrs have been measured in the code domain and are reported in table 2. then, a channel power measurement was performed on the same signal. the mpe and cp measurements are reported in table 3. results show that the use of eq. (3) and (4) gives very close values, and confirm the conservative value obtained using mpe compared to the channel power result. 5. conclusions the emf human exposure to dss signals is a topic which is gaining growing interest among scientific community. in this paper the results of a study regarding the use of 4g or 5g mpe procedures for dss signal are reported and found in good agreement with those reported in [19]. the dss signals were generated by a base station emulator and transferred to the measurement instruments through an air interface adapter to obtain a controlled environment. this allowed to focus the analysis on the effect of the frame structure on the mpe procedure, excluding the random effects associated to fading phenomena affecting signals received in real environments. the analysis confirms that both the 4g and the proposed 5g mpe procedures can be used for mpe of dss signals, provided that the correct number of subcarriers in the dss frame is considered. references [1] iec 62232. determination of rf field strength, power density and sar in the vicinity of radiocommunication base stations for the purpose of evaluating human exposure. iec international electrotechnical commission, 2017. 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[14] m. d. migliore, f. schettino, power reduction estimation of 5g active antenna systems for human exposure assessment in realistic scenarios. ieee access 2020, 8, 220095–220107. 331 doi: 10.1109/access.2020.3042002. [15] a. hirata, y. diao, t. onishi, k. sasaki, s. ahn, d. colombi, v. de santis, i. laakso, l. giaccone, w. joseph, et al. assessment of human exposure to electromagnetic fields: review and future directions. ieee transactions on electromagnetic compatibility 2021. [16] m. d. migliore, d. franci, s. pavoncello, e. grillo, t. aureli, s. adda, r. suman, s. d’elia, f. schettino, a new paradigm in 5g maximum power extrapolation for human exposure assessment: forcing gnb traffic toward the measurement equipment. ieee access 2021, 9, 101946–101958. doi: 10.1109/access.2021.3092704. table 2. rs and pbch-dmrs power per re. prs -69.94 dbm ppbch-dmrs -70.20 dbm table 3. comparison of mpe and cp measurement. 4g mpe -38.90 dbm 5g mpe -39.15 dbm cp -41.24 dbm https://doi.org/10.1109/access.2019.2961225 https://doi.org/10.3390/environments7030022 https://doi.org/10.3390/electronics9020223 https://doi.org/10.1109/access.2020.2998448 https://doi.org/10.1109/access.2021.3092704 https://doi.org/10.1109/access.2017.2753459 https://doi.org/10.1109/access.2020.3028471 https://doi.org/10.3390/app10155280 https://doi.org/10.1109/access.2020.3042002 https://doi.org/10.1109/access.2021.3092704 acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 7 [17] d. franci, s. coltellacci, e. grillo, s. pavoncello, t. aureli, r. cintoli, m.d. migliore, experimental procedure for fifth generation (5g) electromagnetic field (emf) measurement and maximum power extrapolation for human exposure assessment. environments 2020, 7, 342. doi: 10.3390/environments7030022 [18] ts 36.213. evolved universal terrestrial radio access (eutra); physical layer procedures. 3rd generation partnership project (3gpp), 2015 [19] l. m. schilling, c. bornkessel, m. a. hein, analysis of instantaneous and maximal rf exposure in 4g/5g networks with dynamic spectrum sharing, 16th european conference on antennas and propagation (eucap), 2022, pp. 1-5. doi: 10.23919/eucap53622.2022.9769680. https://doi.org/10.3390/environments7030022 https://doi.org/10.23919/eucap53622.2022.9769680 the importance of physiological data variability in wearable devices for digital health applications acta imeko issn: 2221-870x june 2022, volume 11, number 2, 1 8 acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 1 the importance of physiological data variability in wearable devices for digital health applications gloria cosoli1, angelica poli2, susanna spinsante2, lorenzo scalise1 1 department of industrial engineering and mathematical sciences, università politecnica delle marche, v. brecce bianche, 60131 ancona, italy 2 department of information engineering, università politecnica delle marche, v. brecce bianche, 60131 ancona, italy section: research paper keywords: wearable devices; physiological measurements; data variability; physiological monitoring citation: gloria cosoli, angelica poli, susanna spinsante, lorenzo scalise, the importance of physiological data variability in wearable devices for digital health applications, acta imeko, vol. 11, no. 2, article 25, citation: gloria cosoli, angelica poli, susanna spinsante, lorenzo scalise, the importance of physiological data variability in wearable devices for digital health applications, acta imeko, vol. 11, no. 2, article 25, june 2022, identifier: imeko-acta11 (2022)-02-25 section editor: francesco lamonaca, university of calabria, italy received july 13, 2021; in final form march 21, 2022; published june 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: gloria cosoli, e-mail: g.cosoli@staff.univpm.it 1. introduction the use of wearable devices is constantly spreading all over the world, thanks to their wide accessibility and high ease of use [1] (even if further actions and improvements are still needed to overcome barriers for a larger adoption by older adults [2]). nowadays a continuously growing number of people wear a smartwatch monitoring a plethora of physiological parameters: heart rate (hr) [3], energy expenditure (ee) [4], blood volume pulse signal (bvp) [5], electrodermal activity (eda) [6], acceleration signal [7], sleep quality [8], respiration rate [9], stressrelated indices [10], etc. these measurements can be useful for different purposes, from cardiovascular monitoring [11] to sleep tracking [12], through activity assessment [13], fitness-oriented applications [14] and blood pressure observation [15], just to cite some. furthermore, in the recent months wearable devices have expanded their application also to the possible detection of early symptoms related to sars-cov-2 pandemic [16], since this virus has stressed the importance of remote monitoring both to limit contagion and for “testing, tracking and tracing” strategies [17]. however, there are also critical aspects that should be thoroughly considered, pertaining to health-related data privacy issues and measurement accuracy of these innovative wearable instruments [5], which undoubtedly play important roles in the era of personalized medicine and digital health [18], [19]. physiological signals can be collected through wearable devices 24 hours a day, 7 days a week, producing big amounts of data, which are analysed through artificial intelligence (ai) algorithms more and more frequently, in order to provide useful information for the so-called decision-making processes [20], abstract this paper aims at characterizing the variability of physiological data collected through a wearable device (empatica e4), given that both intraand inter-subject variability play a pivotal role in digital health applications, where artificial intelligence (ai) techniques have become popular. inter-beat intervals (ibis), electrodermal activity (eda) and skin temperature (skt) signals have been considered and variability has been evaluated in terms of general statistics (mean and standard deviation) and coefficient of variation. results show that both intraand inter-subject variability values are significant, especially when considering those parameters describing how the signals vary over time. moreover, eda seems to be the signal characterized by the highest variability, followed by ibis, contrary to skt that results more stable. this variability could affect ai algorithms in classifying signals according to particular discriminants (e.g. emotions, daily activities, etc.), taking into account the dual role of variability: hindering a net distinction between classes, but also making algorithms more robust for deep learning purposes thanks to the consideration of a wide test population. indeed, it is worthy to note that variability plays a fundamental role in the whole measurement chain, characterizing data reliability and impacting on the final results accuracy and consequently on decision-making processes. mailto:g.cosoli@staff.univpm.it acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 2 [21], thus supporting human choices in different fields, from industry 4.0 [22], [23] to ehealth [24]. the purposes can be different: emotion classification [25], activity recognition [26], hypertension management [27], fall detection [28], smart living environments and well-being assessment [29], and so on. in order to be able to develop robust models, capable to provide reliable information, data quality is fundamental [30]; in this perspective, not only hardware and acquisition options (e.g. sampling frequency, signal-to-noise ratio (snr), resolution, etc.) have a big impact, but also data variability, linked both to different sources to collect data [31], but also to the physiological variability itself. indeed, the classification performance of ai algorithms surely depends on the variability observed in the data collected on the test population: if it is true that (physiological) variability somehow hinders a perfect discrimination among classes, on the other hand it is necessary to test a wide population in order to include its variability and avoid overfitting issues. these aspects should be thoroughly considered when developing ai algorithms for digital health applications, which cannot neglect physiological variability characterising the involved population, and consequently the measured data. the study reported in this manuscript aims at evaluating the intraand inter-subject variability of different physiological signals collected through a wearable wrist-worn device (empatica e4). in particular, the authors have analysed cardiacrelated parameters (i.e. heart rate variability – hrv – parameters computed on the bvp signal measured through a photoplethysmographic – ppg – sensor), features computed on eda signal and skin temperature (skt) values. mean, standard deviation and coefficient of variation have been computed for each extracted parameter, considering the repeated tests on a same subject to evaluate intra-subject variability, and the whole acquired data for inter-subject variability. the rest of the paper is organized as follows: section 2 describes the materials and methods employed for data acquisitions and for the evaluation of data variability, section 3 reports the intraand inter-subject variability results, and finally in section 4 the authors provide their considerations and conclusions. 2. materials and methods 2.1. participants the study was conducted on 10 healthy volunteers: 3 males, 7 females; age of (33 ± 16) years with a range of (15 59) years; height of (169.78 ± 8.83) cm; weight of (66.55 ± 12.00) kg; bmi of (22.92 ± 2.14) kg⁄m2 – data are reported as mean ± standard deviation. they declared they did not take any medication in the 24 hours preceding the tests, nor had particular clinical histories possibly influencing the results. before starting the tests, each participant was informed on the test purpose and procedure and signed an informed consent according to the european regulation 2016/679, i.e., the general data protection regulation (gdpr) to obtain the permission for processing personal data. 2.2. data collection in order to assess the inter-subject and intra-subject variability of physiological parameters, each subject repeated the acquisitions six times, for a total of 60 recordings, each lasting 5 minutes. ambient temperature and relative humidity were equal to (20 ± 2) °c and (50 ± 5) %, respectively, to be perceived as comfortable by most of the involved individuals. the participants (with a skin colour classification of type ii – fitzpatrick scale), laying comfortably in a supine position (i.e., in rest condition) in a quiet room, were instructed to relax as much as possible, breathe normally, and not talk during recordings, in order to minimize movement artifacts. as shown in figure 1, the physiological signals were simultaneously collected through a multisensory wearable device, namely empatica e4 [32], placed on the dominant wrist. this acquisition device was chosen as it provides the raw data, thus resulting particularly suitable for research purposes. firstly, the participants were allowed to adjust the device positioning to increase the comfort feeling. then, the device placement was verified to ensure the optimal skin contact (not worn too tightly or too loosely), and consequently to guarantee the optimal conditions for reliable ppg sensor acquisition [33] and, therefore, as high as possible data quality. 2.3. data acquisition device individual physiological signals were recorded with the multimodal device empatica e4 (class iia medical device according to the 93/42/eec directive) – firmware version: fw 3.1.0.7124. such a device captures the inter-beat-interval (ibi), bvp, eda, human skt, and 3-axis accelerometer signals. in particular, bvp and ibi signals, both sampled at 64 hz with a resolution of 0.9 nw/digit, are derived from the ppg sensor. on the bottom of the wristband, there are two green light emitting diodes (leds) enabling the measurements of blood volume changes and heartbeats, and two red leds for reducing the motion artifacts. additionally, two units of photodiodes (total 14 mm2 sensitive area) measure the reflected light. on the bracelet band of empatica e4, two ag/agcl electrodes allow to pass a small amount of alternating current (frequency 8 hz, with a maximum peak-to-peak value of 100 µa) for measuring the skin conductance in µs, sampled at 4 hz with a resolution of 900 ps in the range of [0.01, 100] µs. at the same sampling frequency (4 hz), an infrared thermopile, placed on the back of the case, records the skt data in °c with an accuracy of ± 0.20 °c (within the range 36 °c 39 °c), and a resolution of 0.02 °c. calibration is valid in the range [-40, 115] °c. the last sensor is a 3-axial mems accelerometer used to collect the acceleration along the three dimensions x, y, z with a 32 hz sampling frequency and a default measurement range of ± 2 g. in this case the resolution of the output signal is 0.015 g (8 bit). a dedicated mobile application (e4 realtime) was used to stream and view data in real-time on a mobile device connected with empatica e4 via bluetooth low energy (ble). following each measurement session, data were automatically transferred to a cloud repository (empatica connect) to view, manage, and download raw data in .csv format in the post-processing phase of the study. figure 1. measurement setup. acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 3 2.4. data analysis as mentioned above, in this study the data variability analysis was conducted on hrv (or, more precisely, on pulse rate variability [34], [35]), eda and skt signals, previously processed in matlab environment in order to extract relevant features. regarding the hrv evaluation, after applying a previously developed artifact correction method [36], the analysis was performed on ibis signals by using the kubios toolbox [37]. seven meaningful hrv-related parameters were extracted from the corrected ibis signals in time domain (table 1), namely: mean and standard deviation of ibis; mean, standard deviation, minimum and maximum values of hr; root mean square of successive rr interval differences. frequency domain parameters were not considered in the present work, to limit the number of parameters extracted from the same signal, and also because the parameters in frequency domain can be strongly affected by spurious components linked to movement artifacts, to which wrist-worn wearable devices are prone [38], even more during intense physical activities [39]. concerning eda data, the bio-sp toolbox [40] was used to pre-process the signals and to extract all the features that the toolbox permits to compute. indeed, eda signal is composed by the superimposition of two components, specifically skin conductance response (scr) and skin conductance level (scl), related to the fast response to external stimuli events and the slow changes in baseline levels, respectively. this means that the scl depends on the individual characteristics (e.g. skin condition), and can differ markedly between individuals. consequently, under rest condition with no external stimuli, the scl has a higher impact than the scr component on both eda signal trend and amplitude. according to the literature [41], a gaussian low-pass filter, with a 40-point window and a sigma of 400 ms, was applied to reduce noise and motion artifacts due to potential subject’s wrist movements. in order to characterize the eda signal, the following five features were computed within bio-sp toolbox in time domain (table 1): scr mean duration, scr mean amplitude, scr mean rise-time, eda mean signal, number of scrs. finally, since an inspection of the skt data revealed slight and slow °c changes at rest, no filters were applied. therefore, from the raw skt signal the following parameters were extracted (table 1): mean and standard deviation, minimum and maximum of skin temperature. once the whole set of features was computed and extracted from the considered signals, both intraand inter-subject variability was evaluated for each metric. more specifically, data variability was estimated by computing the mean (𝜇), standard deviation (𝜎) and coefficient of variation (𝑐v = 𝜎 𝜇⁄ ) for all the extracted features. furthermore, the normality of the parameters distributions was verified by means of shapiro-wilk test [42] (null hypothesis: the test population is normally distributed; pvalue ≤ 0.05 considered as statistically significant). 3. results in this section, results are reported by grouping them according to data type: cardiac related parameters (i.e. hrv analysis parameters, subsection 3.1), eda-related parameters (subsection 3.2) and skin temperature parameters (subsection 3.3). results are reported in tables as 𝜇 ± 𝜎 (𝑐v); some examples of mean distributions are also shown by using the histogram representation. 3.1. hrv parameters the authors analysed the variability of hrv signal at parameters level, focusing on those extracted in time domain. the shapiro-wilk test evidenced that rr_mean, hr_mean, hr_min and hr_max can be considered as normally distributed (p-value ≥ 0.05). an example of the distribution is reported in the histogram (figure 2) related to rr_mean parameter. for the others (i.e. rr_std and rmssd), the null hypothesis cannot be rejected; the reason could be found in the limited numerosity of the test population (60 recordings on 6 subjects). similarly, the hr_std resulted to have non-normal distribution, probably also due to the presence of one outlier subject (i.e. subject no. 6, see table 2). observing the variability results in table 2, it is possible to notice a very high variability, in particular for the parameters table 1. time-domain features extracted from the physiological signals acquired in the tests. signal features measurement unit description hrv rr_mean ms mean value of inter-beat intervals rr_std ms standard deviation of inter-beat intervals hr_mean bpm mean value of heart rate hr_std bpm standard deviation of heart rate hr_min bpm minimum value of heart rate hr_max bpm maximum value of heart rate rrmsd ms root mean square of successive inter-beat intervals eda scr_d_mean s mean duration of skin conductance response signal scr_a_mean µs mean amplitude of skin conductance response signal scr_rt_mean s mean rise time of skin conductance response signal eda_mean µs mean value of eda signal scr_n no. of skin conductance response peaks skt skt_mean °c mean value of skin temperature skt_std °c standard deviation of skin temperature skt_min °c minimum value of skin temperature skt_max °c maximum value of skin temperature acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 4 describing how the measurement oscillates around its mean value, i.e. the standard deviation values of rr, hr and rmssd, reporting inter-subject variabilities of 55.8 %, 126.9 % and 65.7 %, respectively. this seems to underline the physiological variability, hence a subject’s condition of interest cannot be described (and classified) without properly considering such data variability. a particular remark should be made on the extremely high inter-variability of hr_std parameter; indeed, this could be linked to the subject no. 6 reporting an extremely high variability (i.e., 125.1 %), as already mentioned above. indeed, by performing a visual inspection of data collected on subject no. 6, among the tests conducted, one measurement on this subject resulted particularly noisy, hindering a reliable hrv analysis despite the use of proper artifact correction methods in the preprocessing phase. however, if this test is discarded from the variability analysis, the intra-variability of hr_std reduces from (12 ± 15) bpm (125.1 %) to (6 ± 3) bpm (50.0 %) – while the remaining parameters do not vary substantially; in this way, the inter-subject variability related to hr_std parameter would be (4 ± 3) bpm (81.1 %). the observed noise, which quite often characterises signals acquired through ppg sensors of wearable devices, could be an effect of subjects’ wrist movements [38]. intra-subject variability shows similar results, evidencing a very high variability, especially for the standard deviation parameters, describing the variations over time. on the other hand, mean value parameters show a quite low intra-subject variability, with values often lower than 10 % (e.g. for rr_mean parameter, lower than 10 % with the exception of 3 subjects out of 10). 3.2. eda parameters as stated above, in rest conditions scl is the predominant component of eda signal; this can result in very low intensity signals related to scr component (more linked to eventual stimuli), and consequently the eda_mean parameter values are expected to be low. in fact, in table 3, eda_mean parameters show very low values, up to 0.0005 μs for the subject no. 9. such very low mean values, together with high signal variability (i.e. high standard deviation), result in extremely high coefficients of variation (see for example subjects no. 3 and 9, where 𝑐v is extremely high due to the fact that the mean value of signal is an order of magnitude lower than its standard deviation). more in general, the parameters related to the eda signals show a very high variability, with coefficient of variation values related to inter-subject variability often over 100 %. also, intrasubject variability seems to be extremely high, evidencing that eda signal is not stable over time, hence it should be considered in this long-term evolution, instead of limiting to use descriptive statistics. such a high variability could be attributable to the fact that eda measurements at total rest, with no external stimuli, seem to be quite complicated, especially when performed by means of wearable devices. in fact, there are multiple subjective causes influencing the measurement results. furthermore, it should be considered that wrist eda results to be quite different from standard finger eda [43]. regarding the type of distribution, no features extracted from eda can be considered as normally distributed. the reason could be attributed again to the restricted test population. an example of distribution is reported in the histogram (figure 3) for eda_mean parameter. 3.3. skin temperature parameters contrarily to the previously reported parameters, skin temperature (table 4) shows measures slowly varying over time (with the exception of the standard deviation value, evidencing a higher variability – up to 87.1 % in intra-subject results), hence providing a more precise footprint of a subject in a determined condition. on the other hand, this could mean that the wrist skin temperature has a slow dynamic, thus it could be not suitable to rapidly mirror possible changes in the subject’s psycho-physical table 2. variability of hrv parameters in time domain. results are reported as µ ± σ (cv). subject rr_mean in ms rr_std in ms hr_mean in bpm hr_std in bpm hr_min in bpm hr_max in bpm rmssd in ms 1 1044 ± 66 (6.3 %) 70 ± 43 (61.0 %) 58 ± 4 (6.5 %) 4 ± 2 (49.9 %) 50 ± 4 (8.8 %) 65 ± 6 (8.7 %) 94 ± 64 (68.6 %) 2 1152 ± 30 (2.6 %) 36 ± 12 (33.7 %) 52 ± 1 (2.5 %) 2 ± 1 (48.5 %) 49 ± 2 (3.3 %) 58 ± 4 (6.1 %) 46 ± 16 (33.7 %) 3 934 ± 44 (4.7 %) 40 ± 13 (33.6 %) 64 ± 3 (4.8 %) 3 ± 1 (39.9 %) 59 ± 5 (7.8 %) 76 ± 5 (6.1 %) 49 ± 18 (36.8 %) 4 1008 ± 80 (8.0 %) 83 ± 51 (61.1 %) 60 ± 5 (8.1 %) 6 ± 5 (87.3 %) 49 ± 7 (13.6 %) 70 ± 5 (7.8 %) 114 ± 69 (60.1 %) 5 1027 ± 80 (7.8 %) 39 ± 20 (51.9 %) 59 ± 5 (8.0 %) 3 ± 2 (72.0 %) 53 ± 3 (5.0 %) 64 ± 6 (9.7 %) 54 ± 30 (56.1 %) 6 991 ± 133 (13.5 %) 72 ± 26 (35.5 %) 62 ± 9 (14.2 %) 12 ± 15 (125.1 %)* 52 ± 8 (15.1 %) 74 ± 12 (16.5 %) 97 ± 42 (43.2 %) 7 938 ± 31 (3.4 %) 68 ± 28 (40.5 %) 64 ± 2 (3.4 %) 6 ± 5 (81.3 %) 55 ± 5 (9.1 %) 74 ± 4 (4.8 %) 90 ± 44 (48.9 %) 8 873 ± 44 (5.0 %) 48 ± 2 (4.9 %) 69 ± 3 (5.0 %) 4 ± 1 (10.4 %) 61 ± 3 (4.2 %) 79 ± 6 (6.9 %) 45 ± 2 (4.7 %) 9 1083 ± 139 (12.9 %) 31 ± 7 (23.7 %) 56 ± 7 (12.8 %) 2 ± 1 (24.8 %) 53 ± 7 (12.8 %) 61 ± 7 (11.5 %) 39 ± 7 (18.6 %) 10 885 ± 130 (14.7 %) 49 ± 19 (39.6 %) 69 ± 10 (14.7 %) 4 ± 1 (16.9 %) 62 ± 9 (15.3 %) 83 ± 7 (8.7 %) 43 ± 66 (39.0 %) tot. 993 ± 117 (11.8 %) 54 ± 30 (55.8 %) 61 ± 7 (12.0 %) 4 ± 6 (126.9 %)* 54 ± 7 (12.9 %) 70 ± 10 (14.2 %) 67 ± 4 (65.7 %) * results affected by a particularly noisy test performed on subject no.6 figure 2. histogram related to rr_mean parameter (hrv signal). acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 5 conditions. none of the parameters extracted from skt signal can be considered normally distributed, according to the shapiro-wilk test. the reason could be the same indicated for the other signal (i.e. narrow test population). an example is reported in the histogram (figure 4) for skt_mean (where the distribution skewness is markedly < 0). 4. discussion and conclusions the use of wearable devices in a growing number of application fields emphasizes the need of considering the metrological aspects determining the reliability of measurement results. in recent years, ai algorithms have known unprecedent developments, providing extremely powerful tools to support decision-making processes and thus prevent serious health-issues in a variety of digital health applications, among which affective states classification, ehealth, smart living environments and ambient assisted living. in order to obtain good performances from ai algorithms, data accuracy and data quality are of uttermost importance, along with data variability that undoubtedly represents a key factor in this scenario. furthermore, only a part of variability can be minimised (e.g. by correcting the sensor positioning in the data acquisition phase), but another part is inevitable and uncontrollable, given that there is a physiological variability, whose values cannot be disregarded. it is a matter of fact that all the steps of the measurement chain influence the final results of ai algorithms: from the sensors uncertainties to the data variability and accuracy, all influencing the reliability of the output information. in a real-life context, this contributes to reveal a corrupted output with a poor information quality, which could be used for different final purposes (e.g. support to decision-making processes in digital health scenarios) [29]. the results obtained in this study have highlighted the physiological data variability among different subjects and intrasubject, considering data acquired by means of a wearable device. in particular, hrv and eda signals have been firstly analysed, observing that hrv parameters in time domain exhibit higher inter-subject variability when considering measures describing their variations over time (i.e. standard deviation values), with respect to average values, which seem more stable. furthermore, eda signals appear to be extremely changeable even in a same subject, evidencing the intrinsic variable nature of this type of data. indeed, this type of signal is referred to wrist skin conductance, instead of finger one, which is the site generally used for standard measurements. previous studies underlined that the measurement is not reliable if compared to finger/palm skin conductivity [44]; in fact, thermoregulatory processes would affect the results more than psychophysiological phenomena, which on the contrary are more influencing in the standard measurement sites [45]. on the other hand, other types of physiological data, such as skt, can show a quite limited variability, resulting more stable than hrv and eda. however, the slow changes could be table 3. variability of eda parameters. results are reported as µ ± σ (cv). subject scr_d_mean in s scr_a_mean in μs scr_rt_mean in s eda_mean in μs scr_n 1 10.2 ± 5.0 (49.4 %) 0.0097 ± 0.0034 (34.8 %) 5.3 ± 2.6 (48.7 %) 0.0022 ± 0.0032 (144.5 %) 23 ± 7 (30.5 %) 2 26.1 ± 26.1 (99.8 %) 0.0138 ± 0.0056 (72.7 %) 13.1 ± 10.6 (80.3 %) 0.0030 ± 0.0052 (177.1 %) 14 ± 10 (71.2 %) 3 8.3 ± 5.4 (65.0 %) 0.0095 ± 0.0056 (59.3 %) 4.3 ± 2.9 (68.6 %) 0.0010 ± 0.019 (1980.1 %) 12 ± 9 (77.7 %) 4 12.4 ± 17.5 (140.9 %) 0.0098 ± 0.0079 (80.8 %) 9.4 ± 15.8 (167.9 %) 0.0078 ± 0.010 (133.0 %) 21 ± 16 (76.6 %) 5 18.9 ± 19.2 (101.7 %) 0.0115 ± 0.0044 (38.1 %) 5.2 ± 3.4 (65.5 %) 0.0027 ± 0.0044 (131.4 %) 22 ± 13 (59.5 %) 6 15.2 ± 10.0 (65.6 %) 0.0112 ± 0.0051 (45.6 %) 9.3 ± 7.4 (79.5 %) 0.0043 ± 0.0051 (50.1 %) 18 ± 7 (41.0 %) 7 14.5 ± 14.4 (99.2 %) 0.0105 ± 0.0090 (85.7 %) 10.6 ± 12.2 (115.1 %) 0.0066 ± 0.0090 (97.4 %) 16 ± 14 (85.5 %) 8 5.7 ± 1.3 (22.5 %) 0.0257 ± 0.0160 (62.7 %) 3.1 ± 0.7 (23.1 %) 0.0029 ± 0.0160 (126.9 %) 23 ± 9 (38.0 %) 9 4.9 ± 0.9 (18.1 %) 0.0116 ± 0.0061 (52.9 %) 2.7 ± 0.5 (18.4 %) 0.0005 ± 0.0061 (862.7 %) 30 ± 9 (31.3 %) 10 5.6 ± 1.2 (20.5 %) 0.0144 ± 0.0065 (45.6 %) 2.9 ± 0.6 (19.3 %) 0.0029 ± 0.0065 (137.9 %) 30 ± 8 (27.9 %) tot. 12.2 ± 13.7 (112.3 %) 0.0128 ± 0.0089 (69.3 %) 6.6 ± 7.9 (120.0 %) 0.0034 ± 0.0076 (224.3 %) 21 ± 11(54.6 %) figure 3. histogram related to eda_mean parameter (eda signal). figure 4. histogram related to skt_mean parameter (skt signal). acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 6 problematic in following, for example, the subject’s reactions to external stimuli. the observed variability can represent a double sword edge: on one hand, the subjective diversity can hinder a net classification by means of ai algorithms; on the other hand, considering a test population sufficiently wide to include all the characteristic variability is required to develop robust ai algorithms, not suffering from overfitting issues. it is worthy to underline that the test population of this study is quite limited (10 subjects), therefore the normality condition could be non-optimally satisfied (verification through shapirowilk test). it would be interesting to repeat this kind of analysis on some publicly available large-scale databases (e.g. wesad [46], k-emocon [47], tiles [48], etc.), in order to examine the data variability results on wider populations (possibly including also different age groups) and also considering longer acquisition intervals and different measuring devices and acquisition conditions (e.g. free-living conditions, which probably remark variability). additionally, future studies may include one or more ai algorithms to compare the achieved performance on two datasets with different variabilities, for demonstrating the high impact of data variability on ai algorithms outputs, which can consequently impact on decision-making processes. acknowledgement a. p. and s. s. gratefully acknowledge the support of the italian ministry for economic development (mise) in implementation of the financial programme "research and development projects for the implementation of the national smart specilization strategy – “dm mise 5 marzo 2018", project "chaalenge", proposal no. 493, project nr. f/180016/0105/x43. references [1] g. cosoli, s. spinsante, l. scalise, wrist-worn and chest-strap wearable devices: systematic review on accuracy and metrological characteristics, measurement, apr. 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of electrical engineering and information technology, slovak university of technology in bratislava, ilkovicova 3, 812 19 bratislava, slovakia 2 vuez, a. s., hviezdoslavova 35, 934 39 levice, slovakia section: research paper keywords: industrial robot; simulation; digital twin; robotized welding; sql server; hmi; database; 2d scanner; 3d scanner; industrial web; automatized measuring system; industry 4.0 citation: zuzana kovarikova, frantisek duchon, andrej babinec, dusan labat, digital tools as part of a robotic system for adaptive manipulation and welding of objects, acta imeko, vol. 11, no. 3, article 11, september 2022, identifier: imeko-acta-11 (2022)-03-11 section editor: zafar taqvi, usa received march 26, 2022; in final form july 22, 2022; published september 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this article was written thanks to the support under the operational program of integrated infrastructure for the following projects: "robotic workplace for intelligent welding of small volume production (izvar)", code itms2014 +:313012p386, and "digitization of robotic welding workplace (diroz)", code itms2014 +:313012s686 (both co-financed by the european regional development fund). corresponding author: zuzana kovarikova, e-mail: kovarikova@vuez.sk 1. introduction traditionally it has been difficult to use automation in small batch production with hight variation in volumes and high mix of products [1]. there is a great potential for small batch producers to use flexible automation in manufacturing operation to remain competitive [1]. to achieve flexibility, it is crucial to design the structure of digital tools so that the greatest possible adaptability is achieved with tools that require a minimum time of conversion of technological units. the interaction between automation controllers and computer-aided design/manufacturing (cad/cam) systems capable of offline programming, is generally a way to decrease production down time due to programming [1]. the technology components modelled in cad tools are an important input for the creation of the digital twin. digital twin (dt) is the technical core for establishing cyberphysical production system (cpps) in the context of industry 4.0 [2]. the importance of digital twin (dt), which is characterized by the cyber–physical integration, is increasingly emphasized by both academia and industry [3]. through data modelling, data are stored according to certain criteria and logic, which can facilitate data processing [3]. theories of service modelling are useful for the identification, analysis, and upgrade of services [3]. simulation theories are useful for operation analysis (e.g., structural strength analysis and kinetic analysis) in a simulation environment [3]. abstract the aim of this article is to describe the design and verification of digital tools usable for sharing information within a team of workers and machines that manage and execute production carried out by a robotic system. the basic method is to define the structure of the digital tool and data flows necessary to enable an exchange of data needed to perform robotic manipulation and robotic welding of variable products minimizing at the same time strenuous human activity. the proposed structure of data interconnects a set of intelligent sensors with control of 18 degrees of freedom of 3 robotic manipulators, a welding device, and a production information system. part of the work was also to verify the functionality of the proposed structure of the digital tools. in the first phase, simulations using a digital twin prototype of the workplace for robotic manipulation and robotic welding were performed to verify the functionality the digital tools. subsequently, a digital tool was tested in the environment of a real prototype workplace for robotic manipulation and robotic welding. simulation results and data obtained from the prototype tests proved the functionality of the digital tool inclusive of the production information system. https://www.fei.stuba.sk/ mailto:kovarikova@vuez.sk acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 2 digital twins are more than just pure data, they include algorithms, which describe their real counterpart and take a decision in the production system. [4]. dts can make a production process more reliable, flexible, and predictable [3]. above all, dts can visualize and update the real-time status, which is useful for monitoring a production process [3]. 2. robotic system for adaptive manipulation and welding of objects the robotic system for adaptive manipulation and welding of objects shown in figure 1 consists of three robots. there are two robots for manipulating the to-be-welded parts and one robot for tungsten inert gas welding. the welding robot can scan the welding gap and the surface parameters of the final weld using a 2d laser scanner installed on the robot body. there are also two warehouses of parts equipped with 3d laser scanners, each of them installed above the concerned warehouse. the 3d laser scanners give feedback to the control system of robotically positioned to-be-welded parts. the source of energy needed for welding in the robotic system for adaptive manipulation and welding of objects is a robotic welding machine. the robotic welder is equipped with a digital interface enabling to set its parameters from the central control system by digital communication. adaptation of the workplace for handling of parts and products of various shapes is enabled by a quick-change system of robotic grippers. each robotic manipulator has one robotic gripper stand with six positions for setting up effectors of different types. the robotic system for adaptive manipulation and welding of objects also includes an automated system for measuring process quantities which is connected to the production and quality information system by digital communication. coordination of workplace components is ensured by a central control system which is a mediator and provider of process variables scanned at the workplace. the process variables are provided to the human-machine interface and the sql database where digital data are registered and archived. power and media distribution systems provide operating power distribution, air distribution for operation of grippers and inert gas distribution to create a protective atmosphere. 3. digital tools and flow of digital data the diagram in figure 2 shows the digital tools and the flow of digital data. an important source as well as consumer of the data is the prototype of the workplace itself. the data of the figure 1. robotic system for adaptive manipulation and welding of objects. figure 2. digital tools and data flow in prototype of the robotic system for adaptive manipulation and welding of objects. acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 3 robotized process is read, and the required parameters are set using a programmable logic controller. the programmable logic controller is a central control system of the robotic system for adaptive manipulation and welding of objects. 3.1. robot controllers there are three robot controllers in the prototype of the robotic system for adaptive manipulation and welding of objects. two controllers control robots for manipulation of to-be-welded objects, one of them also manipulation of products after welding operation. one robot controller controls trajectory of the tool centre point, i.e. tungsten electrode tip. 3.2. robotic removal of to-be-welded parts from 3d laser scanners robotic positioning of the to-be-welded parts removed from the parts warehouse to a position close to the welding position is performed automatically based on the feedback from 3d laser scanners. each robotic manipulator has one 3d laser scanner able to obtain point cloud data characterizing the state of the warehouse of the stored parts before every single removal. this automatic robotic manipulation process is shown in figure 3. 3.3. 2d laser measurement system the welding robot is equipped with a 2d laser sensor that reads data needed to evaluate the geometry of the weld gap. the measured data are used to correct the weld gap, to generate the welding trajectory, and to automate the quality control of the performed welds. 3.4. ftp server the ftp server contains data in the file format. cad data corresponding to the design of the prototype workplace (including the design of variable robotic grippers and support constructions for setting up parts in warehouses) is stored in this data repository. data on the required design of the positioned parts, to-be-welded parts, and welded parts defining the desired product are also stored here. the ftp server contains also robotic system simulation files for adaptive manipulation and welding of objects as well as thermodynamic simulation files. in addition, the ftp server stores files with measured data obtained from the 2d laser scanner, thermovision camera measurements as well as historical trends from an automated system for process measurements, and photo documentation. 3.5. sql server the database of the robotic system for adaptive manipulation and welding of objects is implemented and operated in the ms sql server environment. the database contains records of robotic welds, data on required technological parameters of robotic welding and measured data of process variables. 3.6. web server the web server of the robotic system for adaptive manipulation and welding of objects provides data from the database in ms sql through web pages in the form of tables and behaviour of selected quantities in graphical form. it also allows the welding technologist to enter the required values of welding parameters to optimize robotic welding. 3.7. human-machine interface the human-machine interface (figure 4) for controlling and monitoring the state of the robotic system for adaptive manipulation and welding of objects provides windows for setting the robotic welding parameters and for monitoring the robotic welding process, a control panel for controlling the prototype workplace, and tools for viewing measured process variables in both tabular and graphical forms. the humanmachine interface is installed on operator panels and computers with visualization immediately next to the prototype workplace. 3.8. quality control the quality control of the final product is implemented in two ways. the first one is an automated weld inspection performed immediately after welding by robots using the 2d laser scanner installed on the welding robot. automated quality control data is recorded and archived automatically via measurement data files. the second one is a manual quality control of the final product performed by a person who enters the control results into preprepared protocols archived in the ftp server. 4. simulation of prototype robotic system for adaptive manipulation and welding of objects simulation of both the robotic positioning of parts and the robotic welding is preceded by testing of the feasibility to obtain a quality product in the prototype. for this simulation, a digital twin of the robotic system for adaptive manipulation and welding of objects is used. figure 3. robotic removal of to-be-welded parts from 3d laser scanners. figure 4. human-machine interface of the robotic system for adaptive manipulation and welding of objects. acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 4 the advantage of verifying the design by means of a digital twin consists in the possibility of tuning a correct synchronization of the movements of three robotic manipulators which operate in a common working space during robotic positioning and welding of the product. in this way, it is possible to prevent potential collision events and to optimize the cycle time of welding and handling processes. in the digital twin, it is also possible to verify correctness of the design of robotic grippers. when simulating the robotic manipulation of a given part, it is necessary to take into account its geometry and weight. in the digital twin, dynamic events that occur during the positioning of welded parts can also be simulated. figure 5 shows the verification of the automated robotic grip of a selected robotic gripper from a quick-change robotic gripper system. attachments of robotic grippers of various types can be verified by simulation via a digital twin which contains a quickchange system of robot effectors. before testing the robotic welding in the prototype workplace, a thermodynamic simulation is performed simulating the propagation of heat from the weld site into the welded body. a render of the thermodynamic simulation is shown in figure 6. thermodynamic simulations are compared with temperature data measured by a thermovision camera during tests of welding in the prototype workplace and help to optimize preparation of specifications of robotic tig welding parameters. 5. simulation and testing outputs the simulation of the process of robotic positioning and robotic welding for both fillet welds and butt welds was verified by the simulation. the design of the robotic positioning of the final product into the robotic ultrasound diagnostic workplace was also verified by means of a digital twin. figure 7 shows robotic manipulation of two to-be-welded flat sheets (up) and robotic welding during performing a fillet weld of them (down). with the same robotic fingers, the sheets were robotically positioned and held also when performing a butt weld as shown in figure 8. figure 5. simulation of changing robot’s grippers in digital twin. figure 6. thermodynamic simulation of tig-welding two tubes together. figure 7. up: robotic manipulation of two to-be-welded flat sheets simulation. down: robotic holding of two to-be-welded flat sheets simulation. figure 8. robotic positioning of to-be-welded parts in a prototype robotic workplace. position before weld gap correction from data obtain by 2d laser scanner. acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 5 testing of robotic positioning of to-be-welded parts in a prototype robotic workplace showed sliding of the sheets in this type of fingers, which had a negative impact on the feasibility and quality of the weld. for this reason, a different type of fingers was designed, as shown in figure 9. during the design of these fingers, the ansys calculation program was used in which the design was optimized so that the finger was as light as possible and at the same time sufficiently strong. figure 9 on the left shows the designed robotic finger before performing the optimization. the finger after optimization is shown on the right. the described robotic system was designed for adaptive manipulation and welding. thus, with the help of robotic manipulators, it is possible to position and hold objects of different sizes and shapes during robotic welding. the designs of all considered scenarios of robotic positioning are verified in advance by the digital twin of the workplace of robotic manipulation and robotic welding. figure 10 shows the output of the simulation of robotic manipulation and robotic welding of cylindrical objects by realization of butt welding. the upper part shows a simulation of the simultaneous positioning of three robotic manipulators during the welding. after welding, the welding robot is repositioned to its home position by the central control system. the first robotic manipulator is instructed to open the gripper that held the part during the welding process. subsequently, with the second robotic manipulator, the final product is robotically positioned in the output warehouse. the output of the robotic positioning simulation of the final product by the second robotic manipulator is shown in figure 10 below. after obtaining suitable robotic trajectories of both robotic manipulators for positioning and holding to-be-welded parts as well as trajectories of the welding robot, the parameters obtained in the digital twin technology were verified in a prototype robotic positioning and welding workplace. photographs from the testing of the robotic process in the workplace prototype are shown in figure 11. the upper part shows the robotic positioning of to-be-welded parts and the automated measurement of the weld gap by a laser scanner before its correction. after automatic correction of the weld gap, figure 9. design of the robotic fingers. left – before optimization in ansys. right – after optimization in ansys. figure 10. up: robotic holding during welding of two to-be-welded cylindrical objects simulation. below: robotic manipulation of welded cylindrical object simulation. figure 11. cylindrical objects before their automatic weld-gap correction and during robotized welding in the workspace. acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 6 synchronized positioning of robotic manipulators holding parts and robotic welding is performed as shown in figure 11 below. the robotic workplace of handling and welding is followed by the workplace of robotic ultrasonic inspection for proving the quality of welds by a non-destructive method. one of the robotic manipulators of the welding workplace is used for positioning of the welded and cooled products in the workplace of robotic ultrasound diagnostics as it is shown in figure 12. positioning processes are synchronized with each other. after performing robotic ultrasound diagnostics, the tested product is again positioned by a robotic manipulator and placed in a warehouse of quality products or failures, depending on the results of the implemented non-destructive inspection. figure 13 shows robotic manipulation of the to-be-tested object in real workspace. 6. testing the functionality of prototype digital data tools testing of robotic manipulation and robotic welding as well as testing of functionality of digital tools as an effective means of data exchange between the workplace and the workplace operators, welding technologists, quality control workers, designers, robot programmers, and the central control system and the production management was carried out in the prototype figure 12. robotic manipulation of the to-be-ultrasonic-robotic-diagnostictesting objects in digital twin. figure 13. robotic manipulation of the tested objects in the robotic ultrasonic diagnostic workspace. figure 14. testing of robotic manipulation and robotic welding in prototype of the robotic system for adaptive manipulation and welding of objects. acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 7 workplace of the robotic system for adaptive manipulation and welding of objects. the prototype of the robotic system for adaptive manipulation and welding of objects is shown in figure 14. in the prototype of the robotic system for adaptive manipulation and welding of objects, several welded objects of different sizes and shapes were tested. one of them is shown in figure 15. using a digital twin, it was possible to simulate robotic positioning and welding using a digital twin of the workspace. digital tools as part of the robotic system for adaptive manipulation and welding of objects allow to automatically record data about each tested sample and store them in an sql database. from the database, measured values of process variables can be displayed directly at the prototype workplace on the hmi system monitor. at the same time, these measured values are available to the welding technologist to optimize future robotic welding procedures as well as to the quality control staff via a web interface in both tabular and graphical forms. measured values of welding voltage and welding current from the robotic welding of the test sample in a graphical form are shown in figure 16. the graph is drawn from web application of the prototype. 7. conclusion the research presented in the article shows that digital tools as part of a robotic system for adaptive manipulation and welding of objects can be effectively used in modelling or design verification by simulations through a digital twin prototype of the robotic workplace. the exchange of digital data takes place between the components of the prototype consisting of one welding robot, two handling robots equipped with a quickchange robotic gripper system, 3d scanners, a 2d scanner, an automated process variable measurement system, a sql database, a ftp server, and a web portal. implementation of the figure 15. one type of welded samples. figure 16. graph of measured values of welding voltage (blue) ang welding current (violet) for testing sample in figure 15. acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 8 digital tools into the prototype enables to adapt the workplace for production of various products of different shapes and sizes. in this prototype, the web portal allows comfortable entering and mining of data characterizing the process of robotic manipulation and robotic welding. acknowledgement this article was written thanks to the support under the operational program of integrated infrastructure for the following projects: "robotic workplace for intelligent welding of small volume production (izvar)", code itms2014 +:313012p386, and "digitization of robotic welding workplace (diroz)", code itms2014 +:313012s686 (both co-financed by the european regional development fund). references [1] m. lofving, p. almstrom, c. jarebrant, b. wadman, m. widfeldt, evaluation of flexible automation for small batch production, sciencedirect, 2018, pp. 177-184. doi: 10.1016/j.promfg.2018.06.072 [2] ch. liu, p. jiang, w. jiang, web-based digital twin modeling and remote control of cyber-physical production systems, robotics and computer integrated manufacturing, 2020, pp. 1-16. doi: 10.1016/j.rcim.2020.101956 [3] f. tao, h. zhang, a. liu, a.y.c. nee, digital twin in industry: state-of-the-art, ieee transactions on industrial informatics, vol. 15, no 4, april 2019, pp. 2405-2415. doi: 10.1109/tii.2018.2873186 [4] r. ferro, h. sajjad, r. e. c. ordonez, steps for data exchange between real environment and virtual simulation environment, iccms ’21, 25-27 june 2021, melbourne, vic, australia, isbn 978-1-4503-8979-2/21/06. doi: 10.1145/3474963.3474988 https://doi.org/10.1016/j.promfg.2018.06.072 https://doi.org/10.1016/j.rcim.2020.101956 https://doi.org/10.1109/tii.2018.2873186 https://doi.org/10.1145/3474963.3474988 introductory notes for the acta imeko special issue on ‘innovative signal processing and communication techniques for measurement and sensing systems’ acta imeko issn: 2221-870x march 2022, volume 11, number 1, 1 3 acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 1 introductory notes for the acta imeko special issue on ‘innovative signal processing and communication techniques for measurement and sensing systems’ md. zia ur rahman1 1 department of electronics and communication engineering, koneru lakshmaiah education eoundation, k l university, green fields, guntur-522303, a.p., india section: editorial citation: md. zia ur rahman, introductory notes for the acta imeko special issue on ‘innovative signal processing and communication techniques for measurement and sensing systems’, acta imeko, vol. 11, no. 1, article 3, march 2022, identifier: imeko-acta-11 (2022)-01-03 received march 30, 2022; in final form march 30, 2022; published march 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: md. zia ur rahman, e-mail: mdzr55@gmail.com dear readers, the rapid developments in signal processing and communication techniques enable measurement and sensing systems more accurate and facilitates high resolution sensing data output. this broadens the market prospects and demands, as a result the measurement and sensing systems become a key element in any complex engineering systems. innovations in signal processing and communication methods make the sensing systems fast, intelligent and knowledge-based systems. these innovations enable the measurement and sensing systems higher accuracy, faster detection ability and lower the implementation cost. this has led to the possibility of development of dynamic, smart sensing systems. the scope of this special issue will focus on innovative signal processing and communication techniques for measurement and sensing systems by identifying new perspectives and highlighting potential research issues and challenges. specifically, this special issue will demonstrate how the emerging technologies could be used in future smart sensing systems. the topics of interest in the context of measurement and sensing systems includes: antenna measurements, artificial intelligence, beam forming techniques, body area networks, embedded processors, image sensors and processing, internet of things, knowledge based systems, machine learning algorithms, medical signal analysis, sensor data processing, vlsi architectures and many more thrust areas. this special issue consists of 15 full length papers discussed in the context of measurement and sensing suitable for publishing in acta imeko. in the paper ‘mitigation of spectrum sensing data falsification attack using multilayer perception in cognitive radio networks’ the issue of spectrum scarcity and low spectrum utilization in wireless communication systems is addressed. malicious users receive spectrum sensing data, resulting in inaccurate global decisions about spectrum availability. this work proposes multilayer perception and then measures statistical aspects of incoming signals to identify false data in cooperative spectrum sensing. the paper titled ‘evaluation on effect of alkaline activator on compaction properties of red mud stabilized by ground granulated blast slag’ details about virgin red mud (rm), ground granulated blast slag (ggbs), stabilized rm, and alkaline activated ggbs stabilized rm samples, comprehensive compaction tests were performed. the effect of an alkaline activator on the compaction properties of ggbs stabilized rm is investigated in this research. the compaction curves indicated a substantial difference in maximum dry density and optimal moisture content with the modification of ggbs percentage and varied ratios of naoh to na2sio3 when standard and modified proctor compaction tests were conducted for various combinations of rm and ggbs. the paper ‘multipath routing protocol based on backward routing with optimal fuzzy logic in medical telemetry systems for physiological data measurements,’ by suryadevara kranthi et al., proposes a novel safe multi-way approach for trustworthy data transfer that is dependent on service quality. multi-path routing is also supported by the ad hoc on demand backward routing protocol with optimal fuzzy logic (ofl). by generating rules in ofl and thus choosing an optimal rule, the hybridization of the bat optimization strategy delivers the best route. the final delay, the packet delivery ratio, and other criteria are used to assess the efficiency of the proposed technique. the fourth paper ‘a machine learning based sensing and measurement framework for timing of volcanic eruption and categorization of seismic data’ authored by vijay souri maddila et al., investigates the circumstances and factors that govern the volcanic explosive ejection are unclear, and there is currently no efficient approach mailto:mdzr55@gmail.com acta imeko issn: 2221-870x march 2022, volume 11, number 1, 2 4 acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 2 to predict when a volcanic explosive ejection will terminate. create a decisiveness measure d to analyze the uniformity of the groups supplied by these machine learning models using controlled machine learning approaches such as support vector machine (svm), random forest (rf), logistic regression (lr), and gaussian process classifiers (gpc). the measured end-date derived by seismic information classification for both volcanic systems is two to four months later than the end-dates determined by the earliest instance of visual eruption. the paper ‘fire sm: new dataset for anomaly detection of fire in video surveillance,’ seeks to aid the growth of this particular research by including the fire sm dataset, which is a large and diverse new dataset. furthermore, a precise estimation in early fire detection utilising an indicator, average precision, can yield extra information. in this paper, two existing common methodologies were compared to different anomaly detection methods that give an efficient solution to discover fire incidents. in the paper ‘extended buffer zone algorithm to reduce rerouting time in biotelemetry systems using sensing,’ routing strategies for mobile adhoc networks (manets) with connection breakdowns induced by frequent node movement, measurement, and a dynamic network topology are discussed. many ideas have been presented by researchers to shorten the rerouting time. buffer zone routing (bzr) is one such solution, which splits a node transmission region into a safe zone adjacent to the node and a hazardous zone towards the broadcast range's end. the energy consumption of the nodes is reduced when routing decisions are made promptly. transfer time is lowered and routing efficiency is improved in the wider bzr safe region. it corrects flaws in the current algorithm and fills in gaps, reducing the time necessary for manet rerouting. in the paper ‘multi-input multi-output antenna measurements with super wide bandwidth for wireless applications using isolated t stub and defected ground structure,’ pradeep vinaik kodavanti et al., offers the concept of faulty ground structures for improving the antenna's radiation properties, particularly in a multi-input multi-output (mimo) arrangement. in both single and array configurations, the suggested antenna architecture with slightly flared out feed system is constructed and analyzed with defective ground. the t stub is a t-shaped stub that was employed in this project with a defective ground construction. to improve the mimo configuration features, a t stub is introduced, as well as a faulty ground. simulations are run on an electromagnetic modeling tool, and parameters such as reflection coefficient, voltage standing wave ratio, gain, radiation pattern, and current distribution plots are measured. in the paper ‘analysis of multiband rectangular patch antenna with defected ground structure using reflection coefficient measurement,’ the computer simulation technology (cst) microwave studio software is used to develop and simulate a new quad band antenna that can function in four different frequency bands. various parameters were improved using parametric analysis to improve the antenna's performance. various antenna parameters have to be measured throughout this investigation. in the paper entitled ‘analysis of peak-to-average power ratio in filter bank multicarrier with offset quadrature amplitude modulation systems using partial transmit sequence with shuffled frog leap optimization technique’ addressed about the filter bank multicarrier with offset quadrature amplitude modulation (fbmc-oqam), which tackles the problem of low adjacent channel leakage ratio, has recently stimulated the interest of numerous researchers. however, the fbmc system's energy measuring efficiency is harmed by the problem of high peak-to-average power ratio (papr) measurement. this paper proposes the partial transmit sequence (pts) with shuffled frog leap (sfl) phase optimization method to reduce the larger papr measurement, which is a major drawback of the filter bank multicarrier with offset quadrature amplitude modulation (fbmc-oqam) system. matlab is used to measure the experimental parameters and assess the results. the paper ‘beamforming in cognitive radio networks using partial update adaptive learning algorithm’ investigates cognitive radio technology as a means of increasing bandwidth efficiency. frequency that isn't used in any way will be employed in this cognitive radio by utilising some of the most powerful resources. one of the key advantages of cognitive radio signals is that they can identify different channels in the spectrum and change the frequencies that are often used. in this research, cognitive radio was developed utilising the beamforming approach, with power allocation as a strategy for the unlicensed transmitter that is completely based on sensing results. it is based on the status of the principal user in a different cognitive radio network, whereas an unlicensed transmitter uses a single antenna and changes the power transmitted. in the paper, ‘efficient deep learning based data augmentation techniques for enhanced learning on inadequate medical imaging data,’ a unique strategy to data augmentation for medical imaging was developed, which could partially solve the problem of limited availability of chest x-ray data. on the original data, a preprocessing step was performed to reduce the image size from 1024x1024x1 to 128x128x1. from datasets generated by a simple generative adversarial network (gan) and a transfer learning gan, the cnn learnt considerably faster and had improved accuracy, and this could be a one-stop solution for the limited availability of chest x-ray data. the paper ‘image reconstruction using refined res-unet model for mir,’ proposes reconstruction of the input medical image, a unique content-based res-unet framework is proposed, which performs an efficient image retrieval task. resnet50 is used as an encoder in the proposed work to conduct feature vector encoding. the proposed model's performance is assessed using the two benchmark datasets, ild and via/elcap-ct. the suggested model outperforms traditional approaches, as evidenced by the comparison findings. in the paper, ‘multilayer feature fusion using covariance for remote sensing scene classification,’ stacked covariance is a new technique for scene categorization utilising remote sensing data that combines visual information from various layers of a cnn. in the current staked covariance (sc) based classification framework, feature extraction is conducted first using a pretrained cnn model, followed by feature fusion using covariance. each feature is the covariance of two separate feature maps, and these features are used to classify data using svm. the proposed sc approach regularly outperforms other classification methods and delivers better results. acta imeko issn: 2221-870x march 2022, volume 11, number 1, 3 5 acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 3 university of bridgeport prof. navarun gupta presented a paper entitled, ‘spectrum sensing using energy measurement in wireless telemetry networks using logarithmic adaptive learning,’ the spectrum sensing method is used to identify primary user signals in cognitive radios. the least logarithmic absolute difference (llad) algorithm, in which noise strengths are modified at licenced users' sensing points, is proposed to avoid interferences between primary and secondary users. estimated noise signals are removed using the proposed approach. to determine the threshold value, the probability of detection (pod) and probability of false alarm (pofa) are assessed. by sharing the un-used spectrum, the proposed energy measurement-based spectrum sensing method is effective in remote health care monitoring and medical telemetry applications. finally, the paper ‘classification of brain tumours using artificial neural networks,’ deals with magnetic resonance (mr) brain image for medical analysis and diagnosis. these images are typically measured in radiology departments to assess images of anatomy as well as the human body's general physiological processes. magnetic resonance imaging measurements are employed using a strong magnetic field, its gradients, also radio waves to create images of human organs in this process. blood clots or damaged blood vessels in the brain can also be detected using an mr brain imaging. artificial neural networks (ann) are used to classify whether an mr brain image contains a benign or malignant tumor. finally, ann is used to classify the information into benign and malignant tumors. the major goal and purpose of the study is to determine if the tumors are benign or malignant. we thank all the authors who contributed to this special issue, as well as all the reviewers, and special thanks and gratitude for prof. francesco lamonaca, acta imeko's editor in chief, for his tireless and patient assistance in making this special issue possible. at the same time, our sincere thanks are extended to dr. dirk röske, associate editor, for his assistance and contribution at various levels in the production process. i am honoured to have served as guest editor for this issue, and i hope that by doing so, we will be able to bring the recent advancements in the singnal processing and communication techniques in the contest of measurement and sensing systems. md. zia ur rahman, guest editor. a preliminary study on an image analysis based method for lowest detectable signal measurements in pulsed wave doppler ultrasounds acta imeko issn: 2221-870x june 2021, volume 10, number 2, 126 132 acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 126 a preliminary study on an image analysis based method for lowest detectable signal measurements in pulsed wave doppler ultrasounds giorgia fiori1, fabio fuiano1, andrea scorza1, jan galo2, silvia conforto1, salvatore a. sciuto1 1 engineering department, roma tre university, via della vasca navale 79, 00146 rome, italy 2 clinical engineering service, irccs children hospital bambino gesù, piazza di sant’onofrio 4, 00165 rome, italy section: research paper keywords: lowest detectable signal; pw doppler; automatic doppler sensitivity measurement method; quality control; doppler flow phantom citation: giorgia fiori, fabio fuiano, andrea scorza, jan galo, silvia conforto, salvatore andrea sciuto, a preliminary study on an image analysis based method for lowest detectable signal measurements in pulsed wave doppler ultrasounds, acta imeko, vol. 10, no. 2, article 18, june 2021, identifier: imekoacta-10 (2021)-02-18 section editor: giuseppe caravello, università degli studi di palermo, italy received january 18, 2021; in final form march 8, 2021; published june 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: giorgia fiori, e-mail: giorgia.fiori@uniroma3.it 1. introduction pulsed wave (pw) doppler ultrasound (us) is a doppler technique that allows to display the spectrograms of blood flow velocity from selected depths in tissues. in particular, real-time blood velocity can be accurately measured within a sample volume (sv) in correspondence of a specified depth (adjustable by the operator) from the transducer/medium interface. in this regard, despite the lack of a shared worldwide standard on us equipment testing, performance evaluation of doppler systems is a currently investigated issue in the scientific research field [1][9]. a great number of doppler test parameters is recommended by the main medical us professional bodies, to be included in quality control (qc) protocols [10]-[12]. among these parameters, the lowest detectable signal may be considered mandatory to assess pw doppler system performance, since in scientific literature it is identified as an index of pw doppler sensitivity [1], [10], [12]-[14]. in particular, the lowest detectable signal in the spectrogram image (ldsimg) has been defined by the authors as the minimum signal level that can be clearly distinguished from noise [15]. in [16], the maximum sensitivity has been referred to as the measurement of the weakest doppler shift signal (linked to the ldsimg through the cosine of the insonification angle) that a us system can detect and display on pw image above the electronic noise. in clinical practice, sensitivity outlines the ability to detect doppler signals from small vessels for increasing distances from the us probe. the goals of the present study are (a) the improvement of a novel automatic algorithm, namely automatic doppler sensitivity measurement method (adsmm), firstly presented in [15], for the ldsimg evaluation by means of a commercial flow phantom; abstract nowadays, doppler system performance evaluation is a widespread issue because a shared worldwide standard is still awaited. among the recommended doppler test parameters, the lowest detectable signal could be considered mandatory in quality control (qc) protocols for pulsed wave (pw) doppler. such parameter is defined as the minimum signal level that can be clearly distinguished from noise and therefore, it is considered as related to pw doppler sensitivity. the present study focuses on proposing and validating a novel image analysis based method for the estimation of the lowest detectable signal in the spectrogram image (ldsimg), namely automatic doppler sensitivity measurement method (adsmm), as well as to compare its results with the outcomes retrieved from the naked eye doppler sensitivity method (nedsm), based on the mean judgment of three independent observers. data have been collected from a doppler flow phantom, through three ultrasound systems for general purpose imaging, equipped with two linear array probes each and with two configuration settings. results are globally compatible among the proposed methods, us systems and settings. further studies could be carried out on a higher number of us diagnostic systems, doppler frequencies and observers, as well as with different probe and phantom models. mailto:giorgia.fiori@uniroma3.it acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 127 (b) its validation through the comparison with the outcomes provided by the naked eye doppler sensitivity method (nedsm) carried out by three observers without clinical expertise. data have been collected with different settings from three us systems equipped with two linear array us probes each, which worked at similar doppler frequencies. in section 2 the estimation rationale underlying the ldsimg parameter definition will be described. in section 3 the experimental setup used in this study, the adsmm and the nedsm implementation will be discussed. in section 4 the uncertainty analysis of both methods through monte carlo simulation (mcs) will be carried out. in section 5 results will be presented and discussed on the basis of the comparison between the ones obtained from the adsmm and the nedsm, respectively. finally, in the concluding section the major achievements and the future developments of the research hereby presented will be reported. 2. ldsimg estimation in current scientific literature, a shared consensus on the test protocols for the ldsimg estimation is still awaited. nevertheless, factors that affect the lowest detectable signal can be objectively identified. they can be classified in two main groups, according to the device from which they can be adjusted: a) us system (doppler frequency f0, system settings, sample volume length svl, sample volume depth svd, insonification angle ); b) test device (blood mimicking fluid bmf, velocity v and reflectors density). more in detail f0, v and  directly affect the doppler shift fd that determines the pw spectrogram, according to the wellknown (approximated) relationship: 𝑓𝐷 ≅ 2 𝑓0 𝑣 𝑐 cos 𝜃. (1) furthermore, in correspondence of the svl increase within the flow, the doppler shift spread increases, as echoes from a higher number of reflectors of different velocities are produced for the same flow. on the other hand, a bmf with higher particles density produces a more intense doppler signal for the same flow velocity, due to the higher number of reflectors. moreover, the spectrogram intensity depends on the position of the sv into the tube, depending on the flow velocity profile. finally, svd determines the spectrogram attenuation: echoes from higher depths are affected by higher attenuation, therefore they are represented in a weaker spectrogram until it can no longer be distinguishable from noise. from the consideration of the abovementioned factors, it is possible to evaluate the ldsimg from the sum of two contributions: (a) the attenuation , due to the echoes path length into the phantom tissue mimicking material (tmm) and (b) the doppler signal attenuation g due to the echoes reduction into the spectrogram, from maximum intensity to minimum. therefore, ldsimg can be expressed through the following mathematical formulation: 𝐿𝐷𝑆img = ∆𝛼 + ∆𝐺 ≅ 2 𝛼 𝑓0 𝑆𝑉𝐷 + (𝐺max − 𝐺min), (2) where  is the (mean) attenuation coefficient in the tmm (usually expressed in dbcm-1mhz-1), f0 is the probe doppler frequency, gmax is the maximum doppler gain before no negligible noise appears in the spectrogram image and gmin is the minimum doppler gain corresponding to the lack of signal (i.e., the spectrogram intensity is very close to zero). moreover, if the us system does not provide the doppler gain in db (e.g., arbitrary units, au), a unit conversion is needed. 3. materials and methods in this work, the image analysis based method for the ldsimg estimation, namely automatic doppler sensitivity measurement method (adsmm), implemented in matlab environment, has been improved and validated. this has been carried out from the spectrogram images collected from three us systems, equipped with two linear array us probes each, by means of a commercial flow phantom. 3.1. experimental setup the gammex, optimizer® 1425a [17] doppler flow phantom has been used to acquire the pw doppler images from a sloped tube within a tmm at a specified continuous flow rate. the device consists of a hydraulic circuit filled with a bmf, a tmm and an electric flow controller (table 1). the ldsimg measurement has been carried out at two different pw doppler settings, i.e., set i and set ii, and a single doppler frequency for each linear array probe (table 2). the lowest and stable flow phantom velocity has been set, and sv size as well as insonification angle have been kept constant. conversely, the tube inner diameter could not be changed because of the phantom design. therefore, during the spectrogram acquisitions the svd only has been varied. data have been collected for six svd values spaced of 3 mm each. figure 1 shows the sample volume depths for each probe. table 1. doppler flow phantom characteristics. ultrasound phantom[17] us phantom model gammex optimizer® 1425a scanning material water-based mimicking gel tube inner diameter (nominal) 5 mm attenuation (0.50 ± 0.05) db·cm-1·mhz-1 tmm speed of sound (1540 ± 10) m·s-1 bmf speed of sound (1550 ± 10) m·s-1 flow rate (nominal) 2.6 ml·s-1 velocity setting (nominal) 30 cm·s-1 table 2. b-mode and pw doppler us systems settings. parameter set i set ii dynamic range (db) maximum a: maximum b: 0.8·maximum c: 0.6·maximum doppler frequency (mhz) a1: 6.25; a2: 6.25 b1: 5; b2: 6.3 c1: 5; c2: 6.2 wall filter (hz) minimum 100 – 150 svl (mm) a: 1.5; b: 2.0; c: 1.5 svd range (mm) a1: 64-79; a2: 52-67 b1: 61-76; b2: 58-73 c1: 73-88; c2: 70-85 insonification angle (°)  51 set i = raw working conditions; set ii = best working conditions as provided from the specialist. number 1, associated to the corresponding us system a, b or c, indicates the probe with the lower doppler frequency, while number 2 indicates the probe with the higher doppler frequency. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 128 such depths have been set at different values because the nondetectability of the spectrogram intensity varies according to the probe. in the acquisition process, the us probes have been maintained still on the phantom scanning surface through a holder ensuring the insonification angle to be constant throughout the whole acquisition time. 3.2. automatic doppler sensitivity measurement method an ad hoc acquisition protocol has been designed and implemented to estimate ldsimg parameter through the automatic determination of gmin and gmax for each svd value. pw images have been acquired by varying doppler gain from 0 db (or 0 au) to the us system maximum adjustable gain with steps of 2 db (or 2 au). after the data acquisition, the adsmm processes the acquired spectrograms for the automatic determination of gmin and gmax values through two steps: 1) for gmin computation, two regions of interest, rois (signal) and roin (noise), with the same size (810 px × 144 px) are drawn on the pw image in correspondence of the spectrogram and noise, respectively (figure 2a). the mean gray level value μn inside the roin is calculated to estimate the noise level, while rois is firstly subdivided in 3240 cells of 6 px × 6 px and the mean gray level values μs,i of each cell are computed, resulting in a new matrix rois2. afterwards, a snr matrix is obtained whose elements are given by the following expression: 𝑆𝑁𝑅𝑖 = 𝜇𝑠,𝑖 𝜇𝑛 . (3) snr matrix is computed for increasing doppler gain values. among them, the adsmm determines gmin as the first gain value for which the number of cells with snri ≥ 2 is higher than 1 % of the total number of cells. 2) gmax is determined from the roin computed in the first step (figure 2b). similarly, it is firstly subdivided into cells of 6×6 px and the mean gray level values μn,i of each cell are evaluated, resulting in a new matrix roin2 obtained for increasing doppler gain values. among them, the adsmm determines gmax as the first gain value for which the number of cells with μn,i ≥ 3 is higher than 1 % of the total number of cells. 3.3. naked eye doppler sensitivity method the nedsm is based on an in-house matlab function implemented to allow the three observers to express their judgment on gmin and gmax values from the acquired pw spectrograms. tests have been independently performed without variations of environmental lightening conditions, as well as by keeping a fixed monitor distance. more in detail, the nedsm randomly provides the observers with the pw spectrograms allowing them to indicate which pw images are associated to gmin and gmax values for each svd according to the six linear probes. the pw images order has been randomized and the observers have been requested to repeat the test six times for the study of subjects’ interand intra-variability. the compatibility between the results obtained through the adsmm and the nedsm has been evaluated according to the following condition [18]: |𝜇adsmm − 𝜇nedsm| ≤ 𝛿adsmm + 𝛿nedsm, (4) where adsmm and nedsm are the mean ldsimg values estimated through the adsmm and the nedsm, respectively, while adsmm and nedsm are the corresponding uncertainties. 4. monte carlo simulation as already experienced in other studies [19]-[25], mcs is a powerful tool to estimate the uncertainty and assess the robustness of measurements processed by software. two different mcs series have been carried out to estimate ldsimg uncertainty for the adsmm and the nedsm, respectively. the number of iterations for each mcs has been set at 105 cycles. the doppler probe frequency f0 uncertainty has been considered negligible because of the narrow bandwidth of the transmitted pulse. in table 3 all the distributions assigned to the variables influencing the ldsimg expressed in (2), are listed: the tmm figure 1. schematic representation of the sample volume depths according to each us probe. number 1, associated to the corresponding us system a, b or c, indicates the probe with the lower doppler frequency, while number 2 indicates the probe with the higher doppler frequency. a b figure 2. example of rois and roin on pw spectrograms with svd at 73 mm, in set i configuration, for the automatic determination of a) gmin and b) gmax. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 129 attenuation  has been set as a normal distribution whose standard deviation (sd) has been retrieved from the phantom datasheet by supposing a 95% confidence level, while the depth z has been set as a uniform distribution with mean value equal to the svd and sd estimated from the sample volume depth resolution. on the other hand, uniform distributions have been assigned to the adsmm and the nedsm minimum and maximum gains. gmin,adsmm and gmax,adsmm have been automatically determined by the adsmm, while gmin,nedsm and gmax,nedsm are the mean values of the three observers judgement. as regards the adsmm gains standard deviations, they have been estimated considering the doppler gain resolution. on the table 3. mcs distributions assigned to the variables influencing the ldsimg. parameter distribution unit mean ± sd tmm attenuation  normal db·cm-1·mhz-1 0.500 ± 0.025 depth z uniform cm svd ± 0.3 gmin and gmax (adsmm) uniform db or au (*) gmin,adsmm ± 0.6 uniform gmax,adsmm ± 0.6 gmin and gmax (nedsm) uniform db or au (*) gmin,nedsm ± min,nedsm uniform gmax,nedsm ± max,nedsm (*) the doppler gain measurement unit is db for us systems a and b, while au for us system c. table 4. ldsimg results for the adsmm and the nedsm according to the us system, probe and configuration setting. us system and probe sample volume depth svd (mm) ldsimg (db) set i set ii adsmm nedsm adsmm nedsm a1 64 54 ± 4 55 ± 4 52 ± 4 55 ± 5 67 54 ± 4 55 ± 5 54 ± 4 56 ± 5 70 54 ± 5 54 ± 5 54 ± 5 55 ± 5 73 54 ± 5 53 ± 5 54 ± 5 54 ± 5 76 53 ± 5 53 ± 5 54 ± 5 53 ± 5 79 55 ± 5 54 ± 6 55 ± 5 53 ± 5 ldsimg,a1 54 ± 5 54 ± 5 54 ± 5 54 ± 5 a2 52 49 ± 4 51 ± 4 51 ± 3 50 ± 4 55 48 ± 4 51 ± 5 48 ± 4 51 ± 4 58 48 ± 4 50 ± 4 50 ± 4 51 ± 5 61 48 ± 4 50 ± 5 46 ± 4 49 ± 5 64 46 ± 4 48 ± 5 46 ± 4 47 ± 5 67 48 ± 4 50 ± 5 48 ± 4 49 ± 5 ldsimg,a2 48 ± 4 50 ± 5 48 ± 4 49 ± 5 b1 61 53 ± 3 52 ± 4 51 ± 3 52 ± 4 64 54 ± 3 52 ± 4 56 ± 3 58 ± 4 67 56 ± 4 54 ± 5 57 ± 4 59 ± 4 70 55 ± 4 55 ± 5 57 ± 4 59 ± 5 73 54 ± 4 56 ± 4 55 ± 4 57 ± 5 76 54 ± 4 57 ± 5 54 ± 4 56 ± 4 ldsimg,b1 54 ± 4 54 ± 5 55 ± 4 57 ± 4 b2 58 59 ± 4 57 ± 4 62 ± 4 64 ± 5 61 54 ± 4 56 ± 5 56 ± 4 58 ± 5 64 56 ± 4 58 ± 5 56 ± 4 58 ± 5 67 58 ± 4 60 ± 5 58 ± 4 60 ± 5 70 58 ± 5 60 ± 5 60 ± 5 61 ± 5 73 58 ± 5 60 ± 5 60 ± 5 61 ± 5 ldsimg,b2 57 ± 4 58 ± 5 59 ± 4 60 ± 5 c1 73 61 ± 15 67 ± 19 62 ± 17 58 ± 17 76 61 ± 16 67 ± 19 62 ± 17 58 ± 17 79 61 ± 15 60 ± 17 62 ± 16 63 ± 18 82 69 ± 15 70 ± 18 71 ± 16 67 ± 18 85 61 ± 14 58 ± 15 56 ± 9 57 ± 13 88 61 ± 13 60 ± 15 62 ± 13 61 ± 15 ldsimg,c1 62 ± 15 64 ± 17 63 ± 15 61 ± 16 c2 70 63 ± 13 64 ± 16 57 ± 8 59 ± 14 73 63 ± 13 64 ± 16 57 ± 9 59 ± 14 76 57 ± 8 57 ± 10 52 ± 6 54 ± 7 79 54 ± 6 55 ± 7 56 ± 7 56 ± 7 82 54 ± 6 56 ± 7 55 ± 6 56 ± 7 85 55 ± 6 55 ± 6 55 ± 6 56 ± 7 ldsimg,c2 58 ± 9 59 ± 11 55 ± 7 57 ± 10 acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 130 other hand, the standard deviations min,nedsm and max,nedsm due to subjects’ interand intra-variability have been obtained through one-way anova statistical test (p < 0.01) at each sample volume depth for each us probe. 5. results and discussion the ldsimg results for the three us diagnostic systems equipped with two linear array probes obtained through the adsmm and the nedsm for set i and set ii have been reported in table 4 and shown in figure 3. the uncertainty values have been retrieved through 2.5 and 97.5 percentiles from the data distributions obtained through mcss. firstly, by considering the adsmm, ldsimg results are globally compatible for each us system and linear probe, independently from set i and set ii (figure 3 a, c), as well as from svd at which the pw spectrograms have been acquired. more in detail, the results are always compatible in us system a for both its probes throughout the configuration settings, and the same applies for us system c. conversely, scanner b always shows compatible results for set i only, nevertheless results preserve a global compatibility for set ii. the same considerations can be made for the nedsm, whose ldsimg results are globally compatible (figure 3 b, d). as regards the comparison between the adsmm and the nedsm, it can be noticed that for every single probe, ldsimg values obtained in set i are always compatible for both the methods applied and independently from the sample volume depth. such compatibility is preserved by changing the configuration setting (set ii), except for the first probe of us scanner b, because of the sample volume depth dependency. secondly, it’s worthy noticing that ldsimg outcomes obtained through the adsmm and the nedsm, in both set i and set ii, do not significantly deviate from the constant trend, differently from [15]. such issue could derive from the reduction of doppler gain step (2 db or 2 au). in fact, the non-constant trend in [15], could have been probably due to the combination of both the further attenuation caused by the presence of nylon pins above the tube sections where pw spectrograms were acquired, and the higher doppler gain step (5 au) applied, resulting in a wrong identification of gmin and gmax. as regards the methods measurement uncertainty, it can be assessed that the low nedsm uncertainty values are probably due to an adaptation process that the observer went through during the tests despite the randomization of the pw spectrogram images. such issue led to a lowering of the intraobserver variability with respect to the inter-observer one. on the other hand, us system c shows the highest measurement uncertainties, independently from the method applied, because its doppler gain measurement unit is not provided in db, leading to the necessity of a specific conversion procedure that introduces a further uncertainty contribution. finally, the mean ldsimg (ldsimg,a1, ldsimg,a2, ldsimg,b1, ldsimg,b2, ldsimg,c1 and ldsimg,c2) and the corresponding uncertainty values for each linear array probe, computed for both the adsmm and the nedsm as well as for set i and set ii, have been reported in table 4 and shown in figure 4. it should be pointed out that such results always show compatibility among the two methods investigated and the configuration settings. the highest mean ldsimg value has been achieved for probe 1 of scanner c for both methods and configuration settings. besides, the lowest mean ldsimg value has been found in correspondence of the probe 2 of scanner a for both methods and configuration settings. therefore, the us system c seems to be the most sensitive among the diagnostic systems involved in the present study. a. adsmm – set i b. nedsm – set i c. adsmm – set ii d. nedsm – set ii figure 3. ldsimg outcomes obtained through the adsmm and the nedsm, for both linear array probes of each us system a, b and c according to the configuration settings (set i and set ii). acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 131 as a final remark, it should be pointed out that the physical model underlying both the adsmm and the nedsm proposed in the present work, is a differential model as it relies on the doppler gains difference, as in (2). this is undoubtedly an advantage when the observer expresses its own judgment looking at the pw spectrograms: in fact, the model differential nature allows the observer test to be carried out on a monitor with a different dynamic scale with respect to the diagnostic us system one, without affecting the overall results. such feature improves the nedsm robustness because tests can be carried out on different monitors. further considerations should be addressed about the adsmm and the nedsm application in qcs for the assessment of medical us systems, since a significant ldsimg reduction with respect to a baseline can be related to both a sensitivity loss (gmin tends to increase) and a noise rise (gmax tends to decrease). to this aim, the us scanner settings should be adjusted with care, together with the test object speed v in the phantom (e.g., the maximum velocity of the bmf in the flow phantom) and the measurement depth svd. in particular, both v and svd should be selected in order to assure the proper evaluation of gmin, since the doppler signal can be displayed anyway into the spectrogram for higher values of v and low attenuations. in this regard, high sensitivity probes may need deeper test objects embedded in high attenuation tmms (e.g., steady flows of bmf at higher depths). on the other hand, care should be paid to the test setup and its functioning, in order to avoid any deterioration in its components (e.g., variation in the tmm and/or bmf characteristics) to be confused with performance reduction in the us system. although the results shown in this work are very promising, all the above issues are worthy of further studies, aiming to provide a suitable and robust tool for qc technicians. 6. conclusions in the present work, the lowest detectable signal in the spectrogram image (ldsimg), an index for pw doppler qc performance test, has been proposed and investigated as it provides an indication about pw doppler sensitivity. its estimation has been carried out through an improved image analysis based algorithm, namely automatic doppler sensitivity measurement method (adsmm), developed in matlab environment whose physical model has already been proposed in literature. the adsmm validation has been performed by comparing its outcomes with the ones provided by the naked eye doppler sensitivity method (nedsm), carried out by three independent observers without clinical expertise. data have been collected from a doppler flow phantom, by means of three diagnostic systems equipped with two linear probes each, in two different us system settings (set i and set ii). globally, the results obtained through the adsmm and the nedsm are compatible, independently from the configuration setting. such compatibility suggests a good reliability of the adsmm in ldsimg parameter estimation. among the future developments, further tests could be carried out (a) on a wider observers’ sample, (b) on a higher number of us diagnostic systems available in the market, (c) with different probe doppler frequencies and (d) with different probe models (e.g., convex and phased array). in particular, the latter could be tested on different doppler phantom models with higher depth size and/or attenuation coefficient to guarantee the correct estimation of the minimum doppler gain in the ldsimg expression. on the other hand, a physical model improvement could be expected in order to retrieve ldsimg values even if the doppler spectrogram is still displayed on the image in correspondence of the zero doppler gain. despite the promising results, further studies are going to be carried out in order to assess and improve the robustness and suitability of the ldsimg measurement in ultrasound qc. acknowledgement the authors wish to thank ge healthcare, hitachi healthcare and mindray medical for hardware supply and technical assistance in data collection. a) b) figure 4. mean ldsimg values (ldsimg,a1, ldsimg,a2, ldsimg,b1, ldsimg,b2, ldsimg,c1 and ldsimg,c2) obtained 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acta imeko is an e-journal reporting on the contributions on the state and progress of the science and technology of measurement. the articles are based on presentations presented at imeko workshops, symposia and congresses. the journal is published by imeko, the international measurement confederation. the issn, the international identifier for serials, is 2221-870x. editorial and publication board  prof. paolo carbone (italy – editor in chief) dr. dirk röske (germany – information officer) prof. pasquale daponte (italy – president of imeko) prof. paul p.l. regtien (the netherlands – vice president for publications) prof. francisco alegria (portugal – editorial office) prof. sergio rapuano (italy – editorial office) imeko technical committee chairmen (ex officio) about imeko  the international measurement confederation, imeko, is an international federation of actually 39 national member organisations individually concerned with the advancement of measurement technology. its fundamental objectives are the promotion of international interchange of scientific and technical information in the field of measurement, and the enhancement of international co-operation among scientists and engineers from research and industry. addresses  principal contact prof. paolo carbone university of perugia dept. electr. inform. engineering (diei) via g.duranti, 93, 06125 perugia italy email: paolo.carbone@unipg.it acta imeko prof. paul p. l. regtien measurement science consultancy (msc) julia culpstraat 66, 7558 jb hengelo (ov) the netherlands email: paul@regtien.net support contact dr. dirk röske physikalisch-technische bundesanstalt (ptb) bundesallee 100, 38116 braunschweig germany email: dirk.roeske@ptb.de fire sm: new dataset for anomaly detection of fire in video surveillance acta imeko issn: 2221-870x march 2022, volume 11, number 1, 1 6 acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 1 fire sm: new dataset for anomaly detection of fire in video surveillance shital mali1, uday khot1 1 department of electronics and telecommunication , st. francis institute of technology, mumbai university, mumbai, india section: research paper keywords: anomalous; convolutional neural network; dataset; fire; smoke citation: shital mali, uday khot, fire sm: new dataset for anomaly detection of fire in video surveillance, acta imeko, vol. 11, no. 1, article 25, march 2022, identifier: imeko-acta-11 (2022)-01-25 section editor: md zia ur rahman, koneru lakshmaiah education foundation, guntur, india received november 29, 2021; in final form march 6, 2022; published march 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: shital mali, e-mail: shital.mali@rait.ac.in 1. introduction surveillance cameras are widespread, and it is not feasible to have people actively tracking them. in most instances, nearly all footage of surveillance camera is unimportant. only rare pieces of video are of the main concern. thus, the key inspiration for creating video anomaly detection along with image-based is, to automatically locate areas of video/image which are irregular. this would mark those for human inspection. recently, the research study of the identification of video/image anomaly has been characterised by two parameters. the training videos are made using a secure event. the anomalous event identification would be task followed after examining the video. in order to define what is usual for a specific scene, it is important to have training footage of normal behaviour. by an anomalous case that implies the localized video section, which is substantially dissimilar, happen inside a training video. more difficult is to choose very different attributes, that have been handled in point of interest application. such disparity would be due to many causes, the majority usually remarkable presence of the things inside video. interestingly note, most researchers conferred on anomaly detection after experimentation [1], [2], [3], [4] and some have published their findings with different techniques [5], [6], [7]. few studies have discussed about the usual anomaly video which either coming from one or two same scene. the attribute that may attach in identification purpose, the unique index of geographical space in an instance of video. for certain instances, the detection algorithm one can identify the anomalous scene that for other instance may not be a case of anomalous. the problem in little moment was needed to take care under such research study. the quality-wise distinct issue in one scene may create a difficulty in superimposed multiple-scenes. the identification and analysis of single instance indicates to very uniquely handle feature in surveillance system, this study focused on such aspect. so, measurement technology plays a vital role in anomaly detection and surveillance applications. the formulation explains about the differences which lastly act spatially. the detection of anomalous activity in video/image directly related to performance and accuracy of detection algorithm. there would always be a scope of improvement in anomalous detection algorithm. abstract tiny datasets of restricted range operations, as well as flawed assessment criteria, are currently stifling progress in video anomaly detection science. this paper aims at assisting the progress of this research topic, incorporating a wide and diverse new dataset known as fire sm. further, additional information can be derived by a precise estimation in early fire detection using an indicator, average precision. in addition to the proposed dataset, the investigations under anomaly situations have been supported by results. in this paper different anomaly detection methods that offer efficient way to detect fire incidences have been compared with two existing popular techniques. the findings were analysed using average precision (ap) as a performance measure. it indicates about 78 % accuracy on the proposed dataset, compared to 71 % and 61 % on foggia dataset, for inceptionnet and firenet algorithm, respectively. the proposed dataset can be useful in a variety of cases. findings show that the crucial advantage is its diversity. mailto:shital.mali@rait.ac.in acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 2 the number of challenges would come to place while dealing with anomalous detection in fire related datasets. the shortcoming involves the unique kind dataset solely based on fire anomalous instances, low resolution of existing datasets, variations in anomalous. few more cases, in which uncertainty, inconsistencies, and loss in quality have been identified. the main focus of the paper has been on the detection of anomaly, analysis of obtained result after application on experimental dataset with the help of few assessment indices. the induction of new dataset of early fire and smoke (refer to table 1) would be helpful in many applications. maintaining diversities in dataset would be a key point, which checks anomalous things in different direction and more complex way. 2. existing dataset fires are man-made hazards that inflict human. it causes social, and economic damage. early fire alarm and an automatic approach are important and useful to emergency recovery services to minimize these losses. existing fire alarm devices have been shown to be unreliable in terms of numerous real-world situations. the vital disadvantage of the sensor-based framework is that it should be situated close to a fire or warmth source. however, this makes them impractical to use in a variety of frequently occurring scenarios such as long-distance fire occurrences as seen in figure 1. due to this, the traditional approach has failed to avoid a number of fire deaths. solutions to this usually involve a reasonable amount of fire or heat sensation to stimulate the alarm. in addition, the fire or smoke regions are not precisely located. due to shortcomings of fire detection, researchers have been researching computer vision related approaches that have become alternatives for improving the fire and smoke detection system. existing vision-based approaches focusing solely on the transformation of colour space for fire area detection [1], [2]. rule-based methodology, along with colour space, has a promising future in delivering improved results. however, such devices are also vulnerable to other lit items such as streetlights. additional methods applied to the decision-making algorithm additional features to colour-based methods such as location, boundary and motion cues [3], [4]. classifiers such as bayes classifier, dual optical flow and multi-expert scheme have been used to minimize false detection or misclassification. however, these strategies are vulnerable to error and fail in many complex real-world scenarios as seen in figure 2. however, due to the complexities of the condition, fire detection is a challenging task. as it does not have a definite form, area of incidence, complex temporal behaviour so as to extract the function. the hand-crafted collection of features involves a considerable amount of domain information. table 2 listed details of existing dataset. the foggia video dataset [8] and the chino dataset [9] were the two basic datasets. the first dataset includes of 31 enclosed environment and open-air videos. in that, seventeen are with not fire related while fourteen categorized of fire. as a result, colourbased methods are incapable for recognizing genuine fire and table 1. fire instances in fire sm dataset. anomaly class instances 1. outside offices 78 2. outside apartment 88 3. in bushes 26 4. outside light 15 5. street light 13 6. decorative lighting 11 7. bon-fire 9 8. cooking gas 25 table 2. existing datasets. type size per image rate no. of frames related fire remark or observations fire1 320x240 15 705 yes see [10] fire2 320x240 29 116 yes refer [10] and [11] fire3 400x256 15 255 yes fire4 400x256 15 240 yes fire5 400x256 15 195 yes fire6 320x240 10 1200 yes fire7 400x256 15 195 yes fire8 400x256 15 240 yes fire9 400x256 15 240 yes fire10 400x256 15 210 yes fire11 400x256 15 210 yes fire12 400x256 15 210 yes fire13 320x240 25 1650 yes fire14 320x240 15 5535 yes foggia et al. [8] fire15 320x240 15 240 no refer [11] and [10] fire16 320x240 10 900 no fire17 320x240 25 1725 no fire18 352x288 10 600 no fire19 320x240 10 630 no fire20 320x240 9 5958 no fire21 720x480 10 80 no fire22 480x272 25 22500 no foggia et al. [8] fire23 720x576 7 6097 no refer [11] and [10] fire24 320x240 10 342 no fire25 352x288 10 140 no fire26 720x576 7 847 no fire27 320x240 10 1400 no fire28 352x288 25 6025 no fire29 720x576 10 600 no fire30 800x600 15 1920 no foggia et al. [8] fire31 800x600 15 1485 no figure 1. test images of training data. figure 2. sample confusing images which look like fire or smoke. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 3 scenes with red shading parts. additionally, movement-based strategies may mistakenly portray a mountain scene of smoke, fog, or haze. these pieces have made the informational collection more troublesome, empowering us to push our engineering and assess its exhibition in different genuine settings. another issue that arises during data processing is the difference between the fire and the non-fire. at a greater distance, for example, fire2 [10] video contains so little fire. in the other hand, the fire13 [10] video shows no fire, but only within a very small range. thus, red designs and grounds like billboard (fire14) and radish grass (fire6) are available in numerous photos, making the dataset hard to decipher. the second dataset is relatively limited but very difficult. this dataset contains a total of 226 images, 119 of which contain fire while the other 107 are fire-like pictures including night falls, fire-like stars, and daylight coming through windows and so on. an enormous amount of data is required in training for convolutional neural networks (cnns). conversely, the current image/video fire collections are insufficient to meet the demands. table 3 displays some limited scale fire picture/video information repositories. the data collection includes 13,400 fire images in all. these photographs were taken both outside and inside. there are 9695 "fire" and 7442 "smoke" facets in the data collection. in addition, the dataset includes 15,780 images that do not have flames. these data were acquired from 16 separate user environments and involve 49,614 distorted images. each picture usually involves distortion such as due to surrounding noise or climatic condition. for this investigation, half of the pictures in the information assortment are utilized as the preparation/approval set. the remaining half is utilized as the test set. 3. experimental details experiments were carried out using a deep neural network technique has been applied on proposed dataset. for which the system was used, the nvidia rtx 2080 processor with 10 gb on-board ram, as well as ubuntu os16.04 based on system. the cpu would be of intel core i5. this system was having ram of 64gb. the analyses utilized 68,457 pictures acquired from notable fire datasets. this includes foggia et al. [8] of 62,690 pictures. the planning and testing periods of the tests followed the trial system, where 20 % and 80 % of the information were utilized for preparing and testing, separately. the technique was applied with a qualified proposed updated efficientdet [13]. the modified efficientdet algorithm incorporated with leaky relu as activation function has been replaced hardswish. a training data of 2717 pictures was generated by using a model of 2529 fire pictures and 190 non-fire pictures. the planned network, however, with only 2-classes, i.e. fire and not fire class. data sets are one of the essential components for evaluating the output of any given device. evaluating the algorithm against a regular data set is one of the most difficult activities. in the suggested datasets, all photographs are original and taken by real people. this dataset is therefore the most demanding and diversified dataset ever produced. this handcrafted research dataset was designed to explain the generalization of a qualified model. this involves an average of 2 boxes per picture of varying size and aspect ratio. activation mapping has been exploited. this was required to get the approximate bounding box. the loss function was used as a binary cross-entropy during the study of this dataset. in addition, the optimizer is found to be rmsprop with an early learning score of 0.001s. the number of 300 epochs was taken into account. the accompanying segments present subtleties of results got utilizing different fire datasets and their correlation with cutting edge fire information base methodologies. 4. fire sm dataset description and results this proposed fire sm dataset was verified for density of occurrences of actual fire location in an image. the dataset contains images of the fire which was located not only at centre of image but also at corner, top, bottom side as well included. the density of fire location in an image was shown in figure 3. this figure shows the fire location in relative coordinate plane. in this, red colour signifies fire at middle, orange, yellow colour table 3. number of fire image/video datasets present [12]. institution format object website bilkent university video fire, smoke, disturbance http://signal.ee.bilkent.edu.tr/visitfire/index.html cvpr lab, at keimyung university video fire, smoke, disturbance https://cvpr.kmu.ac.kr umrcnrs 6134 spe, corsica university dataset fire http://cfdb.univ-crse.fr/index.php?menu=1 faculty of electrical engineering, split university image, video smoke http://wildfire.fesb.hr/ institute of microelectronics, seville, spain image, video smoke https://www2.imse-cnm.csic.es/vmote/english_version/ national fire research laboratory, nist video fire https://www.nist.gov/topics/fire state key laboratory of fire science, university of science and technology of china image, video smoke http://smoke.ustc.edu.en/datasets.htm figure 3. fire location in images with distribution in relative coordinate in fire sm dataset (red-at middle, orange, yellowat corner, sky blue, dark blue not at middle and corner). http://signal.ee.bilkent.edu.tr/visitfire/index.html https://cvpr.kmu.ac.kr/ http://cfdb.univ-crse.fr/index.php?menu=1 http://wildfire.fesb.hr/ https://www2.imse-cnm.csic.es/vmote/english_version/ https://www.nist.gov/topics/fire http://smoke.ustc.edu.en/datasets.htm acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 4 signifies fire at corner, while sky blue, dark blue colour signifies fire not at middle and corner position. this figure supported to the diversified distribution dataset of fire was proposed for anomaly fire detection technique. in reality, with a phenomenon that appears over several frames, it is necessary to discover an irregularity in probably a portion of the images. however, confirming the area in every frame of the track is typically needless. this is especially true where there is uncertainty regarding when to start and finish the previously described phenomenon, as well as when anomalous activity is heavily occluded for a few frames. below mentioned feature are nothing but a measure of classification quality. 4.1. feature indices 4.1.1. localized detection index/rate the localized detection rate ldr is defined as ldr = (number of true regions detected)/(total number of regions). true region in an image detected if intersection of true area and recognized local portion is more or equivalent to  as shown in figure 4. 4.1.2. region based detection rate the region based detection rate rbdr is defined as rbdr= (number of positive image detected)/(total number of regions).  ranges between 0 to 1. default, =0.1. the negative region rate nrr is defined as nrr= (total non-positive regions) / (total frames or images) the average detection rate for negative region rate, nrr, will ranging from 0 to 1. there is a compromise between the discovery rate (genuine positive rate) and the bogus positive rate, likewise with any location rule. this can be caught in the roc bend determined by changing the inconsistency score edge that characterises what locales are seen as abnormality. figure 5 and figure 6, show characteristic curves for inceptionnet method, foggia and fire sm datasets. the nature of the curve signifies those values favorable to proposed dataset i.e., fire sm compared to foggia. khan et al. [14] described inceptionnet method on fire instance dataset. the dataset mentioned was less diversified as compared to proposed fire sm dataset. khan et al. [14] and firenet [15] proposed approaches focused on the classification of the leave-one-out strategy to be used for each level. in comparison to these algorithms, the updated efficientdet [16]-[19] based more on the degree of detection. this paper considered the average precision (ap) indicator for quantitative analysis. results were collected and 1 1 1 1 1 1 1 1 1 figure 4. representation of framework of areas petitioning frame. ‘1’ defines depicted as true region. gives an idea of detection method. a) b) figure 5. a, b nrr per frame characteristic curves for different dataset. a) b) figure 6. a, b nrr frame level characteristic curves for different dataset. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 5 seen in table 4 for the proposed dataset relative to the foggia dataset. activation mapping has been used to get an estimated bounding box. table 4 shows the updated efficientdet performance better relative to other algorithms. at present, both average precision (ap) at 50 and ap at 75 were compared. the efficientdet result obtained given the proposed early fire dataset was approximately 73 per cent and 71 per cent compared to approximately 53 per cent, 51 per cent and approximately 68 per cent respectively, and 58 per cent for inceptionnet and firenet. on the other hand, when taking into account the foggia dataset, the findings obtained were approximately 82 per cent, 78 per cent. these have been compared to about 65 %, 61 % and around 73 %, and 71 % for inceptionnet and firenet respectively, for ap@50 and ap@75 both. figure 7 shows the detection of fire and smoke. 5. conclusions this paper introduces a new fire sm database of fire anomaly scenarios. the database has been therefore the most demanding and diversified dataset ever produced. this hand-crafted research dataset was designed to explain the generalization of a qualified model. this research study proposes novel lightweight and realtime for detecting smoke and fire in videos or photographs. exiting datasets are either restricted or produced synthetically for testing purposes. in this study, validation was carried out on a real-world challenging proposed dataset that includes the majority of fire and smoke event scenarios. further, the weighted bi-directional feature pyramid network (bifpn) as well as compound scaling, consistently achieve better efficiency in efficientdet. experiment findings show that google's newest model, efficientdet, outperforms foggia on the proposed dataset. these results were obtained using average precision (ap) as an indicator; on the proposed dataset, it shows around 78 %, compared to 71 % and 61 % for inceptionnet and firenet, respectively, on the foggia dataset. the new assessment criteria address the shortcomings of the traditional criteria in this field. it provides a more accurate picture of how well an algorithm performs in real environment. furthermore, in this study, two variants of a latest fire anomaly detection algorithm used as a benchmark to which future work was measured. the new database would be helpful to encourage new novel techniques under this research field. references [1] t. celik, h. demirel, h. ozkaramanl, m. uyguroglu, fire detection using statistical color model in video sequences. journal of visual communication and image representation. 2007 apr 1;18(2):176-85. doi: 10.1016/j.jvcir.2006.12.003 [2] b. c. ko, s. j. ham, j. y. nam. modeling and formalization of fuzzy finite automata for detection of irregular fire flames. ieee transactions on circuits and systems for video technology. 2011 may 19; 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http://dx.doi.org/10.21014/acta_imeko.v9i1.732 http://dx.doi.org/10.21014/acta_imeko.v10i3.1020 journal contacts acta imeko issn: 2221-870x september 2021, volume 10, number 3 acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 0 journal contacts about the journal acta imeko is an e-journal reporting on the contributions on the state and progress of the science and technology of measurement. the articles are mainly based on presentations presented at imeko workshops, symposia and congresses. the journal is published by imeko, the international measurement confederation. the issn, the international identifier for serials, is 2221-870x. editor‐in‐chief francesco lamonaca, italy founding editor‐in‐chief paul p. l. regtien, netherlands associate editor dirk röske, germany editorial board section editors (vol. 7 10) leopoldo angrisani, italy filippo attivissimo, italy eulalia balestieri, italy eric benoit, france paolo carbone, italy lorenzo ciani, italy catalin damian, romania pasquale daponte, italy luca de vito, italy luigi ferrigno, italy edoardo fiorucci, italy alistair forbes, united kingdom helena geirinhas ramos, portugal sabrina grassini, italy fernando janeiro, portugal konrad jedrzejewski, poland andy knott, united kingdom yasuharu koike, japan francesco lamonaca, italy massimo lazzaroni, italy fabio leccese, italy rosario morello, italy michele norgia, italy franco pavese, italy pedro miguel pinto ramos, portugal nicola pompeo, italy sergio rapuano, italy gustavo ripper, brazil maik rosenberger, germany dirk röske, germany alexandru salceanu, romania constantin sarmasanu, romania lorenzo scalise, italy emiliano schena, italy enrico silva, italy krzysztof stepien, poland ronald summers, uk marco tarabini, italy yvan baudoin, belgium piotr bilski, poland francesco bonavolonta, italy giuseppe caravello, italy carlo carobbi, italy marcantonio catelani, italy mauro d’arco, italy egidio de benedeto, italy alessandro depari, italy alessandro germak, italy istván harmati, hungary min-seok kim, korea bálint kiss, hungary momoko kojima, japan koji ogushi, japan vilmos palfi, hungary jeerasak pitakarnnop, thailand jan saliga, slovakia emiliano sisinni, italy ciro spataro, italy oscar tamburis, italy jorge c. torres-guzman, mexico ioan tudosa, italy ian veldman, south africa rugkanawan wongpithayadisai, thailand claudia zoani, italy about imeko the international measurement confederation, imeko, is an international federation of actually 42 national member organisations individually concerned with the advancement of measurement technology. its fundamental objectives are the promotion of international interchange of scientific and technical information in the field of measurement, and the enhancement of international co-operation among scientists and engineers from research and industry. addresses principal contact prof. francesco lamonaca university of calabria department of computer science, modelling, electronic and system science via p. bucci, 41c, vi floor, arcavacata di rende, 87036 (cs), italy e-mail: editorinchief.actaimeko@hunmeko.org acta imeko bálint kiss, e-mail: bkiss@iit.bme.hu piotr bilski, e-mail: pbilski@ire.pw.edu.pl support contact dr. dirk röske physikalisch-technische bundesanstalt (ptb) bundesallee 100, 38116 braunschweig, germany e-mail: dirk.roeske@ptb.de mailto:editorinchief.actaimeko@hunmeko.org mailto:bkiss@iit.bme.hu mailto:pbilski@ire.pw.edu.pl mailto:dirk.roeske@ptb.de microsoft word article 16 113-807-1-ga.docx acta imeko december 2013, volume 2, number 2, 91 – 95 www.imeko.org acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 91 challenges and perspectives of regional cooperation within coomet – the euro-asian regional metrology organization pavel neyezhmakov1, klaus-dieter sommer2 1 national scientific centre “institute of metrology”, mironositskaya 42, 61002 kharkov, ukraine 2 physikalisch-technische bundesanstalt, bundesallee 100, 38116 braunschweig, germany section: technical note keywords: metre convention; rmo; jcrb; cipm mra; quality management system citation: pavel neyezhmakov, klaus-dieter sommer, challenges and perspectives of regional cooperation within coomet – the euro-asian regional metrology organization, acta imeko, vol. 2, no. 2, article 16, december 2013, identifier: imeko-acta-02 (2013)-02-16 editor: paolo carbone, university of perugia received april 17th, 2013; in final form december 14th, 2013; published december 2013 copyright: © 2013 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: pavel neyezhmakov, email: pavel.neyezhmakov@rambler.ru 1. introduction coomet (coomet – abridged form from “cooperation in metrology”) is a regional metrology organization (rmo) [1] that establishes cooperation between the national metrology institutions of the countries of central and eastern europe. it was founded in june 1991 and renamed euro-asian cooperation of national metrology institutions in 2000. coomet is open to any metrology institution from other regions; they can join as associate members. the basic activity of coomet is cooperation in the following areas: measurement standards of physical quantities, legal metrology, quality management systems (qms), information and training. the participation in coomet activities gives an opportunity for the member countries to solve metrological tasks in a more efficient way on the basis of approved rules and procedures. the current 15 members of coomet are the metrology institutions from armenia, azerbaijan, belarus, bulgaria, georgia, kazakhstan, kyrgyzstan, lithuania, moldova, russia, romania, slovakia, tajikistan, ukraine and uzbekistan. in addition, there are 3 associate members: germany, dpr of korea, and cuba. the objectives of coomet are the following:  provide assistance in effectively addressing any problems relating to the uniformity of measures, uniformity of measurements and the required accuracy of their results;  provide assistance in promoting cooperation between national economies and eliminating technical barriers for international trade;  harmonize activities of metrology services of the euro-asian countries with similar activities in other regions.  these objectives are accomplished by cooperation between interested coomet member countries with regards to supporting activities related to the accreditation of national metrology institutes (nmis), as well as calibration and measurement laboratories.  at today’s stage of progress, the tasks of coomet are aimed at strengthening the links between the nmis in order to solve common problems and to create effective mechanisms that will meet the following objectives: abstract the creation of the metrological infrastructure providing traceable results of the measurement is one of the major tasks of coomet member-countries from central asia and to caucasian region. coomet successfully cooperates in developing the basic metrology infrastructures of these countries. in accordance with strategic aims coomet also supports metrological knowledge transfer and developing technical competence for innovations and scientific research. for the purpose of implementing joint research projects a tc 5 was established. cooperation within tc 5 aims at activating the nmis of coomet member countries in the global integration process in science, technology and high-end manufacturing. acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 92  achieve compatibility of measurement standards and harmonize the requirements imposed on measuring instruments and methods for their metrological control;  recognize the equivalence of national certificates authenticating the results of metrology activities;  exchange information on the current status of national metrology services and their development;  collaborate in developing metrology projects; and  promote the exchange of metrology services. a considerable advance in the formation of the current aims and tasks of the organization occurred as a result of several coomet committee meetings. at these meetings, decisions were made to increase the effectiveness of coomet activities. it meant that planning of cooperation and informing about activities, interaction with other international and regional metrology organizations, etc. should be improved. according to the coomet first development program for 2001-2002 there was developed and approved the organizational structure of coomet, according to which structural and working bodies in all fields of cooperation covered by the mou were created (see figure 1). this structure provided wide involvement of qualified specialists of nmis of member countries into coomet activities. transformations directed toward the improvement of organization activities were reflected in the mou and rules of procedure of coomet, developed and fixed in the appropriate documents. in 2005 the mou and rules of procedure were amended with respect to the election of the coomet president, according to which in the year before the end of the acting president’s term, the future president is elected and authorized for the next 3 years. one more important branch of coomet activity is the development and adoption of the conception of coomet in 2005. conception of cooperation and activity of coomet determines the strategy tasks from a mediumterm and long-term perspective and provides for their implementation. among these tasks are the following:  strengthening innovation and technology components of member countries in the global system of economic and society development;  competent and economically effective participation of coomet member countries in global integration processes in the fields of science, technology, science intensive production and the economy in general;  increasing the level of competitiveness of member countries in the fields of science and technology through participation in the world market of intellectual products, science intensive products and services. 2. implementation of the cipm mra the implementation of the cipm mra [2] is directed at the fulfillment of the following tasks:  organization and holding of regional comparison of measurement standards of coomet member countries in order to assure the traceability of measurement standards to reference values of the si units;  determination of the degree of equivalence of national measurement standards;  regional and interregional reviews of the calibration and measurement capabilities (cmc) and their figure 1. current organizational structure of coomet. acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 93 publication in the bipm key comparison database (kcdb); and  evaluation of the quality management systems (qms) of nmis through external review of each qms. today, nmis from 12 coomet member countries are participants in the cipm mra. six countries among them are members of the international bureau of weights and measures (bipm) and six are associates of the general conference on weights and measures (cgpm). with the aim of providing support for nmis in coomet for the cipm mra purpose, several recommendations have been successfully developed and approved. according to the cipm mra d-04 “calibration and measurement capabilities in the context of the cipm mra,” [3] the joint committee of regional metrology organizations and bipm (jcrb) requires that cmc data presented for publication in annex c should be completely supported by an implemented quality system, controlled and approved by the local rmo, and the range and uncertainty of the cmc shouldn’t contradict the information obtained from the results of key and supplementary comparisons. for the implementation of this jcrb requirement, coomet has instituted the program of comparisons (document coomet d9). the joint committee for measurement standards (jcms) annually updates the program of comparisons and presents the results at the president’s council meeting for approval by the coomet president. according to the cipm mra d-04 prior to be submitted for the inter-regional review, the cmcs should be reviewed and approved by the rmo. coomet has established the process for the intra-rmo review. this process follows the cipm mra-d-04 and assures that the cmcs submitted for the inter-regional review have sufficient technical support. the technical committees form groups of technical experts for the review of the comparison’s results and cmcs-data. starting in 2002, the coomet quality forum directed the cipm mra realization. the evaluation of the quality management system in coomet nmis is carried out during the conduction of peer reviews by coomet auditors and technical experts once every 5 years and is coordinated by the technical committee of coomet quality forum (tc qf). the peer reviews are carried out in accordance with the rmo coomet recommendations. the tc qf is conducting monitoring of quality management systems of nmis of member countries on the basis of cross analysis of annual reports, directed to the secretariat of tc qf. currently 22 nmis from 18 coomet member countries are cooperating within the quality forum. 3. involving new parties in the cipm mra providing for the traceability in the coomet region has required great attention and support for countries of central asia and the caucasus region in the development of national metrology infrastructures, harmonization with international requirements, improving the national standards of these countries, and preparing these nmis for signing the cipm mra. countries which are on the way to signing: azerbaijan, armenia, kyrgyzstan, tajikistan and uzbekistan. aiming for stable development and cooperation, in the near future coomet plans to provide for the preparation of these countries to sign and realize the cipm mra, for active participation of their national measurement standards in international comparisons, for the creation of quality systems for their nmis, and for implementing the requirements of iso/iec 17025. in order to support these countries, subcommittee “support in developing basic metrological infrastructures of coomet member countries” was established in 2008 within coomet tc 4 for “information and training.” this sc has to solve the following tasks in the countries of the region: assistance in the preparation for signing the cipm mra; preparation of the staff for qms application according to iso/iec 17025; assistance in conducting comparisons and preparing cmcs; training for national metrology staff; and the organization of training workshops. with the financial support within the realized ртвcoomet technical cooperation project titled, “support of cooperation between member countries of the regional metrology organization coomet,” several workshops were organized in 2008 – 2012 by coomet tc 4 for directors and experts of the nmis. for example, workshop for coomet nmi’s internal auditors (see figure 2) according to iso/iec 17025 was held 7–8 of november 2012 in nism (chisinau, republic of moldova). workshop participants: 36 representatives from 12 countries azerbaijan, armenia, belarus, germany, georgia, kazakhstan, lithuania, moldova, russia, slovakia, tajikistan, ukraine. the workshop consisted of theoretical and practical parts. next items were presented and discussed:  the basics of qms in nmi;  coomet regulations and procedures on the assessment of qms nmi;  requirements according to iso/iec 17025. the “model” internal audit of nism laboratories was carried out. after “model” internal audit the participants were tested and received the certificates. considering the great importance of the cipm mra in broadening the economic, scientific, technical, and international cooperation, as well as eliminating technical barriers to trade, the activity performed in this field will surely result in signing of the metre convention by the governments of the above-mentioned countries or acquiring the status of an associate to the general conference of weights and measures (cgpm) in order to participate in the figure 2. participants of the workshop in moldova. acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 94 implementation of the cipm mra in the near future. 4. cooperating in the field of legal metrology coomet mou contains a number of regulations which are absent in the by-law documents of other regional organizations. for example, coomet activities concern not only scientific but also legal metrology. legal metrology is one of the well-developed areas of cooperation in coomet member countries. at the 20th coomet committee meeting in 2010 [4], a new structure of tc 2 for legal metrology was approved. the new structure consists of the following subcommittees:  sc 2.1 harmonization of regulations and norms  sc 2.2 technologies of measuring devices and systems in legal metrology  sc 2.3 competence assessment of bodies in legal metrology  sc 2.4 legal metrological control (lmc)  the tasks of these scs are the following:  sc 2.1: acceptance or adaptation of approved international or regional documents (e.g: viml, oiml documents, welmec guides, ec recommendations).  sc 2.2: development of test procedures for measuring instruments (mi), including software and measuring systems, but also data transfer and other future technologies.  sc 2.3: development of criteria for the assessment of verification laboratories and other parties.  sc 2.4: establishment of projects for the elements of lmc (surveillance qm, market surveillance, field surveillance). the main purpose of the changes in the structure of tc 2 was to improve the efficiency and optimization of the legal metrology activities based on the experience of the international organization of legal metrology (oiml) and other regional legal metrology organizations. the new structure of tc 2 allows for further expansion of cooperation in the field of legal metrology. the discussion on the new content of work within tc 2 shows that the countryspecific interests should be considered. all tc 2 member states have the same aim, which is to build and enforce an operational, effective system for legal metrology. the new subcommittee structure is open for different realizations and for future changes in the methods for legal metrology. one example is the current low interest in market surveillance of several states, while others use this system more than the preventive verification system. so the question for all is how mutual acceptance can be created in the case of free trade of products, i.e., prepackaged goods or measuring instruments. in this context, the growing importance of conformity assessment was noted. 5. joint research in metrology as a result of many years of discussion of the possibility to realize joint research projects and their sources of funding, a technical committee for joint research in metrology, tc 5, was established in 2009. the tasks of tc 5 for the near future are:  to identify common research areas;  to determine priority fields in research and development;  to determine the efficiency of projects for the economy of coomet member countries; and  to identify those interested groups that benefit the most from the implementation of the projects. it should be noted that there are a number of research projects that are currently implemented within coomet: earth rotation period (erp) determination on the basis of data from observatories of coomet countries; metrology of nanotechnology, standardization of eu-152 radionuclide solution; etc. for example, within erp coomet project, in 2010 the observatories in russia, ukraine, uzbekistan, bulgaria, poland, and the czech republic made routine star and satellite observations and then transmitted the observation data to the erp processing and calculating centre at vniiftri. an exchange of erp observing data and calculating results was made between the participating countries and the international and national centres for erp determination. the calculations of the pole coordinates and duration of the day by the results of gps observations at the stations on the territory of russia were made on a regular basis. the accuracy of erp determination by means of all the techniques of the country participants was about 0.0002″ and 0.02 ms with regard to the pole coordinates and to the universal time, respectively. these values closely approach the accuracy of products of the international earth rotation service (iers). however, the number of these projects is rather small. therefore, for realization of significant tc 5 tasks, the following project was initiated: coomet 492/de/10 “development of a concept for joint metrology research in coomet member countries”. within this project the working group (wg) prepared the questionnaires for conducting the following:  state-of-art analysis;  research needs of participating countries;  structuring and prioritizing of potential research projects;  identification of research subjects of common interest (e.g., metrology for energy, for environment, for health, security and safety, etc. – grand challenges in the field of metrology);  expected social and economic impact of the research and development outcome. the meeting of the wg was held in july 2012 in nsc “institute of metrology”, ukraine. at this meeting questions connected with the need of joint research in coomet were considered, common tasks and the scope of the europeanasian metrology research venture (eamrv) were discussed, as well as the aspired social economic impact, possible styles of cooperation, procedures for determining and application of eamrv etc. further, the results from three questionnaires were evaluated by the wg and were sent to the representatives of coomet member-countries:  questionnaire no.1 “nmi’s current research-anddevelopment resources, capabilities, capacities and international cooperation”; acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 95  questionnaire no.2 “aspired capacity, capability and quality of the national calibration, measurement and testing infrastructure”;  questionnaire no.3 “metrology for the future and demands on joint metrological research”. in order to realize this project, coomet proceeds from world achievements in the field of metrology related to science, industry and economy of all cooperating countries, and, at the same time, establishes a self-contained metrology research strategy for the european-asian transition area, including central asian and caucasian regions, tailored to particular economical needs of its member countries. the development and implementation of joint projects in coomet will contribute to basic science and technology, as well as stimulate innovations to solve metrology problems in coomet member countries. references [1] http://www.coomet.net, http://www.coomet.org. [2] http://www.bipm.org/en/cipm-mra/mra_online.html. [3] http://www.bipm.org/utils/common/cipm_ mra/cipm mra-d-04.pdf. [4] p. neyezhmakov. “20 years of coomet: we measure together for a better tomorrow”, oiml bulletin. – july 2011, vol. lii № 3. – pp. 32-37. solutions and limitations of the geomatic survey of an archaeological site in hard to access areas with a latest generation smartphone: the example of the intihuatana stone in machu picchu (peru) acta imeko issn: 2221-870x march 2022, volume 11, number 1, 1 8 acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 1 solutions and limitations of the geomatic survey of an archaeological site in hard to access areas with a latest generation smartphone: the example of the intihuatana stone in machu picchu (peru) valerio baiocchi1, silvio del pizzo2, felicia monti1, giovanni pugliano3, matteo onori1, umberto robustelli4, salvatore troisi2, felicia vatore1, francisco james león trujillo5 1 dipartimento ingegneria civile, edile ed ambientale(dicea), sapienza università di roma, via eudossiana 18, i00184, italy 2 dipartimento di scienze e tecnologie, università degli studi di napoli "parthenope", centro direzionale isola c4, i80143 napoli, italy 3 dipartimento ingegneria civile, edile ed ambientale, università degli studi di napoli "federico ii", via claudio 21, i80125 napoli, italy 4 dipartimento di ingegneria, università degli studi di napoli "parthenope", centro direzionale isola c4, i80143 napoli, italy 5 carrera de ingeniería civil, facultad de ingeniería y arquitectura, universidad de lima, perù section: research paper keywords: intihuatana stone; machu picchu; gnss; gps; sfm; xiaomi; dsm citation: valerio baiocchi, silvio del pizzo, felicia monti, giovanni pugliano, matteo onori, umberto robustelli, salvatore troisi, felicia vatore, francisco james león trujillo, solutions and limitations of the geomatic survey of an archaeological site in hard to access areas with a latest generation smartphone: the example of the intihuatana stone in machu picchu (peru), acta imeko, vol. 11, no. 1, article 20, march 2022, identifier: imeko-acta-11 (2022)-01-20 section editor: fabio santaniello, university of trento, italy received april 7, 2021; in final form february 27, 2022; published march 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: valerio baiocchi, e-mail: valerio.baiocchi@uniroma1.it 1. introduction the geomatic survey of archaeological remains is always a difficult task due to the complex accessibility of sites whether they are located in urban or more remote areas. in fact, remains in densely populated areas have problems of accessibility because they are often located in underground areas with limited space [1]. on the other hand, remains in sparsely populated areas are often remote and difficult to reach with bulky equipment [2]-[6]. in the present work we wanted to test the geometric accuracy that can be obtained by creating a three-dimensional point cloud model from images acquired using a latest generation smartphone (xiaomi mi9). the three-dimensional model was framed in the geodetic datum wgs84 by means of differential measurements performed with the same smartphone thanks to the recent possibility of writing gnss rinex observation files made possible by the android operating system (from version 7). abstract archaeological remains need to be geometrically surveyed and set in absolute reference systems in order to allow a "virtual visit" and to create "digital twins" useful in case of deterioration for proper restoration. some countries (e.g., peru) have a vast archaeological heritage whose survey requires optimized procedures that allow high productivity while maintaining high standards of geometric accuracy. a large part of peru's cultural heritage is located in remote areas, at high altitudes and not easily accessible. for this reason, it is of great interest to study the possible applications of easily transportable instruments. in this study it was verified how the capabilities of the latest smartphones in terms of absolute differential positioning and photogrammetric acquisition can allow the acquisition of a geometrically correct and georeferenced three-dimensional model. the experimentation concerned a new survey of the intihuatana stones at machu picchu and its comparison with a previous survey carried out with a much more complex laser scanning instrumentation. it is important to note that both the photogrammetric survey and the gps/gnss survey were carried out with the same smartphone taking full advantage of both features of the same mobile phone. relative comparison to an existing point cloud provided differences of 2 millimeters in mean with an rmse of 2 cm. the absolute positioning accuracy compared to a very large-scale cartography appears to be of the order of one metre as was expected mainly due to the high distance of the gps/gnss permanent stations. mailto:valerio.baiocchi@uniroma1.it acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 2 1.1. what are intihuatana stones? all the important inca cities had an intihuatana (or intiwatana) stone, whose most accredited translation is "the place where the sun binds", it is believed that they indicated in some way the dates of the solstices. there are many theories concerning the history of the city of machu picchu and the meaning of the intihuatana stone itself, but scholars are yet to reach a consensus. giulio magli [7], an italian archaeological astronomer, proposes that the inca ritualistically travelled from cusco to machu picchu. the inca took this pilgrimage to replicate the mythical journey that the first inca thought their ancestors took from the island of the sun in lake titicaca. magli believes that the pilgrimage concluded at the highest peak in the main ruins, the steps leading to the intihuatana stone. in any case, the spanish invaders, wanting to abolish the inca religious beliefs, soon after their arrival in peru in the fourteenth century destroyed almost all the intihuatana stones they found in the various cities except for some of them, including machu picchu, probably because of the city's difficult accessibility (figure 1). this last theory, however, contrasts with the fact that the city of machu picchu was never really completely "lost" but only abandoned, the inhabitants of the area have always known its existence and led the first scientific research missions at the beginning of the twentieth century [8]. 1.2. latest smartphone geomatic capabilities mobile phone technology is producing cameras with increasing resolution, with some of the latest smartphone models reaching 100 megapixels [9]. it is important to consider that resolution alone is no guarantee of correct photogrammetric reconstruction; the limited size of a mobile phone's camera does not allow to achieve the geometric characteristics of a digital single lens reflex as well as of a photogrammetric camera. liquid lens technology is also soon to be released, which will make the optical sensors of smartphones much more versatile but, on the other hand, calibration or self-calibration of the optics will be more complex. in any case, smartphone images have provided interesting results, especially considering their easy transportation [10], [11]. in these and other works in the literature on the use of smartphones for photogrammetry, quite variable precision and accuracy have been observed, ranging from tens of centimetres [10] to a few centimetres [11], the causes of this variability are still being studied by the scientific community. a very important factor is the use of ground control points (gcps) without which there are poor and uncontrolled results [12], [13]. other factors that influence the final results can be: the size and shape of the object to be detected, the acquisition distance and, obviously, the quality of the camera optics; it is precisely because of this last factor that it seems that the most recent smartphones are getting closer and closer to the classic professional cameras, which in any case almost always give more accurate results [10]. at the same time the android operating system has recently (august 2016) released the possibility to access to raw global navigation satellite systems (gnss) measurements on several (but not all) android devices, allowing to write phase and/or code observations in a rinex file similar to the procedure employed by professional receivers, such as it happens for geodetic measurements [14]. mobile phone manufacturers, stimulated by this availability, have made terminals containing dual frequency capable chips which potentially allow for improved accuracy due to the possibility of estimating, as is known, the delay of the satellite signal due to the crossing of the ionosphere. unfortunately, till now, double frequency gnss chips embedded in mobile phone limit this possibility to gps (“l1/l5” frequencies) and galilelo constellations (“e1/e5” frequencies) while glonass and beidou, can be observed only in single frequency. this limitation combined with gnss smart phone antennas once again of reduced dimensions, and other hardware limitations, do not allow to reach the millimetric accuracies of the geodetic receivers but metric and decimetric accuracies are possible [15], [16]. the combined use of these two features of modern mobile terminals is of particular interest, because what most affects the quality of three-dimensional models from photogrammetry is the presence and accuracy of control points that are needed to improve the intrinsic geometry of the camera, to georeference the model and scale it correctly. in other words, without gcps, the model is generally deformed and only roughly georeferenced and scaled. structure from motion (sfm) algorithms typically use the positions that they read on the single frames that are acquired by the cameras through the built-in gps/gnss receivers in point positioning mode which produces accuracies in the range of tens of meters. the possibility to acquire ground control points (gcps) and sfm images from one single device with potentially decimetric or centimetric accuracy disclose new possibilities for prompt surveys, especially in areas with access difficulties such as archaeological ones. 2. materials and surveys performed as already mentioned, the intihuatana stone of machu picchu is one of the few that can still be observed, however, given its historical and cultural importance as well as damages caused by tourists in the past, access is restricted. furthermore, the site of machu picchu is, as is well known, not easy to reach, since it is an entire inca city within which the accessibility routes are those of the time and obviously cannot be modified. it should be noted (again figure 1) that the intihuatana stone is practically at the highest and most central point of the site, and therefore at the most difficult point to reach with heavy instruments. all of the above shows that the survey of such an archaeological remain may require a complex logistical and authorisation process to be carried out using traditional techniques. the interest in studying the geometric characteristics of the intihuatana stone, and in particular its alignment with respect to the geographic north, suggested that it would be advantageous to use a recently released smartphone, which has photographic and positioning characteristics useful for the reconstruction of an accurate three-dimensional model. in particular, smartphones with up to five different focal lengths have recently become popular, allowing surveyors to select the most suitable optics for the survey at hand without having to use optical zooms, which require varying calibration parameters, making the process much more complex. nevertheless, it is necessary to calibrate the terminal for each of the optics in use and care must be taken to ensure that the specific calibration for that smartphone lens is applied to each acquisition. it is actually quite simple to change the optics, for example in the android operating system, in the "pro" mode of the camera application, it can be changed instantly between the various optics available (three in the smartphone used in this test, but up to five in more recent smartphones) much more quickly than with traditional cameras. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 3 in this regard, it is perhaps worth mentioning the announced (by xiaomi company) imminent release of the first smartphones with liquid lenses, which promise to significantly improve the quality of leisure photography by allowing different focuses to be used in the same image, but these same features could create difficulties for the correct use of the photogrammetric equations that have traditionally been written for solid-state optics. anyway, the latest generation devices allow to set the camera focus mode as well as the aperture and the shooting time like a professional camera, furthermore the latest generation high-end smartphone are equipped with several cameras characterized by different focal length. such configuration allows to conduct a survey using an adaptive and swift approach, specifically the user can easily modify the focal length switching from a camera to another, in order to choose the suitable one in according to the environment conditions. the focal length is an important parameter, since it set the field of view of the camera, moreover, even others camera calibration parameters such as distortion coefficients are strictly related to the focal length [17]. a xiaomi mi9 terminal was used for this experiment, which is equipped with sony 48 mpix ultra wide-angle ai triple camera: a 48 mpix primary camera with a pixel size of 0.8 µm ƒ/1.75 aperture, 12 mpix telephoto camera with a pixel size of 1.0 µm ƒ/2.2 aperture and a16 mpix ultra wide-angle lens with a pixel size of 1.0 µm ƒ/2.2 aperture [18]. at the time of the present experimentation (april 2019) this camera was considered at the top range of mobile phone cameras, although it has been overtaken by other models in recent months [9]. its size (157.5 x 74.7 x 7.6 mm) and weight (173 g.) do not generate any transportation problems and it is even possible to take more than one terminal with you for specific needs. at the time of experimentation its cost was that of an average mobile phone, just over 500 euros, but now it is possible to buy it refurbished for less than 300 euros, to give an idea of the investment required, which is certainly lower than that of other instruments that could be certainly more accurate. in the present experiment, the feature of interest was recorded figure 1. the intihuatana stone location in machu picchu. figure 2. three-dimensional model of intihuatana stone, acquired and georeferenced with xiaomi mi9 smart phone. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 4 in its entirety with intermediate optics. additional images were acquired using wide angle optics for comparison. the geometry and sequence of the images to be acquired for correct photogrammetric reconstruction using smartphones is the subject of lively debate in the scientific community, which has also developed useful guidelines [19]. for this experiment, the authors have decided to proceed according to their experience gained in previous experiments [1], [2], [10] and therefore to use mainly the intermediate focal length with a single horizontal strip, maintaining an overlap between one frame and the next never minor than 60 % in the longitudinal direction. the images taken from different points of view, along the entire accessible area near the intihuatana stone, were subsequently processed with agisoft metashape software version 1.5.0 [20], obtaining a complete three-dimensional model of the visible part of the stone itself (figure 2). figure 3. reference laser scanning model. figure 4. reference laser scanning model and photogrammetric model co-registered. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 5 image processing in the metashape software requires personnel with at least basic photogrammetric knowledge, but many of the functions are automated, so the time required by the operator is reduced to a few hours, while processing time can be up to a few days on average performing hardware and for very detailed models. obviously, one of the most important factors for processing time is the number and resolution of the images. the smartphone itself was used in static mode to collect ground control points from the only natural points measurable on the 3d-model obtained by sfm which were the tops of the fence posts (figure 2). we used three points on the fences; the fourth point had in fact a less extensive view of the sky and gave results that were not entirely congruent with the other three and is a likely outlier; it is therefore only included in the photogrammetric software as a check point (cp) and consequently not used to estimate the rototranslation of the model. we could not use artificially marked points because this would have required specific permissions and the blocking of tourist flow. of course, from a photogrammetric point of view, it is much more correct to survey points all around the monument even with artificial targets but in this case, it was not possible. however, we decided to carry out the experimentation even in these unfavourable conditions because they are very similar to those encountered in real field situations due to access difficulties or morphologies that are unfavourable for gps/gnss surveying, such as the presence of steep slopes near the monuments to be surveyed due to excavation works. the results are nevertheless interesting despite the less-than-optimal geometry of the points. the survey with raw gps/gnss data acquisition has been possible thanks to the app rinex on version 1.3; rinex on utilizes the measurements of the new android raw measurements api to produce rinex observation and navigation message files. the app was written by nsl as part of the flamingo project [21]. it should be noted that, as mentioned above, the dual frequency is only observable for galileo satellites (e5 frequency) and the latest generation gps satellites (l5 frequency). in addition, at least on the terminal we used, the possibility of writing phase observations in the postprocessing files seems to be disabled, limiting the possibilities of processing observations both in "classic" post-processing mode and in "precise point positioning" (ppp) mode, which would be very useful in these areas given the great distance from the permanent stations of the peruvian correction network. in other words, since only code observations can be recorded, acquisition times have to be prudently extended, the achievable accuracy is reduced to about 1 metre and the usefulness of the dual frequency is practically lost [22]. this limitation is even more incomprehensible given that it was possible to write phase observations with the previous model "xiaomi mi8" [23] of which the "mi9" is the evolution. this determined the necessity to process the rinex files in post processing code using the open-source software rtklib 2.4.2 [24], which would have potentially allowed to process all four gnss constellations. unfortunately, since it was only possible to operate the code difference relative to the permanent stations of the national geodetic network of peru, it was necessary to limit ourselves to the use of only the gps and glonass constellations. in particular, the observations were differentiated with respect to the stations of abancay (ap01) and cusco (cs01), both about 80 km away with significant differences in altitude [25]. by differentiating with respect to both stations, results were figure 5. reference laser scanning model and photogrammetric model compared with c2m distance signed tool (distances are in metres). acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 6 obtained with an estimated accuracy of 0.8 1 metre, which, considering the relatively short measurement sessions possible (so as not to hinder the flow of tourists) of about 20 minutes and the distance from the permanent stations can be considered absolutely satisfactory. the whole survey took less than an hour and a half, most of which was spent surveying the gps/gnss points, while the photogrammetric survey took less than ten minutes. in this regard, it must be further specified that according to recent research [26] such times could be considerably shortened (about 5 minutes per point) if permanent real time gps/gnss stations (rtk) were available, as for example in europe; on the other hand, in areas even more remote than those under study, it would be necessary to use ppp techniques [27] which require about one hour per point. all survey operations do not require any particular specialisation and can be carried out by one person. 3. results and discussion in order to check the results obtained from the smartphone device a comparison between the photogrammetric model and a 3d model from a lidar survey was performed. indeed, the university of arkansas [28] has released on the web a 3d model of the archaeological site of machu picchu performed using an optech ilris-3d laser scanner, that is a long-range lidar (figure 3). the laser scanner survey can be considered as reference, to estimate the accuracy as well as the completeness achieved by the photogrammetric solution. unfortunately, the lidar model (figure 3) is not georeferenced, indeed, it is linked to a local reference system while the scale is correct. however, from the university of arkansas web page for the specific project [29] it can be seen that the scanning resolution was set at 3 cm and that at that distance the estimated accuracy is 7 mm. using the classical icp (iterative closest point) technique the two models have been registered in a common reference system (figure 4). therefore, a comparison between the two models was performed using the tool c2m present in the open-source software cloudcompare ver. 2.10.3 [30]. for each 3d point extracted by the photogrammetric procedure was computed the signed distance from the surface derived by the lidar survey. the result is reported in figure 5 using a coloured map. a statistical analysis, of the result obtained from the comparison between the two models, was conducted. the computed distance showed a remarkable agreement as reported in the histogram of the figure 6. specifically, a gaussian curve fitting was carried out to the signed distance, obtaining a curve with a mean of 2 millimetres and a standard deviation of 1 centimetre. however, it can be observed that the average is very close to zero (although there is a small amount of systematic shift), that the maximum deviations are below 5 centimetres and that most of these are in the 2.5 cm range. these results are absolutely interesting, particularly when compared with previous research [10]-[12], but it should be noted that in our case the lidar cloud was adapted to the photogrammetric cloud because the lidar cloud was not georeferenced. these results can therefore confirm a relative coherence between the two clouds, but not absolute. finally, we wanted to evaluate the possibilities of some software to reconstruct missing parts, in particular scann3d [31] in android environment and trnio [32] in ios environment. this evaluation was interesting in this case because the accessibility to the monument was not complete all around figure 6. histogram of differences between reference laser scanning model and photogrammetric model compared with c2m tool and the fitted gaussian curve computed (distances are in metres). acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 7 the monument itself due to the already mentioned protection needs. the 3d model obtained by the use of the first software was unsatisfactory and is not shown here; on the other hand, the result obtained with trnio is shown in figure 4 where also some parts that are badly reconstructed can be easily detected. it was much more complex to verify gnss measurements accuracy, defined as the difference between the results obtained and the actual values of the same point coordinates. in order to be able to calculate this statistical parameter it is in fact, as is well known, necessary to know the true values with an accuracy higher than that expected for the measurement system on a suitable set of points. usually, in geomatics and geodesy, such verifications are carried out using trigonometric points whose coordinates are known a priori with geodetic accuracy. unfortunately, at the machu picchu site there is only one trigonometric point on maps, but it is not unambiguously identifiable in the field, which suggests that it may have been removed, as often happens at archaeological sites of considerable interest. it is obviously not possible to compare with the coverage available on the web such as google earth whose estimated planimetric accuracies are generally lower than those expected for our survey methodology [33]. official large-scale cartographies of the area are not available, with the exception of a 1:750 nominal scale map produced by the ministry of culture of peru in 2014 (figure 7); considering a graphic error of 0.2 mm, its planimetric accuracy could be 15 cm, but since there are no metadata of the cartography itself, this must be considered only as a hypothesis. in any case, the comparison with this cartography showed a good agreement in line with what was verified in previous experiments [10], [11], [12], [13] even if the measured points cannot be identified with certainty because they didn’t exist in 2014 and so they are not reported on the map. 4. conclusions and future perspectives the present experimentation has shown that it is possible to carry out a complete and expeditious georeferenced geomatic survey with the latest generation smartphone. the ease of transport and the simplicity of the operations greatly facilitates the survey work both in terms of logistics and authorisation. the final results are at least comparable with those previously carried out on the same site with laser scanning. the possible applications of such surveys are numerous, ranging from rapid and efficient documentation to the valorisation of sites that are difficult for the general public to access, but also to true photogrammetric surveys where logistical situations make it very costly or impossible to survey with more rigorous instruments. it is interesting to study in the future the possibilities offered by smartphones that also acquire dual frequency phase gnss observations; this would also allow the application of ppp postprocessing, which could 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http://wayback.archive-it.org/6471/20150825200335/http:/cast.uark.edu/home/research/archaeology-and-historic-preservation/archaeological-geomatics/archaeological-laser-scanning/laser-scanning-at-machu-picchu.html http://www.cloudcompare.org/ http://www.cloudcompare.org/ http://www.cloudcompare.org/ https://play.google.com/store/apps/details?id=com.smartmobilevision.scann3d&hl=it https://play.google.com/store/apps/details?id=com.smartmobilevision.scann3d&hl=it http://www.trnio.com/ https://doi.org/10.1080/17538947.2015.1031716 an innovative correction method of wind speed for efficiency evaluation of wind turbines acta imeko issn: 2221-870x june 2021, volume 10, number 2, 46 53 acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 46 an innovative correction method of wind speed for efficiency evaluation of wind turbines alessio carullo1, alessandro ciocia2, gabriele malgaroli2, filippo spertino2 1 dipartimento di elettronica e telecomunicazioni, politecnico di torino, corso duca degli abruzzi 24, 10129 turin, italy 2 dipartimento energia, politecnico di torino, corso duca degli abruzzi 24, 10129 turin, italy section: research paper keywords: renewable energy sources; wind energy systems; wind speed; power measurement; uncertainty citation: alessio carullo, alessandro ciocia, gabriele malgaroli, filippo spertino, an innovative correction method of wind speed for efficiency evaluation of wind turbines, acta imeko, vol. 10, no. 2, article 8, june 2021, identifier: imeko-acta-10 (2021)-02-08 section editor: ciro spataro, university of palermo, italy received january 15, 2021; in final form april 28, 2021; published june 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: alessio carullo, e-mail: alessio.carullo@polito.it 1. introduction the increasing energy demand and the requirements of minimal environmental impact have pushed towards to a huge increase of renewable energy sources (res). a drawback of these sources is their intermittency, which can be mitigated by means of the integration of storage units, e.g. electrochemical batteries [1]-[3]. among the res, wind turbines (wts) represent a reliable and clean source of electricity with low marginal costs [3]. new wind power plants have been installed in europe in 2020, with a cumulative rated power of about 7 gw, and an increase of about 10 gw is expected in 2021, thus reaching a cumulative capacity of about 250 gw [4]. in this framework, offshore applications will represent about 20 % of new installations in the period 2020-2023, especially in the netherlands, ireland, norway and france. wts can work at fixed speed or variable speed, and the latter are able to adjust the rotor speed, thus following the maximum aerodynamic power of the wind [5]. on the other hand, their control requires the wind speed to be measured through an anemometer, thus increasing the overall cost and the size of the system. the anemometer is usually located on the back of the turbine, where a wind speed that is lower than the wind speed entering in the rotor is measured. for this reason, the use of the measurement of this anemometer leads wts to exhibit experimental performance that seems better than their nameplate specification, since the manufacturer states the power curve with reference to the wind speed at the entrance of the rotor. in addition, manufacturer-stated performance refers to ideal conditions of minimum turbulence, flat terrain and absence of wakes due to obstacles [6]. a reliable estimation of wt performance then requires the measured wind speed to be corrected and, for this reason, two different correction methods have been defined in technical specifications and international standards. the first method does not take into account the effects of the wakes of other turbines and obstacles, while the second method filters the considered direction of the wind in order to remove these wake effects. wake is a sort of loading effect, as occurs in electric circuits. wakes are long trails of wind abstract the performance of horizontal axis wind turbines (wts) is strongly affected by the wind speed entering in their rotor. generally, this quantity is not available, because the wind speed is measured on the nacelle behind the turbine rotor, providing a lower value. therefore, two correction methods are usually employed, requiring two input quantities: the wind speed on the back of the turbine nacelle and the wind speed measured by a meteorological mast close to the turbines under analysis. however, the presence of this station in wind farms is rare and the number of wts in the wind farm is high. this paper proposes an innovative correction, named “statistical method” (sm), that evaluates the efficiency of wts by estimating the wind speed entering in the wts rotor. this method relies on the manufacturer power curve and the data measured by the wt anemometer only, thus having the possibility to be employed also in wind farms without a meteorological station. the effectiveness of such a method is discussed by comparing the results obtained through the standard methods implemented on two turbines (rated power = 1.5 mw and 2.5 mw) of a wind power plant (nominal power = 80 mw) in southern italy. mailto:alessio.carullo@polito.it acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 47 in turbulent regime and with lower speed with respect to the entrance of turbines rotor. this effect needs to be minimized because if the wind flow entering in the wt rotor is affected by the wake of another turbine, its speed is lower than in the wakefree conditions, i.e. energy production is reduced. a common requirement of these methods is the availability of two quantities: the wind speed vwt measured by the anemometer and the wind speed vstat detected by a meteorological mast, which has to be close to the turbine under investigation. unfortunately, a meteorological mast is not always present in wind power plants, thus preventing the implementation of these correction methods. to overcome this limitation, the present work proposes an alternative method that relies on the manufacturer power curve and the wind speed detected by the turbine anemometer, thus not requiring the measurements provided by a meteorological mast. in section 2, a review of the two standard methods is presented and the new correction method is described. section 3 defines the concept of yearly average efficiency (which takes into account the energy generated by a wt), and describes the parameters availability and capacity factor. in section 4, a case study is presented related to a wind power plant in southern italy. section 5 reports the obtained results for two different wts that are located in two different areas of the considered power plant. finally, section 6 summarizes the main outcomes of this work. 2. correction methods an expansion of the stream tube occurs before and after the passage in the three-blade rotor. therefore, the cross section of the wind flow increases, while its kinetic energy (thus, its speed) decreases. the presented methods aim to correct the wind speed detected by the anemometer behind the turbine rotor, calculating the corresponding speed at its entrance. before the application of one of the proposed correction methods, a preliminary normalization to the reference air density ρref = 1.225 kg/m3 is performed, since manufacturer specifications refer to this condition. in particular, for wts with active power control, experimental results are corrected according to the following expression [6]: 𝑣cor = 𝑣𝑒𝑥𝑝 ∙ ( 𝜌air 𝜌ref ) 1 3⁄ , (1) where vcor is the corrected wind speed, vexp is the measured wind speed and ρair is the air density during the measurement. 2.1. method #1 – straight line method (slm) the first method requires vwt and vstat as input quantities, and consists of the following steps: 1) step a selection of the wind-speed direction. the wind direction β is properly selected in order to consider valid the assumption vstat ≈ ventr, where ventr is the wind speed that enters in the turbine rotor. in particular, experimental results are filtered in order to analyse the wind contributions flowing from the station to the wt. assuming to simplify the problem as a 2d system without the vertical coordinate, a straight line is traced between the anemometric station and the wt under test and its orientation βwt with respect to the north direction is calculated. however, if the set of experimental data is limited, a low number of experimental points is available. in this case, it is generally convenient to extend the analysis to wind speeds with orientations β = βwt ± δβ, where 2 δβ is the top angle of a triangle whose base is the rotor diameter d of the wt (d = 2 ∙ rd, by assumption, where rd is the length of a blade, neglecting the hub radius) and the third vertex of the triangle is the meteorological mast. 2) step b selection of data with ventr > vwt. as described in the step a, the wind speeds of interest flow from the anemometric station to the wt. therefore, ventr has to be larger than vwt because the kinetic energy of the wind decreases when it flows through the meteorological mast. 3) step c removal of experimental data with turbulence larger than 10 %. in each time interval, the turbulence is the ratio between the standard deviation of the wind speed and its average value in the interval. the power curve provided by the manufacturer is measured in conditions of minimum turbulence, which is generally lower than 10 % [7]. 4) step d linear regression of experimental data. in this step, a linear equation that describes vstat as a function of vwt is identified in order to estimate ventr by the line of regression of vstat on vwt, where the measurement of vwt is corrected thanks to the measurement of vstat. the goodness-of-fit of the linear regression to the experimental data is measured through the parameter r2, which ranges from 0 (no suitable model) to 1 (best model). during the design of a wind power plant, the position of the turbines has to be optimized in order to minimize their mutual wakes and maximize their energy production. however, due to different constraints, such as terrains and land morphology, these effects cannot be always minimized. therefore, the first method needs to be modified in order to remove possible errors due to mutual wakes effect. 2.2. method #2 – no wakes method (nwm) this method is similar to and consists of the same steps as the slm. however, since nwm aims to avoid the mutual wakes that affect the measurements, step a is modified. indeed, nwm does not focus the correction on the direction joining the meteorological mast and the wt; on the contrary, it investigates all the directions in which the anemometric station and the wt are not affected by the wakes of other turbines. the procedure used to determine the wind directions disengaged from any obstacles is based on the document [6]. in particular, for each obstacle in the neighbourhood of the wt, such as other operating wts or a meteorological station, the wind direction angles α that must be excluded from the analysis are calculated according to this expression: 𝛼 = 1.3 ∙ arctan ( 2.5 ∙ 𝐷 𝐿 + 0.15) + 10 , (2) where d is the rotor diameter and l is the mutual distance between the obstacle and the wt under test. after the selection of the proper wind direction, it is possible to verify the validity of the results thanks to a more sophisticated analytical model, which is named the “jensen model” or “park model” [8]. it permits to estimate the wind speed v* perturbed by the wake of a turbine using the following expression: acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 48 𝑣∗ = 𝑣0 ∙ [ 1 − 1 − √1 − 𝐶𝑇 (1 + 𝑘 ∙ 𝑥 𝑟𝑑 ) 2 ] , (3) where v0 is the wind speed not affected by wakes, ct is the thrust coefficient of a wt and depends on the wind intensity, rd is the radius of the turbine rotor, and x is the downwind distance. the parameter k is the decay constant of the wake that is estimated according to the following equation: 𝑘 = 0.5 ln ( ℎ 𝑧0 ) , (4) where h is the hub height of the wt and z0 is the roughness of the ground. according to jensen model, the wake increases linearly with x and its diffusion radius rx can be estimated as: 𝑟𝑥 = 𝑟𝑑 + 𝑘 ∙ 𝑥 . (5) it should be noted that the model considers the perturbation of the flow profile along the direction of the wind, while its perpendicular component is assumed constant (1-d model). finally, this model assumes that k is a constant parameter that depends only on h and z0. 2.3. method #3 – statistical method (sm) the alternative method does not require experimental data provided by a meteorological station [9]: the input quantities are the wind speed measured by the wt anemometer and the power curve provided by the manufacturer. the assumption behind this methodology is that the power curve of the wt manufacturer is the locus of the points where the generator operates with the best performance. therefore, the analytic relation between vwt and the wind speed provided by the wt manufacturer (for the same output electric power pk) is derived. figure 1 reports the scheme of the aforementioned methodology. more in detail, the methodology consists of the following steps: 1) step a removal of experimental data with turbulence larger than 10 % [7]; 2) step b selection of the experimental set sk. one of the available working point pk = p(vk) is selected on the power curve provided by the wt manufacturer. then, a set sk of experimental data is identified such that the electric output power lies in the neighbourhood of pk, i.e. in the interval between pk∙(1-ε) and pk∙(1+ε). in this work, the value of ε was set to 0.01, based on the consideration that output powers within a ± 1 % interval are not distinguishable due to the typical uncertainty of the equipment that is used to measure this quantity. the set sk is described as: 𝑆𝑘 = {[𝑣wt,𝑖, 𝑃(𝑣wt,𝑖)]: 𝑃(𝑣wt,𝑖) ∈ [𝑃𝑘 ∙ (1 − 𝜀) ÷ 𝑃𝑘 ∙ (1 + 𝜀)] } . (6) 3) step c calculation of the empiric cumulative distribution function (ecdf) of the wind speed. the ecdf of the wind speed corresponding to the selected value of vk is calculated, as shown in figure 2 (blue dots), which refers to the value vk = 12 m/s. the same figure also highlights how the calculated ecdf is well approximated by the cdf f(vwt) (red line) corresponding to the probability density function (pdf) f(vwt) of the known factorial function γ [10]: 𝑓(𝑣wt) = 𝑣wt 𝑎−1 𝑏𝑎 ∙ 𝛤(𝑎) ∙ 𝑒 − 𝑣wt 𝑏 (𝑣wt ≥ 0) , (7) where the parameter a is estimated as the square ratio between the mean value and the standard deviation of sk, while the parameter b is derived as the ratio between the mean value of sk and a; 4) step d estimation of the wind-speed fifth percentile. starting from the pdf f(vwt), the fifth percentile 𝑣wt 5 % of the wind speed, i.e. the value that has the 5 % of probability to not be exceeded in sk, is selected; steps from b to d are repeated for each available working point p(vk) in the power curve provided by the manufacturer. 5) step e linear regression of experimental data. this step is similar to step d of the other methods, but in this case a linear equation is obtained between 𝑣wt 5 % and the corresponding vk. figure 1. scheme of the methodology for the sm. figure 2. example of calculated ecdf for vk = 12 m/s. 0 0.2 0.4 0.6 0.8 1 10 11 12 13 14 15 c d f wind speed (m/s) ecdf gamma cdf 0 1 2 3 0 5 10 15 20 p o w e r o u tp u t (m w ) wind speed (m/s) manufacturer curve experimental data set sk pk 0 2 4 6 8 10 12 14 2 4 6 8 10 12 v st a t (m /s ) vstat = 0.926 ∙ + 0.936 r2 ≈ 1 ) steps a-b step e steps c-d 0 0.2 0.4 0.6 0.8 1 11 12 13 14 15 vwt (m/s) ecdf f(vwt) vk = 12 m/s acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 49 one should note that when the wt reaches its nominal power, the correspondence between the wind speed and the output power is not unique. the output power is indeed limited to the rated value and it can be obtained with several values of the wind speed. this represents a limit of the proposed method, which is not applicable in the range of high wind speeds due to its strong dependence on the power curve of the wt manufacturer. 3. estimation of wt efficiency the efficiency of a wt is the ratio between the electrical power it produces and the aerodynamic power of the wind at the entrance of the rotor. the aerodynamic power paer of the wind can be calculated as [11]: 𝑃aer = 1 2 ∙ 𝜌air ∙ π 4 ∙ 𝐷2 ∙ 𝑣entr 3 . (8) the efficiency can be also estimated as the ratio between electrical and wind energies in a certain time interval δt. indicating the measured wt output power as pout, the efficiency can be obtained as [12]-[13]: 𝜂 = 𝑃out 𝑃aer = 𝑃out ∙ δ𝑡 𝑃aer ∙ δ𝑡 = 𝐸el 𝐸aer , (9) where eel and eaer are electrical and aerodynamic energies, respectively. with the aim of comparing the three proposed correction methods, the results will be expressed in terms of weighted yearly efficiency η*: 𝜂∗ = ∑ (𝜂𝑘 ∙ 𝐸𝑘)year ∑ (𝐸𝑘)year = ∑ (𝜂𝑘 ∙ 𝐸𝑘)year 𝐸y,exp (10) where ηk is the wt efficiency, ek is the output energy in the k th time interval (δt = 10 min), and ey,exp is the experimental yearly energy generated by the wt. thanks to the availability of an anemometric station and to the accurate selection of the wind direction, the nwm is considered as the reference method. the other two methods will be then compared to the nwm by means of the efficiency deviation δη*, which is defined as: δ𝜂∗ = 100 % ∙ 𝜂∗ − 𝜂nwm ∗ 𝜂nwm ∗ (11) where 𝜂nwm ∗ is the average efficiency estimated with the nwm. moreover, the availability factor and the capacity factor are calculated for the turbines under analysis. the first quantity provides information regarding the probability that a system is operational at a specific time: in particular, it is the ratio between the uptime of wts and their total operation time, which includes non-operation periods due to failures or maintenance actions [14]-[15]. on the other hand, the capacity factor is, for a given time interval, the ratio between the real energy produced and injected into the grid, and the ideal energy that could be generated by the turbine working continuously at its rated power. an example of centralized power station with very high capacity factor (about 90 % of one year) are units for base load operation. 4. case study the three methods previously described were applied to two wts of a wind farm in southern italy (with 43 wts, global nominal power of 80 mw and altitude between 1100 m and 1200 m) using data collected during a measurement campaign in 2017. the turbines of the wind farm have hub height of 80 m and a three-bladed rotor. their wind speed range is the following: cut-in speed vc-in = 3.5 m/s, cut-out speed vc-out = 25 m/s. however, the wind farm is divided in two parts, with two different models of turbines. in the first area, wts have a nominal power of 2.5 mw, a rotor diameter of 80 m and rated wind speed of 15 m/s. in this part, a meteorological mast (height of about 80 m) is present, measuring the quantities of interest. in particular, it is equipped with: 1) a first class cup anemometer, which acquires the horizontal component of the wind speed according to the requirements provided in [6]; 2) first class sensors that detect the wind direction according to [7]; 3) pressure, humidity and temperature sensors, which measure the environmental quantities that are used to estimate the air density at the height of meteorological mast and turbine. the anemometer provides a resolution of 0.05 m/s and its stated uncertainty is ± 1 % of the measured value in the range (0.3 ÷ 50) m/s with a minimum uncertainty of ± 0.2 m/s. the environmental quantities are measured with uncertainties of ± 2 °c for the temperature, ± 5 %rh for the relative humidity and ± 1 kpa for the pressure. in the second area of the wind farm, wts have a rated power of 1.5 mw, a rotor diameter of 77 m and a rated wind speed of 12 m/s. however, in this area, there is no meteorological station installed; hence, the only data available are provided by the anemometer located on the back of the turbines. regarding the wts of the entire plant, they are equipped with an ultrasonic anemometer that measures the absolute value and the direction of the wind speed, providing a resolution of 0.01 m/s and an uncertainty of ± 2 % of the measured value in the range (0 ÷ 60) m/s (minimum uncertainty ± 0.25 m/s). the electrical output power of the wts is measured with a relative standard uncertainty of 1 %. 5. results 5.1. results of one turbine with the meteorological station in this subsection, the results for a wt in the first area of the wind farm (which is the one equipped with a meteorological mast) are presented. the electrical power measurements pout obtained at the output of the wt (average values within 10 min time intervals) are shown in figure 3 (blue points) with respect to the measured wind speed, which is corrected according to equation (1). in the same figure, which refers to results that are collected during a time interval of approximately one year, the manufacturer power curve (red line) is also reported. one should note that a high number of observations are on the left of the manufacturer power curve, since no correction methods are applied to these experimental results. this behaviour is not realistic, since the experimental performance of the wt cannot be higher than the manufacturer's specifications. furthermore, the cut-in and cut-out wind speeds are about 3 m/s and 24 m/s, respectively, which are lower than the corresponding nominal values (vc-in = 3.5 m/s, vc-out = 25 m/s). according to the correction methods described in section ii, experimental results that show turbulence larger than 10 % are removed. furthermore, also results showing null output power for wind speed in the range (vc-in ÷ vc-out) are removed, since they refer to failure conditions of the investigated plant. before acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 50 applying the described correction methods, a preliminary uncertainty estimation is performed, taking into account the instrumental uncertainty of wattmeter and anemometer of the wt and the contribution related to the repeatability of the measured output power. as a first step, the method of bins [16] is applied: power measurements are grouped according to the corresponding wind speed measured by the wt anemometer. since its uncertainty has a maximum value of 0.5 m/s for wind speed value of 25 m/s, experimental results are grouped in uniform wind-speed bins with a width of ± 0.5 m/s. then, the mean output power is estimated for each identified group and the standard deviation of the mean is considered as the estimation of the measurement repeatability. this contribution is combined to the instrumental standard uncertainty (1 % of the measured value), thus obtaining the combined standard uncertainty u(p). the obtained results are summarized in figure 4, where the red bars refer to the manufacturer power curve, while the grey bars represent the experimental means of each group centred around integer values of wind speed. the error bars superimposed to each grey bar are the intervals pmean,i ± u(pi). even though the anomalous data points are removed, the uncorrected experimental results are not fully consistent with the manufacturer specifications yet: for wind-speeds up to 6 m/s and at 21 m/s, the electrical output power is higher than the manufacturer specifications, and cut-in and cut-out wind speeds remain the same previously estimated. implementing the slm, the wind speed direction considered in the correction is β= (231 ± 13)° and the linear regression (r2 = 0.969) results in the following equation: 𝑣𝑠𝑡𝑎𝑡 = 0.971 ∙ 𝑣𝑊𝑇 + 0.758 (12) the results after the correction with equation (12) are reported in figure 5. for wind speeds lower than 21 m/s, the corrected output power is lower than the manufacturer curve. regarding cut-in and cut-out wind speeds, the slm correction leads to an estimation of vc-in that is comparable to the rated specification (≈ 3.5 m/s), while vc-out remains lower (≈ 24 m/s). moreover, for wind speeds higher than 13 m/s, the manufacturer power curve reaches a saturation power of about 2.5 mw, while the experimental data reach a higher saturation power. this behaviour is realistic, being due to the pitch regulation of the wt: in fact, the turbine is allowed to work with a maximum power of about 104 % of rated data. this performance of the wt results in a higher energy production; however, an earlier aging of the turbine due to a higher degradation of the materials may occur as well. the direction of the wind speeds considered in the slm may be affected by turbulence, mainly due to the wakes of other turbines or obstacles. the nwm permits to solve this issue by identifying the wind speed directions in which the turbine and the meteorological mast are not affected by wakes. according to jensen model, the angular section not affected by wakes corresponds to β ranging between -26.8° and 12.7°: thus, the north direction (β = 0°) is selected for the nwm. figure 6 presents the results of jensen model for the wind directions considered in the slm and the nwm using k = 0.075, h = 80 m, rd = 40 m, and z0 = 0.1 m. the blue and the red circles represent the wt under analysis and the meteorological mast, respectively; the grey circles indicate the other wind generators, and the cones represent the areas affected by wakes. with the wind direction assumed for the slm, the wt and the station are affected by the wake of another turbine; on the other hand, with north direction, they are wake-free. the resulting regression equation (r2 = 0.979) for the nwm is the following: 𝑣stat = 0.998 ∙ 𝑣wt + 0.550 (13) figure 7 reports the results after the correction. the nwm correctly estimates the cut-in and cut-out wind speeds as they coincide with the values provided by the manufacturer (≈ 3.5 m/s and ≈ 25 m/s, respectively). the sm does not require experimental data from a meteorological mast, thus it can be used also in wind power plants without weather stations close to the wts. however, as described in section 2, this correction cannot be applied to the figure 3. turbine #1 uncorrected raw experimental data (blue dots) and manufacturer power curve (red line). figure 4. turbine #1 uncorrected experimental data after the preliminary data processing. figure 5. turbine #1 slm corrected results (grey bars) and manufacturer power curve (red bars). acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 51 last part of the power curve, where the electrical power reaches a saturation value (the nominal power). thus, the sm is applied to wind speeds in the range (4 ÷ 15) m/s, which is the rated wind speed by the manufacturer. however, the local distribution of experimental wind speeds for the year under study (red bars in figure 8) shows that this range contains most of experimental data. actually, for wind speeds of ≈ 15 m/s, the corresponding cumulative function (blue curve) is higher than 0.93, i.e. more than 93 % of wind speeds are ≤ 15 m/s. in order to achieve a better accuracy, regression equations are identified for two wind speed ranges: lower than 6 m/s, and between 6 m/s and 15 m/s. the resulting equations are the following (r2  1): 𝑣stat = 0.779 ∙ 𝑣wt + 1.493; 𝑣wt < 6 m s (14) 𝑣stat = 0.923 ∙ 𝑣wt + 0.961; 6 m s ≤ 𝑣wt < 15 m s (15) the results of the sm, which are reported in figure 9, highlight that the performance of the wt is now realistic, since it is lower than manufacturer power curve. the weighted yearly efficiencies are calculated on a yearly basis to evaluate the effectiveness of the corrections (figure 10). for wind speeds higher than the rated value, the curves converge into the manufacturer data (blue curve), while at lower wind speeds uncorrected data (red curve) have a different shape from the manufacturer curve. moreover, at low wind speeds, the necessity to correct raw data is evident: in fact, at a wind speed of ≈5 m/s, the raw weighted efficiency (η* ≈0.4) is higher than manufacturer value. actually, uncorrected data are based on the wind speed detected on the back of the turbine rotor: this value is lower than the wind speed entering the turbine, leading to an overestimation of its efficiency. regarding the corrections, the slm (green curve) estimates the lowest efficiencies, but, as previously described, its results are affected by wakes. indeed, the comparison with the nwm curve shows that the presence of wakes leads to an underestimation of the efficiency. the shape of the efficiency curve of the sm, which is based on the manufacturer data, is the closest to the shape of the manufacturer one. table 1 reports the weighted yearly efficiencies and the deviations of the slm and the sm with respect to the nwm, which is assumed as the reference. for wind speeds < 6 m/s, the two methods underestimate the efficiency, with similar deviations from the nwm of about -9.1 % (slm) and -7.9 % (sm). in the intermediate wind speeds range (6 ÷ 15) m/s, the slm underestimates the efficiency with a spread of about -3.5 %, while the performance of the wt is overestimated by the sm with a deviation of about 5.2 %. the turbine under analysis has very high energy performance in the area of the farm with the meteorological mast. its availability and capacity factors are evaluated: the wt under study is in operation for 97.2 % of time, with an average capacity factor of 29.3 %. among the turbines in this part of the wind farm, the performance of this wt is one of the best, being the average availability and capacity factors of the plant equal to 96.5 % and 22.3 %, respectively. figure 6. turbine #1 jensen model with wind directions assumed for slm (top) and nwm (bottom). figure 7. turbine #1 nwm corrected results (grey bars) and manufacturer power curve (red bars). figure 8. cumulative function and pdf of wind speed distribution. figure 9. turbine #1 sm corrected results (grey bars) and manufacturer power curve (red bars). 0 0.1 0.2 0.3 0 0.2 0.4 0.6 0.8 1 4 10 16 22 p d f c u m u la ti v e f u n c ti o n wind speed (m/s) acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 52 5.2. results of one turbine without the meteorological station this subsection presents the results for a wt in the second area of the wind farm (i.e., without the meteorological mast). the power curve provided by the manufacturer (red line) and experimental observations (blue points, corresponding to average pout values within 10 min time intervals) are presented in figure 11; the wind speeds are corrected according to equation (1). in figure 11, a high number of data are shifted on the left of the manufacturer power curve. moreover, the cut-in and the cut-out wind speeds (≈ 3 m/s and ≈ 20 m/s) are lower than the nominal values (vc-in = 3.5 m/s, vc-out = 25 m/s): thus, a correction of these data is required. first, failure conditions are excluded from the analysis by removing observations exhibiting turbulence higher than 10 % and null output power. as described in the previous subsection, a preliminary uncertainty estimation is performed starting from the instrumental uncertainty of the wt wattmeter and anemometer, and the repeatability of the measured output power. figure 12 shows the results of this preliminary analysis: the red bars correspond to the manufacturer power curve, while the grey bars represent the experimental means of each group centered around integer values of wind speed. the error bars superimposed on each grey bar are the intervals pmean,i ± u(pi). the figure confirms the preliminary results of the other turbine under study: despite data corresponding to abnormal operation are excluded, the electrical output power remains higher than the manufacturer specifications. however, cut-in and cut-out wind speeds (≈ 4 m/s and ≈ 21 m/s) are closer to the values provided by the manufacturer. this wt is in the area of the wind farm without a meteorological station, thus only the sm can be applied and the data after the correction are presented in figure 13. as described in section ii, this method cannot be applied to the part of the power curve where the power is constant (nominal power); hence, wind speeds higher than rated value (12 m/s) are excluded from the analysis. after applying the correction, the performance of the wt (grey bars) is realistic, being lower than manufacturer curve (red bars). two regression equations (r2  1) are determined for the wind speed intervals < 6 m/s, and between 6 m/s and 12 m/s: 𝑣stat = 0.946 ∙ 𝑣wt + 1.258; 𝑣wt < 6 m s (16) 𝑣stat = 0.893 ∙ 𝑣wt + 1.923; 6 m s ≤ 𝑣wt < 12 m s (17) the weighted yearly efficiencies for raw data are about 33.5 % (wind speeds < 6 m/s) and 42.6 % (wind speeds between 6 m/s and 12 m/s). after the sm correction, the weighted efficiencies are of about 27.1 % (< 6 m/s) and 34.3 % (6 m/s ÷ 12 m/s), with a decrease of about 19 % for both wind speed ranges. this turbine exhibits very high energy performance in the area of the plant without the weather station. indeed, it has the highest availability of the wind farm (99.1 %), while the average value in the second part of the plant is 97.4 %; moreover, its capacity factor is 27.9 %. this value is lower than the other wt under analysis, despite being higher than the average quantity (20.1 %) in the second area of the plant. 6. conclusions this work proposes an innovative method, named “statistical method” (sm), to evaluate the average efficiency of wind turbines by correcting the wind speed at the entrance of the rotor from nacelle anemometer. in the literature, other two methods (straight line method, slm, and no wakes method, nwm) are defined to perform this correction, taking into account technical table 1. weighted yearly efficiencies and deviations for the correction methods (turbine with meteorological station). wind speed range 𝜼𝐫𝐚𝐰 ∗ 𝜼𝐒𝐋𝐌 ∗ 𝜼𝐍𝐖𝐌 ∗ 𝜼𝐒𝐌 ∗ 𝜟𝜼𝐒𝐋𝐌 ∗ 𝜟𝜼𝐒𝐌 ∗ < 6 m/s 39.7 30.0 % 33.0 % 30.0 % -9.1 % -7.9 % 6 – 15 m/s 39.9 33.7 % 34.8 % 37.0 % -3.5 % 5.2 % figure 10. turbine #1 weighted yearly efficiencies for the proposed corrections. figure 11. turbine #2 uncorrected raw experimental data (blue dots) and manufacturer power curve (red line). figure 12. turbine #2 uncorrected experimental data after the preliminary data processing. 0 0.1 0.2 0.3 0.4 0.5 0 5 10 15 20 25 w e ig h te d y e a rl y e ff ic ie n c y η * wind speed (m/s) uncorrected data manufacturer data sm nwm slm acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 53 specifications and international standards. these correction methods require data measured by a meteorological mast close to the turbines, but the presence of this station in wind farms is rare. conversely, the correction proposed in this paper evaluates the wind speed entering in the wts rotor only relying on the manufacturer power curve and the data measured by the wt anemometer. indeed, it may be applied also in wind farms that are not equipped with a meteorological station. in the present work, these three methods were applied to a one-year experimental campaign on a wind farm in southern italy. in particular, two turbines located in two different areas of the same plant were analysed: only the first turbine was close to a meteorological mast. the effects of the corrections were evaluated representing the electrical output power by means of the method of bins and setting the width of the bins according to the uncertainty of the used anemometer. an uncertainty estimation was also performed for the output wt power, taking into account the power measurement uncertainty and the repeatability within each wind-speed bin. the results of the nwm were considered as a reference. regarding the turbine in the part including the mast, the sm performed similarly to the slm, providing comparable absolute deviations in terms of weighted efficiencies with respect to the reference. in fact, the deviations of the sm are about ± 7 % in the total range, while the quantities corresponding to the slm are ± 9 % with respect to the nwm. after the sm correction, the weighted yearly efficiency decreased between about 10 % and 20 % with respect to raw data before correction in the usual wind speed range. references [1] f. spertino, a. ciocia, v. cocina, p. di leo, renewable sources with storage for cost-effective solutions to supply commercial loads, proc. of 2016 international symposium on power electronics, electrical drives, automation and motion (speedam), anacapri, italy, 22-24 june 2016, pp. 242-247. doi: https://doi.org/10.1109/speedam.2016.7525987 [2] a. mahesh, k.s. sandhu, hybrid wind/photovoltaic energy system developments: critical review and findings, renewable and sustainable energy reviews 52 (2015) pp. 1135-1147. doi: https://doi.org/10.1016/j.rser.2015.08.008 [3] z. zhang, y. zhang, q. huang, w. lee, market-oriented optimal dispatching strategy for a wind farm with a multiple stage hybrid energy storage system, csee journal of power and energy systems 4(4) (2018) pp. 417-424. doi: https://doi.org/10.17775/cseejpes.2018.00130 [4] wind energy in europe: outlook to 2023. online [accessed 09 june 2021] https://windeurope.org/about-wind/reports/wind-energy-ineurope-outlook-to-2023/ [5] p. w. carlin, a.s. laxson, e. b. muljadi, the history and state of the art of variable-speed wind turbine 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[17] a. carullo, a. ciocia, p. di leo, f. giordano, g. malgaroli, l. peraga, f. spertino, a. vallan, comparison of correction methods of wind speed for performance evaluation of wind turbines, proc. of 24th imeko tc4 international symposium, 14-16 sept. 2020, pp. 291-296. online [accessed 09 june 2021] https://www.imeko.org/publications/tc4-2020/imeko-tc42020-55.pdf figure 13. turbine #2 sm corrected results (grey bars) and manufacturer power curve (red bars). https://doi.org/10.1109/speedam.2016.7525987 https://doi.org/10.1016/j.rser.2015.08.008 https://doi.org/10.17775/cseejpes.2018.00130 https://windeurope.org/about-wind/reports/wind-energy-in-europe-outlook-to-2023/ https://windeurope.org/about-wind/reports/wind-energy-in-europe-outlook-to-2023/ https://doi.org/10.1002/we.77 https://doi.org/10.1109/icrera.2015.7418682 https://doi.org/10.1002/we.1863 https://doi.org/10.1016/j.renene.2012.01.002 https://doi.org/10.1080/00029890.1959.11989422 https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.589.2982&rep=rep1&type=pdf https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.589.2982&rep=rep1&type=pdf https://doi.org/10.1109/mepcon.2017.8301377 https://www.imeko.org/publications/tc4-2017/imeko-tc4-2017-080.pdf https://www.imeko.org/publications/tc4-2017/imeko-tc4-2017-080.pdf https://doi.org/10.1109/eeeic.2019.8783240 https://doi.org/10.1109/tia.2020.3031547 https://www.imeko.org/publications/tc4-2020/imeko-tc4-2020-55.pdf https://www.imeko.org/publications/tc4-2020/imeko-tc4-2020-55.pdf a training centre for intraocular pressure metrology acta imeko issn: 2221-870x december 2022, volume 11, number 4, 1 6 acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 1 a training centre for intraocular pressure metrology dominik pražák1, vítězslav suchý1, markéta šafaříková-pštroszová1,2, kateřina drbálková1,2, václav sedlák1,2, šejla ališić3, anatolii bescupscii4, vanco kacarski5 1 czech metrology institute, okružní 31, 63800 brno, czechia 2 slovak university of technology, f. of mechanical engineering, nám. slobody 2910/17, 81231 bratislava, slovakia 3 institut za mjeriteljstvo bosne i hercegovine, branilaca sarajeva 25, 71000 sarajevo, bosnia and herzegovina 4 i. p. institutul naţional de metrologie, eugen coca 28, 2064 chişinău, moldova 5 bureau of metrology, bull. jane sandanski 109a, 1000 skopje, north macedonia section: research paper keywords: training centre; metrological traceability; eye-tonometer; intraocular pressure citation: dominik pražák, vítězslav suchý, markéta šafaříková-pštroszová, kateřina drbálková, václav sedlák, šejla ališić, anatolii bescupscii, vanco kacarski, a training centre for intraocular pressure metrology, acta imeko, vol. 11, no. 4, article 4, december 2022, identifier: imeko-acta-11 (2022)-04-04 section editor: eric benoit, université savoie mont blanc, france received june 27, 2022; in final form august 17, 2022; published december 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: empir project 20scp02 cefton corresponding author: dominik pražák, e-mail: dprazak@cmi.cz 1. introduction the iop belongs to the basic diagnostic indicators in ophthalmology and optometry. although this quantity is monitored also in the veterinary medicine, this paper deals exclusively with the human medicine. the screenings of the intraocular hypertension serve, first of all, for the prevention and early diagnosis of the glaucoma. hence, there is a high societal interest in the correctness of these measurements which are performed by the eye-tonometers, [1]-[4]. some european countries (czechia, germany and lithuania) consider these instruments so crucial that they are a subject of the legal metrology. however, harmonization in this area is relatively low across europe, [5]. the iop is still measured and referenced in the millimetres of mercury (1 mmhg ≈ 133.3 pa) for the historical and practical reasons, [6]. there is a consensus that the normal values should be within the range from 10 mmhg to 20 mmhg. however, the task of the metrology is to ensure traceability in the complete physiological and pathophysiological range up to 80 mmhg. 2. project history to overcome the obstacles in building the iop metrology, the national metrology institutes (nmi) of austria, czechia, germany, poland, slovakia and turkey, together with technical university in bratislava and palacký university in olomouc formed a consortium within empir programme, solving project intense, which ran from june 2017 to may 2020, [5]-[9]. the scope was much broader, of course, but the main results from the point of view of the cooperation of the european nmis in this field is the foundation of a smart specialisation concept (ssc) for the iop metrology, [10], and a training centre for iop metrology in the premises of the cmi in the city of most which was built with an essential help of the german colleagues. the advanced trainings of the cmi personnel were also accomplished and the needed technical expertise was successfully audited. an important part was a satisfying bilateral comparison in the iop between the cmi and the slovak technical university (stu) in the beginning of 2020 during which two clinically tested noncontact tonometers nidek nt-2000 served as the laboratory standards and a set of the silicone eyes and an artificial eye served abstract the eye-tonometers are the important medical devices with measuring function which are necessary for the screenings of the intraocular hypertension (a serious risk factor for the glaucoma). however, it is not an easy task to ensure their correct metrological traceability. there is needed not only a wide range of various equipment but also the relevant know-how. hence, a training centre for this quantity was established at the czech metrology institute (cmi) within the framework of a smart specialisation concept for the intraocular pressure (iop) metrology. the paper briefly outlines its history, scope, methodologies and future development plans. mailto:dprazak@cmi.cz acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 2 as the transfer-standards. the centre is now able to provide the metrological traceability and the relevant trainings to the other european nmis. from the beginning it was envisaged that the ssc would be extended in future geographically beyond the central europe and thematically beyond the iop metrology. hence, the training centre also plays a crucial role in the follow-up empir project cefton, which runs from september 2021 to february 2023, [11]-[12], which focuses on the iop metrology know-how transfer to the nmis of the selected central europe free trade agreement (cefta) countries. cmi joined the forces with the nmis of bosnia and hercegovina, moldova, and north macedonia that will serve as the pathfinders for the remaining cefta countries. the project has no research ambitions being entirely focused on the capacity building and engaging in the ssc. the scope and content of the offered trainings as well as the plans for future will be presented. 3. outline of the training centre 3.1. scope of the trainings first, it must be highlighted that the centre does not provide training of use of the eye-tonometers on the patients. this is the task of the medical doctors and nurses. the centre aims to provide training for the metrologists, i.e., the ways how to establish a correct traceability (calibrations and verifications) of the eye-tonometers and of the respective instrumental standards. the training is based on the good practice guidelines developed during the project intense, [7], which in turn reflect the relevant international standards and recommendations as well as the german and czech regulations, [13]-[17]. in con-temporary, the scope of the training centre covers the contact (impression and applanation) tonometers [18]-[22], the non-contact tonometers [23]-[25], the rebound tonometers [25], [26] and the contour tonometers [21], [27]. 3.2. impression tonometers impression (or indentation or schiøtz) tonometer is the oldest (more than 120 years) eye-tonometry principle still in practical utilisation, see figure 1. it determines the iop by the depth of corneal indentation caused by a plunger with the exactly defined weight and dimensions. in order to measure the very high iops, extra weights can be loaded. all these instruments are manufactured following the common standardized specifications. hence, their traceability consists of the checks of all the prescribed geometrical (e.g., the curvatures of the contact areas) and mechanical (e.g., weights and friction) requirements and tolerance limits, see figure 1. the weights can be checked by a mechanical or by an electronic balance in a special set-up. the laboratory is equipped with both. 3.3. applanation tonometers applanation (or goldmann) tonometer is also a long-time established principle but still considered to be a “golden standard.” it determines the iop by measuring a force needed to reach an applanation (i.e., flattening) of a cornea caused by a transparent probe with a known contact area (a circle of 3.06 mm diameter). the traceability of these instruments is again ensured in a classical way, by checking their geometrical specifications and optical quality and by calibrating their force sensor. also in this case, the force can be defined by a mechanical balance or by an electronic sensor, see figure 2. again, the laboratory is equipped with both. the local acceleration due to gravity in the laboratory must be known with a sufficient precision. 3.4. non-contact tonometers the non-contact (or air-puff) tonometers are the most widely utilized tonometers in contemporary, because there is no mechanical contact with the eye during measurement resulting in no need of a topical eye anaesthesia. these instruments also aim at an applanation of a cornea, but they do not reach it by a direct mechanical contact (as goldmann tonometers do), using instead a short and rapid pulse of air directed from a nozzle to the middle of a cornea. the moment of reaching the applanation is detected by a reflection of an infrared beam from the cornea. (in fact, we should speak about reaching a slightly concave shape instead of a real applanation.) the state-of-the-art devices are able to determine also other important ophthalmological measurands (e.g., central corneal thickness). in contrast to the contact tonometers, there is no possibility of a direct classical traceability in this case. their traceability must be ensured to another non-contact tonometer which is clinically tested (the training laboratory is equipped with one) via a suitable transfer standard. there are three possible types of transferstandards available: a set of rubber (silicone) eyes, see figure 3, an electronic eye and a flapper (ptb-jig), see figure 4. the laboratory is equipped with all these devices, because none of these can be utilized universally with all the types of the non figure 1. an impression tonometer placed on a precise calibration sphere. figure 2. calibration of an applanation tonometer (detail). acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 3 contact tonometers produced by the various manufacturers. the laboratory also took part in the mentioned above interlaboratory comparison in this quantity with the stu, [9]. a virtual digital model of the eye cornea was created at the stu during intense project. then a real mechanical model (artificial eye) corresponding to the virtual model was constructed for the experimental verifications. the stu used this artificial eye as one of the transfer-standards in the mentioned above comparison. the target is to develop a „universal iop transfer-standard“ with exchangeble artificial corneas of various thicknesses with a hydraulical or pneumatically regulated inner pressure, [8], [9], [24], [28]-[33], figure 5 and figure 6. 3.5. rebound tonometers the rebound tonometers emerged in the beginning of the 21st century and are becoming popular due to their ease of use (home diagnostic also possible). in this case, a very light and nonharming probe (plastic coated metal core with a spherical plastic tip) is ejected from the instrument against a cornea and is then reflected back into it. the probe movement can be monitored inductively, and the time response is used to calculate a value of the measured iop. the traceability of these instruments must be again ensured to a clinically tested rebound tonometer via a testing bench consisting of a silicone membrane surrogating a cornea with an inner pressure regulated by a water column which enables to compare the readings of a clinically tested device and a calibrated device, see figure 7 and figure 8. figure 3. a detail of a non-contact tonometer with a set of the rubber eyes. figure 4. a detail of a non-contact tonometer with a flapper. figure 5. the new transfer-standard during the comparison. figure 6. the new transfer-standard during the comparison. figure 7. a rebound tonometer on its test bench. acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 4 3.6. contour tonometers the contour tonometer is another modern device. the head of this instrument has a concave shape corresponding to the typical shape and size of human cornea. the head is pressed to a cornea with a constant force (i.e., being in contact with the cornea but not applanating it). a miniature piezoresistive pressure sensor mounted in the head is then able to detect the iop with such a sensitivity that it is even able to detect the minor iop fluctuations caused by the cardiac cycle. the principle is less influenced by a corneal thickness or a corneal rigidity, but it is rather sensitive to a corneal curvature. the traceability of this device can be relatively easily and straightforwardly ensured by a direct calibration of its internal pressure sensor, see figure 9. 4. history of the trainings the first training for two experts of a german stakeholder took place during the project intense in march 2020. then these activities were interrupted due to the covid-19 pandemic. however, the trainings were resumed again in january 2022 within the project cefton when two colleagues from bosnia and herzegovina took part. it was followed by a training of six people of the nmis of bosnia and herzegovina, moldova and north macedonia in june 2022. all the trainings took place at the training centre in most and covered all the principles described above. it was found useful that both the instruments and the instrumental standards to which these are traceable are concentrated at one place. hence, the attendees could more easily distinguish between “construction principles of the tonometers” and “traceability principles of the tonometers” which used to be a stumblingblock during theoretical lectures. the only shortcoming found was the fact that the training does not cover maklakoff tonometer. this predecessor and a very simplified variant of goldmann tonometer has not been used in practice in the central europe for years but is still widely utilized in the area of the former soviet union. 5. tasks for future as it was mentioned in 3.4, some modern non-contact tonometers are able to determine more eye characteristics than the sole iop value (e.g., corneal thickness, rigidity or hysteresis). however, it is still not solved how to ensure a traceability for these extra measurands. we remain in contact with the academic partners to solve these problems to. the artificial eye of the stu seems to be a good starting device for the studies of corneal thickness influence because it allows to utilize the artificial corneas of various thicknesses. initial research in this direction has already started, [8]-[10] and figure 10. also, we search for a possibility to obtain a sample of a maklakoff tonometer and to establish a procedure for its traceability. moreover, as a greater emphasis is being given to the accuracy and reliability of the medical devices with a measuring function, [34]-[41], we consider the training centre a starting nucleus of further cooperation activities in the sector of medical metrology. 6. conclusions as a result of the fruitful cooperation of the european nmis, the training centre for the iop metrology at the cmi covers the most common eye tonometry principles, has the state-of-the-art equipment and remains in the intensive contacts with the nmi and academic partners to broaden its scope in the future. figure 8. a rebound tonometer on its test bench (detail). figure 9. a contour tonometer connected to a pressure standard. figure 10. experimentally found influence of the thickness of the artificial cornea on the non-contact tonometer response as found by the stu. acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 5 acknowledgement this work was funded by the european metrology programme for innovation and research (empir) project 20scp02 cefton. the empir initiative is co-funded by the european union horizon 2020 research and innovation programme and the empir participating states. references [1] b. thylefors, a.-d. négrel, the global impact of glaucoma, bulletin of the world health organization, 72 (1994), pp. 323329. 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75. doi: 10.1186/s12938-020-00820-0 https://doi.org/10.1063/1.5066529 https://doi.org/10.1109/i2mtc.2019.8827028 http://dx.doi.org/10.2478/scjme-2019-0021 https://doi.org/10.1109/sas48726.2020.9220014 http://dx.doi.org/10.1007/978-3-030-33146-7_33 https://doi.org/10.1016/j.healthpol.2009.03.016 https://doi.org/10.1051/ijmqe/2011101 https://doi.org/10.1007/s00769-015-1149-9 https://doi.org/10.5455/medarh.2016.70.140-141 http://dx.doi.org/10.1007/978-981-10-4166-2_88 http://dx.doi.org/10.1007/978-3-030-17971-7_67 https://doi.org/10.1109/mim.2020.9126071 https://doi.org/10.1186/s12938-020-00820-0 mathmet measurement uncertainty training activity – overview of courses, software, and classroom examples acta imeko issn: 2221-870x june 2023, volume 12, number 2, 1 6 acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 1 mathmet measurement uncertainty training activity – overview of courses, software, and classroom examples francesca r. pennecchi1, peter m. harris2 1 istituto nazionale di ricerca metrologica (inrim), strada delle cacce 91, 10135 torino, italy 2 national physical laboratory (npl), hampton road, teddington tw11 0lw, uk section: technical note keywords: measurement uncertainty (mu); training; mathmet; overview; mu courses; classroom examples and software citation: francesca r. pennecchi, peter m. harris, mathmet measurement uncertainty training activity – overview of courses, software, and classroom examples, acta imeko, vol. 12, no. 2, article 12, june 2023, identifier: imeko-acta-12 (2023)-02-12 section editor: eric benoit, université savoie mont blanc, france received july 1, 2022; in final form march 14, 2023; published june 2023 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: npl’s work was supported by the uk government’s department for business, energy, and industrial strategy (beis) as part of its national measurement system (nms) programme. corresponding author: francesca pennecchi, e-mail: f.pennecchi@inrim.it 1. introduction in october 2021, a two-year mathmet [1] activity was launched [2] with the aim of developing new training material and establishing an active community for those involved in teaching measurement uncertainty. this “measurement uncertainty training activity” [3] is conducted by a consortium of mathmet and non-mathmet members who committed themselves, on a voluntary basis, to develop new training material on measurement uncertainty (mu) and to strengthen collaborations among experts and interested people at metrology institutes, universities, industry and within accreditation and legal metrology communities. concerning the development of new material for mu training, this will include an overview of existing courses, software and examples, which can guide trainees across the tools and materials already available at different levels and in different fields of application. it is also planned to prepare some short videos explaining the need for, and common difficulties in, evaluating mu. all material will be made publicly available on the dedicated webpage [3] of the mathmet website and will be actively disseminated to a large set of practitioners in metrology, academia, and industry. in the present abstract, we will focus on the survey of existing courses and software for mu evaluation, together with the review of selected examples, suitable for mu training, which will be revisited in the form of proper classroom examples. further new training material that will be developed from scratch by the mu training activity is presented separately at this joint symposium [4] and will not be detailed here. 2. overview of existing courses and software starting from the plethora of courses on mu usually offered by the partners of the consortium, the first step was to undertake a review of such courses, in order to inform the wider audience about their availability and characteristics. in this respect, mathmet will serve as a reference point to make connections among trainers and trainees at a european level and beyond. the abstract a collaborative activity on “measurement uncertainty (mu) training” under the auspices of the european metrology network for mathematics and statistics (mathmet) is underway. this abstract reports on the progress to undertake surveys of existing training courses on mu, software for mu evaluation, and classroom examples to support the understanding of methods for mu evaluation. an appreciable number of training courses, software and examples have been identified and are currently under review. these tools and materials will be analysed and categorised according to their main features and characteristics. special attention will be given to their adherence to the jcgm guidelines (i.e., jcgm 100:2008, jcgm 101:2008 and jcgm 102:2011). it is hoped that the knowledge assembled in this activity will help practitioners to make good choices about appropriate material to support their training needs, as well as help developers of training material to ensure good coverage of their training products and target them at user needs. mailto:f.pennecchi@ acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 2 courses will be categorized according to their main features to enable the audience to easily identify which course would fit the best with their need. analogously, based on the availability of a variety of software (sw) performing mu evaluation, a critical overview of such tools is currently underway to analyse their characteristics, such as the status, the kind of methods they implement and the main operating conditions. 2.1. existing courses on mu so far, information on 41 training courses taught by 14 partners and 1 stakeholder of the activity consortium has been collected, for a total amount of more than 880 hours of lessons per year. for each course, a specifically developed template was completed by a reference contact, who was free to provide the following details: general information (title of the course, its integration into a training framework or project, specific field of application, organizing body, website advertising the course, duration, frequency, language(s), location, material provided to attendees, attendance fee, final examination, kind of certification, etc.) audience (target audience, specific constraints and prerequisites, average number of attendees) teacher/s or technical contact/s technical contents classroom examples from a preliminary analysis of the features undertaken it is worth noting that most of the courses are specifically dedicated to mu evaluation but a good number have a broader scope (covering, for example, metrology in general or for a specific si quantity). these courses were included in the review as they make a strong effort in teaching mu evaluation. figure 1 gives an idea of their specific fields of application. the majority of courses (78 %) is given on a recurring basis but some are elearning courses and are available on demand. 15 % are offered in more than one language (see the languages distribution in figure 2). 50 % require some sort of final examination and 50 % have an enrolment fee. 15 % of courses are aimed at legal metrology, 37 % at nmis, 46 % at calibration and testing laboratories and 24 % at academia (with overlapping categories). concerning the “technical contents”, the contact person completing the template was required to describe the main topics of the course and the extent to which they comply with the prescriptions of the jcgm wg1 suite of documents [5], with a special focus on the teaching of the law of propagation of uncertainty (lpu) and the monte carlo method (mcm) for propagation of distributions: review of mathematical tools (linear algebra, partial derivatives, linear regression, …) review of probability concepts (random variables, distributions, …) basic metrological concepts (measurand, measurement model, error, accuracy, precision, repeatability, reproducibility, …) input standard uncertainties and covariances (gum type a and type b) lpu (gum 1st or also higher-order taylor series expansion, expanded uncertainty) lpu (jcgm 102 multivariate models) mcm for propagation of distributions (jcgm 101 univariate models) mcm for propagation of distributions (jcgm 102 multivariate models) validating lpu against mcm reporting the measurement result as a result, 34 % of courses provide a review of mathematical concepts, 85 % of probabilistic topics and 95 % of metrological topics. almost all discuss how to model and evaluate input standard uncertainties and covariances, as well as the application of lpu to univariate models. interestingly, though, only 20 % address lpu for multivariate models (jcgm 102). concerning the teaching of mcm, 44 % of courses treat mcm for univariate models (jcgm 101) but only 15 % for multivariate models (jcgm 102): see figure 3 and figure 4. moreover, the training on mcm is not homogenous across the audience: it is almost never taught in courses for the legal metrology community, a third of the time to calibration and testing laboratory personnel, half of the time to nmi employees and most of the time in courses for academia. as a general comment, it seems there is a gap in the treatment of multivariate models, both from the side of lpu and even worse for what concerns application of mcm. this implies that little attention is given to the training on calculation of covariances among measurands depending on some common input quantities and hence being correlated. this seems in figure 1. fields of application of the courses. figure 2. languages used in the courses (the total number of occurrences of languages used in the courses, also considering the various combinations of languages, was 49). acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 3 contrast with the fact that the main target audience (46 %) are calibration and testing laboratories and calibration procedures often involve multivariate models. an encouraging result is that 75 % of the courses dealing with lpu for multivariate models address the problem by teaching also the corresponding mcm. in the questionnaire, it was also possible to specify which references are used and if any software is applied or mentioned in the course. the references mainly reported are documents of the jcgm wg1 suite, iso and oiml standards, euramet and ilac guides, as well as documents by ea, ukas, din, eurachem/citac, etc. 68 % of the courses rely on, or at least mention, use of some sw or programming language, like excel, matlab, r, labview, origin, the nist uncertainty machine and the gum workbench. among the technical topics treated on top of standard ones (i.e., lpu and mcm), the following are also mentioned: bayesian inference, conformity assessment, linear regression, and quality control. comments concerning the “classroom examples” are left until section 3. 2.2. software for mu evaluation a further survey was initiated by a subset of the activity partners, i.e., inrim, npl, lne, ipq, imbih, metas and polito, with the aim of categorizing available software related to mu and summarizing the methods offered by such software to the end users. a list of software was agreed within the consortium encompassing 50 sw for mu evaluation, coming from several sources (mainly from a wikipedia webpage [6]). 35 sw were already analysed by involved partners, by filling in an agreed list of characteristics/features. the software ranged from basic uncertainty calculators to quite complex, broad-scope software, and from user-friendly web applications to comprehensive collections of libraries and tools for uncertainty quantification. some of those sw are currently under analysis by the partners, considering the following characteristics: general information technical features adherence to jcgm 100:2008 adherence to jcgm 101:2008 adherence to jcgm 102:2011 for the “general information” and “technical features” items, information is reported on license, version, programming language, whether the sw is computer-based or a web application, its language(s), documentation, and evidence of verification and validation. concerning the sw so far analysed, 74 % of them are cross-platform, 85 % computer-based (15 % web application), 54 % provide some evidence of validation, and all are available in english version (some also in other languages). the distribution of the programming languages is shown in figure 5. in the questionnaire, moreover, it has to be stated whether the sw is able to handle correlated input quantities, nonlinear models, more than one output quantity (most of the analysed sw have these features, i.e., 86 %, 89 % and 57 %, respectively), complex-valued quantities, implicit models, symbolic uncertainty evaluation, repeated input observations, and input imported from previous analyses (these features, instead, are generally less covered by the sw). information will be also given on the output results and their format. the adherence of the implemented methods with the jcgm documents [5] is investigated in some detail. the aim is to assess the metrological relevance of each sw and its level of compliance with recognized guidelines. concerning jcgm 100:2008, the sw is checked against its ability to implement the lpu (without or with correlation among input quantities and implementing the first or higher-order taylor series approximation), to (analytically or numerically) calculate the sensitivity coefficients, to provide a summary of standard uncertainty components, and to calculate the effective degrees of freedom and the expanded uncertainty at a prescribed coverage probability. in this respect, the majority of the sw implement lpu based on the first-order taylor approximation of the model (71 %) and provide sensitivity coefficients (55 %) and expanded uncertainties (54 %). the remaining capabilities are less frequently addressed. concerning the jcgm 101:2008 and jcgm 102:2011 documents, the main features under investigation are the maximum numbers of monte carlo trials and of input quantities, figure 3. courses teaching mcm (jcgm 101). figure 4. courses teaching mcm (jcgm 102). figure 5. programming languages. acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 4 the gallery of available (univariate or multivariate) input probability density functions, the application of lpu to explicit or implicit (univariate or multivariate) measurement models, the ability to provide a coverage interval for the output quantity at a prescribed coverage probability (also considering probabilistically symmetric and shortest coverage intervals), to perform an adaptive monte carlo procedure, and to validate the gum uncertainty framework against the monte carlo method. as concerns the adherence to jcgm 101:2008, most of the analysed sw respect the document prescriptions on how to assign the input probability density functions (60 %) and calculate an estimate and the associated uncertainty from the simulated output distribution (60 %). the other features are less well covered. as concerns the adherence to jcgm 102:2011, it is evident a large gap exists in this respect: only 20 % of sw implement lpu for explicit multivariate models, and a meagre 6 % for implicit ones; 31 % of sw apply a monte carlo procedure to calculate an estimate and the associated covariance matrix from the simulated multivariate distribution of the measurand. in parallel with the above-described revision of the available sw for mu evaluation, it is worth mentioning that mathmet is developing a quality management system (qms) for software, data and guidelines as one of the main outputs of the mathmetrelated joint network project. relevant output is available in a dedicated publication [7] and uploaded on a mathmet webpage dedicated to quality assurance tools [8]. 3. overview of classroom examples concerning the “classroom examples” offered in existing courses on mu, contact persons were asked to provide information about some of the main examples treated and their characteristics, comprising: title short description application area (calibration, testing, conformity assessment, etc.) metrology area (mass, length, etc.) approach to mu evaluation (jcgm 100, jcgm 101, etc.) level of difficulty (simple, medium, difficult) existing supporting material exists or to be developed so far, 69 examples have been collected from 36 training courses, of which 46 identify “calibration” as the main application area. the examples are spread over 15 different metrology areas (including “not specified”), with the top two listed as “dimensional” (18/69) and “temperature” (13/69) accounting for almost one-half of the examples. there is a focus on applying the lpu approach of jcgm 100:2008, either on its own (40/69) or in combination with other approaches (55/69) for comparison. very few examples are classified as “difficult” (4/69) with most classified as “simple” (33/69), but in this regard the classification for the different levels of difficulty is likely to be quite subjective. it is planned to review other sources of examples used for teaching the principles of mu and for demonstrating different methods for mu evaluation. one such source is the compendium [9] of examples that was the main output of the emue project [10]. the compendium presents 41 examples from six broad application areas: industry and society quality of life energy environment conformity assessment calibration, measurement, and testing figure 6, figure 7 and figure 8 present graphically a comparison of the examples taken from the two sources in terms of the categories of metrology area, approach (to mu evaluation) and level (of difficulty). the examples taken from the emue project are spread over 19 different metrology areas, with a more uniform distribution across them, and the top two listed are “chemistry” (8/41) and “flow metrology” (6/41). in terms of metrology area, the examples from the two sources appear complementary, perhaps reflecting in existing courses how examples from the “dimensional” and “temperature” areas are more accessible to a general audience and easier to teach, whereas the examples collected in the emue project reflect the wider interests of the partners involved in the project. there is again a focus on applying the lpu approach of jcgm 100:2008, either on its own or in combination with other approaches, but the examples from the emue project offer a wider range of approaches, including bayesian, regression and “top-down” approaches to mu evaluation. finally, a judgment about the level of difficulty of each example was made by one of the authors regarding how a “non-expert” faced with the example in a training course might perceive the example. the result was that all examples were classified as “medium” to “difficult” and so, in this regard, the examples from the sources again can be considered complementary. the data collected from these different sources serve as a basis for identification of interesting cases to be further developed in the form of classroom examples. in general, the analysis of the results, and the comparison for different sources, will support the identification of needs not covered by existing training courses, or deficiencies in those courses, and it will facilitate the exchange of knowledge between people teaching mu. 4. conclusions a collaborative activity on “measurement uncertainty training” under the auspices of the european metrology network for mathematics and statistics (mathmet) is underway. this abstract reports on the progress to undertake surveys of existing training courses on mu, software for mu evaluation, and examples to support the understanding of methods for mu evaluation. it appears that an appreciable amount of material is available: 41 training courses, 69 examples, and 50 items of software. it is hoped that the knowledge assembled in this activity will help practitioners to make good choices about appropriate material to support their training needs, as well as help developers of training material to ensure good coverage of their training products and target them at user needs. actual and future updated versions of the three surveys will be published on a dedicated webpage [11]. acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 5 figure 6. classification according to “metrology area” for the examples taken from existing training courses (top) and developed in the emue project (bottom). acknowledgement the authors of this abstract wish to thank the colleagues of all the partners of the activity consortium for participating in the survey of courses, software and classroom examples. the consortium comprises [3]: ptb (coordination), cem, gum, imbih, ims sas, inrim, ipq, lne, metas, npl, smd, accredia ente italiano di accreditamento, deutsche akademie für metrologie (dam), national standards authority of ireland (nsai), politecnico di torino, university of konstanz. figure 7. as figure 6 but using the classification of “approach (to mu evaluation)”. figure 8. as figure 6 but using the classification of “level (of difficulty)”. references [1] european metrology network for mathematics and statistics home page. online [accessed 27 february 2023] https://www.euramet.org/european-metrologynetworks/mathmet [2] emn mathmet starts initiative on measurement uncertainty training news story. online [accessed 27 february 2023] https://www.euramet.org/european-metrologynetworks/mathmet/bugermenu/news [3] mathmet measurement uncertainty training activity home page. online [accessed 27 february 2023] https://www.euramet.org/goto/g1-e34bee [4] k. klauenberg, introduction to the mathmet activity mu training, presented at the joint imeko tc1-tc7-tc13-tc18 & 0 5 10 15 20 biology chemistry dimensional electrical force frequency general illuminance mass material properties pressure radiometry, photometry temperature time not specified course examples atmospheric measurement chemistry dimensional electrical flow metrology gas metrology general hardness imaging key comparisons mass material combustion precipitation pressure temperature thermal comfort thrust torque viscosity emue examples https://www.euramet.org/european-metrology-networks/mathmet https://www.euramet.org/european-metrology-networks/mathmet https://www.euramet.org/european-metrology-networks/mathmet/bugermenu/news https://www.euramet.org/european-metrology-networks/mathmet/bugermenu/news https://www.euramet.org/goto/g1-e34bee acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 6 mathmet symposium, porto, portugal, 31 august-2 september 2022. [5] jcgm publications: guides in metrology home page. online [accessed 27 february 2023] https://www.bipm.org/en/committees/jc/jcgm/publications [6] list of uncertainty propagation software webpage. online [accessed 27 february 2023] https://en.wikipedia.org/wiki/list_of_uncertainty_propagation _software [7] keith lines, jean-laurent hippolyte, indhu george and peter harris, a mathmet quality management system for data, software, and guidelines, acta imeko vol. 11 no. 4 (2022). doi: 10.21014/actaimeko.v11i4.1348 [8] quality assurance tools webpage. online [accessed 27 february 2023] https://www.euramet.org/european-metrologynetworks/mathmet/activities/quality-assurance-tools [9] good practice in evaluating measurement uncertainty, compendium of examples, adriaan m.h. van der veen and maurice g. cox (editors), 27 july 2021. online [accessed 27 february 2023] http://empir.npl.co.uk/emue/wpcontent/uploads/sites/49/2021/07/compendium_m36.pdf [10] examples of measurement uncertainty evaluation project home page. online [accessed 27 february 2023] http://empir.npl.co.uk/emue/ [11] mathmet measurement uncertainty training activity, for trainees measurement uncertainty training webpage. online [accessed 27 february 2023] https://www.euramet.org/european-metrologynetworks/mathmet/activities/measurement-uncertainty-trainingactivity/for-trainees-measurement-uncertainty-training https://www.bipm.org/en/committees/jc/jcgm/publications https://en.wikipedia.org/wiki/list_of_uncertainty_propagation_software https://en.wikipedia.org/wiki/list_of_uncertainty_propagation_software https://doi.org/10.21014/actaimeko.v11i4.1348 https://www.euramet.org/european-metrology-networks/mathmet/activities/quality-assurance-tools https://www.euramet.org/european-metrology-networks/mathmet/activities/quality-assurance-tools http://empir.npl.co.uk/emue/wp-content/uploads/sites/49/2021/07/compendium_m36.pdf http://empir.npl.co.uk/emue/wp-content/uploads/sites/49/2021/07/compendium_m36.pdf http://empir.npl.co.uk/emue/ https://www.euramet.org/european-metrology-networks/mathmet/activities/measurement-uncertainty-training-activity/for-trainees-measurement-uncertainty-training https://www.euramet.org/european-metrology-networks/mathmet/activities/measurement-uncertainty-training-activity/for-trainees-measurement-uncertainty-training https://www.euramet.org/european-metrology-networks/mathmet/activities/measurement-uncertainty-training-activity/for-trainees-measurement-uncertainty-training a mathmet quality management system for data, software, and guidelines acta imeko issn: 2221-870x december 2022, volume 11, number 4, 1 6 acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 1 a mathmet quality management system for data, software, and guidelines keith lines1, jean-laurent hippolyte1, indhu george1, peter harris1 1 national physical laboratory, hampton road, teddington tw11 0lw, uk section: research paper keywords: quality management system; mathmet; data; software; guideline citation: keith lines, jean-laurent hippolyte, indhu george, peter harris, a mathmet quality management system for data, software, and guidelines, acta imeko, vol. 11, no. 4, article 8, december 2022, identifier: imeko-acta-11 (2022)-04-08 section editor: eric benoit, université savoie mont blanc, france received july 18, 2022; in final form december 2, 2022; published december 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: the project 18net05 mathmet has received funding from the empir programme co-financed by the participating states and from the european union’s horizon 2020 research and innovation programme. corresponding author: keith lines, e-mail: keith.lines@npl.co.uk 1. introduction the european metrology network for mathematics and statistics (mathmet) [1] has been established to help bring a collaborative approach to addressing the needs of measurement scientists for expertise in applied mathematics, statistics, and computational tools. ever more accurate, consistent, and traceable measurements will be vital to meeting challenges such as climate monitoring, clean energy, modern-day health care and sustainability. these measurements are often underpinned by new and increasingly complex mathematical and statistical techniques that are reliant on data sets and software. fit for purpose data sets, software, and guidelines to meet the requirements of the national measurement institutes (nmis) and other stakeholder organisations and individuals, that will both draw on and contribute to mathmet, will be vital to mathmet’s success. an outline of a mathmet quality management system (qms) for research outputs in the form of data, software, and guidelines was presented at the mathematical and statistical methods for metrology virtual workshop 2021 (msmm 2021) [2]. feedback from delegates helped confirm that the iso process-based approach taken, and described below, was appropriate. this paper outlines the current version of the qms, which has benefited from the feedback from msmm 2021 and the input of other mathmet members and is organised as follows. in section 2 the essential components of the qms for all three research outputs are described, as well as on-line risk assessment tools that guide a user through the process of assigning an integrity level for the research outputs of data and software. in section 3, some examples of case studies that are being used to refine and demonstrate the qms are indicated. the lessons learned from these case studies will be reported separately to this paper. finally, conclusions are given in section 4. 2. components of the qms 2.1. background the qms follows a process-based approach as defined in iso 9001:2015 [3] and related standards. this approach incorporates the “plan-do-check-act” (pdca) cycle and risk-based thinking. over a million organisations are certified to iso 9001, abstract the european metrology network for mathematics and statistics (mathmet) is creating a quality management system (qms) to ensure that research outputs in the forms of data, software and guidelines are fit-for-purpose, achieve a sufficient level of quality, and are consistent with the aims of national measurement institutes to provide quality-assured and trusted outputs. the essential components of the qms for all three forms of research output are discussed. on-line, interactive risk assessment tools that guide a user through the process of assigning an integrity level for the research outputs of data and software, are described. examples of case studies that have been used to demonstrate the qms are indicated. mailto:keith.lines@npl.co.uk acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 2 and npl has held iso 9001 certification for more than 25 years. npl has also successfully applied tickitplus certification [4] (and its predecessor scheme tickit), which builds on iso 9001, to its software development activities. the experience gained from applying these schemes, plus the large number of iso and ieee standards and related documents supporting them, strongly implies that this approach is the right one to take. 2.2. data and software a quality management plan is key to the qms components for data and software. this plan lists the quality management activities needed for a particular dataset or piece of software. these activities follow a typical life cycle from requirements capture to design and development, verification, and validation through to maintenance. review is an important activity that is carried out throughout the life cycle. as noted in section 1, risk assessment is a key element of the qms, and risk is quantified using a value called an integrity level. the integrity level is a number between 1 and 4, where 1 indicates the lowest level of risk (for example, prototypes of software for internal use within an organisation) and 4 indicates the highest level (for example, software that is safety critical). the concept is analogous to, but should not be confused with, the safety integrity level of iec 61508 [5]. the integrity level is used to decide the quality management activities and interventions to be listed on the plan. higher integrity levels have a greater associated risk and therefore need more activities and interventions (for example, review by a thirdparty, independent of the team that developed the data or software). for software, the qms can include established quality procedures and templates from the mathmet members. the development of metrology software is usually a much smaller scale exercise than would normally be addressed using such a qms. there are no large teams of developers to manage, a typical team may consist of no more than one or two people. the software may have a small number of highly specialist users, rather than an app distributed to many thousands of users with varying levels of technical expertise. however, there are issues that a qms can help manage. for example: • such software is typically developed by metrologists rather than software engineers. it is strongly arguable that software engineering good practice should be a part of every modern-day scientist’s toolkit of skills. however, analogous to how the guidance of a numerical analyst should be sought for certain mathematical problems, there are situations in which it is strongly advisable to consult a software engineer (e.g., safety-critical software). the qms provides a framework to help make, document and review such decisions. • non-trivial mathematics is at the heart of metrology software. even the simplest equations can become difficult to implement and maintain if an inappropriate implementation platform is selected. • for large scale software development projects, roles such as user and developer are distinct and held by different people. that situation is often not the case for metrology software. also, it is not always easy to define who the customer is for this software. is the customer somebody within a funding body? is the customer somebody internal to the organisation acting as a proxy for someone in an external organisation? • lack of clarity of roles can lead to serious issues that could be prevented easily, not least finding the software after the original developers have left the organisation. if these developers also provide the service for which the software was developed (or were authors of the paper for which the software generated results) they will know where it’s located. could others find the software, and be sure the correct version has been accessed (not an older version that contains some serious bugs)? • a related point is that some metrology software can be in use for a long time. can the correct versions of the source code and documentation, for example explaining how the equations were derived, be found 20 or more years after initial release? • perhaps the software itself will never be released outside of the organisation in which it was developed. however, results such as calibration certificates and research papers, will be released outside of the organisation. software quality management is no less important in such situations as it is when the software itself is released. • even the smallest piece of software must be traceable to the results it produces. the ongoing reproducibility crisis [6] would be eased if questions such as the following could be always answered easily “which version of which script produced those results? the exact version please, not one containing subsequent modifications”, “which versions of which libraries did the script call?” and “what were the reasons these libraries were considered appropriate for this work?”. perhaps journals should be asking for the upload of scripts as well as the data the scripts processed. what may seem like tedious, unnecessary and timeconsuming bureaucracy (particularly during some interesting and exciting research) could save considerably more tedium in the longer term. • following on from the above point, the provenance of packages and libraries is a key point to consider. a richly featured, but new and experimental, library may be appropriate for a prototype but not for generating certificates for customers. a proprietary closed-source package from a long-established supplier with a strong reputation for well-engineered software may be the right option. alternatively, what better guarantee of quality can there be than an open-source package that has the input of many experts in a particular field? there are often no “right” or “wrong” answers, just decisions to made, documented and reviewed. again, the qms provides a framework to help with these tasks. • it should never, ever be necessary to have to look at the code of even the smallest script to work out what it does. the mathematics must be documented in a way that can be independently verified without having to examine code. in some circumstances code comments, and perhaps an accompanying readme file, will be sufficient. in other circumstances more thorough documentation will be required. again, the qms provides a framework to help decide what documentation will be necessary. acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 3 in summary, for software the qms aims to minimize chances of errors and manage issues such as traceability and accountability, transferability, maintenance and reproducibility. the final three points need to be managed without the original developers being available. some activities listed in the quality plan are considered mandatory to ensure the software is of the required quality, and the qms provides templates to cover, for example, requirements capture. however, if an alternative template is considered more appropriate for a particular project, then this template can be used instead. in this case, it is mandatory that the reasons for such decisions are recorded. quality management of data is less well established than quality management of software. accordingly, developing this component of the qms was arguably a research activity to some extent. a key concept is that integrity levels are adapted to apply to data as well as software. also, the iso 8000 series of standards [7] applies the same pdca and risk-based paradigms to data quality management on-line risk assessment tools, called quality assurance tools, will assist with assigning integrity levels and generating quality management plans. further details are provided in section 2.4 2.3. guidelines the aim of this component of the qms is to ensure a sufficient level of quality in the development, assessment, and recommendation of existing and future guidelines for mathematics and statistics in metrology. it is intended to include: 1. a rigorous review process for existing and new guidelines involving external experts and mathmet stakeholders 2. quality management activities that shall accompany and improve guidelines that will be developed in the future in projects involving one or more members of mathmet 3. a process to provide advice and feedback that will enable third parties to adapt and improve mathematical and statistical guidelines to the needs and requirements of the european metrology community and its stakeholders. the processes applied by organisations such as iso/iec for standards development [8], eurachem for its development of guidance documents [9], and npl for its review and approval of documents, have been reviewed and used to steer the design of the qms. however, the approach taken has been to present a qms in ‘skeleton’ form that sets only high-level requirements on users into which the processes adopted by individual mathmet members can comfortably fit. for example, for a future guideline, the process comprises the stages of development, review and approval, publication, and maintenance. the stage of review and approval can be iterative and can involve separate steps that focus on different aspects, such as technical correctness or presentation and style. the stage of maintenance depends on the guideline being provided with appropriate metadata allowing versions (and changes between versions) to be tracked correctly. for both types of guidelines (future and existing), the qms involves completing a checklist comprising a set of questions, and making a recommendation based on the answers to those questions. for an existing guideline, the checklist considers: • whether the origin of the document is an established organisation • whether the document has been independently reviewed and approved to be issued • whether the document comes with appropriate metadata (such as title, author, unique identifier or version number, issuer, review date, etc.) • whether the document is adequately protected with respect to copyright and intellectual property rights • what is the language of the document and whether it needs translation (for example, to english) • whether the document states the targeted audience or readership • whether the technical content of the document is relevant to the focus of mathmet on mathematics and statistics for metrology • whether conclusions are clearly stated, appropriate and relevant • whether complete and appropriate acknowledgments are made (for example, to originating projects and funding sources) • whether complete, appropriate, and primary references are listed • whether the overall presentation of the material in the document is clear and understandable. for a future guideline, additional questions are included covering: • whether the document is technically sound • whether the document has undergone adequate review relating to both technical and presentational aspects • whether notation and abbreviations have been adequately and clearly defined. the qms will be applied to five metrology case studies identified at the outset (see, for example, section 3 and [19]). the results of those will be assessed in terms of effectiveness, risk assessment and quality interventions by the qms. the qms will also be presented here at this workshop to stakeholders to gain feedback and to understand its effectiveness in meeting the needs of stakeholders. 2.4. on-line risk assessment tools for data and software as noted in section 1, integrity levels for data and software are essentially a calculation involving criticality and complexity. other factors may also need to be considered, such as the availability of suitably qualified developers. mathmet will provide on-line risk assessment tools to guide the user through the process of calculating an integrity level and generating a quality management plan. the tool will be illustrated here at this workshop to stakeholders to gain feedback. for the case of software, table 1 and table 3 list the different classifications relating to criticality of usage (cu) and complexity (cp; table 2 concerns only data and will be discussed later). the choice of the classifications, between ‘not critical’ (1) and ‘life critical’ (4) for cu and between ‘very simple’ (1) and ‘complex’ (4) for cp, are quite subjective. however, the calculation of the software integrity level (swil), as detailed in table 4, is then deterministic and undertaken automatically by the assessment tool. the user has the possibility to moderate the calculated swil, considering factors that influence the associated risk. for example, the swil might be reduced if there is an alternative acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 4 means of verification or increased if there is reliance on key staff. see table 5 for examples of moderating factors. the reasons for such moderation of the swil should be recorded. once the swil is fixed, the assessment tool automatically generates a software quality management plan identifying those activities and interventions that are mandatory, recommended and not required. table 6 lists activities related to the capture of user requirements for software. for example, for a swil of 2, documented user requirements are mandatory, and their review by the project team and (a proxy for) the customer is also mandatory but review by an independent person is not required. in contrast, an independent review is recommended for a swil of 3 and mandatory for a swil of 4. software quality requirements are set related to the stages in the software development cycle of: • capturing software functional requirements • software design • software coding • verification and validation • delivery, use and maintenance. for the case of data, the assessment tool operates in a similar manner to that for software. it takes the user through a series of questions collected into the following sections: • dataset details • responsibilities (in terms of data managers, data administrators, data stewards and data technicians) • document control • complexity and criticality leading to the assignment of a data integrity level that can be moderated by the user • fitness for purpose • quality planning • quality monitoring, control and improvement • quality assurance • data understandability • metrological soundness. table 1. classifying data and software according to the criticality of usage (cu). cu criticality of usage explanation 1 not critical • no danger of loss of income or reputation • short life, will not require maintenance in future 2 significant • potential for loss of income or reputation 3 substantial • likely to lead to loss of income or reputation 4 life critical • may result in personal injury or loss of life table 2. classifying data according to complexity (cp). cp complexity of data typical features 1 very simple • commonly used datatypes • few datatypes • small amount of data • simple/unexpensive data infrastructure • simple uncertainty budget 2 simple • easy to visualise • moderate number of datatypes • moderate amount of data • intermediate data infrastructure • intermediate uncertainty budget 3 moderate • non-trivial datatypes • fair number of datatypes • large dataset • complex/expensive data infrastructure • complicated uncertainty budget 4 complex • non-trivial datatypes • combination of many non-trivial datatypes • very complex/expensive data infrastructure • very complicated uncertainty budget table 3. classifying software according to complexity (cp). cp complexity of program typical features 1 very simple • elementary functionality, easy to understand • little or no control of an external system • simple mathematics 2 simple • simple functionality • straightforward control of a system • intermediate mathematics 3 moderate • large or very large programs • difficult to modify • complicated mathematics 4 complex • extremely complex functionality • complex feedback systems • very complicated mathematics table 4. calculating the integrity level for data (dil) and software (swil). cp1 cp2 cp3 cp4 cu1 1 1 1 1 cu2 2 2 3 4 cu3 3 3 3 4 cu4 4 4 4 4 table 5. moderating factors for a calculated software integrity level (swil). moderating factors possible effect on swil alternative means of verification decrease modular approach decrease suitably trained staff available decrease difficult to test increase reliant on key staff increase inexperienced staff increase ambitious timescales increase ambitious requirements increase new technology increase novel design increase table 6. quality interventions for capturing software user requirements and their dependence on the calculated integrity level (x, r and m denote not required, recommended and mandatory, respectively) quality requirement swil1 swil2 swil3 swil4 documented user requirements m m m m review by team r m m m review by suitably qualified independent person x x r m review by customer or proxy m m m m acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 5 in the above, ‘metrological soundness’ considers whether the dataset contains measured data or results derived from measurements or simulated (measured) data or combinations of those, whether the generation of the dataset is intended to be repeatable and reproducible (and how the repeatability and reproducibility conditions for the measurements are documented), how measurement uncertainty is evaluated and expressed, and how confidence in the generation of the dataset is demonstrated. these issues are important for datasets generated for, and to be used in, a metrological context. the calculated data integrity level (dil) determines the comprehensiveness of the data plan. the dil, in turn, depends on the risks associated with the criticality of usage (see table 1) and complexity (see table 2) of the dataset. as with software, the dil is calculated as detailed in table 4. for example, for a dil of 1, it is not necessary to provide, under the section on fitness for purpose, information about how the data life cycle will be documented whereas such information is mandatory for a dataset having a dil of 4. considering the section on ‘metrological soundness’, there is no difference between datasets having different data integrity levels. for a dataset having the highest dil, the associated plan comprises information relating to 41 questions in total. figure 1 sumarises the qms processes for data and software. 3. case studies the acceptance and success of the mathmet qms will depend on its ability to address a variety of needs set both by the ‘owner’ of the research output as well as the user or customer at which the research output is aimed. to this end, case studies are being undertaken to support the development of the qms and help to promote it to mathmet members and stakeholders. they are chosen to illustrate the wide range of possible research outputs and are undertaken by mathmet members having different experience of using a qms. case studies focussed on the application of the qms to software, e.g., [10], [11], [12], and those focussed on data, e.g., [13], [14], [15], were described at msmm 2021 [2]. additional case studies focussed on guidelines have since been chosen to demonstrate the application of the qms to that form of research output, and include: • a eurachem document on the use of uncertainty in compliance assessment [16] • a good practice guide describing methods of metrological data processing for industrial process optimization, focusing on aspects of redundancy, synchronization and feature selection applied to data affected by measurement [17] • best practice guides on bayesian inference for regression problems, uncertainty evaluation for computationally expensive models, and decisionmaking and conformity assessment [18] • an internal mathmet document containing a glossary and ontology of terms to support the qms described in this abstract. in [19] the application of the qms to several of these case studies by different mathmet members is presented, and the ease of use and possible pitfalls of the qms are discussed. the lessons learned from the different case studies will be reported elsewhere. 4. conclusions the components of a quality management system (qms), created by the european metrology network for mathematics and statistics (mathmet), have been described. the aim of the qms is to ensure that research outputs in the forms of data, software and guidelines are fit-for-purpose, achieve a sufficient level of quality, and are consistent with the aims of national measurement institutes to provide quality-assured and trusted outputs. a pragmatic approach has been taken to the development of the qms, which sets only high-level requirements on users to ensure there are no conflicts with the processes in-place and adopted by individual mathmet members. a key element of the qms is a risk assessment, and on-line tools that guide a user through the process of assigning an integrity level for the research outputs of data and software has also been presented. acknowledgement the project 18net05 mathmet has received funding from the empir programme co-financed by the participating states and from the european union’s horizon 2020 research and innovation programme. we thank the other members of the emn mathmet for their support in the development of the qms described here. figure 1. mathmet qms process flowchart. acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 6 references [1] mathmet: european metrology network for mathematics and statistics home page. online [accessed 01 december 2022] https://www.euramet.org/european-metrologynetworks/mathmet/ [2] msmm 2021 mathematical and statistical methods for metrology. online [accessed 01 december 2022] http://www.msmm2021.polito.it/programme [3] iso 9001: quality management systems – requirements, 2015. online [accessed 01 december 2022] https://www.iso.org/standard/62085.html [4] 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[9] proposed spherical and local direction-based feature descriptors for medical and natural image retrieval. a directional edge local ternary pattern based on reference pixel is proposed by vipparthi et al. [10]. in [11], a three-stage local feature descriptor is proposed with multidimension and multi-direction for image retrieval. moreover, directional mask is utilized to extract the directional edges. lastly, the image is retrieved using the maximum directional edge-based features. in further work, a abstract measurement of medical data and extraction of medical images with various sensory systems have become a crucial part of diagnosing and treating various diseases. in this contest measurement technology plays a key role in diagnosis. medical experts usually use previous case studies to identify and deal with the current medical condition. in this context, an expert required to explore the large medical database to search relevant images for the required analysis. searching such large database and retrieving an image efficiently becomes very tedious task. therefore, this paper proposed a measurement-based image reconstruction using the refined res-unet framework for medical image retrieval (mir) for managing such crucial tasks and reduce complexities. the proposed two stage framework consists of an image reconstruction using res-unet and index similarity matching of query image with history images. res-unet is a vanilla combination of resnet50 as an encoder that gives latent information of input image, and the decoder from unet reconstructs the image using latent information. further, those latent features match similar image features and retrieve indexed images from the medical image database. the efficacy of proposed method was confirmed on benchmark medical image databases such as ild and via/elcapct for mir. the proposed framework outperforms the existing methods in the task of mir. mailto:rohinilp11@gmail.com acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 2 reference block gradient directions-based feature descriptor is proposed for image retrieval [12]. a wavelet-based 3d circular difference pattern is proposed for medical image retrieval. thresholdbased lbp and local adjacent neighbourhood average difference information is combined for medical image retrieval in [13]. previous works utilized hand crafted feature descriptors for feature extraction in cbmir modelling. however, the handcrafted feature descriptors own to the limitation of relying on hand-written assumptions [14], which limits the efficiency of the overall model. these features become less effective in encoding the robust and complex features. currently, learning based feature descriptors are resolving the above limitation by utilizing the power of learning complex features based on their different model architectures. convolution neural networks (cnn) shows remarkable results in the defined area because of the high learning ability of these network. many works are proposed utilizing the cnn network architecture for diverse applications like objection detection, image classification, anomaly detection etc. [15]-[18]. the feature learning capability is also utilized by researchers in medical domain for medical image analysis, medical image segmentation, image de-hazing etc., [15], [19]-[22]. however, even after the remarkable performance on small network, the cnn based architectures perform badly when the depth of the network increases giving rise to the vanishing gradient issues. to resolve the above limitation, a residual learning (resnet) based architecture is proposed in [23] for image classification. motivated from this, an image retrieval approach is proposed based on image-to-image reconstruction for image retrieval. gans are utilized with image-to-image translation approach provides outperforming results. this inspired the researchers to utilize gan network for various diverse computer vision applications like image enhancement [15], moving object segmentation [16], depth estimation [24], etc. gan is composed of two component generator and discriminator. adversarial training includes the training of both generator and discriminator. in [25], an inception architecture is performed which significantly improved the performance with relatively low computational cost. motivated from the significant performance of adversarial learning and inception architecture in diverse fields, this paper proposed refined network architecture utilizing residual network for image reconstruction based on measurement technology. our network removes the skip connection which makes the reconstructed image more robust in addition to the high-quality reconstruction. the paper defines the following as major contribution: 1. an end-to-end refined content-based image reconstruction learning framework is proposed for medical image retrieval. 2. proposed a novel res-unet framework for reconstruction of input medical image exploits novel pretrained resnet50 model with cnn up-sampler. 3. robust features generated using resnet encoder for index matching and retrieval of image from the database. 4. this paper utilized two benchmark datasets to quantify proposed res-unet in cbmir. 2. proposed work a novel content-based medical image retrieval framework named refined res-unet model is proposed for effective and efficient image retrieval as shown in figure 1. the framework incorporates two functioning blocks: reconstruction block and indexing block. the reconstruction block is inspired by the standard pix2pix [26] architecture where the unet architecture comprises an encoder and decoder. proposed framework includes resnet50 [23] network as an encoder with five blocks. firstly, the input image is processed to the resnet50 block of the novel proposed model, which performs the encoding to figure 1. refined res-unet framework for cbmir. stage 1: reconstruct image using res-unet. stage 2: encoder resnet50 feature extraction for index matching and image retrieval. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 3 generate the set of features taking advantage of self-attention process. further, the features are decoded to reconstruct the original input medical image. the accurate and robust reconstruction of the original image from the encoded features justifies the conceptual representation of input image as encoded features [27], [28]. hence, the encoded features are further processed for index matching and retrieval task. for the retrieval of best matches from the available database corresponding to any input query image, conventional index matching and retrieval module is used. the proposed model functioning is processed in three steps as follows: i) refined image reconstruction network ii) robust feature generation iii) medical image retrieval module 2.1. refined image reconstruction network the deployment of generative adversarial networks (gans) for various problems like object segmentation, image enhancement, image super-resolution, image reconstruction, depth estimation, etc. gans are utilized to produce the synthetic data resembling with the original data. the algorithmic architecture of gan consists of two neural networks that are called generator and discriminator. the generator module generates unique plausible examples from the sample domain and the discriminator classifies it as real or fake content. motivated by the successful results from gans in literature, we proposed res-unet model for image restoration with selfattention for robust feature learning. the proposed network initially encodes the input into encoded features exploiting selfattention process. consequently, the features are decoded back to restore the original image. as shown in figure 1, the proposed model designed with a encoder and decoder part. for the encoder part resnet is utilized with its 50-layer architecture named resnet50. the resnet50 [23] network is considered till layer conv5_x. the encoder part takes input (i256×256×3) as image and output a latent feature vector (feat8×8×2048). the input/output relation of the proposed encoder is defined in (1) as follows: 𝐹𝑒𝑎𝑡8x8x2048 = 𝑅𝑒𝑠𝑁𝑒𝑡50(𝐼256x256x3) . (1) encoder is followed by a decoder while training the model. decoder includes five layers each of which is comprises of convotranspose layer, batch normalization layer and relu layer. decoder up-samples the feature vector of the input image. the output of encoder (feat8×8×2048) is fed to the decoder to which finally outputs the reconstructed image. the output from the decoder is given in (2) as follows: 𝑂𝑢𝑡 = 𝐶𝑇3x3(𝐹𝑒𝑎𝑡)(2,2,2,2,2) (512,256,128,64,3) , (2) where 𝐶𝑇3x3 (. )𝑆 𝐹 represents convtranspose and f: 512, 256, 128, 64, 3 is the number of filters used at each layer with corresponding stride s: 2, 2, 2, 2, 2. reconstruction loss proposed res-unet is an unsupervised learning technique, which generates output image as an input replica, i.e., reconstruction of input image. hence to learn proposed architecture for reconstruction of image. we utilize the l1 loss as shown in (3) as follows 𝑙𝑜𝑠𝑠 = 𝐼256x256x3 − 𝑂𝑢𝑡 , (3) where i256×256×3 is an input image and out is reconstructed image of i. 2.2. robust feature generation the accuracy of the image retrieval model highly depends on different representation of features through essential feature extraction method. two of the major methods for feature extraction are hand-crafted feature descriptors and learning based feature descriptor. hand-crafted feature extractors are rule based descriptors like ss3d [9], ltcop [29], ltrp [15]. the learning based feature descriptors make use of the characteristic of input image like shape, size, texture, to extract the features (alexnet [30], resnet [23], vgg-16 [31]). the proposed res-unet model uses the learning-based feature descriptor for image reconstruction process. figure 2 represents the performance of the model for reconstruction of the image from the abstract features of an encoder. the results justify the ability of the encoder in the model for generating effective features from the input medical image. hence, the figure 2. upper row depicts the original database image. lower row depicts the reconstructed image using proposed res-unet model. ild and via/elcap-ct databases are used in column 1-2 and column 3-4 respectively. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 4 output from the encoder, (feat2×2×2048) is utilized as robust features of the corresponding input image. further, these features are used for index matching and retrieval tasks from the database. in figure 1, the lower side depicts the indexing and image retrieval from the extracted features of the query image. 2.3. medical image retrieval module let’s consider, 𝑣𝑓𝑄 = [𝑣𝑓𝑄1, 𝑣𝑓𝑄2, … , 𝑣𝑓𝑄𝑁] defines the feature vector of input query image. further, the feature vector of ith image in datasets db is, 𝑣𝑓𝐷𝐵𝑖 = [𝑣𝑓𝐷𝐵𝑖1, 𝑣𝑓𝐷𝐵𝑖2, … , 𝑣𝑓𝐷𝐵𝑖𝑁 ], 𝑖 = (1, 2, … , 𝑁) represents dataset images. for retrieving the similar images from database as the query image, a similarity index d is used between the input query image 𝑣𝑓𝑄 and each image of the dataset 𝑣𝑓𝐷𝐵𝑖 . for our model similarity evaluation, d1 distance is used for query image as most of the existing state-of -the art methods have used the same. the d distance is given as in (4) 𝐷(𝑄, 𝐷𝐵) = ∑ | 𝑣𝑓𝐷𝐵𝑚,𝑛 − 𝑣𝑓𝑄𝑛 1 + 𝑣𝑓𝐷𝐵𝑚,𝑛 + 𝑣𝑓𝑄𝑛 | , 𝑁 𝑛=1 (4) where, q is the query image, n is the length of feature vector, db is database image, 𝑣𝑓𝐷𝐵𝑚,𝑛 is n th feature of nth image in the database, 𝑣𝑓𝑄𝑛 is n th feature of query image. 3. training details brats-2015 [32] database is considered for training the proposed model. the database consists of 220 high-grade gliomas and 54 low-grade gliomas. database has scans of t1weighted (t1), t1-weighted imaging with gadolinium enhancing contrast (t1c), t2-weighted (t2), and fluid attenuated inversion recovery scans. 14,415 brain magnetic resonance imaging (mri) slices are taken from the entire brats-2015 database and 4155 slices are selected to train the model over 20 epochs. the time taken for training is 553 s per epoch and the batch size is 1. 4. results and discussion this section provides the elaborated performance analysis of the proposed model res-unet and gives a comparison with the conventional hand-crafted and learning based algorithm in the literature. for comparison, the work considered three benchmark medical image database with different modalities like mri and computer tomography (ct). 4.1. retrieval accuracy on ild database two image sets interstitial images and clinical data for lung disease are examined for performing the experimental study from the integrated interstitial lung disease (ild) dataset [33]. ild is known to be the finest database which is used for research and studying of lung disease with radiological and clinical data. the dataset is utilized as a referral resource to train new radiologists. the efficiency of ild diagnosis can be improved by using a reduced invasive x-ray methodology, as x-ray and ct of a particular patient is kept in the same resource. the standard ild dataset includes ct scans of lungs for 130 patients which has underlined disease regions by the experienced radiologists for each ild [33]. a small part of the dataset is available with 658 regions with marking of patches/regions for different classes such as fibrosis (187 regions), micro-nodules (173 regions), and healthy (139 regions), ground glass (106 regions), emphysema (53 regions). table 1 provides the comparison evaluation of the proposed method and existing methods on the metric average retrieval precision applied on the topmost 10 images from the ild dataset. table 2 provides the comparison on the metric group-wise average retrieval precision on the ild dataset. results from table 1 and table 2 proves the out-performance of the proposed model from all the conventional methods. table 1. comparison evaluation of the proposed method and existing methods on the metric average retrieval precision applied on the topmost 10 images from the ild dataset. method 1 2 3 4 5 6 7 8 9 10 ss3d [9] 100 83.21 76.85 72.34 69.27 66.44 64.48 62.71 61.43 60.40 ltcop [29] 100 84.95 76.29 71.77 67.44 64.79 62.57 60.83 59.14 58.13 ltrp [34] 100 85.26 77.56 71.96 68.54 65.02 62.48 60.75 59.08 57.61 mdmep [11] 100 86.17 79.43 75.87 72.58 70.09 68.32 67.21 66.18 65.4 alexnet [30] 100 88.91 83.99 79.6 76.66 74.54 72.38 71.12 69.79 68.45 resnet [23] 100 89.67 84.75 81.31 78.84 76.55 75.51 74.34 73.44 72.67 vgg-16 [31] 100 89.21 83.94 80.24 77.96 75.94 74.4 73.21 72.26 71.34 res-unet 100 98 97.33 97.25 97.20 97.00 96.85 96.75 96.66 96.60 table 2. comparison on the metric group-wise average retrieval precision on the ild dataset. method group 1 group 2 group 3 group 4 group 5 average ss3d [9] 47.17 72.62 53.21 52.82 75.68 60.30 ltcop [29] 62.64 69.95 53.96 48.72 78.97 62.85 ltrp [34] 54.72 72.41 50.94 53.85 77.51 61.88 mdmep [11] 75.09 70.37 68.11 43.08 79.56 67.24 alexnet [30] 65.28 83.10 65.85 48.72 82.64 69.12 resnet [23] 71.70 88.24 61.51 46.67 85.13 70.65 vgg-16 [31] 73.96 87.49 57.17 57.44 83.22 71.86 res-unet 93.3 100 97.3 96.09 97.3 96.79 acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 5 4.2. retrieval accuracy on via/i-elcap-cp database the proposed model is evaluated for cbmir on the via/i-elcap-ct [35] dataset. this dataset is widely used for the cbmir model evaluation. the dataset comprises 1000 images splitted into 10 categories having 100 images per category. table 3 depicts the comparison result in terms of average retrieval precision on the uppermost 10 images of database. likewise, table 4 provides the average retrieval recall comparison between the proposed and existing methods. table 3 and table 4 shows that the proposed method exceeds in the retrieval performance. the accuracy of conventional approaches reached till 93 %. however, the proposed method shows 99 % accuracy for cbmir. the produced results can be justified as the power of network in adversarial feature learning. 5. conclusion in this paper, a novel content-based res-unet framework is proposed for reconstruction of the 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arcavacata di rende, 87036 (cs), italy section: editorial citation: pasquale arpaia, umberto cesaro, francesco lamonaca, editorial to selected papers from the international excellence phd school ‘i. gorini’, acta imeko, vol. 10, no. 4, article 3, december 2021, identifier: imeko-acta-10 (2021)-04-03 received december 14, 2021; in final form december 14, 2021; published december 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: umberto cesaro, e-mail: ucesaro@unina.it dear readers, this acta imeko special issue collects the best works presented by young researchers attending the international ph.d. school "italo gorini 2021", held in naples in the period 6-10 september 2021. the international ph.d. school "italo gorini 2021" is the doctoral school promoted by the italian associations "electrical and electronic measurements group" (gmee) and "mechanical and thermal measurements group" (gmmt). the school activity deals with a wide variety of issues related to measurement. the school is aimed at phd students, as well as young people from research and industry fields. the school addresses both methodological issues, in the field of science and technology related to the measures and instrumentation, and advanced problems of practical interest. then, special attention is paid also to the impact of measures on the scientific and engineering context, which is strongly influenced by the evolution of technology in different sectors. this year the “italo gorini” school has received the patronage of imeko and we are honoured to promote imeko among young brilliant scientists who will be the future of measurement science. the works presented in this special issue are the extended version of those that won the "best presentation", "best scientific contribute" and "best application" awards during the school. simone mari et al., in the paper entitled ‘measurements for nonintrusive load monitoring through machine learning approaches’, propose several possible approaches for the non-intrusive load monitoring systems operating in real time, analysing them from the measurement point of view. the investigated measurement and post-processing techniques are illustrated and the results discussed. emanuele buchicchio et al., in the paper ‘gesture recognition of sign language alphabet with a convolutional neural network using a magnetic positioning system’ introduce a system that combines sensor-based gesture acquisition and deep learning techniques for gesture recognition providing a 100% classification accuracy. the social impact of the proposal is wide, since gesture recognition is a fundamental step to enable efficient communication for the deaf through the automated translation of sign language. mattia alessandro ragolia et al., in the paper ‘a virtual platform for real-time performance analysis of electromagnetic tracking systems for surgical navigation’ propose a virtual platform for assessing the performance of electromagnetic tracking systems (emtss) for surgical navigation, showing in real time the effects of the various sources affecting the distance estimation accuracy. the implemented measurement platform provides a useful tool for supporting engineers during design and prototyping of emtss. a particular effort was dedicated to the development of an efficient and robust algorithm, to obtain an accurate estimation of the instrument position for distances from the magnetic field generator beyond 0.5 m. indeed, the main goal of the paper is to improve the limited range of current commercial systems, which strongly affects the freedom of movement of the medical team. the paper by leila es sebar et al. ‘a metrological approach for multispectral photogrammetry’ presents the design and development of a three-dimensional reference object for the metrological quality assessment of photogrammetry-based techniques. such techniques are typically used in the cultural heritage field. the reference object was a 3d printed specimen, with a nominal manufacturing uncertainty in the order of 0.01 mm. the object has been realized as a dodecahedron, and in each face, a different pictorial preparation has been inserted. the preparations include several pigments, binders, and varnishes, to be representative of the materials and techniques used historically by artists. pasquale arpaia, umberto cesaro, guest editors francesco lamonaca, editor-in-chief mailto:ucesaro@unina.it a virtual platform for real-time performance analysis of electromagnetic tracking systems for surgical navigation acta imeko issn: 2221-870x december 2021, volume 10, number 4, 103 110 acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 103 a virtual platform for real-time performance analysis of electromagnetic tracking systems for surgical navigation mattia alessandro ragolia1, filippo attivissimo1, attilio di nisio1, anna m. l. lanzolla1, marco scarpetta1 1 department of electrical and information engineering, polytechnic of bari, via e.orabona 4, 70125 bari, italy section: research paper keywords: electromagnetic tracking systems; image guided surgery; surgical navigation; real-time virtual platform; system design citation: mattia alessandro ragolia, filippo attivissimo, attilio di nisio, anna maria lucia lanzolla, marco scarpetta, a virtual platform for real-time performance analysis of electromagnetic tracking systems for surgical navigation, acta imeko, vol. 10, no. 4, article 18, december 2021, identifier: imekoacta-10 (2021)-04-18 section editors: umberto cesaro and pasquale arpaia, university of naples federico ii, italy received october 27, 2021; in final form december 5, 2021; published december 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: mattia alessandro ragolia, e-mail: mattiaalessandro.ragolia@poliba.it 1. introduction tracking systems are widely used in many applications, providing information about the localization of a target object in a defined area. the medical field is among the ones that are mostly taking advantage form the research on tracking systems. different kinds of technologies are used to develop such systems, depending on accuracy requirements, tracking environment, and tracking distance. the applications range from optical and inertial systems for motion tracking and rehabilitation [1]-[2], up to more complex systems for surgical navigation. the last one is a procedure that relies on tracking systems to guide the surgeon during interventions, it allows to reduce the invasiveness of the intervention, thus improving its accuracy and safety, reducing risks of complications and hospitalization time [3]-[6]. surgical navigation mainly relies on optical and electromagnetic (em) technologies [7]. optical tracking systems are very accurate and reliable, and they are employed in many medical applications [8]-[11], but they constantly require a direct line-of-sight, which prevents their use in presence of obstacles during intracorporal tracking. electromagnetic tracking systems (emtss) overcome this limitation [12]: a very small magnetic sensor, which measures the magnetic field produced by a known field generator (fg), is inserted into the surgical instrument (e.g., a flexible instrument such as an endoscope or a needle [6]), and the position of the sensor is estimated by means of a suitable algorithm. the intraoperative localization of the instruments is shown on a screen in front of the surgeon, and the anatomical area is reconstructed by merging information obtained by different medical imaging techniques, such as computed tomography, ultrasounds, and nuclear magnetic resonance[13], [14], which are acquired during the pre-operative phase. abstract electromagnetic tracking systems (emtss) are widely used in surgical navigation, allowing to improve the outcome of diagnosis and surgical interventions, by providing the surgeon with real-time position of surgical instruments during medical procedures. however, particular effort was dedicated to the development of efficient and robust algorithms, to obtain an accurate estimation of the instrument position for distances from the magnetic field generator beyond 0.5 m. indeed, the main goal is to improve the limited range of current commercial systems, which strongly affects the freedom of movement of the medical team. studies are currently being conducted to optimize the magnetic field generator configuration (both geometrical arrangements and electrical properties) since it affects tracking accuracy. in this paper, we propose a virtual platform for assessing the performance of emtss for surgical navigation, providing realtime results and statistics, and allowing to track instruments both in real and simulated environments. simulations and experimental tests are performed to validate the proposed virtual platform, by employing it to assess the performance of a real emts. the platform offers a real-time tool to analyze emts components and field generator configurations, for a deeper understanding of emts technology, thus supporting engineers during system design and characterization. mailto:mattiaalessandro.ragolia@poliba.it acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 104 the main limitation of em technology is the short tracking distance, which is generally no longer than 0.5 m from the fg in current commercial systems, due to the reduced amplitude of the magnetic field with distance, and the high sensitivity to em interferences and magnetic field distortions, thus limiting tracking accuracy far from the fg [15]. tracking distance and accuracy are crucial, and they should be taken into account during the development of a surgical navigation systems. many aspects affect system performance, and engineers and manufacturers should consider all of them, since also a small accuracy or distance increase is a valuable achievement in this field. in this paper, we propose a virtual platform for assessing the performance of emtss for surgical navigation, showing in real time how the various sources of error affect the accuracy of tracking distance estimation. this platform provides a useful tool for supporting engineers during design and prototyping of emtss. the paper is structured as follows: the main sources of error in emtss, and the importance of knowing them during system developing, are discussed in section 2; the virtual platform, developed to provide a tool to analyse system performance during the prototyping phase, is illustrated in section 3; in section 4, emts prototype architecture is described and the developed virtual platform is evaluated by simulated and experimental tests performed on the emts prototype; conclusions are drawn in section 5. 2. sources of error modelling the magnetic field and the various sources of error is crucial in many applications, allowing to implement ad-hoc algorithms for automated error compensation [16], [17]. emtss tracking accuracy can be affected by several sources of error, which can be divided into static errors and dynamic errors [7], [12], [15]. static errors occur when the sensor is placed in a given position, maintaining a fixed orientation. they are in turn classified as follows. • systematic errors: they are due to distortions of the magnetic field generated by i) the presence of metal objects in the surrounding environment, which can produce eddy currents induced by the variable field (mainly in ac systems), which generate secondary magnetic fields that add up to the main magnetic field; ii) ferromagnetic materials which, immersed in the main field, orient their domains causing a magnetization that modifies the field lines; and iii) power supply currents of the emts itself or of other electronic medical devices present in the operating room that can also cause a distortion of the magnetic field. these errors can be reduced by appropriate calibration techniques [18]. • random errors (also referred to as jitter [12],[19]): these errors, mainly due to noise, reduce the repeatability of the system. dynamic errors change over time, and they are mainly caused by variations in external em fields, due to the movement of external organs, such as conductive, ferromagnetic, and electrical materials, which cause field distortions that are extremely difficult to compensate. the movement of the sensor itself is also a source of dynamic error, depending on its speed. it must also be considered that tracking accuracy depends on the design of the fg and the choice of the position reconstruction algorithm. moreover, the non-ideality of the electronic components of the tracking system itself affects the performance of the system. in fact, the generated field is never perfectly stable due to the intrinsic limits of the fg, and the measurement and acquisition process is subject to noise, which cannot be totally eliminated. for the reduction of random and dynamic errors, suitable filtering techniques and synchronization of the sampling frequency are particularly useful [20]. the implementation of the kalman filter can also significantly reduce random errors [21]. 3. virtual platform the virtual platform is developed in labview® software (by national instruments corp.), which is largely used to control and monitor industrial equipment and processes, and for the creation of test and measurement systems [22], [23]. it offers a real-time feedback of tracking accuracy (figure 1) and it provides an intuitive and user-friendly interface. it is designed to be used in combination with a robot to move the sensor and provide accurate position references. the platform is composed of six main sections, which are described in the following subsections. the functioning of the platform is illustrated in figure 2. the model of the emts if defined in an external file and imported into the platform, and the user defines the trajectory for sensor movement. two different modalities can be performed: i) in the experimental mode, the platform connects to the daq device and the induced signal in the magnetic sensor is acquired as it is moved by the robot along the defined trajectory, ii) in the simulation mode, the signal of the magnetic sensor is simulated by employing a model of the magnetic field; in both cases, noise can be added to the signal. finally, the position of the sensor is estimated by means of a suitable reconstruction algorithm, providing real-time 3d representation and error statistics. 3.1. 3d view and real-time tracking statistic tracking systems provide the surgeon with the real-time estimate of sensor position, which is shown on a screen in front of the surgeon, where is also displayed the patient’s anatomic area. 2d and 3d views are commonly used; in particular, the latter is more difficult to interpret, but seems to guarantee greater precision [24]. hence, the platform provides a 3d view of tracking, where the actual and estimated position are displayed. moreover, real-time feedback of system performance becomes particularly useful when analyzing how a system responds to different inputs. many design errors can be quickly avoided by real-time feedback. hence, real-time plot and statistics of position tracking errors are provided during experiments. in particular, the position error along each cartesian axis, computed as the difference between the estimated position and the one provided by the robot, is shown on a graph, and its mean value and standard deviation are displayed. for example, the peak error in figure 1 suggests performing a deeper exploration of the correspondent space region. all tracking results and statistics can be easily exported for further elaboration in matlab or other software. 3.2. emts model import • the number and arrangement of the transmitting coils, as well as their electrical properties, highly affect system performance [25]. often the transmitting coils must be placed inside a well-defined space due to practical needs, such as the configuration of the clinical environment, weight acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 105 limitations, or application requirements. moreover, the tracking volume is usually proportional to the dimension of the fg (i.e., the magnetic field intensity) [12]. hence the platform provides the import of a mat-file (binary matlab® file) containing the geometrical arrangement and the electrical properties of the transmitting coils of the fg, in order to test different fg layouts, as well as the parameters of the sensor coil. in section 4.2, two different fg configurations will be compared to illustrate this functionality. • the assessment of tracking accuracy is a mandatory step in emtss developing, and different types of protocols have been defined [12], most commonly employing phantoms such as board and cube phantoms, as well as moving phantoms to assess dynamic performance. in addition, robots are also used to move the sensor, providing accurate position references, and allowing automatic and repeatable test; on the other side, robotic components can cause interference in the tracking volume, and they are quite expensive. in [26], the authors used a carbon fiber rod, held by the robot gripper, with the magnetic sensor positioned at the tip, in order to distance the sensor from the metallic components of the robot. the cinematics of the simulated robot (shown in figure 1) is based on a real robot, model rv-2fb-d from mitsubishi, which was employed in this research to move the sensor; however the platform provides the import of a file containing the model (i.e., the geometry of joints and links) of any robot. both the fg and the robot are displayed in the 3d scene of the platform, by means of the labview robotics toolkit. the fg shown in figure 1 represents the emts described in section 4.3. 3.3. reconstruction algorithm and magnetic field model different techniques can be used to reconstruct the position of the sensor in an emts based on frequency division multiplexing. figure 1. virtual platform developed in labview, during the execution of a simulation. on the left: the model of the melfa robot is shown during the movement; the green point is the position estimate provided by the algorithm, and fg reference system is shown in red. on the top: the noise settings section and the trajectory definition are shown. at the bottom: real-time statistics of position error are provided. figure 2. scheme of the functioning of the virtual platform. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 106 in [27], a suitable interpolation algorithm has been used to reconstruct the position of the sensor in a small space. the sensor is placed in 𝑀 different calibration positions, and the voltage from the sensors are measured; then, position estimation is based on interpolation between calibration points by using delaunay triangulation and linear interpolation. this technique requires measurements of the magnetic field in a dense grid to reach adequate accuracy, it does not allow extrapolation, and it is time-consuming, thus it could be applied only to small regions in the tracking volume. other algorithms are based on i) a model of the magnetic field obtained by approximating the coils as magnetic dipoles, or on ii) a model obtained by considering the mutual inductance between the transmitting coil and the sensor coil, which are considered as circular filaments [28]. both models require the knowledge of the geometrical parameters of the coils, and the electrical quantities (i.e., current and voltage) of the transmitting coils, but do not need as many measurements as the interpolation method. moreover, they could be used to compute the magnetic field (and therefore the induced voltage) in the whole tracking volume, allowing to perform experiments in a simulated environment. hence, the platform provides the possibility to choose an arbitrary reconstruction algorithm (developed in matlab), or to track the sensor simultaneously with two or more reconstruction techniques, to compare their performance in different scenarios. in this paper, to model the magnetic field produced by the fg and to reconstruct sensor position, we employ the dipole model explained in [21], [26], [28]. it is obtained by considering the magnetic moment generated by the i-th transmitting coil, expressed as: 𝒎𝑡𝑥,𝑖 = 𝑚𝑡𝑥,𝑖 �̂�𝑡𝑥,𝑖 , 𝑚𝑡𝑥,𝑖 = 𝑁𝑡𝑥,𝑖 𝑆𝑡𝑥,𝑖 𝐼𝑖𝑖 , 𝑆𝑡𝑥,𝑖 = π 𝑟𝑡𝑥,𝑖 2 , (1) where �̂�𝑡𝑥,𝑖 is the versor orthogonal to the surface 𝑆𝑡𝑥,𝑖 of the i-th transmitting coil, and 𝑟𝑡𝑥,𝑖 , 𝑁𝑡𝑥,𝑖 , and 𝐼𝑖𝑖 are the coil radius, the number of turns, and the rms value of the excitation current, respectively. the subscript i takes into account the differences of real parameters among each transmitting coil [26]. the rms magnetic field generated by the i-th transmitting coil in a generic point 𝒑𝒔 = [𝑥, 𝑦, 𝑧] t is 𝐁𝑖 (𝒑𝒔, 𝐼𝑖𝑖 ) = b𝑖 𝑥 𝒙 + b𝑖 𝑦 �̂� + b𝑖 𝑧 �̂� = µ0 4 π 𝑚𝑡𝑥,𝑖 𝑑𝑖 3 [3(�̂�𝑡𝑥,𝑖 · �̂�𝑑,𝑖 )�̂�𝑑,𝑖 − �̂�𝑡𝑥,𝑖 ] , (2) where 𝑑𝑖 = |𝒅𝒊|, with 𝒅𝒊 = 𝒑𝒔 − 𝒑𝒕𝒙,𝒊 represents the vector distance between 𝒑𝒔 and the center 𝒑𝒕𝒙,𝒊 of the i-th transmitting coil, and �̂�𝑑,𝑖 is its associated versor. if the magnetic flux is considered homogeneous on the surface of sensing coil 𝑆𝑠, the induced voltage related to the i-th coil can be expressed as �̃�𝑖 = 2 π 𝑓𝑖 𝑁𝑠 𝑆𝑠 𝐁𝑖 · �̂�𝑠 (3) where 𝑁𝑠 is the number of sensor coil turns and �̂�𝑠 = [𝑐𝑜𝑠(𝛼𝑠 ) 𝑐𝑜𝑠(𝛽𝑠 ), 𝑠𝑖𝑛( 𝛼𝑠 ) 𝑐𝑜𝑠(𝛽𝑠 ), 𝑠𝑖𝑛(𝛽𝑠 )] 𝑇 is the versor orthogonal to the sensor surface, where 𝛼𝑠 and 𝛽𝑠 define the orientation of the sensor coil. the position is estimated by minimizing the following cost function [29]: 𝐹(𝜽, 𝑰𝒕𝒙) = ‖𝒗 − �̃�(𝜽, 𝑰𝒕𝒙)‖2 2 , (4) which represents the squared error between the induced voltage 𝒗 measured from sensor coil and the voltage �̃� obtained by applying (3); this latter depends on 𝜽 = [𝒑𝒔 t, 𝛼𝑠 , 𝛽𝑠 ] t, and on the vector of the currents 𝑰𝒕𝒙. the minimum of (4), i.e. �̂� = arg min 𝐹(𝜽, 𝑰𝒕𝒙), is obtained by using the levenberg– marquardt algorithm. for the details, see [29]. 3.4. experimental or simulation mode the platform allows to control and perform both simulation and experimental tests. simulation mode the aforementioned models allow to carry out experiments on a simulated environment, resulting in a valuable tool for emtss design. it is possible to define the fg and the sensor coil (section 3.2), the position reconstruction algorithm (section 3.3), and custom trajectories along which to move the sensor. experimental mode – it is possible to define a trajectory, move the robot and acquire data from the data acquisition (daq) device. the 3d scene will show the real-time movement of the robot, along with the tracked position (the green dot in figure 1). hybrid mode – it is possible to import experimental data acquired during past experiments, and to run a simulation test showing the tracking results, also simulating the actual acquisition time of the related experiment. 3.5. noise section the voltage noise in the sensor signal highly affects tracking accuracy. several error sources contribute to sensor voltage noise, and two main contributions can be considered (all noise components are intended as standard deviation of rms quantities): i) the measurement and acquisition noise 𝜎𝑎𝑐𝑞 , and ii) the fg noise 𝜎𝐵 (𝒑𝒔, 𝜎𝐼 )the last one depends on the position 𝒑𝒔 of the sensor relative to the fg and is due to excitation current noise 𝜎𝐼 . the voltage noise 𝜎𝑣 can be expressed as: 𝜎𝑣 = √𝜎𝐵 2 + 𝜎𝑎𝑐𝑞 2 , (5) where it has been assumed that 𝜎𝐵 and 𝜎𝑎𝑐𝑞 both contribute independently. note that • 𝜎𝑎𝑐𝑞 is approximately constant in the whole working volume, since it depends on the measurement devices and on the johnson noise of the sensor. experimentally, it has been measured 𝜎𝑎𝑐𝑞 ≅ 20 nv for each frequency component. • 𝜎𝐵 depends on sensor pose and excitation currents, hence it is related to 𝜎𝐼 , and its contribution is higher when the sensor is closer to the fg. moreover, 𝜎𝐼 can differ between each transmitting coil. experimentally, it has been measured 𝜎𝐼 ≅ 0.07 ma as an average value among transmitting coils. in [29], a technique to compensate the effect of 𝜎𝐼 on the position error has been proposed. the effect of these noise components must be considered during simulations. hence, the platform includes a section to set the noise components (𝜎𝑎𝑐𝑞 is a scalar, 𝜎𝐼 is a 𝑛𝑥1 vector), to be added in simulations and also during real-time experiments, to investigate how a certain source of error affects tracking accuracy. for instance, a discussion about the selection of the daq device depending on the noise is carried out in section 4.1. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 107 4. validation the proposed platform is suitable for the assessment of virtual emtss during simulation, as well as the developed emts prototypes. in sections 4.1 and 4.2 we employ the platform for a practical case, showing its usefulness in assisting engineers during emts design and characterization, and in section 4.3 we illustrate some tests performed on the real emts. 4.1. daq device selection the platform can support engineers during daq device selection by using the noise section described in section 3.5. when setting up the measurement chain to acquire the signal form the sensor coil, it is important to select the daq device according to the accuracy requirements of the system. frequency sampling and noise are two main parameters to be considered [20], which affect system accuracy. in particular, the noise floor indicated in the datasheet of the daq device is added to the induced voltage, thus it directly affects position repeatability and accuracy [20]. in this way, the choice of a low-noise daq device can be evaluated for the purpose of improving performance. in this section, we performed some simulations: the rms induced voltage was simulated by applying (3), then adding a voltage noise component to simulate the noise floor of the daq device. figure 3 shows the results, where the mean of the euclidean position error over a linear trajectory (101 points at 600 mm from the fg) is plotted versus a range of selected 𝜎𝑎𝑐𝑞 , considering a fixed current noise of 𝜎𝐼 = 0.07 ma. as expected, it can be observed an increasing error with 𝜎acq; moreover, the behaviour is quite linear. a mean euclidean error below 2 mm is obtained if using a daq device with 𝜎acq lower than 40 nv. this information could be particularly useful when choosing components, considering the trade-off between increased cost and required accuracy. 4.2. fg configuration optimization as said in section 3.2, the platform allows to test different fg configurations, to evaluate the influence of the number, arrangement, and electrical properties of the transmitting coils on system performance. in this section we compare the performance of two fg configurations: one representing the emts prototype (figure 1), and one flat fg composed of six coplanar transmitting coils (figure 4). the coils of the two configurations are identical in their geometrical and electrical parameters, except for their position and orientation in space. figure 5 shows the position error along each axes, obtained by keeping the sensor with a fixed orientation along the z-axis, and moving it along the x-axis along a linear trajectory of 101 points, with a step of 1 mm, from point (x, y, z)=(-50, 0, 600) to (x, y, z)=(50, 0, 600), considering the reference system of the fgs. the rms induced voltage was simulated by applying (3), assuming an acquisition frequency of 20 hz. current and voltage noise of 𝜎𝐼 = 0.07 ma and 𝜎acq = 20 nv were added to each channel. it can be noted higher accuracy in the 5-coils fg configuration, whereas the 6-coils fg exhibits higher position error, in particular along xand y-axes. this suggests that further investigation should be performed to understand the cause of the error in that configuration, to avoid it during the realization of the fg. figure 3. mean euclidean position error vs. 𝜎𝑎𝑐𝑞 , assuming 𝜎𝐼 =0.07 ma. figure 4. flat fg configuration, obtained by modifying the number of transmitting coils ant their position and orientation. the fg reference system is shown in red. figure 5. comparison of position error of the two fg configurations. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 108 4.3. developed emts and experimental test the results obtained from the simulations performed with the platform must be comparable with the ones obtained with an actual fg, in order to validate it effectiveness in assisting the system designers. in collaboration with the company masmec biomed (modugno, bari, italy), an emts prototype was developed to obtain accurate estimation of sensor pose beyond 0.5 m from the fg, thus improving the state-of-the-art of commercial systems [20], [26], [27], [29]. it consists of three main components (figure 6): i. a magnetic field generator to generate em signals (the same shown in figure 1); ii. a small em sensor coil from aurora system; iii. a control unit for data acquisition and signal processing. the fg is composed of five transmitting coils, whose arrangement minimizes mutual inductances. each coil is powered with a sinusoidal current at different frequencies (approximately from 1 to 5 khz), thus generating an ac magnetic field, whose amplitude does not exceed 0.2 mt, which is the threshold value set in the ieee standard c95.1-2005. the whole magnetic flux generates an induced voltage on the em sensor (to be inserted into the surgical instrument), which is acquired, digitalized, and filtered by means of five band-pass filters obtaining five rms voltage components, related to the different excitation frequencies. these components are used to estimate sensor position by means of a suitable reconstruction algorithm. moreover, the current in each transmitting coil is measured by using five hall effect sensors (la 55-p lem), for the purpose of i) ensuring stability of the magnetic field by means of a current control loop, and ii) reducing error due to variations in the magnetic field, as shown in [29]. the control software was developed in labview®, and the sensor was moved by means of an industrial robot (by mitsubishi), which provided accurate position reference. an experimental test was performed to show the potentialities of the proposed platform when tracking a real sensor coil. the em sensor coil was moved by the robot along the trajectory defined in section 4.2, and the rms induced voltage was measured with a frequency of 20 hz, as suitable for real-time surgical applications (the sampling frequency of the daq device is set to 50 khz, with 2500 samples, thus computing the rms value every 50 ms, i.e., 20 hz [20]). the same trajectory was performed on both simulated and experimental data. for the simulation, 𝜎𝐼 = 0.07 ma and 𝜎𝐼 = 20 nv were considered for each channel, as quantified from the experimental data. figure 7 shows the obtained results. the position error obtained in both simulated and experimental case are comparable, with a mean euclidean position error of about 1 mm and 2 mm, respectively, which is suitable for many surgical procedures [12]. the difference is due to the approximation of the coils with magnetic dipoles, and to uncertainty in parameters. this result validates the performance of the platform in simulating real tracking, providing a valuable tool during system design and prototyping. 5. conclusions several sources of error affect emtss, and the high accuracy required from surgical application is highly influenced by the design and arrangement of the transmitting coils of the fg. many design errors can be quickly identified and avoided by realtime feedback. in this paper we illustrated the main features of a virtual platform, which permits to analyse system performance by adding noise components and simulating error sources, hence the robustness and the accuracy of the system and its weaknesses can be studied. moreover, it can be particularly useful for system prototyping, by investigating the effects of system parameters (geometrical and electrical ones). the usefulness of the platform was demonstrated by performing simulations related to some practical cases. finally, it was validated by performing some tests on a real emts, obtaining a mean euclidean position error of about 2 mm at a distance of 600 mm from the fg, comparable with the position error of 1 mm obtained by simulations, which is suitable for many surgical procedures. further development will regard an improved graphic and user interface, the inclusion of other sources of error (magnetic field distortion, em interferences), as well as a dynamic system model, in order to evaluate position error in fast varying conditions; the kalman filter will also implemented to obtain smooth trajectories. moreover, in this first version, the algorithm is developed in matlab, but other programming languages -e.g., pythonwill be considered in further versions. figure 6. experimental setup for system characterization. figure 7. position error from simulated (blue line) and experimental data (red line) obtained by moving the melfa robot along a linear trajectory at a distance of 600 mm from the fg. the sensor is aligned along z-axis. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 109 references [1] l. romeo, r. marani, m. malosio, a. g. perri, t. d’orazio, performance analysis of body tracking with the microsoft azure kinect, 2021 29th mediterranean 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https://doi.org/10.1109/memea52024.2021.9478723 ancient metrology in architecture: a new approach in the study of the roman bridge of canosa di puglia (italy) acta imeko issn: 2221-870x march 2022, volume 11, number 1, 1 7 acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 1 ancient metrology in architecture: a new approach in the study of the roman bridge of canosa di puglia (italy) germano germano’1 1 scuola superiore meridionale, university of naples federico ii, 80138, italy section: research paper keywords: ancient metrology; heritage; archaeology; roman architecture; bridge citation: germano germano’, ancient metrology in architecture: a new approach in the study of the roman bridge of canosa di puglia (italy), acta imeko, vol. 11, no. 1, article 16, march 2022, identifier: imeko-acta-11 (2022)-01-16 section editor: fabio santaniello, university of trento, italy received march 7, 2021; in final form march 26, 2022; published march 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: germano germano’, e-mail: germano.germano@live.com 1. introduction a metrological approach is essential when studying ancient monuments, as much as latin is in the study of roman civilization. the language of architecture makes use of numbers and measurements that are specific to each culture or geographical area, and their interpretation is key to developing hypotheses about the original look of an artifact and unlocking its meaning. this interpretation is often made difficult by the frequent encounter of gaps in the structural stratigraphy of the monument itself. this difficulty also lies in the use of a different construction vocabulary in the ancient world, made up of measurement systems that cannot be superimposed on the ones used nowadays, globally recognized in the international system of units. an interpretative criterion can therefore consist of detecting the recurrence of round numbers or multiples and submultiples of the numeral system of a certain culture, starting from today's metric data, comparing it with known ancient units of measurement and cross-referencing the data to verify the existence of correspondences. this approach [1] has proven to be useful in the study of a masonry bridge dating back to the time of the roman emperor trajan (2nd century ce), located along the ofanto river near canosa di puglia, in southern italy. before reaching its present conformation, the bridge is described in ancient documents as having only three arches, of which the central one, wider and higher than the others, gave impressiveness to the whole structure. this characteristic is common to many roman bridges, such as the nearby ascoli satriano bridge, and is due to the intent to obstruct the flow of the river as little as possible. the first (as well as the only) systematic study of the bridge dates back to 1985, when an archaeological excavation was carried out under the scientific direction of professor raffaella cassano of the university of bari [2], [3]. on that occasion, the archaeological investigation focused in particular on the study of the platea and its construction techniques. after more than thirty years, this research embraces the heritage of those studies, adding new data on the architecture of the bridge and on the basis of this data formulating new hypotheses with the aim of shedding new light on the millennial history of the monument [4]. abstract the bridge of canosa di puglia (italy) was originally built in the 2nd century ce to cross the ofanto river along the via traiana, the route built at the behest of emperor trajan that connected rome with the port of brindisi, on the adriatic sea. restorations, collapses and architectural transformations have deeply altered its original structure over the centuries, making it lose the traces of a monumental central arch. archival and field research, conducted through various surveys, has produced new data that has provided an update of the bridge's history. the aim of this dissertation is to show the results of a research conducted with a new methodological approach to the monument, applying ancient metrology to the interpretation of its architectural evolution. this method has proven to be indispensable to formulate hypotheses about the original configuration of the bridge, whose central arch would result to be one of the widest among the bridges of the roman architecture. mailto:germano.germano@live.com acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 2 2. historical background the history of the bridge follows the events of the road network established in this large flat area of apulia, called tavoliere, which thanks to its geographical characteristics was considered ideal not only for transhumance practices but also for the passage of routes to reach the south of the region and therefore the east. 2.1. the bridge in the roman age the original structure was erected on the occasion of the construction of the via traiana (trajan way) ordered by emperor trajan in 109 ce, starting from the city of benevento, to create an alternative to the via appia (figure 1) that would allow a faster connection between rome and brindisi, the main port towards the east. it is not clear if there was already a bridge here prior to the imperial age, for the structure was constructed on a route already in use before, the via minucia [5]. in any case, the presence of a well-preserved foundation stone paved platea, reveals an ex novo construction which is consistent with the construction techniques of the imperial age. the first works of restoration are documented in roman times through inscriptions [6] that attest to repairs under septimius severus and caracalla, in the tetrarchic period, between the end of the 3rd century ce and the beginning of the 4th century ce and in the constantinian age. however, the epigraphs only report these operations in a generic and celebrative way, without providing any useful data to determine measurements. 2.2. the bridge in middle ages in the middle ages the bridge was still in use, since it was located along the via francigena, a road through by which christian pilgrims from all over europe reached the ports of puglia to the holy land. in the middle ages even flocks, herds and shepherds used it when they seasonally migrated from the altitudes of abruzzo and molise to the plains of the tavoliere with a milder climate, through the so called tratturi (drover’s roads), one of which was passing right through here. during this long period new works were certainly necessary, but these were not documented until 1521, as the fragment of an inscription, reused in the mausoleum of boemondo d'altavilla in canosa, would evidence. in 1541 a polish traveller described it as altissimum pontem muratum (“a very high masonry bridge”) [7], while the first known information on its dimensions has been provided by an italian traveler who at the end of the 16th century, in addition to describing the bridge as ponte bellissimo fatto (“a very well-made bridge”), reported the size of the central arch: 128 palms long and 40 palms high [8]. 2.3. 18th century: first survey, the collapse and the reconstruction more than a century later, earthquakes, floods and wear and tear would weaken the structure of the bridge, making further restoration work necessary. the institution in charge of its maintenance, as belonging to the area of its domain, was the regia dogana delle pecore (sheep customs office), established in nearby foggia in 1480. the documents relating to the interventions of the eighteenth century are still preserved in its archives. in these archives it is stated that in 1749 a technical expert's report was commissioned to francesco delfino in which he warned of the dangers of stability. attached to it, he schematically drew an architectural representation of the bridge in which he indicated the possible breaking points (figure 2). in the same document [9] he also reported the exact measurements of the arches: "[…] the main arch (is) 112 palms wide, high from the floor to the top (of) 44 palms, with a front of 5 palms, (while) the two lateral arches are wide 50 palms each, and high 25 palms". due to the stalling of a targeted action that inevitably would have involved large costs, which none of the parties involved wanted to bear (the dogana, the crown, local administrators and landowners), the central arch collapsed on 11 february 1751. several considerations were made by technical experts who immediately intervened after the matter, among which, in the end, the proposal of a safer reconstruction prevailed, but which would have definitively changed the millenary aspect of the bridge. in fact, it was decided not to rebuild the central arch but instead to put two smaller ones in its place resting on a newly built central pillar, thus lowering the profile of the bridge to a height similar to that visible today. 2.4. the bridge today the latter, however, does not date back to these interventions because the current structure is the result of a reconstruction carried out in the mid-twentieth century, particularly concerning the central arches, after the retreating german troops in world war ii bombed the bridge [10], of which only the piers and the abutments were saved. the bridge today (figure 3) features five arches of different sizes and morphology (starting from the east: 6.50 m, 13 m, 12.10 m, 12.10 m, 13 m) based on four piers of different sizes, ranging from a minimum of 6.2 m to a maximum of 8.4 m. these figure 1. the route of via appia (the appian way) and via traiana (the trajan way) with the major cities along the road and, in red, the city of canusium (canosa di puglia) near the ofanto river. figure 2. drawing of the bridge by francesco delfino, 1749, detail. (archivio di statto di foggia) acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 3 are composed of square blocks in isodomic work and equipped with triangular starlings and prismatic cones, upstream and downstream. the walkway above is developed for a length of 170 m and a width of 4.5 m, its trend is not straight and nor in correspondence with the abutments. the piers, the abutments, and the foundation platea are the only surviving elements of the original structure [3]. when the river is dry, it is also possible to see the concrete walkway built for the passage of american tanks in the last phase of world war ii [10] (figure 4). today, the bridge is only accessible on foot, so as not to overburden the structure with the passage of vehicles, for which instead was built a bridge in the 20th century a few hundred meters further upriver. 3. method thanks to the diffusion of new digital technology and the constant decrease of its cost it has been possible to develop methodologies able to detect, interpret and preserve very important data regarding works of archaeological and architectural heritage [11], [12]. all these operations have been a fundamental support to the global process of knowledge and analysis, revealing the need for multidisciplinary expertise, both theoretical and technical, in the approach to archaeological and architectural heritage. once the research aim has been set up and the parts of the artefact considered to be original had been ascertained, the following steps were taken: archival research, through published sources and unstudied documents, comparison with historical photographs, on-site survey with manual and drone technology, photogrammetry operations, identification and recording of the different ssu (structural stratigraphic units) [13], [14], 3d modeling, graphic restitution and reconstruction hypotheses. leaving aside the part about archival research, which has been dealt with in detail elsewhere [15], all the on-site operations highlighted the complexity of data collection in a context such as the fluvial one. in particular, the photographic campaign and the acquisition of the metric data with the integrated survey were found to be difficult due to the high flow of the river and the dense vegetation. for this reason, survey operations must take into account the natural context in which they are carried out and must also be appropriately planned according to the season. 3.1. survey with drone technology and photogrammetry thanks to drone technology, it was possible to overcome some of the obstacles described above and record a set of data useful for the creation of the virtual model. the drone used is a dji mavic 2 pro equipped with a high resolution camera with 78.8° shooting angle of 26.6 mm and 1/2.3'' cmos sensor of 12.7 megapixel and glonass gps system with vertical accuracy error of ± 0.1 m and ± 0.3 m figure 3. the bridge of canosa di puglia. view from west. figure 4. the bridge during the dry season, when the paving blocks of the roman platea and the concrete boardwalk built in the first half of the 20th century are visible. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 4 horizontal. proceeding with an aerial photo acquisition in manual flight, 3 photo acquisitions were made, of which 2 in rotation around the artifact at an altitude of 5 m and 15 m and one top view at an altitude of 25 m. the rotation photographic acquisition was made with a camera inclination of 0° (the one at 5 m), then with a view perpendicular to the building in order to reduce the aberrations during data processing, the one at 15 m with a camera inclination of 45° for a photo acquisition necessary to receive data of the intersection node between the vertical and horizontal surfaces, and finally the one above 25 m with nadiral camera inclination for the acquisition of data from the upper part of the bridge, producing a total of 154 photographs with gsd (ground simple distance) of 4. 55mm/pix. even if ground control points (gcps) are usually necessary to guarantee that the shape of the model is geometrically correct in all three dimensions, during the acquisitions it was not necessary to use gcps for a subsequent scaling of the model, as the measurements were taken using the architectural elements present in the structure such as the widths of the spans and the width of its the extrados, through direct survey operations and the use of laser level and laser rangefinder. the photogrammetry operations [16] were carried out with the help of agisoft photoscan software, through which a dense point cloud composed of 12,490,415 points was generated, on which a textured mesh composed of 396,783 polygons and 396,783 vertices was produced (figure 5). the model was scaled according to the points detected in the campaign phases. the combined use of the survey methodologies constituted a system of verification of the overall process in the acquisition of the monument's dimensional information and its interpretation. in addition to the textured model (figure 6) generated by the photogrammetry software, a simplified model has been elaborated for the virtual reconstruction operations [17]. the former then served as a comparison and verification of the latter, and vice versa. 3.2. ancient metrology and data analysis a metrological approach has been adopted in the interpretation of data resulting from the survey, comparing it with dimensional measures mentioned in ancient sources and cross-referencing them by synchronic or diachronic conversion of numerical data. the first case applies for historically and culturally related contexts, while the second case implements a multi-layered reading of different ages. discarding the parts ascertained as added or reconstructed, only the original elements still in situ were taken into consideration. finally, a reconstructive hypothesis of the original morphology of the monument has been formulated, based on roman construction and measurement methods. a certain degree of approximation is to be expected both in reporting the measures and, consequently, in converting them to reconstructive hypotheses. however, the margin of error is limited enough to consider the data as valid for the purpose of the research. figure 5. a) survey report: camera locations and image overlap; b) camera locations and error estimates. z error is represented by ellipse colour. x,y errors are represented by ellipse shape. estimated camera locations are marked with a black dot; c) survey report. reconstructed digital elevation model. figure 6. axonometric view of the bridge elaborated using drone technology and aerial photogrammetry. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 5 while the fundamental unit of measurement used during the roman age was the foot (pes) and followed closely the measures of its greek counterpart, the attic foot, corresponding to 0.296 m [18], the authors of the two reports describe the bridge using the same unit of measurement, i.e. the palmo (table 1). as the two cases, 1584 and 1749, fall within the time span of the dominion of the kingdom of naples, which included the whole of southern italy, we have therefore to assume that they were referring to the neapolitan palm, corresponding to 0.26367 m [19], calculated on the basis of the measurement of an oxidized bar kept in castel capuano in naples used as the governmental standard [20], according to an edict (lost) issued on 6 april 1480 by ferdinand i of aragon and in force until 1840. by applying a metrological approach, it is possible to derive different hypotheses about the construction phases of the bridge. 3.3. synchronic conversion: what happened between the two key dates (1584 1749)? a first conversion, of synchronic type, is made within the same measurement system to hypothesize the changes between the two sources key dates: it is interesting to note that both measurements are a multiple of 8. in the kingdom of naples, the unit of measurement that follows that of the palmo is the canna, which measures exactly 8 palms. reading it in multiples, the 1749 survey describes an arch span reduced from 16 to 14 canne, exactly one canna per side (2.1 m). a margin of error of 4.2 m appears too large for a width of about 30 m. one possibility is that consolidation works were necessary following one or both of the devastating earthquakes, the first in 1627 and the second in 1731 [21], which struck the capitanata area and in particular canosa. in this sense, the work may have involved the overlaying or covering of the inner part of the pillars with a masonry reinforcement designed on the basis of a standard building measure, i.e. one canna. within delfino's design there are two elements protruding from the piers (figure 7), an unusual fact that supports this hypothesis, as an expert surveyor would hardly have invented or exaggerated, despite the evident schematization of the work. 3.4. diachronic conversion: reconstruction hypothesis on the other side, through a diachronic conversion of the units of measurement (table 2) it is possible to read the dimensions of the monument with the same measuring standards of the roman builders. as previously said, the central arch was imposing and larger than the other two, as shown by delfino's drawing and, even if with a more simplified style, other graphic sources preserved in the archives. therefore, if we accept the dimensions reported in the document (112 palms) and hypothesize that they are unchanged from the roman age, having assumed the lateral piers as dating back to the imperial age, we obtain a measure (29.49 m) that is, with the due margin of error, exactly corresponding to 100 roman feet. this measurement is an architectural constant of the monumental roman architecture [22], present in one of the most famous monuments of the emperor trajan in rome, the column called "centenaria" for its height, and then in the aurelian column, but also verifiable in the diameter of many monuments such as mausoleums (emperor augustus, cecilia metella, l. m. plancus and sempronius atratinus and the most famous pyramid tomb of caius cestius, whose side measures exactly 100 roman feet), public buildings (the hall of the palatine basilica of constantine in trier), theater orchestras (aquileia), bridges (narni, alcantara) and many others, just to mention a few [23]-[26]. also the height, 44 palms, would correspond to about 40 roman feet, still a multiple decimal number, but this data is subject to more variables since in the case of restoration or collapse the section of the arches is the part most exposed to sensitive alterations. moreover, it must be considered that it is not indicated in the sources whether the measurements were taken at the height of the water or the foundation platea. given these measurements, therefore the arc of the circle tangent was traced to the ideal segment at the level of the piers, about 3 m high, obtaining a figure that outlines a double-inclined slopes profile, based on a comparison with similar bridges, such as that of ascoli satriano on the carapelle river (2nd century ce) and the pont julien at bonnieux, in france (augustan age). the inclination of this profile corresponds to that found in the joints of the abutments (figure 8), especially on the western side, which would confirm the presence of the original outline of the bridge (figure 9). further information comes from some inscribed slabs found in cerignola [27] that could belong to the bridge. this type of inscribed slab bore the inscription relating to the work, exalting its completion, and was part of the construction program of the via traiana. also in this case, even if we do not have any information on what the original parapet looked like, the metric data relative to the "front" (5 neapolitan palms, that is 132 cm) table 1. ancient units and their mutual correspondence. unit roman feet neapolitan palms meters pes 1 1.1225 0.2960 palmo 0.8909 1 0.2637 canna 7.1270 8 2.1096 meter 3.3784 3.7922 1 figure 7. drawing of the bridge by francesco delfino, 1749, detail of the central arch (archivio di stato di foggia). possible traces of structural changes between 1584 (a) and 1749 (b) which would explain the different dimensional data of the two reports mentioned in the text. table 2. diachronic measures conversion chart. element of the bridge palms meters roman feet main arch (span) 112 29.53 99.78 ≃ 100 main arch (height) 44 11.60 39.20 ≃ 40 lateral arches (span) 50 13.19 44.54 ≃ 45 lateral arches (height) 25 6.59 22.27 front 5 1.31 4.45 acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 6 corresponds exactly to the height of the slab of cerignola, so is possible that it was set along the balustrade that delimited the passage on the bridge. thanks to this large amount of details, the reconstructive hypothesis of the bridge during the original phase of the structure and the subsequent ones has been elaborated, according to a representation technique that, combining technical drawing with watercolor effects, was both consistent with the data and useful for dissemination (figure 10). this phase of representation, based on processing and elaboration of raw data obtained from the previous survey, requires a logical process [28] that can lead us to reconstruct an accurate and complete 3d model in order to be used as a support for the research. in order for the representation to be complete and intelligible, it is necessary indeed to understand the geometry and shape of the elements to be represented, as well as their reciprocal relationships [29]. 4. conclusions in an impervious context such as that of a river, challenging due to the frequent lack of sources and archaeological data, the research has shown how useful metrology has proved to be in formulating the reconstructive hypothesis with a high degree of reliability on a monument that has been strongly compromised over the centuries. beyond the theories on the exact morphology of the bridge, the research brings to light an architectural reality whose scope has been unfairly underestimated, and which places the bridge back in the history of ancient architecture, counting it among those with a central span among the longest in roman times, according to a comparison with the monumental study carried out by professor vittorio galliazzo [30], which is the most important work on roman bridges, together with those of colin o'connor [31] and piero gazzola [32]. the results of this study would confirm the tendency in roman monumental architecture to use a 100-foot module, figure 8. upper, the bridge with the geometrical construction of the hypothesized shape. lower, northwest front of the bridge, abutment. the red line shows the probable original incline still identifiable in the grade of the rows of blocks. figure 9. graphical reconstruction of the bridge of canosa in roman times. figure 10. illustration of the two main phases: the roman bridge (top); the bridge nowadays showing the foundation platea (bottom). acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 7 although the variables involved encourage a careful approach, in order to prevent the temptation of a “metrological pareidolia”. references [1] g. germano’, the roman bridge of canosa di puglia: a metrological approach, proc. of the 2020 imeko tc-4 international conference on metrology for archaeology and cultural heritage, trento, italy, 22-24 october 2020, pp. 605-610. online [accessed 20 march 2022] https://www.imeko.org/publications/tc4-archaeo2020/imeko-tc4-metroarchaeo2020-115.pdf [2] r. cassano, canosa. campagna di scavo 1985: il ponte romano sull’ofanto, in: le rassegne archeologiche in puglia. atti del xxv convegno di studi sulla magna grecia, taranto, 3-7 ottobre 1985, napoli 1986, pp. 408-142. 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[in italian] https://www.imeko.org/publications/tc4-archaeo-2020/imeko-tc4-metroarchaeo2020-115.pdf https://www.imeko.org/publications/tc4-archaeo-2020/imeko-tc4-metroarchaeo2020-115.pdf https://doi.org/10.1016/j.jas.2014.01.010 http://dx.doi.org/10.22201/fa.14058901p.2020.45.77630 https://doi.org/10.3390/rs3061104 https://doi.org/10.14434/sdh.v5i1.30812 https://doi.org/10.6092/ingv.it-cpti11 https://hal.archives-ouvertes.fr/hal-01435853/document http://www.fastionline.org/docs/folder-it-2021-495.pdf http://dx.doi.org/10.21014/acta_imeko.v10i1.842 http://dx.doi.org/10.1007/s12518-011-0076-7 feasibility and performance analysis in 3d printing of artworks using laser scanning microprofilometry acta imeko issn: 2221-870x march 2022, volume 11, number 1, 1 7 acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 1 feasibility and performance analysis in 3d printing of artworks using laser scanning microprofilometry sara mazzocato1, giacomo marchioro2, claudia daffara1 1 department of computer science, university of verona, str. le grazie 15, i-37134, verona, italy 2 department of cultures and civilisations, university of verona, v.le dell'università 4, i-37129, verona, italy section: research paper keywords: optical profilometry; 3d printing; conoscopic holography; surface analysis; non-destructive testing citation: sara mazzocato, giacomo marchioro, claudia daffara, feasibility and performance analysis in 3d printing of artworks using laser scanning microprofilometry, acta imeko, vol. 11, no. 1, article 19, march 2022, identifier: imeko-acta-11 (2022)-01-19 section editor: fabio santaniello, university of trento, italy received march 7, 2021; in final form march 22, 2022; published march 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was partially supported by scan4reco project, european union horizon 2020 framework programme for research and innovation, grant agreement no 665091 and by temart project, por fesr 2014-2020. corresponding author: sara mazzocato, e-mail: sara.mazzocato@univr.it 1. introduction 3d sensors and 3d printing technologies are gaining more and more attention in many fields, from industry to medicine, to cultural heritage [1]-[5] also thanks to fact that 3d printers are being easily accessible and have gradually gained better levels of accuracy. in the field of cultural heritage, these technologies arouse interest because of the possibility to reproduce artworks or part of them for museum exhibition, restoration and conservation reasons, haptic fruition, and other purposes [6]-[8]. clearly, the first step to obtain realistic and accurate 3d printed objects is the data acquisition process. in this context, non-contact 3d optical systems play an important role in such applications, from quality control to robotics, where a remote measurement is preferable and/or the object is fragile. their capability to measure the surfaces in contact-less and noninvasive way led them to be key instruments especially in the field of cultural heritage, where the surface of the object or the 3d shape at the various scales is the central and essential part of the artwork itself [9]-[14] and integrated diagnostics is performed [15]. each surface has an intrinsic multiscale nature that can be represented as a superimposition of a large number of spatial wavelengths. the natural question that arises spontaneously is if (and the extent to which) the 3d printing process preserves this intrinsic multiscale nature of the surface. despite the rapid growth of the use of 3d printing technologies, the accuracy of the 3d printed models has not been thoroughly investigated and not all the technologies are studied [16]-[19]. in this paper, we first present the prototype of the optical scanning microprofilometer as scanning system that allows high accuracy acquisition of the object [20]. the system is based on the interferometric method of conoscopic holography that enables to acquire surface data with micrometric resolution. thanks to the adaptability of the conoscopic holography sensors and the scanning setup, the system is able to measure irregular shapes, composite materials, and polychrome surfaces, thus abstract we investigated optical scanning microprofilometry and conoscopic holography sensors as nondestructive testing and evaluation tools in archeology for obtaining an accurate 3d printed reproduction of the data. the modular microprofilometer prototype allows a versatile acquisition of different materials and shapes producing a high-quality dataset that enables surface modelling at micrometric scales from which a "scientific" replica can be obtained through 3d printing technologies. as exemplar case study, an archeological amphora was acquired and 3d printed. in order to test the feasibility and the performance of the whole process chain from the acquisition to the reproduction, we propose a statistical multiscale analysis of the surface signal of object and replica based on metrological parameters. this approach allows to demonstrate that the accuracy of the 3d printing process preserves the range of spatial wavelengths that characterizes the surface features of interest within the technology capabilities. this work extends the usefulness of the replicas from museum exposition to scientific applications. mailto:sara.mazzocato@univr.it acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 2 leading to a multiscale and multimaterial approach in surface analysis [21]. here, the scanning profilometry is used to acquire accurate data of an archaeological object, which are then processed in order to obtain a 3d printed replica of the object itself. thus, we have come full circle by measuring the 3d printed object and by comparing the surfaces on a metrological basis. after a brief presentation of the optical microprofilometer developed in our laboratory (section 2), we demonstrate the application in a real case study, processing the dataset acquired on an ancient amphora and obtaining a mesh file suitable for the 3d printer (section 3). in section 4 we propose a statistical and multiscale analysis of the original object and its 3d printed replica in order to assess the accuracy of the whole process from the acquisition to the 3d printing. finally, in the concluding section, the main results are discussed. 2. optical microprofilometry the optical microprofilometer is based on the conoscopic holography technique that exploits interference patterns to obtain a pointwise measurement of distances with micrometric accuracy [22], [23]. in detail, a laser backscattered from the surface is splitted into ordinary and extraordinary rays after passing through an optically anisotropic crystal. the two beams share the same geometric paths but have orthogonal polarization modes. the refractive index of the crystal depends on the angle of incidence of the beam and on its polarization state determining a difference in the optical path lengths. once the two beams exit the crystal, they interfere to each other, and the pattern is recorded. the analysis of the generated pattern allows to obtain information about the distance of the sampled surface from the light source [5], [6]. conoscopic holography sensors enable to perform contactless measurements at sub-millimeter spatial resolution with a precision down to a few micrometers on different kind of materials, from the specular reflective surfaces to the diffusive ones. the developed system has a multi-probe module including a sensor for diffusive materials, a sensor for reflective materials, and a sensor for specular or transparent surfaces (all sensors by optimet). interchangeable lenses allow performing acquisitions in different working ranges, i.e., the maximum scanned height, tailored to the scale of the object. this aspect is very important in profilometry applied to the variegate 3d archaeological manufacts. the different combination of sensors and lenses allows the analysis of reflective materials with a maximum accuracy of 1 µm and working range of 1 mm up to a working range of 9 mm with an accuracy of 4.5 µm. while for diffusive material it is possible to achieve an accuracy of 2 µm with a working range limited to 0.6 mm or extending the working range up to 180 mm maintaining a sub-millimeter accuracy of 100 µm. the surface dataset is obtained as sequence of single point measurements of the depth distance (z) in the x-y plane, with the probe triggered to a set of motorized stages moving the object plate, as can be seen in figure 1. the advantage is the creation of profiles and surface maps with custom “field of view” that is only limited by the axis range. our scanning setup is composed by a motion system with linear micrometric axis stages (by pi) orthogonally mounted to form the acquisition grid (x, y axis). the axes have a maximum travel range of 300 mm and a step precision of 0.1 µm with an accuracy of 1 µm over the entire length. the probe operates in pulse-mode, receiving pulses from an external trigger sent by the scanning system: for each pulse the sensor measures the distance to the sampled object. as depicted in figure 1 the object surface is effectively measured within maximum the working range of the depth sensor. in order to improve the capability of scanning complex shapes, a third motorized axis controlling the position of the probe along the line of sight can be added. 3. surface acquisition and 3d printing the case study concerns a portion of an archaeological amphora (figure 2). the object is acquired with the conopoint-3 sensor with a 75mm lens. this probe-lens coupling allows a working range of 18 mm with a stand-off distance of 70 mm and a laser spot size of 47 µm. we acquired a selected region of interest of 55.2 × 80.1 mm2 with a scanning step (x-y sampling grid) of 100 µm. the interest in this case arises from its surface geometrical structure being a superimposition of a large number of scales. the usual approach in surface metrology is to separate the surface in three main scales: the roughness, that represents the irregularities at smaller wavelengths and exhibits a random nature often related to the behaviour of the material; the waviness, i.e. the more widely spaced variation often associated with the traces left by the tools used for the creation of the object; and the form, i.e. the 3d shape of the object. obviously, even if these features are intrinsic characteristics of the object, the acquired surface signals depend on the bandwidth of the measurement: the distribution of peaks and valleys in a surface profile is influenced figure 1. scanning setup of the microprofilometer with the specification of the working range and an exemplification of a scan line. figure 2. part of the archaeological amphora with the scanned region highlighted. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 3 by the sampling step that, together with the nyquist criterium, determines the smallest spatial structure. surface signal decomposition is of particular interest in archaeology, as the waviness pattern can be put in relation with the manufacturing tools. a polynomial fitting enables to separate the form and the texture to obtain the so-called conditioned surface (with zero mean), while the roughness is separated from the waviness using a gaussian filter. 3.1. 3d printing from the acquired data we can tailor the creation of a mesh to be used for reproducing the object with 3d printing technology. most of the profilometers do not store the data as a point clouds or mesh so they cannot be printed directly. therefore, we developed our own tools for building a mesh from the height maps collected by the microprofilometer. in detail, from the generated grid of equally spaced point we obtain the point cloud data with the triplet (x, y, z) representing the vertices of the mesh, we create the faces and hence a cuboid with the same dimension of the scan and we substitute the top face with the scan. eventually, we can programmatically create and export the mesh to a stl file (using, for example, the trimesh library [24]), the typical file format used by the 3d printers, storing the triangulated surface. figure 3 shows the 3d printed object, created using stereolithography technology with a resolution of 50 µm in the z-axis of the printer. the resolution in the x-y printer plane is not specified even if it is specified the laser spot size of 140 µm [25]. in this kind of printing process, the object is printed layer-bylayer by polymerization of a liquid resin thanks to the exposure to laser light. each single layer in the x-y plane is created by a raster-scanning of the focused laser light spot within the plane. the accuracy of the process depends also on the printing direction [26]. on this regard, it is worth to note that the best orientation of the model during the printing process should be the one that reflects the acquisition process. in fact, the optical microprofilometer is a line-of-sight technique that provides a height map as result of the measure and, like most of the conventional surface measurement techniques, it is not able to acquire re-entering features (like a “c-shape” along the scan directions). 4. surface analysis and comparison as mentioned above, a surface can be represented as a superimposition of different frequencies; adding a process in the chain can alter the resulting surface. in particular, the 3d printing process could be seen as a filter that cuts or badly passes some components. therefore, the question to which we are interested, that naturally arises after the printing, regards the level of accuracy of the printing process in the creation of the replica. to answer this question, the 3d printed object was scanned with the microprofilometer in the same measurement condition, i.e. with the same probe, lens, and scan velocity. the comparative analysis of the surfaces was performed on the basis of surface metrology parameters [27]-[30]. figure 4 shows the comparison between the amplitude parameters of the original object and the 3d printed replica. in particular, the root mean square roughness sq, the average roughness sa, the skewness ssk and the kurtosis sku are evaluated. it is worth noting that the sku parameter, representing the sharpness of the surface peaks, deviates from 0.7 to 175.4, while the averaged texture amplitude sa varies from 112 µm to 110 µm, sq changes from 134 µm to 142 µm and the skewness ssk varies from 0 to -5. in order to understand the differences between the surface datasets of the real object and the replica, the same signal decomposition is carried out, namely the form is separated from the texture through a second order polynomial fitting and then the roughness is highlighted through a gaussian filter keeping the same parameters in both cases. the figures below (figure 5, figure 6) show the texture of the original and 3d printed objects. once the decomposition is done, the texture and the roughness are analysed using a multiscale approach [31]. this way, the texture and the roughness features are inspected with the variation of the observation scale. in order to have an insight of the average behaviour of the surface, the amplitude parameter sq and the extreme values parameters sz (maximum height of the peaks), sv (maximum depth of the valleys) and st (maximum peak to valley) are calculated as a function of the evaluation length. in particular, the multiscale analysis is developed as follows: the surface is divided in square subregions of specific side, i.e. the evaluation length, skipping a margin around the surface to avoid artefacts due to edge effects. in each subregion the aforementioned parameters are calculated and are then averaged on the patches. plotting the texture parameters against the evaluation length gives an idea of their variation with the observation scale. here, the evaluation length starts from 2 mm (400 sampled points in each subregion) and increases of 2 mm each time. from figure 7 it emerges that the root mean square sq of the original and the 3d printed object follows a similar behaviour with the length scale. as expected, sq first increases with the evaluation length and then it converges to a stable value. figure 3. 3d printed region of the amphora. figure 4. comparison of amplitude parameters between original and 3d printed object: root mean square deviation (sq), mean absolute deviation (sa), skewness (ssk) and kurtosis (sku) of the surface heights distribution. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 4 figures below show the variation of the extreme parameters with the evaluation length. as can be seen in figure 8, the maximum peak to valley distance st follows a similar evolution, but it is interesting to note how the values regarding the 3d printed object are lower than those concerning the original object. more in details, while the maximum height of the peaks sz (figure 9) has a similar trend, the maximum depth of valleys sv (figure 10) differs. beside resolution limits, a possible explanation is the combined effects of the gravity and the washing that is performed after the 3d printing process. in fact, during the printing process, the object grew upside down and after the object was completed, it was subjected to washing and post-curing processes. the washing has the goal of removing uncured resin from the surface of printed parts by simultaneously soaking and moving them in a solvent, but it is possible that some particles of liquid remain trapped within the valleys. another comparative analysis was performed, in term of frequencies, through the power spectrum density function (psd). the psd is calculated as the averaged one-dimensional psds along the scan direction of the surface height profiles. in order to have a double comparison, the psd is calculated also in a second scan of the 3d printed object rotated of 180°. the psd is plotted versus the wavevectors defined as qi = 2/i, where i is the wavelength of the surface component. figure 11 shows the power spectrum of the whole surfaces while figure 12 shows the power spectrum of the texture, i.e. the previous surfaces once the form is removed. as can be seen, the power spectrum of the total signal shows that the information is preserved for the lower frequencies, with a variation in the figure 5. texture of the original object acquired with a scan step of 100 µm and an accuracy in the z-axis of 10 µm. figure 6. texture of the 3d printed object acquired with a scan step of 100 µm and an accuracy in the z-axis of 10 µm. figure 7. variation of the sq of the textures calculated at different scan size. figure 8. variation of the peak-to-valley distances of the texture (st) calculated at different scan size. figure 9. variation of the maximum peak height of the texture (sz) calculated at different scan size. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 5 behaviour of the higher frequencies at q  15 mm-1 (  0.42 mm). this can be better analysed in the texture signal (figure 12) where the power spectra of the real and printed objects intersect. up to that point, the signal of the real object is higher, while from that point onward it emerges that the psd of the printed object is no more informative. the roughness analysis is done once the roughness signal is decomposed from the texture. the critical issues related to the use of the gaussian filter is the choice of the cut-off wavelength. the figure below shows the variation of sq of the roughness signal of the original and the printed object versus the cut-off wavelength. as can be seen, the trend follows a similar behaviour with a lower sq in the replica due the smoothing of the 3d printing process (figure 13a). as the cut-off becomes longer, the sq increases including also the waviness contribution (figure 13b). figure 14 shows an example of the texture decomposition with a cut-off of 6 mm. it emerges that the waviness signal is well preserved while the roughness signal highlights the smoothing effect described above. 5. conclusions in this work we presented the application of scanning optical microprofilometry for the acquisition, the analysis, and the 3d printer reproduction of archaeological objects. thanks to the capability to perform contact-less and full-field measurements with micrometric precision, this technique is a powerful tool for non-destructive testing and evaluation in cultural heritage. the versatile instrument configuration based on scanning stages and conoscopic holography sensors allows for setting optimal sampling parameters for different needs, enabling multiscale and multi-material surface measurements. the “field of view” of the figure 10. variation of the maximum valley depth of the texture (sv) calculated at different scan size. figure 11. comparison between the average power spectra of the total surface of original object and 3d printed object. figure 12. comparison between the average power spectra of the texture signals of the original object and the 3d printed object. a) b) figure 13. variation of the sq with the cut-off wavelength of the gaussian filter. the top plot a) represents the roughness signal extracted with a cut-off step of 100 µm while the second plot b) shows the increase in sq by varying the cut-off from 1 mm to 45 mm. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 6 x-y scanning (up to 30 × 30 cm2) was designed for acquiring microsurface data in medium-sized objects, within a z depth range that is determined by the lens (e.g. the range can vary from 0.6 mm to 180 mm maintaining the same probe), thus allowing the surface modelling of “flat” objects like ancient coins or significant part of 3d objects in a measuring session. reaching a micrometric resolution is of fundamental importance in the acquisition and in the analysis of the material surface. in this study we focused on the measurement of an archaeological object representing a complex and exemplar case study, with sharp evidence of the three main surface signal components: shape, waviness, and roughness. we demonstrated how we obtained a high-fidelity and high-resolution 3d printed replica of the object starting from the microprofilometry dataset. moreover, we have come full circle acquiring the printed object and assessing how the process preserves the surface signals. in order to perform this task, a multiscale analysis of the original and 3d printed object is proposed by inspecting the inband roughness through the power spectrum, as well as the variation of the areal amplitude parameters (sa, sq, ssk, and sku) with the observation length. it was demonstrated how the printing process is accurate even if the replica is affected by less marked valleys and by a loss of signal in the high frequency surface components, thus resulting in a smoothing effect. using a scanning step of 100 µm in the microprofilometer with the conoscopic probe setting a laser spot of 47 µm and a depth accuracy of 10 µm, and using a printing resolution of 50 µm (along the printer z-axis), it is shown that, beside the form of the archaeological manufact, the surface texture is accurately acquired and 3d printed without artifacts affecting the waviness and the roughness appearance. the focus of this work was the feasibility and performance analysis in 3d printing of artworks using laser scanning microprofilometry. we have turned our attention to a 3d replica obtained with the stereolithography technology and a photopolymer resin. however, the method could be used to compare the performance of different kind of 3d printer technologies as well as various 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https://doi.org/10.1111/1556-4029.13917 https://doi.org/10.3390/su13020680 https://doi.org/10.1117/12.2270307 https://doi.org/10.1109/eeeic.2017.7977779 https://doi.org/10.1364/josaa.9.000070 https://doi.org/10.3390/s90907021 https://trimsh.org/ https://formlabs.com/3d-printers/form-2/tech-specs/ https://doi.org/10.3390/ma13153405 https://doi.org/10.1007/978-3-642-36458-7_2 https://doi.org/10.1016/j.wear.2011.08.006 iomt-based biomedical measurement systems for healthcare monitoring: a review acta imeko issn: 2221-870x june 2021, volume 10, number 2, 174 184 acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 174 iomt-based biomedical measurement systems for healthcare monitoring: a review imran ahmed1, eulalia balestrieri1, francesco lamonaca2 1 university of sannio, 82100 benevento, bn, italy. 2 university of calabria, 87036 arcavacata, rende (cs), italy. section: research paper keywords: internet of things; internet of medical things; iot devices; biomedical devices; biomedical measurement systems; non-invasive medical devices citation: imran ahmed, eulalia balestrieri, francesco lamonaca, iomt-based biomedical measurement systems for healthcare monitoring: a review, acta imeko, vol. 10, no. 2, article 24, june 2021, identifier: imeko-acta-10 (2021)-02-24 section editor: ciro spataro, university of palermo, italy received march 6, 2021; in final form april 30, 2021; published june 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: imran ahmed, e-mail: iahmed@unisannio.it 1. introduction biomedical measurement systems (bms) play a key role in the detection and diagnosis of various diseases, providing new solutions for healthcare monitoring and improving bioprocesses and technology for biomedical equipment. generally, bms use measurement devices to collect vital signs, such as heart rate, pulse rate and body temperature, from the human body, and then these vital signs are processed by a processing unit. finally, the results are displayed to aid doctors in the diagnosis of various diseases. however, for old-fashioned bms (for example, nonportable and non-smart ultrasound machines), acquiring vital signs without disturbing the patient’s routine activities is challenging. moreover, these old-fashioned bms require patients to visit the hospital for their check-up, which takes a great deal of time out of their busy lives. therefore, in order to utilise bms without disturbing routine activities, these systems must be able to acquire data in different scenarios [1]. this means not only being limited to situations that require the presence of patients inside hospitals but also outside, for example, industry workers, miners and sports professionals in their working environments as well as military personnel and individuals in their home environment. hence, the use of bms for today’s lifestyle demands that devices belonging to these systems should be compact, user friendly and comfortable for the wearer, with adequate measurement accuracy even in a harsh or complex environment [1], [3]. as a result of these emerging requirements, recent research activities have been directed towards improving bms by using the internet of things (iot) [4]-[8] and by creating a new paradigm, the internet of medical things (iomt) [1], [9]. these iomt solutions are mainly based on wearable and implantable biomedical measurement devices [12] using different sensors, such as tactile [13], silicon [14], polymer [15] and opticalbased sensors [16], [17] or sensors already integrated into commonly used devices, such as smartphones [3], [18]-[23]. wearable iomt bms typically include devices such as smartwatches, armbands, glasses, smart helmets and digital hearing devices [1], [25]. today, many wearable devices are smart in the sense that they can locally process signals acquired from sensors and transmit measurement data through the network to other connected devices (on a mobile phone or through hospital abstract biomedical measurement systems (bms) have provided new solutions for healthcare monitoring and the diagnosis of various chronic diseases. with a growing demand for bms in the field of medical applications, researchers are focusing on advancing these systems, including internet of medical things (iomt)-based bms, with the aim of improving bioprocesses, healthcare systems and technologies for biomedical equipment. this paper presents an overview of recent activities towards the development of iomt-based bms for various healthcare applications. different methods and approaches used in the development of these systems are presented and discussed, taking into account some metrological aspects related to the requirement for accuracy, reliability and calibration. the presented iomtbased bms are applied to healthcare applications concerning, in particular, heart, brain and blood sugar diseases as well as internal body sound and blood pressure measurements. finally, the paper provides a discussion about the shortcomings and challenges that need to be addressed along with some possible directions for future research activities. mailto:iahmed@unisannio.it acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 175 systems) so that doctors can promptly monitor and analyse the patient’s data to make effective decisions, especially in case of emergency [5], [8]. figure 1 shows some smart wearable and implantable devices that are used for measuring different vital signs. these devices acquire measurement data and then process and send the data to local elaboration units for further processing and for the presentation of the resulting information to clinicians or patients [12]. therefore, they are considered a substratum for the development of iomt bms. in order to stimulate research for designing innovative bms, this paper presents an overview of iomt-based bms. it is an extended version of a previous contribution to technical committee 4 of the 2020 international measurement confederation (imeko tc4) conference held in palermo, italy [9], and takes into account further developments in measurement devices and available techniques used for different medical applications. a discussion concerning each example described is reported, including the advantages, working principles and technology usage. in addition, some general issues and challenges related to the metrological aspects of iomt-based bms are highlighted. the organisation of this paper is as follows. section ii explains the basic architecture of iomt-based bms, and section iii presents the five main categories of existing iomt-based bms while also introducing some important metrological issues in relation to measurement devices used in iomt. section iv discusses the metrological challenges for existing bms, and finally, the conclusions are presented in section v. 2. iot-based biomedical measurement systems (iomt) the main advantage of iomt-based bms is that these systems provide the online monitoring of a patient's health to facilitate a quick response in an emergency and to offer remote access to doctors as well as relatives and the patients themselves for monitoring targeted vital signs (blood pressure, heart rate, glucose level and so on) [26]. to this end, iomt-based bms are usually designed to offer the following features: (i) the continuous monitoring of parameters without disturbing the patient’s daily routine, (ii) an alarm triggered in an emergency, and (iii) the use of low-cost measurement devices. as a consequence, the final aims of an iomt-based bms include the following: (i) a reduction in the cost of hospitalisation, (ii) the optimisation of public health costs, (iii) an increase in the independence and quality of life of older adults and (iv) an improvement in the monitoring of hospitalised and/or critical patients. the general architecture for iomt-based systems is shown in figure 2 [27]. compared with architectures [28]-[30] that are specifically designed for particular applications, such as heart disease, blood pressure and blood sugar, the system shown in figure 2 is more general and demonstrates the common components belonging to complete iot-based bms: (i) a physical layer, (ii) a data integration layer and (iii) an application service/presentation layer. in the physical layer, iomt-based bms mostly use wearable devices to measure the vital signs (heart rate, pulse rate, body temperature, blood pressure, oxygen concentration, lung contraction volume, blood sugar level, respiration rate and so on) of the subjects being monitored. these measurement data are first stored in the storage memory and then transferred to the data integration layer (figure 2) through the internet/bluetooth or any other communication protocol. in the data integration layer, the received data are processed and then sent to the application service/presentation layer. nowadays, various types of software are available to extract useful information from the measurement data. at the application service/presentation layer, data are analysed by the doctor or experts, enabling them to take effective decisions about the disease. in the following sections, some recently developed iomt-based systems applied to the diagnosis of various diseases are discussed along with some metrology-related issues. 3. existing iomt-bms classification overview iomt bms have various medical applications, including healthcare monitoring and the diagnosis of various diseases. based on these applications, this section has classified the existing iomt bms into five main categories: (i) iomt bms for heart disease, (ii) iomt bms for examining internal body a) b) figure 1. smart biomedical measurement devices: (a) smart wearable measurement devices [24], (b) smart implantable measurement devices. figure 2. general architecture of iomt systems. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 176 sounds, (iii) iomt bms for blood pressure, (iv) iomt bms for brain diseases and (v) iomt bms for blood sugar disease. for each category, various examples are discussed, examining their advantages, working principles and technology usage as well as their reliability and accuracy. in the literature, it has been observed that researchers in the field of iomt bms are usually concerned with applying iot technologies to different medical applications without focusing on developing suitable solutions in terms of metrology and requirements. since iomt bms rely on various types of measurement to acquire information about vital signs of the human body, therefore the reliability and accuracy of these systems play a critical and essential role in their actual ability to provide correct and suitable information that can be used by doctors for their diagnoses. it is also essential that iomt-bms devices are properly calibrated to ensure that they are accurate and perform in an appropriate and timely manner. vital-sign measurements that do not have the required accuracy can result in the misdiagnosis of patients’ diseases. accurate and reliable measurements not only ensure effective treatment but also save time and costs related to misdiagnosed patients. thus, the metrological characterisation and calibration of devices are very important for validating the reliability of iomt systems, and researchers must consider these characteristics, starting at the initial design phase and continuing to the final testing phase of the iomt bms. the following subsections discuss existing examples of iomt bms and related metrological issues. 3.1. iomt bms for heart disease the early detection of heart disease is very important for saving lives, and iomt could play a vital role in achieving this goal. in the physical layer, iomt-based systems for heart-disease detection generally take numerous measurements, such as sugar concentration levels, cholesterol levels, heart rate and pulse rate as well as other vital signs, using sensors. these measurements are usually taken by iomt devices, such as smartwatches, electrocardiogram (ecg) monitors and other ecg or opticalsensor based heart-monitoring medical devices. smartwatches mostly use optical sensors to scan the blood flow near the wrist to measure these vital signs, while ecg monitors use electrodes to acquire the electrical signals moving through the heart, record the strength and the timing of these signals and then display the acquired measurements in graphical form. however, there are few limitations to measure these vital signs due to the measurement conditions (for example, a patient sweating during the ecg measurement) and the accuracy of the measuring device [31]. once the measurements are taken, they are sent to the data integration layer through the internet and may use cloud-based servers for further processing [32]. after processing, the results are analysed by doctors in the application service/presentation layer by means of a mobile app or web page. additionally, further algorithms based on artificial intelligence (ai) are now available and are integrated into the data integration layer in order to further aid doctors in the diagnostic process [28]. for example, in [29], an iomt-based detection system for the monitoring of heart-related diseases based on the deep belief network model and a higher order boltzmann machine is presented. this system uses iot devices, such as embedded sensors and a wearable watch, to measure vital signs, such as heart rate and blood pressure, and to record other physical activities. however, the authors do not provide any details on the types of sensor and wearable watch used in the collection of these data or how accurate these measurements are. after collection, the required data is transmitted to the healthcare centre to be processed using the higher order boltzmann deep belief neural network (hobdbnn), which learns the features of heart disease from previous analyses. to evaluate the diseaseprediction accuracy, the system is implemented using matlab, and the collected data is divided into two sets: 70 % of the total data is for training the network and 30 % for testing purposes. the hobdbnn performance is evaluated by different metrics, such as the sensitivity and specificity, precision and f-measure, [29] which are defined as 𝑆𝑒𝑛𝑠𝑖𝑡𝑖𝑣𝑖𝑡𝑦/𝑅𝑒𝑐𝑎𝑙𝑙 = 𝑇𝑟𝑢𝑒 𝑝𝑜𝑠𝑖𝑡𝑖𝑣𝑒 𝑇𝑟𝑢𝑒 𝑝𝑜𝑠𝑖𝑡𝑖𝑣𝑒 + 𝐹𝑎𝑙𝑠𝑒 𝑛𝑒𝑔𝑎𝑡𝑖𝑣𝑒 , (1) 𝑆𝑝𝑒𝑐𝑖𝑓𝑖𝑐𝑖𝑡𝑦 = 𝑇𝑟𝑢𝑒 𝑛𝑒𝑔𝑎𝑡𝑖𝑣𝑒 𝑇𝑟𝑢𝑒 𝑛𝑒𝑔𝑎𝑡𝑖𝑣𝑒 + 𝐹𝑎𝑙𝑠𝑒 𝑝𝑜𝑠𝑖𝑡𝑖𝑣𝑒 , (2) 𝑃𝑟𝑒𝑐𝑖𝑠𝑖𝑜𝑛 = 𝑇𝑟𝑢𝑒 𝑝𝑜𝑠𝑖𝑡𝑖𝑣𝑒 𝑇𝑟𝑢𝑒 𝑛𝑒𝑔𝑎𝑡𝑖𝑣𝑒 + 𝐹𝑎𝑙𝑠𝑒 𝑝𝑜𝑠𝑖𝑡𝑖𝑣𝑒 , (3) 𝐹 𝑚𝑒𝑎𝑠𝑢𝑟𝑒 = 2 × 𝑃𝑟𝑒𝑐𝑖𝑠𝑖𝑜𝑛 × 𝑅𝑒𝑐𝑎𝑙𝑙 𝑃𝑟𝑒𝑐𝑖𝑠𝑖𝑜𝑛 + 𝑅𝑒𝑐𝑎𝑙𝑙 , (4) where true positive is the model’s outcome that shows correct prediction of positive class, true negative refers to the correct prediction of a negative class, false positive is the model’s outcome that shows incorrect prediction of positive class and false negative is the incorrect prediction of a negative class. these hobdbnn performance metrics are then compared with other optimised classifiers, such as genetic algorithm-based trained recurrent fuzzy neural networks, swarm-optimised convolution neural networks with a support vector algorithm, particle-optimised feed-forward back-propagated neural networks and particle swarm-optimised radial-basis function networks. the results demonstrate that the performance metrics of the proposed hobdbnn have better values than those achieved using the other above-mentioned methods. the overall prediction rate of the deep network of the proposed system is reported to be about 99.03 %. similarly, an ecg-based heart-disease recognition system is presented in [33]. this system measures the heart data by using a commercially available device called the pulse sensor amped, which consists of a simple optical heart-rate sensor with amplification and noise cancellation hardware components to collect noise-free heart-pulse readings. the collected data are then transmitted wirelessly to the mobile application via an arduino microcontroller. a monitoring algorithm is implemented in a mobile application to detect any variances from the normal heart rate. this mobile application also raises the alarm whenever an emergency occurs. the system is capable of predicting heart disease by using an intelligent classifier and a machine-learning algorithm, which are pre-trained using clinical data. the authors have reported a 100 % detection rate for monitoring the algorithm and an 85 % correct-classification rate for the classifier. in [34], an iomt-based low-power cardiovascular healthcare system with cross-layer optimisation from a sensing patch to a cloud platform is presented. it uses a wearable ecg patch with custom system-on-chip technology that is integrated with a wireless connectivity to connect with mobile devices and a cloud acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 177 platform. to measure and process the ecg signals, the sensing patch needs to be placed directly on the human body. the system performance is evaluated by first checking the signal denoising and compression capability and then by evaluating the correct disease prediction rate using the mobile device and the cloud platform. to evaluate the signal denoising and compression capability of the proposed system, it is tested on an mit-bih database (an open-source dataset that provides standard investigation material for the detection of heart arrhythmia [35]). in particular, various types of noise (a baseline drift noise, power-line noise and electromyography noise) are added to the dataset signals, and, under three different scenarios, the proposed system is evaluated: (i) signal denoising, (ii) signal compression and (iii) combined signal denoising and compression. in this context, some metrics are used, such as the denoised mean square error (mse), the denoised signal-to-noise ratio (snr, db), improvement in the mse in percentage, the percentage root mean square difference (prd), the signal compression ratio (cr) and the quality score (qs), which are defined as 𝑀𝑆𝐸 = ∑[𝑓𝑐 (𝑛) − 𝑓𝑟 (𝑛)] 2 𝑁𝑖 1 𝑁𝑖⁄ , (5) 𝑆𝑁𝑅 = 10 log [ ∑ 𝑓𝑐 2(𝑛) 𝑁𝑖 1 ∑ [𝑓𝑐 (𝑛) − 𝑓𝑟 (𝑛)] 2𝑁𝑖 1 ] , (6) 𝑃𝑅𝐷 = √∑[𝑓𝑖 (𝑛) − 𝑓𝑟 (𝑛)] 2 𝑁𝑖 1 , (7) where n is the number of samples of a signal, fc(n) is the noiseless reference signal, fr(n) is the reconstructed signal after denoising, fi(n) is the input signal and ni is the total length of the input signal. 𝐶𝑅 = 𝑁𝑖 /𝑀𝛿 , (8) 𝑄𝑆 = 𝐶𝑅 𝑃𝑅𝐷⁄ , (9) where mδ is the number of resolved coefficients after compression. in the case of signal denoising, the results show that the average improvement in snr is 12.63 db and the improvement in mse is 94.47 %. for signal compression, the results show that the average cr is 7.89, the average prd is 0.61 % and the average qs is 13.06. with regard to combined signal denoising and compression, the average improvement in mse and snr is 94.47 % and 12.63 db, respectively, and the average cr, prd and qs are 9.8, 6.14 % and 1.62, respectively. in this case, compressed and denoised signals are directly generated with only one iteration of the whole system, which can improve the system efficiency at the cost of sacrificing signal performance (prd increases and qs decreases). to test the disease-prediction accuracy executed on the cloud platform and mobile device, four kinds of ecg signal are analysed to detect arrhythmias in disease [36]: (i) a normal ecg signal, (ii) a left bundle-branch block (lbbb), (iii) a right bundle-branch block (rbbb) and (iv) paced beats (pb). the five-fold cross validation [36] results for classifying these four kinds of ecg signal, using the mobile device and the cloud platform, are presented. for a normal ecg, the correct classification rate is 96 %, for lbbb it is 98 %, for rbbb it is 100 % and for pb it is 94 %. the average correct classification rate of the proposed disease-prediction system executed on the cloud platform is 97 %, which is calculated using the mean average correct detection values of all five-fold results [36]. 3.2. iomt bms for internal body sounds auscultation is a process for examining internal body sounds, such as from the heart, lungs or other organs, for medical diagnoses. typically, a stethoscope is used to examine these sounds [46], which can help to detect abnormalities that occur in the human body and provide information about various diseases [46]. however, the auscultation process has some important limitations: it requires the doctor to have good hearing acuity and expertise in order to accurately detect abnormal sounds, the improper placement of the stethoscope on the body results in the improper acquisition of sound, the patient should be in a relaxed position for correct sound acquisition and the presence of background noise can affect the process. in this regard, aibased auscultation systems have been proved to be very helpful for professionals/doctors in determining abnormal sounds [47], [48]. however, iot-based auscultation systems allow doctors to remotely monitor their patients and record the sounds, and they offer features for sharing information with other professionals to obtain an immediate second opinion [49]. in this paper, some recently developed iomt devices related to the measurement of internal body sounds are discussed. for example, ekuore pro [37] is an iot-based wireless stethoscope that acquires and measures internal body sounds through devices placed on the body. it can be connected to a mobile app using wi-fi to show phonograms in real time and keep track of the patient’s medical history, which can be easily shared with professionals and doctors. however, the manufacturer has not provided any information on the accuracy of the system. another iomt device for the measurement of internal body sounds currently available on the market, stethee, is presented in [38]. it is an ai-based wireless stethoscope that uses aida technology (an aiand machine-learning-based solution to analyse data [39]) to analyse the heartbeat and lung sounds acquired by placing the device on the body. it uses the stethee app, which displays clinical information such as heart rate, average systole, average diastole and respiratory rate in only 20 seconds, and it also allows the live streaming of sound data so that it can be visualised in real time for the easy evaluation of vital signs. although the manufacturer claims that the device has a number of powerful features, but information about measurement methods for the calculation of parameters and their accuracy is not provided. similarly, the iomt-based hd steth system [40] has been introduced by the hd medical group for cardiac auscultation. hd steth is a medical stethoscope approved by the food and drug administration (fda), and it is composed of integrated ecg electrodes, four microprocessors, an ai-enabled detection system for detecting cardiovascular disease and a display screen to visualise phonocardiographs and electrocardiographs. it provides the real-time visualisation of cardiac waveforms via the bluetooth low energy (ble) mobile app, and patients can easily share data with specialists via a cloud platform for a remote diagnosis. this device has been patented with noise cancellation and smart amplification for high-fidelity auscultation. as with the previously mentioned devices, the manufacturer has not provided any information on measurement accuracy. another iot-based smart stethoscope, stethome [41], is a ce certified and intelligent medical device able to measure and classify abnormal sounds in the respiratory system, and it can remotely analyse other internal sounds in the body. stethome is able to connect to a smartphone via bluetooth, and the patient can place acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 178 the device on the points indicated by the mobile app for the recording to start automatically. the mobile app then stores the medical history in the cloud, notifying patients if there are any abnormal sounds, and sends the examination results to a doctor, who will take effective decisions accordingly. it uses ai algorithms to verify the examination process, which makes, in the manufacturer’s opinion, the stethome about 29 % more accurate than a specialist in the detection and classification of abnormal sounds. however, the accuracy evaluation process is not provided by the manufacturer, so there is no information on how the 29 % figure has been obtained. in [42], an iomt-based wireless digital stethoscope with mobile integration for sound auscultation is presented. the system first acquires the sound signals from the human body by using a traditional stethoscope chest piece that has an integrated microcontroller unit and a bluetooth communication device. the acquired data are then processed and finally transmitted to a mobile device for recording and listening to and for the visual display of sound waveforms. this system can be used for monitoring patients in remote locations, especially in quarantine units, and can also be utilised for remotely training healthcare staff through the broadcasting of the recorded signals. however, the authors have not provided any information on the measurement accuracy of the system. in [43], an iomt-based novel cardiac auscultation monitoring system based on wireless sensing for healthcare is presented. in this system, the cardiac-sound auscultation sensing unit consists of two main components. the first is the hky-06b heart-sound sensor, manufactured by huake electronic, which converts the weak heart vibration signal into electrical signals. it also has integrated micro-sound components made from polymer materials. it has the capability of detecting all kinds of heart and acoustic sounds from the body’s surface. the second component is the data acquisition module consisting of cc2540 system-on-chip technology with an external antenna [44], an 8051-based micro-controller [45] and other auxiliary components. this module’s main function is analogue-to-digital conversion and bluetooth transmission. it uses a ble bluetooth protocol to offer power efficiency and moderate the data transmission rate. the proposed system is used to monitor cardiovascular health, and the acquired information is sent to caregivers as well as medical practitioners using the iot network and an android mobile app. in particular, pre-processing, segmentation and clustering techniques are performed to gather any significant health information. the system also features a hilbert–huang transform to reduce interference signals and help extract features of the first heart sound, s1, and the second heart sound, s2. in healthy people, s1 and s2 are produced by the closure of the atrioventricular valves and semilunar valves, respectively. the detection rate of the proposed system for s1 and s2 is 88.4 % and 82.7 %, respectively, and the overall detection rate of s1 and s2 for irregular heart sounds is 86.66 %, as reported in the article. in [46], an iomt-based smartphone auscultation system is presented. it is a low-cost stethoscope connected to a mobile phone that can record lung sounds and detect abnormal sounds from recorded data. the system uses a support vector machine to identify the sound of wheezes and crackles by extracting features from the spectrogram of each sound signal. the system is trained using recorded data consisting of lung sounds from 155 patients suffering from wheezes or crackles. the system is validated by evaluating the performance of detection algorithms, taking into account the area-under-the-curve (auc) parameters. this auc is calculated by plotting the receiver-operating-characteristic (roc) curve between the false positive rate and the true positive rate, as shown in figure 3. for the crackle detector algorithm, the auc value is 0.87, and for the wheeze algorithm, the auc value is 0.71. in [49], the iot-based smartphone monitoring of a second heart sound split is presented. the heart sounds are recorded using a customised external microphone consisting of an acoustic stethoscope and a 3.5-mm mini-plug condenser microphone with adapter that connects wirelessly to a mobile app to record the heartbeat. the system detects s1 and s2 by converting the recorded heartbeat signal into a frequency domain using the fast fourier transform. s2 is then fed to a discrete wavelet transform (dwt) and a continuous wavelet transform to extract the aortic and pulmonic components. this system can be very useful for remotely monitoring s2. however, these authors have also not provided any information on the measurement accuracy of the system. 3.3. iomt bms for blood pressure high blood pressure (bp) is a serious issue that affects older people as well as young adults; it is important for patients to control their bp with repeated check-ups, otherwise serious conditions such as heart failure or strokes can occur. therefore, patients that suffer hypertension need a bp check-up several times a day. iomt-based bp measurement systems can help to make this task easier for the patients. an automatic bp measurement system using the oscillometric technique is presented in [50]. this system is capable of monitoring both systolic and diastolic pressure, which are used to define arterial bp. the values are continuously updated through wi-fi on a database that can be accessed remotely, where these data are compared with already existing data to improve the accuracy of the results. the authors have stated the accuracy of the system to be 7 mmhg. this accuracy has been calculated by means of standards or protocols (defined for bp measuring devices) that are based on the general consensus of several organisations, such as the us association for the advancement of medical figure 3. receiver operating characteristic (roc) curves for crackles and wheezes [46]. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 179 instrumentation (aami), the british hypertension society and the european society of hypertension (esh), which are active working groups on bp monitoring, as well as the international organization for standardization (iso) [51]. however, these protocols and the accuracy of the oscillometric bp instrument continue to be the subject of discussion in the scientific field. this is due, for example, to the bp oscillometric device’s tendency to provide inaccurate measurements for certain patient groups and to be prone to noise and artifacts. moreover, the difficulty in reproducibility of the adopted calibrating methods [52]-[54] allows monitors to pass validation tests when there are clinically significant differences in bp estimated values in some individuals [52]. in [55], the qardio arm system is used to develop a smart bp measurement system, in which the acquired oscillometric data are transferred to a smartphone app for analysis and visualisation. the accuracy of the system has been evaluated by comparing its results with the omron m3 device, as it has been clinically validated according to the existing esh international protocol. however, the same concerns about the accuracy and calibration of the oscillometric bp measurement expressed for the previous device exist for the qardio arm system. in [56], the omron heartguide is introduced. it is an fdaapproved iomt smartwatch for bp measurement. this device can measure bp by using an inflatable cuff within the smartwatch bracelet. the smartwatch sends the data to the data integration layer via the internet, and then sends it on to the application service/presentation layer, where it is available for the doctor to access it in real time. the measurement accuracy of the device is about 3 mmhg, but the validation of this device has also been carried out under the protocols for bp devices, with the same limitations reported above. similarly, the iomt-based instant blood pressure (ibp) auralife app is presented in [57] for bp measurement using a mobile phone and without the use of any external hardware. ibp auralife extracts the bp values from the photoplethysmogram (ppg) signal acquired with a flash led light and a mobile camera. the accuracy of the system is evaluated by using the aami/iso 81060-2:2013 protocol to compare ibp with other reference upper-arm cuffless bp monitors and oscillometric blood pressure cuff devices. a result analysis shows that the device’s systolic bp mean accuracy is 2.7 mmhg and diastolic bp mean accuracy is 2.6 mmhg. however, the system only delivers results at this level of accuracy for individuals whose bp lies in a specific range; therefore, it is not suitable for use by individuals whose bp falls outside of the systolic range of 83–178 mmhg or diastolic range of 58–107 mmhg. the manufacturer states that it is not recommended for medical use and not a substitute for cuff-based or other bp monitors because it provides an estimate of bp only. another device, the asus vivowatch bp, is reported in [58]. this device has an ecg sensor on the back that receives an ecg signal from the wrist and an optical sensor on the front for the measurement of ppg signals from the index finger. this data is then automatically processed, and the results are displayed to the user. however, the manufacturer has not provided any information on the accuracy of the device. 3.4. iomt bms for brain diseases many people are affected by brain diseases, such as brain tumours, dementia, headaches, brain strokes, chronic pains in the head, tourette’s syndrome, alzheimer’s, parkinson’s and epilepsy. the development of iomt-based bms in the field of brain-related diseases is a promising solution for the monitoring of patients and timely detection of such diseases. typical devices that are used in iomt-based bms for brain-related diseases are electroencephalogram (eeg) electrodes, smartwatches, galvanic skin response sensors and cameras. these devices are used to monitor brain-disease patients. for example, by using eeg electrodes, it is possible to measure eeg signals to monitor brain activities, with galvanic skin response sensors, the changes in sweat glands can be measured to monitor stress, and by using cameras, it is possible to monitor the daily physical activities of patient (e.g. neuro-degenerative disease patients). in this context, some iomt brain-related bms are used to monitor and measure vital signs (for example stress levels and eeg signals) and can generate an alarm in the case of a crisis. in [59], an iomt smart sensor for stress-level detection is presented. this is a novel stress-detection system called istress. it monitors stress levels by measuring parameters such as the rate of body motion, body temperature and levels of sweat during physical activity using sensors for temperature and humidity and also a three-axis accelerometer. the collected data are then processed using a neural network based on a mamdani-type fuzzy logic controller, which is based on a fuzzy technique. the proposed system is very efficient in terms of power consumption and allows the real-time remote monitoring of stress levels by transmitting the collected data to the cloud, thus helping to improve the detection of the patient’s health status. the system classifies stress into three levels: low, normal and medium. the outputs of the sensors are fed to a fuzzy logic controller designed in matlab to detect stress levels. the authors report that this system has a stress detection rate of 97 %. in [60], a high-definition camera is used to analyse the motion of the patients with neuro-degenerative diseases (nd). the system is based on remote video monitoring that measures the quantity and quality of two clinically relevant motor symptoms (impairment in step length and arm-swing angle). the system has been evaluated by mean absolute error (mae), which gives an indication of how close the measurements are to the ground truth. for this evaluation test, a video of a healthy individual in a walking position is recorded in two scenarios: (i) just walking and (ii) sitting on a chair, followed by standing up and walking. the camera is setup to capture the lateral view for the correct detection of the required nd parameters. a total of 23 valid step lengths and 10 arm-swing angles are recorded in both cases. the ground-truth measurements are marked/annotated using the kinovea software package, which provides a set of tools to capture, slow down, study, compare, annotate and measure the technical performance of a video [61]. the authors state that the system is able to measure nd parameters with a tolerance ranging from 2 % to 5 %. in [62], an iot-based system is reported that predicts the parkinson’s brain disorder. the system uses wearable iot-based deep brain simulation (dbs) to collect patient brain activities and assess the condition of cells to predict brain functionality changes. dbs is a smart device that collects brain data by placing electrodes on individuals to conduct continuous monitoring. by means of an heuristic tubu-optimised sequence modular neural network (htsmnn), it is possible to predict the changes present in the human brain and its functions. to validate the proposed system, the authors have used the dataset in [63] that contains parkinson’s disease-related information. the performance of the system is analysed using mae, mse, precision, recall, classification accuracy (ca) and the auc, which are defined as acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 180 𝑀𝐴𝐸 = 1 𝑁 ∑ 𝑦𝑖 − ỹ𝑖 𝑁𝑖 1 , (10) 𝑀𝑆𝐸 = 1 𝑁 ∑(𝑦𝑖 − �̃�𝑖 ) 2 , 𝑁𝑖 1 (11) 𝐶𝐴 = 𝑇𝑃 + 𝑇𝑁 𝑇𝑃 + 𝑇𝑁 + 𝐹𝑃 + 𝐹𝑁 , (12) where n is the number of parkinson’s features and y is the actual and �̃� the predicted output; tp is the true positive, tn the true negative, fn the false negative and fp the false positive, while precision and recall have already been defined in equations (2) and (4). the above system parameters are calculated by first calculating the system deviation to identify the errors present in the parkinson’s disease recognition process. the deviation is computed by considering the difference between the actual and predicted output. the htsmnn system deviation value is compared with other methods, such as particle-swarm-optimised neural networks, particle-swarm-optimised radial basis neural networks, genetic algorithm-based extreme machine-learning networks and tubu-optimised deep neural networks. after computing the above parameters, it is reported by the authors that the system ensures an mae equal to 0.284, an mse equal to 0.273 and a ca equal to 98.07 %. in [64], an iomt-based bms is presented using the deep learning approach, called stress-lysis, which is used to measure stress levels. the deep learning system is developed and tested with three different datasets containing activities of daily living (adl) and physical activities. adl are collected with an accelerometer and humidity and temperature sensors worn on the wrist. a physical-activity monitoring dataset is developed by collecting 18 different activities from nine volunteers wearing three inertial measurement sensors and a heart-rate monitor. the system learns the stress-level parameters obtained by the wrist band, such as skin temperature, heart rate and sweat during physical activity. the authors have conducted the validation of this proposed solution by analysing the collected dataset using python and tensorflow. the dataset consists of 2,000 samples, of which 1,334 samples are for training purposes and 667 are for testing. the results are displayed in the form of a loss function that demonstrates that the correct classification rate is in the range of 98.3 % to 99.7 %. in [9], a seizure-detection iomt system is demonstrated by using a dwt, hjorth parameters and a k-nn classifier. this system is based on an iomt device called neuro-thing, which is capable of accurately detecting seizure-related diseases. in this method, eeg electrodes are used to acquire eeg signals, which contain information on the physiological state of the brain to understand and monitor brain function. these eeg signals are decomposed in sub-bands using dwt, and then hjorth parameters are extracted from the decomposed signals, which are classified using the k-nn method. the device is capable of sending the information to the iot cloud, where it can be accessed by doctors/physicians so that they can take effective decisions about the disease. in order to validate the proposed method, the authors perform system-level simulations, implemented in a simulink environment, where dwt and the k-nn classifier are developed by user-defined functions. iomt implementation is done using thingspeak, which is an open data platform that enables iot applications to gather and analyse data in the cloud. in addition, the system is validated by experimental results in which open-source eeg data [11] is utilised to validate the classification capability of the k-nn model by calculating sensitivity and specificity, as defined in equations (1) and (2). the reported results show 100% classification accuracy for normal vs. ictal eeg and 97.9% for normal and interictal vs. ictal eeg. 3.5. iomt bms for blood sugar disease diabetic or blood sugar disease occurs when the human body is not able to properly process blood sugar [65]. generally, blood sugar is measured by determining the concentration of glucose in the blood. most devices are based on electrochemical technology, which uses electrochemical strips to perform measurements. there are some limitations to obtaining accurate measurements due to the variance in strip manufacturing and the use of old or out-of-date strips. other limitations arise from environmental factors, such as temperature or altitude (in hilly areas), or from patient factors, such as improper handwashing [66]. patients suffering from this disease usually require their blood glucose levels to be checked regularly and to manage their diet to keep the effects of this disease under control. recent research focuses on using iomt to improve the sharing of measurement data with physicians and then giving prompt feedback to patients. an iomt system with a novel framework to measure and monitor glucose levels is presented in [65]. the system is used for remotely powered implantable glucose monitoring, in which the signal, retrieved from the interaction of radio frequency signals with biological tissue, is first characterised and then monitored. a low-power bluetooth protocol is used for the transmission of measurement data to the user’s mobile. however, the authors do not discuss the accuracy of the proposed system. in [67], an iomt-based system for glucose monitoring is presented. the article presents a non-invasive blood glucose measurement system based on optical detection and an optimised regression model. a system for light absorbance at a wavelength of 940 nm with a prediction model is designed, and the technique is validated through measurements taken from human fingertips. the evaluation of the method is performed by comparing the achieved results with referenced blood glucose concentrations using an sd-check one-touch glucometer. the results are evaluated by calculating the mean absolute difference, which is found to be about 5.82 mg/dl, while the mean absolute relative difference (mard) is 5.20 %, the average error (avge) is 5.14 % and rmse is 7.50 mg/dl. the test samples of 43 healthy people and diabetic patients are collected for a clarke error grid analysis, which is used to quantify the clinical accuracy of blood glucose devices with reference values [68]. the overall results show that better measurements have been achieved using the proposed approach than using the non-invasive measurement methods presented in [69]-[71]. in [72], iglu 2.0 is presented. this is a new iomt-based wearable device that is used for measuring blood glucose levels. the device uses infrared spectroscopy and iomt paradigms for the remote access of data by doctors/users. analysis of the optimised regression model is performed, and the system is validated on healthy, pre-diabetic and diabetic patients. the robust regression models of serum glucose levels are deployed as the mechanism for measurement for this proposed solution. in particular, a total of 50 different samples of capillary glucose and acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 181 37 samples of serum glucose are taken from pre-diabetic, diabetic and healthy people for the testing and analysis of the proposed iglu 2.0 device. the obtained results are then compared with the reference values for serum glucose obtained from a laboratory. in particular, the reference values for capillary glucose are measured using an invasive glucometer sd check, which is the gold standard for validation purposes. in terms of capillary blood glucose, avge is found to be about 6.07 % and mard is 6.08 %, whereas for serum glucose, avge is 4.88 % and mard is 4.86 %. in [73], a reliable iot-based embedded healthcare system that uses the alaris 8100 infusion pump, keil lpc-1768 board and iot cloud to monitor diabetic patients is proposed. the infusion pump delivers medical liquid (insulin) to the patient on a timely basis, and the keil lpc-1768 board is responsible for delivering the control commands and daily patient readings and providing a secure connection layer. the system is capable of storing patient records in the cloud, and a secure hash algorithm and secure socket shell are employed in this system to achieve the reliability components of the proposed scheme. the article reports that the system has a 99.3 % probability of continuing to operate normally. also the authors claim that the proposed system design is reliable, secure and authentic in relation to security and privacy. however, the metrological aspects of the proposed system are not discussed. 4. metrological iomt-bms challenges there are several metrological issues related to iot-based bms that should be addressed when considering their large-scale implementation in the healthcare sector. as reported in the previous sections, several devices have been proposed by researchers in this field, but the majority of researchers do not focus on the device’s metrological characterisation, and they provide questionable validation, calibration and/or accuracy parameters. the treatment and monitoring of diseases are based on measurements provided by the devices used in iomt bms. if iomt bms are not investigated from a metrological perspective, there can be no certainty about their capacity for providing reliable, accurate, precise and repeatable measurements of vital signs, with the serious risk of delivering incorrect information, leading to the misdiagnosis or incorrect treatment of diseases [73]. therefore, it is important to consider all aspects relating to the measurement accuracy of the devices (measurement nodes) used in the system. in addition, it is essential that the measurement devices used in iomt bms are properly calibrated [75] using suitable reproducible procedures and references. there should be appropriate guidelines for the common user on the calibration process of the iomt device or properly accredited laboratories that provide these services, either onsite or remotely. in practice, calibrations are usually made by switching the device on and off or zeroing or resetting the device, which is not recommended [73]. due to the presence of various measurement parameters (i.e. pressure flow, temperature, sound pressure), it is difficult to calibrate these devices. moreover, one of the major challenges is that there is no general consensus among different laboratories (from different disciplines, such as pressure flow, temperature, sound pressure) on how to regulate calibration traceability on a single platform for the different kinds of measurement system [76]. 5. conclusion iomt bms play an important role in the diagnosis of diseases, such as abnormal blood pressure, heart attacks, brain tumours, alzheimer’s, parkinson’s and epilepsy as well as in healthcare monitoring, the monitoring of disease progression and biomedical research. the rapid growth of and increasing demand for iomt bms that suit the modern lifestyle make it essential for these systems to be accurate, fast, user friendly and comfortable for the wearer and to provide 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field of medical devices, int. j. metrol. qual. eng. 2 (2011) pp. 135-140. doi: 10.1051/ijmqe/2011101 https://doi.org/10.1109/tce.2020.3011966 http://www.iariajournals.org/internet_technology/inttech_v12_n12_2019_paged.pdf http://www.iariajournals.org/internet_technology/inttech_v12_n12_2019_paged.pdf https://doi.org/10.1109/mim.2020.9126071 https://doi.org/10.1109/tim.2010.2050978 https://doi.org/10.1051/ijmqe/2011101 diagnosis of rotating machine defects by vibration analysis acta imeko issn: 2221-870x march 2023, volume 12, number 1, 1 6 acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 1 diagnosis of rotating machine defects by vibration analysis karim bouaouiche1, yamina menasria1, dalila khalfa1 1 electromechanical engineering laboratory, badji mokhtar university annaba b.p.12 23000, algeria section: research paper keywords: vibration signal; kurtosis; fmd; hilbert transform; filtering citation: karim bouaouiche,yamina menasria, dalila khalfa, diagnosis of rotating machine defects by vibration analysis, acta imeko, vol. 12, no. 1, article 26, march 2023, identifier: imeko-acta-12 (2023)-01-26 section editor: francesco lamonaca, university of calabria, italy received december 27, 2022; in final form march 16, 2023; published march 2023 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was supported by electromechanical engineering laboratory funding, algeria. corresponding author: karim bouaouiche, e-mail: karimbouaouiche@gmail.com 1. introduction rotating machines such as engine and gearbox produce a vibration which characterized by a set of parameters such as amplitude, velocity and acceleration [1]. the measurement of the parameters depends on the frequency of the vibration such as displacement sensor used at low frequency and velocity sensor at medium frequency and accelerometer at high frequency, vibration monitoring is a technique more used in maintenance, which allows the detection of defects [1]. the method of machine monitoring divided into two types: the first is the methods of fault detection based on the creation of an analytical model to correlate the signature and detect the damage parameters such as hidden markov model (hmm), artificial neural network (ann) [1]. the second method is the processing and extraction of the characteristics of the vibration signal in the time domain, frequency domain, and time-frequency domain [1]. data acquisition is an important step in the diagnosis of machine faults and the information collected divided into two types: data from events dependent on installation, repair, but monitoring data collected by measurements such as vibration data, the next step after the acquisition is the analysis of the data through models, algorithms, tools, which ensures a better interpretation [2]. the algorithms of decomposition of a signal are the most used in data analysis, such as emd, eemd, ceemd, empirical mode decomposition (emd) created by huang in 1998 but this method presents the problem of mixing modes [3]. through its emd improved by wu in 2009 to ensemble empirical mode decomposition (eemd) and in 2011 improved by torres to complete ensemble empirical mode decomposition (ceemd) [3]. the last decomposition method created by yonghao miao in 2022 is called feature mode decomposition (fmd) [4]. in this work, we propose an approach to analyse the real signals of the ball bearing 6025-skf, the experimental signals available on the platform (cwru) in the form of matlab files, see figure 1. 2. method in this study, we propose an approach that allows the processing of vibratory signals of a machine in an efficient way and gives a result that directly indicates the kinematic frequency figure 1. parts of the proposed approach. abstract this work presents the analysis of the vibration signals of a ball bearing, the signals available on the platform case western reserve university in the form of matlab files. by proposing an approach for the diagnosis of any rotating machine and detects the failed components in a simple way, the proposed approach consists of several methods and steps each complementary to the other. first on decomposing the signal by the feature mode decomposition (fmd) method and then, according to the kurtosis values by selecting the useful signals after the selection by computing the sum of the useful signals called the residual of the original signal. the simplicity of the form of the residual makes through the application of band pass filtering and hilbert transform, but the analysis of the peaks represents the frequency of the failed components. acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 2 of the failed components. the approach consists of two parts, the first is the experimental part of signal acquisition by a measurement chain and the second part is the processing of the vibration signal by a matlab code compatible with a proposed approach that provides the frequencies of defects: figure 2 illustrates the proposed approach that consists of several steps each complementary to the other: step1: after insertion of the signal in fact the decomposition by fmd through the use of a finite impulse response (fir) filter bank the signal decomposes into different modes, the algorithm of fmd consists of the following steps [4]: enter the number of modes 𝑛 and the length of the filter 𝐿. initialization of filter banks by a hanning window using 𝐾 filter. obtain the modes (filtered signals) by the following equation: 𝑢𝑘 𝑖 = 𝑥 ∗ 𝑓𝑘 𝑖 (1) 𝑢𝑘 𝑖 : modes, 𝑥: signal, 𝑓𝑘 𝑖 : filter coefficient, 𝑖 : iteration, ∗ : convolution. updating the filter coefficients by using the signal 𝑥 and the modes 𝑢 and the estimated period 𝑇, which chosen as the point where the autocorrelation spectrum reaches the maximum value 𝑅𝑘 𝑖 after the point zero crossing. judges the number of iterations 𝑖 if it reaches the pre-interaction number by going to the next step. calculation of correlation coefficient 𝐶𝐶, both modes build a matrix 𝐶𝐶𝐾×𝐾 , then calculating the correlated kurtosis 𝐶𝐾 using the estimated period. judges if the number of modes 𝑛 reaches the specified number. obtain the final modes. figure 3 shows the flowchart of the fmd method [4]: the input parameters to the fmd algorithm are the number of modes 𝑛, the length of the filter 𝐿 and the number of frequency band segments 𝐾: the filter length 𝐿 has an influence on the filtering performance, the fmd performance test consists of the variation of the correlated kurtosis 𝐶𝐾 values of the filtered signals as a function of the filter length 𝐿, and the test result defines the optimal fmd filter length range, 𝐿 ∈ [30; 100], [4]. the parameter 𝐾 must be greater than 𝑛 to ensure decomposition, the fmd performance test which consists of the variation of 𝐶𝐾 values as a function of 𝐾 defines the optimal parameter interval 𝐾 ∈ [5; 10], [4]. however, we in your study propose criteria to define the values of the parameters of fmd: the number equals half the number of modes obtained by emd: 𝑛(fmd) = 𝑛(emd) 2 . (2) the parameter 𝐾 is half of the optimal interval: 𝐾 = 5 + 10 2 ≅ 8 . (3) the number (𝐿) defined according to the variation of cross correlation (𝐶) between the modes obtained by fmd and the original signal, the case that represents a maximum total value (𝑇𝐶) of cross correlation (𝐶) chosen. figure 2. the signal processing approach. figure 3. fmd algorithm. acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 3 the cross correlation function applied to measure the similarity between two functions and calculated by the following formula [5]: 𝐶(𝜏) = ∫ 𝑥(𝑡) 𝐼𝑀𝐹𝑖 (𝑡 − 𝜏) d𝑡 +∞ −∞ (4) 𝑖 = 1, 2, … , 𝑛(fmd): the number of modes, 𝐼𝑀𝐹: the modes, 𝜏: time shift parameter. 𝑇𝐶 = ∑ 𝐶𝑖 (𝜏) 𝑛(fmd) 𝑖=1 . (5) step2: the kurtosis is a moment of order 4 and used to identify the non-periodic impacts and operation of a machine, the value there is a threshold in the detection of faults when the kurtosis greater than three the signal is impulsive [6]. as the case of bearing in good condition, the kurtosis is less than three and the distribution of the gaussian signal [7]. 𝑘𝑢 = 1 𝑁 ∑ ( 𝑥𝑖 − �̅� 𝜎 ) 4𝑁 𝑖=1 , (6) �̅�: the mean, 𝜎: standard deviation. step3 : the sum of the imf that illustrates a kurtosis value greater than three called the signal residual 𝑥res [8], after the determination of residual on calculating the kurtogram to define the two pass frequencies of the digital band pass filter, the limits of the interval that illustrate a high value of spectral kurtosis 𝐾(𝑓) is the pass frequencies. the kurtogram represents the variation of spectral kurtosis values as a function of frequency 𝑓 and width ∆𝑓, [9]. 𝐾(𝑓) is calculated by the following formula, [9]: 𝐾(𝑓) = < 𝐻4 (𝑡, 𝑓) > < 𝐻2(𝑡, 𝑓) >2 − 2 (7) 𝐻(𝑡, 𝑓): complex envelope of the signal at frequency 𝑓, estimated by the short-term fourier transform [9]. step 4: calculation of the envelope spectrum of the filtered signal 𝑥resf by the hilbert 𝐻[𝑥resf(𝑡)] and fourier transform, the absolute value 𝐸(𝑡) of the analytical signal 𝑧(𝑡) is the envelope of 𝑥resf and 𝐸(𝑓) is the spectrum, 𝐸(𝑡) and 𝐸(𝑓) calculated by the following equations [10]: 𝐻[𝑥resf(𝑡)] = 𝑥resf(𝑡) ∗ 1 π 𝑡 (8) ∗: convolution product. 𝑧(𝑡) = 𝑥resf + 𝑗 𝐻[𝑥resf(𝑡)] (9) 𝑧(𝑡) = 𝐸(𝑡) 𝑒𝑗 𝜑(𝑡) (10) 𝑥resf(𝑡) = 𝑅{𝑧(𝑡)} = 𝐸(𝑡) cos (𝜑(𝑡)) (11) 𝐸(𝑡) = |𝑥resf(𝑡) + 𝑗 𝐻[𝑥resf(𝑡)]| (12) 𝐸(𝑓) = ∫ 𝐸(𝑡)𝑒 −𝑗 2 π 𝑓 𝑡 d𝑡 +∞ −∞ . (13) step 5: the peak is the maximum amplitude value of a signal found by the following formula [11]: 𝑃𝑒𝑎𝑘 = max|𝐸(𝑓)| (14) the relationship between the peak frequency and the kinematic frequency of the components of a machine is the result of the diagnosis that ensures the detection of the faulty component. 3. vibration signals in this section, analysing the real vibratory signal of ball bearing type 6025-skf that supports the shaft of an electric motor at the drive end and subjected to different speeds and loads, among the signals available in the platform cwru taking two signals of defects, the characteristics of each signal represented in table 1, [12]. fault frequencies of bearing components are the multiple of the rotation frequency in hz with a coefficient as shown in table 2 [12]. 4. results and discussions 4.1. case 1 by applying the proposed approach on the inner race fault signal (107.mat), starting with the estimation of the parameters of the fmd algorithm: the number of modes obtained by applying the emd algorithm on the signal 107.mat equals 13 so 𝑛fmd ≅ 6. the variation of 𝑇𝐶 as a function of 𝐿 is represented in figure 4. the maximum value of cross-correlation 𝑇𝐶 = 893.8 has 𝐿 = 32, the step 𝑃 of variation of the interval 𝐿 equals two, 𝑃 = 2. the decomposition of the vibration signal (107.mat) by the fmd method gives six modes represented in figure 5, and the spectrum of each mode illustrated in figure 6. the kurtosis values for each mode shown in the table 3. the original signal residual is the sum of the modes (2, 3, … , 6): 𝑥res(𝑡) = ∑ 𝐼𝑀𝐹𝑖 (𝑡) 6 𝑖=2 . (15) the spectrum and the temporal signal of 𝑥res(𝑡) are represented in figure 7. table 1. vibration signals. signal defect diameter sampling frequency inner race (107.mat) load (1491.4 n m/s) speed (1750 rpm) 0.1778 mm 12 khz inner race (211.mat) load (1491.4 n m/s) speed (1750 rpm) 0.5334 mm 12 khz table 2. defect frequencies. components coefficients frequency (hz) inner race 5.4152 157.94 outer race 3.5848 104.55 cage 0.39828 11.61 rolling element 4.7135 137.47 acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 4 the residual spectrum is complex and rich with information as shown in figure 7, the interpretation of 𝑥res(𝑓) is difficult for this reason, in fact the pass band filtering through the buttworth filter, the two pass frequencies of the filter defined from kurtogram of 𝑥res(𝑡). the kurtogram in figure 8 shows a maximum value of spectral kurtosis 𝐾max = 0.5 at the centered frequency 𝑓c = 3750 hz with a width 𝐵𝑤 = 500 hz at the level 𝑙 = 3.5 so the two passing frequencies 𝐹𝑝 = [3250 ; 4250] (hz). the spectrum of the envelope 𝐸(𝑓) of the filtered signal 𝑥resf(𝑡) in figure 9, illustrates a peak max|𝐸(𝑓)| = 0.661 at frequency 𝑓 = 157.5 hz, the value of 𝑓 closer to the fault frequency of the inner race (𝐹inner race = 𝑓). 4.2. case 2 in this case by analysing the vibration signal (211.mat) by the proposed approach, but summarizes the different steps: figure 4. the variation of tc. figure 5. the temporal variation of the modes. figure 6. the spectrum of the modes. figure 7. the residual. figure 8. kurtogram of 𝑥𝑟𝑒𝑠 (𝑡). table 3. kurtosis values. imf kurtosis 1 2.1 2 5.8 3 6.3 4 8.8 5 5.7 6 5 acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 5 the emd decomposition of the signal yields 14 modes, the maximum cross-correlation value 𝑇𝐶 = 329.55 at 𝐿 = 30 as shown in figure 10, the estimation of fmd parameters according to the proposed criteria is: 𝑛fmd = 7; 𝐿 = 30; 𝐾 = 8. the residual is equal to the sum of all modes (1, 2, … ,7) as shown in the table 4. we observe on the spectrum 𝑥res(𝑓) (figure 11) a very high amplitude peak at the inner race fault frequency. 𝑥res(𝑡) = ∑ 𝐼𝑀𝐹𝑖 (𝑡) 7 𝑖=1 . (16) from the kurtogram of 𝑥res(𝑡) find the two frequencies of passage 𝐹𝑝 = [5375 ; 5875] hz. on the envelope spectrum 𝐸(𝑓) (figure 12), a high amplitude peak is found in the rotation frequency (29 hz) and the second peak at the inner race fault frequency (157.5 hz). 5. conclusion in this paper, we analyse two fault vibration signals of the inner ring of the ball bearing by a new proposed approach, which consists of a set of signal processing methods organized in the form of steps. from the final result of the proposed approach, we conclude that the new criteria formulated to estimate the input parameter of the fmd algorithm show a good efficiency since the number of modes and the length of the filter are two essential parameters to ensure the signal decomposition by fmd. as well as the figure 9. 𝐸(𝑡) and 𝐸(𝑓). figure 10. 𝑇𝐶 as a function of 𝐿 figure 11. 𝑥res(𝑡) and 𝑥res(𝑓). figure 12. 𝐸(𝑡) and 𝐸(𝑓). table 4. the values of kurtosis. imf kurtosis 1 3.5 2 9.7 3 10 4 10.6 5 10.2 6 10.2 7 16 acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 6 parameters guarantee the decrease of the equality constraint between the original signal and the set of modes obtained after the decomposition. the selection of useful modes through the use of parameters such as kurtosis is a very important step to extract the information depends on defects. the envelope analysis is the last step of the proposed approach that represents the peaks of high amplitudes at the default frequency, envelope analysis is a combination of band pass filtering and hilbert transform and fourier transform, in your study the two pass frequencies of the filter defined by the maximum values of spectral kurtosis in the kurtogram. references [1] deepam goyal, b. s. pabla, condition based maintenance of machine tools—a review, cirp journal of manufacturing science and technology 10 (2015), pp. 24-35. doi: 10.1016/j.cirpj.2015.05.004 [2] a. k. s. jardine, daming lin, d. banjevic, a review on machinery diagnostics and prognostics implementing condition-based maintenance, mechanical systems and signal processing 20.7 (2006), pp. 1483-1510. doi: 10.1016/j.ymssp.2005.09.012 [3] fereshteh yousefi rizi, a review of notable studies on using empirical mode decomposition for biomedical signal and image processing." signal processing and renewable energy 3.4 (2019): 89-113. online [accessed 16 march 2023] https://spre.stb.iau.ir/article_669673.html [4] y. miao, b. zhang, c. li, j. lin, d. zhang, feature mode decomposition: new decomposition theory for rotating machinery fault diagnosis, ieee transactions on industrial electronics 70.2 (2022), pp. 1949-1960. doi: 10.1109/tie.2022.3156156 [5] yang liu, jigou liu, r. kennel, frequency measurement method of signals with low signal-to-noise-ratio using cross-correlation, machines 9.6 (2021): 123. doi: 10.3390/machines9060123 [6] shaopeng liu, shumin hou, kongde he, weihua yang, lkurtosis and its application for fault detection of rolling element bearings, measurement 116 (2018), pp. 523-532. doi: 10.1016/j.measurement.2017.11.049 [7] a. j. oyobé okassa, c. welba, j. p. ngantcha, extraction of vibration signal parameters by dwt for a new approach to using kurtosis, 2021. online [accessed 16 march 2023] https://www.researchgate.net/publication/353546792 [8] hafida mahgoun, bekka rais elhadi, ahmed felkaoui, gearbox fault diagnosis using ensemble empirical mode decomposition (eemd) and residual signal, mechanics & industry 13.1 (2012), pp. 33-44. doi: 10.1051/meca/2011150 [9] j. antoni, fast computation of the kurtogram for the detection of transient faults, mechanical systems and signal processing 21.1 (2007), pp. 108-124. doi: 10.1016/j.ymssp.2005.12.002 [10] d. wang, q. miao, x. fan, h.-zh. huang, rolling element bearing fault detection using an improved combination of hilbert and wavelet transforms, journal of mechanical science and technology 23.12 (2009), pp. 3292-3301. doi: 10.1007/s12206-009-0807-4 [11] mohamad hazwan mohd ghazali, wan rahiman, vibration analysis for machine monitoring and diagnosis: a systematic review, shock and vibration 2021, article id 9469318, 25 pp. doi: 10.1155/2021/9469318 [12] case western reserve university, bearing database. online [accessed 16 march 2023] https://engineering.case.edu/bearingdatacenter https://doi.org/10.1016/j.cirpj.2015.05.004 https://doi.org/10.1016/j.ymssp.2005.09.012 https://spre.stb.iau.ir/article_669673.html https://doi.org/10.1109/tie.2022.3156156 https://doi.org/10.3390/machines9060123 https://doi.org/10.1016/j.measurement.2017.11.049 https://www.researchgate.net/publication/353546792 https://doi.org/10.1051/meca/2011150 https://doi.org/10.1016/j.ymssp.2005.12.002 https://doi.org/10.1007/s12206-009-0807-4 https://doi.org/10.1155/2021/9469318 metrological characterization of instruments for body impedance analysis acta imeko issn: 2221-870x september 2022, volume 11, number 3, 1 7 acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 1 metrological characterization of instruments for body impedance analysis valerio marcotuli1, matteo zago2, alex p. moorhead1, marco vespasiani3, giacomo vespasiani3, marco tarabini1 1 department of mechanical engineering, politecnico di milano, via privata giuseppe la masa 1, 20156 milan, italy 2 faculty of exercise and sports science, università degli studi di milano, via festa del perdono 7 20122 milan, italy 3 technical department, metadieta s.r.l., via antonio bosio, 2, 00161 rome, italy section: research paper keywords: bioimpedance; body composition; measurement uncertainty; calibration; multivariate linear regression citation: valerio marcotuli, matteo zago, alex p. moorhead, marco vespasiani, giacomo vespasiani, marco tarabini, metrological characterization of instruments for body impedance analysis, acta imeko, vol. 11, no. 3, article 14, september 2022, identifier: imeko-acta-11 (2022)-03-14 section editor: francesco lamonaca, university of calabria, italy received october 7, 2021; in final form august 31, 2022; published september 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: valerio marcotuli, e-mail: valerio.marcotuli@polimi.it 1. introduction body composition describes the main components of the human body in terms of free fat mass (ffm), fat mass (fm) or their ratio ffm/fm. the analysis of body composition is used in different fields such as biology and medicine to estimate the nutritional status, muscular volume variations and potentially event pathological status. for example, physiological aging leads to a reduction of ffm and muscular mass, while fat increases and is redistributed over the body areas [1]. different levels of body composition, atomic, molecular cellular, tissular and global, can be analyzed depending on the measurement methods [2]. body mass index (bmi) is a generic indicator of the body composition, but it tends to give inaccurate information when subjects are highly overweight or obese; in fact, it is possible that malnutrition exists yet is masked by the high amount of fat mass [3]. a solution for measuring body composition is represented by the dual-energy x-ray absorptiometry (dxa). this is an imaging technique, similar to magnetic resonance imaging (mri), which scans the patient with two beams of x-rays with different energy (usually 40 and 70 kv). in recent years, dxa has become recognized as the “gold standard” for measuring body composition [4]. it evaluates both the global and the regional distribution of the three main body components: bone mineral content (bmc), fm and ffm. the accuracy of dxa makes it very effective in studying patient composition within specific body regions and evaluating their effect on the patient health [5]. unfortunately, a dxa machine is expensive ($20,000+) making it typically available only at big infrastructure such as clinics and hospitals. an alternative technique is the bioelectrical impedance analysis (bia): this employs a low alternate current (ac) with high frequency at 50 khz transmitted across the body to estimate its composition based on the hydration level of tissues [6]. bia allows for quick examinations, and it is much less expensive than dxa. additionally, bia is less dangerous than dxa as it does abstract body impedance analysis (bia) is used to evaluate the human body composition by measuring the resistance and reactance of human tissues with a high-frequency, low-intensity electric current. nonetheless, the estimation of the body composition is influenced by many factors: body status, environmental conditions, instrumentation, and measurement procedure. this work studies the effect of the connection cables, conductive electrodes, adhesive gel, and bia device characteristics on the measurement uncertainty. tests were initially performed on electric circuits with passive elements and on a jelly phantom simulating the body characteristics. results showed that the cables mainly contribute to increase the error on the resistance measurement, while the electrodes and the adhesive introduce a negligible disturbance on the measurement chain. this paper also proposes a calibration procedure based on a multivariate linear regression to compensate for the systematic error effect of bia devices. mailto:valerio.marcotuli@polimi.it acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 2 not use x-rays, meaning it can be also repeated several times with no contraindications. nevertheless, bia can be highly affected by many factors such as altered hydration of the subject, measurement conditions, ethnic background, and health conditions [7]. bia devices measure the magnitude of the impedance opposed to the current that varies with respect to the body anatomy. specifically, the physical principle assumes that the body is made up of tissues with different composition. some tissues are good conductors due to their water content while others are insulators. the water content is inversely related to the resistance that opposes the current flow. on the other hand, cellular membranes, able to accumulate electrical loads, can be considered capacitors. the presence of capacitors is directly proportional to reactance and introduces an observable delay on the current flow. the sum of the resistance and reactance defines the impedance. its evaluation indicates the body hydration and provides an estimate of the nutritional state equivalent to the cellular amount. since water is the main component of the cells and it is almost absent in fat, it is possible to deduce the amount of ffm from the water content. consequently, fm is evaluated by simply subtracting the ffm to the total weight [8]. 1.1. fricke’s circuit: a human body electrical model the human body can be modeled as a set of resistance and capacitance connected in parallel or in series. the most common body model used in the field of bia is the fricke’s circuit, whose two parallel branches represent the intracellular and extracellular components. in this model, a high-frequency current passes through the intracellular water, while at low frequencies through the extracellular space. this is because at zero or low frequency, the current does not penetrate the cell membrane (acting as an insulator), while it passes through the extracellular medium made of water and sodium [9]. the intracellular behavior, in turn, can be modeled as a resistance ri (due to the water and potassium content) and a capacitance 𝑋𝑐 of the cell membrane, while the extracellular behavior is described by a single resistance re as shown in figure 1. the total body resistance r measured by a bia instrument is in turn a combination of the two resistances ri and re which indicate the real part of a complex number [10]. generally, the phasor and other indices such the ratio ri/re can be good estimators of diseases presence, nutritional status, and hydration condition [11]. 1.2. the calibration plots the cole-cole plot is commonly used to visualize the electrical response of body measurements with the resistance r on the x-axis and the negative reactance 𝑋𝑐 on the y-axis. at extremely high or ideally infinite frequency, the intracellular branch is the only one with the minimum resistance value ri. at low or zero frequency, the current passes only in the extracellular space since the cell membranes act as insulators. consequently, re is the maximum value of resistance. the relationship between the capacitance 𝑋𝑐 and the total resistance r of a body can be expressed by a phase angle φ [12]. therefore, the resulting phasor ranging from ri and re describes an arc segment as shown in figure 2 and all the measured values would lie below it. this plot can be standardized with respect to height, gender, and ethnicity, to form a calibration model divided into adjacent areas contained in tolerance ellipses at 50 %, 75 %, and 95 % belonging to a certain population group (as seen in figure 3, which shows an example of a calibration model standardized by the height, h) [13]. the plot is used as a calibration map by companies for converting a measurement performed by means of a device into a body status information [14]. if the bia device displays a low measurement accuracy, measurement results could be misleading. 1.3. measurement uncertainty the biasing factors on bioimpedance estimation can be attributed to the subject (the measurand is not constant), to the measurement protocols, and to the instrumentation [15]. in this figure 1. fricke's circuit model for body composition consisting into two branches related to the intracellular and extracellular behaviors. figure 2. example of cole-cole plot of the fricke's circuit. figure 3. example of a calibration model standardized by the height (h). acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 3 study we investigate the possible source of errors of the bia instrumentation, consisting in a control unit, cables, and electrodes. the control unit is composed of electronic circuitry placed in a case with one or more ports for connecting the cables. even if protected, the circuitry is subjected to thermal, electrical, and magnetic disturbances [16], [17]. the identification of these disturbances is essential for the performances of the devices and to improve competitivity in the market. for this reason, the control unit and the accessories should be metrologically characterized through a specific test for each possible sources of error [18], [19]. moreover, if the disturbances are properly identified, a corrective calibration strategy can be applied [20], [21]. 2. materials and methods 2.1. instrumental equipment the instrumentation selected for this study consists of a bia device (metadieta bia), three sets of cables, three sets of electrodes of different manufacturers, a series of resistances and capacitors, and a breadboard. metadieta bia (figure 4) is an electromedical device for the evaluation of the corporal composition manufactured by the company meteda s.r.l. (rome, italy). the bioimpedance is measured by placing four electrodes on the hands and feet, with a single cable connected to the main unit. the impedance value is computed from the response to a sinusoidal current of 350 μa with a frequency of 50 khz (a standard de facto for most bia devices with a single frequency). the device has a size of 43 × 43 × 12 mm3 and a mass of 50 g; a lithium battery can supply the device up to 14 hours in working conditions. it does not have a screen on the control unit, but it can be managed by an application running on phones, tablets, and computers with a bluetooth connection. the device is designed to be used in clinics by physicians, nutritional biologists and qualified sanitary personnel but also by consumers in home environment. the application provides the user with information about the preparation and the execution of the test measurement, then it sends and stores the data on the cloud for later analyses. data measurements are processed on the cloud application and the results can be either quantitative for clinical personnel or qualitative with displayed information in graphs along with the tendencies for individual users. the additional equipment for the test is represented by three cables of the same model between the main unit and the four electrodes clamps and a series of electrodes of three producers: biatrodes® by akern slr (firenze, italy), bia electrodes by rjl systems inc (clinton twp, mi, usa), and regal™ resting ecg by vermed® inc (bells falls, vt, usa). 2.2. proposed method the first operation to perform with a measurement device regards the metrological characterization in terms of repeatability and reproducibility after the identification of the possible sources of error [22]. generally, this kind of device makes use of empirical equations whose parameters are established by means of a calibration operation performed in laboratory [23]. since the calibration curves can assume a large set of values, a simplification of the process can rely on the study of a group of key values. this research proposes a data selection based on six values of resistance between 200 ω and 900 ω with a step of 140 ω, combined with six values of reactance between 15 ω and 115 ω with a step of 20 ω. these values are represented in the grid in figure 5. to assemble a physical circuit starting from the reactance values, suitable capacitors can be identified by converting 𝑋𝑐 into a capacitance c with the formula: 𝐶 = 1 2 ∙ 𝜋 ∙ 𝑓 ∙ 𝑋𝑐 , (1) where f is the frequency of ac generated by the metadieta bia device, i.e. 50 khz. the capacitance values obtained after the conversion are therefore: 212 nf, 91 nf, 58 nf, 42 nf, 34 nf, and 28 nf. by combining the values of resistance and capacitance, we defined a grid of 36 combinations and we evaluated the measurements’ repeatability and reproducibility in each condition. the procedure also allows identifying compensation functions allowing to reduce the systematic errors affecting the reading [24]. 2.3. experimental design the metadieta bia is turned on when the cables are inserted in the miniusb port and the connection is initiated by the application on a master device. measurements are typically performed by placing four electrodes on the hands and feet. the electrodes are silver plated for a low resistance and attached to the skin using an adhesive gel. however, for consistency, all the experiments were performed on laboratory instrumentation with electric circuits figure 4. picture of the metadieta bia control unit. figure 5. calibration grid with 36 combinations of the key values selected. acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 4 representing the body composition through the fricke’s model, so the electrodes were included only in specific tests. the tests were performed in metrospace lab of politecnico di milano and can be divided into: 1. preliminary tests for the metrological characterization of metadieta bia device, cables, electrodes, and adhesive gel. 2. test for systematic error compensation based on the calibration grid in figure 5. a high precision lcr meter, model lcr-819 gw instek (good will instrument co., ltd, taiwan), was used as a reference system for measuring the impedance of the test components, while a multimeter, model agilent 34401a, was used for the only resistance measurements of the electrical components. 2.4. preliminary tests first, the measurement repeatability of the control unit was tested by performing 30 measurements of the resistance r and reactance 𝑋𝑐 repeated on five different electric circuits connecting the cable clamps directly to the circuit with no other modifications between each test and the next. the three different cables of the same model were tested with 30 measurements each with the lcr meter, on the same electric circuit directly connecting the clamps of the cables. keeping the same configuration, the effect of the electrodes was studied applying these components without the adhesive material between the clamps and the electric circuit with passive elements. a total of 30 different sets of electrodes of the three manufacturers were tested, 4 electrodes for each set. at the same time, the resistance r of the cables and the electrodes was measured 30 times for each component through the multimeter. the variability of electrical resistance of the electrodes was estimated by placing the multimeter terminals in two positions, on the tab and on an opposite area far from it (circled in figure 6). the effect of the adhesive gel, which determines the interaction with the bia device and a biological tissue, was simulated by means of a jelly phantom (figure 7) with nominal resistance of 𝑅𝑝ℎ = (571.2 ± 1.2) ω (c.i. = 68 %) and nominal reactance of 𝑋𝑐 𝑝ℎ = (75.1 ± 1.9) ω (c.i. = 68 %) [25]. for this test, 30 measurements for each manufacturer’s electrode were performed to calculate the mean value of the resistance �̅� and reactance �̅�𝑐 and the relative standard deviation. the four electrodes were positioned at the edges of the container, one couple on the left side and the other couple on the right side with a distance of about 30 cm. the distance between the two electrodes of each couple was of about 10 cm as recommended by the manufacturer. this configuration with the dominant distance (30 cm > 10 cm) between the two couples of electrodes aimed to replicate the measurement behaviour on a human body, avoiding uncontrolled dispersion of the electric charge. 2.5. tests for systematic error compensation a set of 36 circuits with passive elements was built by combining selected components with the resistance and capacitance collected in table 1, to the key values of the calibration grid in figure 5. table 1 also includes the reactance values after the conversion obtained by inverting the eq.1. the resistances components have a manufacturing tolerance of 0.1%, whereas the capacitors have a value of 1%. the circuits were mounted on a breadboard and the values read by metadieta bia device were compared to the values read by the lcr meter as references [26]. the differences between the measured and the reference allowed to calculate the rmse and control for the presence of defined patterns related to systematic disturbances. part of these disturbances was removed by adding two corrections terms 𝑅𝑎 and x𝑐 𝑎, obtained by a least square minimization of a multivariate linear model, to the generic measurements r and 𝑋𝑐 in the form: 𝑅𝑎𝑑𝑗 = 𝑅 + 𝑅𝑎 (2) and 𝑋𝑐 𝑎𝑑𝑗 = 𝑋𝑐 + x𝑐 𝑎 , (3) where 𝑅𝑎𝑑𝑗 and 𝑋𝑐 𝑎𝑑𝑗 are the compensated results. figure 6. area of the electrode for measuring the resistance. figure 7. preliminary test of the electrodes on a jelly phantom. table 1. resistances and capacitances of the selected components and the reactance values after conversion for the calibration map experiments. component 1 2 3 4 5 6 r in ω 200 330 470 615 780 910 c in nf 225 92 51 36 32 27 xc in ω 14 35 56 89 99 120 acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 5 3. results 3.1. preliminary tests the results of the repeatability test of the control unit on the 5 electric circuits with 30 measurements performed on each circuit are shown in table 2: r and 𝑋𝑐 are the key values chosen for the experiments, 𝑅𝑟𝑒𝑓 and 𝑋𝑐 𝑟𝑒𝑓 are the reference values read by the lcr meter, �̅� and �̅�𝑐 are the mean values read by the metadieta bia device with 𝜎𝑅 and 𝜎𝑋𝑐 the relative standard deviations. the three tested cables showed a standard deviation of the resistance of 𝜎𝑅 = 1.8 ω, while the standard deviation of the reactance is 𝜎𝑋𝑐 = 0.1 ω. from these values it was possible to evaluate the uncertainty values 𝑢𝑅 = 𝜎𝑅 √30 =⁄ 0.33 ω and 𝑢𝑋𝑐 = 𝜎𝑋𝑐 √30 =⁄ 0.018 ω (c.i. = 68 %). the electrode without the adhesive gel were tested on a circuit with the nominal resistance r = (617.812 ± 0.011) ω (c.i. = 68 %) and the equivalent reactance 𝑋𝑐= (90.137 ± 0.019) ω (c.i. = 68 %) with the metadieta bia device. the mean and the standard deviation of the resistance and the reactance are reported in table 3. the maximum standard deviation values were reported by the rjl systems electrodes equal to 𝜎𝑅 = 0.5 ω and 𝜎𝑋𝑐 = 0.1 ω with the correspondent uncertainties equal to 𝑢𝑅 = 𝜎𝑅 √30 =⁄ 0.091 ω and 𝑢𝑋𝑐 = 𝜎𝑋𝑐 √30 =⁄ 0.018 ω (c.i. = 68 %). the resistance-only measurements of the same electrodes performed through the multimeter are reported in table 4. in this case both rjl systems and vermed® electrodes reported a maximum standard deviation of 𝜎𝑅 = 0.4 ω and an uncertainty of 𝑢𝑅 = 𝜎𝑅 √30 =⁄ 0.073 ω (c.i. = 68 %). the last experiment of the preliminary test on the jelly phantom are reported in table 5. all three electrode samples showed a standard deviation of 𝜎𝑅 = 0.1 ω with an uncertainty of 𝑢𝑅 = 𝜎𝑅 √30 =⁄ 0.018 ω (c.i. = 68 %), whereas akern and vermed® electrodes reported a standard deviation different from zero and equal to 𝜎𝑋𝑐 = 0.1 ω corresponding to an uncertainty of 𝑢𝑋𝑐 = 𝜎𝑋𝑐 √30 =⁄ 0.018 ω (c.i. = 68 %). 3.2. systematic error compensation the measurements on 36 electric combinations with the metadieta bia device and the reference values are depicted in figure 8. from these data, the rmse of the 36 configurations resulted 𝑅𝑅𝑀𝑆𝐸 = 4.17 ω and 𝑋𝑐,𝑅𝑀𝑆𝐸 = 7.28 ω. the minimization of the least square on the multivariate linear regression returned the following correction terms: 𝑅𝑎 = −1.592 + 0.994 ∙ 𝑅 + 0.002 ∙ 𝑋𝑐 + 2.45 ∙ 10−5 ∙ 𝑅 ∙ 𝑋𝑐 (4) and x𝑐 𝑎 = −3.412 + 0.010 ∙ 𝑅 + 1.079 ∙ 𝑋𝑐 − 2.19 ∙ 10−5 ∙ 𝑅 ∙ 𝑋𝑐 . (5) with r and 𝑋𝑐 the actual values read by the bia device. furthermore, the multivariate linear regression reported the table 2. results of the repeatability test of the control unit on 5 electric circuits. r in ω xc in ω rref in ω xcref in ω �̅� in ω σr in ω �̅�𝐜 in ω σxc in ω 200 15 200.1 17.9 202.7 0.0 18.9 0.1 200 75 191.4 92.3 193.5 0.1 88.7 0.1 340 115 330.5 124.4 333.4 0.1 116.1 0.0 620 75 617.8 90.1 622.1 0.0 80.2 0.0 900 95 910.9 101.1 916.9 0.0 98.3 0.0 table 3. results of the repeatability test of the three producer’s electrodes without the adhesive gel by the metadieta bia device. manufacturer �̅� in ω σr in ω �̅�𝐜 in ω σxc in ω akern 619.2 0.2 91.4 0.0 rjl systems 619.5 0.5 91.4 0.1 vermed® 619.4 0.1 91.5 0.0 table 4. results of the repeatability test of the three producer’s electrodes without the adhesive gel by the multimeter agilent 34401a. manufacturer �̅� in ω σr in ω akern 1.6 0.3 rjl systems 2.1 0.4 vermed® 2.1 0.4 table 5. results of the repeatability test of the three producer’s electrodes with the adhesive gel on the jelly phantom by the metadieta bia device. manufacturer �̅� in ω σr in ω �̅�𝐜 in ω σxc in ω akern 573.2 0.1 79.1 0.1 rjl systems 573.3 0.1 78.5 0.0 vermed® 573.2 0.1 78.9 0.1 acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 6 adjusted r2 values of �̅�𝑅 2 =0.947 for the resistance and �̅�𝑋𝑐 2 = 0.696 for the reactance. compensating for the values in figure 8 with the terms 𝑅𝑎 and x𝑐 𝑎, the values of rmse decrease to 𝑅𝑅𝑀𝑆𝐸 =1.16 ω and 𝑋𝑐,𝑅𝑀𝑆𝐸 =1.28 ω. 4. discussion the tests on the metadieta bia device revealed that the cables, the silver-plated electrodes, and the gel have a negligible influence on the overall measurement chain: the cables showed an uncertainty of 𝑢𝑅 = 3.3 ∙ 10 -1 ω (c.i. = 68 %) and 𝑢𝑋𝑐 = 1.8 ∙ 10-2 ω (c.i. = 68 %), while the maximum uncertainties introduced by the electrodes were 𝑢𝑅 = 8.6 ∙ 10 -2 ω (c.i. = 68 %) and 𝑢𝑋𝑐 = 1.7 ∙ 10 -2 ω (c.i. = 68 %). the comparison between the three electrode models also showed that these elements have the same electric characteristics for which the device performance does not change, as proved by sanchez et al. [27]. also, the tests for the gel on the jelly phantom did not report any significant influence since the maximum uncertainties were 𝑢𝑅 = 1.7 ∙ 10-2 ω (c.i. = 68 %) and 𝑢𝑋𝑐 = 1.7 ∙ 10 -2 ω (c.i. = 68 %). this means that the adhesive gel is essential for keeping the contact between the electrodes and the skin but it does not add any relevant disturbance to the measurement process [28]. the comparison between the reference values and the measurements with the bia device in figure 8 showed that the uncertainties of the reactance and resistance tend to increase for the combinations with higher values. nonetheless, the trend was corrected effectively by the multivariate linear regression. in fact, the two terms 𝑅𝑎 and x𝑐 𝑎 can decrease the uncertainties to 𝑅𝑅𝑀𝑆𝐸 =1.16 ω and 𝑋𝑐,𝑅𝑀𝑆𝐸 =1.28 ω. moreover, by observing the expressions of 𝑅𝑎, it is evident that the read reactance contribution is negligible. conversely, the read resistance value has a relevant influence on the compensation procedure. 5. conclusions bia is an effective and valid tool to estimate body composition from a fast and safe single measurement. nonetheless, the estimation can fail when the measurement conditions change or if there is a poor calibration of the bia device. in this paper, we evaluated the causes of variability of bioimpedance measurements. first, the equipment was metrologically characterized showing that it does not influence the measurements significantly with uncertainties lower than 0.35 ω (c.i. = 68 %) for both resistance and reactance. for what concerns the validation of bia equations, it must be carried out against gold standards, even though they exhibit limitations due to hydration conditions, age, and ethnicity. this study proposed a calibration grid made of 36 configurations of key values. the grid allowed to calculate multivariate linear models minimizing the least square errors which can be used to calibrate the metadieta bia device. in the case-study presented in this work, the bias error compensation reduced the rmse from 4.2 ω to 1.2 ω for the resistance and from 7.3 ω to 1.3 ω for the reactance with the adjusted r2 values respectively of 0.947 and 0.696. prospectively, the calibration maps can be extended to higher values and the key points grid can be further populated for more robust results. references [1] u. g. kyle, l. genton, d. o. slosman, c. pichard, fat-free and fat mass percentiles in 5225 healthy subjects aged 15 to 98 years, nutrition. 17 (2001), pp. 534-541. doi: 10.1016/s0899-9007(01)00555-x [2] h. c. lukaski, methods for the assessment of human body composition: traditional and new, am. j. clin. nutr. 46 (1987), pp. 537-556. doi: 10.1093/ajcn/46.4.537 [3] a. talluri, r. liedtke, e. i. mohamed, c. maiolo, r. martinoli, a. de lorenzo, the application of body cell mass index for studying muscle mass changes in health and disease conditions, acta diabetol. 40 (2003). doi: 10.1007/s00592-003-0088-9 [4] a. choi, j. y. kim, s. jo, j. h. 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(2019). doi: 10.1109/ieecon45304.2019.8939016 https://doi.org/10.3265/nefrologia.pre2011.oct.11108 https://doi.org/10.1159/000077042 https://doi.org/10.1109/tbme.2007.906516 https://doi.org/10.1109/tim.2004.840237 https://doi.org/10.3390/s140610895 https://doi.org/10.1016/j.measurement.2006.03.004 https://doi.org/10.1016/j.measurement.2018.08.070 https://doi.org/10.1016/j.measen.2021.100067 https://doi.org/10.1016/j.measurement.2018.02.032 https://doi.org/10.1109/19.843082 https://doi.org/10.1109/tdei.2016.005507 https://doi.org/10.1109/memea.2014.6860044 https://doi.org/10.21014/acta_imeko.v10i2.1042 https://doi.org/10.1109/tim.2002.803293 https://doi.org/10.3390/s21155195 https://doi.org/10.1109/tim.2002.803300 https://doi.org/10.1109/tim.2013.2292272 https://doi.org/10.1109/ieecon45304.2019.8939016 primary calibration of mechanical sensors with digital output for dynamic applications acta imeko issn: 2221-870x september 2021, volume 10, number 3, 177 184 acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 177 primary calibration of mechanical sensors with digital output for dynamic applications benedikt seeger1, thomas bruns1 1 physikalisch-technische bundesanstalt, bundesallee 100, 38116 braunschweig, germany section: research paper keywords: primary calibration; mems; complex frequency response; digital interface; dynamic measurement citation: benedikt seeger, thomas bruns, primary calibration of mechanical sensors with digital output for dynamic applications, acta imeko, vol. 10, no. 3, article 24, september 2021, identifier: imeko-acta-10 (2021)-03-24 section editor: francesco lamonaca, university of calabria, italy received february 26, 2021; in final form july 9, 2021; published september 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this project 17ind12 met4fof has received funding from the empir programme co-financed by the participating states and from the european union’s horizon 2020 research and innovation programme. corresponding author: thomas bruns, e-mail: thomas.bruns@ptb.de 1. introduction more and more applications and appliances in both the industrial and the consumer fields are integrating micromechanical sensors with a direct digital output. that means the measured value of a quantity like acceleration, air pressure or force is accessible via a digital interface like spi, i2c or can in terms of a binary number. those values can be directly fed into control units or digital displays for further processing without any need for traditional signal conditioning by amplifiers or sampling by discrete ad converters. this is very beneficial for the integration of measurement capabilities in all kinds of devices and systems, from simple telephones to uavs1 and beyond. accordingly, the market for mems sensors has been booming for several years. in condition monitoring applications, e.g., on production machines, these sensors can be used to measure vibrations, also using information from the frequency domain [1], which makes 1 unmanned aerial vehicles also known as drones. dynamic calibration and absolute timestamping advantageous [2], [3]. from the metrology perspective, such sensors are rather inconspicuous as long as the focus is on a static measurement. as soon as the focus shifts onto dynamic signals, however, the timing of signal acquisition is of high importance, and the integrated sampling and signal conditioning of such sensors creates new challenges for the calibration lab. this starts with the fact that existing documentary standards and guidelines for dynamic calibration, like the current iso 16063 series or [4] rely on the synchronised timing of the (analogue input) data acquisition channels, which is under the control of the calibration system. recent work on the calibration of sensors for other mechanical quantities like force, torque or pressure relies on the same principles. while the calibration of the magnitude of a sensor's complex transfer function only needs the technical interfaces and a stationary excitation signal [5], the phase or group delay calibration is strongly dependent on either the synchronicity of the channels or other precise knowledge of the sample timing. abstract this article tackles the challenge of the dynamic calibration of modern sensors with integrated data sampling and purely digital output for the measurement of mechanical quantities like acceleration, angular velocity, force, pressure, or torque. based on the established calibration methods using sine excitation, it describes an extension of the established methods and devices that yields primary calibration results for the magnitude and phase of the complex transfer function. the system is demonstrated with a focus on primary accelerometer calibrations but can easily be transferred to the other mechanical quantities. furthermore, it is shown that the method can be used to investigate the quality and characteristics of the timing for the internal sampling of such digital output sensors. thus, it is able to gain crucial information for any subsequent phase-related measurements with such sensors. mailto:thomas.bruns@ptb.de acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 178 2. extension to conventional primary calibration a conventional analogue calibration system (acs) in compliance with iso 16063-11 or iso 16063-21 is based on two data acquisition channels with common timing usually provided by a central clock signal in a single data acquisition system. the two channels record the reference quantity (𝑦ref ), like acceleration, velocity, or displacement) and the calibration quantity (voltage, charge) as the output of the device under test (dut). the clock is usually quite accurate and the sampling of the time series from the acquisition can be considered equidistant. using a mono-frequent sinusoidal drive signal with angular frequency 𝜔 for the mechanical excitation (e.g., via a shaker), a linear sine approximation is used to quantify the reference input (𝑦ref) and the response (𝑦dut) of the dut. via linear least squares fitting, the parameters in the following equations are determined: 𝑦ref(𝑡𝑖 ) = 𝐴ref sin(𝜔𝑡𝑖) + 𝐵ref cos(𝜔𝑡𝑖) + 𝐶ref 𝑦dut (𝑡𝑖 ) = 𝐴dut sin(𝜔𝑡𝑖 ) + 𝐵dut cos(𝜔𝑡𝑖 ) + 𝐶dut , (1) where 𝐴 and 𝐵 can be considered as quadrature components of each signal, while 𝐶 is introduced to cover any potential bias introduced into the measurement. the complex transfer function s of the dut is then given in terms of magnitude �̂� and phase δ𝜑 by 𝑆(𝜔) = �̂�(𝜔) ⋅ e−𝑖 δ𝜑(𝜔) with �̂�(𝜔) = √ 𝐴dut 2 + 𝐵dut 2 𝐴ref 2 + 𝐵ref 2 and (2) 𝜑(𝜔) = arctan ( 𝐵dut 𝐴dut ) − arctan ( 𝐵ref 𝐴ref ) . although not self-evident from the equations, it is crucial for the correct calibration of the phase delay that the time base for the data acquisition (represented by 𝑡𝑖 in equation (1) is the same for both channels. in figure 1, such an acs is represented on the top left. the channel for the dut called "sync" in this schematic for reasons that will become clear, soon. 2.1. sample timestamping for the case of a digital output sensor (dos), the prerequisite last mentioned in the previous paragraph is no longer fulfilled. neither is the sample clock frequency (or the actual instance of sampling) under the control of the laboratory's acs nor is the exact instance of sampling even known to the measurement system. a reliable phase calibration under such circumstances requires an extension of the acs in order to mitigate the lack of knowledge and control. for the rest of this section it is presumed that the dos provides a "data ready signal", that is, it provides a means to signal to any connected recording system that a new sample has been acquired and is ready for delivery. this signal is active whenever a new sample was acquired by the sensor. the term "sample" in relation to the dos relates to one set of values of the measurands of the dos. these values are nominally associated with the same single point in time, e.g., for a three-component accelerometer, a sample could be a tuple (𝑎𝑥 , 𝑎𝑦 , 𝑎𝑧 ). the complications compared to a conventional analogue sensor are threefold: 1. the data interface is digital and cannot be read out by the classic acs. 2. the precise sample clock is unknown and asynchronous in relation to the acs's internal clock. 3. the sampling instances of the dos are not necessarily equidistant. to overcome these complications and accomplish a phase calibration of a dos with data ready output, the acs is extended with a separate digital acquisition unit (dau) as shown in the top right part of figure 1. figure 1. the conventional analogue calibration system (acs) on the top left extended by the digital acquisition unit (dau) on the top right. at the bottom the respective signal traces over time as acquired by the two systems. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 179 the core of this dau is the sampling and timestamping of the data from the digital interface of the dos related to a common time base, which it also uses for the sampling of an additional analogue channel called "sync" (orange arrows) in figure 1. in the implementation used for the investigations in this publication, the sampling of dos data and sync were performed synchronously. this, however, is not a strict requirement for the procedure. whenever the dos signals the availability of a new sample via the data ready output (upper green arrow in figure 1, the dau immediately samples the sync signal, (𝑦sync) (right orange input to dau in figure 1), reads the digital data sample, (𝑦dos) from the dos interface (lower green arrow in figure 1) and marks this data tuple with a precise timestamp based on a gpsaligned clock of the microprocessor used. repeating this procedure, the dau acquires a time series of digital and analogue data with a sample clock as close to the dos internal sample clock as possible but with timing information as accurate as the external clock provided to the dau. the relation to the reference signal for the calibration, which is still acquired with the acs (blue arrow in figure 1), is provided by a sync signal (orange arrow) and acs input in figure 1, hence the naming of this channel. the sync signal is a mono-frequent sinusoidal voltage with the identical frequency to the excitation signal. it is fed into the dut channel of the acs and to the sync input of the dau. such a signal can be easily derived directly from the excitation signal or taken from the reference channel of the acs. as a consequence of this setup and procedure, the originally independent systems of the acs and the dau can now be linked in time via the common sync signal which is sampled by both systems based on their respective time bases. this is depicted in the time traces in the lower part of figure 1, where the same sync signal (in orange) is given twice: once as acquired by the acs and secondly as acquired by the dau. 2.2. implementation details of the dau the implementation of the dau used in this study was based on an stm32f767® microcontroller and a related development board (nucleo-f767zi®). the acquisition of dos output and the adc was handled in a single interrupt service routine, which was triggered by the data ready pin of the dos. the timing information was generated by an internal 108 mhz counter, which, in turn, was synchronised with an externally provided pulse-per-second (pps) clock from a gps receiver. the details of the implementation are available under open access conditions [11], [12] the uncertainty of the absolute timestamp was evaluated by measuring a reference pps signal stabilised to ptb's atomic clock with the dau. figure 2 shows the deviation of the timestamps from the expected value as determined by the dau. that deviation is mainly caused by the short-term drift of the microcontroller's phase locked loop and the intrinsic jitter of the gps's pps signal. overall, an accuracy better than 150 ns was achieved. 2.3. data analysis for sample time stamping based on the described signal generation and data acquisition scheme, the system generates four times series of timestamped samples related to three different signal sources. the amplitudes and initial phases of these can be determined by appropriate sine approximation methods (described below). those time-series are: i. the reference signal acquired by the acs: 𝑦𝑖,ref(𝑡𝑖 ) = �̂�ref ⋅ sin(𝜔 𝑡𝑖,acs − 𝜑ref). ii. the sync signal acquired by the acs: 𝑦𝑖,sync(𝑡𝑖,acs) = �̂�sync,acs ⋅ sin(𝜔 𝑡𝑖,acs − 𝜑sync,acs). iii. the sync signal acquired by the dau: 𝑦𝑘,sync(𝑡𝑘,dau) = �̂�sync,dau ⋅ sin(𝜔 𝑡𝑘,dau − 𝜑sync,dau). iv. the dos signal acquired by the dau: 𝑦𝑙,dos(𝑡𝑙,dos) = �̂�dos ⋅ sin(𝜔 𝑡𝑙,dos − 𝜑dos) . here, the index i indicates that (i) and (ii) have the same time base and are synchronously sampled (by the acs). in contrast (iii) and (iv) do need the same time base (from the dau) but not necessarily synchronous sampling, hence the indices are k and l. the magnitude of the transfer function of the dos is independent of the actual timing situation and can be directly calculated from (i) and (iv) as �̂�(𝜔) = �̂�dos �̂�ref . (3) since (ii) and (iii) are records of the same source with only apparent but no real existing delay between 𝜑sync,acs and 𝜑sync,dau, the time bases of acs and dau and thus the initial phases of the reference and the dos can be linked. the phase difference between 𝑦ref and 𝑦dos is accordingly given by δ𝜑 = 𝜑dos − 𝜑sync,dau + 𝜑sync,acs − 𝜑ref . (4) it should be noted that the accuracy of that link is dependent on the timestamping accuracy of the dau and not on the precision of the dos's sample clock. as well as the uncertainty of the sync signal (iii) sampled by the dau. the transfer function of the dau adc must be calibrated and taken into account [13]. on the contrary, the measurement scheme described above provides additional information on the precision of the dos's sample clock, as described in section 3. 2.4. series timestamping some digital sensors/measuring systems do not provide a data ready signal for each sample but instead record a full time series of data after a given trigger signal. to calibrate such a kind of sensor (we will call it "time series sensor”, tss), precise knowledge of the relative time between the trigger and the excitation signal is needed. if the acs does not provide such a figure 2. distribution of the timestamp deviations determined using the dau with reference to an atomic clock stabilised pps signal over a monitoring period of 45 hours. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 180 synchronised trigger signal, separate analogue trigger logic, e.g. from a digital storage oscilloscope (dso), can be used to derive and generate the trigger signal from an analogue sync signal like that mentioned in section 2.1. figure 3 shows a schematic diagram of a setup to calibrate such a tss. the acs again acquires the generator signal 𝑦sync (𝑡𝑖 ), see section 2.1, as for an analogue dut (orange arrow in figure 3 into the acs). the dso (in our case a rigol mso5072) triggers, upon zero crossing of the same analogue sync signal (orange arrow in figure 3 into the dso) and generates a trigger signal which is connected to the tss's trigger input (brown arrow in figure 3). this starts the recording of the time series 𝑦tss(𝑡tss). note that due to noise, the dso does not trigger perfectly at the zero crossing of the sync signal, and there is some additional processing delay within the dso. however, the resulting actual trigger delay can be determined and corrected for, e.g. by sine approximation of the sync signal acquired simultaneously by the dso and determining the actual trigger phase δ𝜑dso,triggernoise. to this end, in a dedicated setup, the dso is used to sample its own trigger output simultaneously to the sine wave of the sync signal, which causes that trigger output. based on the two simultaneously acquired traces, the delay between the zero crossing of the sync signal (e.g. determined by sine approximation) and the rising edge of the trigger can be determined. due to inevitable noise, there will be a certain variation in the determined trigger delay. in the actual setup, the trigger signal is emitted with a mean delay of 𝜏dso = 236(1) ns. this was previously calibrated with a rectangle generator and the feedback of the trigger output to a second input channel of the dso. the use of a rectangle generator, providing a steep rising edge, was considered more precise than the sine approximation approach applied to the sync signal. however, it requires additional equipment. 2.5. data analysis for series timestamping subsequent to the data acquisition described, a fourparameter sine approximation (fpsa) according to [6] is used to determine the angular frequency 𝜔, amplitude �̂�𝑇𝑆𝑆, initial phase 𝜑𝑇𝑆𝑆 and offset 𝐶𝑇𝑆𝑆 of the tss's output 𝑦tss,𝜔 (𝑡tss) (note that due to the operation principle of a tss, the time instances are unknown.) with these parameters, the magnitude response �̂�(𝜔) of its complex transfer function at the excitation frequency 𝜔 can be obtained directly from the amplitude of the reference signal �̂�ref(𝜔) and the amplitude of the time-series signal �̂�tss(𝜔) �̂�(𝜔) = �̂�tss(𝜔) �̂�ref(𝜔) . (5) note that the internal sampling frequency of the tss, 𝑓s,tss may differ from its specified nominal sampling frequency 𝑓s,tss,nominal. such differences lead to a deviation of the apparent excitation frequency determined by the fpsa as compared to the applied excitation frequency 𝜔. a reduced internal sample rate 𝑓s,tss leads to an apparent increase of 𝜔tss, and accordingly an increased 𝑓s,tss leads to an apparent reduction of this frequency. the actual mean internal sample rate 𝑓s,tss can then be determined as follows: 𝑓s,tss = 𝑓s,tss,nominal ⋅ 𝜔 𝜔tss , (6) provided that the excitation frequency from the acs, 𝜔, is well known. the initial phase of the tss signal 𝜑tss was determined using the four-parameter approximation and is thus linked to the actual period of the excitation signal. therefore, no correction related to sample rate deviations of the measured initial phase is necessary. figure 3. the acs with a digital storage oscilloscope (dso) as a trigger generator for the tss. the dso triggers on the zero crossing of the sync signal. the trigger output of the dso, in turn, triggers the data acquisition of the tss. the traces are the reference signal (blue), the analogue sinusoidal sync signal (orange), the trigger (brown) and the sampled time series of the tss (green). acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 181 consequently, the phase response δ𝜑 of the complex transfer function of a tss can be calculated as follows, δ𝜑 = 𝜑tss − δ𝜑dso − δ𝜑acs , (7) using the phase difference δ𝜑acs between the reference and sync signal obtained by the analogue calibration system2. consequently, the complex transfer function �̂�(𝜔) is given by: 𝑆(𝜔) = �̂�(𝜔) ⋅ e−𝑖 δ𝜑 = �̂�tss (𝜔) �̂�ref(𝜔) ⋅ 𝑒 −𝑖(𝜑tss−δ𝜑dso−δ𝜑acs). (8) 3. sample rate problems and mitigation strategies typical doss generate their internal sample rate with an internal clock unit based e.g. on an rc oscillator. such simple clock generators are subject to drift depending on the operational temperature or supply voltage etc. accordingly, the stability and precision of such clocks should be checked. in [7] the effect of a biased sample frequency was described, and a rather elaborate correction method was provided. the calibration procedures presented in this article are independent of the internal sample clock by design, because they make use of external gps-aligned timestamping. even more, these procedures provide the direct means for a quantitative determination of sample clock accuracy and stability. this can be achieved at least in two ways: • direct sample interval evaluation based on the timestamps 𝑡𝑙,dos, the sample interval between each dos sample pair can be calculated as 𝑡𝑙+1,dos − 𝑡𝑙,dos and compared to the nominal sample interval. this procedure and the resulting insights, of course, depend on the availability of independent precise and stable timestamps for each sample, which makes it unfeasible for the series timestamping. • nominal frequency evaluation to get a more general estimate of the dos sample rate precision and stability, it is sufficient to have a known, stable and precise single frequency excitation for the dos. accordingly, this method can be applied even without the complete calibration set-up. if the internal clock of the dos deviates by a factor 𝜆 from the laboratory clock, the dos senses an external excitation with frequency 𝑓vib with an apparent frequency 𝑓vib,dos. the relation between the two is 𝑓vib,dos = 𝑓vib 𝜆 . (9) this is the equivalent situation to that described by equation (6) for series timestamping. the apparent frequency sensed by the dos can easily be determined by applying an fpsa [4] to the time series measured by the dos. this algorithm does not only fit the amplitude and phase of a sinusoidal signal but iteratively reduces apparent frequency deviations starting with an initial frequency guess. note that in the case of sample timestamping the timestamps 𝑡𝑙 used for that fit have to be the nominal (deviating) 2 it should be noted that any additional phase delays within the acs known from conventional calibration have to be considered in the same fashion for the digitally enhanced system. timestamps calculated from the nominal sample rate of the dos. then, equation (9) yields the following for the real mean sample rate 𝑓dos,real of the dos 𝑓dos,real = 𝑓dos,nom 𝜆 . (10) 4. first primary calibration results 4.1. the transfer function in order to test the above-mentioned concept, the dau was used to perform a primary calibration of the z-axis of a 6-degreeof-freedom mems sensor of the type mpu-9250 following [6]. figure 4 shows the situation in the acs, i.e. the dos mounted on the shaker's armature and the connected dau. for the determined complex transfer function, the reference acceleration was measured by laser interferometry on top of the mems's housing. a second laser was targeted at the surface of the carrier breakout board right next to the dos in order to investigate any relative motion due to the solder joints. this, however, is of no concern in this manuscript. the generator signal driving the power amplifier for the shaker was used as the sync signal to link the acs and the dau. the calibration was performed in the frequency range from 10 hz to 200 hz with single frequency sine excitation and excitation levels between 2.5 m/s² and 100 m/s². the mpu-9250 sensor is one of those that provide a "data ready" signal whenever the internal sampling process is finished. according to the previous description, the dau acquired a sample of three acceleration values, three angular rate values and one temperature value for each trigger from the data ready signal and timestamped this set of values with an absolute time value. the sensor was running at a nominal sampling rate of 1000 s/s. figure 4. primary calibration setup for z-axis of an mpu-9250 sensor. the bottom half shows the dau with the stm32f7 development board in piggyback. in the top half, the mpu-9250-breakout board mounted on a steel adapter and screwed to the armature of an se-09 shaker is shown. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 182 the analysis focused on the single (nominal) axis of excitation, which was the z-axis. by using the sync signal as described in section 2.3 and sine fitting of the z-axis values related to the absolute timestamps, it was possible to attain a traceable primary calibration of the magnitude and phase of the respective complex transfer function. the results are depicted in figure 5 for three repeated measurements of the same unit. a linear regression to the phase results for the lower frequencies suggests a group delay of the sensor of 8,1 ms. 4.2. the internal sampling in addition to this classic evaluation, the timestamping enables the evaluation of the stability of the sensor's internal sample clock and possible influencing quantities. in direct comparison to a 10 mhz reference frequency distributed within ptb and traceable to the atomic fountain clocks of ptb, the utilised gps-based timestamping revealed a mean sample interval of 996,96 µs for a measurement time of approx. 8000 s. over this time the sample interval length yielded a standard deviation of 21 ns and a spread of 335 ns, where admittedly the quantization of the individual timestamps is 10 ns due to the counting frequency of 108 mhz of the dau. the distribution of the sample intervals is shown in the histogram in figure 6. from the visual appearance, it is evident that the distribution is not gaussian and therefore not the usually supposed normal distribution. rather than that, certain values of the sample interval are apparently strongly favoured over others. one cause for such an effect may be found in sensor-internal digital compensation processes in relation to temperature. as the mpu9250 also features an internal temperature probe, the investigation of the correlation of the sample interval versus the temperature is an obvious next step. figure 7 depicts the relation of the sample interval over the apparent temperature as measured internally. it is evident that three to four of the preferred sample durations can be associated with a certain temperature and that, in general, the average sampling frequency falls (slightly) with rising temperature. for an in-depth understanding of this peculiar relation, a detailed knowledge of the internal processes of the sensor is probably necessary. nevertheless, it remains to be noted that the application of sample timestamping with the newly presented dau could demonstrate that the statistical properties of the sampling unit of this sensor are far from the typically expected uniform or normal distribution. in order to compare this finding with some other dos, a bosch bma280 was calibrated in an identical setting, and the measured sample intervals were subject to the same processing. figure 8 depicts the results. in this case the distribution is split into three parts with a prioritised centre part which has three orders of magnitude more counts than the side lobes. such a shape makes the usual characterisation with a standard deviation almost meaningless. why such a peculiar distribution is generated becomes clearer on closer inspection of the time series of sample intervals given in figure 9. figure 5. magnitude (top) and phase (bottom) of the complex transfer function of an mpu-9250 sensor measured in 3 repetitions (#1, #2, #3). the insert show details at low frequencies. figure 6. distribution of sample intervals of an mpu-9250 dos determined by sample timestamping. counts are in logarithmic scale. figure 7. relation of sample interval and sensed temperature of the mpu9250 sensor. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 183 as seen in the upper chart, the vast majorities of the sample intervals are close to the mean value of 485 µs. however, periodically, i.e. approximately every 1.1 s or 2200 samples, there is a double spike. the close-up of one of those features in the lower chart of the figure discloses a combination of a delayed followed by a premature sample. presumably, some internal process of the digital part of the sensor, like an adaptive digital filter or an adjustable compensation, takes some excessive computation time, slowing down either the sampling process or the delivery (data ready) of the analogue quantity. in order to keep the mean sample rate constant, the delay is followed by an abridged sample interval compensating the delay. 5. measurement uncertainty this article is not dedicated to an in-depth discussion and evaluation of the measurement uncertainty budget associated with the new hardware and methodology. this will need more research in the various components involved and probably is worth an article on its own. nevertheless, we want to briefly address a few issues that were found during the research work so far. 5.1. magnitude of the transfer function the magnitude calibration is a rather straight forward process for the dos and in terms of measurement uncertainty very similar to conventional work with ad-converted output voltages. a significant difference is, however, that opposed to conventional set-ups, no gain adjustment by using external amplifiers is possible. this results in a fixed signal resolution that is linked to the embedded adc and has to be considered in the measurement uncertainty of the dos especially for excitations in the lower amplitude range. 5.2. phase of the transfer function the phase of the transfer function is the actual challenge in calibration and therefore the focus of this article. internally the measurement uncertainty of the phase is relying on the quality of the embedded oscillator and some firmware aspects of the dos. this was demonstrated by the measurement-analysis of the previous section 4. while the influence of a low-quality oscillator may be mitigated by precise timestamping during calibration, such an approach is usually not feasible for the typical application of dos. here, the nominal sampling rate is assumed. in addition to these components of measurement uncertainty of phase intrinsic to the dos, the hardware of the dau and the methodology add their own components. of course, the phase or group delay associated with the sampling of the sync-signal by the dau has to be calibrated, as this is no longer controlled by the acs. the best estimate of this group delay can be corrected, but an uncertainty remains from this calibration. this was briefly addressed in sections 2.2 and 2.4 for the work presented here. additional phase measurement uncertainty is inflicted by the noise on the sync signal which links the time-bases of acs and dau. this is strongly dependent on the source available as sync signal and in particular its signal-to-noise ratio. this noise degrades the phase accuracy on both sides, the acs and the dau. however, in the described set-up it can hardly be avoided. 6. conclusion and outlook conventional analogue accelerometer calibration systems are not capable of calibrating modern digital output sensors. however, an extension providing the necessary digital interface and analogue input for a suitable reference signal can extend the capabilities to the magnitude and phase calibration of the complex frequency response of such sensors. the typically independent internal sampling of the dos poses a special challenge in the calibration as well as a new source of measurement uncertainty and a new specification that has to become part of the calibration routine and the calibration result. with two different approaches, it is possible to calibrate devices that deliver separate samples as well as devices that buffer a series of samples after a single trigger and deliver the whole series at once. provided that the calibration laboratory is equipped with a precise and stable timestamp source, like a gpsaligned clock, simple procedures lead to detailed insights concerning the internal timing of the dos and their dependencies on environmental conditions. it turns out that simple classical assumptions about characteristics like the accuracy and jitter of the internal sampling units can be misleading. however, the application of external timestamping, as demonstrated, gives a clear and traceable quantification of these properties. within the european project titled met4fof, a cross validation in terms of an interlaboratory comparison between different participants is planned. in order to address the question of consistency of the results, a further evaluation of additional measurement uncertainty components involved with the use of the dos will be included. an existing calibration system will be equipped with a gps based absolute time base so that the phase of the transfer function can be determined without a reference figure 8. distribution of sample interval lengths of a bma280 dos determined by sample timestamping. counts are in logarithmic scale. figure 9. sample interval length over time plot for the bma280 sensor. the lower plot is a zoom-in on the upper overview. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 184 signal, but by means of the absolute time. by comparison with this system, the solution presented here will be evaluated with respect to uncertainty. the intrinsic timing properties, in particular, add a new dimension to measurement uncertainty estimation. in addition, most mems accelerometers on the market are multi-component devices. the question of dynamic crosstalk may therefore add another complication, where the static (dc) crosstalk has already been studied in other publications such as [7], [8]. acknowledgement this project 17ind12 met4fof has received funding from the empir programme co-financed by the participating states and from the european union’s horizon 2020 research and innovation programme references [1] n. helwig, e. pignanelli, a. schütze, condition monitoring of a complex hydraulic system using multivariate statistics, proc. i2mtc-2015 2015 ieee international instrumentation and measurement technology conference, paper pps1-39, pisa, italy, may 11-14 (2015), https://ieeexplore.ieee.org/document/7151267 [2] b. seeger, th. bruns s. eichstädt, methods for dynamic calibration and augmentation of digital acceleration mems sensors. 19th international congress of metrology, 22003, paris, france (2019), https://cfmetrologie.edpsciences.org/articles/metrology/pdf/20 19/01/metrology_cim2019_22003.pdf [3] s. eichstädt, m. gruber, a.p. vedurmudi, b. seeger, th. bruns, g. kok, toward smart traceability for digital sensors and the industrial internet of things. sensors 21(6) (2021), doi: 10.3390/s21062019 [4] guideline dkd-r 3-2: calibration of conditioning amplifiers for dynamic application, physikalisch-technische bundesanstalt, dkd, braunschweig, gemany (2019), doi: 10.7795/550.20190425en [5] g. d'emilia, a. gaspari, f. mazzoleni, e. natale, a. schiavi, calibration of tri-axial mems accelerometers in the lowfrequency range – part 1: comparison among methods j. sens. sens. syst., 7, 245–257, 2018, doi: 10.5194/jsss-7-245-2018 [6] ieee 1057-2017 ieee standard for digitizing waveform recorders, standard, institute of electrical and electronics engineers, new york, ny, usa (2018). online [accessed 8 september 2021] https://standards.ieee.org/standard/1057-2017.html [7] w.-s. cheung, effects of the sample clock of digital-output mems accelerometers on vibration amplitude and phase measurements, metrologia, 57 (1) (2020), doi: 10.1088/1681-7575/ab5505 [8] iso 16063-11:1999, methods for the calibration of vibration and shock transducers – part 11: primary vibration calibration by laser interferometry, international organization for standardization, geneva, ch (1999). [9] a. prato, f. mazzoleni, a. schiavi, traceability of digital 3-axis mems accelerometer: simultaneous determination of main and transverse sensitivities in the frequency domain, metrologia, 57 (3) (2020) p. 035013. doi: 10.1088/1681-7575/ab79be [10] m. y. a. michael gaitan, j. geist, analysis and protocol for characterizing intrinsic properties of three-axis mems accelerometers using a gimbal rotated in the gravitational field, proc. of the imeko 23rd tc3, 13th tc5 and 4th tc22 international conference, helsinki, finland, 30 may 1 june 2017, 3 pp. online [accessed 8 september 2021] https://www.imeko.org/publications/tc22-2017/imekotc22-2017-015.pdf [11] software for the met4fof smartup unit v2. online [accessed 8 september 2021] https://github.com/met4fof/met4fof-smartupunit [12] pcb layout of the met4fof interface board. online [accessed 8 september 2021] https://circuitmaker.com/projects/details/benedikt-seeger2/met4fof-interface-board [13] b. seeger, l. klaus d. nordmann, dynamic calibration of digital angular rate sensors, acta imeko 9 (2020) 5, pp. 394-400. doi: 10.21014/acta_imeko.v9i5.1008 https://ieeexplore.ieee.org/document/7151267 https://cfmetrologie.edpsciences.org/articles/metrology/pdf/2019/01/metrology_cim2019_22003.pdf https://cfmetrologie.edpsciences.org/articles/metrology/pdf/2019/01/metrology_cim2019_22003.pdf https://doi.org/10.3390/s21062019 https://doi.org/10.7795/550.20190425en https://doi.org/10.5194/jsss-7-245-2018 https://standards.ieee.org/standard/1057-2017.html https://doi.org/10.1088/1681-7575/ab5505 https://doi.org/10.1088/1681-7575/ab79be https://www.imeko.org/publications/tc22-2017/imeko-tc22-2017-015.pdf https://www.imeko.org/publications/tc22-2017/imeko-tc22-2017-015.pdf https://github.com/met4fof/met4fof-smartupunit https://circuitmaker.com/projects/details/benedikt-seeger-2/met4fof-interface-board https://circuitmaker.com/projects/details/benedikt-seeger-2/met4fof-interface-board http://dx.doi.org/10.21014/acta_imeko.v9i5.1008 microsoft word article 11 116-485-1-galley finale.doc acta imeko december 2013, volume 2, number 2, 61 – 66 www.imeko.org acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 61 digital demodulator unit of laser vibrometer standard for in situ measurement akihiro oota1, hideaki nozato1, wataru kokuyama1, yoshinori kobayashi2, osamu takano2, naoki kasai2 1 national institute of advanced industrial science and technology, central 3, 1-1-1 umezono, tsukuba, japan 2 neoark corporation, 2062-21 nakanomachi, hachioji, tokyo, japan section: research paper keywords: laser vibrometer; analog demodulator; digital demodulator; iso 16063-41 citation: akihiro oota, hideaki nozato, wataru kokuyama, yoshinori kobayashi, osamu takano, naoki kasai, digital demodulator unit of laser vibrometer standard for in situ measurement, acta imeko, vol. 2, no. 2, article 11, december 2013, identifier: imeko-acta-02 (2013)-02-11 editor: paolo carbone, university of perugia received february 18th, 2013; in final form september 23th, 2013; published december 2013 copyright: © 2013 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: this work was financially supported by meti (ministry of economy, trade and industry), japan corresponding author: akihiro oota, e-mail: a-oota@aist.go.jp 1. introduction laser vibrometers have been frequently used for precise vibration measurements to ensure the safety of commercial products in industry. the reliability and accuracy of such measurements should be guaranteed by a traceability system, which is a series of calibrations meeting national or international metrology standards regarding the use of laser vibrometers. to establish such a traceability system for laser vibrometers, a novel calibration method has been published as a new international standard (iso 16063-41 [1]) for laser vibrometers. some demonstrations have been attempted in accordance with this standard [2-5]. laser vibrometers consist of two key components, a laser optics unit and a demodulator unit. a heterodyne laser interferometer or homodyne laser interferometer can be applied with a laser optics unit. the interferometry signal detected by a laser optics unit is transformed into an acceleration, velocity, or displacement signal by the demodulator unit. two kinds of demodulator unit, analog and digital types, can be applied to realize this transformation. for many years, many analog demodulator units have been used to transform interferometry signals into velocity signals. these units enable the direct conversion from doppler signals to velocity signals, which can be continuously obtained over a wide velocity or frequency range. such a laser vibrometer based on doppler signals is referred to as a laser doppler vibrometer. in an analog demodulator unit, the characteristics of the electric components such as the frequency-to-voltage converter, lowpass filter, and scaling amplifier strongly affect the measurement accuracy [6]. on the other hand, the digital demodulator unit applies a new signal processing method that does not depend on the characteristics of analog components. because of the wide bandwidth of the interferometry signals, the only analog device required is an analog-to-digital converter (adc) with high speed and high resolution. in the digital demodulator unit, the measurement accuracy depends mainly on the characteristics of the adc and the calculation algorithm. therefore, the digital demodulator unit would enable users to easily achieve high measurement accuracy, in contrast with that obtained with the analog demodulator unit. however, abstract to establish an easy-to-use traceability system for laser vibrometers in vibration acceleration standard, a highly accurate reference standard is required. in this study, we investigated the measurement reliability of a commercial laser vibrometer with an analog demodulator. the results confirmed a large measurement deviation from nominal sensitivity values due to characteristics of the analog demodulator. to reduce such deviation, a high-accuracy digital demodulator, which can be utilized as a reference standard, was developed. the experimental results confirmed the improved measurement accuracy of the developed digital demodulator. acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 62 digital demodulator units cannot always provide continuous measurements over the wide velocity or frequency range achieved by analog demodulator units because of technical barriers such as the limited number of samples available for digital processing and the limited sampling rate [6]. in this study, characteristics of various types of commercial laser vibrometer were investigated, and we clarify the disadvantages of the analog demodulator unit used. in addition, a novel digital demodulator unit, which enables us to continuously provide output data for in situ measurement, was developed to realize high measurement reliability, and we confirmed its capability experimentally. 2. characteristics of commercial laser vibrometer with analog demodulator unit 2.1. characteristics of three commercial laser vibrometers to confirm the potential of commercial laser vibrometers, we performed the calibration of a laser vibrometer and its demodulator unit. three different kinds of commercial laser vibrometers with analog demodulators were applied to the calibration, which are in this work denominated lv a, lv b and lv c. these laser vibrometers are made by three manufacturers in japan. the primary calibration of the laser vibrometer using practical vibrations was performed in accordance with iso 16063-41 (fringe counting method and sine approximation method). figure 1 shows the experimental setup used for the primary calibration of the laser vibrometer. the laser interferometry system used as a reference standard is the homodyne laser interferometry system included in the japanese national standard for primary calibration of accelerometers. in this setup, the fringe counting method was applied for the frequency range 20 hz – 80 hz, and the sine approximation method was applied for the frequency range 100 hz – 5 khz. in addition, their demodulator units are electrically calibrated with simulated frequency modulated signals, which are equivalent to output signals obtained from the laser optics unit in the calibration in accordance with iso 16063-41. figure 2 shows the experimental setup used for the demodulator unit calibration. a radio frequency (rf) signal generator was applied to generate the simulated frequency-modulated signal. an r.m.s. voltmeter (or digitizer) measures the output voltage of the demodulator unit. the rf signal generator and r.m.s. voltmeter can be calibrated through a traceability system of the time standard and voltage standard, respectively. figure 3 shows primary calibration results of the laser vibrometers lv-a, lv-b and lv-c, and also the electrical calibration results of their respective demodulator units. these demodulators were set for a nominal sensitivity of 10 (mm/s)/v. the calibration results of the demodulator units show very similar characteristics to the laser vibrometer calibration results. figure 4 shows the corrected primary calibration results based on the demodulator unit calibration results. although the results from two different calibration methods had a large deviation of more than 1.0 % from nominal sensitivities as shown in figure 3, a small deviation of less than 0.5 % was figure 1. experimental setup of primary calibration of a laser vibrometer. rf signal generator analogue demodulator r.m.s. voltmeter rf in out rf signal generator analogue demodulator r.m.s. voltmeter rf in out figure 2. experimental setup of demodulator unit calibration. acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 63 almost achieved by performing a correction based on their demodulator unit calibration results. the small deviation after correction in figure 4 may normally be due to the effect of the optics unit of the laser vibrometer. however, all results after correction would be expected to be similar to each other. therefore, the deviation after correction would be due more to the systematic error from common components in the demodulator unit calibration, such as the simulated frequency-modulated signal from the radio frequency signal generator, and not so much by the effect of the optics unit as by the systematic error from the common component in the demodulator unit calibration. 2.2. linearity and frequency dependency of the laser vibrometer the experiments mentioned above were carried out for a specific frequency series as accelerometer calibration. however, as the basic principle of a laser vibrometer, the frequency shift of light scattered from a moving surface by the doppler-effect is proportional to the velocity of the moving surface. therefore, the nonlinearity of the velocity amplitude may be more significant than the frequency characteristics. in this study, both typical characteristics were evaluated in detail for the lv-c. the setting of the nominal sensitivity value was fixed during the evaluation. figure 5 shows the relative deviation from nominal sensitivity obtained while the velocity amplitude is adjusted from 3 mm/s to 100 mm/s at a calibration frequency of 160 hz. nonlinearity higher than 5 % was observed in this case. figure 6 shows the frequency characteristic of lv-c and its demodulator c at constant velocity amplitude of 10 mm/s. the relative deviation from nominal sensitivity changes approximately 0.7 % between 20 hz and 5 khz. as the sensitivity fluctuation due to the velocity amplitude is much larger than that due to the frequency, the linearity for the velocity amplitude may be as important for in situ measurements as the sensitivity of the laser vibrometer. the evaluation results of the demodulator unit are very similar to the evaluation results of the laser vibrometer. as a result, we can conclude that the drastic fluctuation of the laser vibrometer sensitivity would be mainly due to the analog demodulator unit. -6 -4 -2 0 2 4 6 10 100 1000 1000 d ev ia ti on fr om n om in al s en si ti vi ty [ % ]' frequency [hz] demodulator a lv a demodulator b lv b demodulator c lv c figure 3. comparison between calibrations of commercial laser vibrometers (lv) and electrical calibration of its demodulator unit calibration. the lvs of three different manufacturers were set at a nominal sensitivity of 10 (mm/s)/v. a, b and c: different manufacturers. lv: laser vibrometer. -6 -4 -2 0 2 4 6 10 100 1000 10000 d ev ia tio n fr om n om in al s en si tiv ity [% ]' frequency [hz] lv a after correction lv b after correction lv c after correction figure 4. correction results based on demodulator unit calibration. -5 -4 -3 -2 -1 0 1 2 1 10 100 d ev ia tio n fr om n om in al s en si tiv ity [% ] velocity amplitude [mm/sec] lv c demodulator c > 5 % figure 5. typical nonlinearity for velocity amplitude at 160 hz for a nominal sensitivity of 10 (mm/s)/v. -5.5 -5.3 -5.1 -4.9 -4.7 -4.5 -4.3 -4.1 -3.9 -3.7 -3.5 10 100 1000 1000 d ev ia tio n fr om n om in al s en si tiv ity [% ] ' frequency [hz] lv c demodulator c figure 6. typical frequency characteristic for nominal sensitivity of 10 (mm/s)/v (velocity amplitude of sinusoidal vibration given by vibration exciter is fixed at 10 mm/s). acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 64 consequently, to ensure the measurement reliability of the laser vibrometer in measurement ranges not covered by the primary calibration of the laser vibrometer in accordance with iso 16063-41, the appropriate electrical calibration for analog demodulator units would be very useful to extrapolate the sensitivity. however, if a more precise vibration measurement is required, the measurement accuracy of the demodulator unit should be improved. 3. development of the digital demodulator unit 3.1. preliminary comparison between the digital demodulator unit and the analog demodulator unit in iso 16063-41, the digital signal processing for the decoding of the doppler signal is required in the definition of laser vibrometer standards. therefore, to develop a higher accuracy demodulator unit, the potential of digital demodulation should be investigated by comparing it with analog demodulation. this comparison was carried out using electrical calibration and primary calibration explained above in section 2. in this experiment, lv-c was used as the laser vibrometer with the analog demodulator unit. the optics part of lv-c can be detached from the analog demodulator unit because it was specially designed for this purpose. on the other hand, to conveniently achieve digital demodulation, a commercial signal analyzer (anritsu ms2690a) was applied. by using this instrument, i & q signals (in-phase and quadrature-phase signal pair) can be easily obtained, and the decoding of the i & q signals is then performed by our proprietary software using labview. the signal analyzer has an adc resolution of 16 bits, a maximum sampling rate of 50 mhz, and an rf downconversion function. in this experiment, the signal analyzer and rf signal generator used to generate the simulated fm signal use a common sampling clock signal for their own processing. to implement a laser vibrometer with a digital demodulator unit, the optics part of lv-c was connected to the signal analyzer. figure 7 shows the evaluation results obtained from both the primary calibration and electrical calibration. it compares the primary calibration results obtained for lv-c with the analog demodulator c, with the analog demodulator c after correction, with the digital demodulator, and the electrical calibration result of only the digital demodulator unit. as a result, the potential of the digital demodulator unit to achieve higher measurement accuracy is confirmed. 3.2. development of the digital demodulator unit for in situ measurements based on the results shown in section 3.1, the digital demodulator unit with higher accuracy for laser vibrometer is now discussed. although the application of the signal analyzer led to good results in the preliminary experiment, the signal analyzer would be unsuitable as it cannot continuously output data in real time, as required for in situ measurements. therefore, a dedicated digital demodulator unit, which has a smaller deviation than commercial analog demodulator units, and which generates continuous output in real time, was designed for in situ measurements. the developed digital demodulator unit is constructed with an adc, digital to analog converter (dac), and fieldprogrammable gate array (fpga) at low cost. a he-ne laser is used as the laser source in almost all commercial laser vibrometers in japan. the driving frequency of its acousto optic modulator (bragg cell) in the laser optics unit is fixed at 80 mhz. the maximum frequency shift due to the doppler effect, which is equivalent to the maximum measurable velocity, is set to 10 mhz to cover the velocity range below 3 m/s. thus the maximum frequency of the signal to be detected by the adc is equivalent to 90 mhz. to satisfy the nyquist sampling theorem, a sampling rate of more than 200 mhz is required as the maximum sampling rate of the adc at least. to implement the measurement with higher accuracy in this study, the adc is selected to have two channel inputs, a high sampling rate of 500 ms/s, and a high resolution of 12 bits. the dac has a high sampling rate of 250 ms/s and a high resolution of 16 bits. high-speed signal processing is required to obtain the demodulation signal in as close to real time as possible. fpga can process data faster than dsp. therefore, an fpga was applied to achieve high-speed signal processing for demodulation at low cost. the data processing including the digital demodulation algorithm embedded in the fpga was developed to satisfy the requirement for the laser vibrometer standard described in iso 16063-41. additionally, adc, dac and fpga are driven with common clock signal. the specifications of the adc, dac and fpga are given to enable continuous measurements over a wide frequency range of up to several hundred khz. this digital demodulator unit can receive a carrier frequency signal of either 80 mhz or 10 mhz, which will enable it to immediately process the frequency signal after down conversion. in addition, the higher accuracy measurement can be achieved by applying a highaccuracy 10 mhz external clock to this demodulator unit. figure 8 shows a photograph of the developed digital demodulator unit. the developed digital demodulator unit was evaluated using the same experimental setup in the previous section under the same experimental conditions as the analog demodulator unit. figure 9 and figure 10 show typical evaluation results obtained. experimental results showed that the relative deviation is confirmed to be below 0.3% for three frequency series in the applicable measurement range, and the time delay of the output due to the signal processing is evaluated to be about 30 μs. these results indicate that the developed digital demodulator -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 10 100 1000 1000 r el at iv e de vi at io n fr om n om in al s en si tiv ity [ % ]' frequency [hz] digital demodulator unit lv c with analog demodulator unit lv c after correction lv with digital demodulator unit figure 7. preliminary comparison of digital demodulation with analog demodulation. acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 65 unit has a sufficiently high measurement accuracy to be applicable as part of a laser vibrometer standard. 3.3. practical shock measurement using a laser vibrometer with the developed digital demodulator unit the prototype of the developed digital demodulator unit must be combined with the laser optics unit to work as a laser vibrometer for in situ measurements. therefore, the performance of the laser vibrometer, which consists of the developed digital demodulator unit and a commercial laser optics unit, was demonstrated using a shock acceleration exciter. to generate a rectilinear motion for sensing, a shock acceleration exciter was used for primary shock acceleration calibration as a national standard in japan [7]. to validate the capability for sensing, in this experiment, a homodyne laser interferometer in a shock acceleration calibration system was used as the reference standard. figure 11 shows typical experimental results. the peak -1 0 1 2 3 4 5 6 7 8 9 0.01 0.1 1 10 d ev ia tio n fr om n om in al s en si tiv ity [% ] velocity amplitude [m/sec] dd at 1000hz dd at 160hz dd at 5000hz ad at 1000hz ad at 160hz ad at 5000hz applicable measurement range figure 9. typical evaluation results of the relative sensitivity deviation for the developed digital demodulator unit. dd: digital demodulator unit. ad: analog demodulator unit. -0.25 -0.2 -0.15 -0.1 -0.05 0 0.05 0 2000 4000 6000 8000 10000 ph as e sh if t [ ra d] frequency [hz] dd ad figure 10. typical evaluation results of the phase shift for the developed digital demodulator unit (phase shift). dd: digital demodulator unit. ad: analog demodulator unit. (a) overview of the developed digital demodulator unit (b) internal view of the developed digital demodulator unit figure 8. digital demodulator unit for the laser vibrometer developed in this study. -0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 -500 0 500 1000 1500 2000 2500 3000 0.0036 0.0038 0.004 0.0042 0.0044 0.0046 v el oc ity [m /s ec ] a cc el er at io n [m /s 2 ] time [sec] acceleration sensed by ref. std. velocity sensed by ref. std. velocity sensed by dd figure 11. comparison of typical measurement results of the laser vibrometer with prototype of the developed digital demodulator unit and shock acceleration standard with homodyne laser interferometer in nmij. dd: laser vibrometer with prototype of the developed digital demodulator unit. ref.std.: homodyne laser interferometer for the shock acceleration standard in nmij. acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 66 acceleration sensed by the reference standard was about 1700 m/s2. from a comparison of both velocity waveforms, a slight time delay was observed in the laser vibrometer with the developed digital demodulator unit. this result is in agreement with the result of the phase shift obtained in figure 10. in addition, the waveform that was obtained indicates unstable behavior after the application of shock acceleration. this instability may be due to the nonlinearity of the sensitivity to small velocity changes, and is a future technical issue to be investigated. 4. conclusions the characteristics of a laser vibrometer with an analog demodulator unit were investigated using three different commercial laser vibrometers. the calibration results of these demodulator units show very similar characteristics to those of the laser vibrometer calibration. most of the deviation from the nominal sensitivity is due to the characteristics of the demodulator unit used. the measurement accuracy of the laser vibrometer with a digital demodulator unit was compared with that of a laser vibrometer with an analog demodulator unit using the same laser optics unit. the results confirmed the higher measurement accuracy of the digital demodulator unit. to overcome the difficulty of realizing a continuous signal processing for in situ measurement, the digital demodulator unit was developed using an fpga, an adc with a high sampling rate, and a dac with a high sampling rate. the excellent potential of the developed digital demodulator unit was experimentally confirmed. on the other hand, we observed unstable behavior on the application of shock acceleration. this instability should be addressed to ensure greater usefulness for a laser vibrometer standard. acknowledgement this work was performed by aist, and is a research project aimed at supporting small and medium companies. we acknowledge the financial support provided by the meti (ministry of economy, trade and industry). we deeply appreciate the kind cooperation of the two main japanese manufacturers of laser vibrometers. we also wish to thank dr. h-j.von martens, who drafted iso 16063-41, for the useful discussions. references [1] iso 16063-41: methods for the calibration of vibration and shock transducers – part 41: calibration of laser vibrometers, international organization for standardization (2011). [2] h.-j. von martens, t. bruns, a. taubüner, w. wabinski, u. göbel, recent progress in accurate vibration measurements by laser techniques, 7th international conference on vibration measurements by laser techniques: advances and applications, ancona, italy, 2006, vol. 6345, pp. 634501-1-23. [3] a. oota, t. usuda, t. ishigami, h. nozato, h. aoyama, and s. sato, preliminary implementation of primary calibration system for laser vibrometer, 7th international conference on vibration measurements by laser techniques: advances and applications, ancona, italy, 2006, vol. 6345, pp. 634503-1-8. [4] u. buehn, and h. nicklich, “calibration of laser vibrometer standards according to the standardization project iso 1606341”, 7th international conference on vibration measurements by laser techniques: advances and applications, ancona, italy, 2006, vol. 6345, pp.63451f-1-9. [5] g. p. ripper, g. a. garcia, and r. s. dias, the development of a new primary calibration system for laser vibrometer at inmetro, imeko 20th tc-3, 3rd tc-16 and 1st tc-22 international conference, mérida, mexico, 2007, paper id-105. [6] m. bauer, f. ritter, and g. siegmund, high-precision laser vibrometers based on digital doppler-signal processing, 5th international conference on vibration measurements by laser techniques: advances and applications, 2002, vol. 4827 pp. 50-61. [7] h. nozato, t. usuda, a. oota, t. ishigami, calibration of vibration pick-ups with laser interferometry part iv: development of shock acceleration exciter and calibration system, measurement science and technology, vol. 21, no. 6, (2010), paper id 065107, pp. 1-10. first considerations on post processing kinematic gnss data during a geophysical oceanographic cruise acta imeko issn: 2221-870x december 2021, volume 10, number 4, 10 16 acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 10 first considerations on post processing kinematic gnss data during a geophysical oceanographic cruise valerio baiocchi1, alessandro bosman2, gino dardanelli3, francesca giannone4 1 dipartimento ingegneria civile, edile ed ambientale(dicea), sapienza università di roma, via eudossiana 18, i00184, italy 2 istituto di geologia ambientale e geoigegneria, consiglio nazionale delle ricerche (cnrigag), rome, italy 3 department of civil, environmental, aerospace, materials engineering (dicam), university of palermo, italy 4 department of engineering, niccolò cusano university, via don carlo gnocchi 3, rome, i 00166, italy section: research paper keywords: gnss; bathymetry survey; rtk-lib; geoid; tyrrhenian sea citation: valerio baiocchi, alessandro bosman, gino dardanelli, francesca giannone, first considerations on post processing kinematic gnss data during a geophysical oceanographic cruise, acta imeko, vol. 10, no. 4, article 6, december 2021, identifier: imeko-acta-10 (2021)-04-06 section editor: silvio del pizzo, university of naples 'parhenope', italy received june 1, 2021; in final form december 6, 2021; published december 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was supported by italian national research council (cnr) corresponding author: valerio baiocchi, e-mail: valerio.baiocchi@uniroma1.it 1. introduction the problem of calculating the connection between on-shore and off-shore heights is still very open and debated. in fact, even though they are all heights, they are referred to reference systems with non-equivalent definitions, so conceptually and numerically very different, and this often makes the connection between the two types of heights complex [1]. the ground altimetric reference system is generally based on the definition of a specific equipotential surface of the gravitational field, identified by the mean level of a reference tide gauge. this method has been used in italy both historically [2] and currently [3] and various national terrestrial altimetric systems have been defined, for this reason the possibility of unifying them at a regional [4] and global level [5] is currently subject of research. gnss systems have made it possible for the first time to obtain highly accurate planoaltimetric measurements (and therefore also elevation measurements) both on land and at sea; however, these measurements do not refer to a physical reality but to a mathematical surface, the ellipsoid of rotation. on land, ellipsoid elevations are transformed into orthometric elevations using local geoid models [6], which are generally more accurate, or global models, which are generally less accurate [7]. the problem of elevations at sea is even more complex because the sea surface is not an equipotential surface, which is "... characterized by uniform temperature and density and free of perturbations related to currents, winds and tides." [8] unfortunately, these are ideal conditions that are never found in nature, and for this reason the materialisation of altimetric reference systems at sea is profoundly different from terrestrial altimetric reference systems [9]. it is important to underline that the interest in this case is mainly in seafloor depth in order to construct high resolution digital elevation model (dems) for seafloor, habitat abstract differential gnss positioning on vessels is of considerable interest in various fields of application as navigation aids, precision positioning for geophysical surveys or sampling purposes especially when high resolution bathymetric surveys are conducted. however ship positioning must be considered a kinematic survey with all the associated problems. the possibility of using high-precision differential gnss receivers in navigation is of increasing interest, also due to the very recent availability of low-cost differential receivers that may soon replace classic navigation ones based on the less accurate point positioning technique. the availability of greater plano-altimetric accuracy, however, requires an increasingly better understanding of planimetric and altimetric reference systems. in particular, the results allow preliminary considerations on the congruence between terrestrial reference systems (which the gnss survey can easily refer to) and marine reference systems (connected to national tidegauge network). in spite of the fluctuations due to the physiological continuous variation of the ship's attitude, gnss plot faithfully followed the trend of the tidal variations and highlighted the shifts between gnss plot and the tide gauges due to the different materialization of the relative reference systems. mailto:valerio.baiocchi@uniroma1.it acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 11 mapping and bathymetric cartographies which are mainly used for navigational, safety and scientific bathymetric surveys [10][12]. the elevations at sea are often referred to a conventional local zero identified with a local tide gauge, conventionally the tide gauge's "zero" has no connection with the "zero" of the national altimetric system; bathymetries are referred to the tide gauges reference system for prudential reasons related to navigation and nautical charts [9]. therefore, there is no congruence between the zero of the tide gauges and the national altimetric system, not even between the various tide gauges at a given time. it must also be taken into account that the connection between the national altimetric system and the elevations on the islands cannot be made by precision geometric levelling but it is often made by trigonometric levelling or gnss levelling corrected later with the same geoid models [13]. the interest in the correct relationship between terrestrial and tidal altimetry systems is constantly increasing both for the growing interest in the automatic extraction of coastlines [14]-[16] and for the very recent availability of low cost gnss receivers that can acquire in differential mode, allowing centimetric and potentially also millimetric [17] planimetric accuracy even at sea. in this paper, the first results of a post processing kinematic (ppk) survey performed during the “thygraf – tyrrhenian gravity flow” oceanographic campaign in the southern tyrrhenian sea conducted on board of the urania r/v (research vessel) are reported. the aim was to make an initial comparison between the altitudes acquired by the gnss device and corrected with a geoid model and those recorded simultaneously by the tide gauges present in the area. in the paragraph "materials and methods" the instruments used and the measurements carried out will be illustrated, in the paragraph "gnss data processing" the processing carried out and the different strategies used will be reported and finally, in the paragraph "results and discussion" the results obtained will be compared with the tidal data and the conclusions will be illustrated. 2. materials and methods during the thygraf oceanographic survey conducted on board of the urania r/v from 12th to 19th of february 2013, the geodetic team of the scientific crew was engaged in experimenting and validating some innovative techniques of satellite survey in navigation. for this purpose, the geodetic class gps-gnss receiver topcon legacy-e was used and the antenna (topcon pga) was installed on the top of the ship itself, thanks to the collaboration of the ship's personnel (in figure 1 square antenna to the right of the main mast). before departure, a survey was carried out using a total station with the aim to measure the antenna height respect to the waterline. the result was 18.132 m with respect to the bottom of antenna mount. the installation height was necessary to avoid cycle slip and electromagnetic disturbances from the ship's machinery, but it certainly amplified the variability of the three-dimensional coordinates recorded due to the continuous and physiological variation of the ship's attitude. so, the antenna was installed on one of the highest points of the ship where the only obstruction was the highest part of the mast. the antenna was in basic configuration without multipath limiting devices because these could have increased the effect of the wind, so for this first experiment it was decided not to use them. no extensive analysis of antenna effects was carried out in this first experiment. the data surveyed (acquired in double frequency with sampling interval of one second) have subsequently been postprocessed with respect to the permanent stations of the new national dynamic network, these reference measurements have been made available by various agencies (university of palermo, calabria region etc.). the survey and the subsequent postprocessing allowed to evaluate the three-dimensional position of the antenna itself, statically fixed to the body of the ship throughout the oceanographic survey, with centimetre accuracy. it is important to underline that the antenna was obviously affected by all the displacements and attitude variations (heave, roll, pitch and yaw) that affected the ship during its navigation. this series of measurements allowed us to evaluate their variations with great accuracy. the gnss survey allowed the altimetric comparison between the data provided by the tidal networks (referred to a conventional local zero with respect to a local tide gauge) and the ones observed in navigation (ellipsoidal heigh that must be corrected with geoid model italgeo05). this analysis highlights a fundamental aspect linked to the compatibility between different altimetric reference systems, in order to allow a connection between on-shore and off-shore heights. 3. gnss data processing the gnss receiver positioned on the urania r/v (figure 1) acquired dual frequency data with 1s sampling interval from 14th to 18th of february 2013; during the research survey the marine weather conditions were optimal. the permanent station of tropea (table 1) belonging to the permanent network of calabria region [18] was selected for data post-processing, hourly rinex files with a sampling interval of 1 s were downloaded. figure 1. gnss antenna and its installation on the urania ship. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 12 3.1. rtklib the first step of the data processing was performed with rtklib ver. 2.4.2, open-source package for gnss positioning developed by tomoji takasu in 2007 [20], [21]. rtklib can process data from different satellite systems (gps, glonass, galileo, qzss and beidou) considering approaches both in realtime and post-processing and various position modes: single, dgps/dgnss, kinematic, static, moving-baseline, fixed, ppp-kinematic, ppp-static and ppp-fixed. unfortunately, rtklib provides a graphical interface where it is possible to upload only two rinex files: one for the rover and a second for the base station; furthermore, the software does not manage the raw format of the topcon receiver (figure 2). then, to facilitate the processing operations the data were managed with the open-source package teqc by unavco [22]. teqc is a toolkit with three main functions: translation, editing, and quality checking, from which it gets its name. for our purposes, the two functions of translation and editing were exploited: the first function allows to convert (translate) gnss raw receiver files into rinex format (observation and navigational files); the second one allows to cut or splice rinex files. the data, in tps format, acquired by the receiver positioned on the urania r/v were then converted to rinex and reorganised into daily files with the teqc software. in addition, the data from the trop permanent station, already in rinex format, were edited with teqc to create a single daily file. the output files from the teqc software were imported into rtklib, together with the precise ephemerides released by crustal dynamics data information system (cddis) [4] and processed in kinematic mode. for the doy046 (acquisition period from 15/02/13 00:00:00 to 15/02/13 23:59:59) and doy047 (acquisition period from 16/02/13 00:00:00 to 16/02/13 23:59:59), the number of fixed/float solution (table 2) and the estimated standard deviations (table 3) were reported. unfortunately, the trop permanent station acquisitions for the period 19:00:00 – 19:59:59 was not available, then rtklib software could only process the solution in single position (table 2). as a consequence, the coordinates related to these 2412 observations were not considered in the results (table 3) and in all figures related to doy047 because of very low and not reliable solution precision. the positions measured in kinematic mode, obtained processing the gnss data acquired during the two days of navigation, show a similar level of precision with a mean value of 0.005 m for the planimetric coordinates and 0.012 m for heights. the planimetric navigation paths of the ship is represented in gis software (qgis): on the first day (doy046) the path was mainly a round trip between the two islands of stromboli and lipari and therefore away from the mainland. on the second day (doy047) navigation started with a long “transfer” section from the islands to the mainland in a north-west -> south-east direction and then navigation along the coast and some grids in specific areas. in figure 3 the navigation paths doy046 (red track) and doy047 (yellow track) and the permanent gnss station trop location (red triangle) are represented. the daily trend of heights, despite high variability due to the vessel attitude, shows a periodic variation almost certainly due to the tidal effect (figure 4a). during the second day (figure 4b), a discontinuity is observed in the central hours of the day, probably table 1. coordinates and characteristics of the trop station as reported on the operator's website (reference system wgs84-etrs89; epsg: 4937 [19]). name station trop latitude 38°40' 45.6525" n longitude 15°53' 48.2067" e h in m 100.086 antenna type leiat504gg leis receiver leica grx1200ggpro figure 2. rtklib options for the ppk data processing. table 2. position estimated and their solutions. position estimated doy046 doy047 total 85563 85233 fixed solution 47573 (56 %) 39609 (46 %) float solution 37990 (56 %) 43212 (51 %) single solution 0 2412 (3 %) table 3. position estimated and their solutions. doy046 doy047 sdn (m) sde (m) sdu (m) sdn (m) sde (m) sdu (m) avg 0.006 0.005 0.012 0.005 0.005 0.012 max 0.543 0.407 0.988 0.637 0.486 1.065 min 0.003 0.003 0.008 0.003 0.002 0.008 figure 3. navigation paths on doy046 (red track) and doy047 (yellow track) and the permanent gnss station trop location (red triangle). acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 13 this trend is due to the well-known effects of local tidal disturbance observed in the messina strait. 3.2. topcon tools the results of the heights measured in kinematic mode obtained from the processing of gnss data in rtklib was repeated for verification with a commercial software, topcon tools. this verification was carried out because rtklib sometimes has small “bugs” [21] and because, according to some authors, topcon tools would show very accurate results, in some cases even comparable with those of scientific software. this is probably due to a complete configurability of the processing [23] the topcon tools package ver. 8.2.3 by topcon corporation was used for the kinematic measurements. the software allows the data processing from different devices such as total stations, digital levels and gnss receivers, and it was used in several technical-scientific applications [24], [25]. topcon tools uses the modified hopfield model for the tropospheric corrections [26]. the employed positioning mode was code-based differential (“code diff”), the time range and the cut-off angle were set to 15 seconds and 10 degrees, respectively. recently dardanelli et al. [27] showed that the hypothesis of a normal distribution is confirmed in most of the pairs and, specifically, the static vs. nrtk pair seems to achieve the best congruence, while involving the ppp approach, pairs obtained with csrs software achieve better congruence than those involving rtklib software. although the lowest congruencies seem characterizing the pairs involving rtklib, this result should not be considered a criticism on the performance of this well-known open access program, which undoubtedly is one among of the most useful gnss processing software available, given its very straightforward applicability, considering also that our analysis is limited to few hours of data. the results obtained from the topcon tools processing are different from those obtained with rtklib, but the general trend that seems to mainly reproduce the tidal effects is very similar (figure 5a and b) as well as the numerical results (table 4). 4. results and discussion the ellipsoidal heights processed with rtklib and topcon tools were then compared with the hydrometric levels of some a) b) figure 4. ellipsoidal heigh variations during doy046 (a in figure) and during doy047 (b in figure) table 4. positions estimated with both packages and their estimated standard deviations. doy046 doy047 sdn (m) sde (m) sdu (m) sdn (m) sde (m) sdu (m) avg 0.101 0.062 0.118 0.078 0.078 0.051 max 0.140 0.106 0.150 0.283 0.283 0.112 min 0.070 0.032 0.100 0.042 0.042 0.013 doy046 doy047 sdn (m) sde (m) sdu (m) sdn (m) sde (m) sdu (m) avg 0.006 0.005 0.012 0.005 0.005 0.012 max 0.543 0.407 0.988 0.637 0.486 1.065 min 0.003 0.003 0.008 0.003 0.002 0.008 a) b) figure 5. ellipsoidal height variations during doy046 (a in figure) and during doy047 (b in figure), comparison between rtklib (blue dots) and topcon tools (orange dots) results. in red is the 10-minute moving average from topcon tools and in grey that of rtklib. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 14 stations belonging to the rmn – national tidegauge network [28]. rmn stations close to the study area are: “ginostra”, “strombolicchio”, “reggio calabria” and “messina” (figure 6).unfortunately, the data from strombolicchio station are not available for the period under examination. we decided to compare the hydrometric level with the ellipsoidal height, only on the tide gauge station of ginostra and palinuro, while reggio calabria and messina were not selected because they showed a very different trend probably due to the well-known local effects near the messina strait (figure 7). at first, the variations measured by the tide gauges were compared only in terms of trend with the heights measured by the gnss receiver on board the ship. in order to filter the heights and limit the effect of the oscillations caused by navigation, a moving average over ten-minute periods was adopted (in the figure is the grey trend). as already observed, there is an apparent similar trend between the tide gauge and the averaged gnss altitudes but also a constant shift between them. for doy046 the trend agreement continues very well for the whole day while for doy047 there seems to be a noticeable discontinuity at a certain point. the causes may be various but probably the aforementioned tidal effects near the strait are the main cause (figure 8). for the reasons outlined above, it was decided to continue the analysis only on doy046, which seems more significant. it is important to remember that the orthometric correction operated with a geoid model brings the elevations to an equipotential surface of the gravitational field at a given point on the national territory (for italy, genoa) that in general does not figure 6. tidegauge stations available in the study area. figure 7. tide gauges and path of the navigation during doy046 and doy047. a) b) figure 8. tide gauges trend and mean heights from gnss receiver for doy046 (a) and doy047 (b), note the different origin of height of the two series. figure 9. orthometric heights for doy046 (a) converted using italgeo2005 geoid model. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 15 coincide with the sea level in another position of the national territory at a given time [29], [30]. moreover, there is a problem of connection of the various tide gauges to the national altimetric system and conventionally the "zero" of the tide gauge has no connection with the "zero" of the national altimetric system. the ellipsoidal elevations measured with the gnss receiver were converted into orthometric elevations (h in figure 9) with respect to the national altimetric system using the geoid model italgeo2005, which is accredited with 2.5 cm of accuracy [5]. however, it should be noted that the reported accuracy is estimated on land where geoid–ellipsoid separation values measured on altimetric benchmarks are used to improve the estimate of geoid–ellipsoid separation itself. in our case some of the survey points are close to the coast and therefore the estimate of the geoid–ellipsoid separation should be reliable, while for offshore points the estimate is certainly less reliable. moreover, the connection between the national and islands elevation systems cannot be made by precision geometric levelling but it is often made by trigonometric or gnss levelling corrected later with the same geoid models. the elevations were then converted from ellipsoidal to orthometric using the software "geotrasformer" [31] that applies the resampling algorithms of the gridded geoid–ellipsoid separation values provided by the italian military geographic institute (igmi), the official national geodetic agency that released the grids of the italgeo2005 model. to compare surveyed data with the tide gauges information it is necessary subtract estimated antenna elevation (section 2) from the orthometric values themselves (gnss measurements corrected with italgeo2005); it must be considered, however, that during navigation the ship can progressively but significantly change its height due to the discharge of waste water and fuel consumption, such variations can reach several centimetres during a campaign and their variation may not be constant, therefore an average value during the day can still give reliable information. it was decided to take an average of the orthometric heights for the entire day to reduce the effect of the tide, obtaining an average height of 18.276 m. (including the height of the pga2 antenna). considering the approximations mentioned above, a comparison was made between the heights obtained with gnss measurements corrected with italgeo2005 and antenna elevation (𝐻 in figure 10) and the heights reported at the same time by the two tide gauges considered significant: "ginostra" and "palinuro" (hydrostatic level in figure 10). the main trend of all three tracks (figure 10) follows the same tidal repetitions. the gap in height between ship and tidal gauge data is due to the different definition of reference altimetric system, but the tidal effect is still predominant. there is also a systematic shift between the heights of the two tide gauges, which may be due to local reasons of a different average sea level or, more likely, to a different materialisation during the installation of the tide gauge itself, but also to an imperfect connection between the tide gauges due to the difficulty of connecting the heights of an island (ginostra) to the national altimetric system on the mainland. 5. conclusions and further developments considering the fluctuations due to the physiological continuous variation of the ship's attitude, the gnss plot faithfully followed the trend of the tidal variations and highlighted the shifts between the gnss data and the tide gauges due to the different materialisation of the relative reference systems. in fact, even if the installation height amplified the variability due to the continuous variation of the ship's attitude, the average trend of the gnss plot showed a relative trend very similar to that of the neighbouring tide gauges considered significant. after the orthometric correction of the heights and the estimation of the antenna height, it was possible to compare the data also "in absolute" (without forgetting the different altimetric references) and this comparison showed a remarkable agreement between the heights measured by the gnss and the tide gauges, highlighting, at the same time, the effects of the different altimetric references. this experimentation highlighted the need to rethink or update marine altimeter datums especially in view of the possible imminent diffusion of low-cost differential gnss receivers in navigation. to study the possibility of highlighting any anomalies in operation, usually due to momentary dysfunctions of the satellite segment, the experimentation of the innovative approach "multiconstellation" will be tested. this approach, designed by the same research group, can facilitate the detection of these anomalies while highlighting the corrections to be made. if this is verified, it would be possible to make the positioning measurements made during navigation more "robust" (i.e. less prone to errors), significantly improving their reliability. this could also be verified by a comparison with the information provided by the ship's dgps-ins navigation system. acknowledgement this research was funded by the national research council (cnr) and carried out in the framework of the flagship project ritmare (ricerca italiana per il mare). the authors would like to special thank the technical and scientific crews of the thygraf mission for their support in the installation of the receiver and for their continuous support in carrying out the measurement operations and its recordings. references [1] e. alcaras, c. parente, a. vallario,. the importance of the coordinate transformation process in using heterogeneous data in coastal and marine geographic information system, journal of marine science and engineering. 8(9) (2020) 708. doi: 10.3390/jmse8090708 [2] a. mori, la cartografia ufficiale in italia e l’istituto geografico militare nel cinquantenario dell’istituto geografico militare figure 10. orthometric heights for doy046 (a) comparison with the heights reported at the same time by the two tide gauges considered significant: "ginostra" and "palinuro". https://doi.org/10.3390/jmse8090708 acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 16 (1872-1922); istituto geografico militare, stabilimento poligrafico per l’amministrazione della guerra: roma, italy, 1922, 425 pp. [3] l. surace, i sistemi di riferimento geotopocatografici in italia. bollettino di geodesia e scienze affini 57(1996) pp. 181–234, in italian. 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http://www.assoporti.it/sites/www.assoporti.it/files/eventiesterni/pierozzi.pdf estimate the useful life for a heating, ventilation, and air conditioning system on a high-speed train using failure models acta imeko issn: 2221-870x september 2021, volume 10, number 3, 100 107 acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 100 estimate the useful life for a heating, ventilation, and air conditioning system on a high-speed train using failure models marcantonio catelani1, lorenzo ciani1, giulia guidi1, gabriele patrizi1, diego galar2 1 department of information engineering, university of florence via di s. marta 3, 50139, florence (italy) 2 luleå university of technology, lulea, sweden section: research paper keywords: reliability; diagnostic; railway engineering; failure rate; hvac; useful life citation: marcantonio catelani, lorenzo ciani, giulia guidi, gabriele patrizi, diego galar, estimate the useful life for a heating, ventilation, and air conditioning system on a high-speed train using failure models, acta imeko, vol. 10, no. 3, article 10, september 2021, identifier: imeko-acta-10 (2021)-0310 section editor: lorenzo ciani, university of florence, italy received january 29, 2021; in final form august 2, 2021; published september 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: giulia guidi, e-mail: giulia.guidi@unifi.it 1. introduction all devices are constituted from materials that will tend to degrade with time. the materials degradation will continue until some critical device parameter can no longer meet the required specification for proper device functionality [1]-[8]. for this reason, as well as the growing complexity of equipment and the rapidly increasing cost incurred by loss of operation and for maintenance, the interest in reliability is growing in many industrial fields. generally reliability could be assessed through different methods, such as reliability prediction, fault tree analysis , reliability block diagram etc (see for instance [9]-[12]). fault tree analysis (fta) [13], [14] is an analytical and deductive (topdown) method. it is an organized graphical representation of the conditions or other factors causing or contributing to the occurrence of a defined outcome, referred to as the "top event". while, reliability block diagram (rbd) [15] is a functional diagram of all the components making up the system that shows how component reliability contributes to failure or success of the whole system. these above-mentioned techniques need input data to be performed but sometimes data are not available, and they need to be predicted. an accurate reliability prediction should be performed in the early stages of a development program to support the design process [16]-[21]. a reliability prediction of electronic components could be assessed following the guidelines of several handbooks, while the prediction of mechanical components is more challenging because of the following reasons [16], [22]: • individual mechanical components such as valves and gearboxes often perform more than one function and abstract heating, ventilation, and air conditioning (hvac) is a widely used system used to guarantee an acceptable level of occupancy comfort, to maintain good indoor air quality, and to minimize system costs and energy requirements. if failure data coming from company database are not available, then a reliability prediction based on failure rate model and handbook data must be carried out. performing a reliability prediction provides an awareness of potential equipment degradation during the equipment life cycle. otherwise, if field data regarding the component failures are available, then classical reliability assessment techniques such as fault tree analysis and reliability block diagram should be carried out. reliability prediction of mechanical components is a challenging task that must be carefully assessed during the design of a system. for these reasons, this paper deals with the reliability assessment of an hvac using both failure rate model for mechanical components and field data. the reliability obtained using the field data is compared to the one achieved using the failure rate models in order to assess a model which includes all the mechanical parts. the study highlights how it is fundamental to analyze the reliability of complex system integrating both field data and mathematical model. mailto:giulia.guidi@unifi.it acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 101 failure data for specific applications of nonstandard components are seldom available. • failure rates of mechanical components are not usually described by a constant failure rate distribution because of wear, fatigue and other stress-related failure mechanisms resulting in equipment degradation. data gathering is complicated when the constant failure rate distribution cannot be assumed and individual times to failure must be recorded in addition to total operating hours and total failures. • mechanical equipment reliability is more sensitive to loading, operating mode and utilization rate than electronic equipment reliability. failure rate data based on operating time alone are usually inadequate for a reliability prediction of mechanical equipment • definition of failure for mechanical equipment depends upon its application. lack of such information in a failure rate data bank limits its usefulness. the above listed problems demonstrates the need for reliability prediction models that do not rely solely on existing failure rate data banks [23], [24]. trying to solve these needs, this paper aims to introduce a reliability assessment procedure which integrates failure rate models and field data to optimize the reliability analysis of a railway heating, ventilation and air conditioning (hvac) system. the paper uses both fta and rbd techniques to estimate the system reliability based on realistic failure rate models for mechanical components. the rest of the paper is organized as follow: section 2 illustrates the aim of an hvac and it presents the high-level taxonomy of the system under test; section 3 presents the failure rate prediction of three mechanical components (compressor, heat exchanger and blower) using failure models; section 4 shows the results of the reliability assessment carried out using fta and rbd techniques and finally section 5 compares the results achieved with the different techniques. 2. hvac for high-speed train underground transport and rail systems become more and more frequent as they allow rapid transit times while transporting a large number of users [25]. consequently, rams (reliability, availability, maintainability and safety) analysis has become a fundamental tool during the design of railway systems [25]-[27]. the network of high-speed trains and also standard rails are more and more transferred to underground tunnels in order to mitigate the environmental impact. both applications need ventilation rates. in metros the influx of a large number of people and the presence of moving trains generate a reduction of oxygen and an increase in heat and pollutant. mechanical ventilation is then required to achieve the necessary air exchange and grant users of the underground train systems comfortable conditions. ventilation systems have a second and even more important purpose. that is to guarantee safety in case of fire emergency. in order to create a safe and clean environment for escaping mechanical ventilation both in tunnels and in the stations is activated. in rails the ventilation of tunnels is mainly dedicated to fire emergencies where it is vital to keep under control the smoke propagation and create safe areas and clear environment for the users. furthermore, efficient temperature regulation is becoming a necessity to face overcrowded carriages [28]-[30]. hvac is the best way of regulating temperature and air quality on crowded trains [31]. one of the most important guarantees that rail manufacturers should look for during the design of an air conditioning systems is reliability under the actual operating conditions [28], [32]. during the design of an hvac system it is necessary to achieve information about the hvac equipment and their uses[33]. the taxonomy is a systematic classification of items into generic groups based on factors possibly common to several of the items (location, use, equipment subdivision, etc.). referring to figure 1, levels 1 to 5 represent a high-level categorization that relates to industries and plant application regardless of the equipment units (see level 6) involved. this is because an equipment unit (e.g., air conditioning unit) can be used in many different industries and plant configurations and, for analysing the failure/reliability/maintainability of similar equipment, it is necessary to have information about the operating context. taxonomic information on these levels (1 to 5) shall be included in the database for each equipment unit as “use/location data”. levels 6 to 9 are related to the equipment unit (inventory) with the subdivision in lower indenture levels corresponding to a parent-child relationship. the taxonomy of the system under test, from level 1 to level 5 is reported in table 1. the levels from 6 to 9 are very structured and include the level of the components divided also in the part sections. 3. failure rate models predicting the life of a mechanical element is not easy, it includes mathematical equations to estimate the design life of mechanical components [16]. these reliability equations consider the design parameters, environmental extremes and operational stresses to predict the reliability parameters. the total failure rate of the figure 1. taxonomy classification with taxonomic levels (source iso 14224 2016 [34]). table 1. taxonomy of the system from level 1 to level 5. taxonomy level description level 1 industry railway level 2 business category high speed level 3 installation s121 level 4 unit front car level 5 system hvac system acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 102 component is the sum of the failure rates for the parts for a particular time period in question. the equations rely on a base failure rate derived from laboratory test data where the exact stress levels are known. more information about the failure rate data used in this work could be found in [19]. the most critical components of an heating, ventilation and air conditioning (hvac) system are the compressor, the heat exchanger and the blower[25], [35]. in order to improve the failure rate of these items, the relative failure models have been analysed in the following sections. 3.1. compressor model a compressor system is made up of one or more stages. the compressor compresses the gas, increasing its temperature and pressure [16], [36]. the total compressor may be comprised of elements or groups of elements in series to form a multistage compressor based on the change in temperature and pressure across each stage. every compressor to be analyzed will be characterized by a unique design and it will be comprised of many different components. according to [16] and to the compressor datasheet, the designed hvac compressor is a reciprocating type compressor. the following equation has been obtained in order to estimate the failure rate of the actual compressor used in the considered hvac design. 𝜆c = (𝜆fd ∙ 𝐶sf)+𝜆ca + 𝜆be + 𝜆va + 𝜆se + 𝜆sh , (1) where • 𝜆c is the total failure rate of compressor • 𝜆fd is failure rate of fluid driver • 𝐶sf is the compressor service multiplying factor • 𝜆ca is the failure rate of the compressor casing • 𝜆be is the total failure rate of compressor shaft bearings • 𝜆va is the total failure rate of control valve assemblies • 𝜆se is the total failure rate of compressor seals • 𝜆sh is the failure rate of compressor shaft. different compressor configurations such as piston, rotary screw and centrifugal have different parts within the total compressor and it is important to obtain a parts list for the compressor prior to estimating its reliability. the failure rate for each part comprising the compressor must be determined before the entire compressor assembly failure rate, λc, can be determined. failure rates for each part will depend on expected operational and environmental factors that exist during compressor operation. the total failure rate of compressor shaft bearings is: 𝜆be = 𝜆be,b ∙ 𝐶r ∙ 𝐶v ∙ 𝐶cw ∙ 𝐶t ∙ 𝐶sf ∙ 𝐶c , (2) where • 𝜆be is the total failure rate of bearing • 𝜆be,b is base failure rate • 𝐶r is life adjustment factor for reliability • 𝐶v is multiplying factor for lubricant • 𝐶cw is multiplying factor for water contaminant level • 𝐶t is multiplying factor for operating temperature • 𝐶sf is multiplying factor for operating service conditions • 𝐶c is multiplying factor for lubrication contamination level. the total failure rate of control valve assemblies is given by: 𝜆va = 𝜆po + 𝜆se + 𝜆sp + 𝜆so + 𝜆ho , (3) where • 𝜆va is the total failure rate of total valve assemblies • 𝜆po is the failure rate of poppet assembly • 𝜆se is the failure rate of the seals • 𝜆sp is the failure rate of spring(s) • 𝜆so is the failure rate of solenoid • 𝜆ho is the failure rate of valve housing consequently, using the failure data illustrated in [19] it is possible to solve equation (2)-(3). then, the compressor failure rate could be estimated integrating these results into equation (1), as follow: 𝜆c = 1.56 ∙ 10 −5 failure/h (4) usually, failure rates of components implemented in railway applications are expressed in failure/km or for sake of simplicity fpmk (failure per million kilometers). moreover, the duty cycle of the compressor must be taken into account in order to obtain a more accurate evaluation. consequently, considering an approximate annual distance for high-speed train of half a million kilometers and a duty cycle of 30 %, the failure rate of the compressor becomes: 𝜆compressor = 8.22 ∙ 10 −2 fpmk . (5) the failure rate achieved above must be compared with the failure rate provided by the manufacturer of the component (merak) which is obtained integrating field data and internal company tests. 𝜆compressor,merak = 7.79 ∙ 10 −2 fpmk . (6) comparing equations (5) and (6) it is possible to note that the failure rate obtained using the compressor model provides a value slightly higher than the one provided by the compressor manufacturer. the several variables considered by the failure model in equation (1) produce the different results since the merak value is based mainly on field data. figure 2 shows the reliability curves relative to the failure rates of equations (5) and (6). the blue line represents the reliability calculated with the manufacturer “merak” failure rate while the red one the reliability calculated using the failure model of the compressor. the curve obtained using the model decreases faster because its failure rate is higher than the merak failure rate. anyway, the difference of the two curves is limited, at the beginning the figure 2. reliability curves of the compressor assembly calculated through merak data and compressor model given by equation (1). acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 103 curves are approximately equal then they decrease with different exponential decay rates. 3.2. heat exchanger model heat exchangers are essential part of any kind of hvac system nowadays. the main function of a heat recovery system is to increase the energy efficiency by reducing energy consumption and also by reducing the cost of operating by transferring heat between two gases or fluids, thus reducing the energy consumptions. in heat exchangers, as the name suggests, there is a transfer of energy from one fluid to another. both these fluids are physically separated and there is no direct contact between the fluids. there are different types of heat exchangers such as shell and tube, u tube, shell and coil, helical, plate etc. the transfer of heat can be between steam and water, water and steam, refrigerant and water, refrigerant and air, water and water. the hvac system’s compressor generates heat by compressing refrigerant. this heat can be captured and used for heating domestic water. for this purpose, a heat exchanger is placed in between the compressor and the condenser. the water that is to be heated is circulated though this heat exchanger with the help of a pump whenever the hvac system is on. the heat exchanger included in the system under test is composed by a tube and an expansion valve. the failure rate of a fluid conductor is extremely sensitive to the operating environment of the system in which it is installed as compared to the design of the pipe. each application must be evaluated individually because of the many installation, usage and maintenance variables that affect the failure rate. the failure of a piping assembly depends primarily on the connection joints and it can be estimated with the following equation [16]: 𝜆p = 𝜆p,b ∙ 𝐶e = 2.2 ∙ 10 −6 failure/h (7) where • 𝜆p is the failure rate of pipe assembly • 𝜆p,b is the base failure rate of pipe assembly, which is 1.57 ∙ 10−6 failure/h • 𝐶e is the environmental factor equal to 1.4 in case of a railway application. for the expansion valve the failure rate is provided by [16] and it is 𝜆va = 4.5 ∙ 10 −6 failure/h. therefore, the whole failure rate of the heat exchanger is given by the sum of the failure rate of the pipe and the failure rate of the valve, each one weighted on its own duty cycle. in particular, the duty cycle of the pipe is 80 % while the duty cycle of the valve is 30 %. consequently, the failure rate of the heat exchanger is given by: 𝜆heat exchanger = 3.11 ∙ 10 −6 failure h⁄ . (8) quite the same as the compressor, also the failure rate of the heat exchanger must be converted from failure/h into fpmk. the heat exchanger failure rate in case of railway application is the following: 𝜆heat exchanger = 5.4 ∙ 10 −2 fpmk . (9) also in this case, the failure rate based on field data has been provided by the component manufacturer merak and it is equal to: 𝜆heat exchanger,merak = 4.124 ∙ 10 −2 fpmk . (10) figure 3 shows the two different reliability curves, the blue one is related to merak data while the red one is related to the model results achieved in equation (9). the model line results a pessimistic estimation also for this component. the difference between the two reliability trends is extremely low. this is mainly due to the fact that for a simpler element like the heat exchanger, the manufacturer merak and model lead to a similar result. 3.3. blower model one of the most common downfalls of installed hvac systems is their inability to distribute the correct amount of air to where it’s needed most. when systems are restrictive, or blowers aren’t powerful enough, the air simply doesn’t make it to where it needs to go. a blower is composed by: • an ac motor; • two bearings; • a fan. the failure rate of a motor is affected by such factors as insulation deterioration, wear of sliding parts, bearing deterioration, torque, load size and type, overhung loads, thrust loads and rotational speed. the failure rate model developed is based on a fractional or integral horsepower ac type motor. the reliability of an electric motor is dependent upon the reliability of its parts, which may include bearings, electrical windings, armature/shaft, housing, gears and brushes. failure mechanisms resulting in part degradation and failure rate distribution (as a function of time) are considered to be independent in each failure rate model. the total motor system failure rate is the sum of the failure rate of each of the parts in the motor: 𝜆motor = 𝜆m,b ∙ 𝐶sf + 𝜆wi + 𝜆st + 𝜆as + 𝜆be + 𝜆gr + 𝜆c, (11) where • 𝜆motor is the total failure rate for the motor system • 𝜆m,b is the base failure rate of motor • 𝐶sf is the motor load service factor • 𝜆wi is the failure rate of electric motor windings • 𝜆st is the failure rate of the stator housing • 𝜆as is the failure rate of the armature shaft • 𝜆be is the failure rate of the bearing evaluated using equation (2) and the suitable factors • 𝜆gr is the failure rate of gears • 𝜆c is the failure rate of capacitor. the bearings failure rate could be estimated following the guidelines in section 3.1. the fans are modelled in according to mil-std-217f [37] by: figure 3. reliability curves of the heat exchanger assembly calculated through merak data and heat exchanger model according to equation (9). acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 104 𝜆fan = [ 𝑡 2 𝛼𝐵 3 + 1 𝛼𝑊 ] failure/h (12) where • t is the motor operating time period • 𝛼b the weibull characteristics life for motor bearing • 𝛼w the weibull characteristics life for motor windings finally, the whole blower failure rate is given by the sum of the failure rates of its components, so: 𝜆blower = 𝜆motor + 2 ∙ 𝜆bearing + 𝜆fan = 1.328 ∙ 10−5 failure/h . (13) then considering the duty cycle of 100 % and the required conversion from failure/h into fpmk, the blower failure rate become: 𝜆blower = 2.33 ∙ 10 −1 fpmk (14) the failure rate achieved analyzing field data has been provided by the component manufacturer merak, and it is illustrated in the following: 𝜆blower,merak = 7.97 ∙ 10 −2 fpmk . (15) figure 4 shows the two reliability curves, the blue one is related to the merak data and the red one is related to the model data. like for the other components, the reliability calculated through the models provides a pessimistic reliability trend respect the reliability calculated with the field data provided by merak. this time, the differences between field data and model data is quite remarkable. this could be due to the harsh operating condition considered by the failure rate model in [16]. 4. reliability analysis when the data (coming from tests or from the manufacturer) are available, techniques such as fta or rbd could be used to estimate the useful life of the system. 4.1. fault tree analysis fault tree diagrams consist of gates and events connected with lines. the and and or gates are the two most commonly used gates in a fault tree. to illustrate the use of these gates, consider two events (called "input events") that can lead to another event (called the "output event")[14], [35]. if the occurrence of either input event causes the output event to occur, then these input events are connected using an or gate. alternatively, if both input events must occur in order to the occurrence of the output event, then they are connected by an and gate. fault tree analysis gates can also be combined to create more complex representations. in case of the hvac system under analysis, the top event is “hvac failure” and it’s caused by four different events, as illustrate in figure 5: • “possible fire”, when some events could involve a risk of fire in the railway cabin. • “loss of emergency ventilation”, when the emergency ventilation doesn’t work. • “loss of functions caused by a single event”, when a single event causes a direct loss of all the cooling, heating, ventilation function • “indirect loss of cooling heating and ventilation”, when some events cause independently a loss of cooling, heating and ventilation functions. the top event “hvac failure” is linked to the abovedescribed input events trough an or gate, that means if at least one of the four input events happens the whole system fails. every one of the input events in figure 5 is in turn caused by an extremely complex combinations of several events. the complete fta diagram is very large and structured, and it is not possible to show it entirely. so, for the sake of simplicity, figure 5 shows only an extract of these fta. the reliability trend considering the fta configuration is shown in figure 6. the curve is a decreasing exponential, it starts from a unitary reliability and it tends to zero. the analysis is simulated starting from 0 km up to 6∙106 km. according to an annual forecast distance traversed of about 487∙103 km, the simulation in term of time is over 12 years. at distance 0.5∙106 km (approximately 1 year) the reliability is around the 80 %, while after 1∙106 km (approximately 2 years) is decrease approximately to the 60 % and then it tends to zero at 5∙106 km (approximately 10 years). these results are justified by two reasons: • the mechanical nature of the whole system, that contributes to a fast decrease of the reliability. • the or gate that lead to the top event, which is the worst-case scenario between the several ones considered during the design. 4.2. reliability block diagram an overall system reliability prediction can be made by looking at the reliabilities of the components that make up the whole system or product. in order to construct a reliability block diagram, the reliability-wise configuration of the components must be determined. consequently, the analysis method used for computing the reliability of a system will also depend on the reliability-wise configuration of the components/subsystems. figure 4. reliability curves of the blower assembly calculated through merak data and blower model as in equation (14). figure 5. extract of the fta diagram for the hvac system under analysis. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 105 that configuration can be as simple as units arranged in a pure series or parallel configuration. there can also be systems of combined series/parallel configurations or complex systems that cannot be decomposed into groups of series and parallel configurations. the hvac system under analysis could be described using a series of three main blocks (see figure 7): • cooling system • heating system • ventilation system. therefore, supposing the exponential distribution for all the items [18], [38], [39], the reliability equation of the whole system is: 𝑅sys(𝑡) = e −(𝜆cooling+𝜆heating+𝜆ventilation)∙𝑡 . (16) figure 8 shows a comparison between cooling, heating and ventilation reliability curves. the figure also shows the whole system reliability trend calculated with equation (16), which is illustrated using a dashed black line. the red line represents the heating system, the blue line the ventilation system and the green line the cooling system. the worst system, in reliability terms, is the cooling system, because it contains a lot of series elements and most of them are mechanical items. the three systems are connected in a series configuration, where the component with the least reliability has the biggest effect on the system's reliability. as a result, the reliability of a series system is always less than the reliability of the least reliable component. that’s why the black line, representing the whole system reliability is lower than the cooling curve (the least reliable). 4.3. comparison between fta and rbd results table 2 shows the comparison of the reliability trends between the two proposed methods: reliability block diagram and fault tree analysis. the two curves are very similar, but the rbd reliability is always higher than the fta results (at every distance). the differences could be caused by: • different algorithm used by the software for the calculation of the reliability. • the fta analysis results are more complete and they consider all the possible path that lead to the top event, so it considers also the relationship between failures. the first column of table 2 reports the distance travelled by the train, the second the corresponding time, the third and the fourth the reliability values of the fta and rbd respectively. the last one reports the absolute percentage difference of the two previous columns. all the values relative to the rbd are higher than the fta, but their difference is lower than 6 %. figure 9 shows the trend of the difference between the two curves, it illustrates that the maximum value is 6.5 %, so the difference is very low. at the beginning the difference is not remarkable, in particular before 1000000 km is lower than 4 %, then it increases, and the peak is between 1500000 km and 2500000 km where the difference is 6 %. after that, it decreases slowly and it reaches the value of 2 % at 6000000 km. therefore, the two methods provide comparable results, and both the outcomes are valid. 5. comparison between field data and model data a comparison between the failure estimation of the previous paragraphs and the failure data provided by the manufacturer of the hvac has been carried out in order to investigate how the model-based failure rates affect the reliability trend of the whole system. the model-based failure rates of compressor, blower and heat exchanger are used to calculate the whole hvac reliability together with the failure rate estimation of the other components, which make up the system. figure 6. reliability trend of the fta. figure 7. reliability block diagram of the hvac system. figure 8. reliability trends of cooling, heating and ventilation systems (continuous lines) compared with the hvac system reliability (dashed line). table 2. reliability data of or type fta and rbd. distance km time rfta rrbd difference 0.5 ∙ 106 1 year 0.78 0.80 3 % 1 ∙ 106 2 years 0.60 0.64 4 % 2 ∙ 106 4 years 0.34 0.40 6 % 3 ∙ 106 6 years 0.18 0.24 6 % 4 ∙ 106 8 years 0.1 0.14 4 % 5 ∙ 106 10 years 0.05 0.08 3 % 6 ∙ 106 12 years 0.03 0.05 2 % acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 106 figure 10 shows three reliability curves, the blue trend is related to the fta reliability, the red one is calculated with the rbd analysis, while the green one is related to the failure rate of the components calculated using the failure models. it’s possible to note that the model-based failure rates contribute to reduce the reliability and have an important contribution to the whole system reliability. the failure rate models provide a pessimistic reliability results for the three components analyzed before. consequently, their reliability curves affect the whole system reliability, producing a trend lower than the ones calculated with the manufacturer data (in case of both fta and rbd techniques. 6. conclusion the paper deals with a heating, ventilation and air conditioning system mounted on a high-speed train. the first part of the paper illustrates the taxonomy of the system under study. the architecture of an hvac system includes several critical components, such as: a fan (blower), a heat exchanger and a compressor. a detailed study on the failure rates of the hvac most critical components is presented in this paper. compressor, heat exchanger and blower show a model-based reliability lower than the reliability achieved using the field data provided by the manufacturer of the hvac “merak”. then, the reliability of the complete hvac system has been estimated using two wellknown techniques: fault tree analysis and reliability block diagram. fta and rbd methods take the field data provided by merak as input to evaluate the system reliability over distance travelled by the train. the final analysis shows how the model failure rates affect the whole hvac reliability comparing the results achieved using fta and rbd with the one obtained using the failure rate models. the model-based failure rate provides a pessimistic result because it considers every possible failure modes and failure mechanisms of each subitem that make up the component. despite this. it could be not so realistic since it doesn’t properly consider the real operating conditions of the system under test. quite the opposite, the reliability evaluated using the field data takes into account the real context of the hvac but some of the failure mechanisms might be not occur during the observed time interval. for these reasons. it is fundamental to analyze the reliability of such complex system integrating both techniques. references [1] g. d’emilia, a. gaspari, e. 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studies for the mathmet quality management system at vsl, the dutch national metrology institute acta imeko issn: 2221-870x june 2023, volume 12, number 2, 1 5 acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 1 case studies for the mathmet quality management system at vsl, the dutch national metrology institute gertjan kok1 1 vsl, thijsseweg 11, 2629 ja, delft, the netherlands section: research paper keywords: european metrology network; mathmet; quality management system; validation citation: gertjan kok, case studies for the mathmet quality management system at vsl, the dutch national metrology institute, acta imeko, vol. 12, no. 2, article 21, june 2023, identifier: imeko-acta-12 (2023)-02-21 section editor: eric benoit, université savoie mont blanc, france received july 11, 2022; in final form april 21, 2023; published june 2023 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: the work presented in this article has been performed in the empir project “support for a european metrology network for mathematics and statistics” (15net05 mathmet). this project has received funding from the empir programme co-financed by the participating states and from the european union’s horizon 2020 research and innovation programme. corresponding author: gertjan kok, e-mail: gkok@vsl.nl 1. introduction metrology is the science of measurement founded on the si system of units [1]. the metrological traceability of measurement results is an essential part of metrology. it is defined [2] as the property of a measurement result whereby the result can be related to a reference through a documented unbroken chain of calibrations, each contributing to the measurement uncertainty. for this chain to work properly, all constituent parts should be carefully assessed and validated, and the results of the validation should be properly documented. measurement results play a dominant role in this chain, but that does not mean that hardware and instrumentation are the only things that matter. mathematical calculations implemented in software can form an essential part of the measurement. these mathematical calculations will almost certainly be implemented in software, which may have been validated using some reference datasets. the whole data analysis procedure may be based on a written guideline. for complying with metrological traceability, it is therefore essential that the used software, data and guidelines are also under quality control. this requirement means that their working and content is checked for correctness, and that storing meta-information like version control data is properly managed. as there is worldwide cooperation within metrological applications, it is logical to organize this type of quality control at an international level. the emn mathmet [3] is therefore developing a lightweight quality management system (qms) against which the existing procedures at national metrology institutes (nmis) can be benchmarked and which can help to complement them to get more uniformity in assessing the quality of software, data and guidelines by different nmis. a full description of this qms is presented in [4], which has recently been published in acta imeko. this article is an accompaniment to it and supports [4]. in this contribution we will report on the application of the qms to several use cases concerning software, reference data and guidelines by vsl. we will discuss the usefulness, advantages, and disadvantages of the qms and possible pitfalls in sections 3 to 5, after shortly introducing the qms in section 2. finally, in section 6 some overall conclusions will be formulated. note that all these viewpoints and conclusions are abstract the european metrology network mathmet is a network in which a large number of european national metrology institutes combine their forces in the area of mathematics and statistics applied to metrological problems. one underlying principle of such a cooperation is to have a common understanding of the ‘quality’ of software, data and guidelines. to this purpose a flexible, lightweight quality management system (qms), also referred to as quality assessment tools (qat), is under development by the emn. in this contribution the application of the qms to several use cases of different nature by vsl is presented. the benefits and usefulness of the current version of the qms are discussed from the particular viewpoint of a particular employee of vsl, and an outlook for possible future extensions and usage of the qms is given. mailto:gkok@vsl.nl acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 2 from the perspective of one employee of vsl only, relate to the particular version of the qms of march 2022, and they are not necessarily shared by other nmis or by emn mathmet itself. 2. short overview of the qms for software, data and guidelines a thorough overview of the qms for software, data and guidelines is given in [4]. in this section a summary is given, starting with some remarks regarding its scope. 2.1. goal of the qms originally there was the idea that the emn itself would ‘recommend’ software, reference data and guidelines. assessment of these items by means of the qms would ensure that the emn mathmet recommendations meet the highest quality levels and achieve wide use and substantial impact. for various reasons this is currently not seen as realistic. one important reason is the fact that an emn is part of the larger entity euramet [5] and that the decision-making authority, responsibility and liability for such recommendations is not entirely clear. the second reason is the scope of the emn, which is now seen as a platform to interact with stakeholders and to define future research directions, fostering collaboration and preventing duplication of work. actual technical work should be done inside other forms of cooperation. linked to this reason is that the required budget for performing an assessment is not available within the emn itself. the qms might therefore be seen as a tool to help individual nmis assessing software, data and guidelines, rather than a tool for the emn itself. performed reviews will not be published on the mathmet website but could be put on the website of an individual nmi if an nmi would wish to do so. it therefore seems reasonable to assess the mathmet qms from the perspective of a single nmi. various nmis in mathmet have announced that they will indeed discuss the benefits of the mathmet qms with people directly responsible for quality control at their respective nmis. this will be done in the near future at vsl as well. this article presents an initial assessment of the qms by the author. 2.2. qms for software the qms for software consists of an interactive pdf-file of 5 pages. based on the calculated risk level, specific fields are visible and need to be filled out. the qms for software requires that the project team provides information and evidence of documents covering the following aspects and activities: • some meta-data • a risk level analysis resulting in a “software integrity level” that determines the quality interventions needed • user requirements • functional requirements • design • coding • verification • validation • delivery, use and maintenance. 2.3. qms for data the qms for data consists of an interactive pdf-file with 41 questions, which are again only visible if they are deemed relevant for the selected risk level. for data the team should provide information regarding: • general details and responsibilities • a risk level analysis, resulting in a “data integrity level” that determines the quality interventions needed • user requirements documentation and approval • data life cycle documentation • quality planning • quality monitoring, control and improvement • quality assurance • understandability • metrological soundness most questions to be answered are of general nature. only the last set of questions explicitly involve some metrological aspects. there are no questions explicitly addressing the mathematical aspect the data may have. 2.4. qms for guidelines the qms for guidelines consists of two different checklists: one checklist for existing guidelines and one for future guidelines. at the moment of writing of this manuscript these checklists are still out for review by the project partners, but preliminary versions have been assessed by vsl. the checklists are quite similar. they ask for information regarding: • organization generating the document • independent review and approval available • appropriate metadata available • copyright and ip protection • language • mentioning of target audience • relevance for mathematics and statistics in metrology and the target audience • clearly stated conclusions • appropriate references • presentation easy to understand what is noteworthy, is that the qms checklist is not asking to perform a thorough review of all mathematics by the user of the checklist, but rather to assess if this has been done (and documented!) already and by whom. for some questions, e.g., regarding the presentation and conclusions, it would of course be beneficial to read through the complete document. however, these questions can also be answered with a reasonable level of confidence by only reading small parts of the document. 3. uses case 1: qms applied to software the usefulness of the qms has been assessed by applying it to two pieces of software. 3.1. context the first piece of software was a library of mathematical routines written in python which can be used to take advantage of redundancy in sensor network data [6]. it was developed in the research project met4fof [7]. at a first stage a chi-squared based consistency check is performed to assess the statistical consistency of the sensor data. in the case of consistency, the measurement data is combined into a best estimate of the measurand respecting sensor uncertainties and covariances, whereas in the second case the largest consistent subset of acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 3 sensors is constructed. this is not only done for the case that the sensors directly measure values of the measurand, but also if there is a linear relationship between the vector of sensor values 𝒙 and a vector of values 𝒚 for the measurand. this vector 𝒚 reflects the availability of multiple, redundant estimates of the measurand. the relationship takes the form 𝒚 = 𝒂 + 𝐵 𝒙 , (1) in which 𝒂 is a vector and 𝐵 a matrix. the second piece of software that was used to evaluate the qms consisted of a recently developed calculation module based on the written standard iso 6142-1 [8]. this software is used at vsl in the production of certified reference materials (i.e., gas mixtures) for customers. the input to the software consists of atomic weights, chemical formulas of the mixture components, amount of substance fractions of the components in the parent gases and the added gas mass from each parent gas mixture to the target gas mixture based on weighing of the cylinder. the outputs of the calculation module are the amount of the substance fractions of the components in the target gas mixture, including their uncertainties and covariances. the quality system at vsl requires that software should be version controlled, documented and validated. however, there are no uniform, detailed procedures or templates to this purpose. in practice different groups assure the quality in different ways. 3.2. benefits and usefulness of the qms the qms for software was applied to these pieces of software with the aim of assessing the qms, rather than constructing all required information that might not be readily available. the following parts of the qms for software were especially appreciated: • the templates help to give a uniform description of the software. • the templates help to avoid overlooking important aspects of software quality. • at vsl there is a focus on version control, documentation, and validation of software and storing these properly. user and functional requirements as well as software design may be lost after the software has been released. it would be good to properly control these documents as well. especially the documentation of software design could be useful for future improvements of the software, possibly by new personnel. the following parts of the qms for software seemed to be less appropriate for the vsl context: • the ‘review by customer or proxy’ may need some flexible interpretation, as vsl does not sell any software to external customers. there could be a ‘vsl internal customer’ for the software, and/ or the envisaged outputs of the software could be assessed against known requirements of customers. in the case of research projects that are, e.g., funded by the eu, the ‘review by customer’ is usually difficult to achieve. • the number of up to three required reviews for some aspects is quite large and can be burdensome, especially for a small nmi like vsl. • the document asks for requirement and design documents at the moment of filling out the form. at vsl, often a gradual, agile based, approach is used for software development. it is not so clear for the author of this paper how the qms should be used in that context. should the qms forms and all implied documentation and reviews be repeated at each ‘sprint’ (development cycle) or at each new release of the software? some more guidance and clarity would be beneficial. as a general observation it would be helpful if the qms indicates some examples of ict tools (preferably open source) that could be used in combination with an agile development process while assuring the traceability (in an administrative sense) of all choices made. in this way, possible requirements of the mathmet qms that may not be directly accommodated by the quality system and available software systems at an nmi, could more readily be implemented at an nmi and used in the context of work related to emn mathmet. 4. use case 2: qms applied to data in this section we will assess the qms for data by applying it to some mathematical reference datasets that were generated in an earlier project. 4.1. context in the tracim project [9] reference datasets were produced for various mathematical problems, e.g., for non-linear least squares fitting problems. the precise definition of the computational aims, together with the datasets were stored in a database [10] accessible from the internet for registered users. an example of such a fit problem is the determination of the best fit parameters 𝑎′, 𝑏′ and 𝑐′ of a function 𝑦 = 𝑓(𝑥, 𝑎, 𝑏, 𝑐) modelling exponential decay to 𝑛 datapoints (𝑥𝑖 , 𝑦𝑖 ) 1 ≤ 𝑖 ≤ 𝑛 such that 𝐷(𝑎, 𝑏, 𝑐) = ∑(𝑦𝑖 − 𝑓(𝑥𝑖 , 𝑎, 𝑏, 𝑐)) 2 → min 𝑛 𝑖=1 . (2) in the database [10] reference datasets are available for this and other fit problems. at vsl there is no specific quality guidance for the generation and documentation of such datasets, other than the requirements mentioned in section 3 for software. 4.2. benefits and usefulness of the qms the interactive pdf file with 37 pages and at most 42 questions was filled out for the application described in section 4.1. the following parts of the qms for data were especially appreciated: • with the help of the data quality management plan template a uniform plan for all applications can be created. • the questions cover a large range of quality aspects, which might be forgotten if the qms tool would not be used. the following parts of the qms for data seem to be less appropriate for vsl’s needs: • the mathematical aspect of the data is not particularly addressed. there could be more guidance with respect to how to assess the correctness of numerical data. • four responsibilities related to data are mentioned: data manager, data administrator, data steward, data technician. these roles do not always seem to exist acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 4 at vsl, especially not for data generated in research projects. in many cases the situation seems to be much simpler. • similar to what was mentioned in the qms for software, it would be nice if more guidance could be given regarding how to implement all mentioned quality aspects by means of some ict tools, preferably open source. 5. use case 3: qms applied to guidelines in this last use case, we will discuss the application of the qms to a set of mathematical guidelines which was produced in the emrp project new04 [11], and to which vsl contributed. 5.1. context in the new04 project three best practice guides (bpgs) were produced [12]: 1. a guide to bayesian inference for regression problems (bpg1) 2. best practice guide to uncertainty evaluation for computationally expensive models (bpg2) 3. a guide to decision-making and conformity assessment (bpg3) except for the formatting of the title page, bpg1 and bpg2 are very similar in document structure. bpg3 consists of a set of four different loosely connected documents. we applied the qms checklist for existing guidelines to bpg2. this document of 84 pages provides a summary of current best practice in uncertainty evaluation for computationally expensive models. in the first part of the document the methods are explained. in the second part three case studies are presented. 5.2. benefits and usefulness of the qms the qms checklist is mainly asking some questions about the existence of specific information like ‘version number’, ‘independently reviewed’, ‘target audience’ and ‘appropriate references’. the benefit of this approach is that the assessment can be done fairly quickly without having to read, study and check the document itself. the qms checklist verifies that some formal quality criteria are fulfilled, and it is not requiring a tedious scientific review of the content. these simple checks can give a good indication of the overall care with which the document has been prepared in a very quick way, which is the main benefit of this qms for guidelines in our opinion. if the conclusion is that the document hasn’t been independently reviewed, then this job is still out to be done, but this is not directly in the scope of the qms. the application of the qms checklist to bpg2 yielded some interesting deficits. bpg2 has no version number, it doesn’t say anything about ‘copyright’, or ‘independent review’ and there are no ‘clearly stated conclusions’. the document simply ends with the last use case. this is particularly interesting, because several of the authors of bpg2 are mathmet members and even involved in the creation of the qms. the mathematical content of the bpg may be impeccable, but it doesn’t fulfil all quality metrics of the mathmet qms. 6. conclusions and outlook as over time the scope of the emn has become clearer, also the place of the qms in it has been reassessed. initial ideas about a mathmet qms that ensure that ‘emn mathmet recommendations meet the highest quality levels and achieve wide use and substantial impact’ [13] seem to have been replaced in practice by a qms that can help individual nmis with their quality assessment, at least from the perception from vsl. in this paper it has been assessed how this worked out for vsl, and which aspects of the qms proved useful and which less appropriate for the vsl context. the overall conclusion is that the collaboration within the emn on the qms gave useful insights with respect to assuring the quality of software, data and guidelines, and which aspects could matter. at the same time a proper assessment of the different parts and questions of the qms is needed in order to best align it with vsl’s requirements and working field. a discussion with the quality coordinators at vsl is still outstanding. when more nmis assess the mathmet qms and reflect on its implementation in nmi specific quality procedures, the common ground of most useful aspects of the qms will become clearer. this may lead to a next step in the development of the qms, which should lead to a greater uniformity in quality assessment of software, data and guidelines by nmis, and to a reduction of costs to set-up the system. also, there might be additional guidance for the usage of modern ict tools to assure the quality of software, data and guidelines in a more efficient way. as guaranteeing the quality of software, data and guidelines (cf. the attention paid to research papers with open software and data) is getting nowadays more and more attention, the creation of a common qms framework by emn mathmet for nmis seems to come at the right moment. this and similar initiatives will help to maintain and increase the trustworthiness in services provided by nmis. acknowledgement we thank the npl team for leading the development of the mathmet qms and in particular peter harris for fruitful discussions and feedback to an earlier draft of this paper. we also thank the referees for their comments which helped to improve this paper. the work presented in this article has been performed in the empir project “support for a european metrology network for mathematics and statistics” (15net05 mathmet). this project has received funding from the empir programme cofinanced by the participating states and from the european union’s horizon 2020 research and innovation programme. references [1] euramet european association of national metrology institutes, european metrology network (emn) mathmet. online [accessed 6 june 2023] www.euramet.org/mathmet [2] bipm, si system of units. online [accessed 6 june 2023] https://www.bipm.org/en/measurement-units [3] bipm, vim3, definition of traceability. online [accessed 6 june 2023] https://jcgm.bipm.org/vim/en/2.41.html [4] keith lines, jean-laurent hippolyte, indhu george, peter harris, a mathmet quality management system for data, software, and guidelines, acta imeko, vol. 11, no. 4, article 8, december 2022, pp. 1-6. doi: 10.21014/actaimeko.v11i4.1348 [5] euramet european asssociation of metrology institutes. online [accessed 6 june 2023] www.euramet.org http://www.euramet.org/mathmet https://www.bipm.org/en/measurement-units https://jcgm.bipm.org/vim/en/2.41.html https://doi.org/10.21014/actaimeko.v11i4.1348 http://www.euramet.org/ acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 5 [6] metrology for the factory of the future, github software repository. online [accessed 6 june 2023] https://github.com/met4fof/met4fof-redundancy [7] empir project 17ind12 met4fof metrology for the factory of the future, 2018 – 2021. online [accessed 6 june 2023] https://www.ptb.de/empir2018/met4fof/home/ [8] iso 6142-1:2015, gas analysis — preparation of calibration gas mixtures — part 1: gravimetric method for class i mixtures, iso, geneva, 2015 [9] emrp project new06 tracim traceability for computationally-intensive metrology, 06/2012 05/2015; web site. online [accessed 6 june 2023] https://www.tracim.eu [10] tracim database with reference datasets. online [accessed 6 june 2023] http://www.tracim-cadb.npl.co.uk/ [11] emrp project new04 novel mathematical and statistical approaches to uncertainty evaluation, 08/2012 07/2015, web site. online [accessed 6 june 2023] https://www.ptb.de/emrp/new04-home.html [12] emrp project new04 novel mathematical and statistical approaches to uncertainty evaluation: best practice guides. online [accessed 6 june 2023] https://www.ptb.de/emrp/2976.html [13] project protocol of empir project 15net05 mathmet support for a european metrology network for mathematics and statistics, 2019 2023 https://github.com/met4fof/met4fof-redundancy https://www.ptb.de/empir2018/met4fof/home/ https://www.tracim.eu/ http://www.tracim-cadb.npl.co.uk/ https://www.ptb.de/emrp/new04-home.html https://www.ptb.de/emrp/2976.html magnetic circuit optimization of linear dynamic actuators acta imeko issn: 2221-870x september 2021, volume 10, number 3, 134 141 acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 134 magnetic circuit optimization of linear dynamic actuators laszlo kazup1, angela varadine szarka1 1 reseach institute of electronics and information technology, university of miskolc, miskolc, hungary section: research paper keywords: magnetic brake; linear brake; magnetic circuit calculation; dynamic braking citation: laszlo kazup, angela varadine szarka, magnetic circuit optimization of linear dynamic actuators, acta imeko, vol. 10, no. 3, article 19, september 2021, identifier: imeko-acta-10 (2021)-03-19 section editor: lorenzo ciani, university of florence, italy received february 5, 2021; in final form april 27, 2021; published september 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: the described article/presentation/study was carried out as part of the efop-3.6.1-16-2016-00011 “younger and renewing university – innovative knowledge city – institutional development of the university of miskolc aiming at intelligent specialisation” project implemented in the framework of the szechenyi 2020 program. the realization of t this project is supported by the european union, co-financed by the european social fund. corresponding author: laszlo kazup, e-mail: laszlo.kazup@eiki.hu 1. introduction in the field of manufacturing, many methods of lifetime testing are used. in case of electronic devices, the stressand lifetime testing is quite simple in most cases compared to the mechanical tests, when mechanical load emulation is very difficult and expensive. for example, power tools are tested with different loads and some of them have alternating linear movements which should be loaded. there are not fully developed contactless load emulating methods for this application. sometimes hydraulic system is used with low efficiency. in case of traditional practice test methods, an operator works with the device under test (dut), he/she cuts, sands, or planes different materials. this type of testing is expensive, not reliable, and less repeatable than the automated test solutions. in some tests the operators were replaced by industrial robots, but the test is still expensive and dirty due to the using of real materials. the best result would be a system which can emulate the loading force without any physical contact, abrasion, and dirt. research of braking method for fast alternating linear movements by using contactless magnetic methods is in the focus of our project. this work is the second phase of the research and development project, which was started in 2008. the original goal of the project was to develop a special magnetic brake which can simulate the real operation of an electric jig saw to replace the traditional practice test method in which operators perform the full test process by cutting different types of material such as wood, steel, aluminium, etc. the repeatability of this test method is very low, as well as the reliability of the documented test circumstances. also a lot of waste and dirt in the test centre was produced. in those days a special hydraulic brake was developed in switzerland to solve this problem, but its performance, controllability and reliability was poor. the analysis of different dynamic brake constructions and methods have proved that the most reliable and efficient solution can be achieved by using a magnetic construction. the test equipment should perform a special braking characteristic which can be controlled even in a single moving period, and in case of using an electromagnetic solution, these properties can be abstract contactless braking methods (with capability of energy recuperation) are more and more widely used and they replace the traditional abrasive and dissipative braking techniques. in case of rotating motion, the method is trivial and often used nowadays. but when the movement is linear and fast alternating, there are only a few possibilities to break the movement. the basic goal of research project is to develop a linear braking method based on the magnetic principle, which enables the efficient and highly controllable braking of alternating movements. frequency of the alternating movement can be in wide range, aim of the research to develop contactless braking method for vibrating movement for as higher as possible frequency. the research includes examination and further development of possible magnetic implementations and existing methods, so that an efficient construction suitable for the effective linear movement control can be created. the first problem to be solved is design a well-constructed magnetic circuit with high air gap induction, which provides effective and good dynamic parameters for the braking devices. the present paper summarizes the magnetostatics design of “voice-coil linear actuator” type actuators and the effects of structure-related flux scattering and its compensation. mailto:laszlo.kazup@eiki.hu acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 135 realized. the development of this test equipment needed both practical and theoretical improvement. the prototype testing and evaluation originated the further hypotheses. our research group aims to create general theoretical and modelling methodology supporting reliable practical realization of dynamically controlled magnetic fields which can be used in many different industries to optimize the performance of voice-coil type linear actuators and brakes. my research work includes systematic analysis of all possible and realistic magnetic circuit constructions, optimization of the magnetic circuits, and also performing static and dynamic simulations to maximize the efficiency. the research includes analysis of voice-coil type magnetic actuators, design of magnetic circuit to maximize efficiency and reliability and minimize weight and size of the brake. the first part of this paper includes introduction of a method for transformation of 2-dimension magnetic calculations to the cylindrical coordinate system and presents the analysis of the flux leakage and its effects to the results of calculations. the calculations are confirmed by finite element simulations, and the results are also used to correct the differences caused by the flux leakage. these calculations, simulations and corrections were solved for different shapes to realize a shapeand size independent model to calculate the correct average flux density in the air gap. the second part of paper presents the results of dynamic simulations, by which dynamic behaviour (relationship between the current and the force in dynamic cases, eddy currentand solid losses, etc.) of the voice coil-type actuator is analysed. this research is the extension of the air-gap induction distortion analysis, which work is described in paper “a. diagnostics of air-gap induction’s distortion in linear magnetic brake for dynamic applications”, [1] [3]. 2. design of cylindrical magnetic circuit with twodimensional, plane cross-section model calculations the aim of the first part of the work was to theoretically establish, develop, and validate a method that transforms the dimensions of a cylindrical symmetrical magnetic circuit of a given size into an equivalent two-dimensional cross-sectional and constant depth model. in this way, cylindrical magnetic circuits can also be calculated. in this method the cylindrical magnetic circuit is “spread out” so that the vertical (z) dimensions are leaved unchanged, and the r values are transformed into x values to create a flat-section, fixed-depth model (“cubic model”) in which the volume of each part is the same as the volume of the corresponding parts in the cylindrical model, so the two models is connected to each other by the unchanged value of the flux. however, inductions calculated in the planar model is also valid for the transformation, the calculated values correspond to the average values in the cylindrical model, since values of the magnetic induction in the cylindrical model are changing in radial direction. as a result, higher induction values are observed on the inner half of the cylindrical parts and lower on the outer half. figure 1. shows the dimensions of a typical cylindrical model, and figure 2 illustrates the corresponding x and z dimensions in a planar cross-sectional model for transformation. [6], [7] the depth d of the plane cross-section fixed-depth model should be taken so that the x dimensions of the plane model are close to the radius differences of the cylindrical model in the area most affected by the test (this is practically the air gap). thus, for practical reasons, depth dimension d is selected equal to the length of line at the air gap center circle which is as follows: 𝑑 = (𝑟2 + 𝑟3) π (1) to determine the relationship between the radius and the x values, the volume equality was used as already mentioned above, which is the following: 𝑉𝑛𝑟ℎ = 𝑉𝑛𝑥𝑦 (𝑟𝑛 2 − 𝑟𝑛−1 2) π ℎ𝑚 = 𝑥𝑛 𝑦𝑚 𝑑 . (2) since the given h and y values in the two models are the same based on a previous condition, the final correlation for the x values after transformations is as follows: 𝑥𝑛 = (𝑟𝑛 2 − 𝑟𝑛−1 2) π 𝑑 . (3) in the above relation if n = 1, then r0 is 0, assuming that inner bar (iron cores and also magnets) is cylindrical. if the inner bar has ring section, value of r0 is equal to radius of inner ring. 3. verification of the relationships received by finite element simulation to verify transformation relationships introduced above, a transformation of a cylindrical magnetic circuit of a given size was performed. in determining the depth dimension d, the length figure 1. the mechanical drawing of the first experimental dynamic magnetic brake prototype, [3]. figure 2. the half cross-section of a typical cylindrical magnetic circuit. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 136 of the central circle of the air gap was considered, which will result nearly equal air gap induction in plane model and in the cylindrical model. the dimensions of the initial cylindrical model and the plane model which is the result of the transformation are summarized in table 1. the next step was to determine the air gap induction based on static magnetic calculations in which dimensions of the plane model were used. initial data had to be determined, these are the main magnetic parameters of the applied soft ferromagnetic and permanent magnetic material. these data were received from the finite element simulation software database for better comparison. these material properties are as follows (the operating point values of the magnet have been determined graphically): permanent magnet o material: y25 ferrite magnet o residual induction: 0.378 t o coercive force: 153035 a/m o relative permeability at the operating point section of the demagnetization curve: 1.9 soft steel parts o material: 1010 low carbon soft steel o relative permeability: ~103 o maximum induction: 1 t (in linear part of bh graph) the summarized relative permeability of the permanent magnet and the air gap is three order of magnitude smaller than relative permeability of the body, therefore the permeability of the soft iron was neglected in the calculations. in this phase of the research the magnetic calculations focused to the permanent magnet and the air gap. based on the above described initial data, the main steps of the calculations were as follows. 1.) determining the operating point of the magnet by equation of the air gap line (based on data of the transformed geometry): 𝐻𝑚 = −1 𝜇0 ∙ 𝑥1 𝑦1 ∙ 𝑥3 𝑦2 ∙ 𝐵𝑚 𝐻𝑚 = −7000 a m 𝐵𝑚 = 0.3726 t (4) 2.) determination of the flux: 𝛷 = 𝐵 ∙ 𝐴 = 𝐵𝑚 ∙ 𝐴𝑚 = 𝐵𝑚 ∙ 𝑥1 ∙ 𝑑 = 1.873 ∙ 10−3 wb . (5) 3.) determination of the air gap induction: 𝐵 = 𝛷 𝐴 → 𝐵𝛿 = 𝛷 𝐴𝛿 = 𝛷 𝑦1 𝑑 = 0.2092 t . (6) after performing the calculations, both the original cylindrical and the transformed magnetic circuits were simulated by femm 4.0 finite element simulation software [7]. the simulation results and the calculated values are summarized in table 2. the air gap induction value was approx. 75 % compared to the calculated one in the simulation (the reason is the leakage flux around the air gap). in spite of this, the average value of the induction in the magnet and the flux of the magnetic circuit showed only a very small difference from the calculated data. according to these results we can state that the model and transformation provided good results and can be used in the next stage of the research. most of the differences from the simulated values (the finite element simulation is the validation of the calculations in this stage) are due to the leakage flux, correction of which needs further studies. although flux in the air gap is also 72 % less, including leakage induction lines, simulation gives the original calculated flux value. the further paragraphs show that in case of flux leakage compensation the calculations will approximate the simulated results very close. 4. calculation of magnetic circuit for required air gap induction and air gap depth while the previous calculations illustrated the transformation of a magnetic circuit with given dimensions, in practice, developing a so-called “voice-coil-actuator”, a much more common problem is adjusting dimensions of the structure, especially dimensions of the magnet to be used, to the given air gap height and air gap induction value. in such a case, the minimum air gap diameter has to be defined at which the given induction can be performed at the specified air gap height. also, if the diameter of the air gap is fixed, feasibility of the desired induction with the given permanent magnet type at the specified sizes should be checked. in addition, the effect of table 1. basic dimensions of the cylindrical model and calculated dimensions of the transformed model. parameter description value (mm) original values of the cylindrical model 𝑟0 internal radius of the ring magnet 40 𝑟1 external radius of the ring magnet 70 𝑟2 internal radius of the air gap 𝑟3 external radius of the air gap 72.5 𝑟4 internal radius of the outer ring 130 𝑟5 external radius of the outer ring 150 𝑧1 height of the air gap 20 𝑧2 height of the magnet/outer ring 60 𝑧3 height of the bottom cylinder 20 transformed values 𝑥1 width of the transformed magnet 11.23 𝑥2 distance between the air gap and the magnet 23.16 𝑥3 air gap length 2.49998 (~2.5) 𝑥4 distance between the air gap and the outer mild iron part 81.71 𝑥5 width of the outer mild iron part (transformed one of the original outer ring) 39.3 𝑧1 height of the air gap 20 𝑧2 height of the magnet/ outer mild iron part 60 𝑧3 height of the bottom mild iron part 20 𝑑 depth of the magnetic circuit 447.68 table 2. comparison of the calculated and the simulated results parameter calculated value simulated value (cylindrical model) simulated value (plane model) bm 0.3726 t 0.3739 t 0.3744 t φm 1.873 · 10-3 wb 1.879 · 10-3 wb 1.886 · 10-3 wb bδ 0.2092 t 0.1509 t 0.138 t φδ 1.873 · 10-3 wb 1.351 · 10-3 wb 1.24 · 10-3 wb acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 137 leakage flux must be considered when calculating these data (the study and correction of leakage is discussed in chapter vi. 4.1. calculation steps 1.) definition of operating point values of magnets the real demagnetization curve of permanent magnets is linear in a relatively wide range, it has nonlinearity only near the coercive force. therefore, the operating point of the magnet should be defined to provide maximum value of the product b · h, which is in the point 𝐵𝑚 = 𝐵𝑟 / 2 in the practice. 2.) determination of the cross section of magnets perpendicular to flux the air gap flux can be determined from the air gap cross section and the desired induction. since the air gap and the flux of the magnet are the same in theory, the cross section of the magnet can be determined from the equation φ = b · a. the value of the radius of this surface must be checked to ensure that it is smaller than the inner circle line of the air gap (in the case of a plane model the test can also be done, in which case the x value at the beginning of the air gap must be greater than or equal to x). if the evaluation shows that the desired induction is not feasible at a given air gap circle, the air gap diameter must be chosen to be higher, or if it is not possible, the magnet can be used with a different (higher) induction than the optimal operating point, which will result an increase in the length of the magnet (see point 3). in practice, the air gap and the flux of the magnet do not match due to scattering, so the correction described later should be applied. 3.) determination of the optimal length of permanent magnets since the reluctance of the iron body is neglected according to an earlier condition, the equation ∮ 𝐻𝑑𝑙 in the magnetic circuit can be defined as follows (without considering leakage correction): 𝐻𝛿 𝛿 + 𝐻𝑚 𝑦𝑚 = 0, 𝐵𝑚 𝜇0 𝜇𝑚 𝑦𝑚 = −𝐵𝛿 𝜇0 𝛿 (7) where 𝜇𝑚 is the relative permeability of the permanent magnet (typically will be between 1 and 2), 𝐵𝑚 is the operating point induction of the permanent magnet, 𝑦𝑚 is the length of permanent magnet to be defined, 𝐵𝛿 required induction, 𝛿 length of air gap. calculation example shows that the required air gap induction can be achieved without operating the magnet at the operating point. the demagnetizing field strength is less than the operating point field strength, but in this case the magnet length must be greater than the optimal value for the equation to be realized. if the goal is to use a permanent magnet with the smallest possible volume (and at the same time the lowest cost), it is advisable to use the optimal dimensions determined by the operating point. to achieve this, custom-made permanent magnets are necessary in practice. experience shows that when using commercial permanent magnets from catalogs some we should accept some comptonization. 4.) determination of cross section of soft iron body based on the data sheets of various commonly used soft iron materials, we can state that approx. up to 1 t, their b-h curve is linear, so it is not recommended to design above this value. otherwise, especially near saturation, the relative permeability of the iron decreases, and in this case the reluctance of the given section is no longer negligible. [6] the previously determined flux value and the maximum induction can be calculated based on the cross-sectional dimensions. 5. application of ferrite magnets in the outer ring as a flux conductor the results of the dynamic simulations proved, that the reluctance of the magnetic circuit and the properties of the materials in the vertical columns and rings greatly influence the value of the inductivity of the moving coil and the magnitude of the eddy current losses during dynamic operation. we have also examined an initial experimental design that included a permanent magnet both inside and outside. a static study of this construction was also carried out, during which it was found that the external permanent magnet does not substantially increase the air gap induction in the magnetic-air gap-magnet series magnetic circuit, it only increases the demagnetization field strength of the inner, so-called “working” magnet, which results operating point down shifting of this magnet. however, dynamic studies have discovered some advantages, which are the reduced inductance of the moving coil and reduced power dissipation of iron loss. results show that the inductance is approx. half of the level when using soft iron instead of an external magnet in such a structure. the conclusion is that if the dimensions of the external magnet are determined so that the magnetic field strength inside of it is close to 0, then the magnetic ring behaves as a “flux conductor” like iron with low relative permeability. this operating state is also characteristic of soft iron materials in the near-saturation state, with significant flux leakage. however, in the case of permanent magnets, the leakage is minimal. the results are better dynamic parameters caused by reduced inductance of the moving coil as well as reduced eddy current losses, but in turn it and does not reduce the performance of the correctly calculated dimensions working magnet. if this solution is used for the sizing of the magnetic circuit, the last point of the design steps is modified as follows: the cross section of the external magnetic ring at which the induction is equal to the value of the residual induction of the magnet must be determined. for practical reasons, it is recommended to choose the length of the external magnet as equal to the length of the inner magnet (this simplifies the construction). while experience show that neodymium iron-boron (or samarium cobalt at higher operating temperatures) is the most suitable material for the internal working magnet due to its high energy density, conventional strontium ferrite magnets can also be used for the external magnetic ring used as a flux conductor. they have lower prices than the two types of magnets listed above and, since they are used as external elements, their relatively large size is not limited by critical parameters affecting the moving mass, such as the diameter of the moving coil. 6. analysis and correction of flux leakage the leakage of magnetic induction lines in a magnetic circuit with an air gap is a complex problem that depends on several design and operating parameters. practical experiences show that in optimal situation more than 90% of the leakage flux is present around the air gap, but in case of permanent magnets operating out of the optimal operating point or at soft iron sections near acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 138 the saturation, significant part of lines may close outside the magnetic circuit. there are several estimation work-help documents of leakage calculations, including air-gap leakage calculations are available from permanent magnet manufacturers to magnetic circuit designers. in these documents, the air gap and the field around it are divided into several areas depending on the type of magnetic circuit, which can include semicylindrical, semispherical, quarter-spherical and/or prismatic areas. the magnetic reluctances of these parallel fields are defined using exact, empirical formulas. however, these definitions help only in the design of certain magnetic circuits with frequently used structures. [9], [10] in general cases, the effects of leakage in an arbitrary magnetic circuit can be estimated most accurately by finite element simulation. this requires exact and accurate information about geometric model and characteristics of the applied materials. in order to make method outlined earlier applicable in practice for design “voice-coil actuator” type electromagnetic actuators, correct levels of leakage flux should be estimated. in the first step we have worked out an algorithm for automated static magnetic calculations. the calculations are based on the basic magnetostatic calculations mentioned in chapter 4. the main input parameters of this algorithm are the residual induction of the applied magnet types (internal and external), its coercive force, the height of the air gap, and the radius of its internal and external sections. the required air gap induction can be given by the following values: start value stop value number of steps the algorithm can generate a table including the most important geometric parameters of the parametric simulation sequences, which are the following: 𝑟0: the inner radius of the inner magnet 𝑟1: the outer radius of the inner magnet, equal to the inner radius of the air gap 𝑟2: the inner radius of the outer magnet, equal to the outer radius of the air gap 𝑟3: the outer radius of the outer magnet several simulation series were performed to analyze the effect of flux leakage and determine a correction relationship. these simulations were built by using ansys maxwell 2d finite element simulation software [8]. the models in the simulation were axis-symmetrical models. in the first simulation a magnetic circuit according to figure 1. was used, in which the radius of the central circle of the air gap is 70 mm and the height of the air gap is 20 mm. the start value of induction is 0.1 t and the stop value is 1 t, with 0.05 t steps. the 1 t stop value is maximized by the given air gap center circle radius. using the received geometric dimensions, a parametric simulation was prepared to investigate the difference between the average induction value experienced in the air gap and the initial air gap induction value due to flux leakage. in the simulation deviation of working range from the operating point was examined for internal magnets (induction value is 0.67 t for n35 type magnets) and deviation of the average induction from the residual induction value (br = 0.38 t) was analyzed. the cross-sections of the soft iron elements of the construction were set large enough to avoid saturation or close to saturation operation. initial values and the results of the simulation are summarized in table 3. simulation results prove slight increase (between 72 % and 78 %) in the ratio of theoretical and simulated air gap induction when varying the value of the air gap induction from 0.1 t to 1 t. examining the simulation results, we can find that that the operating point of the working magnet on the demagnetization curve b-h always shifts to the right of the ideal operating point. this phenomenon is caused by the modelling error, that is the computational models do not consider either the alternative reluctances caused by leakage or the real magnetization curve of the body. however, the difference in practice is small enough to be neglected in the general case. for this reason, the operating point of the external magnets also differs slightly from h = 0. the next step of the work is to determine a correction factor for the required initial air gap induction, resulting corrected geometric parameters at which the value of the simulated air gap induction will be equal to the originally required (not corrected) air gap induction. the correction relationship determined from the simulation results is expressed by the following equation 𝐵δkorr = 𝐵δ ∙ 1.281 + 0.018 . (8) including this correction into the original algorithm, the parametric simulation was repeated with initial values of 0.1 t and 0.75 t. the higher values is selected according to the maximum possible 1 t corrected initial value of induction. the results are shown in table 4. 7. the effect of permanent magnets used as flux conductors on dynamic behaviour experience has shown that the method mentioned in the previous chapter, discussing the use of permanent magnets instead of mild steel in the outer ring of cylindrical magnet circuits, improves the dynamic behaviour of such magnetic actuators. this improvement is detectable in both permanent magnet and electromagnetically excited constructions. due to the structure of the above-mentioned constructions, the magnetic field created by the vertically arranged voice coil current has an impact on the magnetic field of the stator (the still table 3. comparison of the calculated and the simulated results required air-gap induction in t simulated air-gap induction in t ratio simulated vs. required induction in % operating point induction of inner magnet in t operating point induction of external magnet in t 0.1 0.071 71.00 0.793 0.335 0.15 0.109 72.66 0.784 0.344 0.2 0.147 73.50 0.779 0.351 0.25 0.185 74.00 0.774 0.356 0.3 0.223 74.33 0.770 0.362 0.35 0.261 74.57 0.767 0.367 0.4 0.299 74.75 0.764 0.371 0.45 0.338 75.11 0.761 0.375 0.5 0.376 75.20 0.758 0.379 0.55 0.415 75.45 0.755 0.382 0.6 0.453 75.50 0.752 0.386 0.65 0.492 75.69 0.750 0.389 0.7 0.532 76.00 0.746 0.393 0.75 0.571 76.13 0.743 0.395 0.8 0.611 76.37 0.740 0.399 0.85 0.651 76.59 0.736 0.402 0.9 0.692 76.88 0.732 0.405 0.95 0.734 77.26 0.727 0.409 1 0.778 77.80 0.717 0.413 acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 139 part of the actuator, which means the overall magnetic circuit) as well. previous tests and simulations in the field have verified that in certain applications, when a high current flows through the voice coil, the magnetic field created by the coil modifies, distorts the originally homogeneous magnetic field of the air gap. the distortion of induction in the air gap is particularly high in the case of voice coils excited with direct current. as a result, since the length of the voice coil is finite, during operation the force applied to the turns exceeds the mechanical limits the construction of the voice coil was designed to withstand in the first place. thus, if the actuator is designed to exert constant or slowly changing high force, this distortion must be taken into consideration when developing the construction of the voice coil. figure 3 illustrates the character of distortion of induction in the air gap in the case of direct current controlling. furthermore, the tests have revealed that since in most cases the mild steel parts of such actuators are made of solid mild steel units, locally induced eddy currents compensate the distortion of induction in the air gap in the case of rapidly changing, dynamic voice coil currents. as indicated by tests on the first, electromagnetically excited prototype of the braking system to be developed, even in case of 20hz current components, the distortion of the induction in the air gap considerably decreases as compared to excitation with direct current. the results of a series of such tests are shown in figure 4. on the other hand, later finite element simulation tests on further developed magnetic constructions have revealed another problem: the momentarily self-inductance of the voice coil is connected to the reluctance of the magnetic ring of the stator. for dynamic operation, it is essential that the self-inductance of the voice coil be as low as possible in order to achieve the adequate impulse response. when excitation is carried out with ndfeb permanent magnets, the self-inductance of the voice coil in the construction examined (a magnetic ring with a 55 mm middle/mean diameter air gap) is relatively low, approximately 13 µh, which may be appropriate from the aspect of dynamic behaviour. in this case, finite element simulations have resulted in a linear connection with a fairly good approximation between the current in the coil and the force generated in the coil. this is caused by the high reluctance of the magnetic ring in the environment of the voice coil, for the magnetic field of the voice coil can considerably influence the volume and direction of inductance in permanent table 4. comparison of the calculated and the simulated results required induction in t corrected value of induction for simulation input in t simulated induction in t 0.1 0.139 0.106 0.15 0.205 0.154 0.2 0.271 0.202 0.25 0.337 0.251 0.3 0.402 0.3 0.35 0.468 0.35 0.4 0.533 0.399 0.45 0.598 0.449 0.5 0.663 0.499 0.55 0.728 0.55 0.6 0.792 0.6 0.65 0.857 0.652 0.7 0.919 0.705 0.75 0.984 0.759 figure 3. the cross-section of the 2-d model. figure 4. average ac component of the air gap induction as a function of the frequency in a voice-coil type linear actuator excited by dc current. figure 5. the simulation model of an improved construction of an ndfebmagnet based voice coil actuator (brake) including permanent magnets in the outer ring. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 140 magnets only in case of a much higher value of excitation than the optimal operational value in the bias point, due to the character of demagnetization curve of the permanent magnet. however, further dynamic simulations indicate that with the help of the flux conductor solution, discussed in the previous chapter, the self-inductance of the voice coil can be decreased even more (about 10 % 12 %). figure 5 demonstrates the a forementioned construction solution, while the relevant results of the finite element simulations are shown in figure 6 and figure 7. the dynamic analysis of the construction including permanent magnets has shown that this buildup can have quite excellent dynamic behaviour even without extra magnets. however, in the case of electromagnetic excitation, the entire magnetic ring is by default made of mild steel of high relative permeability, owing to which the reluctance of the magnetic ring is low and the magnetic field generated locally due to the current in the voice coil causes greater distortion in the original magnetic field of the stator excited with current pulse / stator supplied with excitation current. result of a finite element simulation on an electromagnetically excited construction (the size of the buildup is the same) indicate that in this case the self-inductance of the voice coil (identical in the case of voice coil dimensions) is about 100 times higher than the same value of the ndfeb magnet-based construction, and the exciting electric current changes considerably and non-linearly between the average value and depending on time. this simulation result is shown on figure 8. the reason for this is that the greater part of the metallic body is magnetized to almost full saturation for the sake of the greatest air gap induction possible, and in certain sections, due to the magnetic field of the voice coil, the temporary operating point shifts to the non-linear part of the magnetization curve. this results in a non-linear connection between the voice coil current and the force generated, which, together with a highvalue induction impair the dynamic behaviour of the construction. if certain parts of the outer ring of the stator consist of permanent magnets with the operation point settings defined in the previous chapter, this non-linearity and the inductance of the voice coil may be decreased significantly, thus improving dynamic behaviour. consequently, the solution discussed in the previous chapter is capable of improving the dynamic behaviour of a so-called “voice-coil-actuator” by decreasing the inductance of the voice coil and its non-linear nature, especially if the excitation of the magnetic ring of the stator is carried out with an electromagnet. 8. conclusions and outlook results of the research show that air gap induction correction is an effective method to calculate geometry of magnets and checking the real air gap induction for the calculated geometry for magnetic circuits of so-called “speaker-type voice coil actuator” actuators. accuracy of air gap induction simulation can be increased if considering further construction details, like join deviations or detailed material properties. difference between the magnetic properties of real soft magnetic material and simulated material may also cause some simulation error. converting cylindrical, axially symmetrical magnetic constructions to plane model by defined geometric transformations, the induction and field strength values of the magnetic circuit’s sections can be determined with acceptable accuracy. the acceptable accuracy highly depends on the compensation capacity of the control system to be used in the system, therefore checking and correcting the calculations by finite element simulations can be still useful. validation of the developed calculation and simulation methods is in progress. a prototype is designed and built with the geometrical dimensions defined by the described methods; tests will be performed in the near future. the validated methods will be used for development and optimization of industrial testing processes. acknowledgement this research was supported by the european union and the hungarian state, co-financed by the european regional development fund in the framework of the ginop-2.3.4-152016-00004 project, aimed to promote the cooperation between the higher education and the industry. references [1] a. váradiné szarka, linear magnetic break of special test requirements with dynamic performance, journal of electrical and electronics engineering, vol. 3, no. 2, 2010, pp. 237‒240. online [accessed 2 september 2021] https://www.ingentaconnect.com/content/doaj/18446035/201 0/00000003/00000002/art00031 [2] c. h. chen, a. k. higgins, r. m. strnat, effect of geometry on magnetization distortion in closed-circuit magnetic measurements, journal of magnetism and magnetic materials, vol. 320, no. 9, figure 6. moving coil inductance of the original construction (current flow starts at 50ms) figure 7. moving coil inductance of the improved construction (current flow starts at 50ms) figure 8. dynamic finite element simulation of a construction excited by electromagnet https://www.ingentaconnect.com/content/doaj/18446035/2010/00000003/00000002/art00031 https://www.ingentaconnect.com/content/doaj/18446035/2010/00000003/00000002/art00031 acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 141 2008, pp. 1597‒1656. doi: 10.1016/j.jmmm.2008.01.035 [3] l. kazup, a. váradiné szarka, diagnostics of air-gap induction’s distortion in linear magnetic brake for dynamic applications, xxi imeko world congress on measurement in research and industry, prague, czech 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https://product.tdk.com/en/products/magnet/technote/designguide.html https://product.tdk.com/en/products/magnet/technote/designguide.html https://www.tokyoferrite-ho.co.jp/en/wordpress/wp-content/uploads/2017/03/technical_02.pdf https://www.tokyoferrite-ho.co.jp/en/wordpress/wp-content/uploads/2017/03/technical_02.pdf microsoft word article 2 editorial carbone.docx acta imeko  august 2013, volume 2, number 1, 2  www.imeko.org    acta imeko | www.imeko.org  august 2013 | volume 2 | number 1 | 2  acta imeko: gaining momentum  paolo carbone  department of electronics and information engineering, university of perugia, via g. duranti, 93 – 06125 perugia, italy    citation: paolo carbone, acta imeko: gaining momentum, acta imeko, vol. 2, no. 1, article 2, august 2013, identifier: imeko‐acta‐02(2013)‐01‐02  editor: paolo carbone, university of perugia, italy  copyright: © 2013 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  corresponding author: paolo carbone, e‐mail: paolo.carbone@unipg.it    1. introduction  after the journal kick-off last year, much has been done to produce this issue: many new people were involved to process incoming papers and an improved software infrastructure now guarantees sustainability of all operations. the publication process has been fine-tuned and many new papers are currently entering the production process. acta imeko publishes extended papers that have been previously presented at workshops, congresses and symposia organized by imeko worldwide. it offers the opportunity to publish the outcomes of research activities, presented and discussed at conferences, to favour improvements. the amplitude of the topics treated within imeko, by its 24 technical committees guarantees a full coverage of the subjects of interest in metrology and of their applications. so, if you happen to present your scientific work in an imeko sponsored event, consider also the possibility to access acta imeko as your supporting medium for the extended version of your paper. the open access nature of this publication will guarantee maximum availability of your results, looking at the entire scientific world as a possible stakeholder. as first editor-in-chief of this scientific journal, i am very proud to introduce this issue, full of interesting contributions by scientists in the areas of measurement of electrical quantities (tc4), education and training in measurement and instrumentation (tc1), measurement science (tc7), and measurements in biology and medicine (tc13). papers refer to two major imeko events that took place recently in korea and germany. details on these symposia are provided in the introductions by the section editors of this issue, ján šaliga, dušan agrež and gerhard linß. many people have contributed to this issue under the supervision of the imeko vice-president for publications, paul regtien. beyond the valuable contributions of all involved reviewers, in particular francisco alegria, pedro ramos, sergio rapuano and dirk röske did all an excellent and effective job in making this publication possible. thank you to all of them. keep in touch with our journal and enjoy its scientific content. introduction to the special section of the 2019 apmf, the asia pacific measurement forum on mechanical quantities acta imeko issn: 2221-870x march 2021, volume 10, number 1, 5 acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 5 introduction to the special section of the 2019 apmf, the asia pacific measurement forum on mechanical quantities koji ogushi1, momoko kojima1 1 national metrology institute of japan, national institute of advanced industrial science and technology, tsukuba central 3, 1-1-1 umezono, tsukuba, 305-8563 ibaraki, japan section: editorial citation: koji ogushi, momoko kojima, introduction to the special section of the 2019 apmf, the asia pacific measurement forum on mechanical quantities, acta imeko, vol. 10, no. 1, article 2, march 2021, identifier: imeko-acta-10 (2021)-01-02 editor: francesco lamonaca, university of calabria, italy received march 17, 2021; in final form march 17, 2021; published march 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding authors: koji ogushi, e-mail: kji.ogushi@aist.go.jp momoko kojima, e-mail: m.kojima@aist.go.jp dear readers, measurement technology on mass, force, torque constitutes an integral part of intellectual infrastructure for a diverse range of human activities such as quality and safety assurance of industrial products, fair trade, energy-saving, and environmental protection. the asia pacific symposium on measurement of mass, force and torque (apmf), since its initiation in 1992, has been offering participants the opportunity of exchanging the latest information on r&d in these fields. it has been growing steadily as a not-to-miss event for metrologists, researchers, and engineers, especially those actively working in the asia-pacific region. the name "asia pacific measurement forum on mechanical quantities (apmf)" has been considered and changed from "asia pacific symposium on measurement of mass force and torque (apmf)" in 2017. besides, apmf activities' scope has been extended into different mechanical quantities such as density, hardness, pressure, vacuum, and others. the apmf 2019 has been held in niigata, japan, from 17th to 21st november 2019. it has been sponsored by the society of instrument and control engineers (sice), co-sponsored by the international measurement confederation (imeko) and niigata university, and organized by the national metrology institute of japan (nmij), a division of the national institute of advanced industrial science and technology (aist). the successful forum was contributed by more than 140 participants from 14 countries and economies, presenting 71 scientific papers. in this special issue, you can find four articles selected by the international program committee of the apmf, which is considered worthy of publishing in the acta imeko journal after peer-reviewing. the papers included in the special section show recent progress of the research on the measurements in the fields of mass, pressure, and flow in the asia-pacific region. we briefly introduce those papers as follows. in the field of mass metrology, one paper was selected. yuhsin wu and her colleagues presented the combined x-ray fluorescence (xrf) / x-ray photoelectron spectroscopy (xps) surface analysis system for quantitative surface layer analysis of si spheres in order to realize the new kg definition by x-ray crystal density (xrcd) method in cms/itri. ptb cooperated with cms by transmitting the information and technology of the xrcd method. in the field of pressure metrology, two papers are included in the issue. ahmed s. hashad and ptb team reported the evaluation of ptb force-balanced piston gauge (fpg), which is a non-rotating piston gauge. they have compared fpg with three different ptb pressure standards ranging from 3 pa to 15 kpa and confirmed the theoretically obtained effective area. the other is the topic about improvement of yokogawa's silicon resonant pressure transducer reported by hideaki yamashita. the characteristics of the pressure sensors are refined excellently based on the calibration results from nmij and yokogawa, which aimed to be used as a transfer standard in future key comparisons. in the flow field, masanao kaneko has numerically investigated the effect of a single groove on the flow behaviour and loss generation in a linear compressor cascade. analysis was performed by changing the tip clearance, which will be beneficial for the improvement of the compressor aerodynamic performance in the future. we are deeply grateful to all contributors, editors, authors, and reviewers who make this issue possible and hope you will enjoy reading this special section. koji ogushi, momoko kojima guest editors mailto:kji.ogushi@aist.go.jp mailto:m.kojima@aist.go.jp development and metrological characterization of cement-based elements with self-sensing capabilities for structural health monitoring purposes acta imeko issn: 2221-870x june 2023, volume 12, number 2, 1 12 acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 1 development and metrological characterization of cementbased elements with self-sensing capabilities for structural health monitoring purposes gloria cosoli1, alessandra mobili2, elisa blasi2, francesca tittarelli2,3, milena martarelli1, gian marco revel1 1 dip. di ingegneria industriale e scienze matematiche, università politecnica delle marche, via brecce bianche snc 60131 ancona, italy 2 dip. di scienze e ingegneria della materia, dell'ambiente ed urbanistica, università politecnica delle marche, via brecce bianche snc 60131 ancona-instm research unit, italy 3 istituto di scienze dell’atmosfera e del clima, consiglio nazionale delle ricerche, via gobetti 101 40129 bologna, italy section: research paper keywords: structural health monitoring; piezoresistivity; self-sensing materials; resilience; metrological characterization citation: gloria cosoli, alessandra mobili, elisa blasi, francesca tittarelli, milena martarelli, gian marco revel, development and metrological characterization of cement-based elements with self-sensing capabilities for structural health monitoring purposes, acta imeko, vol. 12, no. 2, article 31, june 2023, identifier: imeko-acta-12 (2023)-02-31 section editor: alfredo cigada, politecnico di milano, italy, andrea scorza, università degli studi roma tre, italy, roberto montanini, università degli studi di messina, italy received november 30, 2022; in final form april 22, 2023; published june 2023 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this research activity was carried out within the framework of the recity project “resilient city – everyday revolution”, identification code ars01_00592. corresponding author: gloria cosoli, e-mail: g.cosoli@staff.univpm.it 1. introduction the life cycle of cement-based structures can be optimized through proper measurements, in particular in the field of structural health monitoring (shm). indeed, the advantage of continuous monitoring is unquestionable with respect to periodic inspections, which can occur when it is already too late to intervene effectively. adequate interventions strategies can be adopted if measurement results timely highlight a potential damage in a structure/infrastructure [1]. accordingly, it is possible to minimize management costs [2], which are higher as the time between damage occurrence and intervention increases (de sitter’s law [3]). in this way, the public administrations can prioritize the interventions on structures and infrastructures abstract mortar specimens containing conductive additions (i.e., biochar and recycled carbon fibres – both alone and together, and graphene nanoplatelets) were characterized from a metrological point of view. their piezoresistive capability was evaluated, exploiting the 4electrode wenner’s method to measure electrical impedance in alternating current (ac); in this way, both material and electrodematerial polarization issues were avoided. the selected mix-design was used to manufacture scaled concrete beams serving as demonstrators. additionally, fem-based models were realized for a preliminary analysis of the modal parameters that will be investigated through impact tests conducted after different loading tests, simulating potential seismic effects. the results show that the combined use of recycled carbon fibers and biochar provide the best performance in terms of piezoresistivity (with a sensitivity of 0.109 (µm/m)-1 vs 0.003 (µm/m)-1 of reference mortar). conductive additions improve the signal-to-noise ratio (snr) and increase the material electrical conductivity, providing suitable tools to develop a distributed sensor network for structural health monitoring (shm). such a monitoring system could be exploited to enhance the resilience of strategic structures and infrastructures towards natural hazards. a homogeneous distribution of conductive additions during casting is fundamental to enhance the measurement repeatability. in fact, both concrete intrinsic properties and curing effect (hydration phenomena, increasing electrical impedance) cause a high variability. mailto:g.cosoli@staff.univpm.it acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 2 identified through proper shm technologies combined with an early warning system [4]. sensors play a pivotal role in this field [5], especially if they have internet of things (iot) capabilities that endorse data sharing, cloud services for computation (also with artificial intelligence – ai – technologies), and remote monitoring systems [6]. the best solution is probably represented by distributed sensor networks, able to gather data in many locations within the same structure, thus mapping the whole system and highlighting eventual criticalities in almost real-time [7]. moreover, the collected data are exploited to constitute a dedicated database, which can be used to feed ai and machine learning (ml) algorithms for both prediction and classification purposes [8]–[10]. furthermore, these data can be interfaced with the structure bim (building information model), hence keeping track of all the changes occurring over years [11]. nondestructive techniques (ndts) are among the most used systems for shm, since they allow to measure significant parameters without taking samples to be analysed in laboratory. standard sensors can be employed, such as accelerometers, load cells, inclinometers, gps, environmental sensors, and also noncontact systems (e.g. laser-based sensors) [12]–[14]. the spatial resolution of the measurement results clearly depends on the sensors positioning; as mentioned above, distributed sensor networks are particularly relevant in this field, since they allow an actual mapping of the whole structure to be monitored, with a level of detail related to the final aims of the monitoring. in this context, also the cost of the hardware should be properly considered, being almost proportional to the nodes number. even better results can be achieved combining monitoring and inspections procedures; the latter can be performed also with advanced techniques, such as sensorized drones (also known as unmanned aerial vehicles, uavs [15]). amongst possible sensors, uavs can have onboard environmental sensors for air quality assessment [16] or high-resolution multispectral visionbased systems to detect possible structural damages or degradation phenomena (e.g., caused by a prolonged exposure in an aggressive environment – such as chloride-rich solutions/aerosols). when inspection identifies an event of relevance, further tests can be planned, for example to assess the severity of the phenomenon (e.g., cracks aperture) [17], [18]. what is more, the scanning of a structure with a vision-based system embedded on a uav allows to obtain a 3d model, showing the positioning and the severity of the identified defects or damages. in recent years, shm sensors are often coupled to self-sensing materials [19] (even better if eco-compatible and sustainable, as by-products or recycled materials). in this way, it is possible to develop distributed sensor networks with iot capabilities, able to continuously gather data from remote buildings and infrastructures. this is particularly relevant in case of critical structures, which should be always operational, even after a catastrophic event, when the management of aftershock emergencies is pivotal. in this context, monitoring systems feeding early warning systems are very important to ensure public safety [13], [20]. indeed, self-sensing materials confer many capabilities to the structure, easing its monitoring. indeed, through self-sensing materials the structure perceives its own health status [21], being able not only to sense external loads (i.e., phenomena related to piezoresistive capacity), but also to detect the penetration of contaminants or identify defects and cracks. many materials have been recently applied to pursue this aim, both in form of fibres and fillers. among the others we can mention steel fibres [22], carbon fibres (both virgin and recycled), nickel powder [23], carbon nanotubes [24], graphene [25], graphite [26], foundry sand [27], carbon black [28], char, and biochar [29]. in a view of green and circular economy, recently particular interest has been shown on recycled material and byproducts potentially having self-sensing capabilities; this strategy allows not only to reuse materials, but also to limit production costs. for example, in the european project endurcrete (new environmental friendly and durable concrete, integrating industrial by-products and hybrid systems, for civil, industrial, and offshore applications, ga n° 760639, http://www.endurcrete.eu/) some of the present authors have developed self-sensing mix-designs for mortar and concrete including carbon-based additions, namely recycled carbon fibres and char or biochar. also a patent (https://www.knowledgeshare.eu/en/patent/eco-friendly-and-self-sensing-mortar/) has been granted on this invention, together with the related measurement system for electrical impedance (“eco-compatible and self-sensing mortar and concrete compositions for manufacturing reinforced and non-reinforced constructive elements, related construction element and methods for the realization of self-monitorable building structures”, patent n° 102020000022024). this can be exploited for shm purposes in self-monitorable structures, whose life cycle will result optimised thanks to the continuous assessment of health status. the activities of endurcrete project are being followed up within the framework of the national project recity (resilient city – everyday revolution, pon r&i 2014-2020, identification code: ars01_00592, http://www.ponricerca.gov.it/media/396378/ars01_00592decreto-concessione-prot369_10feb21.pdf), whose objective is to realize a multimodal monitoring system (modular and interoperable as much as possible) that can enhance the resilience of critical structures/infrastructures with respect to natural hazards, along with the resilience of energy distribution systems. among natural threats, it is worth mentioning earthquakes and landslides (also interacting each other). indeed, in italy particular attention is paid to seismic risk, being italy a seismic area, where important earthquakes often manifest in different regions, such as the crater of the centre of italy. in the perspective of seismic protection of buildings and infrastructures, on the one hand, the strains caused by external loads should be assessed to analyse possible structural damages; on the other hand, the analysis of vibrations is pivotal to characterise the dynamic behaviour of the whole structure and identify possible criticalities. lacanna et al. [30] considered a bell tower (giotto’s bell tower, firenze, italy) and studied its dynamic response through the combination of operational modal analysis and seismic interferometry. they evaluated frequency, shape modes, and seismic wave velocity, using a seismic sensor network capable to promptly identify eventual structural damages. the results showed that the analysed bell tower is a dispersive structure with bending deformation. induced vibrations represent a great concern for concrete-based lifelines, such as bridges [31]: hence, control and mitigation of vibrations is fundamental. in the context of seismic monitoring, accelerometers are the most common sensors; just as an example, oliveira et alegre [32] applied accelerometers in the monitoring of dams. this way, they were able to describe natural frequencies, mode shapes, and seismic response over time. indeed, in a seismic context shm surely plays a pivotal role, even more when monitoring is sided by an early warning system based on the measured signals. in this way, the public administrations can be supported in decision-making strategies acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 3 definition, essential for risk management and for the prioritization of emergency interventions. in the recity project, the authors of this paper utilize data-fusion strategies together with artificial intelligence (ai) technologies to extract meaningful parameters related to the structural health status of a structure. this information can be exploited to set up an early warning system, promptly highlighting critical situations that should be timely considered, providing an efficient intervention. furthermore, ai algorithms will be useful for prediction purposes as well, made possible through the ingestion of longtime series data for model training. finally, the recity project aims to valorise good practices for resilience, supporting citizens community during emergency situations. to this aim, the data gathered through the monitoring system will be shared in dedicated platforms with user-friendly interfaces, thus allowing the creation of a formed, informed, trained, and active community, which is aware of the city status and has a proper sense of community. this paper presents the results of the metrological characterization of different types of mortar with self-sensing capabilities, embedding sensing electrodes for electrical impedance measurement, thus proving their piezoresistive ability. the results were compared to standard measurements performed with traditional strain gages. moreover, this work reports the first results on monitoring of demonstrative scaled prototypes realized with the best performing conductive additions in terms of self-sensing capability. in the near future, these prototypes will be subjected to loading tests and will be analysed in terms of dynamic response, hence demonstrating their potentiality for application within the monitoring platform, especially in a seismic context. then, durability tests will be carried out and data will be acquired for a long time, thus collecting data useful for the training of ai algorithms in a view of early warning system. indeed, this is pivotal to enhance the resilience of structures and infrastructures to natural hazards (like earthquakes). the paper is organised as follows: the materials and methods are reported in section 2, section 3 shows the results, and the authors give their discussions and conclusions in section 4, together with possible future developments. 2. materials and methods the main aim of the recity project is to develop a flexible and interoperable platform for the collection of multimodal signals and their sharing on a cloud to deliver different services (e.g., data processing and ai algorithms exploitation for setting up early warnings and deriving significant indices for shm purposes). the potentialities of this platform are relevant for managing emergencies and adopting adequate policies in the seismic context, thus improving the resilience of structures and infrastructures, especially when they are critical constructions. data from different sensors will be gathered from the project demonstrators, which will be described in the following sections. the pipeline related to the platform is reported in figure 1. the collected data will be stored both in a dedicated database and in fiware ecosystem (https://www.fiware.org/); fiware can deal with different types of data and can also merge new and existent data models, in a view of developing smart cities and systems capable to share information and knowledge with diverse stakeholders (e.g., institutions and public decision-makers, but also common citizens). in this way, the researchers’ community can arise the awareness on the city structures and infrastructures, hence also improving the resilience towards possible emergency situations. 2.1. metrological characterization of piezoresistive capacity in a preliminary phase of the recity project, the authors considered mortar specimens to identify the best mix-design in terms of piezoresistive capacity. different types of conductive additions were employed, and their behaviour was evaluated under laboratory conditions. these tests were performed to select the best performing carbon-based additions to be used for the casting of the project demonstrators (concrete beams), which will be further detailed. indeed, those concrete specimens will be subjected to both loading tests and vibrational analyses, simulating the effect of a seismic event. the recity platform for shm will include, among the others, electrical impedance sensors and accelerometers; in particular, the signals will be acquired through a low-cost system, the eval-ad5940 board (by analog device). within the context of piezoresistivity tests on mortar specimens, electrical impedance signals were compared with traditional strain gages deformation, to characterize these selfsensing materials in terms of metrological performance. for sure, this performance is affected by the conductive additions included in the mix-design. prismatic mortar specimens (40 mm × 40 mm × 160 mm, figure 2) were realized according to 5 different mix-designs: • reference mortar specimens (ref), without conductive additions, to be considered as reference mixture; • biochar-based mortar (bch). the by-product was provided by res (reliable environmental solutions) in pellet format, then was grinded and sieved at 75 µm before addition in mix-design (0.5 vol.%) in order to facilitate its distribution within the mixture; figure 1. scheme of data acquisition from demonstrators, sharing, processing and results. acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 4 • mortar containing 6-mm long recycled carbon fibres (rcf). the fibres were supplied by procotex belgium sa and were obtained mixing carbon fibres of different origins and graphite from pure carbon fibres coils. a mean average density of 1.85 g/cm3 was considered and the fibres were added in 0.05 vol.%; • mortar with both biochar and recycled carbon fibres (bch+rcf), at the same dosages used in bch and rcf specimens (i.e., 0.5 vol.% and 0.05 vol.%, respectively); • mortar manufactured with graphene nanoplatelets (gnp), having a thickness of 6-8 nm and a size lower than 5 µm, in 0.5 vol.%. in particular, pentagraf 30 graphene nanoplatelets (produced by pentachem s.r.l.) were used; their specific surface area (measured with bet adsorbance method, brunauer, emmett, teller) was equal to 30 m2/g. to manufacture mortar specimens, we used portland cement (cem ii/a-ll) and a calcareous sand (0-8 mm) as fine aggregate, mixing it with 5.5 wt.% water to reach saturated surface dried (s.s.d.) conditions. the water/cement ratio (w/c) was the same for all the mortar specimens and was equal to 0.55 by mass, whereas the aggregate/cement ratio (a/c) was equal to 3 by mass. the mortars workability (x, measured according to uni en 1015-3 standard) was equal to 140 mm ≤ x ≤ 200 mm (i.e., plastic workability). the mix-designs referred to each type of mortar are reported in table 1. to characterize the mortars in terms of piezoresistive capacity, electrical impedance measurements should be carried out. hence, 4 stainless steel rods (diameter: 3 mm; length: 40 mm; inter-electrode distance: 12 mm), acting as electrodes, were embedded (half length) in the specimens centreline, in order to exploit the wenner’s configuration method [33] in alternating current (ac, in particular with a measurement frequency higher than 1 khz), hence avoiding both electrode-interface and material polarization effects, which would affect the measurement result with a significant uncertainty value. after the casting phase, specimens were cured in a temperature (t) and relative humidity (rh) controlled environment (t = (20 ± 1) °c; rh = (95 ± 5) %) for 7 days, being wrapped by plastic sheets. then, they were left at t = (20 ± 1) °c and rh = (50 ± 5) % without any cover. during the curing phase, the mortar specimens were regularly monitored in terms of both mechanical resistance and electrical impedance, with measurement carried out at 2, 7, and 28 days. compressive strength was assessed with a hydraulic press (galdabini, with an applicable maximum load of 200 kn), considering the average value obtained on 3 dedicated specimens of the same type. on the other hand, electrical impedance was assessed through electrical impedance spectroscopy method, employing a potentiostat/galvanostat (metrohm) with a 4electrode configuration (figure 3), on the prismatic mortar specimens with electrodes. the loading tests were performed after the completion of curing phase. a mechanical press (zwick roell z050) was employed to apply a maximum load equal to 11.5 kn. the value of the applied load was set in order to remain in the elastic range of the material [34], which was measured on the ref mortar during curing; in particular, 20% of the obtained compressive strength of the ref specimen was chosen. hence, the formation of cracks was avoided, as well as the alteration of the specimen mechanical properties. each specimen was subjected to 5 loading cycles per test and the test was repeated three times in different weeks (in a time interval of 8 weeks), for a total of 15 loading cycles per specimen. with this test protocol, also the variability due to hydration phenomena plays a relevant role on the measured electrical impedance. the complete test setup is reported in figure 4, where it is possible to observe the zwickroell mechanical press, equipped with a load cell (full scale: 50 kn), used to load the mortar specimen, on which a strain gage specific for cement-based materials (hbk, net grid length: 50 mm) was installed. spider8 system by hbm was employed to acquire strain gages signals, adopting a half-bridge configuration to compensate eventual external disturbs. moreover, a preliminary test was performed to compare the results obtained with halfand full-bridge configurations. to this aim, a bch specimen was subjected to 5 figure 2. geometry of the sensorized mortar specimen (strain gage and electrical impedance electrodes can be appreciated in the figure). table 1. mortars mix-design (kg/m3). mix cement water sand rcf bch gnp ref 499 274 1497 rcf 499 274 1496 1 bch 496 273 1489 10 bch+rcf 496 273 1488 1 10 gnp 496 273 1489 10 figure 3. wenner’s method applied on a mortar specimen: electric current is applied between the external electrodes (i.e. we and ce) and the corresponding electric potential drop is measured between the internal ones (i.e. s and re). acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 5 repeated loading cycles and the strain was assessed with both the wheatstone’s bridge configurations. data analysis focused on the real part of electrical impedance, which in literature is the most associated one to the structural conditions of the material [35]. 2.2. design and realization of the project demonstrators to verify the behaviour of self-sensing materials in a seismic context, the authors designed loading tests and vibrational analyses on scaled demonstrators. in particular, 1:5 scaled reinforced concrete beams (10 cm × 10 cm × 50 cm) were planned in detail, both in terms of materials and embedded sensors. for the former, the best conductive additions and dosages resulting from piezoresistivity tests were chosen; for the latter, different types of sensors were selected, namely: • electrical impedance sensors, which are fundamental to show the piezoresistive capacity of the concrete elements; • accelerometers, mounted on specific bases fixed on the upper specimen surface to measure the dynamic response of the structure to external excitation (provided with an impact hammer, as it will be described in detail); • sensors for the monitoring of rebar free corrosion potential, useful to early detect the concrete deterioration as cracking or water penetration occur [38] (cosmonet concrete structures monitoring network, università politecnica delle marche, patent n° 0001364988). indeed, after loading and vibrational tests, the specimens will be subjected to accelerated degradation tests; hence, the presence of cracks generated during loading could ease the penetration of contaminants into the material. the geometry of the prototype is reported in figure 5; 20 degrees of freedom for the measurement of the specimen dynamic response are foreseen (stainless-steel washers are positioned on the specimen upper surface through bicomponent acrylic resin, in order to easily install the accelerometers during experimental modal analysis through beeswax), whereas the excitation point is set on the specimen centreline. the specimen has a reinforcing steel rebar at its centre, where different sensors are placed: • electrode arrays for electrical impedance measurement; • pseudo-reference electrode for the measurement of the rebar free corrosion potential (cosmonet sensor [36]). the impact test will be carried out with a sensorized impact hammer (pcb 086 b04), equipped with a load cell for the measurement of the provided force. in this way, possible effects of earthquake are simulated, including cracking phenomena linked to external forces acting on the structural element. hence, the dynamic response is evaluated at time 0 (as-is conditions) and after each applied load, to observe possible modifications in the modal parameters of the element. figure 4. measurement setup for piezoresistivity tests: loads are applied on the tested mortar specimen through a mechanical press; hence, strain and electrical impedance are measured by means of a wheatstone’s bridge and a galvanostat/potentiostat, respectively. figure 5. geometry of the demonstrator prototype with sensors, namely electrode array for electrical impedance measurement, electrode for the measurement of the rebar free corrosion potential (i.e. cosmonet sensor), as well as excitation and acceleration measurement points for impact test. acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 6 2.2.1. manufacturing of concrete demonstrators no. 9 concrete specimens were manufactured as follows: • 3 sensorized concrete beams, to be subjected to loading tests and vibrational analyses; • 3 sensorized concrete beams, to maintain undamaged (i.e., reference specimens); • 3 non-sensorized concrete beams, to be subjected to loading tests and vibrational analyses. these serve to evaluate the effect of the embedded sensors, representing discontinuities for the material. to manufacture concrete specimens, we used portland cement (cem ii/a-ll) and a calcareous sand (0-4 mm) as fine aggregate, whereas intermediate (5-10 mm) and coarse (10-15 mm) river gravels were used as coarse aggregates. rcf and bch were added at 0.5 vol.% and 0.05 vol.% on the total, respectively, as conductive additions (being them resulted the best performing additions in terms of piezoresistive capability, see section 3.1.2). the w/c ratio was set at 0.50 by mass to reach the s5 workability class. the mix-design is reported in table 2. the casting phase was carried out using a concrete mixer; at first, the solid components were mixed together for 8 minutes, then water was added and mixed for additional 10 minutes. to manufacture sensorized/non-sensorized reinforced specimens, the fresh mix was poured in prismatic moulds (10 cm × 10 cm × 50 cm); in addition, cubic specimens (side: 10 cm) were realized for compressive strength tests (performed at 1, 7, and 28 days according to the en 12390-3 standard). moreover, also flexural strength was assessed on dedicated 10 cm × 10 cm × 50 cm nonsensorized reinforced specimens, according to the en 12390-5. 2.2.2. fem numerical model in order to carry out a preliminary test in terms of modal analysis, numerical simulations were made in comsol multiphysics® environment, exploiting finite element method (fem). in particular, the designed concrete beam was simulated in different configurations: • scaled (10 cm × 10 cm × 50 cm) and life-size (50 cm × 50 cm × 250 cm) concrete specimens, without reinforcing rebar; • scaled (10 cm × 10 cm × 50 cm) concrete specimen, with reinforcing rebar; • scaled (10 cm × 10 cm × 50 cm) concrete specimen, with reinforcing rebar and embedded sensors. indeed, the embedded sensors represent discontinuities inside the specimen; analogously, the plastic tubes used for installing the reinforcement rebar and the sensors influence the modal parameters of the structural element. hence, these preliminary models help to better understand the behaviour of the element in loading and vibrational tests, as well as to identify the natural frequencies of interest and the related modal shapes in both sensorized and non-sensorized beams. the geometry of the sensorized and non-sensorized reinforced concrete models is reported in figure 6 and in figure 7, respectively. 3. results this section reports the results related to the different research activities described in this paper, namely: • the results related to mortar specimens (section 3.1), including the monitoring of mechanical strength and electrical impedance of the different mix-design mortar specimens during curing (sub-section 3.1.1) and the results from piezoresistivity tests, together with the results of preliminary comparison tests between halfbridge and full-bridge configurations (sub-section 3.1.2); • the preliminary results related to the concrete beam demonstrators (section 3.2) in terms of mechanical strength characterization (sub-section 3.2.1), the monitoring of electrical impedance during curing (subsection 3.2.2), and the mode shapes resulting from fembased numerical simulations on both sensorized and nonsensorized concrete specimens (sub-section 3.2.3). table 2. concrete mix-design (kg/m3). cement 470 water 235 air (%) 2.5 sand 795 intermediate gravel 321 coarse gravel 476 rcf 1 bch 10 figure 6. geometry of the fem model related to the scaled sensorized concrete specimen. acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 7 3.1. monitoring and piezoresistivity tests on mortar specimens in this section, the results related to the monitoring during curing phase of mortar specimens (in terms of both mechanical strength and electrical impedance) and the piezoresistivity tests, together with the comparison between half-bridge and fullbridge configurations for the strain measurement, are reported. 3.1.1. monitoring during curing phase the results in terms of compressive strength and real part of electrical impedance are reported in figure 8 and figure 9, respectively. it is possible to observe an increase in both values, as expected during curing, since hydration of the material is occurring, hence electrical impedance and mechanical strength increase [37]. furthermore, mechanical strength at 28 days is enhanced through the addition of rcf to the mortar mix-design, passing from 36 mpa (ref specimen) to 40 mpa and 43 mpa for bch+rcf and rcf specimens, respectively. thus, it can be stated that the addition of rcf improves the mechanical performance of the material, thanks to the presence of carbon micro-particles on their surface acting as nucleation points for the formation of c–s–h crystals [38], [39]. also the self-sensing properties should be enhanced, given that rcf contribute to decrease the material electrical resistivity; in particular, a decrease of 85 % and 92 % is obtained for rcf and bch+rcf mixtures, respectively. in this way, it is possible to exploit relatively lowcost sensors for the monitoring of electrical impedance, hence enabling the realization of multiple modes sensor networks for shm purposes [2]. observing figure 9, it is possible to notice relevant differences in terms of electrical impedance among diverse mix-designs. this was expected and is attributable to the different conductive additions employed to realize the mortar specimens. indeed, rcf significantly decrease the electrical resistivity of the final material, thanks to their good electrical conductivity properties. 3.1.2. piezoresistivity tests preliminary tests were carried out to compare the results obtainable with a half-bridge and a full-bridge configuration for the measurement of strain of a mortar specimen (namely a bch specimen) subjected to cyclic loading tests. the results provided a repeatability range of 18 µε and 4 µε for half-bridge and fullbridge configurations, respectively; moreover, a repeatability deviation of 7 µε and 2 µε was obtained in the two cases, respectively. given that these values are acceptable for the infield application of interest, the half-bridge configuration was selected for the rest of piezoresistivity tests. indeed, it is an optimal compromise between metrological performance (accuracy and sensitivity – gage factor) and ease of installation as well as cost (also in view of distributed sensor networks realization). the results of the piezoresistivity tests are reported in table 3; in particular, the mean (µ) and standard deviation (σ) values are reported for applied maximum loading force (fmax), maximum strain (εmax), variation of the real part of electrical impedance (∆zre) and related electrical impedance at 0-time (zre_t0), and sensitivity of electrical impedance real part towards strain. results are reported for all the tested mix-designs and are figure 7. geometry of the fem model related to the scaled non-sensorized concrete specimen. figure 8. mechanical strength of mortar specimens during curing phase. figure 9. real part of electrical impedance of mortar specimens during curing phase. acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 8 averaged on the 15 loading cycles applied on each specimen. as expected, quite high values of standard deviation were obtained for electrical impedance. they can be attributed to the ageing process of the specimens (causing the material hydration – tests were performed in a time span of 8 weeks), which on the other hand cause also significant variations in terms of mechanical elasticity, reflecting into high standard deviation values for strain parameter. the sensitivity of electrical impedance towards strain (and, hence, external load) is improved by conductive additions. sensitivity passes from 0.003 µε-1 for ref mortar specimen to 0.109 µε-1 for bch+rcf one; in this case, in fact, the lower electrical resistivity leads to a higher percentage variation of electrical impedance. for the sake of completeness, it should be noted that rcf alone do not provide the same performance to mortar, at least at the considered load values; for this reason, biochar plays a key role in the provision of piezoresistive properties. moreover, high variability is observed also in the response of self-sensing materials in terms of electrical impedance variations; for example, considering bch+rcf mortar, a standard deviation of 26.46 ω for a mean value of 70.26 ω is reported for ∆zre quantity. this fact could be due to both hydration phenomena occurring over time (thus changing the material morphology and composition), as well as to the fact that cement-based materials (e.g., mortar and concrete) are inhomogeneous by definition. for this reason, significant variability could be observed also among specimens manufactured according to the same mix-design. in any case, the variations of the real part of electrical impedance mirror quite well the applied load and, hence, the strain of the specimen. an example of this behaviour is reported in figure 10 for bch+rcf specimen (5 loading cycles are considered); in particular, it is possible to observe that zre decreases with increasing applied load, since compression causes a decrease of the specimen length and, hence, of the sensing volume interested by the sensing electrodes. however, a lowmoderate strength of linear correlation was evidenced for all the tested mortar specimens, with the exception of bch+rcf mortar, where the pearson’s correlation coefficient was equal to 0.8 (figure 11). 3.2. the concrete beam project demonstrators in this section preliminary results related to the manufactured concrete beams as recity project demonstrators (figure 12) are reported. 3.2.1. mechanical strength the compressive strength measured on dedicated specimens is reported in figure 14. as expected, the compressive strength increases over time, reaching an average value of 40 mpa at 28 days, with a standard deviation of 1 mpa. concerning the flexural strength, an average value of 14 mpa was obtained, with a standard deviation of 1 mpa. 3.2.2. monitoring of electrical impedance during curing the electrical impedance data (in particular, in terms of real part, zre) is reported in figure 15. as expected, zre increases over time while only a specimen (i.e., c) is quite different from the others; this may be due to some particularly big aggregates present within the sensing volume. 3.2.3. fem numerical model the results obtained from scaled and life-size non-reinforced beams show that the natural frequencies vary together with the scaling factor; in particular, the natural frequencies will be 5 times those of the life-size element. for example, considering the first mode shape (figure 13), the natural frequency is estimated at 241 hz for the life-size structure (fn,real) and at 1205 hz for the scaled beam (fn,scaled); this means that fn,scaled is approximately 5 times fn,real. for this reason, it is necessary to evaluate the effects of a seismic event at frequencies higher than those typical of an figure 10. results in terms of loading force (f, top), strain (ε, centre), and real part of electrical impedance (zre, bottom) – example for bch+rcf mortar specimen. table 3. results obtained for the different mix-designs (reported as mean (standard deviation)). mix-design fmax in n εmax in µm/m ∆zre in ω zre_t0 in ω sensitivity in (µm/m)-1 ref 11516.20 (6.25) 157 (73) 23.16 (18.25) 5673.10 (3572.57) 0.003 (0.002) rcf 11512.16 (2.05) 181 (78) 0.92 (0.33) 203.14 (24.71) 0.004 (0.003) bch 11511.57 (2.11) 217 (119) 22.25 (15.30) 5725.73 (6071.60) 0.008 (0.009) bch+rcf 11512.76 (1.26) 155 (49) 70.26 (26.46) 419.07 (71.15) 0.109 (0.026) gnp 11511.88 (2.04) 226 (88) 9.86 (8.08) 6605.27 (4999.12) 0.001 (0.000) figure 11. evaluation of the linear correlation between the real part of electrical impedance (zre) and strain – the red line is the interpolating line (rcf+bch mortar specimen). acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 9 earthquake, which are in the range of 1-10 hz [40], [41]. the reinforcing rebar seems to not influence the natural frequencies of the concrete beam, at least at frequencies up to 4000 hz, which will be the spectral range considered in the experimental modal analysis; this means that the geometry of the rebar is not particularly influencing in terms of the element rigidity. however, the presence of the external tubes modifies the dynamic behaviour of the structural element; in particular, the nodal lines of the first mode shape (figure 16) move on the tubes themselves and the related natural frequency increases up to 1529 hz (approximately + 27 %). this means that the structural element is slightly stiffer because of the embedded components, which also influence the deformation, as well as making the specimen less homogeneous. considering the second mode shape, it is possible to observe that the presence of plastic tubes introduces two additional nodal lines located on the tubes themselves, even if the associated natural frequency is almost the same (i.e., 2841 hz for the nonsensorized specimen, figure 17, against 2846 hz for the sensorized one, figure 18). 4. discussion and conclusions this paper introduced the monitoring platform being developed in the framework of the recity project (identification code: ars01_00592); in particular, the resilience of cement-based structures against seismic events are considered in the presented research activities. at first, the authors investigate different mix-designs of mortar in terms of piezoresistive capability; hence, sensorized concrete beams are designed and realized with the better mix-design to serve as the project demonstrators. preliminary fem numerical models are realized to analyse the modal parameters of the structural elements and the effect of the discontinuities represented by the embedded sensors. figure 12. example of the manufactured concrete beam demonstrators; tubes for installing the steel rebars and sensors are visible at the specimen extremities, whereas stainless-steel washers for the installation of accelerometers for modal analysis can be observed on the specimens top face. figure 13. first mode shape for life-size beam (natural frequency: 241 hz, corresponding to 1205 hz for the 1:5 scaled beam). figure 14. compressive strength of concrete specimens during curing phase. figure 15. real part of electrical impedance measured during curing on the 6 sensorized specimens. acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 10 the results show that carbon-based conductive additions in form of filler and fibres (namely biochar, bch, and recycled carbon fibres, rcf) allow to obtain the best performance in terms of sensitivity to external loads. in particular, the measured electrical impedance shows a trend mirroring the one of the applied loads and, consequently, of the strain induced to the specimen. in this way, an electrical quantity (electrical impedance) reflects the behaviour of a mechanical quantity (strain), hence a sensor with self-sensing capacities is obtained. the metrological characterization of the phenomenon is pivotal and evidences the key role played by the type of conductive materials added to the mix-design. in fact, conductive additions have a twofold role: on the one hand, they decrease the material electrical resistivity, thus improving the circulation of electric current and easing the electrical impedance measurement; on the other hand, they improve the quality of the electric signal, decreasing the noise effect and, thus, enhancing the signal-tonoise ratio (snr). the best performance in terms of piezoresistive capability was obtained by the mix-design containing both bch and rcf, resulting in the highest sensitivity towards strain; in particular, the average sensitivity of bch+rcf mortar was equal to 0.109 (µm/m)-1, against 0.003 (µm/m)-1 of ref specimen. for this reason, these types and dosages of conductive additions were chosen for the realization of concrete scaled beams to serve as demonstrators of the recity project. furthermore, the selected conductive additions are green sustainable by-products, so they can be fruitfully exploited also in a view of an environmentally friendly circular economy. it is worthy to underline that a homogeneous distribution of conductive additions is fundamental. indeed, cement-based materials are inhomogeneous by definition; hence, the distribution of components during casting phase is pivotal. moreover, the electrical impedance measurement is local and is related to a limited sensing volume (depending on the interelectrode spacing chosen according to the wenner’s method, in the 4-electrode ac configuration). thus, it would be fundamental to optimize the manufacturing procedure to enhance the metrological performance of sensors based on selfsensing materials, especially in terms of measurement repeatability. furthermore, it should be considered also the fact that electrical impedance depends not only on the external loads, but also on several different variables, such as environmental parameters (temperature and relative humidity), damages and cracks, penetration of contaminants, carbonation phenomena, etc. for this reason, electrical impedance should be analysed not in absolute values, but in terms of trend variations, so as to be able to detect unexpected peaks or variations (differing from the normal daily changes [2]) that require ad hoc investigations (e.g. specific inspections). electrical impedance measurements can provide a lot of information on the structure health status and boundary conditions, resulting particularly suitable for data fusion techniques used in a view of extracting meaningful indicators in the context of shm. the sensing electrodes used for electrical impedance assessment could somehow substitute the traditional strain gages, which are much more expensive and difficult to install, besides being more delicate and requiring a more sophisticated acquisition circuit (i.e., wheatstone’s bridge). in the future, the realized concrete specimens will be subjected to loading tests with increasing load values (starting from 50% of the concrete flexural strength until failure load), in order to progressively drive cracking phenomena. modal analysis will be performed on the specimen as-is (time 0) and just after the execution of each load test. in this way, it will be possible to evaluate the effects of external loads and cracks on the modal parameters of the element, representing its “footprint”. both variations of natural frequencies and changes in the mode shape or mode curvature will be analysed, with the objective to both detect cracking onset and assess the severity of the damage. moreover, vision-based techniques will be exploited for the detection and the quantitative assessment of cracking phenomena; an automated measurement system developed within the framework of the endurcrete european project will be exploited to this aim. moreover, after the execution of loading figure 16. first mode shape for scaled sensorized beam (natural frequency: 1529 hz). figure 17. second mode shape for scaled non-sensorized beam (natural frequency: 2841 hz). figure 18. second mode shape for scaled sensorized beam (natural frequency: 2846 hz). acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 11 tests and related experimental modal analysis, all the concrete specimens will be subjected to accelerated durability tests, in particular to the exposure in water solutions. the aim is to evaluate how damages caused by seismic events can impact on the material durability. in the recity project the electrical impedance data will be combined to the signals measured by means of standard transducers; they will be exploited also together with modal parameters coming from vibrational analyses, thus contributing to characterize a cement-based structure from a broader perspective. in fact, the multidomain information, properly analysed through ai-bases algorithms, can support decisionmaking processes and management procedures regarding critical structures. this allows to prioritize the interventions needed to guarantee the community safety and wellbeing, also enhancing the resilience towards natural hazards and emergency situations. moreover, the recity platform will enable data sharing, so as to arise the community awareness on these aspects, making a society not only informed but also formed and active in the management of the (smart) city structures in an environment that inevitably becomes more and more urbanized. acronyms ac alternating current ai artificial intelligence bch biochar bim building information model fem finite element model gnp graphene nanoplatelets ml machine learning ndt non-destructive technique rcf recycled carbon fibers rh relative humidity shm structural health monitoring shr signal-to-noise ratio ssd saturated surface dried uav unmanned aerial vehicle w/c water-to-cement ratio acknowledgement this research activity was carried out within the framework of the recity project “resilient city – everyday revolution” (recity) – pon r&i 2014-2020 e fsc “avviso per la presentazione di ricerca industriale e sviluppo sperimentale nelle 12 aree di specializzazione individuate dal pnr 20152020”, identification code ars01_00592. references [1] dipartimento della protezione civile, indicazioni operative per il raccordo e il coordinamento delle attività di sopralluogo tecnico speditivo, italy, 2018, p. 29. online [accessed 22 june 2023] [in italian] https://www.lazio.beniculturali.it/wpcontent/uploads/2021/03/dpc-indicazioni-operative.pdf [2] n. giulietti, p. chiariotti, g. cosoli, g. giacometti, l. violini, a. mobili, g. pandarese, f. tittarelli, g. m. revel, continuous monitoring of the health status of cement-based structures: electrical impedance measurements and remote monitoring solutions, acta imeko, vol. 10, no. 4, dec. 2021, pp. 132–139. doi: 10.21014/acta_imeko.v10i4.1140 [3] w. r. de sitter, costs of service life optimization ‘the law of fives’, comité euro-international du béton, 1984, pp. 131–134. 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https://www.libreriauniversitaria.it/new-concrete-collepardi-mario-tintoretto/libro/9788890377723 https://www.libreriauniversitaria.it/new-concrete-collepardi-mario-tintoretto/libro/9788890377723 https://doi.org/10.1016/j.jobe.2022.104237 https://doi.org/10.1016/j.compositesb.2016.12.029 https://doi.org/10.1186/s40623-016-0544-8 https://doi.org/10.1029/2012gl054382 microsoft word article 15 102-789-1-galley final.docx acta imeko december 2013, volume 2, number 2, 86 – 90 www.imeko.org acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 86 establishing a metrological infrastructure and traceability of electrical power and energy in the r. macedonia ljupco arsov, marija cundeva-blajer ss. cyril and methodius university, faculty of electrical engineering and information technologies-skopje, ruger boskovic b.b., pob 574, 1000 skopje, r. macedonia section: technical note keywords: metrological infrastructure; power and energy measurements; calibrations; standards citation: ljupco arsov, marija cundeva-blajer, establishing metrology infrastructure and traceability of electrical power and energy in r. macedonia, acta imeko, vol. 2, no. 2, article 15, december 2013, identifier: imeko-acta-02 (2013)-02-15 editor: paolo carbone, university of perugia received april 12nd, 2013; in final form december 2nd, 2013; published december 2013 copyright: © 2013 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: this work was supported by faculty of electrical engineering and information technologies-skopje corresponding author: marija cundeva-blajer, e-mail: mcundeva@feit.ukim.edu.mk 1. introduction after gaining the independence of the r. macedonia, the process of creating a metrological system and an infrastructure necessary for the establishment of measurement standards and metrological traceability has begun. the development has started with the creation of own legislative, institutions, equipment and infrastructure for the application of legal and industrial metrology. for the measurement of electrical energy and power, in the frame of legal metrology, nothing was inherited from the former yugoslav system. however, the significance of these measurements impose quick measures to be undertaken, because of the large needs and quantity, the large finances connected to the trade of electrical energy, the need of consumers’ protection and insurance of conditions of fair trade. the adequacy of these solutions, the need of their upgrading and improvement, as well as the model which would create conditions for fair trade and electrical energy consumers’ protection are further discussed. 2. current state of the art the field of measurements of electrical energy and power is legaly regulated by the law on metrology from 2002 [1]. beside this law a set of rulebooks [2-4] are adopted. currently, there is a process of a transposition of the eu documents in the field of metrology, which are connected to certain types of measuring instruments (sector directives), [5]. through the law on metrology [1], as one of the basic objectives in the legal regulation of the measurements of electrical energy, the condition on insurance of fair trade is posed. this fair trade will be insured by exact measurements of the electrical energy, with legally aproved instruments in compliance to the rulebook on measuring instruments [3], calibrated and verified, and with measurements traceable to the national standard. the traceability requirement, defined in article 3 of the law on metrology [1], although it is not explicitly stated, should insure measurement traceability to the national standard of the r. macedonia, to national standards of other states and to international standards. abstract in the paper the current state and the establishment of a metrological infrastructure and traceability of the measurements of electrical power and energy, i.e. the creation of conditions for unity of power and energy measurement results, international comparability of the results and measurements which insure fair trade and consumers’ protection are elaborated. beside the legal aspects, also other features are discussed, like the needs for calibration and verification in the field of electrical power and energy, participants in the chain of measurements and trade with electrical energy, the organization, the infrastructure, the methods and the systems of calibration and verification. an organization and certain documents which will contribute to the establishment of a system in accordance with the international standards and practice, as well as traceability and fair trade, are proposed. acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 87 by taking into account that the trade of electrical energy is not only internal, but in a large part international, it is necessary to insure traceability and comparability of the results not only on a national, but also on an international level. the same addresses also to the usage of electricity meters, which should comply with the specifications of the domestic regulation [3] as well as with the european regulation [5]. from the aspect of law regulation, this requirment is fulfilled through transposition of the eu directives and harmonization of the national laws and regulation with the eu directives in the field of metrology. in respect to the compliance of the electricity meters to the requirements of the regulation, which is declared through posing of the „ce“ mark, and which presumes existence of a notified body which will participate in the procedure of approval and marking, it is unclear how the domestic producers could address this question, since the r. macedonia is not a member of the eu. according to the law on metrology [1], in order to insure the conditions for fair trade, the electricity meters should be verified, which is done by legal and physical entities selling electrical energy, and formally by the bureau of metrology of the r. macedonia. if the requirements of the international standard for bodies performing inspection [6] and the international standard for testing and calibration laboratories [7] are taken into account, this solution, inherited from the former system, is in contradiction to the requirements of these standards in respect to the independence, impartiality, integrity and confidentiality of the bodies performing verification (control), e.g. calibration. this has become especially obvious after the process of privatization of the distribution of electrical energy in 2006. the control performed by the interested body is in conflict with the requirement for independent, impartial, and confidential control of the electrical energy measurements. the market of electrical energy in the r. macedonia is not well developed yet. main participants in the trade of electrical energy are evn-macedonia, mepso, elem, licenced trade houses as well as the large industrial consumers (mak-steel skopje, feni industry kavadarci, mital steel-skopje etc.), light industry and the households. the bureau of metrology of the r. macedonia is in charge to insure the unity of the measurements of electrical energy, national standard of electrical energy, type approvals of the electricity meters produced according to the legal requirements, periodical verification/calibration, metrological surveilance of the electricity meters and measurement traceability to the national standard. the bureau of metrology of rm, by the help of the eu, has formed and partially equipped its laboratories in the last years. however in the laboratory for electrical energy there is no electrical power standard, so the bureau is not in a position to insure traceability of the electrical energy measurements i.e. is not in a position to calibrate standards and instruments for electrical power. the legal requirement for verification of the electricity meters is realized through verification (calibration) of electricity meters in the evn-macedonia laboratory for verification of electricity meters, which has 6 verification systems emh/mte on disposal equipped with a power standard, accuracy class 0.05. after the control/verification by the evn-macedonia, the bureau of metrology on the basis of the control results formally performs the verification and sealing. the bureau of metrology performs the metrological surveilance and verification through its department for verification of electricity meters. however, the methods of surveillance of the verification (metrological control) according to the equipment for control, sampling frequency, number of samples and the application of proper statistical methods according to the standards (iec 62058-11, [8], iec 62058-21, [9], iec 62058-31, [10]), do not give enough confidence of possible deviations from the requirements of the rulebook on measuring instruments [3]. the fee for verification of electricity meters which is paid to the bureau of metrology of rm (7 euro for three-phase electricity meters and 4 euro for single-phase meters), could be used for significant improvement of the equipment and the other preconditions and resources for confident metrological control as well as for insurance of traceability to the national and international standards [11], [12]. the electrical energy consumption in the r. macedonia, like in other countries, is connected to the life standard and is in constant increase. according to the state statistical office in the r. macedonia 82.6% (2009) and 83.6% (2010), out of 8265 837 mwh (2009), i.e. 8 677 969 mwh (2010), gross domestic consumption of electrical energy is of domestic production, and around 17% is imported [13, 14]. the biggest consumers of electricity in 2010 were the households with a share of 37.3%, the industrial sectors (energy sector plus industry) with 25.3%, and the other sectors with 17.7% of the gross national electricity consumption. own consumption (in production, transmission and distribution) of electricity in 2010 was 5%, while distribution losses were 14.7% of the gross national electricity consumption [14]. all this energy is measured at the level of system interconnections, at the level of large consumers, at the level of industry, at the level of small consumers and households at different voltage levels, different locations, and different types of electricity meters for direct and indirect electrical energy measurements. currently, in the r. macedonia approximately 800,000 electricity meters in the households and 7500 meters in the industry are in usage. the electricity meters used in the households of rm were in a large scale three-phase meters, accuracy class 2, produced by iskra-kranj, slovenia and in smaller scale of domestic production by video inzeneringohrid (by licence of siemens) and energetika-vds strumica (own development). the electricity meters used in the industry are mainly for indirect measurements, accuracy class 1 and 0.5, while the electricity meters at the system interconnections are of accuracy class 0.1. roughly, it can be estimated that these meters are used with approximately 30,000 instrument transformers of accuracy class 0.5. according to the law on metrology it is foreseen first, periodical and, if necessary, extra verification of the electricity meters to be done. the legal period for periodical verification table 1. annual verifications of electricity meters performed by bom. year 2007 year 2008 year 2009 year 2010 year 2011 101 400 99 800 231 900 180 000 120 000 acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 88 of the electricity meters is 10 years, which imposes that approximately 80,000 meters should be verified annually. according to the data of the bureau of metrology [15], in the last years the number of electricity meters verifications performed by the bom is given in table 1. these figures comprise the number of first verifications performed by the meter producers (foreign producers), as well as the number of periodically verified electricity meters by evn-macedonia. the legal period of verification of instrument transformers is 5 years, so the annual number of verified instrument transformers should be 6000. all the meters used for billing the electrical energy must have a type approval and verification. the type approval means compliance of the meter type to the technical standards and the legal regulation. in the procedure of type approval about. 30 different tests, including testing of the electrical and metrological characteristics, isolation tests, mechanical tests as well as some newer tests: software validation, life cycle and confidence, are performed. practically, it is impossible to test each meter, therefore samples of the newly developed meters are taken, and if all the tests are satisfying, the type approval is issued. the meters identical to the approved ones are considered to comply to the standards. according to the law, each electricity meter used for billing of electrical energy mus be verified. for verification of the meters and instrument transformers a limited set of tests is required, which is mainly testing of the meter error at different working conditions. 3. establishing of legal requirements, traceability and confidence of electrical power measurements the current legislation which regulates the measurements of electrical energy in the r. macedonia is discussed and referred to in section 2. if this regulation is compared with the regulation for measurement of electrical energy used in the eu countries, it can be noticed that a transposition of the mid directive, [5] is done, but for full coverage of the field of measurements of electrical energy and power, it is necessary to apply the european standards en 50470-1 [16] and en 504703 [17] for classes a, b, c (classes а, b and c correspond to accuracy classes 2, 1 and 0.5, respectively). the standards iec 62052-11 [18], iec 62053-21 [19] and iec 62053-22 [20], for classes 2, 1, 0.5, 0.5s and 0.2s should be applied too. one of the possible problems in the legal harmonization would be the insurance of indepence, impartiality, integrity and confidence of the bodies performing type testings, calibration and verification. therefore, changes in the law on metrology [1] and in the rulebook [4] are necessary. the goal of these changes is to enable the activity of independent accredited and authorized bodies for control of electricity meters, independent accredited calibration laboratories and independent services for electricity meters. one of the possible options of organization of the legal metrology for electricity meters is given in figure 1. the verification of the electricity meters and the instrument transformers in an authorized inspection body would be realized according to the sequence in figure 2. the traceability of the electrical energy measurements to national and international standards could be realized through a three-scale hierarchy chain shown in figure 3. for the realization of the traceability chain it is necessary to create a national laboratory which will be equipped with a national primary standard for electrical energy. this laboratory should fulfil the requirements of the standards iso 17025 [7] as well as to participate at international comparisons with other national laboratories, i.e. to calibrate its standard at national laboratories of other states with standards of a higher accuracy class. this national laboratory and the national standard could be in the frame of the bureau of metrology of the r. macedonia, but also other solutions are possible. the laboratory can be created where staff, equipment and space for such a laboratory already exist. according to the practice in most of the countries, the calibration laboratories for electricity meters would serve the figure 1. scheme-proposal on legal metrology of measurements of electrical energy. ministry of economy of rm, bom application approval authorization notification producers and importers independent control body verification of electricity meters and instrument transformers services application for type approval application for verification own inspection type testing application for verification of repaired meters verification/ inspection power companies consumers acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 89 industry and other owners of instruments for electrical energy and power for calibration and type testing, and should be independent laboratories which fulfil the requirements of the standard iso 17025 [7]. one of the important conditions for insurance of fair trade of electrical energy is the creation of independent and confident control/inspection body for verification of electricity meters. according to the international standard for control bodies iso 17020 [6], such bodies of type a should fulfil the following requirements: 1. the inspection body and its staff shall be independent to the parties involved, i.e. shall not be the designer, manufacturer, supplier, installer, and purchaser, owner, user or maintainer of the items which are inspected, nor authorized, representative of any of these parties. 2. the inspection body and its staff shall not be engaged in any activities that may conflict with their independence of judgment and integrity in relation to their inspection activities. in particular they shall not become directly involved in the design, manufacture, supply, installation, use or maintenance of the items inspected or similar competitive items. 3. all interested parties shall have access to the services of the inspection body. there shall not be undue financial or other conditions. the procedures under which the body operates shall be administrated in a non-discriminatory manner. beside the organizational, management and documentation requirements, the inspection body must be equipped with competent and responsible staff, which will respect the inspection procedures, criteria and ethics as well as proper equipment which must be calibrated with traceability to the national electrical energy standard. the inspection body should have on disposal proper procedures and protocols of testing and procedures for processing of the results, as well as statistical indicators for the pool of verified meters. for the activities of this inspection body, beside the standard iso 17020 and the national legislation, there are more international standards and international guides. 4. conclusions the current state and the importance of the measurements of electrical energy for billing and other purposes requires quick measures for harmonization and upgrading of the macedonian system of legal and industrial metrology for electrical energy. it is necessary to establish a national standard of electrical energy and a national laboratory for electrical energy, as well as traceability of the electrical energy measurements to them. it is necessary also to create independent competent calibration and testing laboratories, i.e. independent, impartial and confident inspection bodies for control/verification of electricity meters in compliance to the international standards iso 17025 and iso 17020, the standards for electricity meters and the practice in the other countries in europe and the world. the proposed schemes for practicing the legal metrology and establishing of figure 2. verification process. figure 3. traceability chain of the electrical energy measurements. international comparisons and calibrations national laboratory of electrical energy primary national standard (class 0.01) accredited laboratory for electrical energy standards (class 0.05) inspection body for verification of electricity meters standards (class 0.05) consumers electricity meters (class 2, 1, 0.5s, 0.2s) review and preparation testing assessment sealing document for refusal return/ delivery acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 90 the traceability chain in the measurements of electrical energy are possible options which should be further elaborated by taking into account all the aspects: legal, current state, staff, technical requirements, as well as the economic aspects. references [1] law on metrology, official gazette of r. macedonia no. 55/02, 84/07 and 120/09. [2] rulebook of the definitions, nomenclature and symbols, the scope, application and obligation for usage and writing of the legal measurement units, official gazette of r. macedonia, no. 104/2007. [3] rulebook on measuring instruments, official gazette of r. macedonia no. 17/10 [4] rulebook on determination of the categories and types of measuring instruments for which the verification is obligatory, procedures of verification, deadlines of periodical verification and the categories and types of measuring instruments on which an authorization for verification can be obtained, official gazette of r. macedonia”, no. 102/2007. [5] directive 2004/22/ec on measuring instruments (mid), official journal of the european union, 2004. [6] en iso 17020, general criteria for the operation of various types of bodies performing inspection, cenelec, brussels, 2004. [7] en iso/iec 17025, general requirements for the competence of testing and calibration laboratories, cenelec, brussels, 2005. [8] iec 62058-11, electricity metering equipment (ac) acceptance inspection, part 11: general acceptance inspection methods, international electrotechnical commission, geneve, 2008. [9] iec 62058-21, electricity metering equipment (ac) acceptance inspection part 21: particular requirements for electromechanical meters for active energy (classes 0,5, 1 and 2), international electrotechnical commission, geneve, 2008. [10] iec 62058-31, electricity metering equipment (ac) acceptance inspection part 31: particular requirements for static meters for active energy (classes 0,2 s, 0,5 s, 1 and 2), international electrotechnical commission, geneve, 2008. [11] decision on the amount and form of payment of the fee for services of the bureau of metrology and the authorized legal entity, official gazette of r. macedonia, no. 51/2004, no. 64/2008, no. 121/2010. [12] press release no. 0302-864/1 of bureau of metrology of r. macedonia from 1. 03. 2010. [13] press release no. 6.1.10.83, state statistical office of r. macedonia from 02.12.2010. [14] press release no. 6.1.11.91, state statistical office of r. macedonia from 30.11.2011. [15] strategic plan for the development of the bureau of metrology and the metrological infrastructure of r. macedonia 2010-2012, bureau of metrology of r. macedonia, 2010. [16] cen en 50470-1, electricity metering equipment (ac) general requirements, tests and test conditions. metering equipment (class indices a, b and c), cenelec, brussels, 2006. [17] cen en 50470-3: electricity metering equipment (ac) part 3: particular requirements static meters for active energy (class indices a, b and c), cenelec, brussels, 2006. [18] iec 62052-11, electricity metering equipment (ac) general requirements, tests and test conditions part 11: metering equipment, international electrotechnical commission, geneve, 2003. [19] iec 62053-21, electricity metering equipment (ac) particular requirements part 21: static meters for active energy (classes 1 and 2), international electrotechnical commission, geneve, 2003. [20] iec 62053-22, electricity metering equipment (ac) particular requirements part 22: static meters for active energy (classes 0.2 s and 0.5 s), international electrotechnical commission, geneve, 2003. 10-22-3-ce acta imeko december 2011, issue 0, 2 www.imeko.org acta imeko | www.imeko.org december 2011 | issue 0 | 2 journal contacts keywords: acta imeko, editorial, contacts citation: paul p.l. regtien, journal contacts, acta imeko, no. 0, december 2011, p. 2, identifier: imeko-acta-00(2011)-01-02 editor: paul regtien, measurement science consultancy, the netherlands received december 28, 2011; in final form december 29, 2011; published december 30, 2011 copyright: © 2011 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: this work was supported by measurement science consultancy, the netherlands corresponding author: paul p. l. regtien, e-mail: paul@regtien.net about the journal acta imeko is an e-journal reporting on the contributions on the state and progress of the science and technology of measurement. the articles are based on presentations presented at imeko workshops, symposia and congresses. the journal is published by imeko, the international measurement confederation. the issn, the international identifier for serials, is: 2221-870x. editorial and publication board professor paul p.l. regtien (netherlands vice president for publications) dr dirk röske (germany information officer) professor antónio da cruz serra (portugal chairman of the advisory board) professor pasquale daponte (italy chairman of the technical board) francisco allegria (portugal – editorial office) sergio rapuao (italy – editorial office) imeko technical committee chairmen (ex officio) about imeko the international measurement confederation, imeko, is an international federation of actually 39 national member organisations individually concerned with the advancement of measurement technology. its fundamental objectives are the promotion of international interchange of scientific and technical information in the field of measurement, and the enhancement of international co-operation among scientists and engineers from research and industry. addresses principal contact: paul p. l. regtien measurement science consultancy (msc) julia culpstraat 66 7558 jb hengelo (ov) the netherlands email: paul@regtien.net acta imeko attn. dr. dirk röske physikalisch-technische bundesanstalt (ptb) bundesallee 100, 38116 braunschweig germany support contact dirk röske email: dirk.roeske@ptb.de multipath routing protocol based on backward routing with optimal fuzzy logic in medical telemetry systems for physiological data measurements acta imeko issn: 2221-870x march 2022, volume 11, number 1, 1 -8 acta imeko | www.imeko.org march 2022| volume 11 | number 1 | 1 multipath routing protocol based on backward routing with optimal fuzzy logic in medical telemetry systems for physiological data measurements suryadevara kranthi1, pallavi deshpande2, shadab pasha khan3, amedapu srinivas4 1 department of it, v r siddhartha engineering college, vijayawada-520007, andhra pradesh, india 2 department of electronics and telecommunication, bharati vidyapeeth (deemed to be university) college of engineering, pune-411043, maharastra, india 3 department of it, oriental institute of science & technology, bhopal-462021, madya pradesh, india 4 department of cse, sreenidhi institute of science and technology, hyderabad-501301, telangana, india section: research paper keywords: authentication; quality of service; mobile ad hoc network; multipath routing; optimal fuzzy logic citation: suryadevara kranthi, pallavi deshpande, shadab pasha khan, amedapu srinivas, multipath routing protocol based on backward routing with optimal fuzzy logic in medical telemetry systems for physiological data measurements, acta imeko, vol. 11, no. 1, article 23, march 2022, identifier: imekoacta-11 (2022)-01-23 section editor: md zia ur rahman, koneru lakshmaiah education foundation, guntur, india received november 20, 2021; in final form february 22, 2022; published march 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: suryadevara kranthi, e-mail: kranthisri41@gmail.com 1. introduction a mobile ad hoc network (manet) has several sensor nodes [1] that are connected without central infrastructure via wireless media. to execute a fundamental routing activity like transferring data packets from one source to a location, routing techniques such as dynamic basis [2] and ad hoc remote vector [3] were devised. the manet properties should be taken into account in routing methods. the fundamental requirements of a manet are data dependability, multipath selection [4] and security [5] that improve network presentation. to attain this purpose many of the investigations have been developed. one of the important manet functionalities is on-demand routing [6]. in an ad hoc setting, the routing arrangements [7] must be confident, resilient and flexible. dynamic topology and node connection failure restricts the routing function. the mobility of the node makes it harder to route because it causes the nodes' connection failure. this frequent link failure leads to expensive routing and topology management, degrades reliability of data abstract mobile ad hoc network (manet) performance is critically affected by the mobility and resource constraints of nodes. node mobility will impact on connecting stability, and node resource limitations will result in congestion, so the development of a routing protocol that promotes quality of service (qos) in manet is quite difficult. in particular, frequent interrupting connection may degrade qos performance in the high-speed node drive scenario, so it is required to build the manet protocol routing which can be adapted for changes in the networking architecture to support qos. moreover, manet's multipath routing is the most necessary for secure transmission that can be achieved if self-centered nodes in the manet network are ignored. secure routing guarantees the reliability of data, confidentiality, authorization and authentication and non-denial. a novel safe multi-way method for dependable data transfer, which depends on the quality of service, is offered in this study. the ad hoc on demand backward routing protocol with optimal fuzzy logic (ofl) is also designed to provide multi-path routing. the hybridization of the bat optimization approach provides the optimum route by generating rules in ofl, and hence chooses an optimal rule. the efficiency of the planned technique is evaluated for factors such as final delay, the packet delivery ratio and so on. when the node is 150, the proposed hybrid bat algorithm achieved 0.61 of average packet latency, where the existing technique particle swarm optimization achieved nearly 1.2 to 1.3 packet latency. in this communication network, measurement of packet delivery ratio is an important task for establishing the communication network. the results show that our suggested work has more energy efficiency and network lifetime than existing ones. mailto:kranthisri41@gmail.com acta imeko | www.imeko.org march 2022| volume 11 | number 1 | 2 transfer and reduces network efficiency. the failure of the connection in manet therefore becomes a serious issue. furthermore, such a link failure often results in a trajectory of tragedy. as a result, data transmission reliability is diminished; the lower the packet transmission ratio the greater the delay from end to end. the measurement of data packets in a manet increases the overhead control message pricey and reduces routing function performance. this is in turn more useful method in physiological data measurement and transmission via cognitive radio-based networks. therefore, the selection of an alternative way or the creation of several paths is vital to ensure that the data transfer is reliable when the link is unsuccessful. and there is a lot of interest in loop-free paths to determine an ideal path between several routes between the source and destination of a network. in order to improve the reliability of the data transmission, multipath routing is constructed in a manet [8], providing for a load balance between the nodes. using several split pathways, the data has been sent parallel and the delivery rate is greatly improved. the problem of scalability, confidentiality, integrity and the life of the network is addressed in the multi-routing systems [9]. the dependability of data communication in a manet is ensured by multi-way [10] routing between source and destination. existing multipath routing systems in a manet cause flooding, empty neighbourhoods, flat management, widespread data, high energy consumption, meddling and load balance concerns. the efficient multi-way routing strategy is therefore presented to tackle one or more of these problems. in the dynamic environment changes and frequent path failures [11]-[12] also the existing multi-path routers are not good. they also produce an overhead route in the network. the overhead routing takes up a significant part of the bandwidth of the network and the power of the mobile node quickly exhausts. therefore, in order to limit the mobile node's participation in a route discovery phase to maintain dependability of data communication, with minimum overall costs, a reliable multi-path protocol is essential [13]. multi-routing protocols are continuously improved for one decade by updating the quality of service (qos) support demand. as a matter of fact, the multi path routing protocol may liken the available node resources in various paths, such as the available bandwidth, battery level, etc., and choose the optimum path for data to satisfy qos. today, in the manet path selection parameters not only the available path node resources but path stability is included. researchers are looking for different parameters to measure path stability effectively. however, the routing protocol for adaptive link state, which is changing quickly to enable qos, has not yet been fully researched in the highspeed movements of resource limited nodes scenario. finding adequate criteria for road stability would also be harder. the major goal of this study is the development of a safe, nearby position verification protocol for the qos performance of highspeed manet, by measuring the packet delivery ratio. a backward routing technique is devised in this routing protocol to determine the confident nodes. in this work, the optimum fuzzy [14]-[15] system is used to improve the qos routing procedure. in fuzzy logic system, the optimal rule generation is identified by using the hybrid bat algorithm (hbat) technique and compared its efficiency with other optimization techniques. the paper is structured as follows: literary review is presented in section 2. section 3 delivers an explanation of the methods proposed. section 4 provides validation of the methods proposed with current methodologies and measuring the packet delivery ratio. finally, section 5 offerings the conclusion of this study and its future effort. 2. literature review this section mainly examines several routing methods in order to deal with changes in network topology and analyses their quality of service. gomes et al. [16] introduces the industrial wireless sensor network (wsn) link quality estimator, and the dedicated wsn node assesses connection quality depending on the signal intensity received. the estimate of connecting stability by distance alone, however, is not adequate for scenarios with high speeds such nodes that move with an average speed of 20m/s. since node speed can be reflected in qos performance through path life. long lifetime paths can give superior qos due to the long-term stability of the broadcast path. to improve the qos, cross-layer multicast routing (clmr) protocol based on a tree is proposed in [17]. they take advantage of data on battery life, physical layer bandwidth, routing layer stability and application layer overhead to upgrade the tree and then calculate the cost function based on the attributes of each layer. to develop robust routing pathways, clmr selects low-cost nodes. a topological change adaptive ad-hoc multi-path distance vector protocol is developed in [18], which aims to minimize the data traffic by utilizing qos. the protocol's limits are that it does not function well in dynamic arrangements requiring stability and node density both on routes. in general, this protocol improves a little when other protocols do far better in many circumstances. the notion of finding the efficient route with least energy consumption and shortest distance was proposed for the fitness function adaptive ad-hoc on-demand multipath distance vector protocol (ff-aomdv) [19]. this protocol employed aomdv, and the broadcast will be performed over the alternate route with the shortest path in the routing table in the event of any breakdown or break in connection. the ff-aomdv model took into account few qos characteristics, which means that its presentation is not as high as aomdv, while network improvement is very limited. the authors of [20] devised a routing technique which improves the network's quality with ga. this work took into account several circumstances including mobility speed and failure of the nodes. compared to other protocols, the network performance was enhanced. however, the energy consumption issue was not taken into account. in the multi-track transmission control protocol in [21], the energy-efficient mobbing control technique was projected. the results show that this algorithm works better than multipath transmission control protocol because of its reduced energy use and improved throughput. the suggested approach did not however take into account the random loss regularly occurring in wireless connections in loss networks. therefore, loss of packets is supposed to decrease the congestion window and degrade data performance as a result of congestion lost. a routing procedure that takes into account the energy efficiency and certain qos routing factors was proposed by the authors in [22]. in order to update nodes to qod state, the method relies on the transmission of topological information to the whole network. this influx of information would raise the traffic overhead, especially with big networks. a protocol known as the dynamic energy ad-hoc on-demand distance vector (aodv) protocol was suggested in [23]. the major goal is to reduce the time required to transmit packets, reduce the consumption of energy as well as to maximize the life of the network. it determines the shortest distance path and selects the acta imeko | www.imeko.org march 2022| volume 11 | number 1 | 3 intermediate nodes with high residual power and network authority. in case of any inattentive link breaking or low-energy nodes during packet delivery, this procedure gives external energy to nodes. this enhances the reliability of the route and the lifespan. this external energy supply can also be considered to minimize the cost of the network. authors of [24] proposed to do away with congestion and clustering in wsn a new protocol called congestion and clustering routing. congestion control and model-based bandwidth estimates were presented in sender side to tcptraffic via containment-free multiple-access time division based manets [25]. both protocols offer acceptable routing options based on preventing congestion but do not take account of the random loss that would unduly narrow the congestion window. masood ahmad et al. [26] introduced a clustering technique based on honeybee and genetic algorithm. the overhead of topology maintenance is reduced by this structure. dynamic solutions with improved quality are provided by this combined algorithm, therefore, stable and balanced clusters are formed. however, multipath routing is done by this technique with high energy consumption. the energy issues and workload of ch are focused by amutha et al. [27] by developing a cluster manager-based ch selection scheme. the ch execute the packet transmission between sensor nodes in the network. low bandwidth, reliable throughput and low energy are provided by this technique. however, there is a major problem developed in this study, which is use of extra mechanism for selection process that leads automatically low bandwidth. 2.1. problem identification manet's volatility and limited assets make it very difficult to communicate information via such networks. – because of inconsistent wireless channel, integrated device shortages, channel quarrels, portability, node control and affirmation device, assurance of qos to manet applications is also highly complex. – the manet mobile nodes do not provide adequate power and risk repetitive node failures, resulting in a variety of network topologies, network allocations, packet loss problems and the lowest signal quality. – the current key organisation strategy of addressing an awkward node successfully does not deprive users of creating useful identities or the stealing of the individuality of individuals who do not participate in network activities. – moreover, manets generally require fresh limitations on problems related with qos in wire-based networks. this is the projected dynamic behaviour of the respective networks and the controlled assets. 3. proposed methodology in this study, a new optimum multi-track routing protocol is provided for manet to improve the qos, where network failures are reduced and will not use the information of individuals, who are not participate in the network activities. this is the four phases of the suggested model: the trust ideal for routing, the multipath routing, the optimal rule and data transmission. the phases of the proposed model are shown in figure 1. the secure adjacent position trust verification protocol (saptv) for analysis of trust value process is considered here. in our trust model each node maintains a value of trust for each of its neighbours. the ad-hoc on-demand distance vector backward routing (aodv-br) is employed for the development of confidential packets in the multi-way outing process. if source s wishes to connect with destination d, the route delivery is initiated by the source at that time. to improve the qos routing procedure, the optimum fuzzy system is used. the rules are made in a fuzzy sourceand target-based system. an inspired optimization model is proposed to optimize these rules. 3.1. trust model for multipath routing the source node begins the network-wide flood with the distribution of the route query packet, the most important waiting packets for the route response. the simulation process, the saptv protocol, is now intended. the data clustering updates the entry value. as demonstrated in (1), the trust measure is then taken into account 𝑇𝑣1~𝑇𝑣2 = 1 − (1 − 𝑇𝑣2) 𝑇1 , (1) where t 1 is the trust value suggestion and t 2 is the trust value direct. the values are between 0 and 1. the innovative technique specifies an explicit trust relationship as the trust suggestion among two nodes in the similar collection, but the trust references are regarded by deliberation as a confidence association among nodes in the diverging collection. the trust warning for pathways is always active if it is less than the trust value. 3.2. multipath routing (mr) process in order to ensure the source-destination route, the routing protocols must show significant energy efficiency levels. in this respect, there are now a number of energy-efficient routing algorithms. the protocols in this document propose that a considerable effort be made to depart from origin to endpoint figure 1. proposed model schematic diagram. figure 2. fuzzy logic process. acta imeko | www.imeko.org march 2022| volume 11 | number 1 | 4 node on the most energy-efficient path. a new routing method for manet is developed called aodv-br that identifies the least node’s residual energy in the paths and classifies the descending path in the descendant order of nodal residual energy. figure 2 provides an ideal fuzzy logic (fl) method block diagram. after several interactions, the connection quality of a route is likely to be significantly diminished. thus, the hbat algorithm, which works admirably efficiently, selects the best way among the traditional route in the network. 3.2.1. aodv-br protocol the response messages and data packets are checked, and local repair is conducted in the aodv-br by a backup route. the source and endpoint nodes have the ability to change the data in the aodv-br. thus, with the assistance of this strategy, the finest overheard backup route is taken up. when the data packet is eradicated, it is transmitted via a substitute path in the aodv-br. 3.3. routing process on optimal fuzzy logic in this study, qos is improved by providing the shortest route using the optimal fuzzy logic (ofl) algorithm, where hbat is developed for optimal rule generation in ofl algorithm. initially, the description of ofl is provided, where fl consists of four phases: flushing, generating a rule, membership, and defuzzifying. the values of a number of rules are properly translated into linguistic variables which in fluid logic describe the strength of settlement with the arithmetic value of the targets. three separate kinds of values of trust like friends, acquaintance and strangers are characterized correspondingly by the three fuzzy as (high, medium and low). 3.3.1. fuzzification the hard value of any fuzzy collection is subjected to fluctuation to describe the value of the relationship. it also demonstrates the ability of a higher system to adjust the real scalar quality to a fluffy value. a floating x-collector partition represents a job a: x an l, where l reaches [0, 1], otherwise known as the membership feature. 3.3.2. membership function this function is educated by many forms in order to evaluate the very comfortable member company, by put on the easiest separate member function from one another, through straight lines. 3.3.3. rule generation process the collection of if-then rules builds on the data of a human expert to detect the preferred performance of the scheme. the rules can be maintained with the aim of achieving the entire functionality of the system. under table 1 below are presented sample fuzzy rules. 3.4. data transmission after receipt of the message from the destination, the data packet is sent to the destination node. there are several transmissions in preferred pathways, and the link quality is likely to be lost. thus, because of loss of the connection value, the source node does not accept any recognitions from the target node. the network opts for an ideal way from the basis routes. the best way to reach the goal should be energy-effective and must be the shortest. therefore, in order to identify the shortest route in an optimal way, the research study uses hbat algorithm in ofl, which is explained as follows: 3.4.1. bat algorithm bat algorithm (bat), which was projected by xin-she yang in 2010, is a swarm-intelligence algorithm. in the algorithm, the search process is motivated by the fats, by the ability to search and locate the proofs. the bats emit noise and can make a difference between the food and beasts, and estimate the distance, using the reflected sound waves. the following equation (2) is used to update the location of the bats (solution) during the optimization procedure: 𝑥𝑖 𝑡 = 𝑥𝑖 𝑡−1 + 𝑣𝑖 𝑡 , (2) where the current solution is signified by 𝑥𝑖 𝑡−1 and the new, updated position at iteration 𝑖 − 𝑡ℎ solution is represented by 𝑥𝑖 𝑡 . the velocity is represented by 𝑣𝑖 𝑡 . the velocity at time step 𝑡 is designed as follows in (3): 𝑣𝑖 𝑡 = 𝑣𝑖 𝑡−1 + (𝑥𝑖 𝑡−1 − 𝑥∗)𝑓𝑖 , (3) where 𝑥∗, denotes the current best global location, and 𝑓𝑖 specifies the 𝑖 − 𝑡ℎ bat frequency. the solution frequency is consistently taken from the specified range from the least to the maximum frequency and is evaluated as follows in (4): 𝑓𝑖 = 𝑓min + (𝑓max − 𝑓min)𝛽, (4) where 𝑓min and 𝑓max are represented as the minimum and maximum frequency, correspondingly and 𝛽 is represented as a random number, 𝛽 ∈ [0, 1, which is defined as follows in (5): 𝑥new = 𝑥old + 𝜖𝐴 𝑡 , (5) where 𝛽 is a scaling factor with random values among 0 and 1, when the average loudness of all bats is given by 𝐴𝑡 . the latch is updated as follows, after the prey is found by a bat and that are shown in (6) and (7): 𝐴𝑖 𝑡 = 𝛼𝐴𝑖 𝑡−1, 𝑟𝑖 𝑡 = 𝑟𝑖 0[1 − e−𝛾 𝑡 ] (6) 𝐴𝑖 𝑡 → 0, 𝑟𝑖 𝑡 → 𝑟𝑖 0, while 𝑡 → ∞ , (7) where 𝐴𝑖 𝑡 designates to the loudness of 𝑖 − 𝑡ℎ bat, at iteration 𝑡, and 𝑟 is the pulse emission level. the values of 𝛼 and 𝛾 are constant. 3.4.2. hybridized bat algorithm the process of exploitation is strong in the unique bat algorithm, but it is caught in local optimum when exploring the search space. in this paper, the hybridized bat algorithm is used to improve the exploration phase of the bat algorithm with the search of bees from the algorithm for an artificial bee colony, in this way, it is prevented to get trapped to the local optima. randomly inside the lower and lower limits are generated the initial solution populations as in (8); table 1. fuzzy rules. s.no mobility trust value delay route 1 low low low below optimal 2 medium low medium below optimal 3 high high low sub optimal 4 high medium low sub optimal 5 low high low low optimal 6 low high low optimal 7 medium medium low optimal acta imeko | www.imeko.org march 2022| volume 11 | number 1 | 5 𝑥𝑖,𝑗 = 𝑙𝑏𝑗 + rand ∙ (𝑢𝑏𝑗 − 𝑙𝑏𝑗 ) , (8) where 𝑙𝑏𝑗 and 𝑢𝑏𝑗 are referenced to both the lower and the upper bound; rand is a random number of uniform distribution, rand ∈ [0, 1]; and 𝑥𝑖,𝑗 is a solution where the items are referenced by 𝑗. the fitness value is calculated as shown in (9) for each individual in the population after the generation of the starting population. 𝐹𝑥𝑖 = { 1 𝑓𝑥𝑖 𝑖𝑓 𝑓𝑥𝑖 ≥ 0 1 + |𝑓𝑥𝑖 | otherwise, (9) where the fitness function of 𝑖 − 𝑡ℎ individual is denoted by 𝐹𝑥𝑖 , and 𝑓𝑥𝑖 is the objective function of the 𝑖 − 𝑡ℎ individual. table 2 shows the parameters used in proposed algorithm. two alternative methods improve the exploration of search space. the counter (t) determines if the search procedure of the bat algorithm or the observer bee mechanism will be used. at each step, the persons move and update the position in accordance with the eq. (2), however the observer technique uses the following equation (10) if the value of t is odd: 𝑥𝑖,𝑗 𝑡 = 𝑥𝑖,𝑗 𝑡−1 + rand ∙ (𝑥𝑖,𝑗 𝑡−1 − 𝑥𝑘,𝑗 𝑡−1), (10) where the novel solution at time step 𝑡 is denoted by 𝑥𝑖,𝑗 𝑡 . the 𝑗 − 𝑡ℎ element of the 𝑖 − 𝑡ℎ individual is denoted by 𝑥𝑖,𝑗 𝑡−1and 𝑥𝑘,𝑗 𝑡−1 denotes the 𝑘 − 𝑡ℎ neighbor solution, rand ∈ [0, 1]. the random walk of the solution according to (5) will utilize the promising area. the selection probability that is expressed as follows in (11) determines whether the newly produced solution is established or discarded: 𝑝𝑖 = 𝐹avg ∑ 𝐹𝑥𝑖 𝑛 𝑖=1 , (11) where 𝑝𝑖 means the selection possibility and 𝐹 as the fitness value. 4. results and discussion in a series of simulated saptv scenarios, testing is performed with the proposed saptv-hbat algorithm, existing saptv particle swarm optimization, whale optimization algorithm (woa) and bat algorithms. a c++-based ns-2 simulator implements the planned algorithm and uses the amount of node, node mobility, traffic load and halt time for several simulation scenarios. table 3 lists the simulation parameters. nodes are randomly positioned within a 1000-meter radius in all circumstances. every mobile node's maximum transmission range is 250 m. nodes often move with speed in the range [0, 10] m/s, based on the random mobility point model. in this concept of mobility, all nodes migrate to a new destination point and remain there for a certain duration termed a pause. the channel is 2 mbps and the ieee 802.21 power save mode is used by the mac protocol. window size and beacon interval ad hoc traffic indication mode are 0.05s and 0.25s. each simulation is performed for 900 s. two ray floor models are employed to propagate. average control messages, power consumption, packet delivery ratio (pdr), network service life and delay are involved in the investigation. simulation outcomes were obtained after 10 runs to achieve stable status. table 2. hybridized bat algorithm control factors. notation parameter value n population size 20 𝛾 constant parameter 0.9 𝐴𝑚𝑖𝑛 constant minimum loudness 1 α constant parameter 0.9 𝐴0 maximum initial loudness 100 𝑓min minimum frequency 0 𝑓max maximum frequency 1 maxiter maximum iteration 30 algorithm 1. hybridized bat algorithm 𝑂𝑏𝑗𝑒𝑐𝑡𝑖𝑣𝑒 𝑓𝑢𝑛𝑐𝑡𝑖𝑜𝑛 𝑓(𝑥) 𝐼𝑛𝑖𝑡𝑖𝑎𝑙𝑖𝑧𝑒 𝑡ℎ𝑒 𝑝𝑜𝑝𝑢𝑙𝑎𝑡𝑖𝑜𝑛 𝑜𝑓 𝑏𝑎𝑡𝑠, 𝑡ℎ𝑒 𝑣𝑎𝑙𝑢𝑒𝑠 𝑜𝑓 𝑣𝑖 , 𝑟𝑖 𝑎𝑛𝑑 𝐴𝑖, 𝑑𝑒𝑓𝑖𝑛𝑒 𝑡ℎ𝑒 𝑓𝑟𝑒𝑞𝑢𝑒𝑛𝑐𝑦 𝑜𝑓 𝑝𝑢𝑙𝑠𝑒 (𝑓𝑖 )𝑎𝑡 𝑥𝑖 , 𝑡ℎ𝑒 𝑣𝑎𝑙𝑢𝑒 𝑜𝑓 𝑡ℎ𝑒 𝑚𝑎𝑥𝑖𝑚𝑢𝑚 𝑖𝑡𝑒𝑟𝑎𝑡𝑖𝑜𝑛 (𝑀𝑎𝑥𝐼𝑡𝑒𝑟), 𝑎𝑛𝑑 𝑠𝑒𝑡 𝑡ℎ𝑒 𝑖𝑡𝑒𝑟𝑎𝑡𝑖𝑜𝑛 𝑐𝑜𝑢𝑛𝑡𝑒𝑟 (𝑡) 𝑡𝑜 0 𝒘𝒉𝒊𝒍𝒆 𝑡 < 𝑀𝑎𝑥𝐼𝑡𝑒𝑟 𝒅𝒐 𝒇𝒐𝒓 𝑖 = 1 𝑡𝑜 𝑁 (𝑒𝑎𝑐ℎ 𝑁 𝑖𝑛𝑑𝑖𝑣𝑖𝑑𝑢𝑎𝑙𝑠 𝑖𝑛 𝑡ℎ𝑒 𝑝𝑜𝑝𝑢𝑙𝑎𝑡𝑖𝑜𝑛) 𝒅𝒐 𝒊𝒇 𝑡 𝑖𝑠 𝑒𝑣𝑒𝑛 𝒕𝒉𝒆𝒏 𝐶𝑎𝑙𝑐𝑢𝑙𝑎𝑡𝑒 𝑡ℎ𝑒 𝑣𝑒𝑙𝑜𝑐𝑖𝑡𝑦 𝑎𝑛𝑑 𝑓𝑟𝑒𝑞𝑢𝑒𝑛𝑐𝑦 𝑣𝑎𝑙𝑢𝑒 𝑢𝑠𝑖𝑛𝑔 (3) 𝑎𝑛𝑑 (4) 𝑃𝑒𝑟𝑓𝑜𝑟𝑚 𝑡ℎ𝑒 𝑏𝑎𝑡 𝑠𝑒𝑎𝑟𝑐ℎ 𝑝𝑟𝑜𝑐𝑒𝑑𝑢𝑟𝑒 𝑢𝑠𝑖𝑛𝑔 (2) 𝒆𝒍𝒔𝒆 𝑃𝑒𝑟𝑓𝑜𝑟𝑚 𝑡ℎ𝑒 𝑜𝑛𝑙𝑜𝑜𝑘𝑒𝑟 𝑠𝑒𝑎𝑟𝑐ℎ 𝑝𝑟𝑜𝑐𝑒𝑑𝑢𝑟𝑒 𝑢𝑠𝑖𝑛𝑔 (11) 𝒆𝒏𝒅 𝒊𝒇 𝒊𝒇 𝑟𝑎𝑛𝑑 > 𝑟𝑖 𝒕𝒉𝒆𝒏 𝑆𝑒𝑙𝑒𝑐𝑡 𝑡ℎ𝑒 𝑓𝑖𝑡𝑡𝑒𝑠𝑡 𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛 𝑃𝑒𝑟𝑓𝑜𝑟𝑚 𝑡ℎ𝑒 𝑟𝑎𝑛𝑑𝑜𝑚 𝑤𝑎𝑙𝑘 𝑝𝑟𝑜𝑐𝑒𝑠𝑠 𝑢𝑠𝑖𝑛𝑔 (5) 𝒆𝒏𝒅 𝒊𝒇 𝒊𝒇𝑝𝑖 < 𝐴𝑖 𝑎𝑛𝑑 𝑓(𝑥𝑖 ) < 𝑓(𝑥∗) 𝒕𝒉𝒆𝒏 𝑇ℎ𝑒 𝑛𝑒𝑤𝑙𝑦 𝑔𝑒𝑛𝑒𝑟𝑎𝑡𝑒𝑑 𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛 𝑖𝑠 𝑎𝑐𝑐𝑒𝑝𝑡𝑒𝑑 𝑅𝑒𝑑𝑢𝑐𝑒 𝐴𝑖 𝑎𝑛𝑑 𝑖𝑛𝑐𝑟𝑒𝑎𝑠𝑒 𝑟𝑖 𝑢𝑡𝑖𝑙𝑖𝑧𝑖𝑛𝑔 (6) 𝒆𝒏𝒅 𝒊𝒇 𝒆𝒏𝒅 𝒇𝒐𝒓 𝐹𝑖𝑛𝑑 𝑎𝑛𝑑 𝑠𝑎𝑣𝑒 𝑡ℎ𝑒 𝑐𝑢𝑟𝑟𝑒𝑛𝑡 𝑏𝑒𝑠𝑡 𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛 𝑥 ∗ 𝒆𝒏𝒅 𝒘𝒉𝒊𝒍𝒆 𝑅𝑒𝑡𝑢𝑟𝑛 𝑡ℎ𝑒 𝑏𝑒𝑠𝑡 𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛. table 3. simulation structure. parameter value packet size 256 bytes pause time 0–600 s transmission range (r) 250 m data size 5 mbytes data rate 2 mbits/sec simulation period 900 s traffic load 1 to 5 packets traffic type cbr sum of nodes 100–150 initial energy 180 j maximum node speed 10 m/s network area 1000 m × 1000 m acta imeko | www.imeko.org march 2022| volume 11 | number 1 | 6 initially, the energy consumption of each model is test and the graphical results are shown in figure 3. the particle swarm optimization (pso) algorithm, woa, bat and the proposed hbat of saptv achieved 4.9 j, 3.7 j, 3.5 j and only 2.4 j, when the node is 120. however, the energy consumption is increased, when the number of nodes upsurges. for instance, when the node is 150, the pso, woa, bat and proposed hbat of saptv achieved 5.8 j, 4.8 j, 3.9 j and only 2.8 j. this decrease is due to a decrease in energy, hops and traffic consumption in data transmission over the defined path. moreover, hop-proportional energy is also available. traffic loads hence reduce energy consumption when hop numbers are optimized. the packet size for the proposed saptv-hbat protocol is shown in figure 4. it is experiential that the proposed saptv-hbat has better pdr than saptv-pso saptv-woa and saptv-bat. the pdr increases when the number of nodes upsurges. when the node is 100, the pso, woa, bat and proposed hbat with saptv achieved pdr of 65, 79, 87 and 96. in addition, these techniques achieved pdr of 72, 80, 88 and 97, when the node is 130. the increased sum of nodes makes sure the optimal route from the source to the destination is selected. the amount of backup and duration, i.e., path life, is also increased. figure 5 displays the average packet latency in diverse network sizes for the proposed system. when the node is 110, the average packet latency of saptvpso, saptv-woa, saptv-bat and saptv-hbat is 0.66, 0.61, 0.47 and 0.42. the same techniques achieved 0.91, 0.81, 0.67 and 0.54 of average packet latency, when the node reaches 130th level. but, the existing technique pso and woa with saptv reaches high packet latency i.e. nearly 1.2 to 1.3, where bat achieves 0.93 and proposed hbat with saptv achieved 0.61 of average packet latency, when the node is 150. compared to other approaches, the proposed saptv-hbat is observed to be less delay. the growing number of paths between nodes results in the selection of the best path with fewer hop counts and node sharing. therefore, this approach has a reduced latency and uses bandwidth better. figure 6 displays the average sum of figure 3. graphical representation of proposed model in terms of energy consumption. figure 4. graphical representation of proposed saptvhbat on the basis of packet delivery ratio. figure 5. graphical representation of proposed saptv-hbat in terms of average packet latency. figure 6. graphical representation of proposed saptv-hbat in terms of average number of control packet. 0 1 2 3 4 5 6 100 110 120 130 140 150 e n e r g y c o n su m p ti o n number of nodes saptvpso saptvwoa saptvbat saptvhbat 0 20 40 60 80 100 1 0 0 1 1 0 1 2 0 1 3 0 1 4 0 1 5 0 p a c k e t d e li v e r y r a ti o number of nodes saptv-pso saptv-woa saptv-bat saptv-hbat 0 0,2 0,4 0,6 0,8 1 1,2 1,4 100 110 120 130 140 150 a v e r a g e p a c k e t l a te n c y number of nodes saptvpso saptvwoa saptvbat saptvhbat 0 5 10 15 20 25 1 0 0 1 1 0 1 2 0 1 3 0 1 4 0 1 5 0 n e tw o r k l if e ti m e number of nodes saptv-pso saptv-woa saptv-bat saptv-hbat acta imeko | www.imeko.org march 2022| volume 11 | number 1 | 7 control packets for the saptv-hbat design for various network sizes. when the node is 100, the average sum of control packets of pso, woa, bat and hbat with saptv is 1150, 1020, 950 and 830. the same techniques achieved 1250, 1158, 1059 and 950 of average number of control packets, when the node is 120. but, the existing technique pso and woa with saptv reaches high control packets i.e. nearly 1400 to 1500, where bat achieves nearly 1200 to 1300 and proposed hbat achieved 1120 and 1210 of average control packets, when the node is 140 and 150. it shows that control packets are increasing with the growth in the sum of nodes. the hbat optimization approach used to locate paths from source to destination is employed in a multipath network. figure 7 shows the study of network life. the lifetime of the network shall be measured as any of the nodes in the track are determined to be dead among the start of the data transmission into a particular path. when the node is 100, the average network lifetime of pso, woa, bat and hbat with saptv is 7, 9, 12 and 14. the same techniques achieved 8, 11, 13 and 15 of average network lifetime, when the node is 110. the average network lifetime of saptv-pso, saptv-woa, saptv-bat and saptv-hbat is 13, 15, 17 and 19, when the node is 130. but, the existing technique pso and woa reaches less network life time i.e., nearly 14 to 17, where bat achieves nearly 18 and 20 and proposed hbat achieved 21 and 23 of average path life time, when the node is 140 and 150. the lifetime of the network is better than everyone in the projected saptv-hbat. it is because the path selection is based on a multi-target function. the data transmission will not involve the node with excessive electricity consumption and traffic backlog. high traffic losses lead to packet loss and needless energy use. the hbat in saptv also displays better network lifetime than pso, woa and bat due to the consideration of trustworthy routes. 5. conclusion manet is an autonomous mobile node approach. qos protocols which can be adapted to quick topology modifications would assist high-speed manet network applications. a route may lose its link quality after the number of broadcasts. in this work, a multi-path routing system for the dependable exchange of data was suggested for qos. ofl is developed to find the shortest route in multipath routing, where the optimal rules are generated by hbat. the performance of the proposed model is compared with existing optimization approaches in terms of network life, pdr, energy consumption, average control packages and latency. the proposed protocol can be employed as an effective high-speed manet solution with qos necessities and limitations on resources such as multimedia communication between car and vehicle, the mobile video surveillance system etc. in the context of the previous energysensitive routing protocols the energy usage of the suggested routing protocol was quite small. as a future effort, the author will further examine the development of high-speed routing protocols that take into account both trajectory stability and node density. furthermore, the next research will create the way forward by implementing effective protocols and maximizing the performance, the qos and lowering communication delays. references [1] l. ciani, a. bartolini, g. guidi, g. 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csaba gór2, róbert moni3, istván harmati1 1 bme, dept. of control engineering and information technology, budapest, hungary 2 continental adas ai, budapest, hungary 3 bme, dept. of telecommunications and media informatics, budapest, hungary section: research paper keywords: artificial intelligence, machine learning, mobile robot, reinforcement learning, simulation-to-reality, transfer learning citation: andrás kalapos, csaba gór, róbert moni, istván harmati, vision-based reinforcement learning for lane-tracking control, acta imeko, vol. 10, no. 3, article 4, september 2021, identifier: imeko-acta-10 (2021)-03-04 section editor: bálint kiss, budapest university of technology and economics, hungary received january 17, 2021; in final form september 22, 2021; published september 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: andrás kalapos, e-mail: andras.kalapos.research@gmail.com 1. introduction reinforcement learning has been used to solve many control and robotics tasks. however, only a handful of papers have been published that apply this technique to end-to-end driving [1]-[7], and even fewer studies have focused on reinforcement learningbased driving, trained only in simulations and then applied to real-world problems. generally, bridging the gap between simulation and the real world is an important transfer-learning problem related to reinforcement learning, and it is an unresolved task for researchers. mnih et al. [1] proposed a method to train vehicle controller policies that predict discrete control actions based on a single image of a forward-facing camera. jaritz et al. [2] used wrc6, a realistic racing simulator, to train a vision-based road-following policy. they assessed the policy's generalisation capability by testing it on previously unseen tracks and on real driving videos in an open-loop configuration; but their work did not extend to an evaluation of real vehicles in closed-loop control. kendall et al. [3] demonstrated real-world driving by training a lanefollowing policy exclusively on a real vehicle under the supervision of a safety driver. shi et al. [4] presented research that involved training reinforcement learning agents in duckietown, in a similar way to that presented here; however, the focus was mainly on presenting a method that explained the reasoning behind the trained agents rather than the training methods. also similar to the present study, balaji et al. [5] presented a method for training a road-following policy in a simulator using reinforcement learning and tested the trained agent in the real world, yet their primary contribution is the deepracer platform rather than an in-depth analysis of the road-following policy. almási et al. [7] also used reinforcement learning to solve lane following in the duckietown environment, but their work differs from the present study in the use of an off-policy reinforcement learning algorithm (deep q-networks (dqns) [8]); in this study an on-policy algorithm (proximal policy optimization [9]) is used, which achieves significantly better sample efficiency and shorter training times. another important difference is that almási et al. applied hand-crafted colour threshold-based segmentation to the input images, whereas the method presented here takes the ‘raw’ images as inputs, which allows for a more robust real performance. abstract the present study focused on vision-based end-to-end reinforcement learning in relation to vehicle control problems such as lane following and collision avoidance. the controller policy presented in this paper is able to control a small-scale robot to follow the righthand lane of a real two-lane road, although its training has only been carried out in a simulation. this model, realised by a simple, convolutional network, relies on images of a forward-facing monocular camera and generates continuous actions that directly control the vehicle. to train this policy, proximal policy optimization was used, and to achieve the generalisation capability requir ed for real performance, domain randomisation was used. a thorough analysis of the trained policy was conducted by measuring multiple performance metrics and comparing these to baselines that rely on other methods. to assess the quality of the simulation-to-reality transfer learning process and the performance of the controller in the real world, simple metrics were measured on a real track and compared with results from a matching simulation. further analysis was carried out by visualising salient object maps. mailto:andras.kalapos.research@gmail.com acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 8 this paper is an extended version of the authors’ original contribution [10]. it includes the results of the 5th ai driving olympics [11] and aims to improve the description of the methods. in both works, vision-based end-to-end reinforcement learning relating to vehicle control problems is studied and a solution is proposed that performs lane following in the real world, using continuous actions, without any real data provided by an expert (as in [3]). also, validation of the trained policies in both the real and simulated domains is conducted. the training and evaluation code for this paper is available on github1. 2. methods in this study, a neural-network-based controller was trained that takes images from a forward-looking monocular camera and produces control signals to drive a vehicle in the right-hand lane of a two-way road. the vehicle to be controlled was a small differential-wheeled mobile robot, a duckiebot, which is part of the duckietown ecosystem [11], a simple and accessible platform for research and education on mobile robotics and autonomous vehicles. the primary objective was to travel as far as possible within a given time without leaving the road. lane departure was allowed but not preferred. although the latest version of the duckiebot is equipped with wheel encoders, for this method, the vehicle was solely reliant on data from the robot's forward-facing monocular camera. 2.1. reinforcement learning algorithm in reinforcement learning, an agent interacts with the environment by taking 𝑎𝑡 action, then the environment returns 𝑠𝑡+1 observation and 𝑟𝑡+1 reward. the agent computes the next 𝑎𝑡+1 action based on 𝑠𝑡+1 and so on. the policy is the parametric controller of the agent, and it is tuned during the reinforcement learning training. sequences of actions, observations and rewards (𝜏 trajectories) are used to train the parameters of the policy to maximise the expected reward over a finite number of steps (agent–environment interactions). for vehicle control problems, the actions are the signals that control the vehicle, such as the steering and throttle, and the observations are the sensor data relating to the environment of the vehicle, such as the camera, lidar data or higher-level environment models. in this research, the observations were images from the robot's forward-facing camera, and the actions were the velocity signals for the two wheels of the robot. policy optimisation algorithms are on-policy reinforcement learning methods that optimise the parameters of the πθ(𝑎𝑡|𝑠𝑡) policy based on the 𝑎𝑡 actions and the 𝑟𝑡 reward received for them; 𝜃 denotes the trainable parameters of the policy. on-policy reinforcement learning algorithms optimise the πθ(𝑎𝑡|𝑠𝑡) policy based on trajectories in which the actions have been computed by πθ(𝑎𝑡|𝑠𝑡). in contrast, off-policy algorithms (such as dqns [8]) compute actions based on the estimate of the action-value function of the environment, which they learn using data from a large number of (earlier) trajectories, making these algorithms less stable in some environments. in policy optimisation algorithms, the πθ(𝑎𝑡|𝑠𝑡) policy is stochastic, and in the case of deep reinforcement learning, it is implemented by a neural network, which is updated using a gradient method. the policy is stochastic because, instead of computing the actions directly, 1 https://github.com/kaland313/duckietown-rl (accessed 23 september 2021) the policy network predicts the parameters of a probability distribution (see 𝜇 and 𝜎 in figure 1) that is sampled to acquire the 𝑎�̃� predicted actions (here, predicted refers to this action being predicted by the policy). in the present study, to train the policy, the proximal policy optimization algorithm [9] was used because of its stability, sample-complexity and ability to take advantage of multiple parallel workers. proximal policy optimization performs the weight updates using a special loss function to keep the new policy close to the old, thereby improving the stability of the training. two loss functions were proposed by schulman et al. [9]: 𝔏clip(𝜃) = �̂�[min(𝜌𝑡(𝜃)�̂�𝑡,clip(𝜌𝑡(𝜃),1 − 𝜖,1 + 𝜖)�̂�𝑡)], (1) 𝔏klpen(𝜃) = �̂�[𝜌𝑡(𝜃)�̂� − 𝛽 kl[πθold(⋅ |𝑠𝑡),πθ(⋅ |𝑠𝑡)]], (2) where clip(⋅) and kl[⋅] refer to the clipping function and the kullback–leibler (kl) divergence, respectively, while �̂� is calculated as the generalised advantage estimate [12]. in these loss functions, 𝜖 is usually a constant in the [0.1,0.3] range, while 𝛽 is an adaptive parameter, and 𝜌 𝑡 (𝜃) = πθ(𝑎𝑡|𝑠𝑡) πθold(𝑎𝑡|𝑠𝑡) . (3) an open-source implementation of proximal policy optimization from rllib [13] was used, which performs the gradient updates based on the weighted sum of these loss functions. the pseudo code and additional details for the algorithm are provided in the appendix. 2.2. policy architecture the controller policy was realised by a shallow (4-layer) convolutional neural network. both the policy and the value network used the architecture presented by mnih et al. [1], with the only difference being the use of linear activation in the output of the policy network. no weights were shared between the policy and the value network. this policy is considered to be endto-end because the only learning component is the neural network, which directly computes actions based on observations from the environment. some preand post-processing was applied to the observations and actions, but these only performed very simple transformations (explained in the next paragraph and section 2.3). the aim of these preand post-processing steps was to transform the 𝑠𝑡 observations and 𝑎𝑡 actions into representations that enabled faster convergence without losing figure 1. illustration of the policy architecture with the notations used. the agent is represented jointly by the ‘policy network’ and ‘sampling action distribution’ blocks; 𝑠𝑡: ‘raw’ observation, 𝑠�̃�: pre-processed observation, 𝑎�̃�: predicted action, 𝑎𝑡: post-processed action. https://github.com/kaland313/duckietown-rl acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 9 any important features in the observations or restricting necessary actions. the input of the policy network consisted of the last three observations (images) scaled, cropped and stacked (along the depth axis). the observations returned by the environment (𝑠𝑡 on figure 1) were 640 × 480 (width, height) rgb images, the top third of which mainly showed the sky, which was therefore cropped. the cropped images were then scaled down to 84 × 84 resolution (note the uneven scaling), which were then stacked along the depth axis, resulting in 84 × 84 × 9 input tensors (𝑠�̃� in figure 1). the last three images were stacked to provide the policy with information about the robot's speed and acceleration. multiple action representations were experimented with (see section 2.3). based on these representations, the policy outputs 𝒂�̃� predicted an action vector of two or a scalar value that controlled the vehicle. the policy was stochastic, and the output of the neural network therefore produced the 𝜇 and logσ parameters of a multivariate diagonal normal distribution. during training, this distribution was sampled to acquire the 𝑎�̃� actions, which improved the exploration of the action space. during these evaluations, the sampling step was skipped by using the predicted 𝜇 mean value as the 𝑎�̃� policy output. 2.3. action representations the action mapping step transformed the 𝑎�̃� predicted actions, which could be implemented using many representations, to 𝑎𝑡 = [𝜔𝑙,𝜔𝑟] wheel velocities (see figure 1). the vehicle to be controlled was a differential-wheeled robot; the most basic action representation was therefore to directly compute the angular velocities of the two wheels as continuous values in the 𝜔𝑙,𝑟 ∈ [−1;1] range (where 1 and −1 corresponded to forward and backward rotation at full speed). however, this action space allowed for actions that were not necessary for the manoeuvres examined in this paper. moreover, as the reinforcement learning algorithm ruled out unnecessary actions, exploration of the action space was potentially made more difficult, and the number of steps required to train an agent was therefore increased. several methods can be used to constrain and simplify the action space, such as discretisation, clipping some actions or mapping to a lower-dimensional space. most previous studies [1],[2],[5],[7] have used discrete action spaces, thus the neural network in these policies selected one from a set of hand-crafted actions (steering, throttle combinations), while kendall et al. [3] utilised continuous actions, as has been used in this study. in order to test the reinforcement learning algorithm's ability to address general tasks, multiple action mappings and simplifications of the action space were experimented with. these are described in the following paragraphs. wheel velocity: wheel velocities were a direct output of the policy; 𝑎𝑡 = [𝜔𝑙,𝜔𝑟] = 𝑎�̃�, therefore 𝜔𝑙,𝑟 ∈ [−1;1]. wheel velocity positive only: only positive wheel velocities were allowed because only these were required to move forward. values predicted outside the 𝜔𝑙,𝑟 ∈ [0;1] interval were clipped: 𝑎𝑡 = [𝜔𝑙,𝜔𝑟] = clip(𝑎�̃�,0,1). wheel velocity braking: wheel velocities were still only able to fall within the 𝜔𝑙,𝑟 ∈ [0;1] interval, but the predicted values were interpreted as the amount of braking from the maximum speed. the main differentiating factor from the ‘positive only’ option was the bias towards moving forward at full speed: 𝑎𝑡 = [𝜔𝑙,𝜔𝑟] = clip(1 − 𝑎�̃�,0,1). steering: predicting a scalar value that was continuously mapped to combinations of wheel velocities. the 0.0 scalar value corresponds to moving straight (at full speed), while −1.0 and 1.0 refer to turning left or right with one wheel completely stopped and the other going at full speed. intermediate values are computed using linear interpolation between these values. the speed of the robot is always maximal for a particular steering value. below is the formula that implements this action mapping: 𝑎𝑡 = [𝜔𝑙,𝜔𝑟] = clip([1 + 𝑎�̃�,1 − 𝑎�̃�],0,1). 2.4. reward shaping the reward function is a fundamental element of every reinforcement learning problem, as it serves the important role of converting a task from a textual description to a mathematical optimisation problem. the primary objective for the agent is to travel as far as possible within a given time in the right-hand lane; therefore, two rewards that promote this behaviour were proposed. distance travelled: the agent’s reward was directly proportional to the distance it moved along the right-hand lane at each step. only longitudinal motion was counted and only if the robot stayed in the right-hand lane. orientation: the agent was rewarded if it was facing and moving in the desired orientation, which was determined based on its lateral position. in simple terms, it received the largest reward if it faced towards the centre of the right-hand lane (some example configurations are shown in figure 2 d). a term proportional to the angular velocity of the faster moving wheel was also added to encourage fast motion. this reward was calculated as 𝑟 = 𝜆ψ 𝑟ψ(𝛹,𝑑) + λ𝑣 𝑟𝑣(𝜔𝑙,𝜔𝑟), where 𝑟ψ(⋅),𝑟𝑣(⋅) are the orientation and velocity-based components (explained below), while the 𝜆ψ,𝜆𝑣 constants scale these to [-1,1]. 𝛹,𝑑 are the orientation and lateral error from the desired trajectory, which is the centreline of the right-hand lane (see figure 2 a). the orientation-based term was calculated as 𝑟ψ(𝛹,𝑑) = λ(𝛹𝑒𝑟𝑟) = λ(𝛹 − 𝛹des(𝑑)), where 𝛹des(𝑑) is the desired orientation calculated using the lateral distance from the desired trajectory (see figure 2 b for the illustration of 𝛹des(𝑑)). the λ function achieves the promotion of the |𝛹𝑒𝑟𝑟| < 𝜑 error, while an error larger than 𝜑 leads to a small negative reward (a plot of λ(𝑥) is shown in figure 2 c): figure 2. explanation of the proposed orientation reward: (a) explains 𝛹,d, (b) shows how the desired orientation depends on the lateral error, (c) shows the λ(𝑥) function and (d) provides some examples of desired configurations. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 10 λ(𝑥) = { 1 2 + 1 2 cos(π 𝑥 𝜑 ) if − 1 ≤ 𝑥 ≤ 1 (1 − | 𝑥 𝜑 |) otherwise , (4) where the ε ∈ [10−1,10−2] and 𝜑 = 50° hyperparameters are selected arbitrarily. the velocity-based component was calculated as 𝑟𝑣(𝜔𝑙,𝜔𝑟) = max(𝜔𝑙,𝜔𝑟) to reward an equally high-speed motion in both straight and curved sections. in the curved sections, only the outer wheel was able to rotate at maximal speed, while on a straight road, both wheels were able to do so. 2.5. simulation-to-reality transfer to train the agents, an open-source simulation of the duckietown environment was used [14]. this simulation models certain physical properties of the real environment accurately (dimensions of the robot, camera parameters, dynamic properties, etc.), but several other effects (textures, objects at the side of the road) and light simulation are less realistic (e.g. compared to modern computer games). these inaccuracies create a gap between simulation and reality that makes it challenging for any reinforcement learning agent to be trained only in simulation but operate in reality. to bridge the simulation-to-reality gap and to achieve the generalisation capability required for real performance, domain randomisation was used. this involves training the policy in many different variants of a simulated environment by varying lighting conditions, object textures, the camera, vehicle dynamics parameters and road structures (see figure 3 for examples of domain randomised observations). in addition to the ‘built-in’ randomisation options of gym-duckietown, this study used a diverse set of maps to train on in order to further improve the agent's generalisation capability. 2.6. collision avoidance collision avoidance with other vehicles greatly increases the complexity of the lane-following task. these problems can be solved in different ways, for example, by overtaking or following at a safe distance. however, the sensing capability of the vehicle and the complexity of the policy determine the solution it can learn. images from the forward-facing camera of a duckiebot only have a 160 ° horizontal field of view; therefore, the policy controlling the vehicle has no information about objects moving next to or behind the robot. for simplicity, in this study, the same convolutional network for collision avoidance as for lane following was used, which does not feature a long short-term memory cell or any other sequence modelling component (in contrast to [2]). for these reasons, it is unable to plan long manoeuvres, such as overtaking, which also requires side vision to check when it is safe to return to the right-hand lane. the policy was therefore trained in situations where there was a slow vehicle ahead, and the agent had to learn to perform lane following at full speed until it had caught up with the vehicle in front, at which point it had to reduce its speed and maintain a safe distance to avoid collision. in these experiments, the wheel velocity braking action representation was used as the policy's output because this allowed the agent to slow down or even stop the vehicle if necessary (unlike the steering action). both the orientation and the distance travelled reward functions were used to train agents for collision avoidance. the former was supplemented with a term that promoted collision avoidance, while the latter was used unchanged. the simulation used provided a 𝑝coll penalty if the safety circles around the two vehicles overlapped. the 𝑟𝑐𝑜𝑙𝑙 reward component that promoted collision avoidance was calculated using this penalty. if the penalty decreased because the robot was able to increase its distance from an obstacle, the reward term was proportional to the change in penalty; otherwise, it was 0: 𝑟coll = { −𝜆coll ⋅ δ𝑝coll if δ𝑝coll < 0 0 otherwise . (5) this term was added to the orientation reward, and it aimed to encourage the policy to increase the distance from the vehicle ahead if it got too close. collisions were only penalised by terminating the episode without giving any negative rewards. 2.7. evaluation to assess the performance of the reinforcement learningbased controller, multiple performance metrics in the simulation were measured and compared against two baselines, one using a classical control theory approach and the other being human driving. survival time (𝑡survive) in s: the time until the robot left the road or the duration of an evaluation episode. distance travelled in ego-lane (𝑠ego) in m: the distance travelled along the right-hand lane within a fixed time period. only longitudinal motion was counted; tangential movement therefore counted the most towards this metric. distance travelled both lanes (𝑠both) in m: both the distance travelled along the right-hand-lane within a fixed time period and sections where the agent moved into the oncoming lane counted towards this metric. lateral deviation (𝑑𝑑) in m·s: lateral deviation from the lane’s centreline integrated over the time of an episode. orientation deviation (𝑑ψ) in rad·s: the robot orientation's deviation from the tangent of the lane centreline integrated over the time of an episode. figure 3. examples of domain randomised observations. a) simulated b) simulated c) real figure 4. a) test track used for simulated reinforcement learning and baseline evaluations; b) and c) real and simulated test track used for the evaluation of the simulation-to-reality transfer. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 11 time outside ego-lane (𝑡𝑜𝑢𝑡) in s: time spent outside the ego-lane. even though duckietown is intended to be a standardised platform, it is still under development, and the official evaluation methods and baselines have not been adopted widely in the research community. the ai driving olympics provided a great opportunity to benchmark the solution presented here to others; however, the methods behind these solutions have not yet been published in the scientific literature. for this reason, this method was analysed primarily by comparing it with baselines that could be evaluated under the same conditions. the classical control theory baseline relies on information about the robot’s relative location and orientation to the centreline of the lane, which is available in the simulator. this baseline works by controlling the robot to orient itself towards a point on its desired path ahead and calculating wheel velocities using a proportional-derivative (pd) controller based on the orientation error of the robot. the parameters of this controller are hand-tuned to achieve a sufficiently good performance, but more advanced control schemes could offer better results. in many reinforcement learning problems (e.g. the atari 2600 games [15]) the agents are compared to human baselines. motivated by this benchmark, a method to measure how well humans were able to control duckiebots was proposed, which was then used as a baseline. the values shown in table 1 were recorded by controlling the simulated robot using the arrow keys on a keyboard (therefore via discrete actions), while the observations seen by the human driver were very similar to the observations of the reinforcement learning agent. 2.8. methods to improve results at the ai driving olympics the agents in this study were trained to solve autonomous driving problems in the duckietown environment and not to maximise scores at the ai driving olympics. therefore, some hyperparameters and methods had to be modified to match the competitions' evaluation procedures. it was found that training on lower frame rates (0.1 ms step time) improved the scores even though the evaluation simulation was stepped more frequently. in addition, implementing the same motion blur simulation that was applied in the official evaluation improved the results significantly compared with agents that were trained on nonblurred observations. 3. results 3.1. simulation even though multiple papers have demonstrated the feasibility of training vision-based driving policies using reinforcement learning, adapting to a new environment still poses many challenges. due to the high dimensionality of the image-like observations, many algorithms converge slowly and are very sensitive to hyperparameter selection. the method presented in this study, using proximal policy optimization, is able to converge with good lane-following policies in 1-million timesteps thanks to the high sample complexity of the algorithm. this training takes 2–2.5 hours on five cores of an intel xeon e5-2698 v4 2.2 ghz cpu and an nvidia tesla v100 gpu if 16 parallel environments are used. 3.1.1. comparison against baselines table 1 compares the reinforcement learning agent from this study with the baselines. the performance of the trained policy is measurable to the classical control theory baseline as well as to how well humans are able to control the robot in the simulation. most metrics indicate similarly good or equal performance even though the pd-controller baseline relies on high-level data such as position and orientation error rather than images. 3.1.2. comparison against other solutions at the ai driving olympics table 2 shows the top-ranking solutions of the simulated lane-following (validation) challenge at the 5th ai driving olympics. all top-performing solutions were able to control the robot reliably in the simulation for the duration of an episode (60 s); however, the distances travelled were different. the method in this study is able to control the robot reliably at the highest speed, so it therefore achieves the highest distance-travelled value while also showing good lateral deviation and rarely departing from the ego-lane. 3.1.3. action representation and reward shaping experiments with different action representations show that constrained and preferably biased action spaces allow convergence with good policies (wheel velocity braking and steering). however, more general action spaces (wheel velocity and its clipped version) can only converge with inferior policies during the same number of steps (see figure 5). the proposed orientation-based table 1. comparison of the reinforcement learning agent with two baselines in simulation. mean metrics over 5 episodes rl agent pd baseline human baseline survival time in s ↑ 15 15 15 distance travelled both lanes in m ↑ 7.1 7.6 7.0 distance travelled ego-lane in m ↑ 7.0 7.6 6.7 lateral deviation in m ·s ↓ 0.5 0.5 0.9 orientation deviation in rad·s ↓ 1.5 1.1 2.8 table 2. comparing the method in this study with other solutions at the ai driving olympics author 𝒕𝐬𝐮𝐫𝐯𝐢𝐯𝐞 in s ↑ 𝒔𝐞𝐠𝐨 in m ↑ 𝒅𝒅 in m·s ↓ 𝒕𝐨𝐮𝐭 in s ↓ a. kalapos [10], [16] 60 30.38 2.65 0 a. béres [16] 60 29.14 4.10 1.4 m. tim [16] 60 28.52 3.45 0.4 a. nikolskaya 60 24.80 3.15 1.6 r. moni [16] 60 18.60 1.78 0 z. lorincz [16] 60 18.6 3.5 0.8 m. sazanovich 60 16.12 4.35 3.4 r. jean 60 15.5 3.28 0 y. belousov 60 14.88 5.41 9.8 m. teng 60 11.78 2.92 0 p. almási [7], [16] 60 11.16 1.32 0 a) orientation reward b) distance travelled reward figure 5. learning curves for the reinforcement learning agent with different action representations and reward functions. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 12 reward function also leads to as good a final performance as one that is ‘trivially’ rewarding based on the distance travelled; however, the latter seems to perform better on more general action representations (because policies using these action spaces and trained with the orientation reward do not learn to move fast). 3.2. real-world driving to measure the quality of the transfer learning process and the performance of the controller in the real world, performance metrics that were easily measurable both in reality and simulation were selected. these were recorded in both domains in matching experiments and compared against each other. the geometry of the tracks, the dimensions and the speed of the robot were simulated accurately to evaluate the robustness of the policy against all the inaccurately simulated effects and those that were not simulated. using this method, policies trained in the domainrandomised simulation were tested as well as those that were trained only in the ‘nominal’ simulation. this allows for the evaluation of the transfer learning process and the highlighting of the effects of training with domain randomisation. the real and simulated version of the test track used in this analysis is shown in figure 4 b and figure 4 c. during real evaluations, it was generally found that under ideal circumstances (no distracting objects at the side of the road and good lighting conditions), the policy trained in the ‘nominal’ simulation was able to drive reasonably well. however, training with domain randomisation led to a more reliable and robust performance in the real world. table 1 shows the quantitative results of this evaluation. the two policies seemed to perform equally well when compared based on their performance in the simulation. however, metrics recorded in the real environment show that the policy trained with domain randomisation performed almost as well as in the simulation, while the other policy performed noticeably worse. the lower distance travelled ego-lane metric of the domainrandomised policy can be explained by the fact that the vehicle tended to drift to the left-hand lane at sharp turns but returned to the right-hand lane afterwards, while the nominal policy usually made more serious mistakes. note that in these experiments the orientation-based reward and the steering action representation were used, as this configuration learns to control the robot in the minimum number of steps and the shortest training time. an online video demonstrates the performance of the trained agent from this study: https://youtu.be/kz7ywemg1is (accessed 23 september 2021). an important limitation for the method presented in this study is that during real evaluations, the speed of the robot had to be decreased to half of the simulated value. the policy evaluations were executed on a pc connected to the robot via wireless lan; therefore, the observations and the actions were transmitted between the two devices at every step. this introduced delays in the order of 10 – 100 ms, making the control loop unstable when the robot was moving at full speed. however, at half speed, a stable operation was achieved. it was noticed that models trained with motion blur and longer step times for the ai driving olympics performed more reliably in the real world regardless of whether they used domain randomisation. however, further analysis and retraining of these agents multiple times is needed to firmly support these presumptions. 3.3. collision avoidance figure 6 demonstrates the learned collision avoidance behaviour. in the first few seconds of the simulation, the robot controlled by the reinforcement learning policy accelerates to full speed. then, as it approaches the slower, non-learning robot, it reduces its speed and maintains an approximately constant distance from the vehicle ahead (see figure 6). from the simple, fully convolutional network of this policy, learning, planning and executing a more complex behaviour, such as overtaking, cannot be expected. table 4 shows that training with both reward functions leads to functional lane-following behaviour. however, the nonmaximal survival time values indicate that neither of the policies are capable of performing lane following reliably with the presence of an obstacle robot for 60 s. all metrics in table 4 indicate that the modified orientation reward leads to better lanefollowing metrics than the simpler distance travelled reward. it should be noted that these metrics were mainly selected to evaluate the lane-following capabilities of an agent; a more intable 3. evaluation results of reinforcement learning agent in the real environment and in matching simulations. eval. domain mean metrics over 6 episodes domain rand. policy nominal policy real survival time in s ↑ 54 45 distance travelled both lanes in m ↑ 15.6 11.4 distance travelled ego-lane in m ↑ 7.0 8.4 sim. survival time in s ↑ 60 60 distance travelled in m ↑ 15.5 15.0 a) 𝑡 = 0 s b) 𝑡 = 6 s c) 𝑡 = 8 s d) 𝑡 = 24 s e) approximate distance between the vehicles initial positions catching up following the vehicle ahead figure 6. sequence of robot positions in a collision avoidance experiment with a policy trained using the modified orientation reward. after 𝑡 = 6 s, the controlled robot follows the vehicle in front at a short but safe distance until the end of the episode (approximate distance is calculated as the distance between the centre points of the robots minus the length of a robot). table 4. evaluation results of policies trained for collision avoidance with different reward functions. mean metrics over 15 episodes distance travelled orientation +𝑟coll survival time (max. 60) in s ↑ 46 52 distance travelled both lanes in m ↑ 22.5 22.9 distance travelled ego-lane in m ↑ 22.7 23.1 lateral deviation in m·s ↓ 1.9 1.6 orientation deviation in rad·s ↓ 6.3 5.8 https://youtu.be/kz7ywemg1is acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 13 depth analysis of collision avoidance with a vehicle in front calls for more specific metrics. an online video demonstrates the performance of the agent trained in this study: https://youtu.be/8gqauvty1po (accessed 23 september 2021) 3.4. salient object maps visualising which parts of the input image contribute the most to a particular output (action) is important because it provides some explanation of the network's inner workings. figure 7 shows salient object maps in different scenarios generated using the method proposed in [17]. all of these images indicate high activations on lane markings, which is expected. 4. conclusions this work presented a solution to the problem of complex, vision-based lane following in the duckietown environment using reinforcement learning to train an end-to-end steering policy capable of simulation-to-real transfer learning. it was found that the training is sensitive to problem formulation, such as the representation of actions. this study has demonstrated that by using domain randomisation, a moderately detailed and accurate simulation is sufficient for training end-to-end lanefollowing agents that operate in a real environment. the performance of these agents was evaluated by comparing some basic metrics to match real and simulated scenarios. agents were also successfully trained to perform collision avoidance in addition to lane following. finally, salient object visualisation was used to give an illustrative explanation of the inner workings of the policies in both the real and simulated domains. acknowledgement we would like to show our gratitude to professor bálint gyires-tóth (bme, dept. of telecommunications and media informatics) for his assistance and comments on the progress of our research. the research reported in this paper and carried out at the budapest university of technology and economics was supported by continental automotive hungary ltd. and the ‘tkp2020, institutional excellence programme’ of the national research development and innovation office in the field of artificial intelligence (bme ie-mi-sc tkp2020). references [1] v. mnih, a. p. badia, m. mirza, a. graves, t. lillicrap, t. harley, d. silver, k. kavukcuoglu, asynchronous methods for deep reinforcement learning, proc. of the international conference on machine learning, new york, united states, 19–24 june 2016, pp. 1928-1937. [2] m. jaritz, r. de charette, m. toromanoff, e. perot, f. nashashibi, end-to-end race driving with deep reinforcement learning, proc. of the ieee international conference on robotics and automation (icra), brisbane, australia, 21–25 may 2018, pp. 2070-2075. [3] a. kendall, j. hawke, d. janz, p. mazur, d. reda, j. allen, v. lam, a. bewley, a. shah, learning to drive in a day, proc. of the international conference on robotics and automation (icra), montreal, canada, 20–24 may 2019, pp. 8248-8254. [4] w. shi, s. song, z. wang, g. huang, self-supervised discovering of causal features: towards interpretable reinforcement learning, 2020. online [accessed 3 august 2020] https://arxiv.org/abs/2003.07069 [5] b. balaji, s. mallya, s. genc, s. gupta, l. dirac, v. khare, g. roy, t. sun, y. tao, b. townsend, e. calleja, s. muralidhara, d. karuppasamy, deepracer: educational autonomous racing platform for experimentation with sim2real reinforcement learning, 2019. online [accessed 13 april 2020] https://arxiv.org/abs/1911.01562 [6] m. szemenyei, p. reizinger, attention-based curiosity in multiagent reinforcement learning environments, proc. of the international conference on control, artificial intelligence, robotics & optimization (iccairo), majorca island, spain, 3–5 may 2019, pp. 176-181. [7] p. almási, r. moni, b. gyires-tóth, robust reinforcement learning-based autonomous driving agent for simulation and real world, proc. of the international joint conference on neural networks (ijcnn), glasgow, united kingdom, 19–24 july 2020, pp. 1-8. [8] v. mnih, k. kavukcuoglu, d. silver, a. graves, i. antonoglou, d. wierstra, m. riedmiller, playing atari with deep reinforcement learning, 2013. online [accessed 13 april 2020] https://arxiv.org/abs/1312.5602 [9] j. schulman, f. wolski, p. dhariwal, a. radford, o. klimov, proximal policy optimization algorithms, 2017. online [accessed 2 december 2019] https://arxiv.org/abs/1707.06347 [10] a. kalapos, c. gór, r. moni, i. harmati, sim-to-real reinforcement learning applied to end-to-end vehicle control, proc. of the 23rd international symposium on measurement and control in robotics (ismcr), budapest, hungary, 15–17 october 2020, pp. 1-6. [11] j. zilly, j. tani, b. considine, b. mehta, a. f. daniele, m. diaz, g. bernasconi, c. ruch, j. hakenberg, f. golemo, a. k. bowser, m. r. walter, r. hristov, s. mallya, e. frazzoli, a. censi, l. paull, the ai driving olympics at neurips, 2018. online [accessed 13 april 2020] https://arxiv.org/abs/1903.02503 [12] j. schulman, p. moritz, s. levine, m. jordan, p. abbeel, highdimensional continuous control using generalized advantage estimation, proc. of the international conference on learning representations (iclr), san juan, puerto rico, 2–4 may 2016, 14 pp. online [accessed 23 september 2021] http://arxiv.org/abs/1506.02438 [13] e. liang, r. liaw, r. nishihara, p. moritz, r. fox, k. goldberg, j. gonzalez, m. jordan, i. stoica, rllib: abstractions for distributed reinforcement learning, proc. of the international conference on machine learning, stockholm, sweden, 10–15 july 2018, pp. 3053-3062. [14] m. chevalier-boisvert, f. golemo, y. cao, b. mehta, l. paull, duckietown environments for openai gym, 2018. online [accessed 15 january 2021] https://github.com/duckietown/ gym-duckietown [15] m. g. bellemare, y. naddaf, j. veness, m. bowling, the arcade learning environment: an evaluation platform for general agents, j. artif. intell. res. 47 (2013), pp. 253-279. doi: 10.1613/jair.3912 [16] r. moni, a. kalapos, a. béres, m. tim, p. almási, z. lőrincz, pia project achievements at aido5, 2020. online [accessed 15 january 2021] https://medium.com/@smartlabai/pia-project-achievementsat-aido5-a441a24484ef a) simulated b) real c) collision avoidance figure 7. salient objects highlighted on observations in different domains and tasks. blue regions represent high activations throughout the network. https://youtu.be/8gqauvty1po https://arxiv.org/abs/2003.07069 https://arxiv.org/abs/1911.01562 https://arxiv.org/abs/1312.5602 https://arxiv.org/abs/1707.06347 https://arxiv.org/abs/1903.02503 http://arxiv.org/abs/1506.02438 https://github.com/duckietown/%20gym-duckietown https://doi.org/10.1613/jair.3912 https://medium.com/@smartlabai/pia-project-achievements-at-aido5-a441a24484ef https://medium.com/@smartlabai/pia-project-achievements-at-aido5-a441a24484ef acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 14 [17] m. bojarski, p. yeres, a. choromanska, k. choromanski, b. firner, l. d. jackel, u. muller, explaining how a deep neural network trained with end-to-end learning steers a car, 2017. online [accessed 15 april 2020] https://arxiv.org/abs/1704.07911 appendix proximal policy optimization the pseudo code for proximal policy optimization (ppo) is as follows: algorithm ppo, actor-critic style (based on [9]) input: initial policy with 𝜃0 parameters and initial value function estimator with 𝜙0 parameters for iteration = 1,2,... do for actor=1,2,...,n do run πθold in the environment for t timesteps to collect τ𝑖 trajectory compute advantage estimates �̂�1,… ,â𝑇 based on the current value function end optimise 𝔏clip(𝜃) + 𝔏klpen(𝜃) wrt. 𝜃, for k epochs and minibatch size 𝑀 ≤ 𝑁𝑇 fit the value function estimate by regression on mean-squared error 𝜃old ← 𝜃, 𝜙old ← 𝜙 end the 𝛽 adaptive parameter mentioned in section 2.1 is updated according to the following rule: 𝛽 ← { 𝛽/2, if 𝑑 < 𝑑targ/1.5 𝛽 × 2, if 𝑑 > 𝑑targ × 1.5, (6) where 𝑑targ is a hyperparameter and 𝑑 is the kl-divergence of the old and the updated policy 𝑑 = �̂�[𝐾𝐿[πθold(⋅ |𝑠𝑡),πθ(⋅ |𝑠𝑡)]]. (7) the �̂�𝑡 generalised advantage estimate [12] is calculated as �̂�𝑡 = ∑(γλ) 𝑙δ𝑡 𝑉 ∞ 𝑙 (8) 𝛿𝑡 𝑉 = 𝑟𝑡 + 𝛾 𝑉(𝑠𝑡+1) − 𝑉(𝑠𝑡) , (9) where 𝑉(𝑠𝑡) and 𝑉(𝑠𝑡+1) are the value function estimates calculated by the value network at steps 𝑡 and 𝑡 + 1; 𝛾 is the discount factor, while 𝜆 is a hyperparameter of the generalised advantage estimate. to assure reproducibility, the hyperparameters of the algorithm are provided in the table 5. table 5. hyperparameters of the algorithm. the description of some parameters is from the rllib documentation [13]. description value number of parallel environments 𝑁 = 16 learning rate α = 5 × 10−5 discount factor for return calculation 𝛾 = 0.99 𝜆 parameter for the generalised advantage estimate 𝜆 = 0.95 ppo clip parameter ϵ = 0.2 sample batch size 𝑇 = 256 sgd minibatch size 𝑀 = 128 number of epochs executed in every iteration 𝐾 = 30 target kl-divergence for the calculation of 𝛽 𝑑targ = 0.01 https://arxiv.org/abs/1704.07911 measurements of virial coefficients of helium, argon and nitrogen for the needs of static expansion method acta imeko issn: 2221-870x june 2022, volume 11, number 2, 1 4 acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 1 measurements of virial coefficients of helium, argon and nitrogen for the needs of static expansion method sefer avdiaj1,2, yllka delija1 1 department of physics, university of prishtina, prishtina 10000, kosovo 2 nanophysics, outgassing and diffusion research group, nanoalb-unit of albanian nanoscience and nanotechnology, 1000 tirana, albania section: research paper keywords: real gases; virial equation of state; compressibility factor; virial coefficients citation: sefer avdiaj, yllka delija, measurements of virial coefficients of helium, argon and nitrogen for the needs of static expansion method, acta imeko, vol. 11, no. 2, article 26, june 2022, identifier: imeko-acta-11 (2022)-02-26 section editor: sabrina grassini, politecnico di torino, italy received august 4, 2021; in final form march 15, 2022; published june 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: yllka delija, e-mail: delijayllka@gmail.com 1. introduction the deviations of real gases from ideal gas behaviour are best seen by determining the compressibility factor [1]: 𝑍 = 𝑃 𝑉𝑚 𝑅 𝑇 , (1) where 𝑃 is the pressure, 𝑉𝑚 (𝑉/𝑛) is the molar volume, 𝑇 is the temperature, and 𝑅 is the universal gas constant. at low pressures and high temperatures, the ideal gas law can adequately predict the behaviour of natural gas. however, at high pressures and low temperatures, the gas deviates from the ideal gas behaviour and is described as real gas. this deviation reflects the character and strength of the intermolecular forces between the particles making up the gas. several equations of state have been suggested to account for the deviations from ideality. a very handy expression that allows for deviations from ideal behaviour is the virial equation of state [2]. this is a simple power series expansion in either the inverse molar volume, 1/𝑉𝑚 ,: 𝑍 = 𝑝 𝑉𝑚 𝑅 𝑇 = 𝐴 + 𝐵 𝑉𝑚 + 𝐶 𝑉𝑚 2 + ⋯, (2) or the pressure, 𝑝,: 𝑍 = 𝑝 𝑉𝑚 𝑅 𝑇 = 𝐴′ + 𝐵′ 𝑝 + 𝐶 ′𝑝2 + ⋯, (3) with 𝐴, 𝐴’first virial coefficient, 𝐵, 𝐵’second virial coefficient, 𝐶, 𝐶’third virial coefficient. all virial coefficients are temperature-dependent. according to statistical mechanics, the first virial coefficient is related to one-body interactions, the second virial coefficient is related to two-body interactions and the higher virial coefficients are related to multi-body interactions [1], [3]. the main goal of this experiment is to determine values for the second virial coefficient b of gases helium, argon and nitrogen at room temperature. the second virial coefficient has a theoretical relationship with the intermolecular forces between a pair of molecules which can provide quantitative information on these forces. abstract generally, there are three primary methods in vacuum metrology: mercury manometer, static expansion method, and continuous expansion method. for the pressure below 10 pa, the idea of the primary standard is that the gas is measured precisely at a pressure as high as possible, and then the gas is expanded to the bigger volumes; this allows to calculate the expanded pressure. an important parameter that needs to be taken care of in primary vacuum calibration methods is the compressibility factor of the working gas. the influence of virial coefficients on the realization of primary standards in vacuum metrology, especially in the realization of the static expansion method is very important. in this paper we will present the measured data for virial coefficients of three gases helium, argon and nitrogen measured at room temperature and a pressure range from 3 kpa to 130 kpa. the dominating term due to real gas properties arises from the second virial coefficient. the influence of higher orders of virial coefficients drops rapidly with lower pressure, particularly for gas pressures values lower than one atmosphere. hence, in our calculation, the series of real gas was used for the first and second virial coefficients but not for higher-order virial coefficients. mailto:delijayllka@gmail.com acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 2 the results will serve the scientific community in the field of metrology for the most accurate measurement standards in vacuum metrology, in atomic physics will show the level of interaction of atoms and molecules of certain gases, and in the chemical aspect we will gain knowledge about behaviour of gases at different pressures; in this way, it will be possible to obtain information about the limit of transition from ‘ideal gas’ to ‘real gas’ and vice versa. 2. experimental setup a schematic illustration of the experimental setup is shown in figure 1. our system comprises five chambers (𝑉0, 𝑉1, 𝑉2, 𝑉3 and 𝑉4) of significantly different sizes, the volumes of which have been determined by using the gas expansion method. the vacuum isolation valves are installed between the chambers, and a turbomolecular pump (hicube eco) is connected to valve 5. a capacitance diaphragm gauge (cdg025d) is attached to the chamber 𝑉0 to measure the precise pressure before and after expansion. the whole experiment was performed under wellcontrolled ambient temperature in the air-conditioned laboratory. in order to reduce the temperature drifts and transient local heating during the working day, we kept all electrical equipment (pump and lights) switched on at all times (24 h/day). the average temperature was 296 k and the pressure range was from 3 kpa to 130 kpa [4]. 3. volume determination to determine the volume of vacuum chambers, we must know the value of one of the volumes. we measured the geometrical dimensions of the known volume 𝑉0 by using a calibrated caliper. a cylinder with radius 𝑟 and height ℎ has a volume given by 𝑉0 = π 𝑟 2ℎ. using the dimensions of the volume 𝑉0 we have calculated mean value of the volume 𝑉0 = 0.71 l with associate uncertainty 0.14 % for coverage factor 𝑘 = 1. all the uncertainties throughout this paper are given for coverage factor 𝑘 = 1 [1], [4]. therefore, to determine volumes 𝑉1, 𝑉2, 𝑉3 and 𝑉4 we used the static expansion method. in this method, the pressures before and after the expansion are used for determining the expansion ratio. the gas is first enclosed in the smaller volume, then it is allowed to enter the larger volumes to expand under nearly perfect isothermal conditions [5]. this procedure is applicable only if the pressure after expansion can be measured with about the same accuracy as the pressure before expansion. argon is used as a gas for this type of calibration, although helium and nitrogen could also be used. the purity of the argon gas was 5.0 𝑁 (99.999 %). the mean values, standard deviation and uncertainty of volumes of the five vacuum chambers are given in table 1. the measurement uncertainty depends on the ratio of volumes (pressures before and after expansions) [6]. 4. compressibility factor in this section, an analytical approach for calculating the compressibility factor of gases is presented. in this experiment, the gas was expanded four times in different volumes. the experimental procedure can be described as follows. in the beginning, all valves are opened and the entire system is pumped down to less than 10−5 pa. valves 2, 3, 4, 5 are closed, valves 0, 1 are opened and gas comes out very quickly into 𝑉0 + 𝑉1. with the left hand, a knob has adjusted the regulator to fill the system at a pressure of a little over 130 kpa. the pressure is monitored for a few minutes until is stable, then is recorded as 𝑝1. the ambient temperature t is also recorded. then valve 2 is opened and the gas expanded into 𝑉0 + 𝑉1 + 𝑉2. again, after equilibrium, the pressure is recorded as 𝑝2. then valve 3 is opened and gas is expanded into 𝑉0 + 𝑉1 + 𝑉2 + 𝑉3. when equilibrium is reached the pressure is recorded as 𝑝3. for the final measurement, valve 4 is left open and the gas expanded into all volumes 𝑉0 + 𝑉1 + 𝑉2 + 𝑉3 + 𝑉4. after the gas is expanded in the entire system, pressure dropped below 3 kpa and is recorded as 𝑝4. this procedure is repeated nine times, and the equilibrated pressures are recorded at each stage [7], [8]. for pressure measurements, we have used cdg025d with an uncertainty of 0.2 % [9]. once each gas transfer pressure is measured, the volumes and temperature are also known, we need only the number of moles of gas to find directly its compressibility factor 𝑍. from the general gas equation, we obtain the expression for the amount of substance: 𝑛 = 𝑝 𝑉 𝑇 𝑇0 𝑃0 𝑉0 (4) where 𝑇0 is the standard temperature, 𝑃0 is the standard pressure and 𝑉0 is the molar volume at stp [1]. after we obtained the complete data set 𝑃, 𝑉, 𝑇 and 𝑛, the compressibility factor is calculated by using equation (1). plots of compressibility factor (𝑍) as a function of inverse molar volume (1/𝑉𝑚 ) for (a) helium, (b) argon and (c) nitrogen are presented in figure 2. for these real gases in the graphs, we notice that the shapes of the curves look a little different for each gas, and most of the curves only approximately resemble the ideal gas line at 𝑍 = 1 over a limited pressure range [10]-[12]. figure 1. schematic illustration of the experimental setup. valves 1, 2, 3 and 4 are installed between the chambers 𝑉0, 𝑉1, 𝑉2, 𝑉3 and 𝑉4. the capacitance diaphragm gauge (cdg) is attached to the chamber 𝑉0. a black rubber hosing connects the gas regulator to valve 0 and a flexible metal hose is used from valve 5 to the turbomolecular vacuum pump. table 1. mean value, standard deviation and uncertainty of volumes. �̅� 𝜹 𝒖 𝑉0(l) 0.7100 9.61 · 10 -4 1.36 · 10-3 𝑉1(l) 0.0668 5.82 · 10 -5 8.71 · 10-4 𝑉2(l) 0.7879 2.04 · 10 -3 2.59 · 10-3 𝑉3(l) 0.1652 2.82 · 10 -3 1.71 · 10-2 𝑉4(l) 0.5416 5.48 · 10 -3 1.01 · 10-2 acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 3 5. results 5.1. calculation of virial coefficients at low pressures, e.g. below two atmospheres, the third and all higher virial terms usually may be neglected; hence, the equation of state (2) becomes: 𝑍 = 𝑝 𝑉 𝑛 𝑅 𝑇 = 𝐴 + 𝐵 𝑉𝑚 + ⋯ (5) using spreadsheet software such as excel, we plotted 𝑍 as a function of 1/𝑉𝑚 for each trial. for each gas, some curvature is observed, and we determined values for both the first (a) and second (b) virial coefficients. using the linest function in excel, we determined the values and standard deviations of the slope and y-intercept in a linear fit. by properly combining values of b and their uncertainties from multiple trials, an average value of this coefficient and its standard deviation for each gas was calculated [1], [3]. the mean value, standard deviation and uncertainty of the first and second virial coefficients for these three gasses are summarized in table 2. figure 3 shows values of the second virial coefficient for (a) helium, (b) argon and (c) nitrogen at room temperature. 6. discussion in table 3, the experimental values obtained in this work for second virial coefficients are compared with values from the literature [1], [13]-[17]. it can be seen the experimental results show good agreement with the data in the literature. according to the above results, it is possible to make the following considerations: a. the compressibility factor of helium is greater than 1, which shows that repulsive forces dominate between molecules and atoms. whereas for argon and nitrogen the value of z is lower than 1, in this case, the attractive forces are stronger (figure 2). b. at room temperature, the second virial coefficient is positive for helium and negative for argon and nitrogen (figure 3). figure 2. the compressibility factor, z, as a function of inverse molar volume (1/𝑉𝑚 ) for (a) helium, (b) argon and (c) nitrogen. figure 3. values of 𝐵 coefficient for (a) helium, (b) argon and (c) nitrogen at 296 k. table 2. mean value, standard deviation and uncertainty of 𝑨 and 𝑩 coefficients for helium, argon and nitrogen. 𝐻𝑒 �̅� 𝛿 𝑢 𝐴 1.00120 1.26 · 10-5 4.22 · 10-6 𝐵 (m3/mol) 1.16 · 10-5 2.33 · 10-7 7.78 · 10-8 𝐴𝑟 �̅� 𝛿 𝑢 𝐴 0.99871 1.63 · 10-4 5.42 · 10-5 𝐵 (m3/mol) -1.61 · 10-5 3.75 · 10-7 1.24 · 10-7 𝑁2 �̅� 𝛿 𝑢 𝐴 0.9998 3.75 · 10-5 1.25 · 10-5 𝐵 (m3/mol) -5.1 · 10-6 7.25 · 10-8 2.41 · 10-8 1,00119 1,00121 1,00123 1,00125 1,00127 1,4 2,4 3,4 4,4 𝑦 = 𝑍 𝑥 = 1/𝑉𝑚 (𝑎) 0,99845 0,99855 0,99865 0,99875 0,99885 0,99895 0,99905 1,4 2,4 3,4 4,4 𝑦 = 𝑍 𝑥 = 1/𝑉𝑚 (𝑏) 0,99975 0,99977 0,99979 0,99981 0,99983 0,99985 0,99987 0,99989 1,5 2,0 2,5 3,0 3,5 4,0 4,5 𝑦 = 𝑍 𝑥 = 1/𝑉𝑚 (𝑐) 1,12e-05 1,14e-05 1,16e-05 1,18e-05 1,20e-05 1,22e-05 1 2 3 4 5 6 7 8 9 𝐵 (m 3 / m o l) (𝑎) -1,70e-05 -1,65e-05 -1,60e-05 -1,55e-05 -1,50e-05 -1,45e-05 1 2 3 4 5 6 7 8 9 𝐵 (m 3 / m o l) (𝑏) -5,30e-06 -5,20e-06 -5,10e-06 -5,00e-06 -4,90e-06 -4,80e-06 1 2 3 4 5 6 7 8 𝐵 (m 3 / m o l) (𝑐) acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 4 7. conclusions based on the obtained experimental data, the pressure that is generated in the primary pressure standard for these initial pressure values must be corrected at most up to 0.005% since the deviation of the real gas from the ideal behaviour is very small. the gas with the smallest deviation from the ideal behaviour was nitrogen with a correction of about 0.001%: based on these results, we are going to use nitrogen in future calibration procedures. acknowledgement the present work was partly financed by the ministry of education, science and technology of the republic of kosovo (mest) for the project: developing of primary metrology in the republic of kosovomeasurement of the compressibility factor of noble gases and nitrogen. most of the experimental equipment was donated by the u.s. embassy in kosovo. references [1] j. m. h. levelt sengers, m. klein, j. s. gallagher, pressurevolume-temperature relationships of gases; virial coefficients, national bureau of standards, washington d.c. (1971). online [accessed 14 march 2022] doi: https://apps.dtic.mil/sti/citations/ad0719749 [2] t. d. varberg, a. j. bendelsmith, k. t. kuwata, measurement of the compressibility factor of gases: a physical chemistry laboratory experiment, journal of chemical education (2011), art. 1591. doi: 10.1021/ed100788r [3] p. j. mcelroy, r. battino, m. k. dowd, compression-factor measurements on methane, carbon dioxide, and (methane + carbon dioxide) using a weighing method, j. chem. thermodynamics 21(12) (1989), pp 1287-1300 doi: 10.1016/0021-9614(89)90117-1 [4] s. avdiaj, j. šetina, b. erjavec, volume determination of vacuum vessels by gas expansion method, metrology society of india, mapan 30 (2015), pp. 175-178. doi: 10.1007/s12647-015-0137-1 [5] a. kumar, v. n. thakur, h. kumar, characterization of spinning rotor gauge-3 using orific flow system and static expansion system, acta imeko 9(5) (2020), pp 325. doi: 10.21014/acta_imeko.v9i5.993 [6] c. mauricio villamizar mora, j. j. duarte franco, v. j. manrique moreno, c. e. garcía sánchez, analysis of the mathematical modelling of a static expansion system, acta imeko 10(3) (2021), pp. 185-191. doi: 10.21014/acta_imeko.v10i3.1061 [7] k. jousten, handbook of vacuum technology, wiley-vch, germany (2016), isbn 978-3-527-41338-6, pp 710-715. doi: 10.1002/9783527688265.ch2 [8] john w. moore, conrad l. stanitski, peter c. jurs, chemistry_ the molecular science-brooks cole, cengage learning; 4th edition (march 5, 2010), isbn-10: 1-4390-4930-0, pp 440-150. [9] inficon sky cdg025d capacitance diaphragm gauge on eurovaccum. online [accessed 20 april 2022] https://eurovacuum.eu/products/gauges/cdg025d/ [10] h. boschi-filho, c.c.buthers, second virial coefficient for real gases at high temperature, j. phys. chem. 73(3) 1969, pp 608– 615, art. 380. doi: 10.1021/j100723a022 [11] r. balasubramanian, x. ramya rayen, r. murugesan, second virial coefficients of noble gases, international journal of science and research (ijsr), 6(10) (2017). online [accessed 14 march 2022] https://www.ijsr.net/archive/v6i10/art20177192.pdf [12] c. gaiser, b. fellmuth, highly-accurate density-virial-coefficient values for helium, neon, and argon at 0.01℃ determined by dielectric-constant gas thermometry, the journal of chemical physics 150 (2019), art. no. 134303. doi: 10.1063/1.5090224 [13] b. schramm, e. elias, l. kern, gh. natour, a. schmitt, c. weber, precise measurements of second virial coefficients of simple gases and gas mixtures in the temperature range below 300 k, 95 (1991), pp. 615-621. doi: 10.1002/bbpc.19910950513 [14] a. hutem, s. boonchui, numerical evaluation of second and third virial coefficients of some inert gases via classical cluster expansion, journal of mathematical chemistry 50 (2012), pp 1262–1276. doi: 10.1007/s10910-011-9966-5 [15] e. bich, r. hellmann, e. vogel, ab initio potential energy curve for the helium atom pair and thermophysical properties of the dilute helium gas. ii. thermophysical standard values for lowdensity helium, an international journal at the interface between chemistry and physics 105(23-24) (2007). doi: 10.1080/00268970701730096 [16] d. w. rogers, concise physical chemistry, wiley; 1st edition, march, 2011, pp 18-34, isbn: 978-0-470-52264-6. [17] d. white, t. rubin, p. camky, h. l. johnston, the virial coefficients of helium from 20 to 300 k, j. phys. chem. 64(11) 1960, pp. 1607-1612, art. 774. doi: 10.1021/j100840a002 table 3. comparison between experimental values of second virial coefficients and literature values. 𝑇 (k) 𝐻𝑒 𝐴𝑟 𝑁2 literature 293 11.2 [14] -16.9 [14] -6.1 [14] 296 11.7 [13] -16.0 [13] -5.0 [13] 298 11.8 [15] -15.8 [16] -4.5 [16] 300 11.9 [17] -15.7 [1] -4.7 [1] this work 296 11.6 -16.1 -5.1 𝐵 in 10−6 (m3/mol) https://apps.dtic.mil/sti/citations/ad0719749 https://doi.org/10.1021/ed100788r https://doi.org/10.1016/0021-9614(89)90117-1 https://doi.org/10.1007/s12647-015-0137-1 http://dx.doi.org/10.21014/acta_imeko.v9i5.993 http://dx.doi.org/10.21014/acta_imeko.v10i3.1061 https://doi.org/10.1002/9783527688265.ch2 https://eurovacuum.eu/products/gauges/cdg025d/ https://doi.org/10.1021/j100723a022 https://www.ijsr.net/archive/v6i10/art20177192.pdf https://doi.org/10.1063/1.5090224 https://doi.org/10.1002/bbpc.19910950513 https://doi.org/10.1007/s10910-011-9966-5 https://doi.org/10.1080/00268970701730096 https://doi.org/10.1021/j100840a002 tracking and viewing modifications in digital calibration certificates acta imeko issn: 2221-870x march 2023, volume 12, number 1, 1 7 acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 1 tracking and viewing modifications in digital calibration certificates vashti galpin1, ian smith2, jean-laurent hippolyte2 1 school of informatics, university of edinburgh, united kingdom 2 national physical laboratory, united kingdom section: research paper keywords: digital calibration certificates, provenance, temporal databases, xml, prototype citation: vashti galpin, ian smith, jean-laurent hippolyte, tracking and viewing modifications in digital calibration certificates, acta imeko, vol. 12, no. 1, article 11, march 2023, identifier: imeko-acta-12 (2023)-01-11 section editor: daniel hutzschenreuter, ptb, germany received november 20, 2022; in final form march 20, 2023; published march 2023 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was supported by erc consolidator grant skye [grant number 682315] and by an iscf metrology fellowship grant provided by the uk government’s department for business, energy and industrial strategy (beis). corresponding author: vashti galpin, e-mail: vashti.galpin@ed.ac.uk 1. introduction information relating to the calibration of an instrument or artefact is captured in documents referred to as calibration certificates. these documents are typically provided either as physical paper-based documents or in electronic form, for example, in archivable portable document format (pdf-a). a downside of both approaches is that the information they contain is not machine-readable, that is it is not available in a form that can be read and processed by computer. using the information therefore requires, to some degree, human involvement and as such is prone to error, for example, arising from the transcription of numerical information from paper to computer. recent initiatives have considered the replacement of paperbased and electronic calibration certificates by fully machinereadable calibration certificates, referred to as “digital calibration certificates”, often abbreviated to “dccs”. the european metrology programme for innovation and research (empir) [1] funded the “smartcom” joint research project (2018-2021) [2] which developed a framework for dccs. the framework allows the key calibration certificate components of administrative data and measurement results to be stored. the recipient of a dcc will have, or be able to develop, software to ingest and use the information stored therein. ensuring the integrity and longevity of current and historical calibration data is essential for calibration laboratories to maintain long-term trusted relationships with their customers. it is also a requirement of iso/iec 17025 accreditation for these laboratories [3], along with the preservation of information about the methods, processes, software, equipment, and staff involved in the calibration task. this accompanying information, which is essential to trace and potentially repeat the conditions under which the calibration is performed, is more generally known as “provenance information” [4]. laboratories must be able to track and compare the differences between successive calibrations of the same artefact in data (for example, newly observed measurement results) or in provenance (for example, a different employee now signs off the calibration results). since dccs are containers for the trusted exchange of calibration data, their potential to increase the productivity of laboratories relies on abstract trust in current and historical calibration data is crucial. the recently proposed xml schema for digital calibration certificates (dccs) provides machine-readability and a common exchange format to enhance this trust. we present a prototype web application developed in the programming language links for storing and displaying a dcc using a relational database. in particular, we leverage the temporal database features that links provides to capture different versions of a certificate and inspect differences between versions. the prototype is the starting point for developing software to support dccs and the data with which they are populated and has underlined that dccs are the tip of the iceberg in automating the management of digital calibration data, activity that includes data provenance and tracking of modifications. mailto:vashti.galpin@ed.ac.uk acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 2 automated, integrated, and time-based curation of this data and associated provenance. the structure of this paper is as follows: we provide background on dccs before we go on to describe our prototype. we discuss the choices made in the development of the prototype and illustrate its usage with a screenshot. we conclude with ideas for further work. this paper is an extended version of an earlier conference paper [5]. 2. digital calibration certificates the structure of a dcc [6] reflects the information that is required by iso/iec 17025 [3] for reporting calibration results. a dcc comprises two compulsory sections. the first compulsory section contains the administrative information including that which is generally presented on the first page of a paper-based certificate. the second compulsory section contains measurement results. the measurement results section itself relies upon a framework, referred to as the “d-si”, developed specifically for the storage of measurement data. the framework ensures that quantity values, units of measurement and uncertainty information can all be represented. a dcc may also include two optional sections. the first optional section contains information presented purely for humans, for example, calibration-specific data sheets, and other auxiliary, machineinterpretable data, for example, relational tables. the second optional section contains an encoded version of a humanreadable version of the certificate. the d-si and dcc frameworks specify, for example, how numerical values and date and time information should be presented and uses the bipm si brochure [7] and the siunitx package for latex [8] as the basis for the provision of units of measurement. the d-si and the dcc may be implemented in a language chosen by the user, for example, extensible markup language (xml) or javascript object notation (json). the smartcom project has developed and made available an xml schema for the d-si framework [9] while the physikalisch-technische bundesanstalt (ptb), the national metrology institute (nmi) of germany, has made available an xml schema for the dcc [10]. demonstrators have been developed by nmis using excel and python [11], [12] and as a web application (gemimeg) [13] to illustrate the use of these schema. a different approach considers embedding calibration data in pdf calibration certificates to support machine-readability [14]. nmis have also been investigating good practice for dccs [15] and usage in specific domains [16], [17]. developers of commercial software for calibration are also starting to support the use of dccs [18], [19]. the first imeko tc6 m4dconf, held in berlin in 2022, focussed on the digital transformation of metrology and many submissions considered dccs [20]. we are aware that the frameworks (and related schemas) may be subject to future updates, for example, if the bipm si brochure is updated, or if the requirements for the contents of a calibration certificate change. in the approach we have taken, dealing with xml schema changes is straightforward, and in fact occurred during our development when a newer version of the dcc schema became available, and we discovered that the dummy certificate with which we were working was missing a compulsory xml element. 3. the dcc protoytpe our prototype application brings together concepts from metrology (specifically, the xml schema for dccs) and academic research in databases and programming languages and enables the transfer of research ideas from academia. on the computer science research side, there are two distinct concepts: temporal databases and a technique for storing xml documents in a relational database, that are combined using the links programming language, a tierless language that supports language-integrated query and provides a single language for the development of web applications with database back-ends [21], [22]. we now consider these three components in detail: links, temporal databases, and storage of xml documents; before considering some details of the implementation of the application. 3.1. the links programming language links is a strongly-typed and statically-typed functional programming language, that is cross-tier (or tierless): it removes the need for the developer to write javascript or a particular database query language (in this case, postgresql [23]) for the different tiers of a web application providing data from a database. in particular, links assists in the writing of correct software by providing language-integrated query which allows the programmer to write high-level queries that are transformed to correct sql queries with known performance for execution on the database. furthermore, the type-checking of variables and functions before program execution reduces the likelihood of run-time errors. figure 1 illustrates the links architecture. the links interpreter takes the links code and generates correct html and javascript to run in the user’s browser, and to support exchange of data between the browser and links program, thus allowing an interactive webpage. the interpreter also generates correct sql queries which are passed to a database server for execution which responds with the results. using this approach, data intensive operations can take place on the database server, using the fact that database systems such as postgresql employ efficient data structures and algorithms for queries. in particular, indexes on key values can make search operations very efficient. links provides syntax to specify database tables as well as comprehension syntax to iterate over each row of a database table. we use the mvu (model-view-update) library [24] for the development of the web interface as it provides functions to use the element of the bootstrap toolkit for website development [25], thereby affording a higher level of abstraction by which to achieve an appealing web frontend. figure 1. the links architecture. acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 3 3.2. temporal databases temporal databases capture when a piece of data is valid. in the case of a relational database management system (rdbms), we are interested in the time period for which a row in a database table is valid, and the inclusion of additional fields (columns) allows this time period to be recorded [26], [27]. we are interested in the provenance of dccs and hence we want to capture when data changes in the database, namely transaction-time information. temporal databases can also record valid-time information which is information about when something is true in the real world. figure 2 demonstrates the difference between an update in a standard relational database versus one in a temporal database supporting transaction time. in the standard case, the new data overwrites the old data and there is no record of the fact that there was a previous value for the field. in a temporal database, two additional fields capture information about when the data was changed, permitting queries such as “what were the values on this day last year?”, “has this field ever had a different value?”, and “which fields have the most frequent changes?”. in the example, the date fields are given informally; however, in the database, the fields are of sql data type timestamp with time zone and cannot be null. in terms of performance in the dcc scenario, since updates are not frequent occurrences, the additional database operations required in the temporal case will not impose a significant overhead. links has recently been extended with temporal features that have been demonstrated in the context of curated scientific databases [28] and we use these features in the development of our prototype. 3.3. xml documents for this paper, we have used a synthetic dcc in xml, containing dummy administrative data and measurement results, formatted according to the xml schema [10] made available by ptb. xml is a tree-structured language, and an xml document is an (inverted) tree of nodes starting from a root node. each node can have any number of child nodes. nodes without children are called leaves. each node in an xml document is either an element or a text node (a string of characters). an element consists of an opening figure 2. non-temporal update of a field versus a temporal update. <dcc:digitalcalibrationcertificate xmlns:dcc="https://ptb.de/dcc" xmlns:si="https://ptb.de/si" schemaversion="3.1.2" ... > <dcc:administrativedata> <dcc:dccsoftware> ... </dcc:dccsoftware> <dcc:coredata> <dcc:countrycodeiso3166_1>gb</dcc:countrycodeiso3166_1> <dcc:usedlangcodeiso639_1>en</dcc:usedlangcodeiso639_1> ... <dcc:performancelocation>laboratory</dcc:performancelocation> </dcc:coredata> <dcc:items> <dcc:item> <dcc:name> <dcc:content>digital thermometer</dcc:content> </dcc:name> ... </dcc:item> </dcc:items> <dcc:calibrationlaboratory> ... </dcc:calibrationlaboratory> <dcc:resppersons> ... </dcc:resppersons> <dcc:customer> ... </dcc:customer> <dcc:statements> ... </dcc:statements> </dcc:administrativedata> <dcc:measurementresults> ... </dcc:measurementresults> </dcc:digitalcalibrationcertificate> figure 3. an example dcc in xml: text nodes are shown by white text on a dark blue background, attributes by black text on a grey background and the other components are xml elements defined by opening and closing tags. the ellipses indicate where elements have been omitted. acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 4 tag and closing tag (or a combined opening and closing tag). the child nodes of the element (if any) appear in between a pair of opening and closing tags of that element. these child nodes can be elements themselves or text (strings of characters). a text node has no children (and hence is a leaf). an xml schema describes the tags that can be used in a document and how they can be nested [29]. additionally, opening tags may contain attributes. example nodes and attributes are given in table 1 and table 2. figure 3 provides an example dcc using the xml schema. in the example, <dcc:performancelocation> is an opening tag and </dcc:performancelocation> is a closing tag, together providing an xml element, which contains the text node laboratory. in the figure, the opening tag for the dcc:digitalcalibrationcertificate element contains three attributes which are shown and these attributes provide sources for the xml schema and give the schema version used in the document. the children of this initial element are the elements dcc:administrativedata and dcc:measurementresults. the temporal features of links are currently supported by an rdbms, hence it is necessary to map from the xml schema to this database. there were two choices here: either to use the dcc xml schema to create the appropriate tables in the rdbms or to treat the data in a schema-agnostic manner and capture it as an xml document. we choose the latter approach because changes to the xml schema will not require changes to the rdbms schema. the dynamic dewey (dde) labelling scheme supports the labelling of xml documents. it uniquely labels each node in an xml document and thereby provides a key for storing the node in a relational table. it has good query and insertion performance [30]. it is an extension of the dewey labelling scheme which has good query performance but differs from it by it removing the need for relabelling (which reduces update performance) when the xml document is updated because dynamic dewey can provide unique labels for new nodes. to store an xml document using this approach, only two tables are required [30] (although a third table can be added to store path information separately – we omit these details here). this approach requires the generation of a unique value to serve as a key for each node (an element or a text node) in an xml document. the dde algorithm determines this unique label (referred to as dde) which consists of a sequence of integers (the nodes can be labelled with negative numbers in certain cases) separated by dots – so-called dot decimal notation. a total order is defined for the labels (which differs from the standard ordering over dot-decimal notation), and this ordering can be used to reconstruct the nesting of the xml document as well as to determine relationships between nodes such as parent, child, and sibling. the main table stores nodes and is similar to the one illustrated in the right-hand side of figure 2. its schema is (dde, tag, text, path, tt-from, tt-to). each row has the dde value for the node as the key attribute. if the node is an element, the element tag will appear in the tag column, otherwise if it is a text node, its text string will appear in the text column (one of these two columns is always empty). the path column stores the list of the element tags followed from the root element to the parent of the node. the last two columns tt-from and tt-to store the timestamps that describe when the node is valid. because the dde value captures the relationships between nodes, there is no need to explicitly refer to other nodes in a row of this table. the second table contains attribute information and has the schema (dde, attr, value, tt-from, tt-to). in an element, each attribute can only appear at most once, hence the attribute name and the dde value are sufficient to uniquely identify the attribute’s value. consider the following xml fragment: <el1 attr1=”a value”> <el2>text<\el2> <el3 attr2=”another value”><\el3> <\el1>. the dde algorithm will generate the content shown in tables 1 and 2. for this example, the length of each segment of the key is 3 characters. in the implementation, this length is a parameter, and it can be set appropriately for document size. as can be seen from this explanation, the actual details of the xml schema are not used for the database schema – there is no table for customer details, for example. by using the dde ordering with the appropriate links code to specify joins over the relations, documents can be stored, retrieved and displayed, as shown by our implementation. 3.4. implementation the prototype provides an interface for viewing a single dcc. straightforward extensions include the ability to edit the dcc and to validate that it adheres to the dcc and d-si xml schemas. further obvious extensions are the ability to work with many dccs, to compare different dccs, and to support the signing of dccs. we are interested in provenance beyond tracking changes in the data, such as recording which person and what software made the changes. in the longer term, we will require systems that support the whole calibration workflow including automated collection and processing of machine-readable calibration data. using the interface, it is possible to view the current state of the document, the state at a previously specified date and time, and to compare two versions at two specified dates and times. it is possible to hide or show specific subtrees of the document, and when doing the comparison, to see the subtrees where changes have been made. the change analysis option allows the user to see a history of all changes over the life of a dcc. figure 4 shows how changes are highlighted as well as cumulative totals of insertions and changes for each tag and text node in the document. there are various options to view subtrees for a specific tab or to control the display of the whole document. the figure also shows how details of changes are displayed. (note that this screenshot is of a different dcc to the one presented in figure 3.) the links code for the prototype is available at https://github.com/vcgalpin/dcc-xml-temporal [31]. table 1. example node table. dde tag text path tt-from tt-to 001 el1 \ … … 001.001 el2 \el1\ … … 001.001.001 text \el1\el2\ … … 001.002 el3 \el1\ … … table 2. example attribute table. dde attr value tt-from tt-to 001 attr1 a value … … 001.002 attr2 another value … … https://github.com/vcgalpin/dcc-xml-temporal acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 5 as mentioned above, a more complete application should be able to validate against the dcc and d-si xml schemas. the level of validation that is provided by these schemas is very high but not complete. in some cases, patterns are used to ensure that values are within bounds. for example, the si:probabilityvaluetype [9] has the following associated pattern <xs:pattern value="\+?((0(\.\d*)?)|(1(\.0*)?))”/>. this pattern specifies that a value of this type is an element of the interval [0,1], as required for a probability value. however, not all aspects of the input for a dcc are specified in this manner and it would be useful to be able to check for correct spelling of measurement units and probability distribution names, as well as requirements that are specific to a particular domain of measurement or instrument. good practice guides for dccs in specific domains are being prepared [14] and another possibility is to provide additional schemas that prescribe requirements for specific groups of instruments. moreover, not all functionality needs to be supported directly in links. for example, schema validation, which could be provided by different software such as the tracim system 2.0 developed by ptb as part of the smartcom project [32]. the application validates xml documents for the d-si schema, and also undertakes additional compliance checking of the units used and the minimum metrological information required by vim [33] and gum [34]. however, the provision of a suitable interface for users will require the integration of the different pieces of software. 3.5. discussion considering our choice to work with a relational database, links may in future support the storage of xml documents directly with temporal features, and hence the translation would no longer be necessary. in that case, the schema be used more directly to store data, such as the customer details, for example. since the dcc is only one component of the envisioned digital calibration workflow and can be seen as output and data exchange from some form of data storage, there is no need to use an xml storage format. dealing with temporal data in a relational database is better understood and less experimental so we believe it makes sense to use a rdbms for a system for digital figure 4. a screenshot of the change analysis interface showing an insertion and a change of email – both are highlighted. the insertions and changes columns provide information about the number of modifications. the first number shows the number of changes for the particular tag or text node, and the number in brackets shows the number of changes that have happened below and including the current tag or text node. acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 6 calibration data. working in a schema-less fashion with the xml document provides the ability to cope with changes in the dcc xml schema without affecting the storage in the rdbms, and we have already found this approach to be of benefit when changes were made to the dcc schema. another approach to deal with update provenance is provided by databases that support versioning such as orpheusdb [35]. this relational database is based on the concepts that are used in software versioning systems such as git [36] and is motivated by the requirement to track changes in curated scientific databases. temporal databases can be seen as providing a linear model of time whereas versioning supports a non-linear model of time. in the non-linear model, it is possible for separate branches to have different updates that then must be merged together at a later time point. it seems unlikely that dccs require this level of complexity. moreover, versioning usually requires users to explicitly create versions, whereas temporal database provide an automatic approach to tracking changes. furthermore, querying in the linear model is more straightforward than in the non-linear model. 4. future work and conclusions the prototype we have developed demonstrates that the combination of dccs and temporal databases results in streamlined curation of calibration data. the prototype leverages features of the links language (a) to facilitate the development of a user-friendly web frontend, (b) to integrate relational database queries on a database that maps with the dcc xml tree structure, and (c) to enable automated tracking of changes made to calibration records over time, as well as the explanation and timeline for those changes. looking to the future, further advances in the area of calibration certificates are in the pipeline, such as digital schemax [37] which will include envelope digital certificates for grouping related calibration certificates together, as well as schema for digital reference materials and digital test certificates, and these advances will require appropriate digital solutions. another possibility that results from the digital storage of multiple certificates is to link information across certificates, and to develop new aspects of traceability, and related to this development, certificate validity and frequency of calibration. in many cases, nmis and calibration laboratories might not know the exact needs of the users at the end of the measurement chain. to meet those needs, a comprehensive traceability chain including the specification of the dependence between consecutive measurement stages is therefore required [38]. our temporal approach to the curation of calibration data could be used to retain detailed information that the end-users would want to process in a way that was not anticipated by the nmi and calibration laboratories, for example, analysing the risks associated with factors that impact multiple stages across the measurement chain. to conclude, dccs are the tip of the iceberg (figure 5) when considering the whole lifecycle of machine-readable calibration data. their creation and use depend on software to support storage and provenance of the underlying calibration data. the development of the prototype has raised interesting questions about what such a system should be able to provide, and we will continue with our investigations and development. acknowledgement funding: [vg] this work was supported by erc consolidator grant skye [grant number 682315] and by an iscf metrology fellowship grant provided by the uk government’s department for business, energy and industrial strategy (beis). 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in final form november 18th, 2013; published december 2013 copyright: © 2013 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: (none reported) corresponding author: fabien ezedine, email: ezedinefabien@gmail.com 1. introduction carolina cruz-neira et al. [1] defined vr in 1992 as a "system which provides real-time viewer-centered head-tracking perspective with a large angle of view, interactive control, and binocular display". the concept and the relation between the vr system and the user can be summarized as follows [2]:  capture of user's actions;  computation of these data and creation of a tailored response;  transmission of the response toward the user. this concept was greatly improved during the 80's thanks to the parallel evolution of computer science, and the development of new external devices in haptic interaction and visualisation. since then, more and more applications are developed in manufacturing, research, teaching or therapeutics fields, through multiple devices such as the head-mounted display (hmd), the boom or the cave automatic virtual environment (cave). this research work deals with the cave. figure 1. model of location and orientations of the eight-cameras system in the cave. abstract this paper studies the factors that influence the accuracy of virtual reality (vr) systems, in particular for applications in a cave automatic virtual environment (cave). the cave can be used to train surgeon students. for this purpose, an application for a total knee arthroplasty surgery is investigated. to meet the requirements for a high quality training, the accuracy of the tracking system in the cave has to be improved. first, a complete model of the tracking system is created based on the extrinsic and intrinsic parameters of the eight-camera system. with this model, the uncertainty of the tracking system is determined for one location in the cave. next, a hybrid method, comprising the monte carlo method and design of experiment, is used to find the most important factors influencing the tracking accuracy in the cave. acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 21 our cave consists of a four projection-sides environment (three walls and a ground), a set of cameras (eight cameras in the studied system) and four stereoscopic projectors. figure 1 models approximate locations and orientations of cameras in the cave structure. interactions between a numerical model, a system of projection, a system of tracking and a set of tracked spheres hung to user are created. a user which is immersed in this virtual world wears shutter glasses, in order to get a depth perception of the numerical model projected by projectors, via mirrors. cameras are able to locate glasses, thus defining the field of vision of the user, through spherical markers tracked by these cameras shown in figure 2. then, this information is computed and sent back to the user. the technology used by the system in place strongly influences the quality of the immersion experienced and sensed by the user. these devices provide information in order to get the location of the user in the cave, using the tracking of optical markers [3]. applications require a more or less good accuracy: for example, a visitor in a museum travelling inside an architecture, or a medical student training on a knee surgery, are differentiated by the precision needed to really feel the immersion inside this virtual world. therefore, before loading an application inside a vr system, checking the compatibility between the precision required by the application and the real accuracy delivered is a compulsory step. if this process does not bring any convincing results, then two options are possible: either the application cannot be loaded in the cave with its proper features, or a method has to be found to improve the accuracy of the tracking system, which is the purpose of this paper. the method is, first, to model the tracking system in order to define the uncertainties of the captured coordinates: these uncertainties will be derived from a covariance matrix which is found using monte carlo simulations; and second, to identify the most influential factors using a hybrid method involving monte carlo simulations, design of experiment, hadamard matrix and bayesian analysis. 2. the pin-hole model in the tracking system, arttrack cameras are modelled using the principle of the pin-hole camera and projective geometry theory, which is shown in figure 3. linear algebra is used to provide the cartesian coordinates m(ui, vi) in the ccd frame of the image of a marker tracked by each camera i. these coordinates are derived from the position s(xi,yi,zi) of the marker in the cave global coordinate system. these coordinates are given as a function of the extrinsic and intrinsic factors of the camera [4]. extrinsic factors define the position and the orientation of each camera. the position of each camera is characterized by the coordinates of the lens centre oi. the orientation of each camera is defined by azimuth and inclination angles. intrinsic factors define the proper characteristics of the camera, such as the focus distance f, the skew coefficient α, defining the angular error between the horizontal and vertical directions of pixels and the coordinates (u0i, v0i) of the intercept ci (figure 3) between the focus axis and the image plane of the camera i. the reference frames used in the model are (i refers to the index of a camera, 1 < i < 8):  ( , , , )wr o x y z    : bound to the cave, the global coordinates system ;  ( , , , )cai i cai cai cair o x y z    : bound to the camera i ;  im ( , , )ii i ir a u v   : bound to the image plane of the camera i ; the characteristic points are defined as following:  ( , , )s x y z : coordinates of s into wr ;  ),,( iii zyxs : coordinates of s into cair ;  ( , )i i im u v : coordinates of im into irim ;  0 0( , )i i ic u v : coordinates of ic into im ir ; the pin-hole model is used to provide a realistic description of a camera. in this approach, the real image plane is shifted to the location zci = f = oici and reversed, making the analysis easier [5]. the pin-hole model allows defining the image mi of figure 2. arttrack tracking cameras. figure 3. pin-hole model with f, the focus distance. acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 22 any tracked point s in the local reference frame of the camera. the coordinates (ui, vi) of mi are given as a function of the focus distance f, the skew coefficient α and the scale factor h (equation 1): 0 0 . 0 . 0 0 . 0 0 1 0 1 i i i i i i i x h u f u y h v f v z h                                (1) classical matrix transformations are used to derive the local coordinates of s from the fixed coordinates in the reference frame rw of the cave:     11 12 13 21 22 23 31 32 33 1 0 0 0 1 0 . . 0 0 1 1 1 0 0 0 1 0 0 . 0 0 0 0 1 1 i xi i yi i i i zi i i i i i i i i i x x t y y t t r z z t r r r x r r r y r r r z                                                              (2) and      .i yi xir r r considering, in cartesian coordinates:  it : relative to the translation vector io o   xir : relative to the orientation of the camera around x-axis:   1 0 0 0 cos( ) sin( ) 0 sin( ) cos( ) xi xi xi xi xi r r r r r          (3)  yir : relative to the orientation of the camera around yaxis, using axial rotation (quaternion theory). considering ( , , )i xi yi zin n n n  , the transformation matrix is:   2 2 2 1 0 0 cos( ). 0 1 0 0 0 1 . . (1 cos( )). . . . . 0 sin( ). 0 0 yi yi xi xi yi yi zi yi xi yi yi yi zi xi zi yi zi zi zi yi yi zi xi yi xi r r n n n n n r n n n n n n n n n n n n r n n n n                                 (4) some distortions caused by the lens have to be considered in order to extend the pine-hole model. the vector distortion is ( , )txi yi  . these distortions can be radial, image decentring, and prismatic. equation (5) describes the detailed model r3d1p1, a non-linear polynomial distortion model for cameras [6]: 3 2. 2 2 1 1 2 2 1 3 2. 2 2 2 1 2 2 2 .[ . .( 2. ) 2. . . . ] .[ . .( 2. ) 2. . . . ] n xi mi ni mi i mi mi n i mi mi i mi n yi mi ni mi i mi mi n i mi mi i mi x r d x d x y p y r d y d x y p                              where: 2 2 2( )mi mi mix y   is the squared distance between ci and mi ; rni is the radial distortion factor (1 3)n  dmi is the image decentring distortion factor (1 2)m  relatives to xcai-axis for m = 1 and to ycai-axis for m = 2 ; pmi is the prismatic distortion factor (1 2)m  relatives to xcai-axis for m = 1 and to ycai-axis for m = 2 ; ( ; )i i mi mic m x y  . then, equation (6) describes the full model providing the coordinates of the image of a tracked marker, inside the ccd frame of the camera i, as a function of s(x,y,z) : 11 12 13 31 32 33 21 22 23 0 31 32 33 21 22 23 0 31 32 33 . . . . . . . . . . . . . . . . . . . . . i i i xi i i i i zi i i i yi xi i i i i zi i i i yi i yi i i i i zi x r y r z r t u f x r y r z r t x r y r z r t u x r y r z r t x r y r z r t v f v x r y r z r t                                  (6) 3. tracking uncertainty computation in the pin-hole model, the intrinsic and extrinsic parameters are assumed to be perfectly defined but it is not really the case. in fact, these factors are calibrated by preliminary experiments and, therefore, are only ranged in a given uncertainty interval. for this reason, the real images of any marker captured by the camera do not fit the position calculated from the nominal parameters but are subjected to some deviations. therefore, the approximated 3d coordinates of the tracked point computed by the multiple camera system do not correspond to the real ones. our aim is consequently to appraise the uncertainties of the 3d coordinates evaluated by the tracking system. for that purpose, the intrinsic and extrinsic parameters are randomly perturbed assuming a uniform distribution in their uncertainty intervals, in order to be placed in the most critical case [7]. each simulation is assumed to correspond to a real configuration of the tracking system. for this end, the coordinates of the images mi as they would be captured by the eight cameras are (5) acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 23 computed. the theoretical positions of mi are also derived from the nominal mean parameters. the difference between both points is called projection error (ei). as in a real experimental configuration, the coordinates of the tracked point s are supposed unknown. after computing the projection error for each camera using the full model, the least squares method is, therefore, used to evaluate the location of s in rw. the deviation ( , , )dm dx dy dz  between the real and evaluated locations of s is also computed. this calculation procedure is repeated 30000 times using a monte carlo simulation approach [8]. this method provides the variance covariance matrix mv of the deviations dm  . the error zone, an ellipsoidal shape, is also created. if the eigenvalues and eigenvectors of the variance covariance matrix are calculated, the dimension and orientation of the error zone can be computed. a cave with eight tracking cameras counts until one hundred and twenty eight significant factors, sixteen per camera [5]. in order to improve the accuracy of the tracking system and consequently the quality of the user immersion in the cave, it is important to target adjustable factors which have a significant influence. 3.1. the hybrid method as shown in figure 4, the flowchart explains the first step of this hybrid method which is to detect adjustable factors. it is assumed that intrinsic factors cannot be modified and/or adjusted by the user. indeed, the cameras used are off-the-shelf ones from arttrack and the only factors we can work on are the extrinsic ones. moreover, preliminary calculations have shown that the distortion factors of the camera do not have significant effect on the components of the variance covariance matrix vm. therefore, only effects of the position and the orientations of the cameras are studied. the cameras are fixed on the framework of the cave. the position referring to the camera i is defined as the location of lens optic centre using cartesian coordinates in rw (camitx, camity, camitz). the orientation of the optic axis is defined by camirx and camiry. depending on the location of cameras inside the framework, all parameters cannot be adjusted. then, only twenty six extrinsic factors are adjustable (cam1ty, cam2tz, cam3tx, cam3tz, cam4tx, cam5tx, cam6tx, cam6tz, cam7tz, cam8ty) and camirx, camiry for 1 8i  . these factors of position and orientation of cameras are detailed in figure 5, where ci refers to the location of the camera i. the processing for adjustable and non-adjustable factors is different. in the real configuration of the cave, the nonadjustable factors xi are fixed and calibrated during manufacturing. however in order to compare the effects of the adjustable factors and the unknown influences of the internal parameters of the cameras, the later values were generated randomly in the monte carlo simulation, leading to random perturbations of the calculated results. this approach permitted the application of statistical tests to discriminate the significant adjustable factors. the effects of the adjustable factors xi are studied by a doe methodology [10]. for this purpose, only the lower and higher values of the factors were considered and normalized to (-1,1). in a classical approach, it would lead to 226 simulations. but, in order to decrease the number of random generations, an optimized strategy is used, based on hadamard design matrix of dimension 28x26 [11]. the doe analysis leads to twenty eight monte carlo simulations. two responses (8), function of factors xi, are provided by the covariance matrix after monte carlo simulations: the average dimension of the error zone, y1; the distortion of the error zone, y2, such as: figure 4. flowchart of hybrid method (mcm, doe). figure 5. translation and rotation adjustments available for each camera. acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 24 1 2 2 1 . ( ) 3 3 . 2 m ij y m tr v y d         where .md v m i  (8) a screening process is used to detect the influential factors. the screening models are linear systems which link y1 and y2, the response vectors to x, the experimental matrix. vectors b1, b2 of components bi1, bi2 defined the coefficients of the model and e1, e2 the error vectors: 1 1 1 2 2 2 . . y x b e y x b e      (9) the error vectors e1, e2 describe two kinds of deviations: the best fit approximation error and the random perturbations introduced by the non-adjustable factors. due to the large number of intrinsic and extrinsic parameters of the monte carlo simulation, the distribution of these errors becomes practically gaussian, even if the random generations used a uniform repartition. these systems get a number of equations greater than the number of unknowns. the least squares method is therefore used to solve the system. the estimates 1 2 ˆ ˆ,b b of vectors b1, b2 are obtained: 1 1 1 1 2 2 ˆ ( . ) . . ˆ ( . ) . . t t t t b x x x y b x x x y       (10) then, statistical tools are used to create a statement of factors, concerning their influence on responses. figure 5 summarizes the whole hybrid method used to highlight the most influential factors of the tracking system in the cave. 3.2. statistical tools two tools are used to analyse and state the influence of the factors: the graph of effects and the bayesian analysis. regarding the graph of effects, it is assumed that bi1, bi2 coefficients are propagated using a student law. the degree of freedom is one, because twenty seven factors bi and twenty eight different equations given by the hadamard matrix are considered. with 5% risk, the student table provides: 0.025 12.7t  . 95% of bij values belong to this range of confidence: 0,025 0,025 ˆ ˆ. var( ); . var( )ij ijt b t b      . (11) then, in order to look after the range of confidence of bi1, bi2 coefficients, the error matrix of two answers y1 and y2 1 2( , )e e e can be computed as follow: 1 1 1 2 2 2 ˆ. ˆ. e y x b e y x b       (12) 1 2 ˆ ˆ( ), ( )var b var b are computed using these equations : 1 2 1 2 1 1 2 2 ˆ( ) ( . ) . ˆ( ) ( . ) . t e t e var b x x var b x x         (13) regarding the bayesian analysis, the principle of the test is to consider a posteriori that every factor is active [12][13]. two parameters are considered: the probability that a factor is active a priori (p) ; the ratio between variances of active factors and variances of non-active factors (q). then, the aim is to calculate probabilities a posteriori for any combination (p,q) ranging : 0.1 0.4p  for p and 5 20q  for q. table 1. domain of factor concerning position of cameras given in millimetres. table 2. domain of factor concerning orientation of cameras, 1 8i  , i integer given in degrees. figure 6. bayesian analysis of y1. acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 25 3.3. adjustable factor settings the location of the marker studied in monte carlo simulations is (0, 1200, 0), all coordinates are given in millimetres. the model was programmed in visual basic. depending on the characteristics of the computers used, a simulation may take four to six hours. the domain of every factor has to be chosen. as said previously and concerning the positions of cameras, their location influences this domain, whereas the domain of factors about orientation remains the same. the domains per adjustable factor are shown in table 1 and table 2. the unit used for the position is the millimetre and the one used for orientation is the degree. these variations are given as a function of the theoretical location and orientation of cameras inside the reference frame of the cave as shown in figure 5. 4. result analysis 4.1. analysis for y1 these two statistical analyses are strongly similar and lead to the same findings: the positions of camera inside the cave do not influence the dimension of the error zone ; only the orientations of cameras are adjustable factors influencing the dimension of the error zone. the symmetry of the cave is respected ; the influential factors over y1 are the rotations : cam1ry, cam2rx, cam3ry, cam4rx, cam5rx, cam6ry, cam7rx and cam8ry. in the case of cam4ry and cam5ry, a reflection has to be done as the geometrical symmetry of the cave is not respected. as per figure 6 and figure 7, the statistical analysis shows that the orientations of each camera might have an influence on the dimension of the error zone. figure 7. graph of effects of y1. figure 8. bayesian analysis of y2. acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 26 4.2. analysis for y2 as the response mentioned above, the two analyses lead to the same findings: as the response mentioned above, the two analyses lead to the same findings: the positions of camera inside the cave do not influence the dimension of the error zone ; only the orientations of cameras are adjustable factors influencing the dimension of the error zone. the symmetry of the cave is respected ; the influential factors over y2 are the rotations : cam1rx and cam8rx. as per figure 8 and figure 9, the statistical analysis shows that the orientations of cameras 2, 5, 6 and 7 might have a low influence over the distortion of the error zone. cameras 1 and 8 strongly influence the distortion of the error zone in comparison to others. 5. conclusion this study focused on the tracking system of a cave automatic virtual environment. in order to improve the accuracy of any position captured by the system a complete modelling of the cameras has been developed. this approach permitted the simulation of the tracking device of the cave. using a hybrid method based on a doe and monte carlo simulations, the effect of the positions and the orientations of the cameras over distortion and average dimension of the error zone were studied. the final result revealed that the only adjustable factors which has to be considered for a strong influence on the dimension and the distortion of the error zone are the orientations of cameras. the next step of this research will be to analyse the possible interactions between the key factors and to create the response surfaces. acknowledgement this work was realized in collaboration with the centre de réalité virtuelle de marseille (crvm) in france. we thank all members of crvm team, especially its director prof. daniel mestre and engineers pierre mallet, vincent perrot and jeanmarie pergandi, for their help and great support. this research work was also supported by an strg grant from the universiti kuala lumpur, unikl. references [1] cruz-neira, carolina, daniel j. sandin, and thomas a. defanti. "surround-screen projection-based virtual reality: the design and implementation of the cave." proceedings of the 20th annual conference on computer graphics and interactive techniques. acm, 1993. [2] sylvain jubertie, "modèles et outils pour le déploiement d'applications de réalité virtuelle sur les architectures distribuées hétérogènes", thesis from université d'orléans, france, december 2007. [3] thomas a. defanti, gregory dawe, daniel j. sandin, jurgen p. schulze, peter otto, javier girado, falko kuester, larry smarr, ramesh rao, "the starcave, a third-generation cave and virtual reality optiportal", future generation computer systems, volume 25, issue 2, february 2009, pp. 169-178. [4] f. ezedine, w. m. wan muhamad, j.m. linares, “uncertainty calculation of a multicamera tracking system in a cave”, advanced mathematical and computational tools in metrology and testing, vol.9 (f pavese, m bär, j-r filtz, a b forbes, l pendrill, h. shirono, eds.), series on advances in mathematics for applied sciences vol. 84, world scientific, singapore, pp. 151-158, 2012. [5] a. g. buaes, "a low cost one camera tracking system for indoor wide-area augmented and virtual reality environments", post graduation program in electrical engineering, 2006. [6] c. ricolfe-viala, a.j. sanchez-salmeron, "robust metric calibration of non-linear camera lens distortion", pattern recognition 43, 2010, pp. 1688-1699. [7] m. matsumoto, t. nishimura, "mersenne twister: a 623dimensionally equidistributed uniform pseudo-random number generator", acm transactions on modeling and computer simulation, vol. 8, no. 1, january 1998, pp. 3–30. [8] m. douilly, n. anwer, p. bourdet, n. chevassus, p. le vacon, uncertainty evaluation for pose estimation by multiple camera measurement system, advanced mathematical and computational tools in metrology and testing, serie on advances in mathematics for applied science, vol. 78, pp. 73-79. figure 9. graph of effects of y2. acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 27 [9] j.m. linares, j.m. sprauel, p. bourdet, "uncertainty of reference frames characterized by real time optical measurements: application to computer assisted orthopaedic surgery", cirp annals manufacturing technology, volume 58, issue 1, 2009, pp. 447-450. [10] j. chaves-jacob, j.m. linares, j.m. sprauel, “using statistical confidence boundary of a d.o.e. response surface to estimate optimal factors”, advanced mathematical and computational tools in metrology and testing, vol.9 (f pavese, m bär, j-r filtz, a b forbes, l pendrill, h. shirono, eds.), series on advances in mathematics for applied sciences vol. 84, world scientific, singapore, pp. 74-81, 2012. [11] c. koukouvinos, s. stylianou, "on skew-hadamard matrices", discrete mathematics, volume 308, issue 13, july 2008, pp. 2723-2731. [12] r. kacker, r. kessel, k.-d. sommer, "only non-informative bayesian prior distribution agree with the gum type a : evaluations of input quantities", advanced mathematical and computational tools in metrology and testing, vol.9 (f pavese, m bär, j-r filtz, a b forbes, l pendrill, h. shirono, eds.), series on advances in mathematics for applied sciences vol. 84, world scientific, singapore, pp. 216-223, 2012. [13] g.a. kyriazis, "bayesian inference in waveform metrology", advanced mathematical and computational tools in metrology and testing, vol.9 (f pavese, m bär, j-r filtz, a b forbes, l pendrill, h. shirono, eds.), series on advances in mathematics for applied sciences vol. 84, world scientific, singapore, pp. 232-243, 2012. analysing and simulating electronic devices as antennas acta imeko issn: 2221-870x september 2021, volume 10, number 3, 150 155 acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 150 analysing and simulating electronic devices as antennas dániel erdősy1, tamás bodolai2, angéla váradiné szarka2 1 university of miskolc, institute of electrical and electronic engineering, miskolc-egyetemváros 3515, hungary 2 university of miskolc, research institute of electronics and information technology, miskolc-egyetemváros 3515, hungary section: research paper keywords: emc simulation, devices as antennas, device simulation citation: dániel erdősy, tamás bodolai, angéla váradiné szarka, analysing and simulating electronic devices as antennas, acta imeko, vol. 10, no. 3, article 21, september 2021, identifier: imeko-acta-10 (2021)-03-21 section editor: lorenzo ciani, university of florence, italy received february 8, 2021; in final form august 4, 2021; published september 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: dániel erdősy, e-mail: elkedani@uni-miskolc.hu 1. introduction in all departments of electronic design and manufacturing, electromagnetic compatibility (emc) is one of the most important factors. all the devices need emc to work properly. in fact, some devices are prone to interfere with others, while others radiate a considerable amount of electromagnetic waves. nowadays, emc has become increasingly important, because of the growing number of electronic devices, gadgets, smart devices we use. the basic idea is that if we know some radiation or immunity properties of the product, we could feed these into the simulation software. if we have the proper structural model of the product, then we could simulate and analyse the properties; therefore, with the results, we can consider what changes must be made. an example is when a bigger structure, which contains metal parts, magnets, external wires and electronics, may be too complicated for complex simulations, but the different parts can be analysed as simple individual antennas. this paper explains the theory behind a new method to reduce costs on electronic design stages, simulations and emc measurements. 2. emc problems in long terms a common approach to achieve the required emc levels starts with the design of the device. at this stage, most of the work that affect emc properties can be carried out easily, without further costs. after the design, prototypes can be manufactured and, if required, emc measurements are performed. if everything goes right, final manufacturing starts, and the product could get on the market. but what happens if after some time, we want this product to achieve higher emc limits in special appliances, due to local laws, or other devices working next to it, [1]? the costs of a whole new process easily get high but improving the good working product can be worth it. here comes a special idea about this improvement: if we know what we want to improve in the product, for example, in a frequency range it emits more em waves than we want, we could analyse the structure directly as some antennas on that specific range. emc measurements can declare the radiation characteristic, but it could be cost effective, if we could easily simulate that. simulation is one side of determining the radiation properties of abstract electromagnetic compatibility (emc) is getting more and more interest with the ever-increasing adoption of electronic devices. emc must be planned, calculated, simulated, and measured; this is necessary to ensure that the device can work in the environment of other devices and does not disturb other devices through undesired interference. in complex devices, simulation methods due to the large number of elements and complicated operations can be extremely costly and time consuming. we should consider how is it possible to reduce these costs, for example with simplifying our simulation terminology. this paper explains a general theory about using premeasured emc properties in simulation to predict how a complex system (containing many devices) will work. this theory is about how different radiated electromagnetic waves affect other devices. emc standards have been developed to test and limit unintentional coupling to and from electronic devices (interference), but not developed to deal with more devices crowded in a smaller environment. if we handle the different devices as antennas, in the simulation environment we must not deal with their internal structure and operation (unless we need information of their internal structure). in this way, simulation time can be drastically reduced. mailto:elkedani@uni-miskolc.hu acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 151 the product, but in function of internal structure and operation parameters, simulation time and computer requirements increase [2]. 3. devices as antennas antennas can be made of a simple wire or complex metal structures, maybe some smaller blocks that act like a grid antenna. it is possible to look at our device as an antenna. they are not optimized to work as antennas (in general conditions), but as unintentional antennas. 3.1. gain and radiation pattern antennas are typically designed for specific usage with needed characteristics (like gain and pattern). this means they have a known frequency response, directivity, or pattern. on the other hand, simple devices are not designed like this, so the mentioned parameters work on a smaller scale. this means that measurements on these unintentional antennas present some challenges compared to normal antenna tests. for example, commercial emc tests measure the entire 3d radiation, while in automotive tests only a few directions must be measured. this is one reason why alternative methods to estimate the directivity, radiation pattern and other needed parameters are considered [3]. for real-life antennas used nowadays, well-known methods are developed to determine the required parameters and these methods (with some modifications) should be use on unintentional antennas also [4]. 3.2. measuring the device to measure the radiated power pattern (of any device) one method is to use a fully or semi anechoic chamber (for example a sac-3) (figure 1) [5]. the receiving antenna must be equally far from the source. the equipment under test (eut) is mounted on a rotating table, while the receiving antenna mounted on vertical sliding base to move up and down. each measurement is carried out in one position, then rotating the eut or lifting the antenna. from the measurements, a ratio of the maximum to average power can be calculated for each planar cut. another method uses a reverberation chamber (figure 2). this type of chamber is generally employed for immunity testing; however, it can also provide an efficient method for radiated emission measurements. results of these measurements yield the total radiated power, not directivity properties. 4. simulating emc properties without emc measuring technology, electromagnetic simulation of the device can also provide us some useful, antenna-like properties [7]. there are several types of simulating software on the market. numerical simulations can provide full emc properties, but to inspect a bigger complex system, it needs lots of computing time. even though precise and complex simulation is not the most cost-effective solution, on a lower, individual parts level can be worth it. numerical simulations are useful for both purposes, i.e. not only to represent the observed phenomena and find effective countermeasures early, but also to predict phenomena without measurement in the design stage [8]. modelling a bldc motor especially for emc simulation is presented in [9]. bldc motors are a good point in the emc topic, because they get more and more popular, but also due to their working conditions, can suffer from lots of electromagnetic problems. the sum of instantaneous voltage of three-phase windings of a motor (also 2-phase windings), or common-mode voltage, is not zero in this system. furthermore, the switching frequency is increasing with the development of switching devices such as igbts. these facts cause high-frequency leakage current, which leads to emi/emc issues. the common-mode voltage also provokes a shaft voltage, which causes a bearing current. the bearing current damages a bearing and makes its lifetime shorter. in addition, high-frequency switching causes overvoltage and turn-to-turn voltage stress in stator windings, which affects its insulation seriously. it is possible that modern simulation software on a fast-enough computer can simulate the most required parameters, but in [9] the entire simulation process is taken in different parts to increase efficiency. although models are made for electromagnetic field analysis, capacitance is independently calculated by static electric field analysis and inductance with resistance is calculated by magnetic field analysis. the motor with laminated iron core radiates different em frequencies, because of different reasons. the effect of eddy current becomes significant only above 1 khz. magnetic flux is not completely pushed out of the iron core even at 1 mhz. the resistance in case of laminated iron core is larger than that in case of bulk iron core above 20 khz. this is because the surface area where eddy current flows is larger and thus the total power loss is larger in laminated iron core for such high frequencies. in this way, with 2-d mesh data, it is likely to overestimate the shielding effect of eddy current in laminated iron core. therefore, we should use 3-d mesh data to calculate inductance and resistance of stator windings accurately (figure 3) [9]. this example shows figure 1. sac-3 chamber with automotive table and bilog antenna at the university of miskolc. figure 2. the reverberation chamber as described in iec-61000-4-21 [6]. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 152 that precise simulation requires different approaches; there is no straightforward solution for it. with the different simulation results it is possible to construct a high-frequency motor equivalent circuit (or model) (figure 3 shows an example for a bldc motor) [10]. this also means that 2d and 3d simulation software is no longer needed, because we only need the results of previous simulations. the equivalent circuit contains the data of resistance, inductance, and capacitance of the stator windings, so after some “basic” calculation we can achieve a frequency characteristic of the motor impedance (figure 4). both the magnitude and the phase angle can be calculated; therefore, the whole process can result in the required antenna parameters. these numerical simulations are not relevant in fine tuning a real motor, because there are more phenomena (such as hysteresis, permeability, magnetic saturation) and lack of factors like manufacturing imperfection, temperature, aging. nonetheless, it is possible to use the results further for our needs [11]-[13]. 5. estimation of directivity after the emc measurements or similar simulations mentioned in section 3 and 4, enough data is gained to move to the next step. this section is based on [15], [16], where a theory of estimating not all antenna parameters, but the directivity was presented (note that if directivity measurements are done, this step can be skipped.) directivity of an antenna (or device as antenna) is the evaluation of far-field form of the spherical mode expansion of the emitter. the assumption is the expansion coefficients are independent random variables. equation (1) shows the general form for the far-field pattern (in 3d spherical coordinate system expressed with polar coordinates). �̅�(𝑟, 𝜃, 𝜗) = 𝑘√𝜂 1 √4 π e𝑗 𝑘 𝑟 𝑘 𝑟 ∑ 𝑄𝑠𝑚𝑛 (3) 𝐾𝑠𝑚𝑛̅̅ ̅̅ ̅̅ ̅(𝜃, 𝜗) 𝑠𝑚𝑛 , (1) where the spherical coordinate system is defined as usual, η is the free space wave impedance, ∑ = 𝑠𝑚𝑛 ∑ ∑ ∑ , 𝑛 𝑚=−𝑛 𝑁 𝑛=1 2 𝑠=1 𝑁 ≈ 𝑘 𝑎 , (2) where a is the radius of the minimum sphere enclosing the emitter (it is assumed the spherical coordinate system is centred in this sphere), k a is called the electrical size, 𝑄𝑠𝑚𝑛 (3) are the wave coefficients, and 𝐾𝑠𝑚𝑛̅̅ ̅̅ ̅̅ ̅(𝜃, 𝜗) are the far-field large argument, forms of the dimensionless power-normalized spherical wave functions [17]. this convention yields the expression (3) 𝑃rad = 1 2 ∑ |𝑄𝑠𝑚𝑛 (3) |2 𝑠𝑚𝑛 . (3) expression (3) shows the total radiated power (prad) from the emitter. in general, the summation over n goes to infinity; however, spherical wave functions with indices n > k a are cut off and will not contribute in the far-field and, because of this, the series can be truncated. 𝑁𝑚 = 2 ∑(2 𝑛 + 1) 𝑁 𝑛=1 = 2(𝑁 2 + 2 𝑁) , (4) where nm is the total number of modes. each coefficient has an independent real and imaginary part. with this expression, an upper bound can be found. directivity is the ration of radiated power (at a given angle) and distance to the total radiated power divided by the total solid angle. the caucy-schwartz inequality yields a sum over the number of modes (nm), which is reduced by a factor of two. the result expression below once (ka) is substituted for n: 𝐷max ≈ { 3, 𝑘 𝑎 ≤ 1 (𝑘 𝑎)2 + 2 𝑘 𝑎, 𝑘 𝑎 > 1 . (5) the only problem here is dmax is an overestimation of the directivity of unwanted antennas. a better estimation can be derived by assuming that the spherical-wave coefficients are independent random variables. for unintentional emitters, the mean of coand crosspolarized terms will be equal, so the mean value of d is 1. 𝐷co = 𝐷cross = 1/2 (6) the field components of expression (1) is the following: figure 3. equivalent circuit of a three-phase y-connected bldc motor [14]. figure 4. calculated (a) resistance and (b) inductance of stator windings per phase for common mode and differential mode [9]. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 153 |𝐸𝜃̅̅ ̅(𝑟, 𝜃, 𝜗)| 2 = 𝜂 4 π 𝑟2 | ∑ 𝑄𝑠𝑚𝑛 (3) 𝐾𝑠𝑚𝑛 (𝜃, 𝜗) ∙ �̅�𝜃 𝑠𝑚𝑛 | 2 (7) |𝐸𝜗̅̅̅̅ (𝑟, 𝜃, 𝜗)| 2 = 𝜂 4 π 𝑟2 | ∑ 𝑄𝑠𝑚𝑛 (3) 𝐾𝑠𝑚𝑛 (𝜃, 𝜗) ∙ �̅�𝜗 𝑠𝑚𝑛 | 2 (8) as mentioned earlier, the real and imaginary parts of 𝑄𝑠𝑚𝑛 (3) are independent and gaussian distributed with zero mean. the directivities dco and dcross is similarly distributed. the expected value for the maximum over ns samples of a χ 2 with two degrees of freedom distribution is: 𝐷co,max̅̅ ̅̅ ̅̅ ̅̅ ̅ = 𝐷co̅̅ ̅̅ ̅ ∑ 1 𝑛 𝑁𝑠 𝑛=1 . (9) this summation can be approximated and substituting the expected value of dco from (6) yields expression (10). 𝐷co,max̅̅ ̅̅ ̅̅ ̅̅ ̅ ≈ 1 2 [0.577 + ln(𝑁𝑠 ) + 1 2 𝑁𝑠 ] (10) if 𝑘 𝑎 > 1, ns = 2 nm and n = k a expression (10) results in (4). for 𝑘 𝑎 = 1 (ns = 12), (10) yields 𝐷co,max̅̅ ̅̅ ̅̅ ̅̅ ̅ ≈ 1.55. a physical interpretation of ns = 12 for 𝑘 𝑎 ≤ 1 is that the real and imaginary parts of six dipole moments (three electric and three magnetic) yield 12 independent source contributions. the directivity estimate is also very near the directivity of a single short dipole (d = 1.5). thus, using 1.55 for electrically small emitters in (10) results in a continuous function and should give good estimates for directivity: 𝐷max̅̅ ̅̅ ̅̅ ≈ { 1.55, 𝑘 𝑎 ≤ 1 1 2 [0.577 + ln(4(𝑘 𝑎)2 + 8 𝑘 𝑎) + 1 8(𝑘 𝑎)2 + 16 𝑘 𝑎 ] , 𝑘 𝑎 > 1 . (11) the main difference between the intentional emitter upper bound given by (5) and the unintentional emitter expected value given by (11) is that the upper bound increases rapidly as the square of the electrical size k a, while the expected value increases only as the natural logarithm (ln) of electrical size k a. the estimated, measured and simulated total radiated power ratio for an example eut are compared in [3]. figure 5 shows that the measurement result and simulation result are in optimal agreement; also, the theoretical estimation is adequately good. this means that estimation can be used instead of measurement and simulation, but if higher precision is needed, they cannot be ignored. the theory also shows that, in earlier stages, only estimation can be enough to determine the overall emc performance. this can further reduce costs, because we do not need actual measurements or time-consuming simulation processes to get antenna like properties. on the other hand, a large and complex device containing many different parts is usually made of such individual components that their emc properties are measured by the manufacturer (for example, in automotive industry, every electronic component should pass emc restrictions before building into the actual vehicle), so this type of estimation or simulation process can be skipped [18]. 6. antenna arrays at his point, we should have all the parameters to handle our individual devices as antennas. measurements, simulations, and estimations must give enough information about the product. when assembling the final, complex device made of the individual devices we can treat it like an antenna array. a classical antenna array (often called a 'phased array') is a set of two or more antennas. the signals from the antennas are combined or processed to achieve improved performance over that of a single antenna. these types of antennas can be used to increase overall gain, maximize the signal to interference plus noise ratio, direct the array so that it is most sensitive in a direction, cancel out interference and so on [19]. these applications (some of them with reciprocity) are also the basics of decreasing overall emc performance of a complex system. in a phased array it is not so hard to add together the different antennas, because they are usually consisting of similar ones and made symmetrically [4], [20]. classical antenna arrays are made for specific reasons, but in our case, the different devices according to their operation, orientation, and distance from each other act like transmitters, but also receivers [21]. if we choose one operation where we handle all of them as receivers, we can calculate the emi properties. next to the weighting, we need to include distance and directivity factors to get accurate results. we can calculate the overall power of radiated electromagnetic waves if we handle all of them like transmitters. and finally, if we want a mixed operation calculation, because we know that some of the individual devices work more like transmitters (emits more em waves than they are immune to) and some of them work more like receivers (they are sensitive to em waves but does not radiate so much) we need calculate them one by one. this method is like superposition in electrotechnics but with antennas [18]. weighting, antenna factors, distances, directivity properties must stay, but they can be “switched off” one-by-one and in a round calculate the effect on all the others, then finally sum-up the results of each round. in each iteration, it is possible to inspect the effect of only one device on the others or choosing several iterations to select a group of devices. it yields a similar result if we take the complex system; put it in an emc chamber and measuring. after one complete measurement, we switch off some devices, or just leave a little group of devices and measuring figure 5. the estimated (theory), measured, and simulated ratio for an example eut [3]. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 154 again the emc. the cost of only calculations and measurements cannot be compared [22]. 7. possible errors this general theory is based on devices as antennas so one thing must be mentioned is conducted emc [23]. it is acceptable to measure the connecting wires, but real-life application may use cables different in length, diameter, shielding properties, etc. although we know the radiation pattern and radiated immunity of the product, but if we want proper results of the simulation, we must inspect the connecting wires also. it is not the most important in all applications but for example in automotive industry where lots of wires inside a metal body crowded with electronics, the problem should be solved [24], [25]. another possible factor is the passive, shielding like parts in the complex system. mountings, covers, design elements made from metal can easily distort the results. if we take them in count like the “active” devices, the risk of error decreases, but this procedure highly enlarges the number of devices. the less precise properties we use, the less precise results we get. if emc measurements or proper simulations are not available, we can estimate the performance of the device but in the more cases we calculate with estimations, the results may not meet our expectations in precision. 8. conclusions and outlook this paper provided a theory of handling devices as antennas to easily calculate emc properties if more devices are used in a closer area. the individual steps are used commonly, but until this point, we have not found this combined procedure. the scenario appears easily in electrical boxes, vehicles or in our house because we use more and more electronic devices. to achieve results, we need antenna like properties of the device, like overall gain, frequency chart, directivity, and other factors. these can be gained from measurements in proper emc chamber, 3d simulations and calculations or estimations. with these we must calculate the final performance of this antenna array with different methods depend on what performance parameter we need. the whole concept is illustrated in figure 6. further research on this theory must be done because the whole process (in ideal circumstances) can easily cut-off costs in designing and emc testing. also, to prove the concept example systems should be build up to try the different approaches. references [1] r. g: jobava, a. l. gheonjian, j. hippeli, g. chiqovani, d. d. karkashadze, f. g. bogdanov, b. khvitia, a. g. bzhalava, simulation of low-frequency magnetic fields in automotive emc problems, ieee transactions on electromagnetic compatibility 56(6) (2014) pp. 1420-1430. doi: 10.1109/temc.2014.2325134 [2] a. barchanski, emc simulation of consumer electronic devices, high frequency electronics, july 2013. 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https://doi.org/org/10.1109/emceco.2005.1513077 https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=5306818 https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=5306818 characterisation of glue behaviour under thermal and mechanical stress conditions acta imeko issn: 2221-870x december 2021, volume 10, number 4, 162 168 acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 162 characterisation of glue behaviour under thermal and mechanical stress conditions gianluca caposciutti1, bernardo tellini1, alfredo cigada2, stefano manzoni2 1 dip. di ingegneria dell’energia, dei sistemi, del territorio e delle costruzioni, università di pisa, largo lucio lazzarino, 56125 pisa, italy 2 dip. di meccanica., politecnico di milano, via la masa 1, 20156 milano, italy section: research paper keywords: temperature cycles; glue bonding; mechanical stress; thermal stabilization citation: gianluca caposciutti, bernardo tellini, alfredo cigada, stefano manzoni, characterization of glue behaviour under thermal and mechanical stress conditions, acta imeko, vol. 10, no. 4, article 23, december 2021, identifier: imeko-acta-10 (2021)-04-23 section editor: roberto montanini, università di messina and alfredo cigada, politecnico di milano, italy received july 29, 2021; in final form november 30, 2021; published december 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: gianluca caposciutti, e-mail: gianluca.caposciutti@ing.unipi.it 1. introduction attention about structural health monitoring (shm) systems, to monitor any possible infrastructure damage, is growing a lot these days [1]: unfortunately, the same attention is not being gained by metrology issues, dealing with the quality of measurements and their reliability. new low-cost measurement nodes, monitoring the structure motion, also require that the box housing the electronics and the elements fixing the boxes to the structure have costs aligned with those sensing nodes. the most common solution today is the use of plastic boxes, granting the required ip protection grade; these are fixed to the concrete structure by means of dowels and/or glue. also, the boards hosting sensors, microcontrollers and the needed electronics, today designed with a flat bottom to grant the same motion of the box and the sensor, preventing from the dynamic behaviour of the board to affect measurements, are connected to the enclosing box by means of metallic clips or glue. thus, there is a chain between the structure surface and the sensing element with many interfaces, influencing the metrological performances of the whole system. the presence of these interfaces can generate significant consequences on the measurements, especially when mems clinometers and accelerometers are taken into consideration. indeed, their working principle is based on the estimation of the angle between the sensing axis and the gravity vector. being the sensor glued to the enclosing box, when temperature changes (e.g., due to usual environmental thermal shifts), the glue can change its behaviour and geometrical layout. regarding the latter aspect, this implies that a significant systematic error can affect the readout e.g., by introducing a temperature dependant offset, while this effect must be avoided at best to preserve the quality of the measurement. such a goal can be achieved by properly choosing the type of glue through a study of its thermomechanical behaviour. in this paper, this is accomplished by capacitive measurements on tailored set-ups, as outlined below. sensors fixed to a structure for monitoring purposes, for instance on a bridge, can undergo important temperature changes, very high during summer and very low during winter. in order to better define the glue behaviour, a reference to capacitive displacement sensors has been adopted for a thorough understanding of the temperature related phenomena. capacitive abstract new low-cost measuring devices require that the box housing and electronics have the cost aligned with the sensing system. nowadays, metallic clips and/or glue are commonly used to fix the electronics to the box, thus providing the same motion of the structure to the sensing element. however, these systems may undergo daily or seasonal thermal cycles, and the combined effect of thermal and mechanical stress can determine significant uncertainties in the measurand evaluation. to study these effects, we prepared some parallel plates capacitors by using glue as a dielectric material. we used different types of fixing and sample assembly to separate the effects of glue softening on the capacitor active area and plates distance. therefore, we assessed the sample modification by measuring the capacitance variation during controlled temperature cycles. we explored possible non-linear behaviour of the capacitance vs. temperature, and possible effects of thermal cycles on the glue geometry. further work is still needed to properly assess the nature of this phenomenon and to study the effect of mechanical stress on the sample’s capacitance. mailto:gianluca.caposciutti@ing.unipi.it acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 163 displacement sensors are those exhibiting the highest sensitivity (up to hundreds of volt per millimetre), so it has been decided to use the glue as a dielectric interposed between two metallic plates: even the smallest change in the plate distance due to temperature can therefore be easily detected. capacitance measurements have therefore been carried out during varying temperature cycles, trying to split any change in the glue/dielectric features out of a real change of the plate distance: the latter is the main concern for shm issues. according to the theory, the capacitance between two conductors is defined as the ratio between the charge on one conductor and the potential difference between them. such a parameter depends on the geometry of the conductors, on their relative distances, and on the characteristic of the interposed dielectric medium. apart from simple geometries and ideal dielectric medium, an accurate calculation and measurement of the capacitance is a difficult task. for slowly varying fields, the electric permittivity ε is a macroscopic constitutive parameter which relates the macroscopic fields electric flux density �⃗⃗� and electric field �⃗� [2], [3]. a comprehensive discussion of the physics of frequency dispersion and of the effective time constants in dielectric media remains a complex issue, as well as a clear operative meaning of the measurement of ε in static conditions [4]. more in general, the properties of the dielectric material can vary as a function of several other parameters. for instance, temperature, and mechanical stress generally play an important role and should be properly taken into account for a valid description [5], [6]. regarding the temperature, the number of polarizable ions per unit of volume is a direct consequence of volume expansion, the ions polarizability depends itself on their thermal energy, which also influences the number of defects and disorder in the material, and the effective relaxation time. the total stress on dielectric material also affects its physical properties [2], [7], for which three different contributions are identified: mechanical stress, maxwell stress, and electrostriction component. in more detail, in absence of electric field, the material (assumed in an elastic regime) finally obeys to hooke’s law [8]. moreover, it is shown that the maxwell stress in solid dielectrics, such as many polymers, directly affects the material stress status, and causes molecular deformation, thus modifying the dielectric and electrical properties of the material [9]. the main purpose of the current work is to characterize the effect of the glue bonding between a sensor or its housing, and the target surface. particular attention is given to the glue thermal behaviour, which may significantly affect the sensor readout due to bonding deformation, e.g., in the case of a tilt or strainsensitive element. indeed, the temperature variation can lead to the deformation of the glue, possibly altering the relative distance between the plates and affecting the reference position of a sensor with respect to the base surface. to this aim, the mechanical and thermal variation of the glue is studied by using an electrical model of the system. in a first approximation, the bonding between the sensor or its housing and the target surface can be modelled as a flat plates capacitor, where the glue plays the role of the dielectric medium. the paper is structured as follows. section 2 shows the model used in this study, and section 3 reports the experimental analysis performed. moreover, section 4 shows the results obtained with the devices described in section 3, and finally, some conclusions are drawn in section 5. 2. modelling let us assume a capacitor having parallel plane electrodes with surface s, separation distance d, and dielectric medium with constant permittivity ε. according to the theory, for slowlyvarying and uniform electric field distribution between the two electrodes, the capacitance c between them reads as: 𝐶 = 𝜀 𝑆 𝑑 (1) it is worth to point out that for finite electrodes, the surface charge distribution is indeed not uniform [10], and an accurate estimation of the field distribution is not straightforward due to the fringing field effect and possible divergent field values at the electrode edges. metrological institutes commonly adopt guard rings as a method for the mitigation of such an effect [11], [12]. an accurate characterization of the dielectric material bases on a high control of the electric field distribution, and, in analogy with the characterization of magnetic properties of a material, we can more properly refer to characterizing the capacitance of the sample [13]. as a consequence, we assume for our analysis the following capacitance model: 𝐶(𝜀, 𝑆, 𝑑, 𝑓, 𝑇) = 𝜀(𝑓, 𝑇) 𝑆(𝑇) 𝑑(𝑇) , (2) for which we refer to capacitors made of two parallel metal disks, filled by a glue substance as interposed dielectric material and capacitance c. in equation (2), we made explicit for the capacitance parameters the dependence on the frequency f and the temperature t. thus, as mentioned in the introduction, ε generally presents frequency dispersion and depends on the temperature. the frequency dispersion represents a variation of ε vs. frequency and, following a classical approach, it is described by spring-mass models. for a review on such a topic, clausius-mossotti’s, debye’s expressions, and their novel modified versions are recognized models of such a phenomenon. the temperature dependence of the dielectric properties finds a general description for relatively rarefied media, such as the description of the orientation polarization based on maxwell-boltzmann statistics. on the other hand, such a relationship can significantly vary depending on the temperature range, structure, and physical status of the material. a conceptual scheme of the modelled configuration is shown in figure 1. the values of d and s are constant in the ideal configuration. however, these parameters vary with possible material contraction and dilatation because of the temperature variation. furthermore, glue softening can modify the material structure, influencing its dielectric and mechanical properties (e.g., viscosity reduction when temperature increase). thus, as made explicit in equation (2), the geometry of the system and the capacitance c are expected to vary with the temperature. as a consequence of such behaviours, the capacitor plates can get closer one each other under the effect of their weight and the electrostatic forces, thus reducing their relative distance. moreover, a viscosity reduction of the glue can let it slide over the plates, thus varying the geometry of the sample. these two main phenomena, together with the proper dielectric material temperature characteristic, generate a complex behaviour of the sample capacitance as a function of the temperature. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 164 as an example for raising the temperature, the glue softening might reduce d, determining an increase of c according to equation (2). moreover, the possibility of glue sliding also contributes to the variation of the c at high temperatures. after the heating process, the values of d and s can be eventually stabilized and, in a cooling stage, the behaviour of c(t) might follow a different curve, thus showing a hysteresis behaviour with the temperature. this complex phenomenon is expected to be more evident at high temperatures and it may exhibit some relaxation toward a stable condition after a few thermal cycles. to carry out a consistent characterization of c, we built several capacitor samples with specific features to distinguish the above-mentioned effects. firstly, the overall characteristic c(t) is evaluated by using a parallel face capacitor with circular electrodes. the geometry of the device is left free to evolve as a function of the temperature. the dielectric glue is positioned in the centre of the plates, far from the edges of electrodes, thus reducing the fringing field effect at the electrode’s edge. a second sample is built as the first one, but keeping fixed d to a minimum value by using ad-hoc glass spacers. the use of glass spacers with a low thermal dilatation coefficient allows for neglecting the electrode distance variation as a function of the temperature. however, for such a configuration, the glue can still possibly slide through the inter-electrode region, and finally affect the geometry and the active volume of the dielectric medium of the capacitor. a third sample is made with glass spacers, and by covering the border of the electrodes with dielectric glue. in this last case, the glue geometrical variation is expected not to significantly affect the material within the plates, thus further reducing the possible effective variation of both s(t) and d(t). therefore, temperature cycles are applied to the different samples and the behaviour of their capacitance is assessed at different operating frequencies. 3. experimental setup different capacitor samples are built with plain faces configuration, by adopting circular plates with 60 mm diameter and 3 mm thickness made by aluminium. a layer of glue is placed at the centre of the capacitor as the dielectric. particularly, the glue is placed in a 40 mm internal disk using proper pla spacers, which also kept the plates at the desired distance d of 1 mm, 1.4 mm, and 2.8 mm, respectively. the central glue positioning allows reduction as much as possible of the edge effect due to the electrode's borders. the distance d of the obtained samples can be further fixed to a minimum extent utilizing some 0.2 mm thickness glass spacers, placed at about 120° angular distance around the capacitor. the spacer's material is chosen to present a negligible thermal dilatation coefficient value with respect to the aluminium of the electrodes. the capacitor forming process and a final sample are shown in figure 2. the capacitors made as described, with d = 1 mm, d = 1.4 mm, and d = 2.8 mm, are namely c1, c2, and c3, respectively. the second type of capacitor, namely c4, is built with a distance d of 1.4 mm, kept by a 7 glass spacer with 0.2 mm thickness each, placed as the previous device. however, in this case, the glue is placed to fill all the plates and their borders. moreover, the upper circular plate has 60 mm diameter and figure 1. scheme of the modelled configuration. a) b) c) d) e) figure 2. spacers used for glue deposition (a) and final spacers for glue hardening (b). internal glue disk area (c), final capacitor assembly (d) and its main scheme (e). a) b) figure 3. capacitor sample c4 with glue covering the plate border (a) and its scheme (b). acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 165 3 mm thickness, while the bottom plate is 180 mm diameter and 1 mm thickness. the sample c4 is shown in figure 3. to evaluate the sample's capacitance variation as a function of the temperature, the devices undergo to thermal cycling between -70 °c to +70 °c. the samples are placed in a genviro 060lc climate chamber and cycled with a maximum heating rate of 7 °c/h. such a low value is necessary to ensure thermal homogeneity of the sample, and to provide symmetric cycles during heating and cooling, thus limiting the influence of the thermal dynamic on the capacitance of the sample. the temperature is monitored on the top plate and on the climate chamber environment by using 2 rtd 1/10 din pt100 sensors, which signals are acquired through a ni9219 board mounted on a ni9178 chassis. the capacitance c and the conductance g of the samples are measured using an lrc meter e4980a in the 20 hz – 2 mhz range. proper compensation of the connecting wire is made before each test. the model used for calculating c and g is a parallel between the capacitance c and a resistance, which value is equal to the reciprocal of g. instruments management and data acquisition are performed through a labview software made on purpose. the scheme of the experimental setup is shown in figure 4. 4. results and discussion the frequency response of the sample is reported at the temperature of (24.0 ± 0.9) °c in figure 5 for the sample c1. figure 5 shows that the capacitance c decreases with the frequency, while g increases with it. c and g are in the order of 100 pf and 100 to 300 ns, respectively. data of g above 20 khz are not reported due to high uncertainty in their value. in particular, a significant reduction of the capacitance occurs at around 20 hz. in liquids, low excitation frequencies, typically in the order of some tenth of hertz, can cause ionic transport and layering phenomena, thus producing a double layer capacitance which largely increases the value of c. besides, the c and g isothermal curves are consistent with the recent literature regarding polymers and amorphous materials for the studied frequency range [14]. the capacitance values obtained by c1, c2 and c3 at (27.2 ± 0.6) °c are reported with respect to the value of c3 in figure 6. therefore, figure 6 shows that the capacitance values vary as a function of 1/d. indeed, c1/c3 and c2/c3 agree with d3/d1 and d3/d2 according to the model shown in equations (1) and (2). however, for frequencies in the order of a tenth of hertz, the capacitance results in a different behaviour as discussed in relation to figure 5a. in figure 7, we show c1 vs. t at 10 khz frequency, with no glass spacers used. the c1 device (d = 1 mm) undergoes a quick cooling to 70 °c, where for 10 hours a steady-state temperature is maintained. the initial thermal settlement is used to make uniform the temperature between the environment and the sample, and to reduce thermal gradients within the dielectric material. the first cooling branch in figure 7b shows that, when the temperature is figure 4. scheme of the experimental setup. a) b) figure 5. capacitance c (a) and conductance g (b) as a function of the analysis frequency for c1. uncertainties are given with 95% confidence interval. figure 6. capacity ratio at around 30 °c for c1, c2 and c3 capacitors as a function of the frequency. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 166 constant, the capacitance slightly varies by increasing its value by around 3 % (i.e., 66 pf to 68 pf). this variation can be attributed to some stabilization phenomena. the next cycles clearly show a hysteresis on the c-t plot on the high-temperature side. in the present case, the hysteresis could be generated by the effect of the mechanical instability of the sample, as the combination of the effect of the glue softening, collapsing, and sliding, together with the characteristic permittivity evolution as a function of the temperature, proper of the dielectric material. however, at this study stage, hysteresis can also derive from more complex phenomena involving the physical characteristics of the dielectric material and depending on the permittivity behaviour vs. t [15], [16]. figure 8 shows the capacitance behaviour of c1, c2 and c3 samples with fixed minimum distance d under the thermal cycles shown by figure 7a. results are provided for 10 khz frequency value. figure 8 shows that hysteretic behaviour occurs in all the different samples with no significant difference concerning the case without spacers, shown in figure 7. furthermore, the samples c2 and c3 present a significant instability in the measure of c above 40 °c. this effect can be attributed to a major impact of the glue softening on the capacitor effective geometry. moreover, in the cases shown by figures 8b and 8c, second cycles (i.e. the second cooling and heating branches) present a shifted and lower value of c, indicating possible glue settlements. according to these observations, the test is repeated on the case c3 by stabilizing the sample at + 70 °c for 20 hours. the temperature profile over the time and the results in terms of capacitance at 10 khz vs. temperature are shown in figure 9. in figure 9, the hysteretic behaviour is significantly reduced and barely observable, while the measurement instability disappeared. this evidence highlights that the observed phenomena are determined by thermo-mechanical effects on the sample geometry. moreover, the high-temperature treatment a) temperature profile b) capacitance behaviour figure 7. measured temperature profile (a) and behaviour of c1 sample (d = 1 mm) (b) with no fixing to the geometric sample parameters. uncertainties are given with 95% confidence interval through the coloured strips. a) c1 (d = 1 mm) b) c2 (d = 1.4 mm) c) c3 (d = 2.8 mm) figure 8. behaviour of c1 (a), c2 (b) and c3 (c) capacity as a function of the temperature during thermal cycling with fixed minimum plate distance and stabilization at -70 °c. uncertainties are given with 95% confidence interval through the coloured strips. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 167 process (red path in figure 9) contributes to significantly mitigate these effects. the last test is carried out on sample c4, where the dielectric glue is placed over the electrodes and their borders, thus reducing the glue softening on the active region in between the electrodes. thus, a 20 hours stabilization is performed at +70 °c and several cycles are provided between + 70 °c and -70 °c to further observe the capacitance behaviour. the results are shown in figure 10, together with the performed temperature cycles. moreover, figure 11 shows the capacitance behaviour at 10 khz as a function of the temperature for the 7th cycle (i.e. the last performed cycle) and different operating frequencies. figure 10 shows that the first 3 cycles exhibit an accommodation behaviour. however, the capacitancetemperature curve does not present any significant difference among the cycles after the 4th thermal cycle. moreover, the capacitance hysteresis practically disappeared at this level of approximation, as also supported by figure 11, for all the frequencies studied, and the c-t curve appears mainly monotone in the studied temperature range. the growth of c with the temperature is consistent with the recent literature regarding polymers and amorphous materials [14]. according to the presented results, the hysteresis shown in figure 7, figure 8, and figure 9 is significantly reduced adopting thermal treatment and the c4 configuration. possible thermaldynamic and thermo-mechanic phenomena still could affect the hysteretic behaviour of the capacitance. further experiments are in progress aimed at investigating such behaviour. as discussed in the previous section, it is worth highlighting that figure 11 reports the behaviour of c(t) of the c4 sample for which the geometry variation can be practically neglected as a function of the temperature. therefore, the results in figure 11 can represent the behaviour of the permittivity ε(t) according to equation (2). 5. conclusions the accurate positioning and referencing of sensors, such as strain or tilt sensors, is crucial for the proper measurand evaluation. in many cases, the sensor and the measuring body are glue bonded together, ensuring a solid and cheap fixing. on the other hand, the time stability of the bonding may be an issue especially when temperature cycles and the forces into play can affect the glue's physical and geometrical features. therefore, thermal and mechanical analysis of a glue bonding is made via electrical measurement technique, by studying the behaviour of the capacitance in a device using glue as the dielectric material. several types of fixing and pre-processing are used to separate the effects of the temperature cycles over the glue geometry variation. to assess such a behaviour, sample capacitance was analysed as a function of the electric field frequency and the environment temperature. it turned out that when no geometrical fixing is used, temperature cycles cause hysteretic a) temperature profile b) capacitance behaviour figure 9. measured temperature profile (a) and behaviour of c3 sample (d = 2.8 mm) (b) with fixed minimum plate distance and stabilization at +70 °c. uncertainties are given with 95 % confidence interval through the coloured strips. a) temperature profile b) capacitance behaviour figure 10. measured temperature profile (a) and behaviour of c4 sample (d = 1.4 mm) (b) with fixed minimum plate distance and stabilization at +70 °c. uncertainties are given with 95 % confidence interval through the coloured strips. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 168 behaviour of measured capacitance. when the capacitor geometry is fixed independently of the temperature, and after thermal stabilization of the dielectric, the hysteresis disappeared, and only a monotone behaviour c-t remains for all the tested frequencies. therefore, we shown that the glue deformation is possibly responsible for the capacitance hysteretic behaviour. despite this could be kept under control by e.g. proper thermal stabilization of the glue, an underestimation of this occurrence can lead to significant systematic errors in the evaluation of measurands through glue-fixed sensors. references [1] d. w. ha, h. s. park, s. w. choi, y. kim, a wireless memsbased inclinometer sensor node for structural health monitoring, sensors . 13 (2013). doi: 10.3390/s131216090 [2] l. d. landau, e. m. lifshitz, the classical theory of field, 1971. [3] j. d. jackson , classical electrodynamics, third edition. new york : wiley, 1999. online [accessed 15 december 2021] https://search.library.wisc.edu/catalog/999849741702121 [4] m. bologna, b. tellini, remarks on the measurement of static permittivity through a classical description, prog. electromagn. res. c. 33 (2012), pp. 95–108. [5] m. r. mahboob, z. h. zargar, t. islam, a sensitive and highly linear capacitive thin film sensor for trace moisture measurement in gases, sensors actuators b chem. 228 (2016), pp. 658–664. doi: 10.1016/j.snb.2016.01.088 [6] a. g. cockbain, p. j. harrop, the temperature coefficient of capacitance, j. phys. d. appl. phys. 1 (1968), pp. 1109–1115. doi: 10.1088/0022-3727/1/9/302 [7] h. y. lee, y. peng, y. m. shkel, strain-dielectric response of dielectrics as foundation for electrostriction stresses, j. appl. phys. 98 (2005) 74104. doi: 10.1063/1.2073977 [8] y. m. shkel, n.j. ferrier, electrostriction enhancement of solidstate capacitance sensing, ieee/asme trans. mechatronics. 8 (2003), pp. 318–325. doi: 10.1109/tmech.2003.816805 [9] j.-. crine, influence of electro-mechanical stress on electrical properties of dielectric polymers, ieee trans. dielectr. electr. insul. 12 (2005), pp. 791–800. doi: 10.1109/tdei.2005.1511104 [10] m. dhamodaran, r. dhanasekaran, s. ammal, evaluation of the capacitance and charge distribution of metallic objects by electrostatic analysis, in: journal of scientific & industrial research, vol. 75, 2016, pp. 552-556. [11] w. c. heerens, f.c. vermeulen, capacitance of kelvin guard‐ring capacitors with modified edge geometry, j. appl. phys. 46 (1975), pp. 2486–2490. doi: 10.1063/1.322234 [12] s. dado, capacitive sensors with pre-calculable capacitance, in: transactions on electrical engineering, vol. 2, 2013. [13] fausto fiorillo, characterization and measurement of magnetic materials, academic press, 2005. [14] j.d. menczel, r.b. prime, thermal analysis of polymers: fundamentals and applications, 2009. [15] a. bousseksou, g. molnár, p. demont, j. menegotto, observation of a thermal hysteresis loop in the dielectric constant of spin crossover complexes: towards molecular memory devices, j. mater. chem. 13 (2003), pp. 2069–2071. doi: 10.1039/b306638j [16] s. saadaoui, o. fathallah, h. maaref, fermi level pinning, capacitance hysteresis, tunnel effect, and deep level in algan/gan high-electron-mobility transistor, superlattices microstruct. 156 (2021), 106959. doi: 10.1016/j.spmi.2021.106959 figure 11. behaviour of c4 sample (d = 1.4 mm) at different temperature and excitation frequencies https://doi.org/10.3390/s131216090 https://search.library.wisc.edu/catalog/999849741702121 https://doi.org/10.1016/j.snb.2016.01.088 https://doi.org/10.1088/0022-3727/1/9/302 https://doi.org/10.1063/1.2073977 https://doi.org/10.1109/tmech.2003.816805 https://doi.org/10.1109/tdei.2005.1511104 https://doi.org/10.1063/1.322234 https://doi.org/10.1039/b306638j https://doi.org/10.1016/j.spmi.2021.106959 results of study of quantization and discretization error of digital tachometers with encoder acta imeko issn: 2221-870x june 2023, volume 12, number 2, 1 6 acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 1 results of study of quantization and discretization error of digital tachometers with encoder vasyl kukharchuk1, oleksandr vasilevskyi2, volodymyr holodiuk1 1 vinnytsia national technical university, 95 khmelnitsky shose str., 21021, vinnytsia, ukraine 2 university of texas at austin, austin, texas, usa section: research paper keywords: angular velocity; encoder; quantization; sampling; microprocessor tachometer; quantization and sampling error equation; "adjoining interval" citation: vasyl kukharchuk, oleksandr vasilevskyi, volodymyr holodiuk, results of study of quantization and discretization error of digital tachometers with encoder, acta imeko, vol. 12, no. 2, article 19, june 2023, identifier: imeko-acta-12 (2023)-02-19 section editor: francesco lamonaca, university of calabria, italy received january 20, 2023; in final form february 19, 2023; published june 2023 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: vasyl kukharchuk, e-mail: bkuch@ukr.net 1. introduction currently, to intensify the testing of electric machines, the vast majority of research is focused on the acceleration of tests carried out in the no-load experiment [1]-[4]. the main one here is the transient characteristic (variable angular velocity over time 𝑛(𝑡)), which is obtained in the dynamic mode of operation of the measuring object (electric machine) with practically zero moment of resistance on its shaft (мс ≅ 0). to ensure the maximum number of measured values of angular velocity 𝑛 during the transition process (3 ÷ 5) 𝜏, most researchers [5]-[8] focused their choice on digital measuring channels with encoders, the main components of which are the following: measurements object ob, coupling shaft mc, encoder e, shaper f, device for the period selection t, f0 frequency generator g, "and" logic gate for the tx period quantization, binary counter ct2, programmable pi interface (parallel or serial), mps microprocessor system. in the generalized structural diagram shown in figure 1, the hardware sequence of measurement data transformations is carried out in the vast majority [9]-[12]. 1. encoder e converts the non-electric value angular velocity n into a sequence of electrical signals, the frequency of which is determined as 𝑓𝑥 = 𝑛 𝑍 60 , (1) figure 1. generalized structural diagram of the angular velocity measuring channel. abstract the analysis of measuring channels of angular velocity with an encoder given by the authors made it possible for the first time to obtain an equation for estimating the quantization and sampling error for an exponential mathematical model describing the transient process of operation of electrical machines. the components of the mathematical model of this dynamic error are the sampling step and the derivative, which characterizes the rate of change of the measured value over time. it was found that the errors of quantization and sampling significantly depend on the value of the resolution z of the encoder. moreover, an increase in z leads to a decrease in the sampling error, but the relative quantization error increases. to reconcile these components of errors, the laws of change in the distinguishing ability z of the encoder are adaptive to the dynamic properties of the change in angular velocity over time. proved that to ensure the maximum speed of measuring the angular velocity during the transient process, it is advisable to implement the method of changing the distinguishing ability of the encoder on the internal timers of the microcontroller proposed adapt ive to its dynamic properties, and the quantization of informative periods proportional to the measured angular velocity should be carried out in "adjoining intervals". mailto:bkuch@ukr.net acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 2 where 𝑍 is the resolution of the encoder. 2. the shaper f from the output quasi-sinusoidal encoder output signals forms pulses of a rectangular shape, the logic levels of which correspond to the levels of ttl logic. 3. the counting trigger t from the output frequency signals 𝑓𝑥 of the encoder, selects informative periods 𝑇𝑥 , proportional to the angular velocity 𝑛. 4. in the logical "and" gate, quantization takes place [10] by comparing the measured physical value of the period 𝑇𝑥 and the sample period 𝑇0. as a result of a such comparison, conversion equations for the frequency measurement channel of the instantaneous values given by such a transformation function are obtained 𝑁 = 𝑇𝑥 𝑇0 = 𝑇𝑥 ∙ 𝑓0 = 𝑓0 𝑓𝑥 . (2) 5. binary counter ct2 carries out the procedure of counting the number of 𝑁 sample periods 𝑇0, which were quantized by periods 𝑇𝑥 . 6. the programmable interface provides the transfer of binary codes of the number of pulses 𝑁[00. . .15] from the parallel outputs of the binary counter ct2 to the accumulator of the mps microprocessor system, the main components of which are the mcu microcontroller, ram, and permanent rom memory. the exchange of measurement information between the programmable interface and the microprocessor system accumulator is implemented in program mode, interrupt mode or direct memory access. 7. an array of measurement information about the transient characteristic of the measurement object is accumulated and memorized in the operational ram of the mps in the form of binary codes n, proportional to the instantaneous values of the periods 𝑇𝑥 of the frequency 𝑓𝑥 from the output of the encoder e. 8. to present the measured information in angular velocity values, the conversion equation for this type of non-electric quantity measuring channel is obtained by substituting the 𝑓𝑥 value from equation (1) into (2), which unambiguously links the input angular velocity 𝑛𝑋 with the output value, the number of pulses n on digital outputs of binary counter ct2 𝑁 = 60 ⋅ 𝑓0 𝑛𝑥 𝑧 . (3) 9. from the conversion equation (3), the array of instantaneous angular velocity values is calculated by software 𝑛𝑥 = 60 ⋅ 𝑓0 𝑁 𝑧 . (4) the imperfection of the given approach is explained by the shortcomings [13]-[17] inherent in digital measuring devices, the circuitry of which is implemented according to a "hard" control scheme. 2. measuring channels with microprocessor control the duality of the hardware and software implementation of microprocessor devices (figure 2) potentially provides greater flexibility and adaptability to the dynamic properties of measurement objects [10], [16], [18], which significantly expands their functional capabilities and improves static and dynamic metrological characteristics. the disadvantage of this development direction of microprocessor tachometers is the quantization of only even 𝑇𝑥 or only odd periods. in the above "quasi-instantaneous" means, measurement information is obtained not in each period, but after a period, which does not allow for the transient characteristic 𝑛(𝑡) with the required accuracy: determine the numerical values of the amplitude and duration of synchronous dips in angular velocity; to differentiate the experimentally obtained numerical values of the periods to construct the functional dependence of the change in acceleration over time; to indirectly determine the value of the dynamic moment and dynamic mechanical characteristic the phase "portrait" of the object of measurement. the solution to the problems highlighted above is the implementation of quantization [10], [12], [19]-[21] of each even 𝑇𝑥 and each odd periods (figure 3), which are proportional to the instantaneous value of the angular velocity, which is implemented by the hardware and software of the microcontroller. it is advisable to implement the quantization method in "adjacent" intervals on two internal timers of the microcontroller, and the analysis of metrological characteristics will allow to determine the parameter, knowledge of which will ensure its adaptation to the dynamic properties of the measurement object. 3. main metrological characteristics for further research, as an initial mathematical model of the angular velocity of electric machines, we will use a trivial (figure 4) exponential model 𝑛(𝑡) =  ω ∙ (1 − e − 𝑡 𝜏), (5) where ω – synchronous speed of the electric motor rotation, 𝜏 – electromechanical constant. as a result of the interaction of the hardware and software of the microcontroller, the analogue value 𝑛 is replaced, which has figure 2. generalized structural diagram of the microprocessor tachometer. figure 3. to the issue of quantization in "adjacent" intervals. acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 3 an infinite number of values in the specified measurement range (from 𝑛min to 𝑛max), due to the limited number of its instantaneous values (because 𝑇д ≠ 0), a discretization error occurs [22] 𝛥д(𝑡) = 1 2 𝑇д ⋅ d𝑛 d𝑡 (6) thus, obtain a mathematical model for estimating the sampling error. the angular acceleration of the shaft movement of the object of measurement in the transient mode of its operation is determined as ε(𝑡) = d𝑛 d𝑡 = ω ⋅ e − 𝑡 𝜏 𝜏 (7) the sampling step for a digital tachometer with microprocessor control is determined as follows [10] 𝑇д = 𝑡adc + 𝑡fl + 𝑡dr . here, 𝑡adc is the duration of the analog-to-digital conversion, which is equal to the measured period 𝑡adc = 𝑇𝑋 , 𝑡fl is the time required to execute the flag subroutine waiting for the flag, 𝑡dr is the time to execute the driver software driver. considering the fact that during the measurement of the angular velocity in the "adjacent" intervals, the waiting subroutines for the flag and the driver software driver are executed after the completion of 𝑇𝑥 quantization, then 𝑇д = 𝑡adc = 𝑇𝑥 . (8) in this regard, the sampling frequency [23] of the angular velocity measuring channel is defined as 𝑓д = 1 𝑇д = 1 𝑇𝑋 . (9) now present the encoder conversion equation (1) in the following form: 𝑓д = ω ∙ 𝑧 60 (10) and the discretization step, respectively 𝑇д(𝑡) = 60 𝛺 ∙ 𝑧 = 60 𝛺 ∙ (1 − e − 𝑡 𝜏) ∙ 𝑧 . (11) substitute (7), (8), (11) into (6) and obtain a mathematical model for estimating (figure 5) the sampling error of the digital angular velocity measurement channel in "adjacent" intervals 𝛥д(𝑡) = 1 2 𝑇д ⋅ d𝜔 d𝑡 = 30 e − 𝑡 𝜏 (1 − e − 𝑡 𝜏) ∙ 𝑧 𝜏 (12) to analyse the relative error of quantization, we will first obtain the transfer function of the microprocessor tachometer 𝑁(𝑡) = 𝑓0 𝑓𝑥 = 60 ∙ 𝑓0 𝛺 ∙ (1 − e − 𝑡 𝜏) 𝑧 (13) considering (13), the mathematical model for estimating the quantization error in the transient mode of operation of the measurement object will have the form 𝛿к(𝑡) = 1 𝑁 ∙ 100% = 5 ∙ 𝛺 ∙ (1 − e − 𝑡 𝜏) ∙ 𝑧 3 ∙ 𝑓0 . (14) analysis of the quantization error equation (figure 6) shows that its reduction can be ensured by two methods: 1. reducing resolution z of the encoder; 2. increasing the quantization frequency 𝑓0 of the quartz resonator g. the second approach has the limitation 𝑓0 ≤ 𝑓𝑔𝑟 . the quantization frequency is limited by the limit frequency of the crystal resonator of the microcontroller mcu. in turn, decreasing in the resolution z of the encoder leads to an increasing in the sampling error (figure 6), which is not acceptable for dynamic measurements. the following conclusion can be drawn from the above graphic dependences of quantization and discretization errors: quantization and discretization errors significantly depend on the resolution value z of the encoder. moreover, increasing z leads to decreasing in the sampling error, but the relative quantization error increases. to reconcile these component errors, it is necessary to obtain the laws of change of the encoder’s resolution z, adapted to the law of angular velocity change in time. using (12), we obtain the law of change in the resolution of the encoder in the process of increasing the angular velocity from 0 to ω, compliance with which will ensure the value of the dynamic sampling error, which does not exceed the normalized value ∆д≤ ∆дн zд(t) = 30 e − 𝑡 𝜏 ∆дн 𝜏 ∙ (1 − e − 𝑡 𝜏) . (15) figure 4. dependence 𝑛 = (𝑡): 𝜏 = 0.5; 𝛺 = 1500 rpm. figure 5. laws of encoder resolution changing. acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 4 similarly, from (14) we will get the law of the encoder resolution changing, which implementation will ensure the normalization 𝛿к ≤ 𝛿кн of the quantization error 𝑍к(𝑡) = 3 ∙ 𝛿кн ∙ 𝑓0 5 ∙ 𝛺 (1 − e − 𝑡 𝜏) . (16) graphic dependences of the change in encoder resolution 𝑧 = 𝑓(𝑡) during dynamic measurements of angular velocity 𝑛 = 𝑓(𝑡) during the transient process of the measurement object are shown in figure 5. to present the quality of measurement results based on the concept of measurement uncertainty, which is currently recommended by international standards [24]-[28], the following method of calculating the standard uncertainty of type b, which arises due to the existence of the discredit error 𝑢bd(𝑡), is proposed in the assumption on the normal distribution law of the components of the digital angular velocity measurement channel 𝑢bd(𝑡) = 𝛥д(𝑡) 𝑘p = 1 2 𝑇д d𝜔 d𝑡 𝑘p −1, (17) where 𝑘p is the coverage coefficient, which for a normal distribution law is taken as equal to 1.96 at a confidence level of p=0.95 [29], [30]; 𝛥д(𝑡) is the discrediting error of the digital angular velocity measurement channel. substituting the maximum value of the sampling error ∆дн ≤ 0.2 rpm (figure 6, b) into equation (17), we obtain the standard sampling uncertainty of type b, which is equal to 𝑢bd(𝑡) = 0.1 rpm for a confidence level of 0.95. the relative standard uncertainty of discretization can be estimated by the expression [29], [31] 𝑢�̃� = 𝑢bd(𝑡) 𝑛(𝑡) 100 % = 0.1 1500 100 % ≈ 0.01 % . (18) as follows from the conducted studies of the quantization error (figure 6, a), its value does not exceed 0.5 % at an angular speed of 1500 rpm. the standard uncertainty of type b, due to the presence of the quantization error [32], assuming its uniform distribution law, is calculated using the expression 𝑢𝐵𝑘 (𝑡) = 𝛿𝑘 (𝑡) 100 % √12 𝑛max(𝑡) = 0.5 % 346 % 1500 rpm = 2.17 rpm . (19) for the given object of measurement (for example, a threephase asynchronous motor uad-34 with a nominal rotation speed ω = 1500 rpm and 𝜏 = 0.5, a coupling of the membrane type, an encoder lir-120a (z=65536) and a debugging a board based on a dual-core 32-bit microcontroller tms320f28379d, containing 1 mb of flash memory, 128 kb of ram and having a sample frequency 𝑓0=8 mhz formed from the clock frequency of a quartz resonator, a method of normalizing the error [22] of the 𝑍д(𝑡) discretization and a method of normalization is proposed [33], [34] quantization errors 𝑍k(𝑡) by determining the change in resolution 𝑧 of the encoder, which corresponds to the change in the time coordinate 𝑡 in the transient process of measurement object: 𝑍д(𝑡) = 300 e − 2 𝑡 𝜏 (1 − e − 2 𝑡 𝜏 ) (20) 𝑍к(𝑡) = 1600 (1 − e − 2 𝑡 𝜏 ) . (21) a microcontroller such as the tms320f28379d has twelve 32-bit general-purpose timers in its structure, which is quite enough to carry out this kind of dynamic measurements with the adaptation of the z resolution of the encoder to the dynamic properties of the measurement object: timer 𝑇0 is programmed to the mode of the real-time counter, which every 0.01 s generates a control signal at its output, according to which the value of the binary code of the coefficient k of its list is recorded in the counter of timer 𝑇1; every 0.01 s, the value of the list coefficient k is recorded in timer 𝑇1 to form at its output a frequency signal 𝑓𝑋 𝑘 ⁄ , proportional to the resolution 𝑍д(𝑡) or 𝑍к(𝑡) according to (20) or (21); in timer 𝑇2, even 𝑇𝑥 periods of the frequency signal 𝑓𝑋 𝑘 ⁄ from the direct output of timer 𝑇1 are quantized. in timer 𝑇3, odd 𝑇𝑥 periods of the frequency signal 𝑓𝑋 𝑘 ⁄ from the inverse output of timer 𝑇1 are quantized. therefore, the method of quantization in "adjacent" intervals takes place in two timers 𝑇2 and 𝑇3 of the microcontroller. quantization of 𝑇𝑥 and 𝑇𝑥 periods occurs in both timers. first, as a result of counting the periods 𝑇0 of the sample frequency of the quartz resonator 𝑓0 from the leading edge to the trailing edge of the even period 𝑇𝑥 in the second 𝑇2 timer, and then from the leading edge to the trailing edge of the odd period 𝑇𝑥 in the third 𝑇3 timer. thus, each of the programmable timers of the microcontroller works in two modes: quantization (from rising edge to falling edge); transfer and memorization of measurement information (from falling edge to rising edge); additional speed of the proposed method is provided by the transfer and recording of measured information to the ram in dma mode (direct access to memory) without the participation of the computing core, freeing its resources for other, priority tasks during the measurement process. this hardware and software implementation of quantization in " adjacent " intervals ensures maximum speed of measurement and guarantees sufficient time for transferring and memorizing a figure 6. to the issue of quantization and discretization error. acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 5 large volume of quantization results in the mps ram. and the implementation of the method of error normalization [22] of discretization 𝑍д(𝑡) or the quantization error [23], [34] 𝑍к(𝑡) by changing the resolution z of the encoder in the real-time measuring mode of operation allows to increase the accuracy of dynamic angular velocity measurements. 4. conclusions the analysis of well-known digital angular velocity measurement channels with an encoder made it possible to solve two extremely important tasks for metrology increasing the accuracy and speed of dynamic measurements of the angular velocity of the measurement object operating in real time. for the first time, an equation for estimating the quantization and discretization error was obtained for an exponential mathematical model that describes the transient process of the electric machines operation. the components of the mathematical model of these dynamic errors are the quantization and discretization steps and the derivative, which characterizes the rate of change of the measured quantity over time. it is proved that quantization and discretization errors significantly depend on the value of encoder resolution z. moreover, an increasing in z leads to a decreasing in the sampling error, but the relative quantization error increases. in order to reconcile these component errors, the laws of changing the resolution z of the encoder were obtained, which make it possible to adapt to the dynamic properties of the change in angular velocity over time for the selected object of measurement (for example, a threephase asynchronous machine uad-34 with a nominal speed of rotation ω = 1500 𝑟𝑝𝑚 and τ = 0.5), a coupling coupling of the membrane type, an encoder lir-120 (z=65536) and dualcore 32-bit microcontroller tms320f28379d, methods of normalizing the value of the sampling error ∆дн and the quantization error 𝛿кн are proposed. the normalization of these error components is carried out in the process of dynamic measurements of the angular velocity n during the transient process of the measurement object, changing the resolution z of the encoder, thus ensuring the fulfilment of the condition: ∆д≤ ∆дн or 𝛿к ≤ 𝛿кн it was established that in order to ensure the maximum speed of the angular velocity measuring during the transient process of the measurement object, it is advisable to implement the proposed adaptive to its dynamic properties method of changing the resolution of encoder on the internal timers of the microcontroller, and to carry out the quantization of informative periods proportional to the measured angular velocity in "adjacent intervals". references [1] m. zhao j. lin, health assessment of rotating machinery using a rotary encoder, ieee transactions on industrial electronics, vol. 65, no. 3, march 2018, pp. 2548-2556. doi: 10.1109/tie.2017.2739689 [2] h. li, g. cheng, a comparative study of speed estimation methods for motor rotor. in: y. jia, w. zhang, y. fu, z. yu, s. zheng (eds), proc. of the 17th chinese intelligent systems conference, fuzhou, china, 16-17 october 2021. lecture notes in electrical engineering, vol 803. springer, singapore. doi: 10.1007/978-981-16-6328-4_86 [3] v. v. kukharchuk, elements of the control theory of electric machines dynamic parameters: monograph, vinnytsia: universum-vinnytsia, 1998, 125 p. 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https://doi.org/10.35784/iapgos.2080 https://doi.org/10.21014/acta_imeko.v10i4.814 https://doi.org/10.21014/acta_imeko.v10i2.810 https://doi.org/10.1051/ijmqe/2017005 https://doi.org/10.17973/mmsj.2021_10_2021030 https://doi.org/10.1117/12.2501578 https://doi.org/10.1007/978-3-030-91327-4_41 https://doi.org/10.1117/12.2249195 https://doi.org/10.1051/ijmqe/2017019 the results of atmospheric parameters measurements in the millimeter wavelength range on the radio astronomy observatory “suffa plateau” acta imeko issn: 2221-870x june 2023, volume 12, number 2, 1 5 acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 1 the results of atmospheric parameters measurements in the millimeter wavelength range on the radio astronomy observatory “suffa plateau” dilshod raupov1,2, s. ilyasov1,2, g. i. shanin3 1 ulugh beg astronomical institute, uzbek academy of sciences, tashkent, uzbekistan 2 jizzah state pedagogical university, jizzah, usbekistan 3 radio observatory rt-70, uzbek academy of sciences, tashkent, uzbekistan section: research paper keywords: astroclimate; turbulence; transparency windows; atmospheric absorption; precipitated water; radio range; millimeter wave; neper citation: dilshod raupov, s. ilyasov, g. i. shanin, the results of atmospheric parameters measurements in the millimeter wavelength range on the radio astronomy observatory “suffa plateau”, acta imeko, vol. 12, no. 2, article 18, june 2023, identifier: imeko-acta-12 (2023)-02-18 section editor: francesco lamonaca, university of calabria, italy received december 12, 2022; in final form january 30, 2023; published june 2023 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: dilshod raupov, e-mail: dilshod@astrin.uz 1. introduction recently, thanks to the rapid development of microwave technology, the creation of adaptive large-aperture radio telescopes and the possibility of combining them into a system of ground-based and ground-space interferometers with very long bases, has gained considerable interest in fact, this represents a real possibility to fully realize the advantage of the millimetre range in solving fundamental problems of cosmology, as well as in solving a number of applied problems at a completely new level. at the same time, such a rapid technological breakthrough in the development of microwave technology inevitably required the solution of new problems in a detailed study of the parameters of the medium for the propagation of mm-wave radio waves. significant factors, in addition to insignificant attenuation in space plasma due to scattering and absorption, affecting the propagation of mm waves from the source of radiation to terrestrial observation are distortions introduced by the terrestrial atmosphere. in addition, the earth's atmosphere absorbs electromagnetic radiation of most wavelengths. however, there are frequency bands (radio windows) in which the atmosphere is substantially transparent. such a radio window in the mm region of the spectrum is a region from 1 to 15 mm, in which several absorption lines of oxygen and water vapor are located. a large number of papers and monographs have been devoted to theoretical studies of molecular absorption of mm waves. in these papers, it is emphasized that, in theoretical absorption calculations, special attention should be paid to the feasibility of the physical approximations used and to the adequacy of the description of the corresponding processes. from this point of view, interesting results were obtained in [1], where it was possible to achieve agreement between the theory of molecular absorption in water vapor in transparency windows and experimental data by taking into account the duration of molecular collisions in the framework of the memory function method. abstract the results of measurements of atmospheric absorption and the amount of precipitated water on the suffa plateau for the period from january, 2015 to november, 2020, are presented. the measurements of atmospheric parameters in the 2 and 3 mm range of the radio waves spectrum were carried out using the miap-2 radiometer. the results of more than six years of measurements have show that on the suffa plateau, atmospheric parameters in the above range remain fairly stable. the median value of the atmospheric absorption and the amount of precipitated water over the entire observation period were 0.14 and 0.12 nep and 5.91 and 9.83 mm, respectively, for the ranges of 2 and 3 mm. mailto:dilshod@astrin.uz acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 2 noteworthy are the calculations of absorption in water vapor and in molecular oxygen [2], [3] using natural (from a balloon) and laboratory measurements [4], taking into account the influence of the earth's magnetic field on the characteristics of the propagation of millimeter waves in the region λ = 5 mm. however, in theoretical calculations, there is still a problem of correct comparison of the theory of molecular absorption of millimeter waves with experimental and laboratory data, as well as with insufficiently studied statistical characteristics of fluctuations of atmospheric meteorological parameters. recently, interest has been growing in the construction of radio models of the atmosphere, with the help of which it would be possible to determine quickly (and with fair reliability) the characteristics of the propagation of millimeter waves along paths of various orientations and lengths. in the radio model constructed in [5], the input parameters are meteorological elements (pressure, temperature, humidity), the concentration of water particles suspended in the air, and the intensity of rain. the model makes it possible to calculate the influence of the attenuating and refractive characteristics of the medium on the propagation of radio waves in various atmospheric conditions. however, the authors note the need to refine and supplement the model and, first of all, the need for a much more thorough meteorological support. recently, scientists have paid much attention to the study of the influence of atmospheric parameters on the characteristics of microwave radio waves both in areas that are promising for the installation of large radiometric systems, and at operating complexes. one of such radiometric complexes being created on the plato de chajnantor in the central andes (chile) is alma (atacama large millimeter array) [6], [7]. another potential site for installing mm-submm telescopes is antarctica (south pole), where the dome-c antarctic observatory is being created on a plateau at the altitude of 2835 m [8]-[10]. detailed studies of transparency and stability of atmosphere in this range were also carried out at mauna kea (hawaiian islands, usa) [11], where submillimeter telescopes (in particular, the sma complex submillimeter array) are installed. radio transparency studies for mm-submm astronomy were also carried out at the indian astronomical observatory hanle in the himalayas in 2000-2003. [12], pampa la bola (8 km ne from chajnantor) [13], rio frio [14]. water in different phases and mixtures is a key element of the climate system. long-term chemical and isotopic changes of oceanic and atmospheric water mixtures must be monitored regularly and precisely [15]. the study of the atmospheric absorption at the radio astronomy observatory "suffa plateau" (uzbekistan) has been carried out from 2015 to 2020 in atmospheric transparency windows of 2 and 3 mm using the miap-2 measuring complex. a detailed description of the radiometer, its functional diagram, basic principles of measurements and calculations, estimates of permissible errors are given in [16]. 2. characteristics of the measuring complex the miap-2 measuring complex is a radiometric system that includes two radiometers for frequencies of 84-99 ghz (λav = 3 mm) and 132-148 ghz (λav = 2 mm), a turntable and a control, acquisition and processing system based on a personal computer. the receiver in the range (84-99) ghz is made according to the direct amplification scheme with detection at the fundamental frequency. the modulator at the input of the receiver is made on the basis of chains of series-parallel connected diodes with a schottky barrier (sbb) installed in the waveguide of the main section. the modulator control and synchronous data acquisition are carried out using a pc via the usb-4716 module. the modulation frequency is about 36 hz. the noise temperature of the receiver is 1300 k. the solid-state receiver of the radiometer in the range (132148) ghz is made according to the super heterodyne circuit and includes a local oscillator based on the gunn diode, a balanced mixer on the dbsh and an if in the range (4-8) ghz. the modulator is similar to the one described above, it is also controlled and data is collected via the usb-4716 module. the modulation frequency is also about 36 hz. the noise temperature of the receiver is 6300 k. the turntable consists of a support frame for mounting the radiometer module, a swivel mirror, and a mirror drive system based on a stepper motor. the weather protection housing is made of stainless steel and has a radio-transparent ptfe window. the limits of change of the angle of observation are 0º90º. the drive is controlled according to a given program via the usb-4716 module. the control, data collection and processing system ensure fully automatic operation of the complex in the mode of cyclic observations at a given time. both radiometers are equipped with lens antennas with a conical feed. the lenses are made of teflon and are limited on one side by a hyperbolic surface and flat on the other. each of the boundary surfaces has an enlightenment, which is made in the form of periodic circular concentric grooves ("corrugations"), providing a reflection coefficient in the entire operating range of each of the radiometers no more than 0.5 %. the irradiators are in the form of circular cones with a break. the antenna beam width at half power level in both bands is about 2.5º. atmospheric absorption measurements of radio waves in the millimetres range carried out by the method of vertical cuts were described in [17]. the method is based on measuring the intrinsic thermal radiation of the atmosphere, while comparing the brightness temperature increments of two parts of the atmosphere at different zenith angles relative to the known temperature of a certain reference region. in spite of all its advantages, this method is limited by the choice of one or another model of the structure of the atmosphere. in figure 1. location of the suffa plateau (λ = 65°26, ϕ = 39°37, h = 2500). acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 3 particular, it is assumed that the earth's atmosphere is isothermal in horizontal coordinates, which can lead to errors associated with drifting temperature in homogeneities of the surface layers of the atmosphere. the measurement principle is illustrated in figure 2. the left side shows a typical temperature distribution in the troposphere with respect to height (the so-called temperature profile). in the middle a typical distribution of humidity in height. these profiles are subject to significant seasonal and daily changes, in fact, from them it is possible to calculate the amount of deposited water and extrapolate it to absorption. on the right are the zenith angles (that is, the number of atmospheres) by which the radiometer approximates the brightness temperature profile. before starting, the device is set to a strictly horizontal position according to the hydraulic level. the measurement cycle begins with the rotation of the mirror along the photo sensor exactly in the "zenith" direction. next, the mirror is moved using a stepper motor (the “step” value is 0.72 degrees) to a certain angle, the brightness temperature of the “spot” of the sky in the direction of observation is measured, the mirror moves to the next angle, the next measurement is made, etc. the signal averaging time can be changed from 1 s (with a “calm” atmosphere) to tens of seconds. the signal through the amplitude-to-digital converter (adc) enters the computer. next, a system of two (measurement at two angles) and five (measurement at five angles) equations is solved. the calculated value of atmospheric absorption is displayed on the graph. the cycle lasts about a minute, then the mirror is brought to its original position, the calculated transparency value is entered into the database and marked on the graph, after which the device is ready for the next measurement cycle. the time interval between cycles also varies widely, depending on the task set by the observer. with the help of the control program of the complex, it is possible to select any observation angles in the range from 0° (zenith) to 90° (horizon). however, this choice should take into account both the sensitivity of the device (it is necessary that the "neighbouring" corners have a difference in brightness temperatures distinguishable by the device), and some features of its design. the point is that the error introduced into the measurements and associated with the finite width of the radiation pattern and a fairly wide bandwidth has a complex dependence on the observation angle. an appropriate selection of angles can minimize this error. when processing the results from two angles, these angles are 60.5° and 76.3° (corresponding to approximately two and four atmospheric thicknesses overcome by the signal). when processing five corners, the most rational choice is: 60.5°, 76.3°, 81.4°, 84.2°, 88.6°. the last corner is as close as possible to the horizon. close absorption values obtained by processing the results of measurements at 2 and 5 angles indicate both the normal operation of the device and the absence of interference in the form of clouds or ground objects (at the “lowest” angle) in the direction of observation, as well as “calm state of the atmosphere. after processing each cycle of measurements, we directly obtain the value of atmospheric absorption at the zenith, expressed in neper (1 nep = 8.686 db), i.e. the so-called optical thickness of the atmosphere. this value is subject to significant seasonal and daily changes, which is primarily due to changes in the total content of water vapor in the atmosphere. at the end of the observation session, we obtain a chronology of absorption changes in two bands during the observation time. any observation period can be chosen. the possibility of roundthe-clock monitoring of radio transparency is provided. the total absorption of radio waves of the specified range includes absorption by oxygen molecules, water vapor contained in the atmosphere, and absorption in clouds. in clear time, the third component naturally vanishes. oxygen absorption varies little for a particular observation site over time due to its relatively constant content in the atmosphere. it is almost constant and depends only on the height of the observation site. thus, the change in radio transparency is mainly due to a change in the amount of water vapor in the atmosphere, or, in other words, a change in the amount of precipitated water. 3. observational statistics the significant array of measurements of atmospheric absorption in the mm spectral range has been accumulated on the suffa plateau for the period from january 2015 to november 2020. as an example, the full time series of atmospheric absorption obtained on the suffa plateau for the above period are shown in the 2 mm radio wave band is shown in figure 3. some gaps of data in observations is due to technical reasons. as can be seen from the figure, the value of atmospheric absorption over the entire observation period remains stable. a similar trend is also observed in the values of atmospheric absorption and the amount of atmospheric deposited water in the 2 and 3 mm ranges. it should be noted that equipment malfunctions appeared figure 2. the principle of measurement by the method of vertical cuts. the left side shows a typical temperature distribution in the troposphere with respect to height (the so-called temperature profile). in the middle a typical distribution of humidity in height. these profiles are subject to significant seasonal and daily changes, in fact, from them it is possible to calculate the amount of precipitated water and extrapolate it to absorption. on the right are the zenith angles (that is, the number of atmospheres) by which the radiometer approximates the brightness temperature profile [16]. figure 3. the atmospheric absorption time series obtained from january 2015 to november 2020 on the suffa plateau during the entire period in the 2 mm range acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 4 in 2018, and the data obtained were very different from the previous values of atmospheric parameters. therefore, when calculating atmospheric parameters, the data of 2018 were discarded. the statistical distribution of atmospheric absorption and the amount of precipitated water is shown in figure 4. the median value of atmospheric absorption over the entire observation period was 0.13 and 0.11 nep for the 2 mm and 3 mm radio range, respectively. the median value of the atmospheric amount of precipitable water for the entire period of observations in the 2 mm radio band was 4.96 mm, and for the 3 mm band, 9.25 mm. the sesonal trends are shown in table 1, which reports the average monthly values of atmospheric absorption (abs) and the amount of precipitable water vapor (pwv) computed after merging different years. as can be seen from the data, in the 2015-2020 period, the average monthly values of atmospheric absorption and the amount of deposited water on the suffa plateau remain fairly stable. the main range of changes in atmospheric absorption and the amount of deposited water lies within 0.11-0.19 nep and 4.84-7.40 mm for the 2 mm range of the radio wave spectrum, and for the 3 mm range ‒ 0.110.14 nep and 8.81 -13.38 mm, respectively. for a visual representation of the statistics of atmospheric parameters and its trend of change, the seasons were chosen according to the principle of weather conditions: november, december, january and february belong to the winter season; the transitional season includes march, april, september and october, the summer season may, june, july and august. in the winter season, the average value of atmospheric absorption is less than 0.10 nep, in the transitional season it is less than 0.12 nep, and in the summer season it is more than 0.12 to 0.15 nep, for the 3 mm range. for the 2 mm range in the winter season, the average value of atmospheric absorption is less than 0.12 nep, in the transitional season it is less than 0.14 nep, and in the summer season it is more than 0.14 to 0.15 nep. the temporal dynamics of precipitated water in january and july are characteristic of extreme climate conditions. the average value of deposited water in january is about 4.84 mm for the 2 mm range, 10.0 mm for the 3 mm range, and in july 7.40 and 13.38 mm for the 2 and 3 mm ranges, respectively. the diurnal variations of the deposited water in the summer period are more significant than in winter. on some nights in december and january, the amount of precipitated water drops to a minimum of about 2 mm, in summer it rises to 12 mm. 4. conclusions based on this study, it can be concluded that over a six-year period of time, the atmospheric parameters on the suffa plateau remain fairly stable. the values of atmospheric absorption and deposited water presented here correspond to the values of the entire thickness of the atmosphere at the zenith. measurements of atmospheric parameters on the suffa plateau showed that the value of atmospheric absorption at the zenith at a wavelength of 3 mm, sometimes for several is was within 0.06–0.08 nep, and at a wavelength of 2 mm within 0.08-0.10 nep. at shorter time intervals, for several hours, the absorption of the 3 mm wave sometimes drops to 0.06–0.08 nep, and at a wavelength of 2 mm to 0.05–0.06 nep. such cases occur in the winter. the amount of precipitated water for several days is in the range of 1.62.0 mm. they can be reduced to parameters at any angle, as well as extrapolated to any height, taking into account the standard atmosphere model. references [1] yu. p. kalmykov, s. v. titov, application of the method of memory functions to calculate the rotational absorption spectrum of water vapour, radio engineering and electronics, 33(1) (1989), pp. 13-20. figure 4. statistical distribution of atmospheric absorption (on top) and amount of deposited water (below) on the suffa plateau, calculated for the radio wave bands of 2 and 3 mm, from january 2015 to november 2020. the y-axis of the histogram is on the right, the cumulative distribution is on the left. table 1. monthly average values of atmospheric absorption and integral amount of deposited water from january 2015 to december 2020. months absorption 3 mm absorption 2 mm pwv 3 mm pwv 2 mm january 0,11 0,11 8,81 4,84 february 0,12 0,14 10,44 5,05 march 0,12 0,15 11,42 5,89 april 0,13 0,16 12,22 6,22 may 0,14 0,18 12,82 7,21 june 0,14 0,19 13,26 7,27 july 0,14 0,17 13,38 7,40 august 0,14 0,16 13,25 7,28 september 0,14 0,14 12,57 7,09 october 0,13 0,14 12,05 6,62 november 0,12 0,12 11,11 5,56 december 0,12 0,11 9,25 4,69 mean 0,12 0,14 11,30 5,91 acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 5 [2] yu. p. kalmykov, s. v. titov, on the absorption spectrum of molecular oxygen in the o-hz frequency range, radio physics 32(8) (1989), pp. 933-944. [3] a. r. l. yasmin, theoretical modeling of microwave absorption by water vapour, appl. optics 29(13) (1990), pp. 1979-1983. doi: 10.1364/ao.29.001979 [4] a. a. vlasov, e. n. kadygrov, e. a. kuklin, laboratory measurements of the absorption of 5 mm radio waves in atmospheric oxygen, xvi all-union conference on radio wave propagation. abstracts of reports. kharkov, soviet union, khpi. 1990, part 2, p. 35. [5] a. a. vlasov, e. n. kadygrov, a. n. shaposhnikov, selection of the model for calculating the o2 absorption coefficient for determining the temperature profile of the atmosphere from satellite microwave measurements, earth exploration from space, 1 (1990), pp. 36-39. [6] s. j. e. radford, the atacama large millimeter array: observing the distant universe, in observing dark energy, asp conf. ser. 339, s. c. wolff, t. r. lauer (eds.), san francisco: asp, usa, 2005, isbn: 1-58381-206-7, p. 177. [7] s. j. e. radford, alma site and configurations, national radio science meeting, washington, d. c.,usa, national academy of sciences, 2002, p. 364. [8] p. g. calisse, m. c. b. ashley, m. g. burton, m. a. phillips, j. w. v. storey, s. j. e. radford, j. b. peterson, submillimeter site testing at dome c, antarctica in third international workshop on astrophysics at dome c, pasa, 21(3) (2004), pp. 256 – 263. doi: 10.1071/as03018 [9] p. g. calisse, m. c. b. ashley, m. g. burton, j. s. lawrence, t. travouillon, j. b. peterson, m. a. phillips, s. j. e. radford, j. w. v. storey, c. dome, antarctica: the best accessible submillimeter site on the planet? the dense interstellar medium in galaxies, springer proceedings in physics 91, ed. pfalzner, s., kramer, c., staubmeier, c., & heithausen, a., (berlin: springer, isbn: 3-540-21254-x), 2004, pp. 353-356. doi: 10.1007/978-3-642-18902-9_64 [10] p. g. calisse, m. c. b. ashley, m. g. burton, j. r. lawrence, m. a. phillips, j. w. v. storey, s. j. e radford, j. b. peterson, new submillimeter site testing results from dome c, antarctica, astronomy in antarctica, ed. burton, m., highlights of astronomy, 13 = astronomy in antarctica, 25th ga of the iau, ss 2, 18 july, 2003 in sydney, australia, meeting abstract, p. 33. [11] j. b. peterson, s. j. e. radford, p. a. r. ade, r. a. chamberlin, m. j. o'kelly, k. m. peterson, e. schartman, stability of the submillimeter brightness of the atmosphere above mauna kea, chajnantor and the south pole, pasp 115 (2003), art. 383. doi: 10.1086/368101 [12] p. g. ananthasubramanian, s. yamamoto, t. p. prabhu, d. angchuk, measurements of 220 ghz atmospheric transparency at iao, hanle, during 2000-2003, bull. astr. soc. india, 32(2) (2004), pp. 99-111. [13] s. matsushita, h. matsuo, j. r. pardo, s. j. e. radford, fts measurements of submillimeter-wave atmospheric opacity at pampa la bola ii, supra-terahertz windows and model fitting, publications of the astronomical society of japan 51 (1999), art. no. 603. doi: 10.1093/pasj/51.5.603 [14] m. a. holdaway, m. ishiguro, s. m. foster, r. kawabe, k. kohno, f. n. owen, s. j. e. radford, m. saito, comparison of rio frio and chajnantor site testing data, mma memo #152. [15] r. feistel, salinity and relative humidity: climatological relevance and metrological needs, acta imeko 4(4) (2015), pp. 57-61. doi: 10.21014/acta_imeko.v4i4.216 [16] g. m. bubnov, yu. n. artemenko, v. f. vdovin, d. b. danilevsky, i. i. zinchenko, v. i. nosov, p. l. nikiforov, g. i. shanin, d. a. raupov, the results of astroclimate observations in the short-wave length interval of the millimeter-wave range on the suffa plateau, radio physics and quantum electronics 59(8-9) 2017, pp. 763-771. doi: 10.1007/s11141-017-9745-7 [17] a. g. kislyakov, on the distribution of the absorption of radio waves in the atmosphere according to its own radio emission ii, radio engineering and electronics 13(7) (1968), pp. 1161-1168. https://doi.org/10.1364/ao.29.001979 https://ui.adsabs.harvard.edu/link_gateway/2004pasa...21..256c/doi:10.1071/as03018 http://www.springer.de/ https://doi.org/10.1007/978-3-642-18902-9_64 https://doi.org/10.1086/368101 http://adsabs.harvard.edu/cgi-bin/author_form?author=matsuo,+h&fullauthor=matsuo,%20hiroshi&charset=iso-8859-1&db_key=ast http://adsabs.harvard.edu/cgi-bin/author_form?author=radford,+s&fullauthor=radford,%20simon%20j.%20e.&charset=iso-8859-1&db_key=ast https://doi.org/10.1093/pasj/51.5.603 https://doi.org/10.21014/acta_imeko.v4i4.216 https://doi.org/10.1007/s11141-017-9745-7 measurements for non-intrusive load monitoring through machine learning approaches acta imeko issn: 2221-870x december 2021, volume 10, number 4, 90 96 acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 90 measurements for non-intrusive load monitoring through machine learning approaches giovanni bucci1, fabrizio ciancetta1, edoardo fiorucci1, simone mari1, andrea fioravanti1 1 university of l’aquila, piazzale e. pontieri 1, 67100 l’aquila, italy section: research paper keywords: non-intrusive load monitoring (nilm); energy management; deep learning (dl); sweep frequency response analysis (sfra) citation: giovanni bucci, fabrizio ciancetta, edoardo fiorucci, simone mari, andrea fioravanti, measurements for non-intrusive load monitoring through machine learning approaches, acta imeko, vol. 10, no. 4, article 16, december 2021, identifier: imeko-acta-10 (2021)-04-16 section editors: umberto cesaro and pasquale arpaia, university of naples federico ii, italy received october 12, 2020; in final form december 6, 2021; published december 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: simone mari, e-mail: simone.mari@graduate.univaq.it 1. introduction nowadays, economic development has led to a steady increase in the demand for electricity and related advanced services. companies are therefore moving towards the conversion of their traditional electrical systems into smart systems. one of the advantages that can be achieved is an efficient use of energy through the programming of loads and the awareness of consumption by users. to this end it is necessary to know the information on the status and consumption of the various loads powered by the system. this can be achieved through intrusive monitoring, i.e. by installing individual sensors for each load. but also through non-intrusive monitoring, i.e. measuring the total power absorbed by the system and deducing the contributions of the individual loads from this, through the use of specific algorithms. in this second case, an extremely simple and compact measurement system is obtained, at the expense of greater complexity from the point of view of processing [1]. these non-intrusive monitoring systems, however, have proven effective on a wide range of applications, which go beyond energy management alone [2]. non-intrusive load monitoring systems (nilms) are used successfully in many applications, including demand response programs, where consumers can generate profits based on their flexibility [3], [4]. other applications are those of anomaly detection, to detect malfunctions based on the profiles of power absorbed by individual loads [5] and of condition-based maintenance, which has allowed the creation of monitoring systems capable of helping operators in maintenance planning [6], [7]. finally, the ambient assisted living is also very important, where the nilm system monitors the switching on and off of household appliances to infer the position and activities of people, detecting the space-time context and, therefore, the activities of daily life. of the subject [8]-[10]. the first nilm system was proposed by g. hart in 1985 [11]. this algorithm was based on a detection of the edges in the aggregate power profile, followed by a clustering operation and subsequent matching based on the value of the absorbed power and on the on and off time. clearly this approach, while being abstract the topic of non-intrusive load monitoring (nilm) has seen a significant increase in research interest over the past decade, which has led to a significant increase in the performance of these systems. nowadays, nilm systems are used in numerous applications, in particular by energy companies that provide users with an advanced management service of different consumption. these systems are mainly based on artificial intelligence algorithms that allow the disaggregation of energy by processing the absorbed power signal over more or less long time intervals (generally from fractions of an hour up to 24 h). less attention was paid to the search for solutions that allow non-intrusive monitoring of the load in (almost) real time, that is, systems that make it possible to determine the variations in loads in extremely short times (seconds or fractions of a second). this paper proposes possible approaches for non-intrusive load monitoring systems operating in real time, analysing them from the point of view of measurement. the measurement and postprocessing techniques used are illustrated and the results discussed. in addition, the work discusses the use of the results obtained to train machine learning algorithms that allow you to convert the measurement results into useful information for the user. mailto:simone.mari@graduate.univaq.it acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 91 functional in certain situations, showed significant limitations as a multi-state appliance had to be managed as a set of distinct on / off appliances. conversely, continuously variable appliances and appliances with permanent consumption could not be detected correctly. it also included strong manual feature extraction requirements. subsequently other algorithms based on combinatorial optimization were proposed [12], whose main assumption is that each load can be in a given state (one of a reduced number k), in which each state is associated with a different energy consumption. the goal of the algorithm is to assign states to household appliances in such a way as to minimize the difference between the aggregate power reading and the sum of the energy consumption of the different loads. in the last decade, thanks to the increase in available computing powers, attention has shifted to artificial intelligence algorithms, such as hidden-state markov chains [13]-[15] and deep learning models [16]-[18]. in particular, the use of deep learning algorithms has overcome many of the limits that have characterized previous methods, thus allowing measurement systems to adapt to homes never analysed during the training phase. furthermore, in terms of accuracy, systems based on convolutional neural networks have overcome other state-ofthe-art methods, such as those based on factorial hidden markov models [19]. however, the state of the art of these systems is represented almost exclusively by monitoring systems that process signals over time intervals of the order of hours, consequently the resulting feedback will not be in real time. instead, it is often necessary to know in real time the status changes of the monitored loads. non-intrusive load monitoring systems are therefore required, capable of recognizing the different powered devices based on signal processing, during intervals of seconds or fractions of seconds. in this paper, two approaches for the recognition of electrical loads in real time will be presented. the first is a passive measurement system, based on the acquisition and processing of the current absorbed by the system. the second is an active measurement system, based on the measurement of the response to a variable frequency signal injected into the system. 2. nilm system based on passive measurements a first attempt was made by creating a recognition system for electrical loads based on the analysis of the total absorbed current. a system of this type allows to obtain a low cost and galvanically isolated measuring system. in steady state conditions, the absorbed current does not provide sufficient information to characterize a wide range of different loads. this is because the waveform of the current absorbed by domestic loads hardly has a significant harmonic content. therefore, the only considerations that can be made are based on the difference in amplitude. it was therefore decided to characterize the loads on the basis of their transient characteristics. previous studies have tried to create nilm systems based on transient characteristics [20]-[23], but all have limited the analysis to a reduced number of loads and particular cases. in this study, on the other hand, tests were conducted on signals acquired from a test plant in which five commonly used household appliances were activated and deactivated, but above all the performance was also evaluated on the basis of the building-level fully-labeled public dataset for electricity disaggregation (blued) [24]. first, the rms value of the current is calculated by processing the acquired raw current with a sliding window technique, as follows: 𝐼rms (𝑘) = √ 1 𝑁 ∑ 𝑖(𝑛) 2 𝑘+(𝑁−1) 𝑛=𝑘 , (1) where 𝑘 is the 𝑘-th measured current sample, 𝑁 is the number of samples per cycle, 𝑖(𝑛) is the sampled signal, and 𝑛 is the summation index. the rms value is then derived, and the resulting signal 𝐼rms(𝑘) ′ is an impulsive signal in which each pulse corresponds to a change in state of one of the powered loads. an example is shown in figure 1. 𝐼rms(𝑘) ′ = 𝐼rms (𝑘) − 𝐼rms (𝑘−1) (2) the position of the pulse in the derived signal identifies the moment in which a certain event occurred. in this way, the information relating to the steady state is filtered and only the information relating to the transients is kept. this impulsive signal is successively processed by the short-time fourier transform (stft), through the following known transformation [25], [26]: 𝑆𝑇𝐹𝑇(𝑚,𝜔) = ∑ 𝐼rms(𝑛) ′ 𝑤(𝑛−𝑚) 𝑒 −𝑗𝜔𝑛 ∞ 𝑛=−∞ (3) each change in the states of the powered loads is characterized and discriminated on the basis of the spectral content of the derived signal. the current is processed cyclically figure 1. variation with time of the rms current 𝐼rms (k) (top) and its derivative 𝐼rms(k) ′ (bottom). acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 92 at 1 second acquisition intervals, following the described procedure. each acquisition slot is processed (to calculate rms and the derivative) by adopting an overlap of 500 ms to ensure correct analysis. it is also processed for transient events, which can be fragmented into two successive slots. the stft is implemented by processing 10-cycles (200 ms) windows with an overlap of 4/5 of the processing window. this results in a spectrogram with 101 points in frequency at 26 different instants of time. to take into account the sign of the change (switching on, switching off or passing to a different consumption state), the spectrogram is multiplied by the sign of the cumulative sum, evaluated on the rms signal as follows: 𝑆𝑁 = ∑(𝐼rms (𝑛) − 𝐼rms (𝑛−1)) 𝑁 𝑛=1 (4) where 𝐼rms (𝑛) is the rms value of the current described in (1), 𝑁 is the number of samples, and 𝑆𝑁 is the value of the cumulative sum. the final signal 𝑆(𝑖, 𝑗) can be obtained in the form of a 101 × 26 matrix, as follows: 𝑆(𝑖, 𝑗) = 𝑆𝑇𝐹𝑇(𝑚,𝜔) ∙ sgn(𝑆𝑁 ) = ∑ 𝐼rms(𝑛) ′ 𝑤(𝑛−𝑚) e −𝑗𝜔𝑛 ∞ 𝑛=−∞ ∙ sgn (∑(𝐼rms (𝑛) − 𝐼rms (𝑛−1)) 𝑁 𝑛=1 ) (5) an example of the spectrogram obtained from this procedure is shown in figure 2. this spectrogram is used as inputs to a neural network which provides a response every 500 ms, indicating the presence or absence of events in the signal, and the type of device involved. 2.1. the adopted artificial neural network the deduction of the loads, starting from the spectrogram described above, is traced back to a multiclass classification problem, that is, a single unique label must be associated with each spectrogram. analysing the current using such a small sliding window (1 second with 0.5 second overlap) makes it possible to state that within a single window there is no change of state of more than one load. artificial neural networks (anns) are an example of algorithms that natively support multiclass classification problems. in this work, a particular ann type, namely, the convolutional neural network (cnn), is adopted [27] because of its capability of processing complex inputs such as multidimensional arrays. more specifically, cnns are designed to exploit the intrinsic properties of some two-dimensional data structures, in which there is a correlation between spatially close elements (local connectivity). the proposed system [28] includes different layers: an input level (for signal loading), three groups of layers, each of which consisting of convolution, relu, and max pooling layers (for feature extraction from the input), and a group of flatten, fully connected, and softmax layers, which uses data from convolution layers to generate the output. 2.2. the proposed system setup the proposed measurement system uses an agilent u2542a data acquisition module with a 16-bit resolution. the current signal was acquired using a ta sct-013 current transducer and the sampling frequency was set to 10 khz. the cnn network was implemented on a desktop computer (based on the windows 10 x64 operating system) using the open-source python 3.7 from anaconda. tests were conducted on signals acquired directly from a real system, in order to have flexibility both as regards the sampling frequency and for the generation of multiple events. other tests were conducted on signals belonging to the public blued dataset, which features 34 different types of devices. the proposed measurement system was installed on a test system, designed to generate electrical loads made by domestic users, as part of the research project "non-intrusive infrastructures for monitoring loads in residential users". the system, which is located in the electrical engineering laboratory of the university of l'aquila (i), allows the generation of electrical loads in a single or simultaneous way. these loads correspond to the loads generated by the most common household appliances and are integrated in a structure similar to that of a residential building to reproduce the real problems of conditioning and measurement of the signals. 2.3. the obtained results the performance of the nilm system was assessed by conducting acquisitions, during which various loads were turned on and off for a total of over 519 events. next, blued, a public dataset on residential electricity usage, was used. this dataset includes voltage and current measurements for a single-family house in the united states, sampled at 12 khz for an entire week. regarding nilm systems, no standard and consolidated techniques can be found in the literature to evaluate the performance of event detectors. since the purpose of a nilm system is to disaggregate consumption for each of the devices in question, their performances were analysed to verify the achievement of these objectives, which in summary are correct identification and classification of the events. these parameters were obtained using the number of true positive (tp), false positive (fp), true negative (tn), and false negative (fn). in addition, the accuracy was assessed as follows: 𝐴𝑐𝑐𝑢𝑟𝑎𝑐𝑦 % = 𝐶𝑜𝑟𝑟𝑒𝑐𝑡 𝑚𝑎𝑡𝑐ℎ𝑒𝑠 𝑇𝑜𝑡𝑎𝑙 𝑝𝑜𝑠𝑠𝑖𝑏𝑙𝑒 𝑚𝑎𝑡𝑐ℎ𝑒𝑠 100 % (6) the obtained results are summarized in table 1. figure 2. spectrogram obtained during the switch on of a microwave oven. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 93 3. nilm system based on active measurements subsequently, we tried to recognize the loads powered by an electrical system through the sweep frequency response analysis (sfra). the sfra is a non-destructive diagnostic technique that detects the displacement and deformation of windings, among other mechanical and electrical failures, in power and distribution transformers. sfra proceeds by applying a sinusoidal voltage signal of constant amplitude and variable frequency between one terminal of the bipole under test and ground. the response is measured between the other bipole terminal and ground. both input and output signals are acquired and processed. the obtained result is the transfer function (tf) of the bipole over a wide frequency range. a failure is detected when a change in the tf is observed. the possibility of using these traces to identify which devices are powered at the time of measurement was evaluated [29]. the basic idea is to detect a change in the load, starting from the change in the measured tf. for this purpose, a variable frequency sinusoidal signal is applied between the terminal of the power phase conductor and ground, by means of the instrumentation shown in figure 3, then both the applied input signal and the output signal between the neutral conductor terminal and ground are measured and processed. the test instrument generates a sinusoidal input signal of constant amplitude (a few volt) and frequency variable in the range between 10 khz and 1.5 mhz. the results obtained on the electrical system are analysed on a temporal basis, comparing them with those previously obtained on the same system. the measurement techniques follow the iec 60076-18 standard [30], which regulates the test execution methods, the characteristics of the instruments used, the connection methods and the analysis of the results. figure 4 shows the signatures obtained for the different types of loads. tests were also conducted in order to evaluate the ability to discriminate through these traces different loads when they are powered simultaneously. from figure 5 it is possible to see the possibility to discriminate, through these traces, if the heater is powered individually or in combination with other loads. 3.1. the machine learning approaches in order to translate these traces into useful information for the users, the problem was formulated as a multi-label classification problem. this is a variant of the classification problem, where multiple labels (or multiple classes) may be assigned to each instance. multi-label classification is a generalization of multiclass classification, which is the singlelabel problem of categorizing instances into precisely one of more than two classes. in the multi-label problem, there is no constraint on how many of the classes the instance can be assigned to. the problem was initially addressed with an ann [31] similar to the one described in the previous section, with good results. however, a limitation of the anns is the large amount of training data required, which makes it difficult to apply them in real cases. an attempt was therefore made to use another machine learning algorithm, the support vector machine (svm). svm is one of the most popular artificial intelligence algorithms and is a supervised learning algorithm used primarily for solving classification problems. unlike generic classification algorithms that discriminate on the basis of characteristics common to each class, svm focuses on the samples that are most similar to each other but belonging to different classes, which are therefore the most difficult samples to discriminate. on the basis of these samples, the algorithm constructs an optimal hyperplane capable of separating them, and which can then be used to discriminate the new samples. these samples are table 1. scores achieved with the acquired signal and the blued dataset. acquired signal blued dataset precision 0.981 0.998 recall 0.998 0.998 f1-score 0.989 0.998 accuracy % 98.0% 87.9% figure 3. instrumentation used for the sfra. figure 4. sfra tests of different household appliances. figure 5. sfra tests with simultaneous loads powered. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 94 called support vectors, because they are the only samples that support model creation, while all other samples are useless. in a two-dimensional case, where the examples to be classified are defined by only two characteristics, the optimal hyperplane is reduced to a straight line as shown in figure 6. the algorithm searches for the line that maximizes the margin between the two examples indicated as support vectors. if it is not possible to separate the classes with a straight line, as in the case of nonlinear classification problems, the algorithm uses the kernel trick [32]. in particular, a polynomial nucleus was chosen for this work, thus examining not only the given characteristics of the input samples to determine their similarity, but also their combinations. in the case of the proposed nilm system, the problem is obviously not two-dimensional. in fact, the sfra measuring system returns an array of 320 points, which represent the transfer functions corresponding to the different frequency values. therefore, the inputs of the svm is 320, and consequently also the size of the problem is 320. to solve the problem, to identify which devices are powered starting from the result of the sfra measurement, four svm classifiers are used, each of which performs a binary classification, identifying the presence or absence of the device associated with it. 3.2. the obtained results unlike the nilm system based on passive measurements, in this case there is no public dataset that has the necessary characteristics, i. e. there is no public dataset of measurements obtained through the sfra technique. therefore, the performance evaluation was made solely on the basis of our acquired measurements. as for the previously described system, also in this case the same test parameters were used. the proposed algorithm was subjected to different scenarios, each for a certain number of tests, in which the different appliances were powered individually or simultaneously. since, as already explained above, each appliance has an associated svm algorithm that reveals its presence or not, the performance of the four algorithms were assessed individually. to allow a comparison with the algorithms developed by other researchers, precision, recall, f1-score and accuracy during classification were evaluated [33]. the results are shown in table 2. as far as the ann is concerned, the evaluation measures for a multiclass, hence single-label classification problem, are generally different from those for the multiple label. in singlelabel classification we can use simple metrics such as precision, recall, and accuracy [34]. however, in the multi-label classification, an incorrect classification is no longer a real error, as a forecast containing a subset of the actual classes is certainly better than a forecast that does not, i.e. correctly predicting two of the four labels is better than foresee the absence of labels. to evaluate the performance of a multi-label classifier we have to calculate the average of the classes. there are two different methods of doing this called micro-averaging and macroaveraging [35]. the current is processed cyclically at 1 second acquisition the metric independently for each class and then take the average hence treating all classes equally, whereas the microaverage will aggregate the contributions of all classes to compute the average metric. in a multi-label classification setup, microaverage is preferable if there is a suspicion that there may be a class imbalance (i.e. the possibility of having many more examples of a class than other classes). in the cases under examination, this problem does not exist as the examples used for training and testing are sufficiently uniform, so micro-average and macro-average can both be considered reliable. 𝑃𝑟𝑒𝑐𝑖𝑠𝑖𝑜𝑛micro−averaging = ∑ 𝑇𝑃𝑛 𝑁 𝑛=1 ∑ 𝑇𝑃𝑛 + 𝐹𝑃𝑛 𝑁 𝑛=1 (7) 𝑅𝑒𝑐𝑎𝑙𝑙micro−averaging = ∑ 𝑇𝑃𝑛 𝑁 𝑛=1 ∑ 𝑇𝑃𝑛 + 𝐹𝑁𝑛 𝑁 𝑛=1 (8) 𝐹1 − 𝑠𝑐𝑜𝑟𝑒micro−averaging = 2 × 𝑃𝑟𝑒𝑐𝑖𝑠𝑖𝑜𝑛micro−averaging × 𝑅𝑒𝑐𝑎𝑙𝑙micro−averaging 𝑃𝑟𝑒𝑐𝑖𝑠𝑖𝑜𝑛micro−averaging + 𝑅𝑒𝑐𝑎𝑙𝑙micro−averaging (9) 𝑃𝑟𝑒𝑐𝑖𝑠𝑖𝑜𝑛macro−averaging = ∑ 𝑃𝑟𝑒𝑐𝑖𝑠𝑖𝑜𝑛𝑛 𝑁 𝑛=1 𝑁 (10) 𝑅𝑒𝑐𝑎𝑙𝑙macro−averaging = ∑ 𝑅𝑒𝑐𝑎𝑙𝑙𝑛 𝑁 𝑛=1 𝑁 (11) 𝐹1 − 𝑠𝑐𝑜𝑟𝑒macro−averaging = 2 × 𝑃𝑟𝑒𝑐𝑖𝑠𝑖𝑜𝑛macro−averaging × 𝑅𝑒𝑐𝑎𝑙𝑙macro−averaging 𝑃𝑟𝑒𝑐𝑖𝑠𝑖𝑜𝑛macro−averaging + 𝑅𝑒𝑐𝑎𝑙𝑙macro−averaging (12) figure 6. representation of a linear classification problem (top) and a nonlinear classification problem (bottom) in which the samples are defined by only two features. table 2. achieved scores with the svm. svm lamp svm hairdryer svm induction hob svm heater tp 42 200 200 200 fp 0 0 0 2 tn 400 250 250 248 fn 8 0 0 0 precision 1 1 1 0.99 recall 0.84 1 1 1 f1-score 0.91 1 1 0.99 acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 95 table 3 shows the scores achieved according to the two criteria both with the ann and with the svm. 4. conclusions and final remarks in this paper a brief introduction of the state of the art of nilm systems has been presented. two different types of systems for real-time identification of electrical loads, based on different measurement techniques, were then presented. both systems were proven excellent identification performance. more in detail, the first system, based on the spectrogram analysis of the effective current through the cnn, has been proven excellent performance in correspondence with both the acquired measurements and those available in the blued dataset, reaching f1-score respectively equal to at 0.989 and 0.998 and accuracy% respectively equal to 98.0% and 87.9%. the greatest difficulty encountered in the classification phase with the blued dataset is attributable to the significantly greater number of devices that the network is required to recognize, compared to those used for the acquired measurements. furthermore, the value obtained for the f1-score is higher than that obtained with other systems using the same dataset, as those proposed in [36] (0.915) and [37] (0.932). traditional nilm systems perform the loads classification based on the analysis of quantities also related to voltage (e.g. analysis in the p-q or v-i plan [38]). the proposed system has the advantage of measuring only the overall current in a house. as a result, the complexity of the processing system is reduced. another advantage is that the measuring system can be implemented with a galvanically isolated at low-cost system, using a clamp current transducer. the second proposed system, based on the trace analysis provided by the sfra, also proved excellent performance. the traces were initially processed through an artificial neural network similar to that used for the previous system, reaching f1-score of 0.96. in order to reduce the number of training examples needed, it was decided to use a support vector machine. despite a significant reduction in the examples needed for training (from over 2000 to 90), the f1-score achieved with this second machine learning structure was even higher than that obtained with the artificial neural network. a system of this type is particularly interesting as it allows the creation of a plug-in solution that can be installed in any domestic, industrial or commercial environment. furthermore, the detection technique takes into account the physical characteristics of household appliances and the resulting transfer function. consequently, the identification of multi-state or continuously variable appliances is simplified, compared to processing time-varying signals such as real power, current, etc. references [1] g. bucci, f. ciancetta, e. fiorucci and s. mari, load identification system for residential applications based on the nilm technique, 2020 ieee international instrumentation and measurement technology conference (i2mtc), 25-28 may 2020, pp. 1-6. doi: 10.1109/i2mtc43012.2020.9128599 [2] g. bucci, f. ciancetta, e. fiorucci, s. mari, a. fioravanti, state of art overview of non-intrusive load monitoring applications in smart grids, measurement: sensors 18(2021), art. no. 100145. doi: 10.1016/j.measen.2021.100145 [3] a. lucas, l. jansen, n. andreadou, e. kotsakis, m. masera, load flexibility forecast for dr using non-intrusive load monitoring in the residential sector, energies 12(14) (2019), art. no. 2725. doi: 10.3390/en12142725 [4] w. schneider, f. campello de souza, non-intrusive load monitoring for smart grids, 2018. technical report. dell emc. 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https://doi.org/10.1109/61.400897 https://doi.org/10.1109/icemi.2007.4350914 https://doi.org/10.1109/apccas.2006.342241 https://doi.org/10.1109/icengtechnol.2017.8308186 https://doi.org/10.1109/tim.2020.3035193 https://doi.org/10.1109/speedam48782.2020.9161856 https://doi.org/10.1145/3467707.3467753 https://doi.org/10.1214/009053607000000677 https://doi.org/10.1109/metroind4.0iot51437.2021.9488472 https://doi.org/10.1007/s12053-014-9306-2 https://doi.org/10.1016/j.schres.2018.08.020 https://doi.org/10.1109/urtc.2017.8284200 https://doi.org/10.1109/pesgm.2014.6938824 improved uncertainty evaluation for a long distance measurement by means of a temperature sensor network acta imeko issn: 2221-870x march 2023, volume 12, number 1, 1 6 acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 1 improved uncertainty evaluation for a long distance measurement by means of a temperature sensor network gertjan kok1, federica gugole1, aaron seymour1, richard koops1 1 vsl b.v., thijsseweg 11, 2629 ja delft, the netherlands section: research paper keywords: sensor network; uncertainty; interferometry; temperature; refractive index citation: gertjan kok, federica gugole, aaron seymour, richard koops, improved uncertainty evaluation for a long distance measurement by means of a temperature sensor network, acta imeko, vol. 12, no. 1, article 14, march 2023, identifier: imeko-acta-12 (2023)-01-14 section editor: daniel hutzschenreuter, ptb, germany received november 19, 2022; in final form february 14, 2023; published march 2023 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this project (17ind12 met4fof) has received funding from the empir programme co-financed by the participating states and from the european union’s horizon 2020 research and innovation programme. corresponding author: gertjan kok, e-mail: gkok@vsl.nl 1. introduction one of the many aspects of the digital transformation is that sensor networks come at an affordable cost, are relatively easy to set up, and can measure with good accuracy once properly calibrated. as such they can be helpful to support other measurements by giving a greater understanding of the set-up, validate assumptions and improve uncertainty calculations. examples of sensor networks supplementing high-grade measurements are sensor networks for measuring outdoor air quality [1] and for noise measurements [2]. sensor networks are also advantageously being used in agriculture [3] and in aerospace applications [4]. this paper deals with a long-distance interferometric measurement, which will be supported by means of a sensor network for measuring ambient temperature. various measurement schemes exists for interferometric distance measurements [5] and some of them have been implemented at vsl [6], the dutch national metrology institute (nmi). in this paper we consider the classical fringe counting michelson interferometer [5]. the research question addressed by this paper was to validate a particular assumption that is currently used in the uncertainty calculation for interferometric measurements over long distances. this assumption was that the inhomogeneity of the air temperature was small enough that the current approach of using 5 temperature sensors over a total distance of 50 meters is sufficient. this assumption and the meaning of ‘sufficient’ will be made quantitative in section 2. in order to be able to validate this assumption two activities were undertaken. the first activity was the extension of the mathematical model describing the measurement from considering only the mean temperature over a long-distance path to account for the temperature profile over a path. this naturally led to additional theoretical questions regarding the exact shape of the light path, which were addressed by performing simulations, see section 2. secondly, we installed a sensor abstract the aim of the research described in this paper was to more accurately determine the measurement uncertainty of an interferometric long distance measurement. the chosen method was to install a temperature sensor network in the laboratory to measure in more detail the ambient temperature profile to more accurately determine the local values of the refractive index on the path travelled by the laser light. the experimental measurements were supplemented by a theoretical analysis of the mathematical model being used. the outcome of the performed work was that the claimed measurement uncertainty of the distance measurement, which is based on using only 5 temperature sensors, was justified. however, if in the future a lower uncertainty would be needed, a sensor network like the one that was temporarily installed would be needed. during the measurement campaign an offset in the mean temperature of 0.2 °c was found, which was equal to the maximum allowed bias in view of the claimed uncertainty for the long distance measurement. at a more general level, it was concluded that such sensor networks provide a useful new tool to increase the understanding of other measurements, to validate assumptions and optimize existing measurements. mailto:gkok@vsl.nl acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 2 network with 51 calibrated temperature sensors in vsl’s 50 meter long climatised corridor in which highly accurate long distance measurements are performed. the knowledge of the air temperature along a path is essential for this application due to its effect on the refractive index of air. with the help of the temperature sensor network the uncertainty evaluation of the long distance measurement was improved. this work was performed in the empir project "metrology for the factory of the future" in a task dedicated to redundant measurements of ambient conditions [7]. in this project several aspects related to sensor networks were studied, like redundancy aspects [8] or the effect of synchronization errors [9], based on various case studies in industrial testbeds. the outline of this paper is as follows. in section 22 the problem will be formulated with more mathematical detail, followed by a detailed presentation of the sensor network in section 3. in section 4 the results of both the simulations and the measurements will be presented. in the last section the overall conclusions are summarized. 2. problem description 2.1. measurement problem and approximations the distance measurement takes place in a climatised laboratory in which the temperature is kept between 19.5 °c and 20.5 °c. in the set up distances up to 50 m are being measured by means of a michelson interferometer with a laser light source with a vacuum wavelength 𝜆vac = 633 nm. a measurement is performed by moving an optical target (retroreflector) from the initial position to the final position. the optical target is mounted on a cart which moves smoothly in a straight line on a rail such that the electronics can count the number of fringes 𝑚 (not necessarily an integer). to convert this number to a measured distance 𝐷, it needs to be divided by two and multiplied by the actual wavelength in air 𝜆. this wavelength is derived from the vacuum wavelength by multiplication with the refractive index 𝑛. the refractive index is evaluated using edlen's formula [10] at the laser wavelength 𝜆vac, the measured mean air pressure, the measured mean relative humidity and the measured mean air temperature 𝑇0. finally, using these mean values the resulting estimate 𝐷0 of the distance is calculated by 𝐷0 = 𝑚 2 𝜆vac 𝑛(𝑇0). (1) the claimed expanded uncertainty for 𝐷0 with coverage factor 𝑘 = 2 is 1 ppm relative. in absolute terms this amounts to 50 µm at a measured distance of 50 m. equation (1) is based on the following assumptions: 1. it is assumed that the non-linearity of the function 𝑛(𝑇) is weak and that it is sufficient to evaluate 𝑛(𝑇) at the mean temperature value only. 2. due to local temperature variations, the light will be refracted and not travel in a perfect straight line, but on a curved path, leading to an overestimate of the distance. it's assumed that this effect is small. 3. it is assumed that the mean temperature can be estimated sufficiently well by measuring spot temperatures at only 5 positions. the validity of assumptions 1 and 2 have been analysed in a theoretical way and the results are presented in section 3.1 and 3.2. the validity of assumption 3 was assessed by means of installing a dedicated temperature sensor network. this network will be presented in more detail in the next section. if the assumptions 1 and 3 are not fulfilled, a more complex model of the form 𝐷1 = 𝑚 2 𝜆vac 1 𝐿 ∫ 𝑛(𝑇(𝑥)) d𝑥 𝐿 0 (2) will be needed in which the full variation of the refractive index over the travel path of the laser light is considered. here 𝐿 denotes the nominal distance and 𝑇(𝑥) the temperature at position 𝑥 along a path. in optics, the integral quantity in equation (2) is called the optical path length. if assumption 2 is not verified, then the model would become even more complex as the measured distance would depend on a three-dimensional profile of the refractive index. in that case a map of the temperature in three-dimensional space would be required. 2.2. target uncertainty in generic terms, an additional uncertainty contribution is not considered significant if it contributes less than about 1/5 of the current combined uncertainty. this is because the new combined uncertainty including the additional uncertainty component will have essentially the same size as √12 + 0.22 ≈ 1. having this in mind, we will now assess what the maximum measurement bias of the mean ambient temperature can be, without significantly affecting the combined uncertainty of the long distance measurement. the claimed combined expanded uncertainty at 50 m is 50 µm (𝑘 = 2). in view of the discussion of the last paragraph, the expanded uncertainty or maximum bias in the measured mean temperature should not contribute more than 1/5 of 50 µm, which is 10 µm. using edlen's formula, it can be calculated that a change of 1 °c in temperature induces a change in the calculated refractive index of slightly lower than 1 ppm, which results in a change of the calculated distance of 48 µm at a total measured distance of 50 m by means of equation (1). this indicates that the expanded uncertainty of the mean temperature should not be more than 10 48 ∙ 1 ℃ = 0.21 ℃ . (3) for a potential systematic bias in the measurement temperature we are using the same threshold of 0.21 °c. the goal of the sensor network is to assess if the measurement bias when determining the mean temperature using only 5 temperature sensors compared to the improved estimate when using 51 sensors lies below this threshold. if this is indeed the case, the usual procedure using only 5 temperature sensors can be retained without a need for increasing the claimed uncertainty to account for a larger than expected uncertainty in the measured mean temperature. 3. temperature sensor network 3.1. construction of the network fifty-one temperature sensors were assembled in house in order to get the lowest measurement uncertainty. the sensing part consisted of a 10 kω ntc thermistor placed in series with a resistance of 12 kω. the communicating part was formed by texas instruments cc2531 usb zigbee modules, communicating wirelessly at a frequency of 2.4 ghz. the voltage over the thermistor was measured by an analogue voltage input acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 3 of the module. to enhance the stability of the network, 5 additional cc2531 chips with external antennae were used. whereas the sensor nodes had only a meandered antenna on the chip itself, the external antennae more than tripled the link quality. the routers were used to repeat the network signal over the full distance of the measurement. to supply power to the nodes and routers a long wire with usb plugs at 1 meter spacing was used, as this seemed more practical than 51 individual battery packs. a texas instruments cc1352p chip was used as a coordinator in the network. finally, a raspberry pi 3 model a+ was used to run the zigbee server to which voltage readings were also logged in real time. in figure 1 a photo is shown with a temperature sensor mounted below the rail of the interferometric measurement cart set-up, together with a router node with external antenna. the total cost of the hardware was around 1000 eur. 3.2. calibration of the sensors the sensors were calibrated in the following way. the sensors, together with a set of traceable reference sensors, were mounted on an aluminium block which could be brought to a desired temperature by means of a water-based heating and cooling device. the block and sensors were well isolated from the environment by means of an insulating box. the digital voltage output which was transmitted wirelessly, was calibrated and a calibration function relating voltage output to temperature was established. calibration of all sensors took place before and after the measurement campaigns. after calibration, the expanded uncertainty of each of the sensors was 0.04 °c (k = 2), mainly limited by the resolution of the digital voltage output of the sensors. this uncertainty includes a component for the repeatability over two days, but doesn't include a component for the expected drift of the sensors over the entire measurement period. 3.3. software to operate the zigbee network by means of the raspberry pi two pieces of software were used. the zigbee2mqtt software [11] was used to start the zigbee server. the zigbee mosquitto software [12] provides a messaging protocol which was used to interface with the coordinator. these two pieces of software ultimately allow for the text message transmissions containing the voltage readings from the adcs of each of the sensor nodes to be stored on the raspberry pi. appropriate firmware was installed at the sensor nodes [13] and at the coordinator node [14]. 3.4. test procedure the main aim of the measurement campaign was to gain insight into the overall temperature profile along the corridor, and specifically, to assess if the mean value of the 5 climate system sensors was sufficiently close to the mean temperature of the 51 sensor network sensors, having the numerical target required for the interferometric distance measurement in mind as stated in equation (3). in order to get some further insights, a test plan with the following five test cases was defined: 1. undisturbed profile with nobody present in the corridor 2. person sitting at a fixed spot in the corridor 3. person walking around in the corridor all the time 4. person walking around in the corridor and then leaving the corridor 5. profile during a long distance measurement with the moving cart all sensors were calibrated in week 1. then the test plan was executed in week 2 and repeated in week 3. in week 4 all sensors were recalibrated in order to assess their drift. 4. results in this section the results will be presented. they are ordered according to the assumptions to be verified as listed in section 2.1. 4.1. assumption 1 on non-linearity of refractive index formula the effect of the neglected non-linearity of the refractive index as a function of temperature in equation (1) was analysed using a worst-case approach. if the mean temperature is 20.0 °c, then the worst case is that half of the path length is at 19.5 °c and the other half at 20.5 °c. the effective average refractive index in this case then amounts to (𝑛(19.5 ℃) + 𝑛(20.5 ℃)) 2⁄ which has to be compared with the value 𝑛(20.0 ℃) that is obtained from first averaging the temperature instead of averaging refractive indices, the latter begin what is currently used. the difference is only 6∙10-10 both in absolute and in relative sense, as 𝑛 ≈ 1.0003. the induced measurement error by averaging temperatures as done in equation (1) instead of averaging refractive indices therefore amounts to only -6∙10-10 × 50 m = -0.03 µm at a measurement distance of 50 m, which is very minor. the non-linearity of the refractive index dependence on temperature can therefore be neglected and for the uncertainty calculation of the interferometric long distance measurement it is sufficient to focus on the correct determination of the mean temperature. figure 1. the horizontal black parts are the rails for the cart used in the interferometer set-up. in the red circle at the lower left corner the sensing part of a temperature sensor node is visible. in the yellow circle in the top right corner a router node with external antenna is visible, connected to the wire supplying power by means of a usb connector. acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 4 4.2. assumption 2 on curved light paths if a light beam travels from one medium into another medium with a different refractive index (e.g., air with a different temperature), it will change its direction of propagation following snell's law [15]: 𝑛1 sin 𝜃1 = 𝑛2 sin 𝜃2 , (4) where 𝑛𝑖 and 𝜃𝑖 are respectively the refractive index and the angle of the propagation direction of the beam to the surface normal in medium 𝑖 = 1 and 2. so the magnitude of the direction change 𝜃2 − 𝜃1 depends on the difference of the refractive indices and on the angle of incidence 𝜃1. equation (1) is based on the assumption that the light travels in a perfectly straight line without any curvature due to diffraction. to assess how much this simplification affects the final measurement result 𝐷0 two-dimensional curves were simulated by modelling the temperature distribution by a onedimensional gaussian process with a squared exponential kernel of the form 𝜎t 2 exp (− (𝑥1 − 𝑥2) 2 2 ℓ2 ) . (5) in equation (5), the parameters 𝑥1 and 𝑥2 denote the horizontal position of points 1 and 2 (horizontal distance from the laser emitting the light), ℓ the characteristic length-scale of the kernel, and 𝜎t the standard deviation of the temperature. this kernel defines the correlation strength between the temperatures at different positions in the corridor. the normal vectors of the interfaces between two neighbouring zones of uniform temperature were chosen randomly. in reality the situation is probably more favourable than this, as the interfaces will be more similar to planes perpendicular to the propagation direction of the light and not fully randomly oriented. in figure 2, a schematic sketch of the simulated temperature profile together with the light path is shown. in our simulation we used 1000 zones of uniform temperature over a distance of 50 m. in this simulation only the geometric path length was assessed, thus the effect of different actual wavelengths due to changes in the refractive index in different path sections is not included in the results in this section. this latter effect is covered by the results of sections 4.1 and 4.3 it was found that the maximum path increase significantly depended on the used parameter values. as a reasonable choice for the kernel parameters the characteristic length-scale of correlation parameter was set to ℓ = 1 m and the standard deviation of the temperature was set to 𝜎𝑇 = 0.25 °c, as the climate system kept the laboratory temperature between 19.50 °c and 20.50 °c. for this choice the simulated increase in path length remained below 1 µm, thus being negligible. in figure 3, an example of a simulated path of the laser beam is shown. note nevertheless that one can mathematically construct worst-case temperature profiles with worst-case normal vectors in which the light travels an almost arbitrary path of arbitrary large distance, e.g., the light can change direction by 180°. for this to happen, the normal vectors should be chosen close to vertical and the temperature differences between zones should be chosen as large as possible. 4.3. assumption 3 on the measured overall mean temperature the test procedure with the temperature sensor network as specified in section 3.4 was executed twice in two consecutive weeks. the main points of attention during the data analysis were the changes of the measurement values of the individual sensors over time, the spatial temperature profile of the mean sensor values in the corridor, and the comparison of the measured values by the 5 sensors of the fixed installed climate system and the 5 sensors of the sensor network that were in the closest proximity. after the first test campaign it turned out that some of the 51 sensors reported readings only sporadically, or not at all. therefore fewer sensors were used during the second campaign. in the first campaign data from 40 sensors were available, whereas in the second campaign data from 30 sensors were used. the temperature traces of single sensors over time were generally very stable, although incidentally some higher and/or lower values were measured. in figure 4, the temperature profile in space for test case 5 simulating a real measurement is shown. as some measurement instruments and computers are present near the start of the corridor the temperature is slightly higher in that region. the temperature profiles for the other cases look similar. we'll now first present the conclusions for the various test cases as defined in section 3.4 before addressing our main question based on the uncertainty of the long distance measurement induced by the non-constant temperature profile. figure 2. two-dimensional simulation of curved light paths due to refraction effects. the vertical curvy black dashed lines delimit zones with different temperatures. the slanted orange lines indicate the assumed direction of the local interface between these zones. the light is assumed to travel from the red spot on the left to the green spot on the right and back. figure 3. blue: simulated curved path of the light using 1000 zones of 50 mm width in which the temperature was assumed to be uniform. red: temperature profile over the distance of 50 m. not shown are the angles defining the interfaces between the zones, which are completely random. acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 5 it turned out that the measurement results for the various test cases were not significantly different, as compared with the unperturbed case 1. in test case 2, a person sitting between two sensors did not cause any significant perturbation in the temperature measurements of the sensors. in test cases 3 and 4, a person walking around the lab caused significant perturbations in the sensors’ reading during the first execution but not during the second. the results on this aspect were therefore inconclusive. during test case 5, a battery powered temperature sensor was mounted on the moving cart and its measured values were compared with the values of the non-moving sensors mounted closest by at the moment when the cart passed. the differences between the moving sensor network sensor and the non-moving sensor network sensors varied from 0.05 °c to a few 0.1 °c with a maximum of even 0.6 °c, which is higher than expected. furthermore, the values of the 5 fixed installed climate system sensors were compared with the values of the nearest sensors of the sensor network, both placed on the 50 meter long granite support for the interferometric measurement set-up. two of the five climate sensors agreed well with the closest sensor network sensors within the mutual uncertainties, whereas a third sensor agreed part of the time. two sensors didn't agree and differences of 0.15 to 0.20 °c were present between the mean values of the climate system sensors and the sensor network sensors. the differences can be due to the sensor hanging in free air or not, or to locally warmer air flows (one such a flow was detected close to an electrical device mounted half way the corridor). this indicates that the location of the sensors can be an important factor when measuring the temperature of the lab. in figure 5, the temperature plots over time of two climate sensors together with the plots of the nearest sensor network sensors are shown. we’ll now turn to the results for the main purpose of our measurement campaigns, which was to determine if the mean temperature of the corridor was measured sufficiently well, with a bias below 0.21 °c. in table 1, the mean temperatures as measured by the climate system and as measured by the sensor network including the uncertainties and their differences are shown. the uncertainty of both the climate system and the sensor network was dominated by a systematic part due to the calibration, supplemented by a part for the variation during the measurement. based on the recalibration of the sensor network sensors after the measurement campaigns a standard uncertainty for sensor drift of 0.05 °c was added per sensor. however, as it was a random contribution, it averaged out when calculating the mean temperature and it didn’t affect the uncertainty of the mean value. as can be seen from table 1, it turned out that the difference between the mean corridor temperature as measured by the sensor network and the climate system was at most 0.25 °c with an expanded uncertainty of 0.06 °c. when using the expanded measurement uncertainty as a tolerance, the threshold of 0.21 °c of equation (3) was not exceeded. it was therefore decided that it is not needed to improve the temperature measurement in the corridor for this application in a permanent way, nor to increase the claimed uncertainty. however, the results also indicated that if the claimed uncertainty were to be reduced in future, then a better understanding of the temperature profile would be needed. this could then be accomplished by the sensor network, albeit that its potential drift over time would need a more careful study. figure 4. temperature profile in space for test case 5 simulating a real measurement for the two repetitions a (top) and b (bottom). the circular marker is at the mean value and the error bars are at plus minus one standard deviation. the lines above and below connect the maximum and minimum measured values during the entire time span. table 1. mean temperature and expanded uncertainty (k = 2) in parentheses as measured by the climate system and by the sensor network, and their differences, as measured in the 5 test cases and the two test campaigns a and b. case number climate system / °c sensor network / °c difference / °c 1-a 19.91 (0.06) 19.67 (0.03) 0.24 (0.06) 1-b 19.89 (0.06) 19.73 (0.03) 0.16 (0.06) 2-a 19.92 (0.06) 19.69 (0.03) 0.24 (0.06) 2-b 19.93 (0.06) 19.75 (0.03) 0.18 (0.06) 3-a 19.94 (0.06) 19.70 (0.03) 0.25 (0.06) 3-b 19.91 (0.06) 19.76 (0.03) 0.15 (0.06) 4-a 19.91 (0.06) 19.68 (0.03) 0.23 (0.06) 4-b 19.90 (0.06) 19.75 (0.03) 0.15 (0.06) 5-a 19.93 (0.06) 19.73 (0.02) 0.20 (0.06) 5-b 19.92 (0.06) 10.76 (0.03) 0.16 (0.06) figure 5. measured temperature profile over time for two sensors of the climate system and the sensors of the sensor network that was installed at the closest position for test case 5-a. for the climate system sensor displayed in the upper plot there is a difference of about 0.15 °c, whereas for the measurement at the bottom there is complete agreement. acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 6 5. conclusions the digital transformation makes it economically and practically possible to perform a larger number of measurements in a much finer spatial grid by means of sensor networks. this can help to better monitor the ambient measurement conditions of a measurement set-up and as a consequence potentially reduce its measurement uncertainty. in this contribution a sensor network with 51 sensors for measuring the ambient temperature in a corridor used for interferometric long distance measurements was presented. this network enabled the analysis of the temperature profile in much more detail than what was possible until recently, together with a more accurate measurement of the mean temperature. simulation results showed that it is sufficient to focus on the mean temperature only, as well as that the effect of a non-straight optical path due to refraction effects is negligible. the measurement results using the network showed that the mean temperature measured by the 5 climate system sensors was on average about 0.2 °c higher than the more accurate measurement by the sensor network. based on this result it was decided that the current practice with only five temperature measurements and a claimed relative measurement uncertainty of 1 ppm is valid. in future work the network can be used in other applications where temperature measurement is of critical importance. furthermore, employment and recalibration at longer time scales will give more information about the metrological stability of such sensor networks. acknowledgements this project (17ind12 met4fof) has received funding from the empir programme co-financed by the participating states and from the european union's horizon 2020 research and innovation programme. we thank the independent reviewers for their comments, which helped to significantly improve the paper. references [1] d. r. peters, o. a. m. popoola, r. l. jones, n. a. martin, j. mills, e. r. fonseca, a. stidworthy, e. forsyth, d. carruthers, m. dupuy-todd, f. douglas, k. moore, r. u. shah, l. e. padilla, r. a. alvarez, evaluating uncertainty in sensor networks for urban air pollution insights, atmos. meas. tech., 15, 2022, pp. 321–334. doi: 10.5194/amt-15-321-2022, [2] l. luo, h. qin, x. song, m. wang h. qiu, z. zhou, wireless sensor networks for noise measurement and acoustic event recognitions in urban environments. sensors. 2020; 20(7): 2093. doi: 10.3390/s20072093. [3] l. maiolo, d. polese, advances in sensing technologies for smart monitoring in precise agriculture, proc. of the 10th int. conference on sensor networks sensornets 2021, virtual, 9-10 february 2021, 151-158. online [accessed 26022023] https://www.scitepress.org/publishedpapers/2021/104154/10 4154.pdf [4] f. leccese, m. cagnetti, s. sciuto, a. scorza, k. torokhtii, e. silva, (2017). analysis, design, realization and test of a sensor network for aerospace applications, proc. of the 2017 ieee int. instrumentation and measurement technology conference i2mtc 2017, turin, italy, 22-25 may 2017, pp. 1-6. doi: 10.1109/i2mtc.2017.7969946 [5] p. hariharan (2007). basics of interferometry, second edition. elsevier. isbn 978-0-12-373589-8. [6] s. van den berg, s. van eldik,, n. bhattacharya, mode-resolved frequency comb interferometry for high-accuracy long distance measurement. sci rep 5, 2015, 14661. doi: 10.1038/srep14661 [7] empir project 17ind12 metfof metrology for the factory of the future, 2018 – 2021. online [accessed 21 february 2023] www.met4fof.eu [8] g. kok, quantifizierung von redundanz in sensornetzwerken und die beziehung zur messunsicherheit, tm technisches messen, 89(10), 2022, pp. 647-657, [in german]. doi: 10.1515/teme-2022-0012 [9] t. dorst, influence of synchronization within a sensor network on machine learning results. j. sens. sens. syst., 10, 2021, pp. 233–245. doi: 10.5194/jsss-10-233-2021 [10] b. edlén, the refractive index of air, metrologia vol. 2, no. 2, 1966, pp. 71–80. doi: 10.1088/0026-1394/2/2/002 [11] zigbee mqtt software. online [accessed 21 february 2023] https://www.zigbee2mqtt.io/guide/getting-started/ [12] zigbee mosquitto software. online [accessed 21 february 2023] https://www.mosquitto.org/download/ [13] zigbee sensor node firmware. online [accessed 21 february 2023] https://github.com/ptvoinfo/zigbee-configurable-firmware [14] zigbee coordinator firmware. online [accessed 21 february 2023] https://github.com/koenkk/z-stackfirmware/raw/master/coordinator/z-stack_3.x.0/bin/. [15] m. born, e. wolf, principles of optics, 1959, new york, ny: pergamon press inc. https://doi.org/10.5194/amt-15-321-2022 https://doi.org/10.3390/s20072093 https://www.scitepress.org/publishedpapers/2021/104154/104154.pdf https://www.scitepress.org/publishedpapers/2021/104154/104154.pdf https://doi.org/10.1109/i2mtc.2017.7969946 https://doi.org/10.1038/srep14661 http://www.met4fof.eu/ https://doi.org/10.1515/teme-2022-0012 https://doi.org/10.5194/jsss-10-233-2021 https://doi.org/10.1088/0026-1394/2/2/002 https://www.zigbee2mqtt.io/guide/getting-started/ https://www.mosquitto.org/download/ https://github.com/ptvoinfo/zigbee-configurable-firmware https://github.com/koenkk/z-stack-firmware/raw/master/coordinator/z-stack_3.x.0/bin/ https://github.com/koenkk/z-stack-firmware/raw/master/coordinator/z-stack_3.x.0/bin/ aeronautic pilot training and augmented reality acta imeko issn: 2221-870x september 2021, volume 10, number 3, 66 71 acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 66 aeronautic pilot training and augmented reality simone keller füchter1, mário sergio schlichting1, george salazar2 1 university of estácio de santa catarina pesquisa produtividade program, av. leoberto leal, 431, são josé, santa catarina, brazil 2 nasa, national aeronautics and space administration johnson space center, 2101 nasa parkway houston, 77058, texas usa section: research paper keywords: augmented reality; virtual training; flight panel; pilot training citation: simone keller füchter, mário sergio schlichting, george salazar, aeronautic pilot training and augmented reality, acta imeko, vol. 10, no. 3, article 11, september 2021, identifier: imeko-acta-10 (2021)-03-11 section editor: bálint kiss, budapest university of technology and economics, hungary received january 15, 2021; in final form september 14, 2021; published september 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: simone keller füchter, e-mail: simonekf.2011@gmail.com 1. introduction various different materials are used for pilot training on the equipment used in various types of aircraft. traditionally, the aircraft manual has been an important source of information on each part of the aircraft’s structure, as well as its mechanical, electronic and digital components. while the operating manual remains the main learning resource, a number of multimedia resources have been added for better visualisation of certain tasks and commands during the panel checklist. the array of available printed books can also be a good knowledge resource for students aspiring to be private pilots, also known as pps. this pilot category involves certification that allows the individual to fly an airplane and carry baggage and passengers without being paid or hired.[1] videos, photographs, printed posters, flight simulators and virtual reality glasses can also be used to help learn [2] and memorise the components of the panels. each resource has its advantages and disadvantages. for example, the simulator is fairly complete and close to reality but is not available 24 hours a day for everyone, while its cost per hour of use is relatively high since it involves sophisticated equipment. meanwhile, while videos present a great deal of detail, they do not involve interaction, and in an effort to find the information one is looking for, it is sometimes necessary to watch a long video to be able to visualise a specific procedure. software such as microsoft's flight simulator [3] and x-plane [4], among others, have a wide base of information and procedures as well as highquality graphics. however, the purpose of the present work is to create a prototype app for smartphones that is easy to use and is specifically focused on familiarising the pilot with the panel and the respective checklists. thus, the learning and memorisation of a new aircraft, as well as the attendant training, can be faster, bringing great savings and, above all, enhanced flight safety. checklists are a critical part of preparing for a flight. apps such as that presented in this paper can be used anywhere without the need for computers, screens, joysticks, or any other hardware; in fact, all that is required is a smartphone. the app can be used, for example, before the pilot goes into a flight simulator. indeed, the app is not intended for use as a kind of flight simulator, but a specific and cheap tool to help pilots or students to memorise the flight panel components and study them continuously, be it at home, in open spaces or even in rooms without an internet abstract a pre-flight checklist requires in-depth technical knowledge of the aircraft, including its dashboard, avionics, instruments, functions and cabin layout. to obtain up-to-date certification, students training to be a pilot or advanced pilot must be completely familiar with each instrument and its position on the flight panel. every second spent searching for the location of an instrument, switch or indicator can waste time, resulting in a poor start-up procedure and possibly a safety hazard. the objective of this research was to obtain preliminary data to determine whether the use of augmented reality as a human interface for training can help pilots improve their skills and learn new flight panel layouts of different aircraft. the methodology used was the human-centred design method, which is a multidisciplinary process that involves various actors who collaborate in terms of design skills, which includes individuals such as flight instructors, students, and pilots. a mobile/tablet application prototype was created with sufficient detail on the flight panel of a cessna150, an aircraft used in training flights at the aero club of santa catarina. the tests were applied in brazil and the results indicated good response and acceptance from the users. mailto:simonekf.2011@gmail.com acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 67 connection. indeed, the main objective of this work was to attempt to represent, as realistically as possible, the details of the flight panel without the need of computers and in a cost-effective and efficient way using readily available technology. 2. methodology the methodology used for this research with augmented reality (ar) training is the human-centred design (hcd) method. this method is a multidisciplinary process that involves various stakeholders who collaborate in terms of design skills, including the individuals who specifically pertain to the process [5], such as flight instructors, pilots and students. interactivity is a key point in this field and comes in the form of continuous testing and evaluation of the ar system and the related concepts. the hcd method is a key component of the human-systems integration (hsi) process, which is defined as an interdisciplinary technical and management process that integrates human considerations within and across all system elements [6]. in this context, iso 13407:1999, part of ics 13.180 on ergonomics, has been withdrawn and revised in terms of iso 9241-210:2019 ergonomics of human-system interaction-part 210: humancentred design for interactive systems. [7] the hsi domains define (a) how human capabilities or limitations impact the hardware and software and, conversely, (b) how the system’s software, hardware, and environment influence human performance.[8] meanwhile, there are four principles of hcd [9]: • function allocation between user and technology • design iteration • multidisciplinary design • active involvement of users and a clear understanding of user and task requirements. these principles are crucial for the concept of this research since they are focused on the ergonomics, comfort and acceptance of the pilots and students. for this study, a prototype mobile app related to ar was built based on the cessna150 aircraft panel, which is used for the training of new pps in a number of flying clubs in brazil, specifically here, the aero club of santa catarina. this prototype provides the pilot with the opportunity to familiarise themselves with the panel of a new plane that he/she may fly, accessing it on their smartphone anytime, anywhere, and viewing the panel in a highly realistic way. furthermore, perhaps more importantly, the pilot can use the main checklist procedures in 3d view, which enables a better understanding of certain movements of push/pull, rotate, and other movements. 3. augmented reality the technology pertaining to ar involves overlaying virtual components in a real-world environment with users viewing it using a specific device. for the most part, virtual objects are added to the real world in real-time during the user experience [10]. to connect with this technology, digital devices such as smartphones, tablets and virtual reality glasses (e.g., microsoft hololens) [11] can be used. the difference between ar and virtual reality (vr) is that the latter creates a completely synthetic and artificial world in which the user is completely immersed, while in the former, the user can observe a real environment with virtual objects overlaid. with ar, the pilot can observe a virtual flight panel in front of them and, using a smartphone, can visualise and study each component before moving on to a real physical aircraft. the combination of ar and smartphone provides a useful tool for training, with the professional able to practice in a manufacturing environment [9], a garage or a hangar. when used correctly, the ar technology can present a real environment in terms of creating virtual objects that mimic real-time applications [12]. however, the future of ar depends on huge innovations in all fields [13], and this paper presents a new way of learning and training aircraft-related checklist panels. 3.1. augmented reality as a training tool this technology is used in various industries [14], military environments, and schools and is highly useful in the health field, making it possible to visualise the inside of a human body. furthermore, this type of digital interface is suitable for the new generations comfortable with the devices used for vr and ar [15]. when combined with gamification, educational technologies can be improved, largely because the new generations are open to experimenting with new virtual competencies and stimuli and are highly motivated to win [16]. the same idea can be applied to ar, that is, using challenges similar to videogames to promote learning and pleasure, as explained in terms of the dopamine cycle, which pertains to challenges, achievement and pleasure [17]. a recent study conducted on a multi-national european sample of pilots regarding the use of ar and gamification [11] demonstrated that 72.25 % of the women pilots and 56.25 % of the male pilots considered it satisfactory in terms of successfully finishing a task, while, overall, 70.74 % of the pilots regarded the feedback received for corrective actions as satisfactory. this demonstrates that interactivity is important for users. for this reason, the third step of the proof of concept presented in this paper was created to clarify the number of possibilities this prototype can offer. 3.2. augmented reality and aviation according to the aviation instructors handbook, [18] vr is already part of pilot training, while ar is not mentioned. therefore, this field requires more research and more applications to find new ways to use this tool, such as external inspection, maintenance, procedures using the flight panel and various other aspects [19], [20], [21]. 4. the prototype in this paper, a life-size prototype was created and tested by six users made up of flight instructors, pilots and students, who applied the visualisation of a virtual flight panel employing ar to their smartphones or tablets. bringing together the information from the manual, the experience of the instructors and pilots who were studying how to use different aircrafts, and the proofs of concept, various evaluations were created, as shown in figure 1. a full description follows. 4.1. preliminary proof of concept the prototype was built in unity 3d, a game development platform [22], using vuforia [23] as the ar technology. here, it was used as a printed marker in banner size (a0), as shown in figure 2. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 68 after the first test, a small ‘pocket’ marker was printed, as shown in figure 3. the app was developed after several meetings with the instructors and several practice flights with the students. the assessment was made based on how pilots perform a checklist before departure in the aircraft. a sketch was shown with the first placement of the instruments and buttons. the size of the panel was presented, and the quality of the generated 3d image was tested. the result exhibited good aesthetic quality and high definition, but still presented little interactivity. after the first phase, an enhancement was applied where various animations were created and placed in the first menu item, the idea being to present each item in the manual according to the order in which it appears from left to right, following the checklist found in the manual. for each instrument or button displayed, a green highlight appears, as shown in figure 4. this step was not planned with this type of indication, but with the feedback given in the meetings and discussions, we determined that the first step in the study of the checklist was not to start with the checklist steps. in fact, it was more interesting to present all the instruments and buttons first before moving on to the checklist, using various animations of objects and instruments. the prototype, which ended up acquiring the unexpected new feature described above, now featured the item in the menu that makes it possible to understand not only the names and functions of the buttons but also the kind of movement it performs. if this movement is a turn, it is evidenced in terms of three dimensions: whether it is, for example, 45 ⁰ or 90 ⁰, whether it is a push/pull-type button, or whether it is an on/off switch. as such, it is possible to closely visualise the movement of the mixture control, which consists of a button and a lever that must be pressed at the same time. therefore, in this second proof of concept, the goal was to demonstrate the panel in a better way than the pictures in the aircraft manual, where it is difficult to clearly observe the controls’ design, format and movements, as shown in figure 5. in short, an aircraft manual presents 2d images, making it difficult to visualise the array of controls and understand whether each is a rotate, push/pull or press button. thus, the goal was to demonstrate the panel in high definition and to include animation and interaction, as shown in figure 6. this app is very specific to a checklist approach and the panel components, which makes it different to other applications such as x-plane. the focus of our app includes a didactical figure 1. proof of concept. figure 2. a life-size panel that can be viewed at home using a smartphone and a banner printed marker. figure 3. a small-size ‘pocket’ panel that can be visualised anywhere, anytime in a ubiquitous approach. figure 4. each instrument and controls are presented to the user. figure 5. typical images presented in aircraft manuals, where full visualisation can be difficult (cessna 150 aircraft manual). acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 69 explanation for each instrument. the cessna 150 starting engine checklist [24] was used here, as shown in figure 7. 4.2. enhanced proof of concept at this point, greater interaction with the user was pursued, with the same checklist presented and the user asked to perform the movements of the lower control panel. it will only be possible to start the aircraft (start the engine) in this ar simulation if all the buttons have been executed with the correct movement and in the correct order. at the end of this step, the engine is started, and the audio will confirm the complete verification of the checklist in addition to a message. following discussions with all those involved, it was understood that these features would bring gamification to the app, as the pilot feels challenged ‘to hit’ the movements and finish the checklist perfectly, and as a reward, the engine will start. this stage is still under development, but its concept and technologies have already been finalised. flight instructors, pilots and students were involved in this experiment. 5. the experience the prototype was tested by students, pilots and instructors. the experience involved the following steps: • explaining the objectives of the study; • obtaining the participants’ consent; • reading the orientation on how to use the app; • using the app in front of a banner as a marker for the ar. the user can be standing or seated and can move in front of the panel; • using the app with a banner marker in a0 size and with a little marker printed on half an a4 page. in the first case, the user can see the ar panel in full scale (lifesize). in the second case, the user can move the little marker using their hands and manipulate the panel position so that it can be visualised comfortably; • filling out a questionnaire for reflection and sharing points of view the tests were performed with instructors, as can be seen in figure 8, while the experience also involved students preparing to gain their pp license, as shown in figure 9. 6. results the results demonstrated that the virtual 3d model is highly realistic and will prove useful for the pilot through the feature of simulating failures in the instruments in order to check whether the pilot had paid attention to the flight indicators or even whether the aircraft has deficiencies in the human interface design, which should be corrected or controlled during the flight. the prototype features various animated controls and items that promote interactive and complex tasks in different situations. the app can help the pilot to be more confident, faster and more secure when flying. meanwhile, ensuring that less time is spent on the checklist in a real aircraft or a flight simulator will reduce the costs of the process and increase the safety with ar training. furthermore, the use of ar improves the pilot’s situational awareness (s-a) [25] in perceiving, comprehending and projecting future actions in scenarios in which s-a and the figure 6. in an ar app, it is possible to identify that a specific control is not a button but a lever. the animation clearly shows how to execute the procedure. figure 7. the checklist items used in this experience (cessna150 aircraft manual). figure 8. flight instructor testing the ar app. figure 9. student testing the ar app. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 70 system’s mental models are important for minimising human error. table 1 presents example information and comments that allowed us to conclude how useful this application can be for pilots and students, and even for companies that need to improve their training resources. 7. conclusion this was merely a preliminary study, and more students, pilots and instructors will be invited to participate in future tests of the system. in addition, flight engineers and physicians will also become involved so as to increase the diversity in the evaluation of the system as part of the hcd method. it will be important to ensure that this application goes beyond the data and information that can be found in aircraft manuals and the existing procedures. it will also be crucial to improve the app such that it can capture and share the knowledge and background of qualified pilots, instructors and all stakeholders who may have novice pilots. this will help promote a complete experience, improve security, ensure better quality flights and good tolls, and will present mutually beneficial teaching methods, transferring the knowledge and experience of flying. unquestionably, this study can be useful for the aircraft industries and for professionals such as those operating in the medical industry, where medical practitioners can receive more information to support the human cognitive and health fields in terms of, for example, medical operation procedures. essentially, the ar method is independent of the field of application. acknowledgements s. k. füchter thanks the aero club of santa catarina, the instructors, claudia thofehrn and thiago bortholotti, the pilots and the students. we also thank dr orly pedroso, physician specialist in aerospace medicine, development in health, astronaut, and increased vr in cosmic space. he is accredited by the national civil aviation agency (anac), brazil; postgraduate in the course for aerospatiale medical examiners, and nasa, johnson space center, houston, texas, usa: physiology and aerospatiale medicine course. s. k. füchter also thanks the university of estácio de santa catarina center and the programa pesquisa produtividade (ppp). references [1] i. n. gleim and g. w. gleim, private pilot faa knowledge test (1st ed.), university station, 2011, pp 2. [2] j. tustain, tudo sobre realidade virtual & fotografia 3600. são paulo editora senac, 2019. [3] microsoft. microsoft flight simulator. xbox. online [accessed 18 september 2020] http://xbox.com/games/microsoft-flight-simulator [4] laminar research. x-plane 11. x-plane. online [accessed 18 september 2020] https://www.x-plane.com [5] international standards organisation (iso), iso 13407. humancentered design processes for interactive systems. online [accessed 15 september 2020] fehler! linkreferenz ungültig.https://www.iso.org/obp/ui/#iso:std:iso:13407:en [6] j. s. martinez, n. schoenstein, g. salazar, t. m. swarmer, h. silva, n. russi-vigoya, a. baturoni-cortez, j. love, d. wong, r. walker, implementation of human-system integration workshop at nasa for human spaceflight, 70th international astronautical congress iac washington d. c., usa, 21-25 october 2019, 11 pp. online [accessed 20 sept. 2020] https://ntrs.nasa.gov/api/citations/20190032353/downloads/2 0190032353.pdf table 1. users comments. profile/function comments pros cons flight instructor/ coordinator ‘the app has great potential, not only for new students studying to get their private pilot license, but also for experienced pilots when they want to upgrade and get their jet license, for example’. ‘easy, it can be replicated to other airplanes, high definition’. ‘check list could have all items, it could be more complete’. flight instructors ‘cool. we can see in detail’. ‘it is a good way to learn about the instruments and controls. good to memorise’. ‘cool. we can see in detail’. ‘it is a good way to learn about the instruments and controls. good to memorise’. ‘the prototype is just for android. various instructors use iphone’. students ‘there are illustrations in the manuals that do not allow us to understand the real movement that is made when activating a certain switch, lever, button, or selector. with ar, it is possible to visualise things as if the panel were really in front of us, and the items are easily seen’. ‘it would have been great to have had an app like this one when i was preparing myself to be a pilot and upgrading my licenses and airplanes’. ‘functional, detailed, easy’. ‘it could include more procedures’. students ‘i found it very realistic and i liked the part where i could have it at home and use the panel whenever i wanted’. ‘super. i can study at home and see the panel in detail. when i use this airplane ( i will be a new student) it will be familiar to me’. ‘nowadays, i´m learning to fly in an aeroboeiro aircraft, i would like to have one of these for this airplane’. ‘cool, portable, useful in training’. ‘the prototype is just for android. various students use iphone; the screen is too small to click on it’. aerospace physician ‘brilliant project’ ‘useful and safe in training’. http://xbox.com/games/microsoft-flight-simulator https://www.x-plane.com/ https://www.iso.org/obp/ui/#iso:std:iso:13407:en https://ntrs.nasa.gov/api/citations/20190032353/downloads/20190032353.pdf https://ntrs.nasa.gov/api/citations/20190032353/downloads/20190032353.pdf acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 71 [7] iso international standalisation organisation, documentation. online [accessed 25 september 2020] https://www.iso.org/standard/52075.html [8] nasa/sp–2015-3709 human systems integration (hsi) practitioner’s guide. national aeronautics and space administration lyndon b. johnson space center houston, texas. 2015. online [accessed 18 december 2020] https://ntrs.nasa.gov/api/citations/20150022283/downloads/2 0150022283.pdf [9] nasa/tp-2014-218556. human integration design processes (hidp). national aeronautics and space administration international space station program johnson space center houston, texas. usa, 2014. online [accessed 18 december 2020] https://www.nasa.gov/sites/default/files/atoms/files/human_i ntegration_design_processes.pdf [10] p. milgram, f. kishino, a taxonomy of mixed reality visual displays. ieice trans. inform. syst. 77 (1994), pp. 1321-1329. [11] h. schaffernak, potential augmented reality application areas for pilot education: an exploratory study. educ. sci. 2020, 10, p. 86. doi: 10.3390/educsci10040086 [12] c. kirner, and r. tori, introdução à realidade virtual, realidade misturada e hiper-realidade. realidade virtual: conceitos, tecnologia e tendências. 1 ed. sã paulo, senac, 2004, v. 1. [13] p. cipresso, i. a. giglioli, m. a. raya, g. riva, the past, present, and future of virtual and augmented reality research. front psychol (2018). doi: 10.3389/fpsyg.2018.02086 [14] s. füchter, t. pham, a. perecin, l. ramos, a. k. füchter, m. schlichting, o uso do game como ferramenta de educação e sensibilisação sobre a reciclagem de lixo. revista educação e cultura contemporânea 13(31) (2016), pp. 56-81. doi: 10.5935/2238-1279.20160022 [15] l. brown, the next generation classroom: transforming aviation training with augmented reality. in: proceedings of the national training aircraft symposium (ntas), embry-riddle aeronautical university, daytona beach, fl, usa, 14–16 august 2017. online [accessed 8 jan. 2021] https://commons.erau.edu/ntas/2017/presentations/40/?utms ource=commons.erau.edu%2fntas%2f2017%2fpresentations% 2f40utmmedium=pdfutmcampaign=pdfcoverpages [16] s. k. füchter, m. s. schlichting, utilisação da realidade virtual voltada para o treinamento industrial. revista científica multidisciplinar núcleo do conhecimento. ano 03 10(07) (2019), pp. 113-120. [17] g. zichermann, j. linder, the gamification revolution: how leaders leverage game mechanics to crush the competition. new york: mcgraw-hill, 2013. [18] aviation instructor’s handbook: 2008, department of transportation, federal aviation administration, flight standards service: oklahoma city, ok, usa, 2008. [19] s. k. fuchter, augmented reality in aviation for pilots and technicians. october 2018 xr community of practice. johnson space center – nasa. houston, 2018. [20] s. k. fuchter, pre-flight inspection and briefing for training aircraft pilots using augmented reality. ieee galveston bay section. joint meeting of yp group and new i&s section chapter. university of houston clear lake, houston, tx, usa, 2018. [21] s. k. fuchter. augmented reality as a tool for technical training. galveston bay section meeting: the institute of electrical and electronics engineers (ieee). gilruth center nasa-jsc, 17 august 2017. online [accessed 18 december 2020] https://site.ieee.org/gb/files/2017/06/ieee-gbs-081717.pdf [22] unity. documentation. online [accessed 18 september 2020] https://learn.unity.com/?_ga=2.143231477.1523753934.163121 9945-1791299178.1627660886 [23] ptc. vuforia: market-leading enterprise ar. online [accessed 22 september 2020] https://www.ptc.com/en/products/vuforia [24] cessna pilot’s operating handbook. 150 commuter cessna model 150m, 1977. [25] m. r. endsley, toward a theory of situation awareness in dynamic systems. human factors journal 37(1) (1995), pp. 32-64. https://www.iso.org/standard/52075.html https://ntrs.nasa.gov/api/citations/20150022283/downloads/20150022283.pdf https://ntrs.nasa.gov/api/citations/20150022283/downloads/20150022283.pdf https://www.nasa.gov/sites/default/files/atoms/files/human_integration_design_processes.pdf https://www.nasa.gov/sites/default/files/atoms/files/human_integration_design_processes.pdf https://doi.org/10.3390/educsci10040086 https://doi.org/10.3389/fpsyg.2018.02086 http://dx.doi.org/10.5935/2238-1279.20160022 https://commons.erau.edu/ntas/2017/presentations/40/?utmsource=commons.erau.edu%2fntas%2f2017%2fpresentations%2f40utmmedium=pdfutmcampaign=pdfcoverpages https://commons.erau.edu/ntas/2017/presentations/40/?utmsource=commons.erau.edu%2fntas%2f2017%2fpresentations%2f40utmmedium=pdfutmcampaign=pdfcoverpages https://commons.erau.edu/ntas/2017/presentations/40/?utmsource=commons.erau.edu%2fntas%2f2017%2fpresentations%2f40utmmedium=pdfutmcampaign=pdfcoverpages https://site.ieee.org/gb/files/2017/06/ieee-gbs-081717.pdf https://learn.unity.com/?_ga=2.143231477.1523753934.1631219945-1791299178.1627660886 https://learn.unity.com/?_ga=2.143231477.1523753934.1631219945-1791299178.1627660886 https://www.ptc.com/en/products/vuforia macro x-ray fluorescence analysis of xvi-xvii century italian paintings and preliminary test for developing a combined fluorescence apparatus with digital radiography acta imeko issn: 2221-870x march 2022, volume 11, number 1, 1 8 acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 1 macro x-ray fluorescence analysis of xvi-xvii century italian paintings and preliminary test for developing a combined fluorescence apparatus with digital radiography leandro sottili1,2, laura guidorzi1,2, alessandro lo giudice1,2, anna mazzinghi3,4, chiara ruberto3,4, lisa castelli4, caroline czelusniak4, lorenzo giuntini3,4, mirko massi4, francesco taccetti4, marco nervo5, rodrigo torres6, francesco arneodo6, alessandro re1,2 1 dipartimento di fisica, università degli studi di torino, via pietro giuria 1, 10125 torino, italy 2 istituto nazionale di fisica nucleare (infn), sezione di torino, via pietro giuria 1, 10125 torino, italy 3 dipartimento di fisica e astronomia, università degli studi di firenze, via giovanni sansone 1, sesto fiorentino, 50019 firenze, italy 4 istituto nazionale di fisica nucleare (infn), sezione di firenze, via giovanni sansone 1, sesto fiorentino, 50019 firenze, italy 5 centro conservazione e restauro “la venaria reale”, piazza della repubblica, venaria reale, 10078 torino, italy 6 new york university abu dhabi, division of science, p.o. box 129188, saadiyat island, abu dhabi, united arab emirates section: research paper keywords: ma-xrf; digital radiography; pigments identification; paintings citation: leandro sottili, laura guidorzi, alessandro lo giudice, anna mazzinghi, chiara ruberto, lisa castelli, caroline czelusniak, lorenzo giuntini, mirko massi, francesco taccetti, marco nervo, rodrigo torres, francesco arneodo, alessandro re, macro x-ray fluorescence analysis of xvi-xvii century italian paintings and preliminary test for developing a combined fluorescence apparatus with digital radiography, acta imeko, vol. 11, no. 1, article 6, march 2022, identifier: imeko-acta-11 (2022)-01-06 section editor: fabio santaniello, university of trento, italy received march 7, 2021; in final form december 13, 2021; published march 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this project has received funding from: the european union’s horizon 2020 research and innovation programme under the marie skłodowskacurie grant agreement no 754511 (phd technologies driven sciences: technologies for cultural heritage – t4c); infn-chnet and compagnia di san paolo (nexto project, progetti di ateneo 2017). corresponding author: alessandro lo giudice, e-mail: alessandro.logiudice@unito.it 1. introduction nowadays, the use of non-destructive non-invasive x-ray based techniques is well established in heritage science for analysis and conservation of artworks [1]-[3]. x-ray fluorescence (xrf) technique plays a fundamental role since it provides information on the elemental composition of painted surfaces, contributing to identify the materials employed in artworks. whenever xrf is combined with scanning capability on macroscopic surfaces, the technique is indicated as macro xray fluorescence (ma-xrf) [4]. conversely, due to the impossibility to transport most of the artworks inside a laboratory to undertake scientific analyses, e.g., for their preciousness or considerable weight, an important class of instruments is made up of portable and transportable scanners [5]. a number of ma-xrf scanners are nowadays in use in heritage science, both commercial [6] and built in-house [7]-[9]. despite the high analytical capabilities of the ma-xrf abstract using portable instruments for the preservation of artworks in heritage science is more and more common. among the techniques, macro x-ray fluorescence (ma-xrf) and digital radiography (dr) play a key-role in the field, therefore a number of ma-xrf scanners and radiographic apparatuses have been developed for this scope. recently, the infn-chnet group, the network of the infn devoted to cultural heritage, has developed a ma-xrf scanner for in-situ analyses. the instrument is fully operative, and it has already been employed in museums, conservation centres and out-door fields. in the present paper, the ma-xrf analysis conducted with the instrument on four italian artworks undertaking conservation treatments at the conservation centre ccr “la venaria reale” are presented. results on the preliminary test to combine dr with ma-xrf in a single apparatus are also shown. mailto:alessandro.logiudice@unito.it acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 2 technique, it is worth underlining the importance of a thorough multi-analytical approach for a better comprehension of the artworks. another well-established non-destructive non-invasive and transportable x-ray technique is the digital radiography (dr) whose potentialities are widely known [10] as a tool for conservators and art historians [11]. it is frequently used in combination with ma-xrf by means of a dedicated instrument for a more complete information of artworks, as in the case of painting on canvas and on wooden panels [12], [13]. however, the possibility to employ a single apparatus integrating xrf and dr is not yet well investigated [14]. the advantage would be to have a single x-ray tube for a straightforward combined analysis in the same area. in this work, the ma-xrf scanner [15] developed in-house by the cultural heritage network of the national institute of nuclear physics (infn-chnet) was used to analyse xvi-xvii century paintings under conservation at the centro conservazione e restauro (ccr) “la venaria reale” [16], located nearby torino. to date, the infn-chnet network gathers 18 local divisions, 4 italian partners among which the ccr “la venaria reale”, that is a second level node in the network, and international partners as the new york university of abu dhabi (uae) [17]. moreover, a flat panel detector for dr coupled with a minix-ray tube that will be used in a modified version of the infnchnet ma-xrf was tested on a painting on canvas. information obtained by means of elemental mapping and radiography were combined for a better comprehension of the realisation of the artwork. 2. experimental set-up for the measurements presented in this paper two set-up were used: the ma-xrf scanner developed by the infn-chnet group for compositional information and a mini-x-ray tube combined with a flat panel detector for dr, that will be added in a modified version of the ma-xrf scanner in the near future. 2.1. the infn-chnet ma-xrf scanner the infn-chnet ma-xrf scanner (figure 1) is a compact (60 × 50 × 50 cm3) and lightweight (around 10 kg) instrument. its main parts are the measuring head, a three axes motor stage and a case containing all the electronics for acquisition and control. the measuring head is composed by an x-ray tube (moxtek©, 40 kv maximum voltage, 0.1 ma maximum anode current, 4 w maximum power, mo anode) with a brass collimator (typically 800 µm of diameter), a silicon drift detector (amptek© xr100 sdd, 50 mm2 effective active surface, 12.5 µm thickness be window) and a telemeter (keyence ia100). the motor stage (physik instrumente©, travel ranges 30 cm horizontally (x axis), 15 cm vertically (y axis) and 5 cm in z direction) holding the measuring head is screwed on the carbonfibre case. typical operating voltage is around 30 kv. signals are collected with a multi-channel analyser (model caen dt5780) and the whole system is controlled by a laptop. the control-acquisition-analysis software is developed within the infn-chnet network and allows both an on-line and an off-line analysis. the output of the acquisition process is a file containing the scanning coordinates and, for each position, the spectrum acquired. for each map, a single element can be selected and shown in the scanned area, or in a part of it. using the raw data, for each element the relative intensities are shown in grey scale, in which the maximum intensity is in white and the lower is in black. scan is carried out on the x axis, and a step size of typically 1 mm is set on the y axis resulting in a pixel size of 1 mm2. a complete review on the instrument can be found in [15]. the instrument has already been used for a number of different applications, i.e. paintings [18]-[21], illuminated manuscripts [22], coins [23], ceramics [24], and furniture [25]. 2.2. the digital radiography set-up structural information of artworks can be obtained by a radiographic approach. although a radiography could be carried out in principle using the same x-ray tube employed in the present infn-chnet ma-xrf apparatus, for future applications a modified version with a different source will be considered. considering the higher distance from the object needed to obtain a radiography than xrf maps, and the thickness of artworks to be passed through, an x-ray tube with a slightly higher voltage and power was used. in particular, the measurements were made with a moxtek©, 60 kv x-ray tube (1 ma maximum anode current, 12 w maximum power, 0.4 mm diameter nominal focal spot size, rh anode). if not collimated, it generates a 20 cm diameter beam at about 25 cm of distance. as the present ma-xrf apparatus only has a 5 cm z travel range, the future version will be capable of a translation in z up to 30 cm to avoid the handling of the artwork between xrf and dr measurements. about x-ray imaging, a shad-o-box hs detector by teledyne, model 6k was selected. the detector contains a large active area (11,4 cm × 14,6 cm) that is fully covered by the x-ray beam at 25 cm of distance from the source; the pixel size is 49.5 µm and the maximum integration time 65 s. the video signal is digitised to 14 bits, reassembled within the camera’s fpga, and then transferred to a computer via a high-speed gigabit ethernet interface. the cmos sensor inside the detector contains a direct-contact csi scintillator, that converts x-ray photons into visible light that is sensed by the cmos photodiodes. a thin graphite cover protects the sensor from accidental damage as well as from ambient light. the shad-o-box hs camera also contains lead and steel shielding to protect its electronics from x-ray radiation. the cameras are sensitive to x-ray energies as low as 15 kev, and may be used with generators up to 225 kvp. the detector, that has already been used for x-ray imaging with conventional tubes [26], is part of the nexto project that has the aim to integrate ma-xrf, dr and x-ray luminescence (xrl) [27] in a single portable instrument. figure 1. infn-chnet ma-xrf scanner placed in front of the panel painting madonna e i santi by cristoforo roncalli, known as il pomarancio. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 3 3. applications at the ccr “la venaria reale” in this section different applications of the instrumentation on paintings are presented. the works of art represent case studies from italian central regions of different periods (from the beginning of the 16th to the beginning of the 17th century). they were analysed during conservation processes carried out at the ccr “la venaria reale” [28]. for the ma-xrf measurements, a collimator of 800 µm diameter was used. the vertical step was set to 1 mm and a scanning speed of 3 mm/s. furthermore, the keyence ia-100 telemeter was switched on to maintain the sample distance during the scanning process. 3.1. madonna di san rocco by francesco sparapane the first painting presented is the oil on panel madonna di san rocco depicting the virgin with the child, saint antonio from padua and saint rocco by francesco sparapane (preci, umbria region, 1530 ca.). the importance of the work of art is related to the lack of documented paintings by the author, thereby its study represents a key-feature for understanding the painting technique of the artist [29]. the ma-xrf measurements were conducted on two areas as shown in figure 2, from which a number of maps were created. the source voltage was set to 30 kv and its anode current to 20 µa. the maps around saint rocco (figure 3) show the use of lead white, most likely due to the imprimatur layer and as proper pigment in the flesh tones. from the map of copper, the green part of the hat was realised with copper-based compounds [30]. the presence of tin is also detected in this same region, although in moderate amounts, and might be due to the use of lead-tin yellow in mixture with the copper-based pigment. a more precise identification of the material cannot be made with xrf technique: for instance, it is not possible to distinguish between a mixture of tin-based yellow with malachite rather than with azurite [31]. the shadows of the flesh tones were realised with ochreearths, as can be inferred from the match between iron and manganese maps [32]. corresponding to the red tone in the checks, a high signal of mercury is present, most likely due to the use of vermilion-cinnabar (hgs) [33]. furthermore, calcium is present in the strings of the hat, the dark strips and in the eyes, that may indicate the use of bone black for darkening [34] as well as manganese in the same areas may indicate the use of manganese black [35]. the halo was made with gold (figure 3), while the corresponding presence of calcium and iron is probably due to a calcium/iron-based preparation, as discussed in [36]. the second area around the upper part of the head of s. antonio (figure 4) presents similar results. however, a marked difference is related to the sky, that is made with a copper-based compound (most likely azurite [31]) with a glaze realised with smalt, a material rarely used as a pigment from the 15th century and which was widespread from the 17th c. onwards. concisely, smalt is a blue potash glass (thus characterised also by presence of potassium and aluminium) where the chromophore is cobalt and it usually contains impurities, among others, of bismuth when produced after 1520 [37]. its presence is thus hypothesised by the maps of the corresponding elements. a similar palette was probably used for the sky in the first area; however, due to the bad conservation condition, only traces of the characteristic elements are present in the maps of copper and cobalt. 3.2. madonna con bambino e santi by pomarancio the oil on canvas madonna con bambino e santi by cristoforo roncalli, known as il pomarancio, was made in the first decade of the 17th century, and it is placed in the santa maria argentea church in norcia (umbria region, italy). the virgin and the child are depicted with the saints eutizio, fiorenzo, santolo, and spes. figure 2. painting madonna con bambino e s. antonio e s. rocco by francesco sparapane. the scanned areas are indicated in the white boxes. figure 3. ma-xrf maps of area 1 of the madonna di san rocco by francesco sparapane (size 280 mm × 70 mm). figure 4. ma-xrf maps of the area 2 of the madonna di san rocco by francesco sparapane (size 120 mm × 70 mm). acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 4 the focus of the analysis was the painting palette used in the flesh tones by the author [38], of which two representative areas were scanned, as shown in figure 5. the source voltage was set to 28 kv and its anode current to 20 µa. the maps realised in the first area (figure 6) show the use of lead white for the flesh tones and the book, while a high signal of iron is present corresponding to the shadows. the blue cope of saint fiorenzo shows an intense signal of copper, due to a copper-based compound, leading to the hypothesis of azurite [31]. the darkest colour is related to a high signal of calcium, that, by only means of the xrf technique, cannot lead to a precise hypothesis on the material used. furthermore, the presence of tin was detected in the squiggles as well as a higher intensity of lead, most likely due to the use of lead-tin yellow [32]. the second area (figure 7) shows a different composition: the map of mercury matches with the hand, leading to the hypothesis of vermilion-cinnabar for the glove. as opposed to the previous area, the map of iron does not show an intense signal in the hand of saint spes. the main signal of iron comes from the stick and the cope in correspondence to the yellow colour. by comparing the maps of iron, manganese, mercury, and tin, it can be noted that all of them are present in the crosier, even if tin and mercury are in the highlights, whereas the manganese and iron are in the shadows. this result can be explained by the use of vermilioncinnabar mixed with lead-tin yellow [32] in the highlights, and the use of ochre-earths [32] in the shading. iron, manganese, and tin are also present in the yellow cope. furthermore, iron and manganese are present in the green medallion, which present a strong signal from copper, related to copper-based pigments. from the map of copper, it may be seen that all the green colours in the area are related to its presence. however, as in the hat of saint rocco seen in the previous section, it is not possible to hypothesise a conclusion on the material used. 3.3. adorazione dei magi by sante peranda the oil on canvas adorazione dei magi by sante peranda (figure 8) is dated around the first decade of the 17th century. in this case the interest was focused on the blue colours. the measurements were conducted in three areas: one on the robe of the virgin, one behind the magus on the far left wearing the white dress, and the last one behind the kneeling magus. the composition detected is different for each area. the source voltage was set to 28 kv and its anode current to 30 µa. in the first area, the virgin’s robe (figure 9), cobalt is present. as in the previous sections, this may suggest the use of smalt as figure 5. painting madonna con bambino e i santi by il pomarancio. the scanned areas are indicated in white boxes. the saints are, from left, s.eutizio, s. fiorenzo, s. santolo, and s. spes. figure 6. maps of the area 1 of madonna con bambino e i santi by il pomarancio (size 130 mm × 110 mm). figure 7. ma-xrf maps of area 2 of madonna con bambino e i santi by il pomarancio (size 150 mm × 137 mm). acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 5 blue pigment [37]. the match between the cobalt and the silicon maps most likely indicates the use of such blue glass in this area. furthermore, the localised lack of these two elements in the area is related with the presence of a conservation intervention with a titanium-based material [32]. in the second area, shown in figure 10, the composition of the blue is similar to the previous, despite the presence of a significant iron signal, probably due to the use of ochre-earths for the shading [34]. however, with this technique alone a later retouch with prussian blue cannot be excluded [39]. beside the map of cobalt, the map of bismuth is also reported to confirm the hypothesis of smalt and, consequently, the dating of the painting [34]. moreover, it can be seen that the lα-line of lead (10.55 kev) is detected in the whole area, whereas the m-lines (2.34 kev) are present only in the robe. this is due to the different absorption for different x-ray energies (the lower is the energy and the higher is the absorption), therefore the comparison of the two maps suggests that lead white was used for the imprimatur, as well as for the white robe of the magus on the far left. a different composition is detected in the last blue area (figure 11). in this case a strong signal of copper is present, whereas no presence of cobalt was detected. for this reason, conversely to the previous cases, the use of azurite can be hypothesised for the blue tone in the area. it is also clearly visible from the map of copper its presence beneath the yellow robe, probably due to a pentimento in the back of the kneeling magus. the last hypothesis can be also supported by the maps of lead, in which its use up on the copper can be hypothesised by the detection of the m-line only in the region of the robe, whereas the rest of the area shows only the l-lines of lead. the yellow robe shows the presence of iron and lead, which may suggest a combined use of white lead and yellow ochreearths [31]. the hair of the servant present in the area shows a signal of iron and manganese, probably due to the employment of ochreearths. 3.4. madonna con bambino ed i santi crescentino e donnino by timoteo viti the last painting presented is the madonna con bambino ed i santi crescentino e donnino by timoteo viti (figure 12), dated between 1500 and 1510. the work is a tempera on canvas, its size is 168 cm × 165 cm. the painting presented bad conservation conditions on the areas around the faces of the virgin and the child. the painting technique is tempera magra [40], in which the binder tends to be absorbed by the preparatory layer. moreover, the application of a protective varnish was not envisaged, leaving the paint in direct contact with the external environment. figure 8. adorazione dei magi by sante peranda. the scanned areas are indicated in white boxes. figure 9. maps of area 1 in the robe of the virgin in figure 8 (size 100 mm × 100 mm). figure 10. maps of area 2 in the robe of the magus wearing the white dress in figure 8 (size 85 mm × 40 mm). figure 11. maps of area 3 in the blue robe behind the kneeling magus in figure 8 (size 70 mm × 65 mm). acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 6 the source voltage was set to 28 kv and the anode current to 30 µa. the maps of the area around the child’s head are presented in figure 14. as can be seen from figure 13, the flesh tones are characterised by a strong signal of calcium, whereas no evidence of the use of lead was detected. moreover, the shading was made most likely with ochre-earths, according to the map of iron. in addition, the highlights of the mouth and the cheeks show a signal of mercury, most likely due to the use of cinnabarvermilion. the high presence of calcium can be explained with the use of white of san giovanni (white lime) pigment or other calcium-based compounds [41]. furthermore, by creating a spectrum in the area of the face, and comparing it with a spectrum obtained outside (figure13), it can be noted a higher intensity of the 2.0 kev line compared with the kα of calcium, that can be explained with the presence of phosphorus in the flesh tone. this may be explained with the presence of bone black, a pigment used for shading [34]. the signal of lead is present in the hair of the child. furthermore, the match of the spatial distribution of tin with lead may indicate the use of lead-tin yellow for it. the landscape of the background was realised with copperbased compounds mixed with ochre-earths, while the halo was realised with gold. in addition to the ma-xrf measurements, a radiographic investigation was carried out in the same area using the set-up described in section 2.2. the voltage was set to 20 kv, the anode current to 0.6 ma and the integration time to 2 seconds. as shown in figure 15, for example in jesus christ’s hair, the image is more detailed than the ma-xrf map: this allows the visualization of warp and weft threads of the canvas. moreover, it can be observed to match with the ma-xrf map distributions of the heavy metals (pb, sn, au, cu), and only partially with the distribution of calcium. this result is due to the very thin thickness of the painting layer, typical of the tempera magra painting technique. 4. conclusions the infn-chnet ma-xrf scanner was applied on four italian paintings at the ccr “la venaria reale”. for each application, different queries were advanced during the conservation processes and the described analysis achieved important information on the painting layers. in the madonna di san rocco by francesco sparapane, the figure 12. madonna con bambino e i santi crescentino e donnino by timoteo viti. the results presented are from the area in the white box. figure 13. comparison between two spectra, one obtained selecting an area inside the face (black) and outside (red). figure 14. ma-xrf maps of the area around the face of jesus christ in madonna ed i santi crescentini e donnino painting (size 140 mm × 110 mm). figure 15. radiography of the area around jesus christ’s face. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 7 composition of the flesh tones of s. antonio and s. rocco were identified, even though a definite composition cannot be measured, due to the limitation of the xrf technique to detect elements lighter than sodium. a similar conclusion has been made for other parts of the painting (the sky and s. rocco’s clothes). in the madonna con bambino e i santi by il pomarancio, the wide painting palette (lead white, lead-tin yellow, cinnabar-vermilion, copper-based compounds) was measured, confirming the skills of the author. for the adorazione dei magi by sante peranda, the blue colours in the areas under study have shown different compositions. it is worth noting that a more precise identification of the materials employed in the painting layers is not possible with only xrf technique, but a further investigation with other techniques such as fiber optics reflectance spectroscopy (fors) or raman spectroscopy is needed. in the last painting, the presence of calcium-based white in the face of the child was detected. however, no signal of lead is present in that area, whereas it is present in the background. in addition, dr was conducted in the same area using a new set-up that was proven to be suitable to be combined with xrf in a single instrument. the test carried out at the ccr “la venaria reale'' is the first step for the development of this multitechnique device. the complete realisation will rely on the expertise of the infn-chnet group, which has already allowed to achieve several important technological results in heritage science applications [42]-[46]. acknowledgement this project has received funding from compagnia di san paolo (nexto project, progetto di ateneo 2017) and the infn-chnet project. this project has received funding from the european union’s horizon 2020 research and innovation programme under the marie skłodowska-curie grant agreement no 754511 (phd technologies driven sciences: technologies for cultural heritage – t4c). the authors wish to warmly thank marco manetti of the infn-fi for his invaluable technical support, the students giulia corrada, francesca erbetta, daniele dutto, giulia dilecce and their supervisors, prof.ssa gianna ferraris di celle, prof. alessandro gatti, prof. bernadette ventura, prof. antonio iaccarino idelson and the staff of the ccr “la venaria reale” for its support. references [1] v. gonzalez, m. cotte, f. vanmeert, w. de nolf, k. janssens, x‐ ray diffraction mapping for cultural heritage science: a review of experimental configurations and applications, chem. eur. j., 2020, 26, 1703. doi: 10.1002/chem.201903284 [2] m. p. morigi, f. casali, radiography and computed tomography for works of art, in handbook of x-ray imaging: physics and technology, editor p. russo, boca raton: crc press, 2018, pp. 1185-1210. 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dip coating; ethanol sensors; concentration evaluation in liquids; pmma coating citation: antonino quattrocchi, roberto montanini, mariangela latino, nicola donato, pmma-coated fiber bragg grating sensor for measurement of ethanol in liquid solution: manufacturing and metrological evaluation, acta imeko, vol. 10, no. 2, article 19, june 2021, identifier: imeko-acta-10 (2021)-02-19 section editor: ciro spataro, university of palermo, italy received january 18, 2021; in final form april 30, 2021; published june 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: antonino quattrocchi, e-mail: antonino.quattrocchi@unime.it 1. introduction fiber bragg grating (fbg) sensing is a mature technology that has been used so far to develop measurement solutions for a wide range of applications. currently, they are commonly employed for health structural monitoring, as well as to fabricate distributed sensing devices to be used in different fields such as medicine and biomedical research, automotive, aerospace, maritime and civil engineering, and many others. fbgs are largely preferred over other types of optical sensors [1] thanks to their unique advantages. they present small sizes, immunity from electromagnetic disturbances, good resistance in terms of temperature and humidity and, above all, they are wavelength encoded and chemically inert. for these reasons, they often represent the favorite choice for applications in critical environments, e.g. explosive or radioactive [2]. the sensitivity of fbg to the variation of a specific parameter can be improved by choosing a suitable coating material. the basic principle relies in the swallowing of the coating material which eventually induces a change in the grating period and in the refractive index along the fiber length. several types of materials and deposition techniques can be used to create a fiber coating for a specific application. fbg-based humidity sensors has been realized by dip coating the fiber in a polyamide solution [3], by depositing a layer of graphene oxide on the fiber surface [4] and by embedding the fbg in a steel and carbon fiber reinforced polymer [5]. montanini et al. [6] compared results obtained on two polymer-coated fbg humidity sensors, using poly(methyl methacrylate) (pmma) and poly(vinyl acetate) (pvac) as sensing materials. the calibration of the two sensors was carried out in the 20 70 %rh range at three different temperatures (15 °c, 30 °c and 45 °c) by a two-pressure humidity generator. the pmma-coated sensor prototype displayed overall much better performance when compared with abstract the sensitivity of fiber bragg gratings (fbgs) to the variation of a specific parameter can be improved by choosing a suitable coating material. this paper explores the possibility of measuring the concentration of ethanol in aqueous solutions by using poly(methyl methacrylate) (pmma) as coating material of a single-mode fbg. the basic measurement principle relies in the absorption of liquid ethanol which produces controlled swallowing of the coating, straining the underneath bragg grating. the pmma coating was deposited on the fbg by means of a specifically designed dip coating test bench, able to accurately control the thickness of the applied layer. metrological performances of the developed pmma-coated fbg ethanol sensor have been assessed for ethanol concentrations in the range of 0-38 % in distilled water at a constant temperature of 25 °c. the prototype shows good repeatability and better sensitivity when compared to a traditional fbg, especially at low ethanol concentrations. the dynamic behavior of the sensor (which depends on the kinetics of the absorption mechanism) has been assessed as well, showing a response time of about 290 s, while the recovery time (660 s) was almost twice. mailto:antonino.quattrocchi@unime.it acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 134 the pvac-coated one, although it manifested a higher temperature sensitivity. instead, fbg-based cryogenic thermometers were developed using coatings of metals (copper, zinc, tin and lead) for the fiber by electroplating and casting [7], by bonding the fbg onto teflon substrates [8] and by placing a layer of ormocer on the fiber surface by gravity-coating [9]. another specific application was developed by campopiano et al. [10]. the authors produced several types of hydrophones, using plastic-coated fbgs. they evaluated how the use of polymers, which exhibits great variability in terms of obtainable geometries and acoustic and mechanical properties, guarantees the achievement of customized performances. an important field of application is that of the chemical industry. cong et al. [11] proposed an optical salinity sensor based on a hydrogel-coated fbg. the detection mechanism relies on the mechanical stress induced into the coating material when water comes out of it. the main limitation of this device lies in the slow response time due to the balance between the forces acting at the intermolecular level of the coating material. aldaba et al. [12] investigated a ph sensor consisting of a polyaniline layer polymerized on the surface of a fbg. in this case, the ph detection occurs thanks to the interaction between the optical absorption spectrum of polyaniline, which is sensitive to ph, and the bragg grating. the device exhibited fast response, biochemical compatibility, temperature independence and longterm stability. dinia et al. [13] proposed a fbg multifunctional ph sensor, covered with a ph-sensitive hydrogel, to monitor the ph rain on critical and prestigious monuments. ethanol is a widely used chemical in many fields, such as medical, food processing and automotive [13]. however, because of its high volatility and flammability, ethanol can be dangerous for living organisms and, therefore, it is very important to have disposal effective detection systems [15]. several sensor typologies are available, basing on transduction effect, spanning from resistive sensors [16], electrochemical [17], gas and resonant ones [18, 19]. in contrast, fbg represents an intrinsically safe device, which can be profitable used to measure ethanol concentration either in vapor or liquid phase. morisawa et al. [17] analyzed a plastic optical fiber sensor, coated with a layer of novolac resin, for detecting alcohol vapors, including ethanol. they putted into evidence that the transmitted light intensity is a function of the coating thickness, of the exposure time, of the alcohol type and of the vapor pressure. latino et al. [21] presented some preliminary results concerning the assessment of low concentration of ethanol (0-3 %) in aqueous vapor, using a fbg coated with thin layers of pmma with different thicknesses. in this case, pmma was distributed onto the fbg surface with a simple dip coating technique. coradin et al. [22] evaluated the metrological performance of four fbgs, assembled in several configurations, for analysis of water-ethanol mixtures. raikar et al. [23] employed a ge-b doped fbg to show as the wavelength shift decreases with the increase in liquid solution concentrations, moving from 0 % to 50 %. arasu et al. [24] developed a fbg coated with a thin gold film deposited by sputtering. their results displayed as the concentration of ethanol in water from 0 % to 100 % is better assessed measuring the change in absorbance with the thickest gold layer. recently, kumar et al. [25] analyzed a graphene oxide-coated fbg for ethanol detection in petrol. this sensor was manufactured covering a partially chemical etched fbg with a layer of an oxidize graphite powder by means of a drop casting technique. the detection of ethanol proportion in petrol, from 0 to 100 %, was investigated and a calibration curve was obtained. the main result shows as the graphene oxide-coated fbg allows a significant enhancement (by a factor of ten) in the detection of ethanol in comparison with un-coated fbg. in this paper, which is an extended version of the one presented at the 24th imeko tc4 international symposium and the 22nd international workshop on adc and dac modeling and testing (imeko tc4 2020) [26], starting from our previous experience gained with pmma-coated fbgs designed for sensing ethanol concentration in aqueous vapor [21], we aim to explore the possibility to measure the concentration of ethanol in liquid solutions too. the performance of the pmma-coated fbg has been investigated by evaluating its response to ethanol concentration at a constant temperature of 25 °c and a calibration curve was obtained and compared with that of a traditional fbg. the main metrological features of the prototype, in terms of sensitivity and response/recovery time, have also been assessed and discussed. 2. material and methods 2.1. fbg ethanol sensor manufacturing the fbg ethanol sensor has been realized by recoating a single-mode silica fbg with a thin layer of pmma. the device has an active length of the bragg grating of about 14 mm and a total thickness of (0.44 ± 0.03) mm. this coating thickness value was chosen in order to find the best balance between sensing properties and stability ones. the fbg is of the smf-28 type with a diameter of 250 µm, of which 9 µm of core, 125 µm of cladding and 250 µm of an acrylate coating, a reflectivity > 90 % and a full-width at half maximum (fwhm) < 0.30 nm. figure 1 reports a magnification of the sensor structure. the measurement principle relies on the selective absorption of ethanol by the coating layer made of pmma. this interaction causes the swelling of the sensing polymer and a consequent mechanical effect on the fbg, which produces a wavelength shift related with the ethanol concentration in the liquid solution. in order to develop the fbg ethanol sensor, the pre-existent coating of the single-mode optical fiber was removed, then, the pmma deposition was carried out by a controlled dip coating procedure. the fbg was dipped under controlled conditions (constant dipping/rising velocity) in a solution of 560 mg pmma figure 1. fbg ethanol sensor with pmma sensing layer. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 135 and 3 ml of acetone. the block diagram of the deposition system is depicted in figure 2. it consisted of a precision dc servo motor, equipped with an agilent e3632a power supply, an adaptor, and a stripe holder where the sample was held. by means of the power modulation delivered to the servo motor, it was possible to control the rising/dipping rate of the fbg in the pmma-acetone solution. figure 3 reports the relationship between the voltage applied to the servo motor and the resulting dipping rate, showing a clear linear trend. hence, by simply varying the rising/dipping rate, it was possible to accurately control the thickness of the coating layer. 2.2. measurement setup the measurement setup consisted of an optical spectrum analyzer (osa) (micron optics mod. si 720) with ± 3 pm accuracy, wired to the fbg ethanol sensor and to a pc. a dedicated graphical user interface, developed in labview™ environment, was implemented to display in real time and to store the data, using a national instruments gpib-usb-hs ieee-488.2 communication protocol. the fbg ethanol sensor was placed into a cylindrical metallic container, filled with the target solution of distilled water and ethanol. the temperature of the metallic container was kept constant through a thermostatic bath, with the aim to minimize the temperature dependence of the fbg and better relate the sensor response with the ethanol concentration. figure 4 illustrates a block diagram of the measurement setup. 3. results 3.1. response of the fbg ethanol sensor figure 5 shows the power spectra of the fbg ethanol sensor obtained for three different concentrations of ethanol in water at a constant temperature of 25 °c. the peak of each curve shifts to the right as the ethanol concentration increases, while the maximum amplitude of the power can be considered sufficiently constant. this highlights how the interaction between the pmma coating and ethanol results only in a shift of the wavelength of the bragg grating, without substantial modification of the shape of the reflected signal. hence, the fbg sensor response at constant temperature can be defined considering the relative shift of the bragg wavelength δλ with respect to the baseline wavelength in distilled water λh 2 o (i.e. 0 % v/v of ethanol, reference temperature t0 of 25 °c): ∆𝜆 = 𝜆r − 𝜆h2o, (1) being λr the actual wavelength of the fbg sensor as it is exposed to a definite concentration of ethanol in distilled water. figure 6 displays the fbg response as it is repeatedly immersed firstly in distilled water and eventually in a water/ethanol solution of 7.89% v/v at a constant temperature of 25 °c. the obtained data highlight a large variation of the fbg wavelength: in air, the wavelength λ0 is 1554.73 nm and it rapidly increases to 1555.99 nm (λh 2 o) as the fbg ethanol sensor was immersed in distillated water. this occurs because the pmma layer hydrates in contact with water, and it begins to expand until a steady state condition has been reached. as the sensor is exposed to the water/ethanol solution, ethanol is absorbed by the pmma layer and the swelling process restarts until a new steady state condition is gained. the described interaction between pmma and figure 2. block diagram of the deposition setup. 1 2 3 4 5 6 7 8 9 2 4 6 8 10 12 14 16 v e lo c it y ( m m /m in ) supply voltage (v) y = -0.0870 + 1.8811 x r 2 = 0.9982 figure 3. relationship between the supply voltage to the dc servo motor and the resulting dipping rate. figure 4. block diagram of the measurement setup. 1553 1554 1555 1556 1557 1558 -40 -30 -20 -10 0 h 2 o 2.78% 14.63% 32.69% p o w e r (d b m ) wavelength (nm) l h 2 0 = 1554.99 nm coating: pmma reference: h 2 o (t h 2 o = 25 °c) target: ethanol (t ethanol = 25 °c) figure 5. power spectra of the fbg ethanol sensor for different concentrations of ethanol in pure water. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 136 ethanol is reversible, since the baseline is reached when the sensor is exposed to distilled water. then, a proper calibration procedure allows to evaluate ethanol concentration in the whole characterization range. figure 7 presents an example of data recorded in real-time, keeping the temperature constant at 25 °c and varying the ethanol concentration from 2.78 % v/v to 5.40 % v/v, triggering the osa on the maximum marker. even though the fbg ethanol sensor response is subject to a transient before reaching the steady state condition, it is evident that a greater value of λr is correlated to an increase of ethanol concentration. furthermore, the measurement process is reversible since the acquired wavelength coincides with the basic one (λh 2 o = 1554.99 nm), when the fbg ethanol sensor is placed into distilled water. figure 8 puts into evidence the response of the fbg ethanol sensor by increasing ethanol concentrations at a constant temperature of 25 °c. it can be clearly seen how an increasing in ethanol concentration brings to a corresponding increasing of the lr in the whole ethanol investigation range. 3.2. metrological evaluation of the fbg ethanol sensor in this section will be evaluated the metrological performance of the fbg ethanol sensor, such as repeatability and the calibration curve. figure 9 reports the repeatability of the fbg ethanol sensor, when it is exposed to three pulses of 7.89 % v/v of ethanol and again to distilled water at a constant temperature of 25 °c. the repeatability was estimated at an ethanol concentration of 7.89 % v/v and for the distilled water (base line); here are reported a mean value of 0.075 nm with a standard deviation of 0.005 nm, and 0.071 nm and 0.003 nm, respectively. therefore, peak values of λr and λh 2 o show a good repeatability and reversibility. figure 10 a) compares the calibration curves of the fbg and the pmma-coated fbg (i.e. the fbg ethanol sensor) for ethanol concentrations ranging from 0 to 38 % v/v at a constant temperature of 25 °c. it can be seen a linear trend for the fbg vs. ethanol concentration. this is probably due to ethanol absorption by the acrylate coating of the fbg itself. on the other hand, the pmma-coated fbg exhibits more complex behavior. in the latter case, although a linear branch can be observed for low concentration values, a polynomial curve is able to fit the data in the whole ethanol range. a such nonlinear trend could be caused by the binding forces between the pmma molecules, which could then limit the swelling of the coating layer (i.e. the ethanol absorption) above a defined threshold. however, the sensitivity of the pmma-coated sensor is much greater (more than one order of magnitude) than the fbg one. while figure 10 b) reports the residual values of both the previous fitting curves. 0 2000 4000 6000 8000 10000 1554.70 1554.75 1554.80 1554.85 1554.90 1554.95 1555.00 1555.05 1555.10 l r ( n m ) time (s) l 0 = 1554.71 nm l h 2 0 = 1554.99 nm coating: pmma reference: h 2 o (t h 2 o = 25 °c) target: 7.89 % v/v ethanol (t ethanol = 25 °c) in a ir in h 2 o figure 6. different response of the fbg ethanol sensor in air, into distilled water and exposed at a fixed ethanol concentration (7.89 % v/v). 3000 4000 5000 6000 7000 8000 9000 1554.85 1554.90 1554.95 1555.00 1555.05 1555.10 in h 2 oin h2o 5.40% 2.78% in h 2 o l r ( n m ) time (s) l 0 = 1554.73 nm l h 2 o = 1554.99 nm coating: pmma reference: h 2 o (t h 2 o = 25°c) target: ethanol (t ethanol = 25°c) figure 7. wavelength shifts after exposition of the fbg ethanol sensor to different concentrations of ethanol. 19500 20000 20500 21000 21500 22000 22500 23000 1554.95 1555.00 1555.05 1555.10 1555.15 1555.20 1555.25 l r ( n m ) time (s) l 0 = 1554,73 nm l h 2 o = 1554.99 nm coating: pmma reference: h 2 o (t h 2 o =25°c) target: ethanol (t ethanol =25°c) 38% 13% 27% 33% 21% enhance ethanol concentration in h 2 o figure 8. response of the fbg ethanol sensor at progressive ethanol concentrations. 2500 5000 7500 10000 12500 15000 17500 20000 1554.80 1554.85 1554.90 1554.95 1555.00 1555.05 1555.10 l r ( n m ) time (s) l 0 = 1554.73 nm l h 2 0 = 1554.99 nm coating: pmma reference: h 2 o (t h 2 o = 25 °c) target: 7.89% ethanol (t ethanol = 25 °c) figure 9. repeatability of the fbg ethanol sensor exposed at a fixed ethanol concentration of 7.89 % v/v. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 137 figure 11 displays the first derivatives of the calibration curves towards time, for both the fgb and the pmma-coated one. these curves give the sensitivity of the devices, highlighting that the pmma coating leads to a significant increase of the fbg sensitivity to ethanol. it is also noteworthy to observe that the sensitivity of the pmma-coated fbg is not constant, but it strongly depends on the specific ethanol concentration. on the other hand, the sensitivity of the fbg can be considered constant. furthermore, the acrylate coating of the fbg has a considerably lower absorption of ethanol compared to the pmma coating, as already highlighted by figure 10 a). 3.3. behavior of the fbg ethanol sensor the fbg ethanol sensor exhibits a dynamic behavior as a function of the kinetics of the swelling phenomenon of the pmma layer. the typical values of the response and the recovery time are identified in figure 12 for an ethanol concentration of 7.89 % v/v at a temperature of 25 ° c. these parameters are calculated as the time to reach the 90 % of the final value, when the sensor is exposed to ethanol, and the time to reach the 90 % of the baseline wavelength, when the sensor is exposed to distilled water, respectively. the response time value is of 290 s, while the recovery one is of 660 s. by considering these results, it is possible to note how the sensing of ethanol (i.e. the absorbing process) is faster than the desorbing one (the process to reach again the baseline wavelength in presence of only distilled water). 4. conclusions this paper describes the manufacturing process and the metrological characterization of a fbg-based ethanol sensor obtained by covering the bragg grating with a layer of pmma. the obtained device is able to evaluate the different ethanol concentrations in distilled water at a constant temperature of 25 °c. in fact, the measurement data have shown a clear shift in the wavelength of the bragg grating, strictly related to the increasing in ethanol concentration. the fbg ethanol sensor identifies specific and progressive concentrations of ethanol, showing good repeatability. its calibration curve highlights a polynomial trend that easily describes the whole range of considered concentration (from 0 % to 38 % in v/v of ethanol in distilled water). furthermore, compared to the calibration curve for fbg, better sensitivity has been achieved, especially for the low concentration of ethanol. finally, dynamic behavior for a fixed ethanol concentration (7.89 %) has also been assessed. in this case, the response time (290 s) is more than half of the recovery one (660 s). future work will be focused on the cross-sensitivity investigation of the analyzed pmma-coated fbg, in order to evaluate the selectivity of the device to other chemical compounds. 0 5 10 15 20 25 30 35 40 0 50 100 150 200 250 fbg pmma-coated fbg linear fit of fbg polynomial fit of pmma-coated fbg d l (p m ) ethanol (% v/v) y = -0.2244 + 0.6389 x r 2 = 0.9991 y = 6.2643 + 9.2300 x 0.1169 x 2 r 2 = 0.9932 l 0 = 1554.73 nm l h 2 o = 1554.99 nm reference: h 2 o (t h 2 o = 25°c) target: ethanol (t ethanol = 25°c) a) 0 5 10 15 20 25 30 35 40 -12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12 linear fit of fbg polynomial fit of pmma-coated fbg r e s id u a l o f d l (p m ) ethanol (% v/v)b) figure 10. a) comparison between the calibration curves of the fbg and the pmma-coated one and b) residual analysis of the fittings. 0 5 10 15 20 25 30 35 40 0 5 10 15 y = -0.0001 + 0.6479 x y = -0.1993 + 8.2098 x fbg pmma-coated fbg linear fit of fbg linear fit of pmma-coated fbg d d l /d t (p m /s ) ethanol (% v/v) l 0 = 1554.73 nm l h 2 0 = 1554.99 nm reference: h 2 o (t h 2 o = 25°c) target: ethanol (t ethanol = 25°c) figure 11. sensitivity vs. ethanol concentration for fbg and for pmma-coated one. 3000 4000 5000 6000 1554.90 1554.95 1555.00 1555.05 1555.10 recovery in 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non-contact measurement; low-cost technology; customised orthopaedic brace; non-collaborative patient; multimodal approach; 3d printing citation: iva xhimitiku, giulia pascoletti, elisabetta m. zanetti, gianluca rossi, 3d shape measurement techniques for human body reconstruction, acta imeko, vol. 11, no. 2, article 33, june 2022, identifier: imeko-acta-11 (2022)-02-33 section editor: francesco lamonaca, university of calabria, italy received december 20, 2021; in final form march 15, 2022; published june 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: iva xhimitiku, e-mail: vaxhimitiku@gmail.com 1. introduction biomedical applications often required device customization: as well known, no patient is identical to another one, and this is even truer referring to pathologic conditions. in the past, customization has often been sacrificed in favour of manufacturability, however, with the advent of 3d printing [1], this shortcoming is being overcome [2], [3], and more and more emphasis is being given to the necessity of providing fast and accurate systems to obtain the geometry of the whole body [4], [5], [6] or of specific body segments [7]. traditional techniques are based on plaster moulds and are affected by some major limitations such as: the invasiveness, the need to keep the patient still for the curing time [8], a limited accuracy (over 15 mm, according to[9], [10]), and the impossibility of acquiring undercut geometries. more recently, and as a viable alternative, various non-contact instruments have been developed in order to perform digital scanning [11], [12], [13] and the respective performances have been extensively reported in literature [14], [15]. however, the application introduced in this work was somehow peculiar due to the young age of the patient [16] which led to add some requirements to the scanning methodology that are a time limit to perform the whole acquisition, and the possibility to compensate motions since the patient was not collaborative due to his young age [17], [18]. the final aim was to obtain the 3d geometry of his trunk in order to gather input data for brace design [19]. prior attempts had been made with traditional moulding techniques and they did not succeed due to frequent patient movements [20], [21], [22]. a specific methodology has been here developed, tested and discussed, which is based on a multimodal approach [23] where the benefits of different scanning technique are merged in order to optimize the final result. in section 2, three common scanning techniques are briefly described, reporting their specifications, and highlighting the respective advantages and disadvantages in relation to their application to human body scanning. these technologies are photogrammetry, light detection and ranging (lidar) and structured light scan. the performances of these shape measurement techniques have been assessed reconstructing the torso of two adults (one abstract in this work the performances of three different techniques for 3d scanning have been investigated. in particular two commercial tools (smartphone camera and ipad pro lidar) and a structured light scanner (go!scan 50) have been used for the analysis. first, two different subjects have been scanned with the three different techniques and the obtained 3d model were analysed in order to evaluate the respective reconstruction accuracy. a case study involving a child was then considered, with the main aim of providing useful information on performances of scanning techniques for clinical applications, where boundary conditions are often challenging (i.e., non-collaborative patient). finally, a full procedure for the 3d reconstruction of a human shape is proposed, in order to setup a helpful workflow for clinical applications. mailto:vaxhimitiku@gmail.com acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 2 male and one female); the main objective of this first analysis was to evaluate the performances of two low-cost tools [16], [24], [25] (smartphone camera and ipad pro lidar), in relation to the accurate reconstruction obtainable with the structured light scanner, used as reference measurement system [26]. once the performances of these tools have been defined under ‘ideal’ scanning conditions (collaborative subject able to maintain a position throughout the scan process), the same techniques have been used to obtain a set of 3d scans of a 4-year-old boy’s torso, at an orthopaedic laboratory (officina ortopedica semidoro srl, perugia, italy). for both these analyses, the process of 3d reconstruction and structure extraction is described in detail in section 5. the accuracy and correlation among the geometries reconstructed with different visual devices, are evaluated and discussed, and the bias given by a non-collaborative patient is illustrated, leading to introduce a new methodology based on a multimodal approach, whose benefits are outlined and quantified. in section 6, it is demonstrated how this methodology can be applied in orthopaedics [1], [8], [11], and on least collaborative patients, making it possible to obtain body scans where the alternative approach based on plaster of paris moulds would fail or would result in lower accuracy and longer execution times. 2. background there are several techniques to perform body scans; herein photogrammetry, structured light and lidar will be considered (figure 1). a) photogrammetry (ph): it is an imaging method used to capture pictures of objects from different perspectives with calibrated cameras. the feature points obtained by overlapped images are used to calculate shooting position through specific algorithms which allow individuating automatically chromatic analogies between two images [21], [27], [28]. b) lidar: recent developments of commercial devices such as smartphones and tablets have led to very fast scanning with lidar. this technique is based on the time of flight [29] measurements, that is the time taken by an object, a particle or a wave to travel a certain distance. more specifically, lidar emits a pulse or modulated light signal and measures the time difference in the returning wavefront [25], [30]; this allows estimating distance from signal propagation speed. c) structured light scanner (sl): this technology is based on the projection of a known light pattern (grid) on the object and, according to the deformation of this projected grid on the curved surface of the object, reconstructs its geometry. moreover, triangulation is used for the location of each point on the object, thanks to two cameras placed at known angles. these measurement techniques have some specific advantages over contact measuring techniques, such as fast acquisition, high accuracy, and minimal invasiveness. depending on the application, some specifications may become more relevant than others. with reference to clinical applications, in some cases high resolution and accuracy must be prioritized, while in other cases a good representation of the colour and structure is mandatory. 3. instruments 3.1. photogrammetry for this application a commercial smartphone redmi note 10 with an average cost of 190 € was used. it is equipped with a duo camera system dedicated for commercial use. its technology includes a digital stabilizer, 30 fps video speed and video rec. 4k (2160p). this tool has been paired to zephyr software (3d flow, v. aerial 3.1) in order to obtain a 3d reconstruction. this software uses structure from motion (sfm) algorithms for photogrammetric processing of digital images to create 3d spatial data. 3.2. structured light scanner the go!scan 50 (15 k€) is a hand-held scanner based on structured light with high speed. in total, this scanner uses three cameras positioned at various angles and depths. in the centre of the device, an rgb camera is installed surrounded by led flashlight to capture textures without the need for special light setup. the scanner works at a rate of 550000 measurements per second, covering a scanning area of 380 × 380 mm² with a resolution of 0.5 mm and a point accuracy up to 0.1 mm. a lamp guidance system helps to set the scanning distance between 0.3 m and 3.0 m. the surface is captured while moving the hand-held scanner over the object. moreover, it is possible to reduce the noise arising from movement, by setting the appropriate parameters on the acquisition software (vx element by creaform, v. 0.9) [31]. the go!scan 50 is the only certified instrument among those used for this work; for this reason, the respective reconstructed 3d geometries have been considered as the most accurate for replicating the actual torso shape and used as reference to evaluate the reconstruction accuracy of the other techniques [32], [33]. 3.3. lidar the ipad pro lidar scanner is a pulsed laser able to capture surroundings up to 5 m through a photon-level reading since it works at time of flight, the time required for data acquisition is strictly related to the speed of light and distance. apple inc. itself does not specify the accuracy of the respective technologies or hardware [25]. this tool allows scanning objects and exporting scans as 3d textured cad models. the scanning resolution used for our applications was 0.2 mm. the scanning time for a particular subject varies from operator to operator since using each scanner is an acquired skill. in general, scanning could take about 15 min depending on the desired accuracy level of the resulting scan [14]. as a rule of thumb, the fastest technique is the lidar scanning and the slowest one is the sl system. subject comfort is comparable among the reviewed scanners. figure 1. instruments and operation scheme: a) structured light; b) photogrammetry; c) lidar. acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 3 4. methodology in the first part of this work the accuracy of reconstruction of the considered scanning techniques was investigated through the trunk reconstruction of a male and a female human subject. scanning results coming from this analysis have been used as reference for techniques comparison, since the subjects can be considered in a stable configuration, with the exception of the intrinsic deformability of the trunk (micro movements due to breathing). in the following step, the same analysis has been repeated on a 4-year-old trunk, adding one more bias given by subject’s macro-movements. ph is characterised by a timing video acquisition of about 50 s for each subject. using zephyr software, the geometry of the torso was reconstructed taking a total of 7 h with high software settings: up to 15000 keypoints per image and “pairwise image matching” setting on at least 10 images. keypoints are specific points in a picture that zephyr can understand and recognize in different pictures. matching stage depth (pairwise image matching) controls how many pairwise image matching to perform. usually, the more is the better; however, this comes at a computational cost. the mesh given by this scan technique can result in shape’s topological errors due to shadow areas and object’s movement. the shape complexity and the macro movements led to sudden changes of curvature, making the reconstruction difficult and resulting so in missing parts and loss of details. the mesh obtained from the scan performed with go!scan 50 required higher manual processing times, given the computational heaviness due to the high resolution. the scan parameters were set directly in the vx elements software according to the manufacturer, with a resolution of 2 mm. targets, semi-rigid positioning, and natural features were used for placement parameters. the acquisition and processing data both required an average time of 15 min under ideal conditions (collaborative subject). scanning with the ipad is the fastest technique. accurate colour information (texture) can be obtained from the two rear cameras whose images are managed by proprietary algorithms. output meshes are of low quality, due to the limited number of triangles used for the surface discretization. for both adults and child scanning analyses, the procedure consists of four main steps: 1) scanning: trunk acquisition required the scanners to rotate around the subject; adhesive circular reference targets with a diameter of 10 mm have been used in order to facilitate the alignment and matching between scans on the post-processing phase (figure 2 b); these targets have been positioned over the trunk considering that, as well known, at least three tie-points must be present in two neighbouring scans in order to allow the respective alignment. 2) geometry reconstruction (post-processing): post-processing was performed using geomagic studio software (3d system, v. 12) [34]. sometimes, data acquisition results in more than one point cloud; hence, these point clouds have to be registered and then merged in order to obtain one single cloud. a cleaning phase follows, where spurious points are eliminated; these points are generated by environment noise and by subject motion or by the camera resolution being close to the size of geometric details. a triangulated mesh is then generated, and it is smoothed to obtain a more regular geometry. the smoothing phase must be performed carefully in order to avoid losing relevant information. finally, the mesh is edited to avoid double vertices, discontinuities of face’s normal, holes, internal faces, so obtaining a manifold geometry. at the end of editing, the mesh is optimised to reduce the number of triangles. 3) comparison among measurement techniques: first of all, scanners’ performances were evaluated in terms of times required to obtain the final geometry. the geometries were then compared through a fully automated operation, performed by dedicated software: geomagic studio. it should be reminded that mesh coming from different scanning are not iso-topological [25] and this can make this operation more critical in addition to the 288389, 431000, 158000 triangles being processed for male, female and child torso respectively. more in detail, for the adults’ scans case, the reference geometry obtained from go!scan 50 was compared to output geometries from ph and from lidar, analysing the distribution of distances both before and after mesh filtering. a software-coded mapping analysis between pairwise scans was performed: results of this analysis are represented by the standard deviation of the statistical distribution of the shortest distance between two scans, along with the mean value of this distance. this analysis is a signed type analysis; for this reason, in the following positive and negative values of the mean distance will be provided, representing deviations towards the outer or inner scanned volume, respectively (figure 3). for the child torso (figure 4), this deviation analysis was performed twice. in the first instance, lidar scans were compared, analysing the deviation distribution at different threshold levels (10, 20, 80, 120 and 180 mm), where the threshold parameter represents the distance value (in mm) beyond which the mesh points are considered as outliers. this analysis was performed because three lidar scans figure 2. data obtained by acquisition with the three instruments; a) body reconstruction of a female and a male (red and blue); b) marker details. acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 4 were obtained: a full body scan (longer acquisition time) and two partial body scans (shorter acquisition time). over the acquisition time of the two partial scans, the subject’s torso could be reasonably considered still, while the full body scan, due to the longer acquisition time, was more biased by macro movements. the multimodal approach used for the child torso consists in the reconstruction of the 3d geometry using the two partial sl scans after their alignment with the lidar scan, which was used as a reference for the global alignment, since it was the only technique which allowed obtaining a full body scan. the deviation analysis among lidar models has allowed quantifying full scan’s macro movements and the respective reconstruction uncertainty for the final torso 3d model, if this scan was used as a reference for the go!scan 50 model positioning. 4) measurement: prior to subjects’ trunk scanning, some main measurements were taken with a seamstress meter in order to have a reference when checking the scanned geometry scale. 5. results 5.1. adult subjects scan scans from sl scanner are the most accurate and are certified; therefore, as aforementioned, they were used as a reference. for ph, it was possible to reconstruct only a portion of the surface for the female subject, while meshes related to the male test has resulted without detail: 94 % of points were too far from sl points to be used for the calculation of geometric deviation (figure 3). with reference to the female subject, 57 % of points resulted to be far from the sl model. this is due to male subject movement, colour and reflection of clothes. the best matching points were located at the torso back. lidar scans were more complete: only 17 % of points had to be discarded for the female subject and 38 % for the male one (table 1). in terms of triangles number, which is closely related to the geometry accuracy, sl scan has given a total of 350614 triangles, ph has resulted in 14430 triangles, and lidar has provided 37792 triangles for the male subject and 46811 triangles for the female subject. two reference points have been tracked through apposite markers (figure 2 b). the respective distance was equal to 100 mm with reference to the male subject, and 90 mm for the female subject. in the male subject this same distance was evaluated equal to 98.6 mm with sl (1.49 % uncertainty); 109 mm with lidar (9 % uncertainty). with reference to the female subject, the respective distance was evaluated equal to 89.9 mm with sl (1.11 % uncertainty), and 104 mm with lidar (15.5 % uncertainty). 5.2. young boy’s scans the following information has been obtained: a) 1 ph scan, with partial covering of the subject’s trunk, obtained in 18 s with 110244 triangles (referred as ‘ph’ in the following); b) 3 lidar scans: one full-body scan (biased by the movement of the subject) with 8420 triangles and two partial scans of the left (4310 triangles) and right side (4303 triangles), minimally affected by child’s movements. these scans required 4 s and 10 s for the figure 3. example of distances’ distributions between the outputs obtained with sl and lidar instruments: male torso. figure 4. a) young child torso and detail of scan’s output given by b) ph, c) lidar and d) sl techniques table 1. comparison of distances’ distributions between lidar and sl scans, for the adult case. pairwise comparison between techniques reference scan max (mm) mean +/ (mm) dev. std. (mm) distant point (%) sl_lidar (female subject) sl 20 7.61/5.42 8.63 17 sl_ph (female subject) sl 20 12.52/11.06 13.14 57 sl_lidar (male subject) sl 20 6.00/7.00 9.00 38 sl_ph (male subject) sl 20 15.00/14.00 16.00 94 acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 5 left and the right side, and 20 s for the full trunk. in the following, these scans are referred to as lidar ‘1’, ‘2’ and ‘3’ (left, right and full-body respectively), according to the respective position in the scanning sequence (figure 5); c) 2 partial sl scans from go!scan 50: these are much more accurate (47294 triangles) and required about 5 min for the back side and 4 min for front side. ph failed to reconstruct the trunk because the legs were the only still part of the child’s body (figure 4 b). 5.2.1. analysis of lidar’s results in figure 6 a detail of lidar scans alignment is shown. the three scans were compared through three pairwise combinations, varying the threshold distance: the distribution of distances between two scans has been obtained on a limited set of points, whose distance laid below this given threshold value. value. this threshold was varied in order to assess its influence on final results (figure 7). according to figure 8 a, 10 mm or 20 mm threshold values have to be chosen in order to keep the standard deviation below 60 mm. however, 10 mm threshold would produce a too high percentage of outliers, as shown in figure 8 b. therefore, a threshold value equal to 20 mm has been chosen: it represents the trade-off between the standard deviation and the percentage of retained points. having chosen the 20 mm threshold as reference, the mean values for the standard deviation of distances among lidar scans have been analysed. these values are reported in table 2, they show that the minimum value of the deviation is associated to lidar 2 (which is the fastest scan) versus lidar 3 comparison and lidar 3 (which is the only full body scan available) versus lidar 1 comparison (bolded values in table 2). for this reason, lidar 3 has been chosen as a reference for the following alignment procedure in multimodal scans. 5.2.2. lidar versus structured light scans from sl have been considered as a reference since the respective scanner has been certified and this technique is known to be the most accurate [33]. an optimized geometric alignment was performed by geomagic studio software, which is based on iterative closest point algorithms. figure 9 show the displacement between scans after alignment. the sections are evaluated by a level curves measurement tool that returns the circumferences of trunk. three combinations have been studied: three scans form lidar were compared to both sl scans (figure 10). the maximum standard deviation has resulted to be equal to 6.93 mm with mean values equal to +6.32 mm and -6.36 mm (where positive and negative values represent deviations towards the outer or inner scanned volume, respectively) obtained from sl-lidar 3 combination, corresponding to the overlap between the full lidar and the sl scans (reference). on the other hand, the minimum standard deviation is represented by figure 5. lidar acquisition: a) right side scan detail of three lidar scan acquisitions referred to as ‘lidar 1’; b) left side scan without movement, referred to as ‘lidar 2’; c) total body scan movement, referred to as ‘lidar 3’. figure 6. example of lidar scans alignment: a) point selection for alignment; b) alignment; c) top view of alignment. figure 7. example of pairwise comparisons between lidar 1 and lidar 6 scans with a threshold value of 20 mm. green colour indicates areas with below-threshold distances, red areas are above the reference surface and blue ones are below the reference surface, grey areas are out of range (outlier). figure 8. trend of a) standard deviation and b) percentage of outliers versus threshold values for different pairwise comparisons. acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 6 overlapping the fastest lidar scan (lidar 2) and both partial sl scans (references). the value of standard deviation in this case is 6.65 mm with mean values equal to +5.71 mm and -5.49 mm (table 3). as noted, the full body lidar scan (lidar 3) has the closest values to both sl scans, and it is the best suited to replicate the actual back shape and to be used as reference for sl scans alignment. 5.2.3. multimodal space procedure the full body scan obtained from lidar was used as reference for both sl scans (chest and back) positioning, while ph provided an incomplete result which could not be merged to obtain a full trunk scan. looking at lidar results, the lines of movement can be outlined in texture scans (figure 11). the two sl scans were overlapped on the 3d lidar full scan and in the next step a topological optimization of the trunk was performed with 3-matic (materialise, v. 12) [35], a software used for clinical application (figure 12). finally, a comparison between the actual trunk measurements (circumferences at chest and waist levels) and the corresponding measurements taken on the reconstructed geometry was performed, resulting in a difference of 5.9 mm (1.25 %) at the waist level, and an uncertainty of 8.2 mm (1.64 %) at the chest level (figure 13). the plaster mould accuracy, acceptable for medical applications is above 15 mm [9], [10]. the uncertainty of the reconstruction for this multimodal non-contact measurement methodology is within this limit in fact the maximum uncertainty is 8.2 mm. 6. discussion all instruments, photogrammetry, structured light scanner and lidar have been proved to be able to capture trunk geometry in a still patient. when results coming from all three table 2. comparison of distances’ distributions between lidar scans for the child case. pairwise comparison between three lidar scans reference scan max (mm) mean +/ (mm) dev. std. (mm) distant point (%) lidar 1 (young boy) lidar 2 20 7.60/9.54 10.00 57 lidar 2 (young boy) lidar 3 20 8.27/7.52 9.16 44 lidar 3 (young boy) lidar 1 20 6.05/5.97 7.90 13 table 3. comparisons between structured light and lidar scans for the child. techniques reference scan max (mm) mean +/ (mm) dev. std. (mm) distant point (%) sl_lidar 1 sl 20 6.73/5.83 6.68 55 sl_lidar 2 sl 20 5.71/5.49 6.65 75 sl_lidar 3 sl 20 6.32/6.93 6.93 62 a) b) figure 9. a) level curves (blue curves) for distance evaluation between lidar and sl scans: b) example of lidar 3 to sl scan alignment. figure 10. example of distances distribution between sl and lidar 3 scan. figure 11. lidar movement texture detail. upper row: side view. lower row: back view. a) column: lidar 1; b) column: lidar 2 scan; c) column: lidar 3. figure 12. reconstruction of torso in 3-matic materialise, using the full lidar scan as a reference. acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 7 instruments were compared to those coming from traditional techniques based on plaster moulding, they proved to be more accurate with the advantage of producing a digital editable model; structured light scanner produced the most accurate results. when a non-collaborative patient is considered, new specifications must be taken into account such as the time required for scanning the whole geometry and the robustness of reconstruction algorithms. as a result, lidar technique was proved to be the only technique able to provide a full scan, thanks to the lowest acquisition time. however, the respective accuracy was quite low, and lidar could not be used alone; however, it could be used as reference for structured light scans registration, so removing the major source of noise in sl, that is non-collaborative patient’s movement. from this, it can be pointed out that a multimodal methodology was needed in order to overcome the limited accuracy of lidar, recovering information from partial scans obtained from sl. the whole methodology has been set up and tested with encouraging results: the final outcome has an acceptable accuracy (8.2 mm), where the only alternative would be taking a limited number of measurements on the noncollaborative patient body. compared to plaster moulding, the accuracy is greatly improved (8.2 mm against 15 mm), and the bias given by dermal tissue compressibility [36], [37] is totally absent. once the scans were cleaned, simplified and merged, the standard triangulation language (stl) model was exported and 3d printed, to evaluate the viability of this workflow to produce a customised brace. finally, the brace was manufactured with traditional method on the 3d printed volume, without any contact with the subject (figure 14), after having been virtually tested through mock up techniques [38]. 7. conclusions in this work a multimodal scanning approach was proposed. the uncertainty given by movement was analysed and compensated. a full procedure for the reconstruction of the 3d external shape was developed by integration of different 3d measurement techniques. the shape of the human torso of a child was finally measured, 3d printed and used for the creation of a patient-specific brace. future developments will focus on combining fast and lowcost techniques and algorithms with 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messina, italy 2 university of messina, department of engineering, c.da di dio, vill. s.agata, 98166 messina, italy section: research paper keywords: machine learning; electrochemical impedance spectroscopy eis; lithium-ion battery; state of charge; state of health citation: davide aloisio, giuseppe campobello, salvatore gianluca leonardi, francesco sergi, giovanni brunaccini, marco ferraro, vincenzo antonucci, antonino segreto, nicola donato, comparison of machine learning techniques for soc and soh evaluation from impedance data of an aged lithium ion battery, acta imeko, vol. 10, no. 2, article 12, june 2021, identifier: imeko-acta-10 (2021)-02-12 section editor: ciro spataro, university of palermo, italy received january 18, 2021; in final form april 29, 2021; published june 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was funded by the italian ministry of economic development under the programme “ricerca di sistema”, project electrochemical storage corresponding author: davide aloisio, e-mail: aloisio@itae.cnr.it 1. introduction as well known, machine learning (ml) is a subfield of computing, an artificial intelligence (ai) technique that provides machines with the ability to learn from the field data without explicit programming [1]. in particular, ml can be really useful in applications that try to extract some information or unknown properties (‘features’) from the dataset (usually called ‘training set’) coming from data warehouses or data lakes. information extracted from this kind of data analyses can be used to develop prediction models for systems behaviour (subject to certain operative conditions and under some constraints). in particular, the battery behaviour characterisation is quite complex to be described through analytical models, mainly due to many parameters act in determining the ageing evolution (e.g. charge and discharge current rates, operative temperature, depth of discharge (dod) reached, state of charge (soc) during rest periods and so on. therefore, the combination of the aforementioned parameters makes the systems hard to model via analytical equations. this is particularly evident for li-ion batteries, for which it is more difficult to describe electrochemical processes with analytical equations, due to the nonlinearities present in their behaviours. the analytical models require, in addition to input data of the actual working conditions (current, temperature, etc.), the knowledge of many parameters (geometry, density and porosity of materials, etc.) these data are not always available or easily measurable and can vary over time (e.g. due to ageing). therefore, the analytical models can be affected by inaccuracy. abstract state of charge estimation and ageing evolution of lithium ion (li-ion) batteries are key points for their massive applications in the market. however, the battery behavior is very complex to understand because many parameters act in determining their ageing evolution. therefore, traditional analytical models employed for this purpose are often affected by inaccuracy. in this context, machine learning techniques can provide a viable alternative to traditional models and a useful tool to characterize the batteries behavior. in this work, different machine learning techniques were applied to model the impedance evolution over time of an aged cobalt based li-ion battery, cycled under a stationary frequency regulation profile for grid application. the different ml techniques were compared in terms of accuracy to determine the state of charge and the state of health over the battery ageing phenomena. experimental results showed that ml based on random forest algorithm can be profitably used for this purpose. mailto:aloisio@itae.cnr.it acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 81 in this context, ml techniques represent a viable alternative and a useful tool for modelling the battery behaviour. ml algorithms learn directly from experimental data, reducing the complexity of modelling, usually due to the high number of parameters and empirical adjustments needed. in addition, according to the recent literature, the application of ml techniques in the prediction of the ageing of li-ion batteries shows errors in the range between 0.5% 5.5% [2]-[4]. this range of accuracy is considered a good compromise among algorithm complexity, effort spent on model development and reliability of results. many physical and electrical parameters are characteristic of the chemical reactions inside li-ion batteries. therefore, these relations could be used as tools for the battery state modelling [5], [6]. typically, these features are derived from charging and discharging curves, since typical battery management systems (bms) are able to collect current-voltage data. hence these are the most commonly used parameters for real time battery monitoring [7], [8]. various approaches based on the use of different parameters were proposed in the literature to train machine learning models. among battery parameters, singlepoints terminal voltage, current, temperature, charge/discharge profiles [2], [9], [10] or their geometrical characteristics [11] were employed for this purpose. however, much more information about the status of the battery can be extracted from the impedance spectra recorded by means of electrochemical impedance spectroscopy (eis) [12]. indeed, the impedance spectrum of a lithium cell contains rich information on all materials properties, interfacial phenomena and electrochemical reactions. from a practical point of view, many of these can be extrapolated by the nyquist diagram in which inverse imaginary part of impedance is plotted against the real one for each investigated frequency (of solicitations). in the case of li-ion batteries, the nyquist diagram consists of four distinct regions typically belonging to the frequency range between 10 mhz to 10 khz [13]. in the low frequency region, an almost linear trend in the nyquist plot is representative of the solid diffusion of lithium ions through the electrodes material. in the medium-high frequencies range, one or more semicircles usually represent the impedance of either charge transfer phenomena or passivation layers on the electrodes surface (solid electrolyte interphase-sei). the intersection of the impedance spectrum with the real axis (pure ohmic impedance) represents the cell internal resistance. finally, the high frequency region is representative of inductive phenomena. since each one of these phenomena are strictly related to temperature, to soc and to the state of health (soh) of the cell, then the analysis of the impedance data can be used to monitor the status of the battery [14]. however, due to the large number of data involved in a single eis spectrum and the amount of information it can contain, the use of conventional data analysis methods may be difficult. also, because of the difficulties in measuring the impedance while the cells are active, eis is not widely used [15]. to overcome this drawback, increasing attention is paid to the implementation of ml approaches, either to aid the fitting of the parameters of equivalent circuits able to describe the battery impedance [16], or by directly modelling the entire impedance spectrum [17]. in this paper, ml is addressed to identify possible methodologies to estimate the soc and soh of li-ion battery from eis data, mainly aiming at developing a feasible model easy to be integrated in a battery management system (bms). implementation in bmss of techniques able to extend batteries useful life, estimating the possible replacement time (estimation of remaining useful life, or rul), is considered a key research activity in the field [18]. in section 2, some state-of-the-art of ml techniques applied to soh and rul estimation are reviewed. section 3 describes the experimental procedures employed to age the li-ion cell; the main parameters extrapolated to create the dataset for the algorithm; and the methods for their collection. section 4 describes the methodology used to carry out the first selection of ml algorithm and the validation of the model. section 5 presents the main results related to the use of different classifiers to model both soc and capacity loss of the li-ion cell. finally, in section 6, the main observations are summarised. 2. ml algorithms for state of health (soh) and remaining useful life (rul) evaluation: a brief review thanks to the remarkable computational capabilities of today’s systems, learning algorithms applied to large quantities of data have often become the preferred approach in the search and identification of complex system behaviour, and therefore represent a valid tool for soh estimation of batteries. in these techniques, a large amount of data, constituted by main battery parameters, are collected continuously up to the end of their life. the dataset analysis of the battery life, performed by learning algorithms, allows extracting non-linear relationships among the various parameters. the knowledge derived from this kind of information can allow a careful management of the battery, helping to extend the useful life and giving reliable prediction on possible replacement times, with obvious positive impact on costs and investments. ml techniques such as fuzzy logic (fl), support vector machine (svm) and artificial neural networks (ann) have extensively been applied for the estimation of the health of batteries, and a brief review can be found in [3]. in most cases, soh is estimated by determining battery capacity and internal resistance, parameters strictly related to soh, from input variables behaviour analysis (current, temperature, voltage, etc.) an application of fuzzy logic with a potential use in portable devices is reported in [19], where electrochemical impedance spectroscopy (eis) technique was used for the dataset creation. however, improper hypotheses in the fuzzy rules [3] and reduced set of observations can lead to substantial errors. the support vector machine is a regression algorithm which converts nonlinearities in a lower dimension space to a linear model developed in a higher-dimensional one [20]. examples of application of this technique applied to soh are reported in [21][25]. in particular, in [25] an online method for soh estimation was developed determining support vectors by means of pieces of charging curves. soh with less than 2% error for 80% of all the cases for commercial nmc li-ion batteries was achieved. the accuracy of the results is strongly dependent on the noise and operational conditions; hence, other data manipulation techniques (particle filter, bayesian technique) have to be used in conjunction with svm to increase the robustness of the estimation [26], thus increasing the complexity of implementation. relevance vector machine (rvm) is suggested as a possible improvement of this approach in [20]. artificial neural networks (anns) are probably the most used approach, inspired by the biological functioning of the human brain, for modeling nonlinear systems. soh estimation using an independently recurrent neural network (in rnn) was realised in acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 82 [3]. here soh was predicted accurately with a root means square error (rmse) of 1.33% and mean absolute error (mae) of 1.14%. the main limitation is the need of a detailed analysis on experimental dataset. different chemistries can require a precise identification and understanding of input parameters. in [27], an improved neural network method based on the combination of a lstm (long-short-term memory) and (pso) particle swarm optimisation was developed. the methodology proposed here uses some additional techniques in each part of the learning process, such as pso for optimisation of the weights, dynamic incremental learning for soh model updating, ceemdan method to denoise raw data, with the aim of increasing the accuracy of the model [27]. another hybrid approach can be found in [28], where false nearest neighbour method was used in conjunction with a mixed lstm and convolutional neural network (cnn) as a solution for unreliable sliding window sizes, a problem commonly present in data-driven rul evaluation approaches. the complexity and topology of ann used in these works is actually classified as deep learning, an evolution of the machine learning concept coined for neural networks which exploits the concept of multilayer perceptron (mlp). a comparison of deep learning and different other common techniques showing its potential and advantages of data driven approaches was presented in [4]. the outcomes showed the goodness of deep neural networks (dnn), which are suitable when high accuracy is needed. however, also this technique is not easy to be implemented due to higher computational complexity and resources needed [4]. a lot of other techniques and approaches can be found in the literature. although out of the scope of the present work, the goal of a possible implementation in bms suggests the choice of low-complexity approaches to reduce computational resources needed and thus leading to lower energy consumption [29]. a possible alternative is given by random forest algorithms. they generally use reduced computation resources, and thus can result preferable in comparison to other techniques analysed, based on svm and nn. in general, linear regressors or random forest response is faster than complex model and is easily interpreted. however, it has to be underlined that the accuracy in random forest models is related to the number and size of trees and therefore to the availability of memory [1], [30]. 3. dataset collection and creation the present work was aimed at the development of a method to identify the degradation level induced by the use of li-ion batteries in a primary frequency regulation (fr) service. more precisely, the activity was focused on the identification of the main parameters indicating the state of battery degradation. for this purpose, cylindrical-type 18650 li-ion-ion cells (table 1) were cycle aged according to a test profile extrapolated by the standard iec 61427-2 [31]. the standard profile requires that the storage system is able to provide symmetrical charging and discharging phases at constant power of 500 kw and 1000 kw, respectively, with a voltage range of 400–600 v. therefore, the profile was adapted to the single cell characteristics. moreover, in order to enhance the degradation of the cell (thus limiting the overall duration required for data collection) fr ageing tests were accelerated by operating at an ambient temperature of 45 °c. in fact, the degradation processes of li-ion batteries are accelerated by temperature increase [32]. the ageing tests were performed by a dual-channel bitrode ftv1 battery cycler. in addition, the cell was tested under temperature-controlled atmosphere in an angelantoni discovery dm 340 bt climatic chamber. the fr ageing profile with actual power steps imposed to the cell is shown in figure 1. the full ageing protocol consisted of a first charge of the cell up to 100% soc and then an execution of the fr profile. once the cell reached the lower voltage cut-off threshold (discharged), a recharge up to 100% soc was performed and then the cycle was restarted. the ageing level was defined in terms of residual capacity retained by the cell. this information was obtained from periodic check-ups carried out on the cell, approximately every 10 days of operation. parametric check-ups of the cells performed the extraction of residual capacity and impedance evaluations by means of eis technique. both analyses were carried out through a high reliability autolab 302n potentiostat/galvanostat (whose potential accuracy and current accuracy are both ±0.2% of the full-scale value). it is worth noting that, due to instruments calibration and performance, measurements were considered reliable and having no impact of uncertainty on the model. the robustness of the model will be investigated in a future work. capacity tests, constituted by a galvanostatic discharge at nominal c-rate and room temperature, allowed to extrapolate the characteristic parameters indicative of the soh of the cell. the recorded discharge curves at the begin of life (bol) and different soh levels are reported in figure 2a. in particular, residual capacity (cd) and residual energy (ed) were collected and used as output variables of the database. the value of cd was obtained by integrating the actual current (id) between begin of discharge (t0) and end of discharge (tf), within the upper and lower voltage cut off limit 𝐶𝑑 = ∫ 𝐼𝑑(𝑡)d𝑡 𝑡𝑓 𝑡𝑜 . (1) table 1. main characteristics of the tested li-ion cell. description value nominal voltage 3.7 v nominal capacity 1.1 ah max charge current 4 a max discharge current 10 a maximum voltage 4.2 v minimum voltage 2.5 v discharge temperature -30 ÷ 60 °c charge temperature 0 ÷ 60 °c chemistry licoo2-linicomno2/graphite figure 1. power profile used to age the battery according to a frequency regulation profile extrapolated from the international standard iec 61427-2. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 83 the quantity ed was obtained by integrating the actual power (id) between the begin of discharge (t0) and the end of discharge (tf), within the upper and lower voltage cut off limits. 𝐸𝑑 = ∫ 𝑃𝑑(𝑡) d𝑡 𝑡𝑓 𝑡𝑜 , (2) where 𝑃𝑑(𝑡) = 𝑉(𝑡) ∙ 𝐼𝑑(𝑡), with 𝑉(𝑡) and 𝐼𝑑(𝑡) representing the instantaneous values of voltage and current, respectively. the recorded discharge curves at begin of life (bol) and different soh levels are reported in figure 2a. soh levels were defined as capacity loss of the cell identified during each parametric check-up. as input variables of the algorithm, the complex impedance values were collected at different frequencies and soh levels of the cell. such information came from eis analysis carried out in correspondence of parametric check-ups. to create the database, the impedance of the cell was recorded at different socs (100%, 75%, 50%, 25%, 0%) at bol and every ten days of operation under fr cycle, until a loss of capacity (closs) of about 8% was reached. moreover, the loss of capacity (closs, as effect of ageing) was used as indicative parameter of the cell soh. nyquist plots of the impedance recorded for different socs at bol and five different soh levels are reported in figure 2b. the impedance was recorded in the frequency range between 10 mhz to 10 khz with ten points per decade, which leads to 61 values for each soc. finally, the data set used for the case study consists of 1830 impedance measurements. table 2 contains some statistical information on the dataset used. 4. methodology the above-mentioned dataset has been used to test various classification and regression techniques through the scikit-learn tool [33], an open-source library for machine learning developed in python. among them, k-nearest neighbors (knn), linear discriminant analysis (lda), gaussian naive bayes (gnb), support vector classification (svc), decision tree (dt), linear regressor, lasso, ridge and random forest were considered for performances comparison. in order to avoid an influence on the results by the particular previous partitioning, a cross-validation technique was also used. in particular, in this phase the original data set was partitioned into 5 subsets (folds) used for tests and training. in the case of regressors, the value of the mae and the determination coefficient (r2) was calculated for each round. similarly, the accuracy (acc) was measured for the classifiers. the models were then compared on the basis of the average values of the aforementioned metrics obtained in the 5 validation rounds. the standard deviation (std) was also determined from the same metrics, which provides information on the robustness of the model (in fact, lower values of std generally correspond to more robust models). 5. results 5.1. data analysis first, correlation coefficients were analysed to investigate relationships among impedance measurements and the corresponding soc and closs values. correlation coefficients of soc and closs, specifically achieved for the rf cycle, are summarised in table 3 for both rectangular and polar forms of the impedance. the analysis of the correlation coefficients shows that the highest correlation value is between the closs measurements and the real part of the impedance (re(z)), for which a correlation coefficient of 0.471 was obtained. it is also possible to observe that the correlation coefficient obtained between closs and the impedance module (abs(z)) is just smaller (0.456). the similarity between these two correlation coefficients suggests that, for the purpose of closs modelling, it is possible to use either the module or the real part of the impedance. in the case of soc, the highest correlation value is obtained with the impedance phase values (arg(z)) for which a correlation coefficient of 0.337 was obtained. rectangular coordinate values, on the other hand, are uncorrelated to soc. therefore, it can be assumed that, for soc modelling, the phase values of impedance are the most useful, at least for this set of data. as a consequence, it is to be expected that machine learning algorithms will perform better with the use of impedance values represented in polar coordinates rather than rectangular ones. table 2. statistical data of used dataset. f (hz) re{z} (ω) im{z} (ω) soc (%) c_loss (%) range min-max 0.01-10000 0.041-0.207 -0.072-0.067 0-100 0-8.27 mean 797 0.0762 0.0019 50.00 4.54 std 1951.6 0.0212 0.0175 35.36 2.69 figure 2. a) discharge curves for extrapolation of output variables; b) nyquist plot of the impedance used as input variables of the database. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 84 the above analysis was repeated considering only eis impedance data corresponding to frequency values lower than 350 hz. henceforward, in this work, we will refer to this data set as filtered data. indeed, as also reported in [34] where a similar lithium cell was used, the most important features induced by ageing on physico-chemical processes were observed only for the negative imaginary part of the impedance, which, in our case, matches with the selected filtered frequency range. also, it is well known that eis at moderate and high frequency is strongly dependent on the experimental setup and cables, thus leading to measurement errors and scattered data [35]. accordingly, the comparison of correlation coefficients reported in table 3 and table 4 (for original and filtered data, respectively) reveals a marked improvement in soc correlation when only low frequency measures are considered. in particular, in the case of filtered data, a correlation coefficient of 0.706 was obtained between the soc and the impedance phase, which is relatively higher in comparison to the value of 0.337 obtained for the original dataset. as a consequence, machine learning algorithms are expected to provide higher performance results when trained with the filtered dataset. 5.2. comparison of machine learning algorithms performance of several machine learning classifiers and regressors were evaluated and compared. among them, knearest neighbors (knn), linear discriminant analysis (lda), gaussian naive bayes (gnb), support vector classification (svc) and decision tree (dt), were considered as representative classification algorithms. the aforementioned algorithms were compared in terms of accuracy achieved on both original dataset and filtered dataset, using a cross-validation method on 5 folds. table 5 shows the average values and the standard deviation of the accuracy obtained for the above classifiers in the case of soc prediction obtained by training the algorithms with the original dataset. it is possible to observe how the use of polar representation leads to an improvement in accuracy for all classifiers with an increase between 40% and 270%, depending on the classifier. for both representations (rectangular/polar), the decision tree (dt) exhibits a better performance, obtaining an average accuracy equal to 0.915 with the polar representation. this analysis was repeated considering the filtered dataset, i.e. removing high frequency points from the original dataset. as can be seen from table 6, filtering improves accuracy of almost all classifiers (for the sake of clarity, in table 6 only results for polar coordinates are reported). for better comparison, in figure 3, a box plot shows the median value (orange line), quartiles, and range of accuracy values (minimum and maximum values) in the case of algorithm trained considering filtered (figure 3b) and original (figure 3a) datasets. from the comparison between figure 3a and figure 3b it can be observed that for the lda classifier the use of filtered values leads to a marked improvement in performance. moreover, in the case of dt, in addition to an increase of the average accuracy value, there is also a significant reduction of data dispersion that justifies the reduction of the standard deviation in table 6, obtained in the case of filtered data. similar considerations can be carried out using for comparison purpose the f1 metric [36]. in fact, as shown in figure 4 where the macro-average f1 score obtained for the same classifiers (for both filtered and un-filtered datasets) is reported, dt classifier achieves better results even considering the macro-averaged f1 metric instead of the accuracy. finally, figure 5 shows the confusion matrix obtained for the dt classifier in the case of the filtered dataset for an 80/20 distribution, i.e. with 80% of the data used for training and 20% for testing. a total of 272 soc values were tested and only 16 of them were wrongly classified, thus obtaining an accuracy on the specific test set of 94.31%. therefore, the achieved classification can be effectively used to evaluate the state of charge of the battery starting from the impedance and, in particular, to predict when the state of charge is below 50%. it is worth mentioning that, the choice of using classifiers instead of regressors is related to the specific application. in a few cases, in fact, classifiers able to simply detect discrete values of soc can be useful for detecting when specific critical threshold levels have been reached, i.e. the 20% of capacity reduction commonly used for automotive applications. it is worth noting that, in the previous analysis, the default values of scikit-learn was used for all classifiers, i.e. all classifiers table 3. correlation matrix for impedance measures evaluated on original/unfiltered data re{z} (ω) im{z} (ω) abs{z} (ω) arg{z} (ω) closs +0.471 +0.044 +0.456 -0.002 soc -0.166 -0.103 -0.170 -0.337 table 4. correlation matrix for impedance measures evaluated only on lowfrequency data. re{z} (ω) im{z} (ω) abs{z} (ω) arg{z} (ω) closs +0.477 +0.119 +0.458 -0.001 soc -0.213 -0.1239 -0.215 -0.706 table 5. accuracy of some classifiers used for modeling the soc starting for unfiltered data in rectangular and polar representation. classifier representation z mean std lda rectangular 0.234 0.027 lda polar 0.333 0.045 gnb rectangular 0.196 0.020 gnb polar 0.370 0.053 svc rectangular 0.192 0.014 svc polar 0.380 0.068 knn rectangular 0.222 0.072 knn polar 0.383 0.088 dt rectangular 0.329 0.020 dt polar 0.915 0.047 table 6. accuracy of some classifiers used for modeling the soc in polar representation for filter and unfiltered data. classifier mean std mean std (original data) (filtered data) lda 0.333 0.045 0.602 0.192 gnb 0.370 0.053 0.397 0.027 svc 0.380 0.068 0.374 0.066 knn 0.383 0.088 0.392 0.084 dt 0.915 0.047 0.938 0.024 acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 85 were applied without any optimisation. this fact partially justifies why most classifiers exhibit poor performances. in addition, it is well known that lda, like other linear classifiers and regressors such as ridge and lasso, adapts well to linear models while the dependence of soc on impedance curves does not. nevertheless, it is generally better to test and compare them due to their lower computational complexity. therefore, a similar analysis was carried out for linear, lasso, elastic, ridge, gradient boosting, ada boost and random forest regressors with the main difference that, in the case of regressors, performance was measured in terms of mae and the determination coefficient (r2). the regressor with the best performance in terms of both r2 and mae was the random forest. the distributions of the predicted values by random forest regressor, when trained with the filtered data, are reported in figure 6a and figure 6b for the modeling of soc and closs, respectively. figure 6 also reports the average values and standard deviations of r2 and mae. in particular, in the case of soc, an average value of r2 of 0.98 was achieved (see figure 6a). in comparison to [37], which considered unfiltered data, a significant reduction was obtained in the mae, from 2.65 to 1.87. in addition, as discussed in the following subsection, the use of filtered data leads to models with lower complexity. 5.3. analysis of the random forest parameters different tradeoffs between performance and complexity of machine learning algorithms can be obtained by a proper tuning of related parameters. in the specific case of the random forest, the most important parameters that impact on both performance and overall complexity are the number of trees (n_estimators) and the maximum depth of trees (max_depth). generally, increasing one or both of such parameters improve performance at the cost of greater complexity and estimation time. table 7 shows the r2 and mae metrics obtained with random forest for some combinations of n_estimators and max_depth considering the original set, i.e. unfiltered data. it is possible to observe that r2 and mae metrics are most affected by the max_depth parameter. in particular, a maximum value of r2 equal to 0.97 is achieved by setting max_depth = 30. the use of higher values increases computational complexity without significant performance advantages. as regards the other parameter investigated (i.e., n_estimators), there is no substantial difference in the values of r2 and mae obtained by fixing max_depth = 30 and using n_estimators values higher than 100. this analysis leads to conclude that, in the case of unfiltered data, the optimal values of the random forest parameters that maximise performance are max_depth = 30 and n_estimators = 100, which are the parameters used in [37]. the same analysis was conducted for filtered data, and the related results are summarised in table 8. in this case, better a) b) figure 3. accuracy of machine learning algorithms on the soc estimation a) unfiltered, b) filtered data. a) b) figure 4. f1 metric results for a) unfiltered and b) filtered data. figure 5. confusion matrix of dt classifier. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 86 results are achieved even with lower values of the parameters. for instance, the performance obtained using filtered data with max_depth=10 and n_estimators=10 is better than when using unfiltered data for max_depth=30 and n_estimators=100. thus, by training the algorithm with filtered data we obtained models with better performance and lower complexity. 6. conclusions starting from impedance measurements, different machine learning techniques were analysed as predictors of the state of charge and the loss of capacity of a lithium battery, subjected to a frequency regulation profile for grid applications. according to the results, the following conclusions can be drawn: for the training of machine learning techniques, the use of impedance values expressed in polar form is to be preferred; decision trees and random forest provided superior performance compared to the other machine learning techniques analysed; using low frequency data for training random forest regressor improved performance in terms of r2 and mae for both state of charge and capacity loss prediction and largely reduced overall complexity. acknowledgement special thanks to the italian ministry of economic development for funding this activity. references [1] g. hackeling, mastering machine learning with scikit-learn: packt 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table 7. r2 and mae values obtained by random forest technique varying parameters n_estimators and max_depth (on unfiltered data). n_estimators max_depth r2 mae 10 5 0.79 (0.01) 10.47 (0.29) 10 10 0.93 (0.01) 4.49 (0.38) 10 30 0.96 (0.01) 3.32 (0.50) 10 50 0.96 (0.01) 3.34 (0.26) 25 30 0.97 (0.01) 3.11 (0.24) 50 50 0.97 (0.01) 3.02 (0.34) 100 5 0.80 (0.01) 10.43 (0.24) 100 10 0.93 (0.01) 4.43 (0.29) 100 30 0.97 (0.01) 3.02 (0.36) 100 50 0.97 (0.01) 3.03 (0.34) 1000 30 0.97 (0.01) 2.99 (0.30) table 8. r2 and mae values obtained by random forest technique varying parameters n_estimators and max_depth (on filtered data). n_estimators max_depth r2 mae 10 5 0.95 (0.01) 4.62 (0.30) 10 10 0.98 (0.00) 1.94 (0.22) 10 30 0.98 (0.00) 1.94 (0.22) 10 50 0.98 (0.00) 1.84 (0.29) 25 30 0.98 (0.00) 1.89 (0.15) 50 50 0.98 (0.00) 1.91 (0.18) 100 5 0.95 (0.01) 4.50 (0.20) 100 10 0.98 (0.00) 1.89 (0.13) 100 30 0.98 (0.00) 1.8 (0.21) 100 50 0.98 (0.00) 1.87 (0.22) 1000 30 0.98 (0.00) 1.83 (0.18) a) b) 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acute respiratory syndrome coronavirus 2 infection control: a feasibility study acta imeko issn: 2221-870x june 2022, volume 11, number 2, 1 8 acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 1 development of a contactless operation system for radiographic consoles using an eye tracker for severe acute respiratory syndrome coronavirus 2 infection control: a feasibility study mitsuru sato1, mizuki narita2, naoya takahashi1, yohan kondo1, masashi okamoto1, toshihiro ogura2 1 department of radiological technology, school of health sciences, niigata university, niigata, japan 2 department of radiology, gunma prefectural college of health sciences, gunma, japan section: research paper keywords: infection control; sars-cov-2; eye-tracking manipulation; contactless device; radiographic console citation: mitsuru sato, mizuki narita, naoya takahashi, yohan kondo, masashi okamoto, toshihiro ogura, development of a contactless operation system for radiographic consoles using an eye tracker for severe acute respiratory syndrome coronavirus 2 infection control: a feasibility study, acta imeko, vol. 11, no. 2, article 38, june 2022, identifier: imeko-acta-11 (2022)-02-38 section editor: francesco lamonaca, university of calabria, italy received march 29, 2022; in final form june 8, 2022; published june 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: mitsuru sato, e-mail: mitu-sato@clg.niigata-u.ac.jp 1. introduction an outbreak of severe acute respiratory syndrome coronavirus (sars-cov-2) infection occurred in wuhan, china, in december 2019 [1]–[5]. since then, the virus has been transmitted worldwide, and consequently, the world health organization declared it a pandemic on march 11, 2020. this infection can be primarily transmitted via droplets and contact routes [6]-[8]. currently, infection control measures, including social distancing, using masks or face shields, and frequent handwashing and disinfection, are important [9], [10], particularly in the medical field. hospitals with isolation wards for patients with coronavirus disease 2019 (covid-19) are taking various measures to prevent infection transmission. the environment where patients with covid-19 are treated is zoned as clean (cold zone), intermediate (warm zone), and unclean (hot zone) areas [11]-[13] to prevent hospital-wide infection. however, it is difficult to completely control infection despite such isolation measures [14]. chest radiography is important for the management of covid-19, and portable x-ray machines are used in isolation wards. in most hospitals, after imaging patients in the isolation ward is completed, all areas that may have come in contact with the patient or been exposed to droplets, including the device, the flat panel detector, and imaging console attached to the flat panel detector system, should be disinfected [13]. however, medical staff can be mentally and physically exhausted during a abstract sterilization of medical equipment in isolation wards is essential to prevent the transmission of severe acute respiratory syndrome coronavirus 2 (sars–cov-2) infection. particularly, the radiographic console of portable x-ray machines requires frequent disinfection because it is regularly moved; this requires considerable infection control effort as the number of patients with coronavirus disease 2019 (covid-19) increases. to evaluate the application of a system facilitating noncontact operation of radiographic consoles for patients with covid-19 to reduce the need for frequent disinfection. we developed a noncontact operation system for radiographic consoles that used a common eye tracker. we compared calibration errors between with and without face shield conditions. moreover, the use of console operation among 41 participants was investigated. the calibration error of the eye tracker between with and without face shield conditions did not significantly differ. all (n = 41) observers completed the console operation. pearson’s correlation coefficient analysis showed a strong correlation (r = 0.92, p < 0.001) between the average operation time and the average number of misoperations. our system that used an eye tracker can be applied even if the operator uses a face shield. thus, its application is important in preventing the transmission of infection. mailto:mitu-sato@clg.niigata-u.ac.jp acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 2 pandemic, making it difficult to perform appropriate disinfection. moreover, disinfection may not have been strictly practiced prior to the pandemic [15]-[17]. when several patients with covid-19 undergo imaging, the console should be disinfected to prevent secondary infections, such as due to methicillin-resistant staphylococcus aureus and vancomycinresistant enterococci infections, even if imaging was conducted in the same ward. nevertheless, it is difficult to disinfect a plastic bag covering a complex-structures medical device while wearing personal protective equipment. therefore, the radiographic console, which is touched frequently, can be a source of infection [18]. reducing the frequency of touching the imaging equipment, which can be achieved using contactless input devices, is important in addressing these issues. currently, several contactless devices are available. however, their use for protection against sars-cov-2 infection has not been reported. previous studies have assessed the use of contactless input devices to operate medical devices without touching them in the clinical setting [19]-[24]. such devices are effective in maintaining sterile rooms [19]. therefore, this study aimed to assess the use of an eye tracker, which does not require body movement, as a contactless input device. image display systems have been successfully manipulated via eye tracking during interventional radiology, thereby allowing images to be paged and magnified using the observer’s eye movements alone [25]. in this study, we applied such technology to develop a radiographic console operation system for infection control during portable x-ray imaging of patients with covid19. face shields are used as personal protective equipment in the management of patients with covid-19, and they create an obstruction between the eye tracker and the eyes. therefore, we evaluated our operating system by assessing calibration errors between with and without face shield conditions, average time required for console operation, and average number of misoperations. 2. material and methods 2.1. development of a contactless operation system using an eye tracker in this study, we used tobii pceye mini (tobii, stockholm, sweden) as the eye tracker for our contactless operation system (figure 1). this small and light weight devices has the following measurements: width, 169.5 mm; height, 17.8 mm; thickness, 12.4 mm; and weight, 59 g. it can be easily installed on the radiographic console used in portable x-ray systems. the usable distance from the eye detector ranges from 45 to 85 cm, the sampling rate of the eye tracker is 60 hz, and the recommended screen size is up to 19 in. we used a computer with the following specifications: windows 10 home 64 bit, intel core i7-6700hq central processing unit, and nvidia geforce gtx 960m; its screen size was 17 in (monitor size: width, 38.4 cm; height, 21.6 cm). the eye tracker could be easily used with a universal serial bus connection; however, it requires prior calibration. the system provided with the tobii pceye mini was used for calibration. there was a function in this system to inform the observer whether the position of the observer is appropriate in order to properly calibrate the system. in addition, instructions are displayed on the screen regarding the location to be gazed for calibration so that it can be facilitated. specifically, we entered the gaze detection range and gazed at seven points on the screen, i.e., centre, upper right, upper centre, upper left, lower right, lower centre, and lower left. we used the pupil centre corneal reflection method to calibrate the operator’s gaze point by measuring the corneal reflection point of the irradiated infrared light and the position of the pupil when gazing at each point, which improved with the eye tracker [26]. we developed a contactless operation system for radiographic consoles that used an eye tracker to prevent the transmission of sars-cov-2 infection. we used microsoft visual studio (microsoft, redmond, wa, usa) as an integrated development environment, c# (microsoft), and the nuget package of tobii.interaction v.0.7.3 (tobii core sdk, tobii). the principle of the operation was based on the characteristics of eye movement. the two types of fixations are saccade, which is a quick movement of the gaze, and fixation, which requires the maintenance of gaze on the same spot [27], [28]. the vector was calculated as the amount of gaze point movement on the screen for 0.02 s to detect the fixation state. in this study, we considered the saccade state if the amount of movement exceeded 200 pixels (0.02 cm per pixel, which is approximately 4 cm). an amount of movement of 200 pixels in 0.02 s could be considered a saccade [25]. if the amount of movement was below this threshold, the system was considered in fixation state. the only commands required to operate the radiographic console were moving the cursor and clicking. therefore, we developed a method to move the cursor in accordance with the movement of the gaze point and to click when the fixation state is reached. figure 2 shows the use of the console operation system for imaging. in total, 41 students from radiological technology participated, and they were briefed in advance about the operation system. table 1 shows the characteristics of the observers. 2.2. considerations of the system because eye tracking may not be possible when a face shield is worn, the developed system was used with and without face shield. we used a face shield (logi meister, osaka, japan) made of polyethylene terephthalate with following measurements: height, 22 cm; width, 33 cm; and thickness, 0.25 mm. the distance from the eyeball to the face shield is approximately 4 cm (figure 3). we performed (1) a comparison of calibration errors between the two face shield conditions and (2) an analysis of console operation. in experiment 1, calibration of the eye detector was performed with and without a face shield. subsequently, the error between the actual gaze point coordinates on the screen and the detected gaze point coordinates on the screen was figure 1. overview of tobii pceye mini (tobii, stockholm, sweden). acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 3 measured at the following nine points on the monitor: top left, top, top right, left, centre, right, bottom left, bottom, and bottom right. a gazing point was displayed on the screen to measure the error (figure 4). the points on the four corners of the screen were displayed at a distance of 150 pixels each in the x and y coordinates from the screen edge (resolution: 1920 pixels × 1080 pixels) to assess the calibration error in the periphery as much as possible. the other five points were placed at the centre of the x and y coordinates. the coordinates of the mouse cursor while the operator was gazing at each point were measured five times. the measurement results were averaged to reduce measurement error due to nystagmus. nystagmus is a cyclic and involuntary oscillatory movement of the eyeball. the effect of physiological nystagmus was reduced. the distance was calculated from the obtained coordinates. the calibration error was defined as the distance between actual gaze point which is the location coordinates of gazed by observer and the detection point coordinates for each of the nine points. figure 2. operation of the radiographic console used in isolation wards. real-time gaze analysis allows operations required in conducting examinations, such as clicking buttons based on gaze duration. table 1. characteristics of the participants observer eye condition 1 bare 2 with glasses 3 with soft contact lenses 4 with soft contact lenses 5 with soft contact lenses 6 with soft contact lenses 7 with soft contact lenses 8 with soft contact lenses 9 with soft contact lenses 10 with soft contact lenses 11 with soft contact lenses 12 with glasses 13 with soft contact lenses 14 with soft contact lenses 15 with soft contact lenses 16 bare 17 with soft contact lenses 18 bare 19 with soft contact lenses 20 with glasses 21 with soft contact lenses 22 with soft contact lenses 23 with soft contact lenses 24 with soft contact lenses 25 bare 26 bare 27 bare 28 with soft contact lenses 29 with soft contact lenses 30 with soft contact lenses 31 with soft contact lenses 32 with soft contact lenses 33 with soft contact lenses 34 with soft contact lenses 35 with soft contact lenses 36 with soft contact lenses 37 with soft contact lenses 38 with glasses 39 with soft contact lenses 40 with soft contact lenses 41 with soft contact lenses figure 3. image of the face shield made of polyethylene terephthalate (logi meister, osaka, japan) used in this study. acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 4 in experiment 2, the developed system was used to evaluate the operability of the console simulating the radiographic console attached to the flat panel detector system while the operator is wearing a face shield. this radiographic console simulated that used in the clinical setting (console advance dr-id300cl; fujifilm, tokyo, japan) and was divided into the patient selection (figure 5 a), radiographic item confirmation (figure 5 b), and (3) radiographic screens (figure 5 c). the clicked locations are listed below. • (1) patient selection screen: select a patient from the list (figure 5 a-1) • select button (figure 5 a-2) • (2) radiographic item confirmation screen: start examination button (figure 5 b-3) • (3) radiographic screen: select the radiographic item (figure 5 c-4) • re-imaging process (figure 5 c-5) • add the same type of imaging (figure 5 c-6) • click the end button (figure 5 c-7) these buttons measured 26 cm × 1 cm, 3 cm × 1 cm, 4 cm × 2 cm, 10 cm × 2 cm, 2 cm × 2 cm, 2 cm × 2 cm, and 4 cm × 4 cm, respectively. the experimental procedure was based on the actual examination procedure, and the console was operated in the order of patient selection, confirmation of radiographic items, and operation of the radiographic screen (figure 6). the time between the start of the operation and the completion of clicking the end button was measured. the number of clicks on the screen was recorded, as was the number of clicks caused by accidental eye pauses. the procedure mentioned above was performed five times with each observer. 2.3. statistical analysis the data obtained were the calibration errors at each of the nine positions under the two face shield conditions in experiment 1. we used the paired t test to investigate whether significant differences existed in the average calibration error of all observers. we also investigated whether significant differences existed in the average calibration error at each point for all observers. a value of p < 0.05 was considered statistically significant. the average operation time and the average misoperation time for the radiographic console were obtained in experiment 2. moreover, we investigated whether the calibration errors obtained from experiment 1 was correlated with the average operation time and the average number of misoperations. furthermore, the correlation between the average operation time and the average number of misoperations was evaluated via pearson’s product–moment correlation coefficient analysis. the results ranged from −1.0 to 1.0, with −1.0 and 1.0 representing a negative and positive correlation, respectively. an absolute value of < 0.2 was defined as almost no correlation; 0.2 – 0.4, weak correlation; 0.4 – 0.7, medium correlation; and  0.7, strong correlation. a p value of < 0.05 indicated a correlation between the two face shield conditions. figure 4. illustration of the nine measurement points evaluated in this study. the calibration error was measured by calculating the difference in the coordinates between the detection and actual gaze points. figure 5 a. components of the radiographic console used in this study (patient selection). the numerals in boldface indicate the button click steps. figure 5 b. components of the radiographic console used in this study (radiographic item confirmation). the numerals in boldface indicate the button click steps. figure 5 c. components of the radiographic console used in this study (radiographic screens). the numerals in boldface indicate the button click steps. acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 5 3. results 3.1. experiment 1 the average ± standard deviation (sd) calibration errors at all points for all observers with a face shield and those without a face shield were 1.22 ± 0.94 cm and 1.19 ± 0.79 cm, respectively (figure 7). no significant difference between the two face shield conditions was observed. data about the average calibration error at each point for all observers are shown in figure 8. the nine measurement points corresponded to top left, top, top right, left, centre, right, bottom left, bottom, and bottom right. only measurement point 7, which represented the bottom left, had a significantly larger calibration error in the no face shield condition. data about the average calibration error for all points for each observer are shown in figure 9. there was no tendency for operation either with a face shield or that without a face shield to be consistently larger, although significant differences were observed for 16 of the 41 observers. 3.2. experiment 2 although students and not radiologists participated in this study, they were able to operate the console easily. there was no significant difference between the calibration error of the eye tracker with and without the face shield. the results of the experiment revealed that all observers (n = 41) were able to operate the console. the average operation time was 37.89 ± 24.22 s, and the average number of misoperations was 5.4 ± 4.1. pearson’s product–moment correlation coefficient analysis found a very strong positive correlation (r = 0.92, p < 0.001) between the average time required to complete the operation and the average number of misoperations (figure 10 a). it found no correlation (r = 0.24, p = 0.13) between the average operation time and the calibration error (figure 10 b) also between the average number of misoperations and the calibration error (r = 0.28, p = 0.08) (figure 10 c). 4. discussion there have been no studies to utilize eye tracker for infection control of sars-cov-2. in previous study, there are methods to manipulate image display systems using motion sensors [19]-[24]. however, there are very few cases in which manipulation was achieved using an eye tracker. therefore, the study by use of an figure 6. overall scheme of the experimental procedure. figure 7. comparison of calibration errors between the use and nonuse of a face shield at all measurement points for all observers. figure 8. calibration errors according to measurement point. figure 9. calibration errors of all measurement points according to observer. significant differences were detected between some observers. acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 6 eye tracker was state-of-the-art. furthermore, it is versatile and useful because it can be used not only for sars-cov-2, but also for infection control against other viruses and pathogenic bacteria. the method proposed in this study can not only prevent the risk of contact infection but also save supplies and improve the time efficiency of disinfection. the mean calibration errors at all points for all observers did not significantly differ with and without the use of face shield. forty-one observers participated in this study, representing a relatively large number of people. one of the primary characteristics of eye detectors is that the four corners of the screen are prone to errors during calibration. in this study, we followed the guidance of the calibration system attached to the eye tracker, made the distance between the computer screen and the observer constant, and calibrated the system to have the same geometric arrangement. however, because the pupil centre corneal reflection method was used to detect the shape of the eyeball and the position of the reflection of infrared light on the eyeball, even a slight shift in position may have affected the calibration error. nevertheless, poor operability on the four corners of the screen is not an uncommon outcome. in such a case, operability can be improved by placing the buttons closer to the centre and making them appear with gazing time or eye movement. the larger the calibration error, the larger is the gap between the gazed position and the coordinates of the mouse cursor. therefore, there was a possibility of accidental clicking on a position other than the button owing to calibration errors. moreover, the larger the calibration error, the more difficult it was to adjust the click position with the gaze point, which could increase the average operation time. pearson’s product–moment correlation coefficient analysis showed that there was a very strong correlation between the average operation time and the average number of misoperations. however, there was no correlation between the average calibration error and the average operation time and average number of misoperations. although it is natural for the operation time to increase with the number of misoperations, because the average calibration error does not correlate with the average number of misoperations, it is possible that a small calibration error, such as that in this study, would not have a significant impact on usability. nevertheless, when we obtained feedback on the system’s operability from observers after the experiment, several stated that the buttons were difficult to operate because of their small size and that they were able to click on the button by moving their eyes according to the amount of the shift when they clicked on a position different from the one they were looking at. therefore, it is possible to reduce the effect of calibration error via the operator’s effort, although the operation time will tend to increase because of misoperation. despite the system’s potential, its user interface needs to be improved because it can be easily operated at the same level as the current touch pad or touch panel operation for clinical use. however, the average operation time was longer (20–80 s) than that when the same operation was performed with a mouse (approximately 10 s). the eye tracker was affected by the calibration error, and the detection results showed coordinates that were slightly different from the actual gaze point. therefore, even a calibration error of a level that would not be problematic in such studies as a gaze analysis would be problematic in such cases as this one that require detailed button operation of the imaging console. we considered that this was a result of the small size of the buttons. if the button size is smaller than the calibration error, then it could be difficult to operate (figure 11). this study showed that it is possible to operate imaging consoles while wearing a face shield. although it is difficult to immediately introduce this technology to the clinical setting, it can be used in clinical practice in the near future because the usability of the radiographic console can be improved by simply increasing the size of the buttons. furthermore, the observers in this study were students; the system we developed is useful as it can be operated by observers who are not familiar with radiographic consoles. taken together, the proposed manipulation method that used an eye tracker for infection control is not ready for clinical use. however, because it can be used even when a face shield is worn, its clinical application is feasible with improvements in the radiographic console. it is necessary to improve the eye tracker and the ui in order to use the method in actual clinical situation. the first priority should be improvement of the ui. as mentioned above, the influence of calibration error can be reduced by improving the button size. although the observer determined the operating position according to the system's instructions during the experiment, operation was possible even if the observer's position was slightly moved. however, there were cases when the gazing position could not be detected correctly while the operating position differed significantly from the position at the calibration. as a mentioned above, the usable distance of the eye figure 10. a) – c) results of pearson’s product–moment correlation coefficient analysis of the manipulation experiment data obtained for each observer. figure 11. effect of button size characteristics on difficulty in operating our contactless system that used an eye tracker. if the button size is larger than the calibration error, then the misoperation can be reduced if the button is pressed. acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 7 tracker ranges from 45 to 85 cm, which is approximately equal to the distance from which touch panel operation and/or mouse operation is performed. therefore, it is usable within the range of normal use. it was possible that the eye tracker would be unusable in the existence of natural light. however, experiments in this study were conducted in a laboratory where natural light existed and under fluorescent lighting. in other words, the experiments were conducted in an environment similar to a hospital room. in addition, there are cases in which eye tracker was used in hospital rooms in previous studies, although these studies were not directly operating computers [29], [30]. therefore, we believe that the system in this study can be used without problems in the isolation ward where it is planned to be used. 5. conclusions feasibility of a contactless operation system for radiographic consoles using an eye tracker for sars-cov-2 infection control has been demonstrated by this study. the system developed in this study is extremely useful even when the operator uses a face shield. however, because of its long average operation time, a radiographic console designed with an eye tracker should be developed for daily clinical use. thus, our proposed method can be useful for controlling not only sars-cov-2 infection but also other potential conditions in the future. 6. acknowledgement the authors thank the students of gunma prefectural college of health sciences who participated as study observers for their assistance in the measurements as well as haruka hirasawa and madoka hasegawa of gunma prefectural college of health sciences for their helpful assistance. 7. research ethics and patient consent all procedures involving human participants were performed in accordance with the ethical standards of the relevant institutional and/or national research committee and the declaration of helsinki and its later amendments or comparable ethical standards. informed consent was obtained from all participants. this research, which involves the development of a medical device operation system, specifically an image display system, using a contactless device, was approved by the ethical review committee of gunma prefectural college of health sciences (approval no.: 2020-16). this work was not previously published in part or its entirety. 8. declaration of conflict of interests the authors declare that there is no conflict of interest. 9. funding this research did not receive any specific grant from any funding agency in the public, commercial, or not-for-profit sector. references [1] c. huang, y. wang, x. li, (another 26 authors), clinical features of patients infected with 2019 novel coronavirus in wuhan, china, lancet 395 (2020), pp. 497–506. doi: 10.1016/s0140-6736(20)30183-5 [2] h. shi, x. han, n. jiang, y. cao, o. alwalid, j. gu, y. fan, c. zheng, radiological findings from 81 patients with covid-19 pneumonia in wuhan, china: a descriptive study, lancet infect dis. 20 (2020), pp.425–434. doi: 10.1016/s1473-3099(20)30086-4 [3] x. yang, y. yu, j. xu, (another 13 authors), clinical course and outcomes of critically ill patients with sars-cov-2 pneumonia in wuhan, china: a single-centered, retrospective, observational study, lancet respir med. 8 (2020), pp. 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[28] a. van der gijp, c. j. ravesloot, h. jarodzka, m. f. van der schaaf, i. c. van der schaaf, j. p. j. van schaik, th. j. ten cate, how visual search relates to visual diagnostic performance: a narrative systematic review of eye-tracking research in radiology, adv health sci educ theory pract. 22 (2017), pp. 765–787. doi: 10.1007/s10459-016-9698-1 [29] r. bates, m. donegan, h. o. istance, j. p. hansen, k.-j. räihä, introducing cogain: communication by gaze interaction, univ access inf soc, 6(2007), 159–166. doi: 10.1007/s10209-007-0077-9 [30] m. debeljak, j. ocepek, a. zupan, eye controlled human computer interaction for severely motor disabled children. computers helping people with special needs, icchp 2012. lecture notes in computer science, 7383(2012), pp. 153-156. https://doi.org/10.1086/501777 https://doi.org/10.1016/s0195-6701(01)90015-5 https://doi.org/10.1016/s0195-6701(01)90012-x https://doi.org/10.1186/s40201-015-0160-8 https://doi.org/10.1148/rg.332125101 https://doi.org/10.1007/s00345-012-0879-0 https://doi.org/10.1136/amiajnl-2012-001212 https://doi.org/10.1007/s12194-014-0259-0 https://doi.org/10.1007/s12194-015-0313-6 https://doi.org/10.1007/s11548-016-1480-6 https://doi.org/10.1016/j.acra.2020.09.027 https://www.tobiipro.com/learn-and-support/learn/eye-tracking-essentials/how-do-tobii-eye-trackers-work/ https://www.tobiipro.com/learn-and-support/learn/eye-tracking-essentials/how-do-tobii-eye-trackers-work/ https://doi.org/10.1007/s10459-016-9698-1 https://doi.org/10.1007/s10209-007-0077-9 jelly-z: twisted and coiled polymer muscle actuated jellyfish robot for environmental monitoring acta imeko issn: 2221-870x september 2022, volume 11, number 3, 1 7 acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 1 jelly-z: twisted and coiled polymer muscle actuated jellyfish robot for environmental monitoring pawandeep singh matharu1, akash ashok ghadge1, yara almubarak2, yonas tadesse1,3,4,5 1 humanoids, biorobotics, and smart systems laboratory, mechanical engineering department, the university of texas at dallas, richardson, tx 78705, usa 2 sorobotics laboratory, mechanical engineering department, wayne state university, detroit, mi 48202, usa 3 biomedical engineering department, the university of texas at dallas, richardson, tx 78705, usa 4 electrical and computer engineering department, the university of texas at dallas, richardson, tx 78705, usa 5 alan g. macdiarmid nanotech institute, the university of texas at dallas, richardson, tx 78705, usa section: research paper keywords: artificial muscles; underwater robots; biomimetics; computer vision; jellyfish; smart materials; tcp citation: pawandeep singh matharu, akash ashok ghadge, yara almubarak, yonas tadesse, jelly-z: twisted and coiled polymer muscle actuated jellyfish robot for environmental monitoring, acta imeko, vol. 11, no. 3, article 6, september 2022, identifier: imeko-acta-11 (2022)-03-06 section editor: zafar taqvi, usa received february 27, 2022; in final form august 25, 2022; published september 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was partially supported by office of naval research, usa. corresponding author: pawandeep singh matharu, e-mail: pawandeep.matharu@utdallas.edu 1. introduction in recent times dealing with inhospitable environments has become inevitable. the advancement of technology and increase in demand have forced humans to consider exploration, operation, and data collection in places that are nearly impossible for them to operate in unless equipped with expensive, and in some cases, heavy protective equipment. the human activities are restrained especially in an underwater medium. they are faced with environment limitation such as extreme temperatures (4 °c to 1 °c), radiation, and extreme pressure at depths of over (10,000 ft). moreover, facing physical limitations such as time spent underwater, size, danger when faced with underwater creatures. adaptability is needed to deal with the numerous obstacles presented for humans in an aquatic environment, thus the integration of underwater soft robotics is highly critical and essential for the success of human exploration of the ocean. any soft robot’s development requires improvement in its locomotion, size, weight, flexibility, and control. combining soft materials and artificial muscles along with sensors is the steppingstone of the next generation of smart biomimetic robots making them highly compliant. this part of the development stage is critical and requires many iterations and analysis. the key challenges now are control, movement, and stiffness modulation. control of stiffness is highly essential in soft robots [1]-[4]. abstract silent underwater actuation and object detection are desired for certain applications in environmental monitoring. however, several challenges need to be faced when addressing simultaneously the issues of actuation and object detection using vision system. this paper presents a swimming underwater soft robot inspired by the moon jellyfish (aurelia aurita) species and other similar robots; however, this robot uniquely utilizes novel artificial muscles and incorporates camera for visual information processing. the actuation characteristics of the novel artificial muscles in water are presented which can be used for any other applications. the bio-inspired robot, jelly-z, has the following characteristics: (1) the integration of three 60 mm-long twisted, and coiled polymer fishing line (tcpfl) muscles in a silicone bell to achieve contraction and expansion motions for swimming; (2) a jevois camera is mounted on jelly-z to perform object detection while swimming using a pre-trained neural network; (3) jelly-z weighs a total of 215 g with all its components and is capable of swimming 360 mm in 63 seconds. the present work shows, for the first time, the integration of camera detection and tcpfl actuators in an underwater soft jellyfish robot, and the associated performance characteristics. this kind of robot can be a good platform for monitoring of aquatic environment either for detection of objects by estimating the percentage of similarity to pre-trained network or by mounting sensors to monitor water quality when fully developed. mailto:pawandeep.matharu@utdallas.edu acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 2 recently, researchers have also taken advantage of using soft material to explore the mariana trench (depth of 11,034 m underwater) [5]. many works have been presented which start replicating the movement and behaviour of animals using synthetic materials to illustrate a high degree of freedom robot and variability in stiffness such as 3d printed musculoskeletal joints [6], [7], biomimetic octopus like tentacles [8]-[10], robotic fish [11]-[13], and most importantly jellyfish like robots [14]-[16]. the jellyfish is considered as the most efficient swimmer in the ocean [17], with its highly flexible and deformable bell, it can propel itself long distances while exerting very low energy. robertson et al. presented a robot with jet propulsion similar to the jellyfish inspired by scallops [18]. origami inspired robots have also been investigated [19]. others have presented “soft growing” robot that can be controlled and actuated with a pneumatic actuation mechanism[20]. reviews on the challenges of maintaining linear assets by effectively utilizing autonomous robots have been shown to reduce maintenance costs, human involvement, etc. [21]. however, the research focus in our work is on the jellyfish for its geometrical simplicity and its swimming advantages. many have also attempted to integrate artificial muscles in various configurations to achieve their desired biomimetic jellyfish design. some include jellyfish like robots actuated by shape memory alloys (smas) [14], [22], twisted and coiled polymers (tcps) [15], dielectric elastomers (des) [23], [24], pneumatics [25], ionic polymer metal composites (ipmcs) [26], and hydrogen fuel powered [16]. one aspect that is missing in the literature is the integration of sensors and the lack of object detection capabilities underwater, except for a katzschmann et al. [13] and lm-jelly that utilizes magnetic field and electromagnetic actuator [27]. haines et al. showed that twisted and coiled polymer muscles from fishing line (tcpfl) can be very promising for robotic applications [28]. these muscles can exhibit large displacements in response to heating, which considerably decreases the stiffness of the tcp muscles [29]. the thermally induced fiber untwists in the coiled structure allowing for tensile and torsional actuation in tcp muscles [30]. wu et al. [31] developed a novel mandrel coiled tcpfl muscle for actuating the musculoskeletal system and showed that these types of actuators can be used for other soft robotic applications. however, these mandrel coiled tcpfl actuators have low blocking force/ high strain; hence, taking into account to application considered in this work, we fabricated self-coiled tcpfl actuators [6], [10] with large blocking force and enough displacement to actuate the soft robot. we seek to present a fully functional swimming underwater robot, jelly-z, that is used for applications such as underwater monitoring and data collection. to this extent, in this work, we present jelly-z as shown in figure 1, inspired by the geometry of the moon jellyfish and other similar robots, but this robot is actuated by twisted and coiled polymer fishing line muscles. it is the first jellyfish-like soft robot equipped with a camera with object detection capabilities and actuated by artificial muscles at the same time. these two features make this robot stand out as it attempts to address the fundamental problems in actuation, design, and the use of vision system of soft robots to be deployed for eco-friendly underwater mission. the main features of this robot are: • mimics the bow (jellyfish bell) and string (tcpfl) arrangement for the actuation mechanism. • noiseless and vibration free swimming underwater. • easy to fabricate and is lightweight. • equipped with a camera for surveillance and object detection underwater. the highlights of the paper can be listed as follows. first, the detailed design and fabrication of jelly-z using tcpfl to mimic the movement of moon jelly is shown. second, the fabrication process and characterization results of twisted and coiled polymer fishing line muscles (tcpfl) in underwater environment. third, successful vertical swimming experiment of soft structure of jelly-z robot which includes swimming analysis and object detection results. the contribution of this work in measurement and estimation is that the proposed small bioinspired underwater soft robot equipped with a small camera utilizes unique artificial muscles (which are silent in actuation, easily manufacturable and have sufficient actuation properties) for swimming. it is able to predict and detect different kinds of objects in the surrounding while swimming in water and estimating the percentage of similarity to the object the camera/robot is trained for. in addition, we provided the characteristics of the artificial muscles (tcpfl) in water which can be used for other similar applications. 2. design and fabrication of jelly-z the main body of jelly-z is fabricated from a round silicone bell (diameter 130 mm). spring steels are added to provide stiffness to the attached artificial muscle [15, 32]. it also contains a jevois smart camera and a piece of foam for buoyancy. a rendered image shows the assembled version of the bot in figure 1(a and b). figure 1. (a) cad design showing major components of the jelly-z robot, (b) top view. acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 3 2.1. assembly of jelly-z robot unlike many rigid and complex underwater robots and rovs jelly-z can be fully assembled with six simple steps. first, prepare the steel springs. the 120-micron steel spring is cut into 120 mm x 10 mm strips. figure 2(1) shows a snapshot of one of the steel springs used in the robot. for this robot, we require a total 6 strips. second, since the tcpfl muscles are directly attached to the steel strip by crimping, an insulating tape is added to prevent any current transfer between the actuating muscles and steel strip. moreover, electrical wires are placed in between the tape and the steel strips to keep them fixed within the robot as shown in figure 2(2). third, a 3d printed (abs plastic) mould (figure 2(3)) is used to fabricate the silicone bell. the mould is prepared by cleaning its surface and spraying it with a non-stick compound. fourth, align the steel strips in the designed direction as shown in figure 2(4). fifth, pour the silicone mix (ecoflex 00-10, 1:1 ratio of part a and part b) into the mould (figure 2(5)) and allow it to cure for a minimum of four hours. finally, integrate the tcpfl muscles into the bell by attaching them to the steel springs. the artificial muscles must be stretched (prestressed) to create gaps between each pitch which allows it to contract while heating. then attach both the foam and camera on the robot of the jellyfish as shown in figure 2(6). the foam is used to set the neutral buoyancy of jelly-z which was calculated based on its experimental volume. the camera is also sealed and waterproofed before attaching. 2.2. fabrication of twisted and coiled polymer fishing line (tcpfl) muscles the fabrication process of tcpfl actuators is simple, scalable, and allows the user to easily manipulate the actuator’s properties such as resistance, diameter, and length. this fabrication process previously presented in detail in our previous work [6], [7], [10]. it is done in-house using a setup which includes two stepper motors, a controller, a power supply, and a computer. the fabrication consists of four major steps (1) inserting a full twist on the fishing line fibre; (2) incorporating the nichrome wire; (3) coiling the wire and nichrome together; and (4) annealing in the oven to allow the actuator to maintain its coiled structure. to assure that all the actuators perform and behave the same, two long fishing line actuator were fabricated and later cut into the desired shorter length for the jellyfish. in this case, two 130 mm fishing line actuators were made and cut into shorter 60 mm length. the precursor fibre is an 80-lb nylon 6,6 monofilament (0.8 mm in diameter) purchased from eagleclaw. the conductive nichrome wire is 160 µm in diameter purchased from mcmaster carr. the coiling speed was kept at 150 rpm. figure 3 shows the fabrication set up. stepper motor 1 (sm1) located at the top is used to insert a twist and coil the fibres. stepper motor 2 (sm2) is used to guide the coiling of the nichrome wire throughout the length of the fishing line fibre. the speed of sm2 (150 rpm) is critical as it controls the pitch and amount of nichrome that is being incorporated which will affect the final electrical resistance of the actuator. 3. isotonic testing of tcpfl in an underwater environment 3.1. characterization setup isotonic testing is one of the most important characterization processes to identify how the muscle behaves when its heated and then cooled under a constant load condition. this test can be performed in both air and underwater environment to fully mimic the muscle’s true actuation condition. this set up, shown in figure 4 (left), includes a power supply for joule heating, ni daq 9219 to record the temperature change using thermocouples, and a camera along with tracker physics program to measure the displacement for an underwater testing environment. 3.2. characterization results figure 5 presents the results of the characterization experiments conducted for the tcpfl muscles in water. figure 4 (right) shows a zoomed in snapshot of the tcpfl in its unloaded and loaded state. the length of the muscle is 60 mm (same as that in the robot), diameter d is 2.5 mm and resistance r is 60 ω. the aim of this experiment is to test the effect of different input currents on the tcpfl actuator in an underwater environment. the properties of tcpfl muscles in water is shown in table 1. figure 2. schematic showing fabrication steps of jelly-z robot. figure 3. schematic diagram of the tcpfl muscle fabrication process (left) twisting process, (middle) nichrome incorporation process and (right) coiling process. twisting and coiling protocol similar to wu et al. [7], hamidi et al. [6] and almubarak et al. [10]. figure 4. (left) isotonic test experimental setup, (right) zoomed-in image of the tcp fishing line muscle; no pre-stress and with pre-stress. acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 4 a 500 g weight attached to the free end of the tcpfl muscle while the other end (top end) is fixed in a glass cylinder filled with 5.5 gallons of water. copper wires are connected to both ends on the muscle and to a power supply. the thermocouple is directly connected to the actuator on one end and into the ni daq 9219 on the other. lastly, a camera is placed at the free end on the muscle to record the displacement. a labview program collects and saves all the data at a frequency of 10 hz for analysis by the user. the open-source tracking program (tracker) captures the actuation displacement from the recording and the data is plotted in matlab 2021b as shown in figure 6. the maximum temperature and actuation strain are measured experimentally at three different input currents (0.45 a, 0.55 a, 0.75 a) as shown in figure 5 (a). the maximum actuation strain is noted at ~7 % (figure 5 (d)) while the highest temperature reached is ~62 °c (figure 5 (c)). the highest voltage consumption is ~65 v (figure 5 (b)), which makes the power consumption ~48 w. 4. jelly-z swimming experiment for efficient propulsion of the robot, pressure gradients are generated during each contraction and relaxation cycle across the bell margin of jelly-z bot that allows an upward movement. during each relaxation cycle there is a slight sinking of the robot due to the reverse movement of the bell. figure 6 shows the total vertical distance jelly-z swam in a 70-gallon fish tank (92 x 58.5 x 46 cm3). the jellyfish robot takes up to 21 actuation cycles to swim 360 mm vertically in 63 seconds. a camera mounted on a tripod, working at 60 fps, is used to track the jelly-z while swimming. the open-source tracking program (tracker physics) is employed to measure the distance travelled by the robot and the data is plotted in matlab 2021b. the background stripes are taken as a measurement reference as each layer (white and black in figure 6(a)) are 8 mm wide. three muscles of 60 mm length (60 ω resistance each) are used to make the robot swim with an input current of 1.8 a for 1.5 s heating (contraction cycle) and 1.5 s cooling (relaxation cycle – 0 a). the velocity of the robot is slower for the first 10 cycles of operation while the velocity increases as the robot reaches closer to the surface of water. this is due to the air bubbles that are formed underneath the bell as the water reacts with the high power supplied to the muscles. figure 6(a) and figure 6(b) show the full assembly of the jelly-z robot through the front and bottom perspective views. figure 6(c) (i, ii, iii) show the zoomed-in graphs showing the first 3 cycles (i) current, (ii) velocity (velocity amplitude for each actuation cycle reaches to ~20 mm/sec.), (iii) displacement for 0.33 hz of actuation frequency (duty cycle – 67 %). the total electrical power used to actuate the jelly-z is 109.8 w, but generally these muscles consume much lesser power in air (1/3rd). this is due to the thermal energy being consumed in the surrounding environment (water); hence, to reach the actuation temperature for muscle movement, the input power is higher in water than in air. water as a medium also allows rapid cooling of the muscles. as this is the first application shown with tcpfl muscles in water, more research must be conducted to reduce the heat lost in the surrounding water medium, thus reducing input power. 5. underwater object detection the experimental set up to test the design consists of a large fish tank (920 mm x 585 mm x 460 mm) made of transparent glass, a dc power supply unit, a standard laptop, a fully assembled jelly-z bot prototype mounted with a jevois smart camera, and the appropriate wirings/cables for connections. for this set up, we have used jevois a-33 smart camera solution, by jevois smart machine vision (jsmv) weighing ~17 g (figure 7, bottom(a)). the camera (hardware specifications given in table 2) is mounted on top of the jelly-z bot as shown in full assembly in figure 7 (top right). the camera combines with a camera sensor, embedded quad-core computer, and usb video link in a tiny package. the advantage of using jsmv can be explained with the help of schematic in figure 7(bottom, (b)). a standard camera displays the output graphics without any processing and leave the analysis of data to the receiver. while a jsmv includes a processing unit which processes the video to interpret contents and provide instant results to the receiver. the hardware specifications are given in table 2 [34]. figure 5. experimental characterization of tcp fishing line muscle in an underwater environment and two-step input sequence with a pre-stress loading of 500 gm for 3 different input currents (0.45 amps, 0.55 amps, 0.75 amps); (a) current vs time of two-step power input, time domain plots for two-step power input: (b) output voltage, (c) temperature, (d) strain. table 1. twisted and coiled polymer fishing line actuator mechanical and electrical properties. property characteristic/quantity material nylon (6,6) fishing line type of actuation electrothermal type of resistance wire nichrome (nickel 70 %, chromium 30 %) nichrome temperature coefficient of resistance (1/°c) 579 · 10−6 [33] fishing line diameter (mm) 0.8 nichrome diameter (µm) 160 length of the actuator after coiling (mm) 60 diameter of the actuator after coiling (mm) 2 mass of the actuator (kg) 0.4 · 10-3 resistance () 60 heating time (s) 5 cooling time (s) 25 duty cycle (%) 16.6 actuation frequency (hz) 0.33 free strain in water (%) ~7 % blocking force in water (n) 5.88 current in water (a) 0.45-0.75 voltage (v)/ power in water (w) 50 v / 30 w acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 5 the jsmv does not come with waterproof properties and is not suitable for underwater applications. so, in order to make the camera waterproof we have embedded the camera unit inside a silicone rubber mould, can be seen in figure 7 (b). this allows camera to function underwater, however, the camera unit has a cooling fan which has to be removed in the process and that limits the functioning of camera up to 2 mins due to overheating. while the software specifications include already uploaded object detection software such as tensorflow, yolo darknet, matlab module and python object recognition. for this set up, we have used yolo darknet set up. standard module of yolo provided by jsmv detects upto 1000 different types of objects using deep neural network, darknet. darknet is an open source neural network framework written in ‘c’ and ‘cuda’ [35]. figure 7(c) shows architecture of yolo framework. yolo’s architecture is very similar to fcnn (fully connected neural network). this is a neural network that applies multiple convolutional layers to extract features from images to create learning models. the max pool layer sub-samples the image properties for each of the smaller segments in the image and trains the model accordingly. many such layers are implemented in the neural network of yolo. other layers like the fully connected layer combines the weight associated to the feature properties of the image [36]. figure 6. (top) underwater swimming analysis for jelly-z robot at 1.8 a input current / voltage – 60 v, duty cycle – 50 % for an actuation frequency of 0.33 hz at different time intervals. (a) front perspective view. (b) bottom perspective view. (bottom) (c, i-iii) input current, velocity profile and distance vs time graphs for first three cycles. table 2. hardware specifications of jevois a-33 smart camera. parameters specifications weight 17 g size 28 cm³ processor allwinner a33 quad core arm cortex a7 processor @ 1.34ghz with vfpv4 and neon, and a dual core mali-400 gpu supporting opengl-es 2.0 memory 256 mb ddr3 sdram camera sensor 1.3 mp camera with sxga (1280 x 1024) up to 15 fps (frames/s) hardware serial port 5v or 3.3v (selected through vcc-io pin) micro serial port connector to communicate with arduino or other embedded controllers power 3.5 watts maximum from usb port. requires usb 3.0 port or y-cable to two usb 2.0 ports. led one two-color led: green: power is good. orange: power is good, and camera is streaming video frames figure 7. (a) underwater object detection setup. (b) waterproofing of a jevois camera. (c) autonomous object detection in jevoise camera vs standard camera. (d)the cnn architecture of yolo deep learning model. (e)experimental results of object detection from jevois camera. (i) 65 % human (ii) 63% laptop, (iii) 76% human, (iv) multiple humansat 57 % & 35 %. conv. layer 7x7x64 conv. layer 3x3x192 conv. layer 1x1x128 conv. layer 1x1x256 conv. layer 1x1x512 conv. layer 1x1x1024 conv. layer (a) (b) (c) (d) (e) acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 6 the architecture divides the sample image into a grid size of s*s, known as a residual block. each grid cell is responsible to detect the object cantered within itself. each grid cell predicts the bounding boxes with help of their confidence score. if there is no object in the grid cell, the confidence score would remain zero. each bounding box provides five predictions (x, y, w, h) and confidence score. (x, y) represent the centre of the box and (w, h) represent the dimensions of the box. the predicted bounding boxes are equivalent to the true boxes of the objects when intersection over union is used. this phenomenon gets rid of any unnecessary bounding boxes that don't complement the objects' characteristics (like height and width). the final detection will be unique bounding boxes that correctly fit the objects [37]. the results of object detection experiment can be seen in figure 7(e). the camera mounted on jelly-z bot while being submerged under water, can detect humans passing in-front of fish tank. the camera is also able to identify a laptop during this experiment. to test the detection of multiple objects, two people walked next to fish tank, and as seen in figure 7(e) the camera was able to distinguish between two people and were able to detect them both separately. capabilities of underwater detection are enormous along with application in civil and military domains. 6. conclusions and future work we presented a fully-functional underwater jellyfish like robot, jelly-z. this work shows the first implementation of actuation solely by self-coiled twisted and coiled polymer with fishing line artificial muscles (tcpfl) and the integration of object detection system presented for a soft underwater robot. we showed the unique design of jelly-z inspired by the moon jellyfish and the actuation mechanism using tcps. this design iteration enabled the integration of three tcpfl muscles for the bell contraction and relaxation motion to allow for swimming. jelly-z achieved swimming at a velocity of 5.7 mm/s traveling 360 mm vertically in 63 s. it could generate an instantaneous velocity of ~20 mm/s per cycle, while carrying its own weight of 215 g. it is to be noted that the vertical swimming of this robot can be controlled by turning off the actuation, which allows the jelly-z to essentially sink under the action of its own weight. moreover, we presented the fabrication of tcpfl and underwater isotonic testing. the tcpfl can actuate up to ~7% in underwater condition at a power of 109 w while carrying a load of 500 g, almost 1,000 times heavier than its weight. the tcp muscles are manufactured in-house, and all the muscle integration processes are simple. some of the challenges to improve upon is the life cycle of these (tcpfl) artificial muscles in water, the high-power consumption of the muscles and the operation time of the jevois camera when it is underwater. these aspects need further work and should be done before considering for deployment. as for the system’s behaviour in non-ideal conditions a control system will need to be developed equipped with an internal gps and a 3d imu which will help the robot maintain its specific position when it is pushed or exposed to high underwater currents. autonomous underwater seakeeping is beyond the scope of the presented work but it is worth consideration for research in future projects and applications. we also equipped jelly-z with the jevois smart camera and tested for object and human detection in an underwater environment using the yolo darknet object detection algorithm. future work will include theoretical modelling and comparison with experimental characterization results for tcpfl actuators. improving the efficiency of tcpfl actuators is another major work that has to be done in future. water tunnel tests have to be conducted to study the generation of vortices while the robot is swimming during the contraction and relaxation motion of each cycle. characterization of thrust force generated and simulations for the same have to be carried out from the linear robot motion for improving the efficiency of the robot design. work has to be done on enhancing underwater imaging [37] and object detection capabilities for practical applications. the work presented here is an attempt towards developing a better design of life-like soft robotic jellyfish with no motors or 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cls; helical; airborne; capillary effect; conditioning circuit citation: filippo attivissimo, francesco adamo, luisa de palma, daniel lotano, attilio di nisio, first experimental tests on the prototype of a capacitive oil level sensor for aeronautical applications, acta imeko, vol. 12, no. 1, article 27, march 2023, identifier: imeko-acta-12 (2023)-01-27 section editor: francesco lamonaca, university of calabria, italy received february 3, 2023; in final form february 16, 2023; published march 2023 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this research was funded by the italian ministry of university and research (mur) on the “pon r&i 2014-2020 – area di specializzazione “aerospazio”, project “further”. corresponding author: francesco adamo, e-mail: francesco.adamo@poliba.it 1. introduction aircrafts require highly reliable level monitoring for fluids like lubricant oils for the main engine as well as for other mechanical subsystems. these levels can be sensed using microwave sensors [1]-[4], tdr-based (time domain reflectometry) sensors [5][6], optical sensors, and many other methods. however, these techniques usually require very complex and expensive signal conditioning hardware. moreover, it is important to note that level monitoring in aeronautical applications requires a continuous measurement over the entire dynamic range. a capacitive level sensor (cls) perfectly fits this requirement because its output is not only continuous but even highly linear. a cls carries even more benefits, as it needs a limited planned maintenance, has long mtbf (mean time between failures) and most importantly, it requires low-cost conditioning electronics. capacitive sensors are commonly used in a large variety of industrial applications for measuring several parameters, e.g., pressures, mechanical strains, displacements, ecg signals, facial movements, sound levels, fluid levels, object presence, and many others [7]-[18]. this paper investigates a novel approach to the design of a capacitive sensor for level sensing, with the proposal of a new geometry design for the sensing probes. the design is presented in section 2, starting from the work presented in [19]. several problems will be discussed, such as vibrational stresses and capillarity effects of the fluid in the gap between probes. in section 3 the details of the conditioning circuit (already presented in the previous paper [20]), the readout electronic and the test methodology will be discussed. finally, in section 4 the preliminary experimental results of the characterization of the prototypes will be provided. 2. sensor probes design to develop a new sensor for lubricant oil level monitoring for aerospace applications, a first look at the “standard” or “conventional design” cls probes is needed. for reference purposes, we have considered the commercially available cls model 8tj209 from ametek aerospace [21]; it is made of two cylindrical and concentric metallic plates with a gap of about 4.3 mm and has a declared sensitivity of about 8 pf/inch ≅ 315 pf/m. due to the relatively wide gap between plates, the dynamic response to a step variation of the abstract in this paper, the design, the prototyping, and the first results of experimental tests of a capacitive oil level sensor (cls) intended for aeronautical applications are described. due to potentially high vibrational stresses and the presence of high electromagnetic interferences (emi), the working conditions on aircraft can be considered quite harsh. hence, both the sensing part and the conditioning circuit should meet strict constraints. for this reason, in the design phase, great attention has been paid also to the mechanical characteristics of the probes. all the design aspects are exposed and the main advantages concerning alternative level sensing techniques are discussed, and the preliminary experimental results of sensitivity, linearity, hysteresis, and settling time tests are presented and commented. mailto:francesco.adamo@poliba.it acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 2 liquid level is of the second order, and thus prone to oscillations, as demonstrated in [22] and explained in the following. the aim of the novel design here presented is to increase the sensitivity and, at the same time, to improve the dynamic response reducing the overshoot and the settling time. the sensitivity of a cls is simply defined as the first partial derivative of the output capacitance vs. the liquid level: 𝑆 ≝ 𝜗𝐶 𝜗ℎ⁄ . in working conditions these kinds of clss can be seen as the parallel connection of two capacitors: the first one corresponding to the section immersed into the liquid and the second one corresponding to the remaining section where the dielectric is the air; so the total output capacitance is given by: 𝐶 = 𝐶0 𝐿 [𝜀𝑟,𝑙𝑖𝑞 ∙ ℎ + (𝐿 − ℎ)] , (1) where ℎ is the liquid level, 𝐿 is the length of the probe, 𝜀𝑟,𝑙𝑖𝑞 is the relative permittivity of the sensed liquid and, finally, c0 is the “dry capacitance” of the sensor, i.e. the capacitance shown at the output terminals when the fluid level is ℎ = 0. for simplicity, it has been assumed that relative permittivity of air is one. following the definition of sensitivity, we can write: 𝑆 = 𝐶0 𝐿 (𝜀𝑟,𝑙𝑖𝑞 − 1) . (2) equations (1) and (2) are generic and can be adapted to clss with concentric cylindrical armatures simply considering that for this kind of geometry the dry capacitance per unit length 𝐶0 𝐿 ⁄ is given by: 𝐶0 𝐿 = 2𝜋𝜀0 ln 𝑟𝑒 𝑅𝑖 = 2𝜋𝜀0 ln 1 1 − 𝑤 𝑟𝑒 , (3) where 𝜀0 is the vacuum/air permittivity, 𝑟𝑒 is the inner radius of the external electrode, 𝑅𝑖 is the outer radius of the internal electrode and 𝑤 = 𝑟𝑒 − 𝑅𝑖 is the gap between the electrodes. from equations (2) and (3) it’s easy to see that the sensitivity 𝑆 increases when the ratio w/re decreases or, in other words, when the gap w between the electrodes is narrowed. narrowing the gap has also the additional effect of improving the dynamic response of the sensor, as the mechanical behaviour of the liquid in between the plates of the probe changes. in fact, performing numerical simulations for decreasing values of w, the dynamic response to a step input level changes from a second order one to a first-order approximation when 𝑤 ≤ 1.35 mm [22]. this result has been obtained by considering that the system is modelled by communicating vessels, and can be described by the second-order non-linear equation deeply investigated as equation (12) in the mentioned manuscript. the analysis takes into account several pressure variables, such as hydrostatic pressure, electrostatic pressure, pressure drop due to friction losses and so on. however, some negative aspects must also be carefully considered: i) with such a marked gap-width reduction, a significant capillarity rise starts to happen, and ii) the oil is held on the plates for a longer period due to its viscosity. the capillarity effect can be numerically modelled and addressed by engraving slits into at least one of the concentric probes so that no column above the bath level can build up in the channel. two kinds of contour have been considered for the slits, as shown in figure 1: helicoidal and straight. both were carefully modelled and studied in comsol multiphysics® to calculate the physical characteristics, in particular their sensitivities [22]. furthermore, also the mechanical behaviour of these new kinds of electrodes was carefully studied. in fact, in the typical operating condition aboard helicopters, the vibrational stresses are very significant, and, if not modelled and counteracted the resulting behaviour of the sensor could be undetermined. in fact, in presence of intense mechanical vibrations, the electrodes are set in relative motion to each other, so capacitance fluctuations and, at least potentially, also possible short circuits can occur. the highest values of vibration frequencies occur on helicopters with many blades in the main rotor. as an example, consider a rotor with 7 blades spinning at 225 rpm; in these conditions, the fundamental excitation frequency is 𝑓 = 225 ∙ 7 60⁄ ≅ 25 hz. it is then important that the vibrational modes of the proposed probes have higher frequencies than the ones occurring on the helicopter where it is used. for our analysis, only the flexural vibrations were considered, because longitudinal and torsional ones do not alter the relative position of the electrodes due to the mechanical constraints [22]. due to this constraint as well as to the chemically highly aggressive environment in which the sensor must work, we choose to make the probe prototypes in marine-grade stainless steel (aisi-316); materials like copper or bronze have been excluded a-priori due to their poor resistances to corrosion phenomena [23]-[24]. performing a fem (finite elements method) analysis of the modes of vibration, the probe with the longitudinal slit shows the lowest resonant frequencies at 67 hz for the internal electrode and 83 hz for the external electrode, while the resonant frequencies for the helicoidal slit probe are 37 hz and 44 hz respectively. hence, both probes have higher modes than the vibrations on the helicopter, so no resonances will occur in working conditions. after these preliminary considerations, three different probes were manufactured and tested: one with a helicoidal slit, one with a straight longitudinal slit, and a third one with the internal electrode without any cut and the external electrode with the same previous helical slit. table 1. sensitivities of the proposed cls calculated in comsol multiphysics®. corresponding sensitivity concerning an uncut probe. slit contour sensitivity (pf/m) no slit 725 helicoidal 718 straight 723 a) b) c) figure 1. electrodes prototypes used in the experiments: a) without slits b) with helical slits and b) with a longitudinal straight slit. acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 3 3. experimental setup and methods the purposely designed and prototyped readout electronics as well as the methodology followed for the experiments and the sampling of the data is explained in the following. a look at the mechatronic setup used for the sliding of the probes in and out of the oil is given too. 3.1. conditioning circuit one of the most common and immediate ways to measure the output capacitance of a capacitive sensor is to insert it in a low-pass series rc filter and then measure its time constant in response to a step voltage input stimulus. in such a circuit, the current is given by: 𝑖(𝑡) = 𝑉 𝑅 𝑒 − 𝑡 𝜏 , (4) where 𝜏 = 𝑅 𝐶 is the time constant. hence, the capacity can be calculated by measuring the time needed to charge the capacitor through the resistor from a given start voltage to an end voltage. unfortunately, this method is unreliable because of two main reasons: 1) the effective resolution of low-cost adcs or even of the adcs integrated into typical microcontrollers is too low (typically less than 12 bits) to achieve the requested measurement accuracy, and 2) the time needed to complete a measurement can be too long. in this work a different approach was pursued: we used an lc tank oscillator circuit for the measurement of the frequency shift due to capacitance variations. a parallel lc oscillator has a wellknown resonant oscillation frequency where the capacitor reactance matches the inductor reactance: 𝑋ctot = 1 2 π 𝑓 𝐶tot = 2 π 𝑓 𝐿 = 𝑋𝐿 , (5) where: 𝑋ctot and 𝑋l are the capacitor and the inductor reactance respectively. 𝐶tot is the total capacitance in the circuit, i.e. the sum of a fixed capacity value 𝐶 and of the sensor’s capacitance 𝐶𝑥; to improve the measurement accuracy, also the parasitic capacitances 𝐶par due to the printed circuit board (pcb) traces and to the connecting cables must be included. l is the inductance in the oscillator circuit and 𝑓 is the resonant frequency. equation (5) can be rewritten with the resonant frequency on the left-hand side as: 𝑓 = 1 2 π √𝐿 𝐶tot . (6) hence the sensor’s capacitance 𝐶𝑥 can be calculated from (6) as: 𝐶𝑥 (𝑓) = 1 𝐿(2 π 𝑓)2 − (𝐶 + 𝐶par) . (7) the tank oscillator was implemented using standard commercial values for inductor 𝐿 and capacitor 𝐶, namely 𝐿 = 10 μh and 𝐶 = 270 pf. the pcb parasitic capacitance was measured to be 𝐶par ≅ 13 pf, i.e. a value well in line with typical mean values of capacitance on a pcb. if we assume that the sensor’s capacitance spans the range 500 nf – 1000 pf, the expected frequency span is about 393 khz. 3.2. measurement system the sensor readout is performed by the fdc2214 by texas instruments, a specialized capacitance-to-frequency converter (cfc) integrated circuit (ic) [25]-[27]. the fdc2214 was selected among many alternative ics because it has 4 independent input channels with a full-scale range (fsr) of up to 250 nf, a nominal resolution of 28 bits, an rms noise of 0.3 ff, and an output data rate of 4.08 ksa/s. the prototype of the pcb with the fdc2214 and the minimum electronics needed to interface it with an external microcontroller is shown in figure 2. the fdc2214 communicates with the external microcontroller through an i2c (inter integrated circuits) interface. to interface it with a pc, we used a simple arduino uno development system with an atmega328p mcu (micro controller unit) by microchip corp. measurement data are acquired from the fdc2214 by the mcu and transferred to the pc where they are processed in mathworks matlab. to simplify the test procedures, the probes being characterized were tied to a high-precision linear actuator and suspended over a 125 mm diameter pvc pipe filled with lubricant oil (figure 3). the probes were mounted on the moving cart of the system driven by a closed-loop controlled stepper motor and a high-precision trapezoidal screw. this setup allowed the positioning of the sensor with a nominal resolution of 50 µm. for the sake of completeness, it must be said that this setup has been preferred to the one on which the sensor probes are set figure 2. the prototype of the sensor readout electronics based on the fdc2214. figure 3 . the mechatronic system developed for the automated tests on the cls prototypes; the four main parts of the system are evidenced. eado t electronics ega p on rd ino losed loop stepper otor driver ro e nder test acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 4 in position in a tank and a pump is used to fill and empty the tank with the lubricant oil, due to the limited maximum flow of the available pumps. the lubricant oil used for the tests is an sae 15w40 standard, semi-synthetic one by viskoil®. its characteristics were deeply studied and described in [28]. some assumptions for the experimental tests were done: the room temperature in the laboratory was considered constant. the lubricant is not worn as it is not working in a motor, so there is no contamination by water nor coolant, or metallic debris. from [28], the dielectric constant reduces by only 1.5% when the temperature rises from 60 °c to 120 °c; so, in a first approximation, its changes can be considered linear in the working temperature range of a typical high-performance aeronautical motor (about 100 °c). in the following, the relative dielectric constant is assumed to be 𝜀r,oil = 2.2. in perspective, the effects of temperature on the dielectric constant could also be measured and compensated by taking advantage of one of the residual 3 channels available on the fdc2214; in fact, it can be used to measure the capacitance of a reference capacitor with known geometry permanently and fully immersed in the same oil tank where the cls is installed. 3.3. test methods for each sensor, sensitivity, hysteresis, rise time, and root mean square non-linearity error (rmse) have been studied and evaluated to carry out a comparison. hysteresis experiments were done with three immersion and extraction cycles of the probes with increasing pause times between the moving and the sampling phase. the experiments were performed over a full run of 400 mm in 10 mm steps. for each step, after a delay time (if added), 100 samples were taken in a time interval of about 2 s. the mean absolute hysteresis is calculated as the mean of the differences between the immersion and extraction measures. the sensitivity is calculated by fitting the 10 s cycle data with a first-grade polynomial function. the rise time experiments were performed by measuring the sensor’s output capacitance for a minimum of 20 s, after rapidly dipping or extracting the probe into/from the oil. the time constant 𝜏63 was calculated as 63 % of the time from the moment the sensor stopped to the moment its output capacitance reached its settled value. settling time was then assumed to be 𝑡𝑟 = 5 ∙ 𝜏63. the same sampled data were also used to calculate the sensitivity of the sensor and the rmse, both for the immersion and for the extraction movements. 4. results according to the hysteresis experiment, the sensor with mixed electrodes, i.e., the probe with the internal plate without slits and the external plate with helicoidal slits, showed overall the least residual non-linearity. with respect to this aspect, when given enough time for the settling of the output capacitance, the longitudinal slit probe appears to be the worst-performing one. the sensitivity shown by all the sensors is about 25% higher than the one expected from the simulations run in comsol multiphysics®. the highest sensitivity was achieved with the helicoidal slits probe: this is not in accordance with the physics and could be due to a misalignment of plates or to the small defects of the manufacturing process. other prototypes of the electrodes will be prepared but with different and more accurate manufacturing processes to study this phenomenon. with respect to the rise time evaluation experiments, the main issues arose with the helicoidal probe; in fact, in this case, we observed a very high 𝜏63 time constant since i) the gap between the two electrodes is very narrow and this, as already explained, increases the time needed for the oil to rise and fall in the gap, and ii) the slits are narrow too, allowing a thin membrane of oil to form in them. hence, the oil is held on the electrodes for a longer time. moreover, the time constants are different for increasing and decreasing oil levels; indeed, the time constant is higher for the extraction paths due to the oil retention phenomenon. in this case, the vertical slit electrodes performed best, even though the results are comparable to the mixed electrodes ones. finally, a preliminary look was given at the dynamic performance of the sensors. the residual non-linearity rmse is comparable for all three probes. results for the immersion and extraction are similar too. however, when fitting the sampled data and calculating the sensitivity, a clear difference between the extraction and immersion characteristics arises, with the sensitivity for the immersion being lower than the extraction. figure 4. results of the hysteresis experiment on probes with helicoidal slits a) linear fit and b) residuals. table 2. comparison of performances of the cls with the three different kinds of electrodes probe sensitivity pf/mm 𝑴𝒂𝒙 |𝑵𝑳𝑬|(*) pf 𝑴𝒂𝒙 |𝒉𝒚𝒔𝒕𝒆𝒓𝒆𝒔𝒊𝒔| pf rise time s helicoidal 0.949 4.59 2.57 4.85 mixed 0.888 1.93 1.24 3.01 straight 0.879 11.58 5.10 3.01 (*) nle = non-linearity error acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 5 5. conclusions in this manuscript, the authors presented a novel design strategy for oil-level monitoring aboard aircraft. after a first indepth study of a novel geometry for the probes, with accurate numerical simulations on the electrical aspects, also the effects of mechanical stresses on the electrodes were modelled and addressed. the probes have been prototyped and tested using an in-house developed pc-based mechatronic system and readout electronic. the tests showed positive results in line with the expected behaviour predicted through simulations. in particular, the proposed solution proved superior to the other level sensing technologies in the industry nowadays. concerning the sensitivity, the proposed sensor showed an improvement of more than three times with respect to other commercial clss. furthermore, the issue of the second-order response in response to a step change in the oil level was overcome, with an improvement in the settling time too. even the comparison with an optical level sensor is promising, as the readout electronic is simpler, cheaper, and most important more reliable. the readout electronic was prototyped in a small series with a cost of about 50 eur/unit while, the cost of a single set of probes of the csl is around 1,500 eur, due to the complexities of the manufacturing process. of course, both these cost components could be reduced in the mass production of the sensor. however, it can be estimated that also with a limited number of units the overall cost of the proposed system could be lower than that of the corresponding optical or microwave solutions. 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https://doi.org/10.1016/j.measurement.2023.112476 https://www.doi.org/10.1109/i2mtc.2019.8826965 https://doi.org/10.1109/tim.2019.2905734 https://www.ti.com/lit/gpn/fdc2214 https://www.ti.com/lit/pdf/snoa935 https://www.ti.com/lit/ug/snou138a/snou138a.pdf https://doi.org/10.3390/app11125642 analysis of peak-to-average power ratio in filter bank multicarrier with offset quadrature amplitude modulation systems using partial transmit sequence with shuffled frog leap optimization technique acta imeko issn: 2221-870x march 2022, volume 11, number 1, 1 5 acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 1 analysis of peak-to-average power ratio in filter bank multicarrier with offset quadrature amplitude modulation systems using partial transmit sequence with shuffled frog leap optimization technique ch. thejesh kumar1, amit bindaj karpurapu2, mayank mathur3 1 department of ece,raghu institute of technology,visakhapatnam-531162, andhra pradesh, india 2 department of ece,swarnabharathi institute of science and technology, khammam-507002, telangana, india 3 university of technology, jaipore 303903, rajasthan, india section: research paper keywords: fbmc-oqam; pts; leap frog optimization; papr; ber citation: ch. thejesh kumar, amit bindaj karpurapu, mayank mathur, analysis of peak-to-average power ratio in filter bank multicarrier with offset quadrature amplitude modulation systems using partial transmit sequence with shuffled frog leap optimization technique, acta imeko, vol. 11, no. 1, article 29, march 2022, identifier: imeko-acta-11 (2022)-01-29 section editor: md zia ur rahman, koneru lakshmaiah education foundation, guntur, india received november 29, 2021; in final form march 5, 2022; published march 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: ch. thejesh kumar, e-mail: thejeshkumar.ch@gmail.com 1. introduction the technology of orthogonal frequency division multiplexing (ofdm) technology emerged as the suitable multicarrier modulation technique for modern wireless communications [1]. they are responsible for the effective spectral frequency usage while minimizing inter-symbol interface. this is usually performed employing the guard interval insertion. in the multipath channel. adding the guard interval, however, reduces the efficiency of spectra and power. in order to tackle the issue, the filter bank multicarrier with offset quadrature amplitude modulation (fbmc-oqam) based technique has been proposed. the technique improves spectral efficiency. however, this comes at the cost of computational complexity. further, it has a noticeable impact yielding to reduced out-of-band power leakage [2]. the fbmc-oqam is being evaluated as a contender for the 5g mobile communication standard because to its benefits like excellent temporal and frequency localisation. further, other significant features are the minimization of out-of-band emission and resistance to phase noise [3]. in the fbmc-oqam approach [4], a prototype filter is used to reduce the orthogonality of subcarriers. this is often used to avoid the gi insertion. as a result, this can achieve greater spectral efficiency. citing the recent developments in wireless communication, the demand for mobile and broad-band service in 5g is evergrowing. in this line, the fbmc-oqam approach can be considered as the effective to support the ofdm technique [5]. abstract because of its low adjacent channel leakage ratio, the filter bank multicarrier with offset quadrature amplitude modulation (fbmcoqam) has sparked the attention of many researchers in recent times. however, the problem of high peak-to-average power ratio (papr) measurement has a detrimental influence upon the fbmc system's energy measurement efficiency. we study papr reduction of fbmc-oqam signals using the partial transmit sequence (pts) methods in this work. the pts with shuffled frog leap phase optimization method is proposed in this paper to reduce the larger papr measurement, which is the major disadvantage of the filter bank multicarrier with offset quadrature amplitude modulation system. according to the simulation software findings, the suggested approach has a considerably superior papr, which may reduce the papr by about 2 db at complementary cumulative distribution function 10-3 when compared to the traditional pts method, and it has a much lower computational complexity than the previous ways. the experimental parameters are measured, and results are evaluated using matlab tool. mailto:thejeshkumar.ch@gmail.com acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 2 the significant problem persistent in ofdm and fbmcoqam is the peak-to-average power ratio (papr) enhancement. as a result of this issue, greater non-linear signal distortion has developed at the output of a power amplifier, resulting in a catastrophic reduction of bit-error rate (ber) performance. to address this issue, various papr reduction techniques for multicarrier modulation systems have been developed, including partial transmit sequence (pts) [6], selective mapping [7], and others. further, the manuscript is organized to provide the information on papr reduction techniques in section 2. the algorithm is explained in section 3 while the demonstration of the application is given in section 4. the simulated results, computations and discussions are presented in section 5. overall conclusions are given in section 6. 2. papr reduction methods initially, the concept of papr reduction has been considered as an essential step in communications especially pointing to applications like radar and speech synthesis. typical radar systems have several limitations in terms of peak power. this is very common issue in every communication system. similarly, the speech synthesis application suffers degradation as it is severely bounded by peak power. this is due to the reason that the larger the signal, land up in the situation where the computing machine voice sounds very hard. these features should be avoided especially while handling human speech. when a multicarrier-based communication system is considered, the primary issue to deal with is the inherent papr. hence, the papr reduction can be considered as the essential part of improving the quality of the communication. several schemes and methods are proposed in due course of time to handle the papr reduction process with ease. it is essential to mention that the papr should be dealt through a non-complex phenomenon. for an ofdm system, several papr reduction schemes are proposed precisely following this rule. to list some of the most successful schemes so far, are clipping, tone injection and reservation. similarly, other techniques based on constellations and transmit sequences are also popular. along with these techniques like selective mapping along with block coding are some to mention in the popular list. a comparative study of the techniques mentioned above essentially gave a deep insight on the potential application and strength of each and every method. one of the most common papr reduction methods is the pts method, which is characterized as a distortion-less approach. the minimal papr value in the pts technique is obtained by multiplying the data signal cluster of each data symbol choosing the available and suitable phase combination. while demodulating, the corresponding side information provided by the transmitter is essential. challenging this, a solution to this papr enhancement issue has been provided in specific to the fbmc-oqam system. this can outperform the standard pts technique in terms of papr performance while reducing the computational cost of optimizing phase combination. the quiet characteristics are that the bigger papr is minimized using the overlapping-pts technique [8], [9] and the phase combination is optimised utilizing the shuffled frog leap (sfl) phase optimization. the receiver, like the pts technique, need transmitters to provide relevant side information which is useful for further the data demodulation. 3. shuffled frog leap optimization the sfl algorithm (sfla) belongs to the class of metaheuristic algorithms. it performs a learned heuristic approach for searching a solution. especially, it performs a combinatorial optimization employing several heuristic functions. these heuristic functions are non-complex mathematical models. the evolution of the memes is the factor of inspiration while structuring the algorithm. it mimics the way established to exchange the information among them. the sfla is a hybrid of deterministic and random methods. as in particle swarm optimization, the deterministic method enables the algorithm to effectively employ the surface data which is considered to be highly responsive. this has often been the potential driving source for the heuristic search. the random elements enable the search pattern's flexibility and resilience. like any other algorithm, the initial step to choose the population. this refers to number of frogs which are randomly generated. this consistently includes the whole swamp. the population has been classified into multiple parallel communities. these are referred as memeplexes. independent evolution of the individual is encouraged to search the space in multiple directions. the frogs inside each memeplex are infected by the ideas of other frogs, resulting in memetic development. memetic evolution increases the quality of an individual's meme and improves the individual frog's performance toward a goal. to keep the infection process competitive, frogs with better memes (ideas) must contribute more to the creation of new ideas than frogs with weak ideas. using a triangular probability distribution to choose frogs gives superior ideas a competitive edge. during evolution, frogs do change their memes. this is usually taking in to account the corresponding best available information. this information can be usually obtained from the memeplex. however, in many cases it can be fetched from the whole population basing on the quality. this change refers to the jump and the corresponding magnitude is its step size. this yields the new meme relative to the updated position of the frog. every frog is reintroduced to the community once it has improved its status. the knowledge acquired from a position shift is instantly available for improvement. this rapid access to fresh information distinguishes this technique from genetic algorithm. usually in the later, the whole population is affected. however, in the sfla, the scenario is different. every frog in the population is considered to be the potential solution. this depicts the concepts of idea dissemination. the potential part of the algorithm refers to the comparative study as drawn by the team of innovators suitably developing the concept. this can also be visualised as the engineer who continuously works towards bettering the designs. 3.1. sfla steps step1. initialization. selection of 𝑚 and 𝑛. here 𝑚 refers to number of memeplexes [9]. similarly, 𝑛 denotes number of frogs available every memeplex. the value of 𝐹 is computed as overall swamp size 𝐹 = 𝑚 × 𝑛. step2. generate a virtual population. step3. sort basing on ranks of the frogs. the sorting should follow the process to keep the best on top. step4. partition of frogs into memeplexes. partition array x into 𝑚 memeplexes 𝑌1, 𝑌2, … . , 𝑌m , each containing 𝑛 frogs, such that acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 3 𝑌𝑘 = 𝑈(j)k, f(𝑗)𝒌|u(𝑗)𝒌 = 𝑈(𝑘 + 𝑚(𝑗 − 1)), f(𝑗)𝒌 = f(𝑘 + 𝑚(𝑗 − 1)), 𝑗 = 1, … , 𝑛; 𝑘 = 1, … , 𝑚 . (1) for instance, if 𝑚 = 3, the corresponding rank 1 goes to memeplex-1. similarly, the rank-2 is assigned to memeplex-2. subsequently the rank 3 awarded to meplex-3 while the 4th rank designated to memeplex-1. the process goes on. step5. this step refers to memetic evolution. this takes place inside every memeplex. yield every memeplex 𝑌𝑘 , 𝑘 = 1, … , 𝑚. this is in accordance with the sfla. step6. all the memes are shuffled. following the finite iterative process, 𝑌1, … , 𝑌m should be replacing into 𝑋. this is with respect to 𝑋 = {𝑌𝑘 , 𝑘 = 1, … , 𝑚}. further, the step also includes sorting 𝑋 in the decreasing order basing on the respective performance index. this finally updates the position of the best frog to 𝑃x. step7. check whether convergence is achieved. termination should be called in once the conference is witnessed. if not go to step#3 once again. the termination criterion can be the computational time, number of iterations or the convergence. the convergence refers to ‘best memetic pattern’ without change. basing on this the computation of the function evaluation takes place. 4. papr reduction using pts-sfl 4.1. papr papr is the ratio of the peak power of a multicarrier signal to its average power. as the papr value rises, it shows that the system requires high-power amplifiers (hpas) to function in their saturation area, which is essential since any additional increase in signal amplitude forces the hpa to operate in its nonlinear region, resulting in signal distortion. the reduction methods in ofdm systems are performed separately to each symbol, and the papr of each symbol is computed independently. the overlapping of data blocks is not considered for fbmc/oqam signal which is good sign. to compute the papr correctly, the overlapping of the symbol blocks must be taken into account [10]. the papr of pure fbmc/oqam in db can be expressed as follows: 𝑃𝐴𝑃𝑅(db) = 10 log10 max (𝑚 − 1) ≤ 𝑡 ≤ 𝑚 𝑇 |𝑠(𝑡)m| 2 e[|𝑠(𝑡)|2] (2) from above equation (2) the numerator represents the peak power in the duration of the input 𝑚thdata block, 𝑇 (𝑚 − 1) ≤ 𝑡 ≤ 𝑚 𝑇, and the denominator e[|𝑠(𝑡)|2] represents the average power of the fbmc signal. 4.2. ptps in the conventional pts method, an input block is divided into 𝑉 sub-blocks. each sub-block is zero-padded to create a vector of length 𝑁. inverse fast fourier transform is performed separately for each sub-block, significantly increasing the computation complexity. adjacent, interleaving, and pseudorandom are some of the widely used partitioning schemes [11]. the adjacent method is used in the proposed method, owing to its simplicity and effectiveness. from figure 1, it can be observed that the resulting sequences are optimized by phase rotation factors 𝑏 = [𝑏1, 𝑏2, … , 𝑏v], where 𝑏v = ej 2 π 𝑣/𝑊 and 𝑣 = 0, 1, … , 𝑊 − 1 , to create symbol candidates referred to as partial transmit sequences. this operation results in a variation of the peak values for the signal candidates, and the one with the minimum papr is chosen for transmission. the total number of signal candidates depends on v and w, where w is the number of the phase factors allowed for a single sub-block. the process of producing the optimum phase factor vector that reduces the papr is as follows [12]-[17] {�̃�1, �̃�2, … �̃�v} = arg min [𝑏1, … , 𝑏v] max 0 ≤ 𝑡 ≤ 𝑇 |∑ 𝑠m 𝑣 𝑏𝑣 𝑉 𝑣=1 | 2 . (3) the transmit signal after reducing the papr can be expressed as follows: �̃�m(𝑡) = ∑ 𝑠m 𝑣 �̃�𝑣 𝑉 𝑣=1 . (4) direct application of this method to each fbmc symbol separately is not effective. as the symbols overlap, the parts where the symbols overlap will have a peak regrowth, increasing the papr again [18]. 4.3. pts-slf it is evident that the proposed yielded excellent papr reduction. this comes with a performance that reported lower computational complexity [19]-[22]. the process of sfl in optimizing the phase is performed according to the algorithm discussed in section 2. the proposed block diagram is shown in figure 1. 5. results and discussion following the implementation of the sfla to the problem of papr reduction, the simulation-based experiment has been carried out and the results are presented in this section. the experimental computations are responsible for the deep insights which can be drawn on the concept of papr reduction. clearly, the induction of the pts-sfl and the effect on the papr are the part of study. the probabilistic approach and the computations based on complementary cumulative distribution function (ccdf) are performed as a part of the simulation. it is evident that the corresponding parameter has a direct impact on the papr. the results demonstrate various papr reduction techniques and are compared with proposed model. figure 1. the proposed pts-sfl technique. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 4 the signals are optimized independently inconsideration of the overlapping signals, the complexity of the algorithm can be reduced using fewer combinations of phase factors. herein, we explore the effects of different combinations of phase factors on the performance of the proposed technique. figure 2 shows the ccdf curves for the proposed technique without clipping and with four sub-blocks. from figure 2 it is shown that papr is around 8.1db using sfl optimization technique. further, 8 subblocks are considered and papr results are evaluated and is shown in figure 3. by considering 𝑉 = 8, the threshold papr obtained using pts is 7.5 db, using segment-based optimization the papr value is around 7.4 db and using ant colony optimization technique is around 7.1 db. the papr value obtained using sfl optimization is 6.5 db which is almost 38 % reduction of papr compared to original fbmc model. from figure 4, it is shown that for different 𝑉 values our proposed sfla performance is better comparing to other existing techniques. the performance of ber is shown in figure 5. at snr of 10 db, the ber is very low for proposed method compared to clipping signal and original fbmc and also other two techniques i.e., satin bowerbird optimization (sbo) and ant colony optimization (aco). the bit error rate values at snr of 10 s db are tabulated and are shown in table 1. 6. conclusions in this paper, the technique of overlapping-pts has been formulated and demonstrate. the technique has been successfully simulated and applied to the problem of papr reduction in the considered system. the proposed technique featured with reduced computational complexity. it is also reported that the phase optimization can be achieved in the perceived technique using the artificial bee colony phase optimization. the analysis of the simulated reports yielded an excellent insight which is sufficient to consider that the technique can achieve dominating papr performance. this can subsequently suppress the papr by 2 db at least. the corresponding suppression reported as per the experimentation seems to be having an average of 38 %. this significant part is that the papr performance does not degrades the ber. references [1] t. hwang, c. yang, g. wu, s. li, g. y. li, ofdm and its wireless applications: a survey, ieee transactions on vehicular technology, 58(4) (2009), pp. 1673-1694. doi: 10.1109/tvt.2008.2004555 figure 2. comparison of papr for different techniques. figure 3. comparison of papr with respect to threshold values. figure 4. comparison of papr for v = 4 and v = 8 using different methods. figure 5. comparison of ber with respect to snr. table 1. ber obtained using different techniques for snr of 10 db (s surge of information). technique ber clipping at s = 90 10-2* clipping at s = 30 10-2.8 clipping at s = 15 10-3 fbmc original 10-3.4 sbo 10-3.45 aco 10-3.6 proposed sfla 10-3.8 https://doi.org/10.1109/tvt.2008.2004555 acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 5 [2] h.q. wei, a. schmeink, comparison and evaluation between fbmc and ofdm systems, proc. of international itg workshop on smart antennas, ilmenau, germany, 3-5 march 2015, pp. 1-7. 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https://doi.org/10.1109/access.2016.2605008 https://doi.org/10.1109/tbc.2005.856727 https://doi.org/10.1109/pimrc.2007.4393989 https://doi.org/10.1109/twc.2010.062310.100191 https://doi.org/10.1109/tbc.2010.2079691 https://doi.org/10.1109/glocom.2000.891239 https://doi.org/10.1109/vetecf.2003.1285976 https://doi.org/10.1109/pimrc.2003.1264266 https://doi.org/10.1016/j.measurement.2021.109267 http://dx.doi.org/10.21014/acta_imeko.v10i3.1020 http://dx.doi.org/10.21014/acta_imeko.v9i2.802 microsoft word article 17 mikes advertisement.docx acta imeko december 2013, volume 2, number 2, 96 www.imeko.org acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 96 section: advertisement arctic metrology centre for metrology and accreditation – mikes mikes-kajaani, tehdaskatu 15, puristamo 9p19, fi-87100 kajaani, finland staff of mikes-kajaani (from left): sauli kilponen, petri koponen, kari kyllönen, jani korhonen, timo nissilä and aimo pusa. second from right dr. rainer engel from the ptb germany. mikes-kajaani, the world’s northernmost national standards laboratory located near the arctic circle, has integrated well to the local research community, and serves both national and international customers with calibrations and special services. premises in the newly designed, renovated building have proper temperature, vibration and humidity control as well as precisely determined g-values. the group running the facility is built up from eight local experts. force, torque and heavy masses are in full service and serve industry in its calibration needs. the assembled three different water flow calibration rigs are also in service. mikes is a partner in a joint research centre cemis (centre for measurement and information systems; the umbrella organisation of measurement technology in kajaani) together with the universities of oulu and jyväskylä, kajaani university of applied sciences, and vtt technical research centre of finland. cemis specialises in research and training in the field of measurement and information systems. in this cooperation, mikes has been active in applied research varying from optical emission based measurement methods of heavy metals in water to ski base topography measurements. new thinking of reliable measurement results is implemented in every project. mikes-kajaani is also participating in a european metrology research project (emrp) aiming for better measurements of larger forces. calibration services at mikes-kajaani. contact information: group manager petri koponen, tel. +358 29 5054 453, firstname.lastname@mikes.fi, www.mikes.fi unit range relative uncertainty (k=2) remarks force 1 n – 1.1 mn 210-5 110-4 8 devices torque 0.1 nm – 20 knm 210-5 510-4 7 devices mass 50 kg – 2000 kg 210-6 3 devices water flow 0.2 l/s – 750 l/s 0.03 % 0.5 % gravimetric or reference consistency 0 – 12 % 0.2 % flow rate 0.5 m/s – 4.0 m/s (consistency dependent) real-time power quality measurement audit of government building – a case study in compliance acta imeko issn: 2221-870x march 2023, volume 12, number 1, 1 9 acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 1 real-time power quality measurement audit of government building – a case study in compliance with ieee 1159 d. v. n. ananth1, naeem hannoon2, mohamed shahriman bin mohamed yunus3, mohd hanif bin jamaludin3, v. v. s. s. sameer charavarthy4, p. s. r. chowdary4 1 department of electrical and electronics engineering, raghu institute of technology, visakhapatnam-531162, andhra pradesh, india 2 faculty of electrical engineering, uitm, 40450 shah alam, malaysia 3 centre of excellent for engineering & technology, melaka, malaysia 4 department of electronics and communication engineering, raghu institute of technology, visakhapatnam-531162, andhra pradesh, india section: research paper keywords: ieee 1159 compliance; power quality measurement; switchgear; fluke device; facts devices; custom-power devices; power auditing citation: d. v. n. ananth, naeem hannoon, mohamed shahriman bin mohamed yunus, mohamed hanif b. jamaaludin, v. v. s. s. sameer charavarthy, p. s. r. chowdary, real-time power quality measurement audit of government building – a case study in compliance, acta imeko, vol. 12, no. 1, article 21, march 2023, identifier: imeko-acta-12 (2023)-01-21 section editor: francesco lamonaca, university of calabria, italy received may 2, 2022; in final form february 7, 2023; published march 2023 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: d. v. n. ananth, e-mail: dvnananth1@gmail.com 1. introduction the power quality (pq) plays a vital role in utility systems, with emphasis on a) continuity deliver power supply, and b) quality or purity of voltage [1], [2]. this pq can be defined as the problem taking place in voltage, current or frequency deviations that result in failure or disoperation of electrical equipment or maintain the integrity of power supplied to the system. poor pq ‘events’ such as voltage sag, swells, transients, harmonics, unbalance, interruption, flickering, etc., are the various issues on pq [3]-[5]. it is crucial to perform a power quality audit to identify any pq problems in a system. the audit helps to understand the more sensitive types of equipment and devices in the industrial plant and associated issues they face, due to the current and voltage disturbances. many commercial pq monitoring instruments have sampling rates of 256 samples per cycle since the majority of pq events have frequency contents below 5 khz as observes in the reference paper [6]. the availability of high-end instruments to capture infrequent very high frequency events is limited due to technical and economical hurdles. the pq disturbances are mainly classified as short-term and long-term variation. higher load switching devices like drives are short-term and disturbances such as over-voltage, short-voltage faults come under long term [7]. there are specific standards that are followed to maintain equipment and personnel safety, and it depends on the country and the industry [8]-[9]. some associations accountable for emergent pq standards comprise the institute of electrical and electronics engineers (ieee), the abstract power quality (pq) measurements and auditing play a vital role for smart grid applications, industrial safety and reliability. the major electrical pq characteristics and parameters, like voltage sags/swells, harmonic distortion, voltage unbalance, voltage variation & flicker, and supply interruptions, are studied with the aim of maintaining the international or national standards. the electrical characteristics are studied and analyzed for single-phase and poly-phase systems in this ieee 1159 recommended practice. the significant pq constraints and parameters, like electromagnetic interference phenomenon deviation from the regular operation due to load equipment or source to load interaction, are described. further, it is discussed pq different monitoring devices, roles, and applications. in this paper, power quality monitoring of the equipment using fluke instrument and accessories to identify the equipment performance, audit, pq issues are discussed practically for a two-panel boards switchgear equipment as a case study in malaysia. here, trends in voltage and current changes with current imbalances were observed on 1st may 2019 morning 8'o clock and on 7th may 2019 evening at 9'0 clock. mailto:dvnananth1@gmail.com acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 2 international electronic commission (iec), national electrical manufacturers association (nema), national fire protection association (nfpa), national environmental policy act (nepa), and american national standards institute (ansi), etc., [10]. the true value of any pq monitoring program lies in its ability to analyse and interpret voluminous raw data and generate actionable information to prevent pq problems or improve the overall power quality performance. ieee 1100-1999 and ieee 1159 are one of the finest sources for describing pq concerns and providing solutions to equipment and system difficulties. pq refers to the powering and grounding of electronic equipment in a way that is appropriate for the device's functioning and compatible with the property's wiring system and other connected equipment. moreover, practice for powering and grounding is ieee std. 1100-1999 and monitoring electrical power quality is ieee std. 1159-2009 [11]. the iec also provides a platform to companies, industries to discuss and develop the standards required like iec std. 61000-2-1, electromagnetic compatibility (emc environmental description for low frequency carries out disturbances and signalling in the municipal power supply system [11]. the standard definition of ieee std. 1159-2009 is “any power problem manifested in voltage, current, or frequency deviations that results in failure or disoperation of customer equipment depending on deficient power quality” [12]. there is research on ieee std. 1159 and different authors suggested notable remarks on maintenance of the standards for equipment and personnel safety. the s-transform method used as input to this hybrid classifier of pq disturbances shows pleasing results of efficient recognition and classification with the proposed real-time method [13]. an s-transform based semisupervised approach is discussed to solve classification crises without the necessity of sample data. first, pq disturbances detection using the s-transform chosen to exact enough pq time-frequency events characteristic with s-matrix for later categorization under noise, and traits are attained. in the next step, seven to eight binary classifiers are created with the normalized ruler to recognize these disturbances. finally, these multiple pq disturbances are classified using this method [14]. a sampling rate of 15 to 18 khz and de-noised with the help of discrete wavelet transform (dwt) based technique requires signals with superior signal-to-noise ratio classification pq features with an accuracy of 98.18 % as contrasted to the generalized wavelet algorithm [15]. many practical types of equipment like flexible ac transmission system (facts) devices improve the overall pq standards and norms for companies and industries. two types of facts devices were conducted in india in the real-time environment on the distribution power system under the case studies with devices like distribution static compensator (dstatcom) and static var compensator (svc). this helped to exemplify the modelling technique and the efficacy of these facts and custom power devices in curtailing financial losses [16]. another case study using a dvr, for voltage sag mitigation with a conventional pi controller [17]. here, better dynamic restoration under the voltage disturbance and the ability of postfault resurgence as a case study is observed on an extensive electrical distribution system. appropriate evaluation techniques are verified for the outcome without, and with-compensation devices are studied to evaluate the role of heat-maps. in a similar way, soft computing-based techniques are alternatives for pq parameter enhancement like voltage regulation, power factor control, harmonics, dynamic loads, and non-linear loads in spite of conventional techniques. the softcomputing techniques for pq applications are particle swarm optimization (pso) algorithm [18], security and cost-optimal allocation of multifacts devices using multi-objective (pso) [19], few advanced non-linear methods like pq theory & fractional order pid controller in dpfc for pq improvement [20]. the pq performance using facts and renewable energy sources (res) with the technical and economic evaluation is discussed [21], [22]. the payback period calculation applies facts devices in the case study with the metallurgical industry [23]. a malaysian case study on distributed generation: a review on current energy status, grid-interconnected pq issues, and implementation constraints [24]. in this paper, carbon emissions, res, different countries' electrical pq standards and regulations, additional facts and custom-power devices, techniques for pq enhancements as a review is made exclusively in detail. the major objectives of the present work are to record and interpret raw data into useful information, analyse the pq measurement results and understand the characteristics of pq variations on the power supply. many pq events are organized worldwide to improve the pq standards for the company and industry [25]-[28]. these are done by american power laboratory, canadian electrical associations, unipede europe, and many standard organizations. the major objectives of the work are to record and interpret raw data into useful information, analyse the power quality measurement results and understand the characteristics of power quality variations on the power supply. books discussing on various pq issues, harmonic analysis, sources, distortion and monitoring and further study on equipment overheating, motor failure, capacitor failure, and incorrect power metering are examples of issues with harmonic distortion [29]-[30]. measurements are made of the components of a single-phase induction motor with closed or semi-closed rotor slots that is capacitor-start and capacitor-run, and a model is developed for the examination of the machine's performance in the frequency domain under the effect of harmonic voltages [31]. the current versions of ieee and iec are too restrictive for low-frequency voltage and current (integer) harmonics as they apply to residential power systems, according to experimental and analytical studies of the effects of voltage and current harmonics on induction machines, transformers, appliances, and relays. it is advised that flexible, rather than strict, rules be set up such that various harmonic spectra be allowed that cause the same extra temperature to rise in transformers and induction machines [32]-[33]. power quality factors are evaluated using a variety of computer algorithms, each with unique benefits. in order to compare the perceived power quality of complicated electric systems, this study improved the curve fitting algorithm (cfa), which has excellent accuracy in the assessment of the signal's power quality characteristics [34]. the tracking of pq metrics by biggest italian telecommunications company had it happen inside four medium/low voltage transformer rooms of telecommunication power substations. the tests were carried out using a suitable home-made apparatus that samples the mains' three-phases and neutral lines over the course of around two years and the results are presented [35][36]. in this paper, the objective is not to describe the signal processing or artificial intelligent methods, other than quite to discuss challenges and probable appliance of signal processing techniques in spiralling raw pq measurement data to a much acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 3 more valuable commodity-knowledge and information to improve pq performance. section 2 of the paper presents research methodology with field measurement approach, while sections 3 and 4 provide descriptions on case study background with equipment installation and erection locations and results and analysis of the proposed method with fluke measuring device to analyse raw pq measurement data. the applications described provide the basis for research efforts done in centre of excellent for engineering & technology (create), malaysia (many of which are under way around the world) to identify new and improved methods for the data analysis and development of important conclusions from the measurement data. the main key research queries explored are: • what affects the pq in the switchgear terminal unit? • what existing standards help in pq improvements in malaysia? • what is the solution/enhancement for the current imbalance and other pq issues? in this reverence, the explicit research objectives on pq events are: 1. analyse the pq events and their impact on the supply system and sensitive loads; 2. explore the precise pq events distressing create switchgear terminal unit and the distribution units; 3. perform a pilot survey on pq events to review the contentment level with the services accessible; 4. monitor the pq events and suggest suitable monitoring instruments. 2. research methodology (field measurement) the pq monitoring and analysis in a practical environment using the field-measurement method in the flowchart representation are shown in figure 1a. a closed-loop circular diagram representation is shown in figure 1b. in this diagram, we can observe the equipment location at one point or multiple points has to be chosen initially. later, these pq monitoring devices are installed based on voltage and power specifications. the data is stored and retrieved for the pq analysis, with any deviation from typical values under current unbalance or voltage disturbances are analysed and recorded. after this, the pq equipment and devices are changed in other switchboards, and the process is repeated for auditing and measuring. finally, characteristics tables are drawn, deviations in parameters shown in tables, and graphs are stored for records. the figure 1a and figure 1b depict, analyse and classify the power quality problems. this paper focuses on field measurement analysis technique to analyse pq waveforms and signals like voltage and current. with the installation of the pq measuring devices, the pq events are recorded, summarized and analysed to understand the sequence of the event, influence on the nearest, farthest and sensitive loads are examined in detail. field measurement based auding technique with fluke like pq measuring device is very popular in understanding the pq events. this method presents a good time resolution for narrow window while the large window is useful for good frequency resolution. in addition, this method has advantages as the system parameters dependency is decreased, mathematical modelling, reduced complex erections and therefore it is easy to implement and also the measuring devices involved are all not necessary. this paper presents the development of pq monitoring system by using field measurement analysis technique which is an auding based using pq measuring devices. from fluke device, parameters of the power quality signals are estimated such as rms and fundamental value, total harmonic distortion (thd), current to neutral and current through ground for voltage and current. then, characteristics of the signals are measured using these devices and calculations are made from the signal parameters based on ieee std. 1159-2009 as shown in flow chart in figure 1a. from the time illustration, pq parameters are measured such as instantaneous of rms voltage, rms fundamental voltage, total waveform distortion, total harmonic distortion and total non-harmonic distortion. then, characteristics of power quality signals are calculated from the signal parameters and are used for signal classification. the signal parameters and characteristics and their influence on the system are studied in detail in the further sections. 3. case study background the centre of excellent for engineering & technology (create), malaysia industrial erection to understand the pq auditing, recommended practice, current disturbance, and impact, location of the pq devices is studied. two sets of pq a) b) figure 1. a) flowchart showing practical pq monitoring equipment installation and b) process flow schematic closed-loop operation for pq analysis. result power quality analyzer identify the site locations installation of advance signal processing equipments records the data from the analyzer for some period of time.analyze the data of voltage, current and frequency. observe any pq issues such as voltage fluctuations etc. if pq issues are observed, install pq regulators to improve the system performance. acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 4 fluke devices are located at point of measurement 1, and 2 with layout diagram is shown in figure 2a. here, the parameter symbols db ha/ g/ p and e represent distribution generator (dg), ground (g), phase (p), earth (e) for our switchgear electrical medium voltage switch board-1 (emsb) as shown in figure 2a. the total pq devices, individual components, and accessories used for the measurement and data storage are shown in figure 2b. after connection and erection of the pq devices, the switchgear board unit looks as in figure 2c. the fluke device and other pq devices after erection at the switchgear unit at create, malaysia in figure 2d. the point of measurement-1 erection unit equipped is shown in figure 2e. the model and type of the pq equipment used in paper are shown with their purpose and type number in table 1. the create electrical medium voltage switch board-1 (emsb-1), malaysia looks to improve the pq standards and norms and reliability of supply to the building for research analysis. between may to september 2019, create used this project with self-study analysis to install power quality measuring devices and parameter values in various selected sub-locations in the switchgear terminal. in april 2019, complete quality of service came to the picture, and the results are recorded for retrieving and analysis. it is established to track the pq monitoring and reporting features under normal and abnormal current balance conditions. the research proposes that, if put into practice will provide a better solution in solving the pq events and maintenance of the ieee 1159 compliance recommended in table 2. based on the ieee 1159 compliance, table 2 describes the range and nominal operating conditions constraints. the power interruption must be more than 1 minute of time with voltage less than 10 % of its nominal value of 230 v (< 23 v). the over-voltage is defined as voltage greater than 10 % and under-voltage is less than 6 % of nominal 230 v. the voltage unbalance is referred to 2 % change in any phase voltage from the nominal value and current unbalance is 30 % of operating load current. the highest neutral a) b) c) d) e) figure 2. a) layout of the indoor switchgear board with the pq equipment erection, b) fluke pq based components used in our case-study, c) the indoor switchgear board at pc-1 ready for erection, d) fluke pq based components erected and ready for testing, e) point of measurement-1 erection unit. table 1. model and type of the pq equipment. meter used purpose model no. fluke 1750 power recorder power quality analyzer 1750 multi clamp leaker measurement of leakage currents 140 fluke true rms multimeter voltage measurement 117 fluke clamp meter current measurement 376 table 2. categories and compliance standards with ieee 1159. categories compliance ieee 1159 power interruption more than 1 minute, 10 % (23 v) overvoltage > 253 v (+10 %) under-voltage < 216.2 v (-6 %) voltage unbalance 2 % current unbalance 30 % high neutral to earth voltage 3 v ne impulse > 20 v transient voltage dip (sag) 23 v – 207 v voltage swell > 253 v voltage fluctuations 6 % thd (voltage) 5 % thd (current) 3 % dc offset 0.10 % acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 5 (n) to earth (e) voltage must be within 3 v and the impulse voltage is said to be greater than 20 v between these ne terminals. the voltage sag is defined for a nominal voltage of 230 v is between 23 to 207 volts and swell if the voltage is greater than 253 volts. the voltage fluctuation is said be when there is ±6 % variation in voltage range from 230 volts and this repeats for every millisecond of time with the nominal voltage. the maximum allowable total harmonic distortion (thd) voltage is 5 % and current is 3 % and the dc offset or dc component on the ac supply should be less than 0.10 %. these are the allowable ranges for the standard ieee 1159/ 2009. the pq event under voltage-time stamp is shown in the pictorial form in figure 3. here, 90-110 % of the voltage event magnitude on the y-axis represents regular operation, and 100 % is ideal voltage. the voltage between 0 to 90 % is under-voltage and beyond 110 % is over-voltage. in the x-axis, the time up to 0.5 cycles considered to be notch or transient for under-voltage and over-voltage. a momentary fault is from 0.5 cycles to 3s, and to 1 minute is called the temporary fault, or beyond 1 minute is called sustained interruption. after discussing the practical layout and basic background on the work, results are discussed based on practical case studies in the next section. 4. results and discussion in this section, different case studies are studied and analysed at (emsb1 point)-voltage values based on the practical reading. here, voltage waveforms and data recorded are explored in the first case under steady-state analysis. later in the second case, load variation is dealt and in the last case, under very light load conditions is analysed. 4.1. case 1: voltage flow diagram of the three phases under normal operating conditions the three phases voltage distribution at emsb discussed in figure 2 switchgear terminal location is shown in figure 4. here, the rms voltage variation in all the three phases shown in the red, blue, and black waveform is between 248 to 240 volts under current imbalance regular operation in an industrial/company location. voltage surges observed in the fluke meter having amplitudes 221.8 v and 219.4 v because of load shifting observed in the figure. this current imbalance leads to voltage sag-like behaviour but is within limits. table 3 shows the deviation from the expected value with maximum and minimum values in three different phases and the neutral point at various times from 1st may to 6th may 2019. the system can also be utilized to capture continuous sampling as well as collecting and recording the real time signal. the data is then analysed to be stored in database. this system can set specific time to store data into computer automatically. figure 8 shows examples of data which has been stored in notepad by using this system. based on the measured values for voltage and current using the system, the results obtained are comparable to the measured values using the existing equipment which is fluke power quality analyser. the development of the real-time power quality monitoring system is shown in figure 2a to figure 2f. the system is capable of measuring all standard power line measurements such as voltage (rms), current (rms), frequency, real power, reactive power, apparent power and power factor that are also plotted in graph as shown in figure 4. in addition, the voltage and current signals are shown in time and frequency domain as illustrated and tabulated. based on table 3, the significant observations found are maximum average rms voltage vl1n, vl2n & vl3n at p1 are 246.2 v (7.04 %), 246.37 v (7.12 %), and 247.47 v (7.60 %), respectively, which refer to a nominal voltage of 230 v. the average rms vng is 2.91 v, which is lower than the maximum requirement (3 v). the voltage fluctuation is 5.45 %, which complies with the standard requirement (< 6 %) for a trend of 10 minutes of data. 4.2. voltage flow diagram of the three phases under load variation conditions a fluctuating balanced load and the voltage behaviour at the emsb-1 are analysed in this case study here. the current is varying from 50 a to 160 a under light load to full load usage condition is shown in the right side of the graph plat, and voltage rms parameters are shown in the left side of the plot. the voltage deviation is observed between 223 volts to 247 volts for all three phases, neutral and ground, shown in figure 5. it can be observed, the maximum and minimum rms current of all the figure 3. pq voltage event description with the duration time stamp. figure 4. rms voltage of the three phases recorded in may 2019 at emsb-1 at the point of connection-1. table 3. maximum and minimum rms voltage of three phases and neutral from 1st may to 6th may 2019. phase max (rms) time min (rms) time v rms avg l1n 246.20 v 01/05/2019 08:00:00 233.59 v 03/05/2019 10:50:00 v rms avg l2n 246.37 v 01/05/2019 07:30:00 234.06 v 03/05/2019 10:50:00 v rms avg l3n 247.47 v 01/05/2019 07:30:00 234.93 v 03/05/2019 10:50:00 v rms avg ng 2.91 v 06/05/2019 07:10:00 1.29 v 03/05/2019 17:10:00 acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 6 three phases and also in the neutral line, the current imbalance is observed at the switchboard emsb-1 during the that during the period 30th april may to 8th may 2019. the details are tabulated as shown in the table 4. the maximum and minimum 10 min trends data for currents at emsb1(p1) is recorded for the analysis. the maximum average rms current l1, l2 & l3 at p1 is 158.81 a, 181.25 a and 158.51 a, respectively. the current unbalance is 14.97 % is lower than the maximum permissible value (< 30 %). the foremost concern for malaysian electrical power system is to sustain reliable power supply, maintain almost constant voltage with variable load condition. the present real-time study plays a vital role in understanding the effect of line or phase of the power system, timing at which it influences, unbalance, neutral current concern and grounding technique are the main factors observed in this case study. 4.3. voltage flow diagram of the three phases under very light load variations conditions in this case, light loads like lighting only are applied, and the voltage fluctuations are analysed. a light-load is in general, results in increase in the voltage and hence to be regulated to reach the nominal value. the voltage behaviour at the emsb-2is analysed for 10-minute time scale between 1st to 5th may 2019 in this present case study. the voltage varies from 232 v to 248 v under light load usage condition is shown with voltage rms parameters recorded at db ha/g/p/e (p2) for all the three phases, neutral and ground, at rms current of l1, l2 dan l3 phases shown in figure 6 and table 5. the normal rms current of three phases is 18 a, and neutral current is 6 a is observed at emsb-1 is summarized as depicted in figure 7 and table 6. in this case study, a 10-minute trend in the voltage at the point of connection (p2) is analysed and shown in figure 7. the maximum average rms voltage vl1n, vl2n & vl3n at p2 are 245.07 v (6.55 %), 245.30 v (6.65 %), and 247.59 v (7.65 %), respectively, which refer to a nominal voltage of 230 v. the average rms vng is 3.5 v, which is higher than the maximum requirement (3 v). the voltage fluctuates 7.2 %, which is higher than the standard ieee 1159 requirement (< 6 %) and within limits. by managerial control, harmonic spread among areas and the system unbalance are dealt with and general pq development of the proposed field measurement is provided. the maximum and minimum of 10 min trends data for currents at db ha/g/p/e at the point of connection (p2) are analysed in this case study. the average rms current maximum figure 5. rms voltage of the three phases at emsb-1 at the point of connection-1 under various loading conditions. table 4. maximum and minimum rms current of three phases and neutral from 30th april, may to 8th may 2019 under current imbalance operation observed at emsb-1. phase max (rms) time min (rms) time a rms avg l1n 158.81 a 07/05/2019 07:40:00 51.54 a 01/05/2019 10:30:00 a rms avg l2n 181.25 a 01/05/2019 21:10:00 50.86 a 30/04/2019 18:40:00 a rms avg l3n 158.51 a 08/05/2019 00:20:00 53.58 a 01/05/2019 10:20:00 a rms avg n 1.57 a 01/05/2019 04:00:00 1.41 a 01/05/2019 09:40:00 a rms avg g 0.12 a 01/05/2019 08:30:00 0.07 a 30/04/2019 17:20:00 % a unbalance 14.94 03/05/2019 19:10:00 2.40 06/05/2019 07:20:00 figure 6. rms voltage of the three phases at emsb-1 at the point of connection-1 under very light load conditions. table 5. maximum and minimum rms current of three phases and neutral from 1st may to 7th may 2019 under light load operation at emsb-1 phase max (rms) time min (rms) time v rms avg l1n 245.07 v 01/05/2019 08:30:00 232.15 v 03/05/2019 10:50:00 v rms avg l2n 245.30 v 01/05/2019 08:30:00 232.86 v 03/05/2019 10:50:00 v rms avg l3n 247.59 v 01/05/2019 07:30:00 234.05 v 03/05/2019 10:50:00 v rms avg ng 3.50 v 02/05/2019 08:00:00 1.56 v 07/05/2019 19:30:00 figure 7. rms current for the three phases at db ha/g/p/e point-current values at emsb-2 from 30th april to 4th may 2019. v o lt s r m s a m p s r m s recorded volts/amps/hz ref erence timeplot: 30/04/2019 12:21:56 08/05/2019 10:16:23 click on any axis and drag to scroll acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 7 l1, l2 & l3 at p2 are 1.11 a, 2.94 a, and 7.04 a. the current unbalance is 54.85 %, which is higher than the requirement by ieee 1159 (< 30 %), which is also within limits. the final summarized form of the computer and business equipment manufacturer's association (cbema) curve methodology (field measurement) is described in figure 8. in this graph, the y-axis represents voltage deviation, and the xaxis denotes a time scale. it is observed that 289 under-voltage disturbances for a maximum of 8.3 ms are observed in one year from may 2019 to april 2020. also, 98 under-voltage (sags), 18 interruptions, 164 over-voltage (swell) and 19 transients are observed under the cbema curve. hence, this type of analysis with pq equipment and chart-based analysis with the help of the cbema curve aid in understanding the pq disturbances and formulations to be done for the company/industry based on the ieee 1159 standards. consequently, it is observed that the pq problem of create electrical medium voltage switch board-1 (emsb-1) and emsb2, system requires to be concentrated more as the pq condition in one sub-area in one sub-station or one electrical terminal will definitely influence the other electrical terminals pq level due to unbalance, neutral current flow and harmonic propagation. the proposed method is recommended and the use of fluke and other pq devices with necessary testing equipment needs to be installed at the electrical sub-system or main point where the sensitive loads are connected at the point of the system. the pq assessment, practices are observed with the auditing technique involved and observed under the existing unbalance and sudden load varying condition. in this paper, the development of real-time monitoring system for power quality signal is developed by using field measurement method. the system performs power line measurement such as voltage and current in root mean square (rms), real power, apparent power, reactive power, frequency and power factor. from the tfr of power quality signal, parameters of the signal are estimated such as rms voltage, rms fundamental voltage, total waveform distortion, total harmonic distortion and total non-harmonic distortion. based on the monitoring testing, the system exhibits excellent accuracy in calculation due to low percentage error compared to the existing equipment. 5. conclusions a real-time power auditing with field measurement proposed scheme in this paper to advance the pq of a sub-station when the pq requirements and challenges are different. the voltage fluctuation rate at db ha/g/p/e is 7.2 %, which is higher than 6 % (ieee 1159), which is expected due to high impedance in the installation. voltage unbalance, current sudden changing with load variation, harmonics, are observed using pq measurement devices like fluke etc., and auding is done and then our technique is applied in this paper. high neutral to the ground voltage (average vng rms) at db ha/g/p/e is more than 3 v (3.5 v at the site) which is expected due to intermittent termination or loose contact at the grounding system. the activity of high vng impulse observed at installation, which is expected due to intermittent single line-to-ground fault, which can affect electrical equipment. from the real time observation, it is found that, we can increase the efficiency of power distribution in an industrial network is possible with proper electrical auditing. it is found that, the solution for harmonic and unbalancing in a power sub-station to the load point. this will mainly influence the sensitive loads like computer, scada, small lighting systems etc. in addition, the proposed technique is useful to enhance the pq of existing sensitive load buses (or cps) without installation of a new active power filters or external devices in the respective buses. it is due to incorporated pq upgrading of the sub-system is addressed employed with the available devices. pq should also be adopted as part of a preventive maintenance program. this type of data analysis done in create electrical medium voltage switch board-1 (emsb-1 and 2), practically helps in studying environmental, economic, and engineering impacts on the power systems. the main contribution of the proposed technique is pertaining an effective array of available pq devices, advanced cbema methods in the electrical power auditing process so that the effect of one parameter on the other parameter, grid influence, role of neutral and ground are understood and helped in pq improvement. to this aim, pq events and suggest suitable monitoring instruments are done in this paper. the effect of light load, current imbalance, and standard operating conditions also considerably influences the system's voltage deviations. because of the harmonic propagation, current unbalance and mutual effects of unbalances between sub-stations and the load points, the loading order of each sub-station is of vast significance and this concern is addressed by using a scheduling framework and regular power auditing is observed with the proposed process. if non-linear and unbalanced loads are considered, the impact of voltage deviations is very high, which is to be carefully investigated in future studies. the cost of pq devices, location, type of loading, disturbance play a vital role in pq improvement. the concluding section contains the major achievements of the research presented in the manuscript. it should be concise but table 6. maximum and minimum rms current of three phases and neutral from 30th april may to 8th may 2019 under light load operation observed at emsb-2. phase max (rms) time min (rms) time a rms avg l1 1.11 a 3/05/2019 11:20:00 0.55 a 02/05/2019 10:30:00 a rms avg l2 2.94 a 0/04/2019 17:00:00 2.31 a 01/05/2019 11:00:00 a rms avg l3 7.04 a 2/05/2019 16:10:00 5.84 a 04/05/2019 07:50:00 a rms avg n 6.68 a 0/04/2019 17:00:00 5.60 a 04/05/2019 18:30:00 a rms avg g 0.16 a 0/04/2019 17:00:00 0.05 a 04/05/2019 18:00:00 % a unbalance 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https://www.scopus.com/record/display.uri?eid=2-s2.0-0023327247&origin=reflist https://www.scopus.com/record/display.uri?eid=2-s2.0-0023327247&origin=reflist https://www.scopus.com/record/display.uri?eid=2-s2.0-0023327247&origin=reflist https://doi.org/10.1109/tpwrd.1987.4308128 https://doi.org/10.1109/intlec.2007.4448800 https://doi.org/10.3390/electronics5040064 editorial to selected papers from imeko tc1-tc7-tc13-tc18 joint symposium and mathmet (european metrology network for mathematics and statistics) workshop 2022 acta imeko issn: 2221-870x december 2022, volume 11, number 4, 1 2 editorial to selected papers from imeko tc1-tc7-tc13-tc18 joint symposium and mathmet (european metrology network for mathematics and statistics) workshop 2022 eric benoit1 1 université savoie mont blanc, polytech'savoie, lab. d'informatique, systèmes, traitement de l'information et de la connaissance (listic) b.p. 80439 annecy-le-vieux cedex 74944, france section: editorial citation: eric benoit, editorial to selected papers from imeko tc1-tc7-tc13-tc18 joint symposium and mathmet (european metrology network for mathematics and statistics) workshop 2022, acta imeko, vol. 11, no. 4, article 2, december 2022, identifier: imeko-acta-11 (2022)-04-02 received december 28, 2022; in final form december 28, 2022; published december 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: eric benoit, e-mail: eric.benoit@univ-smb.fr dear readers, this issue includes a first selection of papers issued from the imeko tc1-tc7-tc13-tc18 joint symposium and mathmet (european metrology network for mathematics and statistics) workshop “cutting-edge measurement science for the future” held in porto in august 2022. due to the pandemic, theses four imeko communities didn’t have the possibility to meet face to face since 2019 and the enthusiasm of all authors to expose their studies definitely contributed to the success of this event. for this reason a second selection will be presented in a future issue. three papers related to education and training in measurement and instrumentation are presented in this issue. jakub svatos and jan holub explored the impact of the pandemic on education of measurement with the paper “how the covid19 changed the hands-on laboratory classes of electrical measurement” [1]. dominik pražák et al present “a training centre for intraocular pressure metrology” [2]. on another field of measurement education and training, raik illmann, maik rosenberger and gunther notni expose their “training program for the metric specification of imaging sensors” [3]. the measurement of biophysical properties is a topic shared by several teams during the symposium. it is considered, first by francesco crenna et al with their paper on “biomechanics in crutch assisted walking” [4], and by jakub wagner and roman z. morawski with a “spatiotemporal analysis of human gait, based on feet trajectories estimated by means of depth sensors” [5]. finally, this first selection ends with 2 papers on knowledge management considerations related to measurement science: in “contrasting roles of measurement knowledge systems in confounding or creating sustainable change” [6], william p. fisher compare the scientific modelling and the statistical modeling in the context of promoting sustainable change. in “a mathmet quality management system for data, software, and guidelines” keith lines et al present the essential components of the mathmet qms that has the potential to become a reference in the measurement community [7]. i hope you will enjoy your reading. eric benoit section editor references [1] jakub svatos, jan holub, how the covid-19 changed the handson laboratory classes of electrical measurement acta imeko 11 (2022) 4, pp. 1-7. doi: 10.21014/actaimeko.v11i4.1301 [2] dominik pražák, vítězslav suchý, markéta šafaříková-pštroszová, kateřina drbálková, václav sedlák, šejla ališić, anatolii bescupscii, vanco kacarski, a training centre for intraocular pressure metrology, acta imeko 11 (2022) 4, pp. 1-7. doi: 10.21014/actaimeko.v11i4.1300 [3] raik illmann, maik rosenberger, gunther notni, training program for the metric specification of imaging sensors acta imeko 11 (2022) 4, pp. 1-6. doi: 10.21014/actaimeko.v11i4.1361 [4] francesco crenna, matteo lancini, marco ghidelli, giovanni b. rossi, marta berardengo biomechanics in crutch assisted walking, acta imeko 11 (2022) 4, pp. 1-5. doi: 10.21014/actaimeko.v11i4.1328 [5] jakub wagner, roman z. morawski, spatiotemporal analysis of human gait, based on feet trajectories estimated by means of depth sensors acta imeko 11 (2022) 4, pp. 1-7. doi: 10.21014/actaimeko.v11i4.1349 mailto:eric.benoit@univ-smb.fr https://doi.org/10.21014/actaimeko.v11i4.1301 https://doi.org/10.21014/actaimeko.v11i4.1300 https://doi.org/10.21014/actaimeko.v11i4.1361 https://doi.org/10.21014/actaimeko.v11i4.1328 https://doi.org/10.21014/actaimeko.v11i4.1349 acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 2 [6] william p. fisher, jr., contrasting roles of measurement knowledge systems in confounding or creating sustainable change acta imeko 11 (2022) 4, pp. 1-6. doi: 10.21014/actaimeko.v11i4.1330 [7] keith lines, jean-laurent hippolyte, indhu george, peter harris, a mathmet quality management system for data, software, and guidelines acta imeko 11 (2022) 4, pp. 1-6. doi: 10.21014/actaimeko.v11i4.1348 https://doi.org/10.21014/actaimeko.v11i4.1330 https://doi.org/10.21014/actaimeko.v11i4.1348 the experience gained from implementing an iso 56000-based innovation management system acta imeko issn: 2221-870x june 2023, volume 12, number 2, 1 6 acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 1 the experience gained from implementing an iso 56000based innovation management system tzvetelin gueorguiev1 1 university of ruse “angel kanchev”, 8 studentska street, 7004 ruse, bulgaria section: research paper keywords: innovation management system; iso 56000 series; intellectual property management; best practice citation: tzvetelin gueorguiev, the experience gained from implementing an iso 56000-based innovation management system, acta imeko, vol. 12, no. 2, article 37, june 2023, identifier: imeko-acta-12 (2023)-02-37 section editor: leonardo iannucci, politecnico di torino, italy received january 29, 2023; in final form may 27, 2023; published june 2023 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: tzvetelin gueorguiev, e-mail: tzgeorgiev@uni-ruse.bg 1. introduction innovations are a key factor for organizational success. the ever more frequent use of the word ‘innovation’ threatens to cause lack of sensitivity towards its true essence. in a way this is similar to the use of the word ‘quality’ which now represents anything meaning perfection, excellence, conformity, satisfaction, etc. the iso 9000 series of standards is globally recognized as a model of a successful management system. the fundamentals, vocabulary and the quality management principles are established in iso 9000:2015. the requirements for a quality management system are specified in iso 9001:2015. the guidance on the implementation of iso 9001 requirements, as well as some examples are given in iso/ts 9002:2016. the guidance to achieve sustained success of the organization is given in iso 9004:2018. this standard also contains a 5-level self-assessment tool for various quality management processes. clause 11.4 defines the guidance for processes related to innovation. in time, the upward trend of iso 9001 certified quality management systems has somewhat slowed down thus giving way to other management systems, such as information security, energy, and more specificallyto innovation management systems [1]. this is reaffirmed by lines et al. along with highlighting the importance of the process approach and the “plan-do-check-act” (pdca) cycle for all management system standards published by the international organization for standardization, iso [2]. the pdca cycle and innovations are also part of the standard iso 21001:2018. the factor which practically makes the difference in quality management systems is the rate of improvementbe it incremental or continual. any innovation may be considered to be an improvement, but not every improvement is innovative. this paper presents an overview of iso 56000 based innovation management systems in order to better understand the significance of such management systems and the perceived benefits from their implementation. the current state of the iso 56000 series of standards for innovation management systems is discussed in section 2. in section 3 some examples of the implementation of guidance standards from the iso 56000 series at the university of ruse are described. finally, in the concluding section, some of the proven practices for innovation management are summarized. abstract the aim of this paper is to share the experience gained from an early adoption of the iso 56000 series of standards for innovation management systems. the university of ruse is among the first universities in bulgaria to implement an iso 9001 quality management system. later this system is updated and extended with elements of iso 21001:2018management systems for educational organizations. since 2020 the technology transfer and intellectual property centre (ttipc) at the university of ruse has implemented a number of requirements and guidelines of the iso 56000 series of standards. the foundation of the integrated management system is iso 56002:2019 with guidance for innovation management. the principles for innovation management from iso 56000:2020 are being followed while implementing key methods and tools from iso 56003:2019 for innovation partnership, iso/tr 56004:2019 for innovation management assessment, iso 56005:2020 for intellectual property management, and iso 56006:2021 for strategic intelligence. mailto:tzgeorgiev@uni-ruse.bg acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 2 2. the iso 56000 series of standards the need for a specific standard for innovation management systems became evident at the turn of the 21-st century. a detailed chronology of the development of innovation management standards is presented by da silva [3]. in 2008 the european committee for standardization (cen) has created a specific technical committee on innovation managementcen/tc 389 which published the first set of international standards on innovation management – the cen/ts 16555 series. this initiative is continued by the international organization for standardization which established a similar technical committeeiso/tc 279 “innovation management”. until january 2023, iso has published six standards for innovation management, part of the iso 56000 series: • iso 56000:2020 innovation management fundamentals and vocabulary [4]; • iso 56002:2019 innovation management innovation management system guidance [5]; • iso 56003:2019 innovation management tools and methods for innovation partnership guidance [6]; • iso/tr 56004:2019 innovation management assessment guidance [7]; • iso 56005:2020 innovation management tools and methods for intellectual property management guidance [8]; • iso 56006:2021 innovation management tools and methods for strategic intelligence management guidance [9]. four additional iso standards are at different stages of development: • iso/awi 56000 innovation management fundamentals and vocabulary (an update of the standard iso 56000:2020); • iso/cd 56001 innovation management innovation management system requirements [10]; • iso/fdis 56007 innovation management system tools and methods for managing opportunities and ideas guidance [11]; • iso/dis 56008 innovation management tools and methods for innovation operation measurements guidance [12]; • iso/cd ts 56010 innovation management illustrative examples of iso 56000 [13]. the similarity of the concepts behind iso 9000 and iso 56000, and iso/ts 9002 and iso 56002 is noticeable. yet, the approach applied for developing innovation management standards is substantially different. instead of publishing the standard with requirements for innovation management systemsiso 56001 first, and then giving guidance on their implementation, iso/tc 279 has chosen a more customer-friendly approach. this “innovation” allows the potential developers, implementers and auditors of innovation management systems to establish their systems based on the guidance and supporting standards. only when sufficient practice in maintaining such systems is in place, the requirements standard will be used to audit and possibly certify for conformance to iso 56001. 2.1. iso 56000:2020 innovation management fundamentals and vocabulary this standard is prepared by iso/tc 279 “innovation management” with the purpose to clarify the terminology, concepts and principles to be used in the whole iso 56000 series of standards. 8 groups of terms are defined in clause 3. they are related to innovation, organization, objective, knowledge, intellectual property, innovation initiative, performance, and assessment. the alignment of these terms and definitions with the oslo manual and the definitions of intellectual property in trips (agreement on trade-related aspects of intellectual property rights) and wipo (world intellectual property organization) is made clear in annex b of iso 56000:2020. in addition to clarifying the impact of innovations, clause 4 outlines the fundamental concepts and the 8 innovation management principles: • realization of value; • future-focused leaders; • strategic direction; • culture; • exploiting insights; • managing uncertainty; • adaptability; • systems approach. each of these principles is detailed using a structure of 4 elements: statement, rationale, key benefits, and possible actions. this allows the innovation managers in the organization to have a clear purpose (the statement), some helpful information to raise awareness and achieve motivation of the interested parties (rationale and key benefits), as well as an outline of an action plan to establish the foundations of a solid innovation management system (possible actions). 2.2. iso 56002:2019 innovation management system guidance this standard follows the high-level structure of annex sl. this makes the clauses of iso 56002, parts of the text, some terms and definitions identical with the ones used in other management systems standards, and more specificallyiso 9001 for quality management systems. some elements are added to the “backbone” of annex sl such as: innovation culture, innovation vision, innovation strategy, innovation portfolios, etc. in clause 7 “support”, important additions are: tools and methods, strategic intelligence management, and intellectual property management. these elements are further explained and extensively discussed in the supporting standards of the iso 56000 series. new structural elements in clause 8 “operations” are the innovation initiatives and the innovation processes comprising of identifying opportunities, creating concepts, validating concepts, developing solutions, and deploying solutions. these steps of the innovation process are also aligned with the steps of the pdca cycle, just like the overall arrangement of the clauses of iso 56002. the role of metrology is clearly outlined in clause 9 “performance evaluation”, and more specifically in 9.1 “monitoring, measurement, analysis and evaluation”. clause 10 “improvement” completes the pdca cycle with deviation, nonconformity and corrective action, and continual improvement of the innovation management system. this general analysis of the standard iso 56002:2019 should not be misleading. the in-depth analysis of the content of each clause shows a specific focus on innovation management which cannot be found in other management system standards. acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 3 2.3. iso 56003:2019 tools and methods for innovation partnership innovations are often the result of teamwork and partnerships. the guidance of iso 56002:2019 refers to partners on a number of occasions, such as: • a tool for managing uncertainty and risk; • an element of the context of the organization; • a key interested party; • an element to consider when determining the scope of the innovation management system; • a possibility for collaboration (clause 4.4.3); • an outsourcing opportunity; • a source of knowledge, competence, finances, and infrastructure; • a recipient and a source of information in the communication process; • a source and a user of intellectual property rights; • a counterpart in innovation initiatives and processes. the core of this standard can be found between clauses 4 to 8 and includes: • the innovation partnership framework (clause 4); • making the decision whether to enter an innovation partnership or not (clause 5 and annexes a and d); • selecting internal and external partners by generating a long list, and thena short list of potential partners, and ultimately deciding based on objective criteria (clause 6 and annex b); • partnership alignment and signing a non-disclosure agreement (nda), a memorandum of understanding, a letter of intent or other legally binding agreements (clause 7 and annex c), and • managing the interactions between the partners throughout the lifetime of the innovation partnership (clause 8 and annex d). all the annexes provide guidance on the applicable tools and methods for the clauses of iso 56003:2019. annex e presents a guidance on monitoring, measurement, analysis and evaluation of a set of quantitative and qualitative innovation performance indicators, and links them to the performance evaluation criteria set out in clause 9.1 of iso 56002:2019. 2.4. iso/tr 56004:2019 innovation management assessment this standard continues the measurement of the innovation performance indicators. as outlined in clause 4, this is done in order to: • gain a better understanding of innovation management; • determine the performance of the current innovation management; • meet internal and/or external requirements, and • improve the performance and increase the value of the organization. in clause 5 “choosing the innovation management assessment (ima) approach” organizations can find different ima approaches, examples of quantitative and qualitative measures that serve as performance criteria for innovation management, as well as guidance on the type, quality and format of the ima outputs. clause 6 “the ima process” visualizes the rest of the clauses and how they interconnect within the plan-do-check-act cycle: • prepare (plan) the ima (clause 7)aligning the strategic intent and the scope of the ima, defining the design, the expected results and the performance metrics of the ima, clarifying the resources needed and the organization’s ability and willingness to change, and setting up the ima; • conduct (do) the ima (clause 8)setting up the necessary tools, collecting quantitative and qualitative data, analysing data and identifying gaps in innovation management and ima; • conclude (check) the ima (clause 7)documenting ima findings, structuring the ima report content, communicating findings to relevant top management and interested parties, and recommending actions for improvement; • improvement (act) of the ima itselfdetermining and implementing a roadmap for enhancing future imas. about half of the iso 56003:2019 standard is devoted to the 7 principles that facilitate the design and implementation of the ima (annex a), and visual examples how to present the aggregated results (dashboard and radar diagram) and detailed results (histogram, bar chart, benchmarking of key performance indicators, scoreboard, etc.) from the ima (annex b). 2.5. iso 56005:2020 intellectual property management innovation is intrinsically related to intellectual property (ip) whether or not it is protected by a patent, utility model, trademark, industrial design or other type of intellectual property rights (ipr). the effective and efficient management of ip and ipr can substantially increase the innovative potential of the organization by improving the competence of its people and enhancing organizational knowledge. this standard builds on the foundations set in clause 7.8 “intellectual property management” of iso 56002:2019 and supports the innovation strategy of the organization. the ip management framework (clause 4) encompasses: the context of the organization, the commitment of top management, leaders and ip managers, the innovative culture, the knowledge, competence, education and training of human capital, the financial and legal considerations. clause 5 of iso 56005:2020 defines the interrelationship between the business strategy, the innovation strategy, and the ip strategy of the organization. clause 6 “ip management in the innovation process” has identical structure to that of clause 8.3 “innovation process” in iso 56002:2019, also aligned with the pdca cycle: 1) general; 2) identify opportunities (plan); 3) create concepts (do); 4) validate concepts (check); 5) develop solutions (act); 6) deploy solutions (act). this sequence of steps in the innovation process is logical but by no means it is linear. whatever the case, this process is proven to lead to more efficient innovations by streamlining the efforts of researcher teams and innovators, and innovation partnerships. by combining elements from “the golden circle” (why? – how? – what?) and sipoc diagrams (suppliers – inputs – process steps – outputs – customers) for each of the abovementioned steps, clauses from 6.2 to 6.6 provide detailed guidance on: • why this is important for ip management? acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 4 • what are the necessary inputs to be considered in the innovation process? • how should this be done throughout the lifecycle of the innovation initiative? • the outputs resulting from these activities. in most cases, the outputs from the previous step(s) should serve as inputs for the next step(s) in the innovation process. the iso 56005:2020 supports the implementation of clause 6 by providing 6 annexes with tools and methods for: • invention record and disclosure (annex a); • ip generation, acquisition and maintenance (annex b); • ip search (annex c); • ipr evaluation (annex d); • ip risk management (annex e); • ip exploitation (annex f). 2.6. iso 56006:2021 strategic intelligence management clause 7.7 “strategic intelligence management” of iso 56002:2019 states that “strategic intelligence can include activities to acquire, collect, interpret, analyse, evaluate, apply, and deliver to, or share between, decision-makers and other relevant interested parties, the necessary data, information, and knowledge.” [5]. this information shall be communicated to the top management of the organization in order to align it to the strategic direction, to anticipate and manage change, and to navigate the organization successfully into a volatile, uncertain, complex and ambiguous (vuca) environment. the standard iso 56006:2021 shares the same 8 innovation management principles as iso 56000:2020 and adds aspects of strategic intelligence to the statement of each principle. clause 3 “terms and definitions” has two elements3.1 “intelligence”, and 3.2 “strategic intelligence”. when the two terms are combined, their definitions would convey the following meaning of strategic intelligence: the result of gathering, analysing and interpreting data (related to market, technology, competition, intellectual property or business), information and knowledge directed to top management with recommendations to make decisions impacting the vision, strategy, policy and objectives as well as innovation activities of the organization. clause 4 presents the fundamentals of strategic intelligence such as: purpose, needs, core process, timing, expected outcomes, and essential support in terms of infrastructure and competencies. the strategic intelligence cycle follows the diki model: data – information – knowledge – intelligence. this model is presented in more detail in clause 5 and consists of the following steps: • framingdefining the criteria and scope for intelligence generation; • data gathering and analysiswith main outcome information, or analysed data; • interpretationwith outcome knowledge, or interpreted information; • recommendation, based on knowledge with outcome intelligence, i.e., communicated knowledge. clause 6 “intelligence communication” consists of 6.1 “recommendations to top management”, and 6.2 “documentation, communication and distribution control”. the other standards of the iso 56000 series are at different stages of development. undoubtedly, upon their publication, they will have a major impact on the understanding and implementation of innovation management systems. 3. implementation of iso 56000 guidance at the university of ruse the implementation of management systems in higher educational organizations began by adapting iso 9001 requirements to the actual educational environment. naturally, the pre-existing management systems and the international and national regulatory documents served as bases for the additional requirements. the technology transfer and intellectual property centre (ttipc) at the university of ruse “angel kanchev” is integrating elements of innovation management systems. this improvement to the existing quality management systems integrates the requirements of iso 9001:2015 and of iso 21001:2018 with the guidance of iso 56002:2019. the process has started in 2020 with the election of the current manager of the ttipc. the value of the books “iso 56000: building an innovation management system: bring creativity and curiosity to your qms”” by peter merrill [14], and “managing innovation: integrating technological, market and organizational change” by joe tidd and john bessant [15] cannot be overestimated. the implementation of the iso 56000 series of standards encompasses the elements from 3.1 to 3.8. 3.1. updated innovation vision, mission, strategy, policy, and objectives these strategic documents specified in iso 56002:2019 are developed by the manager of the ttipc and approved by the academic council and the rector of the university of ruse. the innovation mission of the ttipc is to promote the protection of intellectual property of the researchers at the university of ruse, and to expand the opportunities for realization of modern technologies by partners of the university of ruse. the innovation vision is to ensure that the university of ruse is established and a regional leader in the realization of its intellectual property. the innovation strategy being followed is to determine the existing opportunities for progress and to seek out “burning issues” that require innovative solutions which can be developed by multidisciplinary teams of researchers at the university of ruse and its partner organizations. the innovation policy states that the top management is committed to achieving the status and recognition of a research university while always striving for continual improvement of the innovation management system. the innovation objectives are consistent with the innovation policy. there is a requirement for all professors and research staff to participate in research projects, as well as to develop and register ip with a stronger focus on international and bulgarian patents, utility models, and submitted applications for ipr protection. 3.2. streamlined innovation management process prior to 2020, there were 3 different centres focusing on different aspects of the ttipc. these were: • the centre for intellectual property; • the centre for technology transfer; • the danube transfer centre. by using the guidance of iso 56002:2019 and iso 56005:2020, the ttipc now implements a series of innovation management processes that encompass the identification of opportunities, creation and validation of concepts, developing and deploying solutions. since 2022, the prevention and monitoring of infringement of ip and copyright are added to the scope of activities of the ttipc by implementing the system acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 5 approved by the ministry of education and science of bulgaria strikeplagiarism. 3.3. innovation portfolio the first innovation portfolio of the university of ruse meeting the intent of the guidance of clause 6.4 of iso 56002:2019 is presented on figure 1. the ttipc has developed and published innovation portfolios yearly. currently there are the 2021 and the 2022 editions which contain all existing and newly registered intellectual property rights (ipr)patents, utility models, trademarks, designs, etc. this allows the realization of synergies, including possibilities for re-use and optimization. an added benefit from the innovation portfolio is that it is used to raise interested parties’ awareness of the ipr of the university and its researcher community, and last but not leastto improve the visibility of innovators and their success stories. 3.4. ipr database the initial stages of ipr management can be characterised as utterly basic and passive. there used to a very general list of patents and utility models with no specific information. the newly created and continuously updated database of ipr includes the following elements: • type of ipr with colour coding for different ipr; • date and number of the application; • date and number of the registration by the patent office; • ipr owner(s) and/or inventor(s); • period of validity, including a calendar of the dates of validity allowing to have objective evidence (a scanned certificate) on any given date (see figure 2); • payment status. the ipr database is used as an input for reporting the performance of the innovation management system at top management reviews and/or responding to external inquiries. 3.5. assessments of the performance of the innovation management system these assessments are done periodically depending on the level of reporting. the ttipc presents quarterly reports to the director and the managing council of the research & development centre (r&dc) at the university of ruse. in addition, the detailed annual report contains all the major activities and key performance indicators of the innovation management system. this is done in line with: • the criteria and procedures of the ministry of education and science, and the national evaluation and accreditation agency; • the guidance of iso/tr 56004:2019 about performance criteria (clause 5.2.1) and performance metrics (clause 7.4); • the internal regulations of the university of ruse “angel kanchev” [16], [17]. the continual improvement of the implemented innovation management system is also assessed for positive change in the maturity level of the system [18]. the performance is communicated to interested parties by aligning the innovation management system with the iso 56000 series of standards and the united nations sustainable development goals [19]. 3.6. innovation partnerships the innovation portfolio has made the application of the guidance of iso 56003:2019 a priority. some teams of innovators have existed prior to the implementation of the innovation management system. they generally comprised of researchers from the same scientific field. rarely they involved inventors from other departments and faculties, and even less oftencollaborators from external organizations. these partnerships generate better overall outcomes and outputs. the implementation of iso 56003:2019 at the university of ruse is presented in [20]. initially, the long list of potential partners was created based on the information contained in the innovation portfolio, and the discussion with interested parties. using the criteria, described in iso 56003:2019, the long list was reduced. the result of the partnership selection has produced an interdisciplinary team of researchers from 3 faculties, 5 different departments, phd, msc and bsc students, and an external practitioner. 3.7. management of strategic intelligence iso 56006:2021 is published as a bulgarian standard on 23 march 2022 but is not available in bulgarian. this fact hinders its smooth implementation. to avoid waiting for an official translation, the ttipc has made a working document based on the guidance of this standard. it is communicated to the vicedeans in charge of research so that strategic intelligence from various fields can support the decision-makers in the university. 3.8. additional innovation management initiatives and activities some of the activities arising from the integrated quality, educational and innovation management system, which has placed the university of ruse among the top research institutions in bulgaria, are: figure 1. innovation portfolio of the university of ruse for the period 01 january 2020 – 31 december 2020 [in bulgarian]. figure 2. calendar of the ipr database [in bulgarian]. https://strikeplagiarism.com/en/ acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 6 • active membership in professional organizations such as ispimthe international society for professional innovation management, and special interest groups (sigs); • participation in conferences, seminars, webinars and workshops aiming to exchange best practices; • organizing seminars to obtain buy-in from internal and external interested parties; • promoting best practices by making interviews with successful innovators; • participation in promotional events such as the annual innovative youth expo (see figure 3), etc. 4. conclusions the experience of the university of ruse with the innovation management system shows that the best results and performance of the management system are achieved when the organization: • carefully considers the current context and emerging trends and technologies, as well as the requirements of relevant interested parties; • successfully manages the competence, awareness and involvement of key representatives of top management, researchers, professors, laboratory technicians and administrative staff; • actively engages in professional organizations, projects and trainings; • timely aligns its strategic direction with international and national regulatory documents and frameworks; • embraces change and creates its own the future. the guidance, tools and methods of the iso 56000 series of standards are creatively implemented and have contributed to a significant improvement of the innovation management system. references [1] the iso survey of management system standard certifications 2021. 24 january 2023. online [accessed 28 january 2023] https://www.iso.org/home.isodocumentsdownload.do?t=9z0j vmcb1svkz9z0zejqmegpesczmgl_ggsogk7yes3dbsznygu 5-tlkaplnmw68&csrftoken=sr21-gstr-4lvnqg1k-negg-s2pa-vmfo-txer. [2] k. lines, j.-l. hippolyte, i. george, p. harris, a mathmet quality management system for data, software, and guidelines, acta imeko, vol. 11, no. 4, (2022), pp. 1-6. doi: 10.21014/actaimeko.v11i4.1348 . [3] s. b. da silva, improving the firm innovation capacity through the adoption of standardized innovation management systems: a comparative analysis of the iso 56002:2019 with the literature on firm innovation capacity, international journal of innovation – iji, 9(2), (2021), pp. 389-413. doi: 10.5585/iji.v9i2.19273. [4] iso 56000:2020, innovation management fundamentals and vocabulary. [5] iso 56002:2019, innovation management innovation management system guidance. [6] iso 56003:2019, innovation management tools and methods for innovation partnership guidance. [7] iso/tr 56004:2019, innovation management assessment guidance. [8] iso 56005:2020, innovation management tools and methods for intellectual property management guidance. [9] iso 56006:2021, innovation management tools and methods for strategic intelligence management guidance. [10] iso/cd 56001 innovation management innovation management system requirements. online [accessed 28 january 2023] https://www.iso.org/standard/79278.html. [11] iso/fdis 56007 innovation management system tools and methods for managing opportunities and ideas guidance. online [accessed 28 january 2023] https://www.iso.org/standard/75068.html. [12] iso/dis 56008 innovation management tools and methods for innovation operation measurements guidance. online [accessed 28 january 2023] https://www.iso.org/standard/78485.html. [13] iso/cd ts 56010 innovation management illustrative examples of iso 56000. online [accessed 28 january 2023] https://www.iso.org/standard/80612.html. [14] p. merrill, iso 56000: building an innovation management system: bring creativity and curiosity to your qms, quality press, milwaukee, 2020, isbn 978-1-951058-26-9. [15] j. tidd, j. r. bessant, managing innovation: integrating technological, market and organizational change, wiley, hoboken, 2021, isbn 978-1-119-71319-7. [16] tz. gueorguiev, b. sakakushev, b. evstatiev, improving educational management systems by integrating quality and innovations, 59 annual scientific conference of angel kanchev university of ruse and union of scientists – ruse “new industries, ruse, bulgaria, 2020, pp. 27-32. online [accessed 28 january 2023] https://conf.uni-ruse.bg/bg/docs/cp20/9.1/9.1-3.pdf. [17] tz. gueorguiev, assessment of innovation management and intellectual property, intellectual property in universities – new horizons of academic dialogue, sofia, bulgaria, 2022, pp. 217-230 [in bulgarian]. online [accessed 28 january 2023] https://unyka.unibit.bg/images/pdf/2022-1/15-c-georgiev.pdf. [18] tz. gueorguiev, continual improvement and innovations, 6th int. conf. on advances in mechanical engineering (icame 2021), istanbul, turkey, 20-22 october 2021, pp. 320-325. [19] tz. gueorguiev, innovation management systems – reality and perspectives, international scientific journal “innovations”, vol. 9 (2021), issue 2, pp. 48-50. online [accessed 28 january 2023] https://stumejournals.com/journals/innovations/2021/2/48.ful l.pdf. 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https://www.iso.org/home.isodocumentsdownload.do?t=9z0jvmcb1svkz9z0zejqmegpesczmgl_ggsogk7yes3dbsznygu5-tlkaplnmw68&csrftoken=sr21-gstr-4lvn-qg1k-negg-s2pa-vmfo-txer https://doi.org/10.21014/actaimeko.v11i4.1348 http://dx.doi.org/10.5585/iji.v9i2.19273 https://www.iso.org/standard/79278.html https://www.iso.org/standard/75068.html https://www.iso.org/standard/78485.html https://www.iso.org/standard/80612.html https://conf.uni-ruse.bg/bg/docs/cp20/9.1/9.1-3.pdf https://unyka.unibit.bg/images/pdf/2022-1/15-c-georgiev.pdf https://stumejournals.com/journals/innovations/2021/2/48.full.pdf https://stumejournals.com/journals/innovations/2021/2/48.full.pdf https://conf.uni-ruse.bg/bg/docs/cp22/9.1/9.1-1.pdf an iot-based handheld environmental and air quality monitoring station acta imeko issn: 2221-870x september 2023, volume 12, number 3, 1 8 acta imeko | www.imeko.org september 2023 | volume 12 | number 3 | 1 an iot-based handheld environmental and air quality monitoring station m. n. m. aashiq1, w. t. c. c. kurera1, m. g. s. p. thilekaratne1, a. m. a. saja1, m. r. m. rouzin1, navod neranjan2, hayti yassin2 1 faculty of engineering, south eastern university of sri lanka, sri lanka 2 faculty of integrated technologies, university brunei darussalam, brunei darussalam section: research paper keywords: environmental monitoring; air quality; internet of things; cloud; sensor network citation: m. n. m. aashiq, w. t. c. c. kurera, m. g. s. p. thilekaratne, a. m. a. saja, m. r. m. rouzin, navod neranjan, hayti yassin, an iot-based handheld environmental and air quality monitoring station, acta imeko, vol. 12, no. 3, article 6, september 2023, identifier: imeko-acta-12 (2023)-03-06 section editor: susanna spinsante, grazia iadarola, polytechnic university of marche, ancona, italy received february 27, 2023; in final form june 11, 2023; published september 2023 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: m. n. m. aashiq, e-mail: aashiqmnm@gmail.com 1. introduction in the early days, people used larger devices to measure a single weather parameter. they are used to place weather stations in a location to collect and analyse information, and to make predictions on future weather changes. in the past years the devices were large, complex, and analogic type. then, simpler and less power-consuming digital systems were introduced [1]. the collection and the storage of data with analogic and digital devices is a tedious and time-consuming activity, as manual involvement was significantly needed for that. the need of collecting real-time data and their storage led researchers to pay more attention to obtaining accurate and fast weather parameters in a large area using simple on-set devices [2]. these devices are considered automated devices that measure and record weather information using sensors. with the real-time data monitoring ability of iot, these devices become more reliable. hence the iot is considered as the modern state-of-theart method for measuring the weather and air quality parameters. overall, iot technology provides the capability to monitor, collect and exchange data using a device consisting of sensors or actuators, controllers, etc. the main reason for using iot in smart weather monitoring is due to the vast range of options that exist in this technology. on the other hand, iot enables the connection of a larger number of sensors to a cloud network using various connectivity methods based on connection range. to date, an iot is considered the best way to collect, maintain, and analyse large amount of data [3]. abstract weather and air quality play an important role in determining environmental pollution. fluctuation of these parameters not only causes environmental pollution but also causes severe injuries to human health. with the emergence of the internet of things (iot), sensorbased weather devices with easy observation facilities started to develop. in this regard, this study focused on developing an iot-based portable weather monitoring gadget that can measure weather and air parameters which are more often required in our day-to-day life. the proposed system is capable to measure temperature, pressure, humidity, altitude, pm2.5, pm10 level, voc, and co level. it consists of a portable display and a mobile app with a thing speak cloud platform. further, the system has a wi-fi and gsm connection to communicate data. a mobile application was developed to monitor the readings in real-time which are stored in the cloud platform. the developed hardware was carefully calibrated in the national meteorological department to make sure our system is practically usable. compared to existing models, our prototype is very handholdable, easily installable which does not require trained technicians, and is easily maintainable. also, it is possible to access the data from anywhere in the world through wi-fi connectivity, and possible to make data visualization and analysis. on the other hand, it is very difficult to find a single, portable size, and low-cost device to collect all these parameters together. nonetheless, our prototype has the potential of connecting with multiple similar devices to create a larger iot network grid. overall, our proposed product has a combination of weather and air quality parameters in a portable and handholdable size with low-cost and low power consumption which other devices do not have according to the latest literature. mailto:paul@regtien.net acta imeko | www.imeko.org september 2023 | volume 12 | number 3 | 2 this study aimed to design a small portable weather monitoring device suitable for environments such as schools, laboratories, and industries to monitor real-time parameters. the device can give all the required environmental and air quality parameters at once with a reasonable time range and with the possibility to customize the data acquisition. hence, this device could be used to create a nationwide network to collect the required weather and air quality parameters accessible at any place. overall, the weather station gets seven important parameters at once with easy monitoring methods. the system could be used in agricultural and field monitoring activities, environmental controlling activities in places such as greenhouses, environmental and air pollution monitoring, damage control activities, climate change study, etc. further, this network would generate big data which could be used for machine learning research. this system could also be usable as a home automation control device in which we can control the home automation sensors and actuators based on the weather and air quality readings that we are obtaining. additionally, air pollution causes severe health injuries to human health, which ultimately result in death [4]. hence, this system could be used to monitor the air quality parameters in the close living environment and to accordingly take countermeasures to avoid life-threatening risks. the rest of the paper is organised as follows. section 2 describes the previous works, section 3 provides existing models, section 4 and 5 explains the hardware and software layouts, section 6 analyses the results, and finally, section 7 concludes our work. 2. related work in general, smart weather stations are automated systems designed by connecting one or more sensors using a microcontroller. with the ability of iot to connect devices to a cloud network using wireless communication, the data could be accessed from any place in the world. in [2] an iot-based system was designed using an lpc1768 microcontroller and four sensors to measure temperature, pressure, relative humidity, and light intensity. the data was collected and sent to a base station using an esp8266 wi-fi module. in [3], a system was implemented using an arduino uno board, dht11 sensor, esp8266, and wi-fi module which transmit data to open iot api service thing speak where the data can be stored and analysed. rather than weather monitoring, air quality measuring devices are also designed using arduino uno, mq135, and mq7 sensors [5]. also, there are systems designed [6] using raspberry pi to monitor temperature, humidity, pm2.5, and pm10 concentration, and air quality index. later projects were using the arduino uno based system [7], to measure even gas content and earthquake information with a combination of moisture, temperature, humidity, and rainfall parameters. there were even single sensors such as mq135 which were able to measure carbon dioxide, sulphur dioxide, nitrogen dioxide, smoke, and lpg gas [8]. there are many challenges related to iot-based systems such as complex topology design, privacy and security, power backup, and high memory requirements, in [9], the researchers have designed an indoor and outdoor pollution monitoring system using esp8266 node mcu controller, sds021 sensor, and ze07co sensor. the system is a low-cost, simple topology system. further the gsm/ gprs modules could also be used in iot systems to build a mobile communication systems [10], [11]. overall, these systems used an lcd to display real-time information, and systems could use wireless modules such as esp8266 to connect the system to the internet using a wireless connection. then the data was transferred to a remote server or iot platform such as “thing speak” or to a mobile application [12]. wireless sensor networks have carried out a selfconfiguration and reconfigurations, and can adapt well to mobility as well as to a remote control [13]. rather than webbased technologies, messaging technologies such as mqtt can be used in iot systems [14]. this can help to separate system migration complexity and make it easier to build a distributed information system. in the global market, there are iot-based weather devices that can measure only a few parameters such as temperature, humidity, pressure, or some on-set devices that can use as domestic or industrial weather stations. many of them are powered by batteries and they use solar power systems as a backup power supply. in [15], the authors describe an iot-based microclimate monitoring weather station that can be installed in any place and can be easily modified to suit any environment. the data is transmitted to the cloud at a fixed range using a mobile communication network. a microprocessor, local storage unit, communication module, lcd display, power controller, and sensor panel comprise the system whereas it has only the temperature and the humidity sensor to get only those two parameters. further, in [16] there was another work carried out by international water management institute (iwmi) to upgrade the climate monitoring systems in sri lanka, with the aim of designing and programming a device using arduino, using opensource software and hardware. the aim of this equipment was to monitor the variability of rainfall. the system was able to measure parameters such as wind speed, pressure, humidity, water pollutants, wind direction, rainfall, and water level. the device consists of an arduino lakduino board, a weather shield, a gps receiver, a data logger, an anemometer, and a rain gauge with solar-powered rechargeable battery. further [17]-[19] are addressing similar devices used in environmental and agricultural monitoring applications. however, most of the existing devices were developed only for either weather monitoring or air quality monitoring separately. only recent researches have started to incorporate both types of devices [20]-[29], but there is still a wide combination of parameters to include. our objective was to measure multiple parameters with a single device, given that they are needed by environmental researchers for machine learningbased forecasting models [30]-[37]. moreover, most of the existing works did not indicate the calibration procedure and the results after obtaining the calibration report. hence, this would question the reliability of the hardware for practical use. most researchers have carried out laboratory tests to show the variations in the readings by suddenly creating some variations in the atmosphere and checking if the device readings were changed or not. apart from that, the sensor components are being improved and enhanced with the latest technologies, leading to a better performance and a less power consumption. the majority of the previous works were carried out through raspberry pi, a more complicated device if compared to arduino uno, because of its programming structure and the nature of the raspbian operating system [3]. in table 1 we compare the existing models with the one we propose in this study. based on acta imeko | www.imeko.org september 2023 | volume 12 | number 3 | 3 the cost of the equipment, we have also categorized if the cost for the development of the setup is low, high, or moderate. based on the comparison with the literature, it is evident that our proposed setup enables weather, air quality, and environmental monitoring facilities for a low cost, with cloud, and mobile connection facilities. 3. system requisites in order to build the hardware and the software platforms we needed to have microcontrollers, weather parameter reading sensors, air pollution monitoring sensors, a 3d printer, a wi-fi module, a gsm module, a display unit, a cloud platform for data storage, data visualization, and data analysis, android application development kit, and finally, a power source for the continuous operation of the system. 4. system hardware the proposed portable smart weather monitoring station is designed as a weather station that should be able to measure both weather and air quality parameters using an iot-based system. the system has an arduino uno board as the main microcontroller and a node mcu with an esp8266 wi-fi module. the device can measure temperature, humidity, pressure, pm2.5, pm10, o3, co, and voc levels. the data is sent to a thing speak web platform and to a portable display using a wi-fi connection. these data can be accessed and analysed in an iot platform “thing speak” or can be accessed using a mobile application or web server for other activities. the system also has a gsm connection to be used in the absence of wireless connectivity. the next subsections present a brief description of the components of the system. 4.1. microcontroller microcontroller consists of an arduino uno and a node mcu board. arduino uno is an open-source microcontroller board based on the microchip atmega328 microcontroller. this will be a 14-pin board powered by a usb cable or by a 9-volt external battery. arduino uno board has a voltage range of 720 v and can be programmable using arduino ide. the node mcu board is used as a supported microcontroller as well as the esp8266 wi-fi module enhances to provide the wireless connectivity requirements of the device. 4.2. sensors we have selected the sensors according to three main features, necessary to address our main objective: i) less power consumption, ii) small dimension, and iii) low cost. aht10 was selected to get temperature and humidity measurements. it has a small volume, a less power consumption, and a better antiinterference capability. the maximum voltage requirement for its operation is 3.6v dc. mq-7 is having more sensitivity to the carbon monoxide gas. it can detect up to 10-500 ppm range of co gas. it also complies with our three major concerns and provides suitable digital outputs. mq-7 was interfaced with the arduino-uno controller. furthermore, it produces low conductance with clean air. bmp280 was selected for the pressure sensor due to its robustness, high reliability, and digital outputs along with the above three primary concerns. it can be interfaced with mobile devices, gps modules, and even with watches. bmp280 was connected to the node mcu controller since arduino uno is fully loaded with the previous sensors. zp07-mp503 is a volatile organic components detection sensor that is sensitive to formaldehyde, alcohol, cigarette smoke, ammonia, benzene, hydrogen, essence, etc. it is also very costeffective, has a long life, and pre-shipment calibrated sensor. this sensor was connected to our main arduino uno controller and can be operated with 5vdc power. conversely, it is not suggested to expose the module to a high concentration of organic gases for an extended period. the laser scattering principle is utilized here, namely, the induction of scattering by using a laser to radiate suspended particles in the air, then collect scattering light to a specific degree, and lastly acquire the dispersion of light changes throughout the time. finally, using mie theory [38], a microprocessor can calculate the equivalent particle diameter as well as the number of particles of different sizes per unit volume. this sensor is linked to the node mcu since the connections are full in the arduino uno. this sensor also requires around 5vdc power for its continuous operation. after the complete connection of all the sensors and configurations, two outer cases were printed using the 3d printer. one is for the main equipment and the other one is for the digital display unit. table 2 illustrates the details of the sensors that were used. aht10, pms 5003, and bmp280 sensors are described by an uncertainty value which indicates the maximum deviation values from the actual readings. they can be considered as the worstcase uncertainties (maximum errors). but for the other sensors, mq-07 and zp07-mp503 are described by a sensitive value that defines the smallest physical parameter input required to produce a discernible output change or it can be interpreted as the input parameter change needed to form a standardized output change. consequently, figure 1 depicts the block diagram for the hardware implementation. figure 2 shows the internal and external physical organization of the device. a detailed wiring and schematic diagram have been depicted in figure 3. 4.3. wireless connectivity a wireless connection was established using esp 8266 wi-fi module. this connection is used to send data to the thingspeak channel and portable display unit. this wi-fi module uses the table 1. comparison of similar research. reference w e a th e r m o n it o ri n g a ir q u a lit y m o n it o ri n g e n vi ro n m e n t m o n it o ri n g c lo u d e n a b le d m o b ile c o n n e ct iv it y c o st ( lo w (l ), m o d e ra te (m ), h ig h (h )) [2] ✓  ✓   l [3] ✓   ✓  l [4]  ✓  ✓  l [9]  ✓  ✓  l [10] ✓ ✓    l [20] ✓     l [23]  ✓    m [26]  ✓    l [28] ✓   ✓ ✓ h [29] ✓     h our work ✓ ✓ ✓ ✓ ✓ l acta imeko | www.imeko.org september 2023 | volume 12 | number 3 | 4 full stack of tcp/ip. when this wi-fi module is connected to the internet it will get the ip address automatically or dynamically. after establishing the wi-fi connection, it would be possible to transmit the equipment readings to the thingspeak cloud in which real-time monitoring, data visualizations, and data analysis are possible in one place. it requires only 3.3 vdc power for its function and it is also an inexpensive device. this works under the 2.4ghz (802.11 b/g/n) regular wi-fi standard. this wi-fi module was also connected to the node mcu controller. 4.4. portable display unit a display unit with wi-fi connectivity was designed along with the prototype to display the sensors’ data using wi-fi connectivity. a node mcu with esp 8266 wi-fi module and an oled 0.96' display is used in the portable display. a lithium battery powered up the device. the required power supply for this unit is provided by the arduino uno in the system. 5. system software software is critical to the integration and operation of our hardware design. there are two software components in our design. the first component is driving the operation of the hardware components including the sensors. it was completed by microcontroller programming using arduino ide. the second component is the android-based graphical user interface (gui) which can be used by multiple mobile devices to get the weather and air quality readings. 5.1. thingspeak channel a thingspeak channel was created using a commercial license which contains 8 channels to display temperature, humidity, pressure, relative altitude, voc, co and pm2.5, and pm10 levels. table 3 describes the channel information of the obtained account. thingspeak uses traditional http/https connectivity via the internet. this cloud-based analytics platform is used to gather, view, and analyse live data streams. figure 4 shows the dashboard visualization layout of the real-time data values. 5.2. mobile application an android app with graphical user interface (gui) was designed using the android studio to enable users to retrieve the data from the thingspeak platform easily. the application is more flexible and efficient to use, and the user authentication features help users to log in using their username and password. mysql has been used to store the required details. android table 2. specification of the sensors. module/ sensor parameter uncertainty/ sensitivity range of measurement aht10 humidity, temperature ±2 % rh ±0.3 % ℃ 0 … 100 % rh, −40 … + 85℃ pms 5003 pm2.5, pm10 ± 10μ g/m3 0.3 … 1.0, 1.0 … 2.5, 2.5 … 10 bmp280 pressure, temperature ±1 hpa ±1.0 ℃ 300 … 1100 hpa −40 … + 85℃ mq 131 ozone 𝑅𝑠(in air) 𝑅𝑠(200 ppm) ) ≥ 2 10 … 1000 ppb mq-7 carbon monoxide 𝑅𝑠(in air) 𝑅𝑠(100 ppm) ) ≥ 5 10 … 500 ppm zp07-mp503 volatile organic materials ≤ 1 % /𝑦𝑒𝑎𝑟 figure 1. block diagram figure 2. enclosure and wiring of the hardware system includes the sensors aht10, mq-07, voc, bmp280, esp 8266, pms5003, zp07-mp503, and the battery pack unit. figure 3. detailed wiring and schematic diagram of the device showing the sensor interfaces to the arduino uno and node mcu controllers. table 3. thingspeak channel information. channel name quick weather channel id 1675898 author spt4725 access public/ private acta imeko | www.imeko.org september 2023 | volume 12 | number 3 | 5 studio uses java script object notation (json) parser to enable sending get requests to the thingspeak channel to collect the necessary data by using the rest api request methods. figure 5 illustrates the login page and the selection of parameter options menu page of the developed mobile application. 6. results and discussion it is very important to have higher precision or lower uncertainty values when building measurement equipment. the accuracy value is defined by the combination of the resolution and precision of that sensor. resolution is the measurable incremental variation and precision is the nature of giving the same output the for same input values [39]. in weather monitoring gadgets, uncertainty is the most important feature which should be given much priority as well. calibration could be done in many ways such as by homogenous mutual test, heterogenous mutual test, combined mutual test, and by using standard sensors [40]. we have used the fourth method for calibration which is using standard sensors. after designing and configuring the device, data collection was carried out through a calibrated device available at the sri lanka meteorology department headquarters to check the uncertainty of device readings and to calculate error percentages. for that purpose, a mounted ambient weather station is used. according to those data, the error calculation and plotting are done for a selected time interval using the obtained data. data was recorded from morning 10.00 am until afternoon 4.00 pm at 30-minute intervals. figure 6 shows the humidity variation, figure 7 shows the pressure variation, and figure 8 shows the figure 4. real-time visualization from the thingspeak channel figure 5. mobile application. figure 6. rh measurements from 10.00 am to 4.00 pm: comparison between the presented device and the reference device. figure 7. pressure variation from 10.00 am to 4.00 pm between our device and the calibration reference device. figure 8. temperature measurements from 10.00 am to 4.00 pm: comparison between the presented device and the reference device. acta imeko | www.imeko.org september 2023 | volume 12 | number 3 | 6 temperature variation. relative humidity error varies from 0.32 % to 5.92 %. but two outlier values caused errors of 9.39 % and 11.15 %. this might have occurred due to some temporary malfunctioning of the sensor at that instance. the error of the pressure parameter varies between 0.38 hpa and 2.03 hpa. similarly, temperature error values are deviating from 0.10 °c and 4.79 °c values. deviation values for humidity, pressure, and temperature parameters are shown in detail in table 4. figure 9 and figure 10 are showing similar graphs for pm2.5, and pm10 reading values. co level readings are very close to the standard reading values with less than a 5 % error percentage. pm10 sensor also indicates the lower uncertainty values close to the standard reading with less than 7 % of error values. pm2.5 sensor is also showing a lesser deviation from the actual reading values. in many instances error percentage is 0 %, but in a few instances, they are reaching 10 % to 11 % with a little higher deviation. in any way, the overall accuracy values are good in comparison with the standard values. table 5 shows the final summary of the sensor accuracy values in the mean absolute percentage error (mape) metric. it is obvious from the summary table that the pressure sensor, co sensor, pm2.5, and pm10 sensor are having reliable reading values. but the temperature sensor and pressure sensor should be replaced or rechecked. these variations might have occurred due to various reasons such as there is always a fluctuation range provided in any type of sensor as indicated in table 2. manufacturing variation, environmental effects over time, the presence of signal-to-noise ratio, improper installation practices while using very thin and sensitive cables, discontinuity in reading the values (temporary malfunction) or non-responsiveness, sporadic events, and some human planned attacks such as malicious attacks, and tampering are some of the potential reasons for getting the error values [40], [41]. calibration is helpful in removing the structure errors of the sensors. by using the error values which are shown in figure 6, figure 7, figure 8, and table 3, the characteristic curves could be obtained for correcting the reading values. this curve would be linear in the ideal situation only, most of the time, it will be non-linear. singlepoint, two-point, and multi-point adjustments are done for nonlinear cases [40]. 7. conclusion this paper describes an iot-based environmental and air quality monitoring station that could be a part of a larger weather data acquisition network. this platform could be used to collect and analyze the readings of various parameters in multiple geographical locations and this could help us to get the smaller variations of the environmental and air quality parameters among the various villages and cities. the designed prototype is consisting of 5 sensors and a wi-fi connection. we connected bmp280, aht10, mq7, pm2.5, and voc sensors which give us a total of 8 parameters as temperature, humidity, pressure, co level, altitude, pm2.5, pm10 level, and voc level. also, we expected to design a very smaller and handholdable size equipment with very low cost and successfully made it. if we want to use the market products, we need to use multiple devices and some of them are very larger and need trained technicians for installation. this is the main novelty of our research study. the wi-fi connection is set up using esp8266 in the node mcu board. the parameters can be successfully updated using the thing speak platform with an interval of 30 seconds. the data could be also observed using a portable handheld display unit or the mobile app as well. the display unit is updated every 1 minute using wi-fi connectivity. this system paves the way for a crucial step in understanding the creation and execution of iot applications and serves as a basis for several figure 9. pm2.5 measurements from 10.00 am to 4.00 pm: comparison between the presented device and the reference device. figure 10. pm10 measurements from 10.00 am to 4.00 pm: comparison between the presented device and the reference device. table 4. comparison of temperature, humidity, and pressure sensors’ reading with standard sensors. temperature (°c) relative humidity (%) pressure (hpa) time standard sensor1 error standard sensor2 error standard sensor3 error 10.00 am 28.72 30.03 1.31 72.39 73.38 0.99 1011.86 1013.67 1.81 10.30 am 28.98 33.77 4.79 70.07 64.15 -5.92 1011.74 1013.58 1.84 11.00 am 29.52 31.16 1.64 67.53 71.18 3.65 1011.57 1013.54 1.97 11.30 am 28.23 30.32 2.09 75.13 75.45 0.32 1011.02 1013.00 1.98 12.00 pm 28.39 30.57 2.18 75.10 75.66 0.56 1010.39 1012.36 1.97 12.30 pm 28.88 29.45 0.57 72.88 78.38 5.50 1009.8 1011.83 2.03 01.00 pm 26.50 28.51 2.01 87.56 84.20 -3.36 1009.73 1011.63 1.90 01.30 pm 24.15 28.38 4.23 93.79 84.40 -9.39 1008.95 1010.61 1.66 02.00 pm 25.53 26.21 0.68 95.11 91.44 -3.67 1008.36 1009.61 1.25 02.30 pm 25.82 25.64 -0.18 92.91 91.56 -1.35 1007.85 1008.62 0.77 03.00 pm 26.70 26.80 0.10 90.11 92.20 2.09 1007.45 1007.93 0.48 03.30 pm 27.44 27.23 -0.21 82.39 93.54 11.15 1007.45 1007.83 0.38 04.00 pm 27.33 27.55 0.22 84.17 92.22 8.05 1006.97 1007.92 0.95 acta imeko | www.imeko.org september 2023 | volume 12 | number 3 | 7 advancements in building centralized climate control entities. our main contribution to the existing literature is proposing a handheld weather station with air quality measurements which could be helpful in building a larger network. moreover, most of the previous researchers did not provide the calibration details which might question the reliability of their research work. on the other hand, this system could help the authorities to take necessary actions to control environmental and air pollution which ultimately protects humans from severe life-threatening health injuries. our calibration results have shown a good indication of the practical working ability of our equipment. continuous power supply to the equipment is another crucial factor for such kinds of iot devices. since the main goal of this research was to build handheldsize equipment, it is not preferable to allow for carrying bigger size power resources along with the gadget. hence, we have used a power bank with a rechargeable battery. 24 hours of continuous operation is not possible with our equipment since the equipment should be switched off while recharging the battery. this is one of the limitations of our work. there could be much research carried out to design very efficient and handheld power supplies for such systems. this could be explored in the future. moreover, somehow the temperature-humidity sensor which is ah10 has produced unreliable results with more than 5 % of mape value. this should be checked and rectified. we can test this by using different types of the latest iot sensor models and should compare the results. it is a real challenge to get guidance from the previous studies since majority of the previous researchers have not produced their calibration and comparison results. further, we should be sure that there are no noise effects and signal interferences are generated in the environment. on the other hand, other sensors have produced extremely better results. acknowledgment we acknowledge the contribution of the sri lanka meteorological department in helping us to get the calibration of our equipment. their support in this work should be highly appreciated. references [1] w. y. yi, k. m. lo, t. mak, 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https://doi.org/10.1109/sas48726.2020.9220079 https://doi.org/10.3390/s17122856 https://doi.org/10.3390/s18041056 https://doi.org/10.3390/s20040998 https://doi.org/10.3390/s21010181 https://doi.org/10.3390/s21113831 https://doi.org/10.1016/j.apr.2022.101547 https://doi.org/10.1016/j.scitotenv.2020.139656 https://doi.org/10.4209/aaqr.2019.08.0408 https://doi.org/10.3390/app11167326 https://doi.org/10.3390/atmos10070373 https://doi.org/10.3390/atmos13040538 https://doi.org/10.3390/en15176428 https://doi.org/10.1007/s00521-021-06082-8 https://doi.org/10.1109/i2mtc48687.2022.9806516 https://doi.org/10.1109/i2mtc.2015.7151413 https://doi.org/10.1109/memea52024.2021.9478673 https://doi.org/10.1109/i2mtc.2017.7969884 introductory notes for the acta imeko thematic issue on measurement systems and instruments based on iot technologies for health acta imeko issn: 2221-870x september 2023, volume 12, number 3, 1 2 acta imeko | www.imeko.org september 2023 | volume 12 | number 3 | 1 introductory notes for the acta imeko thematic issue on measurement systems and instruments based on iot technologies for health grazia iadarola1, susanna spinsante1, francesco lamonaca2 1 department of information engineering (dii), polytechnic university of marche, via brecce bianche 12, 60131, ancona, italy 2 department of computer science, modeling, electronics and systems engineering (dimes), university of calabria, 87036, arcavacata di rende, italy section: editorial citation: grazia iadarola, susanna spinsante, francesco lamonaca, introductory notes for the acta imeko thematic issue on measurement systems and instruments based on iot technologies for health, acta imeko, vol. 12, no. 3, article 2, september 2023, identifier: imeko-acta-12 (2023)-03-02 received june 28, 2023; in final form june 30, 2023; published september 2023 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: grazia iadarola, e-mail: g.iadarola@staff.univpm.it dear readers, the internet of things (iot) has revolutionized many services people rely on in everyday life. of course, there is no exception in the healthcare sector, where the impact of iot grows rapidly. indeed, the use of iot technologies helps healthcare, by connecting more easily patients to medical staff for continuous and long-term monitoring, through smart data management and innovative wearables as well. with these premises, we are delighted to present to you this new thematic issue “measurement systems and instruments based on iot technologies for health”. the papers accepted in this issue contemplate, under different points of view, the importance of measurement systems and instruments for iot applications related to health. common aspects can be traced in these papers, from low-cost monitoring to wireless interfaces, both fundamental for iot. an important theme underlined by these papers is air quality monitoring, which should be analyzed in everyday life to avoid serious health risks. a first example of low-cost monitoring for an iot application related to health is presented in [1]. specifically, the low-cost monitoring system is proposed for skin conductance signals. the comparison with the signals provided by a reference desk equipment points out the ability of the low-cost system in providing the same relevant information for stimulus detection, despite its simpler design and hardware limitations. in fact, the increase of both baseline and peaks are detected by the proposed low-cost system after stimulation. a low-cost system for air quality is then described in [2]. the system monitors co2 and volatile organic compounds (vocs) inside protective equipment such as ffp2 masks (commonly adopted during the covid-19 pandemic), aggregates data over a 15-minute window, and calculates average values for each measured parameter. by comparing average values to reference thresholds, the monitoring system can suggest removing the mask when necessary. an innovative aspect is the personalized monitoring of exhaled breath, as customized and reliable information is provided to doctors, thanks to the integration of removable memories. air quality in living environments or outdoor is discussed also in [3], where a portable monitoring station is presented to measure parameters such as co2 and vocs, in the presence of pollutants. the portable station acquires a combination of weather and air quality parameters, with low-cost and reduced power consumption. data connectivity is ensured by wireless interfaces and the portable station can be part of a network, for possible distributed monitoring and alert delivery, in case of critical weather and air conditions. similarly, wireless networks can be exploited in indoor location identification and tracking. in [4], a method for remote rehabilitation that requires only the access to wi-fi points (routers) is presented without the hotspot mode for user mobile devices. the wi-fi access points with non-standard firmware are proposed as measuring equipment to determine in real time the coordinates of the patient location within a medical institution. mailto:g.iadarola@staff.univpm.it acta imeko issn: 2221-870x september 2023, volume 12, number 3, 2 3 acta imeko | www.imeko.org september 2023 | volume 12 | number 3 | 2 finally, the paper [5] presents a mechatronic automatism based on near-field communication that allows the identification of user garments by sensor data, to improve the quality of life of fragile people, such as blind or disabled ones. with the help of an integrated interface to manage the requests from the user, a proper algorithm classifies the garments depending on their predominant color. we hope you will enjoy your reading. grazia iadarola, susanna spinsante and francesco lamonaca, guest editors references [1] g. iadarola, v. bruschi, s. cecchi, n. dourou, s. spinsante, lowcost monitoring for stimulus detection in skin conductance, acta imeko, vol. 12, no. 3, pp. 1-6. doi: https://doi.org/10.21014/actaimeko.v12i3.1540 [2] f. ruffa, m. lugarà, g. fulco, c. de capua, an iot measurement system for a tailored monitoring of co2 and total volatile organic compounds inside face masks, acta imeko, vol. 12, no. 3, pp. 18. doi: https://doi.org/10.21014/actaimeko.v12i3.1536 [3] m. n. m. aashiq, w. t. c. c. kurera, m. g. s. p. thilekaratne, a. m. a. saja, m. r. m. rouzin, n. neranjan, h. yassin, an iotbased handheld environmental and air quality monitoring station, acta imeko, vol. 12, no. 3, pp. 1-8. doi: https://doi.org/10.21014/actaimeko.v12i3.1487 [4] o. tohoiev, determining the location of patients in remote rehabilitation by means of a wireless network, acta imeko, vol. 12, no. 3, pp. 1-5. doi: https://doi.org/10.21014/actaimeko.v12i3.1495 [5] l. silva, d. rocha, f. soares, j. sena esteves, v. carvalho, automatic system to identify and manage garments for blind people, acta imeko, vol. 12, no. 3, pp. 1-10. doi: https://doi.org/10.21014/actaimeko.v12i3.1490 https://doi.org/10.21014/actaimeko.v12i3.1540 https://doi.org/10.21014/actaimeko.v12i3.1536 https://doi.org/10.21014/actaimeko.v12i3.1487 https://doi.org/10.21014/actaimeko.v12i3.1495 https://doi.org/10.21014/actaimeko.v12i3.1490 introductory notes for the acta imeko first issue 2023, special issue on metrology and digital transformation acta imeko issn: 2221-870x march 2023, volume 12, number 1, 1 3 acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 1 introductory notes for the acta imeko first issue 2023, special issue on metrology and digital transformation jon bartholomew1, daniel hutzschenreuter2, siniša prugovečki3, cristian zet4 1 emirates metrology institute, abu dhabi quality and conformity council, p.o box 853, abu dhabi, united arab emirates 2 department 9.4 metrology for the digital transfromation, physikalisch-technische bundesanstalt (ptb), bundesallee 100, 38116 braunschweig, germany 3 lorisq inc., 99 wall st., new york 10005, u.s.a. / metroteka d.o.o., kreše golika 3, 10000 zagreb, croatia 4 gheorghe asachi technical university of iasi, faculty of electrical engineering, 23 d. mangeron blvd, 700050 iasi, romania section: editorial citation: daniel hutzschenreuter, introductory notes for the acta imeko first issue 2023, special issue on metrology and digital transformation, acta imeko, vol. 12, no. 1, article 2, march 2023, identifier: imeko-acta-12 (2023)-01-02 received march 22, 2023; in final form march 30, 2023; published march 2023 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: francesco lamonaca, e-mail: editorinchief.actaimeko@hunmeko.org dear readers, the transition of metrology into the emerging digital era has grown to become one of the key challenges in the 21st century for metrologists worldwide. we already know that its highly cross-functional and interdisciplinary nature will lead to many major changes in the field, where not only expertise in measurement science but also skills for digital data, digital infrastructures, and machine-driven processes will play an important role. imeko accompanied by acta imeko is playing an important role in interconnecting experts around the world to share knowledge to foster the transition into the digital future. as a result, imeko tc6 was started in 2021 [1]. the aim of tc6 is to develop, organise and disseminate fundamental concepts of measurement science that relate to digitalization and digital transformation in science, industry, and society. the tc promotes the accumulation and curation of knowledge in various forms relating to the digitalization of measurement methodologies and measurement outcomes. its purpose is to provide a robust body of knowledge to support digital transformation whenever measurement is involved. digitalization's multidisciplinary nature is expected to overlap with other imeko groups' interests, therefore tc6 encourages collaborations and joint activities with other tcs and partner organizations around the globe. on 19 – 21 september 2022 tc6 organized the first international conference on metrology and digital transformation – the m4dconf 2022. with over 50 presentations and more than 200 participants from around the world the conference received great attention. highlights were the sessions by patron organisations: a session organized by the international committee on weights and measures (cipm) on the digital transformation of the international system of units, a session organized by the international organization in legal metrology (oiml), a session on metrological traceability in digital applications by imeko tc8, as well as sessions supported by euramet and eurolab. with further support by apmp, gulfmet and sim, many more high levels sessions were held providing a broad spectrum of hands-on contributions. topics included: pathways to digital transformation for small and emerging organizations, representation and use of fair (findable, accessible, interoperable, reusable) metrological information, digital infrastructures and technologies, metrology for advanced and sustainable manufacturing, and applications of artificial intelligence methods. in almost 50 % of all presentations, the development and use of machine-readable digital certificates in metrology was presented as an important technology. on behalf of imeko tc6 and the editors, it is a delight that this special issue of acta imeko is dedicated to the 16 best contributions on digitalization in metrology from m4dconf2022. it wouldn’t have been possible without the dedication of the authors, reviewers, conference committee members, editors and the exceptional support of the acta imeko team (francesco lamonaco, dirk röske and the copyeditors). the usability and interoperability of conventional quantityunit system builds the basis for reliable metrological data representation and exchange between both humans and machines. while today’s analogue systems are suitable for human users a direct translation into digital formats is posing difficulties to digitalization. the metrology-information layer (m-layer) is a new approach from research to overcome the limitations by providing profound metadata describing quantity kinds (aspects), units and scales to disambiguate transition between unit systems. mailto:editorinchief.actaimeko@hunmeko.org acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 2 blair hall and mark kuster have a major contribution to the development of the m-layer concept. the paper by mark kuster [2] reports work toward developing the m-layer’s current abstract conceptualization into a concrete model, working prototype, and demonstration software, with the eventual goal to create a fair resource. sensor networks (sn) are becoming an inherent part of many upcoming technologies, including smart cities, smart grids, complex processes monitoring in many industries, autonomous driving, healthcare and many other applications. martin koval et al. [3] describe the general approach for sns in their paper and deal with the principle components of sn architecture, plus the opportunities for the implementation of current and new technologies. together with other technologies such as ai and utilization of big data, the sn is becoming an important tool for optimization of many processes. the sn can help to push the limits in metrology with new effective algorithms in the ai or helping to solve challenges in uncertainty evaluation. however, sns as measuring systems underpin many developments in digital transformation, with applications ranging from regulated utility networks to low-cost internet of things (iot). the metrological assessment of sensor networks necessitates a fundamental revision of calibration, uncertainty propagation and performance assessment and new approaches for information and data handling regarding the individual sensors and their interactions in the network to allow a systems metrology approach to be established. the contribution by sascha eichstädt et al. [4] introduces some initial findings from recent research and gives an outlook into future developments. early in 2022 the national institute of standards and technology embarked on a pilot project to produce digital calibration reports and digital certificates of analysis for reference materials. william dinis camara et al. [5] present a paper reporting on the progress of the effort for reference material certificates and discuss some of the challenges and solutions in creating digital certificates. challenges include the diverse and complex information presently contained in certificates, conversion of values to non-si units of measurement to match the needs of stakeholders, format updates necessary for machine generation, and the wide variety of reference materials offered. in automated digital systems and environments such as industry 4.0 measurement data, including the calibration information, will need to flow through the whole process chain in digital format. the study presented by juho nummiluikki et al. [6] demonstrates a fully digitalized environment for calibration data generation, transfer, and usage in a proof of concept (poc) project. it is an outcome from a collaborative approach showing an exchange of calibration information by dccs from a national metrology institute through an accredited laboratory to endusers of calibrated measuring instruments. digital technologies have been providing their usefulness in instrumentation and measurements for many years. the ability to use software in measuring instruments became a common practice, bringing the advantage of signal and information processing and interfacing. instruments can be calibrated and verified in an automated calibration system which can perform all the process without the operator interference. the paper by cristian zet et al. [7] presents the possibility of joining an automated calibration system and the creation of dccs with the blockchain technology for storing and validating data. demonstrated benefits are digital traceability of dcc content by preservation of the full history of calibration information in a digital wallet and mechanisms to protect data from change. degradation of temperature sensors in harsh environments such as high temperature, contamination, vibration, and ionising radiation causes a progressive loss of accuracy that is not apparent. new developments to overcome the problem of ‘calibration drift’, include self-validating thermocouples and embedded phase-change cells which self-calibrate in situ by means of a built-in temperature reference and practical primary thermometers, such as the johnson noise thermometer, which measure temperature directly and do not suffer from calibration drift. all these developments provide measurement assurance which is an essential part of digitalization to ensure that sensor output is always ‘right’, as well as providing essential ‘points of truth’ in a sensor network. jonathan pearce et al. [8] present in an overview to some state-of-the art developments giving an excellent insight on how data quality can benefit from combining new measurement concepts with digital technologies. sensor networks could provide useful new tools for laboratories to increase the understanding of other measurements, to validate assumptions on and optimize existing measurements. a practical application for more accurate determination of the measurement uncertainty of an interferometric long-distance measurement is presented in the work by gertjan kok at al. [9]. a network with five temperature sensors was installed in a laboratory to measure a more detailed profile of the ambient temperature. it aims for more accurate determination of values for the refractive index on the path travelled by the laser light. during the measurement campaign an offset in the mean temperature of 0.2 °c was found, which was equal to the maximum allowed bias in view of the claimed uncertainty for the long-distance measurement. in the framework of empir project comtraforce, a digital twin (dt) concept of force measurement device was developed. dt aims to cover static, continuous, as well as dynamic calibration processes, preserving data quality and collecting calibration data for improved decision-making. to illustrate the dt concept, a prototype realization for static and continuous force calibration processes was developed, involving a simulation with ansys engineering software. oksana baer et al. [10] report on the current progress of the work in their paper. it is focused on data connection between a physical device and the dt. the dt model is validated using traceable measurements. in recent years, the need for better data science and data engineering have raised many challenges for metrology concerning the increasing amount of data that needs to be made available in ‘appropriate’ ways. to ensure findability, accessibility, interoperability, and reusability (fair) of digital resources, digital objects as a synthesis of data and metadata with persistent and unique identifiers should be used. in this context, the fair data principles formulate requirements that research data and, ideally, also industrial data should fulfil to make full use of them, particularly when machine learning or other datadriven methods are under consideration. in the contribution by tanja dorst et al. [11], the process of providing scientific data of an industrial testbed in a traceable and fair manner is documented as a descriptive example. ‘data metrology’, i.e., the evaluation of data quality and its fitness-for-purpose, is an inherent part of many disciplines including physics and engineering. in other domains such as life sciences, health, and pharmaceutical manufacturing these tools are often added as an afterthought, if considered at all. the use of data-driven decision-making and the advent of machine learning in these industries has created an urgent demand for harmonized, high-quality, and instantly available datasets across acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 3 domains. fair principles alone do not guarantee that data is fitfor-purpose. issues such as missing data and metadata, insufficient knowledge of measurement conditions or data provenance are well known and can be aided by applying metrological concepts to data preparation to increase confidence. a showcase for life science and health care projects where data metrology has been used to improve data quality is presented by paul m. duncan et al. [12]. digitalization will also affect the ways metrologists will handle data in their everyday work. an example of a modern approach to ease annotating, archiving, retrieving, and searching measurement data from a large-scale data archival system is described by frederic brochu et al. [13]. their tool extends and simplifies the interaction with their database and is implemented in popular scientific applications used for data analysis, namely matlab and python. it allows scientists to execute complex interactions with the database for data curation and retrieval tasks in a few simple lines of accessible templated code. shifting from paper-based data archives to purely digital ones raises a need for proper tracing of changes in data records without accidentally deleting or overwriting content. vashti galpin et al [14] have developed a framework allowing traceable updates in relational databases. they present a prototype web application developed in the programming language links for storing and displaying dcc using a relational database. their work leverages the temporal database features that links provides to capture different versions of a certificate and inspect differences between versions. modern programming language type systems help programmers write correct software, and furthermore help them write the software they intended to write. such type systems could be a powerful tool in the digitalization of metrology. by exploiting advances in dependent type systems, it is possible to strengthen the ability of software to reason about dimensional correctness of metrology data and bridge the gap between human-readable semantic specifications of data, and the actual code representing it in a specific programming environment. conor mcbride et al. [15] show in their paper how expressive types can be used to encode dimension and units of measurement information, which can be used to avoid dimensional mistakes and guide software construction. an automatic creation of source code is considered to further help to eliminate a whole class of potential bugs in software. introducing new information systems in organisations is often disruptive for individual departments. alexander oppermann et al. [16] show how application of an operation layer concept can allow departments to maintain their existing process and infrastructures and still harmonise data through the use of uniform representational state transfer interfaces. the automatic data transfer implemented reduces the workload for employees, increases the productivity, integrity and availability of data and greatly reduces the susceptibility to errors. we hope you will enjoy your reading. jon bartholomew, daniel hutzschenreuter, siniša prugovečki and cristian zet guest editors for the special issue on metrology and digital transformation references [1] imeko, imeko tc6 homepage. online [accessed 24 march 2023] https://www.imeko.org/index.php/tc6-homepage [2] mark j. kuster, toward a metrology-information layer for digital systems, acta imeko, vol. 12 (2023) no.1, pp. 1-4. doi: 10.21014/actaimeko.v12i1.1416 [3] martin koval et al., general sensors network application approach, acta imeko, vol. 12 (2023) no.1, pp. 1-5. doi: 10.21014/actaimeko.v12i1.1409 [4] sascha eichstädt et al., fundamental aspects in sensor network metrology, acta imeko, vol. 12 (2023) no.1, pp. 1-6. doi: 10.21014/actaimeko.v12i1.1417 [5] william dinis camara et al., digital nist: an examination of the obstacles and opportunities in the digital transformation of nist’s reference materials, acta imeko, vol. 12 (2023) no.1, pp. 1-4. doi: 10.21014/actaimeko.v12i1.1403 [6] juho nummiluikki et al., digital calibration certificate in an industrial application, acta imeko, vol. 12 (2023) no.1, pp. 1-6. doi: 10.21014/actaimeko.v12i1.1402 [7] cristian zet et al., automated calibration and dcc generation system with storage in private permissioned blockchain network, acta imeko, vol. 12 (2023) no.1, pp. 1-7. doi: 10.21014/actaimeko.v12i1.1414 [8] jonathan pearce et al., progress towards in-situ traceability and digitalization of temperature measurements, acta imeko, vol. 12 (2023) no.1, pp. 1-6. doi: 10.21014/actaimeko.v12i1.1386 [9] gertjan kok et al., improved uncertainty evaluation for a long distance measurement by means of a temperature sensor network, acta imeko, vol. 12 (2023) no.1, pp. 1-6. doi: 10.21014/actaimeko.v12i1.1411 [10] oksana baer at al., digital twin concept of a force measuring device based on the finite element method, acta imeko, vol. 12 (2023) no.1, pp. 1-5. doi: 10.21014/actaimeko.v12i1.1404 [11] tanja dorst et al.: a case study on providing fair and metrologically traceable data sets, acta imeko, vol. 12 (2023) no.1, pp. 1-6. doi: 10.21014/actaimeko.v12i1.1401 [12] paul m. duncan et al., metrology for data in life sciences, healthcare and pharmaceutical manufacturing: case studies from the national physical laboratory, acta imeko, vol. 12 (2023) no.1, pp. 1-5. doi: 10.21014/actaimeko.v12i1.1406 [13] frederic brochu et al., a tool for curating and searching databases proving traceable analysis of data and workflows, acta imeko, vol. 12 (2023) no.1, pp. 1-6. doi: 10.21014/actaimeko.v12i1.1408 [14] vashti galpin et al., tracking and viewing modifications in digital calibration certificates, acta imeko, vol. 12 (2023) no.1, pp. 1-7. doi: 10.21014/actaimeko.v12i1.1407 [15] conor mcbride et al., measuring with confidence: leveraging expressive type systems for correct-by-construction software, acta imeko, vol. 12 (2023) no.1, pp. 1-5. doi: 10.21014/actaimeko.v12i1.1412 [16] alexander oppermann et al., digital transformation: towards a cloud native architecture for highly automated and event driven processes, acta imeko, vol. 12 (2023) no.1, pp. 1-6. doi: 10.21014/actaimeko.v12i1.1410 https://www.imeko.org/index.php/tc6-homepage https://doi.org/10.21014/actaimeko.v12i1.1416 https://doi.org/10.21014/actaimeko.v12i1.1409 https://doi.org/10.21014/actaimeko.v12i1.1417 https://doi.org/10.21014/actaimeko.v12i1.1403 https://doi.org/10.21014/actaimeko.v12i1.1402 https://doi.org/10.21014/actaimeko.v12i1.1414 https://doi.org/10.21014/actaimeko.v12i1.1386 https://doi.org/10.21014/actaimeko.v12i1.1411 https://doi.org/10.21014/actaimeko.v12i1.1404 https://doi.org/10.21014/actaimeko.v12i1.1401 https://doi.org/10.21014/actaimeko.v12i1.1406 https://doi.org/10.21014/actaimeko.v12i1.1408 https://doi.org/10.21014/actaimeko.v12i1.1407 https://doi.org/10.21014/actaimeko.v12i1.1412 https://doi.org/10.21014/actaimeko.v12i1.1410 multilayer feature fusion using covariance for remote sensing scene classification acta imeko issn: 2221-870x march 2022, volume 11, number 1, 1 8 acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 1 multilayer feature fusion using covariance for remote sensing scene classification s. thirumaladevi1, k. veera swamy2, m. sailaja3 1 ece department, jawaharlal nehru technological university, kakinada 533003, andhra pradesh, india 2 ece department, vasavi college of engineering, ibrahimbagh, hyderabad 500 031, telangana, india 3 ece department, jawaharlal nehru technological university, kakinada 533003, andhra pradesh, india section: research paper keywords: feature extraction; pre-trained convolutional neural networks; support vector machine; scene classification citation: s. thirumaladevi, k. veera swamy, m. sailaja, multilayer feature fusion using covariance for remote sensing scene classification, acta imeko, vol. 11, no. 1, article 33, march 2022, identifier: imeko-acta-11 (2022)-01-33 section editor: md zia ur rahman, koneru lakshmaiah education foundation, guntur, india received december 25, 2021; in final form february 20, 2022; published march 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: s. thirumaladevi, e-mail: thirumaladeviece1@gmail.com 1. introduction remote sensing scene categorization has gotten a lot of attention recently, and it may be utilized in a variety of practical applications like urban planning, defence, space applications, in which measurement technology plays a key role [1]. on the other hand, it is a difficult challenge, since scene images often have complicated spatial structures with great intra-class and slight inter-class variability. to solve this problem, numerous strategies for scene classification have been advised in recent years [2]. recently, inspired by the tremendous achievement of convolutional neural networks (cnns) relating to the computer vision field [3]. deep neural networks have gained prominence in the remote sensing community due to their exceptional performance, particularly in scene classification and computer vision applications [4]. developing a deep cnn model from scratch, on the other hand, frequently necessitates a large amount of training data, whereas commercially available scene image data sets of remote sensing scene image data sets are typically tiny. deep cnn models have a high degree of generalization on a wide range of tasks because they are commonly trained on imagenet [5], which contains millions of images (e.g., scene classification and object detection [6]). in this context, the idea of using off-the-shelf pretrained cnn models for example alexnet [7], visual geometry group (vgg) 16 [8], and vgg19 as feature extractors for scene categorization using remote sensing has gained attraction. the success is due to these models representing images using hierarchical architecture and can extract more representative features. while these models can achieve categorization performance is excellent. hu et al. [9] looked into two scenarios for using a pretrained cnn model abstract remote sensing images are obtained by electromagnetic measurement from the terrain of interest. in high-resolution remote sensing imageries extraction measurement technology plays a vital role. the scene classification is one of the interesting and challenging problems due to the similarity of image structure and the available hrrs image datasets are all small. training new convolutional neural networks (cnn) using small datasets is prone to overfitting and poor attainability. to overcome this situation using the features produced by pre-trained convolutional nets and using those features to train an image classifier. to retrieve informative features from these images we use the existing alex net, vgg16, and vgg19 frameworks as a feature extractor. to increase classification performance further makes an innovative contribution fusion of multilayer features obtained by using covariance. first, to extract multilayer features, a pre-trained cnn model is used. the features are then stacked, downsampling is used to stack features of different spatial dimensions together and the covariance for the stacked features is calculated. finally, the resulting covariance matrices are employed as features in a support vector machine classification. the results of the experiments, which were conducted on two difficult data sets, uc merced and siri-whu. the proposed staked covariance method consistently outperforms and achieves better classification performance. achieves accuracy by an average of 6 % and 4 %, respectively, when compared to corresponding pre-trained cnn scene classification methods. mailto:thirumaladeviece1@gmail.com acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 2 (vgg16). the final few fully connected layers are portrayed as final image attributes for scene classification in the first scenario. in the second case, the final convolutional layer's feature maps are encoded to represent the input image using a standard method of feature encoding, such as the improved fisher kernel [10]. the vector support machine (svm) is used as the final classifier in both cases. to improve the efficacy of the proposed method, the features were extracted from multiple cnns of the same image combined by xue et al. [11] for classification. for feature fusion, sun et al. [12] have used the gated bidirectional connection method. in [13], the image is represented by combining the last two fully connected (fc) layers of a cnn model. here we propose an innovative method, called the stacked covariance strategy to fuse features from different layers of a pre-trained cnn to classify remote sensing scenes. in the first phase, a pre-trained cnn model is used to extract multi-layered features and concatenate them. the covariance approach is used to aggregate the concatenated multiple feature vectors extracted from different layers. in contrast to traditional strategies, which only use first-order statistics to integrate feature vectors, the proposed strategy allows for the use of second-order statistics information. more representative features can thus be learned as a result. then, the features are stacked, and covariance is calculated. finally, for classification using an svm classifier and improved the classification performance. this is how the rest of the paper is organized. section 2 explains the intended scene classification framework, novel aspects of our proposed technique. section 3 contains the full experimental results for two data sets, section 4 of this work concludes with some observations. 2. proposed technique description the process of transforming the raw image into numerical features that can be processed while retaining the original information is referred to as feature extraction. with the upsurge of deep learning, the first layers of deep networks have largely replaced feature extraction, particularly for image data. pretrained networks with hierarchical architecture can extract a large number of features from an image, which is thought to transmit additional information that can be put to much better use to increase categorization accuracy. the learned image characteristics are first retrieved from a pre-trained convolutional neural network and then used to train an image classifier. all pretrained cnns requisite fixed-size input images and specify the desired image size, as well as create augmented image data stores and use these data stores as input arguments to activations to automatically resize the training and test images before they are submitted to the network. remove the pre-trained cnn's last fc layer fc8 and consider the rest as a fixed feature extractor. we feed an input image scene into the cnn and using the vector as a representation of global features of the input image, generate in advance a dimensional activation vector from the first or second fc layer. finally, use the dimensional features to train a linear svm classifier for scene classification. figure 1 shows an illustration of this. to improve the classification accuracy further proposed a modified pre-trained network design by combining information from several convolution layers. the shallower levels of the cnn model are more likely to represent low-level visual components (such as edges), whereas the deeper layers exemplify more abstract information in the images. furthermore, in certain computer vision applications, combining different levels, from shallow to deep, can provide state-of-the-art performance, meaning that merging different layers of cnn can be very helpful. our proposed approach uses a similar strategy to take advantage of the information held by multiple layers in this way. this is represented in figure 2. here convolutional layers of the last three blocks of pretrained networks are adopted and concatenated the features extracted from these layers. namely conv3, conv4, conv5 in case of alexnet and "conv3-3", "conv43", and "conv5-3" in case of vgg16, vgg19. different convolutional layers predominantly have distinctive spatial dimensions, they cannot be directly concatenated. to address this issue, downsampling with bilinear interpolation is used in conjunction with channel-wise average fusion. obtained features that were reformed into a matrix along the channel dimension and aggregated using covariance. the proposed technique is described below. cnn model is a collection of functions in which each function 𝑓𝑛 takes data samples 𝑋𝑛 and a filter bank 𝑏𝑛 as inputs and outputs 𝑋𝑛+1 , where 𝑛 = 1, 2, … 𝑁 and 𝑁 is the number of layers represented as a number (1) 𝐹(𝑋) = 𝑓𝑁(… 𝑓2(𝑓1(𝑋; 𝑏1); 𝑏2) … 𝑏𝑁 ) . (1) the filter bank 𝑏𝑛 for a pretrained cnn model was learned from a large data collection. the multiplayer characteristics are retrieved from an input image 𝑋 as follows: 𝐿1 = 𝑓1(𝑋; 𝑏1), 𝐿2 = 𝑓2(𝑋; 𝑏2), and so on. as pretrained models, alexnet, vgg16, and vgg19 are employed in this paper. the features produced from the convolutional layers of the last three blocks of pretrained networks are adopted and utilized. different convolutional layers typically have different spatial dimensions; therefore, they can't be concatenated directly. direct concatenation is not allowed when conv3 may have 𝐿1 ∈ 𝑅1 ℎ×𝑤×𝑑1 , conv4 𝐿2 ∈ 𝑅2 ℎ×𝑤×𝑑2 , and conv5 has 𝐿3 ∈ 𝑅3 ℎ×𝑤×𝑑3 . downsampling with bilinear interpolation, as well as channel-wise average fusion, are used to solve this problem. using downsampling with a 𝑑 number of dimensions three convolutional layers that have been pre-processed have been obtained and channel-wise average fusion is performed then the stacked feature set is acquired as follows: 𝐿 = [𝐿1, 𝐿2, 𝐿3] ∈ 𝑅𝑖 𝑆×𝑆×𝐷 , where 𝐷 = 7 𝑑 and 𝑆 is the predefined down-sampled spatial dimension. the covariance-based pooling can be written as [14] figure 1. classification using single-layered pre-trained cnn as a feature extractor. figure 2. classification using stacked multilayer pre-trained cnn as a feature extractor. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 3 𝑃 = 1 𝑁 − 1 ∑(𝑌𝑖 − 𝜇) (𝑌𝑖 − 𝜇) t 𝑁 𝑖=1 ∈ 𝑅𝑖 𝐷×𝐷 , (2) where [𝑌1, 𝑌2, … 𝑌𝑁 ] ∈ 𝑅𝑖 𝐷×𝑁 is the vectorization of 𝐿, 𝑁 = 𝑆 2 and 𝜇 = (1 𝑁⁄ ) ∑ 𝑌𝑖 ∈ 𝑅 𝐷𝑁 𝑖=1 , the covariance between the two separate features makes is represented by p, while the variance of each map is represented by diagonal entries. this method incorporates covariance (i.e., second-order statistics) to produce a more dense and discriminative exemplification. second, the correlation between two distinct feature maps is represented by each entry in the covariance matrix. this is an easy approach to merge data from different feature maps that complement each other. the suggested method varies from existing cnn-based algorithms that are pre-trained. concatenating the cnn's unique convolutional features (from shallow to deep layers), feature maps from several layers are merged. as a result, the suggested technique performs much better in terms of categorization. furthermore, because the covariance matrices do not lie in euclidean space, they cannot be processed by the svm. the covariance matrix, on the other hand, can be mapped into euclidean space using the matrix logarithm operation [15]. �̂� = logm(𝑃) = 𝑈 log ∑ 𝑈t ∈ 𝑅𝐷×𝐷 , (3) where, 𝑃 = 𝑈 ∑ 𝑈t is the eigen decomposition equation of 𝑃. the preceding operations are carried out both on the samples of training and testing. {𝑉𝑖 , 𝑆𝑖 }, 𝑖 = 1, 2, … 𝑛 and the testing sets are now taken into account where 𝑆𝑖 represents the number of training samples and 𝑛 represents the number of corresponding labels. {𝑉𝑖 , 𝑆𝑖 }, 𝑖 = 1, 2, … 𝑛 is exploited to train an svm model as min𝑎,ζ ,𝑏 {1 2⁄ ‖𝑎‖2 + 𝑃 ∑ ζ𝑖 } 𝑆𝑖 (∠φ(𝑉𝑖 , 𝑎 + 𝑏)) ≥ 1 − ζ𝑖 𝜀𝑖 > 0, 𝑉𝑖 = 1, 2, … 𝑛 (4) where 𝑎 and 𝑏 are the parameters of a linear classifier (.) is the mapping function and 𝜀𝑖 are positive slack variables to assert with outliers in the training set. with k(𝑉𝑖 , 𝑉𝑗 ) = 𝑉𝑖 t𝑉𝑗 𝑓(𝑥) = sgn (∑ 𝑆𝑖 𝜆𝑖 𝑘(𝑣𝑖 , 𝑣) 𝑛 𝑖=1 + 𝑏 ). (5) 3. experimental results analysis discussion 3.1 experimental data sets we run tests on two tough image data sets related to remote sensing scene images to see how well the suggested approach performs. 1) land use data set from uc merced [16]: 2100 pictures are classified into 21 scene groups in the uc merced land use (uc) [17] data set. each class consists of 100 images in the rgb space with a size of 256 × 256 pixels. each image has a one-foot pixel resolution. figure 3. depicts sample images a) b) c) d) e) f) g) h) i) j) k) l) m) n) o) p) q) r) s) t) u) figure 3. land-use categories of 21 example classes representation of uc merced data set : a) agricultural13, b) airplane19, c) baseball diamond3, d) beach33, e) buildings21, f) chaparral13, g) denseresidential40, h) forest23, i) freeway23, j) golfcourse41, k) harbour31, l) intersection3, m) mediumresidential12, n) mobilehomepark12, o) overpass45, p) parkinglot32, q) river32, r) runway26, s) sparse residential64, t) storagetanks54, u) tenniscourt16. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 4 from each class, some categories (forest and sparse residential, for example) exhibit a significant level of interclass similarity, making the uc data set a difficult one to work with. siri-whu was obtained from google earth (google inc.) and covers urban regions in china, as well as siri-whu [18]. there are 12 items in the data set. each class has 200 photos, each cropped to 200 × 200 pixels and with a spatial resolution of 2 meters. in this study, 80 % of the training samples were chosen from the siri-whu [19] google data set, while the remaining amount of samples was kept for testing. the sample images of the siri-whu data set are shown in figure 4. 3.2 experimental setup in our approach, multilayer features are extracted using three well-known cnn pretrained models: alexnet [7], vgg-19 [8], and vgg-16 [8]. vgg-16 and vgg-19 three convolutional layers (e.g., "conv3–3," "conv4–3," and "conv5–3") , alexnet's three convolutional layers (i.e., "conv3," "conv4", and "conv5") are used. for feature extraction, the scene images are preset to the size of the input layer such as 227 × 227 × 3 in the case of alex net and 224 × 224 × 3 forvgg-vg16 and 19. imagenet is used to train both models. for illustration purposes, the uc data set and the siri-whu data set are used, with 80 % training samples and 20 % testing samples selected. the most frequently used image classification assessment criteria are oa and confusion matrix, f1-score. confusion matrix: this is a special matrix that is commonly used g to visualize the output. each column in this matrix represents the estimated value, while each row signifies an authentic category. as a result, evaluating is relatively simple. overall accuracy (oa): the number of appropriately categorized images divide by the total number of images in the data set, regardless of which class they belong to. f1 score: the f1 score is a metric used to determine how accurate a test is. the harmonic mean of recall and precision is calculated using the test's precision and recall. based on the combination of real and anticipated categories, classification problem with several 𝑀, which comprises 𝑃 positive instances and 𝑁 negative instances, there are four types of cases: true positives (𝑇𝑃), false positives (𝐹𝑃), true negatives (𝑇𝑁), and false negatives (𝐹𝑁). the positive sample 𝑃 = 𝑇𝑃 + 𝐹𝑁 provides a positive sample that is expected to be positive, while 𝑇𝑃 represents a positive sample that is forecast to be negative. similarly, 𝑇𝑁 denotes the number of negative cases that are identified as negative, while 𝐹𝑃 denotes the number of negative incidences that are predicted to be positive; thus, 𝑁 = 𝑇𝑁 + 𝐹𝑃 denotes the total number of negative samples. the fraction of correct instances is the accuracy, and the calculation equation is 𝐴𝑐𝑐𝑢𝑟𝑎𝑐𝑦 = 𝑇𝑃 𝑇𝑃 + 𝐹𝑁 + 𝑇𝑁 + 𝐹𝑃 . (6) the fraction of actually positive instances in all cases projected to be positive elements is called precision. the formula is as follows: 𝑃𝑟𝑒𝑐𝑖𝑠𝑖𝑜𝑛 = 𝑇𝑃 𝑇𝑃 + 𝐹𝑃 . (7) the recall calculation equation is the fraction of all positive samples that are projected to be positive. 𝑅𝑒𝑐𝑎𝑙𝑙 = 𝑇𝑃 𝑇𝑃 + 𝐹𝑁 . (8) the f1-score is a precision and recall evaluation indicator with a comprehensive calculation equation. 𝑅𝑒𝑐𝑎𝑙𝑙 = 2 ∙ 𝑃𝑟𝑒𝑐𝑖𝑠𝑖𝑜𝑛 ∙ 𝑅𝑒𝑐𝑎𝑙𝑙 𝑃𝑟𝑒𝑐𝑖𝑠𝑖𝑜𝑛 + 𝑅𝑒𝑐𝑎𝑙𝑙 . (9) the confusion matrix, along with accuracy, is shown in figure 5. the first column depicts a single fully connected layer is used as the final feature extractor for scene classification, whereas the second column depicts the proposed sc-based network classification. experiments on the uc data set revealed that the oas of pre-trained alexnets is 79.76 %, vgg-19 is 81.19 %, and vgg-16 is 83.81 % while using sc and combining the last a) b) c) d) e) f) g) h) i) j) k) l) figure 4. example class representation of the siri-whu dataset: a) agriculture1, b) commercial50, c) harbor64, d) idle_land76, e) industrial111, f) meadow120, g) overpass15, h) park29, i) pond37, j) residential1, k) river106, l) water97. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 5 three conv layers increases accuracy by 85 %, 87.14 %, and 88.33 %, respectively. the proposed method accomplishes perfect classification performance on the majority of classes, such as agricultural, beach, chaparral, forest, harbour, parking lot, runway and improved classes are buildings, dense residential, baseball diamond, tennis court and on average 6 % accuracy is increased in the case of ucm. similar results can be obtained in the experiments conducted on the siri-whu data set, confusion matrix for the fully connected layer is treated as a final feature extractor for scene classification, proposed sc-based classification are shown in figure 6, where the pre-processed single layer of alexnet is 86.52 %%, vgg-19 is 87.6 %, and vgg-16 is 88.04 % while using the proposed strategy increases the accuracy by 90 %, a) d) b) e) c) f) figure 5. confusion matrix of uc merced dataset using three pre-trained networks. first column corresponding to single-layered, a) alex net, b) vgg19, c) vgg16, second column corresponding to multi-layered fusion, d) sc-alex net, e) sc-vgg19, f) sc-vgg16. table 1. comparison results for the two data sets ucm and siri-whu. ucm dataset with 80% training (overall accuracy %) siri-whu dataset with 80% training (overall accuracy %) network/method pre-trained network fc7 as a feature extractor proposed sc network as a feature extractor pre-trained network fc7 as a feature extractor proposed sc network as a feature extractor alexnet 79.76 85 86.52 90 vgg-vd19 81.19 87.14 87.60 91.08 vgg-vd16 83.81 88.33 88.04 92.60 acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 6 91.08 %, and 92.60 %, respectively. in most classes, the suggested technique achieves optimal classification performance like agriculture, commercial, harbour, meadow, meadow and improved classes are industrial, overpass, pond, overall 4 % accuracy is increased and comparison graphs are shown in figure 7. the related comparison results for the two data sets are shown in table 1. the proposed scenario shows a clear improvement in oa when several conv layers are combined. as illustrated in figure 8, f1 scores of improved classes of ucm data set. by using the proposed strategy, a considerable number of classes show noticeable improvement such as agricultural, beach, harbour, runway reached 100 % and dense residential-class improves approximately 40 %. likewise, figure 9, shows the corresponding f1 scores of pretrained single-layered and proposed networks on the siri-whu data set. as can be witnessed the proposed strategy, exhibits obvious improvements in most of the classes for example, on the siri-whu data set water reached 100% and harbour, idle_land, industrial, overpass, park classes reach above 90%. 4. conclusion in this research, we portray stacked covariance, a new technique for fusing image features from multiple layers of a cnn for scene categorization using remote sensing data. feature extraction is performed initially with a pre-trained cnn model, followed by feature fusion with covariance in the presented scbased classification framework. more dense features are recovered for classification because the proposed scenario takes into account second-order data. each feature represents the covariance of two distinct feature maps and these features are applied to svm for classification. our extensive suggested sc method's effectiveness is validated by comparison with state-ofthe-art methodologies using two publicly accessible remote sensing image scene categorization data sets. we recognize that utilizing the proposed sc technique, the accuracy attained for most classes shows obvious enhancements, indicating that this is a viable improvement strategy. a) d) b) e) c) f) figure 6. confusion matrix of siri-whu dataset using three pre-trained networks. first column corresponding to single-layered, a) alex net, b) vgg19, c) vgg16, second column corresponding to multi-layered fusion, d) sc-alex net, e) sc-vgg19, f) sc-vgg16. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 7 references [1] l. fang, n. he, s. li, p. ghamisi, j. a. benediktsson, extinction profiles fusion for hyperspectral images classification ieee 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http://www.lmars.whu.edu.cn/prof_web/zhongyanfei/num/google.html https://doi.org/10.3390/rs10030444 acta imeko  december 2014, volume 3, number 4, 10 – 16  www.imeko.org    acta imeko | www.imeko.org  december 2014 | volume 3 | number 4 | 10  two‐functional µbist for testing and self‐diagnosis of analog  circuits in electronic embedded systems  dariusz załęski, romuald zielonko  gdansk university of technology, faculty of electronics, telecommunications and informatics, department of metrology and  optoelectronics, ul. gabriela narutowicza 11/12, 80‐233 gdansk ,poland      section: research paper   keywords: electronic embedded systems; built‐in self‐testers (bist); functional testing; analog fault diagnostics  citation: dariusz załęski, romuald zielonko, two‐functional µbist for testing and self‐diagnosis of analog circuits in electronic embedded systems, acta  imeko, vol. 3, no. 4, article 4, december 2014, identifier: imeko‐acta‐03 (2014)‐04‐04  editor: paolo carbone, university of perugia   received october 10 th , 2013; in final form october 10 th , 2013; published december 2014  copyright: © 2014 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: (none reported)  corresponding author: romuald zielonko, e‐mail: zielonko@eti.pg.gda.pl    1. introduction  electronic embedded systems (eess) with an embedded intelligent unit in a structure (usually in the form of a microcontroller), are widely used in many branches of industry, technology and science. the dominant group of eess are mixed-signal systems. a common fact of these systems is the presence of analog circuits, since there always must be an interface to the real world. mixed-signal eess bring new challenges to the test problem of the analog circuits and blocks, because there is lack of general test methods and strategies. the main direction of development of testing analog circuits in eess is the built-in self-test (bist) technique. in last years many specific solutions of bists dedicated for concrete circuits have been reported: oscillation-based bist (obist) [1], digital reuse [2], histogram based [3], [4] dedicated for fully-differential stages [5], [6],  bist [7], [8] and adc bist for ad converters. the common feature of these solutions is hardware excess, i.e. the necessity to build an overhead hardware into the ees’s structure, which introduces additional costs. one promising solution to reduce the hardware overhead and testing costs is reuse of signal blocks already present in the ees (such as processors and memories) for a bist creation. the new generations of microcontrollers (e.g. at91sam, aduc814) have hardware resources (adcs, dacs, timers, counters, analog comparators) that allow to generate stimulating signals and to measure responses of a circuit under test (cut), as well as the computing power to realize of testing procedures. therefore, the hardware and software resources of modern microcontrollers are sufficient for creation of microbists (µbists). these resources are sufficient, but simultaneously they are rather modest, thus special signals and procedures adequate to microcontroller resources must be used. some solution of the µbist for functional testing based on shape designed complementary signals (css) [9], [10], abstract  the paper concerns the testing of analog circuits and blocks in mixed‐signal electronic embedded systems (eess), using the built‐in  self‐test  (bist)  technique.  an  integrated,  two‐functional,  embedded  microtester  (µbist)  based  on  reuse  of  signal  blocks  already  present in an ees, such as microprocessors, memories, adcs, dacs, is presented. the novelty of the µbist solution is its extended  functionality. it can perform 2 testing functions: functional testing and fault diagnosis on the level of localization of a faulty element.  for functional testing the complementary signals (css), and for fault diagnosis the simulation before test (sbt) vocabulary techniques  have been used. in the fault vocabulary the graphical signatures in the form of identification curves in multidimensional spaces have  been applied.   acta imeko | www.imeko.org  december 2014 | volume 3 | number 4 | 11  [11] has been proposed by the authors in [12]. the disadvantage of this solution is poor functionality, limited only to functional testing. the novelty of the solution presented in this paper is the extended functionality of the µbist, including both functional and diagnostic testing. in the proposed solution (named the integrated, two-functional µbist) for functional testing, complementary signals and for fault diagnosis sbt (simulation before test) techniques have been used. the paper is organized as follows: section ii presents the cs method for functional testing. section iii describes a vocabulary sbt technique for diagnostic testing. section iv presents an experimental realization of the integrated twofunctional µbist and its verification in an exemplary ees. 2. description and investigation of the cs method  for functional testing  the essence of this method is stimulation of the cut by a special shape-designed cs signal, whose parameters are matched to poles of the cut’s transfer function. two kinds of cs signals has been investigated in [14]: �iparameters cs (with variable impulse amplitude �i) and tiparameters cs (with variable impulse width ti). a typical example of ti-parameters cs signal is shown in figure 1a. the testing principle is explained in figure 1b. the first impulse of the cs signal drives the cut into an initial state yt0(t), whereas the remaining impulses yt1(t), yt2(t) compensate it, in the manner shown in the figure, that the cut response y(te) reaches a zero state at the signal end time te. such situation takes place when the cut is in its nominal state. in a faulty state, when transfer function poles are deviating from nominal values, the cut response after the end of stimulation is not compensated (y(te)≠0). the sample value of the cut response y(te) depends on deviation of transfer function parameters from the nominal state. so, it can be used for functional testing of the cut transfer function. the cs method has been widely investigated, at first via simulations and then via physical experiments, on the example of testing of a 4th order butterworth low-pass filter (lpf). the structure of the filter, as well as nominal functional parameters and component values are shown in figure 2. the filter was composed with two biquad sections realized on the integrated circuit uaf42. the transfer function of the 4th order butterworth lpf is described by the following formula:                        2 2 1 2 2 2 2 21 2 1 2 1 2 ( ) n niv n n n n g s s s s s q q (1) where: n1, n2 are filter cut-off angular frequencies, q1, q2 are quality factors of the 1st and 2nd section. the filter was stimulated by a matched ti-parameter cs signal with 1 v impulses and with following ti-parameters: t0 = 15.916 ms, t1 = 37.393 ms, t2 = 57.185 ms, t3 = 70.397 ms, t4 = te = 75.027 ms. the investigation results of the deviation influence of both: quality factors q1, q2 and angular frequencies n1, n2 on the cut’s output signal, are presented in figure 3. it is seen from plots shown in the figure, relations y(te)=f(qi/qi) and y(te)=f(ni/ni) are convenient to implement. the y(te)=f(qi/qi) relations are linear in a wide range of qi/qi deviation (±50%). the testing t t0 t1 t2 t3 -  u(t) te (a)  (b) figure 1. an example of the 3 rd  order ti‐parameter cs (a) and cut responses  yti(t) to each impulse of ti‐parameter cs and its resultant response y(t) (b).  r14 50 k r12 50 k rf11 r11 50 k c12 1000 pf c11 1000 pf rf12 rg1 rq1 12 13 8 7 14 1 3 11 uaf42 vin c11a c12a r24 50 k r22 50 k rf21 r21 50 k c22 1000 pf c21 1000 pf rf22 rg2 rq2 12 13 8 7 14 1 3 11 uaf42 lp out c21a c22a   components  values  of  the  1 st   section:  rg1=52.22 k  rq1=83.56 k  rf11,rf12=15.28 k c11a=1052 nf  c12a=1054 nf        components  values  of  the  2 nd   section:  rg2=51.84 k  rq2=17.43 k  rf21,rf22=14.95 k c11a=1040 nf  c12a=1041 nf    functional parameters:  fc=10 hz  c=62.8319 rad/s  q1=0.54119  q2=1.30655  s1, s2=‐58.0491  4.0447i   s3, s4=‐24.0447  8.0491i figure 2. schematic diagram of investigated 4 th  order butterworth low‐pass  filter and its component values.  -50 -40 -30 -20 -10 0 10 20 30 40 50 -40 -30 -20 -10 10 20 30 40 y (t e) [mv] qi/ qi [%] (a) -50 -40 -30 -20 -10 0 10 20 30 40 50 -40 -20 20 40 60 80 y(t e) [mv] n/n [%] (b) figure  3.  responses  y(te)=f(qi/qi)  and  y(te)=f(ni/ni)  of  the  tested  4 th   order lpf as a function of q1 and q2 factors deviation (a) and n1 and n2  angular frequencies deviation (b).  y(t) acta imeko | www.imeko.org  december 2014 | volume 3 | number 4 | 12  sensitivity is about 0.8 mv/1%. the y(te)=f(ni/ni) relations are slightly nonlinear and the testing range for positive deviations of (ni/ni) is limited to 25%. testing sensitivities are not equal for different biquad sections of the filter. for the 1st section the sensitivity is about 0.5 mv/1% and for the 2nd section it is about 1 mv/1%. in all investigated cases testing sensitivities are high enough for cut functional testing. the cut’s output response signals y(te) are easy to measure using the ad converter available in the microcontroller aduc814, chosen for the realization of the two-functional µbist. hardware resources of this microcontroller (12-bit adc and dacs and 3x16-bit programmable counters) are good enough for the generation of cs signals with adequate precision and for accurate measurement of cut’s output responses. in [14] the metrological properties of the �i-parameters and ti-parameters css have been widely investigated and compared. the following conclusions should be emphasized: both signals have similar sensitivities to deviations of cut functional parameters, more predestined for application in the µbist are ti-parameters css. they are shorter, easier to generate using microcontroller resources and can be optimized to the cut structure by matching the width of the 1st impulse. the most useful is a unipolar ti-parameter cs. generally, we can conclude that unipolar ti-parameters css signals are suitable for application in a µbist for functional testing of analog circuits and blocks in eess, which appears as the first function of an integrated µbist. the cs method might be useful in the low frequency range, up to about 10 khz. 3. description and investigation of the sbt  diagnostic method  for diagnostic testing of analog circuits in the eess, which appears as the second function of an integrated µbist, the vocabulary sbt method has been chosen, with graphical fault signatures in the form of identification curves in multidimensional measurement spaces. such a form of the fault vocabulary has two advantages: identification curves can be scaled, thus they make fault localization and also identification possible, it is possible to introduce an additional distinctive feature of the measurement signal to increase the dimension of the measurement space, and in consequence rarefying curves in the space. in result it enables to increase distinctivity of fault localization and to decrease the fault masking effect of parameter tolerances. we investigated this method using a 2d and 3d measurement space based on the samples u1(t1), u2(t2),..., uk(tk) of a cut output response at properly chosen time moments, taking into account 2 cases: toleranceless cuts and cuts with real tolerances of elements. at first, for simplification we explain this method on the example of fault diagnosis of the 2nd order butterworth lpf (figure 4), without taking into account tolerances. for the filter, on the basis of 2 or 3 samples of its output response to a single square-impulse stimulation, we can r1 = r2 = 10 k   c1 = 70.44 nf   c2 = 146.35 nf  figure 4. the 2nd order low‐pass filter.  (a) (b)  figure 5. the families of identification curves on the plane (a) and in the 3d  space (b).  figure 6. the identification curves for range 0.5pi nompi1.5pi nom.  (a) r1, r2, r3 = 5 k,  c1 = 44.5 nf   c2 = 110 nf  c3 = 6.42 nf  fc= 1 khz   (b) figure 7. the circuit diagram the 3 rd  order butterworth lpf (a) and families  of dispersed identification curves (belts) in the 3d space (b).  acta imeko | www.imeko.org  december 2014 | volume 3 | number 4 | 13  obtain the fault vocabulary in the form of a family of identification curves on a plane (for 2 samples u1(t1), u2(t2)) or in 3d space (for 3 samples u1(t1), u2(t2), u3(t3)) as shown in figure 5, for wide a range of parameters change: 0.1pinompi10pinom. in practice, the soft faults are diagnosed in narrower ranges: 50% (0.5pinompi1.5pinom) or even 25%. the family of scaled identification curves in the range 50% is shown in figure 6. in this range, as it is seen, the curves are smooth and easy to point interpolation. for such fault signature classification a simple conventional point distance classifier can be used. the required memory size for a fault vocabulary is moderate. for example, in case of a fault vocabulary in 3d space with 10 identification curves and 50 interpolation points for each, the memory size is about 3750 bytes. the toleranceless version of the sbt method can be useful for fault diagnosis of circuits with small parameter tolerances <0.5%. it seems to be of little use in practice. in practice, analog circuits have higher tolerances (in real circuits 2%-5%), which cause dispersion of identification curves in the measurement space. the dispersed curves form in the 2d space a kind of identification belts, and in the 3d space they form the identification snakes as shown in figure 7. such families can be obtained from simulations using the monte carlo method. identification belts and snakes in the general case have variable cross sections, but for moderate cut’s tolerances (2%) they can be assumed constant. the identification belts and snakes can be used for soft fault diagnosis, but their more sophisticated description as graphic signatures in the fault vocabulary is needed. an appropriate classifier fitted to these signatures must be used. for modeling of the identification belts and snakes and description in a vocabulary, we have used models shown in figure 8, for 2d and 3d spaces. the belts are modeled by the set of circles with radius of  around the interpolation points pi of the nominal identification curve. the sets of spheres around the interpolation points, with radius of , model the identification snakes. they are described by two factors: co-ordinates of interpolating points of the nominal identification curve, and the scaling factor , which characterizes the dispersion of the curve in these points. different versions of the method were investigated, but for implementation in the µbist based on the aduc814 microcontroller, we have used the 3d version of the method with the following assumptions: all identification snakes have a constant scaling factor  = 2, where  is the standard deviation and the distance between interpolation points pi is 1.5 = 3. the confidence coefficient  depends on the distance between adjacent interpolation points pi. for a distance equal to 3, based on the drawing in figure 9 that shows a piece of the identification belt model in 2d space, the value of the confidence coefficient  has been estimated as  =0.954-0.0350.92 [14]. this value is sufficient for using this model in practice. the testing procedure is simple and based on sequential checking of the distance between the measurement point pm and the interpolating points pi of identification curves with the use of a conventional point distance classifier. description of the identification snakes in the 3d fault vocabulary (using co-ordinates of interpolation points pi and scaling factor ) for the cut with 10 elements requires 3000 bytes, for a fault range of 50% in a testing procedure, 9 operations must be performed for the checking of each interpolating point pi of the identification curve in the fault vocabulary. thus, about 5000 operations are required for testing a 10 element cut. it can easily be realized in an acceptable time of <0.1 seconds. based on simulation investigations, we can conclude that the real tolerance version of the sbt method can be easily realized in the aduc814 microcontroller, but it needs experimental verification in a physically realized µbist. (a) (b) figure 8. models of identification belts and snakes in 2d (a) and 3d (b)  spaces.  figure 9. a piece of identification belt model for estimation of confidence  coefficient α.  figure 10. the block diagram of the realized µbist.  acta imeko | www.imeko.org  december 2014 | volume 3 | number 4 | 14  4. experimental realization and verification of an  integrated two‐function µbist  on the basis of hardware and software resources of the analog devices microcontroller aduc814, an integrated µbist for functional and fault diagnostic testing of analog circuits in the eess has been physically realized and investigated in an experimental embedded system. the structure of the µbist is shown in figure 10 and its circuit diagram is shown in figure 11. almost all additional elements surrounding the aduc814 chip are present in each ees and they are necessary for proper functioning of the microcontroller. the microcontroller was programmed with software realizing two testing procedures of functional testing (using unipolar ti-parameter cs signals) and the diagnostic testing using the sbt method, in a 3d version. the µbist was investigated experimentally, on the basis of physical testing of some real analog circuits, mainly filters. different variants of the cs method for functional testing and the sbt method for fault diagnosis have been experimentally investigated. various deviations of qi, n figure 11. the circuit diagram and the picture of the realized µbist.  -50 -40 -30 -20 -10 0 10 20 30 40 50 -40 -30 -20 -10 10 20 30 y(te) [mv] q1/q1 [%] measured cut response simulated cut response (a) -50 -40 -30 -20 -10 0 10 20 30 40 50 -40 -30 -20 -10 10 20 30 40 50 y(te) [mv] measured cut r esponse simulated cut r esponse q2/q2 [%] (b) figure  12.  the  simulated  and  measured  cut  responses  y(te)=f(qi/qi)  as  function of q1 (a) and q2 (b) deviations.  -50 -40 -30 -20 -10 0 10 20 30 40 50 -30 -20 -10 10 20 30 40 50 y(te) [mv] measured cut response simulated cut response n1/n 1 [%] (a) -50 -40 -30 -20 -10 0 10 20 30 50 -40 -20 20 40 60 80 40 measured cut response simulated cut response y(te) [mv] n2/n2 [%] (b) figure 13. the simulated and measured cut responses y(te)=f(ni/ni) as a  function of n1 (a) and n2 (b) deviations.  figure  14.  the  testing  results  on  the  background  of  the  family  of  identification curves for the 2 nd  order lpf.  acta imeko | www.imeko.org  december 2014 | volume 3 | number 4 | 15  parameters and values of soft faults for each pi element were physically entered to real cuts. the experimental results were compared with simulation ones. for the cs method of functional testing a comparison between experimental and simulation results is shown in figure 12. in simulation and experimental investigations the same cs signal level of 1 v has been used. figure 12 shows experimental (blue curve) and simulated (red curve) plots: y(te)=f(q1/q1) and y(te)=f(q2/q2) concerning dependences of the cut response signal on q1 and q2 deviations. analogous plots of y(te)=f(n1/n1) and y(te)=f(n2/n2), concerning deviations of n1 and n2, are shown in figure 13. as it is seen in both figures, experimental results are very close to the simulated ones. experimental and simulated plots have the same shapes and they are very close, especially in the range of ±10% deviations. higher differences out of this range do not matter in practice, because relations cut response – parameter deviations can be modeled on the basis of experimental data. in simulations and experimental investigations of this method, cs signals with a level of 1 v have been used. for such signals the threshold level of functional testing is on the level of a few percent for qi/qi and ni/ni deviations. in practice cut stimulation with higher level of cs, up to 5 v, can be used. it enables to obtain a lower threshold level for both parameter deviations qi/qi and ni/ni, below 1%. the convergence of the simulation and experimental results confirms the previous conclusions from simulation investigations. the method can be used in a frequency range up to 10 khz. for the sbt method of fault diagnostics the 2nd order lpf shown in figure 4 was experimentally investigated. figure 14 shows the results of the investigations (in the form of crosses) in comparison with simulated identification curves. as it is seen, in this case the experimental results and simulated curves are very close in a wide investigated range 0.1pinompi10pinom.. this example and other investigations presented in [14] confirm the usefulness of this sbt diagnostic method for application in a µbist. it can be utilized for fault localization in analog circuits with a moderate number of components of about 10, like: filters, equivalent circuits of signal transmission lines or nonelectrical objects (e.g. sensors) modeled in the form of multi-element two-terminals or two-ports. the method makes localization of single soft faults and ambiguity groups possible as well as detection of multiple faults. on the basis of both above methods of functional and diagnostic testing, the final version of the two-functional µbist has been programmed in the microcontroller aduc814. in both methods unipolar input signals with an extended impulse level of 5 v have been used. some µbist properties are given below: threshold level for functional testing deviations qi/qi is 1% and for deviations ni/ni is about 0.8%. mean functional testing time of the 4th order lpf is about 90 ms. mean time of diagnostic testing of the same filter is about 600 ms. the required microcontroller memory size for functional testing is 476 bytes and for diagnostic testing it is 2469 bytes (total 2945 bytes). there are many possibilities of improving the µbist, mainly by using more advanced 16or 32-bit microcontrollers and a neural network classifier of fault signatures. recently, a neural classifier with two-central basis functions has been elaborated and reported in [16]. it is well suited to application in bists. we are going to implement this classifier in the new version of the integrated bist. 5. conclusions  in this paper the new concept, methodology and solution of an integrated, two-functional bist, dedicated to testing of analog parts of electronic embedded systems, have been presented. the bist integrates two functions: functional testing and fault diagnosis on the level of localization of soft faulty component. the bist has been constructed on the basis of reusing a microcontroller and other blocks already present in each ees. we showed that contemporary microcontrollers, like the aduc814, have sufficient hardware and software resources to design bists for functional and diagnostic testing of linear analog circuits in the low frequency range (0.1 hz 10 khz) such as: filters, signal transmission paths or some non-electrical objects (e.g. sensors) modeled by electrical schemes. there are perspectives of further improvement of the presented bist, using 16or 32-bit microcontrollers and a neural classifier. references  [1] toczek w.: “an oscillation-based built-in test scheme with agc loop”. measurement, vol. 41, no. 2 pp. 160-168, 2008. [2] andrade a. q.:”test planning for mixed-signal socs and analog bist: a case study”. ieee transact. on instrumentation and measurement, vol.54, no. 4, 2005. [3] toczek w., kowalewski m., zielonko r.: “histogram-based feature extraction technique applied for fault diagnosis of electronic circuits”. proceedings of the 10th int. conference on technical diagnosis, imeko tc-10, pp. 27-32, budapest, 2005. [4] ren j., h. ye: “a novel linear histogram bist for adc”. ninth international conference on solid-state and integrated-circuit technology, pp. 2099-2102, beijing, 2008. [5] stratigopoulos h. g. d.: “an adaptive checker for fully differential analog code”. ieee journal of solid-state circuits, vol. 41, no. 6, pp. 1421-1429, 2006. [6] toczek w.: “self-testing of fully differential multistage circuits using common-mode excitation”. microelectronics reliability, pp. 1890-1899, 2008. 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[12] załęski d., bartosiński b., zielonko r.: “application of complementary signals in built-in self testers for mixedsignal embedded electronic systems”. ieee transactions on instrumentation and measurement, vol. 59, 2010, pp. 345352. [13] czaja z., zielonko r.: “fault diagnosis in electronic circuits based on bilinear transformation in 3-d and 4-d spaces”. ieee transactions on instrumentation and measurement, vol. 52, no. 1, pp. 97-102, 2003. [14] załęski d., the autodiagnostics of electronic embedded systems, phd dissertation, gdansk university of technology, 2013. (in polish). [15] czaja z., załęski d.: ”implementation of an input-output method of diagnosis of analog electronic circuits in embedded systems”. proceedings of the 10th imeko tc10 international conference on technical diagnostics, pp. 145150, budapest, 9-10 june 2005. [16] kowalewski m., the tolerance-proof vocabulary methods of fault diagnostic in electronic circuits using dedicated faults classifier with neural network, phd dissertation, gdansk university of technology, 2012 (in polish). non-destructive investigation of the kyathos (6th-4th centuries bce) from the necropolis volna 1 on the taman peninsula by neutron resonance capture and x-ray fluorescence analysis acta imeko issn: 2221-870x september 2022, volume 11, number 3, 1 6 acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 1 non-destructive investigation of the kyathos (6th-4th centuries bce) from the necropolis volna 1 on the taman peninsula by neutron resonance capture and x-ray fluorescence analysis nina simbirtseva1,2, pavel v. sedyshev1, saltanat mazhen1,2, almat yergashov1,2, andrei yu. dmitriev1, irina a. saprykina3, roman a. mimokhod3 1 frank laboratory of neutron physics, joint institute for nuclear research, dubna, russia 2 institute of nuclear physics, almaty, 050032, the republic of kazakhstan 3 institute of archaeology of the russian academy of sciences, moscow, russia section: research paper keywords: neutron resonance capture analysis; non-destructive neutron analysis; xrf analysis citation: nina simbirtseva, pavel v. sedyshev, saltanat mazhen, almat yergashov, andrei yu. dmitriev, irina a. saprykina, roman a. mimokhod, nondestructive investigation of the kyathos (6th-4th centuries bce) from the necropolis volna 1 on the taman peninsula by neutron resonance capture and xray fluorescence analysis, acta imeko, vol. 11, no. 3, article 20, september 2022, identifier: imeko-acta-11 (2022)-03-20 section editor: francesco lamonaca, university of calabria, italy received march 5, 2021; in final form august 31, 2022; published september 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: nina v. simbirtseva, e-mail: simbirtseva@jinr.ru 1. introduction neutron resonance capture analysis (nrca) is known as technique for non-destructive investigations of various objects, including archeological artifacts and objects of cultural heritage. the induced activity in experiments with bronze artefacts is practically absent, what was evaluated in several works one of them you can find in ref. [1]. the method relies on registration neutron resonances in radiative capture and measurement the yield of reaction products in these resonances. the energy positions of resonances give information about isotope and elemental composition of an object. the area under the resonances can be used to calculate the number of the element or isotope’s nuclei. nrca is applied for various studies at different institutes and sources, such as the gelina pulsed neutron source of the institute of reference materials and measurements of the joint research center (gel, belgium) [2] the isis pulsed neutron and muon source in the united kingdom [3] and the j-parc pulsed neutron source in japan [4]. at present, the nrca is also used at the frank laboratory of neutron physics (flnp) in the joint institute nuclear research (jinr), dubna, russia [5]. the experiments are carried out at the intense resonance neutron source (iren) facility [6]-[8] with multi-sectional liquid scintillator detector (210 liters), which is used for the registration prompt gamma-quanta [9]. one of these experiments at the iren facility was carried out for the kyathos which was transferred by the institute of archeology of the russian academy of sciences. abstract the method of neutron resonance capture analysis (nrca) is currently being developed at the frank laboratory of neutron physi cs. the analysis determines the elemental and isotope compositions of objects non-destructively, which makes it suitable measurement tools for artefacts without sampling. nrca is based on the registration of neutron resonances in radiative capture and the measurement of the yield of reaction products in these resonances. the potential of nrca at the intense resonance neutron source facility is demonstrated on the investigation of a kyathos (6th-4th centuries bce) from the necropolis volna 1 on the taman peninsula. in addition, x-ray analysis was applied to the same archeological object. the element composition determined by nrca and xrf data is in agreement. mailto:simbirtseva@jinr.ru https://context.reverso.net/%d0%bf%d0%b5%d1%80%d0%b5%d0%b2%d0%be%d0%b4/%d0%b0%d0%bd%d0%b3%d0%bb%d0%b8%d0%b9%d1%81%d0%ba%d0%b8%d0%b9-%d1%80%d1%83%d1%81%d1%81%d0%ba%d0%b8%d0%b9/russian+academy+of+sciences acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 2 2. the kyathos in 2016-2018 a sochi expedition group of institute of archaeology of the russian academy of sciences (ia ras) under the leadership of roman a. mimokhod [10] conducted excavations of an antique town soil necropolis volna 1 on the taman peninsula (figure 1). the necropolis is dated to the middle / second quarter of the 6th century bc the beginning of the 3rd century bc, the main period of its use dates back to the second half of the 6th-5th centuries bc. the burials of the volna 1 necropolis were supposedly left by the greek and barbarian population; the earliest burials may have been left by settlers who arrived from the territory of magna graecia (children's burials in amphorae, burials in the "rider's pose", etc.). the burial ground volna-1 is an important monument for studying of a problem of greekbarbarian relations on the territory of the borderland the northern black sea region, the clash and interaction of two different ethnocultural layers of the population. the social structure, economic and political position of the first greek colonies on the territory of the bosporus depended on the nature of these contacts, their duration and strength, and the development of adaptive mechanisms. this interaction process was extremely complex, multifaceted and contradictory. on the one hand, with the emergence of the greek colonies, confrontational relations with the local population inevitably developed, on the other hand, the greek colonists and the local population influenced on each other. this process was superimposed on the specificity and uniqueness of the northern black sea region, where for a long time societies of fundamentally different economic structures, the level of development of social, political and economic life came into contact. more than 2,000 burials have been uncovered within the boundaries of the necropolis. an anthropological material, representative and significant for the history and archeology of the northern black sea region, was obtained. the collection of items obtained during the excavations is represented by ceramic materials, incl. production of ceramic centers of ancient greece (container amphoras, kiliks, skyphoses, drinking bowls, lekythoses, askoses, etc.), phoenician glass, weapons (spears, swords, arrows), protective weapons (full armor, helmet of the corinthian type), jewelry (bronze, silver and gold earrings and rings, etc.), coins and other categories of burial objects. all this testifies to the fact that the volna 1 burial ground is a city necropolis. this places it in the category of the most prestigious necropolises of the bosporan kingdom. the necropolis was associated with the settlement of the same name, to which it adjoins from the north. in a settlement, existed from the pre-greek period to the 3rd century bc, the systems of urban planning, stone house-building were studied. the fact that it is not a rural settlement, but a polis is evidenced by a number of prestigious finds, including, for example, a ceramic mask, which most likely indicates the presence of a theater in the city. in the burials, objects rare for the territory of the northern black sea region were found: a bronze prosthesis with a wooden support structure for a leg, an iron plate armor, a bronze corinthian helmet of the "hermione" type, musical instruments (cithara, lyre), a wreath on a gilded bone base with bronze petals and gold beads, as well as a series of kiathoi ancient greek vessels for pouring wine [11]. there are in total 17 burials from the materials of the excavations of ia ras, where bronze kyathoi or their fragments were found. their planigraphy is illustrative. burials with these items are located in the early section of the necropolis, in the northwestern part (figure 2). figure 1. location of the volna 1 necropolis. figure 2. the scheme of the necropolis of excavations 2017 2018 and a location of the burials with kiathoi. acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 3 in the excavations of the ia ras in 2016, which is located to the south, kiathoi were not found. a burial 656, in which the item, considered in the article, was found (figure 3, 1), was paired. one skeleton belonged to a man 45-55 years old, the second to a woman 20-25 years old. the burial was done in a box, which was made of mud blocks. the dead were laid on a klinai a wooden bed, from the legs of which characteristic recesses remained in the grave. the burial had pronounced military attributes. fragments of an iron sword have been found near the male skeleton; an accompanying burial of a horse was done next to the pit. the kyathos was found in the filling of the burial pit. its original position is unclear. however, the grave contains vessels that are clearly associated with wine drinking: skyphos (figure 3, 3) and kilik (figure 3, 4). the kyathos, as an item for scooping and pouring wine, complements this set. the volume of the scoop was 0.045 liters, that is, a quarter of a sextarius. in contrast to various similar finds made of clay (their production from clay began at the end of the 6th century bc), the kiathoi found on the territory of the necropolis volna 1 were made of metal. it moves them in to the category of special objects. presumably, these kiathoi refer to greek imports that entered the northern black sea region along with the greek colonists. 3. nrca experiment the investigations were carried out at iren facility. the main part of the iren facility is a linear electron accelerator. the facility parameters: the average energy of electrons was ~ 60 mev, the peak current was ~ 1.5 а, the width of electron pulse was ~ 100 ns, and the repetition rate was 25 hz. neutron producing target is made of tungsten-based alloy and represents a cylinder 40 mm in diameter and 100 mm in height placed within an aluminum can 160 mm in diameter and 200 mm in height. distilled water is circulated inside the can, providing target cooling and neutron moderation. water layer thickness in a radial direction is 50 mm. the total neutron yield was about 3∙1011 s-1. the measurements were carried out at the 58.6 meters flight path of the 3rd channel of the iren. the big liquid scintillator detector was used for the registration of γ-quanta. the sample was placed inside the detector. the neutron flux was permanently monitored by the snm-17 neutron counter. the signals from the detector and the monitor counter were simultaneously fed to the two independent inputs of time-todigital converter (tdc). the measurements with the sample lasted about 136 hours. the resonance energies were determined according to the formula: 𝐸 = 5227 𝐿2 𝑡 2 , (1) where 𝑡 is time of flight in microseconds, 𝐿 flight path in meters, 𝐸 kinetic energy of a neutron in ev. the resonances of silver, tin, copper and arsenic were identified on the time-of-flight spectrum (figure 4, figure 5) [12], [13]. figure 3. volna 1. items from burial 656: 1 – kyathos, 2 bowl, 3 – skyphos, 4 kilik, 5 lekythos. figure 4. the part of time-of-flight spectrum of (n,γ) reactions on the kyathos material. figure 5. the part of time-of-flight spectrum of (n,γ) reactions on the kyathos material. acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 4 the measurements with standard samples of identified elements were made in addition to the measurement with the investigated sample. parts of time-of-flight spectra of (n, γ) reactions on the material of standard silver, tin, copper and arsenic samples are shown in figure 6 to figure 9. 4. data analysis and results five resonances of tin, two resonances of copper, one resonance of silver and one resonance of arsenic were selected during the analysis of the experimental data. only well-resolved, without overlapping, with sufficient statistics and unambiguously known parameters resonances were analyzed. the sum of the detector counts in resonance is expressed by the formula: ∑ 𝑁 = 𝑓(𝐸0) ⋅ 𝑆 ⋅ 𝑡 ⋅ 𝜀𝛾 ⋅ 𝛤𝛾 𝛤 𝐴 . (2) here, 𝑓(𝐸0) is the neutron flux density at the resonance energy 𝐸0, 𝑆 – the sample area, 𝑡 – measuring time, 𝜀𝛾 – the detection efficiency of the detector radiative capture, 𝛤𝛾 , 𝛤 – the radiative and total resonance widths. 𝐴 = ∫ [1 − 𝑇(𝐸)] 𝐸2 𝐸1 d𝐸 (3) – resonance area on the transmission curve, where 𝐸1, 𝐸2 – initial and final values of energy range near resonance. 𝑇(𝐸) = 𝑒 −𝑛 𝜎(𝐸) (4) – the energy dependence of the neutron transmission by the sample; 𝜎(𝐸) – the total cross section at this energy with doppler broadening, 𝑛 – the number of isotope nuclei per unit area. the value 𝐴 was determined from experimental data for investigated sample by the formula: 𝐴𝑥 = ∑ 𝑁𝑥 ⋅ 𝑀𝑠 ⋅ 𝑆𝑠 ∑ 𝑁𝑠 ⋅ 𝑀𝑥 ⋅ 𝑆𝑥 ⋅ 𝐴𝑠 . (5) here, ∑ 𝑁𝑥 , ∑ 𝑁𝑠 counts under the resonance peak of the investigated and standard samples, 𝑆𝑥 , 𝑆𝑠 – the area of the investigated and standard samples. 𝑀𝑥 , 𝑀𝑠 – the number of monitor counts during the measurement of the investigated and standard samples. we used a program written according to the algorithm given in [14] for calculation of 𝐴𝑠 (resonance area on the transmission curve of the standard sample) and 𝑛𝑥 (the number of isotope nuclei per unit area of the investigated sample) values. this procedure is schematically shown in (figure 10). the 𝐴𝑠 value was calculated by means of known resonances parameters and figure 6. the part of time-of-flight spectrum of (n,γ) reactions of the standard silver sample. figure 7. the part of time-of-flight spectrum of (n,γ) reactions of the standard tin sample. figure 8. the part of time-of-flight spectrum of (n, γ) reactions of the standard copper sample. figure 9. the part of time-of-flight spectrum of (n,γ) reactions of the standard arsenic sample. acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 5 𝑛𝑥 parameters for the standard sample. the 𝑛𝑥 value was determined from the investigated sample 𝐴𝑥 value. the first measurement with the kyathos [16] hadn’t shown satisfactory results for copper and arsenic. it was decided to repeat the measurement but take into account archeological object and neutron flux features. the kyathos is a relatively long object (a length is about 16 cm) and has uneven thickness distribution (a handle and a bucket). as we know the neutron flux intensity decrease from a center to an edge it makes sense to measure the investigated object in parts and place every part to the center of the beam line. this experiment was carried out and we have obtained two spectrums (with the handle and with the bucket separately) that were summarized taking into account monitoring coefficients (figure 4, figure 5). the analysis results are presented in the table 1. the nrca has limitations correlated to the neutron flux intensity at iren facility at the present time, using of certain type of a detector system in an experiment and a matrix of elements in an investigated object. and besides that, this method has low sensitivity to elements lighter than iron and to elements that have atomic mass close to magic mass numbers (bismuth, lead, etc.) additional measurements therefore were carried out by x-ray fluorescence (xrf) on 4 points of archeological object by means portable spectrometer 5i tracer (bruker) (table 2). the element composition of the kyathos determined by nrca and xrf are in agreement; some differences in wt% could be well understood due to the different nature of the analysis. the application of the xrf analysis allows determining the elemental composition on the surface. nrca, in turn, does not measure the element and isotope compositions just on the surface but rather in the bulk (through the whole volume of the object), the analytical results are not significantly affected by surface corrosion. 5. conclusion nrca carried out at the intense resonance neutron source (iren) facility for the kyathos from the necropolis volna 1 on the taman peninsula showed that this method delivers satisfactory results. the mass of kyathos, determined by weighing is 86.7 g. according to the result of nrca the value of determining total elements mass coincides with the kyathos mass within the margin of error and considering the presence of silicon, aluminum and iron on the surface of the object. the element composition of the kyathos determined by nrca and xrf are in agreement. the results obtained confirm the using of tin bronze to make the investigated kyathos. the recorded presence of tin in the composition of the alloy, its quantitative characteristics, along with the established presence of arsenic, refers us to the type of alloy characteristic of archaic greece [17]. the presence of such elements as arsenic, silver point to the conclusion that copper was obtained from polymetallic ores. nrca allows not only identifying the elemental and isotopic composition of the sample but also makes it possible to determine the amounts of elements and isotopes in the whole volume of the object. the method is non-destructive; the induced activity of the bronze samples is practically absent. all this makes it promising for research of archaeological artifacts and objects of cultural heritage. although the number of facilities that can provide suitable neutron beams is limited, nrca might be a useful additional analyzing technique. acknowledgement the authors express their gratitude to the staff of the iren facility and to a. p. sumbaev, the head of the development of the facility, to v. v. kobets, the head of sector no.5 of the scientific and experimental injection and ring division of the nuclotron (veksler and baldin laboratory of high energy physics), for the supporting with uninterrupted operation of the facility in the process of measurements. references [1] h. postma, m. blauw, p. bode, p. mutti, f.corvi, p. siegler, neutron-resonance capture analysis of materials, journal of radioaanalitical and nuclear chemistry 248(1) (2001), pp. 115120. doi: 10.1023/a:1010690428025 [2] h. postma, p. schillebeeckx, neutron resonance analysis, in neutron methods for archaeology and cultural heritage. neutron scattering applications and techniques, ed. by n. kardjilov and g. festa (springer, cham, 2017), pp. 235–283. [3] g. gorini (ancient charm collab.), ancient charm: a research project for neutron-based investigation of cultural-heritage figure 10. dependence of value a on a number of nuclei and resonance parameters with taking into account δ (doppler effect) [15]. table 1. the results of measurements with the kyathos by nrca (the bulk). № element mass, g weight, % 1 cu 59.7 ± 3.9 68.8 ± 4.5 2 sn 5.29 ± 0.23 6.10 ± 0.26 3 as 0.1892 ± 0.0081 0.02179 ± 0.0094 4 ag 0.0131 ± 0.0014 0.0151 ± 0.0016 table 2. the results of measurements with the kyathos by xrf, averaged over four points (the surface). № element weight, % 1 cu 61.63 ± 0.31 2 si 21.57 ± 0.15 3 al 7.02 ± 0.19 4 sn 4.58 ± 0.25 5 fe 4.254 ± 0.075 6 pb 0.684 ± 0.073 7 ti 0.327 ± 0.057 8 as 0.050 ± 0.017 https://doi.org/10.1023/a:1010690428025 acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 6 objects, nuovo cim. c 30, 2007, pp. 47–58. doi: 10.1393/ncc/i2006-10035-9 [4] h. hasemi, m. harada, t. kai, t. shinohara, m. ooi, h. sato, k. kino, m. segawa, t. kamiyama, y. kiyanagi, evaluation of nuclide density by neutron transmission at the noboru instrument in j-parc/mlf, nucl. instrum. methods phys. res., sect. a 773 (2015), pp. 137–149. doi: 10.1016/j.nima.2014.11.036 [5] n. v. bazhazhina, yu. d. mareev, l. b. pikelner, p. v. sedyshev, v. n. shvetsov, analysis of element and isotope composition of samples by neutron spectroscopy at the iren facility, physics of particles and nuclei letters 12 (2015), pp. 578–583. doi: 10.1134/s1547477115040081 [6] o. v. belikov, a. v. belozerov, yu. becher, yu. bulycheva, a. a. fateev, a. a. galt, a. s. kayukov, a. r. krylov, v. v. kobetz, p. v. logachev, a. s. medvedko, i. n. meshkov, v. f. minashkin, v. m. pavlov, v. a. petrov, v. g. pyataev, a. d. rogov, p. v. sedyshev, v. g. shabratov, v. a. shvec, v. n. shvetsov, a. v. skrypnik, a. p. sumbaev, a. v. ufimtsev, v. n. zamrij, physical start-up of the first stage of iren facility, journal of physics: conf. ser. 205, 2010, 012053. doi: 10.1088/1742-6596/205/1/012053 [7] o. v. belikov, a. v. belozerov, yu. becher, yu. bulycheva, a. a. fateev, a. a. galt, a. s. kayukov, a. r. krylov, v. v. kobetz, p. v. logachev, a. s. medvedko, i.n.meshkov, v. f. minashkin, v. m. pavlov, v. a. petrov, et al., physical start-up of the first stage of iren facility, j. phys.: conf. ser. 205 (2019), art. no. 012053. doi: 10.1088/1742-6596/205/1/012053 [8] e. a. golubkov, v. v. kobets, v. f. minashkin, k. i. mikhailov, a. n. repkin, a. p. sumbaev, k. v. udovichenko, v. n. shvetsov, the first results of the commissioning of the second accelerating section of the lue-200 accelerator of the iren installation, soobshch. oiyai r9-2017-77 (dubna, oiyai, 2017). [9] h. maletsky, l. b. pikelner, k. g. rodionov, i. m. salamatin, e. i. sharapov, detector of neutrons and gamma rays for work in the field of neutron spectroscopy, communication of jinr 13-6609 (dubna, jinr, 1972) 1-15 (in russian). [10] r. a. mimokhod, n. i. sudarev, p. s. uspensky, the necropolis volna 1(2017) (krasnodar territory, taman peninsula), rescue archaeological research materials 25 (2018), pp. 220–231. [11] r. a. mimokhod, n. i. sudarev, p. s. uspensky, necropolis volna-1 on the taman peninsula (2019), new archaeological projects. recreating the past. to the 100th anniversary of russian academic archeology. edited by makarov m., ia ran, 2019, pp. 80–83. [12] s. f. mughabghab, neutron gross sections, neutron resonance parameters and thermal gross sections, academic press, new york, 1984, isbn: 9780125097017. [13] s. i. sukhoruchkin, z. n. soroko, v. v. deriglazov, low energy neutron physics. landolt-bornstein.v. i/16b, berlin: springer verlag, 1998, isbn: 3540608575. 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[15] p. v. sedyshev, n. v. simbirtseva, a. m. yergashov, s. t. mazhen, yu. d. mareev, v. n. shvetsov, m. g. abramzon, i. a. saprykina, determining the elemental composition of antique coins of phanagorian treasure by neutron spectroscopy at the pulsed neutron source iren in flnp jinr, pis’ma v zhurnal fizika elementarnykh chastits i atomnogo yadra, issn:1547-4771, physics of particles and nuclei letters 17(3), pp. 389-400. doi: 10.1134/s1547477120030139 [16] n. v. simbirtseva, p. v. sedyshev, s. t. mazhen, a. m. yergashov, i. a. saprykina, r. a. mimokhod, preliminary result of investigation of element composition of kyathos (6th-4th centuries bce) from the necropolis volna 1 on the taman peninsula by neutron resonance capture analysis. imeko tc4 international conference on metrology for archaeology and cultural heritage, 22-24 october 2020, trento, italy, proceedings, 2020. online [accessed 24 september 2022] 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https://discovery.ucl.ac.uk/id/eprint/1471577/1/orfanou_orfanou%202015%20early%20iron%20age%20greek%20copper-based%20technology.pdf https://discovery.ucl.ac.uk/id/eprint/1471577/1/orfanou_orfanou%202015%20early%20iron%20age%20greek%20copper-based%20technology.pdf https://discovery.ucl.ac.uk/id/eprint/1471577/1/orfanou_orfanou%202015%20early%20iron%20age%20greek%20copper-based%20technology.pdf the importance of sound velocity determination for bathymetric survey acta imeko issn: 2221-870x december 2021, volume 10, number 4, 46 53 acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 46 the importance of sound velocity determination for bathymetric survey pier paolo amoroso1, claudio parente2 1 international phd programme “environment, resources and sustainable development”, department of science and technology, parthenope university of naples, centro direzionale, isola c4, (80143) naples, italy 2 department of science and technology, parthenope university of naples, centro direzionale, isola c4, (80143) naples, italy section: research paper keywords: bathymetric survey; single beam; multi beam; sound velocity in water; depth measurements citation: pier paolo amoroso, claudio parente, the importance of sound velocity determination for bathymetric survey, acta imeko, vol. 10, no. 4, article 10, december 2021, identifier: imeko-acta-10 (2021)-04-10 section editor: silvio del pizzo, university of naples 'parhenope', italy received may 1, 2021; in final form december 6, 2021; published december 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was supported by parthenope university of naples, italy. corresponding author: claudio parente, e-mail: claudio.parente@uniparthenope.it 1. introduction as reported in huet (2009), “hydrography is the branch of applied sciences, which deals with the measurement, and description of the physical features of oceans, seas coastal areas, lakes and rivers, as well as with the prediction of their evolution, for the primary purpose of safety of navigation and all other marine activities, including economic development, security and defence, scientific research, and environmental protection” [1]. according to iho, international hydrographic organization [2], hydrographic survey can be defined as the survey of an area of water, but in modern use it includes many other objectives such as measurements of tides, currents, and determination of the physical and chemical properties of water. the main objective is to obtain essential data for the edition of nautical charts with particular interest in the characteristics that may influence navigation [3]. in addition, the hydrographic surveys aim to acquire the information necessary for marine navigation products and for the management, engineering, and science of coastal areas. the bathymetric surveys belong to the family of hydrographic surveys and are carried out whenever there is a need to precisely know the morphological trend of the seabed [4], [5]. they are therefore preliminary to the realization of maritime and river works and in the case of works already supplied, they are indispensable to verify continuously water heads and dredging volumes [6]. generally, hydrographic surveys are carried out using a vessel equipped with a precision echo sounder, which uses the principle of acoustic waves to sound the bottom and determine the depth. an accurate determination of the vessel position and attitude as well as a correct functioning of the echo sounder are both fundamental for the quality of the survey results. using satellites techniques, differential corrections with code measurements allow accuracies estimated to a few meters. currently, one of the abstract bathymetric surveys are carried out whenever there is a need to know the exact morphological trend of the seabed. for a correct operation of the echo sounder, which uses the principle of acoustic waves to scan the bottom and determine the depth, it is important to accurately determine the sound velocity in water, as it varies according to specific parameters (density, temperature, and pressure). in this work, we want to analyse the role of sound velocity determination in bathymetric survey and its impact on the accuracy of depth measurement. the experiments are conducted on data set provided by “istituto idrografico della marina militare italiana” (iim), the official hydrographic office for italy, and acquired in the ligurian sea. in our case, the formulas of chen & millero (unesco), medwin, and mackenzie were applied. the introduction of errors on chemical-physical parameters of the water column (temperature, pressure, salinity, depth) simulating inaccurate measurements, produces considerable impacts on sound velocity determination and subsequently a decrease of the depth value accuracy. the results remark the need to use precise probes and accurate procedures to obtain reliable depth data. mailto:claudio.parente@uniparthenope.it acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 47 most widely used techniques for high-precision positioning of vessels in bathymetric survey is the rtk mode (phase measurements) [7] which allow to achieve centimetre accuracy on the horizontal and vertical plane. since the effects of the vessel motion on the accuracy of the observed depths and their positioning, attitude (roll, pitch, and heading) and heave must be measured using appropriate instruments, such as inertial sensors that can be also integrated with gps [8]. a correct functioning of the echo sounder is very important for accurate depth measurement, but an accurate determination of the sound velocity in water is necessary [9]. once the instrument is activated, the transmitter sends a small amount of electrical energy towards the transducer, which switches it into a sound pulse and sends it towards the seabed. once it reaches the seabottom, the signal goes back to the transducer. therefore, the instrument is used to measure the time interval between the transmission and reception of the signal [10], [11]. two principal types of echo sounder are available, namely the single beam and the multibeam. the substantial difference is that the single beam emits a single sound impulse, while the multibeam emits a beam of sound impulses, which allow to obtain a greater and more detailed acquisition of the seabed [12]. the single beam and multibeam are basic instruments of acoustical oceanography, the discipline that describes the role of the ocean as an acoustic medium by relating oceanic properties to the behaviour of underwater acoustic propagation, noise, etc [13]. the sound velocity in a medium is mostly influenced by the medium itself [14], [15], so it is affected by the conditions of the sea-bottom boundaries as well as by the variation of the chemical-physical parameters of the water volume [16]. in fact, the sound velocity in seawater is defined as a function of the isothermal compressibility, the ratio of specific heats of seawater at constant pressure and constant volume, and the density of seawater [17]. particularly, the sound velocity in sea water increases with an increase in temperature, salinity or pressure [18]. temperature decreases from the sea-surface to the seabed, but there are different local variations. the sound velocity profile is very variable near the surface according to the seasons and the hours of the day, due to the heat exchange with the atmosphere, which modifies the temperature and salinity of the sea [16]. if temperature is constant, the velocity sound increases with depth due to the pressure gradient [19]. normally in literature, the average value of the sound velocity in water is accepted as 1500 m/s, calculated taking as reference the nominal conditions of the water, characterized by a temperature of 0 ° c, a salinity of 35 ppt (parts per thousand) and a pressure of 760 mmhg [20], [21]. this average value, however, can oscillate according to the characteristics of the water, varying between 1387 m/s and 1529 m/s [17]. local velocity measurements are quite difficult to perform accurately, whereas its constitutive parameters are more easily quantified. particular probes, bar check method and empirical formulas, permit to determine the sound velocity. empirical formulas require physical and chemical parameters, such as depth (d), temperature (t), salinity (s), pressure (p). these parameters can be measured with different types of instruments. there are many different formulas available to calculate the sound velocity in water, and the most popular and accurate are chen & millero (1977) [22]-[25], del grosso (1974) [26]-[28], mackenzie (1981) [29]-[31] and medwin (1975) [32], [33]. 2. sound velocity determination the determination of the sound velocity in water can be obtained through direct or indirect measurements. among the systems commonly used to measure the sound velocity in situ, we find the depth velocimeter, which directly measures the sound velocity for a high frequency wave transmitted over an accurately regulated distance. among the systems commonly used to determine the sound velocity in indirect way, we find specific probes capable of measuring chemical-physical parameters of water as input data. for example, the bathythermograph or the xbt probe [34], [35] measures the water temperature only as a function of depth; to deduce the sound velocity, it is necessary to have the salinity data independently, so it is measured simultaneously by a conductivity meter, integrated in the same device. the echo sounder is calibrated for water temperature and salinity, or directly to a known depth using the "bar check" method (measurement of the immersion depth of a metal bar or disc lowered below the transducer and suspended a graduated cable). this method consists in immersing under the echo sounder transducer, a plate with a square base of edge equal to 60 cm supported by a chain that has centimetre subdivisions. dive depths are generally set at 5 m, 10 m, 15 m, 20 m [36]. then you go to operate on the settings of the sound velocity in the water proceeding with the measurements until the correct depth value is obtained. this type of operation is repeated for the various control depths at least twice on each depth, and an arithmetic average is calculated on the obtained sound velocity values and set on the instrument. some systems, however, use all the values read at the various depths. this method is conveniently carried out in shallow water with low currents, gives a mean velocity to the observed depth and simultaneously checks the calibration of the sounder [37]. when indirect measurements are carried out and chemicalphysical parameters of water are available, the application of formulas that permit to calculate sound velocity is necessary. different formulas are used depending on the depth values, i.e.: 1. chen & millero is only used for depths less than 1000 m; 2. del grosso is used only for depths greater than 1000 m; 3. mackenzie for quick calculations in ocean waters up to 8000 m depth; 4. medwin is used for quick calculations in ocean waters up to 1000 m depth [38]. a range of validity of physical and chemical parameters characterizes every empirical formula. normally, depths are measured in meters, temperatures are measured in °c, salinity is measured in ppt (part for thousand) and pressure is measured in bar. a brief description of each formula is reported below. the formula chen & millero is denominated as the international algorithm adopted by unesco; accurate models than others characterize it, and the equation is the following: 𝑐 = 𝐶𝑤(𝑇, 𝑃) + 𝐴(𝑇, 𝑃)𝑆 + 𝐵(𝑇, 𝑃)𝑆 3 2⁄ + 𝐷(𝑇, 𝑃)𝑆 2 , (1) where 𝐶𝑤 (𝑇, 𝑃) = c00+c01 t+c02 t 2+c03 t 3+c04t 4 + +c05t 5 + (c10+c11t+c12t 2 +c13t 3+ c14 t 4)p+ +(c20+c21t+c22t 2+c23t 3 +c24t 4)p2+ +(c30+c31t+c32t 2)p3 (2) acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 48 𝐴(𝑇, 𝑃) = a00 + a01t + a02t 2 + a03t 3 + a04t 4 + +(𝐴10 + 𝐴11𝑇 + 𝐴12𝑇 2 + 𝐴13𝑇 3 + +𝐴14𝑇 4)𝑃 + +(𝐴20 + 𝐴21𝑇 + 𝐴22𝑇 2 + 𝐴23𝑇 3)𝑃2 + +(𝐴30 + 𝐴31𝑇 + 𝐴32𝑇 2)𝑃3, (3) 𝐵(𝑇, 𝑃) = 𝐵00 + 𝐵01𝑇 + (𝐵10 + 𝐵11 𝑇)𝑃 , (4) 𝐷(𝑇, 𝑃) = 𝐷00 + 𝐷10𝑃 . (5) this formula is valid for temperature values included in the range 0 °c < t < 40 °c, salinity values included in the range 0 ppt < s < 40 ppt, and pressure values included in the range 0 bar < p < 1000 bar [39]. the coefficients in the equations are reported in table 1. the del grosso equation is used as an alternative to unesco algorithm. the formula is the following: 𝑐(𝑆, 𝑇, 𝑃) = 𝐶000 + ∆𝐶t + ∆𝐶s + ∆𝐶p + ∆𝐶stp, (6) where ∆𝐶t(𝑇) = 𝐶t1𝑇 + 𝐶t2𝑇 2 + 𝐶t3𝑇 3, (7) ∆𝐶s(𝑆) = 𝐶s1𝑆 + 𝐶s2𝑆 2 , (8) ∆𝐶p(p) = 𝐶p1𝑃 + 𝐶p2𝑃 2 + 𝐶p3𝑃 3 , (9) ∆cstp(𝑆, 𝑇, 𝑃) = 𝐶tp𝑇 𝑃 + 𝐶t3p𝑇 3𝑃 + 𝐶tp2𝑇 𝑃 2 + +𝐶t2p2𝑇 2𝑃2 + 𝐶tp3𝑇 𝑃 3 + 𝐶st𝑆 𝑇+ 𝐶st2𝑆 𝑇 2 + +𝐶stp𝑆 𝑇 𝑃 + +𝐶s2tp𝑆 2𝑇 𝑃 + 𝐶s2p2𝑆 2𝑃2 . (10) the coefficient values are shown in table 2. this formula is valid for temperature values included in the range 0 °c < t < 30 °c, salinity values included in the range 30 ppt < s < 40 ppt, and pressure values included in the range 0 bar < p < 980.665 bar. unlike chen & millero and del grosso, mackenzie uses depth in the formula for velocity calculation. the formula is: 𝑐(𝐷, 𝑆, 𝑇) = 1448.96 + 4.591𝑇 − 5.304 ∙ 10−2𝑇2 + +2.374 ∙ 10−4𝑇3 + 1.340(𝑆 − 35) + 1.630 ∙ 10−2𝐷 + 1.675 ∙ 10−7𝐷2 − +1.25 ∙ 10−2𝑇(𝑆 − 35) − 7.139 ∙ 10−13𝑇𝐷3. (11) this formula is valid for temperature values included in the range 2 °c < t < 30 °c, salinity values included in the range 25 ppt < s < 40 ppt, and depth values included in the range 0 m < d < 8000 m [38]. medwin is the simplest formula, and it is given as: 𝑐 = 1449.2 + 4.6 𝑇 − 0.055 𝑇2 + 0.00029 𝑇3 + (1.34 − 0.010 𝑇)(𝑆 − 35) + 0.016 𝐷 . (12) this formula, instead, is valid for temperature values included in the range 0 °c < t < 35 °c, salinity values included in the range 0 ppt < s < 40 ppt and depth values included in the range 0 m < d < 1000 m. 3. applications 3.1. hydrographic data the “istituto idrografico della marina militare italiana (iim)” provided the data set for this work. the ship used for the survey is the nave magnaghi, a hydro-oceanographic ship. the hydrographic activity carried out by the unit is concretized in the realization of port, coastal and offshore reliefs through sounding operations, research of minimum depths and wrecks, determination of the topography of the coastline and port works, study of the nature of the seabed etc. the sampling point is localized near the ligurian coast named “riviera di levante” and has the following coordinates referred to world geodetic system 84 (wgs-84): φ = 43°55’17’’.53 n, λ = 9°40’55’’.4 e (figure 1). this type of data set was obtained by using the "ctd idronaut-ocean seven 316 plus" probe which measures chemicalphysical parameters of water providing in output the values of pressure, temperature, conductivity, salinity, depth and velocity. particularly, data are provided about every meter of depth, in order to be able to determine the correct vertical profile of the sound velocity. the analysed depths range from 0.99 m to 324.12 m, the pressure values are provided by the probe in db (decibar), the temperature in degrees celsius (° c) and the salinity in parts per thousand (ppt). as reported in the probe technical specification document, pressure and temperature are directed measured as well as other parameters e. g. conductivity and ph. table 1. coefficients of chen & millero formula. a00 1.389 b00 -0.01922 c22 2.60e-08 a01 -0.01262 b01 -4.40e-05 c23 -2.50e-10 a02 7.16e-05 b10 7.36e-05 c24 1.04e-12 a03 2.01e-06 b11 1.79e-07 c30 -9.80e-09 a04 -3.20e-08 c00 1402.388 c31 3.85e-10 a10 9.47e-05 c01 5.03711 c32 -2.40e-12 a11 -1.30e-05 c02 -0.05809 d00 0.001727 a12 -6.50e-08 c03 0.000334 d10 -8.00e-06 a13 1.05e-08 c04 -1.50e-06 a14 -2.00e-10 c05 3.15e-09 a20 -3.90e-07 c10 0.153563 a21 9.10e-09 c11 0.00069 a22 -1.60e-10 c12 -8.20e-06 a23 7.99e-12 c13 1.36e-07 a30 1.10e-10 c14 -6.10e-10 a31 6.65e-12 c20 3.13e-05 a32 -3.40e-13 c21 -1.70e-06 table 2. coefficients of del grosso formula. c000 1402.392 ct1 5.01e+00 ct2 -5.51e-02 ct3 2.22e-04 cs1 1.33e+00 cs2 1.29e-04 cp1 0.1560592 cp2 2.45e-05 cp3 -8.83e-09 cst -1.28e-02 ctp 6.35e-03 ct2p2 2.66e-08 ctp2 -1.59e-06 ctp3 5.22e-10 ct3p -4.38e-07 cs2p2 -1.62e-09 cst2 9.69e-05 cs2tp 4.86e-06 cstp -3.41e-04 acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 49 other parameters such as salinity and water density are obtained using specific algorithms available in literature [40]. the probe is characterized by the following accuracy values for direct measurements: 5.00e-04 bar for pressure; 2.00e-03 °c for temperature. in consideration of the adopted approach of the indirect measurement of the salinity, the accuracy is better than 0.005 ppt. 3.2. results in this work, we proceeded with the calculation of the sound velocity in water using three of the formulas previously described (del grosso formula is not applicable because the analyzed depths are less than 1000 m). table 3 shows a selection of data supplied by “idronaut ctd” acquired at different depths along the investigated water column, as well as the relative sound velocity values cu, cme, cma, calculated using respectively the unesco formula, the medwin formula and the mackenzie formula. table 4 shows the statistics (mean, standard deviation, minimum and maximum values) of sound velocity values supplied by the adopted formulas. the sound velocity calculated with the unesco formula oscillates between a minimum equal to 1508.988 m/s and a maximum equal to 1533.994 m/s, with an average value equal to 1513.543 m/s. the velocity calculated with the medwin formula oscillates between a minimum equal to 1509.142 m/s and a maximum equal to 1534.026 m/s, with an average value of 1513.636 m/s. finally, the sound velocity calculated with the mackenzie formula oscillates between a minimum equal to 1509.455 m/s and a maximum equal to 1534.676 m/s, with an average value equal to 1514.043 m/s. in the table 5, the statistical values of the residuals produced by the comparison between the adopted formulas are reported. once the sound velocity was calculated, a systematic error was introduced in each chemical-physical parameter of the water, i.e. temperature, salinity, pressure and depth, in order to evaluate the impact on the determination of the sound velocity. by systematic error we mean, in this case, an incorrect sampling of the parameters in question, simulating probe or human errors during the measurement. the introduced errors are: 0.1, 0.5 and 1 °c figure 1. localization of the sample point: visualization on the italy map (upper) and on sentinel-2 satellite image of the ligurian sea (lower). table 3. a selection of values of pressure, temperature and salinity supplied by ctd probe and corresponding velocity of the sound in water calculated by using unesco, medwin and mackenzie formulas. p (bar) d (m) t (°c) s (ppt) cu (m/s) cme (m/s) cma (m/s) 0.1 0.990 23.397 38.1408 1533.889 1533.924 1534.569 0.2 1.980 23.396 38.147 1533.910 1533.945 1534.591 0.3 2.980 23.398 38.151 1533.934 1533.968 1534.615 0.4 3.970 23.397 38.154 1533.953 1533.987 1534.635 0.5 4.960 23.398 38.156 1533.974 1534.006 1534.655 0.6 5.950 23.397 38.160 1533.994 1534.026 1534.676 0.7 6.940 23.389 38.163 1533.994 1534.024 1534.676 0.8 7.940 23.380 38.166 1533.990 1534.020 1534.672 0.9 8.930 23.369 38.171 1533.987 1534.016 1534.669 1 9.920 23.361 38.176 1533.988 1534.017 1534.671 5.1 50.580 16.426 38.046 1515.445 1515.598 1515.984 10.1 100.170 14.034 38.180 1508.991 1509.145 1509.458 20.2 200.280 13.944 38.519 1510.773 1510.865 1511.252 32.7 324.120 14.007 38.660 1513.210 1513.216 1513.672 table 4. statistics of sound velocity values supplied by the adopted formulas. cu (m/s) cme (m/s) cma (m/s) mean 1513.543 1513.636 1514.043 st. dev. 6.686 6.666 6.747 min 1508.988 1509.142 1509.455 max 1533.994 1534.026 1534.676 table 5. statistical values of the residuals produced by the comparison between the adopted formulas. cme – cu (m/s) cma – cu (m/s) cma – cme (m/s) mean 0.094 0.501 0.407 st. dev. 0.050 0.063 0.087 rmse 0.106 0.505 0.416 min 0.006 0.462 0.312 max 0.165 0.694 0.666 acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 50 for temperature, 0.1, 0.5 and 1 ppt for salinity, 0.1, 0.5 and 1 bar for pressure, 0.1 0.5 and 1 m for depth. however, those errors are worse than the accuracy values reported for the probe in subchapter 3.1, so to simulate combination of unfavourable environmental situations and poorly accurate operations. we then proceeded with the calculation of the residuals obtained by comparing the values of sound velocity produced by systematic error with the initial values of sound velocity considered as a reference. subsequently, the statistical values (mean, standard deviation, rmse, minimum and maximum values) are calculated for each formula, so to define the impact of the systematic errors on the accuracy of sound velocity determination. table 6, table 7, table 8 and table 9 show the statistical values of the residual produced by the injection of systematic errors in, respectively, the temperature, salinity, pressure and depth values in each formula. we finally proceeded with the calculation of the statistical values of the residuals generated by the worst possible combination of the systematic errors on temperature and salinity. the results of this calculation are shown in the table 10. in the end, the estimate of the errors on the depths was calculated. to provide the reader with an idea of this error dimension, average values of sound velocity in water were taken, calculated using the three formulas, every 100 m, 200 m and a single average value over the entire column of water, both with and without systematic errors. figure 2 shows the variability of the sound velocity in the case of unesco formula application, when we introduced systematic errors on temperature (0.5 °c), salinity (0.5 ppt) and both (0.5 °c-0.5 ppt). table 11 shows the rmse values of the differences between the known and the calculated depths by using different sound table 6. statistical values of the residuals produced by systematic errors for the temperature (0.1-0.5-1 °c) introduced in the adopted formulas. mean st. dev. rmse min max δc u (m/s) (0.1 °c) 0.311 0.020 0.312 0.249 0.321 δc me (m/s) (0.1 °c) 0.310 0.021 0.311 0.247 0.320 δc ma (m/s) (0.1 °c) 0.315 0.021 0.315 0.250 0.325 δc u (m/s) (0.5 °c) 1.549 0.102 1.553 1.239 1.597 δc me (m/s) (0.5 °c) 1.542 0.104 1.546 1.227 1.591 δc ma (m/s) (0.5 °c) 1.565 0.107 1.569 1.240 1.616 δc u (m/s) (1 °c) 3.079 0.203 3.085 2.461 3.174 δc me (m/s) (1 °c) 3.063 0.207 3.070 2.437 3.161 δc ma (m/s) (1 °c) 3.110 0.213 3.117 2.463 3.210 table 7. statistical values of the residuals produced by systematic errors for the salinity (0.1-0.5-1 ppt) introduced in the adopted formulas. mean st. dev. rmse min max δc u (m/s) (0.1 ppt) 0.117 0.002 0.117 0.109 0.118 δc me (m/s) (0.1 ppt) 0.119 0.003 0.119 0.111 0.120 δc ma (m/s) (0.1 ppt) 0.133 0.000 0.133 0.133 0.133 δc u (m/s) (0.5 ppt) 0.584 0.012 0.584 0.547 0.590 δc me (m/s) (0.5 ppt) 0.594 0.013 0.594 0.553 0.600 δc ma (m/s) (0.5 ppt) 0.664 0.000 0.664 0.664 0.664 δc u (m/s) (1 ppt) 1.169 0.025 1.169 1.094 1.181 δc me (m/s) (1 ppt) 1.188 0.027 1.189 1.106 1.201 δc ma (m/s) (1 ppt) 1.328 0.000 1.328 1.328 1.328 table 8. statistical values of the residuals produced by systematic errors for the pressure (0.1-0.5-1 bar) introduced in the adopted formulas. mean st. dev. rmse min max δc u (m/s) (0.1 bar) 0.017 0.000 0.017 0.017 0.017 δc me (m/s) (0.1 bar) δc ma (m/s) (0.1 bar) δc u (m/s) (0.5 bar) 0.083 0.000 0.083 0.083 0.083 δc me (m/s) (0.5 bar) δc ma (m/s) (0.5 bar) δc u (m/s) (1 bar) 0.166 0.001 0.166 0.166 0.167 δc me (m/s) (1 bar) δc ma (m/s) (1 bar) table 9. statistical values of the residuals produced by systematic errors for the depth (0.1-0.5-1 m) introduced in the adopted formulas. mean st. dev. rmse min max δc u (m/s) (0.1 m) δc me (m/s) (0.1 m) 0.002 0.000 0.002 0.002 0.002 δc ma (m/s) (0.1 m) 0.002 0.000 0.002 0.002 0.002 δc u (m/s) (0.5 m) δc me (m/s) (0.5 m) 0.008 0.000 0.008 0.008 0.008 δc ma (m/s) (0.5 m) 0.008 0.000 0.008 0.008 0.008 δc u (m/s) (1 m) δc me (m/s) (1 m) 0.016 0.000 0.016 0.016 0.016 δc ma(m/s) (1 m) 0.016 0.000 0.016 0.016 0.016 table 10. statistical values of the residuals produced by combination of systematic errors for temperature and salinity (0.1 °c-0.1 ppt, 0.5 °c-0.5 ppt, 1 °c-1 ppt), introduced in the adopted formulas. mean st. dev. rmse min max δc u (m/s) (0.1 °c-0.1 ppt) 0.428 0.023 0.429 0.358 0.439 δc me (m/s) (0.1 °c-0.1 ppt) 0.429 0.024 0.429 0.357 0.440 δc ma (m/s) (0.1 °c-0.1 ppt) 0.447 0.021 0.448 0.382 0.458 δc u (m/s) (0.5 °c-0.5 ppt) 2.131 0.114 2.134 1.784 2.185 δc me (m/s) (0.5 °c-0.5 ppt) 2.134 0.117 2.137 1.777 2.189 δc ma (m/s) (0.5 °c-0.5 ppt) 2.229 0.107 2.232 1.904 2.280 δc u (m/s) (1 °c-1 ppt) 4.237 0.227 4.244 3.547 4.344 δc me (m/s) (1 °c-1 ppt) 4.242 0.233 4.248 3.533 4.352 δc ma (m/s) (1 °c-1 ppt) 4.437 0.213 4.442 3.790 4.538 figure 2. impact on the sound velocity in water caused by systematic errors on t, s and t-s. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 51 velocity values, those obtained from different approaches, i.e. using unesco formula (rmseu), medwin formula (rmseme) and mackenzie formula (rmsema), averaging the sound velocity values on sections (100 m) or on the entire water column, injecting errors in temperature (0.1-0.5-1 °c), salinity (0.1-0.5-1 ppt) or both. 3.3. discussions the results highlight how the values of sound velocity in water obtained by means of the three formulas are like each other. in particular, as reported in table 5, it is possible to see how the unesco formula and the medwin formula are very similar to each other, as they show a very small rmse, while the mackenzie formula, compared to the other two, approximates the sound velocity values differently in the same conditions. from the results shown in the previous tables, temperature and salinity are the parameters that have the greatest effect on the determination of the sound velocity in water. in fact, the introduction of systematic errors on them produces impacts greater than depth and pressure. as regards the temperature, the parameter that has the greatest influence compared to the others, there is an increase in the rmse value, in particular with the presence of an error of 1 °c, an rmse equal to 3.088 m/s is obtained, as shown in table 6. these results are approximately similar for all formulas. for salinity, another parameter with strong influence, as shown in table 7, we can see that the results obtained by using the unesco formula and the medwin formula are very similar, while regarding the mackenzie formula, we can see how an important variation of the velocity values is obtained, but this remains constant along the water column. this result is highlighted by the standard deviation values. for the pressure and the depth, the situation is the opposite, as they are the parameters that have minor effects on the determination of the velocity, in fact the impact produced by the introduction of systematic errors can be considered almost negligible. for pressure, we have an rmse much smaller compared to the one seen previously. it can be noted that even with the presence of a systematic error equal to 1 bar, the rmse is almost negligible, showing a value of 0.16 m/s as shown in table 8. finally, for the depth, as shown in table 9, we get the smallest values of rmse. also for these two parameters, the formulas show similar values. in the case of the worst combination generated by the simultaneous introduction of systematic errors on temperature and salinity, table 10 shows very important results. the incorrect determination of these parameters, combined with each other, leads to an rmse ranging from 0.428 m/s (for a systematic error of 0.1 °c 0.1 ppt) up to 4.248 m/s (for a systematic error of 1 °c – 1 ppt). also in this case, the values obtained by the formulas of unesco and medwin are similar, while the mackenzie formula shows a slight increase in the obtained values. in the end, table 11 shows the rmse values for the depth associated with the resulting velocity values. in particular, it can be noted that, as more velocity values are taken along the water column, the rmse value tends to decrease. in fact, for a single velocity value over the entire water column, an rmse equal to 0.260 m is obtained, while for velocity values taken every 100 m, an rmse equal to 0.110 m is obtained. we can also note that a systematic error on velocity naturally leads to an error on depth, even taking multiple velocity values along the water column, generating an rmse equal to 0.560 m in the worst possible case, i.e. with the presence of a systematic error on both temperature and salinity. 4. conclusions bathymetric surveys are typically carried out using techniques that exploit the propagation of acoustic waves in water. therefore, the correct determination of the sound velocity in water is of fundamental importance. it was found that an error on the chemical-physical parameters of the water (temperature, pressure, salinity, depth) due to an inaccurate instrument calibration, or, rather, to the use of a wrong model for one of them, can impact significantly on sound velocity determination. in particular, this article provides a measurement of the errors that can be produced. for our application, the formulas of unesco, medwin and mackenzie have been taken into consideration and systematic errors on the four parameters have been simulated. the inaccuracy of temperature and salinity measurements produces the greatest effects. particularly, the results remark that the sound velocity is very sensitive to the variation of the temperature. an error on the determination of the sound velocity in water leads to a non-negligible error on the depth. it has been seen that using a single sound velocity value over the entire water column, affected by a combination of systematic errors for temperature and salinity, generates errors that can reach about 0.5 m. of course, taking more different velocity values as reference along the water column allows you to determine the depth of the bottom more accurately, but even in this case, you risk having non negligible errors. it is possible to conclude that the sound velocity in water represents a very important parameter in bathymetric surveys, and therefore it must necessarily be determined with the highest possible accuracy. our experiments do not permit to evaluate the best method for indirect measurement of sound in water: the table 11. rmse values of the differences between the known and the calculated depths using velocity sound values derived from different approaches. velocity values taken on: rmseu (m) rmseme (m) rmsema (m) entire water column 0.260 0.250 0.263 every 100 m 0.111 0.107 0.121 entire water column (0.1 °c) 0.246 0.236 0.243 entire water column (0.5 °c) 0.245 0.243 0.248 entire water column (1 °c) 0.353 0.356 0.360 every 100 m (0.1 °c) 0.116 0.116 0.128 every 100 m (0.5 °c) 0.216 0.224 0.229 every 100 m (1°c) 0.388 0.397 0.401 entire water column (0.1 ppt) 0.254 0.244 0.251 entire water column (0.5 ppt) 0.237 0.229 0.234 entire water column (1 ppt) 0.235 0.231 0.239 every 100 m (0.1 ppt) 0.111 0.109 0.123 every 100 m (0.5 ppt) 0.130 0.134 0.148 every 100 m (1 ppt) 0.178 0.188 0.207 entire water column (0.1 °c-0.1 ppt) 0.241 0.233 0.239 entire water column (0.5 °c-0.5 ppt) 0.276 0.277 0.286 entire water column (1 °c-1 ppt) 0.470 0.477 0.497 every 100 m (0.1 °c-0.1 ppt) 0.121 0.123 0.134 every 100 m (0.5 °c-0.5 ppt) 0.279 0.290 0.302 every 100 m (1°c-1 ppt) 0.526 0.538 0.560 acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 52 paper highlights the impact of inaccurate determination of temperature, pressure and salinity on the bathymetric survey results. extremely precise probes for direct measurement now available are to prefer to improve the depth determination. acknowledgement this work synthesizes results of experiments executed within a research project performed in the laboratory of geomatics, remote sensing and gis of the “parthenope” university of naples. we would like to thank the technical staff for their support. references [1] m. huet, marine spatial data infrastructure, an iho perspective: data, products, standards and policies, international hydrographic bureau, monaco (2009). 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[40] n. p. fofonoff, r.c. millard jr., algorithms for computation of fundamental properties of seawater, unesco technical papers in marine science, 44, (1983). https://doi.org/10.1175/bams-d-15-00031.1 https://doi.org/10.3133/sir20095110 https://doi.org/10.1109/cspa.2011.5759882 acta imeko  september 2014, volume 3, number 3, 73  www.imeko.org    acta imeko | www.imeko.org  september 2014 | volume 3 | number 3 | 73  section: advertisement  article 15 204 template advertisement_hbm application of wearable eeg sensors for indoor thermal comfort measurements acta imeko issn: 2221-870x december 2021, volume 10, number 4, 214 220 acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 214 application of wearable eeg sensors for indoor thermal comfort measurements silvia angela mansi1, ilaria pigliautile2, camillo porcaro3,4, anna laura pisello2, marco arnesano1 1 università telematica ecampus, 22060 novedrate (co), italy 2 ciriaf – centro interuniversitario sull’inquinamento e l’ambiente mauro felli, dipartimento di ingegneria, università di perugia, 06125 perugia, italy 3 department of neuroscience and padova neuroscience center (pnc), university of padova, padova, italy 4 institute of cognitive sciences and technologies (istc) national research council (cnr), rome, italy section: research paper keywords: thermal comfort measurement, electroencephalography (eeg); wearable sensors, signal processing citation: silvia angela mansi, ilaria pigliautile, camillo porcaro, anna laura pisello, marco arnesano, application of wearable eeg sensors for indoor thermal comfort measurements, acta imeko, vol. 10, no. 4, article 33, december 2021, identifier: imeko-acta-10 (2021)-04-33 section editors: carlo carobbi, university of florence, gian marco revel, università politecnica delle marche and nicola giaquinto, politecnico di bari, italy received october 7, 2021; in final form december 6, 2021; published december 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was supported by the italian ministry of research through the next:com (prot.20172fsch4) ‘’towards the next generation of multiphysics and multidomain environmental comfort models: theory elaboration and validation experiment’’ project, within the prin 2017 program. corresponding author: marco arnesano, e-mail: marco.arnesano@uniecampus.it 1. introduction human-building interaction is an essential subject studied in the last decades, aimed at ensuring the occupants’ wellbeing and the correlated influence on buildings’ energy consumption. an important aspect of the built environment is indoor thermal comfort and its impact on occupants. from the 1970s, different thermal comfort models have been introduced. firstly, the twonode model [1] was presented by j. b. pierce, which represents the human body into two-layer: skin and core. each layer is a heat transfer node with thermal physiological parameters controlled by energy and mass conservation low. then, fanger [2] proposed the predicted mean vote (pmv) model to predict the thermal comfort of a large sample of individuals. in the last 3 decades, the adaptive model was widely used [3]. it is based on findings from the simultaneous collection of data on the thermal environment and thermal response of subjects, to determine the indoor thermal states, and the influencing parameters, that satisfy occupants’ sensations. the resulting adaptive models provide experimental relationships between the thermal comfort temperature and the outdoor air temperature. however, neither of those models encompass the personal physiological and psychophysics influence on the individual thermal perception. the concept of the personal comfort model has been introduced as a novel approach to predict individual-specific thermal comfort based on the measurements of environmental quantities, occupants’ behaviour, and physiological responses. abstract multidomain comfort theories have been demonstrated to interpret human thermal comfort in buildings by employing human-centered physiological measurements coupled with environmental sensing techniques. thermal comfort has been correlated with brain activity through electroencephalographic (eeg) measurements. however, the application of low-cost wearable eeg sensors for measuring thermal comfort has not been thoroughly investigated. wearable eeg devices provide several advantages in terms of reduced intrusiveness and application in real-life contexts. however, they are prone to measurement uncertainties. this study presents results from the application of an eeg wearable device to investigate changes in the eeg frequency domain at different indoor temperatures. twenty-three participants were enrolled, and the eeg signals were recorded at three ambient temperatures: cold (16 °c), neutral (24 °c), and warm (31 °c). then, the analysis of brain power spectral densities (psds) was performed, to investigate features correlated with thermal sensations. statistically significant differences of several eeg features, measured on both frontal and temporal electrodes, were found between the three thermal conditions. results bring to the conclusion that wearable sensors could be used for eeg acquisition applied to thermal comfort measurement, but only after a dedicated signal processing to remove the uncertainty due to artifacts. mailto:marco.arnesano@uniecampus.it acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 215 the usage of different physiological signals has been discussed in the literature, in the field of thermal comfort measurements. four of them have been identified as highly correlated with the perceived thermal comfort: electroencephalogram (eeg), electrocardiogram (ecg), skin temperature (st), and galvanic skin response (gsr), being a part of different processes involved in the thermoregulatory system activities [4]. several studies evaluated the thermal status of individuals acquiring different physiological signals simultaneously, but analysed them separately [5]. only a few of those provide a regression equation that relates physiological parameters to each other [6]. in addition, wearable sensors, for measuring real-time physiological signals, have been assessed as a promising technology in the field of personal thermal comfort estimation [7]. among physiological signals, measurable via wearable devices, the eeg has instigated interest, in the field of thermal comfort, for the possibility to monitor the human physiological responses changes in real-time [8]. generally, the brain’s electrical activity changes in response to the process of perception and cognition of environmental stimuli. eeg represents the measure of voltage fluctuation resulting from ionic current within the neurons of the brain [9]. brainwaves are detected using sensors placed on the scalp according to the 10-20 system [10]. they are divided into bandwidths. each of those corresponds to a particular state of mind. in general, delta (0.1 hz – 4 hz) is associated with deep sleep, theta waves (4 hz – 7.5 hz) are related to consciousness sleep towards drowsiness, alpha waves (7.5 hz – 12 hz) are the prominent rhythm in relaxing and passive attention activities, beta (12 hz – 30 hz) is associated with active thinking and gamma waves (30 hz – 45 hz) are prominent during high mental activities. recent studies showed how eeg power spectral densities (psds) were influenced by changes in environmental temperature. lv and colleagues [11] correlated the eeg frequency bands to the neutral and warm air temperatures. they showed higher delta-band activity in the warm condition. yao and colleagues [10] measured eeg signals from 20 subjects exposed at low, neutral, and high temperatures, showing that the relative eeg power of the beta band was significantly higher in the cold and warm environment compared with the neutral condition. lim et al. [12] found a connection between the eeg and thermal comfort, alpha/beta ratio (rab) increased in a comfortable environment, the opposite trend was highlighted by the relative beta (rb). other studies [12], [17] revealed how eeg frequencies change in conjunction with body temperature variations and how the ambient temperature influences eeg psds. son et al. [13] investigated a correlation between psychological and physiological measures to evaluate thermal comfort. their results showed an increase of theta band and a decrease of the beta band according to the thermal pleasure. wu et al. [14] classified thermal comfort under different conditions, showing an increase of delta and a decrease of beta power in a warm environment. zhu [15] examined changes in eeg responses during cognitive activity at different air temperatures. findings indicated a high value of relative delta and a low value of relative theta, alpha, and beta at high temperatures. in the above-mentioned eeg studies, measurements were performed using a traditional cap or medical devices; these devices can provide good quality of data, but they require a long time of applications and often they are perceived as unpleasant by the users. moreover, they are not applicable in a real-life context. such issues could be solved by the advent of wearable sensors. they offer several advantages: they are low cost, simple to be used, their comfortable design allows to reduce the time of application and, more importantly, they considerably attenuate the obstructiveness of measurements, making the experimental time not unpleasant for the participants. however, they are strongly prone to collect environmental noise and artifacts due to subject movements (e.g., eyeblink, muscular artifacts), which means that acquired data need to be processed before they became reliable for thermal comfort measurement [21]. several studies performed a metrological characterization of wearable devices for testing their accuracy. arpaia et al. [8] proposed a human stress detection method based on eeg signal acquired by a highly wearable single channel instrument. the results of their study demonstrated that the four standard machine learning (ml) classifiers used, reached more than 90 % of accuracy in distinguishing stress conditions of participants. in another study, arpaia and colleagues [16] present the calibration and the metrological characterization of a low-cost wearable device (olimex eeg-smt). preliminary calibration results showed good linearity, however, a magnitude of error around 8 % with a dependence on frequency was detected. in general, studies revealed how the commercial low-cost wearable devices used in conjunction with ml classifiers in an experimental context, can reach an accuracy between 83.3 % and 99.1 % [17]. thus, findings revealed that eeg wearable sensors can provide the required accuracy for the classification of human mental states. this paper presents the application of eeg wearable sensors for thermal comfort measurement. the experimental protocol, the signal processing procedure, and the statistical analysis are illustrated together with results from the measurement campaign performed in a controlled environment. results demonstrate the feasibility of the proposed approach, that could be used to build personalized comfort models based on eeg measurement. 2. materials and methods 2.1. eeg data collection and processing 2.1.1 eeg measurement device in this study, the eeg signal acquisition was done using a commercial wearable device: the interaxon muse headband [18]. the reference electrode fpz (cms/drl) is located on the forehead, the input electrodes are two front (left and right of the reference: af7, af8, silver made) and two posteriors, above each ear (tp9 and tp10, conductive silicone -rubber) (figure 1). the device acquires signals at 256 hz sampling frequency. raw eeg data were collected using the muse application [19], paired with a smartphone through bluetooth low energy (ble). however, the reliability of muse can be questioned. the limited number of electrodes can preclude the multi-networks figure 1. a) muse 2 headband sensors overview. b) top-down view of the eeg electrode positions on the subject’s head. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 216 evaluation from being focused on a specific area of the brain. the frontal electrodes are more prone to collect eye-blink and movement artifacts, and they can disrupt the measurements of actual brain waves [4]. dry electrodes may also be more prone to result in discomfort over time and pose a higher risk of misplacement on the forehead, resulting in lower accuracy of the signal acquired. in addition, certain head shapes, head sizes, and hairstyles made data collection difficult, the poor contact with the head surface does not allow a proper data acquisition [20], [21]. nevertheless, many studies demonstrated its ability to be applied in research experimental context. krigolson et al. [18] demonstrated that muse can be used successfully for eventrelated potential (erp) study applications. they tested the reliability of erp data collected with muse using a resampling analysis, obtaining reliable erp components with muse (especially the n200) with a minimal number of participants. youssef et al. [22], in their study for lies detection using muse, showed great success in their experimental purpose. ratti and colleagues [23] compared eeg medical devices with consumer muse portable devices. this study demonstrated that muse psds were similar to medical systems, but with higher variation (the power spectral ratio was between 0.975 and 1.025 for medical equipment, ratios between 1.125 and 1.225 for muse). this broadband increase in the power spectrum of muse data may reflect artifacts in data recorded by dry electrodes. however, muse is simple to set up, the applicability is quick (less than 10 min) and simple, which is significantly convenient for the selfhelp applications. 2.1.2 eeg data processing eeg recording is prone to collect noise and physiological artifacts, such as eye blinking, movements, and non-physiological artifacts such as electrical interference. therefore, it is very essential to apply processing and denoising to the recorded eeg data. based on the literature background, a processing custom code was implemented to filter and isolate the signal of interest: notch filter was used for removing the power line noise (50/60 hz), a high-pass filter at 0.1 hz to remove dc offset and low-frequency skin potential artifacts, a low-pass filter at 45 hz to remove high-frequency noise [24]. the independent component analysis (ica), which decomposes the signal into maximally independent components and artifacts components, was applied. after a visual inspection, the eyeblink component was removed, and the data were reconstructed (figure 2). epochs of 2.000 ms with 1.000 ms overlap were extracted from artifacts-free continuous data. then the power spectrum analysis (psd) was computed using a pwelch function with hamming 256-samples window, with 50 % overlap. the output was normalized using the z-score normalization method. 2.1.3 eeg features extraction to establish the correlation between eeg frequency bands and the subjective thermal sensation, retrieved through a questionnaire, the features extraction was performed. the output of pre-processing step represents the five major brain waves in the different frequency ranges: delta, theta, alpha, beta, and gamma waves. once the five brain waves were computed the main eeg features were calculated, based on their relevance in the context of the thermal comfort assessment using eegs. 2.2. experimental campaign 2.2.1 participants twenty-three healthy volunteers were enrolled for the experiment. they were informed about the experimental protocol and data management. collected data were anonymized. two experimental sessions were conducted during wintertime from january to february 2021, and summertime in july 2021. the selected group included 9 males and 14 females. all volunteers were local students at the ‘’university of perugia’’, where the experiment took place. none of them had a pathological history. personal information was collected with a survey filled by all subjects at the end of each test. table 1 summarizes the information about the participants involved in figure 2. normalized eeg data before eyeblink component removal; corrected eeg data after eyeblink component removal and all the components estimated by ica are reported. component 3 was rejected. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 217 the campaigns: range of ages, mean and standard deviation (std) of height and weight. the average clothing thermal insulation was 1.1 clo and 0.43, typical for wintertime and summertime respectively. the metabolic rate of participants was 1.1 met according to standard iso 7730 [25]. the second part of the survey aims at collecting data about thermal perception. in particular, the sensation vote for each thermal condition is given through a 5-points scale going from -2 to 2, where 0 corresponds to neutrality. 2.2.2 experimental setup: the controlled test room the experiments were carried out in the next.room (4 × 4 × 2.7 m3), a novel test room built up at the engineering campus of perugia university (italy) for human-comfort studies. the next.room ambient temperature was controlled through the installed heating, ventilation & air conditioning (hvac) system based on a heat pump with an inverter and providing four levels of fan speed. the room has a window that was shaded during all the tests of the experimental campaign here described, while the internal level of illuminance was kept constant thanks to the installed artificial lighting system. the environmental parameters inside the next.room were continuously monitored employing a fixed microclimate station located in the centre of the room and the associated data logging system [26]. table 2 provides information about the sensors installed in the test room, their accuracies, and the environmental boundaries monitored during cold (16 °c), neutral (24 °c), and warm (31 °c) tests accounting for both the winter and summertime series. 2.2.3 experimental setup: environmental condition during the experiments (both winter and summer seasons) the air velocity was always below 0.1 m/s, the relative humidity of air was not dependently controlled, while the air temperature was set at 16 °c (cold), 24 °c (neutral), and 31 °c (warm) according to the thermal sensation perceived by the participants collected with the survey (table 3). a specific schedule for the experiment was adopted. the measurements were done from 10:00 a.m. to 1:00 p.m. and from 3:00 p.m. to 6:30 p.m. subjects were asked to sit down and keep relaxed; no activity was allowed. the experimental procedure was carried out sequentially for 2 days for each thermal condition, in both seasons. each experiment lasted 20 min, 15 min for thermal adaptation (the previous study affirmed that people need 15 min to adapt to a new environment [27]), and 5 for recording data. (figure 3). 3. results the capability of discriminating the cold, warm and neutral sensations starting from the portable eeg measurements was evaluated in terms of statistical significance between brain waves features. eeg features were divided into three groups (cold, neutral, and warm) according to the thermal sensation scores expressed by the participants in the questionnaire. mean (𝑓)̅ and standard uncertainty of the mean (𝑢𝑓 ) of each feature was then calculated according to [28]. the normality of each group of features was evaluated with the shapiro test [29]. given that all groups presented a nongaussian distribution, the statistical significance was determined with the non-parametric kruskal-wallies test [30]. table 4 reports the features that turned out to provide a significant statistical difference (p-value < 0.05). the results showed how brain activities were altered by the thermal sensation perceived by occupants to the different environmental temperatures they were exposed to. in particular, the outcomes revealed that eeg features connected to high-frequency bands, such as beta and gamma for both frontal and temporal electrodes, tended to decrease with a warm sensation. instead, features that express the mean power of low-frequency bands registered an opposite trend (alpha-beta ratio af8, relative alpha tp9, theta beta ratio af8). the capability of a feature to discriminate between two different thermal conditions was further investigated with a posthoc analysis based on the dwass-steel-critchlow-fligner pairwise comparison test [31]. results from the test are reported in table 5. the pairwise comparison results demonstrated that table 1. details of the subjects participating in experiments. information details total number of subjects 23 (9 males and 14 females) age (min-max) (27 32) years height (mean ± std) (162 ± 12) cm mean of weight (mean ± std) (50.7 ± 15.3) kg table 2. technical information of the sensors for environmental parameters monitoring sensor environmental parameter accuracy thermal-hygrometer air temperature, °c relative humidity, % ± 0.1 °c ± 1.5 % black globe radiant temperature sensor mean radiant temperature, °c ± 0.15 °c hotwire anemometer air velocity, m/s ± 0.05 m/s co2sensor co2 concentration, ppm ± 50 ppm (+ 2 %) luxmeter illuminance, lx ± 5 % figure 3. measurements of eeg signals with muse headband interaxon in climate chamber. a facial mask was not used during the experiments. table 3. mean values and std of environmental parameters monitored in the climate chamber during the experimental sessions. measured parameters cold neutral warm air temperature, °c 16.7 ± 0.3 24.3 ± 0.5 31.4 ± 0.5 relative humidity, % 25.9 ± 0.5 20.1 ± 0.2 18.2 ± 0.4 air velocity, m/s 0.1 ± 0.02 0.07 ± 0.04 0.09 ± 0.03 mean radiant temperature, °c 16.4 ± 0.6 23.9 ± 0.5 30.2 ± 0.4 co2 concentration, ppm 487 ± 12 492 ± 4 502 ± 6 illuminance, lx 287 ± 22 290 ± 25 281 ± 23 acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 218 all the features (except for alpha-beta ratio tp9, beta tp10, and gamma tp9) were different between cold and warm thermal sensations. beta af8, gamma af8, and relative beta af8 also showed differences between neutral and warm conditions, as shown in figure 4. none of the measured features showed differences between cold and neutral thermal sensations. in general, eeg measurements performed with the portable device showed a correlation with the thermal sensations in terms of increase or decrease of power of brain waves. the warm sensation could be correlated to an increase of alpha and theta waves, indicating that when subjects are exposed to a warm environment tend to be less concentrated and unable to keep focused. on the other hand, in cold conditions, there is an increase in the activity of beta and gamma waves, which are the main brain waves connected to high mental activity. at the same time, high levels of beta and gamma can be synonyms of high stress, indicating that the cold condition is perceived as more stressful than the warm one. results obtained with the muse turned out to be aligned with the state of the art concerning thermal comfort measurements based on eeg data. 4. conclusions as it is already known the prominence of an eeg frequency band is correlated to a certain type of mental state. for example, the high power of gamma waves corresponds to high mental activity, vice versa dominant alpha corresponds to relaxed conditions. some studies revealed that eeg theta waves increased while beta waves decreased with a comfortable thermal state, others showed that high values of theta band correspond to a high state of arousal level and vice versa. all studies have been performed with medical and non-portable devices, providing accurate eeg measurements, but with poor wearability and not applicable in real-life situations. for this reason, the proposed study aims at demonstrating whether a lowcost portable eeg device (muse) could be used to perform thermal comfort measurements, providing better wearability but with lower measurement accuracy. the experiments were performed in a controlled environment, where eeg was measured on 23 subjects exposed to warm, neutral, and cold conditions. considering the different thermal sensations perceived by subjects exposed to the same environmental condition, before performing the statistical analysis, the physiological features were divided and classified according to the thermal sensation questionnaire-based results into cold, table 4. mean feature (𝑓̅), standard uncertainty of the mean (u), h-statistic, and the significative p-value (p-value<0.05) for each of the statistically significant eeg features are reported. eeg features 𝒇 ̅ ± 𝒖𝒇 (cold) 𝒇 ̅ ± 𝒖𝒇 (neutral) 𝒇 ̅ ± 𝒖𝒇 (warm) h-statistic p-value alpa beta ratio tp10 1.2 ± 0.08 1.3 ± 0.07 1.3 ± 0.07 3.3 0.04 alpha beta product tp9 in db 1.3 ± 0.12 1.5 ± 0.14 1.6 ± 0.10 8.1 0.02 alpha beta product af8 in db 0.1 ± 0.05 0.1 ± 0.03 0.1 ± 0.02 8.2 0.02 beta af8 in db 0.4 ± 0.10 0.3 ± 0.07 0.1 ± 0.06 13.9 0.001 beta tp9 in db 0.5 ± 0.05 0.0 ± 0.09 0.4 ± 0.5 5.3 0.006 gamma af7 in db 0.1 ± 0.09 0.1 ± 0.05 -0.1 ± 0.09 6.7 0.03 gamma af8 in db 0.1 ± 0.1 0.0 ± 0.06 -0.1 ± 0.07 12.4 0.002 gamma tp10 in db 0.2 ± 0.06 0.2 ± 0.02 0.1 ± 0.04 7.9 0.02 gamma tp9 in db 0.2 ± 0.07 0.1 ± 0.05 0.1 ± 0.05 8.9 0.01 relative alpha tp9 0.3 ± 0.17 0.3 ± 0.02 0.3 ± 0.07 10.3 0.006 relative beta af8 0.3 ± 0.06 0.3 ± 0.05 0.1 ± 0.07 16.9 < 0.001 relative gamma tp10 0.1 ± 0.02 0.1 ± 0.01 0.1 ± 0.02 6.1 0.04 relative gamma tp9 0.1 ± 0.03 0.0 ± 0.02 0.0 ± 0.03 4.8 0.09 relative theta af8 0.1 ± 0.06 0.1 ± 0.05 0.2 ± 0.07 9.2 0.01 temporal asymmetry alpha in db 0.1 ± 0.03 0.0 ± 0.02 0.0 ± 0.02 4.5 0.01 temporal asymmetry delta in db 0.1 ± 0.03 0.0 ± 0.03 0.0 ± 0.02 6.7 0.03 temporal asymmetry theta in db 0.1 ± 0.03 0.0 ± 0.4 0.0 ± 0.03 8.7 0.01 theta beta ratio af8 -0.3 ± 0.71 0.1 ± 0.24 2.3 ± 2.31 7.2 0.02 table 5. dwass-steel-critchlow-fligner pairwise comparison between cold (c), neutral (n), and warm (w) thermal sensation results (* for p-value < 0.05). eeg features c-n (p-value) c-w (p-value) n-w (p-value) alpa beta ratio tp10 0.3 0.1 0.7 alpha beta product tp9 in db 0.2 0.01* 0.6 alpha beta product af8 in db 0.5 0.02* 0.1 beta af8 in db 0.5 0.001* 0.03* beta tp9 in db 0.1 0.02* 0.9 gamma af7 in db 0.2 0.04* 0.6 gamma af8 in db 0.5 0.003* 0.04* gamma tp10 in db 0.5 0.02* 0.3 gamma tp9 in db 0.1 0.2 0.8 relative alpha tp9 0.1 0.004* 0.5 relative beta af8 0.9 0.004* 0.001* relative gamma tp10 0.4 0.04* 0.6 relative gamma tp9 0.2 0.02* 0.3 relative theta af8 0.6 0.2 0.007* temporal asymmetry alpha in db 0.5 0.02* 0.2 temporal asymmetry delta in db 0.3 0.02* 0.6 temporal asymmetry theta in db 0.6 0.01* 0.2 theta beta ratio af8 0.2 0.03* 0.4 acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 219 neutral, and warm groups. this approach allows investigating the thermal perception of occupants overcoming the limits due to the different nationalities and personal thermal history, focusing on just the perception felt by the subjects during experiments. results showed that all the eeg features calculated presented a statistical difference between cold and warm sensations. beta af8, gamma af8 and, relative beta af8 showed significant differences also between neutral and warm sensations. in general, the results demonstrate that muse eeg wearable device could be used for thermal comfort measurements, discriminating cold and warm thermal sensation. due to its comfortable design, the muse considerably reduces the measurement’s intrusiveness, reducing consequently the time of application and discomfort due to wearability. the presence of dry electrodes facilitates the recording procedure but, the absence of a conductivity gel increases the electrical impedance between the dry electrodes and the skin, making the devices more prone to register artifacts, such as muscular movements and eyeblink, with the risk of reducing the signal-to-noise ratio. therefore, signal processing for data cleaning and artifacts removal, based on the approach proposed in this paper, is required. in addition, the usage of water solution, to improve the conductivity of muse electrodes during its application, could introduce a further source of uncertainty in the case of longlasting experiments. the water evaporation can lead to drifts of the measured voltage, resulting in a misleading features interpretation. for longer experiments, future investigations are required to estimate the measurement uncertainty in the case of the usage of portable eeg sensors for real-life applications. nevertheless, for future developments, the idea to study the eeg signal in conjunction with other physiological signals, such as ecg, st, and gsr has been considered. this approach could allow the generation of comprehensive knowledge about the physiological response of the mechanisms involved in the human thermoregulatory system. new experimental campaigns will be conducted, increasing the number of samples to give consistency to the statistical results. an investigation about how the gender and age of subjects can affect thermal perception will be considered, as an additional aspect of the study. the final scope is to identify the most relevant physiological features to be used as input for the creation of predictive models, based on ml techniques, capable of thermal comfort level classification. acknowledgment the research has been co-founded by the italian ministry of research through the next:com (prot.20172fsch4) ‘’towards the next generation of multiphysics and multidomain environmental comfort models: theory elaboration and validation experiment’’ project, within the prin 2017 program. references [1] q. zhao, z. lian, d. lai, thermal comfort models and their developments: a review, energy built environ., vol. 2, no. 1, pp. 21–33, jan. 2021. doi: 10.1016/j.enbenv.2020.05.007 [2] p. o. fanger, assessment 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epistemology; philosophy of probability; measurement modelling; probabilistic models citation: giovanni battista rossi, francesco crenna, marta berardengo, probability theory as a logic for modelling the measurement process, acta imeko, vol. 12, no. 2, article 13, june 2023, identifier: imeko-acta-12 (2023)-02-13 section editor: eric benoit, université savoie mont blanc, france received july 2, 2022; in final form march 16, 2023; published june 2023 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: giovanni battista rossi, e-mail: g.b.rossi@unige.it 1. introduction the problem of the nature of probability has become a topic in measurement science for over twenty years, as a part of the debate on the expression and evaluation of measurement uncertainty, significantly raised by the publication of the guide to the expression of uncertainty in measurement (gum) [1] and of its long lasting and still ongoing revision process [2]. in the debate, the opposition between the bayesian and the frequentist schools of thoughts in statistics soon emerged, which involves the consideration of the nature of probability. in this regard, some authors pursue an explicit adoption of a bayesian paradigm for the overall context of uncertainty evaluation [3], [4], others instead suggest maintaining a more open attitude [5], [6], even when expressing a preference for the bayesian view [7], or to include the frequentist approach [8], when appropriate. here the focus is put on measurement modelling and probability is regarded as a logical and mathematical tool for developing such models, in such a way as to account for uncertainty. alternative choices could be done, for example those based on the evidence theory [9], [10], but here only probability is discussed and investigated. measurement modelling has been recently the subject of investigation, not only in respect to practical issues [11], but also to theoretical and foundational aspects [12], [13]. yet the “nature” of probability in such modelling seems to have not been discussed explicitly, which is instead the goal of this communication. basically, it is here suggested that probability can be regarded as an appropriate logic for developing models of measurement when uncertainty must be accounted for. therefore, in section 2 deterministic measurement modelling will be firstly addressed. then, in section 3, the logical approach to probability here proposed will be presented. its application to probabilistic measurement modelling will be addressed in section 4 and conclusions will be drawn in section 5. 2. deterministic measurement modelling 2.1. generic modelling issues it is here suggested that probability can be understood as a logic for developing measurement models. the notion of model thus needs reviewing. to establish some terminology, let us consider a system as a set of entities with relations among them [14]. a model can be thus understood as an abstract system, capable of describing a class of real systems. for example, if we abstract the problem of the nature of probability has been drawn to the attention of the measurement community in the comparison between the frequentist and the bayesian views, in the expression and the evaluation of measurement uncertainty. in this regard, it is here suggested that probability can be interpreted as a logic for developing models of measurement capable of accounting for uncertainty. this contributes to regard measurement theory as an autonomous discipline, rather than a mere application field for statistics. following a previous work in this line of research, where only measurement representations, through the various kinds of scales, were considered, here the modelling of the measurement process is discussed and the validity of the approach is confirmed, which suggests that the vision of probability as a logic could be adopted for the entire measurement theory. with this approach, a deterministic model can be turned into probabilistic by simply shifting from a deterministic to a probabilistic semantic. mailto:g.b.rossi@unige.it acta imeko | www.imeko.org june 2023 | volume a | number b | 5 consider the height of the inhabitants of a generic town, the model, 𝑀, can be expressed by a function, ℎ: 𝑈 → 𝑋, that associate to each inhabitant his/her height, on a proper height scale. for maximum simplicity, in the following illustrative examples height will be considered as a purely ordinal property, and 𝑋 the set of the numbers expressing height on an ordinal scale. therefore, the model can be synthetically expressed by the triple: 𝑀 = (𝑈, 𝑋, ℎ). (1) let us now introduce the distinction between deterministic and probabilistic models. a typical statement related to model 𝑀 is: ℎ(𝑢) = 𝑥 , (2) with 𝑢 ∈ 𝑈 and 𝑥 ∈ 𝑋. yet the truth of this statement is undefined, till a specific town, 𝑇, is considered. with reference to 𝑇, instead, if 𝐴 denotes the set of its inhabitants, 𝑋𝐴 the set of their height values and ℎ𝐴 the corresponding height function, the model is now specialised to 𝑇, that is: 𝑀(𝑇) = (𝐴, 𝑋𝐴, ℎ𝐴). (3) suppose for example that in 𝑇 there are just 3 inhabitants, 𝐴 = {𝑎, 𝑏, 𝑐}, that 𝑋 = {1,2}, and ℎ𝐴 = {(𝑎, 2), (𝑏, 1), (𝑐, 1)}, then (𝐴, 𝑋𝐴 , ℎ𝐴) = ({𝑎, 𝑏, 𝑐}, {1,2}, {(𝑎, 2), (𝑏, 1), (𝑐, 1)}). (4) the structure in equation (4) provides a sematic, that is a criterion of truth for the deterministic model 𝑀, since it allows us to ascertain the truth of any statement involved in the model. for example, ℎ(𝑎) = 2 is true, whilst ℎ(𝑏) = 2 is false. the general truth criterion is thus, for town 𝑇, 𝑢 ∈ 𝐴, 𝑥 ∈ 𝑋𝐴: ℎ(𝑢) = 𝑥 ↔ (𝑢, 𝑥) ∈ ℎ𝐴. (5) let us call 𝑇 an instance of the model 𝑀: then a model is deterministic if for any instance of the model all the statements concerning the model are either true or false. conversely, we will call probabilistic a model where for at least one of its instances there is at least one statement concerning the model for which its state of truth cannot be ascertain, but only a probability can be assigned to it. the transition from a deterministic to a probabilistic description will be discussed in section 3. 2.2. modelling the measurand measurement modelling concerns both the measurand and the measurement process. the modelling of the measurand aims at ensuring that the property of interest can be measured and it is thus closely related to the measurability issue [15]. at a foundational level, this implies assuming that the quantity1 under consideration can be measured on an appropriate measurement scale, i.e., that it possesses the required empirical properties. for example, (empirical) order is required for an ordinal scale, whilst order and difference are needed in the case of an interval scale. at a more operational level, modelling the measurand may account for the interactions it has with the environment and with the measuring system, to ensure that they do not hinder measurement, to compensate them, if possible, and to account for them in the uncertainty budget. 1 “quantity” here stands for “measurable property”. 2 note that here the term “object” has to be understood as “the carrier of the property to be measured” irrespectively of it being a concrete object, here only the first aspect, that is the possess of proper empirical properties, is briefly discussed. this is typically the scope of the so-called representational theory and it can be summarised by one or more representation theorems. for example, taking again the case of height of persons, still considering it as an ordinal property, an operation of empirical comparison needs considering, that allows us to determine, for any pairs of persons, 𝑢 and 𝑣, whether 𝑢 is taller than 𝑣, 𝑢 ≻ℎ 𝑣, or 𝑣 is taller than 𝑢, 𝑣 ≻ℎ 𝑢, or they are equally tall, 𝑢 ∼ℎ 𝑣. one such operation, provided that is transitive, ensures that the function “height of persons”, introduced in the previous section, exists. the corresponding model is now: 𝑀′ = (𝑈, 𝑋, ≽, 𝑚) , (6) where ℎ, height, has been replaced by the more general symbol 𝑚, “measure”, and the subscript ℎ has been dropped accordingly. then, the corresponding representation theorem reads: 𝑢 ≽ 𝑣 ↔ 𝑚(𝑢) ≥ 𝑚(𝑣) . (7) yet, although the existence of the function ℎ is mathematically ensured by the properties of the empirical relation ≽ℎ, its actual experimental determination requires a measurement process, which is to be modelled now. 2.3. modelling the measurement process for modelling the measurement process, the approach proposed in reference [16] is here followed and only very briefly recalled. it is suggested that measurement can be parsed in two phases, called observation and restitution. in the observation phase the “object”2 carrying the property to be measured interacts with the measurement system, in such a way that an observable output, called instrument indication, is produced, based on which a measurement value can be assigned to the measurand. the successive phase, where the result is produced, based on the instrument indication and accounting for calibration results (calibration curve), is here called restitution. this approach is essentially in agreement with others recently proposed in the literature [17]-[20]. for example, in the case of persons’ height, the measuring device may consist of a platform, on which the subject to be measured must stand erect, and of an ultrasonic sensor, placed at a fixed height over the head of the subject. the instrument generates a signal whose intensity, 𝑦, is proportional to the distance of the sensor from the top of the head of the subject, which constitutes the instrument indication. let us call 𝜑 the function that describes this phase: thus, if 𝑎 is the object to be measured, 𝑦 = 𝜑(𝑎). (8) calibration requires the pre-constitution of a reference (measurement) scale, 𝑅 = {(𝑠1, 𝑥1), (𝑠2, 𝑥2), … , (𝑠𝑛 , 𝑥𝑛)}, which includes a set of standards, 𝑆 = {𝑠1, 𝑠2, … , 𝑠𝑛 }, and their corresponding measurement values, 𝑋 = {𝑥1, 𝑥2, … , 𝑥𝑛 }. calibration can be done by inputting such standards to the measuring system and recording their corresponding indications, thus forming the function 𝜑𝑠 = {(𝑠1, 𝑦1), (𝑠2, 𝑦2), … , (𝑠𝑛 , 𝑦𝑛 )}, which is a subset of 𝜑, defined above. based on this information, it is possible to obtain a calibration function, 𝑓 = like a workpiece, or an event, such as a sound or a shock, or even a person, in the case of psychometrics [13]. acta imeko | www.imeko.org june 2023 | volume a | number b | 5 {(𝑥1, 𝑦1), (𝑥2, 𝑦2), … , (𝑥𝑛 , 𝑦𝑛 )}, that establishes a correspondence between the value of each standard and the corresponding output of the measuring device. calibration allows us to perform measurement, since, once the instrument indication has been obtained, it is possible to assign to the measurand, in the restitution phase, the value of the standard that would have produced the same output, that is �̂� = 𝑓 −1(𝑦) ≜ 𝑔(𝑦). (9) lastly, we obtain a description of the overall measurement process, by combining observation and restitution: �̂� = 𝛾(𝑎) = 𝑔(𝜑(𝑎)) = 𝑓 −1(𝜑(𝑎)). (10) this equation constitutes a basic deterministic model of the measurement process. in [20], a more detailed model was presented, where the generation of instrument indication was more deeply investigated. yet the structure of that model is compatible with the one just recalled, that will be used in the following, for the sake of simplicity. let us now show how this model can be turned into probabilistic by just shifting from a deterministic to a probabilistic semantic, which is the main goal of this communication. prior to doing so, the present approach to considering probability theory as a logic must be presented, with a special focus on the notions of probabilistic function, with associated operations of inversion and composition, which are necessary for treating equation (10). 3. probability as a logic for models formulated through first-order languages 3.1. probabilistic semantic let us consider models formulated in a first order language, 𝑳, that is a language whose elementary propositions concern properties of, or relations among, individuals, whilst more complex ones can be formed by combining the elementary ones through logical operators, such as conjunction, ∧, disjunction, ∨, or negation, ¬ [21]. such a language is rich enough for our purposes, as it will appear in the following. once a statement is made, it is of interest to assess its truth or falsity. this is the object of semantic, and the basis of a deterministic semantic, for statements of our interest, has already been presented in previous section 2.1. the purpose of the proposed theory is to replace a deterministic semantic with a probabilistic one [22]. as we have seen, a deterministic model, 𝑀𝑑 , may be expressed by a structure, 𝐻 = (𝐶, 𝑅), where 𝐶 = 𝐴1 × 𝐴2 … × 𝐴𝑝 is a cartesian product of sets and 𝑅 = (𝑅1, 𝑅2, … , 𝑅𝑞 ), where each 𝑅𝑖 is a 𝑚𝑖 -ary relation on 𝐶, expressed in the language 𝑳. the truth of a generic statement, 𝜙, concerning 𝑀𝑑 , can be assessed in the following way: • if 𝜙 is an elementary proposition, it is true if for some 𝑅𝑖 ∈ 𝑅, 𝜙 ∈ 𝑅𝑖 , • if instead it is the combination of elementary propositions, through logical operators, it is true if it satisfies the truth condition of the operators combined with the truth state of the elementary propositions involved. a probabilistic model, 𝑀𝑝, instead is constituted by a finite collection of structures, 𝐸 = {𝐻1 , 𝐻2 , … 𝐻𝑀 }, all associated to the same collection of sets, 𝐶, and a probability distribution 𝑃(𝐻𝑖 ) over 𝐸, such that 𝑃(𝐸) = ∑ 𝑃(𝐻𝑖 ) = 1 𝑀 𝑖=1 . (11) such structures constitute a set of possible realisations of the same basic underlying structure and are sometimes suggestively called “possible worlds”. then, the probability of any statement 𝜙 associated to 𝑀𝑝 is 𝑃(𝜙) = 𝑃{𝐻 ∈ 𝐸|𝜙} = ∑ 𝑃(𝐻𝑖 ) 𝐻𝑖∈𝐸|𝜙 , (12) where 𝐻 ∈ 𝐸|𝜙 denotes a structure where 𝜙 is true, {𝐻 ∈ 𝐸|𝜙} is the subset of 𝐸 that includes all the structures in which 𝜙 is true and the sought probability is the sum of the probabilities of such structures. to apply this approach to measurement, its application to probabilistic m-ary relations and to probabilistic functions must be investigated, with a special focus on probabilistic inversion. 3.2. probabilistic relations if 𝑅(𝑢1, 𝑢2, … 𝑢𝑚 ) is an m-ary relation and 𝐸 is a finite collection of structures, 𝐻𝑖 (𝐶, 𝑅𝑖 ), where the truth of 𝑅 can be ascertained, we obtain: 𝑃(𝑅(𝑎1, 𝑎2, … 𝑎𝑚 )) = 𝑃{𝐻 ∈ 𝐸|𝑅(𝑎1, 𝑎2, … 𝑎𝑚 )} = ∑ 𝑃(𝐻𝑖 )𝐻𝑖∈𝐸|𝑅(𝑎1,𝑎2,…𝑎𝑚) . (13) probabilistic relations were treated in detail in reference [22] and thus are not pursued further here. 3.3. probabilistic functions considering a function 𝑓: 𝐴 → 𝐵, the associated structure is 𝐻 = (𝐴 × 𝐵, 𝑓), and the generic statement 𝑣 = 𝑓(𝑢) denotes a binary relation on 𝐴 × 𝐵 such that ∀𝑢 ∈ 𝐴, ∃𝑣 ∈ 𝐵(𝑣 = 𝑓(𝑢) ), and ∀𝑢 ∈ 𝐴∀𝑣, 𝑧 ∈ 𝐵(𝑣 = 𝑓(𝑢) ∧ 𝑧 = 𝑓(𝑢) → 𝑣 = 𝑧). let us then consider a finite collection, 𝐸, of such structures and an associated probability distribution on 𝐸. then the probability that the above statement holds true for a pair (𝑎, 𝑏), 𝑎 ∈ 𝐴, 𝑏 ∈ 𝐵, can be calculated by: 𝑃(𝑓(𝑎) = 𝑏) = 𝑃{𝐻 ∈ 𝐸|𝑓(𝑎) = 𝑏} = ∑ 𝑃(𝐻𝑖 )𝐻𝑖|𝑓(𝑎)=𝑏 . (14) 3.4. probabilistic inversion consider now the probabilistic inverse to the function 𝑓 in the previous subsection, i.e., 𝑔: 𝐵 → 𝐴. let us consider first the possibility of calculating directly the probability associated to each value of 𝑔 from the knowledge of the corresponding direct function 𝑓, through the very definition of inverse function, by establishing the following rule: 𝑃(𝑔(𝑏) = 𝑎) ∝ 𝑃{𝐻 ∈ 𝐸|𝑓(𝑎) = 𝑏} = ∑ 𝑃(𝐻𝑖 )𝐻𝑖|𝑓(𝑎)=𝑏 . (15) after imposing the closure condition ∑ 𝑃(𝑔(𝑏) = 𝑢)𝑢∈𝐴 =1, we obtain the rule: 𝑃(𝑔(𝑏) = 𝑎) = ∑ 𝑃(𝐻𝑖)𝐻𝑖|𝑓(𝑎)=𝑏 ∑ 𝑃(𝑓(𝑢)=𝑏)𝑢∈𝐴 . (16) let us briefly discuss the relationship between probabilistic inversion, as here presented, and the bayes-laplace rule. to do that, let now 𝑢 and 𝑣 be two variables that denotes generic elements of 𝐴 and 𝐵, respectively, and let 𝑎 and 𝑏 be two specific elements of 𝐴 and 𝐵, respectively. then we can form the atomic statements 𝜙 = (𝑢 = 𝑎) and 𝜓 = (𝑣 = 𝑏), that means, for example, that, in some circumstance, the element 𝑎 ∈ 𝐴 acta imeko | www.imeko.org june 2023 | volume a | number b | 5 occurred and the element 𝑏 ∈ 𝐵 occurred. then function 𝑓 induces a conditional probability measure on 𝐴 × 𝐵, defined by: 𝑃(𝜓|𝜙) = 𝑃((𝑣 = 𝑏)|(𝑢 = 𝑎) ) = 𝑃(𝑏 = 𝑓(𝑎)). (17) then the (inverse) conditional probability 𝑃(𝜙|𝜓), equals the probability of the inverse function, and can be calculated through the bayes-laplace rule, with a uniform prior, that is 𝑃(𝜙|𝜓) = 𝑃((𝑢 = 𝑎)|(𝑣 = 𝑏)) = 𝑃((𝑣 = 𝑏)|(𝑢 = 𝑎) ) 𝑃((𝑣 = 𝑏)) = 𝑃(𝑎 = 𝑔(𝑏)) (18) therefore, in this context, the bayes laplace rule can be interpreted as a procedure for calculating the inverse of a probabilistic function. consequently, its use in measurement can be presented just as a step in measurement modelling, as it will be shown in the next section, without taking any commitment to bayesian statistics, with its philosophical and epistemological implications [23]. 3.5. composition of probabilistic functions lastly, let 𝑓, 𝑔, and ℎ be three probabilistic functions, 𝑓: 𝐴 → 𝐵, 𝑔: 𝐵 → 𝐶 and ℎ: 𝐴 → 𝐶, where for 𝑢 ∈ 𝐴, ℎ(𝑢) = 𝑔(𝑓(𝑢)). then the probability of statements concerning ℎ can be assessed through the rule: 𝑃(𝑤 = ℎ(𝑢)) = ∑ 𝑃 𝑣∈𝐵 (𝑤 = 𝑔(𝑣))𝑃(𝑣 = 𝑓(𝑢)) (19) where 𝑤 ∈ 𝐶. let us now apply the above rules to the probabilistic modelling of measurement processes. 4. probability as a logic for measurement modelling 4.1. modelling the measurand in section 2.2 a deterministic model was developed, based on equations (6) and (7). such model implies that empirical relations appearing in it are uncertainty-free. if, instead, the intrinsic uncertainty of the measurand, which basically corresponds to the “definitional uncertainty” in the vim, needs considering, such model must be turned into probabilistic. this can be done, by applying equation (13), to equation (7), which ultimately yields 𝑃(𝑢 ≽ 𝑣) = 𝑃(ℎ(𝑢) ≥ ℎ(𝑣)), (20) as proved in reference [22] and similar results can be obtained for all the scales of practical interest. 4.2. modelling the measurement process the overall modelling of the measurement process has been outlined in section 2.3, where it was suggested that the overall measurement process can be described by the measurement function 𝛾: 𝐴 → 𝑋, characterised by equation (10). therefore, a proper structure for the measurement process is 𝑀" = (𝐴 × 𝑌 × 𝑋, 𝜑, 𝑓, 𝛾). (21) yet, this description does not include the modelling of the measurand and does not allow to account for the associated intrinsic or definitional uncertainty, as previously discussed. this is acceptable in practice when such uncertainty is considered 3 see reference [22] for additional details of this representational side of the question. negligible. yet in the general case, models 𝑀′ and 𝑀" must be merged, yielding (for a purely ordinal quantity) the structure: 𝑁 = (𝐴 × 𝑌 × 𝑋, ≽, 𝑚, 𝜑, 𝑓, 𝛾). (22) as anticipated, this overall model can be interpreted either as deterministic of probabilistic, after interpreting the relations, variable and/or functions involved accordingly. recalling the previously presented equations, we obtain for a generic probabilistic statement concerning the measurement function 𝛾, in model 𝑀′: 𝑃(�̂� = 𝛾(𝑎)) = 𝑃 (�̂� = 𝑓 −1(𝜑(𝑎))) = ∑ 𝑃(𝑦=𝑓(𝑥)) ∑ 𝑃(𝑦=𝑓(𝑤)) 𝑤 𝑃(𝑦 = 𝜑(𝑎)) 𝑦 . (23) on the other hand, if we want to account for intrinsic uncertainty also, we should refer to model 𝑁 and consider 𝑚 as a probabilistic function as well. note, in this regard, that the function 𝜑: 𝐴 → 𝑌, only depends (at least ideally) on the way in which the object 𝑎 realises and manifests the quantity, 𝑥, of interest. let us call it 𝑥𝑎 = 𝑚(𝑎). therefore, 𝑦 = 𝜑(𝑎) = 𝑓(𝑚(𝑎)). (24) 4.3. a very simple numerical illustrative example let us finally illustrate the entire procedure by a very simple numerical example, concerning the (purely ordinal) height of three subjects, call them john (𝑎), paul (𝑏) and evelyn (𝑐). suppose john is definitely taller than the other two, so that 𝑃(𝑎 ≻ 𝑏) = 𝑃(𝑎 ≻ 𝑐) = 1.0. let instead paul be almost as tall as evelyn”, with 𝑃(𝑏~𝑐) = 0.6, 𝑃(𝑏 ≻ 𝑐) = 0.1 and 𝑃(𝑐 ≻ 𝑏) = 0.3. then it is easy to check that a proper function 𝑚: {𝑎, 𝑏, 𝑐} → {1,2} will have3: 𝑃(𝑚(𝑎) = 1) = 0.0; 𝑃(𝑚(𝑎) = 2) = 1.0; 𝑃(𝑚(𝑏) = 1) = 0.9; 𝑃(𝑚(𝑏) = 2) = 0.1; 𝑃(𝑚(𝑐) = 1) = 0.7; 𝑃(𝑚(𝑐) = 2) = 0.3. let us now consider the calibration function, 𝑓: 𝑋 → 𝑋 and let 𝑋 = {1,2} and the probability of 𝑓 be such that: 𝑃(𝑓(1) = 1) = 0.8; 𝑃(𝑓(1) = 2) = 0.2; 𝑃(𝑓(2) = 1) = 0.1; 𝑃(𝑓(2) = 2) = 0.9. then the probability of the inverse function 𝑔 is such that: 𝑃(𝑔(1) = 1) = 8/9; 𝑃(𝑔(1) = 2) = 1/9; 𝑃(𝑔(2) = 1) = 2/11; 𝑃(𝑔(2) = 2) = 9/11. the observation function 𝜑, is obtained by composing 𝑓 and 𝑚, according to equation (19), which yields: 𝑃(𝜑(𝑎) = 1) = 0.10; 𝑃(𝜑(𝑎) = 2) = 0.90; 𝑃(𝜑(𝑏) = 1) = 0.73; 𝑃(𝜑(𝑏) = 2) = 0.27; 𝑃(𝜑(𝑐) = 1) = 0.59; 𝑃(𝜑(𝑐) = 2) = 0.41. lastly, the measurement function 𝛾 outcomes from the composition of 𝑔 with 𝜑, yielding: 𝑃(𝛾(𝑎) = 1) = 0.251; 𝑃(𝛾(𝑎) = 2) = 0.749; 𝑃(𝛾(𝑏) = 1) = 0.698; 𝑃(𝛾(𝑏) = 2) = 0.302; 𝑃(𝛾(𝑐) = 1) = 0.599; 𝑃(𝛾(𝑐) = 2) = 0.401. 5. conclusion the problem of the interpretation of probability in measurement has been considered and it was suggested to regard acta imeko | www.imeko.org june 2023 | volume a | number b | 5 probability theory as a logic for developing probabilistic models. a remarkable feature of this approach is that after modelling measurement through the relations holding among the transformations involved, the model can be treated as either deterministic or probabilistic, depending upon the chosen sematic. alternative approaches can be considered, such as the fuzzy logic or the possibility theory [9], [10]. all these approaches have their merits and limitations, and the choice may be done depending upon the assumptions made in the development of the model. the logicistic approach here developed may overcome some reservations about probability theory, related to the limits of the frequentistic and the subjectivistic approaches, and may thus contribute to a wider use of the probabilistic approach. references [1] bipm, iec, ifcc, iso, iupac, iupap, oiml 1993 guide to the expression of uncertainty in measurement (iso, geneva, switzerland) corrected and reprinted, 1995, isbn 92-67-10188 [2] w. bich, m. cox, c. michotte, towards a new gum – an update, metrologia, 53, 2016, s149-s159. doi: 10.1088/0026-1394/53/5/s149 [3] w. bich, from errors to probability density functions. evolution of the concept of measurement uncertainty, ieee trans. instr. meas., 61, 2012, pp. 2153-2159. doi: 10.1109/tim.2012.2193696 [4] i. lira, the gum revision: the bayesian view toward the expression of measurement uncertainty, european journal of physics, 37, 2016, 025803. doi: 10.1088/0143-0807/37/2/025803 [5] d. r. white, in pursuit of a fit-for-purpose uncertainty guide, metrologia, 53, 2016, s107-s124. doi: 10.1088/0026-1394/53/4/s107 [6] a. possolo, a. l. pintar, plurality of type a evaluations of uncertainty, metrologia, 54, 2017, pp. 617-632. doi: 10.1088/1681-7575/aa7e4a [7] a. possolo, c. elster, evaluating the uncertainty of input quantities, in measurement models, metrologia, 51, 2014, pp. 339353. doi: 10.1088/0026-1394/51/3/339 [8] w. f. guthie, h.-k. liu, a. l. rukhin, b. toman, c.-m. jack wang, n.-f. zhang, three statistical paradigms for the assessment and interpretation of measurement uncertainty, in f. pavese. a. b. forbes (eds.) data modelling for metrology and testing in measurement science, boston, birkhauser, 2009. 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[14] a. backlund, the definition of system, kybernetes 29, 2000, pp. 444-451. doi: 10.1108/03684920010322055 [15] g. b. rossi, measurability, measurement, 40, 2007, pp. 545-562. doi: 10.1016/j.measurement.2007.02.003 [16] g. b. rossi, toward an interdisciplinary probabilistic theory of measurement, ieee trans. instrumentation and measurement, 61, 2012, pp. 2097-2106. doi: 10.1109/tim.2012.2197071 [17] k. d. sommer, b. r. l. siebert, systematic approach to the modelling of measurement for uncertainty evaluation, metrologia, 43, 2006, s200-s210. doi: 10.1088/1742-6596/13/1/052 [18] a. giordani, l. mari, a structural model of direct measurement, measurement, 145, 2019, pp. 535-550. doi: 10.1016/j.measurement.2019.05.060 [19] r. z. morawski, an application oriented mathematical meta model of measurement, measurement, 46, 2013, pp. 3753-3765 doi: 10.1016/j.measurement.2013.04.004 [20] g. b. rossi, f. crenna, a formal theory of the measurement system, measurement, 116, 2018, pp. 644-651. doi: 10.1016/j.measurement.2017.10.062 [21] g. rigamonti, corso di logica, torino, boringhieri, 2005. [in italian] [22] g. b. rossi, f. crenna, a first-order probabilistic logic with application to measurement representations, measurement 79, 2016, pp. 251-259. doi: 10.1016/j.measurement.2015.04.024 [23] g. b. rossi, f. crenna, beyond the opposition between the bayesian and the frequentistic views in measurement, measurement, 151, 2020, 107157. doi: 10.1016/j.measurement.2019.107157 https://doi.org/10.1088/0026-1394/53/5/s149 https://doi.org/10.1109/tim.2012.2193696 https://doi.org/10.1088/0143-0807/37/2/025803 http://dx.doi.org/10.1088/0026-1394/53/4/s107 https://doi.org/10.1088/1681-7575/aa7e4a https://doi.org/10.1088/0026-1394/51/3/339 http://dx.doi.org/10.1016/j.measurement.2013.04.006 https://doi.org/10.1108/03684920010322055 https://doi.org/10.1016/j.measurement.2007.02.003 https://doi.org/10.1109/tim.2012.2197071 https://doi.org/10.1088/1742-6596/13/1/052 about:blank about:blank about:blank https://doi.org/10.1016/j.measurement.2015.04.024 https://doi.org/10.1016/j.measurement.2019.107157 a 3d-printed soft orthotic hand actuated with twisted and coiled polymer muscles triggered by electromyography signals acta imeko issn: 2221-870x september 2022, volume 11, number 3, 1 8 acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 1 a 3d-printed soft orthotic hand actuated with twisted and coiled polymer muscles triggered by electromyography signals irfan zobayed1,2, drew miles1, yonas tadesse1,2,3,4 1 humanoid, biorobotics, and smart systems (hbs) lab, mechanical engineering department, the university of texas at dallas 2 biomedical engineering department, the university of texas at dallas 3 electrical and computer engineering department, the university of texas at dallas 4 alan g. macdiarmid nanotech institute, the university of texas at dallas section: research paper keywords: actuators; soft robotics; emg; rehabilitation measurement instrument; powered hand orthosis citation: irfan zobayed, drew miles, yonas tadesse, a 3d-printed soft orthotic hand actuated with twisted and coiled polymer muscles triggered by electromyography signals, acta imeko, vol. 11, no. 3, article 9, september 2022, identifier: imeko-acta-11 (2022)-03-09 section editor: zafar taqvi, usa received march 15, 2022; in final form september 23, 2022; published september 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: yonas tadesse, e-mail: yonas.tadesse@utdallas.edu 1. introduction one of the major groups of patients who experience an overall decrease in hand function and ability are those who suffer from ischemic stroke, which makes up for a majority of all strokes suffered worldwide. in fact, the number of strokes that occur annually worldwide is approximately 17 million, justifying why it is one of the most common causes of death in many countries, often trailing coronary artery disease [1]. specifically in the united states, strokes are the fifth leading cause of death, yielding to more than 125,000 casualties annually as well as leaving long-lasting effects for nearly one million more patients [2]. alongside the decreased physical ability of patients, strokes also hold a strong economic toll as well, due to health care services, medications, loss of productivity, and required rehabilitation, which can not only bring economic issues to patients but cost the economy billions of dollars [1]. worldwide, approximately 50 to 60 million people suffer from some form of hand impairment after suffering not only a stroke but also spinal cord injury (sci) and skeletal muscle atrophy. this often results in a loss of independence, leading to a decrease in the quality of activities in daily life for such patients [3]. the prevalence of upper-extremity neuromotor impairments because of ischemic strokes and sci is statistically inevitable – when combined with the economic challenges that come when attempting to manage a stroke, there is an overwhelming issue in access to affordable treatment to treat any related impairments. performing rehabilitation on a patient as soon as possible after suffering a stroke gives the best chance of regaining any lost hand functionality and dexterity back to the patient [4]. however, traditional methods of rehabilitation are not only expensive but abstract various wearable robotic hands, prosthetic hands, and orthotic exoskeletons developed in the last decade aim to rehabilitate patients whose daily quality of life is affected from hand impairments – however, a majority of these devices are controlled by bulky, expensive, noisy, and uncomfortable actuators. twisted and coiled polymer (tcp) muscles are novel smart actuators that address these key drawbacks. they have been utilized in soft robotics, hand orthosis exoskeletons, and powered hand orthotic devices; they are also light weight, high-performance, and inexpensive to manufacture. previously, tcp muscles have been controlled via power supplies with mechanical switches that are not portable, hence making it unfeasible for long term applications. in this work, a portable control system for tcp muscles via electromyography (emg) signals that are captured through electrodes placed on the arm of the user and processed through a channel of electrical components to actuate 4-ply tcp muscles, which is demonstrated on a 3d-printed soft orthotic hand. with portable emg control, orthotic devices can become more independently accessible to the user, making these devices novel instruments for measuring, aiding, and expediting the progress of hand impairment rehabilitation. acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 2 also not possible to perform since patients need to be fully conscious and at a minimum state of health post-stroke or injury, which is very uncommon in most cases. to tackle the expenses and lack of accessibility that comes with rehabilitation for hand impairments, medical robotics have been a refreshing addition to the few options patients have when selecting their path to recovery post-stroke. specifically, the introduction of powered hand orthotic exoskeletons has made great strides in easing the difficulty that defames neuromotor rehab from both an engineering and clinical perspective. the quality and efficiency of the current common rehabilitation process improvement as well as expediting the overall process that helps retain maximal motor function for patients within the critical period post-stroke. design parameters for hand exoskeleton devices are imperative to the user experience as well, including a focus on key mechanical aspects that relate to hand anatomy, user comfort, effective force transmission via an actuator, and affordability for the common consumer [5]. the critical combination of such parameters within an exoskeleton device must not only aim to meet user demands but also needs to meet the demands of the industry. one of the current industry leaders in assistive hand orthotic devices is the myopro device that was developed by myomo inc. – electromyography, better known as emg, signals are captured by electrodes upon muscle contractions and filtered through their patented software that is onboard the device itself. the filtered signal then triggers the actuators, which are motors, within the device [6]. unique due to its combination of portability, self-control mechanism by processing emg signals, and minimalist look, the myopro device exhibits the key combination of design parameters within an exoskeleton device. although the device can exhibit the dexterity and function of a human hand, the cost to purchase the device for a consumer will yield nearly $80,000 after insurance, which is unaffordable for a majority of patients who have hand motor impairments. another industry-leading alternative assistive hand orthotic device is the nasa roboglove, a humanoid robotic that was designed to perform and enhance human-scale work, which is an alternative approach to the perspective of rehabilitative applications for stroke patients [7]. the device was developed in a partnership between general motors and nasa in an attempt to improve and assist healthy humans and their hand mechanical capabilities in applications that may require it, such as performing highintensity work that requires strength in the international space station (iss) or other zero-gravity environments. although innovative in its design and application intentions, the roboglove is not available to average consumers for purchase, further justifying the need for the accessibility and affordability of hand orthotic devices. as a result of both the need for affordable soft robotic stroke rehabilitation alternatives and research for accessible optimized combinations of critical parameters for building exoskeletons, there have been multiple attempts to build these devices. over recent years, there have been many attempts at developing reliable hand orthotic devices, including 165 different separate iterations of dynamic hand orthosis that have been studied and documented by bos et al. by sifting through 296 different sources of literature, with 109 of the devices developed since 2011. bos found that a majority of the dynamic hand orthosis is currently powered by dc motors and that devices are powered with traditional power supplies and trigger actuation via manual transmission, force, pressure, pre-tension loads, and in some advanced cases, emg signals [8]. several hand orthotic devices are able to perform some of the key functionalities that are presented by the myopro and roboglove. these hands are much more affordable but are not commercially available. developed by butzer et al. is the relab tenoexo, which can actuate on spring pre-loaded tension mechanisms via a dc motor that is triggered by processed emg signals [3]. a similar powered hand orthotic device is also actuated by dc motors that are triggered via emg signals, which was developed by park et al. out of columbia [9]. another implementation of a hand orthotic device that was partially 3-d printed with pla filament was developed by yoo et al., which is also actuated by a dc motor and significantly cheaper than other similar devices. an overwhelming majority of the devices mentioned as well as other similar devices utilize some form of a traditional or bulky actuator or emg triggering for the actuation of the device [3], [8], [10]-[14]. traditional actuators, although affordable, are bulky, noisy, and not user-friendly. a cost-effective, high performance, and lightweight soft artificial actuator called twisted and coiled polymer (tcp) muscles was developed by haines et al. 2014, who founded a new class of smart actuators that have high power to weight ratios and also alleviate many of the issues that are founded with traditional actuators [15]. tcp muscles are fabricated from precursor fiber conductive multifilament silver-coated nylon 6,6 sewing thread and can exhibit linear strokes as high as 50 % and is easy to both fabricate and manufacture – the first-ever soft robotic powered hand orthotic device to utilize this new class of smart actuators was the igrab, which is a 3d printed hand orthotic exoskeleton, developed by saharan et al., that is actuated via 2-ply tcp muscles [16]. like tcp muscles, other types of smart actuators that also exhibit some of the similar key thermomechanical and electrical properties, which include shape memory alloy (sma) and fishing line muscles. both are suitable for various applications given their unique mechanical properties. smas are a promising alternative to traditional actuators due to their shape retention abilities when actuating [17]. furthermore, their muscle-like properties and various methods of cooling them for faster movement prove their relevance in robotic applications [18]. fishing line muscles (tcpfl) are similar to silver-coated tcp (tcpag) muscles in terms of fabrication and manufacturing, but they are developed from a monofilament nylon 6 fishing line rather than a thread, allowing them to produce higher actuation in particular cases, which have been demonstrated in underwater soft robotics [19]. the new class of smart actuators founded by haines et al. exhibit excellent thermomechanical properties in various soft robotic applications and address most of the key issues that are found within current hand orthotic exoskeleton devices, including affordability and accessibility, as well as user experience, bulky designs, and noisy applications. however, actuation of these muscles was performed manually via a traditional power supply – hence, there is no portable or userfocused method of triggering actuation for them, specifically tcp muscles. as depicted in earlier works related to hand orthotic devices, emg triggered actuation of dc motors were evaluated and applied to devices, yet no such control exists for tcp muscles. traditional emg signal actuation methods utilize advanced signal processing and pattern recognition as discussed by jafarzadeh et al. [20], who introduces a novel method of emg signal processing via deep learning with convolutional neural networks (cnn) for more accurate signal processing. a derivation of emg signal processing from the cnn can be acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 3 implemented for accurate emg results to trigger tcp muscle actuation in a soft robotic application. in the following study, a novel method of portable tcp muscle control will be introduced, specifically triggering the actuation of these novel smart actuators via processed emg signals on a 3d printed soft robotic hand and gathering data to evaluate the performance of the system. 2. methods & materials the following section will thoroughly address the proper methods that were taken place to perform the experimentation and implementation of emg control of novel artificial tcp muscles concerning the control of a soft robotic, hand orthotic exoskeleton as a complete system as depicted in figure 1 above. 2.1. tcp muscle manufacturing twisted and coiled polymer (tcp) muscles are made of precursor fiber conductive multifilament silver-coated nylon 6,6 sewing thread – they can be made as 2-ply, 4-ply, or 6-ply strands, which are each depicted in figure 2b. the thread can be purchased commercially from shieldex trading inc. in both small cones and large spools. thicker tcp muscles will require a larger current to actuate as resistance has a direct correlation with length and ply on the muscle [21]. the fabrication of the tcp muscles is prepared in a similar way as described in saharan et al. [17]. briefly, a 170 cm long, 200 µm diameter silver-coated nylon fiber strand is cut from the cone or spool – a 2-ply tcp muscle needs 1 strand of fiber; 2 and 3 strands are needed for 4-ply and 6-ply, respectively. the strand(s) will be connected at each end (2-ply muscles strands are not connected since they are a single thread), which will then have one end attached to a 450 rpm motor and the other end attached to a pre-set load to undergo twist insertion method as seen in figure 2a. after the fiber strands begin to coil, plying is performed by folding the coiled muscle in half, which will twist in the opposite direction upon release and will then be crimped with a size 6 or 8 gold-plated ring terminal to maintain its shape and tension. the pre-set loads are 175 g, 350 g, and 525 g for 2ply, 4-ply, and 6-ply muscles, respectively. for the twisted and plied fibers to actuate properly, electrothermal annealing and training of the tcp muscles are required – each muscle underwent pulse actuation cycles while holding pre-set loads that are slightly heavier than the loads for fabrication – 300 g, 600 g, 900 g are used for 2-ply, 4-ply, and 6ply, respectively. the exact cycle type, electrical specifications, duration, and repetition that each muscle undergoes are explicitly figure 1. (a) simplified emg signal algorithm block diagram (blue arrows) and (b) hardware diagram (orange arrows) for proposed method to control orthotic hands, such as the igrab hand orthotic device by lokesh et al. [16], via raw emg signals. table 1. tcp training cycle specifications. figure 2. (a) fabrication set-up for tcp muscles; fibers spun on dc motor at 450 rpm with pre-tension load at the bottom, (b) examples of 2-ply, 4-ply, 6ply tcp at 10x scale and (c) characterization setup for tcp muscles; the power supply sends current values from steps 3 and 4 of table 1; ni daq device derives temperature from k-type thermocouples, displacement, current, and voltage to calculate the actuation strain and power consumption. acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 4 listed in table 1. tcp muscles elongate during this process due to the strain placed on the muscle to retain its mechanical properties; the two 4-ply tcp muscles used for this study are ~21.0 cm long post-trained. to characterize the tcp muscles that are manufactured through the fabrication, training, and annealing phases, the characterization set up, as depicted in figure 2c, gathers key thermomechanical properties of each muscle placed within the set-up. a single trained tcp muscle is secured onto the set-up platform and has a pre-tension weight added by a defined load cell (300 g, 600 g, and 900 g for 2-ply, 4-ply, and 6-ply muscles respectively). a benchtop power supply then applies a constant current into the muscle, which provides a variable voltage due to the varying resistance of each muscle, hence actuating the tcp muscle. a laser sensor is mounted and secured onto the set up to calculate the displacement of the muscle in real-time when it is actuated. two thermocouples (omega, k-type 36 awg) are attached at two different points on the muscle to follow thermal changes throughout the actuation and cooling process of the muscle. all data from the thermocouples, laser sensor, voltage, and current are captured by a national instruments (ni) rio data acquisition (daq) device, which consisted of an analog output module (ni-9263), differential analog input module (ni9219), voltage input module (ni-9201), and another singlechannel analog input module (ni-9221). the data is then processed into a ni labview application and outputted into a spreadsheet, whose data is plotted and visualized by an automated matlab script for muscle characterization. the data points that are assessed are actuation strain, temperature, current, voltage, and power of each tcp that is manufactured. actuation results for 4-ply and 6-ply are shown in figure 3. all fabrication, training, annealing, and characterization of tcp muscles were performed in the humanoid, biorobitcs, and smart systems (hbs) laboratory at the mechanical engineering department at the university of texas at dallas. for the following article, two fully trained and characterized 4-ply tcp muscles were utilized. 2.2. emg signal algorithm the algorithm that is depicted in figure 1a captures raw emg signals from the electrodes, then filters excess noise and amplifies the raw filtered signal. the input signal then passes through threshold parameters from the electrodes that must be satisfied – these values must be within a certain period of time based on actuation frequencies and between 2 to 3.5 mv for a single finger or 3 to 4.5 mv for two fingers. once the signal satisfies the threshold, a scaled output signal will be generated to trigger the controller board, which will communicate to the driver board to send a voltage signal to actuate the 4-ply tcp muscles on the robotic hand. 2.3. emg control setup the emg control set up, depicted in figure 4, is composed of four main components: the (1) signal acquisition board, (2) power driver control board, (3) sensor data board, and (4) the nvidia jetson tx2 module controller. for testing purposes, not all board components were connected. the signal acquisition board, seen in figure 4d has a total of eight inputs for a maximum of eight electrode pairs and one reference electrode – each electrode pair monitors a different muscle or grouping of muscles. these electrode pairs take raw emg signals from the subject and send them into the myoware muscle sensor (pn# sen-13723) seen in figure 4b and figure 5a, which will collect the raw emg signals and input the signal into the board. the myoware sensor acts in between the electrodes and signal acquisition board. the power control board, seen in figure 4e, regulates the voltage and current that is inputted from the battery to power the entire system safely as well as output the voltage to actuate the muscles. the board has a voltage rating of around 20v. the operational amplifiers and dac chips can output a maximum of 5.7v from each terminal. the board can be fully powered by an operational voltage of only 6v to output an accumulated 45.6v through all eight terminals. the board has a negligible current rating relative to the current it uses. for experimental purposes, a benchtop power supply was utilized in this study. the sensor data board, seen in figure 4c, has a total of eight analog inputs to input sensor voltage signals for multiple purposes, including data and performance analytics as well as the implementation of future feedback control systems. all three boards were designed in eagle and manufactured by a local pcb vendor. each board is directly connected to the nvidia jetson tx2, seen in figure 4a, which acts as the controller and processor for the input data from the sensor data board and power board. the controller was selected for its ability to filter copious quantities of data within the emg processing algorithm for multiple fingers and output proper commands to the power control board to output the necessary voltage and current to move the hand orthotic device from its pre-trained position to the desired position seen in figure 5e and in figure 5f. figure 3. example characterization data of a 16.7 cm 4-ply and a 17.1 cm 6-ply muscle at 600g for 50 seconds. the current is not depicted since it is a constant input of 1.5 a and 2.1 a for 4-ply and 6-ply, respectively. (a) the strain peaks at 5.5 % for 4-ply and 6.4 % for 6-ply; (b) the voltage peaks at 8.2 v for 4-ply and 8.0 v for 6-ply; (c) the power used peaks at 12.8 w for 4-ply and 16.7 w for 6-ply; (d) the temperature peaks at 129 0c for 4-ply and 140 0c for 6-ply. the number of plies has a direct relationship to the maximum strain a muscle can exhibit. acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 5 2.4. orthotic hand depicted in figure 4f, the 3-d printed orthotic hand is a prototype developed in-house and was set up as a testing apparatus for experimental purposes. the material of the 3-d printed hand orthotic device is made of thermoplastic polyurethane (tpu) material and is actuated by two 21 cm posttrained 4-ply tcp muscles, which are each connected to the index finger and ring finger of the orthotic hand by commercial fishing line that will mimic the anatomical functionality of a tendon in the human hand. figure 4. the components that power the emg triggered actuation system consists of the (a) nvidia jetson tx2, (b) myoware muscle sensor, (c) sensor data board, (d) signal acquisition board, (e) power control board, and (f) a 3-d printed orthotic hand, which is actuated by 4-ply tcp muscles connected by a commercial fishing line tendon to each the index and ring finger. figure 5. (a) electrode placement used for emg controlled actuation in relation to key anatomical muscle structure of the anterior left forearm for clear signal reception; (b) an open hand, which is the rest position for the user, will not actuate the hand and keep the (c) device in its pre-trained rest position; (d) a curled of closed hand by the user will trigger a signal to actuate the tcp muscles, which (e) actuates the ring finger or (f) both the ring and index finger. acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 6 2.5. test subject to trigger actuation of the hand orthotic device via emg signals, a verified subject was required, hence an institutional review board or irb approval was applied for and granted. electrodes from the emg control setup were placed on the anterior surface of the subjects’ arm at the base of the forearm, distal to the elbow joint with a reference on the medial bony protrusion of the elbow. the muscle of primary interest is the flexor digitorum profundus, shown in figure 5a. 3. results the following section depicts the results of the experimentation conducted for the actuation of a single finger and dual finger setup, as well as a brief overview of the muscle performance and potential areas of experimental improvement for the data seen in figure 6. 3.1. single finger actuation depicted in figure 6a is the acquisition of the filtered emg signal, denoted by the red-triangle-line for a single finger set-up. initially reading a voltage of 1.72 mv from the electrode while the user was in rest position (seen in figure 5b), the user then closed their hand (seen in figure 5d), yielding a spike in voltage to 3.42 mv for a duration of 0.35 seconds and causing the orthotic hand to also close before returning to a steady voltage around 1.72 mv in rest position. the full flexion yielded from the actuation strain is depicted by the index finger seen on the orthotic hand in figure 5e. the output voltage from the power control board to the muscle was ~5.8 v, which was expected given the electrical specifications of the board and verified with a multimeter. in figure 6b, the acquisition of the single finger unfiltered emg signal that was processed by the myoware muscle sensor is depicted, showing a similar trend to that of the filtered signal. 3.2. dual finger actuation portrayed in figure 6a is the acquisition of the filtered emg signal for a double finger set-up, denoted by the blue-circle-line. similar to the single finger, the initial voltage when the user's hand was at rest was at 1.78 mv before the user closed their hand, which then yielded a spike in the filtered voltage to 3.42 mv for a duration of 0.35 seconds before returning to a steady voltage around 1.74 mv. the full flexion of the two fingers, which were observed on the index finger and ring finger simultaneously, is depicted in figure 5f, which were both connected via the same set-up seen in the single finger set-up for both fingers. in figure 6b, the acquisition of the dual finger unfiltered emg signal that was processed by the myoware muscle sensor is depicted, showing a similar trend to that of the filtered signal. 3.3. signal comparison when plotted side by side, the filtered emg signals that were acquired from the electrode on the user, which are seen in figure 6a, depict similar shapes and trends – the same can be said for the side-by-side comparison of the unfiltered signals depicted in figure 6b. when comparing the statistical data discussed earlier in this section, there are similar power sources of voltage from the electrode, which should be expected given that the same hand gestures were performed. this verifies that the emg input is acquired correctly by the signal acquisition board and converted properly by the microprocessor because the output voltage from the power control board was able to reach peak voltage for a maximum actuation strain and flexion. it is important to note that during the rest phases (before the initial spike of the voltage at 0.4s and 0.35s for single and dual finger set-ups, respectfully) that for the two different set-ups, it is evident that the emg voltage of the dual finger set-up has a very slight variance when compared to that of the single finger set-up – this is because there are two output channels opened from the power control board for the actuation of the orthotic hand. as more channels are opened, the filtering of the acquisition signal from the electrodes becomes slightly weaker, causing the variation seen for the dual finger actuation. although there is the presence of a slight variance, it is not enough to determine a significant impact on the acquisition and filtering of the signals. furthermore, the signals acquired are solely that of the subject approved via the irb process and no one else, as emg signals can vary amongst each individual, commonly ranging between 1 mv to 10 mv, which is comfortably within the range of acquisition seen in the study [22]. additional subject tests will optimize the current acquisition and filtration algorithm. 3.4. response time the single finger actuation began 0.4 seconds from the initial start of the emg signal capture whereas the dual finger actuation began 0.05 seconds earlier at 0.35 seconds. although the figure 6. the emg acquisition signals depicted are a result of the raw emg signals captured by the three electrodes on the myoware muscle sensor, which were filtered and adjusted by the controller; (a) filtered emg signals and (b) unfiltered emg signals. acta imeko | www.imeko.org september 2022 | volume 11 | number 3 | 7 response time is faster as the number of open channels increase, studies with more actuated fingers should be performed to validate this relationship. further improvements to the current raw signal capture methods can also improve response time. however, the response time of the tcp muscle is slower due to the time it needs to cool, which is a typical behaviour of thermal muscles (see figure 3a). 3.5. performance the muscles perform with a delayed actuation producing an assistive push to the orthotic hand, approximately 0.4 seconds for the single finger actuation and 0.35 seconds for the dual finger actuation. through coupling with muscles on the posterior side of the arm, movement will become slower, smoother, and have a higher degree of control. drawing from the conclusions seen in previous studies [17], the utilization of 2-ply muscles instead of 4-ply would increase the speed of actuation with lesser power consumption but might not be strong enough to further develop into a compact device. likewise, 6-ply muscles would have the necessary strength (as seen by the example muscles in figure 3) to actuate but will require greater than the 5.7 volts provided in the current experiment. furthermore, testing more channels might allow for further insight into variations of the emg signal acquisition, which can then be addressed by updating the current processing and filtering algorithm. 4. discussion tcp muscles are a quality candidate for an effective low-cost orthotic device. the muscles used in this device were of short length without a pulley system, hence resulting in a much shorter strain per actuation with a higher degree of precision and control. if the individual muscles were combined with many others similar to that of human muscles, smoother and more powerful motion could be obtained. the performance of these muscles is also affected by the power limitations of the board. while tcp muscles perform optimally at around 6-13 v depending on the specific ply, the highest deliverable voltage with the current setup design is approximately 5.7 v [20]. sma muscles can perform well at the provided voltage but increase the production cost greatly and potential hysteresis during the cooling process can also slow down the efficiency of actuation. fishing line muscles are another powerful soft artificial muscle that actuates based on temperature changes. the heating and cooling of these muscles can be achieved through water or fans depending on the size and containment of the system [17], [23]. however, the amount of actuation currently produced is not inherently undesirable for an assistive motion and the following study supports that this motion can be increased temporarily for the rehabilitation phase of product use. an important distinction is the degree and type of care needed for rehabilitation versus assistance in daily activities. robotic devices have been used in research studies for motor function rehabilitation quite extensively in the past twenty years. these training periods typically last around one hour and are conducted with at least one day between each session [24]. assistive devices are designed to be worn during all hours of daily life to assist with essential tasks and functions. once a person’s functionality is restored to their individual peak, an assistive device works to fill the gaps between their current functionality and their preincident functionality [25], which can potentially become more accessible to patients as more iterations of portable and soft orthotic hand devices are investigated. the performance of this device in its current form has not yet been determined through standard human subject tests. this is appropriate as the initial tests performed verified the efficacy and need for further testing through credited existing tests. the fugl meyer test is a classic functionality test that examines the degree of functionality that a person possesses in their upper extremities [26]. when performed with and without an assistive device, the respective scores can be compared to assess the effectiveness of the device. additional tests such as the action research arm test (arat), wolf motor test, and box and block test can provide a more holistic characterization of soft robotic assistive device performance [27]-[29]. emg controlled actuation, through soft or hard actuators, requires precise processing and filtering of the emg signal in real-time. the methods required for such processing depend on the system used for emg acquisition, the muscles being monitored, and the sensor used. the emg acquisition of the flexor digitorum profundus results in desirable levels of control for hand orthotic devices [30]. this could be improved through the addition of a complementary sensor placed on the posterior surface of the forearm in the same position as the original sensor. likewise, the addition of further processing techniques can serve as an alternative to additional electrode placement [31]. such research can be performed and further investigated from the given study, potentially enabling the development of more novel solutions that are tailored to both affordability and accessibility of the device for patients as well as thorough user experience. 5. conclusion loss of hand movement or functionality is a widespread problem in the world today. the orthotic hand discussed would restore functionality to the user at a fraction of the cost of currently available systems. furthermore, it can be utilized a measurement tool for rehabilitative progress for patients who are trying to regain functionality in their hand. with human subject trials, the device can be further improved for better portability and convenience. additional testing will also yield more emg data, allowing for more precise control of the device. integrating existing gesture-based emg databases with our current algorithms will allow for complex mechanical movements of the orthotic device [32]. this same technology can apply not only well to low-cost prosthetics and rehabilitative robotics for trans-ulnar amputees, but also as assistive devices for the general use of able-bodied individuals [33]. with modifications, this technology could have useful applications for restoring functionality in those with parkinson’s disease or who suffer from foot drops. the orthotic device discussed 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https://doi.org/10.1056/nejmoa0911341 https://doi.org/10.1177/2055668316670553 http://dx.doi.org/10.1016/s0004-9514(08)70034-5 https://doi.org/10.1191/0269215505cr832oa https://doi.org/10.1109/icorr.2005.1501122 https://jneuroengrehab.biomedcentral.com/articles/10.1186/s12984-019-0633-6 https://jneuroengrehab.biomedcentral.com/articles/10.1186/s12984-019-0633-6 https://www.nature.com/articles/s41597-019-0285-1 remote video monitoring for offshore sea farms: reliability and availability evaluation and image quality assessment via laboratory tests acta imeko issn: 2221-870x december 2021, volume 10, number 4, 17 24 acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 17 remote video monitoring for offshore sea farms: reliability and availability evaluation and image quality assessment via laboratory tests david baldo1, gabriele di renzone1, ada fort1, marco mugnaini1, giacomo peruzzi1, alessandro pozzebon2, valerio vignoli1 1 department of information engineering and mathematics, university of siena, via roma 56, 53100 siena, italy 2 department of information engineering, university of padova, via gradenigo 6/b, 35131 padova, italy section: research paper keywords: video monitoring; offshore; reliability; image quality; climatic chamber citation: david baldo, gabriele di renzone, ada fort, marco mugnaini, giacomo peruzzi, alessandro pozzebon, valerio vignoli, remote video monitoring for offshore sea farms: reliability and availability evaluation and image quality assessment via laboratory tests, acta imeko, vol. 10, no. 4, article 7, december 2021, identifier: imeko-acta-10 (2021)-04-07 section editor: silvio del pizzo, university of naples 'parhenope', italy received june 1, 2021; in final form december 6, 2021; published december 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this research was funded by regione toscana, project seafactory, por fesr 2014–2020 bando n.2: progetti strategici di ricerca e sviluppo delle mpmi. corresponding author: alessandro pozzebon, e-mail: alessandro.pozzebon@unipd.it 1. introduction fish farming has undergone a massive growth for years now, mainly owing to various causes. primarily, bred seafood could take part within the fight against world hunger without entailing an increase in costs, thus proving to be a cheap and valuable alternative for global food supply. furthermore, bred seafood quality may be easily certified since the complete fish lifetime can be promptly traced throughout the breeding cycle. the upcoming affluence of this sector is also validated by some studies: a prediction on worldwide fishing market in 2030 [1] foresees that 62 % of fish for human consumption will be produced via aquaculture by that year. in addition, the food and agriculture organization (fao) foretold a hopeful situation presuming an aquaculture production expansion up to 58% by 2022 [2]. concerning europe, a future discord among fish demand and supply was guessed [3], therefore fostering fish farming may turn to be an advisable initiative. finally, in italy aquaculture includes more than 800 companies, and most of them are situated in the mediterranean sea, where more than 5000 plants are located. abstract in this article, the availability and reliability of a remote video monitoring system for offshore sea farming plants are studied and tested in laboratory. the scope of the system is to ensure a video surveillance infrastructure so to supervise breeding cages along with the fish inside them, in order to contrast undesired phenomena like fish poaching as well as cages damages. the system is installed on a cage floating structure: it is mainly composed of an ip camera that is controlled by a raspberry pi zero which is the core of the system. images are streamed thanks to a 3g/4g dongle, while the overall system is powered via two photovoltaic panels charging a backup battery. simulations are carried out considering two seasonal functioning periods (i.e., winter and summer): each of them is characterised by temperature trends defined according to the average temperatures of the system deployment site, 8 km offshore the city of piombino, italy. in order to optimise power consumption without hindering application scenario requirements, the system operates according to a duty cycle of 2 minutes out of 15 (i.e., 8 minutes of operation per hour). the performances of the system are then tested in laboratory exploiting a climatic chamber so to simulate different environmental conditions: variations on image quality are then analysed in order to identify possible dependencies on critical situations related to specific temperature and relative humidity values and to the presence of salt in the air. mailto:alessandro.pozzebon@unipd.it acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 18 offshore sea farms need to rely on surveillance systems so to contrast undesired phenomena like fish poaching as well as breeding cages damages. therefore, in this paper an autonomous remote video monitoring system for offshore sea farms is presented along with simulations and laboratory tests whose outcomes are exploited to study its availability and reliability along with assessing performance variations and its overall effectiveness. the system is designed so to include off-the-shelf components, and in order to be energy efficient since it is powered via an energy harvesting system (i.e., two photovoltaic panels and a backup battery). eventually, the system is installed on a cage floating structure. since environmental conditions may severely affect the reliability of the system in terms of quality of the acquired images, tests are performed exploiting a climatic chamber, thus allowing to simulate different environmental conditions in terms of temperature and humidity. system behaviour in extremely high and low temperatures as well as high humidity levels was tested, analysing the possible degradation in images: in particular, the environmental parameters for the various tests were identified considering the meteorological conditions of the final deployment site, as well as the ones of any general marine site in the temperate climate zone. this paper is an extension of [4] and it is drawn up as follows. some related works are reported in section 2, while section 3 shows the video monitoring system architecture. the reliability configurations on which simulations are carried out are outlined in section 4, and simulations results are presented in section 5. section 6 is devoted to the description of the laboratory tests setup, while in section 7 the tests results are presented and discussed. eventually, section 8 points out remarks and conclusions. 2. related works autonomous systems for the monitoring of fish behaviour within offshore sea farms during feeding phases was reviewed in [5]: amid sundry enabling technologies and processing techniques, the use of video recordings thorough ad-hoc systems and cameras were pointed out, thus showing their feasibility. video monitoring systems that are deployed in marine contexts are mainly designed for coastal safeguard so to assess erosion [6]-[9] rather than for sea farms surveillance. such systems make use either of standard ip cameras [6], [7], like the one within the system presented in this paper, or of embedded cameras directly controlled by a single-board computer as raspberry pi [8], which is the control unit of the system that will be presented in the following section. similarly, remote video monitoring systems may be also exploited to assess post-storm recoveries within beaches. to this end, in [9] such a system is developed and tested in australia. therein, a video monitoring framework was employed for the assessment of erosion levels of sandy coastlines that experienced severe storms. at the same time, such facility was used as well to evaluate the recovery level after storms, highlighting adequate system reliability and effectiveness. nonetheless, these sorts of monitoring infrastructures are usually ashore installed thus easing, for instance, data transmission (which can be accomplished without resorting to wireless solutions) and system maintenance. however, the literature also comprehends works implementing video monitoring systems that are installed on offshore buoys within breeding plants: in [10], cameras are set up within the fish cages, while video streaming is ensured by a radio frequency system that is installed on board of the buoy. similarly, works reported in [11], [12] extend such a surveillance system. for what concerns the application scenario, [13] shares a similar context with respect to the one of this paper, since the authors propose a remote monitoring system for offshore oceanic sea farms made of floating cages. the system includes cameras, sonars, and telemetry devices, and it is installed on cage feed buoys (which are like the one that will be introduced later on). unfortunately, though, images are not ashore streamed because they are recorded and stored on board of a boat. on the other hand, marine video monitoring systems are additionally devised so to operate underwater, fulfill submarine investigation and exploration purposes [14], as well as for proper aquaculture ponds characterised by turbid water [15]. concerning video monitoring systems in a broad sense (i.e., which are employed in diverse contexts with respect to the marine one) that rely on photovoltaic energy harvesting systems and a backup battery, works within [16], [17] confirm the suitability of such a technique thus underlining the potential effectiveness of the solution proposed in this paper. image quality may be judged by means of several metrics so to measure the grade of similarity between two pictures. indeed, cameras capabilities can be objectively evaluated by resorting to the aforesaid metrics. this paper makes use of a well-known technique, that will be introduced later, which received a wide approval in the academic world. however, and for the sake of completeness, some newer approaches are also cited below so to provide readers with additional references. for instance, in [18] image quality assessment is performed from a vectorial perspective by making recourse to the vector root mean squared error which allows for an alternative to the biggest limitations (e.g., noise cancellation or detail preservation) of mean-squarederror-based techniques. on the other hand, image quality assessment may be also accomplished by means of machine learning techniques, and in particular [19] tackles the problem by falling back on deep neural networks. they can capture highorder statistics of image feature descriptors and storing them into a dictionary thus turning out into a deep dictionary encoding network. finally, captured images from remote video monitoring systems may suffer from artifacts stemming from equipment wear as well as from bad weather. in order to enhance image quality, it is favourable setting up methods aiming at mitigating such shortcomings. therefore, in [20] a solution to the problem is proposed by establishing an algorithm for dehazing images which can be applied to a myriad of contexts, especially the harsher ones like offshore. 3. system architecture the block diagram of the video monitoring system is depicted in figure 1, and it is composed of 3 main building blocks (i.e., power supply, control and communications and camera). the power supply block contains 2 photovoltaic panels providing 20 w each, whose task is either to power up the whole system and to recharge a 12 v, 25 ah lead-acid backup battery through a solar charge controller. the power supply block is only composed of off-the-shelf components: such a decision was made so to develop the prototype as soon as possible to test its effectiveness. the core of the power supply system is the solar charge controller. it is responsible for correctly powering up the control and communications block and the camera in function of both the battery charge level and the power coming from the photovoltaic panels. indeed, whenever they are exposed to acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 19 enough sunlight, the solar charge controller manages the harvested energy for running the whole system as well as for recharging the backup battery. on the contrary, whenever the harvested energy is scant (e.g., during the night), the solar charge controller draws energy from the battery so to supply the system. photovoltaic panels are fundamental for the long-term functioning of the system. indeed, such a prototype is offshore installed, and it is supposed to operate for at least a 6-month timespan, while the mere backup battery only ensures a 48-hour autonomy. thus, additionally highlights the low-power feature of the system components (that will be introduced below) along with the effectiveness of the duty-cycling functioning policy (that will be addressed in the next section). however, a complete battery discharge is extremely unlikely since it would be the consequence of several hours of darkness. the control and communications block is the core of the system since it manages the duty cycling of the camera along with images capturing and transmitting towards a remote server, while minimizing power consumption by the activation of the inner elements only for the minimum amount of time needed. such a working flow is obtained by the following off-the-shelf components: • dc-dc converter which filters out the power supply coming from the appropriate building block so to correctly power each of the system elements; • raspberry pi zero is the control unit of the system due to python scripts managing both the camera and the duty cycling of all of the other components; • raspberry pi relays board contains relay switches that are directly controlled by the raspberry pi so to turn on and off the camera and the other system elements whenever they are needed and only for the strictly necessary time in order to limit the overall power consumption; • 3g/4g dongle provides internet connectivity that is exploited to send the captured images and the debug logs to a remote server in order to perform diagnostic; • reset switch performs a daily hardware reset of the whole system acting as a sort of long-term watchdog timer so to overcome software issues or unexpected behaviours. the camera is an off-the-shelf outdoor ip one produced by hikvision, which is especially designed so to resist to marine environments. all the elements composing the control and communications block are housed within an ip56 box, while the complete system is mounted on a support pole (see figure 2) which is offshore installed on a breeding cage floating structure (see figure 3). 4. reliability configurations the application scenario for this video monitoring system does not necessitate of real-time image streaming. in particular, only snapshots on a regular basis (i.e., one every 15 minutes) are required. therefore, in order to meet functioning requirements, the system operates for a time span of 2 minutes every quarter of an hour within which the picture is taken and remotely sent via the internet. in so doing, only 8 minutes per hour of system activity is experienced (i.e., 172 minutes per day), thus also optimizing power consumption. for what concerns weather conditions the system is exposed, two seasonal functioning periods are identified on which availability and reliability simulations are carried out: • winter, that is made up of 3 8-hour time slots, that are daily repeated, which are in turn characterised by a temperature of 5°c, 10°c and 15°c; • summer, which accounts for 3 8-hour time slots, which are daily repeated, that are respectively characterised by a temperature of 20°c, 25°c and 30°c. such temperatures are considered because of the future system deployment scenario: 8 km offshore the city of piombino, italy. simulation parameters are summarised in figure 4, while availability and reliability simulation schemes are shown in figure 5 [21]-[24]. the mil hdbk 217f database was selected to evaluate individual component failure rates at different temperatures considering an environment of the kind naval unsheltered (nu). unfortunately, such parameters are usually not available from component producers, therefore a conservative approach given by the mentioned database results in worst condition scenarios was followed. 5. simulation results as shown in figure 6, simulations were performed by means of the blocksim software (by reliasoft) on the two scenarios previously described. in the upper picture the availability, that is the ratio between the mean time between failures and the sum of the overmentioned time plus the mean time to restoration (mtbf/(mtbf+mttr)), over time (considering a restoration time of 48 hours) is shown, while in the bottom picture the reliability trend over time is represented considering as failure rates the inverses of the figures cited in table of figure 4. both results prove that the summer period presents a decrease in the final time values with respect to the winter one. such results agree with what expected by the theory connected to the exploitation of the mil hdbk 217f database where figure 1. video monitoring system block scheme. figure 2. realization of the video monitoring system: photovoltaic panels (left) and pole with camera and ip56 box containing the electronics (right). acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 20 temperature-based degradation, under a fixed environment, takes place. to simulate the availability model, a fixed 48 h repair time was considered. of course, due to the very small number of electronic components and to the low system complexity, the system has to be tested especially for wintertime on long periods since the overall mtbf is considerably high at those temperatures. moreover, both the selected duty cycle for system exploitation and the power consumption reduction limit the internal temperature raise keeping it almost constant, without significant degradation rates apparently induced on individual electronics. simulations do not include condensation due to temperature excursions. therefore, the rusting due to salty environment condensation is not considered in the results. the overall system is housed within an ip56 box presenting holes to which siphons are connected so to allow heat dissipation via air circulation. however, this also implies that salty air enters within the box coming into contact with system components. this is far from being an optimal solution, albeit it is sufficient for the prototype. indeed, the latter is supposed to operate just for 6 months during which damages due to saltiness should be limited. consequently, salty air from the sea is assumed to be present around the electronics especially if maintenance activities will be performed. hence, such environment is by far challenging for what concerns junctions and soldering rather than single component performances. in the future, in case of failure, a root cause analysis could be applied on single assets so to verify whether this hypothesis is confirmed or not. the system availability without the charging station is not taken into consideration because the system would not work enough time to gather the requested data, and thus it is excluded by the simulations. 6. experimental setup following the simulations concerning the availability and reliability of the system, a set of tests was planned and performed in laboratory, with the aim of identifying possible negative effects on the acquired images due to the severe variations in terms of environmental conditions that the system, and the video camera in particular, is subject to. so to expose the system to different environmental conditions, the control and communication block, as well as the camera itself, were detached from the energy harvesting system and positioned inside a climatic chamber, figure 3. autonomous remote video monitoring system prototype offshore installed on a cage floating structure. figure 4. simulation parameters. figure 5. availability and reliability simulation schemes: winter and summer working periods. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 21 whose size did not allow the insertion of the complete system. indeed, an acs angelantoni hygros 250 environmental test chamber, whose internal volume is 600 × 535 × 700 mm3 (w × d × h), was used to perform tests. the climatic chamber is characterised by a temperature range from -40 °c to +180 °c, and a relative humidity range from 10 % to 98 % in the temperature range from +5 °c to +95 °c. it is important to point out that the relative humidity concentration depends on the psychrometric principle; therefore, the relative humidity value strictly depends on the temperature values. the minimum relative humidity values 𝑅𝐻min are expressed in the following bulleted list: • 𝑅𝐻min = 𝑓𝑙𝑜𝑎𝑡𝑖𝑛𝑔 for 𝑇 ≤ 0 °c; • 𝑅𝐻min = 55% for 𝑇 = +10 °c; • 𝑅𝐻min = 30% for 𝑇 = +20 °c; • 𝑅𝐻min = 17% for 𝑇 = +30 °c; • 𝑅𝐻min = 10% for 𝑇 ≥ +40 °c; where 𝑇 is the temperature value. tests were performed following the subsequent approach. the climatic chamber was programmed so to simulate the maritime weather, which has a relative humidity value around 80%; therefore, it performs the following temperature and relative humidity pairs, which for the sake of simplicity are numbered from the first to the last executed test: 1. 𝑇 = −10 °c with 𝑅𝐻 = 𝑓𝑙𝑜𝑎𝑡𝑖𝑛𝑔; 2. 𝑇 = 0 °c with 𝑅𝐻 = 𝑓𝑙𝑜𝑎𝑡𝑖𝑛𝑔; 3. 𝑇 = +10 °c with 𝑅𝐻 = 80 %; 4. 𝑇 = +20 °c with 𝑅𝐻 = 80 %; 5. 𝑇 = +30 °c with 𝑅𝐻 = 80 %; 6. 𝑇 = +40 °c with 𝑅𝐻 = 80 %; 7. 𝑇 = +50 °c with 𝑅𝐻 = 80 %. the whole system was positioned inside the climatic chamber, and it was cycled for each temperature humidity pair. moreover, 300 ml of saturated solution of salt in water was put along with the system to simulate the real conditions of the final deployment site. for the sake of completeness, it must be said that the battery and the 3g/4g dongle are exempt from the test. the first one was completely removed since lead-acid batteries are subjected to breakdown when extreme and abrupt changes of temperature values occur; thus, the system was mains powered exploiting a cable gland. the latter one, instead, was positioned outside the climatic chamber since the chassis of the hygros 250 is made of metal (except for the porthole), so it compromises the connection capability of the dongle. test setup can be seen in figure 7. the camera was fixed to the climatic chamber so to avoid misplacements of the pictures, while tests were in progress. since the aim of the tests was to identify possible image degradations, a frame control image, shown in figure 8, was selected. such image was chosen with the aim of comparing the details and chromatic differences that may occur when the camera is subjected to different environments. the control image was positioned at the porthole, inside the climatic chamber; then, a picture of the control image was taken at a temperature of 25 °c and a relative humidity of 50 %, with the light of the climatic chamber switched on. the test started setting the climatic chamber as introduced in the aforesaid numbered list. as soon as the climatic chamber reached the defined temperature and relative humidity pair, the whole system was held at those levels for 10 minutes so to allow the electronics inside the climatic chamber to reach a steady state. at this point the inner light was switched on; the system was powered up, and 500 pictures were taken at a rate of approximately one picture every two seconds. tests and related results are presented in the following section. figure 6. availability and reliability simulation results during winter and summer working periods. figure 7. test setup: control image fixed at the porthole can be seen on the left, while the camera and the system are positioned inside the climatic chamber together with the glass containing the saturated solution of salt and water. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 22 7. test results and discussion tests aimed at assessing the video surveillance monitoring system performances, under different environmental conditions, from the point of view of two different benchmarks: picture acquisition time and picture quality. acquisition times were directly sampled by the video monitoring system. then, for each of the experimental set, those times were averaged: the relative results are reported in figure 9. the maximum mean acquisition time (i.e., 1173 ms) was experienced at -10 °c, while the minimum one (i.e., 1126 ms) was recoded at 30 °c and 80 % of relative humidity. however, despite the mean acquisition time trend is not constant, it could be considered so since the difference amid the maximum and the minimum is just 47 ms, thus underlying the fact that no significant performances variation is present. the assessment of the variation of picture quality was carried out by resorting to the multiscale structural similarity (msssim) index for image quality [25]. this method compares two images and provides as output an index (i.e., the ms-ssim) representing how much these pictures are similar: the closer the ms-ssim to 1, the more similar the images. if ms-ssim for two images is 1, then they are identical. therefore, each of the pictures taken by the camera during the experimental sets were compared to the control one by means of the ms-ssim, and then the resulting ms-ssims were averaged for each experimental set. mean ms-ssim trend is reported in figure 10, while figure 11 shows the control picture along with one picture for each of the experimental sets. although all the pictures seem to look the same, as the mean ms-ssim trend shows, they slightly differ. as expected, the maximum mean ms-ssim (i.e., 0.799) was experienced at 20 °c and 80 % of relative humidity: indeed, the control picture was taken at room temperature, therefore the most significant environmental variation was the relative humidity content. on the other hand, the minimum mean msssim (i.e., 0.685) was recorded at -10 °c. qualitatively, the system better performed at higher temperatures, indeed meaningful discrepancy amid lower temperatures experimental sets and higher temperatures ones took place. this behaviour can be justified by the mechanical stress induced on the camera infrastructure. electronics cannot be responsible for picture variability over temperature and humidity stress for two reasons. the first one is that the timing and temperature range of experiments are so limited that electronics are not likely to be affected. the second one is that relative humidity can affect both the optical transparency and the plastic support can be deformed in such temperature range. therefore, it is likely that in such testing condition the plastic support holding the camera optics can be affected influencing camera acquisition and therefore the images. 8. conclusions the aim of this paper was to describe the architecture of an autonomous video monitoring system to be employed for the remote control of offshore breeding cages in aquaculture plants. the proposed solution is characterised by energy self-sufficiency thanks to an energy harvesting system based on the use of photovoltaic panels. in order to demonstrate the usability of the system, as a first step simulations were performed in order to validate its reliability and availability. then, a set of tests were carried out, exploiting a climatic chamber, so to identify possible image degradations due to different environmental conditions. results prove that the system can be successfully employed in the proposed application scenario for both winter and summer environmental settings, while degradations occurring at extreme climatic conditions are still compliant with the video monitoring purpose of the whole system. the current analysis is not including considerations on the possible rusting caused by the operation scenario either on soldering, junctions and connectors which should be included in the future in order perform an utter system reliability and availability analysis. moreover, while the figure 8. image used for the tests. figure 9. mean acquisition times. figure 10. mean ms-ssim trend. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 23 tests demonstrated the reliability of the image acquisition procedure, a long-term field test, performed on site, is expected to be carried out in the near future in order to study the behaviour of the system in its real deployment scenario. acknowledgement the 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427–430. doi: 10.1109/i2mtc.2013.6555453 [25] z. wang, e. p. simoncelli and a. c. bovik, multiscale structural similarity for image quality assessment, proceedings of the thrityseventh asilomar conference on signals, systems & computers, pacific grove, ca, usa, 9-12 november 2003, pp. 1398-1402. doi: 10.1109/acssc.2003.1292216 https://doi.org/10.1109/tim.2020.2984928 https://doi.org/10.1109/tim.2020.3024335 https://doi.org/10.1016/s0026-2714(02)00153-1 https://doi.org/10.1109/imtc.2007.379086 https://www.scopus.com/authid/detail.uri?authorid=6505763334 https://doi.org/10.1016/s0026-2714(02)00152-x https://doi.org/10.1109/i2mtc.2013.6555453 https://doi.org/10.1109/acssc.2003.1292216 a metrological approach for multispectral photogrammetry acta imeko issn: 2221-870x december 2021, volume 10, number 4, 111 116 acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 111 a metrological approach for multispectral photogrammetry leila es sebar1, luca lombardo2, marco parvis2, emma angelini1, alessandro re3,4, sabrina grassini1 1 dipartimento di scienza applicata e tecnologia, politecnico di torino, corso duca degli abruzzi 24, 10129, turin, italy 2 dipartimento di elettronica e telecomunicazioni, politecnico di torino, corso duca degli abruzzi 24, 10129, turin, italy 3 dipartimento di fisica, università degli studi di torino, via pietro giuria 1, 10125, turin, italy 4 infn, sezione di torino, via pietro giuria 1, 10125, turin, italy section: research paper keywords: photogrammetry; multispectral imaging; reference object; metrology; cultural heritage citation: leila es sebar, luca lombardo, marco parvis, emma angelini, alessandro re, sabrina grassini, a metrological approach for multispectral photogrammetry, acta imeko, vol. 10, no. 4, article 19, december 2021, identifier: imeko-acta-10 (2021)-04-19 section editors: umberto cesaro and pasquale arpaia, university of naples federico ii, italy received november 4, 2021; in final form december 6, 2021; published december 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: leila es sebar, e-mail: leila.essebar@polito.it 1. introduction in the last few years, digitalization techniques and related 3d imaging systems have acquired major importance in several fields, like industry, medicine, civil engineering, architecture, and cultural heritage. for the last mentioned one, in particular, such technologies can provide multiple contributions, in terms of conservation, data archiving, enhancement, and web sharing [1][3]. the existing three-dimensional imaging systems, which acquire measurements through light waves, can be discriminated on the basis of the ranging principle employed [4]. among the several techniques, photogrammetry is a remote image-based technique, that became widely diffused. in particular, this technique allows for the collection of reliable 3d data of an object, regarding its surface (color and texture) and its geometrical features without requiring any mechanical interaction with the object itself [5]. indeed, a 3d model is constructed starting from digital images of the object, leading to the creation of its virtual replica. with the increasing diffusion of digitalization techniques and the aims of many users to the creation of 3d models, several concerns have been raised about the results that can be achieved. therefore, even though digitalization practices are widely diffused and they can provide realistic replicas of an object, the factors that impact the uncertainty of the final 3d models are several and they must be further investigated. some authors have summarized the most important factors that affect the uncertainty in 3d imaging and modeling systems [4], [6]. nevertheless, precision and accuracy evaluation of 3d models has not been supported by internationally recognized standards which are of major importance to avoid archiving and sharing wrong information [7]. some publications have presented different test artifacts or new systems that could be used to test the performances of the photogrammetry approach [6],[8]. in some cases, the accuracy of a final model is determined by comparing the results with some reference data, acquired with active systems such as laser scanners [8]-[10]. otherwise, the results are also evaluated on the basis of statistical parameters generated by the employed reconstruction software [11]. abstract this paper presents the design and development of a three-dimensional reference object for the metrological quality assessment of photogrammetry-based techniques, for application in the cultural heritage field. the reference object was 3d printed, with nominal manufacturing uncertainty of the order of 0.01 mm. the object was realized as a dodecahedron, and in each face, a different pictorial preparation was inserted. the preparations include several pigments, binders, and varnishes, to be representative of the materials and techniques used historically by artists. since the reference object’s shape, size and uncertainty are known, it is possible to use this object as a reference to evaluate the quality of a 3d model from the metric point of view. in particular, verification of dimensional precision and accuracy are performed using the standard deviation on measurements acquired on the reference object and the final 3d model. in addition, the object can be used as a reference for uv-induced visible luminescence (uvl) acquisition, being the materials employed uv-fluorescent. results obtained with visible-reflected and uvl images are presented and discussed. mailto:leila.essebar@polito.it acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 112 nevertheless, there is no unique and recognized way to define the quality of a reconstructed model. this paper presents preliminary but promising results, achieved through the design and realization of a low-cost 3d printed reference object specifically designed for assessing position accuracy and dimensional uncertainty of photogrammetric reliefs. moreover, the reference object, realized in collaboration with the “centro conservazione e restauro la venaria reale”, was created with special insets in which different pictorial preparations were inserted in order to be representative of the materials and techniques used historically by artists. the proposed object could be therefore proposed also as a reference sample for multispectral imaging applications, a widely diffused 2d technique for the characterization and identification of historic-artistic materials [12], [13]. generally, photogrammetry and multispectral imaging are applied as separate techniques, but their combined application is becoming more and more frequent in the cultural heritage field. indeed, this approach exploits the benefit of mapping multispectral imaging data onto 3d models for complete documentation of the conservation state of an object [14], [15]. even though several references can be used in different fields [16]-[18], in this case a specific reference object has to be employed. the reference object was tested using a photogrammetric measuring system, that allows acquisition of both the visible-reflected (vis) and the uv-induced luminescence (uvl) images. in particular, the experimental setup is composed by an ad-hoc modified digital camera capable of working in a wide spectral range (350-1100 nm), several different lighting sources and filters, and an automatic rotating platform. meshroom [19], an open-source software, was used to make the photogrammetric reconstruction of the reference object. the obtained results were then compared to the physical 3d object in order to estimate the accuracy of the final 3d replica. in addition, a comparison of two different approaches for the realization of the uvl model is presented. 2. 3d reference object the reference object consists of a 3d printed polymeric dodecahedron. figure 1 shows the prototype, that was designed with the wings 3d [20] software. then, the reference object was realized with a projet 2500 plus (3d systems) printer, employing the visijet® m2r-gry resin. the printer allows obtaining an object with a nominal uncertainty of the order of 0.01 mm. the reference object was designed to be suitable for photogrammetry survey, indeed its shape is properly created for achieving specific information regarding the reconstruction geometrical accuracy. furthermore, the object was designed in order to have twelve pentagonal slots, in which different pigment preparations can be inserted. in particular, twelve different pigments were chosen to be representative of the principal artistic materials. all the pigments employed are provided by kremer pigmente gmbh & co. kg [21]. in particular, the pigments employed are lead white, white barium sulfate, bone black, magnetite black, raw sienna italian, lead-tin yellow, minium, lac dye, azurite, malachite, verdigris, and lapis lazuli. the twelve painting preparations were realized in order to reproduce the techniques employed in real historical artifacts. therefore, each one consists of several consecutive layers: support, preparation layer, underdrawings, pictorial layer, and varnishes. the preparation layer is made of stucco, realized adding gypsum to saturate a solution of water and animal glue (14:1 ratio in weight). then, three different underdrawings were applied directly on top of the stucco layer. in particular, the materials employed are charcoal, sanguine, and iron gall ink. these first two layers, namely stucco preparation, and underdrawings are the same for all the twelve mock-ups. each of the twelve sections was designed to host one single pigment/dye in nine different combinations. indeed, each section was divided into three subsections, based on the binder employed: arabic gum, egg tempera, and linseed oil. then each section is further divided into three subsections: one with a historical varnish (i.e. mastic), one with a modern one, and one left unprotected. this choice is of particular interest for the uvl imaging techniques because it allows discriminating between the fluorescence of the different pictorial preparations with and without varnish. figure 2 shows the proposed reference object and a scheme of the pictorial preparation. figure 1. drawing of the reference object realized by means of wings 3d software. figure 2. on the left: top view and scheme of the pictorial preparation. on the right, the proposed reference object. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 113 3. 3d acquisition system the system employed to acquire the images of the reference object is composed of a modified digital camera, a set of suitable light sources, and an automatic rotating platform. 3.1. acquisition setup the images were acquired with a fujifilm xt-30 digital camera coupled with a minolta mc rokkor-pf 50mm f/1.7 lens. the camera is modified to be suitable for ultraviolet-visibleinfrared photography acquisition, in the range from 350 nm to 1100 nm. for both vis and uvl measurements the camera was equipped with a hoya ir uv-cut filter and a schott bg40 filter. the image acquisition was performed in a room where it was possible to create absolute darkness, in order to avoid any possible interference from unwanted light sources. led uv a365 nm filtered sources were employed as sources for the acquisition of uvl images, while standard halogen lamps were used for vis images. table 1 reports the main parameters employed in the image acquisition and figure 3 shows the complete acquisition setup [22]. the acquisition system is completed by the rotating platform which allows to automatically take images of the object at specified rotation angles. the platform is composed of a circular rotating plate that hosts the object. the plate is connected to a stepper motor (type nema 17) by employing a suitable gearbox (1:18 ratio) in order to increase the torque and the angular resolution and to reduce the rotation speed. a stepper motor driver chip (a4988, allegro microsystems) is used to drive the motor and to move the platform at specified angular positions with a resolution of 0.1°. the platform is controlled by an arduinouno development board connected to a computer, where a dedicated application allows the users to set up all the acquisition parameters (such as image number, angle, speed, etc.) and to carry out some basic image pre-processing on the acquired images. furthermore, the platform features a camera shot trigger output which, connected to the camera, allows the platform to automatically trigger the camera shot at each one of the specified object positions. this greatly improves and simplifies the image acquisition procedure dramatically reducing the user manual intervention. 3.2. data processing the reconstruction of the 3d models was performed by means of meshroom (version 2021.1.0). this software has a particular embedded feature, called live reconstruction that allows to directly import images while they are acquired and to augment the previous structure from motion coverage with an iterative process. in this study, the images were iteratively added in groups of four for each step. the first block of images was acquired frontally respect to the artifact with an angular step of 15°. subsequently, two additional sets of images were acquired after flipping the artifact on different sides in order to improve the reconstruction of all the faces and their details. hence, a total of 72 reflected vis images plus 72 uvl images were collected and processed with a standard pipeline in meshroom. in particular, the following steps were carried out: camera initialization, natural feature extraction, image matching, features matching, structure from motion (sfm) creation, prepare dense scene, depth map estimation, depth map filtering, meshing, meshing filtering, texturing. the parameters of reconstruction were all set to default values. figure 3. photogrammetry system for multispectral images acquisitions. figure 4. from left to right: 3d model from reflected vis images; 3d model obtained after re-texturing of the reflected-vis model with uvl images; and 3d model obtained using uvl images directly. table 1. the employed acquisition parameters. parameter value image size 6240 × 4160 sensor size/type 23.5mm × 15.6mm (aps-c)/ x-trans cmos effective pixels 26 megapixels image format .raf iso 200 focal length 50 mm aperture f/16 shutter speed 2.0 s acquired images 72 (3 revolutions of 24 images) acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 114 the aforementioned procedure was applied to reconstruct models from both vis and uvl images. therefore, two different models were obtained. nevertheless, meshroom allows to texture a 3d model also using a different set of images with respect to the ones used to generate the point cloud and the mesh. indeed, it is possible to duplicate the computed dense scene node and import a new folder of images. in this way, the software will generate a texture for the model, with a different set of images. in order to test this feature, the procedure was applied to the vis model, to retexture it with the uvl images. the three textured 3d models were exported in the obj format and properly scaled by employing the open-source software wings 3d. to scale each model, three different dimensions of the object were measured, and the mean scaling factor was computed. figure 4 shows an image of the final models. 4. experimental validation in order to assess the uncertainty of the 3d reconstructed models from reflected vis images, several distances on the real artifact were taken by using a caliper and compared with the same distances on the 3d models. figure 5 shows the distances measured, which are distributed all around the artifact itself. the distances were chosen in correspondence of the edges and between opposite faces of the artifact since they can be easily measured. the measurements of the artifact were collected with a 1/20 mm caliper. whereas the software wings 3d was employed to measure the corresponding distances on the virtual replica. the model uncertainty was estimated according to the difference δ as reported in (1):  = |𝐷𝑟 −𝐷𝑚 | , (1) where 𝐷𝑟 is the distance on the reference object and 𝐷𝑚 is the one on the 3d model. in addition, also the relative uncertainty was calculated as shown in (2): % = |𝐷𝑟 −𝐷𝑚 | 𝐷𝑟 × 100 % . (2) finally, the overall standard deviation was evaluated, as in (3): table 2. uncertainty estimation of the 3d model from vis images. the measurement on the reference object, on the vis model, and on the uvl model are reported.  indicates the difference between the measurements (equation (1)), and  is the relative uncertainty (equation(2)). dimension reference object (mm) vis-3d model (mm) δ (mm) ε (%) uvl-3d model (mm) δ (mm) ε (%) a 44.2 44.0 0.23 0.52 44.1 0.13 0.30 b 44.3 44.5 0.19 0.43 44.8 0.31 0.70 c 44.1 44.2 0.11 0.25 44.7 0.491 1.11 d 99.0 98.7 0.31 0.31 100.6 1.91 1.94 e 71.4 71.2 0.25 0.35 71.6 0.45 0.63 f 44.3 44.0 0.35 0.79 44.8 0.85 1.93 g 44.3 44.6 0.33 0.74 44.6 0.03 0.07 h 44.4 44.5 0.08 0.18 44.8 0.32 0.72 i 121.6 122.2 0.55 0.45 125.3 3.15 2.58 l 71.4 71.4 0.05 0.07 72 0.65 0.91 m 71.2 71.3 0.14 0.20 71.4 0.06 0.08 n 98.8 98.9 0.12 0.12 100.8 1.88 1.90 o 115.0 114.9 0.14 0.12 116.7 1.84 1.60 p 71.2 70.5 0.67 0.94 71.2 0.67 0.95 q 114.5 114.8 0.34 0.30 116.6 1.76 1.53 r 70.8 70.9 0.19 0.27 71.8 0.86 1.21 s 70.8 70.5 0.27 0.38 71.9 1.37 1.94 t 70.9 70.9 0.03 0.04 71.7 0.77 1.09 u 44.2 44.2 0.06 0.14 44.5 0.29 0.66 v 70.9 70.8 0.02 0.03 71.9 1.07 1.51 figure 5. original vis images of the reference object with validation measurement distances. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 115 𝜎 = √ 1 𝑁 − 1 ∑(𝛿𝑖 2 ) 𝑁 𝑖=1 (3) regarding the multispectral reconstruction, two different approaches were tested. one uvl model was obtained retexturing the mesh already obtained for the vis model. this means that the coordinates of the measured points did not change and that the uvl-model is perfectly superimposable. therefore, there is no need to perform a metric evaluation of this model. the second uvl-model was instead reconstructed starting directly from uvl images. for the estimation of the quality of the model, the procedure previously presented was applied. in particular, the differences between the distances measured on the uvl 3d model and the ones measured on the reference object were computed. in table 2, the distance difference δ in mm and the corresponding relative error ε are reported. on the basis of these results, the reconstructed visible models are quite reliable with maximum dimensional uncertainties lower than 1 % for the visible model and lower than 2 % for the uv model. nevertheless, the average uncertainties are lower, reaching about 0.33 % and 1.17 % for the visible and the uv models, respectively, while the standard deviations  of the differences between the real model and the reconstructed one are of about 170 µm and 690 µm, respectively. the higher uncertainty obtained for the uv model is probably due to a higher color uniformity of the acquired images which affected the reconstruction process. therefore, the reconstruction accuracy can be probably improved by tuning the image acquisition procedure and the reconstruction parameters. 5. conclusions this paper presented the design and development of an artifact, that can be used as a metric reference object for the assessment of the accuracy and dimensional uncertainty of the 3d model obtained through photogrammetry. the object is 3d printed and has twelve insets, in which several pictorial preparations were inserted. indeed, the object is suitable also to be a reference for multispectral imaging. the reference object was employed in order to test the photogrammetric measurement system employed, from a metric point of view. a comparison between several distances acquired both on the mechanical reference object and on the reconstructed vis 3d model was realized. the maximum dimensional uncertainty is lower than 1 % and the average uncertainty reached about 0.33 %. moreover, the reconstruction of the model from uvl images was performed, using two different approaches. the obtained results were compared with the real object. from the comparison between the obtained results, it is possible to state that the approach that involves the creation of a vis model and the subsequent re-texturing with uvl images achieved the best results. acknowledgement the authors would like to acknowledge dr. paola buscaglia from centro conservazione e restaura “la venaria reale” for the support related to the realization of the pictorial preparation. references [1] m. russo, f. remondino, g. guidi, principali tecniche e strumenti per il rilievo tridimensionale in ambito archeologico, archeologia e calcolatori, 22 (2011) (in italian), pp. 169-198, issn 1120-6861. [2] l. es sebar, l. iannucci, c. gori, a. re, m. parvis, e. angelini, s. grassini, in-situ multi-analytical study of ongoing corrosion processes on bronze artworks exposed outdoors, acta imeko 10(1) (2021) pp. 241-249. doi: 10.21014/acta_imeko.v10i1.894 [3] i. m. e. zaragoza, g. caroti, a. piemonte, the use of image and laser scanner survey archives for cultural heritage 3d modelling and change analysis, acta imeko 10(1) (2021) pp. 114-121. doi: 10.21014/acta_imeko.v10i1.847 [4] j. a. beraldin, m. rioux, l. cournoyer, f. blais, m. picard, j. pekelsky, traceable 3d imaging metrology, proc. spie videometrics ix 6491 (2007) doi: 10.1117/12.698381 [5] t. schenk, introduction to photogrammetry, the ohio state university, columbus, 2005, 106. online [accessed 3 december 2021] https://www.mat.uc.pt/~gil/downloads/introphoto.pdf [6] j. a. beraldin, f. blais, s. el-hakim, l. cournoyer, m. picard, traceable 3d imaging metrology: evaluation of 3d digitizing techniques in a dedicated metrology laboratory, proceedings of the 8th conference on optical 3-d measurement techniques, july 912, 2007, zurich, switzerland 2007, pp. 310-318. [7] i. toschi, a. capra, l. de luca, j. a. beraldin, on the evaluation of photogrammetric methods for dense 3d surface reconstruction in a metrological context, in isprs technical commission v symposium, wg1 2(5) (2014) pp. 371-378. [8] g. j. higinio, b riveiro, j. armesto, p. arias, verification artifact for photogrammetric measurement systems, optical engineering 50(7) (2011), art. 073603. doi: 10.1117/1.3598868 [9] c. buzi, i. micarelli, a. profico, j. conti, r. grassetti, w. cristiano, f. di vincenzo, m. a. tafuri, g. manzi, measuring the shape: performance evaluation of a photogrammetry improvement applied to the neanderthal skull saccopastore 1, acta imeko 7(3) (2018). doi: 10.21014/acta_imeko.v7i3.597 [10] a. koutsoudis, b. vidmar, g. ioannakis, f. arnaoutoglou, g. pavlidis, c. chamzas, multi-image 3d reconstruction data evaluation, journal of cultural heritage 15(1) (2014) pp. 73-79. doi: 10.1016/j.culher.2012.12.003 [11] a. calantropio, m.p. deseilligny, f. rinaudo, e. rupnik, evaluation of photogrammetric block orientation using quality descriptors from statistically filtered tie points, international archives of the photogrammetry, remote sensing & spatial information sciences 42(2) (2018). [12] j. dyer, g. verri, j. cupitt, multispectral imaging in reflectance and photo-induced luminscence modes: a user manual, british museum, 2013. [13] a. cosentino, identification of pigments by multispectral imaging; a flowchart method, herit sci 2(8) (2014). doi: 10.1186/2050-7445-2-8 [14] s. b. hedeaard, c. brøns, i. drug, p. saulins, c. bercu, a. jakovlev, l. kjær, multispectral photogrammetry: 3d models highlighting traces of paint on ancient sculptures, in dhn (2019), pp. 181-189. [15] e., nocerino, d. h. rieke-zapp, e. trinkl, r. rosenbauer, e. m. farella, d. morabito, f. remondino, mapping vis and uvl imagery on 3d geometry for non-invasive, non-contact analysis of a vase, international archives of the photogrammetry, remote sensing and spatial information sciences isprs archives, 42 (2), (2018) pp. 773-780. doi: 10.5194/isprs-archives-xlii-2-773-2018 [16] m. parvis, s. corbellini, l. lombardo, l. iannucci, s. grassini, e. angelini, inertial measurement system for swimming rehabilitation, 2017 ieee international symposium on medical http://dx.doi.org/10.21014/acta_imeko.v10i1.894 http://dx.doi.org/10.21014/acta_imeko.v10i1.847 https://doi.org/10.1117/12.698381 https://www.mat.uc.pt/~gil/downloads/introphoto.pdf https://doi.org/10.1117/1.3598868 http://dx.doi.org/10.21014/acta_imeko.v7i3.597 https://doi.org/10.1016/j.culher.2012.12.003 https://doi.org/10.1186/2050-7445-2-8 http://dx.doi.org/10.5194/isprs-archives-xlii-2-773-2018 acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 116 measurements and applications (memea), rochester, mn, usa, 8-10 may 2017, pp. 361-366. doi: 10.1109/memea.2017.7985903 [17] a. gullino, m. parvis, l. lombardo, s. grassini, n. donato, k. moulaee, g. neri, employment of nb2o5 thin-films for ethanol sensing, 2020 ieee international instrumentation and measurement technology conference (i2mtc), dubrovnik, croatia, may 25-28, 2020, pp. 1-6. doi: 10.1109/i2mtc43012.2020.9128457 [18] l. iannucci, l. lombardo, m. parvis, p. cristiani, r. basseguy, e. angelini, s. grassini, an imaging system for microbial corrosion analysis, 2019 ieee international instrumentation and measurement technology conference (i2mtc), auckland, new zealand, may 20-23, 2019, pp. 1-6. doi: 10.1109/i2mtc.2019.8826965 [19] alicevision, meshroom 3d reconstruction software. online [accessed 3 december 2021] https://alicevision.org/#meshroom [20] wings 3d. online [accessed 3 december 2021] http://www.wings3d.com [21] kremer pigmente. online [accessed 3 december 2021] https://www.kremer-pigmente.com/en [22] l. es sebar, s. grassini, m. parvis, l. lombardo, a low-cost automatic acquisition system for photogrammetry, 2021 ieee international instrumentation and measurement technology conference-i2mtc, (2021) pp. 1-6. doi: 10.1109/i2mtc50364.2021.9459991 https://doi.org/10.1109/memea.2017.7985903 https://doi.org/10.1109/i2mtc43012.2020.9128457 https://doi.org/10.1109/i2mtc.2019.8826965 https://alicevision.org/#meshroom http://www.wings3d.com/ https://www.kremer-pigmente.com/en https://doi.org/10.1109/i2mtc50364.2021.9459991 an informed type a evaluation of standard uncertainty valid for any sample size greater than or equal to 1 acta imeko issn: 2221-870x june 2022, volume 11, number 2, 1 5 acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 1 an informed type a evaluation of standard uncertainty valid for any sample size greater than or equal to 1 carlo carobbi1 1 departement of information engineering, università degli studi di firenze, via santa marta 3, 50139 firenze, italy section: research paper keywords: measurement uncertainty; type a evaluation; pooled variance; bayesian inference; informative prior citation: carlo carobbi, an informed type a evaluation of standard uncertainty valid for any sample size greater than or equal to 1, acta imeko, vol. 11, no. 2, article 29, june 2022, identifier: imeko-acta-11 (2022)-02-29 section editor: francesco lamonaca, university of calabria, italy received october 1, 2021; in final form february 23, 2022; published june 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: carlo carobbi, e-mail: carlo.carobbi@unifi.it 1. introduction the quantification of the type a uncertainty contribution in the case of a small sample ( = 1, 2, 3n ) is a subject of research and passionate debate in the working group 1 of the joint committee for guides in metrology (jcgm wg1), the standards working group involved in the maintenance and development of the guide to the expression of uncertainty in measurement (gum, [1]) and its supplements. the topic is so felt that, at the end of 2019, the “jcgm wg1 workshop on type a evaluation of measurement uncertainty for a small set of observations” was held at the bureau international des poids et mesures (bipm, sèvres, paris). the problem arose following the negative reaction to the committee draft (cd) of the review of the gum, circulated at the end of 2014 [2]. one of the most criticized issues of the draft of the “new gum” is the type a evaluation of uncertainty based on the use of a student’s t probability density function having −1n degrees of freedom, shifted by the mean y of the n observations i y , = 1, 2,...,i n , and scaled by the standard deviation of the mean s n , where (1) and . (2) by following this approach, the type a evaluation of standard uncertainty is , (3) which is not valid for a sample having a size of less than = 4n . such solution originates from a bayesian approach to inference, where improper priors (jeffreys prior) are adopted for the mean  and variance  2 parameters of the parent normal probability density function (pdf), i.e. (4) and = =  1 1 n i i y y n ( ) = = − −  22 1 1 1 n i i s y y n ( ) − = − 1 3 n s u y n n ~ const. abstract an informed type a evaluation of standard uncertainty is here derived based on bayesian analysis. the result is mathematically simple, easily interpretable, applicable both in the theoretical framework of the guide to the expression of uncertainty in measurement (propagation of standard uncertainties) and in that of the supplement 1 of the guide (propagation of distributions), valid for any size greater than or equal to 1 of the sample of present observations. the evaluation consistently addresses prior information in the form of the sample variance of a series of recorded experimental observations and in the form of an educated guess based on expert’s experience. it turns out that distinction between type a and type b evaluation is, in this context, contrived. mailto:carlo.carobbi@unifi.it acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 2 , (5) where ( ) 20p represents the improper prior adopted for  2 , namely . (6) note that the information conveyed by these priors is the one strictly relevant to the character of the two parameters:  is the location parameter and  2 is the scale parameter. in contrast, practitioners in testing and calibration have much richer information about the variability of the measurement process that is represented by (6). the bayesian approach is the one followed by the supplement 1 of the gum (gums1, [3]) and the intent of jcgm wg1 was precisely to align the gum to gums1 by attributing the same student's t probability density to a sample of repeated observations. the problem is that, by doing so, it is possible to propagate the distributions (as foreseen by gums1) but it is not possible to propagate the standard uncertainties (as foreseen by the gum) if the sample size is less than = 4n . this is generally not acceptable (e.g., in destructive), particularly if implemented as a standard (mandatory) method. the gum and the gums1 approaches are therefore inconsistent. they produce substantially different results when random variability is a significant contribution to measurement uncertainty and the number of measurements used for its estimate is low [4]. the jcgm wg 1 did not seemingly yet identify a way out of the inconsistence between the gum and the gums1. both frequentists and bayesians can agree on the fact that the estimate of the average value obtainable from such a small sample is not very reliable. in favor of the bayesian approach to inference, one can observe that no other way to enrich the estimate is available than the use of prior information on the variability of the measurement process that integrates the meagre experimental observation. in this sense, a bayesian approach is useful because, differently from the frequentist approach, it provides us with a method for combining prior information with experimental observation. from the applicative point of view these concepts have relevance to the evaluation of measurement repeatability. measurement repeatability quantifies the variability of measurement results obtained under specified repeatability conditions. measurement repeatability is an essential contribution to measurement uncertainty in every field of experimental activity. in the context of testing and calibration if a stable item is retested or re-calibrated, the new measurement results are expected to be compatible with the old ones. two distinct operators should provide compatible measurement results when testing or calibrating the same item. measurement repeatability is then a reference for qualification of personnel. monitoring measurement repeatability contributes to assuring the validity of test and calibration results. in an accreditation regime [5], measurement repeatability must be kept under statistical control. periodic assessments are carried out by the accreditation body aimed at verifying, through an appropriate experimental check, the robustness of the estimate of measurement repeatability, see [6], equation (6), p. 5 (in italian), and [7], clause 6.6.3. the gum provides type a evaluation of standard uncertainty as the tool to quantify measurement repeatability. type a evaluation is based on a frequentist approach, thus implying that information on the quality of the estimate of measurement uncertainty must be conveyed to the user. this is done in terms of effective degrees of freedom. the gums1 adopts a knowledge based (in contrast to frequentist) approach to model measurement repeatability. the quality of the estimate of measurement uncertainty is accounted for by the available prior knowledge, which eventually determines the width of the coverage interval. the use of numerical methods for professional (accredited) evaluation of measurement uncertainty is expected to increase in the future. indeed, the gums1 numerical method, which is based on the propagation of probability distributions, accounts for possible non-linearity of the measurement model, is simple, less prone to mistakes (partial derivatives are not required), provides all the available information about the measurand in terms of its probability distribution. further, the use of numerical methods is practically unavoidable when the measurement model is complex and/or the measurand is an ensemble of scalar quantities (vector). on the other extreme, the analytical method (based on the law of propagation of uncertainty) is consolidated and the one predominantly adopted nowadays. a further point of strength of the analytical method is its great pedagogical value. achieving consistence between the analytical and numerical approaches to measurement uncertainty quantification is therefore desirable since both have arguments of strength and are expected to coexist in the future. what is proposed here is a knowledge-based approach to the type a evaluation of measurement uncertainty and, specifically, measurement repeatability. an estimate of the repeatability of a measurement system may be available, representative of its performance in testing. this knowledge may be derived from: • systematic recording of periodic verifications of the measurement system • analysis and quantification of the individual sources of variability in the measurement chain • normative reference (for standard measurement systems used in testing) • information from manufacturers of measuring instruments • experience with the specific measurement chain or similar ones. as in the gums1, use is made here of bayesian inference since it provides a straightforward method to incorporate prior knowledge. differently from the gums1 bayesian approach, here an informative prior pdf is assigned to  2 . to obtain analytical results, useful in the framework of the law of propagation of uncertainty, a normal probability model is assumed with a non-informative prior pdf for the mean and a conjugate prior pdf for the variance. in section 2 the theoretical approach is described and in subsection 2.1 is compared with another one [8] previously presented in the scientific literature and proposed by a member of jcgm wg1. in section 3 theoretical results are applied to a practical case, based on the experience of the author, as an assessor of accredited testing laboratories. conclusions follow in section 4. finally, an appendix is devoted to the mathematical derivations supporting the results presented in section 2. ( )~ 2 20p ( )   2 0 2 1 p acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 3 2. type a evaluation when prior information is available by prior information we mean here information on the variability of the measurement process obtained before that a certain test (or calibration) is carried out. let us consider the case in which the a priori information consists of a relatively long series of experimental observations. the important hypothesis that must be verified is that the previous experimental observations are obtained under repeatability conditions that are representative of those that occur during the test, both as regards the measurement system and the measurand. if this is not verified, the a priori information is not valid to represent the variability observed during the test. this hypothesis is necessarily realized following an experimental procedure based on a physical modeling aimed at identifying the causes of the variability and at limiting its effects. it is the experimenter's task to ensure that the hypothesis is verified in practice. in mathematical terms, the bayesian inference is made on the mean value  and the variance  2 of a gaussian pdf assuming an improper uniform pdf for  and a scaled inverse  2 pdf [9], table a.1, p. 576, for  2 . the choice of the improper uniform pdf for  is justified by the desire to avoid introducing an a-priori bias on the best estimate of the measurand value, which in this way depends solely on the experimental observation obtained during the test. the choice of the scaled inverse  2 pdf for  2 is justified by the desire to incorporate prior information while retaining the well-known student’s t as the posterior pdf of  [9], section 3.3, p. 67. the parameters of the scaled inverse  2 pdf are the prior variance  2 0 and the associated degrees of freedom  0 . another advantage stemming from the use of the scaled inverse  2 pdf is the immediate physical interpretation of the degrees of freedom  0 as the number of measurements that have been necessary to derive the prior estimate  2 0 minus 1. at the same time  0 can be linked to the degree of credibility trusted to  2 0 as an estimate of  2 , as it is demonstrated here, through the use of (11). with this choice of the prior pdfs (see the appendix for the derivation) we obtain, for the posterior marginal pdf of  a student’s t pdf with degrees of freedom , (7) shifted in (8) and with scaling factor  2 n n , where . (9) according to this approach, the type a evaluation of standard uncertainty will be . (10) we observe from (7) that the number of degrees of freedom  0 of the prior evaluation of variability,  0 , add up to the number of degrees of freedom −1n with which the variability s is evaluated during testing. the result is valid if the assumption that repeatability conditions are kept the same both in the prior investigation and testing is verified. the estimate (8) is determined by repeated observations obtained during the testing phase because a constant and improper prior pdf for  has been chosen. the result (9) is particularly simple and convincing: the variance  2 n which quantifies the variability of the measurement process is the result of the pooling of the prior variance  2 0 and the sample variance observed in testing 2 s through a weighted average, the weights being the corresponding degrees of freedom. the type a evaluation of standard uncertainty passes from (3), in absence of prior information, to (10), which is valid also for = 1n provided that   0 3 . the following consideration is also of interest. the prior information about the variability of the measurement process may be derived, for example, from the assessment of an expert. a simple form of this prior information is a best estimate  0 and a quantile   that the expert judges to be exceeded with a small probability  . a link can be established among   ,  and  0 for a given  0 . this can be done through the cumulative distribution function of the scaled inverse  2 prior of  2 evaluated at  2 , namely , (11) where is the upper incomplete gamma function with parameters  and z , and ( ) z is the gamma function. if  0 is known then (11), for any given  , implicitly provides a value for  0 . this relationship can be represented through a plot such as the one in figure 1. note from figure 1 that the larger is    0 the smaller is  0 for a given  . the smaller is  for a given    0 the larger is  0 . the idea of pooling prior variability is not new in the context of the gum. it is indeed briefly mentioned in clause 6.4.9.6 of the gums1 and in 9.2.6 of the cd of gum review [2]. 2.1. comparison with the type a evaluation obtained truncating the improper prior for 𝝈𝟐 in a recent paper [8] cox and shirono propose a solution to the problem of the type a evaluation in case of small sample where  t is an upper bound (truncation) value for the improper prior of  2 , i.e.  = − + 0 ( 1) n n  = n y ( )     − + = − + 2 2 0 02 0 1 ( 1) n n s n      = − 2 n n n n              = −       2 0 0 0 2 0 , 2 2 1 2 ( ) ( )  −  = − 1 , exp z z t t dt acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 4 . (12) the prior pdf of  is in [8], as in this work, a constant improper prior. by following [8], the type a evaluation of standard uncertainty can be expressed as  s n , where . (13)   0 if  2n is a function of s , n and  t . as shown in figure 2, it results that    t s also when  t s and n is arbitrarily large. this is problematic because, when observed variability is more credible (larger number of degrees of freedom) than prior knowledge of variability, then the observed variability, not its prior estimate, should dominate the type a evaluation. in other words, setting an upper bound on  2 is acceptable provided that irrefutable evidence is available of an upper truncation value. otherwise, setting a large value with an associated small probability of being exceeded is a more cautionary approach. another limitation of the approach in [8] is that necessarily  2n (see (13),  = 0 if = 1n ) while, according to the solution here proposed, also the case = 1n is tractable. 3. application in the context of accreditation to iso/iec 17025 national accreditation bodies require evaluation of measurement repeatability of the test methods in the scope of accreditation. such evaluation is carried out by testing laboratories through periodic recording of measurement results obtained in representative conditions of actual testing. an estimate  0 with corresponding degrees of freedom  0 is thus obtained. how to incorporate this prior knowledge into test outcome? we here provide a numerical example in the context of electromagnetic compatibility (emc) testing. suppose that the estimate of the non-repeatability of the radiated emission measurement chain is  = 0 0.8 db and  = 0 9 . testing two times ( = 2n ) an absolute deviation between measured values of 1.5 db is obtained, then = 1.5 2s db = 1.06 db. by pooling standard deviations  0 and s we have ( ) = − + = + =01 1 9 10n n , ( )      + − + = = = − + + 2 0 2 22 0 0 1 db 0.76 db 1 1 0 ( 1) 8 1 9 .06 9 . n n s n and      = = = − − 10 0.76 0.60 db 2 10 2 2 n n n n as a second example consider the case where an expert of the specific test method provides a guess  = 0 1 db, based on experience with similar test systems. the expert is also confident that, with a low probability  = 5 %,  exceeds   = 2.5 db. this state of knowledge corresponds to approximately (see figure 1)  = 0 4 , from which  = 5 n (instead of 10, as in the previous example),  = 0.86 n db (instead of 0.76 db), and   = 0.78 db (instead of 0.60 db). 4. conclusions reliable statistical techniques to incorporate prior knowledge into the so-called “type a” evaluation of standard uncertainty should be identified to make evaluation more robust in case of small sample. the use of these statistical techniques should be promoted and confidently accepted in accredited testing if competence requirements are fulfilled. gums1 already provides such a tool by pooling prior variance and sample variance. a bayesian derivation of the gums1 pooled variance is here illustrated along with and a more flexible interpretation aimed at addressing expert’s knowledge as a useful source of reliable information. ( ) ~             2 2 0 1 0 0 t t p ( ) ( )      −    −−    =   −    −    1 2 2 2 2 2 3 , 2 2 11 2 1 , 2 2 1 t t n n sn n n s figure 1: plots of the degrees of freedom as a function of the ratio obtained by solving the implicit equation (11) for three values of probability (see the legend). figure 2: plots of as a function of and for selected values of (see the legend). note that for any value of and for any value of .  0  0     t  t s n    ts  ts n acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 5 according to the results described in this work there is no need to distinguish between type a and type b evaluations since a homogeneous mathematical treatment is used to address prior information about variability (notwithstanding is originated from experimental evidence or expert’s experience) and its pooling with present observation. the main ideas and results in this work were presented by the author of this paper, during the 2019 jcgm-wg1 workshop mentioned in the introduction section. i would like to acknowledge, that during the same workshop, also anthony o’hagan (emeritus professor, university of sheffield), proposed the use of the scaled inverse  2 pdf to solve the problem of the type a evaluation in case of small sample size. his formulation of the solution (still unpublished) was different from mine, but it is remarkable that two researchers, having a completely different background, arrived at similar proposal. the concluding section contains the major achievements of the research presented in the manuscript. it should be concise but informative. when numerical results are an essential part of the research, for instance a wider measurement range, higher uncertainty (6), they should be included in the conclusions. notice that conclusions are not the same as an abstract. references [1] gum: bipm, iec, ifcc, ilac, iso, iupac, iupap and oiml, 2008 guide to the expression of uncertainty in measurement, jcgm 100:2008, gum 1995 with minor corrections. [2] jcgm 100 201x cd (committee draft), evaluation of measurement data guide to uncertainty in measurement, circulated in december 2014. [3] gums1: bipm, iec, ifcc, ilac, iso, iupac, iupap and oiml, 2008 supplement 1 to the ‘guide to the expression of uncertainty in measurement’ – propagation of distributions using a monte carlo method jcgm 101:2008. [4] w. bich, m. cox, r. dybkaer, c. elster, revision of the ‘guide to the expression of uncertainty in measurement’, metrologia 49 (2012) 702–705. doi: 10.1088/0026-1394/49/6/702 [5] iso/iec 17025, conformity assessment–general requirements for the competence of testing and calibration laboratories, int. org. standardization, geneva, switzerland (2017). [6] sinal dt-0002/6, guida al calcolo della ripetibilità di un metodo di prova ed alla sua verifica nel tempo, rev. 0, dicembre 2007. [in italian] [7] b. magnusson, u. örnemark (eds.) eurachem guide: the fitness for purpose of analytical methods – a laboratory guide to method validation and related topics, (2nd ed. 2014). isbn 97891-87461-59-0. online [accessed 22 april 2022] https://www.eurachem.org/index.php/publications/guides/mv [8] m. cox, t. shirono, informative bayesian type a uncertainty evaluation, especially applicable to a small number of observations, metrologia 54 (2017), pp. 642–652. doi: 10.1088/1681-7575/aa787f [9] a. gelman, a. vehtari, j. b. carlin, h. stern, d. b. dunson, d. b. rubin, bayesian data analysis, third edition, crc press, 2014, isbn 9781439840955. appendix we here derive the marginal posterior pdf of  given prior information in terms of the prior pdfs of  and  2 and the set of observations i y , where = 1, 2,...,i n . a uniform prior pdf is assigned to  as , (14) while the prior of  2 is a scaled inverse  2 pdf with prior variance  2 0 and associated degrees of freedom  0 . (15)  and are a-priori independent, then the joint prior pdf of and is, from (14) and (15), . (16) the likelihood of the observations is easily obtained as [9] , (17) where y is a vector representing the set of observations i y , = 1, 2,...,i n . due to bayes theorem the joint posterior pdf of  and  2 is given by . (18) substituting (16) and (17) into (18) and marginalizing with respect to  2 it is readily obtained (19) where ( )|p y represents the marginal posterior pdf of  . it is evident from (19) that ( )|p y is a student’s t pdf shifted in y and scaled by  2 n n , where  2 n is given by (9). ~  2 const. ( )~   −2 2 20 0inv ,  2   2 ( ) ( ) ( )       − +    −    0 2 2 1 2 2 0 0 2 , exp 2 p ( ) ( ) ( ) ( ) ( ) ( )         = −  − −        − + − = −       2 2 2 1 21 2 22 2 2 2 exp 2 ; , 1 exp 2 i n i n y l n s n y y ( ) ( ) ( )      2 2 2, | ; , ,p l py y ( ) ( ) ( )      + −  −  +   − +   0 2 2 2 2 0 0 | 1 1 n n y p y n s http://dx.doi.org/10.1088/0026-1394/49/6/702 https://www.eurachem.org/index.php/publications/guides/mv https://doi.org/10.1088/1681-7575/aa787f measurements of helium permeation in zerodur glass used for the realisation of quantum pascal acta imeko issn: 2221-870x june 2022, volume 11, number 2, 1 4 acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 1 measurements of helium permeation in zerodur glass used for the realisation of quantum pascal ardita kurtishaj1, ibrahim hameli1, arber zeqiraj2, sefer avdiaj1 1 department of physics, university of prishtina “hasan prishtina”, 10000 prishtinë, kosovo 2 department of materials and metallurgy, university of mitrovica “isa boletini”, 40000 mitrovicë, kosovo section: research paper keywords: permeation; helium; diffusion; vacuum; metrology citation: ardita kurtishaj, ibrahim hameli, arber zeqiraj, sefer avdiaj, measurements of helium permeation in zerodur glass used for the realisation of quantum pascal, acta imeko, vol. 11, no. 2, article 27, june 2022, identifier: imeko-acta-11 (2022)-02-27 section editor: sabrina grassini, politecnico di torino, italy received august 3, 2021; in final form march 1, 2022; published june 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: sefer avdiaj, e-mail: sefer.avdiaj@uni-pr.edu 1. introduction pressure is traditionally defined as force per unit area. therefore, to realise the unit of pressure pascal, the most apparent method is to apply a known force to a known surface. essentially, this is how pressure is defined since 1640, when evangelista torricelli invented the mercury barometer [1]. nonetheless, since pressures in the vacuum range do not exert great forces, it becomes more convenient to formulate pascal as the amount of energy per unit volume [2]. consequently, at low pressures, pascal is realized through the law of ideal gases, utilizing the optical measurement of gas density [3]. one of the methods that serves for the latter purpose, relies on fabryperot optical cavities for the measurement of refractivity of the gas being used [4], [5]. cavities made out of ultra-low expansion (ule) glass were initially proposed to measure helium refractivity for the new realisation of pascal [1], [4], [6]. however, the usage of this material has shown some difficulties, as reported in references [2] and [7]. one of these difficulties is the permeability of helium into the cavity material. for this reason, the 18sib04 quantumpascal empir project “towards quantum-based realisations of the pascal” proposed the testing of zerodur as potential cavity material. to estimate whether zerodur is more suitable than the ule glass, different studies are being made, such as the one reported in reference [8]. this evaluation requires the modelling of gas transport dynamics in the material. such modelling, on the other hand, requires knowledge of diffusion and permeability coefficients. therefore, as collaborators in the above-mentioned project, we have studied the permeability of helium into the zerodur material. the measurements were performed in the temperature range 27 °c – 120 °c. determined values of helium gas permeability in the zerodur sample are given in the temperature range 80 °c – 120 °c. 2. materials and methods the vacuum system, in which the measurements were made, consists of two separate volumes (high-pressure volume 𝑉1 and abstract in the new optical pressure standard ultra-low expansion glass cavities were proposed to measure helium refractivity for a new realisation of the unit of pressure, pascal. however, it was noticed that the use of this type of material causes some difficulties. one of the main problems of ule glass is the pumping effect for helium. therefore, instead of ule, zerodur glass was proposed as a material for the cavity. this proposal was given by the vacuum metrology team of the physikalisch-technische bundesanstalt ptb in the quantumpascal project. in order to calculate the flow of helium gas through zerodur glass one has to know the permeation constant k. moreover, the modelling of time dependency of the flow requires the knowledge of diffusion constant d as well. the relation between them is given by k = s · d, where s is the solubility of helium in glass. in our research work we measured permeation of helium gas in zerodur. measurements were performed in the temperature range 80 °c – 120 °c. based on our results, we consider that the zerodur material has potential to be used as cavity material for the new quantum standard of pressure. mailto:sefer.avdiaj@uni-pr.edu acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 2 low-pressure volume 𝑉2 + 𝑉3, as shown in figure 1) having in common the sample wall. each of these volumes has its own vacuum pump, vacuum gauge and valves. in this setup, gas diffuses through the thin sample wall directly into the chamber where the quadrupole mass spectrometer was mounted. the vacuum chamber’s material is stainless steel, the valves used are kf valves with stainless steel body and elastomer seal, whereas the pumps used in this research work are pfeiffer turbopumps. with this vacuum system, we investigated the diffusion of helium in a zerodur sample. the sample (a squared plate with a thickness of 0.2 cm and an area of 2.27 cm2) was received by the physikalisch-technische bundesanstalt in berlin (charge number "105080201"). aluminium joints, iso-kf flanges and elastomer o-rings were used to mount the sample into the system. the orings were used to prevent he permeation and were placed on both sides of the sample, whereas kf flanges were placed next to the o-rings and then were tightened using aluminium joints. to regulate and control the temperature we used htc-5500 /5500pro temperature control unit and heating tapes. the sample was wrapped several times with heating tapes, which were then wrapped with aluminium foil. such sample insulation provided temperature regulation of about ± 1 ℃. temperature changes were recorded using a thermocouple connected to the labview software. before running measurements with the investigated sample, a leak test was performed, to make sure there was no he leakage through the elastomer parts of our system. during the measurements, the following procedures were pursued: the high-pressure chamber and the vacuum chamber were evacuated with the aid of two turbomolecular pumps. when the vacuum chamber had been pumped down to 10-4 pa, helium gas was admitted to the high-pressure side of the mounted sample, at a pressure of 1.32·105 pa and temperature 27.1 ℃. data on the he partial pressure and the chemical composition of the gas species in the vacuum system were obtained by pfeiffer’s prismapro qmg 250 f1 quadrupole mass spectrometer, qms. data recording and analysis was done using qms software pv massspec. the qms was factory calibrated, nonetheless, its calibration expired by the time we conducted the study. to investigate helium diffusion through the zerodur sample we used two main methods. the first method aimed to determine permeation, diffusion and solubility coefficients by recording he partial pressure increase in the vacuum chamber vs. time, until steady state is achieved. after the steady state is reached, the permeated amount of substance vs. time graph is a straight line. the intercept of this straight line with the time axis can be used to calculate the diffusion coefficient, d [9]. the permeation coefficient, k, can be determined from the helium gas flow in the steady state, the known thickness and area of the sample used, and the known pressure in the high-pressure chamber, as described in reference [9]. the relation between the diffusion and permeation coefficient is given by k = s · d, where s is the solubility of he in glass. therefore, with the known diffusion and permeation coefficients, solubility coefficient can be calculated as well. the second method is a modification of the so-called accumulation method, which is described in detail below. following the two methods, our study lasted 70 days. from the first day to the twelfth day the sample was kept at a temperature of 27 °c. then, for the next nineteen days, temperature was raised and adjusted to be 50 ℃. fourteen consecutive days temperature has been changed to 80 ℃, then the next nine days to 110 ℃, the next seven days to 115 ℃ and the last nine days to 120 ℃. to reduce the noise in the recorded signal, dwell time was changed from 32 ms to 1024 ms on the 49th day of the study. also, due to the reduction of he pressure in the high-pressure volume, on the 66th day of the study pressure was increased from 8.87·104 pa to 1.31·105 pa. on the 69th day of the study he gas was pumped from the high-pressure volume, to finally record changes in the he signal. 3. results and discussion modified accumulation method with the previously described procedure, he signal was recorded after its permeation into the zerodur sample. graphs presented at figure 2 display some of the measurements of this signal (representing the partial pressure of he, in pa) at different times and temperatures during the study. as can be noticed from the graphs listed in figure 2, even with the increased dwell time, the recorded he signal was still very low. this may be because he permeability in zerodur material is so small that it cannot be recorded with our experimental scheme. therefore, we have conducted figure 1. schematic representation of the vacuum system used to study the permeability of helium gas into the zerodur material. acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 3 measurements with a modified version of the accumulation method as well [10]. to record measurements by this method, the valve separating the pump and the vacuum system (valve 3, in figure 1), closes. this enables the accumulation of helium gas that has permeated the sample and allows a longer time for the qms to record the signal, before it is pumped. while keeping valve 3 closed, the total pressure is monitored in the full range gauge (gauge 5, figure 1). as soon as pressure reaches a value of 10-2 pa the valve opens to prevent damage to the qms filament. this procedure is repeated several times, for different times and temperatures. in the meantime, we tried to conduct measurements with the same method, while keeping valve 2 closed (the valve separating volume 𝑉2 from volume 𝑉3, in figure 1). this enables for the gas that permeates the zerodur sample to not glide into the vacuum chamber where the qms is located, and it allows us to make comparisons between the signal recorded in these two states. if after closing valve 2 the signal value drops, it indicates that the recorded helium signal is actually the one permeating the sample. since no significant values of the helium signal were recorded up to the temperature of 80℃, the results of the measurements conducted with the accumulation method are presented for the temperatures of 80 ℃, 110 ℃, 115 ℃ and 120 ℃. an example of recorded measurements is shown in figure 3. the first three measurements represent the helium signal recorded by the accumulation method when valve 2 is open while the last three measurements indicate the helium signal recorded by the accumulation method when valve 2 is closed. thus, by comparing the recorded measurements, with valve 2 open or closed, the helium gas flow can be determined. with the known gas flow, the coefficient of permeability can be determined utilizing the known mathematics of the problem. the latter has been discussed in many articles and books, such as [11], [12], and [13]. here we refer to reference [9] where the gas flow q, through a plane sample with pressure gradient ∆p, cross-sectional area a and thickness l, is given by: 𝑞 = 𝐾 𝐴 𝑝 − 𝑝0 𝑙 , (1) figure 2. recorded he signal at different times and temperatures during the study. p(cdg) indicates the he pressure at the high-pressure volume, measured with the capacitance diaphragm gauge – cdg. in addition to the date of measurements recording, the start time of their registration and dwell time are marked as well. acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 4 where k is the permeability coefficient, p is helium pressure in the high-pressure volume and p0 is the helium partial pressure in the low-pressure volume. as the low-pressure side of our sample is a high vacuum, it was assumed that the partial pressure of helium at this side is zero. the volume of the low-pressure part of the system was determined by gas expansion method [14]. the sample surface and thickness were determined geometrically. values of the permeability coefficient for different temperatures and at different study times are listed in table 1. 4. conclusions after 70 days of studying the helium permeability in the zerodur material, the stationary state of the diffusion process was not recorded. this is because, the helium signal is so weak that it cannot be fully recorded with our vacuum system. as it was not possible to analyse the stationary state of helium diffusion in the zerodur material, the time constant was not recorded, and consequently the helium diffusion coefficient in the zerodur material was not determined. on the other hand, with the accumulation method, we determined the helium permeability coefficient in the zerodur material, for different temperatures. to our knowledge, there are no data in literature concerning the helium permeability in zerodur. from the obtained results we estimate that the value of this coefficient is quite low and that it does not change much with increased temperature. for this and for the fact that the time constant of this material is observed to be long, we can conclude that zerodur material has the potential to be used as cavity material for the quantum standard of pressure measurement. overcoming other problems regarding the performance of refractometers is among the objectives of the "quantumpascal" empir project. acknowledgement the research equipment in our laboratory was financed by usa embassy in kosovo and the ministry of education, science and innovation of kosovo. we also thank prof. lars westerberg from uppsala university for providing us with some of the vacuum components used in this research. references [1] j. hendricks, quantum for pressure, nat. phys., vol. 14, no. 1, pp. 100–100, 2018. doi: https://doi.org/10.1038/nphys4338 [2] k. jousten, j. hendricks, d. barker, k. douglas, s. eckel, p. egan, j. fedchak, j. flügge, ch. gaiser, d. olson, j. ricker, t. rubin, w. sabuga, j. scherschligt, r. schödel, u. sterr, j. stone, g. strouse, perspectives for a new realization of the pascal by optical methods, metrologia, vol. 54, no. 6, 2017, pp. s146–s161. doi: https://doi.org/10.1088/1681-7575/aa8a4d [3] k. jousten, a unit for nothing, nat. phys., vol. 15, no. 6, 2019, p. 618. doi: https://doi.org/10.1038/s41567-019-0530-8 [4] p. f. egan, j. a. stone, j. h. hendricks, j. e. ricker, g. e. scace, g. f. strouse, performance of a dual fabry–perot cavity refractometer, opt. lett., vol. 40, no. 17, 2015, p. 3945. doi: https://doi.org/10.1364/ol.40.003945 [5] z. silvestri, d. bentouati, p. otal, j. p. wallerand, towards an improved helium-based refractometer for pressure measurents, acta imeko, vol. 9, 2020, no. 5, pp. 305-309. doi: http://dx.doi.org/10.21014/acta_imeko.v9i5.989 [6] j. scherschligt, j. a. fedchak, z. ahmed, d. s. barker, k. douglass, s. eckel, e. hanson, j. hendricks, n. klimov, t. purdy, j. ricker, ro. singh, j. stone, quantum-based vacuum metrology at nist, arxiv, 2018, pp. 1-49. doi: https://doi.org/10.1116/1.5033568 [7] s. avdiaj, y. yang, k. jousten, t. rubin, note: diffusion constant and solubility of helium in ule glass at 23 °c, j. chem. phys., vol. 148, no. 11, 2018, pp. 3–5. doi: https://doi.org/10.1063/1.5019015 [8] j. zakrisson, i. silander, c. forssén, z. silvestri, d. mari, s. pasqualin, a. kussicke, p. asbahr, t. rubin, o. axner, simulation of pressure-induced cavity deformation – the 18sib04 quantumpascal empir project, acta imeko, vol. 9, 2020, no. 5, pp. 281-286. doi: http://dx.doi.org/10.21014/acta_imeko.v9i5.985 [9] b. sebok, m. schülke, f. réti, g. kiss, diffusivity, permeability and solubility of h2, ar, n2, and co2 in poly(tetrafluoroethylene) between room temperature and 180 °c, polym. test., vol. 49, 2016, pp. 66-72. doi: https://doi.org/10.1016/j.polymertesting.2015.10.016 [10] technical specification iso/ts — procedures to measure and report, vol. 2018, 2018. [11] j. crank, the mathematics of diffusion, oxford university press, london and new york, 1975. [12] v. o. altemose, helium diffusion through glass, j. appl. phys., vol. 32, no. 7, 1961, pp. 1309-1316. doi: https://doi.org/10.1063/1.1736226 [13] f. j. nortonn, helium diffusion through glass, j. am. ceram. soc., vol. 36, no. 3, 1953, pp. 90-96. doi: https://doi.org/10.1111/j.1151-2916.1953.tb12843.x [14] s. avdiaj, j. setina, b. erjavec, volume determination of vacuum vessels by gas expansion method, mapan j. metrol. soc. india, vol. 30, no. 3, 2015, pp. 175-178. doi: https://doi.org/10.1007/s12647-015-0137-1. table 1. determined values of the permeability coefficient of helium gas in the zerodur sample, for different temperatures. date and time of commencement of data registration temperature t in °c permeation coefficient k in cm² / s 31.08.2020 12:51h 80 1.12 ∙ 10−15 05.09.2020 18:24h 110 1.30 ∙ 10−14 08.09.2020 17:20h 110 1.59 ∙ 10−14 09.09.2020 17:18h 115 2.09 ∙ 10−14 12.09.2020 15:27h 115 5.20 ∙ 10−14 19.09.2020 15:54h 120 6.47 ∙ 10−14 figure 3. an example of the data conducted with the accumulation method at temperature 110 °c. the first three measurements represent the helium signal recorded by the accumulation method when valve 2 is open while the last three measurements show the helium signal recorded by the accumulation method when valve 2 is closed. https://doi.org/10.1038/nphys4338 https://doi.org/10.1088/1681-7575/aa8a4d https://doi.org/10.1038/s41567-019-0530-8 https://doi.org/10.1364/ol.40.003945 http://dx.doi.org/10.21014/acta_imeko.v9i5.989 https://doi.org/10.1116/1.5033568 https://doi.org/10.1063/1.5019015 http://dx.doi.org/10.21014/acta_imeko.v9i5.985 https://doi.org/10.1016/j.polymertesting.2015.10.016 https://doi.org/10.1063/1.1736226 https://doi.org/10.1111/j.1151-2916.1953.tb12843.x https://doi.org/10.1007/s12647-015-0137-1 microsoft word 18-143-1-ga.doc acta imeko  july 2012, volume 1, number 1, 36‐41  www.imeko.org    acta imeko | www.imeko.org    july 2012 | volume 1 | number 1 | 36  high‐speed multichannel impedance measuring system  marco j. da silva, eduardo n. dos santos, tiago p. vendruscolo  department of electrical engineering, universidade tecnológica federal do paraná, av. sete de setembro 3165, 80230‐901 curitiba‐pr,  brazil    keywords: electrical impedance; multichannel; high‐speed measurement; fluid distribution measurement; planar sensor   citation: marco j. da silva, eduardo n. dos santos, tiago p. vendruscolo, high‐speed multichannel impedance measuring system, acta imeko, vol. 1, no. 1,  article 9, july 2012, identifier: imeko‐acta‐01(2012)‐01‐09  editor: pedro ramos, instituto de telecomunicações and instituto superior técnico/universidade técnica de lisboa, portugal  received january 24 th , 2012; in final form june 1 st , 2012; published july 2012  copyright: © 2012 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: this work was partially funded by bg group, brazil and petrobras, brazil  corresponding author: marco j. da silva, e‐mail: mdasilva@utfpr.edu.br  1. introduction  impedance sensors, in which the measurand causes a variation of an electrical characteristics such as resistance or capacitance, have found widespread use in industrial applications mainly due to their simplicity, low fabrication costs and robustness [1,2]. impedance measurement is a common tool for the characterization of electrical properties of materials and substances, in which measurement times of seconds to minutes are used to achieve high measurement accuracy in the chemical analysis [3,4]. however, in industrial applications, measuring times in the range of microseconds to milliseconds are required in order to investigate dynamic processes, e.g. mixing of substances in chemical reactors, or multiphase flow in pipelines [5]. accuracy requirements in such applications are less critical, since substances involved, and consequently their electrical properties, are known a-priori. in addition, multipoint impedance measurement is also often required in order to obtain spatial or distributed information, for instance, in the imaging of multiphase flows [6,7] or for the measurement of distributed sensors [8-14]. hitherto used systems for multichannel measurement are limited to the evaluation of a single electric parameter such as resistance or capacitance [7-14]. commercial measuring instruments such as lcr meters can only reach repetition rates of few measurements per second and are, therefore, not suitable for high-speed measurements which are necessary in some industrial application, such as the investigation of fluid flow. in recent years some efforts have been made by applying dual-modality measuring techniques for enhancing the range of application in which two different sensing techniques are used to distinguish more than two substances. furthermore, the use of single-point electrical impedance/admittance measurements, i.e. the measurement of both real and imaginary parts (or amplitude and phase), has been reported in the past [15-18]. in this paper, we introduce a novel high-speed multichannel electronics being able to simultaneously determine the conductive and the capacitive component of a multipoint sensor which is arranged in a matrix-like form such as wire-mesh sensors [6,7] or planar array sensors [12-14]. circuits are based on classical impedance measurement techniques and the basic idea of two components evaluation is to apply an excitation signal composed of two distinct frequencies. based on amplitude measurements only and spectral analysis of data it is possible to determine the resistive and capacitive components at each sensing point. multichannel interrogation is achieved by a multiplexed excitation-sensing scheme. in  this  paper,  a  novel  high‐speed  multichannel  impedance  measuring  system  in  presented.  the  measurements  are  based  on  simultaneous excitation with two distinct frequencies to interrogate the multiple sensing point of a given sensor. received signals are  analogue‐to‐digital converted (with a daq card) and the amplitudes of each frequency are determined using fft  implemented  in  labview. the capacitive and conductive parts of impedance are calculated based on amplitude measurements. the developed system  can operate 8 transmitter and 8 receiver electrodes at a frame repetition frequency of up to 781 hz, i.e. single channels are sampled at  6,248 hz. the system has been evaluated by measuring reference components. deviations from references values are below 10%  which considering the fast repetition frequency of measurements is satisfactory. the developed system was applied to visualize the  fluid distribution over the surface of planar multipoint sensor. two different liquids (oil and water) and air were evaluated and their  spatial distribution over the sensor’s surface was correctly visualized.   acta imeko | www.imeko.org    july 2012 | volume 1 | number 1 | 37  in section 2 the basic measuring principle is first described and then the multichannel electronics is presented. the new electronics is evaluated in section 3. an application example is given in section 4, in which a planar array sensor is used to visualize the spatial distribution of different fluids over the sensor surface. the main achievements are summarized in the end. 2. system development  2.1. one‐channel circuit analysis  for impedance measurements we chose the circuit synonymously known as transimpedance amplifier, autobalancing bridge or current-voltage converter, as depicted in figure 1. since the developed system is meant to be used to identify different fluids in an approach similar to [13], the electric model that better represents the characteristics of a fluid is a parallel rc circuit [19, 20]. thus, the unknown impedance was assumed to be capacitive only. in figure 1, vi is the excitation voltage, zx represents the unknown impedance and cf along with rf the feedback network. furthermore, cs1 and cs2 represent the stray capacitances to ground which are caused, for instance, by cables used to connect the circuit with a sensor. in principle these stray capacitances have no influence in the circuit since cs1 is directly driven by the source voltage and cs2 is virtually grounded by the opamp. the impedance is determined by measuring the voltage at the opamp output. assuming that the opamp is ideal the complex-value output voltage vo is determined by o x x i f f g j c g j c          v v , (1) where ω = 2πf, f is the frequency of the sinusoidal excitation signal and j is the imaginary unit ( j2 = −1 ). the bode diagram for the amplitude of eq. (1), using typical r and c values, is shown in figure 2. two plateaus can be readily identified – one at low frequencies and the other at high frequencies. knowing that a capacitor works as an open circuit in dc, the smaller the frequency, the lower the influence of the capacitive part, that is, the impedance is purely resistive. on the other hand, the higher the frequency, the higher the influence of the capacitive part and the lower is the resistive part relative contribution. the magnitude of each plateau is given by the quotient of gx/gf and cx/cf which are obtained taking the limit for f → 0 and f → ∞ of the modulus of equation (1), in the form     22 2 x xo 22 2i f f 2 2 g f c g f c      v v . (2) a simplified way to view the behaviour of an auto-balancing bridge is according to the graph showing in the figure 3. further shown in the figure, is the operational amplifier frequency response. in principle any two frequencies may be chosen for determining the unknown components. the simplest choice, however, is to select two frequencies located exactly in each plateau. hence, the two unknown parameters are found by low o x i f f f g g   v v (3) and high o x i f f f c c   v v . (4) since the resistance r is directly proportional to the conductivity of the material, and the capacitance c is directly proportional to the electric permittivity of the material, the measurement of the resistance and capacitance is an indication of the electrical conductivity and permittivity of the substance in question. substances can thus be distinguished from each other and thus correctly indentified based on these measurements. 2.2. multichannel system  the developed multichannel system is schematically shown in figure 4. the developed hardware is basically divided into four blocks. the first is the transmitter printed-circuit board (pcb), in which there are two direct digital synthesizers (ad9833 from analog devices) for generating the two figure  1.  practical  circuit  for  measuring  impedances  formed  as  parallel circuit of a capacitor and a resistor.  figure 2. frequency response for components values cf = 10 pf, gf = 10 µs  (100 kω), cx and gx (rx) are indicated in the plots.  figure 3. asymptotic and simplified frequency response of a practical circuit  for  impedance  measurement  considering  the  opamp  non‐ideal  frequency  response.  acta imeko | www.imeko.org    july 2012 | volume 1 | number 1 | 38  sinusoidal signals at different frequencies. the direct digital synthesizer (dds) is a programmable integrated circuit, which can generate signals up to 12.5 mhz. the dds are programmed via the employed data acquisition card (pcie-7841r from national instruments) using a serial peripheral interface (spi) and are user-defined via software developed in labview. as described in the previous section, two frequencies are sufficient for the evaluation of parameters r and c. in this way, a signal composed by two single frequencies is generated by summing together the signals of two separated dds circuits by means of an adder circuit using operational amplifiers. the excitation signal (containing two frequencies) is connected to up to eight excitation electrodes with the help of analogue switches (adg1434). the non-selected channels are grounded in order to allow for a multiplexed excitation scheme. buffers (lmh6722) at the output assure that the excitation signals have low impedance. the signal is than connected to the transmitter electrodes of a given sensor. the receiver electrodes of the sensor are connected to the receiver board, where transimpedance amplifiers (figure 2) convert the signals into proportional voltage, as described in section 2.1, whereby the operational amplifier used is the opa656 (500 mhz unity gain bandwidth) with feedback components of 4.7 pf and 100 kω. the voltage signals from the opamp are a/d-converted by the acquisition card. the digitalized signals are processed in the host pc with a program created in labview. the amplitudes of the excitation signals are determined using fast fourier transform (fft) processing tools. in order to allow for fast response, each channel is sampled at 200 khz (maximum possible sampling frequency – simultaneous sampling) and 32 samples are processed. for the reason that for one single image to be obtained, 8 transmitter electrodes must be activated, up to 781 hz frame rate is possible for sensors arranged in an 8 × 8 matrix configuration. although the pcie-7841r analogue input channels have a bandwidth (−3db) of 1 mhz, high frequency sinusoidal signal was configured to 1.825 mhz in order to reach the capacitance plateau as schematically shown in figure 3. the voltage signal at 1.825 mhz is attenuated by −6.45 db. the low frequency signal used was 75 khz. since the sampling frequency is 200 khz (nyquist frequency = 100 khz), the 1.825 mhz signal is undersampled, appearing as 25 khz component in the fft spectrum, as schematically shown in figure 5. though undersampled and attenuated, the amplitude of the high frequency component can be correctly evaluated after calibration measurements, i.e. measurements with known impedances or fluids. 3. system evaluation  the developed system was evaluated to verify the performance of step and frequency responses. further, accuracy of the developed system was evaluated by measuring different impedances with known values consisting of a parallel rc circuit, as described below. 3.1. frequency response  a sinusoidal voltage signal swept in frequency from 2 khz to 20 mhz was used to test the frequency response of the auto-balancing bridge circuit. the signal was applied to the input of a known rc circuit (measurand) which was connected to the auto-balancing bridge circuit. the output signal of the receiver module was measured with an oscilloscope. thus, we surveyed the frequency response curves for four different combinations of resistors and capacitors with values similar to the ones in figure 2 for comparison. the obtained frequency responses are depicted in figure 6. the two plateaus can be clearly seen in the experimental results. also, the shape of the curves fits very well with the theoretical response, as anticipated in section 2.1. only the absolute measured values (figure 6) slightly deviate from the theoretical ones (figure 2). this fact implies that the circuit figure  4.  block  diagram  of  the  developed  multichannel  impedance  measuring system.  10075 1,82525 frequency (khz) a m p lit u d e figure 5. schematic representation of amplitude spectrum obtained via fft analysis.  figure  6.  frequency  response  in  circuit  for  components  values  cf  =  10pf,  gf = 10µs (100 kω), cx and gx (rx) are indicated in the plots.  acta imeko | www.imeko.org    july 2012 | volume 1 | number 1 | 39  input-output response must be adjusted by using some reference components. 3.2. step response  in order to verify the maximum achievable repetition in a multichannel system, the step response of the auto-balancing bridge circuit was evaluated. using a waveform generator capable of amplitude modulation (agilent 33220a), in which a carrier sine wave of 1 mhz was modulated with a square wave signal of 50 khz, the step response of the circuit can be evaluated. an oscilloscope at the output shows the resulting waveform, as depicted in figure 7. in this experiment the rc circuit used was a 2.2 pf capacitance in parallel with a 1 mω resistance. as shown in the figure 7, the time response of the carrier (1 mhz) is almost instantaneous for the time scale used, so that the system can be properly used in the intended kilohertz repetition range with no loss in measurement accuracy. 3.3. static impedance measurement  in order to evaluate the accuracy of the measuring circuit, different combinations of known resistor and capacitors were measured for a single transmitter-receiver pair, which is representative for all channels. excitation amplitude of two components was kept constant, and the amplitude of output signals was determined via 32-point fft and at a repetition rate of 6.248 khz (single channel measurement), as described earlier. twenty two combinations of resistors and capacitors were measured, with values between 100 kω 2.2 mω and 1 pf 10 pf. the parity plots of measured (eq. 3 and 4) and reference values are shown in figures 8 and 9. deviations from references values are below 10% which is satisfactory considering the fast repetition frequency. higher accuracy may be achieved by lowering the repetition frequency of measurements, thus decreasing random errors. 4. example of an application  in this section, the developed electronics is used to operate a planar array sensor. the sensor is composed by a number of identical sensing elements which are individually interrogated by the electronics. as an example of application, the spatial distribution of two different liquids and of air is visualized based on their electrical properties. 4.1. planar array sensor  the sensor employed in this section is composed of 256 single interdigital sensing structures which are multiplexed in a matrix with 16 driver (rows) and 16 sensing electrodes (columns). figure 10 depicts the sensor. since the developed electronics can handle 8 x 8 electrodes, only one quarter of the sensor was interrogated (region indicated by the yellow square in figure 10). 4.2. data processing  since the resistance r is directly proportional to the conductivity of the material, and the capacitance c is directly proportional to the electric permittivity of the material, from equations (3) and (4) it is possible to show that the amplitudes of the voltage measured in each plateau is proportional to the electrical properties of the fluid at the sensing elements of a planar sensor. for simplicity we will define vκ as the voltage of low-frequency component and vε, as the voltage of high-frequency component. the measured output voltages vκ correspond to the conductivity value κ at the crossing points according to κv k   , (5) where k is a proportionality factor which depends on electronic circuit and sensor constants. in this way, the individual substance present over a sensing point can be identified by evaluating the output voltage vκ and thus an electrically conducting and a non-conducting fluid (e.g. air and water) can be distinguished. in order to discriminate two non-conducting fluids like air and oil, the capacitance or the permittivity of a sensing element must be evaluated. the voltage vε is proportional to the relative permittivity εr of the fluid present at a crossing point according to figure 7. step response in circuit for a rate of 50 khz.  figure 8. parity plot of measured and reference values of capacitance. the  dotted lines show the +/‐10% deviation from ideal line.  figure 9. parity plot of measured and reference values of conductance. the dotted lines show the +/‐10% deviation from ideal line.  acta imeko | www.imeko.org    july 2012 | volume 1 | number 1 | 40  ε rv a b   , (6) where a and b are constants that encompass the specific parameters of the electronics and sensor. repeated successive activation of all transmitter electrodes and measurement of currents for all receiver channels gives a three-dimensional data matrix with electrical voltage values denoted by v(i,j,k) which corresponds either to conductivity or permittivity distributions over the pipe’s cross section. here, i and j are the spatial indices (corresponding to the wire numbers 1 to 8) and k is the temporal index of each image. equations (5) and (6) hold for every sensing element in the matrix of a planar sensor. a calibration of all transmitter-receiver pairs is necessary to obtain accurate output readings. calibration may be performed by measuring the output voltages for two known substances (for instance water and air) and calculating parameters k(i,j) or a(i,j) and b(i,j) for each transmitterreceiver channel. 4.3. substance distribution measurement  in order to show the imaging capability of the spatial distribution of a substance over the sensor surface, a simple experiment was conducted. tap water and silicone oil were intentionally distributed as two column arrangement over the sensor surface, as illustrated in figure 11. the electrical conductivity of water, measured with a conductivity meter, was 398 μs/cm. the conductivity of oil can be neglected. reference relative permittivities of air, oil and water are 1, 2.8, and 80, respectively [19, 20]. prior to the start of the experiment a two-point calibration was realized with air and water as references. in this test, 100 frames were acquired and averaged. figure 12 shows the resulting images, in which a logarithmic colour scale for the permittivity and linear colour scale for conductivity values was used. all three substances involved (air, oil and water) can be clearly recognized in the permittivity image, while in the conductivity one only the conducting from the non-conducting phase is distinguished, as expected. comparison via visual inspection of the distribution shows good agreement with the measurements. the measured values of conductivity and permittivity for the sensing elements at row number 4 (middle line of images) were figure  10.  photography  of  planar  array  sensor  employed  for  substance distribution  measurement.  region  in  the  sensor  indicated  by  the  dotted yellow square is the interrogating area (8 x 8 single sensors).   water oil figure  11.  distribution  of  substances  imposed  over  the  surface  of  planar sensor.  figure 12. measured distributions of electrical permittivity and conductivity. 1 2 3 4 5 6 7 8 10 -2 10 -1 10 0 10 1 10 2 10 3 mearuring point # c o n d u c ti v it y (  s /c m ) 1 2 3 4 5 6 7 8 10 0 10 1 10 2 mearuring point # r e la ti v e p e rm it ti v it y ( -) figure 13. values of measured electrical conductivity and permittivity taken from sensors at line number 4 (central line of interrogating area).   acta imeko | www.imeko.org    july 2012 | volume 1 | number 1 | 41  taken and plotted in figure 13, in order to show quantitative data. a logarithmic scale for both permittivity and conductivity is used for comparison purposes. it can be seen that the conductivity values of air and oil are very low (around 10-2 μs/cm) and the conductivity value of water is about 400 μs/cm, as expected. permittivity values for air, oil and water are in good agreement with reference ones. all three substances can be properly distinguished from each other based on their electrical properties. 5. conclusions  a novel measuring system for multipoint impedance measurement of capacitive and resistive components was presented and tested. the impedance is determined based on the measurement of the amplitudes at two distinct frequencies. the system was evaluated showing appropriated accuracy and fast response. imaging capability of the multichannel system was explored by operating a planar array sensor. conductivity and permittivity images are in good agreement with the imposed distribution over sensor surface. further work will focus on data fusion of the two measured parameter and application to the imaging of dynamic flow. acknowledgement  authors acknowledge financial support from petrobras, brazil and bg group, brazil for partially funding this project. e.n. santos thanks prh10/anp/petrobras for doctoral degree scholarship and t.p. vendruscolo thanks ibp and bg group, brazil for a master degree scholarship. references  [1] r.pallas-areny and j.g.webster, sensors and signal conditioning, 2nd ed., new york: willey, 2001, isbn 0471332321. 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[20] d.r.lide, crc handbook of chemistry and physics, 85th ed., boca raton, fl: crc press, 2005, pp. 6-155−6-177, isbn 978-0849304859. spatiotemporal analysis of human gait, based on feet trajectories estimated by means of depth sensors acta imeko issn: 2221-870x december 2022, volume 11, number 4, 1 7 acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 1 spatiotemporal analysis of human gait, based on feet trajectories estimated by means of depth sensors jakub wagner1, roman z. morawski1 1 warsaw university of technology, faculty of electronics and information technology, institute of radioelectronics and multimedia technology, nowowiejska 15/19, 00-665 warsaw, poland section: research paper keywords: gait analysis; gait asymmetry; healthcare; depth sensor; measurement data processing citation: jakub wagner, roman z. morawski , spatiotemporal analysis of human gait, based on feet trajectories estimated by means of depth sensors, acta imeko, vol. 11, no. 4, article 9, december 2022, identifier: imeko-acta-11 (2022)-04-09 section editor: eric benoit, université savoie mont blanc, france received july 19, 2022; in final form november 4, 2022; published december 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was supported by the institute of radioelectronics and multimedia technology at the faculty of electronics and information technology, warsaw university of technology. corresponding author: jakub wagner, e-mail: jakub.wagner@pw.edu.pl 1. introduction the analysis of human gait is a fertile research field with various applications related to healthcare, in particular – to monitoring of elderly persons. gait analysis can be used, e.g., for estimating the risk of falling [1], diagnosing cognitive impairments [2] and optimising the rehabilitation after a stroke [3]. such analysis may involve the estimation of [4] – angles between body segments, – ground-reaction forces, – electromyographic signals, – spatiotemporal gait parameters. this paper is focused on the estimation of the latter, and in particular of [5]: – length and duration of steps and strides, – walking speed, – cadence (or the number of steps per minute), – duration of the phases of a gait cycle. the gait cycle is the time interval encompassing the repeatable pattern of movement performed during walking, i.e. a single stride or two steps (one by each foot). within the gait cycle, the so-called swing phase and stance phase can be identified. these phases correspond to the time intervals when a given foot is moving or resting on the floor, respectively. the duration of the so-called double-support phase, during which both feet contact the floor, is also of interest for healthcare practitioners [2]. important information, useful from the healthcare perspective, can be extracted from the average values of the aforementioned parameters and from some indicators characterising their variability [6]. estimates of those parameters can also be used for obtaining information about gait asymmetry, which is considered particularly useful in the treatment of strokeinduced hemiplegia [7]. the gait asymmetry is typically quantified by comparing the values of some parameters, characterising the left and right sides of the body, e.g. by computing the ratio between the duration of the stance phases of the two feet [3]. the spatiotemporal gait parameters can be estimated by a trained clinician using a stopwatch; the simplicity and low cost of such an examination method are behind its prevalence, although its accuracy and repeatability are quite limited [8]. a technological solution that allows for more reliable analysis of gait, relatively widespread in research laboratories and clinical facilities, involves abstract this paper addresses a methodology for the analysis of human gait, based on the data acquired by means of depth sensors. this methodology is dedicated to healthcare-related applications and involves the identification of the phases of the gait cycle by thresholding estimates of velocities of the examined person’s feet. in order to assess its performance, a series of experiments was carried out using a reference gait-analysis system based on a pressure-distribution-measurement platform. an original method for quantifying gait asymmetry, based on a quasi-correlation between the feet trajectories, was also proposed and tested experimentally. the results of the reported experiments seem to promise a high applicability potential of the considered methodology. mailto:jakub.wagner@pw.edu.pl acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 2 the use of platforms or treadmills equipped with pressure sensors. even more detailed and useful information can be obtained by means of optoelectronic systems based on multiple video cameras, tracking the motion of special markers attached to the person’s body; however, such systems are quite expensive and need to be installed in a large room. the last decade has brought about the development of other gait-analysis techniques based on various types of sensors, including wearable sensors, which seem to be applied most frequently. this paper is devoted to a technique whose applicability potential has not yet been fully explored, viz. a technique based on depth sensors. in the last decade, the possibility of using depth sensors for the analysis of human gait has attracted considerable interest of the scientific community. this interest seems to be justified by the facts that 1) such sensors are relatively inexpensive and commercially available, and 2) the acquisition of data representative of human movement can be quite fast and convenient, viz. it can be performed in the natural conditions of overground walking, without requiring the person to wear any devices or markers on the body or clothes. moreover, the data acquired by means of such sensors convey quite detailed and rich information about human movement, allowing for both its spatiotemporal and kinematic analysis. such sensors do not seem, however, as reliable – in terms of the attainable measurement uncertainty – as marker-based optoelectronic systems or pressure-measuring platforms. furthermore, unlike wearable sensors, depth sensors have a limited field of view to which the examination must be confined, and their reliability depends – to some extent – on the angle at which the examined person is observed. given their advantages and disadvantages with respect to other technologies applicable for gait analysis, the depth sensors are often considered potentially useful for rapid screening of patients prior to more detailed diagnostic procedures. the gait-analysis systems based on depth sensors may, therefore, become quite common in clinical practice and inhome monitoring; the development of suitable data-processing methods and research aimed at assessing the validity of those methods is thus necessary [9]. a recent review of issues related to the development of gait-analysis systems based on depth sensors and other techniques can be found in [10], chapter 8. a depth sensor typically consists of a projector and a camera, both operating in the infrared range of electromagnetic radiation. the data acquired by means of such a sensor represent its distance to the objects present in its field of view. those data are organised in sequences of so-called depth images. in a depth image, each pixel represents the three-dimensional position of a point belonging to a surface reflecting infrared radiation. an exemplary depth image is shown in figure 1a. what makes the depth sensors particularly promising for gait analysis is the existence of algorithms for automatic identification of human silhouettes and for the localisation of various anatomical landmarks, such as the head, the feet, selected joints etc. such an algorithm is implemented in one of the most popular devices comprising depth sensors, viz. the microsoft kinect device [11], including its kinect v2 model which has been used in the experiments reported in this paper. similar algorithms, which can be used for processing data from other depth sensors, are available commercially. an exemplary human silhouette, identified using the algorithm implemented in the kinect v2 device, is shown in figure 1b. various approaches to the application of depth sensors in the systems for gait analysis have been reported in the last decade. the difficulties in developing such systems are mainly related to the uncertainty of localisation of feet and other anatomical landmarks, and from the limitations of the depth sensors’ field of view. the techniques proposed so far differ in terms of the examination setup (which may involve the use of one or more depth sensors, a treadmill, wearable devices etc.), the dataprocessing methods used for the identification of the gait-cycle phases, and other aspects [12]–[14]. the methodology for spatiotemporal gait analysis considered in this work consists in obtaining some estimates of feet positions using a depth sensor, followed by processing them by means of a procedure which comprises the following operations: – estimation of feet velocities; – identification of the swing and stance phases by thresholding feet velocities; – estimation of selected spatiotemporal gait parameters; and – computation of indicators of gait asymmetry. the above-listed operations are described in more detail in section 2. section 3 is devoted to the description of the experiments that were carried out in order to assess the applicability potential of the considered methodology for gait analysis. the results of these experiments are presented in section 4 and discussed in section 5 where conclusions are drawn. the following acronyms appear in the rest of this paper: – tvr – total variation regularisation; – str – stance time ratio; – gct – gait cycle time; – spm – steps per minute. 2. data processing procedure 2.1. estimation of feet velocities the estimates of the positions of feet, obtained by means of a kinect v2 device, are subject to non-negligible measurement uncertainty. among the 25 anatomical landmarks, whose positions can be estimated using the algorithm implemented in that device, the feet and ankles are usually localised with the least accuracy [15]. that algorithm performs poorly in distinguishing a foot from the ground when the foot is resting; hence, the estimates of feet positions are more accurate at the moments when the feet are off the ground [16]. the antero-posterior component of those estimates (i.e. the one corresponding to the walking direction) is more accurate than the vertical one and the medio-lateral one; furthermore, those estimates are more accurate if the person is walking rather than when the range of movement of the feet is small (e.g. in sit-to-stand tests) [15]. a) b) figure 1. a depth image in which brighter pixels represent larger distance from the sensor (a); a human silhouette, identified in that image by means of the kinect device (b). acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 3 when compared with the reference data acquired by means of marker-based optoelectronic movement-analysis systems, the foot position estimates obtained by means of kinect v2 devices have turned out to be somewhat biased [17]. this is, however, of lesser practical importance for the gait-analysis methodology proposed here, because this methodology refers to the differences among the estimates of the positions of feet rather than to their absolute values. on the other hand, those estimates are subject to random errors. the mean euclidean distance between those estimates and the corresponding reference values has been reported to be ca. 8.0 cm [18]. although such distance may be considered negligible in various applications, in the case of the proposed methodology it could – if not remedied – significantly distort the results of data processing; that is because this methodology involves the numerical differentiation of sequences of foot position estimates, and differentiation is numerically ill-conditioned, which means that small errors can be very significantly amplified in its course. an exemplary foot trajectory and the results of its differentiation by means of the central-difference method, without any prior denoising, are shown in figure 2. as discussed in the following subsections, the visible abrupt changes in the estimates of foot velocity would hinder the identification of the stance and swing phases according to the proposed methodology. in order to prevent this effect, the foot position estimates were denoised prior to their numerical differentiation. in the context of the analysis of human movement, the technique most commonly used for denoising depth-sensorbased estimates of positions of anatomical landmarks is low-pass filtering by means of a butterworth filter of order 2, 3 or 4 with a cut-off frequency from the range [2, 10] hz (cf., e.g., [13], [18], [19]). in this study, this denoising technique was compared with two other techniques, less frequently considered in this context: the savitzky-golay filter and the total-variation regularisation technique (the tvr technique). filtering by means of a savitzky-golay filter is equivalent to approximating the data – within a moving window – by means of a fixed-degree polynomial [20]; both the length of the moving window and the degree of the approximating polynomial need to be optimised empirically. this technique is particularly useful for denoising data representative of a smooth signal – i.e. a signal adequately modelled by a mathematical function which has some continuous derivatives – while, at the same time, preserving the width and height of the peaks present in that signal [21]. the savitzky-golay filter is a low-pass filter whose cut-off frequency increases with the increasing polynomial degree and with the decreasing window length. unlike the butterworth filter, the savitzky-golay filter is characterised by a linear phase response. it has similarly flat passband but less attenuation in the stopband than the butterworth filter [22]. the frequency characteristics of exemplary filters of both kinds are presented in figure 3, their impulse responses – in figure 4. the tvr technique is well-suited for processing piecewiselinear rather than smooth signals [23]. it seems potentially useful for denoising the estimates of the antero-posterior position of a foot during walking, because that position is constant during the stance phase and changing approximately linearly during the swing phase. this technique is characterised by a single scalar regularisation parameter whose value needs to be optimised empirically. if the savitzky-golay filter or the tvr technique is applied, the estimates of the derivative can be obtained directly, without a) b) figure 2. an exemplary sequence of data acquired by means of a kinect v2 device, representative of the antero-posterior position of the left foot of a person who was walking toward that device (a), and the results of numerical differentiation of that sequence using the central-difference method (b). a) b) figure 3. the frequency characteristics of the savitzky-golay filter with 19sample window and 2-degree approximating polynomial, and of the butterworth filter of order 2 with the cut-off frequency 1.4 hz. a) b) figure 4. the impulse response of the savitzky-golay filter with 19-sample window and 2-degree approximating polynomial (a), and of the butterworth filter of order 2 with the cut-off frequency 1.4 hz (b). acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 4 computing the denoised sequence of data. however, in the experiments reported herein, better results were obtained by computing the velocity estimates using the central-difference method on the basis of the denoised position estimates. a recent review of methods of numerical differentiation, which can be used in this context, can be found in [10], chapter 5. exemplary estimates of feet velocities, obtained by means of the three above-described denoising techniques, are shown in figure 5. 2.2. identification of gait-cycle phases the swing phase and the stance phase were identified as the time intervals in which the velocity of the examined foot is above or below – respectively – an empirically selected threshold. two variants of threshold values were considered here: – a fixed absolute value; – a value relative to the maximum velocity estimate present in the set of data under analysis. exemplary results of this operation are shown in figure 6. 2.3. estimation of spatiotemporal gait parameters the length and duration of the steps were estimated according to the definitions provided in the documentation of the zebris fdm gait analysis system [24]. the estimates of the duration of the left and right stance phase and the doublesupport phase were divided by the duration of the gait cycle. the average walking speed was estimated by dividing the total distance, travelled during the experiment, by the total walking time. the cadence was estimated by computing the inverse of the duration of the steps, averaged over all observed left and right steps. the estimates of each spatiotemporal gait parameter, differing from their median value by more than an empirically selected threshold value, were identified as outliers and removed from the set of results. for brevity, some spatiotemporal gait parameters whose values can be inferred from the values of the parameters mentioned above – such as the stride length or the duration of the swing phase – are omitted here. 2.4. quantification of gait asymmetry the following indicator – which, to the best authors’ knowledge, has not been considered in any previous studies – can be used for quantifying gait asymmetry: ( ) ( ) ( ) ( )       +    − +   +      l r 0 lr 2 2 l r 0 0 max 1, 1 t t t v t v t dt r v t dt v t dt , (1) where ( )lv t and ( )rv t denote the horizontal speed of the left and right foot – respectively – at a time instant t belonging to the time interval under analysis [0, t]. this indicator can be interpreted as the maximum of a quasi-correlation between the trajectories of both feet; its values close to 1 indicate near-perfect symmetry, and smaller values – lesser symmetry. the stance time ratio (str), defined as follows:          l r l r r l if 0,1 otherwise st st st st str st st (2) – where lst and rst denote the stance time of the left and right foot, respectively – was used as a reference indicator of gait asymmetry [3]. 3. experimentation programme a set of experiments was completed in order to assess the applicabilitiy potential of the considered methodology for spatiotemporal gait analysis. the data representative of the human gait were acquired by means of a kinect v2 device and a zebris fdm pressure-measurement platform simultaneously. three persons walked over the platform 33 times in total. the depth sensor’s line of sight coincided with the walking direction. the sketch of the experimental setup is shown in figure 7. a) b) c) figure 5. the estimates of the feet velocities, obtained by means of numerical differentiation of some exemplary feet trajectories denoised using the butterworth filter (a), the savitzky-golay filter (b) and the tvr technique (c). figure 6. the exemplary results of the identification of the gait-cycle phases by thresholding the estimates of the foot velocity. figure 7. sketch of the experimental setup. acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 5 subsets of data from the depth sensor, representative of the 2.8 m displacement which comprised the space covered by the platform – with a certain margin – were extracted. according to the examination procedure associated with the zebris platform, the examined persons’ passages across the platform were grouped into triplets and the results corresponding to each triplet of passages were averaged; thus, 11 pairs of the estimates of each parameter were obtained (i.e. the pairs comprising one depthsensor-based estimate and one zebris-platform-based estimate). the mean error and the standard deviation of errors of the depth-sensor-based estimates were evaluated by treating the zebris-platform-based estimates as the reference. the dataprocessing software associated with the zebris platform was used for evaluating the standard deviation of the zebrisplatform-based estimates of spatiotemporal gait parameters. the cycle of data processing was repeated six times, according to six variants of the procedure described in section 2 – the variants being the combinations of the three denoising techniques (the butterworth filter, the savitzky-golay filter and the tvr technique) and two options of velocity threshold values (absolute or relative to the maximum velocity estimate). 4. results the values of the parameters of data-processing methods, optimised empirically in such a way as to minimise the differences between the depth-sensor-based and zebrisplatform-based estimates of the spatiotemporal gait parameters, are presented in table 1. the selected values of the parameters of the savitzky-golay filter correspond to the cut-off frequency of 1.53 hz. the mean errors and standard deviations of the estimates of spatiotemporal gait parameters are presented in table 2. the use of the absolute velocity threshold consistently yielded better results than the use of the relative threshold in all cases; hence, for brevity, only the results obtained using the absolute threshold are presented in table 2. the values of the indicators of gait asymmetry lrr and str, determined for all the recorded passages (the latter – on the basis of the data from both the depth sensor and the zebris platform), are shown in figure 8. the reported experiments involved only healthy persons whose gait was quite symmetric; thus, all these values are close to 1. to further assess the informative value of the proposed indicator lrr , another experiment, including emulation of asymmetric gait, was performed. during this experiment, a single healthy person walked first naturally and next – making fast steps with the right foot and slow steps with the left foot. the values 1.00 lr r = and 0.99str = were obtained for natural gait, whereas lr 0.86r = and 0.77str = – for emulated asymmetric gait. the results of this experiment are illustrated in figure 9. 5. discussion and conclusion all three denoising techniques, introduced in section 2.1, allowed for obtaining estimates of spatiotemporal gait parameters subject to quite similar uncertainty. in the case of the step times, the tvr technique yielded more biased estimates but less dispersed than the low-pass filters. in the cases of other parameters, differences in the uncertainty indicators could be noticed among the different denoising techniques, but the reported results do not justify the indication of any of them as capable of providing the most reliable results. the absolute (rather than the relative) value of the velocity threshold can be recommended for further study. table 1. empirically selected values of the parameters of data-processing methods in six variants of the procedure for estimation of spatiotemporal gait parameters. butterworth filter absolute threshold relative threshold order 2 4 cut-off frequency 1.4 hz 3.6 hz velocity threshold 1.7 m/s 40 % savitzky-golay filter absolute threshold relative threshold window length 19 samples 19 samples polynomial degree 2 2 velocity threshold 1.7 m/s 42 % tvr technique absolute threshold relative threshold regularisation parameter 8 · 10–4 1 · 10–3 velocity threshold 1.6 m/s 50 % table 2. mean values and the standard deviations of the errors corrupting the estimates of spatiotemporal gait parameters, obtained by means of the depth sensor, and the standard deviations of the corresponding reference estimates obtained by means of the zebris platform; the symbols l and r indicate the left and right side of the body; gct is the acronym of gait-cycle time; spm – of steps per minute. depth-sensor-based estimates zebris-platformbased estimates mean error standard deviation of errors standard deviation of estimates butterworth filter savitzky-golay filter tvr technique butterworth filter savitzky-golay filter tvr technique step time (l) –0.004 s –0.004 s –0.009 s 0.019 s 0.022 s 0.016 s 0.009 s step time (r) 0.000 s 0.001 s 0.008 s 0.020 s 0.023 s 0.021 s 0.013 s step length (l) –1.4 cm –0.8 cm –2.1 cm 1.8 cm 3.4 cm 3.2 cm 1.4 cm step length (r) –0.4 cm –0.9 cm 0.1 cm 3.8 cm 3.6 cm 4.1 cm 1.5 cm stance time (l) 0.1 % gct 0.9 % gct –0.5 % gct 2.5 % gct 2.1 % gct 2.3 % gct 0.5 % gct stance time (r) –0.2 % gct –0.3 % gct –2.1 % gct 1.0 % gct 1.1 % gct 2.1 % gct 0.8 % gct double-support time 0.6 % gct 0.9 % gct –1.9 % gct 4.1 % gct 2.6 % gct 3.6 % gct 0.8 % gct walking speed –1.4 cm/s –1.4 cm/s –1.4 cm/s 2.9 cm/s 2.9 cm/s 2.9 cm/s 5.1 cm/s cadence 0.70 spm 0.54 spm 0.31 spm 2.25 spm 2.37 spm 2.12 spm 2.46 spm acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 6 the mean differences between the depth-sensor-based estimates of spatiotemporal gait parameters and the corresponding reference values were quite small, whereas the standard deviations of the depth-sensor-based estimates were somewhat larger than those of the reference values but within the same order of magnitude. these results indicate a high applicability potential of the considered methodology for gait analysis: the somewhat larger uncertainty of the estimates may be justified by the considerably smaller cost and complexity of the examination setup and procedure, if compared to the use of the zebris platform. moreover, the considered methodology enables one to quantify gait asymmetry. in the experiments involving healthy persons, the values of all considered indicators of gait asymmetry – presented in figure 8 – are, as expected, close to 1; the dispersion of the values of the proposed indicator rlr, defined by equation (1), is the smallest. in the experiment including the emulation of asymmetric gait – whose results are presented in figure 9 – significantly different values of the indicators were obtained for symmetric and asymmetric gait. the aforementioned experimental results suggest that the proposed indicator may become quite useful in clinical practice; however, more experimental work is needed to reliably assess its capability to properly characterise the degree of gait asymmetry. the following practical considerations can be made based on the experiments: – the value of the foot velocity threshold has a significant impact on the uncertainty of the estimates of spatiotemporal gait parameters; in most experiments, the values 1.6–1.7 m/s have yielded the best results. the choice of the type of the low-pass filter used for denoising the foot position estimates does not seem to significantly affect the results, but the values of that filter’s parameters need to be selected carefully. – the best results can be obtained if the examined person walks towards the depth sensor along its line of sight; if the walking direction does not parallel that line, the feet occlude each other from time to time and, consequently, the reliability of the obtained results is significantly reduced. – certain kinds of examined person’s clothing, such as skirts or wide trousers, significantly hinder the localisation of the feet on the basis of depth-sensor data, making it impossible to obtain reliable estimates of spatiotemporal gait parameters. the authors’ plans for future work include: – the implementation and testing of other data-processing methods aimed at identifying gait-cycle phases, including the methods based on the distances between the examined person’s knees [13] and on the vertical oscillations of that person’s centre of mass [14]; – the experiments aimed at assessing the uncertainty of identification of gait-cycle phases, involving the use of a reference optoelectronic gait-analysis system (which, in contrast to the zebris platform, is capable of providing not only the reference values of spatiotemporal gait parameters, but also the reference three-dimensional trajectories of the feet); – the experiments involving persons whose ability to walk is impaired, in particular – whose gait is significantly asymmetrical. acknowledgement the authors express their sincere gratitude to dr. katarzyna kaczmarczyk and dr. michalina błażkiewicz from the university of physical education in warsaw for their help in the acquisition of data used for experimentation reported in this paper. references [1] g. allali, c. p. launay, h. m. blumen, m. l. callisaya, a.-m. de cock, r. w. kressig, v. srikanth, j.-p. steinmetz, j. verghese, o. beauchet, the biomathics consortium, falls, cognitive impairment, and gait performance: results from the good initiative, journal of the american medical directors association 18 (2017), pp. 335–340. doi: 10.1016/j.jamda.2016.10.008 [2] i. mulas, v. putzu, g. asoni, d. viale, i. mameli, m. pau, clinical assessment of gait and functional mobility in italian healthy and cognitively impaired older persons using wearable inertial sensors, aging clinical and experimental research 33 (2021), pp. 1853– 1864. doi: 10.1007/s40520-020-01715-9 [3] f. parafita, p. ferreira, d. raab, p. flores, h. hefter, m. siebler, a. kecskeméthy, evaluating balance, stability and gait symmetry of stroke patients using instrumented gait analysis techniques, proc. of the 4th joint international conference on multibody system dynamics, montréal, canada, 2016. figure 8. the values of the indicators of gait asymmetry, obtained 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1627–1639. doi: 10.1021/ac60214a047 [22] r. w. schafer, on the frequency-domain properties of savitzkygolay filters, in proc. of the 2011 digital signal processing and signal processing education meeting, sedona, usa, 2011, pp. 54– 59. doi: 10.1109/dsp-spe.2011.5739186 [23] j. wagner, r. z. morawski, gait-analysis-oriented processing of one-dimensional data with total-variation regularisation, measurement: sensors 18 (2021), p. 100287. doi: 10.1016/j.measen.2021.100287 [24] zebris medical gmbh, zebris fdm 1.16.x software manual. online [accessed 12 december 2022] https://www.zebris.de/en/service/downloads https://doi.org/10.1016/j.jbiomech.2013.08.017 https://doi.org/10.1016/j.gaitpost.2011.03.024 https://doi.org/10.1007/978-3-030-24233-6_7 https://doi.org/10.1186/s12883-021-02072-4 https://doi.org/10.1155/2019/2085039 https://doi.org/10.1016/j.gaitpost.2018.11.029 https://doi.org/10.1007/978-3-030-96009-4 https://doi.org/10.1109/cvpr.2011.5995316 https://doi.org/10.1109/jbhi.2020.3024925 https://doi.org/10.1016/j.gaitpost.2016.08.022 https://doi.org/10.3233/ais-170444 https://doi.org/10.1371/journal.pone.0166532 https://doi.org/10.1109/ichi.2015.54 https://doi.org/10.3390/s17102301 https://doi.org/10.3390/s20185104 https://doi.org./10.3390/s21175945 https://doi.org/10.1109/msp.2011.941097 https://doi.org/10.1021/ac60214a047 https://doi.org/10.1109/dsp-spe.2011.5739186 https://doi.org/10.1016/j.measen.2021.100287 https://www.zebris.de/en/service/downloads microsoft word 19-149-1-ga.doc acta imeko  july 2012, volume 1, number 1, 42‐48  www.imeko.org    acta imeko | www.imeko.org  july 2012 | volume 1 | number 1 | 42  characterization of the isdb‐tb critical spectrum mask  pablo n. de césare, marcelo tenorio  national institute of industrial technology (inti), radiocommunication laboratory, av gral. paz 5445, san martín, buenos aires argentina    keywords: isdb‐t, spectrum mask, intermodulation, spectrum analyzer  citation: pablo n. de césare, marcelo tenorio characterization of the isdb‐tb critical spectrum mask, acta imeko, vol. 1, no. 1, article 10, july 2012,  identifier: imeko‐acta‐01(2012)‐01‐10  editor: pedro ramos, instituto de telecomunicações and instituto superior técnico/universidade técnica de lisboa, portugal  received january 9 th , 2012; in final form july 6 th , 2012; published july 2012  copyright: © 2012 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: no information available  corresponding author: pablo de césare, e‐mail: decesare@inti.gob.ar  1. introduction  the transmission spectrum of the integrated services digital broadcasting-terrestrial (isdb-t) signal [1] consists of 13 successive ofdm segments. the nature of ofdm modulation is to send the information parallelized in thousands of modulated carriers very close to each other, obtained through an ifft algorithm. these carriers are then amplified by a power amplifier chain with a non-linear behaviour that generates intermodulation products that increase the out of band emission interfering with the adjacent channels and also degrading signal quality due to inter-carrier interference. reducing out-of-band emissions demands the incorporation of an external filter at the output of the power transmitter to eliminate the intermodulation products generated [2][3]. as defined in the brazilian digital terrestrial television standard abnt nbr 15601, there are three different spectrum masks: non-critical; sub-critical and critical. the difference between them are the attenuation of the out-side emission being the most difficult to achieve and test. transmission spectrum masks are commonly tested with spectrum analysers, but in isdb-t, the requirements are too high and more sophisticated measurement test methods must be incorporated into digital tv analysers. in this paper the transmission spectrum mask measurement as an alternative methodology is proposed and compared with a multi-propose spectrum analyser and other dedicated digital tv analysers, the characteristics of the test equipment required is described and the uncertainty calculation for the proposed method is included. 2. test analisys  2.1. isdb‐t signal analysis  isdb-t modulation can be configured to operate in three different modes: mode 1 or 2k, mode 2 or 4k and mode 3 or 8k that modify the number of carriers per ofdm segment. the frequency bandwidth shall be 5.7 mhz when the ofdm carrier bandwidth is 5.572 mhz with 4 khz spacing between carrier frequencies in mode 1. this bandwidth shall apply regardless of the mode which is chosen, and selected to ensure that the bandwidth of 5.610 mhz has a gap where each carrier of the uppermost and lowermost frequencies in the 5.572mhz bandwidth includes 99 % of the energy. according with standards abnt nbr 15601 and arib std-b31 if the spectrum mask is tested with a resolution bandwidth of 10 khz, the power density is reduced by 5.572 mhz 10 log 27.46 db 10 khz       . (1) abnt nbr 15601 table 41 defines the limits of the outof-band emission indicating the attenuations in relation to the transmitter average power. the final purpose of this test it to characterize the power density distributions, thus the limits non‐linearity  in the  isdb‐t transmission chain causes  intermodulation products that widens the spectrum emission and should be  taken into account when assigning frequency channels. the characterization of the transmission spectrum mask is one of the most  important measurements in order to achieve the best use of the electromagnetic spectrum. the use of critical mask defined in [1]  allows allocations of co‐site adjacent channels for an efficient use of the electromagnetic spectrum that is a finite and limited resource.  this paper discusses different test procedures using spectrum analyzers and dedicated digital tv analyzers in order to measure the  isdb‐t transmission mask.  acta imeko | www.imeko.org  july 2012 | volume 1 | number 1 | 43  given in [1] should be specified as attenuation in relation to the power density at 10 khz as shown in figure 1. 2.2. two tone intermodulation phenomena  a two tone intermodulation phenomenon can be observed when two cw sources are combined at the input of an amplifier and their frequencies are inside the passband of the device being tested. nonlinearities in the amplifier will lead to intermodulation products of the form (n1-m2). the components (21-2),(22-1) and (31-22),(32-21) are known respectively as third and fifth order intermodulation products and their frequencies and levels are close to the fundamental tones 1 and 2. higher order products are generally negligible in comparison. this phenomenon is also present in the testing equipment and is one of the limits of the available dynamic range. the input mixer of a spectrum analyser is a non-linear device, so it always generates distortion by itself. most spectrum analysers use diode mixers and their current intensity can be expressed as . 1 qv kt si i e          , (2) where is is the diode’s saturation current, q is the electron charge (1.60×10–19 c), v is the instantaneous voltage, k is the boltzmann’s constant (1.38×10–23 joule/°k), t is the temperature in degrees kelvin. expanding into a power series leads to 2 3 1 2 3 ( ...)si i k v k v k v    . (3) consider that two tones and the local oscillator are input into the mixer. in this case the input voltage is given by 1 1 2 2 sin( ) sin( ) sin( )lo lov v t v t v t     . (4) the following unwanted mixing products are also generated in the output mixer: 2 4 1 2 1 2 2 4 1 2 2 1 ( / 8) cos[ – ( 2 – )] ( / 8) cos [ – ( 2 – ] lo lo lo lo k v v v t k v vv t       (5) where 4 4 4 ! q kt k       . (6) if v1 and v2 have the same amplitude, their products can be considered as cubic terms. the third order products rise at rate 3 db/db as is shown in figure 2. 2.3. instrument available dynamic range  dynamic range is the ratio, expressed in db, of the largest to the smallest signals simultaneously present at the input of the instrument that allows measurement of the smaller signal to a given degree of uncertainty. there are three factors that limit the dynamic range: internal noise, internal intermodulation performance and the phase noise of a local oscillator. the instrument internally generates noise and distortion that affects the measurement’s accuracy. spectrum analysers’ noise floor is described by danl and signals below this level cannot be seen. the input attenuator strongly affects the sensitivity of the analyser to display low level signals by attenuating the input signal and reducing the signal-to-noise ratio (snr). resolution bandwidth also affects snr. the total noise power is determined by the width of the if filter. the noise level displayed varies following the relation 2 1,2 1 10 log rbw n rbw        . (7) since danl is often referred to a specific if-filter bandwidth, it is easy to obtain the displayed noise for any if-filter bandwidth. isdb-t spectrum mask measurement transmission-spectrum limit masks -140 -120 -100 -80 -60 -40 -20 0 -15 -10 -5 0 5 10 15 difference from the center frequency [mhz] a tt e n u a ti o n [ d b /1 0 k h z] channel power critical figure 1. isdb‐t critical mask.  figure. 3 spectral growth caused by the input mixer level.  figure 2. two‐tone intermodulation.  acta imeko | www.imeko.org  july 2012 | volume 1 | number 1 | 44  defines a 10 khz if-filter. the unwanted distortion products generated by the input mixer fall at the same frequencies as the distortion products under measure on the isdb-t input signal. the effect of the intermodulation can be seen as a spectral growth usually described as “spectrum shoulders” as can be seen in the top trace of figure 3. the phase relationship generates additions and cancelations between the generated intermodulation products and the signal that we want to measure. the lack of knowledge of the phase relationship between them is a source of uncertainty that also depends on their amplitude relationship through the relation 20 internal intermodulation 20 log(1 10 ) d error   , (8) where d is the difference in db between the amplitude of the intermodulation products generated internally and the amplitude of the intermodulation of the incoming signal. the probability function distribution can be assumed as being rectangular. the third order performance is given by the third order intercept point (toi) and represents the mixer level at which the internally generated intermodulation would be equal to the fundamental tones. this point is obtained by sweeping the levels of the fundamental tones 1 and 2 and observing the levels of the third order components as a function of the input power level. the dynamic range can be plotted as a function of the mixer input signal level. low levels reduce the snr and causes lower intermodulation products. high levels lead to more noise immunity but increase the intermodulation products. figure 4 shows the hp8564 [4] available dynamic range for isdb-t spectrum mask, where 78db is the best condition obtained for an input level of -27 dbm. as was shown in figure 1, the required dynamic range for transmission spectrum mask is obtained by addition of the total power channel to the power density in a 10 khz band and the highest attenuation defined in [1]: required dynamic range 27.46 db 97 db 124.46 db   . (9) actually, there is no available spectrum analyser with a dynamic range of 124.46db. most digital tv analysers have been equipped to perform the spectrum transmission mask test. however yhey do not have the better toi level and the lower danl required. they only incorporate the facility to add the spectrum emission with the filter mask response. table 1 gives a comparison between a spectrum analyser hp8564 [5] and the digital tv analysers anritsu ms8911b [5] and rohde & schwarz eth [6] and etl [7]. 3. test procedure  3.1. using a spectrum analyser  to solve the excessive dynamic range requirement, out of band emissions can be tested by measuring the isdb-t signal before filter mask and the filter response separately and then multiply both curves (or add them if they are expressed in db). figure 5a shows the filter characterization block diagram using a tracking generator synchronized with a spectrum analyser using the same setting defined in abnt nbr 15601. the filter characterization should be saved in an internal trace memory as figure 5b shows, where it is necessary that the top of the trace has the same reference level value of the spectrum analyser. figure 6 shows one of the possible block diagrams of the isdb-t signal measurement system. the directional coupler before the mask filter must be previously characterized. to   a)  b)  figure  5.  filter  characterization  a)  block  diagram  and  b)  characterization  trace. table 1. instruments’ specifications.  figure 4. hp8564 dynamic range.   figure 6. isdb‐t characterization block diagram. acta imeko | www.imeko.org  july 2012 | volume 1 | number 1 | 45  obtain the best dynamic range it is necessary to add internal and external attenuation. insufficient attenuation will cause intermodulation products close to the signal under test and an excessive one will increase the noise level. in practice it is desirable to adjust the attenuation in 1 db steps until one obtains the best dynamic range. most spectrum analysers have many functions like the addition and subtraction of traces. the peak power density should have the same value as the reference level of the spectrum analyser. when emissions cannot be measured due to the dynamic range, it is possible to recall the filter curve from an auxiliary trace and add it with the isdb-t spectrum obtained before the mask filter as is shown in figure 7. filter correction is very important for frequencies of  3 mhz from the centre frequency. in offset of 3 mhz from the centre frequency the available dynamic range should be enough to determine the compliance of the transmission spectrum mask, but results with lower uncertainties can be obtained applying the filter corrections only out of these limits with the configuration shown in figure 6. between 3 mhz from the centre frequency lower uncertainties are obtained with the block diagram shown in figure 8, without any correction of the filter transference. 3.2. using dedicated dtv analysers  when using anritsu ms8911b with option 030 for isdb-t analysis a pre-load default filter response can be used. comparison between the ms8911b pre-load default filter ant that of a commercial rfs [5] response is shown in figure 9. the eth tv analyser from rohde & schwarz has an integrated tracking generator for filter characterization that allows saving to an internal trace. the etl tv analyser from rohde & schwarz also has an integrated tracking generator for filter characterization but the filter response should be manually entered into a register. 4. uncertainty analisys  4.1. uncertainty due to amplitude accuracy  when the signal under test is applied to the instrument, the first source of uncertainty is given by impedance mismatch which causes the incident and reflected signals to add constructively or destructively. this causes the signal received by the analyser to be larger or smaller than the original one. this measurement is made with the same attenuator settings so the input attenuation switching uncertainty does not need to be considered. the input signal is mixed with the local oscillator and their flatness contributes to the frequency response uncertainty. spectrum analysers have a band switching uncertainty but in measurements with a30 mhz span they do not usually need to be added. after the input signal is converted to an if, it passes through the if gain amplifier and if attenuator that references the input signal amplitudes to the reference level. reference level accuracy and resolution bandwidth switching do not add uncertainty in relative measurements such as spectrum mask, but the display scale fidelity should be included. in table 2 the sources of uncertainties, in relative measurement using spectrum analyser, are summarized. uncertainty due to impedance mismatch can be calculated in the same way like power measurements [8][9].the systematic error is 12 l while the limits of mismatch uncertainty are 2 g l   caused by the lack of the phase difference between g and l . in most cases, uncertainty due to mismatch is relatively small. figure 7. transmission spectrum mask result.   figure 9. anritsu 8911b default filter comparison.    figure 8. isdb‐t characterization block diagram between the 3 mhz from  the centre frequency.  table 2. amplitude uncertainty in relative measurements. source of uncertainty  probability distribution  divisor  impedance mismatch  u‐shape  2   frequency response  rectangular  3   display scale fidelity  rectangular  3   -80 -70 -60 -50 -40 -30 -20 -10 0 -15 -10 -5 0 5 10 15 offset from center frequency [mhz] a tt e n u a ti o n [ d b ] rfs 8pxwe characterization anritsu ms8911b default filter characteristic normalized to -27,4db acta imeko | www.imeko.org  july 2012 | volume 1 | number 1 | 46  4.2. uncertainty due to filter characterization  following the connections described in figure 5a, filter characterization uncertainty will include the same sources described in table 2, in addition with the frequency response of the tracking generator and mismatch in the input and output ports. that implies that the filter response trace, like figure 5b shows, is obtained with a given level of uncertainty that will be carried out until the final result. the attenuation of a filter inserted between a generator and a spectrum analyser that are not perfectly matched [3], has a standard deviation of mismatch, m [db], which can be approximated by   0.52 22 2 11 22 42 2 21 . . 8.686 [db] 2 . 1 g f l f g l f s s m s                  (10) where s11f and s21f can be obtained from the filter specifications [5]. table 3 shows the uncertainty sources of the filter characterization measurement. 5. results  the device under test and the instruments used during the test are show in table 4. 5.1. measuring transmission spectrum mask after filter  between 3.5 mhz offset from centre frequency using  spectrum analyser  figure 10a shows the transmission mask measurement measured as figure 8 describes. figure 10b represents the same information as a function of frequency offset from the centre frequency channel. on table 6 the uncertainty budget is shown. the internal intermodulation error shown in table 5 is obtained using the spectrum analyser settings and the calculated available dynamic range. table 3. amplitude uncertainty in filter characterization.  source of uncertainty  probability distribution  divisor  uncertainty for filter mismatch  normal  1  generator amplitude linearity  rectangular  3   display scale fidelity  rectangular  3   frequency response  rectangular  3   table 4. devices under test and test instruments.  isdb‐t transmitter  nec dtl‐10/1r6p 1kw uhf digital tv transmitter system  output filter  rfs 8pxwe  spectrum analyser  hp8564e  sweep generator  r&s smp02  variable attenuator 1 db step  hp84904l  table 5. internal intermodulation error calculation.  analyzer parameter  value  reference level [dbm]  ‐26.7  internal attenuator  [db]  30  power/power density [db]  27.46  mixer level [dbm]  ‐29.24  available dynamic range [db]  ‐78  d [db]  18  errorinternal intermodulation [db]  1.1  table  6.  transmission  mask  measurement  uncertainty  for 3.5mhz  offset  from the centre frequency.  source of  uncertainty  value  [db]  probability  distribution  divisor  ui  [db]  veff  impedance  mismatch  0.12  u‐shape  2   0.08   frequency  response  0.80  rectangular  3   0.46   display scale  fidelity  0.85  rectangular  3   0.49   internal  intermodulation   1.10  rectangular  3   0.64   combined standard  uncertainty     normal    0.93  >1000  expanded  uncertainty     k = 2    1.86  >1000    a)    b)    figure 10. critical mask measurement between 3.5 mhz from the channel  centre frequency.  acta imeko | www.imeko.org  july 2012 | volume 1 | number 1 | 47  5.2 uncertainty budget  the uncertainty budget for the transmission mask can be seen in table 6 for the case of 3.5 mhz offset from the centre frequency. 5.3 measuring transmission spectrum mask before filter  beside 3.5 mhz offset from centre frequency using  spectrum analyser  for testing the transmission spectrum mask on frequencies offset higher than 3.5 mhz, two measurements must be done: filter characterization and spectrum emissions measures before the filter. the results of filter characterization measurement following the connections described in figure 5a, are shown in figure 11 and the uncertainty budget on table 7. 5.4 filter characterization uncertainty budget  figure 12 shows the filter characterization trace, the spectrum emission trace, measured before the filter and the result obtained as the addition of these two traces. the results are compared with the limits of the critical mask defined in [1]. table 7. filter characterization uncertainty. source of  uncertainty  value  [db]  probability  distribution  divisor  ui  [db]  veff  filter  impedance  mismatch  0.31  normal  1  0.31   generator  amplitude  linearity  0.60  rectangular  3   0.35   frequency  response  0.80  rectangular  3   0.46   display scale  fidelity  0.85  rectangular  3   0.49   combined  standard  uncertainty     normal    0.74  >1000  expanded  uncertainty     k = 2    1.49  >1000  table 8. internal intermodulation error calculation.  analyzer parameter  value  mixer level [dbm]  ‐29.24  available dynamic range [db]  ‐78  d [db]  22  errorinternal intermodulation [db]  0.7  table 9. transmission mask measurement uncertainty.  source of  uncertainty  value  [db]  probability  distribution  divisor  ui  [db]  veff  impedance  mismatch  0.12  u‐shape  2   0.08   frequency  response  0.80  rectangular  3   0.46   display scale  fidelity  0.85  rectangular  3   0.49   internal  intermodulation  0.7  rectangular  3   0.20   filter  characterization  0.74  normal  1  0.74  >1000 combined standard  uncertainty    normal    1.11  >1000 expanded  uncertainty    k = 2    2.22  >1000   figure 11. filter characterization measurement.  figure 12. transmission spectrum mask measurement results.  figure 13. transmission spectrum mask measurement results.  acta imeko | www.imeko.org  july 2012 | volume 1 | number 1 | 48  5.5. uncertainty due to internal intermodulation  to measure the isdb-t signal before the filter, the available dynamic range is 22 db. table 8 shows the spectrum analyser parameters and the obtained error generated by the internal intermodulation. 5.6. measuring transmission spectrum using anritsu  ms8911b  the results of critical mask transmission measurements using anritsu ms8911b are shown in figure 13, where the filter characterization measurement described before was replaced by the default filter response pre-load. 6. conclusions  it is possible to reduce measurement uncertainty of the transmission spectrum mask between 3.5 mhz from the centre frequency using the available dynamic range when the internal intermodulation error is relative low. out of 3.5 mhz from the centre frequency, where the requirement of dynamic range exceeds the available one, a filter response correction is applied and the results are obtained by addition the mask filter response to the isdb-t signal measured before it. in this case, the uncertainty is higher due to the contribution of filter characterization uncertainty. isdb-t testers have the similar performance than multipurpose spectrum analyser for testing the transmission spectrum mask. the use of pre-load default filter response gives a good approximation of the final measurement result but the results obtained should not be used for approval test being the mask filter characterization the more accurate method. digital tv analysers incorporate facilities to add the mask filter response to the isdb-t signal but the main advantage of them are the base band analysis, allowing measurement like modulation error ratio (m.e.r.), echo pattern, imbalance i-q, and bit error rate performance that characterize the signal quality. references  [1] abnt nbr 15601a – digital terrestrial television – transmission system (in portuguese), associação brasileria de normas técnicas, 2007. isbn: 978-85-07-00539-1. [2] hossein yamini and vijaya kumar devabhaktuni “cad of dual-mode elliptic filters exploiting segmentation”, 36th european microwave conference, 10-15 sept. 2006, pp. 560-563. [3] rfs, technical data sheet pxw series. [4] agilent 8560 e-series spectrum analysers product specification. [5] anritsu ms8911b product specification. [6] rohde & schwarz eth product specification. [7] rohde & schwarz etl product specification. [8] i.a. harris, f.l. warner, “re-examination of mismatch uncertainty when measuring microwave power and attenuation”, iee proceedings, part h microwaves, optics and antennas, vol. 128, pt. h, no. 1, february 1981, pp. 35-41. [9] glenn f. engen, “microwave circuit theory and foundations of microwave metrology”, iet 1992, isbn 0-86341-287-4, pp. 33-83. microsoft word article 13 168-1322-1-le.docx acta imeko  february 2015, volume 4, number 1, 82 – 89  www.imeko.org    acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 82  fpga‐based real time compensation method for medium  voltage transducers  gabriella crotti 1 , daniele gallo 2 , domenico giordano 1 , carmine landi 2 , mario luiso 2   1  inrim, strada delle cacce 91, 10135 torino, italy  2  department of industrial and information engineering, second university of naples via roma 29, 81031 aversa (ce), italy      section: research paper   keywords: voltage transducer, voltage measurement, power system measurement, power quality measurement, optimization  citation: gabriella crotti, daniele gallo, domenico giordano, carmine landi, mario luiso, fpga‐based real time compensation method for medium voltage  transducers, acta imeko, vol. 4, no. 1, article 13, february 2015, identifier: imeko‐acta‐04 (2015)‐01‐13  editor: paolo carbone, university of perugia   received december 31 st , 2013; in final form november 12 th , 2014; published february 2015  copyright: © 2014 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: this work was supported by measurement science consultancy, the netherlands  corresponding author: mario luiso, e‐mail: mario.luiso@unina2.it    1. introduction  the growing need of power quality analysis in medium voltage (mv) grids, required by the increasing diffusion of distributed generation sources, has led to the employment of transducers and measurement techniques able to convert and acquire voltage signals with improved accuracy. many applications require the use of transducers with wide frequency bandwidth ([1]-[3]), like power quality measurements ([4]), measurements on photovoltaic plants ([5]), energy and power meter calibration ([6]). for this purpose, voltage dividers which show higher frequency bandwidth and better linearity than conventional voltage transformers are becoming the most used transducers in power quality assessments. however, disadvantages like a higher dependency on the divider transfer function from environmental conditions (temperature, proximity effects) are introduced. the proximity effects can be reduced by shielding the device ([7]); this produces an increase of the stray capacitive coupling among the elements whose strength varies with the insulation medium. many dividers, for economic and safety reasons, make use of resin as an insulating medium whose electric characteristics can strongly depend on temperature, voltage and frequency ([8], [9]). with the aim of improving the performances of these dividers, so allowing their applications to grids with severe electrical and environmental conditions like the railway feed systems and on board mv circuits, a real time compensation method is proposed. in scientific literature several papers face the issue of characterizing and compensating measurement instrument transformers ([10]-[16]). most presented techniques perform compensation of measuring transformers only at industrial frequency; moreover, at best author’s knowledge, none of them apply the compensation to transducers intended for medium voltage power systems. so, in this paper, an improved technique for compensating voltage transducers in a wide frequency range is presented. it is based on the identification of a digital filter, with frequency response equal to the inverse of the divider, which is executed on a field programmable gate array (fpga), equipped with a/d and d/a converters. as an application, the transfer abstract  since the increase of the distributed power connected to the medium voltage networks, a capillary monitoring of the power quality  becomes essential. this entails the spread of transducers with suitable frequency bandwidths, as required by the relevant standards.  the paper describes a real time compensation method for the extension of the frequency bandwidth of medium voltage dividers  whose performances do not allow to perform measurements over a wide frequency range. this approach will contribute to keep the  costs of this innovation low. acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 83  function for the frequency compensation of two mv dividers is identified. the two chosen dividers have strongly different frequency behaviours; therefore the application of the proposed technique to the compensation of their frequency responses represents an interesting test bench. the paper is organized as follows. in section 2 there is a description of the operation of the two dividers. in section 3 the measurement setup used to characterize the dividers is presented. section 4 deals with the compensation technique. section 5 presents the optimization results and finally section 6 shows the experimental results related to the compensated dividers. 2. divider features  the compensation technique is applied to two dividers with different frequency behaviour. one is a pure resistive divider (rd) and the other is a resistive-capacitive divider (rcd). the highest voltage for equipment (um) is 24 kv for both. the resistive divider has a rated scale factor knom of 10000 and a 30 mω high voltage arm. the rcd, whose knom is 1000, is made, for the mv arm, of a series of 20 cells. each cell consists of a 10 mω non-inductive thin film resistor ([8], [9]) and a 1.5 nf ceramic class 1 capacitor, parallel connected. for the low voltage arm an equivalent resistance of 155 kω and 116.4 nf ceramic capacitors are used. the mv cells are disposed in four layers, each one made of five cells, which are series connected following an opposite path to minimize the stray inductances. a two sections cylindrical shield allows the control of the capacitive coupling due to the surrounding structures (figure 1). both the dividers are resin insulated, with a consequent downgrade of their performances. the transfer functions of the two dividers show strongly different dynamic behaviours (figures 2 and 3). the scale factor and phase error of rd (figure 2) have an overall variation of 30 db and about 1.2 rad respectively over 4 decades, while the rcd (figure 3) shows 12% and 100 mrad of variation over the same frequency range. the so different frequency behaviour of the two dividers represents an interesting test bench for the compensation technique. in fact the identification of the parameters of the compensation transfer function requires a growing effort from rd to rcd. 3. divider characterization  the complex frequency response of a voltage divider with a scale factor higher than 1000 requires a characterised measurement system able to compare voltage phasors whose amplitude differs three orders of magnitude. this can be performed by means of digitisers coupled with an attenuator probe or a reference divider, with high scale factor and frequency response up to at least 1 mhz. as an alternative, to perform the frequency sweep at low voltage (e.g. hundreds of volt), without making use of a reference voltage transducer, two agilent 3458 multimeters (dmms) are employed as digitizers. they have full scale ranges from 100 mv to 1000 v and a frequency bandwidth up to 12 mhz, depending on the selected digitizing technique. the synchronization between the two multimeters is provided by an external trigger supplied by a waveform generator. before introducing the measurement circuit, some aspects on dmm digitizing methods are discussed in the following. 3.1. multimeter digitizing methods  the multimeters provide three digitizing methods: dcv, direct sampling and subsampling. two of them, dcv and sub-sampling, are implemented for the divider frequency characterisation. by dcv technique, the signal is acquired just in dc mode, and the sample is obtained by integration of the input signal, over a specified integration time interval, that can be varied from 500 ns up to 1 s. the technique offers speed and resolution tradeoffs from 18 bits (5½ digits) at 6 khz to 16 bits (4½ digits) at 100 khz, as well a high input impedance. however, the maximum sample rate of 100 khz associated with a bandwidth limited     figure 1. resistive‐capacitive shielded divider during the assembly step. acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 84  to 30 khz for 100 v and 1000 v full scale makes this digitizing technique not adequate to perform the frequency analysis up to 100 khz, but it ensures a high magnitude and phase accuracy up to tens of hertz. the sub-sampling technique employs a track and hold (t&h) circuit which performs the signal integration over a very small interval (2 ns) and holds the integrated sample during the slower a/d conversion. the required a/d conversion time limits the actual sample frequency to 50 khz, but thanks to the subsampling algorithm the sampling frequency reaches 100 mhz. the bandwidth at 100 v peak full scale is limited to 1∙108 v∙hz which means that at 100 v the bandwidth drops to 1 mhz. table i summarizes the frequency bandwidth, the input impedance and the best accuracy for the employed scales and digitizing methods. 3.2. measurement set‐up  the measurement set-ups for the subsampling and dcv digitising methods are shown in figure 4a and 4b respectively. in both cases a waveform generator gives the trigger signal. for the dcv mode, a 5 v square wave signal triggers the simultaneous acquisitions of each sample by the two multimeters and the applied voltage is given by a power amplifier (dc – 500 khz, 120 v) supplied by a calibrator. when the subsampling technique is used (a picture of this configuration is shown in figure 5), the output of the waveform generator supplies the power amplifier and the available sync output triggers the two multimeter acquisition sequences. since with this approach the multimeter time base is used, errors can be introduced because of slight differences in the two timebases. a) b) figure 2. frequency behaviour of rd divider between 20 hz and 90 khz (a)  and zoom between 20 hz and 300 hz (b). a) b) figure 3. frequency behaviour of rcd divider between 10 hz and 90 khz (a)  and zoom between 10 hz and 300 hz (b).  table 1. multimeter specifications.    peak full scale  bandwidth  input impedance  best accuracy  dcv  100 mv  80 khz  >1010 ω  0.00005 – 0.01%  1 v  150 khz  >1010 ω  100 v  30 khz  >10 mω  1000 v  30 khz  >10 mω  sub‐sampling  100 mv  12 mhz  1 mω 144 pf  0.02% 1 v  12 mhz  100 v  12 mhz (1 mhz at 100 v) 10 100 1000 10000 90000 0 10 20 30 frequency [hz] m a g n itu d e *k n o m [ v /v ] rd divider frequency response 10 100 1000 10000 90000 0 0.5 1 1.5 frequency [hz] p h a se [ ra d ] magnitude phase 10 50 100 150 200 250 300 0.9 0.95 1 1.05 1.1 frequency [hz] m a g n itu d e *k n o m [ v /v ] rd divider frequency response 10 50 100 150 200 250 300 0 25 50 75 100 frequency [hz] p h a se [ m ra d ] magnitude phase 10 100 1000 10000 90000 0.88 0.9 0.92 0.94 0.96 0.98 1 frequency [hz] m a g n itu d e *k n o m [ v /v ] rcd divider frequency response 10 100 1000 10000 90000 -40 -20 0 20 40 60 80 frequency [hz] p h a se [ m ra d ] magnitude phase 10 50 100 150 200 250 300 0.9 0.92 0.94 0.96 0.98 1 frequency [hz] m a g n itu d e *k n o m [ v /v ] rcd divider frequency response 10 50 100 150 200 250 300 -50 -20 10 40 70 100 frequency [hz] p h a se [ m ra d ] magnitude phase acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 85  because of its better accuracy at low frequency (within 400 µrad up to some hundreds of hertz for the phase, when introducing corrections for the use of scales with different bandwidth), the dcv method is employed from 10 hz to 60 hz. as to the subsampling technique, a time delay of 25 ns, due to the different latency on the trigger for the two dmms, becomes relevant in terms of angle for frequencies higher than 10 khz. this systematic error is corrected but the overall accuracy degrades. moreover, approaching 100 khz the accuracy further decreases because of the bandwidth which is limited to 1 mhz for the 1000 v scale. for what concerns the uncertainty on the frequency of the generated signal, the utilized calibrator has an uncertainty of 25 ppm±1 mhz between 0.01 hz and 11 khz, while it has an uncertainty of 25 ppm±15 mhz between 12 khz and 120 khz. summing up all the uncertainty contributions of the measurement setup, the measurement uncertainty is conservatively estimated to be 500 ppm and 5 mrad at 100 khz for the scale factors and phase errors. the magnitude, phase and dc component of each voltage signal is estimated by the application of a fitting algorithm to the acquired samples over 10 periods. since the waveform generator and the calibrator provides signals with very low distortions, no higher harmonics besides the dc and the fundamental one have to be inserted in the fitting procedure, which provides accurate estimation of the amplitude and phase within tens of ppm and microradians respectively. 4. compensation technique  since a divider can be considered as a linear system, its frequency response can be expressed by: (1) where x and y are the spectra of the signals, both before and after the transduction. r and  are systematic modifications introduced by the transduction in amplitude and in phase, respectively, on spectral components of the input signal. once the divider has been metrologically characterized and its frequency response found over a certain frequency range, cascading a device with a frequency response equal to the inverse of the divider frequency response, systematic deviations are compensated over all the considered frequency range. for this purpose, a filter can be adopted, whose frequency response, hd(f), should be exactly given by: (2) for any frequency, f, in the range of interest. the analog implementation of transfer function (2) is not easily practicable and it can lead to acceptable results only if applied to a very limited frequency range. better results can be obtained with digital filtering. obviously, for a real-time compensation, a digital processor has to implement such digital filtering. in the following the procedure for identifying the filter is described, while the hardware which executes it in real time is shown in the next sections. two main implementations for digital filters exist: fir and iir. fir filters are relatively simple to compute and inherently stable but their main drawback, compared with   a)    b)  figure 4. measurement set‐up for sub‐sampling digitizing method (a) and  for dcv method (b).  figure 5. measurement set‐up for the subsampling configuration. broadband amplifier dmm1 dmm2 voltage divider input output gpib controller sync out input wavefor generator output trigger calibrator outputtrigger output waveform generator input outsync controller gpib output input voltage divider dmm2 dmm1 broadband amplifier waveform multimeters broad band amplifier waveform generator acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 86  those of iir filters, is that they may need a large number of coefficients to approximate a desired response; moreover they can only introduce a delay in phase frequency response. this makes them ineffective for the aim of this paper. an iir filter is generally modeled by a transfer function in the z-domain that can be written as: ⋯ ⋯ . (3) with this approach, filter design requires the choice of the best values for parameters a1, …, an and b0, b1, …, bm so that the transfer function of the filter approximates a desired frequency characteristic. the problem of choosing the best coefficients can be formulated, from a mathematical point of view, as an inverse problem ([18], [19]) and solved by adopting optimization techniques ([16], [17]). an objective function, describing the difference among desired frequency response and obtained values has to be defined and minimized by an optimization algorithm. the choice of the objective (or cost) function affects the optimality and the computational complexity of the solution. a robust design should account for filter response over its entire bandwidth. as illustrated in the previous section, the automated station used for divider characterization operates in the range of dc 100 khz: therefore, the best filter is identified in this range. anyway, identifying a filter, assigning frequency response data over its entire bandwidth, is a more complicated task, since its transfer function has fewer degrees of freedom. an efficient solution has been found choosing different expressions for filter transfer function and objective function. as it is said in [20], if the filter transfer function, h(z), is factorized in second order sections (sos), the frequency response is less sensitive to changes in coefficient values. this factorization can be expressed as: ∏ , , , , (4) in addition, for the objective function ([21]), the following expression is used: ∑ ∙ |log , log | ∙ log log (5) where p is the vector of the 4n+1 variables of (4), m is the number of the frequency points involved in the identification procedure and w is the vector of the weights. the cost function (5) weighs the ratio, rather than the difference, between the model frequency response and the frequency response data at each frequency. in addition, a logarithmically spaced frequency interval has been used. practically it is like including bode’s concept in the cost function, in fact, it weighs the difference between bode diagrams of the model frequency response and the frequency response data. it is important to note that this exactly addresses the real problem as what is really requested, it is to minimize the difference between the bode diagrams obtained by the model and the experimental data. the optimization problem here studied has a non-linear objective function with 4n+1 independent variables. therefore, the research space should be r4n+1, with r the whole set of real numbers. nevertheless, this interval can be reduced adopting some constraints on solution characteristics. the constraints divide the research space into feasible and infeasible regions with remarkable reduction of computational burden ([22]). considering this a constraint results in filter stability: the poles of the digital filter must have a modulus smaller than one, so nonlinear inequality constraints are imposed. in order to numerically study equation (5), a hybrid scheme based on the combination of a stochastic and deterministic approach has been adopted ([23], [24]). the two approaches are used in a combined way to take advantages of their complementary characteristics. in fact, the deterministic approach is the fastest way to work out a solution but the quality of the results strongly depends on the choice of the starting point. non-deterministic approaches do not depend on the initial choice and they are usually slow in finding out optimal solutions. starting from these considerations, an initial exploration of space of solutions is made by a genetic algorithm having a population size greater than the number of coefficients chosen as target. then, the obtained values are used as initial points to run a constrained deterministic approach based on the sequential quadratic programming (sqp) ([22]) to find out the optimal solution. the sqp algorithm was preferred over simpler algorithms (such as zero-order methods) taking into account the information about the derivative of the objective function and, in addition, to include in a direct way the above mentioned constraints. the described procedure has two parameters, which can be arbitrarily chosen before solution research starts: the sampling frequency and the number of soss. since the frequency response of the filter is strictly related to the sampling frequency, its value has to be carefully chosen. in fact, if the chosen sampling frequency differs from the actual sampling frequency of the utilized fpga boards, then the actual frequency response of the executed filter differs from the one found in the identification procedure. for the case at hand, the utilized fpga boards (described in detail in the next section) have a sampling period resolution equal to 1 µs, while the timebase accuracy is equal to ±100 ppm with a peak-to-peak jitter of 250 ps. according to such considerations, taking into account the frequency range involved in the compensation procedure, the sampling frequency has been chosen equal to 200 khz. the number of soss should be fixed very carefully because it is directly proportional to the number of independent variables of the objective function and to the complexity of the obtained filter. typically, better results are obtained increasing the filter order. anyway, it is fundamental to keep filter computational burden low. for this reason, the procedure is repeated a certain number of times, varying for each run the number of soss, in order to find out the best values for them. acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 87  the identification procedure is made in this way: first of all, hd(f), defined in (2), is constructed in a numerical way, linearly interpolating experimental data obtained from calibration. the optimization algorithm runs in two nested loops, varying the number of soss; in the inner loop the procedure is repeated a certain number of times. this is required based on the fact that the utilized hybrid optimization technique includes a stochastic algorithm which returns different results every run. the number of frequency points is chosen equal to four times the total number of variables [25]. among the solutions, referring to the same number of soss and coming from the inner loop, the solution that minimizes the cost function is chosen. 5. optimization results  data available from characterization of uncompensated dividers are used in the optimization procedure, in order to find the digital filters that, minimizing ratio error and phase displacement, could compensate the divider frequency response. as it is previously said, the sampling frequency of the digital filter is chosen equal to 200 khz. the number of soss has been chosen from the range of 1 5. the reason of this choice is that higher orders would require a high computational burden, not compatible with the hardware used for implementing the compensation filter. the inner loop is repeated 10 times. to evaluate the filter’s performance, over the whole input frequencies range, ratio errors and phase displacements have to be evaluated before and after filter introduction. those before the filter introduction are reported in (6) and (7), while those after the filter introduction are in (8) and (9), where h(f) is the frequency response of the implemented filter. ∆ 100 1 (6) ∆ (7) ∆ 100 | | 1 (8) ∆ (9) starting from the definitions (6)-(9), the two indices in (10) and (11) have been used for characterizing improvements introduced by the filter: they describe the relative mean quadratic improvements in ratio error and phase displacement, respectively. ∑ ∆ ∑ ∆ (10) ∑ ∆ ∑ ∆ (11) ir and i are shown in figure 6, for the rd divider, and in figure 7, for the rcd divider, as functions of the number of soss. for the rd divider, the improvements are about 31.4 figure 6. compensation improvement for rd divider.  table 2. coefficients of the best compensating filter for rd divider, expressed referring to the filter representation in (3).  b0  b1  b2  b3  b4  b5  b6  626.80  749.95  ‐415.10  ‐230.50  35.625  ‐4.3313  ‐6.7867  ‐  a1  a2  a3  a4  a5  a6  ‐  ‐0.46864  ‐0.96937  0.50844  0.0063371  ‐0.0026524  0.00017674      table 3. coefficients of the best compensating filter for rcd divider, expressed referring to the filter representation in (3).  b0  b1  b2  b3  b4  1111.8  ‐615.8  ‐1090.4  626.8  ‐9.1506  ‐  a1  a2  a3  a4  ‐  ‐0.53772  ‐0.98070  0.54920  ‐0.0078201  1 2 3 4 5 0 10 20 30 40 sos number r a tio e rr o r im p ro ve m e n t compensation improvement for rd divider 1 2 3 4 5 0 10 20 30 40 sos number p h a se d is p la ce m e n t im p ro ve m e n t ratio phase acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 88  and 22.8 for ratio error and phase displacement, respectively. for the rcd divider, the improvements are about 161.4 and 1.4 for ratio error and phase displacement, respectively. table 2 and table 3 show the coefficients of the best compensating filters for, respectively, the rd and the rcd dividers; the coefficient refers to the standard representation for an iir filter, that is the form (3). 6. experimental results  in order to perform experimental verification of the filter, designed in the previous section, a compensating device has been implemented with a real-time digital processor, opportunely equipped with analog to digital and digital to analog converters. the case in question, an fpga board has been used. its features are: 1) xilinx virtex-ii 1 megagate fpga with 16 bufgmuxs, 324 external iobs, 227 loced iobs, 40 mult18x18s, 40 ramb16s, 5120 slices [26]; 2) clock rate equal to 40 mhz; 3) 96 digital lines; 4) 8 analog inputs, independent sampling rates up to 200 khz, 16-bit resolution, ±10 v; 5) 8 analog outputs, independent update rates up to 1 mhz, 16-bit resolution, ±10 v. the block scheme of the compensated divider is shown in figure 8, where vin is the input voltage, vout the divider output voltage, vout,k the sampled version of vout, vout,k* the filtered version of vout,k, vout* the analog version of vout,k* and it is the output of the compensated divider. in this way, the compensated divider continues to be an analog device, offering thus the possibility of being employed in whatever measuring system. using the presented compensation device, the two dividers were characterized through the measurement setup figure 7. compensation improvement for rcd divider.  a) b) figure  8.  a)  inverse  frequency  response  of  rd  divider  and  frequency  response of its compensating filter. b) ratio error and phase displacement  of compensated rd divider.  figure 9. block scheme of the compensated divider.  a) b) figure  10  a).  inverse  frequency  response  of  rcd  divider  and  frequency  response of its compensating filter. b) ratio error and phase displacement  of compensated rcd divider.  1 2 3 4 5 0 50 100 150 200 sos number r a tio e rr o r im p ro ve m e n t compensation improvement for rcd divider 1 2 3 4 5 0 0.5 1 1.5 2 sos number p h a se d is p la ce m e n t im p ro ve m e n t ratio phase 10 100 1000 10000 90000 0 5000 10000 15000 rd divider inverse's and filter frequency responses frequency [hz] m a g n itu d e [ v /v ] divider inverse's filter 10 100 1000 10000 90000 -1.5 -1 -0.5 0 frequency [hz] p h a se [ ra d ] 10 100 1000 10000 90000 -15 -10 -5 0 5 compensated rd divider frequency errors frequency [hz] r a tio e rr o r [% ] 10 100 1000 10000 90000 -0.1 -0.05 0 0.05 0.1 frequency [hz] p h a se d is p la ce m e n t [r a d ] 10 100 1000 10000 90000 1000 1050 1100 1150 rcd divider inverse's and filter frequency responses frequency [hz] m a g n itu d e [ v /v ] divider inverse's filter 10 100 1000 10000 90000 -0.1 -0.05 0 0.05 0.1 frequency [hz] p h a se [ ra d ] 10 100 1000 10000 90000 -2 -1 0 1 2 compensated rcd divider frequency errors frequency [hz] r a tio e rr o r [% ] 10 100 1000 10000 90000 -0.05 0 0.05 0.1 0.15 frequency [hz] p h a se d is p la ce m e n t [r a d ] a/d fpg compensation device d/a divide compensated divider vin vout vout,k vout,k * vout* acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 89  shown in the previous sections. figure 9 a) shows the rd divider inverses and its compensating filter frequency responses, while figure 9 b) shows the ratio error and phase displacement of the compensated rd divider. figure 10 a) shows the rcd divider inverses and its compensating filter frequency responses, while figure 10 b) shows the ratio error and phase displacement of the compensated rcd divider. 7. conclusions  since the increase of the distributed power connected to the medium voltage networks, a capillary monitoring of the power quality becomes essential. this entails the spread of transducers with suitable frequency bandwidths, as required by the relevant standards. the paper describes a real time compensation method for the extension of the frequency bandwidth of mv voltage dividers whose performances would not have satisfied the requirements. the adoption of the compensation technique allows to reach improvement in dividers performance up to a factor 160. in this way, the performance of the compensated dividers is comparable with that of dividers of a better accuracy class, but the cost is kept low. references  [1] en 50160, “voltage characteristics of electricity supplied by public distribution systems”, cenelec, 1999. [2] iec en 61000-4-30: “testing and measurement techniques – power quality measurement methods”, 11/2003. [3] d. gallo, c. landi, m. luiso, e. fiorucci, g. bucci, f. ciancetta, “realization and characterization of an electronic instrument transducer for mv networks with fiber optic insulation”, wseas transactions on power systems, issn: 1790-5060, e-issn: 2224-350x, issue 1, volume 8, january 2013. [4] a. delle femine, d. gallo, c. landi, m. luiso, “power quality monitoring instrument with fpga transducer compensation”, ieee transactions on instrumentation and measurement, volume 58, issue 9, sept. 2009 page(s):3149 – 3158, issn: 00189456. [5] d. gallo, c. landi, m. luiso, e. fiorucci, “analysis of a photovoltaic system: ac and dc power quality”, wseas transactions on power systems, issn: 17905060, e-issn: 2224-350x, issue 4, volume 8, october 2013, pp. 45-55. [6] a. delle femine, d. gallo, c. landi, m. luiso, “advanced instrument for field calibration of electrical energy meters”, ieee transactions on instrumentation and measurement, volume 58, issue 3, march 2009 page(s):618 – 625, issn: 00189456. [7] j. merev, o. yilmaz, and o. kalenderli, “selecting resistors for high voltage divider,” in proc. xiiith international symposium on high voltage engineering, delft, the netherlands, aug. 2003. [8] g. crotti, d. giordano, a. sardi. “design of a rc medium voltage divider for on-site calibration”. in: proceeding of the 16th international symposium on high voltage engineering. cape town, south africa, august 2009, p. 210-214, isbn/issn: 9780620445849. [9] g. crotti, d. giordano, a. sardi, “development and use of a medium voltage rc divider for on-site calibration”. in: international workshop on applied measurements for power systems (amps), 2011 ieee . aachen, germany, september 2011, p. 53-57, isbn/issn: 9781612849454. [10] daponte, p.; “electronically compensated current transformer modeling”, measurement, 15 (4), p.213-222, jul 1995. [11] locci, n.; muscas, c.; “a digital compensation method for improving current transformer accuracy” power delivery, ieee transactions on, volume 15, issue 4, oct. 2000 page(s):1104 – 1109. [12] locci, n.; muscas, c.; “hysteresis and eddy currents compensation in current transformers”, power delivery, ieee transactions on, volume 16, issue 2, april 2001 page(s):154 – 159. [13] slomovitz, d.; “electronic system for increasing the accuracy of in-service instrument-current transformers”, instrumentation and measurement, ieee transactions on, volume 52, issue 2, april 2003 page(s):408 – 410. [14] baccigalupi, a.; liccardo, a.; “low-cost prototype for the electronically compensation of current transformers”, sensors journal, ieee, volume 9, issue 6, june 2009 page(s):641 – 647. [15] cataliotti, a.; di cara, d.; emanuel, a.e.; nuccio, s.; “a novel approach to current transformer characterization in the presence of harmonic distortion”, instrumentation and measurement, ieee transactions on, volume 58, issue 5, may 2009 page(s):1446 – 1453. [16] d. gallo, c. landi, m. luiso, “large bandwidth compensation of current transformers”, proceedings of ieee international instrumentation and measurement technology conference i2mtc 2009, singapore, may 5-7 2009, e-isbn :978-1-4244-3353-7. [17] d. gallo, c. landi, m. luiso, “compensation of current transformers by means of field programmable gate array”, metrology and measurement systems, volume xvi number 2/2009, pp. 279-288, index 330930, issn 0860-8229, www.metrology.pg.gda.pl [18] j. kaipio and e. somersalo, statistical and computational inverse problems, ser. applied mathematic sciences. new york: springer-verlag, 2005. [19] k. deb, multiobjective optimization using evolutionary algorithms, hoboken, nj: wiley, 2001. [20] a. v. oppenheim, r. w. schafer, “digital signal processing” englewood cliffs, nj: prenticehall, 1975. [21] banos, a.; gomez, f.; “parametric identification of transfer functions from frequency response data”, iet computing & control engineering journal, volume 6, issue 3, june 1995 page(s):137 – 144. [22] t. f. coleman and y. li, “an interior, trust region approach for nonlinear minimization subject to bounds,” siam j. optim., vol. 6, no. 2, pp. 418–445, may 1996. [23] j. h. holland, “genetic algorithms,” sci. amer., vol. 267, pp. 66–73, 1992. [24] d. e. goldberg, genetic algorithms in search, optimization, and machine learning. reading, ma: addison-wesley, 1989. [25] i. kollar, y. rolain, “complex correction of data acquisition channels using fir equalizer filters”, instrumentation and measurement, ieee transactions on, volume 42, issue 5, oct. 1993 page(s):920 – 924. [26] xilinx web site, www.xilinx.com. microsoft word article 13 106-705-1-le-final.doc acta imeko december 2013, volume 2, number 2, 73 – 77 www.imeko.org acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 73 machine integrated telecentric surface metrology in laser structuring systems robert schmitt1 2, tilo pfeifer1 2, guilherme mallmann2 1 laboratory for machine tools and production engineering wzl, rwth aachen university, aachen, germany 2 fraunhofer institute for production technology (ipt) – department of production metrology, aachen, germany section: research paper keywords: inline metrology, frequency-domain optical coherence tomography, surface inspection, laser structuring citation: robert schmitt, tilo pfeifer, guilherme mallmann, machine integrated telecentric surface metrology in laser structuring systems, acta imeko, vol. 2, no. 2, article 13, december 2013, identifier: imeko-acta-02 (2013)-02-13 editor: paolo carbone, university of perugia received april 15th, 2013; in final form october 8th, 2013; published december 2013 copyright: © 2013 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: this work was supported by german federal ministry of education and research (bmbf), germany corresponding author: guilherme mallmann, e-mail: guilherme.mallmann@ipt.fraunhofer.de 1. introduction the functionalization of surfaces based on laser micromachining is an innovative technology used in a broad spectrum of industrial branches. its main advantage against other processes is the high machining flexibility. this technique permits the structuring of different workpieces (different form complexity and materials) with the same machine tool. examples can be found in minimizing air resistance, operation noise and friction losses [1] as well as in the surface structuring of tools and moulds [2]. there is, however, a trend towards the improvement of the overall process quality and automation as well as smaller microstructures in laser micro-machining. the fulfilment of these demands are at present limited by the non-existence of a sufficiently described process model as well as the absence of a robust and accurate inline process monitoring technique. on the one hand the missing process model leads to a time consuming effort to initialize the laser structuring of new products. this procedure depends e.g. on the applied material composition, product form and surface roughness. if the process behaviour is unknown for a determined workpiece, laser parameters and suitable machining strategies have to be identified in a trial and error testing before the real machining process can start. in this context reference geometries need to be structured and analysed outside the machine tool until a suitable parameter set is found. on the other hand the absence of a process control based on the real machined surface causes a high degree of inefficiency. this is explained by the inability of the machine to identify process defects during the machining procedure, leading to an increased possibility of rejected parts with a high degree of added value. for solving this task with a high level of compatibility and integration, an optical distance measurement system based on the frequency domain optical coherence tomography (fd-oct) was developed. the described telecentric measurement through the laser machining optical system enables a fast and highly accurate surface inspection in machine coordinates before, during and after the structuring process. based on this process monitoring a machining control can be set-up, leading to a fully automated process adjustment and manufacture procedure. abstract the laser structuring is an innovative technology used in a broad spectrum of industrial branches. there is, however, a market trend to smaller and more accurate micro structures, which demands a higher level of precision and efficiency in this process. in this terms, an inline inspection is necessary, in order to improve the process through a closed-loop control and early defect detection. within this paper an optical measurement system for inline inspection of micro and macro surface structures is described. measurements on standards and laser structured surfaces are presented, which underline the potential of this technique for inline surface inspection of laser structured surfaces. acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 74 2. state of the art 2.1. laser structuring process the laser structuring technology utilizes thermal mechanisms to machine a workpiece, which are induced through the absorption of high amounts of energy. the physical interaction of laser radiation and matter is therefore a crucial point in this process. its efficiency is associated with laser and material properties, as applied wavelength, focal radius, angle of incidence, material light absorption, surface roughness, metal temperature, laser pulse repetition frequency and energy [1]. typical working wavelengths for the machining of metals and their alloys are 1064 nm (infra-red light) or 532 nm (green light). the guidance of the laser beam to the part’s surface is accomplished in most systems using a rastering system or a laser scanner [3]. this configuration uses computer numerically controlled galvanometer mirrors to deflect and an f-theta lens to focus the laser beam over a working area. this lens is wavelength optimized to focus the chief ray of the laser beam normal to the scanning field regardless of the scan angle, as well as to make the traveled distance of the laser spot on the focal plan directly proportional to the scan angle [4]. 2.2. laser structuring process monitoring inline process monitoring solutions for laser based structuring systems currently being developed in the academy and industry show technical limitations. the technologies available present in part low accuracy, no depth information or are not able to measure directly in machine coordinates. for example an approach using conoscopic holography is used. this technique is not able to measure directly in machine coordinates, as it uses a different optical path as the process laser, leading to complex calibration steps and inserting transformation errors as well as measurement displacement. in [5] a technology based on the acquisition of process generated electro-magnetic emissions is presented for the monitoring of the selective laser melting process. a similar approach can be also applied on laser structuring systems. this technique is however not able to deliver any direct depth information, just being able to monitor the amount of energy absorbed in the machining procedure and based on this evaluate the removed depth. 3. solution concept the solution concept for the machine integrated process monitoring system was designed based on the rastering system machine layout (figure 1). as measurement system an optical distance measurement technique based on the frequency domain optical coherence tomography (fd-oct) was used. the system integration is accomplished through an optical element as beam splitter. 3.1. frequency domain optical coherence tomography (fd-oct) the fd-oct is a technique based on low-coherence interferometry. differently from normal low-coherence interferometers, which use a piezo element to find the maximum interference point, in the fd-oct the depth information is gained by analyzing the spectrum of the acquired interferogram. the calculation of the fourier transformation of the acquired spectrum provides a back reflection profile as a function of the depth. for the generation of the interference pattern a measurement and a reference path are used, where the optical path difference between these arms is detected. the higher the optical path difference between reference and measuring arm, the higher the resulting interference modulation (figure 2). the total interference signal i(k) is given by the spectral intensity distribution of the light source (g(k)) times the square of the sum of the two back reflected signals (ar as the reflection amplitude of the coefficient reference arm and a(z) as the backscattering coefficient of the object, with regard to the offset z0), where k is the optical wavenumber [6]:   0 ( ) ( ) exp( 2 ) ( ) exp 2 ( )( )r zi k g k a i kr a z i kn z r z dz    (1) where n is the refractive index, 2r is the path length in the reference arm, 2(r+z) is the path length in the object arm and 2z the difference in path length between both arms. by finding the maximum amplitude at the spectrum’s fourier transformation, the absolute optical path difference can be detected. the max. measuring depth (zmax) is described by [7]  2max 0 4z n n  (2) where is the central wavelength,  is the bandwidth, n is the sample’s refractive index and n is the number of detector units covered by the light source’s spectrum. the axial resolution of an fd-oct is described by [7] 2 02 0.44fd oct car l     . (3) for the measurement of single distance (a single back reflection) the axial resolution can be increased to a submicrometric resolution by the usage of signal processing techniques, such as gauss fit. figure 1. concept of the inline process monitoring system – (a) dispersion compensator / glass rod. figure 2. frequency domain optical coherence tomography (ft-oct) set-up. acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 75 3.2. measurement system prototype the spectrometer for the interference signal acquisition was developed with a wavelength measuring range of 107 nm, which can be adjusted in the absolute frequency range from 900 to 1100 nm depending on the light source to be used (figure 3). as a detector an indium gallium arsenide (ingaas) line camera was used. standard silicon based detectors present quantum efficiencies of less than 20% for the applied wavelength range against values between 60%-80% for ingaas (1.7 µm) based detectors [8]. the light source used in the system is a superluminescent diode with central wavelength at 1017 nm and wavelength range of 101 nm. the theoretical measuring range (maximum depth scan) of the developed fd-oct using the presented sld light source was calculated using equation (2) to be of 1.31 mm. the available measurement range was evaluated using a precision linear translation table. the results showed a maximum distance measurement of 1.25 mm using a specimen of aluminium with a technical surface, which simulates the workpieces used for the laser structuring. the theoretical axial resolution of the system calculated based on equation (3) is 4.51 µm. the usage of a gauss fitting algorithm to find the modulation frequency after the fourier transformation of the acquired light spectrum increases the axial resolution by calculating a sub-pixel accurate curve maximum. based on this technique an increased axial resolution could be achieved. the standard deviation of the distance measurement values acquired in the centre of the scanner field was 218 nm. a detailed analysis of the measurement system is presented in [9]. 3.3. beam coupling the concept chosen for the integration of the developed fd-oct in a laser structuring machine is based on an optical filter or a dichroic beam splitter used as an optical coupler. the coupling is accomplished by using this optical element in the reflective area for the measurement wavelengths and in the transmissive area for the structuring laser wavelength. a very important requirement on this system is the laser beam’s transmission efficiency. the laser beam coupling performance will be directly connected to the machine process energy efficiency as well as to the overall heat development on the coupling system. in order to warrant a robust system with little long time deviations, e.g. by component wear, and little energy losses an optical element with a transmission of near to 100% needs to be chosen. another important system requirement is a highly accurate beam alignment. a misalignment between laser and measurement beam will lead to a displacement between the laser and the measurement spot, causing a mismatch / uncertainty in the measurement results. the developed coupling system meets the described demands and enables a system integration by an unique machine hardware change. the insertion of a coupling optical element in the laser beam path with a determined angle (45° for a dichroic beam splitter or a wavelength dependent angle for an optical filter) fulfills the complete integration. the component disposal for the beam coupling can be seen in the prototype setup presented in figure 4. the coupling efficiency of the concept was evaluated using an optical edge filter for the wavelength of 1064 nm. by changing the angle of the edge filter, the edge frequency between reflection and transmission is displaced. by an angle of 23° the edge frequency is adjusted in such a way, that the filter reflects the wavelength bandwidth of the measuring system and transmits the wavelength of the laser beam (figure 4). an overall coupling efficiency of over 95% for the laser beam and over 93% for the measurement beam was evaluated in laboratory tests. these results validate the concept for the machine integration. 3.4. telecentric f-theta scanning lens as presented in figure 1 a typical scanning system used in laser structuring systems is composed mostly of a scanning unit based on galvanometric moved mirrors and an f-theta scanning lens. the scanning objective used in the presented system prototype is a telecentric f-theta scanning lens. this lens type is wavelength optimized through the addition of a targeted optical distortion in the lens system. the aim of this optimization is to create a focal plane for the laser beam, as well as to create a direct proportional relation between scanning angle and laser spot position [4]. the designed optical system of a telecentric f-theta lens causes optical aberration in wavelengths other than the machining laser wavelength. this aberration introduces systematic errors to the measurement beam, such as a figure 4. system prototype with detailed view of the beam coupling unit (laser and measurement beam coupling). figure 3. measurement system based on fourier domain low coherence tomography. acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 76 distortion of the focal plane, changes in the optical path and form deformation of the measurement spot. the evaluated dispersion could be compensated on the measurement system by the usage of a special designed glass rod, which was inserted on the system’s reference path (figure 1). this glass rod represents the same optical dispersion generated by the f-theta in the main optical path. hereby the interference conditions are fulfilled for all field angles of the scanner. the remaining effects of the f-theta in the optical path for other beam entrance angles can be seen in the measurement results as slight deviations of the real object form or a small decrease of the lateral accuracy in the border of the scanning field. the specific optical path deformation over the scanning field could be evaluated through a system simulation. the results show a form error in the shape of a saddle (figure 5). 3.5. laser structuring system prototype in order to evaluate the proposed inline measurement technique previously to a machine integration, a laser structuring system prototype was constructed. for the deflection of the laser and measurement beam a galvanometer based scanning unit was used. a telecentric f-theta lens with focal distance of 80 mm was applied for focusing both beams on the structuring plane (figure 6). the combination of both optical elements enables a working field of 30 mm × 30 mm. as machining laser a nanosecond pulsed fiber laser with central wavelength at 1064 nm was used. the controlling of the scanner and laser units is executed by a specially developed piece of software. a laser security compliant housing completes the prototype. 4. results the evaluation of the system prototype was carried out through a series of test measurements in flatness and step standards, as well as in laser structured workpieces. for these tests the processing laser was turned off. by measuring a flatness and a step standard the remaining amount of optical aberration introduced by the telecentric f-theta lens could be investigated. alterations in the optical path length of the measuring beam affect the surface form inspection and need to be characterized. the measurement of a flatness standard shows a slight distorted plane (figure 7), as expected by the system simulation results (figure 5). for a measurement area of 20 mm × 20 mm a parabolic distortion could be detected in x and y directions. after the subtraction of the plane inclination a maximal deformation of 48 µm in the x-axis and of 65 µm in the y-axis was measured. as already shown in the system simulation, this measurement distortion is caused by an optical dispersion in dependency of the beam entrance angle. a correction of this effect can be achieved by a system calibration, which is based on a measurement of a reference surface (e.g. flatness standard). the evaluated form error is modeled by a 3d polynomial and used in the system software to compensate the measurement results. to evaluate the measurement range as well as a possible non-linearity after the calibration process a step standard with steps of about 100 µm was measured (figure 8). the measured area of the workpiece was 6.5 mm × 2 mm. the overall height variation measured by the prototype was 925 µm over the 10 steps. a reference measurement with a chromatic sensor acquired a height variation of 917 µm over the 10 steps. an overall non-linearity of about 8 µm over a measurement range of 1 mm could be evaluated. this represents a non-linearity of figure 5. simulation of the optical path length of the measurement beam through the machine optical system over the complete working field. figure 6. prototype of a laser structuring system with the developed inline measurement system. figure 7. 3d measurement of a reference surface (unit in mm). acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 77 less than 1% for the presented prototype in the used scanning area. regarding the application of the presented measurement system for laser structuring processes, e.g. in the manufacturing of tools and moulds [1], a laser machined structure (20 mm × 20 mm) was measured and analyzed (figure 9). the resulting measured surface demonstrates the robustness of the inline system for the inspection of complex surfaces. 5. conclusions within this paper an optical measurement system for the surface inspection of micro and macro structures with sub-micron accuracy within a laser structuring machine was described and evaluated. a standard deviation of the distance measurements in the z-axis of fewer than 218 nm and a nonlinearity of less than 1% for a measurement range of 1 mm was determined. measurements on standards and laser structured surfaces are presented, which validate the potential of this technique for a telecentric surface inspection within laser structuring machines. future investigations, especially in the usage of this technique for a feedback control of adaptive laser micro processing machines, will be carried out. the effects of process emissions in the measurement results are also an important research task. acknowledgement we gratefully acknowledge the financial support by the german ministry of education for the project ‘scan4surf ’ (02po2861), which is the basis for the proposed achievements. references [1] s. schreck et. al., “laser-assisted structuring of ceramic and steel surfaces for improving tribological properties”, proc. of the european materials research society, applied surface science, 2005, vol. 247, pp. 616-622. [2] f. klocke, et al., “reproduzierbare designoberflächen im werkzeugbau: laserstrahlstrukturieren als alternatives fertigungsverfahren zur oberflächenstrukturierung”, werkstatttechnik, 2009, no. 11/12, pp. 844-850. [3] j. c. ion, laser processing of engineering materials – principles, procedure and industrial application, elsevier, 2005, isbn 978-0-7506-6079-2, p. 389. [4] b. furlong and s. motakef, “scanning lenses and systems”, photonik international, no. 2, 2008, pp. 20-23. [5] t. craeghs et. al., “online quality control of selective laser melting”, proceedings of the 20th solid freeform fabrication (sff) symposium, 2011. [6] m. brezinski, optical coherence tomography – principles and applications, elsevier, 2006, isbn 978-0121335700 pp. 130-134. [7] p. tomlings and r. wang, “theory, developments and applications of optical coherence tomography”, applied physics, no. 38, 2005, pp. 2519-2535. [8] a. rogalski, infrared detectors, crc press, 2010, isbn 978-1420076714, pp. 315-317. [9] r. schmitt, g. mallmann and p. peterka, “development of a fd-oct for the inline process metrology in laser structuring systems”, proc. spie 8082, 2011, 808228. figure 8. 3d measurement of a step standard (unit in mm). figure 9. 3d measurement of a laser structured surface (unit in mm). bringing optical metrology to testing and inspection activities in civil engineering acta imeko issn: 2221-870x september 2021, volume 10, number 3, 108 116 acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 108 bringing optical metrology to testing and inspection activities in civil engineering luís martins1, álvaro ribeiro1, maria do céu almeida1, joão alves e sousa2 1 lnec national laboratory for civil engineering, avenida do brasil 101, 1700-066 lisbon, portugal 2 ipq portuguese institute for quality, rua antónio gião 2, 2829-513 caparica, portugal section: research paper keywords: optical metrology; civil engineering; testing; inspection citation: luis martins, álvaro ribeiro, maria do céu almeida, joão alves e sousa, bringing optical metrology to testing and inspection activities in civil engineering, acta imeko, vol. 10, no. 3, article 16, september 2021, identifier: imeko-acta-10 (2021)-03-16 section editor: lorenzo ciani, university of florence, italy received february 8, 2021; in final form august 5, 2021; published september 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was supported by lnec national laboratory for civil engineering, portugal. corresponding author: luís martins, e-mail: lfmartins@lnec.pt 1. introduction optical metrology has a large scientific and technological scope of application, providing a wide range of measurement methods, from interferometry to photometry, radiometry and, more recently, to applications using digital, video and vision systems, which combined with computational algorithms, allow obtaining traceable and accurate measurements. increasing accuracy of optical measurement instruments creates new opportunities for applications in civil engineering, namely, for testing and inspection activities. these new methodologies open broader possibilities in civil engineering domains where dimensional and geometrical quantities are major sources of information on infrastructures and construction materials. the assessment of their performance and behaviour, often involves monitoring and analysis under dynamic regimes [1], [2]. in many cases, the development of new technologies, based on the use of methods combining optics and digital algorithms, have recognized advantages, namely, those using non-invasive techniques in harsh environments and remote observation [3]. moreover, the need for accurate measurements related to infrastructures management, e.g., in early detection of damage or in safety monitoring, is growing. the contribution of metrology in this area is key to increase the confidence in decision-making processes. r&di activities in the optical metrology domain in recent years at the portuguese national laboratory for civil engineering (lnec) led to the development of innovative applications, many of them related to doctoral academic research. the main objectives are: (i) to design and develop optical solutions for applications where conventional instrumentation does not provide satisfactory results; (ii) to establish si (international system of units) traceability of measurements undertaken with optical instruments; (iii) to develop advanced mathematical and numerical tools, namely based on monte carlo methods (mcm) and bayesian methods, abstract optical metrology has an increasing impact on observation and experimental activities in civil engineering, contributing to the research and development of innovative, non-invasive techniques applied in testing and inspection of infrastructures and construction materials to ensure safety and quality of life. advances in specific applications are presented in the paper, highlighting the application cases carried out by lnec (the portuguese national laboratory for civil engineering). the examples include: (i) structural monitoring of a long-span suspension bridge; (ii) use of close circuit television (cctv) cameras in drain and sewer inspection; (iii) calibration of a large-scale seismic shaking table with laser interferometry; (iv) destructive mechanical testing of masonry specimens. current and future research work in this field is emphasized in the final section. examples given are related to the use of m oiré techniques for digital modelling of reduced-scale hydraulic surfaces and to the use of laser interferometry for calibration of strain measurement standard for the geometrical evaluation of concrete testing machines. mailto:lfmartins@lnec.pt acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 109 bringing benefits to the evaluation of measurement uncertainty in complex and non-linear optical problems. this paper exemplifies how new methods enable traceable and accurate solutions to assess conformity with safety requirements, providing support to the measurement uncertainty evaluation as a tool to use decision rules. in addition, the applications described emphasize the role of digital and optical systems, as a basis for robust techniques able to provide measurement estimates for dimensional quantities, replacing conventional invasive measurement approaches. to illustrate these achievements, results of r&di in the civil engineering context are presented, including examples of application in: (i) structural monitoring of a long-span suspension bridge; (ii) drain and sewer inspection using cctv cameras; (iii) calibration of a large-scale seismic shaking table with laser interferometry; (iv) destructive testing of masonry specimens. 2. overview of optical metrology optical metrology is a specific scientific area of metrology, defined as the science of measurement and its applications [4], in which experimental measurement processes are supported by light. currently, it has a significant contribution in multiple scientific and engineering domains, improving measurement methods and instruments, to assess their limits and increasing their capabilities in order to improve the knowledge of the studied phenomena. in recent years, the technological development of computational tools has extended the optical metrology activity scope, by increasing the number of measurement processes supported in digital processing of images obtained from optical systems [5]. this activity is characterized by the detection and record abilities, without physical contact with the object and in minor time interval, of a large amount of information (dimensional, geometrical, radiometric, photometric, colour, thermal, among others), overcoming human vision limitations, reaching information imperceptible for human eyes and, therefore, improving knowledge about phenomena. although this paper is focused on dimensional measurements, optical metrology also reaches other domains of activity, namely, temperature, mechanical and chemical quantities. optical metrology covers a wide range of dimensional measurement intervals, from nanometer magnitude up to the dimension of celestial bodies and space distances. in this context, measurement principles are usually grouped in three categories [6]: (i) geometrical optics – related to the refraction, reflection and linear propagation of light phenomena, which are the functional support of several instruments and measurement systems composed by light sources, lenses, diaphragms, mirrors, prisms, beam splitters, filters and optical electronic components; (ii) wave optics – where the wave nature of light is explored, namely, the interference of electromagnetic waves with similar or identical wavelength, being present in a wide range of instruments and measurement systems which use polarized and holographic optical components and diffraction gratings; and (iii) quantum optics – supports the generation of laser beams which correspond to high intensity and monochromatic coherent light sources used, e.g., in sub-nanometer interferometry and scanning microscopy. in the case of civil engineering, two main areas for applications of optical metrology are identified: space and aerial observation; and terrestrial observation. space observation, supported by optical systems, equipped with panchromatic and multi-spectral sensors integrated in remote sensing satellites, is gradually more frequent in the context of civil engineering, due to the growing access to temporal and spatial collections of digital images of the earth’s surface with increasing spatial resolution. aerial observation is generally focused on photogrammetric activities undertaken from aircrafts, aiming at the production of geographic information to be included in topographic charts or geographical information systems, namely, through orthophotos and three-dimensional models (realistic or graphical) representing a certain region of the earth’s surface. moreover, optical systems are also installed in uav unmanned aerial vehicles, used in the visual inspection of large constructions, contributing to the detection and mapping of observations (e.g. cracks, infiltrations, among others) and analysis of their progression with time (see example in figure 1) [7]. 3. structural monitoring of a long-span suspension bridge optical metrology has been successively applied by lnec to the monitoring of a long-span suspension bridge, allowing the development of non-contact measurement systems, capable of determining three-dimensional displacements of critical regions, namely, in the bridge’s main span central section. optical systems are an interesting solution for this class of measurement problems, especially in the observation of metallic bridges, where the accuracy of microwave interferometric radar systems [8] and global navigation satellite systems [9], [10] can be affected, for instance, by the multi-path effect resulting from electromagnetic wave reflections in the bridge’s structural components. the measurement approach developed consists in the use of a digital camera rigidly installed beneath the bridge’s stiffness girder, oriented towards a set of four active targets placed at a tower foundation, materializing the world three-dimensional system. provided that the camera’s intrinsic parameters (focal length, principal point coordinates and lens distortion coefficients) and the targets relative coordinates are accurately known (by previous testing), non-linear optimization methods can be used to determine the position of the camera’s projection centre. the temporal evolution of this quantity is considered representative of the bridge’s dynamic displacement at the location of the camera. since distances can be quite high in this type of observation context, thus the use of high focal length lenses is required to figure 1. digital image processing of concrete wall surface image showing crack. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 110 achieve a suitable spatial image resolution. however, conventional camera parameterization methods were mainly developed for small focal length cameras (below 100 mm). when applied to high focal length cameras, such methods can reveal numeric instability related to over-parameterization and ill-conditioned matrices. a suitable solution for this problem is found in [11], where the intrinsic parametrization method is described, supported in the use of diffractive optical elements (doe). this approach was implemented in the 25th of april long-span suspension bridge (p25a) in lisbon (portugal), for an observation distance near 500 m. to obtain suitable sensitivity of three-dimensional displacement measurement, a 600 mm high focal length lens (composed by a 300 mm telephoto lens and a 2x teleconverter) was used. a set of four active targets was placed in the p25a bridge south tower foundation (figure 2), facing the bridge’s main span where the camera was installed (figure 3). each of the four targets was composed by 16 leds, distributed in a circular geometrical pattern capable of emitting a narrow near-infrared beam (875 nm wavelength) and compatible with the camera’s spectral sensitivity. an optical filter on the camera reduced the environment visible irradiance from many other elements in the observation scenario, thus improving contrast in the target image. several field validation tests were performed, aiming at the quantification of the optical phenomena influence, such as atmospheric refraction and turbulence in the dimensional measurement accuracy. a calibration device was used for this purpose [11], [12], allowing to install the set of targets in four reference positions. by placing the camera in the p25a south anchorage, orientated toward the calibration device in the p25a south tower foundation (both considered static structural regions), the systematic effect caused by refraction and the beam wandering effect originated turbulence, mainly in the summer season, were quantified as explained in [12]. since the p25a bridge has two main decks (an upper road deck and a lower train deck), two types of displacement records with and without train circulation were obtained during field testing of the displacement measurement system. due to the reduced measurement sensitivity in the longitudinal direction, demonstrated in the validation tests, only transverse and vertical displacements were recorded. an image acquisition frequency of 15 hz was defined for an observation time interval of three minutes. the collected image sequences were digitally processed afterward, using the same techniques applied in the validation tests. figure 4 exemplifies a typical displacement record obtained for a passengers train passage on the p25a main span central section. for the operational condition mentioned train and road traffic the observed maximum (peak-to-peak) displacement were 0.39 m and 1.69 m, respectively, in the transverse and vertical directions. high-measurement sensitivity is noticed in the vertical displacement record where the number of train carriages (four) can be temporally discriminated four small spikes around t = 120 s, with a 95 % expanded measurement uncertainty of 8.8 mm. the distributed passengers train load was estimated between 20.7 kn/m (empty train) and 28.8 kn/m (overload train), which is considerably lower than the distributed load applied in the p25a static loading test performed in 1999, where a 3.15 m vertical displacement value was recorded for a 77.5 kn/m distributed load. as expected, in the absence of train circulation in the p25a, the observed maximum displacements were less significant, namely 0.53 m and 0.29 m, respectively, for the vertical and transverse directions, as shown in figure 5. 4. drain and sewer inspection using cctv cameras another recent example of the application of optical metrology to the civil engineering inspection context is the study carried out on the metrological quality of dimensional measurements based on images from cctv inspections in drain and sewer systems (example shown in figure 6). in this context, investigations are carried out using several sources of information, including external and internal inspection activities for the detection and characterization of anomalies which can negatively affect the performance of the drain or sewer system. cctv inspection is a largely used visual inspection technique for non-man entry components. this type of indirect visual inspection is characterized by the quantification of a significant number of absolute and relative dimensional quantities, which contribute to the characterization of the inspection observations and, consequently, to the analysis of the performance of drain and sewer systems outside buildings. unfavourable environmental factors and conditions in the drain or sewer components pose difficulties in the estimation of the quantities of interest and the quality of the recorded images can be quite poor (lighting, lack of reference points, geometric irregularities and subjective assessments, among others). the study [14] stresses the need of proper metrological characterization of the optical system the cctv camera used in drain or sewer inspections, namely, the geometrical characterization and quantification of intrinsic parameters using traceable reference dimensional patterns and applying known algorithms. the standard radiometric characterization, aiming at figure 2. active targets on the south tower foundation. figure 3. digital camera installed in the stiffness girder. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 111 the determination of the cctv camera sensitivity, linearity, noise, dark current, spatial non-uniformity and defecting pixels, is also mentioned [15]. two measurement models were studied to be applied in this context the perspective camera model and the orthographic projection camera model [16]. the first model implies having input knowledge about the camera’s intrinsic parameters and the extrinsic parameters (the camera position and orientation in the local or global coordinate system), which must be obtained from instrumentation of the cctv camera. the second model is a less rigorous approach that can be followed, assuming a parallel geometrical relation between the image plane and the crosssection plane in the drain or sewer to define a scale coefficient between real dimension (in millimetres) and image dimension (in pixels). research efforts were directed towards the evaluation of the measurement uncertainty following the gum framework [17], [18]. particular attention was given to the influence of lens distortion in the results obtained from the perspective camera model. in a typical inspection of a drain or sewer system, a reduced focal distance lens is generally used to have a wider angle. in this type of lens, distortion can cause geometrical figure 4. p25a main span central section displacement train and road traffic. figure 5. p25a main span central section displacement road traffic only. figure 6. inspection image showing dimensional reduction by deformation effect [13]. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 112 deformation of the image, thus affecting the accuracy of dimensional measurements. for this purpose, intrinsic parameters’ estimates and standard uncertainties were obtained [14] for the case of a camera with a 4 mm nominal focal length and an image sensor with 480 × 640 squared pixels, considering a pixel linear dimension equal to 6.5 µm. high-order radial distortion coefficients were considered negligible. the standard uncertainty related to the image coordinates, resulting from the performed intrinsic parametrization, was equal to 0.04 pixel. to assess the impact of distortion in the image coordinate measurement accuracy, a monte carlo method [18] was used, given the complex and non-linear lens distortion model [19]. figure 7 shows the estimates of the image variation due to the combined effect of radial and tangential distortions and figure 8 presents the corresponding 95 % measurement uncertainty. as shown in figure 7 and figure 8, the distortion impact in the images coordinates is quite low. as expected, a higher distortion is observed in the extreme regions of the image, especially in the corners. the maximum distortion estimate is close to 0.050 pixel with a 95 % expanded uncertainty of 0.001 pixel. these results allow to remove the distortion component from the perspective camera model, making it less complex and numerically more stable. due to the non-linear and complex mathematical models related to the perspective camera model, a monte carlo method was again applied in numerical simulation, in order to obtain the dispersion of values related to the local dimensional coordinates which support dimensional measurement in inspection images. a 95 % computational accuracy level lower than 1 mm was obtained. the simulation results showed that a dimensional accuracy level lower than 10 mm can only be achieved for a camera and plane location standard uncertainties of 1 mm and an image coordinate standard uncertainty bellow 3 pixels. in a sensitivity point of view, the camera and plane standard uncertainty showed a stronger contribution to the dimensional accuracy level, rather than the image coordinate measurement uncertainty. when compared with the global dimensions of the corresponding camera field-of-view (974 mm x 731 mm), the 95 % expanded uncertainty of the dimensional coordinates is comprised between 0.2 % and 4.1 %. the measurement uncertainty related to the adoption of the orthographic projection model was also studied in [14] using the uncertainty propagation law [17], considering the linearity of the applied mathematical models. for the worst case, related to the scale coefficient with the highest measurement uncertainty, the obtained dimensional measurement accuracy was always above 5 %. better accuracy levels are possible, namely, in the case of the lowest measurement uncertainty of the scale coefficient, for standard uncertainties of 1.3 pixel (for dimensional measurements close to 100 mm) and 2.5 pixels (for dimensional measurements of 200 mm), 5. calibration of a large-scale seismic shaking table with laser interferometry laser interferometry was applied for the calibration of a large-scale seismic shaking table, used by lnec’s earthquake engineering research centre in r&di activities related to seismic risk analysis and experimental and analytical dynamic modelling of structures, components and equipment. this european seismic engineering research infrastructure (shown in figure 9) is composed by a high stiffness testing platform with 4.6 m x 5.6 m dimensions and a maximum payload capacity of 392 kn, connected to hydraulic actuators, allowing to test real or reduced-scale models up to extreme collapse conditions, between 0 hz and 40 hz [20]. the control system used allows the active application of the displacement to the testing platform in three independent orthogonal axis, while its rotation is passively restricted using torsion bars. the performed calibration is included in the introduction of quality management systems in large experimental infrastructures with r&di [21], aiming the recognition of technical competence for testing and measurement and the formal definition of management processes, which can be regularly assessed by an independent entity. the compliance with metrological requirements is a key issue in this context, being related, for example, with traceability and calibration procedures, conformity assessment, measurement correction and uncertainty evaluation, data record management and data analysis procedures. laser interferometry was used to evaluate the dimensional cross-axis motion, as well as the rotation motion across axis performances of lnec’s shaking table, using specific experimental setups and optical components, as shown in figure 10 and figure 11. this experimental work allowed performing remote and non-invasive measurements with a high accuracy level in a harsh environment, being composed by two stages: the laser beam figure 7. image distortion estimates in pixels. figure 8. image distortion 95 % expanded uncertainties in pixels. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 113 alignment and data acquisition (500 sampling pairs from both the interferometer and the dimensional sensors of the seismic shaking table, having a gaussian representation of the probability distribution). the main identified uncertainty components were related to misalignment of optical elements, time synchronization and influence quantities such as air and material temperature, relative humidity and atmospheric pressure. specific actions were taken in order to minimize these uncertainty components, namely, full-range preliminary tests with adaptative adjustment of the main optical components, the application of a signal synchronization procedure and the use of compensation algorithms for the correction of the material thermal expansion and of the air refraction index [23]. one of the developed tests was defined in order to evaluate the dimensional scale calibration errors and reversibility, using input dynamic series with low variance 30 mm calibration steps, within a measurement interval of ± 120 mm. examples of obtained results are shown in figure 12 and figure 13. a measurement discrimination test was also developed, considering transition steps of 0.5 mm, 0.1 mm and 5.0 mm given at 20 mm, 50 mm and 80 mm linear positions. an example of the obtained results is shown in figure 14. the obtained results show calibration errors ranging, approximately between -0.4 mm and 0.7 mm, with a reduced reversibility close to 0.1 mm. these results were included in the measurement uncertainty evaluation, from which an instrumental measurement accuracy of 0.31 mm was obtained considering a confidence interval of 95 %. the corresponding target instrumental measurement uncertainty, defined as a metrological requirement for the seismic shaking table, is equal to 1 mm. additional dynamical tests and the corresponding discussion of results can be found in [21]. figure 9. top view of lnec’s earthquake engineering testing room [22]. figure 10. experimental setup for cross-axis motion testing. figure 11. experimental setup for rotation motion testing. figure 12. calibration errors and reversibility for the static position test of axis 1-t-a. figure 13. calibration errors for the dynamic position test of axis 1-t-a. figure 14. results of the discrimination test of axis 1-t-a at the 80 mm position. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 114 6. destructive mechanical testing of masonry specimens the application of optical metrology to the destructive mechanical testing of masonry specimens was motivated by the possibility of obtaining non-contact dimensional measurements. in a destructive test, the use of classical invasive instrumentation, such as deformeters, electrical strain gauges and contact displacement sensors, is considered not suitable for some applications due to dynamic effect in the experimental setup and to the high risk of damaging the equipment. knowledge of mechanical characteristics of resistant masonry walls is one of the aspects that still have gaps, mainly due to the difficulty in obtaining representative specimens. in addition, the growing interest in the rehabilitation of old buildings contributes to the search for new reinforcement solutions that are compatible with the original building construction techniques [24], [25]. it is equally important to ensure that these reinforcement techniques, in addition to the aesthetic and functional aspects, also reduce the seismic vulnerability of these buildings [26]. from an experimental point view, dimensional measurements have a strong contribution for the determination of key mechanical characteristics since they support the indirect strain measurement in the tested specimens [27], [28]. afterwards, these measurements are used for characterizing the masonry specimen mechanical behaviour in terms of its elasticity modulus and poisson ratio. the optical measurement solution proposed [29] is based in the use of a single camera with a spatial position and orientation allowing visualization of a set of passive targets evenly distributed in different regions, both in the static region surrounding the specimen and in the dynamic region of the tested specimen surface. the weak perspective model or the orthographic model with uniform scaling was adopted [29] allowing to establish a functional relation of the three-dimensional point georeferenced (expressed in millimetres, for example) with the corresponding bi-dimensional position in the image (usually expressed in pixels). a measurement referential, composed of reference targets, was placed in front of the observation region in the masonry specimen at the minimum distance from the specimen surface (without contact), thus minimizing the observation depth difference to the monitoring targets fixed and scattered in the observation region (in the inner region of the referential), as shown in figure 15. the mentioned referential was subjected to dimensional measurement in an optical measuring machine, before the specimen testing, aiming at the determination of the three-dimensional georeferenced position of each reference target. the knowledge of these spatial coordinates supported the calculation of the scale coefficient in each acquired image, since the measurement referential is placed in a static region of the experimental setup (ensuring that it does not touch the specimen and it is not subjected to vibrations produced by the testing machine). solid and hollow ceramic brick masonry specimens were retrieved from the walls of one building built in the beginning of the 20th century in the city of lisbon (portugal), which was undergoing rehabilitation. the proposed optical approach was implemented by fixing monitoring targets in the specimen’s ceramic bricks and placing the measurement referential with the reference targets close to the observation surfaces as shown in figure 16 (displacement sensors are also visible, being used for validation purposes, without specimen collapse). the recorded images were subjected to tailored digital image processing algorithm, in order to retrieve the image coordinates of both reference and monitoring targets, as shown in figure 17. the first stage of obtained results is related to the scale coefficient measurement samples (with a dimension equal to 28), from which an average value was obtained. figure 18 illustrates the dispersion of scale coefficient values obtained for one of the used measurement referential. based on the specimen’s length and width measurements, as well as the axial compression force readings obtained from the used universal testing machine, vertical and horizonal figure 15. schematic representation of the proposed optical measurement method. figure 16. instrumentation of the masonry specimen. figure 17. example of targets image after digital processing, showing the determined centroids. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 115 dimensional measurements were performed in the frontal and rear surfaces of the specimen, noticing the existence of both contact and optical measurement points not spatially coincident. from the collected data, stress vs. strain curves were obtained for the loading and unloading cycle corresponding to 1/3 of the fracture stress, as shown in figure 19 and figure 20. figure 20 shows the effect of noise in the strain measurements obtained by the optical dimensional measurements, when compared with the strain measurements obtained by the contact measurement chain (figure 19). this is justified by the low spatial resolution of the acquired images, which affects the targets image coordinates which support the deformation measurement. a higher spatial resolution can be achieved with an image sensor composed by smaller pixels or by using a different lens that is capable of producing a higher image magnification with an acceptable narrow field-of-view. these results were use in the determination of mechanical properties estimates and measurement uncertainties in tested masonry specimens. a detail discussion is shown in [29]. 7. conclusions this paper describes relevant contributions of optical metrology when applied in different testing and inspection activities in civil engineering, providing significant added-value in decision-making processes. the wide diversity of testing and inspection activities in this context, together with the versatility of the measurement solutions and tools provided by optical metrology, motivates the development of new interdisciplinary r&di work at lnec, so far with promising results. one of these fields is the development of moiré techniques [30] applied in the digital modelling of reduced-scale hydraulic surfaces. hydraulic experimental activities are frequently carried out in a dynamic regime; however, conventional invasive instrumentation is often unsuitable for real-time observations, making these experiments time-consuming and with reduced acquisition frequency. moiré techniques have been successively applied in other scientific and technical areas, however, their application in the civil engineering context is still quite reduced. another research field being developed by lnec in this context is the application of laser interferometry in the calibration of a strain measurement standard used for the geometrical evaluation of concrete testing machines (self-alignment and movement restriction) [31]. this measurement standard a strain gauged column is required to have a reduced instrumental measurement uncertainty (0.1 % or 5x10-6), making laser interferometry an interesting suitable solution for this objective. acknowledgement the authors acknowledge the financial support provided by lnec national laboratory for civil engineering. references [1] g. di leo, c. liguori, a. paolillo, a. pietrosanto, machine vision systems for on line quality monitoring in industrial applications, acta imeko 4 (2015) 1, pp. 121-127. doi: 10.21014/acta_imeko.v1i1.7 [2] g. d’emilia, d. di gasbarro, e. natale, optical system for online monitoring of welding: a machine learning approach for optimal set up giulio, acta imeko 5 (2016) 4, pp. 4-11. doi: 10.21014/acta_imeko.v5i4.420 [3] m. lo brutto, g. dardanelli, vision metrology and structure from motion for archaeological heritage 3d reconstruction: a case 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[31] en 12390-4 testing hardened concrete part 4: compressive strength specifications for testing machines, cen european committee for standardization, 2019. https://doi.org/10.1016/j.ndteint.2004.02.004 https://doi.org/10.1117/1.oe.54.1.014105 https://doi.org/10.1007/s10765-014-1607-3 http://dx.doi.org/10.21014/acta_imeko.v9i1.744 http://dx.doi.org/10.21014/acta_imeko.v5i4.356 https://doi.org/10.1088/1742-6596/772/1/012006 https://doi.org/10.1016/j.compstruct.2014.02.020 https://doi.org/10.1016/j.compstruct.2016.06.066 https://doi.org/10.3390/app10010371 acta imeko  issn: 2221‐870x  june 2015, volume 4, number 2, 100‐106    acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 100  least square procedures to improve the results of the three‐ parameter sine‐fitting algorithm  aldo baccigalupi, guido d’alessandro, mauro d’arco, rosario schiano lo moriello   dipartimento di ingegneria elettrica e delle tecnologie dell’informazione, università degli studi di napoli federico ii, via claudio 21, 80125  napoli, italy      section: research paper   keywords: model identification; sine fitting; least square minimization; parameters estimation; bias; uncertainty; computational burden; running time  citation: a. baccigalupi, g. d’alessandro, m. d’arco, r. schiano lo moriello, least square procedures to improve the results of the three‐parameter sine  fitting algorithm, acta imeko, vol. 4, no. 2, article 17, june 2015, identifier: imeko‐acta‐04 (2015)‐02‐17  editor: paolo carbone, university of perugia, italy  received january 7, 2014; in final form june 9, 2014; published june  2015  copyright: © 2015 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  corresponding author: mauro d’arco, e‐mail: darco@unina.it    1. introduction  many powerful methods aimed at measuring the parameters of digital signals corrupted by noise rely on model fitting approaches [1]. fitting consists in adjusting the parameters of the model in order to minimize a distance measurement between the model and the digital signal. when the distance measurement consists in the root mean square difference, the fitting is recognized as the best fit in a least square sense [2]-[3]. sine fitting is aimed at accurately estimating the frequency, amplitude, phase and dc value of a sinusoidal signal corrupted by noise. also, the estimation of the variance of the noise can be gained from the residue of the fit [4]-[7]. for instance, displacement sensors [8] and digital impedance meters [9] exploit sine fitting to estimate the amplitude of the signal, whereas digital instruments devoted to system monitoring and control use sine fitting to obtain accurate frequency [10]-[12] or phase measurements [13]-[16]. in metrological applications the chief purpose of the fitting is the accurate estimation of the variance of the noise [17]. the most common sine-fitting algorithm is the three parameter algorithm [17], which is based on a linear algebra framework, and it is utilized when the frequency parameter is known and stable. the accuracy characterizing the parameters estimates offered by the three parameter algorithm is mainly limited by the exact knowledge of the frequency of the signal. if the fitting result is unsatisfactory a four parameter algorithm can be adopted to improve the estimates. the four parameter algorithm starts from the results of a pre-fit, obtained through the three parameter algorithm, and runs a least square procedure to (i) update the estimates of amplitude, phase, and dc value and (ii) identify a frequency correction term to improve the frequency estimate. in order to further improve the results, the four-parameter algorithm can be recursively invoked until the target accuracy is gained, using the estimates gained at the previous execution as inputs for the next execution. in the paper, starting form an analysis of the effects produced by a frequency error in the use of the three parameter sine fitting algorithm, two least square methods capable of complementing the three parameter sine fitting algorithm and improve its results are proposed as alternative to the four parameter algorithm. the proposed approaches quantifies both the frequency error, to offer an accurate estimate of the frequency of the input signal, and the variance of the noise. abstract  the  paper  presents  two  approaches  to  improve  the  three  parameter  sine  fitting  algorithm  and  attain  accurate  estimates  of  the  parameters of a sinusoidal signal corrupted by noise. as the four parameter sine fitting algorithm, which is usually adopted to improve  the estimates of the three parameter algorithm, in theory, the proposed ones can be inserted into iterative schemes and repeated  until a target precision is gained. anyway, the use of the proposed algorithms is particularly suggested for those applications in which  the results must be gained in very short times, which are in contrast to long iterative procedures. in these cases, when a single run of  the algorithms has to offer the required improvement, the proposed ones are valid alternatives with respect to the four parameter  algorithm, sharing with it almost comparable accuracy.  acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 101  their performance in terms of accuracy and running times are thoroughly analysed and compared to that offered by the four parameter algorithm. although both the proposed algorithms and the four parameter algorithm can be inserted into iterative schemes and repeated until a target precision is gained, the single shot results are considered in the comparison. the proposed algorithms are in fact intended for those applications in which the estimates of the frequency and of the variance of the noise must be gained in very short times, which are in contrast to long iterative approaches. 2. sine fitting approaches  2.1. three parameter sine fitting  the standard three parameter sine fitting algorithm estimates the amplitude, phase and dc value of the sinusoidal signal that best fits in a least square sense a digital signal yk. named the value of the digital frequency of the input data (i.e. the frequency in hertz normalized to the sample frequency), a, , and c, respectively, the amplitude, phase and dc value, nk the noise, and k the digital time ranging from 0 up to m-1, the following model can be adopted:   kk nckay  2cos (1) the three-parameters sine-fitting algorithm requires as input parameter the value of the digital frequency or an estimate  of it. if the value  coincides with , the values of the parameters a, and c can be gained considering the equation:     kk nckbkay  00000 2sin2cos  (2) which is equivalent to equation (1) but linear in the parameters a0, b0 and c0, respectively defined as: a0 = acos, b0 = -asin, and c0 = c. discarding the noise nk and ranging k from 0 up to m-1, equation (2) produces a linear system made up of m equations, which can be represented in matrix form by: d0x0 = y (3) where x0 is a column array with the unknown components a0, b0 and c0, y is a column array containing the samples of the input digital signal, yk, and d0 is the matrix of the coefficients:                   1 1 1 1010 00 0000 0 mm kk sc sc sc   d (4) in which  kc k 00 2cos  , and  ks k 00 2sin  . the three parameter algorithm determines x0 by means of the pseudoinverse method, namely:   ydddx t00t0 10  (5) as it is well known the solution represented by equation (5) minimizes the mean square value of the residue of the fitting, which should coincide with the noise. the parameters of the model given in (1) are then estimated through:            0 00 2 0 2 0 ,atan2 cc ba baa  (6) in which atan2( . , . ) represents a four quadrant inverse tangent function. 2.2. four parameter sine fitting  the four parameter sine fitting algorithm works on the hypothesis that the estimated value of the digital frequency  approaches the exact one , but it is biased by a small amount . it linearizes the model given in (2), which is non-linear with respect to , by approximating it with a first order taylor expansion centred in i-1 = 0. thus it considers for the acquired digital signal the representation:          kiiiii iiiiik nkkbkka ckbkay     1111 11 2cos2sin2 2sin2cos   (7) in which there are four unknown parameters, namely ai, bi, ci, and i. the term i represents a correction term that added to i-1 produces an improved estimate of the digital frequency, i=i-1+i. equation (7) represents an iterative model that includes the values of the parameters estimated at step (i-1)-th and the unknown parameters to be estimated at step i-th. in order to start the iterations, the four parameter algorithm uses at step i = 1 the results of a three-parameter pre-fit to initialize ai-1 = a0, bi-1 = b0 and i-1 = 0; for the next iterations, the parameters estimated at step i-th are reused at step (i+1)-th. the method to solve (7) is formally identical to that discussed for the three-parameter algorithm. in fact, discarding the noise nk and ranging k from 0 up to m-1, equation (7) produces a linear system made up of m equations. the estimates of the parameters ai, bi, ci and i that minimize the mean square value of the residue in (7) can thus be attained solving the system by means of the pseudo-inverse method. in matrix form, collecting the parameters ai, bi, ci and i, in the column array xi, it results:   ydddx tt iiii 1 (8) where di is defined by:                              1,111,111,11,1 ,11,11,1,1 0,110,110,10,1 121 21 021 miimiimimi kiikiikiki iiiiii i cbsamsc cbsaksc cbsasc    d (9) the four-parameter algorithm is usually run until the distance between successive estimates is smaller than a target threshold. it typically converges in less than ten iterations, but can diverge if the initialization step is coarse. 3. analysis of the effects of a frequency error  in order to improve the results of the fitting, an appropriate model is proposed to analytically describe the effects produced by a frequency error. in particular, the digital signal can be represented according to:        k kk ncika nckay   00 0 2cos 2cos   (10) in which i0 satisfies the constraint 20i0 =  + n2 for an unknown integer n, and ∆ is much smaller than . from equation (10) it follows that:             k k ncikika ikikay   000 000 2sin2sin 2cos2cos   (11) and, in the hypothesis that the frequency error  is small: acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 102        k k nckika kay     00 0 2sin2 2cos (12) according to (12), the residue       k kk nkika ckay     00 0 2sin2 2cos (13) does not coincide with the noise but includes an additional contribution, which is described by linearly damped oscillations characterized by frequency 0; the parameters  and i0 characterize the envelope of the damped oscillations. this contribution is often visible in the typical graph of the k data resulting in real tests: an example of a possible scenario is given in figure 1. 4. proposed methods  the removal of the deterministic contribution present in (13) can be obtained by least square estimation procedures. in particular, two methods can be considered. 4.1. method ‘a’  a first method consists in finding the values of the parameters  and i0 that produce the least square residue in (13). the values can be obtained by solving through the pseudo-inverse method the over-determined system:                                             1 0 0 1 0 1 0 1 0 m k m k m k i s s s sm ks s            (14) where    kask 02sin2 . specifically, named a the column array with components  and i0: a = (ata)-1at (15) where  is the column array containing the values k, and a is the matrix in (14). it is worth noting that (15) is formally the solution of a straight line fit problem. the estimate of the frequency error can be therefore straightforwardly gained by applying the formula:           1 0 1 0 2 1 0 1 0 22 m k kk m k k m k kk m k k skskssk  (16) in order to measure the random noise nk, the deterministic contribution present in (13), identified through the estimation of  and i0, has to be subtracted from the residue k. 4.2. method ‘b’  alternatively the frequency correction term  and the variance 2n of the noise corrupting the signal can be estimated from the squared values 2k of the residue throughout a further least square procedure. in this case a simple and linear model can be utilized to fit the data. specifically, taking the square values of equation (13), it follows:             k kk nkika nkika     00 2 0 2 0 22 2sin22 24cos12 (17) the squared values of the noise 2kn include both a constant term equal to 2n , which is the variance of the original noise nk, and a zero mean random term mk. grouping all the contributions that have zero mean value in (17), both if characterized by deterministic or random nature, in the term lk:            k kk nkika mkikal     00 0 2 0 2 2sin22 24cos2 (18) it can be stated that:     knk likika  2200222 22  (19) to gain estimates of , i0, and 2n , and get rid of lk, equation (19) is first rewritten in a more compact form: kk lcbkak  22 (20) and the parameters a, b, and c are estimated by solving through the pseudo-inverse method the system:                                                    2 1 2 2 0 2 2 111 1 100 m k c b a mm kk        (21) specifically, named b the column array with components a, b, and c it results: b = (btb)-1bt (22) where  is a column array containing the values 2k and b is the matrix in (21). the typical results obtained fitting the 2k data are given in figure 2. figure 1. residue characterized by linearly damped oscillations masked by  noise.  figure 1. squared values of the residue and polynomial fitting results.  acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 103  finally, taking into account that a, b, and c are related to , i0 and 2n by:               22 0 22 0 0 2 0 2 2 42 2 nn iaaic iaaib aa    (23) the parameters of interest can be gained from:                   2 0 2 20 2 4 2 1 iac a b i a a n     (24) in order to estimate the term that compensates the frequency error, the sign of the correction has to be gained throughout further considerations. for instance, if the initial frequency estimate 0 has been obtained by applying a peak location algorithm to the results y(m), m = 0, …, m-1, of a m points discrete fourier transform (m-dft), according to:      n j jymynm  020 ;max  (25) the correction can be given without sign ambiguity as:        11 jyjysign (26) which is valid for rectangular window at large values of j (typically more than 10 cycles in the measurement interval). the high signal to noise ratio in the neighbouring of the peak value should assure the minimum sensitivity to the effects of the noise.  5. performance evaluation  the performance of the proposed methods has been assessed by means of several tests. the main goal of the tests is evaluating the accuracy of the methods at estimating both the frequency of the input signal and the variance of the superimposed noise. in this case the accuracy can be decomposed into two terms, i.e. bias and repeatability, and can be quantified as square root of their quadratic summation. each test consists in repeating n times two fundamental steps: running a three-parameter sine-fitting algorithm to attain initial estimates of the parameters of the sine model; applying the proposed methods and the four-parameter algorithm to attain improved estimates of the parameters of interest. the difference between the mean value of the repeated estimates and the reference value represents the experimental bias, while the experimental standard deviation of the repeated estimates represents the repeatability. the signals under test are made up of m samples that represent a sinusoid corrupted by noise. the digital frequency of the test signals  is obtained adding to a basic value 0, expressed with finite resolution rbw = 1/m, an offset , i.e.  = 0 + . the basic value 0 is given in input to the initial three-parameter sine-fitting algorithm to obtain an initial estimate of the signals parameters. the offset  makes the methods under test work in the presence of a frequency error. figure 3 shows the typical results related to digital frequency estimates, obtained for a signal under test characterized by m = 100, 0 = 0.310, and signal to noise ratio (snr) equal to 37 db. to highlight both the bias and repeatability contributions to the overall accuracy the results are shown by means of scatterplots; different markers are associated to the different methods. in particular, the results offered by the peak location algorithm applied to the outcomes of the m-dft are represented by marker ‘*’, those offered by method a by marker ‘+’, those offered by method b by marker ‘o’, and those offered by the four-parameter sine fitting algorithm in single run mode by marker ‘x’. the considered frequency offsets  are positive and equal to 10%, 20%, 30% 40% and 50% of rbw; it has been observed that positive and negative frequency errors produce the same statistics. for each method a set of 15 estimates has been considered to put in evidence bias and repeatability. the results highlight that methods a and b exhibit comparable performance, both in terms of bias and repeatability, with respect to the four-parameter sine-fitting algorithm in single run mode. note that the results produced by the peak location applied to the m-dft results are characterized by utmost repeatability, which is explained by the minimum sensitivity to the effects of the noise in the neighbouring of the peak value of the signal spectrum, as well as the high snr equal to 37 db. figure 4 shows the typical results related to the variance estimates obtained in the same test conditions. specifically, the results are given by means of a scatterplot that makes visible both bias and repeatability. the peak-to-peak amplitude of the simulated signal is 1 v, the noise variance is 0.005 v2. in this case marker ‘*’ is associated to the results offered by the three parameter sine fitting algorithm. methods a and b exhibit almost comparable performance with respect to the four parameter sine fitting algorithm utilized in single run mode. similar tests have been repeated for signals characterized by reduced snr = 17 db; figure 5 and figure 6 show the results. concerning the digital frequency estimations, methods a and b still exhibit comparable performance in terms of bias and repeatability with respect to the four parameter sine fitting algorithm. figure  3.  results  related  to  the  estimates  of  the  digital  frequency  of  sinusoidal signals corrupted by noise when the snr is equal to 37 db.  acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 104  further tests have been performed in order to verify any influence of the length of the processed data on the results. in particular, the estimates of the digital frequency and noise variance have been performed upon records made up of 100, 250, 500, and 1000 samples. the results show that the accuracy characterizing the digital frequency estimates improves upon the increasing of the record size m. the performance in terms of bias and repeatability related to the variance estimates produced by the proposed methods show the same sensitivity on the record size with respect to that offered by the four parameter algorithm. figure 7 and figure 8 show a typical case: they are related to a signal corrupted by noise characterized by snr = 17 db. it is worth noting that the resolution rbw needed to measure the exact digital frequency is 0.001, thus for the record characterized by size 100, 250 and 500, the m-dft approach offers a digital frequency estimate which is shifted from the real one, because the available resolutions are respectively equal to 0.01, 0.004, 0.002. the results obtained for m = 1000 are very accurate because the digital frequency of the signal is exactly estimated by the mdft approach.  6. conclusions  the paper has presented two methods to improve the three parameter sine fitting algorithm. the performance of the two methods, intended as an additional step of the three parameter figure  5.  results  related  to  the  estimates  of  the  digital  frequency  of sinusoidal signals corrupted by noise when snr is equal to 17 db.  figure  6.  results  related  to  the  estimates  of  the  digital  frequency  of sinusoidal signals corrupted by noise when snr is equal to 20 db.  figure 7. estimates of the digital frequency of the signal corrupted by noise:  the accuracy improves upon the increasing of the record size m.  figure 8. typical estimates of the variance of the noise: the performance of  the  proposed  methods  are  comparable  to  those  offered  by  the  four  parameter algorithm.  figure 4. results related to the estimates of the noise variance carried out  when the digital frequency of the signal is affected by error.  acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 105  sine fitting algorithm, has been compared to that of a four parameter sine fitting algorithm used in single run mode. the proposed methods represent nice alternatives to the four parameter sine fitting algorithm for the identification of the parameters of the sinusoidal model and the estimation of the variance of the superimposed noise. in addition, they are suitable to function in a large variety of noise conditions. it is worth noting that one of the simplest ways to improve the results of a three parameter sine fitting algorithm is preliminary computing an accurate initial estimate of the frequency of the input signal. to this end, high-accuracy spectrum analysis techniques, capable of providing analytical leakage compensation and improving frequency estimation, can be employed; one of these simply requires interpolating the dft results related to the signal under test. in several practical applications the proposed methods are likely rejected in favor of interpolated dft approaches, while in particular cases can be considered for their low computational burden. anyhow, the proposed methods represent two different lines of attack to refine the three-parameter sine fitting results, to be valued from a conceptual and methodological point of view. appendix a  the computational burden of the different approaches that have been considered can be evaluated by counting the number of the most expensive calculus operations, i.e. the nonlinear operations and multiplications, which are required to obtain the results. the overall computational burden of the proposed methods and the four parameter algorithm considered for estimating the digital frequency and the variance of the noise includes that of the three parameter algorithm. the three parameter sine fitting algorithm requires the definition of matrix d0 that involves the calculation of cos(2k) and sin(2k), with k ranging from 0 up to m-1, and has a cost equal to 2m. then a least square minimization has to be performed by processing m equations to gain the values of p parameters. in detail, mp(p+1)/2 multiplications are needed to compute the symmetrical square matrix d0td0 and mp multiplications are required to condition the input data by computing d0ty. the other calculations have a negligible cost. in fact, the number of multiplications needed to perform the p-square matrix inversion, i.e. (d0td0)-1, in the case that the inversion is attained by calculating the algebraic complements, which is the most expensive method and requires p! multiplications, is negligible since p is equal to 3; similarly, the final step that consists in calculating the inner product between the inverted matrix and the conditioned data, counts only p2 multiplications, which is negligible. the four parameter algorithm adds the computational burden of including in matrix di a fourth column, i.e. 2a0kcos(20k) 2b0ksin(20k), which requires 4m multiplications, aside the three already present in d0. moreover it demands least square minimization upon m equations to gain q = 4 parameters. method a instead involves the computational burden of defining matrix a, which consists in calculating the values sk, which have a cost equal to m, and performing the m multiplications k·sk, and, furthermore, the computational burden of performing least square minimization upon m equations to gain r = 2 parameters. nonetheless, to estimate the variance of the noise, the parameters  and i0 have to be used to build up the fitted model of the residue k, extract the noise nk, and calculate its variance. method b is the best one from a computational point of view because it does not add any computational burden for defining matrix b, the values of which are independent from unknown parameters: to define matrix b only the size m of the acquired data is needed. since the square matrix (btb)-1 can be calculated and stored previously, the remaining computational burden is just that of data conditioning, which consists in calculating bty and requires 3m multiplications. moreover, the method makes ready both an estimate of the frequency error and of the variance of the noise without requiring building up models. the running times of the three and four parameter algorithms as well as of the methods a and b have also been measured. in particular, the algorithms have been run on a comex xp.520 core 2duo t7200 machine, which is a windows based multitasking processor. due to the multitasking functioning of the operative system, the running times have shown different values in repeated tests. to clear the effects of interruptions and holding-ups due to the operative system, the same test signals have been processed a thousand times and the average running time observed for each algorithm has been considered as a reasonable measurement. the test results highlight that the four parameter sine fitting algorithm lengthens the running time of the three parameter algorithm. in particular, table 1 gives in the first column the results related to the average processing time of a record of 1000 points in milliseconds. it also summarizes in the second column the approximate estimations of the computational burden of the four different approaches, expressed in terms of number of most expensive calculus operations. references  [1] steven m. kay , “fundamentals of statistical signal processing”, volume i: estimation theory, april 1993, prentice hall. [2] ramos, p.m., fonseca da silva, m., martins, r.c., serra, a.m.c., “simulation and experimental results of multiharmonic leastsquares fitting algorithms applied to periodic signals”, ieee trans. on instrum. and measurements, vol.55, n.2, 2006, pp.646-651. [3] ramos, p.m., serra, a.c., “least squares multiharmonic fitting: convergence improvements”, ieee trans. on instrum. and measurements, vol.56, n.4, 2007, pp.1412-1418. [4] quatieri, t.f., danisewicz, r.g., “an approach to co-channel talker interference suppression using a sinusoidal model for speech”, ieee trans, on acoustics, speech and signal processing, vol.38, n.1, 1990, pp.56-69. [5] kollar, i., blair, j.j., “improved determination of the best fitting sine wave in adc testing”, ieee trans. on instrum. and measurements, vol.54, n.5, 2005, pp.1978-1983. [6] pintelon, r., schoukens, j., “an improved sine-wave fitting procedure for characterizing data acquisition channels”, ieee table 1. average running times determined upon 1000 runs and theoretical  computational burden.    average  running  time [ms]  theoretical  computational burden  3 par. 12.8 [ 2 + p(p+1)/2 + p ] m   4 par. 37.7 [ 4+2 + p(p+1)/2 + p q(q+1)/2 + q ] m method ‘a’ 25.7 [ 2 + 2 + p(p+1)/2 + r(r+1)/2 + r] m method ‘b’ 19.1 [ 3 + 2 + p(p+1)/2 + p ] m  acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 106  trans. on instrum. and measurements, vol.45, n.2, 1996, pp.588-593. [7] bertocco, m., narduzzi, c., “sine-fit versus discrete fourier transform-based algorithms in snr testing of waveform digitizers,’ ieee trans. on instrum. and measurements, vol.46, n.2, 1995, pp.445-448. [8] masi, a., brielmann, a., losito, r., martino, m., “lvdt conditioning on the lhc collimators”, ieee transactions on nuclear science, vol.55, n.1, 2008, pp.67-75. [9] radil, t., ramos, p.m., serra, a.c., “impedance measurement with sine-fitting algorithms implemented in a dsp portable device”, ieee trans. on instrum. and measurements, vol.57, n.1, 2008, pp.197-204. [10] bakhtiari, s., shaolin liao, elmer, t.w., gopalsami, n.s., raptis, a.c., “a real-time heart rate analysis for a remote millimeter wave i-q sensor”, ieee trans. on biomedical engineering, vol.58, n.6, 2011, pp.1839-1845. [11] mahata, k., “subspace fitting approaches for frequency estimation using real-valued data”, ieee trans. on signal processing, vol.53, n.2, 2005, pp.3099-3110. [12] radil, t., ramos, p.m., serra, a.c., “new spectrum leakage correction algorithm for frequency estimation of power system signals”, ieee trans. on instrum. and measurements, vol.58, n.5, 2009, pp.1670-1679. [13] stenbakken, g., ming zhou, “dynamic phasor measurement unit test system”, ieee power engineering society general meeting, 2007, pp.1-8. [14] yang kuojun, tian shulin, “algorithm based on tdc to estimate and calibrate delay between channels of high-speed data acquisition system”, 10th international conference on electronic measurement & instruments (icemi), vol.3, 2011, pp.221-224. [15] vucijak, n.m., saranovac, l.v., “a simple algorithm for the estimation of phase difference between two sinusoidal voltages”, ieee trans. on instrum. and measurements, vol.59, n.12, 2010, pp.3152-3158. [16] fenster, s., dann, t., moretti, a., “synchrotron improvements with shed waveform”, ieee transactions on nuclear science, vol.28, n.3, 1981, pp.2577-2579. [17] ieee std 1241-2010 (revision of ieee std 1241-2000), “ieee standard for terminology and test methods for analog-todigital converters”, 2011, pp.1-139. acta imeko  issn: 2221‐870x  june 2015, volume 4, number 2, 10‐17    acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 10  development of a knowledge base for the planning of  prismatic parts inspection on cmm  vidosav d. majstorovic 1 , slavenko m. stojadinovic 1 , tatjana v. sibalija 2   1  university of belgrade, faculty of mechanical engineering, department for production enginnering, kraljice marije 16, 11120 belgrade    2  metropolitan university, belgrade, faculty of information technology, tadeusa koscuska 63, 11 000 belgrade        section: research paper  keywords: inspection planning; cmm; knowledge base; feature – based system  citation: vidosav d. majstorovic, slavenko m. stojadinovic, tatjana v. sibalija, development of a knowledge base for the planning of prismatic parts  inspection on cmm, acta imeko, vol. 4, no. 2, article 3, june 2015, identifier: imeko‐acta‐04 (2015)‐02‐03  editor: paolo carbone, university of perugia, italy  received june 26, 2014; in final form january 28, 2015; published june  2015  copyright: © 2015 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  corresponding author: slavenko m. stojadinovic, e‐mail: slavenkostojadinovic@gmail.com    1. introduction  the development of an intelligent system for inspection planning is the imperative and prerequisite for a new generation of metrological systems and their applications within the digital quality concept, based on the global interoperability model [13]. the interoperability model integrates cad-cam-cai data on a digital platform [4, 5] and presents a basis for virtual simulation and knowledge-based inspection planning, especially for prismatic parts. digital manufacturing is a framework for the development of a new generation of technological systems, based on virtual simulation, digital modelling of a product and cloud computing application [2, 5]. expert systems (ess), being an advanced approach for modelling and application of engineering knowledge in technological systems, have been frequently used in the last few decades [6-9]. since 2000, ess have passed through the mature stage of development and application [10-14]. influenced by ess for the technological processes planning (capp/cam) and step standardisation [4,15,16], the metrological primitives modelling is based on the modelling of metrological characteristics (features) [17-19]. our research is based on this approach [20, 21]. in the cai model, consideration of metrological features implies the recognition and extraction from the cad model and their definition and modelling [18]. our approach has evolved from the definition and modelling [21] and application of ontology for defining a knowledge hierarchy for inspection [20], toward an integrated approach [22] which uses a digital product model in interoperability environment (autodesk inventor professional 2011, solidworks, pc-dmis, protege). our model of the system for intelligent design of a conceptual inspection plan for cmms (i.e. intelligent system for automatic planning of inspection – isapi), in the artificial intelligence environment, is presented in this paper. special attention is dedicated to the knowledge base model that was developed and applied within the proposed approach [22]. 2. development of a knowledge base model   the conceptual inspection plan presents a basis for cmm programming. usually, based on experience, knowledge and skills, an engineer (inspection planner) generates a conceptual plan for the inspection on cmms. however, this approach should be avoided in a modern manufacturing, especially due to the fact that today cmms work in a digital environment and abstract  inspection  on  coordinate  measuring  machines (cmms)  is  based  on  software  support  for  various  classes  of  metrological  tasks,  i.e.  tolerances. today, the design of a uniform inspection plan for a measuring part presents a rather complex issue due to the following:  (i) metrological complexity of a measuring part; (ii) skills and knowledge of a designer / inspection planner; and (iii) software for cai  model, considered as a part of an integrated cad‐capp‐cam‐cai system. this issue could be addressed by the usage of expert systems  that generate a conceptual inspection plan for a measuring part, based on which the inspection plan for a selected cmm could be  automatically developed. this paper presents the development of a model of an automatic inspection planning system for cmms, and,  in particular, the developed knowledge base model.     acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 11  with huge numbers of different parts. in contrast to the old approach, a new concept is based on the usage of ess that generate conceptual inspection plans. the first investigations in this domain have been conducted a long time ago [6], defining a framework for the design of an inspection plan for cmms using ess. since then, owing to the development of artificial intelligence tools, a new generation of ess has been developed. the basis of each es model is a knowledge base, its organisation, scope and content in terms of factual and heuristic knowledge. a knowledge base must contain necessary knowledge and information about the measuring part (mp), its tolerances, geometric features (gf) and metrological features (mf), inspection sequences (is), measuring probe configuration (pc), measuring machine (mm), and fixture tools (ft) and accessories, as presented in figure 1 [21]. the basic steps for generating a conceptual inspection plan for cmm are: (i) analysis and synthesis of metrological tasks (tolerances) according to the digital product model (the nodes mp, mf and gf presented in figure 1); (ii) definition and orientation of measuring coordinate systems (mp, mf, gf, is, pc and mm); (iii) selection of a measuring probe configuration (is, pc and mm); and (iv) definition of a measuring strategy (decomposition of metrological features, development of geometric features and inspection planning). therefore, our approach is based on the geometrical (cad), technological (capp/cam) and metrological (cai) integration, as presented in the knowledge base model in figure 1). the proposed model generates a conceptual inspection plan with the following elements: (i) measuring part with metrological / geometric features and inspection sequences; (ii) measuring coordinate systems; (iii) measuring probe configuration; (iv) fixture tools and accessories; and (v) necessary characteristics of cmm. the proposed model supports the cai system [23, 24] developed as isapi [22], and also supports the realisation of the following approaches: (i) measurement (measuring part – cmm – measuring results), (ii) inspection (measuring part – cad – cmm – inspection results), (iii) reversible engineering (real part model – cmm – cad), and (iv) inspection planning (cad – cai – inspection plan). therefore, our model is based on the following axiom: geometrical – technological metrological integration (figure 2), compatible with the state-of-the-art approaches in digital manufacturing [4]. previous analyses show that the measuring and inspection on cmm require a wider scope of knowledge: (i) knowledge about the part processing (for the analysis of tolerances and definition of measuring coordinate systems); (ii) general knowledge about tolerances (analysis and synthesis of metrological and geometric features); (iii) specific mathematical knowledge about modelling and relations between geometric and metrological features (e.g. curves modelling); (iv) knowledge about cmm and its working principles; (v) knowledge about software installed on cmm; and (vi) heuristic knowledge about this domain. 2.1. an experimental example  an experiment is performed on a real measuring part [25]. for the observed measuring part a graph of knowledge base model is developed with 4 nodes and their interrelations, as presented in figure 3. the nodes are non-terminated symbols that present knowledge entities: mp – knowledge about the measuring part and its tolerances, mf – knowledge about metrological features, gf – knowledge about geometric features for the defined metrological feature. non-terminated symbols as knowledge entities are connected with terminated symbols: a0 = (tolerances, feature), a1 = (metrological feature, feature, tolerances), a2 = (geometric feature, features), b = (tolerance, geometric feature, features), and a = (measuring part, geometric features, features). they are presented as ontological structures with hierarchical relations that define all elements of knowledge in this domain. for example, relations between mp and gf could be as follows: a0 a1 a2 (reasoning line for generating a measuring probe path), a0 – b (reasoning line for generating point coordinates at the geometric feature), and a (reasoning line for geometric features at the measuring part). relevant investigations on the modelling based on tolerance characteristics have already been performed [4,15,16,21]. our approach integrates metrological – geometrical information from the cad digital model, where geometrical information are parameters of geometric features taken from the iges file after modelling of the prismatic measuring part using software autodesk inventor professional 2011. figure  2.  elements  for  building  the  knowledge  based  model  for  prismatic parts inspection on cmm.   figure 1. the knowledge base graph for the  isapi model.    acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 12  therefore, the tolerances defined by iso 1101 standard are presented by the graph that illustrates ontological decomposition into metrological features, and then – into geometric features. this way the reasoning line is specified, and, by extracting parameters, each geometric feature is uniquely defined. this geometric feature presents a basis for the analysis of accessibility of the measuring probe, path planning, and for generating measuring protocols for the input measuring requirements. terminated symbol a0 defines the decomposition of a prismatic measuring part into the tolerances type defined by the iso standard: length tolerances (tl), form/shape tolerances (tf), orientation tolerances (to), and location tolerances (tlc). terminated symbol a1 defines the continuation of decomposition into specific sub-types of tolerances defined in the iso standard. for example, the orientation tolerance could be decomposed into the following sub-types: parallelism (to1), perpendicularity (to2) and angularity (to3). terminated symbol a2 also defines the continuation of tolerances decomposition into the specific forms/shapes. these forms are denoted as metrological characteristics (features). from the metrological aspect, one metrological feature is composed of one or more geometric features. figure 4 shows a graph of an ontological structure (developed using protege software), that presents rules for the decomposition in a general case. figure 3. the graph of a knowledge base and its decomposition.    figure 4. the rules for decomposition of a knowledge base graph, developed by using protege software.    acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 13  for example, the tolerance tl is composed of seven metrological features: tl1-1, tl1-2, tl1-3, tl1-4, tl1-5, tl1-6 and tl1-7. further decomposition of each of them could be presented by five geometric features (gf100-points, gf200-lines, gf500-planes, gf600-cylinders and gf700spheres). initially, the geometric feature concept has been defined in analytical geometry. based on this definition, it was later used in engineering modelling. in manufacturing engineering, the geometric features present a basis for the definition of certain aspects of a product design, process design, inspection design, etc. [24]. in our model, the geometric feature presents the lowest level of tolerances definition that refers to the generation of measuring probe points on the measuring part. the systematisation of geometric features is presented in figure 5. geometric features shown in figure 5 present basic features used for the development and explanation of all types of metrological features. each geometric feature is uniquely defined by the set of parameters with respect to the local coordinate system oxyz (presented by the red colour in figure 5) and the coordinate system of a measuring part оxмyмzм (presented by the black colour in figure 5). these parameters could be: coordinates (x, y, z), diameter (d, d1), height (h, h1), width (a), length (b), vector of a primitive (n), parameter of the fullness of a feature (np=1 full, np=-1 empty). vector n determines the orientation of primitives in a space. the position of a primitive is defined by the coordinates x0, y0, z0. the parameter of fullness is defined by the unit vector of the x-axis of a feature: the value of a parameter np=1 implies a full feature and the value np=-1 implies an empty feature. the parameter of fullness and the vector of a feature define the direction of a measuring probe access during the definition of metrological feature coordinates. the extraction of parameters of a geometric feature cylinder from the iges file is based on the recognition of its structure. the part of a structure needed for the analysis in our model is presented in table 1. an iges file is composed of five sections in the following order: start section, global section, directory entry section, parameter data section, and terminate section, as shown in figure 6. all geometric entities are given in the directory entry section and parameter data section. the extraction of parameters is performed based on the number of sequences of an entity (geometric feature), as in table 2. figure 5. the geometric features and their parameters (geometric primitives).    acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 14  the extraction of parameters for the other geometric features could be conducted using the same procedure that was used for a cylinder. the primary objective is decomposition of a prismatic measuring part into metrological features (mf) that indirectly participate in the inspection planning. the secondary objective is decomposition of tolerances into geometric features (gf), and, from a metrological aspect, they give a full set of information to define a conceptual inspection plan. an application of the aforementioned concept of a knowledge base is performed on a real measuring part (a housing of the main spindle of a lathe), presented in figure 7. according to the knowledge base model, tolerances of a part are reduced to geometric features, including all metrological features that are involved in part tolerances. the measuring part implies the following tolerances: tolerance of length (tl), tolerance of shape / form (tf), tolerance of orientation (to) and tolerance of location (tlc). the length tolerance is composed of four length tolerances (tl1-61, tl1-62, tl1-63 and tl1-7) and five diameter tolerances (tl2-11, tl2-12 tl2-13 tl2-14 and tl2-15). table 1. extraction of iges parameters of a cylinder.  entity  1  1   2   3 5   6   7  73‐80 line  (generatrix)  110  x1, y1, z1 (start point)  x2, y2, z2  (end point)  4 seq. number  line (axis)  110  x3, y3, z3 (start point)  x4, y4, z4  (end point)  5 seq. number  surface of  revolution  120  seq. no. 1,  seq. no. 2  α 1, α2  (start and  end angle)  3 seq. number  direction  123  i1, j1, k1  (unit vector)    18 seq. number  direction  123  i2, j2, k2  (unit vector)    28 seq. number  figure 6. iges file of a cylinder geometric feature.  table 2. calculation of parameters.  sequence number  parameter of cylinder  4,5  d= ((x6‐x1) 2 +( y6‐y1) 2 +( z6‐z1) 2 ) 0.5 5  xo=x1,  yo=y1,  zo=z1  5  h= ((x2‐x1) 2 +( y2‐y1) 2 +( z2‐z1) 2 ) 0.5 18  n= i1  j1  k1  figure 7. the real measuring part.    acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 15  the form tolerance is tf2-1, while the orientation tolerances are designated as to2-11, to2-12, to2-13 to2-14, to2-15 and to2-16. the location tolerance is marked as tlc1-4. figure 8 shows the ontological hierarchy (class hierarchy, object property hierarchy and data property hierarchy), i.e. the decomposition of a prismatic measuring part into features. it was based on the general tolerance types as defined by the iso standard. in this case, the tolerances are tl, tf, to and tlc. these tolerance types are further decomposed into specific tolerances types that are also defined by the standard. it is necessary to follow this procedure in order to connect specific tolerance types with the real tolerances of real parts. the next step implies further decomposition into specific tolerance types presented in the technical drawing of the part. metrological features are composed of a few geometric features, and present the relations between tolerance types and geometric features of the part. in other words, if we know that the technical drawing of the part is a real source of metrological information then the metrological features are introduced as a link between tolerances and the part geometry whose carrier is the digital model of the part (cad model). cad software allows us to input only one type of tolerance – the tl type. this shows that, in inspection planning, the cad model of the part could be used only from the geometrical aspect. that is why software for cmm loads geometrical information from the iges file. inspection planning of prismatic parts on cmms is usually performed with respect to three mutually orthogonal directions, depending on the number, position and orientation of measuring stylus in a measuring sensor. figure 9. the direction of probe accessibility.  figure 8. decomposition of the real measuring part into metrological features.    acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 16  this assumption serves as a basis for the development of a sequence of metrological features inspection for prismatic parts in our model. from the mentioned three directions, in general it is possible to derive six directions of a measuring probe access (dpa). since a measuring part must be set up at the machine table, one direction of access is disregarded. hence, there are five possible directions: dpa 1, dpa 2, dpa 3, dpa 4 and dpa 5, as presented in figure 9. each of these directions corresponds to some of the directions of the coordinate system of the cmm: dpa 1 = z; dpa 2 =  x; dpa 3 = y; dpa 4 = +x; and dpa 5 = +y. at the level of a geometric feature, dpa is determined by parameters n and np. for the inspection of a primitive, the possible directions are dpa 1, dpa 2, dpa 3, dpa 4 and dpa 5. a special case is when the direction of a vector n does not match with dpa 1, dpa 2, dpa 3, dpa 4 and/or dpa 5. then, the movement of a measuring probe should be decomposed into movements in two dpa’s: the first is the closest one from dpa 1, dpa 2, dpa 3, dpa 4 and dpa 5 (according to the minimal angle criteria) for the movement immediately beneath the primitive, and the second is n vector of the primitive. this approach automatically defines an inspection sequence for the measuring part. dpa 1,…, dpa 5 differ from the movement of machine axes. dpa defines the access to a feature, and machine axes accomplish acquisition of measuring points in the desired directions. verification of this approach has been performed on a cmm, and results were reported in [26]. 3. conclusion  the development and application of the uniform inspection plan for cmms are important issues that depend on metrological complexity of a part and skills and knowledge of an engineer (inspection planner). the presented research shows a model of the knowledge base for cmm inspection planning that aims to offer a solution for the development of an intelligent system for inspection planning (isapi). the presented knowledge base is defined by entities and relationships among them. the result of this approach is a definition of relations between geometric features and certain types of tolerances that could be found by searching through the graph of a knowledge base model. by searching through the graph, the general tolerance types defined by the standard are linked to the geometric features in order to allow the definition of a metrological sequence and planning of a measuring probe path. analysis of this method could be performed in respect to the step nc standard which contains information about tolerances and geometry. however, the existing cad interfaces do not allow us to specify tolerances on the cad model of a part. therefore, it could be said that our model presents an efficient way to provide all necessary data for an automatic inspection, which is its main advantage. acknowledgement  the work presented in this paper has been performed within the technological development project tr 35022, supported by the ministry of education, science and technological development of the republic of serbia. references  [1] e. westkämper, “digital manufacturing in the global era”, in pedro f. cunha and paul g. maropoulos (eds.): digital enterprise technology, perspectives and future challenges, springer, (2007), p. 3-14. [2] e. westkämper, (2010): factories of the future beyond 2013 a view from research: the role of ict. http://cordis.europa.eu/fp7/ict/micro-nanosystems/docs/fofbeyond-2013-workshop/westkaemper-manufuture_en.pdf (accessed july 2013) [3] http://www.steptools.com/library/stepnc/ (accessed december 2013) [4] r. laguionie, m. rauch, j.y. hascoet, s. h. suh, “an extended manufacturing integrated system for feature – based manufacturing with step-nc”, international journal of computer integrated manufacturing, 24 (2011), pp. 785–799. [5] http://ec.europa.eu/research/industrial_technologies/pdf/pppfactories-of-the-future-strategic-multiannual-roadmap-infoday_en.pdf (accessed august 2013). [6] h.a. elmaraghy, p.h. gu, “expert system for inspection planning”, annals of the cirp, 36 (1987), pp. 85–89. [7] c.w. ziemian, d.j. medeiros, “automated feature accessibility for inspection on a coordinate measuring machine”, 35 (1997), pp. 2839–2856. 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[18] s.g. zhang, a. ajmal, j. wootton, a. chisholm, “a featurebased inspection process planning system for co-ordinate measuring machine (cmm)”, journal of materials processing technology, 107 (2000), pp. 111-118. [19] w.c. myeong, l. honghee, s.y. gil, c. jinhwa, “a feature-based inspection planning system for coordinate measuring machines”, int. j. adv. manuf. technol., 26 (2005), pp. 1078–1087.   acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 17  [20] s.m. stojadinovic, v.d. majstorovic, “towards the development of feature – based ontology for inspection planning system on cmm”, journal of machine engineering, 12 (2012), pp. 89-98. [21] majstorović, v., “inspection planning on cmm based expert system”, proceedings of 36th cirp international seminar on manufacturing systems, pp. 48 – 56, saarbrucken, germany. [22] s. stojadinović, “intelligent concept for the inspection planning of prismatic parts on measuring machines”, (phd in progress). mechanical engineering faculty, belgrade, 2014. [23] v. majstorović, p. bojanić, v. milačić, “expert system for inspection planning on cmm”, the first world congress on intelligent manufacturing process & systems, proceedings, pp. 120-126, san juan, 1995. [24] v. majstorović, p. bojanić, s. vraneš, “intelligent environment for product and process design”, proceeding of 29th cirp conference “manufacturing systems”, pp.145-148, osaka, 1997. [25] v. majstorovic, t. sibalija, m. ercevic, b. ercevic, “cai model for prismatic parts in digital manufacturing”, proceedings of det 2014, the 8th international conference on digital enterprise technology, pp. 214-220, stuttgart. [26] v. majstorovic, t. sibalija, b. ercevic, m. ercevic, “capp model for prismatic parts in digital manufacturing”, proceedings of digital product and process development systems ifip tc 5 international conference, new prolamat 2013, pp. 142148, dresden. on the suitability of redundant accelerometers for the implementation of smart oscillation monitoring system: preliminary assessment acta imeko issn: 2221-870x june 2023, volume 12, number 2, 1 9 acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 1 on the suitability of redundant accelerometers for the implementation of smart oscillation monitoring system: preliminary assessment giorgio de alteriis1, enzo caputo1, rosario schiano lo moriello1 1 department of industrial engineering, university of naples federico ii, piazzale tecchio 80, 80125, naples, italy section: research paper keywords: structural monitoring systems; oscillation measurements; kalman filter; zero-velocity update; accelerometers; internet of things citation: giorgio de alteriis, enzo caputo, rosario schiano lo moriello, on the suitability of redundant accelerometers for the implementation of smart oscillation monitoring system: preliminary assessment, acta imeko, vol. 12, no. 2, article 11, june 2023, identifier: imeko-acta-12 (2023)-02-11 section editor: laura fabbiano, politecnico di bari, italy received april 4, 2023; in final form april 18, 2023; published june 2023 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was supported by pon "research and innovation" 2014-2020 and fsc ", project: "intelligent monitoring system for the safety of urban infrastructures (insist)", cup: e64e18000110005. corresponding author: giorgio de alteriis, e-mail: giorgio.dealteriis@unina.it 1. introduction the detection of the stress state of structures is an important problem in the field of structural engineering as it provides crucial information for monitoring their health and detecting the development and propagation of damage within them. structural health monitoring (shm), which encompasses a variety of techniques and technologies, has been developed to address this issue and is widely used in both aerospace and civil engineering [1]–[7]. in recent decades, shm has extended its applications to infrastructure and civil engineering, including historic and new buildings, bridges, tunnels, industrial plants, manufacturing facilities, offshore platforms, port structures, foundations, and excavations [8]–[11]. existing bridge structures are exposed to various environmental and operational stresses during their lifetime, and the influences of these external loads can lead to the acceleration of structural damage. in addition, extreme events such as earthquakes can occur during the life of a bridge, emphasizing the need for timely detection of the structure's condition to ensure safety. while visual inspection has traditionally played an important role in detecting defects on the structure surface and assessing the structural condition, it is labor-intensive, timeconsuming, and subjective. to address this issue, shm techniques have been proposed and are increasingly applied to long-span bridges [12]–[14]. shm is defined as the use of sensing techniques and analysis of structural features to detect structural damage or deterioration; it is suggested that shm should be considered in the context of condition assessment and as a damage detection technique [15]. in summary, the main abstract structural health monitoring (shm) is an essential aspect to ensure the safety and longevity of civil infrastructure. in recent years, there has been a growing interest in developing shm systems based on micro-electro-mechanical systems (mems) technology. mems-based sensors are small, low-power, and cost-effective, making them ideal for large-scale deployment in structural monitoring systems. however, the use of mems-based sensors in shm systems can be challenging due to their inherent errors, such as drift, noise, and bias instability; these errors can affect the accuracy and reliability of the measured data, leading to false alarms or missed detections. therefore, several methods have been proposed to compensate for these errors and improve the performance of mems-based shm systems. for this purpose, the authors propose the combined of a redundant configuration of cost-effective mems accelerometers and a kalman filter approach to compensate mems inertial sensor errors and data filtering; the performance of the method is preliminarily assessed by means of a custom-controlled oscillation generator and compared with that granted by a high-cost, high-performance mems reference system where amplitude differences of 0.02 m/s2 have been experienced. finally, a sensor node for real-time monitoring has been proposed that exploits lorawan and nfc protocols to access the structure information to be monitored. mailto:giorgio.dealteriis@unina.it acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 2 objectives of shm are damage detection and condition assessment. shm techniques have been applied to bridges for almost 40 years through the installation of structural health monitoring systems (shms) and have been increasingly applied to long-span bridges worldwide. in summary, shm techniques, particularly those based on vibration measurements, have great potential in the monitoring and maintenance of bridges to ensure their safety and longevity [16]–[20]. among the various sensor techniques used in shm, vibration-based measurement using accelerometers and data acquisition systems is one of the most widely studied and applied methods. in this approach, the response of the structure is measured under a given excitation, and its modal parameters, such as natural frequencies, damping ratios, mode shapes, and modal scaling factors, are determined [21]–[23]. the performance of the accelerometer is critical to the quality of the acquired data and the accuracy of the results. therefore, the proper selection of the accelerometer should consider the balance between the cost and performance of the overall shm system [24], [25]. in recent years, the use of micro-electromechanical systems (mems) sensors in shm systems has gained significant attention. mems sensors are tiny devices that can measure a wide range of physical quantities, such as acceleration, strain, pressure, and temperature, with high accuracy and precision [26]. they are also compact, low-power, and cost-effective, making them ideal for large-scale and long-term monitoring of structures. the integration of mems sensors into shm systems brings several benefits, such as improved accuracy and reliability, reduced power consumption, and enhanced data acquisition and processing capabilities. mems sensors can provide real-time data on the stress state of a structure, enabling early detection of damage or deterioration and allowing for timely and costeffective maintenance and repair. they can also be used to identify potential failure modes and predict the remaining useful life of the structure. in the case of bridges, the use of mems sensors in shm systems can provide a comprehensive and continuous monitoring solution that can detect even subtle changes in the structure's condition due to external loads, traffic, or environmental factors [24], [27]. this information can be used to optimize the bridge's performance and extend its service life while reducing maintenance and repair costs and ensuring the safety of the users. overall, the use of mems sensors in shm systems represents a promising approach for the monitoring and maintenance of structures, particularly in the case of large-scale and critical infrastructure such as bridges [28]. with further advances in mems technology and data analytics, it is expected that shm systems will become even more efficient, accurate, and costeffective, providing greater benefits to both engineers and endusers. mems inertial sensors offer several advantages, but it is important to consider their drawbacks as well. their small size makes them highly sensitive to environmental changes, and random noise can make error compensation procedures more complex and limit their applicability [29], [30]. an increasing bias drift with non-linear characteristics is a significant factor to consider, as it demonstrates that while mems imus can provide remarkable accuracy at high rates, angular velocity, and acceleration data can easily degrade over longer periods. therefore, special attention must be paid to these sensors, which are typically classified based on their bias instability and random walk parameters, both of which characterize their performance and suitability for specific applications [31], [32]. for example, accelerometers with a bias instability lower than 0.01 mg are considered "marine-grade," while more cost-effective "consumer-grade" sensors have lower performance but also lower power consumption [33]–[35]. to overcome the mems limitations, several studies have been conducted on the adoption of a kalman filter (kf) to compensate the mems errors. in particular, for shm applications, high-frequency noise could be a key element to accurately estimate the structure acceleration. so, the kalman filter algorithm can be used to reduce noise in data obtained from mems accelerometers. it works by estimating the state of a system based on a series of noisy measurements. in the case of mems accelerometer data, the kalman filter can be used to estimate the true acceleration of an object by filtering out highfrequency noise [36]–[39]. in this research, a redundant prototype of low-cost accelerometers that exploits a kalman filter algorithm for filtering purposes has been proposed. then, the performance of the proposed solution is assessed by means of a comparison with a high-performance accelerometer sensor by means of a controlled oscillation generator that is specifically realized to obtain oscillations with known frequency and amplitude. finally, the authors propose a sensor node that could be adopted for shm applications. the paper is organized as follows; the proposed method, the implementation of the controlled oscillation generator, and the sensor node realized are described in section 2, while in section 3, the system architecture that includes both the hardware and software implementation has been described. finally, in section 0 the obtained results are presented as advantages introduced by the proposed approach, the overall performance reached in oscillation measurements, and the proposed iot architecture before drawing the conclusions in section 5. 2. proposed methods and smart monitoring architecture a platform based on the internet of things (iot) exploiting redundant low-cost mems accelerometers for monitoring large structures such as bridges and tunnels has been evaluated, where the redundant prototype is already presented in [40]. the research activities have been focused mainly on two main aspects: (i) a prototype of redundant low-cost mems accelerometers evaluation for oscillation measurements by exploiting the kalman filter for data filtering, where the performance is assessed by means of custom testing setup for controlled oscillation measurements and a result comparisons with a reference system, i.e., high-performance accelerometer sensor; (ii) an iot solution for a real-time system that acquires and visualizes data based on the adoption of the redundant prototype and the typical protocol of iot. 2.1. proposed method regarding the first point, the main goal was to use a redundant configuration of accelerometers to reduce the typical errors that affect low-cost mems accelerometer sensors, such as bias instability, in-run bias instability, and velocity random walk. in this way, thanks to the redundant prototype performances, it could be adopted for oscillation measurements. to this aim, the authors propose an initialization procedure based on the adoption of a zero-velocity update (zvu) filter that allows to estimate of the initial error values and the initial alignment. in acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 3 this way, the acceleration measurements are compensated by the estimated noise values and then are processed by means of a kalman filter for high-frequency noise reduction and, finally, a fast fourier transform (fft) to evaluate the oscillation frequency and amplitude, as shown in figure 1. the performance validation of the proposed method is assessed by a suitably controlled oscillation generator based on a crank-rod system capable to emulate different oscillation frequencies. finally, a marine-grade mems sensor has been adopted as a reference system to assess and compare the performance reached by the proposed solution. 2.1.1. zero-velocity update filter and initial alignment the kalman filter is a recursive algorithm that can be used to estimate the state of a dynamic system based on a series of noisy measurements. it works by predicting the state of the system at the next time step based on a model of the system dynamics and then updating this prediction based on the actual measurements. the algorithm includes two key steps: prediction and update. in the prediction step, the state estimate and error covariance matrix is predicted based on the previous state estimate, control input, and system dynamics. in the update step, these predictions are updated based on the actual measurements and the measurement noise covariance matrix [32]. the zvu filter is based on an error-state kalman filter (eskf). this approach is based on the assumption that the system output is in a standing condition, i.e., the velocity is equal to zero. the zvu filter is realized according to the following steps: 1) the position vector is provided by a gnss source; 2) the velocity vector is set equal to zero; 3) roll and pitch angles are obtained by means of a coarse leveling procedure according to (1) and (2): 𝜃 = arctan ( −𝑓𝑖𝑏, 𝑏 𝑥 √(𝑓𝑖𝑏, 𝑏 𝑦 ) 2 + (𝑓𝑖𝑏, 𝑏 𝑧 ) 2 ) (1) 𝜙 = arctan2(−𝑓𝑖𝑏, 𝑏 𝑦 ,−𝑓𝑖𝑏, 𝑏 𝑧 ) , (2) where 𝜃 is the pitch angle, 𝜙 is the roll angle, 𝑓𝑖𝑏 𝑏 represents the raw acceleration measurements among the x, y, and z axis referred to as the body reference frame. the predict/correct phases are realized according to the eskf implementation [40]; in this way, noise terms are estimated during the state vector correction and exploited in the successive prediction stages. the residual error values are then adopted to correct the acceleration measurements to obtain better performance in frequency and amplitude oscillation measurements. for the sake of clarity, in this specific application, the gnss position is only once evaluated; in fact, the accelerometer sensors are mounted on a structure where the gnss position variations are not significant or not observable by most gnss modules. regardless, knowledge of the position is necessary to accurately compensate for the gravity vector. 2.1.2. kalman filter approach once obtained, the initial noise parameters, thanks to the application of the zvu, and a further kf has been applied to the acquired acceleration samples in order to filter out highfrequency noise. to suitably reduce the computational burden, the update matrix is reduced to the identity matrix (i.e., in the prediction stage, the so-called a-priori estimated value �̂�− is equal to the corrected value obtained in the previous iteration). as for the correction stage, it is obtained according to the following equations: 𝐾 = 𝑃/(𝑃 +𝑅) (3) �̂�+ = �̂�− +𝐾(𝑥 − �̂�−) (4) 𝑃 = (𝐼 −𝐾) 𝑃 + (�̂�+ − �̂�−) 𝑄 , (5) where 𝐾 is the kalman gain, 𝑃 is the error covariance matrix, 𝑅 is the noise covariance matrix, �̂� and �̂�−are the state vector and estimated state vector, respectively, i.e., the acceleration measurements, 𝐼 is the identity matrix, and 𝑄 is the system noise covariance matrix. figure 1. proposed method based on the adoption of a zero velocity update filter. acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 4 2.1.3. controlled oscillation system and measurement setup the measurement setup is composed of a controlled stepper motor that rotates a cylinder that is connected to the beam by means of a linkage in such a way as to convert the rotary motion of the stepper motor into linear motion. the sensor modules are placed on a plate rigidly constrained to the beam; in this way, harmonic motion is realized. the nominal acceleration values (𝑎(𝑡)) can be derived from the maximum imposed displacement and frequency by the equation: 𝑎(𝑡) = −𝜔2𝐴 sin(𝜔 𝑡) , (6) where 𝐴 is the vertical displacement amplitude, 𝜔 = 2 π 𝑓 is the angular frequency. the geometrical dimensions of the realized system are reported in table 1, where it is also highlighted the maximum displacement that is evaluated as the difference between the highest and lower points of the plate (among the z-axis). figure 2 shows the controlled oscillation generator and the measurement setup. in particular, the mechanical parts are realized by the 3d printing process [41], [42] and are highlighted in black color, while the stepper motor and motor driver are highlighted in orange color. the measurement setup is mainly composed of the sensor modules, i.e., the proposed redundant low-cost accelerometers (red) as well as the stim318 by sensonortm, the marine-grade (green) accelerometer exploited as a comparison reference. both modules are connected to a microcontroller (placed under the plate), specifically, the stm32f446re from stmicroelectronicstm, that acquire data via the i2c (inter integrated circuit) and via the uart protocol, respectively. to this aim, an interface between the rs422 and uart protocols was realized using dual differential drivers and receivers (sn75c1167 from texas instruments). to synchronize the two systems, the microcontroller provides an external trigger to sample the marine-grade accelerometer data. the stim318 acquires data at 2 khz and is triggered at 125 hz, resulting in a mean delay of 250 µs between the request and sampling of measured quantities. finally, the acquired data is sent to a personal computer for further processing via a bluetooth module connected to the microcontroller through the uart interface. 2.2. proposed smart monitoring architecture as for the second point, the prototype device is completed with a wireless communication system based on the lorawan protocol and a nfc module for remote and in-situ monitoring, respectively, as shown in figure 3. once the operations for determining the quantities of interest have been locally performed, the microcontroller sends the result via lorawan protocol to a gateway on which a node-red-flow is implemented that forwards the measured data via mqtt to a thingsboardbased cloud dashboard. moreover, the data are also accessible through nfc protocol. in particular, using a mobile phone as nfc reader, the identity of the node is detected and exploited to access the iot platform and display measured data on the mobile application. the proposed architecture aims to transmit only the maximum amplitude value of the signal, which is evaluated by the microcontroller as the peak in the frequency domain. the microcontroller then sends both the frequency and amplitude values, effectively reducing the number of samples transmitted and allowing for real-time monitoring. this ensures that only relevant information is transmitted, resulting in a more efficient system protocol with fewer packets, for forwarding measured data via the lora and nfc protocol so as to speed up nfc and dashboard communications. 3. system architecture the hardware components used in performance assessment and wireless sensor node development for real-time monitoring are reported in table 2 and table 3, respectively, while the corresponding operations flow-chart are shown in figure 4a and figure 4b. for the sake of clarity, the redundant prototype is composed of six inemos from stmicroelectronicstm in a cubic configuration. figure 2. realized oscillating system and measurement setup for performance assessment; thanks to the stepper motor oscillations with known frequency and amplitude can be applied to the low-cost redundant accelerometers. table 1. geometrical dimension of the controlled oscillating system. system dimension (cm) beam 22.5 × 1 × 1 (l × w × h) linkage 9.5 × 0.2 × 0.5 (l × w × h) cylinder radius 5.5 max displacement 0.5 acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 5 figure 3. sensor node and proposed smart monitoring architecture. figure 4. data acquisition flow-chart for: a) performance evaluation; b) realtime monitoring. table 2. performance evaluation equipment. hardware part number manufacturer redundant accelerometers lsm6dsm (six inemos) stmicroelectronics marine-grade accelerometers stim318 sensonor microcontroller nucleo-f446re stmicroelectronics motor stepsyn sanyo denki motor driver board x-nucleo-ihm04a1 stmicroelectronics bluetooth module rn41 roving rs422-uart sn75c1167 texas instruments table 3. sensor node realization hardware. hardware part number manufacturer microcontroller nucleo-f446re stmicroelectronics sensor board x-nucleo-iks02a1 nfc module x-nucleo-nfc04a1 lora module i-nucleo-lrwan spi-micro sd-card 410-380 diligent inc. acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 6 as stated above, the communication between the microcontroller and the pc is realized by means of a bluetooth protocol (figure 4.a); this allows control of the start of the acquisition and data collection. once the start is sent, the microcontroller generates a trigger signal for the sensor modules and starts to acquire data from both. during the tests, it is necessary to ensure accurate measurement synchronizations. therefore, the data is processed offline to avoid introducing delays from each data acquisition in both systems. the software architecture is designed with a focus on synchronizing both systems to obtain comparable measurements between the proposed solutions and the reference system. this approach ensures that the results are reliable and can be compared accurately. as for the wireless sensor node, the software architecture is realized to real-time process the acceleration measurements, as shown in figure 4b, where the microcontroller acquires data and elaborates them according to the proposed method with the aim to evaluate the maximum frequency peak value and send by means of the lorawan protocol and nfc the oscillation frequency and the associated amplitude values. the proposed solution is based on the zvu filter for initial alignment and noise terms evaluation; to enhance parameter estimations require the gnss position information. to this aim, the realized iot architecture includes the setting of these parameters that can be stored on the sd-card; this procedure could be achieved when the sensor node is mounted on the structure that will not change its position. finally, the zvu and initial alignment that requires 60 s is activated in two conditions: 1) each time the device is turned on to correct the random bias values; 2) whenever the internal temperature of the accelerometer changes by ± 3 °c, in such a way as to correct the bias values changes due to the temperature. once the error values are estimated, the microcontroller removes the high-frequency noise from the raw acceleration measurements adopting the kalman filter and then processes them by means of a fft algorithm to evaluate the maximum frequency peak value and associated amplitude values that are sent through the iot protocols, i.e., lorawan and nfc. 4. results the proposed solution is evaluated according to the measurement setup realized and described in section 2.1. to this aim, the oscillation measurements in terms of oscillation amplitude of both systems have been compared at different controlled frequencies from 1 hz to 5 hz. the performance of the proposed method has been assessed by comparing the measurement results provided by the realized system with those guaranteed by a marine-grade mems sensor, i.e., the stim318. the raw acceleration measurements are acquired by the concentrator and sent to a pc that collects and processes data by means of a matlab code, according to section 2.2. the performance reached by the proposed method highlights the benefits introduced by exploiting the zvu filter and initial alignment for measurement calibrations, i.e., the compensation of accelerometer bias drift values. in fact, as shown in figure 5, the signal spectrum of both systems have been compared, where the amplitude value differences associated with the component at 1 hz between the proposed method (𝐴est) and the reference (𝐴ref) are equal to 0.021 m/s², while the amplitude differences between the best sensor i.e., one cube face (𝐴one) measures and reference system are equal to 0.137 m/s². for the sake of clarity, a comparison at frequency oscillation of 1hz is shown, but similar results have been experienced, which are reported in table 4. moreover, the theoretical amplitude values evaluated according to (6) have been reported. the benefit introduced by the proposed method is highlighted in table 5, where the differences between the reference system and estimated values (∆𝐴est) and the reference system and one sensor value (∆𝐴one) have been reported. these results are also shown in figure 6, where the differences between the reference system and uncompensated measures rise as the frequency increases due to the typical lowcost accelerometer errors, while by adopting the proposed method, the differences are constant. in fact, by exploiting the zvu calibration, the bias drift and bias stability are compensated, as shown by the stability of the system as the frequency increases. finally, to assess the proposed solution, figure 7 shows the second-order polynomial fitting to evaluate the quadratic trend of the acceleration amplitude as the frequency increases. the comparison highlights that the reference system and estimated acceleration amplitude values present a comparable trend with the ideal trend, while instead, the uncompensated values show a significant drift over frequency. figure 5. acceleration amplitude comparison at 1hz: reference system (red), proposed method (blue) and one sensor measures (orange). table 4. acceleration amplitude results from 1hz to 5hz. amplitude (m/s²) oscillation frequency (hz) 1 2 3 4 5 reference system 0.173 0.767 1.754 3.123 4.914 proposed method 0.152 0.744 1.733 3.101 4.891 one sensor 0.31 1.243 2.254 4.63 6.823 theoretical acceleration 0.197 0.789 1.776 3.158 4.934 table 5. amplitude differences between the reference system and (i) the proposed method; (ii) one accelerometer sensor measures. amplitude differences (m/s²) oscillation frequency (hz) 1 2 3 4 5 ∆𝐴𝑒𝑠𝑡 0.021 0.023 0.021 0.022 0.022 ∆𝐴𝑜𝑛𝑒 0.137 0.476 0.5 1.507 1.909 acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 7 once the performance has been verified, an example of sensor node operation, presented in section 2.2, is shown in figure 8. the monitoring dashboard can be accessed from both smartphones and pc, and information regarding the amplitude of the oscillation in g and the frequency is shown. for the sake of clarity, the full dashboard also shows all the sensor axes. 5. conclusions the use of mems-based sensors in shm systems has been shown to be an effective and cost-efficient approach for largescale structural monitoring. however, the inherent errors in mems-based sensors can affect the accuracy and reliability of shm systems. to overcome these challenges, this research evaluates the use of a redundant configuration of low-cost mems sensors, then proposes (i) an initialization procedure based on the adoption of a zero-velocity update filter to compensate the bias components i.e., in-run stability, bias instability, and thermal effects; (ii) an initial alignment procedure, and (iii) a kalman filter for high-frequency noise reduction that has proven to be effective in improving the performance of mems-based shm systems keeping advantages in terms of computational load. in particular, the system performance is evaluated by means of a custom oscillating platform that provides oscillation at constant displacement and variable frequency. in this way, the proposed method is tested, and a comparison with a reference system, i.e., a marine-grade accelerometer, theoretical values, and the one accelerometer sensor measures, has been evaluated. the proposed method highlights a remarkable agreement with the reference system, which respects the theoretical trend, and the performance improvements over one accelerometer sensor measurements are deducted from the results obtained; in fact, amplitude differences of 0.02 m/s2 and 0.137 m/s2 have been experienced between the reference system and the proposed solution and one sensor respectively. moreover, it is observed that drift and bias stability errors are not present in the system as the frequency increases. finally, a smart monitoring architecture has been proposed based on the adoption of a sensor node capable of real-time monitoring of the structure by exploiting the iot communication protocols, i.e., lorawan and nfc. the use of these systems, combined with the proposed solution, can figure 6. differences as the frequency changes (from 1h z to 5 hz), between the reference system and (i) the proposed method (blue); (ii) one sensor measures (orange). figure 7. polynomial fitting comparison: ideal trend (black), reference system (red), proposed method (blue) and, one sensor measure (orange). figure 8. monitoring dashboard example of operation. acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 8 provide accurate and reliable data, leading to timely maintenance and cost savings. as future work, additional tests will be executed by means of different beam sizes (the acceleration amplitude could be changed by modifying the beam length) and a vibrodyne to suitably assess the combined effects of amplitude and frequency by evaluating the minimum and maximum operating values. references [1] l. sun, z. shang, y. xia, s. bhowmick, s. nagarajaiah, review of bridge structural health monitoring aided by big data and artificial intelligence: from condition assessment to damage detection, journal of structural engineering, vol. 146, no. 5, 2020, p. 4020073. doi: 10.1061/(asce)st.1943-541x.0002535 [2] j. seo, j. w. hu, j. lee, summary review of structural health monitoring applications for highway bridges, journal of performance of constructed facilities, vol. 30, no. 4, 2016, p. 4015072. doi: 10.1061/(asce)cf.1943-5509.0000824 [3] 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3dsurvey; gis citation: lorenzo fornaciari, emanuele brienza, the masonry of the terme di elagabalo at the palatine hill (rome). survey, analysis and quantification of a roman empire architecture, acta imeko, vol. 11, no. 1, article 11, march 2022, identifier: imeko-acta-11 (2022)-01-11 section editor: fabio santaniello, university of trento, italy received march 1, 2021; in final form march 24, 2022; published march 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: lorenzo fornaciari, e-mail: lorenzo.fornaciari88@gmail.com 1. introduction the area of the northeast slopes of the palatine hill that faces the colosseum valley has been the subject of a long archaeological research since 1986, carried out by the department of scienze dell’antichità of sapienza, university of rome (figure 1). during more than 30 years of excavations, the material remains of major building and monumental interventions have been unearthed, testifying an environmental and topographical continuum where the development of diversified urban systems has involved a complex physical overlap of structures and architectural complexes distributed over time [1]-[4]. today the actual floor and appearance of the zone reflects the aspect reached during the 4th century c.e with some main differences: first of all, the first big excavations made during 19th century included the destruction of many medieval remains; secondly the building during the fascist regime of two new large streets, via dell’impero and via dei trionfi, caused the dismantling of the velia hill and the destruction of several roman monuments, like the meta sudans fountain and the basement of the famous colossus. the target of this investigation was to reconstruct the ancient topography and to restore the connection between the colosseum valley and the palatine hill; the research had to take into consideration the previous literature and studies about the zone and the huge amount of data produced during annual stratigraphic excavations: dealing with this purpose, we had to use it tools for data collection and management, articulated into distinct chronologies and graphic outputs able to reproduce the ancient spaces and their transformations over time, on a small and large representation scale. finally, in several zones of the investigated area previous digs carried from xvii to xix centuries compromised the stratigraphic earth basins, making difficult to read the archaeological record: here, in order to obtain other keys of interpretation, we decided to implement the stratigraphic and abstract the ne slopes of the palatine and the colosseum valley area have been the place of a long archaeological research; here the continuous urban development produced an overlap of architectural complexes distributed over time. the huge amount of archaeological documentation produced during research is managed by an intra-site gis. for ancient walls analysis we have introduced the use of image-based-modelling photogrammetry in order to create a very detailed 3d documentation, marked by an increasing and progressive high-level-autopsy, linked to a dbms dedicated to ancient structural features. in this procedure we also decided to compare two different approaches to check results of both: one based on taking brick measures directly by hand, the other taking same measures on photogrammetric elaborations by gis. through this methodology, counting measures and dimensional aspect of wall facades features, we can evaluate specific aspects of the ancient construction yards for each period; we can also refine the chronological sequences of the architectures and verify the contextual relationships of the surrounding buildings in order to formulate wide-ranging reconstructive hypotheses. mailto:lorenzo.fornaciari88@gmail.com acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 2 detailed analysis of ancient architectures, according to the most current research criteria. in section 2 we will discuss about the archaeological framework; in section 3 we will give a brief description of data management methods, techniques and procedures; in section 4 we will illustrate our particular approach for the structural analysis of ancient walls; in section 5 we will give some quantitative data about constructive features of the ancient walls. finally, in the concluding section, we will account the preliminary results of our research. 2. the archaeological framework an area of about 4500 square meters has been excavated reaching a depth, were possible, of 8/9 meters, chasing the millenary history the city, from first settlement to modern age, including all kind of evidences like medieval spoliations and modern or contemporary interventions. in order to reconstruct the original geomorphology, we have conducted also geophysical investigations tools such as ground-penetrating radar and resistivity [5]-[7]. to make an extreme synthesis of the main results, starting from the remains of iron age huts, found along the slope, we move on to an early urban planning witnessed by the presence of two sanctuaries dating to the roman kingdom (8th-7th century bce) located along both sides of the ancient road leading to the roman forum: one of them can be identified with the curiae veteres and has been frequented until the affirmation of christianity. the installation of a residential district, along the road, is documented since the archaic period: subsequently this area has been periodically rebuilt in the following centuries, until augustus age. in this period, at the meeting point of five of the new 14 city zones planned by the emperor, the first meta sudans fountain was built, in front of the curiae veteres which were also reconstructed in monumental shape during the years of the emperor claudius. the real break-up here happened in conjunction with the great nero’s fire: after this disaster nero decided to carry out in this area a deep urban transformation that would end with the realization of his majestic palace, the domus aurea. in the years between 64 and 68 ce a total reorganization of the road system was made, with a regular and orthogonal shape, according to the guidelines dictated by the palace project. in the colosseum valley the new architectural complex was characterized by columned porticoes around an artificial pond over which the flavian dynasty later will build the colosseum; the palatine hill slopes were regularized by artificial terraces on arcades while the new climbing way to the forum was flanked by arched porticoes. the urban planning of flavian emperors, focused on restoring a public dimension to the urban spaces occupied by the domus aurea, can be emblematically summarized, together with colosseum building, in the reconstruction of the curiae and of the meta sudans fountain, both burned in the fire. the area will be modified again by hadrian with the construction of the venus and rome temple and, a of a long building flanking the porticoes on the other side of the street going to the forum. after another catastrophic fire, at the end of the 2nd century ce the area was rebuilt again by the severian dynasty: in close connection with the monumental project of the new palace of the emperor, the whole front of northeast palatine’s substructures was heavily transformed while the constructions at its feet were completely dismantled and replaced by a new courtyard building, commonly called terme di elagabalo (figure 2). inside this monument, in the 4th century ce, a large banquet hall was obtained, with gardens and fountains and a small bath in the backyard. finally, with the construction of the constantine’s arch and the restorations at the venus and rome temple the ancient urban history of the area was completed [8]-[11]. 3. archaeological record collection: methods techniques and procedures the huge amount of documentation produced in front of such a complex stratigraphic sequence required the development of a data storage and management system, dedicated to contextualize information and capable of proposing new elements useful for research. the entire archive is therefore managed by an intra-site gis (designed since 2001), used for data-retrieving, spatial analysis and for the elaboration of archaeological themes and/or reconstructive models [12]. the historical sequence reconstructed for the excavation areas was therefore contextualized in a wider urban historical framework, traced through the study of previous bibliographies, archive data and historical cartography. the archaeological records collected during these operations were linked to a general map developed in a gis environment, made up of several distinct layers combined into a unique reference system by overlay mapping procedures: the vector cadastre of rome, the digital cartography of the municipality plus several rectified aerial photos and satellite images. over this base-map we have georeferenced several historic cartographies, in raster and vector format, such as nolli’s nuova figure 1. the investigated area in the historic centre of rome. figure 2. general plan of the terme di elagabalo complex: the severian phase is highlighted in red. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 3 pianta di roma (1748), some sheets of lanciani’s forma urbis romae and the cartographic atlas media pars urbis made by v. reina in 1911 (figure 3) [13], [14]. we have updated the territorial study producing a digital elevation model in order to compare the current state with ancient surfaces: this was produced by interpolation of the altimetric data available at the geoportale della regione lazio (figure 4) [15] and subsequently recalibrated on the basis of the altimetry detected during the annual topographic campaigns; it is an additional basis where to place stratigraphic evidence of structures and infrastructures (ancient roads, sewers, terraces) connected to the original morphological configuration; here we can verify, using the deepest stratification data, our environmental reconstructions for the most ancient periods. in fact, using all the data collected from stratigraphic and geoarchaeological investigations, from surveys and historical cartography, two digital terrain models have been developed: one related to the augustan age and the other suggesting the situation after the 64 big fire and nero’s massive interventions. those dtms are the basis of virtual urban scenarios, modelled in a 3d environment, that can be navigated following a diachronic exposition and associating each monument to its period, with the option to converse interactively with each architecture in its specific temporal version [16]. over the years this system has been implemented in software, for the advent of new it products, and in the stored contents: in our spatial database, today, digital and analogical documentation (in particular handmade archaeological detailed drawings and onpaper archaeological forms) are managed together, in order to maintain the integrity of the research archive and its history (figure 5) [17], [18]. to have the best results in this operation a memorandum of agreement has been approved by the sapienza, university of roma, the kore university of enna and the i.s.p.c.-c.n.r. (institute of sciences for cultural heritage of national council of research), institution that since 2007 has collaborated with the research carried out at the palatine hill and the colosseum valley, in particular in 3d surveying and integrated geophysical figure 3. collection of historic cartographies reproducing our investigation area in different periods of modern age (e. brienza and m. fano). figure 4. the new digital elevation model produced from the data of the geoportale della regione lazio. figure 5. intra site gis: data management. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 4 prospecting [19], [20]. during these years a big amount of raw data has been preserved in two different repositories, while only final elaborations were shared by the research group: since 2019 we have finally started to unify the separate archives in a single spatial database, for investigation purposes but also for deontological instances in order to leave a complete and single testimony of all activities carried out in this important archaeological place site during these decades. our intention is focused on giving access to the scientific community but also to interested people, not only to the data (both synthetic and in-depth format) but also to the analysis system itself: paying great attention to the issues of open-data, archeofoss [21] and public archaeology [22] we have tested the migration of the entire dataset and its interrogation criteria and tools to an open-source web-gis platform, using web-oriented dbms like postgresql + postgis and gis software like qgis server+lizmap). in this digital environment, starting from the general site map, it is possible to decompose the single architectures into their structural contexts and features and verify the cognitive process for each one of them: passing from photos to 3d models, then to elevations, wall-samples up to the general synthesis of filecards and records of ancient structures (figure 6). 4. stratigraphic analysis and autopsy of ancient walls the study of ancient architectures today can adopt survey tools able to detect objects in 3d with a certain precision and quickly: through these tools we have produced ortho-photoplans that, gradually, have joined the traditional 2d documentation but we have also proposed three-dimensional sequences of excavated stratigraphic sequences as well as the reproduction of some ancient artifacts, suggesting their virtualdigital restoration. in the study of ancient walls, the use of new image-based-modeling photogrammetry techniques based on structure from motion (having accuracy and photographic texturing) brought us to the realization of a new detailed 3d documentation of the ancient walls [23]-[30]. a 24.2 megapixel canon eos d200 reflex camera, with image stabilizer and a focal length of 18 mm, was used for the photos; in addition, a tripod and a telescopic rod were used to ensure better quality; an average of 30-40 shoots were taken for each acquisition not to burden the processing times. we have produced 3d models of the structures, adopting three approaches based on differentiated analysis scales: a general and less accurate one (mainly aerial), for architectural complexes (figure 7) [31]; a second one, more accurate, for sectors and rooms of ancient buildings (close-range); a third one of in-depth analysis of the walls and samples, using a maximum resolution and accuracy approach (very-close-range, figure 8). in this last case each wall was taken up completely, maintaining a constant distance: particular attention was paid to have a maximum accuracy survey of 1sqm area samples, taken from facades, maintaining a maximum distance of 1 m, in order to obtain images with a high level of detail; in this way it was possible to use very-high quality orthophotos, having an average figure 6. intra site gis: 3d data management. figure 7. aerial 3d survey of the architectural complexes (g. caratelli and c. giorgi). figure 8. very-close-range digital photogrammetry of ancient walls. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 5 detail level between 0.2-0.4 mm / pixel, for the stratigraphic reading and for samples quantifications: in the selection of these samples, we proceeded analytically, choosing 1sqm wallvestment areas with an adequate level of conservation and free of discontinuity elements [32]. vertical high-resolution ortho-photomaps were extrapolated from the textured meshes and, acting a careful verification of the measurements, subjected to a vector design process. the new documentation included also a dbms recording updating the usual formats for ancient structural features following the guidelines suggested by the archeology of construction and by the archaeology of architecture [33]-[39]. we have planned a new file-card format dedicated to register information about the logistic of the ancient construction yards and the related dynamics on material production and ancient building organization, in addition to data relating to their measures, composition and nature (figure 9). in this way the chrono-typological analysis, which traditionally focuses on the recognition of the single construction features by material aspect, has been expanded with the collection of information related to building methods such as, for example, structural expedients for static stability, specific materials selection in relation to particular needs or quantification of the work in terms of time and number of the workers. defining trends, measures and treatments of specific building materials can help us to identify diachronically the processes and resources of the ancient construction yards, while the stratigraphic analysis of the walls, with its identification of constructive temporal sequences, is crucial to understand the formative dynamics of the ancient architectures and must be done through observation of details on the basis of a precise and clearly legible survey. obviously, in order to normalize the data entry and editing, we have encoded standard glossaries while the detailed morphometric information, derived from autoptic analysis of samples taken from wall facades (normally their size is 1 square meter), is managed by sub-cards where each “constituent" (i.e. brick, block, etc.) is organized by type, use/reuse, material, manufacture, finishing, and measures. starting from these assumptions, the analysis of ancient architecture was carried out with a workflow that, as usual, started from the autopsy analysis and survey of each wall. elaborations from photogrammetry have been vectorized in gis environment. figure 9. dbms file-card for structural and construction data. figure 10. vectorization of ancient walls samples. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 6 for this purpose, next to the module dedicated to the analytical database of the ancient walls, a new apparatus has been created for the collection of all the data relevant to the documentary base. here, photographs, 3d models acquired from scratch, sections and elevations, drawings and all the graphic documentation produced during the excavations, have found their place. in this way, through a simple query, it is possible to trace the whole corollary of raw and elaborated data that constitute the starting point for the analysis of each context. for the quantifications of information coming from wallfacades-samples we have performed differentiated gis analysis procedures, comparing the dbms data taken directly on the field (counting bricks and measures on wall facades) and those obtained automatically on spatial vector drawings made on very detailed orthophotos (figure 10); these measures were taken on the same sampled wall-facades but in different way: despite the different tools and procedures the results of 20 samples were indeed very similar, giving us a good indicator of a correct method [40]. through a series of expressions specifically prepared, it is also possible to calculate automatically and expeditiously the variable of the constituent / conglomerate ratio, but also the dimensions of the components of the facades with their degree of homogeneity and variability (figure 11). in particular, the gis for calculating the dimensions of each brick involves the use of two expressions whose common principle is based on the construction of a regular polygon circumscribed to each geometry returning the maximum length along the x and y axes [41], [42]. in this way length is calculated as follows: 𝑏𝑜𝑢𝑛𝑑_𝑤𝑖𝑑𝑡ℎ ($𝑔𝑒𝑜𝑚𝑒𝑡𝑟𝑦); (1) while the thickness: 𝑏𝑜𝑢𝑛𝑑_ℎ𝑒𝑖𝑔ℎ𝑡 ($𝑔𝑒𝑜𝑚𝑒𝑡𝑟𝑦). (2) as mentioned, the measurements made through these expressions agreed with those taken by direct autopsy producing a great advantage in terms of time. 5. some quantification data focusing on the construction process, the first value that must be verified in the analysis is represented by the percentage ratio between constituents, in this case bricks, and binder; we must then add the values of the average of the lengths, thicknesses and areas of the bricks which, being strongly influenced by anomalies, can be misleading in describing the overall character of the masonry; regard this, the standard deviation, i.e. the index according to which the measures of each constituent deviate from their average value, can be very effective [43]; from an archaeological point of view we could say that if the standard deviation index is low this may indicate a certain homogeneity of the bricks, referable to the availability of homogeneous lots of materials; on the contrary, we could imagine very inhomogeneous bricks due the availability of different lots or reused material, recovered from pre-existing buildings, and therefore more fragmented [44], [45]. taking in consideration the peculiar characters of 60 brickwalls samples of our complex a specific feature is the extreme thinness of vertical joints: this observation indicates a high level of specialization of the workers but does not exclude the use of recycled materials. in fact, same samples show a certain lack of homogeneity in lengths and thicknesses of the bricks, highlighting a combined use of new and recycled material, we know that there were three standard brick types, based on multiples of the ancient roman foot (about 29.6 cm): these were produced in a square shape and then cut into a triangle whose longest side was on the facade: by measuring this side it is possible to reconstruct the used brick types, also calculating the parts lost during cutting [46]. from our analysis of about 5700 bricks, it seems that standard type used in masonry is 67% bessales (2/3 of roman foot) divisible into two groups of measures (1822 and 22-25 cm) while 25% of the material is fragmented and probably reused (figure 12) [47], [48]. it is maybe possible that, at the beginning of the construction, it was planned the recovery of the remained material of the previous, destroyed, building. on the other hand, the supply of newly made material is confirmed by presence of special brick types like double-feet bricks (bipedales) used for horizontal planes in masonry and relieving arches in the facades, following constructive and static procedures typical of severian architecture, with a homogeneity and number that suggests a great availability of new material. 6. conclusions through the methodologies and tools described above it is now possible to evaluate specific aspects of the ancient construction yards for each period, such as the extent of resources supply, the reuse index and building materials selection level and consequently refine the chronological sequence of the construction phases of the individual buildings. furthermore, being able to have such a complex sequence of figure 11. vector and dbms data analysis and verification. figure 12. quantification of bricks measures and types in masonries. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 7 building interventions and referring them to architectures of public and private character, it is possible to plan a comparative analysis of the construction techniques adopted in relation to both chrono-typological aspects and to particular contingencies. finally, we have a bigger chance to clarify the structural and contextual relationships of each construction yards with the surrounding buildings, in order to formulate wide-ranging and multi-temporal reconstructive hypotheses. some first outcomes of our work can be briefly exposed here. first of all, our “very-close-range” photogrammetry approach to structural archaeological evidences, using very high definition shoots of samples and supported by total station, with automatic measurement of vectorial features, has given results very encouraging if compared to measures of samples directly taken, with a tape and one by one, on the ancient walls: the dimensions of bricks and other building components of the archaeological structure are always almost the same in both cases and mismatches are very few and very little. in addiction the colours and the type of materials can be clearly distinguished. this means that a mensio-chronological approach using our method can be correct; obviously a total analysis of building materials and their treatments (specially for mortars, concretes and conglomerates) still needs a direct autopsy, physical and material, of archaeological evidence. in particular cases, anyway, for emergency or during seasonal researches in foreigner countries, this expeditive approach can be adopted on the field, obtaining reliable results in the subsequent laboratory study. another result is of archaeological and architectural nature and concerns the accentuate reuse of building materials during the first severian age which, compared to the topographical context (we are next emperor's palace in rome) and the chronology (generally this building practice in the capital is peculiar of late antiquity), might seem an unusual phenomenon. however, this has been already partially detected by previous study of brick stamps found in situ which show the reuse of hadrian's bricks in severian masonry in the terme di elagabalo complex [49] but also in other sectors of the rebuilt emperor’s palace on the palatine, in particular during the first severian phase [50]-[52]. this phenomenon, associated with a new prevalent use of bessales bricks, seems typical of the years of septimius severus, when the first emperor of the new dynasty had to reactivate the intensive production of bricks to manage the reconstruction of a large part of the destroyed city from the 192 c. e. fire [53]. we can imagine that at the initial moment, while waiting for new material stocks in large and continuous quantities, it was decided to reuse bricks from burnt walls, which must have cost about 1/5 of that of first choice, also solving part of rubble removal problem [54], [55]. this would confirm the hypothesis that the construction of our complex should be dated to the years of septimius severus (before 211 c. e.) rather than to the later period of severus alexander (222-235 c. e.) [56], [57]. acknowledgement for support in the realization of this paper we want to thank: clementina panella (former full professor of la sapienza università di roma and scientific director of meta sudans and nepalatine slopes excavations), the parco archeologico del colosseo (in particular dr.sa giulia giovanetti), giovanni caratelli and cecilia giorgi (i.s.p.c.-c.n.r.); a. f. ferrandes (department of scienze dell’antichità of la sapienza università di roma). references [1] c. panella (editor), meta sudans i. un’area sacra in palatio e la valle del colosseo prima e dopo nerone, istituto poligrafico e zecca dello stato, rome, 1996, isbn 9788824039505. [in italian] [2] c. panella (editor), i segni del potere. realtà e immaginario della sovranità nella roma imperiale, edipuglia, bari, 2011, isbn 9788872286166. [in italian] [3] c. panella (editor), scavare nel centro di roma. storie uomini e paesaggi, quasar, rome, 2013 isbn 9788871405117. 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[in italian] [48] m. serlorenzi, s. camporeale, anatomia di un muro romano: dati preliminari sullo smontaggio e quantificazione di alcune strutture in laterizio di epoca adrianea dallo scavo di piazza dante a roma, archeologia dell’architettura 22 (2017) pp. 21-33. [in italian] [49] d. botticelli, epigrafia del costruito. bolli laterizi dallo scavo delle pendici nord-orientali del palatino (area iv, anni 2007-2008) in: valle del colosseo e pendici nord-orientali del palatino. materiali e contesti 3, c. panella, v. cardarelli (editors), scienze e lettere, roma, 2017, isbn 9788866870739, pp. 49–96. [in italian] [50] f. villedieu (editor), la vigna barberini, ii. domus, palais impérial et temples. stratigraphie du secteur nord-est du palatin, école française de rome, rome, 200, isbn 9782728307852. [in french] [51] f. villedieu, la vigna barberini à l’époque sévérienne, in: palast und stadt im severischen rom, n. sojc, a. winterling, u. wulfrheidt (editors), steiner, stuttgart, 2013, isbn 9783515103008 pp. 157-180. [in french] [52] j. pflug, u. wulf-rheidt, la domus severiana sul palatino. gli interventi nel palazzo imperiale, in: roma universalis. i severi: l'impero e la dinastia venuta dall'africa, c. panella, a. d’alessio, r. rea (editors), electa, milano, 2018, isbn 9788891820723, pp. 158-169. [in italian] http://archivio.archeofoss.org/ https://doi.org/10.3390/drones4030046 acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 9 [53] e. bukowiecki, u. wulf-rheidt, l’industria laterizia e l’organizzazione dei cantieri urbani in: roma universalis i severi: l'impero e la dinastia venuta dall'africa, c. panella, a. d’alessio, r. rea (editors), electa, milano, 2018, isbn 9788891820723, pp. pp. 220-227. [in italian] [54] s. barker, roman builders – pillagers or salvagers? the economics of deconstruction and reuse, in: arqueología de la construcción ii. los procesos constructivos en el mundo romano: italia y provincias orientales, s. camporeale, h. dessales, a. pizzo (editors), madrid-mérida, 2010, isbn 9788400092795pp. 127142. [55] c. n. duckworth, a. wilson (editors), recycling and reuse in the roman economy, oxford university press, oxford, 2020, isbn 9780198860846. [56] c. panella, i contesti e le stratigrafie, in: i reperti scultorei dalle “terme di elagabalo”. il ritrovamento, il restauro, l’edizione, m. papini (editor) quasar, roma, 2019 978887140-981-8, pp. 9-54. [in italian] [57] a later dating, during severus alexander reign is proposed in r. mar, el palatí. la formació dels palaus imperials a roma, universitat rovira i virgili, tarragona, 2005, isbn 9788493469801. microsoft word article 1 contacts.docx acta imeko  february 2015, volume 4, number 1, 1  www.imeko.org    acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 1  journal contacts    about the journal  acta imeko is an e-journal reporting on the contributions on the state and progress of the science and technology of measurement. the articles are based on presentations presented at imeko workshops, symposia and congresses. the journal is published by imeko, the international measurement confederation. the issn, the international identifier for serials, is 2221-870x. editor‐in‐chief  paolo carbone, italy honorary editor‐in‐chief  paul p.l. regtien, netherlands editorial board francisco alegria, portugal leopoldo angrisani, italy filippo attivissimo, italy eulalia balestieri, italy eric benoit, france catalin damian, romania pasquale daponte, italy luigi ferrigno, italy alistair forbes, united kingdom fernando janeiro, portugal konrad jedrzejewski, poland andy knott, united kingdom fabio leccese, italy helena geirinhas ramos, portugal pedro ramos, portugal sergio rapuano, italy dirk röske, germany alexandru salceanu, romania constantin sarmasanu, romania lorenzo scalise, italy emiliano schena, italy enrico silva, italy marco tarabini, italy susanne toepfer, germany rainer tutsch, germany luca de vito, italy about imeko  the international measurement confederation, imeko, is an international federation of actually 39 national member organisations individually concerned with the advancement of measurement technology. its fundamental objectives are the promotion of international interchange of scientific and technical information in the field of measurement, and the enhancement of international co-operation among scientists and engineers from research and industry. addresses  principal contact prof. paolo carbone university of perugia dept. electr. inform. engineering (diei) via g.duranti, 93, 06125 perugia, italy email: paolo.carbone@unipg.it acta imeko prof. paul p. l. regtien measurement science consultancy (msc) julia culpstraat 66, 7558 jb hengelo (ov), the netherlands email: paul@regtien.net support contact dr. dirk röske physikalisch-technische bundesanstalt (ptb) bundesallee 100, 38116 braunschweig, germany email: dirk.roeske@ptb.de iot enviromental quality monitoring in smart buildings in presence of measurement uncertainty: a decision making approach acta imeko issn: 2221-870x june 2023, volume 12, number 2, 1 8 acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 1 iot enviromental quality monitoring in smart buildings in presence of measurement uncertainty: a decision making approach damiano alizzio1, claudio de capua1, gaetano fulco1, mariacarla lugarà1, valentina palco1, filippo ruffa1 1 departments diies and dieceam, università ‘mediterranea’ di reggio calabria, 89122, reggio calabria, italy section: research paper keywords: smart building; innovation-iot; security; sensoring; monitoring; building heritage citation: damiano alizzio, claudio de capua, gaetano fulco, mariacarla lugarà, valentina palco, filippo ruffa , iot enviromental quality monitoring in smart buildings in presence of measurement uncertainty: a decision making approach, acta imeko, vol. 12, no. 2, article 24, june 2023, identifier: imeko-acta12 (2023)-02-24 section editor: alfredo cigada, politecnico di milano, italy, andrea scorza, università degli studi roma tre, italy, roberto montanini, università degli studi di messina, italy received december 2, 2022; in final form february 22, 2023; published june 2023 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: gaetano fulco, e-mail: gaetano.fulco@unirc.it 1. introduction going through the historical evolution of living, it emerges that the house, considered as a "safe place", has had to adapt to multiple cultural, economic-political, climatic changes, but also to the needs of individuals and family groups [1]. the changes do not only concern the construction process and the technologies used for the realization of spaces, which certainly, nowadays, are closer to very high-performance standards and able to lower environmental impact as much as possible. the determining factor for the development of the concept of living has always been the relationship between home and family structure [2]. the different patterns and family statuses introduced by modernity necessarily exceed the functionalist vision of the home as a pure "machine for living" [3], i.e. the "house" with minimum standards that guarantee a good quality of life for the occupants. in this scenario, it is important to intersect the digital and real world, so that digital is at the service of the user, in order to improve living comfort and well-being, understood as better living conditions [4]. the emergency to which we are called to respond today as a scientific community, professionals and technicians in the construction sector and beyond, is the fragility to which the vast majority of the italian abstract the change of living concept from "traditional" to "smart” concerns how we live and relate in spaces that today, more than ever, are "sensitive" (i.e. spaces where digital technologies occupy a prominent place in the monitoring and control of buildings, with the aim of achieving high levels of quality of life). we are therefore witnessing the creation of new declinations of living and, in this context, the internet of things (iot) represents the starting point for the creation of connected products that "share" the information they detect with other objects or people on the network. in this scenario, the authors propose an original approach to measurements for the assessment of comfort in living environments. the work consists in the design and implementation of a measurement station, which acquires and analyses data collected by the network of distributed sensors and activates forced ventilation if the level of comfort is below the desired threshold. in such situations where measurement data are compared with a threshold value, it is necessary to consider how measurement uncertainty affects the decision taken; in this particular context, since the activation of actuators involves energy consumption, the decision on the effective threshold crossing should be well thought. for this reason, the aim of this work is to propose a smart monitoring system that through the setup and calibration of two decision-making algorithms, can decide if the measured value is below or over the threshold set with a known probability. in this way, the end user can chose an appropriate strategy, calibrated on the specific living environment, which allows to maximize either environmental comfort or energy saving, depending on the specific needs. mailto:gaetano.fulco@unirc.it acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 2 building heritage is subjected. in fact, in the italian territory, over time, the buildings have shown an insufficient degree of resilience from both a seismic and energetic point of view. in italy, over 22% of buildings are in a mediocre or very bad state of conservation and the construction sector is the most energyintensive in terms of consumption, maintainability and habitability of built environments [5]. we define “fragile” a building that does not fit into an environmental context and compromises the life of its occupants because it is not very resistant or not very liveable, a space without the minimum services of smart living [6]. smart living is a concept that in the last years is becoming even more important in the entire construction sector, covering the different phases: from design to the construction process, from monitoring and maintenance to management by end users. this concept is based on the idea that the use of technology allows to create environments that are able to substantially improve and simplify the quality of life of the occupants [7]. the current technical-scientific debate is focused on what tools, currently in use, can be considered reliable and capable of responding to important needs, such as: the use of renewable sources and strategies to contain energy consumption, the protection of human lives and last but not least the levels of indoor comfort. living, in this perspective is becoming "intelligent and sustainable", with continuous interrelation between human action and technology through increasingly digitized services aimed to considerably increase the levels of quality of life of the occupants. a statistic reported by cisco [8] about the use of the iot (internet of things) points out that in 2010 there were over 12.5 billion connected devices, with an increase of more than 400% in 2020 (about 50 billion). there are several experiments and applications currently underway in the construction sector [9], [10], including "dynamic facades with high-performance envelope for the energy efficiency of the building", "the optimization of iaq (indoor air quality) levels in buildings through intelligent ventilation systems", "systems for monitoring building safety and risk mitigation", etc. technology, connected to open source-hardware and sensing systems allows the detection of any changes in conditions and status [11]. this leads to meet the occupant comfort, energy consumption and cost efficiency needs. an important aspect concerns indoor air quality. according to the world health organization (who) the sick building syndrome (sbs) affects people who are subject to prolonged exposure to chemical, biological and / or physical agents in buildings with a low level of indoor air quality, generally due to poor ventilation. in fact, from literature, we know that iaq levels are generally 2 to 5 times worse than outdoor [12], [13]. this means that the occupants of closed spaces that do not enjoy good natural ventilation (and/or mechanical), have an increased risk of developing psychophysical malaise that often over time, leads to devastating effects on the human body such as the onset of diseases related to the respiratory tract, the central nervous system and even cancer [14]. since in recent years the lifestyle has changed considerably and people spend indoor much more time than before (about 90%), several research activities in the ict (information and communication technologies) sector, aim at investigating how multi-sensor iot platforms can optimize iaq levels in buildings [15], [16]. if we also consider the recent changes in people's lifestyle, who have increased the time spent at home, it is clear the importance of investing in iot platforms, which allow to dialogue with the home and to adapt the environment to the user needs [17]. among the different aspects of living, the present work aims to provide technological tools for efficient and effective monitoring of the living environment, in particular through the development of an automatic measurement and control station aimed at optimizing ieq (indoor environmental quality) levels. the proposed station is able to guarantee safety and living comfort through the continuous monitoring of iaq levels, temperature and relative humidity and, controlling actuators, is capable to restore the baseline conditions. the implemented system considers the uncertainty associated to the measurement process in order to make appropriate decisions about the actual exceeding of the thresholds set. 2. methods and applications in this paragraph, we will describe the acquisition and processing system that using a sensors network is able to detect the quantities of interest to monitor the environmental quality, in particular the iaq and the parameters related to thermal comfort, such as temperature and humidity. to make the system more usable, the monitoring station makes data easily accessible through the network. with regard to the iaq, the monitoring focuses on the assessment of the level of pollutants and the consequent management of the forced ventilation system. to maximize the flexibility, the system implements two different control strategies: one optimizes safety and activates the ventilation system more often, the other aims at obtaining maximum energy efficiency and, therefore, activates ventilation only in relevant cases. the system considers also the quality of the outdoor air, using a network of sensors placed outside the monitored environment, because in some cases it may be lower than the internal one. the comfort matching criteria described above are applied also to the monitoring of thermal quantities to decide whether it is the case to activate the ventilation system. 2.1. monitored parameters the level of indoor air quality refers to the concentration of pollutants, which can come both from sources inside the building, and outside, especially in urban contexts [18]. however, apart from temporary and exceptional situations, it is always possible to consider the air quality outside the building as a baseline, since the presence of polluting sources inside can only worsen the standard situation. in this context, it is of great importance to have real-time monitoring of these pollutants to guarantee timely air changes if air quality levels are no longer satisfactory. the parameters for the evaluation of iaq levels were selected from the state-of-the-art [19]-[26] and, in particular, the study was focused on three commonly studied air pollutants: carbon dioxide (co2), and particulate matter pm2.5 and pm10, as shown in table 1. if the concentration of these pollutants exceeds the safety limits imposed by the community standards, this would entail both immediate and long-term risks to the health of exposed people and, this is even more relevant the higher is their concentration. focusing on the pollutants examined, co2 can be considered as an indicator of the effectiveness of ventilation and excessive population density [27] and, in indoor environments with limited ventilation, should never exceed the limit of 1000 ppm [28] to guarantee adequate safety. the concentration of pm2.5 and pm10 particulate matter in the air has also been acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 3 directly associated with respiratory tract diseases [29], [30]. therefore, it is necessary, in closed spaces, to monitor these quantities in order to plan appropriate intervention strategies that aim to restore the most optimal air quality. it is possible, in fact, if their concentration exceeds the safety limits imposed by current regulations, or rather, limits that are more conservative and include a safety margin, even for long-term exposures, to intervene by activating a ventilation system to guarantee an adequate air change. both for the healthiness of the rooms and to optimize living comfort, it is also necessary to monitor quantities such as temperature and, above all, humidity. the presence of excessive humidity leads to the proliferation of mites and moulds, and the latter can be an important factor in the onset of rheumatic diseases and also increases the thermal perception of both heat and cold. on the other hand, excessive dryness of the air can cause breathing difficulties and increase the transmissibility of certain diseases [31]. several studies have shown that a relative humidity between 40 % and 60% is optimal to maximize comfort [32], [33]. for what concerns the temperature, if it is too high it impacts both on the symptoms of sbs and on the productivity of the occupants. the ideal temperature should be between 21 and 25 °c [34]. to set the thresholds levels of pollutants present in the monitored environment, among the technical regulations, we considered the thresholds established by the world health organization (who). for what concerns thermal comfort, we have referred to the iso 7730 standard [35], which also refers to the iso 17772-1: 2017 standard [36], as shown in table 2. to control the level of iaq the proposed system regulates forced ventilation, reducing so the concentrations of pollutants and regulating temperature and humidity. 2.2. monitoring and control system the monitoring and control system, schematized in figure 1, consists of a central unit that acquires information from the network of distributed wireless sensors (both indoor and outdoor) and if it is necessary, activates the ventilation system, in order to maintain a defined level of comfort in the living environment. the sensor network consists of sensors placed both inside and outside the building, so to monitor indoor and outdoor environmental conditions. the parameters monitored indoor are co2, pm2.5, pm10, temperature and humidity, while outdoor only pm, temperature and humidity. the activation of the ventilation system, instead, is implemented through the control of a relay to bring power to the internal conditioning system. the technical characteristics of the sensors used are shown in table 3. the central control unit is implemented with a national instruments single-board rio, with its expansion board. its function is to take measurement data from the different sensors in real time and to control the actuators of the ventilation system, through the opening of a relay, implementing the algorithm described in the flowchart of figure 2. using a graphical interface, users can view all measurement data and, according to personal needs, they can make changes to the configuration parameters of the smart management system. the system also allows to choose between two different operating strategies that aim respectively at maximizing safety related to the healthiness of the environments or at maximizing energy saving. based on the selection, the system defines the thresholds and decision criteria. 2.3. data analysis and decision-making for each pollutant, we calculate a quality index, which depends on the concentration of the single parameter measured and the reference level taken from the legislation [37] table 1. iaq standards. standard co2 pm10 pm2.5 bes x breeam dgmb en 16798 x x x hqe x x klima x leed x x x nabers x x osmoz x x x well x x table 2. pollutants reference level. parameter threshold value co2 1000 ppm pm10 50 μg/m3 pm2.5 10 μg/m3 temperature 24.5 – 28 °c in summer 20 – 24 °c in winter humidity 40 – 60 % in summer 30 – 60 % in winter parameter threshold value figure 1. control system. table 3. sensors specifications. sensor range accuracy temperature in °c -60 +75 ± 0.5 relative humidity in % 5 95 ± 7 co2 in ppm 0 10000 ± 40 pm2.5 in µg/m3 0 1000 ± 10 pm10 in µg/m3 0 1000 ± 25 acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 4 𝐼x = 100 % − 𝛼 log 𝐶x 𝐶x|0 , (1) where 𝐶x is the measured quantity of the single pollutant x, 𝐶x|0 is the concentration of the same pollutant under ideal conditions and 𝛼 is a constant that considers the toxic levels of the pollutant defined by the standard. the value of  is therefore imposed by placing 𝐼x = 0, when 𝐶x = 𝐶xmax, where 𝐶xmax is the maximum concentration of the pollutant taken as a threshold. the overall iaq level is equal to the minimum of the indices calculated for the single monitored parameters. 𝐼iaq = min(𝐼co2 ,  𝐼pm10,  𝐼pm2.5) , (2) where 𝐼co2 ,  𝐼pm10 and 𝐼pm2.5 are the quality indexes of co2, pm10 and pm2.5. this method lets to have a global and rapid information on the level of air quality in indoor environments and can be easily implemented without great computational efforts. the algorithm compares the estimated level of iaq with the threshold taken as a reference and, if it does not meet the user needs, it activates the ventilation actuators. in the same way the system monitors the temperature and relative humidity in the environment and compares their measured values with the reference thresholds in order to maintain the desired quality standards. as said in section 2.2, configuring the system, it is possible to choose between two different strategies, i.e. maximization of safety or energy saving and to set, consequently, the reference thresholds. in the second case, in fact, we consider as thresholds the levels indicated by the regulations, while in the first case we set thresholds that are more conservative, considering also the number of occupants. in particular, the new thresholds are: 𝑇𝑟𝑒𝑠ℎ𝑜𝑙𝑑  = 𝑁𝐿 𝐾 , (3) where 𝑁𝐿 is the level imposed by normative and 𝐾 depends on the number of occupants in relation to the size of the environment. in situations like this, where there is a need to make decisions about exceeding a certain threshold, the algorithm has to analyse experimental data, considering not only the measured value, but also the uncertainty associated with the measurement process [38]-[41]. in this case we face to a particular class of problems that pertains to the field of decision-making. as known, in fact, the result of a measurement does not provide a single value, but an interval, centred in the measured value, inside which it is possible to find the value of the measurand with a given level of confidence. this means that, when the result of the measurement falls in a certain band around the threshold level identified: 𝑚 = 𝑣 ± 𝜀 , (4) where 𝑚 is the measured value, 𝑣 is the value of the measurand and 𝜀 is the value of the extended uncertainty relating to the measurement, we cannot be certain about the result of the comparison with the reference threshold and therefore probabilistic assessments must be made to associate a known level of risk with the decision that will be taken. taking from datasheet the type b uncertainty associated to each sensor and, considering a measurement error with a gaussian distribution, the probability that the measured value falls within a band of ± 3  around the measurand, where  is the standard deviation, is 99.7 %. clearly, taking into account the extended uncertainty, the wider the region of ambiguity, the more times the decision-making algorithm will intervene, as can be easily understood from figure 3. in the implementation of the decision-making algorithm, it is possible to consider a smaller band of uncertainty, cutting the tails of the gaussian curve, but this means choosing a smaller coverage factor and therefore decreasing the correctness of the decision taken. for what concerns the decision-making algorithms, among the various proposed in the literature, we decided to implement the utility cost test and the fixed risk [42], [43]. to address the decision problem by considering the impact of measurement uncertainty, the utility cost test considers the potential consequences of the different possible decisions. the algorithm evaluates four possible situations and their associated costs, i.e. positive, false positive, false negative, negative. by suitably weighing these costs, the algorithm makes a decision on whether the threshold is actually exceeded; evaluating which of the two possibilities has the lower cost. the fixed risk algorithm, on the other hand, works by initially setting the maximum level of risk acceptable for a wrong decision. this means that the threshold is dynamically set, so that the decision made by the algorithm is right or wrong with a known probability once an lth threshold has been set, comparing the measurement result with it, if the measured value falls within the ambiguity band, there is a risk, highlighted in figure 4 (blue area), of considering the measured value below the threshold when it is above. since the probability density function associated with figure 2. flow chart of the algorithm. figure 3. decision making region acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 5 the measurement is known, it is possible to set the maximum risk that is considered acceptable (mra) in making the decision. through a change of variable, the threshold is repositioned in order to match the following equation: 𝑀𝑅𝐴 = ∫ 𝑓𝑢(𝑥−𝑚) d𝑥 +∞ 𝐿𝑡ℎ , (5) where 𝑓𝑢(𝑥−𝑚) is the probability density function centred in 𝑚. knowing the measurement uncertainty of the sensor, if the process is characterized by a normal distribution, it is possible to set the parameters of the algorithm. the algorithm will then be able to make a decision on exceeding the threshold within the ambiguity zone and give an answer to the decision-making problem, together with a quantification of the risk associated with the decision taken. the implementation of a decisionmaking criterion is essential for those systems, such as the one described in this work, in which the incorrect evaluation of the operating conditions, i.e. considering the parameters within the safety zone when they are not, can affect people health and comfort. the proposed system implements both decision-making algorithms, whose parameters are calibrated for the two different strategies, i.e. maximization of environmental quality and maximization of energy savings. 3. evaluation of decision-making algorithms performances in this paragraph, we talk about the evaluation of the performances of the decision-making algorithms described above. with the simulations performed, we wanted to evaluate how they respond to the two different management strategies: highest air quality and healthiness maximum energy savings once the operational strategy in the management system has been selected, the system calculates the cost functions for the application of the utility cost test and sets the maximum percentage of risk allowed considering the reference threshold exceeded in the fixed risk. the two algorithms thus calibrated will make appropriate decisions in considering the results of measurements that fall within the ambiguity range, above or below the threshold set, based on the overall strategy chosen. to verify the correct functioning of the algorithms, we used simulated datasets, considering a generic sensor with its associated uncertainty and a fixed threshold level. evaluating a region of ± 3 σ around the threshold level, the intervention of the algorithms was tested with different coverage factors and the number of interventions that considered the threshold exceeded was compared with the number of activations that would have occurred without the implementation of the decision-making algorithms. the results obtained are summarized in table 4 and table 5. as we can see in the results reported, considering either a linear or a sinusoidal trend of the measurand, without decision-making algorithms, 50% of the comparisons result above the threshold. this because we perform a direct comparison between the measured value and the threshold, without considering the impact of measurement uncertainty. this means that in operative conditions, where the measured value is given by eq. 4, the result of the comparison can be alternately positive or negative with a probability that is the more similar, the more the measurand is near to the threshold. using decision-making algorithms it is possible to adopt a more or less conservative approach to measurement uncertainty. if the strategy chosen aims at maximizing air quality and minimizing health risk, there is a greater number of activations, therefore a better ventilation of the rooms, compared to what would have happened with a direct comparison with the threshold. in the specific case examined, we can observe an increase in the number of activations of 15% and 10% with utility cost test, respectively for linear and sinusoidal trend of the dataset, and 32% and 29.5% with fixed risk. with a coverage factor k = 1, using the fixed-risk algorithm, we observe 10% less activations than with the same algorithm and a greater coverage factor, this because in this case the decisionmaking band is narrower and therefore the algorithm intervenes fewer times. in the other case, which instead aims at maximizing energy savings by minimizing the number of activations of the ventilation system, we observe a strong reduction in interventions. in fact, using utility cost test algorithm, we have 16.5% and 10.5% less activations than in operation without decision-making algorithms, respectively for linear and sinusoidal trend of the dataset, and 22% and 20% less, using fixed risk. this is just a general case implemented to test the impact of the decision-making approach, since, in real applications, implementing a correct risk assessment during the system design phase, it is possible to re-calibrate the response of the algorithms to match the specific environments needs. 4. conclusions technologies applied to living environments are significant to ensure their smart evolution. the italian scenario still lacks of applications for the redevelopment of buildings (especially those for public use) aimed at improving the quality of life, through targeted actions, which let to contain energy consumption using iot or smart systems. designing and building according to these paradigms means that we seriously need to face with the requirements for balance between resources and environmental impact. the building is an artefact that changes and adapts itself according to user needs. the current challenge consists in considering the building through a multidisciplinary approach that manages the technological and digital system in order to obtain high performance responses in terms of both process control, product maintainability and liveability. in this work, the authors proposed an automatic measurement system to be integrated into the context of smart homes, with the aim of improving aspects related to the safety and quality of living environments. by monitoring air quality, ambient temperature, relative humidity and, consequently, managing the ventilation system, it is possible to optimize the level of ieq in the observed environment. in these contexts where measured data are figure 4. risk level. acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 6 compared with a reference threshold, it is important to consider how measurement uncertainty affects the decision on threshold crossing. this is much more relevant in applications like the one described in this paper where a wrong decision implies a waste of energy. in fact, if the value of the measurand is near to the reference level, the contribution of measurement uncertainty can be determinant to assess if the measured value is above or below the threshold. for this reason, the measurement and control station proposed acquires and analyses the measurement data collected by the network of distributed sensors and, with the support of decision-making algorithms to verify the actual exceeding of the thresholds, activates the actuators of the system to optimize the ieq levels. in this system, the decision-making approach covers a role of primary importance, since it let to be more or less conservative on measurement uncertainty, inherently to the choice on the activation of the ventilation system. in fact, the proposed algorithm works on two levels: one related to the safety of indoor environments, and the other linked to the optimization of comfort. the first level is met within the safety thresholds imposed by the current regulations, for each monitored parameter. environmental comfort, on the other hand, is guaranteed through careful monitoring of ieq levels and the operation of actuators that allow to restore the desired quality levels, if the index falls below the desired comfort threshold. the system allows, through an interactive user interface, to record the user's preferences about the strategy to be adopted. in this way the algorithm will be able to dynamically move the thresholds adapting to the needs of the single households. to verify the functioning of the comparison algorithm, two predefined datasets were used. the decision-making algorithms were calibrated on two different strategies: one aimed at maximizing the quality of the environments and the other aimed at maximizing energy savings. from the tests performed it was possible to observe an increase in activations of the actuators between 10 and 15% using the utility cost test algorithm and between 29 and 32% using the fixed risk algorithm, with respect to direct comparison with the threshold, if the chosen strategy is to maximize quality. in the other case, however, we observed a reduction in the number of activations between 10.5 and 16.5% with the utility cost test algorithm and between 20 and 22% with fixed risk compared to the reference case. the results obtained with these two strategies served to verify the functioning of the decision-making algorithm. in practice, the system, installed in a specific living environment, can be recalibrated to make smart decisions, which best fit the user needs. future developments will concern the integration of other aspects related to smart buildings design, such as the evaluation of the energy balance in a context in which electricity is produced on site from renewable sources. in this sense, it is possible to increase the efficiency and safety of living spaces through the measurement of energy flows and a consequent adaptation of the algorithms that should consider this aspect. references [1] j. hoof, w. zeiler, r. bergen, architectural and building services requirements for smart homes, handbook of smart homes, health care and well-being, 2014, pp. 1-7. doi: 10.1007/978-3-319-01904-8_29-1 [2] l. van den berg, m. kalmijn, t. leopold, family structure and early home leaving: a mediation analysis, european journal of population 34 (2018), pp. 873–900. doi: 10.1007/s10680-017-9461-1 [3] r. suryantini, d. p. kristanti, a. y. yandi, a healthy machine for living: investigating the fluidity of open spaces in the domestic environment during the pandemic, aip conference proceedings 2376(1) (2021). doi: 10.1063/5.0063938 table 4. sinusoidal data variation. sinusoidal data maximizing safety/ quality maximizing energy savings k = 1 n. total threshold comparisons 200 200 n. interventions decision making algorithms 80 80 n. detections above utility cost test threshold 60 19 n. detections above fixed risk threshold 80 0 n. above threshold with direct comparison 40 40 k = 2 n. total threshold comparisons 200 200 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[30] m. a. zoran, r. s. savastru, d. m. savastru, m. n. tautan, assessing the relationship between surface levels of pm2.5 and pm10 particulate matter impact on covid-19 in milan, italy, science of the total environment 738 (2020). doi: 10.1016/j.scitotenv.2020.139825 [31] a. ahlawat, a. wiedensohler, s. mishra, an overview on the role of relative humidity in airborne transmission of sars-cov-2 in indoor environments, aerosol and air quality research, special issue on covid-19 aerosol drivers, impacts and mitigation 20 (2020), pp 1856–1861. doi: 10.4209/aaqr.2020.06.0302 [32] k. h. chan, j. s. malik peiris, s. y. lam, l.m. poon, k. y. yuen, w. h. seto, the effects of temperature and relative humidity on the viability of the sars coronavirus, advances in virology (2011) art. no. 734690. doi: 10.1155/2011/734690 [33] p. wolkoff, indoor air humidity, air quality, and health – an overview, international journal of hygiene and environmental health 221 (2018), pp 376-390. doi: 10.1016/j.ijheh.2018.01.015 [34] a. olli, w. seppänen, some quantitative relations between indoor environmental quality and work performance or health, ashrae https://doi.org/10.1080/17508975.2019.1613219 https://doi.org/10.3390/su10010254 https://doi.org/10.1109/metrolivenv54405.2022.9826946 https://doi.org/10.1109/metrolivenv54405.2022.9826913 https://doi.org/10.1109/metrolivenv54405.2022.9826923 https://doi.org/10.1016/j.rser.2018.05.057 https://doi.org/10.1016/j.renene.2019.01.111 https://doi.org/10.3389/fpubh.2020.00014 https://doi.org/10.3390/su11205777 https://doi.org/10.1016/j.measurement.2013.09.040 https://doi.org/10.3390/s18082660 https://doi.org/10.1016/j.renene.2019.01.111 https://doi.org/10.1016/j.rser.2018.05.057 https://doi.org/10.5094/apr.2015.084 https://doi.org/10.3390/toxics10030136 https://doi.org/10.1109/icetas48360.2019.9117518 https://doi.org/10.3390/ijerph18084060 https://doi.org/10.5194/jsss-7-373-2018 https://doi.org/10.1007/s43621-021-00027-w https://doi.org/10.3390/su11226225 https://doi.org/10.1109/access.2019.2937320 https://doi.org/10.1016/j.scitotenv.2020.139825 https://doi.org/10.4209/aaqr.2020.06.0302 https://doi.org/10.1155/2011/734690 https://doi.org/10.1016/j.ijheh.2018.01.015 acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 8 research journal 12(4) (2006), pp 957-973. online [accessed 22 april 2023]. https://escholarship.org/content/qt80v061jx/qt80v061jx.pdf?t =lnpsal [35] iso 7730:2005 ergonomics of the thermal environment — analytical determination and interpretation of thermal comfort using calculation of the pmv and ppd indices and local thermal comfort criteria. 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[in italian] https://escholarship.org/content/qt80v061jx/qt80v061jx.pdf?t=lnpsal https://escholarship.org/content/qt80v061jx/qt80v061jx.pdf?t=lnpsal https://doi.org/10.1016/j.jclepro.2021.127846 https://doi.org/10.1109/tr.2012.2194174 https://doi.org/10.1109/tim.2008.2003340 https://doi.org/10.1109/jsen.2014.2361697 https://doi.org/10.21014/acta_imeko.v10i4.1181 https://doi.org/10.6148/ijitas.2008.0101.01 microsoft word article 8 162-1309-1-le.docx acta imeko  february 2015, volume 4, number 1, 44 – 52  www.imeko.org    acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 44  statistical characterization of the 2.45 ghz propagation  channel aboard trains  annalisa liccardo  1 , andrea mariscotti  2 , attilio marrese  1 , nicola pasquino  1 , rosario schiano lo  moriello  1   1  dept. of electrical engineering and information technologies, university of naples federico ii, via claudio 21, naples italy  2  dept. of naval and electrical engineering, university of genoa, via all'opera pia 11, genoa italy       section: research paper   keywords: radio propagation channel; path loss; multipath propagation; delay spread; coherence bandwidth; statistical analysis of measurement data  citation: a. liccardo, a. mariscotti, a. marrese, n. pasquino, r. schiano lo moriello, statistical characterization of the 2.45 ghz propagation channel aboard  trains, acta imeko, vol. 4, no. 1, article 8, february 2015, identifier: imeko‐acta‐04 (2015)‐01‐08  editor: paolo carbone, university of perugia   received december 13 th , 2013; in final form november 17 th , 2014; published february 2015  copyright: © 2014 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: this work was supported by measurement science consultancy, the netherlands  corresponding author: nicola pasquino, e‐mail: npasquin@unina.it    1. introduction  use of telecommunication services aboard trains is becoming more and more common for providing information and entertainment services to passengers during trips. recently, wifi access points began to be installed on board with the aim of providing passengers with wide-band access to the internet. trains are a possibly harsh environment to wireless propagation, given the presence of metal walls, seats, moving and steady passengers, besides emissions due to electric and electronic equipment and the power quality phenomena that may cause interference with electronic devices [1]. there are two main areas of interest for researchers in order to reduce performance degradation: the study of the propagation characteristics aboard trains to determine the attenuation figure 1. train layout.  abstract  the propagation channel aboard trains is investigated with reference to the propagation path loss within cars, the delay spread and  the coherence bandwidth. results show that the path loss exponent is slightly smaller than in free space, possibly due to reflections by  metal walls, and that it does not depend significantly on the position of transmitter and receiver. the delay spread and coherence  bandwidth  depend  on  both  the  polarization  and  distance  between  transmitter  and  receiver  while  the  effect  of  interaction  is  not  statistically significant. the best fit for both delay spread’s and coherence bandwidth’s experimental distribution is also investigated.  results show that it does not always match models suggested in the literature and that the fit changes with the values of the input  parameters.  finally,  the  functional  law  between  coherence  bandwidth  and  delay  spread  is  determined.  results  typically  match  expectations although the specific measurement configuration effects the model parameters.  acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 45  law and multipath properties, and the investigation of the effects of external disturbance onto the telecommunication signal. in [2]–[4] the results of an extensive measurement campaign to characterize the behavior of the disturbance radiated by the electric arc generated by the interaction between pantograph and overhead wire can be found, while the effects of pulses onto some quality-of-service parameters have been investigated in [5]. with reference to the characterization of the propagation channel, literature about this topic usually includes research papers about the propagation in different scenarios and at different frequencies. in [6], [7] it is shown that the use of a directive antenna reduces the doppler spread and increases the received power when train runs towards the base transmitting station (bts); when train moves away from bts the omnidirectional antenna provides better results instead. in [8]–[11] it is shown that the classical models for propagation loss (such as hata model and two-ray model) are inadequate for attenuation prediction when propagation occurs under the effect of structures like viaducts and terrain cuttings (canyons). moreover, the presence of canyons, especially if topped by bridges, determines higher path losses than in the case of viaducts. with specific reference to propagation on board, in [12], [13] a narrow-band approach has shown that the transmitted signal can re-enter cars through windows and that its contribution to inter-car propagation is more relevant than the line-of-sight (los) signal. in [14] propagation on board has been analyzed with a 2.35 ghz continuous-wave signal and both a planar and an omnidirectional antenna, placed at different locations. the path loss and the ricean k-factor are shown to be related to the antenna type, while the delay spread is independent of the measurement configuration. studies performed aboard ships [15]–[18] only partially can be applied to trains due to differences in the geometrical and electromagnetic configurations. the paper is organized as follows: in section ii the measurement setup and methodology for the experimental studies are described; in section iii the results are presented. 2. measurement setup  measurements were run on an etr200 train owned by circumvesuviana s.r.l., an italian local transportation service. figure 1 shows a portion of the train layout, with dimension in millimeters. the properties of the propagation channel were investigated with a narrow-band approach. the transmitting port of a fieldfox n9918a vector network analyzer by agilent has been connected to a bbha 9120d horn antenna by schwarzbeck, while the receiving port to an em6865 biconical antenna by electrometrics. automated calibration was run to normalize the attenuation introduced by cables and connections. also, care has been taken to keep polarization coupling and to maximize the received signal by a proper antenna alignment. it was not necessary to take into account antenna factors because the main focus has been on the propagation loss exponent and not on the absolute value of loss itself. measurements were executed in a steady train, without significant reflectors in close proximity outside the car. figure 1 also shows distances from the wall (in mm) for the two propagation configurations for path loss measurements, which were taken along the centerline of the car and away from it (off-axis measurements), both at 120 cm height. samples were taken moving the receiving antenna away from the transmitting one starting at 1 m up to 20 m in 10 cm steps. figure 2 shows a view of the propagation environment for centerline measurements. it is apparent that propagation is affected by reflection on the walls, seats and vertical handrails, because of which the number of measurement points was reduced from 190 to  figure 2. measurement setup.  figure 3. path loss measurements and regression with 95% confidence intervals. left: centerline and off‐axis; upper right: centerline; lower right: off‐axis.  acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 46  171 and 151 for the centerline and off-axis configuration, respectively. delay spread was measured along the centerline, both in vertical and horizontal polarization, at 120 cm height and three distances, i.e. 5, 10 and 15 m, moving the whole system at 10 cm steps. again, some measurement points were missed, this time because of the presence of seats along the off-axis positions besides the ones already excluded because of the handrails. 3. results  3.1. relative path loss plr  since internet access on board is expected to be granted through wifi access points, channel propagation has been investigated in the 2.45 ghz band. figure 3 shows the relative path loss plr obtained by normalizing the received power to that measured at 1 m from the transmitting antennas [19], [20]. the graph on the left shows both centerline and off-axis samples, and regression is carried out without distinction between the two configurations. variability about the regression line is typically caused by the external noise affecting the measurement setup [21]. because of the inhomogeneity of the propagation environment (see figure 1), different exponents are expected to rule the power-decay law along the channel. therefore the approach shown in [22] should have been adopted. however, the limited number of points assigned to each propagation layer would have resulted in a reduced significance of the regression analysis. for this reason, no distinction was made, and the exponent n can be thought of as the average one over the whole distance. the model for the relative path loss plr is [23]:   0 10 logr dpl d n x d       (1) where d0 = 1 m is the reference distance and x is a random variable describing the effect of multipath propagation onto received power, expected to be distributed according to a gaussian random variable. with the obtained experimental data, the path loss 86420-2-4-6-8 100 80 60 40 20 0 residue [db] pe rc en t normal fit residual a) residuals of the whole set of propagation measurement data  840-4-8 100 80 60 40 20 0 840-4-8 residual [db] pe rc en t centerline off-axis normal fit residual b) residuals for centerline and off‐axis propagation  figure 4. residuals of regression.  a) magnitude and phase frequency response for minimum and maximum  delay spread propagation  b) impulse response for minimum and maximum delay spread propagation   figure 5. frequency and impulse response.   figure 6. τrms vs   for each combination of input factors.  acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 47  exponent turns out to be n = 1.71, smaller than that for free-space due to reflection on metal walls. to check that differences between the two propagation directions were indeed statistically insignificant, we also applied the regression procedure separately to the two propagation scenarios. the exponent for centerline and off-axis measurements turned out to be nc = 1.67 and no = 1.73 respectively. the associated 95% confidence intervals are shown in table 1. because of the overlap between them and with the confidence interval for the indistinct regression, propagation in the two configurations can be assumed to be very similar. figure 4 shows that the gaussian model fits the experimental distribution of residuals from linear regression as expected by model (1). residuals from the whole set of propagation data (“overall” propagation) are normal with a p-value p = 10% (figure 4.a), while p = 12.7% and p > 15% for centerline and off-axis propagation, respectively. mean values of residuals are zero with p = 99.7%, p = 85.6% and p = 68.1% for the whole measurement set, centerline and off-axis propagation respectively. the 95% confidence intervals for the mean values are [-0.304;0.305], [-0.39;0.47] and [-0.50;0.33] respectively. moreover, we cannot reject the hypothesis of equal variances for the centerline and offaxis scenarios unless we accept a risk at least equal to 29.1%. the 95% confidence interval for the ratio of the two variances is [0.87;1.61]. 3.2. frequency and impulse response  figure 5.a shows the frequency response (magnitude and phase) of the propagation channel for horizontal polarization with 10 m separation. in the figure the responses with the smallest and largest τrms delay spread, calculated according to [23] and described in the next section, are plotted. the square magnitude of the associated impulse responses are plotted in figure 5.b. 3.3. rms delay spread τrms  the mean excess delay and the rms delay spread τrms are quantities used to estimate the time dispersion properties of the multipath channel. the mean excess delay is the first central moment of the power delay profile:     k kk kk p p        (2) while the rms delay spread τrms is defined as the square root of the second central moment of the power delay profile:   table 1. path loss exponent’s 95% confidence intervals.     n, overall nc, centerline no, off-axis [1.61;1,81] [1.53;1.81] [1.60;1.86] a) main effects plot of polarization and distance for τrms b) interaction plot between polarization and distance for τrms  figure 7. main effects and interaction plots for τrms.  d [m] pol. 15 10 5 v h v h v h 9876543 95% c.i. for standard deviation s [ns] test statistic 39,59 p-value 0,000 test statistic 8,61 p-value 0,000 bartlett's test levene's test a) 95% confidence interval for standard deviation  100 75 50 25 0 403020100 100 75 50 25 0 403020100 403020100 h; 5 t _rms [ns] pe rc en t h; 10 h; 15 v; 5 v; 10 v; 15 normal fit t _rms [ns] b) experimental cdf’s and normal fit  figure 8. validation of anova hypothesis for τrms.  acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 48   22rms    (3) where 2 is the mean square value:     2 2 k kk kk p p        (4) table 2 shows the sample mean values and the sample variance for τrms and . for both parameters the mean value increases with separation between antennas, but for the mean excess delay the increment has a larger magnitude. the relation between rms delay spread and mean excess delay, shown in figure 6, is linear and follows the model: rms      , (5) where it can also be seen that for vertical polarization the correlation coefficient ρ is usually smaller: its values are 76.2%, 80.3% and 78,9% for 5, 10, 15 m horizontal polarization and 53.7%, 65.0% and 66.7% for the same distances, vertical polarization. table 3 reports the values of the α and β parameters. figure 7.a shows the main effects of distance and polarization on τrms, while figure 7.b shows the interaction plot between the two quantities. figure 7.a shows that on average, the two polarizations cause a significant change in the amplitude of τrms, namely the vertical polarization typically suffers from a larger contribution from secondary peaks of the impulse response, that is reflection contributions are more numerous or present higher amplitudes. this may be due to the different boundary conditions encountered by vertically and horizontally polarized waves. the plot on the right of figure 7.a shows that at greater distances the delay spread increases on average, which is due to the larger number of reflected signals arriving at the receiving antenna. the interaction plot in figure 7.b is used to determine if the effect of polarization also depends on distance. it can be seen that the increase in the delay spread when the distance changes between 5 m and 15 m is different for the two polarizations. the interaction plot therefore seems to indicate that there is indeed interaction between the two factors. for a more complete analysis, the analysis of variance (anova) was run on experimental data. results show that while distance and polarization are indeed statistically significant as a cause for variations observed in experimental data (p < 0.1%), interaction cannot be considered significant unless an error probability of at least 20% is accepted. however, the low value of the r2adj statistics (r2adj = 27%) tells that only about 27% of the variability in experimental data can be explained by the linear model including distance, polarization and their interaction. more factors should therefore be included in the model. this is not unexpected at all, since propagation in such a complex scenario as a train car is not only ruled by polarization and distance between transmitting and receiving systems. furthermore, it is necessary to verify that hypotheses for applicability of the anova methodology are satisfied.  figure 9. frequency correlation functions.  a) main effects plot of polarization and distance for bc b) interaction plot between polarization and distance for bc  figure 10. main effects and interactions for bc.  table 2. sample mean and sample variance for   and  rms .  dist. pol. rms s2 s2 5 v 28.05 23.23 16.77 23.43 10 51.07 61.78 20.31 23.33 15 63.91 49.84 21.63 39.56 5 h 22.44 10.96 12.51 13.03 10 40.78 9.92 15.24 18.15 15 62.69 65.61 18.64 42.25 acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 49  figure 8 shows results of such validation. in figure 8.a we see that although variances σ2 can be assumed to be equal between horizontal and vertical polarization at a given distance, they cannot be considered to be equal in general assuming either normal (bartlett’s test) or continuous distributions (levene’s test). in figure 8.b it is shown that the hypothesis of normal distribution of data for each combination of the input factors, unlike expected [24], is only verified for 10 m separation. both conditions pose a limit to the validity of results of anova although the very small p-values associated to the effect of distance and polarization may compensate for it. to further test validity of results, the non-parametric kruskal-wallis test has been applied. it is equivalent to a one-way anova, therefore interaction of factors cannot be determined. it does not require normality of data but does rely on equal variances assumption. by application of the test on τrms it turns out that both distance and polarization are significant at p < 0.1%, confirming the aforementioned outcomes from anova. it must be said however that variances as a function of distance cannot be considered equal (p < 0.1%) while they can be considered equal as a function of polarization (levene’s test, p = 46.9%). 3.4. coherence bandwidth bc  coherence bandwidth bc is the frequency separation at which the frequency correlation function crosses a certain level c. typical values of c are 0.9 and 0.5 [23]. figure 9erro! a origem da referência não foi ncontrada. shows a typical behavior of such functions for each polarization and distance. it must be noted that oscillations in the curves are caused by the multipath propagations and directly related to the amplitude of τrms. in table 4 the sample mean and the sample variance of bc for each configuration are reported. figure 10.a shows the main effects of distance and polarization on bc, while figure 10.b shows the interaction plot between the two quantities. figure 10.a shows that on average, the two polarizations cause a significant change in the amplitude of bc, namely the vertical polarization typically presents a lower value. again, this may be due to the different boundary conditions encountered by vertically and horizontally polarized waves. the plot on the right of figure 10.a shows that at greater distances the coherence bandwidth decreases, in accordance to the increase of the delay spread shown in figure 7 and the behavior shown in figure 9. the interaction plot in figure 10.b shows that the reduction in bc when the distance changes between 5 m and 15 m is slightly different for the two polarizations. the interaction plot therefore seems to indicate that there is small interaction between the two factors. again, for further analysis the analysis of variance (anova) was run on experimental data. results show that while distance and polarization are indeed statistically significant as a cause for variations observed in experimental data (p < 0.1%), interaction can be considered significant at a level of 3.6%. like for τrms the value of the r2adj statistics (r2adj = 23%) suggests that to explain table 3. parameters of linear regression  rms  vs  .  dist. pol. α β 5 v 1.63 0.54 10 -0.07 0.40 15 -16.36 0.59 5 h -9.19 0.83 10 -29.16 1.09 15 -21.08 0.63 d [m] pol. 15 10 5 v h v h v h 3,53,02,52,01,51,0 95% c.i. for standard deviation s [mhz] test statistic 60,82 p-value 0,000 test statistic 10,28 p-value 0,000 bartlett's test lev ene's test a) 95% confidence intervals for standard deviations  100 75 50 25 0 1612840 100 75 50 25 0 1612840 1612840 h; 5 bc [mhz] pe rc en t h; 10 h; 15 v; 5 v; 10 v; 15 normal fit bc [mhz] b) experimental cdf’s and log‐normal fit  figure 11. validation of anova hypothesis for bc.  table 4. sample mean and sample variance for  cb .  dist. pol.  x s2 5 v 4.92 2.47 10 4.44 2.28 15 4.02 2.10 5 h 7.03 2.21 10 6.75 4.88 15 5.21 5.90 acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 50  variability in experimental data more factors should be included in the model. again, this is expected just like for τrms, for the same reasons. validation of the hypotheses for applicability of the anova methodology is shown in figure 11. figure 11.a shows that although variances σ2 can be assumed to be equal at each polarization for the different distances, they cannot be considered to be equal in general assuming either normal (bartlett’s test) or continuous distributions (levene’s test). as for normality of bc’s data, it must be said that there is no evidence in scientific literature about a specific distribution. we found (see figure 11.b) that the best fit is with a lognormal cdf in all cases except for the horizontal polarization at 5 and 15 m. like for delay spread, both conditions pose a limit to the validity of results of anova, although the very small p-values associated to the effect of distance and polarization may compensate for it. the non-parametric kruskal-wallis test has been applied also to bc data. again, results show that both distance and polarization are significant at p < 0.1%. in this case, however, variances as a function of distance can be considered equal (levene’s test, p = 29%) while they cannot be considered equal (p < 0.1%) as a function of polarization. 3.5. coherence bandwidth bc vs. delay spread τrms  often in literature the functional relation between bc and τrms is identified by the following expression [23]: 1 50 c rms b   (6) by the more generic model [25]: c rms b    (7) or by an even more generic one 1,71,61,51,41,31,21,11,00,90,8 1,2 1,0 0,8 0,6 0,4 0,2 log10(t _rms) lo g1 0( bc ) linear fit (t _rms,bc) a) overall behavior of bc vs. τrms  a) bc vs. τrms for each combination of input factors   figure 12. bc vs. τrms.  table 5. values and 95% confidence intervals for a and b.  dist. pol. a b value 95 % c.i. value 95 % c.i. 5 v 1,90 1,79; 2,01 1,02 0,93; 1,11 10 2,28 2,13; 2,43 1,28 1,16; 1,39 15 2,06 1,92; 2,21 1,13 1,02; 1,24 5 h 2,03 1,89; 2,18 1,12 0,99; 1,25 10 2,28 2,11; 2,45 1,27 1,13; 1,42 15 2,16 2,02; 2,30 1,20 1,08; 1,31 overall 2,06 2,01; 2,12 1,13 1,08; 1,17 a) 95% confidence intervals for parameter a  b) 95% confidence intervals for parameter b  figure 13. 95% confidence interval for a and b.  table 6. values of ρ.  overall hor. 5 m hor. 10 m hor. 15 m vert. 5 m vert. 10 m vert. 15 m 91.2% 88.0% 89.4% 92.6% 92.5% 93.0% 91.5% acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 51  c b rms b    (8) in agreement with [26] and [27]. values of the coherence bandwidth bc versus the rms delay spread τrms are shown in figure 12 together with the linear fit: the log-log representation changes eq. (4) to: log log logc rmsb rms b a b       . (9) in figure 12.a regression has been applied to all available data, while in figure 12.b each combination of polarization and distance has been considered separately. values for a and b, and their 95% confidence intervals are shown in table 5 and in figure 13. values are typically compatible, although the configuration at d = 5 m with vertical polarization does not overlap to the other intervals. the large values of the correlation coefficient ρ reported in table 6 for measurement data in both figure 12.a and figure 12.b show that τrms and bc are strongly correlated and that their relationship is very close to a linear function. 4. conclusion  the characterization of the propagation channel aboard train with a narrow-band methodology has been presented. results show that the propagation law is very close to the free-space one, without significant differences between the propagation along the centerline and off-axis. the delays spread and coherence bandwidth are strongly dependent on the polarization of the propagating signal and on the distance between transmitting and receiving antenna, horizontal polarization showing typically smaller spreads and thus larger coherence bandwidth. however, a detailed statistical analysis shows that variability in the data cannot be explained only by the contribution of polarization and distance, and therefore more factors should be included in the model. this is not unexpected given the very complex propagation environment. finally, the parameters of the model describing bc as a function of τrms typically do not depend on the propagation configuration. references  [1] d. gallo, c. landi, n. pasquino, “an instrument for objective measurement of light flicker,” measurement, vol. 41, no. 3, pp. 334–340, april 2008. 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[27] s. howard, k. pahlavan, “measurement and analysis of the indoor radio channel in the frequency domain,” instrumentation and measure-ment, ieee transactions on, vol. 39, no. 5, pp. 751–755, 1990. acta imeko  december 2014, volume 3, number 4, 52  www.imeko.org    acta imeko | www.imeko.org  december 2014 | volume 3 | number 4 | 52  section: advertisement  article 10 204 template advertisement_hbm microsoft word article 10 194-1076-1-pb.docx acta imeko may 2014, volume 3, number 1, 41 – 46 www.imeko.org acta imeko | www.imeko.org may 2014 | volume 3 | number 1 | 41 impact of modern instrumentation on the system of basic concepts in metrology j.m. jaworski, j. bek, a.j. fiok warsaw university of technology, warsaw, poland section: research paper keywords: metrology, measurement model, error citation: j.m. jaworski, j. bek, a.j. fiok, impact of modern instrumentation on the system of basic concepts in metrology, acta imeko, vol. 3, no. 1, article 10, may 2014, identifier: imeko-acta-03 (2014)-01-10 editor: luca mari, università carlo cattaneo received may 1st, 2014; in final form may 1st, 2014; published may 2014 copyright: © 2014 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited 1. introduction every field of human activity has its own system of basic concepts. if the development of a discipline has been relatively slow and uniform, then its system of basic concepts is complete, well-ordered and well-defined. usually, fast progress in the discipline destroys completeness and orderliness of that system; some concepts lose their sense, the sense of many others has to be changed and there is a need for introduction of new ones. in the last decade the progress in measurement science has been generally very significant and fast, but not uniform. the greatest and most rapid it has been in instrumentation mainly due to: (i) proliferation of measurement needs in all the fields of science and technology, (ii) increase of complexity of measurement tasks to be performed in practice, (iii) great advances in technology, mostly in electronics (microprocessors, lsi and vlsi circuits and computers) which make possible implementation of instruments and systems realizing automatically nearly all physical and mathematical operations which are necessary in measurement practice. new instruments (e.g. dynamic signal analysers, digital oscilloscopes) have introduced in metrology changes of not only quantitative character, but also of qualitative one (new types of measurements e.g. of spectrum and cepstrum) – measurements giving deeper knowledge about the measured object than it was possible before. in the paper the authors have tried to prove that the existing system of basic metrological concepts does not correspond to the contemporary state and expected development of instrumentation and up-to-date level of theoretical knowledge. the new system is necessary in everyday work of all metrologists; designers and users as well as teachers of measurement and instrumentation at all levels of education. as the reference for our considerations we have the “international vocabulary of basic and general terms in metrology” [1] which is one of the latest texts dealing with the system of basic concepts. in the paper general rules of creation of a complete system of basic concepts as well as definitions of the most important new concepts and new definitions of some old concepts have been proposed. these proposals have been based both on the own experience of the authors and on the review of up-to-date technical literature, conference proceedings and catalogues of modern instrumentation. this is a reissue of a paper which appeared in acta imeko acta imeko 1988, proceedings of the proceedings of the 11th triennial world congress of the international measurement confederation (imeko), “instrumentation for the 21st century”, 16-21.10.1988, houston, pp. 125–134. the paper is devoted to the new system of basic concepts in metrology which better fits to the development of contemporary instrumentation and measurement science. the existing concepts are confronted with the current needs; their modifications or new concepts are proposed. in particular the concepts of quantity and measurement method have been broadened and generalised. the proposed system is based on understanding of measurement as an experiment of parameter identification of the model of the measured object. one of the most important new concepts introduced here is the concept of measuring metasystem which comprises not only instrumentation but also the measured object, a “measurement interface” between the object and instrumentation and the operator. acta imeko | www.imeko.org may 2014 | volume 3 | number 1 | 42 2. an outline of a development of instrumentation and measurement methods in the development of measuring instruments and measurement methods three partially overlapping periods may be distinguished. first there was a period of mechanical instruments and direct-comparison methods of measurement. the overwhelming majority of instruments were deflection type e.g. pressure gauge, rotameter, liquid thermometer, electromechanical voltmeter. typical measurements by direct comparison methods were measuring of length by means of a gauge, of angle by means of a protractor, of voltage by means of a potentiometer or measurement of resistance by means of an electrical bridge. in the second period electrical and electronic instruments and conversion-based measurement methods were the dominant ones. measuring transducers enabled one to measure non-electrical quantities using electrical methods. electronic instruments became the most important in measurement at first of electrical and next also other quantities. in the culmination point of that era digital instruments were introduced into everyday use. the third stage of instrumentation development, closely connected with the “electronic” one, is the stage of computerization. the dominant measurement methods are indirect ones based on computerized signal processing and calculations. the computer becomes the “heart” of a measuring instrument or a system; it controls the whole measurement process and processes the data carried by the signals from the sensors. the function of traditional readout devices has been taken over by monitors. knowledge about designing measurement algorithms and computer programming becomes of major importance for anybody who wants to measure. 3. contemporary tasks of metrology in classical metrology a measurement has been mainly treated as an experimental determination of the value of a quantity. but in everyday practice we are mainly interested in obtaining a quantitative information about the state or properties of the given object, while a physical quantity accessible for instruments is only a carrier of the required information. the modern instrumentation offers new technical facilities enabling us to extract this information. to achieve it, the tasks of modern metrology have been extended in comparison to the classical one. contemporary measurements are aimed at determining not only the value of a quantity but [2, 3] also: – variation of a physical quantity with time, e.g. its temporal distribution or “waveform”, – distribution of a quantity in space, e.g. in measurement of surface geometry, – mathematical representations of quantities, e.g. spectral density, – relations between quantities, e.g. current-voltage characteristic of an electric element, their distributions or representations, – parameters of such relations, e.g. attenuation. moreover, in many cases we do now measurements which differ from the traditional ones in their basic general character. to such “new types” of measurements belong: (i) complex concurrent measurements of several interrelated quantities which through the relations between these quantities characterize the state and properties of the investigated object; this aspect of contemporary measurements is sometimes called “complexity of measurement”, (ii) measurements in which obtaining of information about the investigated object requires exciting it with an appropriate stimulus and sensing a response signal [2, 3, 6]; such measurements, common in investigation of many electrical, mechanical, optical and physico-chemical properties of great variety of measured objects can be called “active” measurements, (iii) measurements which include operations typical for diagnostics, quality control, image recognition, model identification, a.s.o. 4. assumptions of the proposed system of basic concepts of metrology in the result of analysis of the contemporary measurements we decided to propose a new system of basic concepts in metrology. let us mention some important assumptions of the proposed system. structure of the system of basic concepts in a particular science should correspond with the structure of this science. we have assumed that [2, 5] metrology or using other words – science of measurement and instrumentation – can be divided into four parts: theory, technique, instrumentation and legislation. theory of metrology (we shall call it measurement theory) is the system of general laws of metrology. technique in metrology (we shall call it measuring technique) includes the knowledge concerning purposeful, usually theorybased, activity connected with planning, organization and execution of measurement as well as evaluation, verification and interpretation of measurement results. instrumentation connotes all the knowledge on measuring instruments, systems end devices. legislation (i.e. legal metrology) is a system of law regulations concerning measurements. we proposed to group the basic concepts into six parts: 1) fundamentals of metrology 2) measurement and its component operations 3) errors 4) measuring instruments and their properties 5) measuring technique 6) legal metrology up to now our works have been concentrated on the first three parts and the fifth one. so the concepts dealing with measuring instruments and legal metrology have not been considered in this paper. the proposed system covers in principle the concepts connected with measurements in exact sciences (first of all in physics and technology). for other fields like biology, psychology or economics, perhaps some modifications of the system may be needed. our considerations have been aimed at the system of concepts. the concepts are of primary importance; the terms (names of concepts) are also of importance, but in the stage reported here, our work has been concentrated on the concepts, not on the names of them. of course it has been necessary to use names, but in a lot of cases they are only tentative ones; in some cases no existing word fits with the needed meaning, so we have tried to propose neologisms. the classical approach to the metrology has assumed that the subject of measurement is a physical quantity [1]. the basic assumption of our approach is that the subject of measurement acta imeko | www.imeko.org may 2014 | volume 3 | number 1 | 43 is a physical object and measurement is treated as an experiment of parameter identification of the model of the measured object [2, 3, 5, 6]. 5. measured object, its model, quantities classical metrology defines [1] measurement as “the set of operations having the object of determining the value of a quantity” and quantity as “an attribute of phenomenon, body or substance, which may be distinguished qualitatively and determined quantitatively”. many metrologists consider quantities as real entities being the objects of measurement or experiment. in fact, quantity understood in classical sense is an abstract concept defined for idealized (abstract) objects, different from real objects which are to be measured. in our opinion no measurement can be separated from a part of objective reality – a physical object (called measured object) whose chosen properties are to be quantitatively determined in effect of the measurement. a quantitative determination is a determination by means of mathematical categories. the categories describing properties of the object must be exactly defined. to do it we must choose, assume or create a mathematical model of the measured object. measurement-oriented mathematical model of the measured object is a mathematical or equivalent (e.g. circuit-type) description of all its properties and relations between them which are relevant to the measurement task. the concept of quantity is and will remain one of primary concepts in metrology and in mathematical modelling of physical objects. we propose only to broaden and generalize this basic concept. our approach takes into account that: (i) generally, quantity itself is not a property of real objects; it only models its particular property within bounds of the assumed mathematical model of the object, (ii) temporal and/or space aspects of object properties can be of major importance, (iii) it is necessary to distinguish clearly different concepts named with the same term “quantity”. in classical sense quantities are defined as families of some properties of idealized entities abstracted from relations of equivalence – verifiable by idealized experiments. for instance, ajdukiewicz [8] gives the definition of length: “length is a family of ideal properties abstracted from the relation of congruence of line sections”. the definition of quantity cited at the beginning of this clause given in [1], common in classical metrology, has the same character and fails in many practical measurement situations. for example it is not too easy to utilize the above definition of length, very good for “measurement” of abstract object (line section), in measurement of real 3dimensional object as a cylinder. real objects are always “imperfect” in comparison with abstract objects for which physical quantities (in classical meaning) have been defined. in our opinion quantities should be treated as elements of assumed mathematical model of measured objects. in this sense quantities model elementary properties of the object and of phenomena in it. to distinguish clearly two meanings of the term, we will call quantities being part of the model of real object “modelling quantities” in contrary to those defined for abstract objects called by us (it necessary) “abstract quantities”. mathematical model of the object consists in general of: (i) set of modelling quantities, (ii) set of equations which model relations between properties of objects and phenomena in it (we shall call them model equations). sometimes model may consist of modelling quantities only. the set of modelling quantities and their mathematical character and type of model equations give the structure of the model. the coefficients of model equations are parameters of the model. it is possible to generalize the concept of model parameter as in [6]. the mathematical model of a physical object is always an approximation; if accuracy of the model is high (for the particular object measured for a given aim in given conditions) we may ignore this fact. but in some cases errors of this approximation can be of major importance from the point of view of accuracy of the whole measurement; sometimes the measurement results may be even completely without sense due to assumption of oversimplified model e.g. one dimensional (when measuring the diameter of a cylinder) or linear (when measuring the capacity of ferroelectric condenser in the range of high voltages). for a particular class of objects it is possible to construct or choose a series of models of different accuracy. the utilized model should be optimized from the point of view of the particular measurement task: as simple as possible to make measurement easier and cheaper; as complicated as necessary to include all the factors which may preclude obtaining reliable measurement results of sufficient accuracy. different meanings of the term “quantity” which should be distinguished are as follows: (i) quantity as a property of abstract objects (abstract quantity), (ii) manifestation of a quantity (in an abstract sense) (perhaps it would be better to call it manifestation of a property [9]) i.e. a specific state of property; if two objects are equivalent in the sense of the property considered, they give the same manifestation of this property; the property can be treated as a set of manifestations and relations between them, (iii) mathematical model of a quantity, (iv) values of quantity manifestations, i.e. mathematical categories (e.g. numbers) which are mapping manifestations of the quantity; the mathematical model of a quantity is a set of these categories and relations between them (mapping relations between manifestations). unfortunately these different concepts are usually not distinguished by different terms; the term “quantity” is commonly used for the first three concepts and term “value of a quantity” for the concepts (ii) and (iv). the world of physical quantities (treated as properties of physical objects) given by the classical metrological system of quantities is very simplified; it neglects temporal and spatial aspects of properties of objects. the quantities defined in this world do not depend on the time and space coordinates; we shall call them the static-lumped quantities. such concept of static lumped quantity should be generalized. we propose to introduce concepts of a staticdistributed quantity, a dynamic-lumped quantity and a dynamicdistributed quantity. such generalized quantities are also attributes of objects. their manifestations are mapped into different mathematical categories. images of manifestations (we shall call them values of a quantity, spatial distributions of a quantity, temporal distributions and spatio-temporal distributions) are real numbers, functions of the spatial coordinates, functions of time and functions of time and spatial coordinates. acta imeko | www.imeko.org may 2014 | volume 3 | number 1 | 44 the next step in generalization of quantity is concept of the mathematical representation of quantity which is a transformation of quantity values or distributions into the domain of certain mathematical categories. signal energy, fourier transform, autocorrelation function are typical examples of representations. our system of basic concepts defines also concepts of: measurement scale, additive quantity, interval quantity, vector quantity, unit of quantity, system of quantities, system of units, basic quantity etc. 6. measurement and its component operations classical definition of measurement [1] is: “the set of operations having the object of determining the value of a quantity”. the quantity in the definition is “static lumped”, its values are real numbers. we would like to change the definition to: “measurement is the experiment carried out upon a physical object and aimed at determination of: (i) values or distributions of quantities modelling an object, (ii) values of mathematical representations of quantities modelling an object, (iii) relations between quantities modelling an object or between representations of these quantities”. the term “quantity” used in the proposed definition means a quantity in generalized sense. quantities, their representations, relations between quantities or representations (the relations may be given by parameters of the standard equations), mentioned in definition, are measurands. the fact of great importance for practice is that “values” of these measurands may be not only real numbers but also other mathematical categories as complex numbers, functions of any kinds, sets of numbers, etc. the definition proposed above is closer to “identification” concept of measurement then the classical one. if we only broaden the meaning of parameters of model, treating the values and distributions of quantities or their representations (if they are measurands) and the specific forms of relations between them as values of parameters of the model, then we can define measurement as “an experiment of parameter identification of mathematical model of an object to be measured” [6, 7]. this approach to measurement has many theoretical and practical consequences. the most important ones of them seem to be [2]: (i) widespread interpretation of measurement as a reliable method of objective cognition of reality is no more strictly valid. by means of measurement we investigate reality but only within the bounds of accepted structures of its mathematical models. (ii) the parameter identification approach to the measurement enables one to reinterpret the problem of “true value” as an absolute aim and reference of measurement. (iii) this approach to measurement entails relevant changes in measuring technique, what will be discussed later on. in the classical model of measurement shown in figure l measurement is considered as consisting mainly of operation of comparison of the measured quantity (or a quantity dependent on the measured quantity) with the standard quantity of known value. much less attention has been paid to other operations: sensing of the signal from the measured object (in most cases the concept of measurement signal was not utilized or the signal was treated as the same as the measured quantity itself), conversion (in most cases aimed on signal conditioning) and display of measurement result. in engineering practice measurements of complex objects aimed at determination of many measurands are very often executed at present and in most of cases the measurands are not directly accessible for measuring instruments. one of many examples are measurements of magnetic materials. contemporary measuring instruments and systems require use of a modified model of measurement. the model shown in figure 2 seems to describe well modern measurements [7]. the “inputs” of a measuring instrument (or a system) are measurement signals generated by the measured object or exciting the object. exciting (test) signals, even if generated within the instrument, are from the informative point of view the inputs for the instrument. the measurement signals are in general multidimensional ones. the measured object is described by its mathematical model. the structure of the model (the set of modelling quantities and the type of model equations) is given a priori, the model parameters (in the generalized sense) are the measurands. the measuring system accomplishes first of all operations of sensing and comparison. the operation of sensing is often associated with operation of actuating the measured object with the appropriate stimulus (test signal). the other basic operations in the system are: signal and information processing (enhanced version of classical conversion covering all computerized data processing) and formation of an output signal whose particular case is visualization of the results. the purpose of signal and information processing is the determination of measurands based on the information carried by the measurement signals. to accomplish this task the relations between the measurands and the measurement signals must be strictly determined what is equivalent to the acceptance of the model of the measurement interface between the object and the system. both the structure of the object model and the structure figure 2. model proposed for modern measurements and illustration of the concept “measuring metasystem”. figure 1. classical model of the measurement. acta imeko | www.imeko.org may 2014 | volume 3 | number 1 | 45 and parameters of an interface model are decided on the base of a-priori knowledge about the object and the system. the same is true in relation to the model of measuring system. in fact each description of metrological properties of measuring instrument or system of instruments is its model. mathematical model of classical instrument was sometimes so simple that in some cases it was possible to give it on the front panel of the instrument. model of contemporary instrument is more complex, it needs many pages of the text to be given. the structure and parameters of the model of measuring instruments or system must be known to its user before the execution of the measurement. the classical metrology has put the main stress on the operation of comparison; measurement methods have been distinguished mainly from the point of view of the way of comparison used (e.g. direct-comparison method, substitution method and null-method [1]). our system of basic concepts saves these concepts but takes into account that the measurement comprises also other operations and the measurement method is determined not only by the method of comparison but also by the methods of signal sensing, signal conversion as well as the method(s) of signal and information processing. each operation utilized in the measurement can have “own” methods of realization of its task and some of them should be included in the system of concepts. functioning of contemporary instruments, especially computerized and intelligent ones, is based on sensing and processing of signals. classical metrology paid a very little attention to informative and signal aspects of measurement. so it is necessary to define – from the point of view of metrology – several general concepts (e.g. measurement information, measurement signal, parameters of a measurement signal) as well as some specific concepts connected with forms of measurement signal, types of carried information, types of signal conversion etc. the classical model of measurement shown in figure 1 is a particular, very simplified case of the proposed more general model which was discussed above. in the classical model of measurement the model of measured object is reduced to the quantity modelling the object. 7. errors the insufficiency of the set of concepts connected with errors in measurement considered by classical metrology seems to be now evident. the classical error theory deals only with the inaccuracy of measurements whose results are real numbers. results of contemporary measurements may be also complex numbers, series of real numbers or complex ones, functions (both real and complex ones) etc., as well as parameters of functions or equations. the new set of concepts should also refer to such situations. each operation accomplished in a measuring system is associated with errors, because some of these operations were not considered by classical metrology, these errors have not been considered. for our system of basic concepts new concepts in the error theory have been needed. an attempt to formulate a broadened and modified system of concepts of error theory has been presented in [4]. the most important elements of the system are: 1) the word “error” has two meanings: – an event of discrepancy between two entities compared, the first one being the reference for the other, the other being the realization of the first one, – a mathematical model of the discrepancy which represents it in mathematical categories. 2) in metrology, compared entities (measurands, signal parameters) are mathematical categories or have their own mathematical models so that the error is determined by comparison of mathematical categories in a proper space of compared categories (of e.g. complex numbers, functions of given type etc.). there are two main kinds of errors: – difference error defined by means of algebraic difference between categories compared e.g. “classical” absolute and relative errors of measurement [1], – metric error defined by means of “the distance” (in mathematical sense) between categories compared e.g. mean square error as a measure of a discrepancy between two functions. errors in the domain of real numbers remain very important; their field of application is broad and in many cases it is possible to reduce other types of errors to this type. 3) two kinds of errors as mathematical models must be strictly distinguished: – true error (more precisely conventionally true error) defined as mathematical model of discrepancy between two categories compared, – boundary error defined as the parameter (or parameters) determining a subspace (in space of categories compared) which surrounds one of the category compared and contains the other one (or for which there is a probability large enough that it may contain the other category). in classical metrology reference is a true value; in metrology based on identification concept of measurement in many cases reference can be given by a model more exact than the used one. 4) set of errors distinguished in classical metrology should be broadened. among others there is a need to take into account also: – error of the model of measured object, – error of the model of interface between the object and the measuring system, – error of data processing. 8. measuring technique measuring technique is understood as the system of reasonable and theoretically based methods of carrying out measurement. our system of basic concepts connected with measuring technique bases on two concepts: measuring process and measuring metasystem [4]. measuring process is a connected series of actions and operations deliberately undertaken to carry out given measurement. up to now the concept of measuring system has been limited to “a complete set of measuring instruments and other equipment assembled to carry out a specified measurement task” [1]. we would like to broaden the concept to embrace (figure 2) all the things and ideas needed to carry out given measurement i.e.: (i) object to be measured, (ii) mathematical model of the object, exactly the structure of model and definitions of its parameters which are measurands for measurement, (iii) set of measuring instruments, its mathematical model and programs (instrumental part of the system), acta imeko | www.imeko.org may 2014 | volume 3 | number 1 | 46 (iv) mathematical model of the interface object – instrumental part of the system, (v) program of measurement, (vi) operator. to avoid confusion of the new meaning of the term “measuring system” with the traditional one, we propose to introduce the new term “measuring metasystem” for the measuring system understood in the new broadened sense. the instrumental part of the metasystem might be called, as before “measuring system”. measuring process has some typical stages. we can group them in three main parts of this process: – preparation of measurement, – execution of the measurement, – postprocessing of measurement results. the object to be measured is an operand for measurement. specification of the measurement is a starting point of measuring process. the specification above all consists of: – mathematical model of the object to be measured, exactly the structure of model and definitions of measurands, – permissible errors of measurement, – permissible costs and time of the measurement. in typical situation specification is given a priori but in practice it may be necessary to correct elements of specification. the main parts of measuring process may be subdivided, e.g. the first part, preparation of measurement, into: – creation of the conception of measurement (fixing model of interface object-instruments, choosing the methods of signal sensing, conversion and processing) – design of measuring system and of measurement program, – implementation of measuring system, – making the measuring system ready for proper functioning (including the identification of the mathematical model and errors of the system). the characteristic feature of the modern measuring process seems to be a need of validation and correction of partial results. the effect of each operation in measuring process has to be examined from the point of view of its validity and congruency with the preliminary specification. the needed correction of effect of each operation may be accomplished by a proper modification of this operation and, if necessary, of the previous ones. in a limit case even a change of elements of the measurements specification (e.g. of the object model or of requirements dealing with errors or costs) may be necessary. the broadening of the concept of measuring system is necessary not from the theoretical but from the very practical point of view. it is an effect of the real situation existing in contemporary measurements. it is now impossible to separate hardware from software in modern instrumentation and an interaction (both in hardware and software sense) between the measured object and the measuring system is of major importance. 9. summary and conclusions let us underline once more that the aim of our work reported here is rather practical; we would like to work out the theoretical tools which could be useful for designers and users of modern instrumentation and would facilitate their everyday work. we did not intend to present in this paper a complete system of basic concepts in metrology. it is impossible because creation of a new system is not a trivial task; it requires much effort, profound discussions with the outstanding experts in measurement science and instrumentation. the work has not been completed yet. we presented here only some proposals concerning the scope and the selected most important concepts connected with chosen parts of the system. your valuable opinion will be of major importance for our further work. the most important elements of our approach to the system of basic concepts in metrology are: – observation that the aim of measurement is to obtain not the value of an abstract quantity but an image of the measured real object mapping its properties by means of proper mathematical categories, – conclusion that one of the first stages of the measurement process must be the choice of the mathematical model of the measured object, – treating the measurement as an experiment of parameter identification of the model of the measured object, – generalization of the concept of quantity, – introduction of the concept of measuring metasystem which consists not only of instrumentation but also of: the measured object, the measurement interface between the object and instruments, the whole “software” (in a very broad sense) and the operator, – understanding the major importance of parts of measuring process prior to the execution of the measurement and of the validation of effects of each component operation and, if necessary, corrections of them. we hope we have proved that contemporary measurements need new system of basic concepts. the system reported in the paper is one of possible proposals. acknowledgements the authors would like to express their gratitude to many colleagues – metrologists whose views presented in various discussions seriously influenced the presented work. special thanks to prof. roman morawaki whose opinions and help in preparation of the final version of this paper were of great importance. references [1] international vocabulary of basic and general terms in metrology, bipm, iec, iso, oiml, 1983. [2] a.j. fiok, j.m. jaworski, r.z. morawski, j.s. oledzki, a.c. urban,theory of measurement in teaching metrology at the engineering faculties, proc. 8th int. imeko—tc1 colloquium on higher education, warsaw, 1986. [3] a.j. fiok, j.m. jaworski, what to teach in measurement and instrumentation, 8th int. imeko—tc1 colloquium on higher education, warsaw, 1986. [4] j.m. jaworski, j. bek, theory of errors in metrology. an attempt to formulate the system of basic concepts, 10th imeko congress, prague, 1985, vol. 1, pp. 124–130. [5] j. bek, a.j. fiok, j.m. jaworski, j.s. oledzki, structure of metrology and its system of basic concepts, pomiary, automatyka, kontrola, 1987, no. 8, pp. 175–177 (in polish). [6] j.m. jaworski, a new parameter model of measurement. proc. 5th int. imeko—tc7 symposium on intelligent measurement (ed. by d.hofmann), jena, 1986, vol. 2, pp. 250–252. [7] j.m. jaworski, errors, noise and disturbances in measuring system, 1st int. imeko—tc4 symposium on measurement, como (italy) 1986, pp. 27–32. [8] k. ajdukiewicz, pragmatic logic, pwn, warszawa 1985 (in polish). [9] l. finkelstein, theory and philosophy of measurement, chapter 1 in: handbook of measurement science ed. by p.sydenham, j.wiley, chichester, new york 1982. microsoft word article 5 156-1030-1-le.docx acta imeko  february 2015, volume 4, number 1, 19 – 25  www.imeko.org    acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 19  development, implementation and characterization of a dsp  based data acquisition system with on‐board processing   pedro m. pinto 1 , josé gouveia 2 , pedro m. ramos 3   1  instituto superior técnico/universidade de lisboa, av. rovisco pais 1, 1049‐001 lisbon, portugal   2  instituto de telecomunicações, av. rovisco pais 1, 1049‐001 lisbon, portugal   3  instituto de telecomunicações and instituto superior técnico/universidade de lisboa, av. rovisco pais 1, 1049‐001 lisbon, portugal      section: research paper   keywords: data acquisition; digital signal processing; on‐board processing  citation: pedro m. pinto, josé gouveia, pedro m. ramos,  development, implementation and characterization of a dsp based data acquisition system with  on‐board processing, acta imeko, vol. 4, no. 1, article 5, february 2015, identifier: imeko‐acta‐04 (2015)‐01‐05  editor: paolo carbone, university of perugia   received december 9 th , 2013; in final form april 13 th , 2014; published february 2015  copyright: © 2014 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: work supported by fundação para a ciência e tecnologia, portugal  corresponding author: pedro m. ramos, e‐mail: pedro.m.ramos@tecnico.ulisboa.pt    1. introduction  the development of analog to digital converters (adcs) was the cornerstone of a massive shift in the architectures of measuring systems and devices. in fact, the rapid development of fast adcs capable of achieving resolutions above 10-12 bit has revolutionized the instrumentation and measurement field and can be considered as one of the crucial building blocks to our worldwide digital content driven society. the possibility to measure just about anything, sometimes with extremely reduced costs, has increased many times the deployment of sensor based measurement systems as well as the total amount of acquired data, the rapid development of signal processing algorithms to extract real-time information of the raw data, the use of data mining to analyze the results [1], sensor fusion to combine the measurements from multiple sources [2]. a data acquisition system is typically a system or device that can digitize an input electrical quantity at a given sampling rate. it can have multiple input channels with multiple or single input ranges. by far, most commercial systems are designed to acquire voltages. for multi-channel devices, the cheaper solution is to have a single adc and sequentially connect (using a multiplexer) the input signals to the adc input, thus enabling the acquisition of multiple channels with only one adc. this solution, well suited for many applications [3] and frequently implemented even within multipurpose micro-controllers, reduces cost but is incapable of simultaneous acquisition and the inter-channel interference (cross-talk) is usually non-neglectable. the main alternative is to have a independent adc for each channel and thus a unique signal path to the adc for each input channel. this solution is more expensive but can have lower cross-talk and can perform simultaneous acquisition. abstract  this paper presents the development, implementation and characterization of a data acquisition (daq) system capable of on‐board  processing  the  acquired  data.  the  system  features  four  differential  channels,  with  1  mhz  bandwidth,  simultaneous  acquisition,  9  independent  bipolar  ranges,  and  a  maximum  sampling  rate  of  600  ks/s.  the  analog  daq  inputs  are  protected  against  incorrect  connections even direct connection to the power grid voltage. this protection ensures that the daq can recover to full operation  without the need to replace any damaged components or fuses. a 450 mhz sharc digital signal processor (adsp 21489) is used to  control  the  system  and  perform  on‐board  processing.  interface  between  the  system  and  a  personal  computer  is  through  a  usb  hi‐speed connection.  acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 20  commercial of the shelf (cots) data acquisition devices are used together with a computer or in some sort of specialized integrated system such as vxi [4], pxi [5], vme [6], axie [7] or compactrio [8]. the main advantage of systems that are not computer based is that stricter timing issues and higher throughput can be achieved in those architectures which were specifically developed to deal with the specificity of measurements systems. computer based systems are either pci or usb based. while the former are used in desktop computers, the latter are more flexible and can also be used in portable computers. however, the throughput when using usb connections can limit the range of the combined set of number of acquired samples, sampling rate and adc number of bits. lan based devices (of which lxi [9] is a particular case) are more frequently available but their usage as not reached the initially predicted dissemination. data acquisition systems are currently the most important blocks in just about every measurement system. for example, they are used in power quality monitoring [10, 11], in impedance measurement and spectroscopy devices [12], in the characterization of energy-harvesting devices [13], in environment monitoring [14], in real-time reflectometry diagnostic [15], in fusion experiments [16], in power metering [17] and many other applications. most cots data acquisition devices include some sort of micro-controller or processing unit that manages the communication with the adcs, controls the input range and communicates with the system main processor (e.g., the computer). for applications where the signal processing is done in the computer, this local processor acts mainly as a gateway with little or no processing tasks executed in the actual system device (see for example, [18]). however, in some applications, processing in the acquisition system is required. the reason behind this requirement is for example in large measurement systems with multiple channels, some data reduction must be made before the data can be managed and processed in a timely manner. for example, in power quality monitoring or power quality qos (quality of service), the measurement nodes must acquire the samples continuously without interruption. this constitutes a challenge since sending the raw data to a computer or central processing unit, for processing is hardly possible when the grid can have hundreds of such devices spread out in a large area. collecting all the data in one or more data centers and still achieve real-time processing is not feasible. in this situation, the best approach is to have each node with a processor capable of performing digital signal processing to detect events (in power quality monitoring) and store/transmit only the data of such events. in power quality qos, the nodes should quantify the qos in terms defined by the local power regulator and transmit only the final aggregate parameters at a given periodic interval. another system example that requires processing near the acquisition level are the detectors in the large hadron collider (lhc). the atlas detector [19] has about 90 million channels which requires multiple customized trigger levels and processing algorithms to reduce the amount of data that it collects so as to discard non-relevant or non-events and focus centralized storing and processing on the most promising data. the goal of this work is to develop, implement and characterize a 16-bit four-channel multi-range simultaneous data acquisition system with on-board signal processing capabilities. to achieve this goal, a digital signal processor is used to control the entire system and perform the on-board processing. the system has to be able to execute simultaneous acquisitions at a maximum rate of 600 ks/s and have an analog bandwidth of 1 mhz. multiple independent voltage ranges must be available to allow accurate measurement of signals with different dynamic ranges. the input channels are acquired differentially with impedance at low frequencies of 1 mω (single-ended) and 2 mω (for differential acquisitions) and must withstand a considerable level of incorrect usage (i.e., incorrect direct connection to the power grid) without damaging the system circuitry and still ensure operability (this means that no fuses should be used). 2. system architecture  generically, a data acquisition system can be divided in three blocks. in the first block, the signal conditioning circuit adapts the input signal before digitalization. this includes attenuation or amplification to account for different input ranges. another important function of the conditioning circuit is to protect the system against nonideal conditions, like overload, incorrect usage or electrostatic discharge (esd). in the second block, after the conditioning circuit, the analog to digital converter (adc) digitalizes the signal. in the third block, to control the spi2 spi b sdram adsp-21489 serial/parallel converter adc dac input protection ia switch 3 adc dac input protection ia switch 4 spi spi spi spi spi serial-parallel converter adc dac input protection ia switch 1 adc dac input protection ia switch 2 spi spi spi spi spi spi a usb device figure 1. system architecture.  acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 21  system a microcontroller is used. it can be a pic peripheral interface controller [18], dsp digital signal processor [20], arm [21] or fpga field programmable gate array [22, 23]. this device is responsible for the configuration of the range of each channel, by programming the amplifiers’ gain, controlling the sampling frequency, collecting and transferring the sampled data. in some cases, the system has the ability to process data and control other systems, like actuators for example. in the system described in this paper, the control unit must be able to perform on-board processing such as fft calculation for example and so, a dsp was selected. in figure 1, the proposed system architecture is shown. the system has four identical input analog channels, with independent gain setting and offset compensation, and individual adcs. in addition to the described modules, the system has a 16 mbit flash module to store the dsp program which is loaded upon reset or booting. a dsp external memory module (sdram with 256 mb capacity in 16-bit words) was added to increase storage capacity and further increase flexibility in the processing of large amounts of data. usb 2.0 hi-speed is used in the communication with an external computer. in the system, it is implemented using a ft2232h device. the speed of this interface limits the maximum sampling rate in continuous acquisition mode, when all the samples are to be transferred to the computer. 2.1. analog interface circuitry  the input attenuation/protection circuit is shown in figure 2. its purpose is to attenuate the input signals 5 times and to protect the amplifiers, the adcs and the dsp from incorrect usage [24]. it is dimensioned to sustain input direct connection into the power grid without any system damage and complete operability afterwards. this means that the user can incorrectly connect the system inputs into the power grid without damaging the device. note that, the system will only operate correctly with input voltages up to ±10 v. the purpose of the input attenuation/protection circuit is so that the system is not damaged if it is incorrectly used (up to the power grid voltage). therefore, the maximum operating range is ±10 v and the absolute maximum rating input voltage is ±325 v ( 230 2 v ). the set r2, r3, cvar and the equivalent input capacitance from r3 onward, form the basis of a compensated attenuator as is traditional for instance in the analog input attenuation of oscilloscopes. the schottky diodes (d1) are included to protect over voltages from reaching the programmable amplifiers. r1 and r4 are included to ensure that during transient connection, and while the capacitors are discharged, the maximum current in the protection diodes do not cause their failure. although the inclusion of r1 (as shown in the equivalent circuit for single-ended acquisitions represented in figure 3 where cd are the equivalent capacitance of the diodes and c2 is the combined input capacitance of ia1 and of the offset compensation controlled switch) makes the voltage divider uncompensated, its value is dimensioned so that the extra poles (located at 10 mhz and 887 mhz) and extra zero (located at 796 mhz) are at frequencies well above the specified target input analog bandwidth (1 mhz). note that the value of r1 is selected as a compromise between the location of the lowest pole (at 10 mhz) and the current limit of the diodes. with this set of parameters, the amplitude response of the input attenuator (including r1 and the influence of the diodes) remains essentially constant until at least 1 mhz. the frequency response of the circuit was simulated and the amplitude and phase response are shown in figure 4. the dc gain of -14 db corresponds to the 1/5 gain desired for the input attenuator and the first pole is located at 10 mhz. the phase does not reach -90º because, at 100 mhz, the influence of the other pole and the zero is already changing the initial 1st order low pass filter like response. as shown in figure 2, two instrumentation amplifiers (ia) are used in each channel. each of the ia are ad8250 var c inv var c figure 2. analog signal conditioning circuit.  figure 3. equivalent analog input circuitry for single‐ended acquisitions.  figure 4. simulation results of the gain of the analog input circuit before the  adc.  10 1 10 3 10 5 10 7 10 9 -60 -40 -20 0 frequency [hz] g a in [ d b ] -90 -60 -30 0 p h a se [ º] gain phase acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 22  with programmable gains of 1, 2, 5 and 10, slew-rate of 20 v/µs and cmrr of 80 db. the gain combinations from the two amplifiers enable the implementation of the nine input ranges listed in table 1. note that, for all ranges, the input signal is always attenuated five times by the input resistive divisor (see figure 2). for example a ±1 v input sine signal becomes a ±0.2 v sine signal after the fixed gain attenuator and, in the ±1 v input range, is amplified 10 times (fourth column of table 1) to become a ±2 v sine signal. note that, in the last ia a 2.048 v dc component is added before the adc converts the signal into the digital domain. although it is counterproductive to attenuate a signal and then amplify it, this solution ensures the level of protection against misuse of the device that the system is capable of withstanding. each ia gain is set by the dsp using a spi connection and a serial/parallel converter (one for each channel pair) as shown in figure 5 and as it is directly related with the input range desired by the user. the ia gains are set before acquisition starts which means that this process is not time critical as the range is not changed during an acquisition. therefore, the use of a slow serial to parallel converter is well suited as it also reduces the usage of i/o ports of the dsp. three bits are used for each ia (two for the gain bits and one for the write enable). 2.2. conversion into the digital domain  the selected adcs (analog devices ad7980) are 16-bit sar with maximum sampling rate of 1 ms/s, unipolar input voltage, -3 db input bandwidth of 10 mhz, spi interface with the ability of operation in daisy-chain configuration (as shown in figure 6) to share a spi connection. note that, due to the method of operation of the selected sar adcs, no sample and hold is required. the adc range is set to [0 ; 4.096] v and as mentioned, the vref input of the final ia (see figure 2) is used to add the dc component (2.048 v) necessary to ensure a unipolar signal at the adc inputs. in the digital domain, this fixed value dc component is removed, and the gain introduced in the analog amplification stage is removed from the adc output as each 16-bit sample is converted into the voltage at the channel input taking also into account the gain introduced by the ias. 2.3. final prototype implementation  the final prototype implemented in a printed circuit board (pcb) is shown in figure 7. in the bottom part of the picture, the four bnc connectors for each channel are visible. in the top middle section of the pcb is the dsp figure 5. detail of gain programming stage.   figure 6. adc daisy‐chain connection. each pair of adcs shares the same spi  connection to the dsp.  figure 7. implemented system. the final pcb has 145 mm length and 75 mm  width.  table 1. list of input ranges and corresponding gains of the instrumentation  amplifiers and total analog gain (including also the input attenuator).  input  range  ia 1  gain  ia 2  gain  ia total  gain  total  gain  ±0.1 v  10  10  100  20  ±0.2 v  10  5  50  10  ±0.4 v  5  5  25  5  ±0.5 v  10  2  20  4  ±1 v  5  2  10  2  ±2 v  5  1  5  1  ±2.5 v  2  2  4  0.8  ±5 v  2  1  2  0.4  ±10 v  1  1  1  0.2    adsp-21489 spi serial/parallel converter (mcp23s18) input protection ia input protection adc ia gnd input protection ia input protection adc ia gnd 3 3 3 3 a0 – a1 wren a0 – a1 wren a0 – a1 wren a0 – a1 wren channel a channel a channel b channel b acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 23  while in the left-hand side is the usb communication integrated circuit (the usb connector is just to the left of the ic in the board edge). the system jtag connector is located in the top center of the pcb. note that, once the system 16 mbit flash module is programmed, the jtag is no longer needed since, upon restart, the dsp program is loaded from the flash and executed. the jtag is used for loading new programs, for testing and for programming the flash once the final firmware is ready. 3. system characterization  the frequency response of the acquisition channel (including the analog circuitry and the adc) was measured with a tti tg1010a function generator controlled with ieee 488.2 and an application specifically developed in labview to control the function generator, set the channel input range, retrieve the daq samples and process the results. the processing stage includes the estimation of the measured signal amplitude using a simple, singlechannel sine-fitting algorithm [25]. this characterization application then changes the stimulus frequency and automatically repeats the process for all measurement frequencies (selectable in the labview front panel) and all daq input ranges (also changing the signal generator output amplitude to match 95% of the dynamic input range for each daq range). the final results for one of the input channels and for all the input ranges are shown in figure 8. it can be seen that the overall input channel bandwidth is limited by the response of the lowest range and corresponds to about 2 mhz. however, a slight distortion was measured for signals with frequencies above 1.6 mhz due to the slew-rate of the amplifiers. nevertheless, these limits ensure that the specified 1 mhz input signal bandwidth, for all ranges, is achieved. the results for the remaining three channels are similar. the delay/rising time response of one of the channels (before the adc) was measured with a tti tg1010a function generator and a 350 mhz tds5034 oscilloscope. the results are depicted in figure 9. the rise time is 57.4 ns while the delay is 112 ns. as will be shown in the simultaneous acquisition measurement, these values are within the order of magnitude of the interchannel delay and therefore are considered acceptable. note that the main cause of the rise time and delay are the capacitors used in the analog input compensated attenuator cvar and of the equivalent capacitance c2. to characterize the inter-channel cross-talk, a sine signal near the full-input range of the daq is applied to one channel while the others are short-circuited. the ratio between the generated sine amplitude and the amplitude measured at that same frequency in the other channels is a quantification of the inter-channel cross-talk. to improve the estimation of the cross-talk value, multiple acquisitions are performed and the average spectrums are used. in figure figure  8.  input  frequency  responses  for  all  ranges  in  one  channel.  the  results for the other channels are identical.  figure 9. delay/rising time measurement.  figure  10.  measured  average  spectrum  cross‐talk  measurements  for  91.5523 khz.  the  top  average  spectrum  represents  the  input  sinusoidal  signal while the bottom average spectrum is the one obtained on the non‐ stimulated channel.  10 2 10 3 10 4 10 5 10 6 10 7 -20 -10 0 10 20 30 frequency [hz] g a in [ d b ] ±0.1 v ±0.2 v ±0.4 v ±0.5 v ±1 v ±2 v ±2.5 v ±5 v ±10 v 90 91 92 93 94 95 96 97 98 99 100 -100 -80 -60 -40 -20 0 20 frequency [khz] s p e ct ru m [ d b ] 90 91 92 93 94 95 96 97 98 99 100 -100 -80 -60 -40 -20 0 20 frequency [khz] s p e ct ru m [ d b ] acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 24  10, the results from one of the cross-talk measurements are presented. the stimulus signal is a 91.5523 khz sinusoidal signal. in this case, the cross-talk is -86.3 db. typical crosstalks of around -80 db were achieved for all tested situations. the frequency value was selected to minimize spectral leakage (500 ks/s with 65536 samples correspond to a frequency resolution of 7.63 hz and with this frequency, the fundamental will appear in the fft at the 12000 bin). also the frequency was selected to be representative of the input frequency range and to avoid any overlap (caused by aliasing) from any input signal higher harmonics. note that, multiple tests at different frequencies were also performed and it is from these tests that the overall -80 db crosstalk value was obtained. in addition to the cross-talk evaluation, a study in the harmonic distortion introduced by the system was also performed. this characterization was done with a lowdistortion function generator from stanford research systems (a ds360) with thd better than -100 db (until 20 khz). the results from two situations are shown in figure 11. the first situation corresponds to a 1250 hz sine signal with 1 v amplitude and acquired at 40 ks/s. the measured thd (using 10 records to average the amplitude spectrum) is -79 db. in the second situation, the frequency is changed to 12.5 khz (the sampling rate is also increased 10 fold) and the resulting thd is -72.1 db. 4. measurement results  to demonstrate the simultaneous acquisition of the developed daq, a 10 khz sinusoidal signal was applied to all channels as they were acquired with a sampling rate of 400 ks/s. in figure 12, two of the channels are depicted. in this case, the interchannel delay estimated using the sevenparameter sine-fitting algorithm [26], is under 125 ns. in figure 13 one example of an acquisition, as shown in the front panel of the developed labview interface application, is shown. the application enables the selection of the channels to be acquired, their independent ranges, the number of samples and the sampling rate. in figure 14, the amplitude of the fft calculated within the device is shown. the input signal corresponds to a sinusoidal or triangular signal with 1 khz and amplitude of 0.4 v (generated by a agilent 33210a) sampled at 40 ks/s with 65536 samples. 5. conclusions  the development, implementation and characterization of a simultaneous data acquisition system with fourchannels, multiple independent ranges, analog bandwidth of at least 1 mhz, sampling rate up to 600 ks/s and advanced on-board processing capabilities is presented. multiple results demonstrating the developed system characterization and performance were presented. the device can be used as a development tool for advanced measurement systems requiring embedded processing such as pq monitoring and pq qos measurements.     figure  11.  measured  average  spectrum  thd  measurements.  the  circles  represent  the  detected  harmonic  amplitudes.  the  top  spectrum  corresponds to a 1250 hz sine signal acquired at 40 ks/s and the resulting  thd is ‐79 db. the bottom spectrum corresponds to a 12.5 khz sine signal  acquired at 400 ks/s and the resulting thd is ‐72.1 db.   figure 12. simultaneous acquisition of a 10 khz signal. both channels are  represented (the channel 1 samples are the circles and the samples from  channel 2 are the crosses).  figure 13. front panel of one acquisition example.  0 5 10 15 20 -100 -80 -60 -40 -20 0 frequency [khz] s p e ct ru m [ d b ] 0 50 100 150 200 -100 -80 -60 -40 -20 0 frequency [khz] s p e ct ru m [ d b ] 0 0.05 0.1 0.15 0.2 -0.5 -0.25 0 0.25 0.5 time [ms] c h a n n e l 1 , 2 [ v ] acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 25  references  [1] a. asheibi, d. stirling, d. sutanto, "analyzing harmonic monitoring data using supervised and unsupervised learning," ieee transactions on power delivery, vol. 24, no.1, (2009), pp. 293-301. 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[25] ieee std. 1057-2007, ieee standard for digitizing waveform records, the institute of electrical and electronic engineers, new york, december 2007. [26] p. m. ramos, a. c. serra, “a new sine-fitting algorithm for accurate amplitude and phase measurements in two channel acquisition systems”, measurement, vol. 41, no. 2, (2008), pp. 135-143. figure  14.  example  of  fft  calculation  done  within  the  data  acquisition  device. sinusoidal signal (top) and triangular signal (bottom).  0 5 10 15 20 -120 -100 -80 -60 -40 -20 0 frequency [khz] s p e ct ru m [ d b ] 0 5 10 15 20 -120 -100 -80 -60 -40 -20 0 frequency [khz] s p e ct ru m [ d b ] continuous monitoring of the health status of cement-based structures: electrical impedance measurements and remote monitoring solutions acta imeko issn: 2221-870x december 2021, volume 10, number 4, 132 139 acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 132 continuous monitoring of the health status of cement-based structures: electrical impedance measurements and remote monitoring solutions nicola giulietti1, paolo chiariotti2, gloria cosoli1, giovanni giacometti1, luca violini1, alessandra mobili3, giuseppe pandarese1, francesca tittarelli3,4, gian marco revel1 1 department of engineering and mathematical sciences (diism), v. brecce bianche, 60131, ancona, italy 2 department of mechanical engineering, v. la masa 1, 20156, milano, italy 3 department of materials, environmental sciences and urban planning (simau), v. brecce bianche, instm research unit, 60131, ancona, italy 4 institute of atmospheric sciences and climate, national research council (isac-cnr), v. piero gobetti 101, 40129 bologna, italy section: research paper keywords: concrete health monitoring; electrical impedance; remote monitoring; distributed sensor network citation: nicola giulietti, paolo chiariotti, gloria cosoli, giovanni giacometti, luca violini, alessandra mobili, giuseppe pandarese, francesca tittarelli, gian marco revel, continuous monitoring of the health status of cement-based structures: electrical impedance measurements and remote monitoring solutions, acta imeko, vol. 10, no. 4, article 22, december 2021, identifier: imeko-acta-10 (2021)-04-22 section editor: roberto montanini, università di messina and alfredo cigada, politecnico di milano, italy received july 26, 2021; in final form december 5, 2021; published december 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: endurcrete (new environmental friendly and durable concrete, integrating industrial by-products and hybrid systems, for civil, industrial, and offshore applications) project, funded by the european union’s horizon 2020 research and innovation programme under grant agreement n° 760639. corresponding author: gloria cosoli, e-mail: g.cosoli@staff.univpm.it 1. introduction with a view of optimizing the costs related to maintenance operations necessary to guarantee a service life as long and efficient as possible for cement-based structures, structural health monitoring (shm) is paramount [1]. in fact, inspections alone are not sufficient to carry out timely actions intended to cope with possible damages. monitoring remotely structures and infrastructures is undoubtedly advantageous, since this process provides continuous data, often available in real-time, allowing to promptly act in a proactive way to avoid the risk of damages in the target structure. solutions involving iot-based approaches [2] have been also recently proposed, also in the context of smart cities applications [3]. these approaches demonstrated to be very promising given their distributed nature and connectivity characteristics making them prone to cloud analyses [4]. indeed, it is possible to monitor the factors threatening the durability of a structure, intended as “the ability of concrete to resist abstract the continuous monitoring of cement-based structures and infrastructures is fundamental to optimize their service life and reduce maintenance costs. in the framework of the endurcrete project (ga no. 760639), a remote monitoring system based on electrical impedance measurements was developed. electrical impedance is measured according to the wenner’s method, using 4-electrode arrays embedded in concrete during casting, selecting alternating current as excitation, to avoid the polarization of both electrode/material interface and of material itself. with this measurement, it is possible to promptly identify events related to contaminants ingress or damages (e.g. cracks formation). conductive additions are included in some elements to enhance signal-tonoise ratio, as well as the self-sensing properties of concrete. specifically, a distributed sensor network was implemented, consisting of measurement nodes installed in the elements to be monitored, then connected to a central hub (rs-232 protocol). nodes are realized with an embedded unit for electrical impedance measurements (eval-ad5940bioz board with ad5940 chip, by analog device) and a digital thermometer (ds18b20 by maxim integrated), enclosed in cabinets filled with an ip68 gel against moist-related problems. data are available on a cloud through wi-fi network or lte modem, hence can be accessed remotely via a use-friendly multi-platform interface. mailto:g.cosoli@staff.univpm.it acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 133 weathering action, chemical attack, and abrasion while maintaining its desired engineering properties” (american concrete institute, aci [5]). particular aggressive exposure conditions (e.g. marine environments) and contaminants (e.g. chlorides and sulphates) undermine the structures durability [6], requiring interventions getting more expensive the longer the time since the damage has occurred (“the law of fives” by de sitter states that the repairing costs increase exponentially after the structure damaging [7]). these events modify the concrete element composition and morphology, resulting in changes detectable through different techniques, such as ultrasound [8], [9], computer vision [10], [11], thermography [12], [13], ground penetrating radar (gpr) [14], [15], electrical resistivity [16], only to name a few. the use of electrical resistivity/impedance (cell constant scaling/no scaling) measurements for shm have grown widely [17] and achieved particular success with the introduction of conductive fillers (e.g. char, carbon black, graphene nanoplatelets, nickel powder, graphite powder, iron oxide, titanium dioxide, etc.) and fibres (e.g. virgin and recycled carbon fibres, steel fibres, carbon nanotubes, etc.) to enhance the selfsensing ability of concrete. the resulting lower electrical resistivity of the material allows to obtain a higher signal-tonoise ratio (snr), as well as to use low-cost instrumentation providing levels of accuracy comparable to those levels characterizing laboratory equipment. consequently, the possibility to exploit more affordable electronic equipment paves the way to the development of distributed sensor networks to monitor cement-based structures in those areas most subjected to stress, penetration of contaminants and hence damages and degradation. indeed, the electrical resistivity/impedance measurement is a local measurement, with a “sensing volume” corresponding to the hemisphere with the radius equal to the inter-electrode distance. sensors positioning hence becomes of uttermost importance to get significant data. furthermore, this type of measurement is particularly attractive since allows to monitor a plethora of aspects: the ingress of contaminants [18], [19] (being electrical resistivity linked to the material ability to transport ions, responsible of many degradation processes [20]), the penetration of water (particularly relevant since it carries ions and also aggressive substances, such as chlorides and sulphates), changes in temperature and moisture content, the presence of stress, the formation of cracks, the corrosion of reinforcements, etc. therefore, electrical resistivity/impedance can be assumed as an essential parameter for monitoring the health status of cementbased structures. to perform electrical impedance measurements, a proper excitation signal should be applied to the material; this can be done by means of an electric current/potential, then measuring the corresponding electric potential/current and finally computing the resultant electrical impedance. electrodes are necessary to carry out the measurement; different materials and configurations are reported in literature and both direct or alternating current (dc and ac, respectively) can be used, but the consequent measurement results will be different [6]. indeed, there are no widely accepted standards for this measurement in concrete, even if recommendations are available [21], [22]. in particular, in order to avoid both material and electrode/material interface polarization (the first caused by dc, the second caused by the use of the same two electrodes for both excitation and measurement – giving the so-called “insertion error”), the wenner’s method [23] should be adopted, using four electrodes (two for the material excitation, the other two for the measurement) and ac with a frequency greater than 1 khz [21][24]. in order to remotely monitor all these aspects in a continuous way, a dedicated monitoring system was developed within the endurcrete project (ga no. 760639); in particular, both electrical impedance and temperature of concrete elements are measured. the developed system was tested in a spanish demo site, as it will be described in detail below. the paper is organised as follows: the architecture of the remote measurement system developed to monitor the health status of cement-based structures is presented in section 2, results of laboratory tests for the monitoring system development and preliminary results of long-term monitoring activities are reported in section 3, whereas in section 4 the authors provide their comments and conclude the work. 2. materials and methods 2.1. single node design gold-standard measurement systems to perform electrical impedance measurements are galvanostats/potentiostats; their costs make their in-field use unfeasible, thus alternative sensors should be sought. moreover, given the “local” nature of the measurement, several sensing nodes should be considered to cover those areas prone to damages. the sensing nodes developed go in this direction by exploiting an embedded unit (eval-ad5940bioz board by analog device) controlling the ad5940 chip for electrical impedance measurements (a similar chip, the ad5933, has been already proved to be accurate for damages detection through electrical impedance measurements [25]). the unit was set to carry out electrical impedance measurements according to electrochemical impedance spectroscopy (eis) method in galvanostatic configuration, using ac to excite the material (frequency range: 1 khz – 20 khz; resolution: 1 khz; a signal amplitude of 600 mvpp is used, limiting the flowing electric current through a current limiting resistor to comply to iec 60601 standard – given that the employed acquisition board was originally thought for measurements on the human body). moreover, a digital thermometer was included to monitor the concrete element internal temperature, specifically the ds18b20 by maxim integrated (temperature range: from -55 °c to +125 °c; accuracy: ±0.5 °c from -10 °c to +85 °c; 1-wire bus communication), with stainless steel housing. the electronic board was placed inside an ip66 certified electrical box further filled with a self-sealing polymer-type insulating gel with cold cross-linking targeted to guarantee ip68 performance. the metrological performance of the sensing nodes targeted to the eis measurements was tested at laboratory level by comparison with a potentiostat/galvanostat used in galvanostatic configuration (gamry reference 600 by gamry instruments, considered as the gold-standard instrument). in particular, the comparison was made in terms of the real part of electrical impedance, mainly for two reasons: (i) it is directly related to electrical resistivity, in case of a pure resistive behaviour, according to the second ohm’s law; (ii) it depends on several parameters of interest (e.g. moisture content and water penetration [26], chlorides penetration [18], [27], porosity [28], carbonation depth, cracks formation [29], curing state [30]), given that it is linked to the ionic movement in the material and to the durability itself [6], [31]. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 134 laboratory acquisitions were performed on a concrete block (35 × 35 × 20 cm3) specifically manufactured for this testing activity, using the same mix-design adopted for the in-field validation phase. the concrete mix design was as follows: 375 kg/m3 of cement, 908 kg/m3 of calcareous sand 0 mm – 4 mm, 362 kg/m3 of intermediate gravel 5 mm – 10 mm, 618 kg/m3 of coarse gravel 10 mm – 15 mm, 169 kg/m3 of water, 0.55 kg/m3 and 1.27 kg/m3 of pc2 and pc3 superplasticizers, respectively. the concrete block was manufactured embedding both a temperature sensor and two 4electrode arrays (for performance comparison) for electrical impedance measurement (figure 1). stainless-steel was used for manufacturing the electrodes. furthermore, another 35 × 35 × 20 cm3 concrete specimen was manufactured hosting two couples of electrode arrays with different spacing, namely 1 cm and 4 cm, fixed at a depth of 5 cm and directed downwards and upwards, respectively (figure 2); a temperature sensor was also embedded in the specimen. the results of these laboratory tests allowed to fine-tune the setting parameters of the monitoring system, as well as to better understand the factors influencing the results (e.g. panels positioning). it is worthy to underline the fact that the tests were performed at 20±1 °c ambient temperature and 50±5% relative humidity. 2.2. monitoring system design given the targeted distributed nature of the monitoring system, a star-network architecture was selected. each measurement node is connected to a central gateway acting as data collector (figure 3). serial communication (rs232) was used as communication protocol. the central gateway also hosts a 16 channel usb to db9 rs232 serial adapter hub, an intel® nuc mini-pc and an lte industrial router. a python-based back-end service installed on the nuc sends a serial request to each node every hour and then receives the acquired data. the gateway stores data both locally in sqlite database and on cloud (aws amazon ec2 server) exploiting postrgresql engine. data can be accessed remotely through a dedicated multiplatform application developed in dart language exploiting the google flutter network for android, ios, linux and windows figure 1. first concrete panel prototype: configuration scheme. figure 2. second concrete panel prototype: configuration scheme. figure 3. monitoring system with star-network architecture, with edge devices connected to the central hub, collecting data and sending them to a cloud that can be accessed remotely. figure 4. mobile application for monitoring: home page (left), login and registration (centre, top and bottom) and main page (right). 4 electrode array, 4 cm spacing emperature sensor egend: 2 4 4 electrode array, up ards, 4 cm spacing 4 electrode array, do n ards, cm spacing emperature sensor egend: 2 4 acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 135 environments, developed within the endurcrete project. as example, the application that runs on an android device is shown in the following. at the first access, registration is required with an email address and password; after the login, it is possible to access the main page with different options (figure 4). it is possible to switch between different demo sites (tunnel, tu, and marine, ma – the second has been activated in italy more recently and will be the object of future analyses), then select the panel and the electrode array of interest (for example the label tu_03_3s stands for tunnel demo site, panel n. 3, sensor n. 3 short (1-cm spacing) whereas the label tu_03_1l stands for tunnel demo site, panel n. 3, sensor n. 1 long (4-cm spacing)). furthermore, it is possible to choose the measurement frequency for the electrical impedance measurement and select the desired parameters: temperature, magnitude, phase, real and imaginary parts of electrical impedance; finally, it is possible to figure 5. mobile application for monitoring: selection of demo site, panel and electrode array, measurement frequency and parameters of interest. figure 6. mobile application for monitoring: data saving and graph making. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 136 pick out the monitoring period to be shown (figure 5). it is possible to save data, send them to the email address inserted in the registration phase (with the options to save the data related to all the panels exposed in the demo site), as well as to make a graph with the selected options (figure 6). 2.3. the concrete panels monitored in the project demo sites the concrete panels exposed in the demo sites of the endurcrete project were manufactured with low-clinker cement (aimed at reducing the carbon footprint, given that the production of ordinary portland cement – opc – is responsible for approximately 8 % of global co2 emissions [32]), developed within the same project [33]. two different mix designs were formulated, with and without conductive additions. the monitoring system developed was tested in a spanish demo site, representative of a harsh setting: a tunnel in león, which is an underground environment rich in sulphates. in particular, 2 small panels of 35 × 35 × 20 cm3 (with and without conductive additions), 1 long panel of 200 × 200 × 15 cm3 and another long panel of 200 × 400 × 15 cm3 were exposed (figure 7). in each small panel 4-electrode arrays were embedded, whereas 6 ones in longer panels, in order to have a proper distributed sensor network (figure 8). arrays with different electrode spacings were used, to be installed at depths respecting the literature recommendation (a minimum distance of twice the contact spacing from the element edge should be guaranteed): • 4-cm spacing, installed at a depth of 16 cm and 10 cm in small and long panels, respectively; • 1-cm spacing, to be installed at a depth of 4 cm and 5 cm in small and long panels, respectively. the monitoring system configuration is reported in figure 7, whereas the electrodes configuration in figure 8. it is worthy to mention that temperature sensors were embedded in the monitored concrete specimens, whereas the moisture content was not assessed; indeed, it is out of the scope of the proposed monitoring system to distinguish among the different factors contributing to the results, since the main aim is to identify ingress of contaminants that can possibly threaten the concrete durability. the positioning of the concrete panels in the tunnel is shown in figure 9. figure 7. monitoring system at the demo site in león (spain). figure 8. concrete panels exposed to the demo site in león (spain): no. 2 panels of 35 × 35 × 20cm3 (left), no. 1 panel of 200 × 200 × 15 cm3 (centre) and no. 1 panel of 200 × 400 × 15 cm3 (right). legend: 4-electrode arrays with 4-cm spacing are reported in blue, 4-electrode arrays with 1-cm spacing are reported in red, temperature sensors are reported in yellow. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 137 3. results and discussion 3.1. laboratory tests the results concerning the validation of the measurement system are reported for the real part of electrical impedance measured at 10 khz in figure 10 (single measurements are compared on each day of the time interval considered after the initial concrete curing period); measurements were performed until 15 days after casting. it is possible to notice that the measurement node developed provides results compatible to those obtained from the gold standard, with absolute percentage errors lower than 10%. the advantages of the developed node are multiple: • compactness, thus possibility to be installed close to the element to be monitored: this allows to minimize the cables length and consequently to reduce their influence on the measurement results, as well as to minimize parasitic electrical components; • reduced cost, approximately 3% of the reference equipment; • modularity of the system, which results particularly useful for the development of a distributed sensor network, which can be also modified at a later time. a comparison was made also concerning the electrodes spacing (figure 11). as expected, the real part of electrical impedance is higher with the shorter electrode spacing, since the sensing volume is smaller, resulting in higher opposition to the electric current flow. however, both the measurements show a trend increasing with curing time, as expected from the literature [16], because of the continuous hydration of the cement paste and the gradual water evaporation. in this way, it is possible to monitor different sensing volumes within the same element, also considering surfaces looking out different exposure conditions. some data pre-processing capabilities were also embedded in the system, namely a moving median filter (sliding window: 9 samples) combined to a wavelet decomposition (discrete meyer wavelet of level 5). data were also normalised with respect to the initial value, in order to observe the variations over time, independently from the absolute values (less significant for monitoring purposes, looking for variations due to particular events, such as cracks or contaminants penetration). 3.2. results from tunnel demo site some examples of the results related to the data acquired from 8th october 2020 to 28th june 2021 (approximately 9 months) are reported in this paragraph. in particular, three sensors are considered: 2-electrode arrays with 1-cm spacing embedded in two small panels with (figure 12) and without (figure 13) conductive additions, one electrode array with 4-cm spacing embedded in a long panel without conductive additions (figure 14). it can be observed that the monitoring system did not show any faults during the demo test activities and data were not affected by any issues attributable to the aggressive conditions of the exposure environment since there are no anomalous spikes or particular increase/decrease due to external phenomena. as expected, the electrical impedance module increases with decreasing temperature; however, it is worthy to note that figure 9. positioning of concrete panels in the tunnel at the demo site of león (spain); concrete panels with embedded sensors for the measurement of electrical impedance are 4 (2 small and 2 long). figure 10. comparison between measurement results obtained with the system exploiting ad5940 chip (yellow data) and reference equipment (gamry reference 600, blue data) – measurement frequency: 10 khz. figure 11. comparison between results obtained by electrode arrays with different spacings: 1 cm (orange data) and 4 cm (grey data) – measurement frequency: 10 khz. figure 12. results related to a small panel with conductive additions (1-cm spacing electrode array): temperature, electrical impedance module, electrical impedance phase, real and imaginary parts of electrical impedance (from top to bottom) – measurement frequency: 10 khz. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 138 underground environment makes temperature values quite stable and this reflects on the electrical impedance trend (no day-night cycles can be identified, neither in temperature nor in electrical impedance signals). in addition, environmental sensors were employed in the demo site to continuously monitor environmental temperature and relative humidity, which resulted both quite stable in the considered monitoring time interval (t = 10 °c – 15 °c and rh = 70 % – 80 %, respectively). in line with what obtained in laboratory tests, the electrical impedance is generally higher when electrode spacing is shorter (see figure 13 with respect to figure 14), since the electrical current flows in the sensing volume less easily, probably due to the fact that considering a minor material volume enhances the effects of particularly non-conductive elements (e.g. aggregates). moreover, it is confirmed that conductive additions lower the electrical impedance of concrete panels (see figure 12 in comparison to figure 13), hence allowing a monitoring through the use of relatively low-cost instrumentation, with limited metrological performance. the small panels (figure 12 and figure 13) show a positive phase before normalization of the signal (with respect to the initial value); this results in an inductive imaginary part, contrary to what it is expected from the literature [34]. this is probably due to a capacitive coupling with ground, as it happened during laboratory tests; in fact, this do not happen in long panels (figure 14), which are leaning directly on the ground (figure 9). however, this does not impede the monitoring of those elements and the identification of eventual factors hindering durability. the monitoring phase will continue at least until the project end (december 2021), in order to evaluate possible uncertainty sources linked to the ingress of aggressive substances (e.g. sulphates), which could hinder the concrete elements durability. 4. conclusions the ongoing validation phase of the developed monitoring system at the spanish demo site of león is providing interesting results, even though the small variations recorded so far do not highlight any particular event associated to the ingress of contaminants. the measurement system presented in the paper is continuously generating data from the field application, no matter the aggressive exposure conditions. it is worthy to underline the importance of properly choosing the locations of sensors, being a local measurement with a defined sensing volume; the use of conductive additions allows to employ lowcost modular equipment, with the possibility to realize a distribute sensor network with sensing nodes in correspondence of those areas more prone to damages or contaminants ingress, which could impact on the structure durability and efficiency during the whole life cycle. in the future, the monitoring system will be exploited also in different harsh environments, such as a typical marine site which is rich in chlorides. acknowledgement this research activity was carried out within the endurcrete (new environmental friendly and durable concrete, integrating industrial by-products and hybrid systems, for civil, industrial, and offshore 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a magnetic positioning system acta imeko issn: 2221-870x december 2021, volume 10, number 4, 97 102 acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 97 gesture recognition of sign language alphabet with a convolutional neural network using a magnetic positioning system emanuele buchicchio1, francesco santoni1, alessio de angelis1, antonio moschitta1, paolo carbone1 1 department of engineering university of perugia, italy section: research paper keywords: gesture recognition; sign language; machine learning; cnn citation: emanuele buchicchio, francesco santoni, alessio de angelis, antonio moschitta, paolo carbone, gesture recognition of sign language alphabet with a convolutional neural network using a magnetic positioning system, acta imeko, vol. 10, no. 4, article 17, december 2021, identifier: imeko-acta10 (2021)-04-17 section editors: umberto cesaro and pasquale arpaia, university of naples federico ii, italy received october 15, 2021; in final form december 4, 2021; published december 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: emanuele buchicchio, e-mail: emanuele.buchicchio@studenti.unipg.it 1. introduction sign language recognition (slr) is a research area that involves gesture tracking, pattern matching, computer vision, natural language processing, linguistics, and machine learning [1]. the final goal of slr is to develop methods and algorithms to build an srl system (slrs) capable of identifying signs, decoding their meaning, and producing some output that the intended receiver can understand (figure 1). the general slr problem includes the following tasks: 1) letter/number sign gesture recognition, 2) word sign gesture recognition, and 3) sentence-level sign language translation available literature surveys [2]-[5] report that recent research achieved accuracy in the range of 80–100% for the first two tasks using vision-based and sensor-based approaches. in this paper, we compare the performance of the two systems we developed: a vision-based system and a hybrid system with sensor-based data acquisition and vision-based classification stages. 1.1. slrs performance assessment in the instrumentation and measurement field, machine learning is used for processing indirect measurement results. an indirect measurement is defined in [6] as a “method of measurement in which the value of a quantity is obtained from measurements made by direct methods of measurement of other quantities linked to the measurand by a known relationship.” in the machine learning (ml) common jargon [7], the quantities that can be measured with a direct method are denoted as features x1, x2, …, xn, and the measurand as y. the measurand y is linked to features by a functional relationship y=f(x1, x2, …, xn). the process of estimating f is known as “training.” in the training process, the ml model is trained with the given dataset to find the best possible approximation according to the selected optimality criterion. the trained model produces an estimation of y in response to the vector x=(x1, x2, …, xn). abstract gesture recognition is a fundamental step to enable efficient communication for the deaf through the automated translation of sign language. this work proposes the usage of a high-precision magnetic positioning system for 3d positioning and orientation tracking of the fingers and hands palm. the gesture is reconstructed by the magik (magnetic and inverse kinematics) method and then proce ssed by a deep learning gesture classification model trained to recognize the gestures associated with the sign language alphabet. results confirm the limits of vision-based systems and show that the proposed method based on hand skeleton reconstruction has good generalization properties. the proposed system, which combines sensor-based gesture acquisition and deep learning techniques for gesture recognition, provides a 100% classification accuracy, signer independent, after a few hours of training using transfer learning technique on well-known resnet cnn architecture. the proposed classification model training method can be applied to other sensorbased gesture tracking systems and other applications, regardless of the specific data acquisition technology. mailto:emanuele.buchicchio@studenti.unipg.it acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 98 in the case of classification systems, the measurand y is the class to which an input vector x belongs. the most widely used performance metric for gesture slrs is classification accuracy (defined as the ratio of correct predictions over the total predictions). in this work, accuracy was adopted both for model benchmark and as model optimality criterion. 1.2. sign language sign language (sl) is defined as "any means of communication through bodily movements, especially of the hands and arms, used when spoken communication is impossible or not desirable" [8]. modern sign language originated in the 18th century when charles-michel de l'épée developed a system for spelling out french words with a manual alphabet and expressing whole concepts with simple signs. other national sign languages were developed from this system and became an essential means of communication among the hearing-impaired and deaf communities. according to the world federation of the deaf, today exist over 200 sign languages used by 70 million deaf [9]. sign language involves using facial expressions and different body parts, such as arms, fingers, hands, head, and body. one class of sign languages, also known as fingerspelling, is limited to a set of manual signs that represent the symbols of the letters of an alphabet performed with one hand [10]. the asl signs of the alphabet letters are shown in figure 2. 1.3. vision-based vs. sensor-based approaches for hands tracking and gesture recognition many common devices and applications rely on tracking hands, fingers, or handheld objects. specifically, smartphones and smartwatches track 2d finger position, a mouse tracks 2d hand position, and augmented reality devices like the microsoft hololens 2 track the 3d pose of the finger. in addition to slr, many other applications rely on hand gesture recognition such as augmented reality [12], assistive technology [13], [14], collaborative robotics [15], telerobotics [16], home automation [17], infotainment systems [18], [19], intelligence and espionage [20] and many others [21]. in this paper, we focused on recognizing static hand gestures associated with the letters of the alphabet for fingerspelling. both computer-vision-based and sensor-based approaches were implemented for sign language alphabet recognition. hand features extraction is a significant challenge for vision-based systems [11] because extraction is affected by many factors, such as lighting conditions, complex backgrounds in the image, occlusion, and skin color. sensor-based gesture recognition systems are commonly implemented as gloves featuring various types of sensors. sensor-based approaches have the advantage of simplifying the detection process and can help make the gesture recognition system less dependent on input devices. on the other hand, a disadvantage of sensor-based systems is that they can be expensive and too invasive for real-world deployment. 2. vision-based sign language gesture recognition machine learning techniques are widely adopted for gesture classification tasks. various public datasets are available for system performance assessment and benchmark. the american sign language mnist dataset [22], a flavor of the classic mnist dataset [23], created for sign language gesture, is often used as a baseline. other more complex datasets such as [24], [25] are also available. 2.1. classic machine learning and convolutional neural network on mnist dataset the american sign language mnist dataset is in a tabular format similar to the original mnist dataset. each row in the csv file has a label and 784 pixels values ranging from 0-255, representing a single 28 × 28 pixels greyscale image. in total, there are 27,455 training cases and 7,172 tests cases in this dataset. the classification accuracy was selected as the primary metric for models’ performance assessment and benchmarking with other published comparable works. two different models were trained to accomplish the letter/number gesture recognition task from static images using two different approaches: a classic ml model and a deep neural network (figure 3). the first model was selected among many model candidates obtained by applying different combinations of features engineering techniques, ml algorithms, and ensemble methods using the automated ml (automl) service of azure machine learning. azure machine learning [26] is a cloud-based platform that provides tools for automation and orchestration of all training, scoring, and comparison operations. automl tests hundreds of models in a few hours with parallel job execution with no human interaction after the initial experiment and remote compute target cluster setup. the experiment generates many models that achieve 100% classification accuracy. among figure 1. block diagram of a sign language recognition system (slrs). figure 2. letters of the american sign language (asl) alphabet [11]. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 99 them, the “logistic regression” based model has a smaller memory footprint at runtime. the second model was created with a minimal custom convolutional neural network (cnn) architecture (2d convolution, max pooling, flatten, dense layer, dropout, dense) commonly used for simple deep learning image recognition tasks (figure 4). the model was built and trained with the keras library. model hyperparameters such as the number of neurons in layers, batch size, the number of training epochs, and dropout percentage were tuned using the hyperdrive service from azure machine learning. the best scoring model achieves a classification accuracy score of 99.99%. the best models from the two training pipelines were deployed as web services for production usage. the (zipped) size of the cnn model is about 17 mb when the logistic regression model size is only 0.8 mb. simple and lightweight models should be preferred if there is no performance penalty. 2.2. vision-based classification accuracy the 100% accuracy was confirmed after deployment with test cases from the american sign language mnist. simple classic ml models could not recognize gestures in realistic images with variable backgrounds and light conditions. the cnn model scores over 90% accuracy on a subset of the "asl alphabet" [24] image dataset that includes more "realistic" light and background conditions. however, while deployed as a web service, the performance on image stream from a live camera was not satisfactory for production usage in challenging conditions such as partial line of sight obstruction, presence of shadows in the image, and confusing backgrounds like in the test case of asl alphabet test dataset [25]. 3. sensor-based gesture recognition with deep cnn on visual gesture representation our experiment with a vision-based approach confirms both performance and limitation described in other works. given the result of our experiments and other works, in this paper, we propose an slrs system that combines a sensor-based approach in the acquisition stage and computer vision techniques in the gesture recognition stage (figure 5). 3.1. hand tracking with magnetic position system (mps) the magnetic positioning system (mps) described in [27] is immune from many problems that affect computer vision techniques such as occlusion, light condition, shadows, skin colors. the mps is composed of transmitting nodes and receiving nodes. the transmitting nodes are mounted on the fingers and hand to be tracked (figure 6), whereas the receiving nodes are placed at known positions on the sides of the operational volume. an advantage of the sensor-based systems is that they are not sensitive to illumination conditions and the other factors affecting vision-based systems. furthermore, mps can also operate in the presence of obstructions caused by objects or body parts. therefore, the proposed approach enables robust and reliable tracking of the hand and fingers. it is thus suitable for slr and the other applications of hand gesture recognition, such as human-machine interaction, virtual and augmented reality, robotic telemanipulation, and automation. figure 3. workflow for the comparison of various machine learning models for static gesture recognition using azure skd, automl and hyperdrive for operations automation. figure 4. deep cnn model architecture. figure 5. proposed slrs with sensor-based data acquisition and vision-based gesture recognition. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 100 3.2. gesture recognition using skeleton reconstruction classic machine learning models can achieve 100% accuracy on static sign language recognition tasks on laboratory datasets like [24]. cnn deep learning models score high accuracy (over 90%) on realistic images. classic machine learning models can achieve 100% accuracy on static sign language recognition tasks on laboratory datasets. cnn deep learning models score high accuracy (over 90%) on realistic images with variable light. however, these high performances are not robust and cannot be easily replicated in real-world operating conditions. in our paper [11], we demonstrated that training the classification model on data from a tracking system gives substantial advantages in terms of robustness to environmental conditions and signer variability. the hand gesture is reconstructed using the technique illustrated in [28], with the improvements added in [11], which we called magik (magnetic and inverse kinematics). the method, with some empirical modification introduced in the model to optimize the reconstruction of the gesture among different test subjects, allows reconstructing the movement of the hand with 24 degrees of freedom (dof). positions and orientations of all the magnetic nodes estimated by the mps are sent to a kinematic model of the hand, to obtain the position and flexion of each joint and the position and orientation of the whole hand with respect to the mps reference frame. as the last step, magik produces a visual representation, such as the examples shown in figure 7. we call this technique “skeleton reconstruction”. 3.3. efficient deep cnn training for sign language recognition many pre-trained deep learning models are proven to be adequate for image/video classification tasks. we chose the resnet34 cnn because the resnet (residual network) architecture achieves good results in image classification tasks and is relatively fast to train [29]. figure 8 illustrates the training pipeline implemented with pytorch and fastai [30] library. transfer learning approaches allow fast training of the deep cnn (resnet34) model. the optimal learning rate for training was estimated with the cyclical learning rates method [31] to avoid time-consuming multiple runs to perform hyperparameters sweeps. the rules of thumb for the selection of learning rate value from [31] are: 1) one order of magnitude less than where the minimum loss was achieved; and 2) the last point where the loss was clearly decreasing. the loss estimation plot (figure 9) produced by the algorithm implementation in the fastai library suggested a learning rate in the range 10-2 – 10-3. model fine-tuning was performed using fastai api with a sequence of freeze, fit-one-cycle, unfreeze, and fit-one-cycle operations using the «discriminative learning rate» method. the training continued until error rate, validation loss, and training loss converged to zero after four epochs (figure 10). 3.4. gesture classification inference with mps the trained model, after the fine-tuning process, was developed in an inference pipeline (figure 11) that takes the figure 6. mps transmitting coils mounted on a wearable glove. figure 7. examples of asl letters (y and l) articulated while wearing the glove, and their respective reconstructions obtained through the kinematic model and magik technique. figure 8. training pipeline for resnet43 cnn with transfer learning. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 101 output generated by mps control software and, for each acquired frame: 1) reconstructs the gesture using magik model kinematic model, 2) exports the visual representation as a bitmap image, 3) feeds the cnn model with the generated gesture image and get the array of confidence values associated with each class in the training dataset, and 4) printouts the label of the sign class with the highest confidence value. 4. conclusions classic machine learning models can only achieve 100% accuracy on static sign language recognition tasks on laboratory datasets [22]. deep cnn models can accomplish the task with over 90% accuracy also on more realistic images [24]. however, these high performances are not robust and cannot be replicated in real-world operating conditions. combining sensor-based acquisition, visual reconstruction of the skeleton, and a deep cnn classification model, the proposed system achieves 100% inference accuracy on gestures performed by different people after a few epochs of training. we cannot achieve 100% accuracy with classic machine learning in comparable experimental conditions. the sensor-based approach is immune from many problems that affect computer vision techniques such as occlusion, light condition, shadows, skin colors. building a gesture recognizer on top of a tracking system, instead of direct classification from a sensor 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https://dx.doi.org/10.3390/app9245268 https://doi.org/10.1109/tmc.2019.2893338 https://doi.org/10.1007/s13042-017-0705-5 https://www.kaggle.com/datamunge/sign-language-mnist http://yann.lecun.com/exdb/mnist https://www.kaggle.com/grassknoted/asl-alphabet https://www.kaggle.com/danrasband/asl-alphabet-test https://azure.microsoft.com/it-it/services/machine-learning/#product-overview https://azure.microsoft.com/it-it/services/machine-learning/#product-overview https://doi.org/10.3390/s20216210 https://doi.org/10.1109/tim.2021.3065761 https://doi.org/10.1109/cvpr.2016.90 https://doi.org/10.3390/info11020108 https://arxiv.org/abs/1506.01186 acta imeko  issn: 2221‐870x  june 2015, volume 4, number 2, 2‐3    acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 2  editorial  paolo carbone  university of perugia,  italy   section: editorial   citation: paolo carbone, editorial, acta imeko, vol. 4, no. 2, article 1, june 2015, identifier: imeko‐acta‐04 (2015)‐02‐01  editor: paolo carbone, university of perugia, italy  received june 25, 2015; in final form june 25, 2015; published june 2015  copyright: © 2015 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  corresponding author: paolo carbone, email: paolo.carbone@unipg.it     dear reader, this second issue in 2015 of acta imeko is particularly rich in content. it contains 14 papers related to the 3 imeko events: imeko tc14 ismqc 11th international symposium on measurement and quality control 11 13 september 2013, cracow and kielce, poland joint imeko international tc3, tc5 and tc22 conference 2014 lagoon beach hotel, cape town, south africa february 3, 2014 – february 5, 2014 13th imeko tc10 workshop on technical diagnostics advanced measurement tools in technical diagnostics for systems' reliability and safety warsaw, poland, june 26 27, 2014 moreover, it also contains the very first two papers that were submitted directly to acta imeko without being presented in a prior imeko event. the publication of these two papers testifies a major change in the policy of this journal. while it originally started as one of the two journals of imeko, publishing invited and extended versions of papers presented at imeko conferences and workshops it now also considers papers describing results and advancements in the state-of-theart of measurement science as a whole, freely submitted by their authors. the editors believe that this a major advancement in how this journal serves the community of its readers and contributors. at the same time this is an open invitation to all prospective authors to consider this journal as one journal among which to choose when selecting the publication destination of their researches. this journal is and remains an open access journal serving the large community of researchers in metrology. the first papers in this issue originate from the imeko tc14 event held in cracow and kielce in 2013. the first contribution is authored by researchers working at the leibniz university of hannover in germany and it proposes a new fringe projection method to detect geometry deviations by reducing, at the same time, measurement duration. the second paper covers again the topic of inspection. it is authored by researchers with the university of belgrade and with the metropolitan university in belgrade and it describes a disciplined process, helping practitioners planning inspection of parts on coordinate measuring machines. the last paper associated to this imeko tc14 event is a joint international work between researchers working in ukraine and poland and it covers the topic of designing a four electrode conductivity cell for usage as a primary standard in the area of electrolytic conductivity. the next series of papers is associated to the event held in south africa last year. andrea brüge with the physikalischtechnische bundesanstalt (ptb) in germany, illustrates the characteristics of a calibration facility for torque wrenches. a detailed representation of all uncertainty sources is presented as well as how they contribute to the final uncertainty budget. the second paper in this series is authored by researchers with the national metrology institute of japan. the authors describe the calibration chain for the national torque standard for hand torque screwdrivers. a thorough analysis of the effect of major uncertainty contributions is made and experimental data are presented. leonard klaus and other authors with the german ptb present the following paper on dynamic calibration of torque transducers. this paper describes the issues associated to the modelling of this physical phenomenon and of the identification of the model parameters. the next paper is authored again by researchers with the german ptb. they present a model—based approach to dynamic calibration of force transducers. modelling and identification of model parameters are discussed in this paper and a large set of experimental results is presented, highlighting the difficulties associated to this task and the open issues to still be addressed.   acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 3  the paper by christiaan veldman with the national metrology laboratory of south africa deals with the transverse sensitivity calibration of accelerometers. both the measurement principles are introduced and experimental results presented together with the analysis regarding contributing uncertainty sources. the final paper in this series is authored by christian schlegel and other colleagues working at the german ptb. it covers the topic of force measurements and of the mechanical influences that are present during sinusoidal force measurements. the experimental arrangement is described and modelling of the transducers using fem simulations shows the behaviour of the dynamic modes occurring during periodic excitations. the next 5 papers originate from the imeko event held in warsaw. the first contribution comes from authors with the instituto de telecomunicações and instituto superior técnico in portugal. it deals with non-destructive techniques for testing metallic tubes. the technique used for testing a stainless steel tube using two different magnetic detectors is presented and results discussed to highlight the best solution to solve the test problem. the next paper is authored by lauryna šiaudinyte with the vilnius gediminas technical university. it deals with length measurements and with the description of a linear bench for testing total stations. the topic is clearly described and experimental results are presented. the next paper is authored by researchers with the warsaw university of technology. it covers the topic of power quality in the context of dc traction supply systems. accordingly, the effectiveness of ad-hoc designed filters for reducing the ac components is described along with experimental data regarding the harmonics present in the kv range of the considered supply system. the following paper is the result of a joint collaboration between researchers from china and ukraine. it describes the problem of measuring and correcting the effect of nonlinearities in analog-to-digital converters using multi-resistor dividers. the last paper in this series comes from the federal institute for materials research and testing, located in berlin. it describes two application examples of radio-frequency identification tags in the areas of infrastructure monitoring and of transportation of dangerous goods. this paper shows how innovative applications can still be devised using the rfid technology. the last two papers in this issues are those directly submitted by their authors. the first one is authored by daniele fontanelli et al., with the university of trento in italy. it describes a line tracking measurement system for robotic vehicles. the adopted measurement model is illustrated with great detail and experimental results are presented to validate their proposal. the paper concluding this issue is authored by researchers with the university of naples federico ii. it deals with the 3—parameter sine fitting algorithm. the authors propose two techniques to improve the accuracy of the estimates returned by this algorithm when applied to sinusoidal signals corrupted by noise. let me thank all who made this issue possible. as a concluding remark i would like to thank prof. francisco alegria, with the instituto de telecomunicações, in portugal. he contributed to set up this journal and, starting this issue, he ceases his collaboration. the editors are grateful for the time he spent on improving this publication. have a fruitful reading of the second issue of acta imeko in 2015!   microsoft word 297-1914-1-le-rev acta imeko  issn: 2221‐870x  december 2015, volume 4, number 4, 2    acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 | 2  introduction to the special section of the imeko tc21  “mathematical tools in measurements”  franco pavese   torino, italy     section: editorial   citation: franco pavese, introduction to the special section of the imeko tc21 “mathematical tools in measurements”, acta imeko, vol. 4, no. 4, article 1,  december 2015, identifier: imeko‐acta‐04 (2015)‐04‐01  section editor: franco pavese, torino, italy  received december 10, 2015; in final form december 10, 2015; published december 2015  copyright: © 2015 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  corresponding author: franco pavese, email: frpavese@gmail.com       this special section of the imeko tc21 “mathematical tools in measurements” is formed by a selection of extended papers that was submitted following the presentations made at the international conference “advanced mathematical and computational tools in metrology and testing” (amctm) held in september 2014 in st. petersburg, russia, with an important attendance from russia. for this reason, this section comprises three russian papers out of seven. the paper of mazin proposes the presentation of a physical quantity by a vector of the pseudo-euclidean plane including the quantity value and its degree of uncertainty. the paper of kurekova et al. discusses the positioning uncertainty of kinematic structures, namely of the tricept. the paper of kislitsyna and malykhina deals with the modelling of an altimeter designed for soft landing on the moon, based on gamma rays scattering. the remaining four papers well represent the worldwide attendance to the conference. the paper of barari and ahmadi, from canada, concerns coordinate metrology and introduces a novel approach to predict surface behaviour via distribution of geometric deviations (dgd) to estimate the detailed deviation zone. the paper of gonzalez, estrada and lira, from mexico, discusses the modeling of a combustion chamber of a reference calorimeter used to measure the superior calorific value (scv) of natural gas. the paper of willink, from new zealand, discusses the important issue of the evaluation of the type a component of uncertainty when the sample size is not predetermined, considering both the frequentist and the bayesian methods. the paper by pavese, from italy, starting from a statistics of the results of key comparisons (kc) available on the bipm kcdb, discusses the consequences of the results obtained, of a dominant situation where an intercomparison contains discrepant data, proposing an alternative method to the simple prior correction for the known systematic effects. journal contacts acta imeko issn: 2221-870x june 2022, volume 11, number 2, 1 2 journal contacts about the journal acta imeko is an e-journal reporting on the contributions on the state and progress of the science and technology of measurement. the articles are mainly based on presentations presented at imeko workshops, symposia and congresses. the journal is published by imeko, the international measurement confederation. the issn, the international identifier for serials, is 2221-870x. about imeko the international measurement confederation, imeko, is an international federation of actually 42 national member organisations individually concerned with the advancement of measurement technology. its fundamental objectives are the promotion of international interchange of scientific and technical information in the field of measurement, and the enhancement of international co-operation among scientists and engineers from research and industry. addresses principal contact prof. francesco lamonaca university of calabria department of computer science, modelling, electronic and system science via p. bucci, 41c, vi floor, arcavacata di rende, 87036 (cs), italy e-mail: editorinchief.actaimeko@hunmeko.org support contact dr. dirk röske physikalisch-technische bundesanstalt (ptb) bundesallee 100, 38116 braunschweig, germany e-mail: dirk.roeske@ptb.de editor‐in‐chief francesco lamonaca, italy founding editor‐in‐chief paul p. l. regtien, netherlands associate editor dirk röske, germany copy editors egidio de benedetto, italy silvia sangiovanni, italy layout editors dirk röske, germany leonardo iannucci, italy domenico luca carnì, italy editorial board leopoldo angrisani, italy filippo attivissimo, italy eulalia balestieri, italy eric benoit, france paolo carbone, italy lorenzo ciani, italy catalin damian, romania pasquale daponte, italy luca de vito, italy sascha eichstaedt, germany ravi fernandez, germany luigi ferrigno, italy edoardo fiorucci, italy alistair forbes, united kingdom helena geirinhas ramos, portugal sabrina grassini, italy leonardo iannucci, italy fernando janeiro, portugal konrad jedrzejewski, poland andy knott, united kingdom yasuharu koike, japan dan kytyr, czechia francesco lamonaca, italy aime lay ekuakille, italy massimo lazzaroni, italy fabio leccese, italy rosario morello, italy michele norgia, italy franco pavese, italy pedro miguel pinto ramos, portugal nicola pompeo, italy sergio rapuano, italy renato reis machado, brazil álvaro ribeiro, portugal gustavo ripper, brazil dirk röske, germany maik rosenberger, germany alexandru salceanu, romania constantin sarmasanu, romania lorenzo scalise, italy emiliano schena, italy michela sega, italy enrico silva, italy pier giorgio spazzini, italy krzysztof stepien, poland ronald summers, uk marco tarabini, italy tatjana tomić, croatia joris van loco, belgium zsolt viharos, hungary bernhard zagar, austria davor zvizdic, croatia mailto:editorinchief.actaimeko@hunmeko.org mailto:dirk.roeske@ptb.de acta imeko | www.imeko.org june 2022 | volume 11 | number 2 | 6 section editors (vol. 7 11) yvan baudoin, belgium piotr bilski, poland francesco bonavolonta, italy giuseppe caravello, italy carlo carobbi, italy marcantonio catelani, italy mauro d’arco, italy egidio de benedeto, italy alessandro depari, italy alessandro germak, italy istván harmati, hungary min-seok kim, korea bálint kiss, hungary momoko kojima, japan koji ogushi, japan vilmos palfi, hungary jeerasak pitakarnnop, thailand md zia ur rahman, india fabio santaniello, italy jan saliga, slovakia emiliano sisinni, italy ciro spataro, italy oscar tamburis, italy jorge c. torres-guzman, mexico ioan tudosa, italy ian veldman, south africa rugkanawan wongpithayadisai, thailand claudia zoani, italy microsoft word 216-1842-1-le-rev acta imeko  issn: 2221‐870x  december 2015, volume 4, number 4, 57‐61    acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 | 57  salinity and relative humidity: climatological relevance and  metrological needs   rainer feistel  leibniz institute for baltic sea research, 18119 warnemünde, germany      section: technical note  keywords: teos‐10; salinity; relative humidity; metrology; si‐traceability; climatology; hydrological cycle  citation: rainer feistel, salinity and relative humidity: climatological relevance and metrological needs, acta imeko, vol. 4, no. 4, article 11, november 2015,  identifier: imeko‐acta‐04 (2015)‐04‐11  editor: paolo carbone, university of perugia, italy  received september 29, 2014; in final form september 29, 2014; published december 2015  copyright: © 2015 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  corresponding author: rainer feistel, e‐mail: rainer.feistel@io‐warnemuende.de    1. introduction  between 1910 and 1945, global air temperatures increased by about 0.5 °c, followed by a stagnation period of roughly 30 years, and by another rise by 0.5 °c within 30 years, until quite recently a sudden “hiatus” of global warming was discovered. neither the obvious frequency nor the tipping points of this staircase are explained, let alone predicted, by global climate models. rather, the hunt is open for finding “the lost heat” [1]. among the top “suspects” is ocean-atmosphere interaction with anomalies in humidity and salinity; section 4 will return to this point. another hot favourite is uncertainty of observational data [2]. a rule-of-thumb calculation tells us that warming an atmospheric mass of 104 kg/m2 by 0.5 °c within 30 years requires a mean heat flux of about 5 mw/m2. this estimate may be low by a factor of 10 if the ocean surface is included, but that is another question. on the other hand, oceanic evaporation of 1200 mm/yr exports 100 w/m2 of latent heat to the atmosphere. a typical meteorological uncertainty of 1 %rh of the typical ocean-surface relative humidity (rh) of 80 %rh implies an uncertainty of 6 w/m2 of the latent heat flux [3]. thus, the systematic signal needed to detect the hiatus is 1000 times below the noise level of the data available. in fact the uncertainty of the global latent heat flux amounts to as much as 30 w/m2 for various other reasons [4] even if the uncertainty of the global rh average may likely be less than 1 %rh. atmospheric water flux is a perfect hideout for “the lost heat”. the hydrological cycle is a prominent example for metrological deficiencies in climatology, meteorology and oceanography. scientific analysis must rely on century-long data series of high accuracy and stability, world-wide homogeneity, without spurious trends or technological artefacts, which is by abstract  water plays the  leading thermodynamic role  in earth’s “steam engine” climate. followed by clouds and co2, water vapour  in the  atmosphere is dominating the greenhouse effect. evaporation from the ocean surface is the main route of energy export from the  ocean, the rate of which  is known with poor 20 % uncertainty only. regional climatic trends  in evaporation and precipitation are  reflected in small changes of ocean surface salinity.  observational data of salinity and relative humidity need to be globally comparable within requisite uncertainties over decades and  centuries,  but  both  quantities  rely  on  century‐old  provisional  standards  of  unclear  stability,  or  on  ambiguous  definitions.  this  increasingly urgent and long‐pending problem can only be solved by proper metrological traceability to the international system of  units (si). consistent with such si‐based definitions, state‐of‐the‐art correlation equations for thermophysical properties of water,  seawater,  ice and humid air such as those available from  the  recent oceanographic standard  teos‐10 need to be developed and  adopted as joint international standards for all branches of climate research, in oceanography, meteorology and glaciology for data  analysis and numerical models.   the scor/iapws/iapso joint committee on the properties of seawater jcs is targeting at these aims in cooperation with bipm, wmo  and other international bodies.    acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 | 58  no means granted for the records we possess since meteorological and oceanographic data started to be collected systematically. improvement is urgently needed. anomalies of evaporation and precipitation leave their footprints behind in ocean salinity [5] which can be measured much more precisely than relative humidity or heat flux. but, salinity measurements are not (yet) traceable to the international system of units, si [6] and tiny drifts of the metrological reference materials may remain undetectable, see section 3. the introduction of the new thermodynamic equation of seawater 2010, teos-10 [7] has addressed a number of open questions of salinity measurement and of the definition of relative humidity, as briefly reviewed in section 2. but this work has also shed new light on further fundamental problems which may be solved in close cooperation of a newly established scor1/iapws2/iapso3 joint committee on the properties of seawater, jcs, with international bodies such as bipm4 and wmo5, see section 5. in a recent series of four papers [8], metrological challenges for measurements of key climatological observables are reviewed and future actions are suggested for practical solutions, section 6. 2. seawater standard teos‐10  teos-10, the thermodynamic equation of seawater 2010 [7], was adopted as an international standard for oceanography by the ioc6 in paris in 2009 and by the iugg7 in melbourne in 2011. it provides thermodynamic properties of water, ice, seawater and humid air in a mutually consistent way. its novel axiomatic approach is based on four thermodynamic potentials, subsequently endorsed as iapws documents in 2006, 2008, 2009 and 2010, from which various properties of the pure substances, their mixtures, phase equilibria and composites can be formally derived by mathematical or numerical methods [9], see figure 1. for the first time after a century of international oceanographic standards [10], teos-10 makes use of a chemical composition model for sea salt dissolved in iapso standard seawater (ssw), the standard reference material for oceanographic salinity measurements specified by the practical salinity scale of 1978 (pss-78). the related new referencecomposition salinity scale [11] provides a conversion formula for pss-78 data but additionally permits systematic studies of seawater composition anomalies [12]. teos-10 is highly accurate. within their common ranges of validity, teos-10 is consistent with the cipm-2001 equation for the density of liquid water [13], [14] and with the cipm2007 equation for the density of humid air [15] which are 1 scor: scientific committee on oceanic research, http://www.scor-int.org 2 iapws: international association for the properties of water and steam, http://www.iapws.org 3 iapso: international association for the physical sciences of the oceans, http://iapso.iugg.org 4 bipm: bureau international des poids et mesures, http://www.bipm.org 5 wmo: world meteorological organization, http://www.wmo.int 6 ioc: intergovernmental oceanographic commission of unesco, http://ioc-unesco.org 7 iugg: international union of geodesy and geophysics, http://www.iugg.org recommended for metrology by the cipm8. teos-10 results for the ice point and for the sublimation pressure are the most accurate values currently available [16]. entropy and enthalpy of seawater, ice and humid air available from teos-10 provide precise values of the oceanic heat content [17] and for the latent heat of evaporation and of freezing of seawater [18]. relative fugacity calculated from teos-10 is physically more suitable than relative humidity defined by wmo [19] for the description of non-equilibrium thermodynamic forces at the oceanatmosphere interface [3], [16]. teos-10 was developed in close cooperation between the iapws subcommittee on seawater, scsw, that was established at the 15th international conference on the properties of water and steam in 2008 in berlin, germany, and the scor/iapso working group 127 on thermodynamics and equation of state of seawater, established in 2005 in cairns, australia, and disbanded in 2011 in accordance with the rules governing scor/iapso working groups [10]. wg 127 was chaired by trevor mcdougall, hobart, australia, and scsw by rainer feistel, warnemünde, germany. 3. salinity and relative humidity  conductance measurements relative to a standard reference solution offer higher resolution than si-traceable “absolute” measurements in conductance cells. the difference between the two is crucial for oceanographic density calculations, and so world-wide salinity measurements are carried out with conductivity sensors calibrated with respect to certified ssw specimen, as defined by pss-78. this excellent resolution and comparability prevents the false detection of spurious, virtually significant density gradients in the world ocean but comes at the risk of rising uncertainties in long-term trends [6]. moreover, conductivity measurements alone cannot detect composition anomalies of the solute that may affect the seawater density, such as dissolved silicate in the deep pacific [20]. si-traceability is indispensable for the detection of tiny but climatologically relevant long-term salinity trends. among the surrogate measurands that may substitute conductance for this purpose, density is the most promising candidate, but the 8 cipm: comité international des poids et mesures, http://www.bipm.org/en/committees/cipm/ figure  1.  axiomatic  structure  of  teos‐10.  four  thermodynamic  potentials  (top  row)  contain  all  empirical  coefficients  and  permit  the  calculation  of derived properties in a consistent, independent and complete way.     acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 | 59  intended transition causes numerous practical, technological and metrological problems to be addressed, see section 6. relative humidity is defined by wmo and ashrae9 as the partial pressure of water vapour divided by the same quantity at saturation at the same temperature and pressure. however, several meteorological textbooks, climatological studies or iupac10 offer alternative and mutually inconsistent definitions. most of the definitions fail under certain conditions, for example for humid air in equilibrium with concentrated solutions at low pressures. moreover, different correlation equations for the calculation of dewpoint temperatures or saturation pressures are in practical use, often without explicit specification of uncertainty or range of validity. in order to harmonise this variety and to ensure comparability and stability of measurements, the working group “humidity” of cipm-cct11 is investigating suitable options available for this purpose. among those is relative fugacity [16], [18] which properly describes the thermodynamic driving forces in non-equilibrium situations and naturally covers conditions where other definitions cease to exist. teos-10 is an international standard that offers accurate equations for the calculation of the fugacity of water in humid air [21]. 4. climatological relevance  water is a key player in earth’s climate dynamics; heinrich hertz in 1885 was perhaps the first who painted the physical picture of climate as a “gigantic steam engine” [22]. processes involving water in the atmosphere are very complex, theoretically and observationally demanding, and subject to substantial uncertainties [4]. the fundamental energy conveyor of our climate is the hydrological cycle rather than the radiative "co2 greenhouse" balance often debated in public. water vapour in the atmosphere accounts for 50 60 % of the terrestrial greenhouse effect, in contrast to only about 20 25 % due to carbon dioxide, co2. atmospheric rh also controls the oceanic export of latent heat as well as the formation and distribution of clouds; the latter may contribute another 10 25 % to the greenhouse warming. estimates for the latent-heat fraction of air-sea energy exchange vary between 45 and 90 %. trends in global distributions of humidity, latent heat flux, evaporation and precipitation are closely connected with small but precisely measurable systematic changes in sea-surface salinities [5]. the latter govern oceanic convection processes which may deposit vast amounts of heat in the abyss [1]. as a result of global warming, the amount of tropospheric water is increasing which in turn amplifies the temperature rise via a positive feedback loop, known as the “runaway greenhouse” effect [23]. in contrast, rh over the oceans is rather constant at values of about 80 %rh, almost independently of region, season, or atmospheric warming. 9 ashrae: american society of heating, refrigerating and air-conditioning engineers, http://www.ashrae.org/ 10 iupac: international union of pure and applied chemistry, http://www.iupac.org 11 cct: cipm consultative committee on thermometry, http://www.bipm.org/en/committees/cc/cct/ similarly, no significant trend of the total cloud cover or albedo could be detected yet. the fundamental physical causes for this constancy remain elusive [23] but strong feedback mechanisms between the two “adiabatic invariants” of climate change may be assumed [3], [24]. day-time clouds form the main “input valve” for solar irradiation while surface rh regulates the ocean’s cooling by evaporation. the self-organised loop is closed by the formation of clouds, governed by the amount of atmospheric moisture which originates to 90 % from the ocean. the signature of this process is hidden in the fluctuation spectra of cloudiness and of relative humidity at the sea surface. the characteristic time scales of this feedback are well known and rather short; the residence time of solar energy in the ocean surface layer is between 60 and 90 days, that of water in the troposphere is about 8 days, and that of water in clouds is just 1 hour. short relaxation times indicate high dynamic stability against external fluctuations. small changes of this feedback may easily amplify or compensate effects caused by co2 or other atmospheric perturbations. in contrast to the temperature, small systematic anomalies of the chemical and isotopic compositions of the natural fluid mixtures “seawater” and “humid air” may go almost unnoticed even though they represent key indicators for severe implications of climate change. 5. joint committee on seawater  as long as the understanding of the climate attractor and the predictive capabilities of global circulation models remain insufficient, careful long-term observations extending over decades remain indispensable. however, this urgent need is in striking discrepancy to the fact that the formal definitions and the measurement practice of seawater salinity and atmospheric rh include century-old provisional and ambiguous definitions, in part without proper metrological traceability to the si. such problems were emphasised at the 2010 wmo-bipm workshop on measurement challenges for global observation systems for climate change monitoring in geneva [25]. to overcome this situation and to improve long-term stability and consistency of measurement results, initial meetings between representatives of bipm and iapws took place at the bipm in august 2011 and in february 2012, see figure 2. as a result, a joint effort of iapws, bipm, scor, iapso and wmo has been envisaged to address metrological challenges for measurements of certain key climatological observables, including oceanic salinity and atmospheric relative humidity [8]. in this cooperation of international bodies, figure 2. participants of the bipm‐iapws meeting on 7 february 2012 at the  bipm  headquarter,  pavillon  de  breteuil  at  sèvres  near  paris,  from  left  to  right:  dan  friend,  karol  daucik,  jeff  cooper,  alain  picard,  petra  spitzer,  rainer feistel, michael kühne, andy henson, robert wielgosz.     acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 | 60  iapws may provide highly accurate correlation equations that relate the problematic quantities to suitable surrogate measurands and to other relevant properties, consistent with the teos-10 equations. bipm may support this aim by the endorsement and recommendation of suitable uniform definitions and related metrological standards in the framework of the si. scor, iapso and wmo may guide and support the adoption of the new equations and standards in the atmospheric and oceanographic scientific and technical communities. this important and demanding activity will be coordinated by the joint committee on the properties of seawater12 (jcs) that was founded at the iapws meeting in october 2012 in boulder, colorado. membership and terms of reference were identified in september 2013 during a joint iapws-bipm workshop held at the 16th international conference on the properties of water and steam in greenwich, uk. in return, jcs representatives attended the 2014 meetings of ccqm13 and cct at the bipm in sèvres. jcs is chaired by rich pawlowicz, vancouver, canada. 6. intended activities  jcs in cooperation with ccqm is working toward an oceanic salinity definition traceable to the si. this development may include several subsequent steps: step 1: define salinity as the mass fraction of solute. this was already done by the formal introduction of “absolute salinity” in the framework of teos-10 [11], [12]. step 2: calculate this mass fraction for a chemical reference model. the reference composition introduced in 2008 [11] satisfies this requirement. step 3: by experimental and theoretical data, relate step 2 to si-traceable surrogate measurands of sufficient accuracy: this was achieved by the construction of the teos-10 equation of state [26] which relates seawater density to the absolute salinity, and by studies that demonstrate the insensitivity of this relation with respect to small composition anomalies [12], [20], [27], colloquially known as “millero’s rule” [28]. seawater density relative to pure-water density can be measured in the laboratory within an uncertainty of less than 3 ppm [29], [30], [31], which is sufficiently accurate for oceanography. step 4: officially endorse the relation of step 3 as a standard equation. the introduction of teos-10 as an international standard for oceanography by ioc and iugg served this purpose. step 5: produce reference materials of certified salinity based on the relation defined in step 4. work on this task is in progress. among the various practical problems involved is the stability of certified density in distributed and stored ampoules of standard seawater. step 6: calibrate routine instruments (whatever quantity they actually measure) with the reference materials provided by step 5. it is planned that conventional bathysondes (ctd) equipped with conductivity sensors will be calibrated with respect to 12 jcs: scor/iapws/iapso joint committee on the properties of seawater, http://www.teos-10.org/jcs.htm 13 ccqm: cipm consultative committee for amount of substance, http://www.bipm.org/en/committees/cc/ccqm/ density rather than to salinity. other studies of external groups investigate the sea-going use of in-situ sensors that measure density or refractive index rather than conductivity. step 7: to support step 6, add auxiliary standard equations, such as between salinity and conductivity, or between density and refractive index. these tools may permit an easy in-situ detection of seawater composition anomalies but will in advance require extensive future laboratory work. iapws supports external activities of this kind by certified research needs [32]. cct in cooperation with jcs is working toward a new definition of relative humidity. a tentative series of actions is suggested here, similar to those of salinity, but likely the practice of future scientific and technological developments may take other roads. step 1: develop an si definition of relative humidity. a promising candidate for this quantity is relative fugacity to which all other rh definitions in practical use (by wmo, ashrae, etc.) may be considered as suitable proxies. step 2: calculate the quantity of step 1 for a chemical reference model. the cipm stoichiometric model of dry air [15] is consistent with the teos-10 equation of state for humid air. teos-10 permits the calculation of fugacity and relative fugacity, but more work is needed here on the details of, e.g., the roles of varying amounts of co2 or of the dissolution of air in water. step 3: by experimental and theoretical data, relate step 2 to si-traceable surrogate measurands of sufficient accuracy. this may in a first approach be achieved by the teos-10 equation for humid air but may require further refinement in the future. step 4: officially endorse step 3 as a standard equation. this may take the form of a cipm equation for the relative humidity or relative fugacity. step 5: produce reference materials or calibration devices (such as saturated solutions or humidity generators) of certified rh based on step 4. step 6: calibrate routine instruments (such as the various humidity sensors available) against the standards specified in step 5. 7. conclusions  water in different phases and mixtures is a key element of the climate system. long-term chemical and isotopic changes of oceanic and atmospheric water mixtures must be monitored regularly and precisely. mutual consistency and comparability of observational measurement results are indispensable. future sibased definitions of seawater salinity and relative humidity are necessary and long overdue. iapws formulations for climatologically relevant quantities may support upcoming definitions and measurement standards developed by the bipm. there is a wide and important field for iapws as a standard-developing organisation that is specialised on aqueous systems to engage in climate research, as there is for bipm to further engage in climate research to ensure world-wide uniformity of measurements and their traceability to the si. references  [1] x. chen, k.-k. tung, varying planetary heat sink led to globalwarming slowdown and acceleration, science 345(2014), pp. 897903. [2] j. curry, uncertain temperature trend, nature geosci. 7(2014), pp. 83-84.   acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 | 61  [3] r. feistel, w. ebeling, physics of self-organization and evolution, wiley-vch, weinheim, 2011, isbn 978-3-52740963-1. [4] s.a. josey, s. gulev, l. yu, “exchanges through the ocean surface”, in: ocean circulation and climate. a 21st century perspective. g. siedler, s.m. griffies, j. gould, j.a. church (editors). elsevier, amsterdam, 2013, isbn: 978-0-12-391851-2, pp. 115-140. [5] p.j. durack, s.e. wijffels, t.p. boyer, “long-term salinity changes and implications for the global water cycle”, in: ocean circulation and climate. a 21st century perspective. g. siedler, s.m. griffies, j. gould, j.a. church (editors). elsevier, amsterdam, 2013, isbn: 978-0-12-391851-2, pp. 727-757. [6] s. seitz, r. feistel, d.g. wright, s. weinreben, p. spitzer, p. de bievre, metrological traceability of oceanographic salinity measurement results, ocean sci. 7(2011), pp. 45-62. [7] ioc, scor, iapso, the international thermodynamic equation of seawater 2010: calculation and use of thermodynamic properties. intergovernmental oceanographic commission, manuals and guides no. 56, unesco (english), 196 pp., paris. 2010, http://www.teos-10.org [8] r. feistel, r. wielgosz, s.a. bell, m.f. camões, j.r. cooper, p. dexter, a.g. dickson, p. fisicaro, a.h. harvey, m. heinonen, o. hellmuth, h.-j. kretzschmar, j.w. lovell-smith, t.j. mcdougall, r. pawlowicz, p. ridout, s. seitz, p. spitzer, d. stoica, h. wolf, metrological challenges for measurements of key climatological observables. part 1: overview, part 2: oceanic salinity, part 3: seawater ph, part 4: atmospheric relative humidity, metrologia (2015), in press. [9] r. feistel, d.g. wright, k. miyagawa, a.h. harvey, j. hruby, d.r. jackett, t.j. mcdougall, w. wagner, mutually consistent thermodynamic potentials for fluid water, ice and seawater: a new standard for oceanography, ocean sci. 4(2008), pp. 275-291. [10] r. pawlowicz, t.j. mcdougall, r. feistel, r. tailleux, an historical perspective on the development of the thermodynamic equation of seawater – 2010, ocean sci. 8(2012), pp. 161–174. [11] f.j. millero, r. feistel, d.g. wright, t.j. mcdougall, the composition of standard seawater and the definition of the reference-composition salinity scale, deep-sea res. i 55(2008), pp. 50-72. [12] d.g. wright, r. pawlowicz, t.j. mcdougall, r. feistel, g.m. marion, absolute salinity, "density salinity" and the referencecomposition salinity scale: present and future use in the seawater standard teos-10, ocean sci. 7(2011), pp. 1-26. [13] m. tanaka, g. girard, r. davis, a. peuto, n. bignell, recommended table for the density of water between 0 °c and 40 °c based on recent experimental reports, metrologia 38(2001), pp. 301-309. [14] a.h. harvey, r. span, k., fujii, m., tanaka, r.s. davis, density of water: roles of the cipm and iapws standards, metrologia 46(2009), pp. 196-198. [15] a. picard, r.s. davis, m. gläser, k. fujii, revised formula for the density of moist air (cipm-2007), metrologia 45(2008), pp. 149-155. [16] r. feistel, teos-10: a new international oceanographic standard for seawater, ice, fluid water and humid air, int. j. thermophys. 33(2012), pp. 1335-1351. [17] t.j. mcdougall, potential enthalpy: a conservative oceanic variable for evaluating heat content and heat fluxes, j. phys. oceanogr. 33(2003), pp. 945–963. [18] r. feistel, d.g. wright, h.-j. kretzschmar, e. hagen, s. herrmann, r. span, thermodynamic properties of sea air, ocean sci. 6(2010), pp. 91-141. [19] wmo, guide to meteorological instruments and methods of observation. world meteorological organisation, geneva, 2008, isbn 978-92-63-10008-5. [20] r. pawlowicz, d.g. wright, f.j. millero, the effects of biogeochemical processes on oceanic conductivity/salinity/density relationships and the characterization of real seawater, ocean sci. 7(2011), pp. 363387. [21] r. feistel, j.w. lovell-smith, o. hellmuth, virial approximation of the teos-10 equation for the fugacity of water in humid air. int. j. thermophys. 36(2015), pp. 44-68. [22] j.f. mulligan, g.g. hertz, an unpublished lecture by heinrich hertz: “on the energy balance of the earth”, am. j. phys. 65(1997), pp. 36-45. [23] w. ingram, a very simple model for the water vapour feedback on climate change. quart. j. roy. met. soc. 136(2010), pp. 30-40. [24] r. feistel, “water, steam and climate”, proc. 16th int. conf. prop. water steam, greenwich, uk, september 2013. [25] bipm, “wmo-bipm workshop on measurement challenges for global observation systems for climate change monitoring: traceability, stability and uncertainty”, wmo headquarters, geneva, switzerland, 2010, isbn 13 978-92-822-2239-3. [26] r. feistel, a gibbs function for seawater thermodynamics for –6 to 80 °c and salinity up to 120 g/kg, deep-sea res. i, 55(2008), pp. 1639-1671. [27] r. feistel, g.m. marion, r. pawlowicz, d.g. wright, thermophysical property anomalies of baltic seawater, ocean sci. 6(2010), pp. 949-981. [28] f.j. millero, k. kremling, the densities of baltic waters, deepsea res. 23(1976), pp. 1129-1138. [29] k. kremling, new method for measuring density of seawater, nature 229(1976), pp. 109-110. [30] h. wolf, determination of water density: limitations at the uncertainty level of 1×10−6, accred. qual. assur. 13(2008), pp. 587–591. [31] r. feistel, s. weinreben, h. wolf, s. seitz, p. spitzer, badel, g. nausch, b. schneider, d.g. wright, density and absolute salinity of the baltic sea 2006–2009, ocean sci. 6(2010), pp. 324. [32] iapws, thermophysical properties of seawater. iapws certified research need – icrn 16 (2014), http://www.iapws.org/icrn.html. introductory notes for the acta imeko first issue 2023 general track acta imeko issn: 2221-870x march 2023, volume 12, number 1, 1 2 acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 1 introductory notes for the acta imeko first issue 2023 general track francesco lamonaca1 1 department of department of computer science, modeling, electronics and systems engineering (dimes), university of calabria, ponte p. bucci, 87036, arcavacata di rende, italy section: editorial citation: francesco lamonaca, introductory notes for the acta imeko first issue 2023 general track, acta imeko, vol. 12, no. 1, article 1, march 2023, identifier: imeko-acta-12 (2023)-01-01 received march 29, 2023; in final form march 29, 2023; published march 2023 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: francesco lamonaca, e-mail: editorinchief.actaimeko@hunmeko.org dear readers, as usual also this issue includes a general track aimed to collect contributions that do not relate to a specific event. as editor in chief, it is my pleasure to give to you an overview of these papers, with the aim of encouraging potential authors to consider sharing their research through acta imeko. power quality (pq) measurements and auditing play a vital role for smart grid applications, industrial safety and reliability. the major electrical pq characteristics and parameters are studied and analysed for single-phase and poly-phase systems in the ieee 1159 recommended practice. in [1], a power quality monitoring system to identify the utilisers performance, audit, pq issues is presented practically for a two-panel boards switchgear equipment as a case study in malaysia. today, an innovative leap for wireless sensor networks, leading to the realization of novel and intelligent industrial measurement systems, is represented by the requirements arising from the industry 4.0 and industrial internet of things (iiot) paradigms. in fact, unprecedented challenges to measurement capabilities are being faced, with the ever-increasing need to collect reliable yet accurate data from mobile, battery-powered nodes over potentially large areas. therefore, optimizing energy consumption and predicting battery life are key issues that need to be accurately addressed in such iot-based measurement systems. this is the case for the additive manufacturing application considered in [2], where smart battery-powered sensors embedded in manufactured artifacts need to reliably transmit their measured data to better control production and final use, despite being physically inaccessible. a low power wide area network (lpwan), and in particular lorawan (long range wan), represents a promising solution to ensure sensor connectivity in the aforementioned scenario, being optimized to minimize energy consumption while guaranteeing long-range operation and low-cost deployment. in the application presented in [2], lora-equipped sensors are embedded in artifacts to monitor a set of meaningful parameters throughout their lifetime. in this context, once the sensors are embedded, they are inaccessible, and their only power source is the originally installed battery. therefore, in [2], the battery lifetime prediction and estimation problems are thoroughly investigated. for this purpose, an innovative model based on an artificial neural network (ann) is proposed and developed starting from the discharge curve of lithium-thionyl chloride batteries used in the additive manufacturing application. the results of experimental campaigns carried out on real sensors were compared with those of the model and used to tune it appropriately. the results obtained are encouraging and pave the way for interesting future developments. bouaouiche et al. in [3] present an innovative method for the analysis of the vibration signals of a ball bearing for the diagnosis of rotating machine defects by vibration analysis. the signals are available on the platform case western reserve university in the form of matlab files. the proposed approach consists of several methods and steps including the decomposition of the signals by feature mode decomposition (fmd) method and then, according to the kurtosis values by selecting the signals useful for the defect diagnosis. in [4], the design, the prototyping, and the first results of experimental tests of a capacitive oil level sensor (cls) intended for aeronautical applications are described. due to potentially high vibrational stresses and the presence of high electromagnetic interferences (emi), the working conditions on aircraft can be considered quite harsh. hence, both the sensing part and the conditioning circuit should meet strict constraints. for this reason, in the design phase, great attention has been paid also to the mechanical characteristics of the probes. all the design aspects are exposed and the main advantages concerning alternative level sensing techniques are discussed, and the preliminary experimental results of sensitivity, linearity, hysteresis, and settling time tests are presented and commented. analogue data acquisition is a common task which has application in several fields such as scientific research, industry, food production, safety, and environmental monitoring. it can be carried mailto:editorinchief.actaimeko@hunmeko.org acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 2 out either using systems designed ad-hoc for a specific application or by using general-purpose digital acquisition boards (daq). several daq solutions are nowadays available on the market, however, most of them are extremely expensive and come as commercial closed products, a factor which prevents users to adapt the system to their specific applications and limits the product compatibility to few operating systems or platforms. the paper in [5] describes the design and the preliminary metrological characterisation of a digital data acquisition solution based on the teensyduino development board. the aim of the project presented in [5] is to create a hardware and software infrastructure suitable to be employed on several operating systems and that can be freely modified by the users when required. taking advantage of the teensyduino features, the proposed system is easy to be calibrated and used, and it provides functions and performance comparable to many commercial daqs, but at a significantly lower cost. in recent years, technological innovation has acquired a fundamental role in the agri-food sector, in particular in food quality control. the development of technology allowed to improve the quality of food before it is placed on the market. recently, noninvasive techniques such as those operating in the thz spectral band were applied to the field of food quality control. in the laboratories of the enea centre in frascati, close to rome (italy), a thz imaging system has been developed operating in reflection mode, together with an experimental setup able to measure both reflection and transmission of the samples in the frequency range from 18 ghz 40 ghz. with these two setups, the authors in [6] will distinguish rotten and healthy hazelnuts by acquiring in real time both images of the fruit inside the shell by using the imaging system and the transmission data exploiting the 18-40 ghz system. also this issue shows heterogeneous topics, all connected by the common focus on measurement and instrumentation. i hope you will enjoy your reading. francesco lamonaca editor in chief references [1] d. v. n. ananth, n. hannoon, mohamed shahriman bin mohamed yunus, mohd hanif bin jamaludin, v. v. s. s. sameer charavarthy, p. s. r. chowdary, “real-time power quality measurement audit of government building – a case study in compliance with ieee 1159”, acta imeko, vol. 12 (2023) no. 1, pp. 1-9. doi: 10.21014/actaimeko.v12i1.1287 [2] a. morato, t. fedullo, s. vitturi, l. rovati, f. tramarin, “a learning model for battery lifetime prediction of lora sensors in additive manufacturing”, acta imeko, vol. 12 (2023) no. 1, pp. 1-10. doi: 10.21014/actaimeko.v12i1.1400 [3] k. bouaouiche, y. menasria, d. khalfa, “diagnosis of rotating machine defects by vibration analysis”, acta imeko, vol. 12 (2023) no. 1, pp. 1-6. doi: 10.21014/actaimeko.v12i1.1438 [4] f. attivissimo, f. adamo, l. de palma, d. lotano, a. di nisio, “first experimental tests on the prototype of a capacitive oil level sensor for aeronautical applications”, acta imeko, vol. 12 (2023) no. 1, pp. 1-6. doi: 10.21014/actaimeko.v12i1.1474 [5] l. lombardo, “multi-platform solution for data acquisition”, acta imeko, vol. 12 (2023) no. 1, pp. 1-8. doi: 10.21014/actaimeko.v12i1.1475 [6] manuel greco, fabio leccese, emilio giovenale, andrea doria, “terahertz techniques for better hazelnut quality”, acta imeko, vol. 12 (2023) no. 1, pp. 1-8. doi: 10.21014/actaimeko.v12i1.1477 https://doi.org/10.21014/actaimeko.v12i1.1287 https://doi.org/10.21014/actaimeko.v12i1.1400 https://doi.org/10.21014/actaimeko.v12i1.1438 https://doi.org/10.21014/actaimeko.v12i1.1474 https://doi.org/10.21014/actaimeko.v12i1.1475 https://doi.org/10.21014/actaimeko.v12i1.1477 microsoft word 289-1777-2-le-rev acta imeko  issn: 2221‐870x  september 2015, volume 4, number 3, 2 ‐ 3    acta imeko | www.imeko.org  september 2015 | volume 4 | number 3 | 2  introduction to the special issue on the 20th imeko tc‐4  international symposium and the 18th tc‐4 workshop on  adc and dac modelling and testing  ján šaliga 1 , sergio rapuano 2    1  technical university of kosice, faculty of electrical engineering and informatics, dept. of electronics and multimedia telecommunications,  letná 9 04120 košice, slovakia.  2  university of sannio, department of engineering, corso garibaldi 107, 82100 benevento, italy.    section: editorial   citation: ján šaliga, sergio rapuano, introduction to the special issue on the 20th imeko tc‐4 international symposium and the 18th tc‐4 workshop on adc  and dac modelling and testing, acta imeko, vol. 4, no. 3, article 1, september 2015, identifier: imeko‐acta‐04 (2015)‐03‐01  editor: paolo carbone, university of perugia, italy  received june 25, 2015; in final form june 25, 2015; published june 2015  copyright: © 2015 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  corresponding author: ján šaliga, email: jan.saliga@tuke.sk       the imeko technical committee number 4 on ‘‘measurement of electric quantities’’ comprises researchers from all over the world dealing with electrical and electronic measurements. such research community meets annually at the tc-4 symposia or the imeko world congresses where the most recent original proposals and research results are presented and discussed in several oral and poster sessions focused on specific topics. after having been organized in many countries, from bulgaria in 1990 to barcelona in 2013, the tc-4 symposium came back to italy for the third time in its history. the 2014 edition of the symposium and the 18th tc-4 workshop on adc and dac modelling and testing, in fact, were held in benevento, a historical town amid the southern italy known for the significant memories of its glorious past and the beauty of its unesco world heritage sites. the theme of 2014 was "research on electrical and electronic measurement for the economic upturn". benevento is the site of a small and young but very dynamic university that hosted the tc-4 symposium 2014, which, gathering more than 200 papers presented by researchers of all over the world, was one of the most successful ones. among this relevant number of contributions, the board of tc-4 selected a few papers to be extended and submitted for the peer review of acta imeko journal. in this special issue you can find the 11 articles that, according to the reviewers, were considered worthy of publication in this journal. the double selection of the tc-4 board and the journal reviewers provided a distillate of the most advanced scientific activity in this field. the papers included in the special issue represent a sample of the state of the art of the research on several topics associated with the measurement of electric quantities. more in detail, the traditional research field of analog-todigital converter (adc) metrology is represented in this issue by two papers. the paper reliable, accurate and scalable adc test methods for standard software platforms, by vilmos pálfi, tamás viroztek, and istván kollár, presents some algorithms to overcome some problems arising from the application of the sinewave histogram test, a commonly used method to characterize nonlinear behaviour of adcs. the paper digital reconstruction stage for the fbd ʃ∆–based adc in multistandard receiver: theoretical analysis and design, by rihab lahouli, manel ben-romdhane, chiheb rebai and dominique dallet, presents the design of a digital reconstruction stage for a frequency band decomposition (fbd)-based adc architecture for digitizing multistandard receiver signals. another research topic the tc-4 researchers are focused on is the time synchronization among instruments, always needed when a consistent result is requested from a distributed group of instruments or sensors. the paper time coordination of standalone measurement instruments by synchronized triggering, by francesco lamonaca, domenico luca carnì and domenico grimaldi, proposes a new architecture of hardware interface for the synchronous triggering of the measurement instruments in a distributed measurement system with the aim to avoid the effects of concurrent software processes, and to reduce the causes of delay in detecting the trigger condition. the paper hybrid time synchronization for underwater sensor networks, by oriol pallares valls, pierre-jean bouvet and acta imeko | www.imeko.org  september 2015 | volume 4 | number 3 | 3  joaquin del río, is focused on a particular but significant case of distributed measurement system. it presents a study of time synchronization problems over underwater sensor networks, taking into account the main communication challenges of the water channel and observing its behaviour in simulation and real tests. the current measurement methods and instruments are a core business for tc-4 researchers. in the paper setting up of a floating gate test bench in a low noise environment to measure very low tunneling currents, the authors jérémy postel-pellerin, gilles micolau, philippe chiquet, jeanne melkonian, guillaume just, daniel boyer and cyril ginoux, propose a solution to measure very low tunneling currents in non-volatile memories, based on the floating-gate technique. the proposed key factor is to carry out the measurements in an extremely low-noise environment that allows the authors to reach current levels lower than the ones obtained by direct measurements. the nondestructive testing is a main research field in many areas, including telecommunications, electronics and manufacturing. the research results presented in the two following papers aim at easing the metrological characterization of conductive or superconductive elements by means of microwaves. in particular the paper surface conductance and microwave scattering in semicontinuous gold films, by jan obrzut, presents techniques to study mechanisms of electromagnetic response of randomly structured metallic networks. the paper broadband corbino spectroscopy and stripline resonators to study the microwave properties of superconductors, by marc scheffler, maximilian felger, markus thiemann, daniel hafner, katrin schlegel, martin dressel, konstantin ilin, michael siegel, silvia seiro, christoph geibel and frank steglich, describes two different techniques to study superconductors at microwave frequencies: the broadband corbino approach can be used on a very wide range of frequencies almost continuously but is limited to thin-film samples whereas the stripline resonators are sensitive enough to study low-loss single crystals but can provide results only at a set of discrete resonant frequencies. the design and characterization of measurement transducers is another core activity of tc-4 researchers. several papers were presented in this field during the symposium. this issue includes two papers on this topic. the paper estimation of stepping motor current from long distances through cablelength-adaptive piecewise affine virtual sensor, by alberto oliveri, mark butcher, alessandro masi and marco storace, proposes a piecewise affine virtual sensor, a function of past inputs and measured outputs of a system, for the estimation of the motor-side current of hybrid stepper motors, which actuate the lhc (large hadron collider) collimators at cern. the paper new generation of cage-type current shunts developed using model analysis, by věra nováková zachovalová, martin šíra, pavel bednář and stanislav mašláň, presents a new type of ac/dc current shunt ranging from 30 ma to 10 a developed using a lumped circuit element model. during the tc-4 symposium 2014, several special sessions on interdisciplinary topics were organized gathering researchers from the more traditional tc-4 fields and quite diverse ones that are based on metrology or that are the base for a correct measurement. legal metrology and software quality are two examples of such fields where metrology merges with and supports or is supported by different disciplines. the paper painting authentication by means of a biometric-like approach, by giuseppe schirripa spagnolo, lorenzo cozzella, maurizio caciotta, roberto colasanti and gianluca ferrari, for example, an innovative system based on smartphone acquisition and mobile application is proposed to verify artwork authenticity based on random intrinsic object characteristics. software is an essential component of many measurement systems, while metrology concepts are the base of any method for evaluating the quality of software. the paper some thoughts on quality models: evolution and perspectives, by luigi buglione, discusses from an evolutionary perspective what software quality has been, is and could be perceived and defined during next years, by a measurement perspective. the editors of this issue and the authors are grateful to all the colleagues and institutions that have made these contributions possible. a particular acknowledgement is due to all the tc-4 board members and the reviewers that with their efforts allowed the production of this special issue. we sincerely hope that you will enjoy the reading of this issue. acta imeko  issn: 2221‐870x  june 2015, volume 4, number 2, 80‐84    acta imeko | www.imeko.org  june 2014 | volume 4 | number 2 | 80  method of integral nonlinearity testing and correction of  multi‐range adc by direct measurement of output voltages  of multi‐resistors divider   hu zhengbing 1 , roman kochan 2 , orest kochan 3 , su jun 4 , halyna klym 2   1  huazhong normal university, no.152 luoyu road, wuhan, 430079, p.r.china  2  specialized computer systems department of lviv national polytechnic university, s. bandery str., 12, lviv, 79013, ukraine   3  information measurement technology department of lviv national polytechnic university, s. bandery str., 12, lviv, 79013, ukraine   4  internet of things department of hubei university of technology, no.1 lizhi road, wuhan, 430068, p.r.china       section: research paper   keywords: integral nonlinearity; multi‐resistor voltage divider; residual error  citation: hu zhengbing, roman kochan, orest kochan, su jun, halyna klym, method of integral nonlinearity testing and correction of multi‐range adc by  direct measurement of output voltages of multi‐resistors divider, acta imeko, vol. 4, no. 2, article 14, june 2015, identifier: imeko‐acta‐04 (2015)‐02‐14  editor: paolo carbone, university of perugia, italy  received october 23, 2014; in final form february 10, 2015; published june 2015  copyright: © 2015 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: this work was supported by china international science and technology cooperation project (cu01‐11) and ukrainian ministry of education and  science grant 0115u000446  corresponding author: roman kochan, e‐mail: kochan.roman@gmail.com    1. introduction  the application of digital signal processing algorithms and computer systems in all fields of our life results in the implementation of analogue-to-digital converters (adc) as a component of modern measurement systems. in some cases the adc’s metrology parameters determine the characteristics of the whole measurement system. particularly, this point is important for measurement systems of electrical quantities. therefore improving the adc is an essential task for higher measurement accuracy. the market of precision dc adc’s is led by converters based on sigma-delta modulators sdm [1], [2]. the high accuracy of these components is provided by the implementation of null setting and calibration. these methods provide decrease of additive and multiplicative conversion errors. therefore, the conversion error is composed of the following errors: calibration voltage source, multiplexer and residual adc errors. the most significant component of the residual adc error is its integral nonlinearity. for example, the maximum allowable integral nonlinearity of the 24-bit adc ad7714 [3] is 15 ppm. this nonlinearity corresponds to the 16th bit, that is approximately 8 least significant bits (lsb) are a priori inaccurate and excessive. therefore, for this adc, to obtain a higher accuracy than 15 ppm the integral nonlinearity should be corrected. in the same case, the noise level of this adc does not exceed 2.5 lsb. therefore, the adc has approximately 5.5 stable bits, and therefore it cannot be used when high accuracy is required. moreover, the accuracy of measurement results obtained by the implementation of the substitution method [4] in an adc is defined by the adc’s integral nonlinearity [5]. so correction of the adc’s integral nonlinearity provides a higher accuracy of the measurement results. in [6] a method for the identification of the adc’s integral nonlinearity in the set of testing points conventionally called as basic method was proposed. this method provides the generation of a set of testing points, which correspond to the number sequence run )1( where ru is the range of the adc and n an integer. this means that all testing points are grouped abstract  a method of testing points generation for the identification and correction of the integral nonlinearity of high performance adc’s is   presented. the proposed method is based on averaging all voltages of the multi‐resistor voltage divider. the influence of the resistors  errors and random errors of the adc on the residual error of the integral nonlinearity correction for method based on multi‐resistor  divider is investigated.  acta imeko | www.imeko.org  june 2014 | volume 4 | number 2 | 81  in the lower half of the adc’s range. the main objective of this work is to develop and investigate the method of adc’s integral nonlinearity identification and correction with a uniform distribution of the testing points over the range. 2. approach of testing points generation  the proposed method of testing points generation is based on analog to digital conversion of the output signals of the voltage divider consisting of n serially combined resistors nrrr ,...,, 21 , connected to the reference voltage source refu . the measurement circuit is shown in figure 1. according to kirchhoff’s voltage law, we have the following equation    n i irref uu 1 (1) where niu ir ,1,  is the voltage of the appropriate resistors of the divider. the average voltage of all resistors of the divider u can be computed as:    n i iru n u 1 1 (2) taking into account (1), (2) can be presented as n u u ref (3) it means that the average voltage of all resistors of the divider u does not depend on the voltages of the separate resistors. in addition, according to ohm’s law, the resistances of these resistors do not influence the average voltage. so, we have an indirect measurement described by the function  xhy  , where refuxuy  ,   n x xh  . therefore, the absolute measurement error y is [7]:   xxhy  / (4) where  xh/ is the derivative of the function  xh and x the absolute error of the argument. taking into account that n is a natural number, (4) can be converted to n refu u   (5) where u  is the absolute error of the average voltage u and refu  the absolute error of the voltage source refu . the relative error of the average voltage u – u  is ref ref ref u ref u ref u u u u n u n u         %100%100%100 (6) with refu  the relative error of the voltage source refu . taking into account (6), the next intermediate conclusion can be made: the error caused by the measurement converter based on a multi-resistor voltage divider with averaging voltages of all resistors tends to zero. it provides the opportunity of generating the set of testing signals for the adc with an exactly predefined ratio. in case of adc calibration by the voltage source of the divider refu , the average voltage u as testing point for the identification of the adc’s integral nonlinearity can be used. the result of analog to digital conversion ( c ) of an input voltage u [8] is:  ufu u cc cc ref ref    00 (7) where: 0c – result of analog to digital conversion by the null setting channel; refc – result of analog to digital conversion by the calibration channel (for input connected to voltage refu );  uf – integral nonlinearity of adc’s conversion function, and for     00  fuf ref . taking into account (7), and using (2) and (3) results in   refref n i ref ii u n u cc ufcc n 11 1 0 0      (8) where ic is the result of analog-to-digital conversion of the voltage iru . after simplifying (8) we get    0 1 0 ccufcc ref n i ii   (9) the average voltage of the nonlinear component of the conversion function for all voltages generated by the multiresistor divider  uf can be computed as       n i iuf n uf 1 1 (10) taking into consideration (10), (9) can be transformed as               n i refi cccc n uf 1 00 1 (11) the value of  uf is computed using the value of the integral nonlinearity at the testing point nuu ref . uref ur1 ur2 urn r1 r2 rn і figure 1. circuit of  n ‐resistors voltage divider.  acta imeko | www.imeko.org  june 2014 | volume 4 | number 2 | 82  an analysis of the influence of the tested adc on the residual error [6] shows the potential of this implementation method for an adc with a smooth conversion characteristic. so, it is shown that the multi-resistors voltage divider provides precision identification of the adc’s integral nonlinearity in one testing point without using precision components. the proposed method is called a basic method [6] and it provides increasing the number of generated testing points by choosing n . since n contains the set of natural divisors  tmm ,,1  , it is the set of natural numbers  tkk ,,1  which satisfies the condition tikmn ii ,1;  . it allows the conversion of voltages on the cascades of  tiki ,1 serially connected resistors, which corresponds to (1). therefore, the integral nonlinearity of the adc can be computed in accordance to the set of t voltages: ti m u k n u u i ref i ref i ,1;  (12) the error of all these voltages corresponds to (6), and only one reference voltage source can be used. 3. proposed method of generating testing points  the analysis of testing results of the basic method of identification and correction of the adc’s integral nonlinearity [6] showed a linear dependence of the residual conversion error of the density of testing points concentration. it allows us to separate at least two subranges: lower and higher half of the adc’s range, which has different residual errors after nonlinearity correction using the basic method. the residual nonlinearity error of the upper subrange is approximately one order higher than the residual nonlinearity error of the lower subrange. taking into account these results we propose to generate testing points for nonlinearity identification of a multirange adc (in the simplest case for a double range adc) as the voltages of the serially connected resistors of the multiresistor voltage divider, measured using the lower subrange of the highest range and to use these voltages for calibration and nonlinearity identification/correction for all other ranges. in the case of a voltage divider with n resistors ( nrrr ,...,, 21 ), the following testing points are generated: the first is the voltage on resistor 1r , the second is the voltage on the serially connected resistors 1r and 2r , the third for 31...rr , etc. up to the 2 n th: 21... nrr . in general, this approach provides the generation of 2 n testing points with relative high precision uniformly distributed in the lower half of the highest range of the adc. calibration and nonlinearity identification of the lower ranges provide high accuracy and a uniform distribution (in the case when the ratio of higher range to lower range is not less then two) of all generated testing points. besides, it is possible to generate an arbitrary number of testing points by selecting an appropriate n and increasing the generated testing points with a smaller increase of n in comparison with the basic method. 4. investigation of the residual error  investigation of the residual nonlinear error by experiments demands high precision equipment with errors 3…5 times less then the expected residual nonlinearity, and corresponding to 0.5 ppm. besides, it is necessary to have the opportunity to set the level and the form of the adc’s nonlinearity with the same error level. therefore, the aim is to investigate the proposed method by simulation and to evaluate the influence of resistor errors and adc noise on the residual nonlinear error for various nonlinear functions. generally, the method of investigation results in emulation of the integral nonlinearity by a set of curves and its computing according to the proposed method. the difference between emulated and computed curves is the error for analysis. the algorithm for this investigation consists of the following steps:  definition, by random way, of two curves, which simulate the adc’s nonlinearities in the higher and lower ranges;  definition of resistance of the divider’s resistors nrrr ,...,, 21 with random deviation from the average value for simulation of the resistors’ error;  computation of the results of the analog to digital conversion for appropriate combinations of resistors for the implementation of the basic method for adc’s higher range nonlinearity identification;  noising of these conversion results to simulate the random error of the adc;  computation of parameters of the correction function for the adc’s higher range nonlinearity;  computation of voltages on the serially connected resistors 2 131211 ...;...;...;,; nrrrrrrr using simulated results of the analog-to-digital conversion for these voltages in the higher range and correction of the adc’s nonlinearity in that range;  computation of the results of the analog-to-digital conversion for these voltages using the lower range of the adc;  noising these conversion results for simulation of the random error;  computation of parameters for the correction function for the lower range nonlinearity;  computation of the residual error of the integral nonlinearity correction for the lower range as the difference between the defined curve and the computed correction function for this range. so, accumulating the set of residual error curves we can implement the statistical data manipulation methods for investigation of parameters of the proposed method and their sensitivity to the adc’s random error and resistors error. the nonlinearity simulation curves for the higher and lower ranges are based on the fourth order polynomial functions with random coefficients [6]. the validity criterion of each curve is not exceeding by the absolute maximum value on the range of the adc of the 250 quantums. this constant corresponds to the maximum allowable nonlinearity of the ad7714. examples of such curves are presented in figure 2, with nonlinearity in quantum along the y-axis, and the input voltage in percents of the range along the x-axis. also, two curves with maximum and minimum values for 500 curves generated for our investigation are presented. the software verification was done by setting “ideal” resistors and adc (zero error of all resistors and zero adc’s random error). in this case we obtained a maximum error of 0.05 quantum, which we can explain by error rounding during computation. therefore, we can expect that the developed acta imeko | www.imeko.org  june 2014 | volume 4 | number 2 | 83  models and software are adequate and can be used for investigation of the influence of component errors on the residual error after integral nonlinearity correction. the simulation was done for a 12-resistors voltage divider. this number of resistors was selected based on two contradictory requirements:  maximum number of natural dividers for implementation of the large amount of generated testing points (number 12 totally has six dividers: 1, 2, 3, 4, 6, 12);  minimum number of resistors for decreasing of their channels of multiplexer and simplification (a 12-resistors divider demands 2 switches with 13 channels for the multiplexer). the simulated resistors’ error is ±1 and ±2 %. it corresponds to 10 … 20 years of exploitation of widely used wire-wound-type resistors with allowable error ±0.1 %. the curves of residual error vs input voltage for the lower range in the case of resistor error ±1 % is presented in figure . the five randomly selected curves and two curves with maximum and minimum values for 500 curves are presented. as we can see, the maximum value does not exceed 1.5 quantum in the worst case. this value is comparable with the adc’s resolution. the maximum residual error for resistors with error ±2 % does not exceed 3.5 quantums. this value is comparable with the adc noise. the results testified the residual error for a noise level of ± 6 quantums. they are presented in figure 4. there are five randomly selected curves and two curves with maximum and minimum values for 500 curves. as we can see, the maximum value does not exceed 18 quantums in the worst case. generally, these results are close to the results of the basic method investigation [6] but they correspond to the whole lower range of the adc instead of the lower subrange for the basic method. the form of the curves obtained for other noise levels of the adc is similar to the curves presented in figure 4. figure 5 shows the dependence of the maximum value of the residual error for the lower range versus the noise level. taking into consideration the linear character of this dependence, we can conclude that the noise influence coefficient for the presented method does not exceed three for a 12-resistors voltage divider. 5. conclusions  the investigation of the proposed method of integral nonlinearity identification and correction for the lower range of the adc leads to the following conclusions:  the number of generated testing points depends on the number of resistors in the voltage divider and all of them are evenly distributed over the range. it provides the generation of testing points corresponding to the requirements of the actual standard for metrology verification of adcs with a smooth conversion characteristic [9];  the influence of resistor errors of the voltage divider on the residual error is comparable with the adc’s resolution and it is negligible in comparison with other errors;  the influence of the adc’s random error on the residual error is dominating and proportional to its noise level with proportionality factor three for a 12-resistors voltage divider. the weak sensitivity of the proposed method to resistor errors and adc noise provides the opportunity of its implementation for metrological verification subsystem [10] of adcs using a single channel reference voltage source. the error of such metrological verification is mainly defined by the error of the implemented reference voltage source, therefore, metrology support of such metrological verification subsystem is reduced to the verification of the reference voltage source. it provides the opportunity to embed this metrological verification subsystem with reference voltage source into the adc. -3,00e+02 -2,00e+02 -1,00e+02 0,00e+00 1,00e+02 2,00e+02 3,00e+02 1 6 11 16 21 26 31 36 41 46 51 56 61 66 71 76 81 86 91 96 figure 2. simulated nonlinearity of the adc.      0 5 10 15 20 25 30 2 4 6 8 noise level, quantumsm a x im u m r e si d u a l e rr o r, q u a n tu m s figure 5. maximum residual error vs adc noise level.      figure 4. residual error vs input voltage for adc noise ± 6 quantums.  figure 3 residual error vs input voltage for resistors’ error ± 1 %.  acta imeko | www.imeko.org  june 2014 | volume 4 | number 2 | 84  implementation of proposed method demands special hardware and software. this hardware consists of multiresistors divider, multiplexer and controller. the most complex operation of software is computing of polynomial function with rational coefficients and integer argument. therefore it can be implemented on microcontroller of adc or external data processing module. using external data processing module and external hardware provides implementation of metrological verification subsystem without accessing to adc’s hardware or software. example of universal data processing module, which corresponds to requirements of proposed method, is presented in [11]. it is compatible with wide set of serial peripheral interfaces for connection adc and mentioned hardware. its computing power is enough for implementation proposed method in real time. references  [1] fowler k. part 7: analog-to-digital conversion in real-time systems. ieee instrumentation & measurement magazine. 2003. vol. 6. issue 3. pp. 58-64. [2] kester w, which adc architecture is right for your application?, analog dialogue, 2005, vol. 39, n. 2. рp. 11-19. url:http://www.analog.com/library/analogdialogue/archives/ 39-06/architecture.pdf [3] 24-bit sigma-delta, signal conditioning adc with 2 analog input channels ad7714 data sheets. url: http://www.analog.com/en/analog-to-digitalconverters/ad-converters/ad7714/products/product.html [4] substitution method for measurement of medium resistance url: http://www.myclassbook.org/substitution-method-formeasurement-of-medium-resistance/ [5] kochan r.v, adc implementation for measurement using replacement method, ukrainian metrology journal. – kharkiv, 2010, n. 3, pp. 11-16. (in ukrainian) [6] r. kochan, o. kochan, m. chyrka, s. jun, p. bykovyy, approaches of voltage divider development for metrology verification of adc. proc. of 7-th ieee international conference on intelligent data acquisition and advanced computing systems: technology and applications (idaacs’2013), 12-14 september 2013, berlin, germany, pp. 70 – 75. [7] zhengming wang, dongyun yi, xiaojun duan, jing yao, defeng gu, measurement data modeling and parameter estimation. – crc press, 2011, 553 p. [8] walt kester, data conversion handbook, analog devices, 2004, 952 p. [9] gost 30605-98, interstate standard. digital measurement converters of voltage and current. general technical conditions. actual from 01.01.2004. – moscow.: standards publishing house, 1998, 10 p. (in russian) [10] v. sobolev, a. sachenko, p. daponte, o. aumala, "metrological automatic support in intelligent measurement systems", computer standards & interfaces, elsevier pubs, vol.24, no.2, june 2002, pp.123-131. [11] v. kochan, k. lee, r. kochan, a. sachenko, “approach to improving network capable application processor based on ieee 1451 standard”, computer standards & interfaces, elsevier pubs, vol.28, no.2, december 2005, pp.141-149. microsoft word 304-1952-1-le-rev acta imeko  issn: 2221‐870x  december 2015, volume 4, number 4, 3    acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 | 3  editorial  paolo carbone  university of perugia, italy     section: editorial   citation: paolo carbone, editorial, acta imeko, vol. 4, no. 4, article 2, december 2015, identifier: imeko‐acta‐04 (2015)‐04‐02  editor: paolo carbone, university of perugia, italy  received december 20, 2015; in final form december 20, 2015; published december 2015  copyright: © 2015 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  corresponding author: paolo carbone, email: paolo.carbone@unipg.it      dear reader, the other 6 papers published in this last issue of 2015 are related both to imeko tc1 and tc12 and to another scientific event that took place in italy (the 2015 italian conference on electrical and electronic measurements organized by gmee and gmmt). the last paper in this issue is a freely submitted paper. the paper by crenna et al. covers the topic of measuring human movements in the expanding area of measurements for biomechanics. the authors present a disciplined approach to the assessment of a metrological procedure applied to measurement of forces and kinematic quantities associated to human movements. the aim is that of allowing reproducibility of results by controlling and estimating all major sources of uncertainty. the next paper by rainer feistel is a technical note about the thermodynamic role of water in the determination of climate changes. the role of salinity and relative humidity is of great importance in this case and the metrological requirements for the determination of such quantities must be determined more precisely if data are to be compared over decade long periods of time. this note describes the activities done at the international level to promote traceability of measurement results in this research area. the next paper by rolle et al. is a technical note covering again the topic of measurements for environmental purposes and, in particular, the problem of metrological traceability for the analysis of atmospheric pollutants. it describes the procedures and activities done in this scientific area by the italian metrological institute, inrim. they deal with the preparation of gaseous reference material and with traceability issues for measuring the particulate matter. the paper by lancini et al. is related to reliability of railway components and to the measurements needed for this scope. by using vibration measurements authors show how to detect high wear rates in rolling contacts. the procedure and the experimental results are presented in depth with a large level of detail. it is shown that it is possible to assess wear phenomena of wheel and rail steels by using vibrational analysis. d’emilia et al. consider the uncertainty in the calibration of three-axis low frequency accelerometers. they analyze both static and dynamic calibration up to 4 hz and present both an analysis of the used mathematical model and experimental results. they identify the main sources of uncertainty and provide suggestions for an effective calibration of these sensors. j. bongiorno and a. mariscotti authored the last paper in this issue. it describes the techniques used to measure the realto-earth conductance and the insulating efficiency in railways. in the paper it is reported the application of iec 62128-2 for performing such measurements and experimental results are also described. several practical considerations are made, including description of the effects of measurement uncertainty. this issue contains a very interesting set of papers offering an ever increasingly wider view on the possible applications of metrology and instrumentation. we are looking forward to the issues in 2016, including the extended versions of papers presented at the last imeko world congress held in prague this year. have a fruitful reading of this last issue of acta imeko in 2015! acta imeko issn: 2221-870x july 2017, volume 6, number 2, 89-92 acta imeko | www.imeko.org july 2017 | volume 6 | number 2 | 89 improved evaluation of uncertainty for indirect measurement valeriy didenko department of information-measuring technique, national research university “mpei”, krasnokazarmennaya 14, 105568 moscow, russian federation section: research paper keywords: indirect measurement; accuracy specifications; measurement uncertainty citation: valeriy didenko, improved evaluation of uncertainty for indirect measurement, acta imeko, vol. 6, no. 2, article 16, july 2017, identifier: imekoacta-06 (2017)-02-16 section editor: eric benoit, university savoie mont blanc, france received february 13, 2016; in final form june 30, 2017; published july 2017 copyright: © 2017 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: valeriy didenko, e-mail: didenkovi@mail.ru 1. introduction it is well-known [1] that the result of an indirect measurement is a function of several variables: ),...,2,1( nxxxfx = . (1) the values of these variables are found by direct measurements. the maximum possible absolute error of the indirect measurement as a function of maximum possible errors for the direct measurements ( ...1 x nx ∆∆ ) can be found from (1) approximately as [1]: nx nx f x x f x ∆ ∂ ∂ ++∆ ∂ ∂ =∆ ...1 1 . (2) it is usually suggested that the accuracy of (2) is found by means of high order derivatives in taylor’s expansion of f [1]. another way is to use simulation methods giving changes of , ...,1x x n in (1). according to modern metrology, (2) can be explained as the uncertainty with 100 % confidence level (the worst-case uncertainty) [2]. the worst-case uncertainty means that errors higher than found by (2) are absent. in practice, a real maximum possible absolute error of the indirect measurement can be a little higher than one found by (2). one reason of this fact has already been discussed (errors of taylor’s expansion). other reasons are elevated values of the errors of the direct measurements with regard to , ...,1x x n∆∆ . the worst-case method supposes that elevated errors of the indirect measurements with regard to (2) are negligible, for example, lower than five percent from (2). the result found by (2) is usually much higher than the real values. the reason is that all the errors of the direct measurements are calculated independently. the exception to the rule is given in [2]. all the direct measurements are supposed to use an analog to digital converter (adc) with the same values of the maximum offset error 0u , the maximum gain error gu , and the maximum linearity error inlu . the first two errors are supposed to have the same sign (positive or negative), while the linearity error can change the sign for abstract the paper gives formulae for uncertainty evaluation of an indirect measurement based on direct measurements made by different types of measuring devices. the first type of the measuring devices has the specifications of a total error (e.g. digital instruments), while the second type has the specifications of offset, gain and linearity errors (e.g. analog to digital converters). the choice of a device range and the configuration of measuring circuits for decreasing uncertainty are considered. the conversion of the specifications for the first type to the specifications for the second type is discussed. acta imeko | www.imeko.org july 2017 | volume 6 | number 2 | 90 different direct measurements. the uncertainty of the indirect measurements is found in accordance with [2], [3] for four cases: the standard uncertainty and the worstcase uncertainty (both absolute and relative). the absolute worst-case uncertainty (the maximum possible absolute error) of the indirect measurement with negligible quantization error is then [2]: ∑ = +∑ =+∑ == n i i kinlu n i ixikgu n i ikuxu 1 .110)( (3) coefficients ik can have different signs. for example, the indirect measurement 21 xxx −= gives .121 =−= kk therefore, (3) can show a lower value than the one found by (2) for the devices with the same errors of each direct measurement. direct measurements of nxx ...1 are often fulfilled by data acquisition devices. three most popular sampling architectures are: multiplexed structures, simultaneous sample and hold structures and multi-adc structures [4]. corresponding structures are shown in figure 1. for the same speed of the used adc, the multi-adc architecture gives a higher scan rate per channel and therefore is preferable according to the recommendations in [4]. the following abbreviations are used in figure 1: mux – multiplexer, amp – instrumentation amplifier, adc analog to digital converter, ssh – simultaneous sample and hold. the advantage of the multiplexed architecture in comparison with the multiplexed structure in terms of uncertainty is shown in section 4. besides the values of direct measurements, the values reproduced by material measures (standard electric resistors, standard signal generators etc.) are also included in (1). for several direct measurements, the following variants are discussed in terms of uncertainty: the application of the same adc at the same range, or different ranges; the choice of the same adc, or different adcs at the same range. for simplicity, several phenomena (dynamic errors, e.g.) ignored in [2] are not considered here either. additionally, the quantization error and noise are supposed to be negligible. all these approximations do not usually influence the main conclusions given in the paper. 2. accuracy specifications for different types of measuring devices accuracy specifications of digital instruments (di) are usually presented by the total (maximum) absolute error. as a first approximation, the maximum possible absolute measurement error for each variable ix found by a digital instrument is ( ),x a b xi idi∆ = + (4) where a is a positive number of the same unit as ix and b is a positive non-dimensional number. the maximum absolute error of a digital instrument as a function of an input signal x is shown in figure 2. two values of the input signal (x1 and x2) are considered. the corresponding absolute errors ( 1∆ and 2∆ ) are shown in figure 2. sometimes a is given in % of the full scale ( ifsx . ) and b is given in % of a reading. accuracy specifications of the adc and data acquisition (da) devices are usually presented by the maximum offset, gain and linearity errors. the quantization error and noise (the random error) are usually included in a and b for digital instruments but can be specified separately for other devices. for simplicity, we will not consider them in this paper. then the maximum absolute error of the da is .0x u u x ui ig inlda∆ = + + (5) for example, (5) is used for finding the maximum absolute error of a single measurement in [5]. the maximum absolute error of the adc as a function of input signal x is shown in figure 3. positive errors are considered only as an example. two values of the input signal (x1 and x2) are used. the linearity error is supposed to be zero at the ends of the range (x=0 and x=xfs) but is equal to the maximum value uinl with any sign at any other points. the linearity error is negative near x1 and is positive near x2 (the worst-case method). let us figure 1. simultaneous sampling architecture – simultaneous sample and hold (ssh). figure 2. maximum absolute measurement error vs. input signal for digital instruments. acta imeko | www.imeko.org july 2017 | volume 6 | number 2 | 91 suppose that 1∆ and 2∆ errors are the same for a digital instrument (figure 2) and an adc (figure 3). then the difference between specifications of the two devices mentioned is not important for the evaluation of the direct measurement uncertainty. the situation changes dramatically if we investigate indirect measurements. let us consider the simplest indirect measurement 2 1x x x= − . the uncertainty of the measurement for the digital instrument (figure 2) is .1 2 x di ∆ + ∆∆ = the uncertainty of the measurement for the adc (figure 3) is .2 1 x da ∆ −∆∆ = it is clear from figures 2 and 3 that the second value can be much lower. because of this result it is necessary to find ways to transform the specifications of digital instruments into the form specified for adcs. if (4) and (5) show the same results, then a and b can be found with given inluguu ,,0 as inluua += 0 , (6) gub = . (7) let a and b be specified. if we consider x=0, then 0u = a .it is possible to find the following inequalities (see figure 2): 2 / , 2( ),g fs inlu b a x v a b x≤ + ≤ + (8) where fsx is the full scale of the di. the evaluation of the linearity error by (8) is usually much higher than the true value. therefore (8) does not give any advantages for calculation of the uncertainty of indirect measurements in comparison with initial equation (4). fortunately, some digital instruments have the additional specification of the linearity error. for example, the linearity error is specified in [6] as ,xu a x bl lfsinl = + (9) where al and bl are positive non-dimensional numbers. now the maximum possible absolute error of the digital instrument can be written practically in the same way as it was given for adcs or data acquisition devices: 2 ( . )a inlx fs x a x ui idi b∆ = + ++ (10) in accordance with figure 3, inlu is supposed to be equal to zero at the ends of the scale, but can produce both positive and negative errors at any other points. the signs of errors produced by a and b are constant for the given indirect measurement. the linearity error found by (9) is approximately 10 times less than one found by (8) for the instrument 3401a [6]. if the linearity error for the di is not specified, then it can be found approximately from the experiment [7]. the accuracy of material measures (standard electric resistors, standard signal generators etc.) can be specified by both (4) and (5). for example, the accuracy of voltage calibrators is usually specified as (4), while data acquisition devices with analog output [5] have specifications (5). it means that all the following theory can be used both for results of the direct measurements and quantities reproduced by material measures. 3. general formulae for indirect measurement uncertainty direct measurements n used for the indirect measurement can be divided into three parts. then the absolute worst-case uncertainties of the indirect measurement ,) 121 ( 21 11 . 21 11 . 21 11 .0 1 1 1 1 1 10 )( ikixib n nni i a nn ni i k iinl u nn ni i xikigu nn ni i kiu n i i kinluix n i i kgu n i i kuxu +∑ ++= + +∑ + += ∑ + += +∑ + += ++ ∑ = ++∑ = +∑ = = (11) the simplest indirect measurement is described by the function 2 1x x x= − , where 11 2k k= − = − . the absolute worst-case uncertainty of the indirect measurement when one device is used in the same range ( 21 == nn ) and 012 ≥≥ xx in accordance with (11) is inluxxguxiu 2)12()( +−= . (12) the absolute worst-case uncertainty of the indirect measurement when two devices of the same type are applied in the same range ( 2,2n n= = ,02.01.0 uuu == gugugu == 2.1. , inluinluinlu == 2.1. ) in accordance with (11) is inluxxguuxiiu 2)21(02)( +++= . (13) the maximum difference of the results found by (12) and (13) is at :21 fsxxx ≈≈ inlu fsxguu xiu xiiu ++= 01 )( )( . (14) let us use (14) to compare the uncertainties of the multiplexed and multi-adc structures (discussed in section 1) figure 3. maximum absolute error vs. input signal for adc. acta imeko | www.imeko.org july 2017 | volume 6 | number 2 | 92 for the implementation of the function 2 1x x x= − . model pci-6250, used in the multiplexed structure, includes the adc with the following parameters [5]: =fsx 10 v, =0u 2∙10 -4 v, =gu 6∙10 -5, =inlu 6∙10 -4 v. if we use the same adc in the multi-adc structure, then, according to (14), the worst-case uncertainty is 2.3 times more. it means that recommendations [4] can be not true for the uncertainty of the indirect measurement. if 1x << 2x , then two ranges can be used for each direct measurement. in this case, the absolute worst-case uncertainty of the indirect measurement 2 1x x x= − is .2.1.22.11.2.01.0)( inluinluxguxguuuxiiiu +++++= (15) the application of one range for both direct measurements will be better if (12) gives a lower result in comparison with (15). let us consider the example pci-6250 [5] used for the indirect measurement 2 1x x x= − with 1x ≈ 5 v and 2x ≈10 v. the corresponding specifications for =≈ 1.1 fsxx 5 v are =1.0u 1∙10 -4 v, =1.gu 7∙10 -5 v and =1.inlu 3∙10 -4 v. the specifications for =≈ 1.2 fsxx 10 v were given before. using the pci-6250 at 10 v range only, we get from (12) that )( xiu = 1.5 mv. if we use two ranges (5 v for 1x and 10 v for 2x ), the result is )( xiiiu =2.15 v. it means that the application of only one range gives 1.4 times better result, and the well-known recommendation to use the lowest range (5 v in this example) is valid only for the uncertainty of the direct measurements but can be incorrect for the uncertainty of the indirect measurements. 4. conclusions there are two main types of the specifications for measuring devices: the maximum possible total error (4) and the maximum offset, the gain, and the linearity errors (5). the inequalities (8) are offered to find the maximum gain and linearity errors approximately. the result of the evaluation of the linearity error by (8) can be much higher than the true value. if the linearity error is specified besides (4), then the gain error can be found from (4) by (8). general formulae for the absolute (11) and the relative (12) worst-case uncertainties of the indirect measurement are found as functions of three parts of variables. these parts include application for the indirect measurement of one or several devices in one or several ranges with the specifications of the total error or the maximum offset, gain, and linearity errors separately. only the first parts of (11) can give a lower value of the uncertainty for some types of the indirect measurements in comparison with the approach (2). the formulae published before [1], [2] are special cases of (11). the second part of (11) was not discussed in [1], [2]. formula (11) can also be used to take into account application of material measures (standard electric resistors, standard signal generators etc.) for the indirect measurements. applications of the offered approaches are given in section 4. the advantage of the multiplexed structure vs. the multiadc structure from the uncertainty point of view is shown though the multi-adc structure is better from the dynamics point of view [4]. conditions for the choice of one range of a device instead of two ranges for two direct measurements are found from the uncertainty point of view. references [1] m. sedlacek, v. haasz, “electrical measurement and instrumentation”, vydavatelstvi cvut, prague, 1996. [2] f. attivissimo, n. giaquinto, m. savino, “worst-case uncertainty measurement in adc-based instruments”, computer standard & interfaces, 29 (2007), pp. 5-7. [3] iso, guide to the expression of uncertainty in measurement, 1 st edition 1993, 1995. [4] simultaneous sampl. data acquisition architectures. national instrument, publish date 2010. [5] high-speed data acquisition, national instruments, 2011. [6] agilent technologies, user’s guide, 3401a. [7] v. didenko, a. minin. a. movchan, “polynomial and piece-wise linear approximation of smart transducer errors”, measurement, 31 (2002), pp. 61-69. improved evaluation of uncertainty for indirect measurement the simplest indirect measurement is described by the function, where . the absolute worst-case uncertainty of the indirect measurement when one device is used in the same range () and in accordance with (11) is the absolute worst-case uncertainty of the indirect measurement when two devices of the same type are applied in the same range (, ) in accordance with (11) is the maximum difference of the results found by (12) and (13) is at the application of one range for both direct measurements will be better if (12) gives a lower result in comparison with (15). there are two main types of the specifications for measuring devices: the maximum possible total error (4) and the maximum offset, the gain, and the linearity errors (5). the inequalities (8) are offered to find the maximum gain and linearity errors a... general formulae for the absolute (11) and the relative (12) worst-case uncertainties of the indirect measurement are found as functions of three parts of variables. these parts include application for the indirect measurement of one or several device... applications of the offered approaches are given in section 4. the advantage of the multiplexed structure vs. the multi-adc structure from the uncertainty point of view is shown though the multi-adc structure is better from the dynamics point of view ... predicting and monitoring blood glucose through nutritional factors in type 1 diabetes by artificial neural networks acta imeko issn: 2221-870x june 2023, volume 12, number 2, 1 7 acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 1 predicting and monitoring blood glucose through nutritional factors in type 1 diabetes by artificial neural networks giovanni annuzzi1, lutgarda bozzetto1, andrea cataldo2, sabatina criscuolo3, marisa pesola3 1 department of clinical medicine and surgery, university of naples federico ii, 80125 naples, italy 2 department of engineering for innovation, university of salento, 73100 lecce, italy 3 department of electrical engineering and information technology (dieti), university of naples federico ii, 80125 naples, italy section: research paper keywords: artificial intelligence; monitoring; nutritional factors; postprandial glucose response; type 1 diabetes citation: giovanni annuzzi, lutgarda bozzetto, andrea cataldo, sabatina criscuolo, marisa pesola, predicting and monitoring blood glucose through nutritional factors in type 1 diabetes by artificial neural networks, acta imeko, vol. 12, no. 2, article 7, june 2023, identifier: imeko-acta-12 (2023)-02-07 section editor: francesco lamonaca, university of calabria, italy received january 23, 2023; in final form february 28, 2023; published june 2023 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was carried out as part of the ‘ai4pg’ project, under the fra-finanziamento per la ricerca di ateneo initiative, financially supported by the university of naples federico ii. this work was also supported by the pnrr dm 351/2022 -m4c1, by the european union fse-react-eu, pon research and innovation 2014-2020, dm 1061/2021 contract number dot19x7nyl-2. corresponding author: andrea cataldo, e-mail: andrea.cataldo@unisalento.it 1. introduction type 1 diabetes (t1d) is an autoimmune condition in which the immune system attacks and destroys the pancreatic cells (β cells) that produce insulin [1]. recent epidemiological research (e.g., [2], [3]) estimate that the t1d worldwide prevalence is 9.5 per ten thousand people. along with regular exogenous insulin injection, t1d patients have to lead a healthy lifestyle and carefully manage the levels of blood sugar to prevent complications, such as hypoglycaemia and hyperglycaemia [4]. in particular, the management of the postprandial glucose response is a major issue for t1d patients [5]. the technological advantages in healthcare and the progress in wearable devices [6], [7] have led to the development of artificial pancreas (ap) [8], a closed-loop systems that combines a continuous glucose monitoring (cgm) and, a control algorithm based on heuristics and theoretical knowledge, automating the insulin release via an insulin pump [9]. while the ideal goal is to design a fully closed-loop system, actual clinical scenarios depend on several physiological factors, e.g., delays in insulin assimilation. at the present time, only hybrid closedloop systems (hclss) are available for medical practice. although basal insulin is automatically delivered with little to no issue, these systems are unable to adequately manage the postprandial response, so the patient is forced to manually set up the pre-prandial dose of insulin [10]. thus, a crucial part of abstract the monitoring and management of postprandial glucose response (pgr), by administering an insulin bolus before meals, is a crucial issue in type 1 diabetes (t1d) patients. artificial pancreas (ap), which combines autonomous insulin delivery and blood glucose sensor, is a promising solution; nevertheless, it still requires input from patients about meal carbohydrate intake for bolus administration. this is due to the limited knowledge of the factors that influence pgr. even though meal carbohydrates are regarded as the major factor influencing pgr, medical experience suggests that other nutritional should be considered. to address this issue, in this work, we propose a machine learning (ml)-based approach for a more comprehensive analysis of the impact of nutritional factors (i.e., carbohydrates, protein, lipids, fiber, and energy intake) on the blood glucose levels (bgls). in particular, the proposed ml-model takes into account bgls, insulin doses, and nutritional factors in t1d patients to predict bgls in 60-minute time windows after a meal. a feed-forward neural network was fed with different combinations of bgls, insulin, and nutritional factors, providing a predicted glycaemia curve as output. the validity of the proposed system was demonstrated through tests on public data and on self-produced data, adopting intra and inter-subject approach. results anticipate that patient-specific data about nutritional factors of a meal have a major role in the prediction of postprandial bgls. mailto:andrea.cataldo@unisalento.it acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 2 hcls devices consists of the algorithm responsible of maintaining the levels of blood glucose in a safety range. several control strategies have been designed and presented in literature, spanning from proportional-integral-derivative control to fuzzy logic control [11]. nonetheless, the postprandial glucose response (pgr) still remains a main open issue in aps [12]. research on this topic focused mostly on carbohydrates’ intakes, ignoring additional aspects relative to mealtime, such as other nutritional factors (lipids, proteins, etc.) or psycho-physiological status [13], [14]. artificial intelligence (ai), and in particular machine learning (ml), is increasingly providing new opportunities in ap designs by boosting the extraction of information from big biological data [15]–[17]. an example of promising ai-based strategies for ap is offered by artificial neural networks (anns), which aim to early detect hypoand hyper-glycaemia events, and to consequently enhance insulin administration [18]. however, it is worth pointing out that, at the state of the art, even these anns models considered mostly carbohydrates, without taking into account other nutritional factors [19]. as a matter of fact, the nutritional properties of meals can impact blood glucose level (bgl), significantly affecting pgr. for instance, clinical trials have demonstrated that high-fat/protein meals require more insulin than lower-fat/protein meals with identical carbohydrate content. hence, the design of models based on meal composition rather than just carbohydrates intake seems significant [20]. starting from these considerations, a study of the impact of nutritional factors over the 60 minutes (min) after a meal was conducted by ml methods. in particular, the effect of nutritional factors such as carbohydrates, proteins, lipids, fibres, and meal energy intake, on postprandial blood glucose response was analysed. relying on models presented in [21] and [22], we proposed a model able to predict the glycaemic curve over the 60 minutes from the meal, by considering the nutritional factors. more in detail, the impact of the nutritional factors was investigated by feeding the model with a different combination of bgls, insulin doses, and nutritional factors, by validating the model on both public and self-produced data. the paper is organised as follows. in section 2 an overview of the state-of-the-art of ml solutions in the managing of postprandial (after the meal) blood glucose response was presented. in section 3 the datasets employed, and the proposed method were described. the experimental setup is reported in section 4. section 5 and section 6 show results and discussion, respectively. finally, in the concluding section the key points of the work are summarised, and the future steps are outlined. 1. related work ml has gained increasing attention in several research fields, and especially in health-related tasks [23]–[27]. among the ml techniques, the use of anns in prediction of blood glucose was investigated in several studies [28]–[35], using data from real t1d patients and virtual patients [36], as those obtained with uva/padova simulator [37]. this tool allows the generation of virtual subjects through complex physiological models, enabling users to control the experimental parameters. nonetheless, due to the difficulty of tuning its elements and interpreting the results, actual data from real patients are sometimes preferred for the examination of specific realistic scenarios. as example, in [33] the authors analysed the performance of a feed-forward neural network (ffnn) model for real-time prediction of glucose implementing a prediction horizon (ph) of 75 min. the ffnn was trained using a training set that included cgm values collected in 17 patients. overall, the reported rootmean-square deviation (rmse) was (43.9 ± 6.5) mg/dl. in [34], a glucose prediction algorithm that combines cgm readings and information on carbohydrate intake was proposed, by testing both on virtual patients, and real datasets. results on simulated and real data showed that for a prediction horizon (ph) of 30 min, rmse was calculated as (14.0 ± 4.1) mg/dl and (9.4 ± 1.5) mg/dl, respectively. in [35], a multilayer convolutional neural network (cnn) followed by a recurrent neural network with long short-term memory (lstm) cells was investigated for the prediction of blood glucose with phs of 30 and 60 min. the study was conducted on both virtual patients and t1d real patients with cgm sensors, by achieving rmse results of (21.07 ± 2.35) mg/dl (ph = 30 min) and (33.27 ± 4.79) mg/dl (ph = 60 min) for real data. in [21], the authors proposed a ffnn model, by considering input as a 30 min sliding window across the blood glucose values and associated eight statistics (i.e., minimum, maximum, mean, standard deviation, difference between highest and lowest, median, kurtosis, and skewness). the obtained rmse was (2.82 ± 1.00) mg/dl, (6.31 ± 2.43) mg/dl, (10.65 ± 3.87) mg/dl, and (15.33 ± 5.88 mg/dl), respectively considering different phs (15, 30, 45, and 60 min). however, the information about nutritional factors was not considered in the model. recently, some studies considered the nutritional factors of the meal as an important input of the neural networks [22], [38], [39], forecasting the glycaemia values after the meal. in this regard, in [22] a ffnn to predict post-prandial blood glucose values every 2 min up to 4 hours was designed, by using as input meal information in two ways. in the former, raw nutrient quantities were used. hence, the network was fed by carbohydrates (g), lipids (g), fibers (g), insulin amount (pmol/1000) and bgl (mmol/l). in the second way, a bioinspired model of glucose absorption curve. in particular, numerical parameters such as time elapsed to the peak of the curve, time elapsed to 50 % of the peak of the curve, and rate of absorption at the maximum of the curve were calculated. then, these curve characteristics were exploited to train the network along with insulin amount and bgl. they showed that better performance was achieved when the absorption model was integrated in the model, with an average rmse of 1.12 mmol/l (ph = 60 min), compared to the rmse value for the first approach (1.816 mmol/l). nevertheless, among the involved subjects, only one was a t1d patient. it is worth to note that previous reported studies do not focus on the impact of nutritional factors in blood glucose prediction. considering this, a study of the impact of nutritional factors over a 60-min time window after the meal was conducted by ml methods. 2. dataset characteristics and method 2.1. dataset direcnet dataset the direcnet is a public dataset of cgm measurements, collected by jaeb center for health research [40]. it includes data from child-patients with t1d wearing the medtronic minimed guardian-rt, a hcls device that recorded glucose values at intervals of 5 min. the dataset contains cgm data from 50 patients, aged between 3 to 7 or 12 to 18 years, with acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 3 a t1d for more than 1 year. for approximately 7 days, the blood glucose data was continually collected every 5 minutes. ai4pg dataset the ai4pg dataset, provided by federico ii university hospital (naples, italy), includes information from 25 t1d patients wearing the hcls, medtronic minimed 670g system [41]. the dataset reports data on meals, insulin doses, and cgm measurements for 6/7 days. subjects range age was (40 ± 12) years with a duration of diabetes of (15 ± 12) years. patients completed food diaries with information about meals for 7-days, by obtaining a dataset of 1264 meals (breakfasts, lunches, dinners) represented as time series of preand postprandial glycaemic levels (mg/dl). the dataset includes details about the manual boluses (mbs) administered at mealtime based on carbohydrate intake of the meal, and an estimate of carbohydrates (g), lipids (g), proteins (g), fibers (g) and energy intake (kcal) associated with each meal. the glycaemia levels from cgm every 5 min, from 30 min before meal to 60 min after meal was reported. the data were collected with informed consent from eligible subjects and the protocol was approved by the ethical committee of federico ii university. 2.2. proposed method the proposal of this study is the prediction of post-prandial glycaemia in t1d patients over one hour after the meal, using a ffnn model [42]. the taken approach was inspired by the findings reported in [21] and [22], choosing as inputs a 30-min window of blood glucose values and 8 associated statistical features. more in detail, minimum, maximum, mean, standard deviation, pick-to-pick difference, median, kurtosis, and skewness were calculated on the glycaemia values. the output is the entire glycaemic curve from 5 min to 60 min after the meal, and namely 12 output neurons correspond to these values sampled every 5 minutes. prediction performance was then evaluated on all the 12 output values simultaneously. the number of hidden layers and neurons of the ffnn was set by a grid search strategy. to evaluate the performance of the proposed system in predicting blood glucose levels, a preliminary experiment using direcnet data was conducted. subsequentially, the system was applied to the self-produced ai4pg dataset. since the goal was to examine how nutritional parameters affected postprandial glycaemic response, nine different input configurations were tested: ● #1 only glycaemia: the model took in input: blood glucose levels (mg/dl) from 30 min before meal until mealtime every 5 min glycaemia’s statistical features such as minimum, maximum, mean, standard deviation, difference between highest and lowest, median, kurtosis, and skewness calculated on glycaemia values mentioned above. ● #2 insulin, no nutritional factor: in addition to glycaemia values and associated statistics, the network also took as input the insulin bolus mb (mmol/l). ● single-nutritional factors scenarios: in these scenarios, the inputs were composed of glycaemia values, statistical features, mb, and a single nutritional factor across the following: #3 carbohydrates (g), #4 proteins (g), #5 fibers (g), #6 lipids (g), and #7 energy intake (kcal) associated with each meal. ● #8 insulin, all nutritional factors: the model was supplied simultaneously with glycaemia values, statistical attributes, insulin bolus and all nutritional factors. ● #9 no insulin, all nutritional factors: as the previous one except for the insulin bolus, the model exploited glycaemia values, statistical features, and all nutritional factors. the outputs are the 5-min-step values of the blood glucose curve over 60 min after the meal. root mean square error (rmse), defined by equation (1), was used to evaluate the prediction performance. 𝑅𝑀𝑆𝐸 = √ 1 𝑁 ∑ ( 1 𝑇 ∑ (𝑦𝑝 − 𝑦𝑚 ) 2 𝑇 𝑚=1 ) 𝑁 𝑝=1 , (1) where 𝑦𝑚 and 𝑦𝑝 represents the measured and predicted bgls at the same instant of time, respectively; 𝑇 is the time, while 𝑁 is the total number of blood glucose measurements in the dataset. 3. experiments this section describes the conducted experiments, by reporting the pre-processing and the experimental setup. a set of preliminary experiments were carried out by using the public dataset direcnet to validate the proposed ml system in the bgl prediction task. then, as the goal of this study was analysing the impact of nutritional factors on the bgl prediction capability, the system was used on the ai4pg dataset. figure 1 illustrates the key steps of the proposed pipeline, which are detailed below. figure 1. proposed pipeline. the data pre-processing stage includes the savitzky-golay filtering step and statistical features calculation. then, the dataset is split into training, validation, and test set, and scaled using the min-max scaler strategy. model hyperparameters are tuned by a grid search strategy. finally, the best model is selected. acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 4 3.1. pre-processing for both public direcnet and self-produced ai4pg data, the pre-processing procedure involved filtering and scaling the data. more in detail, from direcnet dataset, the cgm data (mg/dl) of the 12 patients were considered. based on [43], [44], to clean the data from noise and thus improve the performance, the data was filtered with savitzky-golay technique [45], by considering a first-order polynomial with a 15-step filtering window on bgls. then, the model input was constructed by using a 30-minute sliding window across the blood glucose data. moreover, as mentioned, statistical attributes were calculated on each window of bgls data and added as inputs: minimum, maximum, mean, standard deviation, difference between highest and lowest values, median, kurtosis and skewness. as for the self-produced ai4pg data, we considered the blood glucose values (mg/dl), the mb (mmol/l), and the nutritional factors such as energy intake (kcal) of the meal, protein (g), carbohydrates (g), lipids (g), and fibers (g). data from 15 patients were used, for a total 1036 meal records. subsequentially, the bgls were pre-processed by savitzkygolay filter using a first-order polynomial and a 15-step filtering window. therefore, the input of ffnn was composed by glycaemic values from 30 min before meal until mealtime every 5 minutes. in other words, 7 blood glucose values are given in parallel as inputs to the model. in addition, as previously discussed, the 8 statistics computed on pre-prandial bgls were used as input. 3.2. experimental setup a feed forward neural network (ffnn) was investigated as a predictor of postprandial blood glucose levels over a 60-min time window from the meal. in order to set optimal hyperparameters for the proposed ffnn model, a grid search strategy was implemented by considering the search spaces reported in table 1. in particular, discrete ranges of values were chosen for the number of hidden layers (from 1 to 3), the number of neurons per layer (as powers of two) and the learning rate of the process. these values fall within reasonable ranges adopted in previous studies. regardless of each single configuration, relu activation function [46] was chosen for each hidden layer, the regularization term (l2 penalty) with weight decay parameter was set to 0.0001, stochastic gradient descent (sgd) [47] was used as optimization algorithm. the epochs were set to a maximum of 1000 with a patience of 20 [42]. for the experiments, we exploited intra-subjective and inter-subjective approaches on both direcnet and ai4pg data. the intrasubjective approach relies on the inherent physiological variability among different individuals, examining one patient's data at a time and conducting a customized investigation to obtain more accurate considerations. conversely, in the intersubject approach, data from all patients are pooled together to achieve more universal findings. more in detail, for intra-subjective approach, a different model for each patient was built. to validate the method, a holdout validation strategy was performed by splitting the dataset into training, validation, testing sets. in particular, the training set contained 70 % of the data, the 10 % of data was used for validation, and the 20 % for testing. all data were scaled using min-max scaling, by computing the minimum and maximum of the training data. instead, for the inter-subjective case, the data of all patients was used to build a single model to investigate the possibility that a model trained on data from a different subject can generalize to new data. to validate the method, a 5-fold cross-validation (cv) was performed. cv is a technique used in ml to better evaluate the performance of a predictive model on a limited dataset [47]. generally, in k-fold cv, data are divided into k equal-sized folds. the model is trained on k-1 of these folds and evaluated on the remaining fold. this process is repeated k times and then the performance is averaged across all k folds. in this work, for each of the 5 iterations, a portion of the training data was used as the validation set, according to a 70 %/10 % splitting. a min-max scaler was applied considering the minimum and maximum values of the training data. rmse was used for model evaluation on the test set. 4. results in this section, the experimental results both for intraand inter-subjective approaches were reported. direcnet dataset rmse between actual and predicted values every 5 minutes over a 60-min time window was calculated and mean and standard deviation were reported in table 2. in this case, the predictions were based only on bgl, as no information about insulin doses and meals is available. in the intra-subjective approach, the rmse is the average on all the considered patients, whereas, in the inter-subjective case, the rmse is the average on the 5-fold. as observed, the results are similar in the two approaches with a main difference in the standard deviation, that in the intra-subject case is greater due to high performance variability across different patients. instead, since each fold contains data from different patients, the variability of performance between folds was minimal in the inter-subject case. ai4pg dataset – in order to evaluate the impact of nutritional factors and insulin doses on bgl prediction, a statistical paired t-test, with significant level α of 0.05, was exploited. in particular, the t-test was used to compare the #1 only glycaemia results with the other ones. the statistical significance was interpreted through p-value. as can be seen from the results reported in table 3, a positive statistical significance was obtained when the ffnn was fed by nutritional factors individually considered. as expected, carbohydrates are the factor that has the greatest impact on blood glucose prediction [48] in the first 60 minutes after the meal, but also other factors such as proteins, fibers, lipids, and the energy played a key role. instead, for inter-subjective approach shown in table 4, no statistical significance was found as a p-value always greater than the significant level α, reflecting the need to model interindividual variability. as a matter of fact, clinical studies showed a significant role of the individual characteristics in postprandial glucose [49]. table 1. search space adopted during the grid search. hyperparameters search space number of hidden layers [1, 2, 3] number of neurons [32, 64, 128, 256] learning rate [0.0001, 0.0005, 0.001, 0.005, 0.01] table 2. mean rmse with standard deviation for bgl prediction in intrasubjective and inter-subjective approach on direcnet dataset. approach mean ± std (mg/dl) intra-subjective 11.4 ± 3.3 inter-subjective 11.8 ± 0.9 acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 5 5. discussion the goal of this study was to investigate the influence of nutritional factors on bgl forecasts. to do this, a set of preliminary experiments on a direcnet public dataset allowed to verify the capability of proposed mlmodel in bgls prediction with respect to the literature. however, in literature studies the role of nutritional factors, which could help to achieve more effective bgl predictors, is not widely investigated. this study showed, for the intra-subjective case on ai4pg data, that not only carbohydrates but also other nutritional factors such as protein, lipids, fibers, and the caloric intake of the meal have an impact on the prediction of bgl over a 60min time window after a meal. it is interesting to note that considering all the nutritional factors simultaneously leads to a lower effect on the performance with respect to the use of just one nutritional factor. this could be due to the peaking phenomenon [42], [50] of the proposed model, according to which, for finite training sets, the performance of a model does not improve as the increasing of the features number (in this case nutritional factors). another reason could be a negative interaction between the nutritional factors involved. instead, for the inter-subjective case, the nutritional factors do not contribute to appreciably improved bgl predictions. thus, the postprandial glycaemic response seems strongly related to individual subject characteristics and this agrees with clinical studies [48], [49], [51] that have demonstrated a postprandial glucose response almost constant in the same subject, while it changes among different subjects. hence, a better knowledge of how nutritional factors affect bgl prediction could enhance the algorithms controlling insulin infusion in hcls and the calculation of insulin bolus. 6. conclusions the goal of this study was to explore how nutritional factors may affect the prediction of post-prandial bgl in the 60 minutes after mealtime, via machine learning methods. a set of experiments to test bgls prediction by a feed forward neural network was conducted on the self-collected ai4pg dataset, which also contained various data of interest like insulin doses and intakes of nutritional factors. first, the model was validated on the public direcnet dataset, demonstrating capability to be able to obtain an acceptable prediction performance over a 60-minute time window. then, a prediction performance analysis was computed on the ai4pg dataset, considering different nutritional factors as inputs to evaluate the impact of each of them. finally, the case in which all the nutritional factors are considered simultaneously as input was explored. the obtained results show that nutritional factor information can be relevant in bgl forecasts, but this information should be employed in a subject-specific fashion. clearly, this does not exclude the possibility of exploring into alternative machine learning strategies based on transferring knowledge across different datasets, such as transfer learning techniques (e.g. [52], [53]). this study focused on the impact on bgl predictions of different nutritional factors in a 60-minute time window after a meal. in addition to the global rmse performance, one could also evaluate the performance on different time horizons. for instance, this was already investigated in [54], though the temporal resolution was coarser (15 minutes in spite of 5 minutes). in this framework, one could also investigate the impact of nutritional factors at different time scales (greater than 60 minutes). moreover, explainable artificial intelligence (xai) [55]–[59] methods could be help in explaining the input-output relationships and so the impact of different scenarios on bgl prediction. finally, using more sophisticated neural network models, like long short-term memory, could improve the outcome. references [1] a. katsarou, s. gudbjörnsdottir, a. rawshani, d. dabelea, e. 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https://doi.org/10.1016/j.neunet.2021.01.026 https://doi.org/10.2200/s00762ed1v01y201703hlt037 https://doi.org/10.1007/s00125-021-05587-0 https://doi.org/10.1016/j.diabres.2022.110089 https://doi.org/10.1007/11494669_93 https://doi.org/10.2337/dc21-1656 https://doi.org/10.1038/s41598-022-09578-y https://doi.org/10.1109/tkde.2009.191 https://doi.org/10.1109/access.2023.3244712 https://doi.org/10.48550/arxiv.2210.06554 https://doi.org/10.1109/tnnls.2020.3027314 https://doi.org/10.1016/j.knosys.2022.109725 https://doi.org/10.1145/2939672.2939778 microsoft word article 1 contacts.doc acta imeko may 2014, volume 3, number 1, 1 www.imeko.org acta imeko | www.imeko.org may 2014 | volume 3 | number 1 | 1 journal contacts about the journal acta imeko is an e-journal reporting on the contributions on the state and progress of the science and technology of measurement. the articles are based on presentations presented at imeko workshops, symposia and congresses. the journal is published by imeko, the international measurement confederation. the issn, the international identifier for serials, is 2221-870x. editor-in-chief paolo carbone, italy honorary editor-in-chief paul p.l. regtien, netherlands editorial board francisco alegria, portugal leopoldo angrisani, italy filippo attivissimo, italy eulalia balestieri, italy eric benoit, france pasquale daponte, italy luigi ferrigno, italy alistair forbes, united kingdom konrad jedrzejewski, poland andy knott, united kingdom fabio leccese, italy sergio rapuano, italy dirk röske, germany lorenzo scalise, italy emiliano schena, italy enrico silva, italy marco tarabini, italy susanne toepfer, germany luca de vito, italy about imeko the international measurement confederation, imeko, is an international federation of actually 39 national member organisations individually concerned with the advancement of measurement technology. its fundamental objectives are the promotion of international interchange of scientific and technical information in the field of measurement, and the enhancement of international co-operation among scientists and engineers from research and industry. addresses principal contact prof. paolo carbone university of perugia dept. electr. inform. engineering (diei) via g.duranti, 93, 06125 perugia italy email: paolo.carbone@unipg.it acta imeko prof. paul p. l. regtien measurement science consultancy (msc) julia culpstraat 66, 7558 jb hengelo (ov) the netherlands email: paul@regtien.net support contact dr. dirk röske physikalisch-technische bundesanstalt (ptb) bundesallee 100, 38116 braunschweig germany email: dirk.roeske@ptb.de microsoft word 268-1861-1-le-ver2 acta imeko  issn: 2221‐870x  december 2015, volume 4, number 4, 4‐8    acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 | 4  physical quantity as a pseudo‐euclidean vector  valery mazin  st. petersburg state polytechnical university, ul. polytechnicheskaja, 29, 195251  st. petersburg, russia        section: research paper   keywords: pseudo‐euclidean plane; uncertainty; vector of the unit; total vector; pseudo‐euclidean angle  citation: valery mazin, physical quantity as a pseudo‐euclidean vector, acta imeko, vol. 4, no. 4, article 3, december 2015, identifier: imeko‐acta‐04  (2015)‐04‐03  section editor: franco pavese, torino, italy  received april 8, 2015; in final form june 17, 2015; published december 2015  copyright: © 2015 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  corresponding author valery mazin, e‐mail: masin@list.ru     1. introduction  the categories of evolution in theoretical metrology, mentioned in [1], among which developing and substantiating mathematical models should be particularly noted, are still holding their truth today. the geometrical approach plays a special role among other approaches to the understanding of fundamental categories of measurement. in the “encyclopedia of mathematics” [2], the role of geometry is defined as follows: “the development of geometry, its applications, development of the geometric perception of abstract objects in different areas of mathematics and natural science give evidence of the importance of geometry as one of most drastic and productive means of reality cognition in terms of produced ideas and methods”. it is underscored in [3], that the geometric representation of analytic concepts has an infinite heuristic value; it also mentions that geometry “becomes more and more important in . . . physics, simplifying mathematical formalities and deepening physical comprehension. this renaissance of geometry influenced not just special and general theories of relativity, known to be geometrical in their essence, but also other branches of physics, where geometry of physical space is being replaced by the geometry of more abstract spaces”. this article offers for consideration a geometrical representation of a fundamental concept in metrology – the physical quantity. also, a geometric representation integrates uncertainty into the concept of the physical quantity. 2. defining the problem  let a quantity x characterize an object, that allows it to change from xa to xb. this very situation actually takes place in reality, for not a single physical parameter in an imperfect world can be defined by a single value. there is always a probable turn-down range, that exists either due to transition from one kind to another, or due to external conditions change, or due to inaccuracy of a measurement. the mean value and the range of uncertainty characterizes different properties of a quantity (one is indicating the location on the numerical axis, the other – dispersion around this location). on the other hand, a physical quantity contains both, quantitative as well as qualitative aspects. both of these dualities suggest an idea that a representation of a quantity, naturally combining this duality, would be necessary. this research attempts to represent any physical quantity as a two-dimensional vector of a given abstract space. 3. physical quantity as a vector, characterizing its   uncertainty   the following suggestion was proven in [4]: abstract  the aim of this research was an attempt of the finding of a mathematical model for a physical quantity organically including its degree  of  uncertainty.  the  basic  method  is  an  application  of  the  geometrical  approach.  as  the  starting  point  the  basic  equation  of  measurement  is  accepted.  result  was  a  presentation  of  a  physical  quantity  by  a  vector  of  the  pseudo‐euclidean  plane.  four  new  matters appearing by such presentation are discussed.  acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 | 5  a collection of logarithms of physical quantities in any power (integer and fractional) plus zero defines a vector space. each vector of this vector space, per basic equation of measurement x = {x}[x], where x is a physical quantity, {x} is its numerical value, [x] is its unit, can be decomposed into two portions, formed up after taking the logarithm of this equation (strictly speaking, this simple notion is, of course, not an equation): ln x = ln {x} + ln [x] . (1) expression (1) is usually considered lacking mathematical rigor and leading to contradictory statements, for it is commonly adopted that transcendent functions of denominate quantities do not make sense, whereas the units of quantities are exactly what make them as such. nevertheless, formal operation (1), obviously, can be carried out, and we will show that they are not just formal. we shall distinguish between the fundamental possibility of assigning a coordinate system and the method of determining the values of these coordinates. taking the logarithm produces a coordinate system. as for the values of coordinates, multiple approaches are possible. first of all, it is necessary to acknowledge that for as long as the unit of a quantity may have different values, this unit in itself bears a quantitative meaning. this meaning, however, remains indeterminate, for the entire quantitative side of a physical quantity is a result of comparing this quantity with unity. therefore logarithmic transformation can be used. secondly, as it will be shown later, by using the immutable fact of uncertainty of value of a quantity, it will become possible to make sense of ln [x]. and finally, we can consider ln [x] to be simply a symbol of a coordinate, a sign of direction, whereas αi shall be considered the coordinate itself. assuming medxxx ba  , corresponding vectors may be drawn as illustrated in figure 1, where a0 is the axis of numerical values logarithms. we will choose the metric on this plane in such a way, that it reflects the fact of the presence of x in given limits. this can be done on the condition that the vectors magnitudes within their domain of definition were real, and outside the domain of definition imaginary. as a result, we shall arrive to: 0 | |  ba xx lnln , (2) i.e. corresponding vectors turn out to be isotropic. all vectors, located to the left as well as to the right from the isotropic ones, including those, directed alongside the numerical axis, have imaginary lengths. the latter statement meets our condition, whose meaning is clear: what falls short of reality – is imaginary. as abstract numbers don’t exist in nature, a numerical axis must be imaginary. such metric happens to be pseudoeuclidean. every point a of this plane characterizes an equivalent-kind physical quantity, taken in any power and varying within certain limits. we shall name it a representing point. in that case, the meaning of the points population, forming a pseudo-euclidean plane, becomes clear if one can imagine few possible reasons for transitioning from one point to another, i.e. movements in this plane. these reasons are mathematically expressed by parameters, whose variations cause the end of the vector of quantity trace a certain trajectory. it is possible to specify three types of such parameters: coverage probability, power of the quantity and external factors. in such metric  2med02med | | aa x  xx lnln . (3) this leads to |ln xmed| = ab – amed = amed – aa , (4) and   22med | | meda xx lnln . (5) substituting (4) into (5) and taking into consideration that 2 med ba aaa   , (6) we obtain:   ba aax ln . (7) this result is of fundamental importance. it shows that the vector norm, representing the unit of a quantity, is not a permanently set value, but rather is a geometric average of logarithms of the limit values for a given quantity. after simple transformations we also arrive to:   axxb aaaa  00 xln . (8) figure 1. vector representation of a physical quantity.  acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 | 6  as a result of this concept, new entities are coming to life. the first of them – vector norm  x ln . 4. vector of a unit  the vector norm of a unit generally is not equal to zero since it is not a “logarithm of one” in its common sense, but rather is thought to be a logarithm of a certain qualitative content of the quantity. this notion fully agrees with the concept, stating that the numerical value and unit for a quantity reflect its quantitative and qualitative component, respectively. being expressed as a vector, laid in a pseudo-euclidean plane, the unit of quantity thus has two dimensions. one of them is ordinary, with logarithmic coordinate at 0, characterizes the size of the commonly adopted unit. another dimension also gives the size of the unit, except this unit is related to the applicable physical system, and it is defined by the range xa, xb. 5. the full vector of a quantity  another new entity, appearing in connection with the geometrical concept of a physical quantity the length of its full vector x ln . as it can be seen from (6), when either a0x = aa, or a0x = ab , x ln = 0. it can be explained only on the condition that a0x is located at the center of a corresponding probability distribution. in this case, if the center of distribution coincides with its limit, dispersion becomes zero (for there aren’t any values at either side from the center); then a0x becomes constant. no other interpretation is capable to give an informatively sound explanation of this exceptional nature of limit values for a0x . moreover, we can also determine the class of distribution center for a quantity, represented by the coordinate a0x in a given model. since the numerical axis is “common for everyone”, i.e. all values for distribution centers for all physical quantities are being located along it, these centers shall be added up algebraically while the summation itself is a vector addition. it is worth to mention here, that this rule of addition is applicable in case of dealing with population means. consequently, a0x is the population mean of logarithms of quantity x. it is necessary to notice, that in each and every single pseudo-euclidean plane, characterizing a certain quantity, any of known characteristics of the probability distribution center may act as coordinate a0x . having said that, if the quantities are multiplied (which is the same as vector sum of their logarithms), then a rule that governs addition of their logarithmic probability distribution centers shall be defined. likely, this will require establishing different numerical axes for different quantities. in this article, however, we will skip this issue. let's notice that the reason for a physical quantity to be represented as a two-dimensional entity lays in the fundamental uncertainty of its value. such uncertainty causes the averages of a0x and  x ln to have different absolute values. the length of the quantity vector expresses the degree of its uncertainty in a format of geometric average of the two greatest possible deviations of quantity value logarithms from their center of distribution. let's re-write (8) in the following format:         a b a b x x x x x x x x lnlnlnln xln , (9) where x = exp a0x. if the quantity’s variation within the limits xa – xb is reasonably small (owing to say, measurement errors), then the quotients x xb and ax x are approaching 1. then, expanding the logarithms into taylor series and limit the expansion to the first two members, we arrive at:              11 a b x x x x xln , (10) where γ+ and γare positive and negative estimates of the error, respectively. quite often γ+ = γ= γ [5], consequently x ln . (11) quantity variation limits of xa and xb define a level of confidence, specified with a certain probability. consequently, the entire model built as a pseudo-euclidean plane turns out to be probabilistic. 6. the angular characteristic of measurable  property of a physical object  the third new entity that needs much thinking, is the pseudo-euclidean angle between the vector of quantity’s unit and the full vector of the quantity. see figure 2. each point on a straight line, coinciding with the vector of quantity x, including the point, that belongs to x in a fractional power, expresses the same property of the object in both, qualitative and quantitative, terms. for example, electric resistance r, r2, conductivity r-1, and so on express the same property of a conductor, though in various terms. but then vector ln x , as well as any other vector co-linear to it, such as ln x2, characterizes just one image of this general property. it is natural to assume, that there should be a characteristic that unites all of these images. this characteristic, obviously, is the angle at which the line u crosses some chosen direction; for example, the direction of the quantity’s unit vector, as shown in figure 2. the angle between these straight lines, just as the quantity x itself, characterizes the object in the adopted system of measurement; however, more general and more sustainable. it is certainly not the angle that can be seen, but rather a pseudo-euclidean angle. thus, we are forced to consider the angle as a characteristic of a measurable physical property of an object. figure 2. further elaboration of the physical quantity concept.  acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 | 7  for the purpose of making the notes look simpler, we shall write: ln x = x*; ln [x] = [x]*; medlx  * med ;   xa x  e * , where e is a unitary vector, directed along [x]*. to begin with, let’s define the angle θβ between the unitary vector and the geometric average vector. corresponding transformations show:        a b a a ln 2 1  . (12) in other words, the pseudo-euclidean angle between the unitary vector and the geometric average vector for this object is determined by the logarithm of ratio of quantity’s limit values. formula (7) establishes the degree of difference between  *x and lmed and it is significant in the sense that xmed can also be considered a system unit. now let’s define the angle θφ between the vector of geometric average and the full vector of the quantity x, an angle shown in figure 1 as φ. we find: xb ax aa aa 0 0ln 2 1    . (13) in this case, the logarithm is taken from a prime relation, that includes three points aa, a0x, ab – all lying on the same line, a. k. a. an affine invariant. finally, the formula for the angle between the vector of the unit and the full vector of the quantity is expressed as                    0 0 :ln 2 1 ln 2 1 0 0 0 0 b a xb ax xb ax a b a a aa aa aa aa a a   . here the logarithm is taken from a complex relation of four points aa, ab, a0x, and 0 , representing a principal projective invariant. it is remarkable that the last formula exactly matches the formula defining the length 0 a0x in hyperbolic and elliptic geometries. it is analogous to the laguerre formula  2121ln 2 uujj i  , (14) where ψ is the euclidean angle in a complex euclidean space; u1 and u2 are two real straight lines; j1 and j2 are two isotropic imaginary straight lines, whereas all four straight lines pass through the same point;  2121 uujj is the complex relation of four aforementioned straight lines. in our case, the complex relation of four points, obviously, is equal to the complex relation of the straight lines coinciding with vectors *ax , * bx , x* and [x] *. out of these four, the first two are also isotropic. 7. conclusions  the concept representing a physical quantity as a vector of the pseudo-euclidean plane means knowing the center of probability distribution and degree of uncertainty for this quantity. this possibility of natural and, in essence, necessary combination of the center of probability distribution and the degree of uncertainty for a quantity, is a reason to favor this geometrical concept of representing physical quantities. the suggested approach allows to express a certain property of the physical object, that in particular may be expressed via a physical quantity, through a pseudo-euclidean angle. this approach is a natural development of representing a physical quantity as a numerical axis. here comes the question: how exactly shall we use this representation in describing physical processes? in other words, what is a practical value of these results? since the combination of the center of distribution and dispersion around this center for a given quantity is defined by a two-dimensional vector, the equations of interest shall operate with these vectors instead of otherwise commonplace quantities. an example in the appendix illustrates such approach. appendix  example the sensitivity of an induction sensor to magnetic induction is described by the formula: s = wa , where  cyclic frequency of induction varying, w – number of windings in the coil, a – projection of the coil’s area on the plane, perpendicular to induction. let  lay in the range of (5970 ÷ 6590) s-1, with a probability of 0.95; let also w = 1293. in the end, let us take it as given that a lays in the range of (1.22 ÷ 1.24)10-3 m2 with 0.95 probability for different sensors of the same model. the formula for sensitivity in vector form will look as  . (i) for the sake of simplicity, let us call: = s*,  *, = w*, = a*. then let’s break each vector down into components, one of which is directed along the numerical axis and another – along the full vector of the quantity. the result of this transformation is a0s + s* = a0 + * + a0w + w* + a0a + a* , i.e. each quantity is represented by the sum of the distribution center and expanded uncertainty. it can be said that a0s = a0 + a0w + a0a is an algebraic sum of the logarithmic expected values, and s* = * + w* + a* (ii) is a vector sum of the expanded uncertainties. (i) and (ii) are, obviously, identical; however it is important to keep in mind that vectors in (i) are full vectors, whereas in (ii) the same vectors become coordinate vectors, should the coordinate system change. let us determine the relative expanded uncertainty s, taking into account that w = const. according to the vector addition formula: s = | ∗| = | ∗| | ∗| 2 | ∗|| ∗| , where is the coordinate of the metric tensor of the model space. this tensor, according to [6], is determined by the distribution type for  and a, coverage probability, and ratio of the uncertainties of  and a. to find | ∗| and | ∗|, see (11). assuming normal distribution for  and uniform for a , when aforementioned coverage probability and variables are present, following the value tables for the metric tensor developed by one of the authors in [6], we arrive at ≅ 0.05. as a result, it turns out that | ∗| 0.051. here is where methodology from [7] becomes handy. as it follows from earlier stated values for  and a, their relative acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 | 8  expanded uncertainties are 0.05 and 0.01, respectively. matching standard uncertainties at coverage factors of 1.96 and 1.65 are found to be 0.026 and 6.110-3 respectively. the standard uncertainty of s therefore can be found as 0.026 6.1 ∙ 10 0.027. the assumption of a normal distribution for s will produce 0.027 ∙ 1.96 0.053, that in all practicality equals the result, obtained earlier. it is important to notice that we made a supposition regarding the type of resulting distribution. applying the monte carlo method does not require such supposition; however, it requires a considerable increase of computing power. in this case, when statistical sampling of 10,000 values is exercised, s = 0.052, i.e. a comparable result yet again. references  [1] tarbeyev y.v., “development of fundamental research in the area of theoretical metrology”, 3-d all-state conference on theoretical metrology: presentation theses, leningrad, 1986, pages 4-7 (in russian) [2] “encyclopedia of mathematics in five volumes”, vol. 1, moscow, 1977, 1152 col. (in russian) [3] shutz, b., “geometrical methods in mathematical physics”, translation from english, moscow, 1984, 303 p, (in russian) [4] mazin v.d., “description of properties of physical objects in piecewise euclidean and piecewise riemann spaces”, scientific and technical bulletin of spbstu (computing, measuring and operating systems), petersburg, 1993 (in russian) [5] novitsky p.v., zograf i.a., “evaluation of errors estimation in measurement results”, second edition, leningrad, 1991, 303 p. (in russ.) [6] mazin v.d., chepushtanov a.n., “properties and application technology of a vector-analytical method for an estimation of uncertainty in measurement”, 5 international conference on precision measurement, ilmenau, 2008. [7] “guide to the expression of uncertainty in measurement”, jcgm 2008. distributed coverage optimisation for a fleet of unmanned maritime systems acta imeko issn: 2221-870x september 2021, volume 10, number 3, 36 43 acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 36 distributed coverage optimisation for a fleet of unmanned maritime systems geert de cubber1, rihab lahouli2, daniela doroftei1, rob haelterman2 1 royal military academy, department of mechanics, avenue de la renaissance 30, 1000 brussels, belgium 2 royal military academy, department of mathematics, avenue de la renaissance 30, 1000 brussels, belgium section: research paper keywords: unmanned maritime systems; multi-agent systems; maritime surveillance; distributed coverage optimisation citation: geert de cubber, rihab lahouli, daniela doroftei, rob haelterman, distributed coverage optimisation for a fleet of unmanned maritime systems, acta imeko, vol. 10, no. 3, article 7, september 2021, identifier: imeko-acta-10 (2021)-03-07 section editor: bálint kiss, budapest university of technology and economics, hungary received january 15, 2021; in final form september 9, 2021; published september 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: the research presented in this paper has been funded by the belgian royal higher institute of defence, in the framework of the dap19/08 (marsur) project, and by the flemish agency for innovation and entrepreneurship, in the framework of the blue cluster project ssave (hbc.2019.0045). corresponding author: geert de cubber, e-mail: geert.de.cubber@rma.ac.be 1. introduction an ever-increasing percentage of the global population lives in coastal areas. a downside of this evolution is that an increasing number of criminals are turning their attention to our seas and oceans to carry out illegal activities. examples include drug smuggling, human trafficking, illegal fishery and border infringements. the problem for law enforcement agencies is that patrolling and surveilling the vast oceans using traditional means (large, manned vessels) is impossible from an economic and operational point of view. unmanned maritime systems (ums) can potentially provide maritime law enforcement agencies with a valuable tool for increasing their capabilities in relation to maritime surveillance. of course, ums are not the only answer; they are just one part of a much wider maritime situational awareness toolkit [1], which also encompasses satellite monitoring [2], manned and unmanned aerial assets [3] with advanced analytics solutions, allowing the data gathered by all these agents to be turned into information and knowledge. one of the main capabilities the ums require is to be able to operate as a well-coordinated group, working together towards a higher-level goal such as maritime surveillance. however, the practical deployment of these novel smaller-scale ums requires the careful consideration of several aspects related to the operational requirements of the end users [4], the interoperability between the different systems [5] and towards the design of the surveillance architecture. as an example, the traditional approaches towards distributed patrol and surveillance [6]-[8] by manned systems generally do not take into consideration the effects of small waves (which are irrelevant for larger ships but very important for small ums). in this paper, a novel methodology for the real-time control of a fleet of between two and ten ums will therefore be proposed. the presented methodology is cast as a distributed coverage optimisation problem that specifies that the danger level for the ums of overturning is effectively estimated as a function of the potential trajectories and considered in the selection of the optimal movement strategy. as a result, the optimal safe trajectories for all the agents in the fleet can be planned. abstract unmanned maritime systems (ums) can provide important benefits for maritime law enforcement agencies for tasks such as area surveillance and patrolling, especially when they are able to work together as one coordinated system. in this context, this paper proposes a methodology that optimises the coverage of a fleet of ums, thereby maximising the opportunities for identifying threats. unlike traditional approaches to maritime coverage optimisation, which are also used, for example, in search and rescue operations when searching for victims at sea, this approach takes into consideration the limited seaworthiness of small ums, compared with traditional large ships, by incorporating the danger level into the design of the optimiser. mailto:geert.de.cubber@rma.ac.be acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 37 the proposed approach is validated through a simulation in an application scenario [9] connected to the surveillance of belgian offshore wind farms. belgian territorial waters are a very densely populated maritime area, with reserved spaces for all actors, as presented in figure 1, and it is important that all actors stay within the delimited zones. for wind farms (area shaded in red on figure 1), this often presents problems, as other users (e.g. fishing vessels and pleasure yachts) penetrate this zone without permission. in order to police and enforce the exclusion zone, it is necessary to patrol this area, which is on the maritime border with the netherlands and measures about 10 km by 30 km. 2. previous studies multi-agent robotic coverage optimisation is a research topic that has received a considerable amount of attention in recent years, as an increasing number of robotic assets are being deployed; thus, the need to identify strategies to optimise the coordination between these agents has increased. a first distinction to be made between the different methodologies is based upon the type of agents that are taken into consideration. on the one hand, there are approaches that tackle swarms of a high number of less intelligent agents [10]. swarm approaches generally make use of some form of ant colony optimisation algorithm [11] to solve the coverage problem. on the other hand, there are multi-agent approaches that deal with a lower number of more intelligent agents, which is the case for the application in the present study. a second important distinction between methodologies is based upon the assumptions made with regard to the connectivity between the different agents. if continuous broadband access between the agents is assumed, then all agents can obtain perfect localisation and sensor data from one another, and then the approaches can be based on some kind of global optimisation approach [12], with the capability to adapt to a timedependent environment [13]. even though it has been shown that finding a globally optimal solution for the coverage maximisation of a multi-agent fleet is an np-hard problem [14], it is possible to come quite close to this solution within real-time constraints [15], [16]; however, this requires intelligent strategies to guide the optimisation process (see more discussion on this subject later). if, however, unreliable network connections are assumed, then the agents cannot rely on a global planner, and a local optimisation is required. this also entails the need for a distributed approach that still allows for timely coordination between the different agents within the system, as proposed by xin et al. [17]. the methodology presented here adopts a hybrid approach. conceptually, it is based on a global optimisation, but one that is executed separately by each of the agents, taking into consideration the latest known data from the other agents. spatio-temporal memories are used to track and predict the localisation and sensor data from the other agents in order to address communication delays and breakdowns. clearly, these estimations are not perfect, but in this way, the optimisation scheme tries to adopt the best of both types of approach. within the robotics community, most attention has been focused on providing solutions to the multi-agent coverage optimisation problem for unmanned ground vehicles, but there are certainly also approaches that consider unmanned aerial vehicles [18]. however, for maritime systems, the research domain is less developed. fabbri et al. [19] presented a path and decision support system for maritime surveillance vessels, based on multi-objective optimisation algorithms that seek to find an optimal trade-off between several mission objectives. while the concepts are similar, this paper focuses on a high-level decision support system for large, manned vessels. this application aims to develop a solution for small-scale, unmanned patrol vessels, which means that the requirements and constraints are very different. as discussed previously, finding a globally optimal solution for the coverage maximisation of a multi-agent fleet is an nphard problem [14]. this implies that the algorithms scale up traditionally badly for an increasing number of agents. as the hybrid planner proposed here features a mix of global and local optimisation aspects, it also suffers from the drawback of global planning systems. in order to remedy this problem, researchers have suggested particle optimisation [20], as proposed by han et al., or grey wolf optimisation methodologies, as first introduced by mirjalili et al. [21] and later improved for distributed coverage optimisation problems by wang et al. [22]. in short, all these methodologies aim to intelligently prune the number of candidate positions that have to be investigated in order to limit the number of computations to be performed. developing further on these ideas, an optimisation strategy is also proposed that quickly selects the high-probability candidate positions, thereby limiting the computation time. figure 1. maritime spatial plan of belgian territorial waters, showing the very dense occupation of these waters by different actors and for different economic activities. this paper considers the surveillance and patrol of offshore wind farms, indicated on the map as the area with a red overlay (source: belgian federal public service – health, food chain safety and environment). acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 38 3. methodology 3.1. overall framework the proposed methodology draws inspiration from behaviour-based control frameworks [23], in which multiple behaviours actively work together to control the robot, or in this case, the ums. the main problem in behaviour-based control is how to synergise the different individual behaviours into a consistent and optimal global behaviour for the robotic agent. this requires the choice of so-called weight parameters that lead to the expected global behaviour. however, finding these weight parameters is a non-trivial task. therefore, this study proposes an optimisation scheme to find the optimal weights, taking into consideration two objectives: a) increasing the global coverage (and thereby increasing the acquisition of new knowledge about the environment) and b) minimising the danger level (thereby minimising the likelihood of the vessel capsizing). a major design issue for the development of such an optimisation scheme is that the weight parameters to be optimised are subject to a large number of environmental factors, such as visibility and wave height. therefore, for the present study, a dual approach was adopted. • at the offline learning stage, depicted by algorithm 1, an optimisation process was repeatedly run to find the optimal weight parameters 𝑤opt for multiple environmental conditions: 𝑤opt = arg min 𝑤 𝜙(𝑤, 𝛼, 𝑥, 𝑦, 𝜃, 𝜐, 𝛾, 𝑣m, 𝜃m, 𝑤h , 𝑤𝜃 , 𝑜𝑚, 𝜆) , (1) with the following parameters: o 𝑤 represents the weight parameters to be optimised. o 𝛼 is the number of agents. o (𝑥, 𝑦) is the position of the agents in a metric grid. o 𝜃 is the orientation of the agents in radians. o 𝜐 is the visibility in meters. this is a function of the sensorial visibility (which is considered to be static, as the ums sensor package does not change during a mission) and the meteorological visibility, which is dynamic, as the weather conditions may change throughout a mission. o 𝛾 is the sensors’ field of view on board the ums (rad). in this implementation, the sensors are always assumed to be front facing (although the field of view can be set to 360 °). o 𝑣m is the maximum velocity (m/s) that can be reached by the ums. o 𝜃m is the maximum turning rate (rad/s) that can be achieved by the different ums. o 𝑤h is the wave height (m). o 𝑤𝜃 is the wave orientation (rad). o 𝑜𝑚 is an obstacle map, expressed as a probability density function, that expresses the probability of finding an obstacle. o 𝜆 is a dimensionless parameter regulating the relative importance of coverage maximisation and the minimisation of the risk of capsizing. the parameters of the optimisation function 𝜙 and the function itself are further explained in section 3.3. for this optimisation process, the classic nelder–mead simplex algorithm [24] was used. this process typically takes a long time (a few days, depending on the granularity or resolution requested). for this reason, section 3.4 introduces an accelerated optimisation scheme. at the end of this process, the resulting data were stored in a database for later retrieval (during the online stage). • at the online stage, the correct weight parameters for the environmental conditions at hand were retrieved from the database and applied directly to the same optimisation function used before, as depicted by algorithm 2. in the following section, both parts of the optimisation scheme will be discussed in detail. 3.2. offline optimisation algorithm 1 depicts the offline optimisation scheme. as explained, its objective is to develop a database with the optimal weight parameters for each possible combination of environmental factors. this study has focused on four main factors that have been experimentally shown to have an important impact on the choice of the different weight parameters: the number of assets 𝛼, visibility 𝜐, wave height 𝑤h and wave direction 𝑤𝜃 . with regard to the number of assets 𝛼, fleets of between two and ten unmanned systems have been considered for this study. the reason why this number cannot be scaled up further is that the methodology relies on an analysis of the localisation and sensor data from all other assets. the methodology aims to predict the outcome of moving in a number of directions for each of these assets with an 𝒪(𝑁 2) problem. as a result, increasing the number of assets above ten leads to prohibitively long computation times, at least for the non-optimised version of the algorithm (see section 3.4 for details). concerning visibility, as this study is based on the use of small vessels (which have a minimal height and therefore a limited view over the waves), the maximum visibility range is set to 1,000 m. in terms of wave height, the database considers wave heights up to 10 m even though the simulations show that the danger level for such large wave heights is very high, and thus, the seaworthiness of the ums considered in this implementation is not assured. 3.3. online optimisation algorithm 2 depicts the online localisation scheme, which coincides with the optimisation function 𝜙 of algorithm 1. each step of the pseudo-code algorithm is explained here in detail: algorithm 1. offline optimisation scheme. algorithm 1. offline optimisation scheme. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 39 1. first, the relevant weights are extracted from the database. if no exact match can be found, an interpolation is performed taking into consideration the closest matching conditions in the database developed during the offline stage. 2. the assets perform an initial communication to get to know each other's position. an empty coverage map (𝑐𝑚) is constructed (note that no a priori knowledge is assumed). the robotic assets collectively build up a world model (a coverage map indicating areas they have visited and an obstacle map showing areas where they have found obstacles), which is maintained in memory by each of them in order to be able to cope with network outages. this world model is initially empty; the only information the assets have at the start is each other’s position and the boundaries of the working area. 3. the main loop for the simulation timer is created. 4. all the ums in the fleet are interrogated. 5. a set of candidate positions (𝑥c, 𝑦c) that the ums are able to move to is selected, depending on the starting position (𝑥0, 𝑦0 ), orientation 𝜃 and the maximum velocity �̅�max(𝑣m, 𝜃m) of the ums. 6. all possible candidate positions are explored. 7. new information that can be retrieved by moving from the starting position (𝑥0, 𝑦0) to the new position (𝑥, 𝑦) is assessed. this is achieved by adopting a visibility model, indicating, through visibility 𝜐 and the sensor field of view 𝛾, the probability of detecting an object as a result of the vessel’s orientation. the adopted visibility model assumes a mix of infrared, visual and lidar-based sensing and draws upon the heuristically established sensor models established by lahouli et al. [25] (for infrared sensors) and balta et al. [26] (for visual and lidar sensors). figure 2 provides an example of a visibility model for a vessel that is oriented at a 45 ° angle at the position (0,0). this visibility model is compared to the coverage map (𝑐𝑚), resulting in a local map 𝑝1, which can be regarded as a heat map indicating the best locations to move to in order to obtain the maximum amount of new data (i.e. to maximally increase the total value of the coverage map). 8. in order to maximise the chances of finding threats, it is better to move fast. however, the vessel should not move too fast because this would not be fuel efficient, and it could lead to incidents. therefore, another function generates a local heat map favouring a compromise vessel velocity 𝑣. 9. vessels are not able to change their orientation 𝜃 suddenly. therefore, another 'behaviour' generates a local heat map that avoids sharp turns. 10. small vessels are extremely susceptible to waves. both the wave height 𝑤h and the wave direction 𝑤𝜃 play an important role, and these need to be carefully aligned with the vessel speed and orientation. in order to assess this, an empirical ‘wave function’ was compiled, based on sailors’ experiences set out in the literature, that expresses the danger level related to waves. this wave function is expressed as 𝜙wave = (1 − 𝑦) 𝑣 𝑤h , (2) with 𝑦 defined as 𝑦 = 0.35 𝑥 6 − 3.5 𝑥 5 + 12.74 𝑥 4 − 20.75 𝑥 3 +14.36 𝑥 2 − 2.9 𝑥 + 0.1 . (3) figure 3 provides an example of a wave-function equation for a vessel at location (0,0) and with incoming waves from the north. as can be seen, the ideal orientation for the vessel (highest value of 𝜙wave) would be slightly inclined but nearly head on to the waves. orientations that are to be avoided (lowest value of 𝜙wave) are waves coming from the side or from the back. 11. it is important that vessels do not run into any detected obstacles. the ums therefore collectively create and share an obstacle map (𝑜𝑚) and steer away from objects on this map. 12. it would be inefficient for multiple agents in the fleet to investigate the same area. therefore, the swarm optimisation behaviour seeks to maintain an adequate distance between the agents. 13. the different local heat maps are combined into a single map 𝑝 using the weights that were calculated previously (in the offline step). 14. an extra check is performed in order to ensure that the ums do not stray away from the designated surveillance area. 15. an extra check is made in order to avoid revisiting recent locations. this is required not only to speed up the convergence but also to avoid getting stuck at local minima. therefore, a trajectory memory is maintained and checked for pruning the local heat map 𝑝. 16. on the local heat map 𝑝, the optimal position (𝑥𝑏 , 𝑦𝑏 ) is located. 17. all possible positions are then checked. 18. the vessel is steered towards the optimal position. algorithm 2. online optimisation scheme. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 40 19. the danger level for moving to this new position is estimated based on the wave function. the danger level is here defined as 𝑑𝑎𝑛𝑔𝑒𝑟 = 1 − 𝜙wave . 20. the ums performs an update of its sensing cycle, which will update the coverage map as new information is obtained. 21. the iteration for all agents ends. 22. the mean coverage score 𝑓c is recorded. 23. the total (summed) danger score 𝑓d is recorded. for reasons of normalisation, it is divided by the number of assets 𝛼. 24. the temporal loop ends. 25. coverage needs to be maximised while minimising the danger level. therefore, the objective function to be minimised is defined as 𝑓 = 1 𝑓c + 𝜆 𝑓d . (4) the first term of the objective function ensures that the coverage is maximised, while the second term ensures that the danger level is minimised. the parameter 𝜆 regulates the relative importance accorded to both aspects. this parameter is dependent on the type of vessel used. for smaller ums, sea waves present a much higher risk, so 𝜆 should be higher. for larger vessels, 𝜆 can be reduced in order to maximise the coverage mapping more rapidly. 3.4. computational speed optimisation as can be seen in the definition of algorithm 2, the computation time rises exponentially with the number of unmanned systems considered. not only do all of these assets require a separate evaluation (step 4 in algorithm 2), but also the complexity of many of the subprocesses (e.g. swarm optimisation) increases with the number of agents. the main culprits for the long computation time after this are the number of candidate positions that have to be evaluated (step 5 in algorithm 2) and the sometimes-high number of iterations required for the convergence of the nelder–mead simplex algorithm, used for solving (1). the adopted optimisation methodology is geared primarily towards an intelligent pruning of the candidate positions, as follows: • in the first phase, only a very limited subset of the original candidate positions is selected. this is performed by downscaling the selected candidate positions on a lower-scale metric grid. • in the second phase, an analysis (as described in algorithm 2, lines 6 to 17) is performed on the downscaled candidate positions. • a local subgrid is defined at the original resolution around the resulting position. • for all candidate positions within this local subgrid, the analysis (as described in algorithm 2, lines 6 to 17) is performed again and a final position is selected. the double loop may seem to add extra complexity and computation time, but in practice, it avoids the evaluation of a large number of candidate positions that do not have a viable chance of being selected. indeed, as the local maps are mostly continuous functions, it makes sense to evaluate them first at a lower resolution and then to scale up. furthermore, the convergence settings of the nelder–mead simplex algorithm were optimised to match the candidate position pruning approach. 4. validation 4.1. quantitative validation for the validation of the proposed approach, an application was selected that is used for the surveillance of belgian offshore wind farms, which have an area of around 10 km × 30 km that needs to be patrolled. however, the proposed methodology would also be very useful for a maritime search and rescue [27] or a fishery control scenario. figure 2. visibility model for a vessel that is oriented at a 45 ° angle at the position (0,0). figure 3. wave function for a vessel at location (0,0) and with incoming waves from the north. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 41 in order to validate the methodology, it was compared to five state-of-the-art solutions: • a random search, where each agent adopts a completely random movement pattern, • a distributed random search, where the search area is subdivided into equal parts and each agent adopts a random search pattern within the designated subzone, • a lawnmower search, where each agent uses a movement pattern typically adopted by robotic lawnmowers: moving in straight lines and turning a random number of degrees when approaching boundaries, • a distributed lawnmower search, where the search area is subdivided into equal parts and each agent adopts a lawnmower search pattern within the designated subzone, and • distributed greek patterns. this is the search and surveillance approach typically adopted by manned vessels, which has been proven to be very efficient for rapid area coverage. moreover, by subdividing the search area and distributing the search tasks among multiple agents, this approach is quite well suited to maritime coverage optimisation. figure 4 provides an example of the greek pattern. one disadvantage of all these state-of-the-art approaches is that they do not take into consideration the danger the waves pose to the vessel, which is an integral part of the proposed solution. in order to further validate the optimisation scheme, the results were also compared using a non-optimised, nominal version (using a static initial estimate for the weight parameter 𝑤) with the optimised approach. figure 5 presents the results in terms of coverage in a simulation with four agents present. it can be clearly seen that the presented approach (denoted as optimal and indicated in dark red) achieves the highest overall coverage. without using weight optimisation, the distributed greek patterns approach outperforms the baseline nominal approach presented here. all other approaches achieve a performance that is far lower. it can also be observed that the coverage results do not always monotonically increase. this can be explained by the fact that at each iteration, the existing coverage data is ‘aged’ (in practice, the coverage map is multiplied by 0.99) in order to represent the fact that older data have become less valid. the result is that with a limited number of agents, it becomes very difficult to maintain a high overall coverage score. these results can be expected, as the random search and lawnmower search approaches are quite simplistic methodologies, whereas the distributed greek patterns approach has a proven track record for these kinds of applications. still, using weight optimisation, the methodology proposed in this study succeeds in achieving a higher coverage score. however, the major strength of this approach can be seen in figure 6, which indicates the danger level of executing a mission using each of the approaches. the blue portion of the bar chart indicates the mean danger level, whereas the red portion indicates the maximum danger level attained during a particular mission. clearly, both are important for assessing the risk of incidents. it can be clearly seen that both the nominal and the optimal proposed methodology achieve a danger level that is significantly lower than in the other approaches. moreover, for the optimal approach, there is little difference between the mean and the maximum danger levels, indicating that the methodology succeeds in maintaining risk at a constant and low level. figure 4. example of distributed greek patterns. figure 5. evolution of the relative coverage of a surveillance area using seven different approaches. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 42 4.2. scaling and timing in order to assess the effects of the speed optimisation methodology explained in section 3.4, figure 7 shows the evolution of the processing time, relative coverage and relative danger level for the proposed approach with and without the application of the candidate location pruning methodology. as is clearly indicated in figure 7, the computation time is drastically reduced by the incorporation of the candidate position pruning methodology. in general, the accelerated approach is about 9 times faster than the baseline approach. this clearly shows that the proposed methodology of first analysing a limited set of points and then providing a detailed analysis of a dense point set in just one small area has a highly beneficial impact on the global processing time. this also enables the methodology to be used for an increased number of assets even though the global algorithm still scales up to slightly more than 𝒪(𝑁). an important aspect to assess is whether there would be any loss of quality using the accelerated approach. in order to evaluate this, the coverage mapping and danger levels were recorded for both methodologies and for the different numbers of agents, as shown in figure 7. surprisingly, the accelerated approach performed even better than the baseline approach even though the differences were not large. this may seem counter-intuitive at first sight because the accelerated approach evaluates fewer candidate positions and thus, compared with the baseline approach, always has a higher risk of being trapped at local minima. this phenomenon was investigated and found to be caused by the better convergence properties of the accelerated approach. indeed, as the accelerated approach requires fewer time-consuming local map evaluation steps, the nelder–mead simplex optimisation algorithm achieves (slightly) lower values for the optimisation function 𝜙 when using the accelerated method. the ‘side effect’ of accelerated optimisation, as described in section 3.4, is therefore also an improved (higher) coverage mapping and a slightly improved (lower) danger level. 5. conclusions in this paper, an approach towards distributed coverage optimisation for a maritime surveillance application has been presented. the approach is based upon a mix of offline learning and online optimisation. in order to remedy the traditional problem related to the excessive processing time for multi-agent global planning methodologies, an approach for the multi-scale selection of candidate locations has also been proposed. the methodology was validated by comparing it in simulation to multiple state-of-the-art approaches. this comparison demonstrated that the proposed approach performed well in terms of coverage mapping and very well in terms of minimising the danger of capsizing for small, unmanned vessels. moreover, the validation of the performance of the accelerated approach on multi-agent systems demonstrated that the computation time can be drastically reduced while the coverage mapping performance is increased. a next step will be to implement and test the system on the real-life ums that are planned to be deployed to patrol belgian territorial waters. figure 6. relative danger level for executing a maritime surveillance mission using seven different approaches. figure 7. evolution of the processing time, mean relative coverage and mean relative danger level for the proposed approach with and without the application of the candidate location pruning methodology. acta imeko | www.imeko.org september 2021 | volume 10 | number 3 | 43 acknowledgement the research presented in this paper has been funded by the belgian royal higher institute of defence, in the framework of the dap19/08 (marsur) project, and by the flemish agency for innovation and entrepreneurship, in the framework of the blue cluster project ssave (hbc.2019.0045). references [1] l. snidaro, i. visentini, k. bryan, fusing uncertain knowledge and evidence for maritime situational awareness via markov logic networks, information fusion 21 (2015), pp. 159-172. doi: 10.5281/zenodo.22949 [2] e. schwarz, d. krause, h. daedelow, s. voinov, near real time applications for maritime situational awareness, 36th international symposium on remote sensing of environment, berlin, germany, 11–15 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https://doi.org/10.1016/j.ifacol.2015.10.326 https://doi.org/10.1109/cdc.2010.5717883 https://doi.org/10.1109/ismcr47492.2019.8955727 https://doi.org/10.1007/s11721-012-0075-2 https://doi.org/10.1007/978-1-4419-1665-5_8 https://doi.org/10.1177/0278364916688103 https://doi.org/10.1155/2018/3436429 https://doi.org/10.1007/s10846-016-0461-x https://doi.org/10.1109/tac.2010.2040495 https://doi.org/10.1109/icca.2017.8003087 https://doi.org/10.1016/j.compag.2013.09.008 https://doi.org/10.1016/j.advengsoft.2013.12.007 https://doi.org/10.1177/1748302619889498 https://doi.org/10.1137/s1052623496303470 https://doi.org/10.1049/iet-ipr.2017.0221 https://doi.org/10.1016/j.ifacol.2018.11.566 microsoft word 314-2388-1-le-rev acta imeko  issn: 2221‐870x  november 2016, volume 5, number 3, 70‐75    acta imeko | www.imeko.org  november 2016 | volume 5 | number 3| 70  implementations and pilot comparison of primary low  intensity shock calibration  qiao sun, hong‐bo hu  national institute of metrology, beisanhuandonglu 18, 100029 beijing, china       section: research paper   keywords: metrology;  comparison; primary shock calibration;  low intensity shock acceleration   citation: qiao sun, hong‐bo hu, implementations and pilot comparison of primary low intensity shock calibration, acta imeko, vol. 5, no. 3, article 11,  november 2016, identifier: imeko‐acta‐05 (2016)‐03‐11  section editor: konrad jedrzejewski, warsaw university of technology, poland  received january 13, 2016; in final form june 16, 2016; published november 2016  copyright: © 2016 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: this work was supported by general administration of quality supervision, inspection and quarantine,  the people’s republic of china.  corresponding author: qiao sun, e‐mail: sunq@nim.ac.cn    1. introduction  accurate low intensity shock acceleration calibration is of special importance for automobile and civil industries worldwide. the physikalisch-technisches bundesanstalt (ptb) set the first example of a successful implementation of low intensity shock calibration by laser interferometry [1]. recent years have witnessed the establishment of a primary low intensity shock calibration standard and relevant scientific research projects at different metrology institutes of the apmp (asia pacific metrology programme), such as the national metrology institute of japan (nmij) [2], the industrial technology research institute (itri) of taiwan [3], the national institute of metrology of thailand (nimt), and the national institute of metrology (nim), china [4]. the international organization for standardization (iso) 16063-13[5] describes the primary low intensity shock calibration method, algorithm, and technical requirements, with an implementation example of a rigid body collision with excitations from 100 m/s2 to 5000 m/s2 and a pulse duration less than 10 ms. however, in the metrological field of shocks, there was no formal comparison either at a consultative committee (cc) level or a regional metrology organization technical committee (rmo tc) level. therefore, the unification of the shock acceleration quantity was short of direct supporting evidence. the consultative committee of acoustics, ultrasound and vibration (ccauv) has already planned to conduct key comparisons for shock in its strategic planning programme for 2013 to 2023 [6], possibly one key comparison for low intensity and the other for high intensity. abstract  this  paper  first  presents  two  main  technical  concerns  for  a  possible  low  intensity  shock  comparison:  variety  of  primary  shock  calibration  systems  and  feasibility  of  comparison  artifact.  for  the  primary  calibration  system,  the  mechanical  excitation  includes  hammer‐anvil  collision,  pneumatic  driven  projectile  impact  and  the  hopkinson  bar.  the  shock  pulses  generated  have  a  smooth  monopole  shape  by  the  first  two  types  with  air  bearings,  but  a  dipole  shape  by  the  third  type.  for  the  comparison  artifact,  the  standard  accelerometer  of  a  back‐to‐back  type  with  a  charge  amplifier  consists  of  an  accelerometer  measuring  chain  whose  nonlinearity of amplitude and phase frequency responses is investigated. based on the nonlinear fact of comparison artifact in the  frequency domain and the spectrum range difference of mechanical excitations of the calibration system, strict comparison conditions  had to be laid down for the measurement of shock sensitivity at specific acceleration levels and pulse durations.   the pilot comparison, coded as apmp.auv.v‐p1, is successfully organized by the asia pacific metrology programme. some comparison  results of both monopole excitation and dipole excitation are shown with the expanded uncertainty. the completion of this pilot  comparison  can  serve  as  part  of  the  basis  for  a  planned  key  comparison  targeted  at  a  low  intensity  shock  range  at  consultative  committee level.  acta imeko | www.imeko.org  november 2016 | volume 5 | number 3| 71  during the meeting of apmp tcauv in 2011, the decision was taken to make preparations for a pilot comparison targeted at low intensity shock acceleration. despite of scientific investigations of shock standards by individual institutes of the apmp, guest scientist research of the tc initiative project as intra-apmp collaboration on this topic and dedicated sessions of tcauv workshops in japan in 2011 and in thailand in 2014 made a direct contribution to the successful completion of this apmp tcauv pilot comparison of low intensity shock, coded as apmp.auv.v-p1 with participants from nim, itri, nimt and spektra (german dkd lab) [7]. in this paper, the implementations of three different types of shock excitations suitable for laser interferometry are described. rationale of strict comparison conditions are explained in detail. the comparison results are presented in two groups: shock sensitivities of monopole excitation from 500 m/s2 to 5000 m/s2 and shock sensitivities of dipole excitation at 1000 m/s2. the organization of this pilot comparison by the apmp tcauv has proved, among other things, the credibility of the primary low intensity shock calibration capability of the participants. 2. implementations of a low intensity shock calibration system  the primary low intensity shock calibration system consists of three main parts: shock excitation, laser interferometer and signal acquisition and data processing. for the laser interferometer, either homodyne or heterodyne can meet the requirement as primary measurement standard of acceleration for the calibration system. the technology is available in previous research, such as in [1]-[5]. signal acquisition and data processing algorithms and procedures are also covered in detail in [1]-[5]. the different shock excitation devices for primary laser interferometry calibration are of particular interest for the investigation of the pilot comparison in that their frequency spectra can be quite different. therefore, three different types of shock excitation devices which are employed in the pilot comparison are described. 2.1. hammer‐anvil excitation device  the shock excitation device based on a hammer-anvil mechanical collision is called shock machine and its basic working principle is well explained in [5]. the actual implementation is verified. ptb uses a spring unit as exciter to provide the original shock force in [1]. nmij uses an air pressure exciter [2] and itri uses an electromagnetic exciter [3]. the nim’s shock excitation device consists of an electromagnetic exciter and a pneumatic exciter. this combination as a mechanical power supply of the excitation device can deliver a wide range of shock acceleration levels and wider pulse durations [4]. all these four versions of implementation are placed in a horizontal position. it is worth noting that a high resonant frequency airborne hammer and anvil as moving parts of the excitation device is a precondition to generate a high-quality monopole shock pulse. the thin stiff air films can largely reduce mechanical disturbances from other parts of the excitation device, and avoid asymmetric impact forces to the anvil. as a result, the anvil can move rectilinearly with less rotational motion and produce repeatable shock pulses for calibration. figure 1 gives an instance of the primary shock calibration system based on the hammer-anvil excitation device from nim. the structure of the shock excitation device is shown in figure 2. in this figure, the shock exciter is the original part to supply mechanical power in the shock excitation device. the amount of output power is the decisive factor to determine the acceleration level and pulse duration of the shock pulse generated. besides, good controllability of the power supply contributes to good repeatability of the acceleration level generated. two shock exciters are deployed. the electromagnetic exciter is under precise control to produce an expected amount of power. the maximum output force is 9800 n. responding to a voltage input set by the controller, the electromagnetic exciter can drive the hammer at a speed from 0.08 m/s to 4.06 m/s, whose repeatability is better than 1 %. however, limited by its maximum power, the electromagnetic exciter is only used to produce an acceleration level below 5000 m/s2. the pneumatic exciter can produce a power supply corresponding to an air pressure range from 0.49 bar to 4.9 bar. by adjusting the air pressure inside the air tank and regulating the opening time of the electric valve, the pneumatic exciter can drive the hammer at the high speed limit of 5 m/s, which results in the highest generation of shock acceleration level above 10000 m/s2. airborne hammer and anvil are moving parts of the shock machine. they are made of titanium and aluminium alloy respectively, with 30 mm in diameter and 200 mm in length. each titanium hammer and anvil weighs 1.27 kg. their longitudinal resonant frequency of the first order is about 12 khz obtained by special tests. this resonant frequency limits the minimum shock pulse duration to about 0.5 ms generated by the shock machine, with its most energy components in the low frequency range. since the collision caused by their free figure  1.  photo  of  nim’s  primary  shock  calibration  system  based  on  a  hammer‐anvil excitation device.  figure 2. model of the shock excitation device.  acta imeko | www.imeko.org  november 2016 | volume 5 | number 3| 72  motion and direct impact is a precondition for a high-quality shock pulse, both the hammer and the anvil are equipped with air bearings in such a way that their centerlines are aligned within a tolerance of ±0.2 mm. air bearings of 30 mm inner diameter from new way co. are used. stiff air films less than 10 μm can significantly reduce negative influences from resonances of other parts of the shock machine structure, and avoid asymmetric impact forces to the anvil to a large extent. therefore, the anvil can move rectilinearly without rotational motion, and produce repeatable shock pulses for calibration purposes. the shock machine is based on solid body collision of hammer and anvil to produce excitation of shock pulses. but if the airborne hammer and anvil collides directly, it will cause strong resonant motions, and therefore the accelerometer mounted at the end surface of the anvil will provide unreliable output signals which cause inaccurate measurement results. this adverse influence must be avoided for accurate calibration results by attaching a cushioning pad to the hammer. the cushioning pad, likewise a damping isolator to absorb highfrequency noisy motions, acts as a shock pulse generator to produce desired pulse shapes with an expected acceleration level and pulse duration. this object is achieved by applying three sets of cushioning pads of different materials. it is worth noting that different cushion pads of different materials or different hardnesses or thicknesses can generate shock pulses of different shapes, acceleration levels or pulse durations, due to their own damping properties. therefore, each cushioning pad functions well to produce shock pulses at certain acceleration levels. beyond this working range, the pulse shape generated may be distorted and thus cause poor calibration results. 2.2. pneumatic projectile excitation device  the pneumatically driven projectile excitation device is a vertical implementation for good-quality monopole shock pulses in the pilot comparison. a projectile accelerated by pressurized air functions as hammer. while the air pressure remains constant, the kinetic energy of the projectile can be controlled by a motor driven mechanical stop that allows a precise adjustment of the projectiles starting position and thus of the distance over which it is accelerated. therefore, the repeatability of the shock pulse generated by this excitation device is good under the full automatic control of the target acceleration level. to be used as mechanical excitation source for laser interferometry measurements, an air bearing is equipped with the mounting part of the accelerometer to reduce transverse motions caused by the impact of the projectile. figure 3 shows the primary shock calibration system based on a pneumatic projectile excitation device from spektra. 2.3. hopkinson bar excitation device  the hopkinson bar excitation device is based on wave propagation and reflection characteristics inside a long thin bar and can generate dipole shock pulses normally in the acceleration range from 1000 m/s2 to 100000 m/s2 as described in [5]. by careful determination of the dimension and length of the titanium hopkinson bar and application of a piezoelectric actuator as exciter, a shock acceleration range from 200 m/s2 to 40000 m/s2 can be achieved. however, the pulse duration of the half sine shape is narrower than 0.2 ms [8]. the main parts of the hopkinson bar excitation device include a hopkinson bar, a piezoelectric actuator and a reaction mass. when a driving voltage is applied to the actuator, the piezo-stack changes its length and a reaction force is generated by the reaction mass according to newton’s 2nd law. this force acts as the input force on the end surface of the hopkinson bar. the acceleration generated at the other end of the bar can be quite accurate because the driving voltage can be precisely controlled. figure 4 shows the primary shock calibration system based on the pneumatic projectile excitation device from nimt. 3. pilot comparison   3.1. background of the comparison   the accurate measurement of low intensity shock acceleration is vital in certain applications, for example car crash tests. efforts to decrease the losses in human lives on the roads during the 1950s led to an increased research into the biomechanics of the head impact. a break-through was made with the introduction of the wayne state tolerance curve [9]. this curve was interpreted and a weighted injury criterion was developed. this criterion was later transformed into the head injury criterion (hic) to improve the crashworthiness of cars. hic is a measure of the likelihood of head injury arising from an impact. the hic can be used to assess safety related to vehicles, and it is defined as: figure 3. photo of the spektra’s primary shock calibration system based on  a pneumatic projectile excitation device.  figure 4. photo of the nimt’s primary shock calibration system based on a  hopkinson bar excitation device.  acta imeko | www.imeko.org  november 2016 | volume 5 | number 3| 73              2 1 2.5 2 1 2 1 max 1 ( ) ( ) t t hic a t dt t t t t , (1) where t1 and t2 are any two arbitrary times during the acceleration pulse. in 1986, the time interval over which hic is calculated was limited to 36 ms. shock acceleration is measured in standard gravity acceleration, shown in figure 5. normally the variable is derived from the acceleration/time history of an accelerometer mounted at the centre of gravity of a dummy’s head, like in the car crash test photo of figure 5, when the dummy is exposed to crash forces. this means that the hic includes the effects of head acceleration and the duration of the acceleration. large accelerations of up to 5000 m/s2 may be tolerated for very short durations of less than 1 ms. therefore, a low intensity shock comparison is well justified with the acceleration range from 500 m/s² to 5 000 m/s² and a duration of monopole shock pulse from 0.3 to 3 ms. 3.2. comparison feasibility  it was found previously by ptb that different shock sensitivities of the same accelerometer under the same level of shock acceleration were obtained when it was exposed to the excitation of a hammer-anvil device and a hopkinson bar device, respectively. the sensitivity difference was about 4 % at about 5000 m/s2. figure 6 reveals the cause of this sensitivity difference. the excitation of rigid body motion, the impact from hammer-anvil or pneumatic projectile, falls into a narrow low frequency range of the spectrum while the excitation by the hopkinson bar covers a wider spectrum range from low to high frequencies. the sensitivity magnitude frequency response of the accelerometer only has a narrow working range of constant sensitivity at low frequency. this effect plays an increasing role when the shock pulse duration decreases and therefore covers a wide frequency range, which finally results in obvious sensitivity difference by two different excitations. therefore, pulse shape, pulse duration and acceleration level should be restricted for the feasibility of the pilot low intensity shock comparison. hammer-anvil excitation and pneumatic projectile excitation can generate monopole pulses and fall into the same group of rigid body motion. hopkinson bar excitation should be treated into a different group. it should be noted that pulse duration is defined as time width between rising edge point and falling edge point at a 10 % level of peak acceleration. for the monopole group, participants are supposed to measure at the following acceleration levels (all values in m/s²): 500, 1000, 2000, 3000, 4000, 5000. a series of 0.5 ms, 1 ms, 1.5 ms and 2 ms of shock pulses are recommended, with a reference of 2 ms at an acceleration of 1000 m/s². for the dipole group, participants are supposed to measure at 1000 m/s². a series of 0.03 ms, 0.05 ms, 0.07 ms, 0.10 ms (reference), 0.15 ms and 0.20 ms are recommended. 3.3. comparison results   for the purpose of the comparison the pilot laboratory selected an endevco 2270 (sn: 10466) with a brüel & kjær charge amplifier 2692 (sn: 2752215) as accelerometer chain of which monitoring data for 6 months were available and of which data were not included in any published international cooperation work. for this comparison, nim used its hammeranvil calibration system and hopkinson bar calibration system; itri used its hammer-anvil calibration system; nimt used its hopkinson bar calibration system; and spektra used its pneumatic projectile calibration system and hopkinson bar calibration system. in table 1 and 2, the calibration results under monopole and dipole excitations are presented. the weighted mean was agreed upon by all participants to calculate the pilot comparison reference values (pcrv) for the apmp.auv.v-p1 data. pcrvs were calculated separately at each acceleration or pulse duration point for the accelerometer chain. four typical degrees of equivalence with respect to pcrvs of the participants are shown in figure 7 and 8. the degrees of equivalence also support the uncertainty of the measurements of the participants at other acceleration levels and pulse durations. 4. conclusions  based on the investigation work on the primary low intensity shock calibration technique and feasibility of a pilot comparison, tcauv of apmp has successfully conducted a pilot comparison of shock acceleration sensitivity at low intensity shock accelerations from 500 m/s² to 5000 m/s². laser interferometry is a necessity as a measurement standard of the calibration system. three different types of mechanical shock excitations are employed by the participants. the comparison results are divided into two groups by the excitation pulse shape: the monopole and the dipole group. the figure 6. frequency responses of the accelerometer, the rigid body and the  hopkinson bar.  figure 5. wayne state tolerance curve and photo of car crash test in china. acta imeko | www.imeko.org  november 2016 | volume 5 | number 3| 74  reported sensitivities and associated uncertainties from four participants nim, itri, nimt and spektra are used for the calculation of mean values of the pilot comparison results and their associated uncertainties, as well as the deviations from the mean values with associated uncertainties. the degrees of equivalence calculated from the measurement results by the four participants support the measurement uncertainty reported by them at all acceleration levels and pulse durations specified in the technical protocol. the successful completion of this pilot comparison can serve, among other things, as part of the basis for a planned key comparison coded as ccauv.v-k4 targeted at a low intensity shock range at ccauv. acknowledgement  this work was supported in part by the general administration of quality supervision, inspection and quarantine of the people’s republic of china under contract no. alc1201 and was partly funded by the apmp tc initiative project no. tci-2011-01-tcauv and by the apmp dec project 2014. the authors would like to give special table 1. calibration results of the participants for voltage sensitivities sva under monopole shock excitation with expanded relative uncertainty uc (k = 2). acceleration  (m/s²)  pulse duration  (ms)  nim  itri  spektra  sva  (mv/(m/s 2 ))  uc (%)  sva  (mv/(m/s 2 ))  uc (%)  sva  (mv/(m/s 2 ))  uc (%)  500  3.0  0.1967  0.5  0.1966  1.0  0.19727  0.5  1000  2.0  0.1968  0.5  0.1970  1.0  0.19746  0.5  2000  1.5  0.1970  0.5  0.1971  1.0  0.19749  0.7  3000  1.0  0.1973  0.5  0.1971  1.0  0.19755  0.7  4000  1.0  0.1973  0.5  0.1971  1.0  0.19735  0.7  5000  0.8  0.1973  0.5  0.1971  1.0  0.19734  0.7  table 2. calibration results of the participants for voltage sensitivities sva under dipole shock excitation with expanded relative uncertainty uc (k = 2). acceleration  (m/s²)  pulse duration  (ms)  nim  nimt  spektra  sva  (mv/(m/s 2 ))  uc (%)  sva  (mv/(m/s 2 ))  uc (%)  sva  (mv/(m/s 2 ))  uc (%)  1000  0.20  0.1972  0.5  0.1976  0.6  0.19765  0.5  1000  0.15  0.1973  0.5  0.1977  0.6  0.19744  0.5  1000  0.10  0.1976  1.0  0.1976  0.7  0.19765  0.5  1000  0.07  0.1981  1.0  0.1976  1.0  0.19776  0.8  1000  0.05  0.1985  1.5  0.1977  1.0  0.19758  0.8  1000  0.03  0.1990  1.5  0.1985  1.0  0.19718  0.8  figure  7.  degree  of  equivalence  for  voltage  sensitivities  under  monopole shock excitation at 1000 m/s 2 , 2.0 ms and 5000 m/s 2 , 0.8 ms.  figure 8. degree of equivalence for voltage sensitivities under dipole shock  excitation at 1000 m/s 2 , 0.20 ms and 0.03 ms.  acta imeko | www.imeko.org  november 2016 | volume 5 | number 3| 75  thanks to colleagues in itri, nimt, nmij and spektra for their support and cooperation in this comparison, related research work and workshop activities. references  [1] link a, von martens hj, “calibration of accelerometers by shock excitation and laser interferometry”, shock vib, 2000 7:101-112. [2] h nozato, t usuda, a oota, t ishigami, “calibration of vibration pick-ups with laser interferometry: part iv development of a shock acceleration exciter and calibration system”, measurement sci. technol. 2010 21:065107. [3] yu-chung huang, jiun-kai chen, hsin-chia ho, chung-sheng tu, chao-jun chen, “the set up of primary calibration system for shock acceleration in nml”, measurement, 2012 45: 23832387. [4] qiao sun, jian-lin wang, hong-bo hu, “a primary standard for low-g shock calibration by laser interferometry”, measurement sci. technol. 2014 25:075003. [5] iso 16063-13, methods for the calibration of vibration and shock transducers. part 13: primary shock calibration using laser interferometry, 2001. [6] cipm ccauv, “strategy document for rolling programme development for 2013 to 2023”, www.bipm.org/en/committees/cc/ccauv. [7] qiao sun, hongbo hu. “final report on pilot comparison of low intensity shock apmp.auv.v-p1”. metrologia tech. suppl., 2015, 52: 09002. [8] spektra, data sheet of se-220 hop-ms hopkinson bar shock exciter, www.spektra-dresden.com/spektra. [9] lissner hr, lebow m, evans fg 1960 “experimental studies on the relation between acceleration and intracranial pressure change” in men surgery gynecology and obstetrics 111: 329. introductory notes for the acta imeko second issue general track acta imeko issn: 2221-870x june 2023, volume 12, number 2, 1 4 acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 1 introductory notes for the acta imeko second issue general track francesco lamonaca1 1 department of department of computer science, modeling, electronics and systems engineering (dimes), university of calabria, ponte p. bucci, 87036, arcavacata di rende, italy section: editorial citation: francesco lamonaca, introductory notes for the acta imeko second issue general track, acta imeko, vol. 12, no. 2, article 1, june 2023, identifier: imeko-acta-12 (2023)-02-01 received june 28, 2023; in final form june 30, 2023; published june 2023 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: francesco lamonaca, e-mail: editorinchief.actaimeko@hunmeko.org dear readers, as editor in chief of acta imeko, i am pleased to share with you a further success of acta imeko that in the last ranking of scimago increases its position to the third quartile also in instrumentation (https://www.scimagojr.com/journalsearch.php?q =21100407601&tip=sid&clean=0). i would like to thank all the editorial staff, the reviewers, the authors, and each of you for choosing acta imeko as a valuable source to find knowledge and inspiration and an effective media to share your research and results with the scientific community. this general track collects several contributions that do not relate to a specific event. as editor in chief, it is my pleasure to give to you an overview of these papers. accurate, continuous and reliable data gathering and recording about crop growth and state of health, by means of a network of autonomous sensor nodes that require minimal management by the farmer will be essential in future precision agriculture. in [1], a low-cost multi-channel sensor-node architecture is proposed for the distributed monitoring of fruit growth throughout the entire ripening season. the prototype presented is equipped with five independent sensing elements that can be attached each to a sample fruit at the beginning of the season and are capable of estimating the fruit diameter from the first formation up to the harvest. the sensor-node is provided with a lora transceiver for wireless communication with the decision making central, is energetically autonomous thanks to a dedicated energy harvester and an accurate design of power consumption, and each measuring channel provides resolution of a few tenths of a millimeter with a full-scale range of 12 cm. the accurate calibration procedure of the sensor-node and its elements is described in the paper, which allows for the compensation of temperature dispersion, noise and nonlinearities. the prototype was tested on field in real application, in the framework of the research activity for next-generation precision farming performed at the experimental farm of the department of agricultural and food science of the university of bologna, cadriano, italy. the paper in [2] belongs to a line of research known as aerial archaeology and compares some specific visualizations of lidar data (hill-shading, openness, and sky view factor) to understand which of them can provide the best approach to suitably identify and unveil some archaeological permanences as function of different boundary conditions. in the case presented by the authors, such permanences belong to the very special material heritage consisting of the "physical traces" of the great war, although latent, they persist in the present landscapes at different states of preservation and visibility, waiting to be unearthed to express their cultural potential. they represent an indispensable palimpsest of "minor signs" such as, for example, fragments of entrenchments, gun emplacements, shelters, bomb craters, and temporary shelters. such elements made the war machine work at that time while, nowadays, if properly recognized and enhanced, could foster the historical and cultural revitalization of the territories where they are placed. in [3] the results of measurements of atmospheric absorption and the amount of precipitated water on the suffa plateau for the period from january, 2015 to november, 2020, are presented. the measurements of atmospheric parameters in the 2 and 3 mm range of the radio waves spectrum were carried out using the miap-2 radiometer. the results of more than six years of measurements have shown that on the suffa plateau, atmospheric parameters in the above range remain fairly stable. in [4], the analysis of measuring channels of angular velocity with an encoder is given. this analysis has made it possible to obtain an equation for estimating the quantization and sampling error for an exponential mathematical model describing the transient process of operation of electrical machines. the components of the mathematical model of this dynamic error are the sampling step and the derivative, which characterizes the rate of change of the measured value over time. it was found that the errors of quantization and sampling significantly depend on the mailto:editorinchief.actaimeko@hunmeko.org https://www.scimagojr.com/journalsearch.php?q%20=21100407601&tip=sid&clean=0 https://www.scimagojr.com/journalsearch.php?q%20=21100407601&tip=sid&clean=0 acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 2 value of the resolution z of the encoder. moreover, an increase in z leads to a decrease in the sampling error, but the relative quantization error increases. a proposal to reconcile these components of errors is also given. the monitoring and management of postprandial glucose response (pgr), by administering an insulin bolus before meals, is a crucial issue in type 1 diabetes (t1d) patients. artificial pancreas (ap), which combines autonomous insulin delivery and blood glucose sensor, is a promising solution; nevertheless, it still requires input from patients about meal carbohydrate intake for bolus administration. this is due to the limited knowledge of the factors that influence pgr. even though meal carbohydrates are regarded as the major factor influencing pgr, medical experience suggests that other nutritional should be considered. to address this issue, in [5], the authors propose a machine learning (ml)-based approach for a more comprehensive analysis of the impact of nutritional factors (i.e., carbohydrates, protein, lipids, fiber, and energy intake) on the blood glucose levels (bgls). in particular, the proposed ml-model takes into account bgls, insulin doses, and nutritional factors in t1d patients to predict bgls in 60-minute time windows after a meal. a feed-forward neural network was fed with different combinations of bgls, insulin, and nutritional factors, providing a predicted glycaemia curve as output. the validity of the proposed system was demonstrated through tests on public data and on self-produced data, adopting intraand inter-subject approach. results anticipate that patient-specific data about nutritional factors of a meal have a major role in the prediction of postprandial bgls. the cyber-security of an embedded device is a crucial issue especially in the internet of things (iot) paradigm, since the physical accessibility to the smart transducers eases an attacker to eavesdrop the exchanged messages. in [6], the role of metrology in improving the characterization and security testing of embedded devices is discussed in terms of vulnerability testing and robustness evaluation. the presented methods ensure an accurate assessment of the device’s security by relying on statistical analysis and design of experiments. a particular focus is given on power analysis by means of a scatter attack. in this context, the metrological approach contributes to guaranteeing the confidentiality and integrity of the data exchanged by iot transducers. an electroencephalography (eeg)-based classification system of three levels of fear of heights is proposed in [7]. a virtual reality (vr) scenario representing a canyon was exploited to gradually expose the subjects to fear inducing stimuli with increasing intensity. an elevating platform allowed the subjects to reach three different height levels. psychometric tools were employed to initially assess the severity of fear of heights and to assess the effectiveness of fear induction. a feasibility study was conducted on eight subjects who underwent three experimental sessions. the eeg signals were acquired through a 32-channel headset during the exposure to the eliciting vr scenario. the main eeg bands and scalp regions were explored in order to identify which are the most affected by the fear of heights. as a result, the gamma band, followed by the high-beta band, and the frontal area of the scalp resulted the most significant. the average accuracies in the within-subject case for the three-classes fear classification task, were computed. the frontal region of the scalp resulted particularly relevant and an average accuracy of (68.20 ± 11.60) % was achieved using as features the absolute powers in the five eeg bands. considering the frontal region only, the most significant eeg bands resulted to be the highbeta and gamma bands achieving accuracies of (57.90 ± 10.10) % and of (61.30 ± 8.43) %, respectively. the sequential feature selection (sfs) confirmed those results by selecting for the whole set of channels, in the 48.26 % of the cases the gamma band and in the 22.92 % the high-beta band and by achieving an average accuracy of (86.10 ± 8.29) %. in [8], a feasibility study on electroencephalographic monitoring of executive functions during dual (motor and cognitive) task execution is presented. electroencephalographic (eeg) signals are acquired by means of a wearable device with few channels and dry electrodes. the light weight and wireless device allow for walking in a natural way. the most significant eeg features are investigated to classify different levels of activation for two fundamental executive functions (ef) both in sitting and walking conditions. power spectral density in the gamma band resulted in the most relevant feature in discriminating low and high levels of inhibition. power spectral density in the beta and gamma bands resulted the most discriminating the level of activation of working memory. the study poses the basis for (i) monitoring the activation levels of ef during gait allowing loss prevention in the elderly and (ii) specific cognitive rehabilitation aimed at the most relevant executive functions during walking. the paper in [9] deals with the extraction of the fetal electrocardiography (ecg) signal from the raw ecg signals of the mother by the beamforming based algorithms. the foetal ecg sensors bring out signals containing information from the pregnant mother and the infant. detailed and separate signals are already provided by the foetal ecg instruments; but for some specific studies related to the infant conditions, it is necessary to improve the quality of the signal with a dedicated processing. in this paper, four techniques, with some enhancements, are proposed to perform the processing; we have applied the following techniques: least mean square (lms) with adaptive noise cancellation technique, discrete wavelet transform (dwt)-based technique, empirical wavelet transform (ewt) technique, and multiple signal classification (music). the lms and the music pertain to beamforming approach. the techniques were used to decompose and identify the different elements constituting the source signal (mother's signal) and noise cancellation by multivariate empirical mode decomposition (memd) technique. the signal was adaptively decomposed by lms, dwt and music according to optimised parameters to extract some hidden components of the source signal, such as the foetal features, qrs, heartbeat etc. the results have showed that lms, with enhancements, is more effective in identifying and removing useless noise. the techniques were applied to the ecg signal of a 30-year-old healthy pregnant woman, which allowed to verify their applicability. the present research leads to the below main contributions among others: separation of the ecg signal of the foetus from the mother, highlighting the functional state of the foetal heart rhythm (heart rate and heartbeat,) and this can show us if the foetal ecg has malfunctions. the random noise test of analog to digital converters recommended by the ieee 1057 standard for digitizing waveform recorders is studied in [10]. the heuristically derived expression presented for the estimation of the random noise standard deviation is experimentally validated. the standard suggests a triangular stimulus signal. the authors show how to use a sinusoidal stimulus signal to carry out the test. the influence of stimulus signal offset and amplitude on the estimation error is also analysed and an expression is presented acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 3 to compute the minimum amplitude value that guarantees an upper bound on the estimator bias. the paper in [11] presents a theoretical analysis of uncertainty sources in measurement techniques used to determine vibrations of turbomachinery blades using stationary sensors mounted on the casing of the turbomachine. a mathematical model based on fundamental physical principles is proposed, and two different measurement set-ups are evaluated. one set-up uses a reference sensor to measure the passage of an undeformed part of the blades (blade base), while the other set-up does not involve the use of a reference sensor, with both sensors facing the blades tip (deformed part). the intrinsic uncertainty of these methods and the performance of the complete measurement chain are defined. the analysis of the measurement technique leads to conclusions about the practical set-up and possible performances of these measurement techniques. measurement uncertainty plays a very important role ensuring validity of decision-making procedures, since it is the main source of incorrect decisions in conformity assessment. the guidelines given by the actual standards allow one to take a decision of conformity or non-conformity, according to the given limit and measurement uncertainty associated to the measured value. due to measurement uncertainty, a risk of a wrong decision is always present, and the standards also give indications on how to evaluate this risk, although they mostly refer to a normal probability density function to represent the distribution of values that can be reasonably attributed to the measurand. since such a function is not always the one that best represents this distribution of values, the paper in [12] considers some of the most-often used probability density functions and derives simple formulas to set the acceptance (or rejection) limits in such a way that a pre-defined maximum admissible risk is not exceeded. accurate and reliable results in orthodontics heavily depend on selecting the right impression materials. with the rise of digital technology and additive manufacturing techniques, it has become necessary to characterize experimentally the materials used to design prosthetic bases. in the study presented in [13], the mechanical properties of polyetheretherketone, nylon6, nylon12, and polypropylene are analysed, as impression materials commonly used in dentistry applications. specifically, the effect on their flexural elastic modulus of the exposure to working environment conditions is also investigated by means of 3-point bending test performed on virgin materials and samples immersed in saliva for 72 hours. the proposed approach revealed significant behaviour in terms of loss in mechanical performances. these findings have significant implications for the proper selection and use of am materials in dental applications. structural health monitoring (shm) is an essential aspect to ensure the safety and longevity of civil infrastructure. in recent years, there has been a growing interest in developing shm systems based on micro-electro-mechanical systems (mems) technology. mems-based sensors are small, low-power, and cost-effective, making them ideal for large-scale deployment in structural monitoring systems. however, the use of memsbased sensors in shm systems can be challenging due to their inherent errors, such as drift, noise, and bias instability; these errors can affect the accuracy and reliability of the measured data, leading to false alarms or missed detections. therefore, several methods have been proposed to compensate for these errors and improve the performance of mems-based shm systems. for this purpose, the authors in [14] propose the combined of a redundant configuration of cost-effective mems accelerometers and a kalman filter approach to compensate mems inertial sensor errors and data filtering; the performance of the method was preliminarily assessed by means of a custom-controlled oscillation generator and compared with that granted by a highcost, high-performance mems reference system where amplitude differences of 0.02 m/s2 have been experienced. finally, a sensor node for real-time monitoring has been proposed that exploits lorawan and nfc protocols to access the structure information to be monitored. electromagnetic tracking systems (emtss) are widely used in surgical navigation, allowing to improve the outcome of diagnosis and surgical interventions, by providing the surgeon with real-time position of surgical instruments during medical procedures. the main goal is to improve the limited range of current commercial systems, which strongly affects the freedom of movement of the medical team. studies are currently being conducted to optimize the magnetic field generator (fg) configuration (both geometrical arrangements and electrical properties) since it affects tracking accuracy. in [15], authors discuss experimental data from an emts based on a developed 5-coils fg prototype, and they show the correlation between position tracking accuracy and the gradients of the magnetic field. therefore, they optimize the configuration of the fg by employing two different metrics based on i) the maximization of the amplitude of the magnetic field as reported in literature, and ii) the maximization of its gradients. the two optimized configurations are compared in terms of position tracking accuracy, showing that choosing the magnetic field gradients as objective function for optimization leads to higher position tracking accuracy than maximizing the magnetic field amplitude. i hope you will enjoy your readings. francesco lamonaca editor in chief references [1] lorenzo mistral peppi, matteo zauli, luigi manfrini, luca corelli grappadelli, luca de marchi, pier andrea travers, low-cost, high-resolution and no-manning distributed sensing system for the continuous monitoring of fruit growth in precision farming, acta imeko, vol.12, no.2, 2023, pp. 1-11. doi: 10.21014/acta_imeko.v12i2.1342 [2] joel aldrighettoni, maria grazia d’urso, military archaeology and lidar data visualizations: a non-invasive approach to detect historical remains, acta imeko, vol.12, no.2, 2023, pp. 1-10. doi: 10.21014/acta_imeko.v12i2.1395 [3] dilshod raupov, s. ilyasov, g. i. shanin, the results of atmospheric parameters measurements in the millimeter wavelength range on the radio astronomy observatory “suffa plateau”, acta imeko, vol.12, no.2, 2023, pp. 1-10. doi: 10.21014/acta_imeko.v12i2.1430 [4] vasyl kukharchuk, oleksandr vasilevskyi, volodymyr holodiuk results of study of quantization and discretization error of digital tachometers with encoder, acta imeko, vol.12, no.2, 2023, pp. 1-6. doi: 10.21014/acta_imeko.v12i2.1452 [5] giovanni annuzzi, lutgarda bozzetto, andrea cataldo, sabatina criscuolo, marisa pesola, predicting and https://doi.org/10.21014/acta_imeko.v12i2.1342 https://doi.org/10.21014/acta_imeko.v12i2.1395 https://doi.org/10.21014/acta_imeko.v12i2.1430 https://doi.org/10.21014/acta_imeko.v12i2.1452 acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 4 monitoring blood glucose through nutritional factors in type 1 diabetes by artificial neural networks, acta imeko, vol.12, no.2, 2023, pp. 1-7. doi: 10.21014/acta_imeko.v12i2.1453 [6] pasquale arpaia, francesco caputo, antonella cioffi, antonio esposito, the role of metrology in the cybersecurity of embedded devices, acta imeko, vol.12, no.2, 2023, pp. 1-6. doi: 10.21014/acta_imeko.v12i2.1455 [7] andrea apicella, simone barbato, luis alberto barradas chacόn, giovanni d’errico, lucio tommaso de paolis, luigi maffei, patrizia massaro, giovanna mastrati, nicola moccaldi, andrea pollastro, selina christin wriessenegger, electroencephalography correlates of fear of heights in a virtual reality environment, acta imeko, vol.12, no.2, 2023, pp. 1-10. doi: 10.21014/acta_imeko.v12i2.1457 [8] pasquale arpaia, renato cuocolo, paolo de blasiis, anna della calce, allegra fullin, ludovica gargiulo, luigi maffei, nicola moccaldi, electroencephalographic-based wearable instrumentation to monitor the executive functions during gait: a feasibility study, acta imeko, vol.12, no.2, 2023, pp. 1-6. doi: 10.21014/acta_imeko.v12i2.1460 [9] john p. djungha okitadiowo, a. lay-ekuakille, t. isernia, v. bhateja, s.p. singh, beamforming-based algorithms for recovering information from fetal electrocardiographic sensors, acta imeko, vol.12, no.2, 2023, pp. 1-8. doi: 10.21014/acta_imeko.v12i2.1470 [10] francisco alegria, random noise test of analog-to-digital converters, acta imeko, vol.12, no.2, 2023, pp. 1-8. doi: 10.21014/acta_imeko.v12i2.1502 [11] giulio tribbiani, lorenzo capponi, tommaso tocci, tiberio truffarelli, roberto marsili, gianluca rossi, a theoretical model for uncertainty sources identification in tip-timing measurement systems, acta imeko, vol.12, no.2, 2023, pp. 1-6. doi: 10.21014/acta_imeko.v12i2.1510 [12] alessandro ferrero, harsha vardhana jetti, sina ronaghi, simona salicone, a method to consider a maximum admissible risk in decision-making procedures based on measurement results, acta imeko, vol.12, no.2, 2023, pp. 1-9. doi: 10.21014/acta_imeko.v12i2.1518 [13] tommaso zara, lorenzo capponi, francesca masciotti, stefano pagano, roberto marsili, gianluca rossi, effects of saliva on additive manufacturing materials for dentistry applications: experimental research using flexural strength analysis, acta imeko, vol.12, no.2, 2023, pp. 1-6. doi: 10.21014/acta_imeko.v12i2.1519 [14] giorgio de alteriis, enzo caputo, rosario schiano lo moriello, on the suitability of redundant accelerometers for the implementation of smart oscillation monitoring system: preliminary assessment, acta imeko, vol.12, no.2, 2023, pp. 1-9. doi: 10.21014/acta_imeko.v12i2.1532 [15] gregorio andria, filippo attivissimo, attilio di nisio, anna m. l. lanzolla, mattia alessandro ragolia, analysis and optimization of surgical electromagnetic tracking systems by using magnetic field gradients, acta imeko, vol.12, no.2, 2023, pp. 1-8. doi: 10.21014/acta_imeko.v12i2.1589 https://doi.org/10.21014/acta_imeko.v12i2.1453 https://doi.org/10.21014/acta_imeko.v12i2.1455 https://doi.org/10.21014/acta_imeko.v12i2.1457 https://doi.org/10.21014/acta_imeko.v12i2.1460 https://doi.org/10.21014/acta_imeko.v12i2.1470 https://doi.org/10.21014/acta_imeko.v12i2.1502 https://doi.org/10.21014/acta_imeko.v12i2.1510 https://doi.org/10.21014/acta_imeko.v12i2.1518 https://doi.org/10.21014/acta_imeko.v12i2.1519 https://doi.org/10.21014/acta_imeko.v12i2.1532 https://doi.org/10.21014/acta_imeko.v12i2.1589 microsoft word 249-1769-1-le-rev2 acta imeko  issn: 2221‐870x  september 2015, volume 4, number 3, 53 ‐ 58    acta imeko | www.imeko.org  september 2015 | volume 4 | number 3 | 53  estimation of stepping motor current from long distances  through cable‐length‐adaptive piecewise affine virtual sensor  alberto oliveri 1 , mark butcher 2 , alessandro masi 2 , marco storace 1   1  department of electrical, electronic, telecommunications engineering and naval architecture (diten), university of genoa, via opera pia   11a, 16145, genova, italy  2  department of engineering, cern, 1211 geneva, switzerland      section: research paper  keywords: virtual sensor; estimation; fpga; piecewise‐affine functions  citation: alberto oliveri, mark butcher, alessandro masi, marco storace, estimation of stepping motor current from long distances through cable‐length‐ adaptive piecewise affine virtual sensor, acta imeko, vol. 4, no. 3, article 9, september 2015, identifier: imeko‐acta‐04 (2015)‐03‐09  editor: paolo carbone, university of perugia, italy  received february 13, 2015; in final form may 29, 2015; published september 2015  copyright: © 2015 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: work supported by the university of genoa.  corresponding author: marco storace, e‐mail: marco.storace@unige.it    1. introduction  the lhc (large hadron collider) at cern is a circular particle accelerator which, as all machines of this kind, needs a collimation system to block highly energetic particles flying off their trajectories. if these particles are not properly collected, they can seriously damage the accelerator. the module that collects these potentially harmful particles is called collimation system and comprises some moving parts that need to be actuated with high precision inside the tunnel. hybrid stepper motors are often used as the actuators in the collimation system because of their high positioning repeatability and open loop control [1]. these motors, and their electronic drives, are subject to a number of requirements that are relatively unique to accelerators. the environment surrounding the motors is highly radioactive and driver electronics are damaged by this radioactivity, then the drivers are placed in radiation-safe zones at a distance of up to 1 km from the motors. they must therefore be connected to the hybrid stepper motors via long cables, as shown in figure 1. pulse width modulated (pwm) control voltages are largely used to increase power efficiency; however, they generate significant electromagnetic interference (emi) emissions, which can affect neighbouring electronics. high-frequency pwm chopping frequencies must therefore be used to shift the emissions to higher frequencies. these high-frequency voltage signals, nonetheless, cause the long cables to act as transmission lines, and produce a ringing phenomenon in the currents on the driver-side of the cable, the only side where measurements are possible. figure 2 shows a comparison of the current in the driver-cable-motor circuit for a single motor phase. good motor positioning repeatability is of great importance and requires to have real-time knowledge of the motor’s position, in order that compensatory action can be taken to correct any misalignments (i.e., steps losses). radiation-hard resolvers are used to measure the motor position and to detect lost steps. it is, nonetheless, desirable to have sensor redundancy. additionally, even if the stepping motors work at nominal torque, chosen by design to be at least twice the nominal load torque, having an estimate of the real load torque abstract  in this paper a piecewise affine virtual sensor is used for the estimation of the motor‐side current of hybrid stepper motors, which  actuate the lhc (large hadron collider) collimators at cern. the estimation is performed starting from measurements of the current  in the driver, which is connected to the motor by a long cable (up to 720 m). the measured current is therefore affected by noise and  ringing phenomena. the proposed method does not require a model of the cable, since it is only based on measured data and can be  used with cables of different length. a circuit architecture suitable for fpga implementation has been designed and the effects of fixed  point representation of data are analyzed.  acta imeko | www.imeko.org  september 2015 | volume 4 | number 3 | 54  can be useful to warn about degradation of the mechanical parts of the collimation system. a sensorless driver, based on an extended kalman filter (ekf), which can work with arbitrarily long power cables between the driver and motor has been developed in [2]. in order to use exclusively the motor model in the ekf, the algorithm’s inputs and measurements should be the motor-side voltages and currents, respectively. however, as previously stated, these signals are not directly available in ordinary operation since measurements can only be made on the drive side of the cable. in [2] a cable model is used to obtain a transfer function relating the motor-side current to the driverside current. the current is therefore estimated by means of a digital filter whose coefficients can be easily adapted to the used cable. a physical sensor is a hardware device that converts physical phenomena into electric signals. a virtual sensor, instead, uses mathematical models (usually obtained through a black boxapproach) and the reading of other physical sensors to indirectly estimate a given physical variable. virtual sensors are basically nonlinear functions of past inputs and measured outputs of the considered system. they can be useful when some variables are not easily observable or measurable for various reasons, such as availability, cost or maintenance of a physical sensor. in summary, a virtual sensor can be used to estimate an unmeasurable system variable, without needing the knowledge of a mathematical model of the system. a procedure for the design of a virtual sensor is described in [3], which relies on choosing a suitable set of basis functions, so that the resulting virtual sensor satisfies the assumptions required to apply the theoretical results in [4]. in 0, [6] and [7], pwas functions (i.e. piecewise affine functions defined over a simplicial partition) have been proposed as basis functions for the design of virtual sensors. the main advantage of using pwas functions is that they can be implemented very efficiently in digital programmable circuits such as fpgas, providing low power consumption, fast response times, and, at least for high-volume applications, low cost. moreover, convergence and optimality properties of the general approach [3] are maintained. in 0 a single pwas function is used to obtain the estimate (standard virtual sensor), but if a relatively large number of inputs or measurable outputs is available, or if a large number of past data are used, the exponential increase of the complexity (curse of dimensionality) makes the approach impractical. an alternative solution (reduced-complexity virtual sensor) leading to a complexity reduction has been proposed in [6] and [7]. the method used for estimation of the motor-side currents in [2] is based on an approximation of a first principles model of the dynamical system relating the drive-side current to the motorside current. it therefore requires multiple, time intensive steps of modelling, parameter estimation and approximation to produce a real-time implementable, reduced-order estimator. in [8] the reduced-complexity pwas virtual sensor has been used for the estimation of the motor-side current with a fixed cable length of 720 m. here we generalize the sensor in order to make it cable-length-adaptive. the sensor is indeed designed (trained) starting from measurements obtained with cable lengths of 180 m, 360 m and 540 m while the validation is performed on measurements related to a 720 m long cable. a circuit architecture suitable for fpga implementation has been also designed and simulation results considering fixedpoint data representation effects are shown. 2. pwas virtual sensor  in this section the pwas virtual sensor approach is briefly summarized. consider the following nonlinear discrete-time dynamical model: , where ∈ is the state vector, ∈ is the exogenous input vector of manipulated variables, ∈ is the vector of measurable outputs, and k denotes the discrete-time instant. vector ∈ collects a set of variables to be estimated. we assume that the vector can be measured by a real sensor for 0,…, 1. a portion of these measurements, from 0 to 1, , is considered as a training set and is used to design the virtual sensor, which will operate without measuring ; the remaining data, from to 1 is used as a validation set to verify the estimation capabilities of the virtual sensor. we aim to construct a virtual sensor that estimates for , when the real sensor is no more available. the functions ∙,∙ : → , ∙ : → and ∙ : → are assumed to be unknown. for the sake of simplicity, we assume 1; nonscalar can be easily estimated component-wise. since the actual variables , and at past time instants are not available, the noisy measurements of them are assumed to be: , 0 , 0 ̃ , 0 figure 1. illustration of the connection of a 2‐phase hybrid stepper motor to its driver via a long cable, with the corresponding phase currents.  figure 2. comparison of the current in the driver‐cable‐motor circuit for a single motor phase: driver‐side (blue) and motor‐side (red).  acta imeko | www.imeko.org  september 2015 | volume 4 | number 3 | 55  where , and are unmeasured stochastic variables. for given values of , and , the inputs of the virtual sensor will be noisy sequences of measurements of and , and a vector of past values of ̂, namely ≜ ⋯ ≜ ⋯ ≜ ̂ ̂ ⋯ ̂ the values of , , are generally considered as tuning parameters. 2.1. standard virtual sensor  the standard virtual sensor is obtained by estimating in the following way: ̂ , , 1 being a pwa function defined over a simplicial partition made up of vertices and represented through the -basis, i.e., . 2 by denoting as , 1,…, _ the vertices of the simplicial partition, functions are pwas functions such that 1, if 0, if once the basis functions and the simplicial partition are selected, weights uniquely define and, in case the basis is used, they correspond to the value of the function in the partition vertices, i.e., . a detailed discussion about pwas functions can be found in [9]. weights vector … is obtained by solving the following optimization problem: min ̃ , , γw 3 being the number of elements in the training set, the tikhonov regularization parameter and max , , . the first term can be reformulated as a quadratic function of and takes into account the square error between the value of the pwas function (i.e., the estimated data) and the actual data ̂ . the second term performs a tikhonov regularization that depends on the structure of γ. in the simplest case, γ provides the zero-order tikhonov regularization. first-order (or higher order) tikhonov regularizations can be obtained alternatively by considering the gradient of the pwas function (or higher order derivatives) in constructing γ. the choice of the regularization parameter is quite critical since it can highly influence the performances of the virtual sensor. a low value may lead to an ill-conditioned problem, i.e. the solution is sensitive to small changes in data. a high value may lead to an inaccurate estimate of the unmeasurable output. the regularized least squares problem (3) can be recast as an unconstrained quadratic programming (qp) problem in the form: min 1 2 4 a rigorous convergence analysis of the standard virtual sensor is reported in 0. 2.2. reduced‐complexity virtual sensor  the reduced-complexity virtual sensor approach expresses the estimate ̂ as a sum of lower-dimensional pwas functions, in order to mitigate the effects of the curse of dimensionality, which prevents the applicability of the standard virtual sensor in many applications. for the sake of compactness, henceforth the input of the virtual sensor is referred to as ξ ≜ 5 where ≜ . assume to split vector ξ into ∈ subsets ξ ,ξ ,…,ξ , such that all elements of ξ are included in one and only one of these subsets. each subset ξ ( 1,…, ) has dimension equal to , such that 1 and ⋯ . the elements of each ξ are denoted as , , , ,… , , . the proposed reducedcomplexity virtual sensor is defined through a sum of pwas functions , 1,…, , each being the weighted sum of pwas basis functions: ̂ ξ ξ , , ξ 6 where : → (for fixed ), , denotes the -th basis of the -th pwas function and is the number of basis functions in each domain ξ . also, ≜ , ⋯ , , ⋯ , ⋯ , ⋯ , 7 the vector of parameters (which determines the shape of ) is obtained, as for the standard virtual sensor, by solving the least-squares problem min ̃ ξ γw 8 the regularized least squares problem (8) can be recast again as an unconstrained qp problem in the form (4). a rigorous convergence analysis of the reduced-complexity virtual sensor is reported in [7]. 3. design of the specific virtual sensor  in this section, the specific virtual sensor design is described. 3.1. virtual sensor architecture  we exploited the capabilities of moby-dic toolbox for matlab [10] to train a reduced-complexity pwas virtual sensor for the cable-length adaptive estimation of the motorside current, starting from measurements of the drive-side current. the same tool also allowed the automatic generation of vhdl files describing a circuit architecture for the fpga implementation of the sensor. the block scheme of the architecture, described in detail in [7], is shown in figure 3. at each sampling time, the values of the system inputs and measurable outputs are loaded into first-in-first-out (fifo) blocks which behave as buffers, storing the data at current and past time instants (i.e. and ). once the fifo acta imeko | www.imeko.org  september 2015 | volume 4 | number 3 | 56  blocks are full, the computation of the estimated output is performed by the pwas_i ( 1,…, ) blocks, which are responsible for the evaluation of the pwas functions. a detailed description of these blocks is available in [11]. each pwas_i block communicates the end of its computation with a ready_i signal; as soon as all ready_i signals are active, the results of each function evaluation (fpwas_i) are added up by an adder block, which provides the estimation of the unmeasurable output , and a global ready signal is set to logic value ‘1’ indicating that the result is available. if 0, the estimation is brought back to the fifo_z block and is used as input for next estimations. an affine scaling of ̂ (performed by block scale_z) is necessary to bring the value to the correct range needed by circuit inputs. the architecture, has a latency of 9 clock cycles (i.e., 180 ns at 50 mhz). in our case we have 0, 2 and 1, in particular contains the drive-side current and the cable length and is the motor-side current. we designed the sensor by heuristically setting 1 and 2. the domain of the pwas functions constituting the virtual sensor has been partitioned by using two subdivisions along the axis representing the drive-side current and only one subdivision along the axis representing the cable length. 3.2. experimental dataset  a dataset made up of 2,000,000 samples of the drive-side current and the corresponding motor-side current, sampled at 500 ks/s was employed. the samples correspond to 4 different cable lengths: 180 m, 360 m, 540 m and 720 m (500,000 samples for each length). figure 4 shows the drive-side currents for the different cable lengths. in order to acquire simultaneously both the drive-side and motor-side currents, two channels of an 8-bit oscilloscope were used to oversample the signals at 10 ms/s to avoid aliasing. current probes based on combined hall effect and current transformer technologies were used as the sensors. the scope has an effective resolution of 64 ma, which corresponds to a quantization noise variance of 0.0642/12 = 0.000341 a2. the probes have a 3σ accuracy of 0.25 a. considering the noise sources to be independent, the combined variance can be found by summing the individual variances in quadrature. this sum leads to an effective standard deviation of 85.4 ma, which can be considered acceptable compared to the 2√2 a amplitude of current waveform. the 10 ms/s sampled data is then low pass filtered to attenuate all frequency components above 250 khz to below the quantization level before downsampling the signals to 500 ks/s. this lower rate is high enough to capture the significant pwm-excited harmonics, but low enough to be sampled by standard drive adcs, using analog anti-aliasing, thereby allowing real-time estimator implementation. the samples corresponding to cable lengths of 180 m, 360 m and 540 m have been used as a training set, to derive the weights . the remaining 500,000 samples (corresponding to cable length 720 m) have been used to validate the virtual sensor. the weights defining the pwas virtual sensor, see (6), are shown in table 1. 4. results  figures 5 and 6 show the measured motor-side current of the validation dataset (blue) and the estimated current obtained by the fpga in fixed-point precision (red). the root mean squared (rms) estimation error obtained is 0.0858 a in fixed point precision and 0.0794 a in matlab double precision, whilst the overall current waveform has an rms value of 2 a. in [8] a lower rmse value was obtained by employing the pwas virtual sensor, nevertheless that sensor was trained with measurements taken with a 720 m long cable, while here the training is performed with data referred to different cable lengths, and the cable length is a sensor input. this solution is therefore more general, since it is cable-length-adaptive. the motor-side estimated current obtained with the figure 4. drive‐side currents for different cable lengths.  figure 3. block scheme of the circuit implementing the reduced‐complexity  virtual sensor.  table 1: weights of the three pwas functions constituting the reduced‐ complexity virtual sensor. w , w , w , w ,   w , w , 0.3474 0.1560 0.6341  0.2473  0.1559 0.0123 , ,   ,   9.9865 0.1665  10.0544  , ,   ,   3.9596 0.0135  3.7117  acta imeko | www.imeko.org  september 2015 | volume 4 | number 3 | 57  estimator in [2] in matlab double precision is shown in figure 7 for comparison. the rms estimation error is 0.0454 a. despite being lower than that obtained with the proposed method, it requires significantly more effort to obtain the estimator. depending on the application, this extra effort may not be worth the small increase in estimation precision. figure 8 shows the estimation errors for both estimators, pwas virtual sensor (blue curve) and estimator in [2] (red curve). mean value and variance are -0.017 and 0.007, respectively, for the pwas case and 2.5 10-3 and 2.1 10-3, respectively, for the case in [2]. 5. conclusions  we designed a piecewise-affine virtual sensor for the estimation of motor-side current of hybrid stepper motors, starting from measurements of drive-side currents through cables of different length. this sensor is therefore cable-lengthadaptive. a circuit architecture has also been designed for the implementation of the virtual sensor. the proposed solution exhibits slightly poorer performances compared with the ones obtained by standard model-based estimators, but results in a much faster and simpler design, since the identification of a model is not required. furthermore, even this slightly higher estimation error is negligible compared to the waveform amplitude and so allows the desired current levels to be tracked such that a negligible effect occurs on the motor position and torque. this latter is usually chosen to include a significant safety margin anyway. references  [1] p. acarnley, “stepping motors: a guide to theory and practice. control eng”, series 63, 4-th edition, the institution of electrical engineers, london, uk, 2002. [2] a. masi, m. butcher, m. martino, and r. picatoste, “an application of the extended kalman filter for a sensorless stepper motor drive working with long cables”, ieee transactions on industrial electronics, 2012, 59(11), pp. 4217–4225. [3] m. milanese, c. novara, k. hsu, and k. poolla, “filter design from data: direct vs. two-step approaches”, proceedings of the american control conference (acc), 14-16 june 2006, minneapolis, mn, usa, pp. 4466–4470. [4] l. ljung, “convergence analysis of parametric identification methods”, ieee transactions on automatic control, 1978, 23(5), pp. 770–783. [5] t. poggi, m. rubagotti, a. bemporad, and m. storace, “highspeed piecewise affine virtual sensors”, ieee transactions on industrial electronics, 2012, 59(2), pp. 1228–1237. [6] m. rubagotti, t. poggi, a. bemporad, and m. storace, “piecewise affine direct virtual sensors with reduced complexity”, proceedings of the 2012 ieee conference on decision and control (cdc), maui, hawaii (usa), 10-13 dec. 2012, pp. 656– 661. [7] m. rubagotti, t. poggi, a. oliveri, c. a. pascucci, a. bemporad, and m. storace, “low-complexity piecewise-affine virtual sensors: theory and design”, international journal of control, 2014, 87(3), pp. 622–632. [8] a. oliveri, m. butcher, a. masi, and m. storace, “piecewise affine virtual sensor: a case study-estimation of stepping motor current from long distances”, proceedings of the 20th imeko tc4 international symposium and 18th international workshop on adc modelling and testing research on electric and electronic measurement for the economic upturn. benevento, italy, 15-17 sep. 2014. [9] m. parodi, m. storace, and p. julián, “synthesis of multiport resistors with piecewise-linear characteristics: a mixed-signal figure  5.  motor‐side  current:  measured  (blue),  estimated  in  fixed‐point  precision (red).  figure 8. estimation error for the estimator  in [2] (red) and for the pwas  virtual sensor (blue).  figure 6. zoom of figure 5.  figure 7. motor‐side measured current (blue) and current estimated with the estimator in [2] (red).  acta imeko | www.imeko.org  september 2015 | volume 4 | number 3 | 58  architecture”, international journal of circuit theory and applications, 2005, 33(4), pp. 307–319. [10] a. oliveri, d. barcelli, a. bemporad, b. a. g. genuit, w. p. m. h. heemels, t. poggi, m. rubagotti, and m. storace, “mobydic: a matlab toolbox for circuit-oriented design of explicit mpc”, proceedings of the 4th ifac conference on nonlinear model predictive control (nmpc), noordwijkerhout (the netherlands), 23-27 aug. 2012, pp 218–225. [11] m. storace, t. poggi, “digital architectures realizing piecewiselinear multivariate functions: two fpga implementations”, international journal of circuit theory and applications, 2011, 39, pp. 1–15. a theoretical model for uncertainty sources identification in tip-timing measurement systems acta imeko issn: 2221-870x june 2023, volume 12, number 2, 1 6 acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 1 a theoretical model for uncertainty sources identification in tip-timing measurement systems giulio tribbiani1, lorenzo capponi2, tommaso tocci3, tiberio truffarelli3, roberto marsili3, gianluca rossi3 1 cisas g. colombo, university of padova, via venezia 15, 35131 padova, italy 2 department of aerospace engineering, university of illinois urbana-champaign, 104 s. wright street, 61801 urbana, usa 3 department of engineering, university of perugia, via g. duranti 93, 06125 perugia, italy section: research paper keywords: vibrations, blade, turbomachinery, contactless measurement citation: giulio tribbiani, lorenzo capponi, tommaso tocci, tiberio truffarelli, roberto marsili, gianluca rossi, a theoretical model for uncertainty sources identification in tip-timing measurement systems, acta imeko, vol. 12, no. 2, article 28, june 2023, identifier: imeko-acta-12 (2023)-02-28 section editor: laura fabbiano, politecnico di bari, italy received march 14, 2023; in final form april 13, 2023; published june 2023 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: giulio tribbiani, e-mail: giulio.tribbiani@phd.unipd.it 1. introduction in operating conditions, monitoring turbomachinery blades vibrations is necessary for improving structural health techniques and for validating the dynamics of the system [1]. in fact, uncontrolled vibrations at or close to natural frequencies, combined with high thermo-mechanical loads, can lead to damage for the machinery and increase the risk of unexpected failures [2]. traditionally, dynamical analysis and measurement of vibrations of rotating blade have been performed using strain gauge sensors [3]–[5]. however, this technique can have impact on the monitoring procedures [6]. in fact, although strain gauges have high accuracy and their usage is well established, they have relatively limited lifetime in high-temperature conditions, and, as intrusive technique, their installation on rotating systems and their data transmission are complex steps to achieve [5]. to overcome these problematics, non-contact and non-intrusive blade vibration measurement techniques have been developed in the past years, based on vibration, temperature [7] and ultrasound approaches [5], [8]–[12]. one of the most successful and promising on-site techniques for axial turbomachinery blade dynamics measurements is called blade tip-timing (btt) [13], [14]. btt is based on the measurement of instantaneous blade tip deflection by detecting advances or delays in the time of arrival (toa) of the blade tip by means of sensors installed on the casing at fixed angular positions [15]. btt can use different types of sensing probes, such as laser probes [16], [17], microwave sensors [4], [18], capacitive sensors [19], magneto resistive sensors [20], [21], or optical probes [6], [22]–[24]. while the measurement principle is the same for all different sensors, the optical probes are usually preferred for their performances in accuracy and resolution [6], [25]. the blade-sensor interaction generates electrical pulse signals. in ideal conditions, where no blade vibration is considered (i.e., rigid blade), the toa is determined a priori given the geometry and the dynamic of the system [26], [27]. on the other side, considering blades as vibrating flexible structures, blade deflections lead to delays or advances of the blade tip with respect to the expected toa [13], [28], [29]. these shifts in the toa are extracted and used for determining the amplitude of deflection of each blade [30]. for this reason, a thorough identification and definition of sources abstract this paper presents a theoretical analysis of uncertainty sources in measurement techniques used to determine vibrations of turbomachinery blades using stationary sensors mounted on the casing of the turbomachine. a mathematical model based on fundamental physical principles is proposed, and two different measurement set-ups are evaluated. one set-up uses a reference sensor to measure the passage of an undeformed part of the blades (blade base), while the other set-up does not involve the use of a reference sensor, with both sensors facing the blades tip (deformed part). the intrinsic uncertainty of these methods and the performance of the complete measurement chain are defined. the analysis of the measurement technique leads to conclusions about the practical set-up and possible performances of these measurement techniques. mailto:giulio.tribbiani@phd.unipd.it acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 2 of uncertainties for tip timing measurement systems (i.e., uncertainty on the measurement of the toa) is essential for obtaining a detailed information on the dynamics of the system [31]. this will lead to a reliable structural health monitoring, giving insights on instrumentation best practices and information for validating numerical models [32]. different studies have been carried out on btt measurement systems, focusing on the uncertainty of specific cases technique [33]–[36]. this study proposes a complete overview of the uncertainty sources of a generic btt system, and their influence on the estimation of the toa. with regard to this, two different probes configuration are analysed, typically employed in btt measurement systems, either with or without a reference sensor. 2. materials and methods a typical btt measurement system allows the sampling of the relative displacement law s(t), supposed periodic, between two points of the blade, expressed in a reference system rotating with the blade itself, using a non-contact sensor installed on the machine casing. the sensors are usually paired in couple, with one being the reference sensor and the other the sensor used to measure the blade deformation. the first one is typically installed at the base of the blade, where there is no deformation, the other is placed at the blade tip. by analysing the resulting signals, it is possible to obtain the instants in which the blade passes in front of the sensors and measure its deformation. if the tip deflects from the base, there will be a δt, in delay or in advance, between the signals obtained by the reference and the non-reference sensor. in addition to this, the pulse width of the passage signal is representative of the duration of the passage itself. in this way, a displacement and velocity value can be measured for each transition of the blade in front a pair of sensors. the problem lies in the fact that the displacement s(t), as well as the velocity v(t), i.e. its first derivative, have generally higher frequency components than the passage frequency in front of the sensors. hence, s(t) and v(t) are sub-sampled. nevertheless, it is possible, under particular conditions discussed in [37], to calculate the harmonics of s(t), even if the sampling is performed in such conditions. a method is described that allows to obtain the discrete spectrums of the blade displacement and velocity, s(t) and v(t), by solving a system of 2m+1 non-linear equations, with m being the number of harmonics to be evaluated. 3. velocity measurement principle and displacement using reference sensor in this section it will be described the working principle of a btt measuring system using a reference sensor. considering a blade on a rotating drum subject to bending deformations as shown in figure 1. let ω be the angular rotation velocity of the disk. two reference systems are considered: a fixed one (o', s') and a second rotating one (o, s). both s and s' are curvilinear abscissae defined on the circumference containing the blade tips, of radius r. in this reference systems, s identifies the position of the undeformed blade tip, hence s(t) represents the circumferential component of the blade tip displacement over time. it is assumed that this displacement changes in time according to a periodic law: 𝑠(𝑡) = 𝑆 ∙ 𝑓(𝑡), (1) where 𝑓(𝑡) is a periodic function of period t, fundamental frequency f0 = l/t and unitary amplitude, multiplied by a scalar s, greater than 0. the function s(t) is the measurand, sampled by the btt measurement technique. deriving the displacement s(t) the velocity v(t) is obtained. considering the relative motion between drum and casing: 𝑣(𝑡) = 𝑣 ′(𝑡) − 𝑣0′(𝑡) (2) with v’(t) being the velocity in the rotating system and vo’(t) the absolute velocity due to drum rotation. the blade deflection can be measured by analysing the signals generated by the two sensors arranged as shown in figure 1. figure 2 shows the qualitive representation of these two signals. the blade passage is detected when the signal exceeds a threshold value sg. in conditions of undeflected blade, there will be a fixed time delay between the threshold crossing of signals from sensor 1 and 2. if the blade is deformed, the time delay δtac will differ from the one obtained in the previous condition. hence, it is possible to measure the blade deformation by measuring δtac. this method is valid if some key hypotheses are met; these assumptions will inevitably lead to an increase in measurement uncertainty. first, it is considered that the time interval δtac does not vary due to the irregularity of the rotation, hence the angular speed ω of the drum remains strictly constant. second, the δtac in the undeformed configuration is equal to the mean ∆tac̅̅ ̅̅ ̅̅ when the blades are vibrating: ∆𝑡ac̅̅ ̅̅ ̅̅ = 1 𝑁 ∑ ∆𝑡ac(𝑖) 𝑁 𝑖=1 . (3) with these hypotheses, the time delays due to the oscillation s(t) can be calculated from: 𝛿(∆𝑡ac) = ∆𝑡ac − ∆𝑡ac̅̅ ̅̅ ̅̅ . (4) associating the times intervals δtab and δtcd respectively to the distance δab and δcd along which the sensors see the blade tip, defined as distances corresponding to the crossing of the threshold sg as illustrated in figure 2, it is possible to compute the average velocity respectively between a and b and between c and d by the following two equations: figure 1. proposed measuring set-up using a reference sensor (facing the base of the blade) to measure the deformation. acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 3 𝑣0 ′ = 𝛿ab ∆𝑡ab ∙ 𝑅 𝑟 (5) 𝑣′ = 𝛿cd ∆𝑡cd . (6) being the duration of these signals very short, such quantities can be reasonably considered as the instantaneous velocity of the blade tip and base. assuming that both s and the blade tip velocity remain constant during the passage occurred in δtcd, the displacement can be calculated as follows: 𝑠(𝑖) = 𝛿cd ∆𝑡cd ∙ 𝛿(∆𝑡ac) (7) the value of v, assuming that vo’(t) remains constant during δtac using (2) can be expressed as: 𝑣(𝑖) = 𝛿cd ∆𝑡cd − 𝛿ab ∆𝑡ab ∙ 𝑅 𝑟 (8) the usage in (7) and (8) of the instantaneous velocities of rotation allows a considerable extension of the applicability of the btt measurement methods on machines with high degrees of irregularity compared to the ones presented in [15], [30], [38], where velocity is calculated as ω r, hence assumed constant during the entire revolution. in the method proposed here, vo’ remains constant only in the δtac period. in fact, by using (7) and (8) at each instant of passage of the blade in front of a couple of fixed sensors, s(i) and v(i) values are calculated. 4. method without reference sensor in many applications, it is not possible to install a reference sensor in a position useful to detect the passage of a not deformed part of the blade, i.e., the blade base. in these cases, it is possible to install the sensors as shown in figure 3, facing only the blade tip. it is assumed once again that the time interval does not change due to the irregularity of the rotation between the two pulses and that the δt, relative to the passage of the blade in the undeformed configuration, is equal to the average value of δt measured over a certain number of revolutions. the time delays δ(δt(i)) are therefore calculated with respect to these average values. assuming a strictly-constant tangential velocity equal to ωr, the measured blade deflection si, as shown in figure 4, can be calculated by the following relation: 𝑆𝑖 = 𝜔 ∙ 𝑅 ∙ 𝛿(∆𝑡(𝑖)) (9) hence, for each couple of pulses, we can have one pair of displacement samples s and s’. average displacement can be estimated by (s + s’)/2 and a velocity value v can be estimated by (s’-s)/td, where td is the pulse width. to estimate the vibration frequency, we can use the following considerations. if the displacement of the blade is: 𝑠 = 𝐴 ∙ cos(𝜔 𝑡). (10) deriving it, the blad tip velocity v is obtained and can be expressed by: 𝑣 = −𝐴 ∙ 𝜔 ∙ sin(𝜔 𝑡). (11) by computing the ratio between the minimum and maximum value of s and v, it is possible to estimate the vibration frequency: 𝑓 = 𝑣max − 𝑣min 𝑠max − 𝑠min . (12) 5. vibration harmonics calculation the sampling of blade tip vibration by btt technique produces a series of displacement and velocity samples on the rotating reference system: one for each passage of the rotating blade in front of the couple of the sensors. at each instant of figure 2. time history of the outputs of sensors 1 (reference on the base) and 2 (on the tip) typically in volt: the blade is passing when the output exceeds the threshold value sthreshold. figure 3. measuring technique without reference sensor. figure 4. blade vibration and sensor output over time; measured blade deflection at the i-th passage, resulting from the variation δ(δt(i)). acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 4 passage, i.e., at time t(i), a deflection value s(i) and a velocity value v(i) are calculated. the relative motion of the blade tip can be in general described by a periodic function over time. as vibrations, this periodic motion can be effectively described by a relatively small number of harmonics, with the energy being almost completely contained in the first harmonics. the two functions s(t) and v(t) can be in general approximated by a fourier series limited to m harmonics: 𝑠(𝑡) = ∑ 𝐴𝑗 ∙ cos(2 π 𝑣0 𝑗 𝑡) + 𝐵𝑗 ∙ sin(2 π 𝑣0 𝑗 𝑡) 𝑀 𝑗=1 (13) 𝑣(𝑡) = 2 π 𝑣0 ∙ ∙ ∑ −𝐴𝑗 ∙ sin(2 π 𝑣0 𝑗 𝑡) + 𝐵𝑗 ∙ cos(2 π 𝑣0 𝑗 𝑡) 𝑀 𝑗=1 . (14) the values of s(t) and v(t) are measured at the instants t(i), not necessarily equally distributed in time, as the sensors could be placed on the turbine casting at not equi-spaced angles. these values s(i) and v(i) can be used to write a system of nonlinear equations with unknowns that are the fundamental frequency and the 2 m coefficients aj and bj of the harmonics. thus, (2 m + 1) equations can be written; to solve this system, it is necessary to get enough samples of s(t) and v(t), at least (2 m + 1) or more. so, in principle, by the acquisitions of the (2 m + 1) s(i) and v(i) samples and the solution of a non-linear system of equations, it is possible to estimate the discrete spectrum of the harmonics of the vibration. with more than (2 m + 1) samples of s(i) and v(i), a least square approach is also possible. the limits of these ideas have been discussed in [37], where a complete theoretical approach is illustrated. 6. uncertainty sources the hypotheses made for the description of the btt measurement principle of velocity and displacements samples of a rotating blade tip, limit the applicability of the methods. the main parameters of the rotating disk with a vibrating blade are ω, r, i, v, and s. a first uncertainty source can be identified in the variation of the δtac due to the irregularity rotation, expressed by: 𝑖 = 𝜔max − 𝜔min 𝜔average , (15) with ω being the angular speed of the rotating drum. as a first linear approximation, this uncertainty source can be estimated as follows: 𝐸𝑖 = 𝑖 ∙ 𝛥𝑡ac (16) it was previously assumed that s(t) and v(t) do not change in the time period δtac. a linear estimation of changes of s(t) and v(t) during δtac can be expressed by the following relations: 𝐸𝑠 = [ 𝑑𝑠 𝑑𝑡 ] ∙ 𝛥𝑡ac (17) 𝐸𝑣 = [ 𝑑𝑣 𝑑𝑡 ] ∙ 𝛥𝑡ac . (18) assuming the time history of s(t) is a simple sinusoidal vibration: 𝑠(𝑡) = 𝑆 ∙ sin(2 π 𝑣 𝑡) (19) the following relations are obtained: [ 𝑑𝑠 𝑑𝑡 ] max = 2 π ∙ 𝑣 ∙ 𝑠 (20) [ 𝑑𝑣 𝑑𝑡 ] max = 4 π2 ∙ 𝑣2 ∙ 𝑠 . (21) by replacing these values in (17) and (18), it is possible to estimate the maximum effect on uncertainty due to changing of s and v over δtac: 𝐸𝑠 = 2 π ∙ 𝑣 ∙ 𝑠 ∙ 𝛥𝑡ac (22) 𝐸𝑣 = 4 π 2 ∙ 𝑣2 ∙ 𝑠 ∙ 𝛥𝑡ac . (23) considering that the maximum of δtac is obtained when the absolute speed of the blade tip is at minimum, (22) and (23) become: 𝐸𝑠 = 2 π ∙ 𝑣 ∙ 𝑠 ∙ 𝛿ac 𝜔 𝑅 − 2 π 𝑣 𝑠 (24) 𝐸𝑣 = 4 π 2 ∙ 𝑣2 ∙ 𝑠 ∙ 𝛿ac 𝜔 𝑅 − 2 π 𝑣 𝑠 . (25) (16), (24) and (25) estimate the uncertainty amplitude, therefore can be used to identify the conditions of applicability of the measurement methods previously described. knowing this, it is possible to change the parameters of the measurement system, in order to properly choose and install the sensors in convenient locations, as well as find the optimal acquisition set up. the resolution δs of the displacement measurement is given by the δ(δtac ) resolution. starting from: 𝑅𝑠 = ∆𝑠 𝑆 (26) and being trs the resolution in the time measurements, i.e., sampling time, we can write: 𝑡𝑟𝑠 = 𝑅𝑠 (𝛿(∆𝑡ac))max (27) and being: (𝛿(∆𝑡ac))max = 𝑆 𝜔 ∙ 𝑅 . (28) trs strictly affects the resolution on the measurement of the displacement s. a useful equation can be defined to relate trs to rs. this relation becomes fundamental when choosing the resolution of the time measurement, in order to achieve the target resolution of the blade tip displacement s 𝑡𝑟𝑠 = 𝑅𝑠 𝑆 𝜔 ∙ 𝑅 . (29) the resolution on blade tip velocity measurement depends essentially on δ(δtcd) measurement resolution. as for the displacement measurements, the following equation is obtained: 𝑡𝑟𝑣 = 𝑅𝑣 ∙ (𝛿(∆𝑡cd))max (30) and being acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 5 (∆𝑡cd)min = 𝛿cd 𝜔 + 2 𝜋 ∙ 𝑣 ∙ 𝑠 (31) (∆𝑡cd)max = 𝛿cd 𝜔 − 2 π ∙ 𝑣 ∙ 𝑠 (32) hence (𝛿(∆𝑡cd))max = (∆𝑡cd)max − (∆𝑡cd)min 2 . (33) therefore, from (33), it is possible to define another useful formula to choose sampling time of sensor signals in order to have a sufficient resolution for tip time velocity measurement: 𝑡𝑟𝑣 = 2 π ∙ 𝑆 ∙ 𝛿cd ∙ 𝑅𝑣 𝜔2 ∙ 𝑅2 − 4 π2 ∙ 𝑣 2 ∙ 𝑠2 (34) further causes of uncertainty can also be due to: variations of δab and δcd; definition of the threshold sg; relative radial motion between sensor and blade tip; noise in sensors signals. this analysis has been developed in [39]. 7. conclusion some basic models for blade tip timing measurement systems have been defined: the first using one of the two sensors as a reference; the second with both sensors facing the blade tip. although the second approach is more versatile and easier to install, using one sensor as a reference is needed when the analysed machineries have higher degrees of irregularity. uncertainty on the parameters of these models has been theoretically analysed. some simple formulas to relate the uncertainty and resolution obtained with reference to measurement system chosen parameters and sensor installation have been proposed. these formulas could be very useful to make proper choices over measurement system components, in relation to blade tips expected vibration characteristics and turbomachine parameters. so, this work can be used for blade tip timing measurement system design and installation guidelines. references [1] s. l. dixon, fluid mechanics, thermodynamics of turbomachinery, elsevier, 2010, isbn 978-1-85617-793-1. 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december 2014, volume 3, number 4, 46 – 51  www.imeko.org    acta imeko | www.imeko.org  december 2014 | volume 3 | number 4 | 46  surface roughness modeling with edge radius and end  milling parameters on al 7075 alloy using taguchi and  regression methods  m. numan durakbaşa 1 , anıl akdoğan 2 , a. serdar vanlı 2 , aslı günay 2   1 vienna university of technology, institute for production engineering and lasertechnology, department for interchangeable  manufacturing and industrial metrology, 1040 vienna, austria  2  yildiz technical university, faculty of mechanical engineering, department of mechanical engineering, 34349,istanbul,turkey      section: research paper   keywords: edge radius; surface roughness; precise measurements; optimization  citation: m. numan durakbaşa, anıl akdoğan, a. serdar vanlı, aslı günay, surface roughness modeling with edge radius and end milling parameters on al  7075 alloy using taguchi and regression methods, acta imeko, vol. 3, no. 4, article 9, december 2014, identifier: imeko‐acta‐03 (2014)‐04‐09  editor: paolo carbone, university of perugia   received october 10 th , 2013; in final form december 26 th , 2013; published december 2014  copyright: © 2014 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: this work was supported by yildiz technical university, scientific research projects coordination department ‐ project number: 2012‐06‐01‐kap01,  turkey.  corresponding author: anıl akdoğan, e‐mail: nomak@yildiz.edu.tr    1. introduction  despite of recent developments, causes of cutting tool geometry deformations incurred by usage is still unknown and not possible to estimate precisely yet. since the costs of cutting tools and their replacements in machining processes influence the total production costs by 3% and 12%, respectively, accurate measurement of tool geometry is of great importance to delay the tool deformations and to extend tool lifetime period [1]. quality assurance policies are used by many various companies to provide clients with high qualified products and technical support. thus, inprocess inspections should include all steps of manufacturing to meet the demands of related quality management system standards [2]. many different industries set high demands on geometry variation of tools due to the technological developments. these high demands stem from high and ultra-high precision machining of different kinds of materials. for instance, while cutting of brittle materials in a ductile mode using diamond tools, such as ductile cutting of silicon and quartz for wafer fabrication, one of the key conditions for achieving ductile chip formation is to get the right cutting edge radius of the tool to the undeformed chip thickness. it has been shown that the undeformed chip thickness has to be in the order of nanometers and that the tool cutting edge radius has to be smaller than the undeformed chip thickness. therefore, precise measurement of diamond abstract  tool geometry  and edge radius are not only crucial for workpiece surface characteristics determination but  they also have direct  impacts on tool lifetime. they are created in manufacturing and deformations occur in machining processes. with this regard, precise  determination of the geometry is of vital importance. this study focused on process parameters like cutting speed, feed rate, depth of  cut, tool geometry, and different coatings, to indicate the effects on surface roughness of the machined product on the basis of two  and  three‐dimensional  precise  measurements.  this  paper  studies  the  optimization  of  process  parameters  and  different  coating  materials with combined different tool radii to obtain the maximum surface quality for the end milling process of al 7075 alloy by  taguchi and regression methods. the results revealed optimum process parameters with the proper coating type. the calculated  mathematical model predicts average surface roughness value against edge radius wear and processing parameters.  acta imeko | www.imeko.org  december 2014 | volume 3 | number 4 | 47  cutting tools has become a key issue for ductile mode cutting of brittle materials [3]. as emphasized in the literature, cutting tool tip radius deformations during the machining, according to the wear mechanism, is still poorly understood and cannot be accurately predicted [4]. determining significant factors which affect the tool tip radius is crucial for the final surface quality and life of the tool. the precise identification of tool geometry is important for the determination of both tool life and machined material surface conditions. tool geometry identification, including edge radius determination gives not only important information on high precise manufacturing processes, but also explains the wear behavior of the tool. high precision measurement techniques are being used to determine the geometry and especially the edge radius variations of various machining operations tools. recently, 7xxx series al alloy are being used in different experimental studies. al 7076-t6 material was used for investigating high-speed milling effects on the final workpiece surface by means of analytical and experimental methods [5]. al 6061-t6 was used as specimens for micro milling experiments in an experimental study [6]. in another study, al 5083 blocks (140x70x30 mm) were used to investigate parameter effects on tool lifetime. the gray based taguchi method and anova analyses were applied for experimental design and results. this analysis showed that decreasing feed rates increased the tool life [7]. al 7075t6 alloy specimens were applied in an experimental study for dry cutting high-speed milling operations [8]. to model the cutting forces for face milling operations different kinds of al 7050 specimens were used in other experimental studies [9-10]. researchers used al 6061-series aluminum workpieces in a taguchi l9 orthogonal experimental array set to obtain significant factors that affect the workpiece surface roughness. anova was used to analyze outputs from an orthogonal array. according to these analyses, a low feed rate and a minimum rake angle create high surface quality at high speed milling operations [11]. the purpose of this study is to determine the geometries of brand new and used end milling tool tips and the final workpiece surface qualities by means of precise measurement techniques and methods. in addition, we aim at determining the optimal process parameters and tool radius by using the design of experiment (doe) methodology. this paper studies the optimization of processing parameters and different coating materials combined with different tool radii to obtain the minimum ra (arithmetical mean surface roughness parameter) values for the end milling process of al 7075 alloy by using taguchi and regression methods. specifically, the tool edge radius was obtained by modern 3d dimensional methods. the geometry has significant effects on surface quality and there are limited numbers of studies on the subject matter. this experimental study aim was determination of high accuracy mathematical model between the ra and the processing parameters by using the measurement data. the relationship between the control factors and the response factor “surface roughness” is determined. the results revealed proper coating type and the optimum process parameters. in addition, the calculated mathematical model predicts edge radius changes of the tools against workpiece surface damage. 2. materials and methods in this study, al 7075 alloy workpieces were end milled by using 3 different coated milling tools of 3 different initial edge radii. each tool has four flutes with a 30° helix angle and 10 mm nominal diameter. the selected solid carbide cutting tools for experiments were coated with 3 different coating materials by pvd (plasma vapor deposition) technique. aluminum alloys are widely used in aerospace and automotive industries to manufacture products in short processing periods to supply a large number of manufactured products by high-speed machining. the manufacturing of such a significant amount of products requires high productivity with an increased cutting speed and feed rate. al 7075 alloy blocks were used for each set of experiments. the chemical composition of the processed material is given in table 1. the experiments were conducted in two different sets of processing parameters, such that observation of the effects of the harder cutting conditions on tool edge geometry is possible. the experiments were classified into two sets, namely “the first milling operations (1st)” and “the second milling operations (2nd)”. experimental design factors and their levels are given in table 2. the 27 different al alloy blocks had a 5050 mm cross section and a length of 160 mm. the method presented in this study is an experimental design process called the taguchi design by which the inherent variability of materials and manufacturing processes have been taken into account at the design stage. in the taguchi design procedure, the parameter design stage has been widely used for optimization of manufacturing processes [12, 13]. the vital aim of this experimental work is to determine the optimal cutting conditions that yield the minimum ra value. in a significant number of studies, only three common process parameters, i.e. feed rate, cutting speed and depth of cut were experimented [14-17]. manufacturers and customers are expected to test functional geometry and wear characteristics of tools to ensure that only functionally capable tools are used in the production process. for these purposes, the initial geometrical and surface characteristics of the brand new tools are examined in both 2d and 3d measurements. edge radii of the brand new and mill-processed tools are measured by a zoller venturion 450 3d laser scanner, germany. images were captured by a keyence optical microscope, us, and a schut stereo microscope, the netherlands. the milling table 1. chemical composition of al 7075 alloy.   material  si  fe  cu  mn  mg  cr  zn  ti  %  0.08  0.36  1.52  0.08  2.59  0.21  5.62  0.04  acta imeko | www.imeko.org  december 2014 | volume 3 | number 4 | 48  operations were performed on a vertical cnc machining center, mori seiki, milltap 700, japan. a din 6499 er type tool holder was used in all the experiments. after performing two sets of milling operations, the surface roughness of the machined products were measured by using a profilometer from taylor hobson, uk, at five different locations. repetitive measurements increase the conformity of the model accuracy. deformations from previous conditions are provided in table 3 as ∆rε1 and ∆ rε2 respectively for the 1st and 2nd set of milling operations. additionally, table 3 shows the response factors at the end of the 1st and 2nd set of operations as ra measurements, respectively. the arithmetic average of the absolute values of the roughness profile was determined by means of a gauss filter with 0.8 mm cut off value and 4l access length. certainly, tool edge radius stability performance is related with the final surface roughness of the workpiece. therefore, an experimental design was introduced and all test specimen end milling tool radii were measured before and after machining for obtaining deviations from the initial geometry. by this approach changes of surface conditions and tool tip geometry were obtained accurately. figure 1 shows the initial tip geometry figures of the zrn coated tools at 1.0 mm and 1.5 mm nominal radius measurement operations respectively, with ×50 magnification. 3. results and discussions today’s high precise manufacturing technology requires practical solutions to improve surface quality of the machined products. at this point, evaluating cutting performance is an important factor. on the basis of this factor, ra values of the milled products were chosen as response to determine end milling performance of the manufacturing systems. process parameters that affect the characteristics of milled products are tool characteristic parameters; tool radius and coating material and the milling parameters such as the cutting speed, the feed rate, and the depth of cut. the presented results were analyzed in the limits of the selected orthogonal array. the basic quality characteristic for this research is the minimum surface roughness for optimum machining conditions. according to the measurements, the minimum surface roughness was obtained with the third level of radius, the third level of coating, the first level of cutting speed, the second level of depth of cut and the first level of feed rate (table 4). the anova statistical analyzing method was used for investigating the most effective process parameters. the results indicated that coating type is the most effective parameter for the surface roughness in accordance with the selected parameters and their levels. the results were provided at the 95% confidence level. after determining the optimum levels of the processing parameters and tool edge radius, these levels should be verified. repetitive machining processes were applied for the verifications. repetitive surface roughness measurements taken from the work piece are indicated in table 4. the experimental results are in agreement with the predicted results from the proposed method. the experimental results of the surface roughness were comparable with the predicted values. the mathematical model results and the measured surface roughness values in each experiment were similar. the authors also calculated a similar model in a previous study [18]. in this paper, the presented models predict more accurately the final surface quality by using radius wear effects. the authors have conducted an anova test for investigating the most effective process parameters for the first step milling operation results. the coating type was determined as the most significant parameter, according to the anova analysis. analyses showed that the zrn type coating provides the best surface quality with a minimum surface roughness of the workpiece. increasing cutting speed decreases the surface quality, as observed in previous studies [19, 20]. in this experimental work, the minimum cutting speed of 50 m/min resulted in a minimum surface roughness. the s/n ratio graph shows that at 0.75 mm depth of cut the best surface roughness were recorded (figure 2). the depth of cut value has a remarkable impact on the cutting forces. moreover, it introduces thermal changes during the machining process. the depth of cut effects the chip flow angle, which is crucial for heat dissipation. thus, the depth of cut is a significant parameter table 2. experimental control factors and their levels. control  factors  1st milling operation  2nd milling operation level 1  level 2  level 3  level 1  level 2  level 3 radius  (mm)  0.5  1  1.5  0.5  1  1.5  coating   type  altin  alticn  zrn  altin  alticn  zrn  cutting  speed  (m/min)  50  75  100  100  150  200  depth of  cut (mm)  0.50  0.75  1.00  0.50  0.75  1.00  feed rate  (mm/tooth)  0.05 0.10  0.15  0.10  0.20  0.30  figure 2. s/n graph for surface roughness for the 1 st  set operations.  figure 1. optical microscope images of zrn coated tools of 1.0 mm and 1.5  mm nominal radii respectively.  acta imeko | www.imeko.org  december 2014 | volume 3 | number 4 | 49  for the surface quality and the tool lifetime [21-23]. as is indicated, decreasing the feed rate increases the surface quality [24, 25]. the experiments in this study also confirmed that the minimum feed rate leads to a minimum surface roughness of end milled products. the regression analysis was used to understand the relationship between the selected parameters and the surface roughness deviations. the calculated mathematic model for the ra is: 11.1766 11.7502 0.0058 1.1988 4.2456a c p zr r v a f       (1) variables were defined as initial edge radius, edge radius after 1st milling operations, cutting speed, depth of cut and feed rate for the regression analysis. as indicated in eq. (1), the model incorporates the effect of edge radii, cutting speed, depth of cut and feed rate on the surface roughness of the workpiece. the authors strongly recommend the use of the suggested coating type i.e. zrn, when using the regression model considering the taguchi results. the edge radius change, rε1, was also determined as an important parameter during the machining process according to the taguchi method. according to the results, when the edge radius increases, the surface quality improves, related to the depth of the cut values. on the other hand, when the feed rate decreases, the surface quality increases. the feed rate appeared to be the most effective parameter after the coating type for a better surface quality. by the created regression model, 85.53% of the total variability in the deviation can be explained by eq. (1). the 2nd milling operations were conducted using more aggressive processing parameters, doubling vc and fz . the results of the 2nd taguchi analysis verified the optimum process parameters in the 1st operation. the s/n ratio graph indicates that 50 m/min minimum cutting speed, 0.75 mm depth of cut and 0.05 mm/tooth minimum feed rate resulted in a minimum surface roughness values (figure 3). in addition, a tialcn type coating was suggested based on these results. the regression analysis model was run to understand the relationship between the process parameters and the surface roughness for the 2nd milling operation. the calculated mathematical model for the ra is indicated in eq. (2). 88.96% of the total variability in the deviation can be explained by this model. 20.2610 4.7204 0.0054 0.2505 3.9257a c p zr r v a f       (2) table 3. orthogonal array l27 and the results. rε0 :edge radius before milling operation, rε1: edge radius after 1stmilling operation, rε2: edge radius after 2nd milling operation. run  control factors and levels   response factors  1 st milling operation  2 nd  milling operation radius  (mm)  coating  type  cutting speed  vc (m/min)  depth of cut ap (mm)  feed rate fz (mm/tooth)  mean(1 st ) ra (µm)  rε1   rε1‐ rε0  (mm)  mean (2 nd ) ra (µm)  rε2  rε2‐ rε1  (mm)  1  1  1  1  1 1 0.581 0.010  1.010  0.005 2  1  1  1  1 2 1.082 0.002  0.953  0.005 3  1  1  1  1 3 0.617 0.005  1.290  0.010 4  1  2  2  2 1 0.376 0.016  0.590  0.000 5  1  2  2  2 2 0.355 0.008  0.811  0.008 6  1  2  2  2 3 0.639 0.006  1.321  0.007 7  1  3  3  3 1 0.528 0.005  2.161  0.087 8  1  3  3  3 2 0.412 0.038  0.728  0.031 9  1  3  3  3 3 0.648 0.004  0.922  0.005 10  2  1  2  3 1 0.689 0.008  1.360  0.012 11  2  1  2  3 2 1.099 0.338  0.855  0.337 12  2  1  2  3 3 0.992 0.010  1.350  0.009 13  2  2  3  1 1 0.487 0.023  0.694  0.014 14  2  2  3  1 2 0.509 0.003  0.502  0.021 15  2  2  3  1 3 0.684 0.000  0.679  0.003 16  2  3  1  2 1 0.277 0.041  0.274  0.011 17  2  3  1  2 2 0.340 0.031  0.589  0.007 18  2  3  1  2 3 0.839 0.021  1.054  0.009 19  3  1  3  2 1 0.366 0.000  0.448  0.021 20  3  1  3  2 2 0.854 0.009  0.883  0.006 21  3  1  3  2 3 0.758 0.001  0.895  0.002 22  3  2  1  3 1 0.250 0.019  0.504  0.015 23  3  2  1  3 2 0.770 0.013  0.697  0.007 24  3  2  1  3 3 0.472 0.001  1.025  0.013 25  3  3  2  1 1 0.271 0.003  0.419  0.008 26  3  3  2  1 2 0.491 0.007  0.494  0.012 27  3  3  2  1 3 0.697 0.002  0.743  0.004 table 4. confirmation tests results.   control factors and levels  roughness values (µm)  radius (mm) cutting speed (m/min) depth of cut (mm) feed rate (mm/tooth) exp.1 exp. 2  exp.3  predicted 1.5  50  0.75  0.05 0.269 0.262  0.196  0.283 acta imeko | www.imeko.org  december 2014 | volume 3 | number 4 | 50  4. conclusions  this experimental work is about optimizing the processing parameters by using different coating materials which were combined with different tool radii to obtain the minimum ra values for the end milling of al 7075 alloys. the taguchi method was used for the experimental design. tool edge radii and machined surface roughness were measured by means of high precision measurement techniques. surface roughness values were used as output for the analysis. experimental results and the regression models confirmed that the increased wear of edge radii decreased the surface quality of the machined workpiece. according to the proposed optimization method, the optimum process parameters, the minimum cutting speed, the average depth of cut, the minimum feed rate and the maximum edge radius provided the best surface quality for al 7075 alloy. besides, according to the regression analysis results, zrn coating types for relatively low cutting speeds and tialcn coating types for cutting speeds higher than 100 m/min gave the best surface roughness performance. in addition, a relationship between the work piece surface roughness and a cutting tool edge radius were found according to the experimental runs. in conclusion, tool edge radius wear was found to be a crucial parameter, as well as the feed rate, the depth of cut and the cutting speed. this study provides an insight into the relationship between the wear mechanism of the tools and the surface quality of a workpiece, according to the processing parameters. acknowledgement  this research has been supported by yildiz technical university, scientific research projects coordination department, istanbul, turkey, project number: 2012-0601-kap01. the authors are thankful to milmak metal, i̇stanbul, and dmg mori seiki, i̇stanbul, turkey for their support. references  [1] a.weckenmann, k nalkntic, “precision measurement of cutting tools with two matched optical 3d sensors”, manuf. techn., 52, 443–446, 2003, http://dx.doi.org/10.1016/s0007-8506(07)60621-0. 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[25] colak o., kurbanoğlu c., kayacan m. c., “milling surface roughness prediction using evolutionary programming methods”, mater. design, 28, 657–666, 2007, http://dx.doi.org/10.1016/j.matdes.2005.07.004. development and verification of self-sensing structures printed in additive manufacturing: a preliminary study acta imeko issn: 2221-870x june 2023, volume 12, number 2, 1 7 acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 1 development and verification of self-sensing structures printed in additive manufacturing: a preliminary study antonino quattrocchi1, roberto montanini1 1 department of engineering, university of messina, c.da di dio, 98166 messina, italy section: research paper keywords: fiber bragg grating sensor; stereolithography; embedded sensors; 3d printing monitoring; stress-strain measurements citation: antonino quattrocchi, roberto montanini, development and verification of self-sensing structures printed in additive manufacturing: a preliminary study, acta imeko, vol. 12, no. 2, article 25, june 2023, identifier: imeko-acta-12 (2023)-02-25 section editor: alfredo cigada, politecnico di milano, italy, andrea scorza, università degli studi roma tre, italy received december 14, 2022; in final form february 27, 2023; published june 2023 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was supported by the ministry of university and research in italy (mur) under the research project of the national operative program (pon “ricerca e innovazione 2014 e 2020”) “nausica efficient ships through the use of innovative and low carbon technological solutions”. corresponding author: antonino quattrocchi, e-mail: antonino.quattrocchi@unime.it 1. introduction technological innovation is increasingly oriented towards the development of systems that allow monitoring of the health state of structures (structural health monitoring, shm). in this way, structural damage can be early detected and identified by means of sensors integrated into the structure itself, which are therefore able to evaluate the degradation in real time [1]. such sensors are preferred, compared to conventional transduction devices, because they are protected from atmospheric agents and can be placed more easily in critical points of the structure, also in areas that are difficult to reach from the outside [2]. on the other hand, their installation must be foreseen when the structure is designed or as retrofitting, since an easy intervention to restore the malfunctioning sensor is not possible [3]. fiber optical sensors, specifically fiber bragg grating (fbg), are suitable for the production of smart structures [4]. they have numerous advantages, such as low intrusiveness, immunity to electromagnetic interference, self-referentiality, absence of corrosion and the possibility of creating sensor networks with many measurement points. instead, the main drawbacks concern the significant influence from the temperature, the limited opportunity of imparting a curvature to the fiber to avoid signal losses, the relatively high cost of both the sensors and the interrogation devices necessary for the measurement [5]-[7]. a fbg sensor can be installed onto different materials by applying ribbon tapes, glues or epoxy resins. however, these methods are time-consuming, require different molds or tools and induce variability in the measurement of the considered property [8][10]. alternatively, in the case of composites and reinforced concretes, the integration of such sensors can be performed during the stratification of the structure [11], [12]. the rapid diffusion of additive manufacturing (am) in a perspective of industry 4.0 opens new possibilities in the field of shm, enabling the design of complex and topologically abstract additive manufacturing (am) is used today to fabricate complex structures as well as to demonstrate innovative design concepts. this opens new horizons in the field of structural health monitoring (shm), allowing to correlate a high performing design with real-time detection and identification of the structural damage. fiber optical sensors, such as fiber bragg gratings (fbgs), are an effective option for this type of applications. the present work discusses the development of a demonstrative self-sensing structure, obtained by embedding a fbg sensor during the 3d stereolithographic (sla) printing process. the paper reports the strategies developed in order to ensure a correct adhesion of the fbg sensor embedded into the structure and the experimental tests used for validating the structural response of the self-sensing specimen. the output signal of the fbg sensor was continuously recorded during the different stages of the creation phases: this allowed real-time monitoring of the whole am process (i.e. printing, washing and curing stages). the obtained results showed that the self-sensing demonstrative structure was able to effectively monitor the thermo-mechanical behavior of the am process and to guarantee the correct identification and measurement of the strain as the same structure was subjected to a controlled stress. mailto:antonino.quattrocchi@unime.it acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 2 optimized structural elements [13], [14]. nowadays several am methods are available, among which the most popular are fused deposition modeling (fdm), stereolithography (sla) and selective laser sintering (sls). although in all cases the objects are produced by sequential layer deposition of material, there are substantial differences in the stratification process [15], [16]. in this context, the use of a fbg sensor to ensure a selfsensing capability to a 3d printed structure assumes great relevance. among the first researchers, zubel et al. [17] demonstrated the possibility of incorporating some fbg sensors into a patch to estimate strain and temperature. lima et al. [18] proposed a cantilever structure to evaluate displacement, temperature and acceleration. lesiak et al. [19] verified this approach also to produce a measuring head implanted into a mechanical transmission element, while zhang et al. [20] created a plantar pressure sensing platform. a large part of the literature discusses the case of fdmprinted structures, where the incorporation process of the fbg sensors generally follows two different procedures. in the first one [21], the whole structure is printed and equipped with a small channel, where the fbg sensor is inserted and glued. instead, in the second one, the structure is only partially realized, temporarily stopping the printing. then the fbg sensor is placed directly onto the superficial layer [22], [23] or inside a small housing channel [24]. finally, the stratification of the remaining layers goes to completed. in this way, the molding of the polymer filament is exploited to cover the fbg sensor and connect it to the structure. currently few articles discuss the case of sla, whereby pausing the process is more complex. in fact, the manufactured object is alternative immersed in and extracted from the liquid bath in order to create the single layer and to mix the printing material, respectively. such printing material is generally a resin, characterized by a high viscosity at the liquid state. consequently, the solidified layer always remains wet and this is the main cause of the difficulty of correctly placing the fbg sensor [25]. recently manzo et al. [26] have analytically and experimentally investigated two different applications, a reducedscale frame structure and a pressurized cylindrical vessel, obtained by sla. the authors have embedded the fbg sensor filling a thin channel with the printing material (resin). this has involved a considerable attention to avoid air bubbles and the need to cure the resin inserted into the channel. the aim of this work is to discuss the development of a selfsensing structure with embedded fbg, using sla. at first, two different procedures for the integration of the fbg sensor were discussed, highlighting their advantages and disadvantages. after identifying the best configuration, the fbg sensor was exploited to monitor the whole am process (i.e. printing, washing and curing stages) of the self-sensing structure, also making use of additional strategies to compensate for the effects of the temperature. finally, the structural response of the realized selfsensing sample was evaluated by subjecting it to a controlled stress. 2. materials and methods 2.1. printing of the self-sensing structures the representative test sample of the developed self-sensing structure was manufactured by an sla printer (mod. form 2, formlabs), using a photopolymer resin (ppr) (mod. black v4, formlabs) and a layer resolution of 50 µm. its morphology was chosen in accordance with the type iv specimen, reported in the astm d638-14 standard. such a standard is designed to reproduce tensile property data for the evaluation of plastic materials. to allow the self-sensing feature, a fbg sensor (mod. smf28, technica sa) was embedded into the sample. geometrically, it had a diameter of about 242 µm, of which (8.2 ± 0.1) µm of core, (125.0 ± 0.7) µm of cladding and (242 ± 5) µm of an acrylate coating, and a length of the grating of 14 mm. while, optically, it presented a nominal wavelength of (1559.98 ± 0.05) nm in transmission and reflection, a full-width at half maximum (fwhm) of 0.26 nm, a reflective bandwidth (rbw) @-3 db of 0.26 nm and a reflectivity of 85.65 %. specifically, the printing stage was interrupted when the central layer of the test sample was reached and the fbg sensor was carefully placed along the longitudinal axis of the same sample. furthermore, some supports were also printed for pretensioning the fbg sensor and ensuring its correct blocking. once this step was ended, the printing was restarted by completing the structure. at the end of the whole process, the printing supports were removed, and the test sample was first washed in isopropyl alcohol for 20 min and then cured under uv for 30 min (15 min for each side) at 60 °c. together with the test sample, two additional compensation samples (compensation sample a and b), nominally identical to the previous one, were printed (figure 1). a second fbg sensor (mod. smf-28, technica sa, nominal wavelength of (1556.31 ± 0.05) nm), encapsulated into a needle [27], and a k-type thermocouple were respectively embedded, adopting the same procedures already described. their function was to acquire the thermal trend during the manufacturing process, allowing the apparent strain to be assessed. consequently, the choice of this specific fbg was mainly determined by the fact that it was already investigated in [27], although calibrated as a thermometer. 2.2. experimental setup and measurement procedures the optical signal from the two fbg sensors was acquired by a spectrum analyzer (mod. si 720, micron optics) and an optical interrogator (mod. di 410, hbm), characterized by accuracies of ± 3 pm and of ± 1 pm, respectively. instead, a specific data acquisition system (daq, mod. usb-temp, measurement computing) was used to record the output voltage from the thermocouple. after the curing stage, the test sample was left to rest for 30 min at room temperature, to guarantee its thermal stabilization, and subjected to a tensile stress by means of a testing machine (mod. electroplus e3000, instron). a load cell of ± 5 kn and an figure 1. details of the samples during the sla printing process. compensation sample b compensation sample a test sample thermocouple supportsfbg sensor needle acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 3 extensometer (mod. extensometer 12.5 mm class b-2, instron) were used to measure the applied force and the consequent strain, respectively (figure 2). the testing parameters, in accordance with astm d638-14, are shown in table 1. 3. preliminary analysis preliminary tests were carried out to evaluate the aspects related to the embedding process of the fbg sensor into the test sample. 3.1. embedding process of the sensor in the self-sensing structure at the beginning, three compact specimens, i. e. without the fbg sensor, were printed and subjected to a tensile test to determine their mechanical characteristics. then the elastic modulus, equal to about (3.53 ± 0.09) gpa, and the elastic range, up to 300 n were measured. after that, two different ways of embedding the fbg sensor into the test sample were investigated (figure 3). in the first one (configuration with seat) an optical fiber, without bragg grating and with same dimensions as the fbg sensor, was placed and pre-tensioned in a circular seat. such a seat, having a radius of 150 µm, was provided on the longitudinal axis of the test sample in correspondence of its central layer. instead, in the second one (configuration without seat), the optical fiber was simply laid and pre-tensioned in the middle of the central layer. hence, further six specimens, three for each configuration, were tested. these strategies are different to those reported in the literature for sla. in [26], the main drawback that can be found is the difficulty of guaranteeing optimal incorporation of the fbg sensor. in fact, if the thickness and the width of the printed structure is high, there is a significant risk of obtaining an incomplete polymerization process and therefore the imperfect transfer of the load to the fbg sensor itself. 3.2. mechanical characteristics of the embedding process the configuration with seat (figure 4 a) highlighted a lack of continuity, caused by the interruption of the printing to allow the positioning of the optical fiber. after the tensile test (figure 4 c), the optical fiber undergoes a marked dismemberment of its cladding as proof of a good embedding into the printing material. on the other hand, the configuration without seat (figure 4 b), in addition to presenting the same lack of continuity for the reasons already described, generates a relevant distortion of the upper layers of the specimen. finally, at failure (figure 4 d), the strain in the optical fiber suggests a lower adherence with the printing material than in the previous case. figure 5 describes the force-displacement curves, obtained from the tensile test, relating to the compact specimens with seat and without seat. the presence of the optical fiber inside the specimens leads to a slight weakening of the self-sensing structures in terms of load and elongation at failure. generally, it is probably attributable to a higher speed of propagation of the crack, in turn due to the introduction of a discontinuity (optical fiber) into the material. quantitative data are reported in table 2. specifically, the configuration with seat presents a lower reduction of fracture load (4.4 %) than the configuration without seat (7.1 %). that is caused, in an indiscernible way, by the polymerization of the ppr inside the seat and by the distortion of the layers above the optical fiber. however, in the elastic range, all the specimens have a rather limited variation of the elastic modulus (-1.6 % for specimens with seat and -3.5 % without seat referred to the average maximum stress in the elastic figure 2. details of the validation tests. table 1. testing parameters. parameter (unit) value sampling frequency of testing machine (hz) 10 preload (n) 10 loading speed (mm/min) 1 sampling frequency of optical interrogator (hz) 10 sampling frequency of spectrum analyser (hz) 100 sampling frequency of temperature daq (hz) 10 figure 3. scheme of configuration a) with seat and b) without seat. images not in scale. a) b) c) d) figure 4. typical microscope images of two representative specimens of the configuration with seat (a and c) and without seat (b and d) before and after the tensile test, respectively. extensometer sample load cell fbg sensors samples 1st part a) b) seat samples 2nd part acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 4 range of the compact specimens). for this reason, in terms of self-sensing, the different configurations are sufficiently superimposable (figure 6). taking into account the performed preliminary analysis, and considering that the seat also acts as a visual guide during the placing and pre-tensioning phases of the fbg sensor, the choice of the most suitable configuration fell on the one with seat. 4. results in this section, results concerning the monitoring of the 3d printing process and the mechanical behaviour (elastic stage) of the self-sensing assembled structure, are reported. 4.1. monitoring of the stereolithographic printing process figure 7 displays the trends of the characteristic wavelength of the fbg sensors embedded into the test sample and into the compensation sample a as a function of elapsed time during the printing stage. locally, both curves are characterized by a cyclic trend due to the movement of the build platform, i.e., its immersion in the ppr bath, the generation of the single layer and its emersion (figure 8). however, as expected, the fbg sensor embedded into the compensation sample a is less affected by this condition. in fact, its encapsulation inside a needle considerably reduces the strain to which it is subject, favoring the measurement of the apparent strain caused by the variation of the temperature. generally, the fbg sensor into the test sample exhibits a reduction in the pitch of the bragg grating, attributable to a state of compression induced by the polymerization of the ppr. figure 9 compares the trends of the characteristic wavelength of the fbg sensor into the compensation sample a and of the thermocouple into the compensation sample b during the printing stage. in this case, the two signals present a good coherence up to 700 s from to the printing re-start, i.e. from the printing start of the second half of the sample. after that, a visible divergence occurs probably because of a localized strain of the needle due to the excessive heating of the layers above the center line and the consequent state of compression, induced by the polymerization of the ppr. in conclusion, the signal of the fbg sensor into the test sample was suitably compensated by that of the apparent strain recorded by the second fbg into the compensation sample a. figure 10 presents the compensated signal along the whole figure 5. force-displacement curves for the compact specimens (cs) with seat (sws) and without seat (sns). table 2. results of tensile test for different type of specimens. type of specimen average fracture load in n standard deviation in n reduction* of fracture load in n compact specimens 1647 11 specimens with seat 1574 13 4.4 specimens without seat 1530 14 7.1 * referred to the average fracture load of the compact specimens. figure 6. average trends of the stress-strain curves in elastic range for compact specimens (cs) with seat (sws) and without seat (sns). figure 7. time history of the characteristic wavelength of the fbg sensors embedded into the test sample (𝜆fbg) and into the compensation sample a (𝜆tc) during the printing stage. figure 8. detail of the time history of the characteristic wavelength of the fbg sensor embedded into the test sample (𝜆fbg) during the printing stage (i) = start immersion of the build platform in the ppr, (ii) = generation of the single layer and (iii) = start of emersion of the build platform from the ppr. 0.00 0.50 1.00 1.50 2.00 2.50 3.00 0 200 400 600 800 1000 1200 1400 1600 1800 f o rc e / n displacement / mm cs1 cs2 cs3 sws1 sws2 sws3 sns1 sns2 sns3 0.0000 0.0005 0.0010 0.0015 0.0020 0.0025 0.0030 0 2 4 6 8 10 s tr e s s / m p a strain / mm/mm cs sws sns ycs=3519.3782xcs+0.1316 r2cs=0.9993 ysws=3463.9010xsws+0.1290 r2sws=0.9996 ysns=3397.8852xsns+0.1882 r2sns=0.9992 0 500 1000 1500 2000 2500 3000 1553.800 1554.000 1554.200 1554.400 1554.600 1554.800 1555.000 1555.200 1555.400 c h a ra c te ri s ti c w a v e le n g th / n m elapsed time / s lfbg ltc 1140 1150 1160 1170 1180 1190 1200 1210 1220 1554.875 1554.900 1554.925 1554.950 1554.975 1555.000 1555.025 (iii) (ii) l f b g / n m elapsed time / s (i) acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 5 manufacturing process of the self-sensing structure. the major differences between the raw signal (𝜆fbg) and the compensated one (𝜆fbg_compensated) are visible in the printing and curing stages. instead, in the washing and resting stages the curves are sufficiently superimposable after an initial transient. furthermore, all stages, except for the curing one, are subject to a reduction in the characteristic wavelength and therefore to a consequent state of compression. globally, these trends are attributable to the effects of the polymerization of the single layers (compression) and the hardening of the surface of the structure (relaxation). table 3 shows the values of the characteristic wavelengths of the fbg sensor into the test sample at specific time (figure 10). conservatively, the uncertainty of the characteristic wavelength of the fbg sensor embedded into the test sample (𝜆fbg) was estimated as type b [28], based on the accuracy of the spectrum analyzer used for these measurements. therefore, a uniform distribution (i.e. rectangular) with an amplitude of ± 0.003 nm was considered from which a value of type b uncertainty equal to about ± 0.0017 nm was estimated. 4.2. mechanical performance of the self-sensing structure figure 11 exhibits the spectrum of the fbg sensor as the load on the test sample varies, however remaining in elastic range. the tensile phase induces a shift of the peak to greater wavelengths and a reduction of the magnitude of the optical signal due to the increase of the pitch of the bragg grating (table 4). in this case the uncertainty was estimated as type a [28] on the magnitude of the optical signal at 300 n with a value of about 0.01 db. finally, figure 12 reports the calibration curve relating to the tensile strain measured by the test sample. as well-known from the literature [29] and as can be seen also from figure 11, the plotted curve has a good linearity. figure 9. time history of the characteristic wavelength of the fbg sensor embedded into the compensation sample a (𝜆tc) and of the temperature measured by the thermocouple into the compensation sample b during the printing stage. figure 10. time history of the characteristic wavelengths of the fbg sensor embedded into the test sample before (𝜆fbg) and after compensation 𝜆fbg_compensated) during the whole manufacturing process. table 3. characteristic wavelengths of the fbg sensor embedded into the test sample before (𝜆fbg) and after compensation (𝜆fbg_compensated). point elapsed time in s 𝜆fbg in nm 𝜆fbg_compensated in nm δ𝜆* in nm a 1000 1555.020 1554.853 0.167 b 3000 1553.098 1552.938 0.159 c 5000 1551.578 1551.484 0.094 d 7000 1547.718 1547.709 0.009 *computed as δ𝜆 = 𝜆fbg𝜆fbg_compensated figure 11. variation of the magnitude of the optical signal of fbg sensor as a function of the wavelength at a load application on the test sample. table 4. results of tensile tests for different type of specimens. load in n peak wavelength in nm peak magnitude in db standard deviation in db 0 1555.428 -4.62 0.05 100 1557.516 -4.86 0.05 200 1559.504 -5.06 0.05 300 1561.368 -5.27 0.05 figure 12. calibration curve of the fbg sensor used to measure the tensile strain on the test sample. 0 500 1000 1500 2000 2500 3000 1555.100 1555.125 1555.150 1555.175 1555.200 1555.225 1555.250 30.0 32.0 34.0 36.0 38.0 40.0 42.0 l t c / n m elapsed time / s t e m p e ra tu re / ° c 1540.000 1550.000 1560.000 1570.000 1580.000 -60 -50 -40 -30 -20 -10 0 m a g n it u d e / d b lfbg / nm 0 n 100 n 200 n 300 n 0.0000 0.0010 0.0020 0.0030 0.0040 1550.000 1552.000 1554.000 1556.000 1558.000 l f b g / n m strain / mm/mm linear fitting y=1692.098x+1550.407 r2=0.9998 acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 6 5. conclusions the paper discusses the development of self-sensing structures printed in sla and equipped with a fbg sensor. the presented study allows to state that a fbg sensor can be suitably embedded into a structure printed in sla. this integration is optimal when a seat is created, since it guarantees the advantageous placing of the fbg sensor and the determination of a good interface for the transfer of the load without making use of further post-production methods. in addition, such a work shows how a fbg sensor can monitor the stress state induced into a structure throughout its printing process (i.e., printing, washing and curing stages). to obtain this, the use of compensation strategies, implemented through appropriate compensation sensors embedded in similar structures and subjected to the same process, is decisive. this represents an innovative approach of investigation as it allows obtaining information directly related to the printed product without resorting to more complex and often difficult to apply investigations during the printing itself. finally, the self-sensing structure shows an appropriate structural response when subjected to a controlled stress. the main limitation of the proposed work relates to the fact that, because of the movement of the build platform of the sla printer, the fbg sensor could be erroneously embedded into the structure and consequently the quantity of interest could not be correctly measured. such an issue is addressed here by adopting some supports for pre-tensioning the fbg sensor, capable of ensuring its correct blocking. however, this procedure cannot always be easily ensured, especially when the self-sensing structure has a complex geometry. furthermore, the same fbg sensor could require a tortuous positioning such as to disperse the light beam inside it, making the measurement impossible, or to produce its breakage, being constituted by a glass fiber. future developments will be focused on the study of the thermo-mechanical effect due to the printing process variables on the self-sensing structure and on the monitoring of a complex and functional self-sensing structure, equipped with a multisensor fbg system. references [1] e. mesquita, p. antunes, f. coelho, p. andré, a. arêde, h. varum, global overview on advances in structural health monitoring platforms, journal of civil structural health monitoring 6 (2016), pp. 461-47, doi: 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https://doi.org/10.21014/acta_imeko.v10i2.1052 https://doi.org/10.3390/s19010055 https://doi.org/10.1016/j.compstruct.2015.08.100 https://doi.org/10.3390/s110101088 https://doi.org/10.1016/j.measurement.2012.02.026 https://doi.org/10.1016/s0924-4247(02)00429-6 https://doi.org/10.1016/j.measurement.2022.111543 https://doi.org/10.1007/978-1-4614-2413-0_27 https://doi.org/10.1016/j.yofte.2013.11.004 https://doi.org/10.1109/tim.2019.2959293 https://doi.org/10.21014/acta_imeko.v11i3.1268 https://doi.org/doi:10.1117/12.2228753 https://doi.org/10.1117/12.2264600 acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 7 [19] p. lesiak, k. pogorzelec, a. bochenek, p. sobotka, k. bednarska, a. anuszkiewicz, t. osuch, m. sienkiewicz, p. marek, m. nawotka, t. r. woliński, three-dimensional-printed mechanical transmission element with a fiber bragg grating sensor embedded in a replaceable measuring head, sensors 22 (2022) art. no. 3381. doi: 10.3390/s22093381 [20] y. f. zhang, c. y. hong, r. 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(2007), pp. 1718–1726. doi: 10.1088/0964-1726/16/5/026 [28] uncertainty of measurement—part 3: guide to the expression of uncertainty in measurement, document iso/iec guide 983:2008, 2008 [29] w. shen, r. yan, l. xu, g. tang, x. chen, application study on fbg sensor applied to hull structural health monitoring, optik, 126 (2015), pp. 1499-1504. doi: 10.1016/j.ijleo.2015.04.046 https://doi.org/10.3390/s22093381 https://doi.org/10.1016/j.measurement.2017.08.024 https://doi.org/10.1109/jsen.2016.2584141 http://dx.doi.org/10.18494/sam.2016.1189 https://doi.org/10.1016/j.sna.2018.11.024 https://doi.org/10.1016/j.sna.2021.112921 https://doi.org/10.1109/i2mtc43012.2020.9128771 https://doi.org/10.1016/j.yofte.2019.102015 https://doi.org/10.1088/0964-1726/16/5/026 https://doi.org/10.1016/j.ijleo.2015.04.046 metrology infrastructure for radon metrology at the environmental level acta imeko issn: 2221-870x june 2023, volume 12, number 2, 1 7 acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 1 metrology infrastructure for radon metrology at the environmental level annette röttger1, stefan röttger1, daniel rábago2, luis quindós2, katarzyna woloszczuk3, maciej norenberg3, ileana radulescu4, aurelian luca4 1 physikalisch-technische bundesanstalt, bundesallee 100, 38116 braunschweig, germany 2 university of cantabria (uc), spain, 3 central laboratory for radiological protection (clor), poland 4 "horia hulubei" national institute for r&d in physics and nuclear engineering (ifin-hh), romania section: research paper keywords: radon calibration; radon flux calibration; traceradon; radiation protection; climate observation citation: annette röttger, stefan röttger, daniel rábago, luis quindós, katarzyna woloszczuk, maciej norenberg, ileana radulescu, aurelian luca, metrology infrastructure for radon metrology at the environmental level, acta imeko, vol. 12, no. 2, article 35 section editor: leonardo iannucci, politecnico di torino, italy received december 30, 2022; in final form may 24, 2023; published june 2023 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this project 19env01 traceradon has received funding from the empir programme co-financed by the participating states and from the european union's horizon 2020 research and innovation programme. corresponding author: annette röttger, e-mail: annette.roettger@ptb.de 1. introduction radon gas is the largest source of public exposure to naturally occurring radioactivity. radon activity concentration maps, based on atmospheric measurements, as well as radon flux maps can help member states to comply with the eu council directive 2013/59/euratom and, particularly, with the identification of radon priority areas. radon can also be used as a tracer to improve atmospheric transport models and to indirectly estimate greenhouse gas (ghg) fluxes. this is important for supporting successfully ghg mitigation strategies. one approach to estimate ghg fluxes on the local to the regional scale is the so-called radon tracer method (rtm), which is based on the night-time correlation between atmospheric concentrations of radon and ghg measured at a given station together with information on the radon flux data within the station’s footprint. thus, atmospheric monitoring networks are interested or are already measuring atmospheric radon activity concentrations using different techniques but a metrological chain to ensure the traceability of all these measurements has been missing till traceradon [1] started. an overview on the project is available in [2], details on the metrology needs can be found in [3]. this paper presents in section 2 new activity standards which are available to provide new reference atmospheres. the creation of theses atmospheres is a complex task as explained in section 3. to bring all capacities together a reference field for the radon flux from soil to atmosphere is needed. this infrastructure, the so-called exhalation bed facility is presented in section 4. the paper closes with a summary and an outlook. abstract since 2020 a large consortium has been engaged in the project empir 19env01 traceradon to develop the missing traceability chains to improve the sensor networks in climate observation and radiation protection. this paper presents results in the areas of: novel 226ra standard sources with continuous controlled 222rn emanation rate, radon chambers aimed to create a reference radon atmosphere and a reference field for radon flux monitoring. the major challenge lies in the low activity concentrations of radon in outdoor air from 1 bq∙m-3 to 100 bq∙m-3, where below 100 bq∙m-3 there is currently no metrological traceability at all. thus, measured values of different instruments operated at different locations cannot be compared with respect to their results. whin this paper, new infrastructure is presented, capable of filling this gap in traceability. the achieved results make new calibration services, far beyond the state of art, possible. mailto:annette.roettger@ptb.de acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 2 2. new activity standards the metrology of radon (222rn is considered here) with respect to the representation of the unit of activity concentration in bq m-3 can be fundamentally divided into two different approaches: one way to quantify radon absolutely is the method of picolo [4], in which gaseous radon from the decay of radium (226ra) is frozen out at a cold point in vacuum. the alpha particles emitted by the solid radon are measured at a defined solid angle so that the unit of activity can be traced back to the base units of seconds and meters. the radon is then returned to the gas phase and can subsequently be used to produce atmospheres of known activity concentration. other methods are based on liquid scintillation counting [5] or 4πγ method [6]. the activity concentration of an atmosphere produced in this way decreases exponentially with a half-life of 3.8 days, limiting the counting statistics and hence the achievable uncertainty in the calibration of test samples. thus, this approach is not suitable for representing reference atmospheres with activity concentrations comparable to those in outdoor air (< 100 bq∙m-3). the use of this activity concentration for calibration purposes requires that the released activity of radon can be quantified traceable to the si system. since the mechanism causing the release of radon consists of a combination of two processes, on the one hand the recoil of the nuclei resulting from alpha decay and on the other hand the diffusion processes, the amount of radon released usually correlates with environmental parameters, such as relative humidity and temperature, since these influence the effective diffusion coefficient. to investigate these dependencies, novel sources have been developed at ptb with the metrological possibility of continuous determination of the emanation [8]. thus, it can be shown that at low penetration depth of the ionimplanted 226ra, there is no measurable dependence of the emanation on the ambient humidity and only a small dependence on the temperature [9]. the absolute determination of the activity of 226ra is done for this type of sources by spectrometry under a defined solid angle, while the released fraction of 222rn is quantified by -spectrometric comparison with radon-dense sources of the same design. on this basis, several primary 222rn activity standards have been fabricated that are suitable for representing the unit activity in the activity concentration range < 100 bq·m-3 in large-volume climate chambers. this is because typical sizes of walk-in climate chambers are 10 m³ to 30 m³. figure 1 shows three different activity standards in an overview: at the top a source based on electro-deposition of 226ra on stainless steel backing, in the middle mass separated ionimplanted 226ra onto a tungsten backing (tungsten or aluminium are both possible as backing using laser resonance ionization at the risiko 30 kv mass separator of johannes gutenberguniversität mainz) and on the bottom thermal physical vapor deposition of 226racl2 onto 450 mm2 ion-implanted silicon detector. a completely new approach in traceability and thus a milestone in the metrological development is represented by the new device at the bottom of figure 1: it accomplishes several challenges, such as high temporal resolutions and at the same time is able to generate high counting statistics. it also needs to meet specific requirements, like to be able to achieve low figure 1. new kind of activity standard sources for 222rn based on the principle of emanation of 222rn from electrodeposition, implantation or physical vapor deposition of 226ra activity from standards, produced at ptb. from top to bottom: (1) electro-deposited 226ra activity standard: deposition at 30 v < u < 200 v (easy to produce, but still dependant of environmental parameters during utilisation); (2) implanted 226ra activity standard: implantation of 226ra into w / al after mass separation at the risiko mass separator at the university of mainz with about 30 kev (ideal metrological source, but difficult and expensive to produce); (3) pips 226ra activity standard: 450 mm², 300 µm with about 160 bq 226racl2 layer from thermal, physical vapor deposition (new kind of source/detector combination with online emanation traceability). acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 3 uncertainties at low activities, for which the highest possible realizable detection efficiency is needed. the innovative approach was to coat commercially available silicon detectors with a thin layer of 226ra: integrated 226ra source/detector (irsd). after vapor deposition, 226ra is located on the surface of the silicon detector, so that about 50% of the radon produced is released. both, the alpha particles of the 226ra and those of the remaining radon can thus be detected spectrometrically under ambient pressure with a counting yield of about 50%, separately from each other and continuously. since the total amount of radon is a conservation quantity, the continuously recorded alpha spectra allow the amount of released radon to be determined. this type of measurement represents a mathematical inversion and requires special analytical procedures. based on the kalman filter for state estimation in linear dynamic systems, novel algorithms have been designed and implemented, which can be used to calculate the time course of the released activity of radon and the associated uncertainty from the continuously acquired measurement data. using the described method, even the release of one radon atom per second (about 2 µbq∙s-1) can still be determined within a few hours integration time with an uncertainty of around 2 % (k = 1) on average. in the future, the irsd will be used to calibrate radon monitors continuously, automatically and with feedback from radon chambers but even in the field and under changing climatic conditions. 3. new reference atmospheres for calibration of radon monitors the importance of radon exposure to the population and workers was noticed in 2013 by the european union when the basic safety standards directive (bss) was published in the council directive 2013/59/euratom. a significant part of this directive refers to the issues related to the radon hazard in dwellings and workplaces. the eu member states were required to establish national radon action plans addressing long-term risks from radon exposures in dwellings and workplaces. hence, member states needed to establish national reference levels for indoor radon concentrations, which should not be higher than 300 bq·m-3. this created new challenges for the radon dosimetry and calibrations because the previous limit used in certain eu member states had been much higher at 1000 bq∙m-3. among others arose the need for harmonization of existing radon measurement procedures and for creating new ones, ensuring the traceable calibration of radon measurement instruments at low radon activity concentrations with low uncertainties. these aspects could have been solved over the years, inter alia, due to research projects, like for instance metroradon, www.metroradon.eu. radon is not only addressed as the largest natural source of public exposure to ionizing radiation but also as a useful tracer for understanding atmospheric processes and estimating greenhouse gas emissions [10]. that is why reliable measurements of low-level radon activity concentrations, such as those found in the environment (< 20 bq∙m-3), are important both for organizations responsible for radiation protection and climate research. despite the enormous changes in radon metrology that have occurred in recent years, the activity concentrations below 100 bq∙m-3 have not been subject to metrological research so far. this poses new challenges, which are the development of traceable methods and robust technology for measurements of environmental low-level radon activity concentrations and radon fluxes. both are important to derive information from greenhouse gas fluxes in the environment and therefore important for reduction strategy planning. properly defined closed volume is necessary to create the reference radon atmosphere. in calibration laboratories two types of radon chambers are used: the first type is a large container (typically the volume of such chambers is greater than 10 m3). often designed as a walk-in chamber with an air-lock, allowing entry and exit with the minimum disturbance in the radon atmosphere. the second type of radon chambers are small containers only for the equipment under test. the volumes are usually less than 1 m3. the large radon chambers, due to their large volume, allow to perform intercalibrations of active radon monitors or calibration of devices measuring potential alpha energy concentration defined as: “the concentration of short-lived radon-222 or radon-220 progeny in air in terms of the alpha energy emitted during complete decay from radon-222 progeny to lead-210 or from radon-220 progeny to lead-208 of any mixture of short-lived radon-222 or radon-220 in a unit volume of air. the si unit for potential alpha energy concentration is j m-3”, given in icrp 115 [11] the smaller radon chambers are usually used to expose passive detectors in reference radon activity concentration. radon chambers with a volume of about 1 m3 represent an interesting option for calibration laboratories, allowing a successful approach to the calibration of both active and passive radon monitors. a radon chamber is not only a “radon container”, but also a system that ensures the maintenance of radon concentration inside and appropriate environmental conditions. the most frequently monitored parameters are temperature, relative humidity, atmospheric pressure, the ambient aerosol concentration, size distribution of radioactive aerosols, the radon decay products concentration and fractionalization, equilibrium factor of radium-radon gamma-ray dose or dose rate. the system for test atmospheres with radon (star), also called “radon chamber”, has four inseparable parts: the equipment for containing the atmosphere, the equipment for producing the atmosphere, the reference atmosphere thus created, and the equipment and methods for monitoring the atmosphere [12]. devices used to characterise the atmosphere shall be traceable to a primary standard. table 1 shows radon chambers presented in this article. they are compared according to their basic features. in the table the chambers are represented by the name of the institute and country which operating them: central laboratory for radiological protection (clor), poland, "horia hulubei" national institute for r&d in physics and nuclear engineering (ifin-hh), romania and university of cantabria (uc), spain. all these chambers fulfil the requirements of a star. one essential part of the system is the reference radon monitor, used to give the radon concentration inside. in the three cases exposed, the radon monitors have been calibrated at the bundesamt für strahlenschutz (bfs), accredited by the national accreditation body for the federal republic of germany dakks, according to the iso standard 17025, which is traceable to the national standards of the national metrology institute (ptb) [13]. to validate the two types of low-level 222rn emanation sources developed within the traceradon project, comparison measurements will be performed in low-level radon calibration chambers. based on a protocol, these sources are going to be measured also at ifin-hh radon chamber. the ionizing radiation metrology laboratory (lmri, ifinhh, romania) has developed this facility in the framework of acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 4 national and european joint research projects [14]. the chamber is cylindrical in shape, with a volume of 1 m3 and has accessories used to produce radon reference atmospheres they ensure international metrological traceability and equivalence of radon activity measurements, while performing reliable measurements of radon activity concentration. an alphaguard df2000 (saphymo gmbh) radon monitor traceable to bfs is used to take measurements of radon activity concentration inside the chamber. recent testing and improvement of the radon chamber tightness and traceability capacity have been done using the reference instruments and standard radon gas sources, thus obtaining designation by the romanian nuclear authority, national commission for nuclear activities control (cncan) as calibration laboratory for instruments measuring radon activity concentration in air, according to the iso standard 17025. the laboratory is already performing calibrations of active radon measurement instruments belonging to various romanian users. two radon chambers are located in the central laboratory for radiological protection (clor), which is the only laboratory in poland that has accreditation (in accordance to iso/iec 17025) in the field of calibration of radon activity concentration and potential alpha energy concentration monitors. one of them is the walk-in radon chamber with a volume of 12.3 m3. it is an air-tight climatized room made of 100 mm pur sandwich elements, covered outside with zinc-coated steel and plastic and inside with stainless steel. the chamber is built with an air-lock, allowing entry and exit with minimum disturbance to environmental conditions or radon atmosphere. the chamber has the option of setting the temperature and relative humidity within a wide range. it has also several input ports that allow the entry of radon and aerosols, the collection of air samples and the connection of instruments outside. this climatic radon calibration chamber fulfills the requirements of the international standard iec 61577-4. the second one is a small cylindrical shaped, tight container with a volume of 0.47 m3. this newly developed radon chamber is planned to be used for creating low-level reference radon atmospheres using new low-activity sources developed within the traceradon project. so far, to generate the reference radon atmospheres, two certified dry flow-through radon sources by pylon electronic development co. have been used. their activities are 137.3 kbq and 502.5 kbq. the radon chamber of the university of cantabria (uc) spain is the only spanish facility that provides calibration of radon monitors and exposure of passive radon detectors, according to iso/iec 17025 accreditation. it is a cubic shaped, stainless steel container with a wall thickness of 3.25 mm and an internal volume of 1 m3. the access to the chamber is provided by lifting the top lid of the chamber. it also allows access through three circular holes of 80 mm diameter, commonly used to insert and remove passive detectors without disturbing the radon inside during the process. radon measurement equipment can also be inserted inside at different heights. to homogenize the internal concentration, a fan is built in, working non-stop during the exposures. the radon sources used to generate the reference atmosphere are certified sources model pylon rn-1025 (pylon electronics inc.) and sources powdered with high 226ra content and encapsulated in a pvc jar which provides a constant radon diffusion rate. the radon monitors with traceability to international standards are the atmos12 (gammadata instruments ab) and the alphaguard (saphymo gmbh). these devices are connected to a computer located outside the chamber to know the concentration in real time. the radon concentration inside the chamber can be controlled and modified by using an open-air circuit that includes a pump, extracting air from inside the chamber. 4. reference fields for the calibration of radon flux monitors the exhalation bed facility provided by the laboratory of environmental radioactivity, university of cantabria (laruc), acts as a standard flux source of radon activity across a defined area per time. thus, its aim is to be used for calibrating continuous radon flux systems, that can be applied as a transfer standard for test validation of existing radon flux monitors by comparison campaigns in field or on the same exhalation bed under laboratory conditions. the exhalation bed facility consists of two exhalation beds, one with a high radon exhalation rate and another with a low radon exhalation rate (see figure 2). table 1. radon chambers features of clor, ifin-hh and uc from left to right. all chambers provide services like calibration of active monitors, and exposure of passive detectors. clor, poland ifin-hh, romania uc, spain v (m3) 12 0,47 1 1 radon concentration range (bq·m-3) 100 to 50 000 100 to 50 000 100 to 10 000 250 to 11 000 calibration and measurement capability (cmc) (k = 2) 7% 5% 5% accreditation according to iso 17025 designation according to iso 17025 according to iso 17025 reference devices alphaguard alphaguard df2000 alphaguard atmos12 traceability to bfs reference chamber environmental conditions adjustable temperature, humidity, ambient aerosol concentration temperature monitored temperature, humidity, pressure, size distribution of radioactive aerosols temperature, humidity, pressure temperature, humidity, pressure temperature, humidity, pressure acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 5 each exhalation bed has an effective surface of 1 m3 and is made up of five stainless steel welded plates shaping a box, with the upper part open. this configuration avoids the leakages through the plates and forces radon to escape to the top surface. the materials for the exhalation beds were selected according to their radioactive content and properties, such as soil texture, structure, dry bulk density, porosity, etc. the soil for the high exhalation bed was collected from the former spanish uranium mine, located in saelices el chico (salamanca, spain) and managed by the spanish national uranium company enusa. the low exhalation bed soil, however, was taken from the fosbucraa mine (western sahara). the materials were dried, sieved and homogenized before putting them into each container. the radon flux reference value of each exhalation bed has been approached by two ways. firstly, the theoretical one that is obtained from the resolution of the diffusion equation [15]: 𝐸 = 𝜀 · 𝐶ra · 𝜌 · 𝜆 · 𝑧 , (1) where 𝜀 is the soil emanation factor, 𝐶ra is the radium 226 concentration, 𝜌 is the dry bulk density, 𝑧 is the soil thickness and λ the radon decay constant. the equation (1) is an approximation when the soil thickness is much smaller than the radon diffusion length. such assumption was checked from the relationship between the diffusion length l and the porosity p and moisture content m [16]. the second approach is the experimental one, i.e. monitoring the radon increasing in a hermetically closed accumulation chamber [17]. the soil features and the exhalation rate obtained using both approaches are shown in table 2. moreover, during the intercomparison campaigns in the framework of the traceradon project two different areas were tested that could be a reference site to check radon flux systems under environmental conditions [18]. the high radon flux field is in the grounds of a former uranium mine managed by the spanish uranium company (enusa), located in saelices el chico (salamanca, spain). the average soil radium concentration in this area is (814 ± 65) bq∙kg-1 (k = 2). the reference radon flux value obtained during the intercomparison campaign was 2364 bq∙m-2 h-1 with a standard deviation of 1172 bq∙m-2 h-1. the low radon flux field, located in esles de cayón (cantabria, spain), contains an average radium concentration of (29 ± 3) bq∙kg-1 (k = 2) in soil. in the low area, the reference value obtained was 50 bq∙m-2∙h-1 with a standard deviation of 15 bq∙m-2∙h-1. moreover, it was a good agreement between the consensus experimentally obtained values and the output of the model proposed by [19]. the radon flux systems that can be tested in both scenarios, exhalation bed facility and field sites, should have defined protocols, both for installation and for measurement and analysis. in case of the exhalation bed, it is possible to open and close the accumulation chamber manually, if necessary, however in field tests it is preferable to have an automatic system in order to check the correlation with environmental parameters. 5. summary and outlook naturally occurring radon is the cause for most of the population's exposure to ionizing radiation. at the same time, radon is a highly efficient tracer for understanding atmospheric processes, for evaluating the accuracy of chemical transport models, and for providing integrated emission estimates of greenhouse gases. a metrological system for measuring figure 2. picture of the exhalation beds: brown, to the bottom: high radon flux, grey, to the top: low radon flux. table 2. results of the parameters influencing the calculation of radon exhalation rate in each “exhalation bed” for theoretical approach. uncertainties are expressed with the coverage factor (k = 1). exhalation bed parameter symbol (unit) high low moisture content m 0.013 0.05 porosity p 0.36 0.61 emanation factor 𝜀 0.18 ± 0.03 0.53 ± 0.02 radium concentration cra (bq/kg) 19130 ± 350 214 ± 8 bulk density d (kg/m3) 1645 ± 2 893 ± 0.6 radon decay constant λ (h-1) (7.5575 ± 0.0004) ·10-3 thickness z (m) 0.165 ± 0.005 0.130 ± 0.005 diffusion length l (m) 1.29 1.55 exhalation rate (theoretical) etheo (bq m-2 h-1) 6900 ± 1000 100 ± 6 exhalation rate (experimental) eexp (bq m-2 h-1) 6320 ± 240 94 ± 4 acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 6 atmospheric radon activity concentrations as well as the radon flux from the soil is therefore needed for atmospheric, climate, and radiation research. the activities developed and performed within the traceradon project went even further to increase the metrological capabilities of low atmospheric radon concentrations calibration and monitoring in the range below 100 bq m-3. this has included up to now: ― the development of new primary 222rn activity standards suitable for representing the unit activity in the range below 100 bq m-3 in the established walk-in radon chambers. ― the first successful calibrations in the new reference atmospheres radon monitors using the new developed activity standards. ― the development of laboratory exhalation beds tests for the calibration of radon flux monitors. ― intercomparison campaigns in two types of fields, low and high radon exhalation rates. the results obtained in the above-mentioned activities feed into the traceradon project with respect to radionuclide metrology and radiation protection at the environmental level. knowledge of such joint efforts can offer a solid background to providing more accurate and traceable results for these measurement methods, being even more challenging due to the outdoor environment. climate change and radiological protection both affect humankind and the environment worldwide. for the planet to combat both climate change and radiation exposure, measurements must be supported by reliable metrology. by addressing a topic (i.e. the measurement of low levels of radon in the environment) that supports both climate observation and global radiological protection, this project simultaneously supports the long-term economic, social and environmental work of icos, the integrated pollution prevention and control (ippc) directive 2008/1/ec, the iaea, analytical laboratories for the measurement of environmental radioactivity (almera) and who. acknowledgement this project 19env01 traceradon has received funding from the empir programme co-financed by the participating states and from the european union's horizon 2020 research and innovation programme. the following institutes are involved as project partners in empir 19env01 traceradon: physikalischtechnische bundesanstalt, germany, budapest főváros kormányhivatala hungary, cesky metrologicky institut, czech republic, agenzia nazionale per le nuove tecnologie, l'energia e lo sviluppo economico sostenibile, italy, institutul national de cercetare-dezvoltare pentru fizica si inginerie nucleara "horia hulubei", romania, npl management limited, united kingdom, institut za nuklearne nauke vinca, serbia, österreichische agentur für gesundheit und ernährungssicherheit gmbh, austria, centralne laboratorium ochrony radiologicznej, poland, instituto de engenharia de sistemas e computadores, tecnologia e ciência, portugal, joint research centre european commission, lunds universitet, sweden, státní ústav jaderné, chemické a biologické ochrany, czech republic, universidad de cantabria, spain, university of bristol, united kingdom, universitat politècnica de catalunya, spain, université de versailles saint-quentin-en-yvelines, france, ideas hungary betéti társaság, hungary. the consortium is supported by the following collaborating partners: university of heidelberg, germany, ansto, australia's nuclear science and technology organisation, australia, era, european radon association, europe, met office, united kingdom, university of novi sad, serbia, politecnico di milano, italy, university of cordoba, spain, eurados e.v., europe, university of siegen, germany, institut de radioprotection et de sûreté nucléaire, france, arpa piemonte, italy, arpa valle d'aosta, italy as well as the life-respire project, peter bossew, austria and the university of groningen, netherlands. the interest and support for the project extends far beyond europe: the members of the stakeholder committee, for example, come from four different continents. thanks are due to maria sahagia (ifin-hh) for some useful suggestions aimed to improve the manuscript. references [1] a. röttger, s. röttger, c. grossi, a. vargas, r. curcoll (+ another 14 authors), new metrology for radon at the environmental level, meas. sci. technol. 32, 2021, 124008, 13 pp. doi: 10.1088/1361-6501/ac298d [2] s. röttger, a. röttger, c. grossi, a. vargas, u. karstens (+ another 6 authors), radon metrology for use in climate change observation and radiation protection at the environmental level, adv. geosci., 57, 2022, pp. 37–47. doi: 10.5194/adgeo-57-37-2022 [3] s. chambers, a. griffiths, a. g. williams, ot. sisoutham, v. morosh, s. röttger, f. mertes, a. röttger portable two-filter dualflow-loop 222rn detector: stand-alone monitor and calibration transfer device, adv. geosci., 57, 2022, pp. 63–80. doi: 10.5194/adgeo-57-63-2022 [4] j. l. picolo, absolute measurement of radon 222 activity, nucl. instr. meth. a 369, issues 2-3, 1996, pp. 452–457. doi: 10.1016/s0168-9002(96)80029-5 [5] p. cassette, m. sahagia, l. grigorescu, m. c. lépy, j. l. picolo, standardization of 222rn by lsc and comparison with αand γ spectrometry, appl. radiat. isot. 64, 2006, pp. 1465–1470. doi: 10.1016/j.apradiso.2006.02.068 [6] y. nedjadi, ph. spring, c. bailat, m. decombaz, g. triscone, j.-j. gostely, j.-p. laedermann, f. o. bochud primary activity measurements with 4πγ nai(tl) counting and monte carlo calculated efficiencies, appl. rad.. isot. 65, issue 5, 2007, pp. 534538. doi: 10.1016/j.apradiso.2006.10.009 [7] f. mertes, s. röttger, a. röttger, a new primary emanation standard for radon-222, app. rad. and isot., 156, 2020, 108928. doi: 10.1016/j.apradiso.2019.108928 [8] f. mertes s. röttger, a. röttger, development of 222rn emanation sources with integrated, quasi 2pi active monitoring, int. journal of environmental research and public health 2022, 19(2), 2022, 840. doi: 10.3390/ijerph19020840 [9] f. mertes, n. kneip, r. heinke, t. kieck, d. studer, f. weber, s. röttger, a. röttger, k. wendt, c. walther, ion implantation of 226ra for a primary 222rn emanation standard, applied radiation and isotopes, vol. 181, 2022, 110093. doi: 10.1016/j.apradiso.2021.110093 [10] l. quindós, c. sainz fernandez, i. fuente merino, j. l. gutierrez villanueva, a. gonzalez diez, the use of radon as tracer in environmental sciences, acta geophys. 61, 2013, pp. 848-858. doi: 10.2478/s11600-013-0119-z [11] m. tirmarche, j. d. harrison, d. laurier, f. paquet, e. blanchardon, j. w. marsh, icrp publication 115, lung cancer risk from radon and progeny and statement on radon, ann. icrp 40(1), 2015, pp. 1-64. doi: 10.1016/j.icrp.2011.08.011 https://doi.org/10.1088/1361-6501/ac298d https://doi.org/10.5194/adgeo-57-37-2022 https://doi.org/10.5194/adgeo-57-63-2022 https://doi.org/10.1016/s0168-9002(96)80029-5 https://doi.org/10.1016/j.apradiso.2006.02.068 https://doi.org/10.1016/j.apradiso.2006.10.009 https://doi.org/10.1016/j.apradiso.2019.108928 https://doi.org/10.3390/ijerph19020840 https://doi.org/10.1016/j.apradiso.2021.110093 https://doi.org/10.2478/s11600-013-0119-z https://doi.org/10.1016/j.icrp.2011.08.011 acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 7 [12] iec 61577-4, radiation protection instrumentation radon and radon decay product measuring instruments part 4. geneva, switzerland. [13] t. r beck, a. antohe, f. cardellini, a. cucoş, e. fialova (+ another 23 authors), the metrological traceability, performance and precision of european radon calibration facilities, int. journal of environmental research and public health, 18(22), 12150. doi: 10.3390/ijerph182212150 [14] a. luca, i. rădulescu, m.-r. ioan, v. fugaru, c. teodorescu (+ another 7 authors), recent progress in radon metrology at ifinhh romania, atmosphere 2022, 13, 363. doi: 10.3390/atmos13030363 [15] j. porstendörfer, properties and behaviour of radon and thoron and their decay products in the air, journal of aerosol science, 25(2) , 1994, pp. 219-263. doi: 10.1016/0021-8502(94)90077-9 [16] v. c. rogers, k. k. nielson, multiphase radon generation and transport in porous materials, health physics, 60(6), 1991, pp. 807815 doi: 10.1097/00004032-199106000-00006 [17] i. gutiérrez-álvarez, j. e. martín, j. a. adame, c. grossi, a. vargas, j. p. bolívar, applicability of the closed-circuit accumulation chamber technique to measure radon surface exhalation rate under laboratory conditions, radiation measurements, vol. 133, april 2020, art. 106284. doi: 10.1016/j.radmeas.2020.106284 [18] m. fuente, d. rabago, s. herrera, l. quindos, i. fuente, m. foley, c. sainz, performance of radon monitors in a purpose-built radon chamber, journal of radiological protection, 38(3), 2018, 1111. doi: 10.1088/1361-6498/aad969 [19] u. karstens, c. schwingshackl, d. schmithüsen, i. levin, a process-based 222radon flux map for europe and its comparison to long-term observations, atmos. chem. phys., 15, 2015, 12845. doi: 10.5194/acp-15-12845-2015 https://doi.org/10.3390/ijerph182212150 https://doi.org/10.3390/atmos13030363 https://doi.org/10.1016/0021-8502(94)90077-9 https://doi.org/10.1097/00004032-199106000-00006 https://doi.org/10.1016/j.radmeas.2020.106284 https://doi.org/10.1088/1361-6498/aad969 https://doi.org/10.5194/acp-15-12845-2015 acta imeko  issn: 2221‐870x  june 2015, volume 4, number 2, 57‐61    acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 57  mechanical influences in sinusoidal force measurement  christian schlegel, gabriela kiekenap, holger kahmann, rolf kumme  physikalisch‐technische bundesanstalt, bundesallee 100, 38116 braunschweig, germany      section: research paper   keywords: dynamic force; sinusoidal excitation; laser vibrometer; rocking motion; triaxial acceleration; fem modal analysis  citation: c. schlegel, g. kieckenap, b. glöckner, a. buß, r. kumme, mechanical influences in sinusoidal force measurement, acta imeko, vol. 4, no. 2, article  10, june 2015, identifier: imeko‐acta‐04 (2015)‐02‐10  editor: paolo carbone, university of perugia, italy  received august 14, 2014; in final form february 12, 2015; published june 2015  copyright: © 2015 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: this work was supported by euramet  corresponding author: christian schlegel, e‐mail: christian.schlegel@ptb.de    1. introduction  force measurement plays a major role in industrial processes, statically as well as dynamically. in the past a very versatile system of static force calibration was established, which last but not least can be recognized by the many high level force calibration laboratories and services around the globe. nevertheless, most of the processes where force measurement is involved have a dynamic nature. so far only static calibrated force transducers also have been used in dynamic applications. the deviations in measurement associated with this may rise to the order of several percent and, especially in the vicinity of resonances, up to 10 % to 100 %. to close this gap, some developments were carried out in the past to provide dynamic force calibration. these developments have also triggered a project currently running in the european metrology research programme (emrp), the “traceable dynamic measurement of mechanical quantities”, which includes, apart from a work package on dynamic force, also work packages on dynamic pressure, dynamic torque, the electrical characterization of measuring amplifiers and mathematical and statistical methods and modelling [2]. the investigation of the uncertainty contributions in sinusoidal force measurement is crucial for providing a reliable dynamic calibration of force transducers. a sinusoidal calibration is usually performed with an electrodynamic shaker system. thereby, the force transducer, mounted on this shaker, is equipped with a top mass, and the acceleration on the surface of this mass as well as the force transducer signal are measured during the sinusoidal movement. a more detailed description of the whole calibration process can be found in [2]. 2. measuring an acceleration distribution  a big advantage for the acceleration measurement is a scanning vibrometer which offers the opportunity to measure many points, e.g. on the whole surface of the mass block. by averaging the signals measured at these points, one can obtain realistic standard deviations, e.g. uncertainties. the dynamic behaviour during such a sinusoidal excitation depends on the kind of coupling of the top mass on the transducer as well as on the mounting of the transducer on the shaker table. in addition, of course, the internal mechanical structure of the transducer is of fundamental importance. one result of such a dynamic calibration is the dynamic sensitivity, which is the ratio between the force transducer signal and the measured dynamic force, which is the product of the acceleration of the top mass and its mass value. due to the imperfect rigidity of the transducer, which depends on the internal structure, rocking modes may occur at certain frequencies. these rocking modes can be detected, for instance, from the uncertainty as a function abstract  the paper describes mechanical influences which disturb a sinusoidal force calibration and hence have an influence on measurement  uncertainty. the measurements are based on the application of a scanning vibrometer and the use of triaxial accelerometers. the  measuring of many acceleration points on the top mass of the transducer makes it possible to obtain acceleration distributions from  which a standard deviation can be derived; the triaxial accelerometer allows the observance of certain effects, like rocking modes, or  other problems related to specific excitation frequencies of the force transducer. both measurements can be related to each other.  the rocking effects are discussed with fem model calculations.  acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 58  of the frequency drawn for certain measuring channels. figure 1 shows one example of such an analysis. in this special case the uncertainty of the acceleration shows several frequencies where the uncertainty is significantly higher than elsewhere. it should be noted, that this behaviour is not related to rocking modes of the shaker table, which was investigated elsewhere. surprisingly, this behaviour does not correspond to the synchronously measured transducer output signal. it seems that this special kind of force transducer is not so susceptible to interference from external influences. the origin of the relatively large uncertainties of the transducer output signal at 200 hz and 300 hz is unknown; obviously this increase has nothing to do with possible rocking modes. in practice, one could conclude from such a measurement of the uncertainty distribution that it is advisable to avoid the regions where the transducer perhaps shows some irregularities. it should be noted that the uncertainty given above is not the whole uncertainty because the contributions influenced by the measuring equipment are not considered. they yield, depending on the frequency range, an additional 0.3 % to 0.5 %. the scanning of the acceleration can be accompanied by triaxial acceleration measurements, where one can directly measure the transverse acceleration in certain directions. combining the different methods may clarify the question of whether the increased uncertainty at certain frequencies can really be associated with rocking modes. 3. transverse acceleration  in order to investigate the possible rocking modes during the periodic excitation, the transverse acceleration was measured. for this purpose a special arrangement of four triaxial accelerometers on top of a test mass was chosen. figure 2 presents two pictures of the setup. the upper picture shows the force transducer equipped with an 8 kg test mass fixed with a mechanical adapter on the transducer. on top of the test mass the plate with the triaxial accelerometers can be seen. besides the force transducer one can see one additional accelerometer mounted on the shaker table. the force transducer was an interface type with a nominal force range of 25 kn. amplification of the force transducer output was realized with a dewetron conditioning amplifier. the lower picture (figure 3) shows the arrangement of the triaxial accelerometers on the plate which is mounted on the test mass. figure  2.  the  upper  picture  shows  the  whole  arrangement  of  the  force  transducer  with  the  test  mass,  which  was  equipped  with  a  special  plate  where four triaxial accelerometers have been mounted. the lower picture shows the plate with the four triaxial accelerometers.  figure 1. the figure shows the uncertainty of the measured acceleration on the top mass as well as of the force transducer signal. the uncertainty was obtained  by  averaging  62  acceleration  measuring  points  and  their  62  associated force signals at a certain frequency. it should be noted that the uncertainty of the vibrometer contributes to an additional 0.2 % to 0.3 %  and the uncertainty of measurement of the force conditioning amplifier an additional  0.1  %  to  0.2  %.  the  two  bars  reaching  0.04  %  have  to  be multiplied  by  a  factor  of  3.75.  note,  that  the  spatial  acceleration distribution is not taken into account in this analysis; due to the quite small height of the mass block it can be neglected. in addition note the different  scales of both plots.   acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 59  for a better fitting of the accelerometers on the plate, a certain area of the plate was milled out. the sensors themselves were screwed in place from the reverse of the plate. the accelerometers were arranged in such a way that the x and y components have an equidistant separation of 45 degrees. the sensors themselves were from kistler, type 8762a50, which have shear sensor elements that feature extremely low thermal transient responses and have a high immunity to base strain and transverse acceleration. an advanced hybrid charge amplifier is incorporated with a wide frequency range from 0.5 hz – 6 khz. the acceleration range of the sensors is in a scale of ±50 g whereby the peak limit is by ±80 g and the sensitivity is 100 mv/g, where g is the gravitational acceleration. the three outputs of each accelerometer were fed into the kistler 5134b conditioning amplifier which also provides the power supply for the integrated amplifier of the sensor. figure 4 shows the acceleration measurement values according to the eight transverse directions as a function of the excitation frequencies. the behaviour can be quite well demonstrated in a kind of windmill plot. the length of the wings is proportional to the amplitude. the opening angle of the wings can be chosen arbitrarily, but is equal for all directions. on the scale on the left-hand side the amount of the acceleration amplitude in percent in relation to the z-amplitude is given. at first glance one can see that the transverse acceleration distribution changes with the frequency. if one observes at certain frequencies a rocking in the direction of 45°/225°, e.g. at 100 hz and 1250 hz, the picture changes at frequencies of, e.g. 1750 hz and 2000 hz, where the rocking occurs in the direction of 0°/180°. on the other hand, there are quite different amplitudes of the transverse acceleration as a function of frequency, which reach from a few percent at low frequencies up to 100 % at high frequencies, related to the z-amplitude of the acceleration. in figure 5 the vector content of the transverse acceleration was calculated. according to the coordinate system, shown in figure 5, the three vector components give one acceleration vector in space with a certain value, r and an angle of inclination, θ, in respect to the normal of the force vector, which is perpendicular to the upper surface of the top mass (zaxis). the corresponding projection of this vector on the xy-plane as a percentage of r and the angle, θ, is given in figure 5 as a figure  4.  this  figure  shows  the  measured  acceleration,  observed  in  eight  different  directions  at  various  frequencies,  on  top  of  the  test  mass.  the scale on the left is the amplitude of the acceleration given as a percentage  of the z‐component of the acceleration. note the widely‐varying scales.  figure 3 schematic representation of the arrangement of the four triaxial accelerometers with their acceleration vectors in the transverse directions, x and y. in addition there are four accelerations perpendicular to the shown plane, in the z direction.  acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 60  function of frequency. from the figure one can see that quite large rocking motions occur at certain frequencies. the question of what is the origin of such behaviour arises. the origin of the large transverse amplitude at 500 hz is probably connected with a cross resonance of the transducer. the main resonance of the setup is around 930 hz. experimentally, the uncertainties are large beyond the resonance frequency, which is also to be seen in this data, see e.g. figure 1. if one compares both acceleration measurements, the scanning results according to figure 1 and the transverse accelerations according to figure 4 one can see a good correspondence at problematic frequencies, e.g. 500 hz. in summary, one can conclude that distinct frequencies with higher spatial variation of the top mass are often related to rocking modes of the experimental assembly which leads to higher uncertainties. 4. fem simulation of the dynamic system  to get more knowledge about the several dynamic modes which may happen during a periodical excitation an fem modal analysis was performed. in figure 6 the investigated transducer can be seen as a fem model in the upper left corner. the transducer is a shear force transducer, whereby the strain gauges are placed on the thin beams which are in between the holes. the inner part of the transducer is connected via a thread bolt to the outer environment, which provides the force initiation. in the case of the described periodical force measurement this is the additional mass block. by tension or compression of this inner part, the thin beams of the holes are distorted by higher stress, which are detected by the strain gauges. all following sub pictures show the behaviour of the transducer in connection with a 4 kg load mass. the frequencies of the dynamic modes are indicated below the figures, at which the dynamic modes occur. at 261 hz one has a first tilt resonance, where the mass block tilts relative to the transducer body. the red colour is thereby an indication of high stress, whereby the blue colour is a stress free situation. in the special case at 261 hz that means, that these modes initiate additional stress in the mass block. at 374 hz one has a torsion vibration of the mass block against the transducer body. also here the additional stress is transferred to the mass block with the difference that, the stress rises from the inner to the outer diameter of the mass cylinder in cylindrical shells with different stress level. at 1044 hz there is a tilting of both, the mass block and the transducer with respect to each other. the amount of additional stress is more or less the same in the transducer as well as in the mass block. the tilting vibration is non concordant, which leads to the stress maximum at the outer part of the mass block on the bottom side. at 1179 hz there is the main system resonance, which is a vertical vibration of the mass block relative to the transducer. here one can see a homogeneous stress level over the whole mass block. at 1461 hz there is a mode where the body of the transducer makes torsion in respect to the mass block. this mode is similar to the mode at 374 hz, with the difference, that the stress shells now occur in the transducer. figure 5. vector components of the triaxial acceleration measurement. the amplitude of the transverse acceleration as well as the angle of inclination of the acceleration vector in respect to the vertical of the top mass surface  (z‐axis) as a function of the excitation frequency.  figure 6. fem model calculations of the force transducer equipped with a  loading mass. shown are the inner structure of the used transducer, upper left corner and the behaviour of the system at different modal frequencies  the  colours  give  the  amount  of  stress  level  whereby  red  indicates  high  stress and blue no stress.  acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 61  beyond 1500 hz there are several other resonances which cannot be reached by the performed excitations. 5. conclusions  this paper describes some of the mechanical influences which are present during a sinusoidal force measurement. in contrast to single point acceleration measurements, the measurement of the whole acceleration distribution on the surface of the top mass makes it possible to observe effects, like rocking modes or other mechanical deficiencies. surprisingly, the effects seen by the acceleration signal are often suppressed in the force transducer output. it could be shown by transverse acceleration measurements that the measured uncertainty distribution of the acceleration on the top mass has good correspondence with the transverse motions at least below the resonance frequency of the setup. acknowledgement  the emrp is jointly funded by the emrp participating countries within euramet and the european union. references  [1] european metrology research programme (emrp), http://www.emrponline.eu. [2] c. schlegel, g. kieckenap, b. b. glöckner, a. buß, r. kumme “traceable periodic force calibration”, metrologia 2012, 49, n° 3, 224-235. acta imeko  issn: 2221‐870x  june 2015, volume 4, number 2, 39‐44    acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 39  dynamic torque calibration by means of model parameter  identification  leonard klaus 1 , barbora arendacká 2 , michael kobusch 1 , thomas bruns 1   1 physikalisch‐technische bundesanstalt (ptb), bundesallee 100, 38116 braunschweig, germany   2 physikalisch‐technische bundesanstalt (ptb), abbestraße 2‐12, 10587 berlin, germany      section: research paper   keywords: model parameter identification; dynamic torque calibration; dynamic measurement; mechanical model  citation: leonard klaus, barbora arendacká, michael kobusch, thomas bruns, dynamic torque calibration by means of model parameter identification, acta  imeko, vol. 4, no. 2, article 7, june 2015, identifier: imeko‐acta‐04 (2015)‐02‐07  editor: paolo carbone, university of perugia, italy  received september 24, 2014; in final form december 11, 2014; published june 2015  copyright: © 2015 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: this work is part of the european metrology research programme (emrp) joint research project ind09 – “traceable dynamic measurement of  mechanical quantities”. the emrp is jointly funded by the emrp participating countries within euramet and the european union.   corresponding author: leonard klaus, email: leonard.klaus@ptb.de    1. introduction  research in the field of dynamic calibration of torque transducers has been carried out in the context of the european metrology research programme (emrp) joint research project ind09 “traceable dynamic measurement of mechanical quantities” [1], [2]. an existing prototype measuring device [3] was modernised and extended, and a model-based description of the dynamic behaviour of torque transducers was developed [4]. the model of the transducer will be used to describe its dynamic behaviour in a later industrial application. the modelling is necessary, because torque transducers are always coupled on both ends to a given mechanical environment, which may influence the transducer’s dynamic behaviour. in case of the calibration this environment differs from that of a subsequent use in industry. for future dynamic torque calibrations, it will be necessary to identify the model parameters of a transducer to be calibrated from measurement data. this contribution is an extended version of a contribution to the imeko international tc3, tc5 and tc22 conference 2014 [5]. 2. model  the mathematical model used to represent the torque transducer assumes a linear and time invariant (lti) system which consists of two mass moment of inertia (mmoi) elements connected by a torsional spring and a damper in parallel. to be able to include the previously mentioned influence of the mechanical environment in the model-based description of the dynamic behaviour of the transducer, it was necessary to extend the model of the transducer to consider the mounted transducer including the dynamic torque measuring device (i.e. the mechanical environment in case of a calibration). this extended model represents the physical components of the measuring device and the transducer under test while assuming lti behaviour (see figure 1). it consists of elements for the mass moment of inertia, the torsional spring and the abstract  for the dynamic calibration of torque transducers, a model of the unmounted transducer and an extended model of the mounted  transducer including the measuring device have been developed. the dynamic behaviour of a torque transducer under test will be  described by its model parameters. this paper presents the models comprising the known parameters of the measuring device and  the unknown parameters of the transducer and how the calibration measurements are going to be carried out. the principle for the  identification  of  the  transducer’s  model  parameters  from  measurement  data  is  described  using  a  least  squares  approach.  the  influence of a variation of the transducer’s parameters on the frequency response of the expanded model is analysed.   acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 40  torsional damper. the equation of motion is described as an inhomogeneous system of ordinary differential equations: (1) in this equation denotes the mass moment of inertia matrix, the damping matrix, the stiffness matrix and the angle vector and its derivative vectors ( , ), respectively. the forced excitation is described by . for the described model as depicted in figure 1, the model approach leads to the mass moment of inertia matrix 0 0 0 0 0 0 0 0 0 0 0 0 (2a) the damping matrix 0 0 0 0 0 0 (2b) and the corresponding stiffness matrix 0 0 0 0 0 0 (2c) the angle vector and its derivative vectors , , are given by (2d) (2e) (2f) the forced excitation of the rotational exciter is given by 0 0 0 (2g) table 1: model parameters of the measuring device and of the device under  test.  known parameters of  the measuring device  unknown parameters  of the dut  mmoi , ,   , torsional stiffness ,   damping ,   3. known and unknown model parameters  to be able to identify the unknown model parameters of the torque transducer, it was necessary to identify the model parameters of the measuring device first. to this end, dedicated auxiliary measuring set-ups for the determination of the mass moment of inertia, torsional stiffness [4] and torsional damping [6] were developed. based on the measurement results from these set-ups, the previously unknown model parameters of the dynamic torque calibration device have been determined. a similar determination of the model parameters of the transducer under test is not possible. due to the unknown mechanical design and the lack of knowledge about the actual dynamic behaviour, a dynamic calibration remains necessary. the extended model of the measuring device now consists of a set of known model parameters, which represents the components of the measuring device, and of a set of unknown model parameters, representing the transducer under test (see table 1), which need to be identified. 4.  data acquisition and analysis  for the calibration measurement with a transducer under test, a sinusoidal excitation with given frequencies is generated by a rotational exciter. the frequency response function of the drive shaft of the measuring device depends on the device under test (see figure 2). the control of excitation frequency and vibration magnitude, including abort conditions and a predetermination of the frequency response of each set-up, is carried out by means of a closed-loop vibration controller. the excitation frequencies are based on the recommendations from [7]. they are equally spaced in logarithmic scale in the frequency domain. the 1/3 octave series was chosen for low and high frequencies far from the resonance frequency, and the narrower spaced 1/12 octave series was chosen for frequencies close to the resonance frequency, respectively. the frequency range of excitation ranges from 10 hz up to 1 khz. figure 2. frequency response of the measuring device with three duts of  different  torsional  stiffness  and  mmoi  measured  with  random  noise excitation.  figure 1. model of the dynamic torque calibration device (marked in blue)  including the transducer under test (marked in orange).  acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 41  the angle of excitation at the top is measured by means of a laser doppler vibrometer for rotational oscillations. the rotational acceleration at the bottom is measured by means of an integrated angular accelerometer. for calibration measurements, these two quantities and the output of the transducer dut are acquired with a four-channel data acquisition system. two high speed sampling inputs acquire the raw signal of the interferometer with a high sampling rate; two high precision inputs acquire the voltage outputs of the angular accelerometer and of the transducer under test. the frequency modulated interferometer output signal is demodulated by software and down-sampled to the lower sampling rate of the high precision input channels afterwards. the three input quantities are monofrequent sinusoids with the angular frequency 2π , the phase offset and the magnitude as follows ⋅ sin ⋅ (3) such a signal can be described by sine and cosine components as follows ⋅ cos ⋅ ⋅ sin ⋅ (4) the parameters and of eq. 3 can be derived from and of eq. 4 as , arctan (5) the phase angle needs to be derived using a four-quadrant inverse tangent (atan2). if the two-quadrant inverse tangent is used, a correction of for π may be necessary as this function is only defined for –π 2⁄ π 2⁄ . for multi-channel data acquisition with sinusoidal signals, it is advantageous to fit all the channels together with a common frequency [8]. the frequency is not known well enough, so it needs to be estimated as well. the parameters of interest of the three sinusoidal oscillations of the acquired channels ( , , , , , ) can be estimated with a common excitation frequency by the function multsin as follows multsin ⋅ ⋅ cos 2π ⋅ ⋅ sin 2π ⋅ ⋅ ⋅ cos 2π ⋅ ⋅ sin 2π ⋅ ⋅ ⋅ cos 2π ⋅ ⋅ sin 2π ⋅ (6) with the vector of measurement data and the time and assignment matrix consisting of the time vector and the assignment vectors … for the different channels, respectively. the length of the vector and the matrix is 3 , while is the number of measurement values acquired. , , …, , , , …, , , , …, (7) (8) 1 0 0 1 0 0 ⋮ 1 0 0 1 0 0 0 1 0 ⋮ 0 1 0 0 1 0 0 0 1 ⋮ 0 0 1 0 0 1 (9) the unknown parameters , , , , , will be identified by a nonlinear least squares approach as follows argmin multsin (10) if the values of the different acquired channels have different numerical magnitudes (due to units, etc.) the least squares algorithm would unintentionally ‘weight’ the channels according to their numerical magnitudes. to avoid such behaviour, the three channels need to be normalised. a linear least squares regression is applied to each channel based on eq. (4) with the frequency of excitation assumed to be known. based on the fit results for each single channel, the three channels are normalised prior to the combined regression described in eq. (6). the results from the linear regression are used as initial parameters for the iterative combined regression algorithm as described in eq. (10). 5. components influencing the acquired signals  prior to the parameter identification, all the dynamic effects of the different components apart from the dut need to be corrected by means of calibration frequency response functions. this includes the measuring components of the calibration device, as well as the signal conditioning and transmission electronics of the transducer under test. the data acquisition system needs to be calibrated as well. for each component analysed, a complex frequency response function i with the output i and the input i was determined for certain calibration frequencies as follows i i i (11) the magnitude of the frequency response function follows from | i | re i im i (12) with the real part re and the imaginary part im of the frequency response function. the phase of the frequency response function equals arctan im i re i (13) again, as in eq. 5, it is necessary to use a four-quadrant inverse tangent algorithm to derive the correct phase angle. the acquired measuring signals can now be corrected for magnitude and phase. with the corrected magnitude and phase , the measured magnitude and phase and the magnitude and phase frequency response functions , from calibration follows ⋅ (14a) (14b) 6. parameter identification  mounting different devices under test with different properties (mmoi, torsional stiffness, or damping) will influence the frequency response of the measuring device (see figure 2). utilising this variability in the frequency response of acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 42  the measuring device, the properties of the dut may be identified. the output of the transducer is assumed to be proportional to the difference δ , of the torsion angles at the top and at the bottom of the transducer as dut ⋅ h b ⋅ δ (15) because of the measurement principle (strain gauges) and the known linear behaviour. as the transducer under test measures the torque and not the angle difference, a scale factor is introduced. for the parameter identification, all the signals acquired are assumed to be harmonic. in this case we can assume the following relationship for the angle , the angular velocity and the angular acceleration : ⋅ e i ⋅ e i ⋅ e (16) here, i √ 1 denotes the imaginary number. the parameters of the dut will be identified by analysing the output of the transducer and the mechanical input, which is measured as the angular accelerations m and e at the top and at the bottom of the coupling elements (see figure 3). this leads to the following frequency response equations: top i ⋅ δ i ⋅ δ e (17) these equations are based on the ordinary differential equation (ode) system of the model (equations (1), (2a), (2b) (2c), (2d), (2e) (2f), (2g)) and contain the known model parameters of the measuring device, as well as the still unknown parameters of the dut. δ i i i i (18) for the transfer function of the top part top i , there follows i ⋅ ⋅ i i (19) with i i i (20) consisting only of the known model parameters of the measuring device. the expression for is more complex. additionally to eq. (20) we denote i i i (21) which again is not dependent on the dut’s parameters. with i and i we finally obtain , the transfer function of the bottom part of the measuring device examining eq. (22), the simple numerator and complex denominator suggest considering the inverse instead: 1 i i i i i i i i i (23) or alternatively from eq. (19) 1 i i i i i i i ⋅ i ⋅ i (24) which shows the dependencies on the unknown parameters , , , and . to get a closer look into the structure of eq. (24), we denote i , , i i ⋅ i i ⋅ i (25) i i i ⋅ i i i i i ⋅ i i (22) figure 3. transfer function based on the acquired measurement channels.  acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 43  then, using eq. (25) leads to 1 ⋅ i i i ⋅ i , , ⋅ i , , (26) expressing the separability of the model for the parameters 1⁄ , ⁄ and ⁄ . these parameters are conditionally linear. that means, when some of the parameters in the equation are known (in this case , ), we deal with a linear model (with respect to the remaining parameters). the same applies to eq. (19) where i gives 1 i i ⋅ 1 ⋅ i ⋅ 1 ⋅ i (27) with ⁄ and ⁄ being conditionally linear. assuming that and were known in eq. (26), or in eq. (27), additionally to the predetermined parameters of the measuring device, there would be closed-form formulae for the estimation of the remaining parameters 1⁄ , ⁄ and ⁄ (eq. (26)) or ⁄ and ⁄ (eq. (27)) by means of a linear least squares approach. these formulae would of course depend on and which has implications on the estimation procedure for all the unknown model parameters (see table 1). instead of estimating the 5 parameters of eq. (26) or the 4 parameters of eq. (27), respectively, by a least squares approach in one nonlinear optimisation step, the parameters 1⁄ , ⁄ and ⁄ may be replaced by closed-form formulae, and a nonlinear minimisation over two dimensions and can be carried out [9]. consecutively, the estimates for 1⁄ , ⁄ and ⁄ will be obtained from the closed-form formulae, with the estimated values of and . 7. analysis of the sensitivity of parameter  identification by simulation  the expected dynamic behaviour of the measuring device with an installed torque transducer was analysed by simulation. the simulation was carried out based on the model table  2:  parameters  of  hbm  t5  and  hbm  t10f  transducer  from  specifications [10, 11].    torsional stiffness    mmoi  (   mmoi  distribution  ( /   hbm t5 640 n ⋅ m rad⁄ 41 ⋅ 10 kg ⋅ m 0.5/0.5 hbm t10f 160 ⋅ 10 n ⋅ m rad⁄   1.3 ⋅ 10 kg ⋅ m 0.51/0.49 equations of the measuring device (eqs. (1), (2)). the frequency response functions were calculated with chosen parameters for the device under test and with the model parameters of the measuring device. a result of such a simulation is presented in figure 4, giving the magnitude and phase responses of the complex frequency response functions for the top and for the bottom as depicted in figure 3. for the analysis, properties of two typical transducers with a totally different mechanical design – and therefore with different model parameters – were chosen. one transducer is a classical shaft type slip ring transducer of the type hbm t5 (nominal torque 10 n·m) and one is a flange type transducer hbm t10f (nominal torque 50 n·m). both transducers are used for measurements in the dynamic torque calibration device. the specified model parameters of the analysed transducers are given in table 2. the mmoi distribution describes to what amount the mmoi of the transducer is allocated in the head and in the base mmoi element of the model. to find out how well the parameters of transducers under test may be identified, the sensitivity of a change in the parameters of interest , , , and to a change in the theoretical frequency response function (real and imaginary parts) was analysed. parameters which induce only very small changes in the frequency response function might be difficult to estimate or will have large uncertainties. starting from the chosen realistic parameter values, we changed one parameter at a time and compared the resulting frequency response functions with the frequency response functions calculated with the initial set of parameters. this investigation showed that changes in are more pronounced in the real parts of the frequency response functions (see figure 5), while changes in are manifested more in the imaginary parts. to quantify the induced changes, we figure  4.  simulated  transfer  function  top  (top)  and    (bottom)  calculated with the parameters of an hbm t10f transducer.  figure  5.  simulated  change  of  10 %  of    of  a  t10f  transducer  and  its  influence  on  the  real  and  imaginary  part  of  the  two  frequency  response  functions function  top (top) and   (bottom), respectively.  acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 44  considered differences between the real (imaginary) parts as a percentage of the magnitude of the initial inverse frequency response function. it was found that the mass moments of inertia and , of the hbm t5 transducer appear as potentially difficult to identify. a change of 10 % in the value of results in changes of less than 2% relative to the magnitudes in the real and imaginary parts of this frequency response function aside from the resonance case at about 150 hz (see figure 6). a change of 10 % in the value of , which is present only in i , results in changes less than 0.2 % relative to the magnitudes in the real and imaginary parts of this frequency response function, as depicted in figure 7. moreover, the influence is more profound for frequencies above the resonance frequency, which are difficult to measure. the same low sensitivity on a variation of a parameter applies for the damping parameter for the hbm t10f transducer. however, a parameter which is hard to identify means on the other hand that a parameter variation will not influence the dynamic system’s behaviour significantly, e.g. the low mass moment of inertia of the hbm t5 transducer (c.f. table 3) is too small to affect the frequency response of the drive train substantially. 8. conclusions  the presented identification scheme for the model parameters is a necessary component for the dynamic calibration of torque transducers. the dynamic behaviour of torque transducers is described by a physical model. the model parameters of each transducer will be identified from measurement data acquired during calibration. the model parameters of the measuring device have been determined prior to the calibration measurements to be able to identify the parameters of the transducer’s model. for their identification, the angular acceleration at the top and at the bottom of the transducer under test, as well as the transducer’s output, will be analysed. based on this input data, a parameter identification using the method of least squares is presented, and a two stage procedure utilising the separability of the model for a consecutive linear and nonlinear optimisation is described. based on the model of the measuring device, an analysis of the influence of the parameter estimation was carried out. it was shown that some transducer parameters might be difficult to identify if they have little influence on the measuring device’s dynamic behaviour. references  [1] c. bartoli et al., “traceable dynamic measurement of mechanical quantities: objectives and first results of this european project” in international journal of metrology and quality engineering; 3, 127–135 (2012) doi: 10.1051/ijmqe/2012020 [2] c. bartoli et al., “dynamic calibration of force, torque and pressure sensors” in proc. of joint imeko international tc3, tc5 and tc22 conference, 2014, cape town, south africa http://www.imeko.org/publications/tc22-2014/imeko-tc3tc22-2014-007.pdf th. bruns, “sinusoidal torque calibration: a design for traceability in dynamic torque calibration” in proc. of xvii imeko world congress; 2003, dubrovnik, croatia http://www.imeko.org/publications/wc-2003/pwc-2003-tc3008.pdf [3] l. klaus, th. bruns, m. kobusch, “modelling of a dynamic torque calibration device and determination of model parameters” in acta imeko vol. 3, no. 2 (2014), pp. 14-18 http://acta.imeko.org/index.php/actaimeko/article/view/imeko-acta-03%20%282014%29-0205/253 [4] l. klaus, b. arendacká, m. kobusch, th. bruns, “model parameter identification from measurement data for dynamic torque calibration” in proc. of joint imeko international tc3, tc5 and tc22 conference, 2014, cape town, south africa http://www.imeko.org/publications/tc3-2014/imeko-tc32014-018.pdf [5] l. klaus, m. kobusch, “experimental method for the noncontact measurement of rotational damping” in proc. of joint imeko international tc3, tc5 and tc22 conference, 2014, cape town, south africa http://www.imeko.org/publications/tc22-2014/imeko-tc3tc22-2014-003.pdf [6] iso tc 43. “iso 266:1997, acoustics: preferred frequencies for measurements”. international organization for standardization (iso), geneva, switzerland, 1997. [7] p. m. ramos, a. c. serra, “a new sine-fitting algorithm for accurate amplitude and phase measurements in two channel acquisition systems” in measurement vol. 41, no. 2 (2008), pp. 135-143 doi: 10.1016/j.measurement.2006.03.011 [8] g. h. golub, v. pereyra, “the differentiation of pseudo-inverses and nonlinear least squares problems whose variables separate”, siam journal on numerical analysis 10, 1973, pp. 413-432. doi:10.1137/0710036 [9] hottinger baldwin messtechnik gmbh, “t10f torque flange datasheet”, 2009. [10] hottinger baldwin messtechnik gmbh, “t5 torque transducer datasheet”, 2004. figure  6.  simulated  variation  of  10 %  of    of  a  t5  transducer  and  its influence  on  the  real  and  imaginary  parts  of  the  two  frequency  response functions.  figure 7. simulated change of 10 % of   of a t5 transducer and  its only very small influence on the real and imaginary part    (bottom).  multi-analytical approach for the study of an ancient egyptian wooden statuette from the collection of museo egizio of torino acta imeko issn: 2221-870x march 2022, volume 11, number 1, 1 10 acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 1 multi-analytical approach for the study of an ancient egyptian wooden statuette from the collection of museo egizio of torino luisa vigorelli1,2,3, alessandro re2,3, laura guidorzi2,3, tiziana cavaleri4,5, paola buscaglia4, marco nervo3,4, paolo del vesco6, matilde borla7, sabrina grassini8, alessandro lo giudice2,3 1 dipartimento di elettronica e telecomunicazioni, politecnico di torino, c.so duca degli abruzzi 24, 10129 torino, italy 2 dipartimento di fisica, università degli studi di torino, via pietro giuria 1, 10125 torino, italy 3 infn, sezione di torino, via pietro giuria 1, 10125 torino, italy 4 centro conservazione e restauro “la venaria reale”, piazza della repubblica, 10078 venaria reale, torino, italy 5 dipartimento di economia, ingegneria, società e impresa, università degli studi della tuscia, via santa maria in gradi 4, 01100 viterbo, italy 6 fondazione museo delle antichità egizie di torino, via accademia delle scienze 6, 10123 torino, italy 7 soprintendenza abap-to, torino, piazza san giovanni 2, 10122 torino, italy 8 dipartimento di scienza dei materiali e ingegneria chimica, politecnico di torino, c.so duca degli abruzzi 24, 10129 torino, italy section: research paper keywords: egypt; statuette; multi-analytical; museo egizio; tomography citation: luisa vigorelli, alessandro re, laura giudorzi, tiziana cavaleri, paola buscaglia, marco nervo, paolo del vesco, matilde borla, sabrina grassini, alessandro lo giudice, multi-analytical approach for the study of an ancient egyptian wooden statuette from the collection of museo egizio of torino, acta imeko, vol. 11, no. 1, article 15, march 2022, identifier: imeko-acta-11 (2022)-01-15 section editor: fabio santaniello, university of trento, italy received march 7, 2021; in final form march 14, 2022; published march 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: nexto project (progetto di ateneo 2017) funded by compagnia di san paolo, neu art project funded by regione piemonte corresponding author: alessandro re, e-mail: alessandro.re@unito.it 1. introduction scientific research in the cultural heritage field involves, in most cases, the development and use of physical and chemical methods to answer specific questions for a better understanding of objects produced in different contexts during history. through the analyses, it may be possible to reveal and identify materials and technologies used in the past, and also provide more grounded parameters for the preservation and conservation of cultural heritage artefacts [1]. the use of non-invasive methods (with no sampling) is very suitable in this kind of study, allowing the analysis of different abstract in the field of cultural heritage, the interdisciplinary and multi-technique approach to the study of ancient artifacts is widely used, providing more reliable and complementary results. to study these great-value objects, non-invasive approach is always preferred, although micro-invasive techniques may be necessary to answer specific questions. in this work, a study based on both non-invasive and micro-invasive techniques in a two-step approach was applied as a powerful tool to characterise materials and their layering, as well as to get a deeper understanding of the artistic techniques and the conservation history. the object under study is an ancient egyptian wooden statuette, belonging to the collections of the museo egizio of torino. analyses were performed at the centro conservaz ione e restauro “la venaria reale” (ccr), starting from non-invasive multispectral and x-ray imaging on the whole object, in order to obtain information about the technique of assembly and on some aspects of the constituent materials, and up to non-invasive xrf analysis and ft-ir, sem-edx and optical microscopy on micro-samples. this work is intended to lay the groundwork to the study of other wooden objects and statuettes belonging to the same funerary equipment, with the definition of a measuring protocol to study the most significant aspects of the artistic technique. mailto:alessandro.re@unito.it acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 2 types of objects fully respecting their integrity. some of these non-invasive techniques cover a large part of the electromagnetic spectrum, ranging from the analysis with gamma and x-ray radiation going through the ultraviolet, visible and infrared regions [2]-[10]. furthermore, for chemical and compositional analysis, some micro-invasive techniques are often used [11], [12]. together, all these complementary methods are powerful to give valuable information on the elemental composition as well as on the state of preservation, getting to the artistic processes performed by the artists [13], [14]. in this paper a multi-technique approach was used to finalize an in-depth study on an ancient egyptian wooden statuette, belonging to the collection of the museo egizio, in torino. all measurements were performed at centro conservazione e restauro (ccr) “la venaria reale”, where several scientific laboratories for the study and characterization of the different materials of artworks and ancient artefacts are available, before carrying out the required conservation treatments. the approach is based on the combination of imaging techniques, in particular ultraviolet fluorescence (uvf), visible-induced infrared luminescence (vil) and infrared reflectography (ir) that were employed to map the distribution of the different materials, such as the pigments used in the polychromies, their thicknesses and layering, as seen in previous studies [15]-[17]. among imaging analyses, a radiographic (rx) and tomographic (ct) study was also performed in order to investigate the inner part of the objects and to reach a deeper understanding on the execution technique and state of preservation [18], [19]. the next sections show in addition the results of compositional and elemental analysis acquired both directly on the surface with the noninvasive x-ray fluorescence (xrf) technique, and on samples with optical microscopy, fourier transform infrared spectroscopy (ft-ir) and scanning electron microscopy (semedx), largely used also in some other studies [20]-[24]. all the techniques listed above proved to be equally important in this study, each one providing different and complementary information, but essential to get a thorough knowledge of the artefact and to finally implement the best conservation strategy. this work contributes to the creation of a measuring protocol, applicable to other wooden objects and statuettes belonging to the same funerary assemblage, in order to significantly increase our understanding of the entire group of finds retrieved from a specific archaeological context. 2. the statuette the painted wooden statuette, representing an offering bearer (inv. no. s.8795, figure 1), was found during the 1908 excavation season of the italian archaeological mission, directed by ernesto schiaparelli, in the necropolis of asyut (egypt), a site situated some 375 km south of cairo. the statuette was part of the rich funerary assemblage of the so-called “tomb of minhotep”, which included additional statuettes of offering bearers, larger wooden statues, a model of a bakery, boat models, as well as coffins, wooden sticks, a bow with arrows and numerous earthenware jars and bowls [25], [26]. most of the equipment derived from specialized workshops operating in asyut during the early middle kingdom (ca. 1980-1900 bce). according to ancient egyptian religious beliefs, the tomb had to maintain the memory of the deceased, to preserve his/her body and to grant his/her survival in the afterlife thanks to specific rituals and, above all, food offerings. funerary offerings could be real, simply listed on stelae and coffins, or even scale models of servants carrying or processing food. at the beginning of the middle kingdom these scale models, together with models of granaries, boats or artisanal activities, become the main element of tomb assemblages and were placed within the burial chamber, usually near the coffin. the statuette examined for the present study is the typical representation of a female “offering bearer”, carrying a basket on her head, generally held in position with the left hand, and a duck in the other hand. although nowadays the two arms of the statuette are missing, an old photo preserved in the alinari archive shows the object complete of these two parts among the other finds from the same tomb as it was displayed in the museo egizio in the early 20th century [27]. the statuette is structurally composed of three elements (the basket, the human figure and the base) and measures 60.0 (h) × 12.5 (d) × 25.5 (w) cm3. 3. material and methods in the field of cultural heritage, diagnostic protocols usually give priority to non-invasive and imaging analyses because they provide an overview of the main characteristics of the object highlighting some material differences; consequently, they are essential for selecting the most representative subsequent analysis and sampling points. with this approach, the taking of some micro-samples from the artefact is the last step of the diagnostic campaign necessary to answer specific questions that arise during the early stages of the investigation. in this specific case study, non-invasive investigations (imaging and chemical analysis) were initially carried out; the results then led to the need of very small samples (~µm) for more in-depth measurements, such as microscopic investigation and ft-ir spectroscopy, in order to obtain useful information on the state of preservation and previous restorations. figure 1. the “offering bearer” statuette (s. 08795), frontal (left) and lateral (right) views after the conservation treatment. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 3 3.1. techniques and instrumentation uv induced visible fluorescence (uvf) the statuette was irradiated with uv labino® spot lamps (uv light mpxl and uv floodlight), with emission peak at 365 nm. the fluorescence produced in the vis region was captured with a nikon d810 full-frame reflex camera equipped with a peca 916 filter. the post-production of the photographs using adobe's lightroom software provided for the chromatic balance thanks to a 99% spectralon® and a minolta ceramic. this technique allows to evaluate characteristics such as homogeneity and distribution of surface layers based on the colour and intensity of the visible fluorescence induced by uv radiation. infrared reflectography 950 nm (ir1) the investigation was carried out after the first cleaning phase, that provided the removal of superficial dust, to reduce its interference facilitating the study of the artistic technique. the measure was made with ianiro varibeam halogen 800w lamps. the images were acquired in the photographic infrared range (from 750 nm to 950 nm) with a nikon d810 iruv full frame reflex camera, with b + w 093 filter. the post-production of the images using adobe's lightroom software provided for colour and exposure correction. this technique shows the presence of preparatory traces or changes under the painted film. visible-induced infrared luminescence (vil) the vil technique allows the localization of details made with the egyptian blue pigment, even when in poor conservation state, due to its intense fluorescence emission at around 916 nm when illuminated with visible light [28]. the lighting was guaranteed by led lamps without ir emission and the images were acquired in the photographic infrared range (from 750 nm to 950 nm) with a xnite nikon d810 (digital camera uv + vis + ir functionality) modified full frame reflex camera equipped with hoya r72 filter. the post-production carried out with adobe's lightroom software provided for the chromatic balance using colorchecker® classic of 24 colours and a reference standard of egyptian blue (kremer pigmente n° 10060) inserted in the shooting field. radiography (rx) and tomography (ct) the radiographic and tomographic analysis are useful for studying, for example, the techniques used in assembling the structure (e.g. joining elements) or to detect evidence of previous interventions. in particular, ct allows to observe the inner sections of the object, the orientation in space of its constituent parts, the sequence of the layers that compose it and their thickness. in this particular case a first radiographic measurement was performed using a fixed x-ray imaging set-up, developed in the context of the neu_art project [29], [30] and already used on very different kind of artworks [31], [32] and archaeological finds [19], [33]. it consists of a general electric eresco 42mf4 x-ray source, a rotating platform and a hamamatsu c975020tcn linear x-ray detector with 200 μm of pixel dimension that scans at about 0.2-6 m/min over an area of about 2×2 m². since this set-up was optimized for ct of large artefacts, a second rx followed by a ct analysis were performed using a flat panel detector (fp) shad-o-box 6k hs by teledyne dalsa, which, with an area of only about 160 cm2 and a pixel dimension of 49.5 µm, is more suitable for small objects or for part of large objects where a higher resolution is needed. for both measurements, a voltage of 80 kv and a current of 10 ma was set as acquisition parameters. for the radiography, the elaboration of the images was made with the open-source software imagej, whereas for the ct sections reconstruction a filtered back-projection algorithm [34] by means of a non-commercial software-utility developed by dan schneberk of the lawrence livermore national laboratory (usa) was used; the 3d rendering and segmentation was processed using vgstudio max 2.2 from volume graphics. x-ray fluorescence (xrf) the measurements were performed on representative points selected on the basis of the responses of the imaging techniques. the technique identifies the chemical elements present in the analysed area (spot diameter between 0.65 mm and 1.50 mm) for a variable depth based on the chemical nature of the materials present (approximately 150 μm). the collected data are employed to make hypotheses on the inorganic materials (mineral pigments) of the pictorial palette. for the analysis a portable micro-edxrf bruker artax 200 spectrometer was used with a fine focus x-ray source with molybdenum anode and adc with 4096 channels; the anode voltage is adjustable between 0 kv and 50 kv, the anode current between 0 µa and 1500 μa (maximum power 50 w). the measurements were carried out with a voltage of 30 kv and a current of 1300 µa, by fluxing helium on the measurement area in order to optimize the detection limit of the instrument. fourier transform infrared spectroscopy (ft-ir) the analyses were carried out to characterise original organic substances or possibly localized intervention materials previously detected through uv fluorescence imaging. infrared spectrophotometry allows to identify the organic and inorganic components present in the sample. the measurements were conducted on selective micro-samples with a bruker vertex 70 ft-ir spectrophotometer coupled with a bruker hyperion 3000 infrared microscope working in transmission with the aid of a diamond cell. optical microscopy and scanning electron microscopy with edx (om and sem-edx) the stratigraphic samples were collected after the noninvasive analytical campaign, in order to study the materials in depth. the observation with om in stratigraphy allows to obtain information on the executive technique adopted for the realization of the preparations, the polychromies and the finishes; the sem-edx analysis instead allows to obtain compositional information on the elements present in the different layers. following the sampling, the fragments identified for stratigraphic analysis were incorporated in transparent resin (struers epofix epoxy resin). the samples prepared in a polished section are observed by means of an olympus bx51 mineropetrographic microscope, in visible and uv light, interfaced to a pc by means of a digital camera. the acquisition and processing of images is carried out using analysis five proprietary software. for the sem analysis, the samples were observed with a zeiss evo60 electron microscope for morphological investigations (mainly by means of backscattered electrons detector, bsd). the edx bruker microprobe allows semi-quantitative elemental analysis. the analyses were performed in high vacuum for which the sections were carbon-coated. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 4 figure 2. frontal and lateral pictures of the statuette under different illumination for the multispectral analysis: (a) and (e) visible light, (b) and (f) uv fluorescence, (c) ir reflectography, (d) and (g) radiography. in picture (a) the measure points for xrf analysis are signed with numbers 1-8. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 5 4. results and discussion 4.1. assembly and modelling techniques the close observation and the analyses carried out made it possible to describe the assembly technique of the two portions (the basket and the body), which were originally assembled by inserting wooden dowels, with circular section, free of glue or filler material. moreover, a non-uniformity of the surface and the presence of gaps distributed over the entire body could be immediately noticed. in correspondence with some of these, such as on the chest or the hips, it was possible to observe the presence of a double layer of preparation. the first, light brown-coloured and coarser, is directly spread on the wooden material; on this, a second, thinner, white layer was perceivable (figure 2a, figure 3a). in some areas of the sculpture, the thin white layer seems to be applied directly on the wooden material, as observed for example on some gaps in the garment (figure 2e, figure 3b). the pictorial decoration was realized on this white preparation. for a better understanding of the construction technique and the contribution of the preparatory layers in modelling the shape, radiographic and tomographic analyses were carried out [35]. data were acquired on the whole statuette for radiography, while for ct the acquired portion is limited from the basket down to the hips of the statuette. thanks to the x-ray imaging, it was possible to visualize details and features of the object: tomographic data in particular provided important information not only on the assembly, but also on previous structural interventions (discussed in section 4.3). at first glance, it was possible to notice areas of the radiographic images with different radiopacity over the entire volume of the body (figure 2d). this confirm the nonhomogeneity of the preparatory layer distribution, which is more radiopaque than the wooden support. thanks to the ct slices it was also possible to localize the presence of the double layer of preparation mentioned before. the capability of detecting the material of the ground layer allowed us to confirm that it has contributed to partially polish the shape (e.g. head, breasts and hips), and to correct the gaps in volume due to possible defects of the wood or to refine some imprecisions in carving. where the carving was enough refined, a single preparation layer has been laid (figure 4). as regards the assembly technique, the junction between the basket and the head has been realized by means of wooden dowels insertions, as the radiographic, and the tomographic images, have shown (figure 4). the same anchoring system is evident in correspondence of the missing arms, as shown in figure 4e. from the tomographic sections of the head (figure 4c), multiple portions assembled by wooden dowels, peculiarly oversized, are observed, employed in order to achieve the final volume; ct demonstrated to be useful also in understanding the direction of insertion of each dowel. moreover, in correspondence with the right breast, the same type of assembly technique can be seen: the observation of the trend of the inner growth rings allows to recognize this insertion as a remediation for a material detachment probably in the notching phase (figure 4d). to investigate the nature of materials used for the preparation layers (and pigments, as detailed in section 4.2), non-invasive xrf analysis were performed on some representative points (figure 2a and table 1) and two stratigraphic samples, one from the white garment (figure 5) and one from the black wig (figure 6), were analysed by om and sem-edx. the white thin preparation layer results to be made of a calcium carbonate-based material, with a little fraction of quartz. 4.2. pigments and finishing layers for a better understanding of the pictorial materials, imaging analyses have been followed by non-invasive xrf and analysis on two cross-sections, as previously explained. the yellow pale colour used for the skin of the figure results to be made of yellow ochre and/or earth, probably mixed with gypsum and/or calcium carbonate (see figure 2a and table 1). the warm white colour of the garment results to be made of figure 3. detail of the chest (a) and the dress (b) of the statuette; it is visible (a) the preparation layer on which the painted layer is applied; (b) the painted layer applied directly on the wood. figure 4. radiographic image and ct vertical and horizontal slices of the statuette: (a) basket; (b) and (c) head, (d), (e) and (f) body (green arrows: wooden dowels for assemblage; blue arrows: wooden joints; pink arrows: thicker preparation layer). acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 6 gypsum with minimal impurities of iron oxides, as detected in the cross-section (figure 5). considering the rarely documented use of gypsum as a pigment, this data appears very interesting and could be one of the features to be investigated also on the other finds of the funerary equipment. the presence of p could be referred to possible organogenic material in the rock used to produce the pigment (figure 5e and table 1). a black pigment results to be used in profiling the dress, the eyes and the eyebrows, and to colour the wig. the ir reflectography (figure 2c) suggests it is probably a carbon-based material, based on its strong absorption. om and sem-edx analyses of the cross-section from the wig suggest the carbonaceous nature of this black: the black layer is quite thick, nevertheless only very low signals of inorganic elements have been detected in the layer, suggesting it is composed of organic carbon black (figure 6). as regards the wig, the preliminary hypothesis that there could be some egyptian blue pigment mixed with black was ruled out thanks to the vil survey which did not detect any luminescence of the pigment on the statuette. the preliminary close observation of the statuette and the uvf analysis allowed excluding the presence of a finish layer distributed on the surface, rather confirming a strong material non-homogeneity due to the state of preservation and previous interventions (figure 2b,f), as already observed also by the x-ray imaging analysis. a selective sampling from one of the areas with the greatest surface yellowing, that showed yellow-orange uv fluorescence, was taken for further materials investigations with ft-ir analysis (see section 4.3). however, no information about the type of binder used for the preparatory and pictorial layer could be obtained from the analysis, even if, with reference to the technical literature, the most probable hypothesis is the use of a vegetable rubber-based binder [36]. table 1. xrf analysis results (+++ = main chemical elements; ++ = secondary elements; + = trace elements; = not detected). measurement point element mg al si p s cl k ca ti mn fe sr 1_face yellow + + + + ++ + + ++ + + +++ + 2_body yellow + + + ++ + + +++ + + +++ + 3_body yellowish white + + + + ++ + + +++ + + + 4_body clean white + + + ++ + + +++ + + + + 5_leg gap white + + + + + + + +++ + + + + 6_eyebrow black + + + + ++ + + +++ + + +++ + 7_basket dark yellow + + + + + + +++ + + +++ + 8_basket red + + + + + + +++ + + ++ + figure 5. (a) stratigraphic sample of the white preparation, taken from the body of the statuette; (b) cross section under om in visible light (1, preparation layer; 2, warm white pictorial layer); (c) sem-bsd image of the cross section; (d, e) sem-edx spectra respectively of the preparation and pigment layers. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 7 4.3. conservation history and previous interventions as regards the state of preservation of the find, it showed a general greying of tones and some gaps in the preparation and paint layers distributed over the entire surface, as documented by photographs in visible light and x-ray imaging. after the close observation and a careful reading of the imaging analyses, in particular of the uv fluorescence, ft-ir analysis were performed on micro samples taken from (i) an area that showed orange-yellow uv fluorescence and (ii) from the face of the bearer, where a glossy, surface film-forming material seemed to be present. from the resulting spectra, an acrylic resin identified as paraloid was found in both the analysed samples (figure 7). the adhesive material paraloid resulted to be distributed over the entire surface and the data is compatible with the general greying of the surface, due to the typical paraloid absorption of dust and atmospheric particulate in time. figure 7b shows also signals referable to the presence of calcium carbonate and silicates in the pictorial layer, in accordance with the other performed analysis. furthermore, the signal at 1540 cm-1 is attributable to the presence of a protein-based substance, probably due to an ancient securing interventions carried out with animal glues, as traditional conservation practices usually envisaged. in addition, the base of the statuette showed some peculiar characteristics: from a first general observation, it was hypothesized that it could be traced back to a reuse of a wood fragment, perhaps taken from a coffin. this hypothesis could be reasonably confirmed by the evidence of a presumably original joint in correspondence of the lower portion of the base, attributable to the portion of the feet, but no longer in place, that led to think of a reuse. more insights concerning this specific characteristic should be provided, in consideration of a similar element observed in another find of the equipment. figure 6. (a) stratigraphic sample from the black wig on the back of the statuette; (b) cross section under om in visible light (1, white preparation layer; 2, warm white pictorial layer; 3, black pictorial layer); (c) sem-bsd image of the cross section; (d, e, f) sem-edx spectra respectively of the white, warm white and black layers. figure 7. ft-ir spectra (black curves) of the two samples taken from the body (orange-yellow uv fluorescence area) (a) and from the face area (greying of tones) (b). the substance was identified as paraloid (the red curve is the paraloid standard spectrum, for comparison). acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 8 as regards the figure-base anchoring, the complete absence of the feet (generally made of wood, sometimes partially completed with details in modelling material, often equipped with a tenon for the base joint) was immediately noted, and a wooden insert to which the legs were anchored, attributable to a previous intervention, was present (figure 8a). in fact, a complete discrepancy between the main block and this wooden insert was observed (for colour, compactness and grain of the wood); the insert was grouted on the perimeter and hidden on the surface by a pictorial retouching attributable to a previous intervention, rather recent. thanks to the radiographic analysis, this portion in the base was clearly distinguished and it could be observed that the insert reaches about half the total thickness of the base. furthermore, the x-ray images showed how both the legs and the wooden insert were applied and fixed: in fact, x-rays enhanced the presence of two holes created to accommodate the end portions of the two legs. the legs and the big wooden insert in correspondence of the base were fixed by applying a filler material (similar to a mortar), more radiopaque than the wooden material (figure 9). a fracture of the wooden matter in the back of the right leg of the bearer and at the calf level corresponded to a tongue and rebate joint for the completion of the leg anatomy, which can be clearly observed from the radiographic images of the lower part (figure 8b,c). in literature, many of the sculptures represented in a progressive position present an assembly of two separate elements for the back leg, a practical ruse to smoothly carve the internal parts. taking into account both this aspect and the fact that this kind of joint is already documented starting from very ancient chronologies and the characteristics of the wood are rather similar to those of the central body (colour, compactness), the originality of the part could not be excluded in a first phase, despite of some anomalies. in fact, the presence of an intervention mortar at the junction between the leg and the body (attributable to a bonding) and the chromatic and morphological differences between the wooden material of the base insert and that of the leg itself, are not sufficient to attribute the leg portion to an original or subsequent intervention. regarding this issue, a comparison with a wooden sculpture picture (xviii dynasty, collections of the museo egizio of torino), taken before restoration in the late 1980s by the doneux laboratory results to be very interesting: also in this case a wooden insert is observed for the lower portion of one of the two legs, certainly a non-original operation in this instance, but conceptually comparable to the one discussed here [37]. more insights should be provided to ascertain this aspect. as of the basket-head anchoring, evidence of previous intervention was identified. on the basket, in fact, a significant presence of an adhesive material was observed, for whose characterization no specific analysis was conducted; however, due to its mechanical and optical characteristics, in addition to the specific reactivity in contact with polar solvents, the adhesive has probably a synthetic origin (presumably of a vinyl nature, figure 10). finally, both from the radiographic analysis and at the time of disassembly, a metal element inserted to fix the two portions of reduced diameter and size, was identified (figure 11). in consideration of its shape, it is possible to suppose its pertinence to a modern structural intervention. figure 8. the detail of the statuette’s legs, (a) the wooden insert for the legs anchorage, (b) and (c) the radiographic images, frontal and lateral respectively, in which the joint is clearly visible. figure 9. radiographic image of the base of the statuette. figure 10. details of the adhesive residues (yellow arrows) detected at the basket-head interface. acta imeko | www.imeko.org march 2022 | volume 11 | number 1 | 9 5. conclusion the present work reports on the results of a two-step scientific approach useful for the characterization of the materials and the stratigraphy of ancient objects. the combination of imaging and punctual techniques, along with the visual inspection of the artwork, could give indication of the used materials and techniques of assembly, repairs, over paintings, finishes and treatments that have occurred in antiquity and through the centuries. in the specific case under study, the close observation and the tomographic analysis carried out allowed to describe the assembly technique. in fact, the use of several portions assembled using wooden dowels and of a preparation material based on calcium carbonate to achieve the final volume was observed. thanks to chemical investigations, also conducted in consideration of the multispectral imaging results, it was possible to define the nature of the different materials used for the statuette manufacturing. in addition to vil, xrf analysis in combination with om and sem-edx methodology gave the possibility to identify the pigments used for the decoration. taking into account the identification of synthetic materials and the ft-ir analysis results, it has also been possible to distinguish modern interventions, probably dated from the second half of the twentieth century. additionally, more ancient interventions, such as the insertion of wooden elements to complete the figure, seems to be present. all the performed analysis and the consequent evaluations contributed to the definition of the best conservation process for the statuette. in the future, it will be possible to apply the same investigation strategy to other wooden artefacts and statuettes belonging to the same framework, in order to make comparisons among the objects. analogies and differences in terms of materials, manufacturing techniques and state of preservation will support also the egyptological study of specific technical features, aiming at the possible reconstruction of different workshops active in asyut in the early second millennium bce. acknowledgements the nexto project (progetto di ateneo 2017) funded by compagnia di san paolo, the neu_art project funded by regione piemonte, and the infn chnet network are warmly acknowledged. we would like 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la scultura in legno al museo egizio di torino. problemi di conservazione e restauro, materiali e strutture. problemi di conservazione. sulla scultura, nuova serie anno vi, numero 11-12, pp. 9-31, 2008, issn 1121-2373 https://doi.org/10.1016/j.saa.2015.02.052 https://doi.org/10.1002/xrs.3184 https://doi.org/10.14568/cp2017029 https://doi.org/10.14568/cp2017008 http://dx.doi.org/10.5281/zenodo.4007568 http://dx.doi.org/10.1088/1757-899x/37/1/012007 https://doi.org/10.1109/i2mtc.2017.7969985 https://doi.org/10.1186/s40494-014-0019-9 https://doi.org/10.1186/s40494-015-0033-6 template for an acta imeko event paper acta imeko issn: 2221-870x december 2016, volume 5, number 4, 81-87 acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 81 emva 1288 camera characterisation and the influences of radiometric camera characteristics on geometric measurements maik rosenberger, chen zhang, pavel votyakov, marc preißler, rafael celestre, gunther notni ilmenau university of technology, gustav kirchhoff platz 2, 98693 ilmenau, germany section: research paper keywords: camera characterisation; camera parameters; edge transition model; optical geometric measurements citation: maik rosenberger, chen zhang, pavel votyakov, marc preißler, rafael celestre, gunther notni, emva 1288 camera characterisation and the influences of radiometric camera characteristics on geometric measurements, acta imeko, vol. 5, no. 4, article 13, december 2016, identifier: imeko-acta05 (2016)-04-13 section editor: paul regtien, the netherlands received march 22, 2016; in final form december 12, 2016; published december 2016 copyright: © 2016 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: the presented work is the result of the research within the project id2m which is situated at the ilmenau university of technology, germany as part of the research support program innoprofile, funded by the federal ministry of education and research (bmbf) germany corresponding author: maik rosenberger, e-mail: maik.rosenberger@tu-ilmenau.de 1. introduction over the last decades, plenty of different image sensors were developed by a huge variety of companies. presently there is a strong trend to complementary metal oxide semiconductor based image sensors (cmos) in comparison to the charge coupled device sensors (ccd) because of their high resolution and frame rate with lower power consumption [1] as well as their strong ability for integration of signal processing circuits down to pixel level [2]. on the other hand, the cmos sensors are subjected to worse signal to noise ratio (snr), as shown in [1]. furthermore, the cmos sensors have usually a fixed pattern noise (fpn) in the image due to their read out circuit [3], while the fpn in ccd sensors is mostly of a random nature. having this huge variety of sensor characteristics, there is a difficulty in the image sensor choice for practical applications, especially for geometric measurements in regard of the required high measurement stability and accuracy. first of all, the sensor comparison could be meaningfully conducted only under the precondition that the sensors are measured and characterised according to the same standard and with the same measuring instructions. however, the datasheets provided by the industrial camera manufacturers are usually written in their own standards and formats, so it is difficult to draw a comparison between different sensors using them. opposite to this situation, the emva 1288 standard was released to define the measurement and characterisation methods of image sensors and the format of datasheets. abstract over the past decades, a large number of imaging sensors based mostly on ccd or cmos technology were developed. datasheets provided by their developers are usually written on their own standards and no universal figure of merit can be drawn from them for comparison purposes. the emva 1288 is a standard aims to overcome this problem by setting parameters and experimental setup for radiometric characterisation of cameras. an implementation of an experimental setup and software environment for radiometric characterisation of imaging sensors following the guidelines of the emva 1288 is presented here. using simulations, the influences and impact of several emva 1288 parameters on geometric measurements can be estimated. this paper also presents a signal model and image acquisition chain; measurements of radiometric characteristics of an image sensor; and sensor evaluation for geometric measurements, where the aforementioned influences on geometric measurements are discussed. acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 82 furthermore, there is a need for understanding the impact of image sensor parameters on image processing procedures. for high-precise optical 2d geometric measurements based on areas of interest consisting of 1d search lines, the fundament is the determination of edge point location with subpixel accuracy on each search line. up until now, the theoretical investigations on edge detection (e.g. [4]-[6]) focus mainly on the performance comparison between different methods under a simple noise model that is not completely in agreement with the physical camera model and thus cannot present the real sensor characteristics. to estimate the influence of sensor parameters on real measurements, a systematic simulation model is needed. the emva 1288 standard gives a mathematical description of the signal conversion procedure in the camera system which can be also used as the fundament of the simulation model. moreover, the simulation should be designed considering real measurement conditions. in this paper, a measurement setup for radiometric camera characterisation according to the emva 1288 standard is firstly described. subsequently, a method based on a close-to-reality simulation model with the sensor parameters in the emva 1288 standard is proposed to evaluate image sensors in respect of geometric measurements in accordance with measurement metrology. using this simulation method, the influences of several emva 1288 sensor parameters were estimated. 2. signal model and image aquisition chain in general, the task of image sensors is the transformation of electromagnetic radiation into analogue or digital signals. the fundament of this signal conversion process is the photoelectric effect. for image sensors in the ultraviolet-visible (uv-vis) and near-infrared (nir) range, mainly the inner photoelectric effect is utilized for the conversion of photons into electrical signals. figure 1 illustrates the signal conversion stages in the image acquisition. the photons interact with the silicon layer so that a number of electron-hole pairs are generated in dependence on the amount of photons. this effect results in an accumulation of elementary charges in a potential well. the collected charges are converted into voltage output in proportion to the amount of collected electrons in an active sensor area which are called pixels and can be assembled as a ccd or cmos element. finally, the voltage is quantized to discrete digital gray values with an analog-to-digital converter. the saturation of the potential well will lead to saturation of the gray value so that it won’t no longer increase its value with further exposure. figure 1 lists uncertainty sources that are standardised in the emva 1288 camera model, which is illustrated in figure 2. it begins with the generation of electrons during the exposure time. with the wavelength λ dependent total quantum efficiency η(λ), the average number of photons µp hitting the pixel area is converted into an electrical signal µe: 𝜂(𝜆) ∙ 𝜇𝑝 = 𝜇ₑ. (1) for (1), the effects generated by the fill-factor and the influences of the micro lenses mounted on the active sensor area will be included in the total quantum efficiency. the mean number of photons µp can be calculated using the knowledge about the pixel area a, exposure time texp and the irradiance e on the sensor surface according (2): 𝜇𝑝 = 𝐴𝐴𝑡𝑒𝑒𝑒 ℎ𝑣 = 𝐴𝐴𝑡𝑒𝑒𝑒 ℎ 𝑐 𝜆 , (2) where h is the planck’s constant and c the speed of light. for the calculation of µp, a precise irradiance measurement is needed on the same place where the image sensor is under test. besides, a minor number of electrons µd are also accumulated during the exposure time due to the sensor thermal effect as well as the uncertainty from the sensor read out and amplifier circuits. the dark and bright signals are summed and amplified and this behaviour is modelled using (3) with the introduction of the overall system gain factor k: 𝜇𝑦 = 𝐾(𝜇𝑑 + 𝜇𝑒) = 𝜇𝑦.𝑑𝑑𝑑𝑑 + 𝐾𝜇𝑒. (3) in combination with (1) and (2), the mean gray value µy can be calculated as the sum of the mean value of the dark gray value and the product of overall system gain factor k and expected number of electrons µe which is calculated from the number of photons: 𝜇𝑦 = 𝜇𝑦.𝑑𝑑𝑑𝑑 + 𝐾𝜂 𝜆𝐴 ℎ𝑐 𝐸𝑡𝑒𝑒𝑝. (4) with this equation, the linearity of the sensor can be characterized using image gray values. figure 2: schematic of the emva 1288 camera model [8]. figure 1: simplified diagram of the image acquisition process and the principal noise sources [7]. dark noise quantization noise quantum efficiency system gain number of photons number of electrons digital grey value yη k nd nenp σq acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 83 the temporal fluctuation of the number of accumulated charge units, namely the shot noise, is subjected to a poisson distribution [8], hence its variance can be determined with the mean value: 𝜎𝑒2 = 𝜇𝑒 = (𝜇𝑦 − 𝜇𝑦.𝑑𝑑𝑑𝑑) 𝐾⁄ . (5) with the introduction of the quantisation noise σq, a signal independent dark noise σd as well as the photon shot noise σe calculated with (5), the overall noise σy measured with digital images can be characterized with (6): 𝜎𝑦2 = 𝐾2𝜎𝑑 2 + 𝜎𝑞2������� offset + 𝐾⏟ slope �𝜇𝑦 − 𝜇𝑦.𝑑𝑑𝑑𝑑�. (6) as mentioned, the dark noise σd consists of the thermal noise, which is poisson distributed, since it is an accumulative process, and the read out and circuits noise that can be modelled with a gaussian distribution [9]. from the radiation measurement, the dark noise σd and the overall gain factor k can then be measured from (4) to (6) with the photon transfer method [10]. for the determination of these parameters, a measurement setup was developed and is presented in section 3. 3. measurement setup for the determination of radiometric image sensor pamameters 3.1. measurement setup construction the requirements for the measurement setup to calculate the sensor parameters in the emva 1288 standard are given in the sections 6-9 of [8]. with the knowledge of those requirements, the measurement setup in figure 3 was developed. the main criteria for the construction is to ensure the accordance with the pinhole camera model, which is realized with the f-number restriction: 𝑓# = 𝑑 𝐷 = 8. (7) with an f-number restriction of 8 and a given distance d between the light source and the sensor plane, the free radiation diameter d of the light source can be calculated. another major point for this construction is the ideal wall surface behaviour inside the tube. therefore, a special coating which ensures a minor reflection coefficient was used for the inner surface. furthermore, a special camera socket was constructed for the reason to minimize the parasitic reflection inside the mounting. 3.2. software development the software for the emva 1288 measurement setup was developed using the matlab environment. the software handles gige-vision cameras as well as pictures taken by the user with other systems. the complete standard was implemented in this measurement tool. figure 4 illustrates the general procedure of the emva 1288 measurement. first of all, the irradiance which will be then applied to the measurement position of the image sensor under test has to be measured with a radiometer. after the measurement of the radiation power, the test camera can be connected to the setup, afterwards the acquisition is started to measure the radiometric parameters and the non-uniformity of image sensor. at the end of the camera characterisation, a standard compliant emva 1288 datasheet for the test camera can be generated automatically. this datasheet can then be used in the simulation program for the sensor evaluation with regard to its process capability in the high precision 2d optical geometric measurement based on search lines. the simulation program was also developed in matlab with an interface for the import of the camera datasheet. following the camera model in figure 2, a model was developed for the simulation of the 1d edge detection process with subpixel accuracy, which is the crucial process in 2d measurements. with this simulation model and the measured sensor parameters, the sensor can be evaluated according to the measurement uncertainty resulting from the monte carlo simulation [11]. the simulation model and its implementation are described extensively in section 5. moreover, the sensor parameters can be freely scaled in the simulation program in order to estimate the influences of the individual sensor parameters on the measurement. 4. measurements of radiometric image sensor characteristics the first measurements were taken with two ccd camera systems which differ in pixel size and quantum efficiency. the special requirement for this measurement is the equal set of the exposure time levels, so that a comparison between the cameras becomes possible. in figure 5, the different saturation points as well as the difference in the sensitivity coefficient between both cameras are easy to recognise. the saturation point of the ccd sensor icx 415al with higher sensitivity is reached at 34000 photons per pixel, which is significantly lower than icx 424. 5. sensor evaluaton for geometric measurements based on emva 1288 sensor parameters 5.1. simulation model in the majority of previous works about the simulation of 1d edge detection, e.g. [4], [5] and [6], it is simply assumed that figure 4: general procedure for data acquisition and evaluation separated in three stages. figure 3: emva 1288 measurement setup. acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 84 the noise follows a gaussian distribution. but this assumption could not conform to the real radiometric sensor characteristic according to the camera model in emva 1288. in this work, a simulation model based on this camera model was developed with consideration of all noise sources. for the 1d edge detection, a linear image must be generated first. this is considered as a cut from the 2d image. the simulation aims for the evaluation of measurement capability of the whole sensor area, hence the sensor non-uniformity which is characterised with dsnu1288 and prnu1288 is also modelled as random parameters in the 1d image simulation. figure 6 illustrates the process to simulate the measurement uncertainty of edge detection with given radiometric sensor parameters. at the first step, a spatial distribution of irradiation in metric units is given as the light signal on the image sensor. since it is proved in [12] that the edge detection error is strongly dependent on the difference between the edge location and the center of the corresponding sensor pixel, a moving edge is used whose subpixel-part is generated randomly in the value range between 0 and 1 using a uniform distribution function. considering the blurring effect due to the imaging optics, the blurred edge transition model in [13] under the assumption of a gaussian point spread function (psf) is used. this edge model is showed in figure 7 and formulated in (8): 𝐼(𝑥) = 𝑑 2 �𝑒𝑒𝑓 �𝑒−𝑙 √2𝜎 � + 1� + ℎ, (8) where l is the edge location, h the intensity offset, k the edge contrast, and σ the standard deviation of the gaussian psf. this signal is then converted into a discrete distribution of gray values along a defined number of pixels in the following step. at first, the number of photons in each pixel is calculated by integrating (8) over the pixel grid l. the number of photons in the n-th pixel is given by (9): 𝜇𝑝(𝑛) = ∫ 50.34 ∙ 𝑡𝑒𝑒𝑝 ∙ 𝜆 ∙ 𝐼(𝑥) 𝑛∙𝐿 (𝑛−1)∙𝐿 . (9) in this equation, the light wavelength and the exposure time are set constant and their values are 550 nm and 1 ms, respectively. from the number of photons, the gray value of each pixel is simulated according to the system model in figure 2. upon the assumption that the linearity error characteristic is unchanged over the sensor area, the quantum efficiency η is pixel-wisely adjusted to ηl according to the number of irradiated photons and the linearity error curve. in the next step, the spatial nonuniformity of photon response is simulated. according to [14], a gaussian random function with ηl as mean and prnu1288 as variance is used to generate the final quantum efficiency ηl,p for each pixel. then, the bright signal μe with photon shot noise is generated using a random poisson distribution function with mean value ηl,p·μp. in the simulation of the dark signal which is considered as thermally induced electrons, a basic noise signal μd,g is firstly generated from a random poisson distribution function whose mean value is the square of the dark noise value. to simplify the spatial non-uniformity, only the spatial variance is taken into consideration, in disregard of the periodic variations. as in the non-uniformity simulation at the bright signal, a gaussian random function with μd,g as mean and dsnu1288 as variance is used to generate the final dark signal μd. the bright and dark signal are combined with each other, then multiplied with the gain factor k and quantized to gray values. the edge location is detected with an adaptive threshold figure 5: sensitivity measurements at different camera systems. figure 7: edge transition using specific blurred edge model. figure 6: simulation of measurement uncertainty of edge detection. acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 85 that is estimated using the histogram based method in [15]. to achieve the subpixel accuracy, a cubic interpolation is used in the edge transition area and the subpixel edge point is defined as the intersection of the threshold and the fitted function. the complete procedure is repeated according to the monte carlo method [11]. the deviation of the detected edge from the defined edge location in (8) is used as outcome in each iteration. under the assumption that the distribution of the edge detection error is subjected to a symmetric distribution function, the outcomes are evaluated with the quantile method for symmetric distributions [15] to determine the measurement uncertainty (mu), which is defined as half of the interval having a 95 % level of confidence [16]. 5.2. model implementation in the model implementation the parameters in (8) must be determined first. the σ value is to be set according to the characteristic of the optics which will be used in the real measurements. the allowed high level h+k in the irradiation distribution (figure 7) is determined based on the gray value simulation at a single pixel using the sensor data und the camera model. in this process, the irradiation with which the possibility of pixel saturation equals to 95 % is determined by simulation and used as the high level in the light signal. based on the high level h+k, the edge transition contrast in (8) can be flexibly set by varying the offset h to simulate different measurement conditions in the practice. an important parameter in the monte carlo method is the number of iterations n. this number should be as small as possible to save simulation time but ensure a stable simulation result at the same time. the ratio of the standard deviation 𝑠𝑈 of the simulated measurement uncertainty to its mean value 𝑈� is utilized to examine the simulation stability. figure 8 illustrates the dependency of simulation stability on the iteration number. it is shown that this ratio remains nearly constant at 0.13 % from n = 50000, hence all the simulations are performed with 50000 iterations. the simulation result using sensor parameters of the ccd sensor “icx445” is shown in figure 9. it can be shown that the deviation of the detected edge location from its target value agrees very well with a symmetrical distribution. hence, the use of the quantile method can be validated. 5.3. analysis of the influence of sensor data on the uncertainty of measurement based on this simulation model and the real characteristics of the ccd sensor “icx445”, the influences of linearity error, dark noise, dsnu1288 and prnu1288 on the measurement uncertainty were estimated by a gradual raise of individual parameters, while the other system parameters remained unchanged. the σ value in (8) is set to 5, with which the edge transition form nears the real characteristics delivered by optical systems. the tests were at first performed with 100 % contrast, in which the light signal covers the full sensor dynamic range. afterwards, the contrast dependence of the influences of these sensor parameters was investigated with gradually reduced contrast by raising the low level in the light signal. the results of the investigations are represented in figure 10 to figure 13. in the datasheet the dark noise and dsnu1288 are given with the absolute unit [e-], which cannot directly indicate their relation to the 8-bit digitalized image signal. therefore, these parameters are represented with the ratio between them and the saturation capacity of the image sensor. moreover, a change of figure 8: stability of simulation in dependency on the number of iterations. figure 10: measurement uncertainty with magnification of linearity error. figure 9: simulation result with sensor parameters of ccd-sensor “icx445”. figure 11: measurement uncertainty with magnification of dark noise. acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 86 systematic measurement deviation is expected with the magnification of linearity error, hence the variation of the expected value of the absolute measurement deviation were also observed in this case. with the full contrast that refers to the optimal illumination condition, the magnification of linearity error and dark noise cause hardly any significant change of measurement uncertainty (smaller than 0.005 pixel), though the magnification of the linearity error causes a systematic shift of the detected edge location, as shown in figure 14. likewise, the magnification of prnu1288 and dsnu1288 up to 0.5 % brings hardly any changes. with a further magnification of these parameters, a minor rise of measurement uncertainty could be observed. as these parameters expand to 2 %, the uncertainty rises by ca. 0.015 pixel. as a summary, the test results are robust in resistance of a certain deterioration of sensor parameters under optimal contrast conditions, because an adequate dynamic range in the converted gray value image is fundamentally secured in this illumination situation. as the contrast decreases to 60 %, all the measurement uncertainty curves move slowly upwards with only one exception in figure 10. with a further decrease of contrast, this movement becomes significant. the reason for the growth of measurement uncertainty is that the decrease of contrast raises the ratio between the uncertainty of the individual gray values in the signal and its dynamic range by reducing the resulting value. on decreased contrast levels, the magnification of linearity error can cause an irregular change of uncertainty, whereby the value change is lower than 0.04 pixel. the characteristic of the systematic measurement deviation remains nearly the same in 80 % and 60 % contrast, but shows a greatly irregular changes in 40 % and 20 % contrast, as shown in figure 14. the reason is speculated to be that the linearity error of the sensor is not uniformly distributed over the irradiation levels, as shown in figure 15, so the in 40 % and 20 % contrast used sensor ranges have different linearity error characteristics. with the contrast decrease down to 40 %, the measurement uncertainty remains still stable against the magnification of dark noise. a clear relationship between dark noise and uncertainty could only be observed under the 20 % contrast. in this case, the measurement uncertainty rises by 0.063 pixel, as the dark noise expands to 10 % of the saturation capacity. a significant interaction between contrast level and sensor parameters can be observed at prnu1288 and dsnu1288. on the 40 % contrast an obvious rise of measurement uncertainty from the turning points at 0.2 % in both curves can be already observed, whereby the uncertainty rises by 0.1 pixel with the magnification of prnu1288 to 2 % and by 0.06 pixel with the magnification of dsnu1288. at the 20 % contrast, the increase of measurement uncertainty becomes more significantly. it rises by 0.68 pixel with the magnification of prnu1288 and by 0.346 pixel with the magnification of dsnu1288. from the results above, it can be seen that for low light applications and in the case of weak reflective object surfaces the sensor parameters prnu1288 and dsnu1288 have a strong influence on the measurement uncertainty in edge detection and should be considered primarily in sensor selection. figure 15: linearity error curve of the ccd sensor “icx445”. figure 12: measurement uncertainty with magnification of prnu1288. figure 13: measurement uncertainty with magnification of dsnu1288. figure 14: measurement deviation with magnification of linearity error. acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 87 6. discussion of results the magnification of the linearity error curve has generally a relatively weak influence on the measurement uncertainty but a significant influence on the systematic measurement deviation. the reason for it is that the measurement uncertainty results primarily from the uncertainty of the discrete gray values in the edge transition area, whereas the linearity error causes form distortion of the edge transition signal, which has a direct effect on the systematic measurement deviation. the irregularity of the influence on measurement uncertainty lies in the complex relationship between the signal form and the measurement uncertainty. a high dark noise reduces the reachable signal-to-noise ratio of the sensor and thus the dynamic range of signal which correlates with the measurement uncertainty. this effect becomes significant only when the contrast in the light signal is reduced to 20 % and the dark noise is extremely high. the parameters prnu1288 and dsnu1288, which directly refer to the uncertainty of the individual gray values, contribute the most to the measurement uncertainty. the reason for the difference of the curve characteristics shown in figure 12 and figure 13 is that both these parameters represent two different sources of uncertainty. prnu1288 refers to the uncertainty of the photon response so that the bright pixels have a higher uncertainty, while the by dsnu1288 characterized dark noise uncertainty is the same to all pixels. 7. conclusions the presented summary shows an abstract of the possibility to characterize image sensors using the standard emva 1288 and to evaluate image sensors for geometric measurements using the monte carlo method in which the imaging process is simulated using the system model in the emva 1288 standard. with the simulation program, the influences of several essential parameters of image sensor on geometric measurements were investigated. references [1] h. t. beier and b. l. ibey, “experimental comparison of the high-speed imaging performance of an em-ccd and scmos camera in a dynamic live-cell imaging test case”, plos one 9(1):e84614, 2014. [2] a. e. gamal and h. eltoukhy, “cmos image sensors”, ieee circuits and devices magazine, vol. 21, issue: 3, 2005. [3] s. mohammadnejad, s. roshani and m. n. sarvi, “fixed pattern noise reduction method in ccd sensors for leo satellite applications”, proceedings of 11th international conference on telecommunications, 2011. [4] e. p. lyvers, o. r. mitchell, m. l. akey, and a. p. reevs, “subpixel measurements using a moment-based edge operator”, ieee transactions on pattern analysis machine intelligence, vol.11, no.12, pp.1293-1309, 1989. [5] f. bouchara, m. bertrand, s. ramdani and m. haydar, “subpixel edge fitting using b-spline”, proceedings of the 3rd international conference on computer vision, pp. 353-364, 2007. [6] m. hagara and o. ondráček, “moving edge detection with sub pixel accuracy in 1-d images”, proceedings of 26th international conference radioelektronika, 2016. [7] c. aguerrebere, j. delon, y. gousseau and p. musé, “study of the digital camera acquisition process and statistical modeling of the sensor raw data”, 2013. <hal-00733538v4> [8] european machine vision association, emva standard 1288 – standard for characterisation of image sensors and cameras, release 3.0, 2010. [9] r, mancini, op amps for everyone, texas instruments, 2002. [10] j. r. janesick, “ccd characterisation using the photon transfer technique”, solid state imaging arrays, vol. 570 of spie proc., pp. 7-19, 1985. [11] d. p. kroese, t. taimre, z. i. botev, handbook of monte carlo methods, isbn 978-0470177938, wiley; 1. edition, 2015. [12] m. hagara and o. ondráček, “comparison of methods for edge detection with sub-pixel accuracy in 1-d images”, proceedings of 3. mediterranean conference on embedded computing (meco), pp. 124-127, 2014. [13] m. hagara, “edge detection with sub-pixel accuracy based on approximation of edge with erf function”, radio engineering, vol. 20, issue 2, 2011. [14] m. konnik and j. welsh, “high-level numerical simulations of noise in ccd and cmos photosensors: review and tutorial”, eprint arxiv:1412.4031, 2014. [15] k. weißensee, “beitrag zur automatisierbaren messunsicherheitsentwicklung in der präzisionskoordinatenmesstechnik mit bildsensoren”, phdthesis, ilmenau university of technology, 2011. [16] joint committee for guides in metrology, guide to the expression of uncertainty in measurement, 1. edition, 2008. https://hal.archives-ouvertes.fr/hal-00733538v4 microsoft word article 3 190-1060-1-pb.docx acta imeko may 2014, volume 3, number 1, 4 – 9 www.imeko.org acta imeko | www.imeko.org may 2014 | volume 3 | number 1 | 4 the role of measurement in the development of industrial automation s.s. carlisle united kingdom section: research paper keywords: industrial automation citation: s.s. carlisle, the role of measurement in the development of industrial automation, acta imeko, vol. 3, no. 1, article 3, may 2014, identifier: imeko-acta-03 (2014)-01-03 editor: luca mari, università carlo cattaneo received may 1st, 2014; in final form may 1st, 2014; published may 2014 copyright: © 2014 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited 1. introduction throughout the whole history of scientific endeavour the science and technology of measurement has been of paramount importance. our knowledge of physics, chemistry, the nature of matter and of human life itself has progressed according to our ability to measure and so to test our theories. furthermore, it is by measurement that we can communicate experience from one research centre to another; thus a facility is provided for research work to be continued by one worker following another. it also allows workers in different locations to coordinate their efforts in the advancement of scientific knowledge. more recently, measurement in industry has enabled manufacturing and processing skills to be more readily communicated from one worker to another; such communication means that advances in manufacturing practice can proceed and spread more quickly throughout an industry. when operator skill has established optimum conditions for working a process, the instrumentation of the process enables these best conditions to be easily transferred to other operators on similar processes without repeating the work. recent experience by a firm starting up a new oxygen-blown steelmaking process showed that comprehensive instrumentation and data logging on the plant resulted in the commissioning time to get the furnace up to its target production rate being greatly reduced. it is only by measurement in industry that we can really know the efficiency of utilisation of materials and of machines or processes. it is, thus, an essential basis upon which to advance our productivity. measurements are, of course, only of value if we use the results. in basic research we use them to confirm or improve the theory we are testing. in industry we use them, or should use them to alter our production planning or our plant operating conditions to achieve a better result, either as regards productivity or product quality. of course, if we do not use our measurements in industry to test out and compare alternative, and possibly better ways of doing things, we are wasting a valuable facility. measurement combined with human intelligence in interpreting the result and a will to take the action prescribed can hold a process at a constant best working condition in spite of inevitable disturbances in input conditions. when the interpretation of the measurement is simple and the control action easily defined we have for many years now been able to dispense with human interpretation and apply automatic control as in single variable feed-back control systems. nowadays, with the facility of analogue and digital computers we can perform more complex interpretation or processing of the measurement information and determine the required control action automatically in circumstances where previously this is a reissue of a paper which appeared in acta imeko 1967, proceedings of the 4th international measurement congress, 1967, warsaw, vol. 1, pp. 37–50. the paper sets a frame on the role of measurement and control as critical tools to support the automation of manufacturing processes. in this perspective three main requirements for measurement are discussed, i.e., to identify where automation can be most profitably used; to investigate individual process behaviors and hence to formulate process control strategies; and finally to perform quality control of products. acta imeko | www.imeko.org may 2014 | volume 3 | number 1 | 5 we had to employ human judgment and skill. by automation i mean the automatic determination of control action according to measurement information. it is only a matter of degree as to whether a digital computer is involved or not in the processing of the information. measurement science and technology is undoubtedly of importance on its own. i believe that it is by its use in the implementation of automation that it becomes of such unusual significance to industry – and, indeed, to our whole way of life. it is becoming realised in every progressive country that the standard of living prevailing in a community is closely related to the productivity of that community. by productivity, i mean the quantity of a consumable or durable product (such as a can of beans, a motor car), or of a service, such as a telephone connection, that can be produced for a given input of all resources including fuel, raw material, human effort and capital investment. by mechanisation and the use of massive machines, like continuous strip rolling mills, we can produce enormous quantities of steel strip per unit human operator employed. of course in such applications there is a large resource input in the form of fuel and capital investment in machines which basically are produced by human effort. it is important to realise that when discussing productivity we are concerned with output as a function of total resource input, not just output per man on a machine. it is this ratio of output to total resource input which directly determines a national standard of living. it is this ratio which automation can optimise. it is the job of automation to ensure that fuel is efficiently used in a process by applying automatic control to process variables. it is also its job to control raw material conditions and process conditions to ensure that there is a maximum yield of saleable product. furthermore, automation can be used to optimise production planning so that at any time all the components of a production facility are used best in meeting the requirements of the order book at that time. minimisation of inter-process stock is of course an important part of this exercise. the point i wish to emphasise is that automation is concerned with the optimisation of operation of a complete manufacturing complex and not just with providing automatic control of processes nor just with minimising the labour utilisation on operation of a particular plant. it therefore follows that the use of automation in industry is a major controlling factor on our standard of living. 2. impediment to progress in use of automation in industry if one accepts the great significance of this technology to any progressive country, and i think the leaders of nations do, and one notes the outpourings of technological achievement on measurement, automatic process control and computer data processing, which come from conferences like this and ifac and many others, one might well ask: “why don’t we all get on more quickly in using this wonderful tool?” i believe that in general technologically advanced countries find themselves in tile position that their technical competence in automation is taken advantage of by far too small a proportion of their total manufacturing industry. the shining examples of achievement in industries like steel, petro-chemicals, paper production, aircraft and traffic control, get much discussed. but what about automation in food production, brick-making, pottery manufacture, timber products and in the multitude of light engineering industries? these are the industries which in nearly every country are the greatest employers of labour and usually the more backward technically. why are they not so eager to exploit automation? there are many reasons of course. the more obvious ones which get much discussed arise from sociological and capital investment considerations. a less obvious reason but one which i think is far more important and much neglected arises from the fact that automation, more than any other technology, requires a close coordination of effort by people of many different skills and disciplines if it is to be successfully employed. the control engineer cannot make any progress in selling his technology to the building brick manufacturer unless both parties take the trouble to learn something of each other’s business. their disciplines, expertise and language are vastly different and yet it is only by working together that success will be achieved. many of you will have had experience of instruments which you developed for use in an industry – and which you felt confident from trials were satisfactory technically – and yet they fell into disuse by the firm which originally requested them. the reason probably was that in your enthusiasm for technological development of the device you neglected to formulate and communicate to the management of the company a proper conception of the use of the device as a means of improving the productivity and profitability of his business. it is noteworthy that in industries such as steel and modern chemicals, where measurement and control engineers have been introduced alongside production managers, process technologists and plant designers, there have been the most significant developments in the use of automation. this suggests that the use of automation has to be stimulated from within an industry. it cannot be applied by edict or exhortation from outside. the basic problem is one of making those who know the process and product technology of their industry also conscious of the new technology of automation and the use that can be made of it. similarly, the control engineer must learn something of the business and processes of the user industry before he can offer his technology as a viable proposition. i believe that measurement and control engineers must take a share of the blame for too slow progress in the wider use of this technology in industry. research and development people concerned with measurement and control technology too often grasp problems which industry puts to them because the problem gives them an opportunity to extend their research and development expertise. it would be better if their primary motivation was to advance the interests of the user by solving his problem rather than to advance their own disciplines. in other words, we require more research and development people whose enthusiasm is to solve real industrial control problems rather than to develop solutions and then seek problems to justify the solutions. 3. the advancement of the use of automation in the uk taking account of these problems which i have outlined, special facilities have been developed in the uk to promote the wider use of automation throughout our industry [1]. special attention is given to the less technically advanced industries and to the medium to small sized firms. i should like to describe these facilities and some examples of projects undertaken acta imeko | www.imeko.org may 2014 | volume 3 | number 1 | 6 because they demonstrate the role of measurement in promoting industrial automation. in the uk we have a network of 49 research associations, the majority of them serving process industries such as food, baking, textiles, paper manufacture, printing, furniture-making, steelmaking, etc. their job is to advance the technology of the industries they serve. they provide centres of knowledge of their industries’ process and product technology and its trends of development. the procedure now is to form within each of these research associations groups of measurement and control technologists who will develop within the particular industries they serve more technical awareness about control technology. a few of the research associations already have automation groups within their organisations. for example, the british iron and steel research association has a large operational research and control engineering department and has done much to advance automation in the steel industry. to aid the development of this procedure one of tile research associations, the scientific instrument research association (sira) has been appointed as the focal point for development of this automation activity throughout the research associations. sira now has within its organisation a strong industrial measurement and control division which encompasses work on industrial instrument development, control engineering and operational research. we believe that there is special advantage in having a systems engineering expertise in all organisation otherwise concentrated on measurement research and development. it is through industrial systems studies that the requirements for instrument development can be defined most reliably. i think it is wrong to isolate instrument research and development from systems and control engineering. the method of working which has been evolved is to form a joint team of control engineers from sira with experts from the other research association and arrange for this joint team to make a systems study in a factory of the particular industry. the job of the joint team is to study the whole manufacturing complex, the economics and technology of its operation, and out of this study to formulate proposals for the ways ill which automation can be introduced so as to be of greatest advantage in improving productivity. arrangements can then be made to install measurement and control systems to demonstrate economic and technical feasibility. all this work is done in close collaboration with staff of the particular firm chosen for the study. a vital feature in the carrying out of these studies is that control engineers, process technologists and works production staff are united in conducting the operation. 4. the role of measurement in advancing automation the first objective in these production system studies is to acquire data on production performance, plant utilisation and process yields in order to ascertain what parts of tile whole production process can benefit most from improved control. too often one is hampered in conducting these operation studies by the inadequacy of measurement of such things as machine operating time, inter-process stock levels, scrap losses at each process etc. when such matters are studied the opportunities to improve production practice by automation often become apparent and the economic justification can be assessed. for example, in an initial study we made of a biscuit production plant some 18 months ago, the following observations were made [2]. 1. one dough mixer was used to feed each biscuit production line so resulting in a system of 7 mixers feeding 7 lines. the mixers are batch operated and quite separable from the biscuit lines which are continuous flow processes. it was observed that the mixer utilisation was only about 20–50% of their total capacity when keeping all biscuit lines busy. it was thus apparent that by more precise scheduling and control of mixer operation – such as can be done by a computer (parttime use) – the number of mixers could be reduced from 7 to 4 with a potential saving in capital investment of about £ 60,000. 2. some defective biscuits are produced at each stage in the process, e.g. due to faulty dough mixing, malfunctioning of the dough sheeter or the cutters and disturbances in oven operation. also variation of biscuit dimensions causes packaging machine failures and consequent rejection of biscuits. the total production of defectives was estimated to be about 2.5% of production. a reduction of this by say 1% would be worth £ 15,000 per annum. 3. biscuit packages are sold to a specified weight and it is legally necessary that this weight be supplied as a minimum. thus, because of a random variation in individual biscuit weights the average package weight is greater than the weight specified. this excess is known as “give-away” and can be as high as 6–7% (6–10 drams per packet). this can be reduced by reducing the variance in biscuit weight and it is calculated that for each dram reduction in give-away per packet there is a saving to the company of £ 75 per ton of biscuit production. in a typical plant a reduction in give-away of 1.5 drams per packet is worth £ 20,000 per annum to the company. these observations stimulated the development of several in-process measuring devices and control systems. it was apparent that the manual method of sampling biscuits and measuring their weight and dimension by traditional means was unreliable and tedious. furthermore, the making of these measurements was clearly necessary to improve supervision and to give a basis for automatic control of the biscuit machine. consequently a device was developed for automatically sampling the biscuits as they emerge from the baking oven on a belt [3]. figure 1 shows this sampler. it picks up a stream of biscuits from the moving belt and measures their thickness, weight, ovality and colour (brownness) and puts the information through a data logger on to punched tape, while returning the measured biscuits on to the belt in the same place from which they were removed but some distance downstream. some hundreds of thousands of biscuits have been sampled through this machine and distribution curves of weight and dimension variance obtained by computer analysis of the data logger output. the immediate value of this sampler is to show how well or badly a biscuit line is behaving and to enable the effect of changes in process operation to he measured. it will also show the improvement in thickness and weight variation which we expect to get from an automatic thickness control system now being installed on the dough sheeter. ultimately, it will be the master instrument for supervision of several automatic process control systems to be installed on the dough sheeter and on the baking oven. acta imeko | www.imeko.org may 2014 | volume 3 | number 1 | 7 in a similar way we have made a study of a ceramic tilemaking plant which produces some 150,000 sq yds of glazed tiles per week. because of the variability of material, and process parameters, there is considerable production of defective tiles at each stage of the process which includes making green tiles from the clay mix, firing of the tiles and glazing. the percentage of defectives at each stage varies from 5–7% and a rough estimate of the total cost of defectives is about £ 180,000 per annum or about 7% of production costs. even a reduction in defectives of only one-quarter of the present value would be worth about £ 45,000 per annum. assuming a 25% per annum return on capital invested is required, this can justify a capital cost for measurement and control apparatus of £ 180,000. work is now being done to reduce defective production by better control of clay composition, by control of the presses and kilns and by inprocess measurement of the tile dimensions, shape and surface quality. a system for automatic control of the moisture content in clay dust is being developed and it is estimated that this will reduce the cracking of the tiles during firing and bring about a saving of £ 11,000 per annum. so we see that a first requirement for measurement is to ascertain what a production system is doing and so to identify where automation can be most profitably used. the next function of industrial instrumentation is to investigate individual process behaviour and determine the most significant variables and hence formulate a process control strategy. this is the function which has had most attention from control engineers and leads to the sort of process instrumentation and control familiar to us all. the thickness measurement and control of the biscuit dough sheet i referred to earlier is a typical example in this category. it is surprisingly similar to the problem of automatic thickness control in steel rolling mills. i would like to mention a few more unusual examples in this category each of which i think has a message to give. in the manufacture of fan blades for motor car engines the blades are spot welded on to a hub flange and it is common practice to do destructive examination of the welds of a sample of fans taken from the production line. this is wasteful and does not ensure that no defectives get through. in searching for a non-destructive test the british welding research association made a detailed study of the resistance spot welding process and found that while the weld is being made the electrodes are forced apart about ten thousands of an inch and then return as the molten nugget of metal is formed between the parts being welded [4]. figure 2. shows curves of separation of the electrodes for a splash weld with too solid heating (h), a good weld (n), and a weak weld with too little heating (l). it was observed that this curve had a particular form when a good weld was made. thus, it became possible to inspect a weld while it was being made by measuring the time taken for a particular magnitude of electrode separation to occur. this principle is now employed in a resistance weld inspection device which has been developed as an attachment to a welding machine. work is now proceeding to use tile principle as a base for automatic control of the resistance welding process. consider another example. in the manufacture of clay bricks the brick is formed by extruding clay into the required section and then cutting up into bricks ready for firing. it is important that the moisture content of the clay be consistent so as to obtain a uniform contraction of the brick during firing and so reduce incidence of cracking and deformation. for some time a method of measuring the moisture content of the clay has been sought but no really satisfactory solution found. some studies of the extrusion process were made by the ceramics research association and these showed that a measurement of back pressure behind the nozzle of tile extruder is a very good and consistent measure of tile moisture content of the clay. pressure is easy to measure with rugged apparatus suitable for a brick works. this has now been developed as a system for automatic control of the moisture content in tile clay mix. this is a much simpler solution than trying to measure true moisture content. these two simple examples illustrate the need for study of the technology of an industrial process to precede development of process instrumentation and control. they emphasise the responsibility that rests on the control engineer to familiarise himself with the technicalities and mode of operation of a manufacturing process before he applies his specialist technology. failure to recognise these requirements can result in valuable effort being applied to development of the wrong instrument for control of a process. a third function of industrial instrumentation which is of figure 1. automatic sampling machine installed on a biscuit line. figure 2. principle of operation of weld monitor. acta imeko | www.imeko.org may 2014 | volume 3 | number 1 | 8 rapidly growing importance is measurement or inspection of a product, either during the production process or on completion. the need for this can arise for a number of reasons. 1. to remove a limit imposed on production by shortage of labour for product inspection. 2. to provide faster detection of defective product made and so facilitate quicker correction of a faulty process and provide a basis for automatic process control. 3. to detect and remove from a sequence of processes a partially completed product which is defective and which may waste or damage the succeeding process if it is allowed to proceed. 4. to improve quality control of products. very often the inspection of components or products in a factory is one of the most labour intensive functions and is often a tedious and onerous task. consider for example the manufacture of metal coins which in most countries are high precision and high quality objects. it is common practice to inspect visually the blanks coming from the stamping machine to prevent defective blanks being fed to the coining presses where they may cause damage to expensive dies. a coinage plant may produce 20 million coins per week and one will usually find a team of 20 or more inspectors looking at blanks on a moving belt to detect any defectives. this task of visual inspection becomes increasingly expensive as labour pay rates rise and recruitment of suitable labour becomes increasingly difficult. this is an ideal task for the automatic inspection instrument and recently sira has developed a photoelectric device which will inspect the blanks at a speed of up to 40 per second and reject those which have a damaged edge or are out of tolerance on size. figure 3 shows the instrument. the coin blank is passed over an illuminated aperture behind which is an optical system which casts an image of the blank on a photocell. this photocell has a cathode consisting of four quadrants. if this blank is truly symmetrical with no edge damage there will be during the transit of the image over the four photocell quadrants some point at which there is a balance of photo currents in opposing quadrants. this balance point is detected electronically. if no balance is found due to a damaged edge the coin is rejected. this principle may be applied to check dimensions and shape of any two dimensional symmetrical objects. another interesting in-process inspection problem arises in the manufacture of glazed ceramic tiles. after firing the tiles are passed to the glazing plant. some of the defects on the glazed tiles can be attributed to surface defects on the uncoated tile and if these could be detected and rejected by a machine before glazing there could be a significant saving on production costs by avoiding the glazing of defective material. an apparatus is under development for this purpose. it is also aimed at replacing the large labour force required to inspect the finished tiles. numerous new requirements for automatic inspection are introduced as a result of progress in automation of assembly, packaging and filling operations. for example, the introduction of automatic filling machinery in the pharmaceutical industry to put drugs into vials, calls for much closer tolerances to be maintained on the vial dimensions, and so the need arises for automatic dimension checking of the empty vials. furthermore, when the filled vials are packed by machine into cartons these need to be inspected automatically for misfills or particulate matter in the filling. such inspection is done by the human packer as a simultaneous task, but when he or she is replaced by the packaging machine, automatic inspection becomes necessary. this situation arises in many different industries. while packing wrapped confectionery products into cartons the human packer simultaneously checks the quality of the product wrap. however, when the machine does this packing a device for checking the wrap must be introduced. automatic assembly of a wide range of engineering products like watches, gear boxes, electricity meters, electric motors, cycle wheels etc. is being introduced [5]. this is going to impose much stricter discipline on component dimensions and quality and will make objective inspection of components essential. in the simple case of introducing automatic bottle capping machines it becomes necessary to inspect and control the dimensions of the bottle necks and tops much more precisely than is required with manual capping. an automatic assembly machine putting screws in must be protected from being fed with screws without slots in the head. automatic inspection of such things as the surface of tinplate or metal foil or paper presents interesting problems because the decision of acceptance or rejection depends on a judgment which has to take into account the type, size and distribution of defects. detection of the defects is not usually difficult with available technology, and at the high production speeds now common with these materials. the real problems lie in processing the signals from the defect detectors so as to make decisions automatically on suitability or otherwise for the particular use for which the product is required. for this problem we hope to get help from those working on pattern recognition techniques. another problem of product inspection which i think is of great potential interest is in checking items produced in large quantities such as postage stamps, currency notes or similar figure 3. apparatus for detecting defective coin blanks. acta imeko | www.imeko.org may 2014 | volume 3 | number 1 | 9 complex patterns in multicolour print. i doubt if this problem is likely to be solved by traditional scanning methods in which an enormous amount of information is generated in the scan and then compared with a reference standard. it is more likely to be an area of opportunity for techniques using image transformation such as holography. 5. the changing approach to industrial instrumentation measurement technology for use in industry can no longer be treated as a specialisation separable from the situation in which it is utilised. it has become a vital and interwoven constituent in the overall technology of operation and control of an industrial manufacturing process. this has considerable repercussion on the activities of an organisation engaged on instrument development and manufacture and on the training of the instrument technologist. the situation one sees developing is this. the manufacturing and processing industries, as users of instruments, will employ increasing numbers of control engineers. these engineers will concern themselves with improving the control of operation of the processes in their factory so as to improve the efficiency of operation of tile production system as a whole. they will also influence the design and specification of the plant installed to make it inherently more controllable. these control engineers will thus work closely with process technologists, production engineers and plant designers. we also see evidence now of management structure having to be changed in order to permit the fullest utilisation of computers for improved production organisation. it is to the control engineer that the instrument developer will look for advice on the potential usefulness of a new device. i believe the control engineer will act as a vital link between the instrument maker and industrial user. it is due to the too frequent absence of this link that the enthusiastic instrument developer has often been misguided in the past with consequent misplacement of his inventive abilities and the failure of instrument designs to suit the conditions of use. at present very few user industries employ control engineers and this has made it necessary for instrument manufacturers to extend their activities into control engineering and systems design. to do this they have had to learn much of the business and technology of their customers’ industries so that they can offer viable control systems. they have managed to do this for large customer industries, such as steel, chemicals, paper and such like but it is costly for them and particularly difficult to finance as the profit margins on instrument manufacture are modest. the situation is even more difficult when related to the diverse range of smaller unit industries which may in total offer a large potential market but individually offer only small markets for instruments. for these industries there is a need for cooperative work on the development of control technology specific to them. thus, one expects the set up of automation research and development organisations, oriented to the interests of specific potential user industries such as textiles, ceramics, plastics, timber, light engineering etc. within these organisations the special instrument development required to implement control systems will be carried out as part of an activity which encompasses systems engineering and process control engineering. as time progresses these organisations will infuse into their industries control engineers and a philosophy of the significance of control. when that is achieved the job of the industrial instrument manufacturers will be greatly eased. they will be able to devote their research and development resources to the development of the next generation of instruments rather than having to devote it to educating their potential customers in the significance of control and to solving their application engineering problems for them. references [1] s.s. carlisle, getting automation applied, instrument practice, july 1965. [2] r.g. parish, p. wade, d.a. watkin, application of computers and automation techniques to biscuit manufacture, sira confidential research report r339, 1965. [3] p. wade, d.a. watkin, development of some instrumentation for monitoring the production of semi-sweet biscuits, food trade review, april 1966, p. 48. [4] p.m. knowlson, instrumentation for resistance welding, british welding journal supplement, april 1965, p. 183. [5] institution of production engineers journal, volume 43, no. 3, march 1967. bias-induced impedance effect of the current-carrying conductors acta imeko issn: 2221-870x june 2021, volume 10, number 2, 88 97 acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 88 bias-induced impedance effect of the current-carrying conductors sioma baltianski 1 1 wolfson dept. of chemical engineering, technion – israel institute of technology, haifa, israel section: research paper keywords: bias-induced impedance; impedance spectroscopy; zbi-effect citation: sioma baltianski, bias-induced impedance effect of the current-carrying conductors, acta imeko, vol. 10, no. 2, article 13, june 2021, identifier: imeko-acta-10 (2021)-02-13 section editor: giuseppe caravello, università degli studi di palermo, italy received january 18, 2021; in final form april 15, 2021; published june 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: sioma baltianski, e-mail: cesema@technion.ac.il 1. introduction the study of various physical objects using impedance spectroscopy under applied dc bias is widespread. these objects may be of different physical nature: semiconductor structures [1], electroceramic structures [2], [3], electrochemical objects [4], etc. in all cases, the external offset sets the operating point in the vicinity of which the impedance measurements are made. the set offset (bias) makes it possible to tie the parameters obtained from impedance measurements to the physical state of the object under study. the idea to investigate the current-carrying conductors under the influence of bias arose because of a misunderstanding of the behavior of a fairly simple object as a load in the process of testing the different impedance meters. the first research results were published in [5]. possible measurement errors were checked in various ways, and literature sources describing the detected effects were not found. this work relates to impedance spectroscopy for several reasons. on the one hand, based on the described phenomena, sensitive elements can be created that require the use of impedance spectroscopy as a method for extracting informative parameters. on the other hand, it is a challenge to build more sensitive impedance meters (with an appropriate offset function) that allow obtaining reliable data under relatively difficult measurement conditions: low frequency and high tangent of the loss angle. the goal of this work is to reveal the discovered properties, which are important in the context of impedance research. low-frequency impedance spectroscopy was used as a method. the complex conductivity function and its determination by approximating experimental data utilising different models serve as theoretical basis [6]-[8]. the impedance of current-carrying conductors is well known and described in the literature [9]. as an example, we give the behaviour of the impedance of a silver conductor 0.5 m long and 0.25 mm in diameter. the initial experiment in figure 1 without bias (at vdc = 0 v) represents a typical behaviour of the real and imaginary parts of the impedance in the frequency range 1 mhz … 100 mhz. first, we use the full frequency range to verify the processes. later, we will be interested in events only in the low abstract the paper presents the previously unstudied properties of current-carrying conductors utilising impedance spectroscopy. the purpose of the article is to present discovered properties that are the significant context of impedance research. the methodology is based on the superposition of test signals and bias affecting the objects under study. these are the main results obtained in this work: the studied objects have an additional low-frequency impedance during the passage of an electric current; the bias-induced impedance effect (zbieffect) is noticeably manifested in the range of 0.01 hz … 100 hz and it has either capacitive or inductive nature or both types, depending on the bias level (current density) and material types. the experiments in this work were done using open and covered wires made of pure metals, alloys, and non-metal conductors, such as graphite rods. these objects showed the zbi-effect that distinguishes them from other objects, such as standard resistors of the same rating, in which this phenomenon does not occur. the zbi-effect was modeled by equivalent circuits. particular attention is paid to assessing the consistency of experimental data. understanding the nature of this effect can give impetus to the development of a new type of instrument in various fields. mailto:cesema@technion.ac.il acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 89 frequency part of the spectrum. the initial and all subsequent potentiostatic experiments were carried out at the amplitude of the test signal vac = 10 mv. this signal corresponds to a small signal approach. the view of graph results is quite trivial at zero bias. three approximate frequency domains can be distinguished in this graph: the high frequency region (hf) of the spectrum f = 1 mhz … 100 khz; the mid frequency region (mf) f = 100 khz … 100 hz and the low frequency region (lf) f = 100 hz … 0.1 hz. in the hf region there is an increase of a real re(z) and imaginary im(z) part of impedance with increasing frequency. this part is well described by a parallel connection of resistance rp and inductance lp (figure 1). in the mf region, a constant value of re(z) is observed. series resistance rs must be added to the model. a linear decrease in the imaginary component occurs with a decrease in frequency (log-log scale). the relative noise level increases at the same time. this noise is natural and associated with the measuring system capabilities. the lf part of the spectrum at vdc = 0 v demonstrates the constancy of re(z) and the strong noise of im(z). this area is not informative for interpretation using the imaginary part of the impedance. measurements were made using a faraday cage to improve the signal-to-noise ratio mainly for the imaginary impedance component. in this simple model, the specific resistance of the conductor determines the series resistance rs. mainly the length of the conductor determines the inductance lp. the parallel resistance rp that is connected to the inductance characterises the active loss in the conductor due to the skin effect at high frequencies. the experimental values at zero bias correspond to the expected values and are quite common. the situation changes significantly when measurements are carried out under bias. the experimental characteristics are shown in the same figure 1 at biases vdc = 0.09 v … 0.9 v. the increment of bias was 0.09 v, and the measuring test signal was the same, namely vac = 10 mv. the hf and the mf imaginary part of the impedance do not change with bias. however, the lf part changes considerably. this response can be reflected by including an additional non-linear impedance zbi (figure 1). a corresponding increase of the real part of the impedance occurs in the considered region, which meets the kramers–kronig relationships [6]. besides, we observe a monotonic change in the real component of the impedance in the entire frequency range (the model element rs). this change is caused by a shift in the temperature of the conductor due to biases. the model indicated in figure 1 is intuitive, but it well describes and fits the object under study in the specified frequency range. the bias phenomenon is sometimes difficult to detect because experimental data are often on the limit of the sensitivity of measuring instruments, namely, limitations on phase measurement with a high value of the loss tangent. potentiostat/galvanostat biologic sp-240 (biologic science instruments) was used as a measuring instrument. in doubtful cases, data were verified using the potentiostat/galvanostat gamry reference 3000 (gamry instruments). a biologic’s contour plot defines an error not more than 0.3% and 0.3° in the desired measuring range [10]. thus, the experimental data in figure 1 is reliable. moreover, in this case, we are interested in relative changes in the impedance components. a homemade four-wire sample holder was used to connect the samples under test (sut), as shown in figure 2. electrode designation is taken from the manual [10]. this sample holder gives accurate measurement of low-resistance objects as well as negligible influence of contact phenomena. in addition to the devices used in this work (biologic and gamry), which are based on frequency response analyzer, also the devices of the lock-in amplifier type can be used (see for example [11]). we used standard not wire wound resistors as references to verify measuring results and estimate artifacts. this phenomenon was not observed when dealt with standard resistors of the same rating as sut. the impedance change in the lf region can be caused not only by bias using direct current but also by alternating current – by a large amplitude test signal at zero bias. it should be emphasised that in this work, we use a small signal approach in which a change in the lf impedance is not observed at zero bias. thus, the occurrence of the additional impedance in the lf region will be determined solely by the level of bias. this physical phenomenon is named here as a bias-induced impedance (zbieffect). in section 2 we will systematise the experimental results. section 3 is devoted to the interpretation of experimental results lo g ( f r e q /hz ) 50 lo g ( |r e ( z ) /o h m |) 0 -1 -2 -3 -4 -5 lo g ( |im ( z ) /o h m |) 0 -1 -2 -3 -4 -5 re(z) at vdc = 0 v im(z) at vdc = 0 v rs rp lp vdc t zbi lo g (| r e (z )/ o h m |) lo g (|-im (z )/o h m |) log(freq/hz) figure 1. re(z) and im(z) of the silver wire at vdc = 0 v … 0.9 v with bias step 0.09 v; length 500 mm and diameter 0.25 mm; frequency range 1 mhz … 0.1 hz. figure 2. four-wire sample holder. 1 high force-and-sense kelvin clip; 2 – sample under test (wire); 3 – working electrode; 4 – counter electrode; 5 low force-and-sense kelvin clip; 6 – reference electrode; 7working sence electrode. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 90 using electrical models. section 4 discusses significant differences in impedance behavior of open and covered objects. special attention is paid to checking the consistency of experimental data: this is outlined in the section 5. section 6 discusses and proves the main hypotheses that explain the revealed effect. finally, the main results are presented in the concluding section. 2. systematisation of the experimental results we studied pure metals: nickel, copper, silver, tungsten, platinum, gold; alloys: constantan, nichrome, manganin; nonmetals graphite rods. although the frequency scan started from 1 mhz toward low frequencies, the analysis of the results was carried out only for the lf part of the spectrum, where the zbieffect manifested. according to the type of zbi-effect, all studied materials were grouped into three categories: (i) the zbi-effect has a capacitive nature, (ii) an inductive nature, and (iii) mixed when both types of reactance occur. table 1 summarises the properties of the investigated materials. below are the experimental characteristics of one of the representatives of each group. 2.1. pure metals we find significant changes in the behaviour of the imaginary part of the impedance after a critical frequency of about 30 hz (resonance point) when applying bias (figure 1). the inductive nature of reactance sharply changes to a capacitive nature from this point to the direction of lf. we observe a monotonic change in the imaginary and real component of the impedance, depending on the applied bias. changes in the real part of impedance in the mid frequency region also occur; however, this is due to a change in the temperature of the conductor upon bias. for example, with a maximum bias of 0.9 v for this experiment and a conductor resistance of about 0.23 ω, the current flowing through the conductor will be approximately 3.9 a. the power dissipation will be approximately 3.5 w, which will lead to certain heating of the conductor and consequential increase in its resistance. figure 3 shows a nyquist graph of the lf part of the same experiment, which is shown in figure 1. at mid and high frequencies, there is no change in the behaviour of the imaginary component of the impedance under the influence of bias. henceforward, we will limit visualisation within the lf part of experimental data in the form of nyquist plots. similar in appearance, but numerically different in values, the characteristics were obtained in studies of other pure metals: nickel, copper, tungsten, platinum, and gold. 2.2. alloys the impedance characteristics of alloys as a function of bias differ from pure metals. manganin demonstrated the inductive nature of reactance at moderate bias. in our nichrome and constantan samples, the zbi-effect had both capacitive and inductive reactance. the nature of the reactance depends on the level of bias. as an example, studies of a nichrome sample with a diameter of 0.1 mm and a length of 57 mm are presented in figure 4. the experiment was carried out using a bias in the range of 2.8 v… 8.5 v in increments of 0.1 v. the data were taken in the frequency spectrum 1 mhz … 0.1 hz, but only the range of interest is presented here: 100 hz … 0.1 hz. three areas of bias were identified. in the bias of 2.8 v … 4.6 v, the capacitive nature of the reactance was observed. in the range of 4.6 v … 6.7 v, the increasing portion of the inductive nature of the reactance added to the decreasing portion of the capacitive nature of the reactance. with a subsequent increase in bias, the reverse process occurs. also, in the bias range of 6.7 v … 8.5 v, the capacitive nature of the reactance was again re ( z ) / oh m 0.30.250.20.15 im ( z ) /o h m 0.08 0.06 0.04 0.02 0 -0.02 -0.04 -0.06 -0.08 z at vdc = 0 v z at vdc = 0.9 v f < 3 0 h z f > 3 0 h z -i m (z )/ o h m re(z)/ohm figure 3. nyquist plot of silver wire at vdc = 0 v … 0.9 v with bias step 0.09 v; length 500 mm and diameter 0.25 mm. re ( z ) /oh m 9.59 im ( z ) /o h m 0.3 0.2 0.1 0 -0.1 -0.2 -0.3 -0.4 -0.5 z at vdc = 6.7 v z at vdc = 4.6 v re(z)/ohm -i m (z )/ o h m figure 4. nyquist plot of nichrome wire at vdc=4.6 v … 6.7 v with bias step 0.1 v; length 57 mm and diameter 0.1 mm. table 1. systematisation of the investigated materials by the nature of zbieffect. type of conductors pure metals alloys non-metals (graphite) nature of zbi-effect capacitive mixture: capacitive and inductive inductive acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 91 observed, the same as with a small bias. figure 4 shows a transient state when both types of reactance are present. 2.3. non-metals measurements were carried out on graphite rods. samples of various diameters were investigated. figure 5 shows nyquist plots of the impedance of the graphite rod 0.5 mm in diameter and 57 mm in length. the inductive nature of reactance was demonstrated over the entire range of biases. 3. interpretation using electrical models first, we consider a simple case of interpreting experimental data related to pure metals in which the zbi-effect of capacitive nature is manifested. as an example, figure 6 presents a fitting result of the lf part of one of the experiments shown in figure 3, specifically at bias vdc = 0.72 v. the fitting was carried out using the impedance model which consists of serial resistor rs connected to parallel c1 and r1. the resistor rs reflects specific resistance of the sample under test and its geometry. the resistance varies with the applied bias, which affects the temperature of the sample (see a right shift of characteristics in figure 3 with increasing bias). the parallel circuit c1-r1 exactly describes the zbi-effect. figure 6 shows a good fitting quality. a similar approach for fitting can be used for materials in which the zbi-effect is purely inductive (figure 5) by using the lr circuit. the situation becomes more complicated in the case of a complex zbi-effect (figure 4). one of the possible electrical models that satisfactorily approximate the experimental data is embedded in figure 7. a system function as a rational fraction [12] that corresponds to this model has the following form: 𝑍(𝑠) = 𝐴0 + 𝐴1𝑠 + 𝐴2𝑠 2 1 + 𝐵1𝑠 + 𝐵2 𝑠 2 , (1) where 𝑠 = 𝑗 2 π 𝑓 and 𝐴𝑖, 𝐵𝑖 are unknown coefficients. although the system function uniquely approximates the experimental data, its coefficients are difficult to fill with a physical meaning. it is easier to do this using circuit functions which reflect the topology of the corresponding equivalent circuits [12]. the circuit function corresponding to the model in figure 7 is described by the following equation: 𝑍(𝑠) = 𝑅𝑠 + 𝑅1 1 + 𝜏𝐶 ⋅ 𝑠 + 𝐿1 ⋅ 𝑠 1 + 𝜏𝐿 ⋅ 𝑠 , (2) where: 𝜏𝐶 = 𝑅1 ⋅ 𝐶1; 𝜏𝐿 = 𝐿1/𝑅2 and rs, r1, r2, c1, l1 requested parameters from fitting. the system function (1) covers several equivalent circuits. figure 7 represents one of the possible implementations. the results of fitting utilising this circuit for one of the characteristics represented in figure 4, specifically at bias vdc = 5.3 v, are given in figure 7. the selection of a suitable electrical model can be made empirically by iterating through the available set of models determined by the system function (1). to implement this process, experimental data can be approximated using an acceptable set of equivalent circuits and utilising available fitting programs, such as levm [6] or method described in [13]. figure 8 represents the dependencies of the model parameters versus bias of the silver wire corresponding to the data shown in figure 3. re ( z ) /oh m 1.31.2 im ( z ) /o h m 0.05 0 -0.05 -0.1 -0.15 z at vdc = 0v z at vdc = 1v f > 1 0 0 h z -i m (z )/ o h m re(z)/ohm figure 5. nyquist plot of graphite rod at vdc=0 v … 1 v with bias step 0.1 v; length 57 mm and diameter 0.5 mm. r1 rs c1 zbi -i m (z )/ o h m re(z)/ohm figure 6. fitting result of lf part of data (f = 10 hz … 0.1 hz). silver wire at vdc=0.72 v: rs=0.214 ω; r1=0.07 ω; c1=21.34 f. r2 rs c2 zbi r1 l1 -i m (z )/ o h m re(z)/ohm figure 7. fitting result of lf part of data (f = 10 hz … 0.1 hz). nichrome wire at vdc = 5.3 v: rs = 8.261 ω; r1 = 0.622 ω; c1 = 0.373 f; r2 = 1.018 ω; l1 = 1.091 h. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 92 the capacitance с1 increases exponentially with decreasing bias. this leads to a decrease in the contribution of the reactive component into the zbi-effect. at the same time, a monotonic decrease in resistance r1 is observed. as a result, at zero bias, the zbi-effect demonstrated by the vanishingly small magnitude. the resistance rs reflects a change in resistivity as a function of temperature, which in turn depends on the flowing bias current. the temperature value (it got by utilising resistivity) and power dissipation for this experiment shown in figure 9. similar results, but differing in values, were obtained for other pure metals. the graphite rod model behaves quite differently as shown in figure 10. first, the series resistance rs decrease with bias due to a negative temperature coefficient (ntc). it distinguishes them from metal in which there is a positive coefficient of resistance (ptc), see figure 8. secondly, the inductance l1, together with the parallel resistance r1, decreases with decreasing bias. this nullifies the zbi-effect at zero bias. the behaviour of model parameters for alloys is more complex and is beyond the scope of this article. 4. the difference between open and covered objects some of the previously investigated objects were studied using various types of cover. this is necessary to validate the hypotheses put forward to explain the occurrence of the effect as described in section 6. the results of the study of currentcarrying conductors covered with dielectric materials are quite informative for this purpose. here, we present the results employing a shell in the form of thin teflon and ceramic (alumina) tubes that is quite tight adjacent to the object under study. it is necessary to expand the frequency range towards lower frequencies down to 1 mhz to detect the effect of covering onto the impedance results. this requirement significantly increased the time of each experiment. figure 11 shows the real and imaginary parts of the impedance without covering. we used the galvanostatic mode of the measurement device (biologic sp-240) and the same sample holder as previously. the full range of bias current was idc = r1 rs c1 zbi 0.72 v vdc/v r /o h m c /f figure 8. model parameters of silver wire as a function of bias refer to the experiment data in figure 3. vdc/v t p /w figure 9. power dissipation and temperature of silver wire refer to the experiment data in figure 3. rs zbi r1 l1 vdc/v l /h r /o h m figure 10. model parameters of graphite rod as a function of bias refer to the experiment data in figure 5. l o g ( f r e q / hz ) 50 lo g ( |r e ( z ) /o h m |) 0 -1 -2 -3 -4 -5 lo g ( |im ( z ) /o h m |) 0 -1 -2 -3 -4 -5 lo g (| r e (z )/ o h m |) lo g (-|im (z )/o h m |) log(freq/hz) figure 11. re(z) and im(z) of the open silver wire; galvanostatic mode at amplitude iac = 100 ma and bias idc = 0 a … 4a with step 0.4 a; length 500 mm and diameter 0.25 mm; frequency range 1 mhz … 1 mhz. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 93 0 a … 4 a with steps 0.4 a, and the test signal was iac = 100 ma (it satisfies the low signal approach). figure 12 and figure 13 show the results of the silver wire inside alumina and teflon tubes, respectively. a comparison between these experiments in the form of a nyquist plot at one of the biases (at idc = 2 a) is shown in figure 14. it can be seen with the naked eye that an open wire has one time constant while the covered conductors have two time constants. yet teflon covering shows more overlapping and more distributed impedance spectra. the fitting of the low-frequency part of the data (100 hz … 1 mhz) using the equivalent rc-circuits is also presented in figure 14. the fitting results are summarised in table 2. the indexes in the equivalent circuits in the table have the following meaning: p parallel connection and s serial connection. evaluating the fitting results, we can say that these objects are quite satisfactorily approximated by models utilising lumped elements. a better result can be obtained for the case of a conductor surrounded by teflon using the gaussian distribution function, convolved into impedance [14]. but in this case, to demonstrate the zbi-effect when exposed by the covering, this is not essential. a simple calculation of the ratios of the time constants (τ = rc) taken from table 2 gives the following values: τ2 / τ1 = 247 in the case of alumina covering and τ2 / τ1 = 40 in the case of teflon covering. let us cite as an example the behaviour of an object exhibiting an inductive nature of zbi-effect also in a free and covered state. figure 15 shows resulting nyquist graphs of a graphite rod at bias idc = 0.9 a and the test signal iac = 100 ma in the open state and covered with alumina and teflon tubes. it is important to emphasise that the imaginary part has the opposite sign compared to the previous graphs of the silver wire. the fitting results are summarised in table 3. the ratio of the two time constants (τ =l/r) for the graphite rod covered by alumina is τ2 / τ1 = 284. in the case of teflon covering, the ratio is τ2 / τ1 = 22. the difference in the ratios of the time constants is similar to that found earlier in experiments with silver wire covered with the same materials. 5. check of the data consistency current-voltage characteristics were acquired on the same samples to check a data set for internal consistency. a sweep rate of 1 mv/s which is reasonable to our low frequency 0.1 hz (in the first studies) was selected. this speed allows getting quasistatic characteristics. static and differential parameters, namely resistances, were calculated and compared with parameters obtained from impedance measurements. l o g ( f r e q / hz ) 50 lo g ( |r e ( z ) /o h m |) 0 -1 -2 -3 -4 -5 lo g ( |im ( z ) /o h m |) 0 -1 -2 -3 -4 -5 lo g (| r e (z )/ o h m |) lo g (-|im (z )/o h m |) log(freq/hz) figure 12. re(z) and im(z) of the silver wire inside the alumina tube. the same experimental conditions as pointed in figure 11. l o g ( f r e q / hz ) 50 lo g ( |r e ( z ) /o h m |) 0 -1 -2 -3 -4 -5 lo g ( |im ( z ) /o h m |) 0 -1 -2 -3 -4 -5 lo g (| r e (z )/ o h m |) lo g (-|im (z )/o h m |) log(freq/hz) figure 13. re(z) and im(z) of the silver wire inside the teflon tube. the same experimental conditions as pointed in figure 11. re(z)/ohm -i m (z )/ o h m figure 14. nyquist plot of experimental and fitted data of non-covered silver wire, the wire inside alumina, and the wire inside teflon. low frequency part of data: 100 hz … 1mhz at the bias idc = 2 a. table 2. fitting results of non-covered silver wire, the wire inside alumina, and the wire inside teflon (according to figure 14). sut equivalent circuit fit parameters: r/ohm; c/f open wire (r1p + c1p)s + rs r1p = 0.032; c1p = 64.27 rs = 0.185 wire inside alumina (r1p + c1p)s + (r2p + c2p)s + rs r1p = 6.0e-3; c1p = 42.3 r2p = 0.025; c2p = 2.48e3 rs = 0.214 wire inside teflon (r1p + c1p)s + (r2p + c2p)s + rs r1p = 6.2e-3; c1p = 91.8 r2p = 0.013; c2p = 1.86e3 rs = 0.179 acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 94 the i-v characteristics of a silver sample are shown in figure 16. the parameters of the sample under test correspond to the parameters indicated in figure 3. the setup for i-v measurements was identical to the setup for impedance measurements. figure 16 represents i-v curve, static resistance rstat = vdc/idc and differential resistance rdiff = d(vdc)/d(idc). parabolic spline interpolation was used for analytical differentiation. a fairly good accordance was obtained between the model parameters extracted from impedance measurements and the parameters calculated from the current-voltage characteristics. the parameter rs extracted from the impedance zbi and the rstat extracted from the i-v well fit each other in an error of not more than 0.3%. the total resistance rsum=rs+r1 found from the impedance measurements corresponds to the resistance rdiff calculated from the i-v (figure 16). as an example, the bias point vdc = 0.72 v was taken in figure 16 for indicating these correlations. it is corresponding to this bias point in figure 6, figure 8 and figure 9. the dependence of power dissipation pi +δp on the influence of bias and the test signal at an operating point i will have the form: 𝑃𝑖 + 𝛥𝑃 = (𝑉𝑖 + 𝛥𝑉) ⋅ (𝐼𝑖 + 𝛥𝐼), (3) therefore, changes in power due to only the test signal is determined as 𝛥𝑃 = 𝑉𝑖 ⋅ 𝛥𝐼 + 𝐼𝑖 ⋅ 𝛥𝑉 + 𝛥𝑉 ⋅ 𝛥𝐼 , (4) where: vi, ii voltage and current at a working point and δv, δi – amplitudes of voltage and current of the test signal. from (4), it can be noticed that with increasing bias the dissipated power caused by the test signal will increase. hence, the temperature variation will increase, which will lead to an increase in a change of the resistivity by an influence of the test signal. this is an explanation of the magnification of the zbi-effect at all the experiments with increasing a bias. the most mysterious case is the presence of both types of reactivity in experiments with alloys (figure 4 and figure 7). it required an additional consistency check through an independent method. for these purposes, current-voltage characteristics with different sweep rates were used. the sweep rates were chosen to match the transient point around 0.5 hz (figure 7). the corresponding graphs of the i-v characteristics are shown in figure 17. the setup was the same as for impedance measurements. figure 18 shows the graphs of the calculated values of the static resistances rstat obtained from the i-v characteristics with different sweep rates (figure 17). now we can see that, in the vicinity of 5.3 v bias voltage, the static resistance changes the trend from decreasing its value with increasing a bias at the sweep 0.1 mv/s to increasing its value with increasing a bias at the sweep 100 mv/s. the sign of the -i m (z )/ o h m re(z)/ohm figure 15. nyquist plot of experimental and fitted data of non-covered graphite rod, the rod inside alumina, and the rod inside teflon. low frequency part of data 100 hz … 1 mhz at the bias idc = 0.9 a. table 3. fitting results of the non-covered graphite rod, the rod inside alumina, and the rod inside teflon (according to figure 15) sut equivalent circuit fit parameters: r/ohm; l/h open rod (r1p + l1p)s + rs r1p = 0.160; l1p = 0.343 rs = 1.09 rod inside alumina (r1p + l1p)s + (r2p + l2p)s + rs r1p = 0.01; l1p = 1.33e-3 r2p = 0.121; l2p = 4.572 rs = 1.128 rod inside teflon (r1p + l1p)s + (r2p + l2p)s + rs r1p = 0.041; l1p = 0.027 r2p = 0.095; l2p = 1.38 rs = 1.041 r e s is ta n c e /o h m voltage/v c u rre n t/a figure 16. i-v curve; static and differential resistance curves of silver wire with the same dimensions as in figure 3. c u rr e n t/ a voltage/v c u rr e n t/ a voltage/v figure 17. i-v curves with voltage sweeps: 0.1; 1; 10 and 100 mv/s. nichrome wire, the same dimensions as pointed in figure 4. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 95 differential resistance, which is the essence of impedance measurements, will change accordingly. thus, there is a consistency between measurements in the frequency domain (figure 7) and time domain (figure 18). several temperature studies were carried out to check the data consistency. the platinum conductor was heated by a separate heating element at various biases. figure 19 shows the results of the lf part of data 100 hz – 0.1 hz at different values of temperature controller: at room temperature, at 165 c°, and 265 c°. the actual temperature of the wire can be calculated considering the resistivity of the wire – via real part of impedance at frequency 100 hz or using dc – measurement. for qualitative analysis, knowing the actual temperature of the wire is not essential in our case. the main results of this experiment are as follows. we see that the zbi-effect does not occur in the absence of bias at any temperature. at the same time, this effect takes place in the presence of a bias at any temperature. it is also noticeable that the external temperature additively shifts the values of the real impedance component, almost without affecting the imaginary one. a good illustration in this graph is the occasion when, in the case of room temperature and bias idc = 2 a, the actual conductor temperature is practically identical with the case when the temperature controller value shows 265 c° without bias applied to the conductor (idc = 0 a). 6. discussions the impedance of quite ordinary current conductors under bias in the low-frequency region demonstrates remarkable properties named as zbi-effect. the term "current-carrying conductor" refers to extended conductors designed to carry electric current. the research that is outlined in this article is also relevant to other types of objects. these can be conductive or semiconductive materials of various compositions and shapes. an example relates to experiments carried out with thermistors (they are out of the scope of the paper). the zbi-effect is counter-intuitive. it was natural to assume that the measurement of the impedance of objects at the lowest frequency brings it closer to the measurement result at direct current. this is what happens in the absence of bias. yet, if a bias is applied to an object the feeling deceives. it looks paradoxical, but the infra-low frequency is not an asymptotic approximation to dc when measuring the impedance of a conductor under bias. now, the question of how to explain the occurrence of such significant reactive elements in the impedance models of the studied objects. in particular, the capacitance of pure metals reaches the order of farads (figure 8). the inductance of graphite rods reaches about hundreds of millihenry (figure 10). in reality, of course, such reactance does not exist in the studied objects. this phenomenon may be called as a “phantom” reactance. this effect can be explained considering two necessary properties of the studied objects. the first is nonlinearity and the second one is inertia. the nonlinearity of current conductors is the second kind of nonlinearity (indirect). this property distinguishes them from objects of the first kind of nonlinearity (direct), such as p-n junctions or schottky diodes. in the case of the first kind of nonlinearity, the nonlinearity reveals itself directly, without any delay. for the second kind of nonlinearity, it manifests itself due to a resistivity dependence on temperature. this is a factor of the studied material. the nonlinearity of the second kind has a significant delay. the bias sets a specific operating point. the test signal acts in the vicinity of this point. no matter how small the test signal is, it will change the temperature of the investigated object in the locality of the operating point with a certain delay. consequently, the resistance of the material under investigation will cyclically change with the test signal. eventually, the resistance is modulated by the test signal. the difference between the phase of this modulation and the phase of the acting test signal determines the occurrence of phantom reactance. if the investigated object has a ptc property, a capacitive reactance arises. this behavior is typical for pure metals or ptc thermistors. if the object under study has an ntc feature, then an inductive reactance appears. it is specific, for example, to graphite and ntc thermistors. in terms of electrical measurements, impedance is properly defined only for systems satisfying stationarity [6]. in our case, we have a dynamic structure with one exception the system changes cyclically and synchronously with the influence of the test signal. the amplitude and phase response depends on the frequency of the test signal. a purely active resistance, which changes are synchronously according to the test signal, but with r e si st a n ce /o h m voltage/vvoltage/v r es is ta n ce /o h m figure 18. static resistances calculated from the i-v curves represented in figure 17. -i m (z )/ o h m re(z)/ohm figure 19. nyquist plot of platinum wire at different temperatures and biases; length 83 mm and diameter 0.2 mm; frequency range 100 hz … 0.1 hz. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 96 a different phase relative to the test signal, generates a reaction that looks like a complex resistance. as a result, a complex value will be estimated during the measurements as an impedance of the studied object. successive experiments revealed a significant feature. it turned out that the time constant following from the zbi-effect’s model (τ = rc for ptc objects and τ = l / r for ntc objects) weakly depends on the applied bias. it is reasonable to assume that these time constants related to the time constant of the heat exchange between the object under study and the environment (air in our initial study case). studies using a various covering of current-carrying conductors support this hypothesis. there are two time constants (figure 12 and figure 13). the first of which is apparently determined by the thermal properties of the covering and the thermal interaction between the conductor and the covering. the second time constant is determined by the thermal interaction between the covering and the environment. an accurate description of thermal processes requires special knowledge and is beyond the scope of this article. however, it seems possible that the discovered effect could allow to develop specialised sensors to assess the thermal conductivity of various materials. this approach may be an alternative to the methods described in [15], [16]. the experimental results obtained on the nichrome alloy motivates the appearance of additional ideas. in particular, both capacitive and inductive reactive components are observed at a bias vdc = 5.3 v (figure 7) and in the areas close to it (figure 4) depending on the frequency of the test signal. specifically, for figure 7 capacitive nature takes place in the frequency range of 10 hz to 0.5 hz, and inductive nature takes place in the range of 0.5 hz to 0.1 hz. such effects are possible if we assume that the temperature coefficient of resistance (tcr) has dynamic properties. in other words, the tcr changes its character depending on the rate of temperature change. in turn, the rate of temperature variations at the selected operating point will be related to the frequency of the test signal. therefore, in the higher frequency range will be observed a ptc-feature, in the lower frequency range will be observed an ntc-feature (figure 7). this assumption was confirmed using i-v experiments with different sweep rates (figure 17 and figure 18). it looks like impedance spectroscopy may provide a more sensitive tool for assessing the dynamic properties of the tcr. since the dynamic properties of the tcr will depend on the composition of the objects under study, there is a potential possibility of indirect composition estimation by checking the moment of the tcr sign change utilising impedance spectroscopy. the revealed new properties (i.e., the possibility of evaluating the thermal conductivity and estimating the composition of the material by dynamic tcr, arising from the discovered zbi-effect) may represent a significant contribution to the scientific and technical community, in particular, in the development of the theory and practice of impedance spectroscopy of objects that cyclically change their parameters synchronously with the test signal. understanding the nature of this effect can foster the development of a new type of instruments in various fields and various scientific institutions. 7. conclusions in this work, the phenomenon of bias-induced impedance was described. this effect was most evident in the low frequency spectra of the reactive part of the impedance. the manifestation of the different nature of this issue was shown experimentally. the zbi-effect may be capacitive, inductive, or complex, which includes both types of reactance. the nature of the reactance depends on the type of test material. pure metals showed capacitive reactance. graphite rods showed inductive reactance. the alloys showed reactance of both types depending on the level of bias. the investigated objects can be attributed to the inertial nonlinear resistances. the zbi-effect is caused by the thermal interaction between the conductor and the environment under the superposition of the bias combined with a test signal. relatively simple equivalent circuits were found to describe the experimental data. additional studies should be undertaken to better understand the behaviour of alloys and other composites under the bias, especially unexpected dynamic tcr properties. new possibilities arise for assessing the thermal conductivity of various materials. this requires the synthesis of knowledge in the fields of electrical and thermal measurements and the construction of specialised sensor devices. references [1] e. h. nikollian, j. r. brews, mos physics and technology, wiley, new york, 1982. isbn-13: 978-0471430797 [2] s. taibl, g. fafilek, j. fleig, impedance spectra of fe-doped srtio3 thin films upon bias voltage: inductive loops as a trace of ion motion, nanoscale. 8 (2016), pp.13954–13966. doi: 10.1039/c6nr00814c [3] n. kumar, e. a. patterson, t. frömling, d. p. cann, dc-bias dependent impedance spectroscopy of batio3bi(zn1/2ti1/2)o3 ceramics, j. mater. chem. c. 4 (2016), pp. 1782–1786. doi: 10.1039/c5tc04247j [4] z. b. stoynov, b. m. grafov, b. s. savova-stoynov, v. v. elkin, electrochemical impedance, nauka, moscow, 1991. isbn: 5-02001945-3 [5] s. baltianski, low frequency bias-induced impedance, 24th imeko tc4 int. symp. 22nd int. work. adc and dac model. test, palermo, italy, 14-16 september 2020, pp 423-428. online [accessed 18 june 2021] https://www.imeko.org/publications/tc4-2020/imeko-tc42020-79.pdf [6] e. barsoukov, j. r. macdonald (eds.), impedance spectroscopy, theory, experiment, and applications, 2nd ed., new jersey, john wiley & sons. inc., 2005. isbn: 978-0-471-64749-2 [7] m.e. orazem (eds.), b. tribollet, electrochemical impedance spectroscopy, 2nd ed., new jersey, john wiley & sons, inc., 2008. isbn:9780470041406 [8] a. lasia, electrochemical impedance spectroscopy and its applications, electrochem. impedance spectrosc. its appl. 9781461489337 (2014) 1–367. doi: 10.1007/978-1-4614-8933-7 [9] f. w. grover, inductance calculations: working formulas and tables (dover phoenix editions) hardcover – march 29, 2004. isbn-10: 0486495779 [10] n. murer, installation and configuration manual for vmp-300based instruments and boosters. online [accessed 05 june 2021]. https://www.biologic.net/documents/vmp300-based-manuals/ [11] p. baranov, v. borikov, v. ivanova, b. b. duc, s. uchaikin, c. y. liu, lock-in amplifier with a high common-mode rejection ratio in the range of 0.02 to 100 khz, acta imeko 8(1) (2019), pp. 103–110. doi: 10.21014/acta_imeko.v8i1.672 [12] s. s. baltyanskii, measuring the parameters of physical objects by identifying electrical models, meas. tech. 43 (2000), pp. 763–769. doi: 10.1023/a:1026645722396 https://doi.org/10.1039/c6nr00814c https://doi.org/10.1039/c5tc04247j https://www.imeko.org/publications/tc4-2020/imeko-tc4-2020-79.pdf https://www.imeko.org/publications/tc4-2020/imeko-tc4-2020-79.pdf https://doi.org/10.1007/978-1-4614-8933-7 https://www.biologic.net/documents/vmp300-based-manuals/ http://dx.doi.org/10.21014/acta_imeko.v8i1.672 https://doi.org/10.1023/a:1026645722396 acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 97 [13] f. m. janeiro, p. m. ramos, gene expression programming and genetic algorithms in impedance circuit identification, acta imeko 1(1) (2012), pp. 19-25. doi: 10.21014/acta_imeko.v1i1.16 [14] s. baltianski, impedance spectroscopy: separation and asymptotic model interpretation, xxi imeko world congr. measurement res. ind., prague, czech republic, 30 august 04 september 2015, pp. 492-497. online [accessed 05 june 2021]. https://www.imeko.org/publications/wc-2015/imeko-wc2015-tc4-101.pdf [15] e. barsoukov, j. h. jang, h. lee, thermal impedance spectroscopy for li-ion batteries using heat-pulse response analysis, j. power sources 109 (2002), pp. 313–320. doi: 10.1016/s0378-7753(02)00080-0 [16] m. swierczynski, d. i. stroe, t. stanciu, s. k. kær, electrothermal impedance spectroscopy as a cost efficient method for determining thermal parameters of lithium ion batteries: prospects, measurement methods and the state of knowledge, j. clean. prod. 155 (2017), pp. 63–71. doi: 10.1016/j.jclepro.2016.09.109 http://dx.doi.org/10.21014/acta_imeko.v1i1.16 https://www.imeko.org/publications/wc-2015/imeko-wc-2015-tc4-101.pdf https://www.imeko.org/publications/wc-2015/imeko-wc-2015-tc4-101.pdf https://doi.org/10.1016/s0378-7753(02)00080-0 https://doi.org/10.1016/j.jclepro.2016.09.109 microsoft word 257-1778-5-le acta imeko  issn: 2221‐870x  september 2015, volume 4, number 3, 4 ‐ 13    acta imeko | www.imeko.org  september 2015 | volume 4 | number 3 | 4  full information adc test procedures using sinusoidal  excitation, implemented in matlab and labview  vilmos pálfi, tamás virosztek, istván kollár  budapest university of technology and economics, department of measurement and information systems, műegyetem rkp. 3., 1111, budapest, hungary        section: research paper   keywords: adc test; maximum likelihood; sine wave fitting; histogram test; matlab; labview  citation: vilmos pálfi, tamás virosztek, istván kollár, full information adc test procedures using sinusoidal excitation, implemented in matlab and  labview, acta imeko, vol. 4, no. 3, article 2, september 2015, identifier: imeko‐acta‐04 (2015)‐03‐02  editor: paolo carbone, university of perugia, italy  received february 14, 2015; in final form april 8, 2015; published september 2015  copyright: © 2015 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  corresponding author: vilmos pálfi, e‐mail: palfi@mit.bme.hu    1. introduction  testing and characterization of analog-to-digital converters is an important field of measurement technology. a commonly used method for adc testing is using a sine wave excitation signal, e. g. sine wave-based histogram test. in this latter procedure the device under test is excited with a possibly clean sinusoidal input, then a histogram is created which is used after correction for the probability density function of the sine wave to determine the transition levels of the converter. the standard method to estimate the parameters of the excitation signal is the least squares fitting algorithm. if the signal frequency is assumed to be known, the so-called threeparameter fit can be done which estimates the sine and cosine amplitudes and the dc offset level of the signal. in the case of unknown frequency, the four-parameter fit solves the problem. dynamic errors of the adc are often investigated using the fft test which reveals the spurious components and harmonic distortion introduced by the device. the test methods mentioned above are described in detail in the ieee standard [1]. furthermore, this document defines rather strict conditions for the signal parameters which have to be fulfilled to ensure accurate results. however, users have to face some difficulties during the application of the standard procedures:  there is no method proposed to check the fulfilment of the conditions for the signal parameters (e.g. coherence, relative prime condition),  while the proposed methods are sensitive to the signal parameters, they are unable to recognize bad parameter settings which might lead to incorrect characterization of the converter,  correct signal parameters by themselves still do not ensure precise estimation of the sine parameters since the least squares method is sensitive to the nonlinearities of the adc. the main goal of this paper is to present some advanced methods which are able to handle the above problems and provide unbiased information with minimum variance about the signal and adc parameters, using only the measured record. matlab and labview implementation of the methods are freely available on the internet [2], these software tools are also presented here. abstract  analog‐to‐digital  converters  and  the  need  to  test  these  devices  appeared  simultaneously.  thus,  adc  circuit  realizations  and  test  methods evolved also simultaneously. in the last decades several techniques have been elaborated and spread worldwide. these are  available  in  ieee  standards  and  in  the  literature  as  well.  however,  standard  methods  do  not  support  the  recognition  of  incorrect  measurement settings. accurate test results require careful choice of settings and calculated quality parameters of the adc under test  are very sensitive to imperfections of the measurement setup. in addition, the requirements are different for each test technique and  the restrictions can even be contradicting (e.g. overdrive is recommended for histogram test and contraindicated for fft test). this  paper presents solutions to perform the commonly used methods reliably and some advanced methods to increase the performance  of adc quality parameter estimation. implementations of the proposed algorithms are presented as well, with url for download. acta imeko | www.imeko.org  september 2015 | volume 4 | number 3 | 5  section 2 explains in details the standard methods and their disadvantages. the advanced methods are presented in section 3. the software tools are illustrated in section 4. finally, in section 5 some experimental results are shown which confirm the advantages of the proposed methods. this work is based on and extends [5]. 2. standard methods in adc testing  2.1. the sine wave histogram test  the histogram test is an effective way to estimate the code transition levels of an a/d converter. the adc is tested with a pure sine wave which slightly exceeds the input range (see [3]). a histogram is created which shows the number of hits in each code bin. let be the number of hits in code bin ( 0…2 1 for an adc of bits). then the cumulative histogram can be defined as: ∑ . (1) let the model of the excitation signal be cos 2 (2) where , , and are the offset, amplitude, signal frequency and initial phase, respectively. using the parameters , , the number of samples and the cumulative histogram , the th transition level can be estimated with the following formula: cos . (3) the signal parameters and can be estimated in units of the adc quantum step, directly from the cumulative histogram (e.g. from the position of the 10 % and 90 % points), or making a least squares fit of the histogram. this is usually enough to execute the test in adc units (absolute values can be obtained by using estimates of two arbitrary (but not too close) transition levels, matching to the measurement). using such estimators will not introduce any errors in the inl and dnl characteristics of the converter [1]. however, determination of parameters like sinad, enob, etc. requires estimation of all parameters of the input signal (that is, also the phase and frequency). the histogram test is very sensitive to the appropriate ratio of the signal frequency and of the sampling frequency . this ratio defines the relation between the number of samples ( ) and the number of periods ( ) in the record: . (4) standard [1] requires the sampling be coherent (thus has to be an integer number), and and be relative primes. these conditions are very important because they guarantee the unbiased, minimum variance estimation of the transition levels (with respect to the given number of samples, see [4] and [5]). unfortunately, there is no proposed method in the standard about inspecting the fulfilment of the above conditions. the requirements defined in the standard are important because this test technique compares the histogram of the quantized signal to the probability density function (pdf) of the sine wave. if a sine is quantized with an ideal adc, its histogram will be very similar to the pdf, especially if the number of bits is high. figure 1 shows such a histogram. the imperfections of the converter cause distortions in the histogram, since the number of hits in the code bin differs from the ideal case as the code is wider or narrower than the quantization step (see figure 2). however, if the coherence condition is not fulfilled, then the number of periods in the signal is not integer. the samples of the fractional period might also cause distortions in the histogram, so transition levels among these codes will be estimated with systematic errors (see figure 3). figure 1. the histogram of a coherently sampled sine wave quantized by an ideal quantizer.  figure 2. histogram of a coherently sampled sine wave, quantized by a non‐ ideal quantizer.   figure  3.  distortion  in  the  histogram  of  the  sine  wave  caused  by  non‐ coherent sampling. acta imeko | www.imeko.org  september 2015 | volume 4 | number 3 | 6  the conclusion is that only coherent sampling ensures unbiased results in the estimation. however, the relative prime condition still has to be fulfilled, otherwise the quality of the results might be harmed. the distribution of the phases of the samples is uniform in the interval , only if the greatest common divisor of and is 1. in that case every sample excites the adc at a different voltage level, thus the transition levels can be estimated with maximum precision. figure 4 illustrates the case when both coherence and relative prime conditions are fulfilled. the samples are distributed uniformly, the distance between two adjacent samples is everywhere the same. the transition levels can be estimated with the best precision because the uncertainty of their location depends on the distance between the samples close to each other in phase. figure 5 shows the case when the signal is sampled coherently, but the relative prime condition is not fulfilled. the value of the greatest common divisor is 5 instead of 1, thus the samples are arranged into nodes. the distance between two nodes is much higher than its ideal value, thus the uncertainty of the locations of the transition levels is high which significantly increases the variance of the estimation. finally, figure 6 shows the case when the sampling is noncoherent. the distance between the phases varies, which leads to distortions in the histogram of the sine wave (see also figure 3). 2.2. the least squares method  precise estimation of the signal parameters is quite important in adc testing. for example, the rms value of residuals strongly depend on the estimated parameters. equation (3) also shows that the amplitude and dc offset parameters have to be known as exactly as possible to determine the adc characteristics precisely. the least squares method uses the following model of the sine wave: cos 2 sin 2 (5) where cos and sin . the advantage of this modified model is that it is linear in a, b, c, and nonlinear only in the signal frequency. the method minimizes the following quadratic cost function: ∑ cos 2 sin 2 . (6) let denote the matrix of the derivatives of the residuals with respect to the parameters of the sine wave. the solution of the least-squares equation can be expressed in the following form: ̂ . (7) solving the above equation iteratively (e.g. 5-6 times newton-gauss steps) will give the least squares estimator of the sine parameters. despite of the efficiency of the method, it has several disadvantages:  the statistical properties of the estimation depends strongly on the saturation of the adc, e.g. a 10 % overdrive leads to significant errors in the estimation of the sine parameters;  the presence of harmonic components also influence the precision of the estimator negatively;  the least-squares method assumes implicitly an ideal quantizer (so the stochastic model of noise is appropriate). however, this model is not valid for true adcs with nonlinear characteristics which results biased estimation; figure  4.  distribution  of  the  phases  in  [0,  2π]  when  both  coherency  and  relative prime conditions are fulfilled.   figure  5.  distribution  of  the  phases  in  [0,  2π]  when  only  the  coherency condition is fulfilled.   figure  6.  distribution  of  the  phases  in  [0,  2π]  in  case  of  non‐coherent  sampling.   acta imeko | www.imeko.org  september 2015 | volume 4 | number 3 | 7   the computational demands increase rapidly with the record length, however testing high resolution converters requires long measurements. 2.3. the fft test  the purpose of the fft test is to characterize the dynamic behaviour of the adc by identifying spurious and harmonic components introduced by the device. the spurious free dynamic range (sfdr) shows the relation between the carrier and the largest spurious component in the signal. overdrive of the adc or non-coherent sampling significantly decrease the precision of the test results due to spectral leakage and harmonic components caused by clipping the peaks of the sine wave [6]. 3. advanced methods  3.1. main goals  the main goal of this paper is to present some advanced algorithms which are able to handle the problems introduced in section 2. this way the user can perform accurate and reliable adc testing. the methods perform the following tasks:  quality analysis of the measured data is provided by checking saturation and the fulfilment of the conditions required by the histogram test method. this step requires precise estimation of the sine parameters;  if the original record fails to fulfil the conditions, the coherent parts of the record are identified. if the coherent parts are too short compared to the original measurement, a new signal frequency is proposed and the measurement can be repeated following this suggestion. above steps improve significantly the results of the histogram test and the fft test since both methods provide the best results in the case of coherent sampling;  the signal parameters are determined using the maximum likelihood (ml) algorithm. the ml estimator is not influenced negatively by the (possibly) nonlinear characteristics of the adc, thus the signal parameters, the fitting residuals and values such sinad, enob can be determined with the best achievable precision. the algorithms are presented in details in the next subsections. it is important to notice that no a priori information is used or required, the source of the information (transition levels, signal parameters, etc.) is the measured data only. 3.2. overdrive detection  overdrive detection is quite important because distortions in the signal caused by clipping the peaks largely influence the results of sine wave fitting and the fft test. the method identifies the samples in the measured signal which seems to be out of the full-scale range of the adc. for this purpose, first the number of periods ( ) in the signal is determined using ipfft with maximum sidelobe decay window ([7], [8] and [9]). in the next step, the three-parameters sine fit [1] is done to determine the , and parameters. let be the output of the adc, and , be the smallest and the largest output code of the converter. based on [10], only those samples are used during the three-parameters fit algorithm for which the following condition holds true: . (8) then the fit can be expressed as: cos sin . (9) the th sample of the signal is assumed to be overdriven if 1/2 or 1/2 (these are two virtual transition levels at the start and end of the full scale range). the overdriven samples are replaced (as in [6]) with the corresponding samples of : , 1/2 , 1/2 , otherwise . (10) using ′ instead of during the fft test and the sine fitting algorithm improves the results significantly. 3.3. least‐squares fit in the frequency domain  disadvantages of the standard, time domain least squares method (presented in section 2) show that it is not the best method for the determination of the sine parameters. most of the disadvantages can be handled if the fit is performed in the frequency domain. for this purpose, first ′ is windowed with the three-terms blackman-harris windows (see [11]), then the fft of the windowed signal is computed. the blackmanharris window concentrates the information about the sine wave around its frequency very effectively, thus only a few points are used during the iterative estimation method. the method is explained in details in [12] and [13]. the main advantages are the following:  the blackman-harris window compresses the information around the sine and dc frequencies, thus the computational costs are reduced significantly;  the statistical properties of the estimator are the same compared to the original method. on low frequencies the frequency domain method outperforms the original algorithm;  due to the windowing the frequency domain method is much less sensitive to harmonic distortions in the signal. it is important to notice that nonlinearities in the adc characteristics cannot be handled with least squares estimators regardless of the domain of the used samples. however, the influence of the characteristics is much more significant regarding parameters , and in comparison with , the number of periods in the signal. so is approximately unbiased and it was also shown in [13] that its statistical properties allow the estimator be used to check the fulfilment of the coherency and relative prime conditions. 3.4. coherence analysis  the main purpose of the algorithm is to decide the suitability of the measured sine wave for histogram testing. this depends on the exact number of periods in the signal, denoted by . this can be written as 〈 〉 ∆ where 〈 〉 is the rounded value of and ∆ is the residual, thus |∆ | 0.5. the goal is to identify the coherent record parts in the measurement. for this purpose, the condition of carbone and chiorboli is used. it was shown in [14] that if 〈 〉 and are relative primes and |∆ | (11) hold true then the variance of the histogram test method does not increase noticeably in comparison with the ∆ 0 case, acta imeko | www.imeko.org  september 2015 | volume 4 | number 3 | 8  thus the sampling can be assumed coherent from the histogram test’s point of view. since the estimators are probability variables, a probabilistic approach of coherency analysis is recommended and presented. let be the variance of the estimator. as it was shown in [13], is asymptotically gaussian, unbiased and its variance can be determined in close form using the jennrich-theorem (see [15]). using the information above, the following probability can be determined: , . (12) the following probability is the same: , . (13) using the latter form, the probability of coherency can be answered for different record lengths by selecting the values of and properly. let be the rounded value of , thus 〈 〉. to determine the probability of fulfilling the carbonechiorboli condition, the following values of and are required: . (14) if this probability is higher than a previously defined threshold (e.g. 95 %), the sampling can be assumed coherent. in addition, if the relative prime condition is also fulfilled, then the data is optimal for histogram testing. otherwise, additional steps are required to identify the coherent parts of the signal. if the original record of samples fails to fulfil the requirements, then a new has to be determined. let be the number of periods represented by one sample of the signal ( 0.5, this comes from the nyquist-shannon sampling theorem). the exact value of is unknown, but it can be estimated: . (15) let be the number of periods for which the condition of carbone and chiorboli holds true, and the greatest common divisor of and is 1. then can be estimated as: . (16) the variance of is also known: , . (17) this way the probability of coherence can be determined for any record length. using the above procedure, the following steps are proposed to determine the optimal number of samples used for histogram testing:  estimation of the number of periods in the original record using the frequency domain estimator. as a result, we have and ;  determination of the possible number of integer periods in the record. this is stored in the vector, its elements increase from 1 to 〈 〉. the value of is also determined;  determination of the number of samples for each element of . since these have to be integers, these are the rounded values of the ratio of the possible integer number of periods and . the results are stored in the vector;  for the elements of the exact number of periods is calculated and stored in the vector. each element is a probability variable, the standard deviations are stored in / ;  the next step is the calculation of the values of the carbone-chiorboli bound for each element of . once this is done, the probabilities of coherency can be determined for every value of . these probabilities are stored in the vector, and the greatest common divisors of the corresponding elements of and in the vector;  using the above vectors, the optimal number of samples can be determined. the proposed formula is the following: where , and are the th element of the , , vectors, respectively. the histogram test should be performed with that length for which is maximal;  if the value of is too small compared to the original record length, the needed adjustment of the signal frequency can be determined if the sampling frequency is known. from (4) one can derive that ∆ ∆ where ∆ is the required correction in the sampling frequency. note that the frequency resolution of the signal generators is not enough high to adjust the frequency exactly by the proposed value. 3.5. the maximum likelihood method  3.5.1. motivation  the main weakness of the standard ls estimators for the sine wave parameters is the possible bias of them. least-squares estimation can be biased in multiple cases, examples from the field of system identification are itemized in [16]. in adc testing, this problem appears due to the nonideality of the real quantizers: the code transition levels are not distributed uniformly, thus code bins have different widths. however, the ls estimator finds the best fitting sine wave to the output codes of the device under test (occasionally to the nominal voltage values corresponding to the output codes), while the aim is to estimate the parameters of the input sine wave. the nonlinearity of the converter is not modelled in this standard method. the goal of maximum likelihood (ml) estimation of sine wave and adc parameters [17], [21] is to provide minimum-variance unbiased (mvu) estimators for the analog excitation signal considering the non-ideal properties of the quantizer. the theoretical and practical aspects of ml estimation for adc testing are to be itemized in the following subsections. 3.5.2. modelling the measurement setup  for sine wave-based qualification of converter circuits, the measurement setup is simple. the analog side of the adc under test is connected to a sine wave generator, while the digital record of the sine wave is post-processed to achieve the quantities that qualify the converter. however, the quality requirements for the sine wave generator are high. this device shall provide excellent frequency stability even for minutes and acta imeko | www.imeko.org  september 2015 | volume 4 | number 3 | 9  harmonic distortion must be very low. these strict requirements have the following reasons: alteration of frequency, phase noise and multi-harmonic signals can be treated mathematically, nevertheless these options give too many degrees of freedom to the model. e.g. harmonic components in the record provided by the nonlinearity of the converter and provided by the analog generator cannot be distinguished. similarly, a measured sine wave can be described as using additive noise and harmonic components as using frequency alteration and phase noise. the noise of the analog signal, the disturbances of the analog environment and the electronic noise of the adc circuit are handled in a simple, but very lifelike noise model [18]. to examine the statistical and spectral properties of the measurement noise it is expedient to perform long measurements with short circuited analog input or zero excitation on the sine wave generator (the latter one is better to record the electromagnetic disturbances of the environment). evaluating several measurements with different adc circuits, the white noise model is apparently very realistic. this is important from the aspect of the mathematical form of the likelihood function as well, because the samples of the noise are assumed to be independent (see subsection 3.5.3.). regarding the probability distribution of the noise, the results are less straightforward. we tried to confirm or deny the null hypothesis that these samples are from a well-known distribution using the kolmogorov-smirnov test. as the records contained up to 2 million samples, these hypothesis test results are reliable at high confidence level (p = 95-99 %). the distribution of the recorded populations were close to the gaussian normal distribution, but showed significantly higher kurtosis: contained more outliers than it is expected in case of gaussian noise. according to our experience, the best practice is to use a combination of gaussian and laplacian distribution to handle the outliers, but to keep the shape of distribution as well [19]. the model of the measurement and the parameters corresponding to the elements of the setup appear in figure 7. 3.5.3. the likelihood function  the measurement record contains m samples of the digitally recorded sine wave. the observations are these samples of the quantized noisy signal. the likelihood function depends on the following parameters denoted by : , , , , , 1 ,⋯, 2 1 (18) where is the number of bits, denotes the cosine coefficient of the sine wave, is the sine coefficient of the sine wave, denotes the dc offset of the excitation signal. the frequency of the sine wave is denoted by (the sampling frequency is known), denotes the standard deviation of the additive noise on the analog signal, and the code transitions levels of the quantizer (from the lowest to the highest) are denoted by 1 , . . . , 2 1 , respectively. the likelihood of the measurement can be expressed using the following equation: ∏ (19) where denotes the th recorded sample of the sine wave and is a discrete random variable corresponding to the th.sample of the record. to calculate the distribution of , it is necessary to calculate the th sample of a pure sine wave with given parameters , , and : cos 2 sin 2 . (20) the threshold levels of the quantizer (code transition levels) and the noise model appear in the following formula, which gives the discrete distribution of random variable : n , , 1 n , , (21) where n , denotes the cumulative distribution function of the noise with expected value and standard deviation . using the cumulative distribution function (cdf) of gaussian distribution as ncdf is usually a very good approximation (see subsection 3.5.2). in this likelihood function the following a priori information is used:  the noise is white (samples of the noise are independent);  the analog excitation is a sine wave with additive, almost gaussian noise;  the quantizer is described with its sampling frequency ( , constant in a measurement) and code transition levels ( is the voltage value which results digital code 1 with 50 % probability and with 50 % probability as well). the maximum likelihood estimators can be achieved optimizing this likelihood function with respect to the parameters stored in : argmax . (22) 3.5.4. challenges of optimization  the most important problem is the computational demand of the optimization which strongly depends on the number of parameters. using a b-bit quantizer the number of parameters is 2 4, the number of restrictions is 2 1, thus the parameter space increases exponentially with respect to the number of bits. the entire computational demand grows even faster: let n denote the number of parameters, thus depending on the optimization algorithm, the operations have the following computational complexity:  calculating the first-order partial derivatives (the gradient vector): ~ ;  calculating the second order partial derivatives (the hessian matrix): ~ ;  inverting the hessian matrix: ~ . this way performing the optimization process on the entire parameter space requires unacceptable efforts for a regular 12bit, 16-bit or higher resolution adc. to handle this problem one of the following approximations shall be used:  the code transition levels are estimated from the sinusoidal record using histogram test [2]. these values are considered to be fixed and will not be adjusted later. this method reduces the parameter space to 5 dimensions, however brings along all the figure 7. measurement setup for adc testing.   acta imeko | www.imeko.org  september 2015 | volume 4 | number 3 | 10  problems that can appear in sinusoidal histogram testing [4];  the nonlinearity of the quantizer is parameterized: the code transition levels are not estimated one by one, but the entire static transfer characteristics is described using fewer (from 5 up to 15) parameters. this way the number of parameters of the likelihood function remains between 10 and 20, so the optimization can be performed without excessive efforts. for parameterization, chebyshev [20] and taylor polynomials or fourier-coefficients can be used. the information regarding the nonlinearity is described using fewer quantities in this case, however the estimation of these polynomial coefficients brings along less variance than the estimation of the single code transition levels. 3.5.5. numerical recipes to optimize the likelihood function  the likelihood function in the reduced parameter space can be optimized multiple ways to achieve approximate maximum likelihood estimators:  derivative-based methods: the simple gradient descent which only requires the first order partial derivatives, the gauss-method that implies the calculation of the hessian matrix and the generalized levenberg-marquardt method where the hessian is used as well (instead of formula from the jacobian matrix). calculation of the hessian in each iteration cycle can be bypassed using quasi-newton methods (e.g. dfp, bfgs or broyden);  simplex downhill (nelder-mead) method: does not require to calculate derivatives (the objective function must not even be differentiable), however the number of iterations and cost function evaluations can be high (depending on the shape of the minimum/maximum and the termination criteria);  differential evolution: a genetic algorithm used to find the global optimum of the objective function. this method does not require derivatives and is able to escape from local extrema. on the other hand it requires large number of cost function evaluations and the convergence is partially based on heuristics; in our software implementation available on the web [8] a gradient-based method is used, nevertheless other algorithms can be efficient and their efficient usage is also subject of investigation. 4. software tools for adc testing  previous sections demonstrated the importance of the quality analysis on the measured data before the test methods are performed. the algorithms have to be executed in a fixed order to ensure best results. every method on a later stage uses the information provided by previous methods. the main tasks are the fft test, histogram test and estimation of the sine parameters, these are supported by the other algorithms. the data processing chain can be seen on figure 8. first the overdrive detection method is performed which identifies the samples clipped by the adc. these are replaced by their estimated value. this step is done automatically in the labview tool, while in matlab the user is warned (see figure 9). as a result, the results of the fft and the least squares fit in the frequency domain are improved. once the latter is done, the coherency analysis can be performed and the optimal records length can be determined (figure 10). at this point the processed record can be assumed coherent, thus the histogram and fft tests will provide valid results. once the transition levels are known, the maximum likelihood estimation method can be executed. this will estimate the parameters of the sine and noise unbiasedly and with a variance which is very close to the theoretical limit (see figure 11). this serves the accurate determination of some adc quality parameters such as enob, sinad, etc. figure 8. the sequence of methods in the adc testing software.   figure 9. classification of the measurement in the matlab tool.   figure 10. results of the coherency analysis. the user can  choose among  the best 3 options.   figure 11. results of the maximum  likelihood method. towards the signal  and noise parameters, sinad and enob are also determined.   acta imeko | www.imeko.org  september 2015 | volume 4 | number 3 | 11  5. experimental results  5.1. simulation results  this subsection presents the test results of the coherence analysis algorithm. during the test a 12 bit quantizer was used, the characteristics can be seen in figure 12. in the test sine waves were generated and the initial phase and the number of periods were random variables. the initial phase was uniformly distributed in the interval 0,2 . the number of periods, , was also distributed uniformly in 8.95,9.05 . the aim of this selection was to model that users try to fulfil the coherence condition, but due to the errors in the signal and sampling frequencies he fails to do so. the value of the overdrive and was set to 10 % and 215. the model of the sine wave was the following (according to (2)): 1.1 cos . (23) to simulate real-like circumstances and to model the imperfections of the signal generator, independent gaussian noise ( ) was added to the samples of the signal with 0 mean and lsb standard deviation: e 0,var 1 . (24) a harmonic component ( ) was also added with 1 lsb amplitude and double frequency of the carrier: cos . (25) the test signal was the sum of the carrier, the noise and the harmonic component: . (26) 100 tests were run where first the histogram test was performed using the whole record, then using the optimal record length after coherence analysis. figure 13 shows the standard deviation of the error of the histogram test for the case when the whole record was used. figure 14 shows the standard deviation when the record was truncated to the optimal length. the comparison of the figures shows that the coherence analysis method successfully reduced the errors in the estimation. simulation results for the maximum likelihood method and comparison with the ls estimator can be found in [22]. in [13] a comprehensive study is presented about the frequency domain ls estimator. 5.2. experimental results  the presented algorithms were tested with real measurement data. a national instruments adc of 16 bits and =200 khz sampling frequency was used for data quantization. the excitation signal was provided by a brüel&kjaer type 1051 sine wave generator. since a slight overdrive of the adc is recommended for histogram testing the signal amplitude was set to 120 % of the full scale range of the converter. the frequency was set to =97 hz and =220 samples were collected. the nominal values of the frequencies and the record length fulfil both the coherency and relative prime conditions. in the first test, the original least-squares method [1] was compared to the frequency domain method with overdrive detection. the algorithms estimated the four parameters of the sine wave, than sinad and enob were determined using the fitting residuals ( ): ∑ (27) √ (28) . . . (29) in (29) the value of was substituted for for values higher than the full scale of the adc. the results of the comparison can be seen in table 1. figure 12. adc inl and dnl characteristics.   figure 13. standard deviation of the estimation error of the histogram test when the whole record was used.   figure 14. standard deviation of the estimation error of the histogram test after the coherence analysis.   table 1. comparison of the standard and the proposed least‐squares fitting  method.  parameter original method proposed method a [lsb] -30834.5 -30886.4 b [lsb] 12140.2 12160.1 c [lsb] 32789.8 32793.3 j 211.01 211.05 sinad [db] 48.240 79.114 enob 7.721 12.849 acta imeko | www.imeko.org  september 2015 | volume 4 | number 3 | 12  the results show that harmonic components introduced by the saturation of the adc cause huge errors in the amplitude and dc offset parameters of the sine. however, detection and substitution of samples around the peaks improved the results significantly. in the next step the effect of the coherency analysis algorithm was studied. two histogram tests were performed, one for the whole record and one for the coherent part of the signal. coherency analysis showed that the optimal record length for histogram testing is =288227 samples which is only the 27 % of the original length despite the nominal values of the sampling and sine frequency fulfilled every requirement. figure 15 shows the results of the inl estimations and the difference between the results. in the first case the histogram test was performed using a non-coherent record, thus the results are distorted. the error curve showed that 57 transition levels were estimated with an error higher than 3 lsb, 4113 of them were estimated with an error higher than 2 lsb. the value of the average estimation error is 0.957 lsb. it is very important to notice that 288227 samples are not enough to test accurately a 16 bit adc, the presented software would recommend an adjustment in the signal frequency (see section 4). the new record length shows the amount of independent information in the whole record, thus ~70 % of the samples do not provide new information about the transition levels. in addition, it is still better to perform the histogram test with the truncated record since the whole record introduces bias in the estimation, but the variance of the estimation is the same for both records due to the same amount of information. finally, the results of the least-squares method and the maximum likelihood method were compared. the inl curve of the adc shows that the characteristics are nonlinear, thus the least-squares method is not able to provide unbiased results. the ml method uses the transition levels of the converter during the parameter estimation process so the results are precise for nonlinear converters also. table 2 shows the results. the enob and sinad parameters were found to be smaller using the ml method. the explanation if that ls fit minimizes the error, thus the values of above parameters are maximized. however, the true values are provided by the ml estimator which maximizes the probability with respect to the parameters. 6. conclusions  in adc testing, users have to face some difficulties during the application of the standard methods. this paper presented some advanced methods which are able to handle the problems of the original procedures. the implementation of the proposed algorithms was also presented and experimental results showed that the new methods provide accurate and reliable information about the adc and sine wave parameters. references  [1] ieee standard for terminology and test methods for analogto-digital converters," ieee std 1241-2010 (revision of ieee std 1241-2000), pp. 1-139, jan. 14, 2011. doi: 10.1109/ieeestd.2011.5692956 [2] virosztek, t., pálfi v., renczes b., kollár i., balogh l., sárhegyi a., márkus j., bilau z. t., adctest project site: http://www.mit.bme.hu/projects/adctest, 2000-2014 [3] blair, j., "histogram measurement of adc nonlinearities using sine waves," ieee transactions on instrumentation and measurement, vol.43, no.3, pp.373-383, jun 1994. doi: 10.1109/19.293454 [4] vilmos pálfi, istván kollár, “improving the results of the histogram test using a fast sine fit algorithm”, proc. of 19th imeko tc4 symposium, barcelona, spain, 2013. paper 118. url: http://www.imeko.org/publications/tc4-2013/imekotc4-2013-118.pdf [5] vilmos pálfi, istván kollár, “reliable adc testing using labview”, proc. of 20th imeko tc4 symposium, benevento, italy, 2014. pp. 237-241, url: http://www.imeko.org/publications/tc4-2014/imeko-tc42014-259.pdf [6] xu, li, siva kumar sudani, and degang chen. "efficient spectral testing with clipped and noncoherently sampled data." ieee transactions on instrumentation and measurement, 63.6 (2014): 1451-1460. doi: 10.1109/tim.2013.2292273 [7] belega, daniel, and dominique dallet. "frequency estimation via weighted multipoint interpolated dft." science, measurement & technology, iet 2.1 (2008): 1-8. doi.: 10.1049/ietsmt:20070022 [8] belega, daniel, and dominique dallet. "efficiency of the threepoint interpolated dft method on the normalized frequency estimation of a sine-wave." intelligent data acquisition and advanced computing systems: technology and applications, 2009. idaacs 2009. ieee international workshop on. ieee, 2009. doi: 10.1109/idaacs.2009.5343034 [9] belega, daniel, dominique dallet, and dario petri. "accuracy of sine wave frequency estimation by multipoint interpolated dft approach." ieee transactions on instrumentation and measurement, 59.11 (2010): 2808-2815. doi: 10.1109/tim.2010.2060870 [10] kollár, istván, and jerome j. blair. "improved determination of the best fitting sine wave in adc testing." ieee transactions on instrumentation and measurement, 54.5 (2005): 1978-1983. doi: 10.1109/tim.2005.855082 [11] albrecht, hans-helge. "a family of cosine-sum windows for high-resolution measurements." acoustics, speech, and signal table 2. comparison of the least‐squares and maximum likelihood methods. parameter ls ml a [lsb] -30886.4 -30886.4 b [lsb] 12160.1 12160.1 c [lsb] 32793.3 32792.8 j 211.05 211.05 sinad [db] 79.114 78.918 enob 12.849 12.817 figure  15.  comparison  of  results  of  the  histogram  tests  using  the  whole record and the coherent part only.   acta imeko | www.imeko.org  september 2015 | volume 4 | number 3 | 13  processing, ieee international conference on. vol. 5. ieee, 2001. doi: 10.1109/icassp.2001.940309 [12] pálfi, vilmos, and istván kollár. "efficient execution of adc test with sine fitting with verification of excitation signal parameter settings." instrumentation and measurement technology conference (i2mtc), 2012 ieee international. ieee, 2012. doi: 10.1109/i2mtc.2012.6229502 [13] pálfi, vilmos, and istván kollár. "acceleration of the adc test with sine-wave fit." ieee transactions on instrumentation and measurement, 62.5 (2013): 880-888. doi.: 10.1109/tim.2013.2243500 [14] carbone, paolo, and giovanni chiorboli. "adc sinewave histogram testing with quasi-coherent sampling." ieee transactions on instrumentation and measurement, 50.4 (2001): 949-953. doi: 10.1109/19.948305 [15] jennrich, robert i. "asymptotic properties of non-linear least squares estimators." the annals of mathematical statistics (1969): 633-643. doi: 10.1214/aoms/1177697731 [16] schoukens, johan, and rik pintelon. identification of linear systems: a practical guideline to accurate modelling. elsevier, 2014. url.: http://goo.gl/dg7uuv [17] balogh, lászló, istván kollár, and attila sárhegyi. "maximum likelihood estimation of adc parameters." instrumentation and measurement technology conference (i2mtc), 2010 ieee. ieee, 2010. doi.: 10.1109/imtc.2010.5488286 [18] tamás virosztek „adc testing in practice, using maximum likelihood estimation: report in students' scientific circle (tdk)” 49 p., 2013. url: http://mycite.omikk.bme.hu/doc/149632.pdf [19] b renczes, i kollár, p carbone, a moschitta, v pálfi, t virosztek analyzing numerical optimization problems of finite resolution sine wave fitting algorithms – accepted for publication at ieee instrumentation and measurement technology conference (i2mtc), may 11-14 2015, pisa, italy [20] attivissimo, f.; giaquinto, n.; kale, i., "inl reconstruction of a/d converters via parametric spectral estimation," instrumentation and measurement, ieee transactions on , vol.53, no.4, pp.940,946, aug. 2004 doi: 10.1109/tim.2004.831508 [21] šaliga, ján, et al. "a comparison of least squares and maximum likelihood methods using sine fitting in adc testing." measurement 46.10 (2013): 4362-4368. doi.:10.1016/j.measurement.2013.05.004 microsoft word article 16 174-1029-1-le.docx acta imeko  february 2015, volume 4, number 1, 105 – 110  www.imeko.org    acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 105  virtual quasi‐balanced circuits and method of automated  quasi‐balancing  artur skórkowski, adam cichy, sebastian barwinek  institute of measurement science, electronics and control, silesian university of technology, akademicka 10 street, 44‐100 gliwice, poland       section: research paper   keywords: automatic balancing; virtual instrument; quasi‐balanced circuit; labview development  citation: artur skórkowski, adam cichy, sebastian barwinek, virtual quasi‐balanced circuits and method of automated quasi‐balancing, acta imeko, vol. 4,  no. 1, article 16, february 2015, identifier: imeko‐acta‐04 (2015)‐01‐16  editor: paolo carbone, university of perugia   received january 10 th , 2014; in final form march 25 th , 2014; published february 2015  copyright: © 2014 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: this work was supported by institute of measurement science, electronics and control, silesian university of technology, poland  corresponding author: artur skórkowski, e‐mail: artur.skorkowski@polsl.pl    1. introduction  quasi-balanced circuits are ac circuits destined for measuring impedance components. they have a special selected state, the so-called quasi-equilibrium state, which is usually a predetermined phase shift between the selected signals. the advantage of quasi-balanced circuits is the use of only one control element. the quasi-equilibrium state is an a priori assumed non-zero state – generally meant as the achievement of the determined phase shift between the selected signals of the circuit. maximum convergence is the advantage of the circuits under consideration, whereas the lack of possibility of simultaneous measurement of both immitance components is the disadvantage, although the measurement of the second component is usually possible after uncomplicated reconfiguration of the circuit. 2. quasi‐balanced circuit for impedance  components measurements  there are many solutions of quasi-balanced circuits for measuring impedance components, e.g. those presented in [1…7]. figure 1 shows an example of the circuit used for measuring a capacitance modelled by a series combination of rc [8]. modern measuring instruments are more and more often built as virtual instruments. in analog techniques, operations on measurement signals are performed on sampled and quantized signals by software. the block diagram of the circuit (figure 1) describing analog processing becomes then a measurement algorithm (virtual instrument). quasi-balanced circuits can be virtualized very easily, since there are only operations of summing, amplifying or shifting signals by ±π/2 in the discussed circuits. phase-sensitive detection can also be realized with algorithmic methods. the equations describing the selected output signals w1, w2 in the system shown in figure 1 have the form:        2 j 2 2 j 1   ebiw ebiavw x xx (1) where a is the voltage amplifier gain, b is the conversion factor of the current /voltage converter; vx and ix are the voltage and current of the rc object under test, respectively. abstract  a basic purpose of this research was to verify a possibility of automatic balancing in the virtual realization of a quasi‐balanced circuit  for capacitance measurements. the diagrams of a virtual quasi‐balanced instrument are presented in this paper. the tested circuit was  built using a pc computer and the daq card ni‐6009. the daq card and the calculation were controlled by the application developed  in the graphical development platform labview.  acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 106  the complex numbers in eq. 1 can be expressed in polar from as follows:          2 j 2j2j 2 2 j 2j1j1j 1   eeibew eeibevaew x xx (2) where: 21 , ww modules of the selected signals of the circuit; �1, �2 – phases of the selected signals of the circuit; xx iv , modules of the voltage and current of the tested rc two-port; �1, �2 phases of the tested rc two-port. after dividing both sides of the system of equations (2) by each other one obtains the expression:   2 j 2j 2 j 2j1j 21j 2 1   eeib eeibeva e w w x xx     (3) which can be brought to the form: 12 21j 2 1           ez b a e w w x wj (4) where: �w – angle of the phase shift between the selected signals of the circuit, xz modulus of the impedance of the tested rc two-port. the dependence (4) is a complex number equation and can be written as a system of two real number equations in the trigonometric form:          xxw xxw z b a w w z b a w w   cossin 1sincos 2 1 2 1 (5) after dividing both sides of the system of eq. (5) by each other and trigonometric transformation, one obtains the equation describing the signal �w being detected as a function of the circuit parameters a and b as well as the tested impedance components:                          x x xx xx w za zab z b a z b a re im arccotan cos sin1 arccotan   (6) if a ≠ 0 and re(zx) ≠ 0. in the quasi–equilibrium state the conversion equation (6) is reduced to the form:   0 2 cotan im00   xzab (7) from which it is possible to calculate the passive component of the measured impedance   0 0im a b zx  (8) where a0 is the voltage amplifier gain in the quasiequilibrium state, b0 is the conversion factor of the current/voltage converter in the quasi-equilibrium state. since the discussed circuit is destined for capacitance measurements, the capacitance of the capacitor is calculated from eq. (8). in the quasi-equilibrium state the phase angle is set to π/2. then the capacitance of the capacitor can be determined from the relationship:   0 0 im 1 b a z c x x   (9) where a0 and b0 as in equation (8). in the case of using a circuit for capacitance measurements and taking into account that x xx cj rz  1  , (10) equation (6) can be rewritten as:               x x w ar c ab  1 arccotan (11) the detected signal �w is a phase shift between the selected signals w1 and w2. the equation describes the �w signal as a function of the parameters a, b and the measured impedance component. eq. 11 is a conversion equation of the circuit of figure 1. the amplifier’s voltage gain a or the conversion factor of the current/voltage converter’s b can be the adjusted parameter in the circuit of figure 1. the circuit is brought to the quasi-equilibrium state by changing the value of one selected, adjustable parameter a or b. such a process is called the process of quasi–balancing the circuit. if the measuring circuit of figure 1 is destined for measuring the reactance of capacitors, then it is more advantageous to change the setting of the parameter b. change of the parameter a will be more advantageous in circuits for measuring the capacitance. in both cases mentioned above a simple relation between the adjustable parameter and the quantity being measured in the quasi-equilibrium state is obtained. such a feature is not of great importance in modern measuring instruments containing microprocessors, but in some cases (for instance in order to decrease the energy consumption in portable instruments) one still tends to simplify calculations and to reduce the balancing time of the circuit. in the case of the adjustable parameter a, the parameter b remains constant. during the whole measuring process and after achieving the quasi-equilibrium state figure  1.  block  diagram  of  the  quasi‐balanced  circuit  for  capacitance  measurements.  acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 107  const0  bb (12) after substituting eq. (12) in eq. (6) and dividing the numerator and denominator of the argument of the arccotan function in this equation by a0 one obtains                                             x x x x wa z a a z a a z a a z a a a b re im1 arccotan re im arccotan 0 0 0 00 0 (13) where �wa is the signal being detected in the case of the adjustable parameter a. the relation between the active and passive component of the series rc impedance zx is the dielectric loss factor tg �x of this impedance     xx x z z  tg im re  (14) hence equation (13) can be written as follows:               0 0 1 tg 1 arccotan a a a a x wa  (15) figure 2 shows the dependence of the signal being detected, wa, on the adjustable parameter a relative to the value of a0 for different typical values of tg δx. 3. automated quasi‐balancing  figure 3 shows a simplified structure of the virtual instrument executed in the labview graphical programming environment, according to the approach presented in [8].  the quasi–balanced circuit for capacitance measurements shown in figure 1 was executed as a virtual instrument (figure 3). measurement signals, such as a voltage drop across the measured impedance and a current converted into a voltage, were applied to the data acquisition card usb ni 6009. further conversion of the signals in the measuring channels was carried out by a program executed in the labview graphical programming environment. the amplifier voltage gain or the conversion factor of a current/voltage converter may be the adjustable parameter in this system. by amending the value of one selected adjustable parameter a or b, the system is automatically set into the quasi-equilibrium state. in the circuit for capacity measurement it is better to adjust the parameter a at a constant value of the parameter b = b0. the process of the automated quasi-balancing of the circuit shown in figure 1 aiming at determining the capacitance cx given by eq. (9) consists in changing the setting of a at the constant setting of b (b = b0) until the value of the signal being detected achieves π/2. the automated quasi-balancing of the circuit is performed in three steps according to the conversion characteristic presented in figure 4:  for the optional setting a = a1 the indication of a phasesensitive detector wa1 is determined (point 1 in figure 4),  the setting of a is changed and for a2 ≠ a1 the indication of a phase-sensitive detector wa2 is again determined (point 2 in figure 4),  according to the relationships presented in the system of equations (16) the setting a0 corresponding to the selected quasi–equilibrium state wa = π/2 is determined (point 0 in figure 4).                                         0 2 0 2 2 0 1 0 1 1 1 tg 1 arccotan 1 tg 1 arccotan a a a a a a a a x wa x wa   (16)   figure 2.   φwa signal vs. relative parameter a/a0 for different loss factor   tg x values.  figure 3. the labview realization of the virtual capacitance meter.  0 15 30 45 60 75 90 0,85 0,9 0,95 1 1,05 1,1 1,15 wa a/a0 0,001 0,002 0,005 0,01 0,02 0,05 acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 108  for determining the setting a0 it is not necessary to know the loss factor tg �x, since it has a constant value in the system of equations (16) and does not appear in the solution of this system which can be presented as follows:   1122 1221 0 cotan cotan cotan cotan wawa wawa aa aa a    (17) having finished the automated quasi-balancing of the circuit of figure 1, one can determine the capacitance of the tested capacitor from equation (9) based on the known settings b0 and a0. the exemplary results of the tests made for the virtual circuit for capacitance measurements during classical (by changes of the adjustable parameter by a given constant value) and automated quasi-balancing are given in table 1. 4. double quasi‐balanced circuits  in general quasi-balanced circuits only allow the measurement of one impedance component, but it is possible to build circuits to measure two components of impedance, for example in parallel quasi-balanced circuits. some quasi–balanced circuits allow the measurement of the mutual relationship between the components of the impedance, e.g. quality factor. in such systems, double quasi balancing in two successive steps is applied. the circuit of the quasi-balanced bridge, designed to measure the quality factor of real inductors is presented in figure 5. the symbols in figure 1 represent respectively: r3 a standard variable resistor; vs the power supply voltage, r a potentiometer resistance; n a potentiometer setting (0 < n < 1) and i1, i2 the currents of the branches of the bridge. the object under test is modeled as a series connection of resistance rx and inductance lx. the quasi-balancing process requires two steps. in the first state of quasi-equilibrium the phase angle between vad and vdc equals π/2. the slider of the potentiometer r is located in the position for which n = ½ and the regulatory element is a resistor r3. in the second quasi-balance state the phase angle between vdc and vcb also equals π/2. the control element is the potentiometer r. in the second quasibalance state the n parameter is read and then the relationship for the determination of the measured quality factor qc is: n n qc 21  . (18) based on the analysis of the bridge in figure 5 it is possible to build a non-bridge structure, performing the same operations on the current and voltage signals of the tested impedance. the procedure of deriving a non–bridge circuit has been presented in [9]. the non-bridge circuit has the structure shown in figure 6. this circuit processes the measurement signals according to the principle of operation of the bridge from figure 5. the selected signals are phase shifts between w11 and w12 signals and w21 and w22 signals. it can easily be implemented as a virtual instrument. figure 7 shows a view of the prototype of the quality factor meter built according to the previously described figure 4.   φwa signal vs. parameter a for unknown loss factor tg x values  (conversion characteristic).  table  1.  comparison  of  selected  measurement  results  obtained  during  classical  and  automated  quasi‐balancing  of  the  circuit  for  capacitance  measurement.  the classical quasi-balance method a0 wa0 cx, µf 1.0357 90.00 0.3294 the automated quasi-balance method a1 wa1 a2 wa2 a0 wa0 cx, µf 100.0000 15.14 1.0404 89.01 1.0356 90.00 0.3294 3.9401 20.00 1.0875 80.00 1.0360 90.00 0.3295 1.9393 30.09 1.1482 70.09 1.0359 90.00 0.3295 1.5238 40.00 1.2272 60.00 1.0374 89.82 0.3299 figure 5. diagram of the quasi‐balanced bridge for loss factor measurement.  nvx σ σ ix phase detector π/2 phase detector π/2 + + r3 + n w11 w12 w22 w21 figure 6.   block diagram of a quasi‐balanced circuit with dual quasi‐ balancing.   0 15 30 45 60 75 90 wa a a0a1 a2 φwa1 φwa2 1 2 0 acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 109  concept. a coil under test was powered from the rigol dg1022 dds generator. the current of the object was converted to a voltage across the 1 kω standard resistor with accuracy class 0.01. the voltage of the object and the voltage proportional to its current were connected to the 16-bit daq ni usb-6251 [10]. the labview 2011 software package was used to build the virtual instrument [10]. the diagram of the virtual instrument is shown in figure 8 and its front panel in figure 9. the first tests were done as a simulation. the simulations confirmed the usefulness of the system to measure the quality factor of inductors. tests of the circuit were performed for the reference inductance in the range from 0.05 h to 1 h at a frequency of 100 hz. the results were compared with the results obtained from the meter motech mic-4090, for which the manufacturer declares a quality factor accuracy of 0.5%. the exemplary dependence of the errors versus the measured quality factor is shown in figure 10. 5. conclusions  the tests of the presented way of quasi-balancing the circuit for capacitance measurements proved that the proposed procedure is correct and showed the possibility of a significantly faster achievement of the quasi-equilibrium state than in the case of classical balance methods by changes of the adjustable parameter by a given constant value. the presented automated quasi-balance method does not reduce the accuracy of the phase detector operation and does not increase the uncertainty of determining the tested capacitor capacitance significantly. during investigations an insignificant influence of the circuit conversion characteristic shape was observed (figure 4). also the selection of the points on this characteristic had neglible influence on the accuracy of achieving the quasi– equilibrium state. further investigations aim at the detailed determination of the selection of points 1 and 2 during the realization of the procedure of quasi-balancing the circuit on the accuracy of assessing the setting a0 in the quasi-equilibrium state. further, the examination of possibilities of using the presented measuring circuit and the automated quasibalancing procedure for determining the dielectric loss factor tg �x of an rc impedance is planned. the theory and implementation of a non-bridge quasibalanced measuring circuit with dual quasi-balancing, designed for measurements of the quality factor have been presented as well. the main advantage of the circuit is maximum convergence and a simple measuring process. it requires two independent controls. the circuit described above has been implemented as a virtual system, using the labview package. simulation tests and tests carried out on real objects confirmed the usefulness of the proposed figure 9.   front panel of a quasi‐balanced meter with dual quasi‐balancing.   figure 8.   block  diagram  of  a  quasi‐balanced  circuit  with  dual  quasi‐ balancing.  figure 7. view of the prototype of the quality factor meter realized as the  quasi‐balanced circuit with dual quasi‐balancing.   figure 10. error vs. the measured quality factor.  acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 110  solutions. the level of errors reaches 5%, but the study was focused on the prototype, which will be even improved. references  [1] karandeev k. b.: “bridge and potentiometer methods of electrical measurements”, moscow, peace publishers, 1966. [2] atmanand m.a., jagadeesh kumar v., murti v.g.k: “a novel method of measurement of l and c”, ieee transaction on instrumentation & measurement, vol. 44, no. 4, august 1995, pp. 898-903. [3] atmanand m. a., jagadeesh kumar v. and murti v. g. k.: “a microcontroller based quasi-balanced bridge for the measurement of l, c and r”, ieee transaction on instrumentation & measurement, vol. 45, no.3, june 1996, pp. 1-5. [4] burbelo m.i.: “universal quasi-balanced bridges for measuring the parameters of four-element two-terminal networks”, izmeritel'naya tekhnika, vol.44, no.11, november 2001, pp.1130-1133. [5] marcuta c., fosalau c. and petrescu c.: “a virtual impedance measuring instrument based a quasi balanced bridge”, proceedings of the 14th int. symposium imeko tc4, september 2005, pp. 517–523. [6] amira h., hfaiedh m. and valentin m.: “quasi-balanced bridge method for the measurements of the impedances”, iet science, measurement and technology, vol. 3, iss. 6, 2009, pp. 403 – 409. [7] jagadeesh kumar v., sankaran p., sudhakar rao k.: “measurement of c and tan δ of a capacitor employing psds and dual slope dvms”, ieee transaction on instrumentation & measurement, vol.52, no. 5, august 2003, pp.1588-1992. [8] skórkowski a., cichy a.: “testing of the virtual realization of the quasi-balanced circuit for the capacitance measurement”, measurement automation and monitoring, vol. 53, no. 12, 2007, pp. 91-93. [9] cichy a.: “non-bridge circuit with double quasi-balancing for measurement of dielectric loss factor”, science, measurement and technology, vol. 7 iss. 5, 2013, pp. 274-279. [10] sine.ni.com. microsoft word 333-2352-1-le-rev2 acta imeko  issn: 2221‐870x  september 2016, volume 5, number 2, 19‐25    acta imeko | www.imeko.org  september 2016 | volume 5 | number 2 | 19  diet and health in central‐southern italy during the roman  imperial time  luca bondioli 1 , alessia nava 1,2 , paola francesca rossi 1 , alessandra sperduti 1,3   1 section of bioarchaeology, national museum of prehistory and ethnography "luigi pigorini", polo museale del lazio, p.zza marconi 14,  00144 rome, italy  2 department of environmental biology, sapienza university of rome, p.le aldo moro 5, 00185 rome, italy  3 university of naples "l'orientale", p.zza san domenico maggiore 12, 80134 naples, italy      section: research paper   keywords: roman imperial time; diet and health; carbon and nitrogen isotopes  citation: luca bondioli, alessia nava, paola francesca rossi, alessandra sperduti, diet and health in central‐southern italy during the roman imperial time,  acta imeko, vol. 5, no. 2, article 4, september 2016, identifier: imeko‐acta‐05 (2016)‐02‐04  section editors: sabrina grassini, politecnico di torino, italy; alfonso santoriello, università di salerno, italy  received march 15, 2016; in final form april 7, 2016; published september 2016  copyright: © 2016 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: ministero dei beni e delle attività culturali e del turismo, italy   corresponding author: luca bondioli, e‐mail: luca.bondioli@beniculturali.it    1. introduction  the study of the funerary complexes is always and necessarily accompanied by the analysis of human bones and teeth. a fruitful relationship between the two areas of research was in fact established through dialogue and collaboration both on the field as in the laboratory and led to significant advances in the historical reconstruction of the biocultural dynamics that characterized the evolution of human populations in space and time. the final outcome is the attempt to provide a consistent response to the growing demand, by archaeologists and historians, of coherent reconstructions of past adaptations to the environment. skeletal anthropology has matured, since the immediate post-war period, its own disciplinary status, which takes advantage of the development of advanced technologies and methods of investigation. nevertheless, the reconstruction of biological adaptations of past populations, based on odontoskeletal remains, is extremely difficult and complex. this difficulty is even more evident when approaching the subject of dietary and health status reconstructions in the highly stratified roman imperial society. the study of biocultural adaptation scenarios of ancient populations has, in fact, proved to be a difficult challenge. indeed, the validity of the mortality models based on skeletal analyses was deeply criticized by the contributions of boquet and masset [1] and wood and associates [2], who undermined the reconstructive power of paleodemographic and health status indicators in the odontoskeletal record. fortunately, over twenty years after the publication of these basic critical tools, skeletal anthropology is consistently increasing the quantity and especially the quality of the information today extractable from skeletal populations. this contribution illustrates the usefulness abstract  the reconstruction of ancient diets by means of stable isotopes analysis acquires a deeper meaning when their results are compared  with  other  odonto‐skeletal  indicators  which  are  strongly  contextualized  in  the  light  of  historical  and  archaeological  evidence.  nevertheless,  the  outcomes  can  be  contradictory  or,  more  realistically,  they  may  not  completely  satisfy  our  hypotheses  on  how  complex and diverse conditions ‐ such as health status, life style, diet and nutrition ‐ can actually interrelate in the life course of an  individual.  in this study we present and discuss evidences from isola sacra and velia, two roman imperial age coastal towns. the δ 15 n and δ 13 c  values  are  compared  with  demographic  and  health  status  parameters,  such  as  age,  sex,  stature,  auricular  exostoses,  dish,  cribra  orbitalia, enamel defects.   acta imeko | www.imeko.org  september 2016 | volume 5 | number 2 | 20  of collagen carbon and nitrogen isotopic composition studies as proxy of ancient diet. the data from two major italian coastal communities of roman times are used not only to reconstruct diet, but are analysed in the framework of the morphological, histological and paleopathological evidences, opening unexpected interpretive opportunities. 2. materials and methods  2.1. the skeletal material from isola sacra and velia  the two skeletal series come from the roman imperial necropolises of isola sacra (lazio, i-iii cent. ad) and veliaporta marina (campania, i-ii cent. ad) (figure 1). isola sacra has been the main necropolis of the harbor town of portus romae, serving the capital of the empire. the several excavation campaigns have yielded more than 2000 skeletal individuals. porta marina was the southern gate graveyard of velia, a port town on the tyrrhenian sea. more than 300 skeletons were recovered. the high level of preservation and completeness of both skeletal collections has allowed an extensive and highly standardized data recording. in this study the isotopic individual data, available for 117 individuals from velia and 105 individuals from isola sacra, are crossed with other anthropological data. 2.2. sex and age‐at‐death diagnosis  when dealing with well represented skeletal individuals, sex estimates can reach high levels of consistency. sexing on the base of skull and pelvis morphology gives 98 % of correct diagnosis [3]. a recent study has demonstrated a similar discriminative power in the application of univariate and multivariate methods on long bones [4]. the level of reliability of the several aging techniques proposed in literature has been questioned and tested in numerous anthropological studies. the most critical contribution [1] pointed out that skeletal age-at-death estimates tend to mimic the age distribution of the reference sample from which the criteria were developed. this issue can be partly overcome by employing procedures such as the "combined" method [5], the summary age method [6] and the transition analysis [7]. another important issue is the level of consistency among researchers and across laboratories [8]-[10]. in this study, the analyses for sex and age-at-death assessments were performed following the criteria commonly reported in literature [3], [11], [12]. sex was mainly diagnosed on the basis of the morphology of the pelvis and the skull. age at death of adults individuals was estimated by means of many odontoskeletal indicators, which included, among others, degenerative changes of the pubic symphysis and auricular surface of the pelvis; morphological variation of the sternal end of the 4th rib, dental wear and ectoand endocranial suture closure. the subadults were aged according to dental formation and eruption stages [13] and skeletal maturity and dimensions [14]. in our age at-death assessment we followed the approach of the combined method in order to minimize the "reference sample effect". we also tested, in the velia series, the intraobserver level of consistency. a subsample of 241 skeletal individuals was aged by two researchers separately. the level of agreement reaches 88.8 %, while in the remaining cases the discordance never exceeded one age class. as expected, the inter-observer error was very low for subadults (5.8 % of the total estimates) in comparison with the adults (>20 years 18.6 %). the cases of discrepancy have been resolved with an additional assessment performed by a third observer, and using wider age classes. 2.3. stature  adult stature estimation can be considered as a rather good descriptive of the life conditions of a population because, beyond the direct action of the genetic pool, is strongly influenced by the health status and diet during early life [15]. estimation of stature in archaeological samples relies on linear regression formulae derived from reference collections that correlate the length of long bones (femur, tibia, fibula, humerus, radius, ulna) with living stature (i.e. [16]-[19]). the choice of the most proper regression formula to be used is crucial: the equations available in the literature are calibrated on specific modern population samples and the body proportions of the archaeological sample should be consistent with that of the reference one [20]. as suggested by giannecchini and moggicecchi [21], the pearson's regression formulae are the most reliable for italian historical samples, thus adult living stature in our roman italian coastal population was calculated applying this regression method. the maximum length for each bone, with no regard about side, was measured following the standard technique by martin and saller [22]. for each individual, the length of all the available long bones (considering femur, tibia, humerus, radius) was measured and the stature was given as the mean value of all the obtainable estimates. 2.4. pathological assessments  in the last decades, paleopathology has undergone a significant shift from descriptive analysis on individual cases to a population-based epidemiological perspective. the quantification and the cross comparison of past population health status present problems of diagnosis and interpretation [2], [23]-[25], which correlate with the standardization of the scoring methods and with inter-observer consistency [26], [27]. finally, the paleopathological practice has to deal with the   figure 1. geographic location of the port towns of portus romae and velia  porta marina.   acta imeko | www.imeko.org  september 2016 | volume 5 | number 2 | 21  continuum model of pathological skeletal signs and the issue of how to better ascribe the progressive manifestations to clear-cut discrete categories. in our study, the following macroscopic health indicators were recorded: cribra orbitalia; enamel hypoplasia; diffuse idiopathic hyperostosis (dish), external auricular exostosis (eae). all the health indicators were systematically recorded for the whole sample. the general guidelines for diagnosis and assessment of the pathological changes are reported in buikstra and ubelaker [11] and grauer [28]. dish was positively diagnosed by the pathognomonic features of the vertebral column; i.e. the ossification of the anterior longitudinal ligaments of the spine. enamel hypoplasic defects on teeth were counted and chronologically assessed by measuring their relative position from the cervical line (cemento-enamel junction) [29]. a 4 grade recording system was used in the assessment of presence and severity of cribra orbitalia, according to buikstra and ubelaker [11]; while the recording of external auricular exostosis followed the method presented in crowe and colleagues [30]. 2.5. dental enamel histology  histomorphometrical analysis of tooth crowns allows enhancing and interpreting enamel microstructures that are formed during amelogenesis (i.e. weekly retzius lines and daily cross striations [31]) as result of the regular periodicity of enamel matrix secretion. a physiological stress event strong enough to temporary disrupt amelogenesis, causes an abrupt change in the orientation of the enamel prisms, visible in an histological thin section as a broad dark band (accentuated retzius line (arl) or wilson band [32]). thus using the long and short period markings of enamel formation, it is possible to reconstruct the temporal scales of stress event chronology. the scale is zeroed at birth, marked by the first accentuated stria, the neonatal line [33], [34]. to establish the chronology of the stress events in the deciduous teeth the regression formulas proposed by birch and dean [35] were used, while for the first permanent molars the method by guatelli-steinberg and collaborators [36] has been applied, based on the average daily rate of enamel secretion (about 2.85 µm per day) within 200 µm from the enamel dentine junction. starting from the tip of the dentine horn and following an enamel prism toward the outer surface, the first arl was detected and the corresponding length was measured. the same procedure was applied to all the arl encountered in the lateral enamel moving toward the tooth neck. the regression formulas transform distances in days of enamel formation. this methodology has been used to determine with a daily accuracy the age at death of immature individuals in a subsample from velia, presenting the neonatal line and the crown still forming at the time of death. thin sections of dental crowns were obtained using the method proposed by caropreso and colleagues [37]. the teeth, after an ultrasonic bath, were included in bicomponent epoxy resin (epofix buehler) and cut in longitudinal buccolingual sections by means of a diamond blade microtome (leica microtome diamond blade 1600, leica ag). each section was thinned with abrasive paper mounted on a motorized grinder in distilled water (minimet 1000 automatic polishing machine, buehler) up to about 100 µm. further polishing with alumina powder (0.05 µm gamma alumina micropolish b, buehler) allowed the elimination of the cutting marks and the enhancement of the features of the fine structure of the enamel. after assembling the cover glass, each thin section was digitally recorded through a camera (leica dfc 295) attached to an optical microscope (laborlux s, leica ag) under polarized light, with the magnification of 40× (resolution 1600×1200 pixels). to digitally analyze the images, each crown was reconstructed assembling overlapping pictures using dedicated software (fiji plugin mosaic j, [38]). 2.6. isotopes analyses  the isotopes analyses for diet reconstruction were previously performed and published [39]-[41]. full details of the sampling and analytical methodologies, as well as individual data, are given in the above mentioned studies. 3. results and discussion  3.1. δ15n and δ13c values of isola sacra and velia   the diet composition of velia and isola sacra, compared with data available in literature for the roman empire, is presented in figure 2. the two samples under study are characterized by a high intra-site variability. for δ15n, in fact, the values show the coexistence of both low and high protein intake diets, the latter possibly deriving from marine food. as already suggested in [39]-[41], males tend to consume more high trophic foods (meat and/or fish) than females (figure 3), even if this tendency is statistically significant in velia but not in isola sacra. 3.2. velia: diet and stature   adult stature, although being a multifactorial trait influenced by a complex interplay of genetic and environmental factors, is commonly used as a proxy for health and nutritional status during growth [15]. the relationship between stature and diet composition for velia is presented in figure 4. results show a positive correlation between individual height and δ15n values, reaching the significance only in the male series (generalized additive regression model, p=0.022). figure 2. plot of convex hulls of the δ 15 n and δ 13 c individual values in velia  (blue), isola sacra (red) and other roman imperial age skeletal series (grey). acta imeko | www.imeko.org  september 2016 | volume 5 | number 2 | 22  3.3. velia and isola sacra: diet and auricular exostosis  external auricular exostosis (eae) is an abnormal bony growth within the external ear meatus that has proved to be very informative in detecting aquatic working activities in past populations [40]. the frequency of eae in isola sacra and velia adult males is rather high (21.1 % and 35.3 %, respectively). the presence of this specific occupational marker is significantly higher among the marine food consumers in velia, as shown in figure 5. conversely, at isola sacra, the correlation between eae and the c, n isotopic ratio fails to be significantly different from zero. 3.4. velia: diet and cribra orbitalia  cribra orbitalia is a pathological lesion consisting of porosities in the outer table of the orbital roof. it is mainly linked to iron deficiency anemia, of acquired (unbalanced diets, diarrheal disease, intestinal parasites) or hereditary origin (i.e. sickle cell or thalassemia). other possible but rarer causes have also been acknowledged [42]. cribra frequency in velia is 28.4 % (n=74) with no differences between sexes. the combined data of diet and cribra (figure 6) point out that the individuals showing the highest δ15n values do not present the lesions, while for the lower values both affected and unaffected individuals are equally represented. overall, cribra and diet are not correlated at a level significantly different from zero. 3.5.  velia: diet and dish  diffuse idiopathic skeletal hyperostosis (dish) is a pathological condition characterized by vertebral ankylosis and extraspinal bone proliferations. dish has a complex etiology: it is often found associated with different metabolic disorders and it is more frequent in aged males. in the velia male sample, the occurrence of dish is rather high (12.5 %; n=48), when compared with other archeological series [43], and significantly correlates with high δ15n values (figure 7), thus contributing with positive results to the issue of the possible influence of nutritional habits in the onset and manifestation of the pathology [44], [45]. 3.6. velia and isola sacra: health status and weaning  histomorphometrical analysis of the dental crowns was performed in a subsample of 79 subadults from velia and figure  5.  δ 15 n  and  δ 13 c  values  and  presence  of  the  external  auricular  exostosis (eae). figure 6. velia: nitrogen and carbon isotopic delta values and presence of  cribra orbitalia. figure 3. δ 15 n and δ 13 c values in velia and isola sacra by sex.    figure  4.  3d  scatterplot  and  generalized  additive  regression  of  stature  in  males (green dots) and female (blue dots) of velia in reference to n and c  isotopic ratios.  -22 -21 -20 -19 -18 -17 -16 6 8 1 0 1 2 1 4 13c 1 5 n isola sacra isola sacra eae velia velia eae acta imeko | www.imeko.org  september 2016 | volume 5 | number 2 | 23  compared with the results obtained from the histology of 127 deciduous teeth from the isola sacra sample [46]. figure 8 shows the monthly prevalence distribution of the defects in both velia and isola sacra. in both samples, the general trend of the curve is comparable, and shows a steep rise in months 1-5 possibly related, as expectable, to the decline of the maternal buffer. the higher monthly prevalence values observed in isola sacra suggest worst general health conditions during infancy, that could be related to a much broader range of the socio-economic status in reference to the smaller and more homogeneous velian community. such kind of morbility trends is usually associated with the stress of weaning; however, the maximum peak in velia seems to occur too early to be consistently explained with this hypothesis. in isola sacra the high number of physiological stresses (arl), registered from birth to 9 months, seems to do not affect the survivorship of children at later ages (r=-0.037). similarly, the number of arl does not consistently associate with diet (figure 9). 4. conclusions  the carbon and nitrogen stable isotope analysis depicts the dietary habits of velia and portus romae communities, showing a high degree of variability in the diet composition, especially regarding the levels of protein intake. both samples show indication of marine food consumption. these evidences can be explained by the geographical location of the towns, together with the social complexity and differential subsistence strategies within the communities. in the study of past populations, diet is usually considered a good proxy for wellbeing and it is usually presented and discussed as a standing alone evidence. conversely we think that a more in-depth comprehension of ancient life conditions can be obtained through a strong historical and archeological contextualization and through the integration of data from multiple sources. nevertheless, crossing stable isotope data with demographic, morphological and health parameters not always yields univocal results of simple and straightforward interpretation. in fact, we found that nitrogen delta values significantly correlates with a specific occupational activity (eae), higher statures and the manifestation of a skeletal disorder (dish), conversely the association of diet and aspecific stress indicators (arl and cribra orbitalia) was not detected. possible explanations can be suggested by the fact that cribra orbitalia, observed in the adult sample of velia were developed during infancy and adolescence. however, recent calibrated studies on bone remodeling rates [47] suggest that the isotopic signal in the adult bone still reflects most of its later infancy, adolescence and early adulthood. the absence of correlation between diet and arl in the growing segment of isola sacra deserves further consideration and analysis, since it goes straight to the issue of the differential (and hidden) frailty of individuals and its influence in their morbidity and mortality. in conclusion, the identification of physiological stress events and processes (such as weaning), the quantification of timing of formation, maturation and tooth eruption, the reconstruction of the differential diet interand intra population and its changes in the course of life, represent 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[47] r. hedges, j. clement, c. thomas, t. o'connell, collagen turnover in the adult femoral mid-shaft: modeled from anthropogenic radiocarbon tracer measurements, am. j. phys. anthropol. 133 (2007) pp.808-816. decision-making on establishment of re-calibration intervals of testing, inspection or certification measurement equipment by data science acta imeko issn: 2221-870x june 2023, volume 12, number 2, 1 8 acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 1 decision-making on establishment of re-calibration intervals of testing, inspection or certification measurement equipment by data science marija cundeva – blajer1 1 ss. cyril and methodius university in skopje (ukim), faculty of electrical engineering and information technologies (feeit), st. rugjer boskovik no. 18, 1000 skopje, republic of north macedonia section: research paper keywords: decision-making in conformity assessment; testing-inspection-certification; re-calibration interval; data fusion citation: marija cundeva – blajer, decision-making on establishment of re-calibration intervals of testing, inspection or certification measurement equipment by data science, acta imeko, vol. 12, no. 2, article 38, june 2023, identifier: imeko-acta-12 (2023)-02-38 section editor: leonardo iannucci, politecnico di torino, italy received january 31, 2023; in final form june 20, 2023; published june 2023 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: marija cundeva-blajer, e-mail: mcundeva@feit.ukim.edu.mk 1. introduction data science classes of tic problems the conformity assessment processes are mostly finalized with decision delivery, mostly evident in testing, inspection, and certification (tic). these decisions are predominantly based on empirical data derived by measurements. measurements are crucial in various critical societal sectors, such as healthcare, trade, industry, energy sector, environmental protection, etc., where tic activities are commonly conducted. lately, significant impact on the tic sector is created by the spin of the digital transformation. in the centre of the digital transformation there is the enormous quantity of data, which is continuously produced, processed, stored, and used for increasing number of applications. artificial intelligence, machine learning, internet of things, big data analytics etc. are based on data. however, the data quality is an issue not always properly addressed, especially in sectors with traditions based on experimental approaches, as the tic. poor models, incorrect results, and finally wrong decisions might derive from poor quality of data. data science utilizes scientific approaches, protocols, algorithms and systems with interdisciplinarity to extract insights and information from noisy, structured and unstructured data, and deploy knowledge from data in wide scope of applicative solutions [1], [2]. the recent revival of measurement and data science interrelation is induced by the emerging application of sensory devices and significant increase of available data storage, processing, transmission capacities which are variously utilized. one of the results of the high quantity of recorded information and of the theoretical achievements in measurement and data science, is an invention of numerous newly developed products and smart services. this contribution conducts an analysis of the options for utilization of the latest data science achievements in the tic decision making processes, based on conclusions with synchronous usage of “measurements” as completely empirical, and the “data science” as methodology oriented towards modelling and simulation, combined with high complementarity and synergy, i.e., the data fusion approach. the modern scientific abstract this contribution is related to issues on decisions in conformity assessment, especially in testing, inspection, and certification (tic) predominantly based on measurement data. the digital transformation has started to impact the tic sector, which is related to intensified utilization of data science in tic. the quality of the data in big data analytics, is not always sufficiently addressed, especially in sectors with traditionally empirical approaches, such as tic. this paper conducts a survey of options for deployment of data science in the tic decision making processes, based on conclusions with complementary usage of empirical “measurements” and the “data science”. the focus of the discussion is centered over a case study where data fusion is applied for determining the tic instrument calibration frequency, presenting a model to establish recalibration interval with reduced risk in the final decision delivery over the next re-calibration moment. mailto:mcundeva@feit.ukim.edu.mk acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 2 methodologies require sustaining the theory validity, through experimental verification, whenever possible. experimental proof comprises a quantitative measure or non-quantitative (i.e., qualitative measure) of the observed quantities achieved through measurement. the consistency degree of various measurement results, derived by different independent experimenters or by the same experimenter at various moments, provides an indicator for the reliability of the results of the quantity of interest, taking into account that empirical knowledge is mostly imperfect to some degree and the combination of observations are standard and essential practices [3]. several classes of data science problems for which techniques might be developed and evaluated across different domains in the tic sector are [1]: • detection: finding data of interest in given dataset. • anomaly detection: identification of system states that force additional pattern classes in a model. outlier detection is associated with identifying potentially erroneous data items forcing changes in prediction models “influential observations”. • cleaning: elimination of errors, omissions, and inconsistencies in data or across datasets. • alignment: relating different instances of the same object [4], like a word with the corresponding visual object, or time stamps associated with two different time series. data alignment is frequently used for entity resolution, identifying common entities among different data sources. • data fusion: different representations integration of the same real-world object, encoded in a well-defined knowledge base of entity types [5]. • identification and classification: attempt to determine, for each item of interest, the type or class to which the item belongs [6]. • regression: finding functional relationships between variables. • prediction: estimation of a variable or multiple variables of interest at future times. • structured prediction: tasks where the outputs are structured objects, rather than numeric values. a desirable technique to classify a variable in terms of a more complicated structure than producing discrete or realnumber values. • knowledge base construction: construction of a database with a predefined schema, based on any number of diverse inputs. • density estimation: production of a probability density (distribution function), beside a label/value. • joint inference: joint optimization of predictors for different sub-problems using constraints that enforce global consistency used for detection and cleaning for more accurate results. data science involves ranking, clustering, and transcription (“structured prediction”), as in [7]. other classes of problems are based on algorithms and techniques which are applied to raw data at an earlier “pre-processing” stage. different data processing may be activated if evaluation methodology is essential, [1]. 2. main statistical paradigms for decision making in tic the international endorsement of the guide to the expression of uncertainty in measurement (gum), [8] accelerated the need to provide uncertainty statements in measurement results. the laboratory accreditation based on standards such as iso 17025 [9] has amplified this process. as the uncertainty statements have been recognized as essential for effective decision making, various laboratories, from national metrology institutes to commercial test laboratories, insert significant workload into evaluation of measurement uncertainty by applying the gum methods [8], [10], [23], but also the methodologies proposed in the international guideline ilac g8 [22]. the approaches for uncertainty propagation evaluation in the tic applications comprise the frequentist, bayesian, and fiducial statistical paradigms [11], [23]. the first statistical paradigm frequentist, is where uncertainty can be probabilistically assessed, and is based on statistical theory, referred as “classical” or “conventional”. considering the origin of uncertainty in tic, these approaches must be adjusted to derive frequentist uncertainty intervals under practical conditions. in most realistic tic environments, uncertainty intervals must comprise both the uncertainty in quantities estimated using data and the uncertainty in quantities derived from expert knowledge, so the approach of data fusion is indispensable. to gain an uncertainty interval, the measurands which are not under observation are usually treated as random variables with probability distributions of their values, on the other hand measurands with values possible to be assessed by applying statistical data are considered as unknown constants. the traditional frequentist protocols should be altered to achieve the prescribed level of confidence after averaging over the potential quantities’ values evaluated by expert judgment [11]. the second paradigm bayesian approach [11] named after the fundamental theorem, which was proved by the reverend thomas bayes in the mid-1700s, is where the analyst’s knowledge about the measurands is modeled as a set of stochastic variables with a probability distribution in the joint parameter space. the theorem enables the probability distributions to be updated based on the observed data and the inter-relationships of the parameters defined by the function or equivalent statistical models. the probability distribution is obtained by describing the knowledge of measurand given the observed data. the third statistical paradigm fiducial approach, is developed by r. a. fisher in the 1930s [11]. the probability distribution (fiducial distribution) for a measurand conditional on the data is gained from the interrelationship of measurand and the input value described by the function and the distributional assumptions on the data used to estimate. 3. decision-making, and risk based thinking in tic established by data fusion data fusion aim to obtain higher quality information to apply to specific contexts, by profiting from the symbiosis of data collected from diverse sources. data fusion is the process of combining data or information to estimate or predict entity states [12]. applied in many decision-making domains, such as the tic, it encompasses classification and pattern recognition utilized to argument decisions. decision making, especially in conformity assessment is directly linked to introduction of risks in the laboratory or the tic entity’s operations. so, in tic it is crucial acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 3 not only to fuse data obtained from multiple sources, both experimental and theoretical, but also to assess threats and risk [9], [22] and [23]. data fusion enlarges robustness and soundness and diminishes the vulnerability of the system giving arguments for the decision, and enabling decision-making even when some sources of information are missing or are inappropriate. through data fusion better and larger coverage of space and time is achieved, ambiguity is decreased, because better information leads to better distinction among available hypotheses. data fusion is based on experimental data output by sensing devices or instruments, and on information gained by other routes (e.g., the user as a data source for a priori knowledge, experience, and model application). data fusion imposes all data to be represented in the same format (e.g., numeric values in the same units, relative values). if data are diverse in representation, data alignment or data registration is indispensable [12]. measurements, as instrument outputs, produce a signal usually affected by noise, and whose reliability has to be proved (e.g., instrument malfunction, express corruption of measured quantity, like jamming). filtering and validation of the data are necessary in data fusion processes. data fusion comprises activities tackling: data from sources with different quality levels, such as different accuracy, co-related data, inflation of information, and all other issues leading to computational problems, and impose a need to change the context of the observation, like from time to frequency domain, or to extract features or attributes [12]. as an illustration for application of data science in the tic sector, one of the most relevant tic decision-making and riskintroducing issues will be further demonstrated determination of the re-calibration period of the tic measurement equipment by deploying data fusion as a mean for argumentation. 4. planning the tic instrument re-calibration period by deployment of data fusion estimating the re-calibration intervals is an essential issue of the tic sector entities utilizing calibrated instruments in their activities. most of the test equipment in today's laboratory inventories are multi-parameter items or consist of individual single-parameter items. an item-measurand is declared to be outof-tolerance if a single instrument parameter or an item in a set, is found to be out of pre-defined specifications. this is expensive, and introduces risks [13], [14], [23]. most of the published methods for planning the re-calibration period of an instrument, are of statistical origin and can be adequately used only for large inventories of instruments, [15]. as a result of the different performance characteristics of individual instruments and their changeable working conditions, instrument reliability is complex to anticipate. extended calibration intervals might have a consequence in increased potential costs associated with a given instrument, as more operation cycles (tests or calibrations) have been conducted before it is re-calibrated and found to be inor out-of-tolerance. a posteriori costs might encompass a reverse traceability review to identify the items that have been tested by the instrument, a thorough investigation of the level of negative impact on their performance given the scale of the instrument’s out-of-tolerance, leading to customer alert, accreditation suspension, product recall and imperceptible issues like the tic entity’s jeopardized reputation might occur. in this contribution, the focus is on estimating the recalibration period of measuring instruments used by the tic entities by deploying data fusion for reduced decision-making risk. the approach for determining the recalibration range will be validated through a case study on experimental calibration and check data of an electrical measuring instrument, by fusion of data from diverse sources (both a posteriori experimental and a priori knowledge, experience, model application). most of the standards according to which the tic entities are accredited/certified require to have available, suitable, and adequate facilities and equipment to permit all tic activities to be carried out in a competent and safe manner, with the responsibility lying solely on the tic entity. one of the most significant decisions regarding the calibration is “when and how often to do it?” many factors influence the time range between calibrations, and they should be identified and considered by the tic entity. the most important factors are: • uncertainty of measurement required or declared by the tic entity, • risk of a measuring instrument exceeding the limits of the maximum permissible error when in use, • cost of necessary correction measures when it is found that the instrument was out-of-tolerance over a long period of time, • type of instrument, • tendency to wear and drift, • manufacturer’s recommendation, • extent and severity of use, • environmental conditions (climatic conditions, vibration, ionizing radiation, etc.), • trend data obtained from previous calibration records, • recorded history of maintenance and servicing, • frequency of cross-checking against other reference standards/measuring devices (including diverse measures for quality assurance in tic, such as inter-laboratory comparisons or proficiency testing schemes, or repeatability of tests under different operating conditions), • frequency and quality of intermediate checks in meantime, • transportation arrangements and risk, and • degree to which the tic personnel are trained [15]. the ilac-g24 specifies the following methods, [15]: • automatic adjustment or “staircase” (calendar-time), • control chart (calendar-time), • “in-use” time, • “in service” checking, or • “black-box” testing, and • other statistical approaches. the use of statistical methods (i.e., by deploying data science) on an individual instrument or instrument type are of interest, especially if combined with adequate software tools. according to agilent technologies®, prior to the introduction of a new product, [16] the responsible personnel set the initial recommended re-calibration period. data is treated as reliable data if originating from at least three areas: • data from similar instruments, • data for the individual components used in the instrument, • data on any subassemblies deriving from existing mature products (i.e., instruments). acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 4 the usual working environment and the testing results of the surrounding conditions conducted on instrument prototypes are considered as well [18]. several methods for determining the calibration intervals are published, [13], [14], [19], [20] and [21]. some models assume that the calibration condition of the instrument can be traced by monitoring the drift of an observable parameter, [13]. the calibration ranges can be presented according to analysis by parameter variables data, analysis by parameter attributes data, by instrument attributes data, and by class instrument attributes data. other methods, such as an extension by providing a maximum likelihood estimation for the analysis of data characterized by unknown failure times, are given in [13], where the estimation method is using the exponential reliability function. an approach with a review of the instrument’s calibration history is presented in [14], calibration records indicate the history of remaining in tolerance. the instrument might have a higher likelihood of remaining in tolerance, as a result of an algorithm that has been developed calculating calibration ranges based on the condition received from calibration along with a historical weighting. a method from variables data is presented for determining calibration intervals for parameters whose value demonstrate time-drift with constant statistical variance. the method utilizes variables data in the analysis of the timedependence of deviations between as-left and as-found values from calibration. the deviations are from the difference between a parameter's as-found value at a given calibration and as-left value prior to calibration [14]. the choices for the tolerance band for parameter x in the table 1, are derived from other authors publications [14], but also based on the own laboratory metrology experience. in further research other methodologies for this purpose are planned to be deployed, like the suggested decision-making ranges as in ilac g8, [22]. in [19] and [21], a stochastic optimisation approach for determination of the re-calibration period is presented. a genetic algorithm methodology is deployed for estimation of the next calibration period, considering the previous calibration history of the measurement device. the experimental results of last calibration certificate are used for verification of the predicted device measurement time drift in the moment of the estimated moment of next calibration. the modelling is performed by representing the time dependence of the instrument time-drift with lagrange orthogonal polynomials, constructed from experimental calibration history embedded in an algorithm based on the statistical least square method and inclusion of the accompanying uncertainties in the genetic algorithm stochastic optimisation tool for determination of the coefficients of the polynomial model of the function of the time-drift of the instrument. 4.1. estimation of a re-calibration period-model development based on the previous discussions and survey, the following innovative data fusion model for determination of the recalibration period is proposed: ni = eci ⋅ [𝐶1 ∙ x + c2 ⋅ x + c3 ∙ x + c4 ∙ x + ic ∙ 𝑌 + +cfu ∙ 𝑍 + co ∙ 𝑈 + ofh ∙ 𝑉 + ms ∙ 𝑊] (1) where are: ni new interval eci established calibration interval table 1. values of parameters as multipliers. parameter value x “in tolerance” 1 “out of tolerance” 0.8 < 1x the tolerance band “out of tolerance” 0.6 > 1x the tolerance band, < 2x the tolerance band “out of tolerance” 0.4 > 2x the tolerance band, < 4x the tolerance band “out of tolerance” 0.3 > 4x the tolerance band, < 4x the tolerance band y=σyi y1 number of in-service checks between calibrations 1 time 0.1 < 5 times 0.3 < 10 times 0.4 > 10 times 0.5 y2 measured value no difference (<3%) 0.5 difference < 20% 0.4 difference > 20% 0.1 z=σzi z1 frequency of usage dayly 0.1 montly 0.5 yearly 0.7 z2 habit of usage used with caution in laboratory conditions 0.3 used with caution in tendency to wear and drift 0.2 use without special attention in terms of events 0.1 u=σui u1 cost of calibration small 0 medium 0.3 large 0.5 u2 cost of necessary correction measurement (in case the artefact of calibration is out of specifications and further service/adjustment is needed and repeated calibration procedure is imposed) < 0.5 x cost of calibration 0.5 < 1 x cost of calibration 0.1 > 1 x cost of calibration 0 v the operator is trained to handle the instrument and knows the measured items 1 the operator is trained to handle the instrument, but imperfectly acquainted with the measured items 0.5 w service (minor repair or adjustment of the artefact of calibration) performed between previous and last calibration 0.5 no service performed between previous and last calibration 1 acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 5 𝐶1 historical weighting of the most recent calibration (0.2, modification of the simplified method) 𝐶2 historical weighting of the most previous calibration (0.1, modification of the simplified method) 𝐶3 historical weighting of the previous calibration (0.08, modification of the simplified method) 𝐶4 predicted value of next calibration (0.06, newly introduced parameter) ic in-service check between calibrations (0.1, newly introduced parameter) cfu condition and frequency of use (0.2, newly introduced parameter) co costs (0.1, newly introduced parameter) ofh operator factor and habit (0.08, newly introduced parameter) ms maintain and service (0.08, newly introduced parameter). the model in the equation (1) is derived by combing models from [14] an [15], and by deploying the laboratory experience in electrical metrology (i.e., the linearity of the electrical instruments characteristics derived from the instruments technical specification), like the time-drift of the instruments [17]. the main idea is to provide an easy model ready to be used in the tic entities everyday operations. the eci can be specified depending on the experience with the stability of similar instruments, experience, and recommendations. this is a parameter containing the a priori knowledge in the data fusion process. other coefficients of a priori knowledge origin are 𝐶1, 𝐶2, 𝐶3, 𝐶4, ic, cfu, co, ofh and ms. the 𝐶1, 𝐶2, 𝐶3, are historical weighting coefficients, of the previous three calibrations. in case of more than three previous data, for a shorter time history they can be taken into consideration with significantly lower weighting coefficients (less than 0,4 or 0,2, respectively for the fourth and fifth previous calibration), and in case of longer time history (longer periods of re-calibration of more than one year) they can be neglected. the longest possible re-calibration period will be estimated, leading to a conclusion that this approach is more rigorous in comparison to the “simplified method” as defined in [15], only if the estimated period is shorter than the real time range used for validation derived from the last calibration certificate and in which the instrument was found to be in-tolerance. the parameters as multipliers are given in table 1. 4.2. experimental case study for methodology validation a data base containing the historical data of previous calibrations of the instrument must be established and maintained by the tic entity, for further proper implementation in the proposed model. the proposed model is feasible to be utilized after at least two conducted calibrations of the instrument in appropriate time ranges. as a case study for validation of the proposed methodology, a real data base with the calibration history of a digital multimeter used during testing process by a tic body is adopted. the variations of the calibration values should be considered in maximum available measurement points, emphasising points with detected changes. to be on the safe side, the most acceptable value of x is the smallest value among all available points. the expected value of the next calibration time moment can be obtained by deploying sophisticated algorithms previously published in [14], [18], [19], [20], and [21], but for some tic entities their approaches introduce obstacles and risks for implementation. the methodology we present in this study, embedding the statistical tool of the least squares, is a simplified option for the tic entities. this method is chosen because it is embedded in many already available calculators to the tic entities in a very user-friendly form (e.g., ms excel). the in-service checks with another instrument, should be accomplished in time moments and occasions where the uncertainty of calibration is on disposal for both instruments. tic entity’s quality management proposes the extent of factors and habits of the staff, while the instrument operator specifies the frequency and conditions of use, which are another example of a priori knowledge in the data fusion process. depending on the available history data and tracking behaviour of the instrument, the coefficients proposed in the algorithm can be modified and customized for each instrument or group of instruments (i.e., the model is universal and invariant to the type of instrument or the number of instruments). the next calibration period of a metrel® eutotest xe mi 3102 tester is estimated as a case study for the model validation. following the recommendations of the instrument producer metrel® [17], regular 6-months or 1-year calibration of all measurement functions of the instrument should be carried out. this case study has been chosen because of the instrument, the artefact of calibration, and because the laboratory already has sufficient data on disposal. namely, the data used in the modelling and verification process, is for a period of 72 months (i.e., period of 6 years), which is an appropriate time range to derive sound regressive conclusions. in tables 2 and 3 the calibration history for the instrument in a single point of the current and voltage measurement ranges are given, respectively. the data used in tables 2 and 3 are from the calibration certificates of the instrument, conducted by an external accredited calibration laboratory. to the moment of first calibration the zero value is assigned, and the time representation of the further moments of calibration are expressed in month units from the first calibration. the reference calibration value for the current is chosen to be 10 a, while the reference value for the voltage is selected to be 400 v. the measurement uncertainty is divided by a corresponding coverage factor declared in the calibration certificate, as in the history calibrations, they are carried out in different laboratories, some expressing the expanded uncertainty at factor of coverage table 2. the calibration history in a single point of 10 a in current measurement range of metrel® eurotest xe mi 3102. t in month i in a uncertainty in ma 0.00 9.98 0.020 15.00 10.00 4.121 36.80 10.01 14.545 52.60 9.98 14.545 72.90 10.00 0.015 table 3. the calibration history in a single point of voltage measurement range of metrel® eurotest xe mi 3102. t in month u in v uncertainy in v 0.00 399.00 0.42 21.80 401.00 0.36 37.66 401.00 0.36 58.02 400.00 0.50 acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 6 k = 1.65 (for rectangular distribution at probability of 95 %) and some expressing the expanded uncertainty at factor of coverage k = 2 (for normal distribution at probability of 95 %). so, the standard uncertainty is utilized in the calculations. this is the step of data alignment in the process of fusion of heterogeneous data. the trend lines for both quantities current at 10 a represented in equation (2), and voltage at 400 v represented in equation (3) are derived by utilization of the statistical least square method, with exclusion of the experimental value from last calibration, and it is used for predictive verification of the models. the derived predictive models are: 𝐼 = −7 ∙ 10−7 ⋅ 𝑡 3 + 10−5 ⋅ 𝑡 2 + 0.0013 ⋅ 𝑡 + 9.98 𝑅2 = 1 (2) u = −0.0024 ⋅ 𝑡 2 + 0.1448 ⋅ t + 399 𝑅2 = 1 . (3) in figures 1 and 2 the calculated expected values from the function models in (2) and (3) by inserting the last calibration time moment in the models, are shown in figures 1 and 2. so the expected values are 9.85 a for current measuring range and 399.32 v for the voltage measurement range and are in tolerance. the differences between the calculated (theoretical) values and the real measured values, derive from the ranges of the measurements uncertainty in calibration which is one of the main inputs in the modelling, and which is of stochastic and unpredictable nature. in the table 4 are presented the experimental results of inservice check measurements with another instrument of similar type (comprising the same measurement ranges as the object of validation) with established measurement traceability. the results are in limits of errors (i.e., in-tolerance). in case of out of tolerance result derived from the in-service measurement checks, according to the prescribed laboratory procedures the instrument will be subjected to immediate re-calibration or put out of service. other values for the parameters in the algorithm are as follows: eci = 24 months ni = eci ⋅ [𝐶1 ⋅ x + c2 ⋅ x + c3 ⋅ x + c4 ⋅ x + +ic ⋅ y + cfu ⋅ z + co ⋅ u + ofh ⋅ v + ms ⋅ w] = 24 ⋅ [0.2 ⋅ 1 + 0.1 ⋅ 1 + 0.08 ⋅ 1 + +0.06 ⋅ 1 + 0.1 ⋅ 0.8 + 0.2 ⋅ 0.3 + 0.1 ⋅ 0 + +0.08 ⋅ 1 + 0.08 ⋅ 1] = 24 ⋅ 0.74 = 17.76 . (4) the last calibration is not used in the prediction of the next value and is used as a validation point of the algorithm. the real calibration period (between the last two calibrations) is 20 months, while the predicted re-calibration period by the proposed algorithm is 18 months. the values obtained with the last calibration validate the method. shorter value of the recalibration interval is obtained, which is on the safe side, and can figure 1. the calibration history and expected value in a single point 10 a of current measurement range. figure 2. the calibration history and expected value in a single point 400 v of voltage measurement range. table 4. experimental values of the last in-service check with other calibrated instrument with established measurement traceability. instument i in a u in v metrel mi 3102 3.7 224 metrel mi 833 3.7 223 table 5. multipliers used in the case study. parameter value x “in tolerance” 1 y=σyi y1 number of check between calibration < 5 times 0.3 y2 measured value no difference (3%) 0.5 z=σzi z1 frequency of usage dayly 0.1 z2 habit of usage used with caution in tendency to wear and drift 0.2 u=σui u1 coft of calibration small 0 u2 cost of necessary correction measurement > 1 x cost of calibration 0 v the operator is trained to handle the instrument and knows the measured items 1 w no service performed between previous and last calibration 1 acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 7 be accepted as applicable without introduction of additional risks from aspect of the re-calibration period. in fact, the tic operation’s risk is significantly mitigated, i.e. minimized. namely, in case when the derived theoretical value would be outside of the tolerance limits, is could be concluded that the planned recalibration is not appropriately chosen (i.e., it is too long). in that case, new planning should be conducted, in accordance with the period calculated using the model. additional validation of the predictive model for determination of the re-calibration period is the a posteriori experimental approach of verification by executing in-service check measurements with another instrument of similar type (comprising the same measurement ranges as the object of validation) with established measurement traceability. this case study has validated the proposed methodology for prediction of the next moment of instrument calibration. the derived results demonstrate reduced risk arising from out-oftolerance state of the instrument due to prolonged re-calibration period. so, this data fusion methodology, with the proposed simple procedure for application in any tic entity, enables argumented decision making concerning the determination of the instrument re-calibration period. the mitigated risk from this aspect increases the confidence in the reliability in the instruments used by the conformity assessment body. 5. conclusions the proposed methodology for predicting the period of recalibration based on data fusion concept is simple, containing a plenty of data on factors influencing the stability of the instrument derived from diverse sources. it is easily deployable in the daily routine of any tic entity. the model opts to decrease the quality management risk of the occurrence of errors due to inappropriately defined re-calibration period of any instrument used in the tic activities. the presented case study validates and confirms the effectiveness of the proposed methodology, through experimental values verification. an advantage of proposed universal model is its openness which enables the variation of the coefficients and provides means for specialization in case of a group of instruments. one of the options for generalisation of the method in case of a group of large number of instruments of the same type, is the possibility to fix some of the coefficients, and to make variations of certain coefficients. more thorough studies should be conducted in this context, by deploying statistical approaches for random sampling of data from previous calibrations of high number of instruments of same type (i.e., date reduction should be carried out through a data science approach). finally, it can be concluded that data fusion approach is highly adaptable for various decision-making situations in the tic sector, opening possibilities for mitigation and reduction of risks during tic operations. references [1] k. cunha, r. santos. the reliability of data from metrology 4.0. international journal on data science and technology. vol. 6, no. 4, 2020, pp. 66-69. doi: 10.11648/j.ijdst.20200604.11 [2] b. j. dorr, c.s. greenberg, c.s., fontana, p. et al. a new data science research program: evaluation, metrology, standards, and community outreach, int j data sci anal 1, 2016, pp. 177–197. doi: 10.1007/s41060-016-0016-z [3] f. pavese, an introduction to data modelling principles in metrology and testing, ed. f. pavese, a. forbes “data modeling for metrology and testing in 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[9] iso, en iso/iec 17025 general requirements for the competence of testing and calibration laboratories, cenelec, brussels, 2017. [10] bipm, evaluation of measurement data supplement 1 to the “guide to the expression of uncertainty in measurement” propagation of distributions using a monte carlo method, jcgm 101:2008. [11] w. f. guthrie, h. liu, a. l. rukhin, b. toman, j. c. m. wang, n. zhang, three statistical paradigms for the assessment and interpretation of measurement uncertainty, ed. f. pavese, a. forbes “data modeling for metrology and testing in measurement science”, birkhauser boston, springer, llc 2009, pp. 71-116. [12] p. s. girao, o. postolache, j. m. d. pereira, data fusion, decision-making, and risk analysis: mathematical tools and techniques, ed. f. pavese, a. forbes “data modeling for metrology and testing in measurement science”, birkhauser boston, springer, llc 2009, pp. 205-254. [13] a. krleski, m. cundeva-blajer, analysis and contribution to methods for determination of optimal recalibration intervals, proc. of mma’16 26th symposium metrology and metrology assurance, sozopol, bulgaria, 7-11 september 2016, pp. 493-499. online [accessed 26 june 2023] https://metrology-bg.org/fulltextpapers/187.pdf [14] h. castrup, calibration intervals from variables data, ncsli work. & symp., washington, usa, 2005. [15] ilac, ilac-g24:2022 guidelines for the determination of calibration intervals of measuring instruments. online [accessed 25 june 2023] https://ilac.org/?ddownload=124891 [16] agilent technologies, setting and adjusting instrument calibration intervals – app. note, 2013. [17] metrel, eurocheck cs 2099 user manual, 2013. [18] allen bare, simplified calibration interval analysis, ncsli workshop & symposium, 2006. 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[21] m. cundeva-blajer, application of optimization methods for improved electrical metrology, acta imeko november 2016, vol. 5, issue 3, pp. 9-15. doi: 10.21014/acta_imeko.v5i3.378 https://doi.org/10.11648/j.ijdst.20200604.11 https://doi.org/10.1007/s41060-016-0016-z https://doi.org/10.1007/978-3-642-02463-4_12 http://dx.doi.org/10.1609/aimag.v36i1.2569 http://www.kdnuggets.com/2015/01/fundamental-methods-data-science-classification-regression-similarity-matching.html http://www.kdnuggets.com/2015/01/fundamental-methods-data-science-classification-regression-similarity-matching.html https://metrology-bg.org/fulltextpapers/187.pdf https://ilac.org/?ddownload=124891 https://doi.org/10.21014/acta_imeko.v5i3.378 acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 8 [22] ilac, ilac g8: 09/2019 guidelines on decision rules and statements of conformity. online [accessed 25 june 2023] https://ilac.org/?ddownload=122722 [23] s. andonov, m. cundeva-blajer, fmea for tcal: risk analysis in compliance to en iso/iec 17025:2017 requirements, proc. of 24th imeko tc4 int. symp. / 22nd int. workshop on adc and dac modelling and testing, 2020 palermo, italy, 14-16 september 2020. online [accessed 25 june 2023] https://www.imeko.org/publications/tc4-2020/imeko-tc42020-42.pdf [24] m. cundeva-blajer, data science for testing, inspection and certification, proc. of the imeko tc11 & tc24 joint hybrid conference, dubrovnik, croatia, 17-19 october 2022. doi: 10.21014/tc11-2022.25 https://ilac.org/?ddownload=122722 https://www.imeko.org/publications/tc4-2020/imeko-tc4-2020-42.pdf https://www.imeko.org/publications/tc4-2020/imeko-tc4-2020-42.pdf https://doi.org/10.21014/tc11-2022.25 microsoft word 261-1779-2-le-rev3 acta imeko  issn: 2221‐870x  september 2015, volume 4, number 3, 42 ‐ 46    acta imeko | www.imeko.org  september 2015 | volume 4 | number 3 | 42  surface conductance and microwave scattering in  semicontinuous gold films  jan obrzut  material measurement laboratory, national institute of standards and technology, gaithersburg, md 20899‐8542, usa            section: research paper   keywords: coplanar waveguides; scattering parameters; conductance; microwave absorption; percolation; semi‐continuous gold films  citation: jan obrzut, surface conductance and microwave scattering in semicontinuous gold films, acta imeko, vol. 4, no. 3, article 7, september 2015,  identifier: imeko‐acta‐04 (2015)‐03‐07  editor: paolo carbone, university of perugia, italy  received february 24, 2015; in final form april 8, 2015; published september 2015  copyright: © 2015 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: this work was supported by the nist center for nanoscale science and technology, and material measurement laboratory   corresponding author: jan obrzut, e‐mail: jan.obrzut@nist.gov    1. introduction  semicontinuous thin metallic films have attracted attention due to their unusual optical and electronic properties such as enhancement of non-linear optical effects [1] and inhomogeneous localization of electromagnetic eigenmodes [2]. the characteristic fractal morphology of such films develops during deposition, when metallic nanoparticles nucleate to form conducting clusters mixed with dielectric voids. the size of the conducting clusters increases with increasing deposition thickness until a percolation threshold is passed and the film becomes conducting [3], [4]. in the vicinity of the percolation transition, the size of the inhomogeneities is comparable with the mean free path of the electrons and results in partial carrier localization [5]. such films may also exhibit unexpected electromagnetic characteristics, as the carrier response can be dominated by resistive localized activated transport between grains [2]. typically, dc or thz [6] electrical conductivity has been reported, but microwave studies are scarce due to the complexity of such measurements [7], [8]. in this paper, we report measurements of conductance and power absorption over the dielectric to metallic percolation transition using a technique based on coplanar waveguides (cpw) [9]. this approach enables effective impedance matching and allows for measurement of the film propagation characteristics, which can be accurately normalized with the film surface conductance while mitigating uncertainty from the film thickness measurement. the results are presented for gold nanoparticles deposited on a flat surface, which we choose as a model of 2d semicontinuous metallic film. abstract  semicontinuous gold films 4 nm to 12 nm thick were characterized using patterned coplanar waveguides over a frequency range of  100  mhz  to  20  ghz.  such  films  can  form  two  dimensional  fractal  aggregates  mixed  with  dielectric  voids  with  unusually  large  electromagnetic absorption. surface conductance and microwave absorption were obtained from the measured scattering parameters  using  a  microwave  transmission‐reflection  model.  within  the  percolation  coverage  of  gold  nanoparticles,  when  the  surface  conductance  increases  from  10 –5   s  to  10 –3   s,  the  film  properties  transition  from  dielectric  to  metallic,  and  the  corresponding  microwave transmittance falls far more rapidly than the classical skin depth model would suggest. the resulting microwave absorption  attains a peak value in this range, which results from an inhomogeneous localization of an electromagnetic field in fractal structures.  the  dielectric  to  metallic  transition  can  be  easily  identified  experimentally  from  an  abrupt  change  in  the  phase  of  the  reflection  scattering  parameter  s11.  the  results  demonstrate  a  convenient  measurement  technique  to  study  electromagnetic  properties  of  surface‐enhanced semicontinuous metallic films for thin broadband absorbers with minimized reflection, and for other microwave  applications.   acta imeko | www.imeko.org  september 2015 | volume 4 | number 3 | 43  2. methods  2.1. coplanar waveguide testing structure  coplanar waveguides with a nominal impedance value (z0) of 50  and a propagation length (l) ranging from 450 µm to 1800 µm, were made with 10 nm ti and 200 nm au evaporated on 500 m thick, 25 mm by 25 mm electronic grade alumina wafers (figure 1). cpws were patterned by lift off lithography. the width (w) of the central signal strip of these cpws was 50 µm ±0.2 µm, while the signal to ground plane spacing (s) was nominally 22 m. semicontinuous films were deposited by thermal evaporation of gold directly onto cpw through a shadow mask. during deposition the film average mass thickness (d) was monitored by a quartz crystal oscillator with resolution of 1 nm. 2.2. microwave measurement and analysis  measurements of the microwave wave scattering parameters, s11 and s21, were performed using an agilent 8720d network analyzer in the frequency range of 1 ghz to 20 ghz. the analyzer was connected to the cpw test structure with phase preserving cables from agilent (85131-60013) and 50  ground-signal-ground (gsg) air-coplanar probes (acp40, 100 µm pitch) from cascade. the measurement system was calibrated using a 101-190c impedance calibration standard and wincal calibration software from cascade. parameters in bold face denote complex quantities that have both magnitude and phase. after film deposition, the cpw impedance changes from z0 to zs. we consider the cpw test structure as a microwave network consisting of impedance discontinuity z0;zs;z0, that is, zs inserted between two reference transmission lines having a real characteristic impedance z0 (figure 1) where multiple wave reflection takes place at each z0;zs interface, affecting the reflection coefficient, , and transmission coefficient, t . the material’s properties in the specimen section of propagation length l are represented by the complex impedance zs, and complex propagation constant s. the relation between the measured scattering parameters s11, s21, and , zs and t are given by equations (1-3) [10]-[12]: 11 2 21 2 112 2 1 1 s ss b,bbγ   (1) γ γ z    1 1 0zs (2) γss γss t )(1 2111 2111    . (3) in (1) || <= 1.0. the complex transmission coefficient t = e-s l is related to the complex propagation constant by (4): ls )]/( [ln -1tγ  (4) which has multiple solutions. since t = |t |ej , (4) can be rearranged into (5) where the real part of s has an unique value, and only the imaginary part, the phase constant in (3) has multiple values: ]-n[(2|]/| [ln -1s ljl /)tγ  . (5) the ambiguity in the phase constant can be easily eliminated by measurements on two cpws with different propagation lengths. having determined zs and s, the distributed conductance, gs, can be obtained from the conventional transmission line relations [12]: )re( s s sg z γ  . (6) the distributed conductance gs (6) can be scaled with the film surface conductance (s), (in units of siemens per square): sgss 2 1  (7) where (s) is the cpw signal to ground plane spacing, here s = 22 m (figure 1). note, that s is independent of the cpw geometry, but it reflects the physical structure of the film, which depends on the particles surface coverage. in particular, s eliminates ambiguity associated with the film mass thickness measurement, which is commonly used in characterizing 2d metallic films. the microwave absorption, as, is given by (8): sss rta  1 (8) where the transmittance ts, and reflectance rs, are magnitudes of the corresponding transmission and reflection coefficient, respectively. the combined uncertainty of scattering parameters magnitude is 0.1 db and phase angle is 2. the combined relative uncertainty of s and as is within 5 %. 3. results   3.1. scattering parameters  figure 2 shows the magnitude and phase of the complex scattering parameters measured for uncoated cpws and cpws figure 2. magnitude and phase of scattering parameters s11 and s21 for the uncoated cpws (1), and the following au film thickness: (2) 4 nm, (3) 6 nm, (4) 7 nm,  (5) 8 nm, (6) 9 nm, and (7) 10 nm.  figure 1. cpw testing structure. acta imeko | www.imeko.org  september 2015 | volume 4 | number 3 | 44  coated with gold films with thickness increasing from 4 nm to 10 nm. the magnitude of the reflected wave, |s11|, for uncoated cpws is rather small, in the range of –45 db, indicating negligibly small insertion loss. the corresponding phase angle (plot 1 of s11 phase) indicates that the impedance mismatch is not significant. uncoated cpws show highly transmitting characteristics with |s21| slightly decreasing with frequency from its maximum value of 0 db, again indicating that the cpws test structures are well impedance matched. there is a general tendency of conducting au films to affect the magnitude and phase of both the s11 and s21 parameters. with increasing film thickness, the magnitude of s11 increases, while the magnitude of s21 correspondingly decreases. for example, in comparison to uncoated cpws, for a 7 nm thick au film the |s11| value increases by several orders of magnitude, from –45 db to about –10 db, while |s21| decreases from about 0 db to about –4.3 db. these changes are significant, and reflect the increasingly conducting character of the au films. as the film thickness grows, |s11| increases towards the maximum value of 0 db, while the phase angle approaches the value of . an abrupt change in s11 phase, from –135 to +175 when the film thickness increases from 6 nm to 8 nm, indicates a conductivity percolation transition from a dielectric to a conducting metallic state. films thicker than 6 nm are clearly conducting, each showing |s11| values approaching 0 db and similar phase angles of about . this is characteristic for reflection from a highly conducting interface. changes in s21 magnitude and phase show higher sensitivity to these thicker, more conducting films, and our measurement of both s11 and s21 accurately captures this transition from the insulating to the conducting state. the conductance is apparently frequency independent, as inferred from the flat characteristic of the magnitude of scattering parameters. it also increases with increasing film thickness. 3.2. surface conductance  the surface conductance values, s, determined from (7) are listed in table 1 at several frequencies. the observed slight dependence of s on frequency is rather not significant, and for films thicker than 4 nm it is within the measurement uncertainty of 5 %. in comparison the effect of the particle coverage, indicated by the film mass thickness d, is much larger. films 4 nm thick are only weakly conducting with s slightly increasing with frequency, from 210-6 s at 2 ghz to about 4.0  10–6 s at 10 ghz. thicker films show higher s values. s of 6 nm thick films is in the range of about 1.3  10–5 s to 1.4  10–5 s. this value increases further by three orders of magnitude to about 1.5  10–2 s for films 10 nm thick. such a large increase of conductance within the relatively narrow thickness range is indicative of the dielectric to metallic conductor percolation transition. with increasing d the metallic nanoparticles, initially separated in a mix with dielectric voids, nucleate to form conducting clusters. the size of the conducting clusters increases with increasing film thickness until a percolation threshold is passed and the predominantly dielectric film becomes a metallic conductor. in our previous work [13], we applied the generalized effective medium theory (gem) to model the effect of film surface coverage, p, on conductivity and found this model to provide a good estimate of the percolation threshold concentration pc, and critical exponents, s and t. in the gem model, the conductivity, s, below pc scales with the surface conductance of the dielectric, d, as s  d (pc  p)s where the exponent s describes the singular changes in s near the percolation threshold. d = 2 10-7 s is the measured conductance of the alumina substrate. above pc, s scales with the exponent t, s  m (p  pc)t, where m is the surface conductance of pure metal. the universal exponents t and s, both have values of about 1.2  0.2, close to the known ‘universal’ percolation exponent values in two dimensions [14]. similarly, the pc value of about 0.54 is consistent with the value of 0.5 predicted for a 2d square lattice. thus overall, these results were found to be consistent with charge transport in two-dimensions. in comparison, s  0.73 in three-dimensions for thicker, 20 nm films of random metallic particles [15]. the gem model formally “merges” the percolation and effective medium theories and typically captures well the conductivity crossover between the high and low conductivity states that is characteristic of most real dielectric – metallic composite materials in two and three dimensions [16], [17]. the shortcoming of this procedure is that the exact perturbative result is no longer recovered in the additive concentration. the effective medium theory predicts ‘effective’ absorption near pc in a very narrow concentration range when the permittivity diverges from the dielectric to metallic, even if both the dielectric and conducting components are lossless. in the next section we show the experimental evidence of a broad microwave absorption peak that is continuous on the conductivity scale, in apparent contradiction with the effective medium scaling arguments. 3.3. microwave absorption  figure 3 shows the microwave reflectance rs, transmittance ts, and absorption as (8) at 10 ghz plotted directly as a function of s. this approach removes ambiguity associated   figure  3.  microwave  transmittance  ts  (triangles),  reflectance  rs  (solid  squares) and absorption as (circles) of au films at 10 ghz in relation to the  film surface conductance s. table 1. surface conductance (in s), at several frequencies (f) for gold films  having mass thickness (d). d (nm) f (ghz) 2 5 10 15 20 4 1.86 10-6 2.85 10-6 3.92 10-6 3.71 10-6 3.32 10-6 6 1.33 10-5 1.47 10-5 1.46 10-5 1.33 10-5 1.27 10-5 7 1.00 10-3 1.01 10-3 1.10 10-3 1.10 10-3 1.16 10-3 8 3.00 10-3 3.00 10-3 3.01 10-3 3.01 10-3 3.14 10-3 9 6.48 10-3 6.59 10-3 6.68 10-3 6.72 10-3 6.68 10-3 10 1.87 10-2 1.73 10-2 1.55 10-2 1.44 10-2 1.35 10-2 acta imeko | www.imeko.org  september 2015 | volume 4 | number 3 | 45  with correlating the measured mass film thickness with surface coverage in semicontinuous films. reflectance of films with conductance s < 310-4 s is near zero. in this dielectric regime, the transmittance gradually decreases from 100 % to about 70 %, which is balanced by a gradual increase in absorption, as. within percolation transition when s increases from 310-4 s to 10-2 s, the transmittance falls rapidly to zero and as shows a peak reaching a maximum of about 0.5 at s  310-3 s, while rs increases steeply. above the conductivity percolation transition, when s > 10-2 s, as decays reaching a value of about 10 % at s  0.1 s, while rs approaches a value of 90 %. the large value of reflectance above s  0.01 s (d > 10 nm) is consistent with the film forming an electrically conducting mesh. the size of the conducting clusters, estimated from our microscopic study is about 25 nm [13]. thus, the metallic mesh at wavelengths  longer than the cluster size is almost totally reflecting. in figure 3, rs > 90 % when s  0.15 s (d > 12 nm) and the network of au nanoparticles acquires electrical characteristics of a continuous metallic film, with the bulk conductivity v  107 s/m. 4. discussion  the absorption peak that appears within the conductivity percolation transition (figure 3) is an interesting characteristic of the semicontinuous randomly distributed conductive nanoparticles. to our knowledge this is the first experimental evidence of such absorption in the microwave range. the results indicate a non-classical effect where microwave transmittance falls far more rapidly than the classical skin depth model [18] would suggest. table 2 illustrates this situation where attenuation 1-te =1-exp(-2d/e), due to skin penetration depth, e = (fr0(s/d))-1/2, is several orders of magnitude smaller than the actually measured absorption as. for 8 nm thick films, the classical attenuation model predicts absorption of 210-3, since at 10 ghz s  310-3 s and the skin depth e is about 9 m. in comparison, the measured absorption approaches a value of 0.5 and the corresponding classical penetration depth as  2d/ln(1-as) decreases to only about 25 nm. large local electromagnetic field fluctuations have been identified numerically at the percolation transition in 2d metallic-dielectric networks, assuming a strong skin effect, and frequency independent conductivity [2]. the presented microwave scattering data provide experimental evidence that the interaction of the electromagnetic field with randomly distributed conductive particles leads to localized resonant modes within the film, confined in a dimension comparable with the particle grain size, which is in the range of 20 nm to 100 nm (table 2). in an attempt to explore the origin of the microwave absorption in more detail, we examined the phase characteristic of the reflected wave, s11 shown in figure 2. the phase angle () of the scattering parameter s11 is negative, consistent with the predominantly dielectric character of films thinner than 6 nm below the percolation threshold. with film conductance increasing beyond the conductivity percolation threshold, s > 10-3 s, (d > 6 nm) the phase angle changes from negative to positive values by about . this indicates a transition from a capacitive (xc) to inductive reactance (xl) evidencing an lc resonance, which leads to power dissipation over the randomly distributed conducting paths. the film response may be formally described as equivalent to a distribution of resonant lc circuits or electrically shorted lossy transmission lines. the wide range of conductivity values over which the anomalous absorption is observed (figure 3) suggests high damping conditions and that the distribution of the effective guided wavelengths is rather broad. the ratio of the internal film resistance and impedance, |r/z| normalizes the resonant absorption such that if z = r then as = 1.0 [19]. when as attains a peak value  0.5, the film reactance is inductive (z = r + xl) and r/(r + xl)  0.5. the result demonstrates that a portion of the injected microwave power is trapped in the magnetic field. the 2d semicontinuous films composed of metallic gold nanoparticles act within the percolation conductivity transition as a surface reactance with diminishing resistive part. trapped mode resonances are characteristic of 2d metasurfaces, which are periodic arrays of planar resonant circuits with dimensions smaller than the wavelength. metasurfaces or metafilms can be considered as a subset of 3d metamaterials. the properties of classical thin film metamaterials have been traditionally characterized by effective permittivity and permeability with the effective medium theory. since the bulk properties and thickness of metasurfaces cannot be uniquely defined, the surface susceptibilities are used instead, and the scattering from metasurfaces is characterized in terms of the generalized sheet transition conditions (gstc)[20]. the theoretical studies predict trapped mode resonance having a fano-type line shape on the reflection/transmission spectra [21], [22]. one of the attractive features offered by metasurfaces is the possibility of compact electromagnetic absorbers. such devices will scatter in a narrow band, in the vicinity of the resonant frequency range [23]. in contrast to gstc results, the scattering parameters (figure 2) and conductance (table 1) of semicontinuous films are evidently frequency independent in the investigated frequency range suggesting that the characteristic properties of such films can be described by the effective medium (gem) model. the gem universal exponents t  s  1.2 are consistent with 2d percolated network. but, using our experimental data, v  107 s/m, d  10, and 1/(t+s) = 0.42, gem predicts the effective absorption in unrealistically narrow concentration range, |p-pc|< |2f0d/(4v)| 1/(t+s)= 8.6 10-4. to solve these difficulties, a dynamic effective model was introduced and the effect of broadband scattering of microwaves from inhomogeneous surfaces near percolation coverage has been analysed numerically in terms of generalized ohm’s law (gol) [22]. the gol simulations support our observation of an anomalous broadband absorption corresponding to resonating table  2.  surface  conductance  (s),  skin  depth  (e),  attenuation  (1‐te),  measured absorption (as), and the corresponding penetration depth as at  10 ghz for au films having thickness (d).  d   (nm)  s   (s)  e (m)  1‐te  as  as   (nm)  7  1.510‐5  109  1.310‐4  0.05  190 8  3 x10 ‐3   9  2 10‐3  0.5  25 9  1.510‐2  4  5 10‐3  0.35  40 10  510‐2  2.3  910‐3  0.2  90 acta imeko | www.imeko.org  september 2015 | volume 4 | number 3 | 46  structures at the limit of high damping and strong skin penetration. in practice one can easily estimate scattering properties of 2d semicontinuous films using surface conductance and as., ts and rs plots in figure 3. for example, transmission ts of a single layer with s  310-3 s is about 0.5 and rs  0.14. neglecting multiple reflections in an n-layer configuration, transmission will decrease as (ts)n, rs will remain about the same and the total absorption will approach the value of 1(ts)n rs. for a nano-size structure composed of 10 layers, (ts)n  10-3 (30 db) and absorption will increases to about 0.85. in comparison, a classical absorber with rs in the range of 0.14 and total attenuation, 1(ts)n, of 0.997 would have to be macroscopically thick. somewhat more accurate results may be obtained from numerical models discussed above. nevertheless, the presented results demonstrate the practical usefulness and advantage of our experimental approach, which utilizes surface conductance as a constituent parameter in characterization of anomalous microwave scattering from 2d semicontinuous films. 5. conclusions  the presented measurement technique minimizes the impedance mismatch and allows effective coupling of microwaves to nanostructured metallic films over a broad frequency range. the results provide experimental evidence of large resonant absorption in semicontinuous gold films within the conductivity percolation transition. this transition can be easily identified experimentally from the characteristic  phase change of the reflection scattering parameter s11. the effective attenuation distance of microwaves is about 25 nm to 100 nm. this is several orders of magnitude shorter than the classical skin depth model would predict. the films attain a maximum absorption of about 0.5 slightly above the percolation threshold, when the film thickness is about 8 nm and surface conductance is about 2.510-3 s. by tuning the particle concentration towards the percolation threshold, the film reactance becomes inductive and the structure transitions from an electric to magnetic energy concentrator. the effect opens the possibility of controlled scattering of microwave energy in sub-wavelength structures through networks of metallic nanoparticles. conductance of films thicker than 15 nm approaches the value of 0.6 s, which corresponds to volume conductivity of bulk metallic gold of about 107 s/m. disclaimer  certain commercial equipment, instruments, or materials are identified in this paper to foster understanding. such identification does not imply recommendation or endorsement 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[22] a. dhoubi, s. n. burokur, a. lupu, a. de lustrac and a. priou, “excitation of trapped modes from a metasurface composed of only z-shaped meta-atoms”, appl. phys. lett., 103 (2013) p. 184103. [23] o. luukkonen, f. costa, c. r. smovski, a. monorchio and s. a. tretyakov, “a thin electromagnetic absorber for wide incidence angles and both polarizations”, ieee trans. antennas propag., 57 (2009) pp. 3119-3125. multi-platform solution for data acquisition acta imeko issn: 2221-870x march 2023, volume 12, number 1, 1 8 acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 1 multi-platform solution for data acquisition luca lombardo1 1 department of electronics and telecommunications, politecnico di torino, corso duca degli abruzzi 24, 10129 torino, italy section: research paper keywords: daq; signal processing; low-cost instrumentation citation: luca lombardo, multi-platform solution for data acquisition, acta imeko, vol. 12, no. 1, article 28, march 2023, identifier: imeko-acta-12 (2023)01-28 section editor: francesco lamonaca, university of calabria, italy received february 2, 2023; in final form february 27, 2023; published march 2023 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: luca lombardo, e-mail: luca.lombardo@polito.it 1. introduction several applications both in the research and industry fields require the digital acquisition of analogue signals and the subsequent signal processing. this task can be successfully carried out by employing data acquisition boards, usually referred to as daq boards. these systems are typically able to acquire digital and analogue signals sending data to a host computer via usb port, for storage and further processing, and are widely employed in many applications, from imaging processing to acquisition of physical and chemical quantities, [1][3]. many companies and universities daily use such generalpurpose acquisition boards when it is not economically advantageous to develop a dedicated acquisition system. as a matter of fact, the use of daq boards requires an easy interfacing with the host computer in order to carry out several operations including: board calibration [4] and configuration, data acquisition from several sources, and data saving. typically, users are supported in such operations by a control and acquisition software, provided by the board manufacturer, which is installed on the host computer. unfortunately, such software is in most of the cases compatible only with windows os [5]. moreover, the software is usually provided as commercial closedlicense application which cannot be freely modified or updated, so that users are dependent on the board manufacturer for firmware and software updates and for platform support. this factor is extremely important because it prevents the employment of the commercial daq in several applications where, due to other constraints, the use of windows-based computers is not possible. few open-source solutions are available [6]-[8], but usually they lack in performance (especially in terms of accuracy) when compared with commercial daqs. other daqs, instead, are developed for very specific applications [9] or they feature fpga/dsp-based data processing which increases costs [10]. this work tries to tackle this issue, by providing a simple, thought powerful solution suitable for data acquisition regardless of the employed operating system. the proposed system is based on a teensy board [11], easily available off-theshelf and able to connect to host pcs via a serial ’emulated’ usb. the usb port is also used to power both the teensy and all the auxiliary circuits, therefore, no external power supply is required. even though it is not possible to guarantee its compatibility for all future operating systems, the probability of a dropping abstract analogue data acquisition is a common task which has application in several fields such as scientific research, industry, food production, safety, and environmental monitoring. it can be carried out either using systems designed ad-hoc for a specific application or by using general-purpose digital acquisition boards (daq). several daq solutions are nowadays available on the market, however, most of them are extremely expensive and come as commercial closed products, a factor which prevents users to adapt the system to their specific applications and limits the product compatibility to few operating systems or platforms. this paper describes the design and the preliminary metrological characterisation of a digital data acquisition solution based on the teensyduino development board. the aim of the project is to create a hardware and software infrastructure suitable to be employed on several operating systems and that can be freely modified by the users when required. teensyduino board is a well-known development platform which is characterised by high computing performance and usb support. taking advantage of the teensyduino features, the proposed system is easy to be calibrated and used, and it provides functions and performance comparable to many commercial daqs, but at a significantly lower cost. mailto:luca.lombardo@polito.it acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 2 support is minimal due to the use of basic drivers, typically available for all machines. the acquisition system is compact, easy to use and to calibrate. in particular, calibration is carried out either in-house, if a suitable dmm is available, or in a calibration laboratory. the daq provides also analogue and digital outputs which can be used to generate analogue signals and to interface with other systems. all these features are typically available only on extremely expensive commercial daqs. two different solutions have been developed, either with or without external adcs and dacs. the former, based on highperforming adcs and dacs, can be extremely accurate and can provide outstanding performance, while the latter, based on the internal teensy peripherals, has limited performance, but it has a really low cost. in this work both solutions are explored and, in both cases, suitable control software is provided so that the board use is simple and straightforward. 2. software infrastructure in order to have a simple and ready-to-use acquisition system, the board should be able to work on almost any type of platform. this implies the development of a software infrastructure which is compatible with most of the machines and operating systems nowadays available on the market. to address this issue, the chosen solution was to rely on a layered software infrastructure, as shown in figure 1. java has been used for developing the software due to its high compatibility and portability on a very wide range of platforms, including windows, linux, and macos. the teensy board comes with a native usb interface which embeds an ’emulated’ serial port supporting data transfer rates similar the usb 2.0 full-speed specifications. when the teensy is connected to the host computer, it is recognized as a ’virtual serial port’ if a suitable low-level driver is installed. such type of driver is available virtually for all operative systems/platforms and, in most cases, it is natively installed. the proposed solution makes use of the fazecast multi-platform serial driver [12], but several other java serial drivers could be used as well. the next layer is the daq driver, which embeds all the control and data exchange tasks between the virtual serial port and the main application. in particular, the daq driver is based on a java class named tennsytask and all serial data interchange with the teensy board is confined inside another class called usbconnect. any user wishing to employ the daq driver simply has to instantiate a teensytask and add the required channels, either analogue or digital, input or output, using the method createchannel provided by one of the channels (aichannel, ...). the user can set all the timing details by using the method timing. eventually, the user can instantiate one of the reader/writer methods (analogsinglechannelreader, ...) and decide the number of samples to be read and/or written. finally, the main application provides a graphical user interface (gui), which allows the users to configure all the daq parameters, visualize the acquired data and export them in several formats. if such application does not satisfy the requirements of a specific acquisition task, users can freely develop their own software taking advantage from the daq driver. an alternative software architecture using python is currently under development with the aim to further extend the range of platforms supported by the proposed daq solution. 3. teenydaq board without external adc as a first case study, the author developed an ultra-low-cost daq board using only a teensy 3.6 board and few passive external components, as it is shown in figure 2. the board can be proposed for possible applications in different fields, such as the development of generic acquisition of analogue signals, impedance measurements, environmental and structural monitoring, corrosion protection also in the cultural heritage field, motor control for movable equipment and biomedical applications [13]-[16]. the board has a cost in the order of 30 $ and it has dimensions and performance similar to the usb6001, a commercial daq deployed by national instruments [17]. the daq is fully compliant with the software described in section ii. figure 1. the developed software infrastructure which allows the board to be interfaced to a wide range of different platforms, including windows, linux and macos. figure 2. structure of the ultra-low-cost teensy-based daq board. inside the 3d-printed box, there is the teensy 3.6, few passive components and the input/output connectors. acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 3 3.1. board architecture teensy 3.6 contains a micro-controller with two analog-to digital converters and two digital-to-analogue converters, plus several digital input/output pins. therefore, it is possible to arrange a complete solution based only on this development board, adding only few passive components. some possible configurations have been proposed in literature [18]-[19], however, some problems still exist: – the adcs can sample up to the speed of 1 msps, but the inputs are constrained to only positive voltages in the range from 0 v to 3.3 v. usually, daq boards have bipolar input ranges which extend up to 10 v. it is possible to obtain bipolar ranges only with passive components, but at the cost of a reduced input impedance and accuracy. – commercial daqs are able to measure very lowamplitude signals thanks to internal active amplifiers, but this cannot be achieved with passive component. – the adcs have a maximum nominal number of bits equal to 16, but the uncertainty and the noise limit the effective bit number to about 12. in addition, a low input impedance is required to maintain the 12-bit accuracy, so this limits the type of sources which can be used with the daq. – the dacs are quite limited, as well. despite their high sample rate capability, again the output voltage range is limited from 0 v up to 3.3 v, while commercial boards usually have bipolar outputs in the range of +/10 v. – digital input/outputs are limited to a maximum of 3.3 v and applying the usual ttl voltage of 5 v can damage the board. the discussed limitations can be partially overcome. figure 3(a) shows a possible solution for the unipolar teensy adc inputs. basically, this is achieved by adding a fixed offset so that the ain voltage is at the adc range centre when the input voltage 𝑉in is zero. the relation between the adc input voltage ain and the input voltage vin can be written as: 𝐴in = 𝐾 + 𝛼 𝑉in , (1) where 𝛼 is the attenuation factor of the input voltage and 𝐾 is a fixed dc offset which is added to the scaled input voltage: 𝐾 = 𝑉 + 𝑅1𝑅2 (𝑅1𝑅2 + 𝑅1𝑅3 + 𝑅2𝑅3) (2) 𝛼 = 𝑅2𝑅3 (𝑅1𝑅2 + 𝑅1𝑅3 + 𝑅2𝑅3) (3) by using this configuration, virtually any bipolar input voltage range can be obtained while keeping the adc input within the allowed range by selecting the resistors r1, r2, and r3 to satisfy the following equations: 𝐾 = 𝑅21 𝑅3 + 𝑅21 ∙ 𝑉 + = 𝑉ref 2 , 𝑅𝑖𝑗 = 𝑅𝑖 ∙ 𝑅𝑗 𝑅𝑖 + 𝑅𝑗 (4) 𝛼 = 𝑉ref 2 𝑉inmax , (5) where 𝑉ref is the teensy adc reference voltage, usually equal to either 1.2 v or 3.3 v, and 𝑉inmax is the maximum input voltage. the equivalent input impedance can be easily calculated as: 𝑅in = 𝑅1 + 𝑅23 . (6) as an example, by using 𝑉 + = 𝑉ref = 3.3 v and selecting resistors of 𝑅1 = 849 k, 𝑅2 = 300 k and 𝑅3 = 221 k, a maximum input of about 10 v and an input impedance of about 1 m can be obtained without using any active component and selecting the resistors only from the e96, 1% series. however, still some problems are present: – the input impedance is quite high, in the order of 1 m, but still lower than commercial daqs whose typical input impedance is of the order of 1 g. moreover, a voltage appears on the input pins. in principle, this is not a problem, but it could either harm some circuits or interfere with their operation. – the equivalent impedance at ain pins is of the order of 115 k while the teensy specifications state that the analogue input impedance should be kept at the minimum possible value. while this might not be a problem, there are cases where the adc accuracy cannot be guaranteed. this problem could be easily solved by inserting a buffer amplifier, but this would increase the cost and prevent a really simple system to be arranged. figure 3. (a) input passive conditioning network which allows the teensy to measure bipolar signals in a fixed input range. (b) simple configuration to test the adc noise and accuracy performance for different input resistances. figure 4. example of dc voltage acquired with two different input resistances, respectively, of 1 k to 1 m. the inset shows the signal acquired in the same conditions when the dac is programmed to output sine signal. acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 4 – using the same value for 𝑉 + and 𝑉ref makes the contribution of the 𝑉 + uncertainty negligible, but the resistor uncertainty of 1 % may lead to an uncertainty on each input range of the order of 2 %. this means that in the absence of other corrections, at full input range a 2 % uncertainty should be expected (200 mv on the input value). if such measurement uncertainty is not acceptable, there is a simple way of calibrating the system. as the scaling gain for each input may be slightly different, each channel must be individually calibrated. the solution is the use of a known voltage to be measured by each channel. as an example, by employing the texas instruments lm4030amf-2.5 voltage reference featuring an output voltage of 2.5 v with a 0.05 % maximum uncertainty and a temperature coefficient of 10 ppm/˚c, the calibration can be easily performed for each channel. the author decided to put the lm4030amf-2.5 on the pcb and to take its output out on one connector pin without adding any digital switch for the calibration with the aim of maintaining the system cost as low as possible. in this way the calibration has to be done by manually shorting each input to the ground to measure 𝐾 and then connect it to the reference output for measuring 𝛼. the values are then stored on the teensy memory for automatically correcting the acquired data. all these operations do not eliminate the problem related to the impedance on the input analogue pins on the teensy. under the best conditions, i.e., with hardware averaging enabled, differential mode and adc clock slow enough, the accuracy is better than 13 bits, but if all these conditions are not met, the accuracy may severely degrade, especially if the impedance seen by the analogue inputs is high. in order to test such performance deterioration, the configuration shown in figure 3(b) can be used. using a dac output lets users to change the signal amplitude so that it is possible to stimulate the teensy adc with a voltage which is inside the allowed range, while it is possible to observe the degradation of the sampled output signal by changing the input resistance 𝑅in. as an example, figure 4 superposed two acquisitions carried out with a 𝑅in of 1 k and 1 m. the difference in terms of noise performance is clear. moreover, no change was observed by driving the dac with different dc values. the inset shows, instead, the acquired data when the dac is programmed to output a sine signal with the same two input resistances. the signals have a different phase as no phase adjustment was performed and have different amplitudes since the adc loading effect is not negligible when using a resistance of 1 m. in any case, the noise effect on the acquired signal is similar as in the case of dc values. computing the equivalent bits of the acquisition is not straightforward, as it depends on the acquisition purpose. in the worst-case scenario, i.e., if the instantaneous value of the signal is required, the equivalent bits can be obtained as: 𝐸𝑞bits = log2 ( 216 ∆ ) , (7) where ∆ is the maximum difference between the real value and the measured value. the author tested the adc for resistances in the range from 1 k to 1 m by using two different adc configurations: – maximum sampling rate (about 200 khz) and no hardware averaging; – slow sampling rate (about 5 khz) and a hardware averaging of 32. the results are shown in figure 5, where the equivalent bit number is calculated as function of the input resistance. the figure shows how in the case of maximum speed the equivalent bits exceed 10 bits only when the resistance is negligible and severely degrades as the input resistance increases, while the equivalent bits are close to 13 when the speed is limited to 5 khz. it is worth to note that this is the worst case and if, as an example, a sine signal is used and a sine amplitude is required, much better values can be obtained as the noise tends to spread over a wide frequency range. this is confirmed by the fft of the acquired data. figure 6, as an example, shows the fourier transform of the data acquired at a sampling rate of 100 khz with an input resistance of 1 m apart from the peak at 50 hz, which is expected when the source impedance is high and no shielding is done, it is clear how the noise spread on a large frequency range. 3.2. performance the daq which uses only the teensy 3.6 and few external components has several advantages, including an extremely low figure 5. equivalent bit number as function of the input resistance 𝑅in. the two traces refer to the adc acquiring in single ended mode with a sampling rate of 200 khz (no hardware averaging) and 5 khz (with hardware averaging of 32), respectively. figure 6. fft of the acquired data in the case of a sampling rate of 100 khz and an input impedance of 1 m. the inset shows that the peak highlighted by the fft is due to the 50 hz mains noise which couples to the input when source impedance is high. acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 5 cost and a very compact size. unfortunately, the absence of any active component at the input stages involves also several issues which can be addressed only partially. the lack of any gain stage at the input limits the capability of the daq to acquire signals with very low amplitudes. the relatively low input impedance limits the employment of the board to only low-impedance sources. moreover, as shown by figure 5, also at low input resistance (few k) there is an unacceptable signal degradation. better results can be obtained when the teensy 3.6 adc is used with the hardware average and at a low sampling rate. in these cases, the adcs is able to deliver an accuracy of about 12 bits when used on single ended channels. this would turn out in about 1.2  10-4 relative uncertainty at full range, but such an uncertainty can be achieved only with a very low input impedance. 4. teensydaq board and external converters to overcome the performance degradation due to the low input impedance and the lack of active gain stages, the author designed an advanced daq board with performance comparable to high-end commercial systems by adding some external active devices. nevertheless, the cost of the new daq is of about 100 $, still much lower than the price of competing products. figure 7 shows the general block diagram of the high performance daq. the new daq version still employs the teensy 3.6, already used for the ultra-low-cost board. such teensy can be easily replaced with the teensy 4.1 which features quite better performance with respect to teensy 3.6 (larger ram to store the data, higher clock frequency and improved processing capabilities). however, teensy 4.1 lacks the dac converters and, therefore, it cannot be employed on the ultralow-cost daq. the principal task of the teensy is to interface the external adc and dac converters, to acquire and temporary store the data, and to send them to the host computer. moreover, some optional signal processing (such as averaging and filtering) could be implemented on the teensy itself thanks to its high computing capabilities. the daq board is directly powered from the 5 v of the usb port, therefore, no external power supply is required. however, internally the system needs different voltages for properly operating. for this reason, a dedicate power supply module, shown in figure 7, is implemented in the daq. this module employs a boost/inverter dc-dc converter and few ultra-lownoise linear voltage regulators to provide the 15 v rails and the low-noise 3.3 v voltage required by the analogue inputs and outputs and the acquisition chain. eventually, another 3.3 v voltage is generated by the teensy and used to supply only digital circuits, avoiding any noise coupling between analogue and digital devices. the overall performance and functionalities of this daq are comparable to, or even better than, many commercial boards which are available at a much higher cost. 4.1. analogue input channels the section related to the analogue input channels is highlighted with a green colour in figure 7, while figure 8 shows the detailed implementation of the adc acquisition chain. the daq features 8 fully differential inputs which can be alternatively used as single-ended inputs. each channel has an input impedance higher than 1 g and all the channels can be sampled with a user-selectable sampling rate up to 1 msample/s shared between the active channels. the acquisition of multiple channels is carried out sequentially by using an analogue multiplexer which features low noise, low distortion and high bandwidth. a high-performance precision programmable gain amplifier (pga) is employed to maximize the input range capability of the acquisition chain. the pga is connected to the multiplexer output and can be programmed to have 11 different gains in the range of 1/8 v/v to 128 v/v. additionally, the pga can add an optional 1.375 gain factor. therefore, thanks to the 22 different input gain combinations of the pga, the daq is able to acquire analogue signals in bipolar ranges from about  19 mv up to about  12 v. moreover, the pga is designed to add a fixed dc voltage to the output, so that the subsequent stages can be powered by a single-ended power supply. the selected adc is the texas instrument ads8881 chip [20]. this high-performance adc features a resolution of 18 bits and a maximum sampling rate of 1 msample/s. it exhibits very good performance both in terms of linearity and accuracy when its inputs are properly decoupled and buffered. for this reason, a dedicated differential buffer/-driver with an anti-aliasing filter has been inserted between the pga outputs and the adc inputs. the topology selected for the anti-aliasing filter was a classic differential rc first-order low-pass filter. particular attention has been posed when designing the external voltage reference circuit required by the adc. a highaccuracy and low-drift reference voltage with a nominal output value of 2.5 v was employed along with three operational amplifiers. in particular, the reference voltage selected for the figure 7. block diagram of the high-performance daq board which employees the teensy 3.6 and external adc and dacs. figure 8. block diagram of the analogue acquisition chain featuring the ads8881 high-performance adc. acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 6 prototype was the ref5025i, featuring a low-noise output voltage of 2.5 v with an initial maximum uncertainty of 0.05 % and a temperature coefficient of only 3 ppm/˚c. one high-speed operation amplifier is used as buffer for providing the reference voltage to the adc. such an amplifier satisfies the band and stability requirements of the adc operating at its maximum frequency, but it does not have a suitable accuracy. to overcome such a problem, another high-accuracy operational amplified has been inserted in the reaction chain of the first amplifier. this way all the adc requests in terms of accuracy, band and stability are satisfied. a third high-accuracy operation amplifier is instead employed to generate a 1.25 v required for the proper operation of the pga. the data stream provided by the adc is read by the teensy 3.6 via the spi0 interface and temporarily stored in the microcontroller ram together with an accurate timestamp and a channel reference. once acquired, the samples can be sent to the host pc via the usb interface without any significant latency or sampling rate reduction. optionally, the acquired data can be stored on the micro secure digital (µsd) available on the teensy. this is an added value of the proposed solution which can additionally operate as a standalone data logger without connection to any host computer. of course, in this case, the daq board has to be powered by an external power source. as an example, a rechargeable battery can be employed making the daq a portable data logger able to operate also in remote locations. 4.2. analogue output channels and digital input/output the daq can also be equipped with a maximum of four analogue output channels working with a resolution of 16 bits and a maximum sampling rate of 100 khz, as highlighted in yellow in figure 7. the outputs are capable of generating positive and negative voltages and currents in different ranges which can be selected by the user. these dacs can be used along with the input analogue channels and are able to generate arbitrary waveforms up to about 20 khz. in this case, the waveform can be configured from the host pc and programmed into the teensy, which will take charge to send the proper samples to the dacs without any user intervention. additionally, the board has 16 digital input/output channels (shown in orange in figure 7) which are able to operate at 5 v. the channels can be read/written simultaneously in block of 8 bits and can be individually configured either as inputs or outputs. these digital channels are obtained by means of an io expander chip which is connected to the teensy 3.6 via the spi2. eventually, four high-speed digital inputs and four highspeed digital outputs are available as well. these digital lines are directly connected to the teensy 3.6 io pins using a level converter which is able to operate with any logic level between 1.8 v and 3.6 v at a maximum frequency of 1 mhz. the analogue outputs can be useful in several specific applications. as an example, they are able to accurately bias sensors such as load gauges and generate control signals for several systems. moreover, together with the analogue inputs they can be used for implementing impedance measurements and readout units for electrochemical sensors. eventually, the availability of digital lines allows the daq to interface with digital systems and they can be optionally used in pwm mode. 4.3. high-performance teensydaq characterisation the design of the high-performance teensydaq has been carried out trying to obtain an optimal trade-off among performance, cost, size, and functions. therefore, smd packages have been selected both for performance and size constraints. unfortunately, this can be an issue during the first testing stage of the prototype, being smd devices more difficult to be handled and connected on development boards. for this reason, smd-to-tht adapters, test jumpers, and sockets have been used to realize the first prototype of the board. this solution allows one to easily arrange/modify the circuits under test. however, the employment of wires and sockets severely degrades ac performance due to multiple noise coupling sources. nevertheless, the author tried to optimize the single blocks of the daq and carried out a preliminary characterisation of dc and ac performance, as described in the following sections, obtaining very promising results which are comparable with commercial high-end products. 4.4. dc performance initial tests have been carried out in order to assess dc accuracy and repeatability. all tests have been performed using as reference voltmeter the 8.5 digits hp 3458, calibrated less than 2 years ago with a stated dc uncertainty of less than 1 ppm. the adc has been used at a sampling frequency of 100 khz, acquiring about 30000 samples for each measurement (about 0.3 s measuring time). initially, the complete acquisition chain has been verified (adc together pga and driver, see figure 8) to observe linearity and overall gain of the daq. figure 9 shows the experimental results obtained in the input voltage range from -2.5 v to 2.5 v and setting the pga gain to the unity. the plot inset shows an excellent linearity of the entire acquisition chain. the difference between the actual measured values and the expected ones is reported, as well. it is possible to see how the difference is extremely low with an overall gain very close to 1 and a negligible offset. even though the measurement number is quite limited, it is possible to estimate the maximum linearity error for the entire measurement chain in about 200 µv, while maintaining mostly under 40 µv, as visible from figure 9. then, acquisition chain noise and measurement repeatability have been assessed. the test has been carried out in three steps with the aim of quantifying the influence of each stage of the chain. initially, the performance of only the adc have been checked and compared with the specifications of the manufacturer. a stabilized power supply has been used to apply figure 9. transfer characteristic of the analogue acquisition chain obtained in an input voltage range of -2.5 v to 2.5 v by setting the pga gain to 1 and the difference between expected and measured values. acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 7 voltages between -2.5 v and 2.5 v directly to the adc input. then, the same measurement has been repeated including the adc driver and the pga. in particular, the input voltage has been measured at the same time with the daq and the hp 3458 voltmeter, acquiring respectively about 30000 and 20 measurements for each voltage. the standard deviation of the hp 3458 was negligible for all measurements. figure 10 shows the three sets of results: each dot shows the standard deviation obtained by the daq, respectively applying the input voltage to the adc, to the adc together with its driver, and to the whole acquisition chain (including the pga). of course, the lowest standard deviation is obtained with only the adc, with a maximum value of about 35 ppm. it is worth to notice that 1 lsb at 18 bits resolution corresponds to 8 ppm. therefore, the adc standard deviation is of the order of 4 lsb, in-line with the 3 lsb declared by the manufacturer. the standard deviation slightly increases up to about 50 ppm when the adc driver is included, which corresponds to about 6 lsb. eventually, the standard deviation reaches values of about 90 ppm when the complete acquisition chain is used. this increase is partially due to the intrinsic noise of the pga, but also the presence of long connection wires which cannot now be neglected being the input impedance of the pga higher than 1 g. nevertheless, such results are extremely promising because in the same conditions several commercial high-end devices, available at much higher prices, have an uncertainty in the order of 3000 ppm without calibration and more than 100 ppm after calibration [21]. 4.5. ac performance obtaining an ac characterisation of the daq is not straightforward. as an example, the dmm employed for the dc characterisation has an ac uncertainty of 10 ppm  100 ppm depending on range and frequency, therefore, it is quite difficult to have a good transfer uncertainty ratio. for this reason, it has been decided to use a low-distortion sine generator and to compute the fourier transform of the acquired samples. the generator has a stated distortion of 0.01 % but it is not perfectly stable in frequency. therefore, it is not possible to make the transform on an integer number of periods. acquisitions have been carried out at nominal frequencies of 10 hz, 100 hz, 1 khz and 10 khz, while the adc sampling frequency has been set to 100 khz for all measurements. as an example, figure 11 shows the result obtained with 1 khz sine signal. it is possible to note the presence of high-frequency noise as well as components at 100 hz and multiples, probably due to noise and mains coupling with the daq signal path. notably, there are also two symmetrical lines around the main component at 1 khz, probably due to the frequency instability of the signal generator. nevertheless, the tail of the spectrum reaches -110 db at low frequency and more than -90 db at high frequency. therefore, the achieved performance is generally quite promising, also considering that the assembly of this first prototype has multiple noise-coupling paths due to several wired connections and sockets between the daq components. 5. conclusions the paper presents the design and initial characterisation of two daq multi-platform solutions with the aim of addressing several issues currently present on most of the commercial devices: software/hardware limited compatibility and support, high cost and impossibility to freely modify the software to fit any specific applications. the two boards are based on a teensy 3.6 and are able to operate on most of the operative systems thanks to a dedicated software infrastructure designed to simplify the use, the maintenance and the upgrading. the first version is very simple and employs only the teensy 3.6 and few passive devices. it features an extremely low-cost of about 30 $, but it has quite limited performance and can be employed only when the input signal impedance is very low. the second version, instead, uses external high-performance adc and dacs converters and is able to achieve performance comparable with high-end commercial products, but at a cost anyway lower than 100 $. the first prototype of the daq has been tested and several optimizations have been carried out. the achieved results are very interesting and demonstrate the feasibility of the proposed board as an alternative solution to commercial products. nevertheless, due to circuit assembly implemented by using sockets and several connection wires, the ac performance requires further investigation. for this reason, a new prototype is currently under development. the new board will be realized using printed circuit boards in a modular design, where each block (as shown in figure 7) will be implemented as an independent module so that it will be very easy to change or upgrade any part of the system. figure 10. standard deviation obtained from the proposed daq in the range from -2.5 v to 2.5 v, respectively considering the only adc, the adc together with its driver, and the complete acquisition chain. figure 11. fourier transform of the acquired data using 1 khz sine input signal. acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 8 references [1] l. iannucci, m. parvis, p. cristiani, r. ferrero, e. angelini, s. grassini, a novel approach for microbial corrosion assessment, ieee transactions on instrumentation and measurement, vol. 68, 2019, no. 5, pp. 1424-1431. doi: 10.1109/tim.2019.2905734 [2] l. es sebar, s. grassini, m. parvis, l. lombardo, a low-cost automatic acquisition system for photogrammetry, proceedings of ieee instrumentation and measurement technology conference, (2021). doi: 10.1109/i2mtc50364.2021.9459991 [3] l. iannucci, chemometrics for data interpretation: application of principal 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https://doi.org/10.1109/mic.2012.6273385 https://doi.org/10.1016/j.measurement.2017.11.058 https://doi.org/10.21014/acta_imeko.v4i1.156 https://www.pjrc.com/teensy https://fazecast.github.io/jserialcomm https://doi.org/10.1109/i2mtc.2019.8826965 https://doi.org/10.1109/memea52024.2021.9478711 https://doi.org/10.1109/memea49120.2020.9137226 https://doi.org/10.21014/acta_imeko.v10i4.1194 https://www.ni.com/it-it/support/model.usb-6001.html https://doi.org/10.1109/tim.2020.3038005 https://doi.org/10.21014/acta_imeko.v10i1.894 https://www.ti.com/product/ads8881 https://www.ni.com/en-us/support/model.usb-6212.html editorial acta imeko june 2014, volume 3, number 2, 2 www.imeko.org editorial paul regtien measurement science consultancy (msc), julia culpstraat 66, 7558 jb hengelo (ov), the netherlands section: editorial citation: paul regtien, editorial, acta imeko, vol. 3, no. 2, article 2, june 2014, identifier: imeko-acta-03 (2014)-02-02 editor: paolo carbone, university of perugia copyright: © 2014 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: paul regtien, e-mail: paul@regtien.net this issue of acta imeko comprises the third set of articles that are based on papers presented at the xx imeko world congress, busan, republic of korea, 9–12 september, 2012. the issue includes the extended and updated versions of papers from tc3 (measurement of force, mass and torque) and tc16 (pressure and vacuum measurement). the remaining submissions from this world congress to be published by acta imeko will appear in the forthcoming issue. this edition starts with eight research papers on force, mass and torque measurements (tc3). the first one deals with the proposed new si and the consequences for mass metrology. the next two papers discuss models for a checkweigher and a dynamic torque calibrator, respectively, and they are followed by a paper on an improved force standard machine. calibration systems for torque are presented in the next two contributions and, in a subsequent paper, the effect of humidity on torque transducers is discussed. the last contribution from tc3 is about a multi-component facility for torque and force. four articles from tc16 follow on equipment for pressure and vacuum measurements: a pressure balance piston, capacitive diaphragm gauges, pressure measuring multipliers and finally a discussion on rarefied gas flow in pressure and vacuum measurements. the issue closes with an advertisment from hbm, a contributor to the confederation. advertising is open for other supporting members. we would like to thank all those who have contributed to this issue: authors, reviewers, copyeditors, layout editors and journal managers. special thanks go to the section editors yon-kyu park (tc3) and jorge torres-guzman (tc16). we hope you enjoy this issue. acta imeko | www.imeko.org june 2014 | volume 3 | number 2 | 2 editorial microsoft word 17-120-2-ga.doc acta imeko  july 2012, volume 1, number 1, 26‐35  www.imeko.org    acta imeko | www.imeko.org  july 2012 | volume 1 | number 1 | 26  characterization of power quality transient phenomena  of dc railway traction supply  andrea mariscotti  università di genova, via opera pia 11a, 16145 genova, italy    keywords: conducted interference; guideway transportation systems; power quality; spectral analysis; supply transients; time domain analysis  citation: andrea mariscotti, characterization of power quality transient phenomena of dc railway traction supply, acta imeko, vol. 1, no. 1, article 8, july  2012, identifier: imeko‐acta‐01(2012)‐01‐08  editor: pedro ramos, instituto de telecomunicações and instituto superior técnico/universidade técnica de lisboa, portugal  received january 4 th , 2012; in final form may 4 th , 2012; published july 2012  copyright: © 2012 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: information not available  corresponding author: andrea mariscotti, e‐mail: andrea.mariscotti@unige.it  1. introduction  power quality (pq) in railways encompasses a variety of phenomena, steady and transient: harmonic components (characteristic of the substation rectifiers), other similar components (not harmonically related to the supply frequency on the feeding ac side of substations, but caused by on-board converters and drives) and low frequency voltage fluctuations (due to more or less periodic load cycles) are examples of the first kind; steep current absorptions (in the form of the inrush current following the pantograph rise) and line voltage variations (due for example to pantograph bounce), and complex changes of modulation patterns (due to various types of transients, sudden tractive efforts and braking, wheel slip and slide, etc.) are examples of the second kind. pq in railway traction systems is relevant for several reasons: line voltage variations have influence on the pantograph voltage perceived by the single rolling stock and its performance [2]; harmonic distortion in combination with traction line impedance can trigger line voltage instability [2]; uncommon components in the absorbed current spectrum may lead to interference to signalling circuits when flowing back to substation as return current through the rails. more generally the pq compatibility of a rolling stock unit with a railway network is recognized as electrical interoperability, part of the overall interoperability, aiming at ensuring the safe and efficient circulation across different railway networks in different countries, such as those of the european union. pq properties and definitions are reviewed in section 2 with reference to standards and distinguishing those phenomena that are peculiar of dc distribution. in section 3 these elements are revisited and alternative methods are considered for their calculation. in section 4 measured data are used to verify both the considered methods and the relevance of the pq issues occurring on real railway networks. 2. review of power quality phenomena  definition and analysis of pq phenomena in ac railways are easier because they may be derived from ac supply networks [3], notwithstanding the different supply frequency, the distribution scheme and the presence of moving loads. in ac railways the emissions from substations and rolling stock are all of the harmonic type with reference to the common supply frequency [4], featuring a stability that is slightly worse than that of country-wide power grids [5][6]. in dc railways there is no such a synchronizing element and the emissions are harmonically related to independent sources: the substation keeps supplied by the dedicated feeders derived from the country-wide power grid, while for the rolling stock the motor frequency and the dc railways are considered as an example of a complex distribution network, featuring distributed supply and moving  loads. the  power  quality  phenomena  affecting  traction  voltage  are  many,  steady  and  transient,  depending  on  the  converter  operations  at  substations and on‐board vehicles. a classification is proposed and a discussion follows of the correct methods for their quantification,  with reference to related standards and to the vast background related to ac networks. the characterization of pq is related on one  side to ripple and fluctuation of the pantograph variables for supply purposes, and on the other side to the appearance of specific  components  relevant  for  interference  to  signaling  on  the  track.  considerations  and  conclusions  are  based  on  a  large  amount  of  measured  waveforms,  recorded  on  the  italian  railway  networks,  and  are  aimed  at  contributing  to  the  forthcoming  standards  on  traction supply quality.  acta imeko | www.imeko.org  july 2012 | volume 1 | number 1 | 27  switching frequency of dc/dc converters and inverters are the reference. the time window and the frequency resolution for spectrum analysis are thus not immediately following [3]. for ac and dc railways, besides power quality issues related to harmonic disturbance and their composition [4], network resonance and traction line rolling stock interaction [7]-[9], a very basic requirement that impacts on train performance and service efficiency is the voltage level “seen” by the train pantograph, called useful voltage. the useful voltage uav,u is defined in the en 50388 [2] for dc railway systems as the average value of the mean value of the pantograph voltage vp (i.e. dc component) over a well-defined geographical area of the national network and for one or several trains. thus, there is a distinction between the uav,u(zone), for the average operation made over all the circulating trains in a given zone, and uav,u(train), for the average operation made for one train over a predetermined journey or its timetable. the measurement data collected in the past and that will be used for the following analysis were all recorded on a single train and thus allow the calculation of uav,u(train) only. in general, pq concept may be extended to a series of phenomena, steady and transient, that may have relevance for the whole railway network: impact on, and possible interference, to signalling circuits is required to be evaluated by analyzing the pantograph current spectrum against a so-called limit mask [10][11]. the analysis parameters are more or less fixed by the different national procedures adopted by the various administrations, yet there is no agreement on how to distinguish transient events: they bring non-characteristic spectral components and in some cases they are simply “recognized” and excluded from the analysis [12]. the focus here is on the identification of these phenomena on long-term recordings, so that they can be properly treated (in some cases processed separately, or used for the specific analysis of transient interference or simply discarded). at dc it is not possible to use the fundamental period to derive an indication of the correct time window, both for harmonics and for the computation of the mean value. the time window duration has however a strong influence on how different types of transients are included and weighted, and this could lead to some uncertainty. this aspect is considered later on in section 3. so far transient phenomena have been generically considered; the classification proposed in [13] is reported for the discussion:  (type 1) unusual sudden tractive efforts with packet-like current absorptions may trigger oscillations in the onboard filter and thus in the pantograph current ip, but with negligible effect on vp, due to the low short circuit impedance of the network;  (type 2) change over to an adjacent supply section connected to a different substation, while passing under a supply gap (the equivalent for dc of a neutral section for ac systems) and producing a vp step change;  (type 3) pantograph bounces disconnect the sliding contact from the contact wire for a few ms, depending on several factors (speed, mechanical performance of the pantograph frame and dampers, catenary oscillations, ice); this produces a step change of absorbed current ip and a spike like change of vp; vp is namely determined by the free response of the on-board circuitry for the short time interval during which the pantograph is detached from the catenary; it is remembered also that current conduction still occurs through the plasma at the pantograph-contact wire interface and this represents an increase of the supply resistance, thus explaining the vp steep but limited increase;  (type 4) change of the operating conditions and of the spectrum of current emissions of onboard converters due to wheel slip, internal control rules, driving style and applied torque, etc. the next section is dedicated to the calculation methods and their settings to identify and evaluate these phenomena in terms of amplitude, spectral distribution and frequency of occurrence. 3. calculation methods and parameters  without a fundamental frequency for the definition of harmonics, all the narrowband components are relevant for the calculation of the supply distortion. the distortion is thus put in relationship to ripple, defined below and particularly appropriate for the evaluation of the traction supply voltage. the attention is on the pantograph voltage, even if the considered methods may be applied to the pantograph current as well. the ripple index defined below is a measure of the perceived quality applicable to dc networks [13][14]. some types of transients and their influence on the spectral properties are then considered. the subject reveals to be particularly interesting also for the relationship with the interference to signalling systems, in particular if the variable of interest becomes the pantograph current. in any case a distorted pantograph voltage reflects in a distorted pantograph current if the locomotive can be considered simply as a passive load at those frequencies. 3.1. ripple and useful voltage  ripple is defined as the variation of a quantity about its steady state value during steady electric system operation [3]. ripple is interpreted often as a periodic variation around the steady state dc value, but not necessarily so [15][16]: some components are related to steady periodic sources (harmonics of rectifiers and inverters [17]-[19] in steady conditions), but others are caused by transients (interpreted as aperiodic phenomena of limited duration). the expressions defined in [20] are briefly summarized. given the q(t) quantity (in this case the pantograph voltage vp), the peak-to-peak value qpp over mt samples interval is the exact ripple index, ri, that is the maximum peak-to-peak value over a given time interval, and is given by  , , max [ ] [ ]pp t t n m q q n q n m m    . (1) if a spectrum-based approach is followed, a sliding dft over a time window t is computed giving the spectrum q[k] [21][22]. ri is then calculated as the sum of the components of the set kthr, containing values of the index k with corresponding component q[k] with amplitude larger than a given threshold thr; sum-of-amplitudes (sa) and sum-of-amplitudes-andphases (sap) rules, as given in [20] , , [ ] thr dft t sa k k q q k    (2a) and , , [ ] thr dft t sap k k q q k    . (2b) acta imeko | www.imeko.org  july 2012 | volume 1 | number 1 | 28  the thr value must be carefully chosen not to leave out any significant component and to keep the size of kthr as low as possible; normally a 0.1 % relative to the nominal voltage is already a conservative choice, provided that full-scale is correctly set. lower values may be used in the presence of many spectral components, but only in case of a background noise that is located well below. the purpose of thr is to “clean” the spectrum, ruling out all spectral components other than the relevant ones not to artificially magnify the resulting ri value. holding these assumptions a lower thr value, such as 0.01 % used in the next figures, represents only a more cautious choice, but does not change the result. an overlap factor p of 0.5 was shown to adequately track signal dynamics and is compatible in terms of minimum correlation for the large part of used windows. in [13] it is shown that the two indexes differ slightly and therefore only sa will be considered in the following. uav,u is the index that is evaluated to assess the adequacy of the infrastructure (power supply network) to the prescribed performance of the circulating rolling stock and concerns only the average value, while the proposed ri takes account in a similar way of the other spectrum components. for 3 kv dc railways the minimum uav,u is set to 2800 v for high speed lines and 2700 v for conventional lines. 3.2. periodic phenomena  with similarity to harmonics in ac railways, periodic phenomena in dc railways are related to the commutation of static converters either at substations (6-pulse and 12-pulse diode rectifiers) or on-board (the front-end choppers have a switching frequency of some hundreds hz, possibly changing in discrete steps for variable operating conditions; the conducted emissions of traction drives may leak back to the dc line, appearing as sixth multiples of the motor stator frequency and/or higher components around the pwm switching frequency). changes to the operating conditions, such as a change of speed and thus of mechanical rotational speed, may occur leading to a type 4 transient, highly dependent on the type of locomotive and the adopted modulations and control schemes. the requirements on frequency resolution df (and thus on the time window duration t) are set by several national standards for evaluation of interference to signalling, still following a dft approach. in this case df is set to 1 hz [12] or slightly larger and the average of successive spectra may be required; the spectrum is extended up to some khz, 3600 hz for the italian case. the most modern approach envisaged by the recent clc/ts 50238-2 [23] is that of using a bank of band-pass filters followed by an integral calculation of the rms value [24]. it is also underlined that a coarser frequency resolution allows fast tracking of transient components. being the relevant components for interference located in the higher portion of the frequency axis, a df of e.g. 5 hz leads to a few % frequency resolution and allows a 200 ms base time resolution, that can be further reduced by means of overlap. 3.3. aperiodic transients  the complex scenario of variable operating conditions, position of the rolling stock and the possibility of local unstable conditions of traction converters located on different nearby vehicles, give rise to a wide set of transients. as already pointed out in the classification given in section 2, transients of the pantograph voltage and/or current may occur due to the inrush current of the vehicle filter, pantograph bounces, wheel slipping and sliding, etc. the aim is twofold: identifying transients during the measurements of conducted emissions on the pantograph current (subject to the values of the analysis parameters set by the standards for interference to track circuits) and during the evaluation of the useful voltage (with much less constraints on the parameters settings). broadly speaking the former may require a finer frequency resolution to match with the bandwidth and operating frequency range of the victim track circuits and related limit prescriptions. the latter is mostly influenced by slow variations, such as those experienced while moving at the end of a supply section, farthest from the substation (as it will be commented for figure 1 at points (1) and (2)); the analysis can be made with a less demanding frequency resolution and thus a finer time resolution. a 10 hz frequency resolution will be normally used in the following, accompanied by a 50 % (or 75 %) overlap. 4. real cases  recordings taken on the italian 3 kv dc railway lines are considered to evaluate the ri, to identify the transients, their spectral characteristics and their influence on the uav,u and in general on the spectrum components. the following subsections are aimed at evaluating the spectral properties of transients and defining the best indexes to identify and isolate the transients themselves. 4.1. useful voltage and ripple index  the useful voltage has been computed from the measured pantograph voltage vp using a 1 s average interval (as prescribed by the en 50163 [5] and en 50388 [2]) for various test runs performed on different italian lines (details are given in the caption of figure 1). it is underlined that the uav,u computation here includes all the operating conditions of the train rather than only traction, thus explaining the high overvoltages on the chiusi-arezzoorte line, probably occurring during regenerative braking in particular line conditions). the processed vp recordings give only in one case a useful voltage lower than the prescribed limits [2], 2800 v and 2700 v, for high speed and conventional interoperable dc lines. in the arezzo-chiusi-orte recordings some overvoltages are present, with a value much higher than the indicated umax3, the maximum non-permanent voltage as per en 50163. the observed overvoltages last for one or two average samples, so they have a time duration nearly equal to or lower than 1 s and thus fall under the category of transient overvoltages. a 10 min profile of vp and ip is shown in figure 2, together with the sa index, that differs from the one computed in [13] for the lower threshold thr (0.01 %, rather than 0.1 %) and the use of 75 % overlap. this recording shows quite a regular profile of current absorption and the large step changes in vp are due to supply gaps (type 1 transient). the vp box-like increase (3) is probably due to a particular arrangement approaching a station, confirmed by the decrease of ip at the end of the recording. a fairly constant high value ip profile represents medium/high speed travelling on straight lines with rare stops, such as on main lines. even in this case pantograph bounces, wheel slip/slide and transitions in the operating conditions can produce transients with effects visible in the voltage and current spectra. acta imeko | www.imeko.org  july 2012 | volume 1 | number 1 | 29  figure 2 is referred to a heavy loaded line, with the absorbed current in the range of 1700-1800 a, corresponding to an absorbed power of about 5.8 to 6.3 mw. the corresponding voltage ripple is however limited to about 2 %, occurring only at some time intervals. at point (4) ri has a step-like reduction, not only due to the increase in vp dc value, but also due to lower supply impedance. figure 3 (with the same overlap and thr values of figure 2) shows an interval of lightly loaded operations with the ri that is nearly half that of figure 1, as the reduced absorbed current implies. moreover the pantograph voltage is at the maximum values compatible with the 3 kv dc system [5] (the test run began close to a supply substation of a normally highly loaded line, so regulated for the maximum allowed no-load voltage); in this case the ri normalized by its value is further reduced. it is interesting to note that in the first 100 s, characterized by a negligible ri, time-varying harmonics are populating the spectrum (see section 4.2). 4.2. spectral components  when major transients are not visible in the time domain waveforms, this does not imply that other types of transients may be present. these transients are mainly of type 1 (bounded to the low frequency range) or type 4 (occurring over the entire frequency interval). figure 5 through 6 show the spectra at low frequency and around one of the pwm bands of the traction drive for an apparently almost constant ri interval (corresponding to t*=5.8 s in figure 2). low frequency broadband components due to leakage often cover important changes to high frequency components of smaller amplitude. transients of this kind produce several components that may be the reason for noncompliance with signalling interference limits [23] during dedicated tests and must be correctly weighted in the overall operating profile of the converter under test [25]. leakage at very low frequency occurs because of the non-stationary nature of supply voltage fluctuations. the best tool to identify and to track spectrum occupation is the time-frequency spectrogram. figure 5 reports the spectrum of the pantograph current, to 0 500 1000 1500 2000 2500 2500 3000 3500 4000 4500 5000 time [s] u a v ,u [ v ] umax3 = 3900 v umin,hs = 2800 v umin,conv = 2700 v figure 1. uav,u for runs of different duration: arezzo‐chiusi‐orte (black), torino‐modane (blue), gemona‐tarvisio (green), genova‐ventimiglia (red/brown).  0 100 200 300 400 500 600 3400 3600 3800 v o lta g e [ v ] 0 100 200 300 400 500 600 500 1000 1500 2000 c u rr e n t [a ] 0 100 200 300 400 500 600 0 1 2 time [s] r ip p le [ % ] (1) (2) (3) (4) figure 2. ripple index: sa curve for a 10 min test run.  0 100 200 300 400 500 600 700 800 900 1000 3400 3600 3800 4000 v o lta g e [v ] 0 100 200 300 400 500 600 700 800 900 1000 0 200 400 600 c u rr e n t [a ] 0 100 200 300 400 500 600 700 800 900 1000 0 0.5 1 1.5 time [s] r ip p le [ % ] figure 3. ripple index: sa curve for a 17 min test run.  acta imeko | www.imeko.org  july 2012 | volume 1 | number 1 | 30  identify the spectrum components more easily; the components of the pantograph voltage spectrum are immediately related to them by the line impedance seen at the pantograph. in figure 5a two components at 150 and 490 hz definitely increase after t*, besides an evident frequency leakage below 80 hz; zero stuffing is used in figure 5a to enhance the frequency resolution down to 5 hz, identifying the peculiar behaviour below 80 hz probably due to the resonant free response of the on-board filter, but surely influenced by the frequency leakage of the dc component. for the aim of reducing the latter it is advisable to subtract an estimate of the dc component, bringing the analysed signal down to zero mean. in figure 5b at higher frequency the dotted and solid black spectra at or after t* prevail, showing an increase also of high frequency components. two components are almost fix, 300 hz (substation) and 560 hz (on-board chopper). the raw fourier spectra need to be pre-processed before being used for the ri calculation and other computations, since the background noise components may largely influence an overall index like ri or the total rms; a threshold needs to be assigned to zero out the components below it. when evaluating broad peaks with a fine frequency resolution, peak isolation is necessary to avoid counting them more than once, including lateral components due to leakage. these techniques have already been successfully applied to the processing of recordings of electromagnetic field intensity on-board rolling stock [26]. however the use of a threshold, overlap percentage and peak isolation is prone to large deviations, as it is evident, by comparing the results in figure 2 with the preliminary results reported in [13]. it is however recognized that the ri index is a useful tool to locate various types of transients on the time-axis for further processing and analysis. from a graphical point of view, the time-frequency representation of fourier spectra is an effective tool. transients and time-varying components leave a clear signature in the fourier spectra computed by short time fourier transform, or spectrogram. as it is known, the spectrogram is subject to the same constraint on the frequency and time resolution of the basic fourier analysis. the steady characteristic harmonics at 300 hz, 600 hz and 900 hz may be easily seen in figure 4(a); other steady noncharacteristic harmonics at 50 hz, 100 hz and 150 hz are located in the lower part of the frequency range. a comb pattern of harmonics referred to a fundamental around 280 hz is also present (280 hz, 560 hz, 840 hz, 1120 hz), that increases with increasing speed and reaches approximately 460 hz, 940 hz, 1400 hz, 1860 hz after 30 s; the estimated rate of rise is thus 6 hz/s. this is evidently the emission of a traction static converter during acceleration and represents a typical type 4 transient. (a) (b) figure 4. spectrogram  in dbv: (a)  low frequency harmonics are artificially  compressed to highlight time‐varying harmonics; (b) typical non‐stationary  pwm signature from a different test run.  0 100 200 300 400 500 600 700 10 -2 10 -1 10 0 10 1 10 2 frequency [hz] c u rr e n t [a ] 0 100 200 300 400 500 600 700 10 -2 10 -1 10 0 10 1 10 2 frequency [hz] c u rr e n t [a ] t*-3t t*-2t t*-1t t* t*+1t t*+2t 300 hz 560 hz 300 hz 560 hz 100 hz 100 hz 490 hz 150 hz 490 hz 150 hz (a) 500 1000 1500 2000 2500 3000 10 -2 10 -1 10 0 frequency [hz] c u rr e n t [a ] t*-2t, -t t* t*+1t,+2t 560 hz 2750 hz 1130 hz (b) figure 5. analysis of transient behaviour at t*=5.8 s: (a) low frequency, with  base and enhanced frequency resolution; (b) high frequency.  acta imeko | www.imeko.org  july 2012 | volume 1 | number 1 | 31  the spectrogram in figure 4a also shows two vertical lines at 40 and 80 s, corresponding to transients of type 2 or 3. in figure 4b the high frequency pattern of the pwm modulation of a variable speed drive, probably the traction drive, is also clearly visible. the components are located symmetrically around the higher central component. the evolution versus time follows the operating conditions of the rolling stock, i.e. speed and tractive effort. this type of emissions, also of type 4, is particularly relevant when interference to signalling is concerned. limits for track circuits are expressed with a bandpass approach and non-stationary components may fall inside the susceptibility band [10][23][24]. the spectrogram is particularly useful in detecting such transients if a trade-off between frequency and time axis resolution is reached. post-processing of the amplitude and graphical presentation can ease visual detection. two transients have been selected and further analysed in figure 6 and figure 7 with a frequency resolution of 25 hz (40 ms time window) and overlap of 50 %; the overlap is necessary not only to track time varying components, but also to artificially increase the time axis resolution. the periodicity of about 200 hz, visible in figure 8, is related to the two peaks at about 5-6 ms in figure 7; on the contrary the single peak of figure 6 produces a flat spectrum. in both cases the transient components mask the characteristic harmonics, such as 300 hz. the last plot, in figure 8 is obtained with a 90 % overlap, so that the time step is only 4 ms; it is possible to distinguish the spectra before and after the transient event (dashed and dotted black curves), the spectra just at the beginning and the end of it (solid black curves) and during the transient itself (grey curves). overlapping and zero stuffing can improve the time axis resolution, ensuring transient detection; based on normal transient durations (2 to 10 ms), a 25 hz resolution with 50 % overlap ensures a 20 ms time step and a uniform increase of spectrum components by about 30 %. smaller time steps can be used simply increasing the overlap to 75 % (10 ms) or 90 % (4 ms). with the latter time step and the base 25 hz resolution, also the single pantograph voltage spike can be detected and correctly processed with acceptable “dilution” over the 40 ms window. the analysis is repeated for another transient with a finer frequency resolution and a better time axis representation (figure 9). the 300 hz component is visible as the main ripple term both in time and frequency domains; the grey circles identify a three-by-three pattern due to the 100 hz component. the transient occurring at 15.25 ms produces the broad spectrum shown in heavy black, but the extinction of the transient in about 1 ms is followed by the free response of the on-board filter, that lasts for about 10 ms, modifying the regular pattern of 300 hz and 100 hz components (see the two grey arrows on the following two 300 hz cycles) and produces the 75.66 75.67 75.68 75.69 3700 3800 3900 4000 4100 4200 tim e [s] v o lta g e [ v ] 0 200 400 600 800 1000-40 -20 0 20 40 60 80 frequency [hz] v o lta g e [ d b v ] figure  6.  spectrogram:  transient  waveform  and  fourier  spectra  before (dashed), at (solid) and after (dotted) the transient (20 ms time step).  81.75 81.76 81.77 81.78 81.79 81.83700 3800 3900 4000 4100 4200 tim e [s] v o lta g e [ v ] 0 200 400 600 800 1000 frequency [hz] -40 -20 0 20 40 60 80 v o lta g e [ d b v ] figure  7.  spectrogram:  transient  waveform  and  fourier  spectra  before  (dashed), at (solid) and after (dotted) the transient (20 ms time step).  0 200 400 600 800 100020 40 60 80 100 120 140 frequency [hz] v o lta g e [ v ] figure  8.  spectrogram  (same  as  figure  7):  spectra  before  (dashed  black),  partially  including  (solid  black),  centred  at  (solid  grey)  and  after  (dotted  black) the transient (4 ms time step).  acta imeko | www.imeko.org  july 2012 | volume 1 | number 1 | 32  transient spectrum components indicated by the black arrows. the spectrum components during the transient are predominant up to about 500 hz, with an amplitude increase of approximately 10 db. another uncommon type of transient is related to the stability of the supply frequency which, for dc traction systems supplied by the national grids, is 50 hz in europe and 60 hz in a few other countries (e.g. japan, korea, united states, etc.). the analysis of the arezzo-chiusi-orte recordings looking for type 2 or type 3 transients revealed a temporary shift of substation characteristic harmonics due to a supply frequency change, as shown in figure 10. in figure 10a the vp waveform and the ri curve are shown only for reference. the spectrogram in figure 10b using a logarithmic frequency scale reveals repeated low frequency transients of moderate amplitude, indicated by the repeated dark and clear vertical lines, extending up to about the 50 hz component. the most relevant transients at 101.0 s and 101.4 s extend above it, up to the 100 hz component. the linear scale in figure 10c reveals a frequency shift of all substation characteristic harmonics that can be led back to a significant change of the 50 hz supply frequency of nearly 0.7 hz (estimated from the higher order harmonics, where the frequency shift is proportionally larger and more easily evaluated). this event is quite uncommon for the observed relevance and the effect is a shift of various substation characteristic harmonics that might fall inside the operating frequency intervals of signalling devices. frequency stability is not further considered here; a complete discussion of this topic for ac traction supplies can be found in [6][28], where the variations of the fundamental are however much smaller with a span in the order of 0.1 hz. 4.3. wavelet analysis  the considerations on the classical fourier analysis of section 4.2, with the frequency resolution chosen with constraints on the time resolution, suggest the use of a wavelet representation for transients of type 1, 2 and 3. the discrete time wavelet transform (dtwt) is at the moment preferred for its simplicity with respect to the continuous wavelet transform, notwithstanding the better performances of the latter in terms of time and frequency resolution [27]. transients in vp are detected, by applying a threshold to the details dk, and classified, by deriving an 15.2 15.21 15.22 15.23 15.24 15.25 15.26 15.27 15.28 15.29 15.3 3450 3500 3550 3600 3650 3700 3750 time [s] v o lta g e [ v ] 0 200 400 600 800 1000 1200 1400 1600 1800 2000 10 -1 10 0 10 1 frequency [hz] v o lta g e [ v ] figure  9.  spectrogram:  transient  waveform  and  fourier  spectra  before (dashed) and at (solid) the transient (10 ms time step, 50 % overlap); the arrows in the time domain trace identify a change in the regular pattern of the  100  hz  and  300  hz  components,  the  three  arrows  in  the  frequency domain spectrum  identify the frequency intervals where the effect of the transient is evident.  97 98 99 100 101 102 103 104 105 3720 3725 3730 3735 3740 v o lta g e [ v ] 97 98 99 100 101 102 103 104 105 0.65 0.7 0.75 0.8 time [s] v o lta g e [ % ] a)  b)  c)  figure 10.  in a) the pantograph voltage, vp (top) and the ripple  index, ri (bottom). in b) and c) the spectrogram in dbv (10 hz frequency resolution, 75 % overlap) in logarithmic and linear frequency scale respectively.  acta imeko | www.imeko.org  july 2012 | volume 1 | number 1 | 33  empirical characterization of amplitude and oscillations in each detail. figure 11 shows an example of a type 3 transient at 70.29 s; a pantograph bounce is superimposed on the steady vp ripple of substation harmonics (an interpretation of the main ripple components visible in the time domain waveform was done after figure 9). the details d3 and d4 show the 300 hz ripple, that is absent from the adjacent details. wavelet details can be interpreted as band-pass channels with a centre frequency that, for dtwt, is determined by a geometric ratio. the basic frequency fb is determined by the sampling frequency and by the type of the wavelet; the centre frequency values at each scale a (and detail) are readily determined as fb/2 a. details not interested by the substation ripple are the candidates for the transient detection (see table 1). the best mother wavelet is then selected by considering the correct band apportioning, the amplitude of the peaks appearing in the details and the accuracy of their time location. many mother wavelets produce almost identical results [13]; the comparison is carried out by analysing the frequency behaviour of the examined wavelets. in the following “db” will stand for daubechies and “sym” for symlet. the values reported in table 1 are referred to a sampling time of 160 μs, eight times slower than the original sampling time. to cover a wider frequency interval, a lower value is needed and a larger number of details, thus increasing the computational effort. it is remembered that pantograph bounces trigger the free response of the on-board filter, located in the lower portion of the frequency axis (see figure 9), and modify temporarily the conducted emissions of the on-board converters, so that they represent a relevant event for the evaluation of interference to signalling circuits, in particular in the low and medium frequency intervals. in [28] the db4/db6 and db8/db10 were found the best choices for fast and slow transients, but for ac industrial supply networks. two wavelets (db3 and db5) are tested first and the results are shown in figure 12 and figure 13. low order wavelets may be not suitable to track all the variations of the analysed signal, but they feature a slightly larger peak amplitude in the low order details and a nonoscillating behaviour, thus ensuring a better time axis location of the crossing of the chosen threshold value and a nonambiguous behaviour (see figure 14 for a db1 wavelet applied to the same signal trace). without showing the results in the caption of figure 14 it is stated that the results are practically identical to those obtained with “sym1” and “haar” wavelets. given the use of discrete values to define the latter, the same is highly indicated to reduce the required computational effort and implement a real-time function. setting the sampling frequency to a higher value (25 khz, four times larger than the previous one used to derive table 1 and figure 11, figure 12 and figure 13), transients with a steep rise time and a time duration limited to one or few milliseconds may be detected more effectively (as shown in figure 15, that corresponds to figure 13 in [13]). such a sampling time moves all the five details on a band above the main characteristic harmonics, being fc5 located at 778 hz. 5. conclusions  in the present paper a range of transients typical of dc railway systems are considered and classified for their time and frequency behaviour. the target of the analysis is the evaluation of the power quality perceived at the pantograph voltage, but also the identification and location on the time axis of transients, that are relevant for interference to signalling circuits and thus for interoperability. the use of spectrogram and wavelets is proposed for the location of pantograph bounces on long records, that amount to many gigabytes of data and cannot be inspected manually; wavelets types and the behaviour of the details were tested based on real signals. the results available in the literature, that advise the optimal settings for wavelet analysis, are almost always referred to ac distribution networks in industrial systems, while a dc railway system represent a peculiar case study. very simple mother wavelets, such as a daubechies or symlet of order 1 or the haar wavelet, seem preferable (in particular the latter), if accurate time axis location and fast computing are the main requisites. 70.28 70.285 70.29 70.295 70.3 70.305 70.31 70.315 70.32 3750 3800 3850 3900 3950 4000 4050 4100 4150 4200 4250 time [s] v o lta g e [ v ] 70.25 70.3 70.35 -200 0 200 70.25 70.3 70.35 -200 0 200 70.25 70.3 70.35 -200 0 200 70.25 70.3 70.35 -500 0 500 70.25 70.3 70.35 -500 0 500 time [s] d1 d2 d3 d4 d5 figure 11. vp transient analysed with db3, 5 levels.  table 1. frequency axis location of wavelets.    wavelet type    db1  db3  db5  dt [μs]  160  160  160  fc1 [hz]  3113  2500  2083  fc2 [hz]  1556  1250  1042  fc3 [hz]  778.2  625  520.7  fc4 [hz]  389.1  312.5  260.4  fc5 [hz]  194.6  156.3  130.2  acta imeko | www.imeko.org  july 2012 | volume 1 | number 1 | 34                              references  [1] a. mariscotti, direct measurement of power quality over railway networks with results of a 16.7 hz network, ieee transactions on instrumentation and measurement, 60 (2011), pp. 1604-1612. [2] en 50388, railway applications – power supply and rolling stock – technical criteria for the coordination between power supply (substation) and rolling stock to achieve interoperability, aug. 2005. [3] en 61000-4-7, electromagnetic compatibility – part 4-7: testing and measurement techniques – general guide on harmonics and interharmonics measurements and instrumentation, for power supply systems and equipment connected thereto, 2002-08. [4] b. hemmer, a. mariscotti d. wuergler, recommendations for the calculation of the total disturbing return current from electric traction vehicles, ieee transactions on power delivery, 19 (2004), pp. 1190-1197. [5] en 50163, railway applications – supply voltages of traction systems, nov. 2004. [6] a. mariscotti and d. slepicka, “analysis of frequency stability of 16.7 hz railways”, proc. of the ieee intern. instrumentation and measurement technical conference i2mtc, may 10-12, 2011, hangzhou, china. [7] m. meyer and g.j. van alphen, netzresonanzmessungen auf hsl zuid und betuweroute, eisenbahn review, 7-8 (2006), pp. 610-611. [8] a. dolara, m. gualdoni and s. leva, impact of high voltage primary supply lines in the 2×25 kv – 50 hz railway system on the equivalent impedance at pantograph terminals, ieee transactions on power delivery, 27 (2012), pp. 164-175. [9] m. meyer and j. schöning, netzstabilitat in grossen bahnnetzen, eisenbahn review, 7-8 (1999), pp. 312-317. [10] pren 50238-2 (draft, pr. 15360), railway applications – compatibility between rolling stock and train detection systems – part 2: compatibility with track circuits, 2009. [11] clc/tr 50507, railway applications – interference limits of existing track circuits used on european railways, 2007-05. [12] specifica ferrovie dello stato (fs) n. 370582 esp. 1.0. [13] a. mariscotti, “dc railway line voltage ripple for periodic and aperiodic phenomena”, proc. of the imeko tc 04 congress, sept. 27-30, 2011, natal, rn, brazil. [14] m. caserza magro, a. mariscotti and p. pinceti, “definition of power quality indices for dc low voltage distribution networks”, imtc 2006, sorrento, italy, april 20-23, 2006. [15] en 61000-4-17, electromagnetic compatibility – part 4-17: testing and measurement techniques – ripple on d.c. input power port immunity test, 1999-08. 81.76 81.765 81.77 81.775 81.78 81.785 81.79 -200 0 200 81.76 81.765 81.77 81.775 81.78 81.785 81.79 -300 0 300 81.76 81.765 81.77 81.775 81.78 81.785 81.79 -500 -250 0 250 500 81.76 81.765 81.77 81.775 81.78 81.785 81.79 -1000 0 1000 81.76 81.765 81.77 81.775 81.78 81.785 81.79 -1000 0 1000 time [s] figure 12. vp transient of figure 8: db3, 5 levels.  81.76 81.765 81.77 81.775 81.78 81.785 81.79 -200 0 200 81.76 81.765 81.77 81.775 81.78 81.785 81.79 -300 0 300 81.76 81.765 81.77 81.775 81.78 81.785 81.79 -400 0 400 81.76 81.765 81.77 81.775 81.78 81.785 81.79 -500 0 500 81.76 81.765 81.77 81.775 81.78 81.785 81.79 -1000 0 1000 time [s] figure 13. vp transient of figure 8: db3, 5 levels.  81.76 81.765 81.77 81.775 81.78 81.785 81.79 -300 0 300 81.76 81.765 81.77 81.775 81.78 81.785 81.79 -500 0 500 81.76 81.765 81.77 81.775 81.78 81.785 81.79 -500 0 500 81.76 81.765 81.77 81.775 81.78 81.785 81.79 -600 0 600 81.76 81.765 81.77 81.775 81.78 81.785 81.79 -600 0 600 time [s] figure 14. vp transient of figure 8: db1, 5 levels (identical results for sym1 and haar wavelets).  81.76 81.765 81.77 81.775 81.78 81.785 81.79 -300 0 300 81.76 81.765 81.77 81.775 81.78 81.785 81.79 -500 0 500 81.76 81.765 81.77 81.775 81.78 81.785 81.79 -500 0 500 81.76 81.765 81.77 81.775 81.78 81.785 81.79 -800 0 800 81.76 81.765 81.77 81.775 81.78 81.785 81.79 -1000 0 1000 time [s] figure 15. vp transient of figure 8: db1, 5 levels (40 s sample time).  acta imeko | www.imeko.org  july 2012 | volume 1 | number 1 | 35  [16] mil std 704e, aircraft electric power characteristics, may 1991. [17] a. mariscotti, analysis of the dc link current spectrum in voltage source inverters, ieee transactions on circuits and systems – part i, 49 (2002), pp. 484-491. [18] a. mariscotti and p. pozzobon, synthesis of line impedance expressions for railway traction systems, ieee transactions on vehicular technology, 52 (2003), pp. 420-430. [19] e.w. kimbark, direct current transmission, wiley interscience, 1971. [20] a. mariscotti, “methods for ripple index evaluation in dc low voltage distribution networks”, proc. of the ieee instrumentation and measurement technical conference imtc, may 2-4, 2007, warsaw, poland. [21] s. herraiz jaramillo, g.t. heydt and efrain o’neill-carrillo, power quality indexes for aperiodic voltage and currents, ieee transactions on power delivery, 15 (2000), pp. 784-790. [22] a. mariscotti, discussion of power quality indexes for aperiodic voltage and currents, ieee transactions on power delivery, 15 (2000), pp. 1333-1334. [23] clc/ts 50238-2, railway applications – compatibility between rolling stock and train detection systems – part 2: compatibility with track circuits, 2010-07. [24] a. mariscotti, “on the uncertainty of the bandpass filter method for the evaluation of interference on track circuits”, proc. of the 20th imeko world congress, sept. 9-14, 2012, busan, republic of korea. [25] g. armanino, a. mariscotti and m. mazzucchelli, “in-house test of low frequency conducted emissions of static converters for railway application”, proc. of the 17th imeko world congress, june 22-27, 2003, cavtat-dubrovnik, croatia. [26] d. bellan, a. gaggelli, f. maradei, a. mariscotti and s. pignari, time-domain measurement and spectral analysis of nonstationary low-frequency magnetic field emissions on board of rolling stock, ieee transactions on electromagnetic compatibility, 46 (2004), pp. 12-23. [27] l. angrisani, p. daponte, m. d’apuzzo, and a. testa, a measurement method based on the wavelet transform for power quality analysis, ieee transactions on power delivery, 13 (1998), pp. 990-998. [28] a. mariscotti and d. slepicka, “the frequency stability of the 50 hz french railway”, proc. of the ieee international instrumentation and measurement technical conference, i2mtc, may 14-16, 2012, graz, austria. microsoft word 285-2076-2-le-rev acta imeko  issn: 2221‐870x  april 2016, volume 5, number 1, 32‐35    acta imeko | www.imeko.org  april 2016 | volume 5 | number 1 | 32  spectroscopic fingerprinting techniques for food  characterisation  monica casale,   lucia bagnasco, chiara casolino, silvia lanteri, riccardo leardi  univeristy of genoa, departmentof pharmacy – via brigata salerno 13, 16147 genova, italy      section: research paper   keywords: non selective signals; uv‐visible spectroscopy (uv‐vis); mid‐infrared spectroscopy (mirs); near‐infrared spectroscopy (nirs); chemometrics;  food analysis  citation: monica casale, lucia bagnasco, chiara casolino, silvia lanteri, riccardo leardi, spectroscopic fingerprinting techniques for food characterisation,  acta imeko, vol. 5, no. 1, article 7, april 2016, identifier: imeko‐acta‐05 (2016)‐01‐07  section editor: claudia zoani, italian national agency for new technologies, energy and sustainable economic development affiliation, rome, italy  received june 26, 2015; in final form december 17, 2015; published april 2016  copyright: © 2016 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  corresponding author:  riccardo leardi, e‐mail: riclea@difar.unige.it    1. introduction  traditional analytical methods for food analysis have several drawbacks, such as low speed, the necessity for sample pretreatments, a requirement for highly-skilled personnel and destruction of the sample. several fast and non-destructive instrumental methods have been proposed to overcome these hurdles. among them, uvvisible, mid infrared and near infrared spectroscopy have proven to be successful analytical methods for analysis of food since they offer a number of important advantages [1], [2]. they are fast, non-destructive methods and they require a minimal or no sample preparation. moreover, they are less expensive because no reagents are required and thus no wastes are produced. finally, the versatility of these instruments makes them useful tools for online process monitoring. chemical information contained in the spectra resides in the band positions, intensities, and shapes. whereas band positions give information about the molecular structure of chemical compounds, the intensities of the bands are related to the concentration of these compounds as described by the lambert-beer law. the easiest way to determine the content of a chemical compound is to measure the change in the intensity of a well-resolved band that has been unambiguously attributed to this compound [3]. however, this is possible for a pure component system, but foods contain numerous components giving rise to complex spectra with overlapping peaks. in fact, in order to take advantage of these spectroscopic fingerprinting techniques, the analyst must overcome limitations in sensitivity and selectivity that arise from the abstract  the analysis of samples by using spectroscopic fingerprinting techniques is more and more common and widespread. such approaches  are very convenient, since they are usually fast, cheap and non‐destructive. in many applications no sample pretreatment is required,  the acquisition of the spectrum can be performed in about one minute and no solvents are required. as a consequence, the return on  investment of the related technology is very high.   the “disadvantage” of these techniques is that, the signal being non‐selective, simple mathematical approaches (e.g., lambert‐beer  law) cannot be applied. instead, a multivariate treatment must be performed by using chemometrics tools.   in what concerns food analysis, they can be applied in several steps, from the evaluation of the quality and the conformity of raw  material  to  the  assessment  of  the  quality  of  the  final  product,  to  the  monitoring  of  the  shelf  life  of  the  product  itself.  another  interesting  field  of  application  is  the  verification  of  food‐authenticity  claims,  this  being  extremely  important  in  the  case  of  foods  labeled as protected designation of origin (pdo), protected geographical indication (pgi) and traditional speciality guaranteed (tsg).   in the present paper, it is described how non‐selective signals can be used for obtaining useful information about a food. acta imeko | www.imeko.org  april 2016 | volume 5 | number 1 | 33  relatively weak and highly overlapping bands found in the spectra. the result is a unique “fingerprint” that can be used to confirm the identity of a sample. thus, for the implementation of a successful analysis the use of chemometric methods is fundamental to extract from the spectra as much information as possible about the analysed samples. the information extracted from the non-selective signals can be used for two types of analysis: (1) quantitative analysis to link features of the spectra to quantifiable properties of the samples. spectroscopic techniques are commonly used to obtain calibration models able to predict the concentration of a compound or a specific characteristic of a food product. several studies have been performed, for example on the detection of adulterants in foods. (2) qualitative analysis, i.e. classification of samples. recently, a rather specific type of fraud has become important involving claims regarding the geographical origin of food ingredients. it is generally true that deceptions regarding the geographical origin of foods have few health implications, but they may nonetheless represent a serious commercial fraud. in fact, consumers often pay a considerable price premium when a food is labelled with a declaration of production within a specific region, since such a label may be perceived as an implicit guarantee of a traditional and, perhaps, healthier manufacturing process. in response, several national and international institutions have issued directives to support the differentiation of agricultural products and foodstuffs on a regional basis by introducing an integrated framework for the protection of both geographical origin and traditional production techniques. in these cases, non-selective signals can be used to obtain classification models able to discriminate samples according to a quality/characteristic. in this paper, as an example, the construction of a reliable quantitative model for the detection of addition of barley to coffee using nir spectroscopy and chemometrics is shown. 2. how to build a model  figure 1 shows the scheme for building a quantitative or qualitative multivariate model. the main steps to be performed are: 1) sample selection the analysed samples should fully represent the population studied; this means that all the variability sources (or at least the most important ones) should be taken into account. moreover, when a calibration model is developed the samples should cover a wide range of response values. unfortunately, there are many reports based on poor sampling, and this affects the result of the whole analysis. for example, in order to discriminate extra virgin olive oils on the basis of the olive cultivar, oil samples should be collected from different oil mills in order to take into account the different sources of variability, such as geographical origin, production year, harvest period and production technologies (fertilizer, olive fly control tools, extraction process, etc). 2) spectra acquisition and data matrix the acquisition of the spectra is very simple and generally requires few minutes. then, spectra have to be arranged in a data matrix in which each row corresponds to a sample and each column to a variable (wavelength). this trivial operation is not always easy to be performed since many instruments do not allow the direct exportation of a set of spectra. 3) sample pretreatment spectra can be affected by turbidity in liquid samples or different granulometry in solid samples and by variations in the optical path length. to avoid or decrease these interferences, mathematical pretreatments are required. the most current data pretreatments are normalisation methods such as standard normal variate (snv) [4] and derivatives. the snv, or row autoscaling, mainly corrects both baseline shifts and global intensity variations, which are related to the granulometry of the sample. each spectrum is row-centered, by subtracting its mean from each single value, and then scaled by dividing it by its standard deviation. as a result, each spectrum has mean equal to 0 and standard deviation equal to 1. since snv removes the possibly shifting informative regions along the signal range, the interpretation of the results referring to the original signals should be performed with caution. other common pretreatments applied to spectroscopic signals are the derivatives. first derivative provides a correction for baseline shifts, while the second derivative represents a measure of the curvature of the original signal, i.e., the rate of change of its slope. such transform provides a correction for both baseline shifts and drifts. a drawback of derivatives is the increase of random noise. 4) variable selection since the uv-visible, nir and mir spectra are characterized by a very high number of variables, the selection of the informative ones is an important task, both to obtain simpler qualitative or quantitative models and to identify the most useful wavelengths. several algorithms can be applied. among them, some commonly used are the stepwise selection methods (i.e: stepwise linear discriminant analysis [5], iterative stepwise elimination [6]), interval-pls [7] and genetic algorithms [8]. compared with the other techniques, genetic algorithms have the great advantage of selecting well defined spectral regions, often corresponding to relevant spectral features, instead of single wavelengths. 5) model building and validation the choice of the classification or regression method should be performed following the simplicity criterion, since simpler models are also more robust and stable. the most used ones are linear discriminant analysis (lda) [9] and k nearest neighbors (knn) [10] for classification, soft independent figure  1.  scheme  for  building  a  quantitative  or  qualitative  multivariate model.   acta imeko | www.imeko.org  april 2016 | volume 5 | number 1 | 34  modeling of class analogy (simca) [11] as class-modeling technique and partial least square (pls) [12] for multivariate calibration. a chemometrical model must be evaluated on the basis of its predictive ability. cross-validation is the most common validation procedure: the n objects are divided into g cancellation groups, the model is computed g times and each time the objects in the corresponding cancellation group are predicted. at the end of the procedure, each object has been predicted once. 6) external test set the real predictive ability of each model should be tested on an external set of samples. the test set has to be totally independent of the calibration set, not only mathematically as in cross-validation, but also from the ‘spatial’ and ‘temporal’ point of view. for instance, in case of industrial data the samples of the test set must be produced and analysed after the samples of the training set; in case of samples whose origin can be very different the samples of the test set must be produced by producers that are not the same as the producers of the samples of the training set; in case of natural products the samples of the test set must come from crops subsequent to those of the samples of the training set. 3. example of construction of a reliable  quantitative model for food analysis  this study [13] presents an application of near infrared spectroscopy for detection and quantification of the fraudulent addition of barley in roasted and ground coffee samples. nine different types of coffee including pure arabica, robusta and mixtures of them at different roasting degrees were blended with four types of barley. the types of coffee and barley were selected in order to be as representative as possible of the italian market. since it was decided to perform 100 experiments out of the 360 combinations resulting from nine coffees, four barleys and ten different concentrations (from 2 % to 20 % w/w), the calibration set was defined by applying a d-optimal design. the validation was performed on thirty experiments selected by applying a subsequent d-optimal design on the combinations not constituting the training set. after that, a further validation was performed on a completely external set made by mixtures of a type of coffee and a type of barley, different from those used in the previous steps. partial least squares regression (pls) was employed to build the models aimed at predicting the amounts of barley in coffee samples. in order to obtain simplified models, taking into account only informative regions of the spectral profiles, genetic algorithms were applied for feature selection. this allowed to reduce the number of data points from 1501 to 188. the models showed excellent predictive ability with root mean square errors (rmse) for the test and external sets equal to 1.4 % w/w and 1.1 % w/w, respectively. as it can be noticed in figure 2, the model obtained is capable to predict barley concentration with a very satisfactory accuracy, not only in calibration but also on external samples. this application clearly shows that the representativity of the training set is a key point in the success of a calibration model. the achievement of very low prediction errors on a totally external test set (i.e., on mixtures composed by qualities of coffee and barley unknown to the model) has been possible only as a consequence of the fact that the training set was made by taking into account a relatively large number of varieties of coffee and barley. another key point is the application of doptimal design for the selection of a subset of adequate size from the very high set of candidate experiments. moreover, the variable selection by using genetic algorithms helped to determine the spectral regions most useful to identify the adulteration of coffee with barley and to increase calibration model performances. 4. conclusions  in the present paper it has been shown, from a methodological point of view, how non-selective signals can be used for obtaining useful information about food. the chemometrical elaboration is fundamental in order to obtain useful information from the spectral data; the main steps of this approach have been identified and their importance discussed also showing a real application. references  [1] t. woodcock, g. downey, c.p. o’donnell, review: better quality food and beverages: the role of near infrared spectroscopy, journal of near infrared spectroscopy, 16(1), (2008), pp. 1-29. [2] l. wang, f. s.c. lee, x. wang, y. he, feasibility study of quantifying and discriminating soybean oil adulteration in camellia oils by attenuated total reflectance mir and fiber optic diffuse reflectance nir, food chemistry, 95, (2006) pp.529–536. [3] r. karoui, g. downey, c. blecker, mid-infrared spectroscopy coupled with chemometrics: a tool for the analysis of intact food systems and the exploration of their molecular structure-quality relationships a review, chemical reviews, 110, (2010), pp.6144– 6168. [4] r.j. barnes, m.s dhanoa, s.j. lister, standard normal variate transformation and de-trending of near-infrared diffuse reflectance spectra. applied spectroscopy, 43(5), (1989), pp.772777. [5] r.i. jenrich, in k. enselein, a. ralston, h.s. wilf (eds)statistical methods for digital computers, j. wiley and sons, new york, 1960, pp. 76. [6] r. boggia, m. forina, p. fossa, l. mosti, chemometric study and validation strategy in the structure-activity relationships of new cardiotonic agents, quantitative structure-activity relationships, 16, (1997), pp. 201-213. figure  2.  experimental  vs.  predicted  values  of  concentration  (%  w/w)  of  barley  in  the  coffee  samples  of  the  external  test  set  (pls  model  on  the  spectral regions selected by ga).   0 5 10 15 20 0 5 10 15 20 experimental value p re d ic te d v a lu e acta imeko | www.imeko.org  april 2016 | volume 5 | number 1 | 35  [7] l. nørgaard, a. saudland, j. wagner, j.p. nielsen, l. munck and s.b. engelsen, interval partial least squares regression (ipls): a comparative chemometric study with an example from nearinfrared spectroscopy, applied spectroscopy, 54, (2000) pp. 413-419. [8] r. leardi, a.l. gonzalez, genetic algorithms applied to feature selection in pls regression: how and when to use them, chemom. intell. lab. syst. 41 (1998) pp.195–207. [9] d. l massart, b. g. m. vandeginste, l. m. c buydens, s. de jong, p. l lewi, j. smeyers-verbeke, “supervised pattern recognition” in, handbook of chemometrics and qualimetrics: part b, vol. 20b, b.g.m. vandeginste, & s.c. rutan, elsevier, amsterdam, 1998, pp. 213-220. [10] t. m. cover and p. hart, "the nearest neighbor decision rule," ieee trans. inform. theory, vol. it-13, pp. 21-27 1967. [11] s. wold, m. sjostrom, “simca: a method for analysing chemical data in terms of similarity and analogy” in chemometrics, theory and application, b. r. kowalski, acs symposium series 52, american chemical society, washington, dc, 1977, pp. 243. [12] h. wold, “partial least squares” in: s. kotz, n.l. johnson, (eds.), encyclopedia of statistical sciences, wiley, new york, 1985, pp. 581–591. [13] h. ebrahimi-najafabadi, r. leardi, p. oliveri, m.c. casolino, m. jalali-heravi, s. lanteri, detection of addition of barley to coffee using near infrared spectroscopy and chemometric techniques, talanta 99, (2012) pp.175–179. microsoft word article 14 169-1323-1-le.doc acta imeko  february 2015, volume 4, number 1, 90 – 96  www.imeko.org    acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 90  validation of numerical methods for electromagnetic  dosimetry through near‐field measurements   d. giordano 1 , l. zilberti 1 , m. borsero 1 , r. forastiere 2 , w. wang 1   1  inrim, strada delle cacce 91 – 10135 torino, italy  2  dip. energia, politecnico di torino, corso duca degli abruzzi – 10129 torino, italy      section: research paper   keywords: electromagnetic dosimetry; mri; near‐field measurements  citation: d. giordano, l. zilberti, m. borsero, r. forastiere, w. wang, validation of numerical methods for electromagnetic dosimetry through near‐field  measurements, acta imeko, vol. 4, no. 1, article 14, february 2015, identifier: imeko‐acta‐04 (2015)‐01‐14  editor: paolo carbone, university of perugia   received december 31 st , 2013; in final form november 21 st , 2014; published february 2015  copyright: © 2014 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: european metrology research programme (emrp)‐hlt06 joint research project (jrp) “metrology for next‐generation safety standards and  equipment in mri” (2012–2015)  corresponding author: d. giordano, e‐mail: d.giordano@inrim.it    1. introduction  an estimated 8 – 10% of the european population are carrying medical implants. at present, they are excluded from receiving a magnetic resonance imaging (mri) scan as no metrics exists to assess the specific safety risks related to these implants. with the aim of investigating implant associated risks [13], advanced modelling concepts for the evaluation of electromagnetic field inside phantoms and human computer models [4][5] are fundamental tools which must be validated by experimental comparisons to guarantee their reliability. to this end an experimental tool was designed and set up and first comparisons between measurements and numerical results obtained through a boundary element method (bem) algorithm, are presented in the paper. since this preliminary investigation is based on a simple numerical model involving all the physical aspects, a simplified experimental tool is required as well. to do that a system made of a cylindrical phantom with a tissue-like liquid, a loop antenna and a 3d electromagnetic field mapping was set up. the magnetic source and measuring system shall be able to generate and detect radio frequency (rf) electromagnetic fields with magnitude and frequency similar to the ones exploited by magnetic resonance imaging (mri) devices for diagnostic purposes. as regards the frequency, it shall be comparable with the isocentre resonance frequency (larmor frequency) of the most used mri devices. these are characterized by frequencies of about 64 mhz, 128 mhz and 300 mhz related to isocentre static magnetic fields of 1.5 t, 3 t and 7 t respectively. for these preliminary investigations the frequency of interest was set to 64 mhz [6]. as well known from the nuclear magnetic resonance (nmr) theory, a rotation of the nuclear magnetic moment vectors can be impressed by transferring a suitable amount of energy into the anatomy by means of coils tuned at the larmor frequency. such coils have to generate rf magnetic abstract  this paper describes the arrangement of a first experimental set‐up which allows the comparison between the measurement of the  electromagnetic field quantities induced inside a simple cylindrical phantom and the same quantities estimated numerically through a  boundary element method. the reliability of the numerical method has been tested at 64 mhz, the larmor frequency associated to  the magnetic resonance imaging devices with an isocenter magnetic field of 1.5 t. to assess its robustness, the comparison is also  performed by introducing, inside the phantom, a metallic non magnetic element, which roughly simulates a medical implant.   acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 91  fields perpendicular to the static magnetic field direction. consequently, a loop antenna was selected [6-7]. since the aim of this paper is the experimental set-up, just a synthetic description of the field formulation is given in section 2. a description of the experimental set-up is given in section 3, and some critical items due to the simplified model of the loop antenna are pointed out. in particular, the presence of the shield in the actual loop and the absence of the earth in the numerical model are argued. the first comparisons between measured and computed magnetic field components are performed along a vertical and a radial line inside the cylindrical phantom. additional comparisons are performed when a metallic object mimicking an implant is introduced inside the phantom (see section 4). a brief description of the system control and operations, managed by a python programme which automates the generation and acquisition of the field components, is given in section 5, followed by some conclusions (section 6). 2. field formulation    the electromagnetic field problem is described by the electric field integral equation (efie) and magnetic field integral equation (mfie) under sinusoidal conditions (angular frequency ω). the integral equations are solved by the boundary element method (bem), discretizing the external surface (∂ω) of the considered objects (ω) with 2d surface elements. the discretized form of the efie and mfie equations is:       , , , s m m m m i s i mm m m m i m i mm m m m j dv ds ds j ds                         e j n e n e n h         s , , , m m m m i s i mm m m m i m i mm m m m dv ds ds j ds                        h j n h n h n e (1) where js is the impressed current density of the sources (ωs), ξ is the singularity factor (ξ = 0.5 on the surface and ξ = 1 elsewhere) and n is the normal unit vector directed outwards ω. the m-th element is the source point while, during the setting of the matrix, the computational point is the barycentre of the i-th element. the green function is defined as: ' 4 ' ike      r r r r with k j              (2) where r and r’ are the coordinate vectors of the observation and of the source points, while µ, σ and ε are the magnetic permeability, the electric conductivity and the electric permittivity respectively. efie and mfie can be written for any volume belonging to the computational domain (the air region outside the phantom, inside this latter and also inside possible metallic objects). the volume integrals must be included only in volumes involving impressed sources. the integrals of the green function and its gradient are computed by using an adaptive quadrature rule based on kronrod algorithm, which enables high accuracy limiting the computational burden. 3. experimental set‐up  figure 1 shows an overall view of the experimental setup, including the phantom filled with a yellow liquid (the human tissue-like liquid), the 3d probe positioning system made of dielectric material with low permittivity (εr = 2), the electric (or magnetic) field probe and its optical converter, the support for the antennas (more antennas are foreseen). the black loop antenna can be seen on the right side of the phantom. figure 1 experimental set‐up.  acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 92  3.1. phantom  the phantom consists of a cylindrical container made of polymethylmethacrylate whose dielectric permittivity (εr = 2.2), much lower than that of the liquid, does not affect the electric field induced in the phantom. it has both an internal diameter and an internal height equal to 240 mm. the liquid, prepared by the physikalisch technische bundesanstalt laboratories (ptb, berlin), shows electrical characteristics comparable with the ones of some human tissues. to reduce the effects of the support material on the electric field behaviour around the phantom, four slim columns sustain the container, increasing the distance from the structure supporting the whole system. 3.2. magnetic field generation system  the magnetic field source shall be able to induce, inside the phantom, an electric field with a magnitude detectable by the near-field probes employed for this experiment. moreover, the source should not produce spurious electric fields which are not considered in the numerical procedure. the selected source for these purposes is a small loop antenna, consisting of a single turn with 6 cm diameter, inside a balanced e-field shield (figure 2a). through preliminary computations, a magnetic field amplitude of 1 a/m produced by a current of 0.5 a is predicted at 20 mm far from the loop. of course this value is lower than the typical magnetic field value generated by the rf imaging coil of the mri devices [9-11]; nevertheless, this is acceptable because the purpose here is not to cause the nuclear magnetic resonance but to investigate the possible side-effects induced in the human body during mri examination. the estimated electric field induced by such magnetic field is a few tens of volts per meter, enough to be detected by the probe. a first experimental arrangement for the supply and estimation of the current flowing in the antenna is depicted in figure 2b). the supply system consists of a signal generator and a 100 w broadband amplifier (9 khz to 100 mhz). in order to improve the matching between the supply system and the antenna, a series capacitor cs, with a rated value of 8 pf, is introduced. in this way it is possible to obtain the required field without exceeding the maximum power (25 w) of the 50 ω resistive load. this load allows to estimate the current which generates the field. indeed, through the knowledge of the voltage across the resistive load, obtained by the measurement of the incident power p, it is possible to solve the circuital model shown in figure 3a and to estimate the current that flows in the inductance l simulating the loop antenna. the parameter cl simulates the stray electrical coupling between the loop wire and its shield. because of the antenna dimensions and supply frequency, the radiation resistance can be neglected as well as the joule losses and the circuital model can be simplified. the current flowing in the loop antenna (and hence in the inductance l), il, which generates the magnetic field, a)     directional coupler direct power sensor 50 25 w load circular/rectangular antenna coilmatching capacitor 100 w 100 mhz or 400 mhz rf amplifier synthetizer -40 db p reflected power sensor controller  b)  figure 2. a) sketch of the loop antenna [8]. b) sketch of the set‐up for the generation of rf magnetic field.  c l c s lv lv f r l o a d a n t e n n a p   a)   45 50 55 6 0 65 70 0,0 0,2 0,4 0,6 0,8 1,0 t ( s /m ) f (mhz) computed t m easured t   b)  figure 3. a) circuital model for the estimation of the current flowing in the  loop  antenna.  b)  computed  and  measured  frequency  behaviour  of  t  coefficient. figure  4.  investigation  lines  inside  the  phantom:  frontal  and upper view. acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 93  can be directly estimated by knowing the transmitted power to rload and the admittance t defined as:  2 1 with 1 l s res f l s res i c t v l c c                (3) the transmitted power is obtained as difference between the incident and reflected power values measured by means of a directional coupler and a power meter. from that, the voltage vf is easily obtained. the unknown cs and ωres are extrapolated from the experimental frequency behaviour of t from 45 mhz to 70 mhz by means of a least square algorithm between the measured frequency behaviour and the computed one. assuming that the current is uniform along the single turn (i.e., πd << λ/10, where d is the turn diameter and λ is the wavelength of the working frequency), the magnetic field in the loop centre is l c i h d  ; then the following system can be written:     1 2 1 1 2 1 2 2 2 2 1 1 c s f res c s f res h c t d v h c t d v                                    (4) where ω1 and ω2 are arbitrary angular frequencies chosen in the range 45 mhz to 70 mhz. from a series of measured values  c i f h v  evaluated in the same frequency range a couple of such values is randomly extracted to solve the system (4) obtaining a series of couple of values (cs-i, ωres-i). by means of the above cited least square algorithm between the measured t(ω) values and the computed ones, the optimal couple of values (cs, ωres) were found to be equal to 8.3 pf and 70.6 mhz, respectively. figure 3b shows the frequency behaviour of the computed and measured t coefficient. the gap on the e-field shield such as a coupling between the antenna and earth, due to a non-symmetric supply, can compromise the comparison between measurement and numerical results. indeed the two elements introduce spurious electric components which are not taken into account by the numerical procedure. a non-symmetric supply for the antenna generates a common electric coupling with the earth evidenced by an electric field component perpendicular to the earth, not foreseen by the model. the gap problem can be overcome by shielding the gap with a metallic strip. the earth coupling disappears when the antenna is placed close to the phantom. the conductivity of the liquid “shields” the antenna from the earth. the encouraging results shown in the following prove the reliability of the assertions. 3.3. electric and magnetic field probes  since the electromagnetic field measurement should be performed in the near-field region, probes with very small dimensions are chosen. moreover, owing to their usage inside the phantom, resistance to organic solvent must be guaranteed. their main characteristics are resumed in table i [12]. these isotropic probes give the true rms value of the applied field along the three orthogonal axes. thanks to an electro-optic converter the information can be easily carried from the probe to the meter which can be placed a few meters away from the magnetic source. the field generation and detection system is automatically managed by a python program which reduces the measurement time and increases the accuracy of the power measurement, allowing for a thermal equilibrium of the devices. 4. comparison between numerical model and  measurement results  a preliminary calibration phase gives evidence of a rotation of the x/y field arrays around the z direction with respect to the absolute reference system shown in figure 5, for both the electric and magnetic field probes. the estimation of such rotation angle is performed thanks to the symmetry conditions imposed by the loop antenna. the results, accompanied by an uncertainty of about 10%, are 20° and 23.5° for the electric and magnetic probe respectively. the algorithm which manipulates the acquired field components to apply the coordinate rotation is implemented in the software which controls the whole experimental set-up (section 5). as a first comparison between the bem algorithm and table 1. characteristics of the electric and magnetic field probes. section  electric field  magnetic fied  frequency  40 mhz ÷ 6 ghz  10 mhz ÷ 600 mhz (absolute accuracy ± 6.0%);  output linearized  dynamic range  2 v/m ÷1000 v/m  0.08 a/m to 40 a/m at 13.56 mhz   0.01 a/m to 5 a/m at 100 mhz  overall length  337 mm (tip: 40 mm) 337 mm (tip: 40 mm)  distance from   probe tip to dipole centre  2.5 mm  3 mm  tip diameter  8 mm (body: 10 mm) 6 mm (body: 12 mm)  acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 94  measurements, two investigation lines are chosen. line 1 is in parallel to the y axis (figure 4), 30 mm above it. since the induced current effects on the magnetic field at 64 mhz are negligible, the ideal magnetic field array lies on y-z plane, so the hx_bem component is null (figure 5a). the electric field lines induced in the phantom are, at this frequency, circular with the centres on the loop axis, thus, only ex_bem is non-zero along the investigation line (figure 5 b). line 2 is in parallel to the z axis and is placed 110 mm from the reference system centre. since this line lies on the z-y plane which cuts symmetrically the antenna, the hx_bem component is null, hz_bem shows two peaks (figure 6a) in correspondence with the minimum distance between line 2 and the turn of the antenna, while hy_bem reaches the peak in correspondence with the loop antenna axis. the shape of the ex_bem component should be analogous to the shape of hz_bem while ey_bem and ez_bem are zero (figure 6b) because of the circular shape of the electric field lines. when performing electric field measurements the noise level is not always negligible (in the order of 2 to 3 v/m). to reduce the noise effects on the measured values, the following expression is employed: 2 2 64mhz tot noisee e e  (5) where e64mhz is the electric field corrected for the noise, etot is the electric field given by the meter (signal plus noise) and enoise is the mean value of the noise detected when the loop antenna is not supplied. in spite of the problems spotted both in the estimation of the current flowing in the antenna at 64 mhz and the 0,00 0,02 0,04 0,06 0,08 0,10 0,12 0 2 4 6 8 10 h ( a /m ) y (m) h y , h z bem surfaces measurements   a)  0,00 0,02 0,04 0,06 0,08 0,10 0,12 0 10 20 30 40 50 60 70 e ( v /m ) y (m) e x bem surfaces measurements    b)  figure 5.  magnetic a) and electric b) r.m.s. field behaviour along line 1 of  the three components with a unit‐current in the antenna.  0,00 0,04 0,08 0,12 0,16 0,20 0,24 0 2 4 6 8 h ( a /m ) z (m) h y , h z bem surfaces measurements   a)   0,00 0,04 0,08 0,12 0,16 0,20 0,24 0 10 20 30 40 e ( v /m ) z (m) e x bem surfaces measurements    b)  figure 6. magnetic a) and electric b) r.m.s. field behaviour along line 2 of the  three components with a unit‐current in the antenna.  a)             b)  figure 7 a). metallic parallelepiped 200 mm high placed inside the phantom. b) investigation lines and dimensions : frontal and upper view. acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 95  approximation introduced in the numerical representation of the antenna, the measurement-computation comparisons give encouraging results. in particular, figures 6a and 6b show the very good performance of the probes which can detect field having high spatial gradient (close to the antenna axis: z = 0.12 m). the experimental scenario is finally made more complicated by introducing a 40×40×240 mm3, conductive, non-magnetic parallelepiped. the aim is to roughly simulate a medical implant. its large dimensions provide a high distortion field, allowing a first evaluation of the sensitivity of the numerical method. figure 7 shows a picture of the metallic object inserted in the phantom and its position. figures 8 and 9 provide the numerical results compared with the measured ones for the same lines previously investigated. as can be easily seen, a higher discrepancy between measurement and computation is detected, in particular for line 2. this fact deserves further investigations, but probably it can be ascribed to the features of the problem matrix, which, in presence of the metallic object, is badly scaled and gives rise to a nonoptimal convergence of the iterative solver used to solve the bem formulation. moreover, the wavelength in the metallic object is much lower than elsewhere and this would ask for a very fine mesh, with a strong increase of the computational burden needed to guarantee an accurate reconstruction of the electromagnetic quantities. 5. system control and operations  in order to increase the safety, measurement reliability and speed, the supply and measurement systems are managed by a software developed in python environment. python is an interpreted, general purpose, high-level programming language [13]. the software controls the synthesizer and power amplifier by means of a gpib-usb controller. the acquisition of the incident and reflected power is supervised by an usb connection. to download the acquired electric/magnetic field components, a simple network between the remote unit easy4 (server) and the pc (client) by an ethernet connection [14] is set. thanks to a console user interface it is possible to insert the working directory and the output file names. if the directory and/or the files do not exist the program creates them. the measurement results are stored in two output files with the same name but different extensions: “.txt”. they contain the per-unit current electric/magnetic field components, referred to the main reference coordinate, while the second file, with a “.raw” extension, contains the raw field components, affected by noise, and referred to a rotated coordinate system. moreover, the simple user interface requires: the type of the employed probe (electric or magnetic field probe), the geometrical characteristics of the investigation curve and the number of the measurement points.   a)     b)  figure 8 magnetic a) and electric b) r.m.s. field behaviour along line 1 of the  three components with a unit‐current in the antenna.   a)     b)   figure 9. magnetic a) and electric b) r.m.s. field behaviour along line 2 of the  three components with a unit‐current in the antenna. acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 96  once the required measurement attributes are inserted and the electric field probe is connected, the software makes the noise level detection available for each component. then, a warm-up period of 60 s is run to guarantee a thermal equilibrium of the whole system; after that the remote unit provides the field components which are downloaded. in the same time the power information is downloaded and, thanks to the circuital model of the antenna, the flowing current is estimated. a suitable module of the software provides the rotation of the field array in order to refer to the main coordinate system, cancels the noise and evaluates the per-unit current field components. after this phase, the output software gives the coordinate of the new investigation point, the operator places the probe in the indicated point and by pushing enter a new supply, acquisition and manipulation phase is run. in order to protect the matching capacitors, a control structure gives a warning signal if the voltage across the capacitors, estimated on the base of the antenna circuital model and of the voltage and frequency values set on the synthesizer, exceeds a defined limit. 6. conclusions  a description of a first experimental set-up devoted to the assessment of the reliability of numerical models for the estimation of induced electromagnetic fields into a phantom simulating human tissues is presented. a simple model of the transmitting antenna with lumped parameters is implemented and an evidence of the effectiveness of the model is given up to about 70 mhz. a first comparison between computation and measurement along two investigation lines inside the phantom is shown. the results are encouraging. the computed electric field over line 1 shows a maximum deviation lower than 8% for field magnitudes higher than 5 v/m. for line 2, characterized by a higher field gradient value, the computed electric field has a maximum deviation lower than 20% for field magnitudes higher than 5 v/m. the magnetic field comparison between computation and measurement over line 1 shows a deviation higher than 30% for the hy component, while the hz component is better reproduced. the estimated magnetic field over line 2 reproduces with high fidelity the measured one, except for the peak value of the hy component, for which a deviation of 6% is detected. to deeper investigate the reliability of the numerical model in presence of medical implants, the measurements are carried out by placing a metallic object within the tissue-simulating liquid in the phantom. even though the comparison between measurement and numerical method is acceptable, a higher deviation is found, in particular for the peak values of each component. indeed, a metallic object with high conductivity introduces a spatial discontinuity of this parameter and can reduce the accuracy of the numerical simulation. presently, the design of a new antenna system is being developed to increase the value of the electric and magnetic field-strength generated in the phantom, and a further measurement campaign is in progress. acknowledgement  the authors wish to thank dr. gerd weidemann from ptb for his contribution in the production and characterization of the human tissue-like liquid. references  [1] iec 62311 “assessment of electronic and electric equipment related to human exposure restrictions for electromagnetic fields (0 hz – 300 ghz), 2007. [2] koch, k.m. and hargreaves, b.a. and pauly, k. butts and chen, w. and gold, g.e. and king, k.f, “magnetic resonance imaging near metal implants”, journal of magnetic resonance imaging, vol.32, no.4, pp.773-787, 2010. [3] ho, henry s., “safety of metallic implants in magnetic resonance imaging”, journal of magnetic resonance imaging, vol.14, no.4, pp.472—477, 2001. [4] m. borsero, o. bottauscio, l. zilberti, m. chiampi, w. wang “a boundary element estimate of radiated emissions produced by unknown sources”, electromagnetic compatibility (emc europe), 2012 international symposium on. [5] oriano bottauscio, mario chiampi, luca zilberti “boundary element approach to relate surface fields with the specific absorption rate (sar) induced in 3-d human phantoms”, eng. analysis with boundary elements vol 35 (2011) pp. 657–666. [6] “rf coils for mri”, j. thomasvaughon, john r. griffiths, john villy & sons ltd, 2012. [7] joseph p. hornak “the basics of mri”, www.cis.rit.edu/htbooks/mri/, 1996. [8] “near field probe set user’s manual”, emco the electro mechanics company, 1988. [9] stefano pisa, et all. “a study of the interaction between implanted pacemakers and the radio-frequency field produced by magnetic resonance imaging apparatus”, ieee trans. on emc, vol. 50, no.1, pp. 35-42, february 2008. [10] tsuboi, h.; tanaka, h.; misaka, h.; fujita, m., "electromagnetic field analysis of rf antenna for mri," magnetics, ieee transactions on, vol.24, no.6, pp.2591,2593, nov 1988. [11] fujita, m. and higuchi, m. and tsuboi, h. and tanaka, h. and misaki, t., “design of the rf antenna for mri”, magnetics, ieee transactions on, vol.26, no.2, pp.901-904, 1990. [12] speag, “easy4/mri” www.speag.com/products/easy4mri/probes-2. [13] “introduction to python programming and developing gui applications with pyqt”, b.m. harwani, course technology a part of cengage learning, 2012. [14] easy4 exposure acquisition system, schmid & partner engineering ag february 3, 2010. microsoft word 220-1874-1-le-rev acta imeko  issn: 2221‐870x  december 2015, volume 4, number 4, 62‐65    acta imeko | www.imeko.org  december 2015 | volume 4| number 4 | 62  metrological traceability for the analysis of environmental  pollutants in the atmosphere  francesca rolle, enrica pessana, michela sega  istituto nazionale di ricerca metrologica (inrim), strada delle cacce 91, 10135 torino, italy        section: technical note  keywords: metrological traceability; environmental pollution; primary gaseous mixtures; polycyclic aromatic hydrocarbons  citation: francesca rolle, enrica pessana, michela sega, metrological traceability for the analysis of environmental pollutants in the atmosphere, acta  imeko, vol. 4, no. 4, article 12, december 2015, identifier: imeko‐acta‐04 (2015)‐04‐12  editor: paolo carbone, university of perugia, italy  received: september 30, 2014; in final form november 6, 2015; published december 2015  copyright: © 2015 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  corresponding author: francesca rolle, e‐mail: f.rolle@inrim.it    1. introduction  the importance of carrying out accurate and reliable measurements is a fundamental topic in many different fields, in particular for the safeguard of the environment and the climatic conditions of the planet. this is the basis for the planning of correct actions to prevent environmental damages and potential harmful effects for the health of the human beings and biota. in this framework, the application of metrology to chemical monitoring can assure the reliability of measurement results, in particular for the analysis of pollutants in different environmental sectors. metrological traceability of measurement results is a fundamental feature in every measurement field but is crucial when dealing with the environmental chemistry. in this field, the application of metrological concepts is not straightforward, as the analytes are usually present at very low concentrations (even in trace) and the sample matrix can generate interferences during the instrumental identification and quantification of the analytes of interest. at the istituto nazionale di ricerca metrologica (inrim), the italian metrology institute, different activities are carried out for the analysis of gaseous and organic pollutants in the atmosphere. this paper deals with some examples of such activities. on one hand, for gaseous pollutants, an important activity concerns the analysis of two species, carbon dioxide (co2) and nitrogen oxides (nox), the former having great relevance for the greenhouse effect and the latter for the photochemical pollution in urban areas. at inrim primary gravimetric mixtures are produced for the calibration of the instrumentation devoted to the analysis of these gases, with the aim of assuring metrological traceability to the measurements of co2 at ambient level and at the vehicles emission level, and of nox at ambient level. abstract  the importance of carrying out accurate and reliable measurements is a fundamental topic in many different fields, in particular for  the safeguard of the environment and the climatic conditions of the planet. this is the basis for the planning of correct actions to  prevent environmental damages and potential harmful effects for the health of the human beings. the application of metrology to  chemical monitoring can assure the reliability of measurement results.  at the  istituto nazionale di ricerca metrologica (inrim), the  italian metrology  institute, different activities are carried out for the  analysis of gaseous and organic pollutants in the atmosphere. this paper deals with some examples of such activities.   for gaseous pollutants primary gravimetric mixtures are produced for the calibration of the instrumentation devoted to the analysis of  carbon dioxide (co2) and nitrogen oxides (nox), with the aim of assuring traceability to the measurements of co2 at ambient level and  at  the  vehicles  emission  level,  and  of  nox  at  ambient  level.  on  the  other  hand,  research  is  carried  out  in  the  field  of  organic  micropollutants, in particular regarding the establishment of metrological traceability for the atmospheric concentrations of polycyclic  aromatic hydrocarbons (pahs) adsorbed on particulate matter (pm).  acta imeko | www.imeko.org  december 2015 | volume 4| number 4 | 63  on the other hand, research is carried out in the field of organic micropollutants, in particular regarding the establishment of metrological traceability for the atmospheric concentrations of polycyclic aromatic hydrocarbons (pahs) adsorbed on particulate matter (pm). a suitable metrological procedure was developed for the extraction and determination of the priority pahs, as defined by the united states environmental protection agency (us epa), on pm sampled from the air of torino, italy. in section 2, some remarks on the activity carried out for gas analysis are given. section 3 is focused on organic micropollutant analysis. 2. analysis of gaseous environmental pollutants  2.1. inrim activity  at inrim, reference gas mixtures of carbon dioxide (co2) and nitrogen oxides (nox) in the mol/mol range are realised by gravimetry, which is a primary method, described in the international standard iso 6142-1 [1]. such primary mixtures are validated by using different analytical techniques: non-dispersive infrared spectroscopy (ndir) for co2, chemiluminescence analysis and fourier transform infrared spectroscopy (ftir) for nox. stability studies of the prepared mixtures are carried out for their possible use as certified reference materials (crms) for instrumental calibration. a fundamental part of the work is devoted to the measurement uncertainty evaluation and to the study of the matrix effects on the analytical response. 2.2. carbon dioxide and nitrogen oxides  co2 is the primary greenhouse gas emitted through human activities and is naturally present in the atmosphere as part of the earth's carbon cycle. human activities are altering the carbon cycle, both by adding more co2 to the atmosphere and by influencing the ability of natural sinks, like forests, to remove co2 from the atmosphere. the main sources of co2 emissions are power plants, transportation, industry [2] and the most effective way to lower co2 levels in atmosphere is to reduce its emissions and fossil fuel consumption. nox indicates the sum of nitric oxide (no) and nitrogen dioxide (no2). no generates from high temperature combustion processes (e.g in motor vehicles) while no2 generates from no by means of photochemical reactions. nox can be responsible for pulmonary diseases (e.g. asthma, bronchitis) for long periods of exposure at low concentrations. in the troposphere, they are classified as secondary pollutants and participate in the photochemical pollution cycle and in the acidic rains formation. in addition, no has a large influence on both ozone and on the hydroxyl radical. the sum of many oxidised nitrogen species, both organic and inorganic, is referred to as noy, excluding nitrous oxide (n2o), ammonia (nh3), acetonitrile (acn) and hydrocyanic acid (hcn). the official method for nox monitoring is the chemiluminescence analysis, as prescribed in the en 14211 [3]. 2.3. primary method for the preparation of gaseous reference  materials  at inrim, primary gravimetric mixtures are prepared by using a validated procedure. the cylinders are previously conditioned by evacuating and heating them, then filling them with the matrix gas. this process is repeated at least 3 times. the preparation procedure of the mixtures is articulated into three steps: the empty cylinder is weighed and then filled with the analyte gas and the matrix gas. each step is followed by a precision weighing, in order to determine the exact amount of gas introduced in the cylinder. the precision weighing is carried out by comparing the cylinder with a reference one following a double substitution scheme, to take into account also the contribution of the buoyancy effect. 2.4. results  recent activities carried out at inrim concerned the realisation of no2 primary mixtures in synthetic air at few µmol/mol (8-12 µmol/mol range), the realisation of no2 mixtures with different o2 concentrations, for the evaluation of matrix effects and of no mixtures in nitrogen at 500 nmol/mol. chemiluminescence and ftir analysers were calibrated between 5 and 15 µmol/mol of no2 in synthetic air and between 300 and 700 nmol/mol for no (chemiluminescence) for the validation of these mixtures. co2 mixtures were prepared at ambient level (around 400 µmol/mol) and at the vehicle emission levels (11-14 %) and analysed by means of non-dispersive infrared spectroscopy (ndir). chemiluminescence analysis was thought to be a very selective technique for nox, but in recent years it was observed that this technique can give unsatisfactory response due to the interferences of noy species. for this reason, at inrim, the chemiluminescence analysis is compared with ftir analysis, which is less influenced by the interferences of other nitrogen species. at the moment, improvements are under investigation also to reduce interferences of environmental parameters (e.g. ambient temperature, relative humidity, air components) which can influence the ftir analysis. 3. organic micropollutants analysis: polycyclic  aromatic hydrocarbons (pahs)  pahs are a class of organic micropollutants which are present in all the environmental compartments, i.e. soil, water, air and also in food. the latter represents the major source on intake for living beings. in addition, a relevant way of exposure is airborne particulate matter (pm), as pahs can adsorb on pm particles and be carried into the respiratory system, even reaching the alveolar region of the lungs. fine and ultrafine pm are important sources of urban pollution and are potentially responsible for several pathologies (mainly cardiovascular and respiratory). they may lead to harmful effects both for the direct action of the particles (i.e. inflammatory processes) and the action of many pollutants adsorbed on it (e.g. pahs, heavy metals). pahs are a class of organic micropollutants of great concern for their fallouts on human health. indeed scientific evidences showed that some pahs may have carcinogenic effects. in particular the most harmful of them is benzo[a]pyrene (bap), which was classified as carcinogenic agent to humans (group 1) by the international agency for research on cancer (iarc) [4]. the european regulations prescribed the monitoring of this pollutant in ambient air and a target value of concentration in air of 1 ng/m3, for the total content in the pm10 fraction averaged over a calendar year is fixed [5]. acta imeko | www.imeko.org  december 2015 | volume 4| number 4 | 64  in this framework, the use of reliable analytical methods for the determination of pahs levels in the environment is necessary and the guarantee of accurate and comparable analytical results is fundamental to support the planning of preventive actions for the reduction of pm levels and emissions in atmosphere. 3.1. inrim method for the analysis of priority pahs  at inrim a procedure for the quantification of bap in ambient air was developed [6], starting from the guidelines given in the european standard method en 15549 [7] which describes a methodology for the determination of bap in ambient air adsorbed on pm and can be used in the framework of the european directive related to air quality [8]. the developed method was extended to the 16 pahs classified as priority pollutants from us epa and other pahs having toxicological relevance. the procedure was applied to samples of total suspended particles (tsp) sampled in torino. and was validated in all its steps, by evaluating different parameters among which the recovery efficiency, the limit of detection (lod) and the limit of quantification (loq). in our method, a classical extraction technique soxhlet extraction [9] was chosen and the quantification was performed by means of gas chromatography coupled with mass spectrometry (gc-ms). sampling campaigns were carried out in different seasons to evaluate the concentration profiles of pahs during the year. the pm was collected by means of a low volume sampler with a sampling head for tsp, using glass fibre filters as sampling media. the filters were weighed before and after the sampling to determine the real masses of sampled pm. in addition, the same procedure was used for the determination of the final volumes of the extracts, which were calculated from the weighed masses of each extract, taking into account the density of the extraction solvent. in this way, we could assure metrological traceability to the masses of pahs in the sample extracts as they were gravimetrically determined by comparison with calibrated mass standards. the metrological traceability of the quantification step was assured by calibrating the gc-ms with a set of reference standard solutions prepared by gravimetric dilution of a suitable crm nist srm 2260a which contains 36 aromatic hydrocarbons in toluene. the recovery efficiency ranges from 70 to 100 % for the heavier pahs (having more than 4 benzene rings) while for small pahs (2-3 rings) the recovery efficiencies were generally not satisfactory, probably because these analytes are very volatile and can be lost during the extraction step. the regulation [7] fixes a lod value only for bap (lower than 0.04 ng/m3). the lod of our internal method was evaluated between 0.01 and 0.04 ng/m3 for all the pahs, hence in agreement with the regulation. the only exception was phenanthrene with a lod of 0.06 ng/m3. loq was calculated as 10 times the standard deviation of the repeated analyses of the blanks. 3.2. results and discussion  the results obtained for pahs analysis were in accordance with the provisioned seasonal trends. the concentrations measured in pm sampled in spring are below the target value of 1 ng/m3 fixed for bap and are not very high compared with the results of samples collected in winter. the measured concentrations highlight the increasing trend of pahs in atmosphere from spring to winter. these is a typical seasonal trend for pahs, characterized by a lowering of their levels during spring and summer, mainly for the reduction of the emissions, the changing of the meteorological conditions and the increase of the ambient temperatures. during winter, the major emissions in urban centres come from the domestic heating plants and the vehicular outputs. in addition, the lowering of the mixing height in atmosphere forces the pollutants to persist longer in the lower troposphere while the colder temperatures can improve the repartition from vapour phase onto pm particles also for small pahs, because their vapour pressure is strongly reduced by the decrease of the ambient temperatures. bap concentrations measured in winter samples exceeded the target value of 1 ng/m3. these results confirm the abundance of bap and of the other pahs in urban pm and, consequently, highlight the need of resolving actions for the reduction of pahs inlets in atmosphere. in addition, the results show that the ratios of the pahs are almost constant in samples of different periods of the year and the repartition of pahs between vapour and particulate phases is not affected by seasonal changes, but depends on the physico-chemical properties of pahs and on the sources of emission. however, for particulate sampled during some rainy days a lowering of the pahs level was observed. this is an example of the effects of the meteorological conditions on pm and pahs levels in the troposphere. favourable weather conditions can help the deposition of pm particles to the ground, consequently reducing the measured pahs concentrations. 4. conclusions  the metrological approach followed at inrim for the analysis of gaseous and organic pollutants has very important fallouts on these fields of environmental measurements, in particular for the significance of the considered analytes. the aim of the work conducted at inrim is in particular the establishment of correct metrological traceability chains for all the analytes considered. further developments for the preparation of gaseous reference materials concern the use of another primary method for the preparation of reference gas mixtures, namely the dynamic dilution technique. this method has the advantage of preparing primary mixtures prior to the use, hence it is suitable for unstable and reactive molecules and for low concentration levels. in the field of organic analysis, improvements are foreseen in order to give traceability to the pm sampling step and performing sampling campaigns of pm, as requested by the european regulation, extending the research activity also to different analytes of environmental and toxicological interest. references  [1] international standard iso 6142-1:2015 “gas analysis - preparation of calibration gas mixtures -part 1: gravimetric method for class i mixtures” [2] http://www3.epa.gov/climatechange/ghgemissions/sources.htm l [3] en 14211:2012 “ambient air. standard method for the measurement of the concentration of nitrogen dioxide and nitrogen monoxide by chemiluminescence” [4] http://monographs.iarc.fr/eng/classification/index.php acta imeko | www.imeko.org  december 2015 | volume 4| number 4 | 65  [5] directive 2004/107/ec of the european parliament and of the council of 15 december 2004 relating to arsenic, cadmium, mercury, nickel and polycyclic aromatic hydrocarbons in ambient air [6] f. rolle, v. maurino, m. sega “metrological traceability for benzo[a]pyrene quantification in airborne particulate matter”, accreditation and quality assurance, vol. 17, n. 2, 2012, pp. 191-197 [7] en 15549:2008 “air quality standard method for the measurement of the concentration of benzo[a]pyrene in ambient air” [8] directive 2008/50/ec of the european parliament and of the council of 21 may 2008 on ambient air quality and cleaner air for europe [9] us epa method 3540c “soxhlet extraction” (1996). journal contacts acta imeko issn: 2221-870x march 2023, volume 12, number 1, 1 2 acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 1 journal contacts about the journal acta imeko is an e-journal reporting on the contributions on the state and progress of the science and technology of measurement. the articles are mainly based on presentations presented at imeko workshops, symposia and congresses. the journal is published by imeko, the international measurement confederation. the issn, the international identifier for serials, is 2221-870x. about imeko the international measurement confederation, imeko, is an international federation of actually 42 national member organisations individually concerned with the advancement of measurement technology. its fundamental objectives are the promotion of international interchange of scientific and technical information in the field of measurement, and the enhancement of international co-operation among scientists and engineers from research and industry. addresses principal contact prof. francesco lamonaca university of calabria department of computer science, modelling, electronic and system science via p. bucci, 41c, vi floor, arcavacata di rende, 87036 (cs), italy e-mail: editorinchief.actaimeko@hunmeko.org support contact dr. dirk röske physikalisch-technische bundesanstalt (ptb) bundesallee 100, 38116 braunschweig, germany e-mail: dirk.roeske@ptb.de editor‐in‐chief francesco lamonaca, italy founding editor‐in‐chief paul p. l. regtien, netherlands associate editor dirk röske, germany copy editors egidio de benedetto, italy silvia sangiovanni, italy layout editors dirk röske, germany leonardo iannucci, italy domenico luca carnì, italy editorial board leopoldo angrisani, italy filippo attivissimo, italy eulalia balestieri, italy eric benoit, france paolo carbone, italy lorenzo ciani, italy catalin damian, romania pasquale daponte, italy luca de vito, italy sascha eichstaedt, germany laura fabiano, italy ravi fernandez, germany luigi ferrigno, italy edoardo fiorucci, italy alistair forbes, united kingdom helena geirinhas ramos, portugal sabrina grassini, italy leonardo iannucci, italy fernando janeiro, portugal konrad jedrzejewski, poland andy knott, united kingdom yasuharu koike, japan dan kytyr, czechia francesco lamonaca, italy aime lay ekuakille, italy massimo lazzaroni, italy fabio leccese, italy michele norgia, italy franco pavese, italy pedro miguel pinto ramos, portugal nicola pompeo, italy sergio rapuano, italy renato reis machado, brazil álvaro ribeiro, portugal gustavo ripper, brazil dirk röske, germany maik rosenberger, germany alexandru salceanu, romania constantin sarmasanu, romania lorenzo scalise, italy emiliano schena, italy michela sega, italy enrico silva, italy pier giorgio spazzini, italy krzysztof stepien, poland ronald summers, uk marco tarabini, italy tatjana tomić, croatia joris van loco, belgium zsolt viharos, hungary bernhard zagar, austria davor zvizdic, croatia mailto:editorinchief.actaimeko@hunmeko.org mailto:dirk.roeske@ptb.de acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 2 section editors (vol. 7 12) jon bartholomew, uae yvan baudoin, belgium piotr bilski, poland francesco bonavolonta, italy giuseppe caravello, italy carlo carobbi, italy marcantonio catelani, italy mauro d’arco, italy egidio de benedeto, italy alessandro depari, italy alessandro germak, italy istván harmati, hungary daniel hutzschenreuter, germany min-seok kim, korea bálint kiss, hungary momoko kojima, japan koji ogushi, japan vilmos palfi, hungary jeerasak pitakarnnop, thailand siniša prugovečki, usa/croatia md zia ur rahman, india fabio santaniello, italy jan saliga, slovakia emiliano sisinni, italy ciro spataro, italy oscar tamburis, italy zafar taqvi, usa jorge c. torres-guzman, mexico ioan tudosa, italy ian veldman, south africa rugkanawan wongpithayadisai, thailand cristian zet, romania claudia zoani, italy an iot measurement system for a tailored monitoring of co2 and total volatile organic compounds inside face masks acta imeko issn: 2221-870x september 2023, volume 12, number 3, 1 8 acta imeko | www.imeko.org september 2023 | volume 12 | number 3 | 1 an iot measurement system for a tailored monitoring of co2 and total volatile organic compounds inside face masks filippo ruffa1, mariacarla lugarà1, gaetano fulco1, claudio de capua1 1 dip. diies, università ‘mediterranea’ di reggio calabria, 89122, reggio calabria,italy section: research paper keywords: air quality; face mask; tvoc; covid19 citation: filippo ruffa, mariacarla lugarà, gaetano fulco, claudio de capua, an iot measurement system for a tailored monitoring of co2 and total volatile organic compounds inside face masks, acta imeko, vol. 12, no. 3, article 9, september 2023, identifier: imeko-acta-12 (2023)-03-09 section editor: susanna spinsante, grazia iadarola, polytechnic university of marche, ancona, italy received april 14, 2023; in final form june 16, 2023; published september 2023 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: filippo ruffa, e-mail: filippo.ruffa@unirc.it 1. introduction after the declaration of covid-19 as a global pandemic, multiple scientific research publications, acknowledge that sars-cov-2 is mainly transmitted from human to human through close contacts, airborne respiratory droplets, aerosols and contaminated surfaces [1]-[3]. since march 2020 several countermeasures have been taken by the national governments to limit the pandemic spread, whose impact has been devastating in terms of human lives [4]. these countermeasures include restrictive measures to limit interpersonal contacts and the use of personal protective equipment (ppe), i.e., face masks. the introduction of the new mrna covid-19 vaccines strongly reduced morbidity and mortality, anyhow they were not enough to prevent the spread of new sars-cov-2 variants and to fully protect the groups of people most at risk. for this reason, the use of face masks and social distancing are still highly recommended, mainly in those areas with high viral spread [5], [6]. in this context, the use of sensors and the integration of internet of things (iot), can lead to a more efficient and smart usage of ppe, allowing a continuous monitoring of the microenvironment inside them. several studies assess the importance of monitoring air quality in order to reduce exposure to some pollutants that, over time, can increase the risk of psycho-physical discomfort, which often leads to devastating effects on the human organism [7]-[11]. ppes filter the air in both directions (inhaled and exhaled), creating a microenvironment which has higher temperature and humidity than the external environment [12]. moreover, with the use of face masks, it is observed a substantial increase in inhaled co2 concentration, also known as co2 rebreathing [13]-[15]. several studies demonstrated that co2 accumulation in the mask microenvironment varies in function of breath rate and intensity. it was demonstrated that the increase in co2 concentration ranged from 1.5 % at rest, during speech, or at low work rates, up to 3.5 % during low-intensity exercise, which is well beyond the expected in atmospheric air (~0.04 %) [11], [16]. several studies have also analysed the impact of ppe usage during physical exercise [17]-[19]. as stated in [20]: “even though in healthy populations, the potentially life-saving benefits of wearing facemasks seem to outweigh the documented adverse effects, there seems to be widespread abstract this paper proposes an innovative iot-based system for monitoring air quality inside protective equipment such as ffp2 masks, which have become more widespread due to the covid-19 pandemic. the system aims to provide diagnostic elements to specialist doctors and suggest a healthier use of the mask by monitoring the concentration of pollutants. the system aggregates data over a 15-minute window and calculates average values for each measured parameter, comparing them with reference thresholds to suggest removing the mask if necessary. an innovative aspect is personalized monitoring of exhaled breath, specifically volatile organic compounds (vocs), providing a customized and reliable information framework for doctors. this is possible thanks to the integration of removable memories, inside which the user's personal information and metrological characteristics of the system are stored in a standardized form. the proposed platform is accessible to both users and doctors, enabling early diagnosis by providing a complete picture of the patient's specific condition. this solution can have a strong impact on daily life and well-being, especially for diseases that increase the presence of certain compounds in the exhaled breath. mailto:filippo.ruffa@unirc.it acta imeko | www.imeko.org september 2023 | volume 12 | number 3 | 2 agreement on the increased breathing resistance, co2 rebreathing and decreased inhaled o2 concentration caused by the use of facemasks”. in this context, a real-time monitoring of the air quality inside the face mask can help assessing both potential risks for health [20] and the integrity of the mask itself [21]. this is possible nowadays thanks to the technological development, which has allowed the realization of new thin and light sensors used in wearable devices in many medical applications [22]-[27]. such a solution can help in finding a good trade-off between the mitigation of infection transmission risk and minimization of potential harmful effects due to co2 rebreathing, in particular in those contexts, like hospitals, pharmacies, classrooms and workplaces, where face masks are usually worn for many hours. in addition, the integration of sensors and the monitoring of the microenvironment inside the face mask may give important information on the health status of the person who wear it. in particular, a high concentration of some substances, mainly vocs, in exhaled breath, may relate to an increase in oxidative stress and many potential health problems, such as chronic inflammation [28], hypoxia in multi chemical sensitivity (mcs) [29] and even covid-19 [30]. as said in [31] the main techniques used to detect biomarkers in exhaled breath are based either on chromatography and spectroscopy, which is used mainly in laboratory, or on bioelectric mos sensors. using these techniques, it is possible to detect and isolate some specific types of vocs. nowadays it is known that breath contains more than 3500 types of vocs [32] and some of these are particularly related to some specific health issues, such as ammonia, which is related to renal failure, liver dysfunction and cirrhosis, acetone and isoprene to diabetes and hypercholesterolemia, ethane and pentane to gi diseases and asthma, aldehydes to cancer and neurological disease. many other types of vocs are related to specific issues, and many are still under investigation in order to find if there is a correlation with specific health diseases. in this specific context, with the work presented in this paper, the authors aim to propose a tool for monitoring the air quality within the ppe, which allows, albeit without making a diagnosis, to give information indicative of the patient's state of health, based on breath analysis. in particular, the proposed solution consists in an internet of things (iot) face mask with integrated sensors for the monitoring of co2 and total volatile organic compounds (tvoc). the face mask is connected to a personal platform that allows to store all the information related to data monitoring over time and allows the physician to check the information relating their patients on their online medical record. since the masks are approved for use for a few hours, typically 6-8, after which they should be changed, the proposed type of monitoring must provide for the re-configurability of the new masks. this is implemented in the proposed approach using removable memories, which contain all the information relating to the specific patient. in this way, the proposed solution relies on iot to implement a tailored monitoring of the parameters of the patient in an ambient assisted living application [33]-[35], both for prevention of possible health issues and for sharing data on his/her digital medical record. 2. methodology in this section the methodologies used to develop the proposed iot face mask are explained. in detail the discussion is focused on the design of the monitoring system and the iot tools. the proposed system relies on the integration of sensors inside the face mask to continuously monitor co2 and tvoc concentration in the mask microenvironment. in fact, as said in section 1, monitoring the concentration of these two substances is important both for safety reasons, i.e., mitigation of potential harmful effects due to exposure to these substances, and for health monitoring, since tvoc concentration is related to many health issues. the system implements the framework shown in figure 1, consisting of two sensors integrated into the mask, an embedded system for data processing and transmission and an application on a remote device that, together with a local led advises the user in case of anomalies. furthermore, the application, synchronized with a remote smart personal platform, let the user to visualize the monitoring data and the trend over time. a microcontroller (mcu) manages data acquisition phase and, using its internal adc, samples co2 and tvoc sensors data over a 15-minute interval at a rate of 1 s/s (samples for second). data acquired are stored in local memory and, after the time acquisition window has expired, the mcu averages all acquired samples, for each monitored parameter, and compares the average measured value with a reference threshold. the reference threshold is taken by the local regulations and indicates the maximum concentration of the pollutant, which is still considered safe, considering an exposure time of 15 minutes. if the result of the comparison is positive, even for one of the two substances, the microcontroller advises the user of a possible issue turning on a led indicator and sending a warning to the software application on the remote device. after making the comparison with the threshold, the average measured value for each pollutant is sent to a remote device for data collection and visualization on the dedicated application and the internal memory of the mcu is erased and ready to store the new incoming samples. the innovative feature of the proposed system is the monitoring of exhaled tvoc over time, which can provide valuable information about potential health issues. however, the presence of high voc concentrations in exhaled air is not diagnostic by itself, and repeatable measurements must be taken in a controlled environment following standard protocols to compare them with reference thresholds. it is also important to consider that the normal range varies from person to person, figure 1. project framework. acta imeko | www.imeko.org september 2023 | volume 12 | number 3 | 3 making personalized monitoring necessary. monitoring the trend of voc concentration in exhaled breath over time is more important than a single comparison with a reference threshold. for daily use, replicating the measurement in standard conditions is not feasible, nor is it necessary since the objective is to monitor the user's breath parameters over the medium to long term and provide indications for further medical investigation if needed. the personalization of the monitoring process takes place through the use of a removable memory support, that carries general information on the single user, on his/her health status, on average monitoring data over different periods of time and on normal reference range defined by the personal physician. another removable memory support carries calibration and accuracy information on the specific measurement system, allowing to check if data are reliable or if a re-calibration of the system is necessary. since common ffp2 face masks are approved for a use of maximum 6-8 hours, after this time of use, when the user needs to change the mask the entire system, joint with the two memory supports, are removed from the old mask and inserted in the new one to continue the monitoring with the new mask. the microcontroller periodically performs a comparison between the average measured values stored in the memory and the reference range defined by the physician and stored in the same memory. if the measured values are out of range, it turns on a led indicator to inform the user and give suggestion for a medical check. for a better customization of the monitoring process, the use of memories integrates perfectly with the use of a remote smart personal platform, better described in section 2.1, to which only the specialist doctor and the user have access. the smart personal platform is synchronized with the user software application. it is divided into two sections, one dedicated to the air quality monitoring for safety purposes and one dedicated to the automatic processing and custom visualization of exhaled breath parameters. it receives real-time data from the monitoring each 15 minutes, processes the new data, and stores them on the digital medical record of the patient. the platform uses data to create hourly, daily, weekly and monthly statistics regarding the concentration of exhaled tvoc. these statistical data, each time they are re-processed, are sent back to the microcontroller, which stores them on the dedicated removable memory support. a block diagram of the described process is reported in figure 2. 2.1. smart personal platform a key factor that has an important role in improving reliability in the detection of health risk situations, starting from the values of the parameters detected in the breath, is the knowledge of the history of the patient, i.e., what are typical values in known operating conditions. this, together with the knowledge of a reference range determined by the personal physician and fitted to the patient allows a prompt identification of any deviations from standard conditions. therefore, as said above, in order to customize the monitoring system, it becomes essential to integrate memory devices to save personal information useful for assessing the state of breath, the history of previous evaluations and the reference range determined by the specialist doctor. in order to implement an automated access to this information, it was decided to implement two transducers electronic data sheets (teds) that are intended to allow access in a standardized format, according to the indications contained in the ieee 1451 standard, thus facilitating the automatic configuration of the system. this reduces configuration time and allows metrological information to be integrated into data processing, reduces the possibility of error related to manual entry and improves traceability by linking all meta-information to the specific user. teds can be loaded into the system at start stage or on request. in the proposed iot face mask, the presence of this standard data structure allows to access historical information in an easy way and to fit the monitoring system on the specific user. the implemented smart personal platform, synchronized with the software application, can access two teds where, as seen in figure 3, personal data (histo) and sensor-related measurement uncertainty (metro) information are stored and accessible. figure 2. data flow block diagram illustrating the flow of information in the system. figure 3. parameters stored in histo and metro. acta imeko | www.imeko.org september 2023 | volume 12 | number 3 | 4 histo allows the system to adapt the processing algorithm to the specific patient being monitored, it must be patient-specific and updated whenever a new assessment is made. before applying the algorithm for the evaluation of breath, the platform accesses the memory to acquire the statistics of some predefined intervals (mean and standard deviation) and the reference range defined by the specialist doctor. using non-volatile memory offers a simple solution to store patient data and to tailor the assessment made by the algorithm, by providing a comparison baseline for each new assessment. personal information such as: user id, age, sex, statistical data calculated on previous time frames (monthly, weekly, daily or hourly), average percentage of hours of use, presence of past specific diseases that affect breath and reference normal range for tvoc concentration in breath (assessed by the doctor), are all stored in histo. using the stored information, the system facilitates the reading of the measured data. in this way, abnormal breath characteristics or patient-specific pathologies are considered during the evaluation of acquired data, allowing to reduce the occurrence of false positive detections and helping the specialist to have a complete picture of the available information. metro contains information related to the measurement uncertainty of the sensor, because data reliability is of primary importance when dealing with issues related to personal health and safety. the measurement system must be the most accurate and reliable possible. in order to achieve this objective, it is always necessary to consider the calibration intervals, after which the metrological characteristics of the sensor are no longer guaranteed, and a new calibration must be carried out. once a day, the system evaluates whether or not the time suggested for the next calibration has elapsed and, if necessary, informs the user of the need for a technical intervention. the system will continue to operate, but the presence of the alert will be an element to be evaluated in presence of non-normal values, as they could be linked to a drift in sensor performance. all data stored into teds are accessible from the data analysis platform, which, by combining that information, performs a tailored processing and is able to determine if the average tvoc monitored in exhaled breath, over pre-set time windows, is in or out of reference range. furthermore, the platform allows user to plot a graphical representation of the raw data and of the trend line, obtained applying regression techniques. every time the platform receives a new data from the iot mask stores it in a personal medical record, which can be accessed from the specialist. the platform algorithm, whose data flow is shown in figure 4, processes each new received value, updating the hourly, daily, weekly and monthly statistics. after that, it verifies if the average measured tvoc in these time figure 4. smart personal platform algorithm. acta imeko | www.imeko.org september 2023 | volume 12 | number 3 | 5 intervals is within the reference range indicated by the specialist doctor and stored in the histo memory and, if it is not the case, turns on a led on the platform, specific for each time interval, to advise the user that the measured values are out of range. the updated statistics, once calculated, are sent back to the iot mask to be overwritten in the histo memory. the platform allows the user and the specialist doctor, who has full access to the platform, to plot raw measurement data over a selectable time interval and to trace a regression line in order to show the trend over time. furthermore, the platform allows to show on the graph the reference band indicated by the doctor and the average band where the measured data of the patient fall. the last one is identified as the average measured value for the selected interval plus or minus the corresponding standard deviation multiplied for the coverage factor, that is simplified for selection with a simplified label for the user (low, medium, high). as shown in figure 5, “low” indicates the choice of a coverage factor equal to one, which identifies an interval around the average value, where there is approximately 68.3 % probability that a new measured data will fall, medium corresponds to a coverage factor of two, equivalent to 95.4 % probability and high corresponds to a coverage factor of 3, which identifies an interval in which 99.7 % of the measured data will fall, in the absence of anomalies. the user, normally a doctor specialized in respiratory diseases, can obtain different informations by varying these parameters. in fact, based on the choice of the coverage factor, any positioning outside the so-called average range will have a different weight. the graphical visualization of the data, in this way, let the doctor to see if the measurements, in the visualization interval, are compliant or not with the reference range, if there is a drift over time and what is the slope of the regression line and finally to determine the number of occurrences in which the measured value falls out of the average band (considering different coverage factors). the platform is also accessible to the patient, who can see his/her monitoring data and eventual warnings that can advise for a medical check. 3. results to test the proposed monitoring approach, a first prototype of the measurement system, described in section 2 has been realized and tested with ffp2 face masks. the prototype, shown in figure 6, is still a preliminary version and its aim is only to test the functionality of the proposed method. co2 and tvoc sensors, whose characteristics are described in table 1, have been integrated into the face mask making a little cut on the cloth. after introducing the sensors, the cut was stitched up and closed with hot glue to restore the original insulation. the sensors are externally wired to a ni rio acquisition board, which acquires measurement data and implements, on board, the entire monitoring, and control station described in section 2. the smart personal platform, described in section 2.1, has been realized in labview and is reachable through the web by acceding the remote application web page, as shown in figure 7. all the monitoring data are also loaded to a personal ftp folder, which can be remotely acceded by both the user and the personal physician. as it can be seen in figure 7, the user interface of the web application is divided in three panels. the first panel reports the personal information of the user, taken from histo and metro; the second panel shows a series of indicators that warn the user if the average tvoc measured in the last 60 minutes, 24 hours, 7 days or the last month are above a certain tolerance band defined by the physician for the specific user. furthermore, it allows the user and the physician to graphically show the tvoc measured points over a selectable time interval and to figure 5. impact of coverage factor on tolerance band around mean value. figure 6. prototype of the iot face mask. table 1. sensor characteristics. sensor range accuracy co2 in ppm 0 – 40000 ± (40 + 5 % of reading) voc in ppb 0 – 60000 ± 15 % of reading figure 7. graphical interface of the personal smart platform application. acta imeko | www.imeko.org september 2023 | volume 12 | number 3 | 6 trace a regression line implementing a best linear fit function. the information so obtained, in particular the slope of the regression line can give the physician important information on how the exhaled tvoc has changed over the selected time interval. in the same graph it is possible also to graphically show both the limits of the reference range defined by the doctor, and the tolerance band around the average measured value, as defined in section 2.1. in this way, the doctor observing the data can easily assess whether the parameters are out of the normal range or the usual range and, if so, to quantify their extent. furthermore, in this last case, he/she can easily assess whether it is an isolated phenomenon or whether it is part of a process to be observed more carefully. a third panel is dedicated to the monitoring of air quality levels for safety purposes, verifies whether the monitored values of co2 and tvoc are higher than the reference thresholds established by the regulations and, in that case, warns the user of the problem, suggesting, if possible, to go outdoor and remove the mask. ten specimens of the described prototype have been realized and the system was tested on 10 healthy volunteers, 5 males and 5 females, aged between 23 and 35 years over a period of one month. the monitoring was carried out inside the electric and electronic measurement laboratory of the university “mediterranea” of reggio calabria and each day included a monitoring interval of 4 hours. since the face masks are approved for a use of 4-6 hours, at the end of each day they were substituted with new ones. the results obtained from the monitoring campaign over the period of one month are summarized in table 2, which reports for each of the 10 volunteers the average co2 and tvoc measured values and their standard deviation. as you can see in table 2, the average value of co2 measured inside face masks, in all cases, is not far from the safety thresholds defined by the standard regulations in eu, which are 15.000 ppm for an exposure time of 15 minutes and 5000 ppm for an exposure time of 8h. during the monitoring campaign the 15 minutes threshold has never been reached, and the 8h threshold could not be considered, since the monitoring interval is 4 hours per day. anyhow considering a conservative threshold of 10000 ppm for a period of 4 hours, after this time of use, in almost all cases the system turns on the low air quality warning indicator. as it can be seen co2 measurements present also a high standard deviation, which, as the reader can see in figure 8, is due to a drift in co2 concentration inside the mask over the time of use, mainly due to the deterioration of the face mask. for what concerns tvoc measurements no situations of particular relevance were recorded, and the data monitored was almost constant over time. in only one case, a daily variation of expired tvoc deviated from the mean value recorded up to the moment by a value greater than three times the standard deviation. however, as can be seen in figure 9, the phenomenon was episodic and did not impact the slope of the regression line, which is 0.11 ppb/day. in this case, since the phenomenon is episodic and there is no incremental trend in exhaled tvoc, it can be traced back to temporary situations or alimentary reasons. to prove the algorithm functioning in detecting a deviation over the time periods defined before, a montecarlo simulation was performed, obtaining 1000 possible random tvoc paths. the simulation was set to start from a tvoc value of 115 ppb, with an increment between each step chosen randomly from a gaussian distribution centred in zero and with a standard deviation of 10 ppb. the reference range was set, in order that the algorithm can detect a potential anomaly if the average value of the first hour, day, week or month exceeds the average reference value for the same time interval by more than 3 times the standard deviation associated. the results obtained are summarized in table 3. considering one case where the simulated exhaled tvoc over one month period goes out of range, it is shown in figure 10 how the tvoc trend deviates over time, by observing the regression line, which in this case has a slope of 12.1 ppb/day. if a case like this would arise, the personal physician after a medical check can determine if the observed trend is indicative of a possible issue or not. 4. conclusions in conclusion, this work presents an innovative iot measurement system integrated into ppes to monitor air quality in the microenvironment inside them. the proposed system aims to provide an easy-to-use tool that signals the deterioration of air quality inside the mask in the presence of a high exposure risk to two specific pollutants, co2 and tvoc. additionally, the system can be used to monitor the user's health, as voc concentration in exhaled air may be related to various health issues. figure 8. co2 trend over 4 hours of use. table 2. results obtained from one month trial. sex age average co2 in ppm co2 stddev in ppm average tvoc in ppb tvoc stddev in ppb f 23 9599 1156 85 22 f 23 9780 1132 82 23 m 24 10165 1125 100 23 m 23 9130 1210 115 43 f 24 10243 1154 102 23 m 32 10341 1140 115 19 m 31 9136 1132 82 20 f 35 9352 1208 82 21 f 25 9196 1176 81 20 m 28 9656 1147 81 21 table 3. results of algorithm test using montecarlo simulations. time interval number of tvoc paths number of detections above thresholds 1st hour 1000 0 1st day 1000 3 1st week 1000 158 1st month 1000 277 6000 8000 10000 12000 14000 0 1 2 3 4 c o 2 i n p p m time in hours acta imeko | www.imeko.org september 2023 | volume 12 | number 3 | 7 the iot mask becomes a highly personalized health-checking support tool, allowing the specialist doctor and the user to see any warnings and visualize both raw data and trends over selectable time intervals. to account for the variation in normal range among individuals, a standard format file containing personal information is used, and the sensors' calibration status is continuously checked to ensure the reliability of the collected data. ten prototypes were implemented using ni rio acquisition boards and tested on 10 healthy volunteers for one month. the proposed algorithm's capability in detecting anomalies in 1000 random tvoc generated using montecarlo simulations was also evaluated. the results show that after one hour, no detection above thresholds was reported, but after one day, three detections above thresholds were reported. after one week, 158 detections above thresholds were reported, and after one month, 277 detections above thresholds were reported. the iot mask proposed in this work is innovative compared to the state of the art since it becomes a personalized diagnostic support tool that can help obtain early diagnoses for disorders still difficult to identify through traditional protocols. further developments may concern the engineering of the prototype to 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https://doi.org/10.3390/s22239463 https://doi.org/10.1109/i2mtc48687.2022.9806516 https://doi.org/10.1109/memea54994.2022.9856436 https://doi.org/10.1109/i2mtc50364.2021.9459905 https://doi.org/10.1038/nature16521 https://doi.org/10.3390%2fs150203721 https://doi.org/10.1039/c9nr05532k https://doi.org/10.1088/1752-7155/6/2/027108 https://doi.org/10.14814/phy2.15034 https://doi.org/10.1183/13993003.congress-2022.2691 https://doi.org/10.1149/1945-7111/ab67a6 https://doi.org/10.1016/j.anai.2011.02.016 https://doi.org/10.3390/s21217003 https://doi.org/10.1109/metrolivenv54405.2022.9826923 https://doi.org/10.1016/j.measurement.2021.110685 acta imeko  september 2014, volume 3, number 3, 1  www.imeko.org    acta imeko | www.imeko.org  september 2014 | volume 3 | number 3 | 1  journal contacts    about the journal  acta imeko is an e-journal reporting on the contributions on the state and progress of the science and technology of measurement. the articles are based on presentations presented at imeko workshops, symposia and congresses. the journal is published by imeko, the international measurement confederation. the issn, the international identifier for serials, is 2221-870x. editor‐in‐chief  paolo carbone, italy honorary editor‐in‐chief  paul p.l. regtien, netherlands editorial board francisco alegria, portugal leopoldo angrisani, italy filippo attivissimo, italy eulalia balestieri, italy eric benoit, france catalin damian, romania pasquale daponte, italy luigi ferrigno, italy alistair forbes, united kingdom fernando janeiro, portugal konrad jedrzejewski, poland andy knott, united kingdom fabio leccese, italy helena geirinhas ramos, portugal pedro ramos, portugal sergio rapuano, italy dirk röske, germany alexandru salceanu, romania constantin sarmasanu, romania lorenzo scalise, italy emiliano schena, italy enrico silva, italy marco tarabini, italy susanne toepfer, germany rainer tutsch, germany luca de vito, italy about imeko  the international measurement confederation, imeko, is an international federation of actually 39 national member organisations individually concerned with the advancement of measurement technology. its fundamental objectives are the promotion of international interchange of scientific and technical information in the field of measurement, and the enhancement of international co-operation among scientists and engineers from research and industry. addresses  principal contact prof. paolo carbone university of perugia dept. electr. inform. engineering (diei) via g.duranti, 93, 06125 perugia italy email: paolo.carbone@unipg.it acta imeko prof. paul p. l. regtien measurement science consultancy (msc) julia culpstraat 66, 7558 jb hengelo (ov) the netherlands email: paul@regtien.net support contact dr. dirk röske physikalisch-technische bundesanstalt (ptb) bundesallee 100, 38116 braunschweig germany email: dirk.roeske@ptb.de microsoft word 322-2423-1-le-rev3 acta imeko  issn: 2221‐870x  september 2016, volume 5, number 2, 14‐18    acta imeko | www.imeko.org  september 2016 | volume 5 | number 2 | 14  constraining absolute chronologies with the application of  bayesian analysis  francesco maspero, emanuela sibilia, marco martini  cudam, università degli studi di milano bicocca, piazza della scienza 1, 20125 milano, italy  infn ch_net – sezione di milano‐bicocca e dipartimento di scienza dei materiali, università degli studi di milano bicocca, via roberto cozzi  55, 20126 milano, italy      section: research paper   keywords: dating; statistics; bayes; chronology  citation: francesco maspero, emanuela sibilia, marco martini, constraining absolute chronologies with the application of bayesian analysis, acta imeko,  vol. 5, no. 2, article 3, september 2016, identifier: imeko‐acta‐05 (2016)‐05‐03  section editors: sabrina grassini, politecnico di torino, italy; alfonso santoriello, università di salerno, italy  received march 15, 2016; in final form april 7, 2016; published september 2016  copyright: © 2016 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  corresponding author: luca bondioli, francesco maspero, francesco.maspero@unimib.it    1. introduction  absolute dating has become a powerful tool in archaeology. however, the interpretation of the archaeometric data can be sometimes difficult, especially in the case of non-gaussian probability distributions (e.g. radiocarbon dates). in fact, sometimes scientific analysis results are not univocal and must be interpreted. in absence of an impartial instrument to discriminate data, the degrees of freedom in the choice of the right results is often left to archaeologists and historians. moreover, prior information about analyzed samples and provenance sites are not usually taken into account in real synergy with experimental data and historical studies, giving space to subjective interpretations, sometimes in conflict. in order to overcome this misunderstanding it is important to think about the scientific results as a higher concept than “numbers”, and treat the historical data and clues as mathematical terms of an equation. a great help comes from the bayesian statistical approach, a model that combines in a single formal analysis the experimental results coming from scientific analyses together with the present knowledge of an archaeological problem, in order to make inferences that can contextualize the problem in a coherent interpretation [1]-[4]. after a brief remind of the theory related to the method, three case studies are reported and discussed. finally, the potential and advantages of this method will be underlined. 2. theory  the bayes’ theorem states that the posterior probability of an event is proportional to the likelihood times the prior probability, or formally: | | , (1) where are the probabilities of the single events, and | are the likelihood of the two events to be linked. bayesian statistics uses probability as a means of measuring one’s strength of belief in a particular hypothesis being true [5]. in other words, the application of bayesian statistics allows the selection of the most significant data in the experimental set, rejecting the ones not supported by historical evidences or by a relevant likelihood. abstract  in this work the application of bayesian statistics to archaeological problems will be discussed. in particular, three case studies will be  analyzed,  each  presenting  complex  interpretative  scenarios,  and  the  most  suitable  way  to  solve  them.  it  will  be  shown  that  the  bayesian approach allows to refine a dating when in presence of multiple data, even from different dating techniques. the bayesian  approach  is  presented  as  the  common  language  between  physicists,  archaeologists  and  statisticians  to  perform  more  accurate  evaluations on stratigraphies and chronologies.     acta imeko | www.imeko.org  september 2016 | volume 5 | number 2 | 15  the application of this method can bring to some strange and unconventional results, which can be understood only after the comprehension of the bayesian “way of thinking”. a special case may help: in figure 1 a prior probability of an event a (blue curve, labelled r_date standing for “radiocarbon date”) is depicted, which has a maximum probability spanned over 200 years; the likelihood graph (red curve, labelled c_date standing for “calendar date”, normally distributed) states that the probability that event a occurs in conjunction with an event b (such as the use of a coin discovered in the same layer of the material associated to event a) has a normal probability distribution peaked at 920ad with an uncertainty of 20 years. combining this data produces a posterior probability curve extremely different from the prior one, but closer to the real probability distribution, taken into account all the available data (black curve). the powerfulness of this method is well represented by the calibration of radiocarbon dates to obtain the probability density curve a scientist is accustomed to (figure 2). in radiocarbon dating the isotopic concentration of carbon-14 is not univocally related to a single date, but it has to be “calibrated” using a reference curve that links every concentration to a set of calendar ages. in this case the prior information is the existence of the radiocarbon date itself (the result of the isotopic analysis guarantees that this result is associated to at least a date on the calendar timescale); its numerical value is given in the graph title (r_date, expressed in percent of modern radiocarbon pmc). the likelihood parameter is represented by a mix of the uncertainty associated to the isotopic analysis (red curve, figure 2) and the probability distribution given by the calibration curve intcal (blue curve, figure 2). the latter is represented in the graph with its uncertainty, which varies with time depending on the precision and the number of measures used to build the curve. the projection of the gaussian variable on the calibration curve is a density curve (black curve) from which the most credible date intervals (hpd, highest posterior density regions) can be extracted. the table in the graph describes the age ranges with 1-sigma and 2-sigma probability: note that the probability shape changes radically from the gaussian initial one [6], and the probability densities can be splitted in multiple peaks characterized by a fraction of the initial 68 % and 95 %. this approach is extremely useful when there is the need to match multiple dates and prior knowledge about a site or an artifact. in this case, the same formal procedure is used, putting one of the collected dates as a prior information, and using the known connections between the stratigraphy layers or building phases as likelihood parameters. 3. materials and methods  in this work three different examples of bayesian statistics applications will be shown, each of them introducing a more complex approach to archaeological questions that can be solved with the help of prior information. each posterior probability curve is evaluated numerically through a markov monte carlo chain analysis implemented in oxcal 4.2 [6]. this is a software package developed to calibrate radiocarbon concentrations and to calculate the most probable dates associated to a single concentration. it is also possible to apply the most used bayesian models to a set of data distributions, both before and after the calibration process. the monte carlo analysis is performed during the calibration to evaluate the best probability density distribution. more precisely, the metropolishastings algorithm is used, since it only requires relative probability information [7]; in addition, it uses a set of proposal moves which can both result in changes to single elements of the model or changes to the duration and timing of whole groups. this provides much faster convergence for complex models. 4. results  a first example of this approach is the dating of the site of my-son, a cluster of abandoned and partially ruined hindu temples constructed between the 4th and the 14th century ad by the kings of champa in quang nam province (central vietnam) [8]. the available materials included 3 sets of bricks from one of the latest construction phases, and some charcoal pieces embedded in a brick (figure 3a). a memorial stele commemorating the dedication of the temple (1155 ad, c_date t) gives an historical boundary. both thermoluminescence (tl) and radiocarbon were used when possible. the selected samples come from three different sections of the structure walls, and the archaeological question was if all the masonries were contemporary or not. the ages obtained for the bricks allowed to identify three main groups: one (60 %) consisted in reused material, whose dates were well before 1155 ad, the dedication year; another surely posterior to that date; the last (g3 phase) with dates in between. the components of the first group are characterized by a density distribution completely shifted on the left of the stele boundary and their date is univocally assessed before the stele foundation, suggesting a probable reuse from preexisting structures. the components of the second group are completely shifted on its figure 1. representation of a bayesian analysis.  figure 2. example of radiocarbon date calibration.    acta imeko | www.imeko.org  september 2016 | volume 5 | number 2 | 16  right (g4 phase, figure 3a), stressing the possibility of restoration performed in later times. the main question regarding the samples of the third group was if they were reused or purposely made for the edification of the site. unlike the calibration example, here the prior information is represented by a supposed terminus post quem given by the dated stele; this information is formalized by the equation: , , , . (2) this equation and the following don’t take into account the uncertainty associated to the boundary events. this is done to explain the model function, in the algorithm the whole range of probabilities is used, projecting the tales of the boundary distribution and smoothing the step slope of the resulting probability. in figure 3b the same results as in figure 3a are represented, but after the application of the bayesian statistics. the dark grey curves represent the posterior probability density distribution: it is clear that g3h has a large probability to be dated just before 1155 ad (stele), supported by a non-zero probability that comes from the radiocarbon dating of a piece of burned wood embedded in the brick paste. the “combine” distribution is an operation on probability distributions that combines any number of probability distribution functions which give independent information on a parameter. the similarity in the probability distribution allows to extend the production of all g3 bricks before 1155 ad (figure 4). a further modelling takes into account the probable contemporaneity of production of the g3 bricks, adding a prior function: if we assume that the production period is constrained by a start unknown event (ta) and a finish unknown event (tb), the formalization of the model is as follows: ∝ ∏ , , . (3) a second case regards the dating campaign of three burials in the archaeological site of sipan in northern peru. it shows that the precision of the chronological boundaries of an event can be enhanced combining the site stratigraphy with the whole set of dating results. during this campaign, three different tombs were dated, using both tl and radiocarbon techniques. while the stratigraphic evidence clearly stated their relative temporal sequence, the archaeological request was the refinement of the absolute dating of the warrior-priest tomb (t14). looking at the raw results, it appears that the age of all the examined materials, given the experimental uncertainty, is practically the same (range of phase t3: 215-435 ad; range of phase t14: 255-775 ad; range of phase t1-t2: 595-775 ad) (figure 5a). however, using the stratigraphy of the site as the main constraint, the most probable period of t14 construction is severely restricted (figure 5b). the prior information applied here is similar to the one from the previous example, with the difference that here there are three different groups of events, with two unknown intermediate periods that represent the end of construction of a tomb and the start of construction of the next tomb. the formal condition of this model is: ∝ ∏ , , ∏ , , ∏ , , , (4) where ta, tb, tc, and td are the boundaries of the grouped events. as reported in figure 5b, the prior condition of noncontemporaneity of the three phases reduces the span of t14 use by 80 %, especially involving some particularly spanned radiocarbon dates (rc124). the last example regards a neolithic site in southern italy. it shows how the bayesian analysis can extract extended information on a site integrating prior hypothesis and dating. here the information needed is not about the refining of the site chronology, as the collected samples are common wares used in everyday life during all the occupation period, and cannot represent an a priori restrain; instead, it is possible to extrapolate data about the site life span, beside the beginning and end of its occupation. for this settlement, the continuity of site occupation was hypothesized. after a first analysis (figure 6), it was possible to identify two sub-phases, which were further modeled to find a possible hiatus between the two periods. a further complication in the prior probability model   figure  3.  a:  unmodelled  representation  of  radiocarbon  and thermoluminescence  dates  on  my‐son  samples; b: bayesian analysis of the same dates.    figure 4. logical deduction path for g3 phase dating.  analyzed g3 samples probably made before the foundation  stele g3h similar to other g3 samples  g3h charcoal = g3h brick  certainly before the foundation  stele   acta imeko | www.imeko.org  september 2016 | volume 5 | number 2 | 17  is introduced taking into account the presence of a gap between the end of the first event (tb) and the start of the second one (tc): ∝ , ∏ , , ∏ , , . (5) the resulting data are showed in figure 7: the end of the first sub-phase and the beginning of the second are not overlapping, but there is a gap of about 100 years in the probability distribution curves. this can be a signal of a temporary site leaving, as well as of a period of crafting decadence. it is clear that such a result could have not been obtained through a rough qualitative interpretation of the results. 5. discussion and conclusion  the described examples aim to underline the importance of a correct approach during the statistical elaboration of the results of absolute dating techniques. they are not only numbers, but a source of linked information that can be extracted imposing the right conditions and constraints. the right approach is not to put the obtained data in a supposed model, rejecting what doesn’t fit or “doesn’t sound well”; all information should be analyzed and criticized, trying to shape the historical model in a feedback process, involving archaeologists, physicists and statisticians in the discussion.   figure  5.  a:  unmodelled  representation  of  radiocarbon  and thermoluminescence  dates  on  sipan  samples;  b:  bayesian  analysis  of  the same dates.  figure 7. sub‐phases discrimination.  figure 6. distribution of the radiocarbon dates on southern italy samples.     acta imeko | www.imeko.org  september 2016 | volume 5 | number 2 | 18  furthermore, the use of all the available information in the refining process of the statistical data considers the uniqueness of every site, with the great advantage of taking into account the uniqueness of the experimental evidences [9]. the potential of a bayesian approach in archaeology will reach its maximum when a tight interdisciplinary collaboration between archaeologists and staticians will come true. this requires archaeologists to be comfortable in analyzing a situation and defining the archaeological problem in a realistic but not over-refined way. in wider terms, they require to communicate their ideas to statisticians speaking a “shared” language, and to explain the importance of their work in simple terms. references  [1] g. alberti, the aid of bayesian radiocarbon modeling in assessing the chronology of middle bronze age sicily at the site level. a case study, journal of archaeological science: reports, 2 (2015) pp. 246-256. [2] t.s. dye and c.e. buck, archaeological sequence diagrams and bayesian chronological models, journal of archaeological science, 63 (2015) pp. 84-93. [3] w.d. hamilton and j. kenney, multiple bayesian modelling approaches to a suite of radiocarbon dates from ovens excavated at ysgol yr hendre, caernarfon, north wales, quaternary geochronology, 25 (2015) pp. 72-82. [4] a. quiles, e. aubourg, b. berthier, e. delque-količ, g. pierratbonnefois, m.w. dee, g. andreu-lanoë, c. bronk ramsey and c. moreau, bayesian modelling of an absolute chronology for egypt's 18th dynasty by astrophysical and radiocarbon methods, journal of archaeological science, 40 (2013) pp. 423-432. [5] g.a. barnard and t. bayes, studies in the history of probability and statistics: ix. thomas bayes's essay towards solving a problem in the doctrine of chances, biometrika, 45 (1958) pp. 293-315. [6] c.b. ramsey, bayesian analysis of radiocarbon dates, radiocarbon, 51 (2009) pp. 337-360. [7] w.r. gilks, s. richardson and d. spiegelhalter, markov chain monte carlo in practice, chapman & hall, 1996. [8] m. martini, e. sibilia, m. cucarzi and p. zolese, "absolute dating of the my son monuments", in: champa and the archaeology of my son (vietnam). h. a., m. cucarzi and p. zolese (editors). nus press, singapore, 2009, 978-9971-69-451-7, pp. 369-380. [9] c.e. buck, w.g. cavanagh and c.d. litton, bayesian approach to interpreting archaeological data, john wiley & sons, chichester, england, 1996, 0471961973. microsoft word 401-3027-1-le-rv acta imeko  issn: 2221‐8702  july 2017, volume 6, number 2, 54‐58    acta imeko | www.imeko.org  july 2017 | volume 6 | number 2 | 54  molten metal’s reaction force measurement for pressure  estimation and control system construction in press casting  ryosuke tasaki, kazuhiko terashima  department of mechanical engineering, toyohashi university of technology, japan      section: research paper   keywords: force measurement; soft‐sensing; model‐based estimation; casting process  citation: ryosuke tasaki, kazuhiko terashima, molten metal’s reaction force measurement for pressure estimation and control system construction in press  casting, acta imeko, vol. 6, no. 2, article 9, july 2017, identifier: imeko‐acta‐06 (2017)‐02‐09  section editor: min‐seok kim, research institute of standards and science, korea  received june 30, 2017; in final form june 30, 2017; published july 2017  copyright: © 2017 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  corresponding author: ryosuke tasaki, e‐mail: tasaki@me.tut.ac.jp    1. introduction  a new casting method called sand mold press casting, has been developed by our group over recent years. in this casting process, the ladle first pours the molten metal into a lower (drag) mold. after the pouring, an upper (cope) mold is lowered to moderately press the molten metal into the cavity. this process has enabled us to enhance the production yield rate from 70 % of typical gravity casting to over 90 %, because the sprue cup and the runner are not needed for the suggested casting plan [1]. in the casting process, the molten metal can precisely and quickly be poured into the lower mold. liquid weight control during pouring with the tilting ladle has been proposed in very interesting recent studies [2], [3]. however, in the pressing part of this casting process, casting defects are often caused by high pressure inside the mold related to pressing velocity. a major application example of the casting method is metal impeller production, shown in figure 1. high velocity pressing of molten metal induces product defects as a rough surface at the bottom of the upper mold. this type of casting defect, in which molten metal soaks through sand particles of the greensand mold and then solidifies, is called physical metal penetration. the penetration is most likely caused by the high pressure, and it requires an extra process of surface finishing. thus, high quality stable casting must be enabled by excess pressure suppression in the pressing process. to maintain a shorter cycle time in production, a pressing control system, considering the liquid pressure behaviour inside the mold, is highly demanded. pressure control techniques have figure 1. press casting process and its iron product.   abstract  this paper presents a measurement system for molten metal’s reaction force and estimation of  liquid pressure during pressing to  control the iron product quality. we have developed a new type of casting process. in the process, molten metal is quickly filled into  casting molds by high‐speed pressing. casting defects such as physical metal penetration is often caused by excess pressure. hence,  we have constructed a pressure control system using a mathematical model‐based off‐line simulation to derive the ideal feedforward  control input of pressing. however, it is difficult to accurately control the pressure in cases of varying conditions such as liquid volume  and  temperature  changes.  also,  pressure  measurements  by  using  contact‐type  sensors  directly  is  impossible  for  molten  metal,  because of the high temperature of the liquid, over 1400 °c. so, we have proposed a new pressure estimation method with force  measurement data processing. here, the exact reaction force from the molten metal must be accurately observed by a force sensor  set  between  the  upper  mold  and  its  elevating  device.  the  viscosity  coefficient  can  also  be  calculated  on  a  real‐time  basis.  the  proposed force measurement system will realize an improved casting quality due to the effective feed‐back control system. acta imeko | www.imeko.org  july 2017 | volume 6 | number 2 | 55  been proposed for different casting methods, especially the injection molding process with metallic mold [4], [5]. the pressure control problem has been successfully resolved by mathematical modelling of liquid behavior and computing a 3d simulation analysis [6], [7]. furthermore, a feedback pressure control based on pid gain selection has been proposed for the filling process. although the pressure in the mold must be detected in order to control the process adequately using feedback control, it is difficult to measure the fluid pressure directly, because the high temperature of the molten metal over 1400 °c precludes the use of a pressure sensor. thus, in our previous papers [8], [9], the pressure during pressing at a lower pressing velocity was estimated by using a simple model of the molten metal’s pressure based on the analysis results of the computational fluid dynamics (cfd) software. a new sequential pressing control, namely a feedforward method using a novel simplified pressure model, has been reported by the authors [10], [11]. it has been shown that this method is very effective for adjusting the pressure in the mold. as the main work in this paper, we have undertaken several pressing experiments using viscous water to validate a force measurement algorithm for liquid pressure estimation inside the mold, and to confirm the modified pressure control model and a feed-forward control method during pressing. these results indicate that measuring the pressure during pressing is desired to achieve quality casting under a variety of conditions as the poured liquid volume and temperature changes. therefore, we have proposed an indirect pressure measurement for the estimation method using the information about the reaction force from the molten metal during pressing. in the final section, the proposed pressure estimation method is applied to a water pressing experiment using an acrylic mold, shaped as a simple injection type. the effectiveness for the actual flow situation, which means viscous flow depending on varying molten metal temperature, is discussed 2. pressing process in press casting  2.1. servo drive pressing machine  the filling molds consist of greensand in the actual casting production. the convex part of the upper mold has several passages called the overflow area. molten metal that exceeds the product volume flows into the overflow areas during pressing. these areas are the only parts of the casting plan that provide the effect of liquid static pressure. the overflow part is designed as long and narrow channels. when fluid flows into such an area, high pressurization causes casting defects. therefore, it is important to control the pressing velocity in order to suppress the rapid increase in pressure that occurs in high-speed pressing. the upper mold moves up and down by a lifting press cylinder driven by a servomotor, as shown in figure 2. the position of the upper mold can be continuously measured by an encoder mounted on the servo cylinder that controls the molten metal pressure by using the controlled velocity input. the force sensor for measuring the molten metal’s reaction force is mounted between the bottom part of the servo cylinder and the top of the upper metal molding box covering the greensand. 2.2. metal production experiments  casting production tests by press casting have been performed using the previously proposed pressing control. figure 3 shows an overview of the cast product pressed by sequentially switched velocity pattern sv. these two products are cast under the experimental condition of higher temperature ht (1400 °c) on the left side in figure 3, and the right-hand side shows the results tested with lower temperature lt (1350 °c). from these photographs, a better product of sv-ht is clearly verified, because the switch velocity is designed for the higher temperature condition (1400 °c). on the other hand, the case of sv-lt has a tiny penetration part, because a higher pressure is generated with higher viscosity related to the lower temperature. this comparative validation result of the different temperatures indicates that for quality casting an adjustable pressing control depending on the molten metal’s temperature drop during pressing is desired. it can be well performed by using both of model based designs of pressing velocity and the pressure control with a known parameter related to liquid viscosity. 3. reaction force measurement for pressure  estimation  3.1. reaction force measurement for pressure  in actual production, the poured volume and metal temperature are susceptible as a repeating error for each pouring. the optimal higher pressing process is done by sensing the variable volume error and temperature change from the measured liquid pressure inside the mold. therefore, a real figure 2. press casting equipment.         figure 3. casting products in case of different temperature.   acta imeko | www.imeko.org  july 2017 | volume 6 | number 2 | 56  time pressure estimation algorithm and its confirmative experiments are required for the construction of a new control system for press casting. the installation of a contact-type pressure sensor to measure the molten metal pressure is quite difficult due to the high liquid temperature of the molten metal. therefore, an indirect sensor by reaction force is implemented above the upper mold. by using the reaction force from the load-cell, a pressure estimation of the liquid inside the mold is validated. water pressing experiments have been carried out to directly measure the pressure by this contact-type sensor, to observe the flow state and to estimate the pressure by the load-cell. an illustration of the experimental equipment and sensing/control system is shown in figure 4. 3.2. pressure estimation considering frictional resistance  cancellation  pressing motion causes frictional resistance between the upper and lower molds as shown in figure 4. in this section, we propose a pressure estimation method using the reaction force measurement by the load cell. to remove the effect of friction as a disturbance for an accurate pressure estimation, an equation of friction force during pressing is derived. the measurable total force f [n] including the effects of mold friction, inertia of the upper mold and gravity is simply derived, and then the pressure of the base area p [pa] is given by (1), (1) where m [kg] is the mass of the upper mold, g [m/s2] the gravity force, z [m] the upper mold position, μ [-] the wall frictional coefficient, l [m] the girth of the frictional part, and a [m2] is the area of the upper mold base. here μ strongly depends on the pressing velocity, and also switches between both kinetic and static frictions. in (2), μd and μs are coefficients of the kinetic and static frictions respectively, [m/s] is the pressing velocity related to the frictional force. (2) to validate (1) and (2), water pressing experiments have been performed. a cylindrically shaped injection type mold is selected, with a diameter of 0.10 m at the bottom side and a smaller diameter of 0.02 m at the upper side. water inside the mold flows upward during pressing. the velocity reference input and the pressure behavior are shown in figure 5. for each pressing the liquid viscosity is changed, as experimental condition for the pressure estimation. as shown in figure 5, the   (a)  = 0.001[cp]      (b)  = 0.500[cp]      (c) = 1.000[cp]      (d)  = 5.000[cp]  figure 5. pressure estimation results for different viscous liquid. figure 4. block diagram of pressing control system.   acta imeko | www.imeko.org  july 2017 | volume 6 | number 2 | 57  viscosity is adjusted to four values of (a) 0.001, (b) 0.500, (c) 1.000 and (d) 5.000 by cmc(carboxymethylcellulose)-contained water. dashed, solid gray and solid black lines mean the actual pressure measured by the contact-type pressure sensor (ap-10s, keyence corp.), the force measurement data by the load cell (tclz-500ns001, tokyo sokki kenkyujo co., ltd.) and the estimated pressure by (1) and (2), respectively. these results make clear that the actual measured and the estimated pressure behaviors are successfully matched for all values of the liquid viscosity. 3.3. estimation of  unmeasurable liquid viscosity  the cfd based on the exact model such as the navierstokes equation is very effective for offline analysis of fluid behavior and is useful for predicting the behavior and optimization of a casting plan [7], [8]. however, this is not sufficient for the design of a pressing control system, because the complex and exact model calculations take too much time. therefore, a construction of a mathematical pressure model is desired in order to realize an efficient pressing casting control system by estimating the liquid property such as the viscosity depending on varying metal temperature and volume. the mathematical model construction can be considered from the viewpoint of the conservation of energy in the continuous streamline from the bottom to the surface of the liquid. the pressure model construction is considered for only unstationary vertical flow without air entrainment. figure 6 shows the rising flow of the molten metal. the unstationary bernoulli theorem for incompressible fluids, and considering the bottom surface of the upper mold (eh(t) = 0) and the liquid surface (eh(t)), gives us the following pressure model equation:        zvgfz ed el tzz ea ea p p h h hs hm b , 2 1 )( )( 2 2 2                  , (3) where  [kg/m3] is the fluid density and g [m/s2] the acceleration of gravity. the fluid surface position eh [m] and its velocity [m/s] at the free surface as [m2] relate to the mold cross-sectional area am [m2] and the pressing velocity . furthermore,  means the wall frictional coefficient depending on the liquid temperature t [k] on the upward flow. l [m] and d [m] are the vertical flow length and the diameter, respectively. to properly control the pressure behavior inside the casting mold, the unknown temperature depending on parameter  could be identified by the mathematical model (3) as given above and the estimated data mentioned in the previous section. (see [11] for more information about the mathematical model construction). to confirm the proposed viscosity identification by using the mathematical pressure model, several experiments using a simple injection type mold have been carried out. in the experimental results shown in figure 7, the estimated pressure behavior of solid black lines are reshaped with the identified parameter  from the reference output (= 0) related to mathematical model. here, the trapezoidal pressing velocity input is set to the same reference in figure 5. fitting the model based time behavior to the estimated pressure by force sensing, the unknown parameter  has been uniquely derived for each different viscous liquid. the relationship between viscosity and  is almost linear as shown in figure 8. 4. model predictive control approach  the pressure control must satisfy the pressure constraint condition for high-quality sand mold casting. the mpc method first predicts the future output, calculates the optimum input for keeping the constraint, and then realizes a good performance of the real output. in our mpc approach, we have built a predictive model control system with high speed calculation, therefore it is effective for real-time feedback control. equation (4) is the response of the system. the response  should be equal to the reference pressure trajectory  r , suy f  . (4)   figure 7. pressure estimation with identified  .    figure 8. identification parameter of  .  figure 6. illustration of filling flow state shown in cross‐sectional view.   acta imeko | www.imeko.org  july 2017 | volume 6 | number 2 | 58  the optimal control input u is given by    frtt ysssu    1 , (5) where yf is the free output response on the time in the future and s the unit step response matrix. the mpc simulation result is shown in figure 9. this simulation result confirms that the mpc is very effective for the liquid pressure control during pressing in the press casting process. 5. conclusions  for a practical application of the casting quality control system in the press casting process, we have proposed an implementation approach of force measurement, pressure estimation and parameter identification for molten metal during dynamic filling. it is made clear that the molten metal pressure inside the casting mold is accurately estimated by the reaction force from a force sensor mounted between the upper mold and its elevating device. from the estimated pressure behavior, the actual pressure behavior can be calculated with the identified liquid viscosity using the constructed mathematical pressure model. next, the filling behavior of the upward flow liquid can also simultaneously be observed on a real-time basis. in the case of varying casting conditions such as mold shape, molten metal properties, volume and temperature as seen in practical cases, our proposed real-time estimation method in this paper can be effectively integrated with a next-generation supervisory process control system for metal manufacturing. in the near future, optimum quality control experiments using molten metal, based on the model predictive control algorithm, will be demonstrated. references  [1] k. terashima, y. noda, k. kaneto, k. ota, k. hashimoto, j. iwasaki, y. hagata, m. suzuki and y. suzuki, “novel creation and control of sandmold press casting ”post-filled formed casting process”, foundry trade journal international (the journal of the institute of cast metals engineers), no. 183 (3670), (2009) pp. 314-318. [2] y. noda and k.terashima, “modeling and feedforward flow rate control of automatic pouring system with real ladle”, journal of robotics and mechatronics, vol. 19 (2), (2007) pp. 205‐211. [3] y. noda, k. yamamoto and k. terashima, “pouring control with prediction of filling weight in tilting ladle-type automatic pouring system”, international journal of cast metals research, science and engineering of cast metals, solidification and casting processes, afc-10 special issue, vol. 21(1-4), (2008) pp. 287-292. [4] hu, j, vogel. j. h, “dynamic modeling and control of packing pressure in injection molding”, int. journal of engineering materials and technology, vol. 116, no.2, (1994) pp. 244-249. [5] mickowski j. r, teufert, c. e, “the control of impact pressure in the high pressure die casting process”, trans 17th int. die cast congress and exposition, (1993) pp. 349-354. [6] h. devaux, “refining of melts by filtration. a water model study”, in proc. of 53rd international foundry congress, prague, czech, (1986) pp.107-115. [7] c. galaup, u and h. luehr, “3d visualization of foundry molds filling”, in proc. of 53rd international foundry congress, prague, czech, (1986) pp.117-127. [8] i. ohnaka, “development of the innovative foundry simulation technology”, in journal of the materials process technology, ”sokeizai”, vol.45, no.9, (2004) pp. 37-44. [9] k. terashima, r. tasaki, y. noda, k. hashimoto, j. iwasaki, t. atsumi, “optimum pressure control of molten metals for casting production using a novel greensand mold press casting method”, key engineering materials, vol. 457, (2011) pp. 453-458. [10] r. tasaki, y. noda, k. terashima, k. hashimoto, “sequence control of pressing velocity for pressure in press casting process using greensand mold”, journal of cast metal research, vol. 21, no. 1–3, (2008) pp. 369-374. [11] r. tasaki, y. noda, k. hashimoto and k. terashima, “modelling and control of pressurized molten metal in press casting”, journal of mechanics engineering and automation (jmea), vol. 2, no. 2, (2012) pp.77-84.     figure 9. pressure behaviour on mpc simulation.  microsoft word 376-2527-1-le-rev acta imeko  issn: 2221‐870x  november 2016, volume 5, number 3, 16‐23    acta imeko | www.imeko.org  november 2016 | volume 5 | number 3 | 16  proficiency tests with uncertainty information: analysis using  the maximum likelihood method  katsuhiro shirono, masanori shiro, hideyuki tanaka, kensei ehara  national metrology institute of japan, aist central 5, 1‐1‐1 higashi, tsukuba, ibaraki, japan       section: research paper   keywords: proficiency test; en score; uncertainty; iso 13528; maximum likelihood method  citation: katsuhiro shirono, masanori shiro, hideyuki tanaka, kensei ehara, proficiency tests with uncertainty information: analysis using the maximum  likelihood method, acta imeko, vol. 5, no. 3, article 4, november 2016, identifier: imeko‐acta‐05 (2016)‐03‐04  editor: franco pavese, italy  received march 31, 2016; in final form march 31, 2016; published november 2016  copyright: © 2016 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.  funding: this work was supported by a grant‐in‐aid for scientific research (no. 26870899) from the japan society for the promotion of science (jsps), japan.  corresponding author: katsuhiro shirono, e‐mail: k.shirono@aist.go.jp    1. introduction  the proficiency test (pt) using an interlaboratory comparison is an effective tool to assure the quality of the measurements of calibration and testing laboratories. participation in a pt is usually required for a laboratory to be accredited under iso/iec 17025:2005 [1]. for performance evaluation in a pt with information on uncertainty, a comparison with the result of a reference laboratory is implemented using the en score as described in iso 13528:2015 [2], which is referred to as the en number in iso 13528:2005 [3]. iso 13528 is the standard for the statistical methods used in pts. given that the measurement value of laboratory k and its expanded uncertainty are respectively xk and uk while those of the reference laboratory are respectively xref and uref, the en score for laboratory k is defined as follows:     2ref2refn uuxxe kkk  , (1) where superscript (k) means laboratory k. when |en(k)| ≤ 1 and > 1, the performances of laboratory k are evaluated as “satisfactory” and “unsatisfactory”, respectively. because it is difficult to designate the reference laboratory in some testing fields, performance evaluation in a pt with uncertainty information where an appropriate reference laboratory does not exist has not yet been described in iso 13528:2015. it must be noted that when there is no reference laboratory, outliers can seriously impair the quality of the pt. on the other hand, since a pt is conducted to check the proficiency of laboratories, the possible existence of laboratories with inadequate proficiency should be taken into consideration. thus, a robust analysis method is required. although no suggestion can be found in iso 13528:2015, several analysis procedures have already been proposed for key comparison tests, which are a type of pt for national metrology institutes that are basically implemented without a specific reference laboratory. a guideline on statistical methods for key abstract  in this study, we report the application of the maximum likelihood method to the analysis of proficiency test data when uncertainty  information is given and a reference laboratory does not exist. there are two causes that could impair the quality of analysis using the  maximum  likelihood method: the existence of an unknown random effect, and outliers. the conditions under which performance  evaluations can be appropriately conducted are discussed in this study. to avoid serious impacts from these two causes, the maximum  permissible  standard  uncertainty  of  an  unknown  random  effect  and  the  minimum  permissible  standard  uncertainty  of  the  values  reported  by  a  participating  laboratory  are  quantified.  through  simulations,  the  maximum  and  the  minimum  permissible  standard  uncertainties are found to be 0.3 and 0.5 times the  intermediate magnitude of the standard uncertainty that the participants are  expected to report. we believe that our proposed procedure based on these criteria is sufficiently simple to be employed in actual  proficiency tests.   acta imeko | www.imeko.org  november 2016 | volume 5 | number 3 | 17  comparisons was presented by cox [4] in 2002, in which a robust analysis referred to as procedure b is proposed for a case where the results are inconsistent. moreover, analysis with the largest consistent subset (lcs), also proposed by cox [5], has been widely employed in such analysis. the lcs is the subset with the largest data size among the subsets whose consistencies are confirmed through a 2 test. various other methods have also been proposed so far [6]−[10]. it is worth noting that although the statistical models employed in these proposals differ from each other, they are useful in their respective situations. we also developed an analysis method that is robust to outliers [11], [12], which is referred to as the robust method in this paper. this method comprises two steps: (i) the detection of an unknown random effect, and (ii) performance evaluation using the local maximum likelihood (lml) method. the robust method is explained in the appendix, and a brief summary of it is given in subsection 2.1. the advantage of this method is that it lessens the risk of performances being evaluated based on inappropriate data influenced by an unknown random effect, and allows performance evaluations to be conducted with clear statistical meaning. in the present study, the robust method is reduced to a simpler method, which is referred to as the global maximum likelihood (gml) method. the robust method, in which lml estimators are employed, may give the impression of being difficult to implement because of its computational complexity. the approach using the gml method is explained in subsection 2.2, and is, we believe, as simple as algorithm a in iso 13528:2015 appendix c. if the conditions are clearly given, this approach can be employed in an actual pt. corresponding to the two steps in the robust method, conditioning is conducted by two parameters: (i) the maximum permissible standard uncertainty for a random effect, and (ii) the minimum permissible standard uncertainty for the reported values. the first parameter is quantified by examining the magnitude of a random effect that will significantly affect the quality of the pt. the second parameter is considered because, when an outlier has an extremely small uncertainty, the gml method can offer different results from the lml method. these parameters are quantified relative to the intermediate magnitude of the standard uncertainty that the participants are expected to report. this paper is organized as follows: section 2 provides the basic theory of the robust method and the gml method. the qualification of the parameters and the proposal of a practical procedure based on the qualified parameters are then given in section 3. in section 4, the procedure is applied to the data of an actual pt. a brief conclusion is presented in section 5, and information on the robust method is provided in the appendix. 2. robust and global maximum likelihood (gml)  methods  2.1. robust method  suppose that n laboratories participate in a pt. it is assumed that laboratory i reports xi and ui as the reported value and its standard uncertainty for i = 1, 2, …, n. qi is defined as the square of the standard uncertainty ui. as mentioned earlier, the two steps in the robust method are (i) the detection of an unknown random effect, and (ii) performance evaluation using the lml method. it is possible that inhomogeneity or instability of the pt items, or vagueness of the measurand, may cause a large unknown random effect and seriously impair the quality of the pt. the first step is therefore necessary. the second step is important in order to evaluate performances in a manner that is robust to outliers. in the first step, the marginal likelihoods are computed. the marginal likelihood is, simply put, the likelihood of the model. the marginal likelihoods of the models in which an unknown random effect is not considered or is commonly considered for all n laboratories are defined as 0 and n, respectively. when 0 < n, the model with no random effect is more likely, and the data are regarded to be inappropriate for the performance evaluation. in the robust method, the marginal likelihoods of some other models with a random effect are also computed. moreover, an estimation of the measurand is given robustly to outliers through bayesian analysis. two examples are shown in figure 1(a) and (b). in figure 1(a), the data are largely dispersed. in this case, 0 < 7, and an unknown random effect is detected. the data are therefore inappropriate for use in the performance evaluation. on the   (a)    (b)  figure 1. simulated pt data: (a) (x1, x2, x3, x4, x5, x6, x7) and (u1, u2, u3, u4, u5,  u6, u7) are given as (a) (1, 2, 3, 4, 5, 6, 7) and (0.2, 0.2, 0.2, 0.2, 0.2, 0.2, 0.2),  and (b) (1, 2, 4, 4, 4, 6, 6.4) and (1, 1, 1, 1, 1, 1, 0.04). the error bars show  the expanded uncertainties with a coverage factor of 2.   acta imeko | www.imeko.org  november 2016 | volume 5 | number 3 | 18  other hand, most of the data seem consistent in figure 1(b). in this case, an unknown random effect is not detected. the estimation of the measurand is given as 4.0, which is comparable to the values of x3 to x5. in the performance evaluation, the statistical model xi ~ n(, qi + i) (i = 1, 2, …, n) is considered, where  is assumed to be the true value of the measurand in the pt, and i is an additional variance. several local maximums for the likelihood of the model can be observed. to analyse the data robustly, the combination of parameters maximizing the likelihood in which the value of  is closest to the estimate of the measurand is focused on. defining the lml estimators as the parameters in the combination, the symbols lml and 1lml to nlml are introduced to express the lml estimators  and 1 to n. the performance evaluation for laboratory k is then conducted to check the validity of the statistical model xk ~ n(, qk) and xi ~ n(, qi + ilml) for all i ≠ k. the specific form of the statistic is given in the appendix as the extended en score. when |en(k)| ≤ 1 and > 1, the performances of laboratory k are evaluated respectively as “satisfactory” and “unsatisfactory” as described in connection with (1). the extended en scores obtained using the robust method for the data in figure 1(b) are given in table 1. the extended en scores of x3 to x5, whose values are close to the estimation of the measurand, are 0.0, and their performances are evaluated as “satisfactory”. on the other hand, the extended en score of the apparent outlier of x7 is 2.4, and the performance is evaluated as “unsatisfactory”. thus, it is found that the robust method is not significantly influenced by outliers. 2.2. global maximum likelihood (gml) method  although the robust method is flexibly applicable, the computation may be complicated. therefore, it is meaningful to provide a simpler method. analysis using the gml estimator is proposed here. in this section, n, xi, ui, and qi are defined as in subsection 2.1. the statistical model xi ~ n(, qi + i) (i = 1, 2, …, n) is considered in this method as in the robust method. this model is actually identical to model iii proposed by willink [8]. in willink’s paper, the gml estimator of i, igml, is given as igml = max{|xi − gml|2 − qi, 0}, where gml is the gml estimator of . moreover, willink showed that gml can be given through the minimization of the following quantity q():                  n i i i i x q 1 2 log    , (2) where i=max{|xi -| 2, qi}. we apply the same concept as in the robust method to the performance evaluation. from an analogy to en(k) in the robust method given in (a5) in the appendix, the following extended en score is computed for the performance evaluation of laboratory k with igml = igml + qi :   1 gml2 gml 1 gml 12 1                       n ki ik n ki ii n ki ikk n u xx e   , (3) where  kin denotes the summation for i = 1, 2, …, k − 1, k + 1, …, n. this statistic is analogic to the concept of the “exclusive statistics” [13], [14]. when |en(k)| ≤ 1 and > 1, the performances of laboratory k are evaluated as “satisfactory” and “unsatisfactory”, respectively. this analysis method is referred to as the gml method in this study. since no check of an unknown random effect is conducted in this method, the gml method is not appropriate when an unknown random effect has a serious impact on the pt data. therefore, the gml method should not be applied to the data shown in figure 1(a) without any quantitative consideration of the unknown random effect. moreover, gml and lml may differ from each other when an extremely small uncertainty is associated with an outlier. when the gml method is applied to the data in figure 1(b), the extended en scores are reported as shown in table 1. the computed extended en score of laboratory 7 is 0.8, and the performance is evaluated as “satisfactory”. this unnatural result occurs due to the extremely small uncertainty of x7. however, we believe that the gml method is sufficiently simple to be employed in an actual pt analysis if certain conditions are satisfied. specifically, the method is applicable when it is confirmed that the uncertainty of an unknown random effect is negligibly small, and no extremely small standard uncertainties are reported by the participating laboratories. in section 3, the conditions are discussed in quantitative terms and a practical procedure is proposed. 3. conditioning for application of the global  maximum likelihood approach  3.1. concept of the maximum and the minimum permissible  uncertainties  in this section, we focus on the two factors that might cause a problem in the performance evaluation proposed in subsection 2.2: (i) an unknown random effect, and (ii) outliers. the first factor can be neglected when the random effect is much smaller than the uncertainty of the reported values, while the second factor can be neglected when no extremely small uncertainty is reported. once it is clarified how large an uncertainty will be reported by the participants in a pt, criteria to avoid the effects of these factors can be examined. the standard uncertainty that is expected to be averagely reported by the participants in a pt is referred to as the expected standard uncertainty in this study, and expressed as uexp. it is assumed that the expected standard uncertainty can be roughly determined based on technical knowledge in advance of the pt. the maximum permissible standard uncertainty of an unknown random effect and the minimum permissible standard uncertainty of the reported values are introduced to avoid the influences of the random effect and outliers, respectively. these are discussed in subsections 3.2 and 3.3 based on the table  1.  extended  en  scores  computed  using  the  robust  method  and  the  gml method.  laboratory (k)  en (k)  in the robust method  en (k)  in the gml method  1  − 1.4  − 2.7 2  − 1.0  − 2.2 3  0.0  − 1.2 4  0.0  − 1.2 5  0.0  − 1.2 6  0.9  − 0.2 7  2.3  0.8 acta imeko | www.imeko.org  november 2016 | volume 5 | number 3 | 19  assumption that the expected standard uncertainty uexp is appropriately given. in subsection 3.4, a practical procedure is proposed based on these discussions. 3.2. influence of the random effect  only inhomogeneity and instability are considered as sources of an unknown random effect in this study. these random effect sources tend to have a serious impact on the analysis of a pt. it is assumed that it has been confirmed that the definition of the measurand, including the choice of the measurement methods, will not have a serious influence. defining rnd as the standard deviation of the random effect, the case in which rnd is estimated to be 0.3 × uexp is considered. the criterion that the standard deviation of the inhomogeneity must not exceed 0.3 × p, where p is the standard deviation for the proficiency assessment, is given in iso 13528:2015. regarding the instability, almost the same criterion is used. this criterion is applied by extension to a case with uncertainty information in this study. since p could be interpreted as the average value of the dispersion of the reported values, it seems appropriate to replace p with uexp. to characterize these criteria quantitatively, letting −1(.) be the inverse of the cumulative standard normal probability function, it is considered that xi is derived from n(, qi + rnd2) and the data are given as follows:   13.01 1 2     niφ v x i , (4) qi = uexp = 1 for i = 1, 2, …, n, where zi = −1(i /(n+1)), and v =  n i 1 (zi – n i 1 zi/n)2/(n −1). using these data, rnd2 is unbiasedly estimated to be 0.32. it should be noted that this sequence of xi does not necessarily mean that xi is truly derived from n(0, 1 + 0.32), and other statistical models including xi ~ n(, qi + i) (i = 1, 2, …, n) employed in the robust method might yield the same sequence of xi. in figure 2, the dispersion of data when n = 30 is shown. consequently, the unknown random effect is undetected when the data are given by (4). the marginal likelihoods of the models in which an unknown random effect is considered for no and all laboratories, 0 and n, are computed for the cases with n = 2, 5, 10, 20, 100, and 200. figure 3(a) shows 0 and n, implying that 0 is always larger than n. thus, it is concluded based on the discussion in subsection 2.1 that no unknown random effect is detected. since the result could change if different data were provided, the property is checked with more dispersive data. the following data are considered:   15.01 1 2     niφ v x i , (5) qi = uexp = 1 for i = 1, 2, …, n. figure 3(b) shows 0 and n, and 0 > n for all n as well. this means that the unknown random effect is undetectably small even when rnd is roughly estimated to be 0.5 × uexp. therefore, it is concluded that 0.3 × uexp could be a strong candidate for the maximum permissible standard uncertainty. in this discussion, rnd is unbiasedly estimated using the pt data. in iso 13528:2015, on the other hand, inhomogeneity and instability are basically evaluated independently from the pt data. however, since the random effect is undetectably small even with more dispersive data, it can be said to be conservative to set the maximum permissible standard uncertainty as 0.3 × uexp, irrespective of the method used for the estimation of rnd. 3.3. sensitivity to outliers  we recommend that the gml method be employed only when the number of the laboratories with |en| ≤ 1 is 10 or more. when the number of participants is small, the discrimination of outliers from the other values is technically difficult. such discrimination is, however, necessary in the case of the gml method. in the following discussion, the gml method is characterized through simulated data with an outlier and 10 other data. cases with a smaller data size are not taken into consideration. we believe that 0.5 × uexp is appropriate as the minimum permissible standard uncertainty mentioned in subsection 3.1. it   figure 2. simulated pt data given by (4) when n = 30. the error bars show  the expanded uncertainties with a coverage factor of 2.      figure  3.  computed  logarithmic  marginal  likelihoods  per  number  of  laboratories  for  the  data  when  n  =  2  to  200  using  the  data  given  in  the  equations of (a) (4) and (b) (5).  acta imeko | www.imeko.org  november 2016 | volume 5 | number 3 | 20  is considered that the uncertainty should be larger than 1/√10 × uexp ≈ 0.32 × uexp, because the weighted mean of the 10 data with the standard uncertainty of uexp is given as 1/√10 × uexp. when ui < 1/√10 × uexp, the value xi can have a strong impact on the determination of the gml estimator. thus, the minimum uncertainty of 0.5 × uexp seems a possible choice. the robustness of the gml method with this value is consequently examined in this subsection. in discussing the property of the gml method in quantitative terms, it is assumed that uexp = 1, and x1 to x10 are considered to be given as follows:   111 1 iφvx i  , (6) qi = uexp2 = 1, where v is defined in subsection 3.2. in addition to these data, x11 and q11 are basically determined differently so as to be characterized as outliers. see figure 4 as an example with x11 = 2.0 and q11 = 0.52. examples with q11 = 0.32 and 0.52 are described in this study; the analysis with q11 = 0.32 is conducted for comparison. if the simulation with the smaller minimum permissible standard uncertainty of 0.3 × uexp provides a result that is robust to the outliers, it can be said that setting the minimum permissible standard uncertainty as 0.5 × uexp is an adequately conservative choice. consequently, the criterion of a minimum permissible standard uncertainty of 0.5 × uexp seems appropriate. when lml = gml, the identical extended en scores are given in both the lml and the gml methods for all of the laboratories. figure 5(a) and (b) show a comparison between lml and gml for q11 = 0.32 and 0.52, respectively. lml and gml are in perfect agreement with each other under the conditions of both q11 = 0.32 and q11 = 0.52. even in the case of q11 = 0.3, the results are not significantly contaminated by the existence of outliers. thus, the validity of the criterion of 0.5 × uexp is confirmed. it should be noted that the participating laboratories must agree in advance to a pt in which standard uncertainty less than the minimum permissible standard uncertainty is not usually reported, and will not be reported in the pt. this is because the measurements for a pt must be conducted under the usual experimental conditions. it is not recommended that a laboratory that usually reports an uncertainty of less than 0.5 × uexp reports an uncertainty of 0.5 × uexp or more only for the purpose of this pt. 3.4. proposed procedure using the gml method  based on the discussions in subsections 3.2 and 3.3, we suggest the following four conditions for application of the gml method: (i) no unknown random effect other than inhomogeneity and/or instability of the pt items exists, (ii) the random effect caused by the inhomogeneity and/or instability has a standard deviation of less than 0.3 × uexp, (iii) the minimum permissible standard uncertainty from the laboratories is 0.5 × uexp, and (iv) 10 or more laboratories report data for which the performance is evaluated as “satisfactory”. the following procedure entailing 11 steps is proposed to realize the above conditions: 1. it is confirmed that there is no or practically negligible unknown uncertainty in the definition of the measurand, including the choice of the measurement method. 2. the pt provider determines the expected uncertainty, uexp, from the existing technical knowledge. 3. the pt provider checks that the standard deviation of the random effect caused by the inhomogeneity and/or instability of the pt items is less than 0.3 × uexp. 4. all of the participants agree that a standard uncertainty of less than 0.5 × uexp is not usually reported, and will also not be reported in the pt. 5. the pt is implemented and the data of xi and ui (i = 1, 2, …, n) are obtained. 6. it is confirmed that a representative value (e.g., the median) of the reported standard uncertainties is adequately close to the expected uncertainty. 7.                         n i i i i xxx x ni 1 2 ...,, old logminarg 1    .   figure 4. simulated pt data given by (7) when x11 = 2.0 and q11 = 0.5 2 . the  error bars show the expanded uncertainties with a coverage factor of 2.      figure 5. computed gml and lml estimators with q11 = 0.3 2  and 0.5 2  as a function of x11. x1 to x10 yielded by (7).  acta imeko | www.imeko.org  november 2016 | volume 5 | number 3 | 21  8.                                 n i ii n i iii xq xqx 1 2old 1 2old gml ,max/1 ,max/    . 9. if |gml − old| > [ ni 1 {1/max(qi, (xi − gml)2)}]−1/2, where  is a small value like 10–3, old = gml and go to step 8. 10. for k = 1, 2, …, n, en(k) is computed through   1 gml2 gml 1 gml 12 1                       n ki ik n ki ii n ki ikk n u xx e   , where igml = max(qi, (xi − gml)2). 11. it is confirmed that 10 or more laboratories have a performance of |en(k)| ≤ 1. for steps 1, 3, 4, 6, and 10, if the conditions are not satisfied, the robust method should, in principle, be implemented instead of the gml method. however, in step 4, if a laboratory requests permission to report a smaller standard uncertainty than the minimum permissible uncertainty, such a request may be accepted only when the laboratory has sufficient technical evidence. in step 7, the initial value for the estimation of gml is determined as xi with which q(xi) in (2) is the smallest among all q(xi). although it is not mathematically assured that the gml estimator can be obtained through this estimation, we have not found a case in which the gml estimator is not given by this determination of the initial value. the chief advantage of the gml method is that the algorithm is sufficiently simple to be employed in an actual pt; or in other words, to be incorporated into iso 13528:2015. in iso 13528:2015, several algorithms for a pt without uncertainty information are described. it can be said that the above algorithm is as simple as those methods. 4. example of application: measurement of  concentration of copper in water  the gml method is applied to the data from a pt that was conducted by the japan society for analytical chemistry (jsac) from 2014 to 2015 [15]. in this test, the concentration of copper in water was measured and reported in the unit of mg/l. it should be noted that gml analysis was not applied in the actual analysis, and the performances were evaluated through a comparison with the reference laboratory. these data are cited merely as a set of numerical examples. the following explanation is given based on the procedure described in subsection 3.4. step 1: jsac has implemented pts for the measurement of the concentration of metals in water since 2007. thus, information on the uncertainty caused by the measurement method has been shared by the participants, and the uncertainty has been incorporated into the reported uncertainty. hence, it cannot be an unknown component of the uncertainty. step 2: the expected standard uncertainty uexp was yielded as 0.0034 mg/l from the past pt data implemented from 2013 to 2014 [16]. the median of the reported relative standard uncertainties in the pt in 2013 to 2014 was 1.7 %. uexp was therefore given as 1.7 % of the set concentration of copper in the pt in 2014. the set concentration was 0.200 mg/l, and uexp was given as 0.0034 mg/l. step 3: the maximum permissible standard uncertainty of the random effect was given as 0.0010 mg/l, which is 0.3 times the expected uncertainty. inhomogeneity of the pt items was checked in this test. the evaluated standard uncertainty between units was calculated as 0.0003 mg/l through analysis of variance. since this value is much smaller than 0.0010 mg/l, the effect of the inhomogeneity on the pt results was negligible. in actuality, when the robust method is applied, the random effect cannot be detected. step 4: the minimum permissible standard uncertainty of the reported values was given as 0.0017 mg/l, which is 0.5 times the expected uncertainty. since information on the minimum permissible standard uncertainty was not given in the actual pt, there was a laboratory that reported a standard uncertainty of less than 0.0017 mg/l. the data of that laboratory have been removed, because the data are treated only as a numerical example in the present study. of course, it is not recommended in an actual pt that data be removed after the pt without reasonable grounds. step 5: the 22 reported values are shown together with their respective standard uncertainties in table 2 and figure 6. the reported values ranged from 0.1908 mg/l to 0.2417 mg/l. laboratories 6 and 20 reported extremely large standard uncertainties. unlike the case of extremely small uncertainties, these large uncertainties are not considered to impair the quality of the pt. step 6: the median of the standard uncertainties in table 2 is 0.0036 mg/l, which is close to the expected standard uncertainty, 0.0034 mg/l. since the median is larger, it can be table  2.  actual  data  reported  in  a  pt  of  the  concentration  of  copper  in  water conducted by the japan society for analytical chemistry from 2014 to  2015, and the evaluated values of q(xi) and en(i) from the data. one set of  data in which the standard uncertainty exceeded the minimum permissible  standard uncertainty was removed only for the purpose of this study.   laboratory (i) reported  value (xi)  standard  uncertainty  (ui)  q(xi)  en (i)   1 0.1908 0.0088  ‐162.13  ‐0.9  2 0.196 0.0094  ‐179.68  ‐0.5  3 0.1967 0.0033  ‐182.31  ‐1.4  4 0.1987 0.0024  ‐189.10  ‐1.4  5 0.1991 0.0050  ‐190.38  ‐0.7  6 0.202 0.11  ‐199.65  0.0  7 0.2025 0.0022  ‐201.26  ‐0.8  8 0.2025 0.0029  ‐201.26  ‐0.6  9 0.2043 0.0023  ‐205.46  ‐0.4  10 0.2056 0.0036  ‐207.32  0.0  11 0.2059 0.0033  ‐207.43  0.0  12 0.206 0.011  ‐207.42  0.0  13 0.2061 0.0044  ‐207.39  0.0  14 0.2070 0.0018  ‐206.13  0.3  15 0.2071 0.0027  ‐205.89  0.2  16 0.2071 0.0023  ‐205.89 0.3  17 0.2072 0.0018  ‐205.64 0.4  18 0.2116 0.0067  ‐185.02 0.4  19 0.2129 0.0036  ‐180.16 1.0  20 0.214 0.039  ‐176.38 0.1  21 0.216 0.014  ‐169.83 0.4  22 0.2417 0.0036  ‐127.96 4.9  acta imeko | www.imeko.org  november 2016 | volume 5 | number 3 | 22  said that a conservative evaluation was conducted to determine the maximum permissible standard uncertainty for checking the uncertainty of the random effect. on the other hand, there is a possibility such that the minimum permissible standard uncertainty of the reported value might be too small. however, as mentioned in subsection 3.3, when the minimum permissible standard uncertainty is 0.3 times the expected standard uncertainty, the gml method works well in most cases. since 0.0036 (mg/l) / 0.0034 (mg/l) is smaller than 0.5 / 0.3 = 1.67, this slight difference does not seem to have a significant impact on the performance evaluation. steps 7 to 9: the values of q(xi) are shown in table 2, and the minimum is given when i = 11. the value of x11 is therefore employed as the initial value in the repetitive computation to determine gml. the repetitive computation in steps 8 and 9 gives the value of gml as 0.2059 mg/l. in this case, lml = gml, and the performances evaluated using the gml method are identical to those using the robust method. steps 10 and 11: the extended en scores are computed, and these are shown in table 2 together with the reported values. the number of laboratories with a performance of |en(k)| ≤ 1 is 19, which is more than 10. there are three laboratories whose magnitudes of en scores are larger than 1.0. it is found from figure 6 that the values that do not contain gml in the range of their expanded uncertainties are evaluated as “unsatisfactory”. these results seem natural. we have presented a detailed discussion on the validity of the extended en scores in another paper [11]. for comparison, other methods were applied to this example. the robust method shows performance evaluation results identical to those obtained using the gml method. analysis using the largest consistent subset [4] obtains en scores with magnitudes exceeding 1.0 only for laboratories 3, 4, 19, and 22. this difference does not seem to be essential. we have provided further discussion through a comparison with other methods in another paper [12]. 5. conclusion  in this study, the application of maximum likelihood to the analysis of proficiency test data is discussed for cases where uncertainty information is given and a reference laboratory does not exist. to prevent serious effects from an unknown random effect and a few outliers, the following four conditions are suggested: (i) no unknown random effect other than inhomogeneity and/or instability of the pt items exists, (ii) any random effect caused by inhomogeneity and/or instability has a standard deviation of less than 0.3 × uexp, (iii) the minimum permissible standard uncertainty from the laboratories is larger than 0.5 × uexp, and (iv) the performances of 10 or more laboratories are consequently “satisfactory”, where uexp is the standard uncertainty that the participants are expected to report averagely. based on these suggestions, a practical procedure is proposed. moreover, the analysis method is characterized through an actual example. we believe that the analysis method proposed in this study can provide natural results with a computationally simple algorithm. acknowledgement  this work was supported by a grant-in-aid for scientific research (no. 26870899) from the japan society for the promotion of science (jsps). appendix: method proposed in our previous  studies  the method proposed in our previous studies [11], [12], which is referred to as the robust method in the main manuscript, is explained. it should be noted that the meanings of some symbols are different from those in the main manuscript. in this method, the model selection is implemented through a comparison of the marginal likelihood. the statistical model with the following parameters is considered: 1. the number of data to which the common random effect is given, m (m = 0, 2, 3, …, n); 2. the identification vector for correspondence of the laboratory and the data, vk = (k(1), k(2), …, k(n))t (k(1) < k(2) < … < k(m), k(m+1) < k(m+2) < … < k(n)); and 3. the parameters for the priors , m+1, m+2, …, and n. (1 ≤  < +∞, 1 ≤ i (i = 1, 2, …, n)); where n is the number of participating laboratories. suppose that laboratory k(i) reports the measurement value xi and its standard uncertainty ui (i = 1, 2, …, n). let qi = ui2 for simplicity of the description. xi is assumed to be derived from the normal distribution with the same mean of . on the other hand, the variances of the distribution for the reported values of laboratories k(i) (i = 1, 2, …, m) are assumed to be qi + c, where c is the variance caused by an unknown random effect. the variances for the reported values of laboratories k(i) (i = m + 1, m + 2, …, n) are assumed to be qi + i, where i is the additional variance caused by the unskillfulness of these laboratories. thus, the model distributions of xi are given as follows:            .,,1for ,n~ ,,,1for ,n~ c nkmkiqx mkkiqx iii ii       (a1) defining   figure  6.  actual  pt  data  shown  in  table  2.  the  error  bars  show  the  expanded  uncertainties  with  a  coverage  factor  of  2.  the  dotted  line indicates the gml estimator, gml = 0.2059 mg/l. the data denoted by the empty  circles  are  the  values  whose  performances  are  found  to  be “unsatisfactory” in the gml method.  acta imeko | www.imeko.org  november 2016 | volume 5 | number 3 | 23  iii m i i q q                  ,1 1 1 c c , (a2) the priors of , c, and i (i = m + 1, …, n), p(), p(c), and p(i), are given as follows:          . , , 1 1 1 1 c cc iiii m i i qp qp p i                         (a3) the priors of c and i, p(c) and p(i), are given accordingly. the hyperparameters of m, vk,  and i, are optimized to maximize the following modified marginal likelihood:          w c 1 cc ,,,, θvxθ dddppmlλ m mi iik  , (a4) where x = (x1, …, xn)t,  = (m+1, …, n)t, and w= {, c, | − ∞ <  < +∞, 0 < c < +∞, 0 < i < +∞}. l(, c, |x, m, vk) is the likelihood of , c, and  given x, m, and vk. the marginal likelihoods with m = 0 and m = n are referred to as 0 and n, respectively, and employed in the main manuscript. it should be noted that vk is not a parameter when m = 0 and m = n. the point is that if m ≥ 2 is chosen as the optimized parameter, the performance evaluation should not be implemented, because m ≥ 2 means c > 0. c is the variance of a random effect. the effect must be corrected before the performance evaluation. only when m = 0 is chosen, the performance evaluation is given. for the performance evaluation, let the posterior mean of  with the optimized model be rob. several combinations of (, 1, ..., n) locally maximizing the likelihood of the statistical model xi ~ n(, qi + i) (i = 1, 2, …, n) can exist. however, the lml estimators of  and i, lml and ilml, are specifically defined as the values of  and i included in the combinations of (, 1, ..., n) whose value of  is the closest to rob among those several combinations. defining ilml = qi + ilml, the extended en score for laboratory k is proposed as follows:   1 lml2 lml 1 lml 12 1                       n ki ik n ki ii n ki ikk n u xx e   , (a5) to check the validity of the following statistical model:      .,...,1,1,...,1 ,n~ ,,n~ lml nkkix qx ii kk   (a6) references  [1] international organization for standardization (iso)/international electrotechnical commission (iec), “iso/iec 17025:2005: general requirements for the competence of testing and calibration laboratories”, iso, geneva, 2005. [2] iso/iec, “iso/iec 13528:2015: statistical methods for use in proficiency testing by interlaboratory comparisons”, iso, geneva, 2015. [3] iso/iec, “iso/iec 13528:2005: statistical methods for use in proficiency testing by interlaboratory comparisons”, iso, geneva, 2005. [4] m. g. cox, metrologia 39 (2002) pp. 589–595. [5] m. g. cox, metrologia 44 (2007) pp. 187–200. [6] r. c. paule and j. mandel, j. res. natl. inst. stand. technol. 87 (1982) pp. 377–385. [7] b. toman, a. possolo, accred. qual. assur. vol. 14 (2009), pp. 553–563. [8] r. willink, “advanced mathematical and computational tools in metrology and testing x”, world scientific, singapore, 2015, isbn: 978-981-4678-61-2, pp. 78–89. [9] s. k. shirono, h. tanaka, k. ehara, metrologia 47 (2010) pp. 444–452. [10] r. n. kacker, a. forbes, r. kessel, k.-d. sommer, metrologia 45 (2008) pp. 512–23. [11] k. shirono, h. tanaka, m. shiro, k. ehara, measurement 83 (2016) pp. 135-143. [12] k. shirono, h. tanaka, m. shiro, k. ehara, measurement 83 (2016) pp. 144-152. [13] a. g. steele, b. m. wood, r. j. douglas, metrologia 38 (2001) pp. 483–8. [14] m. j. t. milton, m g cox, metrologia 40 (2003) pp. l1-l2. [15] the japan society for analytical chemistry (jsac), jsac/ptp43: report on the proficiency test in accordance with iso/iec 17043, jsac, tokyo, 2015. [16] jsac, jsac/ptp-40: report on the proficiency test in accordance with iso/iec 17043, jsac, tokyo, 2014. editorial to selected papers from imeko tc1-tc7-tc13-tc18 joint symposium and mathmet workshop 2022, 2nd part acta imeko issn: 2221-870x june 2023, volume 12, number 2, 1 2 acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 1 editorial to selected papers from imeko tc1-tc7-tc13-tc18 joint symposium and mathmet workshop 2022, 2nd part eric benoit1 1 université savoie mont blanc, polytech'savoie, lab. d'informatique, systèmes, traitement de l'information et de la connaissance (listic) b.p. 80439 annecy-le-vieux cedex 74944, france section: editorial citation: eric benoit, editorial to selected papers from imeko tc1-tc7-tc13-tc18 joint symposium and mathmet workshop 2022, 2nd part, acta imeko, vol. 12, no. 2, article 2, june 2023, identifier: imeko-acta-12 (2023)-02-02 received june 28, 2023; in final form june 30, 2023; published june 2023 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: francesco lamonaca, e-mail: editorinchief.actaimeko@hunmeko.org dear readers, this issue includes the second part of the selection of papers presented at the imeko tc1-tc7-tc13-tc18 joint symposium and mathmet (european metrology network for mathematics and statistics) workshop “cutting-edge measurement science for the future” held in porto in august 2022. as for the first part [1], all these papers followed an acta imeko regular peerreview process before being included into this issue. i have the pleasure to introduce two first papers devoted to the fundamental and historical aspects of measurement science. in their paper “probability theory as a logic for modelling the measurement process” [2], giovanni rossi, francesco crenna and marta berardengo face a fundamental aspect of measurement with an audacious proposal on the nature of probability in the context of measurement modelling. in this contribution they show that probability can be interpreted as a logic for developing models of measurement and by the way they open multiple doors on future advances in measurement modelling. in “metrology in the early days of social sciences” [3], clara monteiro vieira and elisabeth costa monteiro explore the fundamental aspects of the measurement methods proposed more than a century ago by two founding authors of sociology as a scientific field. they especially seek to identify the possible connections between these preliminary sociological approaches and the current metrological conceptions. the identification of human activity is a crucial topic. paweł mazurek and szymon kruszewski present in their paper “applicability of multiple impulse-radar sensors for the recognition of a person’s action” [4] a highly accurate non-intrusive sensor for the monitoring of elderly persons. artificial intelligence is a major topic of interest related to multiple fields, and it also concerns measurement fields. a first paper presents the results of a study to classify masonry debris by different machine learning methods. elske linß, jurij walz and carsten könke investigate in their paper, “image analysis for the sorting of brick and masonry waste using machine learning methods” [5], the performance of several ai methods (svm, mlp and k-nn) on images of masonry debris. a second paper, “a low-cost table-top robot platform for measurement science education in robotics and artificial intelligence” [6], proposed by hubert zangl, narendiran anandan and ahmed kafrana, is focused on the diffusion of the knowledge in measurement field by the use of dedicated robotic platforms and their digital twins. this second selection of selected papers also includes two contributions related to the european metrology network for mathematics and statistics mathmet, a network including a large number of european national metrology institutes and that aims at fostering the field of mathematical and statistical applications for measurement science in europe. the first one, by francesca r. pennecchi and peter m. harris, is a technical note that concerns the mathmet ‘measurement uncertainty activity’. it provides an analysis of the dissemination of mu training, and a precise report on the progress to undertake surveys of existing training courses on mu: “mathmet measurement uncertainty training activity – overview of courses, software, and classroom examples”[7]. the second one, “case studies for the mathmet quality management system at vsl, the dutch national metrology institute”[8] by gertjan kok presents an early test on two pieces of metrological software, on mathematical reference datasets, and on a set of mathematical guidelines, of the mathmet quality management system previously presented by lines (see [9]). i hope you will enjoy your reading. eric benoit section editor mailto:editorinchief.actaimeko@hunmeko.org acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 2 references [1] eric benoit, editorial to selected papers from imeko tc1-tc7tc13-tc18 joint symposium and mathmet (european metrology network for mathematics and statistics) workshop 2022, acta imeko 11 (2022) 4, pp. 1-2. doi: 10.21014/actaimeko.v11i4.1439 [2] giovanni battista rossi, francesco crenna, marta berardengo, probability theory as a logic for modelling the measurement process, acta imeko, vol.12, no.2, 2023, pp.1-5. doi: 10.21014/acta_imeko.v12i2.1313 [3] clara monteiro vieira, elisabeth costa monteiro, metrology in the early days of social sciences, acta imeko, vol.12, no.2, 2023, pp.1-6. doi: 10.21014/acta_imeko.v12i2.1337 [4] paweł mazurek, szymon kruszewski, applicability of multiple impulse-radar sensors for the recognition of a person’s action, acta imeko, vol.12, no.2, 2023, pp.1-7 doi: 10.21014/acta_imeko.v12i2.1346 [5] elske linß, jurij walz, carsten könke, image analysis for the sorting of brick and masonry waste using machine learning methods, acta imeko, vol.12, no.2, 2023, pp.1-5. doi: 10.21014/acta_imeko.v12i2.1325 [6] hubert zangl, narendiran anandan, ahmed kafrana, a low-cost table-top robot platform for measurement science education in robotics and artificial intelligence, acta imeko, vol.12, no.2, 2023, pp.1-5. doi: 10.21014/acta_imeko.v12i2.1356 [7] francesca r. pennecchi, peter m. harris, mathmet measurement uncertainty training activity – overview of courses, software, and classroom examples, acta imeko, vol.12, no.2, 2023, pp.1-6. doi: 10.21014/acta_imeko.v12i2.1310 [8] gertjan kok, case studies for the mathmet quality management system at vsl, the dutch national metrology institute, acta imeko, vol.12, no.2, 2023, pp.1-5. doi: 10.21014/acta_imeko.v12i2.1339 [9] keith lines, jean-laurent hippolyte, indhu george, peter harris, a mathmet quality management system for data, software, and guidelines, acta imeko, vol.11, no.4, 2022, pp. 1-6. doi: 10.21014/actaimeko.v11i4.1348 doi: 10.21014/actaimeko.v11i4.1348 https://doi.org/10.21014/actaimeko.v11i4.1439 https://doi.org/10.21014/acta_imeko.v12i2.1313 https://doi.org/10.21014/acta_imeko.v12i2.1337 https://doi.org/10.21014/acta_imeko.v12i2.1346 https://doi.org/10.21014/acta_imeko.v12i2.1325 https://doi.org/10.21014/acta_imeko.v12i2.1356 https://doi.org/10.21014/acta_imeko.v12i2.1310 https://doi.org/10.21014/acta_imeko.v12i2.1339 https://doi.org/10.21014/actaimeko.v11i4.1348 acta imeko  september 2014, volume 3, number 3, 2  www.imeko.org    acta imeko | www.imeko.org  september 2014 | volume 3 | number 3 | 2  editorial  paolo carbone  university of perugia    section: editorial   citation: paolo carbone, editorial, acta imeko, vol. 3, no. 3, article 2, september 2014, identifier: imeko‐acta‐03 (2014)‐03‐02  editor: paolo carbone, university of perugia  copyright: © 2014 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  corresponding author: paolo carbone, email: paolo.carbone@unipg.it    this issue of acta imeko collects papers submitted from authors that are active in various areas of imeko and ends the publication of manuscripts submitted after the completion of the world congress in busan (korea), in 2012. in the area tc5 – measurement of hardness we publish 3 contributions. the first of these papers is authored by low et al. and it is a joint experimental work done by authors coming from several major national metrological institutes. it proposes a new definition of the brinell hardness indentation test that proves to be useful when low uncertainties are a goal, at the first levels of the traceability chain. the second paper by ma et al. covers the same topic and highlights the difficulties arising by the usage of the current definition of brinell hardness, when the indentation diameter must be determined. the third paper authored by s. takagi shows how to profile rockwell diamond indenters by means of 3d laser scanners. in the area of nanometrology we publish 2 papers. the first one by schuler et al. deals with the subject of tactile surface measurements and addresses the case when surfaces do not have a limited curvature. experimental results are presented that validate the technique presented in this paper to overcome the curvature problem. the second paper by malinovsky et al. discusses nanometrology capacity at the brazilian national metrology institute. traceability at nanometrological level is the central issue discussed here. an interference microscope is described and its metrological characterization is presented. in the area tc13 – measurements in biology and medicine we publish one paper by fiedler et al. it is the research result of an international team of researchers and it reports about an experimental comparison of dry electrodes for electroencephalography to be applied in brain-machine interfaces. in the area tc20 – energy measurements we publish a contribution by an international team of researchers authored by seitz et al. it covers the important task of improving the si traceability of bioethanol by using electrolytic conductivity measurements. in the area tc7 – measurement science we publish 3 contributions. the first one by zieliński et al. describes a timeinterval measurement system based on multi-tapped delay lines. this component is rapidly becoming an important building block for many practical applications. the authors describe their architecture and show experimental results obtained by using their system. the second contribution by aschenbrenner and zagar presents an inductive absolute position measurement system to be applied in industrial environments. the system is described and characterized from a metrological point of view. e. benoit authors the third paper in this area. it deals with the fundamental issue of color measurements in hard sciences and in behavioral sciences. by using fuzzy scales, the paper shows how to adapt the scale used for color measurements to the measurement context. in the area tc15 – experimental mechanics, we publish a paper by panciroli and minak that describes a methodology to be used for the analysis of the air trapped in a fluid when a deformable structure enters the fluid. the paper closing this issue is authored by chen et al. and it relates to the area tc3 – measurement of force, mass and torque. it describes a built system able to measure microforces. experimental results are obtained by comparison with milligram scale deadweights. i want to thank all authors for their scientific contributions and all people involved in managing and preparing this issue. finally, on behalf of the editorial board i wish you to enjoy the scientific content of this acta imeko issue. microsoft word article 9 163-1352-1-le.doc acta imeko  february 2015, volume 4, number 1, 53 – 60  www.imeko.org    acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 53  simultaneous power quality analysis of feeders in mv utility  power stations  aleksandar nikolic, blagoje babic, aleksandar zigic, nikola miladinovic, srdjan milosavljevic  electrical engineering institute nikola tesla, university of belgrade, koste glavinica 8a, 11000 belgrade, serbia        section: research paper   keywords: power quality; simultaneous measurements; analysis; utility power station  citation: aleksandar nikolic, blagoje babic, aleksandar zigic, nikola miladinovic, srdjan milosavljevic, simultaneous power quality analysis of feeders in mv  utility power stations, acta imeko, vol. 4, no. 1, article 9, february 2015, identifier: imeko‐acta‐04 (2015)‐01‐09  editor: paolo carbone, university of perugia   received december 15 th , 2013; in final form december 11 th , 2014; published february 2015  copyright: © 2014 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: (none reported)  corresponding author: aleksandar nikolic, e‐mail: anikolic@ieent.org    1. introduction  the increased demands for electricity created extensive power generation and distribution grids. industries demand larger and larger shares of the generated power, which, along with the growing use of electricity in the residential sector, stretched electricity generation to the limit. today, electrical utilities are no longer independently operated entities; they are part of a large network of utilities tied together in a complex grid. the combination of these factors has created electrical systems requiring power quality [1],[2]. contemporary concepts of measuring instruments for monitoring power quality are based on sophisticated solutions in the field of information technology with advantages in terms of reliability, conformity, speed of operation, automatic control and management of quality on the principles of distributed intelligent measurement methods using software tools, especially virtual instrumentation. this also becomes important for smart grid solutions, where decisions for different actions in the local network are also based on the power quality analysis [3]-[6]. 2. requirements for data acquisition equipment  the choice of components and equipment in the preparation of the conceptual design of the measuring system which function is to measure and analyse the power quality is defined by the needs of distribution companies [7], [8]. given the needs of distribution substations where the usual numbers of feeders to consumers are 8 or 12, a solution should be found that will allow simultaneous measurements up to 12 feeders. in this way, in the case of an analysis of the impact of changes in consumer power quality on the part of the network that is powered from a substation can unambiguously determines the feeder with the problem. another limiting factor is typical device dimensions. the device shall be mounted on the wall inside the building substation or pole-mounted transformer with abstract  the development of a  low‐cost, space saving device that could simultaneously take measurements from all (usually eight or up to  twelve) outgoing three‐phase feeders in a distribution substation is presented in this paper. to meet these requirements, at least 3  voltage measurements and 36 current measurements should be performed at the same time. in order to save space but not to reduce  the measurement accuracy, a data acquisition system is designed based on real‐time multiprocessing with a microcontroller and an  fpga circuit. voltage and current measurements and their corresponding higher‐order harmonics are calculated using a fast fpga  circuit, while other calculations (power, power factor, voltage and current phase angles, etc.) are performed in the microcontroller.  further savings are obtained using multichannel analog input modules with multiplexed inputs. communication with the supervising  computer  is  done  using  a  gprs  modem  or  wireless  network  module  depending  of  the  station  location.  results  obtained  in  the  laboratory and later in an industrial prototype confirm the proposed solution. acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 54  pole. also, to provide mobility, i.e. portability, the device should be as small as possible just in case that it is used in a number of substations. some problems that could arise are summarized hereafter. the necessity to set some parameters with a frequency at least 10 khz in order to better register voltage and current waveforms, and thus the potential disturbances to the sine waveform, requires that acquisition software should measure values in real time. additionally, because of the need to apply the discrete fourier transform (dft) to calculate the content of higher harmonics [9] of voltage and current of up to the 50th order, programmable fpga circuits and their flexible reprogramming that enables the use of rapid calculations on a hardware level are considered [10],[11]. three phase measurements without phase delay between phases of the same feeder, as a consequence of multiplexing several analogue inputs using one a/d converter, are achieved by connecting each phase of one feeder to a different input module. since measurements on all modules can be initiated in the same time without delay, all three phase currents of one feeder can be measured without phase delay between phases. the phase delay between adjacent phases as an effect of the multiplexer is lower than the acceptable accuracy level. since this phase delay is constant, it could be additionally compensated in software if higher accuracy is requested. the proposed measuring principle is shown in figure 1, where ai denotes analogue inputs (500mv level) on 32 channel modules connected on the same data communication bus. for simplicity, only measurements on two feeders are shown. signals from further feeders (up to 32) are connected in the same manner. the measurement range is selected based on the output of split core current transformers that have 333 mv rated output. such transformers are chosen in order to have a simple installation in the field with an opportunity to move the acquisition system to another power station. also, mv output gives the possibility to directly feed signals to the acquisition system without the need for additional signal conditioning. a way of connecting current transformers on a three-phase feeder is shown in figure 2. it can be observed that the same common line on one three-phase feeder additionaly reduces the overal number of connections, what is a merit for the industrial implementation. 3. prototype development  3.1. laboratory prototype  in order to check the functionality and accuracy of the system, at first a laboratory prototype is developed. it is based on a data acquisition usb card and electronics cards that simulate the three-phase system [10]. this specially developed card has two inputs connected to the usb data acquisition card used for simulating voltage and current. in order to have three voltage and three current signals at the output, the card uses phase shifters to provide additional signals with 120° phase difference. this card needs external power supply of ±15 vdc. the proposed multi-channel power quality acquisition system is built up using a programmable controller ni crio 9076 with 4 modules, one 230 vac three channel voltage measurement module and three 32 channel voltage analogue inputs modules. the same controller configuration is used for the industrial prototype. figure 3 shows the complete laboratory setup. the electronics card that emulates the three phase system is placed on top of the dc power supply. the outputs of the figure 1. proposed measurement principle.  figure 2. connection of split core current transformers with voltage output.  acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 55  emulator are 6 voltages of ±100 mv, where three signals are representing feeder voltages of phases r, s, t and the other three signals are currents of the corresponding phases. 3.2. industrial prototype  the advantage of the hardware-in-the-loop (hil) approach [10], used for the laboratory prototype shown in the previous chapter, is a shorter developing time for the final solution. this relies on the real-time controller, since it is used both in the laboratory and the industrial prototype. furthermore, a real-time algorithm running on the microcontroller and the fpga is retained for the industrial prototype and only changes were made regarding the principle of voltage measurement. in order to further reduce the number of requested measurements, one three-phase voltage is measured using the three channel module with isolated 300 vac inputs. such approach is possible due to the fact that in a utility power station all feeders are supplied from one or more power transformers connected to the same high voltage three-phase system (one feeder) emulator usb daq device rt controller figure 3. laboratory prototype.    figure 4. industrial prototype with one feeder current measurements.  figure 5. first part of real‐time application.  acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 56  line. the difference between laboratory and industrial tests is in the signal introduced to the controller to emulate the three-phase voltage in case of the laboratory prototype. for these tests the same analogue input module used for current measurements is also used for voltage measurement. on the other side, in the industrial prototype real phase voltages of 230 vac are introduced. for that module, initialization and calculations are different. but, that doesn’t influence the other parts of the real-time software, especially higherorder harmonics calculations performed on hardware level using the fpga. figure 6. voltage high‐order harmonics.  figure 7. current high‐order harmonics.  acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 57  in figure 4 an industrial prototype is shown in opened housing. for communication to the supervisory computer ethernet is used or a gprs modem connected to the rs232 port. the controller also has a usb port for external usb memory sticks or even a usb hard disk. in that case a large amount of data could be saved without the necessity for real-time data transfer to another computer. that could be also a merit in cases where the ethernet network is not available or mobile signal strength is poor for the gprs modem. 4. developed software  in order to use all features of the proposed real-time controller, like fpga for computationally high intensive algorithms [11], and at the same time visualisation on a client computer, software is coded using thr graphical programming language labview, a software development environment that contains numerous components, several of which are required for any type of test, measurement, or control application [12],[13]. 4.1. real‐time application  the real-time application, developed in labview and built with real-time and fpga options, consists of three parts and it is the same for both the laboratory and the industrial prototype. this approach provides the possibility to test every aspect that could occur in the real system, while reducing time for final tuning of the software. the first part of the application shows measured or simulated input voltages and currents per feeder, phasor diagram, phase angles and calculated power (active, reactive and apparent) and power factor. it is also possible to get accumulated values (power usage) in wh/kwh/�wh. this part of the application is shown in figure 5. the second and third parts of the application are indicated as vh and ah, respectively. both parts are devoted to high-order harmonics of voltages (vh) and currents (ah) shown in the tables, with a clear indication if values exceeding limits defined in the standards [9], [14]. also, the total harmonic distortion (thd) is shown both for voltage and current, with a colour signal indicating values higher than the limits (8% for voltage and 12% for current). these two parts of the application are shown in figures 6 and 7. 4.2. application for data collecting and analysis  in order to centralize power quality measurements of different consumers, it is required to develop a dedicated database and client application in addition to making the measuring device [15]. the role of the database is to allow storage of measured data, while a client application collects data from the data acquisition software through an appropriate communication medium for the selected protocol. the roles of client applications are to perform the display of archived data and perform when signal processing is needed. the concept of a system that includes a database client application and a power quality measuring device is shown in figure 8. 4.2.1. database structure  the main purpose of the database is to collect the data important for further power quality analysis of supplied electricity. the database is organized regarding to the objects and relations in the power system, but in such a manner to satisfy specific user requirements. modular database organization enables its extension to other power objects and consumers. the database also uses auxiliary tables, like standard limits, measurement methods and instrument technical characteristics. as a system for database control, mysql 5.1 is used. due to the advanced features, the innodb engine has been chosen. 4.2.2. client application structure  for client application development, visual c# is used as a programming language, .net development environment and visual studio as a development tool. development environment .net has a library of base classes for access to different databases, work with xml files, web and windows forms, and connection through remoting and web services. com (component object model), word and excel automatization are also used because most customers like to perform additional calculations in excel and to use word as a report creating tool. c omunication medium save measurement device power quality data software for acquisition central supervision center server computer user software mysql database figure 8. concept of proposed power quality system.  m obile network gprs modem rs 232 data from pow er quality analyzer gprs modem rs 232 client 1 client n server request from control center internet data from pow er quality analyzer request f rom control center figure 9. communication interface using gprs protocol.  acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 58  4.2.3. data transmission from measurement devices  in order to transfer data from the measurement device to the client computer, mostly located in the control centre of the local utility, a different communication medium can be used. after an analysis of different interfaces and protocols [16]-[18], two solutions for communication between local measurement devices mounted in the utility power station and control/diagnostic centre are chosen. the first one is ethernet, for locations that exist in or is nearby a local computer network and a gprs modem for longer distances and for instance pole mounted power transformers. as an upgrade of ethernet, digital radio modems over dnp3 protocol and wireless lan can be used. the protocol modbus is proposed in two versions: modbus rtu and modbus tcp/ip. modbus rtu is used for communication between the metering and the data acquisition device and the local modem (usually gprs), while the modem provides direct translation of data to tcp/ip. in this way it enables further uniformity of data transfer, regardless of whether it uses a gprs wireless interface or the classic ethernet network. also, it is suitable for networking computers to exchange data retrieved from the local measuring points [19]. figure 9 shows a configuration of a communication system with gprs implemented protocol with client-server architecture. 5. experimental results  5.1. laboratory tests  in order to check the proposed multichannel power quality measurement system, first a test signal of 230vac 50hz is used (pure sine signal). in that case we could easily verify correct calculations in software. next,a test signal is superimposed with harmonics (100, 150, 200, 250, 300, 350, 400, 450, 500, 1200 and 1250 hz) with an amplitude of 10 v. the obtained signal is shown in figure 10. it can be observed that total harmonics distortion is higher than the permissive value from the standard [14]. on the bottom diagram a fast-furrier transformation of the signal is shown. several values of higher-order harmonics that exceed limits are indicated as failed in the table shown in figure 10. the results show correctness of the proposed methods for power quality analysis [20], [21]. also, the part of the system based on fpga provides fast, real-time calculations on several channels. 5.2. criteria for selecting typical measurement points  prior to putting the measuring device in an industrial environment, typical measurement points should be selected. one criterion is higher-order harmonics content, since the electricity usage is related to the expected current or voltage distortion in the distribution network. figure 10. measurement results of a test signal with superimposed harmonics.  acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 59  criteria of expected higher-order harmonics are defined by the index of expected higher-order harmonics content, calculated for every power station x/0.4 k: 5 aj, i i hj j 1 ai w ih k w   (1) where khj is the weighted coefficient of power consumption of consumers’ group j, waj,i is the active energy of the consumers group „j“ according to the categorization based on higher-order harmonics and supplied from power station x/0.4kv “i”, and wai is the active energy of all the consumers supplied from power station x/0.4kv “i”. the power station with the highest value of index ihi is the first candidate for placing the power quality supervision system. 5.3. testing of the industrial prototype  using previously defined criteria, as a consumer for testing the industrial prototype an office building is used. such buildings are usually equipped with a lot of computers, air-conditions and fluorescent light. in that case, large office buildings are consumers with high value of total harmonic distortion (thd). in the following figures some results taken from a main supply cabinet in one office building are presented. figure 11 shows the phase voltage rms trend, figure 12 the voltage total harmonic distortion, while figure 13 presents the total harmonic distortion factor of current. 6. conclusions  one solution for solving requirements for simultaneous multi-channel power quality measurement in distribution networks is presented in this paper. it is shown that using state-of-art fpga technology and properly configuring real-time software computationally intensive tasks like dft of signals with harmonics up to 50th can be solved. this enables power quality analysis on 12 channels simultaneously, what is a merit of the proposed system and also requirements by the distributor operator is fulfilled. furthermore, with the proposed measurement arrangement it is possible to measure simultaneously up to 32 feeders or 96 currents along with 3 voltages, while device dimensions and weight are still less than required up to 500x500x200mm (wxhxd) and 5 kg. in this case mobility requirements for the measuring system are also retained. the proposed solution has also reduced developing time. a programmable controller, introduced in the first development stage for the laboratory prototype is used with the same real-time software as in the industrial prototype, thanks to the hil concept applied during laboratory tests. results obtained from both development phases show good correlations. references  [1] c. sankaran, power quality, crc press llc, florida, 2002. [2] kilter, j.; meyer, j.; howe, b.; zavoda, f.; tenti, l.; milanovic, j.v.; bollen, m.; ribeiro, p.f.; doyle, p.; romero gordon, j.m., "current practice and future challenges for power quality monitoring cigre wg c4.112 perspective," 2012 ieee 15th international conference on harmonics and quality of power (ichqp), , pp.390,397, 17-20 june 2012. [3] meyer, j.; kilter, j.; howe, b.; zavoda, f.; tenti, l.; romero gordon, j.m.; milanovic, j.v., "contemporary and future aspects of cost effective power quality monitoring — position paper of cigre wg c4.112," electric power quality and supply reliability conference (pq), 2012 , pp.1,6, 11-13 june 2012. [4] music, m.; bosovic, a.; hasanspahic, n.; avdakovic, s.; becirovic, e., "integrated power quality monitoring systems in smart distribution grids," 2012 ieee international energy conference and exhibition (energycon), pp.501,506, 9-12 sept. 2012. 210,0 215,0 220,0 225,0 230,0 235,0 240,0 245,0 250,0 255,0 v 15:58:00.000 10.10.2013 16:08:00.000 10.10.2013 2 min/div 10:00,000 (m in:s) figure 11. phase voltage trend.  1,800 2,000 2,200 2,400 2,600 % 15:58:00.000 10.10.2013 16:08:00.000 10.10.2013 2 min/div 10:00,000 (m in:s) figure 12. voltage total harmonic distortion trend.  9,500 10,00 10,50 11,00 11,50 12,00 12,50 13,00 13,50 14,00 14,50 15,00 % 15:58:00.000 10.10.2013 16:08:00.000 10.10.2013 2 min/div 10:00,000 (m in:s) figure 13. current total harmonic distortion trend.  acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 60  [5] laskar, s.h.; khan, s.; mohibullah, "power quality monitoring in sustainable energy systems," sustainable systems and technology (issst), 2012 ieee international symposium on, pp.1,6, 16-18 may 2012. [6] cornoiu, m.; barbulescu, c.; jigoria-oprea, d.; kilyeni, s.; prostean, g.; teslovan, r., "power quality monitoring and analysis. case study for 220/110 kv substation," 2011 ieee 3rd international symposium on exploitation of renewable energy sources (expres), pp.133,138, 11-12 march 2011. [7] chen, s.; zhang, c.l.; liu, y.z., "a multi-channel monitoring system for system-wide power quality measurements," power system technology, 2000. proceedings. powercon 2000. international conference on , vol.2, pp.953-958 vol.2, 2000. [8] ruiz-llata, m.; guarnizo, g.; boya, c., "embedded power quality monitoring system based on independent component analysis and svms," the 2011 international joint conference on neural networks (ijcnn), pp.2229,2234, july 31 2011-aug. 5 2011. [9] j. arrillaga, n.r. watson, power system harmonics, john wiley & sons, ltd, new jersey, 2003. [10] cuk, v.; nikolic, a.; zigic, a., “a development system for testing integrated circuits used for power and energy measurements,” in proceedings of 13th international power electronics and motion control conference epe-pemc 2008, pp. 1426-1431, poznan, poland, september 2008. [11] n. kehtarnavaz, s. mahotra, digital signal processing laboratory: labview-based fpga implementation, brownwalker press, florida, 2010. [12] national instruments, getting started with labview, june 2010, publication no. 373427g-01. [13] laskar, shahedul haque; muhammad, mohibullah, "power quality monitoring by virtual instrumentation using labview," proceedings of 2011 46th international universities' power engineering conference (upec), pp.1,6, 5-8 sept. 2011. [14] power quality standard en 50160:2010, voltage characteristics of electricity supplied by public electricity networks, 2010. [15] lee, r.p.k.; lai, l.l.; tse, n., "a web-based multi-channel power quality monitoring system for a large network," power system management and control, 2002. fifth international conference on (conf. publ. no. 488) , vol., no., pp. 112-117, 17-19 april 2002. [16] yue fuchang; shao lin; wu zaijun, "modeling and implementation of power quality monitoring device based on iec 61850," 2012 china international conference on electricity distribution (ciced), pp.1,5, 10-14 sept. 2012. [17] wanfang xu; gang xu; zhijiang xi; chuanyong zhang, "distributed power quality monitoring system based on ethercat," electricity distribution (ciced), 2012 china international conference on , vol., no., pp.1,5, 10-14 sept. 2012. [18] yi zhang; honggeng yang; gong cheng, "application of 3g technology in power quality monitoring system," 2012 asia-pacific power and energy engineering conference (appeec), pp.1,4, 27-29 march 2012. [19] jiang xian-kang; ren jian-wen; he peng-yun, "analysis and application of power quality data from power quality monitoring network," 2011 asia-pacific power and energy engineering conference (appeec), pp.1,4, 25-28 march 2011. [20] nikolic, a.; naumovic-vukovic, d.; skundric, s.; kovacevic, d.; milenkovic, v., “methods for power quality analysis according to en 50160,” in proceedings of the 9th international conference electrical power quality and utilization epqu 2007, pp. 1-6, barcelona, spain, october 2007. [21] nikolic, a.; babic, b.; zigic, a.; miladinovic, n.; milosavljevic, s., “multi-channel system for remote power quality monitoring of electricity supplied by public distribution networks,” in proceedings of the xix imeko tc-4 symposium, pp. 1-6, barcelona, spain, july 2013. microsoft word 342-2440-1-le-rev acta imeko  issn: 2221‐870x  september 2016, volume 5, number 2, 55‐63    acta imeko | www.imeko.org  september 2016 | volume 5 | number 2 | 55  3d  survey  technologies:  investigations  on  accuracy  and  usability  in  archaeology.  the  case  study  of  the  new  “municipio” underground station in naples  luigi fregonese 1 ,  francesco fassi 1 , cristiana achille 1 , andrea adami 1 , sebastiano ackermann 1 , alessia  nobile 1 , daniela giampaola 2 , vittoria carsana 3   1 polytechnic of milan, department abc, hesutech laboratory, campus mantova, piazza d’arco 3 ‐ 46100 – mantova, italy  2 superintendence archaeology campania, piazza museo nazionale, 19 ‐ 80135 ‐ naples, italy  3 assistant of the superintendence archaeology campania, via del marzano, 6 – 80123 – naples, italy      section: research paper   keywords: close range photogrammetry; laser scanner; instruments; accuracy; calibration; 3d‐model; archaeology  citation: luigi fregonese, francesco fassi, cristiana achille, andrea adami, sebastiano ackermann, alessia nobile, daniela giampaola, vittoria carsana, 3d  survey technologies: investigations on accuracy and usability in archaeology. the case study of the new “municipio” underground station in naples, acta  imeko, vol. 5, no. 2, article 8, september 2016, identifier: imeko‐acta‐05 (2016)‐02‐08    section editor: sabrina grassini, politecnico di torino, italy; alfonso santoriello, università di salerno, italy  received march 24, 2016; in final form july 22, 2016; published september 2016  copyright: © 2016 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  corresponding author: luigi fregonese, e‐mail: luigi.fregonese@polimi.it    1. introduction  nowadays, three-dimensional models of objects from images are a standard in a wide range of applications from autonomous robotics to industrial vision and consumer digital entertainment. in addition, it has been a topic of intensive research since the early days of computer vision and in the field of cultural heritage [1], but it has eluded a general solution regarding the accuracy in photogrammetric systems. image based modelling (ibm) is based on multiple 2d image measurements to recover 3d object information through a mathematical model. this method calculates 3d measurements from multiple views with the use of projective geometry and a perspective camera model. in addition, it guarantees a good portability and implies often low cost sensors [2]. however, the surveying results must meet specific criteria in order to provide the required accuracy for certain applications. for that reason, any geometric surveying task, such as the photogrammetric one, includes not only the definition of the relative positions of points and objects but also the estimation of the accuracy of the results [3]. with the least squares adjustment method, based on finding an approximate solution to overdetermined systems, it is abstract  advanced 3d survey technologies, such as digital photogrammetry (imaged based) and laser scanner, are nowadays widely used in  cultural heritage and archaeological fields. the present paper describes the investigations realized by the laboratory hesutech of the  polytechnic of milan in cooperation with the superintendence archaeology campania in order to examine the potentiality of image  based  modeling  (ibm)  systems  applied  to  the  archaeological  field  for  advanced  documentation  purposes.  besides  the  3d  model  production workflow in an uncommon excavation environment, a special consideration about the reached accuracy will be discussed.  in the first part of the research, a comparison between photogrammetric camera parameters obtained with ibm systems and the ones  provided with the calibration certificate by the manufacturer of the camera is performed.   in the second part of the research, the operational phases of the application of such advanced 3d survey technologies are shown. the  test field is the archaeological excavation area for the construction of the new “municipio” underground station in naples. due to its  position in one of the historical area of the city, its construction coexists with the archaeological excavations and it is strictly tied to  their evolution. in such conditions, the need to reduce as much as possible the time to build the public infrastructure is a very relevant  feature together with the ability to produce accurate documentation of what is considered archaeologically important.  acta imeko | www.imeko.org  september 2016 | volume 5 | number 2 | 56  possible to obtain reliable information concerning the accuracy of the results as well as the accuracy of the observations. the building of the new “municipio” underground station in naples has required extended as well as intensive archaeological investigations on almost the entire area of the construction yard. the importance of the findings and the evidences together with the need to speed up the completion of the infrastructure, required to introduce advanced 3d survey technologies to satisfy both the requirements. however, the high amount of almost every day photogrammetric surveys as well as restitutions, has not allowed to follow a rigorous workflow regarding the calibration procedure (e.g. with dedicated calibration test fields). anyhow, a self-calibration has been always performed by using the same images acquired during the survey tasks, obtaining residuals on gcps of few millimetres or 1 to 1.5 cm in the worst cases. in the following section, a comparison between a self-calibration obtained in such way and the one certified by the manufacturer will be descripted and discussed. in the second part, a few numbers of the ibm system applications on different size and complexity objects will be shown. 2. accuracy with ibm method  2.1. method for the determination of the accuracy with ibm  nowadays, high-resolution cameras allow to acquire excellent quality images in terms of resolution and sharpness and can be used to perform precision surveys. the best way to evaluate the efficiency and the quality of this instrument as well as the whole process is to focus on camera calibration, the common issue in photogrammetric applications, especially when the precision for dimensional measurements is a not negligible variable. in order to perform this calibration, the process follows a three steps approach: 1. image acquisition; 2. image alignment with the software photoscan®, to define position of images and camera parameters (internal orientation and lens distortions); 3. comparison of the calculated values with those reported in the metric calibration certificate. compared to the other aberrations, lens distortion is the one that mainly affects photogrammetric measurements in terms of accuracy, and the images must be corrected in photogrammetry. lens distortions are of several kinds, but the radial one is the most significant. it is the radial displacement of a projected image point on the sensor from its theoretically true position or, equivalently, a change in the angle between a ray and the optical axis. in order to define the distortions, the parameters of the internal orientation of a camera are calculated by means of the self-calibration, in which the distortion and camera parameters are included as part of the bundle adjustment solution. the digital camera used in this test is the rollei 6008 (figure 1) with fixed digital 72405433 pixel (49.23236.9444 mm) resolution back sensor (phase one) and with a 40 mm optical lens. this camera has the following features:  it can save loss-free or raw-format images, up to 48 bit colour depth, 32 mb per image;  interchangeable metric lenses pq (fastest shutter speed: 1/500 s) between 40 mm and 350 mm, interchangeable metric lenses pqs (fastest shutter speed: 1/500 s) between 50 mm and 500 mm;  metric calibration certificate for each lens;  pixel size 6.8 μm. the calibration certificate provided by the manufacturer includes the following parameters:  ck: 41.195 mm;  xh: 0.161 mm and yh: +0.460 mm;  a1: 3.6010-5 and a2: 2.4410-8 as parameter for the radial distortion;  r0: 0.0 mm;  r: [0, 31] mm. the curve of distortion is defined by rollei with the following equation: ∆ ∙ ∙ ∙ ∙ . (1) the radial distortion curve reported in figure 2 shows a figure 1. rollei 6008af with phaseone p45.  figure 2. lens radial symmetric distortion values in pixel and μm.  acta imeko | www.imeko.org  september 2016 | volume 5 | number 2 | 57  radial distortion of 374 μm (55 pixels) at 31 mm of radial distance. 2.2. test for accuracy investigation  in order to evaluate the overall accuracy of the ibm system (in particular photoscan) a calibration test was realized in a closed environment set-up in a square room of about 4 meters of length, with a cross vault on the top (figure 3). in this room, a total amount of 108 coded targets (85 coded for photoscan and 23 for leica hds cyclone) were displaced all around and measured with a leica total station ts30. in the first data processing, all acquired targets have been used for camera calibration through the steps of image alignment and subsequently of optimization. in such a way the software photoscan calculated the inner orientation, with the results reported in table 1. these results are congruent with the data provided in the original calibration certificate of the camera and the distortion, as reported on also from the distortion curves of figure 4. as concerning the accuracy of alignment of the photogrammetric model, the process has calculated its solution at high quality and it discovered about 120.000 points (figure 5) with the quality values expressed in table 2. the above described accuracy values clarify that the precision of the photogrammetric model (with an order of 0.5 pixel) well fits the standard deviations obtained for each direction x, y, z. to have a more significant comparison, about 2/3 of all gcp were used as check points (cps) to verify the quality of the alignment of the total model: the results of such test are summarized in table 3. figure 5. gcp and tie points determined in alignment and calibration phase  of the model of investigation.  table 1. camera parameters calculated with photoscan software: exported  in australis format.  camera  parameters  value  camera   rollei 6008 p45 h sensor  7240pixel v sensor  5433pixel pixel size  6.8µm  c  41.150mm xp  ‐0.2549mm yp  0.3618mm k1  ‐3.9628710 ‐5   k2  3.0461610 ‐8   k3  ‐4.7477510 ‐12   p1  3.5966310 ‐6   p2  1.6384410 ‐6   figure  4.  comparison  lens  radial  symmetric  distortion  in  μm  (above)  and  pixel (below).  figure  3.  the  polygon  test  in  which  coded  target  for  photoscan  and  hds laser scanning were positioned.  acta imeko | www.imeko.org  september 2016 | volume 5 | number 2 | 58  as evident from the comparison, the ibm approach, based on photogrammetry and topography, is correct and we can use it also in the documentation of cultural heritage. indeed, the network geometry adopted for the images acquisition in this work is the same as the one adopted during the surveys of architectural and archaeological artifacts; in addition, the order dimension of the acquired objects, for cases of close range survey, are similar. the ibm method is efficient not just for the acquisition stage (in terms of instrumental costs and timing for data acquisition), but also in terms of achievable accuracy. 2.3. test on surface accuracy   the last part of the test concerns the comparison between the point cloud extracted with photogrammetric approach and a second one obtained by employing the laser scanner leica hds 7000 [4]. the laser scan was made by positioning the scanner in the centre of the room and it was registered in the same reference system used for the photogrammetric model by aligning it through the 23 black/white hds targets. the results of the registration, made with leica cyclone, are shown in table 4. the evaluation of surface accuracy has been made with the software cloudcompare which allows to measure the euclidean distance between two pointclouds. in figure 6, it is clear that the distances between photogrammetric and laser scanner pointcloud are included in the interval 0.0006<x<0.0048 mm. these tests confirmed that the photogrammetric approach, ibm, is a reliable survey methodology as it guarantees the same accuracy of other well-established methods such as laser scanner. this method validation allowed us to use both methods, ibm and lasers scanner, and to choose the most suitable one time by time considering also some other aspects such as the possibility to acquire texture, the acquisition time, the operational conditions, etc. the following sections contain a few examples of applications of these survey methods in real, and complex, contexts. 3. case  study:  the  new  underground  station  “municipio” in naples  the construction of the new underground station “municipio” in naples, interchange point between two underground lines and the touristic harbour, has produced time expensive archaeological investigations within the construction areas. these investigations were arranged with the italian cultural and activities heritage ministry, the municipality of naples and “metropolitana di napoli s.p.a.”, concessionaire for the construction of the underground, and represent a precocious example of “planned archaeology” applied to an important public infrastructure: indeed, they have been started a few years before the introduction of the “rescue archaeology” law, issued in 2006. table 2. residuals on gcps.  parameters  value  no. images  73  tie points  117.081 gcp points  108  rms (mm)  0.9  rms (pixel)  0.542 σx (mm)  ±0.61 σy (mm)  ±0.65 σz (mm)  ±0.57 figure 6. euclidean distances between laser scanner and photogrammetric  point clouds.  table 3. estimated accuracy of georeferencing images considering both sets  of points gcps and cps.  parameters  configuration   with gcps only  configuration   with gcps and cps  no. images  73 73 gcps  108 37 cps  0 71 rms (mm)  0.99 0.89 σx (mm)  ±0.63 ±0.61 σy (mm)  ±0.68 ±0.65 σz (mm)  ±0.72 ±0.49 table 4. registration laser scanner data. parameters  value  no. target  23  rms (mm)  1.2  σx (mm)  ±0.95  σy (mm)  ±0.82  σz (mm)  ±1.02  acta imeko | www.imeko.org  september 2016 | volume 5 | number 2 | 59  by following the principles of the “urban archaeology”, the investigations have allowed to explore all the human settlements originated within the area, from the most ancient to the modern ones, denoting an unexampled fact-finding model applied to a critical area of the historical centre of naples, an area that couldn’t be differently investigated due to the complexity of the urban situation, the depth of the evidences and the contextual presence of a phreatic layer. in this second part of the research, survey methodologies verified in the previous part are applied on the below described evidences and adapted to the specific cases will be presented. 3.1. the archaeological evidences the old costal landscape shape in front of the historical neapolitan settlement has been reconstructed thanks to the archaeological investigations. however, the area delimited by the actual via medina, via depretis, piazza municipio and castel nuovo was deeply different by the one known as-is today, outcome of secular natural and anthropic transformations (figure 7). at the beginning, it was the south-west zone of a wide inlet delimited, from south-west to north-east respectively, by the promontory where now castel nuovo is located and by the elevation where now the s.maria di porto salvo church is located. from the end of the 4th century b.c. to the 5th century a.d., the above described area matched with the port basin of neapolis and with the neighbouring seaboard as well [5]. the laying out of the harbour has dated back to the end of the 4th century/beginning of the 3rd century b.c., thanks to the founding of tracks related to an extended dredging operation (about 3750 m2) on the deepest sea bottoms levels, operations that modified the original shape of the inlet. in the same epoch, the above slope was regularized by rising up walls for terracing purposes made with panelled masonry or tuff blocks technique, probably for the protection of the basin. a ramp made with tuff blocks was also built, maybe for mooring purposes. during the augustan age, the area was differently reorganized, and important harbour and street infrastructures were set up. next to the inside edge of the inlet, a quay made with concrete, superimposed over tuff blocks lines and delimited by a reticulated work type wall, was built, and the natural tuff bank was also worked. the presence of an erosional line characterized by the presence of malacofauna (barnacles and oysters) proves that the sea level during the roman imperial age was about 1.70 to 1.80 m below the actual sea level. in the south-east area of the inlet, a much more complex harbour infrastructure, still under investigation, has been recently discovered: it is composed of concrete structures, built by using a wooden formwork, that probably were used as docks or breakwaters to protect the entrance of the port basin. the regularized coastline area was employed to build a thermal structure (1st and 2nd century a.d) along a route, probably the via per cryptam from neapolis to the phlegraean fields. among other things, the stratigraphic sequence of overlapping sea bottoms that included a high number of ceramic and organic evidences (such as waste, ship equipment or lost objects), was also excavated. during these investigations, six shipwrecks were found: poor rests of two shipwrecks dated 2nd century b.c. (named “napoli e” and “napoli h”), two ships dated 1st century a.d. ( “napoli a” and “napoli c”), and other three boats, maybe sunk due to a storm at the end of the 2nd century a.d. (“napoli b”, “napoli f”, “naples g”). all the boats were built according to the shell-first method, with the planks fitted edge to edge and fastened by mortise-andtenon joints technique [6]. shipwrecks a, b and f were sailboat types used for commercial purposes, whereas the c and g ones had a flat vertical stern, belonging to the horeiae class, probably pushed by using both sail and oars with multipurposes employment, details known just from iconographic sources (a mosaic found in althuburus, tunisia) [7]. while the a and c boats were entirely removed by constructing a fiberglass case around them, the f, g and h boats, found in 2015, due to their worse condition, have been accurately disassembled and each component has been stored within water tanks to preserve them. the disassembling process has been set up and executed with the cooperation of the iscr (national institute for conservation and restoration) and the naval archaeologists g.boetto (cnrs – center camille jullian) and c.zazzaro (university of naples “l’orientale”). the archaeological excavations revealed a mutation of the inlet morphology at the beginning of the 5th century a.d.: the coast line moved toward east and so the port basin. in addition, the previously built construction was abandoned or reorganized to be used in different manner. the urban history of the area completely changed from the construction of castel nuovo in the 1279 up to the last square realized in the second half of the 19th century. 3.2. survey method for the archaeology  one of the issues that affects the cultural heritage field is the need to storage a huge number of evidences found during archaeological excavations. most of them are usually preserved in storehouses: the employment of before mentioned digital technologies allows to obtain accurate three dimensional models and to share them through the web network, strongly changing the way to benefit from such ancient treasures. in addition to the possibility to carry out metric and semantic information of an object without physically measuring the real evidence, such technologies are almost a mandatory step especially to document those artefacts that could not be preserved, as in the case of the construction of a public infrastructure. an exclusive use of a traditional two-dimensional drawing approach would bring to a loss of important information [8], metrical as well as semantic, and to a lack of precision. in addition, the total absence of texture information of the figure 7. roman evidences and reconstructed costal landscape shape.  acta imeko | www.imeko.org  september 2016 | volume 5 | number 2 | 60  measured object could make this approach difficult to read if not supported by photographs. because of the threedimensionality of architectonic and archaeological structures, and also of the importance to the appearance quality of the surveyed objects besides the geometrical precision, the best way to represent them is to reproduce them by keeping all the three dimensions and texture information, in order to have a virtual representation extremely similar to the real object. since 2012, the concessionaire for the underground construction, together with the “superintendence archaeology campania”, has set up a cooperation with the “3d survey group” of the “polytechnic of milan”. the main issue that came out in this excavation site was the need to work in a complex environment, with working machinery close to the areas to be surveyed, logistical difficulties, and the necessity to save as much time as possible to avoid construction slowdown. these working conditions has led to further adapt the modus operandi in particular for the acquisition step, but also to better test the adopted survey techniques in a stressing situation. beyond that, the systematic usage of digital photogrammetry and laser scanner (not just for isolated cases) has met the need to produce adequate and detailed documentation of the found evidences within a huge construction area like the one in piazza municipio. the 3d models obtained from each survey session performed in different moments and positions can be successively combined together in order to have a global vision of the found evidences, making possible also the stratigraphic sequence understanding and to reconstruct the ancient scenery and its transformation from the hellenistic era up to the 19th century. the adopting of such new techniques sensibly reduced the acquisition time (and as a consequence improved the efficiency of the excavation process) but guaranteed the accuracy of all acquired data. the strong cooperation between surveyors and archaeologists required to tune up a process schedule among the survey step and the preliminary restitution of the related orthophoto, that permit a practical verification by the archaeologist before let the excavations go on. in a second moment, without the need to halt anymore the excavations, phase drawings, architectonic and stratigraphic sections are carried out in cooperation with the “calcagno architetti associati” company. 3.3. the new underground station  the documentation to produce within the “municipio” construction yard with 3d survey technologies is basically composed by orthophotos and 3d point cloud models [9], [10]. during the last three years, several tests with both survey techniques have been conducted on an elevated number of evidences that differ in terms of type and dimension, reaching the best approach to use depending on the case. at the beginning, digital photogrammetry has been mainly used to generate orthophotos while laser scanner to obtain 3d point clouds. however, the improvements of the actual photogrammetric software and the test validation we exposed, allow now to obtain accurate dense point clouds in an almost fully-automatic way. the obtained results that will follow encourages the use of this image-based technique to extract also 3d point clouds, reserving the use of the laser scanner for particular cases and even lower much more the acquisition time as well as the halt of the excavations. a series of surveys related to particular and different type of evidences and stratigraphy will be presented and discussed. in some case, specific additional solution were adopted to perform the survey and to achieve the requested results in term of precision and resolution [11].  a. dredging tracks  one of the most interesting and particular discoveries has been the finding of the rests of dredging operations on the tuff rock stratum. because these tracks have been found about 7 meters below the actual sea level, it is supposed they were probably realized by employing a dredge mounted on a boat or on a buoyant platform [5]. these tracks have been localized in different but neighbouring excavation areas of the construction yard, and they have been excavated in different moments as well. due to these reasons, it was not possible to have a complete physical global view of the tracks all together. the proposed technologies have been employed in the most part of the area in which the dredging tracks have been found. except for the dredging found in 2004 during the line 1 station shaft excavations, for which less updated survey methodologies were employed, all the remaining areas in which they have been discovered, have been surveyed with actual technologies.  the dredging tracks discovered in the line 6 station shaft have been surveyed in 2014. due to the fact that they were partially found below a reinforced concrete slab, built during the archaeological excavations two years before, the natural light condition was poor, and the laser scanner was preventively preferred to carry out the 3d model. the complexity of the site, due to the irregularity of the elevation profile of the area, and the requested high resolution of the model, required an elevated number of scans and an intensive post-processing work as well. thanks to the high quality of the surveys’ results in terms of level of detail and degree of realism, and to the potentiality allowable by such digital technologies, the archaeologists have now the possibility to observe the site overall on a computer, from almost infinite points of view, to better recognize the directions of the dredging tracks in order to identify the ones belonging to a single dredge passage and also a valid support to study the possible shapes of the tool used to dredge the sea bottom (figure 8). in addition, as the surveys have been topographically georeferenced in a global reference system, the combining of all the surveys performed or to be performed in the neighbouring areas, and the possibility to see all the 3d digital models together, will permit to have an overall view of the whole excavated area. last, all the a posteriori analysis intended to extract in detail measurement information can be performed directly on the 3d model and without a specific urgency, allowing the contractors the prosecution of the station construction and optimizing the infrastructure construction figure 8. dredging tracks found in the line 6 station shaft.  acta imeko | www.imeko.org  september 2016 | volume 5 | number 2 | 61  time as well.  b. sea bottoms stratigraphy  the use of advanced survey technologies to document the stratigraphy sequence of the sea bottoms (figure 9) entailed to plan a process schedule in which a strict interaction between surveyors and archaeologists has been necessary. a new concept of “measurable” stratum in three-dimensions has been adopted, bringing to an important improvement if compared to the traditional survey techniques, mainly based on 2d maps and sparse 3d measured points. the “digital assembling” of all the surveyed stratums together made possible to reconstruct the entire sequence of sediments settled or removed over the centuries, metrically and semantically as well, deleting any possible individual interpretation [12]. within this archaeological area of the line 6 station shaft, a total amount of 16 different sea bottom stratums were surveyed: approximatively, a volume of about 5000 m3 was removed by following the rules of the archaeologic investigations, obtaining a height difference between the first and the last stratum surveyed of about 3.60 m (figure 9 and 10).  c. thermal structure  when a complex ruin has to be excavated, especially if it is characterized by great extensions and big dimensions, or in case of lack of important structure elements, the understanding of the whole structure utility could appear arduous. an orthographic vertical view as well as a 3d model of the area can give a valid help to understand its shape otherwise not always comprehensible just looking at it on the excavation site (figure 11). the survey of the roman thermal structure found in the line 6 station shaft appeared as an extreme difficult task due to the complexity of the site and to the restricted working spaces. beyond the survey of each single walls’ façades of the structure, a global survey of the entire area, by using a bird’s eye view, was necessary. due to the presence of several obstructions that made impossible the employment of auxiliary flying units such as hanging baskets or uavs, a flexible and cheap solution has been designed and manufactured. in order to take vertical photos, the camera was mounted on a horizontally movable aluminium frame controlled in one direction by a sort of “clothesline” and in a second direction by a movable roof previously positioned on two rails to preserve the ruins during the excavation progress [8]. this stratagem, besides its cheapness, turned up to be a good resource to overcome the above described difficulties. thanks to this solution, a true orthophoto (figure 11) of the entire visible parts of the structure was obtained, allowing to have a real perception of the building composition. the 3d model of the same structure instead (figure 12), obtained by employing a laser scanner, allowed to have a good starting point for the prosecution of the excavation activities: indeed, a particularity of this structure is the fact that walls previously used for different purposes in hellenistic epoch were reused to build it in roman epoch. the surveys performed successively, during the prosecution of the excavations and contextually to the disassembling of the thermal structures, permitted to document the more ancient walls, allowing to digitally reconstruct older situations like a backwards travel through the time. figure 11. thermal structure and quay true orthophoto and vector map.  figure 9. perspective view of a 3d model of line 6 station shaft excavation site where the first and last surveyed sea bottoms have been highlighted.  figure 10. stratigraphic section carried out from the combined 3d models of all the surveyed sea bottoms.  acta imeko | www.imeko.org  september 2016 | volume 5 | number 2 | 62  in addition, the possibility to assemble and view together 3d models related to surveys performed in different excavation moments, even in different years, is an exhaustive tool to understand the original uses of the artefacts and the relationship among the structure of different epochs. the accurate documentation of the much important artefacts produced during the excavation progress allowed to carry out graphic two-dimensional representation as well, such as maps, sections and perspective drawings with wall orthophotos façades (figure 13). in addition, stratigraphic sections and 3d multi temporal views have been produced in order to show in a clear manner the relationship between different epochs structures (figure 14).  d. shipwreck “napoli g”  at the end of 2014, the partial bottom of a shipwreck, including edge-joined hull planks and transversal frames, was found (figure 15). due to the particularity of the archaeological evidence and for the location of the finding, there was the necessity to pay serious attention to its excavation and to find a quick solution to provide exhaustive documentation of the evidence and disassemble the shipwreck in as less time as possible. this special case required a strictly cooperation between surveyors and archaeologists, as the start and the prosecution of the disassembling process depended by the preventive and quickly production of maps with orthophotos in specific moments, useful to the archaeologist to take note of each single disassembled part. for these reasons, high resolution orthophotos and accurate 3d models were requested. the high number of corners for the presence of several frames located in their original positon above the bottom of the boat, brought to choose the photogrammetry as the best survey solution in terms of quality of the final model and timeconsuming. indeed, with a range-based instrument like the laser scanner, a huge number of scans to avoid “holes” on the model would have been requested, with a consequent overabundance of unnecessary data, an intensive and time expensive editing activity and several junction zones between the scans to deal with. as additional negative surrounding condition, the boat was found partially cut by the bulkheads built before the beginning of the archaeological excavation activities. the presence of the bulkheads themselves too close to the boat on one side would have made impossible the acquisition with a laser scanner, while the flexibility of the photogrammetry made it possible even if with some difficulty. in order to keep a good range of dof (depth of field) and a suitable degree of sharpness in a low light condition, photos were taken with an elevated f-number and with the camera set on a tripod. normal/nadiral photos were taken together with a set of tilted photos to well reconstruct each particular of the hull and to improve the quality of the camera network. a mobile platform on which to place the camera was also prepared to move the camera itself above the shipwreck. for this evidence, four distinct surveys, which information is summarized in table 5, were performed during the disassembling phases, in order to record as much construction details as possible. other than being a satisfying documentation, an accurate 3d reconstruction of such archaeological found can be a useful tool for historians of ancient naval architecture to better figure 12. perspective view of the 3d model of the roman thermal complex and of the quay.  figure 13. architectonic section of the thermal structure together with the front of the quay.  figure  14.  3d  multi  temporal  view:  roman  structures  in  grayscale, hellenistic ones in magenta.  figure 15. shipwreck “napoli g”.  acta imeko | www.imeko.org  september 2016 | volume 5 | number 2 | 63  understand the typology of the vessel, the employed construction techniques or to have important elements to reconstruct a complete virtual or real model of the vessel reducing reconstruction hypothesis. even a remounting of the shipwreck in a museum, as it was found, could be taken into consideration as each part was mapped in detail before being removed. 4. conclusions  the results of investigations about the accuracy allow to switch between photogrammetry and laser scanner as very really efficient method to document cultural heritage. the above described cases are just a selected group of the most exhaustive and interesting evidences found during the excavations activities to which advanced survey technologies have been applied. even if these cases could appear as a small part of the enormous and continuous survey activity started in 2012 and still under way, they fully give an idea of the potentiality of such methodologies. the thermal structure in particular is a significant example of the possibility to virtually reconstruct the several historical stages that came in succession from hellenistic epoch on. beyond the achievable information that can be extracted from detailed 3d models, the above shown results demonstrate the capability of such technologies to deal with continuous excavation activities that take places simultaneously in different areas of the construction yard and to perfectly satisfy the requirements of all the involved subjects. within the last three years, the improvements introduced in the actual data processing photogrammetric software (parallel computing, automatic point cloud generation, etc.) brought to reorganize the two employed methodologies. according to the results of the first part of this paper, much more space has been reserved to the photogrammetric approach in the last months, sensibly reducing the acquisition time in the perspective of still optimize the work. accurate 3d models in a limited time, depending on the complexity of the object, can now be obtained with the photogrammetry in a fast manner. in addition, this technique demonstrated its capability to be employed even in case of outstanding conditions, thanks to its flexibility. references  [1] l. quan, image-based modeling, 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[2] f. remondino, s. el-hakim, image-based 3d modelling: a review. the photogrammetric record 2006, 2, pp. 269–291. [3] w. boehler, v. bordas, a. marbs, “investigating laser scanner accuracy”, proc. of xixth cipa symposium, 30 september–4 october, 2003, antalya, turkey, pp. 696-702. [4] c. achille, a. adami, s. chiarini, s. cremonesi, f. fassi, l. fregonese, l. taffurelli, uav-based photogrammetry and integrated technologies for architectural applications methodological strategies for the after-quake survey of vertical structures in mantua (italy). sensors 2015, 15, 15520-15539, issn 1424-8220, doi:10.3390/s150715520. [5] v. carsana, s. febbraro, d. giampaola, c. guastaferro, g. irollo, m.r. ruello, “evoluzione del paesaggio costiero tra parthenope e neapolis”, in “people/environment relationships from the mesolithic to the middle ages, recent geoarchaeological findings in southern italy”, méditerranée, no.112, 2009, pp. 14-22. [6] e. allevato, e. russo ermolli, g. boetto, g. di pasquale, “pollen-wood analysis at the neapolis harbour site (1st -3rd century ad, southern italy) and its archaeobotanical implications”, journal of archaeological science, vol. 37, 2010, pp. 2365-2375. [7] d. giampaola, v. carsana, g. boetto, f. crema, c. florio, d. panza, b. pizzo, c. capretti, g. galotta, g. giachi, n. macchioni, m.p. nugari, m. bartolini. “la scoperta del porto di “neapolis”: dalla ricostruzione topografica allo scavo e al recupero dei relitti”, in archaeologia maritima mediterranea, an international journal on underwater archaeology, no.2, 2005 (pisa-roma), pp. 48-91. [8] f. doglioni, “stratigrafia e restauro. tra conoscenza e conservazione dell’architettura”, ed. lint, trieste, 1997. [9] f. fassi, l. fregonese, s. ackermann, v. de troia, “comparison between laser scanning and automated 3d modelling techniques to reconstruct complex and extensive cultural heritage areas”, int. arch. photogramm. remote sens. spatial inf. sci., vol. xl-5/w1, 2013, pp.73-80. [10] f. fassi, c. rossi, a. mandelli, emergency survey of remote and endangered archaeological sites, int. arch. photogramm. remote sens. spatial inf. sci., xl-5/w4, pp. 85-91, doi: 10.5194/isprsarchives-xl-5-w4-85-2015, 2015. [11] f. remondino, heritage recording and 3d modeling with photogrammetry and 3d scanning. remote sensing 2011, 3, 1104-1138, issn 2072-4292, doi:10.3390/rs3061104. [12] m. peripimeno, “sperimentazione di tecniche 3d laser scanning per l’analisi e la conservazione del patrimonio archeologico e storico-monumentale. definizione di procedure e campi di utilizzo”, proc. of national congress sifet integrazione tra le tecniche innovative del rilievo del territorio e dei beni culturali, mondello, 29-31 june 2005. table 5. shipwreck “napoli g” survey report.  survey session  1 st  2 nd  3 rd  4 th  no. photos  334  278  285  213  images resolution  5616*3744  acquisition time  (mins.)  90  55  65  65  orthophoto  restitution time  1 day  no. points 3d model  (millions of points)  117  100  79  74  microsoft word 274-2068-2-le-rev acta imeko  issn: 2221‐870x  april 2016, volume 5, number 1, 22‐31    acta imeko | www.imeko.org  april 2016 | volume 5 | number 1 | 22  variations of phenolic compounds and sensory properties of  virgin olive oils from the variety “istrska belica”  milena bučar‐miklavčič 1 , terezija golob 2 , vasilij valenčič 1 , erika bešter 1 , bojan butinar 1 , ana   miklavčič višnjevec 1   1  up zrs izo; labs d.o.o.,  zelena ulica 8 c, izola, slovenia  2  univerza v ljubljani,  biotehniška fakulteta, jamnikarjeva 101, slovenija      section: research paper   keywords: olive oil; phenolic compounds; sensory properties; “istrska belica”  citation: milena bučar‐miklavčič, terezija golob, vasilij valenčič, erika bešter, bojan butinar, ana  miklavčič višnjevec, variations of phenolic compounds and  sensory properties of virgin olive oils from the variety “istrska belica”, acta imeko, vol. 5, no. 1, article 6, april 2016, identifier: imeko‐acta‐05 (2016)‐01‐06  section editor: claudia zoani, italian national agency for new technologies, energy and sustainable economic development affiliation, rome, italy  received may 29, 2015; in final form november 10, 2015; published april 2016  copyright: © 2016 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: this work was supported by eu founded project uelije  corresponding author: ana m. višnjevec, e‐mail: ana.visnjevecmiklavcic@zrs.upr.si    1. introduction  the olive (olea europaea l.) variety “istrska belica” is the most widely spread olive variety in slovenian olive groves. the quality of the olive oil from “istrska belica” is distinguished by its rich aroma, which reminds the user of the fresh, optimal ripe olive fruit, combined with freshly mown grass. the high content of phenolic compounds gives the “istrska belica” oil its characteristic bitter taste and pungent tactile sensation. diets containing the phenolic compounds found in olive oil can have health benefits, which include the reduction of risk factors for coronary heart disease, protection against several types of cancers, and modification of immune and inflammatory responses [1]-[19]. according to european union regulation (ue) 432/2012, phenolic compounds can be cited according to the indication: “olive oil phenolic compounds contribute to the protection of blood lipids from oxidative stress”. this claim demonstrates the importance of these phenolic compounds. the phenolic compounds in olive oil are secondary metabolites that arise through the conversion of complex substances produced by olive trees, and they can be classified as lignans, flavonoids and secoiridoids. virgin olive oil contains at least 30 different phenolic compounds [1]. the most common lignans in olive oil are pinoresinol, acetoxypinoresinol and hydroxypinoresinol [11], and the most common flavonoids are luteolin and apigenin [20]. while lignans and flavonoids are also in other foods, such as wine, secoiridoids are specific for olive abstract  the olive variety “istrska belica” is well known for its numerous positive properties, such as resistance to low temperature and high oil  content.  the  aim  was  to  determine  the  variations  in  the  levels  of  phenolic  compounds  and  sensory  properties  during  storage  of  “istrska belica” virgin olive oil. the profile of the phenolic compounds and sensory properties of “istrska belica” olive oil were further  compared with those for other varieties,  including “leccino” and “maurino”. the content of phenolic compounds of the olive oils  decreased  after  1  year  and  2  years  of  storage.  after  2  years  of  storage,  the  levels  of  oleuropein  and  the  ligstroside  derivatives  significantly decreased, while the end‐stage compounds tyrosol and hydroxytyrosol increased. these data show that after 1 year of  storage, the “istrska belica” olive oil preserves similar intensities for bitterness and pungency, and similar oleuropein and ligstroside  derivatives levels. in contrast to the other oils, the intensities of bitterness and pungency of “istrska belica” olive oil decreased greatly  only after 2 years of storage. moreover, the phenolic compounds content, and oleuropein and ligstroside derivatives levels, and the  intensities of bitterness and pungency were highest in fresh “istrska belica” olive oil, compared to other olive oils analysed. overall,  “istrska belica” olive oil has important advantages over olive oil from other varieties that are grown in the istria region.  acta imeko | www.imeko.org  april 2016 | volume 5 | number 1 | 23  oil [21], [22]. the two main secoiridoids in olive oil are ligstroside and oleuropein, and their conversion products give olive oil its unique aroma and taste. during the olive-pressing process or if the drupes are injured, ligstroside and oleuropein in the fresh drupes can enter different transformation-reaction pathways, such as their enzymatic and chemical transformation to aldehyde or hydroxy forms [23]. one possible transformation pathway is autooxidative decay as a consequence of the protection of phenolic compounds against harmful oxidative changes [24]. this conversion is gradual and continues in the olive oil throughout its use. during olive oil storage, hydrolytic mechanisms that lead to the release of simple phenols, such as hydroxytyrosol and tyrosol, from the complex phenols, such as the secoiridoids, can occur [25]-[27]. the newly formed substances from these secoiridoids have amphiphilic characteristics, and are thus partitioned between the oily layer and the vegetation water, and are concentrated in the water fraction through their polar functional group [1]. as long as these secoiridoids do not get transformed into their final forms (i.e., the aromatic alcohols tyrosol and hydroxytyrosol), the olive oil preserves its freshness, fruitiness and harmony. it should be noted that when the transformation pathway is reaching its end and the olive oil has already lost its freshness and antioxidative properties, the content of total phenolic compounds can be relatively high. therefore, it is crucial to determine the relative amounts of each of these compounds, and not just the total sum of all of the phenolic compounds. oxidation of olive oil begins as soon as it has been extracted. endogenous enzymes in the olive fruit are involved in the phenolic profile and in its qualitative and quantitative modification during the processing of virgin olive oils [28]. however, olive oil is more severely affected during its storage. the decomposition of the phenolic compounds depends on oxygen, light, temperature, metals, pigments, unsaturated fatty acid content and composition, and quality and kind of natural antioxidants present [29], [30]. part of the low quantity of water in virgin olive oil is free and available for chemical and enzymatic reactions. this low quantity of water keeps hydrophilic phenols in solution, which is where the decomposition process can occur for the phenols and triacylglycerols during olive oil storage [31], [1]. furthermore, according to frankel [30], the hydrophilic antioxidants, such as the polar phenols that are oriented at the air-oil interface, can better protect against oxidation compared to the lipophilic antioxidants, like the tocopherols, which remain in solution in the olive oil. moreover, aparicio et al. [32] measured the correlation between the oxidative stability of virgin olive oil and several compositional variables. they showed that phenols, odiphenols, and the oleic/ linoleic ratio have the highest stability values, followed by chlorophylls, total tocopherols and caratenoids. according to tsimidou et al. [33], hydroxytyrosol is the most active antioxidant compound in virgin olive oil. carrasco-pancorbo et al. [34] showed that the hydroxytyrosol oleuropein-aglycone di-aldehyde (3,4-dhpea-eda; i.e., elenoic acid linked to hydroxytyrosol) and oleuropein aglycon have the strongest antioxidant power. elenoic acid, which does not have a phenolic ring, was one of the compounds investigated in virgin olive oil that has the weakest antioxidant activity. extra virgin olive oil can be characterized by a unique combination of aroma and taste that is highly appreciated [35], [36]. the method of sensory evaluation of virgin olive oils, introduced in 1991 by commission regulation (eec) no. 1348/2013 annex xii (revision of reg. eec 2568/91), lays down the procedure for evaluating the sensory attributes of virgin olive oils and quality classification (categorization). it specifies the criteria for sensory evaluation of virgin olive oil, as well as providing a special vocabulary and standardized conditions for evaluation. however, the panel test, performed according to eu procedures, it is an expensive process that requires accredited assayers and is not easily accessible to the companies that have limited production, and therefore alternative tools have been developed [37]. many studies have tried to clarify the relationships between the sensory attributes in virgin olive oil and the phenolic compounds that are responsible for its aroma and taste [1], [36], [38]-[41]. some studies have suggested that secoiridoid derivatives of hydroxytyrosol are the main contributors to olive oil bitterness [1]. caponio et al. [38] showed that the bitter to pungent taste can be ascribable to oleuropein aglycon. furthermore, oleuropein and its aglycon decrease as the ripening of olives progresses [38]. rotondi et al. [39] confirmed the relationship between the decrease in bitterness and pungency and the reduction in total phenols and diphenol levels. in particular a positive correlation between the content of oleuropein and ligstroside derivatives and the bitterness and pungency was shown. frank and co-authors [40] reported that when an isomer (or isomers) of oleuropein aglycon was prepared by βglucosidase hydrolysis of oleuropein isolated from olives and evaluated by assessors, it was defined as bitter. using the same evaluation technique, no bitterness was observed for hydroxytyrosol or elenolic acid. according to andrewes et al. [41], the dialdehyde form of decarboxymethyl ligstroside aglycone (p-hpea-eda) is the key source of the pungent sensation found in olive oil, while 3,4-dhpea-eda produces very little burning sensation. moreover, beauchamp et al. [5] assessed the pungent intensity of p-hpea-eda isolated from different virgin olive oils, and confirmed that p-hpea-eda is the principal agent responsible for throat irritation. gutierrezrosales et al. [42] concluded that the chromatographic peaks corresponding to 3,4-dhpea-eda, oleuropein-aglycone mono-aldehyde (3,4-dhpea-ea) and p-hpea-eda are mainly responsible for the bitter taste of virgin olive oil. overall, some phenols mainly define the bitterness of olive oil, while others define the perception of pungency, and these might be related to the olive variety. in the present study, the levels of 14 phenolic compounds and the total sum of all of the phenolic compounds included were determined in 167 samples. in all of the samples, an international olive council (ioc) recognized panel tested the sensory properties, as the bitterness, pungency, olive fruity, defects and sensations that resemble olive fruit, green-leaf, tomato, almond, artichoke and vanilla. the aim of the present study was to determine the changes in the phenolic compounds and sensory properties during the storage of the oil of the “istrska belica” variety, which is specific and very important for the region of istria. the phenolic profile and sensory properties of “istrska belica” oil were further compared to the phenolic compounds and sensory properties determined for the oil of other varieties grown in the study area. in addition, the aim was to define the correlations between the content of phenolic compounds and the sensory properties of the fresh oil, and after 1 year and 2 years of storage. to the best of our knowledge, this is the first investigation into the effects of storage on the content of the phenolic compounds and the acta imeko | www.imeko.org  april 2016 | volume 5 | number 1 | 24  sensory properties as tested for the “istrska belica” variety, the oil of which has a characteristic high content of phenolic compounds and a bitter taste. 2. methods  2.1. collection and storage of samples  more than 80 samples of fresh olive oil were randomly collected from slovenian producers. during this 3-year project, the determination of the phenolic compounds and the sensory analyses were carried out on both fresh oil and stored oil produced in slovenia. therefore, the first year samples (2011) were analysed after 1 year and 2 years of storage, and the second year samples (2012) were analysed after 1 year of storage. olive fruit samples were hand-picked from olive trees at the same optimal maturity level for each cultivar taking into consideration the index of maturity and the hardness of the fruit [43]. healthy fruits, without any kind of infection or physical damage, were used for the oil production. the collected samples of extra virgin olive oils were produced using modern production technology with a two-phase decanter. during the production process the temperature was monitored and maintained at 27 °c. the oils were produced according to good manufacturing practice guide [44]. the chemical analyses of fresh olive oil samples were performed after the extraction process in each particular year. all the olive oil samples were stored at 20°c in the closed dark bottles in the same place. 2.2. sensory analysis  the sensory characteristics were determined by a panel composed of eight trained assessors. the sensory properties of fruitiness, bitterness, pungency and other specific characteristics of selected oils were assessed based on the method defined in annex xii of eec regulation no 2568/91, 640/2008, which includes the use of a 10-cm linear scale for intensity determination. 2.3. determination of phenolic compounds  the phenolic compounds in the olive oil, such as the natural and oxidised derivatives of oleuropein and ligstroside, lignans, flavonoids and phenolic acids, were extracted using 60 % (w/w) aqueous methanol solution, and analysed by reverse phase highperformance liquid chromatography (hplc), according to coi/t.20/doc no 29 [42]. the hplc system (agilent 1100) was equipped with a thermostated autosampler, a binary pump system (binpump g1312a), and a diode array detector (g1315b). a phenomenex synergi 4 µm hydro-rp 80 å column (250 × 4.6 mm i.d.; torrance, ca, usa) was used. the analyses were performed according to a modified method published by the ioc [42]. detection was at 280 nm, with the exception of the flavonoids luteolin and apigenin, which were detected at 340 nm. calibration curves for tyrosol (mass fraction from 30 to 800 mg/kg; y = 0.0811a) were constructed using standard compounds. all of the phenolic compounds were determined according to the ioc publication and quantified using the response factor for tyrosol [42]. in this study, the following groups of phenolic compounds were determined: 1) the sum of oleuropein and the ligstroside derivatives; 2) the sum of tyrosol and hydroxytyrosol; 3) the sum of oleuropein, the ligstroside derivatives, tyrosol, hydroxytyrosol, lignans and phenolic acids; and 4) the total phenolic compounds (mg/kg). the term “total phenolic compounds” refers to the “biophenolic minor polar compounds” determined according to the ioc method [45]. 2.4. statistical analysis  all of the data were analysed using the stata13/se software. the distribution of the total phenolic compounds, oleuropein, the ligstroside derivatives, tyrosol and hydroxytyrosol determined in the fresh olive oils and after 1 year and 2 years of storage are given as box plots. the normality of the variable distributions was determined using shapiro–wilk tests. the correlations between the levels of the determined phenolic compounds and the sensory parameters were evaluated. spearman rank correlations were used for bivariate comparisons. due to the significant correlations between the variables of the different phenolic compounds, factor analysis was applied. after running the factor analysis, rotation of the factor loads was performed to provide a clearer pattern. from the different concentraions of the specific phenolic compounds, new variables were created (n=5) for “eigenvalues” >1. wilcoxon–mann–whitney tests were applied for comparisons of two different groups, and kruskal wallis tests were applied for comparisons of three different groups. the level of statistical significance was set to p <0.05. 3. results and discussion   3.1. phenolic compounds  the phenolic compounds were quantified using the response factor for tyrosol [41]. the sum of oleuropein, the ligstroside derivatives, tyrosol, hydroxytyrosol, lignans and phenolic acids (1), the sum of oleuropein and the ligstroside derivatives (2), and the sum of tyrosol and hydroxytyrosol (3) determined in all the fresh olive oils from three years of sampling varied from 145 mg/kg to 966 mg/kg (median, 417 mg/kg), 83 mg/kg to 584 mg/kg (median, 251 mg/kg), and 2 mg/kg to 97 mg/kg (median, 9 mg/kg), respectively. it is important to note that the composition of the phenolic compounds can vary widely according to each olive variety. the contents of the flavonoids luteolin and apigenin in the fresh olive oil from “istrska belica” were in the ranges of 2.6 mg/kg to 5.8 mg/kg (n = 20) and 0.9 mg/kg to 1.9 mg/kg (n = 20), respectively. the same flavonoids contents for “leccino” oil ranged from 1.5 mg/kg to 4.3 mg/kg (n = 11) and 0.3 mg/kg to 0.8 mg/kg (n = 11), and for “maurino” oil they ranged from 0.8 mg/kg to 2.0 mg/kg (n = 4) and 0.2 mg/kg to 0.6 mg/kg (n = 4). the “istrska belica” oil lignans ranged from 22 mg/kg to 70 mg/kg (n = 20), with “leccino” oil as 11 mg/kg to 33 mg/kg (n = 15), and “maurino” oil as 46 mg/kg to 51 mg/kg (n = 6). the dialdehyde forms of both decarboxymethyl oleuropein aglycone (dmo-agl-da) and decarboxymethyl ligstroside aglycone (dml-agl-da) for “istrska belica” oil varied from 23 mg/kg to 124 mg/kg (n = 20), with “leccino” from 17 mg/kg to 274 mg/kg (n = 15), and “maurino” from limit of detection (lod) to 119 mg/kg (n = 6). the oxidised aldehyde and hydroxyl forms of oleuropein aglycone (o-aglda) and ligstroside aglycone (l-agl-da) were lower compared to dmo-agl-da and dml-agl-da, and for “istrska belica” oil they varied from 5.6 mg/kg to 87 mg/kg (n = 20), with “leccino” from 1.5 mg/kg to 21 mg/kg (n = 15), and “maurino” from <lod to 30 mg/kg (n = 6). as expected, the fresh olive oil from “istrska belica” showed the highest median for the total phenolic compounds (median, 616 mg/kg; minimum, 324 mg/kg; maximum, 787 mg/kg; n = 20) and for the sum of oleuropein and the ligstroside derivatives (median, 366 mg/kg; minimum, 165 mg/kg; maximum, 515 mg/kg; n = 20). statistical analysis showed that the differences in the acta imeko | www.imeko.org  april 2016 | volume 5 | number 1 | 25  content of the total phenolic compounds and oleuropein and the ligstroside derivatives in the fresh olive oil from ‘istrska belica” compared to those determined in the fresh olive oil from the other varieties (mediantotal, 360 mg/kg; mediansec, 212 mg/kg; n = 61) were significant (ztotal = -5.049; ptotal <0.005; zsec = -4.063; psec <0.005). this confirms that the high content of phenolic compounds is a specific characteristic of the “istrska belica” variety, compared to the levels determined in the other varieties grown in these slovenian olive groves. the phenolic compounds contents of these olive oils varied greatly according to crop year (table 1, figure 1). the variations in the sum of oleuropein and the ligstroside derivatives, the sum of tyrosol and hydroxytyrosol, and the sum of oleuropein, the ligstroside derivatives, tyrosol, hydroxytyrosol, lignans and phenolic acids in the fresh olive oils from the crop years 2011, 2012 and 2013 are shown in figure 1. these levels (medians, 346 mg/kg, 13 mg/kg, 410 mg/kg, respectively) were highest in the crop year 2012, compared to the years 2011 and 2013. statistical analysis (wilcoxon–mann– whitney tests) showed that these differences were significant for the sum of oleuropein and the ligstroside derivatives (z = 4.655; p <0.005) and for the sum of oleuropein, the ligstroside derivatives, tyrosol, hydroxytyrosol, lignans and phenolic acids (z = -4.572; p <0.005), with marginal significant for the sum of tyrosol and hydroxytyrosol (z = -2.04; p = 0.04). the highest levels of the sum of oleuropein, the ligstroside derivatives, tyrosol, hydroxytyrosol, lignans and phenolic acids in crop year 2012 might have been due to the extreme weather conditions, with a drought for crop year 2012 (figure 1). less than 50% of normal rainfall fell in the crop year 2012 in the study area [46]. the variations in the levels of tyrosol and hydroxytyrosol are not so obvious, because most of these were probably the endproducts of the decomposition pathways of oleuropein and the ligstroside derivatives, and their concentrations might not be directly influenced by the different conditions across the crop years. these variations might be greater after a time of storage longer than 3 years. the sum of oleuropein, the ligstroside derivatives, tyrosol and hydroxytyrosol in the fresh olive oils following 1 year and 2 years of storage are shown in figure 2. while the levels of oleuropein and the ligstroside derivatives decreased significantly across the years, the end-stage compounds tyrosol and hydroxytyrosol only increased after 2 years of storage (figure 2). the sum of oleuropein and the ligstroside derivatives (median, 251 mg/kg), and the sum of oleuropein, the ligstroside derivatives, tyrosol, hydroxytyrosol, lignans and phenolic acids (median, 308 mg/kg) were higher in the fresh oils compared to the levels of oleuropein and the ligstroside derivatives (median, 162 mg/kg) and the sum of oleuropein, the ligstroside derivatives, tyrosol, hydroxytyrosol, lignans and phenolic acids (median, 226 mg/kg) in the oils stored for 1 year and for 2 years. the sum of tyrosol and hydroxytyrosol was higher in the oils stored for 2 years (median, 32 mg/kg), compared to fresh and 1-year-stored oils (median, 14 mg/kg). statistical analysis (wilcoxon–mann–whitney tests) showed that these differences were significant in the case of the sum of oleuropein and the ligstroside derivatives (z = 5.226; p <0.005), the sum of oleuropein, the ligstroside derivatives, tyrosol, hydroxytyrosol, lignans and phenolic acids (z = 4.322; p <0.005), and the sum of tyrosol and hydroxytyrosol (z = -4.970; p <0.005). the relatively high content of the sum of oleuropein, the ligstroside derivatives, tyrosol, hydroxytyrosol, lignans and phenolic acids in the 2-year-stored oils (median, 296 mg/kg; minimum, 138 mg/kg; maximum, 594 mg/kg) is in agreement with the concept that when the transformation pathway of the phenolic compounds is reaching its end, oleuropein and the ligstroside derivatives become substituted with the end-stage compounds tyrosol and hydroxytyrosol. 3.2. sensory properties  a high content of phenolic compounds gives an oil its characteristic bitter taste and pungent tactile sensation. according to the present study, the intensity of bitterness (median, 3.9) and pungency (median, 4.6) in the olive oils was highest in the “istrska belica” oil compared to the oils of the other varieties, such as “leccino” (medianbitterness, 3.3; medianpungency, 3.9) and “maurino” (medianbitterness, 3.5; medianpungency, 4.0). statistical analysis (wilcoxon–mann– whitney tests) showed that the differences between the intensity of bitterness and pungency in the olive oils from “istrska belica” and the other analysed oils were significant (zbitterness = -3.111. pbitterness = 0.002; zpungency = -3.688, ppungency <0.005). moreover, the olive oil from “istrska belica” had a wide range for its sensory profile. in the olive oil from “istrska belica” there were tastes reminiscent of artichoke, almond, tomato, green-leaf and vanilla. in comparison, in olive oil from “maurino” there were only tastes reminiscent of green-leaf and almonds. as for the phenolic compounds in olive oil, the sensory properties of olive oils can vary greatly on a yearly basis. the highest medians of bitterness (median, 3.7; minimum, 1.6; maximum 4.4) and pungency (median, 4.6; minimum, 1.9; maximum, 5.4) in these olive oils were found for the fresh olive oil from crop year 2012. the bitterness and pungency of olive oils are influenced by the phenolic compounds in the olive oil. therefore, the highest median score of bitterness and pungency might be due to the extreme weather conditions, which included a drought in crop year 2012 [46]. however, the highest bitterness and pungency in 2012 were only evident for oils from varieties such as “istrska belica”, “črnica” and “leccio del corno”, while for many varieties included in this study, this was not noted (figure 3). this might be because the specific composition of phenolic compounds of the olive oils depends on the variety. however, due to the relatively low numbers of samples for each variety, with exception of “istrska belica”, “maurino” and “leccino”, further investigations are necessary to confirm these observations. bitterness and pungency are highly dependent on the phenolic compounds in olive oil, and these sensory properties also decreased after 2 years of storage (figure 4). like the variation in bitterness and pungency according to the crop year that depended on the specific varieties (figure 3), the variation in the bitterness and pungency according to the time of storage was also highly dependent on the variety. “istrska belica” preserved a similar intensity of bitterness and pungency after 1 year of storage, compared to the fresh oil. this was not seen for varieties such as “leccino” and “maurino”, where bitterness and pungency decreased greatly after only 1 year of storage (figure 4). the intensities of bitterness and pungency in the olive oil from “istrska belica” decreased greatly only after 2 years of storage. these findings are in agreement with the content of oleuropein and the ligstroside derivatives in “istrska belica” in oil from the crop year 2011 (table 1). therefore, it is important to emphasise the advantages that this domesticated variety provides compared to the other varieties that are grown in this study region. acta imeko | www.imeko.org  april 2016 | volume 5 | number 1 | 26  table 1. mean data (±standard deviation/ range) for the sums of the compounds determined (as indicated) in the fresh olive oils and after 1 and 2 years  of storage, for the oils of the different olive varieties analysed.  storage  (years)  variety  crop  year  n  sum of oleuropein and  the ligstroside derivatives  (mg/kg)  sum of tyroh  and tyr  (mg/kg)  sum of oleuropein and   ligstroside derivatives,  tyr, tyroh, lignans and phenolic  acids  (mg/kg)  total phenolic  compounds  (mg/kg)  fresh  “istrska belica”  2011  6  251 ±58  11 ±2  321 ±68  491 ±105  2012  8  429 ±74  21 ±19  514 ±93  664 ±118  2013  6  373 ±70  12 ±7  449 ±79  604 ±105  “leccino”  2011  6  197 ±78  6 ±2  225 ±82  293 ±105  2012  5  340 ±178  21 ±21  389 ±170  465 ±188  2013  4  211 ±127  6 ±3  239 ±124  302 ±137  “maurino”  2011  2  141 ; 225  8 ; 10  200 ; 289  200 ; 289  2012  2  251 ; 326  7 ; 8  309 ; 383  308 ; 383  2013  2  121 ; 160  2 ; 8  180 ; 210  180 ; 210  “buga”  2011  2  207 ; 240  7 ; 12  253 ; 276  327 ; 349  2012  1  326  9.0  375  461  2013  1  189  6.0  229  292  “črnica”  2011  2  157; 189  20 ; 20  222 ; 271  306 ; 377  2012  1  364  40  469  573  2013  2  142 ; 261  29 ; 97  283 ; 318  358 ; 424  “leccio del corno”  2011  1  159  6.0  219  318  2012  1  367  5  424  516  2013  1  128  2  157  253  “leccione”  2011  1  184  12  227  360  2012  1  285  21  335  450  2013  1  394  19  446  556  “arbequina”  2011  1  187  2  226  271  2012  0  /  /  /  /  2013  0  /  /  /  /  “komuna”  2011  1  266  15  343  492  2012  0  /  /  /  /  2013  0  /  /  /  /  “mata”  2011  1  213  11  249  362  2012  0  /  /  /  /  2013  1  166  21  232  295  mixed  2011  6  171 ±46  8 ±2  226 ±50  320 ±72  2012  5  328 ±46  9 ±3  385 ±51  492 ±62  2013  5  288 ±205  9 ±3  351 ±241  482 ±310  all  2011  31  198 ±59  9 ±4  250 ±67  351 ±108  2012  25  356 ±104  17 ±16  422 ±115  533 ±144  2013  24  259 ±138  14 ±19  318 ±155  426 ±201  1  “istrska belica”  2011  6  247 ±56  37 ±10  345 ±69  508 ±86  2012  8  318 ±83  42 ±25  396 ±103  566 ±124  “leccino”  2011  6  159 ±54  13 ±6  194 ±62  270 ±80  2012  5  232 ±121  23 ±10  273 ±119  375 ±167  “maurino”  2011  2  104 ; 148  24 ; 36  177 ; 234  288 ; 321  2012  2  176 ; 227  20 ; 25  244 ; 285  368 ; 400  “buga”  2011  2  189 ; 214  20 ; 26  242 ; 265  335 ; 376  2012  1  212  17  254  343  “črnica”  2011  2  142 ; 148  27 ; 32  215 ; 233  309 ; 342  2012  1  317  54  412  587  “leccio del corno”  2011  1  107  20  177  282  2012  1  257  19  304  452  “leccione”  2011  1  140  43  217  364  2012  1  180  30  223  339  “arbequina”  2011  1  173  6  220  270  2012  0  /  /  /  /  “komuna”  2011  1  259  46  369  512  2012  0  /  /  /  /  “mata”  2011  1  155  24  198  316  2012  0  /  /  /  /  mixed  2011  6  126 ±45  22 ±7  192 ±48  290 ±70  2012  5  215 ±47  26 ±9  276 ±53  402 ±63  all  2011  31  166 ±64  25 ±12  233 ±80  340 ±113  2012  25  252 ±88  31 ±18  314 ±102  447 ±140  2  “istrska belica”  2011  6  142 ±33  65 ±19  256 ± 56  458 ±108  “leccino”  2011  6  106 ±63  18 ±8  145 ±65  237 ±88  “maurino”  2011  2  48 ; 143  32 ; 48  129 ; 241  212 ; 345  “buga”  2011  2  135 ; 188  27 ; 40  190 ; 237  296 ; 368  “črnica”  2011  2  101 ; 144  32 ; 41  168 ; 239  287 ; 353  “leccio del corno”  2011  1  96  32  178  335  “leccione”  2011  1  78  64  /  /  “arbequina”  2011  1  171  10  225  267  “komuna”  2011  1  173  68  288  473  “mata”  2011  1  102  33  153  257  mixed  2011  6  89 ±30  22 ±7  166 ±35  272 ±49  all  2011  31  114 ±46  25 ±12  189 ±63  312 ±109  acta imeko | www.imeko.org  april 2016 | volume 5 | number 1 | 27  3.3. correlations between sensory profiles and phenolic  compounds in the olive oils managing frames  as indicated in the literature, the relationship between the sensory parameters in virgin olive oils and the levels of total phenolic compounds is well known [36], [1]. the significant positive correlation between the levels of oleuropein and the ligstroside derivatives and bitterness and pungency is in agreement with the literature [1], [39], [40], based on the present data, it is very difficult to make assumptions about which phenolic compound is attributed to the chemo-aesthetic perception of pungency and bitterness. this is mainly because of the significant high correlations between the levels of specific assigned phenolic compounds in the collected samples. due to these significant correlations, factor analysis was applied. the spearman correlations between the pungency and bitternes was increasingly significant for the factors that included oleuropein aglycone, ligstroside aglycone, lignans (factor 1) and the dialdehyde forms of decarboxymethyl oleuropein aglycone and decarboxymethyl ligstroside aglycone (factor 2). 4. conclusions  the content of the phenolic compounds and the sensory properties of these virgin olive oils varied greatly according to variety and crop year. according to our data, the content of total phenolic compounds, the levels of oleuropein and the ligstroside derivatives, the intensity of bitterness, and the pungency were greatest in “istrska belica” fresh olive oil, compared to the other oil varieties analysed here. this study confirms that a high content of phenolic compounds and an astringent and bitter taste is a specific characteristic of “istrska belica” olive oil. moreover, “istrska belica” olive oil has a wide range in its sensory profile, where the tastes are reminiscent of artichoke, almond, tomato, green-leaf, and vanilla. the levels of the phenolic compounds and the sensory properties of these high quality olive oils are closely related. a significant positive correlation was found between the levels of oleuropein and the ligstroside derivatives and the bitterness and pungency. the total phenolic compounds of these olive oils decreased after 1 year and 2 years of storage. while the levels of oleuropein and the ligstroside derivatives significantly decreased with this storage, the end-stage compounds tyrosol and hydroxytyrosol increased after 2 years of storage. “istrska belica” oil maintains similar levels of oleuropein and the ligstroside derivatives and of the intensity of bitterness and pungency after 1 year of storage, compared to the fresh oil. this was not evident for the other varieties involved in the present study. the intensities of bitterness and pungency of the “istrska belica” olive oil decreased greatly only after 2 years of storage. overall, it is important to note the advantages that “istrska belica” olives have compared to the other varieties that are grown in slovenian olive groves. acknowledgement  this work was supported by eu founded project uelije. the authors are grateful to teja hladnik, saša volk and katja fičur for their technical support. references  [1] a. bendini, l. carretani, a. carrasco-pancorbo, a.m. gómezcaravaca, a. segura-carretero, a. fernández-guitérrez, 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[46] agencija rs za okolje, razvoj suše v sloveniji v letu 2012; http://www.arso.gov.si/vode/poro%c4%8dila%20in%20publi kacije/susa%20v%20sloveniji%202012.pdf (accessed november 10, 2015). monte carlo human identification refinement using joints uncertainty acta imeko issn: 2221-870x june 2023, volume 12, number 2, 1 11 acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 1 monte carlo human identification refinement using joints uncertainty mariolino de cecco1, alessandro luchetti1, mattia tavernini2 1 dep. of industrial engineering, university of trento, via sommarive 9 38123 trento (tn), italy 2 robosense srl, viale dante, 300 38057 pergine valsugana (tn), italy section: research paper keywords: human re-identification; uncertainty of human joints; monte carlo method; rgb-d camera citation: alessandro luchetti, mattia tavernini, mariolino de cecco, monte carlo human identification refinement using joints uncertainty, acta imeko, vol. 12, no. 2, article 32, june 2023, identifier: imeko-acta-12 (2023)-02-32 section editor: alfredo cigada, politecnico di milano, italy, roberto montanini, università degli studi di messina, italy received december 3, 2022; in final form february 21, 2023; published june 2023 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: alessandro luchetti, e-mail: alessandro.luchetti@unitn.it 1. introduction nowadays, one challenge still open is the automatic reidentification (re-id) of people, which involves re-identifying the same person in environments populated by multiple people. it consists of robustly re-identifying a person even after changes such as occlusions, lighting variability, different backgrounds, and human poses. most works in this field involve mass surveillance applications [1]. the widespread use of surveillance cameras in public places such as streets, malls, airports, or largescale events makes re-id of people mainly worthwhile to increase public safety standards [2]. other applications range from long-term pedestrian tracking [3] to human activity recognition [4]. for each of them, automatic classification systems can identify and differentiate people based on many prerecorded or real-time videos with low human effort and time. at the same time, however, the need to have a robust choice of the identified person is essential in the industrial environment, where autonomous mobile robots operate in environments populated by humans and interact with them in several ways [5], [6]. the goal is to preserve the safety of the operators while supporting their work: proper identification (id) of operators plays a key role in this context. in this study we explored and selected new descriptors and methods to address the task of re-id. in each frame, we combined color and 3d data acquired by an rgb-d sensor to extract color, biomechanics, and depth information. we designed a new method to select and track a person with high accuracy suitable for industrial applications, considering the uncertainty of human joints. to demonstrate the performance of the proposed method, we first evaluated the monte carlo refinement effects and then the whole method in terms of recognition rate on a public biwi rgbd-id dataset. furthermore, we compared the obtained results with a public re-id neural network provided by the openvino toolkit [7] on the same dataset. the rest of the paper is organized as follows. section 2 outlines the state of the art in re-id techniques. in section 3, we describe the method developed. in section 4, we discuss the effects of monte carlo refinement and validation. in the final section, we draw conclusions. abstract in this work, we propose a new method to re-identify the same individual among different people using rgb-d data. each human signature is a combination of soft biometric traits. in particular, we extract a color-based descriptor and a local feature descriptor through a monte carlo-based algorithm taking into account the uncertainty of human joints and, applied to each descriptor, refines the similarity match against a spatiotemporal database that updates over time. we analyzed the effects of monte carlo refinement in terms of the final maximum matching score obtained for the two descriptors. in addition, we tested the performance of the proposed method on a widely used public dataset against one of the best re-identification methods in the literature. our method achieves an average recognition rate of 99.1 % rank-1 without identification error. its robustness also makes it suitable for industrial applications. mailto:alessandro.luchetti@unitn.it acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 2 2. related work most of the techniques used in literature for re-id exploit soft biometric traits to distinguish humans from their peers. soft biometric traits [8] are physical (e.g., skin color, eye color, hair color, height, weight, gender, race, etc.), behavioral (e.g., gait, keystroke, signature, etc.), or adhered human characteristics (e.g., clothes color, tattoos, accessories, etc.). their advantages include compliance with natural human description labels and nonobtrusiveness in data acquisition. in general, colour is the most common factor affecting the re-id performance and is often encoded in histograms [9], [10]. its inconsistency is the most prominent with respect to viewpoints and poses changes [11]. for these reasons, we also used a color-based descriptor in our method. other approaches use matching based on local feature detectors and descriptors, such as speed up robust features (surf) [12]. in [13], [14], the surf points of interest also cover portions of the background, and their number and position are strongly influenced by the person’s pose, thus making the final re-id less accurate. in our work, we used the same surf descriptor but on points of interest at known positions to avoid that. there are few studies in which descriptors for re-id are based only on 3d information [15]-[18]. in these works, the advantage of using descriptors based only on 3d allows their use even when changing clothes or under conditions of significant illumination change. however, this limits the final performance of re-id due to the limited and inaccurate information available. in [19], the depth features, such as normalized measurements of body parts, are calculated from the positions of joints. their solution is limited by the joint extraction method not discussed in their article and, more generally, by the depth resolution. to overcome these limitations, many works in the literature propose descriptors that integrate colour and 3d data to improve the re-id accuracy. in particular [20] combines clothing appearance descriptors with anthropometric measures extracted from depth data. anthropometric measurements, such limb length, are strongly influenced by the method of joint extraction and the person’s pose, so qualitative feedback on their estimation is insufficient to obtain a robust descriptor. also, in [21], their descriptors do not consider the hand or ankle joints because, in contrast to our work, there is a lack of information about their uncertainty. another example of the combined use of rgd-d data is [22]. the biometric data here involved are colour histograms split between the upper and lower body and on the subjects’ height. in this case, the histograms consider the background reducing the final accuracy of re-id. in the case of occlusions, their method loses the height information, which is the only parameter they extract from the 3d information. more recent work [23] uses a single color-based descriptor generated from a partition grid, the size of which depends on skeletal posture obtained using 3d information. the points in each person's cloud are then grouped according to position in the grid. it requires an updated database for each posture, and the management of these coloured point clouds needs a higher computational cost than our approach, in which masks are used for a colour-based descriptor in each anatomical part from the position of the joints. in addition, because of the imperfect point cloud, some colour information may be lost, especially at the edges of human anatomical parts. finally, using a single colourbased descriptor makes their re-id result less accurate. all the previous works use direct strategies to extract features. the recent spread of deep learning has promoted the use of machine learning techniques to learn similarity models from training samples [24], [25]. however, learning-based strategies are time-consuming for training and testing steps and often unpredictable, which is why they are often used in industry only as support. 3. methodology the main steps of the proposed method are shown in figure 1. using a neural network, we first extrapolate the uncertainty ellipses related to the position of human joints. then we select two descriptors, one based on colour, and one based on local feature, for pseudo-random points within the uncertainty ellipses of each joint. afterwards, we compare the selected descriptors for each joint position to those stored in a spatiotemporal database used as ground truth. a sequential monte carlo method refines the joint positions for each descriptor to obtain the best matching score. finally, the highest descriptor scores are concatenated to get the total re-id score. we performed re-id only when a minimum number of 10 out of 18 joints is found and when the overlapping percentage of the bounding boxes of the selected person with the rest of the people is less than 30 %. in addition, since the re-id of people is also short-term, we assumed that people to be re-identified do not suddenly change clothes or accessories. figure 1. flow chart of the proposed method. acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 3 3.1. human pose estimation with joints uncertainty to estimate the human pose, i.e., joints and connections between them for each person within an image, we applied a neural network provided by intel in the openvino toolkit, called human-pose-estimation-0001. openvino toolkit is a collection of libraries for computer vision. it enables cnnbased deep learning inference and contains an optimized version of opencv [26] libraries for intel hardware. this network is based on openpose [27] approach with tuned mobilenet v1 [28] as a feature extractor. the network is also optimized to run on inference engine, which is a high-performance engine for neural networks developed by intel; it allows inference on many intel hardware such as intel cpu, processor graphics, and fpga. the detection of humans carried out with this network is of the type bottom-up; this means that it does not detect all the human figures in the frame but looks for single interesting points and then, from them, tries to associate together parts/joints to obtain the pose associated to that human. with this net is possible to obtain up to 18 joints per person: ears, eyes, nose, neck, shoulders, elbows, wrists, hips, knees, and ankles. the network is validated on the coco dataset [29]. an rgb image is given as input to the network. the net produces two different outputs. the first consists of 18 probability maps, one for each joint, called heatmaps; the second consists of 19*2 layers, one for the horizontal and one for the vertical direction, called part affinity fields (pafs). with the first set of maps, it is possible to get all the joints in the image by obtaining the positions of the peaks. the second set, on the other hand, provides information on how to match the joints that correspond to a single person. in particular, by taking two possible points and the segment between them, it is possible to determine whether their pair belongs to the same person by checking whether the orientation and position of the segment on the frame match the orientation of the paf unit vector. this is done by evaluating the scalar product of the segment orientation and paf value at a discrete number of points within the same segment. if the orientation of the segment at these points is less than a threshold, the joint pair is saved. finally, this network returns up to 18 joints for each person, assigning them the correct joints by listing pairs of valid joints that share a joint with another pair. we used the heatmap of each joint to extrapolate the ellipse of uncertainty related to the position of the joints and modified the way of extracting the joints from the heatmaps. associating an ellipse of uncertainty with their positions is even more important for how the openvino neural network works for their extraction. it does not consider the joints’ positions in previous frames, which causes their positions to change independently between frames. to extract the ellipse of uncertainty of each joint, we first resize the probability map since the output map of the network is smaller than the input image. in figure 2, there is an example of the non-symmetric probability distribution of a joint. then we cut the probability map at a certain threshold, set it at 50 % probability, and obtain the contour area to select points for descriptors. the threshold percentage value affects the cheesman constant [30], which changes the axes’ dimensions of the ellipses and, accordingly, their final dimensions. assuming a percentage higher than 50 % would result in a larger ellipses size, which for our purpose would result in a higher risk of considering the background instead of looking for the correct position of the joints. this percentage may change depending on the resolution of the image input to the neural network. before proceeding, we mixed the information from the 2d rgb image with the point cloud provided by the depth camera, figure 3, allowing us to better estimate the position in the third dimension of each point within the selected area and remove those that are not on the surface of the human body through median filtering in the depth dimension. with the remaining points, we performed a weighted average with their weights given by the heatmap to find the new positions of each joint, i.e., their centres of mass, and around which to approximate a secondorder surface. we discretely calculated the hessian matrix by taking eight points around the new mean joint positions. each term of the hessian matrix can be approximated by taking the difference between two instances of the gradient vector figure 2. 3d example of non-symmetric probability distribution of the right wrist joint. (a) (b) figure 3. an example of an rgb (a) and depth image (b) (848 × 480 pixels) acquired by realsense depth camera d455. acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 4 evaluated at nearby points of a generic joint i at row ri and columns ci. the hessian matrix terms are replaced by: 𝑯 = [ 𝑯𝟏𝟏 𝑯𝟏𝟐 𝑯𝟐𝟏 𝑯𝟐𝟐 ] = [ 𝐝𝟐𝑯 𝐝𝒄𝟐 𝐝 𝐝𝒄 ( 𝐝𝑯 𝐝𝒓 ) 𝐝 𝐝𝒓 ( 𝐝𝑯 𝐝𝒄 ) 𝐝𝟐𝑯 𝐝𝒓𝟐 ] , (1) where d2𝐻 d𝑐2 = 𝐻(𝑟𝑖,𝑐𝑖+𝑛) − 𝐻(𝑟𝑖,𝑐𝑖) 𝑛 − 𝐻(𝑟𝑖,𝑐𝑖) − 𝐻(𝑟𝑖,𝑐𝑖−𝑛) 𝑛 𝑛 = 1 𝑛2 (𝐻(𝑟𝑖,𝑐𝑖+𝑛) − 2𝐻(𝑟𝑖,𝑐𝑖) + 𝐻(𝑟𝑖,𝑐𝑖−𝑛)) d2𝐻 d𝑟2 = 1 𝑛2 (𝐻(𝑟𝑖+𝑛,𝑐𝑖) − 2𝐻(𝑟𝑖,𝑐𝑖) + 𝐻(𝑟𝑖−𝑛,𝑐𝑖)) d d𝑟 ( d𝐻 d𝑐 ) = d d𝑐 ( d𝐻 d𝑟 ) = 1 4𝑛2 (𝐻(𝑟𝑖+𝑛,𝑐𝑖+𝑛) − 𝐻(𝑟𝑖+𝑛,𝑐𝑖−𝑛) − 𝐻(𝑟𝑖−𝑛,𝑐𝑖+𝑛) + 𝐻(𝑟𝑖−𝑛,𝑐𝑖−𝑛)) with n the difference parameter in pixel. if n is too small there is a risk of considering high noise, while if n is too large, the result may be without meaning because it is averaged. for our application, we chose n = 4 pixels in accordance with the 9 × 9 box filters used in the surf descriptor to approximate gaussian second-order derivatives [12]. finally, the uncertainty ellipse was evaluated with the covariance matrix to obtain a statistical description of the joint positions. the covariance matrix corresponds to the negative of the inverse of the hessian matrix [31]. following this approach, we generate an object for each joint with 3 elements: pixel point, covariance matrix, and probability ellipse, figure 4. the results of the positions of the joints calculated through their centres of mass, as explained above, and through the peaks of the heatmaps, as output from the openvino network, are shown in figure 5. figure 5 shows how, by extracting the position of each joint considering its statistical distribution from the heatmap and applying a depth filter to the selected points around it, the key points result more in the centre of human joints. 3.2. descriptors matching after the human pose estimation, finding the selected user in the frame is necessary. from the result of the joint detections, for each descriptor, we select a fixed number of pseudo-random points that follow a gaussian probability distribution within the ellipse of uncertainty of each joint. to select these points, we first rotated each uncertainty ellipse using the respective eigenvector matrix. then we translated them into the origin through the data of the positions of the centres of mass of the joints, so that the covariance matrices are symmetric and thus the eigenvectors are mutually orthogonal and the pseudo-random points to select could be considered independent along the two semi-axes of the ellipses. once selected, the points were reprojected into the correct reference system. after an initial analysis, we chose a colour-based histogram (hist) descriptor and the surf descriptor for our human signature. each of the two descriptors compared with the ones stored in a database for comparison provides an independent result in terms of joint positions with the best match because to use the histogram-based descriptor, we approximated anatomical parts with geometric figures, which is a simplification not corresponding to reality. in particular, the hist descriptor is evaluated within the selected masks by analysing the hue (h) channel of the hsv colour space. the mask was obtained for the face and torso by connecting joints on their edges. in contrast, for the upper and lower limbs, starting from the direction normal to each pair of joints, quadrilateral masks were found by adding additional points at a fixed length from the selected segment. this length is reduced or increased according to the user's distance from the camera. in the example of figure 6, the width of the limb masks is set to 8 pixels. before using the h-channel histogram as a descriptor, we analysed the most frequently chosen colour spaces, l*a*b* and hsv [32]. the h channel of the hsv colour space is the primary colour attribute and represents the phase angle of the colour, which is the usual name of the colour. compared to l*a*b*, hsv encapsulates colour information in a way more similar to how the human eye perceives colour. instead of defining a colour in terms of a combination of colour, the hsv colour model describes a colour with only one channel. in addition, h values in hsv are more robust to external light changes [33]. also from our tests, where we compared the hchannel and the l*a* channels of hsv and l*a*b* colour spaces, respectively, the h-channel provides more robust behaviour in terms of light changes and colour description for processed images. figure 4. heatmap results superimposed to the rgb image and the probability uncertainty ellipses estimated for each joint (50% confidence level with k=1.177 [30]). figure 5. roi from the initial rgb image with joint positions estimation using peak values (cross symbol in red) or centers of mass (plus symbol in green) of the heatmap. acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 5 to express the similarity between the obtained histogram h1 for each anatomical part and the one selected from the database h2, we used the opencv correlation metric sh(h1,h2) 𝑺𝑯(𝑯𝟏, 𝑯𝟐) = ∑ (𝑯𝟏(𝑰) − �̅�𝟏)(𝑯𝟐(𝑰) − �̅�𝟐)𝑰 √∑ (𝑯𝟏(𝑰) − �̅�𝟏) 𝟐 𝑰 ∑ (𝑯𝟐(𝑰) − �̅�𝟐) 𝟐 𝑰 , (2) where �̅�𝒌 = 𝟏 𝒏𝒃 ∑ 𝑯𝒌(𝑱) 𝑱 (3) and nb is the total number of histogram bins. for our test, we set nb equal to 40 bins. only for the masks of the lower and upper limbs, we considered pseudo-random points around the selected joints because these limbs are the most prone to errors in joint placement, especially in the direction normal to the limb since there is a greater possibility of considering parts of the background as well. each limb is divided into two anatomical regions for a total of eight anatomical parts. the pseudo-normal points for each joint at the end of each anatomical region are projected orthogonally only in the direction normal to the vector linking the two joints. instead, the surf descriptor produces feature vectors of 64 floating elements for each selected point inside the uncertainty ellipse. the obtained descriptor vectors with similar values to those stored in the database are close in cosine similarity distance and far apart for different values. the cosine similarity sc is defined as the cosine of the angle between two vectors, a and b, on dimension ℝ𝑛. the higher the cosine similarity, the higher the probability that the features are similar because the vectors are more aligned. 𝑺𝑪(𝑨, 𝑩) = 𝐜𝐨𝐬(𝜽) = 𝑨 ∙ 𝑩 ||𝑨||||𝑩|| = ∑ 𝑨𝒊𝑩𝒊 𝒏 𝒊=𝟏 √∑ 𝑨𝒊 𝟐𝒏 𝒊=𝟏 √∑ 𝑩𝒊 𝟐𝒏 𝒊=𝟏 (4) where ai and bi are components of vectors a and b respectively. matching for each descriptor is performed only for joints that exist for both the user in the frame being compared and the one in the database. with the constraints imposed for both descriptors, the working domain is 2d. 3.3. monte carlo refinement the selected pseudo-random points for each joint with their similarity scores (sh(h1,h2),sc(a,b)) given by the matches with the database of the two descriptors are used as input for the sequential monte carlo (smc) method, also known as particle filter [34]. the idea behind smc refinement for both descriptors is iteratively changing the joint positions inside the uncertainty ellipses to find the ones with a higher probability of matching in case the subject in the frame chosen to compare with the one in the database is the same. the algorithms used for each of the two descriptors are reported as pseudocodes in appendix a and explained below with an example. for the hist and surf descriptors, in this example, we considered the anatomical region of the right upper arm and left ankle, respectively. for the comparison, two identical frames were taken to be used both as ground truth for the database and as a frame to be compared. it allowed us to (a) (b) figure 6. colour-based descriptor masks for each anatomical part superimposed on the hue channel image (a); in green, an example of a selected anatomical part (a) with its histogram (b). (a) (b) figure 7. an example of monte carlo refinement for the hist descriptor of the right upper arm. in (a), with green dots, the pseudo-random points generated for each joint; red dots show their projection to the normal of the vector linking the two joints. maximum hist descriptor score equal to 0.88. the yellow dots in (b) show the new generated points after 3 iterations. new hist descriptor score of 0.99. acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 6 ensure an exact match between the two frames with a maximum score of 1 for both descriptors. for the hist descriptor applied to the right upper arm are first generated 10 pseudo-random points for both the joint at the end of the anatomical area within the selected joint uncertainty ellipse and filtered in depth, figure 7(a) green dots. all the pseudo-random points are then projected orthogonally in the direction normal to the vector linking the two joints, figure 7(a), red dots. moving to the left and right of each point by a fixed distance, in this case 8 pixels, the right upper arm is simplified into a rectangle and the histogram within the resulting mask is extracted. instead, for the database is used the rectangle obtained from the initial positions of the joint mass centres and compared with each rectangle obtained from each pair of points. then, at each new step, smc takes only half of the first pairs of points with the highest matching score with the database and generates new positions for these points. it moves each point left and right in the direction normal to the linking vector and with a distance proportional to the value of the ellipse's minor semi-axis with a trend of the logarithmic function in base 10: for each point, if the score found is low it moves far away from the previous position, instead if the score is high, it moves slightly away. if a newly generated point ends up outside the uncertainty ellipse, its position is not updated but remains the previous one. each iteration saves the maximum score obtained from the database comparison until the maximum number of iterations is reached. figure 7(b) shows the motion of the joints’ position after 3 iterations. the maximum match for the hist descriptor passes from 0.88 to 0.99. a similar approach is used for the surf descriptor. in this case, we did not consider two joints at a time but a single joint with possible movements in both directions. figure 8 shows an example of 20 pseudo-random points generated for the left ankle within the selected joint uncertainty ellipse. the surf descriptor scores were obtained by comparing each point with the descriptor associated with the joint centre of mass used for the database. then, at each new step, new positions are generated starting from the previous ones, moving them in a random direction and a distance proportional to the value of the ellipse's minor semi-axis with a logarithmic trend. as seen in figure 8(b), after 7 iterations, the points moved to the areas of the highest match from an initial maximum match of 0.94 to 0.99. around the maximum peak, the matching decreases while still having local disturbance peaks, making it impossible to predict the surface pattern with a polynomial. for both descriptors, such a high score already at the first step is also due to the fact that the same frame was used in this example for the database and the one to compare. for the same reason, the score obtained with smc refinement is very close to 1. once the highest match is found for each anatomical part and each joint for hist and surf descriptors, the final score is obtained by concatenating these results for each person with the following expressions: 𝑺𝐇𝐈𝐒𝐓 = || �⃗⃗� 𝑯(𝑯𝟏, 𝑯𝟐)|| √𝒏𝒌 ; 𝑺𝐒𝐔𝐑𝐅 = || �⃗⃗� 𝑪(𝑨, 𝑩)|| √𝒎𝒌 (5) where 𝑆 𝐻(𝐻1, 𝐻2) is the vector containing all pairs of points with the highest matches obtained with the hist descriptor, 𝑆 𝐶(𝐴, 𝐵), on the other hand, contains a point for each joint with the highest matches obtained with the surf descriptor; nk and mk. are the number of anatomical parts and joints found with the hist and surf descriptor, respectively. finally, the total similarity score obtained for person id is the average of the two scores 𝑆hist and 𝑆surf. when a user is identified, the descriptor vectors to be used as ground truth for the database are updated; or when the angle between limbs or light variations exceed 30 %. we monitor the light with the l* channel of l*a*b* spatial color. in addition, descriptor vectors for the database are calculated using the position of the centers of mass of each joint. all databases are also classified based on 3d information about user orientation. the database size selected for each user orientation in our application was set to 10 vectors for both descriptors. 4. validation we assessed the performance of the proposed re-id method by first evaluating the effects of monte carlo refinement and then the re-id results obtained with the whole method. the whole validation was implemented in matlab 2020b. we carried out the assessment on the most widely used dataset for re-identifying people with rgb-d images, such as the biwi rgbd-id dataset [35]. although it is an rgb-d dataset of people targeted to long-term people re-id, we selected 28 people in the training sets in which, for each person, about 300 images were collected on the same day and in the same scene, so (a) (b) figure 8. the mesh is an example of the surf matching score values for all points around the selected joint with the one of the database; the 20 pseudonormal points with their surf matching values at initial configuration (a) with a maximum score of 0.94 and after 7 iterations (b) of sequential monte carlo method with a maximum score of 0.99. acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 7 that the people were dressed in the same way. people moved slightly in the “still set”, while in the “walking set”, each person walked from different viewing angles. in addition, the same number and subject id are present in both sets. 4.1. monte carlo refinement to show the performance of the smc refinement, we tested its effects on both descriptors. we showed an example of the results of the scores obtained with each descriptor using the single image in figure 9(a) from a "still set" of the biwi rgbdid dataset as database. as frames for comparison, however, we used the remaining frames from the "still set" of the user in figure 9(a), another “still set” from a different user in figure 9(b), and their "walking set”, figure 9(c) and figure 9(d), respectively of the two users. we also tested the effect of smc refinement by comparing the database user with another user's dataset to verify that, in the wrong case, the smc refinement does not increase the final scores in a way that would compromise re-id. figure 10 shows the results of the hist descriptor. the results are obtained by concatenating the scores from each limb but not considering the torso and face masks to consider only the anatomical areas where smc refinement acts. by concatenating the final score also with the missing areas, mainly with the torso area, the final solution can only be better and more robust because of the size and location of that area. in particular, figure 10(a) shows the results obtained by comparing the database image of figure 9(a) with the “still set” in which the user is the same or the wrong one. as shown in figure 10(a), in the case of the correct user, the smc refinement improved the final descriptor score in each frame. however, it is high even without the refinement because the user moved slightly in this dataset, and the lighting conditions can be considered constant. in figure 10(b), the same database image is compared with the “walking set”. in this case, the contribution of refinement is greater because of the user’s movement in front of the camera and from a different angle. it demonstrates how refinement is possible with this descriptor, even if lighting conditions and user kinematics change and even with blurred frames. in particular, frame 32 in figure 9 (c) and figure 9 (d) corresponds to the moment when users start walking from a different viewing angle. in both cases in figure 10, comparing the user with the wrong one, refinement increases the final descriptor score but in a way that does not compromise the re-id. it is also true that the lower the score, the easier it is to find a better solution by performing smc refinement. the importance is to keep this increment controlled, as in our case. in contrast, figure 11 shows the results obtained with the surf descriptor by concatenating the results of each joint. this descriptor is strongly influenced by user kinematics. figure 11(a) and figure 11(b) refer to a “still set” with the same user of the database and one with the wrong user, respectively. because of the slight movements of the user, comparing all frames with the single image selected for the database of figure 9(a) is acceptable. if, on the other hand, we compare the database image with a “walking set”, figure 11(c), even if the user is the same between the database and dataset, the scoring trend after the (a) (b) (c) (d) figure 9. (a) frame selected from “still set” to be used as database for comparison; (b) is an example of a frame from “still set” of another user with respect to the one saved in the database; (c)-(d) examples of frames from “walking set” of both users. acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 8 (a) (b) figure 10. results of the hist descriptor score for each frame using the “still set” (a) and “walking set” (b), in both cases comparing the same users (red asterisks) or the wrong one (blue asterisks) without the smc refinement or with it (green symbols). (a) (b) (c) (d) figure 11. results of the surf descriptor score for each frame using the “still set” (a)-(b) and “walking set” (c)-(d). (a)-(c) are obtained by comparing the same users (red asterisks) and (b)-(d) with the wrong one (blue asterisks). all the scores in green are obtained with smc refinement. acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 9 refinement is up and down because comparing the same database image for all possible kinematic poses of the user is a wrong assumption. in this case, the only acceptable comparison can be made for the first few frames where the user is farther from thecamera, but his kinematics does not change. to overcome this problem, we use multiple database images stored in the proposed method, considering not only the user's orientation but also the user's kinematics. 4.2. re-id ranks we assessed the performance of the proposed re-id method using the cumulative matching characteristics (cmc) metric [36], which is commonly used for evaluating re-id algorithms. for every k from 1 to the number of training subjects, the cmc expresses the average person recognition rate computed when the correct person is included in the k best classification scores (rank-k). the most relevant ranks for re-id are the first ranks. in particular, the rank-1 matching rate refers to the probability that the probe image and the image that ranks first in similarity in the search gallery belong to the same subject. therefore, a higher recognition rate at rank-1 means the correct id of the subject in the highest rank and thus better model performance since the chance of having false positives with incorrect subject id is low. we selected the “still set” of images from biwi rgbd-id as the probe, while the “walking set” as the gallery (database). the matching for every subject in the probe with respect to the gallery provides a ranking, figure 12. this procedure is repeated for every subject and then averaged to obtain the final recognition rate. on the same dataset, we ran the re-id openvino demo [37] to compare with our method in terms of cmc curves. the openvino demo detects pedestrians in the frames through the pedestrian detector network person-detection-retail-0013 and reidentifies them through the pedestrian re-id model for a general scenario person-reidentification-retail-0277. the choice of this pre-trained model for the re-id is backed by the superior market-1501 rank-1 [38] accuracy of 96.2 %. this demo was modified to use the re-id network to compare each probe set frame with all the gallery frames. the cmc curves obtained by our method and the re-id of openvino are shown in figure 13. both curves in figure 13 showed a high recognition rate at rank-1. this is due to the goodness of the methods but also to the way the images of the biwi rgbd-id dataset were acquired: the camera was placed in the same position and fixed throughout the time of the tests; also, the same background was used between the probe and gallery images. however, our method gave a better result than that obtained with openvino: 99.1 % vs 98.4 %. this difference can be increased by changing the background between the probe set and the gallery set since the descriptor extracted by the openvino neural network for re-id uses a whole-body image as input and thus goes to consider parts of the background as well. in addition, cmc over-penalizes false positives while ignoring missed re-ids. the decision of whether the selected subject is present in the gallery depends on the choice of the similarity threshold used. in our method, this threshold can be set to a high value without missing re-ids, thanks to the smc refinement process of matching descriptors. 5. conclusions in this paper, an accurate method for the re-id of people was presented. using rgb-d data as a combination of soft biometric traits for similarity matching, we extrapolated a colorbased descriptor and a local feature descriptor considering the uncertainty of human joints. in contrast to the openvino re-id approach, our method does not require any effort for the learning steps, and therefore the features extracted do not depend on the set of images on which the training was performed. it is also more stable than a learning-based strategy because it uses direct strategies to extract features. thanks to its properties, its use is possible in environments where incorrect id is not allowed because it would increase the risk to operators, such as in industrial environments. with our method, this constraint keeps the re-id rate high because, due to the high similarity value obtained by sequential monte carlo refinements in descriptor matching, we can use high similarity thresholds without risking not identifying the selected subject. we achieved a 99.1 % recognition rate at rank-1 on the biwi rgbd-id public dataset. this result represents a significant improvement over previous approaches in the literature obtained using the same public dataset. figure 12. cumulative matching characteristic metric used as validation. figure 13. cumulative matching characteristic curves obtained by our reidentification method and openvino neural network on biwi rgbd-id dataset. acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 10 references [1] s. u. khan, t. hussain, a. ullah, sung wook baik, deep-reid: deep features and autoencoder assisted image patching strategy for person re-identification in smart cities surveillance, multimedia tools and applications springer, 2021, pp. 1-22. doi: 10.1007/s11042-020-10145-8 [2] e. yaghoubi, k. aruna, p. hugo, sss-pr: a short survey of surveys in person re-identification. pattern recognition letters 143, 2021, pp. 50-57. doi: 10.1016/j.patrec.2020.12.017 [3] b. wang, g. wang, k. l. chan, l. wang, tracklet association with online target-specific metric learning, proc. of the ieee conf. on computer vision and pattern recognition, columbus, oh, usa , 23-28 june 2014, pp. 1234-1241. doi: 10.1109/cvpr.2014.161 [4] n. an, s. y. sun, x. g. zhao, z. g. hou, online context-based person re-identification and biometric-based action recognition for service robots, ieee 29th chinese control and decision conference (ccdc), chongqing, china, 28-30 may 2017, pp. 3369-3374. doi: 10.1109/ccdc.2017.7979088 [5] a. luchetti, a. carollo, l. santoro, m. nardello, d. brunelli, p. bosetti, m. de cecco, human identification and tracking using ultra-wideband-vision data fusion in unstructured environments, acta imeko, 10(4), 2021, pp. 124-131. doi: 10.21014/acta_imeko.v10i4.1139 [6] k. koide and j. miura, identification of a specific person using color, height, and gait features for a person following robot, robotics and autonomous systems, 2016, pp. 76-87. 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[27] z. cao, t. simon, s. e. wei, y. sheikh, realtime multi-person 2d pose estimation using part affinity fields, proc. of the ieee conf. on computer vision and pattern recognition, honolulu, 2017, pp. 7291-7299. doi: 10.1109/cvpr.2017.143 [28] a. g. howard, m. zhu, b. chen, d. kalenichenko, w. wang, t. weyand, m. andreetto, h. adam, mobilenets: efficient convolutional neural networks for mobile vision applications. arxiv preprint arxiv:1704.04861, 2017. doi: 10.48550/arxiv.1704.04861 [29] coco consortium, common objects in context. online [accessed 28 nov 2022] https://cocodataset.org/#home [30] r. c. smith, p. cheeseman, on the representation and estimation of spatial uncertainty, the international journal of robotics research, 5(4), 1986, pp. 56-68. doi: 10.1177/027836498600500404 [31] w. c. thacker, the role of the hessian matrix in fitting models to measurements, journal of geophysical research: oceans, 94(c5), https://doi.org/10.1007/s11042-020-10145-8 https://doi.org/10.1016/j.patrec.2020.12.017 https://doi.org/10.1109/cvpr.2014.161 https://doi.org/10.1109/ccdc.2017.7979088 https://doi.org/10.21014/acta_imeko.v10i4.1139 https://docs.openvinotoolkit.org/latest/index.html https://doi.org/10.1007/s11042-010-0635-7 https://doi.org/10.1109/tpami.2012.246 https://doi.org/10.1109/icip.2015.7351146 https://doi.org/10.1109/wacv.2015.120 https://doi.org/10.1007/11744023_32 https://doi.org/10.1109/icdsc.2008.4635689 https://doi.org/10.1109/iciev.2015.7333986 https://doi.org/10.48550/arxiv.1703.09474 http://dx.doi.org/10.1109/icra.2014.6907518 http://dx.doi.org/10.5220/0006155403890397 https://doi.org/10.1007/978-3-642-33863-2_43 https://doi.org/10.1109/tcsvt.2015.2424056 http://dx.doi.org/10.1109/icra.2014.6907689 https://doi.org/10.1109/iwbf.2013.6547322 https://doi.org/10.1016/j.patcog.2019.01.003 https://doi.org/10.1109/tpami.2021.3054775 https://doi.org/10.1016/j.neucom.2019.01.079 http://dx.doi.org/10.1109/cvpr.2017.143 https://doi.org/10.48550/arxiv.1704.04861 https://cocodataset.org/#home https://doi.org/10.1177/027836498600500404 acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 11 1989, pp. 6177-6196. doi: 10.1029/jc094ic05p06177 [32] m. w. schwarz, w. b. cowan, j. c. beatty, an experimental comparison of rgb, yiq, lab, hsv, and opponent color models, acm transactions on graphics (tog), 6(2), 1987, pp. 123158. doi: 10.1145/31336.31338 [33] p.a. marin-reyes, j. lorenzo-navarro, m. castrillón-santana, comparative study of histogram distance measures for reidentification, arxiv preprint arxiv:1611.08134, 2016. doi: 10.48550/arxiv.1611.08134 [34] a. doucet, n. de freitas, j. g. neil gordon, sequential monte carlo methods in practice. ed. arnaud doucet. vol. 1. no. 2. new york: springer, 2001. doi: 10.1007/978-1-4757-3437-9 [35] m. munaro, a. fossati, a. basso, f. menegatti, l. van gool, oneshot person re-identification with a consumer depth camera, in person re-identification, springer, london, 2014, pp. 161-181. doi: 10.1007/978-1-4471-6296-4_8 [36] h. moon, p. j. phillips, computational and performance aspects of pca-based face-recognition algorithms, perception, 30(3), 2001, pp. 303-321. doi: 10.1068/p2896 [37] intel corp., pedestrian tracker c++ demo. online [accessed 28 nov 2022] https://docs.openvino.ai/latest/omz_demos_pedestrian_tracker _demo_cpp.html [38] l. zheng, l. shen, l. tian, s. wang, j. wang, q. tian, scalable person re-identification: a benchmark, proc. of the ieee int. conf. on computer vision, santiago, chile, 07-13 december 2015, pp. 1116-1124. doi: 10.1109/iccv.2015.133 appendix a sequential monte carlo: surf input: 𝑃𝐶𝑀(joint center of mass) 𝑃𝑖 … 𝑃𝑁 (initial pseudo-random points) 𝑆𝑖(𝑃𝑖) … 𝑆𝑁(𝑃𝑁) (descriptor matching scores) output: new point (�̂�𝑖) with 𝑆𝑀𝐴𝑋 for 𝑘 ← 1 to max number of iterations do for 𝑖 ← 1 to 𝑁 do while �̂�𝑖 outside ellipse of uncertainty do dist ← |𝐸𝑙𝑙𝑖𝑝𝑠𝑒𝑚𝑖𝑛𝑜𝑟𝑠𝑒𝑚𝑖𝑎𝑥𝑖𝑠 ∗ log (𝑆𝑖)| if dist > 𝐸𝑙𝑙𝑖𝑝𝑠𝑒𝑚𝑖𝑛𝑜𝑟𝑠𝑒𝑚𝑖𝑎𝑥𝑖𝑠 then dist = 𝐸𝑙𝑙𝑖𝑝𝑠𝑒𝑚𝑖𝑛𝑜𝑟𝑠𝑒𝑚𝑖𝑎𝑥𝑖𝑠 end if rot ← random[0°, 360°] �̂�𝑖 ← 𝑃𝑖 + distance (𝐷𝐼𝑆𝑇) in direction (𝑅𝑂𝑇) end while calculate descriptor matching score: 𝑆𝑖(�̂�𝑖) end for 𝑆 𝑘 = 𝑚𝑎𝑥(𝑆𝑖(�̂�𝑖) … 𝑆𝑁(�̂�𝑁)) end for return 𝑆𝑀𝐴𝑋 = 𝑚𝑎𝑥 (𝑆 ) sequential monte carlo: hist input: 𝑃𝐶𝑀 𝐼 , 𝑃𝐶𝑀 𝐼𝐼 (center of mass of i and ii joint) 𝑃𝑖 𝐼 … 𝑃𝑁 𝐼 (initial pseudo-random points i joint) 𝑃𝑗 𝐼𝐼 … 𝑃𝑁 𝐼𝐼 (initial pseudo-random points ii joint) 𝑆𝑖(𝑃𝑖 𝐼, 𝑃𝑖 𝐼𝐼) … 𝑆𝑁(𝑃𝑗 𝐼, 𝑃𝑗 𝐼𝐼) (descriptor matching scores) output: new points (�̂�𝑖 𝐼, �̂�𝐽 𝐼𝐼) with 𝑆𝑀𝐴𝑋 rot ← normal direction to 𝑃𝐶𝑀 𝐼 𝑃𝐶𝑀 𝐼𝐼̅̅ ̅̅ ̅̅ ̅̅ ̅̅ ̅ 𝑃𝑖 𝐼 … 𝑃𝑁 𝐼 = 𝑃𝑖 𝐼 … 𝑃𝑁 𝐼 ∗ 𝑅𝑂𝑇 𝑃𝑗 𝐼𝐼 … 𝑃𝑁 𝐼𝐼 = 𝑃𝑗 𝐼𝐼 … 𝑃𝑁 𝐼𝐼 ∗ 𝑅𝑂𝑇 for 𝑘 ← 1 to max number of iterations do for 𝑖 ← 1 to 𝑁 do for 𝑗 ← 1 to 𝑁 do saved 𝑆𝑖𝑗(𝑃𝑖 𝐼, 𝑃𝑗 𝐼𝐼) in matrix 𝑆𝑇𝑂𝑇[𝑖, 𝑗] end for end for 𝑠𝑜𝑟𝑡(𝑆𝑇𝑂𝑇) → 𝑆𝑇𝑂𝑇[1. . 𝑁 2 ] dist(i, j) ← |𝐸𝑙𝑙𝑖𝑝𝑠𝑒𝑚𝑖𝑛𝑜𝑟𝑠𝑒𝑚𝑖𝑎𝑥𝑖𝑠 ∗ log (𝑆𝑖𝑗)| if dist > 𝐸𝑙𝑙𝑖𝑝𝑠𝑒𝑚𝑖𝑛𝑜𝑟𝑠𝑒𝑚𝑖𝑎𝑥𝑖𝑠 then dist = 𝐸𝑙𝑙𝑖𝑝𝑠𝑒𝑚𝑖𝑛𝑜𝑟𝑠𝑒𝑚𝑖𝑎𝑥𝑖𝑠 end for 𝑖, 𝑗 ← 1 to 𝑁 do �̂�𝑖 𝐼 ← 𝑃𝑖 𝐼 ± distance (𝐷𝐼𝑆𝑇) in direction (𝑅𝑂𝑇) �̂�𝑗 𝐼𝐼 ← 𝑃𝑗 𝐼𝐼 ± distance (𝐷𝐼𝑆𝑇) in direction (𝑅𝑂𝑇) calculate descriptor matching score: 𝑆𝑖(�̂�𝑖 𝐼, �̂�𝑗 𝐼𝐼) end for 𝑆 𝑘 = 𝑚𝑎𝑥(𝑆𝑖𝑗(�̂�𝑖 𝐼, �̂�𝑗 𝐼𝐼) … 𝑆𝑁𝑁(�̂�𝑁 𝐼 , �̂�𝑁 𝐼𝐼)) end for return 𝑆𝑀𝐴𝑋 = 𝑚𝑎𝑥 (𝑆 ) https://doi.org/10.1029/jc094ic05p06177 https://doi.org/10.1145/31336.31338 https://doi.org/10.48550/arxiv.1611.08134 http://dx.doi.org/10.1007/978-1-4757-3437-9 https://doi.org/10.1007/978-1-4471-6296-4_8 https://doi.org/10.1068/p2896 https://docs.openvino.ai/latest/omz_demos_pedestrian_tracker_demo_cpp.html https://docs.openvino.ai/latest/omz_demos_pedestrian_tracker_demo_cpp.html https://doi.org/10.1109/iccv.2015.133 acta imeko, title acta imeko issn: 2221-870x february 2015, volume 4, number 1, 5 10 template for an imeko event paper thomas bruns1, dirk röske1, paul p.l. regtien2, francisco alegria3 1 physikalisch-technische bundesanstalt, bundesallee 100, 38116 braunschweig, germany 2 measurement science consultancy, julia culpstraat 66, 7558jb hengelo, the netherlands 3 instituto de telecomunicações and instituto superior técnico/universidade técnica de lisboa, av. rovisco pais 1, 1049-001 lisbon,portugal shape \* mergeformat shape \* mergeformat section: research paper keywords: journal; template; imeko; microsoft word citation: thomas bruns, dirk röske, paul p.l. regtien, francisco alegria, template for an imeko event paper, acta imeko, vol. 3, no. 1, article 1, january 2014, identifier: imeko-acta-03 (2014)-01-01 editor: paolo carbone, university of perugia, italy received month day, year; in final form month day, year; published january 2014 copyright: © 2014 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: this work was supported by measurement science consultancy, the netherlands corresponding author: paul p.l. regtien, e-mail: paul@regtien.net shape \* mergeformat 1. introduction the introduction describes the background of the research, a short review of related research published in recent literature, together with the major claims setting the framework of the present publication. references should be related to the present publication, not just a list of papers merrily showing the authors’ knowledge of the literature. this relation must be made explicit. the newly presented method is shortly introduced, as an alternative to previously published methods, with mention of the advantages aimed at. to minimize editing by the editorial team, authors are asked to follow the styles that go with this template. the font size of the whole body text is 10 pt and the font type is garamond. for the header of a section use the style named 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referring to figures and tables in the text if automatic caption numbering is used the author should refer to the figures and tables in the text using automated references. a reference can be inserted in a given point in the text by going to the menu "references" and choosing "insert cross reference" (figure 4 ref _ref312314316 \h \* mergeformat ). select which figure or table are to be cited, the label type ("figure" or "table") and that only the label and number should be used in the citation. keep the option "insert as hyperlink". 5. about equations all equations should be numbered consecutively throughout the paper. do not use outline numbering per section. numbers are placed between parentheses aligned right, and without a label, see equation (1) as an example, expressing the saturation current id in a mosfet transistor [2]: (1) where w is the channel width, l the channel length, (0 the dielectric constant of free space and (ox of the oxide, ( is the mobility in the channel, t the oxide thickness and vgs the gate voltage [2]. make sure that all symbols are defined unambiguously. when confusion may arise, add the units of the parameters between square brackets. use si and derived units only [3]. select the style named “equation” for proper margins between the text and positioning of the equation number (insert a tab right after the equation box). table 1. overview of styles and font sizes used in this template. section font size [pt] format special title calibri 20 bold only first letter is capital authors calibri 12 bold affiliation and email address calibri 9 italic abstract text calibri 9 normal keywords calibri 8 normal label in bold citation calibri 8 normal label in bold editor calibri 8 normal label in bold dates calibri 8 normal label in bold copyright calibri 8 normal label in bold funding calibri 8 normal label in bold corresponding author calibri 8 normal label in bold first-level section headings calibri 10 bold numbered, all caps subsection headings calibri 9 bold outline numbered body text garamond 10 normal justified acknowledgements, appendix garamond 10 normal as body text equations garamond/symbol 10 italic numbered equations (subscript/superscript) garamond/symbol 70% of 10 italic numbered equations (sub-subscript/superscript) garamond/symbol 60% of 10 italic numbered table text calibri 8 normal bold headings figures calibri 9 normal centered captions of figures and tables calibri 8 normal justified references garamond 9 normal numbered long equations that normally span more than one column should be wrapped over more lines, broken at a suitable place by arithmetic symbols (=, +, (, () as separator. an example is equation (2) about the surface heat flux per unit area along a flat plate [4]. (2) equations are considered part of the previous sentence and should, when appropriate, have a period or a comma after them, as in (2). very short equations that are not further referred to may be inserted in line with the text, for instance r = v/i. make sure that all variables are in italic, also when used in the main body. note that the font size for equations is 10 pt and that it must be reduced to 70% and 60% in the case of subscript/ superscript and sub-subscript/superscript respectively. 6. about references and citations references are limited to published works or papers that have been accepted for publication and should give full bibliographical information. they are placed in the section references at the end of the manuscript, in order of their appearance in the text. references are cited in the text by a number between square brackets. ensure that every reference cited in the text is also present in the reference list and vice versa. unpublished results and personal communications may be included in the reference section following the standard reference style and should include a substitution of the publication date with either "unpublished results" or "personal communication". citation of a reference as "in press" implies that the item has been accepted for publication. the format of references is as follows: a. for journal articles: initials and last name(s) of each author, title of article (first word only capitalized), journal title, volume number, (year), pages. b. book references: author(s) as above, title of book (main words capitalized), publisher, city of publication, year, isbn. c. for a chapter in an edited book: author(s) as above, title of article (first word only capitalized), in: title of book (main words capitalized). editor(s). publisher, city of publication, year, isbn, pages. d. conference proceedings: initials and last names of each author, title of article (first word only capitalized), name of the conference, place, country, year, pages. there is a section break at the end of the paper (after the references) so that the content of the last page is equally divided between the two columns. 7. conclusions the concluding section contains the major achievements of the research presented in the manuscript. it should be concise but informative. when numerical results are an essential part of the research, for instance a wider measurement range, higher uncertainty [5], they should be included in the conclusions. notice that conclusions are not the same as an abstract. acknowledgement here persons or institutes may be acknowledged for their technical, scientific or financial support. list them in this section, and not as a footnote or otherwise. references [1] m. fazio, s.l. rota, metrology on stamps, phys. educ. 30 (1995) pp. 289-297. [2] s. middelhoek, s.a. audet, silicon sensors, academic press, london, 1989, isbn 0-12-495-051-5. [3] k.t.v. grattan, “measurement: system of scales and units”, in: concise encyclopedia of measurement and instrumentation. l.finkelstein, k.t.v.grattan (editors). pergamon press, oxford, 1994, isbn 0-08-036212-5, pp. 209-214. [4] m.j. lighthill, “contribution to the theory of heat transfer through a laminar boundary layer”, proc. of roy. soc., london, 1950, a 202, pp. 359-377. [5] v. pop, p.p.l. regtien, h.j. bergveld, p.h.l. notten, j.h.g. op het veld, “uncertainty analysis in a real-time state-of-charge evaluation system for lithium-ion batteries”, proc. of 18th imeko world congress, sept. 17-22, 2006, rio de janeiro, brazil, pp. 164-166. abstract the editorial team of acta imeko encourages authors to follow the instructions as described in this template file to produce their manuscript. the abstract should be composed in a way suitable for publication in the abstract section of electronic journals, and should state concisely what it is written in the paper. important items are the aim of the research, the basic method and the major achievement (also numerically, when applicable). the length should not exceed 200 words. 1 acta imeko | www.imeko.org january 2014 | volume 3 | number 1 | 1 _1465198629.unknown _1465198630.unknown 2 0 2 oxgs d wv i lt mee = ( ) ( ) ( ) ( ) 1 1 1 2 3 '' 1 3 1 1 1 0 pr 0.538 . w ff xx w w x x px tx ddx x t k um tx x m æö æö =´ ç÷ ç÷ èø èø ìü ¶ ïï ´ íý ¶ ïï îþ òò editorial to selected papers from the imeko tc24 special issue "measurements in cultural heritage" acta imeko issn: 2221-870x december 2022, volume 11, number 4, 1 2 acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 1 editorial to selected papers from the imeko tc24 special issue "measurements in cultural heritage" tatjana tomić1, leonardo iannucci2, leila es sebar2 1 ina industrija nafte, lovinčićeva 4, zagreb, croatia 2 department of applied science and technology, politecnico di torino, corso duca degli abruzzi 24, 10129 torino, italy section: editorial citation: tatjana tomić, leonardo iannucci, leila es sebar, editorial to selected papers from the imeko tc24 special issue "measurements in cultural heritage", acta imeko, vol. 11, no. 4, article 3, december 2022, identifier: imeko-acta-11 (2022)-04-03 received december 21, 2022; in final form december 21, 2022; published december 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: leonardo iannucci, e-mail: leonardo.iannucci@polito.it dear readers, this special issue, promoted by the imeko tc24 – chemical measurements, includes four papers focused on the topic of “measurements in cultural heritage”. this field of application is particularly challenging for researchers focused on chemical analyses because it combines traditional issues related to measurement science such as traceability and uncertainty with the constraints related to the world of cultural heritage. actually, when working on historical or archaeological artifacts, sampling is generally not always possible, so non-destructive analyses are preferred. moreover, additional challenges are related to the development and use of portable instrumentation, which is required whenever the artifact under study can not be moved to the laboratory. for these reasons, many researchers in the field of chemical measurements work specifically on archaeometry and on various topics related to cultural heritage. the aim of this special issue is to present some of the possible approaches for this application field, stimulating new studies and new collaborations in this challenging yet fascinating research topic. in the paper “a morphological and chemical classification of bronze corrosion features from an iron age hoard (tintignac, france): the effect of metallurgical factors” [1], giorgia ghiara and co-authors investigate the corrosion products formed on archaeological bronzes coming from a celtic deposit located in central france. the use of a multi-analytical protocol based on image analysis, electron microscopy, and raman spectroscopy allowed the researchers to identify different corrosion morphologies, which were then related to different metallurgical factors such as alloy composition, microstructure, degree of deformation, and grain size. in the paper “photogrammetry and gis to investigate modern landscape change in an early roman colonial territory in molise (italy)”[2], manuel j. h. peters and tesse d. stek present a composite image-based modelling workflow to generate 3d models, historical orthophotos, and historical digital elevation models from images acquired decades ago. the proposed workflow was then applied to improve the interpretation of survey data from the early roman colony of aesernia (south of italy). in the paper entitled “roman coins at the edge of the negev: characterisation of copper alloy artefacts and soil from rakafot 54 (beer sheva, israel)” [3] manuel j. h. peters and co-authors report about the preliminary nonand semi-destructive analysis of copper alloys, corrosion products, and soil components from a roman archaeological site in israel. the use of a multianalytical approach based on x-ray fluorescence, x-ray diffraction, scanning electron microscopy, and micromorphological analyses allowed the researchers to characterise the corrosion products and to correlate them to the soil composition. the fourth paper is entitled “reversible protective and consolidating coatings for the ancient iron joints at the acropolis monuments” [4]. g. frantzi and co-authors present a corpus of measurement methodologies used during a multidisciplinary project, aiming at the protection and conservation of ancient steel joints of the acropolis monuments, undertaken by the acropolis restoration service (ysma). different coating systems were applied on metal coupons, characterising their physicochemical properties and the protection performance against accelerated corrosion test. then a 1-year test was carried out, exposing the coated samples outdoor at the monument site. results allowed the researchers to propose one of the tested coating systems as candidate to protect metal artifacts from atmospheric corrosion. mailto:leonardo.iannucci@polito.it acta imeko issn: 2221-870x december 2022, volume 11, number 4, 2 3 acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 2 this brief excursus about the papers composing this special issue shows the great variety of themes and approaches encountered when dealing with “measurements in cultural heritage”. we hope that these articles will stimulate your interest and that you will enjoy your reading. tatjana tomić leonardo iannucci leila es sebar special issue editors references [1] giorgia ghiara, christophe maniquet, maria maddalena carnasciali, paolo piccardo, a morphological and chemical classification of bronze corrosion features from an iron age hoard (tintignac, france): the effect of metallurgical factors, acta imeko, vol. 11, no. 4, article 11, december 2022. doi: 10.21014/acta_imeko.v11i4.1278 [2] manuel j. h. peters, tesse d. stek, photogrammetry and gis to investigate modern landscape change in an early roman colonial territory in molise (italy), acta imeko, vol. 11, no. 4, article 12, december 2022. doi: 10.21014/actaimeko.v11i4.1284 [3] manuel j. h. peters, yuval goren, peter fabian, josé mirão, carlo bottaini, sabrina grassini, emma angelini, roman coins at the edge of the negev: characterisation of copper alloy artefacts and soil from rakafot 54 (beer sheva, israel), acta imeko, vol. 11, no. 4, article 13, december 2022. doi: 10.21014/actaimeko.v11i4.1285 [4] giasemi frantzi, michail delagrammatikas, olga papadopoulou, charalampos titakis, eleni aggelakopoulou, panayota vassiliou, reversible protective and consolidating coatings for the ancient iron joints at the acropolis monuments, acta imeko, vol. 11, no. 4, article 14, december 2022. doi: 10.21014/actaimeko.v11i4.1341 https://doi.org/10.21014/acta_imeko.v11i4.1278 https://doi.org/10.21014/actaimeko.v11i4.1284 https://doi.org/10.21014/actaimeko.v11i4.1285 https://doi.org/10.21014/actaimeko.v11i4.1341 microsoft word 270-1892-1-pb (1)-rev2 acta imeko  issn: 2221‐870x  december 2015, volume 4, number 4, 82‐87    acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 | 82  accuracy of railway track conductance and joint efficiency  measurement methods  jacopo bongiorno 1 , andrea mariscotti 2   1  università di genova, via opera pia 11a, 16145 genova, italy  2  astm sagl, via comacini 7, 6830 chiasso, switzerland    section: research paper  keywords: conductivity measurement; dc power systems; electric variables measurement; grounding; guideway transportation power systems; guideway  transportation testing; stray current; uncertainty  citation: jacopo bongiorno and andrea mariscotti, accuracy of railway track conductance and joint efficiency measurement methods, acta imeko, vol. 4,  no. 4, article 15, december 2015, identifier: imeko‐acta‐04 (2015)‐04‐15  editor: paolo carbone, university of perugia, italy  received april 12, 2015; in final form october 29, 2015; published december 2015  copyright: © 2015 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  corresponding author: andrea mariscotti, e‐mail: andrea.mariscotti@astm‐e.ch  1. introduction  the impact of stray current is mitigated first of all by limiting the rail-to-earth conductance (in the following indicated by gre when expressed per unit length, so as conductivity) and this clearly stated in the applicable standard for protection against stray current effects in dc systems iec 62128-2 [1]. the rail-to-earth conductance of a track section depends on several factors, characteristic of the track (type of fasteners and sleepers, use of isolating materials and their performance, type of ballast and sub-ballast, etc.) or external to the system (e.g. type of soil) and also depending on environmental conditions (wet or dry soil, moisture percentage, pollutants, ageing, temperature). all these factors hardly can be thoroughly modelled and accurately predicted, and the measurement assumes a paramount importance, not only to determine if the system meets the limits (dictated by standards or set in the contract), but also the suitability of different provisions and countermeasures. the control of track conductance is first implemented by dividing the track into electrically independent sections [2]. the track may feature different constructive characteristics depending on the type of support and infrastructure (tunnel, cut & cover, viaduct, station, etc.), and the provisions adopted for one may not be optimized for the other [3] to [5]. moreover, such track portions shall in principle allow separate testing, with distinct values of gre. for these reasons mechanical insulating rail joints (irjs) are used and their correct and satisfactory operation shall be verified, determining the socalled joint efficiency (je), that is the complement to 100 % of the percentage of rail current leaking through the irj itself. track isolation and track grounding are two opposite requisites when designing for compliance to two different, but connected, problems: stray current protection and electrical safety of track and wayside installations. the two standards iec 62128-2 [1] and iec 62128-1 [6] address them, the former limited to dc systems (stray current is much less relevant for ac systems). to simplify, we might say that corrosion and stray current are relevant in the long term (thus in terms of expected system life, maintenance and monitoring, damage to external third parties), while electrical safety, grounding and bonding, are technical requirements enforced at any time. abstract  the iec 62128‐2 (en 50122‐2) indicates methods for the measurement of rail‐to‐earth conductance and insulating rail joint efficiency  in dc electrified railways. these methods are reviewed, considering the influence of system parameters on the accuracy and variability  of  results.  this  work  reports  characterization  and  quantification  of  uncertainty  and  limits  of  validity,  as  well  as  some  practical  considerations on the execution of measurements. whereas track conductance is characterized by acceptable uncertainty in the range  of one to few %, insulating joint efficiency test is deemed by a larger and variable uncertainty, so that countermeasures and provisions  are necessary.  acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 | 83  the assessment of the compliance of the design and installation of a new railway system is particularly important in the preliminary stage of a project, when pilot tracks are laid down and construction techniques, as well as provisions and countermeasures, are verified, checking effectiveness, cost, implementation and harmonization. to this aim the experimental determination of track quantities to compare with constraints and prescriptions assumes a primary role: suitability of the measurement technique, embodiment in a standard, accuracy and sensitivity are all factors to consider when preparing the test procedures and the test track setup on site, and when technical discussions between contractors and customer take place. this work focuses on the measurement methods for track conductance (intended as per-unit-length conductance, or conductivity, measured in -1m) and joint efficiency appearing in appendix a of iec 62128-2 (equivalent to en 50122-2), considering the spread of results in real conditions for the expected variability of system parameters; technical issues are also considered for the proposed improvements. 2. measurement methods appearing in iec 62128‐2  in this section the two methods for the measurement of track conductivity and joint efficiency are reviewed, setting the basis for the following analysis and discussion. 2.1. standard method for track conductivity measurement  the measurement of track conductivity is described in app. a, sec. 3, of iec 62128-2. the method, whose setup is shown in figure 1, needs two insulating rail joints (irjs) that electrically separate the running rail. the standard requires that a dc voltage source is applied across the first irj (irj-1) and that a nearly constant current flows. the current is proportional to the conductance to earth of the track section under measurement between the two irjs (with the term “track section” we will identify either a single running rail or both rail, without explicit distinction if it is clear or not determinant). the quantities that are measured from the test circuit are: the test current i leaving the source terminal, the voltage to ground vp at the voltage terminal located in p. this voltage is in reality measured against a good enough ground connection (e.g. vertical electrode) or the reference grounding circuit, with respect to which the track conductance is to be determined (e.g. concrete mesh, stray current collector, etc.). due to contingencies and practical issues, the measurement execution may be subject to some constraints, related to: availability of a long enough track behind the source irj (irj-1) or that this track is solidly grounded to close the measurement circuit; the voltage terminal within the section may be moved along it, to ease access to the reference ground potential point, and – it is observed – its position is subject to a minimum distance dmin = 50 m from the first irj, but no maximum length is prescribed. the standard indicates also a maximum length of the section lmax = 2 km. both constraints are aimed at impeding too a favourable configuration, that would lead to underestimation of track conductivity as we will see in section 3.1. the injection point distance from irj x is not constrained, but it is reasonable to assume that it is quite close to the irj under test; by the way, it is possible to show that changing it does not influence appreciably the total amount of current flowing in the left and right rail sections in parallel. rxlxinj re rx re lx zzz xlg z xg z ,,,, // )( 11    . (1) equations in (1) describe the relationship between equivalent impedances seen by injection point, respect right and left parts of the system, and it is evident that zinj keeps almost constant with the left term reducing and the right term increasing, while x moves from left (minimum distance from origin) to right (nearly full length l). the conductivity to earth is estimated, following indications in iec 62128-2 as lv i g p re  , (2) where i is the total current at the injection point, l is the section length, vp is the voltage at the point p. 2.2. standard method for joint efficiency measurement  the method for the measurement of joint efficiency appears in app. a, sec. 5, of iec 62128-2. the method, whose setup is shown in figure 2, needs two insulating rail joints (irjs): one is the joint under test (jut), the other one at the other end electrically separates the rail section. a dc voltage source e is applied across the jut irj-1 and the current flowing behaves as described at the beginning of section 2. the standard specifies that two longitudinal voltage drops across 10 m of rail are measured at the left and the right of the injection point and the ratio of the voltages gives the joint efficiency (je) (3). before using the measured voltages, they are corrected for the off-voltages, i.e. the voltages measured once the source voltage is switched-off. %100 )()( ,2,2,1,1 ,2,2     offonoffon offon uuuu uu je . (3) in reality, because the longitudinal resistance of the rail is a constant, the same equation holds also for the current terms flowing in the two voltmetric sections. the limit for je established by the standard is 95 %. the length lv = 10 m for the measurement of the longitudinal voltage drop is dictated by the standard, as well as figure 2. test setup for joint efficiency as per iec 62128‐2 annex a.5.  figure 1. test setup for track conductivity as per iec 62128‐2 annex a.3.   acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 | 84  the position of the current injection point, immediately after the first two voltage terminals, that is at a distance slightly larger than 10 m. while the resulting je is not influenced by gre, the value of lv is quite relevant. 3. variability of results  the expressions reported in the previous section are now analyzed with respect to plausible parameter variations, solving an equivalent circuit which represents the considered system, in order to check their validity and the spread of results. 3.1. track conductance per‐unit‐length  the expression for the calculation of per-unit-length track conductance from raw measurements (2) relies implicitly on the fact that the whole test current i flows past the voltage terminal, so that the only correction to apply suggested in the standard is for the voltage drop in the running rail. the effect of the fraction of the test current leaving the rail through the conductance-to-earth of the left section before the voltage terminal at p is not taken into account (see figure 3). if the distance of the voltage terminal d from irj-1 were much less than the total track section l, then this phenomenon represents a minor source of error. the rail section is modelled as a ladder network of resistance and conductance elements, rr,i and gr,i, respectively, each determined by the per-unit-length values multiplied by the equivalent length of the circuit cell; r0 represents the equivalent resistance of track sections behind irj-1. the equivalent circuit may be simplified for any point p by calculating the thevenin equivalent (ep in series to zp) for the left part (going iteratively from the source to the point p) and the series-parallel equivalent resistance (zp) for the right part (going backwards from the last element n to the point p). the details of calculations are omitted. the variability of the estimated conductivity with section length l and position p of the voltmetric measurement point is analyzed in figure 4. the attention is focused on five different track-section lengths l, spanning from a minimum of 250 m (typical of depots and some stations with short trains, such as metro and light railways) to the maximum allowed by the standard lmax = 2 km. rail resistivity, ground conductivity and r0, used in the calculation are assumed and kept constant. the equivalent circuit is solved moving the voltmetric measurement position p and varying the distance from the irj from the minimum dmin = 50 m to a practical maximum distance corresponding to half of the length l. the estimated conductivity gre decreases with l and increases with d. assuming an absolute validity of the estimated gre independent on l, the spread of values at the minimum distance of 50 m is nearly 1 % and slightly smaller moving away from the injection point, i.e. with the distance of the voltage terminal d > dmin. the dependency on position p is limited to about 0.2 % relative for each l. the tracks behind the irj under test (irj-1) and closing the circuit for the current to flow through the soil are modelled with an equivalent resistance r0 that is in series to the track section subject to measurement. the influence is evaluated in figure 5, where r0 ranges from 0.1  to 20  and the estimated gre values are plotted for three different track section   (a)    (b)  figure  4.  estimated  conductivity  for  various  test  section  length  values  l (from top to bottom: 250 m black, 500 m blue, 1 000 m green, 1 500 m red,  2 000 m yellow) vs. the position p of the voltmetric v terminal (that moves  from 50 m up to l/2): rail conductivity gre is (a) 10 ms/km, (b) 100 ms/km,  r0 = 1 ω, rr = 20 mω/km.  figure  5.  estimated  conductivity  as  a  function  of  the  assumed  equivalent  resistance  r0  of  tracks  behind  irj‐1  vs.  varying  voltmetric  v  terminal  position p between 50 m and l/2: r0 = 0.1 ω, 1 ω, 10 ω and 20 ω (thinner to  thicker in each group of four curves); gre = 10 ms/km, rr = 20 mω/km, l = 500 m (blue), 1 000 m (green) and 1 500 m (red).  figure  3.  equivalent  circuit  of  the  rail  section  between  irjs  with  voltage source  e  across  irj‐1  and  voltmetric  terminal  at  point  p;  the  reference potential common to all conductance elements and r0 is ground.   acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 | 85  lengths l = 500 m, 1000 m and 1500 m. low values of r0 indicate a well earthed long track section behind the first joint; conversely large values are typical of short track sections (maybe accidentally sectioned) and hence a more difficult path for the return current of the source. in figure 5 it can be seen that for short test sections and large r0 values the estimated gre may be larger than the real one, but in general the error is always to reduce the estimated gre, that is on the wrong side with respect to limits. however, the error is always well below 1 % and r0 = 20 ω represents already a worst case. the estimated conductivity depends slightly also on the rail longitudinal resistance rr, that causes a longitudinal voltage drop due to the flowing test current: this is investigated in figure 6, where three track lengths l are considered fixing r0 = 5 ω and gre=10 ms/km. it is observed that the estimated gre is larger when the rail resistance rr is smaller; the used rr values are characteristic of largeand medium-gauge running rails and the resulting spread of values is around 1 %. 3.2. joint efficiency  assuming an ideal irj with infinite resistance, the application of (3) for different lv brings to values of je (in principle 100 %) that are easily as low as 85 % to 90 %, thus leading to a failed test and a rejected irj. we call this as “setup limit of je” and cannot be lower than 95 %; moreover, it should be higher than that, in order to accommodate some tolerance for less-than-ideal joints, measurement uncertainty and round-offs. it is underlined that while the resistance of a joint is a unique value, joint efficiency depends also on track characteristics: a joint isolates a rail section better if the rail-to-earth conductance is large. a simulation with the simple equivalent circuit shown in figure 3 was done for lv = 2.5 m, 5 m, 10 m and 20 m and the results are reported in figure 7. having reduced the length of the voltmetric sections to 5 m, first, and then to 2.5 m, the measured efficiency was increased above the limit of 95 %. however, reducing lv has the undesired effect of reducing the amplitude of the measured drops u1 and u2, reducing the signal-to-noise ratio and going quite close to the sensitivity of the used instrumentation. preexisting voltages and offsets may be easily compensated for. with lv = 10 m as prescribed, to bring the setup limit of je above 95 %, the required rail section length l shall be at least 200 m. in practical situations this may not be feasible, as for shunting yards and depots of light railways and metros, where trains are never very long and turnouts and shunts are often separated by 150 m to 200 m of track. another practical factor that may in turn limit the reduction of lv is the safety requirement of using selv voltage, which for dc is 60 v. for a good rail with low conductivity to ground (e.g. 10 ms/km or lower [7], [8]) this would imply 0.48 a/km, or 48 ma every 100 m. with a longitudinal rail resistance in the range of 20 /m to 50 /m, the resulting voltage drop across the voltage terminals separated by lv meters, would be 9.6 v to 24 v reading for lv = 10 m and l = 200 m, reducing to 4.8 v to 12 v, when lv is reduced to 5 m. a microvoltmeter and repeated readings shall be adopted, in order to reduce unavoidable fluctuations; it is observed that offvoltages are large enough to affect the accuracy of such readings. it is underlined that the intensities of injected current suggested by the standard are not attainable if the selv voltage limit is enforced; to reach some ampère of intensity for moderate track-section length, the voltage of the dc source shall be increased to more than a hundred volts. the problem is now considered the other way round, that is by assigning irjs a known resistance (1 k, 10 k and 100 k, simulating an increasing efficiency) and a sensitivity study is made to identify which parameters influence the so-determined je value, causing the “setup limit of je” (see figure 8). for high-efficiency irjs (i.e. with equivalent resistance larger than 100 k) large track-conductivity values do not interfere with the test and the resulting je values are sufficiently large, provided that the test track is long enough, i.e. longer than about 200 m minimum. on the contrary, less efficient irjs cannot be satisfactorily measured and variability of track conductance has a negative impact on test accuracy. 4. other practical considerations  besides the evaluation of expressions, also the measurement methods are considered, as far as their applicability and possible improvements are concerned. 4.1. dc source and extraneous voltages  it is required to use a dc voltage source. this is quite convenient for the availability of dc voltages of 12 v, 24 v and figure  7.  joint  efficiency  as  resulting  from  the  application  of  iec  62128‐2  equation and for three different values of lv : 2.5 m and 5 m (brown upper  curves), 10 m (thick red curve), 20 m (yellow lower curve). r0 = 10 ω, rr =  35 mω/km, gre = 10 ms/km.  figure  6.  estimated  conductivity  for  different  rail  longitudinal  resistance values  rr  =  20  (solid),  40  (dashed),  60  (dotted)  mω/km  vs.  voltmetric  v terminal  position  p  between  50  m  and  l/2:  gre  =  10  ms/km,,  l  =  500  m  (blue), 1 000 m (green) and 1 500 m (red), r0 5 ω.  acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 | 86  48 v from batteries of any size, freeing the operator from mains sockets. dc measurements, however, are normally affected by extraneous voltages developing because of electrochemical reactions in the soil and junctions between different metals. two consecutive readings with polarity reversal is the usually adopted solution; the standard unfortunately does not include such possibility, in favour of a simpler additional reading in off conditions, aiming to tackle the problem of pre-existing extraneous voltages. this technique, in principle, is not able to reproduce the same conditions that occur during on-time intervals, when the source voltage is applied and current is flowing. however, practically speaking, off-voltage and reversed polarity readings are quite in agreement, based on experimental observations. for example, for a track-conductance measurement the two positive and negative voltages were +50.4 v and 49.9 v, with an offvoltage of 0.44 v, so quite close to their difference. 4.2. polarization  when applying dc voltage in the “on” interval, quite rapidly polarization occurs in the soil and in the other electrolytic substances that may be dissolved in water and moisture trapped beneath the rails and in the fastening system. when the source voltage is disconnected (as rapidly as possible), the voltage readings during the “off” interval are quite critical. the standard does not say anything for the track-conductivity measurement, but for the joint-efficiency measurement it specifies “directly after the switching off”. practical experience confirms that this might be interpreted by the operator as “a fraction of a second” or “some seconds” after switching off, depending also on his/her speed in storing values, taking notes, etc.. when a data logger is used minimizing delays, it may be observed that the off-voltage due to polarization varies rapidly in the first 3 to 5 seconds, with the exact time instant of the measurement representing a source of uncertainty. when measuring joint efficiency at very low voltage values, the off voltages that are subtracted from the on voltages may become a significant percentage, ranging on average from some % to up to 30 %. varying of a few seconds the instant of off-voltage reading has an overall impact on the final result of about 1 % to 15 % based on experience. in extreme cases the “off” voltage may be nearly equal to the “on” voltage. in table 1 there are reported sample readings made at low temperature (3 °c to 4 °c), with wet soil conditions and humidity around 50 % for the first three measurements (due to soft rain the day before) and the fourth one with some drier soil (measurement taken the following day). observing the values reported for 2 s and 5 s interval after switching off, various behaviours may be outlined: voltage drops by 50 % (meas. 1), by only 30 % (meas. 3), in between these values (meas. 4) or does not appreciably change (meas. 2). this means that there is no rule to follow to compensate for off-voltage readings taken accidentally at different instants of time and that the method of measurement should be more detailed. 5. conclusions  the methods were considered for the experimental determination of per-unit-length track conductance to earth and joint efficiency, as reported in the iec 62128-2 standard. this standard was reiterated until the most recent version in 2012, so that the methods may be considered mature. it was shown that the track-conductance method exhibits a variability with respect to the considered parameters (length of test section, rail resistance, rail-to-earth conductivity and resistance-to-earth of the tracks behind the test section) that is limited to 1 %, in general causing underestimation of the real conductance value. for the joint efficiency method the variability is much larger and the method itself in many cases (especially for short test sections of less than 200 m) is unable to determine satisfactorily joint efficiency, if the limit of 95 % given in the iec 62128-2 is considered, thus causing rejection of good rail joints. the analysis shows which parameters of the setup influence the effectiveness of the method. however, the offered solution of reducing the separation of voltmetric terminals on the two sides of the current injection point reduces the voltage drop intensity, and shall be evaluated against the non-negligible intensity of disturbing external voltages and polarization. the off-voltage reading after the “on” phase is subject to rapid variations in the first seconds after switch off: this is a relevant source of uncertainty and the standard does not establish a clear method, nor the exact amount of time to elapse after switch off to take off-voltage readings. when the length of the test track section is such to cause unavoidable and relevant error, it is proposed to adopt a combination of the following: reduce the voltmetric terminal separation to 5 m and increase the supply voltage to 100 v. references  [1] iec 62128-2, railway applications fixed installations electrical safety, earthing and the return circuit part 2: provisions against the effects of stray currents caused by d.c. traction systems, 2012.   figure  8.  joint  efficiency  with  real  irjs  of  finite  resistance:  5 k,  10 k,   100 k  (from  lighter  to  darkest  line);  hot  and  cold  colours  refer  to gre = 10 ms/km and 100 ms/km, respectively; r0 = 10 ω, rr = 35 m/km.  table 1. polarization for various joint efficiency measurements.     u1 [mv]  u2 [mv]  2s 5s  2s  5s meas. 1  on  0.020  0.290  off  0.006  0.003  0.044  0.025  meas. 2  on 0.021  0.187 off 0.016 0.012  0.040 0.042 meas. 3  on 0.027  0.172 off  0.011  0.08  0.030  0.022  meas. 4  on 0.012  0.095 off  0.011  0.06  0.012  0.09  acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 | 87  [2] k. s. bahra and r. e. catlow, “control of stray currents for d.c. traction systems,” iee conference publication no. 405, electric railways in a united europe, 27-30 march 1995. [3] d. paul, “dc traction power system grounding,” ieee transactions on industrial applications, vol. 38, no. 3, 2002, pp. 818-824. [4] a. ogunsola, a. mariscotti and l. sandrolini, “estimation of stray current from a dc electrified railway and impressed potential on a buried pipe”, ieee transactions on power delivery, vol. 27 no. 4, oct. 2012, pp. 2238-2246. [5] i. cotton, c. charalambous, p. aylott, and p. ernst, “stray current control in dc mass transit systems,” ieee transactions on vehicular technology, vol. 54, no. 2, march 2005, pp. 722-730. [6] iec 62128-1, railway applications fixed installations electrical safety, earthing and the return circuit part 1: protective provisions against electric shock, 2012. [7] shi-lin chen, shih-che hsu, chin-tien tseng, kun-hong yan, huang-yu chou, and tong-ming too, “analysis of rail potential and stray current for taipei metro,” ieee transactions on vehicular technology, vol. 55, no. 1, jan. 2006, pp. 67-75. [8] a. mariscotti and p. pozzobon, “experimental results on low rail-to-rail conductance values”, ieee transactions on vehicular technology, vol. 54, n. 3, may 2005, pp. 1219-1222. template for an acta imeko event paper acta imeko issn: 2221-870x april 2017, volume 6, number 1, 13-19 acta imeko | www.imeko.org april 2017 | volume 6 | number 1 | 13 hybrid backward simulator for determining causal heater state with resolution improvement of measured temperature data through model conformation yukio hiranaka, shinichi miura and toshihiro taketa yamagata university, jonan 4-3-16, yonezawa, 992-8510 yamagata, japan section: research paper keywords: backward simulation; temperature measurement; inverse problem; precision improvement; lsb noise reduction citation: yukio hiranaka, shinichi miura and toshihiro taketa, hybrid backward simulator for determining causal heater state with resolution improvement of measured temperature data through model conformation, acta imeko, vol. 6, no. 1, article 3, april 2017, identifier: imeko-acta-06 (2017)-01-03 section editor: paul regtien, the netherlands received may 28, 2016; in final form march 14, 2017; published april 2017 copyright: © 2017 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: this work was supported by jsps kakenhi grant number 255400006 corresponding author: yukio hiranaka, e-mail: zioi@yz.yamagata-u.ac.jp 1. introduction to estimate heater operation from the data of room temperature change is a typical inverse problem [1], [2]. and it is a kind of ill-conditioned problem because a slight error in the data would extremely disturb the results [3]. however, such a problem is common for measurement and system diagnostics, and an important part of measurement. if the relation between input and output is linear, it is a problem of deconvolution and there are many studies including super-resolution [4]. subtractive deconvolution [5] may be applied if available data are impulsive. unfortunately, temperature change is a long trailing phenomenon and is very sensitive to measurement resolution and error [3]. our idea to tackle this is to consider measurement resolution inevitable and to treat it explicitly by defining a range for each input or output signal. there are two ways to search deconvolution results for range signal models. one is a try and modify method to test some signal within a certain divided range and to judge whether it matches the given output data. it is a forward simulation because it requires many trials, regardless of whether a convergence method is used or not. the other is an inference method starting from the given output data to the input in backward direction. it is a normal method for problem solving, but it is not always possible to find a good solution. however, we may perform a backward simulation in a similar manner to the forward simulation, starting with a range divided output signal, if we can make a backward processing model. there are backward simulation applications in many fields such as process scheduling [6], initial position estimation of physical objects [7], and software debugging [8]. abstract we are developing a backward simulator, which determines the unknown system input from the system output by using a system model. however, its processing time would increase enormously if the simulation model requires the multiple case branching, which is typical for backward simulations. in some target applications, we can use forward simulation processing in the backward data flow with significant reduction of processing time. this paper shows an example of such application to determine system input of heater operation from measured data of room temperature. although the resolution of measurement restricts the performance of the simulation result, we also used the model to improve the resolution of measured data and show its effect to simulation. furthermore, we show the result of reduction of noise caused by quantizing lsb jitters. acta imeko | www.imeko.org april 2017 | volume 6 | number 1 | 14 there exist many difficulties in creating a backward system model. a typical backward simulation requires case branching when it has multiple possible conditions on its backward trace. however, we have experiences to cope with them, and have shown that some physical restriction may effectively reduce the number of case branches [9], [10]. we have been focusing our attention on utilizing temporal model and strict facts such as nonnegative property and causality that any current value must not affect past values. under such conditions, we can perform a backward simulation effectively. in this paper, we show two methods for efficiently performing backward simulation and for effectively improving measurement resolution. the first method is for creating a backward simulator by incorporating a forward simulation model. it extremely suppresses case-branching processing in the example of this paper. the second method is for improving resolution of the given measured data by applying a strict model of the system. hereinafter, we describe the concept of backward simulation and target simulation model in section 1, hybrid implementation of backward integration loop in section 2, measured temperature data and estimation of model parameters in section 3, simulation results for error free temperature data in section 4, effect of quantization error and countermeasure to it in section 5, simulation results for real measurement data in section 6, discussion in section 7, and conclusion in section 8. 2. backward simulation and models here, we explain the concept of backward simulation with an example application of inference of heater activity. figure 1 shows the structure of our forward simulation model for room temperature change caused by an electrical heater. black dots denote branching points, circles with plus sign denote summation points, and arrows denote the direction of physical information flow. a block with z-1 means one sampling time delay, which is a member of the integration loops. we adopted two integration loops because the room temperature increases even after the time the heater is turned off. the first integration loop corresponds to the neighbor of the heater that accumulates heat locally, and the second loop corresponds to the whole room heat reservoir. the parameters a, b are decay factors due to heat transfer, c is the specific heat of the room which converts heat to temperature. figure 1 can be expressed by the following equation, (0 )1 ,1 ( )1 2 1 2 w aq t tiq aq t ti q aq bq  − ≤ < =  − ≤ = −   (1) where 1q and 2q are accumulated heat in the first loop and the second loop, t is time, and the heater of w (w) is on for 0 to ti . we can obtain 2q as { } { } (1 ) (1 ) / ( ) (0 ) 2 ( 1) ( 1) / ( ). ( ) at btw b e a e b b a t ti q at at bt bti iw b e e a e e b b a t ti  − −− + − − ≤ < =  − − − − − − ≤  (2) figure 2 shows the implemented software objects and connection diagram for the simulation of figure 1. each object has a corresponding function, “heater” for heating for a specified time duration, “wsum” for summing heat with wattage input port, “csum” for summing heat, “br” for branching with the same output values, “dly” for one sampling time delay, “co” for coefficient multiplier, and “temp” for temperature recording. these objects are coded in scala classes, which use a java virtual machine, and dly1 and dly2 are instance objects of the class dly, for example. all the objects have independent gui windows which accept local settings and display status and parameters of each object. data flow on the connection links consist of time and values. as an example, heating power 800 w from “heater” at its output port “o” at the time of 10.0 s is shown in a xml style ucf (universal communication format) message [9], [10] <sim><s>heater<s>o</s></s><t>10.0</t>800.0</sim>, where sim is the simulation controller which redirects this message from “o” port of “heater” to “i” port of “sum1” as specified in the simulator’s connection table [10]. the <s> tag indicates the source of the message. the source tag is nested in this case to indicate the port “o” of “heater.”. in the backward simulation (figure 3), ucf messages flow in the backward direction indicated by dashed lines with reversed arrows. the node “temp” is the starting object which sends the time sequenced temperature in the reversed order, one pair of time and temperature data in each ucf message. the temperature in the backward simulation is expressed by a value range which expresses the minimum and the maximum temperature as “0.0, 10.0”, which means that the temperature is in the range from 0.0 inclusive to 10.0 exclusive [10]. by narrowing the range, we can control the resolution of the simulation. simulation parameter “ndiv” set by the starting block “temp” specifies the number of divisions. as an example, when the whole range is from 0.0 to 10.0 and ndiv is equal to 10, the value 4.5 is expressed in the ucf message as “4.0, 5.0” as the divided range width is 1.0. 3. hybrid simulator and implementation two important features of the simulator are time synchronization and hybrid backward simulation. figure 4(a) i i i i i i i i i f heater wsum br1 co1 1-a dly1 a 1z− dly2 b 1z− csum br2 co2 1/c temp o o o o o1 o o o1 o o2 o2 f figure 2. forward simulation objects and connections corresponding to figure 1. heater 1-a 1 c temp a 1z− b 1z− figure 1. forward simulation model of room temperature. heater 1-a 1 c temp a 1z− b 1z− figure 3. backward simulation model corresponding to figure 1. acta imeko | www.imeko.org april 2017 | volume 6 | number 1 | 15 simplifies a forward integration loop in figure 1. the summation node must gather the same time data from the two incoming links, the input x(i) and the feedback f(i)=y(i-1) to form the output y(i)=y(i-1)+x(i). a fully synchronized simulator will do the work naturally. however, we intend to perform our simulation in a distributed processing environment, and we prefer asynchronous processing as long as it is possible. then, the summation node is designed to have an input record table which keeps time and value pairs received at the left input port (triangle arrowed port in figure 1 up to figure 13). the data message from the feedback port (from the delay node) triggers the summation process by picking up the data of the same time from the input record table. figure 4(b) shows the backward version of figure 4(a). we need to calculate two backward outputs from a single backward input at the summing point, satisfying y(i)=x(i)+f(i). the computation intensive solution is to simulate all the pairs of x(i) and f(i), matching the equation. a practical solution is to perform the simulation for a finite number of divided range pairs, e.g. {x,f} pairs of {0.0 to 2.5, 2.5 to 5.0} and {2.5 to 5.0, 0.0 to 2.5} to match the output of 4.5 [10]. a case branching mechanism is needed for such processing. however, if we apply another solution which uses a forward simulation object in the backward simulation as in figure 5, we can avoid the expansion of processing time caused by the case branching. in figure 5, we can formulate the process as x(i)=y(i)–f(i) and f(i)=y(i+1). the branch node sends received backward data in time reverse order through the two links to the sum node and to the delay node. the same input record table used in figure 4 can be utilized to keep y(i) required by the reverse delayed feedback signal of f(i)=y(i+1). we have to describe the detailed processing of the sum node. in the backward simulation, data flows are expressed as a range (the minimum and the maximum). then, the backward calculation must handle the range information. if the range from the right is (a, b), which means that the minimum value is a and the maximum value is b, and the range from the bottom is (c, d), the output through the left port should be calculated as (a-d, b-c) to cover the broadest value range. however, x(i) must be positive or zero as it expresses heat. if a-d is less than zero, it must be substituted by zero. furthermore, if b-c is less than zero, the case simulated is not a feasible one. figure 6 shows the resulting practical hybrid backward simulator, and figure 7 shows implemented objects and connections. the simulation objects in figure 7 are the same objects as in figure 2, with backward processing capability except dly. the objects “co1” and “co2” divide their incoming backward data by their coefficients, “br1” and “br2” pass through their incoming backward data to their two backward outputs. the simulator in figure 7 needs three consecutive backward inputs to start as the first data stops at csum and the second data stops at wsum because there will be no matching time data coming from the other backward port. we describe here the mechanism of model mismatch detection in detail. sum objects (wsum and csum) in figure 7 have two arriving inputs in the backward simulation. we note the backward input from the right of csum at the time sample of t as ( )v t , the other backward input can be expressed as b ( 1)v t − because dly2 node delays the backward flow signal for one sample time and multiplied by a constant coefficient b. the backward output ( )u t from csum can be expressed as equation (3) and must be positive or zero because it expresses heat. ( ) ( ) ( 1) 0.u t v t bv t= − − ≥ (3) the backward output from wsum can be expressed as the following equation, ( ) ( 1) ( ) ( 1) { (t 1) (t 2)} 1 1 ( ) (t 2) ( ) (t 1) 0.1 u t au t v t bv t a v bv a a v t abv a b v a = − − − − − − − − − − + − − + − = ≥− (4) if the both inequalities are not satisfied for the maximum value of the range at any time point, the backward simulation is failed and the starting condition must not occur for the simulation model. 4. real temperature change and model simulation at first, we verified the correctness of the simulator. figure 8 shows the result of a room temperature measurement when the infrared heater (800 w) on the floor was turned on for the duration from 0 s to 180 s in a tiny room of 3.6 m3. the sensor is a sensirion’s sht71, which has 0.01 degree resolution, placed near the heater at 1 m high in the room. figure 8 also shows a simulated temperature change using the parameters a (0.095/10s), b (=0.9938/10s) and c (24200 cal/deg). to determine these parameters, we assume that the heat loss rate of the room (b) is less than that of the heater appliance (a). then the temperature decline after the x(i) y(i) 1z− f(i)=y(i-1) (a) x(i) y(i) 1z− f(i) (b) figure 4. forward integration loop (a) and backward integration loop (b). heater 1-a 1 c temp a 1z− b 1z− figure 6. hybrid backward simulation model. i i i i i i i i f o o o o o1 o o o1 o o2 o2 f heater wsum br1 co1 1-a dly1 a 1z− dly2 b 1z− csum br2 co2 1/c temp i figure 7. hybrid backward simulation objects and their ports. x(i) y(i+1), y(i) 1z− f(i)=y(i+1) (a-d,b-c) (a,b) (c,d) figure 5. hybrid backward integration loop. acta imeko | www.imeko.org april 2017 | volume 6 | number 1 | 16 temperature peak is estimated by the following equation, log . ( )2q bt const t ti= − + ≤ . (5) we define t p as the peak temperature time and estimate a by the following equation derived from equation (2) , ( ) ( 1) / ( 1).p i ia b t at bte e e− = − − (6) the heat capacity parameter c is estimated as the magnification factor for fitting the measured data with the simulated temperature data which was calculated by using the simulator’s forward function with the estimated parameters a and b. those parameters should be fine-tuned to closely fit the measured data. however, the rear part of the curve in figure 8 cannot be fit by our model. we may need the third heat accumulation loop for representing wall temperature change and heat release to the environment. as will be shown in section 6, the above approximation was almost enough because large measured values around the peak are matched. figures 9 and 10 are the results of the backward simulation when the simulated temperature change in figure 8 was fed backwardly from “temp” node in figure 7. the width of the resultant ranges are shown as the difference between min and max values in those figures. the ndiv parameter was set to 1,000 and 10,000 for figures 9 and 10, respectively. the larger ndiv is, the closer the result is to the actual heater operation (800 w for 0-180 s). as we calculated with a float resolution for the temperature data, we can successfully increase ndiv up to the desired resolution to narrow down the min-max difference. 5. effect of measurement resolution we may not obtain such a good result as in figure 10 for usual resolution data. practically, a good resolution of temperature measurement may be 0.01 °c. if we throw away digits smaller than 0.01 °c from the simulated data in figure 8, the backward simulation will stop because of a model mismatch for ndiv larger than 108, which corresponds nearly to 0.01 °c resolution as the maximum temperature change is 1.2 °c (figure 8). figure 11 shows the backward result for ndiv 108. if we intend to obtain better results by increasing ndiv, we have to improve the resolution of the backward temperature data. figure 12 illustrates our method to improve the resolution, where the value div1 is original resolution and value div2 is half of div1. circular points indicate original a/d truncated values for the resolution of div1. if the resolution is figure 8. measured and simulated temperature change caused by heating for 180 s from the start. figure 11. backward simulation result for ndiv=108 for the resolution limited data. div2 div1 1 2 3 4 5 6 figure 12. resolution increase needs to raise values at some points. figure 9. backward model simulation result for ndiv=1,000. figure 10. backward model simulation result for ndiv=10,000. 0 0.2 0.4 0.6 0.8 1 1.2 1.4 0 1000 2000 3000 4000t em pe ra tu re c ha ng e( de g) time(s) measured simulated 0 1000 2000 3000 4000 5000 6000 0 1000 2000 3000 4000 he at er (w ) time(s) min max 0 200 400 600 800 1000 1200 1400 0 1000 2000 3000 4000 he at er (w ) time(s) min max 0 200 400 600 800 1000 0 1000 2000 3000 4000 he at er (w ) time(s) min max acta imeko | www.imeko.org april 2017 | volume 6 | number 1 | 17 improved to half of the original resolution, it is natural to raise the values at the times of 3 and 5 (indicated by triangle points). so, we wrote a shell script to raise the value when the backward simulation is stopped by detecting a model mismatch. as shown in figure 13, if the backward simulation detects a negative value and stops when calculating (3) or (4), we raise the value by div2 at the corresponding time t of v(t) in the case of (3) and u(t) in the case of (4). the process is repeated to get the whole sequence of backward input data pass the model match test, which means until we get a valid backward result. to improve the resolution further, the repetition is to be done for a new resolution value. there may be a case in which the model match test was passed even if the data modification was not fully done as in figure 12. in such cases, further repetition for resolution improvement needs to add more than one resolution value and requires a longer processing time afterward. figure 14 shows the result for ndiv 10,000 after repeating resolution improvement. value ranges other than the duration of heating converge to zero. although values for the duration of heating do not converge to the correct value of 800 w, they clearly indicate heater activity and the average for the duration of 0 s to 180 s is 780 w. it shows that our method of data modification is to suppress the negative value points in (3) and (4), and not to have a resolution improvement effect at positive value points. this means that we will modify data when we detect steeper temperature falls, while we infer heating when we detect steeper temperature rises. 6. real data and backward simulation we show the result of the backward simulator applied to the measured data in figure 8. the backward simulator could only output a model conforming result up to ndiv=50 (figure 15). it shows a large gap between possible minimum and maximum values. still, the real heating power values reside within the minmax pairs. as shown in the previous section, the min value will go up and the max value will go down when the resolution of temperature data improves. the repetitive processing of temperature data modification described in the above section improves the result, shown in figure 16 for ndiv 10,000. we cannot expect further improvement even if we set ndiv larger, because the points of min and max are very close to each other at almost all sample times. checking the detail of the measured data in figure 8, we found that there are fluctuations like the sequence of circle points in figure 17, which would be the result of sampling and truncating quantization of the solid line. those may be lsb (least significant bit) fluctuations caused by noises around digital thresholds. such setbacks are treated as the results of figure 15. backward simulation result for real data from figure 17 for ndiv=50. figure 16. backward simulation result for real data from figure 17 for ndiv=10,000 with resolution improvement. inconsistency1 inconsistency2 accuracy improvement temperatnre data accuracy improvement i i i i i i i i i f o o o o o1 o o o1 o o2 o2 f heater sum1 br1 co1 1-a dly1 a dly2 b sum2 br2 co2 1/c temp figure 13. backward simulation with resolution improvement. figure 14. backward simulation result for ndiv=10,000 with applying resolution improvement. 1 2 3 4 5 6 div figure 17. real a/d data of figure 17 have fluctuating sample points. 0 2000 4000 6000 8000 10000 0 1000 2000 3000 4000 he at er (w ) time(s) min max 0 1000 2000 3000 4000 5000 0 1000 2000 3000 4000 he at er (w ) time(s) min max 0 500 1000 1500 2000 2500 0 1000 2000 3000 4000 he at er (w ) time(s) min max acta imeko | www.imeko.org april 2017 | volume 6 | number 1 | 18 some heating and the simulator making spontaneous wattage rise in the resultant heating estimation in figure 16. if we suppress those fluctuations, we can eliminate spontaneous values. it is natural to estimate the original temperature change as the dashed line in figure 17. then, on probation, we eliminated them, judging by eye with adding only one lsb or subtracting only one lsb around fluctuation points, leaving the points where the fluctuation is more than one lsb. the result is shown in figure 18, slightly smoother than the measured data in figure 8, especially in the tail part.. figure 19 is the result of the backward simulation of the data in figure 18 for ndiv=10,000. it shows a better result than in figure 16. values are large for the period from 70 to 160 seconds and almost zero after 170 seconds. the average heater power for the duration of 0 s to 180 s is 746 w. further improvement with larger ndiv cannot be expected because the min and max points almost coincide for all sample times. 7. discussion by using the hybrid simulation model in a feedback type simulation, no case branching is required in the backward simulation. the backward simulator we use is for functional evaluation and includes gui for monitoring and manual operations. it performs a single simulation for about 3.0 s for 100 sample data and about 3.6 s for 382 sample data. the processing time is not proportional to the number of samples. one cause for it may be the fact that we used multithread java processing on a four-core eight-thread cpu (intel i7-3770). figure 20 shows the processing time for the total repetition of model mismatch and data modification relative to ndiv. processing time in a case of model mismatch depends on the time when the mismatch is detected. average processing time for one backward simulation is from 2.5 to 3 s. we modified the temperature data step by step, which means that we improved the resolution for ndiv 100, and then improved it for 200 by using the result of ndiv=100, as an example. the coordinate of figure 20 shows the total processing time up to the abscissa ndiv values. roughly, the logarithm of the processing time is proportional to the logarithm of ndiv. resolution improvement using model mismatch was successful. although there are many possible methods for such data modification, our method to improve by half of the former resolution when a model mismatch is detected was shown effective. indeed, the curve of figure 18 is in agreement with the measured data in figure 8. our resolution improvement can be considered as a method to search feasible temperature data for a higher resolution. as the estimated parameters a and b may have some errors, they may cause effects on the results of the backward simulation. we have to evaluate such effect, though they may have little effect on the long range of time sequence because those parameters were determined by a relatively long part of the temperature data. although we show here only one real data simulation result, the simulation model is simple, and the result for simulated data is perfect. also, the result for real measured data is almost perfect even if our model and estimated parameters are not perfect. so, we expect similar result would be obtained for other measured data. to improve estimations at periods of heating, we need another simulation model, e.g. a model which restricts heater wattage to one of two ranges. in such case, we need to find the time point where the estimated heating is not feasible and to determine which temperature data should be modified. 8. conclusion we showed that a backward simulation incorporating model conformation and resolution improvement is very effective. it is also shown that the backward simulation can be efficiently performed by using our hybrid method (forward simulation objects in backward simulation structure) by eliminating case figure 19. backward simulation result of fig.18 for ndiv=10,000 with resolution improvement. figure 18. fluctuation eliminated temperture data correspoinding to the measure data in figure 9. figure 20. processing time including resolution improvement vs ndiv for the cases in section 5. 0 1000 2000 3000 4000 5000 0 1000 2000 3000 4000 he at er (w ) time(s) min max 0 0.2 0.4 0.6 0.8 1 1.2 1.4 0 1000 2000 3000 4000 te m pe ra tu re (d eg ) time(s) 1 10 100 1000 10000 100000 100 1000 10000 100000 ac cu m ul at ed p ro ce ss in g tim e( m in ) ndiv acta imeko | www.imeko.org april 2017 | volume 6 | number 1 | 19 branches. causal input changes can be practically determined for the simulated data and for the real measurement data. the internal state of any system can be determined by the backward simulation if we define the system’s model properly for backward simulation. in the case of data with high resolution, the backward simulator outputs almost perfect results. for cases of limited resolution, repetitive resolution improvement responding to model mismatches was successfully done. we also showed that the backward simulation with real measurement data can be effectively done, although noise elimination was needed for errors larger than quantising errors. the results suggest that we can separate noise from signal by using the backward simulation with a model conformation test. the backward simulation offers us a new method to infer causal inputs and internal states of various systems. we need to study the relation among the precision of simulation output, degree of model fitness and snr of data for backward processing. acknowledgement this work was supported by jsps kakenhi grant number 25540006. we thank one of the reviewers who motivated us to improve the simulation. references [1] j. v. beck, b. blackwell and c.r.s. clair jr., inverse heat conduction ill-posed problems, john wiley & sons, 1985, isbn 0-471-08319-4. [2] y. jarny and d. maillet, linear inverse heat conduction problem – two basic examples, http://www.sft.asso.fr/local/sft/dir/user3775/documents/actes/metti5_school/lectures%26tutorialstexts/text-l10-jarny.pdf. [3] k. oguni, inverse problem and instrumentation, ohmsha, tokyo, 2011, isbn 978-4-274-06829-4. [4] t.b.bako and t.daboczi, improved-speed parameter tuning of deconvolution algorithm, ieee trans. instrum. meas., vol.65, no.1, pp.1568-1576, 2016. [5] a.muqaibal, a,safaai-jazi, b.woerner and s.raid, uwb channel impulse response characterization using deconvolution techniques, proc. mwscas, 2002. [6] chueng-chiu huang and his-huang wang, backward simulation with multiple objectives control, proc. imecs (international multiconference of engineers and computer scientist), hong kong, 2009. [7] c.d.twigg and d.l.james, back steps in rigid body simulation, acm trans. graph., vol. 27, no.3, article 25, 2008. [8] j.j.cook, reverse execution of java bytecode, computer journal, vol.45, no.6, pp.608-619, 2002. [9] y. hiranaka and t. taketa, designing, backward range simulator for system diagnoses, proc. xx imeko world congress, 2012. [10] y. hiranaka., h. sakaki, k. ito, t. taketa and s. miura, numerical backward simulation model with case branching capability, proc. 4th international conference on simulation and modeling methodologies, technologies and applications (simultech 2014), pp.225-230, 2014. hybrid backward simulator for determining causal heater state with resolution improvement of measured temperature data through model conformation microsoft word 28-146-1-ga.doc acta imeko  july 2012, volume 1, number 1, 85‐88  www.imeko.org    acta imeko | www.imeko.org  july 2012 | volume 1 | number 1 | 85  two‐stage current transformer with electronic compensation  daniel slomovitz, leonardo trigo, carlos faverio  ute ‐ laboratory, paraguay 2385, montevideo, uruguay      keywords:  current transformer; compensation; measurement; calibration; power; capacitance; two‐stages.  citation: daniel slomovitz, leonardo trigo, carlos faverio, two‐stage current transformer with electronic compensation, acta imeko, vol. 1, no. 1, article 16,  july 2012, identifier: imeko‐acta‐01(2012)‐01‐16  editor: pedro ramos, instituto de telecomunicações and instituto superior técnico/universidade técnica de lisboa, portugal  received january 20 th , 2012; in final form june 8 th , 2012; published july 2012  copyright: © 2012 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: no information provided  corresponding author: daniel slomovitz, e‐mail: dslomo@gmail.com    1. introduction  very high precision wattmeters have been developed in national metrology institutes. some of them are based on adding devices [1], others on bridges [2], and others use two commercial digital sampling voltmeters (dsv), like the model hp 3458 [3, 4]. most of them require a current-to-voltage transducer for the input current. if a simple resistor shunt is used as this transducer, the maximum current is limited by the maximum allowed dissipation. it is not possible to get very low uncertainties if the currents are higher than some miliamperes. this leads to the use of current transformers (ct) to reduce the current through the shunt to a small value, so that the shunt dissipates a small power with low temperature rise. burdens around 10 ω for output voltages between 1 v and 2 v lead to powers lower than 1 w, which are acceptable for this kind of application. when two dsvs are used, one is used to digitize the voltage and the other a voltage proportional to the current. as the best voltage range of the mentioned instrument is 10 v, it is necessary to scale currents that can reach up to 100 a to voltages of few volts. the uncertainty of this model of dsv, using special control programs [5, 6], is in the order of some parts in 106. then, the whole current to voltage transducer must have uncertainties around of a few parts in 106, in order not to degrade the system. as this transducer comprises a resistor shunt and a ct, this last device must have uncertainties around 1 part in 106. to get that, a two-stage transformer [7, 8] is proposed, but this proposal adds electronic compensations. although electronic compensation of two-stage transformers have been proposed in the past [9-18], none of them reaches a complete compensation of the internal transformer error sources. they do not compensate errors caused by internal resistance of the windings and errors coming from internal stray capacitances. for this type of ct, the main source of error is the magnetizing current (im). this current flows through the magnetizing branch, so that it does not flow through the output. figure 1 shows a schematic model of this type of transformer. z1 and z2 are the series impedances with resistive and inductive components (r2, l2), while rm, lm represent the components of the magnetizing branch. c1, c2 represent stray capacitances in the primary and secondary windings. there is no capacitance between primary and secondary due to electrostatic shields that this kind of ct has. to get low errors, the magnetic flux must be as low as possible, reducing in rmlm r2 l2 z1 z2 c1 c2 i vl +ic1 im vm   figure 1. transformer model.  a current transformer with electronic assistance was developed, intended for current to voltage transducers. it is based on the two‐ stage principle, and it includes electronic compensation for magnetizing errors as well as capacitive errors. the nominal input currents  go from 1 a up to 100 a, and the nominal output current is 0.2 a. using a load of 10 ω, the nominal output voltage is 2 v, with 0.4 w of  dissipation.  acta imeko | www.imeko.org  july 2012 | volume 1 | number 1 | 86  this way the magnetizing current. for that purpose, the magnetizing voltage (vm) must be reduced as much as possible. with conventional cts there is a limit due to the voltage drop on the load, which in our case is very high, 2 v at 0.2 a. usually, standard ct loads are around 0.2 ω, that is 50 times lower. to reach lower errors, two-stage cts are used in this application. in two-stage transformers [7], one stage provides the power needed for the core magnetization and for the burden (magnetizing core), while the other stage only supplies the error current of the first stage (ratio core), being its magnetic flux very low but not zero. the exact value of this flux depends on the external burden and the internal series impedance of its secondary winding. the lower the load is, the smaller the magnetic flux. this method requires one to add the currents of both secondary windings, what is done using different methods depending on the application. in current to voltage transducer, this addition can be done using two burden resistors instead of only one [19]. one resistor, of high precision, is the burden of the magnetizing secondary winding and the other one, generally of the same value but lower precision, is the burden of the ratio winding (see figure 2). wm is the magnetizing winding of the two-stage transformer and wc is the ratio one. the core to the left, in figure 2, is the magnetizing one, and the core to the right is the ratio one. as the compensating current through wc is much smaller than the main one (through wm), the same occurs to the voltage drops on rc and rm, reducing the magnetization current of the compensating core. this allows total errors in the order of 10 parts in 106 if burdens around 10 ω are used [19]. for reducing the errors 10 times more, as it is the goal of this proposal, an electronic compensating circuit is added. 2. transformer design  the core for both stages is of a high permeability type (mumetal). both secondary windings (magnetizing and ratio stages) have 500 turns each, as figure 3 shows. there are two groups of primary windings. the first one is formed by 10 groups of 10 turns each. connecting in parallel-series sets, it can be arranged from 10 to 100 effective turns (10, 20, 50 and 100). as the nominal secondary current is 200 ma, the nominal input current ranges are 1 a, 2 a, 5 a and 10 a. for the upper ranges, another primary winding group exists. it has 10 groups of 1 turn each. therefore, using these winding groups, the nominal primary current goes from 10 a up to 100 a. table 1 shows the different ratios available. each group of turns in each winding has the same current whichever were the connection. in this way, the stray magnetic flux does not change when different ratios are selected. this property ensures that the errors are practically the same for all ratios, allowing a reduction in the calibration work. however, error differences can appear between low and high current windings, because they have different turns. to determine the amount of this variation, there is a 10 a range in each winding. connecting both with opposite polarity, ideally, the output must be null. then, measuring the actual output, that ratio error difference can be calculated. 3. electronic compensation  the goal of the compensating circuit is to null the magnetic flux in the compensating stage core. for that, a controlled voltage source generates the same value as the sum of the voltage drops in the internal and external series impedances of the compensating secondary winding, but with opposite polarity. in this way, no magnetizing voltage exists in this winding, neither magnetizing current i. from a theoretic point of view, there is a complete compensation and null errors. this electronic source supplies only the small power required by the compensating stage of the transformer, so that a simple operational amplifier is enough for this application. figure 4 shows a schematic circuit. the purpose of the electronic device is to null the electromotive force (emf) ε in wc. assuming an internal resistive impedance r of this winding, the following equations apply:  1 2i r r v    (1) and wm rm + wc rc vo figure 2. conventional two‐stage current to voltage transducer.  figure 3. windings of the proposed transformer. compensating core to the left, main core to the right.  table 1. available ratios using different series‐parallel connections. parallel  groups  groups  turn/group  ratio  input  current (a)  1  10  10  5  1  2  5  10  10  2  5  2  10  25  5  10  1  10  50  10  1  10  1  50  10  2  5  1  100  20  5  2  1  250  50  10  1  1  500  100  acta imeko | www.imeko.org  july 2012 | volume 1 | number 1 | 87  2 2 1 3   1 r v r i r         , (2) which leads to 1 2 3 r r i r r         . (3) if r = r1r2/r3, then ε = 0. this shows that it is possible to null the emf in wc, nulling in this way the magnetic flux through the auxiliary core. 4. error sources  as mentioned, the first error source is the magnetizing current. in the prototype, it was reduced 100 times with the electronic compensator, reaching errors in the order of 0.1 µa/a (in phase and in quadrature). this value includes ambient temperature variation. the influence of the latter on the compensation is due to the variation of the resistance of the windings, that vary around 0.4%/k, affecting equation (3). second order errors include stray capacitances and stray magnetic fluxes. for the first ones, an electrostatic shield between primary and secondary windings and an electronic compensation to reduce the influence of some stray capacitances were used. the shield was connected to an external guard terminal. in this way, stray capacitances between primary and secondary windings are eliminated. however, stray capacitances between all windings and shields remain. it is possible to reduce this effect using shielded cables for the windings [20], but this technique was not used because of the high cable section required in this design. in the secondaries, these capacitances produce errors in quadrature that only depends on the resistive burden. with 10 ω burden, as is used, this error source cannot be neglected. to compensate it, an electronic device that simulates a negative capacitor was included at the output, in parallel with the burden. its value was adjusted in such way that no error differences exist when the burden was changed between 0 ω and 10 ω. stray capacitances between primary windings themselves, and to shield, changes when different series-parallel connections are used. however, all these capacitances have low values due to the small number of turns that these windings have, leading to errors lower than 1 part in 106. to reduce the effect of stray magnetic fluxes and magnetic shields an equalization winding was added. this winding has 5 partial windings of 5 turns, all connected in parallel, around the core. in this way, it forces the flux to be homogeneous, reducing this error source. the influence of these second order errors in the whole performance was estimated in 0.4 µa/a for the prototype. then, the total error, including the magnetizing current error, is 0.5 µa/a, approximately. to confirm this theoretical error estimation, the prototype was tested against a current comparator, with uncertainties of 0.3 µa/a (in phase and in quadrature). the tested ratios were 5 and 10, and the results are shown in table 2. the uncertainty of this calibration is mainly due to the uncertainty of the current comparator, including its zero detector, previously stated (0.3 µa/a). the error values are basically covered by the estimated errors of the prototype and the uncertainty of the calibration, confirming the theoretic analysis. 5. prototype  figure 5 shows a photo of the prototype. it has two output binding posts (vertical, red at the left) corresponding to the transformer secondary. the four red binding posts in the middle corresponds to the burden resistor, and the two metallic ones, to guard and ground. the banana couple black and red are connected to a thermistor included in the burden resistor to measure its temperature. other switches and leds, at the right, correspond to power supplies. to eliminate interference from power network, batteries are used. at the front panel, it has 10 pairs of terminals of low-current winding (10 groups of 10 turns). each group is connected to each vertical pair of terminals. the combinations in seriesparallel connections are made by changing a circuit printed board (pcb) that is connected at the front of the case. the cooper side of this pcb is on the inner face of the case, so it is not visible. figure 6 shows two pcbs on their cooper sides for 1 a and 5 a. the upper one connects all winding groups in series, forming a winding of 100 turns. the lower pcb connects the first five groups in parallel, in series with the last i + r2r3 zl v2 r1 wm ic1 + i wc +  figure  4.  schematic  circuit  of  the  two‐stage  electronic  compensated transformer.  figure 5. prototype of the proposed transformer. table 2. calibration of the proposed transformer.  ratio  primary current  (a)  error in phase   x10 ‐6   error in quadrature  x10 ‐6   10  2  ‐0.6  ‐0.8  5  1  ‐0.4  ‐0.6  acta imeko | www.imeko.org  july 2012 | volume 1 | number 1 | 88  five groups. in this way, the effective number of turns is 20. there is one printed board for each ratio, with different connections. for 10 a, all groups are connected in parallel. a similar system exists for the high-current winding (10 groups of 1 turn) in the upper side of the case (see figure 7). it also has 10 pairs of terminals that can be connected in different series-parallel configurations. as the currents in these windings are higher than the first ones, the connections are made by using copper bars instead of pcbs. 6. conclusions  a current transformer intended for high precision currentto-voltage transducers was proposed. it uses a two-stage technique plus electronic assistance. this last device cancels the magnetic flux in the compensating core, canceling the error of the transformer due to this cause. it does not depend on the internal winding resistance or the external burden. the primary windings can be arranged in different series-parallel connections covering ranges from 1 a to 100 a. the estimated ratio error is about 0.5 µa/a. references  [1] p. braga, d. slomovitz, “rms voltmeter based power and power-factor measuring system,” international journal of electronics, vol. 75, no 3, set. 1993, pp. 561-565. [2] j. bai, l. yang, s. zhao, j. zong, e. so, “the establishment of power/energy standard at china electric power research institute and its comparison with nrc,” conference on precision electromagnetic measurements, conference digest, june 2010, pp. 275-276. [3] e. tóth, a. m. ribeiro franco, and r. m. debatin, “power and energy reference system, applying dual-channel sampling,” ieee trans. instrum. meas., vol. 54, no. 1, feb. 2005, pp. 404-408. [4] u. pogliano, “use of integrative analog-to-digital converters for high-precision measurement of electrical power,” ieee trans. instrum. meas., vol. 50, no. 5, oct. 2001, pp. 1315-1318. [5] r. l. swerlein, “a 10 ppm accurate digital ac measurement algorithm,” proc. ncsl, albuquerque, usa, aug. 1991, pp. 17-36. [6] g.a. kyriazis, r. swerlein, “evaluation of uncertainty in ac voltage measurement using a digital voltmeter and swerlein´s algorithm,” conference on precision electromagnetic measurements, conference digest, june 2002, pp. 24-25. [7] h. b. brooks, f. c. holtz, “the two-stage current transformer,” aiee trans., vol. 41, june 1922, pp. 382-393. [8] p. j. betts, “two-stage current transformers in differential calibration circuits,” iee proc., vol. 130, pt. a, no. 6, sept. 1983, pp. 324-328. [9] d. l. h. gibbibngs, "a circuit for reducing the exciting current of inductive devices," proc. iee, vol. 108,1961, pp. 339-349. [10] o. peterson, "a self-balancing current comparator," ieee transs on instru. and meas., vol. im-15, 1966, pp. 62-71. [11] r. friedl, "current transformers with electronic error compensation," messkechnik, vol. 76, no 10, 1968, pp. 241-250. [12] t. m. souders, "wide-band two-stage current transformer of high accuracy," ieee trans. instrum. meas., vol. 4, june 1972, pp. 340-349. [13] g. e. bear, "100:1 step-up amplifier-aided two-stage current transformers with small error at 60 hz," ieee trans. instrum. meas., vol. 28, june 1979, pp. 146-152. [14] p. n. miljanic, e. so, w. j. m moore, “an electronically enhanced magnetic core for current transformers,” conference on precision electromagnetic measurements, june 1990, pp. 328. [15] j. l. west, p. n. miljanic, “an improved two-stage current transformer,” ieee trans. instrum. meas., vol. 40, june 1991, pp. 633-635. [16] e. so, s. ren, d.a. bennett, "high-current high-precision openable-core ac and ac/dc current transformers," ieee trans. on instrum. and meas., vol. 42, no. 2, april 1993, pp. 571-576. [17] e. so, d. a. bennett, “a low-current multistage clamp-on current transformer with errors below 50×10-6,” ieee trans. instrum. meas., vol. 46, apr. 1997, pp. 454-458. [18] j. l. west, p. n. miljanic, “an improved two-stage current transformer,” ieee trans. instrum. meas., vol. 40, june 1991, pp. 633-635. [19] conimed, nota de aplicación modelo cct20, conversor corriente tensión de gran exactitud, www.conimed.com. [20] d. slomovitz, h. de souza, “shielded electronic current transformer,” ieee trans. instrum. meas., vol. 54, no. 2, apr. 2005, pp. 500-502. figure 6. circuit printed boards used to connect the groups of low‐current  primary  windings.  the  upper  one  corresponds  to  1  a  of  nominal  current (100 turns), and the lower to 5 a (20 turns).  figure 7. connection terminals for high‐current windings at the upper side  of the case.  microsoft word 305-2374-1-le-rev acta imeko  issn: 2221‐870x  november 2016, volume 5, number 3, 81‐86    acta imeko | www.imeko.org  november 2016 | volume 5 | number 3 | 81  influence of the human body mass in the open‐air mri on  acoustic noise spectrum  jiří přibil 1 , anna přibilová 2 , ivan frollo 1   1 institute of measurement science, slovak academy of sciences, bratislava, slovakia  2 institute of electronics and photonics, faculty of electrical engineering & information technology, slovak university of technology,  bratislava, slovakia      section: research paper   keywords: acoustic noise; noise reduction; signal processing; spectral analysis; statistical evaluation  citation: jiří přibil, anna přibilová, ivan frollo, influence of the human body mass in the open‐air mri on acoustic noise spectrum, acta imeko, vol. 5, no. 3,  article 13, november 2016, identifier: imeko‐acta‐05 (2016)‐03‐13  editor: paolo carbone, university of perugia, italy  received december 21, 2015; in final form may 26, 2016; published september 2016  copyright: © 2016 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: the work has been done in the framework of the cost action ic 1206 and has been supported by the grant agency of the slovak academy of  sciences (vega 2/0013/14)  corresponding author: jiří přibil, e‐mail: jiri.pribil@savba.sk    1. introduction  the magnetic resonance imaging (mri) device usually consists of three gradient coils that produce three orthogonal linear fields for spatial encoding of a scanned object. from the physical principle follows that the rapid changes of the lorentz forces during fast switching inside the weak static field b0 environment [1] result in a significant mechanical vibration of these gradient coils. this process subsequently propagates in the air in the form of a progressive sound wave received by the human auditory system as a noise [2]. due to its harmonic nature and the audio frequency range, the produced acoustic noise of this device can generally be treated as a voiced speech signal, and thus it can be recorded by a microphone and processed in the spectral domain using similar methods as those used for speech signal analysis. the mri technique enables analysis of the human vocal tract structure and its dynamic shaping during speech production while simultaneous recording of a speech signal [3], [4] is performed. the primary volume models of the human acoustic supra-glottal cavities constructed from the mr images can be transformed into three-dimensional (3d) finite element (fe) models [5]. image and audio acquisition must be synchronized and the recorded speech signal must have a good signal-tonoise ratio so that high-quality results are achieved in 3d vocal tract model creation [6]. several approaches to reduce the noise in the mri equipment [7]-[9] are used in practice. one group of these enhancement methods is based on spectral subtraction of the estimated background noise when at least two microphones are used [10]. our method uses only one microphone that picks up the noise of the running mri scan sequence without phonation and then it records the speech signal during phonation with the background mri noise. the following signal processing is carried out: the part of the signal containing only the mri noise is used to calculate the basic spectral envelope using the mean welch periodogram, and then it is analyzed in segments to determine the basic and supplementary spectral properties. the obtained spectral features are subsequently processed statistically and the abstract  the paper analyses changes in spectral properties of the acoustic noise when the examined person lies in the scanning area of the  open‐air magnetic resonance imager (mri). consequently the holder of the lower gradient coils is loaded with the mechanical mass  represented by the person’s weight. the acoustic noise pressure level is mapped in the mri neighborhood, too. obtained results of  spectral analysis will be used for the design of a correction filter to suppress the noise in the simultaneously recorded speech signal for  3d modeling of the human vocal tract.  acta imeko | www.imeko.org  november 2016 | volume 5 | number 3 | 82  achieved values are used to modify the basic spectral envelope of the noise signal that is further subtracted from the spectral envelope of the speech signal with the superimposed mri noise. in general, these noise estimation techniques based on statistical approaches are not able to track real noise variations; thereby they result in an artificial residual fluctuation noise and a distorted speech. therefore, the spectral properties of the acoustic noise generated by the gradient system of the mri device must be analyzed with high precision so that the noise is efficiently suppressed while the maximum quality of the processed speech signal is preserved. to investigate the transmission of vibration through the plastic holder of the mri device scanning area, the measurement arrangement consists of the testing spherical water phantom which is useful for testing of the magnetic field homogeneity [11] in the device calibration phase. the situation changes when the examined person lies in the scanning area of the open-air mri and the holder of the lower gradient coils is loaded with his/her weight. then the mass of the whole mechanical system is altered and a change in spectral properties of the generated acoustic noise is expected, too. to verify this working hypothesis, the noise signal recording and its spectral analysis were performed for different person weights and with the water phantom only (for comparison). the obtained results will be used to devise an improvement of the developed cepstral-based noise reduction method [12] in the speech recorded during mri scanning. 2. analysis of spectral properties of the acoustic  noise signal  the spectrogram can be successfully used for visual comparison of differences in the time / frequency domain – see an example in figure 1a,b. the disadvantage of subjectivity in this method can be eliminated by numerical matching of the spectral envelopes. to obtain the smoothed spectral envelope, the mean periodogram can be computed by the welch method. in general, the periodogram represents an estimate of the power spectral density (psd) of the input signal. using the nfft -point fft to compute the psd as s(e j) / fs and the sampling frequency fs in hz, we obtain the resulting spectral density in logarithmic scale expressed in db/hz – see graphs in figure 1c,d. the basic spectral properties can be determined from the spectral envelope and subsequently the histograms of spectral values can be calculated for objective matching – see figure 1e. further detailed analysis in the frequency domain is done in the region of interest (roi) – see the visualization in figure 2. for numerical comparison it is possible to calculate the rmsbased spectral distances drms between periodograms corresponding to the mri noise signals with different persons lying in the mri device scanning area or using the sole phantom object. in addition, for evaluation of differences between spectral envelope values in the chosen frequency range, the absolute spectral difference sdiff can be calculated by subtraction of their values in db. analyzing this differential signal, the maximum value pmax can be localized and the corresponding frequency fmax can be determined – see the graphs (b) and (c) in figure 2. the supplementary spectral properties are usually determined from the frames (after segmentation and windowing). these properties describe the shape of the magnitude of the power spectrum |s(k)|2 of the noise signal and they can be determined with the help of the additional statistical parameters [13]. the spectral centroid (scentr) is defined as a center of gravity of the spectrum, i. e. the average frequency weighted by the values of the normalized energy of each frequency component in the spectrum     . 2 1 2 2 1 2    fftfft n k n k centr ksksks (1)   figure  2.  the  smoothed  envelopes  by  mean  periodograms  of  two  noise  signals  in  the  full  frequency  range  0fs /2  together  with  calculated  rms  spectral distance (a), selected roi in the low‐frequency band 0~2.5 khz (b),  depicted absolute differential signal sdiff with localization of the frequency  fmax corresponding to the maximum difference pmax (c).  figure 1. spectrograms of two mri noise signals (a, b), power spectral densities and their envelopes  in the selected 250‐ms rois depicted for the  low‐ frequency band 0~2.5 khz (c)‐(d), histograms of these spectral envelopes (e). 0 0.5 1 1.5 2 2.5 0 5 10 15 — › s d iff [ d b ] —› f [khz] s diff p max =5.45 db 0 0.5 1 1.5 2 2.5 -100 -80 -60 -40 -20 — › p [ d b ] —› f [khz] envelope1 envelope2 d rms =1.9278 0 1 2 3 4 5 6 7 8 -100 -80 -60 -40 -20 — › p [ d b ] —› f [khz] envelope1 envelope2 d rms =2.3854 f max = 575 hz a) b) c) roi —› time [s] — › f [k h z ] 0.5 1 1.5 2 2.5 0 2 4 6 8 -120 -100 -80 -60 -40 -20 —› time [s] — › f [k h z ] 0.5 1 1.5 2 2.5 0 2 4 6 8 -120 -100 -80 -60 -40 -20 p [db] p [db] b)a) 0 0.5 1 1.5 2 2.5 -100 -80 -60 -40 -20 — › p o w e r /f re q u e n c y [ d b / h z ] —› f [khz] noise1 0 0.5 1 1.5 2 2.5 -100 -80 -60 -40 -20 — › p o w e r /f re q u e n c y [ d b / h z ] —› f [khz] noise2 -140 -120 -100 -80 -60 -40 -20 0 2 4 6 8 10 —› psd [db] — › o c c u re n c e [ % ] noise1 noise2 c) d) e) acta imeko | www.imeko.org  november 2016 | volume 5 | number 3 | 83  the spectral flatness (sflat) is determined as the ratio of the geometric and the arithmetic mean values of the power spectrum and it also describes the degree of periodicity in the signal [14]     . 1 22 1 2 2          fft fft fftnfft n k n n k flat kskss (2) the spectral spread (sspread) parameter represents the dispersion of the power spectrum around its mean value   ,22   xes spread (3) where  is the first central moment and  is the standard deviation of the spectrum values. the spectral skewness (sskew) is a measure of the asymmetry of the data around the sample mean and can be determined as the third moment    .33  xesskew (4) the spectral kurtosis (skurt) expressed by the fourth central moment represents a measure of peakedness or flatness of the shape of the spectrum relative to the normal distribution for which it is 3 (or 0 after subtraction of 3)    .344  xeskurt (5) 3. subject, experiments, and results  the analyzed open-air mri equipment e-scan opera contains an adjustable bed which can be positioned in the range of 0 to 180 degrees, where the 0 degree represents the left corner near the temperature stabilizer device [15] – see the principal angle diagram of the mri scanning area in figure 3d. this noise has an almost constant sound pressure level (spl) and consequently it can be easily subtracted as a background (spl0). due to the low basic magnetic field b0 (up to 0.2 t) in the scanning area of this mri machine, any interaction with the recording microphone must be eliminated. as the noise properties depend on the microphone position, the optimal recording parameters (the distance between the central point of the mri scanning area and the microphone membrane, the direction angle, the working height, and the type of the microphone pickup pattern) must be found. the chosen type of the sequence together with the basic scan parameters – repetition time (tr) and echo time (te) – have significant influence on the scanning time. values of these parameters result primarily from the chosen type of the scanning sequence. they can also be slightly changed manually but their final values depend on the setting of the other scan parameters – field of view (fov), number of slices, slice thickness, etc. the realization of the experiment with measurement of the acoustic noise produced by the gradient system of the mri device consists of two phases. first, the noise signal is recorded by the pick-up microphone during execution of a scan mr sequence with different testing persons lying in the scanning area of the mri device as documented by a photo in figure 3a, and using only the testing spherical phantom with the diameter 14 cm, filled with the solution of cuso4 in a distilled water [15] (cuso4 shortens the tr time and speeds up the mr data collection) [11]. this phantom is placed in the middle point of the scanning area inside the scanning rf coil – see figure 3b. then, the noise signals are processed as follows: 1. calculation of the basic and supplementary spectral properties of the recorded acoustic noise signals; determination of the main differences between the signals with and without a lying person in the scanning area, and detailed analysis of the influence of different person weights on the spectral properties. 2. visual comparison of the smoothed spectral envelopes in the full frequency range up to fs /2 (0 to 8k), in the low frequency sub-band up to 2.5 khz (0 to 2k), and the middle frequency sub-band of 2~6 khz (2 to 6k); determination of the spectral distances drms of these envelopes between individual persons or the phantom object, and localization of the frequency fmax corresponding to the maximum difference pmax within the low-frequency band 0 to 2k. 3. statistical processing of determined values of the basic and supplementary spectral properties; numerical matching of the obtained results. comparison of spectral envelopes in the low-frequency band up to 2.5 khz together with their histograms for the selected figure 3. an arrangement of noise spl measurement and recording in the  mri opera: a lying person with a pick‐up microphone at the position of 90  degrees (a), the testing water phantom with the recording microphone at the position of 150 degrees (b), the sound level meter at the position of 30 degrees (c), a principal angle diagram of the mri scanning area (d).  acta imeko | www.imeko.org  november 2016 | volume 5 | number 3 | 84  pairs can be seen in figure 4. the observed differences between the selected supplementary spectral properties are visualized by histograms in figure 5, the summarized numerical values are introduced in table 1. the ssf-3d scanning sequence chosen for this comparison experiment can be used for the 3d mr scans of the human vocal tract [12], [16]. the auxiliary parameters of the used sequence were set as follows: te=10 ms, tr=45 ms, 10 slices, 4-mm thick, sagittal orientation. the spherical testing phantom described earlier in this section was inserted in the scanning rf knee coil when the noise signal was recorded without a lying testing person in the mri scanning area. in correspondence with our previous research [12], the mapping of the acoustic noise spl in the mri neighborhood was performed for the mentioned scan sequence. the sound level meter of the multi-function environment meter lafayette dt 8820 (with the range set to 35~100 db) was used for the measurement of the noise spl at directions of 30, 90 and 150 degrees – see the obtained values in a graphical form in figure 6a. the spl meter was located at the distances of 45, 60, and 75 cm from the central point of the scanning area (see figure 3c). subsequently the directional pattern of the acoustic noise spl distribution was measured in the range of <0-180> degrees, per 5 degrees, at the distance of dl=60 cm from the mri device central point with the inserted testing water phantom – see the resulting diagram including the spl0 background values in figure 6b. the noise measurement was practically realized by real-time recoding of the signal by a microphone and transferring it to the external notebook during execution of a chosen scan mr sequence. the recording microphone was located at a distance of 60 cm, at the horizontal positions of 30, 90, and 150 degrees, and vertically in the middle between the both gradient coils. the input analogue noise signal, picked up by the 1" behringer dual diaphragm condenser microphone b-2 pro with the cardioid polar pattern setting, was pre-amplified and processed by the mixer device behringer xenyx 502 connected to the notebook by the usb audio interface u-control uca202. the noise signal was recorded at a sampling frequency of 32 khz, then resampled to 16 khz, and subsequently processed. the stationary signal parts with a time duration of 8 s were selected and normalized to the level of – 16 db using the sound editor program sound forge 8.0. the collected noise database originates from the records of six testing persons lying in the mri scanning area: three males (m1-m3) with figure 4. spectral envelopes in the frequency band up to 2.5 khz and their  histograms for the selected pairs: 78‐kg male m1 / 0.75‐kg phantom wp (a) 50‐kg female f2 / 75‐kg male m2 (b), 50‐kg female f2 / 55‐kg female f3 (c); microphone at 90 degrees.  table 1. summary comparison of mean values of spectral properties of the  acoustic noise for tested pairs of male/female persons and water phantom  (wp) inserted in the scanning area of the mri device e‐scan opera.  tested pairs  fmax     [khz]  pmax    [db]  drms (0_2k)  [db]  drms (2_6k)  [db]  drms (0_8k)  [db]  male/wp  0.577  9.81  4.34  3.85  4.25  female/wp  0.575  8.95  4.18  2.78  3.55  female/male  0.565  5.49  3.25  3.98  3.49  male/male  1.85  3.65  1.62  1.72  1.62  female/female 1.26 3.27  1.29  1.69 1.54 figure 5. histograms of the supplementary spectral properties: centroid (a),  flatness (b), spread (c), skewness (d), and kurtosis (e) for the noise signals  recorded at the position of 150 degrees.  figure 6. bar‐graph of the noise spl values in the directions of 30, 90, and  150  degrees  at  the  distances  of  dl  =  45,  60,  and  75  cm  (a),  detailed  directional pattern of the noise source together with the background noise spl0, dl = 60 cm (b) for the used ssf‐3d mr scan sequence.  300 400 500 600 0 5 10 15 20 —› s centr [hz] — › o c c u re n c e [ % ] male female phantom 0.01 0.015 0.02 0.025 0 5 10 15 20 —› s flat [-] — › o c c u re n c e [ % ] male female phantom 0.1 0.15 0.2 0.25 0 5 10 15 20 —› s spread [-] — › o c c u re n c e [ % ] male female phantom 0.2 0.4 0.6 0.8 0 5 10 15 20 —› s skew [-] — › o c c u re n c e [ % ] male female phantom -2 -1 0 0 5 10 15 20 —› s kurt [-] — › o c c u re n c e [ % ] male female phantom b)a) c) d) e) 30 deg 150 deg 90 deg 0 20 40 60 80 100 — › s p l [ d b ] d l =45 cm d l =60 cm d l =75 cm a) 30 60 90 120 150 1800 —› angle [deg] 60 80 20 400—› spl [db] ssf-3d spl 0 b) acta imeko | www.imeko.org  november 2016 | volume 5 | number 3 | 85  approximate weights about 78 kg, three females (f1-f3) with the mean weight of 53 kg, and the testing water phantom with a holder (wp) weighting about 0.75 kg. the patient’s bed with an examined person was located at 180 degrees – this configuration was chosen since this has a minimal effect on the mentioned temperature stabilizer. 4. discussion of obtained results  the obtained results were evaluated by comparison of the basic spectral properties of the noise signals measured for the pairs with different weights: male/phantom, female/phantom, female/male, male/male, and female/female. the determined spectral differences of the noise signals between the persons with similar weights (female/female – see graphs in figure 4c are small in all three observed frequency ranges. for the male/female pairs the spectral differences are greater and well visible in the spectral envelopes within the low-frequency band up to 2.5 khz – see the graphs in figure 4b (male person m3 with the weight of 80 kg). for the male/phantom pairs the spectral differences are the greatest as can be noticed in the subplots of figure 4a. the achieved results of the supplementary spectral properties of the recoded noise mapped by histograms (see figure 5) document that the range of values represented by the standard deviation is significantly broader for male and female persons than for the testing phantom. it was caused by the fact that the phantom of the same weight was used, but the values of both male and female groups were taken from the measurements using three persons of different weights. next, the results of numerical matching of the noise spectral properties for the tested groups (phantom object, male or female persons) are in correlation with the graphical ones as shown by the summarized mean values in table 2. regarding the supplementary spectral properties of the recoded noise, the achieved results are in correlation with the obtained values of the distribution mapped by histograms (see figure 5) and with numerical matching of the mean values of the noise spectral properties for the tested pairs summarized in table 2. for small differences in the masses (similar weights of the subjects) the frequency of the detected maximum difference between the two spectral envelopes is higher than for the great differences between the masses (the weight of the male subject versus the water phantom giving this frequency about 570 hz). from the performed comparison of the supplementary spectral properties for three tested locations of the recording microphone follows that no significant differences exist, however, the best results are obtained for the microphone position at 90 degrees, i. e. directed at the face of the lying person. at the microphone position of 30 degree the influence of the mri temperature stabilizer can be superimposed as an additive noise with normal distribution. the microphone position at 150 degrees is unnatural from the point of the lying person and, in addition, the distance between the speaker face and the recording microphone was the longest. 5. conclusions  the changes in the spectral properties of the noise signal generated by the gradient coils during the mri scanning sequences while the examined person lay in the scanning area of the open-air mri machine were analyzed. the resulting supplementary spectral features describe also the degree of voicing and the statistical properties of the noise component of the speech signal (type of noise, randomness, distribution, etc.). this information is necessary for correct application of the excitation in the cepstral speech reconstruction after noise suppression [12]. the obtained results will serve to create databases of initial parameters (such as the bank of noise signal pre-processing filters) for a developed cepstrum-based algorithm for noise suppression in the recorded speech. it will be useful in experimental practice, when it often occurs that the basic parameter setting of the used scanning sequence as well as the other scanning parameters must be changed depending on the currently tested person. in addition, it will be very interesting to carry out the detailed analysis and to determine the spectral properties of the mri noise as a function of the mass of the subject. in this stage of our research only six volunteer persons (healthy people – colleagues) took part in our experiment, so it is very difficult or practically impossible. the solution to this issue may be cooperation with some medical centre (in bratislava, brno, etc.) having a certificate for the work with patients. finally, for better knowledge of the acoustic noise conditions in the scanning area and in the vicinity of the mri device, accomplishment of additional measurement and experiments is necessary. to describe how the vibrations induce the acoustic noise and how they travel through the plastic holder of the mri device, the time delay between the vibration impulses caused by the gradient coils and the noise signal must be analyzed, too. acknowledgement  the authors would like to thank the volunteers from the department of imaging methods, institute of measurement science in bratislava, for their help in our experiments. references  [1] a. moelker, p.a. wielopolski, m.t. pattynama, relationship between magnetic field strength and magnetic-resonance-related acoustic noise levels, magnetic resonance materials in physics, biology and medicine 16 (2003) pp. 52-55. [2] g.z. yao, c.k. mechefske, r.k. brian, acoustic noise simulation and measurement of a gradient insert in a 4 t mri, applied acoustics 66 (2005) pp. 957-973. [3] d. aalto et al., large scale data acquisition of simultaneous mri and speech, applied acoustics 83 (2014) pp. 64-75. [4] a.c. freitas, m. wylezinska, m.j. birch, s.e. petersen, m.e. miquel, comparison of cartesian and non-cartesian real-time mri sequences at 1.5t to assess velar motion and velopharyngeal closure during speech, plos one 11 (2016) e0153322. doi: 10.1371/journal.pone.0153322 [5] s.m.r. ventura, d.r.s. freitas, i.m.a.p. ramos, j.m.t.s. tavares, morphologic differences in vocal tract resonance table  2.  summary  mean  values  of  the  supplementary  spectral  properties  including  the  standard  deviation  values  (in  parentheses)  of  the  analyzed  noise signals summarized for all three recoding microphone positions.  property type  male  female  phantom  scentr [hz]  406 (70.8)  425 (71.9)  485 (10.9)  sflat [‐]  0.0145 (0.0032)  0.0138 (0.011)  0.0162 (0.0005)  sspread [‐]  0.221 (0.022)  0.188 (0.098)  0.194 (0.006)  sskew [‐]  0.531 (0.093)  0.452 (0.373)  0.629 (0.016)  skurt [‐]  ‐0.645 (0.376)  ‐1.017 (1.728)  ‐0.226 (0.333)  acta imeko | www.imeko.org  november 2016 | volume 5 | number 3 | 86  cavities of voice professionals: an mri-based study, journal of voice 27 (2013) pp. 132-140. [6] t. vampola, a.m. laukkanen, j. horáček, j.g. švec, vocal tract changes caused by phonation into a tube: a case study using computer tomography and finite element modelling, of the acoustical society of america 129 (2011) pp. 310-315. [7] g. kannan, a.a. milani, i.m.s. panahi, r.w. briggs, an efficient feedback active noise control algorithm based on reduced-order linear predictive modeling of fmri acoustic noise, ieee transactions on biomedical engineering 53 (2011) pp. 33033309. [8] d. aalto et al., large scale data acquisition of simultaneous mri and speech, applied acoustics 83 (2014) pp. 64-75. [9] x. shou, et al., the suppression of selected acoustic frequencies in mri, applied acoustics 71 (2010) pp. 191-200. [10] g. sun, et al., adaptive speech enhancement using directional microphone in a 4-t scanner, magnetic resonance materials in physics, biology and medicine 28 (2015) pp. 473-484. [11] i. frollo et al., measurement and imaging of planar electromagnetic phantoms based on nmr imaging methods, measurement science review 10 (2010) pp. 97-101. [12] j. přibil, j. horáček, p. horák, two methods of mechanical noise reduction of recorded speech during phonation in an mri device, measurement science review 11 (2011) pp. 92-98. [13] y. dodge the concise encyclopedia of statistics, springer, 2008, isbn 978-0-387-32833-1. [14] s. aleinik, o. kudashev, “estimating stochasticity of acoustic signals”, in: speech and computer, lncs 8773. a.ronzhin, r.potapova, v.delic (editors). springer, cham, heidelberg, new york, isbn 978-3-319-11580-1, pp. 192-199. [15] e-scan opera, image quality and sequences manual, 830023522 rev. a, esaote, genova, italy, 2008. [16] j. přibil, d. gogola, t. dermek, i. frollo, design, realization, and experiments with a new rf head probe coil for human vocal tract imaging in an nmr device, measurement science review 12 (2012) pp. 98-103. microsoft word article 6 157-1363-1-le.docx acta imeko  february 2015, volume 4, number 1, 26 – 34  www.imeko.org    acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 26  good practice guide for calibrating a hydrophone "in situ"  with a non‐omnidirectional source at 10 khz  albert garcia‐benadí 1 , javier cadena‐muñoz 2 , joaquín del río fernandez 2 , xavier roset juan 2 , antoni  manuel‐làzaro 2   1  laboratori de metrologia i calibratge, centre tecnològic de vilanova i la geltrú, universitat politècnica de catalunya (upc), rambla  exposició, 24 , 08800 vilanova i la geltrú, barcelona, spain, albert.garcia‐benadi@upc.edu 2  sarti research group. electronics dept. universitat politècnica de catalunya (upc). rambla exposició 24, 08800, vilanova i la geltrú.  barcelona. spain.+(34) 938 967 200, www.cdsarti.org       section: research paper   keywords: hydrophone; uncertainty budget; seawater  citation: albert garcia‐benadí, javier cadena‐muñoz, joaquín del río fernandez, xavier roset juan, antoni manuel‐làzaro, good practice guide for  calibrating a hydrophone "in situ" with a non‐omnidirectional source at 10 khz, acta imeko, vol. 4, no. 1, article 6, february 2015, identifier: imeko‐acta‐04  (2015)‐01‐06  editor: paolo carbone, university of perugia   received december 10 th , 2013; in final form december 16 th , 2014; published february 2015  copyright: © 2014 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: (none reported)  corresponding author: albert garcia‐benadí, e‐mail: albert.garcia‐benadi@upc.edu    1. introduction  nowadays a multitude of tests in the marine environment are performed, such as the measurement of salinity, acidity or basicity (ph) and carbon dioxide (co2). some of these tests include the measurement of noise pollution as well as the study of cetaceans in the marine environment. hydrophones are used to perform such tests. these devices are microphones for the marine environment. the hydrophones can be of different types, but their principal task is the transformation of pressure variation into an electrical variation. most hydrophones are based on a piezoelectric transducer that generates electricity when exposed to a change in pressure. the main parameter that relates the electrical and pressure magnitude is sensitivity. the following equation (1) is used to measure sensitivity            ref ref p p v v s log20 (1) the unit of sensitivity is expressed as db rel 1 v/µpa, because its unit is db (decibels) and then the unit details the reference values, in this case vref is 1 v and pref is 1µpa. the quantity pref is 1 µpa because it is the basic pressure in seawater. in air pref is 20 µpa. in our case the output value is voltage, v, and via the sensitivity the voltage changes to pressure, p. a practical case: the hydrophone has a sensitivity value of -192 db rel 1v/µpa, which gives 15 mv. the real pressure can be described by the equation (2), and the result is 59·106 µpa. in this example the reference values are not used because the voltage value is calculated in v and the result obtained is µpa. 2010 s v p  (2) in (2) the pressure value is calculated, but the interesting parameter is the reception level, rl. equation (3) shows the relation with the pressure and the result is in db. abstract  the aim of this paper is to provide the basis for the calibration of a hydrophone "in situ" with a non‐omnidirectional source at 10 khz,  thus assigning a value of uncertainty, which may be high, but according to requirements, may be sufficient.  acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 27           reference read p p rl log20 (3) the real result for 15 mv is 155.52 db. the sensitivity is a function of frequency, thus the aim of the hydrophone calibration is to obtain the sensitivity as a function of frequency. the calibration of this equipment is detailed in various standards such as [1], where the calibration is done as a function of frequency. the standard calibrations are carried out in laboratories where all parameters are controlled. the calibration method is described in [1]. the standard is divided into 7 topics: free-field reciprocity, free-field calibration by comparison, calibration by hydrostatic excitation, calibration by piezoelectric compensation, acoustical coupler reciprocity, calibration with a pistonphone and calibration with a vibrating column. the uncertainty value obtained in accredited laboratories is less than 1 db rel 1 v/µpa. although standard calibrations of hydrophones are essential to maintain the quality of acoustic monitoring, there is also a need for in-situ calibrations. the high cost for marine observatories to retrieve and redeploy the hydrophones, the requirement for continuous operation and the need for more frequent calibrations are the main factors carried out on in-situ calibrations. therefore, we propose the calibration of the hydrophone in the marine environment at 10 khz with a non-omnidirectional source. the reason for using a nonomnidirectional source is that it has more problems, and the purpose of the paper is to solve those problems by giving clear guidelines. this method of calibration involves a considerable increase in the uncertainty because the main parameters are not controlled, but measured. another problem is the low reproducibility of the test since the same conditions over sea surface and underwater will not occur again. however, in many cases this increment of the uncertainty compensates the little investment in performing the calibration. the objective of the calibration is to obtain the sensitivity as a function of frequency. performing the calibration in a laboratory is expensive because of the logistic involves three steps. the first is the process which involves ships and divers or rov (remote operated vehicle) to bring the hydrophone back to the surface. the second step is the processing time for the calibration and the last one is similar to the first step but for the return process to put the equipment back. all these procedures are expensive, and the processing time could be more than 1 month. for these reasons the aim of the paper is to propose an in situ calibration methodology to reduce the calibration time and costs. in order to achieve this objective it is necessary to evaluate various elements of the environment as well as the spreading factor, the attenuation index and the echo factor. 2. development  in this section, the method used, the equipment required, the basic equations of the sound level in marine environment, the geometrical approach of the scene, all relevant factors in marine environment sound and their uncertainty calculations will be explained in detail. 2.1. equipment  the equipment that is needed to carry out the calibration consists of a sound source, a hydrophone and a gnss (global navigation satellite system) receiver. the properties of the sound source, depicted in figure 1 (left), have to be known, as well as its calibration uncertainty, the tvr (transmit voltage response) as a function of the spl (sound pressure level) and the emission frequency. the hydrophone, depicted in figure 1 (center), (dut, device under test) must be characterized with its sensitivity as a function of frequency. in our case, the hydrophone is an integrated device, including the analogue receiver, the digital converters and the ethernet transmitter. the hydrophone is a bjorge mark, model naxys ethernet 02345. it is composed of an acoustically transparent coverage membrane, where the transducer element is located. figure 2 shows all enclosed elements. the signals sent by the hydrophone are received by an internet protocol suite, in this case user datagram protocol (udp) server. the data structure is detailed on the webpage of the manufacturer. the gnss receiver has to be able to get the raw data from the satellite and to be compatible with an open source program package such as rtklib [2] applications, where the acronym rtk means real time kinetic. the signal generator used in this calibration procedure is a hp 33120a device. it is depicted in figure 1 (right) and provides the signal of amplification. it is connected to the figure 1. the non‐omnidirectional sound pressure generator, lubell model ll9642t,  used  as  acoustic  source  (left),  the  hydrophone  (bjørge  naxyx ethernet  hydrophone  02345  used  as  acoustic  receiver  (center),  and  the signal generator hp33120a (right).   figure 2. main components in the hydrophone.  acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 28  amplifier and a sound pressure generator. the signal generated is a pulse with a frequency of 10 khz, which will be generated once per second. figure 3 shows one of these pulses injected into the environment. the reason why a pulse is generated every second is to avoid any interferences which may arise in reception. if the signal generated is a continuous pulse, there would be interferences in the reception. these interferences will be a surface seawater and seafloor bounce. the preliminary study of the best conditions is detailed in section 2.5. the compass is imbedded in a gnss receiver with bluetooth connection. the gnss is a woxter mark, model bt-tracer100. 2.2. basic equations  the simplified form of the sound wave propagation in environment equation (4), links three acoustic parameters. tlslrl  (4) the source sound level, sl, is the acoustic spectral density produced by an acoustic source recalculated to a distance of 1 meter. the receipt level, rl, is the acoustic signal obtained with the hydrophone. transmission loss, tl, is a decrease in the sound level radiated by a target over a distance. the rl value is obtained with a sensor, and the value sl is obtained at 1 meter with another calibrated hydrophone. the measurements are performed with another hydrophone, in this case the b&k 8103. the calibration is performed at 1 meter deep so as to avoid the initial perturbations of the transient signal from the generator. the output of all generators is detailed at 1 meter. in this case, the source is non-omnidirectional and for this reason it is very important to know the direction of emission. this factor is studied in sections 2.4 and 5.1. the last contribution is the transmission loss, which is composed of other factors, as described in equation (5). echorrrctl  )1000log( (5) where c is the spreading factor, � is the attenuation index [3], r is the distance between source and receiver in figure 3. pulse injected into environment.  figure 4. the attenuation index as a function of temperature and frequency.  the salinity, ph and depth are 30  0 /00, 8 and 20 m, respectively.  figure 5. the attenuation index as a function of depth and frequency. the  salinity, ph and temperature are 30  0 /00, 8 and 15 °c, respectively.  figure  6.  the  attenuation  index  versus  ph  and  frequency.  the  salinity,  depth and temperature are 35  0 /00, 20m and 15 °c respectively.  figure 7. the attenuation index versus salinity and frequency. the depth, ph  and temperature are 20 m, 8 and 15 °c respectively.  acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 29  kilometers, and recho is the echo contribution of the seawater surface and the seabed. the bounce in the water surface implies a phase change. however, there is no phase change in the seafloor. as the distance of the first bounce on the water surface is greater than the rebound on the seabed, we consider only one of the two contributions. another representation of (4) with the simple variables is (6), where v is the voltage received by the hydrophone, ps is the pressure generated and p0 is the basic pressure in the seawater (1 µpa). echorrrcp p vs s  )1000log()log(20)log(20 0 (6) the sensitivity, s, is ready to be calculated in (3). the terms in (6) can be divided into different fields (7), (8) and (9) from (4), the first one being a function of the receiver, the second a function of the source and the last a function of the environment. conceptually the sensitivity is the electrical energy generated minus the pressure energy received, and this pressure energy received is the same as that generated minus the energy absorbed or lost in the environment. svrl  )log(20 (7) )log(20 0p psl s (8) echorrrctl  )1000log( (9) 2.3. transmission loss  the transmission loss depends on other quantities, as can be seen in equation (5). these factors are detailed below. the spreading factor is a function of the geometry and relief of the seabed. in an ideal case, the parameter c has two possible values. the c value is 20 for the spherical propagation, and the c value is 10 for the cylindrical propagation. however, in reality the c value is between 10 and 20. this parameter is very difficult to calculate, being the major source of error and uncertainty. the attenuation index α is a function of other basic parameters, such as the temperature, the salinity, the ph, the depth difference between emission and reception and the emission frequency. the behavior of increasing alpha is due to the increase of the frequency, but the other parameters are also important. the attenuation index α depends solely on environmental conditions, and indicates the attenuation value of any waveform that passes through the environment. the alpha value depends on the frequency of this wave, and increases with the frequency of the impulse waveform. for this reason, in the sea you can hear the low frequencies at greater distances than in the air. alfa can be modeled as a function of the frequency, ph, depth and temperature by equation (10). 2 332 2 2 222 2 1 2 111 fpa ff fpa ff fpa                 (10) the parameters of equation (10) can be obtained from (11). dsatc sa f sa f ddp ddp p ttta t c sa a c a t t ph                     0167,019,121,31412 )35(0018,01 1017,8 10 35 8,2 109,41083,31 102,61037,11 1 105,11011,91059,210937,4 )025,01(44,21 10 86,8 ) 273 1990 8( 2 ) 273 1245 4( 1 2105 3 294 2 1 382754 3 2 )578,0( 1 (11) the variation of α versus frequency and temperature is depicted in figure 4. the variation of α versus frequency and depth is illustrated in figure 5. the variation of � versus frequency and ph is shown in figure 5 and the variation of α versus frequency and salinity is depicted in figure 7. however, the gradients of salinity and ph are null, because the study is in offshore. moreover, because the generator is figure 8. join scheme of generator, pvc triangle and semicircle.  figure 9. source generator and nylon semicircle assembly.  acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 30  located 2 m under the sea surface, the gradient of temperature is also nil although, the temperature gradient in the first meter under the surface is very high because of the solar radiation. the last factor is the echo contribution. this factor is very complex to evaluate, thus if in the geometrical scene the velocity of acquisition and the time between two successive sound pulses are well planned, this factor is not nil, but is detectable graphically. for these reasons it only works with the main signal. these factors are described in the section 2.5. 2.4. non‐omnidirectional source  the aim of this article is to propose a good practice to evaluate the non-omnidirectional sound marine source, in this case at 10 khz. the first step is to characterize the sound generator as a function of angle at 1 meter of distance. the emitted sound is measured using a calibrated hydrophone, in this case a brüel &kjaer (b&k), model 8103, located over a semi sphere with a 1 m radius. figure 8 shows the semi-circle establishment in the source to obtain the directionality generation of the source. given that the source is directional, its directivity is determined by the compass located in the pvc triangle on the water surface. initially the mission of the pvc triangle is to prevent the rotation of the generator that is sunk in water, and to determine the direction of emission. another advantage of the pvc triangle is that it can be used to orient the generator. the generator has a cubic shaped metal skeleton. in the top rear vertices of the cube, the vertices of the pvc triangle are joined, figure 8. the b&k hydrophone is located over the nylon semicircle. 2.5. initial geometrical study  the sound velocity in seawater is a function of the salinity, the temperature and the depth, but in this case the tests are made offshore and around 2 km from dut position. in these conditions the gradients of temperature, salinity and depth are estimated as being nil because when the temperature and salinity changes, the sound speed changes too. another important parameter is the seabed relief. in our case the bathymetry illustrated in figure 10 is known. this approach is correct for calculating the distance because the main signal follows a different path than the first echo. figure 12 shows a schematic with the main parameters. equations (12) and (13) are based on the distance, d0, which is the minimum distance between the sound generator and the hydrophone, and the elapse time, �t, between the main wave and its first echo.              r rr htc hhtc d    sin 3)sin( sin2 1 22 0 (12)           22200 sin)sin( sin 1 rhddhrc t   (13) where: hr = the distance between the hydrophone and the seafloor d0 = distance between the sound generator and the hydrophone on a plane orthogonal to the surface and including the emitter and the receiver c = the sound velocity in the water (about 1500 m/s) θ = angle between the reflected wave and the surface plane in a plane orthogonal to the surface and including the emitter and the receiver he = the distance between the generator and the sea surface. the he and hr are measured with tape measure. the hr was measured by the diver in the maintenance tasks, and he was manually measured when the test was done. the he does not appear because the reflections over the surface are not evaluated since the distance is bigger than 2 m. for this reason, the minimum distance will be hr which is 1 meter. if we fix the time interval to 1 s, the minimum distance between generator and dut must be 750 m. figure 11 shows an example of the real location used for this test. 3. procedure  the calibration process starts with the generation of the sound pulses through a sound generator, and the reception signal by the hydrophone. the main parameters are the generator position, the environmental conditions of the water, the signal amplitude and its frequency. the equipment used and the work method applied are detailed in this section. figure 10. bathymetry of the vilanova coast.  figure 11. distance between emission and reception in a real case.  acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 31  3.1. readings  there are two points for collecting the data. the rl is the dut. the collected data in the ship are: the orientation, measured with a compass situated over the pvc triangle and the gps satellite signal with the raw signal. the sl is measured with a b&k hydrophone located over the nylon semicircle. the temperature, the salinity and the ph are collected through the obsea observatory [4] located where the dut is also positioned. the readings stored aboard ship are: the signal at two frequencies l1 (1575.42 mhz) and l2 (1227.60 mhz) of the reception signal of the gnss receiver. orientation of the source through a compass located at the main vertices of the triangle. emission depth, he. the hydrophone collects the following readings: voltage receipt, which is sent to the internet using the protocol udp (user datagram protocol). environmental conditions such as temperature, salinity and ph are measured in obsea. other required readings for the post-process are the institut català de cartografia (icc) caster file that will be included in the free software real time kinetic (rtk). the motivation to use this system is to carry out a differential correction of the position without the need for our own terrestrial base. for this reason, we use the services provided by different entities such as icc and igs among others. this allows us to make a high quality positioning but without the financial investment. 4. uncertainty calculation  the uncertainty study starts with (6), where the propagation law is applied by the guide of uncertainty (gum) [5]. using the nomenclature detailed in (7), (8) and (9), equation (14) is obtained: )()()( )()()()( 222 2 2 2 2 2 2 2 tluslurlu tlu tl s slu sl s rlu rl s su                           (14) in the next subsections the receiver, the generator and the environment uncertainty are reviewed. the terms of all factors included in (14) are not correlated, because the correlation between the distance and the generator (sl), does not exist, since the generation is done irrespective of the distance. the environment is relevant, in fact it is characterized by three parts, the attenuation index (independent of distance), the spreading factor (element that depends on the morphology and the depth where the test is performed) and the echo contribution (not considered, since we are at a distance greater than the minimum required). 4.1. uncertainty of the receiver   the hydrophone has been considered as a black box where only output values (counts) are known, because we do not analyze the transducer neither the analog to digital conversion. the elimination of the offset of the signal has been done by the difference between maximum and minimum. therefore the possible variation of the offset can be eliminated. thus, the uncertainty of the receiver is a function of the uncertainty in the voltage measurement. the error propagation is shown in (15) 3)10ln( 20 )()( 2 min 2 2 2 2 v v vu v rl rlu                 (15) where the voltage uncertainty has a rectangular probability distribution with a width of the resolution in voltage. the hydrophone sends the values in counts. the voltage is calculated through the conversion factor of the daq and the gain value. in this case, the hydrophone has a converter of 16 bits. as the full scale of the equipment is 5 v, we obtain that the conversion factor between counts and v is 2.5 v/215 counts. the value vmin is the minimum voltage that the hydrophone gives and corresponds to the value of 1 count. the value of the squared uncertainty in voltage is vmin/√6 corresponding to a rectangular probability distribution with a width equal to the minimum value assigned. 4.2. uncertainty of the generator   the uncertainty of the generator is obtained through its calibration certificate. the error propagation is shown in (16) )( )10ln( 20 )()( 2 2 0 2 2 2 s s s s pu pp pu p sl slu                     (16) where u(ps) is obtained in the calibration certificate of the generator as follows (17). 2 g g 2 20 bks bk bk 220 bks s 2 k u 1020 )10ln(v )v(u10)p(u                   (17) the values of ug and kg are obtained from the calibration certificate of the generator. these parameters are calculated by the accredited laboratory following the standard [1] and the parameter sbk and vbk are the sensitivity and the voltage of the b&k hydrophones, respectively. this term is included because the output signal is measured with the b&k hydrophone. 4.3. uncertainty of the environment   the uncertainty of the environment is a function of many parameters and it is the largest contributor to the uncertainty budget. the error propagation is described by equation (18): )()()()( 2 2 2 2 2 2 2 cu c tl ru r tl u tl tlu                            (18) figure 12. schematic test with the main parameters.  acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 32  in (18) the uncertainty of α, the uncertainty of the distance and the uncertainty of spreading appears. the attenuation index, α, is a function of many parameters explained in [3]. equation (10) shows the dependence of α with other factors. the values of parameters of equation (10) are detailed in (11), where c is the sound speed in seawater, sa is the salinity in 0/00, ph is the measure of the acidity or basicity, t is the temperature in degrees celsius, and d is the depth in meters. the uncertainty is a function of the typical uncertainty in temperature, salinity depth, and ph. every term is evaluated through the evaluation of its resolution, its variation and its expanded uncertainty of the calibration certificate with its coverage factor. equation (19) shows the typical uncertainty, with a fictitious parameter w. the parameter w will be finally replaced by the uncertainty of temperature, salinity, ph and depth. 2 ncalibratio ncalibratio 2 iationvar 2 resolution k w 3 w 12 w )w(u                    (19) the spreading uncertainty is calculated using a previous study [6]. this factor is a function of the seabed geometry, the localization and the depth of the generator point, and independent of the other parameters. this factor is a function of the relief of the seabed and the type of generator used. in order to obtain the uncertainty of the distance, the rectangular probability distribution with a 1 m width is used. the reception quality factors of the gps data in raw, after the data integration from the caster, are analyzed offline. we could affirm that the width of the rectangular probability distribution is less 1 m. but this fact is too optimistic; therefore 1 meter is chosen, in this case. it is thus an overestimation. some values calculated are detailed in section 5.3. 5. results  5.1. direction of emission  the direction of emission is calculated through the nylon semicircle and the b&k hydrophone (model 8103); the values obtained are shown in table 1. figure 13 shows the composition graphically. the heights between the emitter and the receiver remain constant, since neither the emitter nor the receiver changes its height regarding the maximum emission angle and the angle between the dut position and the pvc triangle indicates, with the compass measure, the correction value to apply. 5.2. evaluation of position and distance   the use of the rtklib for position correction allows us to know the correct position with an accuracy of, less than 0.5 m. without this correction the position has an accuracy of around 20 m. table 1 and table 2 shows an example of obtained data without correction, and table 3 shows an example with the corrected data. once the emission point is located, then it is necessary to calculate the distance between the generator and the receiver. the distance is calculated with the vicenty [7] conversion. table 4 shows the distance corresponding to the values shown in table 1. the uncertainty of distance is very difficult to obtain because the uncertainty for data of gnss receivers is an independent study, which is being evaluated by many people. in this case, the quality factor included inside the rtklib has been analyzed and the position has been evaluated with a pair of gnss. once the quality factor of the correction data is evaluated and increased with the contribution of vicenty conversion, the value of the distance uncertainty is 0.60 m. this value is obtained for a rectangular distribution probability with a width of 1 m. 5.3. attenuation index and its uncertainty  the environment absorption index is calculated with a high-accuracy recorder for conductivity, temperature and pressure (model sbe 16plus v2), henceforth ctd. the parameters are temperature of 24.04 degrees celsius, salinity of 38.04 0/00 and ph of 8.3. the ctd is located in the obsea. in table 5, some of the values obtained by the ctd during the tests are shown. the variations during the test are negligible because of the short interval of the test time of approximately 15 minutes. the attenuation index is 0.85 db/km. the effective time has been 15 minutes as the test lasted one hour or more, but not all the positions obtained by the gnss receiver were reliable. for a position to be reliable it has to be validated with the data obtained by the caster. this does not always happen because satellites change position and sometimes suitable signals or numbers are not available. if the series of data takes a long time to collect, ambient variables in the water would have to be taken into figure 13. scheme of the reference plane.  acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 33  consideration and so there would be a margin of uncertainty at each point. however, in our case, reliable data is taken every 15 minutes, so we can state that the ambient variables such as temperature, ph and salinity remain constant the uncertainty value for this case is 0.99 db/km. this value is really high in comparison to the absorption index, but it is normal due to uncontrolled parameters. 5.4. spreading value  the spreading value is calculated in another study [6], where the sensitivity uncertainty is known. in this case the value is 16.04±0.67. it is important to note that the spreading factor does not have units. the initial spreading value expected was around 10, because of the shallow depth, about 20 meters. therefore, a cylindrical propagation is expected. but the change is produced because it was initially thought that the distance between receiver and generator would be sufficient to transform the cylindrical propagation into a spherical one, but it is not sufficient. the uncertainty value in this case is 0.67. in this case because the seafloor is flat and so the relief does not change, the value of the spreading factor is constant. 5.5. evaluation of the signal receipt  the generator and reception systems are not synchronized. therefore the received signal timing is independent on the generation. because the sampling frequency is 96000 samples/second, the reception file is very big and the duration of the record is 1.5 hour, even though the test time is 15 minutes. the lack of time table 1. readings received in db at 1 meter from source.  angle over  semicircle  ‐45° regard to 0   plane  0  +45° regard to 0  plane  ‐90 186.34  186.19  186.19 ‐45 186.19  184.06  183.04 0 183.77  193.54  189.06 45 180.69  183.57  183.46 90 186.19  186.99  185.81   table 2. coordinate example without correction, in degrees.   % gpst  latitude n   longitude e 2013/09/19 10:21:21.000 41.17613741  1.765862013 2013/09/19 10:21:22.000 41.17613436  1.765833378 2013/09/19 10:21:23.000 41.17614067  1.765866606 2013/09/19 10:21:24.000 41.17614583  1.765843793 2013/09/19 10:21:25.000 41.17614367  1.765922205   table 3. coordinate example with correction, in degrees.   % gpst  latitude n   longitude e 2013/09/19 10:21:21.000 41.17608642  1.765954648 2013/09/19 10:21:22.000 41.17607901  1.765961776 2013/09/19 10:21:23.000 41.17608325  1.765958510 2013/09/19 10:21:24.000 41.17608555  1.765966284 2013/09/19 10:21:25.000 41.17608986  1.765958881   table 4. example of distance.   % gpst  distance (km)   2013/09/19 10:21:21.000 1.309357 2013/09/19 10:21:22.000 1.310040 2013/09/19 10:21:23.000 1.310051 2013/09/19 10:21:24.000 1.3096851 2013/09/19 10:21:25.000 1.3100011 table 5. example of data obtained from ctd.   day  time  temperature  (°c)  salinity  (‰)  ph  19/09/2013 10:20:57 23.76  38.04  8.30 19/09/2013 10:21:17 23.76  38.04  8.30 19/09/2013 10:21:37 23.76  38.04  8.30 19/09/2013 11:01:37 23.79  38.04  8.29 19/09/2013 12:01:57 24.02  38.05  8.30 figure  14.  temporal  frame  and  frequency  sub‐frame  for  the  acceptation  case.  figure 15. temporal frame and frequency sub‐frame for the decline case.  acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 34  synchronization between the generator and the receptor means that the post-processed signal is a critical task. the signal processing has been made with a matlab® application. the sequence of this application is: first to analyze every frame that arrived from the hydrophone (512 values in int16), then to locate a maximum value and finally to create a sub-frame with the 15 index values before the maximum and 16 index value after the maximum. the subframe, with 32 values, is processed with a fft in order to determine if the maximum value is centered in 10 khz. figure 14 shows the temporal frame (512 values) and the fft of the sub-frame (32 values) for the acceptation case, and the declination case is illustrated in figure 15. the declination case can be produced by many causes such as crustaceans, fish, seaweed and bubbles, among others. the case accepts the maximum and minimum values of the signal, and this value divided by 2 is the amplitude of the received signal. once the amplitude value for the acceptation case and its time are found, another matlab® application is used to overlap the time values. 5.6. sensitivity value  the last step is to assemble all contributions and calculate the sensitivity value and its uncertainty. table 6 shows an example of the different values of the contribution to uncertainty. the step is calculated for every point, table 7 shows an example with some points. the final result is that the sensitivity is –189.25 ±2.95 db rel 1 v/µpa at 10 khz. 6. conclusions  the article gives the basis for considering the realization of an in situ calibration of a hydrophone with nonomnidirectional sound source. the uncertainty contribution is higher in comparison to the uncertainty obtained in the laboratory which is less than 1 db. for this reason, it is necessary to repeat this test to improve the different contributions to uncertainty. improvements could include incrementing the amplitude value of the source generator which lowers the contributions factors to uncertainty of source pressure and of receiver voltage. in the last hydrophone calibration, in february 2010, the sensitivity value was -192 db rel 1 v/µpa at 10 khz, which is inside the confidence interval. acknowledgement  this work was supported by the spanish ministry of economy and competitiveness under the research project: “sistemas inalambricos para la extension de observatorios submarinos” (ctm2010-15459). references  [1] “iec60565underwater acoustics –hydrophones –calibration in the frequency range 0,01 hz to 1 mhz”, iec, 1997. [2] t.takasu, rtklib: open source program package for rtk-gps, foss4g 2009 tokyo, japan, november 2, 2009. [3] r.e. francois and g. r. garrison “sound absorption based on ocean measurements: part ii: part ii: boric acid contribution and equation for total absorption” j. acoust. soc. am. vol. 72, n.6, december 1982. [4] aguzzi, j.; mànuel, a.; condal, f.; guillén, j.; nogueras, m.; del rio, j.; costa, c.; menesatti, p.; puig, p.; sardà, f.; toma, d.; palanques, a. “the new seafloor observatory (obsea) for remote and long-term coastal ecosystem monitoring”. sensors 2011, 11, 5850-5872. [5] “evaluation of measurement data. guide to expression of uncertainty in measurement”, september 2008. [6] garcia-benadí, a.; cadena-muñoz, f.j.; sarria, d.; sitjar, r.; del rio, j. "good practice guide for c calculation". in: 5th martech international workshop on marine technology. pp. 61 66. 0011. isbn 978-84-616-5764-3. [7] t. vicenty. ”direct and inverse solutions of geodesics on the ellipsoid with application of nested equations”. survey review xxii, 176, april 1975. table 6. contribution to uncertainty.   distance [km] u(sl) [db]  u(rl) [db]   u(tl) [db] u [db] 1.362060 0.01 0.01  1.31 2.73 1.400959 0.01 0.01  1.35 2.80 1.402288 0.01 0.01  1.35 2.80 1.438561 0.01 0.01  1.38 2.87 table 7. sensitivity table example.   distance (km)  sensitivity (db)   uncertainty (db)  1.335602 ‐190.17  3.00 1.401709 ‐195.89  2.70 1.525931 ‐190.92  2.93 1.664731 ‐192.31  3.19 microsoft word 237-1859-4-le-rev2 acta imeko  issn: 2221‐870x  december 2015, volume 4, number 4, 66‐74    acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 | 66  abstract  rolling contact fatigue (rcf) plays a critical role in railway components, and the characterization of materials used, in terms of rcf life,  is  still  an  open  task,  made  complex  by  the  interactions  of  different  phenomena.  the  contact  surface  has  a  direct  impact  on  the  pressure exerted and can change during the test, due to wear. the procedure proposed consists in using vibrations of a test bench  during rcf‐life tests to identify when wear increases and causes a quick flattening of the specimen’s surface, and when this process is  complete. the procedure is applied to two case studies regarding wheel and rail steels. in the tests, a wheel steel specimen rotates  against a rail steel specimen, while pressed against each other by a constant force. at regular intervals weight loss and surface analysis  are performed, while vibrations and torque are monitored continuously. destructive tests are carried out at the end of each test.  results from non‐destructive measurements were used to provide input data to a numerical simulation, used to determine the cyclic  plasticity  properties  of  the  material.  the  methodology  proposed  shows  the  potential  application  of  vibration  measurements  for  detecting wear rates thus allowing supporting or partially supplanting destructive testing. using vibration measurements to detect high wear rates in  rolling contact fatigue tests  matteo lancini 1 , ileana bodini 2 , david vetturi 1 , simone pasinetti 1 , angelo mazzù 1 , luigi solazzi 1 ,  candida petrogalli 1 , michela faccoli 1   1  university of brescia, department of mechanical and industrial engineering, via branze 38, brescia, italy  2  university of brescia, department of information engineering, via branze 38, brescia, italy    section: research paper   keywords: damage evaluation; rolling contact fatigue; vibrations   citation: matteo lancini et al, using vibration measurements to detect high wear rates in rolling contact fatigue tests, acta imeko, vol. 4, no. 4, article 13,  december 2015, identifier: imeko‐acta‐04 (2015)‐04‐13  editor: paolo carbone, university of perugia, italy  received december 18, 2014; in final form march 20, 2015; published december 2015  copyright: © 2015 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  corresponding author: matteo lancini, e‐mail: matteo.lancini@unibs.it    1. introduction  characterization of materials is critical in railway applications because cyclic, localized stresses lead to rolling contact fatigue (rcf) phenomena, which can cause severe damage and sudden failure of components. wear, also, is a common phenomenon occurring on both rail and wheels, but its interaction with rcf is not always detrimental: in specific conditions (uniformity, low rate, stability), it could optimize contact geometry, increasing rcf life, while, in non-optimal conditions, it could reduce rcf life. moreover, the high shear stress leads to a complex combination of concurring damage phenomena [1], [2] which should be taken into account. models describing cyclic plasticity are available in literature, but, if material parameters are obtained by laboratory tests in standard stress condition [3], [4], their results are not reliable, especially due to a different propagation mechanism [5]. a reliable procedure to analyze rcf-wear interaction would be to carry on accelerated rcf tests on controlled slip ratio conditions with multiple specimens, interrupting the test at different times and performing destructive analysis on the specimen, which is costly and time consuming. in this paper, an alternative method is proposed: vibrations and torque are monitored throughout the tests and compared with the damage detected by non-destructive methods (contact surface state, wear) and by destructive methods, at the test end (subsurface microstructure and hardness). the correlations found between the measured parameters and damage phenomena, occurring at the surface and subsurface region, such as cyclic plasticity or crack nucleation acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 | 67  and propagation, could be used to replace part of the destructive tests. this would reduce both cost and time needed to perform rcf tests on railway materials. 2. materials  to evaluate the robustness of the proposed approach, different sets of specimens were used. pairs of discs, machined out of wheel rims and railheads respectively, to represent different materials combinations, made up each set. a first set, which was called “primary materials”, was used to investigate the correlation between results from destructive and non-destructive methods and constitutes the primary experimental data source for this work. a second set, here called “secondary materials”, was used for preliminary tests, performed to select which quantity to monitor, out of the six proposed (see paragraph 3.2). a third set, moreover, was employed for early stage tests, using the same materials as in the first set, in order to analyze the behavior in the early stages of the process. 2.1. primary materials  the proposed procedure was applied to pairs of wheel rims and railhead materials (er8 en13262 and uic 900a respectively), as a case study, with conditions reported in table 1 with test id a to f and s3. the wheel specimens were 60 mm wide and 10 mm thick cylindrical discs, while the rail specimens were 59.5 mm in diameter, 10 mm in thickness and had a crowning radius of 200 mm, to prevent any border effect on the contact surface. the mechanical properties of wheel rims (er8 steel) and railheads (900 a steel) tested, as reported by the manufacturer, are shown in table 2. three different contact pressure levels were tested, in two different sliding ratio conditions, totaling six different rolling/sliding parameter combinations, here reported in table 1. vibration recordings from those with the highest sliding ratio (s = 3 %) and with the highest nominal pressure (p = 1 500 mpa) were also examined using the proposed procedure. 2.2. secondary materials  a second set of specimen, here called s2, was used in preliminary tests. sliding ratio, nominal contact pressure and other test condition for this secondary case study are reported in table 1. both wheel and rail specimens had a diameter of 60 mm, and a thickness of 15 mm. wheels discs were made by machining a superlos® steel by lucchini, while rail discs were made by a 900 a steel. mechanical properties of both materials, as reported by the manufacturer, are listed in table 2. 3. methods  3.1. measurement system  the test bench used is a bi-disc machine dedicated to study interactions between two components subjected to cyclic contact in different load conditions, already presented in a previous work by the authors [6], and whose general layout can be found in figure 1. the contact load, up to 70 kn, is maintained constant by means of a servo-hydraulic actuator, enabling the sliding of one of the mandrels, while two independent 33 kw engines provide the specimens rotation. both engines and the actuator are controlled to perform tests at given slip ratio, rotating speed, contact force and engine torque. on both mandrel supports, piezo-accelerometers were mounted using a cyanoacrylate glue, as shown in figure 2: one in the vertical and one in the horizontal plane, both normal to the rotation axis. the transducers used were wilcoxon 736 iepe accelerometers, with a nominal sensitivity of 0.98 v/(m/s²), a full scale of 5 m/s², and a linear bandwidth in the 5 hz to 20 khz range. the signals from the two accelerometers, as well as a torque sensor, positioned on the sliding mandrel, were acquired by means of a configurable data acquisition system at a 5 khz synchronous sampling frequency. table 2. material properties of the steels used.  steel    er8  900a superlos® ultimate tensile stress [mpa] 940  930 980 monotonic yield stress [mpa] 590  470 640 cyclic yield stress  [mpa] 470  390 525 necking [%] 54  26 ‐ elongation [%] 17  14 18 brinell hardness [hb] 230‐255  296 280 figure 1. test bench layout.  table 1. test parameters of the first case study (a to f), of the preliminary test (s2) and of the early stage monitoring test (s3).  test id  nominal contact  pressure  contact load  rail sample  rolling speed  wheel sample  rolling speed  sliding ratio  mpa  n  r.p.m.  r.p.m.  %  a  1 100  1 500 516.0 492.5  3 b  1 300  2 490 516.0 492.5  3 c  1 500  3 830 516.0 492.5  3 d  1 500  3 830 511.0 497.5  1 e  1 300  2 490 511.0 497.5  1 f  1 100  1 500 511.0 497.5  1 s2  1 100  7 557 497.5 502.5  1 s3  1 500  3 830 516.0 492.5  3 acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 | 68  an l101b-2k basler linear array camera was also used to monitor contact surface conditions of both specimens. the camera was acquired in sync with the bench rotation, in a setup leading to a resolution of 1 µm on the specimen’s mid surface. due to the high rotation speed, however, it was only used at regular intervals while the test was on halt, and the specimen was moved at 5 rpm. every 2×105 cycles the test was halted and weighing was performed with a precision scale (0.01 g resolution), to evaluate weight loss due to wear. at the end of each test, the samples were cut in mid-thickness, polished mechanically and etched (using 2 % nital), then inspected using an optical microscope to measure the plastic flow. following this, a leo evo-40xvp scanning electron microscope with an eds probe was used to seek for rcf crack paths, and hv10 micro-hardness was measured at different depths on the cross-section of each sample. 3.2. mechanical model  while vibration analysis is commonly used to detect faults on gears and bearing components [7]-[10], its use in material characterization of continuous discs is limited, as this is usually carried on with standardized methods, generally involving destructive testing or evaluation at the end of test. in previous works [11] a damage indicator, estimated from vibrations and correlated to surface cracks, was proposed. following the same approach the two specimens interactions are synthesized, as it is common in modal analysis, using frequency transfer functions (frf) between torque t and accelerations in the x and y directions, as depicted in figure 3. the idea behind this approach is that any variation in the material or geometry of the samples would be reflected by a simultaneous change in the modal parameters of the system, causing a change in the frfs values. because of this, a change in the dynamic behavior could be used to assess when crowing gets flattened, and the duration of this wear process. frfs were numerically computed as the ha/b cross-spectrum between a signal a and a signal b, divided by the auto-spectrum of signal b. in particular, the following 6 quantities were monitored: a. the transmissibility hy/x, between the y acceleration on the fixed mandrel support along the vertical axis and the x acceleration of the moving mandrel support along the pneumatic actuator axis, b. the rotating inertances hx/t and hy/t between the x and y accelerations and the torque t measured on the rotating axis of the sliding mandrel, c. the reciprocals to the previous three quantities, hx/y, ht/x and ht/y. though all these quantities are related to properties of the system, such as masses and stiffnesses, their connection to any proper mechanical parameter would require an overly complex multiple degree of freedom model, which would exceed the scope of this research. therefore, their absolute values are not meaningful per se, and only their variation from their value at the beginning of each test has been taken into account. 3.3. preliminary tests  preliminary tests were performed, on the specimens described in section 2.2, to select which quantity to monitor and in which frequency range. to avoid misreading due to external or uncorrelated sources of vibrations, the test bench was also monitored while running in jog mode, as well as in full operational mode, but without any load applied between the samples. a power spectral density analysis of x, y and t signals, recorded for about 100 s, and averaged using a 1 hz spectral resolution, revealed that their frequency content was located below 400 hz, while the same analysis performed in the final phases of standardized rcf tests revealed vibrational phenomena up to 1 000 hz. moreover, an experimental modal analysis of the pneumatic actuator, pointed out a resonance frequency lower than 100 hz, therefore, the spectral quantities monitored were treated using a band-pass filter between 400 hz and 1 000 hz. to select which quantity should be monitored, a comparison between the h functions computed, subtracted of their initial value and normalized on their maximum value, was performed during a full test of an unrelated rail/wheel couple, examined in a previous work [11] and here reported in figure 4. this comparison pointed out how the frf between torque and acceleration along the linear actuator axis, here called t/x, and its reciprocal x/t, displayed a sensible change, coherent with other damage indicators. in the initial 60 000 cycles, this quantity is less affected by accidental impacts or other impulsive events, here visible as spikes on the chart, which are frequently occurring due to the uncontrolled environment in which the bench operates. 3.4. vibration analysis  using the acquired data, the transfer function ht/x(ω) between torque and acceleration was computed averaging 5 figure 3. simplified frf based model.  figure 2. specimens mounted on mandrels (the accelerometers measuring axes are highlighted).  acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 | 69  windows of 1 s, thus producing a spectrum of 1 000 lines, with a spectral resolution of 1 hz, every 5 s. the initial dynamic behaviour of the system made up by the test bench and the intact specimens were assessed by averaging the ht/y(ω) values of the first 2 500 cycles, thus assumed as a set of reference values ht/xref(ω). to point out changes in the system, the difference dt/x(ω) between the current status and the reference value, for each frequency resolution ωi, has been computed, then the synthetic index dx/t has been evaluated as the root mean square (rms) summation for all frequencies, as shown in (1).           * 1 ///// 1    n i i ref xtixti ref xtixtxt hhhh n d  . (1) to smooth the resulting signal, given the relative slow damaging process, the rms value was averaged on a 30 s window. the same procedure, here illustrated in figure 5, was performed for the reciprocal hx/t, leading to an indicator dx/t, which was also monitored during the rcf tests. 4. results  4.1. vibrations  the values of dx/t for the four tests under scrutiny are reported in figure 6. all three tests with a high sliding ratio level (3 %) report a steep increase in value (from 10 to more than 100 kg-1m-1), after few thousand cycles, followed by an abrupt decrease after a variable amount of cycles, then settling around a stationary level (30 kg-1m-1). the last test, with a 1 % sliding ratio, displayed only a slight increase after 8 000 cycles to a stationary level (20 kg-1m-1). such behavior could be used to identify a period, characterized by higher dy/t levels, during which the damage process is different from both the initial and the stationary stages. the approximate onset and duration of these periods are reported in table 3, and their values depend on the nominal contact pressure, as visible in figure 7. figure 4. dy/x, dt/x and dt/y, on preliminary tests, where wear rate changes were detected by weight loss measurements [11] after 60 000 and 100 000 cycles. figure 5. signal processing procedure’s steps.  figure 1. dx/t indicators for tests a (black), b (red), c (green) and d (cyan) during the first 20 000 cycles of each test.  30 s averaged rms difference between spectra 400‐1000 hz band pass filter current frf reference frf torque and acceleration measurements 5 s averaged frf 30 s averaged frf acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 | 70  4.2. weight loss  the wear rate, in terms of weight loss as a function of cycle number, with varying nominal contact pressure p, was investigated in tests from a to f. results are shown in figure 8 and figure 9 for sliding ratios s = 1 % and s = 3 % respectively. given the linear relationship displayed between weight loss and cycle number in the steady state period of the wear curves, wear rates were calculated as the angular coefficient of the linear best fit of the experimental points. wear rate values increased as nominal contact pressure and sliding ratio increased. the wear rate in general was higher for the rail steel samples, despite the surface damage was more severe for the wheel steel specimens: this means that wear, in this case, mitigates the effect of surface pitting by removing damaged layers. this linear wear rate behavior suggests a stationary contact geometry already achieved before the first measurement (after 200 000 cycles), while the difference between 3 % and 1 % sliding ratio tests points out a decreased wear rate associated with the lower sliding condition. 4.3. damaged surface  width  shape changes due to wear and rcf were assessed by measuring the worn portion of the wheel specimen as shown on images recorded by the linear camera before test and after 200 000 cycles, here displayed in figure 10 to figure 13. the worn portion was identified by visual inspection of the recorded images and by manually selecting the image area of the specimen surface where linear patterns, due to the specimen machining during its production, were not present, or were only partially visible. the process was repeated on 20 images for each specimen table 3. onset and duration of high dy/t levels.  id  onset  offset  duration  [cycles]  [cycles]  [cycles]  [s] a  700  4 700  4 000  480 b  3 800  8 500  4 700  560 c  6 000  12 700  6 700  800 d  ‐n/a‐  8 200  ‐n/a‐  ‐n/a‐   figure 7. onset and duration of high dy/t levels.  figure 10. wheel contact surfaces before (upper) and after (lower) the first  200 000 cycles for test a.  figure 8. 1 % sliding test weight loss for rail (upper) and wheel (lower).  figure 9. 3 % sliding test weight loss for rail (upper) and wheel (lower).  0 2k 4k 6k 8k 1000 1200 1400 1600 c y cl e s nominal contact pressure [mpa] onset duration 1.61 x 10-063.00 x 10 -06 2.86 x 10-06 0 1 2 3 4 5 6 0 500000 1000000 1500000 2000000 w ei g h t l o ss [ g ] number of cycles 900a rail p = 1100 mpa p = 1300 mpa p = 1500 mpa 2.00 x 10-06 1.96 x 10-06 2.60 x 10-06 0 1 2 3 4 5 6 0 500000 1000000 1500000 2000000 w ei g h t l o ss [ g ] number of cycles er8 wheel p = 1100 mpa p = 1300 mpa p = 1500 mpa 10.0 mm acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 | 71  and then the ratio between damaged and visible area was averaged and multiplied by the nominal thickness of the specimen. when no clear traces of the original surface of the wheel specimen was identified in the image, the whole surface width (10.0 mm) was considered as damaged. 4.4. micro‐hardness  after the tests were completed, micro-hardness measurements were taken on the cross sections of the cut and polished specimens at different depths. the resulting hardness profiles are shown in figure 14. a higher hardening was observed in tests with higher sliding ratios and contact pressures, involving layers between 0.4 mm to 0.8 mm. this behavior could be associated with isotropic hardening. 4.5. scanning electron microscope  the inspection of specimen sections pointed out that the main damage mechanism, in both rail and wheel steels samples, figure 13. wheel contact surfaces before (upper) and after (lower) the first  200 000 cycles for test d.  figure 11. wheel contact surfaces before (upper) and after (lower) the first  150 000 cycles for test b.  figure 12. wheel contact surfaces before (upper) and after (below) the first  200,000 cycles for test c.  figure 14. micro‐hardness profiles on the cross‐section of rail (upper) and  wheel (lower) steel specimen.  10.0 mm 7.8 mm 10.0 mm acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 | 72  was ratcheting, initiating cracks which propagate to cause rcf failure. figure 15 shows the rcf crack path for various test conditions: the rail steel samples (a and c) were always less damaged than the wheel steel ones (b and d) in all tested conditions, and presented only surface cracks. in addition, the wheel steel samples also presented subsurface cracks. all surface and subsurface cracks, which were analyzed, displayed a shallow angle to the surface and follow the plastically deformed material during their growth. 4.6. early stage measurements  to better understand the relationship between the vibrationbased index proposed and wear, in early stages of the damaging process, a separate test has been performed, as indicated in section 2.2. while for previous tests a surface image was available only every 200 000 cycles, this test was halted every 1 500 cycles, and the wheel specimen surface was photographed. this allowed to identify when the flattening due to wear started, and its progression, using the same procedure described in section 4.3, to assess the damaged surface width from surface images, such as the ones displayed in figure 16. the superposition between the two measurements, up to the first 40 000 cycles, is displayed in figure 17, and shows how the dx/t index and the damaged surface width have a similar behavior. during the first 12 000 cycles dx/t is stationary and low (100 kg-1m-1), while the wheel surface does not display any damage. between 13 500 and 25 500 cycles, dx/t quickly increases up to a high stationary value (300 kg-1m-1). in the same period the damaged width rises from 0 mm to 7.5 mm, indicating a rapid flattening of the surface due to a high wear rate. after 25 000 cycles, dx/t displays a stationary behavior, while wear rate decreases to about one third, completing the flattening at 34 500 cycles. 5. discussion  the material plastic behavior was simulated using a numerical simulation for rolling contact [12], already calibrated on er8 en13262 and uic 900a74 [13]. the model predicts plasticization in a two-dimensional plane-strain half-space subjected to contact pressure and friction, also taking into account wear as a concurring phenomenon removing layers from the surface. input parameters for this model are specimen geometry (radius and contact width), load, friction coefficient, material   figure 17. early stage damage progression: damaged surface width from photographs (red, right axis) and vibration‐based dx/t index (blue, left axis).  figure  15.  rcf  crack  path  in:  a)  rail  steel  sample  tested  at  s = 1  %  and  p = 1 500 mpa; b) wheel steel sample tested at s = 1 % and p = 1 500 mpa; c)  rail steel sample tested at s = 3 % and p = 1 300 mpa; d) wheel steel sample  tested at s = 3 % and p = 1 300 mpa.        figure 16. surface of the wheel specimen  in early stage tests after 3 000  cycles (upper), 18 000 cycles (mid) and 27 000 cycles (lower).  acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 | 73  properties (yield stress, elastic constants, isotropic and kinematic hardening constants) and wear rate. the micro-hardness profiles pointed out a behavior associated with isotropic hardening, which allowed for a model simplification, supposing a linear relationship between yield stress and hardness. the images taken after the first 200 000 cycles, displayed in figure 11 to figure 13, proved that wear flattened the rail specimen, thus increasing the contact surface and reducing the actual contact pressure exerted. vibrations analysis, in particular of the first 20 000 cycles, reported in table 3, identified a time frame in which the flattening process was occurring. time and extent of the flattening detected by dx/t allowed to replace the standard point hertz contact in the model with a line contact surface, with a track width of 10 mm (8 mm for the 1 % sliding ratio), occurring at the very beginning of the rcf simulations, as shown in table 4. figure 18 shows the calculated displacements due to plastic strain in the er8 specimen in test d (f = 3 830 n and s = 1 %) of material points that before the deformation process were standing along a vertical line. the simulation allowed predicting that a stationary state was reached in the material strain distribution after 200,000 cycles, coherently with what was experimentally observed in terms of both wear rate and vibration parameters. figure 19 shows the superposition of the observed subsurface microstructure with curves representing the calculated displacements: these curves are in agreement with the general aspect of the microstructure. the simulation correctly predicted that the cracks and plasticized layers could not be deeper than 40-50 µm, even though ratchetting never stops, because wear removes plasticized and cracked layers from the surface, preventing deep crack propagation. the ex-post analysis of the monitored index dx/t showed good correlation with destructive and non-destructive test result. nonetheless, its interpretation could still be difficult, due to the fact that the stationary level (for tests a to d about 30 kg-1m-1 as seen in figure 1) is unknown until it is reached. therefore making deductions, based on the proposed index real time analysis, could still be unfeasible while the test is in its early stage. monitoring the proposed index while the test is still running could be feasible after the first 20 000 cycles, although for its interpretation to be practical still more work is needed, especially towards producing a smoother and more stable dx/t value for the whole rcf life test duration. further tests should be performed, using the same technique on different materials and conditions, to assess the proposed method robustness. moreover, future studies could focus on making the index more easily computed and assess in real time. 6. conclusions  a procedure to assess the presence, onset and duration of collaborative rcf and wear phenomena leading to contact shape alteration was proposed. the approach was applied to a wheel and rail steels characterization procedure based both on rolling-sliding contact tests, on disc specimens, and numerical simulation. an early stage evaluation of the damage process provided evidence that the vibration-based index proposed is able to detect high wear transient states during which the concurring damage phenomena alter the specimen shape. this allows altering the numerical model used for characterization accordingly, without halting tests and removing the specimens from the test bench. the advantage of the proposed method rests, in fact, in its non-destructive and inline nature, therefore allowing for a continuous assessment without interfering with the specimens. this procedure completes the described full rcf failure life test, and allows to correct numerical models required to figure 18. calculated displacements due to plastic strain in er8 specimen  in test d (f = 3 830 n and s = 1 %).  figure 19. superposition of calculated (white) and observed (image) plastic  strain bands in tests with s = 1 % and p = 1 100 mpa (left), and s = 1 % and  p = 1 300 mpa (right), after 2 000 000 cycles.  table  4.  nominal  and  measured  width  of  specimens  as  used  for  simulation.  id  nominal  contact width  contact width  used  width  reported since  [mm]  [mm]  [cycles] a  1.785  10  4 700 b  2.113  10  8 500 c  2.440  10  12 700 d  2.440  8 8 200 acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 | 74  characterize the materials. it points out phenomena whose duration is too short to be taken into account by the same numerical model describing, for the whole rcf life of the specimen, the different damage phenomena and their interactions in a complex case as cyclic contact. acknowledgement  the authors are grateful to mr. silvio bonometti and mrs. valentina ferrari for their support in the experimental activities, and to prof. franco docchio for careful reading and revision. references  [1] zerbst u., beretta s., “failure and damage tolerance aspects of railway components”, engineering failure analysis, 2011, 18 pp. 534-542 [2] donzella g. et al. “progressive damage assessment in the nearsurface layer of railway wheel – rail couple under cyclic contact”, wear, 2011, 271(1-2), pp. 408-416 [3] fedele r., filippini m., maier g., “constitutive model calibration for railway wheel steel through tension-torsion tests”, computers & structures, 2005, 83(12-13), pp. 1005-1020. [4] liu y., stratman b., mahadevan s., “fatigue crack initiation life prediction of railroad wheels”, international journal of fatigue, 2005, 28, pp. 747-756. [5] liu y., liu l., mahadevan s., “analysis of subsurface crack propagation under rolling contact loading in railroad wheels using fem”, engineering fracture mechanics, 2007, 74, pp. 2659-2674. [6] solazzi l. et al. “rolling contact fatigue damage detected by correlation between experimental and numerical analyses”, structural durability and health monitoring, 2012 8(4), pp. 329340 [7] mahfoud, j., breneur, c.., “experimental identification of multiple faults in rotating machines”, smart structures and systems, 2008, 4(4), pp. 429-438. [8] byington, c.s., et al., “shaft coupling model-based prognostics enhanced by vibration diagnostics”, insight: non-destructive testing and condition monitoring, 2009, 51(8), pp. 420-425. [9] roemer, m.j., c.s. byington, and j. sheldon, “advanced vibration analysis to support prognosis of rotating machinery components”, international journal of comadem, 2008, 11(2), pp. 2-11. [10] borghesani, p., pennacchi, p., randall, r. b., sawalhi, n., and ricci, r., 2013, “application of cepstrum pre-whitening for the diagnosis of bearing faults under variable speed conditions”, mechanical systems and signal processing, 36(2), pp. 370-384. [11] solazzi, l., c. petrogalli, and m. lancini, “vibration based diagnostics on rolling contact fatigue test bench”, procedia engineering 2011, 10, pp. 3465–3470. [12] mazzù a., “surface plastic strain in contact problems; prediction by a simplified non-linear kinematic hardening model”, journal of strain analysis, 2009, 44(3), pp. 187-199. [13] mazzù a., et al. “an integrated model for competitive damage mechanisms assessment in railway wheel steels”, wear 322-323, 2015, pp. 181–191. template for an acta imeko event paper acta imeko issn: 2221-870x april 2017, volume 6, number 1, 50-58 acta imeko | www.imeko.org april 2017 | volume 6 | number 1 | 50 statistical experimental design screening strategies for free-monomeric isocyanates determination by uplc in materials used in cork stoppers manufacturing catarina andré1, inês delgado2, isabel castanheira2, joão bordado1, ana sofia matos3 1departamento de engenharia química, instituto superior técnico, universidade de lisboa avenida rovisco pais 1049-001 lisboa, portugal 2departamento de alimentação e nutrição, instituto nacional de saúde doutor ricardo jorge (insa), av. padre cruz 1649-016 lisboa, portugal 3unidemi, departamento de engenharia mecânica e industrial, faculdade de ciências e tecnologia, universidade nova de lisboa, 2829-516 caparica, portugal section: research paper keywords: diisocyanates; derivatization; chromatography; design of experiments; cork stoppers citation: caratina andré, inês delgado, isabel castanheira, joão bordado, ana sofia matos, statistical experimental design screening strategies for freemonomeric isocyanates determination by uplc in materials used in cork stoppers manufacturing, acta imeko, vol. 6, no. 1, article 8, april 2017, identifier: imeko-acta-06 (2017)-01-08 section editor: janaína marques rodrigues, inmetro, brazil received june 15, 2016; in final form november 18, 2016; published april 2017 copyright: © 2017 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: this work was supported by project qren 5012-liracork corresponding author: ana sofia matos, e-mail: asvm@fct.unl.pt 1. introduction cork stoppers are considered the premium material for sealing bottled wine. the main reasons for this are their high compressibility, flexibility and impermeability, which prevent wine from oxidation and deterioration. moreover, cork stoppers are a natural product and therefore environmentally friendly. to avoid wasting residues from the manufacturing process, some sub-products, such as agglomerated cork stoppers, can be produced. the granulated cork is used together with binders that, according to the international code abstract a statistical experimental design was used to screen variables of the analytical procedure to quantify free monomeric isocyanates presented in polyurethane based pre-polymers in trace amounts. for this purpose, diphenylmethane-4,4’-diisocyanate (4,4'-mdi), 2,4-toluene diisocyanate (2,4-tdi) and 2,6-toluene diisocyanate (2,6-tdi) were analysed by ultra performance liquid chromatography with a photo diode array detector (uplc-pda). a preliminary study was performed with three derivatization agents, being 1-(2-piridyl) piperazine (1,2-pp) the most suitable one. column temperature, flow and percentage of ammonium acetate (% nh4ac.) were the factors studied at two levels each. a sequence of experiments was planned according to a 23 full factorial design with three replicates and two repetitions. analysis of variance (anova) was applied for the identification of significant factors and interactions. higher responses were achieved when the column temperature was 30 °c, a flow of 0.3 ml min-1 and a solvent with a percentage of ammonium acetate of 0.1 %. figures of merit were assessed within-laboratory as a preliminary step for method validation. similar values were obtained for tdi and mdi. recoveries are approximately 100 %. in addition, the values of detection limits (lods) for mdi and tdi were 0.08 and 0.11 µg ml−1, respectively, and quantification limits (loqs) were 0.25 and 0.33 µg ml−1 for mdi and tdi, respectively. the working range was between 0.01 and 10.00 µg ml−1 for mdi and 0.01 – 4.95 µg ml−1 for tdi. these figures of merit seemed adequate to detect low amounts of free monomeric isocyanates presented in agglomerates and foams for agglomerated cork stoppers production. this data is suitable to address the optimization of an analytical method by a response surface methodology. acta imeko | www.imeko.org april 2017 | volume 6 | number 1 | 51 of cork stoppers manufacturing practices [1], must be polyurethane based (food grade). during manufacture, residual unpolymerized isocyanate monomer can remain in the polymer and may migrate into wine [2]. isocyanates are very reactive chemical compounds, which contain one or more functional groups of the type –n=c=o. these compounds are classified regarding the number of functional groups contained. they may be mono-, dior polyisocyanates according to containing one, two or more groups, respectively (figure 1 a) b) c) d) e)). there is a critical effect on health associated when these compounds are found in the working environment at high doses that may cause irritation of eyes, skin and respiratory system. for this reason, workers in this type of industry should take special care to protect the eyes, skin and inhalation of vapours. a high care for humans, after long periods of exposure, may become sensitive to extremely low concentrations. coughing, wheezing, chest discomfort, edema and interstitial pulmonary fibrosis are some of the effects that have occurred associated with inhalation of isocyanates. the effect that these compounds have on health are, therefore, very well described in the specific documentation on this subject where the limits for exposure to these compounds are also documented [3]. due to the health effects, these limits are low in some european countries (<20 mg l-1). high performance liquid chromatography has been widely employed for the analysis of isocyanates [2], [4]–[6]. however, this technique requires long run times and large amounts of solvents. the emerging of ultra-performance liquid chromatography (uplc) was an important advance in chromatography performance. the improvement of this new technique was the use of columns packed with particles smaller than 2 µm. the van deemter equation indicates that as the particle size decreases to less than 2.5 µm, there is a significant gain in efficiency and that efficiency does not diminish at increased flow rates or linear velocities [7], [8]. on the other hand, with the decrease of particle size the operating pressure increases and a decreasing of flow rates can be used. with this progress, it is possible to decrease the theoretical plate height that allows overall separation with better resolution. due to the high reactivity of isocyanates with water derivatization of isocyanates is crucial. in literature several derivatization agents were reported. the derivatization reagents most frequently used are mama, 1,2-pp, dba among others (figure 1 f) g) h)) [9]. the ability of a chromatographic method to separate, identify and quantify compounds satisfactorily depends on several factors which can be controlled by the operator. often the use of the design of experiments (doe) can be a powerful tool, which allows the analyst to identify which factors or interaction of factors can affect the chromatographic method, driving to optimization [10]–[14]. whatever the application area, the design of experiments ensure that results are obtained in a more effective way, since it considers the variation of the factors in combination, and their interactions [15]–[17]. the design of experiments allows structuring the order of trials to translate the objectives predefined by the investigator. the doe has two applications in chromatography. the first is to show that no factors in the study are significant, and thus verify the robustness of the method for validation. the second one, used in this study, is to identify which factors are significant and optimize the response taking into account these same factors [12]. analysis of variance (anova) is used for the statistical treatment of results after performing the experiments. this variance analysis allows identifying, objectively, which factors and/or interactions are significantly affecting the answers. after identifying these factors (alone or in interaction), it establishes the best combination of factor levels that will lead to the maximization of the pre-established objectives. as far as we known uplc methods are not used to quantify isocyanates at low concentration levels (< 1 mg kg-1). in our previous work [18], the taguchi method was applied for the determination of free isocyanate analysis in uplc, using mama as a derivatization agent. however, mama seems not suitable for low concentrations due to a higher limit of detection [19]. the main goal of this study was to evaluate the effect of three controllable factors in a chromatographic method to determine free diisocyanates at low concentration levels present in cork stoppers, by a screening process. 2. materials and methods 2.1. reagents and chemicals standards all solvents used were hplc or superior grade with purity higher than 96 %. 2,4-toluene diisocyanate (2,4-tdi) (96 %), 2,6-toluene diisocyanate (2,6-tdi) (98.5 %), hexamethylene diisocyanate (hdi) (99 %) from ehrenstorfer; mixture of 2,4toluene diisocyanate (80 %) and 2,6-toluene diisocyanate (20 %) (t80) from bayer; diphenylmethane-4,4’-diisocyanate (4,4’mdi) (98 %), 9-(methylaminomethyl)anthracene (mama) (99 %), 1-(2-piridyl)piperazine (1,2-pp) (99.5 %), dibutylamine (dba) (99.5 %) from aldrich; triethylamine (tea) (99 %) from alfa aesar; dimethyl sulfoxide (dmso), from sigmaaldrich; dichloromethane (dcm), acetonitrile (acn), ammonium acetate (nh4ac.), formic acid (98-100 %), isooctane, ethanol from merck; orthophosphoric acid (85 %), glacial acetic acid from panreac; n-n’-dimethylformamide (dmf) from emsure. the ultra-pure water used for all purposes was obtained with a milli-q element system from millipore (interface, portugal). 2.2. derivatization and sample preparation procedures 2.2.1. mama – derivatives mama derivatives were prepared at a concentration of 100 µg ml-1 in dcm. working standards were derivatized with figure 1. structure of isocyanates and derivatization agents. a) 4,4' diphenylmethane diisocyanate; b) 2,4-toluene diisocyanate; c) 2,6-toluene diisocyanate; d) hexamethylene diisocyanate; e) 1-naphthyl isocyanate; f) 9-(n-methylaminomethyl) anthracene; g) 1-(2-pyridyl) piperazine; h) dibutylamine. a) b) c) d) e) f) g) h) ocn nco acta imeko | www.imeko.org april 2017 | volume 6 | number 1 | 52 mama for one hour in the dark, evaporated to dryness in nitrogen flow and redissolved in 10 ml of dmf: mobile phase 50:50. 2.2.2. 1,2-pp – derivatives 1,2-pp derivatives were prepared at a concentration of 500 µg ml-1 in dcm and derivatized with 50 µl of 1,2-pp. these stock solutions were derivatized for one hour. intermediate standard solutions were prepared, by dilution, at a concentration of 80 µg ml-1 in acn:dmso (95:5). working standards were prepared, by dilution with acn, to a concentration of 8 µg ml-1. 2.2.3. dba – derivatives a solution of dba was prepared in acn at a concentration of 5000 µg ml-1. isocyanate solution was prepared in isooctane at a concentration of 500 µg ml-1 and added dropwise to the dba solution with continuous stirring. a rotating evaporator was used to evaporate this mixture to dryness. after removing dba surplus under vacuum, the precipitate was recrystallized with 70 % aqueous ethanol. accurately weighed amounts of working solution were dissolved in acetonitrile at a concentration of 1000 µg ml-1 and further diluted in acn: water (50:50) at a concentration of 100 µg ml-1. 2.2.4. internal standard hdi was used as internal standard (is) for mdi determination and 1-naphthyl was used as is for tdis determination. solutions of hdi and 1-naphthyl were prepared with a concentration of 2500 and 5000 µg ml-1 in dcm, respectively. intermediate standard solutions were prepared by dilution in acn:dmso and adding 100 µl of 1,2pp. these solutions were derivatized in the same conditions as the standards and samples. working solutions were prepared by dilution with acn. the same concentration of the internal standard solution was added to all working solutions. 2.2.5. sample treatment samples were prepared by adding adhesive or spraying foam into dcm. intermediate solutions were prepared by dilution in acn:dmso and adding 100 µl of 1,2-pp. these solutions were derivatized for two hours. working solutions were prepared by dilution with acn. all standard solutions were filtered into a vial by a ghp 0.2 µm syringe filter. 2.3. chromatographic condition of uplc-uv-flu acquity ultra performance lc system from waters was used for the analysis of target compounds with an acquity uplc beh c18 (150 × 2.1 mm; 1.7 µm) column. the uplc system has a photo diode array detector (pda) (waters, milford, ma, usa) acquiring at 240 nm and 254 nm. results are expressed in free-monomer isocyanate (%fm). table 1 summarizes the best chromatographic conditions achieved from pre-experiments. 2.3.1. mama the mobile phase for mama derivatives was 80 % acetonitrile and 20 % water with 3 % triethylamine adjusted to ph 3 with orthophosphoric acid. the column temperature was 30 °c and flow rate at 0.3 ml min-1 during 15 minutes. 2.3.2. 1,2-pp for these solutions, the mobile phase consisted in two solvents. the first solution was 0.1 % ammonium acetate adjusted to ph 6 with acetic acid and the second solution was acetonitrile. gradient elution using these two solutions was performed. the gradient was linear from 65:35 to 30:70 until 4.69 minutes, followed by another linear gradient to 5:95 until 5.16 minutes, isocratic elution with 5:95 during additional 1.87 minutes returning to initial condition with a total run time of 10 minutes. the column temperature was 40°c and the flow was 0.4 ml min-1. 2.3.3. dba the mobile phase for these solutions contained acetonitrile/water/formic acid (v/v/v) (a) 5/95/0.05 and (b) 95/5/0.05. elution was performed using a linear gradient from 40 % b to 100 % b in 12 minutes, followed by isocratic elution with 100 % b for the additional 3 minutes. the column temperature was 30 °c and flow was 0.35 ml min-1. 2.4. design of experiments in the present work three variables were identified as potential parameters affecting the chromatographic conditions for isocyanate analysis. the three independent variables (controllable factors) were column temperature (tcol), flow (flow) and percentage of ammonium acetate in solvent a of the mobile phase (sol.). each variable was studied at two levels. in order to evaluate and screen the complete set of main factors and their corresponding interactions a full factorial design with two levels for each factor was used. the levels of each factor, as well as the coded symbol and correspondent units, are summarized in table 2. to control drift and to avoid biases, a full factorial design was carried out in a pseudo-random order, using “flow” as a blocking variable. a total of 8 experiments were performed in triplicate in a random sequence, in which each was injected two times. so, the experiences were reordered taking into account decreasing the total run time and, consequently, the amount of mobile phase. the design matrix is presented by standard order (table 3), together with the results obtained for the two dependent variables analyzed (responses: peak area and resolution between two adjacent peaks). with the study of peak area, the objective was to maximize the response for detecting the higher content possible. to reinforce the screening process, the study of the table 1. overview of styles and font sizes used in this template. derivatization agent detector solvent a solvent b flow (ml min-1) column temperature mama pda 254 nm 3% triethylamine ph = 3.0 acetonitrile 0.33 30 °c 1,2-pp pda 254 nm 0.1% nh4ac. ph = 6.0 acetonitrile 0.4 40 °c dba pda 254 nm acn/h2o/hcooh 5/95/0.05 acn/h2o/hcooh 95/5/0.05 0.35 30 °c acta imeko | www.imeko.org april 2017 | volume 6 | number 1 | 53 resolution between peaks will allow us to identify the best combination of factor levels that conduct to a better peak separation between the three isocyanates in the analysis. the resolution between peaks is defined by: 𝑅𝑠 = 2(𝑡𝑟2−𝑡𝑟1) 𝑤1+𝑤2 (1) where tr1 and tr2 are the retention time (tr) for compounds 1 and 2, in minutes, and w1 and w2 correspond to the peak widths (w) at half peak height for compound 1 and 2, respectively. all the statistical analysis was performed using ‘minitab 16’ (minitab inc., cité paradis, paris, france) statistical software package, which included the analysis of variance (anova) and the determination of the polynomial coefficients to be included in the predicting equations for both dependent variables. all anova assumptions (model errors independent, normally distributed and constant variance) were validated through a complete residual analysis. statistical significance was established at a p-value < 0.05 for all statistical tests applied. 3. results and discussion 3.1. preliminary experiments in this study, only diisocyanates were used so the amount of derivatization reagent had to be at least 2.1 times the amount of isocyanate. a kinetics reaction (results not shown) study was also performed to know the optimum time for the reaction to be complete. to understand which derivatization reagent was most appropriate to determine isocyanates, preliminary experiences were performed. the layout of the chromatogram was used to assess the most suitable derivatizing agent and chromatographic conditions. the finest result for each derivatization reagent was summarized in table 1. preliminary experiences revealed that 1,2-pp was the most appropriate derivatizing agent for isocyanate analysis. for this derivatization agent, the chromatogram layout showed less noise and better peak shape. the screening study was carried out only with 1,2-pp. table 2. controllable factors (independent variables) with the coded symbol, units and the corresponding levels. factor levels variable coded symbol units low (-) high (+) column temperature tcol °c 30 40 flow flow ml min-1 0.3 0.4 solvent sol. % nh4 ac. 0.01 0.1 table 3. factorial design matrix with corresponding results for peak area and resolution between peaks. note: the letters a, b and c correspond respectively to column temperature (tcol), flow (flow) and percentage of ammonium acetate in solvent a of the mobile phase (sol.). low and high levels were denoted by “-“ and “+”, respectively. factors peak area per isocyanate a b c 2,6-tdi 2,4-tdi mdi (1) 955363 956056 961789 4513670 4522689 4502456 1158609 1151364 1150963 a + 964051 965921 943387 4495047 4509529 4486964 1060813 1055479 1076169 b + 721916 717805 721036 3373708 3384527 3384071 861751 866549 818431 ab + + 722704 721636 723888 3377115 3384068 3383109 828270 831404 829637 c + 1006489 1004554 1004636 4613640 4603758 4612637 1166362 1163986 1168146 ac + + 1004989 1004476 1004450 4601103 4592920 4590992 1076628 1089310 1090145 bc + + 744180 738828 751788 3453850 3454082 3458858 872285 870533 874780 abc + + + 750909 750530 748874 3446738 3442698 3444175 837155 830967 836409 a b c resolution between 2,6-tdi and 2,4-tdi resolution between 2,4-tdi and mdi (1) 3.3915 3.4091 3.5397 7.0381 7.0463 7.1022 a + 1.9474 1.9382 2.1732 9.2782 9.1930 9.4726 b + 1.8532 1.8511 1.8973 7.0609 6.8366 6.9113 ab + + 2.0281 2.0343 2.0734 8.9455 8.6981 8.8900 c + 2.2989 2.2892 2.3131 7.4277 7.3511 7.1057 ac + + 2.2180 2.0364 2.0433 9.8617 9.7837 9.7918 bc + + 2.0923 2.1383 1.8169 6.7289 6.6081 6.6600 abc + + + 2.2171 2.1026 2.1767 9.4926 9.7007 9.3122 acta imeko | www.imeko.org april 2017 | volume 6 | number 1 | 54 3.2. design of experiments a full factorial design (ffd) was performed to screen the chromatographic conditions and to provide a better understanding of the interactions between critical factors. the ffd analysis was run with the three factors shown in table 2, namely column temperature (tcol), flow (flow) and ammonium acetate percentage in solvent a of the mobile phase (sol.). results from preliminary experiences were used to select higher level (+). lower level (-) was defined according to uplc characteristics taking into account that uplc columns are packed with solid phase particles smaller than those used in hplc, which yields a lower theoretical plate height and higher analyte separation. two wavelengths for peak detection were selected (osha 254 nm; epa ctm-036a 240 nm) [20], [21]. the quality of each result was defined by comparing peak areas or resolution between peaks under the same chromatographic conditions with detection at 254 nm and 240 nm. the analysis of variance (anova) was chosen and applied directly to the responses (y) shown in table 3, as well as to the transformed response given by a logarithmic transformation of variance (-10 log s2), applied only to peak area. the first analysis allowed identifying which factors can significantly affect the two quality characteristics under study (peak area and resolution between adjacent peaks), obtained at appropriate wavelength (location effects), whereas the second analysis intends to investigate which factors affect the peak area variability (dispersion effects) [16], [23]. tables 4 and 5 present the anova results for the three isocyanates location effects, respectively for the responses peak area and the resolution between peaks. the significance of the factors and interactions, as well as the lack of fit, were evaluated through the p-value and the f-distribution values. the analysis of the lack of fit (smallest p-value of 0.164 for 2,4-tdi peak area) allowed to conclude that the controllable factors chosen for this screening process were correct, allowing to explain the observed data variability expressed in table 3. an exception was identified in table 5 to the response “resolution between 2,6-tdi and 2,4-tdi peaks”, where the lack of this quality parameter (lack of fit) is because all factors and interactions revealed high levels of significance (p-value < 0.000). as can be seen in table 4, and for all isocyanates, the flow was the most contributed factor that allows maximizing the peak area, with a percentage contribution to the total variability of 97.42 %, 99.42 % and 93.56 %, respectively for 2,6-tdi, 2,4tdi and mdi. the strong influence of this factor can be explained by the interaction analyte – mobile phase – stationary phase. also, solvent distribution evidenced a significant effect over peak area, but with significantly less impact. regarding column temperature, this factor does not reveal a significant effect on 2,6-tdi. to select the chromatographic conditions, when two or more isocyanates are present, an evaluation of the resolution between peaks was performed (table 5). with significant table 4. anova results obtained for peak areas of the three isocyanates (2,6-tdi, 2,4-tdi and mdi). (ss sum of squares; dof degrees of freedom; ms – mean squares; fo –statistical value given by factor and error mean squares ratio; p-value level of significance of each factor) source of variation for 2,6-tdi area ss dof ms f0 p-value flow 3.66e+11 1 3.66e+11 14,900 0.000 sol. 8.04e+09 1 8.04e+09 328 0.000 flow x sol. 6.71e+08 1 6.71e+08 27.4 0.000 residual error 4.89e+08 20 2.45e+07 lack of fit 5.00e+07 4 1.25e+07 0.45 0.768 pure error 4.39e+08 16 2.75e+07 total 3.75e+11 23 source of variation for 2,4-tdi area ss dof ms f0 p-value tcol 6.35e+08 1 6.35e+08 12.4 0.002 flow 7.77e+12 1 7.77e+12 1.52e+05 0.000 sol. 4.15e+10 1 4.15e+10 813 0.000 flow x sol. 1.22e+09 1 1.22e+09 23.8 0.000 residual error 9.70e+08 19 5.11e+07 lack of fit 2.59e+08 3 8.63e+07 1.94 0.164 pure error 7.11e+08 16 4.45e+07 total 7.82e+12 23 source of variation for mdi area ss dof ms f0 p-value tcol 1.93e+10 1 1.93e+10 170.4 0.000 flow 4.40e+11 1 4.40e+11 3876 0.000 sol. 1.46e+09 1 1.46e+09 12.9 0.002 tcol x flow 4.83e+09 1 4.83e+09 42.5 0.000 residual error 2.16 e+09 19 1.14 e+08 lack of fit 3.24e+08 3 1.08e+08 0.94 0.443 pure error 1.83e+09 16 1.15e+08 total 4.68e+11 23 acta imeko | www.imeko.org april 2017 | volume 6 | number 1 | 55 contributions, the flow (19.14 %), as well as the interaction between flow and column temperature (23.88 %) have a significant impact over the separation of both tdi isomers. for maximizing the resolution between 2,4-tdi and mdi the most significant factor obtained was column temperature, with a relative contribution of 92.74 %. these two peaks were well defined since they have a resolution value higher than 1.5. thus, column temperature having such significant value, suggests that no other factor is important for this separation. based on the anova results (tables 4 and 5) five polynomial models were constructed, where only factors and interaction with a p-value smaller than 0.05 were chosen to the predictions under varying conditions (table 6). the first three prediction models in coded terms considered the peak area for each of the three isocyanates (2,6-tdi, 2,4-tdi and mdi). regarding peak resolution, the two predictive models obtained, in coded terms, are express in the last two lines of table 6, and corresponds to the peak resolution between 2,6-tdi and 2,4tdi, denoted by res(2,6-tdi – 2,4-tdi), and between 2,4tdi and mdi, denoted by res(2,4-tdi mdi). the quality of the developed models was assessed by the r2, predicted r2 and adjusted r2 values, shown in table 6. since the reported values for these parameters are considerably higher (> 97 %), it means that the models developed through this experimental design strategy were well conducted and performed, allowing to predict the optimum chromatographic conditions of the three isocyanates in the study, at low concentration levels. still, within the same screening strategy, an analysis of variance was done using a log transformation of the peak area variance to identify which factors and/or interactions could reduce significantly the variability present within data (dispersion effects). this analysis could allow us to reach more robustness to peak area determination. the results for 2,4-tdi and mdi are shown in figure 2 through a pareto chart, where the significant factors and interaction bars appear above the vertical line (p-value < 0.05). each bar represents the estimate standardized effect in absolute value. for 2,6-tdi no significant effect was identified, meaning that the data variability due to the dispersion within each experiment couldn´t be explained by any of these factors or interaction between them. regarding 2,4-tdi and mdi, it is clear that all factors (isolated or interactions between them) can contribute significantly to reduce the variability within the data, thus increasing data robustness and improve data accuracy. table 5. anova results obtained for resolution of 2,6-tdi 2,4-tdi and 2,4-tdi mdi. (ss sum of squares; dof degrees of freedom; ms – mean squares; fo –statistical values given by factor and error mean squares ratio; p-value level of significance of each factor) source of variation for res(2,6-tdi – 2,4-tdi) ss dof ms f0 p-value tcol 0.6344 1 0.6344 72.6 0.000 flow 1.1778 1 1.1778 134.9 0.000 sol. 0.2387 1 0.2387 27.3 0.000 tcol x flow 1.4348 1 1.4348 164.3 0.000 tcol x sol. 0.5378 1 0.5378 61.6 0.000 flow x sol. 0.6689 1 0.6689 76.6 0.000 tcol x flow x sol. 0.5901 1 0.5901 67.6 0.000 residual error 0.1397 16 0.0087 total 5.4222 23 source of variation for res(2,4-tdi – mdi) ss dof ms f0 p-value tcol 33.9464 1 33.9464 2099.3 0.000 flow 0.8844 1 0.8844 54.7 0.000 sol. 0.4680 1 0.4680 28.9 0.000 tcol x sol. 0.5338 1 0.5338 33.0 0.000 tcol x flow x sol. 0.1647 1 0.1647 10.2 0.005 residual error 0.2911 18 0.0162 lack of fit 0.0446 2 0.0223 1.4 0.265 pure error 0.2465 16 0.0154 total 36.2884 23 table 6. final model equations with summary statistics. variable final model r2 predicted r2 adjusted r 2 area2,6-tdi 8.58e+5 – 1.23e+5 flow + 1.83e+4 sol. – 5.29e+3 flow x sol. 99.87 % 99.81 % 99.85 % area2,4-tdi 3.98e+6 – 5.14e+3 tcol – 5.69e+5 flow + 4.16 sol. – 7.12e+3 flow x sol. 99.88 % 99.74 % 99.98 % areamdi 9.82e+5 – 2.84e+3 tcol – 1.35e+5 flow + 7.80e+3 sol. + 1.42e+4 tcol x flow 99.54 % 99.26 % 99.44 % res(2,6-tdi – 2,4-tdi) 2.2450 – 0.1626 tcol – 0.2215 flow – 0.0997 sol. +0.2445 tcol x flow + 0.1497 tcol x sol. + 0.1669 flow x sol. – 0.1568 tcol x flow x sol. 97.42 % 94.20 % 96.30 % res(2,4-tdi – mdi) 8.1790 + 1.1893 tcol – 0.1920 flow + 0.1396 sol. + 0.1491 tcol x sol. +0.0828 tcol x flow x sol. 99.20 % 98.57 % 98.98 % acta imeko | www.imeko.org april 2017 | volume 6 | number 1 | 56 note that for all analyses of variance involved in this study a complete residual analysis was performed for each one and models adequacy checked. 3.3. selection of the optimal conditions based on the predictive models shown in table 6, developed to improve free isocyanate determination at low concentrations, together with the pareto charts information (figure 2), a screening process was achieved to identify the best combination of factors level. to reach a more accurate definition of the isocyanates peak, a maximization of the three dependent responses (peak area, resolution and variability) must be achieved. so, the signal of the isolated factors coefficient in the model equations (table 6) indicates, as the best combination: column temperature at 30 ºc; flow at 0.3 ml min-1; and solvent at 0.1 % nh4ac.. the only exceptions identified were for both peaks resolutions, where for res (2,6-tdi – 2,4-tdi) the solvent indicates the use of the low level (0.01 % nh4ac.) and for res (2,4-tdi – mdi) a column temperature of 40 ºc. the first conflict (sol.) could be ignored since the solvent shows the smaller f value within anova table (f0 = 27.3), with a relative contribution to the total variability less than 2 %. regarding the second conflict (tcol at 40 ºc), the authors considered it important to analyse this situation in a different way. while the column temperature has a relative contribution smaller than 4 % for the peak area of both isocyanates (2,4-tdi and mdi), the resolution between these two peaks shows the column temperature to be the most contributive factor with 92.7 %. the retention time when the column temperature was 40 °c; a flow of 0.3 ml min-1; and a solvent of 0.1 % nh4ac. was 2.6 min. for 2,4-tdi and 3.3 min. for mdi at 240 nm. these were considered to be the optimum values for the 2,4-tdi and mdi resolution. to analyse the significant effects of interactions and those considered on the log transformation of the variance, an analysis of means (anom) was performed but no level conflict identified (results not shown). 3.4. method performance since validation guidelines were not established for isocyanates analysis determined by uplc neither 1,2-pp use as a derivatizing agent, an in-house single laboratory validation procedure based on the codex alimentarius [24] was implemented to achieve the performance of the method. parameters such as calibration curve, linearity, working range, limits of detection and quantification, reproducibility and recovery were evaluated and the method to analyze isocyanates present in materials in contact with foodstuff validated. the linearity of the method was evaluated with calibration curves using 40 calibration points for mdi and 15 calibration points for tdi within the corresponding working range (table 7). the correlation coefficient was higher than 0.999 for both compounds. the limit of quantification was used to assess method sensitivity. the calculation of limit of detection (lod) and limit of quantification (loq) values were based on the calibration curve [25]. all experimental values were within the acceptance criteria (residual standard deviation (rsd) <15 %). due to lack of reference material, trueness was estimated, in terms of internal standard recovery, by evaluating the several concentration levels during ten days for mdi analysis and five days for tdi. 3.5. applicability the results of method applicability are presented in table 8. samples, polyurethane adhesives based on tdi and mdi and polyurethane foams mdi based, obtained from the manufactures, produced in different days, were analyzed. in tdi analysis the internal standard used was 1-naphthyl and all quantifications were determined with tdi/1-naphthyl ratio. the peaks were well resolved since the retention time was 2.42 min for 2,6-tdi, 2.66 min for 2,4-tdi and 3.07 min for 1table 7. method performance. calibration curve linearity working range (µg ml-1) lod (µg ml-1) loq (µg ml-1) reproducibility (%) recovery (%) mdi y = 1e+08x + 0.0113 1 0.01 10.00 0.08 0.25 11 101.32 tdi y = 3e+08x + 0.0213 0.9998 0.01 4.95 0.11 0.33 2 100.20 figure 2. pareto plot evidencing the significant main factors and interaction for 2,4-tdi (left) and mdi (right), considering the log transformation of the peak area variance. bars represent the standardized effect estimate in absolute value. 2,4 tdi 0 100 200 300 400 500 600 700 tcol x s ol tcol x flow x s ol tcol tcol x flow s ol flow mdi 0 5 10 p=,05 20 25 30 35 flow x s ol flow s ol tcol x flow x s ol tcol x s ol tcol x flow p = 0.05 acta imeko | www.imeko.org april 2017 | volume 6 | number 1 | 57 naphthyl as shown in figure 3. the validation parameters were adequate with good linearity and recovery between our acceptance criteria (80 %-120 %) can be observed. hdi was the internal standard for mdi analysis. the retention time was, for hdi 2.45 min and 3.34 min for mdi so the peaks were well resolved (figure 3). the recoveries were good for the mdi/hdi ratio so for these peaks there isn’t a matrix or other interferences. the linearity was good for both matrixes mdi based. a 2% fm-tdi maximum was found in tdi based adhesives. fm-mdi content in adhesives ranged from 3 % to 6 %. the amount of fm-mdi in mdi based foams was below 1 %. these values were compared with the expected values theoretically calculated and experimental values obtained by titration carried out in another laboratory. the values were not statistically different. the methods proved to be efficiency because real samples content have the expected value [26]. another advantage is the fact that these pre-polymers can be used in agglomerated cork stoppers industries. 4. conclusion three derivatization agents (mama, dba and 1,2-pp) were studied before the experimental design. 1,2-pp was chosen for table 8. quantification of free isocyanate content from polyurethane prepolymers in several matrices. calibration curve linearity working range (µg ml-1) lod (µg ml-1) loq (µg ml-1) repeatability (%) content (%) recovery (%) tdi adhesive 1 y = 2e+08x + 0.0085 0.9995 0.01 7.87 0.29 0.86 4.3 1.54 ± 0.56 100.65 adhesive 2 y = 3e+08x + 0.1301 0.9992 0.01 9.89 0.46 1.40 5.0 0.76 ± 0.16 100.57 mdi adhesive 3 y = 1e+08x 0.0133 0.9997 0.01 9.96 0.26 0.79 2.8 4.86 ± 1.25 106.57 adhesive 4 y = 1e+08x 0.0133 0.9997 0.01 9.96 0.26 0.79 3.4 5.90 ± 0.83 105.00 adhesive 5 y = 1e+08x 0.0133 0.9997 0.01 9.96 0.26 0.79 0.8 4.57 ± 0.11 111.72 adhesive 6 y = 1e+08x 0.0133 0.9997 0.01 9.96 0.26 0.79 3.0 3.31 ± 1.62 105.49 foam 1 y = 1e+08x 0.0205 0.9995 0.01 9.01 0.33 1.01 0.5 0.93 ± 0.02 100.40 foam 2 y = 1e+08x 0.0205 0.9995 0.01 9.01 0.33 1.01 1.4 1.06 ± 0.07 101.38 foam 3 y = 1e+08x 0.0205 0.9995 0.01 9.01 0.33 1.01 0.4 0.87 ± 0.10 100.62 foam 4 y = 1e+08x 0.0205 0.9995 0.01 9.01 0.33 1.01 0.4 0.83 ± 0.07 101.44 figure 3. chromatogram of three real samples. a) chromatogram of adhesive based on tdis; b) chromatogram of foam based on mdi; c) chromatogram of adhesive based on mdi. a) c) b) acta imeko | www.imeko.org april 2017 | volume 6 | number 1 | 58 doe application since it was the one that provided the best results in preliminary experiments. the chromatographic method to determine free diisocyanates, at low concentration levels, should be performed with a column temperature of 30 °c, a flow of 0.3 ml min-1 and a concentration of ammonium acetate in the mobile phase of 0.1 %nh4ac. the best wave length of the pda detector to quantify mdi was 254 nm and for the tdis it was 240 nm. two methods were developed since the mdi internal standard (hdi) has the same retention time of 2,6-tdi. also, from the design of experiments, the best combination for peak maximization was different for the analysis of mdi and tdis. the results demonstrated that methods are efficient to determine isocyanates at low concentration levels in the material in contact with foodstuff. for both methods, linearity was higher than 0.999 and recovery approximately 100 %. the studied pre-polymers are adequate as “food grade” materials, such as agglomerated cork stoppers among others, since the fm-isocyanate content is under the theoretical levels. acknowledgments catarina andré and inês delgado acknowledge the financial support from the portuguese innovation agency (adi) in the frame of project qren 5012-liracork. this work was supported by the portuguese innovation agency (adi) in the frame of project qren 5012-liracork. references [1] c.e. liège, international code of cork stopper manufacturing practices, 1996. 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[26] i. delgado, c. andré, a. ramos, a.s. matos, k. stockham, s. kumaran, et al., “bracketing versus multipoint calibration in determination of isocyanates in agglomerated cork stoppers”, proc of xx imeko world congress green growth, sept. 9-14, 2012, busan, republic of korea, vol 1, pp. 696-699. statistical experimental design screening strategies for free-monomeric isocyanates determination by uplc in materials used in cork stoppers manufacturing microsoft word 15 acta imeko  issn: 2221‐870x  june 2015, volume 4, number 2, 90‐99    acta imeko | www.imeko.org  june 2015| volume 4 | number 2| 90  a fast and  low‐cost vision‐based  line tracking measurement  system for robotic vehicles   daniele fontanelli 1 , david macii 1 , tizar rizano 2   1  department of industrial engineering, university of trento, via sommarive 9, 38123 trento, italy  2  department of information engineering and computer science, university of trento, via sommarive 5, 38123 trento, italy      section: research paper   keywords: kalman filters; vehicular technologies; image processing; direction estimation; real‐time; random sample and consensus  citation: daniele fontanelli, david macii, tizar rizano, a fast and low‐cost vision‐based line tracking measurement system for robotic vehicles, acta imeko,  vol. 4, no2, article 16, june 2015, identifier: imeko‐acta‐04 (2015)‐02‐16  editor: paolo carbone, university of perugia, italy  received february 9, 2015; in final form june 12, 2015; published june 2015.  copyright: © 2015 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  corresponding author: david macii, e‐mail: david.macii@unitn.it    1. introduction  sensors and measurement science play a key role in the development of industrial automation and robotics [1]. one well-known fundamental and crucial problem in robotics is path planning-and-tracking [2]. in order to address this issue, a robot first has to identify the wanted path by sensing the environment (e.g. by recognizing suitable landmarks and visual cues). secondly, it should be able to estimate steadily its position with respect to the planned trajectory, so that the robot controller can efficiently follow the desired path [3], [4]. one of the simplest and most widely adopted solution to this purpose, especially for automated guided vehicles (agvs) in industrial environments is line tracking [5], [6]. line tracking is also often used to evaluate the ability of new robot prototypes to follow a given trajectory [7]. typical sensors for this kind of applications include reflective infra-red light emitting diodes (leds) coupled with suitable photo-diodes, photo-transistors or light dependent resistors (ldrs) [8], [9], arrays of electric inductance sensors [10], and vision-based systems [11]-[13]. while optical or electric inductance sensors are very cheap, they have also a very limited reading range. moreover, at least two of such sensors are generally needed for correct line detection and to estimate whether the vehicle’s position is left or right of the reference line. the vision-based line detection systems are instead advantageous because much information can be extracted from a sequence of collected pictures. of course, in all cases sensor accuracy, range and speed are essential to ensure good performances and real-time behavior. stemming from industrial robotic applications, the technical solutions developed in this area of research have been also gradually applied to service robots, including autonomous guidance systems [14], [15], intelligent vehicles [16], [17], and abstract  localization  and  tracking  systems  are  nowadays  a  quite  common  solution  for  automated  guided  vehicles  (agv)  in  industrial  environments. the bunch of technological solutions developed in this sector is now being revitalized by their applications in service  robots, e.g. for safe navigation in crowded public spaces or in autonomous cars. related to this field are robotic competitions and  races. in this particular scenario, the robot often has to track a line painted on the ground. line tracking techniques typically rely on  light dependent resistors (ldr), photo‐diodes or photo‐transistors detecting the light generated by normal or infrared light emitting  diodes (leds), arrays of electric inductance sensors or vision systems. crucial issues for line tracking are: accuracy and reliability in  estimating the direction of the moving vehicle with respect to the line and processing speed. such problems are particularly critical  when  high‐speed  mobile  robots  are  considered.  to  address  this  issue,  in  this  paper  a  vision‐based  technique  of  moderate  computational complexity is described. the proposed solution has been implemented on a low‐cost embedded platform and relies on  a high frame rate  light contrast sensor, a tailored ransac‐based algorithm and a kalman filter. the reported experimental results  prove that the proposed solution is able to track the direction of the vehicle in real‐time even when the field of view of the camera is  limited and the vehicle moves at high speed.  acta imeko | www.imeko.org  june 2015| volume 4 | number 2| 91  road vehicles both for hazard detection [18], [19], and fully automatic guidance [20]. in this context, robotic vehicle competitions have considerably thrust the development of smart sensing systems. the main problem that still hinders the effectiveness of sensing solutions for position tracking in challenging scenarios (e.g. in robotic races) is the variability of the environment (especially in outdoor scenarios, where the speed of a robot can be quite higher than indoors) and the possible lack of known cues. in this respect, vision-based solutions have gained an undisputed leading role mainly due to their flexibility and ability to estimate multiple quantities with a single measurement system. unfortunately, this increased flexibility comes at the price of an increased computational load as well. in this paper, we will focus on a novel vision-based measurement technique and on a system prototype that perfectly fits the industrial domain, as it is able to improve accuracy, robustness and speed in estimating both the direction and the position of a robotic vehicle with respect to a painted line, thus supporting efficient control, as preliminary reported in [21]. in general, the problem of line detection is closely related to the classic problem of line recognition in images [22], [23], although it is made more difficult by time-varying light conditions and by the robot’s dynamics. in general, the performances of vision-based line detection systems are limited by robustness and speed issues, which in turn depend on camera frame rate, camera resolution and algorithm complexity. in this respect, one of the most famous and effective algorithms for line recognition is the so-called hough transform [24]. however, this algorithm needs line clustering in the image space and it is quite heavy from the computational point of view, although several optimizations have been proposed in the last years, e.g. through randomization [25], or hierarchical image partitioning [26]. as a result of the availability of increasingly powerful embedded platforms, several other image processing algorithms have been proposed over the last few years, e.g. based on a customized image segmentation [27], fuzzy logic [28], or the viterbi algorithm [29]. all of them rely on standard cameras and are characterized by a significant computational burden. some of the solutions proposed in literature for path detection and tracking combine specifically conceived image features [30], [31], known path models [32], statistical methods [33] or a combination of particle filtering and artificial intelligence [34]. another important field of research, which is somehow related to the problem at hand, but is more focused on lane and obstacle detection rather than on path tracking, is described, for instance, in [35]. the technique described in this paper is instead based on a simple algorithm, which works well also on binary lowresolution images, such as those collected by a special highframe-rate light contrast sensor. this sensor ensures a straightforward detection of the line edges (provided that the colors of the line and of the background are different enough) with minimum bandwidth and latency requirements. it is worth emphasizing that the light contrast sensor is supposed to look at the road surface (i.e. with the camera image plane approximately parallel to the ground), in order to recognize the painted line only. in this way, the probability of recording unwanted objects that may perturb line detection is much smaller than using a front camera. the proposed approach is explicitly conceived to support automated vehicle control over optimal paths [36], although the control problem is out of the scope of this paper. line detection is performed through a random sample and consensus (ransac) algorithm combined with a kalman filter. ransac is a general guess–and–test method that randomly chooses a hypothesis in the measurement space and then scores its trustworthiness a posteriori [37]. ransac has been used in many contexts, including trajectory estimation [38] and, more recently, even in road applications [39], [40]. however, the solution described in this paper is faster and it is expected to be more accurate and more robust than the basic ransac algorithm preliminarily described and analyzed through simulations in [40]. in fact, the additional kalman filter prevents sudden and unnatural jumps in the estimated vehicle direction. the algorithm has been implemented and tested on a simple embedded platform. low cost is indeed a further important feature of the system developed. the rest of the paper is structured as follows. at first in section 2 the model underlying the theoretical problem is described. then, section 3 deals with the description of the estimation algorithm. finally, in section 4 the implementation details as well as several experimental results in different conditions are reported. 2. model description  2.1. vision system model  as shortly explained in section 1, the vision system is supposed to observe the road or floor surface just to detect a painted line. if wp = [wx, wy, wz]t is a point in the reference frame w defined by axes xw, yw and zw (with plane πw=xw x yw lying on the ground as shown in figure 1(a)), the equations of two parallel lines in space (namely the line edges) are given by:          awbwwawrw a w b w a w l w ppopp pppp     (1) where r , wpa and wpb are two given points belonging to the left edge and wo = [wxo, wyo, wzo] t is the offset vector between the two lines. let i be an additional reference frame defined by axes xi, yi and zi so that πi=xi xyi includes the image plane and the origin of i coincides with the principal point of the camera, as shown in figure 1(b). in such a frame, every point lying on the image plane has the zi coordinate equal to zero. if axes xi and yi are parallel to xw and yw, respectively, then the image coordinates ip of a generic point wp in the field of view of the camera are given by (see appendix a for reference):                          z w x w y z w y w x i i i tz tx f tz ty f y x p (2) where fx and fy are the focal lengths along axes xi and yi, respectively, and tw =[tx, ty, tz] t is the translation vector expressing the coordinates of the camera pin-hole in the frame w . therefore, if the line equations (1) are plugged into (2), the coordinates of the points belonging to the line edges projected onto the image plane are:          aibiiairi a i b i a i l i ppopp pppp     (3) acta imeko | www.imeko.org  june 2015| volume 4 | number 2| 92  where ipa = [ ixa, iya] t and ipb = [ ixb, iyb] t are two points of the left edge mapped onto the image plane and io = [ixo, iyo] t is the image offset vector between the left and the right edge. since io can be chosen arbitrarily, in the following we assume that ixo = d ≠0 and iyo = 0. as a consequence, d is the distance between the line edges along axis xi, as depicted in figure 1(b). notice that if the image plane and ground are approximately parallel, threedimensional parallel lines are mapped into two-dimensional parallel lines. the same considerations hold also for the orientation angles of each line assuming that fx = fy, as it often occurs in practice. albeit these two assumptions are not perfectly true in a real scenario [41], a slight change of perspective can be easily addressed through inverse perspective mapping (ipm) [42]. moreover, possible time-varying fluctuations of the image plane (e.g. due to vehicle vibrations or jolts), are generally negligible compared with the intrinsic fast variability of the collected images. 2.2. problem formulation and estimation model  after detecting the line, the goal of the measurement system is to estimate the position and the orientation of the robotic vehicle with respect to the line itself in real-time. in particular, the quantities to be measured are:  h, which is the horizontal coordinate of the intersection point between the line centroid and axis xi, i.e.                  a i b i a i b i a i a i yy xx y d xh 2 (4)  and   ,0 , namely the angle between the parallel line edges and axis yi, i.e.            a i b i a i b i xx yy arctan (5) the meaning of these parameters is shown in figure 1(b). notice that h →∞ for lines that are parallel to xi, axis, whereas (h,α) = (0,0) when the line is perfectly vertical and exactly in the center of the image. as explained in section 3, the values of (h,α) associated with a given image are used as a prior for parallel line detection in a newly acquired image. in this way, the computation time is reduced by constraining the ransacbased line search in a specific subset of the pixels of every collected image. to this purpose, (4) and (5) must be inverted to recover (3). unfortunately, the pair (h,α) alone is not sufficient to this end. indeed, for any given pair, a very large (ideally infinite) number of parallel lines exist. in stricter theoretical terms, the position of the parallel line edges is unobservable using just h and α. in fact, observability is guaranteed only if d is available in (4). therefore, also parameter d needs to be estimated by the algorithm. to this purpose, we can rely on a simple dynamic model, whose state is the vector q=[h, α, d]t. in general, the dynamic of q is a function of both the line to be tracked and the motion of the camera. however, if the field of view of the camera is much shorter than the radius of curvature of the wanted path, the effect of the line curvature in the image plane is negligible, thus greatly simplifying both the line recognition problem and the model. as a consequence, the system dynamic due only to the motion of the camera can be simply modelled as follows: vq v v v d h d h                            (6) where v = [vh, vα, vd] t, namely the input of the system, is the speed vector. note that vh depends mostly on the speed components of the camera along the plane of motion, vα is related to the angular speed of the image plane of the camera, and vd depends on the occasional motion of the camera along the axis orthogonal to the ground surface because of potholes, humps, or sporadic changes of the line width. in any case, the time evolution of the state variables of (6) is affected by the uncertainty associated with the measurement of v = [vh, vα, vd] t. this in turn may require a dedicated inertial platform when involved scenarios are considered. 3. algorithm description  as stated in section 1, the algorithm for line detection and state variable estimation relies on the combination of a tailored ransac algorithm and a kalman filter. the rationale behind the combination of such techniques is related to the different nature of the uncertainty sources affecting the model parameters. on one hand, the unknown probability density function related to the estimation of q can be considered as multimodal for the presence of both outliers and noise in every grabbed image. for instance, if no accurate speed values are available, the components of v in (6) can be described just stochastically (e.g. using the variance of available data). this assumption justifies the choice of a randomized, multihypothesis algorithm like ransac. on the other hand, as a camera cannot move instantaneously in multiple different directions, the distribution of the fluctuations due to its motion must be definitely unimodal. therefore, the multi-modal (a)    (b)  figure 1. vision system model and reference frames: (a) perspective view and (b) top view.   acta imeko | www.imeko.org  june 2015| volume 4 | number 2| 93  ransac approach takes advantage of the kalman filter principal mode defined by (6), since the kalman filter is able to reduce such fluctuations. when the system starts, the kalman filter is initialized as soon as the line is clearly detected in the image plane using ransac without any prior. in this preliminary phase, a longer execution time is tolerated in order to have an accurate first guess. then, the estimation algorithm performs iteratively the following three steps: kalman-based prediction, ransac-based road line recognition and kalman-based update. the flow chart of the proposed algorithm is shown in figure 2. note that ransac operates between the prediction and the update steps of the kalman filter. since the ransac algorithm estimates the parallel lines in the image space, on one hand it benefits from kalman prediction results as a prior; on the other it is used to generate the measurement data for the following update step. in the next subsections, the three steps mentioned above as well as the initialization criteria of the kalman filter are described in detail. 3.1. kalman filter initialization  when the algorithm starts, two quantities are initialized in the kalman filter, i.e.  the initial system state q(0), which is set equal to the values returned by the first iteration of the ransac algorithm on a full image;  the initial value of the state covariance matrix p(0), which is set equal to 1/10 of the input covariance matrix q. the covariance matrix q is a constant diagonal matrix, whose elements are the estimated variances of vh, vα and vd. observe that, unlike how it is commonly done in most kalman filters, in this case the initial state covariance matrix is one order of magnitude smaller than q. this is due to the fact that the initial guess q(0) has usually a high accuracy since it results from the application of the ransac algorithm to a full image. 3.2. kalman filter prediction step  the kalman filter is executed anytime a new image is available. hence, the model reported in (6) is discretized with a sampling period δt equal to the inverse of the frame rate. the discretized system model is then simply given by       ttvtqttq  , where  ttq  is the predicted state of system (6) at time t + δt. since no knowledge about the motion of the camera is assumed, the state prediction equation is    tqttq  . similarly, the predicted covariance matrix is given by     qtpttp  , where p(t) is the covariance matrix of the state variables in (6) and q is the model input covariance matrix defined in section 3.1. 3.3. ransac‐based line recognition  in general, the purpose of ransac is to find a subset s* of a set s containing n data, such that its elements fit an instance m* of a model m (depending on n≤n parameters) within a user-defined tolerance threshold st. in the case considered, m refers to the parallel line equations defined in (3), and s is the set of camera pixels where the probability of finding the road line edges is maximum. in the worst case, s coincides with the whole pixel matrix and n coincides with the number of pixels. however, even if s changes anytime a new frame is collected, it can be properly reduced to the region of interest by using the prior information obtained from the kalman filter prediction   figure 2. flow‐chart of the algorithm. the ransac algorithm runs in two different modes, i.e. either using the prior information returned by the kalman  filter or by processing the whole image for initialization.  acta imeko | www.imeko.org  june 2015| volume 4 | number 2| 94  step. of course, such a reduction does not take place during the initialization of the kalman filter (see figure 2 for reference). the ransac-based line detection relies on (3), which depends on n=3 independent parameters, i.e. ipa, ipb and io = [d, 0]t. on the basis of these assumptions, the main steps of the ransac algorithm conceived for the intended application are briefly summarized in the following.  starting from k=1, n=3 pixels are randomly extracted from s to create a subset sk composed by ipak, ipbk and ipak +iok. if some prior information is available (i.e. the output  ttq  of the prediction step) the choice of the subset sk is constrained in the region determined by  ttq  . the selected points are then used to create a model instance mk, namely two parallel lines based on (3).  afterwards, mk is used to determine the subset of pixels sk*  s that are likely to belong to the line edges within a tolerance interval ±st. sk* is usually referred to as the consensus set or the set of inliers. in practice, this set consists of two disjoint subsets skl* and skr* corresponding to the left and right line edges, respectively. if the prior information provided in the prediction step of the kalman filter is available, threshold st can be modified according to the diagonal elements of the predicted covariance matrix  ttp  ;  finally, this procedure starts over and a new set sk+1 of n random points is chosen from s. in a typical ransac algorithm, the iterative approach ends as soon as the number of elements in sk* exceeds a given threshold. however, due to the time-varying uncertainty contributions affecting the problem at hand, no a priori outlier stochastic description is available. as a consequence, in this case the iterative process stops as soon as a maximum number of iterations k is reached. the value of k in presence of outliers is given by [37]    nw p k    1 1 log log (7) where p is the wanted probability to select the correct model if the set s is sampled k times and w is the probability that one of the chosen points is actually an inlier. in the presented solution the value of k is computed and updated in real-time. let sl* and sr* be the sets with the largest number of inliers for the left and right edges, respectively, after k iterations, with sl*  sr* =ø and sl*  sr* = s*. if l* = ( ipa*, ipb*, io* ) is the corresponding set of parameters to be replaced into (3), the optimal set of values finally results from                22   ** ** minminminarg ~ r ii sp l ii sp l ppppl r i l i (8) where  oppl ibiai ~,~,~~  . given that h~ and ~ are computed from l ~ using (4) and (5), respectively, and recalling that  ti do 0,~~  , the output of the ransac-based line recognition algorithm is  tdhq ~,~,~~  . 3.4. kalman filter update step  the elements of q~ represent the measurement values to be injected into the kalman filter during the update step. in particular, the kalman gain kg is given by      1 rttpttpk g (9) where r is the covariance matrix of q~ and results from the residuals of (8). since the motion of the camera is not included in the model, the kalman filter strongly relies on the available measurement data. in particular, the updated values of both the state and the system covariance matrix result from               tggtgg g rkkkittpkittp ttqqkttqttq   33 ~ (10) where the joseph form has been used to express the updated covariance matrix p(t + δt), thus preventing numerical instability. note that the low computational burden of the kalman filter update step is very suitable for an embedded implementation. it may happen that, if an image is heavily corrupted by structured or unstructured outliers (e.g. illumination problems, shadows, small potholes, faded paint), the ransac algorithm fails in finding a good estimate of the parallel line edges. such situations are tolerated to a certain extent thanks to the information retained by the kalman filter. in such cases, the kalman filter update step simply becomes        ttpttp ttqttq   (11) clearly, expression (11) provides the open-loop dynamic of the kalman estimator. if, for some reason, ransac does not work properly, the covariance matrix p(t + δt) tends to grow indefinitely. for this reason, the trace of p(t + δt) is checked at the end of each update step. if the value of the trace exceeds a user-defined threshold tp (e.g. when the total uncertainty is larger than the image size), the state estimated by the kalman filter is discarded at all and the kalman filter is reinitialized, as soon as the next image is grabbed. it is worth noticing that the case of incomplete measures (e.g. when just one of the line edges is in the field view of the camera) can be handled by the algorithm. indeed, the prior information given by the kalman filter during the prediction step allows ransac to detect correctly even a single line edge, while waiting for the other one reappearing in the image. in this case, just the uncertainty associated with d tends to increase monotonically in the state covariance matrix. 4. experimental results  4.1. experimental setup description  the proposed algorithm has been implemented in a beagleboard xm embedded platform. this platform is equipped with a 1-ghz arm dm3730 by texas instruments, 512 mb of ram, a 4 gb micro secure digital (sd) memory and a linux angstrom distribution. the beagleboard xm provides expansion headers to connect other peripherals to the platform. the adopted vision system relies on a custom light contrast imager driven by an xc2c512 xilinx coolrunner-ii complex programmable logic device (cpld). both board and camera are shown in figure 3. since the pins of the beagleboard expansion headers are rated at 1.8 v while the camera requires 3.3 v, a piggy-back voltage level translator has been used to interface the embedded platform with the camera. the light contrast sensor is a 35-μm cmos imager with a acta imeko | www.imeko.org  june 2015| volume 4 | number 2| 95  resolution of 128 × 64 pixels made at the “fondazione bruno kessler” (fbk), trento, italy [43]. note that the low resolution of the camera is helpful for the application considered, as it greatly reduces the number of iterations of the ransac algorithm at no cost in terms of accuracy, as it will be shown in section 4.2. an important feature of the adopted vision system is that it requires a single bit for each pixel. each pixel generates a voltage value that is proportional to the maximum relative variation of the average light intensity over triples of adjacent pixels during a given integration interval. all output voltage values are quantized by a 1-bit comparator. the resulting bits can be stored into a local on-chip memory or they can be immediately read out and buffered into the cpld. this feature (along with the low resolution of the imager) makes frame acquisition faster than in regular cameras. as a consequence, the camera frame rate can be larger than 100 frame/s, but with low bandwidth requirements for data transfer. in practice, the frame rate is limited by the light integration time of each pixel. this can be as low as 1 ms, but it should be at least 10 ms not to affect the sensitivity of the imager [44]. the on-board cpld is provided with a serial peripheral interface (spi) to read the image frames captured by the camera. the embedded platform communicates with the logic circuitry of the vision system through general purpose inputoutput (gpio) and spi pins. the gpio pins are used to set the camera operating parameters (e.g. operation modes and integration time) and to exchange interrupts and control signals with the vision system. the spi pins are instead used to transfer the image frames. the transmission rate is controlled by the spi clock. in the current implementation the spi clock frequency is 48 mhz. the cpld is designed to hold only one image row at a time. therefore, one spi read per row is needed to read a full image frame. in order to collect such data as quickly as possible, a highly optimized software driver was developed for linux. 4.2. performance evaluation  in order to evaluate the performances of the overall system, several image records of several minutes each have been collected at about 100 frame/s with a pixel integration time of 10 ms. the camera was fastened to the right external rear view mirror of a real vehicle, about 1.2 m above the ground to detect one of the side lines of the road. such lines are typically white and about 15 cm wide. the results of all experiments reported in the following were conducted on urban roads, in a peripheral district of the city of trento. the speed of the vehicle was kept between about 30 km/h and 70 km/h (i.e. 50 km/h on average) depending on the road and traffic conditions. it is worth emphasizing that such testing conditions are more challenging than those adopted in robotic vehicle competitions. in fact, during the experiments, line recognition was hindered and occasionally perturbed by shadows, faded or interrupted lines, gravel and road imperfections (e.g. asphalt patches). usually, the light contrast between the road surface and the lines painted on the street is large enough to enable an easy detection of the line edges. in particular, such edges are depicted as linear clusters of active pixels on a grey background (in the following referred to as data points). some significant examples of collected images are shown in figures 4(a)-(i). figure 4(a) refers to ideal conditions, i.e. a freshly painted white line on a uniform dark grey road background. in this case, the line edges are easily recognizable to the naked eye. in figure 4(b) the right side of the line is partially faded. in figure 4(c) the whole line is faded. in figure 4(d) and 4(e) the line edges are sharp, but some shadows due to nearby objects create some additional contrast points (additional dark pixels). in figure 4(f) one of the line edges is out of the field of view of the camera because the vehicle has slightly departed from the wanted trajectory. in figure 4(g) the road line is covered by some gravel. in figure 4(h) the road is both dusty and in shadow. finally, in figure 4(i) line recognition is perturbed by a manhole cover. in the examples above, the thin, white straight lines plotted in each picture are reconstructed using the values of q = [h,α,d]t estimated in real-time. in all experiments, the threshold value st for the ransac algorithm is a function of the state covariance matrix p, and it is never smaller than 2 pixels. estimation accuracy and robustness to visual artefacts have been evaluated offline. to this purpose, the images collected in about 15 tests of several minutes each have been grouped into three data sets depending on the amount of disturbances. in the following, data set 1 refers to the experiments in which the percentage of images affected by disturbances, such as those shown in figure 4, is moderate (i.e. between about 15 % and 25 %). data set 2 includes the best experiments, namely those where no more than about 15 % of the collected images is perturbed. finally, data set 3 comprises the worst experiments, i.e. those where the line was faded, partially interrupted or perturbed in up to 40 % of images. in all cases, the actual values of the state variables (namely the ground truth parameters denoted as hgt(t), αgt(t) and dgt(t)) were obtained with the following off-line procedure: 1. at first, the so-called projection pursuit algorithm is used to select the coarse-grained histograms with the highest peaks [45]. each set of features generating the histograms’ peaks (one peak for each line) represents a hypothesis; 2. among all possible hypotheses, the one with the highest number of features is chosen; 3. the projection pursuit algorithm is executed again, but with a fine-grained resolution, on the remaining features to detect possible clusters. the cluster with the highest ratio between the number of features and its standard deviation is chosen as the winning hypothesis; 4. the features belonging to such a hypothesis are determined using a voting scheme similar to ransac; 5. finally, a least squares quadratic (lsq) optimization algorithm is used to compute the values of hgt(t), αgt(t) and dgt(t). the results of this ground truth reconstruction procedure were carefully checked a posteriori by visual inspection record figure  3.  vision  system  for  light  contrast  detection  (on  the  left)  and beagleboard xm embedded platform (on the right).   acta imeko | www.imeko.org  june 2015| volume 4 | number 2| 96  by record to remove or to correct possible outliers. a few images whose ground truth parameters could not be clearly estimated and that could not be corrected manually were simply removed from the data sets. figure 5(a)-(c) shows the 0.95-level confidence intervals associated with εh=h-hgt (a), εα =α-αgt (b) and εd =d-dgt (c), for the three data sets described above. in each case the results of two different estimation techniques are shown and compared, i.e. the one described in this paper (which relies on both ransac and kalman filtering) and the solution based on ransac only presented in [40]. we decided to evaluate measurement uncertainty in terms of confidence intervals rather than reporting the standard uncertainty computed with a type-a evaluation approach [46], because the probability density functions of εh, εα and εd are strongly unimodal, but the normal probability plots of the collected data show that they are not normally distributed. an example of such distributions in the case of variable εα for data set 3 is shown in figure 6. the histograms of the other variables are quite similar and are not particularly significant; so they are not reported for the sake of brevity. the use of confidence intervals in figure 5(a)-(c) emphasizes more clearly the benefits of using the kalman filter to improve accuracy, precision and robustness in line detection. this is especially true as far as the α parameter is concerned, which is also the most important for vehicle control purposes. observe that εα ranges approximately between [-1,+1] degree with 95 % probability in all cases. of course, the results related to data set 2 are the best ones. however, even in the presence of more frequent disturbances, the proposed estimation technique is quite robust. this improvement is due to two reasons. first of all, the kalman filter decreases the probability that ransac detects wrong line edges in consecutive images, thus greatly reducing the probability of unnatural “jumps” in the estimation process. secondly, the kalman filter makes the algorithm track angle α even when one of the two line edges is occasionally lost (e.g. because the line is too faded or because it is out of the field of view of the camera, as it is shown in figure 4(f)). the analysis of εd and εh is a bit more complex. first of all, the distribution of such estimation errors in the ransac-only case is asymmetric. this asymmetry is due to the fact that when one of the two line edges is out of view, the “orphan” one is not always detected on the correct side of the line. therefore, the right edge could be recognized as the left one and vice versa. however, the probability of these events is not the same. thus, the estimation errors exhibit an asymmetric distribution, which, in the ransac-only case, can sometimes exceed 15 pixels. given that in the current setup 1 pixel (namely with the vision system about 1.2 m above the ground) corresponds to about 3 cm, the position error can be larger than 45 cm. by using the proposed algorithm, symmetry is usually reestablished because the kalman filter keeps memory of the previous positions of the line edges. in this way, the estimation errors are considerably reduced. indeed, they are generally within ±5 pixels (i.e. about 15 cm), with the only exception of εd in data set 3. this is due to the fact that, when the line is heavily faded, its width can be hardly estimated with good accuracy. nonetheless, the accuracy in estimating vehicle direction is still preserved. it is important to remind that the memory effect of the kalman filter is lost in the case of filter re-initialization, which occurs when the trace of matrix p exceeds the threshold tp defined in section 3.4. a further important aspect that has to be carefully evaluated to appreciate the performances of the system prototype is related to computation time. figure 7 shows the cumulative distribution curves of the execution times of the estimation algorithm running on the chosen low-cost embedded platform. the dashed line refers to the algorithm based on ransac            (a)             (b)          (c)                 (d)          (e)          (f)                (g)          (h)          (i)      figure 4. examples of images collected from the light contrast sensor: (a) ideal situation; (b) partially faded line; (c) considerably faded line; (d)‐(e) effect of  shadows; (f) right edge out of the field of view of the camera; (g) road partially covered by gravel; (h) dusty shady line; (i) line painted over a manhole cover  (i). in each image, the direction of the vehicle estimated by the algorithm with respect to the road line is represented by a white line.    acta imeko | www.imeko.org  june 2015| volume 4 | number 2| 97  only, whereas the solid line corresponds to the proposed solution based on both ransac and kalman filtering. observe that the execution time of the ransac-withkalman-filter approach is faster than the solution based on ransac only. this is due to the fact that generally a smaller number of iterations is needed to detect the line edges due to the prior retained by the kalman filter. in particular, in this case the median execution time of the proposed algorithm is just 4 ms and it is lower than 10 ms with 97 % probability. this means that with a camera frame rate equal to 100 frame/s (so that a new image can be processed every 10 ms), even if the vehicle travels at 100 km/h, the measurement system is able to estimate direction changes with a space resolution of about 30 cm, which is compatible with automated high-speed vehicle control. 5. conclusions  increasingly sophisticated, high-speed automated guided vehicles (agvs) require fast sensors able to ensure accurate and reliable real-time path-following techniques. in this context, this paper presents a simple vision-based technique and a lowcost system able to estimate the direction and the relative position of a vehicle with respect to a line painted on the ground. line-tracking techniques are indeed commonly used in robots for industrial applications. the proposed solution relies on a special camera and on a tailored ransac algorithm enhanced by a kalman filter. even if the system does not address other crucial safety problems, such as collision avoidance for instance, it is robust to manifold uncertainty sources and it is cheaper and faster than other vision-based solutions. this is due not only to the proposed algorithm per se, but also to the low resolution of the adopted light contrast imager, which greatly reduces image processing burden and data transfer latency. in future, the current prototype could be -10 -5 0 5 10 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35   [degrees] p ro b a b ili ty figure 6. histogram of εα obtained when the proposed algorithm is applied  to the images of data set 3.  0 5 10 15 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 execution time [ms] p ro b a b ili ty ransac only ransac & kalman figure  7.  execution  time  cumulative  distribution  curves  of  the  ransac‐ only (dashed  line) and ransac‐with‐kalman‐filter (solid  line) algorithms  running on the beagleboard xm platform.   -25 -20 -15 -10 -5 0 5 10  h [ p ix e l] d at a se t 1 r an sa c o nl y d at a se t 1 r an sa c & k al m an d at a se t 2 r an sa c o nl y d at a se t 2 r an sa c & k al m an d at a se t 3 r an sa c o nl y d at a se t 3 r an sa c & k al m an (a)  -3 -2 -1 0 1 2 3   [ d e g re e s ] d at a se t 1 r an sa c o nl y d at a se t 1 r an sa c & k al m an d at a se t 2 r an sa c o nl y d at a se t 2 r an sa c & k al m an d at a se t 3 r an sa c o nl y d at a se t 3 r an sa c & k al m an (b)  -20 -15 -10 -5 0 5 10 15  d [ p ix e l] d at a se t 1 r an sa c o nl y d at a se t 1 r an sa c & k al m an d at a se t 2 r an sa c o nl y d at a se t 2 r an sa c & k al m an d at a se t 3 r an sa c o nl y d at a se t 3 r an sa c & k al m an (c)   figure 5. 0.95‐level confidence intervals associated to εh (a) and εα (b) and εd (c)  for  three  different  data  sets.  pairs  of  adjacent  intervals  refer  to  the estimation errors resulting from the proposed ransac algorithm with and without kalman filtering, respectively.  acta imeko | www.imeko.org  june 2015| volume 4 | number 2| 98  further improved by using a more sensitive imager and an inertial platform measuring vehicle speed. appendix ‐ derivation of expression (2)  let w be a reference frame, with axes xc, yc and zc defined in such a way that:  xc is directed as the x-axis of the image plane;  yc is oriented as the y-axis of the image plane (according to a right-handed reference frame);  zc is oriented towards the observed scene till intersecting the image plane in the principal point, as shown in figure 1. the origin of c coincides with the camera pin-hole. therefore, c is simply translated with respect to i without any rotation. on the contrary, the orientation of c with respect to w can be arbitrary. therefore, if crw is the rotation matrix of c and tw =[tx, ty, tz] t is the same translation vector, as defined in section 2.1, the coordinates of a generic point wp in the camera frame are given by [41]:   ttwww ptrp 1cc , (a.1) where cp = [cx, cy, cz]t and [crw|tw] is a 3 x 4 matrix which defines the rigid transformation between c and w . in particular, if the axes of c are completely rotated with respect to w as shown in figure 1, (a.1) can be rewritten as               z w y w x w tz ty tx pc (a.2) from the basic theory of vision systems, it is known that the projection of any point in the field of view of the camera onto the image plane results from [41]:                      z y x g z yz xz i i c c c c c c (a.3) where            100 0 yy xx cf csf g (a.4) is the so-called camera calibration matrix, fx and fy represent the focal lengths along axes xi and yi, respectively, s models the radial distortion of the image, and the coordinates cx and cy of the principal point of the camera are both equal to 0 in the frame i . in practice, the terms of the calibration matrix can be estimated using widely known numerical tools [47]. thus, if the radial distortion coefficient is negligible (as it 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[38] d. fontanelli, l. ricciato, s. soatto, “a fast ransac–based registration algorithm for accurate localization in unknown environments using lidar measurements,” proc. ieee international conference on automation science and engineering, sep. 22-25, 2007, scottsdale, az, usa, pp. 597– 602. [39] g. mastorakis, e. davies, “improved line detection algorithm for locating road lane markings,” electronics letters 47 (2011), pp. 183–184. [40] d. fontanelli, m. cappelletti, d. macii, “a ransac-based fast road line detection system for high-speed wheeled vehicles,” proc. of ieee int. instrumentation and measurement technology conference (i2mtc), may 10-12, 2011, hang zhou, china, pp. 186–191. [41] r. hartley, a. zisserman, multiple view geometry in computer vision, cambridge university press, 2003, isbn: 0-52-154-0518. [42] h. mallot, h. blthoff, j. little, and s. bohrer, “inverse perspective mapping simplifies optical flow computation and obstacle detection,” biological cybernetics 64 (1991), pp. 177– 185. [43] m. gottardi, n. massari, s. jawed, “a 100-μw 128×64 pixels contrastbased asynchronous binary vision sensor for sensor networks applications,” ieee journal of solid-state circuits 44 (2009), pp. 1765–1770. [44] l. gasparini, d. macii, m. gottardi, d. fontanelli, “a lowpower data acquisition system for image contrast detection,” in proc. imeko tc-4 international workshop on adc modelling and testing (iwadc), jun. 30jul. 1, 2011, orvieto, italy, pp. 1-6. [45] p. huber, “projection pursuit,” the annals of statistics 13 (1985) 435–475. [46] bipm, and iec, and ifc, and iso, and iupac, oiml, guide to the expression of uncertainty in measurement, geneva, switzerland, 2008. [47] z. zhang, “a flexible new technique for camera calibration,” ieee transactions on pattern analysis and machine intelligence 22 (2002), pp. 1330–1334. template for an acta imeko event paper acta imeko issn: 2221-870x april 2017, volume 6, number 1, 59-63 acta imeko | www.imeko.org april 2017 | volume 6 | number 1 | 59 tools for uncertainty calculations in force measurement dirk röske1, jussi ala-hiiro2, andy knott3, nieves medina4, petr kaspar5, mikołaj woźniak6 1physikalisch-technische bundesanstalt, bundesallee 100, 38116 braunschweig, germany 2vtt mikes metrology, tehdaskatu 15, puristamo 9p19 87100 kajaani, finland 3national physical laboratory, hampton rd, teddington, middlesex tw11 0lw, united kingdom 4centro español de metrología, calle del alfar, 2, 28760 tres cantos, madrid, spain 5cesky metrologicky institut, okružní 31, 638 00 brno, czech republic 6ministerstwo gospodarki, główny urząd miar, ul. elektoralna 2, 00-139 warszawa, poland section: research paper keywords: emrp sib 63; uncertainty; force measurement; calculation tools citation: dirk röske, jussi ala-hiiro, andy knott, nieves medina, petr kaspar, mikołaj woźniak, tools for uncertainty calculations in force measurement, acta imeko, vol. 6, no. 1, article 9, april 2017, identifier: imeko-acta-06 (2017)-01-09 section editor: paul regtien, the netherlands received september 9, 2016; in final form february 13, 2017; published april 2017 copyright: © 2017 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: the research leading to these results has received funding from the european union on the basis of decision no 912/2009/ec. the emrp is jointly funded by the emrp participating countries within euramet and the european union corresponding author: dirk röske, e-mail: dirk.roeske@ptb.de 1. introduction the main objective of the research project “force traceability within the meganewton range” (sib63) in the european metrology research programme (emrp) was to improve the traceability of force measurement from primary standards to calibration services and testing laboratories especially in the meganewton range. the project was started in july 2013 and had a duration of 36 months. the aim of the project was to develop new methods that could be applied by users of measurement devices for large forces in industrial calibration laboratories and applications as well as in testing laboratories. different work packages (wp) had been defined dealing with build-up systems of force transducers and extrapolation methods of measurement results (wp1), with multi-component measurements (wp2) and with time-dependent effects like creep and hysteresis (wp3). the present paper describes the work that was undertaken within work package 4 improved dissemination of the si unit of force (wp4) during the project’s lifetime. the key idea of wp4 was to improve traceability by taking various influencing effects into account. this is done by applying corrections to the measurement results based on mathematical models and known parameter values. for the corrected results, measurement uncertainties can be calculated using the tools provided to the users from the project’s website. the procedure of wp4 was as follows. first, the application conditions and requirements for the calibration depending on the further use of the force measuring instrument were compiled based on an on-line questionnaire. second, the corresponding technical parameters and coefficients describing the effects of application conditions on the measurement result were collected or defined. third, for nine of the parameters and coefficients, detailed models and elaborated uncertainty calculation examples were provided. they are publicly available from the website [1]. fourth, for at least six of the models, online calculations tools were developed. for offline use, a abstract within the framework of the european metrology research programme (emrp), tools for the calculation of measurement uncertainties have been developed. the joint research project (jrp) 63 of the “si broader scope ii” (sib) call of the emrp is entitled: “force traceability within the meganewton range”. the project was started in july 2013 and had a duration of 36 months. the aim of the sib63 project was to improve the traceability of the force measurement especially in the meganewton range. this paper presents the results of work package 4 “improved dissemination of the si unit of force”. the tools developed by the partners are available from the website of the project. acta imeko | www.imeko.org april 2017 | volume 6 | number 1 | 60 spreadsheet file, including the same models, was programmed and is also publicly available from the website of the project. 2. application conditions force transducers are often calibrated under laboratory conditions. in the later use of these instruments the application conditions must be taken into account if they differ from the calibration conditions to an extent that affects the measurement result. in order to receive feedback from the stakeholders about their application conditions, an online form was generated (figure 1). it is still publicly available from the web address http://www.ptb.de/emrp/forcemetrology.html [1]. all graphics are screenshots taken from the project’s website. if the figures are difficult to read, please open – if available the website. as the next step, a survey was undertaken and 66 stakeholders were asked to take part by submitting the filled-in form to the project team. 24 users replied and all the data received was entered into the database and can be found under “work packages/wp4.1 application conditions” on the website. the intention of the survey was to find out under which conditions and in what ranges (for example, temperatures up to 1100 °c) force transducers are sometimes used. it was not intended to develop methods covering all these ranges so users should be aware that, for example, a temperature coefficient that was measured in the range up to 30 °c may not be useful in the range of 100 °c or above. 3. technical parameters and coefficients several parameters or coefficients are already defined in written standards, for example in [2], or in scientific papers. the available information was compiled into one list together with references under “work packages/wp4.2 parameters and coefficients” on the website (figure 2). all parameters are given with their name, symbol and unit, as well as a description. in some cases, it was necessary to adapt the definition given in the guidelines for a better uniformity. this may also serve as a proposal for a future revision of the guidelines. the parameters and coefficients were grouped into geometrical, mechanical, temporal, electrical and environmental effects. for some of the parameters, exact values are given as well. unfortunately, this list is very short and it does not cover most of the parameters due to a lack of corresponding measurement results. if a user needs to know the exact value of a parameter, then it might be necessary to carry out corresponding measurements. on the other hand, depending on the target uncertainty, it may be sufficient to work with parameter ranges in the form of upper limits of their absolute values from data sheets. for example, the temperature effect on the characteristic value (“sensitivity”) of a c4 force transducer from hbm [3] is not higher than 0.01 % per 10 k temperature change (see figure 3). figure 1. questions from the on-line questionnaire about application conditions. figure 2. parameter and coefficient definitions (part of the table). figure 3. parameter ranges from data sheets. http://www.ptb.de/emrp/forcemetrology.html acta imeko | www.imeko.org april 2017 | volume 6 | number 1 | 61 4. detailed models and examples 4.1. models the uncertainty contributions were then evaluated for some of the parameters and their related influencing quantities. in [4] it was shown that the uncertainty calculation can be performed with quite simple software tools. the various influencing effects are given under “work packages/wp4.3 uncertainty contributions” on the website (figure 4), arranged in categories. the intention is that the user applies corrections to the measurement results based on the knowledge of the effect, the mathematical model describing the influence and the values of the parameters including their uncertainties. the models follow directly from the above given definitions of the parameters and coefficients. if new influencing quantities or effects other than those in the list of models have to be considered, the related models can be developed on the basis of the models on the page. 4.2. calculation examples for some of the influencing effects from the list in figure 4, elaborated example calculations are given in the lower part of the wp4.3 page, see figure 5. these examples are organized using an accordion element in the graphical user interface of the page. it consists of a vertically stacked list of alternately expandable header items, meaning a maximum of one item can be expanded at a time. usually [5], for uncertainty calculations only linearized models are applied and the underlying formula is given in (1): 𝑢𝑐2(𝑦) = ∑ � 𝜕𝜕 𝜕𝑥𝑖 � 2 𝑁 𝑖=1 𝑢 2(𝑥𝑖) . (1) in the case of our form, an enhanced formula (2) is used 𝑢𝑐2(𝑦) = �� 𝜕𝜕 𝜕𝑥𝑖 � 2𝑁 𝑖=1 𝑢2(𝑥𝑖) + + ∑ ∑ �1 2 � 𝜕 2𝜕 𝜕𝑥𝑖𝜕𝑥𝑗 � 2 + 𝜕𝜕 𝜕𝑥𝑖 𝜕3𝜕 𝜕𝑥𝑖𝜕𝑥𝑗 2� 𝑁 𝑗=1 𝑁 𝑖=1 𝑢 2(𝑥𝑖)𝑢2�𝑥𝑗� . (2) one of the advantages of this enhanced calculation method is that additional contributions to uncertainty are taken into account. if, for example, the user works at the same temperature as the calibration laboratory (20 °c, see above), the correction due to the temperature coefficient is zero, but due to a supposed higher uncertainty of the temperature measurement (for example, 0.5 k instead of 0.1 k) the result should also have a higher uncertainty. in the case of the example shown in figure 6, the effect of temperature on the characteristic value is calculated when the transducer is used at 35 °c after it was calibrated in an airconditioned laboratory at 20 °c. the standard uncertainty of measurement is 0.25 %. due to the known temperature figure 5. collection of detailed models and example calculations. figure 6. example of a fully elaborated uncertainty calculation including a corrected result. figure 4. list of categories and influencing effects. acta imeko | www.imeko.org april 2017 | volume 6 | number 1 | 62 coefficient of 0.01 % per 10 k, the characteristic values changed from 2.000 mv/v at 1 mn to 2.003 mv/v. the uncorrected result has an additional error of 0.15 %. depending on the application requirements, this can be either low enough to be ignored or it needs to be considered. if the effect is taken into account by applying a correction to the measurement result – which is the preferred way according to the gum [5], the corrected value has an uncertainty of 0.252 % which is slightly higher than the uncertainty of the calibration result (0.25 %). the increase is mainly caused by the uncertainty in the knowledge of the temperature coefficient tkc. the example calculations are also used for the test of correctness and validation of the online and offline tools that are described in the following section. 5. online and offline calculations tools the developed online tools can be found under “work packages/wp4.4 software tool” on the website (figure 7). the user should enter corresponding values into all white input text boxes of the form. after this is done, a click on the “evaluation” button calculates the results and shows them in the grey text boxes together with a list of contributions and an explanation below the form (figure 8). the calculations are performed in the internet browser on the user’s local computer. for this purpose, javascript must be supported by the browser and enabled. the use of this tool is free but without any warranty. the source code is accessible and can be checked. the “reset” button can be used to delete the current results; the values in the white boxes are not affected. this allows the user to obtain a form without any results but it is not necessary to use this reset function, as any input value can be changed at any time and, by pressing the “evaluation” button, the results can be re-calculated using these new values. the user should be aware that the indicated results will not correspond to the input values if the latter have been changed without starting a recalculation using the “evaluation” button. the table below the form shows the contributing variances in descending order. in the example of figure 8 it can be seen that the main contributions come from the uncertainty of the transducer calibration and the uncertainty of the temperature coefficient. in addition to the online tools, an excel file is available for offline use. the first sheet in this file contains a short description together with internal links to the other sheets and some settings for the units in which the values are given. the second sheet offers some explanation of technical terms including their units. each following sheet contains one model and its structure is similar to that on the website. the sheets contain the model and descriptions, input fields (cells) for data, results fields (cells) for the calculation results as well as the “reset” and “evaluation” buttons and the contributing variances, and the explanation. the calculations in this spreadsheet file are based on macros. for the full functionality of the files macros must therefore be allowed. depending on the settings of the local programme, the user may be asked to allow macros to be executed by the programme. the last sheet of the file offers a graphical representation of the different contributing uncertainties (figure 9). the user can select which effects and influences should be figure 8. the filled-in form of figure 7 with calculated results and explanation. figure 9. pie charts of the contributing effects and influences (source: excel file [6]). figure 7. example of an online tool form. acta imeko | www.imeko.org april 2017 | volume 6 | number 1 | 63 taken into account by checking or unchecking the corresponding checkbox in the table above the charts. the graphics will be updated accordingly. 6. conclusions the methods and tools developed within this work package should help the users of calibrated force transducers to better estimate the measurement uncertainty with less effort, thus improving the traceability of force measurement. it should be noted that the methods and tools described in this paper are limited neither to the meganewton force measuring range nor indeed to the quantity force, so they can equally be applied to torque or pressure measurements. acknowledgement the research leading to these results has received funding from the european union on the basis of decision no 912/2009/ec. the emrp is jointly funded by the emrp participating countries within euramet and the european union. references [1] emrp sib63 "force traceability in the meganewton range", [online], http://www.ptb.de/emrp/sib63-home.html. [2] vdi/vde/dkd 2638 kenngrößen für kraftaufnehmer – begriffe, 2008-10, (german and english versions in one document), https://www.vdi.de/richtlinie/vdivdedkd_2638kenngroessen_fuer_kraftaufnehmer_begriffe/. [3] hbm c4 force standard data sheet, special features, http://www.hbm.com.pl/pdf/b0663.pdf. [4] d. röske, “uncertainty calculations using free cas software maxima,” in imeko 22nd tc3, 12th tc5 and 3rd tc22 international conferences, cape town, south africa, february 2014, http://www.imeko.org/publications/tc3-2014/imekotc3-2014-016.pdf. [5] bipm jcgm 100:2008, evaluation of measurement data guide to the expression of uncertainty in measurement, http://www.bipm.org/utils/common/documents/jcgm/jcgm _100_2008_e.pdf. [6] excel file for offline calculation of measurement uncertainties, http://www.ptb.de/emrp/fileadmin/documents/forcemetrology /workpackages/wp4/uploads/wp4.4_spreadsheet_tool_v1.0.1 .xlsm. http://www.ptb.de/emrp/sib63-home.html https://www.vdi.de/richtlinie/vdivdedkd_2638-kenngroessen_fuer_kraftaufnehmer_begriffe/ https://www.vdi.de/richtlinie/vdivdedkd_2638-kenngroessen_fuer_kraftaufnehmer_begriffe/ http://www.hbm.com.pl/pdf/b0663.pdf http://www.imeko.org/publications/tc3-2014/imeko-tc3-2014-016.pdf http://www.imeko.org/publications/tc3-2014/imeko-tc3-2014-016.pdf http://www.bipm.org/utils/common/documents/jcgm/jcgm_100_2008_e.pdf http://www.bipm.org/utils/common/documents/jcgm/jcgm_100_2008_e.pdf http://www.ptb.de/emrp/fileadmin/documents/forcemetrology/workpackages/wp4/uploads/wp4.4_spreadsheet_tool_v1.0.1.xlsm http://www.ptb.de/emrp/fileadmin/documents/forcemetrology/workpackages/wp4/uploads/wp4.4_spreadsheet_tool_v1.0.1.xlsm http://www.ptb.de/emrp/fileadmin/documents/forcemetrology/workpackages/wp4/uploads/wp4.4_spreadsheet_tool_v1.0.1.xlsm microsoft word article 11 195-1080-1-pb.docx acta imeko may 2014, volume 3, number 1, 47 – 55 www.imeko.org acta imeko | www.imeko.org may 2014 | volume 3 | number 1 | 47 intelligent instrumentation: a quality challenge h.j. kohoutek hewlett-packard company, colorado computer quality department, fort collins, colorado, u.s.a. section: research paper keywords: instrumentation design, measurement quality citation: h.j. kohoutek, intelligent instrumentation: a quality challenge, acta imeko, vol. 3, no. 1, article 11, may 2014, identifier: imeko-acta-03 (2014)-01-11 editor: luca mari, università carlo cattaneo received may 1st, 2014; in final form may 1st, 2014; published may 2014 copyright: © 2014 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited 1. introduction regular reports on use of electronic measurement and analytical instrumentation, e.g. [1, 2], indicate that scientists and engineers plan to acquire more and more complex instruments (such as spectrophotometers, chromatographs, electron microscopes) and laboratory data management systems. to maximize the information gain from their application in physical sciences, life sciences, medicine, and engineering, a relatively high level of user knowledge and operational skill is required. this trend toward more complex instrumentation is accompanied by changes in the way professionals fulfill their various tasks [3]. we observe growth of time segments dedicated to study, learning, analysis, reasoning, and judgement in the structure of their jobs, with corresponding decline in data gathering, memorization, and physical activity. there is also a corresponding change in the generic structure of the instrument’s block diagram [4]: from a simple measuring device dedicated to specific physical quantities, followed by transducers interfacing a generic electrical instrument, to current instrumentation (with both analog and digital electronic components) combined with data acquisition and processing. the architecture of contemporary instrumentation permits implementation of complex and abstract models of applicable physical, chemical, and information transformation processes. similar change is observable in the modes of instrumentation controls from mostly manual (allowing operators to adjust the set-up) to stored programs and feedback algorithms, allowing steadily increasing levels of automation in sample handling, repetitive measurements, and self-test. another of the key changes in test and measurement instrumentation is the evolution of small multifunction products (usually an instrument-on-a-card) able to replace an entire rack [5]. the dominating technical issues being addressed by contemporary designers are concentrated around: – requirements for real time measurements [6], reflected in increases of rates of data acquisition, speed of a/d conversion, rates of data processing, careful selection of data format, interrupt priorities, etc. major unsolved problems remain in the areas of multinodal matrix data acquisition and processing of unstructured inputs. – assessing the strategic impact of personal computers [7] which are strengthening their foothold in the field of electronic instrumentation. although pc-based instrumentation does not provide anything substantially new, it handles many data related tasks less expensively and this is a reissue of a paper which appeared in acta imeko acta imeko 1988, proceedings of the proceedings of the 11th triennial world congress of the international measurement confederation (imeko), “instrumentation for the 21st century”, 16-21.10.1988, houston, pp. 337–345. after a review and description of current trends in the design of electronic measurement and analytical instrumentation, changes in its application and use, and of associated quality issues, this paper deals with new quality issues emerging from the expected increase of artificial intelligence impact on system design and implementation strategies. the concept of knowledge quality in all its aspects (i.e. knowledge levels, representation, storage, and processing) is identified as the key new issue. discussion of crucial knowledge quality attributes and associated assurance strategies suggests the need to enrich the assurance sciences and technologies by the methods and tools of applied epistemology. described results from current research and investigation, together with first applications of artificial intelligence to particular analytical instruments, lead to conclusion that the conceptual framework of quality management is, in general, adequate for successful resolution of all quality issues associated with intelligent instrumentation. acta imeko | www.imeko.org may 2014 | volume 3 | number 1 | 48 with more flexibility than dedicated processors. their memory with centrally stored programs and data permits efficient record keeping, cooperation of multiple instruments or measurement processes, and communication. this frees the user from many routine laboratory and reporting chores. – interfacing, stemming from the availability of data processing equipment and need for at least laboratory-wide networking. all this is a contemporary reflection of the intuitively obvious and routine search for more capability (on-line measurements, automation of sample handling, ...), higher performance (detection limits, sensitivity, selectivity, signal-tonoise ratio, speed, ...), ease of use, and lower price. in more specific terms the overall goals of new developments in the various types of instrumentation are: – enhanced productivity and creativity of the analyst and experimenter, – increased information content of experiments, – optimized choice of experiment conditions, – multidimensional and multiparametric measurements, and – an integrated, highly automated laboratory. in general, these instrument design trends [8] seem to be driven by: – need for wide repertoires of techniques and methods in a single laboratory, which is expected to be satisfied by implementations that preclude the prohibitive cost of a large number of dedicated instruments. – recognized power of software leading to more software controlled instrumentation [9] and to blending of computations and measurements, in these systems the instrumentation parts are expected to be fixed, while the software assures needed flexibility and defines how the individual components behave together. this conceptual arrangement permits radical changes in the experiment to be accomplished with minimal hardware changes, in addition, software driven instrumentation opens door for self-tuning, optimization, self-diagnostics, integration into the overall laboratory environment, etc. – availability of new and exotic technologies, such as fiber optics and microfabrication, offering an enormous range of opportunities for chemical microsensors (e.g. chemfets, microdielectrometers, surface acoustic wave sensors), multiwavelength measurements, portability, and cost reduction [10, 11]. the overall pace of changes along these recognized trends will probably quicken and be more dramatic after the introduction of research results of artificial intelligence (ai) into routine instrument design considerations (block diagram of an intelligent instrument is presented in figure 1). experience currently being accumulated in the design of a wide variety of expert systems, complementing the well published work on dendral, macsyma, prospector, mycin, etc., will not only impact the instrumentation design, but will change the jobs of both instrument designers and instrument users [12], and highly probably, create new artificial intelligence technology based jobs in support areas. the expected result will be intelligent instrumentation which will be able to solve problems or produce answers instead of data [13]. the overall objective of introducing ai into instrumentation is to free the professional experimenter from unnecessary involvement with minute implementation details of the experiment and give him or her intelligent assistance in data interpretation, experiment evaluation and, ultimately, experimentation and instrumentation control according to embedded knowledge. results from current research and application implementations, e.g. [14, 15], are presenting a convincing argument about the reality of an expected major impact by ai on the new generation of instruments. embedded artificial intelligence could easily become the dominant design and implementation strategy for 21st century instrumentation. 2. generic quality issues in contemporary instrumentation measurement systems performance, like all technical equipment performance, is a result of sometimes complex interactions among the equipment, the operator, the procedures used, and the environment in which uninterrupted performance is expected. quality management must address all these factors, because they all can be the root cause of imperfect performance representing itself as a detect, an error, or a failure [16]. the quality characteristics of the measurement process have developed around the concepts of precision, the degree of mutual agreement of independent measurements yielded by repeated figure 1. block diagram of an intelligent instrument. acta imeko | www.imeko.org may 2014 | volume 3 | number 1 | 49 applications of the process under specified conditions; and accuracy, the degree of agreement of such measurements with the true value of the magnitude of the quantity under consideration [17]. the concept of precision lends itself to a rigorous mathematical treatment by methods of statistical analysis. for accuracy, unfortunately, there does not exist any single, comprehensive, and satisfactory measure, and in some particular situations the concept itself can become illusive. here statistics can only help, e.g. with tools such as mean square error, as proposed by gauss himself, measurement bias, systematic error, and measurement uncertainty. but even with these tools the steps for analyzing assignable and unassignable causes of error are plagued with many difficulties. to satisfy the statistical character of both concepts multiple measurements are needed, which leads to an empirical concept of the repeatability of a measurement. repeatability implies the same measurement process and the same method of measurement. considered more fully it should also include the same observer, operators, auxiliary equipment, and equipment calibration status. in addition, it should be based on random sampling which takes into account a reasonable range of circumstances. out of the wide variety of measurement equipment quality attributes the ones closest to measurement process quality are performance linearity over the effective range, and instrument stability. linearity (actually the non-linearity or lack of linearity) is usually conveyed by statements about how the signal deviates as it spans the range of measurements. linearity of an instrument is generally determined as the result of a calibration test in which only limited number of values is determined for a system in steady state. only rarely will the same relation apply for dynamic states because the storage parameters will alter the instantaneous transfer characteristics according to the signal amplitude and history of previous signal excursions. pressure gauges, accelerometers, thermometers, etc. will give very different values for instantaneous and steady state measurements. the best expression of an instrument linearity is via the calibration curve showing deviations from the best straight line. drift is a system characteristic, sometimes understood to be synonymous with the term stability, that characterizes how a system variable, which is intended to be constant, varies with time. it also conveys information about very-low-frequency response of the instrument to the measured. drift characteristics are usually specified from the reference point (defined at 25°c) to the extremes of the working range, expanded by safety margins. linearity and stability, traditionally associated with the notion of instrument quality, are mostly a direct function of design choices and goals. other typical design objectives, intended to minimize deterioration due to time or environmental stresses, represent additional product quality attributes which are achieved by proper system structure, component selection, adequate design margins, and built-in redundancy. these attributes are most often known as: – reliability, the probability of a system’s successful function over a given mission period [18]. – environmental ruggedness, defined by a class of environmental stresses under which proper functionality is guaranteed. – electromagnetic compatibility, given by the energy levels, directions, and frequencies of radiated and conducted emissions and by the instrument susceptibility to external electromagnetic fields. – safety level or class, indicating the risks of explosion, fire, or other hazards caused by potential internal defects or failures. – maintainability [19], which is the probability that, when maintenance action is performed under stated conditions, a failed system will be restored to operable condition within a specified total down time. levels of these product quality characteristics, required for business success and customer satisfaction, are given by users’ expectations and set by market conditions, competitive products performance, state-of-the-art of assurance technologies, and by government regulations or international standards. in some situations the overall economic value of these characteristics can be expressed by cost of ownership, which takes into account values of vendor’s goodwill shown in warranty period and conditions, cost of support and service contracts, etc. to the product quality characteristics mentioned above we must add the classical expectations of products free from defects at delivery, compliant to product specifications and data sheets, and evidencing satisfactory level of manufacturing quality controls, adequate workmanship skills, and attention to detail. advances in computing technologies and the related lower cost of computing found their applications in many fields, including analytical, control, and measurement instrumentation. currently the major part of electronic instrumentation is software driven, which brings into focus issues of software quality. the quality of the whole measurement system may, to a significant degree, be dependent on the quality of the code. because computer software is a logical rather than a physical system, it has different characteristics [20], e.g.: – it does not wear out, – maintenance often includes design modifications and enhancements, and – there is no significant manufacturing activity involved. there is also not very much well structured industrial experience to significantly indicate preferable engineering design strategies. many attempts to define software quality characteristics with associated metrics, criteria, and contributing factors have been made, e.g. [21, 22, 23], but only a few members of the proposed family of quality attributes are universally accepted, mainly: – reliability, meaning a measure of the frequency and criticality of unacceptable program behavior under permissible operating conditions; – correctness, now inconclusively demonstrated by validation testing which provides some level of assurance that final software product meets all functional and performance requirements. there is some hope that, in the future, programs will be infallibly proven to be correct. some tools for automated proofs are beginning to emerge from the research work in artificial intelligence. – maintainability, which may be defined qualitatively as the ease with which software can be understood, corrected, adapted, and enhanced. software maintainability is a difficult term to quantify, but gilb [24] proposed a number of measures relating to the maintenance effort. – ease of use assures that the job or task will be successfully completed by personnel at any required stage in system operation within a specified time. usability characteristics are based on observations and understanding of operator acta imeko | www.imeko.org may 2014 | volume 3 | number 1 | 50 behavior, limitations of body characteristics, and reaction times. design approaches and assurance techniques for these quality attributes have not stabilized yet and vary widely from vendor to vendor. in the commercial world, as in science and engineering, these described measurement system quality characteristics are complemented by the ever increasing need to demonstrate that the total measurement uncertainty is sufficiently small to meet users requirements. this measurement assurance, in reality a quality control of measurements, is based on redundancy built into the measurement scheme. this redundancy is usually represented by repeated measurements of a stable standard. measurement assurance is then obtained when verifiable limits of uncertainty have been demonstrated, and measurement process quality control program exists in real time to monitor its performance [25, 26]. 3. quality issues unique to intelligent instrumentation – the definition of quality, which is currently oriented toward conformance to specification. it needs to be replaced by a sociometric definition, which perceives and eventually measures it as a multiattribute product characteristic to be designed in; – the value of quality, now primarily beneficial to the manufacturer via reduced production and support cost, must be recognizable directly at the product level in both classical forms (value in exchange and value in use). this will allow quality to be properly priced, and appreciated as a source of revenue; – the role of the quality department, where quality specialists and managers must shun being critics of manufacturing processes and become contributing partners with all line functions. we understand the fact that scientific research, as well as control and measurement processes are, by nature, processes of goal oriented learning, as evident from the similarity of flow diagrams of basic process steps shown in figures 2a, b. the key objective in making these processes as efficient as possible is frequently achieved by applying scientific analytical instrumentation. further, considering that the learning process itself is a feedback loop between inductive formation of hypotheses, based on accumulated data, and collection of new data according to deductively proposed experiments, we can see that classical instrumentation plays a major role in data gathering and analysis. however, intelligent instrumentation is expected to bring new (expert) levels of efficiency into the figure 2a. the process of experimentation as iterative learning. acta imeko | www.imeko.org may 2014 | volume 3 | number 1 | 51 processes of experimental data interpretation, new hypotheses forming, and designing of new experiments. recognition that all the terms, such as data, information, rules, metarules, heuristics, etc. used in presented flow diagrams, represent different forms of knowledge, brings us to knowledge as the central point of the whole held of artificial intelligence perceived, from the practical viewpoint of the electronic industry, as knowledge-based problem solving by computers. also the empirical and intuitively self-evident observation that the competence, and by that the value, of any intelligent system strongly correlated to the amount and quality of the embedded knowledge, leads us to the central unique quality issue of intelligent instrumentation: the issue of knowledge quality. 3.1. knowledge quality the factual part of our knowledge comes mostly in the form of scientific and technical data in three broad classes [27]: – repeatable measurements on well defined systems (class a); where quality assurance methodologies recommend using data only from reliable sources such as national standard reference data system (nsrds) coordinated by the u.s. national bureau of standards. – observational data, often time or space dependent (class b). the most effective way to assure quality of class b data by careful maintenance and calibration of the measuring instruments prior to data acquisition. recording and preservation of all required auxiliary information is also important. – statistical data (class c); here quality control strategy, based on probability theory, is a more difficult task, often hindered by disagreements in definitions and terminology. one of the greatest benefits of the recent advances in graphics software is increased control over the quality and completeness of experimental data by allowing for its visual inspection via windowing, 3-d modelling, and simultaneous processing of alphanumerical and graphical information. improvements have reached the point where virtually any level of detail from multiple measurements can be observed on the screen and functionally used to enhance data quality. also more frequent acceptance of the need for systematic skepticism and openness to alternative modes of interpretation in data analysis is now leading to application of statistical figure 2b. event-driven control scheme of a general problem solver. acta imeko | www.imeko.org may 2014 | volume 3 | number 1 | 52 methods of exploratory data analysis. these methods reflect recognition of the importance of graphical representation for visual inspection of unprocessed data, which should precede formal statistical calculations even when the latter is the desired end product. this approach opens wider ranges of alternative explanations and allows the researcher to be open to unexpected possibilities, particularly while working with weak theories. assuring quality of higher level knowledge, which is being engineered today and will be embedded in intelligent instrumentation (e.g. definitions, taxonomies, discrete descriptions, constraints, deductive algorithms), presents many epistemological challenges. the philosophical questions about the reliability of scientific knowledge, by themselves a serious intellectual issue, will not significantly impact the design of intelligent instruments. this is so because, in majority of expected applications, the knowledge and application domains will be sufficiently narrow and well defined. this will permit quality assurance to concentrate on issues of unambiguity, consistency coherence, visibility and acceptability of perceived patterns, and methods of justification and validation. a completely new set of quality issues is associated with the problems of knowledge base updates and knowledge maintenance. quality assurance methods here might be simplified by the presence of metaknowledge or metarules controlling automatic learning in embedded knowledge bases. 3.2. quality of knowledge representation the pragmatic approach to embedding knowledge into programs that exhibit intelligent behavior is focused on developing schemes for knowledge representation which permit manipulation of specialized data structures to obtain intelligent inferences. in general, the knowledge representation schemes are combinations of data structures and interpretive procedures. the most frequently used formalisms are: state/space representation, formal logic, procedural representation, semantic nets, production systems, scripts, and frames. there are also special techniques for visual scene and speech representation. the issues of form and notation have occupied most previous quality discussions. currently it is recognized that the issue of what could or could not be done with a representation is more important. so, on the conceptual level, the most important quality attributes of knowledge representation are associated with its three adequacies [28]: – metaphysical, assuring a form that does not contradict the character of facts of interest, – epistemological, which guarantees that the representation can practically express the facts, and – heuristical, that assures the reasoning processes, leading to the problem solution, are expressible in contemplated language. many authors [29] proposed more practical attributes, e.g.: – modularity, which allows adding, deleting, or changing information independently, – uniformity, to assure understandability to all parts of the system achievable via rigid structures, – naturalness, to reflect the ease of expressing important kinds of relevant knowledge, – understandability, the degree to which the representation scheme is clear to humans, – modifiability, which assures context independence. studies of semantic primitives and established representational vocabularies [30] lead to other attributes such as finitude, comprehensiveness, independence, noncircularity, and primitiveness. a completely new quality assurance issue arises when problem solving algorithms require representation change midstream. representation transformations must be then evaluated in terms of isomorphism and homomorphism. 3.3. knowledge base quality issues technical implementations of knowledge bases are benefiting from the design for quality methodologies of data bases, which culminated in fault-tolerant designs. new quality issues, emerging from the special characteristics of relational data bases and current experience with knowledge bases, are bringing into focus needs of assuring: – protections against interferences among the subsets of a knowledge base. this is complicated by the environment of laboratory-wide networks which do not have a central operating system. current “design for quality” research results tend toward making transactions atomic, which could adversely affect the transaction time; – protections against new aspects of pattern sensitivity; – transparency of structural characteristics, a difficult task in conditions of high structural complexity; and – already mentioned knowledge maintenance quality controls. first attempts to formulate adequate assurance strategy for knowledge maintenance have already taken place, as described e.g. in [31], providing prespecified set of control metarules, production rules formalism, and tools for sensitivity analysis, execution tracing, and explanations. but effective implementation of these strategies will be complicated by the problems of scale, speed, and system complexity. the importance of methods for validation of system’s knowledge base and evaluation of its quality is also self-evident, but their development is a very difficult task. 3.4. quality of knowledge processing the basic set of knowledge processing algorithms consists of search processes, deductions, inductive inferences, and learning. – search algorithms, not significantly different from other algorithms implemented by computer programs, do not present new or unique quality problems. the basic questions here still relate to implementation correctness and its demonstrated verification levels. some new challenges may be found in implementation of associative searches, but even here, the main issue is correctness. – deductions are basically chains of statements, each of which is either a premise or something which follows from statements occurring earlier in the chain. mechanization of deductive processes based on predicate logic must, beside the issue of direct correctness, take into account needs for the deduction’s visibility, ease of inspection, backward traceability etc., to improve solution credibility and to allow for correctness verification. – the most difficult task for knowledge processing quality assurance arises when we depart from categorical (or deterministic) reasoning. statistically based inferences, or inferences based on social judgement are plagued with errors of insufficient evidence, illusory correlations, acta imeko | www.imeko.org may 2014 | volume 3 | number 1 | 53 circumstantial prompting, belief perseverance in the face of contrary evidence, etc. research into application of epistemology to solve these types of problems is only beginning. but we do not expect these problems to be encountered during the early generations of intelligent instrumentation. – automated learning systems’ performance can be measured by the rate of knowledge compilation. the quality of such a system is usually described by the system’s generality (the ability to perform successfully in novel situations), its ability to discriminate between critical and peripheral information, its ability to automatically restrict the range of acquired knowledge applicability, and its ability to control the degree of interest of inferences in order to comply with principles of significance and goal satisfaction. but the fundamental conceptual problem of controlling quality of knowledge processing will be resolved only if the problem of adequate modelling of the categorical structure of the thinking process will be resolved. 4. other quality issues associated with the intelligent instrumentation in many aspects the intelligent instruments will be implementations of an advanced generation of the current software driven instrumentation. because of this fact all the traditional software quality issues will be still present and will have to be addressed. the aspects of the new, higher level of intelligent human interface will bring into the field of instrumentation quality issues currently associated with advanced computing and engineering workstations. the most important seem to be the: – quality aspects of the control language; in the field of intelligent instrumentation the interface and control language will have to provide convenient means for implementation of a variety of experimenting strategies and designs. it must also act as a vehicle of experimenter’s thought, and via local area networks allow communication of higher level concepts between experimenters. the minimum quality requirements will have to cover aspects of ease of programming, error handling, automatic backtracking, machine independency of compilers, and level of standardization. similar requirements will apply to languages designed specially for manipulating knowledge bases. – quality of human interface, which must permit the instrumentation user to concentrate fully on his experiment, without being distracted by the uniqueness of the applied computational technology. human interface design strategies will have to go beyond the traditional ergonomic and intuitive approaches to design for system friendliness. they must address the physical, physiological, psychological, and intellectual aspects at the experimenters job and personality. solutions must be based not only on anthropometry but be developed in the context of the measurement system’s lexical, syntactic, and semantic requirements. there is a growing recognition of the need for serious scientific experimentation which will permit both the designer and cognitive psychologist to significantly improve current approaches to human-instrument interface designs. – quality aspects associated with new computing architectures and increased levels of parallelism. – system testability and diagnosis; the growing complexity of the instrumentation, its real-time performance, which will make accurate repetitions of the computational and reasoning processes impossible due to asynchronisms and new levels of freedom for internal intelligent controls, will make these tasks especially difficult but nevertheless extremely important for future fault-tolerant designs. despite expected software dominance there still will be enough hardware in the next generation of instrumentation to keep us worrying about the traditional quality issues associated with product safety, regulatory compliance, environmental ruggedness, reliability, maintainability, supportability, etc. some new quality issues will emerge with new families of vlsi components. the increased levels of their complexity and density will challenge our ability to control qualify of all materials involved in processing, the dimensions of manufactured structures in and below the one micron range, and the many process variables needed to achieve yield economy and final product quality. the expansion of intelligent instrumentation, expected in 1990’s, will be complemented by maturing of some sensor technologies currently being introduced. the new families of sensors solve one set of problems but open risks in other areas of which the quality assurance community must be aware, e.g.: – optical sensors for chemical analysis are an answer to problems of electrical interference, reference electrodes, physical contact with the reagents, and need for multiwavelength measurements. but to ensure the highest possible quality of their application, care must be taken to prevent background light interference, the possibility of photodegradation of reagents, to assure long term stability, etc. the sources of potential error must be identified and compensated for. – electrochemical sensors, where developments are moving away from potentiometric to amperometric sensors. these changes are bringing challenges to techniques of surface modification control required to achieve wanted selectivity, to compensation methods for dealing with increased sensitivity and to design for reliability. for chemfets, especially, challenges will appear in areas of stability and encapsulation technologies. the fundamental quality issues and strategies for assurance of these new families of sensors revolve around: – the two concepts: compound confirmation and compound identification, as well as – the practical need to avoid application surprises, such as interference in sample matrix effects, errors at the reference point, etc. 5. results of current research and applications reports regarding results of ai application in support of instrumented diagnostics and troubleshooting are many. manufacturers, including ibm, digital, tandem, prime, at&t, general electric (for review see e.g. [32]), have devised remote diagnostic expert systems to analyze hardware problems in the field. in 1984 major breakthrough by dr. l.f. pau at battelle memorial institute, switzerland, in development of a set of about 400 application independent metarules for diagnostic expert systems, provided new strategy for both reduction of design cost and increase of capability via a model of a circuit network which describes many types of real electronic faults. also the proliferation of cae tools and workstations has a far reaching impact on the productivity of development and ease of acta imeko | www.imeko.org may 2014 | volume 3 | number 1 | 54 support of maintenance related expert systems. other software based expert system-building tools, such as automated reasoning tool art [33], permits the designer to develop the necessary knowledge base, reflecting his experience. these tools often use frames to represent knowledge about objects of interest, along with their attributes. agenda or menu windows indicate the rules fired together with facts that initiated them. these and command windows, which trace program execution step-by-step, simplify both debugging and use of the system by illustrating the reasoning process flow. less visible, but significant, are the results of the still infrequent applications of ai to upgrade performance of analytical instruments. one of the key reasons for this slow progress is that the majority of known successful projects has been implemented on large computers, often unavailable in current laboratory settings. a microcomputer based expert system for enhancement of electroencephalogram evaluation [15] has demonstrated the usefulness of ai methods in biomedical instrumentation. this system provides on-line evaluation with performance equal to that of a large computer. the most important information is being extracted from the frequency domain, using fft for spectral estimation, hanning windows to reduce spectral leakage, and forty-three assembly language coded rules for basic classification into either normal or abnormal categories. the disadvantage of the present system lies in the need to have the user provide the complete control strategy, i.e. the selection of relevant rules and sequence of their implementation. the development of a totally computerized triple quadrupole mass spectrometer, with prototype knowledge based instrument control [14], is a necessary step toward the goal of intelligent tqms instrumentation. it has the ability to apply structured stored experience, to use power of automatic reasoning to control its own behavior, and to respond to novel situations and new problems. the knowledge including control heuristics is represented in the form of productions (rules). resident, ai guided calibration program assures self-adaptive feedback control process for real-time optimization or tuning of the data acquisition process. studies of data acquisition, knowledge representation, reasoning algorithms, and interpretation of results, applied, e.g., to chromatographic data for the diagnosis of spontaneous bacterial peritonitis [34], indicate that current results can be extended to other applications and could lead to automated intelligent decision systems. honeywell system’s fleet/dras project [35] demonstrated the feasibility of applying expert system technology to automation of data reduction, analysis, and detection of anomalies. although incomplete, work done in investigation and assessment of the mutual relationship between artificial intelligence and assurance sciences and technologies, has identified the key quality strategies necessary to address the new, knowledge quality related issues [36]. these strategies are based on creative application of current assurance expertise enriched by methods of applied epistemology. as described in previous chapters major progress has been achieved in: – controlling quality of the factual part of embedded knowledge; – identification of adequacy (metaphysical, epistemological, and heuristical) as the most important quality attribute of a knowledge representation; – strategies for fault-tolerant design of data and knowledge bases; and – key architectural guidelines for intelligent human interface design. the key quality assurance technologies and skills for the manufacture of key vlsi components, which are expected to be fundamental for the intelligent instrumentation hardware, are identified and in place [37]. new sensor technologies, new non-von neumann architectures, new system parts and functions, as well as support of knowledge quality assurance, will require additional intensive applied research and engineering work by teams of experts from both the r&d and quality communities. 6. summary and conclusions the emergence of successful practical implementations of the research results in artificial intelligence is beginning to impact conceptual thinking about the new generation of electronic measurement and analytical instrumentation. because the competence, and by that the value of an intelligent instrument, will always be strongly correlated to the amount and quality of the embedded knowledge, the issue of knowledge quality is beginning to form the cornerstone of the new concern of quality management. to assure success of quality programs, methods of applied epistemology must be employed. only in this way can the problems of quality at all levels of knowledge, its representation, storage, and processing be solved. the key quality assurance strategies addressing these knowledge related quality attributes have been identified. new quality issues are also emerging because of the increased levels of hardware parallelism, computational speed, system complexity, and new sensor technologies. traditional issues of hardware and software quality, as well as issues of support and vendors goodwill, remain substantially unchanged. current research and application results indicate that artificial intelligence must be considered by instrumentation designers in their search for more capability, higher performance, ease of use, and lower price. they even suggest that embedding artificial intelligence into the measurement and analytical instrumentation could easily become the dominant system design and implementation strategy for the 21st century. the conceptual framework of current quality assurance, if enriched by methods and tools of applied epistemology as indicated, seems to be generally adequate to address the new issues associated with knowledge quality and intelligent user interface. references [1] 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[2] c.j. mosbacher, use of analytical instruments and equipment, research & development, feb. 1986, pp. 90–93. [3] f. hayes-roth, the machine as partner of the new professional, ieee spectrum, jun. 1984, pp. 28–31. [4] c.h. house, perspectives on dedicated and control processing, computer, dec. 1980, pp. 35–49. [5] g. sideris, traditional products get squeezed by newer gear, electronics, feb. 1987, pp. 89–93. [6] j.g. liscouski, selecting instrument interfaces for real-time data acquisition, analytical chemistry, 1982, 54 (7) p. 849. [7] f. guteri, instrumentation (technology 1985), ieee spectrum, jan. 1985, pp. 65–67. [8] w.e. terry, instrument advances will be an intelligent progression, electronic business, dec. 1985, pp.137–138. acta imeko | www.imeko.org may 2014 | volume 3 | number 1 | 55 [9] m.l. salit, m.l.parson, software-driven instrumentation; the new wave, analytical chemistry, 1985, 57 (6) p.715. 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[25] j.w. locks, measurement assurance and accreditation, proc. of the ieee, vol. 74, no. 1, jan. 86, pp. 21–23. [26] r.m. judish, quality control of measurements – measurement assurance”, proc. of the ieee, vol. 74, no 1, jan 86, pp. 23–25. [27] d. lide jr., critical data for critical needs, science, vol. 212, no. 4501, june 1981, pp. 1343–1349. [28] j. mccarthy, p.j. hayes, some philosophical problems from the standpoint of artificial intelligence, readings in artificial intelligence, tioga publishing co., palo alto, california,1981, pp. 431–450. [29] a. barr, e. feigenbaum, editors, handbook of artificial intelligence, vol i, chapter iii, pp. 193–194. [30] y.a. wilks, methodological questions about artificial intelligence; approaches to understanding natural language, journal of pragmatics, vol. 1 (1977), pp. 69–84. [31] p. politakis, s.m. weiss, using empirical analysis to refine expert system knowledge bases, artificial intelligence, vol. 22, no 1 (1984), pp. 23–48. 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[37] h.j. kohoutek, quality assurance technologies for vlsi manufacturing, quality and reliability engineering international, vol. 3 (1987), pp. 107–126. template for an acta imeko event paper acta imeko issn: 2221-870x november 2016, volume 5, number 3, 32-44 acta imeko | www.imeko.org november 2016 | volume 5 | number 3 | 32 numerical inversion of a characteristic function: an alternative tool to form the probability distribution of output quantity in linear measurement models viktor witkovský institute of measurement science, slovak academy of sciences, dúbravská cesta 9, sk-841 04 bratislava, slovakia section: research paper keywords: gum; linear measurement model; probability density function; characteristic function; numerical inversion; gil-pelaez inversion formulae; fft algorithm citation: viktor witkovský, numerical inversion of a characteristic function: an alternative tool to form the probability distribution of output quantity in linear measurement models, acta imeko, vol. 5, no. 3, article 6, november 2016, identifier: imeko-acta-05 (2016)-03-06 section editor: franco pavese, italy received april 27, 2016; in final form april 27, 2016; published november 2016 copyright: © 2016 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: the work was supported by the slovak research and development agency, project apvv-15-0295, and by the scientific grant agency vega of the ministry of education of the slovak republic and the slovak academy of sciences, by the projects vega 2/0047/15 and vega 2/0011/16 corresponding author: viktor witkovský, e-mail: witkovsky@savba.sk 1. introduction the basic working tool in measurement uncertainty analysis, as advocated in the current revision (under preparation) of the guide to the expression of uncertainty in measurement (gum) [1], and consistent with its supplement 1 – propagation of distributions using a monte carlo method [2], is the state-of-knowledge pdf about the quantity (true value of measurand), based on the currently available information. the state-of-knowledge pdf quantifies the degree of belief about the values that can be assigned to the quantity based on the available information. the expectation and the standard deviation of this pdf (if they exist) are used to report the measurement result and the associated (standard) measurement uncertainty. although the latest gum development emphasizes the bayesian view of probability in the evaluation of measurement uncertainty, it should be clearly stated and understood that this approach is not based on the strict bayesian principles of statistical inference (i.e. straightforward application of the abstract measurement uncertainty analysis based on combining the state-of-knowledge distributions requires evaluation of the probability density function (pdf), the cumulative distribution function (cdf), and/or the quantile function (qf) of a random variable reasonably associated with the measurand. this can be derived from the characteristic function (cf), which is defined as a fourier transform of its probability distribution function. working with cfs provides an alternative and frequently much simpler route than working directly with pdfs and/or cdfs. in particular, derivation of the cf of a weighted sum of independent random variables is a simple and trivial task. however, the analytical derivation of the pdf and/or cdf by using the inverse fourier transform is available only in special cases. thus, in most practical situations, a numerical derivation of the pdf/cdf from the cf is an indispensable tool. in metrological applications, such approach can be used to form the probability distribution for the output quantity of a measurement model of additive, linear or generalized linear form. in this paper we propose new original algorithmic implementations of methods for numerical inversion of the characteristic function which are especially suitable for typical metrological applications. the suggested numerical approaches are based on the gil-pelaez inverse formulae and on using the approximation by discrete fourier transform and the fast fourier transform (fft) algorithm for computing pdf/cdf of the univariate continuous random variables. as illustrated here, for typical metrological applications based on linear measurement models the suggested methods are an efficient alternative to the standard monte carlo methods. mailto:witkovsky@savba.sk acta imeko | www.imeko.org november 2016 | volume 5 | number 3 | 33 bayes' theorem). for more details and further discussion see, e.g., [3]-[5], or [6], and also [7] and [8]. in fact, the gum approach is based on using a well-defined functional relationship between the mutually inter-related quantities for propagating the state-of-knowledge pdfs of the input quantities, represented by the random variables (rvs), into the state-of-knowledge pdf of the output quantity – which is believed to be a rv reasonably associated with the measurand. frequently, it is suggested to use a well-known functional relationship (based, e.g., on the physical and/or geometrical laws) between the true value of the measurand and the true values of the other influencing input variables, which is typically expressed by the measurement equation of the measurement model. obviously, such pdf of the output quantity represents currently available knowledge (limited, but hopefully the best to date) about the measurand, i.e. it expresses probability distribution of the values being attributed to a quantity (the measurand), based on information used (which could be rather limited and/or heavily biased). this interpretation is consistent to the original gum definition of the uncertainty in measurement (see [1], clause 2.2.3), which is defined as a parameter, associated with the result of a measurement, that characterizes the dispersion of the values that could be reasonably attributed to the measurand. however, the derived term coverage interval is inconsistent with this interpretation, for more details and discussion see [8]. in fact, without imposing further (well and clearly defined) model assumptions and optimality criteria for selecting and combining the information, it can be only hardly expected that the presented result shall represent the best (in what sense?) estimate of the true measurand value. on the other hand, the proposed gum approach could be well accepted as a (simple) method for combining experimental results with the expert judgment in order to get comprehensive characterization of our knowledge about the true value of measurand, based on all currently available information, albeit without the possibility of guaranteeing the (otherwise naturally) required statistical properties and/or optimality criteria. if this is the goal, other means and/or subsequent analysis should be applied and properly used. as already mentioned, the term coverage interval (introduced in [2], and defined as the interval containing the (true) value of a quantity with a stated probability (say 95 %), based on the information available) is not properly used in this context. hence, as an alternative to the 95 % coverage interval, here we shall use a more appropriate term – the 95 % state-of-knowledge interval. this should read as the interval of 95 % values that could be reasonably attributed to the unknown value of measurand based on the current state-of-knowledge (i.e. based on the measurement model, the currently available information, and method used for combining the information). of course, further study is necessary for characterizing the optimality properties of the used method, e.g., under repeatability conditions. a standard approach to derive the state-of-knowledge pdf is based on the propagation of distributions using a monte carlo method, as suggested in supplement 1 of the gum, [2]. for more details and discussion on applicability of the uncertainty evaluation methods based on the gum and its supplement 1 see, e.g., [9]-[12]. a principal advantage of the monte carlo methods is their simplicity and asymptotic consistency (convergence to the true values with growing number of the monte carlo simulations). a disadvantage of the monte carlo methods is their principal ambiguity (i.e. non-uniqueness/variability of the results under independently repeated computations with given number of simulations) and typically large demand on the computational resources. often, to achieve the pre-selected accuracy level, a need of a very large number of simulations is required. alternatively, in linear measurement models, the state-ofknowledge pdf of the output quantity can be derived by inverting its characteristic function, which can be easily derived from the known characteristic functions of the input quantities. a principal advantage of the methods based on inversion of the characteristic function is their principal exactness (as there is a theoretical one-to-one mapping between the cf and the cdf) and efficiency for all values of the computed cdf/pdf/qf. however, in real situations, the precision of the used computational methods is also subject to possible numerical errors. a typical numerical error of such results is due to particular algorithmic implementation (as e.g., the truncation error and/or the integration error), which can be and should be properly controlled. as we shall illustrate below, in typical metrological applications with linear measurement models and the well behaved probability distributions of the input quantities, the numerical precision and efficiency of the here presented simple methods and algorithms is superior to the standard monte carlo methods. we notice that among other possible alternative approaches to evaluate the propagated probability distribution of the output quantity we can include the advanced methods for arithmetic computations with random variables and their distributions, see e.g. [13], [14], and also [15], [16]. however, applicability of these methods is still limited to a relatively small number of the input random variables. 2. linear measurement model and the characteristic functions as mentioned above, an alternative tool to form the stateof-knowledge probability distribution of the output quantity in a linear measurement model is based on the numerical inversion of its characteristic function (cf), which is defined as a fourier transform of its pdf, see (2). computing the (inverse) fourier transform numerically is a well-known problem, frequently connected with the problem of computing integrals of highly oscillatory (complex) functions. the problem was studied for a long time in general, but also with focus on specific applications, see, e.g., [17]-[22], to show just a few. in particular, the methods suggested for inverting the characteristic function for obtaining the probability distribution function include [23]-[27]. approximations of the continuous fourier transform by the discrete fourier transform and by using the fft algorithm are widely used in different fields of engineering. however, using the fft for evaluation of the pdf/cdf from the characteristic function is not widespread in statistical applications (one important exception is the field of financial mathematics and econometrics), and in general, not well implemented in relevant software packages. in [28], korczynski, cox, and harris suggested and illustrated the use of convolution principles in metrology applications. their suggested approach was based on consecutive replacing of the convolution integrals by the convolution sums evaluated by using the fast fourier transform (i.e. without direct using the exact characteristic functions), to acta imeko | www.imeko.org november 2016 | volume 5 | number 3 | 34 form the probability distribution for the output quantity in the measurement model of additive, linear or generalized linear form. if compared with the here proposed approach based on combining and inverting the exact characteristic functions, the numerical precision of their suggested approach can quickly deteriorate with the growing number of the required convolution integrals. in fact, in metrological applications a number of measurement models used in uncertainty evaluation are, at least approximately (up to reasonable level), of the additive linear form 𝑌 = 𝑐1𝑋1 + ⋯ + 𝑐𝑛𝑋𝑛, (1) where the input quantities 𝑋1, … , 𝑋𝑛 are independent random variables with known probability distributions, 𝑋𝑗 ∼ 𝐹𝑋𝑗, for 𝑗 = 1, … , 𝑛, possibly parametrized by 𝜃𝑗. here, 𝑐1, … , 𝑐𝑛 denote the known constants and 𝑌 represents the univariate output quantity (a random variable with an unknown distribution to be determined). the characteristic function of a continuous univariate random variable 𝑋 ∼ 𝐹𝑋, with its probability density function pdf𝑋(𝑥), is defined as a fourier transform of its pdf, cf𝑋(𝑡) = ∫ 𝑒𝑖𝑖𝑖 ∞ −∞ pdf𝑋(𝑥) 𝑑𝑥, 𝑡 ∈ 𝑹 (2) analytical expressions of the characteristic functions are known for many standard probability distributions, see e.g. [29]-[31], or other available sources. otherwise, cf could be derived either analytically, expressed by using computer-based tools, e.g. by using mathematica, or evaluated numerically. in table 1 we present some selected characteristic functions of the univariate distributions, frequently used in metrological applications. compare the presented distributions with those in table 1 in [2]. notice that the characteristic functions of the symmetric zero-mean distributions are purely real functions of the argument 𝑡 ∈ 𝑹. deriving cf of a weighted sum of independent random variable is a simple and trivial task. let cf𝑋𝑗(𝑡) denote the characteristic function of 𝑋𝑗. the characteristic function of 𝑌 defined by (1) is cf𝑌(𝑡) = cf𝑋1(𝑐1𝑡) ⋯ cf𝑋𝑛(𝑐𝑛𝑡). (3) for illustration, in figure 1 we plotted the cf of a linear combination of two independent chi-squared random variables with 𝜈1 = 1 and 𝜈10 = 10 degrees of freedom, evaluated for 𝑡 ∈ (−1,1). here we shall assume that the considered characteristic functions of the input and/or output quantities, the random variables 𝑋1, … , 𝑋𝑛 and 𝑌 are known or can be easily derived. then, by the fourier inversion theorem, the pdf of the random variable 𝑌 is given by pdf𝑌(𝑦) = 1 2𝜋 ∫ 𝑒−𝑖𝑖𝑖 ∞ −∞ cf𝑌(𝑡) 𝑑𝑡, 𝑦 ∈ 𝑹. (4) analytical derivation of the pdf by using the (inverse) fourier transform (4) is available only in special cases. thus, in most practical situations, a numerical derivation of the pdf/cdf from the cf is an indispensable tool. in the next section we present two simple but frequently very efficient approaches (approximate numerical methods) for inversion of the characteristic function together with a detailed description of their algorithmic implementations, which are suitable for typical metrological applications. 3. numerical inversion of the characteristic function the exact inverse fourier transform (4) can be naturally approximated by the truncated integral form, pdf𝑌(𝑦) = 1 2𝜋 ∫ 𝑒−𝑖𝑖𝑖 𝑇 −𝑇 cf𝑌(𝑡) 𝑑𝑡, (5) table 1. characteristic functions of continuous univariate distributions used in metrological applications (selected symmetric zero-mean distributions and non-negative distributions). here, kν(z) denotes the modified bessel function of the second kind, jν(z) is the bessel function of the first kind, and u(a, b, z) is the confluent hypergeometric function of the second kind. probability distribution characteristic function (cf) gaussian 𝑁(0,1) cf(𝑡) = 𝑒− 1 2 𝑖2 student’s t 𝑡𝜈 cf(𝑡) = 1 2 𝜈 2−1γ�𝜈 2 � �𝜈 1 2 |𝑡|� 𝜈 2 𝐾𝜈 2 �𝜈 1 2 |𝑡|� rectangular 𝑅(−1,1) cf(𝑡) = sin (𝑖) 𝑖 triangular 𝑇(−1,1) cf(𝑡) = 2−2cos (𝑖) 𝑖2 arcsine 𝑈(−1,1) cf(𝑡) = 𝐽0(𝑡) exponential 𝐸𝑥𝐸(𝜆) cf(𝑡) = 𝜆 𝜆−𝑖𝑖 𝜆 > 0 rate gamma γ(𝛼, 𝛽) cf(𝑡) = �1 − 𝑖𝑖 𝛽 � −𝛼 𝛼 > 0 shape, 𝛽 > 0 rate chi-squared 𝜒𝜈2 cf(𝑡) = (1 − 2𝑖𝑡)− 𝜈 2 𝜈 > 0 degrees of freedom fisher-snedecor’s f 𝐹𝜈1,𝜈2 cf(𝑡) = γ�ν1 2 +ν2 2 � γ�𝜈2 2 � 𝑈 �𝜈1 2 , 1 − 𝜈2 2 , − 𝜈2 𝜈1 𝑖𝑡 � 𝜈1 > 0, 𝜈2 > 0 degrees of freedom figure 1. real (blue) and imaginary (red) part of the characteristic function of 𝑌 = 10 𝑋𝜒12 + 𝑋𝜒10 2 — the linear combination of two independent chisquared random variables with 𝜈1 = 1 and 𝜈1 = 10 degrees of freedom, evaluated for 𝑡 ∈ (−1,1). acta imeko | www.imeko.org november 2016 | volume 5 | number 3 | 35 where 𝑇 is a sufficiently large (real) value, and the integrand is a complex (oscillatory) function, which is assumed to decay to zero reasonably fast for |𝑡| → ∞ (as is typical for continuous distributions used in metrological applications). note that the selection of the integration limit 𝑇 and the speed of integrand function decay contribute significantly to the truncation error of this approximation. in general, the required integral can be evaluated by any suitable numerical quadrature method. frequently, a simple trapezoidal rule gives fast and satisfactory results. however, caution is necessary if the integrand is a highly oscillatory function, what is the case if abs(𝑦) is a large value from the tail area of the distribution, as e.g. the 99 % quantile. in such situations a more advanced quadrature method should be used, typically in combination with efficient root-finding algorithms and algorithms for accelerated computing of the limit values of the sums of alternating series (not considered here), see e.g., [21], [32]. note that the selection of the integration method significantly contributes to the integration error of this approximation. figure 2 illustrates the problem of integrating the highly oscillatory integrand function. for typical metrological applications we suggest to consider algorithms based on the gil-pelaez inversion formulae (6)-(7) and the approximate discrete fourier transform (21) resp. (22), based on the simple trapezoidal rule for computing the required sub-integrals. the possible numerical error of the results (i.e. the truncation error and the integration error) can be controlled by selecting a proper truncation limit 𝑇 (sufficiently large, leading to a small truncation error), and by dividing the selected integration interval (0, 𝑇) to a large number, say 𝑁, of small sub-intervals, where the trapezoidal rule provides a satisfactorily good numerical approximation (with small integration error) of the true integral value. for theoretical results on how to control the truncation and integration errors in such and similar situations see, e.g., [35]. 3.1. the gil-pelaez inversion formulae in [33], gil-pelaez derived the inversion formulae, suitable for numerical evaluation of the pdf and/or the cdf, which require integration of a real-valued function, only. the pdf is given by pdf𝑌(𝑦) = 1 𝜋 ∫ ℜ�𝑒−𝑖𝑖𝑖cf𝑌(𝑡)� ∞ 0 𝑑𝑡. (6) further, if 𝑦 is a continuity point of the cumulative distribution function of 𝑌, the cdf is given by cdf𝑌(𝑦) = 1 2 − 1 𝜋 ∫ ℑ� 𝑒−𝑖𝑖𝑖cf𝑌(𝑖) 𝑖 � ∞0 𝑑𝑡. (7) by ℜ(𝑓(𝑡)) and ℑ(𝑓(𝑡)) we denote the real and imaginary part of the complex function 𝑓(𝑡), respectively. numerical inversion of the characteristic function based on (6) and (7) have been successfully implemented for evaluation of the distribution function of a linear combination of independent chi-squared rvs by imhof in [34] and by davies in [35]. further, gil-pelaez's method has been implemented in the algorithm tdist, see [36] and [37], for computing the distribution of a linear combination of independent student's t random variables and/or other symmetric zero-mean random variables, and also for computing the distribution of a linear combination of independent inverted gamma random variables suggested in [38], and the distribution of a linear combination of independent log-lambert 𝑊 × 𝜒𝜈2 rvs, [39]. in [40], the algorithm tdist has been suggested and applied for computing the 95 % state-of-knowledge interval (considered as the approximate 95 % confidence interval) for the common mean value in the inter-laboratory comparisons with systematic effects (biases). in general, the integrals in (6) and (7) can be computed by any numerical quadrature method, possibly in combination with efficient root-finding algorithms and accelerated computing of limits of the alternating series, as considered, e.g., in [20] and [25]. frequently, (6) and (7) can be efficiently approximated by a simple trapezoidal quadrature: pdf𝑌(𝑦) ≈ 𝛿𝑖 𝜋 �𝑤𝑗 𝑁 𝑗=0 ℜ�𝑒−𝑖𝑖𝑗𝑖cf𝑌�𝑡𝑗�� ≈ 𝛿𝑖 𝜋 � 𝑤0 + ∑ 𝑤𝑗𝑁𝑗=1 cos�𝑡𝑗𝑦�ℜ�cf𝑌�𝑡𝑗�� + ∑ 𝑤𝑗𝑁𝑗=1 sin�𝑡𝑗𝑦�ℑ�cf𝑌�𝑡𝑗�� � (8) cdf𝑌(𝑦) ≈ 1 2 − 𝛿𝑖 𝜋 �𝑤𝑗 𝑁 𝑗=0 ℑ� 𝑒−𝑖𝑖𝑗𝑖cf𝑌�𝑡𝑗� 𝑡𝑗 � ≈ 1 2 − 𝛿𝑖 𝜋 ⎝ ⎜ ⎛ 𝑤0(mean(𝑌) − 𝑦) + + ∑ 𝑤𝑗𝑁𝑗=1 cos�𝑡𝑗𝑦�ℑ� cf𝑌�𝑖𝑗� 𝑖𝑗 � + ∑ 𝑤𝑗𝑁𝑗=1 sin�𝑡𝑗𝑦�ℜ� cf𝑌�𝑖𝑗� 𝑖𝑗 � ⎠ ⎟ ⎞ , (9) where 𝑁 is a sufficiently large number of (equidistant) subintervals of (0, 𝑇), 𝑤𝑗 are the appropriate quadrature weights, and 𝑡𝑗 denote the appropriate (equidistant) nodes from the interval (0, 𝑇), for sufficiently large 𝑇. in particular, for the trapezoidal quadrature rule we set • 𝛿𝑖 = 𝑇 𝑁 or alternatively 𝛿𝑖 = 2𝜋 𝐵−𝐴 , which gives 𝑇 = 𝑁 2𝜋 𝐵−𝐴 , • 𝑤0 = 𝑤𝑁 = 1 2 , and 𝑤𝑗 = 1 for 𝑗 = 1, … , 𝑁 − 1, • 𝑡𝑗 = 𝑗𝛿𝑖 for 𝑗 = 0, … , 𝑁, with 𝑇 = 𝑡𝑁 = 𝑁𝛿𝑖. here, the interval (𝐴, 𝐵) specifies the range of typical values 𝑦, i.e. a large part of the distribution support of the random variable 𝑌. figure 2. integrand functions for computing the pdf/cdf of the chi-squared distributed random variable, 𝑌 ∼ 𝜒12, at 𝑦 = 15, computed by the gil-pelaez inversion formulae of its characteristic function cfy(𝑡) = (1 − 2𝑖𝑡) −1 2. the plotted integrand functions of the integrals in (6) and (7) are evaluated for 𝑡 ∈ (0, 𝑇), 𝑇 = 5. the red circles depict the zeros (roots) of the integrand functions on (0, 𝑇). the total integral can be computed as an infinite sum of sub-integrals the limit value of the sum of alternating series. http://www.mathworks.com/matlabcentral/fileexchange/4199-tdist http://www.mathworks.com/matlabcentral/fileexchange/4199-tdist acta imeko | www.imeko.org november 2016 | volume 5 | number 3 | 36 as a simple rule of thumb, if the (optimum) value of 𝑇 is unknown we suggest to start with the application of the sixsigma-rule, i.e. set the typical range (𝐴, 𝐵) as an intersection of the natural parametric space of 𝑌 with the interval (𝐿, 𝑈) (e.g., (𝐴, 𝐵) = (𝐿, 𝑈) ∩ 𝑹 or (𝐴, 𝐵) = (𝐿, 𝑈) ∩ 𝑹+, with • 𝐿 = mean(𝑌) − 6 std(𝑌), • 𝑈 = mean(𝑌) + 6 std(𝑌), where mean(𝑌) and std(𝑌) represent the expectation and the standard deviation of the probability distribution of 𝑌, and define 𝑇 = 𝑁 2𝜋 𝐵−𝐴 for some pre-selected fixed 𝑁. note that the value of 𝑇 should be kept sufficiently large, such that the characteristic function is sufficiently small for all 𝑡 > 𝑇, i.e. 𝑎𝑎𝑎(cf𝑌(𝑡)) < 𝜖 for small 𝜖, say 𝜖 = 10−12. this can be effectively controlled by using a proper value (sufficiently large) of 𝑁. further, for computing the leading term in (9), we use the result from [36]: if the mean (expectation) of 𝑌 exists, then lim𝑖→0 ℑ� 𝑒−𝑖𝑖𝑖cf𝑌(𝑖) 𝑖 � = mean(𝑌) − 𝑦 . (10) the required mean(𝑌) and std(𝑌) can be evaluated analytically, from the moments of the input variables, or approximately, by using numerical differentiation of the characteristic function of 𝑌, cf𝑌(𝑡). in particular, mean(𝑌) ≈ 1 12𝑖ℎ � cf𝑌(−2ℎ) − 8cf𝑌(−ℎ) +8cf𝑌(−ℎ) − cf𝑌(2ℎ) � , (11) std(𝑌) ≈ �m2(𝑌) − mean2(𝑌)� , (12) where m2(𝑌) ≈ 1 144ℎ2 ⎝ ⎜ ⎛ cf𝑌(−4ℎ) − 16cf𝑌(−3ℎ) +64cf𝑌(−2ℎ) + 16cf𝑌(−ℎ) −130 +16cf𝑌(ℎ) + 64cf𝑌(2ℎ) −16cf𝑌(3ℎ) + cf𝑌(4ℎ) ⎠ ⎟ ⎞ , (13) for any small ℎ > 0, e.g., ℎ = 10−4. this numerical approach is applicable even in the cases when the theoretical moments of the considered distribution, defined by cf𝑌(𝑡), do not exist (e.g., for the student’s t distribution with 1 or 2 degrees of freedom). the truncation limit 𝑇 = 𝑁𝛿𝑖 = 𝑁 2𝜋 𝐵−𝐴 depends on 𝑁 and (𝐴, 𝐵). the trade-off between the values of 𝐵 − 𝐴, 𝑁, 𝛿𝑖 and 𝑇, strongly depends on the particular distribution of 𝑌 and its cf. for example, for a highly precise numerical inversion of the standard normal cf, cfy(𝑡) = 𝑒 −1 2 𝑖2, computed in double precision arithmetic with precision better than 𝜖 = 10−14, it is sufficient to set (𝐴, 𝐵) = (−8,8) with 𝐵 − 𝐴 = 16, 𝑁 = 25 = 32, leading to 𝛿𝑖 = 𝜋 8 and 𝑇 = 4 𝜋 with cfy(𝑇) = 𝑒 −1 2 𝑇2 ≈ 5 × 10−35. on the other hand, the numerical inversion of the rectangular cf given by cfy(𝑡) = sin (𝑖) 𝑖 requires in double precision arithmetic the value 𝑇 = 1014 to get 𝑎𝑎𝑎�cf(𝑡)� ≤ 10−14 for 𝑡 > 𝑇. for the natural choice of (𝐴, 𝐵) = (−1,1) with 𝐵 − 𝐴 = 2, this suggests to set 𝑁 ≈ 1013, what is an unacceptable value, and thus it reveals that the simple trapezoidal rule is not a suitable integration method for the highly precise numerical inversion of the rectangular distribution cf. fortunately, cf of the output quantity 𝑌 in typical metrological situations based on linear measurement models is a well behaved function, and the methods based on simple trapezoidal quadrature are efficient to provide good numerical approximations of the true values. in general, the pre-selected values of 𝑇 and 𝑁 should be checked and/or properly corrected. a simple diagnostic check is based on evaluating abs(cf𝑌(𝑇)). for large 𝑇 this should be a small value (smaller that the accepted truncation error). this check allows to control also the level of 𝑁, given the already fixed and sufficiently large interval (𝐴, 𝐵). we note that the presented quadrature method requires only one evaluation of the characteristic function cf𝑌�𝑡𝑗� at 𝑡𝑗, 𝑗 = 1, … , 𝑁, for any 𝑦 ∈ (𝐴, 𝐵) in pdf𝑌(𝑦) and cdf𝑌(𝑦), respectively. moreover, the computation is further simplified if 𝑌 is a continuous random variable with a symmetric zero-mean distribution, i.e. with purely real cf, pdf𝑌(𝑦) ≈ 𝛿𝑖 𝜋 � 1 2 + ∑ cos�𝑡𝑗𝑦�cf𝑌�𝑡𝑗�𝑁−1𝑗=1 + 1 2 cos(𝑡𝑁𝑦)cf𝑌(𝑡𝑁) � , (14) cdf𝑌(𝑦) ≈ 1 2 − 𝛿𝑖 𝜋 � − 𝑖 2 + ∑ sin�𝑡𝑗𝑦� cf𝑌�𝑖𝑗� 𝑖𝑗 𝑁−1𝑗=1 + 1 2 sin(𝑡𝑁𝑦) cf𝑌(𝑖𝑁) 𝑖𝑁 � . (15) finally, the quantile function (qf) can be evaluated by using the iterative newton-raphson scheme, based on repeated evaluations of the pdf/cdf, see (8)-(9) and/or (14)-(15). in particular, for a fixed probability level 𝐸 ∈ (0,1), the 𝐸quantile of the (continuous) distribution of 𝑌, say 𝑞 = qf𝑌(𝐸), is given as a solution (fixed point) of the following iterative scheme, qf𝑌𝑘+1(𝐸) = qf𝑌𝑘(𝐸) − �cdf𝑌�qf𝑌 𝑘(𝑝)�−𝑝� pdf𝑌�qf𝑌 𝑘(𝑝)� , (16) where 𝑘 = 0,1, …, and the starting value qf𝑌0(𝐸) is set as, e.g., qf𝑌0(𝐸) = mean(𝑌), defined by (11). %% example (matlab algorithm cf2distgp) % % pdf and cdf of a linear combination of rvs: % y = c1*x1 + c2*x2 + c3*x3 + c4*x4 + c5*x5, % where, % x1 ~ normal(0,1) with c1=1, % x2 ~ student's t with 1 df and c2=1, % x3 ~ rectangular on (-1,1) with c3=5, % x4 ~ triangular on (-1,1) with c4=1, % x5 ~ u-distribution on (-1,1) with c5=10 cfn = @(t) exp(-t.^2/2); cft = @(t,nu) min(1,besselk(nu/2, ... abs(t).*sqrt(nu),1) .* ... exp(-abs(t).*sqrt(nu)) .* ... (sqrt(nu).*abs(t)).^(nu/2) / ... 2^(nu/2-1)/gamma(nu/2)); cfr = @(t) min(1,sin(t)./t); cft = @(t) min(1,(2-2*cos(t))./t.^2); cfu = @(t) besselj(0,t); c = [1 1 5 1 10]; nu = 1; cfy = @(t) ... cfn(c(1)*t) .* ... cft(c(2)*t,nu) .* ... cfr(c(3)*t) .* ... cft(c(4)*t) .* ... cfu(c(5)*t); y = linspace(-50,50,201)'; [result,cdf,pdf] = cf2distgp(cfy,y); acta imeko | www.imeko.org november 2016 | volume 5 | number 3 | 37 a working version of the matlab algorithm cf2distgp for computing the pdf/cdf by numerical inversion of the characteristic function, based on the gil-pelaez inversion formulae, is presented in appendix a. for illustration, the matlab code presented above evaluates the pdf and cdf of the output variable 𝑌, which is a linear combination of the independent random variables with a normal, student's t, rectangular, triangular and arcsine distributions, i.e. 𝑌 = 𝑋𝑁 + 𝑋𝑖𝜈 + 5𝑋𝑅 + 𝑋𝑇 + 10𝑋𝑈, by using the algorithm cf2distgp with its default settings of 𝑇 and 𝑁. similarly, the pdf/cdf of the random variable 𝑌 = 𝑋𝑁 + 𝑋𝑖𝜈 + 5𝑋𝑅 + 𝑋𝑇 + 10𝑋𝑈 can be evaluated by using the matlab algorithm tdist, see also figure 3: 3.2. numerical inversion of the characteristic function by using the fft algorithm this approach for computing the pdf by numerical inversion of the characteristic function by using the fft algorithm is based on the results by hürlimann in [41]. alternatively, for other applications based on using the fractional fast fourier transform (frft), see [42] and also [43][46]. here we shall approximate the continuous fourier transform (cft), say 𝐹(𝑦) = ∫ 𝑒−𝑖2𝜋𝜋𝑖 ∞ −∞ 𝑓(𝑢) 𝑑𝑢, (17) by a discrete fourier transform (dft). the dft can be efficiently evaluated by using the fft algorithm that computes the same result as the dft, but much faster. for complex numbers 𝑓0, … , 𝑓𝑁−1 the dft is defined as 𝐹𝑘 = ∑ 𝑒 −𝑖2𝜋𝑘𝑗𝑁 𝑓𝑗𝑁−1𝑗=0 , 𝑘 = 0, … , 𝑁 − 1. (18) formally, here we shall use the following notation, 𝐅𝑁 = 𝐹𝐹𝑇(𝐟𝑁), (19) where 𝐟𝑁 = (𝑓0, … , 𝑓𝑁−1) and 𝐅𝑁 = (𝐹0, … , 𝐹𝑁−1). the relationship between the cf and the pdf is given by the (inverse) continuous fourier transform defined by (4). for a sufficiently large interval (−𝑇, 𝑇), it is possible to approximate a pdf by (5). here we shall consider the integral approximation, based on the mid-point integration rule, ∫ 𝑓(𝑥)𝑑𝑥 ≈ 𝑓(𝑎)+𝑓(𝑏) 2 (𝑎 − 𝑎)𝑏𝑎 , which corresponds to the trapezoidal quadrature. for more alternative approaches based on more sophisticated integration rules see, e.g., [39]. similarly as before, let (𝐴, 𝐵) denote a sufficiently large interval, where the distribution of 𝑌 is concentrated. a reasonable rule for determining (𝐴, 𝐵) can be based, for example, on the six-sigma-rule, or its modifications, by using an altered multiplication coefficient (e.g., use 10 instead of 6). let 𝑦𝑘 = 𝐴 + 𝑘𝛿𝑖, with 𝛿𝑖 = 𝐵−𝐴 𝑁 , and 𝑘 = 0, … , 𝑁 − 1. for 𝑁 large, 𝑇 = 𝜋/𝛿𝑖 is also large, and from (5), by using 𝑡 = 2𝜋𝑢, 𝑑𝑡 = 2𝜋𝑑𝑢, and 𝑑𝑢 = 1 𝐵−𝐴 , we get pdf𝑌(𝑦𝑘) ≈ 1 2𝜋 ∫ 𝑒−𝑖2𝜋𝜋𝑖𝑘 1 2𝛿𝑖 − 12𝛿𝑖 cf𝑌(2𝜋𝑢) 𝑑𝑢 . (20) now, we shall approximate the integral (19) by using the approximate trapezoidal quadrature rule, for each of the 𝑁 subintervals. thus, pdf𝑌(𝑦𝑘) ≈ 1 𝐵−𝐴 ∑ 𝑒−𝑖2𝜋𝜋𝑗𝑖𝑘𝑁−1𝑗=0 cf𝑌�2𝜋𝑢𝑗� , (21) figure 3. the probability density function (pdf) and the cumulative distribution function (cdf) of a random variable 𝑌 = ∑ 𝑐𝑗𝑋𝑗 5 𝑗=1 , with 𝑋1 ∼ 𝑁(0,1), 𝑋2 ∼ 𝑡𝜈=1, 𝑋3 ∼ 𝑅(−1,1), 𝑋4 ∼ 𝑇(−1,1), 𝑋5 ∼ 𝑈(−1,1), and coefficients 𝑐 = (𝑐1, … , 𝑐5) = (1,1,5,1,10), evaluated by numerical inversion of its characteristic function by the matlab algorithm tdist, see also the examples. %% example (matlab algorithm tdist) % % tdist at matlab central file exchange: % http://www.mathworks.com/matlabcentral/ % /fileexchange/4199-tdist % % pdf and cdf of a linear combination of rvs % y = c1*x1 + c2*x2 + c3*x3 + c4*x4 + c5*x5 % with: % x1 ~ normal(0,1) [we set df1=inf] with c1=1, % x2 ~ student's t with 1 df [set df2=1], c2=1, % x3 ~ rectangular on (-1,1) [set df3=-1],c3=5, % x4 ~ triangular on (-1,1) [set df4=-2], c4=1, % x5 ~ u-distribution on (-1,1) [df5=-3], c5=10 df = [inf 1 -1 -2 -3]; coefs = [1 1 5 1 10]; [pdf,y] = tdist([],df,coefs,'pdf'); cdf = tdist(y,df,coefs,'cdf'); figure; plot(y,pdf); grid figure; plot(y,cdf); grid http://www.mathworks.com/matlabcentral/fileexchange/4199-tdist http://www.mathworks.com/matlabcentral/fileexchange/4199-tdist acta imeko | www.imeko.org november 2016 | volume 5 | number 3 | 38 where 𝑢𝑗 = 1 2+𝑗− 𝑁 2 𝐵−𝐴 , 𝑗 = 0, … , 𝑁 − 1. from that, by using 𝑒𝑖𝜋 = −1, the expressions for 𝑢𝑗 and 𝑦𝑘, and the dft defined by (17), we finally get the formal relationship 𝐩𝐩𝐟 = 𝐂 ⊙ 𝐹𝐹𝑇(𝐃 ⊙ 𝐜𝐟), (22) where ⊙ denotes the dot product (element wise multiplication), • 𝐩𝐩𝐟 = �pdf𝑌(𝑦0), … , pdf𝑌(𝑦𝑁−1)�, • 𝐂 = (𝐶0, … , 𝐶𝑁−1), with • 𝐶𝑘 = 1 𝐵−𝐴 (−1)�� 1−1 𝑁�� 𝑁𝑁 𝐵−𝑁+𝑘�� , 𝑘 = 0, … , 𝑁 − 1, • 𝐃 = (𝐷0, … , 𝐷𝑁−1), with • 𝐷𝑘 = (−1) − 2𝑁𝐵−𝑁𝑘, 𝑘 = 0, … , 𝑁 − 1, • 𝐜𝐟 = �cf𝑌(𝑡0), … , cf𝑌(𝑡𝑁−1)�, with • 𝑡𝑘 = 2𝜋 𝐵−𝐴 �1 2 + 𝑘 − 𝑁 2 �, 𝑘 = 0, … , 𝑁 − 1. further, cdf is evaluated here by simple cumulative sum from the evaluated pdf values, and qf is evaluated by interpolation from the cdf. a working version of the matlab algorithm cf2distfft for computing the pdf/cdf/qf by numerical inversion of the characteristic function, based on the fft algorithm, is presented in appendix b. for illustration, the matlab code presented below evaluates the pdf/cdf of the output variable 𝑌 = 10 𝑋𝜒12 + 𝑋𝜒102 , by using the algorithm cf2distfft with its default settings of (𝐴, 𝐵) and 𝑁. other specific versions of the algorithm for computing the pdf/cdf/qf of a linear combination of independent random variables with the fisher-snedecor’s f-distributions and the log-normal distributions, by numerical inversion of the characteristic function by using the fft algorithm, are available at the matlab central file exchange as the algorithm fdist, file id: 56262, and the algorithm logndist, file id: 56512, respectively. 4. comparison of the cf approach and the monte carlo method the main advantage of the monte carlo methods lies in their simplicity and asymptotic consistency. a disadvantage of the monte carlo methods lies in the large computational demands, typically required in order to achieve the pre-specified accuracy level. on the other hand, application of the cf approach offers principal theoretical exactness which could be, however, influenced by unacceptable numerical errors (i.e. the truncation and the integration errors), if not properly used. for illustration, here we consider the linear measurement model for calibration of a coaxial step attenuator as considered in [47], a model typical for metrological applications. we consider this example in order to compare results based on the cf approach and the monte carlo method and to illustrate the applicability and/or advantage of the suggested methods for potential users in metrology applications. for more details about this specific example see [47], example s7, and/or [48]. the linear measurement model of the attenuation 𝐿𝑋 of the attenuator to be calibrated is given by 𝐿𝑋 = 𝑐𝑐𝑛𝑎𝑡 + 𝐿𝑆 + 𝛿𝐿𝑆 + 𝛿𝐿𝐷 + 𝛿𝐿𝑀 + 𝛿𝐿𝐾 + 𝛿𝐿𝑖𝑏 − 𝛿𝐿𝑖𝑎 + 𝛿𝐿0𝑏 − 𝛿𝐿0𝑎, (23) with the following information about the distributions of the input quantities (available from the given uncertainty budget, see s7.12 in [47] with the correction as presented in [48]): • 𝑐𝑐𝑛𝑎𝑡 = 30.04 + 0.003 = 30.043, • 𝐿𝑆 ∼ 0.0090 × 𝑁(0,1), • 𝛿𝐿𝑆 ∼ (0.0025/� 1 3 ) × 𝑅(−1,1), • 𝛿𝐿𝐷 ∼ (0.0011/� 1 2 ) × 𝑈(−1,1), • 𝛿𝐿𝑀 ∼ (0.0200/� 1 2 ) × 𝑈(−1,1), • 𝛿𝐿𝐾 ∼ (0.0017/� 1 2 ) × 𝑈(−1,1), • 𝛿𝐿𝑖𝑏 ∼ (0.0003/� 1 3 ) × 𝑅(−1,1) • 𝛿𝐿𝑖𝑎 ∼ (0.0003/� 1 3 ) × 𝑅(−1,1), • 𝛿𝐿0𝑏 ∼ 0.0020 × 𝑁(0,1), • 𝛿𝐿0𝑎 ∼ 0.0020 × 𝑁(0,1). the algorithms cf2distgp, with default option parameters based on the six-sigma-rule and pre-selected 𝑁 = 210 = 1024, evaluated the support interval (𝐴, 𝐵) = (−0.1341,0.1341), and hence 𝑇 = 23989, with 𝛿𝑖 = 𝑇 𝑁 = 23.4. then, the 97.5 %-quantile of 𝑌 = 𝐿𝑋 − 𝑐𝑐𝑛𝑎𝑡 was calculated as 𝑞 = 0.03900448275179. thus, the calculated 95 % state-of-knowledge interval for the attenuation 𝐿𝑋 is given as 30.043 ∓ 0.039. the basic diagnostic check ensures the highest precision of the presented calculations in double precision arithmetic, as abs(cf𝑌(𝑇))=0 (i.e. the value equals to the machine zero, which is here defined as 4.94 × 10−324). the used computer time was 𝑡 = 3.9 × 10−4 s. further, here we also present the estimated values of the required 97.5 % quantile computed by the monte carlo method for sample sizes 𝑀, with 𝑀 = 104, 105, 106, 107, and 108. %% example (matlab algorithm cf2distfft) % % distribution of a linear combination of rvs % (chi-squared rvs with 1 and 10 dfs) % y = 10*x_{\chi^2_1} + x_{\chi^2_10} df1 = 1; df2 = 10; cfchi2_1 = @(t) (1-2i*t).^(-df1/2); cfchi2_10 = @(t) (1-2i*t).^(-df2/2); cfy = @(t) cfchi2_1(10*t) .* cfchi2_10(t); clear options options.ispositivesupport = true; result = cf2distfft(cfy,[],[],options); % plot the cf of y t = linspace(-1,1,501); figure plot(t,real(cfy(t)),t,imag(cfy(t)));grid xlabel('t'); ylabel('characteristic function'); title('y = 10*x_{\chi^2_1}+x_{\chi^2_{10}}') http://www.mathworks.com/matlabcentral/fileexchange/56262-fdist http://www.mathworks.com/matlabcentral/fileexchange/56512-logndist acta imeko | www.imeko.org november 2016 | volume 5 | number 3 | 39 the estimated values of the 97.5 % quantile 𝑞, and the computer time 𝑡 (in seconds), used for the sample size 𝑀: • 𝑀 = 104, 𝑞 = 0.039343812021804, 𝑡 = 0.07, • 𝑀 = 105, 𝑞 = 0.038985244853524, 𝑡 = 0.08, • 𝑀 = 106, 𝑞 = 0.038952503603421, 𝑡 = 0.75, • 𝑀 = 107, 𝑞 = 0.038998144396413, 𝑡 = 7.49, • 𝑀 = 108, 𝑞 = 0.039003626106614, 𝑡 = 153.2. this clearly illustrates the advantages and computational efficiency of the proposed approach over the standard monte carlo methods, especially if high precision of the computed (estimated) quantiles is required. 5. conclusions we suggest to consider numerical methods for derivation of the state-of-knowledge pdf/cdf of the output quantity in linear measurement models from its characteristic function. such approach can be used to form the probability distribution for the output quantity of a measurement model of additive, linear or generalized linear form, and can be considered as an alternative tool to the uncertainty evaluation based on the monte carlo methods. here we have presented two simple but efficient approaches for numerical inversion of the characteristic function, which are especially suitable for metrological applications. the suggested numerical approaches are based on the gilpelaez inverse formula and on the approximation by discrete fourier transform (dft) and the fft algorithm for computing the pdf/cdf of (univariate) continuous random variables. as we have explained and illustrated, the suggested cf approach should be considered as an alternative to the standard approach based on the monte carlo methods (as considered e.g. in supplement 1 – propagation of distributions using a monte carlo method [2]) in specific situations, i.e. for evaluating the state-ofknowledge probability distributions of the output quantity in a linear measurement model, especially if the highest precision of the reported distribution quantiles is required. however, numerical errors of such results should be properly controlled. for illustration purposes, here we present working versions of the matlab codes of the suggested algorithms (cf2distgp and cf2distfft), as well as some simple examples in order to illustrate the applicability of the suggested methods. although the methods for inverting characteristic functions for obtaining the probability distribution functions have been studied for long time, especially in statistical literature, and the possible applications are much more general than those motivated by metrology, surprisingly, such methods are still not widespread and used in engineering applications and only rarely used among statisticians. one possible reason might be that the characteristic functions and the algorithms for numerical inversions are not directly available in standard (statistical) software packages, like e.g., r or matlab. systematic development of the methods, algorithms and the software toolbox (developed for r and/or matlab) for computing, combining and inverting the characteristic functions is our highly desirable goal for the next future. acknowledgement the work was supported by the slovak research and development agency, project apvv-15-0295, and by the scientific grant agency vega of the ministry of education of the slovak republic and the slovak academy of sciences, by the projects vega 2/0047/15 and vega 2/0011/16. %% comparison / cf approach & monte carlo method % cf approach: % distribution and quantile of the output % quantity in linear measurement model % y = l_s + δl_s + δl_d + δl_m + δl_k + % δl_ib δl_ia + δl_0b δl_0a % const = 30.043; prob = 0.975; c = [0.009 0.0025/sqrt(1/3) ... 0.0011/sqrt(1/2) 0.0200/sqrt(1/2) ... 0.0017/sqrt(1/2) 0.0003/sqrt(1/3) ... -0.0003/sqrt(1/3) 0.0020 -0.0020 ]; cfn = @(t) exp(-t.^2/2); cfr = @(t) min(1,sin(t)./t); cfu = @(t) besselj(0,t); cfy = @(t) cfn(c(1)*t) .* cfr(c(2)*t) .* ... cfu(c(3)*t) .* cfu(c(4)*t) .* ... cfu(c(5)*t) .* cfr(c(6)*t) .* ... cfr(c(7)*t) .* cfn(c(8)*t) .* ... cfn(c(9)*t); [result,cdf,pdf,q_cf] = cf2distgp(cfy,[],prob) % 95% state-of-knowledge interval / cf approach: i_cf = const + [-q_cf,q_cf] % monte carlo method: m = [10^4 10^5 10^6 10^7 10^8]; q_mc = zeros(5,1); time = zeros(5,1); rng; for m = 1:5 tic; n = randn(m(m),3); r = 2*rand(m(m),3)-1; u = 2*betarnd(0.5,0.5,m(m),3)-1; y = c(1)*n(:,1) + c(2)*r(:,1) ... + c(3)*u(:,1) + ... + c(4)*u(:,2) + c(5)*u(:,3) ... + c(6)*r(:,2) + c(7)*r(:,3) ... + c(8)*n(:,2) + c(9)*n(:,3); y = sort(y); q_mc(m) = y(ceil(m(m)*prob)); time(m) = toc; end % 95% state-of-knowledge interval / mc method: q = q_mc(5); i_mc = const + [-q,q] acta imeko | www.imeko.org november 2016 | volume 5 | number 3 | 40 appendix a. matlab algorithm cf2distgp for numerical inversion of the characteristic function based on the gil-pelaez inversion formulae function [result,cdf,pdf,qf] = ... cf2distgp(cf,x,prob,options) % calculates cdf/pdf/qf from the characteristic % function by the gil-pelaez inversion formulae, % integrated by a simple trapezoidal quadrature. % % syntax: % [result,cdf,pdf,qf] = ... % cf2distgp(cf,x,prob,options) %% check the input parameters tic; narginchk(1, 4); if nargin < 4, options = []; end if nargin < 3, prob = []; end if nargin < 2, x = []; end if ~isfield(options, 'n') options.n = 2^10; end if ~isfield(options, 't') options.t = []; end if ~isfield(options, 'sixsigmarule') options.sixsigmarule = 6; end if ~isfield(options, 'xmean') options.xmean = []; end if ~isfield(options, 'xstd') options.xstd = []; end if ~isfield(options, 'toldiff') options.toldiff = 1e-4; end if ~isfield(options, 'qf0') options.qf0 = (cf(1e-4)-cf(-1e-4))/(2e-4*1i); end if ~isfield(options, 'crit') options.crit = 1e-13; end if ~isfield(options, 'maxiter') options.maxiter = 1000; end if ~isfield(options, 'isplot') options.isplot = true; end if ~isfield(options, 'dist') options.dist = []; end if ~isempty(options.dist) xmean = options.dist.xmean; cft = options.dist.cft; dt = options.dist.dt; n = length(cft); t = (1:n)' * dt; range = 2*pi / dt; xmin = xmean range/2; xmax = xmean + range/2; xstd = []; else n = options.n; t = options.t; sixsigmarule = options.sixsigmarule; xmean = options.xmean; xstd = options.xstd; h = options.toldiff; cft = cf(h*(1:4)); if isempty(xmean) xmean = real((-cft(2) ... + 8*cft(1)-8*conj(cft(1)) ... + conj(cft(2)))/(1i*12*h)); end if isempty(xstd) xm2 = real(-(conj(cft(4)) ... 16*conj(cft(3)) ... + 64*conj(cft(2)) ... + 16*conj(cft(1)) ... 130 + 16*cft(1) ... + 64*cft(2) ... 16*cft(3)+cft(4))/(144*h^2)); xstd = sqrt(xm2 xmean^2); end if ~isempty(t) dt = t / n; t = (1:n)' * dt; cft = cf(t); cft(n) = cft(n)/2; range = 2*pi / dt; xmin = xmean range/2; xmax = xmean + range/2; xstd = []; else xmin = xmean sixsigmarule * xstd; xmax = xmean + sixsigmarule * xstd; range = xmax xmin; dt = 2*pi / range; t = (1:n)' * dt; cft = cf(t); cft(n) = cft(n)/2; end options.dist.xmean = xmean; options.dist.cft = cft; options.dist.dt = dt; end %% algorithm if isempty(x) x = linspace(xmin,xmax,101)'; end if any(x < xmin) || any(x > xmax) warning(['x out-of-range (the support): ', ... '[xmin, xmax] = [',num2str(xmin),... ', ',num2str(xmax),'] !']); end [n,m] = size(x); x = x(:); e = exp(-1i*x*t'); % cdf cdf = (xmean x)/2 + imag(e * (cft ./ t)); cdf = 0.5 (cdf * dt) / pi; cdf = reshape(max(0,min(1,cdf)),n,m); % pdf pdf = 0.5 + real(e * cft); pdf = (pdf * dt) / pi; pdf = reshape(max(0,pdf),n,m); acta imeko | www.imeko.org november 2016 | volume 5 | number 3 | 41 appendix b. matlab algorithm cf2distfft for numerical inversion of the characteristic function based on the fft algorithm % qf if ~isempty(prob) isplot = options.isplot; options.isplot = false; [n,m] = size(prob); prob = prob(:); maxiter = options.maxiter; crit = options.crit; qf = options.qf0; criterion = true; count = 0; [res,cdfq,pdfq] = ... cf2distgp([],qf,[],options); options = res.options; while criterion count = count + 1; correction = (cdfq prob) ./ pdfq; qf = qf correction; [~,cdfq,pdfq] = ... cf2distgp([],qf,[],options); criterion = any(abs(correction) ... > crit * abs(qf)) ... && max(abs(correction)) ... > crit && count < maxiter; end qf = reshape(qf,n,m); prob = reshape(prob,n,m); options.isplot = isplot; else qf = []; count = []; correction =[]; end %% result result.cdf = cdf; result.pdf = pdf; result.qf = qf; result.x = x; result.xmean = xmean; result.xstd = xstd; result.xmin = xmin; result.xmax = xmax; result.prob = prob; result.sixsigmarule = options.sixsigmarule; result.t = t; result.t = t(end); result.dt = dt; result.cf = cf; result.n = n; result.count = count; result.correction = correction; result.options = options; result.tictoc = toc; %% plot the pdf / cdf if length(x)==1, options.isplot = false; end if options.isplot figure plot(x,pdf,'.-') grid title('pdf specified by the cf') xlabel('x') ylabel('pdf') % figure plot(x,cdf,'.-') grid title('cdf specified by the cf') xlabel('x') ylabel('cdf') end end function [result,cdf,pdf,qf] = ... cf2distfft(cffun,y,prob,options) %cf2distfft calculates the approximate values % of cdf, pdf, and qf by numerical inversion of % the characteristic function cf by using the % fft algorithm. % % syntax: % [result,cdf,pdf,qf] = ... % cf2distfft2(cffun,y,prob,options) % viktor witkovsky (witkovsky@savba.sk) % ver.: 24-apr-2016 17:12:15 %% check the input parameters if nargin < 1 error('too few inputs'); end if nargin < 4, options = []; end if nargin < 3, prob = []; end if nargin < 2, y = []; end if ~isfield(options,'n') options.n = 2^10; end n = options.n; if ~isfield(options,'sixsigmarule') options.sixsigmarule = 6; end if ~isfield(options,'miny') options.miny = []; end if ~isfield(options,'maxy') options.maxy = []; end if ~isfield(options,'isforcedsymmetric') options.isforcedsymmetric = []; end if isempty(options.isforcedsymmetric) isforcedsymmetric = false; else isforcedsymmetric = ... options.isforcedsymmetric; end if ~isfield(options,'iszerosymmetric') options.iszerosymmetric = []; end if isempty(options.iszerosymmetric) if isforcedsymmetric iszerosymmetric = true; else iszerosymmetric = false; end else iszerosymmetric = options.iszerosymmetric; end if ~isfield(options,'ispositivesupport') options.ispositivesupport = []; end acta imeko | www.imeko.org november 2016 | volume 5 | number 3 | 42 if isempty(options.ispositivesupport) ispositivesupport = false; else ispositivesupport = ... options.ispositivesupport; end if ~isfield(options,'isplot') options.isplot = true; end if ~isfield(options,'delta') options.toldiff = 1e-4; end %% moments and support h = options.toldiff; t = h*(1:4); cf = cffun(t); meany = real((-cf(2) + 8*cf(1) – ... 8*conj(cf(1))+ conj(cf(2)))/(1i*12*h)); m2 = real(-(conj(cf(4)) 16*conj(cf(3)) + ... 64*conj(cf(2)) + 16*conj(cf(1)) 130 + ... 16*cf(1) + 64*cf(2) 16*cf(3) + ... cf(4))/(144*h^2)); stdy = sqrt(m2 meany^2); a = meany options.sixsigmarule * stdy; b = meany + options.sixsigmarule * stdy; if ispositivesupport if a <= 0 && ... isempty(options.isforcedsymmetric) a = max(0,a); isforcedsymmetric = true; elseif a > 0 && ... isempty(options.isforcedsymmetric) isforcedsymmetric = false; end end % use the specified values (if available) if ~isempty(options.miny), a = options.miny; end if ~isempty(options.maxy), b = options.maxy; end % symmetric support [-b,b] ? if isforcedsymmetric || iszerosymmetric b = options.sixsigmarule * ... sqrt(stdy^2 + meany^2); if ~isempty(options.maxy) b = options.maxy; end a = -b; end %% characteristic function cf k = (0:(n-1))'; t = 2*pi * (0.5-n/2+k) / (b-a); cf = cffun(t(n/2+1:end)); cf = [conj(cf(end:-1:1));cf]; % cf of the 'symetrized' distribution if isforcedsymmetric cf = real(cf); end %% pdf by the fft algorithm dy = (b-a)/n; c = (-1).^((1-1/n)*(a/dy+k))/(b-a); d = (-1).^(-2*(a/(b-a))*k); pdffft = real(c.*fft(d.*cf)); cdffft = cumsum(pdffft*dy); yfft = a + k * dy; if options.iszerosymmetric cdffft = cdffft + 0.5 ... (cdffft(n/2+1)+cdffft(n/2))/2; end % special treatment for symmetrized distribution if isforcedsymmetric pdffft = max(0,2*pdffft(n/2+1:end)); cdffft = cdffft + 0.5 ... (cdffft(n/2+1)+cdffft(n/2))/2; cdffft = min(1, ... max(0,2*cdffft(n/2+1:end)-1)); yfft = yfft(n/2+1:end); else pdffft = max(0,pdffft); cdffft = min(1,max(0,cdffft)); end ymin = min(yfft); ymax = max(yfft); %% interpolate quantile function : qf(prob) if isempty(prob) prob = [0.9,0.95,0.975,0.99,0.995,0.999]; end [cdfu,id] = unique(cdffft); yyu = yfft(id); szp = size(prob); qffun = @(prob) interp1([-eps;cdfu],... [-eps;yyu+dy/2],prob); qf = reshape(qffun(prob),szp); % interpolate cdf/qf/pdf if isempty(y) y = linspace(a,a+(n-1)*dy,100); end szy = size(y); cdffun = @(x) interp1([-eps;yyu+dy/2],... [-eps;cdfu],x(:)); cdf = reshape(cdffun(y),szy); try pdffun = @(x) interp1(yfft,pdffft,y(:)); pdf = reshape(pdffun(y),szy); catch warning('unable to interpolate') pdf = nan*y; pdffun = []; end %% result result.y = y; result.cdf = cdf; result.pdf = pdf; result.prob = prob; result.quant = qf; result.cdffun = cdffun; result.pdffun = pdffun; result.qffun = qffun; result.ymin = ymin; result.ymax = ymax; result.cdfmin = min(cdffft); result.cdfmax = max(cdffft); result.n = n; result.details.yftt = yfft; result.details.pdffft = pdffft; result.details.cdffft = cdffft; result.details.meany = meany; result.details.stdy = stdy; result.details.a = a; result.details.b = b; result.details.dy = dy; result.details.dt = 2*pi/(b-a); result.details.t = t; result.details.cf = cf; result.details.cffun = cffun; result.options = options; acta imeko | www.imeko.org november 2016 | volume 5 | number 3 | 43 references [1] jcgm100:2008, evaluation of measurement data – guide to the expression of uncertainty in measurement (gum 1995 with minor corrections), in jcgm joint committee for guides in metrology, (iso, bipm, iec, ifcc, ilac, iupac, iupap and oiml, 2008). 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[48] a. possolo, statistical models and computation to evaluate measurement uncertainty, metrologia 51, p. s228 (2014). numerical inversion of a characteristic function: an alternative tool to form the probability distribution of output quantity in linear measurement models microsoft word 298-2198-1-le-rev5 acta imeko  issn: 2221‐870x  april 2016, volume 5, number 1, 69‐80    acta imeko | www.imeko.org  april 2016 | volume 5 | number 1 | 69  nmisa, kebs, bksv tri‐lateral vibration comparison results  anderson maina 1 , ian veldman 2 , henning ploug 3   1  kenyan bereau of standards, bundesallee 100, 38116 braunschweig, kenya  2  national measurement institiute of south africa, csir campus, pretoria, south africa  3  brüel & kjær calibration laboratory, skodsborgvej 307, dk‐2850 nærum, denmark      section: research paper   keywords:  vibration comparison; degrees of equivalence; comparison reference value  citation: anderson maina, ian veldman, henning ploug, nmisa, kebs, bksv tri‐lateral vibration comparison results, acta imeko, vol. 5, no. 1, article 13,  april 2016, identifier: imeko‐acta‐05 (2016)‐01‐13  editor: paolo carbone, university of perugia, italy  received december 14, 2015; in final form december 14, 2015; published april 2016  copyright: © 2016 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  corresponding author: ian veldman, e‐mail: csveldman@nmisa.org    1. introduction  this report presents the results of a comparison in the area of vibration (quantity of acceleration). it was agreed in the technical protocol [1] that the comparison reference values (crvs) be obtained from the nmisa primary vibration calibration system [2] and the degrees of equivalence between the values of the participating laboratories and the crvs be determined. the technical protocol [1] of april 2012 specifies in detail, the aim and the task of the comparison, the conditions of measurement, the transfer standards used, measurement instructions, time schedule and other items. a brief overview of the protocol is given in the sections 4 to 6. section 2 lists the participants with the task of the inter laboratory comparison (ilc) described in section 3. section 7 deals with the measurement results, artefact stability analysis and degrees of equivalence. in section 8, possible correlation between the comparison reference value (crv) and the nmisa results is investigated. 2. participants  three laboratories, namely kenya bureau of standards (kebs), national metrology institute of south africa (nmisa), and brüel & kjær calibration laboratory (bksv) participated in the comparison as shown in table 1. 3. task and purpose of the comparison  the aim of the comparison was to compare measurements of the sensitivity of accelerometers as measured using secondary (back to back) means in accordance with iso 16063-21 “vibration calibration by comparison to a reference transducer” [3]. during the circulation period the three laboratories calibrated two accelerometers (see section 5), as the transfer standards. the laboratories were tasked to measure the magnitude of the sensitivity of the accelerometers at different frequencies and table 1. overview of styles and font sizes used in this template. participating laboratory  acronym  country  calibration  period  brüel & kjær  bksv  denmark  6 to 24 aug 2012 kenyan bureau of standards kebs  kenya  8 to 25 jan 2013  national metrology institute  of south africa  nmisa  south africa  8 to 26 apr 2013 abstract  national  metrology  institutes  (nmis)  and  calibration  laboratories  perform  inter  laboratory  comparisons  (ilc)  as  part  of  their  processes to validate their measurement capabilities (mcs).  in the area of vibration (quantity  acceleration) the kenyan bureau of  standards (kebs) piloted an ilc, inclusive of two additional participating laboratories for this purpose. this technical review documents  the calibration results and findings of the ilc.  acta imeko | www.imeko.org  april 2016 | volume 5 | number 1 | 70  acceleration amplitudes as specified in section 4 and clause 3 of [1]. the reference surface was defined as the mounting surface of the accelerometer. for the double ended accelerometer, the bottom mounting surface was defined as the reference surface. this accelerometer was viewed as a single ended accelerometer for the purpose of this comparison only. dependent on the participating laboratory's measurement capability (mc), the sensitivity reported excluded effects from the applicable conditioning amplifier/power supply unit (psu) used. for all instances, the participating laboratory provided the amplifier to be used. if the sensitivity reported by the laboratory included effects by the amplifier, these effects were taken into account in the reported uncertainty of measurement (uom). no amplifier/psu accompanied the circulation of the transfer standards. 4. conditions of measurement  the participating laboratories observed fully the conditions stated in the technical protocol, i.e. frequencies in hz: 10, 12.5, 16, 20, 25, 31.5, 40, 63, 80, 100, 125, 160, 200, 250, 315, 400, 500, 630, 800, 1000, 1250, 1500, 1600, 2000, 2500, 3000, 3150, 3500, 4000, 4500, 5000, 5500, 6000, 6300, 6500, 7000, 7500, 8000, 8500, 9000, 9500, 10000. amplitudes: the range of acceleration amplitudes was between 1 m/s² and 100 m/s². 1 m/s² to 200 m/s² was admissible (considering the displacement and acceleration limitations of the vibration exciters). ambient temperature and accelerometer temperature during the calibration: (23 ± 3) °c (actual values were stated within tolerances of ± 0.5 °c ). relative humidity: max. 75 % rh mounting torque of the accelerometer: (2 ± 0.1) n·m 5. transfer standards  during the preparatory stage, nmisa investigated the characteristics (long-term stability, linearity, etc.) of the reference standard accelerometers (property of nmisa) to be used as transfer standards. the following accelerometers were selected: accelerometer 1: brüel & kjær 8305 s serial number 1033874, with nominal sensitivity of 0.1 pc/(m/s2). accelerometer 2: pcb 301m15 serial number 1032, with a nominal sensitivity of 10 mv/(m/s2) . stability measurements for both accelerometers were made at the beginning, middle and end of the circulation period using nmisa primary calibration system [2]. the results of these stability measurements are given in section 7. 6. circulation type and transportation  each participating laboratory was accorded three weeks for measurement. at the beginning, middle and the end of the circulation the transfer standards were measured at the nmisa laboratory using primary means [2] in order to determine reference values and to monitor the stability of the transducer sets. the transfer standards were transported in a metal case via courier or by a representative of each participating laboratory in the following order. nmisa (stability & crv) → bksv (measurements) → nmisa (stability & crv) → kebs (measurements) → nmisa (stability & crv) → nmisa (measurements). 7. results of the measurements  7.1. monitoring of stability  the two transfer standards were monitored for stability in the beginning, middle and end of the circulation period using primary means [2]. the laboratory used calculated en values as a stability evaluation measure. the method used for the evaluation of the measurement results was to calculate the deviation, en, normalised with respect to the uom as follows: en = s/uc , (1) where s is the standard deviation of the sensitivity values over the comparison period. uc is the expanded uncertainty of measurement of the nmisa. |en| ≤ 1 were considered acceptable. stability results for the two sensors; brüel & kjær 8305 s and pcb 301m15, are given in table 2 and table 3 respectively and illustrated in figure 1 and figure 2. figure 1. results of monitoring of stability during circulation period for brüel & kjær 8305 s sensor.     figure 2. results of monitoring of stability during circulation period for pcb  301m15 sensor.  acta imeko | www.imeko.org  april 2016 | volume 5 | number 1 | 71  the values reported for the brüel & kjær 8305 s sensor showed good stability over the circulation period. the values reported for the pcb 301m15 sensor suggested a change in sensitivity with time of up to 1 % from the beginning to the end of the circulation period. the effect of this change was compensated for by an increase in the uncertainty of the crv for the pcb accelerometer. no correction was applied to the data reported by the participants. 7.2. comparison reference value  the crvs were obtained from the mean of measurements carried out at the beginning, middle and end of the circulation period by nmisa primary vibration calibration system. the chi-square goodness of fit test was used to check the consistency of the crv(xref) against the results of the participating laboratories (xi) as follows: table 2. results of monitoring of stability during circulation period for brüel & kjær 8305 s sensor. frequency  (hz)  11‐may‐12  sensitivity  (pc/(m/s²))  11‐oct‐12 sensitivity  (pc/(m/s²))  11‐mar‐13 sensitivity  (pc/(m/s²))  s (fc/(m/s²))  uc (fc/(m/s²))  en  10.0  0.124 3  0.124 4  0.124 2  0.10  0.37  0.27  12.5  0.124 3  0.124 4  0.124 2  0.10  0.37  0.27  16.0  0.124 4  0.124 5  0.124 2  0.15  0.37  0.41  20.0  0.124 4  0.124 5  0.124 3  0.10  0.37  0.27  25.0  0.124 4  0.124 5  0.124 3  0.10  0.37  0.27  31.5  0.124 4  0.124 5  0.124 3  0.10  0.37  0.27  40.0  0.124 4  0.124 6  0.124 3  0.15  0.62  0.25  63.0  0.124 4  0.124 6  0.124 3  0.15  0.62  0.25  80.0  0.124 4  0.124 5  0.124 3  0.10  0.62  0.16  100  0.124 5  0.124 3  0.124 3  0.12  0.62  0.19  125  0.124 5  0.124 3  0.124 3  0.12  0.62  0.19  160  0.124 5  0.124 5  0.124 3  0.12  0.62  0.19  200  0.124 4  0.124 2  0.124 3  0.10  0.62  0.16  250  0.124 4  0.124 3  0.124 3  0.06  0.62  0.09  315  0.124 4  0.124 4  0.124 3  0.06  0.62  0.09  400  0.124 5  0.124 4  0.124 4  0.06  0.62  0.09  500  0.124 5  0.124 4  0.124 4  0.06  0.62  0.09  630  0.124 6  0.124 5  0.124 5  0.06  0.62  0.09  800  0.124 7  0.124 6  0.124 6  0.06  0.62  0.09  1 000  0.124 8  0.124 8  0.124 8  0.00  0.62  0.00  1 250  0.125 1  0.125 1  0.125 1  0.00  1.00  0.00  1 500  0.125 6  0.125 6  0.125 5  0.06  1.00  0.06  1 600  0.125 6  0.125 5  0.125 6  0.06  1.00  0.06  2 000  0.126 3  0.126 2  0.126 2  0.06  1.01  0.06  2 500  0.127 4  0.127 3  0.127 3  0.06  1.02  0.06  3 000  0.128 8  0.128 8  0.128 7  0.06  1.03  0.06  3 150  0.129 3  0.129 2  0.129 2  0.06  1.03  0.06  3 500  0.130 4  0.130 3  0.130 3  0.06  1.04  0.06  4 000  0.132 4  0.132 3  0.132 2  0.10  1.06  0.09  4 500  0.134 7  0.134 5  0.134 4  0.15  1.08  0.14  5 000  0.137 3  0.137 1  0.137 0  0.15  1.65  0.09  5 500  0.140 3  0.139 9  0.140 0  0.21  1.68  0.12  6 000  0.143 7  0.143 4  0.143 2  0.25  1.72  0.15  6 300  0.146 3  0.146 4  0.145 7  0.38  1.75  0.22  6 500  0.147 7  0.147 1  0.147 4  0.30  1.77  0.17  7 000  0.152 0  0.151 3  0.151 7  0.35  1.82  0.19  7 500  0.156 8  0.154 5  0.158 1  1.82  1.88  0.97  8 000  0.162 2  0.161 3  0.161 9  0.46  1.94  0.24  8 500  0.168 1  0.167 1  0.167 9  0.53  2.01  0.26  9 000  0.174 5  0.173 4  0.174 4  0.61  2.09  0.29  9 500  0.181 6  0.180 3  0.181 5  0.72  2.17  0.33  10 000  0.190 4  0.189 3  0.190 8  0.78  2.28  0.34  acta imeko | www.imeko.org  april 2016 | volume 5 | number 1 | 72  ⋯ . (2) the consistency check fails if 0.05 . (3) the crvs for both accelerometers as reported by the nmisa using primary accelerometer calibration methods [2] are reported in table 4. table 5 shows the results of the consistency check for the reported stability measurements over time both accelerometers. 7.3. the participants  the participants results were communicated to the pilot laboratory via an electronic spreadsheet circulated with the protocol [1]. the official results were communicated to the pilot laboratory in the form of calibration certificates. the calibration certificates had to be issued in the same format as the laboratory would do for their customers. results of table 6 shows the measurement results of the participants for the sensor brüel & kjær 8305 s while the results for the participants for the pcb 301m15 sensor is reported in table 7. table 3. results of monitoring of stability during circulation period for pcb 301m15 sensor. frequency  (hz)  23‐apr‐12  sensitivity  (mv/(m/s²))  05‐oct‐12 sensitivity  (mv/(m/s²))  14‐mar‐13 sensitivity  (mv/(m/s²))  s  (µv/(m/s²))  uc  (µv/(m/s²))  en  10.0  10.27  10.31  10.37  50  52  0.98  12.5  10.25  10.30  10.35  50  52  0.97  16.0  10.24  10.28  10.33  45  51  0.88  20.0  10.22  10.26  10.31  45  51  0.88  25.0  10.21  10.25  10.29  40  51  0.78  31.5  10.19  10.23  10.27  40  51  0.78  40.0  10.18  10.21  10.25  35  51  0.69  63.0  10.14  10.18  10.22  40  51  0.79  80.0  10.12  10.16  10.20  40  51  0.79  100  10.10  10.12  10.18  42  51  0.82  125  10.08  10.11  10.14  30  51  0.59  160  10.06  10.09  10.13  35  50  0.70  200  10.05  10.07  10.11  31  50  0.61  250  10.03  10.06  10.11  40  50  0.80  315  10.01  10.04  10.08  35  50  0.70  400  9.99  10.02  10.06  35  50  0.70  500  9.97  9.99  10.04  36  50  0.72  630  9.95  9.96  10.02  38  50  0.76  800  9.94  9.95  10.00  32  50  0.65  1 000  9.91  9.93  9.98  36  50  0.73  1 250  9.89  9.91  9.95  31  79  0.39  1 500  9.88  9.90  9.94  31  79  0.39  1 600  9.87  9.89  9.94  36  79  0.46  2 000  9.86  9.86  9.92  35  79  0.44  2 500  9.84  9.86  9.90  31  79  0.39  3 000  9.83  9.84  9.87  21  79  0.26  3 150  9.83  9.84  9.87  21  79  0.26  3 500  9.83  9.83  9.87  23  79  0.29  4 000  9.83  9.83  9.87  23  79  0.29  4 500  9.83  9.83  9.86  17  79  0.22  5 000  9.83  9.83  9.87  23  118  0.20  5 500  9.86  9.85  9.90  26  118  0.22  6 000  9.87  9.87  9.90  17  119  0.15  6 300  9.88  9.87  9.93  32  119  0.27  6 500  9.89  9.88  9.95  38  119  0.32  7 000  9.92  9.91  9.95  21  119  0.17  7 500  9.96  9.94  10.03  47  120  0.39  8 000  9.98  9.97  10.00  15  120  0.13  8 500  10.00  10.01  10.02  10  120  0.08  9 000  10.03  10.05  10.03  12  120  0.10  9 500  10.05  10.09  10.05  23  121  0.19  10 000  10.05  10.12  10.09  35  121  0.29  acta imeko | www.imeko.org  april 2016 | volume 5 | number 1 | 73  7.4. degrees of equivalence between participants and the crvs  table 8 and table 9 show the degrees of equivalence (di) between each of the participating laboratories measurements (xi) and the crvs (xref) for the brüel & kjær 8305 and pcb 301m15 sensors respectively where; (4) and the associated uncertainty of the degree of equivalence (u(di)) where . (5) in table 8, cells with yellow shading represent cases where |di| > u(di). table  4.  crvs  as  by  nmisa  primary  calibration  for  sensors  brüel  &  kjær  8305 s and pcb 301m15.  frequency  (hz)  brüel & kjær 8305 s  pcb 301m15  xref  (pc/(m/s²))  u (xref)   (%)  xref  (mv/(m/s²))  u(xref) (%)  10.0  0.124 3  0.3  10.32  0.5  12.5  0.124 3  0.3  10.30  0.5  16.0  0.124 4  0.3  10.28  0.5  20.0  0.124 4  0.3  10.26  0.5  25.0  0.124 4  0.3  10.25  0.5  31.5  0.124 4  0.3  10.23  0.5  40.0  0.124 4  0.3  10.21  0.5  63.0  0.124 4  0.5  10.18  0.5  80.0  0.124 4  0.5  10.16  0.5  100  0.124 4  0.5  10.13  0.5  125  0.124 4  0.5  10.11  0.5  160  0.124 4  0.5  10.09  0.5  200  0.124 3  0.5  10.08  0.5  250  0.124 3  0.5  10.07  0.5  315  0.124 4  0.5  10.04  0.5  400  0.124 4  0.5  10.02  0.5  500  0.124 4  0.5  10.00  0.5  630  0.124 5  0.5  9.98  0.5  800  0.124 6  0.5  9.96  0.5  1 000  0.124 8  0.5  9.94  0.5  1 250  0.125 1  0.8  9.92  0.8  1 500  0.125 6  0.8  9.91  0.8  1 600  0.125 6  0.8  9.90  0.8  2 000  0.126 2  0.8  9.88  0.8  2 500  0.127 3  0.8  9.87  0.8  3 000  0.128 8  0.8  9.85  0.8  3 150  0.129 2  0.8  9.85  0.8  3 500  0.130 3  0.8  9.84  0.8  4 000  0.132 3  0.8  9.84  0.8  4 500  0.134 5  0.8  9.84  0.8  5 000  0.137 1  1.2  9.84  1.2  5 500  0.140 1  1.2  9.87  1.2  6 000  0.143 4  1.2  9.88  1.2  6 300  0.146 1  1.2  9.89  1.2  6 500  0.147 4  1.2  9.91  1.2  7 000  0.151 7  1.2  9.93  1.2  7 500  0.156 5  1.2  9.98  1.2  8 000  0.161 8  1.2  9.98  1.2  8 500  0.167 7  1.2  10.01  1.2  9 000  0.174 1  1.2  10.04  1.2  9 500  0.181 1  1.2  10.06  1.2  10 000  0.190 2  1.2  10.09  1.2  table 5. the results of the chi‐square goodness of fit test for 2 degrees of  freedom for both accelerometers.  frequency  (hz)  brüel & kjær 8305 s  pcb 301m15        10.0  0.15  ≥0.90  0.66  ≥0.70 12.5  0.54  ≥0.70  0.62  ≥0.70 16.0  0.32  ≥0.80  0.31  ≥0.80 20.0  0.08  ≥0.95  0.63  ≥0.70 25.0  0.18  ≥0.9  0.59  ≥0.70 31.5  0.12  ≥0.90  0.55  ≥0.70 40.0  1.12  ≥0.90  0.55  ≥0.70 63.0  0.10  ≥0.95  0.60  ≥0.70 80.0  0.10  ≥0.95  0.60  ≥0.70 100  0.08  ≥0.95  0.51  ≥0.70 125  0.04  ≥0.95  0.60  ≥0.70 160  0.08  ≥0.95  0.76  ≥0.50 200  0.11  ≥0.90  0.50  ≥0.70 250  0.12  ≥0.90  0.72  ≥0.50 315  0.06  ≥0.95  0.39  ≥0.80 400  0.11  ≥0.90  0.50  ≥0.70 500  0.12  ≥0.90  0.43  ≥0.80 630  0.11  ≥0.90  0.56  ≥0.70 800  0.11  ≥0.90  0.53  ≥0.70 1 000  0.09  ≥0.95  0.58  ≥0.70 1 250  0.01  ≥0.95  0.46  ≥0.70 1 500  0.06  ≥0.95  0.56  ≥0.70 1 600  0.05  ≥0.95  0.51  ≥0.70 2 000  0.04  ≥0.95  0.48  ≥0.70 2 500  0.02  ≥0.95  0.65  ≥0.70 3 000  0.01  ≥0.95  0.45  ≥0.80 3 150  0.01  ≥0.95  0.45  ≥0.80 3 500  0.01  ≥0.95  0.49  ≥0.70 4 000  0.01  ≥0.95  0.36  ≥0.80 4 500  0.07  ≥0.95  0.32  ≥0.80 5 000  0.16  ≥0.90  0.11  ≥0.90 5 500  0.59  ≥0.70  0.25  ≥0.80 6 000  0.99  ≥0.50  0.22  ≥0.80 6 300  1.72  ≥0.30  0.21  ≥0.90 6 500  0.71  ≥0.70  0.09  ≥0.95 7 000  0.33  ≥0.80  0.14  ≥0.90 7 500  0.67  ≥0.70  0.42  ≥0.80 8 000  2.19  ≥0.30  0.68  ≥0.70 8 500  0.94  ≥0.50  0.61  ≥0.70 9 000  1.71  ≥0.30  0.73  ≥0.50 9 500  0.84  ≥0.50  0.26  ≥0.80 10 000  0.90  ≥0.50  0.12  ≥0.90 acta imeko | www.imeko.org  april 2016 | volume 5 | number 1 | 74  sets of degrees of equivalence as reported in table 8 and table 9 for selected frequencies are shown in graphical form for the brüel & kjær 8305 and pcb 301m15 in figure 3 and figure 4 respectively. in section 8 using exclusive statistics [4] it is shown that any correlation between the crv's and the results of nmisa is small as compared to the nmisa uncertainty. 8. considering correlation between crv and nmisa  results  the aim of this comparison was to compare measurements of the sensitivity of accelerometers as measured using secondary (back to back) means in accordance with iso table 6. results of the participants for the sensor brüel & kjær 8305 s. frequency  (hz)  bksv  kebs  nmisa  s  (pc/(m/s²))  u s   (%)  s   (pc/(m/s²))  us   (%)  s   (pc/(m/s²))  us  (%)  10  0.124 5  0.8  0.124 2  0.8  0.124 7  1.0  12.5  0.124 4  0.8  0.124 4  0.8  0.125 2  1.0  16  0.124 5  0.8  0.124 4  0.8  0.125 1  1.0  20  0.124 5  0.8  0.124 3  0.8  0.124 1  1.0  25  0.124 5  0.8  0.124 3  0.8  0.124 9  1.0  31.5  0.124 5  0.8  0.124 3  0.8  0.124 8  1.0  40  0.124 5  0.8  0.124 2  0.8  0.125 7  1.0  63  0.124 4  0.8  0.124 2  0.8  0.124 7  1.0  80  0.124 4  0.8  0.124 2  0.8  0.124 7  1.0  100  0.124 5  0.8  0.124 2  0.8  0.124 6  1.0  125  0.124 4  0.8  0.124 3  0.8  0.124 6  1.0  160  0.124 5  0.6  0.124 2  0.8  0.124 6  1.0  200  0.124 5  0.8  0.124 2  0.8  0.124 6  1.0  250  0.124 6  0.8  0.124 3  0.8  0.124 5  1.0  315  0.124 6  0.8  0.124 3  0.8  0.124 5  1.0  400  0.124 7  0.8  0.124 3  0.8  0.124 5  1.0  500  0.124 7  0.8  0.124 4  0.8  0.124 6  1.0  630  0.124 8  0.8  0.124 4  0.8  0.124 6  1.0  800  0.124 9  0.8  0.124 5  0.8  0.124 7  1.0  1 000  0.125 1  0.8  0.124 8  0.8  0.124 8  1.0  1 250  0.125 2  0.8  0.125 0  1.5  0.125 1  1.3  1 500  0.125 6  0.8  0.125 2  1.5  0.125 4  1.3  1 600  0.125 8  0.8  0.125 4  1.5  0.125 5  1.3  2 000  0.126 4  0.8  0.126 2  1.5  0.126 2  1.3  2 500  0.127 4  0.8  0.127 1  1.5  0.127 3  1.3  3 000  0.128 8  0.8  0.128 7  1.5  0.128 7  1.3  3 150  0.129 3  0.8  0.129 2  1.5  0.129 2  1.3  3 500  0.130 4  0.8  0.130 3  1.5  0.130 4  1.3  4 000  0.132 2  0.8  0.132 3  1.5  0.132 4  1.3  4 500  0.134 4  0.8  0.135 0  1.5  0.134 5  1.3  5 000  0.136 8  0.8  0.137 7  1.5  0.137 1  1.9  5 500  0.139 4  0.8  0.140 9  2.0  0.141 0  1.9  6 000  0.142 3  0.8  0.144 1  2.0  0.143 4  1.9  6 300  0.144 7  0.8  0.147 6  2.0  0.146 1  1.9  6 500  0.146 3  2  0.148 9  2.0  0.149 0  1.9  7 000  0.150 4  2  0.151 6  2.0  0.152 8  1.9  7 500  0.154 8  2  0.157 4  2.0  0.158 1  1.9  8 000  0.159 4  2  0.158 0  2.0  0.160 5  1.9  8 500  0.164 8  2  0.167 6  2.0  0.166 4  1.9  9 000  0.170 6  2  0.172 7  2.0  0.171 8  1.9  9 500  0.178 1  2  0.180 8  2.0  0.179 9  1.9  10 000  0.186 7  2  0.189 6  2.0  0.190 3  1.9  acta imeko | www.imeko.org  april 2016 | volume 5 | number 1 | 75  16063-21: 2003 “vibration calibration by comparison to a reference transducer” [3]. the crvs were obtained from the mean of measurements carried out at the beginning, middle and end of the circulation period by nmisa primary vibration calibration system [2]. however the same primary system was used to calibrate the nmisa reference transducers used in the comparison. we adopt exclusive statistics [4] to show that correlation between the crv's and the results of nmisa is small as compared to the nmisa uncertainty. let χ be the exclusive weighted mean of the results of noncorrelated laboratories, such that ∑ ; ∑ ; , (6) table 7. results of the participants for the sensor pcb 301m15. frequency  (hz)  bksv  kebs  nmisa  s  (mv/(m/s²))  us  (%)  s  (mv/(m/s²))  us   (%)  s  (mv/(m/s²))  us  (%)  10  10.29  0.8  10.30  0.8  10.25  1.0  12.5  10.27  0.8  10.29  0.8  10.23  1.0  16  10.26  0.8  10.27  0.8  10.23  1.0  20  10.25  0.8  10.26  0.8  10.18  1.0  25  10.23  0.8  10.27  0.8  10.18  1.0  31.5  10.22  0.8  10.25  0.8  10.16  1.0  40  10.20  0.8  10.23  0.8  10.14  1.0  63  10.16  0.8  10.20  0.8  10.11  1.0  80  10.14  0.8  10.18  0.8  10.09  1.0  100  10.14  0.8  10.16  0.8  10.07  1.0  125  10.11  0.8  10.15  0.8  10.05  1.0  160  10.10  0.6  10.13  0.8  10.02  1.0  200  10.08  0.8  10.09  0.8  10.01  1.0  250  10.06  0.8  10.09  0.8  9.99  1.0  315  10.04  0.8  10.07  0.8  9.99  1.0  400  10.02  0.8  10.05  0.8  9.96  1.0  500  10.00  0.8  10.02  0.8  9.94  1.0  630  9.98  0.8  10.00  0.8  9.91  1.0  800  9.95  0.8  9.97  0.8  9.89  1.0  1 000  9.93  0.8  9.96  0.8  9.87  1.0  1 250  9.90  0.8  9.93  1.5  9.84  1.3  1 500  9.89  0.8  9.91  1.5  9.82  1.3  1 600  9.89  0.8  9.90  1.5  9.81  1.3  2 000  9.86  0.8  9.90  1.5  9.80  1.3  2 500  9.84  0.8  9.88  1.5  9.78  1.3  3 000  9.84  0.8  9.82  1.5  9.77  1.3  3 150  9.84  0.8  9.82  1.5  9.77  1.3  3 500  9.83  0.8  9.80  1.5  9.76  1.3  4 000  9.83  0.8  9.81  1.5  9.77  1.3  4 500  9.83  0.8  9.85  1.5  9.77  1.3  5 000  9.83  0.8  9.86  1.5  9.79  1.9  5 500  9.84  0.8  9.88  2.0  9.81  1.9  6 000  9.85  0.8  9.89  2.0  9.83  1.9  6 300  9.86  0.8  9.94  2.0  9.88  1.9  6 500  9.87  2.0  9.95  2.0  9.89  1.9  7 000  9.88  2.0  9.94  2.0  9.88  1.9  7 500  9.90  2.0  9.95  2.0  9.89  1.9  8 000  9.93  2.0  9.85  2.0  9.90  1.9  8 500  9.96  2.0  9.92  2.0  9.90  1.9  9 000  10.01  2.0  9.93  2.0  9.92  1.9  9 500  10.05  2.0  9.99  2.0  9.99  1.9  10 000  10.10  2.0  10.03  2.0  10.06  1.9  acta imeko | www.imeko.org  april 2016 | volume 5 | number 1 | 76  where the jth or correlated laboratory (nmisa) is excluded. since the exclusive weighted mean χ is not correlated with the crv, │x crv│ is a simple quantitative measure of the crv bias, also inherent in the nmisa reference transducers. table 10 shows that the absolute values of │x crv│ are small in comparison with the uncertainty of nmisa's comparison results. the value at 8000 hz for the sensor brüel & kjær 8305 being the only exception. table 8. degrees of equivalence participants and the crvs for sensor brüel & kjær 8305 s. frequency  (hz)  bksv  kebs  nmisa  di  (fc/(m/s²))  u(di)  (fc/(m/s²))  di  (fc/(m/s²))  u(di)  (fc/(m/s²))  di  (fc/(m/s²))  u(di)  (fc/(m/s²))  10  0.2  1.1  ‐0.1  1.1  0.4  1.3  12.5  0.1  1.1  0.1  1.1  0.9  1.3  16  0.1  1.1  0.0  1.1  0.7  1.3  20  0.1  1.1  ‐0.1  1.1  ‐0.3  1.3  25  0.1  1.1  ‐0.1  1.1  0.5  1.3  31.5  0.1  1.1  ‐0.1  1.1  0.4  1.3  40  0.1  1.1  ‐0.2  1.1  1.3  1.3  63  0.0  1.2  ‐0.2  1.2  0.3  1.4  80  0.0  1.2  ‐0.2  1.2  0.3  1.4  100  0.1  1.2  ‐0.2  1.2  0.2  1.4  125  0.0  1.2  ‐0.1  1.2  0.2  1.4  160  0.1  1.0  ‐0.2  1.2  0.2  1.4  200  0.2  1.2  ‐0.1  1.2  0.3  1.4  250  0.3  1.2  0.0  1.2  0.2  1.4  315  0.2  1.2  ‐0.1  1.2  0.1  1.4  400  0.3  1.2  ‐0.1  1.2  0.1  1.4  500  0.3  1.2  0.0  1.2  0.2  1.4  630  0.3  1.2  ‐0.1  1.2  0.1  1.4  800  0.3  1.2  ‐0.1  1.2  0.1  1.4  1 000  0.3  1.2  0.0  1.2  0.0  1.4  1 250  0.1  1.4  ‐0.1  2.1  0.0  1.9  1 500  0.0  1.4  ‐0.4  2.1  ‐0.2  1.9  1 600  0.2  1.4  ‐0.2  2.1  ‐0.1  1.9  2 000  0.2  1.4  0.0  2.1  0.0  1.9  2 500  0.1  1.4  ‐0.2  2.2  0.0  1.9  3 000  0.0  1.4  ‐0.1  2.2  ‐0.1  2.0  3 150  0.1  1.4  0.0  2.2  0.0  2.0  3 500  0.1  1.4  0.0  2.2  0.1  2.0  4 000  ‐0.1  1.5  0.0  2.2  0.1  2.0  4 500  ‐0.1  1.5  0.5  2.3  0.0  2.1  5 000  ‐0.3  2.0  0.6  2.6  0.0  3.1  5 500  ‐0.7  2.0  0.8  3.3  0.9  3.2  6 000  ‐1.1  2.0  0.7  3.4  0.0  3.2  6 300  ‐1.4  2.1  1.5  3.4  0.0  3.3  6 500  ‐1.1  3.4  1.5  3.5  1.6  3.3  7 000  ‐1.3  3.5  ‐0.1  3.5  1.1  3.4  7 500  ‐1.7  3.6  0.9  3.7  1.6  3.5  8 000  ‐2.4  3.7  ‐3.8  3.7  ‐1.3  3.6  8 500  ‐2.9  3.9  ‐0.1  3.9  ‐1.3  3.7  9 000  ‐3.5  4.0  ‐1.4  4.0  ‐2.3  3.9  9 500  ‐3.0  4.2  ‐0.3  4.2  ‐1.2  4.1  10 000  ‐3.5  4.3  ‐0.6  4.4  0.1  4.3  acta imeko | www.imeko.org  april 2016 | volume 5 | number 1 | 77  9. conclusions  the ilc reported here was planned as a single event with a protocol equivalent to afrimets.auv.v-s3. it was supposed to provide support and validation of participating laboratories’ mcs in the field of vibration for magnitude sensitivity of accelerometers. the frequency range covered the scope currently implemented by the participating laboratories. the circulation of the artefact went without any complications. the analysis of the crv values and nmisa submitted data indicated no discernible correlation. the reported degrees of equivalence support the individual laboratories’ mcs. table 9. degrees of equivalence participants and the crvs for sensor pcb 301m15. frequency  (hz)  bksv  kebs  nmisa  di   (µv/(m/s²))  u(di)     (µv/(m/s²))  di  (µv/(m/s²))  u(di)   (µv/(m/s²))  di  (µv/(m/s²))  u(di)  (µv/(m/s²))  10  ‐30  100  ‐20  100  ‐70  110  12.5  ‐30  100  ‐10  100  ‐70  110  16  ‐20  100  ‐10  100  ‐50  110  20  ‐10  100  0  100  ‐80  110  25  ‐20  100  20  100  ‐70  110  31.5  ‐10  90  20  100  ‐70  110  40  ‐10  90  20  100  ‐70  110  63  ‐20  90  20  100  ‐70  110  80  ‐20  90  20  100  ‐70  110  100  10  90  30  100  ‐60  110  125  0  90  40  100  ‐60  110  160  10  80  40  100  ‐70  110  200  0  90  10  100  ‐70  110  250  ‐10  90  20  100  ‐80  110  315  0  90  30  90  ‐50  110  400  0  90  30  90  ‐60  110  500  0  90  20  90  ‐60  110  630  0  90  20  90  ‐70  110  800  ‐10  90  10  90  ‐70  110  1 000  ‐10  90  20  90  ‐70  110  1 250  ‐20  110  10  170  ‐80  150  1 500  ‐20  110  0  170  ‐90  150  1 600  ‐10  110  0  170  ‐90  150  2 000  ‐20  110  20  170  ‐80  150  2 500  ‐30  110  10  170  ‐90  150  3 000  ‐10  110  ‐30  170  ‐80  150  3 150  ‐10  110  ‐30  170  ‐80  150  3 500  ‐10  110  ‐40  170  ‐80  150  4 000  ‐10  110  ‐30  170  ‐70  150  4 500  ‐10  110  10  170  ‐70  150  5 000  ‐10  140  20  190  ‐50  220  5 500  ‐30  140  10  230  ‐60  220  6 000  ‐30  140  10  230  ‐50  220  6 300  ‐30  140  50  230  ‐10  220  6 500  ‐40  230  40  230  ‐20  220  7 000  ‐50  230  10  230  ‐50  220  7 500  ‐80  230  ‐30  230  ‐90  220  8 000  ‐50  230  ‐130  230  ‐80  220  8 500  ‐50  230  ‐90  230  ‐110  220  9 000  ‐30  230  ‐110  230  ‐120  220  9 500  ‐10  230  ‐70  230  ‐70  220  10 000  10  230  ‐60  230  ‐30  230  acta imeko | www.imeko.org  april 2016 | volume 5 | number 1 | 78  figure  3.  degrees  of  equivalence  between  the  participating  laboratories  and  the  crvs  for  the  sensor  brüel  &  kjær  8305  in  graphical  form  for  select  frequencies. red lines correspond to cases where |di| > u(di).  acta imeko | www.imeko.org  april 2016 | volume 5 | number 1 | 79    figure 4. degrees of equivalence between the participating laboratories and the crvs for the sensor pcb 301m15 in graphical form for select frequencies.  acta imeko | www.imeko.org  april 2016 | volume 5 | number 1 | 80          references  [1] technical protocol of the comparison nmisa.ilc.auv-03 (vibration). kebs, anderson k. maina, april 2012. [2] iso 16063-11: 1999 “primary vibration calibration by laser interferometry, method 3”. [3] iso 16063-21: 2003 “vibration calibration by comparison to a reference transducer”. [4] steele a. g., wood b. m. and douglas r. j., exclusive statistics; simple treatment of the unavoidable correlations from key comparison reference values, metrologia, vol. 38, pp.483-488, 2001. [5] maina a., veldman i., et al., final report on the supplementary comparison: afrimets.auv.v-s3, metrologia, vol. 51 tech. suppl. 09003, 2014.   table 10. |x ‐ crv | compared to nmisa comparison uncertainty ux. frequency  (hz)  brüel & kjær 8305 s  pcb 301m15  |x ‐crv|  (fc/(m/s²))  us  (fc/(m/s²))  |x ‐crv|  (µv/(m/s²)))  us  (µv/(m/s²))  10  0.0  1.2  26  103  12.5  0.1  1.3  21  102  16  0.0  1.3  15  102  20  0.0  1.2  7  102  25  0.0  1.2  1  102  31.5  0.0  1.2  4  102  40  0.1  1.3  4  101  63  0.1  1.2  0  101  80  0.1  1.2  2  101  100  0.1  1.2  18  101  125  0.1  1.2  19  101  160  0.0  1.2  18  100  200  0.0  1.2  4  100  250  0.1  1.2  6  100  315  0.0  1.2  16  100  400  0.1  1.2  16  100  500  0.1  1.2  10  99  630  0.1  1.2  9  99  800  0.1  1.2  0  99  1 000  0.1  1.2  7  99  1 250  0.1  1.6  12  128  1 500  0.1  1.6  13  128  1 600  0.1  1.6  8  128  2 000  0.2  1.6  8  127  2 500  0.0  1.7  21  127  3 000  0.0  1.7  16  127  3 150  0.1  1.7  15  127  3 500  0.1  1.7  14  127  4 000  0.1  1.7  17  127  4 500  0.0  1.7  7  127  5 000  0.1  2.6  2  186  5 500  0.5  2.7  23  186  6 000  0.9  2.7  22  187  6 300  1.0  2.8  21  188  6 500  0.2  2.8  3  188  7 000  0.7  2.9  19  188  7 500  0.4  3.0  54  188  8 000  3.1  3.0  92  188  8 500  1.5  3.2  68  188  9 000  2.5  3.3  71  188  9 500  1.7  3.4  41  190  10 000  2.1  3.6  26  191  the role of metrology in the cyber-security of embedded devices acta imeko issn: 2221-870x june 2023, volume 12, number 2, 1 6 acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 1 the role of metrology in the cyber-security of embedded devices pasquale arpaia1,2,3, francesco caputo1,2, antonella cioffi1, antonio esposito1,2 1 department of electrical engineering and information technology (dieti), università degli studi di napoli federico ii, naples, italy 2 augmented reality for health monitoring laboratory (arhemlab), università degli studi di napoli federico ii, naples, italy 3 centro interdipartimentale di ricerca in management sanitario e inno vazione in sanità (cirmis), università degli studi di napoli federico ii, naples, italy section: research paper keywords: smart card; vulnerability assessment; side-channel attack; metrology citation: pasquale arpaia, francesco caputo, antonella cioffi, antonio esposito, the role of metrology in the cyber-security of embedded devices, acta imeko, vol. 12, no. 2, article 8, june 2023, identifier: imeko-acta-12 (2023)-02-08 section editor: paolo carbone, university of perugia, italy received january 23, 2023; in final form january 23, 2023; published june 2023 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was supported by the “programma operativo nazionale 2014-2020, dottorati di ricerca su tematiche dell'innovazione e green, 10/08/2021 d.m. 10 agosto 2021, n. 1061, a.a. 2021/2022 ciclo 37 – tematiche green (azione iv.5)”. corresponding author: antonio esposito, e-mail: antonio.esposito9@unina.it 1. introduction the progress in information and communication technology favoured the development of a new paradigm, known as internet of things (iot) [1], [2]. this consists of distributed smart transducers that acquire data continuously and communicate with each other by wireless connection, so as to support everyday tasks and improve the quality of life [3]. these transducer networks concern a broad range of applications, such as environmental monitoring, smart power grids, transportation, healthcare, agriculture, and electronic payments [4]–[8]. most of the iot transducers are physically accessible, and this allows certain types of attacks and security breaches [9]. indeed, an attacker can gain access to the network nodes in order to control them or to eavesdrop on exchanged messages. the security of the processed information should be typically guaranteed by encrypting messages through cryptographic algorithms [10], e.g., based on the well-known advanced encryption standard (aes). such an algorithm is implemented in the iot transducer to encrypt the transmitted messages and decrypt the received ones. however, although mathematically safe, the implementation of these algorithms presents some vulnerabilities to side-channel attacks, namely attacks based on the measurement of physical quantities associated with the encryption/decryption operations [11]. measured quantities may involve power consumption, electromagnetic emissions, execution time, light, or heat associated with device cryptographic operations. this side-channel information, also referred to as “leakages”, can be exploited to discover the secret key of the cryptographic algorithm. the side-channel attacks have been extensively studied by researchers and test laboratories for more than two decades. the attacks that have received most of the attention are based on the measurement of power consumption, which is dissipated by the embedded device during its operations. these are known as power analysis attacks and they were firstly introduced in 1999 by kocher [12], who managed to break a public key cryptographic algorithm by measuring the power consumption of a device. indeed, by exploiting the dependence between power abstract the cyber-security of an embedded device is a crucial issue especially in the internet of things (iot) paradigm, since the physical accessibility to the smart transducers eases an attacker to eavesdrop the exchanged messages. in this manuscript, the role of metrology in improving the characterization and security testing of embedded devices is discussed in terms of vulnerability testing and robustness evaluation. the presented methods ensure an accurate assessment of the device’s security by relying on statistical analysis and design of experiments. a particular focus is given on power analysis by means of a scatter attack. in this context, the metrological approach contributes to guaranteeing the confidentiality and integrity of the data exchanged by iot transducers. mailto:antonio.esposito9@unina.it acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 2 consumption and the internal state of the device during the execution of cryptographic operations, he was able to obtain information about the secret key being used. the implementation of this type of attack simply consists of two main phases: (i) the acquisition of power traces and (ii) a statistical analysis. with regard to the actual statistical analysis, different types of attacks can be distinguished: simple power analysis, differential power analysis [12], correlation power analysis [13], and scatter analysis [14]. moreover, machine learning-based approaches to data analysis are gaining more and more interest due to their capability of decoding patterns generated by complex systems [15]-[17]. in previous studies, many proposals concerned methods to improve the attack efficiency [18]–[20], techniques to make cryptographic algorithms robust against the power attacks [21]– [23], and tests aiming to evaluate the robustness of iot devices with respect to the side-channel attacks. broadly speaking, the appealing sensitive and personal information encourages the attackers, on the one hand, to break a cryptographic system in order to make profits. on the other hand, the security offered by such systems must be increased and tested in order to ensure confidentiality. most cryptography papers present resource usages for breaking the cryptographic scheme analysed in function of the security parameters. this allows the system designers to choose the parameter values in order to make it more expensive to achieve a successful attack [24]. in this context, the rigor offered by metrology can contribute to the characterization of embedded and consumer-grade devices [25]. notably, metrology has been recently allowing substantial progress in the field of information security of smart transducers thanks to a characterization of the aes vulnerability [26] or an evaluation of the robustness offered by countermeasures implemented in many devices [27]. the present work focuses on the role of metrology in improving security testing for embedded devices. in particular, results from previous works on the topic are recalled, integrated, and discussed. in detail, a vulnerability assessment will be presented by also providing some examples of enhanced power analysis attacks. next, the discussion will move to robustness evaluation by means of design of experiments. the overall aim is to highlight the role that metrology can have in cyber-security. the paper is organized as follows. section 2 introduces the instruments and the setup adopted to implement a power analysis scatter attack. section 3 presents the results achieved thanks to the metrological approach to security testing. conclusions are drawn in section 4 along with addressing future works. 2. materials and methods power analysis attacks are side-channel attacks that exploit the variations in the power consumption of a cryptographic device to reveal the secret key [28]. in particular, the data-dependency and the operation dependency are exploited. the following subsections present a power analysis attack known as “scatter attack”, the instruments adopted in power traces acquisition, and the measurement setup. then, how to perform an optimized attack and security testing will be discussed in the next section with inherent results. 2.1. scatter attack the scatter attack was chosen to demonstrate the vulnerability of the aes. the attack was implemented against an iot microcontroller (a.k.a. iot device under test), secured by the aes cryptographic algorithm with a 16-bytes-long secret key, i.e., aes-128. the scatter attack consists of statistical analysis of several power traces, whose variations in amplitude are related to the value of the key. the attack implements a “divide-andconquer” strategy by discovering a single byte of the key at a time. the workflow of the attack is shown in figure 1. for each 𝑖𝑡ℎ byte of the key (𝑖 ∈ [0, 15]), a simple discriminant related to each key byte hypothesis 𝑘 is obtained by means of a pearson’s chi-squared (𝜒2) statistical test. when the key byte hypothesis is correct, the discriminant related to the real value should be characterized by the highest value with respect to other guesses. therefore, high values in the discriminant maximizes the likelihood that the key byte hypothesis coincides with the secret key byte. the secret key is thus discovered by repeating the described procedure for all the key bytes. for the implementation of the attack, a malicious measurement system is wired to the iot microcontroller under test for monitoring the power consumption (power trace acquisition phase) during the encryption. the measured data of the power consumption are processed by a method of signature analysis (statistical analysis phase) in order to reveal the secret key of the aes-128 algorithm. for the advanced encryption standard, the portion of the power traces whose variations in amplitude are related to the key value is represented by the addroundkey and subbytes steps of the first aes round. indeed, the key expansion of the aes algorithm produces 10 round keys, the first of those generated coinciding with the secret key. therefore, the first round employs the secret key in clear. moreover, among the operations computed in the first round, the aes sbox output has a statistical influence on the power consumption [29]. 2.2. instruments the device under test (dut) was the atmega-163, a lowpower cmos 8-bit microcontroller based on the avr architecture, embedded on a smart card. the microcontroller presents 16 kb in-system flash, 512 bytes eeprom, 1024 bytes internal sram, and 8 mhz maximum clock, and implements the advanced encryption standard with a key of 128 bits. the cryptographic algorithm is implemented in software, and it does not include the side-channel countermeasures. the acquisition phase exploited the oscilloscope teledyne lecroy hdo9304, characterized by 3 ghz bandwidth, 40 gsa/s sample rate, and 8-bit analog-to-digital converter (adc). a further hardware figure 1. flow diagram of a scatter attack. acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 3 component, useful for the communication between the dut and the oscilloscope, was employed. this component consisted of power tracer by riscure [30], a low-noise card reader for sidechannel power measurements with precise triggering capabilities. the power tracer supplies the integrated circuit on the smart card and communicates with it via iso/iec 7816-3 protocol. moreover, it contains a low noise amplifier (26 pa/ √ hz @ 1 mhz) with top end low-noise and high bandwidth analogue components that are electrically isolated from digital circuitry. thanks to this circuitry, the power tracer provides the power consumption in output with a good signal-to-noise ratio. the capacitors inside the power tracer are pre-charged to power the smart card during each single measurement to avoid any external noise in the circuit. typically, power consumption of contact smart cards is measured via a resistor inserted between the ground pin of a smart card and the ground of a card reader. the power tracer measured power consumption without measurement resistance in the power chain, thus allowing stable card voltage, maximum signal bandwidth, high sensitivity, and low insertion error [31]. an illustration of the instruments and devices used for the attack is shown in figure 2. 2.3. measurement setup the block diagram of the measurement setup for power traces acquisition, is shown in figure 3. a market-leading and professional tool, namely inspector by riscure, was installed on a personal computer. this sends the initial configuration parameters to the digital oscilloscope by means of a usb protocol. moreover, the software communicates with the smart card through the power tracer. in particular, the pc provides the smart card reader with the plaintexts to be sent to the smart card in the apdu (application protocol data unit) format. note that the apdu is a standard protocol, defined by iso/iec 7816-4, allowing the communication between a smart card reader and a smart card. in the communication between smart card and smart card reader, the inspector tool on the pc appeared as the user interface of the card reader allowing to prepare the commands that will be physically sent by the reader to the smart card. the smart card encrypts the message by using the aes-128 algorithm and returns the encrypted message to the pc by means of the smart card reader. the power tracer is also used to send a trigger signal to the oscilloscope by means of serial i/o line to synchronize the acquisition on the encryption. the oscilloscope acquires the power traces from the output signal of the power tracer, i.e., power consumption with an excellent signal-to-noise ratio. the sample data are sent to the pc by means of a usb interface. before the oscilloscope acquires the power consumption, a bnc coaxial low pass filter (mini circuits blp50+, dc to 48 mhz, 50 ω) cuts the signal frequencies over 48 mhz as they represent no significant information. 3. results in this section, the results achieved by the adoption of the metrology in the security of embedded devices are reported. the discussion is conducted by analyzing the contributions in optimization of power analysis attack and of vulnerability assessment. the overall aim is to give some indications about the current state of metrology role for cybersecurity of embedded devices. 3.1. optimization of power analysis attack the success rate of the power attacks is significantly affected by the signal-to-noise ratio (snr) of the power traces [19]. techniques for noise reduction are particularly important when measuring power consumption because power analysis attacks are very sensitive to the magnitude of these signals to recover the value of the secret key. therefore, it is important to eliminate noise effectively and improve the snr of power traces to extract the secret key with minor effort. indeed, a good level of signal to noise ratio involves in reducing the number of power traces needed to correctly reveal the secret key, and, consequently, in decreasing the time to perform a successful attack. in [32], a filtering operation was employed in order to enhance the test attack effectiveness. the filter adopted is a low pass digital filter, which makes each sample a weighted average of the previous and the current sample. the improvement of the signal to noise ratio is also obtained by the decimation operation applied after an operation of oversampling. in fact, the decimation contributes to improving the signal to noise ratio by reducing the noise floor. an assessment is conducted to establish the best configuration for the filter weight. the experiment proved that the number of disclosed bytes by keeping the number of power tracks fixed increases according to the filter weight. indeed, with 50000 power traces, a filter weight equal to 400 allows to discover the 16 bytes key while a weight of 300 returns only 15 bytes. therefore, a filter weight equal to 400 was considered as the best configuration. the effectiveness of the enhanced scatter attack was proved experimentally in the best configuration by reducing the sample size of power traces. the figure 4, shows the results. the number of bytes disclosed correctly are reported as a function of the number of power traces, where the filter weight is fixed to 400. the plot highlights that, with a filter weight of 400, a number of 30 000 power traces is sufficient to find the encryption key exactly. contrarily, in absence of filtering operation, a successful attack needs 50 000 figure 2. instruments and devices for the scatter attack implementation. figure 3. block diagram of the measurement setup for power traces acquisition. acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 4 power traces. definitely, the use of a filter allows to discover the entire key unlike the lack of pre-processing with the same number of traces. moreover, the filter is able to reduce the number of traces needed for a successful attack. decreasing the number of power traces also reduces the time needed to find the secret key. 3.2. vulnerability assessment enhancement the vulnerability assessment allows us to evaluate the robustness and security of cryptographic devices with respect to “side-channel attacks”. these consist in assessing the effort made to penetrate the device, quantified in terms of computational resources and time necessary for a potential attacker. a good as the robustness of a cryptographic device is important to guarantee, with greater reliability, the confidentiality and integrity of the data. a correct and reliable vulnerability assessment depends on a correct choice of the factors involved in the attack phases, such as sampling frequency, pre-processing techniques and number of traces acquired. indeed, when the parameters are not optimal, an attack could require more effort to reveal the secret key, such as a higher number of traces to acquire and a longer time to reveal the secret key. this occurrence can distort the outcome of the vulnerability analysis. in recent years, metrology has been contributing to the world of cybersecurity in order to improve the characterization of devices in terms of security. in [26], the experimental design method was investigated to evaluate the factors affecting the attack system and to identify the values that maximize the number of bytes correctly identified with a minimum number of experimental tests. the attack system analysed in [26], consists of (i) the measurement devices used to acquire the power traces, (ii) the pre-processing techniques adopted to improve the power traces, and (iii) the statistical analysis of the scatter attack to discover the secret key of the aes-128 implemented in a smart card. the pre-processing techniques employed are a fast bidirectional filter to enhance the signal-to-noise ratio of the power traces and the resampling operation to reduce their dimensionality. the factors chosen for the analysis are the filter weight of the fast bidirectional filter, the resampling rate, and the number of power traces. for each parameter, 3 values are investigated. therefore, a l9 orthogonal array is chosen for the experimental planning. indeed, this design allows us to model a problem of 3 parameters and 3 values. the experimental design method implemented for the attack system under analysis allows to identify the parameters that mostly influence the attack and the values that increase the number of correctly discovered bytes. the results of the statistical significance analysis are shown in the pareto chart of figure 5. the histogram bars report the f-value for each attack parameter, while the line represents the related percentage contribution. the bars presented in descending order highlight the number of power traces as the most important parameter among those considered. the figure 6 exhibits the parameter value effects obtained by and the estimated error (for a confidence level of 99.97 %). in particular, each point is computed as the mean of the objective function values obtained for a fixed value of the factor analyzed. this plot allows to establish the best configuration for the attacking parameters able to maximize the disclosed key bytes. the best configuration for the case under analysis is 500 for the filter weight, 500 ksa/s for the resampling frequency, and 400 for the number of power traces. 3.3. metric for robustness evaluation the security of cryptographic algorithms with respect to power analysis attacks is improved by software and hardware countermeasures. power analysis attacks success in discovering the secret key because the power consumption of cryptographic devices depends on intermediate values of the executed cryptographic algorithms. therefore, the goal of power countermeasures is to make the power consumption of figure 4. number of bytes disclosed by the scatter attack as a function of the number of power traces for a weight of the filter equal to 400 (dotted line: 3rd-order polynomial interpolation). figure 5. pareto chart of the parameters: the histogram bars represent the fvalues (left axis) and the orange line the cumulative percentage contributions (right axis). figure 6. plot summarizing the number of disclosed bytes in each experiment obtained with a fixed number of power traces and weight of the filter. acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 5 cryptographic devices independent of the processed data. the countermeasures include hiding and masking techniques. the hiding countermeasures introduces variations of power consumption in the time domain or amplitude domain, while masking implements a data randomization by concealing each intermediate value. in [27], a method was proposed to assess the security performance among different power countermeasures designed to reinforce a software implementation of aes-128. moreover, the method provides a metric to express the effectiveness of a countermeasure in straightening the iot transducer security. the method consists of conducting a power analysis attack at varying the security measures and in computing, for each combination of attack and countermeasure, the number of traces needed to discover the secret key. this parameter is typically used for assessing the countermeasure effectiveness. the more the countermeasure is effective, the more the number of traces increases. the calculation of the minimum number of power traces needed to succeed in the attack for each countermeasure is obtained as the mean of the minimum number of power traces obtained on n repetitions of the attack on different batches of power traces. a successive-approximation method in a range with extremes determined in a preliminary experimental campaign was adopted for the first repetition of the attack. the successive n − 1 repetitions implement a grid search method with a variable step initialized to the minimum number of power traces found in the first repetition. the step is an increment of a certain number of power traces until the secret key is not fully recovered, and a decrement of an order of magnitude lower until the minimum number of power traces for a particular repetition is identified. the method employs the minimum numbers of power traces needed to discover the secret key obtained by each combination of attack and countermeasure to compute the strength factors (sfs). this parameter quantifies the level of protection for each countermeasure, and it is calculated as 𝑆𝐹𝐶 ∗ = ∑ min𝐴,𝐶 ∗ 𝑁𝐴 𝑖=1 ∑ min𝐴,1 𝑁𝐴 𝑖=1 , (1) where 𝑁𝐴 is the number of implemented power attacks, 𝑚𝑖𝑛𝐴,𝐶 ∗ is the minimum number of power traces for a fixed countermeasure 𝐶 ∗ at varying of the power attack, and 𝑚𝑖𝑛𝐴,1 is the minimum number of power traces for no-countermeasure at varying of the power attack. the proposed method for the robustness evaluation of countermeasures was applied to a case study consisting of a software implementation of the aes-128 reinforced by countermeasures. the countermeasures under analysis are (i) random delay insertion, (ii) random sbox, and (iii) boolean masking. moreover, the configuration with no countermeasures was evaluated. the result of the analysis is reported in table 1. random delay strengthens the aes of a 1.3 factor with respect to no countermeasure condition; the strengthening factor for random sbox is 208, while more than 318 for masking. in case of masking, the non-availability of a minimum number of power traces does not allow to determine a strength factor value but only a low limit. the comparison of strength factors highlights masking as the most effective over other power countermeasures as it increases the number of power traces needed to succeed in the attack to a greater extent than the other countermeasure. 4. conclusions in this work, the role of metrology to improve the characterization and security testing of embedded devices was discussed. this was done by recalling previous works focusing on vulnerability testing and robustness evaluation. such results were recalled, integrated, and discussed. in detail, a method based on the design of experiments was presented to enhance the vulnerability assessment. meanwhile, a metric was introduced to express the effectiveness of a countermeasure in straightening the iot transducer security. overall, the discussion suggests that metrology plays an important role in cyber-security, especially in a contest where iot transducers are spread more and more, and their physical accessibility demands rigorous security testing. future works will continue these investigations by extending the metrological approach to machine learning-based attacks. indeed, these have great potential in security breaches, and they thus deserve further investigation. references [1] l. atzori, a. iera, g. morabito, the internet of things: a survey, computer networks 54(15) (2010), pp. 2787–2805. doi: 10.1016/j.comnet.2010.05.010 [2] i. ahmed, e. balestrieri, f. lamonaca, iomt-based biomedical measurement systems for healthcare monitoring: a review, acta imeko 10(2) (2021), pp. 174-184. doi: 10.21014/acta_imeko.v10i2.1080 [3] t. yousuf, r. mahmoud, f. aloul, i. zualkernan, internet of things (iot) security: current status, challenges and countermeasures, international journal for information security research (ijisr) 5(4) (2015), pp. 608–616. doi: 10.1109/icitst.2015.7412116 [4] w. t. sung, s. j. hsiao, the application of thermal comfort control based on smart house system of iot, 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https://doi.org/10.1109/cis.2014.103 https://doi.org/10.1007/978-3-030-10970-7_22 https://doi.org/10.1109/tdsc.2004.25 https://doi.org/10.1109/mdt.2007.200 https://doi.org/10.1007/11767480_16 https://doi.org/10.1109/tim.2021.3127650 https://doi.org/10.1109/tim.2021.3107610 https://doi.org/10.1109/tim.2021.3088491 https://doi.org/10.1007/s13389-011-0006-y https://www.riscure.com/securitytools/inspector-sca/ https://doi.org/10.1049/iet-cds:20070166 https://doi.org/10.1016/j.measurement.2020.107853 developments of interlaboratory comparisons on pressure measurements in the philippines acta imeko issn: 2221-870x june 2023, volume 12, number 2, 1 6 acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 1 developments of interlaboratory comparisons on pressure measurements in the philippines mary ness i. salazar1,2 1 national metrology laboratory of the philippines – industrial technology development institute dost, philippines 2 measurement science, kriss school university of science and technology, daejeon, republic of korea section: research paper keywords: pressure measurement; intercomparison; proficiency testing; interlaboratory comparison citation: mary ness i. salazar, developments of interlaboratory comparisons on pressure measurements in the philippines, acta imeko, vol. 12, no. 2, article 36, june 2023, identifier: imeko-acta-12 (2023)-02-36 section editor: leonardo iannucci, politecnico di torino, italy received january 13, 2023; in final form may 24, 2023; published june 2023 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was supported by the laboratory proficiency evaluation program of the nml-itdi, dost. corresponding author: mary ness i. salazar, e-mail: marynesssalazar@yahoo.com 1. introduction the national metrology laboratory (nml) of the industrial technology development institute (itdi) under the department of science and technology (dost) conducted interlaboratory comparisons in the field of hydraulic pressure measurement among the local calibration laboratories in the philippines. the provision of interlaboratory comparison, otherwise known as proficiency testing (pt), is a program to strengthen the nml relationship with the said laboratories in establishing scientific metrology in the country. this pt program aims to [1], [2]: (a) determine the technical capabilities and performance of the laboratories; (b) assess the reliability of their measurement results and validate their calibration measurement capabilities; (c) disseminate a harmonized and validated calibration procedure; (d) demonstrate metrological equivalence to the nml and most importantly, (e) provide access to interlaboratory comparisons for their compliance to iso/iec 17025 [[3]] requirements. the pressure standards section (pss) of the nml acted as the program coordinator and reference laboratory, accredited under the terms of iso/iec 17025:2005 (2017 at present). the pss was responsible for providing the artefact, the reference value and its measurement uncertainty, the monitoring of the program as a whole, and preparation of written reports for the two intercomparisons being compared; one was conducted in 2010 while the other was in 2016 the following will be the outline of this paper. in section 2, the comparison process will be discussed followed by the measurement results of the participants. discussion of these results commences in section 4, and the evaluation of both the pts is compared in section 5. finally, the concluding section summarizes the major accomplishment and development based on these experiences. abstract the national metrology laboratory of the philippines, industrial technology development institute under the department of science and technology (itdi-dost) offered first of its kind interlaboratory comparison in pressure measurement in the country following a demand survey identifying the gaps in proving the competency of local calibration laboratories. this paper presents the development in implementing the two interlaboratory comparisons in the philippines with six years interval using the same pressure artefact. with its many objectives, the program also aimed to provide these laboratories access to proficiency testing (pt) to comply with the iso/iec 17025 requirements. comparatively, while both are considered successfully implemented, the improved awareness and commitment to quality of the participants and the enhanced competency of the pilot laboratory in implementing such activity are a few of the enumerated factors instigating the increase in the number of participants as well as those obtaining satisfactory results in the latter pt. the interlaboratory comparison schemes offered aimed at sustaining the demands of metrology stakeholders and continuously develop this service to support further progress in the calibration and measurement capabilities of local laboratories in the country. mailto:marynesssalazar@yahoo.com acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 2 2. comparison process the two intercomparisons followed an almost similar process; differences are, however, emphasized in this paper, specifically the contributing factors that affected the results of the two schemes. it is herein referred to the first pt scheme as 2010 and the second pt scheme as 2016 throughout the discussion. the interlaboratory comparison program was designed as a cycle where the nml, as a reference or pilot laboratory, calibrates the artefact at the program's beginning, middle, and end. one main difference between 2010 and 2016 is the conduct of a preparatory workshop before the start of the pt program. this workshop proved essential in getting to know each laboratory's capabilities, standards, and procedures they follow in calibrating pressure gauges. also, in this workshop, an agreement between the participants and the nml was reached, fulfilling the objective of disseminating a harmonized and validated calibration procedure and the identification of limitations of the participants and the pt program in general. 2.1. participants participants in the two pts are local calibration laboratories with nml as the reference lab. in 2010, five (5) participants were all private laboratories in metro manila, the capital of the philippines. in 2016, the 16 participants comprised private and government laboratories, including some dost regional metrology laboratories from different provinces in the philippines. 2.2. artefact the artefact (figure 1and table 1) used in 2010 and 2016 is a bourdon-tube-type pressure gauge. it was noted that in 2010, two artefacts of different ranges were measured by the participants, but in this paper, comparisons will be based on only one artefact used on both pts, and the description is as follows: the artefact was subjected to initial and subsequent characterization before the pts and maintained a regular interval of calibration and intermediate checks when not used as an artefact. 2.3. calibration method the participants were asked to calibrate the artefact through direct comparison to their standard. in 2010, each participant used the typical calibration procedure in their laboratory, which means their laboratory-developed method. meanwhile, in 2016, after the earlier mentioned preparatory workshop, it was agreed among the participants to follow an international guideline, the dkd-r 6-1 calibration of pressure gauges [4]. which not only guided the calibration procedure but the computation of measurement uncertainty as well. the nml uses the said guideline. 2.4. measurement scheme the nml chose a measurement scheme to monitor the artefact's metrological quality throughout the pt process. in 2010, the artefact was calibrated before and after a trip to a participant, as shown in figure 2. it was hand-carried to and from each participating lab by its representative. in 2016 however, since there were more participants than in 2010 and some were outside metro manila, the nml calibrated the artefact before and after a group of participants, usually 2 to 3 labs, strategically chosen based on location so the sending back and forth to the nml of the artefact are made most efficiently. figure 3 shows this scheme. figure 1. the artefact. table 1. technical specification of the artefact. manufacturer ashcroft serial number / identification s2-w-006 measuring range 25 000 kpa scale division 100 kpa accuracy 0.25 % medium liquid figure 2. pt 2010 measurement scheme. figure 3. pt 2016 measurement scheme. nml lab b nml lab c nml lab d nml lab e start nml end lab a lab4 lab5 lab6 nml lab7 lab8 nml lab9 lab10 lab11 lab12 nml lab13 lab14 lab15 nml lab16 nml end nml start lab1 lab2 lab3 nml acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 3 2.5. report of the participants in 2010, the participants were only asked to submit the filledout nml-provided measurement datasheet and the calibration certificate they usually issue to their customers. other information, such as the uncertainty budget, was only known when required by the nml. this exchange of information could have been more efficient, consequently causing a delay in data evaluation and publishing of the final report. hence, the nml changed this practice in 2016. the participants should submit documents such as the measurement datasheets, a copy of the calibration certificate of their standard proving valid traceability, their usual calibration report, and the uncertainty budget. this change improved the data evaluation process for the nml, making it easier and faster since information transparency was present from the beginning. 2.6. reference values in both pts, the reference values used in evaluating the normalized error (𝐸n ) for each participant were based on the values nearest the participant’s reported results, either before or after the calibration of the reference laboratory. in 2016, this result was given to participants in an interim report, showing only the specific participant’s results compared to nml’s. this interim report was beneficial for participants having to prove their competence to technical peers during their assessment while the intercomparison is not yet completed. it should be noted, however, that in the final report, the reference values reflected are the weighted average of all the measurement results of the nml. 3. measurement results the measurement results of participating laboratories are evaluated using the earlier mentioned normalized error or the 𝐸n ratio [5], calculated using the equation: 𝐸n = 𝑥lab − 𝑥ref √𝑈lab 2 + 𝑈ref 2 , (1) where 𝑥lab is the measured value of the participating laboratory, 𝑥ref is the reference value, 𝑈lab and 𝑈ref are the expanded uncertainty (k = 2) of the participant’s measured value and the reference value, respectively. the reference value in this equation is the deviation of the artefact reading from the nml’s applied pressure at the nominal calibration points. similarly, the measured value of the participating laboratory is the deviation of their reported value to the nominal calibration points. this practice ensures the uniformity of values to be compared. moreover, the expanded uncertainties were reported with a coverage factor of k = 2, indicating a confidence level of approximately 95 %. figure 4 shows the performance of participants in the two pts. more participants joined in 2016 with 88 % (14 out of 16) satisfactory performance compared to 40 % (2 out of 5) in 2010. two participants joined both pts and one participant performed better in 2016 while the other still failed. in 2010, four out of the five participants were able to calibrate the artefact in its full measuring range, while one participant did not submit a result in the last two highest points. meanwhile, in 2016, all the participating laboratories were able to calibrate the artefact as a whole. table 2 and table 3 show the 𝐸𝑛 value of the participants in 2010 and 2016 respectively. figure 4. participants’ performance on two pts. table 2. summary of 2010 participants’ en values relative to the reference values. table 3. summary of 2016 participants’ en values relative to the nominal pressure values 0 5 10 15 20 2010 2016 n o . o f p a rt ic ip a n ts year satisfactory unsatisfactory acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 4 4. discussion of results the laboratory’s satisfactory performance was determined when |𝐸n| ≤ 1 in all the prescribed calibration points. in 2010, only 95 % of the total calibration points with |𝐸𝑛 | ≤ 1 were required to be considered satisfactory laboratory performance, but this was later corrected to 100 % of the calibration points in 2016. some participants interpreted that the 95 % confidence level in the uncertainty budget estimate may also be applied in the inter-laboratory comparison result, thus, assuming that they don’t need to perform well in all the measurement points since there is a 5 % margin of error. the nml had to explain the rationalization that the 5 % margin of error cannot be tolerated in the measurement procedure since the basic requirement of the pt was to calibrate the artefact as a whole and any doubt in their procedure must well be accounted for in their uncertainty budget and not on their measurement value. also, a 95 % satisfactory performance will not be possible in the calibration points prescribed since any one point was already 10 % of the artefact’s ten calibration points. it is, however, emphasized that the laboratory’s performance is considered satisfactory only through the 𝐸n score. accordingly, so as not to abuse the said acceptance criteria, it was later suggested that a limiting value for uncertainty is to be set, and most of the participants followed it in 2016. for illustration purposes, figure 5 and figure 6 show the participant’s deviation from the reference value with its corresponding uncertainties in the non-zero minimum and the maximum calibration points, respectively. the 2010 participants were represented by blue markers and labeled alphabetically (lab a to lab e), while participants in 2016 were of green markers and were labeled with numbers (lab1 to lab16). comparing the participants’ performance in the two pts, all the participants performed satisfactorily in the minimum calibration point in 2010 as opposed to those in 2016, with 1 participant whose value already lies beyond the limit if not with its uncertainty. differences in the computed uncertainty values depended mainly on the standard they used, primarily a digital pressure gauge; only a few used a pressure calibrator and a deadweight tester. moreover, in 2010, while the nml prescribed a guideline for measurement uncertainty calculations, most participating laboratories estimated the expanded uncertainties using their laboratory procedure and technique with the notion that declaring a low uncertainty means better performance. figure 5. participant’s deviation from the reference values and its corresponding uncertainties at 2500 kpa. figure 6. participant’s deviation from the reference values and its corresponding uncertainties at 25 000 kpa. -150 -100 -50 0 50 100 150 200 d e v ia ti o n , k p a 2 500 kpa measurement point -350 -300 -250 -200 -150 -100 -50 0 50 100 150 d e v ia ti o n , k p a 25 000 kpa measurement point acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 5 contrastingly, in 2016, all the participating laboratories followed the agreed guideline on procedure and uncertainty estimates, with the lower limit as the resolution of the artefact. this agreement ensured that measurement uncertainties were not over or under-estimated. a sample calculation of the recommended minimum components of measurement uncertainty of the pilot laboratory is shown in table 4. the minimum components of measurement uncertainty were enumerated as the factors coming from the standard, the height difference between the standard and the artefact or the so-called hydrostatic pressure effect, the artefacts’ resolution, zero deviation, hysteresis, and repeatability. the expanded uncertainty, as earlier mentioned, was evaluated with a 95 % level of confidence with the coverage factor, k = 2. the uncertainty from the standard was taken from the most recent calibration certificate of the artefact before the start of the interlaboratory comparison. the uncertainty due to the hydrostatic pressure effect was evaluated but was found negligible due to minimizing, if not eliminating, the height difference of the reference pressure points of the standard and the artefact during calibration. similarly, the uncertainty due to zero deviation was also found negligible due to the behaviour of the artefact. the major contributor to uncertainty in the reference value was the resolution of the artefact, which was agreed to be the limiting factor for uncertainty evaluation for all the participants, including the pilot laboratory. this decision accommodated the limited accuracy capability of some participants, which caters mainly to industrial calibrations. the uncertainty due to hysteresis and repeatability are the maximum values among the measurement performed by the nml. the nml used these factors in both the 2010 and 2016 pt. 5. evaluation of comparisons comparing the two pts as a whole, defined factors affecting the performance of participating laboratories are summarized in figure 7. the earlier mentioned preparatory workshop, not done in 2010 but conducted in 2016, proved to be one key factor that led to the increased satisfactory performance of participants. in 2016, four laboratories could not attend this workshop; however, two laboratories asked for details after the event and strictly followed the agreed procedure, 1 lab with a satisfactory result but did not follow the agreed uncertainty limit, and one did not perform satisfactorily. in both pts, the main objective of the laboratories' participation was to fulfil the iso/iec 17025 requirement for pt participation. however, the urgency of this requirement was only partially realized by the 2010 participants since the accreditation to iso/iec 17025 was reasonably new in the country during that time. moreover, most 2010 participants have inexperienced or untrained personnel and need to familiarize themselves with the calibration method used by the nml. contrastingly, in 2016, most laboratories had trained personnel and had proven competencies in scopes other than the pressure field. their submitted results showed that knowledge of estimating uncertainty budgets also significantly improved through training. the latter pt also showed an enhanced selection of standard and upgraded facilities by the laboratories. it is most probable that the 2010 pt was considered as a test run by the participating laboratories, with two labs participating satisfactorily on both, 1 lab that improved in 2016, 1 that still unsatisfactorily performed, and one that did not continue to be a calibration laboratory, this is also the participant that did not complete the measurement due to inappropriate standard. the 2016 participants, on the other hand, are mostly maintaining their table 4. the nml’s calculation of the recommended minimum measurement uncertainty components. calibration item test gauge measurement range 25 000 kpa scale division 100 kpa resolution 50 kpa uncertainty of standard used 4.1 kpa coverage factor (k) 2 components of uncertainty nominal pressure uncertainty from the standard uncertainty due to height difference uncertainty due to resolution uncertainty due to zero deviation uncertainty due to hysteresis uncertainty due to repeatability expanded uncertainty kpa kpa kpa kpa kpa kpa kpa kpa 0 2.6 0.0 28.9 0.0 0.0 0.0 58 2500 2.6 0.0 28.9 0.0 0.2 0.1 58 5000 2.6 0.0 28.9 0.0 0.2 0.3 58 7500 2.6 0.0 28.9 0.0 0.1 0.1 58 10000 2.6 0.0 28.9 0.0 0.3 0.8 58 12500 2.6 0.0 28.9 0.0 0.1 0.1 58 15000 2.6 0.0 28.9 0.0 0.2 0.1 58 17500 2.6 0.0 28.9 0.0 0.1 0.1 58 20000 2.6 0.0 28.9 0.0 0.0 0.2 58 22500 2.6 0.0 28.9 0.0 0.1 0.0 58 25000 2.6 0.0 28.9 0.0 0.0 0.1 58 acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 6 iso/iec 17025 accreditation already or are in the process of acquiring their certification in the pressure scope, supported by this intercomparison. in both pts, the nml recommended that all the laboratories with unsatisfactory performance review their calibration method and uncertainty budget analysis, investigate sources of error leading to unsatisfactory results, and initiate corrective actions. the nml, on the other hand, as the reference laboratory, continuously improves as a pt provider learning from experience, from the handling of the artefact to the analysis of data that is most appropriate to all participants. consequently, the nml extended its pt offering regularly, with different pressure ranges and other fields of measurement; also, the conduct of the concluding workshop was planned for the succeeding pts. furthermore, coordination with the local accreditation body as the channel to know the demands for pt in the country for nml's plan on pt provision, and in return, the laboratories are made aware of the nml pt offerings. availability of artefact is still the most significant limitation of the pt provision but was hoping to be resolved to cope-up with ongoing demands on pt. currently, the laboratories' commitment to quality, supported by courses and training which are not only technical but also in the quality management systems, are contributors to the laboratories' handling of intercomparison prominent to satisfactory performance. the participants have become more aware of good laboratory practices and are encouraged to continuously improve through refresher and new metrology awareness courses. 6. summary and conclusion the intercomparisons, 2010 and 2016, are two independent pts and are generally considered successful in terms of results, coordination, and the experience gained by the participants and the reference laboratory. measurement results revealed the calibration and measurement capabilities of each participating laboratory. based on the requirements of iso/iec 17043:2010, the performances are mostly satisfactory, in terms of the 𝐸n values, especially in the 2016 pt. this also indicates that the measurement practices of these participants significantly improved and are aligned and complying with an internationally validated method. the pt schemes offered by the nml will be of continuous improvement to support further progress of the local calibration laboratories in the philippines. acknowledgement funding support for dost and the laboratory proficiency evaluation program (lpep) of the nml, itdi-dost is gratefully acknowledged. the lpep was initially funded by the philippines council for industry, energy and emerging technology for research and development (pcieerd) making it possible to acquire the artefact used in these intercomparisons. references [1] maryness i. salazar, final report for pressure test gauge calibration interlaboratory comparison 07-2010-ptilc-0042, nml, itdi-dost philippines, 2011. unpublished results. [2] maryness i. salazar, interlaboratory comparison on hydraulic pressure standards 01-2016-ptilc(pres)-0002m nml, itdidost, philippines, 2016. unpublished results. [3] international organization for standardization (2017) iso/iec 17025:2017 general requirements for the competence of testing and calibration laboratories. iso geneva, switzerland. [4] ptb deutscher kalibrierdienst, guideline dkd-r 6-1 calibration of pressure gauges. dkd, edition 03/2014. [5] international organization for standardization (2010) iso/iec 17043:2010 conformity assessment – general requirements for proficiency testing. iso, geneva, switzerland. figure 7. defined factors affecting participants’ performance. 0 2 4 6 8 10 12 14 16 preparatory workshop attendance quality management system training of personnel following international guideline use of appropriate standard n o . o f p a rt ic ip a n ts year 2010 2016 a method to consider a maximum admissible risk in decision-making procedures based on measurement results acta imeko issn: 2221-870x june 2023, volume 12, number 2, 1 9 acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 1 a method to consider a maximum admissible risk in decisionmaking procedures based on measurement results alessandro ferrero1, harsha vardhana jetti1, sina ronaghi2, simona salicone1 1 dipartimento di elettronica, informazione e bioingegneria (deib), politecnico di milano, italy 2 dipartimento di energia (deng), politecnico di milano, italy section: research paper keywords: measurement uncertainty; threshold; decision making; risk of wrong decision; maximum admissible risk citation: alessandro ferrero, harsha vardhana jetti, sina ronaghi, simona salicone , a method to consider a maximum permissible risk in decision-making procedures based on measurement results, acta imeko, vol. 12, no. 2, article 41, june 2023, identifier: imeko-acta-12 (2023)-02-41 section editor: laura fabbiano, politecnico di bari, italy received march 17, 2023; in final form may 31, 2023; published june 2023 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: simona salicone, e-mail: simona.salicone@polimi.it 1. introduction measurement results are very often used as input elements in decision-making procedures, which represent the core element of conformity assessment. this is a very critical task in many fields, from the industrial one, where conformity of a product's feature to given specifications must be assessed, to environment protection, health, legal and forensic ones, where decisions are generally related to checking that the presence of a substance (a pollutant, a drug, etc.) or the error of an instrument does not exceed a given threshold or maximum admissible limit. most decisions – if not all of them – are taken by comparing a measurement result with a threshold or a range of admissible values, where the threshold, or the upper and lower limits of the range, are given as simple quantity values [1]. then, according to where the measurement result is located with respect to the threshold or the range, a decision is taken on whether conformity can be declared or not. if measurement uncertainty is not considered, or if it can be assumed to be negligible, this decision can be easily taken by comparing two numerical values: the measured value with the threshold (as shown in figure 1) or the measured value with the upper and lower limits of the range. figure 1 shows that, in such a situation, the decision is apparently taken with no risk of being wrong. however, even if measurement uncertainty has been evaluated and found to be negligible, a risk of wrong decision still exists, because it is widely recognized [2] that “when all of the known or suspected components of error have been evaluated and the appropriate corrections have been applied, there still remains an uncertainty about the correctness of the stated result, that is, a doubt about how well the result of the measurement represents the value of the quantity being measured”. it is also well-known, according to the gum [2], that in many applications “it is often necessary to provide an interval about the measurement result that may be expected to encompass a large fraction of the distribution of values that could reasonably be attributed to the quantity subject to measurement. thus, the ideal method for evaluating and expressing uncertainty in measurement should be capable of readily providing such an interval, in particular, one with a coverage probability or level of confidence that corresponds in a realistic way with that required”. when measurement uncertainty is taken into account, again a decision about conformity can be readily taken if the coverage interval is completely below or above the threshold (figure 2). abstract measurement uncertainty plays a very important role ensuring validity of decision-making procedures, since it is the main source of incorrect decisions in conformity assessment. the guidelines given by the actual standards allow one to take a decision of conformity or non-conformity, according to the given limit and measurement uncertainty associated to the measured value. due to measurement uncertainty, a risk of a wrong decision is always present, and the standards also give indications on how to evaluate this ri sk, although they mostly refer to a normal probability density function to represent the distribution of values that can be reasonably attributed to the measurand. since such a function is not always the one that best represents this distribution of values, this paper considers some of the most-often used probability density functions and derives simple formulas to set the acceptance (or rejection) limits in such a way that a pre-defined maximum admissible risk is not exceeded. mailto:simona.salicone@polimi.it acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 2 on the other hand, the situation represented in figure 3 appears to be quite critical, since the threshold falls inside the coverage interval representing the fraction of the distribution of values that could reasonably be attributed to the quantity subject to measurement (the measurand). this means that there is a probability that some of the values that could reasonably be attributed to the measurand might be greater than threshold t, even if the measured value x̅𝑚 is lower than the threshold. this also means that, if conformity shall be assessed when the measurand is lower than the threshold, a risk exists of declaring the measurand conforming while it is not, and this risk can be evaluated starting from measurement uncertainty [3]. conformity assessment involves, therefore, a decisionmaking process affected by uncertainty. such a problem has been widely covered in the literature [4]-[6], mostly by taking epistemic uncertainty into account [7]. however when the input elements to a decision-making process are measurement results, uncertainty takes a well-defined meaning, defined by the vim [1] and the gum [2], and such a definition and the related evaluation methods cannot be disregarded when evaluating the risk of wrong conformity assessment, as clearly shown in [8]-[13]. this problem is covered by the bipm document jcgm 106:2012 [14], in a very extensive way under a strict metrological perspective, and treating uncertainty according to the gum recommendations [2]. in particular, it covers the problem of stating whether a measured quantity falls inside a given tolerance interval, which is defined in [14] as the “interval of permissible values of a property”. according to the above definition, the tolerance interval can be both a closed interval and a one-sided interval. furthermore, document [14] defines acceptance limits in such a way that, given a measurement uncertainty value, the measurand is declared conforming if the measured value falls inside the acceptance limits and non-conforming when it falls outside these limits. the document considers different decision rules and the way to evaluate the associated risk of incorrect assessment starting from measurement uncertainty. hence, it represents a very useful guide in evaluating the probability of declaring as conforming an item that is not, and vice versa. although this problem is well discussed in [14] from a theoretical perspective, little guidance is provided, from a more practical point of view, on how to set the numerical value of the acceptance limit not to exceed the maximum admissible risk of making a wrong decision (once the measurement uncertainty and the maximum admissible risk are given). this is an important issue when dealing with critical measurements, such as those performed to protect health and environment. this paper, after having quickly reviewed the most used decision rules, proposes a method that, given a threshold (or more in general a tolerance limit), provides the acceptance limit as a function of measurement uncertainty and a predefined maximum admissible risk of exceeding the given threshold. example are given for some of the most used probability distributions. 2. the most common decision rules to correctly evaluate the risk associated with decision rules, it is necessary to identify or assume the probability density function (pdf) representing the distribution of values that could reasonably be attributed to the measurand [2], since this risk can be evaluated only after integrating such pdf from - to the threshold [2], [3]. it is well known that, according to the gum [2], the standard uncertainty u(x) associated with a measurement result x is the standard deviation of the pdf representing the distribution of values that could reasonably be attributed to the measurand. on the other hand, the expanded uncertainty u(x) = ku(x) identifies a coverage interval [x – u(x); x + u(x)], built about the numerical value x of the measurement result, whose coverage probability depends on the assumed probability density function and the considered coverage factor k. it is also worth reminding that the pdf representing the distribution of values that could reasonably be attributed to the measurand depends on the available information. it is generally – and wrongly – considered that the available information comes only from the employed measuring equipment [15], while document jcgm 106:2012 [14] states that such information always has two components: the one available before performing the measurement (called prior information) and the additional information supplied by the measurement. the resulting, or posterior pdf, can be obtained by applying bayes' theorem [14]. keeping in mind the above considerations, it is possible to consider and discuss the two most common and employed decision rules in conformity assessment. it is assumed that the pdfs considered in the following sections are always the posterior pdfs. 2.1. decision rule based on simple acceptance this rule, also known as shared risk, considers accepting as conforming (and reject otherwise) an item whose property has a figure 1. comparison of a measured value with a threshold, when the measured value 𝑥𝑚 is lower (a) and greater (b) than threshold t and measurement uncertainty is not taken into account. figure 2. comparison of an uncertainty interval with a threshold, when the uncertainty interval is completely below (a) and above (b) threshold t. figure 3. comparison of an uncertainty interval with a threshold, when the threshold falls within the interval. acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 3 measured value inside the tolerance interval. in this case, uncertainty is not explicitly considered. in mathematical terms, and assuming that the tolerance interval is given by all values lower or equal than threshold t, an item is accepted as conforming if the measured value xm of property x satisfies to condition 𝑥m ≤ 𝑡. let’s make a few considerations about this decision rule. it can be readily checked that, assuming a symmetrical pdf about xm for the values that could reasonably be attributed to the measurand, the highest probability of exceeding the threshold is obtained in the limit case of xm = t and is 50%, no matter on the evaluated uncertainty value and the pdf. therefore, when this decision rule is applied, measurement uncertainty does not affect the risk: reducing uncertainty only decreases the width of the interval of non-conforming values 𝑥nc that are considered as conforming, but does not reduce the risk of misidentifying nonconforming items as conforming, which still remains 50% (when xm = t). to define a maximum width of the interval of nonconforming values that are considered as conforming, a mutually agreed maximum acceptable expanded uncertainty umax is generally set and it is therefore suggested that the expanded uncertainty u associated to the measured value, for a coverage factor k = 2, must satisfy u  umax [14]. 2.2. decision rule based on guarded acceptance / rejection the simple acceptance rule reported in sec 2.1 shows that the closer the measured value to the threshold, the higher is the probability (up to 50%) of accepting an item as conforming that is not, and vice versa [14]. this probability can be reduced by setting an acceptance limit inside the tolerance interval, as suggested by [14] and as shown in figure 4, when, respectively, the measured value is required to be lower or equal a given threshold (𝑥m ≤ 𝑇𝑈) as in figure 4a and the measured value is required to be within a closed interval (𝑇𝐿 ≤ 𝑥m ≤ 𝑇𝑈) as in figure 4b. figure 4 represents the case of guarded acceptance [14], that is the decision rule for which the risk of accepting a nonconforming item is reduced by setting an acceptance limit au inside the tolerance interval (see sec. 8.3.2 in [14]). according to this rule [14], if the tolerance interval is a onesided interval, upper limited by tu (figure 4a), an acceptance limit au is set inside the tolerance interval. the interval between au and tu (highlighted in yellow in figure 4a) is called the guard band and its width (with sign) is defined as [14]: 𝑤 = 𝑇u − 𝐴𝑈 . (1) in the case of figure 4a, it is w > 0. on the other hand, in the case of a two-sided tolerance interval, two acceptance limits al and au are set, as shown in figure 4b. in this case, two guard bands are obtained, whose widths are defined as wu= tu – au> 0 and wl= tl – al< 0 respectively. from figure 4, it can be conclusded that, when a guarded acceptance decision rule is considered, an acceptance interval smaller than the tolerance interval is obtained. this decision rule is hence in favour of increasing the probability that an accepted item is truly conforming. for the sake of completeness, let’s consider that, with respect to the two cases shown in figure 4 (𝑥m ≤ 𝑇𝑈 and 𝑇𝐿 ≤ 𝑥m ≤ 𝑇𝑈), other two cases exist, that is: • 𝑥m ≥ 𝑇𝐿: in this case, considering guarded acceptance, al is on the right of tl and w= tl – al< 0; • 𝑥m ≤ 𝑇𝐿 ∪ 𝑥m ≥ 𝑇𝑈: in this case, considering guarded acceptance, au is on the right of tu and al is on the left of tl, so that wu < 0 and wl> 0; so that the obtained acceptance interval is smaller than the tolerance interval. a similar, though opposite situation is obtained in the case of guarded rejection [14]. in fact, this decision rule is in favour of increasing the probability that a rejected item is truly nonconforming [14]. in this case, acceptance limits are set, providing acceptance intervals greater than the tolerance interval. without entering the details, as an example, by considering again figure 4a, if the guarded rejection decision rule were applied, then the acceptance limit would be at the right of tu, thus providing a wider acceptance interval. in general, |w| is set as a multiple of the expanded uncertainty: |w|= ru [14]. if the pdf representing the distribution of values that could reasonably be attributed to the measurand is known or assumed, it is also possible to evaluate the risk of declaring a non-conforming value as conforming (or vice versa), as shown in figure 5 in the case 𝑥m ≤ 𝑇𝑈, when a normal pdf is considered and |w| = u = 2u is taken, as suggested by [16]. in particular, figure 5a and figure 5b represent, respectively, the decisions of guarded acceptance and guarded rejection. figure 4. decision rule based on guarded acceptance. in figure 4a a one-sided tolerance interval upper limited by tu is considered, while in figure 4b a twosided tolerance interval is considered between a lower and an upper limit tl and tu. figure 5. example when 𝑥m ≤ 𝑇𝑈 and a normal pdf is supposed. the standard uncertainty 𝑢(𝑥) and the maximum admissible limit tu (red line) are given in table 1, and |𝑤| = 𝑈(𝑥) = 2 𝑢(𝑥) is supposed. the coloured area represents the probability of exceeding tu, when the measured value corresponds to au, in the cases of guarded acceptance (a) and guarded rejection (b). (b) tolerance interval w > 0 acceptance interval acceptance interval tolerance interval wl < 0 wu> 0 (a) guard band acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 4 to understand the relationship between w and the risk of wrong decision, let us consider the numerical example in figure 5, where the maximum admissible limit (mal, or, employing the same notation as the one in [14], tu) for a pollutant in water is assumed to be 50 mg/l. the pollutant concentration is assumed to be measured with a standard uncertainty of 5 mg/l, and the pdf representing the distribution of values that could reasonably be attributed to the measurand is assumed to be normal, as summarized in table 1. according to these values, the expanded uncertainty with k = 2 is u = 10 mg/l. a guard band w = u = 10 mg/l is considered, so that an acceptance limit au = tu – w = 40 mg/l is set, when guarded acceptance is considered. therefore, the concentration of the considered pollutant in water is considered as conforming for every measured value xm  au. if xm = au, the situation shown in figure 5a is obtained, in which the red line is located on tu. since a coverage factor k = 2 has been considered, the coverage probability of interval [xm – u; xm + u] is p = 95.45%. therefore, the risk of exceeding tu, that is the probability pw of taking the wrong decision, when this decision rule is adopted, is 𝑝𝑤 = 1−𝑝 2 = 2.28 %, independently of u. of course, if xm < au, pw < 2.28 %. on the other hand, if the aim of the measurement procedure is to assess, with high probability, that the pollutant concentration is higher than tu, the acceptance limit should be set, according to [14], at au = tu + w. with the same numerical values and assumptions as before, this means that au = 60 mg/l. therefore, the concentration of the considered pollutant in water is considered as non-conforming for every measured value xm  au. if xm = au, the situation shown in figure 5b is obtained, in which the red line is again located on tu. in this case, the risk of declaring that the pollutant exceeds the tolerance limit tu while it does not is again pw = 2.28%. on the other hand, the risk that the pollutant is above the limit is, obviously, 97.7%, 3. the relationship among uncertainty, acceptance limit and the maximum admissible risk the example shown in the previous section relates uncertainty, acceptance limit and the risk of wrong conformity assessment in an implicit way, since it assumes that the distribution of the values that could reasonably be attributed to the measurand obeys to a normal pdf and the generally used coverage factor k = 2 is considered. these assumptions lead to the well-known 2.28 % risk of wrong decision. however, different situations with different pdfs and different values for the acceptance limits may occur in practical cases, where also different values might be desired for the maximum admissible risk (mar) of wrong decisions. therefore, a general formulation relating uncertainty, acceptance limit and mar would be very useful to obtain one of 1 it is worth reminding that a normal pdf is generally obtained when the combined standard uncertainty is obtained as a combination of a sufficiently high number of contributions, so that the central limit theorem applies, as suggested by the gum [2]. on the other hand, the triangular and them, given the other two ones. document jcgm 106:2012 [14] provides some very general indications on how to do this, mostly referring to a normal pdf. attempts were made in the past, especially in the legal metrology domain [17], to set the acceptance limits in such a way that, given the measurement uncertainty, a pre-defined risk could be granted. however, to the authors' knowledge, no practical indications are available to relate acceptance limits, measurement uncertainty, and risk in such a way that, having set two of them, the third one could be found. such a relationship can be obtained starting from the pdf p(x) of the distribution of values that could reasonably be attributed to measurand x. having defined such a pdf, the pertaining cumulative probability distribution function (cdf) can be obtained as: 𝐹𝑋(𝑥) = ∫ 𝑝(𝑡) d𝑡 𝑥 −∞ (2) it can be readily checked that 𝐹𝑋(𝑥) represents the probability that variable x is lower than x. similarly, 1 − 𝐹𝑋(𝑥) represents the probability that variable x is greater than x. therefore, using the same notation as the one used in [14], in the general case shown in figure 4b, given a cdf 𝐹𝑋(𝑥), a tolerance limit tul, and a mar, if, for the considered measurable property, the measured value must be below the given tolerance limit tu, the following inequality must be satisfied: 𝐹𝑋(𝑇u) ≥ 1 − mar , (3) while, if the measured value must be above the given tolerance limit tl, the following inequality must be satisfied: 𝐹𝑋(𝑇l) ≤ mar . (4) therefore, one of the following two equations must be solved to get the value of the acceptance limit au (or al) that ensures that the probability that the tolerance limit tu (or tl) is exceeded is exactly equal to the mar: 𝐴u|𝐹𝑋(𝑇u) = 1 − mar , (5) when the measured value must be below the threshold, or 𝐴l|𝐹𝑋(𝑇l) = mar , (6) when the measured value must be above the threshold. the acceptance limit values ensure that, respectively, if 𝑥𝑚 ≤ 𝐴u (𝑥𝑚 ≥ 𝐴l), then pw  mar, where xm is the measured value and pw is the probability of exceeding the tolerance limit, that is, the probability of wrong decision. of course, solving these equations is strictly related to the shape of the pdf associated with the measurement result, and a solution cannot always be found in closed form. this does not prevent, however, the application of this method, because a numerical solution can be obtained by means of a monte carlo simulation, following the recommendations provided by supplement 1 to the gum [18]. on the other hand, the vast majority of the practical cases consider normal, uniform, triangular or trapezoidal pdfs1. in such cases, a closed-form solution can be readily obtained for au (or al), and, hence, the normal, uniform, triangular and trapezoidal pdfs are considered in the following. trapezoidal pdfs are generally obtained when two uniform pdfs are linearly combined, as in many practical measurement applications. table 1. numerical example. max. admissible limit tu standard uncertainty u pdf type 50 mg/l 5 mg/l normal acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 5 3.1. the measurement results distribute according to a normal posterior pdf when a normal pdf is considered: 𝑝(𝑥) = 1 √2 π σ2 e −(𝑥−𝜇)2 2 σ2 , (7) where 𝜇 is the mean value and 𝜎 is the standard deviation. then, the corresponding cdf is given by: 𝐹𝑋(𝑥) = 1 2 [1 + erf( 𝑥 − μ √2 ⋅ σ )] , (8) where: erf(𝑧) = 2 π ∑ (−1)𝑛 ⋅ 𝑧2𝑛+1 𝑛! ⋅ (2𝑛 + 1) ∞ 𝑛=0 (9) is the error function and can be well approximated with no more than 10 terms in (9). 𝐹𝑋(𝑇u) can, therefore, be written as: 𝐹𝑋(𝑇u) = 1 2 [1 + erf( 𝑇u − 𝜇 √2 ⋅ σ )] , (10) while 𝐹𝑋(𝑇l) can be similarly obtained. in the above equation, 𝜇, the mean value of the normal pdf, represents the measured value 𝑥m of the measurand. therefore, if we want to find au (al), that is the maximum (minimum) value of the measured value such that 𝑝w ≤ mar, 𝜇 = 𝐴u (𝜇 = 𝐴l) must be considered in (10). therefore, according to (5) and (6) 1 2 [1 + erf( 𝑇u,l − 𝐴u,l √2 σ )] ={ 1 − mar if 𝑥m < 𝑇u is required mar if 𝑥m > 𝑇l is required. (11) when 𝑥m < 𝑇u is required, solving equation (11) yields: 𝐴u = 𝑇u − √2 σ ⋅ erfinv(1 − 2 ⋅ mar) , (12) where erfinv is the inverse error function, which is given by: erfinv(𝑧) = ∑ 𝑐𝑘 2 𝑘 + 1 ∞ 𝑘=0 ( √π 2 𝑧) 2 𝑘+1 , (13) where 𝑐0 = 1 and 𝑐𝑘 = ∑ 𝑐𝑚 𝑐𝑘−1−𝑚 (𝑚 + 1)(2𝑚 + 1) 𝑘−1 𝑚=0 . similarly to the error function, also the inverse error function is well approximated with no more than 10 terms in (13). on the other hand, when 𝑥m > 𝑇l is required, solving (11) yields: 𝐴l = 𝑇𝐿 − √2 σ ⋅ erfinv(2 ⋅ mar − 1) . (14) since the inverse error function is an anti-symmetric function, that is: erfinv(−𝑧) = − erfinv(𝑧) , (15) equations (12) and (14) can be grouped into a single equation: 𝐴u,l = 𝑇u,l ∓ √2 σ ⋅ erfinv(1 − 2 ⋅ mar) . (16) therefore, to have a risk lower than mar to exceed the tolerance limit tu (or tl), an acceptance limit au (or al) should be evaluated, obtained by shifting the tolerance limit to the left (or right) by quantity √2 ⋅ σ ⋅ erfinv(1 − 2 ∙ 𝑀𝐴𝑅). in particular: • the limit is shifted to the left when 𝑥𝑚 ≤ 𝑇u is required and guarded acceptance is applied; • the limit is shifted to the right when 𝑥𝑚 ≥ 𝑇l is required and guarded acceptance is applied; • the limit is shifted to the right when 𝑥𝑚 ≤ 𝑇u is required and guarded rejection is applied; • the limit is shifted to the left when 𝑥𝑚 ≥ 𝑇l is required and guarded rejection is applied. to provide a numerical example, let us consider again the example considered in section 2.2 and the values in table 1. let us remember that 𝑥𝑚 < 𝑇u is required and suppose the mar is set to 5 % and guarded acceptance is considered. by applying equation (16), it follows 𝐴u = 41.8 mg/l. figure 6 shows the normal pdf with a mean value equal to the obtained au and standard uncertainty given in table 1. in this figure, the coloured area represents the probability of being above tu, as also reported in table 2. this probability is exactly the set mar. this means that, for every value 𝑥𝑚 < 𝐴u, the probability of exceeding 𝑇u will be lower than 5%. it is therefore possible to set the acceptance limit, given the pdf associated with the estimated measurement uncertainty and the desired mar. 3.2. the measurement results distribute according to a uniform posterior pdf when a uniform pdf is considered: 𝑝(𝑥) = { 1 2𝑎 if 𝜇 − 𝑎 < 𝑥 < 𝜇 + 𝑎 0 otherwise , (17) where 𝜇 is the mean value and 2a is the support of the pdf, which is related to the pdf standard deviation by 𝑎 = 𝜎 ⋅ √3. the corresponding cdf is given by: 𝐹𝑋(𝑥) = ∫ 𝑝(𝑡) d𝑡 = 𝑥 −∞ ∫ 1 2 𝑎 d𝑡 = 𝑥 𝜇−𝑎 1 2 𝑎 .(𝑥 − 𝜇 + 𝑎) (18) figure 6. example when the normal pdf is centered on au = 41.8 mg/l. the coloured area represents the probability of being above tu. table 2. probability of being below or above tu in the case of figure 6. 𝑷(𝒙 < mal) 𝑷(𝒙 > mal) 0.95 0.05 acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 6 and therefore: 𝐹𝑋(𝑇u,l) = 1 2 𝑎 ⋅ (𝑇u,l − 𝜇 + 𝑎) . (19) from (5) and (6), and considering 𝜇 = 𝐴u,l in (19): 1 2a ⋅ (𝑇u,l − 𝐴u,l + a) = = { 1 − mar if 𝑥𝑚 < 𝑇u is required mar if 𝑥𝑚 > 𝑇𝐿 is required . (20) by solving the above equations, the value for the acceptance limit is found: 𝐴u,l = 𝑇u,l ∓ 𝑎 ⋅ (1 − 2 ⋅ mar) (21) which shows that, to have a risk below mar, the acceptance limit must be set to the left or right of the tolerance limit (as detailed in the previou section) by quantity 𝑎 ⋅ (1 − 2 ⋅ mar). to provide a numerical example, let us consider again the example of the pollutant in water considered in section 2.2. let us consider again that tu = 50 mg/l as in table 1, but let us now suppose that the pdf associated with the estimated measurement uncertainty is uniform. let us also suppose that the half-width of this uniform pdf is a = 10 mg/l, that the mar is set to 5 % and that guarded acceptance is applied. by applying equation (21), it follows au = 41 mg/l. figure 7 shows the uniform pdf with a mean value equal to the obtained au. the coloured area represents the probability of being above tu, which is exactly 5 %. this means that every measured value of the pollutant in water lower than au will provide a risk of exceeding tu lower than 5 %. 3.3. the measurement results distribute according to a triangular posterior pdf when a symmetric triangular pdf is considered, its equation is the following: 𝑝(𝑥) = { 𝑦1(𝑥) if 𝜇 − 𝑎 ≤ 𝑥 ≤ 𝜇 𝑦2(𝑥) if 𝜇 < 𝑥 ≤ 𝜇 + 𝑎 0 otherwise, (22) where: 𝑦1(𝑥) = 𝑥 𝑎2 + 𝑎 − 𝜇 𝑎2 (23) and: 𝑦2(𝑥) = − 𝑥 𝑎2 + 𝑎 + 𝜇 𝑎2 (24) and where 𝜇 and 2a are, respectively, the mean value and the support of the pdf. furthermore, 𝑎 = 𝜎 √6 holds, where 𝜎 is the standard deviation of the pdf. to evaluate the corresponding cdf, two situations should be considered, that is the case when 𝑥 ≤ μ and the case when 𝑥 > 𝜇. if 𝑥 ≤ μ, then the cdf is given by: 𝐹𝑋,1(𝑥) = ∫ 𝑦1(𝑡) d𝑡 𝑥 𝜇−𝑎 = ∫ ( 𝑡 𝑎2 + 𝑎 − 𝜇 𝑎2 )d𝑡 𝑥 𝜇−𝑎 = 1 2 𝑎2 ⋅ [𝑥2 + 2𝑥 ⋅ (𝑎 − 𝜇) + (𝑎 − 𝜇)2] = 1 2 𝑎2 ⋅ [𝑥 + (𝑎 − 𝜇)]2 , (25) while, if 𝑥 > μ, the cdf is given by: 𝐹𝑋,2(𝑥) = 1 2 + ∫ 𝑦2(𝑡) d𝑡 = 1 2 𝑥 𝜇 + ∫ (− t a2 + a + 𝜇 a2 ) 𝑥 𝜇 d𝑡 = 1 2 − 1 2 𝑎2 . [𝑥2 − 2 𝑥 ∙ (𝑎 + 𝜇) + 𝜇 ∙ (𝜇 + 2𝑎)] . (26) now, (5) and (6) should be solved for both 𝐹𝑋,1 and 𝐹𝑋,2, thus leading to four equations. however, only the most likely situations are here reported. in fact, when 𝑥 ≤ 𝑇u is required and the mar is supposed to be small (as it should be when environmental, legal or health situations are considered), the situation shown in figure 8a will occur, so that equation (27) must be solved. 𝐴u|𝐹𝑋,2(𝑇u) = 1 − mar . (27) on the other hand, when 𝑥𝑚 ≥ 𝑇l is required and again the mar is supposed to be small, the situation in figure 9b will occur, so that equation (28) must be solved. 𝐴l|𝐹𝑋,1(𝑇l) = mar . (28) equation (27) yields: 1 2 − 1 2 𝑎2 ⋅ [𝑇u 2 − 2 ∙ 𝑇u ⋅ (𝑎 + 𝐴u) + 𝐴u ⋅ (𝐴u + 2𝑎)] = 1 − mar . (29) by solving this simple equation with respect to au, the following second-order equation is found: 𝐴u 2 − 2 ∙ 𝐴u ⋅ (𝑇u − 𝑎) + [(𝑇u − 𝑎) 2 − 2 ∙ 𝑎2 ⋅ mar] = 0 , (30) figure 7. example when the uniform pdf is centered on au = 41 mg/l. the coloured area represents the probability of being above tu. figure 8. example when a triangular pdf is assumed. the coloured area represents the probability of exceeding tu (or tl) when: a) the measured value is required to be below the tolerance limit (𝑥𝑚 ≤ 𝑇u); b) the measured value is required to be above the tolerance limit (𝑥𝑚 ≥ 𝑇l). acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 7 which provides the two solutions: 𝐴u = (𝑇u − 𝑎) ∓ 𝑎 ∙ √2 ∙ mar . (31) among the two above solutions, the one with the minus sign can be discarded. in fact, if we considered a pdf with width 2a and 𝜇 = (𝑇u − 𝑎) − 𝑎 ∙ √2 ∙ mar, this pdf would not cross tu and therefore it would provide a risk of exceeding tu equal to zero. of course, this should be a very lucky situation, but here the limit not to exceed the mar needs to be found and, therefore, the following equation holds, under the assumption that 𝑥 ≤ 𝑇u is required: 𝐴u = (𝑇u − 𝑎) + 𝑎 ∙ √2 ∙ mar . (32) when, on the other hand, 𝑥 ≥ 𝑇l is required, (28) yields: 1 2 𝑎2 ⋅ [𝑇l 2 + 2 ∙ 𝑇𝐿 ⋅ (𝑎 − 𝐴l) + (𝑎 − 𝐴l) 2] = mar . (33) solving this simple equation with respect to al, the following second-order equation is found: 𝐴l 2 − 2 ∙ 𝐴l ⋅ (𝑇l + 𝑎) + [(𝑇l + 𝑎) 2 − 2 ∙ 𝑎2 ⋅ mar] = 0 , (34) which provides the two solutions: 𝐴l = (𝑇l + 𝑎) ∓ 𝑎 ∙ √2 ∙ mar . (35) among the two above solutions, the one with the plus sign can be discarded. in fact, if we considered a pdf with width 2a and 𝜇 = (𝑇l + 𝑎) + 𝑎 ∙ √2 ∙ mar, this pdf would not cross tl and therefore it would provide a risk of exceeding tl equal to zero. of course, this should be a very lucky situation, but here the limit not to exceed the mar needs to be found and, therefore, the following equation holds, under the assumption that 𝑥 ≥ 𝑇l is required: 𝐴𝐿 = (𝑇l + 𝑎) − 𝑎 ∙ √2 ∙ mar . (36) finally, by considering (32) and (36) together, it is: 𝐴u,l = 𝑇𝑈,𝐿 ∓ 𝑎 ⋅ (1 − √2 ∙ mar) , (37) that is, the acceptance limit must be shifted to the left or right of the tolerance limit (as detailed in sec. 3.1) by quantity 𝑎 ⋅ (1 − √2 ∙ mar) to have a risk lower than the mar. as a numerical example, let us consider again the example of the pollutant in water considered in section 2.2. let us consider again that tu = 50 mg/l as in table 1, but let us now assume a triangular pdf associated to the estimated uncertainty. furthermore, the half-width of the triangular pdf is supposed to be a = 10 mg/l, the mar is set to 5% and guarded acceptance is applied. since 𝑥 ≤ 𝑇u is desired, by applying (37), au = 43.2 mg/l is obtained. figure 9 shows the obtained pdf (centered on the obtained au value), where the coloured area represents the probability of being above the tolerance limit tu, which is exactly equal to the pre-set mar (5%). this means that every measured value of the pollutant in water lower than the obtained au value will provide a risk lower than 5 %. 3.4. the measurement results distribute according to a trapezoidal posterior pdf when a symmetric trapezoidal pdf is considered, then the pdf is described by the following equations: 𝑝(𝑥) = { y3(x) if μ − 𝑎 ≤ 𝑥 ≤ μ − 𝑎 β 1 𝑎 ⋅ (1 + β) if μ− 𝑎 β < 𝑥 ≤ μ + 𝑎 β 𝑦4(𝑥) if   μ + 𝑎 β < 𝑥 ≤ μ + 𝑎 0 otherwise, (38) where: 𝑦3(𝑥) = 1 𝑎2 ⋅ (1 − β2) ⋅ (𝑥 + 𝑎 − 𝜇)  (39) 𝑦4(𝑥) = − 1 𝑎2 ⋅ (1 − β2) ⋅ (𝑥 − 𝑎 − 𝜇) , (40) 𝜇 is the mean value of the pdf, 2a is its width and  is the ratio between the two basis. to evaluate the corresponding cdf, three situations should be considered, that is the case when 𝜇 − 𝑎 ≤ 𝑥 ≤ 𝜇 − 𝑎 𝛽, the case when 𝜇 − 𝑎 𝛽 < 𝑥 ≤ 𝜇 + 𝑎 𝛽 and the case when 𝜇 + 𝑎 𝛽 < 𝑥 ≤ 𝜇 + 𝑎. however, similar considerations as the ones made for the triangular pdf apply, so that only the two situations shown in figure 10a and figure 10b are considered. let us call 𝐹𝑋,3(𝑥) the cdf for the case in figure 10a. the following equation must then be solved: 𝐴𝑈|𝐹𝑥,3(𝑇u) = 1 – mar . (41) on the other hand, let us call 𝐹𝑋,4(𝑥) the cdf for the case in figure 10b. the following equation must then be solved: 𝐴𝐿|𝐹𝑋,4(𝑇𝐿) = mar . (42) figure 9. example when the triangular pdf is centered on au = 43.2 mg/l. the coloured area represents the probability of being above tu. figure 10. example when a trapezoidal pdf is assumed. the coloured area represents the probability of exceeding the tolerance limit when: a) the measured value is required to be below the tolerance limit (𝑥𝑚 ≤ 𝑇u); b) the measured value is required to be above the tolerance limit (𝑥𝑚 ≥ 𝑇𝐿). acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 8 it follows: 𝐹𝑥,3(𝑥) = 1 − 𝛽 2 (1 + 𝛽) + 2 𝛽 1 + 𝛽 + ∫ 𝑦4(𝑡) d𝑡 𝑥 𝜇+𝑎𝛽 = 1 + 3 𝛽 2 (1 + 𝛽) + 1 2 𝑎2(1 − 𝛽2) ∙ ⋅ {−𝑥2 + 2(𝜇 + 𝑎) ⋅ [𝑥 − (𝜇 + 𝑎 𝛽)]+(𝜇 + 𝑎 𝛽)2} (43) and, by solving (41), the second-order equation is obtained: 𝐴u 2 − 2 ∙ 𝐴u ⋅ (𝑇u − 𝑎) + (𝑇u − 𝑎) 2 − 2 ∙ 𝑎2 ⋅ mar ⋅ (1 − 𝛽2) = 0, (44) for which the two following solutions can be found: 𝐴u = (𝑇u − 𝑎) ∓ 𝑎 ⋅ √2 ∙ mar ⋅ (1 − 𝛽 2) . (45) according to the same considerations as the ones in the case of a triangular pdf, it can be concluded that, among the two above solutions, the one with the minus sign can be discarded. therefore, the following equation holds, under the assumption that x  tu is required: 𝐴u = (𝑇u − 𝑎) + 𝑎 ⋅ √2 ∙ mar ⋅ (1 − 𝛽 2) . (46) on the other hand: fx,4(x)=∫ y3(t) dt x μ-a = 1 2 𝑎2(1-𝛽2) ⋅[x2+2∙(a-μ)∙𝑥+(𝑎 − 𝜇)2] (47) and, by solving (42) with respect to al, the following secondorder equation is obtained: 𝐴l 2 − 2 ∙ 𝐴l ⋅ (𝑇l + 𝑎) + (𝑇l + 𝑎) 2 − 2 ∙ 𝑎2 ⋅ mar ⋅ (1 − 𝛽2) = 0 (48) equation (48) has two solutions: 𝐴l = (𝑇l + 𝑎) ∓ 𝑎 ⋅ √2 ∙ mar ⋅ (1 − 𝛽 2) , (49) where, according to the same previous considerations, the one with the plus sign can be discarded, so that: 𝐴l = (𝑇l + 𝑎) − 𝑎 ⋅ √2 ∙ mar ⋅ (1 − 𝛽 2) . (50) finally, by considering together (46) and (50), it can be written: 𝐴u,l = 𝑇u,l ∓ 𝑎 ⋅ (1 − √2 ∙ mar ⋅ (1 − 𝛽 2)) , (51) that is, the acceptance limit is simply shifted to the left or right of the tolerance limit (as detailed in sec. 3.1) by quantity 𝑎 ⋅ (1 − √2 ∙ mar ⋅ (1 − 𝛽2)). to provide a numerical example, let us consider again the example of the pollutant in water considered in section 2.2. let us consider again that tu = 50 mg/l as in table 1, but let us now suppose that the pdf is trapezoidal with  = 0.5. furthermore, the half-width of the trapezoidal pdf is supposed to be 𝑎 = 10 mg/l, the mar is set to 5 % and guarded acceptance is applied. since, in the considered example, x  tu is required, (51) yields au = 42.7 mg/l. figure 11 shows the obtained pdf (centered on the obtained au value), where the coloured area represents the probability of being above the tolerance limit tu and it is exactly equal to the pre-set mar (5 %). this means that every measured value of the pollutant in water lower than the obtained au value will provide a risk lower than 5 %. 4. conclusions following the suggestions given in the present standards [14] [16], a measurand is considered as conforming when the measured value falls inside the acceptance interval, as defined in art. 3.3.9 of [14], and is considered non-conforming when the measured value falls inside the rejection interval, as defined in art. 3.3.10 of [14]. the definition of the acceptance interval strongly depends on the measurement uncertainty with which the measurand is measured and the admissible risk of declaring a non-conforming value as conforming and vice versa. while this problem is clearly highlighted in [14], very few practical indications are given on how to define the acceptance limits once the measurement uncertainty is estimated, and the maximum admissible risk (mar) given. this paper has shown how the acceptance limits depend also on the probability density function (pdf) considered to represent the distribution of values that could reasonably be attributed to the measurand, and has proposed a general method to relate them to the considered pdf and the considered mar. the most used normal, uniform, triangular and trapezoidal pdfs have been considered and general formulas have been given to define the acceptance limits given uncertainty and mar. the numerical examples have shown that different results are obtained for the acceptance limits, when the different pdfs are considered, as expected from the theory. the closed-form formulas provided in the paper allow one to evaluate the acceptance limits in a straightforward way, in both situations of guarded acceptance and guarded rejection. should different probability distributions be considered, the general proposed method can still be applied, and a monte carlo simulation can provide the desired acceptance limits. references [1] jcgm 200:2012. international vocabulary of metrology – basic and general concepts and associated terms (vim 2008 with minor corrections). joint committee for guides in metrology. 2012. [2] jcgm 100:2008. evaluation of measurement data – guide to the expression of uncertainty in measurement, (gum 1995 with minor corrections). joint committee for guides in metrology. 2008. figure 11. example when the trapezoidal pdf is centered on au = 42.7 mg/l.. the coloured area represents the probability of being above tu. acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 9 [3] a. ferrero, v. scotti. uncertainty and conscious decisions, in: forensic metrology: an introduction to the fundamentals of metrology for judges, lawyers and forensic scientists. cham: springer international publishing, 2022, pp. 115–124. isbn: 9783-031-14619-0. doi: 10.1007/978-3-031-14619-0_8 [4] m peterson, an introduction to decision theory. cambridge: cambridge university press, 2009, p. 317. isbn: 9780511800917. doi: 10.1017/cbo9780511800917 [5] c yoe, principles of risk analysis: decision making under uncertainty. 2nd ed. boca raton: crc press, 2019, p. 848. isbn: 9780429021121. doi: 10.1201/9780429021121 [6] r. m. peterman, j. l. anderson, decision analysis: a method for taking uncertainties into account in riskbased decision making, human and ecological risk assessment: an int. journal 5.2 (1999), pp. 231–244. doi: 10.1080/10807039991289383 [7] b. solaiman, d. guériot, sh. almouahed, b. alsahwa, é. bossé, a new hybrid possibilistic-probabilistic decision-making scheme for classification, entropy 23.1 (2021). issn: 1099-4300. doi: 10.3390/e23010067 [8] l. r. pendrill, using measurement uncertainty in decision-making and conformity assessment, metrologia 51.4 (2014), pp. 206–218. doi: 10.1088/0026-1394/51/4/s206 [9] a. allard, n. fischer, i. smith, p. harris, l. pendrill, risk calculations for conformity assessment in practice. ed. by array, 19th international congress of metrology cim, paris, france, 2426 september 2019. doi: 10.1051/metrology/201916001 [10] s. puydarrieux, j. m. pou, l. leblond, n. fischer, a. allard, m. feinberg, d. el guennouni, role of measurement uncertainty in conformity assessment. ed. by array, 19th international congress of metrology cim, paris, france, 24-26 september 2019. doi: 10.1051/metrology/201916003 [11] e. cruz de oliveira, f. r. lourenço, risk of false conformity assessment applied to automotive fuel analysis: a multiparameter approach, chemosphere 263 (2021), p. 128265. issn: 0045-6535. doi: 10.1016/j.chemosphere.2020.128265 [12] l. separovic, r. s. simabukuro, a. r. couto, m. l. g. bertanha, f. r.s. dias, a. y. sano, a. m. caffaro& f. r. lourenço, measurement uncertainty and conformity assessment applied to drug and medicine analyses – a review, critical reviews in analytical chemistry 0.0 (2021), pp. 1–16. doi: 10.1080/10408347.2021.1940086 [13] m. h. habibie, o. hedrony, using decision rule in calibration of long gauge block on the conformity assessment scheme, iop conference series: materials science and engineering 673.1 (dec. 2019), p. 012110. doi: 10.1088/1757-899x/673/1/012110 [14] jcgm 106:2012. evaluation of measurement data – the role of measurement uncertainty in conformity assessment. joint committee for guides in metrology. 2012. [15] j. m. pou, l. leblond, smart metrology: from the metrology of instrumentation to the metrology of decisions. ed. by array, 18th international congress of metrology cim, paris, france, 19-21 september 2017. doi: 10.1051/metrology/201701007 [16] iso en 14253-1. geometrical product specification (gps) inspection by measurement of workpieces and measurement instruments part 1: decision rules for proving conformance or non-conformance with specification. iso en 14253-1. 1998 m [17] h. källgren, l. pendrill, b. magnusson. role of measurement uncertainty in conformity assessment in legal metrology and trade, accreditation and quality assurance 8 (2003), pp. 541–547. doi: 10.1007/s00769-003-0707-8 [18] jcgm 101:2008. evaluation of measurement data – supplement 1 to the guide to the expression of uncertainty in measurement – propagation of distributions using a monte carlo method. joint committee for guides in metrology. 2008. https://doi.org/10.1007/978-3-031-14619-0_8 https://doi.org/10.1017/cbo9780511800917 https://doi.org/10.1201/9780429021121 https://doi.org/10.1080/10807039991289383 https://doi.org/10.3390/e23010067 https://doi.org/10.1088/0026-1394/51/4/s206 https://doi.org/10.1051/metrology/201916001 https://doi.org/10.1051/metrology/201916003 https://doi.org/10.1016/j.chemosphere.2020.128265 https://doi.org/10.1080/10408347.2021.1940086 https://dx.doi.org/10.1088/1757-899x/673/1/012110 https://doi.org/10.1051/metrology/201701007 https://doi.org/10.1007/s00769-003-0707-8 acta imeko  issn: 2221‐870x  june 2015, volume 4, number 2, 72‐79    acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 72  effectiveness of filters in 3 kv dc railway traction substations  supplied by distorted voltage–measurements and diagnostics  adam szeląg, tadeusz maciołek, marek patoka   warsaw university of technology, institute of electrical machines, pl. politechniki 1, 00‐661 warsaw, poland         section: research paper   keywords: dc traction substation; harmonics; filters; voltage distortion; railway engineering; measurements   citation: adam szeląg, tadeusz maciołek, marek patoka, effectiveness of filters in 3 kv dc railway traction substations supplied by distorted voltage– measurements and diagnostics, acta imeko, vol. 4, no. 2, article 13, june 2015, identifier: imeko‐acta‐04 (2015)‐02 ‐13  editor: paolo carbone, university of perugia, italy  received november 1, 2014; in final form february 3, 2015; published june 2015  copyright: © 2015 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: this work was supported by research funds of pkp energetyka s.a.  corresponding author: adam szeląg, e‐mail: adam.szelag@ee.pw.edu.pl    1. introduction  quality criteria for a dc traction supply system concern the circuits from the rectifiers installed in traction substations till the vehicle’s current collector. technical effectiveness of the applied supply system as well as quality of the provided energy are assessed mainly by fulfilment of the following requirements [1], [3], [7]-[9], [12]-[16], [18]-[20], [22], [25], [26]: a) required conditions for voltage of rolling stock supply (of values and quality of the provided voltage, including relations to the harmonics content) are fulfilled, and voltage drops in a supplying dc network are low enough not to significantly influence the speed of train traffic; b) admissible levels of shortand long-term loads will not be exceeded in any element of the supply system (supply devices); c) safety of passengers, personnel as well as other persons is provided; d) short-circuit identification and reliability of switching-off (both on the dc and ac side of power supply); e) the system remains open for the increase of traffic – possibility of expansion with further increase in transport demand by its modernisation, depending on a degree in traffic increase until achieving the target traffic anticipated in a forecast. in the analysis of technical effectiveness of the examined load variants, one states conditions for fulfilling the assumed technical standards of a supply system – reliability and dependability of supply, admissible voltage drops and efficiency for the assumed circuits configurations [21]. assumptions regarding train traffic and configuration of a supply system allow for assessment of a supply system functioning during peak traffic hours and correspond to the conditions assumed for dimensioning of system devices in a design process, including equipment on the side of dc voltage [2], [23]. abstract  electric energy quality criteria relating to a dc supply system concern the circuits from the rectifiers installed in traction substations till  the  vehicle's  current  collector.  energy  law  and  related  implementing  provisions  unequivocally  state  that  an  electrified  transport  system, as the energy recipient, shall fulfil energy consumption requirements that are defined in the agreement. it imposes certain  conditions for the railway power supply company. introduction into traffic of the traction vehicles that are equipped with converter  power electronics drive systems  increased requirements regarding voltage quality  in a dc catenary. at the same time,  increase  in  share of non‐linear recipients causes the increase of distortions in ac voltage supplying traction substations, which also transfer to the  dc side. in some cases,  disturbances in operation of low power infrastructure, such as signaling and control  circuits, were observed,   although energy quality parameters at the ac side have been fulfilled. all these factors caused a change in operational conditions of  the resonance smoothing filters hitherto used in the rectifier traction substations that are supplied by ac medium voltage power lines.  this paper presents a research and a case study of the problem of effectiveness of functioning of the applied resonance filters, from  measurements, allowing for problem identification, to the results of the studies of the proposed new solution of a filter and the results  of the observed exploitation of a prototype  with the application of a digital monitoring and diagnostics system.  acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 73  dc output voltage of rectifier traction substations consists of ac components – harmonics, which could influence negatively the operation of various devices, e.g. a railway control and signalling system [3], [10], [23]-[26], and might disturb reception of broadcasting services in the areas close to the substation [22]. ac side current harmonics of rectifiers in traction substations cause distortion of voltage in supplying ac power lines and in a grid point, to which both a railway traction substation and other loads are connected [2], [4]-[6]. 1.1. dc‐side voltage harmonics   the variable component of the dc side voltage is composed of a set of harmonics [2], whose orders n equal: n = c p (1) where c = 1, 2, 3, 4, ... and p=6, 12 (number of pulses of a traction rectifier). therefore, the content of higher harmonics in the rectified voltage will differ in various rectifier systems. for instance, in six-pulse systems the harmonics of the 6th, 12th, 18th and 24th order occur (harmonics of higher orders are usually omitted in technical considerations, due to their low values), whereas in 12-pulse systems only harmonics of the 12th and 24th order occur, thus the 12-pulse system has lower content of higher harmonics than the 6-pulse system. un rms of individual higher harmonics of voltage with a defined value of rectifier voltage is the lower, the higher n harmonics order is. for an idle state of a rectifier operation state the following dependency for un is used [2], [6], [23]: . 12 2 doin u n u   (2) knowing un, a total group variable component can be calculated:  g n n u d nc u 2 (3) where nd, ng – orders, respectively: lower and upper order of a sum of harmonics. the relative value of a variable component of the rectified voltage in the n idle state can also be determined according to: %.100 doi c u u n  (4) under conditions of an idle state, for p = 3 the n coefficient is within 15.3 %, for p = 6 – 4.2 %, while for p = 12 the value is 1.04 %, hence the content of a variable component in the rectified voltage is evidently decreasing with an increase of the number of rectifier pulses. voltage of individual higher harmonics un increases proportionally with the increase of the commutation angle u, thus their rms value un%(u) given in percentage of udoi can be calculated as: )sin()sin(2)cos()cos(2)(sin)1(2 2)1( )( 22 2% nuunnuuun n u uu doin    (5) harmonics values for declared operational overloads of rectifier units that are obtained from this dependency constitute a basis for calculations for smoothing devices installed in traction substations. already at the stage of design, one should therefore determine the means that would enable decreasing disturbing voltages uz to admissible levels, as stated in relevant regulations and recommendations. currently in poland, the admissible level of uz equivalent voltage, taking into account the psophometric weight, is defined by equation (6)[8], [23]:  ) 2 (  nnz uu (6) where: un – rms of nth harmonic, n – coefficient of a psophometric weight of the nth harmonic [8] the value of this coefficient equals 16.5 v in poland, while in germany and in italy it equals 10 v. in some countries, the maximum admissible value of a single harmonic (e.g. 100 v) is determined as well, which in fact concerns the most significant non-characteristic harmonic of 100 hz frequency. however, suitability for use of the psophometric weight criterion for the dc side voltage raises doubts, since in relation to track circuits the use of the current criterion is of more importance. for instance, current limits given in [3] determine assessment of possible disturbances during introduction of a new rolling stock. this criterion does not have restrictions for the frequency range between 601380 hz, that is for a range including the harmonics with higher amplitudes (300, 600, 900, 1200 hz) that are filtered through the resonance contours of dc side filters. therefore, it can be difficult to fulfil current limits for 1500, 1800, 2100 and 2400 hz [3], [26] harmonics (especially for filters with a small inductance), even though the voltage criterion (6) has been fulfilled. 1.2. dc side filters   in order to decrease an ac component on the dc voltage side, there are widely used filters that seek for decreasing higher harmonics that are introduced to a catenary. it is performed by introducing a serial branch and a low-impedance (for certain harmonics) parallel branch that is shunting the traction network. resonance (poland, russia, czech republic, slovakia, spain, slovenia, croatia, japan, republic of south africa, india, algeria, morocco) and aperiodic filters (italy) are commonly used as smoothing devices [2], [23]. a typical scheme of resonance filters used on polish state railways is presented in figure 1. figure 1. a simplified scheme of a filter with two resonance contours (600  and 1200 hz) used in 12‐pulse 3 kv dc traction substations.  acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 74  the effectiveness of filters for each frequency is established using the wn attenuation coefficient (smoothing), which is defined as a ratio of the rms value of voltage uon of nth higher order harmonic at the output of the filter to the rms value of the same voltage higher harmonic un at the output of a rectifier (at the input of the filter), this coefficient assumes the following value: n on n u u w  (7) the basic disadvantages of resonance filters include:  a low level of attenuation with fluctuation of the supplying voltage frequency,  possibility of strengthening certain harmonics as a result of resonance phenomena (especially with supply of vehicles that are equipped with converter systems),  low stability of characteristics in time and necessity of periodic tuning. in case of introduction into traffic of locomotives with power electronic devices (a chopper drive or an inverter with an ac motor), disturbances from higher harmonics in a traction catenary will increase, while additional current harmonics might cause overload of filter elements. furthermore, the phenomenon of transmission of higher harmonics by a traction catenary might happen in case of a substation cooperating with different types of rectifiers. locomotives equipped with converter drives, which generate higher harmonics of current, usually have low-pass filters of lc type as an input. due to the distributed character of passive elements and current (vehicles) as well as voltage (substations) type of harmonics sources, a scheme of a power supply circuit of energy delivery from a 3-phase public network to the 3 kv dc catenary supplied electric vehicle (ev) is significantly complicated (figure 2). so while designing solutions for the main circuit traction vehicles with asynchronous drive, it is necessary to take into consideration and analyse the mutual impact of a supply system (substations, substation’s smoothing filters) on the circuits of signalling, control and communication systems. the frequency spectrum and impact of an ac component of the vehicle’s current on other circuits depend on the method of control, an operating point and converter frequency as well as parameters of other circuits [3], [9], [12], [16], [22]-[24]. in some cases, a locomotive with a converter drive can constitute a lower impedance for harmonics occurring in the dc voltage of a traction substation (especially of lower orders and non-characteristic ones) than the conventional drive with dc motors and resistor start-up [24], [26]. the input filter of a vehicle is a damping circuit, which might cause occurrence of damped oscillations with a frequency corresponding to the frequency of own oscillations in the case of sudden changes of voltage in a traction catenary. when a drive is operating at constant power [12], the phenomenon of excitation of oscillations with a frequency equal to a traction vehicle input filter’s own frequency is known. 2. measurements of the quality of ac voltage  supplying a traction substation  the problem of the quality of electric energy that is delivered to the consumers from the electricity grid concerns both suppliers and consumers of electrical energy. it imposes requirements so as operators of the power supply network supply the recipients with the energy of appropriate quality. the important quality parameters [1]-[7], [15], [25] include the admissible level of non-linear distortions (thd total harmonic distortion) as well as admissible load fluctuations (due to the instability of the receiver) and asymmetry. one should pay particular attention to the criteria of energy quality, which are stated in the standard en 50160 [18], especially with supply of a traction substation with lines having various parameters. traction substation equipped with 12-pulse rectifiers served as an example of the negative influence of a distorted supply voltage on the operation of a 3 kv dc traction system, due to the fact that the phenomenon of fuses deterioration occurred in substation’s resonance type filter. the adjacent substations had also smoothing devices of the resonance type installed, adjusted for harmonics of the 12th and 24th order, which are characteristic for the 12-pulse rectifiers. thus, in order to determine the cause of occurrence of such phenomena, it was important to conduct measurements at 15 kv 50 hz voltage supplying substations. measurements of energy quality were performed in 3-phase 15 kv ac lines supplying a substation, using a specialised meter for measurement of the quality of electrical energy that is consumed by a traction substation. exemplary results waveforms of averaged, 10-min values of voltage harmonics in one of the phases (figure 3a) and thd u values for all phases (figure 3b) in an idle state of a supplying line (without the operating rectifier substation) are presented. one can observe the presence of the 5th harmonic in the supply voltage (up to 5 % at the admissible value of 6 %). the remaining harmonics had far lower values below 1 %. global thd u coefficient exceeded 5 % at the admissible value of 8 %, so the limits imposed by [19] were not exceeded. 3. measurement of effectiveness of a dc side  resonance filter with distorted ac voltage  supplying rectifiers  measurements of rms values of the filter's current on the 3 kv dc side were conducted at various configurations of operation of rectifier units and at different variants of operation of adjacent substations with the use of a specialised fibre optical measurement (figure 4) system established at the warsaw university of technology. in this case, 12-pulse rectifiers were being exploited in the traction substation, thus a filter was devoted to attenuate 600 hz and 1200 hz harmonics (resonance contours) (figure 1) and the harmonics of higher orders by a less significant element an aperiodic element. the fifth harmonic occurring in the supply voltage (figure 3) [2], [20], [23] (although the limit imposed by [19] was not exceeded) causes occurrence, on the dc side, of a 6th voltage harmonic (300 hz), which is non-characteristic for a 12-pulse rectifier (1). figure  2.  a  scheme  of  a  circuit  of  electrical  energy  delivery  from  an  ac power supply network (psn) to a rectifier traction substation with a dc side filter and via catenary to an electric vehicle (ev).  acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 75  therefore, a resonance filter lacks the resonance element for this frequency (this device was designed assuming that this harmonic does not occur in the rectified voltage), and thus it is not attenuated. while in case of operation of two rectifier units the 6th harmonic appeared to be additionally enhanced. this phenomenon causes current of this harmonics (300 hz) in the smoothing devices to reach considerably large values (figure 5a), while the main characteristic harmonic current (600 hz) was much lower, and with the increase in load, the global current flowing was causing a filter fuse to blow.  additionally, what appeared is the 2nd harmonic (100 hz) – figure 5, that is caused by supply voltage asymmetry and which is additionally enhanced by a filter. the undertaken measurements showed:  influence of a supply configuration of a traction substation and the level of distortions coefficient thd u on the energy quality on a dc side;  significance of ac supply voltage asymmetry in the operation of the resonance filters;  influence of the rectifiers configuration in a substation on the value of the filter current and the content of current harmonics of the resonance smoothing device (exemplary waveform of rms values of current harmonics in time is presented in figure 3a). due to exploitation needs, in order to allow temporary operation of the filter, it was decided to increase the value of an aperiodic part (capacitance 100 f – figure 1) from 100 to 350 f, and as a result the 300 hz harmonic current value was reduced significantly (figure 5). figure  4.  measurement  system  in  a  dc  side  filter  cabinet  in  a  3  kv  dc  traction substation (c‐ capacitance, b‐ fuse).                          a)                      b)  figure 3. the level of higher harmonics in a 15 kv line that operates in the idle  state,  participation  (of  individual  voltage  harmonics  a)  thd  u  in particular phases‐b).  a) b) figure 5. rms values of higher harmonics of substation's filter current a) with a  resonance  filter,  b)  with  a  resonance  filter  and  an  expanded  aperiodic element from 100 to 350 f.  0.1 1 10 100 150 250 2400hz 1800hz 1200hz 200hz 100hz 50hz 900hz 600hz 300hz 1800 hz 2400 hz50 hz 900 hz 100 hz 200 hz 1200 hz 600 hz 300 hz t [s] i n [ a ] 0.1 1 10 100 250 500 750 2400hz 1800hz 1200hz 900hz 600hz 300hz 200hz 100hz 50hz 100 hz 1200 hz 200 hz 600 hz 300 hz t[s] in rm s [a ] acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 76  4. modification of a smoothing dc side filter  experience from exploitation of resonance filters and the introduction of a high-voltage supply of 3 kv dc traction substations on polish railways required further analyses and study for several new phenomena that do not occur with substations equipped with previously used resonance filters [11], [17]. this concerned, among others, mutual cooperation (bilateral supply of the same catenary section) of a traction substation and a supply system both on ac and dc sides, taking into account:  states of normal operation (including cooperation with high power vehicles equipped with drives with power electronic converters and adjacent substations with resonance filters);  the most difficult operating and emergency state conditions so the degree of exposure of individual elements to overvoltage and excessive current, e.g. during short-circuit, had to be considered (figure 6). in developing the concept for solutions of a system filter based on the conducted analyses, preliminary work and measurements, the following requirements regarding a dc side filter have been formulated: a) providing proper functioning under operating conditions of a traction substation, so as not to exceed admissible values of distortion; b) limiting the influence of higher non-characteristic harmonics, caused by supply voltage asymmetry, on the value of interference voltage at the dc side; c) limiting the impact of frequency fluctuations of supply voltage on the characteristics of a filter; d) providing proper operation of a filter and limiting its interaction (coupling) with the filters of adjacent substations and input filters of locomotives equipped with converter drives; e) limiting the generated switching overvoltages to the values lower than the withstanding overvoltage of a rectifier and dc circuit elements, e.g. high-speed circuit-breaker. the source of overvoltages in the circuits of substations’ filters is the inductance of a circuit during breaking the load or short-circuit currents by the high-speed circuit breakers of the feeders. the values of overvoltages depend on the current derivative di/dt and the value of inductance in the circuit during shortcircuit (including inductance of a choke and catenary). the higher value of choke inductance decreases the steepness of current rising and subsequently the steepness of current decay with short-circuit breaking; however it linearly increases the value of the generated overvoltage on a choke. switching overvoltages occur in the circuits of a substation on the rectifiers terminals (figure 7), irrespective of the type of the applied filter, while their values and duration depend on inductance of chokes and the values of capacitance (aperiodic branch) connected in parallel behind the choke. another important criterion is the overvoltage withstanding value of a rectifier as well as circuit elements to commutation and switching overvoltages in the circuit of a substation. for instance, in case of short-circuit breaking using the only operating feeder, the overvoltage will appear between the “+” and ““ rectifier terminals, and not in a catenary. therefore, it is so important that the manufacturers define the actual withstanding voltage value of a rectifier, having regard to the transformer and high-speed circuit breaker. it concerned, among others, parameters such as: voltage connected to the “+” and “” terminals from the side of a catenary; rectifier insulation; transformer insulation; overvoltage withstanding value of a chamber of a high-speed circuit breaker after breaking the short-circuit current. the last one is of high importance, especially during possible occurrence of multiple short-circuits across a high-speed breaker chamber when its insulation parameters are lost. what also proved to be beneficial was equipping the filter with the elements limiting the overvoltage value (coordinated with overvoltage withstanding value of a rectifier) and (necessity) discharge resistors for discharging the energy accumulated in the capacitance of a filter as a result of overvoltage as well as discharging the capacitance after decay of rectifier (breaking) voltage [11], [17], [23]. since the direct connection of a not charged set of capacitors to voltage will cause surge charging of a capacitor, hence charging current should be limited so to avoid damaging the capacitor. similarly, high-current short-circuit in a catenary (with short-circuit resistance close to zero) will cause impulse and oscillation discharge of capacitor overload, which may result in its damage. current impulses and oscillations in the circuit of a filter and dc switchgear cause excessive wear-out of contacts and circuit elements, so additional low-value power resistors in series with the filter’s capacitance were added. during trials, ongoing intermittently for a few years, the above mentioned assumed requirements towards the dc side filters were verified by performing the following studies:  before including the filter in the operation: • measurements of filter’s characteristics; • testing voltage insulation and resistance; • checking the correct functioning of the auxiliary circuits;  while putting the filter into operation, checking the correct selection of a filter’s protection fuse;  the test under load was performed (heating of the elements, checking the transient states) and with connecting and switching-off the subsequent rectifier units;  checking discharging time of capacitors with the filter off figure  6.  short‐circuit  current  isc  at  the  output  of  a  3  kv  dc  traction  substation  (lower  figure)  and  capacitance  of  a  filter’s  current  –ic  (upper  figure)  –  the  observed  peak  current  is  caused  by  a  sudden  capacitance  discharge.  figure 7. voltage uc at capacitance terminals during a short‐circuit clearing. acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 77  as well as checking the proper selection of discharging resistors;  checking the elements of a filter and the correct selection of a filter protection fuse with maximum value shortcircuit in a 3 kv catenary, functional testing of a main circuit and auxiliary circuits (signalling of a fuse blow, capacitors damage etc.);  measurements of the filter operation effectiveness (under conditions that were attained on a substation that has been selected for the test study purpose) with one and two rectifier units for existing quality conditions of 15 kv voltage that supplies the substation and with various schemes of bilateral supply of a substation on the 3 kv side with section cabins and adjacent substations operating or not. the assumed specifications of a filter have been confirmed by in-situ measurements of:  characteristics of a filter, before starting operation in the substation,  characteristics of filter attenuation acc. to (4),  harmonics voltages before and behind the filter,  filter’s current spectrum during normal operation,  current of a filter during high-current short-circuit test in a catenary,  calculated disturbing psophometric voltage uz acc. to (6). the presented measurements and theoretical analyses showed, in certain conditions, the increase of the 100 hz harmonic that occurs at the dc side of the substation in case of ac supply voltage asymmetry. filters with the resonance contours can efficiently attenuate the specific harmonic by parallel contours tuned to its frequency, but they may enhance (or do not attenuate) some harmonics (between the frequencies of resonance elements) and they require periodic tuning. hence, what was proposed was a modified filter system according to the patents [11], [17]. lc filters have the appropriate low-pass characteristic of a better attenuation (4) coefficient (figure 8) and they do not require tuning, but the necessary element is the high capacity of the 400÷800 f range, in order to efficiently attenuate lower harmonics. the disturbing voltage (3) for the analysed conditions with lc filters is considerably lower than the admissible value of 16.5 v and lower than for the substations operating with resonance filters thereto used (calculated disturbing voltage for a lc filter is 5 v, while for the thereto used resonance filter it is 17.1 v). lc filters are also efficient with the increase of distortions in the ac voltage supplying the substation [23]. 5. monitoring and diagnostics of filter's  operation in a traction substation  in order to monitor operating conditions of new filters, czat3000plus microprocessor controllers are used for traction substations; they were used in many automation systems developed by pkp-elester [27]. the system of czat 3000plus controllers has a modular structure and is a part of a set with command and registration modules and measurement transducers hvm (high voltage measurement) or voltage transformers and temperature sensors. individual czat3000plus controllers are connected with an object cabinet of remote control via the can-bus/rs485, which eliminates the necessity of laying a large number of control cables from an object cabinet for controlled devices. waveforms of the filter current (figure 9) and the dc voltage at the output of a substation as well as the temperature in a filter’s chamber are measured on-line and are used in the automation and security systems, especially for undervoltage protection, overvoltage protection, overcurrent protection and temperature protection. selected, measured values are registered in a memory card (maximum instantaneous values and averaged values per 1 s), and they are used for evaluation of the filter's operating conditions. one might notice the increased load of a filter with the increased load of a substation (figure 10a) and decrease of the filter's load in the occurrence of regenerative braking of vehicles (figures 10b, 11b). additionally one might observe the increase of the filter's load with decrease of capacitance voltage caused by the increase of the substation’s load by the trains and decrease of current of the filter's load with decrease of the substation's load (figures 10a, 11a). also, the presence of a short term increase of capacitance voltage above the value of 3600 v under the conditions of occurrence of trains regenerative braking has been noted (figures 10b, 11b). figures 10a, 10b, 10c and d present the exemplary results of registration from the czat3000 controller waveforms of u voltage on a filter's capacitance (output voltage from a 3 kv dc substation) upper figures and i current of filter's load (bottom figures). figure 9. exemplary time‐curve of a filter current.  figure  8.  comparison  of  attenuation  characteristic  (4)  of  a  thereto  used resonance filter (signed as: 12‐p rez) with a 12‐pulse unit and lc filter (lc 12‐p),  noticeably  better  attenuation  of  a  lc  filter  with  the  supply  of  a substation with distorted and asymmetric voltage, when 300 hz harmonic is present.  acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 78  6. conclusions  the paper focuses on the analysis, based on measurements and monitoring of the effectiveness of applying dc side filters in 3 kv traction substations supplied by a 3-phase ac medium voltage line with distorted and asymmetrical voltage, but still having energy quality standards [19] fulfilled. using the results of the conducted analyses, studies and measurements as well as experience in filters' exploitation, it can be stated that:  upon measuring the quality of energy supplied from a supplying ac line a selection of the scheme and parameters of a smoothing filter for a dc traction substation should be performed to assess the level of voltage distortions. it is worth underlining that even in case the energy quality standards [19] are fulfilled, it is not enough to ensure proper operation and effectiveness of operating resonance dc side filters and compatibility of track circuits and rolling stock equipped with power electronic converters;  the use in new traction substations supplied by 15 kv voltage of lc filters with higher inductance values than these hitherto used, improved the filters’ efficiency and eliminated problems of fuse burn-out, even in case of distorted ac voltage supplying a traction substation. lc filters do not require tuning and have higher effectiveness (figure 8) than resonance filters in the range of frequencies above the last resonance element a)    b)  figure 10 a, b. exemplary waveforms of u output voltage (upper figure) and  i filter's current (bottom figure) from registration via czat controller  in a traction substation.         a)    b) figure  11.  a,  b.  exemplary  waveforms  of  u  output  voltage  (upper  figure)  and i filter's current (bottom figure) from registration via czat controller in  a traction substation. acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 79  (1.3÷2.4 khz), a range significant due to the accuracy of operation of track circuits and cooperation with modern traction vehicles equipped with converter drive systems) [3], [16], [23], [26]. furthermore, in the ranges between the frequencies of resonance elements, in resonance filters there occur strengthening (or considerable weakening of attenuation) of certain harmonics, which is unfavourable, especially in case of substation supply by distorted and asymmetric ac voltage (currently, these are extremely common phenomena);  for a few years now, in one of the 3 kv dc traction substations operated by pkp energetyka s.a., the supervised exploitation of a prototype of a lc filter with the use of monitoring and diagnostics with a digital czat controller constructed by elester-pkp has been conducted. the exploitation has proved the feasibility of the changes and effectiveness as well as reliability of new solutions. acknowledgement  the authors would like to express their gratitude to colleagues from companies elester pkp and pkp energetyka s.a. for cooperation during measurements and delivery of data from diagnostic systems. references  [1] j. altus, m. novak, a. otcenasova, m. pokorny, a. szeląg, “quality parameters of electricity supplied to electric railways, scientific letters of the university of żilina-communications”, no 2-3/2001. [2] j. arillaga, r. w. neville, “power system harmonics”, second edition, john wiley&sons, ltd, 2003. [3] a. białoń, a. szeląg, w. zając, “disturbing influence of electric traction vehicles on signalling and control circuit on silesian regional railway”, international symposium on electromagnetic compatibility and electromagnetic ecology emc. vol. 95., emc 95 eme, 1995, saint-petersburg, pp. 222-224. [4] s. bolkowski, w. brociek, r.wilanowicz, “comparative analysis of influence of the type line supplying nonlinear load on deformation of voltage and current in the power system”, computer applications in electrical eng, vol.11, 2013, pp. 11 23. [5] w.brociek, r.wilanowicz, z. filipowicz, “frequency characteristics of the power line with nonlinear load”, przegląd elektrotech-niczny, no 4, 2009, pp. 62-64. [6] w. brociek, r. wilanowicz, “estimation of higher voltage and current harmonics generated by nonlinear load”, przegląd elektrotechniczny no 1, 2010, pp.141-143. [7] k. buchta, a. szeląg, “application of statistic and probabilistic methods for assessment of quality of 3kv dc network energy delivery to traction vehicle”, chapt. 4. modern electric traction: volume 2 power supply (ed. k., karwowski, a., szeląg), 2009, politechnika gdańska, pp. 22-46. [8] ccitt directives concerning the protection of telecommunication lines against harmful effects from electric power and electrified railway lines. vol.ii, iii, iv, geneva 1989. [9] p. c. coles, m. fracchia, r.j. hill, p. pozzobon, a. szeląg, “identification of catenary resonance conditions on 3 kv dc traction systems”, 7th mediterranean electrotechnical conf. melcon'94, antalya, turkey, 12-14 april 1994, pp.825-828. [10] w. czuchra, m. kowalczewski, w. zając, “analiza przyczyn odkształceń napięć wyjściowych prostownika 12-pulsowego na podstacji trakcyjnej” (in polish), 6th int. conference “modern electric traction in integrated xxist century europe”, met’ 2003, warsaw, pp. 17-21. [11] gamma filter for 3 kv dc traction substations with 12-pulse rectifiers supplied by medium voltage lines). technical documentation (in polish), 2011, pkp energetyka s.a. [12] k. karwowski, j. skibicki, “analiza stabilności pracy sieciowych pojazdów z napedem energoelektronicznym” (in polish), konf. semtrak 2004, cracow – zakopane 2004, pp. 223-230. [13] m. lewandowski, a. szeląg, “minimizing harmonics of the output voltage of the chopper inverter”, arch. elektrotechnik, vol. 69, 1986, pp.223-226. [14] a. mariscotti, “statistical evaluation of measured voltage spectra in dc railways”, 17th symposium imeko tc 4, 3rd symposium imeko tc 19 and 15th iwadc workshop instrumentation for the ict era sept. 8-10, 2010, kosice, slovakia, pp. 8-10. [15] l. mierzejewski, a. szeląg, “an analysis of disturbances in power utility systems caused by traction rectifier substation”, comprail’96 conference, berlin, 1996, pp. 433-442. [16] a. ogunsola, a. mariscotti, “electromagnetic compatibility in railways-analysis and management”, lecture notes in electrical engineering, isbn: 978-3-642-30280-0, volume 168, 2013, springer. [17] patent republic of poland, “filtr aperiodyczny i sposób wyznaczania parametrów filtra prostowników trakcji elektrycznej” (patent aperiodic filter and a method if determination parameters of traction rectifiers’ filter), nr 186399, 2004. [18] en 50388 standard-2011: “railway applications-power supply and rolling stock-technical criteria for the coordination between power supply (substation) and rolling stock to achieve interoperability”. [19] en 50160 standard-2000: “voltage characteristics of electricity supplied by public distribution systems”. [20] a. szeląg, l. mierzejewski, “ground transportation systems”, chapter in encyclopedia of electrical and electronics eng. suppl. 1, john wiley & sons, inc., (1999), pp. 169-194. [21] a. szeląg, t. maciołek, “a 3 kv dc electric traction system modernisation for increased speed and trains power demand – problems of analysis and synthesis”, przegląd elektrotechniczny, r.89, 3a/2013, pp. 21-28. [22] a. szeląg, m. patoka, “issues of low frequency electromagnetic disturbances measurements in traction vehicles equipped with power electronics drive systems”, przegląd elektrotechniczny 9/2013, pp. 290-296. [23] a. szeląg, m. steczek, “analysis of input impedance frequency characteristic of electric vehicles with a.c. motors supplied by 3 kv dc system for reducing disturbances in signalling track circuits caused by the harmonics in the vehicle’s current”, przegląd elektrotechniczny r.89, 3a/2013, pp. 29-33. [24] a. szeląg, l. mierzejewski, “modelling and verification of simulation results in computer aided analysis of electric traction systems”, comprail 2000, computers in railways vii, bologna, italy, 2000, pp. 599-610. [25] a. szeląg, t. maciołek, m. patoka, “correlation analysis of results of measurements in ac power supply of dc traction substations for identification of harmonic disturbances”, 20th imeko tc4 int. symposium research on electric and electronic measurement for the economic upturn benevento, italy, 2014, pp. 958-963. [26] w. zając, a. szeląg, “harmonic distortion caused by suburban and underground rolling stock with dc motors”, power electronics congress, 1996. technical proceedings. ciep '96., v ieee international, pp. 200-206. [27] url: http://www.elester-pkp.com.pl template for an acta imeko event paper acta imeko issn: 2221-870x april 2017, volume 6, number 1, 20-26 acta imeko | www.imeko.org april 2017 | volume 6 | number 1 | 20 numerical experimental investigation of comparison data evaluation method using preference aggregation sergey v. muravyov, irina a. marinushkina, diana d. garif national research tomsk polytechnic university, pr. lenina 30, 634050 tomsk, russia section: research paper keywords: interlaboratory comparisons; reference value; largest consistent subset; preference aggregation; robust method citation: sergey muravyov, irina marinushkina, diana garif, numerical experimental investigation of comparison data evaluation method using preference aggregation, acta imeko, vol. 6, no. 1, article 4, april 2017, identifier: imeko-acta-06 (2017)-01-04 editor: paolo carbone, university of perugia, italy received july 4, 2016; in final form october 13, 2016; published april 2017 copyright: © 2017 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: this work was supported by the ministry of education and science of russian federation corresponding author: sergey muravyov, e-mail: muravyov@tpu.ru 1. introduction interlaboratory comparisons (ic) are now a quite common and important metrological procedure that is used under key comparisons [1], measurement laboratories proficiency testing [2], etc. the procedure consists in arrangement and implementation of assessment of measurement quality of a given object characteristic by means of several different laboratories in accordance with definite prescribed rules. main task of any kind of interlaboratory comparisons is establishing a reference value of measured quantity xref that characterizes a largest subset of consistent (reliable) measurement results, i.e. so called largest consistent subset (lcs) [3]. for this aim, participating in comparisons laboratories estimates the same nominal value xnom of the measured quantity. laboratories having unreliable measurement results do not participate in establishing the final reference value. it should be noticed that, in contrast to proficiency testing, an official procedure of key comparisons (kcs) of the mra [1] does not allow to discard any of the participant results, even though a result looks like unreliable or outlying. in this paper we will adhere to a hypothetical position that the two types of ics can be treated to be similar actions tolerating exclusion of outliers, understanding the resulting reference value can be biased in the sense that some participants were excluded from its computation. there are different approaches to check consistency of laboratory measurement results and to find the reference value xref, see, for example [3]-[7]. the choice of a particular consistency test method depends on a kind of travelling standard, measurement conditions and number of participating laboratories. widely used methods are statistical ones characterizing ic participant competences to carry out measurements based on, for example, calculation of the difference of laboratory measurement results and assigned by comparison providers, percent differences, percentiles, or ranks [8]. however, these methods usually impose limitations on a feasible ic participating laboratories number. moreover, statistical methods may evince low discriminating ability, that is the capacity to differ truly unreliable laboratories from laboratories providing results to be trusted. in [4]-[5], a rather widely known so called procedure a was presented. the procedure uses the weighted mean value y: − − = = = ∑ ∑2 2 1 1 ( )/ ( ), m m i i i i i y x u x u x (1) abstract an integrated software for experimental testing preference aggregation method for interlaboratory comparison data processing is presented. the data can be obtained by a monte-carlo simulation and/or taken from real comparisons. numerical experimental investigations with the software have shown that, as against traditional techniques of interlaboratory comparison data processing, the preference aggregation method provides a robust comparison reference value to be closer to a nominal value. acta imeko | www.imeko.org april 2017 | volume 6 | number 1 | 21 where xi is the nominal value estimate provided by the i-th laboratory; u(xi) are corresponding standard uncertainties; m is the number of ic participating laboratories. the standard uncertainty of value y has the view: 2 2 1 ( ) 1/ ( ) m i i u y u x− = = ∑ . (2) in this procedure the weighted average value y is accepted as the reference value xref only if its consistency with ic participating laboratories data is confirmed in accordance to the criterion χ2. if the consistency test is not satisfied, it is proposed in [3] to use a strategy of successive exclusion of outliers, that is, results which are not consistent with the others by limits of claimed uncertainties. a result is deemed to be inconsistent if |en| > 2, where − = = ± n 2 2 , 1, ..., . ( ) ( ) i i x y e i m u x u y (3) the process of exclusion of one inconsistent result is repeated until a consistency of results by the criterion χ2 is achieved. for lcs obtained in this way, the reference value is determined by (1), where instead of m number of reliable laboratories m' is used. procedure a can be reasonably applied if measurement results provided by participating laboratories are characterized by a normal probability distribution. that is why there is a need to develop robust methods for interlaboratory comparison data processing that are well-behaved in cases where the law of laboratory measurement results distribution differs from normal or unknown. for example, in paper [9] nielsen proposed the method, the successful application of which has been described in [10]. the method offers to consider the uncertainty range u(xi) as the rectangular distribution and to deem that each participant gives one vote to each value within its uncertainty range and no votes for values outside this range. this produces a robust algorithm for the reference value xref determination that is insensitive to outliers, i.e. results with an uncertainty considerably lower than those of other participants. this paper is devoted to software implementation of the comparison reference value determination method presented in terms of preference aggregation [11]-[13]. in section 2 a way is considered to transform uncertainty intervals provided by participating laboratories into rankings of measured quantity values. then the obtained rankings, constituting an initial preference profile, can serve as input data for determination of consensus ranking by the kemeny rule that allows to find the reference value of the measurand and to assess an ability of participating laboratories to provide reliable measurement results. in section 3 specially developed software is discussed to carry out numerical experimental researches of ic methods including procedure a, the nielsen algorithm and the proposed preference aggregation method. in section 4 processing of real comparison data by the preferences aggregation method is presented. 2. ic data processing on the base of preference aggregation define the procedure of transformation of uncertainty intervals provided by laboratories into rankings. for this aim, designate an uncertainty interval gained by the i-th laboratory through =( ) [ ( ), ( )]i l i u iu x u x u x . define a, a range of actual values (rav), of the measurand for converting uncertainty intervals of m laboratories to rankings. the initial value а1 of a is chosen to be equal to a least lower bound of uncertainty intervals = =1 1min{ ( )| 1, ..., }ia u x i m provided by laboratories. the finite value аn of a is chosen to be equal to a largest upper bound of laboratories uncertainty intervals = =umax{ ( )| 1, ..., }n ia u x i m . divide a into n – 1 equal intervals (divisions) in such a way that their amount guarantees a necessary and sufficient accuracy of the measurand values representation. then there will be n values of the measurand a = {а1, а2, …, аn} corresponding to boundaries of the division intervals (marks), see figure 1. details on the proper selection of a particular value of n can be found in [14]. compose a preference profile λ of m rankings representing the uncertainty intervals of laboratories. each i-th ranking, i = 1, …, m, is a union of binary relations of strict order and equivalence possessing the following properties at k = 1, …, m and i, j = 1, …, n: а) ai  aj if ai ∈ u(xk) ∧ aj ∉ u(xk); b) ai ~ aj if ai, aj ∈ u(xk) ∨ ai, aj ∉ u(xk); c) ai  aj if ai ∉ u(xk) ∧ aj ∈ u(xk). then the measurement result indicated by some laboratories is represented by a ranking of the measurand values where one or more equivalent values which belong to the uncertainty interval of the laboratory are more preferable. all other values of a in this ranking are less preferable and equivalent to each other. thus, each ranking includes a single symbol of strict order  and n – 1 symbols of equivalence ~. to aggregate the m ranking means to determine a single preference relation β ensuring a best compromise between them. such a ranking β is called consensus ranking. in the authors’ works [12], [15], [16] it was shown that the kemeny median can be used in the capacity of consensus ranking. one of the possible algorithms is based on the branch and bound technique and described in [12]. as soon as a consensus ranking β is found, a value ranked first in it can be selected as the reference value xref of the measurand. the lcs consists of laboratories whose uncertainty intervals include the revealed reference value xref. laboratories that do not contain the reference value are ignored when forming the largest consistent subset. a standard uncertainty of the obtained reference value for the lcs is defined as the smallest of the two values, i.e. from the maximum lower bound ≤ ref( )l iu x x and the minimum upper bound ≥u ref( )iu x x of the uncertainty intervals of laboratories. figure 1. an example of shaping a range of actual values a. 11.6 a2 11.8 11.4 12.0 12.2 12.4 a1 a3 a4 a5 a6 range of actual values (rav) = a rav division rav mark 12.6 12.8 a7 a8 initial value finite value acta imeko | www.imeko.org april 2017 | volume 6 | number 1 | 22 3. experimental investigations of ic data processing methods to investigate experimentally the proposed method for ic data processing on the base of preference aggregation special software was developed called interlabcom in the environment microsoft visual с#. the software has a userfriendly interface and, in its current version, implements the following three ic data processing methods: the proposed preference aggregation method (pam), procedure a and the nielsen algorithm. measurement results provided by laboratories can be real and/or simulated by means of a program pseudo-random numbers generator that provides an opportunity to realize various modifications of the monte-carlo method when conducting numerical computing experiments. there is a possibility to choose a uniform or a normal distributions of generated measurement results. uniformly distributed comparison data xi and u(xi) can be generated at a given value хnom using the standard library function rand(). normally distributed data of comparison results are obtained from uniformly distributed data using the well-known box–muller transform [17]. when preparing an experiment, in a special window, one can preset a nominal measurand value xnom, the number of participating laboratories m, and the number of the measurand values n. by pushing the button "generation" the generated measurement result xi and its uncertainty u(xi) are displayed on a monitor screen. the uncertainty u(xi) is represented as the couple of upper and lower bounds. a graph of the initial generated ic data is indicated in a special window (figure 2). uncertainty intervals are shown in a two-dimensional graph with dimensions “measurand” (vertical axis) and “laboratories” (horizontal axis). the software allows to indicate ic data processing of each method in a separate window including a table with initial comparison data (measurand values and corresponding uncertainty intervals), a graph of comparison processed data and conclusion on consistency of each participating laboratory results. all the ic data processing results by means of different methods are reduced to a summary table and graph. an inconsistent result is labelled by a special mark and the corresponding data are removed from the processed set. the graph and final data of comparison can be saved in microsoft excel format for further processing. in order to demonstrate the developed software tool operation, some ic measurement data for 7 participating laboratories are shown in figure 3. in this case the rav with lower and upper bounds 11.43 and 12.73 is divided into 5 equal divisions, bounds of which define 6 values a of the measurand. the appropriate preference profile λ, constructed as described in section 2, has the following view: λ1: a2 ~ a3  a1 ~ a4 ~ a5 ~ a6 λ2: a2 ~ a3  a1 ~ a4 ~ a5 ~ a6 λ3: a3 ~ a4 ~ a5 ~ a6  a1 ~ a2 λ4: a2 ~ a3  a1 ~ a4 ~ a5 ~ a6 λ5: a3 ~ a4 ~ a5  a1 ~ a2 ~ a6 λ6: a2 ~ a3 ~ a4  a1 ~ a5 ~ a6 λ7: a1 ~ a2  a3 ~ a4 ~ a5 ~ a6 for this profile two optimal consensus rankings exist: а3  a2  a4  a5  a6  a1 a3  a2  a4  a5  a1  a6, from where the final consensus ranking is: β = {a3  a2  a4  a5  a6 ~ a1}, where the first position is occupied by the value a3 = 11.95. this value is accepted as the measurand reference value xref. our hypothesis consists in that, as ordinal data are used in the pam, a reference value obtained by means of this method should not significantly depend on the particular probability distribution law of measurement results. for experimental investigations of this hypothesis normally distributed data for 100 individual problems have been generated that were distinguished from each other by random uncertainty intervals; the laboratories number m = 15; хnom = 3. these data were processed by pam, procedure a and the nielsen algorithm. the same steps under similar conditions were undertaken for uniformly distributed generated data. in table 1 and table 2 the numerical experimental investigation results of pam as compared with procedure a and the nielsen algorithm are reduced. the fact that the program model allows to assign and know a nominal value beforehand, gives a possibility to assess a quality of method m intended for ic data processing by means of calculation of the difference ξ = −ref nom( ) .x m x (4) thus, table 1 includes xref and ξ for each individual problem solved by each of the three methods obtained for normal distribution and table 2 includes the values acquired for uniform distribution. figure 3. example of ic measurement results. figure 2. one of the software user interface windows. 11.43 11.69 11.95 12.21 12.47 12.73 λ1 λ2 λ3 λ4 λ5 λ5 λ7 m ea su ra n d , a. u . laboratories https://en.wikipedia.org/wiki/uniform_distribution_(continuous) https://slovari.yandex.ru/hypothesis/en-ru acta imeko | www.imeko.org april 2017 | volume 6 | number 1 | 23 the experimental data were used to plot curves illustrating how values ξ are changed from problem to problem for each comparison method. values ξ were taken for every 100 individual problems and organized in ascending order. figure 4 shows the graph of deviations ξ obtained by the proposed pam compared to procedure a for uniform (u) and normal (n) distributions of comparison data. it should be noticed that procedure a is not intended to be applied for data distributed by laws other than normal. therefore, the experimental results obtained under the uniform law are given here in order to demonstrate the nonrobust method behaviour compared to the robust ones over the same data. one can see in figure 4 that a particular kind of measured results probability distribution practically does not influence the pam (curves 3 and 4) performance. it means that the pam is a robust procedure. over the same data, the procedure a (curves 1 and 2) has shown a considerable increase of ξ when passing from normally to uniformly distributed measurements. figure 5 represents a graph of deviations ξ obtained by the table 2. a fragment comparison generated data procession results for xnom = 3.0 a.u.; uniform distribution. problem number pam procedure a nielsen algorithm xref ξ xref ξ xref ξ 1 3.01 0.01 2.92 0.08 2.95 0.05 2 2.97 0.03 2.92 0.08 2.95 0.05 3 3.12 0.03 2.43 0.57 3.25 0.25 4 3.04 0.04 2.69 0.31 2.67 0.33 5 2.98 0.02 2.65 0.35 2.46 0.54 6 2.98 0.02 2.16 0.84 2.86 0.14 7 2.89 0.11 2.54 0.46 2.86 0.14 8 2.81 0.19 2.57 0.43 2.54 0.46 9 2.91 0.09 2.49 0.51 2.74 0.26 10 3.10 0.10 3.00 0.00 2.71 0.29 11 2.96 0.04 2.62 0.38 3.20 0.20 12 3.04 0.04 2.97 0.03 3.37 0.37 13 3.14 0.14 2.69 0.31 2.73 0.27 14 2.98 0.02 2.90 0.10 3.00 0.00 15 2.90 0.10 2.54 0.46 3.06 0.06 … 86 2.99 0.01 3.01 0.01 2.95 0.05 87 2.84 0.17 2.66 0.34 2.90 0.10 88 3.03 0.03 2.88 0.12 2.85 0.15 89 2.94 0.06 2.77 0.23 2.85 0.15 90 2.86 0.14 2.40 0.60 3.09 0.09 91 2.98 0.02 2.90 0.10 2.79 0.21 92 3.11 0.11 2.38 0.62 3.27 0.27 93 2.97 0.03 2.88 0.12 2.75 0.25 94 2.73 0.27 1.90 1.10 2.69 0.31 95 3.00 0.00 2.49 0.51 3.11 0.11 96 2.96 0.04 2.95 0.05 3.11 0.11 97 2.97 0.03 2.90 0.10 3.12 0.12 98 2.95 0.05 2.62 0.38 3.12 0.12 99 2.98 0.02 2.85 0.15 2.89 0.11 100 3.08 0.08 3.01 0.01 2.93 0.07 table 1. a fragment comparison generated data procession results for xnom = 3.0 arbitrary units (a.u.); normal distribution. problem number pam procedure a nielsen algorithm xref ξ xref ξ xref ξ 1 2.97 0.03 2.92 0.08 2.95 0.05 2 2.91 0.09 2.90 0.10 2.93 0.07 3 2.95 0.05 2.91 0.09 2.91 0.09 4 2.98 0.02 2.98 0.02 2.90 0.12 5 3.05 0.05 2.90 0.10 2.96 0.04 6 2.89 0.11 2.89 0.11 2.86 0.14 7 2.98 0.02 3.00 0.00 2.79 0.21 8 2.93 0.07 2.98 0.02 3.10 0.10 9 2.98 0.02 2.86 0.14 2.91 0.09 10 2.97 0.03 2.97 0.03 2.68 0.32 11 2.98 0.02 2.95 0.05 3.02 0.02 12 2.92 0.08 2.99 0.01 2.85 0.15 13 2.99 0.01 2.97 0.03 2.92 0.08 14 2.96 0.04 2.99 0.01 2.92 0.08 15 2.93 0.07 2.99 0.01 2.99 0.01 … 86 3.03 0.03 2.90 0.11 2.94 0.06 87 2.99 0.01 2.97 0.03 2.85 0.15 88 2.94 0.06 2.97 0.03 2.83 0.17 89 2.98 0.02 2.94 0.06 2,74 0.26 90 2.91 0.09 2.94 0.06 2.88 0.12 91 2.93 0.07 2.97 0.03 2.92 0.08 92 2.98 0.02 2.90 0.10 2.93 0.07 93 2.97 0.03 2.81 0.19 2.70 0.30 94 2.99 0.01 2.99 0.01 2.94 0.06 95 2.99 0.01 2.78 0.22 2.87 0.13 96 2.96 0.04 2.99 0.01 2.83 0.17 97 2.98 0.02 2.97 0.03 3.05 0.05 98 2.97 0.03 2.84 0.16 2.93 0.07 99 2.99 0.01 2.91 0.09 3.11 0.11 100 3.01 0.01 3.01 0.01 2.85 0.15 figure 4. deviations ξ obtained by pam and procedure a for uniform (u) and normal (n) distributions of comparison data. figure 5. deviations ξ obtained by pam and nielsen algorithm for uniform (u) and normal (n) distributions of comparison data. 0 20 40 60 80 100 ξ, a .u . individual problems 1 2 3 4 0.70 0.60 0.50 0.40 0.30 0.20 0.10 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0 20 40 60 80 100 ξ, a .u . individual problems 1 2 3 4 0.70 0.60 0.50 0.40 0.30 0.20 0.10 0.00 1 – nielsen algorithm (u) 2 – nielsen algorithm (n) 3 – pаm (u) 4 – pаm (n) 1 – procedure а (u) 2 – procedure а (n) 3 – pаm (u) 4 – pаm (n) acta imeko | www.imeko.org april 2017 | volume 6 | number 1 | 24 proposed pam compared to the nielsen algorithm for uniform (u) and normal (n) distributions of comparison data. it can be seen from figure 5 that the pam provides estimates of xref closer to the nominal value xnom than the nielsen algorithm. at the same time, the latter method (curves 1 and 2) shows a discrepancy between normally and uniformly distributed data of about 0.18 which is more than twice bigger than pam with a discrepancy 0.08. 4. real comparisons data processing by the method of preferences aggregation let us demonstrate the applicability of the pam to real world examples of comparison data taken from open sources [10], [18]. 4.1. the key comparison on high frequency power participating national metrology institutes (nmi) in key comparison (kc) cipm ccem.rf-k25.w [18] determined the effective efficiency and the calibration factor reference of two waveguide thermistor power sensors in the frequency range from 33 to 50 ghz. the effective efficiency of the travelling standard was determined by formula: dc,sub eff rf,abs p p η = , (5) where pdc,sub is the substituted dc power and prf,abs is the total absorbed rf power. participants of the comparisons calculated also the calibration factor ηcal according to equation: η = −γ η2cal eff(1 ) (6) where г is the input reflection coefficient of the travelling standard which was measured as a complex quantity stated as magnitude and phase at the measuring frequencies. the median absolute deviation was used to identify an outlier: 1 med( mad) median{[ ]},is kσ ≈ ≡ η −η (7) where k1 is a multiplier determined by simulation; ηmed is the median value of measurement results {η}. the value of ηi which differed from the median by more than 2.5·s(mad) has been regarded as an outlier. it has been excluded from the calculation of the reference value. this criterion was used to check each measurement result: η −η > ⋅med 2.5 ( mad)i s . (8) the reference value of kc was determined in accordance with section 8 of the technical report [18] on the basis of the unweighted mean value: ' eff,ref eff, 1 1 . ' m i im = η = η∑ (9) the standard uncertainties were calculated: = η = η∑ ' 2 eff,ref ff, 1 1 ( ) ( ). ' m e i i u u m (10) kc data treatment in accordance with ccem.rf-k25.w. the results of the comparison on effective efficiency at 36 ghz are reduced to table 3. the comparison reference value ηeff,ref = 0.9161 was determined for the effective efficiency ηeff with the uncertainty u(ηeff,ref) = 0.0027. nim, mnia and nrc have not participated in the reference value determination as nim and nrc measurement results were considered to be outliers in accordance with criterion (8). the result of mnia was proved to be traceable to the results of other participants. a graphic illustration of the comparison results and the reference value are shown in figure 6. the results of the comparison on the calibration factor are reduced to table 4. the reference value for the calibration factor ηcal,ref = 0.7942 was determined with the uncertainty u(ηcal,ref) = 0.0024 (figure 7). results of vniiftri and nrc were recognized as outliers. result of nmia turned out to be traceable to the results of other participants. kc data treatment by the pam. data of table 3 were processed using pam at n = 8. then the rav was divided into n – 1 = 7 equal divisions. bounds of divisions corresponded to eight values a of the measurand: a1 = 0.8288, a2 = 0.8462, a3 = 0.8636, a4 = 0.8809, a5 = 0.8983, a6 = 0.9157, a7 = 0.9331, and a8 = 0.9505. the preference profile consisted of nine rankings describing the uncertainty intervals figure 6. uncertainty intervals of effective efficiency value provided by nmis. table 4. key comparisons data on the calibration factor. m nmi calibration factor, ηcal ηcal, i u(ηcal, i) 1 ptb 0.7954 0.0036 2 npl 0.7937 0.0067 3 nist 0.7976 0.0070 4 lne 0.7914 0.0046 5 kriss 0.7935 0.0079 6 nim 0.7936 0.0031 7 vniiftri 0.7820 0.0105 8 mnia 0.7972 0.0073 9 nrc 0.8140 0.0130 table 3. key comparison data on effective efficiency. m nmi effective efficiency, ηeff ηeff, i u(ηeff, i) 1 ptb 0.9153 0.0031 2 npl 0.9167 0.0060 3 nist 0.9184 0.0064 4 lne 0.9157 0.0018 5 kriss 0.9143 0.0104 6 vniiftri 0.9160 0.0079 7 nim 0.8360 0.0072 8 mnia 0.9174 0.0071 9 nrc 0.9375 0.0130 0.8200 0.8400 0.8600 0.8800 0.9000 0.9200 0.9400 0.9600 1 2 3 4 5 6 7 8 9 η e ff national metrology institute 0.9600 0.9400 0.9200 0.9000 0.8800 0.8600 0.8400 0.8200 acta imeko | www.imeko.org april 2017 | volume 6 | number 1 | 25 of the appropriate nmis: λ1: a6  a1 ~ a2 ~ a4 ~ a5 ~ a6 ~ a7 ~ a8 λ2: a6  a1 ~ a2 ~ a4 ~ a5 ~ a6 ~ a7 ~ a8 λ3: a6  a1 ~ a2 ~ a4 ~ a5 ~ a6 ~ a7 ~ a8 λ4: a6  a1 ~ a2 ~ a4 ~ a5 ~ a6 ~ a7 ~ a8 λ5: a6  a1 ~ a2 ~ a4 ~ a5 ~ a6 ~ a7 ~ a8 λ6: a6  a1 ~ a2 ~ a4 ~ a5 ~ a6 ~ a7 ~ a8 λ7: a1  a2 ~ a3 ~ a4 ~ a5 ~ a6 ~ a7 ~ a8 λ8: a6  a1 ~ a2 ~ a4 ~ a5 ~ a6 ~ a7 ~ a8 λ9: a7 ~ a8  a1 ~ a2 ~ a3 ~ a4 ~ a5 ~ a6. the final consensus ranking was determined as βfin = {a6  a1 ~ a7 ~ a8  a2 ~ a3 ~ a4 ~ a5}. the comparison reference value of a6 = ηeff,ref = 0.9157 was obtained with uncertainty u(ηeff,ref) = 0.0018. the cardinality of lcs was m' = 7 as measurement results of nim and nrc were recognized to be outliers because of not containing the obtained reference value (figure 8). data of table 5 were also processed, using the pam, at n = 6. the rav was divided into five equal divisions. bounds of the intervals corresponded to six values of the measurand: a1 = 0.7715, a2 = 0.7826, a3 = 0.7937, a4 = 0.8048, a5 = 0.8159, a6 = 0.8270 (figure 9). the preference profile was shaped of 9 rankings: λ1: a3  a1 ~ a2 ~ a4 ~ a5 ~ a6 λ2: a3  a1 ~ a2 ~ a4 ~ a5 ~ a6 λ3: a3  a1 ~ a2 ~ a4 ~ a5 ~ a6 λ4: a3  a1 ~ a2 ~ a4 ~ a5 ~ a6 λ5: a3  a1 ~ a2 ~ a4 ~ a5 ~ a6 λ6: a3  a1 ~ a2 ~ a4 ~ a5 ~ a6 λ7: a1 ~ a2  a3 ~ a4 ~ a5 ~ a6 λ8: a3  a1 ~ a2 ~ a4 ~ a5 ~ a6 λ9: a4 ~ a5  a1 ~ a2 ~ a3 ~ a6. the final consensus ranking was determined as βfin = {a3  a2 ~ a4 ~ a5 ~ a6  a1}. the reference value of comparison ηcal,ref = 0.7937 was obtained with uncertainty u(ηcal,ref) = 0.0019. the cardinality of lcs was m' = 7 (figure 9). measurement results of vniiftri and nrc were recognized to be outliers. 4.2. interlaboratory power comparison in the microwave region in [5], results of interlaboratory power comparisons in the microwave region (50 mhz–26.5 ghz) on the project sit.af01 were reviewed. they were organized by the inrim (istituto nazionale di ricerca metrologica, italy) in turin. a hewlett packard power meter model 438a as a travelling standard has been sent to 12 laboratories. the comparison aim was to confirm the claimed uncertainties of laboratories accredited in the national system of accreditation in the field of microwave measurements. table 5 and figure 10 show one of the series of comparison data of the power sensor calibration factor k measurements at a frequency of 1 ghz. to process the comparison data the nielsen algorithm (see section 1) was used. according to the data analysis outcomes, figure 7. uncertainty intervals of calibration factor value provided by nmis. figure 8. uncertainty intervals of effective efficiency value provided by nmis and reference value obtained by the pam. table 5. comparisons data on power sensor calibration factor k at 1 ghz. laboratories xi u(xi) 1 0.985 0.013 2 0.989 0.008 3 0.982 0.013 4 0.982 0.035 5 0.984 0.014 6 0.980 0.028 7 0.981 0.017 8 0.990 0.021 9 0.982 0.011 10 0.989 0.017 11 1.017 0.014 12 0.987 0.019 figure 10. uncertainty intervals of calibration factor k value provided by participating laboratories and corresponding reference value. figure 9. uncertainty intervals of the calibration factor value provided by nmis and reference value obtained by the pam. 0.7700 0.7800 0.7900 0.8000 0.8100 0.8200 0.8300 1 2 3 4 5 6 7 8 9 η c al national metrology institute 0.8300 0.8200 0.8100 0.8000 0.7900 0.7800 0.8288 0.8462 0.8636 0.8809 0.8983 0.9157 0.9331 0.9505 1 2 3 4 5 6 7 8 9 η e ff national metrology institute 0.9505 0.9331 0.9157 0.8983 0.8809 0.8636 0.8462 0.8288 0.925 0.945 0.965 0.985 1.005 1.025 1 2 3 4 5 6 7 8 9 10 11 12 ca lib ra ti on fa ct or k laboratories 1.025 1.005 0.985 0.965 0.945 0.7715 0.7826 0.7937 0.8048 0.8159 0.8270 1 2 3 4 5 6 7 8 9 η c al national metrology institute 0.8270 0.8159 0.8048 0.7937 0.7826 0.7715 acta imeko | www.imeko.org april 2017 | volume 6 | number 1 | 26 the lcs, formed as a result of nielsen's algorithm processing, included the eleven laboratories. laboratory 11 was excluded, because its result, in accordance with the algorithm conditions, was deemed to be unreliable. the reference value was obtained as xref = 0.985 in correspondence with the greatest number of laboratory "votes". the data of table 5 were processed using the pam at n = 5. the rav was divided into n – 1 = 4 equal divisions. the bounds of intervals corresponded to five values a of the measurand: a1 = 0.947, a2 = 0.968, a3 = 0.989, a4 = 1.009, a5 = 1.030 (figure 11). the corresponding preference profile was as follows: λ1: a3  a1 ~ a2 ~ a4 ~ a5 λ2: a3  a1 ~ a2 ~ a4 ~ a5 λ3: a3  a1 ~ a2 ~ a4 ~ a5 λ4: a1 ~ a2 ~ a3 ~ a4  a5 λ5: a3  a1 ~ a2 ~ a4 ~ a5 λ6: a2 ~ a3  a1 ~ a4 ~ a5 λ7: a2 ~ a3  a1 ~ a4 ~ a5 λ8: a3 ~ a4  a1 ~ a2 ~ a5 λ9: a3  a1 ~ a2 ~ a4 ~ a5 λ10: a3  a1 ~ a2 ~ a4 ~ a5 λ11: a4 ~ a5  a1 ~ a2 ~ a3 λ12: a2 ~ a3  a1 ~ a4 ~ a5. the final consensus ranking was βfin = {a3  a2  a4  a1 ~ a5}. the value a3 was chosen as the reference value xref = 0.989 with a corresponding uncertainty u(xref) = 0.004. the lcs formed by the pam included 11 laboratories, just as in the project sit.af-01 (figure 11). 5. conclusion a method, called preference aggregation method (pam) has been described, aimed to process ic data. the pam is based on transformation of uncertainty intervals provided by participating laboratories into rankings of measured quantity values. for a preference profile composed in this way, a consensus ranking is determined by the kemeny rule that allows to find the reference value of a measurand. the operation of this method was demonstrated. a software tool has been considered that is intended for experimental investigations of the proposed method and other methods of processing generated normally and uniformly distributed ic data. numerical experiments, carried out with its help, have shown that the pam is indeed a robust procedure that does not depend on the probability distribution of the measurement results. it also follows from numerical experiments that the pam provides an estimate of a reference value being closer to the nominal value than the other robust method (the nielsen algorithm) with half the discrepancy between normally and uniformly distributed comparison data. the pam performance was experimentally verified on real comparison results. in all cases, the reference value and associated uncertainty, determined by the proposed method, were very close to the outcomes obtained by the comparison coordinators. acknowledgement this work was supported in part by the ministry of education and science of russian federation, basic part of the state task in 2014-2016, project 2078 and in 2017-2019, project 4.1763.gzb.2017. the authors would like to thank the anonymous referee for helpful comments. references [1] cipm mra-d-05. measurement comparisons in the cipm mra, version 1.5, p. 28. 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[8] iso 13528 (2005) statistical methods for use in proficiency testing by interlaboratory comparisons. international organization for standardisation, geneva, switzerland. [9] h.s. nielsen, determining consensus values in interlaboratory comparisons and proficiency testing, ncsli newsletter 44(2) (2004) pp. 12-15. [10] l. brunetti, l. oberto, m. sellone, p. terzi, establishing reference value in high frequency power comparisons, measurement 42 (2009), pp. 318-1323. [11] s.v. muravyov, i.a. marinushkina, “largest consistent subsets in interlaboratory comparisons: preference aggregation approach”, proc. of 14th joint international imeko tc1, tc7, tc13 symposium, aug. 31-sept. 2, 2011, jena, germany, pp. 69-73. [12] s.v. muravyov, ordinal measurement, preference aggregation and interlaboratory comparisons, measurement 46(8) (2013) pp. 2927-2935. [13] s.v. murav'ev, aggregation of preferences as a method of solving problems in metrology and measurement technique, meas. tech. 57(2) (2014) pp. 132-138. [14] s.v. muravyov, i.a. marinushkina, processing of interlaboratory comparison data by preference aggregation method, meas. tech. 58(12) (2016) pp. 1285-1291. [15] s.v. muravyov, i.a. marinushkina, intransitivity in multiple solutions of kemeny ranking problem, j. phys. conf. ser. 459(1) (2013) 012006. [16] s.v. muravyov, dealing with chaotic results of kemeny ranking determination, measurement 51 (2014) pp. 328-334. [17] g.e.p. box, m.e. muller, a note on the generation of random normal deviates, ann. math. stat. 29(2) (1958) pp. 610-611. [18] r. judaschke, final report of the pilot laboratory ccem key comparison ccem.rf-k25.w rf power from 33 ghz to 50 ghz in waveguide, physikalisch-technische bundesanstalt, germany, 2014. figure 11. uncertainty intervals provided by participating laboratories and corresponding reference value obtained by the pam. 0.926 0.947 0.968 0.989 1.009 1.030 1 2 3 4 5 6 7 8 9 10 11 12 ca lib ra ti on fa ct or k laboratories 1.030 1.009 0.989 0.968 0.947 0.926 numerical experimental investigation of comparison data evaluation method using preference aggregation microsoft word 24-145-1-ga.doc acta imeko  july 2012, volume 1, number 1, 65‐69  www.imeko.org    acta imeko | www.imeko.org  july 2012 | volume 1 | number 1 | 65  new generation of ac‐dc current transfer standards at  inmetro  m. klonz 1 , r. afonso 2 , r.m. souza 2 , r.p. landim 2   1  retired from physikalisch‐technische bundesanstalt, bundesallee 100, d‐38116 braunschweig, germany  2  instituto nacional de metrologia, qualidade e tecnologia, av. nossa senhora das graças, 50 – xerém, rj25250020 duque de caxias, brazil      keywords: ac‐dc difference; pmjtc; comparison  citation: m. klonz, r. afonso, r.m. souza, r.p. landim, new generation of ac‐dc current transfer standards at inmetro, acta imeko, vol. 1, no. 1, article 13,  july 2012, identifier: imeko‐acta‐01(2012)‐01‐13  editor: pedro ramos, instituto de telecomunicações and instituto superior técnico/universidade técnica de lisboa, portugal  received january 10 th , 2012; in final form may 11 st , 2012; published july 2012  copyright: © 2012 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: information not available  corresponding author: r. m. souza, e‐mail: rmsouza@inmetro.gov.br    1. introduction  inmetro, the brazilian national metrology institute, is responsible for developing new calibration set-ups and standards that will improve the capacity of providing a higher quality calibration service, specially to the brazilian accredited laboratories, which are responsible for providing calibration services to all other laboratories and industries in brazil. the inmetro started to invest in pmjtcs (planar multijunction thermal converters), to replace sjtcs (single junction thermal converters) as primary standard at 10 ma current level. for higher currents the new tccs (thermal current converters) are built from high quality coaxial shunts, model a40b, manufactured by fluke in parallel to pmjtcs. this fluke design of shunts with small current level effect on the ac-dc current transfer difference follows the design by several authors in different national institutes [1, 2, 3, 4, 5]. thermal converters are capable of comparing the joule heating between ac and dc modes at 0.1 µv/v level, and are widely employed as the ac-dc current transfer standards in most of the national metrology institutes. the existing ac-dc current transfer standards of inmetro are made of sjtcs which have one thermocouple at the midpoint of the heater and are enclosed in an evacuated glass bulb to improve its sensitivity [6]. the fundamental limitations of the performance of an sjtc are thermoelectric errors (thomson and peltier effects) in the heater due to the rather large temperature gradient along the heater (about 200 ºc), level dependence of the ac-dc difference, small output voltage and therefore small dynamic range. moreover such an sjtc based ac-dc current transfer system needs to be recalibrated against higher level standards at least every 5 years to obtain small uncertainties. to reduce these thermoelectric errors, the mjtc uses as many as two hundred thermocouples spaced along a much longer heater wire [6], which results in a larger output voltage and negligible temperature gradients along the heater. however, the mjtc fabrication process is complicated and expensive. the design of pmjtcs is suitable to mass production without degradation of the performance of the mjtc. pmjtcs provide long-term stability together with high sensitivity and high dynamic range. they are well known for very small ac–dc current transfer differences at audio frequencies [7, 8]. moreover the shunts used in the former ac-dc current transfer standards (model a40 shunts made by fluke) were replaced by high quality coaxial shunts (model a40b manufactured by fluke). in order to validate the new system, an unofficial comparison was made between ptb (physikalischtechnische bundesanstalt, germany) [9] and inmetro standards. the this  paper  describes  the  new  primary  standard  for  the  ac‐dc  current  transfer  at  inmetro,  based  on  pmjtcs  and  the  new  shunts  manufactured by fluke for rated currents from 10 ma up to 20 a. the build‐up of the ac‐dc current scale is described together with the  uncertainty budgets which result in final uncertainties at 5 a of 6 µa/a to 12 µa/a in the frequency range from 10 hz to 100 khz. the  recalibration of the standards after one year showed very small differences which are included in the uncertainty budget.    acta imeko | www.imeko.org  july 2012 | volume 1 | number 1 | 66  measurements were performed at 10 ma and 5 a, in the whole frequency range from 10 hz to 100 khz. 2. calibration set‐up  the basic standard for ac-dc current transfer is the 10 ma pmjtc providing traceability to ptb. all standards for higher currents contain a shunt associated with a dedicated pmjtc which measures the voltage across the shunt. to build-up the current scale from 10 ma to 20 a, the different current ranges have to be calibrated against each other. in this step-up method starting from 10 ma the next higher current standard for 20 ma is calibrated at the current of 10 ma. under the assumption that it does not change its ac-dc current transfer differences, it is used then at 20 ma. this procedure continues step by step for all frequencies from 10 hz to 100 khz and currents up to 20 a. the calibration set-up used is shown in detail in figure 1. two separate calibrators deliver ac and dc voltages. an ac-dc switch connects the ac and dc voltages to the transconductance amplifier which converts the voltage to the necessary current. both ac-dc current transfer standards are connected in series and therefore get the same current for this comparison. the two nanovoltmeters keithley 182 measure the output voltage of the pmjtcs. the nanovoltmeters are modified because their input amplifiers should be driven at the potential of the ac-dc transfer standards. the basic design of the measurement set-up showing only pmjtcs for 10 ma is given in figure 1. for higher currents, coaxial shunts, model a40b, manufactured by fluke, are associated to them. the different earth connections are chosen in a specific way to avoid any earth loops which may change the measured values in an unknown way. a coaxial choke (cc) has been introduced to suppress earth currents causing common mode voltages at the input of the transconductance amplifier. the introduction of potential driven guards (figure 2) in the comparison circuit of the two ac-dc transfer standards avoids systematic changes of the ac-dc transfer difference of the standard, which is at the higher potential in the series connection of the two standards, especially at higher frequencies [10, 11]. this is necessary because the standard is calibrated at low potential and used at high potential. photos in figure 3 and figure 4 show the calibration set-up. with this calibration set-up, standards for all current ranges have been built-up with a standard deviation of the measurement smaller than 1 µa/a. in figure 5 the different steps in the step-up procedure are shown. all pmjtcs called 90 have heater resistances of 90 ω, whereas the 400 has a heater resistance of 400 ω and the 900 has a 900 ω one. the second name is the shunt for the different currents. in figure 6 some thermal converters are shown. the first one from the left is a 10 ma pmjtc; the second one is a boxed                clarke hess ‐ 8100   hi lo chassis 20 a 20 ma a 2 a    dc calibrator         gnd     output   sense   grd     hi     lo     ac calibrator       gnd   output   sense   grd   hi   lo     external guard off   external guard off      k‐182 pmjtc      k‐182   pmjtc cc   ac‐dc switch   transcoductance  amplifier figure 1. measurement set‐up for ac–dc current transfer difference  measurements.  figure 2. ac‐dc current transfer with potential driven guards (pmjtcs with  shunts for current ranges above 10 ma).  figure 3. ac‐dc current transfer set‐up.  figure 4. connection of the ac‐dc current transfer standards in series.    acta imeko | www.imeko.org  july 2012 | volume 1 | number 1 | 67  50 ma shunt connected to a 90  pmjtc and the others are pmjtcs connected to coaxial shunts for currents up to 100 ma and for higher currents up to 20 a. 3. uncertainty analysis  the model equation is step tep -1 ca c com. mode lev lf diff. step-ups i s i                (1) with  step i – 1: transfer difference of standard at the step i – 1. ca: contribution of the mean of repeated twelve measurements. c: contribution of the measurement set-up. com. mode: transfer difference from the common mode effect in the transconductance amplifier determined from the difference of measurements with and without the choke cc in figure 1. lev: transfer difference due to level dependence of shunts estimated from the design of the shunts. lf: transfer difference due to low frequency behavior of pmjtc. diff.step-ups: correction with the difference of different step-up measurements performed in the same measurement set-up. the sum of the variances of the different contributions results in the variance of the result:                 2 2 2 2 step tep -1 ca c 2 2 2 2 com. mode lev lf diff. step-ups u u u u +u u u u i s i               (2) where u2(x) represents the variance of x. the uncertainty budgets of the different current steps are given in tables 1 to 4. 4. comparison results  an unofficial interlaboratory comparison of ac-dc current transfer standards between ptb and inmetro was performed with a travelling standard for 10 ma and one for 5 a. the current points chosen were 10 ma and 5 a, in the frequency range from 10 hz and 100 khz. in inmetro each current point was measured against inmetro standards in twelve cycles at all frequencies, and the mean was calculated using the results of the sequence which gives the ac-dc current transfer difference, represented by δinmetro. tables 5 and 6 show the results obtained. ptb uses a similar calibration set-up and similar standards. the shunts are manufactured by the norwegian metrology institute justervesenet with a different design [12]. the results obtained in ptb are represented by δptb. the associated expanded uncertainties are given by u. the measured differences inmetro-ptb between both institutes were small compared to the given uncertainties which is also represented by the small en-values. this is a very satisfying result of the comparison. 5. conclusions  in the first step a new step-up procedure was developed to build-up the ac-dc current transfer standards for 10 ma up to 20 a and to perform an uncertainty analysis for the whole build-up. the second step towards the introduction of the new ac-dc current transfer system of inmetro was performing a comparison between inmetro and ptb, which proved that inmetro’s new calibration set-up and standards work as expected.     3 ma  1 ma  900‐2 400‐3  400‐3  900‐2 90‐11  90‐12 5 ma  90‐13 + sh20ma  20 ma  90‐13 + sh20ma  90‐14 + sh50ma  90‐11  90‐12 10 ma  50 ma  90‐14 + sh50ma  90‐15 + sh100ma  100 ma  90‐15 + sh100ma  90‐16 + sh200ma  200 ma  90‐16 + sh200ma  90‐17 + sh500ma  500 ma  90‐17 + sh500ma  90‐18 + sh1a 1 a  90‐18 + sh1a  90‐19 + sh2a 2 a  90‐19 + sh2a  90‐20 + sh5a  5 a  90‐20 + sh5a  90‐21 + sh10a  90‐21 + sh10a  90‐22 + sh20a  10 a  20 a  90‐22 + sh20a  figure 5. schematics of the step‐up procedure.  figure 6. ac‐dc current transfer standards.    acta imeko | www.imeko.org  july 2012 | volume 1 | number 1 | 68  table 1. uncertainty analysis for the step‐up at 10 ma.  influencing quantity  measurement uncertainty in µa/a at the frequency in khz  0.01  0.02  0.03  0.04  0.055  0.12  0.5  1  5  10  20  50  70  100  u( 10 ma)  1.5  1.5  1.5  1.5  1.5  1.5  1.5  1.5  1.5  1.5  1.5  1.5  1.5  1.5  u(ca)  0.5  0.4  0.3  0.5  0.6  0.3  0.3  0.3  0.3  0.3  0.3  0.3  0.4  0.6  u(c)   0.1  0.1  0.1  0.1  0.1  0.1  0.1  0.1  0.1  0.1  0.1  0.1  0.1  0.1  u(com. mode)  1  1  1  1  1  1  0  0  0  0  0  0  0  0  u(lev)  0  0  0  0  0  0  0  0  0  0  0  0  0  0  u(diff. step‐ups)  0.2  0.2  0.0  0.3  0.1  0.0  0.1  0.2  0.1  0.1  0.1  0.1  0.1  0.2  u(lf)  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  u(ma)  1.9  1.9  1.8  1.9  1.9  1.8  1.5  1.5  1.5  1.5  1.5  1.5  1.6  1.6  u(ma) k = 2  3.8  3.8  3.6  3.8  3.8  3.6  3.0  3.0  3.0  3.0  3.0  3.0  3.2  3.2                                table 2. uncertainty analysis for the step‐up at 100 ma.  influencing quantity  measurement uncertainty in µa/a at the frequency in khz  0.01  0.02  0.03  0.04  0.055  0.12  0.5  1  5  10  20  50  70  100  u( 50 ma)  3.0  2.6  2.5  2.5  2.5  2.4  1.7  1.7  1.7  1.7  1.7  1.7  1.8  1.9  u(ca)  0.5  0.3  0.5  0.4  0.3  0.5  0.4  0.4  0.4  0.2  0.4  0.4  0.4  0.5  u(c)   0.2  0.2  0.2  0.2  0.2  0.2  0.2  0.2  0.2  0.2  0.2  0.2  0.2  0.2  u(com. mode)  1.0  1.0  1.0  1.0  1.0  1.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  u(lev)  0.2  0.2  0.2  0.2  0.2  0.2  0.2  0.2  0.2  0.2  0.2  0.2  0.3  0.4  u(diff. step‐ups)  0.1  0.1  0.1  0.1  0.3  0.2  0.1  0.1  0.2  0.4  0.2  0.1  0.1  0.1  u(lf)  0.3  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  u(ma)  3.2  2.9  2.8  2.7  2.8  2.7  1.8  1.8  1.8  1.8  1.8  1.8  1.9  2.0  u(ma) k = 2  6.4  5.8  5.6  5.4  5.6  5.4  3.6  3.6  3.6  3.6  3.6  3.6  3.8  4.0                                table 3. uncertainty analysis for the step‐up at 1 a.  influencing quantity  measurement uncertainty in µa/a at the frequency in khz  0.01  0.02  0.03  0.04  0.055  0.12  0.5  1  5  10  20  50  70  100  u( 500 ma)  3.8  3.5  3.4  3.3  3.4  3.4  2.1  2.0  2.1  2.1  2.1  2.1  2.1  2.5  u(ca)  0.4  0.3  0.3  0.3  0.3  0.4  0.3  0.3  0.2  0.2  0.2  0.2  0.3  0.3  u(c)   0.2  0.2  0.2  0.2  0.2  0.2  0.2  0.2  0.2  0.2  0.2  0.2  0.2  0.2  u(com. mode)  1.0  1.0  1.0  1.0  1.0  1.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  u(lev)  1.0  1.0  1.0  1.0  1.0  1.0  1.0  1.0  1.0  1.0  1.0  1.0  1.0  1.5  u(diff. step‐ups)  0.1  0.1  0.1  0.2  0.2  0.1  0.4  0.1  0.2  0.3  0.1  0.3  0.1  0.4  u(lf)  0.4  0.1  0.1  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  u(a)  4.1  3.8  3.7  3.6  3.7  3.7  2.4  2.3  2.3  2.3  2.3  2.3  2.4  2.9  u(a) k = 2  8.2  7.6  7.4  7.2  7.4  7.4  4.8  4.6  4.6  4.6  4.6  4.6  4.8  5.8    table 4. uncertainty analysis for the step‐up at 5 a.  influencing quantity  measurement uncertainty in µa/a at the frequency in khz  0.01  0.02  0.03  0.04  0.055  0.12  0.5  1  5  10  20  50  70  100  u(  2 a)  4.4  4.1  4.0  3.9  4.0  4.0  2.7  2.6  2.5  2.8  3.1  3.8  3.9  4.6  u(ca)  0.3  0.4  0.3  0.2  0.4  0.4  0.3  0.5  0.4  0.2  0.2  0.4  0.6  0.4  u(c)   0.2  0.2  0.2  0.2  0.2  0.2  0.2  0.2  0.2  0.2  0.2  0.2  0.2  0.2  u(com. mode)  1.0  1.0  1.0  1.0  1.0  1.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  u(lev)  1.0  1.0  1.0  1.0  1.0  1.0  1.0  1.0  1.0  1.5  2.0  3.0  3.0  3.5  u(diff. step‐ups)  0.1  0.2  0.3  0.0  0.1  0.2  0.3  0.1  0.3  0.3  0.2  0.6  0.5  0.7  u(lf)  0.4  0.3  0.1  0.1  0.1  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  u(a)  4.6  4.4  4.3  4.2  4.3  4.3  2.9  2.8  2.8  3.2  3.7  4.9  5.0  5.9  u(a) k = 2  9.2  8.8  8.6  8.4  8.6  8.6  5.8  5.6  5.6  6.4  7.4  9.8  10.0  11.8    acta imeko | www.imeko.org  july 2012 | volume 1 | number 1 | 69  the results of both institutes’ standards agreed within 2 µa/a for 10 ma and 5 µa/a for 5 a between 10 hz and 100 khz. that means that this new system works reliably and is ready for use in the next international comparison of ac-dc current transfer standards (sim.em-k12) [13]. references  [1] i. budovsky, “measurement of phase angle errors of precision current shunts in the frequency range from 40 hz to 200 khz”, ieee trans. instrum. meas., vol. 56, no. 2, april 2007, pp. 284-288. [2] m. garcocz, p. scheibenreiter, w. waldmann, g. heine, “expanding the measurement capabilities for ac-dc current transfer at bev”, 2004 conf. on precision electromagnetic measurements digest, cpem, june 27 to july 2, 2004, pp. 461-462. [3] p.s.filipski and m. boeker, “ac-dc current shunts and system for extended current and frequency ranges”, in proc. imtc, ottawa, on, canada, may 17-19, 2005, pp. 991-995. [4] p. s. filipski and m. boeker, “ac-dc current transfer standards and calibrations at nrc”, simposio de metrologia 2006, 25 al 27 otubre 2006. [5] r. m. souza, r. v. f. ventura, f. a. silveira, “reevaluation of ac-dc current transfer system based on single junction thermal converters at inmetro”, xviii imeko tc4 symposium and ix semetro / metrologia 2011: tc 4 imeko and ix semetro, natal, brazil, 2011. [6] m. klonz, “ac-dc transfer difference of the ptb multijunction thermal converter in the frequency range from 10 hz to 100 khz”, ieee trans. instrum. meas., vol. 36, no. 2, june 1987, pp. 320-329. [7] m. klonz, t. weimann, t., “accurate thin film multijunction thermal converter on a silicon chip”, ieee trans. instrum. meas. im-38, no. 2, april 1989, pp. 335-336, [8] h. laiz, m. klonz, e. kessler, t. spiegel, “new thin-film multijunction thermal converters with negligible low frequency ac-dc transfer differences”, ieee trans. instrum. meas., vol. 50, no. 2, april 2001, pp.333-337. [9] m. klonz, h. laiz, t. spiegel, p. bittel, “ac-dc current transfer step-up and step-down calibration and uncertainty calculation”, ieee trans. instrum. meas., vol. 51, no. 5, 2002, pp. 1027-1034. [10] k. e. rydler, “high precision automated measuring system for ac-dc current transfer standards”, ieee trans. instrum. meas., vol. 42, no.1, april 1993, pp. 608-611. [11] t. funck, and m. klonz, “improved ac-dc current transfer step-up with new current shunts and potential driven guarding”, ieee trans. instrum. meas. vol. 56, april 2007, pp. 361-364. [12] k. lind, t. sorsdal, and h. slinde, “design, modeling, and verification of high-performance ac-dc current shunts from inexpensive components”, ieee trans. instrum. meas., vol. 57, no.1, jan. 2008, pp. 176-181. [13] l. d. lillo, “sim.em-k12 comparison protocol – ac-dc current transfer difference”, 2010. table 5.  result of the comparison between inmetro and ptb at 10 ma. institutes  ac‐dc current transfer differences together with their uncertainties in µa/a at the frequencies in khz  0.01  0.02  0.03  0.04  0.055  0.12  0.5  1  5  10  20  50  70  100  inmetro  5.5  1.3  1.0  0.7  0.3  ‐0.1  ‐0.5  0.1  0.4  0.6  2.2  17.6  34.8  70.9  u inmetro  4  4  4  4  4  4  3  3  3  3  3  3  3  4  ptb  5.3  1.8  0.7  0.7  ‐0.3  ‐0.4  ‐0.1  ‐0.2  0.5  1.5  3.2  18.9  36.4  72.2  u ptb  3  3  3  3  3  3  3  3  3  3  3  3  3  3  difference inmetro ‐ ptb  0.2  ‐0.5  0.3  0.0  0.5  0.3  ‐0.4  0.3  ‐0.1  ‐0.9  ‐1.0  ‐1.3  ‐1.6  ‐1.3  en  0.0  ‐0.1  0.1  0.0  0.1  0.1  ‐0.1  0.1  0.0  ‐0.2  ‐0.2  ‐0.3  ‐0.4  ‐0.3  table 6.  result of the comparison between inmetro and ptb at 5 a.  institutes  ac‐dc current transfer differences together with their uncertainties in µa/a at the frequencies in khz  0.01  0.02  0.03  0.04  0.055  0.12  0.5  1  5  10  20  50  70  100  inmetro  0.0  0.4  0.2  0.1  0.0  ‐0.3  0.0  0.1  2.7  11.6  13.2  ‐88.1  ‐187.0  ‐353.9  u inmetro  9  9  9  9  9  9  6  6  6  7  8  10  10  12  ptb  ‐0.9  ‐0.2  0.4  0.2  ‐0.4  0.5  0.5  0.4  3.0  10.6  10.5  ‐89.9  ‐187.2  ‐349.4  u ptb  5  4  4  4  4  4  4  4  4  5  7  9  10  11  difference inmetro ‐ ptb  0.9  0.6  ‐0.2  0.0  0.4  ‐0.8  ‐0.5  ‐0.3  ‐0.3  1.0  2.7  1.9  0.2  ‐4.5  en  0.1  0.1  0.0  0.0  0.0  ‐0.1  ‐0.1  0.0  0.0  0.1  0.2  0.1  0.0  ‐0.3  acta imeko  issn: 2221‐870x  june 2015, volume 4, number 2, 1    acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 |  1  journal contacts    about the journal    acta imeko is an e-journal reporting on the contributions on the state and progress of the science and technology of measurement. the articles are based on presentations presented at imeko workshops, symposia and congresses. the journal is published by imeko, the international measurement confederation. the issn, the international identifier for serials, is 2221870x.      editor‐in‐chief  paolo carbone, italy    honorary editor‐in‐chief  paul p.l. regtien, netherlands    editorial board      leopoldo angrisani, italy filippo attivissimo, italy eulalia balestieri, italy eric benoit, france catalin damian, romania pasquale daponte, italy luigi ferrigno, italy alistair forbes, united kingdom fernando janeiro, portugal konrad jedrzejewski, poland andy knott, united kingdom fabio leccese, italy rosario morello, italy helena geirinhas ramos, portugal pedro ramos, portugal sergio rapuano, italy dirk röske, germany alexandru salceanu, romania constantin sarmasanu, romania lorenzo scalise, italy emiliano schena, italy enrico silva, italy marco tarabini, italy susanne toepfer, germany rainer tutsch, germany ian veldman, south africa luca de vito, italy     about imeko    the international measurement confederation, imeko, is an international federation of actually 39 national member organisations individually concerned with the advancement of measurement technology. its fundamental objectives are the promotion of international interchange of scientific and technical information in the field of measurement, and the enhancement of international co-operation among scientists and engineers from research and industry.     addresses    principal contact  prof. paolo carbone university of perugia dept. electr. inform. engineering (diei) via g.duranti, 93, 06125 perugia, italy email: paolo.carbone@unipg.it    acta imeko  prof. paul p. l. regtien measurement science consultancy (msc) julia culpstraat 66, 7558 jb hengelo (ov), the netherlands email: paul@regtien.net    support contact  dr. dirk röske physikalisch-technische bundesanstalt (ptb) bundesallee 100, 38116 braunschweig, germany email: dirk.roeske@ptb.de     automatic system to identify and manage garments for blind people acta imeko issn: 2221-870x september 2023, volume 12, number 3, 1 10 acta imeko | www.imeko.org september 2023 | volume 12 | number 3 | 1 automatic system to identify and manage garments for blind people luís silva1, daniel rocha2,3,4, filomena soares2, joão sena esteves2, vítor carvalho2,3 1 university of minho, school of engineering, campus of azurém, guimarães, portugal 2 algoritmi research center, university of minho, campus of azurém, guimarães, portugal 3 2ai-school of technology, ipca, barcelos, portugal 4 inl international iberian nanotechnology laboratory, braga, portugal section: research paper keywords: blind people; automatic system; garments; colour detection; deep learning citation: luís silva, daniel rocha, filomena soares, joão sena esteves, vítor carvalho, automatic system to identify and manage garments for blind people, acta imeko, vol. 12, no. 3, article 7, september 2023, identifier: imeko-acta-12 (2023)-03-07 section editor: susanna spinsante, grazia iadarola, polytechnic university of marche, ancona, italy received february 28, 2023; in final form june 10, 2023; published september 2023 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work has been supported by national funds through fct – fundação para a ciência e tecnologia within the r&d units project scope: uidb/00319/2020, uidb/05549/2020 and uidp/05549/2020 corresponding author: vitor carvalho, e-mail: vcarvalho@ipca.pt 1. introduction in recent years, there has been a concerted effort to develop technologies that provide people with disabilities access to information and knowledge [1]. for individuals who are blind, several technological solutions have been proposed to assist with daily routine activities [2], [3]. despite the advancements in assistive technology, blind individuals still face difficulties in performing basic daily tasks, such as selecting their clothing. the process of identifying features on garments is often slow and challenging, leading to a loss of autonomy in choosing their desired clothing. this study aims to address these challenges by proposing an automatic wardrobe that improves quality of life and well-being of blind people, complementing the continuous work developed under the same project [4]-[11], [14]. in this research, the major contribution is the development of a mechatronic system prototype that facilitates garment selection and management. the prototype is divided into two distinct modules: the first module is dedicated to the physical prototype, while the second module is dedicated to image processing and machine learning algorithms to segment, classify, and extract colours from clothing. furthermore, within this scope, this work had the support of the association of the blind and amblyopes of portugal (acapo) allowing a preliminary validation. this paper comprises seven sections, where section 2 outlines the prior research. following this, section 3 presents a general overview of the system, while sections 4 and 5 discuss the hardware and software architectures, respectively. section 6 provides an initial analysis of the results obtained and presents concluding remarks as well as suggestions for future research. abstract in recent years, there has been increased attention towards the integration of handicapped individuals in society, with significant efforts being made to promote their inclusion. technology has played a critical role in this effort, with several technological solutions emerging to help handicapped people in their daily routines, enabling them to better integrate into society. however, there are still challenges that remain, particularly in the area of basic tasks for blind people, such as managing and identifying personal garments. this study seeks to provide an improvement in the quality of life and well-being of blind people. a mechatronic automatism that allows the identification of user garments using sensors was developed. an interface developed with the implementation of a server responsible for managing the requests from the user is integrated. algorithms were implemented for segmentation and classification of garments and for detecting the predominant colors of each garment. by the results obtained it was evidenced that the system could be an efficient solution to reduce the time taken for garment selection, particularly in terms of color differentiation and the selection of combinations for blind people. mailto:vcarvalho@ipca.pt acta imeko | www.imeko.org september 2023 | volume 12 | number 3 | 2 2. previous work existing solutions for helping blind people choosing their garments rely primarily on the concept of smart wardrobes, which has seen a significant surge in popularity in recent years [12]. furthermore, it was identified by perry et al. the level of acceptance of smart wardrobes and the consumers' opinions about this approach [13]. the study revealed that 84% of the participants identified ease of use and utility as the predominant factors for accepting this technological model. a survey carried out with acapo (portuguese association of blind and amblyope people) allowed the identification of several problems regarding garments identification by blind people [14]. this survey was important to identify that there isn´t a solution to help blind people on garments management. myeyes, proposed in [4], [5] by rocha et al., was developed to overcome this gap. the solution integrates a mobile application with an arduino board, and it allows the users to have a virtual wardrobe with their personal garments. near field communication (nfc) technology is used to allow the addition of garments. as future development of this work, a possible physical implementation of a wardrobe to help blind people was presented in [10]. additionally, r. alabduljabbar et al. presented a system that integrates nfc technology with a smartphone, allowing visually impaired people to choose the desired clothes [15]. a similar solution was proposed in [16] by s. j. v. gatis filho et al. that also explores nfc technology combined with quick response (qr) technology with the main goal of developing a clothing matching system with audio description. solutions whose main target is people without any disability are in a crescent rising with more implementations emerging. goh et al. [17] proposed a system that integrates tags with radio frequency identification (rfid) technology, allowing unique identification of clothing items. this system is controlled by an application that allows garment management and suggests clothing items based on several criteria, such as style, colour, material, and user’s mood. on other hand, some fashion brands present different solutions developed to help people plan what to wear. in 2017, amazon presented the echo look [18] based on a kit with a camera that allows garments photo capture and cataloguing of outfits. this method also suggests combinations based on meteorology and users’ preferences. the mobile application fashion api [19] is a “closet” that plans what to purchase and adds garments based on qr code reading. another mobile application is the smart closet [20], which plans combinations to wear and allows the addition of clothing items based on photo capture. tailortags [21] is a system that uses smart tags to detect garments automatically that suggests combinations based on user’s preferences. the systems presented have the emphasis on solutions based on clothing implementations, not only targeted to blind people, but also showing some examples of what has been made available to the general public, in order to assist and facilitate the selection of clothing pieces. table 1 summarizes the characteristics of the systems analysed. besides all efforts to develop systems to help blind people, except for solutions presented in [4], [5], [15], and [16], the systems in table 1 focus on solutions that help people without any disability. as the project encompasses the recognition and identification of garments’ colours, it was necessary to identify solutions that help blind people in the detection of the colours. in this way, there are some implementations from mobile applications to small electronic devices. regarding to mobile applications, emerged v7 aipoly [22] that allows the identification of not only colours but also objects and texts. the system helps blind people on daily routines tasks and interacts with the user by audio description. relatively to physical devices, there are colortest2000 and colorino. the colortest2000 [23] allows the identification of over 1700 different colours of daily objects and colorino [24] has the capability of distinguish 150 tones of colours. yang, yuan, and tian presented in [25] a prototype that analyses garment and allows the identification of 4 different types of patterns and 11 different colours. the system integrates a camera, a microphone, a computer, and headphones. for the colour identification it is used the hue, saturation, intensity (hsi) model and implemented machine learning algorithms to recognize the patterns. in [26], j. jarin et al. proposed a similar system capable of detecting colours and patterns in clothing pieces. the garments photos captured by the camera are submitted to a support vector machine (svm) algorithm to identify and classify the different colours and patterns on each piece. in the case of the proposal presented in [27] by x. yang et al., it is similar with the previous solution, however, uses statistical feature (sta) and sift (scale invariant feature transform) to identify the colours and patterns in each garment. lastly, medeiros, j. presented in [28] a small wearable device with an endoscopic camera to use in the top of a finger that allows the surface capture of garments images. to identify and classify different textures, there is used classification approach based on the combination of two complementary features that uses imagenet classifier and an svm. the colour detection is based on superpixel segmentation capable of detecting multiple colours simultaneously. regarding the scope of garments segmentation, there are several solutions presented. another capability of the system presented in this paper is the classification and segmentation of garments. for the system implementation the algorithm used was the mask region-based convolution neural network (mask rcnn) that mainly considered image segmentation. it was necessary to identify implementations using this algorithm on garments scope. the solution approached by khurana, t. et al. in [29] allows the distinction of garments with similar colours and patterns. firstly, the cnn is used to detect and segmentate the table 1. analysed solutions overview. solution description myeyes [4], [5] manage garments with rfid “an iot smart clothing system for the visually impaired using nfc technology” [15] manage garments based on nfc technology “my best shirt with the right pants: improving the outfits of visually impaired people with qr codes and nfc tags” [16] manage garments based on the combination of nfc and qr technology “developing a smart wardrobe system” [17] adds clothing items to the wardrobe based on rfid tags reading echo look [18] suggests advice based on weather and personal trend fashion api [19] adds clothing items to the virtual wardrobe based on qr code reading smart closet [20] adds clothing items to the virtual wardrobe based on photo capture tailortags [21] adds clothing items to the virtual wardrobe based on wireless tags detection acta imeko | www.imeko.org september 2023 | volume 12 | number 3 | 3 spatial limits of each garment and next, the features are identified. the results were obtained using two datasets: fashionista with 685 images and cfpd with 2682 images. on one hand, with fashionista dataset, the accuracy and iou obtained was 91.1 % and 42.1 %, respectively. on the other hand, using cfpd dataset, the value obtained as accuracy was 93.5 % and for iou, the value was 58.7 %. the method deepfashion2 proposed in [30] by ge, y. et al. is a method capable of extracting several features of garments images. the dataset used was built with 801 000 images annotated with several data, such as type and style. the model implemented was match r-cnn that is a derivation of mask r-cnn. the algorithm extract features from garments with an average precision of 79.3 % with iou = 0.75 and 83.4 % with iou = 0.5. this solution presented a slight improvement in average precision in comparison with the mask r-cnn implementation. wang, j. et al. presented a solution in [31] which is based in deeplabv3+ model and allows the segmentation of garments in images with complex backgrounds. the neural network architecture was redesigned increasing the adaptability with the limits in each image. concerning to results, the values of 97.26 % in accuracy, 93.23 % in miou and 90.56 % in average precision were slightly higher in comparison with the results obtained previously. in the approach presented in [32] by zhang, x. it is used a convolution neural network to allow the recognition of garments or combinations and identification of the specific year that refers to. the dataset used contained 9339 images of garments of 8 different years and the results presented a good accuracy. regarding to segmentation, the segnet model presented an iou of 0.951. concerning classification, the proposal method presented an accuracy of 0.805, a higher value than 0.785 using the resnet101 model. finally, yang, t. et al. proposed a method in [33] capable of detecting t-shirts and long sleeve shirts silhouettes using instance segmentation with mask r-cnn. the dataset encompasses 9000 images with the annotations. the results showed that the model presented good results in the detection of t-shirts with an average precision of 95 %. furthermore, several studies [34]-[37] presented methods to segment objects using cnns which have similarities to the implementation used and modelized for the garment segmentation and classification. table 2 summarizes the main results of the segmentation and classification methods analysed. the proposed system's primary objective is to identify and classify clothing items for blind individuals to select their clothing with ease. to achieve this goal, a thorough analysis of the current state-of-the-art solutions was conducted, revealing a significant gap to assist visually impaired individuals in this task. thus, a new system is proposed, utilizing a physical prototype with advanced artificial intelligence (ai) algorithms to classify clothing items and detect their predominant colours, enabling blind individuals to make informed clothing choices. the proposed solution represents a significant advancement in assistive technology for the visually impaired and has the potential to improve the daily lives of them. 3. system overview the system presented in this paper consists of a physical prototype and control software, figure 1. the hardware includes an nfc module that reads the garment tags, a stepper motor screwed to the wardrobe roof for circular movement, and a servo motor on each hanger for 180-degree rotation during photo capture. two photos (top and bottom) are taken and stitched together using an algorithm. a raspberry pi serves as the control unit to process data and provide analysis on request. an inside illumination system reduces reflections and creates a controlled environment for photo capture. figure 1 depicts the mobile application as the intermediary between the user and the physical prototype, facilitating the interaction with the raspberry pi through the installation of a server that handles user requests. at the hardware level, the raspberry pi consists of three main components: the nfc reader, cameras, and dc motors, as shown in figure 9. the raspberry pi triggers the corresponding hardware components to capture garment photos and return them to the user in response to their requests. the software module includes two algorithms designed to determine the predominant colours of each garment. a mask region-based convolutional neural network (r-cnn) algorithm is used to carry out garment image segmentation and background removal, using a dataset of 480 annotated garment images. additionally, an algorithm based on opencv analyses the pixels of the images to extract their redgreen-blue (rgb) values and determine the predominant colours. the upgrades between the two systems are significant. while the previous system [4], [5] used nfc communication for garment management, the proposed system combines this functionality with an automation mechanism that can move and rotate garments to capture multiple angles accurately. the table 2. literature overview on garment segmentation and classification. method dataset metrics fully convolutional networks [29] fashionista: 685 images cfpd: 2 682 images accuracy: 91.1 % iou: 42.1 % accuracy: 93.5 % iou: 58.7 % match r-cnn [30] 801 000 images ap75: 79.3 % ap50: 83.4 % deeplabv3+ [31] 491 000 images accuracy: 97.26 % miou: 93.23 % segnet [32] 9 339 images iou: 95.1 % iou: 80.5 % mask r-cnn [33] 9 000 images ap: 95 % figure 1. system overview. acta imeko | www.imeko.org september 2023 | volume 12 | number 3 | 4 integration of a photo capture module further enhances the system's ability to detect and classify garments, allowing the system to determine the predominant colours of each piece accurately. the proposed system's enhanced capabilities represent a significant advancement in the field of garment management and classification, having also potential applications in various industries. 4. hardware architecture the functioning of the system has four distinct phases: nfc reading, photo capturing, hanger rotation, and circular movement. the nfc module is used to read tags attached to each garment, allowing for the collection of the identifier number (uid), which is the user identification code associated with each tag. photo capturing is carried out via a vision system with one camera. capturing a complete photo of a clothing item can be challenging, even with a camera equipped with a larger aperture lens, due to the short distance between the camera and the item in the developed prototype. to address this limitation, a servomotor is attached to the camera to facilitate a 180-degree rotation, allowing the capture of multiple positions of the item, which are subsequently merged using a stitching algorithm to create a complete photo of the garment, combining both top and bottom photos. this innovative approach represents a solution to accurately capturing clothing items in the prototype and can be implemented in various wardrobe sizes, making it a versatile solution. during the photo capture process proper illumination is crucial to avoid dark areas. to achieve this a white led strip consisting of hundreds of leds emitting light at a colour temperature of approximately 6000k is mounted on the wall where the cameras are situated. this type of illumination offers a diffuse, flexible, and cost-effective solution, allowing for a higher focus on each garment. for a circular movement within the wardrobe, a stepper motor is affixed to the roof, with its shaft connected to a circular platform. this platform supports the weight of all servomotors attached to each hanger, responsible for rotating each garment 180 degrees. the servomotors are attached to the underside of the circular platform providing a flexible and cost-effective solution. to establish a proof of concept and create an easily mountable solution, a small ikea wardrobe with dimensions of 50 cm × 30 cm × 80 cm was selected for testing garments of appropriate size [38]. the circular movement of the garments inside the wardrobe is enabled by a stepper motor, installed on the roof, which drives a circular platform capable of supporting the weight of all servo motors attached to each hanger. the servo motors, responsible for rotating each garment 180 degrees, are mounted on the inferior surface of the circular platform. the nfc reader is attached to one of the side walls to allow for collision and detection of the uid associated with each tag. figure 2 represents the hardware overview. the system was implemented using the raspberry pi 3b+ microcontroller board as its base. this board is a popular choice for complex projects due to its versatility and affordability. the board has 40 pins that can be used for various functions such as i2c, spi, and uart communication. it has 1gb of ram and a 16gb sd card with the raspbian operating system installed for storage. to read the tags, a pn532 module that communicates via nfc was used. this module can detect tags up to 4 cm away and can be connected to the raspberry pi board using i2c, spi, or uart. the system considers two types of actuators servomotors and a stepper motor. servomotors are affixed to the hangers, enabling 180-degree garment rotation for photo capture as depicted in figure 3. additionally, a servomotor connected to the camera enables two positions during photo capture. on the other hand, the stepper motor is attached to the wardrobe's roof, facilitating the garment movement. it is controlled by a uln2003 driver, which governs the motor's rotation. to take photos, an ov5647 camera module was used. it integrates the camera and a small board for connection to the bcm2835 processor through channel state information (csi) communication. this camera has a 5 mp resolution and can capture images up to 2592 × 1944 pixels. the csi bus was used to connect this module to the raspberry pi. 5. software architecture regarding the software developed and implemented, the system has three main components: server, control software automatism and image processing algorithms (figure 4). concerning the interaction between the user and the system, a figure 2. hardware overview. figure 3. automatism to rotate the garments. figure 4. server overview. acta imeko | www.imeko.org september 2023 | volume 12 | number 3 | 5 server was implemented on the raspberry pi. the server receives the user’s requests and returns the photo from the respective garment. this communication allows the connection between the user interface and the physical system. the server was implemented using flask framework and hosted on a raspberry pi. the server received photo requests from the user, and a virtual environment was created using docker platform to enable the use of detectron2 on a windows machine for background removal. the client on the docker received garment photos from the user and returned the respective class and predominant colours. a control software was developed to test and validate the wardrobe automation. the state machine, presented in figure 5, described the system's functioning, which included six states: ready, stepper, detection, capture, servo, and stitch. when the user requested a garment, the initial state ready changed to stepper. in this state, the stepper motor controlling the garment movement inside the wardrobe is triggered, moving until the tag with the requested uid is detected and stopped. the state then changed to detection, and the uid tag is displayed. the system then changes to the capture state, and top and bottom front photos of the garment are captured. afterward, the system moves to the servo state, where the servo motor is triggered, performing a 180 degrees rotation. after the rotation, the system returns to the capture state and captures top and bottom back photos of the garment. the system then changes to the stitch state, where a stitching algorithm, which is part of the opencv library, is used to combine the photos. this stitching algorithm solved the issue of not being able to capture the full view of each garment with only one camera shot by combining the images based on keypoint detection and overlapping common points on each picture. 5.1. background removal ai is integrated into the system to accurately classify the type of garment, which is a critical feature since the user may not be aware of all the clothing items inside the closet. nfc communication is utilized only to select the clothing item and identify its respective uid for analysis. after the item is selected, the system performs segmentation and background removal to facilitate identification of the dominant colours of the garment. this process is necessary to determine the colours requested by the user accurately. the integration of ai, nfc communication, and image processing techniques allows the system to identify and classify clothing items accurately, which represents a significant advancement in the field of clothing management and classification. to accomplish this task, a method was needed to remove the image's background while considering the garment's boundaries. after conducting multiple studies and research, a deep learning algorithm called mask r-cnn was employed to segment the images. to model and train the algorithm, google colab framework was utilized, providing high processing power for the neural network training on a local machine. to create the dataset, photos were gathered and divided into six types (classes) of garments: pants, shorts, t-shirts, shirts, polo shirts, and dresses. as there were not enough photos available, several images were obtained from an online dataset [39]. the final dataset comprised a total of 480 garment photos, divided equally among the six classes with 80 photos per class. to ensure the accuracy of the garment boundaries, the images were annotated with the visual geometry group’s annotator tool. following the annotation process, a function was developed to load the annotations in json format from the dataset. this function assigned each annotation to the corresponding class, as well as other attributes, such as region_attributes, shape attributes, and x/y coordinates from the bounding box. transfer learning was employed in the training process, which involved using a pretrained model. the dataset was then loaded, and several parameters were adjusted to control the flow of training. the learning rate was set to adjust the speed at which the neural network converges to the goal. this involved a trade-off between low values, which could slow down the training, and high values, which could cause the model to diverge from the goal. the number of iterations was adjusted to maximize the average precision (ap) and intersection over union (iou) as well as to prevent overfitting. the neural network was trained using specific parameters and further tested with various garments to ensure its accuracy. a function was developed to segment and classify garment images uploaded by the user, which takes an input image and produces three output images: the segmented image, the final image with the background removed, and the corresponding binary mask. additionally, a colour detection algorithm is used to analyse the pixels from the region of interest in each image, as shown in figure 6. figure 5. system state machine. figure 6. image segmentation flowchart. acta imeko | www.imeko.org september 2023 | volume 12 | number 3 | 6 5.2. colour detection after the background removal, the garment image undergoes a colour detection algorithm that detects the predominant colours in the image using the opencv library. the algorithm analyses the rgb values for each pixel in an image, ranging from 0 to 255, to determine the colour based on a combination of the three components. the process begins by defining two arrays containing colour names and their respective rgb values. the algorithm then utilizes a binary mask and final image, generated by a deep learning algorithm that removes the background, to extract the pixel values from the final image, corresponding to the non-black coordinates of the binary mask. to determine the closest colour in the colour array, each pixel within the clothing region of interest is saved, and the frequency of each colour is counted to calculate the percentage of each colour present in the garment. to avoid potential segmentation errors, colours with percentages below 10 % are not considered. by focusing only on the dominant colours of the clothing item while minimizing the impact of segmentation errors on the colour analysis results, this approach ensures that clothing colour analysis is carried out exclusively on the pixels identified as part of the clothing region of interest. this section presents the results obtained to confirm and validate the proper functioning of the different modules previously introduced [40]. in addition, the flowchart of the control software developed, which integrates the hardware and software of the wardrobe automatism, is presented in figure 7. the pictures of a garment captured by the mechatronic system, which are subsequently combined by the stitching algorithm, are represented in figure 8. the automatic wardrobe system was designed to provide controlled photo capture, thereby eliminating dark fields and shadows. a test was conducted on a garment to demonstrate the accuracy of the photo capture control system. figure 9a) and figure 9b) depict photos captured outside the controlled system, which exhibit shadows and dark fields. in contrast, the photo captured inside the automatic wardrobe, as shown in figure 9c), is properly illuminated, highlighting the garments features and eliminating any reflection. the background removal algorithm utilized the mask r-cnn neural network, and several training sessions were carried out to optimize all parameters for maximum mean average precision (map) and iou. initially, the model was trained using a batch size of 128, which was later reduced to 64. during the training process, a large number of iterations were carried out to determine the point at which the model started overfitting. the map results for each class after 5000 iterations are presented in table 3. mean average precision (5000 iterations). . as can be seen from table 3. mean average precision (5000 iterations). , the value obtained for the dress class is not as expected, as the model has difficult in recognizing the dress silhouette correctly, given that the dresses in the dataset do not have a similar shape. subsequently, by adjusting the number of iterations to 3250, the results improved, with the map value figure 7. system flowchart. figure 8. an example of a garment with stitching implemented in the photos: (a) button photo capture; (b) upper photo capture; (c) stitching of both photo captures. figure 9. the effect of lighting conditions on a garment: a) ambient lighting with several reflections and shadows; b) ambient lighting without homogeneous illumination; c) controlled lighting. table 3. mean average precision (5000 iterations). map (%) pants (%) shorts (%) shirt (%) t-shirt (%) polo shirt (%) dress (%) 83.034 91.716 87.129 88.898 84.752 90.908 54.800 table 4. mean average precision (3250 iterations) map (%) pants (%) shorts (%) shirt (%) t-shirt (%) polo shirt (%) dress (%) 85.667 90.565 87.129 91.785 85.941 91.595 66.988 acta imeko | www.imeko.org september 2023 | volume 12 | number 3 | 7 increasing to 85.667 %. this increase was largely due to the improvement in the dress class value, table 4. subsequently, a test was carried out with a slightly lower number of iterations, i.e., 3000 iterations. in this test, the overall map value decreased slightly, however, the value obtained for the dress class increased, approaching the other class values (table 5). thus, two trainings were conducted with a number of iterations of 2750 and 2500 (2nd and 3rd rows, respectively), as presented in table 6. table 6 illustrates that the model's performance starts deteriorating as it enters the underfitting phase after 2500 iterations. although the map value obtained for 2750 iterations was reasonable, the dress class's map fell short of expectations, being considerably lower than the other five classes. furthermore, the iou values were also obtained during the previously performed trainings, as can be seen in table 7. to obtain this metric, a function was used that allows the calculation of iou values with a variable threshold between 0.5 and 0.95, spaced by 0.01. the function goes through the 48 images of the validation dataset and obtains the respective iou value for each photo, comparing the predicted mask with the real mask obtained (from the application of the neural network). in this calculation, the two masks are intersected, and the area of intersection and union between the two masks is obtained, and the corresponding metric is calculated. following the validation of the developed automation, the background removal and colour detection algorithms were tested using several images. three garment photos were used in this testing phase, with one captured within the prototype and the other two taken using a smartphone. the first photo, which is depicted in figure 10, revealed that the segmentation algorithm was not perfect. specifically, the model wrongly identified some background pixels as region of interest, especially around the hanger. to address possible errors resulting from poor segmentation, the colour detection algorithm only considered percentages above 10 %. the percentage analysis revealed a predominance of cadet blue (40 %) with two other colours, slate grey (20 %) and sea green (12 %) (figure 11). the green colour corresponds to the background pixels due to the difficulty in accurately distinguishing the region of interest from the background, especially near the hanger. additionally, two additional photos were captured using a smartphone, as shown in figure 12 and figure 14. figure 12 showcases a precise segmentation, with the colour distribution divided into three colours: dark cyan (32 %), light sea green (27 %), and teal (19 %) (figure 13). the remaining 22 % encompasses colours with low percentages that are irrelevant for this analysis, such as background pixels that were erroneously considered as region of interest, especially around the garment’s edges. in the latest test, the segmentation algorithm was capable of distinguishing the region of interest from the background. the colour analysis shows the following colour distribution: rosy brown (50 %), grey (29 %), and dark grey (17 %) (figure 15). as previously stated, one of the goals of this study was to validate the system with the blind community. to achieve this, a preliminary validation was conducted through an interview with table 5. mean average precision (3000 iterations). map (%) pants (%) shorts (%) shirt (%) t-shirt (%) polo shirt (%) dress (%) 84.094 90.161 87.129 89.810 82.640 81.700 73.124 table 6. mean average precision (2750/2500 iterations). map (%) pants (%) shorts (%) shirt (%) t-shirt (%) polo shirt (%) dress (%) 85.581 91.011 98.57 93.441 83.564 92.702 54.198 79.137 88.520 92.426 82.028 77.355 80.297 54.198 table 7. intersection over union obtained in the sequential training. threshold/iterations 0.5 0.6 0.7 0.8 0.9 average 5000 0.91 0.92 0.91 0.89 0.74 0.87 3250 0.92 0.93 0.89 0.81 0.60 0.83 3000 0.87 0.90 0.86 0.76 0.63 0.80 2750 0.87 0.88 0.84 0.80 0.53 0.78 2500 0.92 0.91 0.86 0.75 0.67 0.82 figure 10. t-shirt segmentation and classification. figure 11. t-shirt colour distribution. figure 12. shorts segmentation: (a) original image; (b) segmented image. figure 13. shorts colour distribution. cadet blue 40% slate gray 20% sea green 12% others 28% dark cyan 32% light sea green 27% teal 19% others 22% acta imeko | www.imeko.org september 2023 | volume 12 | number 3 | 8 a blind representative from the acapo association. the purpose of the interview was to address important questions regarding the use of such systems by visually impaired individuals. the interview began with an overview of the system to introduce all the relevant details. following this, several questions were posed to identify the challenges blind people face when selecting and managing garments. the study revealed that distinguishing between different colours on each garment was a significant challenge. however, insights gained from the validation process suggest that presenting the location of each colour would be more relevant for blind users than showing the percentage of each colour. moreover, it was suggested to include audible feedback during the selection process, such as a gradual rising sound or a voice system to indicate the start and the end of the process. 6. conclusions and future work the scope of this work is integrated in the field of technology that can be used to assist human necessities [41]-[43]. the primary objective of this proposal paper was to present a system that assists blind people in choosing their clothes. the design of the prototype was the first part of the project. it required finding on the market a wardrobe that would allow the proof of concept in a home environment. to develop the system, it was necessary to study the requirements for its operation, particularly with regard to lighting, tag reading, rotation and movement of the clothes, and image capture. to operate these modules, an algorithm was developed to control each of the existing electronic elements. following the validation and individual testing of each module, they were integrated into a single system controlled from a command line interface. to obtain the predominant colours of the clothes, colour detection and background removal algorithms were implemented, with the latter using a neural network. the prototype presented is part of a larger project under development, with its future integration planned for a mobile application as part of the myeyes system [4], [5]. in this regard, a server for user request management has already been implemented. this integration will allow for the replication of the physical wardrobe on a mobile device, enabling clothing selection with a single click. in summary, it was evidenced that the system may be an efficient solution to reduce the time taken for garment selection, particularly in terms of colour differentiation and the selection of combinations for blind people. although, as the developed prototype is a proof of concept, the system has some limitations, including the reduced size of the wardrobe. this feature allowed only for the use of small-sized clothes and a limited number of items inside the wardrobe at a time. all developed electronics were designed for specific use in this small prototype and will need to be resized and relocated for the development of a prototype with larger-sized clothes. when integrating the physical prototype of larger dimensions with myeyes [4], [5], a more comprehensive validation will be conducted with the blind community, allowing to test the developed system with a view to its possible commercialization. acknowledgements this work has the support of association of the blind and amblyopes of portugal (acapo) and association of support for the visually impaired of braga (aadvdb). their considerations allow the first insights to a viable solution for the blind people community. this work has been supported by national funds through fct – fundação para a ciência e tecnologia within the 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https://doi.org/10.1109/tim.2014.2343411 https://doi.org/10.1007/978-3-031-28663-6_1 https://doi.org/10.1109/jsen.2022.3207912 https://doi.org/10.1109/tim.2021.3068173 https://doi.org/10.1109/tim.2014.2343411 statements of conformity provided by laboratories acta imeko issn: 2221-870x june 2023, volume 12, number 2, 1 3 acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 1 statements of conformity provided by laboratories ana čop1 1 croatian metrology society, zagreb, croatia section: technical note keywords: statement of conformity; decision rule; laboratory; ilac g8; iso/iec 17025 citation: ana čop, statements of conformity provided by laboratories, acta imeko, vol. 12, no. 2, article 34, june 2023, identifier: imeko-acta-12 (2023)02-34 section editor: leonardo iannucci, politecnico di torino, italy received january 31, 2023; in final form june 11, 2023; published june 2023 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: ana čop, e-mail: ancica.co@gmail.com 1. introduction laboratories are often required or expected to provide a statement of conformity to specification or standard. specifications are usually defined in the product specification, applicable legislation, or by laboratory customer. they represent acceptable limit that (a property of) a test item shall comply with. because of uncertainty in measurement, when measurement results are close the specification limit, the way how the measurement uncertainty is taken into account can influence the decision on conformity of the test item. the risk of incorrect decision is taken by the side specifying the decision rule to be applied. in the context of laboratory work, this is the laboratory's customer. this note provides information on relevant literature and practice for laboratories to extend their knowledge on statements of conformity and risks associated with decision rules. the starting point is ilac g8 [1] and jcgm 106 [2]. for advanced statistics and calculation, the document [2] should be consulted. 2. decision rule the agreement on how to take into account measurement uncertainty is called a decision rule. the definition of decision rules in iso/iec 17025 [3] is “rule that describes how measurement uncertainty is accounted for when stating conformity with a specified requirement“. the definition in [2], “rule that describes how measurement uncertainty will be accounted for with regard to accepting or rejecting an item, given a specified requirement and the result of a measurement” is based on the consequences of the decision on the conformity of the test item. the definition in [2] also states that this rule shall be documented. statements of conformity can be expressed as a binary decision (conforming/nonconforming, yes/no, pass/fail) or as a non-binary decision (with more than two possible outcomes; e. g. pass, fail, and inconclusive or conditional pass/fail) [1]. in order to provide statements of conformity, a specification or tolerance limit (tl), defined as “specified upper or lower bound of permissible values of a property” [2], and an acceptance limit (al), defined as “specified upper or lower bound of permissible measured quantity values” [2], are established. if there are both upper and lower limits, intervals are considered. a common decision rule known as simple acceptance considers a test item as conforming if the measured value is in the tolerance interval, as shown in figure 1. to keep the probability of incorrect decisions at an acceptable level, this decision rule is associated with the requirement for the value of related acceptable measurement uncertainty [2]. therefore, simple acceptance should not be understood as not considering measurement uncertainty. the risk of providing an incorrect decision of conformity is kept under control by the introduction of guard bands. a guard abstract when required to provide a statement of conformity, the laboratory shall agree with its customer on the specification and decision rule to be applied. employing a decision rule on how to take into account measurement uncertainty carries the risk of providing an incorrect decision. to reduce this risk, the mechanism of guard banding is introduced. this technical note covers the approach presented in ilac g8 and other relevant literature. in some cases, e.g. when assessing conformity with regulatory requirements, measurement uncertainty is taken into account indirectly. when providing statements of conformity, laboratories usually consider the risk related to a particular test item. providing the statement of conformity is also covered by the iso/iec 17025 standard for the competence of laboratories. mailto:ancica.co@gmail.com acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 2 band (𝑤) is an interval between a tolerance limit and a corresponding acceptance limit where length 𝑤 = |𝑇𝐿 − 𝐴𝐿| as defined in [1]. as [1] states, guard bands are a “safety factor built into the measurement decision process”. applying a guard band to reduce the risk of identifying a nonconforming item as conforming (or in other words reducing the probability of falsely accepting the nonconforming item) by setting the acceptance limit inside the tolerance interval is called guarded acceptance [2]. for guarded acceptance the length of the parameter w is taken to be positive [2]. the guarded acceptance is shown in figure 2. guarded rejection is used to increase the probability that item identified as non-conforming item is truly nonconforming (or, in other words, reducing the probability of falsely rejecting a conforming item) [2]. guarded rejection is commonly used “to have evidence that a limit has been exceeded prior to taking a negative action” [2]. the guard band 𝑤 is usually taken to be “a multiple (𝑟)of the expanded uncertainty 𝑈 for a coverage factor 𝑘 = 2, 𝑈 = 2 𝑢, 𝑤 = 𝑟 𝑈” as defined in [2]. eurachem/citac guide [4] provides guidance and examples for determining the guard band and acceptance limit taking into account the measurement uncertainty, distribution of measurand values and required level of the probability that the measurand is within the specification limit. the eurolab guide [5] provides examples where decision rules are defined based on the hypothesis testing approach. in order to assure that the measurement uncertainty is taken into account consistently, especially in the case of assessing conformity to regulatory requirements, the measurement uncertainty is taken into account indirectly. practices in legal metrology [6] include the approach of accounting for plausible measurement uncertainty in maximum permissible errors (mpes), by “establishing conservative (inservice) maximum permissible errors in order to draw “safe” conclusions concerning whether measured errors of indication are within acceptable limits” or “specifying s a fraction, like 1/3 or 1/5, for the maximum allowed ratio of the error (uncertainty) of the standard (reference) measuring instrument to the mpe”. the example where acceptable measurement uncertainty is included in acceptance limit is the wada technical document [7]. this document provides a detailed explanation of defining the decision limit with the guard band defined with maximum acceptable measurement uncertainty obtained from the external quality assessment scheme and k factor corresponding to 95% coverage range for one tailed normal distribution. it is also required that the measurement uncertainty of particular laboratory is lower than this maximum acceptable measurement uncertainty. the document also provides instructions on how to report. an example from law enforcement practice is reduction of the measured speed value by a certain amount (guard band) when calculating a fine for speeding violation, which ensures that the decision to impose a fine for a speeding violation is made with significant confidence that the speed limit has been exceeded. in some cases, like water legislation [8], the performance criteria, limit of detection and acceptable measurement uncertainty are defined, and if ensured, the obtained measurement result is directly compared to the specification limit. the decision rule can also be explicitly stated in regulation or standards. as defined in [9] supporting european official food control, “compliance with the maximal residue level (mrl) is checked by assuming that the mrl is exceeded if the measured value exceeds the mrl by more than the expanded uncertainty (𝑥 − 𝑈 > 𝑀𝑅𝐿). with this decision rule, the value of the measurand should be above the mrl with at least 97.5% confidence”. 3. risk consideration when laboratory considers, the risk of falsely accepting a nonconforming item or falsely rejecting a conforming item is commonly based on the measurement of a particular item. as already mentioned, the risk arising from a measurement decision is on the side that specifies the decision rule to be applied. since the decision rule is specified, or at least accepted, by laboratory customers, it is up to them to cope with the risk of incorrect decisions. laboratory customers also take the responsibility for the consequences of non-conforming items. as [4] points out, there is a need to balance resources to be invested in reducing measure uncertainty against the costs arising from a higher number of incorrect decisions in case of higher measurement uncertainty. as described in [1], different guard bands correspond to different levels of risk. in case of simple acceptance and assuming normal distribution of measurement results, the probability of false accept is up to 50 %. in case of guarded acceptance with a guard band equal to extended measurement uncertainty and assuming the normal distribution of measurement results, the probability of false accept is up to 2.5 %. for larger guard bands, the probability of false accept is additionally reduced. it is up to the laboratory to consider other laboratory risks. a laboratory should invest effort in ensuring that measurement uncertainty is monitored, that it is estimated realistically, fit for purpose and stable in time. with respect to laboratory personnel competence, skills and knowledge should include understanding the context and purpose of measurement requested by the figure 1. illustration of the simple acceptance based on ilac g8 [1]. figure 2. illustration of the guarded acceptance based on ilac g8 [1]. acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 3 laboratory customer, standard or legislation, if applicable; and the risks associated with decision rules. 4. iso/iec 17025 requirements the iso/iec 17025 [3] standard includes requirements related to providing statements of conformity by laboratories. personnel responsible shall be authorised for this activity (iso/iec 17025, §6.2.6) based on the competence criteria set. monitoring of laboratory personnel competence shall also cover competence for providing statements of conformity. in the request review phase, it is up to the laboratory to assure that the customer requesting a statement of conformity is aware and agrees with the decision rule in the request review stage (iso/iec 17025, §7.1.3). when a statement of conformity is provided, the employed decision rule shall be documented and applied. the decision rule shall take into account risks of an incorrect decision, unless it is specified by the customer, legislation, or standard. (iso/iec 17025, §7.8.6.1) the report shall contain both the specification and decision rule applied. (iso/iec 17025, §7.8.6.1). the extent of what the statement of conformity shall identify is prescribed by iso/iec 17025, §8.6.2; and includes “to which results the statement of conformity applies; which specifications, standards or parts thereof are met or not met; and which decision rule is applied (unless it is inherent in the requested specification or standard)”, [3]. 5. conclusions the risk associated with the applied decision rule is to be taken by laboratory customer. to assist the customers who are sometimes not even aware of the risk of incorrect decision, the laboratory shall understand the purpose of the measurement and the level of risk associated with the decision rule applied. to ensure consistent understanding and application, it is necessary that both decision rule and specification are defined and documented in the request review stage, and that the resulting report includes information on both. in order to make a reliable decision on the conformity of the test item, it is an important task of the laboratory to realistically estimate and monitor measurement uncertainty. special care shall be taken in cases where there are several properties measured in the same sample. providing a general statement that the sample is conforming can be misleading. as described in iso/iec 17025, §8.6.2, a statement of conformity shall identify “to which results it applies”, [3]. when sampling activity, as one of laboratory activities recognised by iso/iec 17025, is included in laboratory services, sampling measurement uncertainty contribution to overall measurement uncertainty shall also be considered when providing statements of conformity. acknowledgement i wish to thank višnja gašljević for numerous fruitful discussions on measurement uncertainty and decision rules. references [1] ilac, ilac-g8:09/2019, guidelines on decision rules and statements of conformity. online [accessed 12 june 2023] https://ilac.org/publications-and-resources/ilac-guidance-series/ [2] bipm, jcgm 106:2012, evaluation of measurement data – the role of measurement uncertainty in conformity assessment. online [accessed 12 june 2023] https://www.bipm.org/en/committees/jc/jcgm/publications [3] iso, iso/iec 17025:2017; general requirements for the competence of testing and calibration laboratories [4] a. williams and b. magnusson (eds.) eurachem/citac guide: use of uncertainty information in compliance assessment (2nd ed. 2021). isbn 978-0-948926-38-9. online [accessed 12 june 2023] https://www.eurachem.org/index.php/publications/guides/unc ertcompliance [5] eurolab “cook book” – doc no. 8, rev. n. 3, 09/2018 determination of conformance with specifications using measurement uncertainties – possible strategies. online [accessed 12 june 2023] https://www.eurolab.org/pubs-cookbooks [6] oiml, oiml g 19, the role of measurement uncertainty in conformity assessment decisions in legal metrology, edition 2017. online [accessed 12 june 2023] https://www.oiml.org/en/files/pdf_g/g019-e17.pdf/view [7] wada, wada technical document – td2022dl, decision limits for the confirmatory quantification of exogenous threshold substances by chromatography-based analytical methods, 6 october 2021. online [accessed 12 june 2023] https://www.wada-ama.org/en/resources/labdocuments/td2022dl [8] eu, directive (eu) 2020/2184 of the european parliament and of the council of 16 december 2020 on the quality of water intended for human consumption. online [accessed 12 june 2023] https://eur-lex.europa.eu/eli/dir/2020/2184/oj [9] eu, analytical quality control and method validation procedures for pesticide residues analysis in food and feed, sante/11312/2021. online [accessed 12 june 2023] https://www.eurlpesticides.eu/userfiles/file/eurlall/sante_11312_2021.pdf https://ilac.org/publications-and-resources/ilac-guidance-series/ https://www.bipm.org/en/committees/jc/jcgm/publications https://www.eurachem.org/index.php/publications/guides/uncertcompliance https://www.eurachem.org/index.php/publications/guides/uncertcompliance https://www.eurolab.org/pubs-cookbooks https://www.oiml.org/en/files/pdf_g/g019-e17.pdf/view https://www.wada-ama.org/en/resources/lab-documents/td2022dl https://www.wada-ama.org/en/resources/lab-documents/td2022dl https://eur-lex.europa.eu/eli/dir/2020/2184/oj https://www.eurl-pesticides.eu/userfiles/file/eurlall/sante_11312_2021.pdf https://www.eurl-pesticides.eu/userfiles/file/eurlall/sante_11312_2021.pdf journal contacts acta imeko june 2014, volume 3, number 2, 1 www.imeko.org journal contacts about the journal acta imeko is an e-journal reporting on the contributions on the state and progress of the science and technology of measurement. the articles are based on presentations presented at imeko workshops, symposia and congresses. the journal is published by imeko, the international measurement confederation. the issn, the international identifier for serials, is 2221-870x. editor-in-chief paolo carbone, italy honorary editor-in-chief paul p.l. regtien, netherlands editorial board francisco alegria, portugal leopoldo angrisani, italy filippo attivissimo, italy eulalia balestieri, italy catalin damian, romania eric benoit, france pasquale daponte, italy luigi ferrigno, italy alistair forbes, united kingdom fernando janeiro, portugal konrad jedrzejewski, poland andy knott, united kingdom fabio leccese, italy helena geirinhas ramos, portugal pedro ramos, portugal sergio rapuano, italy dirk röske, germany alexandru salceanu, romania constantin sarmasanu, romania lorenzo scalise, italy emiliano schena, italy enrico silva, italy marco tarabini, italy susanne toepfer, germany luca de vito, italy about imeko the international measurement confederation, imeko, is an international federation of actually 39 national member organisations individually concerned with the advancement of measurement technology. its fundamental objectives are the promotion of international interchange of scientific and technical information in the field of measurement, and the enhancement of international co-operation among scientists and engineers from research and industry. addresses principal contact prof. paolo carbone university of perugia dept. electr. inform. engineering (diei) via g.duranti, 93, 06125 perugia italy email: paolo.carbone@unipg.it acta imeko prof. paul p. l. regtien measurement science consultancy (msc) julia culpstraat 66, 7558 jb hengelo (ov) the netherlands email: paul@regtien.net support contact dr. dirk röske physikalisch-technische bundesanstalt (ptb) bundesallee 100, 38116 braunschweig germany email: dirk.roeske@ptb.de acta imeko | www.imeko.org june 2014 | volume 3 | number 2 | 1 mailto:paul@regtien.net journal contacts microsoft word article 6 tc4 paper 2.docx acta imeko  august 2013, volume 2, number 1, 12 – 15  www.imeko.org    acta imeko | www.imeko.org  august 2013 | volume 2 | number 1 | 12  metrological atomic force microscope and traceable  measurement of nano‐dimension structures  sitian gao, mingzhen lu, wei li, yushu shi, qi li  national institute of metrology, 100013, beijing, china      keywords: nanometrology; afm; traceability; nanostructure; pitch; linewidth; step height;  citation: sitian gao, mingzhen lu, wei li, yushu shi, qi li, “metrological atomic force microscope and traceable measurement of nano‐dimension  structures”, acta imeko, vol. 2, no. 1, article 6, august 2013, identifier: imeko‐acta‐02(2013)‐01‐06  editors: paolo carbone, university of perugia, italy; ján šaliga, technical university of košice, slovakia; dušan agrež, university of ljubljana, slovenia  received january 10 th , 2013; in final form july 11 th , 2013; published august 2013  copyright: © 2013 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: none reported  corresponding author: sitian gao, e‐mail: gaost@nim.ac.cn    1. introduction  with the development of semiconductor industry, the line width of integrated circuits is decreasing to tens of nanometers. the miniaturization in semiconductor manufacturing requires measurements with nanometer accuracy in the lithographic process. accurate dimensions and position of patterns are essential for present industrial quality. the main industrial countries around the world have been working at establishing metrological specifications and instruments relevant to nanotechnology. the discussion group 7 (dg7) for nanometrology under the consultative committee for length’s working group on dimensional metrology (cclwgdm) decided to perform a comparison for five different types of artifacts in order to set up an international nanometrological regime. atomic force microscopes (afms) provide high resolution and quantitative measurements for nanoand micro-scaled structures [1]. afms are engaged in dimensional measurements including distance, pitch, width, diameter, geometry, roughness, and thickness [2]. the probe or the sample is usually scanned by a piezotube and the displacement is measured by capacitive sensors. due to the nonlinearity and hysteresis of the piezo scanner, afms with metrological abilities are required for consistency in the measurement quality. most metrological afms integrate laser interferometers to ensure direct traceability to the si unit of length, and have a full evaluation of uncertainty budget to assign an uncertainty to the measurement results. afms can be calibrated by using transfer standards such as step height, 1d or 2d gratings and 3d pyramids, which are well characterized and traced to metrological afms. at the national institute of metrology (nim), a metrological afm has been developed and improved. in this paper, we report the principle and architecture of the metrological afm. then the calibration applications of the metrological afm in step height and pitch artefacts are demonstrated. 2. metrological afm  2.1. principle of afm  afm is based on the interaction between a sharp tip and samples. when the cantilever is approaching the sample surface, the interaction between the tip and sample will cause bending of the cantilever. the method of cantilever sensing abstract  the  quantity  assurance  in  semiconductor  industry  development  requires  dimensional  measurements  with  nanometer  accuracy.  a  metrological  afm  is  designed  to  establish  a  traceable  standard  with  nanometer  uncertainty.  the  principle  and  design  of  the  instrument are introduced in this paper. the displacement of the sample is traced to the si unit by interferometers. the metrological  afm is applied to step height and line pitch measurements. the results are compared with an optical instrument and a profilometer.  the metrological afm is used for step height measurement in an international comparison and the result shows an uncertainty less  than 2 nm. the application of the metrological afm in pitch measurements is also introduced. acta imeko | www.imeko.org  august 2013 | volume 2 | number 1 | 13  here is a confocal method different from conventional quadrant photodiode detection. the confocal detection principle of the afm veritekt (zeiss) is illustrated in figure 1. the laser beam through the pinhole is focused onto the cantilever and reflected to the beam splitter. then the beam is separated and focused to the two detectors. the two pinholes before the detectors are arranged before and after the focal spots respectively. when the interaction between the tip and sample causes bending of the cantilever, the two detectors have opposite responses to the displacement of the cantilever tip. the reflected laser beam and the incident beam share the same optical path and the structure is stable, so the signal is less disturbed by air. the cantilever is 100 m in length and the tip height is 10-15 m with radius less than 10 nm. to reduce the fluctuation of the laser intensity, the difference of the signals is divided by the summation. the signal shows a linear relation to the displacement of the sample to tip as shown in figure 2. the signal is detected for feedback control of the piezostage, to keep the distance between the tip and sample constant. so the sensor serves as the zero point indicator. 2.2. metrological afm instrument  in the commercial afm using a piezotube to drive the probe, the bending of the tube will change the length of the tube in z-direction when scanning in the xy plane [3]. the scanning surface is curved and requires calibration by standards. the displacements also require calibration for the nonlinearity of the piezoelectric actuators. most of the metrological afms are equipped with interferometers to calibrate the position and trace the displacement to si unit [4]. by the incorporation of three miniature laser interferometers, an afm (zeiss) has been modified to the metrological afm in cooperation with the ilmenau technical university and the physikalisch-technische bundesanstalt [5]. the design is shown in figure 3. the sample stage is moved by a 3-dimensional scanner. the monolithic flexure hinge stage is driven by three linear piezoceramic elements with capacitive sensors in three directions. the flexure hinge is designed with different lengths and deflection angles to achieve a scanning range of 70 μm × 15 μm × 15 μm. the displacements in three axes are traceable by interferometers with a 633 nm laser. the maximum angular motion of the stage is 0.5 arcsec. so the tip of the afm is placed at the intersection of the interferometer beams, to eliminate the abbe error. the displacement signal is sent by a computer to the d/a converter to drive the piezostage. the difference between the expected position p and the real position ps measured by the interferometers is caused by the motion coupling between different axes and the non-linearity error of the motion axes. the compensation function is derived from the measurement. figure 1. afm with confocal sensing technique.  figure 2. the cantilever signal versus the tip displacement.   figure 3. structure of metrological afm.  figure 4.  a sample of a step height standard.  acta imeko | www.imeko.org  august 2013 | volume 2 | number 1 | 14  3. measurement results  the step height, line width and pitch are three important parameters of nano-structure dimensions in integrated circuit manufacturing. to provide consistent and traceable measurements, the transfer standards are calibrated by the metrological afm and then are used to calibrate the instrument in industry and research. 3.1. step height  step height standards are utilized to calibrate the z-axis of microscopes and topography measuring instruments. a step sample is shown in figure 4. it is fabricated on a si substrate with sio2 step squares and bars. six height samples are fabricated with nominal heights 300 nm; 400 nm; 600 nm; 900 nm; 1000 nm; and 1800 nm. the samples are measured with a profilometer and an afm to compare different instruments and methods. the results are shown in table 1. the step height results measured with different instruments show consistent results with deviations less than 10 nm. to establish nanometrological equivalence in the nanoscale regime, international comparison of nano-standards started from the year 2000. step height international comparisons with ptb as the pilot laboratory has been accomplished [6]. the standards are si substrates with sio2 steps coated with cr. the measured region is 100 m×100 m. the scanning area of the instrument is 70 μm×12 μm with 400 × 200 pixels, so three subregions on the standard are scanned from top down in the 100 nm range. the result of a 20 nm step height sample is shown in figure 5. for each profile, the 15 μm lines marked on the upper step profile and the lower surface are fitted with a least square criterion to obtain the step height. the line step heights at different positions are averaged. fifteen national metrology institutes joined the comparison with different instruments. the result of the 70 nm step height is shown in figure 6. the uncertainty of the measured result of nim is no more than 2 nm. the uncertainty is somewhat larger compared with others. in fact the crosstalk in the z-direction caused by movement in the xand y-axes is the main uncertainty component in the step height measurement and this is independent of the step height. the uncertainty for large step height measurement results are relatively small compared with those of the other participant [6]. 3.2. 1d grating pitch  the lateral magnification of the afm can be calibrated by 1d or 2d pitch standards. these standards are calibrated by the metrological afm. for the periodic structure, the gravity centres of the grating structures are calculated and then the position of the centres are averaged to obtain the pitch value. the results of a 3 m pitch are shown in figure 7. the 69.246 m 12.147m 70 nm 0 15 m 15 m 15 m figure 5．image of a 20 nm step sample measured by the metrological afm.  table 1. step height standard results measured with different instruments (nm).  no  profilometer a  metrological afm  afm (ptb) b  1  301.0  300.5  301.3  2  396.0  393.5  391.7  3  593.5  598.9  595.6  4  924.0  922.7  923.6  5  1079.0  1077.0  1075.1  6  1739.0  1730.0  1722.7  a. alpha step 200  b. metrological spm with interferometer to calibrate  figure  6.  measured  step  heights  hi  of  the  participating  institutes  and  reference value href (red line) of the 70 nm step height sample.[6]      (a)    (b)  figure 7. measurement results of a 3 m 1d grating with 35 m × 10 m  scanning range (a) and a 1d scanning profile of the grating (b).  acta imeko | www.imeko.org  august 2013 | volume 2 | number 1 | 15  scanning range is 35 m ×10 m. the profile is shown in figure 7 (b). the height of the grating is about 110 nm. a threshold line across the profile is chosen to separate the grating structures. the gravity centre of each structure is calculated and the average pitch is determined [7]. usually the sample is mounted with an angle deviation relative to the interferometers axes. the incline of the sample causes a cosine error. as shown in figure 8, the error from the angle between the profile and the x-axis α can be corrected by fitting the baseline of the single profile. also the sample orientation in the sample plane relative to the interferometers axes is corrected. a series of measurements along the y-axis is required to calculate the orientation of the grating bars. the real pitch is obtained by 'sin( ) cos( ) x x      , (1) where β is the angle between the measured profile and the direction of the bars. the average pitch is 3.01 m with a standard deviation of 0.05 m due to the inhomogeneity of the sample. the uncertainties of the wavelength and the temperature are negligible. the uncertainty of the instrument due to the coupling of the different axes and the nonlinearity is 1.15 nm. the uncertainty due to the cosine error caused by the sample mounting after correction is 0.03 nm. because the probe tip of the afm has a finite size, the measured profile of the step edge is the convolution of the tip shape and the real step profile. this edge effect is not significant for step heights or line pitch measurements of the periodic structure, for the edges have the same influence and can be eliminated. however the line width is influenced by the tip size of the afm probe. so the definition of the line width is under study to retrieve the real width. 4. conclusions  a metrological afm is designed to establish a traceable standard with nanometer uncertainty. the sample stage is driven by a piezostage and reflectors are mounted on the stage as reference mirrors of interferometers, so that the movements of the stage in 3 directions are traceable. for step height measurements, the metrological afm gives comparable results with other instruments and the afm demonstrates an uncertainty u95 <2 nm. for the pitch measurement, the uncertainty from the instrument is less than 2 nm which is mainly due to the coupling of the axes and the sample incline. the scanning range limits the application of the metrological afm. a large range metrological afm has been developed at the nim to calibrate larger samples [8]. references   [1] a.yacoot, l. koenders, “recent developments in dimensional nanometrology using afms”, meas. sci. technol., 22, (2011), pp. 122001. [2] j. a. kramar, r. dixson, n. g. orji, “scanning probe microscope dimensional metrology at nist”, meas. sci. technol. 22, (2011), pp. 024001. [3] j. kwon, j. hong, y. kim, “atomic force microscope with improved scan accuracy, scan speed and optical vision”, rev sci. instrum., 74, (2003), pp. 4378-4383. [4] g. dai, f. pohlenz, h. danzebrink, k. hasche, g. wilkening, “improving the performance of interferometers in metrological scanning probe microscopes”, meas. sci. technol. 15, (2004), pp. 444-450. [5] m. bienias, s. gao, k. hasche, r. seemann, k. thiele, “a metrological scanning force microscope used for coating thickness and other topographical measurements”, applied physics a., 66, (1998), pp. s837-s842. [6] l. koenders. “wgdm-7: preliminary comparison on nanometrology according to the rules of ccl key comparisons nano 2 step height standards”, ptb braunschweig, (2003). [7] g. dai, l. koenders, f. pohlenz, t. dziomba, h. danzebrink, “accurate and traceable calibration of one-dimensional gratings”, meas. sci. technol. 16, (2005), pp. 1241–1249. [8] m. lu, s. gao, q. li, w. li, y. shi, x. tao, “long range metrological atomic force microscope with versatile measuring head”, proc. spie 8759, eighth international symposium on precision engineering measurement and instrumentation, 2012, 8-11, august, china, pp. 87594y. figure 8. error correction of the sample incline.  microsoft word article 4 191-1064-1-pb.docx acta imeko may 2014, volume 3, number 1, 10 – 15 www.imeko.org acta imeko | www.imeko.org may 2014 | volume 3 | number 1 | 10 fundamental concepts of measurement l. finkelstein the city university, department of systems and automation, st. john street, london e.c.1., england section: research paper keywords: measurement theory, uncertainty citation: l. finkelstein, fundamental concepts of measurement, acta imeko, vol. 3, no. 1, article 4, may 2014, identifier: imeko-acta-03 (2014)-01-04 editor: luca mari, università carlo cattaneo received may 1st, 2014; in final form may 1st, 2014; published may 2014 copyright: © 2014 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited 1. the importance of measurement science the importance of measurement in natural science and technology is undeniable. measurement is the essential tool of scientific investigation and discovery, and it enables the complex phenomena of the universe to be described in the precise, concise and universal language of mathematics, without which it would, in general, be impossible to deduce laws from scientific observation or to formulate useful theoretical constructs. in technology, the increasing complexity and speed of many modern processes and machines, make automatic control essential, and such control is not possible without satisfactory means of measurement. the last three decades have seen spectacular advances in the technology of measurement of physical quantities. the development of electrical sensors and electronic data handling have vastly extended the range of what can be measured. measurement is also making spectacular advances in areas in which it had not hitherto been fruitfully or extensively applied. the social, political, economic and behavioural sciences are to an increasing extent adopting quantitative techniques. the size and complexity of modern society provides difficult problems of planning and control and such activities require data derived from measurement which modern computers make increasingly possible to acquire and handle. yet in spite of the general recognition of the importance of measurement, the progress in its techniques and the spread of its practical application, there is widespread neglect of its fundamental problems. we have not seen a development of measurement science as a widely recognised, distinct branch of knowledge. the teaching of the principles of measurement is neglected in many academic curricula and this neglect is, if anything, increasing. the question must be asked whether a measurement science as an organised, systematic body of knowledge setting out to embrace all aspects and fields of measurement is needed, and, if needed, whether it is possible to build one. it might appear superficially that the impressive development of the technology of measurement achieved by pragmatic approaches shows that a theoretical structure of measurement science is to a large extent superfluous. the opposite, however, is true. the very wealth of subject matter and the explosive growth of scientific information in the field mean that this material must be systematically organised and general principles derived. without a theoretical framework measurement technology would be just a catalogue of techniques and instruments, which a student would find too vast to assimilate, while the practising scientist or engineer would find it impossible to retrieve the right information to lead him to optimal solutions of problems. studies of measurement without a firm theoretical basis do not have the intellectual rigour to justify their place in the academic curriculum, which their practical importance demands. granted that there is a need for a measurement science, is it possible to formulate principles and methods which have general application to all measurement and which are not trivial platitudes? the answer must be, that any measurement problem requires a scientific understanding of the particular phenomenon or process under investigation, and generally the bringing together of a wide range of technologies to its solution such as fine-mechanics, electronics, nucleonics and the like. but, while each field of measurement has its own peculiar this is a reissue of a paper which appeared in acta imeko 1973, proceedings of the 6th congress of the international measurement confederation, “measurement and instrumentation”, 17-23.6.1973, dresden, vol. 1, pp. 11–27. the paper proposes a clear, synthetic presentation of some basics of the measurement science. acta imeko | www.imeko.org may 2014 | volume 3 | number 1 | 11 features, and while measurement in one or other of its applications involves virtually all other technologies, a general principle of measurement, which are valid and useful, can be formulated, and indeed, have been, developed to a high degree of sophistication. the principles of measurement are scattered in the literature of a variety of fields: philosophy of science, statistics and, above all, that body of knowledge which is becoming known as systems science and engineering. it is important to collect these principles together, to synthesize them into a coherent whole, and to bring them to bear in a fruitful way on the problems of the technology of measurement. 2. scope and nature of present survey current development of measurement science by engineers has in the main proceeded along two major lines. one is the treatment of measurement information by the methods of information and signal theory. the other is the systemic analysis of measuring instruments and systems using the approach of dynamic systems analysis and of general network theory. the essence of these approaches is the concept that measuring systems operate by the transformation according to a prescribed functional relation of a measurand into an observable. it would be tempting to try to survey the general structure of measurement science as it stands at present. such an approach, however, would be too superficial to be useful. it is proposed therefore to examine one aspect of the subject only, its logical foundations and epistemology. the paper will attempt to review our understanding of the way in which we create an image of reality in terms of numbers and signals, a step which must precede the subsequent processing of the information. even the relatively limited topic tackled is too wide to be treated in this review with absolute rigour nor can all controversial philosophical points be critically discussed. all that is proposed is to outline key concepts as they are seen by the author. for detailed discussions and comprehensive bibliographies readers are referred to references [1-7]. 3. historical development of the epistemology of measurement the logical foundations of measurement – the relation between the material universe and mathematics – have been studied since the dawn of science. one may perhaps date interest in the subject from pythagoreans and their view that all things are number. aristotle presented the first analysis of the philosophical problems of measurement in his metaphysics. plato made a distinction between pure arithmetic and. its application to the real world and this greatly influenced subsequent thinkers. platonic idealism diverted attention from the problems of logical analysis of measurement. a major landmark in the progress of understanding of the relations between numbers and the real world has been the establishment by descartes of the connection between algebra and geometry. newton was also concerned with the logical foundations of measurement in his development of the theory of fluents. the foundations of the modern study of the epistemology of measurement with reference mainly to physics have been laid down by helmholtz [8] in the last century. this work has been extended and developed in the twenties of this century in the lucid writings of campbell [9, 10]. mathematicians concerned with the logical basis of their subject have naturally been concerned with the axioms of quantification. russell [11], frege [12], hoelder [13] and wiener [14] have among others explored this topic. philosophers of science have also given measurement their attention in recent times within the general context of their subjects. it may be invidious to single out particular contributions but one can perhaps mention the work of carnap [15], cohen, nagel [16], suppes, zinnes [7] and ellis [1]. the logical foundations of measurement have been involved in the studies of the theory of dimensions by such writers as bridgman [17], wallot [18], stille [19] and others. the principal problems that have motivated the modern development of the study of the logical foundations of measurement have been above all the need to develop the methods of measurement in psychology, with which the classical approach of helmholz could not deal, since it was based on the possibility of additive combination of quantities, which cannot be achieved in the case of say psychophysical responses to stimuli. the work of stevens [20], thurstone [21], torgerson [22], suppes and zinnes [7] among others, have in dealing with psychological problems extended the frontiers of our understanding of measurement against, often, the conservative opposition of physical scientists. another area where the need for quantification is growing is that of econometrics and utility theory. this has led among others to the work on measurement theory of pfanzagl [2]. this brief historical note is not intended to be comprehensive or to evaluate critically the individual contributions of the various workers but merely to provide a sketch of the antiquity of the problem, and the wide circles of workers concerned with it, to give credit to some of the principal contributors and to mention a few key publications. 4. epistemology of measurement and the technical sciences in spite of the importance of measurement in technology logical analysis of measurement has received virtually no treatment in the literature of technical measurement. discussions of the subject by engineers are rare. the view is often held that the problems of epistemology of measurement are of no practical interest in technology since the nature of technical measurement is self evident and satisfactorily understood. it is important to assert the opposite in this survey. in automation of industrial processes one of the outstanding practical problems is the monitoring by automatic means of complex product properties, such as flavour of foods or appearance of a decorative surface. it is by no means obvious how one can express such qualities in numbers. the practical solution of the problem depends upon the proper application of the epistemology of measurement. again in the design of engineering systems especially in technical cybernetics it is commonly necessary to formulate performance criteria which quantify various system characteristics and combine these quantities with different weightings so as to form a measure of the total utility of the design. a sound approach to this neglected problem requires an understanding of the fundamental nature of quantification. finally in the problem of error analysis and especially in the bayesian approach, the crucial question is the determination of measures which express correctly the unsatisfactoriness of acta imeko | www.imeko.org may 2014 | volume 3 | number 1 | 12 erroneous estimates. this requires again an understanding of the philosophy of measurement. 5. definition of measurement measurement is the assignment of numbers to entities in such a way as to describe them. i propose to define a number in the sense in which i am going to use the word, as an element n of a set of symbols n, which has a structure or set of relations defined on it. consider a set of extra mathematical entities q and a set of numbers n. measurement is an operation which assigns to an element qiq  , an element nn i  , such that relations between different elements nn i  and nn j  are isomorphic to empirical relations between the corresponding elements of q, qi and qj. 6. conceptual basis measurement presupposes something to be measured. both in the historical development and logical structure of scientific knowledge, measurement requires a clear operational defining concept of the class of entities described by it, that is a rule for determining whether an entity is a member of the class. unless there exists such a clear concept, we cannot start to measure. as precise knowledge derived from measurement accumulates, this usually leads to a clearer and more satisfactory concept definition. the process is iterative and the development is an ascending spiral. in some cases the concept of an entity arises from numerical laws, arrived at by measurement, and the entity is best thought of in such mathematical terms, but in general one attempts to arrive at some qualitative conceptual framework for it, if possible. one of the principal problems of scientific method is to ensure that the scale of measurement established for a class of entities yields measures, which in all contexts, describe the entity in a manner which corresponds to the underlying concept of the class. 7. direct measurement direct measurement is the process of measuring an entity by comparison with entities of the same class and without reference to the measurement of any other class of entities. it relies on the establishment on the class of entities q, upon which a scale of measurement is to be defined, of a system of empirical relations r, which have a formal similarity to relations among members of the class of numbers n. it is proposed to explain the process of direct measurement by examples. the simplest form of r is  q,r = in which we establish on q an equivalence relation  . an equivalence relation is one which is reflexive ( qq  ), symmetrical (if 21 qq  then 12 qq  ) and transitive (if 21 qq  and 32 qq  then 31 qq  ). an equivalence relation has the formal properties of equality. given  q,r = a set of differing entities qsi  may be selected to form a standard set  k21 ,...ss,ss = . numerals or other symbols nn i  are assigned to each is , the same symbol not being assigned to two different standards. in measurement entities qq  are compared with elements of s and those which bear the relation  to a standard are assigned the same symbol as the standard. this assignment constitutes nominal measurement. an example of such measurement is the use of a colour chart based on the empirical relation of “colour matching”. class inclusion is an equivalence relation. consider that q can be divided by empirical operations into n classes qq i  such that qq i n 1i == and  =ji qq if ji  and each subset qi is assigned a unique number say “i”. then if we can determine by an empirical operation that any element qq  belongs to a particular subset iqq  , it is assigned the characteristic symbol or number of the subset “i”. the subset qi may itself often be divided into further subsets, iji, qq  , iji, n 1j qq == and  =ki,ji, qq if kj  . the subsets ji,q are assigned unique numbers, say “ij”. an element for which we assign the class inclusion relation ji,qq  , is assigned the symbol “ij” which denotes its membership of iq and ji,q . this can be continued to an even finer subdivision. classificatory schemes of this kind such as decimal library classifications constitute a form of nominal measurement. it is often disputed whether the processes just described can in fact be called measurement. measures on a nominal scale merely describe whether two entities are identical or different. it is immaterial whether we term nominal measurement proper measurement or not, provided we recognise its similarity, to more sophisticated forms of quantification. it is very different from naming, which is labelling without description. two individuals may have the same name without being in any way similar, but two entities with the sane nominal measure are related. more elaborate than the nominal is the ordinal scale based on the establishment on q of an empirical order system   ,,q,r = .  is an equivalence relation. the set of relations is complementary, that is for any qq,q 21  , one and only one of the following must hold: 21 qq  , 21 qq  or 21 qq  .  and  are converse (that is if 21 qq  then 12 qq  ), irreflexive (that is we cannot have qq  ) transitive and asymmetrical (that is if 21 qq  then not 12 qq  ). the relation system  ,,q,r = enables entities of class q to be arranged in an ordered series. entities that can be so arranged may be called quantities. we can now select a number of differing standard entities qs i  and arrange them in an ordered standard series  k21 ,...ss,ss = . numerals, are assigned to each si, say i, in such a way that the order of numerals corresponds to the order in the standard series of the standards to which they are assigned. any entity qq  can then be measured by comparing it with the elements of s in the same way as in nominal measurement. if q bears the relation  to any element ss i  it is assigned the numeral of is . if an entity is not equivalent to any of the standards it is possible to determine between which two standards the measured entity would lie in the standard series. it is then assigned a numeral lying in between those of the two standard entities. the best example of an ordinal scale of measurement is the mohs’ scale of hardness of minerals. acta imeko | www.imeko.org may 2014 | volume 3 | number 1 | 13 neither nominal nor ordinal measurement as described here constitute entirely satisfactory measurements. in general measurement scales are designed to define a distance concept on the scale of entities to be measured. the analysis and axiomatisation of the problem of defining such a distance concept has received much attention in the literature. those interested are referred to the works listed in the bibliography. it is proposed here to outline the classical approach originating from the work of helmholtz. basically this depends upon defining on the class q, in addition to an order relation system   ,,q,r = , a binary operation c which combines two entities of the class to form a third entity of the class:   321 qq,qc  2313 qqqq  the combination must be commutative:    1221 q,qcq,qc  and associative:      321321 q,q,qccq,qc,qc  these are the formal properties of addition for the class of real numbers. for two masses, for example, rigid connection is a form of combination which meets the requirements specified above, and the equiarm balance is the means by which the relations ‘greater than’, ‘less than’ and ‘equal to’ are established. when such a method of combination has been found, a single entity qs1  is chosen as standard and assigned the number 1. another entity qs11  such that 1 1 1 ss  is then sought and   2111 ss,sc  is assigned the number 2. we then proceed to form   312 ss,sc  and assign it the number 3 and so on. fractional standards are generated by making or seeking two entities qs,s 1 2/12/1  such that 1 2/12/1 ss  and   11 2/12/1 ss,sc  and assigning to 2/1s , the number 1/2. thus we generate an extended set of standards:  ... ,s ,s,...ss 212/1= any quantity qq  can then be measured by seeking the element si to which it bears the relation  . q is assigned the number corresponding to si. thus we can see that using the above procedure we have established a process of assigning numbers to entities in such a way that the numbers tell something of the extent by which one entity differs from another. the above argument has eschewed rigour for the sake of simplicity, but the procedures described can be rigorously axiomatised. scales based on additive combination represent the classical view of fundamental measurement. they give an accurate and penetrating insight into measurement in physics but in many areas of science they cannot be applied. the principal advances of modern theory of measurement have been to show that other methods of arriving at direct scales establishing a distance concept on a class of extra-mathematical entities with the same properties as those described above, are equally valid. one way, for instance, of establishing a scale is by the process of assigning to any pair of psychophysical stimuli a third stimulus judged to be equidistant from them. the essence of all direct measurement scales is an establishment of an isomorphism between empirical relations on the class of measurands and the relations on a class of numbers are not confined to particular isomorphisms. 8. indirect measurement while a wide variety of conceptual entities are capable of direct measurement, there are others which are not. these must be measured by scales which rely upon relations which the entities to be measured bear to other measurable quantities. this is termed indirect measurement. two forms of indirect measurement can be distinguished. the first of these is derived measurement. consider a case in which all systems s associated with a member of the class of entities q, for which we are to define a scale, are also associated with quantities x, y, z ... which can be measured. let there be a numerical law p(x, y, z) = kq in which kq is a constant. let us further suppose that whenever the systems s are arranged in the order of q, they are also arranged in the order of kq. then kq can be taken to be a derived measure of q. density is an example of a physical quantity measured by a derived scale. for all objects of one material the ratio ρ of mass to volume of the object is a constant. whenever objects are arranged in order of density as qualitatively defined, they are also arranged in order of ρ , which is thus a measure of density. the other form of indirect measurement is associative. consider that all systems s, associated with a member of class q, are also associated with some other measurable quantity x. further suppose that whenever the systems s are ordered according to q they are also arranged in the order of x. we can then define f(x) as an associative measure of q, where f( ) is an arbitrary, single-valued, monotonic function. all practical temperature scales are of the associative kind. 9. scales of measurement it should be clear from what has been said that it is not adequate in measurement merely to speak of units, one should speak of scales of measurement, that is of the triplet q, m, n where q is a class of entities, n is a class of numbers and m is a procedure of relating members of 0. to members of n. 10. scale classification forms of scale can be classified according to mathematical transformations which leave the scale invariant. thus if we transform the number n on the scale to a number n’ we have: permissible transformation scale type n’ = f(n) where f any one to one substitution nominal n’ = f(n) f any monotonic function ordinal n’ = an + b a  0 interval n’ = an a  0 ratio a permissible transformation is one which only changes that which has been arbitrarily chosen. thus the scale of mass is a ratio scale because the only thing arbitrarily chosen is the unit mass. similarly associative scales are generally ordinal. 11. measure and concept the point has already been made that the measure of an entity must constitute a description which corresponds to the defining concept of the entity. thus there must be isomorphism between any empirical relations among quantities acta imeko | www.imeko.org may 2014 | volume 3 | number 1 | 14 measured and corresponding relations between their measures. one basic test of this correspondence can be proposed. if, whenever systems associated with an entity of class q are placed in an order according to our quantitative concept of q, the order is the same as that in which the systems would be arranged by the order of some measure of q, the measure can be considered satisfactory. 12. meaningfulness the problem of the meaningfulness of statements made about measures of an entity is important. a statement is meaningful if its truth is unchanged by a permissible transformation of the scales of measurement. we say that a k-ry relation s is meaningful if: s(m(q1) ... m(qk)) = s(m’(q1) ... m’(qk)) where m → m’ is a permissible transformation. thus, as a very simple example, it is meaningful to speak of the ratio of two masses, since that ratio is invariant with respect to changes of the unit of mass, but it is not meaningful to speak of the ratio of two hardnesses measured on the mohs’ scale, since that ratio would be changed by a monotonic transformation of the scale. another view is that only such statements are meaningful which can be logically traced to the empirical operations on which the measurement is founded. 13. systems of scales in physics dimensions there is much that is arbitrary in the choice of scale forms. the guiding principle is that scales are so chosen as to result in the greatest simplicity of the resulting mathematical descriptions of the laws of nature. hence, for instance, the attempt to make practical temperature scale coincide with the thermodynamic temperature scale. in physics we attempt to reduce to the minimum the number of quantities termed primary the scales of which are independently defined. scales for the measurement of other quantities are termed secondary and they are obtained as derived scales from primary quantities by a chain of simple proportionality laws [1, 19]. two points need to be made. there is nothing intrinsically fundamental in the primary quantities: mass, length, time, current and luminous intensity, they are merely conventionally chosen for reason of the practical convenience of the definition of their scales. secondly, the dimension of a quantity in no way embodies any metaphysical essence. it merely denotes the way in which the conventionally chosen scale of the quantity relates to the conventionally chosen scale of the primary quantities and could conceivably be altered by different choice of scales. 14. multidimensional quantities certain entities cannot be described by a single measure, but require an array of numbers for their specification: m(q) = n = [n1, n2, ... nk] such measures are known as vector or multi-dimensional. thus force is described by three numbers: a magnitude and two direction angles, or three components. patterns may be described by an array of numbers specifying features. n may be viewed as specifying a point in a vector space and we may seek to define a metric for a vector space, that is a relation d, such that for any three vectors n1, n2, n3 we have: d(n1, n2) = 0 d(n1, n2) = d(n2, n1) if n1  n2 d(n1, n2) + d(n2, n3) > d(n1, n3) d( ) represents a measure of distance in the vector space. the problem of representing some aspects of n by a single real number is important especially in decision and optimisation theory where we wish to place multidimensional utilities in some order. a typical method is to determine: m2 = n a nt where a is a positive definite weighting matrix. it is important to establish logically what correspondence exists between m and the entity q measured by n. the problems of multidimensional measures offer many still unresolved problems. 15. measurement and uncertainty the essential assumption underlying the logic of measurement is that the operation m forms a single image of q only, and that m is invariant. however, any practical operation defining a scale represents a probabilistic and not a deterministic transformation. m is associated with a probability distribution, which must be specified with it. this subject has not been adequately discussed in the literature. 16. purpose and uses of measurement while measurement is generally recognised to be the foundation of science, in the context of a review of its fundamental concepts there is a need to analyse the purpose and uses of measurement critically. the advantages of measurement as a form of description can be summarised as follows. firstly, measurement represents a description of an entity which is concise, telling us in a single number information which would otherwise need many words. measurement is also a description which is precise, pinpointing by a single number a particular entity where the same verbal description indicates a range of similar but differing things. measurement is objective rather than subjective, that is, it constitutes a description invariant with respect to the observer making it. the language of measurement is universal and commonly understood, though this demands much effort in the establishment and maintenance of good and generally accepted scales and standards. quantitative description involves an ability to make distinctions and to describe relations among entities of the same kind. in particular the ability of ranking objects in an order according to some measure of a relevant attribute, say, size, coat, or efficiency, enables us to make decisions about them in a logical and systematic manner. a measure of an entity gives us an ability to express facts and conventions about it in the formal language of mathematics. without the convenient notation of this language, the complex chains of induction and deduction by which we describe and explain the universe, would be too cumbersome to express. we could not make our thoughts clear either to ourselves or to others. it follows from what has been said that the description of entities by numbers is not good in itself. the only value of measurement lies in the use to which the information is put. science is not just the amassing of numerical data, it depends upon the way in which the data are analysed and organised. finally in relation to technical cybernetics the essential feature of measurement is that it enables the measurand to be acta imeko | www.imeko.org may 2014 | volume 3 | number 1 | 15 expressed in signals which can be handled by machines. qualitative information is not machine intelligible. 17. conclusions the basic conclusions of the present review can be simply stated. the basic concepts of measurement form an essential foundation stone for the erection of a sound measurement science. they now have a sound logical basis which is compatible with the methods of information science. their understanding is essential for the solution of a range of important practical engineering problems. references [1] b. ellis, basic concepts of measurement, cambridge university press, 1966. [2] j. pfanzagl, theory of measurement, physica-verlag wurzburgvienna, 1968. [3] c.w. churchman, p. ratoosh, measurement, definitions and theories, wiley, new york, 1959. [4] akad. nauk., ukrain s.s.r. inst. fil. gnosseogicheskiye aspekty, izmereniyi naukova dumka, kiev, 1968. [5] akad. nauk., ukrain s.s.r. inst. fil. metodologicheskiye problemy teorii, izmereniyi naukova dumka, kiev 1966. [6] o.a. melnikov, o roli uzmereniyi v processe, poznaniya nauka, novosibirsk, 1968. [7] p. suppes, j.l. zinnes, basic measurement theory, in handbook of mathematical psychology, john wiley, 1963. [8] l.v. helmholtz, zaehlen und messen, erkenntnisstheoretisch betrachtet, in philosophische aufsaetze eduard zeller gewidmet, leipzig, 1887. [9] n.r. campbell, physics: the elements, cambridge, 1920. [10] n.r. campbell, an account of the principles of measurement and calculation, longmans and green, london, 1928. [11] b. russel, the principles of mathematics 2nd edition, new york, 1937. [12] g. frege, grundlagen der arithmetik, 1884. [13] o. hoelder, die axiome der quantitaet und die lehre von mass, berichte uber die verhandlungen der koeniglich saechsischen gesselschaft der wissenschaften zu leipzig (math.-phys. kiasse) 53, 1–64. [14] n. wiener, a new theory of measurement: a study in the logic of mathematics, proc. of lond. math. soc. ser 2. 19, pp.181– 205. [15] r. carnap, foundations of logic and measurement, international encyclopedia of unified science, vol. 1, chicago, university of chicago press. [16] m.r. cohen, e. nagel, an introduction to logic and scientific method, harcourt, new york, 1934. [17] p.w. bridgman, dimensional analysis, yale university press, new haven, 1931. [18] j. wallot, groessengleichungen, einheiten und dimensionen, leipzig, 1953. [19] u. stille, messen und rechnen in der physik, vieweg, braunschweig, 1961. [20] s.s. steven, mathematics, measurement and psychophysics, in s.s. steven (ed) handbook of experimental psychology, wiley, new york, 1951. [21] l.l. thurstone, the indifference function, j. soc. psychol 2, 1931, pp 139–167. [22] h.s. torgerson, theory and methods of scaling, wiley, new york, 1958. microsoft word 263-1860-3-le-rev2 acta imeko  issn: 2221‐870x  december 2015, volume 4, number 4, 16‐19    acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 |16  mathematical modeling of an altimeter  irina kislitsyna 1 , galina malykhina 2   1 russian state scientific center for robotics and technical сybernetics, saint‐petersburg, tikhoretsky prospect 21, russian federation  2 peter thegrate saint‐petersburg polytechnic university,  saint‐petersburg, polytechnicheskaya 29, russian federation      section: research paper   keywords: photon altimeter; lunar soil; gamma rays  citation: irina kislitsyna, galina malykhina, mathematical modelling of an altimeter, acta imeko, vol. 4, no. 4, article 5, december2015, identifier: imeko‐ acta‐04 (2015)‐04‐05  section editor: franco pavese, torino, italy  received: march 18, 2015; in final form june 15, 2015; published december 2015  copyright: © 2015 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  corresponding author: galina f. malykhina, e‐mail: g_f_malychina@mail.ru    1. introduction  a photon altimeter is designed for measuring the distance between a lander and the underlying surface. the altimeter should measure the current height of the lander over the lunar surface in the range from 0.3 m to 10 m at a descent rate between 6.5 m/s and 11 m/s, providing a random component of the relative error less than 3 %. for this purpose a photonic altimeter is developed. the altimeter uses the effect of scattering of gamma radiation over the lunar surface. the conditions of a landing on the moon surface seriously differ from conditions on earth due to the lack of the lunar atmosphere, a high background radiation and the soil composition of the lunar surface. there are altimeters of different frequency range such as radio-wave, infrared and optical altimeters, the x-ray and the photon altimeters. the optical altimeters were developed for optical navigation system for lunar landing [1], [2]. we develop the altimeter based on the effect of scattering photons, having the following advantages: the ability to perform measurements through a plasma of engine, the insensitivity to the layer of dust on the surface of the planet, the high measurement accuracy at the low altitudes. the amount of scattered gamma rays depends on the type of the underlying surface of the moon [3]. the lunar surface consists of loose material called regolith, fragments of bedrock and secondary particles.the material was formed as a result of continuous meteoric impact and bombardment by atomic particles over billions of years. the average depth of the regolith that covers the entire surface of the moon ranges from 4 to 5 m in the lunar seas up to 10 to 15 m on the continents. the chemical composition of the regolith depends on the composition of rocks lying below, but it also contains other substances and minerals. the main minerals of lunar rocks are plagioclase (solid solution of albite and an orthite naalsi3o8 caal2si2o8), orthopyroxene (mg,fe)sio3, clinopyroxene (ca,mg,fe)sio3, olivine (mg,fe)2sio4, ilmenite (fetio3) and spinel-group minerals (fecr2o4 fe2tio4 feal2o4), [4]. lunar seas are volcanic plains that fill cavities in the topography of the continents. the predominant types of lunar sea rocks are marine basalts. lunar seas are more suitable for the landing of a spacecraft. 2. intensity of scattering photons  the altimeter uses the effect of gamma-rays scattering in other words, the compton effect. the differential cross-section for compton scattering of one electron per unit solid angle is given by the klein-nishina-tamm formula:                           cos11 cos1 cos1 cos11 1 2 22 2 22 er d d (1) where  is the scattering angle, er is the classical electron radius, m cm arr e ce 15-108179.2 04.137 1   abstract  the aim of this survey  is to simulate a photon altimeter designed for a soft  landing on the  lunar surface. a simulation of the  process of gamma rays scattering from the lunar surface with a typical composition of the lunar soil was implemented.  acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 |17  04.137 1 a is a constant, cm r e c   is the wave length scattering effect, em is the electron mass, c is the speed of light, kev660e is the energy of the primary radiation,  is the ratio of photon energy to the electron rest energy at kev660e , and 129.12  cm e e  . for a mixture of substances on the lunar surface the effective number effz of the underlying soil surface can be calculated by the following formula:     , 1 1 4     m i ii m i ii eff zη zη z (2) where iη is the relative quantity of substance i , having number iz of lunar soil, m is the number of components of the underlying surface. the electron density of the underlying surface is: , 6 1 i i a i ie z a n n     (3) where an is avogadro's number, ia is the atomic mass of substancei. lunar seas are volcanic valleys filling low places of the continent relief, which are the bottom of large craters and pools. a basalt is a dominant type of the sea’s rocks. composition of the ground shown in table 1 was used in the model as an example. the compton cross-section can be calculated by the equation effezn 25 0 1054.6  . the intensity of the scattered photon radiation i is given by the formula:                  , cos1cos11 cos1 1 cos11 cos1 16 3 2 32 2 2 0 0                     r ii (4) where 0i is the primary radiation and  the scattering angle at a distance r from the scattering electron. calculations by (1) to (4) showed that the intensity of the scattered radiation decreases with increasing scattering angle. in the range of angles of interest from 90° to 180° the intensity i of a gamma ray of 660 kev does not change dramatically. therefore, it is advisable to work in this range. the energy of the scattered photons depends on the angle of scattering in accordance with the relationship:   . cos11 2     cm e e e e (5) the photon energy decreases with increasing scattering angle. if the value of the scattering angle is in the range ]180...90[  , then the energy of the scattered gamma rays reduced down to 200 kev. the energy of the scattered photons in the working range depends less on the energy source. the geometry of the gamma radiation source and the gamma radiation detector’s location is shown in figure 1. the photon radiation source (s) is located in the centre of the measuring system, and four detectors (d1-d4) are placed on a distance l from the source. the lander may be tilted relative to the underlying surface, by an slope angle  of the axis formed by a pair of detectors. let h be the current height of the landing apparatus,  is the angle of incidence of the direct beam of the gamma ray,  is the angle between the projection of the axis of the detectors d1 and d2 on the line passing through the point of incidence,  is the scattering angle of photons recorded by the detector d1. the square of the distance between the scattering element and the detector can be calculated by formula:  22222 sintgcos2costg  lhhllhr  (6) the cosine of angle  amounts to:       . tgcos costgsin cos 2222    hl rlhlh   (7) the cosine of the scattering angle:       . 12 1 )cos( 2 2222    tgrh lrtgh    (8) the detector registers photons scattered by the lunar surface. the position of the scattering element may be characterized by angle  of the direct ray and angle  of the position of scattering element.the range of angle  depends on the angle of collimation max , where max0   . the range of angle  is  ., the distance between the centre and the scattering element is defined by the following formula: .tg h table 1. composition of the lunar surface ground.  element  percentage    6...1),(  i i   atomic number    6...1, iz i   atomic mass   6...1, ia i   si  20.4  14  28 o  41.3  8  16 fe  13.2  26  56 ca  0.79  20  40 al  0.68  13  27 mg  0.58  12  24 figure 1. location of the source and the four detectors.  acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 |18  the intensity of photons, detected by d1 can be obtained by integration over the scattering volume, depending on the :,, depth      depth depthe detect dddepthdhlriei      ),,,,,,()( (9) where  e is the coefficient of mass absorption, depending on the photon energy,  is the density of the lunar soil. the intensity of photons, scattered by an element, situated on the distance , the depth (depth) and the angle  , detected by d1, is defined by the following equation:                                     2 22 22 2 2 0 0 cos1cos11 cos1 1 cos11 cos1 16 3 ),,,,,,( r i hlri (10) detectors d1 to d4 receive the gamma rays scattered from the lunar surface in accordance to the angle of collimation of the radiation source. the detectors register the integral of intensity scattered gamma rays over the volume of interaction with the lunar soil. it is useful to describe the effect of the registration as the ratio of primary and scattered fluxes of gamma rays. the integral of ratio 0i i depends on the angle  , the depth of penetration of the gamma rays in the lunar soil ( depth) and on the height h of the landing apparatus.. 3. algorithm of modeling  the algorithm makes it possible to calculate the ratio 0i i of the registered intensity i and the original intensity 0i .it consists of three loops for the triple integral computation. the algorithm includes the following steps.  summary of the algorithm 1. initialize the parameters of the model: cross-section for compton scattering 0 , ratio of photon energy and energy of electron at rest  , mass absorption factor , matter density 0 . 2. set the height 10.0,0  hh , and for ...,,2,1i compute the current hih  . 3. set the angle of slope 32 ,0   , and for ...,,2,1i compute the current   i . 4. set the penetration of photons into lunar material, and for ...,,2,1i compute the current depthidepth  . 5. set the radius of the gamma-ray beam 10.0,0   , and for ...,,2,1i compute the current   i . 6. set the angle of the gamma ray beam 10.0,0   , and for ...,,2,1i compute the current   i . 7. compute the angle of the gamma ray beam and of the scattering element: .       depth arctg   8. compute the distance between detector and scattering element:     .sin tgcos2coscostg 2 1 2 222   ldepth ldepthldepthr   9. compute the cosine of the complementary scattering angle:       . 12 1 )cos( 2 2222    tgdepthr lrtgdepth    10. compute the elementary scattering volume:    .2 2   hv 10. compute the accumulation of the gamma intensity ratio:                  . cos1cos11 cos1 1 cos11 cos1 16 3 2 32 2 2 0 00 v ri i i i                    12. compute the gamma intensity ratio subject to the absorption of photons: .20 deptheii   the mass absorption factorof photons with energy 660 kev by silicon equals 0802.01  cm/g2. the mass absorption factor of the scattering gamma-quantum with energy 200 kev by silicon equals 123.02  cm/g2,  being the density of the matter. 4. numerical experiment  the proposed model is implemented as a simulation program running in matlab. dependences of the average intensity of scattered gamma-ray quanta detected by d1 on a height of the descending module are presented in figure 2.  the gamma-ray flux-noise is characterized by a poisson distribution. therefore, the result of measuring the height and the angle actually is not a smooth function of the intensity, as shown in figure 2. noisy data registered by the four photon detectors should be averaged. figure 3 shows the dependences of the height on the intensity of photon flux for different values of the angle. the measurement result is the average height of the four outputs of the detectors. 0 1 2 3 4 5 6 7 8 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045 r a ti o o f in te n s it ie s ( i/ i0 ) height h gamma=0 degree gamma=5.6 degree gamma=11.25 degree gamma=16.88 degree gamma=22.5 degree figure  2.  the  ratio  of  average  intensity  of  scattered  and  initial  gamma  quanta versus the height of the descending module.   acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 |19  each detector records the angle of the lander.dependences of the measured angle on the height computed using the intensity of the gamma-ray flux are shown in figure 4. the signal received by the detector is stronger when the height of the descending module is small. the height and slope angle can be measured more accurately when the height is small. this fact is an advantage of the photon altimeter. it is more difficult to measure height and slope angle at high-altitude of the descending module. the results of the altimeter model showed that the standard deviation of error of slope angle for low-altitude of the descending module is not greater than 0.02 rad. the standard deviation of the error of the slope angle for high-altitude of the descending module is less than 0.06 rad. 5. conclusions the proposed model is based on initial values:initial height,activity of photon source, topology of photon sources and detectors,composition of the ground. the developed model allows estimate effect of the topology of the measuring system, activity of the source, size and arrangement of the detectors, the composition of the ground. the model allows determine basic characteristics of altimeter with reasonable accuracy. the range of measurement of altitude, the effect of landing apparatus slope, the relative error of measurement may be estimated. the model submit for consideration capability of angle slope measurement. acknowledgement  the scientific research was supported by central research institute of robotics and technical cybernetics. authors are grateful for scientific and financial support. references [1] v. simard bilodeau, s. clerc, r. drai, j. de lafontaine, optical navigation system for pin-point lunar landing, preprints of the 19th world congress the international federation of automatic control cape town, south africa, august 24-29, 2014, pp. 1053510542. [2] xiangyu huang, dayi wang, yingzi he, yifeng guan, autonomous navigation and control for pin point lunar soft landing, 7th international esa conference on guidance, navigation & control systems 2-5 june 2008, tralee, county kerry, ireland. [3] e. i. yurevech, photon technique, saint-petersburg, press of the saint-petersburg polytechnic university, 2003, 235pp, (in russian). [4] v.p. legostaev and v.a. lopota moon-a step towards technology development of the solar system. / m.-rsc "energy", 2011. 684pp, (in russian). 0 1 2 3 4 5 6 7 8 -0.25 -0.2 -0.15 -0.1 -0.05 0 0.05 0.1 height h a n g le o f s lo p e g a m m a < 0 relative error = 0.061682 gamma=0 gamma=-pi/64 radian gamma=-pi/32 radian gamma=-3pi/64 radian gamma=-pi/16 radian figure 4. dependences of slope angle measured for different heights.  500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 0 1 2 3 4 5 6 7 8 gamma intensity (i) h e ig h t h relative error = 0.010311 gamma=0 gamma=-pi/64 рад. gamma=-pi/32 рад. gamma=-3pi/64 рад. gamma=-pi/16 рад. figure  3.  dependences  of  the  height  on  the  intensity  of  photon  flux  for different values of the angle.  journal contacts acta imeko issn: 2221-870x june 2023, volume 12, number 2, 1 2 acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 1 journal contacts about the journal acta imeko is an e-journal reporting on the contributions on the state and progress of the science and technology of measurement. the articles are mainly based on presentations presented at imeko workshops, symposia and congresses. the journal is published by imeko, the international measurement confederation. the issn, the international identifier for serials, is 2221-870x. about imeko the international measurement confederation, imeko, is an international federation of actually 42 national member organisations individually concerned with the advancement of measurement technology. its fundamental objectives are the promotion of international interchange of scientific and technical information in the field of measurement, and the enhancement of international co-operation among scientists and engineers from research and industry. addresses principal contact prof. francesco lamonaca university of calabria department of computer science, modelling, electronic and system science via p. bucci, 41c, vi floor, arcavacata di rende, 87036 (cs), italy e-mail: editorinchief.actaimeko@hunmeko.org support contact dr. dirk röske physikalisch-technische bundesanstalt (ptb) bundesallee 100, 38116 braunschweig, germany e-mail: dirk.roeske@ptb.de editor‐in‐chief francesco lamonaca, italy founding editor‐in‐chief paul p. l. regtien, netherlands associate editor dirk röske, germany copy editors egidio de benedetto, italy luca rossini, italy silvia sangiovanni, italy layout editors dirk röske, germany leonardo iannucci, italy domenico luca carnì, italy editorial board leopoldo angrisani, italy filippo attivissimo, italy eulalia balestieri, italy eric benoit, france paolo carbone, italy lorenzo ciani, italy catalin damian, romania pasquale daponte, italy luca de vito, italy sascha eichstaedt, germany laura fabiano, italy ravi fernandez, germany luigi ferrigno, italy edoardo fiorucci, italy alistair forbes, united kingdom helena geirinhas ramos, portugal sabrina grassini, italy leonardo iannucci, italy fernando janeiro, portugal konrad 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claudia zoani, italy acta imeko, title acta imeko december 2014, volume 3, number 4, 32 – 37 www.imeko.org acta imeko | www.imeko.org december 2014 | volume 3 | number 4 | 32 techniques for on-board vibrational passenger comfort monitoring in public transport ileana bodini, matteo lancini, simone pasinetti, david vetturi university of brescia – department of mechanical and industrial engineering, via branze 38, 25123 brescia, italy section: research paper keywords: vibrational on-board comfort, geo-referenced comfort measurement, thematic maps citation: ileana bodini, matteo lancini, simone pasinetti, david vetturi, techniques for on-board vibrational passenger comfort monitoring in public transport, acta imeko, vol. 3, no. 4, article 7, december 2014, identifier: imeko-acta-03 (2014)-04-07 editor: paolo carbone, university of perugia received october 11 th , 2013; in final form january 14 th , 2014; published december 2014 copyright: © 2014 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: (none reported) corresponding author: ileana bodini, e-mail: ileana.bodini@ing.unibs.it 1. introduction the use and the development of collective transport systems are encouraged by public institutions in order to improve mobility and accessibility in urban areas. these aspects are also key elements in the development of “smart cities”. therefore, the increase of service quality and safety is fundamental in making public transport systems more attractive than private vehicles [1]. service quality is usually evaluated in terms of accessibility to the transport system, timing flexibility and other properties of the transport network, while a comfort analysis is usually focused on the vehicle itself, even if its results depend also on roads and infrastructures. developments in laws [2] and some researches [3, 4, 5] have put in evidence two different aspects that could seem to be in contrast: one is the necessity to improve road safety, by introducing traffic calming measures, such as roundabouts, speed bumps, chicanes, elevated pedestrian crossings or different types of pavements; the second is the attention to the exposure of workers and passengers to vibration also on public transport vehicles, and its effect on transport quality. until now many authors have been studying traffic calming devices and public transport passengers interaction [6, 7, 8, 9]. the purpose of this work is to give useful information to the designers of both vehicles and infrastructures; in particular this research is aimed at defining a technique to evaluate how traffic calming devices affect the acceleration that road public transport passengers perceive and to provide a low-cost device to easily measure and monitor vibration level on road public transport. therefore, taking as a reference the european regulations on rail transports [10], the researchers propose the definition of a vibrational comfort index that allows to characterize and compare different road infrastructures and that could also be useful to compare behaviours of vehicles related to road infrastructure and to monitor vehicle maintenance state. abstract traffic calming devices on urban streets, such as elevated pedestrian crossings, speed bumps and roundabouts, are increasingly used, raising a real problem in relation to the on-board comfort that passengers perceive. to measure vibrational comfort related to traffic calming devices that passengers of the public transport perceive, an acquisition system called asgcm (autonomous system for georeferenced comfort measurements) has been developed, taking as a reference the european regulations on rail transports. asgcm permits to link each measurement of vibration, ground velocity and acceleration with geographical information resulting from a gps. in this way a map of a comfort index, statistical surveys and correlation between on-board comfort and traffic calming, can be directly obtained by any geographic information system (gis), able to query a centralized remote database, which was developed ad-hoc. a large number of experimental tests has been performed to define a vibrational comfort index and to collect a large dataset that allows statistically significant comparisons between different infrastructures and their characterization. the proposed technique can also be useful for diagnostics purposes, such as vehicle comparison and road maintenance state monitoring. acta imeko | www.imeko.org december 2014 | volume 3 | number 4 | 33 to calculate a comfort index and to correlate it with a given location, accelerations and gps positions are needed. in section 2 an asgcm (autonomous system for geo-referenced comfort measurements) measurement system, that consists of the measurement chain and the processing system that allow to collect data and make them available, is described. in section 3 the proposed comfort index is defined, and in section 4 different types of data analysis are proposed. finally, in section 5, the comfort index validation is discussed. 2. measurement and processing system to assess the on-board comfort that bus passengers perceive due to traffic calming devices, an ad-hoc measurement system, called asgcm (autonomous system for geo-referenced comfort measurements) has been developed. this is autonomous for long periods of time and acquires, processes, and records data without requiring specialized staff [11]. to monitor on-board comfort, two quantities are needed: comfort level and geographical location. the measurement chain consists of a capacitive triaxial accelerometer mounted along the three principal orthogonal axes, a ni compact-rio, an analogue input module, supply and connection cables, a commercial gps antenna, and an usb flash memory. both the accelerometer and the ni compact-rio are battery-powered. the comfort level has been calculated taking into account both infrastructural effects, involving low frequencies, and vibration effects. these have been measured using a triaxial accelerometer, with an acquisition rate of 1000 hz and a buffer of 1000 samples. in one second of acquisition the buffer is filled up and data are processed using a band-pass filter, from 0.5 hz to 300 hz, designed according to the human response to vibration filters [10, 12]: acceleration magnitude is computed as a vector sum of the three accelerations acquired in orthogonal directions (the x-axis corresponds to the road direction, the yaxis is in the road plane and orthogonal to the x-axis, and the zaxis is perpendicular to the road plane) and an rms acceleration value is calculated, considering a period of 5 seconds. this period of time represents a compromise between the acquisition of low frequency vibrations and a statistically relevant number of samples. the time needed to fill the buffer is adequate to query the gps: each second a standard nmea (national marine electronics association) rmc (recommended minimum sentence c) string is recorded [13]. this string contains information of position, velocity and time. due to this procedure, illustrated in figure 1, information of vibration, position and velocity related to the same second, are associated with a single geographical point. 3. index choice the european regulation on rail transport [10] defines different types of comfort index; the one for standing passengers is called nvd. this has been adapted for road transport and has been calculated following equation (1), wy a wz a wy a wx anvd  5 22 4 2 163 (1) where wx a is the rms (root mean square) weighed longitudinal acceleration, wy a is the rms weighed lateral acceleration and wz a is the rms weighed vertical acceleration. weighing is performed following european regulations and considering the human response to vibration frequency filters [10, 12]. the higher the nvd value the greater the on-board discomfort. as stated before, each second, this comfort index is linked to vehicle velocity and gps coordinates and the results are updated to a remote database which could then be queried, in order to obtain thematic maps and statistical analyses. 4. analysis results 4.1. analysis of manoeuvres and infrastructures querying the developed database, kinematic parameters, such as speed and accelerations, related to vehicle vibrational comfort, can be analyzed. as a first result these parameters can be plotted as a function of time and position, as shown in figure 2. this type of analysis gives detailed information strictly related to every single passage of a chosen vehicle through a considered infrastructure, which can therefore be very accurately studied. however this kind of analysis is not very appropriate to monitor a complete public transport network or to evaluate an infrastructure design, because many external parameters such as driver, weather, bus crowding and traffic conditions can affect the measurement. these are important figure 1. flow chart of the automated acquisition procedure of asgcm. http://www.nmea.org/ http://www.nmea.org/ acta imeko | www.imeko.org december 2014 | volume 3 | number 4 | 34 factors that can be considered, for example, when discomfort causes have to be investigated. 4.2. geographical analysis to obtain less detailed but more significant information, the acquired data have to be aggregated. a first approach consists in a geographically-based aggregation obtained by dividing the urban map in a grid of a chosen step (for example 1 m), and then associating to each cell the mean value of the parameter under study, in particular the nvd index. to take into account the geographical distribution of the data, the generic parameter is weighed using the inverse of the squared distance between the centre of the cell and the recorded position of each sample, following the widely accepted inverse distance weighing method. in this way a mean value of the chosen parameter has been calculated, taking into account both geographical distribution of the data and different runs of the vehicle, in different conditions, in the same geographical position. figure 3 shows a thematic map of the comfort index of a particular bus route as an average of a large number of acquisitions. in particular the bus route is superimposed on the city cartography: very uncomfortable spots (in red) as well as comfortable ones (in blue) are evidenced. in general, thematic maps can be very useful to correlate a physical quantity, such as vehicle speed, acceleration vector, onboard comfort, with a given infrastructure, for example a roundabout, a speed bump or an elevated pedestrian crossing. figure 2. kinematic parameters (speed and acceleration) and nvd index on the same roundabout. figure 3. thematic map of a portion of a bus route of the nvd comfort index, superimposed on the city cartography. 35302520151050 0.12 0.10 0.08 0.06 0.04 0.02 0.00 nvd d e n s it y mean 9.807 stdev 4.067 n 2125 histogram of nvd normal 35 30 25 20 15 10 5 0 n v d boxplot of nvd figure 4. nvd comfort index statistical analysis. acta imeko | www.imeko.org december 2014 | volume 3 | number 4 | 35 however, using this kind of analysis, it is not possible to define different levels of index nvd related to real situations of comfort or discomfort, therefore thematic maps such as that shown in figure 3 can be only used to define areas of relative discomfort with respect to the acquired data, without being able to point out which is actually the level of criticality itself. 4.3. statistical analysis a second approach to data aggregation consists in isolating some identified infrastructures or situations of interest, such as crossings, elevated pedestrian crossings, roundabouts, straight roads and considering the interesting physical quantities related to these infrastructures (such as vehicle speed, accelerations, on-board comfort) as stochastic variables over which a statistical analysis can be performed. this allows to point out known situations of high comfort levels (straight roads with good pavement conditions) and low comfort levels (roundabouts, raised bumps) and to statistically investigate the corresponding nvd levels to develop a quantitative scale of the index of comfort. the proposed method of analysis consists in selecting a path portion with fixed length, centred on the chosen infrastructure, and in assessing some important statistical parameters of the distribution of the variable of interest, in particular its probability distribution function (pdf), its cumulative distribution function (cdf), a box plot and the 95th percentile of the distribution. this last parameter depends less on impulsive nvd index variations, not relevant from a statistical point of view, than the maximum nvd value, therefore, it has been chosen to summarize the complete distribution with a single number. a box plot is able, in particular, to synthetically indicate the trend of the probability distribution through the identification of its quartiles. as an example, referring to the roundabout proposed in figure 2 and considering the ndv index as a stochastic variable, it is possible to study some statistical parameters, that give brief information about comfort (or discomfort) level related to the chosen infrastructure. figure 4 shows the histogram of the distribution and a box plot of nvd indexes associated with the roundabout. a further example is given in figure 5, which represents the nvd index of a single run as a function of position along the path (inside the inspection radius centred on the roundabout) and puts in evidence (dotted line) that the 95th percentile of the nvd can be used to represent the worst on-board vibrational comfort condition, which occurs when the bus enters the selected infrastructure. the complete statistical population includes all runs through an infrastructure (or through the whole route) of the same vehicle in different traffic, bus crowding, weather conditions and for different drivers. each one of these parameters is a possible analysis criterion and a possible query for the database. following the described method, different statistical analyses have been performed; in particular, it is possible to represent cumulative distribution functions of the comfort index of a complete bus route or of a chosen type of infrastructure, such as roundabouts, elevated pedestrian crossings or straight roads. figure 6 shows these cumulative distributions and allows to compare the comfort index of the whole route with that related to a particular type of infrastructure, giving brief information about on-board comfort of a specific bus route and allowing figure 5. nvd comfort index – 95 th percentile. figure 6. comparison between cumulative distribution functions of the complete route for different infrastructures (straight roads, elevated pedestrian crossings and roundabouts). figure 7. comparison between box plots of different infrastructures (straight roads (sr), elevated pedestrian crossings (pc) and roundabouts (ro)) on the whole route. figure 8. comparison between box plots of different roundabouts. acta imeko | www.imeko.org december 2014 | volume 3 | number 4 | 36 for a comfort-based classification of specific traffic calming devices or road infrastructures. a comparison between comfort indexes related to different types of traffic calming devices and road infrastructures is also shown in figure 7, using box plots as the statistical method. figure 8 shows that it is also possible to compare different traffic calming devices of the same type, such as roundabouts, and to study comfort index distribution of buses crossing such infrastructures. in particular, figure 8 represents the comparison between different roundabouts of the same bus route and box plot are sorted according to diameter size; data were acquired using the same model of bus. due to this kind of graph it is possible to deduce if roundabouts with different diameters present different levels of comfort. another parameter that could be taken into account is the type of manoeuvre on roundabouts with the same geometry. 5. index validation to validate the proposed nvd index, an analysis of correlation between instrumental comfort measurements and individual comfort perception has been performed. the individual perception of the comfort/discomfort level, on a selected bus route, has been evaluated by a test panel jury of more than 30 passengers who were asked repeatedly to express an evaluation of their perception in relation to different infrastructures (straight road, pedestrian crossing, roundabout, etc.), just few moments after their occurrence, for the whole duration of the chosen track. the evaluation is given using a score that varies from 0, when the situation is perceived as very comfortable, to 4, when the situation is very uncomfortable. this ranking system is explained in advance to each subject to avoid misevaluation. for all devices under investigation, mean value and standard deviation have been calculated. as an example, figure 9 shows a distribution of jury responses on subjectively perceived comfort levels: in the graph each judgment (qualitative indication) is associated with a numerical value (quantitative indication) and the average value of judgment has also been reported. to search for a significant correlation, the nvd index related to each studied infrastructure was considered a stochastic variable and the 95th percentile of its distribution was chosen to synthesize the discomfort of the considered infrastructure. a linear regression was the result of a simple least square approach [13], between this indicator and the average of the panel responses. a linear relation has been found (r=96%) between the 95th percentile of the nvd distributions and test panel opinions, associating the best comfort levels found (mean evaluation 0.2 – close to “very good” comfort) with an nvd level of 5 and the highest discomfort (mean evaluation 4 – “very bad”) with an nvd level of 25. this relationship is shown in figure 10 and could be used, as proposed by the authors, to define threshold values for comfort index. 6. conclusions a portable and autonomous measuring device for on-board comfort measurements on local transport buses has been set up, and specific software for retrieving from multiple devices, stocking and localizing data on the whole local transport network has been created. automated tools for a geographical analysis of collected data with a standard gis interface have also been developed and statistical analysis of the influence of specific traffic calming devices on on-board comfort was made feasible using the data collected. a linear correlation between the results obtained by the measurement system and subjective perception of discomfort by a panel jury has been found, validating the authors’ proposal of a modified nvd index for road vehicles for public transportation. in particular, the proposed analysis methodology allows, thanks to comfort thematic maps, to identify critical points, geographically localized, on a particular route, thus making possible a fast maintenance work if needed or pointing out comfort problems related to traffic calming devices or particular traffic and infrastructure conditions. using the proposed index it is possible to compare different bus routes, different types of infrastructures, different types of bus travelling on the same route, different infrastructure of the same type that differs from geometry or for crossing manoeuvres. systematic acquisition of data regarding the whole transport network of a chosen area could allow to monitor the temporal evolution of vibrational comfort of the road network that is related to traffic conditions, road surface damages, presence of traffic calming devices. in this way, not only the general appreciation of the public transport system could be assessed, but also the temporal development of road conditions, as well as the interactions between subsequent traffic calming devices or road infrastructures. figure 9. relation between nvd index and panel evaluation. figure 10. linear relation between 95 th percentile of nvd index and panel evaluation. acta imeko | www.imeko.org december 2014 | volume 3 | number 4 | 37 acknowledgement authors would like to thank brescia trasporti, and its bus drivers, for the support received, prof. g. maternini for comments and insights and prof. f. docchio for careful reading and discussion. references [1] tiboni m., rossetti s., "implementing a road safety review approach for existing bus stops", wit transactions on the built environment, vol. 130, pp. 699 709, wit press. [2] european parliament, “directive 2002/44/ec of the european parliament and of the council of 25 june 2002 on the minimum health and safety requirements regarding the exposure of workers to the risks arising from physical agents (vibrations)”, official journal, l 177 6.7.2002, p. 13. [3] p. eriksson, o. friberg, structural and multidisciplinary optimization 20 (2000) pp. 67–75. [4] i.d. jacobson, r.w. barber, r.d. pepler, l.l. vallerie, transportation research record 646 (1997) pp. 1-6. [5] w.h. park, j.c. wambold, transportation research record 584 (1976), pp. 55-63. [6] g. maternini, silvia foini, tecniche di moderazione del traffico, egaf edizioni, forlì, 2010, isbn 978-88-8482-364-9 [7] p. jönsson, ӧ. johansson, journal of sound and vibration 282 (2005), pp. 1043-1064. [8] s. grudemo, a. ihs, m. wiklund, “the influence of road surface condition on driving comfort” in vti meddelande, 2004, vol. 957. [9] k. ahlin, n.o.j. granlund, international journal of pavement engineering (2002), pp. 207-216. [10] .uni en 12299, “railway applications, ride comfort for passengers, measurement and evaluation” (2009). [11] vettui d., maternini g., lancini m., bodini i., “on-board comfort measuerement system development for buses”, proc. of the seventh international conference on informatics and urban and regional planning input 2012, may 10-12, 2012, cagliari, italy, isbn: 9788856875973, pp 1277-1286iso 8041, “human response to vibration – measuring instrumentation” (2005). [12] sirf, nmea reference manual, sirf technology, inc., california (2007). [13] g.b. rossi, b. berglund, “measurement related to human perception and interpretation – state of the art and challenges”, xix imeko world congress fundamental and applied metrology, sept 6-11, 2009, lisbon. terahertz techniques for better hazelnut quality acta imeko issn: 2221-870x march 2023, volume 12, number 1, 1 8 acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 1 terahertz techniques for better hazelnut quality manuel greco1, fabio leccese1, emilio giovenale2, andrea doria2 1 science department, università degli studi “roma tre”, rome 00146, italy 2 fusion and nuclear dept, enea, frascati, rome 00044, italy section: research paper keywords: terahertz; imaging; transmissions; yig; hazelnut citation: manuel greco, fabio leccese, emilio giovenale, andrea doria, terahertz techniques for better hazelnut quality, acta imeko, vol. 12, no. 1, article 29, march 2023, identifier: imeko-acta-12 (2023)-01-29 section editor: francesco lamonaca, university of calabria, italy received february 6, 2023; in final form february 24, 2023; published march 2023 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: manuel greco, e-mail: manuel.greco@uniroma3.it 1. introduction one of the pillars of food quality is food safety. reducing the presence of pathogens and detecting toxins in food production is a key part of the food production chain. to achieve a high standard of food quality control, a series of methods aimed at initially selecting the product and then placing it on the market, are available. one of the most used techniques in the field of food safety is x-ray imaging [1]-[3], but other methodologies such as ultrasound techniques [4], [5] also find space. one of the main advantages of x-ray techniques is that they produce high resolution images: x-rays can pass through matter undergoing an attenuation which depends, for example, on the type of material crossed. the higher the density of the material crossed, the more the rays are absorbed and the lower the amount of radiation transmitted. the latter contains information relating to the absorption of radiation, which can be used to obtain an image of the structure crossed. at the same time, techniques that use x-rays as an investigative tool are subject to limitations: some materials, such as wood and glass, are weakly detected due to their low density. an additional important factor that needs to be taken into consideration, when using techniques like these, is their ionizing nature. as known, x-radiation having a wavelength capable of interacting directly with the electrons of the orbitals, triggering ionizations and, therefore, modifications at a chemical level. furthermore, x-ray techniques can involve risks both for the operator and for the sample itself, since the x-ray photon is capable to tear electrons from matter and, therefore, can alter the quality of the food. in recent years, techniques operating in the terahertz frequency have appeared on the market and may potentially provide additional information to those obtained with traditional routine techniques. at present, imaging and terahertz spectroscopy techniques have been used in various fields, such as pharmaceutical industries [6], cultural heritage [7], aerospace industry [8] and finally precision agriculture [9], [10]. thz radiation, differently from infrared radiation, excites rotational and roto/vibrational transitions, so polar liquids such as water have a strong absorption in this spectral band. while on one hand this important feature could be exploited to quantify the water content in foods, on the other hand it makes it a limit [11]. as previously reported, unlike x-ray techniques, whose photons ionize matter, thz photons have a much lower energy. by exploiting this characteristic, thz radiation is unable to cause ionizations and, therefore, induce structural changes in biomolecules. therefore, techniques operating in this band can abstract in recent years, technological innovation has acquired a fundamental role in the agri-food sector, in particular in food quality control. the development of technology allowed to improve the quality of the food before it is placed on the market. recently, non-invasive techniques such as those operating in the thz spectral band were applied to the field of food quality control. in the laboratories of the enea centre in frascati, close to rome-italy, has been developed a thz imaging system operating in reflection mode and an experimental setup able to measure both reflection and transmission of the samples in the frequency range between 1840 ghz. with these two setups will distinguish rotten and healthy hazelnuts acquiring in real time both images of the fruit inside the shell by using the imaging system and the transmission data exploiting the 18-40 ghz system. mailto:manuel.greco@uniroma3.it acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 2 be considered safe for operators, but above all for the purposes of food quality. in the food industry, thz imaging technologies have proven useful in detecting food contaminants [12], [13]. as reported in several studies, the presence of chemical products on foods can lead to the onset of pathologies in consumers and therefore constitute a potential risk. recently, a group of researchers applied terahertz time-domain spectroscopy (thz-td) to detect the presence of pesticides in food powders such as sticky rice, sweet potato, and lotus root [14], highlighting specific absorbance peaks ranged from 0.5 to 1.6 thz for some pesticides. foreign bodies can be a problem in food safety as well. in [15], a thz imaging system, operating in transmission mode, was applied to detect foreign bodies, such as stones, glass fragments and metal screws in chocolate bars. all these elements were visible in the thz images. as previously reported, the potential that terahertz technologies can demonstrate in food analysis is enormous. this potential needs to be tested in the laboratory on a large number of samples, possibly combined with artificial intelligence. in this study, thz measurements by using a 97 ghz imaging system and an experimental setup were made on healthy and rotten hazelnuts samples. 2. imaging terahertz scanner these thz measures on healthy and rotten hazelnut were made by using a thz imaging scanner operating in reflection mode. figure 1a shows this system with some components, figure 1b, while a more detail description from a physical point of view is reported in [16]. this system mainly consists of a 97 ghz source, a directional coupler, a truncated waveguide, a laser triangulation system and a schottky diode; this last used as detector of the radiation. the impatt (impact ionization avalanche transit-time) diode, figure 1c, operates at a fixed frequency of 97 ghz with an output power of 70 mw and it was used as source. this diode is based on the avalanche effect. when a strong electric field is applied to a semiconductor or insulator, it can accelerate the free electrons present in the material. after having gained a certain amount of energy and impacting against the atoms of the material, these particles are able to induce ionizations, i.e., lead to the formation of other free electrons. in this way, at the end of this process called the avalanche effect, the number of electrons will be greatly increased. the impatt diode in this setup is connected to an elmika three port directional coupler, based on wr-10 waveguide, with 3 db attenuation as shown in figure 1d. in this study, the directional coupler was used to collect a fraction of the signal reflected back by the sample. a schottky diode, shown in figure 2, was used as detector of the radiation reflected. thanks to this high switching speed, this diode can follow the oscillations of the electric field up to frequencies of the order of hundreds of ghz. in this imaging scanner, the voltage signal generated by the schottky diode is sampled through an analogue-digital converter and sent to pc via usb interface. it is advisable to be able to accurately measure the distance that separates the waveguide from the sample, both to be able to position it correctly and to be able to exploit the capabilities of the system to measure the phase of the reflected radiation. in fact, the phase value also depends on the distance travelled by the reflected radiation, and it is therefore crucial to be able to measure this distance accurately. in order to measure the distance between the waveguide and the sample, a laser triangulation system was used, see figure 3. figure 1 a. imaging terahertz system; 1 b. (a) source, (b) directional coupler, (c) truncated wave guide, (d) laser triangulation system and (e) schottky diode; 1c. impatt diode at 97 ghz; 1d. directional coupler. figure 2. schottky diode. figure 3. laser triangulation system. acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 3 a laser diode emits a laser beam, which is projected onto the surface of the measurement target. subsequently, once the beam has reached the sample, it will be reflected back to be subsequently captured by a ccd/cmos sensor, positioned at a certain angle with respect to the optical axis of the laser beam. the target distance is calculated through a geometric algorithm, while the data is analysed by the internal or external controller and made available in output in different formats. in this work, the laser sensor had a wavelength of 670 nm, with an optical power of less than 1 mw and is capable of measuring distances between 50 and 100 mm with a resolution of 5 µm. furthermore, three stepper motors, figure 4 a-b-c, have been used to move both the source and the detector along the three axes x, y, z. the movement of each motor is regulated by three mercury controllers. the motors are controlled by suitable controllers that regulate all the motor parameters along the axis (x, y, z), translating the movement instructions sent via pc into electrical impulses which activate the physical movement of the motors. the three controllers are identical, but they are programmed by entering the parameters relating to the motor to be controlled (stroke, speed and acceleration characteristics, position of the reference probes, accuracy, etc.). the controllers can be connected to the control pc via usb port or rs232 interface. 3. 18-40 ghz measurement system this measurement system has been designed to measure the transmission of samples in the frequency range between 18 and 40 ghz. it is made up of an yttrium iron garnet (yig) source and a detector used to measure the power of the transmitted radiation. in order to easily measure, the interaction with the radiation takes place in free space, so it is necessary to launch the radiation with the appropriate horns. then, the radiation emitted by the yig source is propagated within a transmission line to the launching horn, and then collected, in this case via a horn, see figure 5. the transmitted signal is detected by means of a schottky diode, and the voltage signal obtained is sent to a control pc via an analogue-digital converter also interfaced via a usb port. considering that each horn reflects a certain percentage of the radiation, the system consisting of the throwing horn, sample and horn used to collect the transmitted signal can be seen as a system composed of three partially reflecting mirrors. each pair of mirrors in fact behaves like an interferometer, producing a series a b c figure 4. a. x-axis motor; 4 b. y-axis motor; 4 c. z-axis motor. figure 5. image of the experimental setup. a) yig source; b) transmission line; c) launching horn; d) collecting horn; e) schottky diode. acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 4 of effects, which allow to obtain information on the sample under examination. this system, therefore, consists of a yig driver system, a yig source, a transmission line made with coaxial cables, a first horn used to launch the radiation, a second horn to collect the transmitted signal and a schottky diode as detector. the yig driver system, shown in figure 6, is able to drive the yig oscillator mlos-184 model. on the front panel, 2 voltages, required for driving the yig oscillator, are displayed. the main voltage (labelled with tn voltage on the display) powers the main coil and can be changed from 0 volt to 10 volts in continuous manner (according with table 1 shown below). the fine voltage (labelled with fine modulation-fm coil voltage on the display) supplies power to the fine modulation coil allowing a little output frequency adjustment and can be changed from -10 v to 10 v. at the enea centre in frascati, close to rome (italy), a broadband yig source was chosen, operating in the region between 18 and 40 ghz with a maximum power of about 20 mw. figure 7 shows the yig source used in this study. spinels, garnets and hexaferrites are among the most common magnetic oxides used in industry. yttrium iron garnet is a synthetic crystal belonging to the garnet group. this material is used for tunable microwave electronic devices [17], circulators [18], insulators, phase shifters, tunable filters, and non-linear devices [19]. this crystal, also, can resonate at microwave frequencies when it is immersed into a static magnetic field. this resonance frequency is directly proportional to the strength of the magnetic field applied to the crystal. the magnetic field can be generated using an electromagnet, a permanent magnet, or a combination of both. the magnetic field of an electromagnet can be tuned using a variable current, thus, by using this crystal, we are to make a tuneable frequency source. a layer of eccosorb was placed inside the petri dish as shown in figure 8. a hole having the same dimensions of the hazelnuts was created in the eccosorb layer to allow the radiation to pass only through the sample. a schottky diode was used to detect the transmitted signal through the sample, figure 9. figure 6. yig driver system. table 1. linear relationship between voltage and frequency. tune voltage (v) frequency (ghz) 0.00 18 ghz 1.00 20 ghz 2.00 22 ghz 3.00 24 ghz 4.00 27 ghz 5.00 29 ghz 6.00 31 ghz 7.00 33 ghz 8.00 36 ghz 9.00 38 ghz 10.00 40 ghz figure 7. yig source. figure 8. petri dish with an eccosorb layer placed inside the dish itself. figure 9. schottky diode acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 5 4. hazelnut samples in this study, thz measurements were made on 5 healthy and 5 rotten hazelnuts belonging to the “tonda gentile romana” species which together with the “nocchione” are the most widespread species in lazio and central italy, figure 10. the tonda gentile romana has a hazelnut colour with shiny dark brown streaks, a smell of toasted hazelnuts, a toasted hazelnut flavor with no bitter or rancid aftertaste and a spheroidal appearance. 5. results at 97 ghz in order to carry out the measurements with the imaging system, the waveguide was positioned very close to the outer shell of each hazelnut. since the thz imaging system was designed to detect the phase shift of the reflected signal, by changing the distance that separates the sample from the waveguide by approximately 1.5 mm, corresponding to half a wavelength, this causes a phase shift, producing different coloured pixels in the image. in figure 11a-b the visible and thz image of the first two healthy hazelnuts is shown. the first healthy hazelnut at first glance did not seem rotten and showed no sign of deterioration, but breaking the outer shell the fruit was rotten. from the terahertz image can be seen that the fruit of the first healthy hazelnut does not have a welldefined, rounded shape if compared to the second healthy one. the second healthy hazelnut, contrarily, shows a well-defined shape. in figure 12a-b the visible and thz image of the third and fourth healthy hazelnut are shown. from the terahertz images can be seen that the fruit of the third and fourth healthy hazelnut has a rounded shape. the figure 10. rotten and healthy hazelnuts. a b figure 11. a. on the left, the first healthy hazelnut; on the right, the second healthy hazelnut. at first glance the first healthy one didn’t seem rotten; 11 b. thz image of the first two healthy hazelnuts. a b figure 12. a. on the left, the third healthy hazelnut; on the right, the fourth healthy hazelnut; 12. b. thz images of the third and fourth healthy hazelnut. a b figure 13. a. visible image of the fifth healthy hazelnut; 13. b. thz image of the fifth healthy hazelnut. acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 6 hazelnut shown on the left is coloured yellow while the one on the right is blue. this is explained by the fact that each hazelnut having a well-defined shape and therefore size, this phenomenon causes a phase shift from a physical point of view. finally, in figure 13a-b, the visible and terahertz image of the fifth healthy hazelnut are shown. the same procedure was also followed for rotten hazelnuts. figure 14, figure 15, figure 16, figure 17, and figure 18 show the visible and thz image of the rotten hazelnuts, respectively. from the thz images, the hazelnuts don’t have a regular shape, i.e., rounded. 6. results at 18-40 ghz after carrying preliminary tests, we realized that it is better to operate in the upper frequency limit of the emission range: when operating below 36 ghz, the diffractive effects due to the higher wavelength, comparable to the hazelnuts dimensions, produce results difficult to analyse, due to the high background radiation transmission. operating at shorter wavelength makes the system able to clearly distinguish between healthy and rotten hazelnuts. focusing on the frequency range between 36 and 40 ghz, with a wavelength range between 8 and 7 millimetres, smaller than the size of hazelnuts, better results were obtained. to test the new setup, we decided to examine five healthy hazelnuts and five rotten ones by modulating the voltage between 8 and 10 volts. in the graph in figure 19, the transmission values relating to healthy and rotten hazelnuts are shown. in figure 19, in each series of data, corresponding to a specific frequency, the transmission of the healthy hazelnuts is shown at figure 14. on the left, visible image of the first rotten hazelnut; on the right thz image of the rotten one. figure 15. on the left, visible image of the first second rotten hazelnut; on the right, thz image of the rotten one. figure 16. on the left, visible image of the third rotten hazelnut; on the right, thz image of the rotten one. figure 17. on the left, visible image of the fourth rotten hazelnut; on the right, thz image of the rotten one. figure 18. on the left, visible image of the fifth rotten hazelnut; on the right, thz image of the rotten one. figure 19. measures performed at different frequencies, corresponding to applied voltages between 8 v and 10 v. figure 20. the orange bar highlights the transmission relating to the first healthy hazelnut. acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 7 the bottom of the graph while the one referring to the rotten is shown above. from the graph, the highest transmission values are those relating to the petri dish used as baseline. a clear difference in transmission between healthy and rotten hazelnuts is evident, apart from the data from the first nut (orange bar in the graph in figure 20), that shows a transmission comparable to that of the rotten hazelnuts. the lower transmission of the healthy hazelnuts is correlated to the higher amount of fatty acids, which absorb the radiation. the anomaly of the first sample gave us a confirmation of this principle and of its possible practical application as a discrimination tool to identify rotten hazelnuts. sample 1 didn’t show, on visual inspection, any sign of deterioration, but when the outer shell was broken, the fruit was rotten as shown in figure 21. this is a clear demonstration that this system is able to identify hazelnuts that would be judged “healthy” as well on the basis of a simple visual inspection (that is the most used method in practice) and demonstrates the absence of any bias in the experiment. 7. conclusions from this study, we have seen how the thz reflection imaging system and the 18-40 ghz experimental system have proved to be non-destructive tools in the food industry. as reported in several scientific papers, terahertz techniques have been used to detect pesticides, fertilizers, and moisture in food. despite these advantages, the technique currently has some limitations, mainly related to its possible use in its practical use in an industrial environment. the terahertz imaging system has shown the capability of differentiating hazelnuts, but at the same time it presents some limitations, the main one of which is given by the scan time required to acquire a single hazelnut (30 seconds to scan a hazelnut), which is too long for a possible application in a production line. a possible solution could be to modify the layout of the instrumentation to carry out the measures in transmission mode, exploiting a thz-sensitive matrix detector, for one-shot image acquisition. to reduce the scanning time, it was therefore thought to design a device operating in transmission by detecting the signal transmitted through the nut. this device is also able to operate in a frequency range between 18 and 40 ghz thanks to the yig source used for this purpose. so, by modulating the magnetic field applied to the yttrium-iron garnet we are able to select the resonance frequency of the crystal. furthermore, this experimental system compared to the thz imaging system is cheaper and faster in terms of data acquisition. finally, from the acquired data it was possible to detect hazelnuts, which at first sight seemed healthy but were rotten. references [1] h. einarsdóttir, m. j. emerson, l. h. clemmensen, k. scherer, k. willer, m. bech, r. larsen, b. k. ersbøll, f. pfeiffer, novelty detection of foreign objects in food using multi-modal x-ray imaging, food control, 67, pp. 39-47. doi: 10.1016/j.foodcont.2016.02.023 [2] r. p. haff, n. toyofuku, x-ray detection of defects and contaminants in the food industry, sensing and instrumentation for food quality and safety, 2(4), pp. 262–273. doi: 10.1007/s11694-008-9059-8 [3] m. s. nielsen, t. lauridsen, l. b. christensen, r. feidenhans’l, xray dark-field imaging for detection of foreign bodies in food. food control, 30(2), pp. 531–535. doi: 10.1016/j.foodcont.2012.08.007 [4] j. chandrapala, c. oliver, s. kentish, m. ashokkumar, ultrasonics in food processing food quality assurance and food safety, trends in food science and technology, 26(2), pp. 88-98. doi: 10.1016/j.tifs.2012.01.010 [5] b. zhao, y. jiang, o. a. basir, g. s. mittal, foreign body detection in foods using the ultrasound pulse/echo method: foreign body detection in thoughout foods, journal of food quality, 27(4), pp. 274–288. doi: 10.1111/j.1745-4557.2004.00651.x [6] d. m. charron, k. ajito, j. kim, y. ueno, chemical mapping of pharmaceutical cocrystals using terahertz spectroscopic imaging, analytical chemistry, 85 (4), pp. 1980-1984. doi: 10.1021/ac302852n [7] a. doria, g. p. gallerano, e. giovenale, m. greco, m. picollo, thz detection of water: applications on mural paintings and mosaics, proc. of the 42nd int. conf. on infrared, millimeter, and terahertz waves, irmmw-thz, cancun, mexico, 27 august-1 september 2017, pp. 1-2. doi: 10.1109/irmmw-thz.2017.8067164 [8] m. greco, e. giovenale, f. leccese, a. doria, e. de francesco, g. piero gallerano, a thz imaging scanner to detect structural and fire damage on glass fiber composite, proc. of the ieee 9th international workshop on metrology for aerospace, metroaerospace, pisa, italy, 27-29 june 2022, pp. 384-389. doi: 10.1109/metroaerospace54187.2022.9856003 [9] m. greco, e. giovenale, f. leccese, a. doria, a discrimination of healthy and rotten hazelnuts using a thz imaging scanner, proc. of the ieee workshop on metrology for agriculture and forestry, metroagrifor, perugia, italy, 3-5 november 2022, pp. 229-233. doi: 10.1109/metroagrifor55389.2022.9964672 [10] m. greco, e. giovenale, f. leccese, a. doria, e. de francesco, g. p. gallerano, a thz imaging scanner to monitor leaf water content, proc. of the ieee int. workshop on metrology for agriculture and forestry, metroagrifor, trento-bolzano, italy, 35 november 2021, pp. 7-11. doi: 10.1109/metroagrifor52389.2021.9628522 [11] a. a. gowen, c. o'sullivan, c. p. o'donnell, terahertz time domain spectroscopy and imaging: emerging techniques for food process monitoring and quality control, trends in food science and technology, 25(1), pp. 40-46. doi: 10.1016/j.tifs.2011.12.006 [12] m. caciotta, s. giarnetti, f. leccese, b. orioni, m. oreggia, c. pucci, s. rametta, flavors mapping by kohonen network classification of panel tests of extra virgin olive oil, measurement: journal of the international measurement confederation, 78, 2016, pp. 366-372. doi: 10.1016/j.measurement.2015.09.051 [13] h. j. shin, s. choi, g. ok, qualitative identification of food materials by complex refractive index mapping in the terahertz range, food chemistry, 245, pp. 282-288. doi: 10.1016/j.foodchem.2017.10.056 figure 21. first healthy hazelnut external visual inspection and status of the inner fruit (degraded). https://doi.org/10.1016/j.foodcont.2016.02.023 https://doi.org/10.1007/s11694-008-9059-8 https://doi.org/10.1016/j.foodcont.2012.08.007 https://doi.org/10.1016/j.tifs.2012.01.010 https://doi.org/10.1111/j.1745-4557.2004.00651.x https://doi.org/10.1021/ac302852n https://doi.org/10.1109/irmmw-thz.2017.8067164 https://doi.org/10.1109/metroaerospace54187.2022.9856003 https://doi.org/10.1109/metroagrifor55389.2022.9964672 https://doi.org/10.1109/metroagrifor52389.2021.9628522 https://doi.org/10.1016/j.tifs.2011.12.006 https://doi.org/10.1016/j.measurement.2015.09.051 https://doi.org/10.1016/j.foodchem.2017.10.056 acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 8 [14] a. ren, a. zahid, d. fan, x. yang, m. a. imran, a. alomainy, q. h. abbasi, state-of-the-art in terahertz sensing for food and water security – a comprehensive review, trends in food science and technology, 85, pp. 241-251. doi: 10.1016/j.tifs.2019.01.019 [15] g. ok, h. j. kim, h. s. chun, s. choi, foreign-body detection in dry food using continuous sub-terahertz wave imaging, food control, 42, pp. 284-289. doi: 10.1016/j.foodcont.2014.02.021 [16] a. doria, g. p. gallerano, e. giovenale, l. senni, m. greco, m. picollo, c. cucci, k. fukunaga, a. c. more, an alternative phasesensitive thz imaging technique for art conservation: history and new developments at the enea center of frascati, applied sciences (switzerland), 10(21), pp. 1-24. doi: 10.3390/app10217661 [17] c.-w. nan, m. i. bichurin, s. dong, d. viehland, g. srinivasan, multiferroic magnetoelectric composites: historical perspective, status, and future directions, progress in advanced dielectrics, pp. 191-293. doi: 10.1142/9789811210433_0005 [18] j. ganne, r. lebourgeois, m. paté, d. dubreuil, l. pinier, h. pascard, the electromagnetic properties of cu-substituted garnets with low sintering temperature, journal of the european ceramic society, 27(8-9) 2007, pp. 2771-2777. doi: 10.1016/j.jeurceramsoc.2006.11.054 [19] j. d. adam, l. e. davis, g. f. dionne, e. f. schloemann, s. n. stitzer, ferrite devices and materials, ieee t. microw. theory. 50 (2002), pp. 721-737. doi: 10.1109/22.989957 https://doi.org/10.1016/j.tifs.2019.01.019 https://doi.org/10.1016/j.foodcont.2014.02.021 https://doi.org/10.3390/app10217661 https://doi.org/10.1142/9789811210433_0005 https://doi.org/10.1016/j.jeurceramsoc.2006.11.054 https://doi.org/10.1109/22.989957 template for an acta imeko event paper acta imeko issn: 2221-870x december 2017, volume 6, number 4, 89-94 acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 89 multi-component measuring device – completion, measurement uncertainty budget and signal crosstalk for combined load conditions sebastian baumgarten, dirk röske and rolf kumme physikalisch-technische bundesanstalt (ptb), bundesallee 100, 38116 braunschweig, germany section: research paper keywords: multi-component measurement; measurement uncertainty budget; torque; force; signal crosstalk; friction coefficient sensor citation: sebastian baumgarten, dirk röske and rolf kumme, multi-component measuring device – completion, measurement uncertainty budget and signal crosstalk for combined load conditions, acta imeko, vol. 6, no. 4, article 14, december 2017, identifier: imeko-acta-06 (2017)-04-14 section editor: eric benoit, university savoie mont blanc, france received march 24, 2016; in final form january 19, 2017; published december 2017 copyright: © 2017 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: sebastian baumgarten, e-mail: sebastian.baumgarten@ptb.de 1. introduction there is an increasing number of measuring systems that can detect more than one force or torque component of these vectorial physical quantities. there is, therefore, an increasing need for traceability with regard to multi-component measurements. realizations of such measuring facilities with sufficient measurement uncertainty and a suitable measuring range are complex and rare. ptb's hexapod [2] and the measuring facility at the instituto nazionale di ricerca metrologica (inrim) [3] are examples of such a realization. the ptb uses the infrastructure that is already available at such measuring facilities, to upgrade one facility by adding additional torque components. as a result of a project in ptb, within the 1 mn force standard machine (1 mn fsm), torques can now be generated by means of a lever/band/mass system. this extension of the fsm allows the combination of a force measuring range from 20 kn to 1 mn with a torque measuring range from 20 n·m to 2 kn·m. this, in turn, extends the service range of the measuring facility, and measuring systems such as friction coefficient sensors or wheel load sensors can, thus, be investigated specifically. the measurement uncertainty budget (mub) for mz is presented. 2. set-up the additional torque device has a modular set-up and can be mounted into or removed from the force flow of the 1-mn fsm. it works on the basis of the principle of a two-armed lever at the ends of which a force couple acts. the force couple is equal value which, although parallel to each other, act in the opposite direction to each other. the cross forces thus neutralize each other and all in all, an active torque mz is realized. the forces are generated via two mass stacks that are located symmetrically on either side of the 1 mn fsm (see figure 1). each of these mass stacks (see figure 2) is composed of a lowerable set of masses. the mass disks are coupled to a metallic band. the metallic band is diverted by means of an airbearing rotor. the vertical gravitational force of the mass stacks becomes a horizontal tensile force. the metallic band is coupled to the lever arm and thus transmits the force onto the abstract this paper presents the completion and the measurement uncertainty budget of a multi-component measuring facility. the new facility is part of the 1 mn force standard machine [1] at the physikalisch-technische bundesanstalt (ptb). it enables the simultaneous generation of a torque in the range from 20 n·m to 2 kn·m in addition to axial forces 20 kn to 1 mn. this allows the characterization of measuring systems which require combined loads of axial forces fz and torques mz, like friction coefficient sensors. the aim is a measurement uncertainty of (k = 2) for mz < 0.01 % and fz < 0.002 %. the physical model yields to extended measurement uncertainties (k = 2) for 20 n·m of 5.9·10-5 and for the maximum load step mz = (2000 ± 0.084) n·m. mailto:sebastian.baumgarten@ptb.de acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 90 system. sensors and step motors stabilize the system position under load and changing load conditions. the synchronous triggering, monitoring and data acquisition are effected by excel macros and a dmp 41. 3. measurement uncertainty budget the following sections are only a summary of the important points of the measurement uncertainty budget, more details about this very comprehensive topic in [9]. a specific measurement uncertainty budget for the additional facility is presented. it includes a model, figure 3, taking physical and geometric influence factors into account. this includes different factors, among other things, environmental influences, geometric characteristics, or the influence of the mass stacks. the influence of different influence factors on the measurement uncertainty and on the signal stability (e.g. friction inside the air bearing) have been investigated. in this case of application, also the realignment process of the mass stacks, the flatness errors of adaption parts and angular deviations must be taken into account. the model therefore encompasses a consideration of the system according to the vectorial components of m (1) and the analysis of the influence factors on the measurement uncertainty. in the coordinate system used, figure 1. multi-component measuring facility: 1 − mass stack a; 2 − mass stack b; 3 − metallic band for force application onto the lever; 4 − masses; 5 − two-armed lever; 6 − coordinate measuring device, mounted onto a column support; 7 − 1 mn fsm. figure 2. mass stack b. both mass stacks exhibit an identical design: 1 − sps control; 2 − support elements resting against the frame of the 1 mn fms; 3 − block with step motors for the displacement and tilting of the airbearing head; 4 − air-bearing head with integrated rotor for force diversion; 5 − metallic band and coupling element for force application; 6 − masses; 7 − rotational and linear table for position displacement of the mass stack. figure 3. overview of the acting influence factors in the form of an ishikawa diagram for mz. acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 91 mz is the torque, ly is the lever length, and fx is the applied force. the ideal case thus consists in the lever and the force vector lying in the x-y-plane and being oriented orthogonal to each other. an additional axial force fz can be applied onto the system by the 1 mn fsm 𝑀��⃗ = �⃗� × 𝑙 = � 𝐹𝑥 𝐹𝑦 𝐹𝑧 � × � 𝑙𝑥 𝑙𝑦 𝑙𝑧 � = � 𝐹𝑦𝑙𝑧 − 𝐹𝑧𝑙𝑦 𝐹𝑧𝑙𝑥 − 𝐹𝑥𝑙𝑧 𝐹𝑥𝑙𝑦 − 𝐹𝑦𝑙𝑥 � = � 𝑀𝑥 𝑀𝑦 𝑀𝑧 � . (1) table 1 shows identified factors and their percentage weighting for the load steps 20 n·m and 2000 n·m. identical measurement uncertainty budgets have been established for each load step. according to the physical model, the measurement uncertainty (k = 2) for the minimum load step mz = (20 ± 1.2·10−3) n·m and for the maximum load step mz = (2000 ± 0.084) n·m. 3.1. local gravitational acceleration the local gravitational acceleration at the measuring station was determined by the institute for earth measurement (ife), hannover, as being gloc = 9.812524 ms−2 with an expanded measurement uncertainty (k = 2) of 10 µms−2. 3.2. density and masses of the weight one set of weights includes the following load steps 1 × 10 n, 2 × 20 n, 1 × 50 n, 3 × 100 n and 3 × 200 n. the density of the material used for the cylindrical weights can be indicated as ρm = 7979.7 kg m−3 ± 2.0 kg m−3 (k = 2). the uncertainty components (k = 2) lie for all masses mm in a range < 5·10−6 kg and are computed separately for each load step. the contribution to the measurement uncertainty budget never exceeds 1.33 %. 3.3. environment for the determination of the acting gravitational force, a buoyancy correction (2) was applied. the measuring facility is located in an air-conditioned hall. changes in the ambient conditions are minimum. the actual values for the air pressure, the humidity and the temperature are acquired to compute the mub. their influence on the mub, however, lies in a range < 0.01 %. the ambient parameters from table 1 for the mub are the humidity hl = 42 % ± 5 %, the temperature tl = 21 °c ± 0.1 °c, and the ambient pressure pl = 1003.4 hpa ± 2 hpa 𝐹𝑥 = 𝑚𝑚 · 𝑔𝑙𝑙𝑙 · �1 − 0,348·𝑝𝐿−0,009·ℎ𝐿·𝑒0,06·𝑇𝐿 (273,15+𝑇𝐿)·𝜌𝑚 � . (2) 3.4. lever length and thermal expansion a two-armed lever is used. a specified value of 999.92 mm applies to both sides. the length of the whole lever was calibrated at ptb's coordinate metrology division; the result obtained was: 1999.882 mm ± 0.028 mm (k = 2). when calculating the total length, also the half of the thickness of the metallic bands for force application must be taken into account. the thickness is 0.08 mm ± 0.001 mm. the measurement uncertainty of the determination of the lever length represents the largest contribution to the mub for mz. due to the geometrical dimension of the lever, this uncertainty cannot be further reduced with the existing coordinate measuring machines. the lever is made of an aluminium alloy. the thermal expansion for this alloy is 2·10−5 k−1. accordingly, temperature fluctuations of 0.1 °c have an influence of 4.92 % on the mub. the lever will later be replaced by another lever made of a temperature-stable invar alloy. 3.5. friction of the air bearings the air bearings do not provide absolutely friction-free force diversion. the influence of the friction inside the air bearing on the torque signal must therefore be investigated [5]. for this investigation, additional weights having a defined mass were applied. the weights are selected in such a way that, with the measuring chain used, a change in signal of practically one digit is expected. the measurements were repeated at all load steps up to 600 n·m and yielded the same result. a change of 1 digit, however, also corresponds to the signal stability of the measuring amplifier (dmp41 with low pass filter 0.04 hz butterworth), the influence has, thus, been estimated as being two digits. this corresponds to a maximum torque proportion of 3.1·10−4 n·m. the contribution to the mub is constant across the load steps. the percentage contribution to the mub for small load steps, 48.3 %, is therefore the largest. 3.6. influence of torsion under load loading the system with a torque leads to a torsion of the adaption/sensor system. torsion, in turn, leads to a reduction of the length of the metallic band between the lever and the unwinding point at the air bearing. the difference represents the overlapping of the metallic band on the side of the force generation fx and, as an additional mass, it contributes accordingly to the torque mz. the proportion directly depends on the load step. the differential length is determined, by means of a laser sensor as being to 10 µm. the change in mass is determined by means of the band thickness 0.080 mm ± 0.001 mm, height 30.0 mm ± 0.1 mm and density 7850 kg m−3 ± 20 kg m−3 and it is taken into account for the torque calculation. the contribution to the mub is < 0.01 %. 4. geometrical characteristics to calculate the mub and the disturbing quantities, the orientation as well as the geometric deviation from the optimal orientation must be detected. parallelism differences, angular table 1. measurement uncertainty budget for 20 n·m and 2000 n·m. influence quantities index of mub (20 ± 1.2·10-3) n·m (2000 ± 0.084) n·m gravitational acceleration 0.05 % 0.12 % ambient pressure < 0.01 % < 0.01 % air humidity < 0.01 % < 0.01 % temperature 4.92 % 9.73 % weight of the masses 1.36 % 0.413 % metallic bands overlap < 0.01 % < 0.01 % lever length 45.24 % 89.53 % air bearing friction 48.31 % < 0.01 % metallic band thickness 0.06 % 0.11 % height discrepancy < 0.01 % < 0.01 % parallelism error 0.04 % 0.07 % angular error of the pressure plate < 0.01 % < 0.01 % angular error of the adaption/sensor system < 0.01 % < 0.01 % acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 92 deviations, tilts of the lever and height differences are part of these deviations. a coordinate measuring device acquires the geometric characteristics. by scanning any given point, the coordinate measuring device, with the aid of various angular encoders, computes the spatial position in relation to the machine coordinate system. the quality of a measurement depends on the measurement process, on the user, on individual errors of the angular encoders as well as on the computation performed by the coordinate measuring device. we have assumed that the accumulation of the individual errors follows a gaussian distribution. the hypothesis was checked – and confirmed – by repeated measurements and by means of a shapiro-wilk test [6] for the individual measurement processes. sine and cosine functions must be used to calculate the mub according to (1). the problem is that the sensitivity coefficient often tends to be zero at small angles. for this reason, an upper estimation is used for the influence [7]. 4.1. deviation in parallelism orientation for an ideal couple, both metallic bands must be exactly parallel to each other. measurement points for the coordinate measuring device on the lever and on the air bearing serve as reference points to determine the angle. the uncertainty across the measurement process was estimated by averaging with (k = 2) 0.02°. together with the fine adjustment of the angular orientation, a parallelism error of 0.072° is obtained. 4.2. deviation in height orientation for the ideal orientation, the height of the force application point at the lever must be in agreement with the band unwinding point at the air bearing. reference points are used for the height orientation of the system; with 45 µm (k = 2) for the normal contribution throughout the measurement process. the stability of the height is given by differential height measurements with a laser sensor at the end of the lever and by the displacement of the air bearing via a step motor. the signal threshold level for the adjustment was laid down as being 100 µm. if the signal threshold level and the contributions due to the uncertainties of differential height measurements and of the displacement by means of the step motor, then one obtains a total contribution of 186 µm. a reduction of the signal threshold level considerably reduces the uncertainty; however, the effort for the adjustment control then increases tremendously. a one-sided height difference of 186 µm, related to a band length of 1440 mm, corresponds, at 1000 n, to a negligible change in torque of 8.3·10−6 n·m. when loading with the 1 mn fsm with fz, the adaptor/sensor/lever set-up lowers itself. this is due to elongations in the 1 mn adjustment control and compression of the adapter/sensor system. for 500 kn this lowering amounts to about 2.6 mm. this height difference to the air bearing is corrected automatically by the adjustment control when the load is changed. the orientation therefore remains stable, even under an fz load, in a range of 186 µm. 4.3. deviation in planarity of the pressure plate, adaptation and sensor the lever's tilt in relation to the ideal x-y-plane depends on the orientation of the adaptor/sensor system. the adaptor parts are mechanical components to mounting the sensors at the multi-component facility. deviations lead to angular errors and, thus, to a tilt of the lever. the standard reference is the pressure plate of the 1 mn fms. averaging over different measurement series provides an estimate of the flatness. this can be specified as αpp = 0° ± 0.0178°. the angle refers to a tilt of the plane in relation to the ideal x-y-plane. correspondingly, an angular deviation αpp = 0° ± 0.0178° also applies to the lever. in addition, the flatness errors accumulate due to the adaption parts, the sensor and their installation. the resulting angular error depends on the quality of the components and must therefore be determined separately for each adaptor/sensor system. in the case of the mub described in table 1, an angular error αap = 0.18° ± 0.0201° can be stated. the mub does not take into account the orientation of the angular error. the error is upper estimated by considering it as being constant for all directions. according to the calibration results obtained by the coordinate metrology division, the deflection of the lever due to its dead weight can be neglected. 5. disturbing quantities the quantities considered as disturbing quantities are the shearing force fy, an additional axial force fz and the bending moments mx and my. a nominal value 0 is the goal for all disturbing quantities. deviations of the geometric orientation (essentially), however, result in an uncertainty for the nominal value; this applies to each quantity. the computation is carried out separately for each torque load step and have to be recalculated for each adaptor/sensor system. for the system to which also table 1 applies, at 2000 n·m, 0 n·m ± 0.4 n·m is obtained for mx and 3.49 n·m ± 1.01 n·m for my. the deviation from the nominal value for my is due to the acting force fx and to an effective lever length lz as a result of the lever's tilt. due to adaption parts with smaller flatness errors, it is possible to reduce the lever's tilt as well as the resulting bending moments significantly. table 2 shows the percentage contribution of the significant influence quantities on the uncertainty of my and mx. the influence quantities that are not mentioned there, see table 1, have a negligibly small influence on the mub amounting to < 0.0001 %. the disturbing quantities fy and fz were also computed from the geometric deviations. at the maximum load step 2000 n·m, one obtains for the system 0 n ± 2.6 n for fy and 0 n ± 0.3 n for fz. disturbing quantities may cause the characteristic curve of a sensor to shift. the signal crosstalk as a function of these quantities is often difficult to describe [8]. with little technical effort, it is possible to use the measuring device asynchronously in order to, for example, estimate the sensitivity of a sensor to a certain disturbing quantity. 6. signal crosstalk for smz the combined load conditions between fz and mz and the signal crosstalk of sfz and smz were investigated based on the table 2. significant influences on the mub of mx and my for 2000 n·m. influence quantities index of mub mx / (0.4 n·m) my / (1.01 n·m) height discrepancy 99.97 % < 0.01 % parallelism error 0.02 % < 0.01 % angular error of the pressure plate < 0.01 % 43.98 % angular error of the adaption/sensor system < 0.01 % 56.02 % acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 93 example of a multi-component sensor (mcs) which is specially adapted to the auxiliary device. the subsequent objective being the traceability of industrial sensors, the measurement results will be used to derive and develop expedient and practical calibration procedures and sequences as well as evaluation and analytical procedures. 6.1. multi-component sensor the sensor has the nominal load ranges fz = 500 kn and mz = 500 n·m. calibration for individual quantities was performed according to en iso 376 and din 51309. for fz, the sensor achieved < 5·10−4 for > 100 kn and < 2·10−3 for < 100 kn, and for mz, the clockand anti-clockwise measurement uncertainty is (k = 2) < 7·10−4. fz is kept constant for a measurement series, whereas mz is varied; the next load step fz is then selected and mz is varied again. this sequence must be observed to prevent the mass disks from coupling asymmetrically into the load frame of the 1 mn force standard machine. figure 4 shows the result of the combined loading for the torque signal smz. the represented signal is only the signal change caused by combined loading. to assess the influence of signal crosstalk quantitatively, figure 5 shows the relative change of the signal based on the signal evolution from the calibration function of mz. without correction, the error share can reach 4 %. this error inherent in the system must therefore be taken into account. the signal crosstalk of mz on the bridges of sfz is negligibly small and is therefore not represented here. 6.2. analysis by means of multiple polynomial regression as shown in figure 4, the signal behaviour can, as a matter of principle, be represented by means of a higher-dimension regression surface. the multiple polynomial regression (mpr) method was applied. equation (3) describes the calculation of a parameter matrix θ from the identity matrix a and a signal matrix z. depending on the order you are aiming at for the solution, a minimum number of data points are required. for a reliable statement with, e.g., a cubic approximate solution, five different fz and mz load steps – i.e. 25 independent data points – are necessary 𝜃 = (𝐴𝑇𝐴)−1𝐴𝑇𝑍. (3) table 3 shows the set of parameters calculated and the coefficient of determination for the signal pattern from figure 4. with (4) the cubic solution describes the signal pattern sufficiently well 𝑆𝑀𝑧 = 𝑎1𝐹𝑧 + 𝑎2𝑀𝑧 + 𝑎3𝐹𝑧 2 + 𝑎4𝐹𝑧𝑀𝑧 + 𝑎5𝑀𝑧 2 + ⋯ 𝑎6𝐹𝑧 3 + 𝑎7𝐹𝑧 2𝑀𝑧 + 𝑎8𝐹𝑧𝑀𝑧 2 + 𝑎9𝑀𝑧 3 . (4) especially for the range > 10 % of the nominal load, the systematic influence can be reduced from 4 % to < 0.5 %. the solution can only be applied to a limited extent to the lower load range. the mpr solution improves significantly with an increasing number of data points. this procedure is applied to both the loading and the unloading range. the inverse transformation of the signal values smz and sfz to the input quantities fz and mz is analogue. the next step is planned to consist of comparison measurements with various friction coefficient sensors in order to determine the specific signal crosstalk. 7. conclusions the extended relative measurement uncertainty (k = 2) of the 1 mn fsm is about 2·10−5. the model provides an expanded relative measurement uncertainty for the additional measuring facility for torque generation of mz = (20 ± 1.2·10−3) n·m up to mz = (2000 ± 0.084) n·m for the maximum load. comparison measurements with different torque reference transducers have shown very good repeatability; the reproducibility, however, is within a range of < 4.1·10−4. the < 2·10−4 goal has, thus, not been achieved yet. most of the time, a measurement uncertainty < 1·10−3 is sufficient for industrial sensors. correspondingly, the measuring device is not yet listed in the catalogue of measuring facilities and ptb's figure 5. relative signal change smz by combined load conditions. table 3. cubic solution parameter for the measurements of figure 4. parameter for the cubic solution a1 a2 a3 a4 a5 a6 a7 a8 a9 r² 1·10-4 -3.4·10-3 -2.41·10-7 -2.98·10-7 -1.17·10-7 2.83·10-10 1.88·10-11 9.2·10-11 1.35·10-10 7.89·10-5 figure 4. torque signal change smz by combined load conditions. acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 94 quality management system. the characterization of the signal crosstalk by means of a specific multi-component sensor has shown that significant errors may occur when signal crosstalk is not taken into account. the multiple polynomial regression method allows the functional relation to be described precisely. comparison measurements with industrial multi-component sensors from the screw industry will have to show whether these findings are applicable to other systems. references [1] w. weiler, m. peters, h. gassmann, h. fricke, w.ackerschott, “die 1-mn-normalmeßeinrichtung der ptb braunschweig.”, vdi-z 120, 1978, pp. 1-6. [2] d. röske, “metrological characterization of a hexapod for a multi-component calibration device”, proc. xvii imeko world congress, dubrovnik, croatia, 2003, pp. 347-351. [3] c. ferrero, li qing zhong, c.marinari and e.martino “new automatic multicomponent calibration system with crossedflexure levers”, proc. of ismcr'93 imeko tc-17, torino, italy, 1993. [4] a. robinson, „the commissioning of the first uk national standard static torque calibration machine“, 19th conference on force, mass and torque measurement, cairo, egypt, 2005 pp.1-6. [5] d. peschel, d. mauersberger, “determination of the friction of aerostatic radial bearings for the lever-mass system of torque standard machines”, proc. of the xiii imeko world congress, turin (italien), sept. 5-9, 1994, vol. 1, pp. 216-220. [6] s. s. shapiro, m. b. wilk “a analysis of variance test for normality”, biometrika, vol. 52, no. ¾, 1965, pp. 591-611. [7] d. röske, “uncertainty contribution in the case of cosine function with zero estimate – a proposal”, imeko 2010 tc3, tc5 and tc22 conferences, pattaya, chonburi, thailand, november 22-25, 2010. [8] a. brüge, d. röske, d. mauersberger, k. adolf “influence of cross forces and bending moments on reference torque sensors for torque wrench calibration”, xix imeko world congress, lisbon (portugal), sept. 6-11, 2009. [9] s.baumgarten, h. kahmann, d. röske “metrological characterization of a 2 kn·m torque standard machine for superposition with axial forces up to 1 mn”, metrologia, vol. 53, 2016, pp. 1165-1176. << /ascii85encodepages false /allowtransparency false /autopositionepsfiles true /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /error /compatibilitylevel 1.4 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.0000 /colorconversionstrategy /cmyk /dothumbnails false /embedallfonts true /embedopentype false /parseiccprofilesincomments true /embedjoboptions true /dscreportinglevel 0 /emitdscwarnings false /endpage -1 /imagememory 1048576 /lockdistillerparams false /maxsubsetpct 100 /optimize true 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/pdfxoutputintentprofileselector /documentcmyk /preserveediting true /untaggedcmykhandling /leaveuntagged /untaggedrgbhandling /usedocumentprofile /usedocumentbleed false >> ] >> setdistillerparams << /hwresolution [2400 2400] /pagesize [612.000 792.000] >> setpagedevice microsoft word article 2 editorial.docx acta imeko  february 2015, volume 4, number 1, 2 – 4  www.imeko.org    acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 2  introductory notes for the acta imeko special issue on the  “19 th  symposium on measurement of electrical quantities”  and the “17 th  workshop on adc/dac modelling and testing”   alexandru salceanu 1 ,  spartacus gomariz 2   1   faculty of electrical engineering,technical university of iasi, str. mangeron 23, iasi (romania)  2  electronics department. universitat politècnica de catalunya, c. comte d'urgell, 187 barcelona (spain)    section: editorial   citation:  alexandru  salceanu,    spartacus  gomariz,  introductory  notes  for  the  acta  imeko  special  issue  on  the  “19 th   symposium  on  measurement  of  electrical quantities” and the “17 th  workshop on adc/dac modelling and testing”, acta imeko, vol. 4, no. 1, article 2, february 2015, identifier: imeko‐ acta‐04 (2015)‐01‐02  editor: paolo carbone, university of perugia   received february 4 th , 2015; in final form february 10 th , 2015; published february 2015  copyright: © 2014 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: this work was supported by the spanish ministry of economy and competitiveness under the research project: cgl2011‐28682‐c02‐02  corresponding author: alexandru salceanu, asalcean@tuiasi.ro    1. introduction  the first issue of volume 4 of acta imeko is in your hands. this issue is centered on the 19th symposium on measurement of electrical quantities and 17th workshop on adc/dac modelling and testing which were held in the city of barcelona on july 2013. the main goals of these time-honored scientific events were to show the latest investigations and exchange of information and points of view on current research in the development and use of electrical and electronic instruments for measuring, monitoring and recording electrical signals. as a novelty, the 19th imeko tc4 expands the topics covered in previous editions in order to address issues related to marine technologies and applications. since august till november 2013, the members of the imeko tc4 board identified 24 valuable contributions presented in the barcelona events, for eventual publication in acta imeko. their authors were invited, in december 2013 to submit extended and updated versions of their selected papers. all of the authors that gave positive responses to this challenge started sending their extended papers into the acta imeko online submission system in early 2014. a significant number of representative reviewers proceeded in successive stages, their task of assessing the papers and submitting their recommendations. the final list of 17 published papers in this issue is a highquality exhibition of the first-class papers submitted to our symposium. all the reviewers involved in the process gave their support aiming to further improve the shape and the intrinsic content of the papers and their work. we sincerely hope that all the contributions that you will find in the next pages provide you information of your interest. 2. about tc4  the essential interest and activity of the imeko tc4 (measurement of electrical quantities) is the development of theoretical and practical research in the topics of electrical and electronic measurements. this technical committee organizes in every year, since its establishment in 1984, specific symposia and workshops. the continuous increase of general relevance and inclusion in everyday life of the electrical and electronic instruments for measuring, monitoring and recording electrical signals has significantly enhanced our activities and enlarged our addressability. the current members of tc4 are: janusz mindykowski, chairperson poland dominique dallet, deputy chairperson france pedro m. ramos, scientific secretary portugal linus michaeli, past chairperson slovak republic miloš sedlacek, past chairperson czech republic pasquale daponte, past chairperson italy antónio cruz serra, past chairperson portugal acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 3  mario savino, honorary chairperson italy joaquin del rio fernández spain raul land estonia graziella kreiseler germany istvan kollar hungary turgay özkan turkey umberto pogliano italy gelson rocha brazil alexandru salceanu romania jan saliga slovak republic sergey g. semenchinsky russia yurij tesyk ukraine vladimir vujicic serbia olfa kanoun germany dušan agrež slovenia victor i. didenko russia izzet kale united kingdom leo van biesen belgium mihai cretu romania christian eugène belgium sohair fakhry egypt vladimir haasz czech republic jouko halttunen finland peter händel sweden damir ilic croatia voicu groza canada jae kap jung – republic of korea elefterios kayafas greece wang xiaofei china he qing china 3. the articles  in the paper by asensio et al (p. 5), the authors, improved a traditional approach, by using a microphone array linked to a noise-monitoring unit, aiming to enable the tracking of the direction of arrival of the sound, thus getting better the classification rates. another advantage is to track the aircraft location along the runway, wanting to transform the sound pressure measurements into sound power level estimations. this novel instrument delivered results with quite good classification rates. the behavior of systems that are prone to noise is investigated in the paper by angrisani et al (p. 11), mainly from the perspective of the noise sources available on the market, with focus on the generators of colored noise. the authors propose an analog generator that exploits an arbitrary waveform generator as noise source, being capable of producing noise signals characterized by arbitrary power spectral densities. the implementation and characterization of a data acquisition (daq) system controlled by a 450 mhz sharc digital signal processor (adsp 21489), being capable of on-board processing the acquired data, is proposed in the next article by pinto et al (p. 19). the system’s main highlights are four differential channels with 1 mhz bandwidth, allowing simultaneous acquisition, 9 independent bipolar ranges and a maximum sampling rate of 600 ks/s. the analog daq inputs are protected against incorrect connections, ensuring full operation recovery, not being necessary to replace the damaged components or fuses. the aim of the paper by garcia-benadí et al (p. 26) is to provide the core for the "in situ", calibration of a hydrophone using a directional 10 khz source. anyway, even if the assigned value of uncertainty is high, the overall results are useful and interesting. the energy management, including the control of the state of charge of the batteries, allowing simultaneous charging of all batteries from outside of the vehicle, and a wireless connection/disconnection mode for an autonomous underwater vehicle is the core of the next article by i. masmitjà et al (p. 35). the methodology for measuring the charge level of the battery is current integration. the associated measured data are sent to the mission control of the vehicle, aiming to optimize and guarantee its navigation. the paper by liccardo et al (p. 44) mainly deals with the propagation channel aboard trains, propagation path loss, the delay spread and the coherence bandwidth. the results sustain that due to reflections by metal walls, the path loss exponent is slightly smaller than in free space, while not being significantly influenced by the position of transmitter and receiver. it is proven that the polarization and the distance between transmitter and receiver have influence on the delay spread and coherence bandwidth. finally the functional law between coherence bandwidth and delay spread was determined. the development of a low-cost, space saving device that could simultaneously take measurements from all (usually eight or up to twelve) outgoing three-phase feeders in distribution substation is the main focus of the contribution by nikolic et al (p. 53). aiming to save space without reducing the measurement accuracy, the authors designed a data acquisition system based on real-time multiprocessing with microcontroller and fpga circuit. a fast fpga circuit is used for voltage and current measurements including high-order harmonics. the proposed solution was validated by the results obtained in the developed laboratory. the properties of the spectrum measured by the time domain electromagnetic interference systems that are influenced by imperfections in the multi-resolution analogto-digital converter are analyzed by kamenský and kováč (p. 61). the authors propose a dedicated process of the identification of discrepancy parameters from experimental data, influencing the expressions of the model and enabling the comparison of experimental with theoretical results. the paper by kızılkaya et al (p. 68) is a study on analyzing and synthesizing new, multi-path, timeinterleaved, digital sigma-delta modulators, able to operate at any arbitrary frequency. dualand quadruple-path fourth-order butterworth, chebyshev, inverse chebyshev and elliptical based digital sigma-delta modulators are discussed, offering to designers the flexibility of specifying the centre-frequency, pass-band/stop-band attenuation as well as the signal bandwidth. detailed simulations acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 4  performed at the behavioral-level in matlab are compared with the experimental results of the fpga implementation of the designed modulators. the paper presents the mathematical modeling and evaluation of the tones caused by the finite word-lengths of these digital multi-path sigma-delta modulators, stimulated by sinusoidal signals. a new architecture of the multi-tap-delay-line timeinterval measurement module of high resolution implemented in single fpga device is proposed in the paper by zieliński et al (p. 77). the here designed measurement module enables to collect sixteen time-stamps during a single measuring cycle. the main improvement is the increased resolution, the reduction of the total duration time of the measurements and the mitigation of the duty cycle of the measurement instrument. a real time and simultaneously low-cost compensation method for the extension of the frequency bandwidth of medium voltage dividers whose performances would not have satisfied the requirements is developed by crotti et al (p. 82). this entails the spread of transducers with suitable frequency bandwidths, as required by the relevant standards. the next paper by giordano et al (p. 90) describes the arrangement of a first experimental set-up which allows the comparison between the measurement of the electromagnetic field quantities induced inside a simple cylindrical phantom and the same quantities estimated numerically through a boundary element method. even if a metallic non-magnetic element which roughly simulates a medical implant was introduced inside the phantom, the method assessed its robustness. a power quality measurement system, intended for measurements in substations at medium voltage level including voltage and current transducers and a selfdeveloped measuring instrument is designed by crotti and giordano (p. 97). while the measuring instrument is based on a reconfigurable i/o-fpga system with embedded software, the used voltage and current transducers are classical: rogowski coil and a voltage divider. an effective application of the measurement system in a private substation is analyzed, connecting an industrial load and two photo-voltaic generation plants to the public medium voltage network. a significant decrease of the current and voltage measurement uncertainty was obtained. the starting point of the paper by skórkowski et al (p. 105) is based on the diagrams of a virtual quasi-balanced instrument, continuing with the verification of a quasibalanced circuit, virtual realization, for capacitance measurements. the daq card ni-6009 and the calculation were controlled by an application developed in labview. bao et al proposed test methods for analog-toinformation converters (p. 111); the main objective was to verify if figures of merit and test methods for traditional analog-to-digital converters might be applied to aics based on the random demodulation architecture. starting from commercially available integrated circuits, an aic prototype was designed. a simulation analysis and an experimental investigation have been carried out to study the additional influencing factors such as the parameters of the reconstruction algorithm. it was demonstrated that standard figures of merit are in general capable of describing the performance of aics, provided they are slightly modified according to the proposals reported by the authors. the main topic of contactless measurements for industrial applications is developed by di leo et al (p. 121). a vision-based system expressly realized for the measurement of geometric and/or chromatic parameters of rubber profiles is designed, for application in the automotive industry. the stereo vision system for the online measurement of the dimensional characteristics of the profile transversal section is described in all details. an improvement of the non-destructive eddy current testing (ect) aiming to detect the presence of thin defects in conductive materials is presented in the last paper, by betta et al (p. 128). an experimental comparison of different excitation signal designed was developed to improve the quality of experimental data associated with small cracks and consequently, to obtain a more reliable determination of the geometrical features of the inspected defects. 4. conclusions  we were deeply honored to act as guest editors for this innovative and challenging issue of acta imeko and we have the satisfaction of finalizing a very useful outcome. the here presented release is the result of the tireless efforts of a large but homogenous team, integrating the authors, the reviewers and, last but not least, paul regtien, paolo carbone and pedro ramos, providing salutary support for all this period. hbm advertisement acta imeko june 2014, volume 3, number 2, 64 www.imeko.org section: advertisement acta imeko | www.imeko.org june 2014 | volume 3 | number 2 | 64 acta imeko, title acta imeko december 2014, volume 3, number 4, 2 – 3 www.imeko.org introductory notes for the acta imeko special issue on “technical diagnostics : new perspectives in measurements, tools and techniques for industrial applications” marcantonio catelani, lorenzo ciani department of information engineering, university of florence, via s. marta 3, 50139, florence (italy) section: editorial keywords: technical diagnostic; quality; industrial application citation: marcantonio catelani, lorenzo ciani, introductory notes for the acta imeko special issue on “technical diagnostics : new perspectives in measurements, tools and techniques for industrial applications”, acta imeko, vol. 3, no. 4, article 2, december 2014, identifier: imeko-acta-03 (2014)-04-02 editor: paolo carbone, university of perugia received november 21tst 2014; in final form november 21st, 2014; published december 2014 copyright: © 2014 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: lorenzo ciani, lorenzo.ciani@unifi.it the concept of quality has undergone important transformations over time. in a first assumption, sometimes passed in many industrial contexts, the term “product quality” was essentially associated with “compliance”. nowadays, the term “conformity to specifications” is widely used but this has to be considered only as an aspect in the measurement of the quality level. in fact, referring to the iso standard 9001 quality management systems, the term quality corresponds with the degree to which a set of characteristics of an object fulfils requirements. concerning characteristics, such standard proposes a classification in physical (e.g. mechanical, electrical, etc.), temporal (e.g. reliability, availability, etc.), and so on. it is interesting to observe that also the concept of product changed over time. now, it is usual to assume the product as the result of a manufacturing process in which a series of activities transforms raw materials, technologies and resources into the output – that is the product. so, it is mandatory to implement different activities concerning monitoring, as well as to collect and analyse appropriate data with the aim to demonstrate the suitability and the achievement of a desired quality levels. the analysis of data provides information about conformity to product requirements but also related to characteristics and trends of processes and products, including opportunities for preventive actions. this last aspect – the preventive action – depends also on the ability to monitor both the functionality of the product and the capability of the process to fulfil requirements. in this scenario imeko tc 10 aims to represent a reference point for scientists and researchers. it can be regarded as a forum for exchanging knowledge and sharing ideas concerning methods, principles, instruments and tools, standards and industrial applications on technical diagnostics as well as their diffusion across the scientific community. in this issue two main aspects are considered as a result of different research activities presented in the workshop. first, the impact of technical diagnostics on the product performances is considered. for this topic different points of view and proposals in the field of fault diagnosis are presented by the authors. b. aubert et al. deal in [1] with an extended kalman filter based fault detection in permanent magnet synchronous generators. being such equipment used often in high technology contexts, it is fundamental to guarantee high performances in terms of reliability and safety. authors propose a method for inter-turn shortcircuit detection based on the identification of the shortcircuited turns ratio in a faulty pmsg model expressed in park frame. several tests are also implemented in order to demonstrate robustness and sensitivity of the approach. in the field of testing for electronic devices an interesting contribution is proposed by d. załęski and r. zielonko in [2]. the paper concerns the testing of analog circuits and blocks in mixed-signal electronic embedded systems (eess) by means of the built-in self-test (bist) technique. in particular, two testing functions are performed: acta imeko | www.imeko.org december 2014 | volume 3 | number 4 | 2 functional testing and fault diagnosis on the level of localization of a faulty element. such proposal can be applied in a wide range of eess. in this first topic of the workshop also the work presented by m. lazzaroni et al. [3] can be located. authors proposed a thermal characterization of a power converter demonstrator used in particularly critical environments where the presence of radiation and magnetic fields are possible. in addition to the previous applications, it is important to underline how technical diagnostics can be considered also to check the operating conditions and the functionality of components (above mechanicals), machines and plants. this is the case of the research presented by g. dinardo in [4] about measurements of vibration affecting rotating components of machines. authors propose a novel configuration of a laser doppler vibrometer (ldv). the instrument was set up for the estimation of the out of plane vibrations of moving (rotating) objects, in order to give a better characterization of the self-tracking technique employed with the use of a 1d single point ldv. on the subject of measurements of vibration, but with different aims, is the paper of i. bodini et al. [5]. the research concerns a monitoring activity for on-board vibration in public transport. this can be considered an example of how technical diagnostics can be implemented as support for the evaluation of quality of service. a set of experimental tests has been performed by the authors to define a vibrational comfort index and to collect a large dataset that allows statistically significant comparisons between different infrastructures and their characterization. the proposed technique can also be useful for diagnostics purposes, such as vehicle comparison and road maintenance state monitoring. as said above, the vast area of technical diagnostics covers also different topics as, for instance, the manufacturing environment. m. scagliarini proposes in [6] a method for assessing multivariate measurement systems. this article describes an approach that, using the data routinely available from the regular activity of the instrument, offers the possibility of assessing multivariate measurement systems without the necessity of performing a multivariate gauge study. the practical application of this research is of interest considering that, often, critical decisions about process and product quality depend on the quality of the measurement systems. the paper [7] of m.n. durakbaşa et al. presents, instead, a particular study on process parameters like cutting speed, feed rate, depth of cut, tool geometry, and different coatings. the goal is to indicate the effects on surface roughness of the machined product on the basis of two and three-dimensional precise measurements. taguchi and regression methods are proposed as methodological approaches in order to predict average surface roughness values against edge radius wear. in conclusion, the papers presented in the 12th imeko tc10 workshop on new perspectives in measurements, tools and techniques for industrial applications, and briefly described here, confirm the vast research area and different interests of scientists in technical diagnostic. references [1] brice aubert et al., stator winding fault diagnosis in permanent magnet synchronous generators based on shortcircuited turns identification using extended kalman filter, acta imeko, vol. 3, no. 4, article 3, december 2014, identifier: imeko-acta-03 (2014)-04-03 [2] dariusz załęski, romuald zielonko, two-functional µbist for testing and self-diagnosis of analog circuits in electronic embedded systems, acta imeko, vol. 3, no. 4, article 4, december 2014, identifier: imeko-acta-03 (2014)-04-04. [3] massimo lazzaroni et al., thermal modeling and characterization for designing reliable power converters for lhc power supplies,acta imeko, vol. 3, no. 4, article 5, december 2014, identifier: imeko-acta-03 (2014)-04-05. [4] giuseppe dinardo et al., how geometric misalignments can affect the accuracy of measurements by a novel configuration of self-tracking ldv, acta imeko, vol. 3, no. 4, article 6, december 2014, identifier: imeko-acta-03 (2014)-04-06. [5] ileana bodini et al., techniques for on-board vibrational passenger comfort monitoring in public transport, acta imeko, vol. 3, no. 4, article 7, december 2014, identifier: imeko-acta-03 (2014)-04-07. [6] michele scagliarini, a method for assessing multivariate measurement systems, acta imeko, vol. 3, no. 4, article 8, december 2014, identifier: imeko-acta-03 (2014)-04-08. [7] m. numan durakbaşa et al., surface roughness modeling with edge radius and end milling parameters on al 7075 alloy using taguchi and regression methods,acta imeko, vol. 3, no. 4, article 9, december 2014, identifier: imekoacta-03 (2014)-04-09. acta imeko | www.imeko.org december 2014 | volume 3 | number 4 | 3 introductory notes for the acta imeko special issue on “technical diagnostics : new perspectives in measurements, tools and techniques for industrial applications” acta imeko  issn: 2221‐870x  june 2015, volume 4, number 2, 4‐9    acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 4  single image geometry inspection using inverse endoscopic  fringe projection  steffen matthias, christoph ohrt, andreas pösch, markus kästner, eduard reithmeier   institute of measurement and automatic control, leibniz universität hannover, germany      section: research paper  keywords: inverse fringe projection; fiberscopy; endoscopy; sheet bulk metal forming  citation: steffen matthias, christoph ohrt, andreas pösch, markus kästner, eduard reithmeier, single image geometry inspection using inverse endoscopic  fringe projection, acta imeko, vol. 4, no2, article 2, june 2015, identifier: imeko‐acta‐04 (2015)‐02‐02  editor: paolo carbone, university of perugia, italy  received april 17, 2014; in final form february 19, 2015; published june  2015  copyright: © 2015 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: this work was funded by the german research foundation (dfg) within the collaborative research center (crc) / tr 73  corresponding author: steffen matthias, e‐mail: steffen.matthias@imr.uni‐hannover.de    1. introduction  the development of a new metal forming process demands several kinds of new technological approaches. starting with new geometrical tool designs, fitted materials have to be found and analyzed. additionally, process parameters such as forming speeds and forces have to be optimized in respect to a maximum output, while maintaining low tool abrasion and long durability. concerning the new method of cold sheet bulk metal forming (sbmf) [3], which is currently being developed in an exemplary manufacturing process, assembly ready geometries shall be generated by a combined process of sheet and bulk forming. this results in both drawing and comprehensive forces on the tool in the same process step. the combination leads to early wearing effects on the sbmf-tool that may cause deficient workpieces. these need to be identified early in the manufacturing process in order to keep rejection costs at a minimum. proper simulations for quality assessments can only be based on measurement data gathered from the experimental process. to generate datasets of the process, measurements obtained in a complete production cycle are necessary. these requirements can only be met with fast areal measurement devices. desired cycle times are in the range of one second and less, while the standard deviation of the measured geometry data should be 100 µm or less compared to the reference. experiences show that fringe projection fulfills these abstract  fringe projection is an important technology for the measurement of free form elements in several application fields. it can be applied  to measure geometry elements smaller than one millimeter. in combination with deviation analysis algorithms, errors in fabrication  lines  can  be  found  promptly  to  minimize  rejections.  however,  some  fields  cannot  be  covered  by  the  classical  fringe  projection  approach. due to shadowing, filigree form elements on narrow or internal carrier geometries cannot be captured. to overcome this  limitation, a fiberscopic micro fringe projection sensor was developed [1]. the new device is capable of resolutions of less than 15 µm  with uncertainties of about 35 µm in a workspace of 3x3x3 mm³.  using standard phase measurement techniques, such as gray‐code and cos²‐patterns, measurement times of over a second are too  long for in‐situ operation. the following work will introduce an approach of applying a new single image measuring method to the  fiberscopic system, based on  inverse fringe projection [2]. the fiberscopic fringe projection system employs a  laser  light source  in  combination with a digital micro‐mirror device (dmd) to generate fringe patterns. fiber optical image bundles (foib) are used as well  as gradient‐index lenses to project these patterns on the specimen. this advanced optical system creates high demands on the pattern  generation algorithms to generate exact inverse patterns for arbitrary cad‐modelled geometries. approaches of optical simulations of  the  complex  beam  path and  the  drawbacks  of the  limited  resolutions  of  the  foibs  are  discussed.  early  results  of  inverse  pattern  simulations using a ray tracing approach of a pinhole system model are presented. acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 5  requirements very well [4]. however, inner geometries such as sbmf-tools are not measureable at optimum angle, which reduces the lateral resolution of the measurement with the cosine of the camera angle towards the surface normal. common fringe projection sensors usually combine fringe generator and camera unit in a fixed housing with a predefined triangulation angle [5]. to overcome these limitations, a fiberscopic fringe projection system was developed [6], which can be positioned close to specifically stressed areas of a sbmftool. these, for example, filigree side form elements are exposed to drawing and bulk-forming forces at the same time. in these areas early abrasion is most likely and therefore needs to be taken care of at the very beginning of the development process. to reduce the time duration of measurements, the inverse fringe projection technique allows the detection of geometry deviations using a single projection of an object-specific inverse fringe pattern. an adapted projector pattern is derived from the specimen’s geometry and the desired camera pattern. deviations in geometry lead to differences in the camera image which can be easily detected. different approaches exist to calculate the inverse pattern. li et al. [7] present an algorithm to calculate inverse patterns by processing measurements of a reference object. following measurements will be able to detect deviations from the initially measured reference geometry. a different approach calculates the inverse pattern by the use of a cad model of the geometry and a mathematical model of the fringe projection system [8]. the latter solution has the advantage of using a virtual reference defined by the tool designer instead of a manufactured reference. in the following sections the principle of the fiberscopic system will be presented together with the adaptation of a model based inverse fringe projection approach. following an introduction of the optical setup, difficulties that arise from the image fibers and special lenses are discussed. in order to verify the pattern generation, parts of the system are simulated using ray-tracing software. the methods to reduce the measurement time to a minimum in order to keep it in the range of the cycle time of the process will be explained. 2. fiberscopic fringe projection  2.1. principle setup  figure 1 shows the basic design of the newly developed setup. to achieve a high depth of field (dof) with foibs, a good fiber coupling at a sufficient intensity is required. by their nature, laser light sources offer high intensities as well as a collimated beam profile, which makes them almost ideal for fiber coupling. only the coherent nature of lasers complicates the use in fringe projection since reflections on a diffuse reflecting specimen, as it is common in structured light measurements, create speckle contrast automatically. sometimes the speckle contrast can be higher than the contrast of the projected fringe pattern and thereby prevents an accurate evaluation [6]. to overcome this, a rotating diffuser is installed in the system together with micro lens arrays that act as beam shapers. since the diffuser changes the speckle formation faster than the frame rate of the ccd, it leads to an integration of randomly distributed patterns which results in a smooth light intensity image [6]. the beam shaper changes the gaussian into a flat top distribution that is fitted to the size of the dmd, which is used for the pattern generation. the pattern is focused to the 1.7 mm input aperture of the foib and guided 1:1 to the measurement area. the image on the specimen is projected through gradient index (grin) rod lenses with high numeric apertures, which are directly attached to the foib. the distorted image of the fringe pattern is captured by a similar grin-foib arrangement in a defined triangulation angle and guided to a 5 megapixel ccd camera for computing. 2.2. data processing  in structured light projection sequences of binary gray-code followed by cos²-phase shift patterns are common in commercial fringe projection systems [4][5]. the method achieves high accuracies and is robust to background light and therefore can be used in a wide spectrum of measurement tasks. however, the projection and acquisition of the sequences take the major time of the whole measurement. additionally, due to the significant loss of resolution in the 100,000 fiber image bundle, binary patterns start to fade on the cost of contrast in fiberscopic fringe projection. additionally, for a robust phase measurement the gray-code sequence needs to have twice the frequency of the continuous cos²-pattern to allow the compensation of errors in the phase-unwrapping process. it has to be noted, that only the information from the cos²-pattern is used in the later process, while the gray-code patterns are only used for phase unwrapping of the continuous pattern. however, due to the limited resolution of the fiber bundles, the maximum pattern frequency is very limited. thus, for the endoscopic system, unwrapping methods using lower frequency cos²-patterns are superior as the highest frequency of the cos²patterns can be higher than by using gray-code assisted unwrapping. also, using the unwrapping technique described by tao peng [9], the number of patterns to project may be reduced, leading to a faster measurement process. to obtain 3d-measurement data from the 2d-phase maps, the system is calibrated using a pinhole camera model and a black box model for the projector [10]. 2.3. resulting data  the generated data of the specimen’s geometry is saved in a point cloud in cartesian coordinates. sets of data in different geometries like plane, spheres and gearings were recorded and compared to data of other measurement systems, such as commercial fringe projection sensors and coordinate measurement machines (cmms), as well as to the original cad design data. for the task the commercial and widely accepted software polyworks (innovmetric software inc) was used. it figure 1. schematic of the fiberscopic fringe projection setup.   acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 6  showed that the presented system achieves accuracies of ± 10 µm for a 3d-measurement area of 3x3x3 mm³ at standard deviations of 35 µm. this made it capable for the indicated task in sbmf. however, even with reduced pattern resolution and the hereby reduced number of images, measuring speeds are not sufficient for measurements in cycle time of the sbmf process for holistic measurements of each part. 3. inverse fiberscopic fringe projection  3.1. inverse fringe projection  inverse fringe projection enables the detection of 3dgeometry deviations utilizing only a single adapted pattern instead of a sequence of patterns [11]. this pattern is adapted to both the geometry of the specimen and the optical characteristics and positioning of the fringe projection system, as seen in figure 2. for the inverse approach, the intended geometry needs to be known as a cad model. the measurement setup, consisting of a camera, a projector and an ideal specimen, needs to be modelled in a virtual environment. this virtual setup must approximate the optical properties of the real setup with sufficient accuracy; therefore, a calibration procedure of the real setup must be undertaken to identify the model parameters of the system. a common approach is to approximate both the camera and the projector using the pinhole model. the pinhole model describes how a point of a 3d object in the camera reference frame is projected on the 2d camera image [12]. the pinhole model consists of a camera matrix k, seen in equation (1), which contains the so-called intrinsic parameters. 0 0 0 0 1 (1) the parameters and describe both pixel size and focal length, while and model the principal point. additional intrinsic parameters may be used to approximate lens distortion effects [13]. extrinsic camera parameters describe the transformation from the world coordinate system, which can for example be the coordinate system of a cad-file, to the camera coordinate system. three parameters describe the rotation, while three additional parameters model the necessary translation. the image of an object point x in the world coordinate system can be expressed in homogenous coordinates by equation (2), where is the camera matrix and the transformation matrix. (2) a complete model includes the intrinsic parameters of both camera and projector as well as a transformation matrix describing the relation of camera and projector and the transformation to the world coordinate system. the inverse pattern can be simulated by ray-tracing using the virtual system. in the first step of the simulation, the inverse projection pattern is calculated by inversion of the path of light propagation. therefore, the camera is modeled as a projector and “emits” a straight, equidistant, structured light pattern onto the specimen (cad model). the projector works in this context as a camera and is used to “capture” the diffuse reflection into a raster image with the dmd-pixels remodeled as sensor pixels. of course, this is only possible utilizing the virtual ray-tracing-based system. when this inverse projection pattern, obtained by simulating the virtual system, is then projected by the real projector onto a real specimen (of the same shape and pose as the virtual specimen), the real camera will capture the beforehand-defined straight, equidistant light intensity pattern. geometry deviations of the real specimen, however, will lead to distortions in the camera image which can be detected robustly using fast 2d image processing techniques. the amount of deviation of the 2d fringe pattern can be related to three dimensional geometry deviations by a linearized defect model called sensitivity map. thus, quantitative information about the geometry defect is obtained. no reconstruction of point cloud data or processing of three dimensional data is required after acquisition of the measurement data resulting in a low latency time from measurement to result. the method is applicable to check for allowable geometry tolerances up to a µm scale. poesch [11] showed the proof of principle on a macroscopic fringe projector. however, the optical path of a fiberscopic fringe projector as described above is far more complex to model for inverse simulation. new artifacts, such as pixelation effects due to the fiber bundle, have to be taken care of. challenges arise from the simulation of the foib and the in and out-coupling, but also from the beam shaping and speckle removal stage. artifacts that arise from the optical elements which are unusual for classic fringe projection systems may complicate the scenario. 3.2. simulation of a fiberscopic fringe projector with ray‐tracing  the simulation of a fiberscopic fringe projection system requires several uncommon components in the beam path, which not only increases the complexity of the projector design, but also extends the average calculation time of one complete simulation run significantly. with the applied simulation software fred (photon engineering llc) and a state of the art multicore computer one simulation cycle with a sufficient number of rays requires a processing time of about one hour. since the optimization includes several kinds of optics and their respective positions numerous runs are inevitable. the setup was created component by component and connected afterwards in a combined simulation environment. in a first step, the telescope and rotating diffuser were optimized for minimal divergence of the beam and a highly randomized speckle distribution at each diffuser rotation. these elements mainly affect the available space in the fringe projection sensor. for the telescope the best simulation results were found with two 60 mm lenses which focus a laser with 2 mm diameter on the rotating diffuser plate. the rays detected behind the last lens are recorded and used as light source for the next simulation step. the flat top generator consists of the fly-eye micro lens arrays and a fourier-lens. figure 2. inverse fringe projection applied to a specimen.  acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 7  the pattern that is generated by the fly-eye optics is shown on “analysis surface 1” in figure 3. the displayed section represents that the fly-eyes successfully break the wave front of the gaussian distribution of the laser and generate a flat top light distribution. the several maxima flatten with increasing distance to the fourier-lens and equalize to a homogeneous level at the stage of the dmd. the optimization criteria were a highly consistent light pattern that exactly fits to the size of the 0.7” dmd. for the dmd in the first place a fixed micro mirror array was generated that projects half of the rays to a dump and the other half to the projecting foib. the pattern is shown on “analysis surface 2” in figure 3. again, the pattern is saved and used as the source for the next, most computation time intensive part of the simulation. the fiber coupling part consists of an in-coupling objective, which focusses the fringe image to a 1.7 mm diameter at the entrance of the foib. in a first attempt, the fiber bundles were simulated by modelling 100,000 individual fibers of 1 m length. the calculation of several hundred reflections in each fiber caused several days of simulation time. for comparison a 10 mm long version was designed. the simulated images of both fiber lengths were very similar, so that for simplification the 10 mm version was chosen for the following simulations (fiber coupling in figure 3). the fringe pattern is projected using an adapted grin-lens that is fitted to the diameter of the foib. under the triangulation angle a second grin-lens acquires the image in the second foib to a second coupling optic, which projects the resulting fringe pattern to the last analysis surface that represents the ccd of the actual setup. with the finished model, the behavior of a projected pattern in the foib can be predicted. especially the decrease of the 1 mp image to 100,000 pixels is very uncommon for fringe projection systems as well as the availability of the comparatively low resolutions. using the image of the last analysis surface in the way described in section 3.1 enables first indications of possible qualities of an inverse fiberscopic fringe projection approach. unfortunately, the number of parameters of this detailed model is too high to allow robust model identification. thus, a simpler approach, similar to the pinhole model, needs to be found in order to calibrate the virtual system to the actual endoscopic fringe projection system. 3.3. practical approach  for the continuous quality control of the abrasion within a sbmf-process it is necessary to compare the measured data to the reference. figure 4 shows the measurement of a sbmfdeep drawing tool with the fiberscopic system. in the first step a reference measurement is obtained by projecting the common gray-code and phase shift sequence which requires at least 12 images. the point cloud derived from the phase-map using the calibration data is calculated in the calibrated coordinate system for the system. to simulate the inverse pattern, it is crucial to know the pose and position of the tool in this system specific coordinate system. both can be obtained by calculating a rigid body transform by aligning measured reference points to the corresponding points in the cad-model using polyworks. from the resulting transformation matrix and the system model an inverse pattern can be generated by the help of ray-tracing. after this initial pattern generation step, inspection can be performed by capturing just one image of the inverse pattern. distortions in the camera image can only be seen at sections where abrasion (either welding on, or wear off effects) has taken place. the measuring procedure only takes several figure 3. complete ray tracing design simulation of the fiberscopic fringe projection system.  acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 8  hundred microseconds, which is below the beforehand made requirement. from the data an easy “out-of-tolerance” study can be derived, providing information about necessary tool exchanges or process stability. desired accuracies in the field of sbmf are in the range of 50 to 100 µm in order to be able to judge wear effects in terms of production quality. to verify the feasibility of this procedure, the system has been calibrated using the common pinhole model for both camera and projector. it was necessary to replace the projector micro lens with a standard grin lens as the adapted lens can only be described with this model with low accuracy. unfortunately, this decreases the depth of field of the endoscopic fringe projection system. as a first proof of principle and verification of the calibrated model parameters, an inverse pattern was simulated for a step calibration standard. the calibration standard is a diffuse plane with a 1 mm deep and 2 mm wide cutout. the standard was positioned arbitrarily in the fringe projection system’s coordinate system. pose and position were estimated by capturing a point cloud with the standard fringe projection technique and fitting it to the cadmodel using polyworks. the simulated inverse projector pattern can be seen in the top left image in figure 5. simulation of the pattern has been performed in a non-optimized matlab ray-tracing environment and required a processing time of about 2 minutes with a model consisting of about 30 triangles. as this simulation needs to be performed only initially, the processing does not limit the measurement time of the system. more advanced software is capable of calculating ray-tracing with complex models in less than one minute. the cutout can be seen in the center of the inverse pattern, while the slight deformation of the pattern towards the outer areas of the geometry is a result of the calibrated camera and projector distortion. in order to assess the quality of this approach, the inverse pattern was projected back onto the calibration standard using the endoscopic system. the grayscale image on the right in figure 5 shows the obtained camera image. it can be seen that the fringe patterns are parallel throughout the camera image, as it has been defined in the inverse pattern generation step. slight deviations are visible near the step in the standards geometry, resulting from slight positioning errors. the used step standard makes these misalignments visible due to its non-continuous geometry. with continuous geometries, such as the gearings on the bulk-sheet metal forming tools, the inverse pattern is less sensitive to misalignments. to enable an automated inspection of the image, a phase map was calculated by processing the fringe pattern in the marked rectangular part of the camera image. as expected, the phase-map shows only small deviations along the vertical axis. the cutout leads to a narrow shadowed area in the camera image, at which the phase unwrapping fails. this leads to the artifact seen in the phase image. 4. conclusions and future activity  the work showed that the principle of fiberscopic fringe projection works with common pattern generation approaches such as gray-code and phase shift or encoded phase shift. with highly collimated light sources and beam shaping a sufficient depth of focus can be achieved even with micro grin optics attached to the foibs. the working system has been mapped into a computer model for the simulation software fred. this model is the basis for the evaluation of virtual model based inverse fringe projection. we showed that the generation of inverse patterns works for the fiberscopic system. for practical evaluations, the simulation model has been simplified by using the pinhole model for projector and camera. arbitrary cad geometry can be used for pattern generation after pose and position of the sensor have been calibrated. the next steps of the work on the fiberscopic fringe projection sensor will be the installation in a sbmf-machine with a high precision positioning system to examine the results of the inverse fiberscopic fringe projection with more complex geometries. possible enhancements of the technique include improvements of the 2d-image processing as well as an improved lighting model for the ray-tracing simulation. apart from lens distortion modelling for the projector grin lens, the figure 4. measurement of a sbmf deep drawing tool (top) with following analyses of the point cloud in respect of abrasion effects (bottom).  figure 5. inverse measurement of a calibration standard.  acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 9  largest limitation of the demonstrated approach is currently its sensitivity to positioning errors and vibrations, as the position of the sensor head to the desired measurement area is defined during the simulation of the inverse patterns. acknowledgement  the authors would like to thank the german research foundation (dfg) for funding the project b6 "endoscopic geometry inspection" within the collaborative research center (crc) / tr 73. references  [1] c. ohrt, s. matthias, m. kästner, e. reithmeier, “fast endoscopic geometry measurement with fiber-based fringe projection for inner geometries”, journal of the cmsc, vol. 7, no. 2, 2012, pp. 10-14. [2] a. pösch, t. vynnyk, m. kästner, o. abo-namous, e. reithmeier, “virtual inverse fringe projection”, cmsc conference, reno, 2010. [3] m. merklein, j.m. allwood, b. behrens, a. brosius, h. hagenah, k. kuzman, k. mori, e. tekkaya, a. weckenmann, “bulk forming of sheet metal”, annals of the cirp, vol. 61, no. 2, 2012, pp. 725-745. [4] m. kästner, “optische geometrieprüfung präzisionsgeschmiedeter hochleistungsbauteile”, shaker, aachen, 2010, isbn 3832294430. [5] g. frankowski, m. chen, t. huth, “real-time 3d shape measurement with digital stripe projection by texas instruments micromirror devices (dmd)”, proceedings of the conference on three-dimensional image capture and applications iii, san jose, ca, usa, 2000. [6] c. ohrt, m. kästner, e. reithmeier, “endoscopic geometry inspection by modular fiber optic sensors with increased depth of focus”, proc. spie 8082, optical measurement systems for industrial inspection vii, 2011. [7] w. li, t. bothe, w. osten, m. kalms, “object adapted pattern projection—part i: generation of inverse patterns”, optics and lasers in engineering, 41(1), 2004, pp. 31-50. [8] a. pösch, t. vynnyk, e. reithmeier, “using inverse fringe projection to speed up the detection of local and global geometry defects on free-form surfaces”, spie optical engineering+ applications, international society for optics and photonics, 2012, pp. 85000b-85000b. [9] t. peng, “algorithms and models for 3-d shape measurement using digital fringe projections”, university of maryland, phdthesis, 2007. [10] j. vargas, m.j. terrón-lópez, j.a. quiroga, “flexible calibration procedure for fringe projection profilometry”, optical engineering, vol. 46, no. 2, 2007, p. 023601. [11] a. pösch, t. vynnyk, e. reithmeier, “fast detection of geometry defects on free-form surfaces using inverse fringe projection”, journal of the cmsc, vol.8, no. 1, 2013. [12] r. hartley, a. zisserman, “multiple view geometry in computer vision”, cambridge university press, 2004, isbn 0521540518. [13] c. brown, “close-range camera calibration”, photogrammetric engineering, vol. 37, no. 8, 1971, pp. 855-866. microsoft word 16-139-1-pb.doc acta imeko  july 2012, volume 1, number 1, 19‐25  www.imeko.org    acta imeko | www.imeko.org  july 2012 | volume 1 | number 1 | 19  gene expression programming and genetic algorithms in  impedance circuit identification  fernando m. janeiro 1 , pedro m. ramos 2   1  instituto de telecomunicações and universidade de évora, largo dos colegiais 2, 7004‐516 évora, portugal  2  instituto de telecomunicações and instituto superior técnico/universidade técnica de lisboa, av. rovisco pais 1, 1049‐001 lisbon,portugal      keywords: impedance spectroscopy; circuit topology identification; genetic algorithms; gene expression programming  citation: fernando m. janeiro, pedro m. ramos, gene expression programming and genetic algorithms in impedance circuit identification, acta imeko,  vol. 1, no. 1, article 7, july 2012, identifier: imeko‐acta‐01(2012)‐01‐07  editor: paul regtien, measurement science consultancy, the netherlands  received january 3 rd , 2012; in final form may 21 st , 2012; published july 2012  copyright: © 2012 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: fct project pest‐oe/eei/la0008/2011  corresponding author: fernando m. janeiro, e‐mail: fmtj@uevora.pt    1. introduction  impedance spectroscopy [1] is used in different fields ranging from biomedical applications [2] to electrochemical applications such as the study of fuel cells [3]. other applications include monitoring of anti-corrosion coatings [4] and sensor modelling, for example, of a humidity sensor [5]. the first step in performing impedance spectroscopy consists on obtaining the impedance spectral response [6]. this can be accomplished for example using an impedance vector-analyser or through a two-channel data acquisition system where a sinefitting algorithm [7] is used to extract the impedance magnitude and phase [8]. an improved version of the basic sine-fitting algorithm, called 7-parameter sine-fitting [9], that is well suited as a digital signal processing algorithm to estimate the impedance parameters has been proposed. usually, to fit the acquired spectral data, some knowledge of the underlying processes is needed to suitably choose a circuit topology that models the process under study. with a known circuit topology, the complex non-linear least squares (cnls) method can be used to obtain the circuit parameters of the chosen model [10]. the cnls implemented in [11] is based on the levenberg-marquardt algorithm and requires the input of the starting search values and then, it will efficiently converge to the local minima near this set of initial parameters. to avoid the convergence to the local minima, a new approach [12] based on a hybrid genetic algorithm (ga) [13] was proposed. this approach has been used to characterize a viscosity measurement system [14]. gene expression programming (gep) [15] was proposed as a method to search for suitable circuit topologies without any prior knowledge of the equivalent circuit [16]. in that work, a basic genetic algorithm was used to estimate the values of the circuit components. a different approach, based on gep and cultural algorithms, was used for the modelling of electrochemical phenomena in [17]. in this work, which is an extended version of [18], gep is interleaved with an improved version of the genetic algorithm to identify the circuit topology and circuit component values. a more effective fitness function, which works well for impedance responses that include resonances, is also used in this paper. in order to make the numerical simulations more realistic, measurement uncertainty is included for the magnitude and phase impedance data. further validation of the algorithm is performed by its application to impedance measurements performed on a circuit that models the humidity sensor presented in [19] and on the viscosity sensor used in [14]. impedance circuit identification through spectroscopy is often used to characterize sensors. when the circuit topology is known, it has  been shown that the component values can be obtained by genetic algorithms. also, gene expression programming can be used to  search for an adequate circuit topology.  in this paper, an  improved version of the  impedance circuit  identification based on gene  expression programming and hybrid genetic algorithm is presented to both identify the circuit and estimate its parameters. simulation  results are used to validate the proposed algorithm in different situations. further validation is presented from measurements on a  circuit that models a humidity sensor and also from measurements on a viscosity sensor. acta imeko | www.imeko.org  july 2012 | volume 1 | number 1 | 20  2. evolutionary algorithms  equivalent circuit identification from impedance spectroscopy involves identifying the circuit topology and then optimizing the values of each component in that circuit. this is accomplished in two interleaved steps; (i) a gep implementation is used to identify potential circuit network topologies that can model the measured impedance; (ii) a hybrid genetic algorithm is applied to each topology to obtain the values of the components that minimize the cost function. in the next subsections these two algorithms are described. 2.1. equivalent impedance parameters estimation  for a given circuit topology, the optimization of the component values is performed using a hybrid genetic algorithm [12]. initially, a population of m chromosomes is created where each chromosome is composed by the values of each component in the current circuit topology. the fitness of each chromosome is usually evaluated through the cost function       2 2 1 1 p i est i i i z z p z        (1) where  iz  is the measured impedance at angular frequencies 2i if  ,  est iz  is the estimated impedance obtained with the component values in the chromosome and p is the number of measurement frequencies. however, in circuits that exhibit resonance-like behaviour, the frequency points in the resonance region could have little weight in the final value of the cost function. this led to having good candidate circuits being discarded by the genetic algorithm. to tackle this issue, a different cost function that does not suffer from these problems was used       2 2 1 1 p i est i i est i z z p z        . (2) with this cost function, the absolute error of the impedance estimation is normalized by the estimated impedance, therefore giving more weight to frequency points in the resonance region. if information regarding the quality of each measurement value is available (e.g., experimental standard deviation), weights can be used in (2) to make sure that the fitting algorithm is more influenced by the measurement values with reduced uncertainty. the fitness of each chromosome is used to evolve the population based on survival of the fittest. just like in a biological population, there is reproduction and mutation. in reproduction, pairs of chromosomes are randomly chosen using a biased roulette wheel selection scheme where the fittest elements have a higher probability of being chosen to reproduce. each pair of chromosomes may create two offspring through the crossover operation or move directly to the next generation. in mutation, randomly chosen positions on some chromosomes are replaced by random generated values. mutation is vital to maintain population diversity and escape local minima of the cost function. traditional minimum search algorithms, such as the levenberg-marquardt algorithm and the gauss-newton method, are very sensitive to the starting search values and are not able to escape local minimums of the cost function. however, genetic algorithms are very efficient in finding the region of the absolute minimum of multi-dimensional cost functions even when the search space is vast and local minima are present nearby [20]. although genetic algorithms are suitable to find the global minimum of the cost function, they take a long time to converge to the actual minimum. therefore, a gauss-newton method is applied using the final results of the genetic algorithm as starting values for the gauss-newton method. 2.2. impedance network topology identification  the identification of the equivalent impedance network topology is performed using gep. each candidate circuit is expressed as a gene which contains components (r, l and c) and operators (series and parallel). the gene is composed of a head of size h and a tail of size t, with 1t h  . the head may contain components and operators while the tail is comprised only of components. this is fundamental to the operation of gep since it guarantees a valid circuit topology under all circumstances. each gene is converted into a binary tree by filling it in a breadth-first fashion as proposed in [15]. this tree can then be recursively run to obtain the equivalent circuit. breadth-first traversal of the tree, also known as width first, consists on filling the tree from left to right and top to bottom. when there are no more node positions to fill in the current level of the tree, the tree filling starts from left to right in the next lower level. in this implementation of gep, the binary tree is completely bypassed and is only used to help visualize the circuit topology. as an example, figure 1 shows an electric circuit that will be used as test impedance in the numerical results. the gep gene that codes this circuit is presented along with the corresponding binary tree. the numbers in the tree leafs correspond to the type of component r, l or c for 1, 2 or 3, respectively. notice that, in the 21 long gene, only the first 9 elements (shown in bold) correspond to the described circuit. the remaining elements are also part of the gene but do not define the circuit. also noticeable is that the last 11 elements (the tail) are all components. initially, a population of n circuit topologies is randomly created according to the previously described rules. the hybrid genetic algorithm is then executed for each candidate topology gene to optimize its component values and obtain its fitness. afterwards, this population is used to create a new generation of circuit topologies trough the gep operations: replication, mutation, transposition and recombination [15]. the new gene + 1 + 2 // // 1 2 3 + 1 1 2 1 3 1 2 1 2 2 3 figure 1. example of circuit topology and corresponding binary tree along  with coding gene for gep with h + t = 21. the coding region is shown in bold  and ends at position 9.  acta imeko | www.imeko.org  july 2012 | volume 1 | number 1 | 21  generation is again evaluated by the hybrid genetic algorithm and the process repeats itself until a valid circuit is found (i.e., until its fitness is better than a predefined threshold) or the maximum number of generations is reached, in which case the algorithm failed to find a suitable circuit. in the creation of a new generation, gep operators are applied sequentially. the replication operator creates an intermediate population by choosing, based on a biased roulette wheel selection scheme, which genes survive from the original population. the fittest elements have a higher probability of being chosen and so multiple copies of them may appear in this intermediate population. genes with low fitness may not be chosen at all. mutation is applied to the intermediate population by changing the value of a random position inside some randomly chosen genes. if the position is inside the head it can be mutated into a circuit component or an operation, while if it is in the tail it is restricted to mutate only into a circuit component. this is mandatory to maintain the structural integrity of the genes. insert sequence transposition (is transposition) is then applied to some randomly chosen genes of the population that resulted from the mutation. in each chosen gene, a sequence of random length is transposed to any position in the head except to the root. an example is shown in figure 2a where, for the sake of simplicity, a smaller gene with h + t = 11 is used. the insertion sequence has length 2 and starts on position 5. the sequence was inserted at the randomly chosen position 2 and the head elements below the insertion point are moved downstream inside the head. however, to maintain gene integrity, the tail remains untouched and the last two positions of the original head are discarded. this operator creates a new intermediate population to which root insertion sequence transposition is applied. root insertion sequence transposition (ris transposition) is very similar to the previous operator, except that the insertion point is always the root. the insertion sequence is also of random length but must start with an operation. figure 2b illustrates this behaviour, where the insertion sequence has length 3 and starts in position 5. it is inserted at the root and the remaining head elements are moved downstream, with the last 3 elements being discarded in order to maintain the original tail. the next operation is the 1 point recombination. two genes and a crossover point are randomly chosen. then, the two genes exchange part of their information as shown in figure 2c. in the example, the crossover point is 3 and therefore positions 1 to 3 are exchanged between the two genes. the last operation is the 2 point recombination, where 2 crossover points are chosen. in the example presented in figure 2d, positions 3 to 7 are exchanged between the two genes. in recombination, the integrity of the gene structure is always assured. at the end of this sequence of operations, a new generation of candidate circuits has been created. also, due to the use of elitism, a copy of the best circuit topology is always included in the new population. these operations can profoundly change the population genes and, although the genes are of fixed length, their circuit coding region varies, resulting in trees of different sizes and complexity. this means that gep can add branches to the circuit or eliminate them, while searching for the circuit that best fits the measured data. before applying the ga, a basic circuit simplify algorithm searches the tree for operations that have, on the two leafs, identical type components. these correspond for example to two identical type components in series or in parallel and can be replaced by a single component. 3. numerical results  in this section, the circuit in figure 1 is used to test the proposed algorithm and the threshold to stop the algorithm is set at 0.0002%  . the spectral response of this impedance with rs = 10 , r = 1000 , ls = l = 1 mh and c = 1 f was calculated for p = 100 linearly spaced frequency points in the 100 hz to 10 khz range and random errors were included in the data to simulate real measurement conditions, with an uncertainty of 0.08% in the impedance magnitude and 0.05º in its phase. the encoding gene chosen by the algorithm is presented in figure 3, along with its corresponding binary tree. the resulting equivalent circuit is presented in figure 4 with the estimated component values. the corresponding frequency responses are shown in figure 5. although the equivalent circuit does not have the same topology as the original impedance circuit, the effect of the figure 2. examples of the main gep operations on genes with h + t = 11.  gene + // // + 3 + // 1 2 2 1 3 2 2 2 1 3 2 2 2 2 figure 3. encoding gene (in bold) and corresponding binary tree obtained by  gep  and  hybrid  genetic  algorithm  for  the  impedance  in  figure  1  with  rs = 10 , r = 1000 , ls = l = 1 mh and c = 1 f.  acta imeko | www.imeko.org  july 2012 | volume 1 | number 1 | 22  capacitor in the first parallel circuit (the 4.934 pf capacitor) and of the inductor in series with the resistance in the second part of the circuit (the 900.3 h inductor) are negligible in the frequency range under study, as can be seen in figure 5 where the spectral response of the two circuits is shown. different runs of the algorithm yield different equivalent circuits, but always in good agreement with the spectral response of the original circuit. it should be noted that, in many applications, an equivalent circuit with physical meaning is needed, which is a requirement that is currently not satisfied by the gep and ga algorithm. in figure 6, the errors of the fit for this situation are presented. in figure 6a, the difference between the magnitudes of the simulated and estimated impedance are shown, while in figure 6b, the difference of the simulated and estimated phase are depicted. note that, the cost function must combine the magnitude and phase information to estimate a single value that quantifies the fit of each circuit and corresponding estimated circuit parameters. so, in figure 6c the magnitude of the difference between the two impedances (corresponding to the distance between the two complex numbers that represent the estimated and simulated impedances) is shown. in order to ensure that higher impedance magnitudes that occur at certain frequencies don’t overly influence the cost function solely due to their higher magnitude, the magnitude of the difference between the estimated and simulated impedance are normalized by the estimated impedance magnitude. this corresponds to each frequency point in cost function (2) and is shown, for this case, in figure 6d. the average of the squared values represented in figure 6d corresponds to the fitting error. in this situation 61.2 10 0.00012%    . since it is not realistic to measure impedance values for 100 different frequencies, the usefulness of the proposed algorithm depends on its performance with fewer measurements. in figures 7, 8 and 9 the results obtained with only 10 frequency points are presented. the circuit topology is quite similar to the original circuit with just the addition of a capacitor in parallel with the correct circuit. as can be seen in figure 8, the magnitude and phase responses of the estimated circuit closely match the original circuit frequency responses. therefore, the extra capacitor relative to the original circuit (the 14.26 pf capacitor), has a negligible effect on the overall impedance at the frequency range under study. note that, even without any measured point in the resonance region, the algorithm still finds a correct equivalent circuit. figure 9 presents the normalized impedance errors for this case, corresponding to a fitting error of 0.00013%  . in the third analysis of the performance of the proposed algorithm, the circuit of figure 1 is considered, but the simulated impedance values are narrowly centred near the resonance. in this case, 11 frequency points are used in the 4 khz to 6 khz range. in figure 10, the magnitude frequency 0 2 4 6 8 10 0 200 400 600 800 1000 1200 frequency [khz] |z | [  ] 0 2 4 6 8 10 -90 -60 -30 0 30 60 90  [ º] figure 5. impedance magnitude and phase of the circuit in figure 1 (lines),  estimated impedance magnitude and phase corresponding to the circuit in figure 4 (dashed lines overlapped with lines) and frequency points used for  estimation (squares for magnitude and circles for phase).  0 2 4 6 8 10 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 frequency [khz] |z m ||z | [ ] 0 2 4 6 8 10 -0.2 -0.1 0 0.1 0.2 frequency [khz]  m - [ º] 0 2 4 6 8 10 0 0.1 0.2 0.3 0.4 frequency [khz] |z m -z | [ ] 0 2 4 6 8 10 0 1 2 3 x 10 -3 frequency [khz] |z m -z |/ |z | a c b d figure 6. four representations of the fitting errors.  figure  4.  circuit  topology  and  component  values  corresponding  to  the  binary tree shown in figure 3.  acta imeko | www.imeko.org  july 2012 | volume 1 | number 1 | 23  response of the original circuit and that of the estimated circuit are compared with the values considered for the circuit estimation. although the frequency points are very close to the resonance, the magnitude estimation is quite good even in the complete frequency range represented. figure 11 shows the results obtained for the phase. in this case, the overall estimation is quite good near the resonance but nearer the edges of the complete frequency range, there are some small noticeable differences. in this case, the error of the fit is 0.00014%  (figure 12). 4. experimental results  the next step in testing the developed algorithm is to evaluate its performance when applied to real measured data. measurements were performed in a circuit that models a humidity sensor [19] and in a previously analysed viscosity sensor [14]. 4.1. humidity sensor circuit model  the circuit that models the humidity sensor is represented in figure 13. the component values of the equivalent circuit of the sensor change with relative humidity and the case corresponding to a relative humidity of 54% is considered. the component values are presented in table 1. the circuit in figure 13 with the component values shown in table 1 was assembled and its spectral response was measured for p = 11 logarithmic spaced frequency points in the 1 hz to 100 khz range. the proposed algorithm was then applied to these measurements to find the equivalent circuit and figure  7.  circuit  topology  and  component  values  obtained  with  the proposed  algorithm  for  10  different  frequencies  in  the  100  hz  to  10  khz range.  0 2 4 6 8 10 0 200 400 600 800 1000 1200 frequency [khz] |z | [ ] 0 2 4 6 8 10 -90 -60 -30 0 30 60 90  [ º] figure 8. impedance magnitude and phase of the circuit in figure 1 (lines), estimated impedance magnitude and phase corresponding to the circuit in figure 7 (dashed lines overlapped with lines) and frequency points used for estimation (squares for magnitude and circles for phase).   0 2 4 6 8 10 0 1 2 3 x 10 -3 frequency [khz] |z m -z |/ |z |   figure 9. normalized impedance errors for 10 linearly spaced frequencies in the 100 hz to 10 khz range.  0 2 4 6 8 10 0 200 400 600 800 1000 1200 frequency [khz] |z | [ ] figure 10. impedance magnitude of the circuit in figure 1 (line), estimated  impedance  magnitude  corresponding  to  the  original  circuit  (dashed  line overlapped  with  line)  and  frequency  points  used  for  the  estimation  (squares) in the 4 khz to 6 khz range.  0 2 4 6 8 10 -90 -60 -30 0 30 60 90 frequency [khz]  [ º] figure  11.  impedance  phase  of  the  circuit  in  figure  1  (line),  estimated  impedance  phase  corresponding  to  the  original  circuit  (dashed  line)  and  frequency  points  used  for  the  estimation  (circles)  in  the  4  khz  to  6  khz range.   4 5 6 0 0.5 1 1.5 2 x 10 -3 frequency [khz] |z m -z |/ |z | figure 12. normalized impedance errors for 11 linearly spaced frequencies  in the 4 khz to 6 khz range.  acta imeko | www.imeko.org  july 2012 | volume 1 | number 1 | 24  respective component values. figure 14 shows an example of the encoding gene and the respective binary tree that was found by the algorithm. the corresponding equivalent circuit with the component values is presented in figure 15. although different runs of the algorithm on the measured data yield different equivalent circuits that do not resemble the original circuit topology shown in figure 13, the spectral response of the resulting circuit closely matches the measurements performed on the original circuit, as shown in figure 16. thus, it is possible to conclude that the circuit in figure 15 is equivalent, at least in the measured frequency range, to the original circuit although it might not be equivalent to the sensor equivalent circuit at different relative humidity values. the error of the fit is 0.032%  (the threshold was set, in this case, to 0.035% due to noisy measurements). 4.2. viscosity sensor  the viscosity sensor consists on a vibrating wire cell whose resonance characteristics change with the viscosity of the liquid in which the wire is immersed and also on its temperature. from the measured frequency response of the sensor, it is possible to obtain its resonance characteristics and in turn obtain the viscosity of the liquid. further details on the working principle of this sensor can be found in [14]. the viscosity sensor wire was immersed in diisodecyl phthalate (didp) liquid at 15ºc. its impedance was then measured for p = 21 frequency values in the range 500 hz to 1.5 khz. from the application of the gep and the hybrid genetic algorithms resulted the equivalent circuit and respective component values presented in figure 17. the impedance frequency response of the equivalent circuit in figure 17 is plotted in figure 18 along with the 21 measured points. both magnitude and phase are in close agreement with the measurements showing that, also in this case, the algorithm was successful in finding an equivalent circuit. the error of the fit is 0.00008%  , while the threshold was set at 0.0001%. 5. conclusions  in this paper, an improved version of impedance spectroscopy using evolutionary algorithms was presented, where gep is used to evolve the target circuit topology and a hybrid genetic algorithm estimates the circuit component values. a detailed description of the gep algorithm and operators is presented as well as a summary of the hybrid genetic algorithm. figure 13. humidity sensor equivalent circuit. the component values change  with relative humidity [19].  gene // + // 1 3 + 3 + // 3 2 2 1 3 3 1 2 2 1 1 3 figure  14.  example  of  encoding  gene  (in  bold)  and  corresponding  binary  tree  obtained  by  gep  and  hybrid  genetic  algorithm  for  the  measured  spectral response of the sensor equivalent circuit with rh = 54%.  table  1.  component  values  for  the  humidity  sensor  equivalent  circuit  at  rh = 54%.  component  value  rs  1.18   rw  624.5   rb  30 m  rp  120.97 k  cb  0.68 f  cp  2.32 f  cpw  5.87 nf  0.4183 f 0.1130 h 620.0 10.76 h 5.711 nf 1.746 m 0.1275 f figure  15.  circuit  topology  and  component  values  corresponding  to  the  binary tree shown in figure 14.  10 -2 10 0 10 210 2 10 3 10 4 10 5 10 6 frequency [khz] |z | [ ] 10 -3 10 -2 10 -1 10 0 10 1 10 2-90 -75 -60 -45 -30 -15 0  [ º] figure 16. estimated impedance magnitude and phase (lines) of the circuit  in  figure  15  versus  the  measured  impedance  magnitude  (squares)  and  phase (circles) of the circuit in figure 13.  acta imeko | www.imeko.org  july 2012 | volume 1 | number 1 | 25  to improve the convergence properties of the algorithm, a different fitness function, which works well with impedance sweeps that include resonances, has been proposed. numerical results show that even with measurement uncertainties and few frequency impedance measured values, the presented algorithm is capable of finding an equivalent circuit. the residual errors in the magnitude and phase of the estimated impedances were used to analyse the performance of the algorithm. the algorithm was also applied to measurements of a circuit that models a humidity sensor at a specific relative humidity and to a viscosity sensor. in both cases it was found that different runs of the algorithm yield different equivalent circuits that nonetheless closely match the measured spectral responses. a method for further automatic simplification of the obtained circuits is currently under development. acknowledgement  the authors would like to thank prof. j. fareleira and prof. f. caetano for the use of their viscosity sensor. references  [1] j.r. macdonald, impedance spectroscopy, annals of biomedical engineering, 20, (1992), pp. 289-305. [2] e. katz, i. willner, probing biomolecular interactions at conductive and semiconductive surfaces by impedance spectroscopy: routes to impedimetric immunosensors, dnasensors, and enzyme biosensors, electroanalysis, 15, (2003), pp. 913–947. [3] a.k. manohar, o. bretschger, k.h. nealson, f. mansfeld, the use of electrochemical impedance spectroscopy (eis) in the evaluation of the electrochemical properties of a microbial fuel cell, bioelectrochemistry, 72, (2008), pp. 149-154. [4] j. hoja, g. lentka, interface circuit for impedance sensors using two specialized single-chip microsystems, sensors and actuators a: physical, 163, (2010), pp. 191-197. [5] j. hoja, g. lentka, method using bilinear transformation for measurement of impedance parameters of a multielement two-terminal network, ieee trans. instrumen. meas., 57, (2008), pp. 1670-1677. [6] e. barsoukov, j. macdonald, impedance spectroscopy theory, experiment, and applications, wiley interscience, hoboken, 2005, isbn 978-0-471-64749-2. [7] ieee std. 1057-2007, ieee standard for digitizing waveform records, the institute of electrical and electronic engineers, (2007), new york, december, e-isbn: 0-7381-4543-2. [8] p.m. ramos, m.f. silva, a.c. serra, low frequency impedance measurement using sine-fitting, measurement, 35, (2004), pp. 89-96. [9] p.m. ramos, a.c. serra, a new sine-fitting algorithm for accurate amplitude and phase measurements in two channel acquisition systems, measurement, 41, (2008), pp. 135-143. [10] j.r. macdonald, j.a. garber, analysis of impedance and admittance data for solids and liquids, j. electrochem. soc., 124, (1977), pp. 1022-1030. [11] r. macdonald, levm/levmw manual, version 8.11, october 2011, available at www.jrossmacdonald.com/ levminfo.html. [12] f.m. janeiro, p.m. ramos, application of genetic algorithms for estimation of impedance parameters of two-terminal networks, i2mtc 09, singapore, 2009, pp. 602-606. [13] f.m. janeiro, p.m. ramos, impedance measurements using genetic algorithms and multiharmonic signals, ieee trans. instrumen. meas., 58, (2009), pp. 383-388. [14] f.m. janeiro, j. fareleira, j. diogo, d. máximo, p. m. ramos, f. caetano, impedance spectroscopy of a vibrating wire for viscosity measurements, i2mtc 10, austin, usa, 2010, pp. 1067-1072. [15] c. ferreira, gene expression programming in problem solving, 6th online world conference on soft computing in industrial applications, 2001. [16] h. cao, j. yu, l. kang, an evolutionary approach for modeling the equivalent circuit for electrochemical impedance spectroscopy, the 2003 congress on evolutionary computation, canberra, australia, 2003, pp. 1819-1825. [17] p. arpaia, a cultural evolutionary programming approach to automatic analytical modeling of electrochemical phenomena through impedance spectroscopy, meas. sci. technol., 20, (2009), pp. 1-10. [18] f.m. janeiro, p.m. ramos, impedance circuit identification through evolutionary algorithms, xviii tc 4 imeko symposium, natal, brazil, 2011. [19] j.a. gonzález, v. lópez, a. bautista, e. otero, x. r. nóvoa, characterization of porous aluminum oxide films from a.c. impedance measurements, j. appl. electrochem., 29, (1999), pp. 229-238. [20] r.l. haupt, s. e. haupt, practical genetic algorithms, wiley-interscience, hoboken, 2004, isbn 978-0-471-45565-3.   figure 17. viscosity sensor equivalent circuit.  0.5 1 1.5 0.6 0.7 0.8 frequency [khz] |z | [ ] 0.5 0.75 1 1.25 1.5 0 2 4 6 8 10  [ º] figure 18. estimated impedance magnitude and phase (lines) of the circuit in  figure  17  versus  the  measured  impedance  magnitude  (squares)  and phase (circles) of the viscosity sensor.  introductory notes for the acta imeko fourth issue 2022 general track acta imeko issn: 2221-870x december 2022, volume 11, number 4, 1 2 acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 1 introductory notes for the acta imeko fourth issue 2022 general track francesco lamonaca1 1 department of department of computer science, modeling, electronics and systems engineering (dimes), university of calabria, ponte p. bucci, 87036, arcavacata di rende, italy section: editorial citation: francesco lamonaca, introductory notes for the acta imeko fourth issue 2022 general track, acta imeko, vol. 11, no. 4, article 1, december 2022, identifier: imeko-acta-11 (2022)-04-01 received december 22, 2022; in final form december 22, 2022; published december 2022 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: francesco lamonaca, e-mail: editorinchief.actaimeko@hunmeko.org dear readers, the end of the year is a time of accounting. thanks to all of you, as a community of authors and reviewers, acta imeko is increasing its reputation. indeed, according to sjr ranking [1], in the last year the journal passed from the fourth to the third quartile both in the field of electrical and electronic engineering and of mechanical engineering. the goal of speeding up the editorial process without reducing the quality of the publications has been achieved too. indeed, the publication time has been reduced by a factor of ten, and the rejection rate is at about 50%. this was done keeping the blind peer-review process based on at least two reviewers. since february 2022 acta imeko is indexed by the directory of open access journals (doaj) [2] and the procedure to be indexed in web of science was started. the editorial board (eb) was enlarged including most of the imeko technical committees chairs. please let me thank all the new eb members for accepting this important role in supporting the journal with their voluntary service. further efforts were done to upgrade the management system of the journal and the website. it was a huge work, especially in transferring the data and the history of the old system to the new one. finally, we have developed a procedure to recognize the voluntary efforts of the reviewers. in agreement with the imeko presidential board we have established an award for the best reviewer of the year and a recognition for the top 10 reviewers. as usual, also this issue includes a general track aimed to collect contributions that do not relate to a specific event. as editor in chief, it is my pleasure to give you an overview of these papers, with the aim of encouraging potential authors to consider sharing their research through acta imeko. natural lighting in building environments is an important aspect for the occupants' mental and physical health. furthermore, the proper exploitation of this resource can bring energy benefits related to the reduced use of artificial lighting. in [3], f. nicoletti et al. provide some estimates of the energy that can be saved by using a lighting system that recognises indoor illuminance. in particular, it is able to manage the switching on of lights according to the daylight detected in the room. the savings from this solution depend on the size and orientation of the window. the analysis is conducted on an office by means of simulations using the inlux-dbr code. the locations have an influence on the luminance characteristics of the sky. the analysis is conducted with reference to one city in the south and one in the north of italy (cosenza and milan). the energy saving is almost independent of latitude and therefore representative of the italian territory. it is highly variable according to exposure, being the highest for southern exposure (97 % with the window size equal to 36 % of the floor area) and between 26 % and 48 % (as a function of window size) for northern exposure. following the revision of the si, kibble balances around the world may now be used to realise the unit of mass. in a kibble balance, the weight of a mass is balanced by the electromagnetic force on a current-carrying coil of wire suspended in a magnetic field. at msl researchers are developing a kibble balance where the coil is connected to the piston of pressure balance in a twin pressure balance arrangement. the piston-cylinder unit of pressure balance provides a repeatable axis for the motion of the coil in the magnetic field. the twin pressure balance arrangement serves as a high-sensitivity force comparator. in the paper entitled “position control for the msl kibble balance coil using a syringe pump” [4], r. j. hawke and m. t. clarkson highlight that in a kibble balance, the position of the coil must be finely controlled. in weighing mode, the coil remains stationary in a location of constant magnetic field. in calibration mode, the coil is moved in the magnetic field to induce a voltage. in particular, they investigate how the piston (and therefore coil) position may be controlled through careful manipulation of the gas column under the piston. they demonstrate the use of a http://? acta imeko issn: 2221-870x december 2022, volume 11, number 4, 2 3 acta imeko | www.imeko.org december 2022 | volume 11 | number 4 | 2 syringe pump as a programmable volume regulator which can provide fall rate compensation as well as controlled motion of the piston. in [4] it is shown that the damped harmonic oscillator response of the pressure balance must be considered when moving the coil. from this initial investigation, authors discuss the implications for use in the msl kibble balance. reliability analysis can be committed to companies by customers willing to verify whether their products comply with the major international standards or simply to verify the design prior of market deployment. nevertheless, these analyses may be required at the very preliminary stages of design or when the design is already in progress due to low organizational capabilities or simple delay in the project implementation process. the results sometime may be far from the market or customer target with a subsequent need to redesign the whole asset. of course, not all the cases fall in the worst scenario and maybe with some additional considerations on mission definition it is possible to comply with the proposed reliability targets. marco mugnaini and ada fort, in the paper entitled “how to stretch system reliability exploiting mission constraints: a practical roadmap for industries” [5] provide an overview on the approach which could be adopted to achieve the reliability target even when the project is still on-going, providing a practical case study. the recent increase in the internet of things and industry 4.0 fields has led many researchers to focus on the innovative technologies that could support these emerging topics in different area of applications. in particular, the current trends are to close the gap between the physical and digital worlds, thus originating the so-called cyber-physical system (cps). a relevant feature of the cps is the digital twin, i.e., a digital replica of a process/product with which the user can interact to operate in the real world. in the paper entitled “digital twins based on augmented reality of measurement instruments for the implementation of a cyber-physical system” [6], a. liccardo et al. propose an innovative approach exploiting an augmented reality solution as digital twin for the electronic instrumentation to obtain a tight connection between the measurements as the physical world and the internet of things as digital applications. actually, by means of the adoption of the 3d scanning strategy, augmented reality software and with the development of a suitable connection between the instrument and the digital world, a cyber-physical system has been realized as an iot platform that collects and controls the real instrumentation and makes it available in augmented reality. an application example involving a digital storage oscilloscope is finally presented to highlight the efficacy of the proposed approach. i hope you will enjoy your reading. francesco lamonaca editor in chief references [1] acta imeko in the scimago journal & country rank. online [accessed 22 december 2022] https://www.scimagojr.com/journalsearch.php?q=21100407601 &tip=sid&clean=0 [2] acta imeko in the directory of open access journals. online [accessed 22 december 2022] https://doaj.org/toc/2221-870x [3] f. nicoletti, v. ferraro, d. kaliakatsos, m. a. cucumo, a. rollo, n. arcuri, “evaluation of the electricity savings resulting from a control system for artificial lights based on the available daylight”, acta imeko, vol.11, no.4, 2022, pp.1-11. doi: 10.21014/actaimeko.v11i4.1309 [4] r. j. hawke, m. t. clarkson, “position control for the msl kibble balance coil using a syringe pump”, acta imeko, vol.11, no.4, 2022, pp.1-7. doi: 10.21014/actaimeko.v11i4.1343 [5] m. mugnaini, a. fort, “how to stretch system reliability exploiting mission constraints: a practical roadmap for industries”, acta imeko, vol.11, no.4, 2022, pp.1-6. doi: 10.21014/actaimeko.v11i4.1358 [6] a. liccardo, f. bonavolontà, r. s. lo moriello, f. lamonaca, l. de vito, a. gloria, e. caputo, g.de alteriis, “digital twins based on augmented reality of measurement instruments for the implementation of a cyber-physical system”, acta imeko, vol.11, no.4, 2022, pp.1-8. doi: 10.21014/actaimeko.v11i4.1397 https://www.scimagojr.com/journalsearch.php?q=21100407601&tip=sid&clean=0 https://www.scimagojr.com/journalsearch.php?q=21100407601&tip=sid&clean=0 https://doaj.org/toc/2221-870x https://doi.org/10.21014/actaimeko.v11i4.1309 https://doi.org/10.21014/actaimeko.v11i4.1343 https://doi.org/10.21014/actaimeko.v11i4.1358 https://doi.org/10.21014/actaimeko.v11i4.1397 microsoft word 213-2175-1-le-rev acta imeko  issn: 2221‐870x  april 2016, volume 5, number 1, 64‐68    acta imeko | www.imeko.org  april 2016 | volume 5 | number 1 | 64  systematic quality control for long term ocean observations  and applications  daniel m. toma 1 , albert garcia‐benadí 2 , bernat‐joan mànuel‐gonzález 1 , joaquín del‐río‐fernández 1    1  sarti research group, electronics dept.,. universitat politècnica de catalunya (upc), rambla exposició 24, 08800, vilanova i la geltrú,  barcelona, spain.   2  laboratori de metrologia i calibratge, centre tecnològic de vilanova i la geltrú, universitat politècnica de catalunya (upc), rambla  exposició, 24, 08800 vilanova i la geltrú, barcelona, spain         section: technical note   keywords: quality control; ocean; long term; observatory; metadata   citation: daniel m. toma, albert garcia‐benadí, bernat‐joan mànuel‐gonzález, joaquín del‐río‐fernández, systematic quality control for long term ocean  observations and applications, acta imeko, vol. 5, no. 1, article 12, april 2016, identifier: imeko‐acta‐05 (2016)‐01‐12  editor: paolo carbone, university of perugia, italy  received september 9, 2014; in final form september 9, 2014; published april 2016  copyright: © 2016 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  corresponding author: daniel m. toma / albert garcia‐benadí, e‐mail: daniel.mihai.toma@upc.edu / albert.garcia‐benadi@upc.edu    1. introduction  recently, marine observations have been rapidly growing. some of the observed parameters that are currently monitored are the sea temperature, level of acidification and noise pollution. the value of each of these parameters gives very valuable information in different areas, for example, the temperature of seawater is currently used to perform better estimations of the climate change, the measure of the acidification of seawater is very important for fishing processes, etc. however, all these data are relevant if there is a continuous recording of them for long periods of time and the measurements have been performed under quality requirements. for this reason, in the last years many permanent seabed observatories as well as floating observatories such as moored or drifters buoys have been deployed to perform long term marine observations which are the basis for environmental modelling and assessment. close examination of these data often reveals a lack of quality that, frequently, happens for extended periods of time. the growing needs for real-time processing of data and the sheer quantity of data produced by these observatories means that automated quality assurance/quality control (qa/qc) is necessary to ensure that the collected data has the quality required for its purpose [1]-[3]. this paper demonstrates the use of well-defined community adopted qa/qc tests and automated data quality assessments [4] and [5] to provide a continuous scale of data quality, the capture of information about the system provenance, sensor and data processing history, and the inclusion of the flag values in metadata stream. an example of this system of implementation and testing the automated data quality assessments on a real time platform is the expandable seafloor observatory, obsea [6] and [7] that is deployed to monitor the barcelona coast, spain. among the parameters observed at obsea, for which these automated data quality controls is applied are the seawater and air temperature, conductivity, underwater and air pressure. abstract  with the advances of last year’s technologies many new observation platforms have been created and connected on networks for the  diffusion of numerous and diverse observations, and also it provided a great possibility to connect all kind of people facilitating the  creations of great scale and  long‐term studies. this paper  is focused on the marine observations and platforms employed for this  scope. real time data and the big data have to accomplish some minimal quality of data requirements. usually, the task to ensure  these quality requirements is accomplished by the platforms responsible. the aim of this paper is to explain the design of these quality  control systems and their implementation in an ocean observation platform.  acta imeko | www.imeko.org  april 2016 | volume 5 | number 1 | 65  2. development  each data measured by the platforms has to pass different filters which are evaluated under some tests established for the customer. these tests have to be applied for every magnitude measured. 2.1. qualification tests  the qualification tests are separated in format tests and behaviour tests. the automated application of these tests is rather straightforward using java programs. however, the estimation of the parameter “thresholds” of these tests poses the greatest challenge. the statistical assumptions dictate that these threshold parameters are ideally defined by having a distribution of values that are objectively considered “reasonable” for every sensor at every site [8]. in the obsea the following automatic quality control tests are implemented with the range, step, delta, sigma, null, and gap parameter thresholds determined by statistical distributions based on existing data over a period of three years:  platform identification. this criterion determinates the location of the equipment, for example: in the laboratory, connected to the obsea platform, or under test.  impossible platform date/time. the date must be greater than the date of the obsea start, and the hour must be from 0 until 23, and the minutes must be from 0 until 59.  regional impossible parameter values. the thresholds of this test are defined for each magnitude. the thresholds establish the extreme values based on the minima and maxima observed for a given sample period (table 1). for example, the seawater temperature of the mediterranean sea cannot exceed 28 degree celsius and cannot be less than 10 degree celsius.  spike test. the tendency between the previous and actual value has to be coherent. for example the step between the previous and actual value of the temperature of seawater cannot exceed 0.1 °c for a sampling interval of less than 1 minute. the threshold of this test must be in accordance with the parameter measured in the environment and the acquisition rate of the instrument.  gradient test. this test fails when the difference between vertically adjacent measurements is too steep. for example the gradient value of the temperature of seawater cannot exceed 0.2 °c for a sampling interval of less than 1 minute. similar to the spike test the threshold of the gradient test must be in accordance with the parameter measured in the environment and the acquisition rate of the instrument. the last test is the visual inspection of the data. this test is not automated, but it is a main test to ensure the quality of the adopted qa/qc. this test performs data verification by means of visual inspection; any flagged data from the previous tests is either verified as being an incorrect value or is accepted as correct data. this test minimizes the risk of inadvertently eliminating the observation of a rare and potentially interesting event for the sake of data quality [9]. 2.2. tools  the obsea automated quality assurance/quality control system was created using the java programming language. the system block diagram in figure 1 provides a general overview of the qa/qc system. the system is composed of three components and their interdependencies, as shown. from this figure, it is clear that dependencies between subsystems are simple and the communication between the subsystem and its dependencies are through their interfaces. the system allows the building of application specific data structures assembled from standard building blocks allowing “on the fly” changes of parameters. therefore, it will be easier to add new algorithms and new parameters to the application. this allows components to be generic and configurable and allows other modules to retrieve information required and query data structures. 2.3. valuation  the result of these automatic quality control tests is a numeric value that is included inside the metadata in the acquired data. these qualifications are detailed in table 2. when the data are collected and properly verified, usually the flag value is 1, but for long time acquisitions, in a hostile table 1. the six tests employed in the obsea data quality control. problem to  be identified  section  test  calculation  data belongs  to platform  platform  identification  test  defined in metadata  timestamp  of data in  observatory  range  impossible  platform  date/time  year greater than 2008   month in range 1 to 12   day in range expected for month   hour in range 0 to 23   minute in range 0 to 59  data outliers regional  impossible  parameter  values  sea water temperature in range 10 °c  to 28 °c   salinity in range 35 to 39  sea level air pressure in range 850 hpa  to 1060 hpa (mbar)  air temperature in range ‐10 °c + 40 °c  wind speed in range 0 m/s to 60 m/s  wind direction in range 0° to 360°  humidity in range 5 % to 95 %  current speed in range 0 m/s to 3 m/s   current direction in range 0° to 360°  wave period in range 0 to 20 s   wave height in range 0 m to 10 m   depth in range 18 m to 21 m  conductivity  in  range  3.5  s/m  to  6.5  s/m  sound velocity in range 1480 m/s to  1550 m/s  jumps in  data values  spike test  | dt – (dt+1 + dt‐1)/2 | –   | (dt+1 – dt‐1) / 2 | > s   (where s defined by sampling)  change in  variance  structure  gradient test  | dt – (dt+1 + dt‐1)/2 | > g   (where g defined by sampling)  a dropped  data point  null test  defined by sampling  data belongs  to platform  platform  identification  test  defined in metadata  acta imeko | www.imeko.org  april 2016 | volume 5 | number 1 | 66  environment, in our case the sea, the equipment can suffer from undesirable variations. 3. results  the implementation of these automated quality control tests is illustrated using salinity and air temperature data from the obsea observatory. figure 2 illustrates the time series of 2 days of salinity data sampled at 10 s intervals in march 2014 and figure 3 illustrates the time series of 4 month of data sampled at 20 s intervals in february–june 2014. it should be noted that these data contain numerous known errors, which is useful for the purposes of this example. the salinity measurement is a function of conductivity, temperature and pressure measurements made with the sbe 37 smp instrument installed in the obsea observatory at a constant depth of approximately 20 m. as can be seen in figure 2, the salinity measurements present numerous errors, which have been flagged as bad data (flag=4) by the automated quality control system. the erroneous salinity measurements have been caused by the incorrect measurements of the conductivity cell or for an excess threshold in the criteria of acceptance. all these considerations are going to improve the automated quality control system. the percentage of incorrect salinity and conductivity measurements of the sbe 37 smp instrument for the 4 month of data sampled at 10 s intervals in february–june 2014 was 0.95 % as shown in table 3. in the same period, there were figure 1. the automated data quality assurance system block diagram.  table 2. quality number included in metadata and its correspondence. flag  meaning  0  no quality control  1  value seems correct  2  value appears inconsistent with other values 3  value seems doubtful  4  value seems erroneous  5  value was modified  6  flagged land test  7‐8  reserved for future use  9  data is missing  table 3. percentages of correct, incorrect and missing values for the obsea  parameters between february 2014 and june 2014.   parameter  measured  value seems  correct  value seems  incorrect  value  missing  sea water  temperature  91.69 %  0.16 %  8.15 %  salinity  90.90 %  0.95 %  8.15 %  depth  91.68 %  0.17 %  8.15 %  conductivity  91.74 %  0.11 %  8.15 %  sound velocity 91.78 %  0.07 %  8.15 %  sea level air  pressure  93.59 %  0.00 %  6.41 %  air  temperature  81,59 %  12.00 %  6.41 %  wind speed  93.59 %  0.00 %  6.41 %  wind direction 93.59 %  0.00 %  6.41 %  acta imeko | www.imeko.org  april 2016 | volume 5 | number 1 | 67  8.15 % of missing data, caused by various factors such as communication problems between the land station and the obsea observatory and rarely caused by instrument outage. 4. conclusion  the automated quality assurance/quality control provides truthfulness for long-term measures as well as the possibility of different states of the equipment, in calibration processes or out of services, among others, without cutting the link with the platform. moreover, the metadata has to be standard for all instruments and sensors; the standardization improves the compatibility of the automatic quality control framework with different platform. it is only through the use of these standardized approach that global scale ecosystem questions can ever be addressed. the future proposals include the uncertainty inside the metadata to know its uncertainty value, and other task will be to improve the top values of the different test to adapt at the place. acknowledgement  this work was partially supported by the projects nexos and fixo3 from the european union’s seventh programme for research, technological development and demonstration under grants agreement no 614102 and no 312463. references  [1] loescher, h. w., ocheltree, t., tanner, b., swiatek, e., dano, b., wong, j., zimmerman, g., campbell, j. l., stock, c., jacobsen, l., shiga, y., kollas, j., liburdy, j., and law, b. e.: comparison of temperature and wind statistics in contrasting environments among different sonic anemometer-thermometers, agr. forest meteorol., 133, pp.119–139. [2] ocheltree, t. o. and loescher, h. w.: design of the ameriflux portable eddy-covariance system and uncertainty analysis of carbon measurements, j. atmos. ocean. tech., 24, pp.1389– 1409, 2007. [3] taylor, j. r. and loescher, h. w.: neon’s fundamental instrument unit dataflow and quality assurance plan, neon.011009, national ecological observatory network, boulder, colorado, 2012. [4] seadatanet, 2007: data quality control procedures, version 0.1, 6th framework of ec dg research. [5] sylvie pouliquen and the data-meq working group, recommendations for in-situ data real time quality control, eg10.19, december 2010. url: http://eurogoos.eu/download/publications/rtqc.pdf [6] daniel toma, ikram bghiel, joaquin del rio, alberto hidalgo, normandino carreras, and antoni manuel 2014. automated data quality assurance using ogc sensor web enablement figure 2. time series of salinity observations between march 15 to march 17 2014 from obsea sea‐bird ctd model (sbe 37 smp) and the corresponding  quality control flags. the sampling rate of the instrument is 10 s. these data contain errors and missing value which are automatically flagged as incorrect  data (4) and missing data (9), respectively.  figure 3. time series of air temperature observations in february–june 2014 from obsea airmar weather station model 150wx and the corresponding  quality control flags. the sampling rate of the instrument is 20 s. these data contain errors and missing value which are automatically flagged as incorrect  data (4) and missing data (9), respectively.  acta imeko | www.imeko.org  april 2016 | volume 5 | number 1 | 68  frameworks for marine observatories, geophysical research abstracts vol. 16, egu2014-11508, 2014 egu general assembly 2014. [7] jacopo aguzzi, antoni mànuel, fernando condal, jorge guillén, marc nogueras, joaquin del rio, corrado costa, paolo menesatti, pere puig, francesc sardà, daniel toma, albert palanques. the new seafloor observatory (obsea) for remote and long-term coastal ecosystem monitoring. in sensors vol. 11, issue 6, 2011. [8] taylor, j. r. and loescher, h. l.: automated quality control methods for sensor data: a novel observatory approach, biogeosciences, 10, 4957-4971, doi:10.5194/bg-10-4957-2013, 2013. [9] essenwanger, o. m.: analytical procedures for the quality control of meteorological data, meteorological observations and instrumentation: meteorological monograph, am. meteorol. soc., 33, 141–147, 1969. acta imeko  issn: 2221‐870x  june 2015, volume 4, number 2, 52‐56    acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 52  accelerometer transverse sensitivity calibration; validation  and uncertainty estimation  christiaan s veldman  national metrology laboratory of south africa, acoustics, ultrasound and vibration laboratory, south africa private bag x34, lynnwood  ridge, 0004, south africa      section: research paper   keywords: accelerometer; calibration; transverse sensitivity; validation; uncertainty of measurement; iso 16063‐31  citation: christiaan s veldman: accelerometer transverse sensitivity calibration; validation and uncertainty estimation, acta imeko, vol. 4, no. 2, article 9,  june 2015, identifier: imeko‐acta‐04 (2015)‐02‐09  editor: paolo carbone, university of perugia, italy  received september 23 rd , 2014; in final form february 14 th , 2015; published june 2015  copyright: © 2015 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: this work was supported by the department of trade and industry, south africa  corresponding author: ian veldman, e‐mail: csveldman@nmisa.org    1. introduction  due to their ease of use and low cost, accelerometers are widely considered as the vibration sensor of choice. a variety of different models are required to cover the wide range of vibration measurement applications. to select the accelerometer best suited for a specific application, the user will typically scrutinize the manufacturer’s specifications. apart from the general (usually the most relevant) specifications such as size, sensitivity, frequencyand acceleration ranges, the manufacturer also specifies the relative transverse sensitivity (rts) of an accelerometer. for specialized application, the transverse sensitivity is of importance. for some applications, a more accurately known value of the transverse sensitivity might be required [1], [2], [3]. in addition, knowledge of the angle (mechanical orientation) of the transverse sensitivity is also required. nmisa developed a capability to accurately measure the transverse sensitivity of accelerometers as part of its research in vibration metrology. nmisa modified its existing low frequency accelerometer calibration system to facilitate the measurement of accelerometer transverse sensitivity and will offer this calibration service to industry in the near future. in section 2, the author defined “transverse sensitivity”. the system hardware and specifications are described in section 3. in section 4, the approach followed to estimate the uncertainty of measurement as well as the major uncertainty of measurement contributors is discussed. the method followed for validating the system, with the validation results are discussed in section 5. finally, in the concluding section the findings are summarized. 2. transverse sensitivity  the transverse sensitivity of an accelerometer is defined as the sensitivity to acceleration applied perpendicular to its sensitive axis [4]. the axis of maximum sensitivity of the transducer is not necessarily aligned with the sensitive axis, as shown in figure 1. as a result, any motion not in line with the sensitive axis will produce an output. if the transducer is placed in a rectangular co-ordinate system, as shown in figure 1, the vector, smax, representing the maximum transducer sensitivity can be resolved into two abstract  the national metrology institute of south africa (nmisa) has implemented a system to measure the transverse sensitivity of vibration  transducers. as a mechanical device, the principle sensing axis of an accelerometer is not 100 % perpendicular to the mounting axis  (surface).  this  gives  rise  to  the  effect  that  the  accelerometer  will  produce  an  electrical  output  even  when  a  mechanical  input  perpendicular to the principle measurement axis is applied. the quantification of this “defect” parameter is of importance when high  accuracy acceleration measurements are performed using accelerometers.  this paper gives a brief overview of the system developed  by the nmisa to measure the transverse sensitivity of vibration transducers. the paper then explores the validation of the system  along with the uncertainty of measurement associated with the calibration system. acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 53  components: the sensitive axis sensitivity, sn, (sensitivity) and the maximum transverse sensitivity, st,max. the theoretical transverse sensitivity curve is shown in figure 2. the transverse sensitivity, expressed as a percentage of the sensitivity, is referred to as the relative transverse sensitivity (rts). the rts is dependent on the excitation angle. for high quality accelerometers, manufacturers supply devices with low rts, typically ≤ 1 %, and with the direction of the lowest transverse sensitivity, βtmin, indicated with a red dot on the accelerometer. the manufacturer supplies these low transverse sensitivity devices through selection. that is, they physically measure the transverse sensitivity and select the units that meet the required rts specification. 3. system description  the transverse sensitivity calibration system of nmisa [5] was developed in compliance with iso 16063-31 [6]. the transverse sensitivity capability was developed as an extension of the existing primary low frequency accelerometer calibration system. a schematic diagram of the system configuration is shown in figure 3. the system utilizes the existing long stroke (152 mm peak to peak) electro-dynamic exciter, connected to an air bearing linear translation stage (abt). a stepper motor controlled turntable is mounted on top of the abt (figure 4). table 1 provides the system parameters. for the vibration generator with turntable system implemented by nmisa, once the unit under test (uut) is mounted on the turntable and all the hardware and cable connections are completed, the in-house developed software is executed. the software performs a set of procedural steps as part of each transverse sensitivity measurement per turntable angular position as follows:  move the turntable to the angular position of interest, (from 0° to 360° in 5° steps);  ramp the exciter to the selected vibration level, in frequency and amplitude;  sample two analogue inputs simultaneously, streaming the data (time series data) directly to computer storage;  apply the three parameter sine fit algorithm (3psf) [7] to the ref and uut output voltages time series to calculate acceleration amplitude and the uut voltage output;  calculate the relative transverse sensitivity (rts) using (1) and (2);  record the rts in the result sheet;  plot the rts on a polar diagram. table 1. parameters of nmisa transverse sensitivity calibration system.  vibration frequency range 5 hz to 20 hz  transverse acceleration range 5 m/s 2  to 50 m/s 2 analogue inputs four simultaneously sampled 12  bit channels  sampling frequency 500 khz  turntable rotation angle 0° to 360°  turntable rotation resolution 1°  reference polytec ofv‐505 heterodyne  laser interferometer system  figure 2. polar plot indicating a theoretical transverse sensitivity curve for an accelerometer.  figure 1. graphical illustration of transverse sensitivity.  figure 3. diagram of the system configuration.  acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 54  these steps are executed for each angular position from 0° to 360° using the selected step size, without the need of any intervention by the metrologist. for the calculation for transverse sensitivity, st, the following formula was used: t out t ˆ ˆ a u s  (1) where st is the transverse sensitivity, ûout is the amplitude of the output signal of the transducer vibrating perpendicularly to its sensitivity axis, and ât is the acceleration level in the test direction. the acceleration level is measured by means of a reference transducer and calculated as: ref ref t ˆ ˆ s u a  (2) where ûref is the voltage output of the transducer; and sref is the sensitivity of the reference transducer. the relative transverse sensitivity, *ts , is calculated from n t* t s s s  (3) where sn is the transducer sensitivity. once all the measurements have been completed, the results are saved using an excel template file. the result sheet also displays the rts in graphical format, similar to figure 2. 4. uncertainty of measurement  a conservative approach was followed w.r.t. the consideration of uncertainty contributors. the “worst case” (but still scientifically valid) values were used for the uncertainty contribution. this allows for a single uncertainty calculation that is valid for a wider range of measurement conditions. it also reduces the need to re-calculate the uncertainty budget for each calibration. however, it does not relieve the metrologist of the responsibility of having to consider the uncertainty for each calibration performed. by using the uncertainty values estimated for a specific calibration, instead of the generalized values, an rts calibration with a smaller uncertainty might be possible. the uncertainty of measurement (uom) was estimated in accordance with the gum [8]. the root uncertainty contributors were identified from the mathematical model in (1), which was expanded to (4) by inserting (1) and (2) into (3): ref out n ref* t ˆ ˆ s s u u s  (4) through consideration of the mathematical model (4) and the measurement procedure, a detailed set of uncertainty contributors was identified. this full set was reduced to a subset containing the uncertainty contributors with a significant contribution. this process produced the list of “dominant” uncertainty contributors listed in table 2. a summary of the uncertainty budget is reported in table 3. the voltages for both the reference signal as well as the accelerometer output signal were captured using an a to d converter and applying the 3psf. the sine approximation method is a well-established and adopted time domain signal processing technique [9]. it requires for the samples to be equidistance sampled, that is that the time between t0, t1, t2…be constant, and if the phase difference between signals is required, that the sampling of the two signals be performed simultaneously. for the system implemented, both these criteria were achieved using a four channel analogue to digital converter (adc) (four individual channels, not multiplexed channels), with a single sample timing clock, thus synchronising the sampling done by the four adcs. it has been established that the 3psf method is influenced by various factors [10-13] for instance:  the sampling frequency;  number of adc bits;  number of samples per period to be an integer number;  number of periods to be a prime number;  signal to noise ratio (snr). of particular relevance to this application is the snr [10] as the accelerometer output is a very small signal due to the low transverse sensitivity. as a result, for this application, the snr is measured. however, for simplicity, a minimum snr limit of figure 4. turntable mounted on top of air‐bearing translation stage.  table 2. significant uncertainty contributors  uncertainty contributor source of uncertainty reference transducer sensitivity calibration certificate acceleration level reference voltage measurement accelerometer output voltage accelerometer voltage  measurement  sensitivity (sensitive axis sensitivity)  calibration certificate  type a uncertainties statistical means (standard  deviation)  acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 55  15 db is set for the purpose of calculating the 3psf uncertainty contribution. for a snr of 15 db, the uncertainty in the magnitude determination, σa, is about 0.3 %. σa is calculated using (5) where σa is the 3psf amplitude precision, σ is the zero mean white gaussian noise, q is the quantization error [10, 11]; n is the number of samples. due to the inherent small transverse sensitivity of accelerometers, additional steps were required to achieve the snr limit requirement. this was accomplished by filtering the time signals using a 4th order butterworth (infinite impulse response (iir)) digital bandpass filter. the forward-reverse filtering model was applied to accomplish zero phase response. the lower and upper cut-off frequencies were selected to be 0.8909·fv and 1.225·fv respectively, where fv is the vibration frequency. 4.1. reference sensitivity uncertainty  for this transverse sensitivity calibration system, the metrologist may choose to use either an accelerometer as the reference device or a laser interferometer (vibrometer). for both options, the sensitivity of the reference device is known from a prior calibration. the uncertainty associated with the sensitivity, along with its coverage factor, is obtained from the calibration of the reference. 4.2. acceleration level uncertainty  the voltage amplitude of the reference signal is determined using the 3psf algorithm [12]. from [13], the uncertainty associated with the amplitude is obtained from (5). 4.3. accelerometer output voltage uncertainty  the uncertainty associated with the accelerometer output voltage is estimated in the same manner as described in section 4.2 since the amplitude is also determined using the 3psf. however, the calculated uncertainty for this parameter is expected to be larger, due to the smaller snr, in light of the smaller output voltage. 4.4. sensitive axis sensitivity  the uncertainty contribution of the sensitivity will depend on the source of the value of sensitivity. generally, this sensitivity will be obtained from a valid calibration certificate. in such an instance, the uncertainty will be taken from the certificate. it is possible to determine the sensitivity of the sensitive axis using the calibration system, prior to performing the rts measurements. in this instance, the uncertainty contribution needs to be calculated separately. 4.5. type a uncertainty  at each measurement point, 0° to 355° in 5° steps, the system captures an equidistant sampled time series, containing 100 vibration cycles for both the referenceas well as the accelerometer channels. to eliminate the undesired effects introduced by the bandpass filtering, the 3psf is applied to the centre 50 cycles only. the final voltage amplitude is calculated as the mean and standard deviation of these 50 voltage amplitudes (per channel). the largest type a uncertainty was determined by calibrating an accelerometer with a low rts (≈ 0.1 %). using this accelerometer the largest standard deviation calculated, considering all the measurement points through the complete 360° rotation was 3.5 %. as was to be expected, the type a uncertainty reached a peak value at an angular position with the lowest rts. 5. validation  the performance of the transverse sensitivity calibration system and procedure was validated by performing a bilateral interlaboratory comparison (ilc). the ilc partner was the deutsche akkreditierungsstelle (dakks) accredited laboratory, spektra gmbh, dresden, germany. the purpose of the ilc was to evaluate and validate the metrological operation of the two participating laboratory’s accelerometer transverse sensitivity calibration systems and relevant procedures. the differences in the rts measurements obtained between the two laboratories would support (or disprove) each laboratory’s measurement capability, within their stated uom. the parameter covered by the ilc was the measurement of the relative transverse sensitivity (rts) of an accelerometer at 16 hz. three accelerometers were used as the ilc transfer devices. the accelerometers that were used are listed in table 4. 5.1. evaluation criteria  for each laboratory i, the data where xi,s, is the maximum relative transverse sensitivity st, reported and u(xi,s), is the reported standard uncertainty associated with the rts. for each of the comparison artefacts (transfer devices) an ilc reference value xr,s was determined as the weighted mean of the results of n laboratories (for this comparison, n = 2) according to , ∑ xi,s u2 xi,s n i 1 ∑ , (6) , ∑ , (7) the degree of equivalence, dlab-wm, and ulab-wm, was table 4. comparison transfer devices.  no accelerometer  serial number 1 endevco 2270m8  16194  2 pcb 3701g2fa3g  8353  3 pcb j353b01  47794  table 3. summary uncertainty budget.  source of uncertainty estimated  uncertainty  probability  distribution  (k)  reference transducer sensitivity 0.5 %  2 voltage measurement accuracy  (sine fitting)  0.2 %  1 uncertainty in the uut  sensitivity value  0.8 %  2 voltage measurement accuracy  (sine fitting)  0.3 %  1 rotation angle accuracy 0.5°  √3 vibration table horizontal  alignment  0.1°  √3  type a evaluation 3.5 %  1 acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 56  determined for the rts measurements using (8) (9) where xlab represents the measurement results obtained by the laboratory for each rts and xwm represents the ilc reference value calculated as the weighted mean (wm) using (8). ulab-wm is the uncertainty of measurement associated with the calculated dlab-wm for k = 2, calculated using (9). 5.2. comparison results  the rtss for the three individual accelerometers as reported by each laboratory (with the associated uncertainties) are reported in table 5. the calculated comparison reference values (weighted mean values) are reported in table 6, while the degrees of equivalence (doe) for each participant is reported in table 7. the doe for the measurement results reported by nmisa are shown in graphical format in figure 5. the graph clearly indicates uncertainties which overlap the reference values indicating nmisa equivalence with spektra. 6. conclusions  a transverse sensitivity calibration system was implemented in compliance with iso 16063-31 by nmisa. the transverse motion is generated using an electro-dynamic vibration exciter with a stepper motor controlled turntable for the angular positioning control. the requirement for a relatively high snr (≥ 15 db) was highlighted. this minimum level of snr is maintained through the use of digital narrow band band-pass filtering. four major sources of uncertainty were identified; the reference transducer sensitivity, the acceleration level, the accelerometer output voltage and the sensitive axis sensitivity. for this system, the upper limit for the type a uncertainty was calculated to be 3.5 %. the system was validated through a bi-lateral ilc. the results for the three different accelerometers used, support the uom estimated by nmisa. the ilc results further indicate that the uom estimated by nmisa could be considered as fairly conservative. references  [1] t. petzsche, “determination of the transverse sensitivity using a mechanical vibration generator with turntable,” iso tc 108/sc 3/wg 6 doc. n153, 2007. [2] j. dosch and m. lally, “automated testing of accelerometer transverse sensitivity,” proceedings of the international modal analysis conference (imac), kissimee, florida, usa, pp. 1-4, 2003. [3] r. sill and e. seller, “accelerometer transverse sensitivity measurement using planar orbital motion,” proceedings of the 77th shock and vibration symposium, monterey, california, usa, pp. 812, november 2006. [4] iso, “mechanical vibration, shock and condition monitoring – vocabulary” iso 2041, 2009. [5] c.s. veldman, “implementation of an accelerometer transverse sensitivity measurement system”, ncsl international, june 2013. [6] iso, “methods for the calibration of vibration and shock transducers -part 31: testing of transverse vibration sensitivity” iso 16063-31. [7] peter händle, “amplitude estimation using ieee-std-1057 three-parameter sine wave fit: statistical distribution, bias and variance”, measurement, vol. 43, pp. 766–770, 2010. [8] bipm, iec, ifcc, ilac, iso, iupac, iupap and oiml 2008 evaluation of measurement data—guide to the expression of uncertainty in measurement joint committee for guides in metrology, jcgm 100:2008. [9] iso, “methods for the calibration of vibration and shock transducers -part 11: primary vibration calibration by laser interferometry” iso 16063-11. [10] m. bertocco, c. narduzi, p. paglierani, d. petri, “accuracy of effective bits estimation methods”, ieee instrumentation and technology conference, brussels, belgium, june 4-6, 1996. [11] konrad heijn, andrzej pacut, “generalized model of the quantization error – a unified approach”, ieee transactions on instrumentation and measurement, vol. 34, n. 1, feb. 1996. [12] f.corrêa alegria, “bias of amplitude estimation using threeparameter sine fitting in the presence of additive noise”, measurement, 42, pp.748 – 756, 2009. [13] f. corrêa alegria, a. cruz serra, “uncertainty of the estimates of sine wave fitting of digital data in the presence of additive noise”, ieee instrumentation and technology conference, sorrento, italy, april 24-27, 2006. table 5. reported ilc results.  accelerometer  nmisa  spektra rts  (%)  uc  (%)  rts  (%)  uc (%)  endevco 2270m8  0.8  3  0.95  0.3 pcb 3701g2fa3g  0.24  3  0.25  0.3 pcb j353b01  0.9  3  0.81  0.3 figure 5. nmisa degrees of equivalence.  table 6. calculated weighted mean values.  accelerometer  weighted mean  rtswm  uwm  (%)  (%)  endevco 2270m8  0.95  0.3  pcb 3701g2fa3g  0.25  0.3  pcb j353b01  0.81  0.3  table 7. calculated degrees of equivalence.  accelerometer  degrees of equivalence  dnmisa‐wm  unmisa‐wm  dspektra‐wm  uspektra‐wm  (%)  (%)  (%)  (%)  endevco   2270m8  ‐0.15  3.15  0.001  0.42  pcb  3701g2fa3g  ‐0.01  3.01  0.0  0.42  pcb  j353b01  0.09  3.11  ‐0.001  0.42  ‐5,0 ‐2,5 0,0 2,5 5,0 0 1 2 3 4d e v ia ti o n  f ro m  w m  ( % ) accelerometer number nmisa doe low-cost monitoring for stimulus detection in skin conductance acta imeko issn: 2221-870x september 2023, volume 12, number 3, 1 6 acta imeko | www.imeko.org september 2023 | volume 12 | number 3 | 1 low-cost monitoring for stimulus detection in skin conductance grazia iadarola1, valeria bruschi1, stefania cecchi1, nefeli dourou1, susanna spinsante1 1 department of information engineering, polytechnic university of marche, 60131 ancona, italy section: research paper keywords: skin conductance; galvanic skin response; stimulus detection; low-cost sensor; active assisted living citation: grazia iadarola, valeria bruschi, stefania cecchi, nefeli dourou, susanna spinsante, low-cost monitoring for stimulus detection in skin conductance, acta imeko, vol. 12, no. 3, article 10, september 2023, identifier: imeko-acta-12 (2023)-03-10 section editor: leonardo iannucci, politecnico di torino, italy received april 16, 2023; in final form june 2, 2023; published september 2023 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this research was funded by the european union – next generation eu. project code: ecs00000041; project cup: c43c22000380007; project title: innovation, digitalisation and sustainability for the diffused economy in central italy – vitality. corresponding author: susanna spinsante, e-mail: s.spinsante@staff.univpm.it 1. introduction several hardware technologies have been developed to integrate different physiological quantities into wearables, enabled with wireless connectivity according to the internet of things (iot) [1], [2]. wearables are often used with the aim of acquiring those parameters that evaluate the subject physiological status, from the physical and health perspective [3]-[8] to the cognitive and emotional one [9]-[11]. indeed, the connection of smart devices to networking systems can reduce unnecessary hospital visits and lower the burden on healthcare services, by transferring data over secure infrastructures. iot devices allow continuous remote monitoring of physiological parameters, thus supporting long-term care at home (active assisted living) [12]. while existing a rich offer of wearables embedding sensors to monitor heart-related parameters, not so many devices are available to collect skin conductance (sc) signals, despite several applications could benefit from them. for example, the possibility to classify different physiological conditions, such as pain or perspiration, by means of sc acquisition system allows a better management of anxiety among patients under treatments, students, workers, or people affected by health problems related to distress [13]-[16]. sc is defined in a seminal work by boucsein [17] as the phenomenon that the skin temporarily becomes a better conductor of electricity due to a change in sweat secretion, when either external or internal stimuli occur. thus, sc signals reflect the changes in electric conductance, which is modulated by sweat gland activity. in fact, an increase in sweating, mostly composed by water, increases the skin capability of conducting electric current. sweat secretion from eccrine glands (spread over the body skin) cannot be consciously controlled, because it is driven by the autonomic nervous system, whose activity is modified by the events involving a subject [18]. consequently, sc signals allow to investigate the relationship between physiological status and stimuli [19], [20]. sc variations may be elicited by the response to stimuli of different nature, such as visual, acoustic [21], [22] or physical ones [23], or by stressful conditions, such as driving [24]-[26]. the so-called skin conductance level (scl) or tonic level reflects the slow changes depending on skin dryness, hydration, and automatic regulation. instead, the skin conductance response (scr) or phasic level represents the dynamic changes associated to stimuli [27], [28]. abstract not so many consumer devices are available for minimally invasive monitoring of skin conductance (sc), differently from what happens for other physiological signals. in this paper, a low-cost monitoring system for sc signals is presented. for comparison purposes, the sc signals are simultaneously acquired by the low-cost monitoring system and by a procomp infiniti desk equipment. the paper shows that, despite the simpler design and hardware limitations exhibited by the low-cost system, the collected sc signals provide the same relevant information for stimulus detection of the sc signals acquired by a much more expensive acquisition board. specifically, the comparison is fulfilled through the ability of the low-cost monitoring system in detecting the increase of both sc baseline and peaks after stimulation. mailto:s.spinsante@staff.univpm.it acta imeko | www.imeko.org september 2023 | volume 12 | number 3 | 2 sc signals may be collected by endosomatic approach, meaning that no external source of electricity is used, or by exosomatic approach, in which electrodes in direct contact with the skin allow to apply a current source. in the exosomatic approach, the ohm’s law is exploited to compute the skin conductance (or resistance) by means of voltage values. despite the device design is simpler in the endosomatic case, the exosomatic approach is more common in wearables, as it does not require to collect the voltage difference between active and inactive sites of the skin. the availability of smart devices for stimulus detection in sc signals can enable several applications regarding human response to specific events [29]. the advantage of using smart devices to collect sc signals consists exactly in the possibility to run different types of experiments with greater flexibility, without bulky equipment, thus in real-life conditions [25]. in such a context, the current work investigates the operation of a low-cost and portable system for sc monitoring. specifically, the current work is intended to evaluate if a low-cost system, although exhibiting simpler design and hardware limitations, can be adopted for stimulus detection. to this purpose, the sc signals acquired by the proposed low-cost system are compared to the sc signals collected by a reference instrument, the procomp infiniti. the comparison is then carried out before and after acoustic stimulation, by analysing the reliability of the low-cost system in detecting event-related changes. the paper is organized as follows. section 2 describes in detail the proposed low-cost system for sc monitoring. section 3 presents, instead, the reference instrument for sc monitoring. the applied test procedure is depicted in section 4. the obtained results are discussed in section 5. finally, section 6 provides conclusive remarks. 2. low-cost system for sc monitoring monitoring for stimulus detection in sc signals is implemented in this study through a low-cost data acquisition system. the proposed low-cost system consists of the grovegsr sensor v1.2 [30], with its signal conditioning circuit, and the arduino uno board, equipped with the atmega328p microcontroller. figure 1 shows the low-cost system. the grove-gsr sensor collects voltage values modulated by a microcurrent, injected by means of two nickel electrodes worn in direct contact to the fingers through small velcro bands. moreover, a signal conditioning circuit, including the texas instruments lm324 operational amplifier, is connected to the electrodes. then, the grove-gsr sensor is connected by means of a 4-wire cable to the arduino uno board, which is useful for low power consumption. the quantity acquired by the low-cost system is the skin resistance (sr), which is the inverse of sc. the voltage signal is acquired at 121 hz and converted into 10-bit digital values, so that such values range from a minimum of 0 to a maximum of 1023. in order to remove glitches from the acquired voltage values, the code embedded into the firmware of the microcontroller computes the sr average over 5 voltage samples. finally, data are sent via universal serial bus (usb) interface from the output port of the microcontroller to a computer, where they are saved into a file. 3. reference system for sc monitoring generally speaking, only a few devices are commercially available to acquire sc signals (see table 1). they exhibit different operation modes and sampling frequencies. as shown in table 1, above all, the accessibility to raw data samples can represent a critical point, causing difficulties in extracting the information of interest. among the available commercial devices for sc monitoring, the procomp infiniti by thought technology is desk equipment considered as a reference system to validate other devices [31], figure 1. the low-cost system for sc monitoring.. figure 2. the procomp infiniti system with the tt-usb interface. table 1. comparison of commercial devices for sc monitoring. device measured value sampling frequency availability of original samples typology empatica e4 conductance 4 hz yes wearable empatica embrace conductance 4 hz yes wearable moodmetric conductance not provided no wearable shimmer3 gsr+ resistance 32 hz yes wearable procomp infiniti conductance 256 hz yes desk equipment biopac mp36r device conductance 500 hz yes desk equipment acta imeko | www.imeko.org september 2023 | volume 12 | number 3 | 3 due to the accuracy of the sc sensor. it consists of a data acquisition system and five channels for monitoring of several biosignals. specifically, the first two channels allow to acquire at a sampling frequency of 2048 hz faster signals, such as electroencephalogram, electrocardiogram, heart rate or blood volume pulse. instead, the remaining three channels are designated to monitor slower signals, such as respiration, temperature, and sc, at a sampling frequency of 256 hz. therefore, one of the last channels is employed for sc monitoring, with a signal input range of [0, 30] μs. in particular, the electrode strap must be fastened around the finger so that the electrode surface is in contact with the pad (but not so tight to limit blood circulation). furthermore, as shown in figure 2, the system comprises a fiber optic cable and a tt-usb interface. the procomp infiniti system encodes and transmits the data via the fiber optic cable to the tt-usb interface, which is in its turn connected to the usb port of the computer. lastly, the biograph infiniti software allows to export the measured values as .csv data files. 4. test procedure the stimulus detection in sc is evaluated on a population of ten subjects, six women and four men, aged between 23 and 49 years (the details are given in table 2). all the involved subjects declared to be in good health and signed an informed consent before participating in the experimental acquisitions. sc signals were simultaneously acquired by the low-cost system and the procomp infiniti. acquisitions were performed inside a laboratory, with subjects comfortably sitting on a chair. in order to simultaneously acquire the sc signals by both the systems on index and middle fingers of the dominant hand, as suggested in [32], the electrodes of the grove-gsr sensor were placed on the fingertips, while the electrodes of the procomp infiniti were placed on the medial phalanx of the fingers, as shown in figure 3. specifically, the electrodes of the low-cost system are located on the fingertips, as suggested in the literature [32], to favor the best possible contact between electrodes and skin, while the electrodes of the procomp infiniti were applied on the medial phalanx of the same fingers. this choice aims to compensate for the smaller amplitude of the signal obtained by the low-cost system, which includes components with lower amplification gain with respect to the procomp infiniti. in any case, as detailed in section 5, the comparison between the two considered systems is carried out by normalizing the amplitudes of the acquired signals. moreover, the subjects were asked to keep their arm in a relaxed position, with the palm of the hand resting on a desk to minimize involuntary movements and favor the contact of the finger skin with the electrodes. the aim of the paper is verifying if the stimulation can be identified in the sc signals collected by both the low-cost system and the procomp infiniti. for such a reason, only a single audio stimulus was considered. in detail, each acquisition session lasted less than 2 minutes (see figure 4). the sc baseline of each subject was firstly acquired for 60 s, in relaxed conditions and in the absence of any external stimulus. then, an audio clip of 6 seconds was played through computer loudspeakers, while the sc acquisition went on for 58 s. thus, the acquisition duration was 1 minute and 58 s overall. the audio clip was chosen with a short duration to elicit a reaction in the listening subject, without generating the so-called “habituation effect” [33]. the audio clip of 6 seconds was extracted from the international affective digitized sounds (iads) database [34]. the iads database is a set of 167 audio clips for experimental investigations on emotion and attention, where at least 100 listeners, equally divided in males and females, rated each sound in the dataset. specifically, the sound clip rocknroll 815 was used in our experiments, since it exhibits the highest average value of the pleasure score (8.13/9.00) over the whole dataset, as reported in the documentation available together with the dataset. the raw data acquired by the low-cost system are firstly converted into sr values according to (1) [30]: sr = (210 + 2 ∙ x) ∙ r / (29 − x), (1) where x is the serial port reading, i.e., the digitized value displayed on the serial port, ranging from 0 to 1023 (10-bits digital values), while r=104 ω is the resistance value of the resistor in the voltage divider circuit used to read the variable resistance of the sensor. thus, the sr data acquired by the low-cost system were saved as local .txt files on the computer, and then processed through matlab environment. in particular, the sc values are calculated as inverse of the sr values. instead, the sc signals acquired by the procomp infiniti are directly provided in siemens and do not need to be preprocessed to be comparable to the sc signals obtained by the low-cost system. the sc data acquired by the procomp infiniti were saved by the biograph infiniti software-sa7900 of thought technology as .csv files and processed through matlab environment. table 2. dataset population details. subject f/m age (years) s1 f 47 s2 m 34 s3 f 33 s4 m 48 s5 f 29 s6 f 40 s7 m 32 s8 f 31 s9 m 49 s10 f 23 figure 3. test setup for simultaneous acquisition of sc signals by the low-cost system and the procomp infiniti. figure 4. time sequence of the sc acquisition protocol. acta imeko | www.imeko.org september 2023 | volume 12 | number 3 | 4 5. results and discussion figure 5 and figure 6 show, for instance, the sc signals of the subjects s4 (male) and s10 (female), acquired by means of both the low-cost system and the procomp infiniti. in order to take into account the different electrode location for the two systems, as well as their different amplification components, the signals are shown with normalized amplitudes, allowing a fair comparison between them. it should be underlined that, looking at the signals in figure 5 and figure 6, clear peaks follow the stimulus event for both the subjects. in fact, as reported in [34], event-related peaks usually happen within 2.5 to 7.0 seconds after the stimulus. stimulus detection by the data acquisition systems is carried out by analyzing the sc signals of the ten subjects, before and after the acoustic stimulation, in terms of variation of scl average and scr peaks. for this reason, the sc signals are filtered, obtaining the two scl and scr components. as concerns the scl component, on the basis of the literature [36], an increase of positive fluctuations from relaxed condition to pleasant stimulation should be clearly evident in the baseline. in table 3, the results of the scl acquired by both the low-cost system and the procomp infiniti are reported before and after the acoustic stimulus. generally, the results provided by the lowcost system confirm the literature findings, as they highlight an increase in the scl average following the stimulus over most of the subjects (eight out of ten). furthermore, in one case out of ten (subject s3), the signals acquired by the two systems lead to different results, to the advantage of the low-cost system. in fact, an increase following the stimulus is recorded in the scl average by the only low-cost system. the data provided in table 4 refer to the analysis of the variation in the number of scr peaks detected before and after the stimulus, for each subject and by both the acquisition systems. for six subjects out of ten, the low-cost system detects an increase in the number of scr peaks following the acoustic stimulus, in accordance with the literature, as reported in [37], [38]. on the other hand, by analyzing the signals collected by the procomp infiniti, the peak increase is found only in four cases out of ten. therefore, the analysis on the acquired sets of signals proves that the low-cost system can contemplate the meaningful variations of the sc signals, as well as the procomp infiniti, taken as a reference device. these results agree with [39], confirming the possibility to implement a low-cost and portable system for sc monitoring aimed at stimulation detection. in fact, the signals collected by the prototypal system shown in this work allow to extract meaningful information in line with the results available in the literature and typically obtained by means of desk equipment, which cannot be suitable for scenarios requiring the mobility and freedom of movements for users. 6. conclusions measuring sc signals by means of minimally invasive and low-cost systems can enable the out-of-lab monitoring of subjects under different types of stimulation without bulky equipment, thus in real-life conditions. in this paper, a low-cost and portable monitoring system for stimulus detection in sc has been proposed. the performance of the proposed low-cost system has been evaluated in comparison to the performance of a reference desk equipment, the procomp infiniti, on ten subjects. the analysis on the sc signals, acquired before and after acoustic stimulation, highlights the capability of the low-cost system to provide their relevant variations, in the slowly varying sc component (scl), as well as in the rapidly varying one (scr). in fact, the increase of scl average and scr peaks after a) low-cost system b) procomp infiniti figure 5. sc signals with normalized amplitude of subject s4 (male) acquired by a) low-cost system, and b) procomp infiniti. a) low-cost system b) procomp infiniti figure 6. normalized amplitude sc signals of subject s10 (female) acquired by a) low-cost system, and b) procomp infiniti acta imeko | www.imeko.org september 2023 | volume 12 | number 3 | 5 stimulation is correctly monitored by the low-cost system, in accordance with the results obtained by means of the desk equipment of greater hardware capabilities. additional investigations aimed at generalizing the promising results obtained in this work will be performed as future work, increasing the dimension of the test population and possibly testing different types of stimulation. references [1] r. morello, f. ruffa, i. jablonski, l. fabbiano, c. de capua, an iot based ecg system to diagnose cardiac pathologies for healthcare applications in smart cities, measurement 190 (2022). doi: 10.1016/j.measurement.2021.110685 [2] k. aparna, d. h. dayajanaki, 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https://doi.org/10.4015/s1016237221500381 https://doi.org/10.1109/metroautomotive54295.2022.9854871 https://doi.org/10.3390/s23084004 https://dx.doi.org/10.1007/978-3-031-28663-6_5 https://doi.org/10.3390/s19081849 https://doi.org/10.1016/j.measurement.2022.111861 https://wiki.seeedstudio.com/grove-gsr_sensor/ https://doi.org/10.1007/978-3-030-97845-7_11 https://doi.org/10.1016/j.physbeh.2012.01.020 https://doi.org/10.1016/j.physbeh.2012.01.020 https://doi.org/10.1111/psyp.14058 https://doi.org/10.1080/19312458.2012.732627 https://doi.org/10.1016/j.jneumeth.2010.04.028 https://doi.org/10.2478/joeb-2018-0010 https://dx.doi.org/10.1007/978-3-031-28663-6_6 microsoft word article 17 175-1305-1-le.docx acta imeko  february 2015, volume 4, number 1, 111 – 120  www.imeko.org    acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 111  frequency‐domain  characterization  of  random  demodulation  analog‐to‐information  converters  doris bao, pasquale daponte, luca de vito, sergio rapuano  department of engineering, university of sannio, piazza roma, 21, 82100 benevento, italy    section: research paper   keywords: analog‐to‐information converter, compressive sampling, testing, frequency domain.  citation: doris bao, pasquale daponte, luca de vito, sergio rapuano, frequency‐domain characterization of random demodulation analog‐to‐information  converters, acta imeko, vol. 4, no. 1, article 17, february 2015, identifier: imeko‐acta‐04 (2015)‐01‐17  editor: paolo carbone, university of perugia   received january 14 th , 2014; in final form april 4 th , 2014; published february 2015  copyright: © 2014 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: (none reported)  corresponding author: luca de vito, e‐mail: devito@unisannio.it  1. introduction  high-speed data acquisition is becoming a relevant topic in advanced applications, such as high-speed radar and communications, signal analysis, high-speed video acquisition, and so on. moreover, it is a relevant challenge in wideband spectrum sensing for software defined radio and cognitive radio applications [1], [2]. such demand often is not met by traditional analog-todigital converters (adcs), due to technological limits in fast sampling rates [3]. the recent studies about compressive sampling (cs) drew a possible solution for signals, that can be represented by a finite number of non-zero elements in a specific domain. they demonstrated that, for such class of signals, it is possible to reconstruct the original waveform, from a set of samples of a lower dimension than that required by the shannon theorem. the idea, underlying the aic, is to spread the frequency content of the input signal. in this way, the high frequency components, folded back to low frequencies, can be acquired by an adc with a lower sampling frequency than that required by the shannon's theorem for the original signal. basing on this concept, different architectures have been proposed, implementing the frequency spreading by exploiting: (i) non-uniform [4] or random sampling [5], (ii) random filters [6], and (iii) random demodulation [3]. the aim of the paper is to define performance parameters and test methods for aics, starting from the state of art of research and the scientific knowledge about adc testing, well summarized in [7] and [8]. to this aim, the first step is the application to aics of standard parameters and test methods, actually defined for adcs in order to study how they are influenced by (i) the aic architecture type, (ii) the aic design parameters, and (iii) the circuit non-idealities. abstract  the paper aims at proposing test methods for analog‐to‐information converters (aics).   in particular, the objective of this work is to verify if figures of merit and test methods, currently defined in standards for traditional  analog‐to‐digital converters, can be applied to aics based on the random demodulation architecture.  for this purpose, an aic prototype has been designed, starting from commercially available integrated circuits. a simulation analysis  and an experimental  investigation have been carried out to study the additional  influencing factors such as the parameters of the  reconstruction algorithm. results show that standard figures of merit are in general capable of describing the performance of aics,  provided  that  they  are  slightly  modified  according  to  the  proposals  reported  in  the  paper.  in  addition,  test  methods  have  to  be  modified in order to take into account the statistical behavior of aics.  acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 112  in the scientific literature few papers can be found, facing the aic testing and most of them take into account only a reduced set of figures of merits (foms) and influencing parameters [9]. in [10], the authors presented a preliminary investigation carried out in simulation and on a first aic prototype, based on a digital oscilloscope, about the application of standard adc foms on the aic. this paper is an extended version of such work, in which new results are presented and a new aic prototype is used, based on commercial integrated circuits. as in [10], the aic architecture considered in this paper is based on the random demodulation, as it does not require a high sampling frequency adc. however, test methods and considerations can be easily extended to the other types of aic architectures. in particular, in the paper, a characterization of a random demodulation aic has been carried out by applying a reconstruction algorithm to the aic output and evaluating the dynamic parameters in the frequency domain, defined for adc testing. to this aim, in a former phase, a behavioural model of the random demodulation aic has been simulated, by considering the non-idealities introduced by its main building blocks. in a latter phase, an aic prototype has been designed, by following the theoretical descriptions found in literature, and an experimental analysis has been conducted on it. the paper is organized as follows: in section 2, an introductory description about compressive sampling theory is given; in section 3, the random demodulation architecture, which was used both in the simulation and the experimental analyses of this work, is described; in section 4, the approach followed for aic testing is explained; then, in section 5, a simulation phase is reported, in which the influence of several factors, such as circuit non-idealities, aic design parameters and reconstruction algorithm parameters, have been investigated. in section 6, the foms defined for traditional adcs are revised. finally, in section 7, the experimental analysis is presented and results are discussed. 2.  theoretical  background  the idea underlying cs approach is that many natural signals have concise representations when expressed in a convenient basis [11]. as an example, audio signals have sparse representations in the short-time fourier transform domain, or in the modified discrete cosine transform domain [12]. another example is given by radar echo signals, that, depending on the radar signal type, can have sparse representations in the time, frequency, wavelet, or time-frequency domains [13]. sparse representations of natural signals, audio, images and videos are currently exploited by transform coding schemes, such as those used by the jpeg, jpeg2000, mpeg, and mp3 standards. however, in signal compression, signals are acquired using nyquist rate converters, then they are transformed in a proper domain, where less significant coefficients are discarded. cs, instead, aims to acquire directly the compressed version of the signal, without wasting acquisition or memory resources, by taking a vector y of observations of the signal to be acquired, where the size of y is lower than that required by the shannon theorem to digitize the signal. in the past, some other techniques have been proposed to overcome the shannon theorem constraints in some specific conditions. the equivalent time sampling of time domain signals is an example of such techniques. however, equivalent time sampling requires the observed portion of the signal to be repetitive. cs, instead is applicable even to a non-repetitive signal, providing that a domain can be found, where the representation of such signal is sparse. for a compressible signal x(t), if x is a the vector of n samples of it acquired according to the shannon theorem, its compressed counterpart is represented by a vector y of size m < n, such that: ,= φxy (1) where,  is a matrix modelling the compression process. it can demonstrated that an estimate of x can be reconstructed from y according to (1) if a matrix transformation  exists, such that: ,= ψcx (2) where c has only k < m non-zero elements. the above defined condition is not rare in reality, since many natural signals are sparse or compressible in the sense that they have concise representations when expressed in the proper basis. by combining (1) and (2), the following expression is obtained: ,= acy (3) where a =  is an m × n matrix and, therefore, (3) is an under-determined linear system in c. the system can be solved by finding the solution of (3) that minimizes the ℓ0 norm, that is having the highest number of non-zero elements in c:   .subject toˆ 0 acycc =argmin= (4) the minimization of the ℓ0 norm is both numerically unstable and np-complete, requiring an exhaustive enumeration of all       k n possible locations of the non-zero entries in c [14]. therefore, the solution is approximated with that obtained by the minimization of the ℓ1 norm:   .subject toˆ 1 acycc =argmin= (5) in presence of additive noise e, equation (3) becomes: eacy += (6) and the minimization problem is modified as:   .subject toˆ 21 τ<argmin= acycc  (7) or as:     .subject toˆ 1 ε<argmin=   acyacc t (8) acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 113  where and are small positive constants. the ℓ1 norm minimization is a convex optimization problem that conveniently can be reduced to a linear program known as basis pursuit, whose computational complexity is about o(n 3) [14]. the minimum number of observations m required to successfully reconstruct x is given by the following expression [11]: ( ) ( )log 2m cμ , k n , ³ × (9) where, c is a positive constant and ( )μ ,  is a quantity, called coherence between  and , having the following expression: ( ) max i j 1 i, j n μ , n φ ,ψ ,  £ £ = (10) with i and j the i-th row of and the j-th column of respectively, and , the dot product. the coherence measures the largest correlation between any two elements of  and ; if  and contain correlated elements, the coherence is large; otherwise, it is small [11]. by looking at (9), it can be easily seen that the lower is the coherence between  and , the fewer observations are needed to reconstruct the original signal. it can be demonstrated that incoherence can be achieved with high probability simply by selecting as a random matrix [14], whose elements are drawn from a suitable distribution, such as gaussian, bernoulli or rademacher. 3.  the  random  demodulation  aic  architecture  the block scheme of the random demodulation aic is shown in figure 1. considering an adc with sampling frequency fs, and a signal x(t), whose frequency content exceeds the first nyquist region       2 0, s f . an example of a three-tone signal having such characteristics is shown in figure 2a. according to the shannon theorem, it would not be possible to sample x(t), since the components located at frequencies greater than fs /2 would produce aliasing. it can be observed that for such signal the condition (2) is verified as soon as x has a fourier representation with a limited number of non-zero coefficients. in that case, in fact, (2) is simply the fourier synthesis formula. therefore, first, a compressed version of x is obtained, and, then, the signal could be reconstructed by solving one of the problems (5), (7) or (8). in order to obtain the compressed version of x(t), it is mixed with a pseudo-random binary sequence (prbs), having a bit rate equal to fp = fs , with positive integer greater than 1. the bit rate of the prbs determines the maximum frequency component of the original signal that the aic is able to acquire, as it is required to be at least twice the maximum frequency component of the signal. the spectrum of the prbs, shown in figure 2b, has a frequency content spread over the whole region  ,f p0, therefore, the output of the mixer contains the information of the original signal, spread over the whole first nyquist band, even if it was originally located outside such region (figure 2c). this allows that after low-pass filtering (figure 2d) and a/d conversion (figure 2e), the information contained in the original signal has not been lost. however, it needs to be recovered from the filter output. a reconstruction algorithm is, then, in charge of extracting this information, by solving one of the problems in (5), (7) or (8), as previously quoted, giving thus a digital representation of the original signal (figure 2f).   figure 1: block scheme of the random demodulation aic.    figure 2. spectra of the signals processed inside the aic structure. the aic is  able of reconstructing a signal even if its spectral content exceeds the first  nyquist region.  acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 114  the capability of the reconstruction algorithm of actually recovering the original signal with high probability depends on the undersampling factor , and on the sparsity rate k / m. a performance analysis for a random demodulator using an integrate and dump filter has been carried out in [15], and the results showed that a high probability of reconstruction could be achieved if:        1log1.7 + k n km (11) therefore, the aic is not able to preserve the information contained in the signal if undersampling is below a certain value, depending on the number of non-zero element in the sparse representation of the signal to be acquired. the architecture can be extended by using several parallel channels, each driven by a different prbs. in this case, the aic is capable of acquiring signals even in the case undersampling causes the overlapping of several signal components, since in each channel, the overlapping appears in different form [16]. for sake of simplicity, in this paper the analysis has been conducted on a single channel architecture. however, results can be easily extended to a multi-channel architecture. a mathematical model of the aic can be obtained as it follows. indicating with x the vector of the signal samples, if it was acquired using a sampling frequency equal to the bit rate of the prbs fp, and with  11,0,  np,pp=p the samples of the prbs, the output of an ideal digital mixer m can be obtained by multiplying x by a diagonal matrix, having the prbs samples on the diagonal: dxm = (12) where:                 1 1 0 00 00 00 np p p=    d (13) instead, the filter can be modelled by the following toeplitz matrix:                    12 121 121 00 00 00 ll ll l hh hhh hhh=    h (14) where,  tlhhh= 110 h is the truncated impulse response of the antialiasing filter of length l. l should be chosen large enough that the truncated response guarantees a good approximation of the actual one. the random demodulation aic can be obtained by choosing a  matrix that decimates the matrix product h d, by taking only 1 every rows. 4.  approach  to  aic  testing  aic testing is a harder task than traditional adc testing, due to the following reasons: (i) the aic output signal has a strong stochastic behaviour, due to the prbs (compare figs.2a and 2e); (ii) the aic output signal is noiselike for every input, thus, it is hard to analyse either in the time or in the frequency domain; and (iii) the parameters of the reconstruction algorithm can affect the aic results. the current research has been focused on frequencydomain testing, as it is mostly used in telecommunications, which is the main application field of aics. the approach followed for aic testing consists of using a sinewave signal as input to the aic, applying a reconstruction algorithm, and characterizing the spectrum of the reconstructed signal, by means of the traditional adc foms. test signal frequencies have been chosen such that the coherent sampling condition is respected for the reconstructed signal, that is they should verify the following condition: π, n j =ω 20 (15) where, j is an integer number, relatively prime to n. the adc testing methods in the frequency domain can be clearly applied to aic as the involved signals are sparse in such domain. as sparsity in the frequency domain has been considered, the matrix in (2) must represent a transformation between time domain and frequency domain. however, since only real-valued test signals have been considered, the discrete hartley transform (dht) basis has been used, which is equivalent to the discrete fourier transform, but is purely real. the generic element i,j of the dht has the following form: .ij n π +ij n π n =ψ ji,                   2 sin 2 cos 1 (16) the dantzig selector [17] has been used as reconstruction algorithm, solving the optimization problem in (8), by means of the implementation given by the ℓ1 -magic matlab toolbox [18]. differently from problem (7), which looks for the solution in the least square sense, the dantzig selector computes the residual r = y – a c, and, then, evaluates its inner products with the columns of a. these inner products form various weighted combinations of the entries in r and all those are expected to be small [19]. the constraint, thus, proceeds by requiring that all these are below the threshold . therefore, the parameter represents the maximum allowable value for the correlation of the residual r with any column of a. in a preliminary phase, the aic has been modelled and some simulations have been carried out in order to understand how the foms, evaluated on the reconstructed signal, can be affected by: (i) circuit non-idealities, (ii) test signal parameters, and (iii) reconstruction algorithm parameters. acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 115  according to the results of the simulation analysis, the following foms, have been extended to aics: spurious free dynamic range (sfdr), signal to noise and distortion ratio (sinad), and total harmonic distortion (thd). for such foms, the definitions in [20] have been used. however, the same approach can be used even considering definitions coming from different standards, such as those considered in [21]. finally, an experimental phase has been conducted by evaluating the revised foms on a specifically designed aic prototype. 5.  simulation  analysis  during simulation analysis, an explicit behavioural model [22] has been considered for each aic block. in figure 3, the steps carried out for the simulation analysis have been drawn. sinewave signals have been generated, with frequency in the range [0.05-0.45] , with a record length of n=512 samples. signal records are then given to a mixer model, together with prbs records of the same size. as any actual mixer will distort the signals, the ideal mixer output yid = x1(t) x2(t) (see eq. 12) has been replaced by a second order polynomial model, as follows:              tdx+tcx+txtx+tbx+tax=ty 22212121 (17) where, x1(t) and x2(t) are the inputs to the mixer, respectively, and a, b, c and d are attenuation coefficients. the output of the mixer is given to a lowpass fir filter, with cut-off frequency equal to /4, modelling the antialiasing filter. the adc, embedded in the aic architecture, has been modelled as a memoryless nonlinearity followed by a downsampling by 4 and an ideal quantizer, where the nonlinearity has been obtained, as in [9], using the following third order polynomial function:       3 3 3 1 c+ txc+tx =ty (18) due to downsampling, the record length is reduced to m=128 samples. the output of the adc model is given to the reconstruction algorithm, able of regenerating a record of n=512 samples containing the reconstructed signal. finally, the foms are evaluated on the averaged spectral magnitude of the reconstructed signal. averaging of spectral magnitude is specified by [20] for the evaluation of the foms in the frequency domain. however, as it is shown in section 5.1, a greater average number is needed than in the case of adcs. the above described steps have been executed on several signals, when the parameters of the models have been varied as shown in the following subsections. figure 3: block scheme of the simulation analysis.           a)  b)      c)  d)  figure 4. spectral  magnitudes obtained by the  fft magnitude on a single  record (a), or by averaging the fft magnitudes evaluated on 10 (b), 100 (c),  and 200 (d) records.             a)  b)  figure 5. averaged spectral magnitudes of the reconstructed signal without  mixer nonlinearity (a), and with mixer nonlinearity (b).  acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 116  influence  of  average  number 5.1. in figure 4, the spectral magnitude is shown, when it is obtained by the fft (fast fourier transform) magnitude of a single record (figure 4a), or when it is obtained by averaging fft magnitudes evaluated on different records. results are shown for a number of averages equal to 10 (figure 4b), 100 (figure 4c), and 200 (figure 4d). as it can be noted, due to the selection of frequency components operated in the reconstruction algorithm, some of them are not identified in a single record. instead, the averaging allows obtaining an estimation of the spectral magnitude for each frequency component. such behaviour is more evident for higher values of , since in this case a higher number of components are discarded by the reconstruction algorithm.  influence  of  mixer  nonlinearity 5.2. the main effect of the mixer nonlinearity is of increasing the level of the noise floor, as shown in figure 5, where the averaged spectral magnitudes of the reconstructed signal when no mixer nonlinearity has been modelled (a=b=c=d=0) (figure5a), and when a nonlinearity is added, with the following values of the attenuation coefficients: a=b=0.1, c=d=0.01 (figure5b). those values have been chosen by looking at the order of magnitude of the typical feedthrough and second order distortion component attenuation for actual mixers. in addition, a particular increase of the level of the components, whose frequencies lie at multiples of the fundamental, has been observed.  influence  of  adc  nonlinearity 5.3. the effect of adc nonlinearity can be observed in figure 6, where the averaged spectral magnitudes of the reconstructed signals are reported in the case the c3 parameter is set to 0 (figure 6a), simulating a linear adc, and when the c3 parameter is set to 0.2 (figure 6b). differently from traditional adcs, the nonlinearity does not act only on a restricted number of components, located at multiples of the fundamental, in fact it causes even an increase of the noise floor. influence  of  adc  quantization 5.4. the adc quantization causes an increase of the noise floor level. as it can be seen in figure 7a, the signal is perfectly reconstructed (with noise floor level depending only on the machine precision) when maximum machine resolution is used. instead, a noise floor arises when quantization is used. however, only a small difference in the level has been observed for different resolutions. during simulation analysis, values of the resolution in the set {8, 10, 12, 14, and 16} bits have been used. in figure 7b, the averaged spectral magnitudes, related to a resolution of 8 bits and 16 bits are shown.  influence  of  reconstruction  parameter   5.5. as the reconstruction parameter  represents a limit to the amount of additive noise that can be tolerated on the output signal, it controls the selection of the frequency components in the reconstructed signal. in fact, for each record, a frequency component will be included in the reconstructed signal, only if its energy exceeds a certain level, depending on the value of . on the averaged spectrum, this causes an increase of the level of both the noise floor and of the harmonics, for decreasing values of . such behaviour can be observed in figure 8a, where the averaged spectral magnitudes of the reconstructed signal are shown, where has been set to 0.01, 0.05, and 0.1.  influence  of  the  filter  length 5.6. the length of the impulse response of the anti-aliasing filter determines the capability of the filter of attenuating the aliasing replicas. the shorter the filter impulse response is, the lower such capability is. figure 8b highlights this behaviour, by showing the averaged spectral magnitudes of the reconstructed signals, when filter lengths of 17 and 49 taps are used. as it can be noted, in the case of a filter length of 17 (blue line), the filter is not able to adequately attenuate the aliasing replicas, located at the frequencies          a)  b)  figure 6. averaged spectral magnitudes of the reconstructed signal without  adc nonlinearity and with a nonlinearity coefficient c3 equal to 0.1, 0.2, and  0.3, respectively.           a)  b)  figure  7.  averaged  spectral  magnitudes  of  the  reconstructed  signal  when  infinite resolution is used (a), and when a quantization of 8 bit or 16 bit is  used (b).  (22) acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 117  0, 0 2 2π π ω +ω        . in the case the filter length is of 49 taps (red line), instead, the aliasing components are well attenuated under the noise floor. 6.  definition  of  the  figures  of  merit  as previously mentioned, the foms used in this work have been defined starting from those of the ieee std. 1241-2010 [20]. however, the following modifications have been introduced: (i) the number of averages of the spectral magnitudes has been set to 200, in order to take into account the stronger stochastic behaviour of the reconstructed signal, and (ii) the frequencies 0, 0 2 2π π ω +ω        have been included in the list of the harmonics, contributing to the thd. the foms have been evaluated by averaging the spectral magnitude   mx as it follows [20]:      1,0,1,2,1 1  n,=m,mxr=mx r =k kavm (19) over a number of r data records. compared to definitions in [20], no changes have been made to the sfdr and sinad:                 mxmax nx =sfdr avm sm iavm 0 1020log (20)           0 2 22 10 3 20log sm avm iavmiavm mx nnx+nx n n =sinad (21) where, ni is the frequency index corresponding to the fundamental component, s0 is the set of frequency indexes from 1 to n -1, excluding the two values corresponding to the fundamental and its image. instead, thd has been modified as shown in (22), where, nh is the number of the highest harmonics considered, mod(∙,∙) is the modulo operator, and nh is the frequency index corresponding to a generic harmonic or its aliasing component:    hih n,±=hn,hnmod=n 2,3, (23) in the considered case nh has been chosen equal to 10. 7.  experimental  investigation  for the purpose of the experimental investigation, an aic prototype has been designed, using commercially available circuits and bench instrumentation. in the following subsections, the aic prototype is described, then, some details, about the calibration phase needed to estimate the  matrix, are given; finally, the experimental results are presented.  the  aic  prototype 7.1. a block scheme of the aic prototype is shown in figure 9. the mixing process is carried out by means of the analog devices ad8342 evaluation board, while the antialiasing filter has been realized by the first channel of the analog devices adrf6510 evaluation board. the adrf6510 is a dual-channel programmable lowpass filter, with cut-off frequency tunable in the range [1-30] mhz.   figure 9. block scheme of the aic prototype and the test setup, used during experimental analysis.         a)  b)  figure  8.  averaged  spectral  magnitudes  of  the  reconstructed  signal  for  values of  equal to 0.01, 0.05, and 0.1. (a), respectively, and the averaged  spectral magnitudes of the reconstructed signal, for filter  lengths equal to  17 and 49 taps (b).  acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 118  after mixing and filtering, signal is acquired by means of an analog devices ad9230 evaluation board, operating at a sampling frequency of 50 ms/s. the adc board is connected with a computer through an analog devices hsc-adc-evalc capture board. for the test purpose, sinewave signals have been generated by an agilent e4438c vector signal generator, while the prbs is generated in the computer and downloaded, through the gpib, to the tektronix awg420 arbitrary waveform generator. an agilent e8663b is used to generate the sampling clock for the adc board. in order to synchronize the prbs generation with the acquisition, the hsc-adc-evalc capture board has been configured with a specific firmware allowing external triggering. in this case, the acquisition is started on the computer, causing a ready signal to be set on the hscadc-evalc capture board. such ready signal is given both to the tektronix awg420, to generate the prbs and again to the hsc-adc-eval capture board to start the acquisition. the agilent e4438c, the tektronix awg420 and the agilent e8663b have been then synchronized by means of a 10 mhz reference, in order to guarantee the coherent sampling condition, and controlled by the computer by means of a gpib interface.   matrix  estimation 7.2. in order to run the reconstruction algorithm, it is necessary to estimate the  matrix in (1). since it is obtained from the prbs and from the impulse response of the anti-aliasing filter through (13) and (14), only the estimation of the impulse response of the anti-aliasing filter is necessary. this is done in a preliminary calibration phase, by connecting the tektronix awg420, generating a prbs, directly to the input of the filter. then, the output of the filter is acquired. as shown in [23], if the input to a system x(t) is a stochastic process, having impulsive autocorrelation function, the impulse response of the system can be measured as:       , r nr =nh xx xy 0 (24) where, rxy(n) is the cross-correlation function between the input and the output and rxx(0) is the autocorrelation of the input, evaluated at a lag equal to 0, corresponding to the input signal energy. since in the considered case the generated prbs signal has impulsive autocorrelation, it is possible to estimate the impulse response of the filter using (24).  experimental  results 7.3. using the aic prototype and the test setup shown in figure 9, a wide experimental investigation has been carried out, by evaluating the sfdr, sinad, and thd, on the reconstructed signal. as in simulation tests, sinewave input signals have been used. in order to guarantee the coherent sampling condition, the test signal frequency has been selected, using the following expression: pf n j =f 0 (25) where, fp is the prbs bit rate, corresponding to the equivalent sampling frequency, and j is an integer coprime to n. sinewave signals have been generated with frequencies in the range [5 95] mhz and prbs bit rate has been fixed to 200 mhz. different values of the cut-off frequency of the antialiasing filter have been used in the range [10 30] mhz, while the sampling frequency of the adc embedded in the aic architecture has been set to 50 ms/s. since the band of the reconstructed signal is in the range [0-100 mhz], and the nyquist frequency of the adc embedded in the aic is 25 mhz, the aic allows extending 4 times the signal bandwidth, compared with the case of the adc embedded in the aic architecture, running at the same sampling frequency. some results of the experimental investigation are shown in figure 10, for a test signal frequency in the range [5-95] mhz, with a step of 5 mhz. the variations of the values of all three foms are in the range of some dbs, even beyond the first nyquist band [0-25] mhz, however, performance show a little decrease, in sfdr and sinad, for frequencies approaching 100 mhz.                   figure 10. experimental results for test signal frequency ranging from 5 to  95 mhz: sfdr (a), sinad (b), and thd (c), for =0.5.  acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 119  it is worth studying the variation of the foms versus the reconstruction parameter , as well as versus the bandwidth of the anti-aliasing filter. as shown by the simulation results, the levels of both the noise floor and the harmonic components decrease with , therefore, an increase in the values of sinad and sfdr is expected, while thd is expected to decrease. this can be easily observed in figure 11, where the trend of the considered foms versus  is shown. in figure 11, the variation of the foms versus the bandwidth of the anti-aliasing filter is shown, too. it can be observed that the values of sinad and thd are generally better for a filter with a wider band. this is probably due to the fact that a greater amount of information passes through the filter and can be used for the reconstruction. in the case of the sfdr, instead, smaller values have been observed with the narrowband filter, and < 1. as it can be seen in figure 12, this due to the aliasing components at fs f0 and fs + f0, that are not attenuated under the noise floor level, by the 20 mhz bandwidth filter. in the case of a 20 mhz bandwidth, in fact, the noise floor level is lower, but the aliasing component are higher than the case of a 14 mhz bandwidth. 8. conclusions  and  further  work  in this paper, a frequency-domain characterization of aics has been presented, by concentrating the attention on the random demodulation architecture. in a former phase, the aic has been simulated by means of explicit behavioural models of its main components, in order to evaluate the influence of the circuit non-idealities, the design parameters and the reconstruction algorithm to the foms, currently defined for adcs. the simulation results show that both mixer and adc nonlinearities contribute to increase the noise floor level, as well as the level of the harmonic components. this is quite different from what happens in traditional adcs, where nonlinearities do not affect the noise floor level. in addition, the antialiasing filter can affect the sfdr, as it can not completely attenuate the aliasing components. as result of the simulation analysis, a modification to the thd definition is proposed by including the energy of the aliasing components. in a latter phase, an experimental analysis has been conducted on a specifically designed aic prototype, using sinewaves, by varying the test input frequency, the reconstruction parameter , and the bandwidth of the antialiasing filter. such analysis showed that the proposed foms are in general capable of describing the performance of the aic. however, the foms should be observed for different values of , since high values of can hide the effects of noise and distortion. further work will be directed to analyse the intermodulation distortion as frequency domain foms. then, it can be interesting to observe the foms even evaluated in the time domain. in addition, the methodology presented in this paper can be extended to other aic architectures, such as those based on the random sampling. finally, an alternative approach to aic testing can be used, without relying on the reconstruction algorithm, but directly comparing the aic output, with the expected output of the aic model.   figure  11. evaluated foms versus  and filter bandwidth, for a test signal  frequency of 10 mhz.    figure 12. averaged spectral magnitude of the reconstructed signal for filter  bandwidth equal to 14 and 20 mhz.  acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 120  references  [1] l.de vito, “methods and technologies for wideband spectrum sensing”, measurement, vol.46, no.9, nov.2013, pp. 3153–3165. 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[6] j.a.tropp, m.b.wakin, m.f.duarte, d.baron, r.g.baraniuk, “random filters for compressive sampling and reconstruction”, proc. of ieee int. conf. on acoustics speed and signal processing, toulouse, france, may 2006, vol.3, pp. iii–872–iii–875. [7] s.rapuano, p.daponte, e.balestrieri, l.de vito, s.j.tilden, s.max, j.blair, “adc parameters and characteristics part 6 in a series of tutorials in instrumentation and measurement”, ieee instrum. meas. magazine, vol.8, no.5, 2005, pp. 44–54. [8] t.e.linnenbrink, j.blair, s.rapuano, p.daponte, e.balestrieri, l.de vito, s.max, s.j.tilden, “adc testing part 7 in a series of tutorials in instrumentation and measurements”, ieee instrum. meas. magazine, vol.9, no.2, 2006, pp. 39–49. [9] z.yu, j.zhou, m.ramirez, s.hoyos, b.m.sadler, “the impact of adc nonlinearity in a mixed-signal compressive sensing system for frequency-domain sparse signals”, physical communication, vol.5, no.2, june 2012, pp. 196-207. 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[23] k.j.keesman, “system identification: an introduction”, springer-verlag, london, 2011. template for an acta imeko event paper acta imeko issn: 2221-870x july 2017, volume 6, number 2, 59-64 acta imeko | www.imeko.org july 2017 | volume 6 | number 2 | 59 torque transducer with check standard combination tassanai sanponpute and chokchai wattong torque laboratory, mechanical department, national institute of metrology (thailand), thailand section: research paper keywords: torque transducer; check standard; intermediate check citation: tassanai sanponpute and chokchai wattong, torque transducer with check standard combination, acta imeko, vol. 6, no. 2, article 10, july 2017, identifier: imeko-acta-06 (2017)-02-10 section editor: min-seok kim, research institute of standards and science, korea received june 30, 2016; in final form june 1, 2017; published july 2017 copyright: © 2017 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: this work was supported by the national institute of metrology (thailand) corresponding author: tassanai sanponpute, e-mail: tassanai@nimt.or.th 1. introduction the calibration status of a torque transducer used in any permanent laboratory or an on-site area shall be periodically confirmed by means of intermediate checks, which is one of the iso/iec 17025:2005 requirements [1]. an intermediate check activity cannot be replaced by calibration. it is an extra activity that should be done during a calibration interval and after using the torque transducer in an uncontrolled area. usually the intermediate check will have a procedure to compare the measurement results with the check standard, which can be another torque transducer. the frequency of the intermediate check basically depends on the number of uses and a torque transducer with high stability must be used as a check standard. the problem is that an intermediate check is a time-consuming process. we intend to reduce the working time for the intermediate check by introducing a new transducer design. torque transducers giving two signals at the same time were designed in order to confirm the calibration status during the measurements. we studied this concept by fabricating the prototype 50 n·m torque transducer with a check standard combination [2]. during the measurements, an intermediate check was also carried out. when torque is applied the transducers, two measurement and check signals are produced at the same time. thus, the comparison between measurement and check signals could be done as a basic principle of the intermediate check. a prototype is shown in figure 1. the ratio of check and measurement signals was calculated using (1), hereafter named the function ratio. it was used to predict the stability or sensitivity change of the transducer. signal tmeasuremen signalcheck = ratio function . (1) the results expressed that the trend of the function ratio was consistent with the trend of stability measured by using a torque standard machine (tsm). thus, the fabricated transducers can simultaneously measure torque and carry out an abstract the aim of this study was to fabricate a torque transducer that has two measurement signals in order to simultaneously measure torque value and check its stability. the transducer has two sensing bodies aligned in the same measurement axis. the first sensing body was designed to be a measurement signal and the second sensing body was designed to be a check signal. the capacity of the check signal was designed at least two times larger than that of the measurement signal. transducers with various capacity ratios of check signal and measurement signal with ratios 2:1, 3:1 and 4:1 were fabricated. to confirm the assumption that the fabricated transducers are able to check stability by themselves, they were overloaded from 100 % to 800 % of their capacity. the experimental results showed that the function ratios of the check signal and measurement signal corresponds with the stability measured by a torque standard machine. thus it can be used to predict stability. stabilities of the check and zero signals are used to find the appropriate capacity ratio. it was found that the stability of check and zero signals showed drift caused by overload. therefore, the capacity ratio should be designed at least 3:1 for transducers made of steel and 2:1 for transducers made of aluminum. a prototype of a torque transfer wrench was also made according to the concept of the torque transducer with check standard combination. experimental results corresponds with results of the torque transducer. it could also check the stability itself. acta imeko | www.imeko.org july 2017 | volume 6 | number 2 | 60 intermediate check by itself. however, this previous research just focused on the preliminary feasibility study. there was no information of an appropriate ratio of check and measurement capacities (hereafter named capacity ratio), zero signal drift and material. this research aims to find out the appropriate capacity ratio, zero signal drift and material to gain a more precise prediction of the measurement stability for practical uses. 2. experimental plan this research started from the design and structure of torque transducers with a capacity ratio between 2:1, 3:1 and 4:1 and made of two different materials: steel and aluminium. two signals, a check and a measurement signal of the fabricated transducers were measured by torque capacity with the tsm. at the time of the measurement, the zero signal was also recorded. then the boundary line of stability was evaluated based on three times the standard deviation. additionally, the function ratio was calculated. to prove how precise the function ratio can predict the stability change of the transducer, it is needed to demonstrate an influence quantity which causes a stability change of the transducer. overload is one of the obvious factors and can quickly create a disturbance. moreover, it is clearly observable. in this study an experimental plan with overload test is presented. previous research [2] showed that a 200 % overload test cannot cause a significant change in stability of the fabricated transducer. therefore, this continuing study aims to extend the overload test up to 800 % where a stability change and zero signal drift can be clearly observed. other influence quantities that affect the stability of the torque transducer were investigated in this research. the overload test started from 100 % to 800 % of capacity and after each overload, the stability of the transducer, the zero signal drift and the function ratio were evaluated. finally, the changed torque sensitivity caused by overload was compared with the function ratio to evaluate the effectiveness of the stability prediction. additionally, an appropriate capacity ratio was evaluated based on the check signal and zero signal drift. 3. design and fabrication 3.1. sensing body design the torque transducer with check standard combination was designed to have two sensing bodies aligned in the same measurement axis so that they undergo the same amount of torque. as per conceptual design, the sensing body of the check signal must be stronger than the body of the measurement signal and consequently the capacity of the check signal will be larger than that of the measurement signal. during the measurement, an intermediate check was also carried out. the conceptual operation is illustrated in figure 2. the sensing body was designed as a cylindrical shape. details of the design including verification with finite element models are given in [2]. this research aimed to discover the effect of the material used to make the sensing body. tool steel grade dc53 [3] and aluminium grade ai 6061-t6 [4] were selected for the fabrication of the transducers. all four transducers were made with different capacity ratios as described in table 1. 3.2. full bridge strain gauge circuit a biaxial strain gauge model fct-2-350-c2-11 manufactured by tml [5] was used in a full bridge strain gauge circuit. the technical specifications were as follows: 2 mm gauge length, 2.07 ± 1 % gauge factor and 350 ω resistance. so, the full bridge circuit was made with nominal sensitivity 2.07 mv/v at 0.001 m/m strain rate. the circuit is illustrated in figure 3. the strain gauges were mounted at positions 0° and 180° along the cylindrical sensing body. however, no temperature compensation and zero balance were included in this circuit [6]. figure 1. prototype of the 50 n·m torque transducer with check standard combination. figure 2. the conceptual operation of the transducer with check standard combination. table 1. details of the designed transducer. model capacity ratio measurement capacity (n·m) check capacity (n·m) material 01 2:1 50s 2:1 50 100 tool steel 02 3:1 30s 3:1 30 100 tool steel 03 4:1 50a 4:1 50 200 aluminum 04 2:1 100a 2:1 100 200 aluminum test signal measurement signal check signal acta imeko | www.imeko.org july 2017 | volume 6 | number 2 | 61 the four completed transducers are shown in figure 4. 4. results and discussion 4.1. stability at normal condition the stability of the transducers was monitored over a period of 60 days. the stability of the measurement signal, the check signal and zero signals are shown in terms of variations from the normal condition. all transducer gave similar results. there were just slight differences in the mean values and variations. the stability trend of transducer model 04 2:1 100a is shown in figure 5 as an example. all stability data were evaluated based on a boundary line or verification criteria of the intermediate check. when the measurement result deviated from the mean value over the boundary line, it assumed this was affected significantly by external influence factors. the mean values and boundary line are shown in table 2. 4.2. stability change vs. function ratio the applied overload tests from 100 % to 800 % capacity impacted directly upon the measurement signal. the sensitivity of the transducer can be stable and the results of both measurement signals and function ratios are within the boundary line (red and green line, respectively) unless it experienced large amounts of overload. when the transducer received a torque over the yield point (approximately 300 % for aluminium and 700 % for heat treated steel), the sensitivity changed correspondingly to the applied overload. the results of the overload test, the measurement signal and the function ratio changes of four transducers are illustrated in figure 6. figure 6a and 6b illustrate that the transducers made of tool steel, model 01 2:1 50s and model 02 3:1 30s, can withstand an figure 3. full bridge strain gauge circuit used in the transducers. figure 5. trend of stability of transducer model 04 2:1 100a, (a) stability of the measurement signal (red line) and function ratio (green line), (b) zero signal drift of the measurement signal (red line) and check signal (blue line). figure 4. fabricated torque transducer with check standard combination. table 2. evaluation results of long-term stability of transducers. stability at capacity zero drift model measurement signal check signal function ratio measurement signal check signal mean boundary mean boundary mean boundary mean boundary mean boundary value line value line value line value line value line (mv/v) +/%rdg (mv/v) +/%rdg per unit +/%rdg (mv/v) +/%fs (mv/v) +/%fs 01 2:1 50s 2.02619 0.03 1.00476 0.03 2.01659 0.02 -0.24995 0.22 -0.09608 0.24 02 3:1 30s 2.04259 0.10 0.60391 0.05 3.38228 0.07 0.45035 0.87 0.07526 2.8 03 4:1 50a 2.09517 0.06 0.47410 0.07 4.41941 0.03 0.59860 0.15 0.62303 1.7 04 2:1 100a 2.10303 0.05 0.95691 0.07 2.19774 0.05 0.57976 0.11 0.20061 0.70 acta imeko | www.imeko.org july 2017 | volume 6 | number 2 | 62 overload up to 500 % of the capacity. within this overload range the variation of the sensitivity was steady within the boundary line. afterwards the sensitivity increased up to 0.6 % at 800 % applied overload. on the other hand, the transducers made of aluminium, model 03 4:1 50a and model 04 2:1 100a, as shown in figure 6c and 6d, can withstand an overload up to 300 % of the capacity. the sensitivity variations were still within the boundary line as long as they received an overload not more than 300 % of the capacity. then the sensitivity abruptly changed after receiving an overload at 340 % and could not be tested anymore because the sensing body was damaged or the material has reached its ultimate strength. therefore, it is like a common torque transducer that cannot be used in case of overload up to the yield or failure point even when it is in the reference type of the torque calibration machine or the deadweight type torque standard machine. considering the trend of the function ratio and stability, it was found that the trend of the function ratio corresponds to the sensitivity change that was obtained by measuring with the tsm. all transducers showed the same agreement even in the case of broken transducers. thus, the results confirmed that the monitoring function ratio was able to predict the stability of the transducers designed and fabricated in this research. these transducers can be used when an ordinary intermediate check process cannot be performed. however, the correlations may not be better than 0.05 % as shown in figures 6c.2 and 6d.2. 4.3. stability of the check signal even though the test results showed that the function ratio can predict the stability of the transducer, the stability of the check signal is still one of the dominant factors that affect the precision of prediction. the stability of the check signal should be within the boundary line during the overload test. the trend of stability of the check signal after receiving overloads of 100 % up to 800 % of the capacity is illustrated in figure 7. figure 7 demonstrates that the check signal obtained a good stability. the sensitivity of the check signal is distributed within the boundary line. it proved that the overload test does not affect the sensitivity of the check signal. the capacity ratios of the fabricated transducers were 2:1, 3:1 and 4:1 as detailed in table 1. therefore, the design of the lowest capacity ratio, 2:1, is effective to ensure the stability at the capacity. 4.4. zero signal drift as per conceptual design, the sensing body of the check signal should be stronger than that of the measurement signal so that the check signal would be more reliable than the measurement signal. moreover, the capacity ratio of the check capacity and measurement capacity were used to define how much stronger the check signal was. although the test result showed that the check signals of four transducers obtained good stability as mentioned above, the zero signal drift was one source affecting the precision of the stability prediction. thus not only the stability of the check signal but also its zero signal drift contributes to the appropriate capacity ratio. test results of the zero signal drift of check and measurement signals are shown in figure 8. also, the zero signal of the check signal of the steel transducers slightly changed, because the sensing bodies were designed to have a capacity larger than that of the measurement. however, transducer capacity 2:1, shown in figure 8a, obtained a change rate more than the capacity ratio 3:1 shown in figure 8b. capacity ratio 3:1 could maintain a zero signal of the check signal within the boundary line, whereas capacity ratio 2:1 could not. consequently, it can be said that transducers made of steel should be designed with a capacity ratio of at least 3:1. figure 6. the stability change after an overload of 100 % to 800 % of the measurement signal (red line) compared with the function ratio (green line) for transducer model (a) 01 2:1 50s, b) 02 3:1 30s, c) 03 4:1 50a and d) 04 2:1 100a. acta imeko | www.imeko.org july 2017 | volume 6 | number 2 | 63 the zero signal of the check signal of the aluminium transducer was quite steady. zero signals of the capacity ratio 4:1 and 2:1 shown in figures 8c and 8d, respectively, were within the boundary line over the overload 40 % of capacity. therefore, the aluminium transducer can be designed with a capacity ratio just 2:1, which is sufficient. 5. the prototype of torque transfer wrench with check standard combination after the successful experiment with the torque transducer, authors were interested to apply this concept to the torque transfer wrench. hence, a prototype of the torque transfer wrench with check standard combination was invented. its sensing body was of the bending type, made of aluminium, capacity 50 n·m and a capacity ratio at 2:1. the prototype is illustrated in figure 9. the investigations followed the experimental plan for torque transducers as stated above. the torque standard machine was still needed for carrying out the stability tests as shown in figure 10. the results showed that the stability at normal condition was similar to the transducer in the previously mentioned experiment. they amount ± 0.08 % for the stability of the measurement signal and the check signal. the calculated function ratio was 2.0034:1 with a boundary within ± 0.03 %. then overload tests were done in order to confirm the function ratio capability to predict the stability of the torque transfer wrench compared with the stability obtained by measurements with the tsm. test results showed a good consistency between figure 7. stability of the check signal after receiving overloads from 100 % to 800 % of the capacity for transducer models (a) 01 2:1 50s, (b) 02 3:1 30s, (c) 03 4:1 50a and (d) 04 2:1 100a. figure 8. zero signal drift of the measurement signal (red line) compared with the check signal (blue line) for transducer models (a) 01 2:1 50s, (b) 02 3:1 30s, (c) 03 4:1 50a and (d) 04 2:1 100a. figure 9. the prototype of the torque transfer wrench with check standard combination. acta imeko | www.imeko.org july 2017 | volume 6 | number 2 | 64 each other as illustrated in figure 11. however, the sensing body of the torque transducer and torque transfer wrench can be damaged at high torques. based on the experiments, the torque transducer damaged at 300 % of the torque capacity while the torque transfer wrench damaged at 450 % of the torque capacity. 6. conclusion transducers made of steel were able to bear overload tests up to 500% of their capacity. the sensitivity and zero signals could be changed according to the amount of overload applied. transducers made of aluminium were able to bear overload tests just at 300 % of the capacity and could be damaged when the overload exceeded 300 %. the function ratio can be used to predict the stability of the fabricated torque transducer. consequently, the torque transducer with check standard combination can perform a measurement and in the meantime carrying an intermediate check itself. a capacity ratio of at least 3:1 for the steel transducer and 2:1 for the aluminium transducer should be considered. the prototype of the torque transfer wrench with check standard combination was additionally made for investigations. the study results obviously showed consistency with the torque transducer that was studied previously. however, the breaking points of the torque transducer and torque transfer wrench were different. they could be caused by the structure and type of sensing bodies. in order to better understand their characteristics, further experiments will be conducted. acknowledgement authors would like to express our sincere thanks to miss apichaya meesaplak and dr. kittisun mongkolsuttirat for proofreading. references [1] iso/iec 17025:2005, general requirements for the competence of testing and calibration laboratories, 2005. [2] tassanai sanponpute, chokchai wattong, “50 n m torque transducer with check standard combination”, proc. of 18th ie network conference, aug. 6-7, 2015, bangkok, thailand. pp.720-725. [3] dc53 high hardness & toughness new general-purpose cold die steel (homepage on the internet). (cited 2017 june 15). available from: http://www.daido.co.jp/en/products/tool/pdf/dc53.pdf [4] aluminium 6061-t6; 6061-t651 (homepage on the internet). (cited 2017 june 15). available from: http://www.matweb.com/search/datasheet_print.aspx?matguid =1b8c06d0ca7c456694c7777d9e10be5b [5] tml strain gauge performance characteristics (homepage on the internet). (cited 2017 june15). available from: http://www.straingage.com/2011/pages/tmlstraingaugeperfo rmancecharacteristics.pdf [6] omega engineering, the pressure, strain and force handbook vol.29 (strain gauges e-5, e-6), 1995. figure 10. the experimental setup for stability of the torque transfer. figure 11. the prototype of torque transfer wrench with check standard combination. http://www.daido.co.jp/en/products/tool/pdf/dc53.pdf http://www.matweb.com/search/datasheet_print.aspx?matguid=1b8c06d0ca7c456694c7777d9e10be5b http://www.matweb.com/search/datasheet_print.aspx?matguid=1b8c06d0ca7c456694c7777d9e10be5b http://www.straingage.com/2011/pages/tmlstraingaugeperformancecharacteristics.pdf http://www.straingage.com/2011/pages/tmlstraingaugeperformancecharacteristics.pdf torque transducer with check standard combination microsoft word 271-1905-1-le-rev acta imeko  issn: 2221‐870x  december 2015, volume 4, number 4, 20‐25    acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 | 20  convergence of a finite difference approach for detailed  deviation zone estimation in coordinate metrology  ahmad barari, saeed jamiolahmadi  faculty of engineering and applied science, university of ontario institute of technology, oshawa, ontario, l1h 4k7, canada      section: research paper   keywords: distribution of geometric deviations; coordinate metrology; finite difference method  citation: ahmad barari, saeed jamiolahmadi, convergence of a finite difference approach for detailed deviation zone estimation in coordinate metrology,  acta imeko, vol. 4, no. 4, article 6, december 2015, identifier: imeko‐acta‐04 (2015)‐04‐06  section editor: franco pavese, torino, italy  received april 13, 2015; in final form october 31, 2015; published december 2015  copyright: © 2015 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: this work was supported by natural sciences and engineering research council (nserc) of canada  corresponding author: ahmad barari, e‐mail: ahmad.barari@uoit.ca    1. introduction  although surface coordinate metrology is a key inspection tool in many different engineering and scientific applications, unfortunately it is still influenced by several concerning sources of uncertainties which reduces the reliability of this commonly assumed precise metrology process. the traditional coordinate metrology is completed by sequential performing of three computational tasks of point measurement planning (pmp), substitute geometry estimation (sge), and deviation zone evaluation (dze). there are several sources of uncertainties experienced and studied by researchers in conducting each one of the above tasks. in such systems, the inspection accuracy is subject to sorts of “plug-in uncertainty”, i.e. the uncertainty due to estimating some aspects of a probability distribution on the basis of sampling. sprauel et al showed that uncertainties in cmm measurements follow a normal distribution. they governed the parameters of the surfaces under inspection to characterize the intrinsic error parameters of cmms [1]. such sources of uncertainties can adversely affect the accuracy and reliability of the inspection. recently, an integrated inspection system has been studied as an efficient solution to combat the inherent plug-in uncertainty in coordinate metrology. choi and kurfess developed a numerical algorithm based on tolerance zone to interpret the data extracted from cmms. they showed that a zone fitting algorithm provides more conformance to the tolerance zone if tolerances are characterized by such span [2], [3]. the number of measured points in any coordinate metrology process is limited in increasing the number of measurements, increases the inspection time and it is costly. the typical hard-probing methods usually are very slow in capturing the points. the number of measurement points using these devices in practice is usually even less than one hundred. it is much less expensive to measure more points using optical abstract  in order to comprehend an entire surface’s deviation zone, infinite measured points are required. using the common measurement  techniques through coordinate metrology, a limited number of surface actual points can be acquired. however, the obtained points  would  not  provide  sufficient  information  to  examine  the  geometry  thoroughly.  reliability  and  convergence  of a  novel  solution  to  predict surface behaviour via distribution of geometric deviations (dgd) is studied in this paper. the methodology governs the mean  value property of the harmonic functions to solve the laplace equation around each measured point. this dgd model can be used to  reconstruct surface deviation values at any unmeasured point of the inspected surface based on a limited number of measured points.  the convergence of the introduced approach is studied in this paper. a complete approach to implement the developed methodology  is described, and the validation process is studied using actual case studies and mathematical functions. this methodology is practical  in closed‐loop inspection and manufacturing processes to form a scheme for compensating the surface errors during manufacturing  process based on the dgd model. acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 | 21  sensors, but unfortunately measurement accuracy in these sensors is still dramatically low. having this limitation, it can be beneficial to design coordinate metrology processes to utilize estimation techniques that helps to model details of the deviation zone for the locations that are not physically measured. the focus of this paper is on evaluation of a detailed deviation zone when a detailed level of information for the distribution of geometric deviations is required. this level of information is important particularly for an upstream process that needs to be performed on the measured surface. a good example of an upstream is a compensating manufacturing phase that removes or compensates the existing geometric deviations of the measured surface to finish a new surface with the desired and acceptable range of characteristics. a successful inspection process evaluates the minimum geometric deviations of the points and their corresponding points on the best substitute geometry. however, this information is only available for the measured points. estimation of geometric deviations at the regions that are not measured by coordinate measurements is necessary for a compensation process in the closed-loop of inspection-machining [3]-[7]. the presented method estimates the distribution of geometric deviations based on the pattern of the available data. this method is developed based on the assumption that the detailed deviation zone has a mean-value property similar to a harmonic function. assigning the surface points to a grid and using the finite difference method derived from the laplace equation, deviation of the rest of the surface points is approximated. the results show an improvement of approximation by adding more points till a convergence threshold is satisfied. the structure of the paper is as follows: after a brief review of the background information, the developed methodology is presented, and then the implementation of the methodology is explained, followed by a validation case study of an industrial part. 2. background  three major concepts utilized in developing the presented methodology are: definition of the laplace equation, meanvalue property, and maximum and minimum principals. a brief review of these concepts is presented. 2.1. laplace equation  a second order partial differential equation, which is often in two dimensions, is written as: 2 2 2 2 0, f f x y       (1) is called laplace equation. if the laplace equation is solved over a boundary condition, function f which satisfies both the laplace equation and its boundary conditions, is called a harmonic function. harmonic functions possess unique properties which are of interest to the developed methodology [8]. 2.2. mean‐value property  if d is a domain of finite measure in the euclidian space rn (where n ≥ 2); and, if there exists a point p0 in d for which every h harmonic function in d and integrable over d, the volume mean of h over d is equal to h(p0). then d is an open ball (or disc for n = 2) with the centre of p0. in other words, if h: u → r is a harmonic function on an open set, it has the mean value property if it satisfies the following relationship [9]: ( ) ( ) ( ) , ( ) . d z h z u w dw d z u   (2) 2.3. maximum‐minimum principal  if h:u → r is a harmonic function on a connected open set u and if there is a point p0 u with the property that 0( ) sup ( )q uu p h q , then u is constant on u. on a corollary of this property the minimum principle is derived in the same way for 0( ) inf ( )q uu p h q [9]. in detail, “sup” and “inf” refer to the superior and inferior of the harmonic function. 3. methodology  a perfect flat surface can be expressed in two dimensions defined by mutually perpendicular u and v axes. any out of flatness deviation of this system in surface can be represented in the e direction which is perpendicular to the u-v 2-d plane. therefore, the flatness coordinate metrology data can be represented in this u-v-e coordinate system, deviation space, expressing the detailed deviations of measured points. as illustrated in figure 1, the deviation of measured point pi in the u-v-e coordinate system is ei and the point pi* with coordinate of ui and vi the corresponding point for pi in the u-v plane. all points pi belong to the actual measured surface while points pi* belong to the best substitute geometry that can represent the actual surface. typically the up-stream design of manufacturing processes uses the best substitute geometry for the required analysis, and considering the overall deviation zones by a deviation boundary over information of the detailed deviation zone can be highly beneficial for processes precision manufacturing and closed-loops of inspection-manufacture. 4. deviation space and grid construction  the substitute geometry is estimated using fitting criteria. the common fitting criteria utilized by industries are the least square fitting and min-max fitting [3]. other fitting criteria can also be utilized upon the mission of inspection and the upstream processes. a good example of other fitting criteria is maximum conformance to design tolerances for closed-loop of inspection and machining [6], [7]. figure 1. u‐v‐e deviation space, detailed deviation zone representation.  acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 | 22  upon estimation of substitute geometry for a group of measured points, it is possible to transfer inspection information to the u-v-e coordinate system described above. after the point measurement process and construction of the deviation space, a grid is devised based on the desired resolution of estimation. this resolution can be decided based on the type and nature of utilized coordinate metrology. in the next step, the deviation space corresponding points, pi*’s, are bijectively assigned to the corresponding grid nodes for further processes. these locations in the deviation space are referred as sites in the rest of this work. values from the e-axis are considered as potential values for local deviation of the surface. these values are the euclidian distance from the measured points to the substitute geometry. 4.1. estimation of deviations  estimation of the geometric deviation for an unmeasured point of the actual surface is required when the geometric deviation values for the measured points are available as the attributes of the sites in the deviation space. it is shown in [3], [5] that the distribution of geometric deviations caused by quasi-static manufacturing errors is continuous because it is a direct function of existence continuity in the nominal geometry. therefore, the geometric deviations can be considered as a continuous variable with known values at sites in the deviation space. with using a proper point measurement process a sufficient number of points from the appropriate locations are measured. therefore, it is assumed that the sites are available to represent the continuity of geometric deviation function. geometric deviation of an arbitrary unmeasured location can be estimated by studying its proximity to the known geometric deviations of the measured sites. in order to estimate the detailed deviation zone the conformal mapping method using a harmonic function with drichlet boundary problem is used. in the current approach, instead of assuming a single shape centre with the potential value of zero, we consider the sites as a group of shape centres and their deviation attributes as the potential function to run the finite different iteration. this way the deviation attributes for the empty nodes between the sites are computed. in order to implement this concept the laplace equation over the grid is solved. the numerical solution for the laplace equation can be obtained using taylor’s series neglecting higher order terms [10]: 2 2 1 12 2 2 1 1 1 1 [ ( , ) ( , ) ( , ) ( , ) 4 ( , )], i i i i i i i i i i f f x y x y x y h x y x y x y                     (3) 2 1 1 2 2 ( , ) 2 ( , ) ( , ) ,i i i i i i x y x y x yf y k        (4) where h and k are the grid step sizes along u and v coordinates respectively, and ф is the detailed deviation zone function. considering equal step sizes in u and v directions, the laplace equation can be rewritten as: 2 2 1 12 2 2 1 1 1 1 [ ( , ) ( , ) ( , ) ( , ) 4 ( , )]. i i i i i i i i i i f f x y x y x y h x y x y x y                     (5) from the subsequent equation a finite different method formula is extracted, if f is considered a harmonic function: 1 1 1 1 1 ( , ) [ ( , ) ( , ) 4 ( , ) ( , )]. i i i i i i i i i i x y x y x y x y x y               (6) to initiate the iteration another equation is defined where j is the iteration index: 1 1 1 1 1 1 ( , ) [ ( , ) ( , ) 4 ( , ) ( , )]. j i i j i i j i i j i i j i i x y x y x y x y x y                (7) in the developed methodology two termination criteria are utilized for the iterations. considering the typical coordinate metrology accuracy between 10-3 mm and 10-6 mm, a threshold is selected. the ultimate goal of the developed method is to find a smooth distribution of attributes all over the grid. therefore, the derivative of attribute for every single grid node is calculated and stored in the attribute derivate matrix. the difference in the attribute derivative matrix is calculated in each iteration with its previous stage. the standard deviation of differences in derivative of attributes for all grid nodes is calculated. if the calculated standard deviation reaches to a level below the decided threshold then the iteration process is terminated. the iteration is costly and takes time; therefore, if the iteration has been running more than a specified amount before converging, the iteration process will be terminated. 5. implementation  the finite difference method can be utilized in the developed methodology only if function ф is a harmonic function; therefore, it is crucial to prove that there exists a function in the domain of the laplace equation answer. since the mean value property is a unique feature of the harmonic functions, if the finite difference method is valid in a domain, then it can be concluded that the governing function in the domain is harmonic and the use of finite difference method is legitimate. this legitimacy is validated for this work by applying the methodology in an actual case study. the implementation process is as follows: a. point measurement process: an industrial flat surface is measured with either an optical sensor or through hard probing to produce a point cloud; b. best plane fit: the best substitute geometry is constructed using the desired fitting criteria, and their euclidian distance from the best fitted plane is calculated and assigned to points; c. mapping to deviation space: measured points are mapped to the deviation space. their corresponding points and their euclidian distances to the best substitute geometry are considered as their coordinates in the constructed deviation space; d. gridding: a grid is created based on the desired resolution of the estimation process. sites are specified in the grid. as a constraint for grid resolution, it is considered that each grid node should not cover more than one site; e. initialization: the deviation attributes of nodes that contain a site are assigned to them and deviation attributes of all other nodes are equal to zero; f. percentile calculation: a series of percentages of points are selected randomly and assigned to the grid node with their potential value. the finite difference method is then applied to the grid; acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 | 23  g. estimation: the finite difference method is applied to find the deviation attribute of all the empty grid nodes through the iterations. it is worthy to mention that the deviation attribute of the sites remain fixed during the iterations; h. termination: to compare the results between various percentages used in the model, the method was iterated for 1000 runs in each case. 6. validatoin and case studies  to validate the process two case studies were studied. the cases consist of one actual manufactured surface and one mathematical surface. the manufactured piece was examined through hard probing to acquire sample points from the surface. in the mathematical case, a sinusoidal function was used to define a 3-d surface; therefore, the deviation error is a function of the position of the points. using the steps defined in the previous section, the methodology is applied on the surfaces to validate the accuracy and efficiency of the dgd model. 6.1. case i: industrial flat surface  as the first case study, an industrial flat surface with over-all dimensions of 30 mm × 30 mm was inspected. the measurement process was conducted using an optical probe on a faro coordinate metrology arm. the point measurement planning was based on stratified grid inspection with a step size of 0.5 mm. through this process 7287 points were captured to generate the point cloud shown in figure 2. as the fitting process a total least square fitting criteria is utilized. the substitute geometry and corresponding deviations of the measured points are mapped to the deviation space which is illustrated in figure 3. a grid with size of 320×240 was created and the 7287 sites were attached to it. the iteration process with the maximum number of iterations equal to 1000 was run. figure 4 to figure 8 illustrate the developed deviation detailed zone after 50 runs, 100 runs, 500 runs and 736 runs, respectively. as it can be seen in figure 4 to figure 8, by increasing the iteration runs the grid is evolving to estimate the deviation attribute of the empty nodes, and it converges to the desired threshold. this convergence is a validation to applicability of the developed methodology for the current problem. since the finite difference method uses the circular neighbourhood to estimate the centre of the circle, it provides a smooth estimation for the empty nodes through progress of the iteration process. based on the number of the runs of the finite difference method the grid converges to find the rest of the potential values. as it can be seen in figure 5 to figure 8, by increasing the iteration runs which in fact means using more and more the finite difference method, the grid is evolving to find the rest of the nodes based on the centre points. it worthy of mention to note that the potential of the centre points remains fixed during the iteration process. since the finite difference method uses the circular neighbour points to estimate the centre of the circle, it provides a smooth estimation of the nodes based on the former points. figure 9 also demonstrates the convergence process of the estimation. based on the termination criterion defined, the estimation process is terminated after 735 iterations.   figure 2. measured points based on point measurement planning.  figure 3. mapping of the measured points to the deviation space.  figure 4. deviation zone distribution before iteration runs.  figure 5. deviation zone distribution after 50 runs.  acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 | 24  6.2. case ii: sinusoidal surface  to test the capability of the proposed methodology in theoretical situations, this case study was considered. the following two directional sinusoidal function was used to generate a hypothetical 3-d surface based upon the assumption that the error deviation is a function of the positions of points as e = f(u,v) [3]. 0.1 sin(2 ) 0.2 sin(2 )e u v       . (8) the surface is constructed on u and v values between -0.5 and 0.5 and then transferred to a grid with the size of 250×250 to build the error model as shown in figure 10. the error values i.e. sites, vary between -0.2 and 0.2. to initialize the finite difference method 10 % of the original is kept in the grid and the rest were erased. assuming a metric unit for the coordinate model as millimeter, the iteration process is initiated based on the stages described in implementation (section 5) to find the 90 % of the sites that were erased. similar to the industrial part, the grid starts to converge to the actual surface. this process is illustrated from figure 11 to figure 15. the convergence process is terminated at iteration number 202, which is demonstrated in figure 16. the convergence is ended at an earlier stage regarding to the industrial surface. this basically shows a good efficiency of the algorithm over mathematical functions. 7. conclusions  estimation of the detailed deviation zone is a key task in integrated computational platforms for coordinate metrology systems. the methodology evaluated in this paper is an figure 6. deviation zone distribution after 100 runs.  figure 7. deviation zone distribution after 500 runs.  figure 8. deviation zone distribution after 735 runs. figure 9. convergence of the estimation process for the industrial part.  figure 10. two directions sinusoidal surface constructed based on (8).  figure 11. deviation zone distribution before iteration runs.  acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 | 25  approach to assess the distribution of geometric deviations on the manufactured surfaces. the convergence of the developed methodology is validated by studying various cases in this paper. the implementation of the detailed deviation zone estimation can be used for upstream manufacturing processes or manufacturing error compensation in closed-loop of inspection-manufacturing. the estimated deviation zone can lead to generate valuable information about the measured surface. this information can be utilized in various design and manufacturing applications. acknowledgement  the authors would like to thank the natural sciences and engineering research council (nserc) of canada for partial funding of this research work. references  [1] sprauel, j. m., et al. "uncertainties in cmm measurements, control of iso specifications." cirp annals-manufacturing technology 52.1 (2003): 423-426. [2] choi, woncheol, and thomas r. kurfess. "dimensional measurement data analysis, part 1: a zone fitting algorithm." journal of manufacturing science and engineering 121.2 (1999): 238-245. [3] a. barari, h. a. elmaraghy, and g. k. knopf, “search-guided sampling to reduce uncertainty of minimum deviation zone estimation,” j. comput. inf. sci. eng., vol. 7, no. 4, pp. 360–371, 2007. [4] a. barari, h. a. elmaraghy, and w. h. elmaraghy, “design for manufacturing of sculptured surfaces: a computational platform,” j. comput. inf. sci. eng., vol. 9, no. 2, p. 21006, 2009. [5] a. barari, h. a. elmaraghy, and p. orban, “nurbs representation of actual machined surfaces,” int. j. comput. integr. manuf., vol. 22, no. 5, pp. 395–410, 2009. [6] a. barari and r. pop-iliev, “reducing rigidity by implementing closed-loop engineering in adaptable design and manufacturing systems,” j. manuf. syst., vol. 28, no. 2, pp. 47–54, 2009. [7] h. a. elmaraghy, a. barari, and g. k. knopf, “integrated inspection and machining for maximum conformance to design tolerances,” in 54th cirp general assembly, 2004. [8] b. epstein and m. m. schiffer, “on the mean-value property of harmonic functions,” j. d’analyse mathématique, vol. 14, no. 1, pp. 109–111, 1965. [9] s. axler, p. bourdon, and r. wade, harmonic function theory, vol. 137. springer science & business media, 2001. [10] h. k. voruganti, b. dasgupta, and g. hommel, “a novel potential field based domain mapping method,” in proceedings of 10th wseas conference on computers (iccompp06), athens, greece, july, 2006, pp. 13–15. figure 16. convergence of the estimation process for the sinusoidal surface. figure 12. deviation zone distribution after 50 runs.  figure 13. deviation zone distribution after 100 runs.  figure 14. deviation zone distribution after 150 runs. figure 15. deviation zone distribution after 202 run.  microsoft word 334-2433-1-le-rv acta imeko  issn: 2221‐870x  september 2016, volume 5, number 2, 26‐32    acta imeko | www.imeko.org  september 2016 | volume 5 | number 2 | 26  paestum dietary habits during the imperial period:  archaeological records and stable isotope measurement   paola ricci 1 , carmina sirignano 2 , simona altieri 1 , mariangela pistillo 3 , alfonso santoriello 3 , carmine  lubritto 1   1 department of environmental, biological and pharmaceutical science and technology, second university of naples, via vivaldi 43, 81100  caserta, italy    2 department of mathematics and phisics, second university of naples, via vivaldi 43, 81100 caserta, italy    3 department of science and cultural heritage, university of salerno, i‐84088, fisciano, salerno, italy         section: research paper   keywords: paleodiet; isotopic analysis; paestum  citation: paola ricci, carmina sirignano, simona altieri, mariangela pistillo, alfonso santoriello, carmine lubritto, paestum dietary habits during the imperial  period: archaeological records and stable isotope measurement, acta imeko, vol. 5, no. 2, article 5, september 2016, identifier: imeko‐acta‐05 (2016)‐02‐ 05  section editors: sabrina grassini, politecnico di torino, italy; alfonso santoriello, università di salerno, italy  received: march 18, 2016; in final form march 20, 2016; published september 2016  copyright: © 2016 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: this work was supported by archaeological superintendence of salerno, italy   corresponding author: paola ricci, paola.ricci@unina2.it    1. introduction  1.1. isotopes definitions  isotopes are nuclides of a single element that differ in their atomic weights (in particular in neutrons number). stable isotopes are generally reported as the measured difference in the isotopic composition of the sample (s) and an accepted standard (std), in term of “delta-notation” (-values) (1): ‰ ∗ 1000 , (1) where r is the isotope ratio between the heavy isotope and the light one of the same element [1]. standards used are internationally well defined: for carbon it is the pdb (peedee belemnite) and for nitrogen it is air [1]. isotopic fractionation is “the partitioning of a sample into two or more parts that have different ratios of heavy and light isotopes than the original ratio”; if one of these parts is “enriched” in the heavy isotope, the other must be “depleted” [1]. 1.2. paleodiet and stable isotopes  the paleodiet, the diet of ancient populations, can be investigated with the consolidated methodology using stable isotopes analysis of human bones collagen [2], [3]. de niro and epstein conducted first studies on animal feeding, indicating that stable isotopes ratios of carbon and nitrogen of whole body and specific tissues reflect that of the consumed food [4], [5]. organic and inorganic constituents of bones represent a record of long-term diet [6] and, in specific, bone collagen carries diet information average of several years because of a turnover of ten years [7]. as the isotopic signature varies among different trophic chains (i.e. marine or terrestrial) and it propagates along a chain, it gives reliable indications to abstract  in historical contexts, analyses of carbon and nitrogen stable isotopes can be useful to answer different question on dietary behavior  and  to  crosscheck  information,  drawn  from  texts  and  classical  archaeological  investigations.  in  this  study  the  isotope  ratio  mass  spectrometry (irms) facility installed at the irms‐sun laboratory of the second university of naples is presented.  moreover, results  coming from application of stable isotope analyses to bone collagen extracted from human remains of the necropolis of “porta sirena”  in paestum will be discussed. finally, a combined analyses of archaeological and historical record and stable isotope measurements  permits to expand our knowledge on diet in roman paestum. acta imeko | www.imeko.org  september 2016 | volume 5 | number 2 | 27  distinguish whether or not an individual belongs to a certain food chain or to a certain level of it [8]-[11]. it must be considered that between diet and bone collagen there is a fractionation of +5 ‰ for carbon and +3 ‰ for nitrogen [12], [13]. about carbon, the 13c signal in human bones reflects the consumption of c3 and c4 plants, that have a different metabolic pathway in the photosynthetic cycles [7], [14], [15], with an enrichment of about 14 ‰ for c4 one. besides, the majority of marine or terrestrial foods provides changes in 13c of human bones (higher signals for prevalence of marine foods) [7]. concerning nitrogen isotopes, the majority of the variations in 15n human collagen derive from trophic level of the food consumed: there is a consistent enrichment in 15n of marine animals, because phytoplankton uses enriched nitrate dissolved in seawater and because larger marine carnivores, usually common food for humans, are in the higher trophic level [16]. for the same reason, small fishes belong to low trophic level and so do not provide great changes in 15n signal of human bones. instead, a big consumption of marine foods is detectable in collagen 15n [17]. therefore, it is possible to establish ranges of 13c and 15n human collagen that indicate the main consumption of terrestrial or marine foods: if marine protein were prevalent in the diet of ancient population, 13c results between -12 and -14 ‰ and 15n is between +12 and + 22 ‰; if they fed more of terrestrial food, 13c values are around –20 ‰ and 15n is between +5 and + 12 ‰ [13], [16], [18], [19]. generally, in a defined historical context, information about foods commonly used in human nutrition is known from archaeological investigations and different models have been implemented in order to untangle the diet of an individual into its different food constituent [20]. but, it appears more difficult to obtain precise information about food consumption as among different social, age and gender groups, within a community or region as a whole. in this case, stable isotopes analysis can be useful to confirm the actual adoption of a certain kind of diet, known to be common at a time. in particular, historical records or textual references about food and diet during the roman imperial period explain that fish or terrestrial animal meat was reserved just to an elite of people [21]. during the last decade, a number of publications have addressed this issue, testing historical accounts by means of stable isotope analysis, in different context in the central and southern italy. they introduced the hypothesis that together with cereals, wine, olive and dry legumes also animal proteins could have been part of the lower classes diet: pork, sheep and goat meat primarily, but also fish when easy accessible for the population, i.e at costal sites. for an introduction to the food availability and diet during the roman imperial time see prowse et al., 2004 [21], killgrove and tykot, 2013 [22], and the references therein. killgrove and tykot, 2013 [22] and rutgers et al., 2009 [23] studied the city of rome itself. the first ones investigated two cemeteries just outside the urban walls, romecasal bertone (2nd-3rd centuries ad) and rome-castellaccio europarco (1st-3rd centuries ad). rutgers et al., 2009 [23] focused their research on early christians buried at the cemetery of st. callixtus (3rd to 5th centuries ad). however, pioneers in this topic have been prowse and co-authors [21], [24]-[26], as they extensively investigated the behaviours of the population from portus romae (trajan’s port, 23 km southwest the city of rome), who was used to bury the corpses in the necropolis of isola sacra (1st-3rd centuries). stable isotope analysis was especially used to look into the diet of the population and how it varied as function of sex, age and status [21], [24] and it revealed that the population generally mixed terrestrial resources with marine food, consuming marine organisms of higher trophic level more than the famous garum, a fish sauce typical of the elite roman diet. the nearby inland cemetery, named anas, was originally intended as reference site for terrestrial based diet. however, final evidences showed that it had been occupied by two different clusters of individuals, identified for their similarity to isola sacra individuals, as one group belonging to the inland site, and another one with individuals that possibly migrated from a coastal zone (anas 1 and anas 2, respectively for our reference) [21]. craig et al. [27] moved their research southwards and they explored the dietary habits at the coastal sites of velia. by means of cluster analysis, they revealed that the majority of the population from the velia necropolis had a diet high in cereals and relatively lower in meat, but they found a group which had consumed more meat and also fish, especially high trophic level fish (velia 1 and velia 2, respectively for our reference). about 30 km to the north of velia, during the excavations conducted in the area of “porta sirena” at paestum (salerno, italy), graves belonging to the roman imperial period came to light. the human remains were studied according to the standards of the funerary archaeology [28] and they have been processed for isotope analysis. in this study, carbon and nitrogen isotope analyses applied to the human remains of the necropolis of “porta sirena” in paestum are presented. primarily, this investigation has aimed to verify the hypothesis of presence of fish in the diet at this shore site during the roman imperial time, especially by comparing coeval sites. furthermore, since more studies on this topic have been invoked as desirable (see for example killgrove et al. 2013 [22]), this work is expected to generally contribute to the knowledge on average lower class diet from this time, which is complex and variable in its distribution among different kind of people and different territories. anthropological assessments are not covered by this study and samples were collected only for paleodiet analysis. but for the purpose of the study itself, it is important to consider that the grave goods found in the site were in no case valuable or interesting, confirming the absence of any elite member of the community interred in this necropolis. 2. materials and methods  2.1. site description  the area of the excavation is located along north-south direction of “porta sirena” (figure 1), while the eastern and western limits of the zone of operation are respectively represented by the modern road and the walls. the area has been divided into different sectors. the excavation of the necropolis, whose tombs are located at about 10-15 cm depth, have so far returned 73 burials, numbered from 21 to 95 (t21 to t95). according to their stratigraphy and the archaeological analysis of the grave goods, the interments have been attributed to a period spanning from the 2nd to the 4th centuries ad. 2.2. paleodiet analysis  for the scope of this study, 23 of the 73 discovered graves have been sampled for paleodiet analysis. the choice fell on those graves where the human remains were better preserved, preferring those among them who by inhumation and grave acta imeko | www.imeko.org  september 2016 | volume 5 | number 2 | 28  goods were more heterogeneous, in order to be as more representative as possible of the whole interred population of the cemetery. the graves in amphorae, were excluded from this study, as they were usually used for very young corpses. long bones, when available, have been preferred to the others, in order to obtain information about lifelong habits of the sampled individuals, and, when possible, the same skeletal element has been chosen for all the individuals, in order to minimize the influence arising from sampling different kind of bones. the classification of sampled bones is reported in table 1, in addition to the age classification, c and n concentrations, c/n ratios, c and n isotopic ratios, collagen yield and presence of grave goods. faunal remains were rare and not properly preserved for classification, for reference purposes two teeth of two different herbivores (possibly a cow indicated as d1 and a goat as d2) have been collected at the site, being coeval with the human individuals under investigation. 2.3. samples preparation  the samples for the analysis were processed to isolate the organic phase of the sample (collagen) adopting a modified procedure from longin method (1971) [29]. a fragment of the sample was selected from each specimen. the bone surface was abraded to remove contaminants and it was pulverized. each sample was then placed in polypropylene test tubes and demineralised in a sequence of acid attacks with hydrochloric acid (hcl) 0.6 m at ambient temperature (20-25 °c), interrupted by one alkali attack (naoh 0.1 m) 30 minutes long. several rinses with de-ionized water were done after each step, before finally oven-drying the samples. finally, the gelatinization protocol was applied [29], [30]. 2.4. elemental analysis (ea) and stable isotopes mass  spectrometry (irms)   the isotope ratio mass spectrometry (irms) is a methodology used for qualitative and quantitative analyses, measuring isotopic ratios of different element of various types of samples (solid, liquid or gas). it is based on the principle of separation of atoms or rather ions with different mass to measure their relative abundances [31]. for solid samples, it is coupled with an elemental analyzer (ea), where the sample is burned and transported by a carrier flow (he) to the irms, as showed in the schema of the instrumentation used at irms-sun laboratory (figure 2). for collagen quality test, c and n fractions of collagen dry table 1. samples classifications, isotopic values and collagen quality indicator of human’s bone and fauna’s teeth samples from the necropolis  “porta  sirena” of paestum.  sample  sample  classification  age   classification  %n %c c/n 13c  (‰vpdb)  15n  (‰air)  collagen  yield  presence of  grave goods  herbivorous d1  tooth  ‐  3.9 10.8 3.3 ‐21.9 4.7 3.1%  herbivorous d2  tooth  ‐  7.1 19.9 3.3 ‐22.1 5.1 1.9%  t24  radius sx  adult  14.1 37.1 3.1 ‐19.0 7.8 2.4%  t26  radius sx  adult  16.1 41.1 3.0 ‐19.5 7.6 3.6%  t34  tibia dx  adult  12.8 33.2 3.0 ‐19.3 8.5 2.7%  x t38  tibia dx  adult  12.8 33.8 3.1 ‐20.9 7.6 2.9%  x t46  femur dx  sub‐adult  13.7 38.6 3.3 ‐20.5 7.1 1.6%  x t47  humerus sx  adult  14.9 38.3 3.0 ‐19.4 7.9 1.5%  x t53  femur dx  sub‐adult  15.8 41.5 3.1 ‐19.1 7.8 2.6%  x t58  ulna  adult  15.7 40.7 3.0 ‐19.0 7.9 2.5%  t64  ulna  adult  16.7 42.9 3.0 ‐18.5 8.2 4.7%  t65  femur dx  sub‐adult  15.5 41.2 3.1 ‐20.4 7.4 3.9%  x t68  humerus dx  adult  8.9 22.9 3.0 ‐18.5 9.4 3.9%  x t69  humerus dx  adult  15.8 41.5 3.1 ‐19.5 8.5 1.6%  x t70  tibia sx  adult  16.0 40.9 3.0 ‐18.8 8.1 3.1%  x t72  tibia sx  sub‐adult  14.4 37.5 3.0 ‐19.4 7.9 1.2%  x t76  femur sx  adult  14.9 38.3 3.0 ‐20.1 7.6 2.4%  t77  tibia dx  adult  14.2 36.5 3.0 ‐20.0 7.8 3.1%  x t80  humerus dx  adult  14.9 38.2 3.0 ‐19.0 7.5 1.0%  x t81  tibia sx  adult  14.0 37.1 3.1 ‐19.6 8.0 1.8%  t82  humerus sx  adult  13.8 36.9 3.1 ‐20.3 8.2 1.0%  x t83  tibia sx  infant  11.3 29.8 3.1 ‐18.4 11.4 2.6%  x t84  humerus dx  adult  15.8 40.6 3.0 ‐19.9 8.8 3.2%  x t85  tibia dx  adult  15.4 40.0 3.0 ‐20.6 8.4 2.7%  x t86  humerus dx  adult  16.3 41.6 3.0 ‐19.3 8.4 3.2%  x figure 1. map of south of italy with the location of the necropolis of “porta sirena  at  paestum  and  the  coeval  sites  considered  in  the  text.  the necropolis of  velia is located to the south of paestum, while to the north,  near rome there is the cemetery of isola sacra (casa bertone, castellaccio europarco   and st callisto are too close to rome   be distinguished at this scale).  acta imeko | www.imeko.org  september 2016 | volume 5 | number 2 | 29  mass (c % and n %) were measured with the elemental analyzer alone (cn flash ea 1112 series, thermo scientific, finningan). in specific, each sample is weighed in a tin capsule (about 3 mg), placed in an auto-sampler and burned in the reaction column of the ea, by high temperature (1020 °c) and oxygen presence. here, the sample derived gas is subjected to red-ox reactions, obtaining co2 and n2 gases, subsequently separated in a chromatographic column, carried by the helium. finally, the gases are captured by a detector: software allows the concentrations calculation, by means of a calibration curve obtained using a standard with certified concentrations. during the gases passage, water is removed by a hygroscopic trap (magnesium perchlorate). the elemental analyzer, in our system, is connected to the irms (delta v advantage, thermo scientific) for isotopic ratios analysis (1 mg of samples), by an interface conflo iii (in continuous flow mode, thermo finningan), that modulates the passage of carrier flow and standard gases. in the irms, the gas sample is ionized by particles coming from a source, the resulting ions are accelerated in an electronic field to a magnetic field, where they have different trajectory depending on their mass/charge ratio. trajectories of lighter and heavier isotopes are deflected in different ways, following the relation (2): , (2) where m is the mass, q the charge, r is the radius of curvature of ions beam trajectory, v the potential differences and b the magnetic field. ions beams are collected by faraday cups (collectors), which are able to measure current intensity generated by incident beam. a mass spectrum is derived, where each line corresponds to a fragment of specific m/q. molecular peak (more intense) is made from molecular ion, that is a non fragmented molecule with a positive charge (for a lost electron). peak position is related to a specific mass value (qualitative analyses), peak intensity is proportional to relative abundance of each fragment (quantitative analyses). samples were retained for isotope analyses when extracted collagen achieved a yield higher than 1 % and an atomic c:n ratio between 2.9 and 3.6 [30]-[34]. for δ15n and δ13c analyses, samples were analysed according to the method used by preston and ovens 1985 [35]. the isotopic measurements were calibrated based on the measurement of standards, aiming to set their values on internationally referenced scales (vpdb for c and air for n) [36]. the analyses were conducted in blocks of 12 samples, maximum. between one block and the next one, three different reference materials were measured: two used to calibrate the measurement, and the last one used to evaluate the proper conduct of the analysis (target) and the repeatability of the measurement itself. the reference materials used for 15n analysis calibration were iaea-n-2 (ammonium sulphate, 15nair = 20.3 ± 0.2 ‰) and iaea-n-1 (ammonium sulphate, 15nair = 0.4 ± 0.2 ‰) [37]. the reference materials used for δ13c analysis were iaea-ch6 (sucrose, 13cvpdb = -10.45 ± 0.03 ‰) and iaea-ch3 (cellulose, 13cvpdb = -24.72 ± 0.04 ‰) [38]. typical analytical precision evaluated from repeated measurements of the standard, used as target, is 0.1 ‰ for 13c and 0.2 ‰ for 15n. 3. results and discussion  the results of the isotopic analysis are shown in table 1 (13c and 15n), together with collagen quality indicators (c/n, collagen yield). the general state of preservation of the sampled bones has been acceptable. testing the quality of collagen extracted assures the absence of potential contamination effects. from the results obtained, all the samples were retained for analysis, presenting yields above 1 % and c/n values spanning from 3.0 to 3.3 [31]-[34]. figure 3 represents the 13c and 15n bi-variate plot which finally allows considerations on the dietary habits. all the results from the necropolis of “porta sirena” in paestum are plotted as compared to the faunal remains, the ones found at the same site (triangles) and the fauna collected at the coeval sites of velia (diamonds) [27] and isola sacra (circles) [21], in particular pigs and herbivores. the isotopic composition of the two fauna samples from paestum falls well within the variability shown by the coeval fauna from the other sites, both isola sacra and velia. regarding to the humans, the collagen extracted from the bone sampled in the tomb t83 has given delta values clearly very different from those found for other samples. the bone fragments extracted from tomb t83 have been attributed to an individual younger than 1 year, an infant, i.e. a breast-feeding baby. breastfeeding individuals can be considered “consumers” of their mother’s tissues, therefore occupying a higher trophic level than the adults and the weaned children. a 15n figure 3. 13c and 15n values for humans (squares) and herbivorous fauna  (triangles) from paestum compared with fauna from coeval costal sites of  isola  sacra  (circles)  and  velia  (diamonds),  average  values  with  standard deviation.  the individuals t82 and t83 are possibly mother and her breast feeding baby.  figure 2. ea/irms facility (2a) and pictures of ea and irms instruments (2b) of the irms‐sun laboratory.  2 2 2 b v r q m  acta imeko | www.imeko.org  september 2016 | volume 5 | number 2 | 30  enrichment of about +2 and +3 ‰ and a 13c enrichment of about 1 ‰ are typical for this trophic shift [39], [40]. this 13c and 15n offset pattern is consistent with what we find for this individual t83. being t83 a child early in life, it is very likely that the individual laying in the same grave (t82) might have been his/her mother. they differ from each other of 1.9 ‰ on the 13c scale and of 3.2 ‰ on the 15n scale, which is a result that could indicate the trophic shift mother-baby. but this could only be confirmed in response to anthropological analysis (i.e. individuals sex). if we exclude the sample belonging to the tomb t83 all the 13c and 15n values are included respectively in the ranges [20.9 ‰, -18.5 ‰] and [+7.1 ‰, +9.4 ‰]. according to these results one can say that the diet of the paestum population buried at the necropolis of “porta sirena” was based on agricultural-pastoral foods, confirming the indications known from independent archaeological studies [28]. the values obtained, in fact, are consistent with a diet based on the consumption of c3 plants, characteristics of temperate climates, with addition of meat as confirmed by the fact that the average of human population (excluding t83) is about 2.4 ‰ and of 3.1 ‰ higher than herbivores samples on the 13c and the 15n scale, respectively. however, this analysis has the limitation of not being able to identify the possible consumption of small fish [17], as explained following in this paragraph. figure 4 and table 2 compare the results from paestum with those from the coeval sites from the central and south of italy shown for their location on the map of figure 1. velia is the closest site and it has been the largest investigated site together with isola sacra, in term of analyzed individuals. in their extensive investigations craig et al. (2009) [27] for velia (diamonds) and prowse et al. (2004) [21] for isola sacra (triangles) have been able to distinguish among groups of the population, which had different consume of meat and, especially, of fish in their diet. here we refer to these categories to further interpret the results of the smaller population sampled at paestum. the groups which very likely introduced fish in their diets have been here defined for simplicity “fish eaters” (isola sacra, anas 2 and velia 2), the others have been named “non fish eaters” (velia 1, anas 1). fish eaters at velia belongs to a small group of 17 individuals (velia 2, empty symbols), while all individuals from isola sacra are thought to introduce to some extent marine food in their diets. in the original study, isola sacra samples were compared with individuals buried in a nearby inland place named anas (triangle). in the anas cemetery just a subset of inhumed individuals was identified with a terrestrial diet (anas 1, full symbols), the rest was formed by costal immigrant or seafarers (anas 2, full symbols). the  13c and 15n of human individuals found at the necropolis of “porta sirena” in paestum averaged (excluding t83) -19.6 ‰ (s=0.7 ‰, n=22) and +8.0 ‰ (s=0.5 ‰, n=22), respectively. on the one hand, the mean 13c of the fish eaters group is -18.9 ‰ (s=0.4 ‰, n=126) and the mean 15n is +10.9 ‰ (s=1.2 ‰, n=125). these values are significantly higher than the values found at paestum (two-sample t-test, p < 0.001; t=4.7 for the difference in 13c, t=18.9 for the difference in 15n). on the other hand, the human individuals found at the necropolis of “porta sirena” in paestum show unequivocal analogy with the group identified as non fish eaters, mainly coming from velia, whose mean 13c is -19.5 ‰ (s=0.2 ‰, n=106) and  15n is +8.2 ‰ (s=0.7 ‰, n=106), suggesting that these individuals had a negligible or no use of sea food in their diets (two-sample t-test, p < 0.001; t=0.6 for the difference in 13c, t=1.3 for the difference in 15n). however, as mentioned before, it remains the hypothesis that “porto sirena” population could eat fish in a minor amount respect to other food, because marine protein could represent up to 20 % of total dietary protein without any appreciable change in 13c values and consuming low trophic level marine resources (e.g. garum) could not change significantly 15n signal [21]. figure  4.  13c  and  15n  bi‐variate  plot  with  the  comparison  between  paestum and other coeval sites (average values with standard deviation) . isola  sacra  (empty  dark  triangle),  anas  2  (empty  light  gray  triangle)  and velia  2  (empty  diamond)  have  been  grouped  to  identify  them  as  “fish  eaters”.  velia1  (full  diamond)  and    anas1  (full  triangle)  are  indicated  as  “non fish eaters”. circles  represent the mean values from the cemeteries  belonging  to  the  city  of  rome  (rome‐casal  bertone,  rome‐castellaccio  europarco and   the  early christian cemetery of st. callixtus). error bars are  standard deviations. table 2. isotopic average values and standard deviations (s) of isotopic results from different sites (see references), where n is the number of samples. site  13c  (‰vpdb)  s 15n  (‰air)  s n  references paestum  ‐19.6  ±0.7  8.0  ±0.5  22  this work  rome casal bertone  ‐18.3  ±0.6 10.1 ±1.5 32  killgrove et al., 2013 rome castellaccio europarco  ‐18.5  ±0.6 9.5 ±1.3 12  killgrove et al., 2014 rome st. callixtus  ‐19.7  ±0.4 10.6 ±0.5 22  rutgers et al., 2009  isola sacra  ‐18.8  ±0.3 10.8 ±1.2 105  craig et al., 2009  anas 1   ‐19.7  ±0.2 7.9 ±0.7 7  craig et al., 2009  anas 2   ‐19.1  ±0.2 11.1 ±0.2 7  craig et al., 2009  velia   ‐19.4  ±0.3 8.7 ±1.3 117  prowse et al., 2004  velia 1   ‐19.5  ±0.2 8.2 ±0.7 100  prowse et al., 2004  velia 2   ‐19.3  ±0.3 11.18 ±1.3 17  prowse et al., 2004 acta imeko | www.imeko.org  september 2016 | volume 5 | number 2 | 31  a third subset of three sites belongs to the city of rome: two cemeteries just outside the urban walls, rome-casal bertone (2nd-3rd centuries ad) and rome-castellaccio europarco (1st-3rd centuries ad) and the early christian cemetery of st. callixtus (3rd to 5th centuries ad). results of the dietary analysis from periurban casal bertone and suburban castellaccio europarco showed evidences that individuals living closer to the city of rome were consuming some aquatic resources, those in the suburbium made greater use of millet [22], as could possibly have done some of the individuals buried at paestum showing higher 13c values. the access to aquatic resources of tiber, instead, were used by rutgers and colleagues (2009) to interpret the comparatively low 13c values as related to freshwater fish consumption. beyond this, there is to be noted that the intra-site and inter-site variability existed in the diet of the common people during the roman imperial period, paestum population, however, does not show signs of this complexity, appearing as a simple rural community, in accordance with the classical paradigm that fish or terrestrial animal meat was reserved to elite people, even when sea resources were so close in space. however, we could not bring any evidences of variability of the dietary behavior according to any hierarchical or social status in paestum. in addition, no particular status distinction among the inhumed individuals was found at the site, as testified, for example, by the kind of tombs and the simple grave goods found in that. 4. conclusions  this work has confirmed that isotopic measurements can integrate classical archaeological investigations to improve the knowledge of diet in imperial roman paestum. as matter of fact, archaeological study allows to place the site chronologically, identifying analogies with the contemporary and vicinal necropolis of velia, especially with respect to homogeneity in the poverty of the materials and rituals of burial. the isotopic analysis outlines uniformity even in the diet, identifying a typical 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[40] b.t. fuller, j.l. fuller, d.a. harris, r.e.m. hedges, “detection of breastfeeding and weaning in modern human infants with carbon and nitrogen stable isotope ratios”, am. j. phys. anthropol., vol.129, 2006, pp.279-293. microsoft word 324-2395-3-le-rev2 acta imeko  issn: 2221‐870x  september 2016, volume 5, number 2, 80‐85    acta imeko | www.imeko.org  september 2016 | volume 5 | number 2 | 80  investigating the urban archaeological sites using ground  penetrating radar. the cases of palatino hill and st john  lateran basilica (roma, italy)  salvatore piro and daniela zamuner  istituto per le tecnologie applicate ai beni culturali, itabc – cnr, rome, italy        section: research paper   keywords: gpr; urban area; palatine hill roma; st. john lateran basilica roma  citation: salvatore piro, daniela zamuner, investigating the urban archaeological sites using ground penetrating radar. the cases of palatino hill and st john  lateran basilica (roma, italy), acta imeko, vol. 5, no. 2, article 11, september 2016, identifier: imeko‐acta‐05 (2016)‐02‐11  section editors: sabrina grassini, politecnico di torino, italy; alfonso santoriello, università di salerno, italy  received: march 2, 2016; in final form may 23, 2016; published september 2016  copyright: : © 2016 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: this work was supported by prin 2012 national project n. nfyhh2, for the palatine site and by the newcastle university (uk), for st. john lateran  basilica  corresponding author: salvatore piro, e‐mail: salvatore.piro@itabc.cnr.it    1. introduction  there are many important research and technical issues related to the investigation in urban areas to locate subsurface cavities and/or archaeological remains, to produce hazard mapping, that is of the highest priority, and archaeological risk maps. this is especially so in civil engineering where it is of key importance for managing safe urban and civil constructions. in many cases, cavities, such as subsidence features, voids and collapses represent disruptions to the geometry of an originally near-horizontal layered system. geophysical techniques can be used to identify the feature geometries by contrasts in the physical properties, but can be greatly impeded by cultural features that interfere with instrument measurements (utilities, structures, surficial debris). site preparation can facilitate the field work and can improve the accuracy of the geophysical surveys and interpretations. the most useful site preparations are performance of a topographic survey to create a site grid, placement of location markers and removal of surficial debris and obstructive vegetation. conditions of the geophysical field project that are subjected to site-specific design include: selection of appropriate techniques, instruments, accessories and settings; performance of dualtechnique surveys for redundant site coverage; performance of follow-up surveys and selection of instrument measurement density. the critical phase of the geophysical survey in an urban area is the interpretation of the collected data and the characterization of the degree of confidence in the interpretations. abstract  the geophysical prospection is generally considered as the attempt to locate structures of archaeological interest buried in the natural  subsoil, but in many cases, when applied in urban centers, this attempt could fail due to the effect and disturbances caused by recent  man‐made structures in the subsoil, covering any signal related to possible archaeological structures. in the present paper the gpr  surveys carried out in two urban archaeological sites in roma, characterised by different targets and environmental conditions, are  presented and discussed. the first site, a portion of palatine hill (archaeological center area of roma) is characterized by natural soil  on the surface and an overlapping of many archaeological structures in its volume. the second site, st. john lateran basilica (roma), is  characterized by artificial medium as road pavement, outside the basilica, and ancient buildings, below the actual basilica. the paper  illustrates the two gpr surveys and the obtained results.  acta imeko | www.imeko.org  september 2016 | volume 5 | number 2 | 81  the urban subsoil consists often of many layers documenting the history of a place, keeping records of alternating phases of construction and destruction. the shallow subsurface of modern cities contains reams of pipes, cellars, wells, cavities, tunnels, graves and foundation walls of former houses, churches and town fortifications. underneath the tarmac of city roads and the paving stones of town squares layers of sand and gravel are criss-crossed with modern fibre optic and telephone cables and century old sewer pipes mixed with the debris of brick buildings. the geophysical prospection of urban centres, which have been abandoned in the past and today are located under barren land, pose different methodological possibilities and challenges compared to surveys conducted in modern city centres. while the first sites may successfully be prospected employing aerial photography, magnetometry, ground penetrating radar, earth resistance and inductive electromagnetic methods [1], many of these methods are difficult or impossible to use in the urban centre. the most promising non-destructive geophysical prospection method for use in urban centres is gpr (ground penetrating radar). gpr measurements are less affected by the presence of metallic structures compared to magnetometer prospection and they result in the largest amount of data of all commonly employed near-surface geophysical methods, providing detailed three-dimensional information about the subsurface [2]. the ground penetrating radar is an electromagnetic impulsive method much suited for shallow depth investigations, as it can supply subsurface profiles grouped in vertical radar sections. the transmitter-receiver antenna is pulled along the surface of a site, signals are sent with a highly directive radiation pattern into the ground and echoes are returned from targets in the ground within a few meters. the emitted radar signal is a pulse of electromagnetic radiation with nominal frequency value in the range 15 to 2500 mhz. the velocity of an electromagnetic wave in air is 30 cm/ns. in soils the velocity is less, as typical values are in the range 5 to 15 cm/ns [2]. while geophysical prospection is generally considered as the attempt to locate structures of archaeological interest, in many cases, when applied in urban centres, this attempt could fail due to the effect and disturbances caused by recent man-made structures in the subsoil, covering any signal related to structures of archaeological interest. modern underground structures (as bars and slabs of reinforced concrete, metallic pipes, cables and associated trenches and building debris) occurring at shallow depth often display a stronger contrast in physical properties relative to the surrounding subsoil than less well expressed archaeological structures, which often are buried at greater depth. challenges for gpr prospection in city centres lie in the large number of obstacles present in the urban environments. traffic islands, metallic gully covers, lamp posts, buildings, trees and parked vehicles cause irregular survey geometries, holes in the surveyed area and disturbing anomalies in the gpr measurements. in section 2 the gpr surveys, carried out in two urban archaeological sites characterised by different environmental conditions, are presented and discussed. 2. archaeological prospection in urban centres  as indicated in the previous section, the archaeological geophysical prospection in the urban area can be limited or affected by recent man-made structures in the sub-soil. in the present paper the surveys made with geophysical methods to investigate two different sites are presented and discussed. the first site, a portion of palatine hill (archaeological centre area of roma) is characterized by natural soil on the surface and an overlapping of many archaeological structures in its volume. the second site, st. john lateran basilica (roma), is characterized by artificial medium as road pavement, outside the basilica, and ancient buildings, below the actual basilica. to investigate these two sites the gpr (ground penetrating radar) system equipped with different antennas (frequencies) and with specific instrumental configurations have been employed. 2.1. palatino hill   to enhance the knowledge of the subsoil between the n-e foot of palatino hill and coliseum valley (rome) as to the location and conservation of unknown buried roman structures, a scientific collaboration between the sapienza university at roma (department of archaeology) and the institute of technologies applied to cultural heritage (itabccnr), was initiated in 2001 and it is still in progress. the study area is characterised by a sequence of complex buildings, related to the roman period between the late republican and severo’s age (200 ad). a thick section of alternating sedimentary and volcanic deposits constitute the framework of the palatine, and locally crops out along the north-western slope of the hill. in this complex site a series of gpr surveys employing different frequencies were carried out. for the field measurements two different gpr sir systems (gssi), one equipped with a 400 and 500 mhz antenna (high-frequency) with constant off-set and the other employing a 70 mhz monostatic antenna (low-frequency), were used. the 400 and 500 mhz antenna were as a compromise between depth penetration to about 2 to 3 meter and resolution of features on the order of 0.15 to 0.20 m in order to define the archaeological features of interest. the 70 mhz antenna was employed to investigate at a depth penetration more than 2 to 3 m and with a resolution more than 0.30 m, comparable with the dimensions of the expected archaeological foundation structures. acquisition was made using a high-resolution approach in which parallel profiles were recorded very closely across the site. signal processing, image processing, and visualization techniques have been used in conjunction with data modelling, elaboration, and interpretation of the recorded subsurface amplitudes [3]. gpr surveys were performed, during september 2001, november 2002, september 2003, may 2004, january 2008 and february 2011 using a 400 and 500 mhz (gssi) bistatic antenna with constant offset, at selected areas on the n-e foot of palatino hill and in the coliseum valley. gpr surveys were also performed during november 2002, 2003 and february 2008 with a 70 mhz (subecho radar) monostatic antenna, to survey an area in the coliseum valley. a total of 243 adjacent profiles at the site of “n-e foot of palatino hill” were collected alternately in reversed and unreversed directions across the survey grids. the horizontal spacing between parallel profiles for both antennas at the site was 0.5 m. radar reflections along the transects were recorded in continuous acquisition mode across the ground at 80 scan s-1, horizontal stacking was set to 4 scans. along each profile acta imeko | www.imeko.org  september 2016 | volume 5 | number 2 | 82  markers were spaced every 1 m to provide spatial reference. all radar reflections within the 95 ns, 105 ns, 113 ns and 150 ns (two-way-travel time) time windows were recorded digitally in the field as 8 and 16 bit data and 512 samples per radar scans. a total of 63 adjacent profiles at the site “coliseum valley” were collected alternately in reversed and un-reversed directions across the survey grids. the horizontal spacing between parallel profiles at the site was also 0.5 m and at a collection rate of 80 scan/s; horizontal stacking was set to 4 scans for both antennas. along each profile markers were spaced every 1 m to provide spatial reference. all radar reflections within the 100 ns and 150 ns (two-way-travel time) time window were recorded digitally in the field as 8 and 16 bit data and 512 samples per radar scans. the data were subsequently processed using standard twodimensional processing techniques employing the gpr-slice v7.0 ground penetrating radar imaging software (goodman, 2011, 2013). the basic radargram signal processing steps included: (i) header editing for inserting the geometrical acquisition information; (ii) dc drift removal; (iii) manual regaining to adjust the acquisition gain function and enhance the visibility of deeper anomalies; (iv) data resampling, (v) frequency analysis; (vi) band pass filtering, (vii) customized background removal filter to attenuate the horizontal banding and (viii) kirckof migration using a constant average velocity [4]. with the aim of obtaining a planimetric vision of all possible anomalous bodies the time-slice representation technique was applied using all field profiles (goodman and piro, 2013). time-slices are calculated by creating 2-d horizontal contour maps of the wave energy from a specified time value across parallel profiles, employing gpr-slice software. time slice data sets were generated by spatially averaging the squared wave amplitudes of radar reflections in the horizontal as well as the vertical, figure 1. background filtering was used to remove line noises that were found to exist in the raw unprocessed images. using these spatial averages, interpolated and solid 3d volumes of reflection amplitudes were generated. using spatially binned and interpolated volumes of radar data show better continuity in mapping subsurface reflections, which are more useful and provide archaeologist an interpretable form of data presentation, then using simple raw or filtered radargrams as volume elements, figures 1, 2 and 3. a nominal microwave velocity of about 10 cm/ns was determined from fitting hyperbolas to the raw field data. this was used in estimating a penetration depth from the gpr survey. during the systematic archaeological investigations made subsequent to the geophysical surveys, from 2001 to 2011, between the n-e foot of palatino hill and the colosseum valley and nearest elagabalo’s thermae, a sequence of complex buildings, related to the roman period between the late republican and severo’s age were discovered [5], [6]. as an example, the structures indicated with usm5008 (flavian era) and usm5045 (nero’s era) (figure 3) are foundations made in cementitious, 0.90 m wide and 12 m long. in the area these reflections individuated at depth 0.60 m, figure 2, continue till figure 2. gpr time slice at a depth of 0.60 m indicates the presence of many  reflections  due  to  remains  of  archaeological  structures  that  have  been found during the first excavations of 2001‐2004.  figure  1.  overlay  analysis  was  applied  to  the  dataset  using  a  map  that collected  the  relative‐strongest‐reflectors  recorded  on  24  individual  time slices from the ground surface to a depth 239 cm. the time slice maps near the surface were underweighted using data transforms to deemphasize the ground surface reflections.   figure  3.  gpr  time  slice  at  a  depth  of  2.40  m  indicates  two  main  walls  (usm5008 and usm5045) which have been found after the archaeological  excavation (violet contour on the map).  acta imeko | www.imeko.org  september 2016 | volume 5 | number 2 | 83  the depth of 2.40 m, figure 3. the results obtained with the gpr surveys have allowed the location of three rooms with rectangular shape, that are clearly visible at 0.60 m depth, figure 2, while in the deeper slices the corresponding images are not so clear. this is due to the combination of many reflected signals due to the presence inside the rooms of the collapse of the adjacent structures. from the excavations the found oldest building is a domus of the late republican period located in front of a line of buildings called tabernae which were used for shops and living quarters. this corridor of buildings connected the colosseum valley and the roman forum. the fire-raising of 64 a.c., signed the destruction of these buildings and the development of nero’s urbanism. the archaeological excavations have located a portion of the foundation of a portico, and a portion of a sewage system with s-n direction and a foundation with e shape, which defines and closes elagabalo’s thermae, [6], [7], figures 3 and 4. 2.2. st. john lateran basilica  the basilica of s. giovanni in laterano (st. john lateran) is the pope’s cathedral and the first public building constructed for christian worship. alongside it lies the first baptistery in western christendom. the complex has been the focus of sundry excavations since the 1730s. these have revealed traces of the earliest phases of both buildings, along with parts of the castra nova of the imperial horseguard, a bath complex and palatial housing. interpretation of these excavations is, however, difficult and most are either undocumented or only partially recorded. this project is undertaking an intensive scientific survey of the entire structure to integrate information from standing buildings, excavated structures and sub-surface features through the collaboration of newcastle university (uk), florence university (italy), musei vaticani and institute for technologies applied to cultural heritage (itabc-cnr, italy). the lateran project is investigating the entire complex to integrate information from standing buildings, excavated structures and sub-surface features. it seeks to understand the stratigraphic, spatial and functional relationships of the different elements underlying the modern complex [8]. the gpr surveys provide information on the spatial context and identification of phasing of subsurface features while the laser scanning provides a spatially accurate, detailed representation of the many materials, textures and structures of the lateran and archaeological remains [9]. during january and july 2012, a series of gpr surveys were conducted below the basilica, inside the archaeological area, and outside the basilica, to demonstrate the potential of this method for this analysis and to locate the expected archaeological structures. the aim of the gpr survey is to identify roman and highmedieval age remains which could enhance understanding of the ancient topography and the urban evolution of the study area. the main goals of this survey are the following: 1. to determine the full plan of the santa croce oratory, built by pope ilaro (v century) and destroyed by sixtus the fifth; part of this building has been identified by olof brandt within the excavated area adjacent to the baptistery. 2. to determine the full extent of the palatial housing found below the western part of the basilica. 3. to determine the limits of castra nova equiutum singularium, the barracks of the imperial horse guards established by the emperor septimius severus. 4. to locate the remains of the buildings of the lateran patriarchy. these are known from renaissance plans but up until now it has not proven possible to locate them all on the ground. for the measurements a gpr sir3000 (gssi), equipped with a 400 mhz (high-frequency) (gssi) bistatic antenna with constant offset and a 70 mhz (low-frequency) (subecho radar) monostatic antenna were employed. some signal processing and representation techniques have been used for data elaboration and interpretation. gpr surveys were performed, employing the indicated system to survey the selected areas outside the basilica in the s. giovanni in laterano square and in front of the basilica. the horizontal spacing between parallel profiles at the site was 0.50 m, employing the two antennas. radar reflections along the transepts were recorded continuously, with different length, across the ground at 40 scans/s; horizontal stacking was set to 3 scans. in the area outside the basilica a total of 777 adjacent profiles across the site were collected alternatively in forward and reverse directions employing the gssi cart system equipped with odometer. all radar reflections within the 90 ns for 400 mhz antenna and 195 ns for 70 mhz antenna (twoway-travel) time window were recorded digitally in the field as 16 bit data and 512 samples per radar scan. a nominal microwave velocity of about 8 cm/ns was determined from fitting hyperbolas to the raw field data. this was used in estimating a penetration depth from the gpr survey. figure  4.  coliseum  valley,  rome.  survey  2008.  gpr  time‐slice  at  the  depth of 2.70 m. the arrows indicate the remains of the front of a temple,  partially found in the excavated area.  acta imeko | www.imeko.org  september 2016 | volume 5 | number 2 | 84  all the gpr data were processed in gpr-slice v7.0 ground penetrating radar imaging software (goodman, 2011, 2013). the basic radargram signal processing steps included: (i) post processing pulse regaining; (ii) dc drift removal; (iii) data resampling, (iv) band pass filtering, (v) migration and (vi) background filter. with the aim of obtaining a planimetric vision of all possible anomalous bodies the time-slice representation technique was applied using all processed profiles [3], [9]. time-slices are calculated by creating 2-d horizontal contour maps of the averaged absolute value of the wave amplitude from a specified time value across parallel profiles. the squared amplitudes were averaged horizontally every 0.25 m along the reflection profiles 3 ns (for 400 mhz antenna) and 6 ns (for 70 mhz antenna) time windows (with a 10 % overlapping of each slice). the resampled amplitudes were gridded using the inverse distance algorithm with a search radius of 0.75 m, figures 5 and 6. in figures 5 and 6 the time-slices (at the estimated depth of 1.40 m in front of the basilica and 2.0 – 2.40 m in the nw of the basilica) for the investigated area are shown. on this map the individuated anomalies are clearly visible. the gpr slice in figure 5 (left) is characterised by structures with different dimensions and shapes. at the estimated depth, the squared structure with l-shape and surface of about 130 m2 is visible. close to this structure, there is a rectangular body characterised on the left edge by a sequence of small squared rooms with dimensions of 1.2 × 1.2 m. in front of these anomalies it is possible to recognize two semi-circular anomalies with a diameter of about 10 m. these anomalies obtained in front of the basilica, figure 5, can be related to the remains of the early christian bishop’s palace (high medieval period) [8]. 3. conclusion  the location, depth, size and vertical overlapping of the buried buildings were effectively estimated from nondestructive ground remote sensing with a ground penetrating radar system, together with information on the geomorphology and on the archaeology of the two sites. the employment, on the same area, of two different frequencies (high and low frequencies) makes possible the integration of the results along the time (depth) vertical scale. this is possible because the high frequency (400 mhz) can detect small features till the depth of 4.5 m and the low frequency (70 mhz) can penetrate deeper detecting biggest features as foundations (as the case of palatine hill). the archaeological excavations made at palatine hill (by prof. clementina panella – sapienza university of roma) during the last years, have confirmed the structures individuated with the gpr method. the results presented in this paper are only related to the first period of the project; new comparison and integration between the gpr images and the archaeological excavation of the last few years are part of the prin2012 project. the new surveys are planned to investigate the area between the coliseum, the costantino’s arc and the via sacra at roman forum. ground penetrating radar (gpr) survey at the st. john lateran basilica has produced significant and fruitful results. the use of two different antennae, as the case of palatine hill, has enabled to reach depths of up to 7 m. the gpr images obtained in front of the basilica, figure 5, are characterised by structures with shape similar to that expected medieval archaeological structures but with different orientation [8]. this result is in accordance with the orientation of the castra and of costantino’s basilica, while it departs from the knowledge of ancient topography. the comparison between the results obtained with gpr surveys with the archaeological knowledge is still in progress. acknowledgement  the authors are very grateful to daniele verrecchia (itabc) for his fruitful collaboration during the surveys, the team of prof. c. panella for the support inside the palatine project, the team of newcastle university for the topographic support and authorities of musei vaticani and st. john lateran basilica for their help during the survey. references  [1] s. campana and s. piro, seeing the unseen. geophysics and landscape archaeology, crc press, taylor & francis group, oxon uk, (2009), isbn 978-0-415-44721-8. [2] i. trinks, p. karlsson, a. biwall and a. hinterlaitner, mapping the urban subsoil using ground penetrating radar – challenges and potentials for archaeological prospection, archaeoscience, revue d’archeometrié, 2009, suppl. 33, pp. 237-240. [3] d. goodman and s. piro, gpr remote sensing in archaeology, 2013, springer (ed), isbn 978-3-642-31856-6, isbn 978-3-64231857-3 (ebook), doi 10.1007/978-3-642-31857-3. springer, berlin, (germany). figure 5. st. john lateran square. gpr time slices in the time window 58‐61  ns twt (estimated depth 1.4 m) for the 400 mhz antenna.  figure 6. st. john lateran square. gpr time slices at the estimated depth 2.0  and 2.4  m, for the 400 mhz antenna.  acta imeko | www.imeko.org  september 2016 | volume 5 | number 2 | 85  [4] s. piro s. and d. goodman, integrated gpr data processing for archaeological surveys in urban area. the case of forum (roma, italy), 2008, 12th international conference on ground penetrating radar, june 16-19, 2008, birmingham, uk. proceedings extanded abstract volume. [5] s. piro and c. panella, geophysical and archaeological investigations on the n-e foot of palatino hill (rome, italy), 2005, proceedings of 6th international conference on archaeological prospection. ed. piro s., (isbn 88-902028-0-7), pp 253-257, cnr, rome. [6] c. panella, s. piro, e. brienza and s. zeggio, indagini geofisiche ed archeologiche in area urbana. il caso delle pendici orientali del palatino (foro romano, roma), 2008, in atti del workshop geofisica per l’archeologia. possibilità e limiti, (cistec, ed., roma), pp 117 – 124. [7] c. panella, scavare nel centro di roma. storie uomini paesaggi, 2013, edizioni quasar, roma. [8] i. haynes, p. liverani, i. peverett, s. piro, g. spinola, progetto laterano: primi risultati. in rendiconti – atti della pontificia accademia romana di archeologia (serie iii), vol. lxxxvi, aa 2013-2014, 2014, pp 125-144, issn 1019-9500. [9] v. gaffney, s. piro, i. haynes, m. watters, s. wilkes, m. lobb., d. zamuner, three-tier visualization of san giovanni in laterano, rome, italy, 2008, 12th international conference on ground penetrating radar, june 16-19, 2008, birmingham, uk. proceedings extanded abstract volume. acta imeko, title acta imeko issn: 2221-870x july 2017, volume 6, number 2, 93-98 acta imeko | www.imeko.org july 2014 | volume 6 | number 2 | 93 decoding of emotional responses to user-unfriendly computer interfaces via electroencephalography signals natsue yoshimura1, 2, 3, osamu koga1, yu katsui1, yousuke ogata1, 2, hiroyuki kambara1, yasuharu koike1, 2 1laboratory for future interdisciplinary research of science and technology, tokyo institute of technology, 4259 nagatsuta-cho, yokohama 226-8503, japan 2integrative brain imaging center, national center of neurology and psychiatry, 4-1-1 ogawa-higashi, kodaira, 187-8551 tokyo, japan 3national institute of neuroscience, national center of neurology and psychiatry, 4-1-1 ogawa-higashi, kodaira, 187-8502 tokyo, japan section: research paper keywords: brain-machine interfaces; human-computer interaction; usability; eeg; emotion citation: natsue yoshimura, osamu koga, yu katsui, yousuke ogata, hiroyuki kambara, yasuharu koike, decoding of emotional responses to user-unfriendly computer interfaces via electroencephalography signals, acta imeko, vol. 6, no. 2, article 17, july 2017, identifier: imeko-acta-06 (2017)-02-17 editor: paolo carbone, university of perugia, italy received may 10, 2015; in final form july 4, 2017; published july 2017 copyright: © 2017 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: this work was supported by the center of innovation program from japan science and technology agency, and jsps kakenhi grants 26112004, 15h01659, and 15k01849. corresponding author: natsue yoshimura, e-mail: yoshimura@pi.titech.ac.jp 1. introduction in our daily lives, we are surrounded by computers and are sometimes forced to use complicated, user-unfriendly interfaces. these situations may, in some cases, be due to a lack of methods for measuring emotional response to usability. therefore, there is a real need for establishing an objective and quantitative evaluation method for interfaces. human activities can be modeled as a decision-making system defined by multiple parameters. a user decides on an action to interact with an interface, such as pressing a button or pulling a lever. this action then results in a change in the interface’s state, such as a change on a screen or an action by a connected machine. this change, in turn, affects parameters for the user, who recognizes the interface change and plans the next action in response. emotion is one such parameter for humans in this model [1]. humans typically feel comfortable with an easy-to-use interface but frustrated with an ill-behaving one. if we could objectively and quantitatively evaluate these emotional states in response to an interface, it would be an effective measurement for usability and may yield an adaptive interface that can change its usability depending on the user’s concurrent feeling. abstract when a user interacts with an interface such as a computer, its effects on specific biosignals may reflect emotional responses to the interface, providing a means to evaluate usability. towards the development of an interface that can adapt its usability based on the user’s emotions, here we decoded electroencephalography (eeg) activity occurring during interaction with a user-unfriendly interface. participants performed target-reaching tasks while irregular transformations were applied to cursor motion to induce frustration. our results showed that differential signals from the frontal electrodes (af3-af4) were sufficient to classify between brain activities during transformed (frustrated) and normal cursor motion (non-frustrated). functional magnetic resonance imaging during the same tasks showed significant activations in the middle frontal gyrus, orbitofrontal gyrus, and inferior parietal cortex, areas found to be related to negative emotions. altogether, these results suggest that the usability of an interface can be measured from eeg signals, which could aid in the development of adaptive interfaces that increase its intuitiveness. acta imeko | www.imeko.org july 2014 | volume 6 | number 2 | 94 numerous studies have been performed on recognition of emotions from biological signals [2]-[8]. among these studies, electroencephalography (eeg) has attracted particular attention [9], as it allows for a highly direct measure of emotional state, with the brain controlling the relevant mechanisms for emotion. li and lu focused on high gamma band eeg and showed that happiness and sadness can be distinguished with over 90 % accuracy [3]. asymmetric activity in the frontal region is also known to occur when emotions are evoked [10], and petrantonakis and hadjileontiadis utilized this phenomenon to distinguish between six emotional states [5], [6]. methods such as those applied in previous studies could also be used to detect emotional responses to user interfaces, providing a novel, objective measure of usability. the goal of this paper was to identify a user’s frustration in using a computer interface to objectively evaluate the userfriendliness of the interface. to do so, we first sought to distinguish between states of frustration and calm. we induced frustration by transforming the user inputs to the interface that made the interface more difficult to use. we acquired eeg data while subjects used the normal (i.e. calm) and transformed (frustrated) interfaces, and applied a machine-learning method to classify between the tasks. to confirm that our eeg data were picking up the emotional signals of frustration, we performed a functional magnetic resonance imaging (fmri) experiment using the same tasks to ensure that brain regions corresponding to negative emotions were elicited. 2. methods 2.1. participants three healthy individuals (two males and one female, 22-25 years old, right-handed) participated in the eeg experiment. one healthy male (50 years old) participated in the fmri experiment. written informed consent was obtained from all the participants prior to the experiment. the experimental protocol was approved by the ethics committee of tokyo institute of technology. 2.2. experiment tasks a red cursor, controlled with a trackball, was initially presented at the center of a computer screen. participants were instructed to manipulate the trackball with their right hand to move the cursor to a target designated by a blue circle. (figure 1). transformations were applied to the trackball output so that cursor movements differed from the trackball’s movements. we expected this transformation to elicit participants’ frustrated feelings as the interface became more difficult to use. we applied two types of transformations: “rotation” and “acceleration.” for rotation tasks, a rotation around the origin was applied; when the trackball pointer was at (x, y) and rotation angle was θ, the cursor was shown at (xcosθ + ysin θ, ycosθ − xsinθ) (figure 2a). for the acceleration task, the speed of the cursor increased with distance between the trackball pointer and the origin. when the distance of the trackball pointer was 𝑑 = �𝑥2 + 𝑦2, (1) the cursor was shown at (kdx, kdy), where k is an acceleration coefficient (figure 2b). we used four transformations for the experiments: rotation (θ= π/3), rotation (θ= −π/2), acceleration (k = 0.02), and normal (no transformation). we chose these transformations specifically to elicit different emotional states in response to the user-unfriendly interface. as we found that rotation transformations were easy to learn after a few sessions, so we employed two rotation tasks of different angles to avoid learning effects. 2.3. eeg experiment eeg signals were recorded using 32 active electrodes (g.usbamp and g.ladybird from g.tec medical engineering, graz, austria) from positions according to the extended international 10-20 system (figure 3). sampling frequency was 256 hz and a 50 hz notch filter was applied during recording. figure 1. positions of the 8 reaching targets (a) and the three periods in a trial, reaching task period (b), “still” period (c), and blinking period (d). figure 2. axis transformation between mouse pointer and cursor. (a) “rotation” transformation, (b) “acceleration” transformation. figure 3. eeg electrode positions. acta imeko | www.imeko.org july 2014 | volume 6 | number 2 | 95 five sessions were performed for each transformation, resulting in 20 sessions per participant. each session consisted of eight trials for the eight target positions in random order (figure 1a). the cursor was initially placed at the origin (a red point in figure 1b). after each reaching task was completed, a 2-second “still” period immediately followed (figure 1c). eeg signals during the still period were used to decode their feelings elicited by the task to avoid the effects of motion intention. we expected that task-related emotional effects would remain during the period. during the still period, participants were instructed to gaze at the same position until the reached target disappeared. then a 3-second “blink” period followed (difure 1d), during which participants were allowed to blink freely. we set the blink period to prevent eye-blinking signals from contaminating the brain activity signals during the task period. 2.4. fmri experiment a 3.0-t signa scanner equipped with a 9-channel array coil (general electric company, fairfield, connecticut) was used for the fmri experiment. functional data were acquired with an echo planar imaging sequence. a functional run consisted of 16 trials for the normal condition, 8 trials for each of the two rotation conditions, and 8 trials for the acceleration condition. we acquired twice the number of normal trials because the participant could perform the reaching task much faster during the normal condition than during other conditions. five runs were performed. during a trial, a target position was randomly chosen from the eight points (figure 1a), and all eight points were chosen twice (for the normal condition) or once (for the other conditions) per run. the cursor was initially placed at the origin (figure 1b). after each reaching task was completed, a still (rest) period immediately followed (figure 1c). the period for the rest was randomly chosen from 1, 2, or 3 s. since eye movements and blinking were not restricted in the fmri experiment, trials did not have a blink period. 2.5. eeg signal processing eeg signals during the 2-s still period were used for further analysis. we used a feature extraction method based on [11]. a 256-point fft was applied to the eeg signals using a 250-ms window and a 125-ms window overlap. we then used the first 40 bins (1-40 hz) of the mean spectrum as features for classification. this feature extraction was performed for signals from single electrodes as well as differential signals between neighbouring electrode pairs. 2.6. classification analysis using eeg signals we conducted a classification analysis to determine if emotional responses to the user-unfriendly interface could be identified from the eeg signals. data for each trial were labelled according to their axis transformations, and the labels were predicted using a machine learning technique. since there were two types of rotation trials, we mixed those trials and randomly eliminated half of them. using a support vector machine [12], 10-fold cross validation was performed to calculate classification accuracy. specifically, with 40 trials acquired for each condition, 36 trials 2 conditions (normal vs. rotation, or normal vs. acceleration) were used for training, and 4 trials 2 conditions were used for verification. following previous studies on negative emotions [10], [13]-[15], we used only frontal and parietal electrodes for classification. net accuracy was calculated by averaging classification accuracy across iterations. welch’s t-test was conducted on the results to test significance. 2.7. fmri image processing and general linear model analysis fmri data were pre-processed and analyzed using statistical parametric mapping software, spm12 (wellcome department of cognitive neurology, uk; available at http://www.fil.ion.ucl.ac.uk/spm). pre-processing steps included slice-timing correction, motion correction, coregistration of the functional and anatomical images, normalization into standard montreal neurological institute (mni) space, and smoothing using an 8-mm gaussian kernel. general linear model analysis was performed for three contrasts: (rotations and acceleration) > normal, rotations > normal, and acceleration > normal. 3. results 3.1. eeg classification results classification accuracies for normal vs. acceleration are shown in figures 4a and 4b. for all participants, accuracies using the differential signal for parietal electrodes (c3 (minus) cz) were significantly higher than chance level of 50 %. feature vectors using (c3 cz) worked well for all participants (participant 1: 76.3 % (p = 2.97 10-4), participant 2: 86.3 % (p = 3.50 10-4 ), participant 3: 80.0 % (p = 1.01 10-4). the differential signal for frontal electrodes (af3 af4) also showed significantly higher accuracy than chance level for all participants (participant 1: 75.0 % (p = 4.39 10--5 ), participant 2: 77.5 % (p = 8.66 10--5), participant 3: 65.0 % (p = 0.0330)). figure 4. classification accuracies using eeg signals. (a) binary classification results comparing acceleration and normal conditions using differential signal c3 – cz; (b) binary classification results comparing acceleration and normal conditions using differential signal af3 – af4; (c) binary classification results comparing rotation and normal conditions using differential signal c3 – cz; (d) binary classification results comparing rotation and normal conditions using differential signal af3 – af4. *p < 0.05, **p < 0.01, ***p < 0.001 by t-test. acta imeko | www.imeko.org july 2014 | volume 6 | number 2 | 96 classification accuracies for normal vs. rotation are shown in figures 4c and 4d). participant 2 did not show significance for either differential signal (c3 cz: 61.3 % (p = 0.0967), af3 af4: 50.0 % (p = 0.50 )). other participants also showed low classification accuracies, except participant 3 for af3 af4 (75.0 %, p = 1.96 10-4 ). 3.2. fmri activation areas figure 5a shows three significant activation areas that were obtained by the contrast (rotations and acceleration) > normal (p < 0.001, uncorrected for multiple comparisons). we expected the contrast to show the net effect of transformed cursor movement on brain activity. the largest cluster was located in the left middle frontal gyrus (mfg) (mni coordinates: [-48, 44, 26], t=3.61), and the second largest cluster was in the right lateral orbitofrontal cortex (lofc) (mni coordinates: [36, 54, -10], t=3.53). the left inferior parietal lobe (ipl) also showed a small activated-cluster (mni coordinates: [-52, -36, 58], t=3.13). looking at the rotations > normal and acceleration > normal contrasts (figures 5b and 5c), the rotation condition showed activated areas in the left ipl (mni coordinates: [-46, 38, 30], t=4.02, p < 0.001, uncorrected) and right mfg (mni coordinates: [28, 6, 56], t=3.24, p < 0.001, uncorrected). the acceleration condition showed many activated areas, and the largest cluster was located in the left and right ipls (mni coordinates: left, [-62, -36, 46], t=4.63, p < 0.05, family-wise error corrected; right, [68, -24, 34], t=4.42, p < 0.001, uncorrected). the right lofc was also the third largest activated area (mni coordinates: [36, 54, -10], t=4.15, p < 0.001, uncorrected). 4. discussion our results showed that significantly high classification accuracies especially in acceleration vs. normal classification figure 5. activation areas in general linear model analysis for contrasts (rotations + acceleration) > normal (a), rotation > normal (b), and acceleration > normal (c). all activated areas shown are significant at a threshold of p<0.001, uncorrected for multiple comparisons. acta imeko | www.imeko.org july 2014 | volume 6 | number 2 | 97 were obtained for all participants using only two eeg electrode signals, suggesting that brain activity varied enough across cursor movement transformations to be discriminated. since the eeg data were not contaminated by electrooculogram or motion artifacts, and all eight target positions were mixed in the classification analysis, the variance in brain activity can be interpreted as a reflection of activity elicited by the cursor transformation. 4.1. difference between rotation and acceleration as shown in figure 4, classification performance for rotation (c and d) was lower than that for acceleration (a and b), though the participants reported that rotation was more frustrating than acceleration. we think this is due to the timing of the occurrence of the frustration. rotation provided difficulty at the initiation of movement, while acceleration posed difficulty towards the end of the task period because it was hard to stop the cursor exactly on the target. we used eeg data from the time period immediately after the reaching tasks were completed to evaluate frustration, to ensure that the data did not contain other effects such as motion and movement directions. thus, it is possible that the time period used included more frustration elicited by acceleration than by rotation. 4.2. validity of the electrode positions providing high accuracies under the pleasure-arousal-dominance (pad) model [16] and its two-dimensional variation [17], [18], which employs arousal (intense calm) and valence (positive negative) as its orthogonal axes, emotions related to user-unfriendliness of interfaces could be considered as high arousal and low valence. in acceleration vs. normal (figure 4a), c3 cz showed high accuracies for all participants. this is comparable with results from previous studies that showed a relation between unpleasant image stimulation and parietal regions [14], [15]. electrodes in frontal regions, af3 af4, were also found to be effective for classification (fig. 4b). kostyunina and kulikov [13] reported that alpha wave power significantly increased at f3, t4, and o1 when feeling anger, which is considered a stronger emotion than annoyance or irritation [19]. disgust, which also has high arousal and low valence, also can be distinguished from a calm state using right frontal electrodes [10]. that may be the reason why af3 af4 showed relatively high accuracy in our results; the differential signal enhanced activity difference between the right and left hemispheres. 4.3. comparison of fmri and eeg results although fmri was performed for only one participant, we believe the data, as it compares with existing findings, does provide some insight into the emotional responses elicited by the tasks. even if we could not obtain high classification accuracies in rotation vs. normal condition due to the time lag between task period and rest period (i.e. data used for analysis), fmri analysis revealed if negative emotions were elicited by both of the cursor transformations. our fmri analysis showed activation mainly in the left and right mfgs, right lateral orbitofrontal gyrus, and left and right ipls. the left and right mfgs, that were mainly activated in the rotation condition, are associated with frustration [8] as well as negative feelings of sadness and anger [7]. the right lofc is associated with negative feelings elicited by angry face observation [20] and stress induction [21] as well as regulation of negative emotions [22]. the ipl relates to various cognitive functions, including attention [23], language [24], action processing, and emotional action observation [25]-[28]. considering our experimental tasks, emotional action observation is unlikely to have been the cause. rather, the ipl activation might have been due to reorienting and spatial attention [23], [29], which are motor planning and action-related functions [30]. controlling the cursor under transformed motion required motor planning for the entire duration of the task. furthermore, ipl activation was lower in the merged contrast (rotation and acceleration) > normal than the other contrasts. this may have occurred because the coordinates of the activated voxels in ipl were not exactly the same between acceleration and rotation, suggesting different strategies were employed for the reorienting or motor planning. comparing the relevant activation areas in fmri with the eeg electrodes positions used for classification, differential signal af3-af4 may have included brain activity around the right and left mfg and right lofc, and c3-cz may have included activity around the right and left ipl. to prioritize activity related to emotional response, it might be better to focus on af3-af4 differential activity in future work. further investigation with an increased sample size and eeg source localization analysis is also needed before more definitive interpretations can be made. 5. conclusions in this study, we used eeg to evaluate the usability of a human interface. we employed target-reaching tasks with transformations applied to cursor motion to introduce userunfriendliness. fft-based feature extraction and support vector machine classification revealed that two electrode signals from frontal regions were sufficiently effective in discriminating between user-friendly and user-unfriendly conditions. an fmri experiment using the same tasks further revealed activation in the left mfg and right lofc, which were previously reported as areas relevant to negative emotions. these results support our future plans to develop an interface with the ability to adapt its usability based on the user’s emotional response. references [1] p. rani, n. sarkar, c.a. smith, and j.a. adams: ‘affective communication for implicit human-machine interaction’, 2003 ieee international conference on systems, man and cybernetics, vols 1-5, conference proceedings, (2003) pp. 48964903. 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[30] s. caspers, k. zilles, a.r. laird, and s.b. eickhoff: ‘ale metaanalysis of action observation and imitation in the human brain’, neuroimage, 50, (3), (2010) pp. 1148-1167. decoding of emotional responses to user-unfriendly computer interfaces via electroencephalography signals template for an acta imeko event paper acta imeko issn: 2221-870x december 2016, volume 5, number 4, 12-18 acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 12 mn·m torque calibration for nacelle test benches using transfer standards christian schlegel, holger kahmann, rolf kumme physikalisch-technische bundesanstalt, bundesallee 100, 38116 braunschweig, germany section: research paper keywords: torque calibration; transfer standard; nacelle test bench citation: christian schlegel, holger kahmann, rolf kumme, mn·m torque calibration for nacelle test benches using transfer standards, acta imeko, vol. 5, no. 4, article 3, december 2016, identifier: imeko-acta-05 (2016)-04-03 section editor: lorenzo ciani, university of florence, italy received september 21, 2016; in final form november 2, 2016; published december 2016 copyright: © 2016 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: the research within the project has received funding from the european union corresponding author: christian schlegel, e-mail:christian.schlegel@ptb.de 1. introduction the portion of renewable energies for electricity production is rising dramatically. for instance, last year the fraction of renewable energies in germany was 32.5 %. the portion of energy which was produced by wind energy was thereby 44.3 %, based on all renewable energies. the german government will increase the share of renewable energy in power generation to 40 to 45 % by 2025. from this development it is clear that wind energy is likely to provide the greatest contribution to this planned expansion. associated with this is a significant increase in the performance of the wind generators. this will lead to individual wind turbines more powerful in height, in wing span as well as in the provided electrical power, as seen in figure 1, taken from [1]. of course, reliable energy production from wind turbines will strongly depend upon their technical reliability. for that reason, several nacelle test benches have been established in the past. one crucial parameter for such a nacelle is the torque load which is initiated in the field according to the strength of the wind field. in nacelle test benches, instead of using wind power, a special motor is used to create the torque. often an additional device is located between the motor and the nacelle to create axial forces as well as parasitic bending forces and moments onto the drive train. one of the most important parameters of such test benches is the torque which is initiated in the nacelle. so far, no traceable calibration to national standards has been performed in such test benches. the paper will show calibration possibilities which already exist and also show future prospects. figure 1. development of the size of wind turbines. abstract to verify all technical aspects of wind turbines, more and more nacelle test benches have come into operation. one crucial parameter is the initiated torque in the nacelles, which amounts to several mn·m. so far, no traceable calibration to national standards has been performed in such test benches. the paper will show calibration possibilities which already exist and also show future prospects. acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 13 the torque m is directly related to the electrical power pel and depends on the revolution speed n, respectively of the frequency f. 60 , 2 n f n p m el = ⋅ = π . (1) it should be noted that the relationship between f and n in (1) is for the case that the revolution speed is given in rounds per minute, rpm. table 1 shows an example of two points of operation. as shown in (1), the torque can be determined by an electrical power measurement. nevertheless, this kind of measurement results in relative uncertainties of several percent; that is why a more precise mechanical measurement of the torque is necessary. special torque transducers were built to measure the torque in the drive train. unfortunately, not all of these transducers are calibrated in the mn·m range due to the lack of a calibration facility. sometimes they are calibrated partially and extrapolated, e.g. via finite element calculations, to the nominal torque [2]-[4]. the paper is organised like this that in chapter 2 an existing standard torque calibration machine will be described where traceable calibrations up to 1.1 mn·m can be performed. in chapter 3 a novel 5 mn·m torque transducer and his calibration according the existing standard din 51309 with the described machine from chapter 2 will be introduced. in addition a proposal will be made how one could extrapolate calibration data to a non measurable range. in chapter 4 a new calibration machine will be introduced which is able to calibrate torques up to 20 mn·m. 2. about the traceable calibration of torque according to the definition of the torque m  , traceability can be realized via a lever arm l on which a force f  which is acting perpendicular, in the simplest case, is the gravitational force m·gloc, whereby m is the mass and gloc is the gravitational constant. ( ) lrlgmfrfrm frm loc =⋅⋅=⋅⋅= ×= ,,sin   . (2) based on this principle, there are torque calibration machines, which, for the force initiation, use mass stacks which can be varied in such a way that various torque steps can be applied. often the masses are coupled to the lever arm by means of thin metal bands (e.g. 20 to 30 µm) to get a very precise contact point for the deadweight force. several aspects of machine design and related measurements can be found in [6]-[12]. unfortunately, this principle cannot be further used by very high torques which are in the mn·m range. in these cases, one can apply a system where the torque is created by an actor lever system and measured by a second force lever system [7]. the principle is illustrated in figure 2 where the levers are indicated in red. between the two levers the torque transducer is mounted indicated in yellow in figure 2. this principle is used, for example, by the ptb 1.1 mn·m standard calibration machine [7], see figure 3. on the measuring side, which is the upper traverse in figures 2 and 3, the lever is connected at both ends with force transducers. in this way, the torque is traced by the length of the lever arm and the measured forces of the calibrated force transducers. in the machine, two different pairs of force transducer can be used, one (120 kn) for a lower range up to 220 knm and one pair (550 kn) for the upper range up to 1.1 mn·m. in the range up to 220 knm one obtains a relative uncertainty of the torque of 1.0·10-3, whereby in the upper range one gets 0.8·10-3. figure 2. working principle of the double lever arm system of the 1.1 mn·m torque calibration machine. figure 3. standard torque calibration machine for nominal torques up to 1.1 mn·m. table 1. examples of the relation of electrical power and torque. electrical power mw revolution speed rpm frequency hz torque mn·m 5 14 0.23 3.4 10 9 0.15 10.6 acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 14 the torque itself is created by two mechanical spindles which are driven by a motor, as main drives which are located between the two lower platforms, see figures 2 and 3. in figure 3 the blue boxes indicate gears, whereby the small boxes represent manually gears and the bigger boxes motor driven gears. in addition, there is a secondary drive to define the horizontal position (left) and to reduce the cross force (fx1, fx2) and the bending moment (mz1, mz2). to compensate for the vertical force fz generated by the transducer weight and the lower lever arm, a hand-operated drive unit (under the lower lever arm) can be used. spring elements (indicated by the blue cylindrical elements in figure 3) are connected to the measuring lever to measure the parasitic mechanical components in the contact area between force transducer and lever arm. with the aid of these spring elements, all parasitic components can be minimized during a calibration procedure. last but not least, a reference torque transducer is mounted in the lower part of the machine (below the red flange) which is, in addition, equipped with measuring bridges for bending forces and moments. this transducer offers additional possibilities to check the adjusted torque and the alignment of the whole measuring axis. the uncertainty of the torque which is provided by the machine mainly depends on the lengths l1 and l2 of the double lever arm, on the measured force values f1 and f2 at both ends of the lever arm and on the remaining parasitic moments mz1 and mz2 which are due to not fully compensated bending components measured on the spring elements that are connected with the force transducers. the model for the uncertainty evaluation of the provided torque is: 212211 zz mmlflfm ++⋅−⋅= . (3) the length of the lever arm was measured with a special coordinate measuring arm within an accuracy of δl = 100 µm. the force transducers can be calibrated in the ptb 1 mn force standard machine. the calibration of the transducers is repeated in a period of two years. thereby, a relative measuring uncertainty of better than 1.0·10-4 can be achieved. nevertheless, in the uncertainty budget, a more conservative value of 1.0·10-3 was taken. for technical reasons, the lever arm cannot be dismounted every two years. therefore, random deformation measurements using laser interferometers are performed to monitor the stability. as seen from table 2, the main uncertainty contribution results from the calibration of the force transducers. the contribution from the parasitic moments mz1 and mz2 are very small and could actually be neglected. the chosen distributions are all rectangular because in most of the cases, where values taken from calibration certificates (here f1, f2 and l1, l2) this distribution is recommended, see also [13]. for the bending moments mz1, mz2 also a rectangle distribution is applied to get a more conservative uncertainty. anyway the rectangle distribution should be applied if no details about a possible distribution of a certain value is known [13], which applies for the case of the bending moment. 3. torque transfer transducers for nacelle test bench calibration in this section, we will show the partial calibration of a 5 mn·m torque transfer transducer and secondly describe a procedure of how to extrapolate torque values if only a partial range was calibrated. this procedure will be illustrated by means of the data of the reference transducer of the 1.1 mnm calibration machine. 3.1. calibration of a 5 mnm torque transducer in the partial range up to 1.1 mnm in order to realize a traceable calibration for nacelle test benches, the empir project “torque measurement in the mn·m range” was started in october 2015 [5]. one of the objectives of this project is to develop novel traceable calibration methods for torque values in nacelle test benches with the use of transfer standards for the range above 1 mn·m. for the realization of this goal, a commercial torque transducer with a nominal range of 5 mn·m will be used, see figure 4. during the above-mentioned empir project, an extrapolation procedure for the range above 1.1 mn·m will be developed. in figure 4, the commercial transducer is depicted together with 2 dmp 41 bridge amplifiers. the transducer is equipped with two independent torque channels, two channels for transverse bending forces, two channels for bending moments and two channels for axial forces. due to these additional channels, an investigation of multi-component loading on the measurement of torque will be possible. in particular, crosstalk effects in the case of 6-component loading (main torque, 3 directional forces, 2 directional bending torques) will be studied to describe effects table 2. uncertainty budget of the 1,1 mnm standard calibration machine. parameter value standard unvertainty distrib. sensitivi ty coeff. uncert. index f1 500.0 0.289 rect. 1.1 0.32 49.6% l1 1.1 52.5·10 -6 rect. 500 0.026 0.3% f2 -500.0 0.289 rect. 1.1 0.32 49.6% l2 1.1 52.5·10 -6 rect. 500 0.026 0.3% mz1 0.02 0.0115 rect. 1.0 0.012 0.0% mz2 0.02 0.0115 rect. 1.0 0.012 0.0% m 1100.04 0.451 result value expanded uncertinty k-factor coverageintervall m 1100.04 0.90 2.00 95% normal-dist rel: 8.2·10-4 figure 4. 5 mn·m torque transducer together with two dmp41 bridge amplifiers for the readout of the 8 channels of the transducer. acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 15 on the torque measurements which occur in large nacelle test benches. finally, a calibration procedure for large nacelle test benches will be developed during the empir program. the calibration procedure will enable the traceability of torque loads up to 20 mn·m and will include an uncertainty model that considers crosstalk effects. first calibration measurements were performed with the shown 5 mn·m torque transducer in the ptb 1.1 mn·m standard calibration machine above described. thereby, the procedure according to the german standard din 51309 was applied [14]. figure 5 shows the calibration procedure according to din 51309. the diagram shows the relationship between the applied torques as a function of time. after three preloads (applying the nominal/maximum torque value of the transducer), a certain number of increasing and decreasing torque steps are performed in three mounting positions. mounting positions means that the transducer is rotated in respect to the axes where the torque acts, thereby often the three mounting positions 0º, 120º and 240º are used. at least 8 torque steps are required to determine a linear or polynomial fit. from the data, several characteristic parameters are derived which are used for the determination of the measurement uncertainty as well as for a classification of the torque transducer. the parameters are indicated in figure 5 in the calibration procedure. for example the hysteresis will be determined by the difference between the maximum and minimum torque value which were obtained in one certain torque step. thereby the data from all mounting positions as well as the steps from increasing and decreasing torque will be taken into account. more details about the indicated parameters of figure 5 and their use for an uncertainty determination can be found in [14]. one characteristic result of the calibration is the deviation from linearity shown in figure 6. the curves reflect the relative deviations of the measured values from a fitting straight line; the deviations are related to the measured mean value of the final value. the fitting straight line is defined by the value of its slope; the axis intercept is zero. the upper diagram in figure 6 shows the data of the 5 mn·m transducer, the lower diagram the data from the reference transducer, which was simultaneously calibrated. note the difference in the shape of the two diagrams. the shape of the reference transducer shows the common behaviour where a transducer is used up to his nominal value (end value). in the case of the 5 mn·m, the transducer was only used up to 22 % of its nominal value, which leads then to a curve shape as shown in figure 6. one of the aims in the mentioned empirproject is to find an extrapolation procedure using the data from the 1.1 mn·m calibration to extrapolate up to 5 mn·m. the significant linearity deviation might be used in this extrapolation procedure. during the calibration of the 5 mn·m transducer, also partial ranges within the 1.1 mn·m range were measured. this data will also be used to develop an extrapolation procedure. in addition, also the signals from the other six parasitic channels were recorded. the analysis of these data will provide information about the correlations of the torque with the bending forces and moments as well as the axial forces. 3.2. extrapolation of calibration data measured in a partial range to the full range in this subsection, a proposal will be made of how the calibration data measured in a partial range has to be extrapolated to the full range. a good starting point for this task is the data of the reference transducer because here, data are available from both a partial range and the full range. this gives us the opportunity to check how precise such an extrapolation will be. in figure 7, the interpolation deviation of the measurement up to 400 kn·m and the interpolation deviation up to the full range of 1.1 mn·m of the reference transducer are compared. in contrast to figure 6, all the three different mounting positions were averaged. as one can be seen from figure 7, the curves for upward or downward measurement exhibit a sinusoidal behaviour. due to this, the attempt is made to make a sinusoidal fit of that data. the equation function which was chosen for such a fit is:       − ⋅⋅+= w xx ayy cπsin0 . (4) thereby, y0 is the offset, a is the amplitude, xc is the zero, and w is the period of the sinusoidal function. these parameters are also illustrated in figure 8, where one can see the sinusoidal fit of the upward row (only increasing torque) of the full-range data of the reference transducer. the uncertainties of the parameters are quite small and lie in the ranges of a few percent. figure 6. the upper curve is the linearity deviation from a fitted straight line through the origin at zero of the 5 mn·m transducer. the lower curve shows the same behaviour of the 1 mn·m reference transducer .the deviations are related to the measured mean values of the final value. figure. 5. calibration procedure according to the german standard din 51309. indicated are several parameters which will be derived from the calibration data. this parameters as well as other additional influences will be used for an uncertainty determination. acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 16 the procedure for the extrapolation is now to fit the partial range data and the full range data with a sinusoidal function according to (4). knowing the fitted parameter, one can then think about a strategy of how to scale the parameters from the partial range function to the full range function. figure 9 shows the two fits of the partial range data and the full range data of the upward torque measurement. the horizontal, coloured dashed lines are the offsets of the two fitted functions. in view of the fact that the difference between the two offsets (y0) is very small, one could neglect these for an extrapolation. the parameters of the fits and their respective uncertainties can be seen in table 3. here one can see that the offsets have the largest uncertainty, whereby the zero and the period of the sinusoidal functions have smaller uncertainties. starting from the parameters of the partial range data, an extrapolated sinusoidal function was calculated which is shown as a dashed curve in figure 9. thereby, the offset y0 from the partial range was chosen, the parameters xc and w were scaled with a factor of 1100/400, (which is just the scaling of the measurement range) and the amplitude was calculated by scaling the amplitude of the partial range with the ratio of the amplitudes of the full range to the partial range. normally one would not know this amplitude ratio, but one could estimate it by measuring several partial ranges. in this way, one would obtain several amplitudes which could be extrapolated linearly to the full measuring range. with the aid of the extrapolated interpolation deviation curve, one can now calculate measuring points which lie outside of the partial range as follows: ex iil ex i yxmy ∆+⋅= . (5) thereby, the extrapolated measuring points yi ex depend from the slope ml of the linear interpolation in the partial range up to 400 kn·m and on the extrapolated interpolation deviation δyi ex which is shown in figure 9. last but not least, one can now compare the measuring points calculated according to (5) with the actually measured points in the full range. as one can be seen from figure 10 the difference between the actually measured points and the extrapolated ones is mainly below 0.1 %. this difference is smaller or just in the order of the uncertainty of the measured torque in this region. using the linear interpolation deviation data for an extrapolation procedure seems to be a feasible way. nevertheless it should be noted that in the actual case of the 5 mn·m transducer this proposed extrapolation procedure will be more uncertain due to the quite scattering data from the partial range, see upper part of figure 6. further measurements of different partial ranges could improve the situation. table 3. fitted parameters of the partial range data and of the full range data with a sinusoidal approximation according to (4). 400 kn·m parameter value uncertainty relative uncertainty % y0 2.93e-05 4.72e-06 16.1 xc 262.53 16.54 6.3 w 300.44 13.93 4.6 a 6.44e-05 6.53e-06 10.1 1.1 mn·m y0 5.74e-05 1.18e-05 20.5 xc 797.63 20.84 2.6 w 852.99 16.48 1.9 a 3.60e-04 1.79e-05 5.0 figure 7. interpolation deviation relative to the nominal torque of the reference transducer for a partial range up to 400 kn·m and the full range up to 1.1 mn·m. the curves are separated depending on whether the torque increased upwards or downwards. figure 8. sinusoidal fit of the interpolation deviation data of the upward torque measurement of the reference transducer in the full range. figure 9. interpolation deviation of the upward row of the part range and the full range data together with a sinusoidal fit. in addition an extrapolated sinusoidal function (dashed curve) is plotted. acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 17 4. development of a torque calibration machine for a range of up to 20 mn·m to extend the torque calibration range above 1.1 mn·m, a complete new calibration machine will be built at ptb. this machine will be part of a wind competence center which is funded by the german federal ministry for economic affairs and energy. this center also includes, besides the new torque calibration machine, a big coordinate measuring machine and a wind channel for the calibration of lidar systems. the coordinate measuring machine should be able, e.g., to geometrically measure the gear parts of the nacelles. the capacity will be sufficient for gearwheels with a diameter of up to 3 m. to realize this new center, two new buildings will be built on the ptb site: one for the coordinate measuring machine and the lidar system, and one especially for the new torque calibration machine. the new torque calibration machine (see figure 11) will be designed in a first stage for torques of up to 5 mn·m, and in a second stage up to 20 mn·m. similar to the 1.1 mn·m calibration machine, the operation principle will also be based on two lever systems: one actor lever and a measuring lever. on the actor lever, the forces will be created by two 1.2 mn servohydraulic cylinders. the lever has a length of 6 m. in addition, also bending moments and axial forces can be applied by a pair of horizontally aligned servo-cylinders. last but not least, the servo cylinders will also be able to operate dynamically in a frequency range of up to 3 hz. for that reason, the foundation of the machine is mounted on air springs. each spring can be individually adjusted in pressure to achieve optimal damping and to avoid resonance frequencies. the measuring lever system includes a pair of force transducers to measure the main force component of the torque and several spring elements for the detection of parasitic bending forces and moments. the base as well as the frame of the machine is designed already for 20 mn·m. by upgrading the machine with 3.6 mn hydraulic cylinders for the generation of the torque an extension to 20 mn·m will be realized. 5. conclusions to increase the reliability of wind turbines, extensive technical tests are performed in nacelle test benches. one important aspect of these tests is the torque in the mn·m range, which is initiated in the nacelle. traceable torque calibration in the mn·m range can so far only be realized by the 1.1 mn·m standard calibration machine at ptb. one solution for a traceable torque calibration of nacelles is to use transfer torque transducers which are calibrated in special standard calibration machines which are traced to the si. currently, a calibration of up to 1.1 mn·m of such a transducers was realized. for the above-mentioned torques, special extrapolation procedures have to be developed. one possibility could be to use in this extrapolation the knowledge of the characteristic sinusoidal shape of the interpolation deviation. to overcome the lack of calibration range, a new machine will be installed insight the ptb´s new wind competence center. this machine will be able to calibrate torques of up to 5 mn·m in a first stage and up to 20 mn·m in a second stage. acknowledgement the author would like to acknowledge the funding of the joint research project “14ind14 mn·m torque – torque measurement in the mn·m range” by the european union´s horizon 2020 research and innovation programme and the empir participating states. references [1] upwind “design limits and solutions for very large wind turbines”, the sixth framework programme for research and development of the european commission (fp6). [2] private communication, d. bosse, center for wind power drives (cwd), rwth-aachen. [3] emrp-program: “force traceability within the meganewton range”, http://www.ptb.de/emrp/2622.html [4] f. tegtmeier et. al., “investigation of transfer standards in the highest range up to 50 mn within emrp project sib 63”, xxi imeko world congress, 2015, prague, czech republic. [5] empir-program: “torque measurement in the mn·m range”, www.euramet.org/research-innovation/empir/empir-calls-andprojects/empir-call-2014/ [6] d. röske et. al., metrological characterization of a 1 n·m torque standard machine at ptb, germany, metrologia 51 (2014) 87-96. [7] d. peschel et. al., the new 1.1 mnm torque standard machine of the ptb braunschweig/germany, proceedings of the imeko-tc3 conference, cairo, egypt 2005, p. 40. [8] d. peschel et. al., “proposal for the design of torque calibration machines using the principle of a component figure 10. relative difference between the extrapolated measuring points and the measured points in the full range up to 1.1 mn·m. the difference is given in percent relative to the measured points. figure 11. design of a new standard torque calibration machine for torques up to 20 mn·m. http://www.ptb.de/emrp/2622.html http://www.euramet.org/research-innovation/empir/empir-calls-and-projects/empir-call-2014/ http://www.euramet.org/research-innovation/empir/empir-calls-and-projects/empir-call-2014/ acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 18 system”, proceedings of the 15th imeko tc3 conference, october 7-11, 1996, madrid, spain, pp. 251-254. [9] d. röske et. al., “key comparisons in the field of torque measurement”, 19th international conference on force, mass and torque, imeko tc3, cairo, 19-23, february, 2005, egypt. [10] d. röske et. al., “realization of the unit of torque determination of the force-acting line position in thin metal belts”, proceedings of the 15th imeko tc3 conference, october 7-11, 1996, madrid, spain, pp. 261-264. [11] d. röske et. al., “on the problem of lever arm lengths in the realization of the torque finite element calculations on lever models”, proceedings of the 14th imeko tc3 conference, september 5-8, 1995, warsaw, poland, pp. 178-182. [12] k. adolf, d. mauersberger, d. peschel, “specifications and uncertainty of measurement of the ptb's 1 knm torque standard machine”, proceedings of the 14th imeko tc3 conference, september 5-8, 1995, warsaw, poland, pp. 174-176. [13] gum, “guide to the expression of uncertainty in measurement”, iso, genf 2008, isbn 92-67-10188-9. [14] din 51309:2005-12, materials testing machines calibration of static torque measuring devices. https://de.wikipedia.org/wiki/spezial:isbn-suche/9267101889 mn m torque calibration for nacelle test benches using transfer standards determining the location of patients in remote rehabilitation by means of a wireless network acta imeko issn: 2221-870x september 2023, volume 12, number 3, 1 5 acta imeko | www.imeko.org september 2023 | volume 12 | number 3 | 1 determining the location of patients in remote rehabilitation by means of a wireless network oleksii tohoiev1 1 petro mohyla black sea national university, 68 desantnykiv str., 10, 54003 mykolaiv, ukraine section: research paper keywords: wi-fi; mac; wireless network; throughput; spatial flow; transport protocol citation: oleksii tohoiev, determining the location of patients in remote rehabilitation by means of a wireless network, acta imeko, vol. 12, no. 3, article 8, september 2023, identifier: imeko-acta-12 (2023)-03-08 section editor: susanna spinsante, grazia iadarola, polytechnic university of marche, ancona, italy received march 10, 2023; in final form june 13, 2023; published september 2023 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was supported by petro mohyla black sea national university (pm bsnu) in mykolaiv (ukraine), and also financed in part of the pm bsnu science-research work by the ministry of education and science of ukraine “development of the modules for automatization of wireless devices for recovery of postinfarction, post-stroke patients in individual conditions of remote individual rehabilitation” (state reg. no. 0121u109898). corresponding author: oleksii tohoiev, e-mail: oleksii.tohoiev@chmnu.edu.ua 1. introduction this article addresses an explanation of indoor location identification method usage developed to be of practical use in medical facilities. it is worth noting the numerous indoor localization methods having limitations, such as complexity of use (the necessity of special equipment being one of the complexities). the method described in this paper does not require additional maintenance or special equipment other than wi-fi access points. the reason why the method is unique lies in the fact that the one not requires the mobile device to be in hotspot mode. but more importantly, wi-fi access points location the only required data. the current paper shows that the acceptable accuracy of the location can be achieved in the real environment exclusively by means of the method described, which relies on simple hardware and software requirements. this paper presents a non-standard innovative way of using wi-fi access points (routers) as measuring equipment to determine the coordinates of a patient's location within a medical institution. to be able to use the described method, the patient must have some device with a built-in wi-fi module (smartphone, smart watch, etc.) or, for example, based on a microcontroller with wifi capabilities (cc3200) for ecg monitoring [1]. the work does not set the task of improving the accuracy of the indicated coordinates, a satisfactory result for the staff is the determination of the room in which the patient is currently located. this is especially important when it is necessary to provide assistance to patients after serious illnesses (who are in rehabilitation), when, for example, they were given an alarm signal or its biomedical signals are unstable. modern scientists are also actively considering the possibility of transferring data from body sensors of patients to remote cloud servers through access points [2]. determining the patient's location near certain telecommunications equipment will facilitate remote monitoring of the patient in the interests of telemedicine. 2. the current state and problems of determining the location of mobile network objects systems for determining the position of moving objects in space using a stationary network (for example, by means of parameters of telecommunications equipment) are effective for abstract this article discusses the indoor object's location identification method that had been developed to use in medical facilities. differently from other similar methods, the one reported in this paper requires access to wi-fi access points only and, most importantly, it does not require the mobile device to be in hotspot mode. it is proposed to use wi-fi access points (routers) with non-standard firmware as measuring equipment to determine the coordinates of the patient's location within the medical institution. thus, it can be claimed that using of the proposed method will help to achieve acceptable accuracy in real environments at real-time identification mobile devices using simple hardware and software requirements. mailto:oleksii.tohoiev@chmnu.edu.ua acta imeko | www.imeko.org september 2023 | volume 12 | number 3 | 2 controlling patient movement over short distances. such "moving objects" can include patients undergoing remote rehabilitation. in any case, such a mobile delivery network should be regarded as a corporate network with a single mac address plan and the ability to listen in (sniffing) on nodes connected via wireless communication means. in the classic sense, a sniffing attack refers to intercepting data by capturing network traffic using a packet sniffer (a program designed to capture network packets). sniffing is typically used by attackers to gain unauthorized access to confidential information, eventually leading to a network outage or damage, or to read messages circulating in the network. therefore, information security measures are aimed at countering sniffing attacks in computer networks [3]. the aforementioned traffic sniffing technology can be applied to determine the location of people moving around within an area where a wireless local network is deployed, if they have a gadget with them (such as a smartphone, tablet, smartwatch, etc.) that has a wi-fi module enabled. such an approach can also be useful for providing individualized conditions for the remote rehabilitation of post-stroke and post-heart attack patients. by using traffic sniffing technology, it is possible to determine the location of both the patient and the doctor, if necessary, to provide timely assistance to patients who are in remote rehabilitation and can move independently. currently, scientists and practitioners have conducted a significant amount of research on the problem of deploying a positioning system for moving objects that have a built-in wireless communication module. analysis of such research indicates that the error of existing methods ranges from 1.78 to 10 meters. scientists have considered various aspects of positioning and navigation methods [4], [5]. the most common technologies for positioning are satellite systems (such as gps and galileo) and terrestrial cellular networks. the tasks of determining the ultimate limits of positioning system accuracy, which are defined by the presence of noise and obstacles, remain completely unsolved and relevant; describing the operation of a number of new methods (such as direct positioning) aimed at enabling gps operation with very weak received radio signals (for example, in mountainous terrain); and methods of optimal combining of measurements based on radio signals and signals from various sensors, such as inertial platforms (including those with gyroscopes). research in new areas of cooperative positioning, where multiple nodes exchange signals and information to improve their positions, is also promising. in the last decade, one of the most popular solutions has been object localization based on wi-fi, which is considered the most promising for investigating open questions in both scientific and industrial communities. the main concept of a wireless wi-fi network is the presence of an access point (ap) that connects to the internet service provider and transmits a radio signal. usually, an ap consists of a receiver, a transmitter, a wired network interface, and software for quick setup. a network is formed around the ap within a radius of 50–100 meters (called a hotspot or wi-fi zone) where the wireless network can be used. the transmission distance depends on the transmitter's power (programmable in some equipment models), the presence and characteristics of obstacles such as antennas. today, the 802.11n standard is widely used, which provides a data transfer speed of up to 320 mbps [6]. methods of wi-fi positioning can be divided into two main groups. one is based on mapping and a map catalog compiled from data from satellite systems (ss) [4], and the other is based on modeling the propagation of radio waves (rf) [7]. the rf model determines the relationship between signal strength and distance. to determine the distance between known points and the patient, trilateration algorithms can be used [8]. however, currently, it takes several dozen measurements to determine the relationship between distance and signal strength. therefore, this model is not entirely dynamic and requires further research and improvement, such as 1) analyze the trends in the development of radio wavebased methods towards the implementation of projects for determining the position of moving objects. 2) substantiate the feasibility and implement the development of a mac-directed approach for monitoring numerous objects in a wireless corporate network. 3) develop an algorithm for detecting the location of moving objects based on a comprehensive approach to determining their positioning and identification. define the limits of using different methods for determining the location of moving objects in systems with a high level of information security. let us consider positioning based on radio wave propagation modeling (rf) [9]. the authors use rssi with bluetooth; however, this is not a suitable method for use at medical institution, because for this you need a separate device or turn on bluetooth separately on the smartphone. the purpose of such modeling is to express the mathematical relationship between the distance from the transmitter to the receiver and the signal strength. the mathematical expression is obtained from a thirdorder polynomial regression. the main advantage of this technology is the speed of positioning. in addition, an important point is that access points (ap) have fixed coordinates (figure 1). the development of methods for determining a patient's location based on a combination of signal characteristics from aps is currently a relevant issue. however, regression requires a large amount of accurate information on signal strength over a fairly long period. this method provides positioning accuracy within 1–3 m. an integral quadratic quality criterion is used to evaluate the effectiveness of positioning technologies [10]. for tracking patient movement, the radar radio frequency system [11] is also used. radar works by recording and processing signal strength information at several base stations located in the area of interest to provide overlapping coverage. it is possible to eliminate unwanted noise in sonar and radar systems by implementing the beamforming method proposed in figure 1. patient in a system with fixed access points api. acta imeko | www.imeko.org september 2023 | volume 12 | number 3 | 3 [12] and well proven in the building of separate infrastructure for medical telemetry applications. using radar technology [13], a mobile device uses a ccmap of the required location. the original cc-map is generated from the coordinates, cc measurements, and patient location. the signal strength from each ap is compared to the corresponding values in the database, and the appropriate location is predicted. the average error of this method is 1.78 m, but the maximum error can be up to 40 m [14]. thus, if it is necessary to determine the location of a moving object with greater accuracy, then it is necessary to turn to other methods of positioning such an object. 3. the method of analyzing the positioning of moving objects based on determining the relative signal strength existing mobile positioning methods designed for stationary objects do not allow to track moving objects, and they will not be picked up by a wi-fi sniffer [15]. the proposed search method can be divided into three stages: data collection, propagation modeling, and location determination, which are presented in figure 2. during the data collection stage, the server periodically requests (position 1 in figure 2) each ap to scan for signals. during the search stage, the terminal investigates the wi-fi spectrum for available access points and determines the received signal strength indication (rssi) that reaches the antenna of each ap (position 2 in figure 2). then this information is sent (position 3 in figure 2) to the search server, which determines the point on the modeled signal propagation map from the device with a wi-fi module on the moving object (patient). at the data collection stage, the search server queries each access point for rssi survey results (figure 3). the results, including the list of access points and their corresponding rssi, are stored in a database (db) on the server. each time the rssi data is obtained from the access points for the current time, new location parameters are calculated. when calculating location parameters for a specific access point, a request is sent to the database to measure the signal received from this access point and detected by another access point. to calculate the distance from the transmitter to the receiver based on the ieee 802.11 standard, rssi is used to measure the relative signal strength, which is typically measured in decibelmilliwatts (dbm). however, there are no specific formulas for calculating accurate distances due to the influence of the real indoor environment and the implementation by manufacturers. according to [9] and using a simple power loss propagation model, the rssi distance measurement is given as: 𝑃𝐿 = 𝑃𝐿0 + 10 𝛾 log10(𝑑 𝑑0⁄ ) + δ, (1) where pl represents the path loss at distance d (in m); pl0 (in dbm) is the total path loss at distance d0(m); 𝛾 is the exponent of path loss depending on the surrounding environment; d is the distance between the patient and the control access point; δ is the variable that takes into account the change in the mean value, it is also often referred to as shadow fading [16]. let us denote each record in the database using the notation 𝑅𝑆𝑆𝐼𝑗,𝑖 , where the subscript j denotes 𝐴𝑃𝑗 , on which the signal was measured, and the index i means 𝐴𝑃𝑖 , from which the measured signal came. when identifying a parameter 𝛾𝑖 for the propagation of the signal coming from 𝐴𝑃𝑖 , it is necessary to query the database for the measurements 𝑅𝑆𝑆𝐼𝑚 , and 𝑅𝑆𝑆𝐼𝑛,𝑖 , collected in the interval δ, where subscripts m, n represent a pair of aps that are not 𝐴𝑃𝑖 . the experimental assessment, as detailed in figure 3, demonstrated that better accuracy could be achieved by excluding measurements from the access point (ap) situated on the same wall as api. specifically, the performance of the parameters for ap2 in figure 1, computed based on measurements acquired at ap3 and ap4, outperformed those generated using measurements from ap1. the observed disparity in parameter estimation was attributed to the reflection of the wall on which the transmitting ap was mounted. knowing the spatial positions of access points, it is possible to obtain distances 𝑑𝑖,𝑚 і 𝑑𝑖,𝑛, which are the distances between 𝐴𝑃𝑖 and 𝐴𝑃𝑚 or 𝐴𝑃𝑛 . rewriting equation (1) with the introduced subscripts, we obtain an equation that can be used to determine 𝛾𝑖 . at the same time, 𝑑0 is taken equal to 1. thus, equation (1) turns into (2): 𝑅𝑆𝑆𝐼𝑚,𝑖 − 𝑅𝑆𝑆𝐼𝑛,𝑖 = 10 𝛾𝑖 log10 𝑑𝑖,𝑚 𝑑𝑖,𝑛 . (2) indicator 𝛾𝑖 loss of signal strength on the path for the access point 𝐴𝑃𝑖 can be calculated using the least-squares regression of equation (2). to explain this phenomenon, it is advisable to extend the signal loss model with a parameter 𝛽𝑖 , which takes into account the effect of the angle difference between the direction of the direct path of the signal and the normal vector to the wall on which the ap is installed. the extended log distance path model can be written as: figure 2. main stages of patient search. figure 3. example of signals emitted from ap1, detected at ap2, ap3, and ap4 and patient. acta imeko | www.imeko.org september 2023 | volume 12 | number 3 | 4 𝑅𝑆𝑆𝐼𝑚,𝑖 − 𝑅𝑆𝑆𝐼𝑛,𝑖 = 10 𝛾𝑖 log 10 𝑑𝑖,𝑚 𝑑𝑖,𝑛 + 10 𝛽𝑖 log 10 𝑑𝑖,𝑚 𝑑𝑖,𝑛 × (𝑎𝑖,𝑚 − 𝑎𝑖,𝑛 ) (3) where the introduced additional symbol αi,j is defined as: 𝑎𝑖,𝑗 = ∢(𝑛𝑖 , 𝑠𝑖,𝑗 ) 𝜋 2⁄ . (4) using (3), it is possible to use data from all surveyed access points as input data to determine the values 𝛾𝑖 and 𝛽𝑖 . it means that there is n×(n−1)/2×h measurement data points to infer signal strength parameters, where n is the number of access points within range 𝐴𝑃𝑖 . the indoor signal propagation model is developed and regularly updated by the international telecommunication union (itu), which is a un agency in the field of information and communication technologies. according to the itu r.1238 standard, for the walls of hospital buildings, γi will always be equal to 20 [17]. the power of the signal strength depends on the ap manufacturer. in the example from part 4, for mikrotik hap2 lite was used, it has a tx dbm of 27. there is also no standard dbm power for mobile devices, in the example, a mobile phone with a tx dbm of 22 antenna was used. hence, from a good signal level of –35 dbm to a weaker signal level of –55 dbm, as shown in the figure 3. as we see that signal strength of ap3 is good (–39 dbm), patient signal is good enough (–43 dbm). ap2 and ap4 signal is weak. 4. signal propagation simulation stage knowing the calculated signal propagation parameters, a model was chosen that was built to estimate the path loss inside the closed area to simulate the signal propagation. therefore, it was decided to use the itu model [17]. this itu model attempts to account for reflection and diffraction caused by objects, energy collisions, movements within the room, multipath effects, etc. their model provides guidance on indoor signal propagation in the 300 mhz to 100 ghz frequency range. the basic model can be expressed as [9]: 𝑃𝐿𝑖,𝑗 = 20 log10(𝑓) + 𝑁 log10(𝑑𝑖,𝑗 ) + 𝐿𝑓 (𝑛) − 28, (5) where 𝑃𝐿𝑖,𝑗 is the total loss on the path of the signal arriving at the access point 𝐴𝑃𝑖 at point j, db; f is the frequency, mhz; n is an indicator of power loss over distance; 𝑑𝑖,𝑗 is the distance between 𝐴𝑃𝑖 and point j,m; 𝐿𝑓 (𝑛) is a factor that takes into account the loss of signal power between floors (its influence is not taken into account in these studies). in the propagation simulation step, the location server calculates the expected power loss in the room for each access point in the mesh pattern and creates a propagation map. to eliminate the need for patient identification, the proposed method only requires the ability of access points to probe wi-fi channels and report the rssi of neighboring access points. most aps on the market today have this feature built in, as it is an integral part of automatically determining the most suitable channel for wi-fi communication. usually, information about these readings is not available to the end user. this is one of the reasons, apart from its popularity, that one of the most common wireless routers, the mikrotik hap2 lite, was chosen for the study. scanning is a periodic event. if its frequency is too high, it leads to an additional load on the wi-fi network, which is undesirable because it affects the performance of data transmission in the wireless network. a scan rate that is too low means that changes to the wi-fi network that is decided to be used to locate moving objects (patients in remote rehabilitation) will take too long to become meaningful and effective. choosing too many iterations will have the same consequences as choosing too long a listening period. too few iterations will result in more influence on the rssi variance than desired. after testing, one minute was chosen as the refresh rate, which ensures that changes due to long-term wi-fi instability and changes in indoor space will have a significant effect in less than 10 minutes, as they will be present in more than 50% of the data points used for calculation. after all the calculations and drawing the map, the developed application will show the doctor in which ward/room of the building the selected patient is located. an example of such a map is shown in figure 4. in the given example, you can see that the patient is in room 1. 5. conclusions the proposed method, based on listening to wi-fi signals, is a further development of the itu-r recommendations [17]. this method can be applied to a variety of wireless telecommunication equipment and requires only ap information. a map of the patient's location is built on the basis of the rssi indicators. to track the location of patients, they (patients) only need to have a smartphone with the wi-fi transmitter turned on. the disadvantages of the proposed method are that the rssi indicator is poorly correlated with the signal quality, but it can be used to approximate the signal quality. a more accurate assessment can be obtained using the link quality indicator (lqi). the expediency of using the specified parameter is justified by the fact that if there are numerous devices in the local network with enabled wi-fi modules in an open environment, such devices are also sources of interference. in this case, the power of the received signal may be high, but the ratio between the received signal and the noise level is lower than necessary to determine the location of the moving object. to use the lqi parameter, the bluetooth module, which is also built into most popular gadgets, or other types of technologies according to the 802.15 standard, are more suitable. figure 4. an example of a patient location map. acta imeko | www.imeko.org september 2023 | volume 12 | number 3 | 5 the implementation of the proposed algorithm does not require specialized antennas, which were used in previous studies [18]. different from previous approaches that relied on specific antenna configurations to achieve optimal results, our algorithm uses built-in signal processing techniques to achieve reliable results using standard antennas commonly found in most devices. this eliminates the need for expensive and specialized antenna systems, making the algorithm affordable and practical for a wide range of applications. acknowledgement this study was funded and supported by petro mohyla black sea national university (pm bsnu) in mykolaiv (ukraine), and also financed in part of the pm bsnu science-research work by the ministry of education and science of ukraine “development of the modules for automatization of wireless devices for recovery of postinfarction, post-stroke patients in individual conditions of remote individual rehabilitation” (state reg. no. 0121u109898). the author declares that there are no conflicts of interest regarding the publication of this article. written & oral informed consent was obtained from all individual participants included in the study but nothing identifying information is included in this paper. references [1] e. balestrieri, p. daponte, l. de vito, f. picariello, s. rapuano, i. tudosa, a wi-fi iot prototype for ecg monitoring exploiting a novel compressed sensing method, acta imeko 9(2) (2020) pp. 38–45. doi: 10.21014/acta_imeko.v9i2.787 [2] j. t. koyazo, m. a. ugwiri, a. lay-ekuakille, m. fazio, 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https://doi.org/10.15587/1729-4061.2017.90863 https://doi.org/10.1109/ipin.2014.7275525 https://doi.org/10.23670/irj.2017.64.012 https://doi.org/10.1109/ipin.2019.8911764 https://doi.org/10.21014/acta_imeko.v11i1.1214 https://doi.org/10.3390/electronics11142209 https://doi.org/10.5772/intechopen.69679 https://doi.org/10.5772/intechopen.75384 https://doi.org/10.3390/mi10030198 https://www.itu.int/rec/r-rec-p.1238 https://doi.org/10.21014/acta_imeko.v11i1.1202 sample volume length and registration accuracy assessment in quality controls of pw doppler diagnostic systems: a comparative study acta imeko issn: 2221-870x june 2023, volume 12, number 2, 1 7 acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 1 sample volume length and registration accuracy assessment in quality controls of pw doppler diagnostic systems: a comparative study giorgia fiori1, gabriele bocchetta1, silvia conforto1, salvatore a. sciuto1, andrea scorza1 1 department of industrial, electronic and mechanical engineering, roma tre university, via della vasca navale 79, 00146 rome, italy section: research paper keywords: quality controls; pw doppler; sample volume; ultrasound diagnostic systems; comparative study citation: giorgia fiori, gabriele bocchetta, silvia conforto, salvatore a. sciuto, andrea scorza, sample volume length and registration accuracy assessment in quality controls of pw doppler diagnostic systems: a comparative study, acta imeko, vol. 12, no. 2, article 6, june 2023, identifier: imeko-acta-12 (2023)02-06 section editor: alfredo cigada, politecnico di milano, italy, roberto montanini, università degli studi di messina, italy received december 5, 2022; in final form april 21, 2023; published june 2023 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: giorgia fiori, e-mail: giorgia.fiori@uniroma3.it 1. introduction pulsed wave (pw) doppler is extensively used in diagnostic imaging since it allows displaying and quantifying the velocity of both arterial and venous flow through time. ultrasound (us) diagnostic systems provide a real-time spectral doppler display by performing the spectral analysis of the doppler signal from the blood vessel [1], [2]. this signal is detected within a sensitive region, known as sample volume (sv), whose length and depth (from the interface between the patient skin and the us probe) can be adjusted by the operator on the b-mode anatomical image [2], [3]. in the literature, two main parameters were used to characterize the sample volume, i.e., size and range gate registration accuracy [4], [5]. in particular, sv size depends on both the effective sample duration and the width of the us beam [6], and it affects the pw doppler spatial resolution. in turn, sample volume size can be split into two main contributions [7]: lateral and axial resolution. nowadays, sv axial resolution is included by the uk's institute of physics and engineering in medicine (ipem) among the basic functional checks for spectral doppler designed for control functioning assessment and faults detection [8]. as regards the range gate registration accuracy, it quantifies the error due to the difference between the sv position displayed on the grayscale image and the actual one. such parameter is one of the quality control (qc) doppler tests recommended by the american institute for ultrasound in medicine (aium) [9]. it is worth noting that the majority of the scientific literature that focused on sample volume size [7], [10]-[12] and range gate registration accuracy [12], [13] dates back to the last years of the xx century. moreover, the qc test procedures recommended by the two international organizations are affected by operator subjectivity. in this regard, performance evaluation of doppler abstract in clinical diagnostics, pulsed wave (pw) doppler is one of the most used spectral doppler techniques since it provides quantitative information about the severity of several cardiac disorders. therefore, routine quality control tests should be scheduled to check whether a proper level of performance is maintained over time. despite continuous research in the field, performance evaluation of doppler equipment is still an open issue. therefore, the present study is focused on the comparative investigation based on a test parameter for the automatic analysis of faults in sample volume length and range gate registration accuracy. the velocity profile discrepancy index (vpdi) provides a quantitative estimation according to the agreement between the theoretical parabolic velo city profile and the measured one. the index was assessed through an automatic method that post-processes pw spectrogram images acquired at six sample volume depths with respect to the vessel radius of a doppler reference device. tests were repeated for three brand-new ultrasound diagnostic systems, equipped with convex and phased array probes, in two working conditions. from the analysis of the results, a lower discrepancy between the measured and the theoretical velocity profile was found for the convex array probes as well as a lower uncertainty contribution. mailto:giorgia.fiori@uniroma3.it acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 2 systems is still an open issue in the scientific research field [4], [14]-[17], despite the increasing demand for proper qc programs. nonetheless, the measurement of blood flow peak velocity is of particular relevance in clinical applications since several clinical parameters are evaluated from the peak velocity envelope. pw doppler provides quantitative information about the severity of many cardiac disorders [18], including the degree of stenosis [19], [20], arterial wall shear stress [21], [22], and pressure drops across cardiac valves [23]. therefore, inaccurate measurements can impact diagnosis and clinical therapy: overestimations ranging from 12 % to 50 % have been estimated as regards the measurement of blood peak velocities [24], [25]. from the abovementioned considerations, the present study would give a contribution to the field of qcs for doppler equipment testing, by proposing a comparative investigation of the velocity profile discrepancy index (vpdi), a quality parameter developed for the objective assessment of both sv length and range gate registration accuracy. the index, preliminarily defined and tested in [26], is estimated from the processing of pw doppler spectrograms by means of a customwritten image analysis method developed in matlab environment. three brand-new us diagnostic systems, each of them equipped with two ultrasound probes (convex and phased array) were tested in two working conditions. in section 2, the estimation rationale underlying the definition of the proposed quality index will be provided. in section 3, the experimental setup components and their main specifications will be described together with the image analysis-based method implemented for vpdi assessment. in section 4, experimental results with their associated measurement uncertainty will be presented and discussed, focusing on the comparison among the us diagnostic systems under test. finally, in the concluding section, the major achievements and future developments will be outlined. 2. velocity profile discrepancy index by considering a cylindrical tube of infinitesimal length d𝑦 exhibiting fully developed laminar flow in the reference system shown in figure 1, the well-known parabolic velocity profile 𝑣(𝑟) of the hagen-poiseuille flow [27], [28] can be written as: 𝑣(𝑟) = − 1 4 𝜇 d𝑃 d𝑦 (𝑅2 − 𝑟2) , (1) where 𝑅 is the tube radius, 𝜇 is the dynamic viscosity coefficient, d𝑃 is the infinitesimal pressure drop and 𝑦-axis is the flow direction. from (1), the maximum velocity value 𝑣max can be reached for 𝑟 = 0, while for 𝑟 = |𝑅| the no-slip boundary condition applies, i.e., the relative tangential and normal velocities between the flow and the tube walls are zero: { 𝑣(𝑟)|𝑟=0 = − 1 4 𝜇 d𝑃 d𝑦 𝑅2 = 𝑣max 𝑣(𝑟)|𝑟=|𝑅| = 0 . (2) from these considerations, it is possible to derive the mathematical expression of the parabolic velocity profile 𝑣(𝑥) as a function of 𝑣max and 𝑅, as follows: { 𝑣(𝑥) = 𝑣max (1 − 𝑥 2 𝑅2 ) |𝑥| ≤ 𝑅 𝑣(𝑥) = 0 |𝑥| > 𝑅 , (3) where 𝑥 is the radial coordinate. by subdividing 𝑣(𝑥) into a fixed number of bins and assuming the limits of the 𝑛-th bin (𝑛 = 1, …, 𝑁) as the sv boundaries (figure 2a), the theoretical velocity �̅�th for each bin can be derived from (3) as follows: { �̅�th = 𝑣max (1 − 𝑥sv 2 𝑅2 + 𝑙2 12 𝑅2 ) |𝑥sv| ≤ 𝑅 �̅�th = 0 |𝑥sv| > 𝑅 , (4) where 𝑥sv is the sv position with respect to the tube radius and 𝑙 is the sample volume length (figure 2b). finally, the velocity profile discrepancy index (vpdi) for the objective assessment of both sample volume length and registration accuracy is derived as [26]: 𝑉𝑃𝐷𝐼 = ∑ 𝑉𝑃𝐷𝐼𝑛 𝑁 𝑛=1 = ∑ (�̅�s,𝑛 − �̅�th,𝑛) 2 𝜎tot,𝑛 2 𝑁 𝑛=1 , (5) figure 1. steady laminar flow in a cylindrical tube of infinitesimal length. a b figure 2. (a) parabolic velocity profile v(x) subdivided in n bins: the limits of each bin are assumed as the sv boundaries; (b) graphical schematization of the sv length and positioning inside the cylindrical tube. acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 3 where �̅�s,𝑛 is the mean peak velocity estimated from the doppler spectrogram collected at pre-set sample volume depth and length, �̅�th,𝑛 is the corresponding theoretical velocity value retrieved by applying (4) for each bin, while 𝜎tot,𝑛 is the total standard deviation estimated for the 𝑛-th bin as follows: { 𝜎tot,𝑛 = √𝜎s,𝑛 2 + 𝜎r,𝑛 2 + 𝜎p,𝑛 2 |𝑥𝑆𝑉 | ≤ 𝑅 𝜎tot,𝑛 = 𝜎r,𝑛 |𝑥𝑆𝑉 | > 𝑅 , (6) where 𝜎s is the standard deviation (sd) of the peak velocities in the doppler spectrogram because of the intrinsic flow dispersion, 𝜎r is the standard deviation of a random distribution associated with the electronic noise superimposed on the pw spectrogram, and 𝜎p is the sd due to the assumption of parabolic profile: 𝜎p = 𝜎sv 2 𝑣max 𝑅2 |𝑥sv| , (7) where 𝜎sv is the sd associated to the sample volume position with respect to r. according to the definition proposed in (5), vpdi is expected to be 0, i.e., the acquired pw spectrograms are not affected by any variation of the sample volume length with respect to the set value and any sv registration error. on the contrary, if vpdi deviates from 0, the pw spectrograms are affected by at least one of the two error sources, i.e., unwanted sv size variation and/or a low sv registration accuracy. 3. materials and methods 3.1. experimental setup the experimental setup was constituted by three high technology level us diagnostic systems, equipped with a convex and a phased array probe each, and a reference test device whose main specifications are listed in table 1. the test device used is a commercial flow phantom model (sun nuclear, doppler 403tm flow phantom [29]) consisting of a continuous flow loop vessel with a horizontal and a diagonal segment having the same inside diameter. a pump and a flow controller are designed by the manufacturer to provide user-selectable flow rates in the range (1.7-12.5) ml·s-1. doppler spectrograms were collected on the phantom diagonal segment set at a constant flow rate of 7.0 ml·s-1 corresponding to a nominal peak velocity of 71.3 cm·s-1. tests were repeated at two different us system working conditions (table 2): the best configuration setting provided by the product specialist (set 1), and the raw configuration setting by reducing preand post-processing settings (set 2). the three ultrasound diagnostic systems, manufactured by different companies, were anonymously addressed as system a, system b and system c. the main setting that distinguishes the two working conditions is the wall filter, a dedicated high-pass filter designed to reject the intense and low-frequency echoes coming from the vessel walls movement (clutter signal) [2]. in the raw configuration setting it was always set at the minimum adjustable table 1. technical specifications of the reference test device. characteristic specification phantom model sun nuclear, doppler 403tm flow phantom attenuation coefficient (0.70 ± 0.05) db·cm-1·mhz-1 tmm (a) sound speed (1540 ± 10) m·s-1 bmf (b) sound speed (1550 ± 10) m·s-1 scanning material aluminium-film composite vessel nominal inside diameter (5.0 ± 0.2) mm horizontal segment depth (2.0 ± 0.3) cm diagonal segment depth from 2.0 to 16.0 cm (± 0.3 cm) at 40° flow mode constant and pulsatile continuous flow mode range [(1.7 – 12.5) ± 0.4] ml·s-1 (a) tmm = tissue mimicking material; (b) bmf = blood mimicking fluid. table 2. main b-mode and pw doppler settings according to the probe model of each ultrasound diagnostic system. b-mode and pw doppler setting probe system a system b system c set 1 set 2 set 1 set 2 set 1 set 2 b-mode frequency range (mhz) b (a) resolution resolution resolution resolution resolution resolution field of view (cm) b 12 12 12 12 12 12 post-processing b linear linear linear linear linear linear doppler frequency (mhz) c (b) 2.2 2.2 2.0 2.0 2.2 2.2 p (c) 1.6 1.6 2.0 2.0 1.8 1.8 wall filter (hz) c medium (d) min (d,e) 120 50 (e) 70 50 (e) p medium (d) min (d,e) 200 50 (e) 125 75 (e) pulse repetition frequency (khz) c 2.63 2.63 2.3 2.3 – (d) – (d) p 2.50 2.50 2.3 2.3 – (d) – (d) nominal svl (mm) b 1 1 1 1 1 1 nominal svdc ± svd (cm) c 11.0 ± 0.1 11.0 ± 0.1 11.0 ± 0.1 11.0 ± 0.1 11.0 ± 0.1 11.0 ± 0.1 p 10.0 ± 0.1 10.0 ± 0.1 10.0 ± 0.1 10.0 ± 0.1 10.0 ± 0.1 10.0 ± 0.1 nominal svdb ± svd (cm) c 10.7 ± 0.1 10.7 ± 0.1 10.7 ± 0.1 10.7 ± 0.1 10.7 ± 0.1 10.7 ± 0.1 p 9.7 ± 0.1 9.7 ± 0.1 9.7 ± 0.1 9.7 ± 0.1 9.7 ± 0.1 9.7 ± 0.1 insonation angle (°) c 43 43 35 35 40 40 p 45 45 36 36 44 44 spectrogram duration (s) b ~ 11 ~ 11 ~ 12 ~ 12 ~ 12 ~ 12 (a) b = both convex and phased array probe models; (b) c = convex array probe model; (c) p = phased array probe model; (d) value not provided; (e) minimum adjustable wall filter value. acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 4 value. conversely, the pulse repetition frequency (prf), i.e., the rate at which us pulses are transmitted [2], was, in turn, determined by the us system since its value depends on the other settings. among the latter, the insonation angle was varied according to both the probe positioning on the scanning surface and the probe model so as to keep the sample volume parallel to the flow axis throughout the acquisitions, while the spectrogram duration was set to at least 10 s (depending on the system availability) in order to allow the comparison of the results retrieved from different system-probe pairs. in addition, the sample volume length (svl) was maintained constant throughout the measurement campaign, while the sample volume depth (svd) was adjusted on the vessel diagonal segment to collect n = 6 doppler spectrograms for each probesystem pair as listed in table 2. first, the sample volume was placed in the centre of the segment diameter, 𝑆𝑉𝐷c in figure 3a, and then it was adjusted at 𝑆𝑉𝐷c ± ∆𝑆𝑉𝐷 (figure 2a), by considering ∆𝑆𝑉𝐷 the minimum sample volume depth variation that can be set in most of the ultrasound systems currently on the market. the sample volume was subsequently placed on the segment boundary, 𝑆𝑉𝐷b in figure 3b, as well as at 𝑆𝑉𝐷b ± ∆𝑆𝑉𝐷 (figure 2b). as in [26], svd was adjusted along the 𝑧-axis by assuming that the velocity does not vary along the flow axis of the tube. 3.2. vpdi assessment the velocity profile discrepancy index defined in [26] was assessed through an image analysis-based method that postprocesses pw doppler images acquired at different sample volume depths. the main steps of the measurement method are described in the following and shown in figure 4, while the specifications assumed in this study are listed in table 3. as a first step, each pw image is cropped to extract the box containing the spectrogram image only, i.e., excluding the patient information, the b-mode grayscale image and the ultrasound settings details. this processing step is automatically carried out based on the metadata included in the dicom (digital imaging and communications in medicine) file. a b figure 3. graphical schematization of the sample volume adjusted (a) at svdc and (b) at svdb on the diagonal segment of the phantom vessel. figure 4. block diagram of the image analysis-based method for velocity profile discrepancy index assessment. acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 5 then, an adaptive grey level threshold 𝑡ℎg is applied to the spectrogram image to detect the pixels associated with the peak velocities. the corresponding mono-dimensional signal 𝑣s through time is retrieved as in [30], [31] and the mean peak velocity �̅�s is computed together with the standard deviation 𝜎s over 𝑀 samples (i.e., spectral lines). on the other hand, the total standard deviation 𝜎tot is estimated. in this study, the following assumptions were made for the estimation of 𝜎tot as in [26]. the standard deviation 𝜎r in (6) was estimated from a randomly generated gaussian distribution consisting of a number of elements equal to 15 % of 𝑀 and mean value half of the spectrogram full scale. in addition, 𝜎sv in (7) was estimated from a uniform distribution within the range [−∆𝑙 2⁄ ; ∆𝑙 2⁄ ] by considering ∆𝑙 as the minimum svl increment. finally, both 𝜎s and 𝜎p in (6) were taken equal to 0 if the number of peak velocities 𝑣s at 0 cm·s -1 was higher than 85 % of 𝑀. therefore, when the above condition occurred, the detected velocities were assumed as electronic noise, i.e., 𝜎tot = 𝜎r. otherwise, the uncertainty propagation law was applied including 𝜎s and 𝜎p in the computation of 𝜎tot even if outside of the vessel segment, i.e., 𝑆𝑉𝐷 at 𝑆𝑉𝐷b − ∆𝑆𝑉𝐷. the processing steps described above are repeated for all the 𝑁 acquisitions corresponding to different sample volume depths. at this point, the theoretical velocity �̅�th is determined for each svd by applying (4), in which the maximum velocity 𝑣max was chosen as the mean peak velocity determined at 𝑆𝑉𝐷c, as in [26]. in the last step, the velocity profile discrepancy index is computed according to (5). vpdi uncertainty can be estimated as the standard deviation of the data distribution obtained through the monte carlo simulation (mcs), a useful tool for the measurement uncertainty estimation of image analysis-based methods [32]-[36]. in this study, a mcs with 104 iterations was run for each probe-system pair in both working conditions. a uniform distribution with ± 2 % bounds was assigned to the adaptive threshold 𝑡ℎg and the 𝑀 spectral lines to be processed were randomized, at each cycle without repetition, among all the spectral lines detected in the spectrogram image. in particular, the uniform distribution was assigned to 𝑡ℎg because, in terms of standard deviation, it is a more cautious approach for the uncertainty estimation, whilst the distribution range was assumed as the maximum threshold dispersion allowing to distinguish the velocity signal from the background noise displayed in the spectrogram, as already experienced in previous studies [31], [37]. 4. results and discussion vpdi experimental outcomes for systems a, b and c equipped with convex and phased array probes at different working conditions are listed in table 4 in terms of mean ± 𝑆𝐷 and shown in figure 5. by comparing the two probe models, the convex array probes showed the best vpdi outcomes (closest to 0) in both working conditions as well as the lowest uncertainty contributions. this suggests that the pw spectrograms acquired through the phased array probes could be most affected by an error in sample volume length (with respect to the pre-set value) and in its registration, (i.e., a low sv registration accuracy). such issue seems to be supported by flow velocity trends still visible outside of the phantom vessel segment, as shown in figure 6. on the other hand, by comparing the two working conditions, higher uncertainty contributions were globally found at raw configuration settings (set 2) independently of the probe model. moreover, higher vdpi values were found in set 2 for both the probe models of system a and system c, while a specific behaviour cannot be inferred for system b. the discrepancy in results between set 1 and set 2 is mostly due to the different filtering of the clutter signal (i.e., wall filter setting in table 2). table 3. parameters setting for vpdi assessment. parameter symbol setting adaptive threshold thg 10 % of maximum gray level value (a) number of spectral lines m 1000 minimum svl increment l 1 mm maximum velocity vmax vs̄ at svdc (a) the threshold setting at 10 % allows to properly distinguish the velocity signal from the background noise displayed in the spectrogram [30], [31]. table 4. vpdi outcomes (mean  sd) for the three ultrasound diagnostic systems equipped with convex and phased array probes at different working conditions. probe model working condition system a system b system c convex set 1 0.14  0.01 0.69  0.04 0.28  0.04 set 2 0.23  0.03 0.68  0.12 0.58  0.12 phased set 1 3.36  0.24 3.46  0.13 1.64  0.18 set 2 4.61  0.37 1.40  0.06 5.71  0.21 a b figure 5. vpdi results for (a) convex and (b) phased array probes according to the ultrasound system and the working condition. figure 6. pw spectrogram acquired through system a equipped with the phased array probe in set 2: sample volume depth adjusted at svdb–svd. acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 6 finally, by focusing on the comparison among the three ultrasound diagnostic systems, it should be noticed that system a equipped with the convex array probe in set 1, and system b equipped with the phased array probe in set 2 showed vpdi outcomes closer to 0. 5. conclusions in the study herein proposed, a comparative investigation on the velocity profile discrepancy index was carried out. the index, preliminarily investigated by the authors, was developed for the objective assessment of sample volume length and range gate registration accuracy. it relies on the assumption of a fully developed laminar flow in order to quantify the discrepancy between the theoretical parabolic profile and the actual velocities. according to the definition, vdpi is expected to be 0 when the processed pw spectrograms are not affected by any variation of the svl with respect to the pre-set nominal value and any range gate registration error. an automatic image analysis-based method post-processes pw spectrograms acquired at different sample volume depths with respect to the vessel radius of a doppler reference device. three brand-new us diagnostic systems, equipped with a convex and phased array probe each, were tested in two working conditions. a commercial reference test device constituted by a continuous flow loop vessel with a horizontal and a diagonal segment was set at a constant flow rate of 7.0 ml·s-1. experimental data were collected on the diagonal segment of the phantom vessel. from the analysis of the results, convex array probes showed vdpi outcomes closer to 0 in both working conditions as well as the lowest uncertainty contributions. on the basis of the promising outcomes obtained and their limitations, further studies and method improvements are going to be carried out to estimate the two error sources separately. moreover, further investigations should be performed including a higher number of diagnostic systems and probe models (e.g., linear array probes). acknowledgement the authors wish to thank jan galo of the clinical engineering service at i.r.c.c.s. children hospital bambino gesù for administrative and technical support; samsung healthcare, philips healthcare and mindray medical for hardware supply and technical assistance in data collection. references [1] d. h. evans, w. n. mcdiken, doppler ultrasound: physics, instrumentation and signal processing, wiley, new york, 2nd edition, 2000, 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[29] sun nuclear corporation, doppler 403™ & mini-doppler 1430™ flow phantoms. online [accessed 20 february 2023] https://www.sunnuclear.com/uploads/documents/datasheets/ diagnostic/dopplerflow_phantoms_113020.pdf [30] g. fiori, f. fuiano, a. scorza, m. schmid, j. galo, s. conforto, s. a. sciuto, a novel sensitivity index from the flow velocity variation in quality control for pw doppler: a preliminary study. proc. of the 2021 ieee international symposium on medical measurements and applications (memea), lausanne, switzerland, 23 – 25 june 2021. doi: 10.1109/memea54994.2022.9856474 [31] g. fiori, f. fuiano, a. scorza, m. schmid, s. conforto, s. a. sciuto, doppler flow phantom failure detection by combining empirical mode decomposition and independent component analysis with short time fourier transform, acta imeko 10 (2021), pp. 185-193. doi: 10.21014/acta_imeko.v10i4.1150 [32] g. fiori, a. pica, s. a. sciuto, f. marinozzi, f. bini, a. scorza. a comparative study on a novel quality 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of the 25th imeko tc4 international symposium & 23rd international workshop on adc and dac modelling and testing, brescia, italy, 12 – 14 september 2022, pp. 151-155. doi: 10.21014/tc4-2022.28 [36] g. fiori, a. scorza, m. schmid, j. galo, s. conforto, s. a. sciuto, a preliminary study on the blind angle estimation for quality assessment of color doppler ultrasound diagnostic systems. proc. of the 25th imeko tc4 international symposium & 23rd international workshop on adc and dac modelling and testing, brescia, italy, 12 – 14 september 2022, pp. 325-329. doi: 10.21014/tc4-2022.60 [37] g. fiori, f. fuiano, a. scorza, m. schmid, j. galo, s. conforto, s. a. sciuto, doppler flow phantom stability assessment through stft technique in medical pw doppler: a preliminary study. proc. of the 2021 ieee international workshop on metrology for industry 4.0 & iot (metroind4.0&iot), rome, italy, 7 – 9 june 2021. doi: 10.1109/metroind4.0iot51437.2021.9488513 https://doi.org/10.1002/jum.15253 https://doi.org/10.1016/j.atherosclerosis.2013.01.026 https://doi.org/10.4103/jmu.jmu_11_17 https://doi.org/10.1016/j.ultrasmedbio.2021.09.006 https://doi.org/10.1016/j.ejmp.2013.10.001 https://doi.org/10.1109/memea54994.2022.9856474 https://www.sunnuclear.com/uploads/documents/datasheets/diagnostic/dopplerflow_phantoms_113020.pdf https://www.sunnuclear.com/uploads/documents/datasheets/diagnostic/dopplerflow_phantoms_113020.pdf https://doi.org/10.1109/memea54994.2022.9856474 https://doi.org/10.21014/acta_imeko.v10i4.1150 https://doi.org/10.3390/s22249868 https://doi.org/10.21014/acta_imeko.v10i2.1051 https://doi.org/10.21014/tc4-2022.59 https://doi.org/10.21014/tc4-2022.28 https://doi.org/10.21014/tc4-2022.60 https://doi.org/10.1109/metroind4.0iot51437.2021.9488513 microsoft word 258-1783-1-le-rev2 acta imeko  issn: 2221‐870x  september 2015, volume 4, number 3, 14 ‐ 22    acta imeko | www.imeko.org  september 2015 | volume 4 | number 3 | 14  mixed baseband architecture based on fbd ʃ∆–based adc  for multistandard receivers  rihab lahouli 1,2 , manel ben‐romdhane 1 , chiheb rebai 1 , dominique dallet 2   1  grescom research lab., sup’com, university of carthage, cité technologique des communications, 2083 el ghazela, ariana, tunisia  2  ims research lab., ipb enseirb‐matmeca, university of bordeaux, 351 cours de la libération, bâtiment a31, 33405 talence cedex,  france      section: research paper   keywords: frequency band decomposition (fbd);  modulators; software defined radio (sdr) receiver   citation: rihab lahouli, manel ben‐romdhane, chiheb rebai, dominique dallet, mixed baseband architecture based on fbd ʃ∆–based adc for  multistandard receivers, acta imeko, vol. 4, no.3, article 3, september 2015, identifier: imeko‐acta‐04 (2015)‐03‐03  editor: paolo carbone, university of perugia, italy  received february 14, 2015; in final form may 18, 2015; published september 2015  copyright: © 2015 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  corresponding authors: rihab lahouli, dominique dallet, e‐mails: rihab.lahouli@supcom.tn, dominique.dallet@ims‐bordeaux.fr    1. introduction  software defined radio (sdr) is a state-of-the-art technology solution of the software radio concept, first introduced by mitola [1]. sdr was proposed by scientists to achieve a feasible multistandard receiver. to ensure software reconfigurability, the received signals must be digitized as near as possible to the antenna in order to reduce analog circuitry. this leads to increased design constraints of the analog-to-digital converter (adc). in fact, in literature, there is no fully integrated adc that covers different coexisting wireless and mobile standards from narrowband to wideband channels with different required dynamic ranges [2]. to deal with this problem, the authors propose the use of parallel architectures of  modulators that ensure high accuracy, in terms of dynamic range, while extending conversion bandwidth. parallel architectures have become an attractive solution for analog-to-digital conversion especially in the context of sdr, where new applications require extended bandwidths. there are three main parallel architectures described in the literature; the hadamard modulated parallel architecture (π) [3], the timeinterleaved architecture (ti) [4], and the frequency band decomposition (fbd) architecture [5]-[7]. in this paper, the authors choose for the fbd architecture because unlike π and ti architectures the fbd architecture is insensitive to gain and offset mismatches [8], [9]. in the fbd architecture, the parallel  modulators are band-pass (bp) and each one converts a part of the total input signal band. there are propositions of fbd architecture designs in the literature, essentially in [5]-[7]. the main drawback of these solutions is abstract  this paper presents the design and simulation results of a novel mixed baseband stage for a frequency band decomposition (fbd)  analog‐to‐digital converter (adc) in a multistandard receiver. the proposed fbd‐based adc architecture is flexible with programmable  parallel branches composed of discrete time (dt) 4 th  order single‐bit ʃ∆ modulators. the mixed baseband architecture uses a single  non‐programmable anti‐aliasing filter (aaf) avoiding the use of an automatic gain control (agc) circuit. system level analysis proved  that  the  proposed  fbd  architecture  satisfies  design  specifications  of  the  software  defined  radio  (sdr)  receiver.  in  this  paper,  the  authors focus on the butterworth aaf filter design for a multistandard receiver. besides, theoretical analysis of the reconstruction  stage for umts test case is discussed. it leads to a complicated system of equations and high digital filter orders. to reduce the digital  reconstruction  stage  complexity,  the  authors  propose  an  optimized  digital  reconstruction  stage  architecture  design.  the  demodulation‐based  digital  reconstruction  stage  using  two  decimation  stages  has  been  implemented  using  matlab/simulink.  technical  choices  and  performances  are  discussed.  the  computed  signal‐to‐noise  ratio  (snr)  of  the  matlab/simulink  fbd  adc  model is equal to at least 75 db which satisfies the dynamic range required for umts signals. next to hardware implementation with  quantized filters coefficients, the authors implemented their proposition in vhdl in a sysgen environment. the measured snr of the  hardware implementation is equal to 74.08 db which satisfies the required dynamic range of umts signals.  acta imeko | www.imeko.org  september 2015 | volume 4 | number 3 | 15  that they are based on continuous-time (ct)- modulators. in fact, ct- modulators bring analog errors that should be handled in the digital reconstruction stage. to overcome this problem, the authors proposed an fbd architecture based on discrete-time (dt)- modulators [10]. they are 4th order  modulators based on single-bit quantizers [10]. the authors choose single-bit quantization to overcome non-linearity errors introduced by multi-bit quantizers [7]. moreover, the novelty in this proposed architecture is the use of six programmable parallel branches with different sub-bandwidths, where only some branches are active according to the selected standard. the multistandard receiver handles e-gsm, umts and ieee802.11a communication standard signals. the outputs of the parallel branches in the fbd adc architecture have to be recombined using a digital reconstruction stage to provide the overall final output. e-gsm signals are not concerned in this stage since they solicit only one branch of the fbd adc architecture. only decimation is required at the  modulator output. however, a digital reconstruction stage is mandatory for the umts and ieee802.11a signals that solicit the three first branches and all the six branches of the adc architecture, respectively. in [11], the authors focused on the design and test of a digital reconstruction stage for the fbd ʃ∆-based adc architecture. it was verified when implementing the whole adc architecture in matlab/simulink that the architecture performances satisfy the standard requirements for the dynamic range of the umts signals in this test case. the choice of this test case has been made because the three first branches of the adc architecture, which are activated for digitizing umts signals, are reused for the digitization of ieee802.11a signals. besides, the umts standard requires a dynamic range of 73.8 db that is higher than the required dynamic range for ieee802.11a which equals 61.8 db. in this paper, interest is focused on the design of the mixed baseband stage for the sdr receiver. indeed, in a conventional baseband receiver stage, an anti-aliasing filter (aaf), an automatic gain control (agc) circuit and an adc are required to analogically process and digitize the received signals. however, in the proposed mixed baseband stage solution in this paper, the authors suggest to suppress the agc, to design a single passive aaf and to digitize the received signal thanks to the multistandard fbd dt -based adc architecture. moreover, the theoretical analysis of the digital reconstruction stage based on demodulation is detailed using multirate theory. the design of the fbd -based adc with demodulationbased digital reconstruction stage is first recalled [11]. then, the novelty comes with the theoretical discussion that justifies the authors’ proposition of an optimized digital reconstruction stage. in this paper, the authors come also with new comparative matlab/simulink simulation results of signal-to-noise ratio regarding frequency position of the input signals. besides, results of hardware implementation with quantized filter coefficients are presented and discussed. the paper is organized as follows. in section 2, the design of an fbd -based mixed baseband stage with a single passive aaf ahead intended for an sdr receiver is presented. section 3 deals with the digital reconstruction stage of the fbd based adc. the two existing approaches in the literature, the direct reconstruction and the demodulation-based reconstruction, are discussed. a demodulation based digital reconstruction stage design for umts test case is proposed and analyzed theoretically using multirate theory. this initial design has been modified and optimized in order to allow its implementation. simulation results of the fbd -based adc model using the matlab/simulink environment are presented in section 4. then, implementation results in vhdl using the sysgen environment are presented and discussed. finally, some conclusions are drawn in section 5. 2. flexible fbd ʃ∆ architecture design  to reach system level specifications of wireless and mobile standards, the authors propose to use a parallel  modulator architecture and modify the conventional mixed baseband stage design [10]. the multistandard receiver processes e-gsm [13], umts [14] and ieee802.11a [15] communication signals [10]. according to these supported communication standard specifications, design specifications for a multistandard sdr receiver have been computed. furthermore, a hybrid homodyne/low-if architecture was proposed in [16] for the sdr receiver front-end. an rf filter selects the received signals. afterward, the signals are amplified by a low-noise amplifier (lna). then, on the one side, the umts and ieee802.11a signals are down-converted by the mixer to baseband frequencies. on the other side, the e-gsm signals are down-converted to a low intermediate frequency of 100 khz to overcome flicker noise disturbance. system level specifications are introduced in sub-section 2.1. then, in sub-section 2.2, the mixed baseband stage design is explained. next, the design of the non-programmable aaf is proposed in sub-section 2.3. afterwards, the design of the fbd -based adc architecture is detailed in sub-section 2.4. 2.1. mixed baseband sdr receiver specifications  according to the specifications of the communication standards handled by the sdr receiver, system level specifications for the baseband receiver are depicted. table 1 summarizes the channel bandwidth chbw, the channel spacing chsp, the reference sensitivity sref, the signal-to-noise ratio (snr) at the receiver input, snrin, the signal-to-noise ratio at the receiver output snrout, the analog gain relative to a 13 dbm adc full scale input gana, the receiver dynamic range drin and the adc dynamic range dradc from which the adc resolution resadc, is deduced. since e-gsm signals are down-converted to a low intermediate frequency of 100 khz to avoid flicker noise, the egsm channel bandwidth is considered equal to 200 khz. the mixed baseband architecture is presented in the next sub-section. table 1. design specifications for the e‐gsm/umts/ieee802.11a receiver.   e‐gsm  umts  ieee802.11a  chbw (mhz) 0.2 3.84  16.6 chsp (mhz) 0.2 5  20 sref (dbm) ‐102 ‐117  ‐65 snrin (db) 18.8 ‐9  36.6 snrout (db) 9 ‐18.2  26.6 gana (db) 28 38  43 drin (db) 87 92  35 dradc (db) 96 73.8  61.8 resadc  (bits) 16 12  10 acta imeko | www.imeko.org  september 2015 | volume 4 | number 3 | 16  2.2. mixed baseband architecture  the mixed baseband stage, presented in figure 1 [10], follows the mixer which is controlled by the local oscillator (lo). the mixed baseband stage is composed of a single passive low-pass (lp) aaf that precedes the fbd -based adc. there is no need for n automatic gain control (agc) circuit before the adc stage since the aaf filters only e-gsm blockers that are outside the ieee802.11a bandwidth [16]. the m parallel single-bit quantizer  modulators are designed using matlab tools. their stability is ensured using a test plan performed in [10].  modulator outputs are combined in the digital reconstruction stage to reconstruct the final output. the design of the non-programmable aaf is explained in the next sub-section. 2.3. design of the non‐programmable aaf  in this sub-section, the authors are interested in the design of a butterworth non-programmable aaf for the sdr receiver. this aaf is unique for the e-gsm, umts and ieee802.11a signals. its role is to attenuate blockers and interfering signals which are susceptible to fold on the useful signal after sampling operation of the adc, while ensuring the required snrout as defined by design specifications presented in table 1. the low-pass aaf is defined by its cut-off frequency fp, its rejection frequency fr, its maximal attenuation in the useful bandwidth amax, and its minimal attenuation amin, beyond the rejection frequency. the cut-off frequency is set equal to half of the channel bandwidth with a conception margin of 30 %. this margin is required to avoid attenuation in the useful channel bandwidth after analog integrated circuit realization but also circuit aging [16]. the rejection frequency is fixed at fs-chbw/2, where fs is the sampling frequency. the fs values for the different supported standards are obtained when designing the fbd ʃ∆-based adc as given in table 2 [10]. these evaluation conditions are explained by figure 2. the minimal attenuation is calculated as given by (1) [16], aafoutrefbl msnrsna min  (1) where sref is the receiver sensitivity whose values for the different supported standards are given in table 1, maaf is a margin of conception equal to 3 db attributed to sref, and nbl is the level of the blocker to attenuate. the blocker level is calculated given the blocker’s profile at the rf filter output of the different supported standards. in fact, lna and mixer linearly amplify the signals in the received bandwidth. values of the aaf parameters are summarized in table 2. the cut-off frequency is considered to be the same for the three standards and corresponds to half of the chbw of ieee802.11a standard with a margin of 30 %. the aaf order is therefore computed given the butterworth attenuation expression as described by (2), )))(110(1(10)( 210/10 max n p a f f logfa  (2) where n is the butterworth filter’s order to compute and amax is set equal to 0.3 db. given the needed amin to attenuate blockers at the rejection frequency of each standard, the required aaf order is computed. computation results are summarized in table 2. for umts and ieee802.11a standards, the required aaf orders are 4 and 3, respectively. however, the e-gsm standard is the most restrictive since it requires a 6th order butterworth aaf. thus, for the sdr receiver the only aaf for the three standards is a 6th order butterworth aaf. the frequency response of the designed aaf is presented in figure 3. 2.4. flexible fbd ʃ∆ architecture  the authors in [10] started from sdr receiver specifications in terms of channel bandwidths and required adc dynamic ranges for the chosen communication standards. the designed discrete-time (dt) fbd  architecture for the adc stage was proposed in [10]. the design realizes a trade-off between increasing the sampling frequency while still operating in discrete time and increasing the number m of parallel branches regarding a low-complexity goal, or increasing  modulator orders while keeping them stable. thus, an fbd  architecture which is composed of 6 programmable parallel aaf digital reconstruction stage 1 bit fin flo frf ∑∆ modulator 1 1 bit ∑∆ modulator m ∑∆ fbd-based adc adc output figure 1. fbd ʃ∆‐based mixed baseband stage.  table 2. design of the lp aaf filter   e‐gsm  umts  ieee802.11a  fs (mhz) 72 72  96 fp (mhz) 10.8 10.8  10.8 fr (mhz) 71.8 70.08  85.2 nbl (db) ‐23 ‐44  ‐47 amin (db) 85 51.8  41.6 aaf order (n) 6 4  3 figure 2. evaluation conditions for the aaf design.  figure  3.  frequency  response  and  specification  mask  of  the  lp  non‐ programmable aaf for the sdr receiver.  0 10 20 30 40 50 60 70 80 90 100 0 20 40 60 80 100 120 frequency (mhz) a tt e n u a ti o n ( d b ) specifications mask 6th order butterworth attenuationamin umts= 51.8 db f r umts =70.08 mhz f p =10.8 mhz amin ieee802.11a= 41.6 db f r ieee802.11a =85.2 mhz f r e-gsm =71.8 mhz amin e-gsm= 85 db acta imeko | www.imeko.org  september 2015 | volume 4 | number 3 | 17  branches was proposed as presented in figure 4(a). according to the e-gsm, umts or ieee802.11a communication standard, from the whole architecture only the needed branches are activated. each branch is composed of a dt 4th order single-bit quantizer  modulator. the  modulator order is defined as the number of integrators or resonators k for low-pass (lp) and band-pass (bp)  modulators, respectively. since the designed fbd architecture is composed of both lp and bp  modulators, the authors designate in this paper k as  modulator order [10]. besides, the  modulators of the proposed fbd architecture are based on a non-unitary signal transfer function (nu-stf) that permits dealing with stability problems and recovering the input signal dynamic range [10]. the branch bandwidths are different and the sampling frequencies vary from one radio communication standard to another in order to optimize the flexible fbd  architecture while fulfilling the theoretically required dynamic ranges. the branch bandwidth and the sampling frequency according to the chosen standard are given by the branch frequency division plan presented in figure 4(b). in the next section, to detail the theoretical analysis and design of the digital reconstruction stage of the fbd -based architecture, the authors select the umts standard as a test case. the choice of this test case has been made because the umts standard uses the three first branches of the adc architecture. these branches are also selected with three more branches for the digitization of ieee802.11a signals. moreover, the required standard dynamic range of the umts is equal to 73.8 db which is higher than the 61.8 db for the required dynamic range of the ieee802.11a. 3. digital reconstruction stage: theoretical  analysis and design  in the literature, there are two main approaches to reconstruct the output signal from the parallel  modulator outputs while ensuring the required dynamic range [5]. for the first solution, the  modulator outputs are directly processed using band-pass filters, then, the selected signals are decimated. however, the second solution demodulates each  modulator output signal by converting it to baseband frequencies, then, the signal is decimated before being processed by a low-pass filter. it was shown in [5] that the digital reconstruction with direct processing presents high complexity due to high required bp filter orders and operating sampling frequencies. the digital reconstruction with demodulation requires lower lp filter orders and operating sampling frequencies. consequently, in this paper, the authors proceed to digital reconstruction with demodulation whose architecture is explained in sub-section 3.1. the theoretical analysis of this architecture is presented in sub-section 3.2. afterwards, an optimized digital reconstruction stage is implemented using matlab/simulink and technology choices are discussed in sub-section 3.3. 3.1. digital reconstruction with demodulation  the digital reconstruction architecture with demodulation is presented in figure 5. in this digital processing, the bp  modulator output signals are first brought to baseband by processing a complex demodulation. this operation consists in multiplying modulator outputs by the complex sequence mk[n] as given by (3) where fck is the central frequency of the kth branch bandwidth, ts is the sampling period which is equal to 1/fs and n is a positive integer:   sck ntfjk enm 2   (3) since the  modulators oversample input signals [5], it is mandatory to proceed to decimation and filtering operations after the complex demodulation operation. hence, each demodulated signal is decimated in order to decrease its sampling frequency and bring it to the nyquist frequency which is defined as the double of the channel bandwidth. the global decimation factor d is equal to the global oversampling ratio, osr, defined as the sampling frequency fs , out of the nyquist frequency. then, each demodulated and decimated signal is processed by a low-pass filter that selects the branch bandwidth before being modulated. the modulation operation consists in frequency up-converting each baseband signal around the corresponding branch central frequency at the nyquist frequency. finally, the output signals of the parallel branches are recombined to form the output signal of the fbd architecture. for the first branch that operates with a lp- (a) (b) figure  4.  (a)  designed  fbd  –based  adc  architecture,  (b)  branch  frequency division plan.  figure 5. digital reconstruction with demodulation of the fbd architecture (general case).  acta imeko | www.imeko.org  september 2015 | volume 4 | number 3 | 18  modulator, there is no need to demodulate and modulate as shown in figure 5. starting from the test case corresponding to the fbd based adc architecture operating to digitize umts signals, the design of a demodulation based digital reconstruction stage is performed. in this chosen test case, only the three first parallel branches of the fbd adc architecture are activated. the sampling frequency is set at 72 mhz and operating branche bandwidths are as explained by the branch frequency division plan for umts signals presented in figure 4(b). the global decimation factor d is chosen equal to 16 which is an integer number that permits digitizing umts signals at a nyquist frequency equal to 4.5 mhz. this nyquist band is between the channel bandwidth chbw and the channel spacing chsp, as given in table 1. the equivalent diagram in the discrete-time domain of the designed digital reconstruction stage is presented in figure 6 where hk(z) is the non-unitary signal transfer function of the kth  modulator, gk(z) the decimation filter of the kth branch, and fk(z) the branch bandwidth selection filter of the kth branch. this model is analyzed analytically in the next sub-section. 3.2. theoretical study of the demodulation‐based digital  reconstruction stage  in this sub-section, based on the multirate theory [12], the theoretical analysis of the digital reconstruction stage model, designed for the umts test case as presented in figure 6, is accomplished. in the first branch, a decimation operation and branch selection filtering process are performed. it is necessary to start by presenting the general expression of the z-transform of a decimated input signal by a decimation factor d as given by (4) [16]:        1 0 /1 1 0 /)2( )( 1 )()( 1 )( d k ld d l dljj wzx d zyorex d ey  (4) with djew /2 and jez  . therefore, the transfer function of the first branch output signal is deduced as expressed by equation (5): )()()()( 1 )( /11 /1 1 /1 1 1 0 /1 1 dldld d l ld zfwzgwzhwzx d zy     (5) in the 2nd and 3rd branche, the digital reconstruction processing contains demodulation and modulation operations that consist, as explained in the previous sub-section, in multiplication of the signal by a discrete exponential signal as given by (3). it is important to note that in this designed digital reconstruction stage, demodulation is operated at the  modulator oversampling frequency fs. however, the modulation is performed at the down sampling frequency equal to fs./d. therefore, the modulation is obtained by multiplying the outputs of branch selection bandwidth filters by the sequence given by (6):   sck ndtfjk enm 2mod_   (6) the z-transform expression of the 2nd branch output signal after demodulation y2dem, is then given by expression (7): )()()( 22 22 2 2 scsc tfjtfj dem ezhezxzy  . (7) then, the expression of the 2nd branch output after decimation is expressed by (8): ).()..( )..( 1 )( /1 2 /2/1 2 1 0 /2/1 2 2 2 lddtfjld d l dtfjld dec wzgewzh ewzx d zy sc sc        . (8) therefore, the z-transform expressions of the 2nd and 3rd branch output signals are determined as given respectively by (9) and (10): )()( )..( )..( 1 )( 22 2 2 2/1 2 2/1 2 /)1(2/1 2 1 0 /)1(2/1 2 scsc sc sc tfjdtfjld ddtfjld d l ddtfjld ezfewzg ewzh ewzx d zy             (9) )()..( )..( )..( 1 )( 33 3 3 2/1 3 2/1 3 /)1(2/1 3 ! 0 /)1(2/1 3 scsc sc sc tfjdtfjld ddtfjld d l ddtfjld ezfewzg ewzh ewzx d zy             (10) the combined output signal of the fbd adc architecture is obtained by summing the three branche output signals as presented by (11): )()()()( 321 zyzyzyzy  . (11) to cancel the aliasing and ensure a perfect reconstruction system, the output signal has to be a delayed version of the input signal and the alias terms should be canceled [12]. in the filter bank architectures, the signal is decimated at the input of the converters and interpolated at their outputs. the main difference between the fbd architecture with demodulationbased digital reconstruction and the filter bank architecture is the presence of demodulation and modulation operations. in fact, the signal at each band-pass branch is frequency shifted through these operations around the corresponding branch’s central frequency. consequently, a part of the input signal which is frequency shifted around the branch central frequency is applied at each branch’s bandwidth. there is a need to recuperate these input signals at the recombined final output. however, the aliasing terms introduced by decimation and corresponding to the input signal terms for l different from zero have to be eliminated to ensure perfect reconstruction. this leads to the expression of the output signal as given by (12) and (13):                                          1 0 1 /1 /)1(/1 /)1(/1 /)1(/1 1 d l m k k d k dd k ld k dd k ld k dd k ld vzf vwzg vwzh vwzx d zy (12) where scktfj k ev 2 with 01 cf figure 6. equivalent diagram of demodulation‐based digital reconstruction stage model for the umts use case.  acta imeko | www.imeko.org  september 2015 | volume 4 | number 3 | 19                0,0 1 1 ,0 /)1(/1 1 1 /1/)1(/1 /)1(/1/)1(/1                          zylfor vzxz d vzfvzg vzhvzx d zy lfor dd k d m k k m k k d k dd k d k dd k d k dd k d k (13) where  and are gain and delay, respectively. this theoretical design of the digital reconstruction stage leads to a complex system of equations and also to very high filter orders when implementing it on matlab/simulink. consequently, it is essential to modify this model to permit an optimized digital implementation solution. 3.3. proposed optimized digital reconstruction stage architecture  design  the digital reconstruction architecture based on demodulation for the umts test case presented in sub-section 3.1 has been modified in order to minimize its implementation complexity. the optimized design is detailed in this sub-section. the model of the fbd -based adc with the proposed digital reconstruction architecture is designed using matlab/simulink as presented in figure 7. the model corresponds to the test case of the fbd architecture intended for umts signals. the corresponding bloc diagram for this model is presented in figure 8. for the first branch, only decimation and filtering operations are needed for the digital reconstruction since it operates at low-pass frequencies as shown in figure 5. the decimation operation is always preceded by a decimation filter that serves as an anti-aliasing filter of the resampling operation. to reduce the complexity of such a decimation filter with a high decimation factor, the authors opt for two-stage decimation. the first stage ensures decimation by a factor of 8 when the second stage decimates by a factor of 2. for the second and third parallel branches that operate in band-pass frequencies, the digital reconstruction is composed of the operations of demodulation, decimation, filtering and modulation as explained in figure 5. the complex demodulation as explained before consists in multiplying the  modulator output by a discrete exponential signal at the branch central frequency as given by (1). in the matlab/simulink model, the authors replace the complex demodulation and modulation by in phase (i) and quadrature (q) paths to ensure better conditions for implementation. the demodulation should then be followed by filtering of the unwanted frequencies which are due to the demodulation operation. this filter presents high complexity since the unwanted frequencies are at low values and the filter operates at the oversampling frequency of the  modulator. for the second branch, the required finite impulse response (fir) filter order after demodulation is equal to 190. to deal with this problem, the authors opt to place the demodulation operation after the first decimation stage with a factor of 8. this solution figure  8.  block  diagram  of  fbd  ʃ∆‐based  adc  architecture  with  demodulation‐based digital reconstruction .  figure 7. proposed fbd‐based adc architecture model with demodulation‐based digital reconstruction.  z 1 unit delay sigma_delta_output1 to workspace3 demodulated_signal_br2 to workspace21 sigma_delta_output3 to workspace2 sigma_delta_output2 to workspace1 final_output to workspace sine wave4 at fin4 sine wave3 at fin3 sine wave2 at fin2 sine wave1 at fin1 sine wave at fc2 sine wave (w_fc3) sine wave at fc3 sine wave at fc2 in1 out1 sigma delta modulator 3 in1 out1 sigma delta modulator 2 in1 out1 sigma delta modulator 1 product9 product8 product7 product4 product3 product2 product11 product1 product fdatool filter q_3 fdatool filter q_2 fdatool filter i_3 fdatool filter i_2 fdatool filter 1 2 downsample8 2 downsample7 2 downsample6 8 downsample3 2 downsample22 2 downsample21 8 downsample2 8 downsample1 fdatool decimation filter1 fdatool decimation filter 3 fdatool decimation filter 2 cosine wave at fc3 cosine wave at fc2 cosine wave at fc3 cosine wave at fc2 -j constant1 butter analog aaf filter add3 add2 i3 q3 i2 q2 acta imeko | www.imeko.org  september 2015 | volume 4 | number 3 | 20  permits to reduce the operating frequency of the filter following the demodulation. moreover, it allows combining this filter with the second stage decimation filter and the low-pass filter that selects the branch bandwidth signals and rejects the quantization noise at the adjacent branches bandwidths. the first decimation stage placed at the  modulator output is composed of an operation of decimation by a factor of 8 preceded by a lp fir decimation filter. the order of these filters at the first, second and third branches are chosen to be 29, 39 and 56, respectively. then, the order of the lp fir filters of branch bandwidth selection are equal to 82. the frequency response of these filters after modulation of the second and third ones is presented in figure 9. the filter responses are overlapping. their intersection is at a level around 6 db and at the frequency limits between adjacent branches as shown in figure 9. the sum of the lp filters of 82nd order after modulation is computed and its frequency response magnitude is presented in figure 10(a). at the higher and lower ends of the bandwidth, the magnitude response presents ripples and attenuations that do not exceed 2 db as shown in figure 10 (b). thus, expected performances of the reconstruction system are not affected. after decimation and filtering operations, the i and q paths are modulated to convert the sub-band signal frequency around its original frequency which is the branch central frequency. finally, the sub-band output signals are recombined to obtain the reconstructed umts signal. simulation results are presented in section 4. 4. simulation results  simulation results are realized by applying a multi-tone signal composed of four sine-wave signals to the fbd model shown in figure 7. the first and last sine-wave frequencies are placed in the bandwidths [0, 600 khz] and [1500 khz, 2500 khz] of the branches 1 and 3, respectively. the selected values are 300 khz and 1900 khz. the first branch central frequency fc1 and the third branch central frequency fc3 are equal to 300 khz and 2000 khz, respectively. the two other sine waves are at frequencies in the 2nd branch bandwidth. they are situated on both sides of the 2nd branch central frequency fc2, which is equal to 1100 khz, and their values are 700 khz and 1300 khz. in fact, the authors tested the umts fbd second branch with a two-tone signal to verify the correct operation of the i/q demodulation and modulation stages. the sine-wave normalized amplitudes are set at 0.5 for the first and last sine waves and at 0.25 for the sine-waves of the 2nd branch where the normalized amplitude is the input amplitude out of the power supply voltage [10]. the zoom in the spectrum over [4.5, 4.5 mhz] of the second branch sigma delta modulator output, sigma_delta_output2, is drawn in figure 11. it is shown that the sine-wave signals are at the frequencies 700 khz and 1300 khz as in the test conditions. to present i/q demodulated signal of the 2nd branch as in figure 11, the authors need to recombine a complex demodulated signal, demodulated_signal_br2, as defined in figure 7. the sampling frequency after the first decimation stage is equal to 9 mhz and the spectrum covers the band [4.5 mhz, 4.5 mhz]. the obtained sine-wave signals have frequencies 200 khz and 400 khz which are the frequencies of the needed demodulated sine-wave signals. however, the sine-wave signals at the frequencies 1800 khz and 2400 khz are unwanted signals that are filtered thanks to the filters filteri_2 and filterq_2 following the demodulation stage. after the second decimation stage, the recombined final output signal spectrum in the band [2.25 mhz, 2.25 mhz] is presented in figure 12. it shows that the modulated signals (a) (b) figure  10.  (a)  magnitude  of  the  sum  of  lp  filters  of  82 nd   order  after  modulation, (b) zoom in [0, 0.6] normalized frequency band to show ripples. figure 9. magnitude of the lp filters of 82 nd  order after modulation.  figure 11. demodulated signal spectrum of the 2 nd  branch.  0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 -90 -80 -70 -60 -50 -40 -30 -20 -10 0 normalized frequency (  rad/sample) m a g n itu d e ( d b ) 0 0.1 0.2 0.3 0.4 0.5 0.6 -6 -4 -2 0 2 4 normalized frequency (  rad/sample) m a g n itu d e ( d b ) -1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 -100 -90 -80 -70 -60 -50 -40 -30 -20 -10 0 normalized frequency: 0.1442871 magnitude: -6.266335 normalized frequency: 0.3444824 magnitude: -6.192487 normalized frequency (  rad/sample) m a g n itu d e ( d b ) filter of branch 1 bandwidth selection filter of branch 2 bandwidth selection filter of branch 3 bandwidth selection -4 -3 -2 -1 0 1 2 3 4 -160 -140 -120 -100 -80 -60 -40 -20 0 20 frequency (mhz) p o w e r (d b m ) sigma delta modulator 2 output demodulated signal acta imeko | www.imeko.org  september 2015 | volume 4 | number 3 | 21  have frequencies equal to ±300 khz, ±700 khz, ±1300 khz and ±1900 khz which corresponds to the chosen values of the test conditions of the simulation results. moreover, the signalto-noise ratio (snr) computed using matlab/simulink is equal to 75.06 db which satisfies the required umts dynamic range which is equal to 73.8 db. performance parameters as snr and effective resolution resadc are computed for different combination of input frequencies of the four sine-wave signals. computation results are summarized in table 3. besides, the designed fbd model is implemented in vhdl using the system generator (sysgen) tool from xilinx inc. in a co-simulation environment with matlab. the implementation is realized on a virtex-6 fpga target from xilinx inc. test conditions for input signals are the same as for the matlab/simulink simulation. the output signal spectrum is presented in figure 13. the computed snr for this spectrum is equal to 74.08 db which satisfies the umts required dynamic range. 5. conclusions  in this paper, the authors proposed a mixed baseband architecture based on a fbd –based adc in a multistandard receiver. the mixed baseband stage architecture is presented and the single non-programmable aaf is designed using butterworth approximation. the theoretical analysis and design of the digital reconstruction stage for the fbd -based adc architecture dedicated to multistandard radio receivers are proposed. the designed digital reconstruction stage is based on demodulation that brings the  modulators outputs to baseband before proceeding to the decimation and lp filtering operations. the parallel signals are then modulated and combined to form a final output signal. however, the theoretical analysis of the digital reconstruction stage does not converge to a solution of filter coefficients. besides, the first proposed design leads to very high filter orders when implemented in matlab/simulink. consequently, it is essential to modify this model to permit an optimized digital implementation solution. finally, the whole fbd -based adc architecture model with the optimized digital reconstruction stage is implemented and tested for the umts test case in matlab/simulink. moreover, hardware implementation and test results in the sysgen environment are presented for quantized coefficient values. all obtained results satisfy at least the required umts dynamic range which is equal to 73.8 db. references  [1] j. mitola, “software radios: survey, critical evaluation and future directions,” ieee aero. and elect. syst. mag., vol.8, no.4, pp.25-36, apr. 1993. [2] j. m. de la rosa, “an empirical and statistical comparison of state-of-the-art sigma-delta-modulators,” ieee int. symp. on circ. and syst., isbn 978-1-4673-5760-9, pp. 825-822, may 2013. [3] i. galton, h.t. jensen, “delta-sigma modulator based a/d conversion without oversampling,” ieee trans. circuits and syst.-ii: analog and digital sig. proc., vol.42, no.12, pp.773-784, dec. 1995. [4] a. eshraghi, t. fiez, “a time-interleaved parallel ∆ʃ a/d converter,” ieee trans. circuits and syst.-ii: analog and digital sig. proc., vol.50, no. 3, pp.118-129, mar. 2003. [5] a. beydoun, p. benabes, “bandpass/wideband adc architecture using parallel delta sigma modulators”, proceedings of the 14th european signal processing conf., sept. 2006. [6] p. benabes, a. beydoun, m. javidan, “frequency-banddecomposition converters using continuous-time sigma delta a/d modulators,” ieee north-east workshop on circuits and syst. and taisa conf., pp. 1 – 4, jun. 2009. [7] p. benabes, “extended frequency-band-decomposition sigmadelta a/d converter”, analog integr. circ. process., springer science+business media, llc 2009. [8] a. eshraghi, t. fiez, “a comparative analysis of parallel delta– sigma adc architectures,” ieee trans. circuits and syst. i: regular papers, vol. 51, no. 3, pp. 450 – 458, mar. 2004. [9] a. blad et al., “a general formulation of analog-to-digital converters using parallel sigma-delta modulators and modulation sequences”, ieee asia pacific conf. circuits and syst. apccas, pp. 438-441, dec. 2006. [10] r. lahouli, m. ben-romdhane, c. rebai, d. dallet, “towards flexible parallel sigma delta modulator for software defined radio receiver”, ieee int. instrum. and meas. technology conf., may 2014. [11] r. lahouli et al., “digital reconstruction stage of the fbd σδbased adc architecture for multistandard receiver”, 20th imeko tc4 international workshop on adc modelling and testing research on electric and electronic measurement for the economic upturn, benevento, italy, sept. 2014. figure 12. recombined output signal spectrum.  table 3. performance parameters of final output signal.  input signals frequencies (khz)  snr (db)    resadc (bits)  300, 700, 1300 and 1900  75.06  12.18  400, 800, 1400 and 2000  74.73  12.12  500, 900, 1400 and 1900  74.43  12.07  100, 1000, 1400 and 1700  75.26  12.21  figure  13.  frequency  spectrum  of  the  recombined  output  signal  using  sysgen implementation.  -3 -2 -1 0 1 2 3 -140 -120 -100 -80 -60 -40 -20 0 20 frequency (mhz) p o w e r (d b m ) -3 -2 -1 0 1 2 3 -150 -100 -50 0 50 frequency (mhz) p o w e r (d b m ) acta imeko | www.imeko.org  september 2015 | volume 4 | number 3 | 22  [12] p. p. vaidyanathan, “multirate systems and filter banks,” eaglewood cliffs, nj: prentice-hall, 1993. [13] gsm. radio transmission and reception gsm 05.05. etsi, 1996. [14] umts. ue. radio transmission and reception (fdd), 3gpp ts 25.101, version 5.2.0 release 5.etsi 2002. [15] ieee 802.11a part 11: wireless lan medium access control (mac), and physical layer specifications, amendment1 high speed physical layer in the 5 ghz band. ieee, 1999. [16] m. ben-romdhane, c. rebai, a. ghazel, p. desgreys, p. loumeau, “nonuniformly controlled analog-to-digital converter for sdr multistandard radio receiver,” ieee trans. circuits and syst. ii: brief papers, vol. 58, no.12, pp. 862 – 866, dec. 2011. microsoft word article 5 196-1084-1-pb.docx acta imeko may 2014, volume 3, number 1, 16 – 18 www.imeko.org acta imeko | www.imeko.org may 2014 | volume 3 | number 1 | 16 the measurement chain and validation of experimental measurements r.j. moffat department of mechanical engineering, stanford university, stanford, california 94305, united states of america section: research paper keywords: measurement chain, uncertainty propagation citation: r.j. moffat, the measurement chain and validation of experimental measurements, acta imeko, vol. 3, no. 1, article 5, may 2014, identifier: imeko-acta-03 (2014)-01-05 editor: luca mari, università carlo cattaneo received may 1st, 2014; in final form may 1st, 2014; published may 2014 copyright: © 2014 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited an experienced human operator can often recognize an anomalous combination of data even upon its first occurrence, and may save an apparatus from a serious failure by this “intuition”. the human operators ability to identify trends and anomalies is based not only upon the instantaneous value of a measurand but upon its context (the other associated measurands including prior values). decisions are made based on groups of data, some of which may even be non-quantitative (i.e. sound, smell, color etc.). the advent of automated data acquisition and computer control opens the door to this same type of decision making by a control system. certain requirements must be met: (1) the measured data must accurately reflect the state of the system (2) the computer must “know what to expect” over the range of normal operation and, (3) the computer must be able to distinguish between allowable deviations due to experimental uncertainty and deviations which signify trouble. these are the same problems which face an experimental research program and it seems likely that the nomenclature and methodology developed for research experiments will be helpful in discussing measurements for computer-aided control. this paper presents three ideas found useful in planning experimental programs: (1) the nomenclature of the measurement chain, (2) the data reduction program considered as a mathematical model of the real system and, (3) the use of uncertainty analysis to predict the allowable scatter in an experimental result. the process of finding the numerical value of a measurand is illustrated in figure 1. five potentially different values exist for each measurement. the various terms will be illustrated in terms of a hypothetical experiment: determining the exhaust gas temperature of a small engine. the principal measurand, in this example, is temperature; all other descriptors of the system are peripheral measurands. the figure 1. the measurement chain. this is a reissue of a paper which appeared in acta imeko 1973, proceedings of the 6th congress of the international measurement confederation, “measurement and instrumentation”, 17-23.6.1973, dresden, vol. 1, pp. 45–53. the paper witnesses the sophisticated discussion that, well before the publication of the guide to the expression of uncertainty in measurement (gum), was active in the measurement science community around the subject of error and uncertainty, and its consequences on the structure of the measuring process and the way it is performed. acta imeko | www.imeko.org may 2014 | volume 3 | number 1 | 17 real value of the principal measurand is the value the measurand would have if the system were not affected by the measurement process. in the present example, the real value would be the temperature of the exhaust in an uninstrumented engine, running at some stated speed and load. the available value is the value of the measurand in the system, at the location of the sensor, while the measurement is being taken. there will always be some difference between the available value and the real value, though it may be small, since it is impossible to change the state of the sensor without also changing the state of the system. in addition, the presence of the sensor may cause the system to move to a new operating point, resulting in a still further change in the value of the measurand. the available value is the one to which the sensor is exposed: a “perfect sensor” would equilibrate at the available value. in the example, the presence of the temperature sensor in the exhaust duct will raise the engine back-pressure, requiring a slight increase in fuel flow to maintain the same nominal speed and load. this will result in an increase in the exhaust gas temperature. the available value will be higher than the real value for this case. the achieved value is the value the measurand has in the sensor, while the measurement is being made. if the calibration of the sensor were perfectly known, then this is the value which would be measured. many sensors, and particularly thermal sensors, respond to more than one aspect of their surroundings. these system/sensor interactions cause the sensor to equilibrate with its entire environment rather than just the principal measurand and gives rise to what is known as “environmental error”. in the present example the temperature sensor is subject to radiation error, conduction error, and velocity error. thus the temperature level in the sensor (the achieved value) will be lower than the temperature of the gas stream at the sensor location (the available value) due to system/sensor interaction. the difference will depend upon the velocity and composition of the exhaust gases as well as the materials and temperatures of the surrounding duct work and hardware. the measured value is the value which is attributed to the measurand when the output of the sensor is interpreted using the best estimate of the calibration of the sensor. if the calibration were without error the measured value would be equal to the achieved value. if the calibration of the sensor is affected by the conditions of use in a manner which is not known to the user, then the measured value will be different from the achieved value. in the present example assume the temperature sensor to be a thermocouple whose elements are exposed directly to the gas streams. after a period of time there may be sufficient chemical reaction with the exhaust gases to cause a change in the calibration of the wire. use of standard tables of emf– temperature on such a thermocouple would result in measured values which might be significantly different from achieved values. the corrected value is the engineer’s best estimate of the real value, accounting for all of the recognized sources of error: system disturbance, system/sensor interactions, and calibration change. in order that experimental data properly describe the state of a system, the corrected values must be acceptably close to the real values. recognition of the many ways in which unwanted effects can enter a measuring chain is important in devising systems which return valid measurements. too often, principal emphasis is placed on the calibration of the sensor (the link between the achieved value and the measured value). the state of the instrumentation art is so well advanced now that, in general, the principal remaining difficulties are caused by the system/sensor interactions. errors due to system/sensor interactions can be controlled by either of two techniques: (1) design of a system which minimizes system disturbance and system/sensor interactions or, (2), use of a data processing program prior to the control mode which applies the required corrections. the data reduction program and the apparatus must be considered together. the data reduction program may require peripheral data to be gathered (i.e. wall temperature, gas velocity etc.) in order to properly correct the control data. this relationship is illustrated in figure 2. consider a general question “what is the value of ?” a test apparatus and its associated data program together must account for all system disturbances and system sensor interactions with a minimum of uncertainty. large corrections tend to be uncertain, hence the system should be designed to minimize the disturbances and interactions. whatever cannot be accomplished by the system design must be done by the data processor. if the combination is properly matched then the corrected value will be independent of the peripheral effects: they will be suppressed by the system and corrected for by the program. only significant information will be passed to the control block. if, for instance, the temperature of the duct walls decreased due to a drop in ambient air temperature, the increased radiation error would cause the measured value of temperature to go down, even though the available value of temperature remained constant. a properly written data processing program would, however, return the same corrected value, since it would properly compute the new radiation correction. one further problem remains: tolerance on the set point. there is an uncertainty in any physical measurement and a result compute from several measurements is affected by the uncertainty in each of its inputs. it is desirable to be able to anticipate the uncertainty interval associated with a computed result, r, which results from the recognized uncertainty in each of its inputs. this describes the interval within which the computed result must lie as a result of purely random variations of each of its input variables. the uncertainty interval represents a “tolerance” on the computed result. for purposes of uncertainty analysis, a single measurement can be regarded as bearing the following information: x x δx (20/1) (1) where: x is the most probable mean value of x which would be figure 2. the experimental loop. acta imeko | www.imeko.org may 2014 | volume 3 | number 1 | 18 observed if it were measured many times x is the presently recorded value of one measurement of x δx is the interval within which the most probable mean is felt to lie (20/1) are the “odds” which the experimenter believes apply to the preceding statement: i.e., a measure of confidence. one frequent technique for estimating the uncertainty in a computed result is that of kline and mcclintock [1] which propagates the uncertainty at constant probability. consider a result, r, computed from several variables, the x , where: (1) each x is independent and (2) each x displays a gaussian distribution of uncertainty. the uncertainty in the result is then given by: δr ∂r∂x1 δx1 2 ∂r∂x2 δx2 2 … ∂r∂xn δxn 2 / (2) the computing equation r=r(x1, x2, x3, ..., xn) is the data reduction equation by which r is calculated from its inputs, x . the various partial derivatives usually have different values in different parts of the operating range. the uncertainty in the result r is, therefore, governed sometimes by one variable and sometimes by another. active computer control permits the use of “variable tolerances” which are consistent with the physical laws governing the uncertainty. the principal problem which arises is: what value of δx should be used? the answer depends upon the use to which the final result will be put. a general answer is shown in figure 3. if an uncertainty calculation is being made in order to plan a system then the only component of δx would be the “resolution” of the sensor or the ability to interpolate data from its output (zeroth order uncertainty). any real system tends to have small disturbances which vary randomly with time (a timewise “jitter” or unsteadiness) and different sensors have different dynamic characteristics and may introduce different phase shifts into their outputs when exposed to the same process stream. one way to deal with this is to treat the unsteadiness as an uncertainty and add its effect to that of the interpolation problem (first order uncertainty). if the final result is to be used in such a way that the absolute level would be important (for example by subtracting two computed results to determine a difference) then the uncertainties in the calibrations must be included (nth order uncertainty). with the use of uncertainty propagation it becomes possible to set floating limits on the control variables to account for the changing sensitivity of the process to its variables. summary in many respects, the advent of computer based control brings closer together the areas of measurement for research and measurement for control. if computer control is carried to its logical end, the control function should be preceded by a data reduction program which corrects for the disturbance effect of the sensor, and all of the recognized interactions between the system and the sensors. a data reduction program which completely models the behavior of the system will return correct measurement data to the control unit, regardless of the peripheral conditions on the system. development of the data reduction program should complement and accompany the development of the hardware system. uncertainties in the measured data will cause uncertainties in the computed result. this requires establishment of “tolerances” on the control parameters. uncertainty analysis techniques based on constant probability propagation provide a rational basis for establishing limits for acceptable excursions. references [1] s.j. kline, f.a. mcclintock, “describing uncertainties in single sample experiments” mechanical engineering, january, 1953. figure 3. the levels of uncertainty analysis. microsoft word 35-177-1-ga.doc acta imeko  july 2012, volume 1, number 1, 1  www.imeko.org    acta imeko | www.imeko.org  july 2012 | volume 1 | number 1 | 1  journal contacts      about the journal  acta imeko is an e-journal reporting on the contributions on the state and progress of the science and technology of measurement. the articles are based on presentations presented at imeko workshops, symposia and congresses. the journal is published by imeko, the international measurement confederation. the issn, the international identifier for serials, is 2221-870x. editorial and publication board  prof. paul p.l. regtien (the netherlands vice president for publications) dr. dirk röske (germany information officer) prof. antónio da cruz serra (portugal chairman of the advisory board) prof. pasquale daponte (italy chairman of the technical board) prof. francisco alegria (portugal – editorial office) prof. sergio rapuano (italy – editorial office) imeko technical committee chairmen (ex officio) about imeko  the international measurement confederation, imeko, is an international federation of actually 39 national member organisations individually concerned with the advancement of measurement technology. its fundamental objectives are the promotion of international interchange of scientific and technical information in the field of measurement, and the enhancement of international co-operation among scientists and engineers from research and industry. addresses  principal contact prof. paul p. l. regtien measurement science consultancy (msc) julia culpstraat 66, 7558 jb hengelo (ov) the netherlands email: paul@regtien.net acta imeko dr. dirk röske physikalisch-technische bundesanstalt (ptb) bundesallee 100, 38116 braunschweig germany email: dirk.roeske@ptb.de support contact dr. dirk röske physikalisch-technische bundesanstalt (ptb) bundesallee 100, 38116 braunschweig germany email: dirk.roeske@ptb.de microsoft word 278-2015-2-le-rev acta imeko  issn: 2221‐870x  april 2016, volume 5, number 1, 5‐9    acta imeko | www.imeko.org  april 2016 | volume 5 | number 1 | 5  lc‐tandem mass spectrometry as a screening tool for  multiple detection of allergenic ingredients in complex foods  linda monaci, rosa pilolli, elisabetta de angelis, rossella carone, michelangelo pascale  institute of sciences of food production (ispa), national research council (cnr), via giovanni amendola 122/o, 70126 bari, italy        section: research paper   keywords: linear ion trap; mass spectrometry; food allergens; multi‐allergen analysis; incurred cookies  citation: linda monaci, rosa pilolli, elisabetta de angelis, rossella carone, michelangelo pascale, lc‐tandem mass spectrometry as a screening tool for  multiple detection of allergenic ingredients in complex foods, acta imeko, vol. 5, no. 1, article 3, april 2016, identifier: imeko‐acta‐05 (2016)‐01‐03  section editor: claudia zoani, italian national agency for new technologies, energy and sustainable economic development affiliation, rome, italy  received 8 june, 2015; in final form december 16, 2015; published april 2016  copyright: © 2016 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: this work was supported by the project safe & smart– nuove tecnologie abilitanti per la food safety e l’integrità delle filiere agro‐alimentari in uno  scenario globale – national cl.an‐cluster agroalimentare nazionale programma area 2  corresponding author: linda monaci , e‐mail: linda.monaci@ispa.cnr.it    1. introduction  food allergies are reported to be on the rise especially in the last decade [1]. apart from the intentional incorporation of allergenic ingredients into foods for manufacturing purposes and regulated by the current legislation enforced in europe (directive 2007/68) [2], contamination of food by hidden allergens at the moment represents a major health problem for allergic patients. allergens are defined hidden when they are not declared on the product label as required by the legislation in place in europe and might unexpectedly reach the end products through several routes of accidental contamination [3]. different analytical methods have been developed for monitoring food allergen contamination along the food chain. methods address either the allergen itself or a marker contained in the allergenic food. to support allergen control within haccp (hazard analysis and critical control points) programs, laboratory immunoassays have been proposed as screening tool due to its ease in use, the relatively high throughputs and the low detection limits reached [4], [5]. on the other hand, an analysis with a higher level of specificity might be required for confirmation of the results obtained. in the last few years mass spectrometry based methods[6], [7] have been considered a promising analytical strategy for food allergens monitoring thanks to the advances made in this technology that enables the reduction of the risk of false positives compared to elisa methods. ms based methods in general enable to overcome the several restrictions issued by antibody-based methods, such as enzyme linked immunosorbent assay. it is renowned that antibody based kits can produce false positives, especially when applied to complex or processed food matrices as a consequence of epitope modification or masking effect. in addition the use of such antibody based kits presents limitations in running multiplex analysis. although a number of papers have been published in this field using mass spectrometry as a screening tool for allergens detection in foods [7]-[15], only a few of them are directed to the simultaneous determination of several classes of food allergens in food commodities[12], [16abstract  in  the  present  investigation,  an  lc‐ms  method  for  sensitive  multiplex  detection  of  five  allergenic  ingredients  in  a  processed  food  matrix is presented. cookie was chosen as complex food model and was incurred with egg, milk, soy, hazelnuts and peanuts before  baking.  extraction,  purification  and  pre‐concentration  protocols  were  applied  to  ground  cookie  basing  on  protocols  described  elsewhere.  specific  instrumental  features  of  a  dual  cell  linear  ion  trap  ms  instrument  were  exploited  to  identify  suitable  peptide  markers  for  each  allergen  and  to  deliver  a  sensitive  multiplex  srm‐based  method  for  the  simultaneous  detection  of  common  allergenic ingredients which might contaminate such a commodity.  acta imeko | www.imeko.org  april 2016 | volume 5 | number 1 | 6  19]. thanks to the innovative configuration and the versatility shown by the dual cell linear ion trap ms used, we have recently developed a method based on micro high performance liquid chromatography (hplc) and esi-ms/ms detection for the screening of egg, milk and soy in cookies by monitoring selected peptide markers for each allergenic ingredient [17]. as a follow up of that work we present herein a step forward of such method that includes other allergenic ingredients to be monitored to deliver a multiple selected reaction monitoring (srm) method for the multi target detection of the allergenic foods like milk, egg, soy, peanut and hazelnut in cookie chosen as complex food matrix. a subset of peptides tracing for five allergenic foods were used to design a multi-target srm method to detect the presence of each allergenic contaminant in food and the two most sensitive peptide markers/protein were selected in order to retrieve quantitative information. 2. materials and methods  2.1. reagents  acetonitrile (lc–ms grade), formic acid, acetic acid, ammonium bicarbonate, trizma base, tween 20, hydrochloric acid, iodoacetamide (iaa), dithiothreitol (dtt), egg powder (ep) and skimmed milk powder (mp) were obtained from sigma–aldrich (milan, italy). trypsin (proteomic grade) was purchased by promega (milan, italy); rapigesttm surfactant was purchased by waters (milford, ma, usa). cellulose acetate syringe filters, 1.2 µm (size 25 mm) were purchased by labochem science s.r.l. (sant’agata di battiati, ct, italy), and polytetrafluoroethylene syringe filters, 0.2 µm (size 4 mm) were purchased by sartorius italy s.r.l. (muggiò, mb, italy). disposable cartridges pd-10 were purchased from ge healthcare life sciences (milan, italy), while ultrafiltration (uf) tubes with 30 kda cut-off membranes were purchased from millipore (billerica, ma, usa). pre-cooked soy flour (sf) was purchased from a local retailer. roasted peanuts and hazelnuts were provided by besana s.p.a. (san gennaro vesuviano, na, italy). 2.2. preparation of incurred cookie samples.  cookies incurred with egg, milk, soy, hazelnut and peanut were prepared according to the following recipe: 418 g of wheat flour, 180 g of sugar, 1 g of salt, 2 g of sodium bicarbonate, 90 g of extra virgin olive oil, 160 g of water, and 6.4 g of each allergenic ingredient (skimmed milk powder, egg powder, precooked soy flour, ground roasted peanut and ground roasted hazelnut). allergenic ingredients were first homogenized and then added to the dry mixture (wheat flour, sugar, salt and sodium bicarbonate) at a final concentration of 10000 µg/g. subsequently, olive oil and water were added and the final total weight was estimated to be approximately 900 g. incurred dough was left mixing 40 min before being further portioning in 10 g aliquots which were spread in open cookie tins of about 7 cm in diameter and approximately 1 cm in height and baked at 200°c for 12 min. after cooling down, cookies were weighted in order to re-scale the actual allergen concentration to the final baked matrix. the highest incurred cookies corresponded to 8756 µg/g (µg of allergenic ingredients per g of matrix). for the production of blank cookie samples the amount of allergenic foods was replaced by wheat flour (a total of 32 g). both blank and incurred cookies were finely milled in a blender at 17,000 rpm (steril mixer 12 model 6805-50, pbi international) by iteration of four cycles of blending (30 s) and rest (10 s) in order to prevent material overheating. ground blank and incurred cookies were passed through a 1 mm sieve, spread on a large tray (50cm × 50cm) and manually mixed for homogeneity purpose. a total of 5 subsamples (10 g each), were taken respectively from each stock powder produced (blank and incurred) and subsequently combined to form a single representative sample (about 50 g each). two serial dry dilutions of the incurred sample were prepared by mixing ground blank and incurred samples in the ratio 1:3, up to reach the final theoretical concentration of 973 µg/g. such concentration level was used as reference sample for all further experiments. 2.3. preparation of calibration curves  four points calibration curves were prepared for incurred cookies to cover the range 20 243 µg/g. calibration curves were prepared by serial dilutions of incurred cookie extracts at 973 µg/g concentration level, with appropriate volumes of a blank cookie extract. all the sample at each concentration levels were purified and pre-concentrated by ultrafiltration with dimensional cut-off membranes basing on protocols detailed elsewhere [17], [19]. 2.4. enzymatic digestion  the finalextract was denatured by heating for 15 min at 95°c and subsequently diluted in the surfactant/denaturing agent rapigesttm (dilution 1:2) to reach a final volume of 100 µl. protein enzymatic digestion was carried out by addition of suitable amount of trypsin as specific cleavage enzyme, to reach the approximately ratio 1/50, enzyme/protein. reaction was stopped after overnight incubation by acidifying the sample with hcl 1 m and the final extract was filtered through 0.2 µm ptfe filters before chromatographic injection. 2.5. hplc–ms/ms analysis and database search.   the hplc–ms system consisted of a uhplc pump provided with an autosampler and an esi interface connected to a linear ion trap mass spectrometer velos protm (thermo fisher scientific, san josè, usa). peptides separation was accomplished on an acclaimtm pepmap100 analytical column (thermo fisher, san josè, us), 1 mm × 15 cm × 3 µm, 100 å porosity at a flow rate of 60 µl/min and the following elution gradient was used: 0–40 min solvent a reduced from 85 % to 45 %, 4042 min further reduction from 45 to 10 %, constant for 10 min, 5254 min back to 85 % and constant at this composition to allow a 15 min column conditioning before next injection (solvent a = h2o + 0.1 % formic acid; solvent b = ch3cn/h2o, 80/20 v/v + 0.1 % formic acid). for srm acquisition mode, a six segments acquisition scheme was set up, screening a total of 21 peptides. each segment counted up to four scan events, in which each selected peptide was isolated and activated by cid with a normalized collision energy of 35 %, and ion current related to the three most intense transitions was recorded within a 3 m/z window. 3. results and discussions  3.1. selection of peptide markers.   one of the most crucial steps in designing a multi-target screening ms-based method for food allergen detection in complex food matrices, is the appropriate selection of target peptides that should fulfil specific requirements to be considered reliable allergens markers. acta imeko | www.imeko.org  april 2016 | volume 5 | number 1 | 7  in order to draw down a list of potential candidate markers for the selected allergens, a preliminary untargeted ms analysis in data dependent acquisition (dda) mode was carried out on the cookies protein extract (sample incurred at 973 µg/g), followed by purification on size-exclusion columns. raw data were processed via commercial software proteome discoverertm (version 1.4) and protein assignment was accomplished via sequest ht scoring algorithm by searching ms/ms spectra against a customized database (db), restricted to food allergens and other contaminants. a list of peptides was generated and the ms/ms spectra providing the highest matching with the predicted fragmentation patterns were further validated by analyst inspection, as already detailed in a previous paper [17] and only ms/ms peptide spectra containing at least three consecutive peptide fragments of either yor b-ions were selected as good candidate. finally, three most intense transitions were chosen for each internally validated peptide and these were further used to build up an srm acquisition scheme aiming at developing a sensitive and selective method for the detection of target proteins in a complex matrix like cookies. in order to design a proper ms acquisition method, parameters such as number of segments along the lc run, number of scan events per segment, acquisition scan rate (and consequently resolution) and acquisition range, were optimized. basing on the expected elution time for each candidate peptide marker, the acquisition run in srm mode was split into six segments, and a maximum of four scan events per segment were recorded for each peptide marker monitored. notably, after peptide isolation and fragmentation, the sum of ion current related to the three most intense transitions was recorded. ms analyses were carried out both in allergen-free and incurred cookie extracts in order to confirm the absence of any interfering peak at the expected retention times and to evaluate the resolution obtained by chromatographic separation; this comparison allowed to highlight some co-elution problems encountered with two candidate peptides, namely peptide esyfvdaqpk, 592.3 m/z belonging to the α-chain of βconglycinin (soy protein), and peptide dqssylqgfsr, 644.3 m/z belonging to a fragment of conarachin (peanut protein) as depicted in figure 1. as a consequence, these peptides were further excluded from the list of candidate markers since not considered a reliable marker in incurred baked cookies. a part from the two mentioned peptides that were excluded from the list, due to the absence of any matrix interferences with the other candidate markers selected, these chromatographic conditions were considered suitable for our purposes and a total of 19 peptides deemed suitable markers for allergen detection in cookies. a typical representation of the multi-target analysis achieved in the srm mode is shown in figure 2 where an overlay of nineteen chromatographic traces is reported. 3.2. assessment of quantitative performances.  in order to select the best quantitative markers within the list of nineteen peptide markers included into the srm-ms/ms instrument method, matrix-matched curves were obtained in incurred cookies and the relevant srm traces were recorded. the two most sensitive peptides for each allergenic food category were then selected as quantifier peptides (table 1). response linearity was assessed on the whole concentration range under investigation (20243 µg/g) with fisher-snedecor f-test. the ratio between regression and residual variance was compared with the critical f value with the proper freedom degrees at 99% of confidence, proving that the linear correlation was significant. limits of detection (lod) and quantification (loq) were estimated as the minimum concentration of an added allergenic ingredient that can be detected, at s/n equals 3 and 10, respectively (the standard deviation of the calibration line intercept was used in this case as an estimate of noise). table 1 reports an overview of method performances for a total of ten peptides selected for the 5 allergenic ingredients. as appearing from the table, some limit values calculated for each allergenic ingredient resulted very similar irrespective of the specific marker monitored, thus confirming the reliability of both peptides selected for quantitative analysis. besides, lods of each allergenic ingredient under consideration in incurred cookies were found to be ranging from 10 µg/g for egg to 8 µg/g for milk and peanut, while the highest sensitivity reached for soy and hazelnut was down to 5 µg/g (allergen/food matrix). figure  1.  comparison  of  srm  chromatographic  traces  recorded  for  blank  and  incurred  cookie  (243  µg/g),  for  two  candidate  peptide  markers  592.3  (esyfvdaqpk) and 644.3 (dqssylqgfsr), belonging to soy and peanut, respectively.  acta imeko | www.imeko.org  april 2016 | volume 5 | number 1 | 8  figure 2. overlay of typical ion chromatograms recorded in srm acquisition mode for the selected peptide markers. the entire chromatographic run was  divided into six acquisition time segments: (i) 0‐4.5 min; (ii) 4.5‐8.5 min; (iii) 8.5‐12.0 min; (iv) 12.0‐14.1 min; (v)14.1‐18.8 min; (vi) 18.8‐30.0 min.  acta imeko | www.imeko.org  april 2016 | volume 5 | number 1 | 9  4. conclusions  in the present investigation, an lc-ms method for sensitive multiplex detection of egg, milk, soy, hazelnuts and peanuts allergenic ingredients in incurred cookie matrix was presented. suitable peptide markers for each allergenic ingredient were selected basing on several parameters which proved the reliability of the identification. specific transitions of ms/ms spectra were identified to build up a highly selective and sensitive lc-multiplex srm-based method for the simultaneous detection of egg, milk, soy, hazelnuts and peanuts cookie. acknowledgement  the work was funded by the project safe & smart– nuove tecnologie abilitanti per la food safety e l’integrità delle filiere agro-alimentari in uno scenario globale – national cl.ancluster agroalimentare nazionale programma area 2. besana group s.p.a. is kindly acknowledged for providing hazelnut and peanuts and for the fruitful discussions. references  [1] l. monaci, r. pilolli, e. de angelis g. mamone, mass spectrometry in food allergen research, in comprehensive analytical chemistry. y. picó editor. publisher, elsevier, amsterdam (netherlands), oxford (uk), waltham (usa), 2015, isbn 978-0-444-63340-8, 359-393. [2] european commission, commission directive 2007/68/ec.off. j. eur. union l130(2007)11–14. [3] s. hattersley, r. ward, a. baka, r.w.r. crevel, advances in the risk management of unintended presence of allergenic foods in manufactured food products an overview. food chem. toxic. 67(2014) 255-261. [4] l. monaci, a. visconti, immunochemical and dna-based methods in food allergen analysis and quality assurance perspectives, tr. food sci. technol. 21 (2010) 272-283. [5] r. pilolli, l. monaci, a. visconti, advances in biosensor development based on integrating nanotechnology and applied to food-allergen management. tr. anal. chem. 47 (2013) 12-26. [6] l. monaci, a. visconti, mass spectrometry-based proteomics methods for analysis of food allergens. tr.anal. chem. 28 (2009) 581-591. [7] p.e.johnson, s. baumgartner, t. aldick, c. bessant, v. giosafatto, j. heick, g. mamone, g. o’connor, r. poms, b. pöpping, a. reuter, f. ulberth, a. watson, l. monaci, e.n. mills, current perspectives and recommendations for the development of mass spectrometry methods for the determination of allergens in foods. j. aoac int. 94 (2011) 1026-1033. [8] a. cryar, c. pritchard, w. burkitt, m. walker, g. o’connor, d.t. burns, m. quaglia, towards absolute quantification of allergenic proteins in food-lysozyme in wine as a model system for metrologically traceable mass spectrometric methods and certified reference materials. j. aoac int. 96 (2013) 1350-1361. [9] b. pöpping, s.b. godefroy, allergen detection by mass spectrometry the new way forward. j. aoac int. 94 (2011)1005-1005. [10] g. picariello, g. mamone, f. addeo, p. ferranti, the frontiers of mass spectrometry-based techniques in food allergenomics. j. chromatogr. a, 1218 (2011) 7386-7398. [11] l. monaci, i. losito, f. palmisano, a. visconti, reliable detection of milk allergens in food using a high-resolution, stand-alone mass spectrometer. j. aoac int. 94 (2011) 10341042. [12] j. heick, m. fischer, b. pöpping, first screening method for the simultaneous detection of seven allergens by liquid chromatography mass spectrometry. j.chromatogr. a 1218 (2011) 938-943. [13] p. lutter, v. parisod, h. weymuth, development and validation of a method for the quantification of milk proteins in food products based on liquid chromatography with mass spectrometric detection. j. aoac int. 94 (2011) 1043-1059. [14] l. monaci, i. losito, f. palmisano, m. godula, a. visconti, towards the quantification of residual milk allergens in caseinate-fined white wines using hplccoupled with singlestage orbitraptm mass spectrometry. food addit. contam. 28 (2011) 1304-1314. [15] l. monaci, e. de angelis, s.l. bavaro, r. pilolli, high resolution-orbitraptm based mass spectrometry for a prompt detection of peanuts in nuts products. food addit. contam. a, 32 (2015) 1607-1616. [16] l. monaci, i. losito, e. de angelis, r. pilolli, a. visconti, multi-allergen quantification of fining-related egg and milk proteins in white wines by high-resolution mass spectrometry. rapid commun. mass spectrom. 27 (2013) 2009-2018. [17] l. monaci, r. pilolli, e. de angelis, m. godula, a. visconti multi-allergen detection in food by micro-hplc coupled to a dualcell linear ion trapmass spectrometer. j chromatogra 1358 (2014) 136–144. [18] r. pilolli, e. de angelis, m. godula, a. visconti,l. monaci, orbitrap™ monostage ms versus hybrid linear ion trap ms: application to multi-allergen screening in wine.j. mass spectrom. 49 (2014) 12541263. [19] r. pilolli, e. de angelis, l. monaci, speeding-up sample handling for multiplex ms/ms allergens detection in a processed food, submitted to food chemistry. table 1. summary of the quantitative performances provided by the lc‐srm method for selected peptide markers of five allergenic ingredients in cookies.  the accession number refers to on‐line uniprot database.  peptide  m/z  allergenic  food  protein  (accession number)  peptide sequence  transitions  retention time  (min)  lod (µg/g)  loq (µg/g)  slope  r 2   844.4  egg  ovalbumin (p01012)  gglepinfqtaadqar  y12 +2 ;  y7 + ;  y10 +   13.3±0.1   10  30  536±11  0.998  761.9  egg  ypilpeylqcvk  b4 + ;  y8 + ;  y9 +   17.5±0.1   15  51  192±6  0.995  692.9  milk  alpha‐s1‐casein (p02662) ffvapfpevfgk   y8 + ;  y9 + ;  y10 +2   22.9±0.1   8  26  4120±70  0.998  634.4  milk  ylgyleqllr   y5 + ;  y6 + ;  y8 +   21.6±0.1   13  40  2140±60  0.997  725.8  soy  glycin g1 (p04776)  sqsdnfeyvsfk  [m+2h] +2 ‐h2o;   y3 + ;  y10 +   12.5±0.1   5  18  1054±13  0.990  793.9  soy    fylagnqeqeflk   y11 +2 ;  y9 + ;  y10 +   14.8±0.1   8  27  1210±20 0.998  815.4  hazelnut  11s globulin‐like  protein  (q8w1c2)  alpddvlanafqisr  y7 + ;  y8 + ;  y13 +2   21.0±0.1   5  16  3330±30 0.999  576.3  hazelnut  adiyteqvgr   [m+2h] +2 ‐h2o;  y6 + ;  y7 +   3.5±0.1   1 0  32  1530±30 0.997  786.9  peanut  conarachin  fragment(q6ps u3)  vlleenaggeqeer y12 +2 ;  y7 + ;  y8 +   3.4±0.1   8  30  389±7  0.998  564.8  peanut  gtgnlelvavr   y3 + ;  y7 + ;  y6 +   9.6±0.1   9  30  526±11  0.997    template for an acta imeko event paper acta imeko issn: 2221-870x december 2017, volume 6, number 4, 80-88 acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 80 simple methods of voltage dip tracking – case study tomasz tarasiuk gdynia maritime university, morska 81-87, 81-225 gdynia, poland section: research paper keywords: voltage short and long term variations, low-cost measurement, low-pass filtering citation: tomasz tarasiuk, simple methods of voltage dip tracking–case study, acta imeko, vol. 6, no. 4, article 13, december 2017, identifier: imeko-acta06 (2017)-04-13 section editor: konrad jedrzejewski, warsaw university of technology, poland received january 26, 2016; in final form november 15, 2017; published december 2017 copyright: © 2017 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: ministry of science and higher education under grant ds/430/2017 corresponding author: tomasz tarasiuk, e-mail: t.tarasiuk@we.am.gdynia.pl 1. introduction the problem of power quality in electric power networks is one of the hottest topics in electrical power engineering over recent years. proliferation of non-linear loads and renewable energy resources have led to notorious voltage disturbances, like voltage and current waveform distortions, voltage dips etc. the intelligent metering and monitoring systems are necessary [1] to monitor power flow and various voltage and current parameters in numerous locations thorough the grid. however, the proper solutions have to be implemented, in order not to increase the cost of the whole infrastructure. in author’s opinion, apart from high grade power quality analysers, a number of low-cost devices will be used, like instruments based on dedicated integrated circuits with fixed signal processing algorithms, application specific integrated circuits (asics). fortunately, such low-cost asics are already available on “the shelf”, e.g. analog devices single phase, multifunction metering ic with neutral current measurement ade7953 [2] or cirrus logic single phase bi-directional power/energy ic cs5461a [3]. they enable the measurement of voltage and current r.m.s. values, power and energy as well as dips or swells assessment. although they are based on various processing principles, the common feature of the devices is signal processing executed by fixed function digital signal processor (dsp) [2]. among the applied algorithms, the input signal filtrations are used for different aims like: elimination of input channels offset, zerocrossing detection or r.m.s. values of the signals and active power measurements [2]. this paper is focused solely on the singular feature of these ics, namely capacity of voltage dip or swell detection. there are several methods of dip detection and characterization, starting from simple solutions as: voltage peak value monitoring [4]-[8] or the traditional voltage r.m.s. value calculation [4]-[8], up to more complex solutions based on d-q transformation [4], [9], short time fourier transform [5], wavelet transform [4], [5], [8], [10], [11], kalman filtering [4], [5], [12] or a combination of both wavelet and kalman filtering [5], to name the most popular. the comparative study on chosen methods performance can be found e.g. in [6] and [8]. unfortunately, the last group of methods lacks simplicity required for implementation in simple, low cost and low energy consuming asics due to complex models and/or the necessity of implementation of a number of conditional and branching operations. therefore, the simpler solutions are used in asics. abstract the paper presents results of an experimental study of two methods of voltage dip tracking. the first is based on half cycle absolute peak value monitoring, whereas the second is based on low-pass filtration of squares of voltage samples. both methods are devised for application in low-cost integrated circuits, dedicated to power quality monitoring. the two real voltage dips have been considered for the aim. the results are compared with the reference method recommended in iec std. 61000-4-30, based on calculation of the r.m.s. voltage refreshed each half cycle. further, the application of the low-pass method for assessment small voltage variations is considered, both short term (r.m.s. voltage refreshed each half cycle) and long term (r.m.s. voltage calculated over 10 cycles of voltage fundamental component). the research confirmed sufficient accuracy of the method based on low-pass filtering for the class a of measurement. acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 81 for instance, the manufactures of above mentioned asics [2], [3] use the method based on peak value monitoring. simply put, if the magnitude of the instantaneous voltage (its absolute value) is below the pre-defined threshold value, a dip is detected. similarly, the swell is to be identified if the absolute value of the magnitude of the instantaneous voltage is above the pre-defined threshold. the method is simple and easy to implement in asics. but the downside of the solution is a notorious impact of noise and distortions of the voltage on the outcome of the dip or swell identification [4], [8]. this can be avoided by superseding this method by an equally simple one based on low-pass filtering of the squares of the input voltage samples, like the method already used by some manufacturers for the r.m.s. value of voltage and current measurements [2], although not used for dip and swell identification. therefore, the paper aim is to explore the possibility of using this solution in its simplest form, which can be easily applied in asics, for dip and swell identification. it is compared with the above mentioned method based on the absolute value of instantaneous voltage monitoring. both solutions are evaluated against the reference measurement procedure laid in iec standard 61000-4-30 for class a measurements, based on the measurement of the r.m.s. voltage over 1 cycle, commencing at a fundamental zero crossing, and refreshed each half-cycle urms(1/2) [13]. this is recommended for voltage dips, swell and interruption detection and evaluation [14]. it has to be added that some documents use the term sag as synonym to the term dip [2], [3], [14]. but since the term dip is used by iec, it will be used consequently thorough this paper. finally, the paper presents a comparative study of the performance of these two methods. the first method is based on monitoring the voltage peak value. the second method is based on the r.m.s. voltage value determined by low pass filtering. because the idea behind the paper is to use real cases for the method’s performance assessment, results obtained by both solutions are compared against quantity values provided by the reference measurement procedure to assess the measurement trueness of the measured quantity values obtained from analysed procedures [15]. the reference procedure is described in detail in section 2. this section includes also basic information about limitations of the standard procedure. all subsequent analyses of experimental data are based on two voltage dips registered in real microgrids, namely the network of a sensitive data centre during its island operation mode as well as a marine microgrid, a network of a ferry, during switchon of the high power electric motor (for driving the bow thruster). further, another marine network with high distortion (voltage thd=11.8 %) and voltage and frequency modulation was considered for determining the method’s performance for tracking short-term voltage variations. the paper is the extended and updated version of the work presented during the xxi imeko world congress, 2015, prague, czech republic [16]. 2. standard framework – reference measurement procedure the voltage dip is defined as the sudden decrease in r.m.s. value of the supply voltage to a value between 10 % and 90 % of the declared voltage (in another standard it is 1 % to 90 % [17]) for durations from 0.5 cycles to 1 min [14]. a voltage dip is to be described by a pair of data: residual voltage (sometimes dip’s depth) and its duration [13]. the residual voltage is the lowest r.m.s. value of the voltage during the considered event, whereas duration is the time difference between the start time (falling of r.m.s. voltage below the dip threshold) and end the time (increase of r.m.s. voltage above the dip threshold plus hysteresis typically equal to 2 % of declared voltage) [13]. in some cases, the voltage dip is followed by a voltage peak (small swell), e.g. during a asynchronous motor start up in the microgrid [18]. typically, a voltage swell is monitored by the same methods like a voltage dip, e.g. voltage instantaneous values [2] or r.m.s. values [13]. it was mentioned above that for class a measurements the considered r.m.s. voltage should be calculated over 1 cycle, commencing at a fundamental zero crossing, and refreshed each half-cycle. it is designated as urms(1/2) [13] and it includes all components like: harmonics, interharmonics, etc. [13]. it has to be added that class s has been defined at the iec standard 61000-4-30 as well. for this class the dip assessment is to be carried out in a similar way like the above described but the voltage r.m.s. value can be refreshed each cycle. manufacturers of measuring instrument should specify which method is used [13]. finally, the r.m.s. value is to be calculated as square root of the mean value of squares of the voltage samples registered over the considered time interval, like required in iec standard 61557-12 [19]. the evaluation of a real dip carried out by the method devised for class a is used in this paper as a reference measurement procedure for evaluations of the other methods, including whole voltage shape assessment during the considered processes, namely bulk load startup in two investigated microgrids and resulted dips. next, it is compared with the same shape determined by the other simple method under investigation based on monitoring the voltage peak value. it must be firmly stressed that the standard approach is contested by some authors. for instance, in [6], the detailed analysis of the standard method’s performance for short dips (duration below two cycles) was presented. authors of [6] pointed out the ambiguity in determining the dip parameters, both duration and residual voltage, by the standard solution for such a short duration phenomenon. this is chiefly related to synchronization based on fundamental zero crossing [6], [8]. the proposed solution can be based on determining the rms value of the voltage over a one-cycle sliding window and computing the rms value at every sampling point [6], [7]. however, the solution entails higher computational burden for ensuring that the window length fits the actual fundamental period [6]. another frequently investigated solution is tracking the energy of the band 0-100 hz after wavelet transformation for detection of short duration dips [10] but this also entails an increase in computational complexity. next, the standard method has another limitation, namely it “does not provide information about the phase angle of voltage supply during the event, which could be of interest for some applications” [8]. reference [20] lists seven dip characterization methods, including four which require phase angle information. despite the above mentioned drawbacks of the standard method, it arguably remains one of the most popular due to its simplicity and sufficient performance for a number of most common applications. 3. the methods under investigation notwithstanding its simplicity, the application of the above described standard procedure for dip detection and evaluation acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 82 can be inconvenient in simple low-cost devices, due to some requirements for hardware resources of the measuring instrument. in shorthand, it requires storing of voltage samples for at least one cycle, conditional operation, some data address generating, etc. obviously, it can be easily implemented in digital signal processors (dsps) but not necessary in low-cost dedicated asics. therefore, manufacturers of low-cost measuring devices implement other principles of dip detection and evaluation [2], [3]. it is permissible for class b measurements if the manufacturer specifies the method used for the aim [13]. so, the paper explores the performance of two arguably simplest methods of voltage dip detection and characterization, which can be used for less demanding applications and both easy to apply in asics. 3.1. dip measurement based on voltage peak value tracking arguably, the simplest solution of the considered problem is detection of time instants when the absolute value of the voltage falls below the programmable threshold [2], [3]. this feature is easy to implement in asics. simply, the dedicated peak register is updated with a new instantaneous voltage value every time that the absolute value of the voltage sample exceeds the sample value already stored in the register. the register can be cleared after reading [2], which can be synchronized with the voltage fundamental component zero-crossing in order to determine the end of each half-cycle. this feature enables continuous recording the maximum value of the voltage waveforms for each half cycle. apart of its simplicity, the obvious disadvantage of the solution is the above mentioned possible impact of voltage distortions or noise [4], [8], since for distorted signals one cannot conclude on its r.m.s. value basing only on the maximum value of the voltage waveforms. it will be proved below that even in the case of low distortion, the results of dip analysis by the method can differ noticeably from the reference method. 3.2. dip measurement based on low-pass filtering of squares of voltage samples the standard dip assessment method is based on the determination of the voltage r.m.s. value over one cycle. it can be easily noted that )( 2 1 0 21 k n k nrms ulpfuu k ≈∑⋅= − = (1) where n is the number of voltage samples recorded over an integer number of cycles (in fact one cycle for dip assessment), uk is the voltage sample, lpf is a low-pass filter the solution consists in superseding the mean filter with n coefficients (each with value of 1/n) by another lpf filter. since n varies depending on the instantaneous frequency it seems much easier to implement in asics a low-pass filter with a constant number of coefficients. because of the ripples of the lpf output, the reading should be synchronised with the voltage fundamental component zero-crossing. it is determined after low-pass filtering of the voltage samples by another lpf. the algorithm [2] after some modifications is shown in figure 1. it should be added that the solution is applied using an ade7953 [2] for the r.m.s. measurement but not for dip detection and evaluation. the signal processing path shown in figure 1 can be used for both: measuring the r.m.s. value of the voltage during steady-state as well as dip monitoring. the reading is to be carried out after each zero-crossing of the voltage fundamental component, similar as in the case of the urms(1/2) measurement. this value is used directly to the dip or swell detection and evaluation. namely, it is to be compared with the assumed threshold level, once again similar as in the case of the standard method. in order to obtain a 10-cycles rms voltage, accumulation of urms(1/2) and averaging is necessary. the block “delay” shown in figure 1 is to account for the lpf1 and lpf2 characteristics (group delays of both filters). it means shifting of the samples of the lpf2 output. it depends on the actual power frequency. so, adaptation of the shift is to be performed. a simpler solution is to use constant sample shifts related to the rated power frequency but it can impact the accuracy of the proposed solution if used in islanded microgrids due to possible power frequency changes. 4. results of experimental research the real examples are used for this paper’s purpose. so, the research consisted in voltage samples registration in real networks and subsequent processing by various signal processing methods. a national instruments controller pxie8106 equipped with two data acquisition boards pxie-6124 was used for voltage samples recording in an office building. the analog input channel consisted of cv3-1500 lem voltage transducers and ltc 1564 anti-aliasing filters. the cut-off frequency of the anti-aliasing filters was equal to 10 khz. in the case of marine systems, data acquisition board pci703-16/a eagle technology, a resistive voltage divider and isolation amplifiers iso 124 of burr brown were used. the cutoff frequency of the anti-aliasing filter was equal to 3.5 khz. figure 1. block diagram of the signal processing path for the measurement of r.m.s. voltage, including dip detection and evaluation. acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 83 finally, the momentary voltages for dip detection are calculated by both investigated methods. the results obtained by analysis of the voltage local peak values (like used in [2] and [3]) are designated as uabs. the recorded absolute values of the voltage waveform are divided by √2 in order to obtain the voltage r.m.s. value and subsequently to compare with the reference method. the results calculated by voltage samples squaring and low-pass filtering of the result are designated as ulpf. it represents the r.m.s. voltage momentary values. for the paper purpose, a third order butterworth filter is used with various cut-off frequencies. it has been mentioned (see figure 1) that, in order to diminish the effect of ripples of the filter output, its reading is to be synchronised with the fundamental component zero-crossing. zero-crossing of the fundamental component of the voltage is determined after low-pass filtering by another third order butterworth filter, but with higher cutoff frequency equal to 80 hz. such a solution is recommended in the iec 61000-4-30 standard for diminishing the impact of higher frequency components. the cut-off frequency is chosen after [2]. to compare the differences between the considered methods and the reference method, the square root of the mean value of squares of the differences is calculated by: ∑ −= − = 1 0 2 21 1 n k rmsmeth kk uu n diffsqr )()( )/( (2) where methu is the method under investigation (uabs or ulpf), n is the number of considered half cycles for the dip assessment. it is assumed that n=146 starting at the dip beginning. 4.1. dip in emergency power system of office building the voltage recording took place in the network of an office building that contained a very important data centre and sensitive for power quality disturbances. particularly, interruptions can lead to severe consequences. therefore, it was equipped with two upss and two generators driven by diesel engines for power backup. the network rated voltage was equal to 230 v and the rated frequency 50 hz. the whole research was carried out during the object island operation mode, due to suspicion of power quality problems during the mode. various parameters of voltage and currents were analysed in various points of the system. during the investigation the process of switching-on the bulk load was recorded. the waveform of the recorded voltage is presented in figure 2. it is easily discernible that the process of switching bulk load on in the power network of the data centre causes a voltage dip up to 85 % of the rated voltage followed by a small voltage swell. it is only 101.6 % of the rated voltage but approximately above 5 % of the registered mean steady-state voltage, which was approximately equal to 96.8 % of the rated voltage (230 v). nevertheless, this phenomenon is considered as well, in order to properly asses the methods under investigation. the details of urms(1/2) calculated according to the iec 61000-4-30 standard and the resulting voltage shape are shown in figure 3. obviously, the processes of switching bulk load on in microgrids cause voltage changes and concurrent momentary frequency changes. for the considered example, the lowest momentary frequency understood as reciprocal of fundamental cycle is equal to 48.61 hz followed by a frequency increase up 51.51 hz. typically, standards related to marine microgrids, e.g. [21], [22] deal with the phenomenon, but it has not been analysed for the paper’s aim. however, both investigated methods, which depend to some respect on the fundamental component zero-crossing, enable concurrent assessment of momentary frequency changes, including their value and duration. in fact, assessment of rapid power frequency changes in marine systems is similar as in the case of voltage dip assessment [21], [22]. finally, the results of the calculation of parameter pairs for the dip assessment (residue voltage umin and duration), completed by the r.m.s. value of the mini-swell umax voltage that followed the considered dip are shown in table 1. moreover, results of determining sqr(diff) for each of the methods are presented in the table as well. analysis of results in table 1 leads to the conclusion that results of dip shape assessment for filters with cut-off frequencies from 15 hz up to 19 hz lead to comparable results. observed minor differences can be neglected. the lowest sqr(diff) value is obtained for a filter with cut-off frequency 17 hz. the analysis confirms that the uabs method leads to worse results than the method based on low-pass filtering of the squares of voltage samples for most of the used filters. next, the capability of tracking the voltage shape for each of the methods is assessed. it is carried out by determining the differences between ulpf and urms(1/2) (they are graphically presented in figure 4 for lpf2 with cut-off frequency equal to 17 hz) and the differences between uabs and urms(1/2). the results of comparison are shown in figure 5. sqr(diff) values for these examples are given in table 1 as well. figure 2. voltage waveform during switching bulk load on in a power network of data centre during island operation. figure 3. variations of urms(1/2) value (reference method) during switching bulk load on in a power network of a data centre during island operation; residue voltage equals 195.86 v (85.16 % of urated), dip duration 18 half cycles, swell equals 233.58 v. -400 -300 -200 -100 0 100 200 300 400 time [ms] u( t) [v ] 800 0 190 200 210 220 230 240 time [s] u rm s( 1/ 2) [v ] 4 0 acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 84 comparison of figures 4 and 5 once again reveals clear superiority of the first solution, i.e. ulpf. the difference between the considered method and the reference method does not exceed 1.2 v. maximal values are observed for the dip beginning. similar differences for the uabs method reach values above 4 v. the reason is distortion of the investigated voltage, although with a relatively low level. the value of the voltage thd was equal to 3.13 % prior to the considered dip and 3.26 % after the dip. it can be noted that even this small increase leads to a significant increase in differences between uabs and urms(1/2) (see figure 5). the differences increase nearly two times under steady-state conditions: before and after dip. the main reason is the increase of the 5th harmonic content by 0.19 % of the fundamental component, completed by a phase shift of some harmonics in relation to the fundamental component after the dip and a resulting increase in the voltage maximal instantaneous value. 4.2. dip in power system of a ferry the second example concerns the dip caused by switching on the high power electric motor with rated power 1.72 mw on board a ferry with a 6.6 kv 60 hz system. the motor start prior to the ship manoeuvring has led to a severe voltage dip, namely the voltage dropped below the permissible limit -20 % of the rated voltage [21], [22]. similar as in the previous example, the dip parameters are determined by the two investigated methods. the waveform of the recorded voltage is presented in figure 6 and the details of urms(1/2) calculated according to the iec 61000-4-30 standard and the resulting voltage shapes are shown in figure 7. the calculation results of the parameter pairs for the dip assessment (residue voltage umin and duration), completed by the r.m.s. voltage of the mini-swell umax that followed the considered dip are shown in table 2 and the differences between ulpf and urms(1/2) and the differences between uabs and urms(1/2) are given in figures 8 and 9, respectively once again, analysis of the above results leads to the conclusion that application of lpf filters with cut-off frequencies 12-20 hz gives satisfactory results. the solution superiority over uabs method is clearly visible, despite the fact that for the ferry voltage thd hardly exceeded 1.2 %. table 1. calculation results of dip (duration and umin) and swell (umax) parameters completed by sqr(diff). fcutoff half cycl. umin umax sqr(diff) [v] [%] [v] [v] urms(1/2) 18 195.86 85.16 233.58 -- uabs 17 197.6 85.91 236.47 3.26 ulpf 1 hz --212.87 92.55 226.99 6.23 2 hz 15 203.64 88.52 228.20 3.51 3 hz 17 198.28 86.20 232.53 1.72 4 hz 17 196.26 85.33 233.46 1.13 5 hz 17 195.20 84.87 233.68 0.88 6 hz 17 194.99 84.77 233.69 0.85 7 hz 18 194.96 84.76 233.72 0.68 8 hz 17 195.16 84.85 233.71 0.55 9 hz 18 195.43 84.97 233.66 0.61 10 hz 17 195.69 85.08 233.69 0.53 11 hz 18 195.87 85.16 233.67 0.36 12 hz 18 195.98 85.20 233.66 0.45 13 hz 18 196.00 85.20 233.67 0.64 14 hz 18 195.93 85.17 233.67 0.51 15 hz 18 195.87 85.16 233.65 0.35 16 hz 18 195.83 85.14 233.62 0.23 17 hz 18 195.82 85.14 233.59 0.20 18 hz 18 195.83 85.14 233.58 0.25 19 hz 18 195.87 85.16 233.59 0.34 20 hz 18 195.87 85.16 233.59 0.88 figure 4. the differences between r.m.s. value of voltage calculated by filtering of square samples ulpf (lpf2 cur-off frequency=17 hz) and reference method urms(1/2). figure 5. the differences between rms value of the voltage calculated on the basis of the absolute peak value of voltage uabs and reference urms(1/2). figure 6. voltage waveform during switch-on of the thruster in power network of ferry. acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 85 4.3. voltage variations tracking in the system with high level of voltage distortion the voltage registered on board of a chemical tanker was chosen for the assessment of the performance under significantly distorted conditions. the vessel is equipped with a shaft generator working on main bus bars via a power converter to obtain a constant frequency. the system rated voltage was 440 v and rated frequency 60 hz. the voltage waveform registered in the system of the chemical tanker is shown in figure 10. for the assessment, the measurement results of the voltage modulation by low-pass filtering ulpf and by the urms(1/2) method were compared. it is to exemplify the capability of tracking of the voltage variations by the ulpf method. it must be stressed that the considered case is very hard to analyze because of concurrent presence of significant waveform distortions as well as fundamental voltage and frequency modulations shown in figure 11. the instantaneous frequency and voltage for figure 11 are calculated by a zoom-dft with kaiser window, refreshed every 0.5 ms and with a frequency resolution 0.001 hz. for this case, a similar modulation was observed for harmonics, e.g. the r.m.s. of the 5th harmonic varied between 15 and 25 v. figure 8. the differences between the r.m.s. value of voltage calculated by filtering of square samples ulpf (lpf2 cur-off frequency=17 hz) and reference method urms(1/2). figure 9. the differences between the rms value of the voltage calculated on the basis of the absolute peak value of voltage uabs and reference urms(1/2). figure 10. voltage waveform in a marine system with a shat generator working via a power converter, thd=11.8 %. figure 7. variations of urms(1/2) (reference method) during switching of the thruster onboard a ferry; residue voltage equals 5015 v (75.98 % of urated), dip duration equals to 51 half cycles, swell equals to 7083.1 v. table 2. results of calculation of dip (duration and umin) and swell (umax) parameters completed by sqr(diff). fcutoff half cycl. umin umax sqr(diff ) [v] [%] [v] [v] urms(1/2) 51 5015.0 75.98 7083.1 -- uabs 49 5211.6 78.96 7299.6 170.03 ulpf 1 hz 52 5554.2 84.15 6671.4 326.89 2 hz 48 4990.2 75.61 6958.7 146.99 3 hz 49 4884.4 74.01 7083 108.95 4 hz 49 4891.1 74.11 7105.6 87.96 5 hz 50 4918.0 74.52 7111.5 76.13 6 hz 50 4926.7 74.65 7109.9 64.24 7 hz 50 4929.8 74.69 7105.7 55.97 8 hz 50 4938.7 74.83 7100.5 51.63 9 hz 51 4953.1 75.05 7093.8 46.56 10 hz 52 4974.3 75.38 7086.9 42.23 11 hz 51 4993.7 75.66 7080.4 38.26 12 hz 51 5016.9 76.01 7074.7 42.65 13 hz 51 5015.5 75.99 7083.1 33.88 14 hz 51 5014.9 75.98 7083.1 28.86 15 hz 51 5013 75.95 7079.6 28.70 16 hz 52 5012.8 75.95 7080.8 32.15 17 hz 51 5014.3 75.97 7082.0 38.19 18 hz 51 5013.6 75.96 7083.3 28.18 19 hz 51 5013.6 75.96 7084.2 22.58 20 hz 51 5014.8 75.98 7085.9 18,43 acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 86 the two voltage tracking methods are graphically compared in figure 12, for a steady-state condition. assumed cut-off frequency of lpf2 filter is equal to 17 hz. analysis of figure 12 once again indirectly proves that the ulpf method would be appropriate for the considered aim, namely tracking of voltage short-term variations, including voltage dip or swell detection and evaluation, even in the case of heavily distorted signals. finally, it has to be added that the r.m.s. value of voltage over 10 or 12 cycles time interval (depending on the power system’s rated frequency) should be calculated for the assessment of magnitude of the supply voltage (apart from dips and swells detection) [13]. because it would be unwise to design a separate signal processing path for the 10 or 12 cycles r.m.s. value determination, it seems that the simplest solution would be determining the average value of readings of the lpf2 filter over a specified number of cycles. it can easily be implemented in ics. to assess the accuracy of the solution, the average values of twenty or twenty four consecutive readings of the lpf2 filter with cut-off frequency equal to 17 hz is determined for the office building (50 hz system) and the chemical tanker (60 hz system), respectively. they are graphically (figure 13) compared with respective r.m.s. values determined for the very same time interval (exactly 10 cycles) by the reference method. the analysis of the results presented in figures 4, 8, 12 and 13 clearly indicates that the same signal processing path can be easily used for assessing voltage dips and swells as well as determining 10/12 cycles voltage magnitude after simple averaging. for example, the maximum difference between reference 10/12 cycle r.m.s. value and the respective value determined by averaging the lpf2 readings (cut-off frequency 17 hz) is below 0.02 v for the case presented in figure 13(a) and it is below 0.5 v for the case presented in figure 13(b). the value of sqr(diff) is below 0.01 v for the former case and 0.3 v for the latter case. 5. conclusions the paper aim was the investigation of some simple algorithms for dip or swell detection and evaluation, easily applicable in low-cost ics dedicated to multifunction electricity figure 11. fluctuations of voltage (a) and its frequency (b) on board of chemical tanker determined by zoom-dft. figure 12. comparison of tracking capabilities of short-term small voltage variations by ulpf method with urms(1/2) method. figure 13. comparison of reference r.m.s. value calculated over 10 or 12 cycles of input voltage and respective value of averaged lpf2 readings over the same time interval, (a) emergency supply of the office building, (b) bow thruster subsystem with non-linear load. acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 87 parameters measurements. it was proved that the method of signal processing based on low-pass filtering of the input voltage samples is more suitable for the aim than the commonly used method based on absolute values of the momentary voltage peak. in the latter case (uabs method) the significant impact of voltage distortions on the measurement accuracy can be observed. it is true even for slightly distorted signals, which are rather norm than exception in nowadays power systems. therefore, using the method based on low-pass filtering is arguably a better solution. since it is already used for r.m.s. value estimation by the same ics, it requires only some design modification , but special attention has to be paid to the cut-off frequency and group delay of implemented lpfs. the used cutoff frequency has to be increased in comparison with that currently used in ics [2] but it inevitably leads to an increase in ripples of the filter output. fortunately, the ripples impact is limited, if synchronisation of reading with voltage zero crossings is implemented. the resulting overall accuracy is better than in the case of the hitherto used solution, based on tracking of the absolute instantaneous value of the input voltage. moreover, the reading of the used lpf2 filters can be used for determining the 10-cycles voltage magnitude with good accuracy after simple averaging. it simplifies the ic design, since the same signal processing path can be used for both aims, dip detection and the 10-cycles voltage magnitude measurement. currently it is determined by two separate signal processing paths, in dedicated ics with fixed dsp. it has to be added that the solution based on low-pass filtering was devised for low-cost and low power consumption applications, with sufficient accuracy for trouble-shooting applications, like defined in the iec standard 61000-4-30 [13]. its application for contractual purposes will require additional research, particularly for other voltage shapes during dips. next, the cut-off frequency of the applied lpf2 filter has to be carefully chosen, since it can affect the uncertainty of the method. if it is too low, it results in an increase in response time and the assessed residue voltage would be higher than the real one. this can affect the accuracy of determining the residue voltage during very short dips. however, a too high cut-off frequency would lead to an increase of ripples of the filter output and increase in the measurement uncertainty since reading exactly at the moments of zero-crossing is hardly possible. in fact, the results are affected by both lpf1 and lpf2 characteristics. their correction can assume discrete values with a resolution equal to the sampling period, since delay means shifting samples of the lpf2 output. so, the accuracy of correction of the group delay for the respective filters is in the range of ±0.5 ts (ts – sampling period, which for the considered research equals 40 µs for the office building and 95 µs for the two later cases, namely the marine microgrids). the remaining factors influencing measurement uncertainty are the same as considered for typical performance monitoring and measuring devices and discussed in standard [19]. particularly, the limitation of the method based on low-pass filtering is the necessity of synchronisation of the reading with the fundamental zero crossing. however, this is also a limitation of the solution based on the peak value, which can be determined once every half cycle. nevertheless, the research carried out for two various dip cases and completed by research of modulated, significantly distorted voltages (network of chemical tanker) proved to have good accuracy of the proposed solution, even under such unfortunate circumstances. the maximum difference between the reference method (for 12 cycles time interval) and the described proposal hardly exceeded 0.11 % of the voltage rated value. summing up, in author’s opinion, the proposed solution based on a simple low-pass filtering gives acceptable accuracy for less demanding applications and can be used for everyday monitoring of power systems performance. however, for more demanding or legal purposes other, more complex solutions, for instance mentioned in the introduction, should be considered. references [1] european commission, “communication from the commission to the european parliament, the council, the european economic and social committee and the committee of the regions, smart grids: from innovation to deployment.” com(2011) 202 final, brussels, 12.4.2011. [2] analog devices, “single phase, multifunction metering ic with neutral current measurement ade7953”, data sheet, 20112013. [3] cirrus logic, “single phase bi-directional power/energy ic cs5461a”, data sheet, 2011. [4] a. khoshkbar sadigh, k.m. smedley, “fast and precise voltage sag detection method for dynamic voltage restorer (dvr) application”, electric power systems research, vol. 130, 2016, pp. 192–207. [5] e. perez, j. barros, “a proposal for on-line detection and classification of voltage events in power systems”, ieee trans. on power delivery vol. 23, no. 4, 2008, pp. 2132-2138. [6] d. gallo, c. landi, m. luiso, e. fiorucci, “survey on voltage dip measurements in standard framework”, ieee trans. on instrumentation and measurement, vol. 63, no. 2, 2014, pp. 374387. [7] m. bollen, i. gu, “signal processing of power quality disturbances”, new york, usa, wiley, 2006. [8] a. moschitta, p. carbone, c. muscas, “performance comparison of advanced techniques for voltage dip detection”, ieee trans. on instrumentation and measurement, vol. 61, no. 5, 2012, pp. 1494-1502. [9] o. hua, b. le-ping, y. zhong-lin, ”voltage sag detection based on dq transform and mathematical morphology filter”, procedia engineering, vol. 23, 2011, pp. 775 – 779. [10] m. apraiz, j. barros, r. diego, “a real-time method for time– frequency detection of transient disturbances in voltage supply systems”, electric power systems research, vol. 108, 2014, pp. 103– 112. [11] j. decanini, m. tonelli-neto, f. malange, c. minussi, “detection and classification of voltage disturbances using a fuzzyartmap-wavelet network”, electric power systems research, vol. 81, 2011, pp. 2057– 2065. [12] e. pérez, j. barros, “an extended kalman filtering approach for detection and analysis of voltage dips in power systems”, electric power systems research, vol. 78, 2008, pp. 618–625. [13] iec standard 61000-4-30, “testing and measurement techniques – power quality measurement methods”, 2015. [14] ieee standard 1159-2009, “ieee recommended practice for monitoring electric power quality”, 2009. [15] joint committee for guides in metrology “international vocabulary of metrology – basic and general concepts and associated terms,” 3rd edition, , jcgm 200:2012. [16] t. tarasiuk, “comparative study on chosen methods of voltage dip tracking based on real example”, xxi imeko world congress “measurement in research and industry” august 30 − september 4, 2015, prague, czech republic. [17] en standard 50160, “voltage characteristics of electricity supplied by public distribution networks”, 2007. [18] a. vicenzutti, d. bosich, g. giadrossii, g. sulligoi, “the role of voltage controls in modern all-electric ships”, ieee acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 88 electrification magazine, vol. 3, no. 2, 2015, pp. 49-65. [19] iec standard 61557-12, “electrical safety in low voltage distribution systems up tp 1000 v a.c. and 1500 v d.c. – equipment for testing, measuring or monitoring of protective measures – part 12: performance measuring and monitoring devices (pmd)”, 2007. [20] y. wang, m. bollen, a. bagheri, x. xiao, m. olofsson, “a quantitative comparison approach for different voltage dip characterization methods”, electric power systems research, vol. 133, 2016, pp. 182–190. [21] det norske veritas, “rules for classification of ships / high speed, light craft and naval surface craft. electrical installations”, part 4, chapter 8, 2011. 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/hrv (za stvaranje adobe pdf dokumenata najpogodnijih za visokokvalitetni ispis prije tiskanja koristite ove postavke. stvoreni pdf dokumenti mogu se otvoriti acrobat i adobe reader 5.0 i kasnijim verzijama.) /hun 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citation: danila moscone, giulia volpe, fabiana arduini, laura micheli, rapid electrochemical screening methods for food safety and quality, acta imeko,  vol. 5, no. 1, article 9, april 2016, identifier: imeko‐acta‐5 (2016)‐1‐09  section editor: claudia zoani, italian national agency for new technologies, energy and sustainable economic development affiliation, rome, italy  received july 29, 2015; in final form december 18, 2015; published april 2016  copyright: © 2016 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  corresponding author: danila moscone, e‐mail: danila.moscone@uniroma2.it    1. introduction  the potential impact of foods on human health is receiving increasing attention by public opinion, scientists and legislators. the interest of consumers, as well as of producers, for food safety and quality testing has increased over the last years. the food production, processing and distribution globalization further complicate the problem. thus, efficient and sensitive analytical methods able to detect food contamination would help to reduce social and health risks. food control analyses require robust, sensitive, and selective detection methods. the most commonly used methods such as chromatography and mass spectrometry require expensive instrumentations and skilled technicians. therefore, the importance of new detection systems, which should be accurate and sensitive, cheap and preferably portable for on-site testing, is evident. sensors and biosensors allow the development of fast, simple and cheap electrochemical screening methods, and some of them are presented in this paper. in particular, screening methods will concern the detection of chemical compounds and microbial contaminants such arsenic and pesticides, salmonella sp, marine toxins such as palitoxin, and viruses such as the hepatitis a virus (hav). in all the methods that will be presented, screen printed electrodes (spes) and/or electrochemical arrays based on an 8screen-printed electrode strips, connected to a cost effective and portable apparatus, have been adopted as electrochemical transducers or assembled as immunosensors. some interesting and useful reviews on spes can be found in literature [1]-[3]. in order to obtain higher sensitivity, in some case spes have been modified with nanostructured materials such as carbon black (cb) or gold nanoparticles (aunps), while in the case of immunosensors the arrays have been coupled with magnetic beads (mb), where the immunological chain occurs. in this paper, we summarize the results of some recent research carried out by our analytical chemistry group, and presented at the 1st imekofood congress held in rome, italy in october 2014. experiments illustrating the optimization and analytical characterization of the developed screening methods and their application in real samples to evaluate matrix effect and recovery, will be presented. abstract  this paper presents some examples of rapid, simple and cost effective screening methods that can be realized by the use of screen  printed  electrodes  (spes)  coupled  with  portable  and  cheap  instrumentation,  for  the  monitoring  of  food  safety  and  quality.  when  necessary, these spes have been modified with nanomaterials in order to improve their analytic performances. arsenic detection, for  example, has been obtained with spes modified with a composite of nanostructured carbon black and au nanoparticles, while for the  pesticide detection the spes were modified with prussian blue nanoparticles in addition to the enzyme butyrylcholinesterase. in the  case of  immunosensors, a high sensitivity has been obtained making the entire  immunological chain to happen on the surface of  magnetic beads (mbs), finally collected on the surface of screen printed arrays with the aid of magnets located just under the working  electrodes. application to real samples are presented, in order to demonstrate the effectiveness of such approaches.    acta imeko | www.imeko.org  april 2016 | volume 5 | number 1 | 46  2. arsenic detection  arsenic has an historical use for its toxic and medicinal properties. in more recent ages, arsenic and its compounds have been widely used in pigments, as insecticides and herbicides, as an alloy in metals, and as chemical warfare agents. due to its wide use, together with its presence in a wide range of minerals, large accumulations now exist in soils and sediments, up to contaminate waters, including drinking waters. the problem of the presence of as in water is known since the 80's, when high levels of as have been found in groundwater resources. thus, contamination of natural water has been identified as a public health problem, due to the mutagenic and carcinogenic effects of this element, especially in the form of as(iii) [4], prompting the world health organization (who) to set the maximum permitted level in drinking water at 10 μg/l [5]. at present, a lot of detection methods have been reported and reviewed [6]. most of them obtain limits of detection below the who arsenic guideline value of 10 g/l, but often they are only suitable for laboratory conditions. therefore, a rapid and sensitive portable system for the screening of as in field could be really useful. we have developed an electrochemical sensor based on spes modified with a nanocomposite of carbon black (cb) and au nanoparticles (cb-aunps/spe) for the detection of as(iii) [7]. actualy, a wide variety of nanomaterials with different properties have found a broad application in several analytical methods. between them, gold nanoparticles (aunps) have received large attention mainly due to their interesting electrocatalytic properties [8], [9]. moreover, recently we demonstrated the advantages of using the cb modified screenprinted electrode (spe) compared to the bare one, for its improved analytic characteristics [10], [11]. further modification of spes with the composite cb-aunps still enhances these characteristics, due to a synergic effect of both nanomaterials. the spes were home produced with graphitebased conductive ink (elettrodag 421) for working and counter electrode and silver-silver chloride conductive ink (elettrodag 477 ss) for pseudo-reference electrode. the diameter of the working electrode was 0.3 cm, resulting in an apparent geometric area of 0.07 cm2. the sensors were prepared by modifying the spe firstly with cb (6 l of a 1 mg/ml solution in dmf/water 1:1) and then with aunps, also homemade, by depositing 6 l of the aunps solution on the cb-modified spe. the chosen electrochemical technique was the linear sweep anodic stripping voltammetry (scan rate=0.8 v/s, tdeposition=300 s, ecleaning= 0.2 v, tcleaning=10 s), applied with the portable potentiostat palmsens. after the electrochemical and analytic optimization, the as was measured with a high sensitivity (674 ma mm-1cm-2) and a lod of 0.4 g/l. finally, cb-aunps/spes has been applied to measure as(iii) traces in drinking water. the analysis requires only few minutes and this sensor provides the detection of as(iii) with high values of percentage recovery (99.9 %) in a tap water sample spiked with the legal limit amount (10 g/l). the electrodes are easily fabricated at low cost, are disposable, and suitable for in situ analysis in real time. 3. pesticides detection  in agricultural or industrial production there is a growing exploitation of chemical compounds, but these substances may pose serious risks on humans, animals and environment. among the various chemicals, pesticides are considered one of the most dangerous, due to their variable nature and to highly toxic effects on living organisms and wildlife. the toxicity of organophosphorus pesticides (ops) mainly arises from their capability to inhibit the acetylcholinesterase enzyme (ache), a crucial enzyme for the central nervous system processes [12], [13]. for these reasons, there is a general concern on pesticide contamination and on their detection in polluted sites. traditional methods for the detection of organophosphorus pesticides employ chromatographic techniques, such as gas chromatography (gc) [14], [15]. biosensors can represent suitable tools for rapid, reliable, cost-effective and in-situ analysis, for screening of environmental samples successively confirmed by traditional laboratory methods. in particular, several biosensors have been described in literature for organophosphorus pesticides based on their capacity to inhibit cholinesterase (che) enzymes [16][20]. in a recent work we developed an amperometric biosensor for the determination of paraoxon, based on the enzyme butyrylcholinesterase (bche) immobilized on screen-printed electrodes modified with prussian blue nanoparticles (pbnps), and embedded in a flow system [21]. prussian blue nanoparticles (pbnps) modification of spes was accomplished by placing a drop (10 μl total volume) of “precursor solution” on the working electrode area. this solution was obtained by mixing 5 μl of 0.1 m potassium ferricyanide in 10 mm hcl with 5 μl of 0.1 m ferric chloride in 10 mm hcl directly on the surface of the working electrode. the solution was left on the electrode for 10 min and then rinsed with a few milliliters of 10 mm hcl. the electrodes were then left 90 min in the oven at 100 °c to obtain a more stable and active layer of pbnps [22]. the pbnps modified electrodes were stored dry at room temperature in dark up to one year. to immobilize the bche enzyme on the pbnps modified electrode surface, 2 µl of 0.25 % glutaraldehyde was applied with a pipette exclusively on the pbnps modified working electrode. the solution was left to evaporate; then, 2 µl of a mixture of bsa, enzyme and nafion was dropped on the working electrode. the mixture was obtained by adding 25 µl of 3 % (w/v) bsa, 25 µl of 0.1 % (v/v) nafion® and 25 µl of a stock enzyme solution (40 u/ml). all solutions were prepared in distilled water. the biosensor was embebbed in a flow system shown in figure 1, and, in order to improve the working stability of the reference electrode, the ag/agcl reference electrode was covered with an acetate cellulose layer by applying 2 µl on its surface. the so prepared amperometric measurements were performed in a carrier solution consisting of 0.05 m phosphate buffer, 0.1 m kcl ph 7.4 at an applied potential of +200 mv vs. ag/agcl. firstly, the carrier buffer was passed through the electrochemical cell for 5 min to register an intensity current (control). then, a carrier buffer containing 5 mm butyrylthiocholine was passed through the flow cell where the biosensor was located. the substrate butyrylthiocholine was hydrolyzed by bche immobilized on the spe-pbnps producing thiocoline, which is electroactive.   acta imeko | www.imeko.org  april 2016 | volume 5 | number 1 | 47  the resulted current signals were continuously recorded and the steady state current values were measured. stabilization of the current in flow conditions was reached in 10 min. the intensity currents were proportional to thiocholine produced, giving information about the enzymatic activity of the immobilized bche. the inhibitory effect of organophosphates (i.e., paraoxon) on the bche biosensor was evaluated by determining the decrease in the current obtained for the oxidation of a lesser amount of thiocholine produced by the enzyme when the sample contaminated with paraoxon was passed through the flow cell for a selected time. thus, the biosensor was exposed to paraoxon, followed by a washing step with distilled water. the enzymatic residual activity was finally determined in a flow buffer, in the presence of the enzymatic substrate. the resulting currents were measured as described above and the degree of inhibition was calculated as a relative decay of the biosensor response through the (1): % 100 (1) where io and ii represent the biosensor responses before and after the incubation procedure, respectively. the introduction of a washing step allowed to avoid electrochemical interferences such as ascorbic acid or phenolic compounds, since the enzymatic activity was always quantified in the phosphate buffer in the absence of any electroactive interfering species eventually present in the sample. the integration in a flow system enables the biosensor to be applied for continuous automatic monitoring of paraoxon in environmental samples. in addition, in a continuous flowsystem biosensor, manual procedures are minimized and analyses can be programmed and remotely delivered, minimizing the operator intervention. in order to optimize the biosensing system, a series of parameters was evaluated and optimized, such as different types of electrochemical cells, the flow rate during the enzymatic measurement and the incubation time. in optimized conditions, storage stability lasted up to 60 days at room temperature in dry conditions, demonstrating an excellent storage stability and making this system highly attractive for commercial use. furthermore, the analytical system was characterized by satisfactory analytical performances in the detection of organophosphate tested (paraoxon) reaching a linear range of concentration between 2 and 10 ppb with a detection limit (lod) of 1 ppb in standard solutions. this system was also challenged in drinking, river and lake water samples with satisfactory recovery values using a dilution step of 1:4 v/v. the advantage of this integrated system is the possibility to measure irreversible inhibitors of cholinesterase enzyme (i.e., organophosphorus and carbammic pesticides) in samples without extended pre-treatments and to automate the analyses reducing costs and time. this analytical system can be used as an alarm system for this class of compounds, followed, in the case of alarm, by the hplc or gc-ms analyses to exactly detect the pesticides present in the sample. 4. palytoxin detection  palytoxin (pltx) is one of the most potent marine toxin known to date [23]. recently blooms of ostreopsis spp. have been reported along the mediterranean coasts, posing serious risks to human health. occurrence of ostreopsis spp may result in palytoxin contamination of seafood (250 µg/kg proposed regulatory limit) and, in order to prevent sanitary risks, there is the need to develop rapid and sensitive monitoring methods of pltx-like compounds in seafood, coupled with an efficient extraction procedure. currently, there are no regulations on pltx-group toxins in shellfish, either in the european union or in other regions of the world, nor there is an official method for their determination. the method most commonly used for detection of pltxs is the mouse bioassay [24] but for reasons of animal welfare, of poor sensitivity and specificity, and long analysis times, other methods detection are required by efsa [25], [26]. we developed an electrochemical sensor, based on an 8screen-printed electrode strip connected to a cost effective and portable apparatus, for palytoxin (pltx) detection [27]. sheep erythrocytes were used to test palytoxin and the degree of haemolysis caused by this toxin was evaluated by measuring the release of the cytosolic lactate dehydrogenase (ldh). the percentage of haemolysis, and therefore the amount of ldh measured, using nadh/pyruvate and appropriate electrochemical mediators, is correlated to the concentration of this toxin. two different electrochemical approaches were investigated for evaluating ldh release, but only one based on the use of a binary redox mediator sequence (phenazine methosulphate in conjugation with hexacyanoferrate (iii)) has proven to be useful for our purpose. the approach involves three sequential reactions: ldh pyruvate + nadh ←→ lactate + nad+ (2) nadh + pms++→nad+ + pmsh (3) pmsh + 2 fe(cn)63→pms+ (which cycles via the reaction 3) + 2 fe(cn)64-+ h+ + 2e (4) after incubation of the ldh (released into the medium) and its substrates for 30 minutes, unreacted nadh (the amount depending on the concentration of ldh) spontaneously interacts with pms+ forming pmsh. this latter, produced by reaction (3) interacts with the second mediator, present in large excess, forming fe cn . in figure 1. scheme of the flow system, [21].    acta imeko | www.imeko.org  april 2016 | volume 5 | number 1 | 48  this approach, when the residual nadh is completely consumed, the reactions stop and reoxidation of fe cn on the spe surface, at an applied potential of +260 mv, gives a current signal inversely proportional to the ldh concentration and therefore to the pltx concentration. no reduction of fe cn and pms+ occurs at this potential. after an analytical/biochemical characterization, the sensor strip was used to measure palytoxin. sheep blood and standard solutions of pltx were let to react using two different incubation times (24 h or 4 h) obtaining working ranges of 7×10-3-0.02 g/l and 0.16-1.3 g/l, respectively. the specificity of the test for palytoxin was evaluated using ouabain which acts as pltx on the na+/k+-atpase pump. a crossreactivity study, using high concentrations of other marine biotoxins was also carried out. experiments to evaluate the matrix effect and recovery on mussel samples were also carried out, and results showed that the matrix effect was dependent on the pltx concentration, and thus it is necessary to use a matrix standard calibration curve for accurate analysis of pltx in experimentally and naturally contaminated samples. this was the first time that a biomolecular method for analysis of pltx in mussels has been evaluated for matrix and recovery effects. compared with the conventional haemolytic– spectrophotometric assay, the method proposed here proved to be faster (4 h blood/pltx incubation instead of 24 h) and uses a cost effective and portable apparatus. 5. salmonella detection  the increased consumption of fresh and ready to use vegetables has recently caused several outbreaks and illness due to their contamination by pathogenic microorganisms, such as salmonella [28], [29]. in italy, the production and processing of fresh vegetables are concentrated in the campania region, and especially in the plain of sele. recently, fresh and ready to eat vegetables from this area have been the cause of repeated food alert in the ec. this problem is mainly due to cultivation practices, manipulation and transformation. since the standard culture method for detecting salmonella [30, iso 6579:2002] requires up to 5 days to produce results, the need to develop rapid methods represents an important issue for the authorities and the producers. the purpose of this study was the development and evaluation of an elime (enzyme-linkedimmuno-magnetic-electrochemical)assay [31], [32] to detect salmonella in vegetables of i and iv gamma. the proposed elime assay is based on the use of magnetic beads (mbs) as support of a sandwich immunological chain, coupled with a strip of 8-magnetized screen-printed electrodes (localized at the bottom of 8 wells). the product of the enzymatic reaction is quickly measured by chronoamperometry at an applied potential of 100 mv for 60 seconds. four different kinds of mbs anti-salmonella were tested: • dynalbeads anti-salmonella (ready to use) • pathatrix anti-salmonella (ready to use) • pathatrix same day anti-salmonella (ready to use) • pan mouse igg mbs coated with a broad reactivity mab anti-salmonella an optimized dilution of pab-hrp (1:100) has been used to complete the sandwich, and the couple tmb + h2o2 as enzyme substrate. after verifying the ability of the system based on the use of dynabeads anti-salmonella to interact with different salmonella serotypes, we focused our attention on s. napoli and s. thompson, recently isolated from vegetables grown in italy. being salmonella, and in general pathogens, a small fraction of a large population of non-target (nt) organisms, present in food, able to adhere to various surfaces including mbs, we also tested several nt bacteria. in order to have the better sensitivity and specificity towards salmonella, different pab-hrp dilutions and several blocking agents (gelatin bovine, ɩ-carrageenan, bsa, pva, dry milk) were investigated. the best results were obtained using pab-hrp=1:100 and dry milk, because of better selectivity (figure 2). among the 20 nt bacteria tested, only e. cloacae, c. freundii, e. aerogenes, e. coli, gave back current signals greater than the zero point. similar results were also obtained using pathatrix mbs. for this reason, different salmonella serotypes were also tested using pan mouse igg mbs coated with a broad reactivity mab antisalmonella (dry milk as blocking agent and pab-hrp=1:100). in figure 2, calibration curves for s. napoli and s. thompson are reported, while figure 3 shows the selectivity study using dry milk and pva as blocking agent. from the results obtained, we can state that these particles are less sensitive but more specific than those ready to use. however, experiments on i and iv gamma vegetables, experimentally contaminated with salmonella, will be performed with both types of particles to evaluate their real capability to distinguish salmonella from endogenous nts. 1e+4 1e+5 1e+6 1e+7 1e+8 0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 salmonella serotypes [cfu/ml] 105 106 107 108 109 figure 2. calibration curves for s. napoli and s. thompson.  figure 3. selectivity studies.    acta imeko | www.imeko.org  april 2016 | volume 5 | number 1 | 49  6. hav detection  hepatitis a virus (hav) causes an acute hepatitis associated with a significant morbidity and occasional mortality. outbreaks of waterborne diseases are certainly underestimated due to the lack of adequate programs for the epidemiological surveillance. current legislation for water, shellfish (ec 2073/2005 ec b53/2004) and plant (ec 2073/2005) does not provide for any limitation due to the presence of hav and other enteric viruses in the irrigation and housing waters [33]. in addition, there is no official method for the detection of these viruses. actually, the presence of viruses in water and/or foods can be just indirectly deduced by the patient's symptoms and confirmed through the search for anti-hav igm and/or anti-hav igg antibodies in patient's blood. immunoanalytical approaches are reported as methods to determine hav in drinking water before its use, thus avoiding the infectious disease. in this work, still in progress, two electrochemical, competitive and sandwich, elime assays were developed. these systems are based on the use of new polydopaminemodified magnetic nanobeads as solid support for the immunochemical chain, and screen printed electrodes as a sensing platform. this rapid and low-cost analysis method involves the use of a portable instrument, able to perform measurements directly in the field. these elime assays showed detection limits equal to 1×10-8 iu/ml and 8×10-7 iu/ml for sandwich and competitive formats, respectively. in order to investigate the applicability of the proposed immunosensors for practical analysis, the immunosensors were used to evaluate the recoveries of different concentrations of hav spiked in different tap water samples. indeed, different hav concentrations from 10-10 to 10-2 iu/ml spiked in tap water samples were analyzed. preliminary results obtained on real samples were compared with those obtained by the qrt-pcr analysis that is a routine technique used to evaluate the hav contamination levels in samples. at moment, electrochemical values do not perfectly match the pcr results, being underestimated. experiments are in progress to elucidate and overcome this problem. 7. conclusions  new screening methods for the control of food safety have been presented. the use of spes in conjunction of portable and quite inexpensive instrumentation, could allow these methods to become popular because quite simple to be carried out also by not specialized operators. examples of determination of arsenic, pesticides and toxins (pltx) at ppb levels have been illustrated, with application to real samples in order to demonstrate the feasibility and effectiveness of these determinations. where necessary, improved analytical characteristics have been obtained modifying the spes with nanostructured materials able to enhance their performances. examples of immunosensors for the detection of bacteria such as salmonella and viruses such as hepatitis a have been also reported. in this case, a large enhancement of the sensitivity of the assay has been obtained using magnetic beads of micrometer dimensions as support for the immunological chain, and arrays of magnetized spes as electrochemical transducers. all the methods presented are intended to be adopted as screening methods, and not used in place of the confirmatory ones. in such way, only samples suspected to be contaminated can be subjected to the required confirmatory analytical methods for residues in foodstuffs. [34], [35]. acknowledgement  the salmonella project was funded by the national project ricerca finalizzata 2009 (rf-2009-1538880). the hav research was supported by grant from eu marie curie fp7 irses (international research staff exchange scheme). references  [1] j.p. metters, r.o. kadara, c.e banks, new directions in screen printed electroanalytical sensors: an overview of recent developments. analyst 136 (2011) pp. 1067-1076. [2] m. li, y.t. li, d.w. li, yt long, recent developments and applications of screen-printed electrodes in environmental assays--a review. anal chim acta. 734 (2012) pp. 31-44. [3] o.d. renedo, m.a. alonso-lomillo, m.j. martínez, recent developments in the field of screen-printed electrodes and their related applications. talanta 73 (2007) pp. 202-219. [4] b.k. mandal, k.t. suzuki, arsenic round the world: a review, talanta 58 (2002) pp. 201-235. [5] s. v. flanagan, r. b. johnston, y. zheng, arsenic in tube well water in bangladesh: health and economic impacts and implications for arsenic mitigation, bull. who 90 (2012) pp. 839-846. [6] d. q. hung, o. nekrassova, r. g. compton analytical methods for inorganic arsenic in water: a review talanta 64 (2004) 269–277. [7] s. cinti, s. politi, d. moscone, g. palleschi and f. arduini, stripping analysis of as(iii) by means of screen-printed electrodes modified with gold nanoparticles and carbon black nanocomposite, 26, (2014) pp. 931–939. [8] k. saha, s. s. agasti, c. kim, x. li, v. m. rotello, gold nanoparticles in chemical and biological sensing, chem.rev., 112 (2012) pp 2739–2779. [9] d. a. schultz, plasmon resonant particles for biological detection, curr.opin. biotech., 14 (2003) pp. 13-22. [10] f. arduini, f. di nardo, a. amine, l. micheli, g. palleschi, d. moscone, carbon black-modified screen-printed electrodes as electroanalytical tools, electroanalysis, 24 (2012) pp. 743-571. [11] f. arduini, c. majorani, a. amine, d. moscone, g. palleschi, hg2+ detection by measuring thiol groups with a highly sensitive screen-printed electrode modified with a nanostructured carbon black film, electrochim. acta 56, (2011) pp. 4209-4215. [12] j.l. sussman, m. harel, f.frolow, c. oefner, a. goldman, l. toker, i. silman, atomic structure of acetylcholinesterase from torpedo californica: a prototypic acetylcholine-binding protein. science 253 (1991) pp. 872–879. [13] j. bajgar, j. fusek, k. kuca, l.bartosova, d. jun, treatment of organophosphate intoxication using cholinesterase reactivators: facts and fiction. mini rev. med. chem. 7 (2007) pp. 461–466. [14] c. bergh, r. torgrip, c. ostman, simultaneous selective detection of organophosphate and phthalate esters using gas chromatography with positive ion chemical ionization tandem mass spectrometry and its application to indoor air and dust. rapid commun. mass spectrom. 24 (2010) pp. 2859–2867. [15] f. de freitas ventura, j. jr. de oliveira, w. dos reis pedreira filho, m. gerardo ribeiro, gc-ms quantification of organophosphorous pesticides extracted from xad-2 sorbent tube and foam patch matrices. anal. methods, 4 (2012) pp. 3666–3673.   acta imeko | www.imeko.org  april 2016 | volume 5 | number 1 | 50  [16] f. arduini, a. amine, d. moscone, g. palleschi, biosensors based on cholinesterase inhibition for insecticides, nerve agents and aflatoxin b1 detection. microchim. acta, 170 (2010) pp. 193–214. [17] s.andreescu, j.l. marty, twenty years research in cholinesterase biosensors: from basic research to practical applications. biomol. eng. 23 (2006) pp. 1–15. [18] m. pohanka, k. musilek, k. kuca, progress of biosensors based on cholinesterase inhibition. curr. med. chem., 16, (2009) pp. 1790–1798. [19] a.p. periasamy, y. umasankar, s.m. chen, nanomaterials— acetylcholinesterase enzyme matrices for organophosphorus pesticides electrochemical sensors: a review. sensors 9 (2009) pp. 4034–4055. [20] w. zhang, a.m. asiri, d. liu, d. du, y. lin. nanomaterialbased biosensors for environmental and biological monitoring of organophosphorus pesticides and nerve agents. trends anal. chem. 54 (2014) pp. 1–10. [21] f. arduini, d. neag, v. scognamiglio, s. patarino, d. moscone, g. palleschi, automatable flow system for paraoxon detection with an embedded screen-printed electrode tailored with butyrylcholinesterase and prussian blue nanoparticles chemosensors, 3 (2015), pp. 129-145. [22] s. cinti, f. arduini, g. vellucci, i. cacciottti, f. nanni, d. moscone, carbon black assisted tailoring of prussian blue nanoparticles to tune sensitivity and detection limit towards h2o2 by using screen-printed electrode. electrochem. commun., 47 (2014) pp. 63–66. [23] m. usami, m. satake, s. ishida, a. inoue, y. kan, t. yasumoto palytoxin analogs from the dinoflagellates ostreopsis siamensis. j am chem soc 117 (1995) pp. 5389–5390. [24] gazzetta ufficiale della repubblica italiana n 165, 16 may (2002) pp 16–19. [25] crlmb eu harmonised standard operating procedure for detection of lipophilic toxins by mouse biomassay. version 5, june (2009). [26] efsa scientific opinion on marine biotoxins in shellfishpalytoxin group. efsa j 7(12) (2009) pp. 1393-1631. [27] g. volpe, l. cozzi, d. migliorelli, l. croci, g. palleschi development of a haemolytic–enzymatic assay with mediated amperometric detection for palytoxin analysis: application to mussels. anal. bioanal. chem. 406 (2014) pp. 2399–2410. [28] m. oliveira, m. abadias, p. colás-medà, j. usall, i. viñas biopreservative methods to control the growth of foodborne pathogens on fresh-cut lettuce, international journal of food microbiology 214 (2015) pp. 4–11. [29] i.h. igbinosa, h. isoken, biofilm formation of salmonella species isolated from fresh cabbage and spinach. journal of applied sciences and environmental management, 19 (2015) pp. 45-50. [30] international standard organization, iso 6579:2002/amd.1:2007 microbiology of food and animal feeding stuffs – horizontal method for the detection of salmonella spp. genève, 2007. 9p. [31] l. croci, e. delibato, g. volpe, d. de medici, g. palleschi, comparison of pcr, electrochemical enzyme-linked immunosorbent assays, and the standard culture method for detecting salmonella in meat products. appl. environ. microbiol. 70 (2004) pp. 1393-1396. [32] e. delibato, g. volpe, d. romanazzo, d. de medici, l. toti, d. moscone, g. palleschi, development and application of an electrochemical plate coupled with immunomagnetic beads (elime) array for salmonella enterica detection in meat samples. j. agric. food chem., 57 (2009) pp 7200–7204. [33] m. koopmans, e. duizer foodborne viruses: an emerging problem, international journal of food microbiology, 90 (2004) pp. 23–41. [34] 2002/657/ec: commission decision of 12 august 2002 implementing council directive 96/23/ec concerning the performance of analytical methods and the interpretation of results (text with eea relevance) (notified under document number c(2002) 3044) official journal l 221 , 17/08/2002 p. 0008 – 0036. [35] sanco/12571/2013 guidance document on analytical quality control and validation procedures for pesticide residues analysis in food and feed. supersedes sanco/12495/2011 implemented by 01/01/2014. acta imeko  issn: 2221‐870x  june 2015, volume 4, number 2, 32‐38    acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 32  direct calibration chain for hand torque screwdrivers from  the national torque standard  koji ogushi, atsuhiro nishino, koji maeda and kazunaga ueda  national metrology institute of japan, aist, tsukuba central 3, 1‐1‐1 umezono, tsukuba, ibaraki, 305‐8563, japan      section: research paper   keywords: torque; standard; screwdriver; calibration  citation: koji ogushi, atsuhiro nishino, koji maeda and kazunaga ueda, direct calibration chain for hand torque screwdrivers from the national torque  standard, acta imeko, vol. 4, no. 2, article 6, june 2015, identifier: imeko‐acta‐04 (2015)‐02‐06  editor: paolo carbone, university of perugia, italy  received october 10, 2014; in final form january 30, 2015; published june 2015  copyright: © 2015 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  corresponding author: koji ogushi, e‐mail: kji.ogushi@aist.go.jp    1. introduction  hand torque screwdrivers (htds) are necessary for the assembly of medical, electrical, and precision devices in the production control process. there has been some progress in the development of calibration chains for hand torque wrenches (htws) in the last two decades [1]–[4]. however, the developmental progress of calibration chains for htds has been less reported. the authors have presented a comparison of the results for torque screwdriver testers (tdts) by using a reference torque screwdriver (rtd) and a conventional weights-and-bar-system (wbs) and have shown some of the advantages of a calibration chain including an rtd [5]. parasitic transverse forces and bending moments necessarily occur superposing on pure torque during calibration using wbs. figure 1 shows the calibration chain for htds with uncertainties (approximate) of calibrations and maximum permissible errors of tests for devices which are subject to. the national metrology institute of japan (nmij) has proposed this direct torque traceability system for hand torque screwdrivers, cooperating with japanese industry. first, a htd should be tested by using torque screwdriver checker (tdc; torque screwdriver testing equipment without loading system) or tdt (torque screwdriver testing equipment with loading system) with the maximum permissible deviation of four percent or six percent according to iso 6789 [6], which is a standard documentation for requirements and test methods of hand torque tools. second, the tdc or the tdt should be calibrated by using the rtd within the uncertainty of one percent. next, the rtd should be calibrated in an jcss (japan calibration service system) accredited laboratory within the uncertainty of approximately 0.2 % to 0.5 %. the accredited laboratory has a torque calibration machine (tcm) for torque measuring devices (pure torque loading). the tcm should have a calibration and measurement capability of approximately 0.04 % to 0.2 %. a high-precision tmd is used for calibration and control of tcm in the accredited laboratory. finally, this high-precision tmd should be calibrated by nmij within uncertainty of approximately 0.01 % to 0.04 %. this paper describes the establishment of a complete calibration chain from the national torque standard to htds, where the calibration and testing methods for the rtd using a national torque standard machine (tsm), for the tdt using the rtd, and for htds using the tdt are also investigated. it is shown that the traceability of the test results for htds is experimentally confirmed by this chain. the torque range used for htds is generally small, e.g., from several centi-newton meters to several newton meters. national torque standards at such a small range had not been abstract  hand torque screwdrivers are used for the fastening control of screws in almost all precise mechanical and electrical parts. this paper  describes the realization of a direct calibration chain for hand torque screwdrivers traceable to the national torque standard. the  calibration methods for a reference torque screwdriver using a primary torque standard machine, a torque screwdriver tester using  the  reference  torque  screwdriver,  and  hand  torque  screwdrivers  using  the  torque  screwdriver  tester  are  described  respectively.  uncertainty evaluation methods for each calibration level are also explained. the effectiveness of the calibration chain for hand torque  screwdrivers could be demonstrated.  acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 33  established in many countries, so the development of a calibration chain for htds has not progressed so much until recently. at nmij, a new deadweight torque standard machine (10-n·m-dwtsm) that is rated for 10 cn·m to 1000 cn·m has been completed and calibration service in this range has begun to the industry [7]. this development has accelerated the establishment of a calibration chain for htds. one user in the medical field requested a digital htd with the widest measuring range possible (for example, from 10 cn·m to 500 cn·m). the normative lower limit of an htd is 20 % of its nominal torque value according to iso 6789: 2003 [6]. this paper also focuses on the problem where the htd is used in the special range beyond the lower limit of 20 % in conjunction with the limit of resolution in the reference device (tdt). 2. experimental conditions  experiments on the calibration chain were conducted as follows: · experiment 1: calibration of the rtd using the tsm as the reference standard. · experiment 2: calibration of the tdt using the rtd as the reference standard. · experiment 3: testing verification of the htd using the tdt as the standard. these experimental conditions are described in the following subsections. 2.1. experiment 1  a newly developed deadweight-type tsm with a rated capacity of 10 n·m was used (10-n·m-dwtsm) [7]. a picture of the machine is shown in figure 2. the calibration range is from 10 cn·m to 1000 cn·m. a torque transducer with a torque screwdriver shape (tp-6n-0707, manufactured by showa measuring instruments inc., cooperating with nmij) was calibrated with a combination of an amplifier/indicator (mgcplus, amplifier type: ml38, manufactured by hbm gmbh) as the rtd. the rated capacity is 600 cn·m, and the observed resolution of the rtd r was 9.4 n·m (referring the definition of resolution in section 3.1). a picture of the tp-6n0707 is shown in figure 3. because of the limited calibration steps realized by the 10n·m-dwtsm, calibration was performed in two ranges: from 10 cn·m to 100 cn·m and from 50 cn·m to 600 cn·m. there were eight calibration steps, as follows: · 10 cn·m, 20 cn·m, 30 cn·m, 40 cn·m, 50 cn·m, 60 cn·m, 80 cn·m, and 100 cn·m, · 50 cn·m, 100 cn·m, 150 cn·m, 200 cn·m, 300 cn·m, 400 cn·m, 500 cn·m, and 600 cn·m. according to jmif015 [8], which is the industrial guideline for the calibration method of tmds issued by japan measuring instruments federation (jmif), at first, two calibration cycles with increasing and decreasing loading steps were conducted after three instances of pre-loading up to the maximum torque at the 0° mounting position. after changing the mounting position, one calibration cycle of increasing and decreasing loading steps was also conducted after one instance of preloading at the 120° and 240° mounting positions. the clockwise (cw) and counterclockwise (ccw) torques were calibrated, respectively. figure 4 shows the loading timetable for experiment 1. the room temperature was 19.6 °c to 20.5 °c, figure  1.  torque  traceability  system  for  htds  proposed  by  nmij (percentages  show  the  maximum  permissible  errors  or  approximate demanded uncertainty).  figure 2. torque standard machine (10‐n∙m‐dwtsm) for calibration of the  reference torque screwdriver (rtd).    figure  3.  torque  transducer  with  torque  screwdriver  shape,  which  was  manufactured by showa measuring instrument, cooperating with nmij.  acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 34  the relative humidity was 36 % to 37 % and the atmospheric pressure was 1013 hpa to 1016 hpa during the calibration. 2.2. experiment 2  the rtd (tp-6n-0707 + mgcplus: ml38) was used as the reference standard. the calibration range was from 10 cn·m to 600 cn·m. a tdt (tdt-600cn, manufactured by tohnichi mfg ltd.) was calibrated by installing the rtd on it (like a htd). a picture of the tdt-600cn with the installed tp-6n0707 is shown in figure 5. the calibration range specified by the manufacturer is from 20 cn·m to 600 cn·m, but the authors investigated the calibration for one range from 10 cn·m to 600 cn·m in this experiment (the lower limit was extended). the resolution of the tdt r was 0.2 cn·m. eight calibration steps were conducted as follows: · 10 cn·m, 20 cn·m, 50 cn·m, 100 cn·m, 200 cn·m, 300 cn·m, 400 cn·m, and 600 cn·m. according to jmif019 [9], which is another industrial guideline for the calibration method of torque wrench testers and torque testing machines issued by jmif, two calibration cycles with increasing loading steps were conducted after three instances of pre-loading up to the maximum torque at the 0° mounting position. after changing the mounting position of the rtd, one calibration cycle of increasing steps was also conducted after one instance of pre-loading at the 120° and 240° mounting positions. cw and ccw torques were calibrated, respectively. figure 6 shows the loading timetable for experiment 2. the room temperature was 20.5 °c to 21.1 °c, the relative humidity was 35 % and the atmospheric pressure was 1001 hpa to 1020 hpa during the calibration. 2.3. experiment 3  the tdt (tdt-600cn) was used as the reference standard. the calibrated range of the tdt was from 10 cn·m to 600 cn·m. the ftd-400cn (indicating type, the resolution: 1 cn·m) and rtd-500cn (setting type, the smallest graduation: 5 cn·m) htds (both manufactured by tohnichi mfg ltd.) were tested by installing them on the tdt. figure 7 shows pictures of the ftd-400cn and rtd-500cn installed on the tdt. the tested ranges were from 80 cn·m to 400 cn·m for the ftd-400cn and from 100 cn·m to 500 cn·m for the rtd 500cn.   figure  6.  loading  timetable  for  the  calibration  of  the  torque  screwdriver  tester (tdt).  figure  4.  loading  timetable  for  the  calibration  of  the  reference  torque screwdriver (rtd).  figure  5.  torque  screwdriver  tester  (tdt)  with  the  reference  torque screwdriver (rtd).  (a) indicating type                                      (b) setting type  figure  7.  hand  torque  screwdrivers  (htds)  installed  on  the  torque  screwdriver tester (tdt).  30 s 0o mes. 1st cycle, preloadings, 2nd cycle, 30 s max. zero 30 s 30 s 120o pre., mes., 240o pre., mes., 30 s 0o mes. 1st cycle, preloadings, 2nd cycle, 30 s max. zero 30 s 30 s 120o pre., mes., 240o pre., mes., acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 35  according to iso 6789 [6], three loading steps of 20 %, 60 %, and 100 % for maximum torque were conducted successively five times after pre-loading at the maximum torque five times. the cw and ccw torques were tested for the ftd400cn, and only the cw torque was tested for the rtd500cn. figure 8 shows the loading timetable for experiment 3. the room temperature was 20.3 °c to 20.5 °c, the relative humidity was 33 % to 34 % and the atmospheric pressure was 1023 hpa during the testing. 3. results and discussions  3.1. experiment 1  the uncertainty was evaluated according to jcg209s11 [10], which is a “guideline for the uncertainty evaluation of calibrations for torque meters and reference torque wrenches” issued by the international accreditation japan at the national institute of testing and evaluation (iajapan/nite: the accreditation body for calibration and testing laboratories in japan). the guideline was made mainly by the authors cooperating with japanese industry, based on the research of calibration methods of tmds [11][12]. detailed explanation of the evaluation method is described as follows. calibration results the measurement values at increasing torque s’ and decreasing torque s” are defined as the differences between the indicated values at the torque steps and the indicated value at the non-loading condition before starting the cycle. the calibration results 's and "s are calculated according to eqs. (1) as the means of the measurement values obtained from different mounting positions. the results measured during the second cycles are not included in the calculations of 's and "s :    rot 1e i1e rot i 1 ' n s n s (1a)    rot 1e i1e rot i " 1 " n s n s (1b) where i, j(=1) and e are the indexes of calibration steps, cycles and different mounting position series, and nrot indicates the total number of mounting positions. reproducibility with different mounting positions the relative reproducibility with different mounting positions bi is calculated according to eq. (2) as the experimental standard deviation of the measurement values during the first cycle of each mounting position:      rot 1e 2 ii1e roti i )'( )1( 1 ' 1 n ss ns b (2) the relative standard uncertainty wrot_rtd,i is calculated according to eq. (3) as the experimental standard deviation of the mean: 2 i rot 2 irot_rtd, 1 b n w  (3) the results measured during the second cycles are not included in the calculation of bi. repeatability with an unchanged mounting position the relative repeatability with an unchanged mounting position b’i is calculated according to eq. (4) when the cycle is repeated twice for only 0° series:     rep 1j ij1 rep i11i21 i ' 1 n s n ss b (4) where the nrep is the number of cycles for the same series, and s’ij1 indicates the measurement value at step i, cycle j and for the first series (e = 1). the relative standard uncertainty wrep_rtd,i is calculated according to eq. (5), by determining b’i as a half-width of the rectangular distribution: 2 i 2 irep_rtd, 3 1 bw  (5) deviation due to interpolation an interpolation equation for the increasing torque is determined via the least squares method using the calibration results i's (i = 1 to n). the interpolation equation relates the measurement value s’ and the torque t as follows: m m 2 210' tatataas  (6a) whether or not the interpolation equation includes a constant term depends on the manual of the calibration laboratory or the specifications from the client although the constant term was included in this experiment. if so be required, the interpolation equation for the decreasing torque may be determined as follows: m m 2 210 ''''" tatataas  (6b) the interpolation equation of the decreasing torque, eq. (6b), must be determined as the calculated values at the maximum torque tmax coincides with the value calculated by the interpolation equation of increasing torque, eq. (6a). the authors also calculated the interpolation equation of the decreasing torque in this experiment. however, the rtd was not subsequently used for the calibration of the tdt in decreasing torque steps. the relative deviation due to the interpolation fa,i is calculated according to eq. (7) as the difference between the calculated   figure  8.  loading  timetable  for  the  testing  verification  of  hand  torque screwdrivers (htds).  20 % loadingspreloadings, max. zero 60 % loadings 100 % loadings acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 36  value obtained by the interpolation equation s’a,i and the calibration result 's : ia, ia,i ia, ' '' s ss f   (7) the relative standard uncertainty associated with interpolation wint_rtd,i is calculated according to eq. (8), by determining fa,i as a half-width of the rectangular distribution: 2 ia, 2 iint_rtd, 3 1 fw  (8) the degree (1st, 2nd or 3rd) of the interpolation equation to be used is recommended to meet the following condition: n < 5; 1st degree, n < 8; 2nd degree, n ≥ 8; 3rd degree. then, the interpolation of 3rd degree was used for this experiment. if the rtd directly indicates values in the unit of torque and the measurement values cannot be electronically fitted to the interpolation curve of the calibration result, then rather than wint_rtd, the relative standard uncertainty of the deviation due to indication wind_rtd should be used. a detailed explanation, however, is not presented herein. zero error the relative zero error f0,e is calculated according to eq. (9) as the absolute value of deviation between the indicated values at the non-loading conditions before starting and after finishing the cycle. the calculation of f0,e does not include values obtained in calibration cycles with only increasing torque: n1e 01e01e e0, ' '" s ss f   (9) where s’01e and s”01e are the zero values indicated before starting and after finishing the cycle for the first cycle and series e. s’n1e shows the measurement value at the maximum torque in series e. the relative standard uncertainty due to zero drift wzer_rtd,i is calculated according to eq. (10), by determining the maximum value of f0,e as a half-width of the rectangular distribution: 2 max,0 2 zer_rtd 3 1 fw  (10) hysteresis the relative reversibility error (hysteresis) hi is calculated according to eq. (12) as the mean of the absolute differences between measurement values at increasing and decreasing torque steps in the same cycle: i 1e i1ei1e rot i ' 1 rot s ss n h n     (11) where s”i1e is the measurement value of the decreasing torque at step i at the first cycle for series e. the values measured in the cycle including only increasing torque are not included in the calculation of hi. the relative standard uncertainty due to reversibility wrev_rtd,i is calculated according to eq. (12), by determining hi as a halfwidth of the rectangular distribution: 2 i 2 irev_rtd, 3 1 hw  (12) the relative reversibility error, however, was not considered in this experiment as mentioned later. resolution the resolution r of the indicator is defined as the smallest fraction of a scale division that is readable in the case of analogue scale. in the case of digital scale, r is defined to be one increment of the last active number of the numerical indicator, provided that the indication does not fluctuate when the rtd is under the non-loading condition. if the indication fluctuates under the non-loading condition by more than one increment, the resolution is equal to a half-width of the fluctuation (here, the fluctuation is two counts when the last active number is fluctuated from 0 to 1, and three counts when it is fluctuated from 0 to 2, for example). r must be converted to and stated in units of torque as expressed in section 2.1. the relative standard uncertainty due to resolution wres_rtd,i is calculated according to eqs. (13), by determining r as a halfwidth or a whole-width of the rectangular distribution depending on the fluctuation: 2 i 2 ires_rtd, 23 2        t r w (non-fluctuation) (13a) 2 i 2 ires_rtd, 3 2        t r w (fluctuation) (13b) where ti is the torque at each torque step. wres_rtd,i is multiplied by 2 considering that the measurement value is obtained as the difference between the indicated values of the loaded torque step and non-loading zero step before starting the cycle (double readings). uncertainty of the calibration result the relative expanded uncertainty wrtd_cal of calibration for the rtd is calculated from the relative combined standard uncertainty wtsm of torque realized by the tsm and the relative combined standard uncertainty wc_rtd ascribable to the measurement of the rtd, according to the following equation: 2 c_rtd 2 tsmic_rtd_cal,irtd_cal, wwkwkw  (14) where the coverage factor k is equal to two and its interval was estimated to have a level of confidence of approximately 95 %. the authors also evaluated the effective degree of freedom and confirmed sufficient large number. wtsm has been evaluated as 3.3 × 10-5 [7]. wc_rtd is calculated as: 2 iint_rtd, 2 irep_rtd, 2 irot_rtd,ic_rtd, wwww  2 ires_rtd, 2 irev_rtd, 2 izer_rtd, www  (15a) in the case of the evaluation of only increasing torque, wc_rtd is calculated from: 2 iint_rtd, 2 irep_rtd, 2 irot_rtd,ic_rtd, wwww  2 ires_rtd, 2 izer_rtd, ww  (15b) eq. (15b) was used for the calculation in this experiment. the results of the uncertainty evaluation are shown in figure 9. the maximum relative expanded uncertainty for each calibration range was as follows: 10 cn·m – 100 cn·m: cw 0.024 %, ccw 0.031 %, 50 cn·m – 600 cn·m: cw 0.021 %, ccw 0.020 %. acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 37  sufficient small uncertainties could be obtained because the rtd was directly calibrated by using the tsm. the relative uncertainty must be larger than the level of 0.02 % when the rtd is calibrated by using the tcm of a calibration laboratory as shown in figure 1. the interpolation equations obtained in the experiment 1 were used for the successive calibration of the tdt described in section 2.2. 3.2. experiment 2 the uncertainty was evaluated according to jcg209s21 [13], which is an another guideline for the uncertainty evaluation of calibrations for torque wrench testers and torque testing machines issued by iajapan/nite. the guideline was also made mainly by the authors cooperating with japanese industry. detailed explanation of the evaluation method is described (but only increasing torque) as follows. uncertainty of the calibration result first, the relative expanded uncertainty of the calibration is expressed by the following equation: 2 iint_tdt, 2 irep_tdt, 2 irot_tdt,ical_tdt, wwwkw  2 rtd 2 ires_tdt, 2 izer_tdt, www  (16) calibration results 's , the uncertainty contribution for the reproducibility with the change in the mounting position wrot_tdt, that for the repeatability without the change in the mounting position wrep_tdt, that due to the interpolation wint_tdt, that due to the resolution wres_tdt and the the coverage factor k are evaluated in the same way as described in section 3.1. zero error the loading timetable was only increasing torque in this calibration. but the relative zero error f0,e and the relative standard uncertainty wzer_tdt is calculated according to eq. (9) and (10) as well. in this case, the uncertainty tends to be larger than the zero error for the cycle of increasing and decreasing torque because unloading from the maximum torque suddenly occurs in the only increasing torque cycle. this would be controversy in the future. however, fortunately, the tdt showed null values of the zero error in all cycles in this experiment because of low resolution in the tdt. uncertainty of rtd as a reference standard the relative combined uncertainty wrtd when the rtd is used as a reference standard is calculated from the relative combined standard uncertainty wc_rtd_cal calculated by eq. (14), the relative standard uncertainty due to temperature variation wrtd_tmp, and the relative standard uncertainty of long term stability of the rtd wrtd_lgstb, according to the following equation: 2 rtd_lgstb 2 rtd_tmp 2 c_rtd_calrtd wwww  (17) wrtd_tmp is calculated by the following equation: 2 meas 2 2 rtd_tmp 23         t w  (18) where  is temperature coefficient of output sensitivity of the rtd and tmeas is temperature variation. the temperature correction of output value of rtd is available if necessary. this term was small in this experiment because the temperature coefficient is -1.4 × 10-4 k-1 and the temperature variation was 1.5 k at the maximum. wrtd_tmp was 6.1 × 10-5. wrtd_lgstb is calculated by the following equation:      cal 1c 2 c_meanc calc_mean rtd_lgstb )( )1( 11 n ss ns w , (19) provided the rtd has been calibrated more than three times (ncal ≥ 3) in the pre-determined calibration period (generally 26 months). here, cs is a calibration result of each time and c_means the mean value of calibration results for all time. it would be acceptable empirically determine wrtd_lgstb if the calibration time is less than three. the authors used empirically 0.02 % for wrtd_lgstb. the result of the uncertainty evaluation of calibration for the tdt is shown in figure 10. the maximum relative expanded uncertainties for the various lower limits of the calibration were as follows: 10 cn·m – 600 cn·m: cw 6.6 %, ccw 2.9 %, 20 cn·m – 600 cn·m: cw 2.6 %, ccw 2.0 %, 50 cn·m – 600 cn·m: cw 1.1 %, ccw 0.62 %, 100 cn·m – 600 cn·m: cw 0.75 %, ccw 0.24 %. in this evaluation, the uncertainty contribution of the interpolation (wint_tdt) was taken into consideration instead of the indication error (wind_tdt); i.e., the authors converted the indicated values into the reference values by using the interpolation equations when the tdt was used for the testing of the htds described in section 2.3 although the tdt has a digital indication display (by the direct torque unit expression). figure 9. uncertainty evaluation results of the calibration for the rtd  figure 10. uncertainty evaluation results of the calibration for the tdt..  r e la ti ve e xp a n d e d u n ce rt a in ty ( k = 2 ) / % . 0.00 0.01 0.02 0.03 0.04 -600 -400 -200 0 200 400 600 torque / cn m larger range, cw larger range, ccw smaller range, cw smaller range, ccw r e la tiv e e xp a n d e d u n ce rt a in ty (k = 2 ) / % . 0 1 2 3 4 5 6 7 -600 -400 -200 0 200 400 600 torque / cn m cw ccw maximum permissible error acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 38  it was found that the uncertainty exceeded 1 %, even if the interpolation equations were used when including a lower limit of 10, 20, or 50 cn·m. both the users and manufacturers of tdts should pay attention to the choice of the lower limit and consider the uncertainty. in addition, the authors insist that the normative description of “the maximum permissible uncertainty of the torque tool calibration devices shall be less than 1 %.” in iso 6789 should be clarified; i.e., the method of uncertainty evaluation for the tdt calibration should be defined and harmonized. 3.3. experiment 3 figure 11 shows test results for two different htds. in all of the five successive measurements at each step, the relative deviations from the reference standard (tdt) were within 6 % of the maximum permissible deviation prescribed in iso 6789 for both the ftd-400cn and rtd-500cn. these htds were recognized to conform with iso 6789 because the maximum uncertainty of the tdt was within 1 % in the tested ranges from 80 cn·m to 500 cn·m. however, if the tdt is used, including the lower range from 10 cn·m to 50 cn·m as described in section 1, the testing results must be out of conformity with iso 6789. the uncertainty was not evaluated for this test results. after discussion with manufacturers and users of htds in japan, it was found that the uncertainty of measurement for the results of testing htds is not yet required. on the other hand, the challenge for calculation of the uncertainty has started [4]. the authors will continue to discuss the problem with related organizations. 4. conclusions  the authors experimentally realized a complete calibration chain from the national torque standard to the hand torque screwdriver (htd), where the appropriate calibration methods and uncertainty evaluations were applied. when torque screwdriver testers (tdts) are used as reference standards for the testing of htds, testing organizations should pay attention to the fact that the calibration uncertainties of the tdts must be within the maximum permissible uncertainty prescribed in iso 6789 for the entire testing range of the htds. here, as shown in figure 1, it would be acceptable even if the first-grade accredited laboratories will calibrate rtds with an uncertainty of approximately 0.2 % for calibrators or users of tdts instead of the values presented in section 3.1 (approx. 0.02 % – 0.03 %). the authors, therefore, insist that it would be sufficiently possible for the japanese industry to establish the hierarchy proposed in figure 1. references  [1] p. d. hohmann, “advantages of traceability by using torque transfer standard tts,” proc. of xiii imeko world congress, sept. 1994, torino, italy, pp. 253-256. [2] a. brüge, d. röske, d. mauersberger and k. adolf, “influence of cross forces and bending moments on reference torque sensors for torque wrench calibration,” proc. xix imeko world congress, sept. 2009, lisbon, portugal, pp.356-361. [3] k. ogushi, a. nishino, k. maeda and k. ueda, “calibration of a torque wrench tester using a reference torque wrench,” sice annual conference 2011, sept, 2011, tokyo, japan, pp. 411-416. [4] d. röske, “iso 6789:2003 calibration results of hand torque tools with measurement uncertainty – some proposals,” proc. xx imeko world congress, sept, 2012, busan, republic of korea,usb flash drive. [5] k. ogushi, a. nishino, k. maeda and k. ueda, “advantages of the calibration chain for hand torque screwdrivers traceable to the national torque standard,” sice annual conference 2012, akita, japan, sept. 2012, usb flash drive. [6] jis b 4652 (iso 6789: 2003), “hand torque tools requirements and test methods,” japanese standards association, 2008 (in japanese). [7] a. nishino, k. ogushi and k. ueda, “uncertainty evaluation of a 10 n·m dead weight torque standard machine and comparison with an 1 kn·m dead weight torque standard machine,” measurement, 49 (2014), pp. 77–90. [8] jmif015, “guideline for calibration laboratories of torque measuring devices”, japan measuring instruments federation, 2004 (in japanese). [9] jmif019, “guideline for calibration laboratories of torque testing machines and/or torque wrench tester,” japan measuring instruments federation, 2007 (in japanese). [10] jcg209s11-05, “jcss guideline on uncertainty estimation – torque meters and reference torque wrenches,” international accreditation japan, national institute of testing and evaluation, 2012 (in japanese). [11] k. ohgushi, t. ota and k. ueda, “experimental study on the uncertainty in static calibration of a torque measuring device – 1st report: verification of repeatability, reproducibility and zero error-,” trans of the sice, 39-7 (2003), pp. 624-630 (in japanese). [12] k. ohgushi, t. ota and k. ueda, “experimental study on the uncertainty in static calibration of a torque measuring device – 2nd report: verification of interpolation error, reversibility and resolution(in japanese),” trans of the sice, 39-7 (2003), pp. 631-636 (in japanese). [13] jcg209s21-02, “jcss guideline on uncertainty estimation – torque wrench testers and torque testing machines,” international accreditation japan, national institute of testing and evaluation, 2012 (in japanese). figure 11. testing results for htds.  -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 -500 -400 -300 -200 -100 0 100 200 300 400 500r e la ti ve d e vi a ti o n s o f te st in g v a lu e s fr o m r e fe re n ce v a lu e s / % torque / cn m ftd-400cn(cw) maximum permissible errorftd-400cn(ccw) rtd-500cn(cw) . microsoft word 377-2372-1-le-rev-prova acta imeko  issn: 2221‐870x  november 2016, volume 5, number 3, 3‐8      acta imeko | www.imeko.org  november 2016 | volume 5 | number 3 | 3  the probability in throwing dices and in measurement   franco pavese  torino, italy      section: research paper   keywords: probability; dices throwing; measurement; uncertainty; aleatory; epistemological; ontological; definitional  citation: franco pavese, the probability in throwing dices and in measurement, acta imeko, vol. 5, no. 3, article 2, november 2016, identifier: imeko‐acta‐ 05 (2016)‐03‐02  section editor: paolo carbone, university of perugia, italy  received april 6, 2016; in final form april 6, 2016; published november 2016  copyright: © 2016 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  corresponding author: franco pavese, e‐mail: frpavese@gmail.com    1. introduction  “it is unanimously agreed that statistics depends somehow on probability. but, as to what probability is and how it is connected with statistics, there has seldom been such complete disagreement and breakdown of communication since the tower of babel. doubtless, much of the disagreement is merely terminological and would disappear under sufficiently sharp analysis.”, [1]. this paper, after a short introduction concerning the several meanings of the terms ‘uncertainty’ and ‘probability’, aims at limiting the illustration to a couple of interpretations of probability, discussing whether application of this concept to dice throwing has exactly the same meaning when it is used for measurement results, or there are differences. the concept of probability is historically born from the speculation about the prediction in gambling problems, like that of the occurrence of a specific face of a fair dice in subsequent throws or of a fair coin in subsequent tossing. in experimental science, when scientists realised that it is impossible to get ideally perfect and full information from measurement [2], the use of the concept of probability proposed itself as the most natural way to circumvent the difficulty and to model chance. the paper illustrates why and how, in general, the conditions of its application are basically different, by reviewing a range of positions. some consequences are drawn.   2. uncertainty interpretations  the term ‘uncertainty’ has several meanings.1 one can find a discussion on this issue from an epistemological point of view in [6]: “uncertainty is pervasive in most of the fields of science and technology (as it is also in real life and ordinary thinking), and in all cases it seems that what really matters and worries is the quantification of the ‘amount’ of uncertainty in some sense attributable to the considered statements, or events, be them possibly submitted to repetition (random events) or not (singular events). all that is done, in general, under a very unclear concept of what the word uncertainty could actually mean, but it is almost always done by aiming at the measurement of the variables affected, in very different contexts, by some kind of uncertainty.” according to that approach, “the linguistic term uncertainty is not only imprecise, but has a very broad applicative spectrum in both ordinary life and science. to scientifically approach what it could mean, it seems necessary to previously capture how their, also imprecise, mother-predicate u = uncertain, the opposite of certain, is used in different contexts and with 1 often the terms "uncertainty" and "error" are used interchangeably, or one of the two is rejected (the second in 14). "we find it convenient to distinguish them thus: ’error’ is the actual difference between a measurement and the value of the quantity it is intended to measure, and is generally unknown at the time of measurement. ’uncertainty’ is a scientist’s assessment of the probable magnitude of that error". 22 abstract  the paper summarises the main differences between the process of throwing dices and the measurement process, and draws some of  the consequences on the meaning and use of the probability concept in the two cases.       acta imeko | www.imeko.org  november 2016 | volume 5 | number 3 | 4  different purposes, and where can consequently be represented by a fuzzy set”. limiting to the scientific field, it actually did not only bring to the concept of ‘probability’, but also to the many other fields of chance, often called “imprecise probability”, first of all to the possibility and fuzzy type of reasoning [7], [8], but also to others, like previsions, lower and upper probabilities, or interval probabilities, belief functions, possibility and necessity measures, lower and upper previsions, comparative probability orderings, partial preference orderings, sets of desirable gambles, p-boxes, robust bayes methods (see references, e.g., in [9]). in particular, uncertainty measures are not necessarily additive [6], [10]-[12]. in measurement, that attribute is added to uncertainty to specify the intended meaning: “measurement uncertainty: nonnegative parameter characterizing the dispersion of the quantity values being attributed to a measurand, based on the information used” (term 2.26 in [13]). it is useful to recall here also its note 1: “measurement uncertainty includes components arising from systematic effects, such as components associated with corrections and the assigned quantity values of measurement standards, as well as the definitional uncertainty. sometimes estimated systematic effects are not corrected for but, instead, associated measurement uncertainty components are incorporated” (emphases added), useful in the subsequent discussion. 3. probability interpretations  probability is one of the concepts born to accommodate the concept of uncertainty, intrinsic in chance, as opposed to the term ‘certainty’ [2]. there are two broad categories of probability interpretations, which can be called "physical" and "evidential" probabilities (the following synthesis is taken from [3], for a full discussion see [4], [5]). physical probabilities, which are also called objective or frequency probabilities, are associated with random physical systems such as roulette wheels, rolling dice and radioactive atoms. in such systems, a given type of event (such as the dice yielding a six) tends to occur at a persistent rate, or "relative frequency", in a long run of trials. physical probabilities either explain, or are invoked to explain, these stable frequencies. thus talking about physical probability makes sense only when dealing with well-defined random experiments. the two main kinds of theory of physical probability are frequentist accounts (such as those of venn, reichenbach and von mises) and propensity accounts (such as those of popper, miller, giere and fetzer). evidential probability, also called bayesian probability (or subjectivist probability), can be assigned to any statement whatsoever, even when no random process is involved, as a way to represent its subjective plausibility, or the degree to which the statement is supported by the available evidence. on most accounts, evidential probabilities are considered to be degrees of belief, defined in terms of dispositions to gamble at certain odds. the four main evidential interpretations are the classical (e.g. laplace's) interpretation, the subjective interpretation (de finetti and savage), the epistemic or inductive interpretation (e.g., ramsey, cox) and the logical interpretation (e.g., keynes, carnap). some interpretations of probability are associated with approaches to statistical inference, including theories of estimation and hypothesis testing. for example, the physical interpretation is taken by followers of "frequentist" statistical methods, such as r.a. fischer, j. neyman and e. pearson. statisticians of the opposing bayesian school typically accept the existence and importance of physical probabilities, but also consider the calculation of evidential probabilities to be both valid and necessary in statistics. this article, however, focuses on the interpretations of probability rather than theories of statistical inference. the terminology of this topic is rather confusing, in part because probabilities are studied within a variety of academic fields. the word "frequentist" is especially tricky. to philosophers it refers to a particular theory of physical probability, one that has more or less been abandoned. to scientists, on the other hand, "frequentist probability" is just another name for physical (or objective) probability. for those who promote bayesian inference view, "frequentist statistics" is an approach to statistical inference that recognises only physical probabilities. also the word "objective", as applied to probability, sometimes means exactly what "physical" means here, but is also used of evidential probabilities that are fixed by rational constraints, such as logical and epistemic probabilities. 4. throwing dices or tossing a coin  the needed approach in this case is a purely mathematical one, since the dice (or coin) is assumed to be ‘fair’ (perfect) and no interaction effect is assumed to occur from the way the throw is performed or with the environment of the dice and of its impact with a surface—nothing about the physics of the process. there are no influence factors affecting the outcome of a toss, which is not even dependent on time. thus, the throws are assumed to be strictly a repeatable ideal process for an indefinitely long time, whence the certain, equal probability of getting each face. in addition, each face is mutually exclusive, and each throw is independent of any past or future throw. this type is also called ‘discrete probability’. the meaning of uncertainty in this framework is that the prediction of the result of the subsequent throws is uncertain because the throwing process is strictly stochastic. no definitional uncertainty exists—for the ideal case. speculations also exist about the effects of deviations on the long run from the strictly ideal assumption, but they enter the arena of experimental science. instead of dices or coins, many other frames are born that can be assimilated to the previous ones (cards, …). 5. the experimental frame  the experimental frame of science is characterised by, at least, two facts: a. the concept of repeatability of the samples is subject to limitations, namely in time, so limiting also the justification to always consider the process as (only) a stochastic one; b. the concept of uncertainty applies not only to the unpredictability—within given (almost always finite) limits—of the next measurement result, but also to the possibility of occurrence of systematic effects, e.g. those born from epistemic reasons. as to a., the vim3 definition of “repeatability condition” is term 2.20 [13]: “condition of measurement, out of a set of conditions that includes the same measurement procedure, same operators, same measuring system, same operating       acta imeko | www.imeko.org  november 2016 | volume 5 | number 3 | 5  conditions and same location, and replicate measurements on the same or similar objects over a short period of time” (emphasis added). in fact, it is basically a tautology, because the expression “short period of time” means ‘so short that the conditions for repeatability hold’. some kind of independent verification of the trueness of the condition is deemed necessary—when possible—concerning two features: (i) the stability in time of the measurand (same or similar objects); (ii) the repeatability of the operating conditions (all the remaining conditions).2 on the contrary, these conditions are both assumed as true for fair dices or coins. as to b., systematic effects induce uncertainty components that are absent, by definition, in the case of fair dices or coins. they are also called ‘bias’,3 indicating the deviations of the measured values from a set of reference values forming what can be called a ‘reference condition’. for example, for a single additive bias bi affecting a quantity xi, is xi = xi + bi (the symbol of ‘standard state’  is borrowed here, for analogy, from physical chemistry to indicate the reference condition for which e(bi) =: 0), and xi + bi = xi – ci , where ci is the so-called ‘correction’.4, [15]. a different category of systematic effects is what is causing the often-called epistemic uncertainty, [16] i.e., the one due to insufficient knowledge of known effects, which reflects onto an imperfect model of the experimental conditions. actually, this category is not exhaustive, and should better be spilt into two distinct categories of uncertainty: – epistemic (imperfect knowledge, namely in science and technique), [17]-[19]; – ontological (ignorance about (some parts of) the phenomenon under study), [20]. the former occurs when an influence quantity is misevaluated or mis-modelled. the latter comprises, e.g., the case where an influence quantity is omitted from the model because the existence of its effect was missed. in both cases, it results into an imperfect modelling. still a distinct category of uncertainty comes from the “definitional uncertainty” [13], [14], which should not be confused with any of the previous ones. it concerns the nonuniqueness of (known) definitions of the measurand: different definitions. it cannot be considered epistemic when it concerns known issues, but it is up to the judgment of the experimenter to take some of the cases into account in the model, or not: this is a model non-uniqueness, not an imperfection of it. ontological, epistemic and definitional uncertainties are not of stochastic nature. figure 1 summarises the different components of uncertainty in experimental science (see, e.g., [20]). the possible time dependence of the process under investigation is another feature of the experimental frame that is useful to consider separately. it can be due to fluctuations in time series, or it may be due to non-repeatability of compounded data series taken subsequent in time. 2 repeatability can be associated to the term “type a uncertainty” of gum, 14 not using the “error approach“. 13 3 “measurement bias: estimate of a systematic measurement error“ (term 2.18 in 13). 4 notice that, in general, e(ci) is what is intended for ‘correction’. in the first case, the terms used to indicate a dependence on time (drifting, dynamic, … systems), or invariance from time (static, stationary, …) may be different in different scientific and technical frames, or be semantically different. about the risk of consequent confusion, the reader is directed to the review performed in [34]. the time scale itself can be a reason for different random effects showing up: an example can be seen in the two-sample variance (allen variance) studies of frequency standards. however, one cannot construe from it that always the fluctuations with time are of random nature: a mixed effect can build up, like in the very common case of a ‘drifting’ characteristics of an instruments from its initial calibrated state. 6. differences between the two frames and some  consequences  “aleatory uncertainty represents an absolute limit. to use the coin toss example, having thrown the coin a thousand times we would be able to express with confidence the probability of a heads occurring, but that is all we can say about the next coin toss” (emphasis added) [20]. probability cannot have the same meaning in experimental science, outside the aleatory components of uncertainty. 5 a useful classification of uncertainty components can be found in [21]: (i) uncertainty in influence quantities, (ii) uncertainty in model, (iii) uncertainty in model parameters. components (i) can be either epistemic if referred to influence quantities properties, or random if referred to their measured values, or both. the influence quantities are normally subdivided into two groups: the “basic” ones (called “input quantities” in [14]) and the “derived” ones (i.e., the ones 5 one can even argue if probability is the only concept that can be used to deal with chance. figure 1. different components of  uncertainty  in experimental science,  in  the four possible combinations of known and unknown information.        acta imeko | www.imeko.org  november 2016 | volume 5 | number 3 | 6  responsible for ‘bias’ [13] and needing ‘corrections’) [14]. the former are measured in indirect measurements (thus having at least one type a uncertainty component), while the latter can be measured, or their values are obtained or inferred from previous knowledge (thus having in the first case at least one type a uncertainty component, or only type b components in the second). 6 component (ii) can be epistemic if referred to model imperfection, or ontological if referred to missed quantities. it also includes the definitional uncertainty. component (iii) can be stochastic if the values are obtained from measurement, or epistemic if the values are computed or inferred. ontological uncertainty is generally not included in the budgets of experimental science,7 and definitional uncertainty is generally resolved by specifying the relevant type of definition—otherwise it becomes an ontological uncertainty component, excluded from treatment in the new gum [32]. epistemic uncertainty is often ‘randomised’, i.e. transformed into a stochastic component, by assuming ignorance about the position parameter, e.g. by assuming a null mean and estimating a range for the resulting uncertainty component. the latter can be set as an interval (e.g. “maximum permissible error” (mpe, term 4.26 in [13]) or “worst case uncertainty” (wcu) [33], or as a non-probabilistic interval, or as the standard deviation, or a multiple of it, set by the chosen confidence interval (or degree of believe), [14]. 7. an example and final remarks  from the previous review it turns out that the case of dices and similar are, in experimental science, an over-simplification of a much more complex structure of uncertainty, such that the former concerns only the intrinsic stochastic component of it. in the latter case, instead, the systematic effects are, in general, the major concern. though they may have a stochastic component, they mainly involve, after all, the need of an assessment that involves basically a subjective judgment requiring a decision. a particularly useful discussion of this situation can be found in [22], where the seminal case (but a wider range of similar cases in measurement applies) of the treatment of uncertainty in the assessment of the values of fundamental constant of physics, is treated from the viewpoint of psychology of measurement and decision theory—an interesting viewpoint basically ‘external’ to the metrology field. different reasons for ‘bias’ in the judgment are considered there, bringing to what is called “overconfidence”: “in several sets of analyzed measurements of physical constants, we have found consistent replication of a robust finding of laboratory studies of human judgment: reported uncertainties are too small. how could this apparent overconfidence arise? experimental studies of human judgment have shown that such biases can arise quite unintentionally from cognitive strategies employed in processing uncertain information”. however, not always this attitude in unintentional: one case “concerns the procedures chosen to assess the uncertainty. the recommended practice in physics is to consider all possible sources of systematic uncertainty when reporting results. 6 prior and posterior are not used in this paper necessarily in the bayesian sense. 7 several decades ago, this was done sparingly but systematically by the ussr school, as “undetected systematic errors”. however, without specific guidelines regarding what to consider and explicit recognition of the subjective elements in uncertainty assessment, one cannot be sure how comprehensively individual scientists have examined the uncertainty surrounding their own experiments. conceivably, some of the apparent overconfidence reflects a deliberate decision to ignore the herder-to-assess sources of uncertainty” (emphasis added); “a second possible source of bias is that, unlike laboratory experiments on judgment, which can take great care to ensure that subjects are motivated to express their uncertainty candidly, real-world settings create other pressures”. in the latter respect, precisely, “having a pre-existing recommended value may particularly encourage investigators to discard or adjust unexpected results, and so induce correlated errors in apparently independent experiments”. this has been observed in several circumstances, according to these authors [22], namely for the speed of light in vacuum, c0 , the basis for the unit of length,8 and the inverse of the fine structure constant, –1, the basis for the value of the electron change e, so of the unit of electrical current. the analysis in [22] in centred only on the uncertainties associated to the recommended values, while the aim of the actual codata task group [23] is also to adjust the values of the constants using the least squares analysis (lsa) method, instead of the use of the mean—or another strictly statistical parameter—of the experimental values (based on probability), [24]. the accent on the subjective side of the uncertainty analysis, (see also [25]) may seem to rule out ‘frequentist’ methodology; however, this does not necessarily mean that the bayesian one can always be better used instead. in chapter 1 of [22], pages 30-31, the authors say: “bayes theorem is an uncontroversial part of probability theory. bayesian inference is more controversial, because it treats probabilities as subjective, thereby allowing inferences that combine diverse kinds of evidence. frequentistic probabilities requires evidence of a single kind (e.g. coin flips). subjective judgements are only probabilities if they pass coherence tests. thus, probabilities are not just any assertion of belief” (emphasis added). actually, codata position had oscillations in the years, starting from a position where its major asset was not the use of the specific analytical treatment (the lsa) [26], but the preliminary critical review of the data and their screening. later they shifted to a position where all the available data were used [27]. that is the position since 2010. possibly, in addition to the criticism about the subjective nature of the screening, an abundance of data prompted the last decision, since a few outlying data could not anymore critically affect the resulting value and associated uncertainty. in [22] a statistical analysis is performed on the reliability of the estimates of the constants values, namely of 40 recommended values for the period 1928-1973. a “surprise index” is computed (percent which fall outside the assessed 98% confidence interval, or outside 2.33s) and found to be an impressive 57%. in more recent times, the studies have dramatically lowered the experimental uncertainty, so the former statistics could have improved a lot. however, the already cited “bandwagon” effect of the recommended values 8 notice that the history of the numerical values of c0 stopped in 1983 with the ‘stipulation’, misinterpreted as a definitive value.       acta imeko | www.imeko.org  november 2016 | volume 5 | number 3 | 7  could now have increased in importance, deserving an urgent update of analyses of this type. in [22] the conclusions about that specific case are: “the underestimation of uncertainty in measurements of physics constants and compilations of recommended values seems to be pervasive. this evidence extends previous findings of overconfidence in laboratory studies of human judgment to a task domain of great practical importance. if reported uncertainties do not reflect the magnitude of actual errors, whether due to incomplete analysis or to judgment biases, the usefulness of those measurements is significantly diminished”. this is critical, e.g., for use in the “new si” [24]. in general, it results that the use of examples concerning the case of dices, coins and similar, has very little value in studying the treatment of experimental uncertainty, and can even be deceiving, so, in general, it should be avoided. 8. appendix: falsification in experimental science  the hereinbefore-asserted subjectivism in experimental sciences, with the need of judgment and decision, entrains the need of “verifications” (from wittgenstein on) or of “falsification” (from popper on) criteria. the first method was later prevalently considered an impossible goal to reach in the lack of a general criterion about the sufficient number of verifications, and the difficulties arising from unavoidable epistemological limitations. the second method was born basically in an uncertainty-free context: however, we learned that in measurement a single occurrence of falsification, even if reasonably proved, cannot be considered sufficient in the frame of uncertain knowledge. not only repeated occurrences are needed, but “falsification is not possible without some threshold deviation which would be considered sufficiently unlikely to reject the theory” [28] — also [29] is interesting, though one not necessarily always may share author’s opinions. early popper (1936) said: “with the idol of certainty (including that of degrees of imperfect certainty or probability) there falls one of the defences of obscurantism which bar the way of scientific advance“, and “the relations between probability and experience are also still in need of clarification. in investigating this problem we shall discover what will at first seem an almost insuperable objection to my methodological views. for although probability statements play such a vitally important role in empirical science, they turn out to be in principle impervious to strict falsification” (emphases added), [30]. later he managed reconciling the probability concept by proposing first the theory of “propensity”, a further interpretation of probability, [31]. references [1] l.j. savage, the foundations of statistics. new york: john wiley & sons, inc. (1954). isbn 0-486-62349-1. [2] f. pavese and p. de bièvre, fostering diversity of thought in measurement science, in advanced mathematical and computational tools in metrology and testing x (f. pavese, w. bremser, a. chunovkina, n. fischer, a.b. forbes, eds.), series on advances in mathematics for applied sciences vol. 86, world scientific, singapore, 2015, pp. 1–8. [3] wikipedia, term “probability interpretations”, as consulted on february 24, 2015, and references therein. [4] hájek, alan, zalta, edward n., ed., interpretations of probability, the stanford encyclopedia of philosophy, online. [5] de elía, ramón; laprise, rené, diversity in interpretations of probability: implications for weather forecasting, monthly weather review 133 (2005) 1129–1143. [6] enric trillas, some uncertain reflections on uncertainty, archives for the philosophy and history of soft computing, international online journal, issue 1 (2013). [7] zadeh, l. a., fuzzy sets as a basis for a theory of possibility. fuzzy sets and systems 1 (1978) 3–28. doi:10.1016/01650114(78)90029-5. [8] dubois, didier; henri prade (1985). théorie des possibilité. masson, paris. [9] wikipedia, term “imprecise probability”, as consulted on june 30, 2015, and references therein. [10] g.l.s. shackle, a non-additive measure of uncertainty, the review of economic studies 17 (1949-1950) 70-74. [11] i. gilboa, d. schmeidler, additive representations of non additive measures and the choquet integral, annals of operations research 52 (1994) 43-65. [12] d. skulj, non-additive probability, 2002. https:// www.stat.aau.at/tagungen/ossiach/skulj.pdf [13] bipm (2012) international vocabulary of basic and general terms in metrology (vim), 3rd edn. bipm/iso, jgcm 200:2012. 2008 version with minor corrections. http://www.bipm.org/en/ publications/guides/vim.html [14] bipm, guide for the expression of uncertainty in measurement; jcgm 00:2008, iso geneva, at http://www.bipm.org/en/ publications/guides/gum.html [15] f. pavese, key comparisons: the chance for discrepant results and some consequences, acta imeko 17 (2015) n.4, 38–47. [16] wikipedia, term “uncertainty quantification”, as consulted on june 30, 2015, and references therein. [17] l.p. swiler, t.l. paez, r. l. mayes, epistemic uncertainty quantification tutorial, proceedings of the imac-xxvii, 2009 orlando, florida usa, 2009 society for experimental mechanics inc. [18] a. der kiureghian, o. ditlevsen, aleatory or epistemic? does it matter?, special workshop on risk acceptance and risk communication, march 26, 2007, stanford university, pp. 1–13 [19] t. o’hagan, dicing with the unknown, significance 2005, 132– 133. [20] m. squair, epistemic, ontological and aleatory risk, 2009, blog http://criticaluncertainties.com/2009/10/11/epistemic-andaleatory-risk/. [21] f. pavese: “mathematical and statistical tools in metrological measurement”, 2013, chapter in physical methods, instruments and measurements, [ed. unesco-eolss joint committee], in encyclopedia of life support systems (eolss), developed under the auspices of the unesco, eolss publishers, oxford, uk, http://www.eolss.net. [22] m. henrion, b. fischhoff, assessing uncertainty in physical constants, ch. 9 in “judgement and decision making“ (b. fischhoff, ed.) routhledge (2013) pp. 172-187, 113649734x, 9781136497346, taken from american journal of physics 54, 791 (1986); doi: 10.1119/1.14447. [23] codata http://physics.nist.gov/cuu/constants/index.html , http://www.bipm.org/extra/codata/. [24] f. pavese, some problems concerning the use of the codata fundamental constants in the definition of measurement units, metrologia 51 (2014) l1–l4, with online supplementary information. [25] f. pavese, subjectively vs. objectively-based uncertainty evaluation in metrology and testing, imeko-tc1-tc7-2008030, http://www.imeko.org/index.php/proceedings# under tc7 (2008); f. pavese, on the degree of objectivity of uncertainty evaluation in metrology and testing” measurement 2009 42 1297–1303.       acta imeko | www.imeko.org  november 2016 | volume 5 | number 3 | 8  [26] b.n. taylor, nbsir 81-2426, 1982, national bureau of standards (today nist), gaithersburg usa. [27] p.j. mohr, b.n. taylor and d.b. newell, codata recommended values of the fundamental physical constants: 2010, rev. modern phys. 84 (2012) 1–94. [28] j.r. grozier, can theories be falsified by experiment?, msc essay (ucl, london, 2012). [29] j.r. grozier, falsificationism, science and uncertainty, msc dissertation, london centre for the history of sciences, technology and medicine, 2013, pp. 1–29. [30] k. popper, the logic of scientific discovery (taylor & francis e-library ed.). london and new york: routledge / taylor & francis e-library (2005). [31] k. popper, the propensity interpretation of the calculus of probability and of the quantum theory. in observation and interpretation, korner & price (eds.), buttersworth scientific publications, 1957, pp. 65–70. [32] jgcm, guide for the expression of uncertainty in measurement, draft of 14 december 2014 delivered for comments to iso tc69. [33] da silva r.j., santos j.r., camões m.f., worst case uncertainty estimates for routine instrumental analysis, observations on the worst case uncertainty, analyst 127 (2002) 957–963; l. fabbiano, n. giaquinto, m. savino, g. vacca, observations on the worst case uncertainty, journal of physics: conference series 459 (2013): 2043, doi:10.1088/1742-6596/459/1/012038. [34] k.h. ruhm, are fluctuating measurement results instable, drifting or non-stationary? a survey on related terms in metrology, journal of physics conference series 459 (2013): 2043, doi: 10.1088/1742-6596/459/1/012043. microsoft word 0 acta imeko  issn: 2221‐870x  november 2016, volume 5, number 3, 1    acta imeko | www.imeko.org  november 2016 | volume 5 | number 3 |  1  journal contacts    about the journal  acta imeko is an e-journal reporting on the contributions on the state and progress of the science and technology of measurement. the articles are mainly based on presentations presented at imeko workshops, symposia and congresses. the journal is published by imeko, the international measurement confederation. the issn, the international identifier for serials, is 2221-870x.      editor‐in‐chief  paolo carbone, italy  honorary editor‐in‐chief  paul p.l. regtien, netherlands                                               editorial board                                       section editors (vol. 4 and 5)  leopoldo angrisani, italy filippo attivissimo, italy eulalia balestieri, italy eric benoit, france catalin damian, romania lorenzo ciani, italy pasquale daponte, italy luca de vito, italy luigi ferrigno, italy edoardo fiorucci, italy alistair forbes, united kingdom helena geirinhas ramos, portugal sabrina grassini, italy fernando janeiro, portugal konrad jedrzejewski, poland andy knott, united kingdom massimo lazzaroni, italy fabio leccese, italy rosario morello, italy michele norgia, italy pedro miguel pinto ramos, portugal sergio rapuano, italy dirk röske, germany alexandru salceanu, romania constantin sarmasanu, romania lorenzo scalise, italy emiliano schena, italy enrico silva, italy marco tarabini, italy eric benoit, france marcantonio catelani, italy paolo carbone, italy lorenzo ciani, italy spartacus gomariz, spain sabrina grassini, italy konrad jędrzejewski, poland min-seok kim, korea franco pavese, italy serge rapuano, italy paul regtien, the netherlands alexandru salceanu, romania jan saliga, slovakia krzysztof stepien, poland marco tarabini, italy ian veldman, south africa claudia zoani, italy final editing    rosario morello, university mediterranea of reggio calabria, italy   about imeko  the international measurement confederation, imeko, is an international federation of actually 40 national member organisations individually concerned with the advancement of measurement technology. its fundamental objectives are the promotion of international interchange of scientific and technical information in the field of measurement, and the enhancement of international co-operation among scientists and engineers from research and industry.     addresses  principal contact  prof. paolo carbone university of perugia dept. electr. inform. engineering (diei) via g.duranti, 93, 06125 perugia, italy email: paolo.carbone@unipg.it    acta imeko  prof. paul p. l. regtien measurement science consultancy (msc) julia culpstraat 66, 7558 jb hengelo (ov), the netherlands email: paul@regtien.net    support contact  dr. dirk röske physikalisch-technische bundesanstalt (ptb) bundesallee 100, 38116 braunschweig, germany email: dirk.roeske@ptb.de acta imeko  december 2014, volume 3, number 4, 4 – 9  www.imeko.org    acta imeko | www.imeko.org  december 2014 | volume 3 | number 4 | 4  stator winding fault diagnosis in permanent magnet  synchronous generators based on short‐circuited turns  identification using extended kalman filter  brice aubert 1,2,3 , jérémi régnier 1,2 , stéphane caux 1,2 , dominique alejo 3   1  université de toulouse ; inpt, ups ; laplace ; enseeiht, 2 rue charles camichel, bp 7122, f‐31071 toulouse cedex 7, france  2  cnrs ; laplace ; f‐31071 toulouse, france  3  aeroconseil ‐ akka technologies group ; 3 rue dieudonné costes; f‐31703 blagnac, france      section: research paper   keywords: extended kalman filter; fault diagnosis; on‐line parameters estimation; permanent magnet synchronous generator; inter‐turn short‐circuit  citation: brice aubert, jérémi régnier, stéphane caux, dominique alejo, stator winding fault diagnosis in permanent magnet synchronous generators based  on short‐circuited turns identification using extended kalman filter, acta imeko, vol. 3, no. 4, article 3, december 2014, identifier: imeko‐acta‐03 (2014)‐ 04‐03  editor: paolo carbone, university of perugia   received october 5 th , 2013; in final form january 4 th , 2014; published december 2014  copyright: © 2014 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: (none reported)  corresponding author: brice aubert, e‐mail: brice.aubert@laplace.univ‐tlse.fr or brice.aubert@aeroconseil.com     1. introduction  permanent magnet synchronous generators (pmsg) are increasingly used in various industrial fields such as aerospace, railway and automotive sectors or renewable energy [1] to take advantage of their high efficiency, small size and easy control for variable-speed applications compared to a wounded rotor design. with regard to safety considerations, the main drawback of pmsg is the critical effect of inter-turn short-circuits in stator windings. indeed, stator faults in pmsg still remain while the machine is rotating because of the permanent rotor flux caused by magnets. more specifically, inter-turn short-circuits in stator windings are tricky to detect and are the root of highest fault currents, especially for a few number of faulty turns [2]. thus, an early on-line detection is required to perform the appropriate safety request and to avoid greater damage. as well as being quick, the detection method should be robust to electrical network variations to avoid false alarms. moreover, the constraints related to on-line monitoring considerations imply a detection algorithm matching the performances of on-board computers. in the literature, three main approaches are commonly used to detect inter-turn short-circuits in stator windings. a first approach, based on signal analysis, often uses spectral tools to underline specific frequency components related to the fault. for example, stator current analysis in park’s [2], concordia [3] or three-phase’s [4] frame and axial flux leakage measurements [5] can be used as an inter-turn shortcircuit fault indicator. even if these methods are compatible with on-line requirements and enable relatively quick fault detection, they are not suitable for variable-speed abstract  this paper deals with an extended kalman filter based fault detection for inter‐turn short‐circuits in permanent magnet synchronous  generators. inter‐turn short‐circuits are among the most critical faults in the pmsg. indeed, due to permanent magnets, the short‐ circuit current is maintained as long as the machine is rotating. thus, a specific pmsg faulty model in the d‐q frame is developed to  estimate the number of short‐circuited turns which are used to build a fault indicator. simulation results demonstrate the sensitivity  and the robustness of the proposed fault indicator against various operation points on an electrical network even for a few number of  short‐circuited turns.  acta imeko | www.imeko.org  december 2014 | volume 3 | number 4 | 5  applications and are not robust with regard to electrical network variations in generator operation. the second approach is related to knowledge-based methods which imply an a priori knowledge of fault signature to allow an a posteriori fault classification during the monitored system operation. artificial intelligence tools as fuzzy logic system [6], artificial neural networks [7] and pattern recognition methods [8] can be used but time response and computation considerations are not matching with on-line detection requirements. the third approach is based on state or parameters estimation and implies the use of mathematical models of the studied system [9]. these methods are a good compromise between robustness with regard to electrical network variations, on-board computation and fast fault detection. to meet these requirements, the chosen method presented in this paper for inter-turn short-circuit detection is based on the identification of the short-circuited turns ratio in a faulty pmsg model expressed in park frame using an extended kalman filter (ekf) algorithm. traditionally used for state or parameter estimation, the ekf algorithm is, in this case, specifically adapted for detection purposes. a basic modelling approach allows defining a generalized algorithm suitable for a large power scale of synchronous generators. no additional measurements are required because the proposed detection scheme is fed with measurements already available for control purposes. this paper is organized as follow: in section 2, the faulty pmsg park model used for fault diagnosis is described. section 3 presents simulation results of the ekf algorithms in both healthy and faulty conditions. the proposed fault indicator built is also described. finally, a robustness and a sensitivity analysis are presented in section 4 to assess the proposed indicator effectiveness. 2. faulty pmsg model for inter‐turn short‐circuit  detection  the faulty pmsg model used for inter-turn short-circuit detection is based on an earlier study [10] with less modelling assumptions (voltage drops due to short-circuit reduction is taken into account) to make the pmsg model more sensitive to windings faults. the model enables the fault localization with the use of angle s/c (equal to 0, 2π/3 or 4π/3 for a short-circuit respectively on phase a, phase b or phase c) and the number of short-circuit turns ns/c (ratio between short-circuited turns and all turns on a stator winding). figure 1 shows this model used for the fault diagnosis with inter-turn short-circuit located on phase c (s/c = 4π/3). according to this faulty model, the stator voltages [vs] and the short-circuit loop equations are respectively given in (1) and (2): vs = rs. is ns/c.rs.ts/c.is/c s (1) 0 = ns/c.rs.ts/c t. is + ns/c.rs.is/c + φs/c (2) where the stator flux [s] and the short-circuit flux s/c are respectively expressed in (3) and (4): φs = l . is + 3 2 .ns/c.lps.t32. cosθs/c sinθs/c .is/c+ + ns/c.lls.ts/c.is/c e (3) φs/c 3 2 .ns/c.lps. t32. cosθs/c sinθs/c t . is ns/c.lls.ts/c t . is ns/c 2 . lps lls .is/c ns/c.ts/c t . e (4) with vs : stator voltages vector is : stator currents vector is/c : short-circuit current e : electromotive forces vector rs : stator resistance of a stator winding lls : leakage inductance of a stator winding lps : magnetizing inductance of a stator winding l = lls+lps lps/2 lps/2 lps/2 lls+lps lps/2 lps/2 lps/2 lls+lps : inductance matrix t32 = 1 √3 . √2 0 1/√2 √3/2 1/√2 √3/2 : concordia transformation matrix ts/c 1 3 . 1 2∙cos θs/c 1 2∙cos θs/c 2π 3 1 2∙cos θs/c 4π 3 : short-circuit matrix after park transformation applied to (1) and (2) and some calculations, the faulty model can be expressed with these two equations (5) and (6): vs dq ‐ rs. i ' s dq ω.ls. 0 ‐1 1 0 . i's dq ls. i's dq e dq (5) is dq = i's dq is/c dq = i's dq 1 zs/c dq ∙ vs dq (6) where: stator synchronous inductance: ls = 3/2∙lps + lls park transformation matrix: p θ = cos sin sin cos figure 1. faulty pmsg model used for inter‐turn short‐circuit diagnosis.  acta imeko | www.imeko.org  december 2014 | volume 3 | number 4 | 6  short-circuit fault impedance: . . . . . . fault localization matrix: cos .sin cos .sin short-circuit current calculation in park frame: ∙ 2 3 . . cos sin . it can be noticed that (5) is the same equation as a classic healthy pmsg model taking into account [is’] as the stator current whereas the inter-turn short-circuit loop is represented in (6) with the equivalent fault impedance [zs/c] at the output of the machine which deflects a part of the stator current. expending this model to each phase, three short-circuit impedances (zs/ca, zs/cb, zs/cc for s/c=0, 2π/3, 4π/3 respectively) are added to the pmsg model as shown in figure 2. this model can be written as a state space representation with [is’] as the state vector. the inter-turn fault is expressed in the feedforward matrix d, depending on the number of short-circuited turns ns/ca, ns/cb, ns/cc. to estimate these parameters in this non-linear model, the ekf algorithm is used. in the ekf algorithm, special attention should be paid to covariance matrices tuning, particularly for the measurement noise covariance matrix r and the state noise covariance matrix q to obtain, at the same time, an accurate estimation and an appropriate dynamic response of the extended parameters [11]. in order to satisfy the fast dynamic response requirement for the fault indicator and according to noise measurements, the covariance matrices used for the estimation of each ns/c are expressed in (7): p0=10 -1 ∙ i5×5; r=10 -2 ∙ i2×2; q = 10 -5∙ i2×2 0 0 10-4.i3×3 (7) simulations results are presented in the following section. 3. simulation procedure  3.1. simulation test bench setup  the pmsg studied in this paper is an existing threephase 3.6 kw four-pole permanent magnet synchronous machine (figure 3). stator windings of pmsg are modified with additional connection points in the stator coils on phase a, allowing the introduction of several inter-turn short-circuit levels using a short-circuit resistance rsc (4%, 8%, 12% and 16% of short-circuit turns respectively corresponding to 3, 6, 9 and 12 turns on 72 whole turns of a stator winding). the faulty pmsg model used for simulation is built using the electrically coupled magnetic circuit (ecmc) method [12]. it consists in a semi-analytical computation of the pmsg inductances from the stator winding layout including additional connections for inter-turn short-circuit generation. the simulation test bench is set with saber software in order to generate various operating points for the faulty pmsg model (figure 4). it is composed of a balanced r-l load to vary the electrical power and the power factor, an unbalanced r load to create power unbalanced and a harmonic load including a three-phase diode bridge rectifier with resistive load to generate current harmonics. 3.2. simulation results  as shown in figure 5, where a 16% inter-turn shortcircuit is generated at t = 0.5 s on phase a, each estimated parameter ns/c is impacted. indeed, a modification of their mean value and large oscillations at twice the electrical frequency appear as soon as the winding fault occurs (figure 5b). it is also noticeable that ns/ca is not the only estimated parameter sensitive to the short-circuit: ns/cb and ns/cc are also modified in a lesser extent due to some simplifying assumption about the faulty pmsg model used for the parameters estimation. however, the fault can clearly be located on phase a in this test. according to this test, the proposed fault indicator to figure 2. pmsg characteristics and its stator windings.  figure 3. inter‐turn faulty pmsg model expressed in park’s frame.  figure 4. simulation test bench scheme.  acta imeko | www.imeko.org  december 2014 | volume 3 | number 4 | 7  detect inter-turn short-circuits expressed in (8) is based on the sum of these three estimated short-circuited turn ratios (figure 5c). a moving average calculation over one electrical half period is added to filter the fault indicator and improve its robustness. fault_indicator = |ns/c|i t/2 i=a,b,c (8) figure 6 shows the evolution of this fault indicator in the healthy case (for t < 0.5 s) and in the faulty case (for t > 0.5 s) with several values of short-circuited turns on phase a. in the healthy case, the fault indicator remains close to 0, reflecting a safe pmsg. in faulty cases, the fault indicator increases with the number of short-circuited turns, which confirms that the fault detection is trickiest for a few number of short-circuited turns. however, shortcircuit detection remains possible even for 4% of shortcircuited turns in comparison with the fault indicator behavior in the healthy case. moreover, time response of the proposed fault indicator remains quick (about 20 ms) in spite of the filtering characteristic of the moving average calculation. in the following section, the effectiveness of this fault indicator is evaluated against several operating points and several inter-turn short-circuit. 4. fault indicator performance assessment  4.1. robustness assessment  in order to evaluate the robustness of each estimated parameter to the inter-turn short-circuit diagnosis, several tests based on figure 4 have been made and are listed below: test 1 frequency variation from 30 hz to 60 hz with 10 hz step and 10 a load current per phase. test 2 power variation from 15 a to 0 a load current per phase with 2.5 a step and 40 hz electrical frequency. test 3 power factor variation from 1 to 0.8 with 0.05 step at 40 hz and 10 a load current per phase. test 4 unbalanced load variation from -5 a to +5 a unbalanced load current with 2.5 a step at 40 hz and 10 a load current in other phases. test 5 harmonic load variation from 0% to 20% of nominal current harmonic distortion variation with 5% step at 40 hz and 10 a load current phases per phase. these scenarios are related to specifications that could be encountered in embedded electrical networks. the robustness is then characterized by the ability of the ekf algorithm to properly separate the healthy and the faulty cases whatever the operating conditions. to quantify this ability, healthy and faulty simulations are performed under the same operating conditions and the worst indicator’s value is kept for each test (i.e. the highest indicator’s value in the healthy case and the lowest indicator’s value in the faulty case). for example, figure 7 shows the evolution of the fault indicator for various operating electrical frequencies. in this test, the highest fault indicator values in healthy cases appear at the lowest frequency whereas the lowest fault indicator values in faulty cases appear at the figure 5. response to a 16% inter‐turn short‐circuit (iload = 10 a per phase,  40  hz).  (a)  short‐circuit  current.  (b)  estimated  parameters.  (c)  fault indicator.  figure 6. fault indicator response to various inter‐turn short‐circuits (iload =  10 a per phase, 40 hz).  figure 7. robustness spider chart for the proposed fault indicator.  acta imeko | www.imeko.org  december 2014 | volume 3 | number 4 | 8  highest frequency. the following spider chart (figure 8) summarizes the robustness of the proposed fault indicator against electrical network variation, using the worst indicator’s values for each test, to compare healthy and faulty operation with several short-circuited turns. it indicates that an inter-turn short-circuit fault can be easily distinguished from the healthy pmsg whatever the operating conditions and even for a few number of short-circuited turns. according to these robustness tests, a detection threshold is set at twice the maximum of the fault indicator in healthy condition for each robustness test, in order to avoid any false alarms in healthy operation. this maximum fault indicator value in healthy case appears at the lowest frequency in the frequency tests. thus, the detection threshold is set to 2.5% and is also represented in figure 8. 4.2. sensitivity assessment  the sensitivity assessment consists in the evaluation of the short-circuit current needed to detect the inter-turn fault. it can be calculated according to the detection threshold previously determined. figure 9 shows this sensitivity test in which the short-circuit resistance rsc is used to vary the inter-turn short-circuit severity. this test confirms that the fault detection is trickier and more serious for a few number of short-circuited turns. indeed, in relation to the 15 a rated current of the studied pmsg, a fault current of 45 a is required to detect a 4% inter-turn short-circuit compared to a 15 a fault current required to detect a 16% inter-turn short-circuit. however, stator windings of a pmsg can easily withstand three times the rated current during the detection time (about 20ms). moreover, these results show that, even for a few number of short-circuited turns, an early detection of a resistive interturn short-circuit is possible with the proposed fault indicator. 5. conclusions  this paper describes a method for inter-turn shortcircuit detection in stator windings based on a short-circuit turns estimation on a faulty pmsg model using an ekf algorithm. this fault diagnosis allows a fast and robust detection of inter-turn short-circuit, whatever the operating conditions on a non ideal electrical network even for a few number of short-circuited turns. moreover, this fault indicator allows to localize the faulty phase and is also sensitive to resistive inter-turn short-circuit which could prevent a dead short-circuit. moreover, experimental tests have also been done with the short-circuit current limited to 25 arms for safety considerations. experimental results, available in [13], are quite similar to simulation ones and confirm the good representativeness of the simulation model even for a resistive inter-turn short-circuit. finally, the prospect of this study is the application of this fault indicator on a 45 kw, 400 hz – 800 hz pmsg. references  [1] k.r. weeber, m.r. shah, k. sivasubramaniam, a. el-refaie, qu ronghai, c. stephens, s. galioto, “advanced permanent magnet machines for a wide range of industrial applications”, ieee power and energy society general meeting, july 2529, 2010, minneapolis, usa, pp. 1-6. [2] s.m.a. cruz, a.j.m. cardoso, multiple reference frames theory: a new method for the diagnosis of stator faults in three-phase induction motors, ieee trans. energy convers. 20 (2005) pp. 611—619. [3] d. diallo, m.e.h. benbouzid, d. hamad, x. pierre, fault detection and diagnosis in an induction machine drive: a pattern recognition approach based on concordia stator mean current vector, ieee trans. energy convers. 20 (2005) pp. 512 -519. [4] m. sahraoui, a. ghoggal, s.e. zouzou, a. aboubou, h. razik, “modelling and detection of inter-turn short circuits in stator windings of induction motor”, proc. int. iecon, 2006, pp. 4981-4986. [5] j. penman, h.g. sedding, b.a. lloyd, w.t. fink, detection and location of interturn short circuits in the stator windings of operating motors, ieee trans. energy convers. 9 (1994) pp. 652-658. figure 8. fault indicator response to various electrical frequencies (iload = 10  a  per  phase).  (a)  electrical  frequency.  (b)  short‐circuit  current.  (c)  fault indicator.  figure 9. sensitiveness assessment for the proposed fault indicator (iload =  10 a per phase, 40 hz). (a) fault indicator. (b) short‐circuit current.  acta imeko | www.imeko.org  december 2014 | volume 3 | number 4 | 9  [6] m.a. awadallah, m.m. morcos, s. gopalakrishnan, t.w. nehl, a neuro-fuzzy approach to automatic diagnosis and location of stator inter-turn faults in csi-fed pm brushless dc motors, ieee trans. energy convers. 20 (2005) pp. 253 259. [7] m.b.k. bouzid, g. champenois, n.m. bellaaj, l. signac, k. jelassi, an effective neural approach for the automatic location of stator interturn faults in induction motor, ieee trans. ind. electron. 55 (2008) pp. 4277-4289. [8] a. soualhi, g. clerc, h. razik, “faults classification of induction machine using an improved ant clustering technique”, ieee international symposium on diagnostics for electric machines, power electronics & drives (sdemped), sept. 5-8, 2011, bologna, italy, pp. 316-321. [9] m. khov, j. regnier, j. faucher, “detection of turn shortcircuit faults in stator of pmsm by on-line parameter estimation”, international symposium on power electronics, electrical drives, automation and motion (speedam), june 11-13, 2008, ischia, italy, pp. 161-166. [10] s. bachir, s. tnani, j.-c. trigeassou and g. champenois, diagnosis by parameter estimation of stator and rotor faults occurring in induction machines, ieee trans. ind. electron. 53 (2006) pp. 963 -973. [11] b. de fornel, j.-p. louis, “electrical actuators: identification and observation”, iste ltd, 2010. [12] a.a. abdallah, j. regnier, j. faucher, “simulation of internal faults in permanent magnet synchronous machines”, 6th international conference on power electronics and drive systems, nov. 28 dec. 1, 2005, kuala lumpur, malaysia, pp. 1390-1395. [13] b. aubert, j. regnier, s. caux, d. alejo, “on-line inter-turn short-circuit detection in permanent magnet synchronous generators”, ieee international symposium on diagnostics for electric machines, power electronics & drives (sdemped), aug. 27-30, 2013, valencia, spain, pp. 329-335. microsoft word 250-1781-1-le-rev acta imeko  issn: 2221‐870x  september 2015, volume 4, number 3, 59 ‐ 64    acta imeko | www.imeko.org  september 2015 | volume 4 | number 3 | 59  new generation of cage‐type current shunts developed using  model analysis  věra nováková zachovalová, martin šíra, pavel bednář, stanislav mašláň  czech metrology institute, okruzni 31 638 00 brno, czech republic      section: research paper   keywords: current shunt; current measurement; phase measurement; electric variables measurement  citation: věra nováková zachovalová, martin šíra, pavel bednář, stanislav mašláň, new generation of cage‐type current shunts developed using model  analysis, acta imeko, vol. 4, no. 3, article 10, september 2015, identifier: imeko‐acta‐04 (2015)‐03‐10  editor: paolo carbone, university of perugia, italy  received february 13, 2015; in final form april 8, 2015; published september 2015  copyright: © 2015 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: this work was supported by czech metrology institute, czech republic.  corresponding author: věra nováková zachovalová, e‐mail: vnovakovazachovalova@cmi.cz    1. introduction  at national metrology institutes (nmi) shunts of special designs are used for high accuracy current measurements converting a current from the working level to a voltage convenient for the input of a meter. these shunts are mainly used in power and power quality measurement systems and in ac-dc current transfer difference measurement systems. the acdc current transfer difference serves for derivation of a root mean square (rms) value of an ac current from a dc current of the same nominal level it can be defined as the relative difference between the rms value of the ac current and the dc current of the same nominal level and can be measured by devices called thermal converters. in recent years the shunts of three different designs for high accuracy current measurements were developed at several nmis:  foil shunts [1] are built using resistive manganin or zeranin foils. they are distinguished by a lower ac-dc difference and phase angle error compared with cage type current shunts but on the other hand their temperature and power coefficient are larger.  cage shunts [2]-[6] are built using a symmetrical construction of printed circuit board (pcb) with insoldered discrete resistors. they are characterized by low temperature and power coefficients, but their ac-dc difference and phase angle error are larger compared with foil shunts.  coaxial shunts [7] are made using disk structures, either with surface mount (smd) resistors or resistive layers. their design is really compact but their frequency characteristic is the worst of all three shunt designs. at the czech metrology institute (cmi) the cage design was used to build a first (original) series of current shunts in 2008 [6]. in subsequent research a calculable model of the cmi cage shunt was developed using lumped circuit elements. the transimpedance, ac-dc difference and also the phase angle error of a shunt can be derived from the model [9]. the goal of this research was to build a new generation of cage shunts from 30 ma up to 10 a based on a model analysis for the purpose of optimizing their construction to achieve the lowest possible ac-dc difference and phase angle error. 2. cmi cage‐type current shunts construction and  modeling  the original cmi’s shunts were developed to be suitable for use with planar multijunction thermal converters (pmjtc) to establish an ac-dc current transfer difference measurement system at cmi in 2008 [6]. abstract  this paper describes a new generation of cage‐type ac‐dc current shunts from 30 ma up to 10 a, developed using a lumped circuit  element model. comparison of the calculated and measured values shows agreement better than 6 ppm in the ac‐dc difference at  frequencies up to 100 khz.  acta imeko | www.imeko.org  september 2015 | volume 4 | number 3 | 60  the voltage drop across every shunt in parallel with a 90 ω pmjtc is 1 v at nominal current. the number, value and type of resistors are different in each shunt (see table 1). the temperature coefficient of the shunts was reduced by using a suitable combination of resistors s102 with different temperature coefficients (one third of s102c and two thirds of s102k), except for the 30 ma shunt with z201 resistors, which temperature coefficient is below 1 ppm/k [6], [8]. the physical design of the original cmi shunts is shown in figure 1 (detailed in figure 2). the shunts are constructed using single-sided and doublesided fibreglass-epoxy pcb material (fr4 with permittivity 5.45 and loss factor 0.0205) of 2 mm thickness with mounted vishay foil z201 or s102 resistors and can be split up in the following construction parts [6], [9]:  input connector;  input part (two disks made of single-sided pcb);  crossbars made of double-sided pcb;  resistors;  output part (circle and disk made of single-sided pcb and an aluminium wire connected between circle and output connector);  output connector. the input connector brings the current to the input disks, which spread the current through the crossbars. each crossbar carries a fraction of the input current to resistors connected in parallel. the output part senses the voltage drop across the resistors. to each construction part a two-port element was assigned and their cascade concatenation led to the development of the shunt model (see figure 3). each two-port was determined based on the physical design of the related construction part, for instance the crossbars are made from double-sided pcb with a leakage capacitance and resistance and between the copper layers, and the copper layer also has some inductance and resistance ) [9]. the product of the cascade matrixes assigned to the twoports defines the cascade matrix of the model [9]: (1) where , , , , and are cascade matrixes of individual two-ports according to figure 3. during measurements the output of the shunt is loaded by the device that measures the voltage drop across the shunt. therefore it is necessary to include this load into the model. then, the cascade matrix of the loaded shunt model is [9]: (2) where is cascade matrix assigned to the load and , , and are elements of matrix . element can be table 1. general parameters of the original shunts.  nominal current resistance (ω) number and  value of  resistors type of  resistors pcb type 30 ma 50 3x 150 ω  z201 fr4 100 ma 10 10x 100 ω  3x s102c 7x s102k  fr4 300 ma 3.3 30x 100 ω  10x s102c 20x s102k fr4 1 a 1 100x 100 ω  33x s102c 67x s102k fr4 10 a 0.1 100x 10 ω  33x s102c 67x s102k fr4 figure 1. the original cmi shunts.   figure 2. the design of the original 100 ma shunt.   figure 3. the lumped element model of the shunts.   current input voltage output acta imeko | www.imeko.org  september 2015 | volume 4 | number 3 | 61  used for calculation of the transimpedance of the shunt [9]: . (3) from the transimpedance the phase-angle error and the acdc difference can be derived easily [9]. this model is based on calculations of all component values from the geometry and material properties. therefore it was necessary to be familiar with the dielectric properties of the pcb (permittivity, loss factor) and the capacitance and inductance of the resistor, which had to be measured [9]. evaluation of associated uncertainties of the model was done by means of the monte carlo method [9]. a detailed description of the model calculation is given in [9]. the model was validated by comparison of the calculated and measured values of the original shunts which showed agreement better than 6 ppm in the ac-dc difference and 110 µrad in the phase angle error at frequencies up to 100 khz [9]. 3. construction improvements  the shunt model can be used to determine the sensitivity of the output quantities to the modification of the input quantities, resulting in improvements to the construction of the shunts [9]. 3.1. theoretical analysis  the sensitivity of the calculated values of the ac-dc difference and phase angle error to modification of the input quantities of the model was investigated using parametric simulation. calculations indicated that some of the input quantities have a significant influence and some have an insignificant influence on the calculated values of the ac-dc difference and phase angle error. the following input quantities of the model were found to have significant influence on the calculated values [9]:  capacitance and inductance of the resistors;  relative permittivity and loss factor of the pcb;  thickness of the pcb;  number of crossbars and geometric dimensions. it was found that the influence of all these input quantities increases with frequency (see the example of the ac-dc difference and phase angle error dependence on frequency with modification of the pcb permittivity in figure 4 and figure 5). by converting the data to a single frequency point it was ascertained that the dependence of the ac-dc difference and phase angle error on the modification of the relevant input quantities is linear except for the dependence on the modification of pcb thickness (see the example of the ac-dc difference and phase angle error dependence on modification of pcb permittivity and pcb thickness in figure 6 to figure 9, calculated at 100 khz). in the next step the calculation of sensitivity coefficients was performed to extract information about the vastness of influence of a single input quantity. the sensitivity coefficients were calculated as: ∆ ∆ (4) figure  6.  ac‐dc  difference  dependence  on  modification  of  the  pcb  permittivity calculated at 100 khz.   figure  7.  phase  angle  error  dependence  on  modification  of  the  pcb permittivity calculated at 100 khz.   figure 4. ac‐dc difference dependence on frequency with modification of pcb permittivity calculated for a 100 ma shunt.   figure  8.  ac‐dc  difference  dependence  on  modification  of  the  pcb  thickness calculated at 100 khz.  figure 5. phase angle error dependence on frequency with modification of  pcb permittivity calculated for a 100 ma shunt.   acta imeko | www.imeko.org  september 2015 | volume 4 | number 3 | 62  where ∆ represents a change of the output quantity (ac-dc difference or phase angle error) corresponding to a change ∆ of the selected input quantity. ac-dc difference and phase angle error sensitivity coefficients calculated at 100 khz are shown in table 2 and table 3. the values of the sensitivity coefficients are different for the low-current and high-current shunts. this indicates that the optimization of the shunts construction depends on different input quantities for the low-current and high-current shunts construction. the ac-dc difference of the low-current shunts is affected by the dielectric properties and thickness of the pcb and slightly also by geometrical dimensions. the ac-dc difference of high-current shunts depends more on the disk size and less on the dielectric properties and thickness of the pcb. the capacitance and inductance of the resistors do not influence the ac-dc difference of all shunts. the phase angle error of the low-current shunts is influenced by the relative permittivity and thickness of the pcb, by geometric dimensions and slightly also by the capacitance and inductance of the resistors. the phase angle error of highcurrent shunts is affected mainly by the inductance of the resistors, but also by the thickness of the pcb and slightly by the relative permittivity. the influence of the loss factor on the phase angle error is negligible for all shunts. 3.2. construction of the new generation cage‐type current shunts  figure 10 shows the new generation of cage-type shunts from 30 ma up to 10 a developed at cmi. the voltage drop across every shunt is 0.6 v at nominal current being convenient for use either with pmjtc in the measurement set up of the acdc current transfer difference established in 2008 [6] or with analogue to digital converters (adc) in view of the development of a new sampling wattmeter measurement system. the improvements to the shunts construction resulted from the theoretical analysis of the lumped element model described above. general information about resistors and pcb material used for their construction are in table 4. the low current shunts (up to 1 a) were constructed using the high frequency pcb material ro4350b with better dielectric properties (permittivity = 3.6, loss factor = 0.0031) than the previously used fr4 material. the thickness of the pcb was reduced from 2 mm down to 1.524 mm. geometrical figure  9.  phase  angle  error  dependence  on  modification  of  the  pcb thickness calculated at 100 khz.   table 2. ac‐dc difference sensitivity coefficients calculated at 100 khz. input quantity 30 ma  shunt 100 ma  shunt 1 a  shunt  10 a  shunt  unit of  sensitivity  coefficient capacitance of resistors  0.4  0.1 0.0  0.0  ppm/pf inductance of resistors  0.0  0.0 0.0  ‐0.1  ppm/nh pcb thickness  ‐44.3  ‐11.3  ‐2.9  3.2  ppm/mm pcb permittivity  30.2  5.1 1.3  ‐3.2  ppm pcb loss factor  57.8  12.9 4.0  1.1  ppm/10 ‐2 with of crossbars  5.5  1.8 0.5  0.1  ppm/mm length of crossbars  2.8  0.4 0.1  0.0  ppm/mm size of disks  21.4  3.1 ‐0.3  ‐5.3  ppm/cm figure 10. the new generation of cage‐type shunts.   table 3. phase angle error sensitivity coefficients calculated at 100 khz. input quantity 30 ma  shunt 100  ma  shunt 1 a  shunt  10 a  shunt  unit of  sensitivity  coefficient capacitance of resistors  ‐94.1  ‐62.8  ‐62.8  ‐6.3  µrad/pf inductance of resistors  4.18  6.3 6.3  62.8  µrad/nh pcb thickness  2673.9  572.9  160.1  41.5  µrad/mm pcb permittivity  ‐1782.0  ‐262.8  ‐73.4  ‐19.1  µrad pcb loss factor  0.8  0.0 0.0  ‐0.2  µrad/10 ‐2 with of crossbars  ‐342.6  ‐90.9  ‐22.7  ‐4.5  µrad/mm length of crossbars  ‐157.4  ‐18.2  ‐4.5  ‐0.9  µrad/mm size of disks  ‐1278.5  ‐239.5  ‐59.5  ‐5.3  µrad/cm table 4. general parameters of the new shunts.  nominal current resistance (ω) number and  value of  resistors type of  resistors pcb type 30 ma 20 3x 60 ω  z201 ro4350b 100 ma 6 10x 60 ω  3x s102c 7x s102k  ro4350b 300 ma 2 30x 60 ω  10x s102c 20x s102k  ro4350b 1 a 0.6 50x 30 ω  17x s102c 33x s102k  ro4350b 10 a 0.06 100x 6 ω  33x s102c 67x s102k  fr4 acta imeko | www.imeko.org  september 2015 | volume 4 | number 3 | 63  dimensions should be kept as small as possible to obtain low frequency dependence of the shunt transimpedance. the dimensions of the 30 ma shunt were slightly reduced to be the same as for the 100 ma shunt. also the number of the 30 ma shunt crossbars was minimized to only three and the 30 ma shunt construction was reinforced using distance struts. the dimensions of the 100 ma, 300 ma and 1 a shunts were not modified. the length of the crossbars was reduced in all shunts. the 10 a shunt was constructed using low frequency pcb (fr4) because of the inconsiderable influence of its dielectric properties to the calculated values. only the thickness of the fr4 was reduced to 1.5 mm. dimensions of the 10 a shunt were reduced to be the same as the original 1 a shunt. the length of the crossbars was also reduced to be as short as possible. resistors mounted in the shunts are the same type as in previous construction because no other resistors with lower inductance were found. including all of these modifications into the model showed a reduction in the ac-dc difference below 3.5 ppm at frequencies up to 100 khz (see table 5). 4. comparison of calculated and measured values  of the new shunts  the ac-dc difference of the shunts was measured using an automated measurement system developed for the ac-dc current transfer difference set up (see figure 11), which was established in 2008 [6] and improved in 2010 [8]. a reference and a calibrated standard are connected in series. ac and dc currents are applied through a transconductance amplifier clarke&hess 8100, which is alternately connected to a dc and ac voltage source (multifunction calibrators fluke or datron) using an automated switch ofmet. the standards are comprised of a shunt loaded by a thermal converter [8]. the uncertainty budget of such a complicated measurement set up covers contributions of the reference standard and its level dependence, the reproducibility of the measured ac-dc difference (standard deviation), influences of the series connection of both standards and measurement set up, the frequency dependence, and the influence of temperature. a detailed description of the uncertainty calculation can be found in [8]. during the measurements of the new generation of the shunts the output of the shunts was loaded by a 90 ω/10 ma pmjtc, which was included in the model. table 5 shows the agreement between the measured and calculated values of the ac-dc differences of the new shunts to be better than 6 ppm at frequencies up to 100 khz. table 6 shows the comparison of the measured ac-dc differences of the original and new shunts. for all shunts a significant reduction of the ac-dc difference was accomplished. the most significant reduction of the ac-dc difference was achieved for the 30 ma shunt, which construction was modified most of all shunts (see section 3.2). following the sensitivity coefficients calculation (see table 2) the 30 ma shunt construction is also most sensitive to the modification of the input quantities in comparison with the other shunts. because of the significant modification of the 30 ma shunt construction and its high sensitivity to these modification its acfigure  11.  the  measurement  system  of  the  ac‐dc  current  transfer  difference.  table 5. calculated and measured values of the ac‐dc difference for the new shunts with associated uncertainties for k=1.  nominal  current  value  ac‐dc difference  frequency 500 hz 1 khz 10 khz 20 khz  50 khz  100 khz 30 ma  20 ω  calculated (ppm)  0.016 0.032 0.320 0.650  1.700  3.400     unc. (ppm)  0.005 0.010 0.096 0.190  0.490  0.980     measured (ppm)  ‐0.4 ‐0.5 1.5 2.9 4.7  7.0     unc. (ppm)  4.4 4.4 4.4 4.4 4.4  4.7 100 ma  6 ω  calculated (ppm)  0.011 0.022 0.022 0.440  1.110  2.240     unc. (ppm)  0.003 0.007 0.067 0.130  0.340  0.680     measured (ppm)  ‐0.9 ‐1.7 0.7 3.4 4.4  6.4     unc. (ppm)  6.4 6.4 6.4 6.5 6.7  7.1 300 ma  2 ω  calculated (ppm)  0.004 0.008 0.075 0.150  0.350  0.620     unc. (ppm)  0.001 0.002 0.023 0.047  0.120  0.230     measured (ppm)  0.9 1.1 1.1 1.9 4.3  5.5     unc. (ppm)  7.9 7.9 7.9 8.0 8.4  8.9 1 a  0.6 ω  calculated (ppm)  0.003 0.006 0.051 0.073  ‐0.039  ‐0.810     unc. (ppm)  0.001 0.002 0.020 0.039  0.098  0.210     measured (ppm)  ‐1.4 ‐0.8 ‐1.5 0.5 1.3  ‐0.3     unc. (ppm)  9.2 9.2 9.2 9.3 9.8  10.4 10 a  0.06 ω  calculated (ppm)  0.003 0.006 0.033 0.004  ‐0.460  ‐2.500     unc. (ppm)  0.000 0.001 0.005 0.013  0.067  0.270     measured (ppm)  0.7 1.5 ‐2.3 ‐1.2  4.0  3.2 unc. (ppm)  11.3  11.3  11.3  11.4  12.1  12.8  acta imeko | www.imeko.org  september 2015 | volume 4 | number 3 | 64  dc difference was decreased by more than 100 ppm. 5. conclusions  a new generation of the cage-type current shunts from 30 ma up to 10 a was developed. construction improvements were done based on the results obtained from the analysis of the lumped element model of the original shunts. comparison of the calculated and measured values of the new shunts showed an agreement better than 6 ppm in the acdc difference at frequencies up to 100 khz. future work will address the comparison of the calculated and measured phase angle errors of the new shunts and the development of high current shunts up to 100 a. references  [1] garcocz, m., scheibenreiter, p., waldmann, w., heine, g., “expanding the measurement capability for ac-dc current transfer at bev”, 2004 digest of cpem conf, june 2004, pp.461,462. [2] filipski, p.s., boecker, m., “ac-dc current shunts and system for extended current and frequency ranges”, ieee trans. on instrum. and meas., vol.55, no.4, aug. 2006, pp.1222-1227. [3] voljc, b., lindic, m., lapuh, r., “direct measurement of ac current by measuring the voltage drop on the coaxial current shunt”, ieee trans. on instrum. and meas., vol.58, no.4, april 2009, pp.863-867. [4] voljc, b., lindic, m., pinter, b., kokalj, m., svetik, z., lapuh, r., “evaluation of a 100 a current shunt for the direct measurement of ac current”, ieee trans. on instrum. and meas., vol.62, no.6, june 2013, pp.1675-1680. [5] lind, k., sorsdal, t., slinde, h., “design, modeling, and verification of high-performance ac-dc current shunts from inexpensive components”, ieee trans. on instrum. and meas., vol. 57, no. 1, jan. 2008, pp. 176-181. [6] zachovalová, v. n., “ac-dc current transfer difference in cmi”, 2008 digest of cpem conf., 2008, pp. 362-363. [7] pogliano, u., bosco, g. c., serazio, d., “coaxial shunts as ac– dc transfer standards of current”, ieee trans. on instrum. and meas., vol. 58, no.4, 2009, p. 872-877. [8] zachovalová, v.n., šira, m., streit, j., “current and frequency range extension of ac-dc current tranfer difference measurement system at cmi”, 2010 digest of cpem conf., 2010, pp.605,606. [9] zachovalová, v.n., “on the current shunts modeling”, ieee trans. on instrum. and meas., vol.63, no.6, june 2014, pp.16201627. table 6. comparison of the measured ac‐dc difference values (ppm) of the original and new shunts generation. nominal  current  generation  value  frequency 500 hz 1 khz 10 khz 20 khz  50 khz  100 khz 30 ma  new  20 ω  0 ‐1 2 3 5  7   original  50 ω  ‐1 0 8 20 57  131 100 ma  new  6 ω  ‐1 ‐2 1 3 4  6   original  10 ω  1 ‐1 4 9 20  39 300 ma  new  2 ω  1 1 1 2 4  6   original  3.3 ω  0 0 4 7 13  22 1 a  new  0.6 ω  ‐1 ‐1 ‐1 1 1  0   original  1 ω  1 ‐3 ‐2 2 6  12 10 a  new  0.06 ω  1 1 ‐2 ‐1 4  3   original  0.1 ω  1 ‐1 ‐2 ‐3 ‐4  ‐23 microsoft word article 10 164-1307-1-le.docx acta imeko  february 2015, volume 4, number 1, 61 – 67  www.imeko.org    acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 61  identification of dominant error parameters in spectra  measured by a tdemi system  miroslav kamenský, karol kováč  slovak university of technology in bratislava, faculty of electrical engineering and information technology, institute of electrical  engineering, ilkovičova 3, 81219 bratislava, slovak republic      section: research paper   keywords: multiresolution quantization, time domain emi measurement, offset and slope errors, spectrum measurement, error identification  citation: miroslav kamenský, karol kováč, identification of dominant error parameters in spectra measured by a tdemi system, acta imeko, vol. 4, no. 1,  article 10, february 2015, identifier: imeko‐acta‐04 (2015)‐01‐10  editor: paolo carbone, university of perugia   received december 16 th , 2013; in final form november 15 th , 2014; published february 2015  copyright: © 2014 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: this work was supported by the slovak research and development agency under grant no. apvv‐0333‐11 and by the slovak grant agency vega  under grant no. 1/0963/12  corresponding author: miroslav kamenský, e‐mail: miroslav.kamensky@stuba.sk    1. introduction  special measuring systems are being developed for different areas of industry improving quality in many aspects. today we often require an automated and fast measuring procedure alongside with high accuracy. measuring of the electromagnetic interference (emi) spectra has been obviously a time consuming procedure. conventional analog emi receivers are based on a superheterodyne principle. here, only one narrow frequency band is transferred to the detector at a time via an intermediate frequency amplifier. therefore timeconsuming sweeping through the whole measured frequency range is needed and several tens of minutes are often required to complete the whole emi spectrum measurement. the advantage of such an arrangement is the high dynamic range of the spectrum measurement required by emi standards. for the commercial production of electronic devices, emi measurements are inevitable but time consuming. automation helps to optimize a measurement process [1] and offers some reduction in the production time. however, it is still limited by the superheterodyne principle of traditional receivers. the introduction of new faster emi measuring principles would bring significant benefits in time to market and costs and could probably support the idea of just in time production [2]. the time domain emi (tdemi) system [3] was introduced quite recently. it is based on a multiresolution analog-to-digital converter (mradc) technology, which engages several parallel input channels to achieve the figure 1. block structure of a tdemi system.   signal power splitter ... ch.x: lim., ampl., adc digital signal processing spectrum ch.1: atten., adc ch.2: lim., ampl., adc abstract  multiresolution  analog‐to‐digital  converters  (mradc)  are  usually  used  in  time  domain  electromagnetic  interference  (tdemi)  measuring systems for very fast signal sampling with a sufficient dynamic range. the properties of the spectrum measured by the  tdemi system influenced by imperfections in the mradc are analyzed in this paper. errors are caused by imperfect matching of the  offset and gain and phase of the circuits used in parallel input channels typical for the mradc. for deep analyses of mradc behavior,  a precise mathematical model has been created using the concept of additive error pulses. furthermore, a dedicated process of the  identification  of  discrepancy  parameters  from  experimental  data  is  proposed.  identified  parameters  enter  the  expressions  of  the  model and enable side to side comparison of experimental and theoretical results. acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 62  required quality of the system. the block structure of a tdemi device is depicted in figure 1. the power splitter distributes the analog signal to all paths and 3 channels are common at present. all adcs are very fast sampling ones of the same type and range. separate amplifiers/attenuators provide different signal ranges and voltage resolutions of individual channels. channel 1 covers the entire range. the range of the next channel is part of the previous one and therefore the input signal will likely exceed the range of subsequent channels. here, the limiter protects the adc input from overvoltage. all channels are simultaneously sampled and converted by identical 8 or 10 bit very fast flash adcs. the final discrete value is created by extracting the output from that adc offering the best resolution but with the range still covering the actual input value. short time fast fourier transform (fft) is finally applied to the sampled data [4] to obtain one representation of the whole frequency spectrum. tdemi devices process the complete spectral content at a time and the parallel structure of the mradc should still allow a high dynamic range of measurements [5]. some influence of the quantization process on the measured spectra [6] remains also for the mradc [7]. however practical experience shows that there are more serious error sources in real systems [8]. it is hardly possible to avoid the offset or phase shift between parallel input channels likewise the gain error differences. similar problems occur in adc systems with time interleaving or with reconfiguration structure [9] where offset or gain mismatch of parallel channels degrades the overall performance. so serious signal discontinuities could arise in points where the system switches between channels. spurious spectrum components generated by those discontinuities significantly restrict the spurious free dynamic range (sfdr) of a real tdemi device. the paper deals with identification of tdemi device errors and with the design of a precise error model. imperfections of a real tdemi system will be explained in chapter 2. a pulse model of the dominant error of spectrum measurements will be discussed in chapter 3. in the next chapter the identification process of channel discrepancy will be presented which offers finding model parameters. finally we will be able to compare the proposed error model with experimental results. 2. imperfections of real tdemi system  the concept of an mradc lies in the use of several parallel adcs each with uniform but different quantization steps. the minimum quantization step for the multiresolution quantization corresponds to the step of the channel with the lowest range. actually, the system range is divided into subranges with different quantization steps. even if perfectly realized such system generates disturbance in the quantized spectrum, as was shown in [7] for a harmonic signal. in the experimental waveform obtained from the tdemi device output and plotted in figure 2a we can observe that in distant parts from the zero axis there is a significant quantization error. however, in a real system there are more serious error sources. the main task of the analog input circuits is to split the input signal into multiple paths with precisely set gains. this is a non-trivial task because ideally a uniform amplification level should be achieved for the frequency range up to several ghz without introducing a different phase shift between channels. channel mismatch compensation techniques have been developed especially for time interleaved adcs [10]. on the other hand, publications describing properties of tdemi measurement systems are dealing mostly with a perfectly matched power splitter and a digital processing module. in a real system amplitude and phase frequency characteristics in each channel are not perfectly matched over the whole frequency range. moreover it is hardly possible to avoid the offset and slope difference between channels as the gain must be different in each channel. in the example of the time representation of the signal (figure 2a, frequency 175 khz and amplitude 0.128 v) such discontinuities are especially evident in the negative portion of the waveform around the threshold of ca. 60 mv between the upper two channels of a three-channel mradc. error components resulting from those discontinuities significantly restrict the sfdr of the spectrum measurement. we will demonstrate that channel discrepancy is the main contribution to spurious figure 2. experimental harmonic signal measured by a real tdemi system:  a) time representation inside the tdemi unit; b) measured spectrum.  acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 63  components visible in the measured spectrum depicted in figure 2b obtained for the same harmonic input signal. experiments suggest that the main spurious components of tdemi measurement results are caused by discontinuities in the time domain signal representation. an harmonic input signal may be considered as suitable for modeling the measured interference of devices operating on the switched mode power supply principle, where the disturbance is like a mixture of sinusoids. we will assume a two-channel mradc as the major influence seems to be the mismatch between the upper two channels. differences between channels result in disturbances similar to timedomain error pulses (and not only for the harmonic input) like sketched in figure 3. discontinuities present in the waveform reconstructed from the sequence of samples obtained by the mradc could be modeled as an additive impulse error signal. then, the spectrum of the impulse disturbance could be used to precisely model the resulting spurious spectral components. this is the fundamental assumption used in our analysis. two rectangular pulses represent the offset between channels while two cosine shaped pulses describe the gain and phase discrepancy. 3. analytical expression of error models  the proposed idea of pulse representation of errors can be utilized in the simulation model or for the evaluation of the analytical model. two error pulses of the model per period of the input signal enable precise estimation of the real error if proper shapes of pulse-forming waveforms are used. for an harmonic input signal the harmonic waveform (yellow “cosine original” in figure 3) with the right offset, gain and phase composes pulses for such accurate simulation model. compared to a real system the model neglects quantization errors, however, it is precise for the estimation of the distortion caused by the discrepancy between channels and can be used as a reference for other approximative models. in a tdemi system the measured spectrum is digitally evaluated from samples. digital systems calculate the spectrum by discrete fourier transform (dft) usually using an fft algorithm. dft output approximates coefficients of fourier series, which decomposes the given periodic function u(t) with frequency f0 into the sum of harmonic functions. therefore, speaking about spectrum, we are trying to find coefficients un = an-jbn (n=0,1,2; b0=0) of the fourier series. the error model is composed of two rectangular and two cosine pulses and at first we need to find the spectral components of all the pulses. rectangular pulses describe the offset between channels, i.e. the offset of the erroneous cosine original (the offset of the yellow waveform in figure 3). the zero coefficient of one rectangular pulse is    rf0fr00,rp 2 ,,, tt a ttaa     (1) while for n>0 we can write       r0f0fr0rp, sinsin 2 ,,, tntn n a ttaa n    , (2)       r0f0fr0rp, coscos 2 ,,, tntn n a ttab n    , (3) where 0 is the angular frequency of pulses, a is the amplitude of the rectangular pulse, tr and tf is the time of rising and falling edge. cosine pulses of the model involve the gain and phase discrepancy. spectral components of one cosine pulse are           r0f0fr00,cp sinsin 2 ,,,, tt a ttaa (4)                    r0f0 rf0 fr0cp,1 2sin2sin 8 cos 4 ,,,, tt a tta ttaa (5)                    r0f0 rf0 fr0cp,1 2cos2cos 8 sin 4 ,,,, tt a tta ttab (6) and for n>1                    ,,spc,,cps 12 ,,spc,,cps 12 ,,,, r0nr0n2 f0nf0n2 fr0,cp ttn n a ttn n a ttaa n       (7)                    ,,sps,,cpc 12 ,,sps,,cpc 12 ,,,, r0nr0n2 f0nf0n2 fr0,cp ttn n a ttn n a ttab n        (8) in this case, besides the frequency, times of edges and the amplitude of the cosine original a also its phase  enters the argument of expressions. for n>1 the formulas are more complicated. therefore we have defined auxiliary figure 3. basic idea of the pulse error model.  table 1. auxiliary functions.  name formula cpsn(,t,)    tnt  sincos  spcn(,t,)    tnt  cossin  cpcn(,t,)    tnt  coscos  spsn(,t,)    tnt  sinsin  ++ = original signal rectangular pulses cosine pulses error error t u tr,1 tr,2tf,1 tf,2 channel 1 channel 2 acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 64  functions (see table 1) which have allowed those simplified notations (7)(8). to write the final expression of spectral error components we follow the idea of two actual error pulses mathematically represented by two pairs of pulses. the time of the rising and falling edge is tr,1, tf,1 for the first error pulse and tr,2, tf,2 for the second. both error pulses are described by the sum of the rectangular pulse and the cosine pulse to cover offset, gain and phase error. considering that an is the real part and -jbn the imaginary part of the spectral coefficient un (use index rp and cp for rectangular and cosine pulse) the error model states          co2f2rco0cp, co1f1rco0,cp 2f2ro0rp, 1f1ro0,rp co1f1rcoo0e, ,,,, ,,,, ,,, ,,, ,,,,,      tta tta tta tta ttaa n n n n n u u u u u     (9) the amplitude ao of rectangular pulses is just the offset of channel 2 towards channel 1. the amplitude and phase of cosine pulses are taken from the erroneous cosine original and could be calculated using phasor rules. if the reference channel 1 has ideal parameters (zero phase and gain of one) the ideal output waveform is a1cos(0t) or a10 in the phasor notation. the amplitude a1 is the amplitude of the input signal or rather the amplitude of the signal obtained solely from channel 1. then the phasor of the erroneous cosine original is 2211coco 0   gaaa (10) where g2 is the gain of channel 2 and 2 is its phase shift. we can make some further simplifications in the proposed model (9). as we assume ideal parameters of channel 1 and if the switching between channels would ideally work according to channel 1, the rectangular pulses in equation (9) will be the same. then they could be replaced by one rectangular pulse signal with double frequency for even n and totally omitted for odd n [8]. in other words, we could expect that the offset discrepancy will impact only even error harmonics. on the other hand if the offset is the dominant mismatch parameter a simplified model can be used with only rectangular pulses (rec model in [8]). then, the amplitude ao should be calculated as mean of the samples within the real error pulse and separately for each rectangular pulse if the real error pulses are not perfectly square. 4. error identification  to be able to apply the proposed model to experimental data we need to extract main error parameters from the measured waveform (figure 2a). the parameters requested by the model are the precise signal frequency f0, the real signal amplitude a1, the offset between channels ao, the gain error described by gain g2 of the second channel and the phase shift between channels 2. also times of rising and falling edges of both error pulses tr,1, tf,1, tr,2, tf,2 should be estimated from the experimental data. to obtain precise results and a repeatable formal process of the estimation we proposed a special procedure of the error identification. it can be summarized into three main steps: 1. identification of edges; 2. curve fitting; 3. determination of parameters of the error model. 4.1. identification of edges  moments of error pulse edges are points where the system switches from one channel to another. from figure 2 it is obvious that noise present in the sampled waveform is a good indication of the channel. unfortunately, we are able to obtain only a screenshot from the commercial tdemi unit. therefore we needed at first to identify data from the original bitmap. our method based on colour shades provided data drawn black in figure 4a. the noise was slightly suppressed here by the data extraction algorithm. however, the difference between channels is still apparent. so, the identification of edges is based on signal noise and is done in the following steps. 1. noise identification to extract noise from the measured signal we smoothed the waveform using moving average (dark gray line in figure 4a). subsequently we separated the noise by subtracting the smoothed waveform from its original. the noise itself is depicted light gray in figure 4a or in figure 4b at a better scale. 2. noise thresholding parts of the noise data of low and high variance relate to the currently used input channel. to identify moments of channel change we need to find points of the variance change. the amount of noise in the actual time point was estimated in figure 4a from the moving root mean squared value (mrmsn, turquoise solid curve). finally by thresholding (with 0.5 mv) we identified edges depicted by turquoise dashed verticals. 3. edge times adjustment edges found by mrmsn thresholding are visibly shifted away from the exact points of noise variance changes. this follows from the nature of squaring which favors higher values. therefore we continued in searching of precise edge times in the vicinity of previously found approximate edges. this time we will use the moving average from the absolute noise values (maan, blue solid curve in figure 4a and figure 4b). maan does not offer as good distances for thresholding as mrmsn. however, in points of exact edges local extremes are located. indeed there is a good chance that exactly in the point of channel change a significant noise value occurs due to the jump in the final waveform caused by the channel discrepancy. to increase the impact of the actual noise value we used a weighted averaging for the maan evaluation. sometimes a special process of final edge discrimination could be needed if some fake edges were also found. in both figure 4a and figure 4b the precise adjusted edges are plotted as blue dashed verticals. 4.2. curve fitting  after identification of edges we are able to separate parts of the waveform samples corresponding to the first or second channel. (at this point we also set the time of the acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 65  first sample to zero.) the basic function for fitting is cosine and we want to identify four parameters. the frequency should be the same for both channels while the amplitude, offset and phase are independent. 1. channel data separation points of separation are the previously adjusted edge moments. in some cases there are fluctuations near the switching point. therefore it is possible and often suitable to omit a few samples from each side of the given edge. we also need to set the channel order: channel 1 data are green and channel 2 data are blue. 2. setting initial conditions nonlinear curve fitting methods require initial data for starting the numeric algorithm. the main input parameter here is a starting frequency which might be the set frequency of the test signal. knowing the frequency the other starting parameters (amplitude, offset, phase) could be calculated by linear fitting applied on the whole data set. 3. nonlinear fitting to identify the frequency of the cosine original we need to use a nonlinear curve fitting algorithm. data from channel 2 contain less noise, therefore we considered the channel 2 output as more suitable for determination of the frequency. we recommend a least squares method for the fitting and we used unweighted non-robust least squares implemented within matlab statistic library functions (gauss-newton algorithm, [11]). from the procedure also the amplitude, offset and phase of the signal from channel 2 are obtained. the cosine fitted to the channel 2 data is depicted as turquoise solid waveform in figure 5. 4. linear fitting if we already know the frequency of the cosine signal being identified a simple linear curve fitting algorithm can be used. the amplitude, offset and phase of the cosine signal approximating channel 1 data could be found from the solution of a pertinent linear system of equations using matrices and matlab. in figure 5 this fitted function is depicted as a light green solid waveform. 4.3. determination of parameters of the error model  identified parameters should be further accommodated for the error model. note that we want to use the channel 1 signal as the reference and therefore e.g. the time values have to be adjusted in the way that we force the zero offset and phase of this channel. discrepancy parameters of channel 2 will be related to channel 1. 1. parameters of input reference signal the frequency identified during the curve fitting process is simply the frequency f0 of the input signal which enters the error model. in our case we calculated f0 = 174.832 khz. as channel 1 is the reference one, amplitude a1 = 0.1079 v identified from this channel will be assigned also to the input or ideal output signal. 2. parameters of channel discrepancy by comparison of both channels we can calculate for channel 2: the offset ao = -6.9920 mv, its gain g2 = 1.2078 and the phase 2 = 3.0647. 3. uniform set of edge times from previously identified edge moments we need to solve relative times toward the start of a given period of the reference signal. the same edge could be identified several times if more input periods are contained in a test interval. four edge times per period are expected for the cosine input signal and from figure 4b one can notice that the first figure 4.  detection  of  times  of  edges.  a)  original  and  smoothed  mradc output  signal  with  detected  switching  points;  b)  noise  data  used  for thresholding and adjusting of edges.  figure 5. curve fitting performed separately for the data from the first and  second channel.  0 2 4 6 8 10 -0.1 -0.05 0 0.05 0.1 0.15 t (s) u (v) a) 0 2 4 6 8 10 -0.01 -0.005 0 0.005 0.01 t (s) u (v) b) 0 2 4 6 8 10 -0.1 -0.05 0 0.05 0.1 0.15 t (s) u (v) acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 66  three are identified two times. in that case we can calculate the final time of a given edge as a simple mean of all corresponding relative edge times. our final uniform set of four edges is: tr,1 = 1.1795 s, tf,1 = 1.8320 s, tr,2 = 3.7667 s, tf,2 = 4.5713 s. 5. results and discussion  parameters of the input signal, the channel discrepancy and switching times were used in the simulation as well as in the analytical model. in the time domain we used the simulation model. error pulses obtained from the model for two periods of the input signal are depicted in red in figure 6. the mradc output signal disturbed by those pulses is in dark gray. the light gray dots in the background are original experimental data only slightly shifted to impose ideality of channel 1. the model waveform visibly fits the experimental data in the time range where they are available. blue dashed verticals represent all 7 edges identified from experiments and adjusted for the used point of zero time. in the simulation model we used one set of four edges for both periods. therefore e.g. the time of the second edge of red error pulses (tf,1 = 1.8320 s) is the mean from time shifts of the second and sixth blue vertical from the start of the given period. after the identification of parameters entering the pulse model and after the verification in the time domain we are ready to confront model results with the experimental spectrum. error spectral components evaluated from the analytical pulse model (9) are depicted in figure 7a as red circles. only the gray “x” represents the measured component i.e. the amplitude of the input harmonic signal. the simulation error spectrum (gray lines with dot markers) is almost identical with results of analytical expressions. black circles are original experimental error spectral components (from figure 2b). apparently there is a visible similarity between the model and experimental results. we can observe a little shift of experimental points from the theoretical ones, however the overall trend or the shape of fluctuations is satisfactory estimated by theoretical expressions. the correlation coefficient is as high as 0.9130 in the compared range of approximately one frequency decade. note that if we used only rectangular pulses in the analytical model the correlation was 0.6928. if we compare experimental results to the own line fit (black solid line), the correlation coefficient is 0.8188. so the overall decay of the error spectrum is not the whole similarity and the number 0.9138 embraces also the similarity in fluctuations. to compare only the shape of fluctuations we decided to subtract the overall decay from spectral components. in figure 7a the lines fitted trough the points are drawn as black or red solid lines for experimental or model results. subsequently in figure 7b only differences from this overall decay are depicted. experimental data are in black here while the model results are red again. the similarity is more visible now and the calculated correlation coefficient of 0.7614 is not influenced by the decay of spectral components with rising frequency. 6. conclusions  the precise pulse error model for the estimation of error spectral components disturbing measurements performed by a time domain electromagnetic interference (tdemi) measuring system was presented in the paper. the analytical model was expressed for a two channel system and the harmonic input signal. furthermore a consistent process of the error parameters identification from experimental data has been proposed based on the signal figure 6. time representation of simulated error pulse signal and distorted mradc output waveform.  figure 7.  comparison  of  experimental  error  spectrum  and  error  components  obtained  from  the  pulse  model:  a)  experimental,  simulation  and theoretical spectra; b) differences from the line fit.  0 2 4 6 8 10 -0.1 -0.05 0 0.05 0.1 0.15 t (s) u (v) 10 0 10 1 30 40 50 60 70 80 90 100 f (mhz) |a| (dbv) a) 10 0 -10 -5 0 5 10 f (mhz) |a| (dbv) b) acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 67  noise. the comparison of model and experimental results has shown that the channel discrepancy represented by the analytical model has a dominant impact on error components occurring in the measured spectrum. shape changes in the experimental spurious components were adequately reproduced by the model while the overall decay was similar too. differences and little shift between compared values could be explained by simplifications used in the model, a limited interval of experimental timedomain data processed during the identification phase, windowing and asynchronous sampling in a real tdemi unit, etc. the designed model could be useful for the further development and improvements of the technology behind tdemi. it could help to find right switching points for expected type of the input signal or indicate critical conditions for the measurement method of tdemi. the proposed process of the error parameter identification helps to find realistic values and intervals of parameters describing the discrepancy. the estimation of the error behaviour is a necessary step towards future searching for methods of error correction. acknowledgement  work presented in this paper was supported by the slovak research and development agency under grant no. apvv-0333-11 and by the slovak grant agency vega under grant no. 1/0963/12. references  [1] o. čičáková, p. štefanička, efficiency of automated measurement systems, measurement science review, 6 (4) (2006) pp. 50-53. [2] b. moric milovanovic, b. sisek, m. kolakovic, just in time concept as a mean for achieving competitive advantage in the virtual economy, proceedings of 22th international daaam symposium, daaam international vienna, austria, 2011, pp. 1105-1106. [3] s. braun, p.russer, the dynamic range of a time-domain emi measurement system using several parallel analog to digital converters, proceedings of the 16th international zurich symposium on electromagnetic compatibility, swiss federal institute of technology zurich, switzerland, 2005, pp. 203-208. [4] f. krug, d. mueller, p. russer, signal processing strategies with the tdemi measurement system, ieee transactions on instrumentation and measurement, 53 (5) 2004, pp. 14021408. [5] s. braun, p. russer, a low-noise multiresolution highdynamic ultra-broad-band time-domain emi measurement system, ieee trans. on microwave theory and techniques, 53 (11), 2005, pp. 33543363. [6] t.a.c.m. claasen, a. jongepier, model for the power spectral density of quantization noise, ieee transactions on acoustics, speech and signal processing, assp-29 (4), 1981, pp. 914-917. [7] m. kamenský, k. kováč, g. války, improvement in spectral properties of quantization noise of harmonic signal using multiresolution quantization, ieee transactions on instrumentation and measurement, 61 (11), 2012, pp. 28882895. [8] m. kamenský, k. kováč, g. války, model of errors caused by discrepancies of input channels in multiresolution adc. imeko 2013 : tc-4 symposium on measurements of electrical quantities and 17th ieadc workshop on advances in instrumentation and sensors interoperability, barcelona, spain, 2013, pp. 178-183. [9] w. carvajal, w. van noije, an optimization-based reconfigurable design for a 6-bit 11-mhz parallel pipeline adc with double-sampling s&h, international journal of reconfigurable computing, article id 786205, 2012, 17 pages. [10] h.a. nawaz, a. sharif, m. sharif, comparative survey on time interleaved analog to digital converter mismatches compensation techniques, research journal of recent sciences, 2 (9), 2013, pp. 95-100. [11] h. wen, j. ma, m. zhang, g. ma, the comparison research of nonlinear curve fitting in matlab and labview. 2012 ieee symposium on electrical & electronics engineering (eeesym), kuala lumpur, malaysia, 2012, pp. 74-77. effects of saliva on additive manufacturing materials for dentistry applications: experimental research using flexural strength analysis acta imeko issn: 2221-870x june 2023, volume 12, number 2, 1 6 acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 1 effects of saliva on additive manufacturing materials for dentistry applications: experimental research using flexural strength analysis tommaso zara1, lorenzo capponi2, francesca masciotti3, stefano pagano3, roberto marsili4, gianluca rossi4 1 cisas g. colombo, university of padova, via venezia 15, 35131 padova, italy 2 department of aerospace engineering, university of illinois at urbana-champaign, 104 s. wright st., 61801 urbana, illinois, usa 3 odontostomatological university centre, department of medicine and surgery, university of perugia, perugia, italy 4 department of engineering, university of perugia, via g. duranti 93, 06122 perugia, italy section: research paper keywords: additive manufacturing; dentistry; flexural strength; polymers; 3d printing citation: tommaso zara, lorenzo capponi, francesca masciotti, stefano pagano, roberto marsili, gianluca rossi, effects of saliva on additive manufacturing materials for dentistry applications: experimental research using flexural strength analysis, acta imeko, vol. 12, no. 2, article 14, june 2023, identifier: imeko-acta-12 (2023)-02-14 section editor: laura fabbiano, politecnico di bari, italy received march 18, 2023; in final form april 13, 2023; published june 2023 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: lorenzo capponi, e-mail: lcapponi@illinois.edu 1. introduction in orthodontics, the accurate depicting of a patient's dentition is essential to properly identify and diagnose malocclusions, to plan out the course of action, to aid with assessments, and to track the effectiveness of medical treatments [1]. for this purpose, in the dentistry field, impression materials are typically used to accurately replicate the shape of the oral structures (including teeth), creating a negative form to generate the cast mold [1]. in the early days of dentistry, materials known as impression compounds were commonly used to make casts of the oral cavity [1]. however, these materials were rigid and did not allow for the creation of accurate casts that could reproduce all the tissues of the oral cavity [2]. as a result, there was a growing need for materials that could remain elastic even after setting. in the mid-20th century, the use of hydrocolloids became widespread due to their ability to provide the required elasticity and to enable the creation of accurate casts of undercuts [3]. over time, other materials, such as pvs have been introduced, which exhibit less shrinkage over time compared to hydrocolloids, and these advancements have greatly improved the accuracy and reliability of dental impressions [4]. in the late 1990s, the most used impression materials in dentistry became addition reaction silicone, polyether, and reversible hydrocolloid [4], [5]. the ideal impression material should possess certain characteristics that are suitable for use in the clinical environment. these materials must be adaptable to the oral structures, be resistant to tearing, and be easily removable without causing trauma to the surrounding tissues [3]. the material's physical and mechanical properties must also be carefully considered to ensure suitability for the clinical application and can provide optimal precision and marginal adaptation [3], [4], [6]. moreover, the ability of the material to abstract accurate and reliable results in orthodontics heavily depend on selecting the right impression materials. with the rise of digital technology and additive manufacturing techniques, it has become necessary to characterize experimentally the materials used to design prosthetic bases. in this study, the mechanical properties of polyetheretherketone, nylon6, nylon12, and polypropylene are analyzed, as impression materials commonly used in dentistry applications. specifically, the effect on their flexural elastic modulus of the exposure to working environment conditions is also investigated by means of 3-point bending test performed on virgin materials and samples immersed in saliva for 72 hours. the proposed approach revealed significant behavior in terms of loss in mechanical performances. these findings have significant implications for the proper selection and use of am materials in dental applications. file://///fs.isb.pad.ptb.de/home/imeko/acta%20imeko%20papers/vol%2012,%20no%202%20(2023)/1_pre-formatted%20copies/lcapponi@illinois.edu acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 2 flow and adapt to the tooth structure is crucial for the creation of a quality restoration. therefore, the impression material must possess good fluidity and moisture tolerance to effectively slip and imprint itself into the tooth structure that needs to be copied [3]. in fact, the marginal precision of an in-vitro dental restoration is typically 50 µm [4]. in summary, the selection of the appropriate impression material is essential to achieving accurate and reliable results in dentistry. although impressions are widely used in dentistry due to their simplicity, ease of use, and cost-effectiveness, during the mixing stage, air bubbles may be incorporated, leading to inaccuracies in the model. furthermore, the typical impression process can be messy, and traces of material may remain on the patient's teeth, necessitating removal [2]. despite the development of new impression materials, it remains challenging to eliminate the possibility of human errors during each stage of the process [7]. additionally, the high fabrication time can impact the chairside or intraoperative period. plaster casts, which are commonly made from impressions, can be damaged and require storage space. patients may also need multiple models to observe the progress of treatment [7]. overall, while impressions have several advantages, they also have limitations and drawbacks that must be considered. because of this, the development of new materials and techniques continues to improve the accuracy and efficiency of the manufacturing, but careful attention must be paid to all stages of the process to achieve optimal results. with the advent of new technologies, it is now possible to obtain a 3d representation of oral structures and tissues using highresolution and high-fidelity scanners. this technology enables the storage of 3d files in digital format, providing the opportunity to reuse them when necessary [3], [4]. this alternative approach offers several advantages over traditional methods, including reduced risk of infection, improved patient comfort, and shorter chair-time. additionally, displaying the 3d images to patients during pre-treatment can simplify the therapy explanation and allow for greater patient involvement in the decision-making process [7]. intraoral scanners capture jaw and tooth geometries in digital format, which can then be processed using computer-aided design (cad) software. this software enables manipulation of the 3d data and facilitates the development of the final product through computer-aided manufacturing (cam). in the dentaltechnical field, cad/cam technology has the potential to bypass laboratory experience and allow for the fabrication of dental restorations directly at the chairside. this approach offers significant time savings and can result in one-appointment dental restorations [8]. in dentistry, there are two main cad/cam techniques: subtractive manufacturing (mm) and additive manufacturing (am). mm involves removing material from a block to produce a prosthetic, typically using metal such as cobalt-chromium (co-cr), zirconia (zro2), titanium (ti), or polymers such as methyl methacrylate (pmma) or polyetheretherketone (peek) [9]. this method provides the highest accuracy in prosthesis production and is commonly used in dentistry. am, on the other hand, is an industrial process that involves the production by adding layer-upon-layer based on a 3d model. this method allows for faster production and is suitable for highly customized manufacturing, with the ability to create irregular grooves, crannies, valleys, and bone-like morphology. as a result, am is ideal for individual dentistry implant [6], [10]. however, due to the inferior flexural strength of printed prostheses compared to mm, am is typically suggested for interim crown and fixed partial dentures to avoid extended mastication periods [9]. studies have shown that am can produce durable dental frameworks and can reduce material waste by up to 40 % compared to mm [10]. additionally, up to 98 % of the waste can be reused in future manufacturing processes, thereby reducing environmental impact [10]. furthermore, compared to milling technology, 3d printing offers a wider range of usable materials and more flexible available machines, making it an attractive technology for new applications in the dentistry field [11], [12]. of all the available techniques, the two most preferred additive manufacturing technologies in dentistry are fused deposition modelling (fdm) and selective laser sintering (sls) [12]. however, the use of metals in dental implants is not recommended [13]. despite their exceptional mechanical properties, the biocompatibility of metals is a known limitation. furthermore, materials such as stainlesssteel exhibit poor corrosion resistance in comparison to titanium, which can lead to allergic reactions. in addition to biocompatibility, aesthetic appearance is an important consideration in dental implant design [14]. for these reasons, polymers have emerged as a promising alternative to metals in this regard, offering improved aesthetics. to further enhance the properties of polymers, ceramic and fiber reinforcements have been incorporated into polymer matrices, resulting in polymer matrix composites. compared to metals, polymer composites have a significantly lower weight, which is one of their major advantages [14]. the elastic modulus of metals is higher than that of bones, which can lead to stress-shielding and failure if the mechanical load is not well-distributed along the adjacent tooth framework [14]. therefore, the use of polymer composites in dental implant design has the potential to offer improved biocompatibility, aesthetics, and mechanical properties [14]-[16]. the literature lacks extensive research where the properties of materials, generally used in am processes, are investigated in working environments for dentistry applications. in fact, saliva in dentistry is often used to estimate the loss of color from an aesthetic point of view, the increase of sample mass, rather than the number of bacteria settling the specimen. the evaluation of the performance in terms of elastic modulus before and after the material’s immersion is a new proposal in dentistry field. this research aims to extend the knowledge on properties and performances of polymers for prosthetic bases in dentistry. by means of flexural strength analysis, this study focuses on the experimental characterization of mechanical properties of four different polymers: polyetheretherketone (peek), nylon6, nylon12, and polypropylene (pp), which samples are obtained by fdm and sls manufacturing processes. as the digestion phase starts in a secreted-saliva environment, a comparison between flexural modules of materials immersed for 72 hours in an artificial-solution of saliva at an ambient temperature of 25 °c to virgin materials is also presented. on the long term, this work will enable the possibility to create customized 3d-printed prostheses directly in the dental office, improving results in orthodontics filed. 2. materials and methods in this research, the properties and the behaviour of peek, nylon6, nylon12, and pp materials are investigated. peek has high processability, and 3d printing makes it suitable for fabricating structures for the oral cavity. peek exhibits remarkable resistance to corrosion and can maintain its mechanical characteristics at temperatures as high as 120 °c. when used in a body fluid environment at 37 °c, acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 3 peek's wear is reduced, thereby minimizing the release of harmful particles, and decreasing the immune response. one of peek's main advantages is its young's modulus, which is similar to that of cortical bone (3-4 gpa) [17]. additionally, by incorporating other compounds, such as carbon fibers, it is possible to enhance the mechanical properties of peek [15]. nylon6 is a thermoplastic material widely used in various engineering applications, including automotive and textiles, due to its ease of processing and excellent mechanical properties [18]. the addition of fillers can further enhance its modulus and strength, resulting in a composite polymer. nylon6's biocompatibility can be attributed to the presence of amide groups in its chemical structure, which resemble the chemical structure of natural peptides. this similarity enables cells to disguise themselves from the immune system, thereby suppressing the host's reaction to a foreign body [19]. nylon12 finds application in various industrial fields. however, compared to nylon6, it has a lower melting point and mechanical strength. nevertheless, nylon12 exhibits excellent flexibility, high pressure resistance, low density, extreme temperature resistance, and thermal stability. moreover, nylon12 demonstrates outstanding impermeability, chemical resistance, and stability in humid environments, making it particularly suitable for water distribution systems that require precise dimensional accuracy [19], [20]. polypropylene is a commercial plastic with a low density and excellent mechanical, physical, and chemical properties. one of the most significant advantages of this material is its high temperature resistance [21]. in the biomedical industry, pp is also used for its hydrophilic characteristics, which promote cell adhesion through the cytokine response [22]. furthermore, the porosity of pp facilitates interaction between the host and the implant, allowing for earlier formation of capillaries [23]. as a preliminary study, where the focus of the research is the material property itself, a simplified geometry of the test samples is preferred over a complex and more realistic design. the specimens used in this study were realized with dimensions of 85 mm in height, 10 mm in width, and 2 mm in thickness, and they are shown in figure 1. the samples were provided by zare, and the printing settings used in the manufacturing process are not given in this paper. while the pp samples were realized using sls printing technique, all the other samples were realized through fdm. the technology behind sls is based on a laser beam that strikes a powdered material lying in a build plate and heats it to a temperature just below the melting point. once the first layer is completed, a rake drags a veil of material from the powder chamber to the build plate waiting for the execution of the successive layer, which has the resolution of 60 µm [24], [25]. even if during the sintering processes the powders remain partially melted resulting in porous internal structures and rough surface finishes, the mechanical properties of sls products are excellent. on the other hand, the fdm is one of the most used printers in the market due to the low-cost availability. a stepper motor driver let a thermoplastic filament to run across a hot nozzle capable to melt the material at a specific temperature previously set. usually, the nozzle can move along x, y and z axes and allows the material to deposit, creating the desired object layer by layer. temperature of the print head, speed of the driven stepper motor and the thickness (approximate accuracy 35–40 µm) of the layers are some of the parameters that can affect the printing quality, the printing-time, and the final costs. the surface roughness and the visible graduation layers are drawbacks of this technology [26]-[28]. a 3-point bending flexural strength test was selected for the material characterization, and the flexural modulus is picked for determining material properties degradation due to exposure to saliva. the flexural modulus, or bending modulus, is an intensive property that is obtained as the ratio of stress to strain in flexural deformation (astm d790). the flexural modulus is determined from the slope of a stress-strain curve produced by a flexural test, assuming a linear stress strain response. a schematic of a flexural modulus measurement setup is shown in figure 2. from figure 2, the flexural modulus is analytically defined as [28]: 𝐸f  =   𝐿3 𝐹 4 𝑤 𝑑 ℎ3 . (1) the experimental research was carried out in the laboratories of the university of perugia. for this study, a lloyo lr 30k was used. a pre-load, whose intensity depended on the specific material, a feed rate of 1.7 mm/min, defined in the iso 178 a, and a maximum deformation of 1 mm were the experimental conditions selected for the experiments (see figure 3). during each test, the stress 𝜎 and the strain 𝜖 measurements are collected, and the flexural modulus can be estimated in the region of linear elasticity by: 𝐸f  =   𝜎 𝜖 . (2) in this study, the flexural modulus is initially determined, by means of the procedure described above, for virgin materials. the second step consists of immersing the samples in an artificial saliva (saliva sintetica cts) in an environmental temperature of 25 °c for 72 hours, time required for the fluid absorption by polymer material [29]. in this way, the effect of the first digestion environment on the properties of materials is obtained. after the figure 1. tested samples materials and geometry. figure 2. flexural modulus measurement: 3-point bending test. acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 4 curing process, the flexural strength test is repeated, and the flexural modulus of samples exposed to saliva is measured and compared to the one obtained in the virgin condition. for the sake of repeatability, this procedure is applied to 3 specimens made of the same material, and, considering the 4 materials analysed, a total of 12 samples are tested for this research. figure 4 shows an example of the results of a flexural strength test measurement for a generic material (differences in behaviour are enlarged for the sake of comprehensiveness). solid lines in figure 4 are obtained as linear interpolation of actual points of measurement (scatter points). from the linear interpolation in the region of elastic behaviour, the flexural elastic modulus is obtained with (2) for each tested sample. in this way, for every material in both virgin and exposed-to-saliva conditions, a mean value of the flexural modulus is given. also, repeatability analysis was conducted for all tests performed, both on virgin samples and those immersed in saliva. 3. discussion an overview of the results obtained as described in section 2 is given in table 1. as expected, the results confirm that the exposure to the saliva degrades the properties of the material. peek shows a better performance in terms of flexural behaviour, with 1794.08 mpa. on the other hand, the nylon12 shows the lowest values, both from the virgin samples (977.86 mpa) and the samples immersed in saliva (921.83 mpa). figure 5 shows the flexural modulus loss for the analysed materials. while nylon6 shows the smallest loss in mechanical performances with a 2 % loss in flexural modulus, pp has the highest decrease among the specimens. the reason for this behaviour can be related to the am printing technology. in fact, as the sls increases the porosity of the sample surface, the ability to absorb liquids also increases [30]. similarly, peek shows a decrease in performance of only 3 %. this is also due to the intrinsic characteristic of the material, which is inherently hydrophobic, with a contact angle of 80-90 degrees [16]. the repeatability analysis, performed with a 2 𝜎 interval at the 95 % of confidence, demonstrate the goodness of the setup and the experiments. in fact, the analysis led to the lowest value of 0.25 %, and a highest of 3.83 %. the results obtained indicate how saliva has a concrete impact in the elastic modulus performance of each material. on a practical level, this study shows a first approach to the possible development of prostheses with complex geometries, printed with these materials using additive manufacturing technologies. this research can provide the means to predict the performance of these prostheses over time in a reconstruction of the daily work environment. 4. conclusions this paper presents a first attempt to broaden the knowledge of properties of materials used for 3d additive manufacturing in dentistry applications. in this sense, the properties of four materials used prosthetic bases in dentistry were studied in terms of flexural strength modulus: polyetheretherketone (peek), nylon6, nylon12, and polypropylene (pp). specifically, the figure 3. example of pp sample mounted on the lloyo flexural strength test machinery. figure 4. example of linear interpolation of measured data for a generic material (differences in behavior are enlarged for the sake of comprehensiveness). table 1. overview of the experimental results. virgin saliva test 1 in mpa test 2 in mpa test 3 in mpa mean in mpa repeatability test 1 in mpa test 2 in mpa test 3 in mpa mean in mpa repeatability nylon6 1360.93 1354.78 1365.72 1327.14 ± 43.50 3.79 % 1282.22 1318.96 1316.30 1305.83 ± 16.72 1.48 % nylon12 1003.62 963.43 966.51 977.85 ± 18.26 2.16 % 910.67 905.02 949.78 921.82 ± 19.90 2.49 % peek 1818.47 1767.71 1799.06 1795.08 ± 20.91 1.35 % 1842.56 1742.04 1734.48 1738.26 ± 3.78 0.25 % pp 1095.33 1070.30 1011.53 1059.05 ± 35.12 3.83 % 988.23 1008.21 935.65 977.36 ± 30.60 3.62 % figure 5. flexural modulus loss for pp, nylon6, nylon12 and peek due to saliva exposure. acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 5 effect of the saliva, as main component of the first digestive environment, on their mechanical performances was investigated by determining the flexural modulus loss in the materials. two different additive techniques were used to manufacture the samples used in the study, and 3-point bending tests were performed for determining flexural elastic moduli. surface material properties and manufacturing techniques were found to be the main factors impacting the properties of the materials exposed to the saliva for 72 hours. future developments will try to overcome the limitation of this research, i.e., involving a parametric study based on different times of exposure [29], immersing materials in different acid-fluids [31], studying the color loss after the immersion for appearance considerations [31], and, mostly, using more complex geometries of samples for representing actual dental prosthetic designs references [1] r. gupta, m. brizuela, dental impression materials, statpearls, sep. 2022. online [accessed 23 april 2023]. https://www.ncbi.nlm.nih.gov/books/nbk574496/ [2] a.-g. gabor, c. zaharia, a. t. stan, a. m. gavrilovici, m.-l. negruțiu, c. sinescu, digital dentistry digital 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affecting factors, and applications, scanning 2021 (2021). doi: 10.1155/2021/9950131 [25] a. dawood, b. m. marti, v. sauret-jackson, a. darwood, 3d printing in dentistry, br dent j 219(11) (2015) pp. 521–529. doi: 10.1038/sj.bdj.2015.914 [26] m. javaid, a. haleem, current status and applications of additive manufacturing in dentistry: a literature-based review, journal of oral biology and craniofacial research 9(3) (2019) pp. 179–185. doi: 10.1016/j.jobcr.2019.04.004 https://www.ncbi.nlm.nih.gov/books/nbk574496/ http://dx.doi.org/10.1515/jim-2017-0033 https://doi.org/10.1016/j.cden.2007.03.006 https://doi.org/10.1111/j.1532-849x.2010.00673.x https://doi.org/10.3390%2fma14071799 https://doi.org/10.1016/s0022-3913(05)80041-1 https://doi.org/10.1016/j.matpr.2020.01.049 https://doi.org/10.1016/j.dental.2019.08.103 https://doi.org/10.1016/j.prosdent.2022.06.008 https://doi.org/10.1111/jerd.12566 https://doi.org/10.1111/jopr.12510 https://doi.org/10.1016/j.dental.2019.02.026 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process in different directions, progress in additive manufacturing 7(5) (2022) pp. 1111–1122 doi: 10.1007/s40964-022-00285-8 [29] f. tamburrino, v. d’antò, r. bucci, g. alessandri-bonetti, s. barone, a. v. razionale, mechanical properties of thermoplastic polymers for aligner manufacturing: in vitro study, dent j 8(2) (2020). doi: 10.3390/dj8020047 [30] l. portilha gomes da costa, f. amorim mendonça alves, a. mazzo, 3d printers in dentistry: a review of additive manufacturing techniques and materials, clin lab res den 2021 (2021) pp. 1–10. doi: 10.11606/issn.2357-8041.clrd.2021.188502 [31] r. s. alsilani, r. m. sherif, n. a. elkhodary, evaluation of colour stability and surface roughness of three cad/cam materials (ips e. max, vita enamic, and peek) after immersion in two beverage solutions: an in vitro study, int j appl dent sci, 8(1) (2022) pp. 439-49. doi: 10.22271/oral.2022.v8.i1g.1460 https://doi.org/10.1016/j.matpr.2022.04.018 https://doi.org/10.1007/s40964-022-00285-8 https://doi.org/10.3390/dj8020047 https://doi.org/10.11606/issn.2357-8041.clrd.2021.188502 https://doi.org/10.22271/oral.2022.v8.i1g.1460 microsoft word 265-1854-1-le-rev acta imeko  issn: 2221‐870x  december 2015, volume 4, number 4, 26‐29    acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 | 26  development of the combustion chamber of a reference  calorimeter by numerical analysis  e. eduardo gonzalez 1 , alejandro estrada 1 , leonel lira 2   1  instituto tecnológico de celaya, antonio garcia cubas 600, av. tecnológico c.p.38010, guanajuato, méxico  2  centro nacional de metrología, km 4.5 carretera a los cués c.p.76246,  el marqués queretaro, méxico        section: research paper   keywords: isoperibolic calorimeter; superior calorific value (scv); computational fluid dynamics (cfd)  citation: e. eduardo gonzález, alejandro estrada, leonel lira, development of combustion chamber of a reference calorimeter by numerical analysis, acta  imeko, vol. 4, no. 4, article 7, december 2015, identifier: imeko‐acta‐04 (2105)‐04‐07  section editor: franco pavese, torino, italy  received march 23, 2015; in final form may 22, 2015; published december 2015  copyright: © 2015 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: this work was supported by instituto tecnológico de celaya, méxico  corresponding author: e. eduardo gonzález, e‐mail: eli.gonzalez@iqcelaya.itc.mx    1. introduction  the three main sources of fuels to generate energy around the world are petroleum with 4059, coal with 3724 and natural gas in the third position with 2906 million tons of crude oil equivalent. it is important to measure the quantity of energy contained in a sample of natural gas, or its scv, for billing natural gas deliveries and it is commercially very important to determine this quantity of energy, because commercial transactions for natural gas are in energy units instead of volume or mass units. on the other hand natural gas is the lowest-cost source of energy and has the lowest co2 emissions of all hydrocarbons, and probably in coming years will become the largest source of energy on the planet. nowadays the energy contained in any combustible gas is calculated on the base of composition via gas chromatography, supported with the standard iso 6976 [1], where the scv values for several pure gases are published. the scv’s in the standard iso 6976 for pure gases were measured in 1931 and 1972. for methane, the uncertainty is specified to amount to 0.12 % [2]. methane is the main constituent of natural gas, so measuring the value of its scv is important because it is used in gas chromatography as a reference material for calibration. however some institutions and researchers have developed special devices to improve this uncertainty and have reduced it to approximately 0.05 % [2]. today some institutions develop their own devices to measure the scv of methane and other gases. some of them are: gerg (groupe européen de recherches gazières) with work from six partners [2]; lne (laboratoire national de métrologie et d’essais) in france [3] and ofgem (office of gas and electricity markets) by dale et al. [4]. due to the commercial importance and the attempt to improve the uncertainty obtained by using gas chromatography supported by standard iso 6976, cenam (centro nacional de metrología) together with the itc (instituto tecnológico de celaya) are developing a reference calorimeter to measure the superior calorific value of natural gas. all this kinds of calorimeters devoted to measure scv named above work very similar and operate using the abstract  the present work focuses on the numerical modeling of two combustion chambers to be used inside an isoperibolic calorimeter to  measure the superior calorific value (scv) from natural gas. this work shows the performance of both chambers working under the  isoperibolic  principle,  through  simulations  based  on  computational  fluid  dynamics  (cfd).  the  aim  of  the  work  is  to  expose  the  performance of combustion chambers published in the literature versus another one proposed in this work, and to show how the  performance  of  the  chamber  proposed  in  this  work  was  improved  by  changing  the  geometry.  it  is  checked  by  analyzing  the  temperature of burned gases at the exit of the combustion chamber. acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 | 27  isoperibolic principle. some differences exist but generally all the apparatuses of this kind have the following main components: 1) the “burner”, which provides and mixes the oxidizer and fuel which generates the flame. the “combustion chamber” and “heat exchanger”, which maximize the heat transfer from the burned gases to the surrounding, generally water. 2) the “calorimeter vessel”, which can contain any fluid, water in this work. its function is to receive and measure the energy generated by the flame and the burned gases, as well as to maintain a homogeneous temperature in the fluid contained. the burner, combustion chamber and heat exchanger are immersed in the calorimeter vessel. 3) the “jacket”, which is a further vessel enclosing the calorimeter vessel and having a temperature either uniform and constant or at least known as regards space and time [5]. the principle under which the calorimeter operates is called isoperibolic. it consists of a rise of temperature from the calorimeter vessel, containing a stirred liquid, observed while the jacket temperature is kept constant [5]. in the literature, specifically in gerg [2], lne [3] and ofgem [4], calorimeters have something in common, the combustion chamber. it has the same geometry: a cylindrical body with hemispherical lid. this work proposes to modify the geometry to an elliptical form of the burner, in order to increase the heat exchange and obtain a lower temperature of the burned gases. to demonstrate this, a numerical analysis was used to simulate the performance of both types of combustion chambers: the elliptical one proposed by this work and the cylindrical one with hemispherical lid found in the literature. 2. methodology  2.1. design  after reviewing and analysing the combustion chambers published by several researchers[2], [3], [4] it was found that they have in common a cylindrical body with hemispherical lid. however, to improve the performance in this work the geometry was modified to an elliptical chamber. we choose this geometry because the area increases in an elliptical chamber without compromising diameter and height. in this way, by increasing the heat transfer area, the temperature of the gases leaving the burner could be reduced. figure 1 shows the combustion chamber from literature [2], [3], [4] with its heat exchanger and the elliptical chamber. 2.2. governing equations  conservation equations for steady state reacting flows are used in this paper. in our case, in order to take into account combustion, it was necessary to include chemical species in the conservation equation, therefore the code solves the conservation equation for chemical species, where the local mass fraction of each species is predicted through the solution of the convection-diffusion equation for the species, by means of the following equation: .)()( 1 iiii srjyvy t     (1) where ir is the net rate of production of species i by chemical reaction and is is the rate of creation by addition from the dispersed phase. in (1) 1j  is the diffusion flux of species i that arises from the gradients of concentration and temperature. the code uses fick’s law to model mass diffusion due to concentration gradients, under which the diffusion flux can be written as: .)( ,,1 t t dy sc dj itimi      (2) in (2), mid , is the mass diffusion coefficient for species i in the mixture, and itd , is the thermal diffusion coefficient. sc is the turbulent schmidt number (where   d/ is the turbulent viscosity and d is the turbulent diffusivity). due to the model used for combustion, the net rate of production of species in (1) is assumed to be controlled by the turbulence with a two-step reaction mechanism: ohcooch 224 25.1  225.0 cooco  due to the non-premixed combustion model used, the code used solves the total enthalpy from the energy equation: .)()()( h p t sh c k hvh t     (3) the conduction and species diffusion terms combine to give the first term on the right-hand of (3) while contribution from viscous dissipation hs appears in the non-conservative form. the total enthalpy h is defined as  j jj hyh (4) where iy is the mass fraction of species j and   t t jrefjjj jref thdtcph , )( , 0 (5) where enthalpy of formation is  jrefj th ,0 of species j at the reference temperature jreft , . to describe the flow of fuel and oxidant, thus buoyancy effect in the water due to large temperature gradients, the code uses the equation of conservation of momentum (6) described by fgpvv t      )()()( τ (6) where p is the static pressure, τ is the stress tensor, g  and f  are the gravitational body force and external body forces, respectively. figure  1.  combustion  chamber  published  in  the  literature,  left  side. combustion chamber from this work, right side.   acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 | 28  the flow simulations were done using fluent. the governing nonlinear equations together with the boundary conditions are solved by an iterative numerical approach using a finite volume method [6]. 2.3. simulation details  grid generation is done using icem. for the meshes an unstructured mesh with tetrahedral elements was used, due to the complex geometry, using elements of about 1 mm in dimension, approximately. in this work, two fluid domains and one solid domain were established. the first fluid domain represents the zone which provides fuel and oxidant, mixes both and generates flame and burned gases. coupled to it, we have one solid domain, which represents the burner and the heat exchanger. the second fluid domain represents the water contained inside the calorimeter vessel, which receives all the heat due to combustion and burned gases, see figure 2. we used default values for all thermophysical and thermochemical properties of fluids included in the code and values from literature [7] for solid properties modelling heat transfer in glass. both models were simulated in 3d and steady state with the same dimensions in diameter and height for both chambers, and with the same number of turns in the heat exchanger for each combustion chamber. the fuel flow is of methane, the oxidant is oxygen and its flow is three times the fuel flow [8]. the molar fractions were established to be 0.96 for methane and 0.90 for oxygen, with temperature inlet of 22.5 °c for both. to try reproducing the physical phenomena as close as possible, the combustion chamber, burner and heat exchanger are assumed to be inside the calorimeter vessel, represented for a water volume of geometry similar to those published in the literature [2], [4]. the walls of the calorimeter vessel were set to 25.0 °c to simulate the isoperibolic enclosure, see figure 3. 3. results  the results of increasing the area of the elliptical chamber was that heat exchange increases and therefore the temperature of burned gases is lower than in the cylindrical chamber with hemispherical lid. the exterior area of the elliptical chamber exposed is 170.0 cm2. on the other hand the cylindrical chamber area is 160.0 cm2 as in figure 4 and 5. in both figures one can see how the zones with highest temperatures are at the top of the burner. there are zones in blank, inside and in the limits of the chambers, because the temperature is higher than 40 °c. the limit among the blank zone and the rest is a red ring, indicating where in the calorimeter the temperature is highest and conversely in blue where the temperature is coldest. it is useful to design the stirring system because in this zone the fluid must recirculate well to get a homogenous temperature inside the calorimeter vessel. table 1 shows, for the proposed chamber (see figure 4) against the type published in literature (see figure 5), that the average temperature of the burned gases at the exit of the heat exchanger is lower by 0.5 °c than the published one. it means an improvement of about 2.0 % of the heat exchanged by the chamber proposed in this work over that in the literature. on the other hand, the average temperature of the water is similar in both cases, but is lower with the elliptical chamber by about 2.7 %. if we analyse the maximum and minimum temperature in the water, the cylindrical chamber has a higher temperature than the elliptical chamber. figure 4. temperature distribution (in celsius). elliptical chamber immersed  in the calorimeter vessel. area with temperatures higher than 40 °c is blank. figure  2.  discretization  of  calorimeter  with  burner,  heat  exchanger  and combustion chamber. right side this work with 1 403 244 nodes; left side from literature with 1 133 433 nodes.  figure 3. sketch analysed through numerical simulation with boundary and initial conditions.  table 1. elliptical chamber versus cylindrical chamber.  schedule  elliptical  chamber  cylindrical  chamber  average temperature of  burned gases at the exit  30.88 °c  31.45 °c  average temperature of water  29.07 °c  29.88 °c  maximum temperature in the water  68.27 °c  79.89 °c  minimum temperature in the water  24.93 °c  24.95 °c  acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 | 29  4. conclusions  we have shown by numerical simulations for the steady state that the heat exchanged by the chamber proposed has improved by around 2.0 % maintaining the same diameter and height for both chambers. it means that the temperature of burned gases at the exit of the heat exchanger was reduced by 0.5 °c. however we still need to validate the results obtained by numerical simulations. numerical simulation is useful when geometries and phenomena that govern the physical model are complicated. developing and evaluating both combustion chambers in the same way lead to possible generated errors that are very similar and therefore results are comparable. we have demonstrated how numerical simulations help to develop measurement apparatuses. acknowledgement  every author who has contributed to the redaction and development of this work is kindly acknowledged. references  [1] iso 6976, natural gas-calculation of calorific values, density, relative density and wobbe index from composition. international standard iso 6976, 2nd edn., 1995-12-01, corrected and reprinted 1996-02-01. [2] p. schley, m. bech, m. uhring, s. m. sarge, j. rauch, f. haloua, j.-r filtz, b. hay, m. yakoubi, j. escande, a. benito, p.l. cremonesi, measurements of the calorific value of methane with the new gerg reference calorimeter, int. journal of thermophysics, 31:665-679 (springer 2010). [3] f. haloua, b. hay and j.-r filtz, new french reference calorimeter for gas calorific value measurements, journal of thermal analysis and calorimetry, vol. 97 2, 676-678, (akadèmiai kiadó, budapest 2009). [4] andrew dale, christopher lythall, john aucott, courtnay sayer, high precision calorimetry to determine the enthalpy of combustion of methane, thermochimica acta 382, 47-54, (elsevier 2001). [5] hobert c. dickinson, combustion calorimetry and heats of combustion of cane sugar, benzoic acid and naphthalene, bulletin of the bureau of standards, (washington 1914). [6] versteeg, h. k., and malalasekera, w., an introduction to computational fluid dynamics: the finite volume method. addison wesley-longman, 1995. [7] incropera and dewitt. fundamentals of heat and mass transfer. fourth edition john wiley & sons. [8] frédérique haloua, jean-noël ponsard, ghislain lartigue, bruno hay, clotilde villermaux, emilie foulon, murès zaréa, thermal behavior modeling of a reference calorimeter for natural gas, international journal of thermal sciences 55, 40-47, (elsevier 2012). [9] ansys® fluent 14.0, user’s guide. © 2011 sas ip inc. [10] ansys® icem cfd 14.0, user’s guide © 2011 sas ip inc. figure  5.  temperature  distribution  (in  celsius).  cylindrical  chamber  with hemispherical  lid  immersed  in  the  calorimeter  vessel.  area  with temperatures higher than 40°c is blank.  microsoft word 299-2475-1-le-rev acta imeko  issn: 2221‐870x  november 2016, volume 5, number 3, 76‐80    acta imeko | www.imeko.org  november 2016 | volume 5 | number 3 | 76  surface analytical model and sorption artifact designing  method  xiao‐ping ren 1,2 , jian wang 1 , ombati wilson 3 ,yong wang 4 ,bai‐fan chen 4,5 , chang‐qing cai 1   1 mass laboratory, division of mechanics and acoustics, national institute of metrology, beijing, p.r.china  2 department of r&d management, national institute of metrology, beijing, p.r.china  3 kenya bureau of standards, nairobi, republic of kenya  4 college of information science & engineering, central south university, changsha, p.r.china  5 department of computer science and engineering, texas a&m university, usa      section: research paper   keywords: mass measurement; surface sorption corrected; mass dissemination; optimization algorithm  citation: xiao‐ping ren, jian wang, ombati wilson ,yong wang, bai‐fan chen, chang‐qing cai, surface analytical model and sorption artifact designing  method, acta imeko, vol. 5, no. 3, article 12, november 2016, identifier: imeko‐acta‐05 (2016)‐03‐12  section editor: marco tarabini, politecnico di milano, italy  received december 15, 2015; in final form august 1, 2016; published november 2016  copyright: © 2016 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding:  this  work  was  supported  by  national  natural  science  funds  of  china  (51405459),  national  science  and  technology  support  program  (2011bak15b06) and special‐funded program on national key scientific instruments and equipment development (2012yq090208).  corresponding author: x.p. ren, e‐mail: renxp@nim.ac.cn    1. introduction  the unit of mass, the kilogram, is the last of the seven base units of the international system of units (si) to be defined according to an invariant of nature rather than a material artefact [1]. both the watt balance method and avogadro method are making the new definition under vacuum conditions whereas the current definition of the unit, from the international prototype kilogram (ipk), is realized and maintained in air. after the new definition, it is still necessary to consider traceability from the new realization in vacuum to the current working standards which are always maintained in air [2]. this is to make an indirect link between air and vacuum mass measurements by measuring mass in vacuum and then characterizing the absorption layers of contaminants during the process of transferring from vacuum to air [3]. during this transition, the mass of the standard is significantly affected by a sorption phenomenon. this phenomenon is caused by atmospheric gases and humidity, which subsequently lead to loss of stability of the mass value of the standard(s) [4]. in 1973, takayoshi studied the problem of surface water on metal artefacts [5]. r. schwartz wrote a series of papers on adsorption isotherms in air [6] and sorption phenomena in vacuum [7]. additional studies focused on pt/ir [8], stainless steel [6], [7], silicon [9] and also other materials (au [8], [10], [11]). for mass dissemination, there are many parameters to be considered in designing the mass standard, like height, diameter, volume, surface area and mass value of the weight. the characteristics of a measurement instrument are also important such as the size of the mass comparator, electronic weighing capacity and volume measurement instrument. in this paper, an adaptive algorithm which can be used to optimize the design of abstract  mass standards with alternative shapes are difficult to design due to the number of complex parameters. an analytical model based  on  surface  sorption  experiments  is  presented  to  study  adsorption.  this  model  is  based  on  an  optimization  algorithm  that  is  conceptualized to help to design the best sorption artefacts. experimental artefacts, cylinder‐weight and stack‐weight, were of the  same volume but different surface areas. this algorithm in essence determines the optimum surface of the artefact. after machining  the artefact, surface sorption measurements were carried out. a sorption experiment was done by transferring the artefact from air to  a vacuum. then the surface sorption model was set up which represented the relationship between sorption coefficient η, time t and  relative humidity h. logarithmic models were used to fit the variation of sorption coefficient η per relative humidity h with time t. acta imeko | www.imeko.org  november 2016 | volume 5 | number 3 | 77  a mass standard based on a surface analytic model of mass standards is described. 2. recent research on surface artifacts  one kilogram weights with different surface areas and same nominal mass values are normally required. samples of different materials (stainless steel, silicon material and platinum-iridium) have to be selected. figure 1 shows an example of designing a stack of weights made of platinum-iridium assembled using a spacer (height of 1 cm and diameter of 2 mm). the volumes, masses and surface areas of the spacers were added to the overall values for the weight stack. for some applications of this kind of stack weight, when taking volume measurement, one disc should be placed into the liquid and then the spacer is placed on its surface. then another disc is put onto the spacer until all the separate discs and spacers are immersed in the volume measurement liquid. during this process, however, the stack of weights may fall down from the magazine and lead to process and system failure. another kind of stack weight was composed of a number of discs [6]. these discs were tightened on carrying rods (d: 10 mm). the discs and carrying rods were polished by häfner company. for this kind of stack weight, there was a hole in the middle of each disc where the rod was screwed in. these discs cannot be totally seamlessly fixed together (as shown in figure 2). thus whenever this stack weight is submerged into the liquid for volume measurement, the volume value would be affected by this gap. a similar design method was described by beer [13]. another kind of artefact, shown in figure 3, consisted of twelve discs separated with pieces of wire and held together with a thin rod. this particular artefact was gold coated with a 6 μm thick layer with an aim of determining the characterizations of the gold surfaces. in section 3 we present a mathematical model that, when optimized through computational methods, provides for the best design parameters of the artefacts to be machined. 3. integral sorption artefact  in mass measurements, the main uncertainties are due to air buoyancy correction and surface sorption correction. in order to improve the accuracy of the surface analytical model of a mass standard, the most important thing is to reduce the influence of the buoyancy correction as much as possible. two prototype models of 1 kg stainless sorption artefacts are shown in figure 4 and figure 5. the model represented by figure 4 is the classical prototype, and it is in cylindrical form with height and diameter being equal. the other model, shown in figure 5, is in the form of a stack and the discs are separated by a rod. this rod is not however separated from the disc; it is a monolith of stainless steel. the surface area (s) and volume (v) of a cylinder weight (figure 4) are shown in (1) and (2), where r, d and h represent the radius, diameter and height of weight respectively. cylinder 2 22 2 2 2 2 2                  d d s r rh h , (1) cylinder 2 2        d v h . (2) the dimensions of the stack prototype (shown in figure 5) are as follows: outer circle’s radius and height being r2 and h2 respectively; inner circle’s radius and height being r1 and h1 respectively. the total surface area and volume of 4-level stack prototype are shown in (3) and (4): 2 2 stack 1 1 2 2 2 16 8 8 6s r h r h r r       , (3) 2 2 stack 1 1 2 23 4v r h r h   . (4) generally, the 4-level stack prototype above is an example of designing the sorption artefact. different levels can be adopted during design of the sorption artefact depending on the sorption effect. the normal formulae for calculating the volume figure 3. 1 kg gold‐plated copper buoyancy artefact.  figure  1.  designing  of  a  platinum‐iridium  weight  set  by  johnson  matthey  and npl [12].  figure 2. the gap between disc and rod.  figure 4. cylinder prototype.           figure 5. discs of stack prototype.  acta imeko | www.imeko.org  november 2016 | volume 5 | number 3 | 78  and surface area of stack weight with different levels n are shown in (5) and (6): 2 2 1 1 2 2( 1)v n r h n r h    , (5) 2 2 1 1 2 2 2 12( 1) 2 2 2( 1)s n r h n r h n r n r         . (6) in this research, the two models of mass standards are expected to be of the same volume but different surface areas i.e. (2) and (4) should be equal. thus, the air buoyancy correction applied during the comparison process will be minimized. the large difference in surface areas between (1) and (3) is better for the mass measurement during transferring from vacuum to ambient. 4. parameter  searching  method  based  on  the  optimization algorithm  for the same material with ideal density ρ, and nominal mass value (1 kg), both height h and radius r of cylinder prototype can be calculated by (2). for sorption purposes, let (5) equals the cylinder volume. thus, there are four parameters which determine the volume of the n-level stack, i.e. r1, r2, h1 and h2. with surface area of cylinder scylinder being also known, these four parameters can therefore be varied until the maximum difference between scylinder and sstack is obtained. however, this design scheme cannot neglect the usage of both mass and volume measurement equipment. all final artefacts therefore can be put into these instruments and their mass and volume can be measured accordingly. during determining the volume and surface area of the cylindrical prototype, a 9-level stack was considered. this design scheme had the following limitations: (a) r2 < 50 mm (size of the weighing magazine); (b) height of weight less than 90 mm (volume measurement instrument space); (c) volume of 9-level stack equal to the volume of cylindrical prototype i.e. vstack = vcylinder ; (d) surface area of 9-level stack > surface area of cylindrical weight i.e. sstack > scylinder; (e) mass difference between cylindrical and 9-level stack prototype less than or equal to 1.5 g (i.e. electronic weighing capacity). if the mass difference exceeds this limitation, the equipment malfunctions or gives misleading results; (f) machining precision equal to 0.01 mm, so the precision of 2 decimal points for each parameter is enough. for example, if the optimal height of the weight h is 53.3725 mm, the value is rounded to 53.37 mm. in this study, there were six parameters [d, h, r1, r2, h1 and h2] and 6 constraint conditions being considered in the sorption artefact. there are several approaches for solving this kind of multi-object optimization. matlab has an optimization toolbox with the function “fgoalattain”, whose graphical user interface is shown in figure 6. the following parameters were determined and programmed: start point, goals, weights, linear inequalities, bounds for variables. objective function and nonlinear constraint function were written into a .m file, which described the formula listed from (1) to (6). more details are given in appendix i and ii. start point and final results are respectively shown in tables 1 and 2. when the optimization algorithm is initially running from the start point, it executes 13 iterations and the algorithm makes the judgement whether volume and surface satisfy the requirement; otherwise, the algorithm is re-executed from the current result until the maximum difference of surface areas is obtained and volume difference is close to zero. optimization trends of surface area, volume and iteration numbers of optimization algorithm at different start points are shown in figure 7. figure 7(b) shows that the optimization sequence of table 1. start point for the optimization algorithm (unit: mm). h r r1 r2 h1 h2 0.01 0.01 0.01 0.01 0.01 0.01     table 2. optimization result for the start point (unit: mm).  h r r1 r2 h1 h2 53.27 26.64 4.04 50 9.50 1.62 figure 7. the trend of surface area, volume and the  iteration numbers of  optimization algorithm at different start points.  figure 6. interface for global optimization toolbox in matlab 2011.   10 20 30 40 50 0 5 10 15 x 10 4 optimization sequence (a) s u rf a ce a re a (m m 2 ) 0 20 40 108 110 112 114 116 118 120 122 optimization sequence (b) v o lu m e (c m 3 ) 0 10 20 30 40 50 0 20 40 60 optimization sequence (c) a lg o ri th m i te ra ti o n n u m b e r cylinder 9 level stack 9 level stack cylinder acta imeko | www.imeko.org  november 2016 | volume 5 | number 3 | 79  volume may have unsatisfactory results (peaks). the curve (red line) in figure 7(c) represents the variation trends, and shows that the algorithm reached stability after 52 optimization sequences. from this particular optimization results, the maximum surface area difference and volume difference were determined to be 133699.71 mm2 and 0 cm3 respectively. 5. surface sorption experiment  after machining the artefacts (in 2014), their surfaces were thoroughly cleaned (using alcohol) to remove any contaminate e.g. oil and dust. they were subsequently set to stabilize for one year within laboratory ambient condition. these artefacts were also cleaned before the sorption measurements in air and vacuum. 5.1. surface measurement in air  during this experiment, the masses of the 9-level stack weight and cylindrical weight described in section 4 were measured using the mettler-toledo m-one system. measurements were performed under normal air condition; their masses were measured from sept. 10th to sept. 14th 2015 and the mass differences thereof computed. this measurement procedure was again repeated at the same condition a month later for two days (i.e. oct. 15th and oct. 16th). results of sorption measurements in air are as shown in figure 8, in which, δi, δm, h, t and ρ respectively represent balance indication, mass difference, relative humidity, temperature in the measurement chamber and air density. the differences between the two series of tests were analysed. 5.2. surface measurement in vacuum  artefacts were transferred from air to vacuum (tm and pm sensor installed in mass measurement system: m-one); the pressure ranged between 7×10-5 pa and 2×10-3 pa. the sorption was measured from oct. 20th to oct. 23th, 2015; results are shown in figure 9. figure 9(a) shows that the measurement values were stable at the end of the tests; the last mass difference was 1.2207 mg. combined with the measurement results in air condition, variation of the relative mass per area a could be determined by mass comparison of the sorption artefacts. 5.3. relation between sorption coefficient and humidity  in 1994, schwartz [6] determined the sorption coefficient η as η=δm/δa+η0. to minimize the effect of η0, the current study ensured that the artefacts were thoroughly cleaned before performing measurements, and hence η0 was assumed to be zero. from the scatter plot of figure 10, the horizontal axis and vertical axis respectively represent real time t (in hours), and sorption coefficient per relative humidity η/h (unit: μg/cm2/rh %). sorption coefficient and relative humidity were fit with the aid of the logarithmic model using the following the function in (7): f(η)= (4.3×10-4+ 3.5×10-5ln (0.43 t +1)) ·h . (7) the results obtained from (7) were similar to the results of schwartz’s model of the sorption coefficient. however, unlike schwartz’s model, the present model introduced humidity into the function and also obeyed the logarithmic rule. this study was congruent with earlier research findings on negligible influence of the effect of roughness condition and temperature of weight surface [6], and did not, therefore, investigate their effects on the sorption behaviour of weight. 6. conclusions  in order to provide a practical approach on disseminating the redefined kilogram, realized in vacuum to the mass scale at ambient, processes such as air to vacuum transferring for figure 8. result of the sorption measurement in air stage   (a) variation  of  the  δi  reading  from  the  balance  with  measuring  times (broken line: long term gap).   (b) variation of mass difference δm with measuring times.   (c) variation of relative humidity h with measuring times.   (d) variation  of  temperature  t  in  the  measurement  chamber  with measuring times.   (e) variation of air density ρ with measuring times.  figure 9. measuring the mass difference under vacuum condition.  0 20 40 60 80 100 120 140 -0.84 -0.835 -0.83 ∆ i( in te rv a l) measurement in air 0 20 40 60 80 100 120 140 -0.8 -0.795 -0.79 ∆ m (m g ) 0 20 40 60 80 100 120 140 35 40 45 h (r h % ) 0 20 40 60 80 100 120 140 20.5 20.6 20.7 t( � ) 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 1.19 1.191 1.192 measuring times ρ (m g /c m 3 ) (a) (b) (d) (e) (c) sep.10th~14th oct.15th~16th 0 10 20 30 40 50 60 70 80 90 -1.222 -1.22 -1.218 -1.216 -1.214 -1.212 （ ） ∆ i in te rv a l measurement in vacuum 0 10 20 30 40 50 60 70 80 90 0 2 4 6 8 x 10 -3 t m 2 (h p a ) 0 10 20 30 40 50 60 70 80 90 0 0.5 1 1.5 x 10 -3 measuring times p m (h p a ) oct 20th (a) (b) (c) oct 23th t (° c ) acta imeko | www.imeko.org  november 2016 | volume 5 | number 3 | 80  standards must be studied. this transfer process and the results thereof are often affected by adsorption and desorption. in this paper, a new surface analytical model combining artefact standards designing method is presented. the model is based on an optimization algorithm which considers important parameters such as suitable design of weights (i.e. cylindrical weight and stacks of discs of weight). these parameters could help the metrologists in improving the accuracy of mass measurements. the preparation of the sorption artefact and the sorption measurement has also been described. sorption coefficient drift was be described by a logarithmic function that includes the effects of the humidity and time. in this experiment, the detailed value of roughness was not given, but the relationship between time, humidity and sorption coefficient was presented. further research may be conducted in future on artefacts to determine their drift for a long time in air condition. appendix i  function f = weight_design(x) density = 8421; n = 9; pi=3.14159265; s_cylinder = 2*pi*x(2)^2+2*pi*x(2)*x(1); v_cylinder = pi*x(1)*x(2)^2/1000; s_stack=2*(n-1)*pi*x(3)*x(5)+2*n*pi*x(4)*x(6)+2*n*pi*x(4) ^2 -2*(n-1)*pi*x(3)^2; v_stack = ((n-1)*pi*x(3)^2*x(5) + n*pi*x(4)^2*x(6))/1000; f(1) = -s_stack + s_cylinder; appendix ii  function [c,ceq] = nonlconstr(x) e1 = 0.5; density = 8421; n = 9; pi=3.14159265; c(1) = -x(1)*x(2)^2 +(1000*(1000000-e1))/pi/density; c(2) = x(1)*x(2)^2-(1000*(1000000+e1))/pi/density; c(3)=(1-n)*x(5)*x(3)^2-n*x(6)*x(4)^2-(1000*(e1-1000000))/ pi/density; c(4)=(n-1)*x(5)*x(3)^2+n*x(6)*x(4)^2-(1000*(e1+1000000))/ pi/density; ceq = pi*x(1)*x(2)^2-(n-1)*pi*x(5)*x(3)^2-n*pi*x(6)*x(4)^2; references  [1] s. davidson, j. berry, z. silvestri, et al, “addressing the requirements for the practical implementation and ongoing maintenance of the redefined kilogram,” proc. of imeko 22nd tc3, 12th tc5 and 3rd tc22 international confer-ences, feb.35, 2014, cape town, republic of south africa. [2] mise en pratique of the definition of the kilogram. http://www.bipm.org/en/si/new_si/mise-en-pratique.html, 2013. [3] p. j. abbott, r.c. dove. “progress on a vacuum-to-air mass calibration using magnetic suspension to disseminate the plankconstant realized kilogram,” proc. of imeko 22nd tc3, 12th tc5 and 3rd tc22 international conferences, feb.3-5, 2014, cape town, republic of south africa. [4] p fuchs, k marti, s russi. “new instrument for the study of ‘the kg, mise en pratique’: first results on the correlation between the change in mass and surface chemical state,” metrologia, 2012, v49, pp. 607–614. [5] takayoshi, shuiti, et al, “coulometric micro-determination of surface water on various metals and glasses and of hydrogen in beryllium metal,” materials transactions, v 14, 1973, pp. 396400. [6] r. schwartz. “precision determination of adsomtion lavers on stainless steel mass standards by mass comparison and ellipsometry. part i: adsorption isotherms in air,” metrologia, v 31, 1994, pp. 117-128. [7] r. schwartz. “precision determination of adsorption layers on stainless steel mass standards by mass comparison and ellipsometry part ii sorption phenomena in vacuum,” metrologia, v31, 1994, pp.129-136. [8] p. fuchs, k. marti , s. russi. “materials for mass standards: longterm stability of pt/ir and au after hydrogen and oxygen lowpressure plasma cleaning,” metrologia, v 49, 2012, pp. 615–627. [9] m. borys, m. mecke, u. kuetgens, et al. “the growth of the oxide layer on silicon spheres and its influence on their mass stability,” proc. of imeko 22nd tc3, 12th tc5 and 3rd tc22 international conferences, feb.3-5, 2014, cape town, republic of south africa. [10] p. fuchs. “low-pressure plasma cleaning of au and ptir noble metal surfaces,” applied surface science, v 256, 2009, pp.1382– 1390. [11] k. marti, p. fuchs , s. russi. “cleaning of mass standards: ii. a comparison of new techniques applied to actual and potential new materials for mass standards,” metrologia, v 50, 2013, pp. 83–92. [12] s. davidson, s. brown, j. berry.”a report on the potential reduction in uncertainty from traceable comparisons of platinum-iridium and stainless steel kilogram mass standards in vacuum”, npl report cmam 88, 2004. [13] w. beer, w. fasel, e. moll, p. richard, et al. “the metas 1 kg vacuum mass comparator – adsorption layer measurements on gold-coated copper buoyancy artefacts,” metrologia, v 39,2002, pp. 263-268. figure 10. relation between time t and η/h.  0 10 20 30 40 50 60 70 4.3 4.4 4.5 4.6 4.7 4.8 x 10 -4 time t(hour) s o rp tio n c o e ffi c ie n t p e r re la tiv e h u m id ity (μ g /c m 2 /% ) η /h vs t logarithmic model microsoft word article 6 192-1068-1-pb.docx acta imeko may 2014, volume 3, number 1, 19 – 22 www.imeko.org acta imeko | www.imeko.org may 2014 | volume 3 | number 1 | 19 problems in theory of measurement today l. gonella istituto di fisica sperimentale, politecnico di torino, c. duca degli abruzzi 24, 10129 torino, italy section: research paper keywords: measurement theory, value, error, uncertainty citation: l. gonella, problems in theory of measurement today, acta imeko, vol. 3, no. 1, article 6, may 2014, identifier: imeko-acta-03 (2014)-01-06 editor: luca mari, università carlo cattaneo received may 1st, 2014; in final form may 1st, 2014; published may 2014 copyright: © 2014 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited 1. subject and scope of the theory of measurement to ask what is (or should be) the subject matter of the theory of measurement is not an idle question, since what seems to be the first problem today is to realize that there are indeed basic problems to solve and to identify then. too many people feel with undue complacency that the theory of measurement is a perfected structure, needing only peripheral work. others feel, instead, that fundamental questions lie unanswered, or even unasked, being pre-emptied by undiscussed assumptions; framing such questions, that i think are to be asked and answered from the operative standpoint [1– 4], is the aim of this paper. the purpose of a theory is to describe reality in a coherent framework – in our case to provide a coherent description of the valid praxis of measurement, suited to its whole range. it is apparent that the current theoretical framework is not up to this task: the rank confusion prevailing in the field is widely recognized and lamented. just try and compare the definitions given in the current literature of basic concepts as value, error, measurement, measurable quantity, etc. (i even saw the lecturers on metrology of an international school declining to hold a seminar on definitions as meaningless in today’s mixedup situation [3]). there is a widening gap between the praxis of measurement and its theory: our measurements agree with one another much better than we agree in talking about them. curiously enough, this is not felt as a flaw in the theory: for most people it is just a matter of ‘terminology’ and the problem is shunted to nomenclature committees, which face up an impossible task. they are supposed to choose the ‘right word’ for designating well-understood, undisputable concepts, but these ones turn out to be hazy or unfitting; a formal redefinition is called for, and in so doing unwitting theoretic work is done: what else is the core of a theory but a crossconnected list of definitions? the ‘current’ theory of measurement is embodied in the definitions, remarks and assumptions (both discussed and taken for granted) of textbooks on one hand and normative literature (standards, codes of practice, etc.) on the other. there is no parallel of the latter source in the other fields of science, unencumbered by the strong normative implications of the measurement problem. what happens is that the ‘normative’ authors feel that it is not up to them to discuss the theory, but only to codify the approved practice using the language of the main line of the approved textbooks; the latter, on their hand, cannot teach differently from the main line of the normative literature without rendering a bad service to their readers. the vicious circle thus generated makes it difficult to update the theory; its haphazard growth disguised as lexical adjustment or conventional ruling cannot touch the basic tenets, while the due concern over its responsibility may well transform some normative body into a low-pass filter against new ideas. this is one reason, i think, why the current theory of measurement is still based on the framework masterly set up by helmholtz [5] on basic ideas of euclidean lineage, which makes it the only field of science treated as yet in full 19th—century terms. a century passed, which shook the very foundations of mechanics and geometry, but new basic concepts like those of signal and this is a reissue of a paper which appeared in acta imeko 1979, proceedings of the 8th imeko congress of the international measurement confederation, “measurement for progress in science and technology”, 21-27.5.1979, moskow, vol. 1, pp. 103–110. the main questions to be answered in the theory of measurement are presented, and their formulation analysed in the classical approach and in a new operative one: the scope and aims of the theory itself, the concepts of measurable quantity, measurement results, error, uncertainty, the role of time. the paper is meant to define problems as the first step toward their solution. acta imeko | www.imeko.org may 2014 | volume 3 | number 1 | 20 information, of quantum indeterminacy, etc., do not seem to have found their proper place in the frame of the measurement theory. meanwhile a more and more sophisticated instrumentation calls for a refined treatment in modern setting; we need a comprehensive theory able to range from the carpenter’s rule to the microprocessor, and the latter can hardly be dealt with as an extrapolation of the former. as technical-scientific people are shy of meddling with ‘philosophical’ issues concerning the meaning of what they do, and grew accustomed to shift most basic issues into terminological channels, the bulk of their work is devoted to mathematical techniques for data elaboration, mostly concerning the probabilistic error analysis. somebody is therefore inclined to identify the theory of measurement with this analysis (blanket statements as “any physical quantity is just a statistical variable” are often heard among physicists), which represents a drastic reduction in scope for the former and leaves the latter without bases (such position is also due to the general, very disputable, inclination to identify ‘theory’ with mathematics). the debate on the basic concepts of measurement has been left mainly to scholars in non-physical sciences [6–9]: almost everybody seems convinced that only there some problem is left, while in physics and the related technical fields all is nicely settled on helmholtz’s lines [8]. in the minds of many people stevens’s ideas are connected with non-technical measurements, though his stand against campbell [6] on psychophysical measurements could well have been taken by any technical scholar on several engineering measurements (e.g. hardness). marvel was expressed for the neglect in which the field is left [7], and it was noted that fundamental issues about measurement in physics are unsolved or plainly ignored [9]. measurement is a knowledge-gathering technique. what its theory is to be concerned with depends on what we expect from a theory. from the operative standpoint the measurement theory is the total body of definitions and rules concerning measuring activities, formulated with the aim of providing a guide to get consistent results. from another it is the argumentation that justifies the measurement itself, thus sanctioning what can be measured and what cannot. the issue of the scope of the measurement theory is coextensive with the question of what is a measurable quantity. 2. measurable quantities in an operative approach we may say a measurable quantity is anything we can describe in an objective and consistent way; the problem then is to define ‘objective and consistent’ in order that we can measure things to our satisfaction and discriminate against illusory knowledge. the classical approach starts from the other end: the consistency is looked for in the formal properties of the class of mathematical entities chosen for the description, the measuring procedure is modelled on a formal operation of this class, and a measurable quantity is what fits the model; the problem then is to justify the measurement of the other things that in practice we measure but do not fit directly in the pre-established model. the universal (though not necessary [9]) choice is to map the measurable quantities on the class of the real numbers, setting up a homomorphism between the empirical juxtaposition of the measured objects and the addition of the numbers assigned to measure them. the obvious paradigm is the measurement of length. all quantities endowed with the ‘additive’ property of the length, called fundamental or extensive (terminology and connotations depend on the author), may be measured directly; the others must be somehow ‘derived’ from them; if some kind of ‘derivation’ is not devised the quantity is not deemed and is excluded from the scientific realm. this way one gets a very good logical consistency for the fundamental quantities, but at the expense of generality, and the approach itself breeds the inclination to reduce the scope of the theory. when a theory meets phenomena which do not fit, either one changes the theory or puts an ‘off limits’ sign on the phenomena; the more clean-cut is the theory, the stronger the temptation to shrug off what does not fit. the ‘derivation’ criteria, either of the quantity or of the measuring procedure, are debatable. considerable ingenuity was spent for deriving the measurement of temperature; other quantities (e.g. hardness) were not so lucky, perhaps because they were not so firmly embedded in the core of physics, and are therefore in a limbo – someone calls them “pseudo-quantities”, which does not deter people from measuring them, but only hampers coherent standards. likewise, nuclear physicists keep using counters as measuring instruments while others show that counting is not a measurement [7]. something is wrong: either the people who keep on measuring “pseudo-quantities”, or the arbitrary requisite of an ‘additive’ property that so many measurands simply do not possess. one must remember that once people believed that all measurements ought to be referred to mass, length, and time: much of this kind of philosophy is still attached to the measurement theory. the ‘derivation’ problem is circumvented by the idea of different ‘scales of measurement’, with different relational properties, but many people do not accept all ‘scales’ as ‘measurement’, and others distinguish between a strict sense and a wide sense of measurement. it is also disturbing that the vectorial character of physical quantities cannot be accounted for in these frameworks. anyhow, if ‘measurement’ is a procedure of limited applicability, other consistency rules, beside another label, must be given to the knowledge-gathering procedure applied to the “pseudo-quantities”, and an overall treatment is in order. is a distinction between strict and wide sense of measurement meaningful, or is it not a question of different types of measurable quantities? the idea itself of looking into mathematics for ‘justifying’ a measuring procedure, for finding out whether or not we are allowed to measure a thing, is disputable: is the theory of measurement a branch of mathematics (albeit applied), or is it a branch of the natural and technical sciences (using suitable mathematical tools)? there is a difference, which lies in where we look for the ultimate criteria of consistency. 3. the data set representing a measurand granted that measurement is, in finkelstein’s well-chosen words, “the process of assignment of numbers to attributes ... in such a way as to describe them” [10], we have yet to decide in which way numbers are to be assigned. in the classical theory the question is not even posed; to represent the measurand one real number is assigned, tied to a unit of measure: the value of the measurand in the chosen scale of measurement. the geometric paradigm indeed leaves no doubt that “measurement demands some one-to-one relation between the numbers and the magnitudes in question” [11]. the fact is that with a single real number to represent the measurand no consistency can be obtained in different measurements of the same measurand, owing to what eisenhart aptly calls the “cussedness of acta imeko | www.imeko.org may 2014 | volume 3 | number 1 | 21 measurement” [12]. the difficulty is bypassed by assuming that the ‘true’ value – the one number that would make all measurements fit – exists, but is unknown and unknowable; individual measurements yield values differing from it by an ‘error’ due to instrumental ‘imperfections’; an analysis of these measurement errors is required to assess the interval within which the true value is supposed to lie. this way theory of measurement and theory of errors are separated: the former is free to pursue its way unencumbered by questions of uncertainty, left in care of the latter. somebody finds it a good arrangement, claiming that the principles of measurement cannot depend on ‘technical’ issues of uncertainty [8]; others feel that ‘essential difficulties in the logic of measurement arise from uncertainty” [10]. to postulate the solution is no way to solve a problem [11]: this course was taken only on the strength of the geometric paradigm. we should ask: is the real number the proper mathematical entity for representing measurands? the real number was invented to solve problems in pure mathematics born of the euclidean mensuration problem, i.e. expressing with a number the proportion of two segments of any geometric figure; the apodictic certainty offered by geometry is so attractive that one forgets it is due to the abstract definition of the geometric figures, that do not belong in the real world. but measurement is meant to describe the real world, and if it follows too closely the tracks of geometry it risks to become itself involved in idealized measurands. the real number is by definition the limit of a convergent sequence, but in measurement there is no such a thing as a sequence with a definite generating rule. we may also ask: is the mathematics of continuum, of which the real number is the main pillar, the proper tool for representing the physical world? atomic and nuclear physics did show matter to be granular; quantum mechanics added a granularity of its own and also an indeterminacy deeply embedded in the very fabric of things physical. this is not the setting meant for real numbers and the mathematics of continua. it would not do to have a different theory of measurement for the macroscopic and microscopic worlds. besides, everybody knows that any real macroscopic measurand is endowed with an intrinsic uncertainty inherent to its very definition (“you don’t measure bricks with a micrometer”); with more and more precise measurements of a given measurand one does not get closer to a real number expressing its value, as the classical theory assumes mimicking sequences, but rather at a certain point one finds oneself measuring something else, because the measurand’s definition has exploded into a finer structure. the implications of this commonplace experience are lost if one postulates descriptions by real numbers, and one also wonders whether possible ties between the quantum indeterminacy of microphysics and this ‘definition indeterminacy’ of macrophysics were not obscured by the slant toward geometric sharpness taken by the theory of measurement. if we look at things from the operative standpoint, we cannot help noting that measurements actually yield numerical intervals, and that upon such intervals we must reason to judge the consistency. even a ‘perfect’ instrument could not overcome the intrinsic uncertainty of a real measurand. the mathematical entity we are actually working with is a set of numbers, and there is nothing wrong in representing a measurand with a full set of numbers tied to a unit of measure: it is yet “assignment of numbers ... in such a way as to describe it”. this set assigned to describe the measurand (let us call it a “value-span”) would include in the description the uncertainty of the measurement, thus reuniting theory of measurement and ‘theory of error’. the one fundamental requirement for consistency is that different valuations of the same parameter yield ‘the same’ result: it is matter of defining rigorously what we mean by ‘the same’ and follow suit. two measurements are consistent if the assigned value-spans overlap: this translates in formal terms the practical judgment of consistency “to stay within the error”. the algebra of sets is to substitute the algebra of real numbers in operating on measurement data, starting with the replacement of the basic relation ‘equal’ with ‘not disjoint’ [1–4]. a new italian reference standard on basic measurement terminology [13] follows this line, that was found easy to practice. one may ask which special properties should have the sets assigned to describe measurands. in particular, the boundary of these sets cannot be sharply defined, if the pitfalls of the realnumber concept are to be avoided. a suitable specialization of the ‘fuzzy set’ concept might well turn out to be the proper mathematical tool for the job [1–3, 14]. 4. error, uncertainty, indifference the problem posed by the cussedness of measurement is tackled by the classical theory in terms of ‘error’. this concept rests on the idea that the instrument ought to indicate directly the true value of the measurand but indicates instead a wrong one; the underlying assumptions (derived straightly from the model of measuring length with a graduated rule) are: (a) the measurand is described by a single real number; (b) the instrumental indication is a value assigned to the measurand. on this basis the questions to ask are: why is the instrument wrong? how much is it wrong? a full answer is impossible: we cannot hope to know the error a priori, we can only judge a posteriori that it is smaller than a certain amount. this ‘maximum error’ is thought to be determinable by combining the different errors due to various causes, separately analysed, and an a priori distinction is made between ‘random’ and ‘systematic’ error; the former may be treated exactly by statistics, but the latter turns out to be an elusive concept [12] that calls for an educated guess. the distinction has some heuristic value, beside historic reasons, but adds more problems than it solves: without a sum rule it is no help for judging the consistency of different measurements, and no logically sound sum rule can be given for such essentially heterogeneous components (as shown by the fact that at least four competing rules are now used). such care was paid in distinguishing between ‘precision’ and ‘accuracy’ that some technical language even lacks a word describing their cumulative effect; so it happens that after defining different errors one often drifts to speak of ‘uncertainty’, without however defining it. the problem may be tackled from a different standpoint if we drop the basic assumptions of above, remembering that the theory had been cast in that mold long before the 20th-century concept of signal were developed. if we look at the instrumental indication as at a signal bearing information on the measurand, and allow that we are to assign a data set describing the latter (not necessarily a single number) by connecting this information with other information pertaining to the instrumental characteristics and to the ‘influence quantities’ affecting the measurement, then the questions to ask are: how do we describe the instrument output? which relation holds between this output and a value-span suited to describe the measurand? this way we keep much closer to the actual practice, and may account in the same framework for all acta imeko | www.imeko.org may 2014 | volume 3 | number 1 | 22 instruments (allowing for the evolution of the output signals from the reading of graduated rules to the string of digits out of on-line computers). this approach calls for a clear distinction between the terms referring to the description and those referring to the instrument’s output. much confusion is thus avoided that the classical approach brings about: many technical languages, e.g., use the same term for both the operation of measurement and the resulting description of the measurand, following a mathematical usage justified only by the uniqueness of mathematical solutions; the statistical treatment of ‘errors’ is also often confused with that of random quantities [4]. the attention is brought on the calibration procedures and the interplay of instrument, measurand, and environment. it is now possible a question that the classical approach cannot ask, i.e., is there a relation between the uncertainty of the measurement and the information gained on the measurand? the error concept reflects only the inability of the instrument to supply the information, assumed to be contained in the true value: the measurement is treated in an all-ornothing way, and we cannot tie a larger error with less information. the uncertainty concept has uncertain meaning. taken as ‘the maximum allowed error’ of the instrument, a given uncertainty associated to the value indicated by the instrument means that we are uncertain on where the true value is in the interval thus defined, and the width of the latter might be construed qualitatively as information on the measurand. on the other hand, if we treat this interval as a set assigned in its whole to describe the measurand, this means we are indifferent on which element of the set represents the measurand: its description is the whole set and two measurands cannot be distinguished from each other if their value-spans overlap; then an instrument which in correspondence to its output signal is able (through its calibration operator) to assign a narrower value-span is also able to discriminate to a higher degree a measurand from others of similar description, which means it supplies more information on the measurand. with this meaning of ‘indifference ranges’ the concept is suited to quantitative treatment and may turn out quite handy in tricky issues as the pattern recognition. 5. time and measurement time affects measurement as an influence quantity like the others, playing two different roles: (a) as the time allowed for the measurement operation; (b) as the lifetime of the instrument (or of its calibration). the classical theory is ill equipped to cope with the first problem, as the true value is surely instantaneous (geometry is time independent) and no question of principle can arise with time-varying measurands. in practice a distinction is made between static and dynamic or stationary and transient measurements (definitions and terms change with the milieu); most standards refer only to the former, as no conceptual tools are available to deal with the latter. with an operative approach in terms of signals and assigned value-spans the measurement time fits snugly in the picture, as one of the influence quantities determining the width of the value-span, and one wonders whether a general relation might be worked out between uncertainty (as indifference range) and measurement time, similar to the one tying the indeterminacy of conjugate variables in quantum mechanics. the second role refers to the time dependence of the calibration, hence to the problem of reliability. the cussedness of measurement depends on time [12] and a distinction is usually made between repeatability and stability, but the current approach does not go much further: the ‘age’ of a calibration is not usually considered an influence quantity though there is no reason for not treating it as such. when we calibrate an instrument we warrant the consistency of the value-spans assigned under the calibration; how long do we mean the warranty to hold? not forever. shall we state that the confidence level of the warranty decreases in time or that the uncertainty increases? the calibration is a procedure of quality control on the instruments, carried out with statistical techniques; the question then arises whether the ergodic theorem may be applied to a population of instruments to predict the behaviour of single ones [4, 15]. references [1] l. gonella, alta frequenza, 4, 622 (1975). [2] l. gonella, imek0 tc7 symposium, enschede 1975. [3] l. gonella, seminar held at the course “metrology and fundamental constants”, int. school of physics “e.fermi”, varenna, july 1976. [4] l. gonella, imek0 tc7 symposium, leningrad 1978. [5] h.v. helmholtz, zahlen und messen erkenntnis – theoretisch betrachtet, philosophische aufsatze eduard zeller gewidmet, leipzig 1887. [6] s.s. stevens, measurement, definitions and theories (c.w. churchman and p. ratoosh eds.), new york 1959, 20. [7] b. ellis, basic concepts of measurement, cambridge 1968. [8] j. pfanzagl, theory of measurement, würzburg 1971. [9] d.h. krantz, r.d. luce, p. suppes, a. tversky, foundations of measurement, vol. 1, new york 1971. [10] l. finkelstein, measurement and control, 8, 105 (1975). [11] b. russell, the principles of mathematics, 2nd ed., new york 1937. [12] c. eisenhart, journal of research nbs, 67c, n°2, 161 (1963). [13] l’unificazione, 30, n°1, 21 (1976). [14] j.l. destouches, imeko tc7 symposium, enschede 1975. [15] j.l. destouches, imeko tc7 symposium, leningrad 1978. microsoft word 409-2586-1-le-rev3 acta imeko  issn: 2221‐870x  november 2016, volume 5, number 3, 95‐100    acta imeko | www.imeko.org  november 2016 | volume 5 | number 3 | 95  visible colour changes estimation in colorimetric  determination of chromium (vi) using polymeric sensors  nataliya a. gavrilenko, sergey v. muravyov, nadezhda v. saranchina, alexey v. sukhanov   national research tomsk polytechnic university, pr. lenina 30, 634050 tomsk, russia      section: research paper   keywords: colorimetric sensor; optode; chromium (vi); transparent polymeric matrix; immobilized reagents; digital colour analysis  citation: nataliya a. gavrilenko, sergey v. muravyov, nadezhda v. saranchina, alexey v. sukhanov, visible colour changes estimation in colorimetric  determination of chromium (vi) using polymeric sensors, acta imeko, vol. 5, no. 3, article 15, november 2016, identifier: imeko‐acta‐05 (2016)‐03‐15  editor: paolo carbone, university of perugia, italy  received july 4, 2016; in final form october 28, 2016; published november 2016  copyright: © 2016 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: this work was supported by russian science foundation (project 14–19–00926)  corresponding author: sergey muravyov, e‐mail: muravyov@tpu.ru      1. introduction  chromium compounds are powerful oxidizing agents and tend to be irritating and corrosive. they appear to be well known toxic substances governing various health and ecological risks [1]. trace levels of chromium can be determined by different analytical techniques most of which are based on atomic absorption spectrometry [2], inductively coupled plasma– mass/atomic emission spectrometry [3], and high-performance liquid chromatography [4]. these methods, in spite of high sensitivity, need expensive instrumentation and skilled staff. optochemical sensors play an important part in industrial, environmental and clinical monitoring thanks to their low cost, possibility for miniaturization and great flexibility [5], [6]. among different types of optochemical sensors, colorimetric sensors (optodes) are especially attractive because they recognize analytes through colour change that allows obtaining the visually observed and easily measurable analytical signal [7], [8]. the analytical signal measurement can be carried out using not only standard spectrophotometric equipment, but also the naked eye without the use of an expensive equipment. naturally, the naked eye techniques cannot be as accurate as spectrophotometry. that is why the visible colour changes should be measured using different chromaticity coordinates (in rgb, xyz, l*a*b*, and/or other systems) and parameters such as total colour difference (colour variation) δe. usually chromaticity parameters are calculated as functions of absorption or reflection spectra by means of computer programs intended for spectral data processing. nowadays, the measurement of colorimetric parameters has become easier due to a wide use of scanners, digital photo and video cameras, etc. [9]-[11]. an image of an optode is captured and transferred to a computer and its colour is interpreted using imaging software. this approach can be implemented also with transformation of an optode colour into an electric signal by means of a diode target array. it provides an opportunity for carrying out rapid chemical analysis in an industrial working environment and/or in the field. it also allows to design a abstract  the  paper  describes  an  application  of  a  kind  of  optical  analytical  method,  digital  colour  analysis  (dca),  using  colorimetric  polymethacrylate sensors (optodes) in order to determine cr (vi). the optodes are made of optically transparent polymethacrylate  matrix (pmm) with 1.5‐diphenylcarbazide immobilized. the developed optode can be used in determination of the analytes using solid  phase  spectrophotometry  and  calculating  colour  coordinates  as  functions  of  absorbance  spectra.  also  colour  coordinates  can  be  represented as basic colour (e.g. rgb) data after the optode image digitizing. then one can determine the content of an analyte in a  sample by an appropriate colour difference calculated for these coordinates. experimental results of cr (vi) determination in tap water  show that the dca relative standard deviation is 8‐17 % and recovery is < 12 % at the range of determined concentrations 0.05‐1.0  mg∙l –1 . the characteristics are comparable with those of the solid phase spectrophotometry.  acta imeko | www.imeko.org  november 2016 | volume 5 | number 3 | 96  reliable and easy-to-use measuring instrument that can be calibrated to be able to measure mass concentration of a whole series of substances. in the tomsk polytechnic university, such an analytical instrument has been created, which is called a digital colorimetric analyzer (dc analyzer) [12]. this portable instrument allows to measure colorimetric parameters of solid transparent optodes with high precision. the dc analyzer is controlled by a laptop or computer through a usb interface. specially developed software enables the instrument calibration for different test systems. due to compactness and ease of use, the dc analyzer can be used for rapid determinations in various practical domains. in the present work, an application of the dc analyser, using colorimetric polymethacrylate sensors as optodes, is considered in order to determine cr (vi). the optodes are made of optically transparent polymethacrylate matrix (pmm) with 1.5diphenylcarbazide immobilized. the developed optodes have been used in determining cr (vi) using solid-phase spectrophotometry and calculating colour coordinates as functions of absorbance spectra. as an alternative approach, colour coordinates can be represented by means of rgb data after the optode image digitizing. subsequent determination of the content of cr (vi) in a sample has been carried out on the base of calculating an appropriate colour difference as a function of the rgb coordinates. additionally, colorimetric parameters of the optode for cr (vi) determination were measured by means of the dc analyser. obtained outcomes are discussed in the present paper which is an extended version of the conference paper [13]. 2. methods  2.1. materials  the pmm is a specially created material containing functional groups which provide the ability to extract both the reagent and determined substance [14]. transparent 10  10 cm polymethacrylate plates, thickness (0.60 ± 0.04) mm, were prepared by the radical block polymerization of methacrylate and (alkyl)acrylates of alkaline (or alkaline earth) metals at the temperature 60-70 ˚c for 3-4 h. the plates were cut to platelets 6.0 × 8.0 mm (weight ca. 0.05 g) intended for analyses. all reagents were of analytical grade and used as purchased without further purification. deionized and distilled water was used in all experiments. the work solutions of 0.025–0.200 % 1.5-diphenylcarbazide were prepared by dissolving precise loads in ethanol by heating in a water bath with subsequent diluting by distilled water. the stock solutions of metals (1 mgml–1) were prepared by dissolving precise loads of their salts in 0.01 m acids. the work standard solutions of the required concentration were prepared by dilution of the stock solution in the day of the experiment. the required ph was adjusted using acid and alkali and controlled using an i-160 ionometer. 2.2. procedure  the immobilization of 1.5-diphenylcarbazide (dpc) into the polymethacrylate matrix was performed by sorption from water-alcohol solution of dpc under bath conditions during 3– 10 min. the tailoring of the optimum conditions of the interaction of immobilized dpc with cr (vi) are described in detail in [15]. briefly, pmm with immobilized dpc was put into 50 ml of cr (vi) solution of different concentration and ph and stirred for 15–30 min. after that absorption spectra or absorbance of pmm was measured. then the polymethacrylate matrix with the immobilized dpc was put into 50.0 ml of an analysed solution of definite concentration of cr (vi) at a ph ≈ 0 and stirred for 15 min. after that absorption spectra of the pmm were measured. then the colour coordinates x, y and z were calculated according to equations [16]:       780 10 380 ( ) ( ) ( )x k s x , (1)       780 10 380 ( ) ( ) ( )y k s y , (2)       780 10 380 ( ) ( ) ( )z k s z , (3) where     780 10 380 100 ( ) ( ) k s y , 10 ( )x , 10 ( )y , 10 ( )z are addition functions of cie 1964 supplementary standard observer, τ() is transmittance of the pmm, s() is relative spectral power distribution of the cie standard illuminant d65. after that, chromaticity coordinates x, y, z were found according to equations: х = х/(х + y + z) , (4) y = y/(x + y + z) , (5) z = z/(x + y + z) , (6) where the condition х + у + z = 1 is valid. then the transition from the xyz to the rgb system was made in accordance to the following equations:   3.2405 1.5371 0.476r x y z , (7)    0.9693 1.8760 0.041g x y z , (8)   0.0556 0.2040 1.0572b x y z . (9) the conversion from the xyz to the cie l*a*b* system was done as described by the following equations [17]:      1 3116( / ) 16 if / > 0.008856 * 903.3( / ) if / 0.008856 n n n n y y y y l y y y y , (10)  * 500[ ( / ) ( / )]n na f y y f y y , (11)  * 200[ ( / ) ( / )]n nb f y y f z z , (12) where       1 3 if 0.008856 ( ) 7.787 16 116 if 0.008856 t t f t t t , xn, yn and zn are values of coordinates of the white point of the system. the total colour differences  abs , xyze  lababse and  rgb abse for the chromaticity coordinates in xyz, cie l*a*b* and rgb colour systems calculated from absorption spectra were estimated according to the equations: 2 2 2 1 2xyz / abs ( )e x y z       , (13) 2 2 2 1 2lab / abs ( )e l a b       , (14) 2 2 2 1 2rgb / abs ( )e r g b       , (15) acta imeko | www.imeko.org  november 2016 | volume 5 | number 3 | 97  where δx, δy, δz; δl, δa, δb and δr, δg, δb are the colour coordinate changes in the xyz, cie l*a*b* and rgb systems, respectively. visible colour changes of the pmm were estimated with digital imaging by means of a scanner and image processing software. the pmm image was captured and transferred to a computer and its colour was interpreted using imaging software where the colorimetric data in rgb format were related to the concentration of the analyte. the total colour difference δescan for the scanned images of the pmm [18] evaluated by means of scanner and software photoshop cs was estimated according to equation: 2 2 2 1 2 0 0 0 / scan [( ) ( ) ( ) ]e r r g g b b       , (16) where r0, g0, b0 are the colour coordinates of samples after contact with a plain solution; r, g, b are the colour coordinates of samples after contact with the solution containing the substance under determination. visible colour changes of the pmm were also estimated with the dc analyzer. for this aim, the pmm after contact with solution under determination was placed into the receiving bin of the analyzer and measurements were carried out. the measurement results were obtained as series of rgb coordinates and the corresponding colour difference δeinstr was calculated by formula similar to (16). 2.3. apparatus  the absorption spectra and absorbencies of the pmm were recorded on the schimadzu uv-mini-1240 (schimadzu corporation, japan) and spekol 21 (carl zeiss jena, germany) spectrophotometers. a non-modified polymethacrylate matrix was used as the reference sample. as the desktop scanner the hewlett packard scanjet 4400 c was used. the personal computer was pentium ii (333 mhz). the software packages were used as follows: photoshop cs for image collection and taking colour intensities, excel spreadsheet for general calculations and origin 7.0 for plotting. the colorimetric parameters were measured by means of the developed dc analyzer (instrumental error < 1%, response time < 25 ms). the ph values were measured by the i-160 ionometer (npo ‘‘izmeritelnaya tekhnika’’, russia) with a glass ph-selective electrode. the ionometer had an absolute error ±0.020 ph and was calibrated at 25 c using buffer solutions with ph 1.00 and 9.18. 3. results and discussion  traditional colorimetric determination of chromium (vi) is based on the formation of the red-violet colored diphenylcarbazonate of chromium (iii) complex as a result of interaction of dpc and cr (vi) ions in acid media. the interaction includes the following two stages (figure 1): redox reaction between chromium (vi) and 1.5-diphenyl-carbazide with formation of chromium (iii) and diphenylcarbazonate (dpco), and complex formation reaction between chromium (iii) and dpco. the exact structure of this complex is not known but it appears to be a cationic complex [cr(iii)dpco](3–n)+ in which an unknown number of protons n are liberated [19]. other colorimetric methods for chromium determination can be found in papers [20]-[23]. 3.1. cr (vi) colorimetric determination using pmm the pmm with immobilized dpc after contact with cr (vi) solution had red-violet colour due to the formation of the cationic complex. the absorption spectra of this complex in the pmm and scanned images of the pmm are presented in figure 2. the absorption spectrum has a maximum at 545 nm. the absorption at the wavelength 545 nm was taken as the analytical signal for solid phase spectrophotometric determination of cr (vi) which was explained in more detail in [15]. 3.2. acidity influence investigation investigations of the influence of analyzed solution acidity on the analytical signal showed that maximum analytical signal figure 2. absorption spectrum of pmm with immobilized dpc after contact  with cr (vi) solution with concentration, mgl–1: curve 1 – 0; curve 2 – 0.05;  curve 3 – 0.10; curve 4 – 0.25; curve 5 – 0.50; curve 6 – 0.75; curve 7 – 1.00. figure 1. scheme of cr(vi) and dpc interaction.  0 0.1 0.2 0.3 0.4 0.5 a 400 500 600 λ, nm 7  6  5  4  3  2  1  7 6 5 4 32 1  n n o nh nh 3   (3 )2[cr(iii)dpcoh] hn n 28h o  8h + dpc dpco n n o nh nh 33 nh o nh nh nh  42hcro  3 2cr 3 2cr acta imeko | www.imeko.org  november 2016 | volume 5 | number 3 | 98  is reached at ph  0. also, the nature of acid plays a significant role in shaping the analytical signal for solid phase spectrophotometric determination of cr (vi). therefore, sulphuric acid and orthophosphoric acid are not suitable for maintaining the medium acidity. among hydrochloric acid and nitric acid the use of the former to maintain the medium acidity provided a maximum analytical signal which was stable during several hours, whereas the latter decreased the analytical signal and increased its stability up to several days. thus, the hydrochloric acid was used in further work due to the maximum analytical signal. 3.3. interfering ions influence investigation  influence of interfering ions such as fe (iii), cu (ii), hg (ii), v (v), co (iii), pb (ii), ni (ii) and mn (ii) on the determination of cr (vi) by using pmm with immobilized dpc was investigated. ions of each of the interfering elements were added individually to a 50 mg·l–1 chromium (vi) solution and relative error δ of cr (vi) determination in presence of the interfering element ions was calculated according to equation δ = [(aς – a)/a]·100, (17) where a is an analytical signal from the pmm with immobilized dpc after contact with cr (vi) solution without interfering ions; aς is the analytical signal after contact with the solution containing interfering ions. the interference investigation results are shown in table 1. 3.4. calibration dependences  the calibration dependences for cr (vi) were obtained for different methods and analytical signals, namely: traditional solid phase spectrophotometry and absorbance maximum a545; the solid phase spectrophotometry and total colour differences abs , xyze lababse and rgb abse computed by equations (14)-(16) on the base of the colour coordinates obtained from the absorption spectra; and the digital colour analysis (dca) [10] with total colour differences δescan for scanned optode images and δeinstr measured by the dc analyzer. the concentration dependences for cr (vi) are described by equations with high correlation coefficient values for all estimation methods of the visible colour changes. analytical performance for the equations of the colour changes are presented in table 2. graphical dependencies of the analytical signal versus concentration of cr (vi) in the analyzed solution are shown in figure 3. one can see in figure 3 that the slope of the calibration line rgbabse is greater than that of the calibration line abs xyze . it means that the use of rgb colour coordinates when computing the total colour difference for absorbance spectrum data provides a better sensitivity compared to xyz coordinates. it should be noticed that, in the case of cr (vi), the calibration dependence for δescan appears to have quadratic character. the calibration dependence for δeinstr could be described by both a second degree polynomial and a linear equation. as soon as the correlation coefficients for the two options were equal to each other, for the sake of simplicity, the linear equation was selected. precision of the procedure was expressed as the relative standard deviation (rsd) for the determination of 0.1 mgl–1 of cr (vi). values of range of determined concentrations (rdc), as well as of rsd, were obtained under conditions of reproducibility for a number of determinations n = 5 according to iso 5725-1994. expressed in percents rsd was estimated by the formula 2 1 1 1 1      ( )n iirsd c cc n , (18) where n is the number of determinations (optodes); ci is the result of i-th concentration determination; and c is the arithmetic mean of the n determination results. expressed in percents recovery q was estimated by the formula   added found added q q q q , (19) where qadded is introduced addition content and qfound is found addition content as an averaged value of n determinations. the accuracy and precision of cr (vi) determination results were verified by the standard addition method using drinking water (table 3). one can see from table 3 that the characteristics of accuracy and precision for analytical signals and δescan of the digital colour analysis are comparable with those of the solid phase spectrophotometry (a545 and δeabs). 4. conclusion  the proposed colorimetric sensor on the base of the pmm with immobilized dpc can be successfully used for the determination of cr (vi) by both solid phase spectrophotometry and digital colour analysis (dca). different analytical signals based on estimation of visible colour changes were studied. metrological performance of the dca method was shown to be comparable with those of the solid phase spectrophotometry. table 2. analytical performance of the cr (vi) colorimetric sensor.  analytical  signal  calibration equation  r 1   rdc 2 , mg∙l –1   a545  0.003 + 0.525  ccr(vi)  0.998  0.05–1.00   rgbabse   0.002 + 0.167  ccr(vi)  0.999  0.05–1.00   lababse 0.4 + 47.8  ccr(vi)  0.999  0.05–1.00   xyzabse   0.003 + 0.103  ccr(vi)  0.998  0.05–1.00  δescan  5.63 + 208.29  ccr(vi)  –  77.24c2cr(vi)  0.997  0.05–1.00  δeinstr  3.335 + 118.85  ccr(vi)  0.992  0.05–1.00  1  r is a correlation coefficient   2  rdc is a range of determined concentrations  table 1. influence of interfering ions.  ions  concentration ratios  δ, % fe (ii)  10  5  50  7  cu (ii)  10  5  50  10  v (v)  10  5  50  13  hg (ii)  10  5  50  13  mn (ii)  10  5  50  13  co (iii)  50  5  pb (ii)  50  5  ni (ii)  50  5  acta imeko | www.imeko.org  november 2016 | volume 5 | number 3 | 99  figure 3. calibration dependences for determination of cr (vi) obtained for estimation of visible colour changes using the analytical signals: a545,  xyz absδe , lab absδe , rgb absδe , δescan and δeinstr.  table 3. determination of cr (vi) in tap water (sample size n = 5, confidence level р = 0.95). technique  analytical signal  added, mg∙l ‐1   found, mgl‐1  rsd1, %  recovery, %  solid‐phase   spectrophotometry         a545  0.070  0.065 ± 0.016  15  –7  0.300  0.29 ± 0.03  7  –3  0.600  0.601 ± 0.023  2  0.2  colour coordinates   rgbabse   0.070  0.066 ± 0.008  14  –6  0.300  0.29 ± 0.02  8  –3  0.600  0.63 ± 0.05  8  5   lababse   0.070  0.068 ± 0.008  14  –3  0.300  0.280 ± 0.023  8  –5  0.600  0.62 ± 0.05  8  3   xyzabse   0.070  0.070 ± 0.013  21  0.2  0.300  0.29 ± 0.03  12  –3  0.600  0.63 ± 0.04  7  4  digital colour analysis   δescan   0.070  0.069 ± 0.012  20  –2  0.300  0.31 ± 0.03  11  2  0.600  0.59 ± 0.06  11  –2  δeinstr   0.070  0.066 ± 0.005  8  –6  0.300  0.34 ± 0.05  17  12  0.600  0.65 ± 0.08   14  9  1  rsd is a relative standard deviation  0 0,03 0,06 0,09 0,12 0,15 0,18 0,0 0,2 0,4 0,6 0,8 1,0 a b so rb a n ce  a t  5 4 5  n m concentration of cr (vi),  mg∙l‐1 0.18 0.15 0.12 0.09 0.06 0.03 0.00 0.0              0.2              0.4             0.6              0.8             1.0 0 10 20 30 40 50 60 0 0,2 0,4 0,6 0,8 1 t o ta l  co lo u r  d if fe re n ce   concentration of cr (vi),  mg∙l‐1 0.0              0.2             0.4             0.6              0.8            1.0 0,00 0,04 0,08 0,12 0,16 0,0 0,2 0,4 0,6 0,8 1,0 t o ta l  co lo u r  d if fe re n ce   concentration of cr (vi),  mg∙l‐1 0.16 0.12 0.08 0.04 0.00 0.0              0.2            0.4             0.6            0.8            1.0 0 30 60 90 120 150 0 0,2 0,4 0,6 0,8 1 t o ta l  co lo u r  d if fe re n ce   concentration of cr (vi),  mg∙l‐1 0.0             0.2            0.4             0.6            0.8            1.0 xyz abse  rgbabse  instre  scane  lababse 545a acta imeko | www.imeko.org  november 2016 | volume 5 | number 3 | 100  the dca for determination of not only cr (vi) but also a wide spectrum of other elements and/or substances can be implemented using an office scanner with graphic software or the dc analyzer. regardless of the implementation mode, it can be applied as a highly sensitive method of chemical analysis in both laboratory and field conditions. use of the dc analyzer facilitates field measurements by means of the dca due to its compactness. acknowledgement  this work was supported by russian science foundation, project 14-19-00926. references  [1] epa/630/p-02/004f, generic ecological assessment endpoints (geae) for ecological risk assessment, u.s. environmental protection agency, washington, dc, october 2003. 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[23] p. parmar, a.k. pillai, v.k. gupta, an improved colorimetric determination of micro amounts of chromium (vi) and chromium (iii) using p-aminoacetophenone and phloroglucinol in different samples, j. anal. chem. 65(6) (2010) pp. 582–587. template for an acta imeko event paper acta imeko issn: 2221-870x december 2016, volume 5, number 4, 4-11 acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 4 optical system for on-line monitoring of welding: a machine learning approach for optimal set up giulio d’emilia, david di gasbarro, emanuela natale università degli studi dell’aquila, via giovanni gronchi 18, 67100 l'aquila, italy section: research paper keywords: p-value; machine learning; vision system; uncertainty; online control; welding citation: giulio d’emilia, david di gasbarro, emanuela natale, optical system for on-line monitoring of welding: a machine learning approach for optimal set up, acta imeko, vol. 5, no. 4, article 2, december 2016, identifier: imeko-acta-05 (2016)-04-02 section editor: lorenzo ciani, university of florence, italy received september 24, 2016; in final form december 14, 2016; published december 2016 copyright: © 2016 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: giulio d’emilia, e-mail: giulio.demilia@univaq.it 1. introduction on-line monitoring of laser welding can be carried out by many inspection techniques, based on acoustic emission, optical detection, image analysis, radiographic testing and ultrasonic [1]. in particular, non-contact optical systems are widely used for on-line monitoring of quality, geometry and position of welding, based on different optical measurement methods, like spectroscopic studies of the welding plasma plume [2], triangulation systems [3] and other types of solutions [4]; remarkable measuring performances could be achieved in terms of measurement uncertainty, even though the cost of these solutions is often high, so that their use is limited to specific applications. furthermore, if reliable and accurate information is needed, more actions have to be provided, in order to improve the efficaciousness of results for geometry and defect classification, in particular for what the image processing is concerned. in fact, careful image analysis algorithms have been developed for defect classification for images obtained by radiographic systems [5], [6] or by measurement sensors of the eddy current type [7]. further approaches appear in literature, often based on neural networks and genetic algorithms, having different objectives: identification of welding parameters for optimization of both welding characteristics and cost [8], [9], classification of defects in general of components for the automotive industry [10] or of welding in particular [11]. furthermore, deep attention has been paid to the use of neural networks for image analysis for quick and effective classification based on a machine learning method [12]. an optical laser sheet is a simple and economical way to carry on non-contact geometrical measurements in different types of applications [3], [4], [13], [14]; a sheet of laser light allows to overcome the problem of low contrast images and to obtain, in a simple manner, information on the shape of surfaces. this is particularly important in case of investigation of untreated metal surfaces. in fact, at the intersection of the laser sheet with the object to be analysed, the laser line appears to be abstract in this paper a methodology is described for continuous checking of the settings of a low-cost vision system for automatic geometrical measurement of welding embedded on components of complicated shape. the measurement system is based on a laser sheet. measuring conditions and the corresponding uncertainty are analyzed by evaluating their p-value and its closeness to an optimal measurement configuration also when working conditions are changed. the method aims to check the holding of optimal measuring conditions by using a machine learning approach for the vision system: based on a such methodology single images can be used to check the settings, therefore allowing a continuous and on-line monitoring of the optical measuring system capabilities. according to this procedure, the optical measuring system is able to reach and to hold uncertainty levels adequate for automatic dimensional checking of welding and of defects, taking into account the effects of system hardware/software incorrect settings and environmental effects, like varying lighting conditions. the paper also studies the effects of process variability on the method for quantitative evaluation, in order to propose on-line solutions for this system. acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 5 well contrasted with respect to the object surface. in this way, the intersection points (circled in red in figure 1) between different surfaces of an object are easily identified. usually the accuracy of these systems has to be optimized, in order to achieve a satisfactory uncertainty level [15]-[17], especially for the control of complicated geometries as in case of automotive applications. furthermore, the best quality level for measurements has to be maintained and this should be assessed on-line with a method not too onerous from the point of view of time, operations and data processing requirements. therefore, it is important to develop both a vision system that works well under certain environmental conditions and a methodology able to automatically maintain good metrological characteristics, also in complex applications and varying operating conditions. in this paper the measurement capability of a measurement system based on a laser sheet is described, with reference to the monitoring of welding having a complicated geometry, for realization of big mechanical components. furthermore, an experimental methodology is presented, in order to improve the possibility of using this simple measuring solution also in difficult situations, aiming to automatically check and maintain conditions of good setting of the vision system, with particular attention to the variations of the lighting level and characteristics. this work aims to individuate the main parameters affecting uncertainty and to evaluate their effect from both a theoretical and experimental point of view. a previous work of the authors [18] developed a first attempt to set a method based on a laser sheet system able to be robust and simply to use, for on-line continuous check of the right optical and procedural settings. the p-value parameter [19] was used to automatically identify the best settings for the vision system, taking into account the main causes of uncertainty. the method demonstrated to be able to detect the effects of a little variation of settings, so strongly supporting the setup of the vision system. the effect of process variability was also studied, in particular dimensional variability of pieces to be welded together, showing that the method is very sensitive to it. the methodology proved to be robust and reliable in practical applications. according to the previous considerations, the paper describes an improved methodology able to improve the accuracy of dimensional measurements and to reduce the processing time of the images of the measured pieces for more efficient use in on-line applications. the method is based on the use of a machine learning approach for optimal set-up and uncertainty reduction, despite of the variation of procedural and environmental conditions. a specific section will present the improved methodology explaining the theoretical and experimental motivations of it. the experimental results and the way the data have been processed will be described, in order to validate the methodology for a class of practical applications. some discussions with reference to the use in field will conclude the paper. 2. materials and methodology position and dimensional check of welding is by a vision system, which is based on a laser sheet, whose architecture is shown in (figure 2 and figure 3). a detailed description of components and the complete procedure for the setup of the system is given in a previous work [20]. in the present paper, the matlab software and machine learning toolbox and computer vision system toolbox have been used, to analyse the illumination conditions in the acquired images. the proposed system uses the same hardware components of the triangulation systems [3], but it provides a different software elaboration; in fact, this approach does not identify the figure 2. scheme of the system for the welding control. figure 3. picture of the system for the welding control. figure 1. example of identification of the intersection point between two lines generated by the laser sheet. acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 6 geometrical surface of the analysed object, but it evaluates, in a simple manner, the distance of characteristic points from a reference. this possibility can be useful in all phases of a welding process: the checking of the line of contact of two pieces to be welded (seam tracking); the driving of the robotic arm that deposits the welding bead; the validation of the position and of the dimensional parameters of the realized welding (positioning and shape of the weld). measurements have been carried out on a piece constituted by two perpendicular steel plates, to be welded together by a linear bead, that is the typical t-joint. the weld seam will have a convex shape and a throat thickness of about 5 mm. in this case, the position of interest is the intersection point between two lines generated by the laser sheet (figure 1), with respect to a reference point of the piece (seam tracking). it is to be noticed that the method is also suitable for other configurations, allowing the measurement of any dimensional parameter based on the evaluation of the distance between two different points. this method is potentially not influenced by the welding parameters. it is important to ensure that characteristics of correct setting of the system are set over all the time, so that the measurement results are reliable and accurate. having a correct setting physically means that all variability causes act in a random and reduced way, so that no systematic and no remarkable effects of any specific variability cause appear. therefore, if the system is properly set, measurements are according a gaussian variability with fixed mean and reduced standard deviation [21]. the methodology described hereinafter is based on this assumption. this condition should be guaranteed both at the installation time of the vision system and during the on-line working. in the previous step [18] the gaussian distribution of repeated measurement of quantities of interest was verified by the p-value [19]. the p-value is widely used in statistical hypothesis testing. a model (the null hypothesis) and a threshold value for p, called the significance level of the test and denoted as α, have to be chosen. if the p-value is less than or equal to the significance level (α), the test suggests that the observed data is inconsistent with the null hypothesis, so the null hypothesis must be rejected [19]. in this case the null hypothesis is the gaussian distribution of measurement results, then, if the p-value, ranging between 0 and 1, is greater than a pre-set threshold, the gaussian distribution of measurements is guaranteed, with the confidence level indicated in the calculation of the p-value itself. of course, before the production activity begins, the time available for checking that the measurement distribution is effectively gaussian is much more than the one available during the production, due to the need of avoiding bottle necks to the production rhythms. therefore, automatically checking these conditions asks different requirements due to differences of time duration available for control before the start of production and during the production itself; this will influence the number of repeated measurements which are possible. the need of assuring that the probability distributions of possible measurements are unchanged (according to the measurement performances) all along the time requires that the closeness of both distributions is evaluated in very short time for on-line control. in the former approach [18] measurements were carried out on each production workpiece, to check that the optimal setup conditions are maintained. each measurement was repeated n times (typically n=30); that takes about 2 s to 3 s depending on the acquisition and image processing rate of the system. the pvalue was calculated on the basis of these n measurements. then, the p-value estimate is compared with a threshold value: if the p-value is above the threshold level, the system is correctly set. the threshold level can be defined in relation to a requested accuracy and to specific environmental conditions by means of a statistical analysis (mean and standard deviation of p-values) based on a sufficient number of p-values (at least 30 p-values) calculated on the first batch of production. being necessary to acquire 30 measurements in static condition and to post-elaborate the p-value, in some cases the production process can be too fast for this method. for these reasons in this paper the procedure has been improved according to actions described in the following: 1. "optical" setup of the vision system and spatial calibration: the optical setup is carried out using a simple calibration pattern. the pattern has to be framed by the camera, and a program for the identification of edges and intersection points is implemented, using the software for the image processing. focusing and exposure time are manually set, until all elements are recognized. a gauge block of known size is used for the evaluation of the ratio pixel/mm. 2. preliminary tests: an experimental and parametric study of the effect of changing the settings of the optical system and of the variation of environmental conditions is carried out. of course, the most relevant aspects are taken into account, like grey level settings and environmental light intensity and typology variation, but other variability sources of the welding process could be considered like, for example, differences in surface reflection. for each test condition the p-value and its statistics are evaluated, in order to correlate estimated ranges of the p-value to different operating conditions, depending on settings and environmental scenario. this step is intended to realize a correspondence between level of p-value, illumination conditions and settings of the vision system. the estimated uncertainty level is also of concern. 3. training and testing of the machine learning classifier: for each test condition 150 images are acquired to train and test a machine learning classifier. the aim of the classification is to quickly identify the operating illumination condition and then the corresponding optimal settings of the vision system. 4. validation: repeated measurements for all the considered working conditions are carried out in order to check the reproducibility of the p-value measurements. this step will be also used to more effectively define the p-value threshold, taking into account the variability in reproducibility conditions. 5. on-line monitoring of the optical system: one image is recorded and classified by the machine learning classifier, to identify the actual operating conditions (illumination condition and settings of the vision system). in this way, image classification allows to measure indirectly the p-value for the system: in case a predefined threshold in not reached, the system settings should be automatically modified in order to improve the p-value for optimal setting. when the p-value is greater than the threshold limit, the measure of the distance of interest can be accepted with known uncertainty level. a synthesis of the methodology is represented in the flow diagram of figure 4. acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 7 in the next section the methodology will be discussed with reference to some practical applications concerning pieces to be welded. 3. results in this section the effects of varying the settings of the vision system with respect to the best ones in different lighting conditions and the application of the machine learning procedure are evaluated. measurements have been carried out on a piece, as an example, constituted by two perpendicular steel plates, to be welded together by a linear bead (figure 1). grey threshold has been identified as the most relevant setting parameter, to be studied, together with the lighting conditions as for the environmental parameters. the change of setting of the vision system, by varying the grey thresholds (variation of the grey level in the transition from dark area to the light one, ranging from 0 to 255) modifies the two boundaries of the laser line, which is a broken line because of the object shape (figure 1). these data should be processed in order to find the distance to be measured. six different conditions of illumination (ic) have been compared. for each lighting different settings of the grey threshold have been examined: ic 1: a neon light system at a 2 m distance from the object. ic 2: the same as ic 1, but the neon light system is partially obscured on the left side at 1.5 m of distance of the steel piece. ic 3: the same as ic 1, but the neon light system is partially obscured at 0.3 m distance. ic 4: the same as ic 3, but the neon light system is partially obscured at 0.5 m distance. ic 5: the same neon system as ic 1, but a led lamp is added at 0.3 m distance. ic 6: the same as ic 2, but the led lamp is placed at 0.5 m distance. the system setting can be optimal or not. this depends on the grey threshold values selected. the conditions are optimal when the measurement result corresponds to the reference one and the standard deviation is based on casual effects and then the calculated p-value is very high (near to 1). for each illumination condition four cases are analysed: three cases have different grey threshold values, the last one is the reproduction of the best case among the previous, based on the p-value. table 1 shows the test plan, having 24 different cases. according to the procedure described in the previous section, a large number of data has been acquired, in the order of 3000 measurements for each case and 150 bitmap images. the results of the “preliminary tests” are discussed in the following subsections. 3.1. p-value analysis the results “preliminary steps” are described in the graphs of figure 5. for all tests n = 30, being n the number of repeated on-line measurements. in particular, the diagrams of figure 5 show the behaviour of the mean p-value (averaging 100 p-values) with respect to the set measurement uncertainty (standard deviation of the normal reference distribution of the vision system measurements) for all the examined cases. the reference distance value is (60.77 +0.01) mm. for the cases with the optimal settings, even though the set uncertainty is reduced, the mean p-value remains practically unchanged, maintaining high values; for the other two cases, the found p-value quickly drops when the requested uncertainty of measurements is reduced. this is true for all the illumination conditions (except for ic 5). this result is very reasonable and it also suggests that a threshold value could be set to separate the best setting case from other ones. the diagrams of figure 5 show that separated p-value ranges occur, between settings optimal or not, if a reduced standard uncertainty is considered (standard uncertainty less than 0.23 mm). this result confirms the ability of this method of distinguishing the correct setting from slightly different configurations of the vision system. in all the conditions of lighting considered, in case the right setting of the grey threshold is fixed, the estimated distance between the reference points is in the range (60.77 ± 0.01) mm. the diagram of figure 5 shows that the p-value is strongly dependent on the grey setting for all the lighting conditions, which have been examined. due to the found differences of pvalue, even though the variability of p-value is taken into account, threshold values could be easily set, in order to quickly identify a condition of right setting. it is interesting to compare for each illumination condition p-values between cases 1 and 4, corresponding to same settings: the p-value estimates are very repeatable for all conditions. 3.2. machine learning classification the results of the “training and testing of the machine learning classifier” phase of the methodology described in section 2 are now discussed. 150 bitmap images, randomly selected from cases 1 to 3 for each illumination condition, have been used for the training and the testing of the machine learning algorithm. the steps for the machine learning system are described in the following: 1. loading the images in the system: 900 images are loaded in the system (150 for each illumination condition). figure 4. flow diagram of the methodology. acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 8 table 1. tests plan. illumination condition case grey threshold 1 grey threshold 2 optimal illumination condition 1 case 1 40 60 illumination condition 1 case 2 45 80 optimal illumination condition 1 case 3 55 100 illumination condition 1 case 4 45 80 optimal illumination condition 2 case 5 45 80 illumination condition 2 case 6 75 40 optimal illumination condition 2 case 7 55 60 illumination condition 2 case 8 75 40 optimal illumination condition 3 case 9 45 80 illumination condition 3 case 10 55 50 optimal illumination condition 3 case 11 65 65 illumination condition 3 case 12 55 50 optimal illumination condition 4 case 13 45 80 illumination condition 4 case 14 50 90 optimal illumination condition 4 case 15 55 60 illumination condition 4 case 16 50 90 optimal illumination condition 5 case 17 45 80 illumination condition 5 case 18 50 75 illumination condition 5 case 19 55 60 optimal illumination condition 5 case 20 55 60 optimal illumination condition 6 case 21 45 80 illumination condition 6 case 22 85 54 optimal illumination condition 6 case 23 65 65 illumination condition 6 case 24 85 54 optimal figure 5. mean p-values and their standard deviation ranges vs. standard uncertainty. acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 9 2. extracting features from the images and keeping 80 % of the strongest features from each image set. a feature is a measurable property or a characteristic of the image; for instance, a geometrical point of discontinuity in the image or an area at a specific grey level. in total the matlab script automatically finds 24491 features for each illumination condition. 3. using k-means clustering to create a 200 words visual vocabulary from the 24491 features. 4. encoding all the 900 images with the features and the clustering method described in point 2 and 3. 5. selecting and training the supervised machine learning classifier: the images used for the training are 50 % of the available ones for each illumination condition. the others ones are used for the testing of the system. a large number of algorithms are compared for the training of the system. the results are described in table 2. most of the algorithms detect the features with 100 % of accuracy. this result confirms the high quality of the images and the uniformity of them in each training set. this is a good result for the training phase but a problem in on-line monitoring. in fact, an algorithm trained with a dataset having low variability generally is unable to work with real data, which are more variable than the original ones. 3.3. validation of the machine learning classifier the results of the “validation” phase of the methodology described in section 2 are now discussed: the images of the fourth case of each illumination condition are used to test the classifier. five images are taken from each of the following cases: 4, 8, 12, 16, 20, 24. these cases are representative of all the illumination conditions (ic 1 to ic 6). these images are taken in the same conditions as the other cases, after a few hours delay. even though a few hours interval occurs between acquisitions, the images to be compared are very similar (figure 6). the classifiers with accuracy 100 % have been used in the validation step. among these, the best results have been reached by means of the “linear discrimination” classifier, described in table 3, in the second column. all the conditions are correctly identified. based on the successful classification, the working situation is identified and the standard deviation of the measurements can be estimated, depending on the actual setting of the grey threshold levels, according to the results of figure 5. in order to check the reproducibility of measurements, the mechanical and the optical systems have been dismounted and reassembled; also all the different illumination conditions were recreated as before, with the aim of obtaining the same working conditions as cases 4-8-12-16-20-24. a preliminary p-value evaluation has been made, to check that optimal working conditions arise for all illumination conditions to be analyzed. a test concerning the classification of images, according to the illumination condition, is carried out; results are shown in the same table 3 in the third column. first image of case 1 last image of case 4 figure 6. comparison of images with same illumination condition. table 3. response of linear discrimination classifier training results. illumination condition of the input image response of linear discrimination classifier response of linear discrimination classifier with new data ic_4 ic_4 ic_3 ic_2 ic_2 ic_2 ic_3 ic_3 ic_2 ic_2 ic_2 ic_2 ic_1 ic_1 ic_1 ic_6 ic_6 ic_6 ic_5 ic_5 ic_5 ic_4 ic_4 ic_3 ic_6 ic_6 ic_6 ic_5 ic_5 ic_5 ic_5 ic_5 ic_5 ic_4 ic_4 ic_3 ic_6 ic_6 ic_6 ic_3 ic_3 ic_2 ic_1 ic_1 ic_1 ic_3 ic_3 ic_2 ic_2 ic_2 ic_2 ic_1 ic_1 ic_1 ic_4 ic_4 ic_3 ic_1 ic_1 ic_1 ic_3 ic_3 ic_2 ic_2 ic_2 ic_2 ic_6 ic_6 ic_6 ic_1 ic_1 ic_1 ic_6 ic_6 ic_6 ic_4 ic_4 ic_3 ic_3 ic_3 ic_2 ic_5 ic_5 ic_5 ic_2 ic_2 ic_2 ic_5 ic_5 ic_5 table 2. training results. classifier accuracy decision trees complex 99.3 % decision trees medium 99.3 % decision trees simple 83.3 % discrimination analysis linear 100 % discrimination analysis quadratic 100 % svm linear 100 % svm quadratic 100 % svm cubic 100 % knn fine 100 % knn medium 100 % knn coarse 100 % knn cosine 100 % knn cubic 100 % knn weighted 100 % ensembles boosted trees 17 % ensembles bagged trees 100 % ensembles subspace discriminant 100 % ensembles subspace knn 100 % acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 10 the classification capability of the classifier is satisfactory: only a few conditions are confused, being very similar with respect to the illumination condition (figure 7). in case of confusion, the uncertainty corresponding to the poorer illumination condition can be considered. 4. conclusion a method has been discussed for on-line validation of position measurements of a monitoring system for welding. the device is based on a vision system and a laser sheet. the p-value parameter has been used to automatically identify the best settings for the vision system, taking into account the main cause of uncertainty. the method is able to detect the effects of a little variation of settings, so strongly supporting the setup of the vision system. a procedure has been proposed with the purpose of applying it on-line; it has been validated for different conditions of lighting. the results proved that the procedure is able to check the holding of optimal measuring conditions by using a machine learning approach for the vision system: based on such a methodology single images can be used to check the settings, for on-line use. once the working situation is identified, the standard deviation of the measurement can be estimated, depending on the actual setting of the grey threshold levels. achievable uncertainty values are in the order of some tenths of a millimeter. as an important improvement of the previous version of the method, the presented solution allowed to make the measurement procedure unaffected by the process variability with a remarkable reduction of measurement uncertainty. acknowledgement the courtesy of vision device srl, torrevecchia teatina, italy, for making available the vision system and the processing software, is gratefully acknowledged. references [1] j. shao, y. yan “review of techniques for on-line monitoring and inspection of laser welding”, journal of physics: conference series 15 (2005) pp. 101-107. [2] m. ferrara, a. ancona, p. m. lugara, m. sibilano, “online quality monitoring of welding processes by means of plasma optical spectroscopy”, p soc photo-opt ins (2000) pp. 750-758. [3] d. acosta, o. garcia, j. aponte, “laser triangulation for shape acquisition in a 3d scanner plus scan electronics”, proc. robotics and automotive mechanics conference, 26-29 sept. 2006, cuernavaca, morelos, mexico, pp. 14-19. [4] f. kong, j. ma, b. carlson, r. kovacevic, “real-time monitoring of laser welding of galvanized high strength steel in lap joint configuration”, opt. laser technol. 44 (2012) vol. 44 pp. 2186– 2196. [5] g. wang, t. warren liao, “automatic identification of different types of welding defects in radiographic images”, ndt &e international 35 (2002) pp. 519-528. [6] k. aoki, y. suga, “application of artificial neural network to discrimination of defect type in automatic radiographic testing of welds”, isij international 39 (1999) pp. 1081-1087. [7] o. postolache, a. lopes ribeiro, h. ramos, “weld testing using eddy current probes and image processing”, proc. of xix imeko world congress fundamental and applied metrology, september 6-11, 2009, lisbon, portugal, pp. 438-442. [8] j. gunther, p.m., pilarski, g. helfrich, h. shen, k. diepold, “intelligent lase welding through representation, prediction and control learning: an architecture with deep neural networks and reinforcement learning”, mechatronics 34 (2016) pp. 1-11. [9] h.y. tseng, “welding parameters optimization for economic design using neural approximation and genetic algorithm”, int. j. adv. manuf. technol. 27 (2006) pp. 897-901. [10] a. sumesh, k. rameshkumar, k. mohandas, r. shyam babu, “use of machine learning algorithms for weld quality monitoring using acoustic signature”, procedia computer science 50 (2015) pp. 316-322. [11] s. ravikumar, k.l. ramachandran, v. sugumaran, “machine learning approach for automated visual inspection of machine components”, expert systems with applications 38 (2011) pp. 3260-3266. [12] a. krizhevsky, i. sutskever, g. e. hinton, “imagenet classification with deep convolutional neural networks”, advance in neural information processing systems 2 (2012) pp. 1097-1105. [13] t. qing-bin, j. chao-qun, h. hui, l. gui-bin, d. zhen-liang, y. feng, “an automatic measuring method and system using laser triangulation scanning for the parameters of a screw thread”, meas. sci. technol. 25 (2014) pp. 035202-035211. [14] p. bellandi, f. docchio, g. sansoni, “roboscan: a combined 2d and 3d vision system for improved speed and flexibility in pickand-place operation”, int j adv manuf tech 69 (2013) pp. 18731886. [15] i. beˇsic´, n. vangestel, j.p. kruth, p. bleys, j. hodolicˇ, “accuracy improvement of laser line scanning for feature measurements on cmm”, opt laser eng, 49 (2011) pp. 1274– 1280. [16] m. mahmud, d. joannic, m. roy, a. isheil, j.f. fontaine, “3d part inspection path planning of a laser scanner with control on the uncertainty”, computer-aided des. 43 (2011) pp. 345–355. [17] f. xi, y. liu, h.-y. feng, “error compensation for threedimensional line laser scanning data”, int j adv manuf tech 18 (2001) pp. 211–216. [18] g. d’emilia, d. di gasbarro, e. natale, “on line control of ic_2 ic_3 ic_4 figure 7. example images from ic_2 ic_3 ic_4. acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 11 optimal set-up of a laser sheet system for real time monitoring of welding”, proc. of 14th imeko tc10 workshop on technical diagnostics, june 27-28, 2016, milan, italy, pp.158-163. [19] d. c. montgomery, “statistical quality control”, mcgraw-hill, 2006, isbn 978-0470169926, pp.116-117. [20] g. d’emilia, d. di gasbarro, e. natale, “a simple and accurate solution based on a laser sheet system for position and size on line monitoring of weldings”, j phys conf ser. 658 (2015). [21] s. a. alfaro, g. c. carvalho, f. r. da cunha, “a statistical approach for monitoring stochastic welding processes”, j. mater. process. technol. 175 (2006) pp. 4-14. optical system for on-line monitoring of welding: a machine learning approach for optimal set up microsoft word 239-1855-2-le-rev acta imeko  issn: 2221‐870x  december 2015, volume 4, number 4, 75‐81    acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 | 75  dynamic calibration uncertainty of three‐axis low frequency  accelerometers  giulio d’emilia, antonella gaspari, emanuela natale   university of l’aquila, dipartimento di ingegneria industriale e dell’informazione e di economia,via g. gronchi 18 , 67100 l’aquila, italy       section: research paper   keywords: three‐axis accelerometer, low frequency vibration, calibration, uncertainty, cross sensitivity   citation: giulio d’emilia, antonella gaspari, emanuela natale, dynamic calibration uncertainty of three‐axis low frequency accelerometers, acta imeko, vol.  4, no. 4, article 14, december 2015, identifier: imeko‐acta‐04 (2015)‐04‐14  editor: paolo carbone, university of perugia, italy  received december 23, 2014; in final form june 12, 2015; published december 2015  copyright: © 2015 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  corresponding author: giulio d’emilia, giulio.demilia@univaq.it    1. introduction  social resilience to disasters is now considered a topic of highest political and technical concern in advanced nations. social safety and building heritage along with sustainable urban development are important issues in setting guide lines which aim to improve this topic. for this purpose, evaluating the conditions and performance of existing civil infrastructures is a crucial aspect that allows the decision-makers to configure the best lines of activity in order to improve the quality of life [1]. to have integrated procedures involving geotechnical and structural aspects finalized to buildings' diagnostics, a distributed sensor network is needed, allowing us to operate in a selective manner and in the most critical situations. this implies the need of a greater number of measuring sensors, for both the three-axis vibration as well as inclination measurements. this innovation, together with the need of covering wide areas through this multi-sensors network in order to include different buildings, sets the requirement of lower-cost solutions that are nevertheless able to ensure the requested uncertainty of measurements. most of the above mentioned measurement and cost requirements could be satisfied by the well-known microelectro-mechanical systems (mems) technology and some examples of its use for these applications can be found in literature [2], even though careful attention should be paid to many possible causes of errors [3], especially when low-cost and low-precision mems accelerometers are considered. due to these facts, many studies can be found in literature, referring to calibration of these sensors, using mechanical reference quantities [3], [4], or using fully electrical methods to estimate the sensitivity of capacitive mems accelerometers in batch fabrication [5]. abstract  in this paper a methodology concerning the static and dynamic calibration of three‐axis low‐cost accelerometers in the (0 to 10) hz  frequency range is described, to be used for evaluation of existing civil infrastructures.   main and cross sensitivities of the accelerometers have been experimentally estimated by means of  the matrix sensitivity concept.  the standard deviation of accelerations obtained along all three axes using different calibration data sets in repeatability conditions  has been calculated and intended as dynamic calibration uncertainty.  the method has been validated by using reference accelerations accurately realized, in order to evaluate the residual bias error.  static  and  dynamic  calibration  test  benches  have  been  used  to  realize  reference  accelerations.  in  order  to  create  a  three‐axis  acceleration field, a mechanical arm is used in static calibration; a rotary device is used in order to test the accelerometers in dynamic  conditions.   according  to  the  procedure  described  in  this  paper,  a  great  improvement  of  the  low  cost  accelerometers’  metrological  characterization could be achieved, especially in dynamic working conditions. acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 | 76  even if different aspects are studied, like bias correction and main and cross sensitivity evaluation, an exhaustive description of calibration uncertainty causes cannot be found, in particular with reference to the dynamic behaviour in the low frequency range (0 to 10 hz). market and literature solutions [2], [6], [7] and characteristics of the calibration test bench designed for this specific application [8], [9], [10], [11], [12], [13], [14] have been considered, but the provided solutions do not appear completely satisfactory if the right trade-off between uncertainty of calibration on one hand, and the cost of the test bench and the calibration procedure duration on the other hand are considered. in fact, only in a few cases the analysis refers to three-axis accelerometers and the test rigs are generally very expensive; they are based on three-dimensional shakers and only consider the high frequency range [13], [14]. when the low frequency range is examined, different devices have been used [2], [6], [7] [8], [9], [10], [11], [12]; in all these cases, the cross sensitivities are not determined and this appears detrimental with respect to the achievement of a satisfactory sensor accuracy. furthermore, bearing in mind these considerations, the first aspect to be considered is the limit that could be achieved with reference to the uncertainty of sensors to be used for acceleration and inclination, taking into account cost requirements as well. this goal involves sensor behaviour, its power supply and conditioning, data acquisition technique, and initial and periodical calibration, especially in the low frequency range of vibrations (0 to 10) hz, but calibration requirements appear to be mandatory. this paper aims to investigate the main technical and procedural aspects to be considered for the definition of a calibration procedure modulated in function of the application, as well as the cost range of the sensors. particular attention is paid to the experiment design, the way in which a mechanical reference acceleration is created and evaluated and how data processing techniques influence the variability of results, retrieving useful information without having to perform superfluous tests, according to the quality level of the sensors that are being selected. it is to be pointed out that straight through the paper the acceleration will be expressed in terms of g, acceleration due to gravity; the authors are aware that this is formally incorrect, but they prefer this solution for better understanding of results in the field of civil and seismic engineering. in section 2 the methodology will be discussed, concerning the experimental solutions able to realize the requested reference acceleration and the procedures for data processing in section 3 results of tests are presented which have been carried out according to the methodology of section 2; the remarkable reduction of systematic errors will be shown, together with the compensation of cross sensitivity effects for measurement uncertainty reduction. the comparison of contributions of different parameters will be used to also identify the specific actions to do for calibration procedure improvement. 2. methodology  scientific evaluations, experts' advice and market analysis led us to define the following ranges of interest for tests:  frequency range: (0 to 40) hz;  maximum amplitude of vibration: about 20 m/s2. in this paper the calibration in the frequency range (0  4) hz will be studied, with the perspective of increasing this range to 10 hz. for the frequency range (10 – 40) hz, other solutions are available, in particular three-axis electro-dynamic shakers. the three-axis sensor will be calibrated both statically and dynamically. in fact, in field applications we are interested to use it as an inclinometer, the steady behaviour, and as a threeaxis accelerometer, involving the dynamic behaviour. for the general calibration of the sensor the following relationship between the input acceleration and the output signals can be set: , (1) where the matrix s = sij is the sensitivity matrix, are the reference acceleration components, are the voltage outputs of the sensor, and is the offset vector. a total of 12 parameters have to be evaluated [3], [13], [14], [15]. depending on conditions (static or dynamic), static sensitivity matrix ss and dynamic sensitivity matrix sd, along with their corresponding offset vectors, qs and qd respectively, should be evaluated. for the static calibration a mechanical arm is used in order to adjust the sensor to different positions, through the gravity acceleration vector components along the three axes. the minimum number of measurement orientations is 4, although more measurements may give a more robust calibration, using a linear least squares optimization. in this article, the effect of the choice of the number of the angular positioning on the calibration uncertainty is investigated in order to provide useful information about the calibration procedure that is more suitable for the characteristics of the selected accelerometers. the behaviour of the main and transverse sensitivities in the frequency range (0 to 10) hz is also a point of interest. for the dynamic calibration purpose, a test bench based on a rotary device is used. it is driven by a brushless servomotor, controlled by a programmable logic controller (plc) by means of a high accuracy angular encoder, which allows us to realize different motion laws (sinusoidal, saw-tooth, ramp, etc..). the test bench is depicted in figure 1. the accelerometer is placed so that x-y or x-z planes do not correspond to the horizontal one, as figure 2 shows: in this way all the measuring axes will be subjected to a variable acceleration at a frequency depending on the repetition rate of oscillations. in particular:  the x and y axes measure components (depending on the angle  in figure 2) of the gravity acceleration g and of the tangential acceleration at;  the z axis measures the centripetal acceleration ac. the tangential acceleration at and the centripetal acceleration ac are defined by the following equations, where ω is the acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 | 77  angular velocity and r is the distance of the sensitive element of the accelerometer with respect to the axis of rotation: ∙ (2) ∙ . (3) in order to build the dynamic sensitivity matrix, a range of different values are assumed for the orientation angle , described in figure 2. a high accuracy three-axis accelerometer could also be employed for further validation purposes, according also to literature indications [12]. as to the uncertainty of radius, r (figure 3), tests are carried out at different radial positions of the sensors, with the purpose of reducing the whole uncertainty on the reference values of acceleration at and ac. in particular, the sensor has been positioned at increasing distances from the axis of rotation using blocks of known thickness, starting from an initial distance, r0. the relationship between the output of the sensor, y, and the increase r of the distance from the rotation axis with respect to the initial distance, r0, has been linearly fitted. the equation of the straight line has been obtained by a least squares linear fitting: y = m · r + b and r = b/m . (4) the graph in figure 4 shows the experimental relationship between the output of the sensor and the increase r of the distance from the rotation axis. the distance r of the measurement point of the accelerometer has been obtained according to (2), being 112.7 mm. the standard uncertainty of r, sr, can be evaluated, according to the following equation [16]: ∙ ∙ , (5) where b and m are the slope and the intercept of equation (2), obtained by a least squares method interpolation. the uncertainty contributions related to b and m, sb and sm respectively, can be calculated according to the following equations [17]: ∑∆ ∑∆ ∑∆⁄ (6) sm 2 = nsv 2 n ∑∆ri 2∑∆ri 2 (7) where: sv 2 = 1 (n-2)⁄ ∑ (m∆ri+b-vi) 2 . (8) ri are the input values of the increase r, and vi are the corresponding output tension values of the accelerometer. from experimental data results: sv = 3.2 · 10-4 v, sb = 2.6 · 10 4 v and sm = 1.2 · 10-5 v/mm. by substituting the relevant values obtained from the experimental test into (5), the relative uncertainty of r is in the order of 0.5 %, which is satisfactory. the effect of the actual shape of motion has to be analysed figure 1. the rotary table, driven by the brushless motor. rotation angle is measured by a high accuracy angular encoder.    figure 2. scheme of tangential acceleration components corresponding to a rotation angle  of the plane x‐z with respect to the horizontal one.  figure 3. distance r of the measuring point from the centre of rotation.  figure 4. method for the determination of the distance r.  acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 | 78  too, in relation to its theoretical setting, as well as the rotary device set-up. in particular the behaviour of the calibration bench is examined in case of both square wave and sinusoidal motion law for periodical oscillations. the whole uncertainty of ac and of at will be evaluated, taking into account the repeatability standard deviation of them, in correspondence of each angular position and the standard uncertainty of r. the absence of systematic effects on the angular position will be verified. tests have been planned in order to consider the following aspects with reference to the uncertainty of ac and of at (or a component of them depending on the position):  motion laws;  rotary device setting;  choosing of the reference;  number of points for calibration. finally it is to be pointed out that the requested calibration procedure and the accuracy to be achieved should be able to fit the sensor characteristics, so that the performances of calibration rig and cost of calibration can be adequately reduced. 3. results  3.1. static calibration  as a first step, a static calibration of a three-axis accelerometer has been done, according to the procedure described in [14], based on (1). in order to obtain a more robust calibration with respect to the case when the minimum necessary number of positions is used being equal to 4, a total of 10 different angular positions have been realized. the results are described in the following static sensitivity matrix: ss = -9.863 -0.006070 -0.04011 -0.1964 -9.936 -0.03548 0.7081 0.03142 -9.863 v/ m/s ) (9) and the offset vector: = 24.32 24.03 26.68 m/s . (10) for geometrical angles a maximum deviation of 1° has been assumed. a maximum relative deviation from the mean values of 2 % has been evaluated for the coefficients of the main sensitivities, taking into account the angle deviation. anyway, measurements have been carried out after calibration in the range ±9.807 m/s2 (±π/2 as for the angle) for each axis, using the local gravity as the source: a maximum deviation of 0.098 m/s2 has been evaluated. this result suggests that the sensitivity matrix should be evaluated on the whole, due to the compensation effect of the variability of the different matrix coefficients between each other. in fact, each acceleration component is corrected by using a combination of three matrix coefficients; therefore the acceleration variability should be reduced with respect to the coefficient one, if casual effects are considered. 3.2. dynamic calibration  3.2.1 identification of the motion law  in case of dynamic calibration of accelerometers, as a preliminary matter, the effect of the motion time law for oscillations on results accuracy has been investigated: in particular saw tooth and sinusoidal velocity profiles of oscillations were considered. at a preliminary evaluation a saw tooth velocity profile, that means square wave profile for at, seemed interesting, having a plateau in at, being very simple to realize on the test bench from the point of view of plc programming, and making able to investigate in the same test many frequencies (the odd harmonic components). this motion law has been discarded, since experimental tests indicated that it is difficult to be realized not only practically but also theoretically, being necessary specific settings, which are meaningless from a physical point of view. furthermore, oscillations in the test bench arise when at is changed, which is a problem also for sensors to be calibrated. all the results hereinafter refer to tests driven with a sinusoidal motion law. tests have been carried out at an oscillation frequency of 0.5, 1.0 and 1.6 hz; in particular the indication will be stressed referring to 1.6 hz oscillation, whose vibration peaks are the highest. 3.2.2 identification of the reference   in a calibration procedure the definition of the reference is a fundamental issue. the possible alternatives that have been considered are:  using a reference accelerometer. it is to be noticed that using a high accuracy accelerometer as the reference for accelerometer calibration introduces problems deriving from the different radial positioning of the reference accelerometer with respect to the other one; this aspect is expected to increase the calibration uncertainty of accelerometers;  using the output signal from the encoder. in particular, the tangential acceleration at and the centripetal one ac can be obtained according to (2) and (3), where ω is the angular velocity, by differentiating the signal of the angular encoder. the reference for the z-axis coincides with the ac value, while the references for the xand yaxes coincide with the sum of the appropriate components of the gravity acceleration g and of the tangential acceleration at, depending on the sensor orientation (figure 2);  using the motion law that manages the motion. this means that the angular velocity and the acceleration at different instants, according to (2) and (3), are obtained not from the real signal of the encoder, but from the theoretical angular velocity, set by the plc. the graphs of figure 5 show a comparison between real and theoretical time behaviour of the acceleration components, with reference to the axes of the accelerometer. it is to be pointed out that the components of at act along the xand y-axes, while ac acts along the z-axis. it can be noticed from the graph of figure 5 that the systematic error of the reference signal on the whole is negligible; in fact, the reference real signal oscillates around the theoretical one, without any offset. the root mean square (rms) of the difference between real axr(t), ayr(t), azr(t) and theoretical values axt(t), ayt(t), azt(t), during the oscillation period, has been evaluated, being 0.28 m/s2, 0.50 m/s2 and 0.12 m/s2 respectively. obviously, most of the oscillations found in axr(t) and ayr(t) depends on numerical differentiation, even though higher order harmonics can be identified with respect to the theoretical acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 | 79  behaviour, due to effective oscillations at points where the motion is inverted. anyway, if the rms is evaluated of the differences between output of the sensor corrected by the “theoretical” matrix (the matrix sd obtained by using the theoretical reference) and the real reference, the following value are obtained: rmsx = 0.36 m/s2; rmsy = 1.3 m/s2; rmsz = 0.35 m/s2 however, the rms of the differences between output of the sensor corrected by the “real” matrix (the matrix sd obtained by using the real reference) and the real reference are: rmsx = 0.35 m/s2; rmsy = 1.2 m/s2; rmsz = 0.32 m/s2. a number n = 20 of points during the cycle were used to evaluate the matrix sd: the best choice for n will be discussed hereinafter. then, a slight improvement using the real reference for the calculation of the matrix is observed, despite the noise produced by the numerical differentiation, since only the real reference can see all the actual vibrations of the calibration bench. for these reasons, the real reference, obtained from the real encoder signal, has been chosen. 3.2.3 number of points for calibration and preliminary  considerations  in table 1 the root mean square of the differences between reference acceleration and corrected sensor data during the oscillation period are evaluated, as a function of the number of points n. in table 1 reference data are those obtained by twice differentiation of the angular encoder data. the effect of building sensitivity matrix with an increasing number of points, n, is studied. furthermore, provided that a sufficient number of points is considered, no effect of increasing it has been acknowledged. it has been set n = 20. the tests so far carried out also suggested some actions to be carried on to improve the calibration procedure performances by reducing calibration uncertainty, in particular of at and its components; this objective will be pursued by making more accurate and repeatable the real reference acceleration profile. these actions on one hand require a mechanical improvement of the rotary table, allowing us to increase the transferability of the angular encoder information to the position of the accelerometer to be calibrated, on the other hand ask for a higher sampling rate of the angular encoder data. it is to be pointed out that the used plc allows us to use a task time of less than 1 ms, allowing a sampling rate of about 1 khz. rather than increasing the number of points on which the sensitivity matrix should be evaluated but provided that a sufficient number of points is considered, it seems more convenient to mix data measured changing the sensor orientation, also in the dynamic calibration. 3.2.4 results of the dynamic calibration and uncertainty  evaluation  considering the observations made in previous paragraphs, 20 different positions of the sensor have been taken into account during every cycle; for each position the reference accelerations along the x-, yand z-axes have been calculated, depending on gravity and motion acceleration components and compared with the corresponding outputs of sensors in all directions. then, the matrix sd and the offset vector qd in the dynamic case have been calculated on the basis of the reference values, being: sd 9.948 0.2576 0.3657 0.5342 10.97 0.6611 0.4454 1.614 10.83 v/ m/s ) (11) q d q q q 24.02 20.10 26.68 m/s (12) the improvement of using the dynamic sensitivity matrix with respect to the static one is clearly shown in diagrams of figures 6a, 6b, 6c, where real encoder data, twice differentiated, used as the reference, sensor data corrected by static sensitivity matrix and data corrected by dynamic sensitivity matrix are compared. a quantitative evaluation of the rms of the differences between the output corrected of the accelerometer and the reference confirm this observation (tables 2 and 3). figure 5. comparison between real and theoretical time behaviour of the  acceleration components, with reference to the axes of the accelerometer. table  2.  rms  of  the  differences  between  the  output  corrected  by  the  dynamic sensitivity matrix and  the reference.  absolute   rmsx [m/s 2 ]  absolute  rmsy [m/s 2 ]  absolute  rmsz[m/s 2 ] 0.35  1.2  0.32    relative     rmsx  relative  rmsy  relative  rmsz  0.15  0.14  0.09  table 3. rms of the differences between the output corrected by the static  sensitivity matrix and  the reference.  absolute   rmsx [m/s 2 ]  absolute  rmsy [m/s 2 ]  absolute  rmsz[m/s 2 ] 0.43  1.3  1.1    relative     rmsx  relative  rmsy  relative  rmsz  0.19  0.14  0.30  table  1.  rms  of  the  differences  between  the  output  corrected  by  the  dynamic sensitivity matrix and  the reference, as a function of n.  n  rmsx [m/s 2 ]  rmsy [m/s 2 ]  rmsz [m/s 2 ]  14  0.762  1.12  0.327  21  0.395  1.10  0.293  28  0.381  1.07  0.288  35  0.379  1.03  0.287  42  0.392  1.08  0.292  acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 | 80  in order to evaluate the variability of the sensor output corrected by the results of the dynamic calibration, 10 different sets of 20 points have been considered, corresponding to different oscillation periods; 10 different matrices sd and offsets qd have been correspondingly calculated. using them to correct the sensor output during a cycle, the variability is found to be 0.15 m/s2, 0.29 m/s2, 0.071 m/s2 for the x-axis, y-axis and z-axis respectively. the same values, expressed as a percentage of the signal amplitude, are: 6.3 %, 3.4 % and 2.0 % respectively. these values can be assumed as standard percentage calibration uncertainties for the x-axis, yaxis and z-axis accelerations. the contribution of the uncertainty of the radius r appears to be negligible in any case, being in the order of 0.5 %. if the variability of the reference is considered too, using 10 cycles of the reference signal (figure 7) an average standard deviation has resulted, equal to: 0.28 m/s2, 1.1 m/s2, 0.16 m/s2, for the x-axis, y-axis and z-axis respectively. these values, expressed as a percentage of the signal amplitude, become: 12 %, 12 %, 4.5 % respectively. using the sensitivity matrix allows to get a corrected output with a reduced variability with respect to the cycle variations of the reference. this result suggests that improving the cycle repeatability of the reference could further improve the correction and reduce uncertainty of the calibrated sensor. a) b) c)  figure 6. comparison among real encoder data, twice differentiated, used as the reference, sensor data corrected by static sensitivity matrix and data corrected  by  dynamic  sensitivity  matrix.  a)  x‐axis  acceleration;  b)  y‐axis  acceleration; c)  z‐axis acceleration.  a) b) c)  figure  7.  variability  of  10  cycles  of  the  reference  acceleration.  a)  x‐axis  acceleration; b) y‐axis acceleration; c) z‐axis acceleration.  acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 | 81  4. conclusions  in this paper some aspects concerning the calibration uncertainty of three-axis low-cost accelerometers for diagnostics of civil buildings are considered. both static conditions and dynamic behaviour in the range (0 to 4) hz as for vibrations have been examined. a frequency varying three-axis field of acceleration has been realized by modulating both gravity acceleration, and tangential/radial acceleration components of a rotary motion created on a test bench driven by a brushless servomotor. reference values for ac and at acceleration (or a component of it depending on position) have been deduced from the indications of a high precision rotary encoder used for axis control, in order to separate the effect on the whole uncertainty of calibration of the test bench characteristics and settings. the effects taken into account refer to :  motion laws;  rotary device setting;  choosing of the reference;  number of points for calibration. experimental results, for both static and dynamic applications, show that both main and cross sensitivities of a three-axis capacitive accelerometer of good quality level from a metrological point of view can be obtained; this allows to correct the effect of cross sensitivities also in dynamic applications, in order to remarkably improve the transducer accuracy for in field applications. using the sensitivity matrix allows to get a corrected output with a reduced variability with respect to the cycle variations of the reference. this result suggests that improving the cycle repeatability of the reference could further improve the correction and reduce uncertainty of the calibrated sensor. therefore, the main actions to be realized should move to improve the mechanical behaviour of the test bench, and the acquisition of the angular encoder data. improving these aspects is expected to further reduce the calibration uncertainty, and also to extend the calibration range up to 10 hz. references  [1] wyss m and rosset p “mapping sismic risk: the current crisis” nat. haz. 68 (2013) pp. 49-52. [2] zhao m and xiong x “a new mems accelerometer applied in civil engineering and its calibration test” the ninth international conference on electronic measurement & instruments icemi-2, 2009, pp.122-125. [3] ren wei, zhang tao, zhang hai-yun, wang lei-gang, zhou yong-jie, luan meng-kai, liu hui-feng, shi jing-wei “a research on calibration of low-precision mems inertial sensors”, 25th chinese conference on control and decision conference (ccdc), 2013, pp. 3243 – 3247 [4] zanjani p n, abraham a a, “a method for calibrating micro electro mechanical systems accelerometer for use as a tilt and seismograph sensor” 12th international conference on computer modelling and simulation (uksim), 2010, pp. 637 – 641 [5] dumas n, azais f, mailly f, nouet p, “study of the electrical set-up for capacitive mems accelerometer test and calibration”, journal of electronic testing 26 (2010) pp. 111-125. [6] milligan d j, homeijer b d and walmsley r g “an ultra-low noise mems accelerometer for seismic imaging” sensors (ieee), 2011, pp. 1281 1284. [7] vallet f and marcou j “a low-frequency optical accelerometer” j. opt. 29 (1998) pp. 152-155. [8] schiefer m i and bono r “improved low frequency accelerometer calibration” xix imeko world congress fundamental and applied metrology, 2009 [9] choi k, jang s and kim y “calibration of inertial measurement units using pendulum motion” int. j. of aeronautical & space sci. 11(3) (2010) pp.234–239. [10] zhu c, qin x, wu z and zheng l “research on calibration for measuring vibration of low frequency” international conference on mechanic automation and control engineering (2010) pp. 3315 3318. [11] mohd-yasin f, nagel d j, ong d s, korman c e and chuah h t “low frequency noise measurement and analysis of capacitive micro-accelerometers” microelectronic engineering 84 (2007) pp. 1788-1791. [12] ohm w, wu l, hanes p and wong g s k “generation of lowfrequency vibration using a cantilever beam for calibration of accelerometer” journal of sound and vibration 289 (2006) pp. 192-209. [13] akira umeda, mike onoe, kohji sakata, takehiro fukushia, kouichi kanari, hiroshi iioka, toshiyuki kobayashi “calibration of three-axis accelerometers using a three-dimensional vibration generator and three laser interferometers” sensors and actuators a: physical 114 (2004) pp. 93-101. [14] nakano a, hirai y, sugano k, tsuchiya t, tabata o, umeda a, “rotational motion effect on sensitivity matrix of mems threeaxis accelerometer for realization of concurrent calibration using vibration table”, ieee 26th international conference on micro electro mechanical systems (mems), 2013, pp. 645 – 648. [15] mark pedley, “high precision calibration of a three-axis accelerometer” freescale semiconductor (2013). [16] giulio d’emilia, antonella gaspari, emanuela natale, “evaluation of aspects affecting measurement of three-axis accelerometers”, measurement 77 (2016) pp. 95-104. [17] ernest o. doebelin, measurement systems, mcgraw-hill , 2004, isbn 0–07–243886–x. microsoft word 349-2408-1-le-rev acta imeko  issn: 2221‐870x  september 2016, volume 5, number 2, 71‐79    acta imeko | www.imeko.org  september 2016 | volume 5 | number 2 | 71  the high official harkhuf and the inscriptions of his tomb in  aswan (egypt). an integrated methodological approach  andrea angelini 1 , giuseppina capriotti vittozzi 2 , marina baldi 3   1 institute for technologies applied to cultural heritage, cnr‐itabc, via salaria km 29,300 ‐ 00015 monterotondo st. (rome), italy   2 institute for the study on ancient mediterranean, cnr‐isma, via salaria km 29,300 ‐ 00015 monterotondo st. (rome), italy  3 institute for biometeorology, cnr‐ibimet, via dei taurini 19 ‐ 00185 rome, italy      section: research paper   keywords: harkhuf tomb; epigraphic numerical model; image based 3d model; virtual scan; aswan climate  citation: andrea angelini, giuseppina capriotti vittozzi, marina baldi, the high official harkhuf and the inscriptions of his tomb in aswan (egypt). an  integrated methodological approach, acta imeko, vol. 5, no. 2, article 10, september 2016, identifier: imeko‐acta‐05 (2016)‐02‐10  section editors: sabrina grassini, politecnico di torino, italy; alfonso santoriello, università di salerno, italy  received march 20, 2016; in final form june 17, 2016; published september 2016  copyright: © 2016 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: this research was supported by cnr in the agreement for scientific cooperation between italy (cnr) and egypt (asrt)  corresponding authors: andrea angelini, e‐mail: andrea.angelini@itabc.cnr.it, giuseppina capriotti vittozzi, e‐mail: giuseppina.capriotti@isma.cnr.it,        marina baldi, e‐mail: m.baldi@ibimet.cnr.it    1. introduction to the project  the cnr – multidisciplinary egyptological mission (cnr – mem) has worked in the tomb of harkhuf in order to document its important hieroglyphic inscriptions. the mission has been carried out in the framework of the project tech (technologies for the egyptian cultural heritage) funded by the national research council of italy (cnr) and the academy of scientific research and technology of egypt (asrt). aswan is the southern gate of egypt: in ancient times caravan routes left the nile valley from this site to reach far away territories and different populations, carrying back exotic and precious goods. the ancient city of elephantine, the river island in the modern town of aswan, was the gateway to africa, the bridgehead towards unknown lands as dangerous as rich in exotic treasures. between the late old kingdom and the early middle kingdom, the tombs of the nobles at qubbet el-hawa testify of ancient journeys, far south explorations, trades and cultural exchanges, in particular, the tomb of harkhuf, a high official of the vi dynasty, who led trading and military expeditions into nubia [1]. the text inscribed on the façade of his tomb is a very important and famous document (figure 1). harkhuf led at least four expeditions to far away countries in the southern or west-southern territories. his tomb testifies of his life, his career, his journeys and explorations towards south, in the nubia and in the lybian desert. harkhuf was a noble of upper egypt, he served the kings merenra and pepi ii, during the vi dynasty in the xxiii cent. bc [1]. this tomb was at first noticed by an italian scholar, ernesto abstract  tech project (technology for the egyptian cultural heritage) aimed to document an egyptian monument for egyptological studies and  researches  but,  at  the  same  time,  to  check  a  new  methodological  approach  for  conservation,  valorisation  and  enhancement.  in  particular, the cnr mission focused the attention on the tomb of harkhuf, a high official of the vi dynasty (xxiii century bc), who led  trading and military expeditions into nubia. the hieroglyphic texts inscribed on the façade of his tomb are very important and famous  documents. the team checked an innovative and integrated methodology. the methodology has been focused mainly on the use of  digital  photogrammetric  systems  in  order  to  generate  an  accurate  numerical  model  (3d)  and  to  facilitate  the  epigraphic  study.  different procedures have been established in the processing and representation steps in order to accomplish the final communication  of the results. moreover climatic measurements have been carried out in order to understand the role of environmental factors on the  deterioration of the monument. finally the data have been crossed in order to check the environmental impact and the decay. acta imeko | www.imeko.org  september 2016 | volume 5 | number 2 | 72  schiaparelli, famous egyptologist who discovered f.i. the tomb of nefertary and other important monuments. schiaparelli, who was director of the egyptian museum in florence and of the egyptian museum of turin, published the inscriptions in the memorie dell’accademia dei lincei in 1892 [2]. schiaparelli was a pioneer of the inter-disciplinary archaeology and egyptology, and he used the photography as an instrument for documentation on the archaeological field. in 1975, miriam lichtheim, who published a famous work on ancient egyptian literature, wrote in her book: cut in soft, flaking stone, the inscription (of harkhuf) is now in a very poor condition. in fact, the wind, the sun and some rainfalls contribute to the decay [3]. tech project aimed to check a non-invasive methodology for documenting egyptian monuments and above all egyptian epigraphy. the tomb of harkhuf has been chosen because of its importance, its status and for the old documentation provided by e. schiaparelli, which represented the starting point for the work. the project aimed at a very good documentation using digital photogrammetric system in order to obtain data on its conditions, to check the decay and the influence of the environmental factors. 2. the acquisition step of the inscriptions  2.1. introduction to the survey  the work presented in the paper is a preliminary note of a wider project on the tomb of harkhuf. since the first survey on the site, in 2014, several and damaged inscriptions preserved on the façade and the pillars inside the tomb, have been noticed. most of them are in different conditions, probably due to the intrinsic characteristics of the stones and to the weathering phenomenon. from the observations made during the mission emerged the importance of documenting the inscriptions, that actually risk to be lost forever. the most important inscriptions are visible on the main façade. they represent an important document of the harkhuf’s life (figure 2). the four pillars inside show reliefs and inscriptions with funerary formulas. the first step of the work was the documentation of the existing texts with a methodology that was able to record the signs and, at the same time, was able to evidence the differential degradation. every inscription was accurately documented through the use of photogrammetric systems. image based 3d methods can be considered a valid instrument of investigation and analysis to improve the knowledge in the archaeological field [4]. in recent years the development of digital photogrammetric systems allowed to define a very accurate working procedure of data acquisition and processing in the archaeological discipline. the extraordinary development of camera sensors, the manufacturing of the lenses and the more accurate algorithms for the recognition of the homologous points on images, allow to reconstruct the whole investigated subject [5]. the methodology was focused on the use of photogrammetric system despite other different approaches for the investigation could be used, such as the laser scanner (triangulation/phase difference) [6] or more recently, polynomial texture mapping (ptm) [7]. this second system is able to display depth details of an object, interpolating information carried out from different lighting of the surface. the limits of ptm consist in a not scaled model, the reduced size of the subject that cannot exceed the meter and the difficulty to apply the system in an open space. the use of digital photogrammetry was justified by the possibility to use a simple device (such as a camera) in almost every condition, collecting important information and data about archaeological artefacts. furthermore, due to the history and evolution of the photogrammetry, it is possible to evaluate its potentiality and extension to different application fields [8]. 2.2. the experimentation on the tomb of harkhuf  the first step of the research focused mainly on the external façades subjected to a greater stress. the survey was performed using two reflex cameras (canon 5d mark ii, canon 60d) with different optic lenses (canon 28 mm/50 mm). the following modalities have been used: structure from motion (sfm/agisoft photoscan); stereoscopic digital photogrammetry (mencisoftware zscan. the first system allows to perform photogrammetric acquisition, starting from unordered images. the software used is a commercial package able to orient and match large dataset of images. there is no published information concerning the matching algorithms employed in the software, even if it seems to be a stereo sgm-method [9]. figure 2. one of the inscription preserved on the main façade of the tomb.  in particular it represents an important document of harkhuf’s life.  figure  1.  the  famous  tomb  of  harkhuf  in  aswan,  egypt.  the  tomb  was completely carved out in the stone.  acta imeko | www.imeko.org  september 2016 | volume 5 | number 2 | 73  in order to achieve accurate data, it is important to acquire images according to an established procedure. the algorithms used from the software are mainly based on the automatic recognition of the homologous points. the geometrical model is well known as epipolar geometry (figure 3). this particular geometry is able to establish a relation between a couple of images. consider two different frames and a point p in the space. the point p generates two homologous projections on the corresponding frames. we define on the frame the corresponding images of the projection centres (epipoles or epipolar points), in order to create the adequate relation. the epipolar plane is identified from the base and the collineation lines. the intersections between the epipolar plane and the two image planes are called epipolar lines. on the epipolar lines it is possible to find the image of the homologous point of the other frame. recent studies on this geometry demonstrated how the correspondences between features can be improved as a function of the position of the frames [10]. normally the epipoles are casually displaced on the frames. their position is much more far from the principal point as the optical axis is parallel. in that condition, when the frames are coplanar, the epipolar lines are in parallel. this is the best condition for the software to recognize the homologous points. the advantage consists in a lesser processing time and a more accurate point clouds (less noises). on the field a 28 mm optic lens with a tripod mounted at 0.50 m from the façade were used, moving the camera along the entire surface (figure 4). the acquisition simulated imaginary tracks in a bustrophedic direction (tape method). they were taken more than 400 pictures only for the letter of harkhuf at very high resolution and with 80 % of overlapping among the pictures. the side lap between each photographic sequence was about 30 %. the choice of the 28 mm optic lens guaranteed a good compromise between radial distortion and the covered area. experimentations were performed also with the other camera in the same condition, to test differences between the camera sensors. canon 5d mark ii is provided with a full frame sensor while canon 60d is equipped with an aps-c sensor. this difference conditioned the number of acquisitions on the face of the tomb. positioning the second camera at the same distance from the subject we needed more images to compensate the overlapping area. the second experimentation was performed with the other system (stereomatching), based on the stereoscopic acquisition of three images, using the same cameras. the canon 5d mark ii was mounted on a little aluminium bar (0.7 m) and fixed on the tripod at 2 m from the inscription. only a single camera, hold on the bar in 3 different positions, has been used. the baseline between 2 adjacent position was set at 200 mm, about 1/10 of the distance from the engravings. in this second application only a limited number of acquisitions were made, even if each one consisted of three camera shots. the co-planarity among the frames for each hat trick must be maintained to achieve a good correspondences of the points. settings of the camera represented an important key factor for avoiding noisy point clouds. in order to achieve good dense point clouds it was important to use fixed optic without aspherical lenses. as we know each optic is suffering from radial distortion, specified by the manufacturer. radial distortions are normally directly proportional to the distance from the centre of the image (principal point). some optics mount asperichal lenses to correct part of this distortion, altering the camera calibration step (internal orientation). in order to have a defined image and a good field depth the camera settings were modified by using automatic shutter, diaphragms at f11/f14, iso sensibility at 100 and infinity figure 4. acquisition step performed with a canon 5d mark  ii and canon  60d mounted on a tripod half a meter from the surface.  figure  3.  epipolar  geometry.  this  particular  geometry  applied  to  digital  photogrammetry  allows  to  recognize  homologous  points  between  a  couple  of frames.  acta imeko | www.imeko.org  september 2016 | volume 5 | number 2 | 74  focusing. for the letter of harkhuf five acquisitions were made to have the complete engravings with different settings and lighting. the position of the tomb conditioned the acquisition steps. they were carried out in 2 different moments of the day: during the early morning, with direct light of the sun on the surface and during the afternoon with diffused illumination. knowing the exact distance among the three camera positions on the bar, it is possible to achieve spatial coordinates of inaccessible points. the system is often used in topography and is known as “space forward intersection”. based on the same principle of the eyesight, space forward intersection allows to reach inaccessible points starting from 2 central projections oriented on the same subject. the geometrical model explains how it is possible to achieve the coordinates (figure 5). the accuracy of the numerical model depends on the angle of the collimation straight lines and on the distance of the cameras from the object. it is however possible to demonstrate that the collimation straight lines are twisted lines, so that interpolation is necessary to find exactly the point. recent studies on this geometrical model evidenced other possible solutions to solve the problem. one of this is represented by the calculation of the middle point between the twisted lines. this is a good solution when the vertical angle of the lines is small. another solution is represented by an innovative geometrical model named “mutual tipping”, developed by carpiceci in 2012 [10]. this geometrical model allows to control 2 important factors: the real phase displacement of the twisted lines; the convergence angle of the triangulation. currently there is no software that uses this solution to generate dense point clouds, indeed the solution adopted is demonstrated only in the projective geometry. it should be interesting to write a specific algorithm able to use this solution and compare the results with the others. the sfm system was used to create a high resolution 3d numerical model (point clouds) even if one of the main problems consisted in a non-scaled result (figure 6). the second system (stereoscopic) generated an accurate and scaled 3d point cloud. in this case the limit consisted in the orientation of the reference system (local) that is in the origin of the perspective pyramid of the camera (the second shot of the hat-trick). in order to solve this problem and process the final graphic restitution it is necessary to transform and rotate the ucs in a desired position such as an orthographic view. aim of the project was to use both the models. the model carried out with the stereoscopic method allowed to extract point coordinates in order to reference the non-scaled model from sfm. the error registered between the two numerical models and the coordinates is about 0.008 m (figure 7). the value of the error is not constant over the entire model and the result is referred to the ground control points used to process the non-scaled model. some factors can change this value such as the exact recognition of the points between the two models, the number of the points used, the different definition of the images taken at different distances from the subject and the resolution step of the two point clouds. the points used for referencing the model (10 points) have been chosen homogeneously over the entire surface, focusing the attention in the corner of the engraved stone. the accuracy of the stereoscopic model and the job condition influenced the final results of the project. the stereoscopic model is composed of 2.5 million of points and figure 6. numerical model (point clouds) of the letter of harkhuf preserved  on  the  main  façade.  the  flags  show  the  ground  control  points  (gcps)  necessary to scale the model. the letter contain 60 million of points.  figure  5.  geometrical  model  used  in  topography  and  implemented  for  digital photogrammetry.  figure  7.  numerical  model  from  stereoscopic  acquisition.  on  the  point  clouds coordinates have been picked in order to reference the sfm model.  acta imeko | www.imeko.org  september 2016 | volume 5 | number 2 | 75  the resolution is about one point every 1 mm (gsd). the accuracy of the data can only be estimated on the basis of a table compiled by mencisoftware. the table evidences the theoretical error of measurements in different condition. with the baseline set to 400 mm and a distance from the subject of 2000 mm the accuracy could be 0.89 mm (table 1). if we had changed the distance of 3 m from the inscription (3000 mm on the table) the accuracy in theory would be 1.84 mm. measurements taken on the inscription (length, height and diagonal) corresponded to those taken on the virtual models. certainly it is not possible to verify the dimension of each hieroglyphic sign, but only testify the entire documentation of inscriptions preserved on the tomb. 3. the processing step   3.1. the point clouds numerical model  as mentioned above the photogrammetric survey was carried out on the entire archaeological complex but only the letter preserved on the main entrance was completely elaborated. a specific elaboration procedure was developed for the inscription and the results are shown below. the processing step was performed with different software for point clouds management. the aim of the study was to create a numerical model of the inscription at high resolution, able to give information about its condition. from the high resolution 3d model (point clouds) it was possible to carry out all the measurements about length, width and depth of the engraving and to get information about the geometry, morphology and the global visualization of the object. with the integration of different point clouds management software it was possible to make a series of elaborations in order to get a better organization and comprehension of the data (figure 8). 3.2. the transformation of the numerical model   the main elaboration was represented from the transformation of the point clouds in the surface. despite it seems very simple and fast, this step is very difficult to process, above all when we handle with complex shapes, such as an egyptian inscription. once the filters were applied on the point clouds, the surface reconstruction was executed through the meshing technique. the meshing process was necessary to: reduce the total of the points; create the connection among the points; measure the connected and continued surfaces (volume, distance); apply a texture map from the bi-dimensional images. the mesh consists of vertices that give positional information and edges that give connective information. by connecting the edges to the vertices it is possible to introduce a concept of "closeness" between vertices and give the topological information. the faces are determined when given the vertices and edges. the faces do not introduce anything at the level of information; rarely they can have some associated attributes. currently a lot of mesh generators exist but the following are the most common, based on the main studies of owen in 1998 [11]. there are three main categories of mesh generators for the reconstruction of the surfaces, starting from a domain of unstructured points; octree; delaunay; advancing front; the most used method for generating triangular meshes is that known as delaunay criterion (figure 9). in the euclidean plane the principle of the method is very simple and it requires that each of the set of the nodes should not be contained in the circle which circumscribes any tetrahedron present in the mesh. for triangulating four vertices abcd, two are the possible options of triangulation. in order to satisfy the criterion, the solution in which the circles circumscribed the two triangles contain a vertex inside them, must be discarded [12]. this is figure 8. on the numerical model  it  is possible to separate the chromatic  characteristics from the shape of the engravings.  figure 9. delaunay criterion showed in the plane. the valid solution allows  to have more regular triangles.  table  1.  table  of  accuracy  of  the  stereoscopic  system  expressed  in  mm taken from mencisfotware/z‐scan.  baseline/distance 100 mm 200mm 300 mm 500 mm 750 mm 1000 mm 2000 mm 3000 mm 20 mm 0.04 0.16 0.37 50 mm 0.15 0.41 0.92 1.64 100 mm 0.20 0.46 0.82 3.27 150 mm 0.31 0.55 2.18 4.91 200 mm 0.41 1.64 3.68 300 mm 1.09 2.46 400 mm 0.82 1.84 500 mm 1.47 600 mm 1.23 700 mm 800 mm acta imeko | www.imeko.org  september 2016 | volume 5 | number 2 | 76  therefore not just a real algorithm, but a selection criterion associated to an algorithm which subsequently generates the triangular surfaces. the effectiveness of this method is demonstrated by the relationship between the delaunay criterion and the voronoi diagram, that illustrated most of tessellation issues present in nature. the delaunay criterion is the dual of the voronoi diagram. it is a suitable system for reconstructing surfaces from unstructured information. unstructured point clouds have been processed with geomagic (wrap algorithm), based always on delaunay triangulation. the sfm point cloud contained 60 million points. the weight of the point cloud made it impossible to triangulate all the points. it was necessary to filter point clouds reducing in percentage the points by using different filters such as the curvature filter. this filter allows to maintain more points in the area where the curvatures are high. after the creation of the triangles the surface was improved through the optimization method. two are the main categories of the optimization method: smoothing algorithms, that maintain the connectivity but re-arrange the nodes; cleanup algorithms, that maintain the position of the nodes but change their connectivity. usually smoothing algorithms alter the original information of the surface, although the final result can be very suitable. cleanup algorithms are very useful for changing the topology of the triangles. sometimes it was very useful to flip the connectivity in order to give the correct shape to the engravings. the model was filtered in different ways to highlight the inscriptions but maintaining possibly original data (figure 10). other procedures were used to improve the final model. the following are only the most used for the letter of harkhuf: the repair of the mesh that is required when the algorithmic operation is not completely successful, so the model can have holes, or topological problems (self intersections or corners and non-manifold vertices); the decimation filter that uses a series of algorithms to simplify the model and generate multi-resolution models (to pyramid levels); the densification or refinement processes to increase the detail of the mesh. countless are the algorithms for the densification processes (such as edge bisection, point insertion, templates) [13]. 3.3. the virtual scan of the numerical model    in order to read the information about the condition of the inscriptions, it has been decided to create maps useful to the investigation and analysis. point clouds carried out from digital photogrammetry are unstructured point clouds. it means that the information is not organized according to a regular grid. normally image-based data presents only information about the spatial coordinates and the rgb value. it was important to add extra information to the numerical model. in the processing step a new procedure was tested in order to transform unstructured point clouds in structured point clouds. how is it possible to change the structure of numerical data? data were processed with a specific software (jrc reconstructor) in order to change the structure of point clouds. the aim was to give the same characteristics of a laser scanner data to the photogrammetric data. this idea was possible thanks to a tool, named “virtual scan”, present in the same software [14]. this tool allows to make a virtual scan of an object in the virtual space. once an orthographic camera that includes the numerical model (unstructured) is established, it is possible to acquire the whole point cloud at a specific resolution, such as a laser scanner. at the same time it is possible to create a spherical or cylindrical camera in any position, simulating the same movement of a panorama scanner. although the resolution of the results depends also on the graphic card mounted, it is possible to create a new structured 3d numerical model of the subject with the same characteristics of a point cloud taken with a laser scanner. for the inscription an orthographic camera was created at a resolution grid of 4.000 × 4.000 (pixel/points). subsequently a new 3d structured numerical model composed by 16 million points was carried out. the new structured model contained fewer points than the original one but the main advantage consisted in the possibility to apply the pre-processing algorithms to photogrammetric data. it was possible to remove the noise and add some useful information such as the normal, the depth and orientation discontinuities. this information has been used to generate different outputs such as digital elevation model (dem), normal maps and other elaborations useful for the final results (figure 11). figure  10.  point  cloud  was  transformed  in  surface  through  the  meshing technique. the final model contained only 3 million of triangles.  figure 11. example of a normal map of a part of an inscription. on the map  it  is  possible  to  evidence  the  difference  between  casual  signs  and  engravings.  acta imeko | www.imeko.org  september 2016 | volume 5 | number 2 | 77  4. the representation of the data   the aim of the work is not only to have a numerical model but also to find a possibility to use the model to draw the hieroglyphics in a bi-dimensional restitution for the epigraphists. the general idea consisted in experimenting different solutions able to solve the dichotomy between innovative acquisitions and graphic representations. usually bi-dimensional representations of the inscriptions are based on direct survey, supported by a photographic restitution on a specific plane. unfortunately this kind of representation does not characterize correctly signs and shape of the engravings. the signs are drawn through an interpretation from the rectified image (figure 12). aim of the project was to find a form of representation suitable for the epigraphic signs. a system derived from cartography and above all in the sculpture has been experimented. the approach is based on the representation of the contour lines. contour lines represent free shape of an object beyond contour outlines and edges of the engravings [15]. the equidistance of the contour lines depends on the representation scale of the graphic restitution. normally it is set to 1/1000 of the representation scale of the object expressed in meter. in theory an equidistance every 1 mm would be enough but in this case a defined restitution of the inscription was necessary. it was decided to use the numerical model (mesh) to generate the sections. sections were set each 0.2 mm (5 times lower) in order to evidence the details of every sign of the inscription. the generation of the sections depends on the quality of the numerical model and also from the main plane chosen from the user. it was necessary to establish a reference plane where the information about the sections are projected. the results evidenced the differences between the old drawings and the new approach. the signs are well defined by the lines and simultaneously the shape and the curvature of the stone material are described (figure 13). 5. the influence of the climate    the tomb of the nobles are carved in the rocks on the west bank of the river nile in aswan, an area characterized by hot and dry weather conditions [16], which are typical of a desert climate. in this region the total rainfall amount per year is about 1 mm, and heavy precipitation is a very rare event occurring once every 1 or 2 years, often resulting in flash flood. these rare, but heavy rainfall episodes have important, and sometimes catastrophic impacts, due to the high intensity and short duration, on population, buildings, infrastructures, ecosystems [17], including wadis discharge [18], and cultural heritage. the meteorological factors affecting the tomb of harkhuf at qubbet el-hawa are air temperature, its diurnal excursion, and wind, and, to some extent, relative humidity. the effects of the environmental factors, including meteorological elements, are evident if the lower part of the façade is compared to the upper part. for centuries, and until the work done by schiaparelli, the lower part has been covered by sand deposited on the façade, therefore remained well sheltered against meteorological factors, while the upper part was always subject to the environmental stress. this resulted in a better status of conservation of the lower part in comparison with the upper, which is still clearly visible. analysis of the meteorological factors in aswan show that night time relative humidity can be more than 30 % during the winter months, which rise rapidly during heavy rainfall episodes. the experiment, designed using portable meteorological instruments, allowed to define if the microclimate around the harkhuf tomb has the same characteristics of the larger aswan area, which can be derived by the meteorological station located at the aswan airport, and to determine the microclimate inside the tomb. in particular, it contributed to: determine the temperature gradient along the façade of the tomb in order to understand if it’s different parts are under the influence of physical stress of different intensity; determine if temperature excursions together with the right level of relative humidity of the air could favour the formation of dew at dawn and if the air can reach high dew point values. preliminary analysis of data collected between 8:00 am and 4:00 pm local time (figure 14) permitted to detect a differential heating of the façade, with the right part reaching temperatures warmer (few degrees c) than the left and for a longer period, being under the direct sunrays until early afternoon. in the interior, during the day, the temperature excursion is much more moderate, although the excursion curve is very well correlated with the air temperature outside the tomb. in addition, some measurements of wind intensity and direction figure 12. hieroglyphics drawn with manual recognition of the signs and a  conceivable support of a rectified image.  figure  13.    the  same  part  drawn  with  the  contour  line  method.  the  equidistance  was  set  to  0.2  mm.  this  adaptation  allowed  to  represent adequately the inscriptions. acta imeko | www.imeko.org  september 2016 | volume 5 | number 2 | 78  show a persistent wind blowing from n-ne starting midmorning, around 10:30 am until late afternoon. relative humidity of the air outside the tomb does not represent a big concern during the day, however it is important in the interior where it can reach larger values and maintain a risky level during night-time which can favour the formation of mold on the ceiling. during the day evaporation from the surface of the nile river is considerable and can increase the air humidity in the lower layer of the atmosphere, the boundary layer. this humid air penetrates into the tomb and remains trapped and starts to condensate and deposit on the internal walls and ceiling increasing the risk of mold formation. all these environmental factors certainly play a key role in the deterioration of the monument and in particular of its façade so magnificently engraved, making its preservation and conservation an absolute necessity. in this respect the methodology described in the previous paragraphs represents a great tool to this purpose. 6. conclusion   the integrated methodology applied to the tomb of harkhuf has given good results. the research evidenced some important information about the characteristics of the letter (the support, the signs, the degradation). the experimentation has given the possibility to investigate the documentation of the tomb from different points of view, testing different elaborations that can evidence new information. particular attention was paid to the procedure known as “virtual scan” able to transform unordered data from imagebased methods in structured information such as range data. this application was very interesting and other experimentation will be made in the future. the team is elaborating the entire data in order to render an accurate documentation and to plan a conservation project. acknowledgements  this paper is precisely the result of tech project (technology for the egyptian cultural heritage), headed by prof. giuseppina capriotti vittozzi and supported by a cnr contribution. the paper is also the result of the collaboration between the authors. in particular giuseppina capriotti vittozzi developed the paragraph introduction to the project, andrea angelini the acquisition step of the inscriptions, the processing step and the representation of the data, marina baldi the influence of the climate. references  [1] 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[14] m. sgrenzaroli, g.p.m. vassena, tecniche di rilevamento tridimensionale tramite laser scanner. volume 1 – introduzione generale, starrylink editrice, brescia, 2007, isbn 978-88-8972073-8. aswan tombs of the nobles hourly temperature and humidity 0 5 10 15 20 25 30 35 40 1 2 :0 0 a m 5 :0 0 a m 1 0 :0 0 a m 3 :0 0 p m 8 :0 0 p m 1 :0 0 a m 6 :0 0 a m 1 1 :0 0 a m 4 :0 0 p m 9 :0 0 p m 2 :0 0 a m 7 :0 0 a m 1 2 :0 0 p m 5 :0 0 p m 1 0 :0 0 p m 3 :0 0 a m 8 :0 0 a m 1 :0 0 p m 6 :0 0 p m 1 1 :0 0 p m 4 :0 0 a m 9 :0 0 a m 2 :0 0 p m 7 :0 0 p m t e m p e ra tu re 0 10 20 30 40 50 60 r e la ti v e h u m id it y temperature °c humidity aswan tombs of the nobles hourly wind speed (km/h) 0 5 10 15 20 25 1 2 :0 0 a m 5 :0 0 a m 1 0 :0 0 a m 3 :0 0 p m 8 :0 0 p m 1 :0 0 a m 6 :0 0 a m 1 1 :0 0 a m 4 :0 0 p m 9 :0 0 p m 2 :0 0 a m 7 :0 0 a m 1 2 :0 0 p m 5 :0 0 p m 1 0 :0 0 p m 3 :0 0 a m 8 :0 0 a m 1 :0 0 p m 6 :0 0 p m 1 1 :0 0 p m 4 :0 0 a m 9 :0 0 a m 2 :0 0 p m 7 :0 0 p m w in d s p e e d figure  14.  diurnal  variation  of  temperature  and  relative  humidity  (top panel) and of wind speed (bottom panel).  acta imeko | www.imeko.org  september 2016 | volume 5 | number 2 | 79  [15] m. carpiceci, g. cresciani, a. angelini, “rock hewn architecture survey: the problem of construction of the geometrical model”, in proc. of international congress of speleology in artifical cavities, hypogea, mar. 11-17, 2015, rome, italy, pp.389-398. [16] l. j. sutton, “the climate of egypt”, weather, 5(2), 1950, pp. 5962. [17] i. springuel, m. sheded, f. darius, r. bornkamm., "vegetation dynamics in an extreme desert wadi under the influence of episodic rainfall." polish botanical studies 22, 2006, pp 459-472. [18] z. el-barbary, g.a. sallam, “optimizing use of rainfall water in east desert of egypt”, in: proc. of the eighth international water technology conference, iwtc8, 2008,alexandria, egypt, pp 7381. acta imeko, title acta imeko issn: 2221-870x december 2017, volume 6, number 4, 69-74 acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 69 reducing the uncertainty of strain gauge amplifier calibration miha hiti slovenian national building and civil engineering institute (zag), dimičeva 12, 1000 ljubljana, slovenija section: research paper keywords: calibration; strain gauge; simulator; amplifier; uncertainty citation: miha hiti, reducing the uncertainty of strain gauge amplifier calibration, vol. 6, no. 4, article 11, december 2017, identifier: imeko-acta-06 (2017)04-11 editor: paolo carbone, university of perugia, italy received june 19, 2016; in final form november 24, 2017; published december 2017 copyright: © 2017 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: the authors acknowledge the financial support from the slovenian research agency (research core funding no. p2-0273) corresponding author: miha hiti, e-mail: miha.hiti@zag.si 1. introduction strain gauge bridge transducers which are used for the measurement of mechanical quantities such as force, torque and pressure, need an instrument for excitation of the strain gauge bridge and for the measurement and indication of the bridge output signal, which depends on the load applied to the transducer. strain gauge bridge measuring amplifiers usually perform these tasks. they indicate the result of the measurement as the ratio of the bridge supply voltage and the bridge output voltage. figure 1 shows a typical transducer– amplifier circuit in a 6-lead configuration. as an important part of the measuring chain (or measuring system), bridge measuring amplifiers should be verified regularly. this is usually performed by calibration with strain gauge bridge simulators, which provide defined voltage ratio values as reference values. such simulators, in turn, need to be calibrated themselves for each reference value they output. for high precision simulators (e.g. 225 hz carrier frequency type), the typical expanded calibration uncertainty u of the 2 mv/v range is about 0.00001 mv/v at nmi level [1], [2] and u=0.00002 mv/v at calibration laboratory level for further dissemination, leading to amplifier calibration uncertainties u=0.00002 mv/v and above. such calibration uncertainties consecutively lead to large uncertainty of strain gauge amplifier measurements in the lower part of the range [3] when expressed in relative terms with respect to the measured value, as is typical for force and torque calibration. figure 2 shows the relative standard uncertainty contribution of the simulator if the amplifier is calibrated with figure 1. strain gauge bridge transducer with 6-lead connection to the bridge amplifier in a force measuring system. abstract the article presents a method for calibration of strain gauge bridge amplifiers with improved uncertainty in low voltage ratio range. the procedure is based on combining traditional calibration of the amplifier at one point and linearity determination of the rest of the range. traditional calibration is performed by a calibrated strain gauge bridge simulator at a reference value where measurement uncertainty is adequate, and the linearity is determined by a combinatorial calibration method with lower uncertainty, employing a special resistance circuit. uncertainty in the lower part of the amplifier range can be significantly improved, resulting in a combined relative standard uncertainty below 2.5x10-5 for the range from 0.04 mv/v to 2.5 mv/v. acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 70 traditional calibrator units, such as hbm k3608 or hbm bn100a simulators. for u=0.00002 mv/v (for k=2) expanded calibration uncertainty and therefore u=0.00001 mv/v standard uncertainty, the relative standard uncertainty w (u divided by the measured value) at 2 mv/v is 5×10-6, rising to w=5×10-5 at 0.2 mv/v, and to w=2.5×10-4 at 0.04 mv/v, which is the typical 2 % low range limit of 2 mv/v nominal range force transducer. the best available relative standard uncertainty of simulator calibration at nmi level with u=0.00001 mv/v expanded uncertainty is also shown, where the relative standard uncertainty w reaches 1.25×10-4 at 0.04 mv/v ratio. such uncertainties significantly exceed the available uncertainty of the realization of mechanical quantities, where, for example, force standard machines offer relative expanded uncertainties w of 1×10-5. calibrating the amplifier together with the transducer as a measuring system is one solution to overcome this limitation, but it limits the transducer to be always used with the same amplifier. the transducer calibration can be void in the case of amplifier replacement if the calibration data of amplifiers is not available. if the transducer is used with another amplifier than the one employed originally during calibration, i.e. with a replacement amplifier, both amplifiers, original and replacement, should be calibrated to assure comparability of results. furthermore, deviations and calibration uncertainties of both amplifiers should be considered [4], [5]. if both amplifiers are not checked with the same simulator, where the simulator is used as a comparator rather than a traceable reference, the uncertainty of the amplifier calibration should be evaluated and taken into account as necessary. according to the international standard iso 376 [4], replacing the amplifier (called simplified an indicator in the standard) is allowed if the following requirements are met: both amplifiers should be calibrated and traceable to national standards. both amplifiers should have the same working parameters (excitation voltage and frequency) and comparable resolution. calibration uncertainties of original and replacement amplifier should not significantly influence the total uncertainty of the force measuring chain. as a recommendation, the uncertainty of the replacement amplifier should not exceed 1/3 of the uncertainty of the entire system. whenever the amplifier is replaced, the contribution of the amplifier calibration should be compared against the calibration uncertainty of the measuring system, for the whole calibrated range, as the calibration uncertainty of the simulator can become the major uncertainty contribution and can increase the total measuring system uncertainty. in section 2 we discuss the effect of the replacement amplifier on the total measuring system uncertainty. further, in section 3, we present a method and a circuit for determining the linearity of the amplifier without the need to first calibrate the circuit, thus allowing lower uncertainties. in section 4 we show the results of the application of the method for linearity determination and calibration on a high precision measuring amplifier. finally, conclusion is given in section 5. 2. uncertainty contribution of the measuring amplifier when a calibrated transducer-amplifier measuring chain is separated, the traceability of the system needs to be reestablished via mv/v voltage ratio standard. a schematics of the calibration of a measuring chain is shown in figure 3a and necessary steps that are required for assuring exchangeability of measuring amplifiers via mv/v traceability is shown in figure 3b. in the first case, the calibration is valid for the whole chain only, as the effects of transducer and amplifier are not individually known and therefore cannot be separated. for such calibration, it is not possible to simply replace the amplifier, as the original amplifier characteristics (e.g. linearity, deviation) are not known. while the calibration results are typically attributed to the transducer, they are actually only valid for the whole force measuring system, the transducer with original amplifier. if the option of replacing the amplifier is required, then the calibration data for the transducer alone need to be provided, and not for the whole system. thus, the effect of the amplifier figure 2. relative standard uncertainty w of a typical calibrated high precision strain gauge bridge simulator with u=0.00002 mv/v (solid line) and the best available relative standard uncertainty at nmi level for u=0.00001 mv/v. (dotted line). figure 3. comparison of (a) calibration of a measuring chain and (b) separate calibration of transducer and amplifier. acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 71 should be eliminated from the measuring chain. for this, the amplifier must first be calibrated, to assess the amplifier characteristics, and then the influence of the amplifier should be considered either the calibrated values of the system corrected for any amplifier deviation or the necessary corrections included as an additional uncertainty component. in any case, the uncertainty of the calibration of the amplifier will have to be included in the transducer calibration uncertainty. before the transducer can be used with a replacement amplifier, the replacement amplifier must also first be calibrated, and any deviations should be taken into account (corrected or included as additional uncertainty component). then, the calibration uncertainty of the replacement amplifier needs be included in the uncertainty budget for the newly assembled measuring system. to illustrate the scale of calibration uncertainty of the amplifier with respect to the uncertainty of the measuring chain (transducer with amplifier) an example is given in figure 4, based on calibration data for a 100 kn force transducer taken from an actual calibration certificate. the calibration uncertainty of the measuring chain is shown with a solid line, as a relative standard uncertainty. the force transducer in this example was calibrated in the range from 5 kn to 100 kn (5 % to 100 % of the nominal range) in a force standard machine with 0.002 % expanded uncertainty. for comparison, the calibration uncertainty of the amplifier is shown with a dashed line, for a nominal output of the transducer of 2 mv/v at nominal load. the calibration uncertainty of the amplifier is specified in its calibration certificate as 0.00002 mv/v from 0.1 mv/v to 2 mv/v. it can be seen that, while the amplifier uncertainty is much lower than the measurement chain uncertainty for the upper range of the transducer, it significantly exceeds the measuring chain calibration uncertainty at lower force values. when replacing the amplifier, the uncertainty of both amplifiers should be taken into account, further increasing the effect of the amplifier calibration uncertainty. as can be seen from this figure, the amplifier uncertainty exceeds 1/3 of the transducer calibration uncertainty for values below 30 % of the nominal range of the transducer. it results in a significant increase of the total uncertainty and should therefore not be neglected. this is also in agreement with the recommendation in iso 376 regarding suitability of replacement amplifiers. figure 5 shows the expanded uncertainty wtrans for the case where the 100 kn force transducer is calibrated together with the amplifier as a measuring chain (solid line), and the resulting expanded uncertainty wtrans+amp when a replacement amplifier is used after calibration (dashed line). for the second case, the combined standard uncertainty wtrans+amp is calculated from contributions of the original calibration uncertainty of the transducer chain wtrans and additional calibration uncertainties of each amplifier (original amplifier – wamp_nmi and replacement amplifier – wamp_lab) according to (1). in this example, both amplifiers have the same calibration uncertainty contribution (best available). 𝑊trans+amp = 2 ∙ �𝑤trans2 + 𝑤amp_nmi2+𝑤amp_lab2. (1) the actual uncertainty contributions from amplifier calibration (absolute uamp and relative wamp) and from transducer chain calibration (wtrans) are shown in table 1. the output of the transducer ranges from 0.04 mv/v at 2 % to 2 mv/v at 100 % nominal range. as the amplifier is not calibrated at 0.04 mv/v, the uncertainty is estimated from the rest of the range. also, calibration of the transducer is not available for 2 kn force step, but is expected to be much less than the amplifier uncertainty. to reduce the effect of the amplifier calibration uncertainty and to keep the low uncertainty of measuring system calibration also at the lower values of the range, an additional evaluation of figure 5. comparison of expanded relative calibration uncertainties of a transducer chain (wtrans) and of a calibration of a transducer with replacement amplifier (wtrans+amp). table 1. comparison of uncertainty contributions. range output [mv/v] uamp [mv/v] wamp [%] wtrans [%] 2 % 0.04 0.00002 (*) 0.050 / 5 % 0.1 0.00002 0.020 0.010 10 % 0.2 0.00002 0.010 0.009 20 % 0.4 0.00002 0.005 0.009 50 % 1.0 0.00002 0.002 0.008 100 % 2.0 0.00002 0.001 0.008 (*) estimate figure 4. comparison of relative standard uncertainty of calibration of a 100 kn force transducer chain (wtrans) and relative uncertainty of amplifier calibration (wamp). acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 72 the amplifier can be performed by determining the linearity of the amplifier (deviation from a straight line response) with low uncertainty. by calibrating the amplifier with a calibrated simulator at one reference value at higher ratio values, where relative uncertainty is adequate and using an alternative linearity check method with low uncertainty to verify the rest of the range, the measurement uncertainty in the lower range of the measuring system can be improved. 3. measuring equipment and procedure 3.1. combinatorial calibration method to solve the problem of evaluation of measuring instruments, where the calibration uncertainty of reference standards is too high, a calibration method that does not require traceably calibrated equipment can be applied [6]. the method, also called combinatorial method, has been in the last 20 years successfully applied in many different fields, e.g. thermometry bridge calibration [7]. the combinatorial calibration method is based on measuring a set of artefacts: each individual artefact separately and also all possible combinations of these artefacts. from the deviation of measured values of combinations of artefacts and calculated results from the same combinations, the non-linearity of the system can be estimated. additionally, if one artefact is calibrated, a full calibration can also be performed. 3.2. circuit for combinatorial method to apply the combinatorial method to evaluation of measuring amplifiers, a suitable set of artefacts in necessary. a circuit fulfilling the requirements, which can also act as a strain gauge bridge simulator, is presented in [8]. a general schematics of the circuit employed for linearity determination is shown in figure 6. the circuit is based on a variable resistive voltage divider comprised of base resistors, with additional resistor networks connected to the output leads of the divider to reduce the variation of the output resistance of the circuit. the output ratio is varied by tapping the output over a single base resistor or a combination of consecutive base resistors. in the case of the circuit in figure 6, the set of artefacts is defined by the four base resistors, which form a part of the voltage divider network. extending the circuit to eight base resistors allows measurements of 36 non-zero output combinations, from which the estimation of the linearity error can be calculated. the output ratios of the divider, as well as input and output resistance of the circuit, can be adapted to meet the requirements. the actual circuit built to check the linearity of the amplifier was designed for a 350 ω input and output resistance and a 2.5 mv/v nominal output ratio. with appropriately selected values for the eight base resistors, it can cover the range from about 0.04 mv/v to 2.5 mv/v. 3.3. linearity determination even without the calibration of the resistors, the circuit can be applied for the determination of linearity of measuring amplifiers. the circuit is connected to the amplifier in place of the transducer and the indication on the amplifier is recorded for each available output combination. when ratio values for all combinations have been measured, deviations of the measured sum of selected resistors and the calculated sum of selected resistors are determined. as the errors for base resistors are not known, they are estimated by a least square fit, based on the error distribution from all measurements. a correction function is calculated for the resulting deviation values which represents the nonlinearity, and residuals of the fit serve as standard uncertainty estimation of the linearity measurement. when the circuit is applied in combination with the combinatorial calibration method, the resulting uncertainty of the linearity determination depends mainly on the quality of the measuring instrument. if it is applied to high precision amplifiers [9], it is possible to reach a standard uncertainty of the linearity determination in the range of 0.000002 mv/v or better [10]. 3.4. reference point calibration the result of the linearity determination alone is not enough to calibrate the amplifier. additional traceable calibration must be performed at least at one non-zero ratio value to establish the absolute linear error of the amplifier indication. as the zero point is usually fixed by definition by setting the indication to zero at the beginning of the measurement or later subtracting the indication at zero load, only one calibration point is necessary to define the slope, as the line is expected to go through zero. the uncertainty of the calibrated absolute value defines the uncertainty of the slope – the linear error, and it scales in proportion to the measured value [6], [11]. for lower values it is proportionally lower and for higher values it is proportionally higher than at the reference point. it is beneficial to choose the largest possible absolute calibration point, as its relative uncertainty contribution will be constant and attributed to the whole range, and therefore provide the lowest uncertainty in the lower part of the range, figure 7. for the purpose of absolute calibration, traditional figure 7. the uncertainty contribution of the absolute calibration point with respect to the selected calibration value. figure 6. schematics of a resistive voltage divider circuit for application of the combinatorial method for amplifier linearity determination. acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 73 simulators can be employed as, depending on the selected range, the simulator relative calibration uncertainty can be adequate for the required task. with zero value set, known linearity correction and its uncertainty, and one absolute calibration point for the slope, the instrument can be fully calibrated. at the expense of increased uncertainty contribution, the calibration range can also be extended above the calibrated point. 4. results the evaluation was performed on an hbm dmp41 highprecision 225 hz carrier frequency amplifier. the selected parameters were 2.5 mv/v range, 5 v excitation voltage and 0.1 hz bessel filter. the expanded calibration uncertainty of the amplifier at 2 mv/v using a traditional simulator was u=0.00002 mv/v, or w=1×10-5 relative uncertainty. in combination with the linearity determination, the calibration of the amplifier in the whole range from 0.04 mv/v to 2.5 mv/v was performed. figure 8 shows the result of the linearity determination with the combinatorial calibration method. the figure shows the resulting deviation of measurement of 36 possible resistor combinations (black dots) and the fit of the linearity error (solid curve) with its estimated standard uncertainty. the calculated standard deviation of the residual errors is about 0.000001 mv/v, shown with solid error bars. the linearity check with a calibrated simulator hbm bn100a is also shown (dashed line with square markers) and the standard uncertainty of the linearity determination of the amplifier performed with the simulator is represented with dashed uncertainty bars. for the comparison, the resulting curves are referenced at 0 mv/v and 2 mv/v. it can be seen that, while both measurements are in good agreement, the uncertainty of the linearity determination with the combinatorial method produces a much lower uncertainty than the linearity measurement with a calibrated simulator. the results of the combinatorial method are sufficient to characterise the nonlinearity, but they do not provide enough information for the calibration of the amplifier, since the linear error is not known. an additional measurement at one ratio value is required to fully calibrate the instrument. if the absolute calibration at one point is made with u=0.00002 mv/v expanded uncertainty, it will define the minimum uncertainty at that point. together with the linearity determination uncertainty, the total calibration uncertainty of the instrument can be calculated. figure 9 shows the standard uncertainty contributions for the combination of absolute calibration of the amplifier at 2 mv/v and linearity determination based on data from figure 8. the dashed line represents the reference point calibration standard uncertainty of u=0.00001 mv/v. for the linear instrument error, this value can be scaled proportionally with the ratio value, its relative contribution remaining constant (dotted line) the contribution due to calibration uncertainty. the second contribution is the standard uncertainty of the linearity determination (thin solid line) performed with the resistor circuit and combinatorial method. the final combined standard uncertainty is shown by the thick solid line. the combined uncertainty is calculated according to (2), where uc is the combined standard uncertainty, usim_prop the proportional part of the standard uncertainty contribution of the absolute point calibration, and ulin_check is the standard uncertainty contribution of the linearity determination employing the combinatorial calibration method: 𝑢c = �𝑢sim_prop2 + 𝑢lin_check2 . (2) it can be seen in figure 9 that the dominant uncertainty contribution for most of the range is the calibration uncertainty arising from the simulator calibration uncertainty. compared to a calibration employing only the simulator, the achieved uncertainty with the proposed procedure has been reduced significantly in the lower range of ratio values, and slightly increased for ratio values above the absolute calibration point. figure 10 shows the final calibration result of the amplifier using the combinatorial calibration method and one reference point calibration with a calibrated simulator. the result of the linearity determination from figure 8 is now combined with the established deviation during the amplifier calibration at 2 mv/v. the uncertainty of the linearity determination is enlarged by the proportional part of the reference point figure 8. amplifier linearity error determination with the combinatorial method (solid error bars) vs. calibration with a simulator (dashed error bars), referenced at 0 mv/v and 2 mv/v. error bars represent standard uncertainty. figure 9. combined standard uncertainty of absolute calibration at 2 mv/v (u=0.00002 mv/v) and linearity determination for the whole range. acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 74 calibration uncertainty. in figure 11 the same measurement example from figure 9 is shown, but standard uncertainty contributions are expressed as relative standard uncertainties. we can see that the relative calibration uncertainty of the simulator increases significantly for lower ratio values (dashed line). the proportional part of the absolute calibration uncertainty taken for the 2 mv/v, when expressed as relative uncertainty, is constant for the whole calibration range (dotted line). the contribution of the linearity determination expressed as relative contribution is shown by the thin solid line. the combined relative standard uncertainty w is below 1×10-5 for the most of the range and below 2.5×10-5 for the whole evaluated range. again, the reduced uncertainty compared to employing only the simulator is evident at low ratio values. above the 2 mv/v absolute reference point, the uncertainty has been slightly increased. in this paper, only the calibration uncertainty of the amplifier calibration at one value and the uncertainty of the linearity check are considered. other contributions, such as resolution of the instrument, drift of the simulator ratio value and other possible contributions are not explicitly taken into account. 5. conclusions the results of the evaluation show improvement in calibration uncertainty of measuring amplifiers in comparison to traditional strain gauge simulator calibration only. combining the traditional simulator based calibration and combinatorial calibration based linearity determination, improved calibration in the lower part of the range can be achieved. with the presented method, the relative standard uncertainty w at 0.04 mv/v could be reduced from typical values of 2.5×10-4 to values below 2.5×10-5. the improved calibration uncertainty allows separate calibration of transducers and amplifiers and thus interchangeable transducer-amplifier combinations, while preserving acceptable calibration uncertainty levels for most scientific and industrial applications. references [1] r. vollmert, g. ramm, “realization, maintenance and dissemination of the measurand “ac voltage ratio in mv/v” for strain gauge measurements”, 18th imeko conference on force, mass and torque 2002, celle, germany, 2002, pp. 145-150. [2] calibration and measurement capabilities cmcs, the bipm key comparison database, http://kcdb.bipm.org, bipm. [3] d. schwind and t. hahn, “investigation of the influence of carrier frequency or direct current voltage in force calibrations”, xix imeko world congress, lisbon, portugal, 2009, pp. 201204. [4] iso 376:2011; metallic materials – calibration of force proving instruments used for the verification of uniaxial testing machines [5] calibration guide euramet cg-4 version 2.0(03/2011): uncertainty of force measurements, euramet, 2011. [6] d.r. white, m.t. clarkson, p. saunders and h.w. yoon “a general technique for calibrating indicating instruments”, metrologia, 45, 2008, pp. 199-210. [7] d.r. white, “a method for calibrating resistance thermometry bridges”, procedings of tempmeko ’96, torino, italy, 1997, pp.129-134. [8] m. hiti, “resistor network for linearity check of voltage ratio meters by combinatorial technique”, meas. sci technol.,vol 26, no. 5, 2015, pp. 1-7. [9] m.m. schäck, “high-precision measurement of strain gauge transducers at the physical limit without any calibration interruption”, imeko 22nd tc3, 12th tc5 and 3rd tc22 international conferences, cape town, republic of south africa, 2014, pp. 118-123. [10] m. hiti, “alternative calibration procedure for strain gauge amplifiers”, xxi imeko world congress 2015, prague, czech republic, 2015. [11] m. borys, r. schwartz, a. reichmuth, r. nater, “fundamentals of mass determination” springer-verlag berlin heidelberg, 2012, isbn 978-3-642-11937-8. figure 10. full calibration of the amplifier with absolute calibration at 2 mv/v and determined linearity for the whole range (solid error bars). result of the traditional calibration is also shown (dashed error bars). both curves are expected to go through zero as initially the indication is set to zero. error bars are for standard uncertainty. figure 11. combined standard uncertainty w of absolute calibration at 2 mv/v (u=0.00002 mv/v or w(2 mv/v)=5x10− 6) and uncertainty of the linearity determination for the whole range. reducing the uncertainty of strain gauge amplifier calibration << /ascii85encodepages false /allowtransparency false /autopositionepsfiles true /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile 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can be opened with acrobat and adobe reader 5.0 and later.) >> /namespace [ (adobe) (common) (1.0) ] /othernamespaces [ << /asreaderspreads false /cropimagestoframes true /errorcontrol /warnandcontinue /flattenerignorespreadoverrides false /includeguidesgrids false /includenonprinting false /includeslug false /namespace [ (adobe) (indesign) (4.0) ] /omitplacedbitmaps false /omitplacedeps false /omitplacedpdf false /simulateoverprint /legacy >> << /addbleedmarks false /addcolorbars false /addcropmarks false /addpageinfo false /addregmarks false /convertcolors /converttocmyk /destinationprofilename () /destinationprofileselector /documentcmyk /downsample16bitimages true /flattenerpreset << /presetselector /mediumresolution >> /formelements false /generatestructure false /includebookmarks false /includehyperlinks false /includeinteractive false /includelayers false /includeprofiles false /multimediahandling /useobjectsettings /namespace [ (adobe) (creativesuite) (2.0) ] /pdfxoutputintentprofileselector /documentcmyk /preserveediting true /untaggedcmykhandling /leaveuntagged /untaggedrgbhandling /usedocumentprofile /usedocumentbleed false >> ] >> setdistillerparams << /hwresolution [2400 2400] /pagesize [612.000 792.000] >> setpagedevice acta imeko  december 2014, volume 3, number 4, 38 – 45  www.imeko.org  acta imeko | www.imeko.org  december 2014 | volume 3 | number 4 | 38  a method for assessing multivariate measurement systems  michele scagliarini  department of statistical sciences, university of bologna, via belle arti 41, 40126 bologna, italy  section: research paper   keywords: covariance matrices; eigenvalues; gauge study; manova; wishart distribution  citation: michele scagliarini, a method for assessing multivariate measurement systems, acta imeko, vol. 3, no. 4, article 8, december 2014, identifier:  imeko‐acta‐03 (2014)‐04‐08  editor: paolo carbone, university of perugia   received december 16 th , 2013; in final form december 16 th , 2013; published december 2014  copyright: © 2014 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: this work was supported by grant from university of bologna  corresponding author: michele scagliarini, e‐mail: michele.scagliarini@unibo.it  1. introduction in a manufacturing environment, critical decisions about process and product quality depend on the quality of the measurement systems. measurement systems analysis (msa) is a set of statistical techniques used to quantify the uncertainty of the measurement instruments. [1] and [2] provided a review of gauge repeatability and reproducibility (r&r) methods for assessing the precision of measurement systems. in the case of univariate measurement systems, several msa-approval metrics are commonly used. for an overview on this topic we suggest [3] and [4]. in current manufacturing industry processes are often characterized by many important characteristics. accordingly, [5] proposed multivariate extensions of three commonly used msa-approval criteria using the volume of constant-density contours to characterize the variability of the measurement system. these multivariate msa-metrics require a multivariate analysis of variance (manova) for estimating the covariance matrices for one factor and twofactor gauge studies. in order to ensure constant flows of reliable data, manufacturers should periodically assess their measurement systems and the costs involved in maintaining well performing measurement systems are normally relevant. this issue motivates the present work. multivariate measurement systems analysis is usually performed by designing suitable gauge (r&r) experiments ignoring available data generated by the measurement system while used for inspection or process control. in recent literature, the use of these measurements from regular use of the instrument has been suggested for univariate msa studies (see e.g, [6]). here we propose the following approach. in the initial set up, after the multivariate measurement instrument is assessed as adequate, its performances are assumed as benchmark. therefore, using the data from the regular activity of the instrument, the periodic assessments of the measurement device are performed by comparing the present precision with the benchmark through a statistical test. since the proposed method does not require a multivariate gauge study, our proposal can be a useful tool for reducing the costs of multivariate msa carried out with a certain frequency. here is the outline of the paper. the next section introduces the multivariate measurement error model, abstract multivariate measurement systems analysis is usually performed by designing suitable gauge r&r experiments ignoring available data  generated by the measurement system while used for inspection or process control. this article proposes an approach that, using the  data  that  are  routinely  available  from  the  regular  activity  of  the  instrument,  offers  the  possibility  of  assessing  multivariate  measurement systems without the necessity of performing a multivariate gauge study. it can be carried out more frequently than a  multivariate  gauge  r&r  experiment,  since  it  can  be  implemented  at  almost  no  additional  cost.  therefore  the  synergic  use  of  the  proposed approach and the traditional multivariate gauge r&r studies can be a useful strategy for  improving the overall quality of  multivariate measurement systems and is effective for reducing the costs of a multivariate msa performed with a certain frequency.  acta imeko | www.imeko.org  december 2014 | volume 3 | number 4 | 39  describes the multivariate msa-approval criteria proposed in recent literature and explains the multivariate analysis of variance (manova) method for estimating the covariance matrices of interest. section 3 develops the test for assessing the multivariate measurement instruments. section 4 studies the performances of the proposed method. finally, the last section contains a discussion and the conclusions. 2. multivariate measurement system analysis  let  1 2, ,..., mx x x x (1) represent the vector of m quality characteristics with mean vector µ and covariance matrix  positive definite. we assume that the multivariate process data are from a multivariate normal distribution. let us denote with  1 2, ,...,  mlsl lsl lsllsl , (2)  1 2, ,...,  musl usl uslusl , (3) and  1 2, ,...,  mt t tt (4) the m-vectors values of the lower specification limits, upper specification limits and target values, respectively. msa methodology assumes the model  y x e (5) where y is the vector of the observable quality characteristics, which is usually obtained from some physical measurements, x is the true quality characteristics vector and e is the multivariate measurement error vector. it is assumed that e  ,0 en σ (6) with c positive definite and that x and e are independent. as a result, y  , yn μ σ (7) where  y eς σ σ . let us denote with i, ei and yi (i=1,2,…,m) the eigenvalues of , e and y, respectively. in the multivariate framework, [5] developed multivariate versions of three univariate gauge-approval criteria. the author proposed the following statistics. the multivariate precision-to-tolerance ratio, which is defined as the m-th root of the ratio of (1-)100% volume of the multivariate error distribution and the volume of the tolerance region. this ratio, according to the specification of the tolerance region, simplifies to   1/ 2 /2 , 1 1 1 1 2 m m m m ei i m m i i p t tol m                           (8) when a hypercube-shaped tolerance region is used, and 1/ 2 , 12 2 m m m ei im i p t tol             (9) for the case of a hyperellipsoid-shaped tolerance region. in the above equations (·) is the gamma function, toli=usli-lsli and 2,m is the 100(1-)-th percentile of the 2 distribution with m degrees of freedom with (1-) usually fixed at 0.99. therefore the p/t1m and p/t2m criteria compare the multivariate instrument variability, computed on the base of the constant-density contour ellipsoid, with the multivariate tolerance region (hypercube or hyperellipsoid). the multivariate percent r&r ratio, which is defined taking the m-th root of the (1-)100% volumes of the gauge error distribution and measured-values distribution. the statistic in question simplifies in 1/ 1 % & 100 m m ei m i yi r r             (10) and expresses the relative widths of the multivariate distributions of the error e and the measured values y. the third multivariate approval-metric is the multivariate-signal-to-noise ratio which compares the (1)100% volume of the gauge-error distribution. this statistic is 1/ 1 2 m m i m i ei snr            . (11) the author in [5] also gave the guidelines for gauge acceptance. approval values for p/t1m and p/t2m range from 0 to 0.3, %r&rm should be ≤30%, while based on snrm a measurement system is adequate when snrm≥5. 2.1. multivariate msa in practice  the covariance matrices , e and y are usually unknown, for this reason [5] also proposes a multivariate analysis of variance (manova) method of estimating the covariance matrices for one-factor and two-factor gauge studies. according to the adopted notation, y′=[y1,y2,...,ym] represents the measured values or data generated by the gauge. let us consider a random-effects manova model [7] where the factors in question are all m-dimensional vectors. let us denote the factors as ip (i=1,2,…,p) for part, jo (j=1,2,…,o) for operator, ijpo for part-operator interaction. the error term is denoted by ijkε , where k=1,2,…,r indicates the repeated reading of the same part by the same operator. therefore ijky is an m-vector containing the k-th reading, by operator j, of the i-th part for the m quality characteristics. in the two-factor gauge study the manova model is:    ijk i j jk ijky p o po ε (12) where the random components ip , jo j, ijpo and ijkε are mutually independent with distributions ip   ,μ pn σ , jo   ,0 on σ , ijpo   ,0 pon σ and ijkε   , 0n σ , respectively. within this framework, acta imeko | www.imeko.org  december 2014 | volume 3 | number 4 | 40    .. ... .. ... 11       p i i i or p msp y y y y (13) is the mean-square for the part matrix, where .. 1 1 1     o r i ijk j kor y y and ... 1 1 1 1      p o r ijk i j kpor y y . the mean square for the operator matrix is   . . ... . . ... 11       o j j j pr o mso y y y y (14) where . . 1 1 1     p r j ijk i kpr y y . the mean square for the partoperator interaction matrix is given by      . .. . . ... . .. . . ... 1 1 1 1             p o ij i j ij i j i j r o p mspo y y y y y y y y (15) and the mse matrix is     ... ...1 1 1 1 1         p o r ijk ijk i j kpo r mse y y y y . (16) the covariance matrices are estimated using the expected mean squares. the parts covariance matrix is estimated by ˆ p or msp mspo σ , (17) the operator factor covariance matrix is estimated by ˆ o pr   mso mspo σ , (18) the part-operator interaction covariance matrix is estimated by ˆ po r mspo mse σ (19) and the covariance matrix of the error terms ijkε is estimated by ˆ  σ mse . (20) by adopting the gauge r&r notation, p corresponds to ,the covariance matrix of the quality characteristic. repeatability and reproducibility are given by σ and o poς σ , respectively. the sum of repeatability and reproducibility gives the gauge measurement error covariance matrix     e o poς σ σ σ . (21) therefore, the estimators of the covariance matrices of interest are: ˆ ˆ pς σ , (22)  ˆ ˆ ˆ ˆ  e o poς σ σ σ , (23) and ˆ ˆ ˆ y eς σ σ . (24) 3. a test for multivariate measurement systems  the multivariate msa-approval criteria described in the previous section are based on constant-density contours of the multivariate normal distribution. a change in the precision of a measurement instrument will cause a change in the corresponding ellipsoid of constant density. therefore, since we are interested in the detection of a worsening in the measurement instrument precision, we will focus on significant reduction of the ellipsoid coverage. let us suppose that at the beginning of the manufacturing activity, which for notation purpose we denote as time t=0, a multivariate msa is performed and that the measurement instrument is assessed as adequate. we denote with e0 the precision of the measurement instrument, with0 the covariance matrix of the true quality characteristic and with y0=0+e0 the covariance matrix of the measurements, at time t=0. assuming the multivariate normality an ellipsoid of constant density is defined by     10 0:    yu cy y μ σ y μ . (25) if we assume c=2,m, then u0 is the boundary of the multivariate region in which 100 (1-)% process fall. after the initial set up, the measurement device is usually used for inspection or process control generating a lot of data at no additional costs. let us consider a time interval in which the instrument has been routinely used. at time t=t the measurement instrument is characterized by a precision et. usually, the process variability is monitored by a suitable control chart thus if no out of control signals occur we can assume the stability of the process i.e. 0=t. under these assumptions the variability of y is 0 yt etς σ σ . (26) in this framework, differences in the variability of the observed measures are only caused by changes in the precision since yt=y0 if and only if et=e0. at time t=t, the 0uy define the quadratic form             1 1 0 . t yt et q           y y μ σ y μ y μ σ σ y μ (27)  tq y under the normality assumption is distributed as a 2 with m degrees of freedom and has several useful properties.  tq y is not constant,   tq cy with equality only when 0yt yς σ , i.e. 0et eς σ . the minimum and the maximum values of  tq y can be determined analytically. using results from [8] we find     max maximum eigenvalue of  tq cy γ (28)     min minimum eigenvalue of  tq cy γ (29) where 10  y ytγ σ σ . therefore, if at time t=t the measurement instrument is worse than at time t=0, then the 0uy define ellipsoids with coverage ranging from    2min pr min m tp q y (30) to acta imeko | www.imeko.org  december 2014 | volume 3 | number 4 | 41     2max pr max m tp q y . (31) the difference between (1 ) , the ellipsoid coverage at time t=0, and minp (or maxp ), the ellipsoid coverage at time t=t, quantifies the possible worsening in the instrument precision at instant t. a test for detecting the decreasing of the coverage can be derived as shown below. let us consider the null hypothesis that the instrument precision at instant t=t is equal to the precision at instant t=0 0 0h : et eς σ (32) if 0h holds, then 0yt yς σ , γ i ,   tq cy and therefore the ellipsoid coverage is min 1  p . let the alternative hypothesis be 1 0h : is positive definiteet eς σ (33) if 1h holds, then   tq cy hence the ellipsoid coverage is smaller than 1  : min 1  p . let 1 2 ,...,       m (34) be the eigenvalues of the matrix γ . under hypothesis 1h we have 1 1m    (35) and the (upper) limit 1 is reached only under 0h . let 1 be the largest eigenvalue of the matrix 1 1 0    yt yω γ σ σ . (36) then, when 1h holds 1 1 1m      (37) with 1 1  only under 0h . let s be the sample covariance matrix of a random sample of size n from y at time t. if 0h holds, then 1/2 1/2 0 0( 1)   y yn σ sς  ( , 1)w ni (38) where ( , 1)w ni denotes a wishart distribution with parameters i and 1n  . from [8] we have that matrix 1/ 2 1/ 2 0 0   y yς sς and matrix 1 0  yς s , have the same eigenvalues. furthermore, also matrices 10  yς s and 1 0  ysς have the same eigenvalues. therefore, if we denote with 1̂ the largest eigenvalue of the matrix 10  ysς , then 0h is not rejected if and only if 1 1 ˆ( 1)n u  , where 1u is the upper percentage point of the largest characteristic root of a wishart matrix. the advantage of this method is that the measurement instrument is assessed by comparing its performance instant t with those at instant 0, without the necessity of performing a multivariate gauge study (manova): the sample covariance matrix s can be estimated using the data available by the routine use of the measurement device at no additional costs. 4. case studies  in this section we discuss the ability of the test for detecting worsening in the measurement instrument performances. before entering in the details of the case studies it is useful to spend a few words reminding that the multivariate msa-metrics are designed thinking at different ways for assessing the measurement precision. the 1mp t and 2mp t criteria compare the multivariate instrument variability with the multivariate tolerance region (hypercube or hyperellipsoid). the remaining metrics do not involve the tolerances: % & mr r expresses the relative widths of the multivariate distributions of the errors e and the measured values y; msnr compares the width of the multivariate distribution of the true quality characteristics x with the corresponding volume of the multivariate errors e. since the test in question does not involve the tolerance regions, we expect a test behaviour similar to those of % & mr r and msnr . for this reason we shall compare the outcomes of the test only with the msa-metrics % & mr r and msnr . we consider as the situation at time t=0 (the benchmark) the case discussed by [5], then we examine a variety of worsening-precision scenarios at time t=t. for each of the proposed scenarios, we compute the multivariate msa-approval metrics presented in section 2 and we design suitable simulation experiments for studying the performances of the proposed test. let us therefore consider the case discussed by [5] where the data come from an automotive body panel gauge-study involving m=4 quality characteristics, with p=5 parts, o=2 operators and r=3 repeated measurements (see table 1 in [5] for the original data). using a two-factor manova method the matrices estimates are 0.01811 0.01600 0.02180 0.00763 0.01600 0.25163 0.15732 0.35463ˆ 0.02180 0.15732 0.20856 0.39249 0.00763 0.35463 0.39249 0.98631                σ 0.00094 0.00168 0.00141 0.00189 0.00168 0.00632 0.00475 0.00702ˆ 0.00141 0.00475 0.00486 0.00581 0.00189 0.00702 0.00581 0.00852              eς and 0.01905 0.01768 0.02321 0.00574 0.01768 0.25795 0.16207 0.36165ˆ 0.02321 0.16207 0.21342 0.39830 0.00574 0.36165 0.38830 0.99483                yς the eigenvalues of the covariance matrices σ̂ , ˆ eς and ˆ yς are reported in table 1. using equations (10) and (11) we obtain % & 12.26061mr r and 11.30385msnr respectively. the results show that the measurement instrument is assessed as acceptable by both the multivariate acta imeko | www.imeko.org  december 2014 | volume 3 | number 4 | 42  criteria, therefore it can be used in the manufacturing process and for our purposes we can assume 0ˆ e eς σ , 0 ˆ σ σ and 0ˆ y yς σ . now we examine several scenarios where a realistic worsening of the measurement instrument after a period of use is considered. we base this discussion on the spectral decomposition of 0eς 0 0 0 0 e e e eς u d u (39) where 0 01 02 0( , ,..., )e e e e mu u u u is the matrix of eigenvectors with columns 0e iu (i=1,2,…,m), and  0 01 02 0, ,...,  e e e e mdiagd is the diagonal matrix of the eigenvalues. the diagonal matrix 0ed is the covariance matrix of the latent independent factors that represent the primary independent sources of variability introduced by the instrument at time t=0. the instrument after a period of use is characterised by a measurement error covariance matrix: et et et etς u d u (40) where 1 2( , ,..., )et et et etmu u u u with columns etiu (i=1,2,…,m) and  1 2, ,...,  et et et etmdiagd . many alternative cases for etς worse than 0eς can be considered, however etς cannot be attained by changing the elements of 0eς arbitrarily. it is realistic to examine for etς cases where changes in the variability are due to changes in variance of the latent independent factors. in other words, the factors that cause the variability in the instrument at instant t remain the same as for instant 0, but with larger variance. this means that cases for etς with practical meaning should be those involving the eigenvalues, 0eit ei  for (i=1,2,…,m), but maintaining unchanged the eigenvectors ( 0 e etu u ).a change in the eigenvectors can be interpreted as a the presence of serious problems in the instrument such that the independent sources of variability become dependent. it is worth noting that this concept has been used also by other authors. for instance [9] used this definition of plausible changes in a process capability analysis framework. in what follows, we consider three cases for etς where the eigenvectors remain unchanged. furthermore, for the sake of completeness, we will also consider the case of a change in the eigenvectors. 4.1. case 1  now we examine the case where the eigenvalues of 0eς are increased by the same additive term  : 0  et ed d i . (41) note that this case is equivalent to add the diagonal matrix  ι directly to 0eς 0  et eς σ i , (42) since from the spectral decomposition of 0eς we get the expression  0 0 0 0 0 0 0 0 0             e e e e e e e e e et u σ i u u σ u u u d i d (43) thus, the eigenvectors remain unchanged, 0 e etu u , and the eigenvalues can be expressed as 0eti e i    for i=1,2,…,m. within this case we consider scenarios where the worsening term  ranges from 0 to 0.006 with a step of 0.0001. therefore, for each value of  : a) we compute the multivariate msa approval criteria % & mr r and msnr and the results are shown in figure 1; b) using the rsoftware we generate 104 samples (n=50,75,100,150) from y   0,  etn μ σ σ where 0  et eς σ i . for each sample we compute the statistic   1ˆ1n  , where 1̂ is the largest eigenvalue of the matrix 10  ysς and s is the sample covariance matrix estimated from the sample. therefore, we evaluate the power of the test computing, for each value of  and n, the proportion of rejections of 0h . note that, fixed 0.05  , we used the function qwishartmax of the rpackage rmtstat [10] for computing the critical values of the test. the simulation results are summarized in figure 2 where, for each sample size, the rejection rates of hypothesis 0h as a function of  are reported. examining the results we note that msnr assesses the instrument as unacceptable for  ≥ 0.0033, while the test concludes, with a power greater than 80%, that instrument at time t=t is worse than the instrument at time t=0 for  ≥ 0.00325, when the sample size is n=75, and for  ≥ 0.00415 when n=50. therefore, pointing out that % & mr r evaluates the instrument as inadequate for  ≥ 0.0059, we can conclude that in this case, for moderate sample sizes (n=50 and n=75), the performances of the test are among the outcomes of metrics msnr and % & mr r . 4.2. case 2  next, we consider the case where the eigenvalues at time t=t are proportional to those at time t=0 0et ed d . (44) table 1. eigenvalues of the estimated covariance matrices. σ̂ 1 2 3 4 1.29428 0.11185 0.05438 0.00410 ˆ eς 1e 2e 3e 4e 0.01908 0.00081 0.00050 0.00025 ˆ yς 1y 2y  3y  4y  1.311189 0.11392 0.05557 0.00457 acta imeko | www.imeko.org  december 2014 | volume 3 | number 4 | 43  note that (44) is equivalent to consider the measurement error covariance matrix of instrument at instant t to be proportional to the instrument precision at instant 0 0et eς σ (45) since we can write    0 1 0 0 0 0 0     e e e e e e e etu d u u d u σ σ . (46) also in this case, the eigenvectors do not change, 0 e etu u , and the eigenvalues are expressed as 0eti e i  , for i=1,2,…,m. we perform our study by considering scenarios where the worsening factor  ranges from 1 to 10 with a step of 0.1. for each value of  we proceed as for case 1: a) we compute the multivariate msa approval metrics, the results are displayed in figure 3; b) we generate for each sample size (n=50, 75, 100, 125), 104 samples from y   0,  etn μ σ σ where 0et eς σ . for each sample we compute the test statistic   1ˆ1n  and, fixed as before an -level of 5%, we evaluate the test power by computing the proportion of the 104 replications where 0h is rejected in favour of 1h . the rejection rates for hypothesis 0h as a function of  are shown in figure 4. in this case % & mr r assesses the measurement instrument as inadequate for  ≥7.4. the test concludes that the instrument at instant t is worse than the instrument m at instant 0, with a power greater than 80%, when δ≥7.2, δ≥6.2, and δ≥5.6 for n=75, n=100 and n=125 respectively. therefore, considering that using snr the instrument is unacceptable for  ≥5.2, we conclude that for sample sizes ranging from n=75 to n=125 the results of the test are among the outcomes of metrics msnr and % & mr r . 4.3. case 3  since in the previous cases we examined cases where all the eigenvalues simultaneously change, now we discuss the case where only several eigenvalues change their values. let us consider the scenario where three eigenvalues are increased by a factor  ranging from 1 to 20 with a step of 0.25:  01 02 03 04, , ,   et e e e ediagd (47) and the covariance matrix etς is given by 0 0et e et eς u d u . for each value of  we have followed the same procedure as for cases 1 and 2: the results are summarized in figures 5 and 6. the pattern of the results is similar to that observed for case 1. msnr assesses the instrument as unacceptable for  ≥9 and % & mr r for  ≥16, while the test concludes, that the instrument at time t=t is worse than the instrument at time 0, with a power greater than 80%, when  ≥8 and  ≥10 for n=75 and n=50 respectively. figure 4. h0 rejection rates versus    for case 2.  figure 2. h0 rejection rates versus    for case 1.  figure 3. multivariate msa‐approval criteria as a function of    for case 2.  figure 1. multivariate msa‐approval criteria as a function of    for case 1.  acta imeko | www.imeko.org  december 2014 | volume 3 | number 4 | 44  4.4. case 4  finally, let us examine the situation where the variations involve also the eigenvectors, which can be interpreted as the presence of a serious problem in the measurement instrument. we examine the case where the first two diagonal elements of etς increase, while the other matrix elements are equal to the corresponding elements of 0eς : 0.00094 0.00168 0.00141 0.00189 0.00168 0.00632 0.00475 0.00702 0.00141 0.00475 0.00486 0.00581 0.00189 0.00702 0.00581 0.00852               etς (48) in the analysis the term  ranges from 1 to 8 with a step of 0.1. figure 7 shows the multivariate msa metrics computed for each values of  . the monte carlo experiment has been performed as in the previous cases and the results are summarized in figure 8. the results show that the test tends to be more sensitive to the increasing of  than % & mr r and msnr . for instance, for n=50 the power of the test is greater than 80% for  ≥6, while msnr evaluates the instrument as inadequate for  ≥6.2. note that in this case, although the multivariate metric % & mr r detects the worsening in the measurement instrument, it assesses the instrument as acceptable for all the values considered of  . summarizing, in the cases examined we aimed to study the performances of the test in realistic worsening scenarios of the measurement instrument after a period of use. the results show that the test provides outcomes with an appreciable level of agreement with the issues of % & mr r and msnr . 5. discussion and conclusions as any activity involving personnel, materials, tools and equipment, msa usually requires a non-negligible financial support. furthermore, the fact that these systems measure more than a single quality characteristic and that periodic assessments of measurement system performance are often required engages manufacturers in important challenges. in this work, we have proposed a method which can be an additional tool for assessing the statistical properties of a multivariate measurement system. the method makes use of the data that are routinely available from the regular activity of the instrument and offers the possibility of assessing multivariate measurement systems without the necessity of performing a multivariate gauge study (manova). since the illustrated strategy can be implemented at almost no additional costs it may carried out more frequently than a manova gauge study. therefore, the synergic use of the proposed approach and figure 5. multivariate msa‐approval criteria as a function of    for case 3.  figure 7. multivariate msa‐approval criteria as a function of    for case 4.  figure 8. h0 rejection rates versus    for case 4.  figure 6. h0 rejection rates versus    for case 3.  acta imeko | www.imeko.org  december 2014 | volume 3 | number 4 | 45  the traditional multivariate gauge r&r studies can be: effective for reducing the costs of a multivariate msa performed with a certain frequency; a useful strategy for improving the overall quality of multivariate measurement systems. acknowledgement  the author would like to thank the two anonymous reviewers for their helpful comments. references  [1] r.k. burdick, c.m. borror, d.c. montgomery, a review of methods for measurements systems capability analysis, j. qual. technol. 35 (2003) pp. 342-354. [2] r.k. burdick, c.m. borror, d.c. montgomery, design and analysis of gauge r&r studies: making decisions with confidence intervals in random mixed anova models, asa–siam series on statistics and applied probability, siam, philadelphia, asa, alexandria, (2005), isbn 089871-588-1. [3] automotive industry action group (aiag), measurement systems analysis, 4th ed. southfield, mi, (2010), isbn 9781605342115 [4] w.h. woodall, c.m. borror, some relationships between gage r&r criteria, qual. reliab. eng. int, 24 (2008) pp. 99106. [5] k.d. majeske, approval criteria for multivariate measurement systems, j. qual. technol. 40 (2008) pp. 140153. [6] o. danila, s.h. steiner, r.j. mackay, assessment of a binary measurement system in current use, j. qual. technol. 42 (2010) pp.152-164. [7] n.h. timm, applied multivariate analysis, springerverlag, new york, 2002, isbn 0-387-95347-7. [8] k.v mardia, j.t. kent, j.m. bibby, multivariate analysis. academic press, san diego, 1979, isbn 9780124712522. [9] i. gonzález, i. sánchez, capability indices and nonconforming proportion in univariate and multivariate processes, int. j. adv. manuf. tech. 44 (2009) pp.1036-1050. [10] i.m., johnstone,. z. ma, p.o. perry, m. shahram, “package rmtstat: distributions, statistics and tests derived from random matrix theory”. (2009) http://cran.rproject. org/web/packages/rmtstat/ electroencephalography correlates of fear of heights in a virtual reality environment acta imeko issn: 2221-870x june 2023, volume 12, number 2, 1 7 acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 1 electroencephalography correlates of fear of heights in a virtual reality environment andrea apicella1, simone barbato2, luis alberto barradas chacόn3, giovanni d’errico4, lucio tommaso de paolis5, luigi maffei6, patrizia massaro6, giovanna mastrati1, nicola moccaldi1, andrea pollastro1, selina christin wriessenegger3 1 department of electrical engineering and information technology, university of naples federico ii, naples, italy 2 idego digital psychology, rome, italy 3 institute of neural engineering, graz university of technology, graz, austria 4 department of applied science and technology, politecnico di torino, turin, italy 5 department of engineering for innovation university of salento, lecce, italy 6 neuroagain neurological and psychiatric center, benevento, italy section: research paper keywords: electroencephalography; brain computer interfaces; fear of heights; virtual reality citation: electroencephalography correlates of fear of heights in a virtual reality environment, andrea apicella, simone barbato, luis alberto barradas chacόn, giovanni d’errico, lucio tommaso de paolis, luigi maffei, patrizia massaro, giovanna mastrati, nicola moccaldi, andrea pollastro, selina christin wriessenegger, acta imeko, vol. 12, no. 2, article 9, june 2023, identifier: imeko-acta-12 (2023)-02-09 section editor: paolo carbone, university of perugia, italy received january 25, 2023; in final form february 21, 2023; published june 2023 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was supported by phd grant “ar4clinicsuraugmented reality for clinical surgery” (inps national social security institution – italy) corresponding author: giovanna mastrati, e-mail: giovanna.mastrati@unina.it 1. introduction the expression fear of heights is often used as a synonym of acrophobia, a specific phobia according to the “diagnostic and statistical manual of mental disorders” (dsm-v) [1]. a detailed definition is provided by the american psychological association (apa) that is, “an excessive, irrational fear of heights, resulting in the avoidance of elevations or marked distress when unable to avoid high places” [2]. among specific phobias, the fear of heights is stated to have one of the highest lifetime prevalence, 6.8 % [1]. fear of heights is characterized by some psychological symptoms: (i) feeling intense anxiety when thinking of or being in high places, (ii) thinking that something bad could happen in a high place, or (iii) desire to escape from the high place. physical symptoms involve: (i) an increased heartbeat, dizziness, and daze abstract an electroencephalography (eeg)-based classification system of three levels of fear of heights is proposed. a virtual reality (vr) scenario representing a canyon was exploited to gradually expose the subjects to fear inducing stimuli with increasing intensity. an elevating platform allowed the subjects to reach three different height levels. psychometric tools were employed to initially assess the severity of fear of heights and to assess the effectiveness of fear induction. a feasibility study was conducted on eight subjects who underwent three experimental sessions. the eeg signals were acquired through a 32-channel headset during the exposure to the eliciting vr scenario. the main eeg bands and scalp regions were explored in order to identify which are the most affected by the fear of heights. as a result, the gamma band, followed by the high-beta band, and the frontal area of the scalp resulted the most significant. the average accuracies in the within-subject case for the three-classes fear classification task, were computed. the frontal region of the scalp resulted particularly relevant and an average accuracy of (68.20 ± 11.60) % was achieved using as features the absolute powers in the five eeg bands. considering the frontal region only, the most significant eeg bands resulted to be the high-beta and gamma bands achieving accuracies of (57.90 ± 10.10) % and of (61.30 ± 8.43) %, respectively. the sequential feature selection (sfs) confirmed those results by selecting for the whole set of channels, in the 48.26 % of the cases the gamma band and in the 22.92 % the high-beta band and by achieving an average accuracy of (86.10 ± 8.29) %. mailto:giovanna.mastrati@unina.it acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 2 when thinking of or being in high places, (ii) nausea, (iii) trembling, and (iv) shortness of breath. very often people suffering from fear of heights do not seek treatment for their condition preferring to avoid the fearful situation. this can cause limitations in performing simple tasks of everyday life. behavioural therapy is mainly used for the treatment of acrophobia. at the basis of the treatment is the exposure of the patient to the anxiogenic stimulus. the subject is slowly encouraged to enter and cope with the anxious situation. the stimulus can be imagined (imaginal modality) or delivered for real (in-vivo modality) [3]. among the exposure approaches, systematic desensitization combines exposure with relaxation techniques [4]; graded exposure, which has been shown to be superior to the former for the treatment of acrophobia [5], involves the gradual exposition of the subject to the phobic source in a managed and controllable environment without the use of relaxation techniques [6]. for more than two decades, the literature and the clinical world have shown how exposure treatment of acrophobia can make use of virtual reality (vr) [7], [8]. vr is a technology that immerses subjects in a computer-generated artificial environment offering a high degree of interactivity, sense of presence and multi-sensory stimulation [9]–[11]. this has led to virtual reality exposure therapy (vret), which shows greater efficacy than imaginal exposure [12], especially for subjects with insufficient imaginative capacity [3] and, in several cases, it is comparable to in-vivo exposure [9]. vret offers greater control to the therapist in the administration of the expository hierarchy and allows customisation to the needs of the subject by generating adjustable stimuli [13], [14]. vret also provides a completely safe environment with respect to the in-vivo exposure. to date, vret-based treatments require the operator intervention to appropriately modulate the phobic stimuli. therefore, the adaptation cannot take place in real-time mode. in recent years, many studies have proposed therapeutic treatments involving extended reality (xr) which are able to adapt to the user on the basis of real-time processing of biosignals [15]–[18]. among the physiological signals (i.e., cerebral blood, electroculographic signals, electrocardiogram, blood volume pulse, galvanic skin response, respiration, phalanx temperature, etc), the non-invasiveness and the high time resolution represent great advantages in the use of the eeg [19]–[21]. thus, the eeg can be successfully employed in realtime brain computer interface (bci) applications [22], [23]. thanks to the advances in the portability and wearability of the eeg devices, vr headsets and the eeg systems are to date compatible and can be simultaneously employed. phobic states can cause an asymmetry in the eeg signal, that is an imbalance in amplitude or power between two sites on the scalp. specifically, an increase in the beta power of the right hemisphere may reflect the presence of phobias [24]. a hyperactivity of the frontal cortex was also found in phobic disorders [25]. a recent study demonstrated that high levels of beta and high-beta activity in the temporal lobes (related to amygdala activation) resulted to be associated with anxiety, fear, panic, and phobia [26]. starting from the eeg signals, fear of heights can be automatically detected by means of machine learning (ml) techniques. the present study aims to identify the eeg signal pro cessing pipeline that maximizes the accuracy of classification between 3 intensity levels (i.e., low, medium and high) of fear of heights in subjects with a different acrophobia severity. thus, the novelty beyond the state of the art consists in focusing on the real time assessment of the intensity levels of fear experienced by a subject rather than the acrophobia severity diagnosis. this is a first step toward a bci adaptive system for the vr-based treatment of fear of heights. 2. related works in [27], the authors aimed to distinguish three groups of subjects affected by different severity of acrophobia by means of the eeg signal. the psychometric tools employed were the acrophobia questionnaire (aq) and the subjective unit of distress (sud) scores. eeg data of 76 subjects were collected during the exposure to a virtual environment reproducing a wooden plank hanging at a height of about 160 m. functional connectivity between each pair of channels was explored and complex networks named functional brain networks (fbns) were obtained. fbns features resulted in being able to distinguish different groups of subjects. ml algorithms and convolutional neural networks (cnns) with fbn features as inputs were trained. the best results were achieved by using cnns and an accuracy of (98.46 ± 0.42) % was obtained. inter and intra-connectivity of cerebral cortex regions resulted able to identify the degree of acrophobia. in [28], the severity of acrophobia was estimated based on eeg, heart rate (hr), and galvanic skin response (gsr). the visual height intolerance questionnaire (vhiq) was employed to preliminary assess the acrophobia intensity of the participants. 8 subjects underwent an in vivo pre-therapy exposure session, followed by a vr therapy and an in vivo evaluation procedure. various machine and deep learning classifiers were implemented and tested in both a userdependent and user-independent setting. in the user-dependent setting, accuracies of 79.12 % and of 52.75 % were achieved in the 2-class (relax vs fear) and 4-class (relax, low, medium, and high) fear level classification, respectively. in the user-independent setting, accuracies of 89.50 % and of 42.50 % were achieved in the same fear level classification tasks. the eeg feature which maximized the classification accuracy resulted to be the log-normalized beta band power. in [29], eeg and ecg biomarkers of 18 subjects were real time monitored in a virtual high-altitude scenario. statistical analysis was employed to explore the relationship between these biomarkers and the height-related stress. perceived stress scale (pss) was employed to record the subjects’ self-assessment of the perceived stress. based on a hr threshold, the sample was divided in two groups and statistically relevant differences emerged between eeg biomarkers. the absolute powers in beta and gamma bands in the occipital region were found to be associated with height-related stress. the following correlation with the pss score were found: (i) the increase in the frontal beta power (ρ = 0.50), (ii) the increase in the parietal beta and gamma powers (ρ = 0.56 and ρ = 0.71), (iii) the increase in the temporal beta and gamma powers (ρ = 0.70 and ρ = 0.60), (iii) the increase in the occipital beta and gamma powers (ρ = 0.53 and ρ = 0.56). 3. experimental validation in this section, the participants, the psychometric tools, the hardware, the software, and the protocol exploited for the experimental validation of the proposed study are presented. acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 3 3.1. participants nine healthy subjects (age 22.63 ± 4.00; 3 males and 6 females) participated in the experiment. they were naive to the task of emotion-related visual stimulation. all participants gave prior written informed consent to participate. 3.2. psychometric tools the aq [30] and the sud scores were used for an initial screening of the sample to evaluate the severity of fear of heights. for each subject, the aq self-report scale allows to assess the anxiety and avoidance levels associated with 20 height-relevant situations. the aq is made of 20 items for each subscale (i.e., anxiety and avoidance). the sud [31] was used to monitor the fear level changes during the ongoing exposure session. it is a visual analogue scale commonly used to measure self-reported habituation of anxiety, agitation, stress or other painful feelings during the exposure therapy. current levels of anxiety are usually rated on a likert scale from 0 (no distress) to 100 (extreme distress). in the present study, a likert scale from 0 to 10 was exploited. in the exposure therapy, the sud is mainly used to develop fear hierarchies and arrange fear-provoking stimuli by order of severity. sud ratings are generally also used to assess the initial fear level of the subjects. subjects simultaneously reporting aq scores > 20 or < 6 on the anxiety and avoidance scales, respectively, and a sud score < 2, were considered not to suffer from fear of heights. only 1 subject belongs to this category and was excluded from the experiments. thus, the eeg data of eight subjects were considered for further analysis. 3.3. hardware eeg signals were recorded through the ultra-light weight, wearable liveamp amplifier from brain products [32] (figure 1). the system is provided with 32 gel-based active electrodes placed following the 10/20 international positioning system. the liveamp is equipped with a 24-bit analog-to-digital converter (measurement range ± 341.6 mv, resolution 40.7 nv lsb). the eeg signals are acquired at a sampling rate of 500 sa/s. the system is wireless and is also provided with a memory card to store the data internally in order to allow greater mobility. the rechargeable battery allows up to 4 hours recording. the brainvision recorder software allows to check the contact impedance between the electrodes and the scalp and the realtime visualization of the eeg data. the meta quest 2 [33], produced by meta platforms (figure 2) is the headset employed for the vr exposure. the quest 2 runs an android-based operating system and can be used both as a standalone device and with the vr software running on a desktop computer. the headset is provided with a fastswitch lcd display with a per-eye resolution of 1832 1920, and refresh rates of 60, 72, 90 hz are supported. the 3d positional audio is built directly into the headset and a data storage of 128 gb or 256 gb is allowed. with a motion tracker with 6 degrees of freedom (dof), the headset is able to track the movements of the head and the body, thus allowing the user a vr experience with realistic precision. 3.4. software the akron application, developed by idego [34], is a vr app designed for the treatment of fear of heights. the app allows to gradually expose the user to fearful situations by increasing the eliciting power of the stimulus. the displayed landscape is a canyon in a rocky desert, where the user can find a river and barren nature, see figure 3. a wooden lift and a platform allow the user to climb, starting from the ground level and reaching three increasing height levels, namely 15 m, 30 m, and 45 m. on each side, the lift is provided with protective barriers to let the user feel safe during the platform raising. frontal barriers are not present when the platform reaches the desired level in order to leave the user in greater eye contact with the sensation of empty space. the app was developed on the android platform using the unity game engine (version 2019.4.16), the c# as a programming language, the opengles3 graphics library, and an il2cpp-type backend configuration. the app has been optimized through an astc compression system to use it on low-end virtual reality viewers such as the meta quest 2. a 72 hz refresh rate was set. the 6 dof allows the user to look around. figure 1. brain vision liveamp and acticap. figure 2. meta quest 2. figure 3. displayed landscape. acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 4 3.5. protocol the experiments were performed at the institute of neural engineering (bci lab) at the graz university of technology. for each subject, after a preparation phase, the three sessions were carried out on the same day. during the preparation phase, participants were carefully instructed on the purpose of the experiment. the researchers mounted the eeg cap on the participants head and the electrodes were filled with conductive gel. electrodes impedance was kept under 25 kω and the quality of the eeg signal was visually inspected. after eeg configuration, participants were asked to wear the vr headset and the eeg signals were again visually inspected to assure no perturbations occurred (figure 4). each session consisted of the same three runs. the subject is required to stand on an elevating platform in a vr environment reproducing a canyon. for each run, a higher height level is reached. before the first run, the subject stands at the ground level to become familiar with the environment and to allow the baseline signals acquisition. each run starts with a visual 5 s countdown followed by the platform raising. once reached the desired level, the subject is asked to keep that position for 90 s. at the end of the run, a message informs the subject the relax phase is starting and they are teleported at the ground level. the participant is then asked to rate the level of discomfort on a scale from 1 to 10, and the relax phase continues for other 60 s. the three sessions last about 45 min. in figure 5, the overall experimental procedure is reported. 4. data analysis 4.1. preprocessing the raw eeg data were pre-processed using matlab v. r2022a. a zero-phase notch iir filter was applied in order to filter out the 50 hz power line noise. a zero-phase digital filtering was performed with a 4th order butterworth band pass iir filter, with cut-off frequencies between 4 and 45 hz in order to extract the eeg frequency bands of interest. both for the notch and the bandpass filters, the zero-phase digital filtering was achieved by using the filtfilt() matlab function. artifact subspace reconstruction (asr) and independent component analysis (ica) [35] were employed to correct the eeg signal from artifacts through the eeglab matlab toolbox version 2019 [36]. the baseline correction was then applied to the eeg signal. next, signals were segmented into 1 s time windows with 50 % overlap between adjacent segments. 4.2. feature extraction, selection and classification the fast fourier transform (fft) with zero-padding was then applied to the eeg signals. the matlab function fft() with a number of points twice the original length of the signal was exploited. the absolute powers were computed in the following frequency bands: theta (4 to 7) hz, alpha (8 to 13) hz, low-beta (14 to 20) hz, high-beta (21 to 29) hz, and gamma (30 to 45) hz. for each frequency band, the absolute powers of channels belonging to the same brain region were averaged according to the subsequent articulation: • frontal: fp1, fp2, f3, f4, f7, f8, fc5, fc6, fc1, fc2, fz • central: c3, c4, cp5, cp6, cp1, cp2, cz • parietal: p3, p4, p7, p8, pz • temporal: t7, t8, tp9, tp10, ft9, ft10 • occipital: o1, o2, oz. the forward sequential feature selector (sfs) algorithm was used to reduce the original dimensional feature space. the sfs belongs to the wrapper methods [37]. for a given classifier, the sfs adds or removes features to form a new feature subset. at each stage, the best feature to add or remove based on the crossvalidation score is identified. the algorithm returned the most significant frequency band for each channel, by reducing the total number of features from 160 to 32. the k-nearest neighbors (k-nn), random forests (rfs), naïve bayes (nb), and support vector machines (svms) were the employed classifiers. the goal was to classify three intensity levels (i.e., low, medium, and high) of fear of heights. for each classifier, a 5-fold stratified cross-validation was employed in a within-subject setting returning a mean classification accuracy for the subject. during the 5 iterations, the test fold was composed of data from randomly selected runs of the 3 sessions. the remaining data (80 %) were used for the training phase. then, the overall mean was calculated, and the standard deviation was computed on the eight subjects considering n-1 degrees of freedom. 5. results the metric used to evaluate the performance of the models is the accuracy that formally refers to the number of correct predictions over the total predictions. three different experiments were carried out in order to find the most significant scalp area, the most significant eeg frequency band, and the most significant frequency band per channel. as reported in table 1, the highest mean classification accuracy was obtained over frontal regions. figure 4. eeg acquisition. figure 5. framework of one experimental session. following the baseline and the exposure phases, the subjective unit of distress (sud) is administered. acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 5 in table 2, the classification accuracies of each subject are shown when the signals acquired over the frontal regions are input to the rf classifier. the achieved results agree with previous findings from the scientific literature: frontal area emerged to be able to distinguish among degrees of acrophobia [27]. in table 3, classification accuracies at varying the frequency bands are reported, considering the only frontal region. the most significant eeg bands resulted to be the gamma and high-beta bands. gamma and beta waves were observed to have a strong correlation with the brain response to highaltitude exposure [29]. these results were confirmed by the sfs when asked to select for each channel the frequency bands maximizing the accuracy. as a result, the gamma band and the high-beta band were selected in 48.26 % and in 22.92 % of channels. in table 4, the performance of classifiers is compared when the sfs algorithm is applied to select the most informative frequency bands and when all the frequency bands are considered. a feature selection process conducted on the eeg data of each single subject allows to achieve a better performance. however, the choice of custom features would require a preliminary calibration of the system for each different subject. to pursue the goal of generalization, the gamma and high beta in the frontal area of the scalp appear to be the most robust features. 6. conclusion and future works an eeg-based detection system of three intensity levels of fear of heights was proposed. a vr scenario reproducing a canyon allowed to gradually expose the subjects to fear eliciting stimuli. by means of an elevating platform, the subjects reached three different height levels. the aq and the sud scores were employed to assess the severity of fear of heights of the experimental sample. the eeg signals of eight subjects were acquired through a 32-channel headset during the exposure to the vr scenario. a detailed analysis was conducted in order to highlight the scalp regions and the eeg bands mostly affected by fear of heights. the average accuracy in the within-subject case for the three-classes fear classification task, was computed. the absolute powers in the five eeg bands extracted from the frontal region of the scalp allowed to achieve average accuracies of (68.20 ± 11.60) %. considering the only frontal, accuracies of (57.90 ± 10.10) % and of (61.30 ± 8.43) % were achieved in the high-beta and gamma bands, respectively. the sfs confirmed those results by selecting for the whole set of channels, in the 48.26 % the gamma band and in the 22.92 % the high-beta band and achieving an average accuracy of 86.10 ± 8.29. in future works the sample size will be enlarged and further eeg features will be tested to improve the generalizability of the results. acknowledgement the authors thank the institute of neural engineering (bci lab) at the graz university of technology for the support in the research activities. the authors also thank the phd grant “ar4clinicsuraugmented reality for clinical surgery” (inps national social security institution – italy). this work was carried out as part of the “ict for health” project, which was financially supported by the italian ministry of education, university and research (miur), under the initiative ‘departments of excellence’ (italian budget law no. 232/2016), through an excellence grant awarded to the department of information technology and electrical engineering of the university of naples federico ii, naples, italy. references [1] american psychiatric association, d., & american psychiatric association. diagnostic and statistical manual of mental disorders: dsm-5 (vol. 5, no. 5). american psychiatric association. washington, dc: 2013. isbn-13: 9780890425558 [2] g. r. vandenbos, apa dictionary of psychology. american psychological association, washington, dc: 2007. isbn-13: 9781433812071 [3] l. f. hodges, r. kooper, t. c. meyer, b. o. rothbaum, d. opdyke, j. j. d. graaff, j. s. williford, m. m. north, virtual environments for treating the fear of heights, ieee computer table 1. within-subject classification accuracy (mean and standard deviation on all the subjects) in % of fear of heights on a 3-level intensity scale. for each scalp region, the absolute powers of the 5 eeg bands were used as input features. brain classifier region knn nb rf svm frontal 53.00 ± 9.86 47.10 ± 7.15 68.20 ± 11.60 63.40 ± 12.80 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grey. frequency classifier band knn nb rf svm theta 39.30 ± 4.76 37.90 ± 3.26 41.60 ± 6.52 39.40 ± 7.66 alpha 41.20 ± 4.52 38.90 ± 6.45 42.70 ± 5.92 43.30 ± 6.43 low-beta 50.80 ± 11.00 45.70 ± 8.70 52.10 ± 10.80 52.00 ± 11.40 high-beta 53.60 ± 13.40 49.70 ± 9.16 57.40 ± 12.50 57.90 ± 10.40 gamma 57.80 ± 9.37 46.60 ± 6.90 61.20 ± 9.76 61.30 ± 8.43 table 4. within-subject classification accuracy (mean and standard deviation) in % of fear of heights on a 3-level intensity scale. the absolute powers of the 5 eeg bands were used as input features. results withand without feature selection are reported. sfs classifier knn nb rf svm with78.60 ± 8.50 55.40 ± 8.29 86.10 ± 8.29 85.70 ± 9.12 without59.50 ± 12.50 52.30 ± 7.09 82.00 ± 11.60 76.70 ± 14.00 acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 6 28(7) (1995), pp. 27–34. doi: 10.1109/2.391038 [4] i. marks, m. gelder, a controlled retrospective study of behaviour therapy in phobic patients, the british journal of psychiatry 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emotion recognition with a low number of channels, ieee access 10 (2022), pp. 117411-117428. doi: 10.1109/access.2022.3219844 https://www.idego.it/ https://doi.org/10.1109/memea54994.2022.9856433 https://doi.org/10.1016/j.jneumeth.2015.01.030 https://doi.org/10.1109/access.2022.3219844 microsoft word 308-2417-3-le-v5 acta imeko  issn: 2221‐870x  november 2016, volume 5, number 3, 87‐94    acta imeko | www.imeko.org  november 2016 | volume 5 | number 3 |87  three‐cycle university studies in metrology ‐ an innovative  development approach  marija cundeva‐blajer  ss. cyril and methodius university, faculty of electrical engineering and information technologies, ul. ruger boskovic br. 18, pobox 574,  1000 skopje, r. macedonia       section: research paper   keywords: metrology education; three‐cycle study; bologna process; international cooperation in education; metrology study programmes  citation: marija cundeva‐blajer, three‐cycle university studies in  metrology – an innovative development approach, acta imeko, vol. 5, no. 3, article 14,  november 2016, identifier: imeko‐acta‐05 (2016)‐03‐14  section editor: paul regtien, the netherlands  received december 25, 2015; in final form july 8, 2016; published november 2016  copyright: © 2016 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: ss. cyril and methodius university, faculty of electrical engineering and information technologies  corresponding author: marija cundeva‐blajer, e‐mail: mcundeva@feit.ukim.edu.mk    1. introduction  the cooperation in the field of education enables development of the technological collaboration. the processes initiated by the bologna declaration from (1999) [1], [2] for internationalization and more intensive cooperation in the higher education, through opening of the national systems of higher education, exchange of academic staff and students mobilities, gives opportunities for more intensive regional and international cooperation in engineering education. however, the development of the metrology infrastructure at the national, regional as well as international level is predetermined by the education and training opportunities [3], [4]. therefore the innovation of metrology education according to the bologna declaration [2] is not only a possibility, but also a necessity because of the demanded highest level of academic staff and research facilities at all three levels of higher education (bsc, msc and phd) [5]. the competitiveness of a company is based on the quality of its products and this, in turn, depends on its measurement capabilities, related to the field of metrology, defined as measurement science and its applications [6]. the metrology provides a substantial basis for fundamental research and development activities. the measuring capability is directly related to the technological level of a country and it is fundamental to the development processes of the emerging countries. measurements have become an increasingly critical tool for national and international trade, and for removing technical barriers to global trade. therefore, the development of a well-structured national as well as regional system of metrology is essential to support scientific progress, innovation, technology and competitiveness of enterprises [6]. the metrology plays an important role in the economical development and in the growth of industry. hence, it is fundamental for developing countries to establish a strong national measurement infrastructure, including the development of human resources in metrology, which supports this abstract  an innovative creation process of three‐cycle university education in the metrology area is presented. the case study of development  of the higher education model in the field of metrology at the ss. cyril &methodius university is analyzed. the realized three‐cycle  university studies in metrology are fully harmonized with the bologna principles. the structures of bsc, msc and the joint international  phd study program initiated by a group of 12 european universities are shown. an international consortium of universities participated  in the creation of the three level university studies in metrology. the specifics, goals, quantitative and qualitative achievements as well  as the impact and contributions at the national, regional and international level of the new and innovated three‐cycle metrology study  programs are given and discussed. acta imeko | www.imeko.org  november 2016 | volume 5 | number 3 |88  economical development and the growth of the national and regional industry [3], [6]. there are different approaches in the development of the metrology studies, depending on the national regulation, current state of the art of the metrology infrastructure as well as metrology traditions [6]-[8]. the bologna declaration of 1999 [2] has led to the process of completely reviewing the higher education system in europe. at the end of the first bologna decade the new twoor three-cycle degree system (bachelor – master – doctoral degree) has been adopted by the majority of the signatory countries. the european credit transfer and accumulation system (ects) has triggered high interest in other major continents, e.g. in the u.s.a., while the u.s. education system is bench-marked against the european one in some studies. also in africa many education systems have been completely altered since the beginning of this century [8]. metrology the measurement science is international and inherited in all engineering fields [3]. it is also represented at the universities in the region of south-east europe (see): 1. ss. cyril & methodius university-skopje, r. macedonia, 2. south-eastern european university-tetovo, r. macedonia, 3. university of zagreb, r. croatia, 4. university of split, r. croatia and 5. university of prishtina, r. kosovo. until 2010 at these universities knowledge in the field of metrology was gained through accredited study programmes at the first (bsc) and the second cycle (msc) of studies according to the european credit transfer system (ects) and in line with the bologna principles. at the ss. cyril and methodius university in skopje, the first international cooperation for higher education reform in the field of metrology was initiated in 2005 through the eu funded project “introduction of two-tier studies of metrology”, a european project financed by the european training foundation, tempus jep cd 19010_2004 with partners from:  the netherlands (technical university of delft)  poland (agh university-krakow)  italy (politecnico di milano), 2005-2007 [9]. innovated bsc courses in metrology were introduced and a completely new msc study program in metrology and quality management was accredited as outcomes of the project. following the process of realization of higher education according to the bologna declaration [2] with three cycles of studies (graduate bsc, master msc and doctoral phd studies), the organization of a third cycle of studies in the multidisciplinary field of metrology as a need was posed [10], [11]. the mutual interest of the above mentioned see universities was expressed in the initiative for the creation and realization of a joint third cycle study program-phd studies in metrology. in the frame of the tempus iv program, this initiative was accepted by the european commission and the education, audiovisual and cultural executive agency (eacea), which financed the realization of the project 158599-tempus-mktempus-jpcr „creation of the third cycle of studiesdoctoral studies in metrology“, [12]. in the creation of the joint phd study program, beside the five above mentioned see universities, the following eu universities participated:  university of pavia, italy,  braunschweig university of technology, germany,  czech technical university in prague,  graz university of technology, austria,  university of zaragoza, spain,  university of gavle, sweden,  superior school of metrology, douai, france and  bureau of metrology of r. macedonia. currently, at the ss. cyril and methodius university in skopje, higher education in metrology is structured according to the bologna three cycles of study as represented in figure 1, where ects denotes european credit transfer system. 2. phase 1: innovation of bsc study program  courses in metrology  after the ratification of the bologna declaration [2] by the parliament of r. macedonia in 2004 [10], the process of higher education reform has intensively started in all study fields. metrology science before the bologna process was presented at the ss. cyril and methodius university in skopje, with the obligatory courses at the third semester of electrical engineering studies through the course of electrical measurements, the course of power measurements at the study program of power engineering and the course of electrical measurements of non-electrical quantities in the 9th semester. in 2005 new reformed bsc studies in electrical engineering, harmonized with the bologna principles, were accredited. the curriculum in measurement education was innovated through some updates, but also through some completely new metrology courses given in table 1. the bsc study programs with the courses programs in metrology were re-accredited in 2011. 3. phase 2: msc study program in metrology and  quality management  before the bologna process reform the metrology education at the ss. cyril and methodius university at a postgraduate level was represented through the curriculum of electrical measurements and materials (with several metrology courses). in 2008 a completely new study program in metrology and quality management was accredited and after some innovation in 2013 it was re-accredited. figure  1.  three‐cycle  studies  in  metrology  at  the  ss.  cyril  and  methodius  university in skopje according to the bologna declaration.  acta imeko | www.imeko.org  november 2016 | volume 5 | number 3 |89  table 1. metrology in bsc studies in electrical engineering and information technologies. course/  study program/  electiveness  competences content electrical  measurements/  all in electrical  engineering/  obligatory  knowledge of methods for measurements  of  electrical  quantities,  principles  and  usages of measurement instruments.     basic terms and definitions in measurement techniques. legal metrology. static  and dynamic characteristics and block structures of measuring devices. theory of  uncertainty,  errors  of  repeatable  and  indirect  measurements.  measurement  transducers,  ac/dc  transducers.  analogue  instruments.  digital  instruments,  structural  elements,  time  interval  and  frequency  measurements.  dmm.  oscilloscope. measurement bridges.  introduction  in remote measurements and  measuring systems.     electrical  measurements of  non‐electrical  quantities/  power engineering/  elective   knowledge on measurement systems and  elements  of  the  measurement  systems,  basic  sensor  technologies,  methods  for  measurement of non‐electrical quantities  by  electrical  procedures,  process  monitoring  systems,  application  of  modern  virtual  instrumentation  and  computer techniques for measurements  introduction to electrical measurements of non‐electrical quantities. application  of  measurement  systems.  elements  of  a  measurement  system.  choice  of  a  proper  measurement  device  or  system.  basic  sensor  technologies  (resistance,  capacitance,  magnetic  sensors,  hall  effect  sensors,  piezoelectric  transducers,  strain  gauges,  piezo‐resistive  sensors,  optical  sensors,  ultrasound  sensors,  nuclear sensors). micro‐sensors and special sensor technologies. smart sensors.  measurement  noise.  measurement  signals  conditioning.  a/d  conversion.  telemetry.  measurement  of  mechanical  quantities  (dimensions,  translation,  translation  and  rotation  velocity,  acceleration,  mass,  force,  torque,  pressure).  measurement of sound, noise and vibrations. measurement of speed, flow and  fluid  level.  humidity  measurement.  temperature  measurement.  radiation  measurement.  systems  for  monitoring  and  process  measurements.  modern  virtual  instrumentation  and  computer  techniques  for  measurement  of  non‐ electrical quantities.  labview practicum/  power engineering/  elective   basic knowledge about labview program,  interfacing  of  measurement  devices  with  pc,  analysis  and  data  acquisition  of  measurement  data.  programming  of  scada  systems,  communication  with  programmable logic controllers.  virtual  instrumentation  and  labview.  labview  environment.  declaration  of  variables,  matrices  and  arrays.  cycles,  state  machines,  clusters  and  arrays.   graphical  user  interface.  file  generation,  data  acquisition,  measurement  and  signal generation. labview advanced features. measurement control systems in  labview.  communication  interfaces  in  labview.  distributed  measurement  systems in labview.    fundamentals of  measurement  systems/  computer  engineering/  elective   knowledge  of  basic  methods  for  measuring  the  electrical  and  non‐ electrical  quantities.  introduction  to  the  principles  of  analogue  and  digital  measuring instruments.  basic  concepts  and  definitions  in  the  measurement  techniques.  static  and  dynamic characteristics of measurement devices. fundamentals of uncertainty,  errors  in  indirect  measurements.  basic  characteristics  of  analogue  and  digital  measuring  instruments.  sensors  and  converters.  classification  of  measurement  sensors.  introduction  to  resistive,  inductive  and  capacitive  sensors  for  measurement  of  various  physical  quantities.  signal  conditioning  and  basic  configurations of measurement circuits.  computerized  measurements/  computer  engineering/  elective   capability  for  development  and  support  of computer‐based measurement systems  and  measurement  data  transmission.  design  and  realization  of  information  measurement systems.  architecture  of  computer  elements  for  data  acquisition.  measurement  signal  conditioning.  a/d  and  d/a  conversion.  standard  interfaces,  serial  and  parallel.  virtual  measurement  systems.  graphical  programming  and  introduction  to  labview. work with variables, their declaration, cycles, areas and clusters. user  interface, measurement data recording and display. examples of measurement  information and measurement controlled pc systems.  power measurements  power engineering/  obligatory   knowledge  for  measuring  methods  and  instrumentation  for  measurements  in  electrical networks.    voltage and current measuring transformers, applications and errors. methods  for  measuring  one‐phase  and  three‐phase  power  and  power  factor.  electronic  and  digital  watt‐meters.  digital  electricity  meters,  principles  and  errors.  introduction  to  remote  measurements  and  measurements  data  transmission.  grounding  grid  measurements  touch  and  step  voltages,  earth  specific  measurements. power quality measurements.  process  measurements/  automatics, system  engineering /  elective   development  of  measurement  systems  applicable in industrial processes.    introduction to measurement systems in processes and their role in automated  systems. classification of sensors and transducers. transmission of measurement  data, voltage and current loops, industrial communication protocols. sources of  errors in processes measurement systems, electromagnetic interferences, active  and passive shields. microprocessor based measurement systems, smart sensors  and mems. materials and technologies of fabrication of smart sensors.  telecommunication  measurements/  telecommunications/  elective  capability  to  work  with  measurement  devices  and  measurement  information  systems in telecommunication     measurement  role  in  quality  control  of  telecommunication  equipment.  programmable  measurement  devices.  microprocessors  and  measurement  applications.  interfaces.  measurement  data  acquisition.  frequency  and  period  measurements.  high  frequency  power  measurements.  measurement  errors  in  high  frequency  power  measurements.  measurements  of  non‐linear  distortions.  pulse generator. signal analyzer. spectral analyzer. logic analyzer. digital storage  oscilloscope.   principles  of  quality  control/  automatics, system  engineering /  elective   introduction  to  the  basic  principles  of  quality  control  and  the  necessary  mathematical  background;  introduction  to the iso 9001 standard.     history and development of quality control. introduction to the basic terms. the  iso 9001 quality standard. total quality management. mathematical models for  quality control, review of basic terms and concepts. introduction to diagrams for  quality  control.  special  control  diagrams  for  attributes  and  variables  in  quality  control.  specification  of  limit  values,  tolerance  and  other  techniques.  process  control. industrial experiments. design of robust systems.  acta imeko | www.imeko.org  november 2016 | volume 5 | number 3 |90  the objectives of the msc study program in metrology and quality management are to educate masters of sciences in the field of metrology and quality management who will be capable to work in:  different companies from industry  companies from the power sector  industrial and scientific laboratories where precise measurements are an important factor in the production  companies which are selling and servicing measurement equipment and instrumentation  the laboratories which are part of the metrological infrastructure, as well as  all others companies where measurements and measurement systems are part of their production process. the study program produces engineers who can work in different sectors and industries, where metrology is a pillar for their successful development. the future masters can work also in scientific institutions and universities, research centres, different laboratories, etc. the recent accredited msc courses in metrology are: 1. principles of metrology and quality management; 2. uncertainty in measurement and calibration; 3. sensors and measurement transducers; 4. microprocessor-based programmable instrumentation; 5. legal and industrial metrology; 6. power systems measurements; 7. processing and transmission of measurement data; 8. computerized measurement systems and virtual instrumentation; 9. digital signal processing; 10. measurement and control systems; 11. environmental monitoring; 12. computer and numerical methods in metrology; 13. quality assurance and quality control; 14. project management; 15. nanometrology and standardization; 16. techniques for non-destructive testing; 17. nanomaterials and nanostructures; 18. metrology of geometrical quantities. the innovation and creation of the bsc as well as the msc courses and curricula were performed through the eu project “introduction of two-tier studies of metrology” and by active support of the:  technical university of delft, the netherlands,  agh university, krakow, poland and  politecnico di milano, italy [9]. 4. phase 3: creation of phd study program in  metrology  the development of the metrology is tightly connected to the development of the industrial production, technical cooperation and trade. in the region of south-eastern europe the development of metrology was behind the needs of the industry, trade and society [1], [3], [5]. the metrological infrastructure and especially the national metrological institutes lack highly educated staff which would be the carriers of the further development [3]. therefore, in 2010, an eu initiative was launched by three south-east european (see) countries jointly to create a study program at doctoral level in metrology. the countries which jointly developed the joint phd study program are r. macedonia, r. croatia and r. kosovo, i.e. the universities:  ss. cyril and methodius university in skopje  university of zagreb  university of split  university of prishtina  south-eastern european university in tetovo. beside in the three above mentioned countries, there is also a lack of staff in the field of metrology in the other balkan countries (greece, serbia, bulgaria, bosnia and herzegovina, montenegro, albania etc.) [5]. by taking into account that metrology is a science represented in all technical disciplines and activities, the candidates which would accomplish the doctoral studies would have wide opportunities for application of their knowledge through activities in numerous fields through problems solving in the industry, health and food sector, environmental protection, energy, transportation and trade sector. so, conditions for development of the metrological infrastructure for the above mentioned areas were created. the wider goals of the joint phd education are:  enhancement of the quality and relevance of the higher education in metrology in macedonia, croatia and kosovo.  to upgrade the capacities of the see universities for international cooperation and for permanent modernisation.  orientation of the see universities to offer high quality education in metrology for the necessary industrial development and economic co-operation with the eu.  to intensify the co-operation of the academic staff of the see and eu universities. the specific objectives are:  harmonisation of the studies in metrology in the three cycle degree system according to the bologna process.  creation of phd studies in metrology at the see universities.  development of new courses and modernisation of the existing ones.  upgrading of laboratories for practical training to phd students in metrology.  establishment of a joint phd study program in metrology among the see universities.  transfer of knowledge and experience in the area of metrology. the creation of the joint study program in metrology was realized through set of joint activities of the see universities, such as: 1. elaboration of a new regulation on the third cycle of phd studies in metrology harmonised with the bologna principles. 2. elaboration and adoption of the joint study programme for phd studies in metrology. 3. creation and maintenance of a web-page of the phd studies in metrology (www.tempus-metrology.ukim.edu.mk) 4. promotion of the phd studies in metrology. 5. development of the content and teaching materials of the new and modernized courses for phd studies in metrology. 6. acquisition of laboratory equipment. acta imeko | www.imeko.org  november 2016 | volume 5 | number 3 |91  7. elaboration of an agreement for the joint phd study program among the see universities. 8. training of academic staff of the see universities by the eu universities through study visits, workshops and invited lectures. 9. enrolment of phd students at the study programme of metrology and students exchange. 10. dissemination of the results and experience from the creation of joint phd study program 11. insurance of sustainability of the phd study program. these activities were jointly and internationally realized. in figures 2 and 3 the joint study visit to the laboratories of the braunschweig university of technology and the swedish national metrology institute sp in boras are shown, respectively. the joint phd study program in metrology consists of four pillars:  ict in metrology;  instrumentation, industrial metrology, quality science;  metrology for life and society;  scientific metrology. the pillars are addressing the most challenging areas in contemporary metrology. each of these pillars comprises several courses given in table 2. the courses are lectured by at least two professors: one from the see universities and one from the eu universities. the students choose four of the elective courses for their curriculum and choose a supervisor from the see universities and a co-supervisor from the eu universities for the phd research. the program is realized by sharing academic staff, laboratories facilities and through a common student’s pool. the student mobility is one of the emphasised components in the realization of the joint phd program in metrology. the mobility is implemented through joint lectures and workshops, phd student conferences and presentations and study visits for accomplishment of the phd research. this concept of realization of the joint phd program implies not only educational cooperation, but also scientific bonding through joint research projects and exchange of researchers. the results of the joint phd study program in metrology are the creation of high-qualified professionals and researchers with the following general competences:  capability of research and development of solutions;  documenting the scientific researches;  working in interdisciplinary scientific research teams;  analysis of scientific and expertise problems;  application of the knowledge into praxis;  application of scientific research procedures and methods;  possibility of systematization of knowledge;  capability of generation of new ideas and solutions;  knowledge of scientific ethics;  presentation of scientific research results. figure  3.  joint  study  visit  of  see  academic  staff  to  laboratories  of  the swedish national metrology institute sp‐boras.  figure  2.  joint  study  visit  of  see  academic  staff  to  laboratories  of  the braunschweig university of technology.  table 2. phd study program in metrology.  pillars  courses  ict in  metrology  1. data acquisition and data processing  2. sensors and sensor networks  3. applicative software for metrology  4. modelling and numerical methods in metrology  5. knowledge discovery and data mining  instrument ation,  industrial  metrology  1. signal conditioning  2. complex monitoring and control systems  3. metrology for energy  4. rf measurements and metrology in  telecommunications  5. diagnostics, ndt and quality control  metrology  for life and  society  1. metrology for life sciences‐environmental  monitoring  2. metrology for chemistry, biochemistry and food  quality and safety  3. electromagnetic fields, electrical safety emc  4. sensor systems for biomedical measurements and  medicine  scientific  metrology  1. quantum metrology and nano‐metrology  2. primary standards, precise measurements and  calibration  3. metrology of mechanical quantities  acta imeko | www.imeko.org  november 2016 | volume 5 | number 3 |92  the specific competences of the doctors of science in the scientific field of metrology are:  expert knowledge in the areas studied through the courses of the study program in metrology,  management of scientific and metrology researches,  design of new products and technologies,  management and design of metrology processes,  capability of management of the functions in a company and their integration through metrology,  generation of innovative metrology approaches,  solving practical problems by using scientific metrology methods and procedures,  activities in metrology consulting services connected to design and engineering of products/processes,  capability of relating theoretical knowledge and practical application of metrology in the engineering processes in the companies,  capability of application of research methods in metrology praxis. by accomplishment of the doctoral studies in metrology, the doctors of sciences in the field of metrology are competent for following job positions:  academic staff in higher education institutions;  researchers in research centers;  researchers in r&d centers in the industry;  researchers and managers in the metrological infrastructure and the national metrology institutes. 5. quantitative and qualitative analysis of the  realized three‐cycle university studies in  metrology  some of the metrology courses in the undergraduate studies in the area of electrical engineering are obligatory as given in table 1. so, all the students enrolling the third semester also enrol the obligatory courses in metrology. the average number of students in the third semester at the faculty of electrical engineering and information technologies at the ss. cyril and methodius university in skopje in the last ten years is app. 200. the success rate of the students who successfully pass the final exam is very high (in average 90 %). the number of students, who enrol the elective bsc courses in metrology, is variable, because the pool of students varies depending on the study program and the semester in the curriculum. for example, the average number of students enrolling the course of power systems measurements is app. 70 in the last five years. however, the interest for some specialized courses, like computerized measurement systems or telecommunication measurements, is low, i.e. app. 6 students per semester. still, the success rate of this courses is very high (almost 100 %), because of the small groups, the individual approach and the possibility for intensive laboratory praxis. the students exchange is not so developed at bsc level, but this opens future possibilities, especially through the eu erasmus program. the academic exchange is done through lectures by visiting professors, appr. 2-3 per semester (figures 4 to 6). since the accreditation of the msc study program in metrology and quality management in 2008 at the ss. cyril and methodius university in skopje,33 students have enrolled this figure 5. eu training of the see academic staff (professors) involved in the  joint phd study program in metrology.  figure 4. joint lectures for phd students in metrology.  figure 6. joint  lectures for phd students  in metrology open for the wider  audience‐with  participation  of  metrologists  from  the  industry  and  other  metrology stake holders.    acta imeko | www.imeko.org  november 2016 | volume 5 | number 3 |93  study program as given in table 3. most of them (70%) are with accomplished bsc degree in electrical engineering. the other candidates are with bsc degrees in physics, chemistry, and mechanical engineering, technology engineering or similar. up to now, 16 students have accomplished their master thesis. at msc level the success rate is lower than at bsc level, which can be concluded from the data in table 3. the main reason is that the candidates are mostly part-time students and are already employed. the topics of the msc theses are mainly in the metrology of electromagnetic quantities and metrology of energy and power, which is to be expected considering the bsc background of the students (e.g. “a contribution to the metrology infrastructure in r. macedonia-national standard of electrical energy and power”). however, some of the msc theses are in other scientific areas, like flow and volume metrology (e.g. “elaboration of national standard of mass flow”), length metrology, mass metrology or some other specialized fields like environmental metrology, metrology for information security or metrology for air traffic security. the exchange of students at msc is more intensive, especially during the life-time of the two tempus projects in 2007-2008 and from 2010-2013. in the frame of the projects app. 10 msc students have realized study stays at the partner universities. also, app. 10 lectures were given by visiting professors from the partner universities. two students have accomplished the experimental part of their msc theses in the laboratories of the partner universities. after the end of the tempus projects, the students mobilities are enabled through the eu erasmus program (in both directions), or through other international or bilateral scientific programs. since the accreditation of the phd studies in metrology in 2013, six students have enrolled the study program, only at the ss. cyril and methodius university in skopje. the number of students is satisfactory, bearing in mind that it is the third cycle and the specifics of the field. the students are with interdisciplinary background (msc in electrical engineering, mechanical engineering, physics...). until now, no candidate has accomplished the phd title. all the students are part-time students already employed in the industry or metrology infrastructure. depending on the year of enrolment they have different success rate in the accomplishments of the phd courses and the scientific progress in their phd research. this is still an early stage of the phd study program to make more specific conclusions on the success rate. since the accreditation of the phd study program in metrology, which was accomplished at the end of the eu tempus-158599 project as a main outcome of the project, the students and professors mobilities are enabled through other programs like the eu erasmus program (in both directions), or through other international or bilateral scientific programs. the international dimension of the joint phd study program in metrology can be seen in table 4, where the academic staff contribution of each of the partner universities is displayed. 6. contribution, international impact and  sustainability of the three‐cycle studies in  metrology  the creation of the three-cycle university studies in metrology and the joint realization by the see and the eu universities potentially contribute to enhancement of the whole scientific and technical cooperation among these countries. the up-to-date experience shows that the results of this cooperation such as upgrading the level of the academic staff, upgrading the laboratories’ facilities or enhancement of exchange of knowledge, is not limited only to the formal university studies. it broadens also positive influence on life-long learning, professional development, and training and generally on the scientific work. it is achieved through mobilities of the academic staff, inclusion of professors from other universities, professionals from the nmis, metrological infrastructure and industry. invited lectures, workshops in metrology are open to all interested institutions and professionals. the network of the 5 see universities and 7 eu universities is extended to other universities, national metrology institutes, laboratories performing testing end calibrations and other research institutions in the region, but also wider in europe. the realization of the joint phd study program developed a common language among the participants in the project, initiation and launching of novel joint ideas for further scientific and metrology cooperation. this is a guarantee of the sustainability of the established three-degree study programs in metrology and for further enhancement of international impact. 7. conclusions  the paper presented an original process of creation of three-cycle university education in the area of metrology. the specifics, qualitative and quantitative achievements and table 4. academic staff of the phd study program in metrology. university  number of  professors engaged  ss. cyril and methodius university in skopje  11  south eastern european university in tetovo  1  university of zagreb  5  university of split  2  university of prishtina  1  czech technical university in prague  2  university of pavia  1  university of zaragoza  1  braunschweig university of technology  1  university of gavle  1  graz university of technology  1  table 3. annual data on msc in metrology and quality management. academic  year  number of  enrolled students  number of  accomplished msc  success  rate [%]  2008  12  7  58.33  2009  4  4  100.00  2010  3  1  33.33  2011  4  1  25.00  2012  2  2  100.00  2013  4  1  25.00  2014  1  0  0.00  2015  3  0  0.00  acta imeko | www.imeko.org  november 2016 | volume 5 | number 3 |94  contribution of the innovated and new study programs were given. the education cooperation through realization of joint study programs, such as presented in the paper, extends also the international scientific, technical and metrological cooperation. the jointly educated staffs represent a bridge for realization and further development of education, scientific, technical and metrological cooperation among the universities, metrology infrastructure and countries. education cooperation through the university programs in metrology is a form which is contributing to the joint technological and metrology infrastructure development. references  [1] m. cundeva-blajer, “development of three-cycle university studies in metrology”, proc. of 21st imeko world congress, aug. 30-sept.4, 2015, prague, czech republic, pp. 42-47. [2] the bologna declaration, 1999. [3] l. arsov, m. cundeva-blajer, “establishing a metrological infrastructure and traceability of electrical power and energy in the r. macedonia”, acta imeko, volume 2, no. 2, december 2013, pp. 86-90. [4] education, audiovisual and culture executive agency, 2012 „overview of the higher education systems in the tempus partner countries: southern mediterranean, a tempus study“, no 14, eacea, brussels, november 2012. [5] l. arsov, m. cundeva-blajer, “internationalization of the phd education in metrology", internationalisation in higher education: evaluating concepts, challenges and strategies ihe 2013, pradec interdisciplinary conf. proc., vol. 2, prague, czech republic, paper no. 7, 25-26 april, 2013. [6] g. m. rocha, r. p. landim, “the brazilian experience in the development of human resources in metrology”, proc. of 20th imeko world congress, sept. 9-14, 2015, busan, south korea, pp. tc1-o2. [7] t. gordiyenko, a. gaber, “occupational education of specialists in field of metrology and instrumentations in ukraine”, proc. of 21st imeko world congress, aug. 30-sept. 4, 2015, prague, czech republic, pp. 56-61. [8] l. p. van biesen, j. l. kaindu, j. m. boyokame, “new tools for training the skills of students in the process of the academisation in the education of engineering degrees: a testimony with application to metrology from the democratic republic congo”, proc. of 21st imeko world congress, aug. 30-sept. 4, 2015, prague, czech republic, pp. 76-78. [9] v. dimcev, m. cundeva-blajer et. al. “introduction of two-tier studies of metrology”, eu project financed by the european training foundation tempus jep cd 19010_2004, 20052007 etf final report, skopje, 2008. [10] higher education law, official gazette of r. macedonia no. 35, 2008. [11] rulebook on the conditions, criteria and rules for enrolment and studying at the third cycle of studies-doctoral studies, ss. cyril and methodius university-skopje, university gazette, no. 150, 2010. [12] l. arsov, m. cundeva-blajer et. al. “creation of the third cycle studies-doctoral studies in metrology”, eu tempus iv joint multi-country project, 2010-2013, eacea final project report, skopje, 2013. analysis and optimization of surgical electromagnetic tracking systems by using magnetic field gradients acta imeko issn: 2221-870x june 2023, volume 12, number 2, 1 8 acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 1 analysis and optimization of surgical electromagnetic tracking systems by using magnetic field gradients gregorio andria1, filippo attivissimo1, attilio di nisio1, anna m. l. lanzolla1, mattia alessandro ragolia1 1 department of electrical and information engineering, polytechnic of bari, via e. orabona 4, 70125 bari, italy section: research paper keywords: electromagnetic tracking; magnetic field generator optimization; magnetic field gradients; dipole model citation: gregorio andria, filippo attivissimo, attilio di nisio, anna m. l. lanzolla, mattia alessandro ragolia, analysis and optimization of surgical electromagnetic tracking systems by using magnetic field gradients, acta imeko, vol. 12, no. 2, article 26, june 2023, identifier: imeko-acta-12 (2023)-0226 section editor: francesco lamonaca, university of calabria, italy received june 14, 2023; in final form june 19, 2023; published june 2023 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: mattia alessandro ragolia, e-mail: mattiaalessandro.ragolia@poliba.it 1. introduction reducing invasiveness of medical measurement systems is driving the research toward the development of new systems and techniques to provide high-quality information with a minimallyinvasive approach, such as diagnostic techniques [1], miniaturized biosensors[2], techniques to monitor vital parameters [3]-[5], and tracking systems for surgical navigation [6]. among these, surgical navigation systems are currently being paid increase attention for the purpose of increasing the tracking distance. two main type of tracking systems are used in surgery: electromagnetic and optical systems [7]. electromagnetic tracking systems (emts) use a small coil sensor, inserted inside the surgical instrument, to determine its position and orientation by measuring the amplitude of the magnetic field of known geometry generated by a field generator (fg) [8]. the small size of the sensor and the independence from a direct line of sight allow to overcome the limitations of optical systems [7], thus allowing the use of flexible instruments such as needles, catheters, and endoscopes [8], [9]. emts technology presents two main limitations: a) it has high sensitivity to em interferences provided by electronic devices and to magnetic field distortions due to metal objects; b) current commercial systems are not able to provide accurate position estimations at a large distance from the signal source, due to the degradation of the magnetic field amplitude with the distance from the fg. hence, the signal source must be placed too much near the operating volume (i.e., patient’s table), thus hindering the medical staff during the operation. currently, commercial emtss provide accurate estimation of surgical instrument position only for limited distance from the fg, generally not more than 0.5 m [8], [10]. it should be noted that a higher tracking distance can be achieved by using some systems, such as abstract background: electromagnetic tracking systems (emtss) are widely used in surgical navigation, allowing to improve the outcome of diagnosis and surgical interventions, by providing the surgeon with real-time position of surgical instruments during medical procedures. objective: the main goal is to improve the limited range of current commercial systems, which strongly affects the freedom of movement of the medical team. studies are currently being conducted to optimize the magnetic field generator (fg) configuration (both geometrical arrangements and electrical properties) since it affects tracking accuracy. methods: in this paper, we discuss experimental data from an emts based on a developed 5-coils fg prototype, and we show the correlation between position tracking accuracy and the gradients of the magnetic field. therefore, we optimize the configuration of the fg by employing two different metrics based on i) the maximization of the amplitude of the magnetic field as reported in literature, and ii) the maximization of its gradients. results: the two optimized configurations are compared in terms of position tracking accuracy, showing that choosing the magnetic field gradients as objective function for optimization leads to higher position tracking accuracy than maximizing the magnetic field amplitude. mailto:mattiaalessandro.ragolia@poliba.it acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 2 polhemus long ranger trackers [11], but they employ larger fgs and bigger sensors, which are not suitable to track surgical instruments such as needles and endoscopes and cannot be used in many applications where a very small size is required. the design of the fg (electrical and geometrical parameters, arrangements of transmitting coils, processing techniques) determines the distribution of the magnetic field in space and the size of the fg itself, and it affects tracking performances [12]. often the transmitting coils must be placed inside a welldefined space due to practical needs, such as the configuration of the clinical environment, weight limitations, or application requirements. moreover, the tracking volume is usually proportional to the size of the fg (i.e., the magnetic field intensity) [8]. four types of generators can therefore be distinguished [8]: • standard fgs: they are the most common type and are produced by different manufacturers. they are small and cover an operating volume with linear sizes of a few tens of centimeters. • flat fgs: they are flat and thin, designed to be placed directly under the operating table where the patient lies. shielding the back of the fg greatly reduces the sensitivity to interference from below, such as induced eddy currents in metal structures. • mobile fgs: they are small fgs, which offer a small operating volume. their advantage is that they can be positioned near the operating area, thus offering good accuracy despite the reduced range. • long-range fgs: available only from polhemus inc., they can cover areas up to 2 m. different manufacturers propose their own fgs, but they do not provide their design principle. hence, ongoing research is focused on designing fg’s parameters with the aim of increasing tracking volume and accuracy [13]-[15]. in [16] we proposed a solution to study the effect of system parameters and fg configuration on tracking accuracy, and we compared the performance of two fg configurations, whose transmitting coils were identical in their geometrical and electrical parameters, except for their number and their pose (i.e., position and orientation) in space. in particular, one configuration consisted of a flat fg with six coplanar transmitting coils, whereas the other one was a representation of the 5-coils fg prototype presented and characterized in [17]-[19], which was developed to overcome the state of the art of current commercial systems by increasing the tracking distance beyond 0.5 m from the fg. in [16] we did not optimize any fg configurations, but we only compared the performances of chosen configurations. in [20] the configuration optimization of the fixed sensor array of an emts was proposed by employing a mobile transmitting coil, by running two-step evolutionary algorithm and using the position rmse (root mean square error) as performance metric. the drawback is that the computational cost is large, and the performance metric is valid only for that specific reconstruction algorithm. in [12], a simulator to evaluate tracking accuracy -according to certain performance metricsby setting system parameters was developed. in this study a test volume of 500 mm × 500 mm × 500 mm, which is typical for medical applications was considered. the authors optimized the configuration of a fg composed of eight transmitting coils, for both a compact fg volume or a wide 3d volume, by defining and comparing two objective functions to i) maximize the voltage induced in the sensor (i.e., the magnetic field) or ii) minimize the positional rmse of the sensor coil within the defined testing volume. the best performances were obtained when choosing the sensor position rmse as objective function, which however depends on that specific used reconstruction algorithm. in this paper we consider the same measurement volume as in [12], and we argue that the fg configuration can be optimized by maximizing the gradients of the induced voltage with respect to sensor position (as highlighted from preliminary experimental results), rather than maximizing the induced voltage as done in [12]; therefore, we follow an approach similar to [21], where the authors defined a metric based on the fisher information matrix. moreover, positional rmse in [12] depended on the chosen reconstruction algorithm, whereas our proposed metric is valid regardless of the position reconstruction algorithms. in [22] we have already discussed the utility of considering the magnetic field gradients to assess system performance, and we supposed position error was influence by gradients, whereas in this paper we perform simulations to employ information from gradients to analyze fg configurations. it should be noted that emtss that employ fixed sensor array and a moving emitting coil (instead of a fixed fg) are also present in literature. the approach that we develop in this paper is general, and it is valid for both cases, i.e., i) multiple fixed transmitting coils and one moving sensor, or ii) one moving transmitting coil and multiple fixed sensors. this paper is structured as follows. in section 2 we describe the model of the magnetic field obtained by approximating the transmitting coils as magnetic dipoles, which leads to define the sensor induced voltage and its spatial gradient. in section 3 we discuss experimental data from an emts based on a novel 5coils fg prototype, and we show the correlation between position error and the gradients of the magnetic field. in section 4 we define two metrics based on i) the amplitude of the magnetic field and ii) the gradients of the magnetic field, and we employ them to optimize the arrangement of the transmitting coils of an 8-coils fg. then, the obtained fg configurations are compared in terms of position tracking accuracy, and the role of the gradient distribution is highlighted. conclusions are drawn in section 5. 2. modeling the magnetic field and its spatial gradients if the wavelength of the generated magnetic fields is greater than the considered tracking volume, the magnetic field produced by the fg can be calculated by treating the transmitting coils as magnetic dipoles. therefore, the magnetic moment produced by the i-th transmitting coil can be described as a function of coil parameters and excitation [18], [23], [24]: 𝒎𝑡𝑥,𝑖 = 𝑚𝑡𝑥,𝑖 �̂�𝑡𝑥,𝑖 , 𝑚𝑡𝑥,𝑖 = 𝑁𝑡𝑥,𝑖 𝑆𝑡𝑥,𝑖 𝐼𝑖𝑖 , 𝑆𝑡𝑥,𝑖 = π 𝑟𝑡𝑥,𝑖 2 (1) where �̂�𝑡𝑥,𝑖 is the versor orthogonal to the surface 𝑆𝑡𝑥,𝑖 of the i-th coil, and 𝑟𝑡𝑥,𝑖 , 𝑁𝑡𝑥,𝑖 and 𝐼𝑖𝑖 denote the coil radius, the number of turns and the rms value of the sinewave excitation current, respectively. the same size and current are considered for all transmitting coils in the fg design, but slight differences in real quantity values have been observed. to account for coils’ acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 3 properties mismatch, these parameters are denoted by the subscript 𝑖. the rms magnetic field generated by the i-th transmitting coil in a generic point 𝒑𝒔 = [𝑥𝑠 , 𝑦𝑠 , 𝑧𝑠 ] t is [18], [23], [24]: 𝐁𝑖 (𝒑𝒔, 𝐼𝑖𝑖 ) = b𝑥𝑖 𝒙 + b𝑦𝑖 �̂� + b𝑧𝑖 �̂� = = µ0 4𝜋 𝑚𝑡𝑥,𝑖 𝑑𝑖 3 [3(�̂�𝑡𝑥,𝑖 · �̂�𝑑,𝑖 )�̂�𝑑,𝑖 − �̂�𝑡𝑥,𝑖 ] , (2) where 𝑑𝑖 = |𝒅𝒊|, with 𝒅𝒊 = 𝒑𝒔 − 𝒑𝒕𝒙,𝒊 represents the vector distance between 𝒑𝒔 and the center 𝒑𝒕𝒙,𝒊 of the i-th transmitting coil, and �̂�𝑑,𝑖 is its associated versor. by considering a homogeneous magnetic flux on the surface of the sensing coil 𝑆𝑠, voltage induced by the i-th transmitting coil may be calculated as: 𝑣𝑖 = 2 π 𝑓𝑖 𝑁𝑠 𝑆𝑠 𝐁𝑖 · �̂�𝑠 , (3) where 𝑁𝑠 is the number of coil sensor turns, 𝑓𝑖 is the excitation current (different for each transmitting coil) and �̂�𝑠 is the versor orthogonal to the sensor surface (figure 1), given by �̂�𝑠 = [ cos(𝛼𝑠 ) cos(𝛽𝑠 ) sin(𝛼𝑠 ) cos(𝛽𝑠 ) sin(𝛽𝑠 ) ] . (4) we can rewrite (3) in matrix notation: 𝒗 = [ 𝑣1 ⋮ 𝑣𝑛 ] = 𝐀𝐁�̂�𝑠 = 𝐇�̂�𝑠 , (5) where 𝐀 = [ 𝛼1 0 ⋯ 0 0 ⋮ 0 𝛼2 ⋯ 0 ⋮ ⋱ ⋮ 0 ⋯ 𝛼𝑛 ] (6) is the 𝑅𝑛×𝑛 matrix of sensor sensitivities at each frequency, with 𝛼𝑖 = 2 π 𝑓𝑖 𝑁𝑠 𝑆𝑠, and 𝐁(𝒑𝒔) = [ b𝑥1 b𝑦1 b𝑧1 ⋮ ⋮ ⋮ b𝑥𝑛 b𝑦𝑛 b𝑧𝑛 ] (7) is the 𝑅𝑛×3 matrix of the generated magnetic fields. it is convenient to define 𝐇 = 𝐀 𝐁. the gradient of the voltage induced by the i-th transmitting coil with respect to the position 𝒑𝒔 of the sensor (i.e., its spatial gradient) can be expressed by [21]: ∇(𝒑𝒔)𝑣𝑖 = [ 𝜕𝑣𝑖 𝜕𝑥𝑠 , 𝜕𝑣𝑖 𝜕𝑦𝑠 , 𝜕𝑣𝑖 𝜕𝑧𝑠 ] 𝑇 = 𝛼 𝜇0 4𝜋 𝑚𝑡𝑥,𝑖 𝑑𝑖 4 {15(�̂�𝑠 · �̂�𝑑,𝑖 )(�̂�𝑡𝑥,𝑖 · �̂�𝑑,𝑖 )�̂�𝑑,𝑖 − 3[(�̂�𝑠 · �̂�𝑑,𝑖 )�̂�𝑡𝑥,𝑖 + (�̂�𝑡𝑥,𝑖 · �̂�𝑑,𝑖 )�̂�𝑠 + (�̂�𝑠 · �̂�𝑡𝑥,𝑖)�̂�𝑑,𝑖 ]} . (8) 3. analysis of magnetic field gradients in [25] experimental tests have been carried out to evaluate position rms error of an emts based on a novel 5-coils fg prototype based on fdm (frequency division multiplexing), which was designed to reduce mutual inductances between transmitting coils, still maintaining a high induced voltage in the test volume, and in compliance with ieee standards for safety levels [26], [27]. in this section we refer to those experimental data and we carry out new analyses to show the correlation between position error and the gradients of the magnetic field. 3.1. error propagation based on magnetic field gradients the standard deviation of repeated measurements of the sensor position 𝒑𝒔 = [𝑥𝑠 , 𝑦𝑠 , 𝑧𝑠 ] t is calculated to evaluate the position repeatability error. we assume that uncorrelated noise affects the voltage components 𝒗 = [𝑣1, … , 𝑣𝑛] t (n=5 for the examined case) induced in the sensor. the diagonal covariance matrix is: 𝑪𝒗 = [ 𝜎𝑣1 2 … 0 ⋮ ⋱ ⋮ 0 … 𝜎𝑣𝑛 2 ] . (9) let g = [g1, … , gn] t: χ ⫃ r3 → rn be the function relating 𝒑𝒔 and 𝒗: 𝒗 = 𝒈(𝒑𝒔) . (10) expression (10) can be linearly approximated in the neighborhood of a generic point 𝒑𝒔, by means of the jacobian matrix 𝑱𝒈(𝒑𝒔) ∈ 𝑅 𝑛×3, whose rows are the gradients 𝛁𝒈𝒋(𝒑𝒔), defined as: 𝛁𝒈𝒋(𝒑𝒔) = [ 𝜕𝑔𝑗 𝜕𝑥 (𝒑𝒔), 𝜕𝑔𝑗 𝜕𝑦 (𝒑𝒔), 𝜕𝑔𝑗 𝜕𝑧 (𝒑𝒔)] (11) with 𝑗 = 1, … , 𝑛, which represents the slope of the function 𝒈(𝒑𝒔) around 𝒑𝒔 along each direction. expression (11) is a general approximation of (8), and it is independent on the chosen magnetic field model, hence can be used for complex systems of when the field model has not been provided or developed. as discussed in [22], [25], the variance 𝝈𝒑𝒔 ∘2 = [𝜎𝑥 2, 𝜎𝑦 2, 𝜎𝑧 2] t of repeated measurements of sensor position 𝒑𝒔 can be estimated by: figure 1. graphical representation and reference system of the 5-coils fg. sensor position is expressed both in cartesian (𝑥𝑠 , 𝑦𝑠 , 𝑧𝑠) and spherical coordinates (𝜌, 𝜙, 𝜃). acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 4 𝝈𝒑𝒔 ∘2 ≃ (𝑱𝒈 +(𝒑𝒔)) ∘2 𝝈𝒗 ∘2, (12) where 𝑱𝒈 +(𝒑𝒔) ∈ 𝑅 3×𝑁 is the moore-penrose pseudoinverse of the jacobian matrix, 𝝈𝒗 ∘2 is a nx1 column vector containing the element of the diagonal of 𝑪𝒗 in (9), ∘ is the hadamard power operator and ∘ 2 denotes element-wise square. the moorepenrose pseudoinverse in (12) provides a least squares solution to the linearized expression of (10). for details, the reader should refer to [22], [25], [28]. equation (10) is experimentally sampled by moving the sensor on a grid of 𝑁𝑝 evenly spaced points 𝒑𝒔𝒋, 𝑗 = 1, … , 𝑁𝑝, and measuring voltages 𝒗𝑖 . the rms error, also known as mean radial spherical error in the context of 3d localization, has been determined for each grid point as 𝜎𝑝𝑠 = √𝜎𝑥 𝑗 2 + 𝜎𝑦 𝑗 2 + 𝜎𝑧 𝑗 2. the partial derivatives are experimentally estimated by the ratios ∆𝑣𝑗 ∆𝑥 , ∆𝑣𝑗 ∆𝑦 , and ∆𝑣𝑗 ∆𝑧 , by considering small variations of sensor position. it should be noticed that this approach is applicable to any position reconstruction algorithm that can be linearized for error propagation. as shown in [22], the same approach can be used to evaluate the effects of residual drift in voltage measurements. 3.2. analysis of the pseudoinverse of the jacobian matrix the jacobian matrix 𝑱𝒈 introduced in section 3.1 gives important information about what to expect in terms of accuracy in different regions of the tracking volume. in this section we evaluate the elements of the pseudoinverse 𝑱𝒈 + of the jacobian matrix in a volume of 400 mm x 500 mm x 400 mm, for different orientations of the magnetic sensor. for each test, the sensor has been placed by means of the industrial robot in a regular grid of 𝑁𝑝 = 𝑛𝑥 · 𝑛𝑦 · 𝑛𝑧 points 𝒑𝑗 , 𝑗 = 1, … , 𝑁𝑝, where 𝑛𝑥 = 5, 𝑛𝑦 = 6, 𝑛𝑧 = 5 are the number of points along x-, yand z-axis, respectively, with a step of 100 mm in each direction, hence each plane normal to z-axis contains 𝑛𝑥 · 𝑛𝑦 = 30 points, for a total of 𝑁𝑝=150 points. the grid has been scanned by increasing the x-coordinate, then the y-coordinate is increased, and finally the z-coordinate, considering the reference system of figure 1. in figure 2, figure 3, and figure 4 the element of 𝑱𝒈 + are shown for the sensor oriented along x-, y-, and z-axis, respectively, thus considering 𝐵𝑥 , 𝐵𝑦 , and 𝐵𝑧 components of the magnetic field. the point order has been changed for figure 3, by increasing first the y-coordinate, then the x-coordinate, in order to highlight the presence of symmetry when switching x and y-coordinate. it can be noted that the elements of 𝑱𝒈 + generally increase when the distance from the fg increases, according to magnetic field model. the elements of 𝑱𝒈 + related to 𝐵𝑧 are much lower than those related to 𝐵𝑥 and 𝐵𝑦 . by comparing figure 2 and figure 3, it can be noted that when the sensor is aligned along x-axis, the elements related to the ycomponent are higher whereas the x-component is lower; viceversa when the sensor is aligned along y-axis; when the sensor is aligned along z-axis all elements assume low values. moreover, it can be noted the presence of peaks in the elements of 𝑱𝒈 + . it should be said that we conducted a related analysis in [22] by investigating the 3ꞏn elements of 𝑱𝒈 + , but in that case only one orientation of the sensor was considered instead of multiple orientations, and the sensor was moved along a single linear trajectory instead of a large volume as done in this case, hence we could not draw any considerations in that case. 3.3. position error evaluation through gradients in [25] we evaluated the position rms error of the 5-coils fg prototype, by applying the procedure shown in section 3.1 in the same volume and by employing the same pseudoinverse values analyzed in section 3.2. hence, in this section we refer to those experimental data and we relate the obtained results to the elements of the pseudoinverse of the jacobian matrix, which is influenced by the arrangement of the transmitting coils of the fg. figure 5 and figure 6 show the position error versus the point index, separately for each z-plane, for better visualization. the point order has been changed for figure 6, by increasing first the y-coordinate, then the x-coordinate, in order to highlight the presence of symmetry when switching xand y-coordinate. the error generally increases when increasing the zcoordinate, in accordance with the magnetic field model and with previous results. when the sensor is aligned along the x-axis, we found that the error of the y-component is higher (not shown figure 2. elements of the pseudoinverse 𝑱𝒈 + of the jacobian matrix with the sensor aligned along x-axis, thus measuring the 𝐵𝑥 component of the magnetic field. figure 3. elements of the pseudoinverse 𝑱𝒈 + of the jacobian matrix with the sensor aligned along y-axis, thus measuring the 𝐵𝑦 component of the magnetic field. acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 5 here, see [25]), whereas the error of the x-component is lower; vice-versa when the sensor is aligned along y-axis. the error of the z-component (not shown here) is very low in both cases, about one order of magnitude lower. this is in accordance with previous results obtained in section 3.2. by considering the elements of 𝑱𝒈 + shown in section 3.2, we can affirm that the increased error obtained for orientation along xand y-axis is due to higher values of the elements of 𝑱𝒈 + (i.e., lower gradients of the magnetic field), rather than to higher voltage noise: in fact, this latter decreases with the distance from the f, whereas the position error increases. in accordance with the results of section 3.2, figure 5 and figure 6 highlight some regions where the position rms error is lower than others, as well as the presence of peaks in correspondence of x = 0 mm for figure 5 (y = ±50 mm for figure 6) when the sensor is aligned along x-axis (y-axis for figure 6); these peaks can be here related to peaks in the elements of 𝑱𝒈 + . except for those peaks, position error is always ≤ 0.8 mm, which is still slightly higher than the values that we obtained in [22]: there, in fact, the sensor was more oriented toward the fg, and not in the xy-plane, thus presenting errors similar to the case of the sensor aligned along z-axis. 4. optimization of field generator configuration following the observations from previous section 3, in this section we optimize the configuration of the fg by employing two different metrics based on i) the maximization of the induced voltage (i.e., the magnetic field) as in [12], and ii) the maximization of the gradients of the magnetic field, and we evaluate the tracking accuracy associated with the optimized configurations. 4.1. objective functions for optimization procedure the fg configuration is optimized by minimizing two different cost functions, namely 𝐹1 and 𝐹2: • 𝐹1 is the same proposed in [12]. it is defined as follows: 𝐹1 = 1 1 𝑛 𝑁𝑝 ∑ ∑ |𝑣𝑖𝑗 | 𝑛 𝑖=1 𝑁𝑝 𝑗=1 = 1 𝑣mean , (13) where 𝑁𝑝 is the number of points of the volume considered for the optimization procedure, and 𝑣𝑖𝑗 is the voltage induced by the i-th transmitting coil when the sensor is in the j-th grid point. in [12] a constant noise level was considered in the whole test volume, and minimizing 𝐹1 equals maximizing the measured induced voltage in the test volume. • 𝐹2 is based on the computation of the spatial gradients expressed by (8). we follow an approach similar to [21], where the authors optimized the allocation of measurements for an emts based on tdm (time-division multiplexing) by using a cost function based on the fisher information matrix to select a subset of a large number (at least 1089) of fixed coplanar sensors, with dipole moments all oriented along z-axis. they considered a constant 𝜎𝑣 2 in the whole test volume. however, they did not evaluate the tracking performance related to their configurations. therefore, we define the following cost function 𝐹2: figure 4. elements of the pseudoinverse 𝑱𝒈 + of the jacobian matrix with the sensor aligned along z-axis, thus measuring the 𝐵𝑧 component of the magnetic field. figure 5. position rms error obtained by propagation through gradients. the sensor is aligned along x-axis, thus measuring the 𝐵𝑥 component of the magnetic field. figure 6. position rms error obtained by propagation through gradients. the sensor is aligned along y-axis, thus measuring the 𝐵𝑦 component of the magnetic field. acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 6 𝐹2 = − ∑ log det 𝐌𝑗 (𝒑𝒔) 𝑁𝑝 𝑗=1 , (14) where: 𝐌𝑗 (𝒑𝒔) = ∑ [∇(𝒑𝒔)𝑣𝑖𝑗 ][∇(𝒑𝒔)𝑣𝑖𝑗 ] 𝑇 𝜎𝑖𝑗 2 𝑛 𝑖=1 (15) is the fisher information matrix (fim), ∇(𝒑𝒔)𝑣𝑖𝑗 is the spatial gradient of 𝑣𝑖𝑗 , defined by expression (8), and 𝜎𝑖𝑗 2 is the variance of the voltage noise at the j-th point and related to the i-th transmitting coil. under the assumption that the noise terms are independent, it follows from the cramérrao inequality that maximizing the fim (in some sense) minimizes the attainable covariance of the estimated position, providing a lower bound [21], [29]: cov �̂�𝑠 ≥ 𝑴 −1 . (16) since it is generally not possible to find an optimal fim, the real-valued function 𝐹2 can be chosen instead [21], [30]. minimizing 𝐹2 is related to the approach followed in section 3.1 by using expression (12): in fact both the covariance (12) and the inverse of the fim can be interpreted as the inverse of the hessian of the weighted least-squares criterion [30]. note that, in contrast to [21], here the model of the voltage noise is composed of two terms, i.e., 𝜎𝑖𝑗 2 = 𝜎𝑎𝑐𝑞 2 + 𝜎𝐵 𝑖𝑗 2 (𝜎𝐼 ), where 𝜎𝑎𝑐𝑞 2 depends on the daq device and it is assumed to be constant in the whole test volume, whereas 𝜎𝐵𝑖𝑗 2 depends on the transmitting coil’s frequency and pose, on the noise 𝜎𝐼 of the excitation currents, and on the sensor position. the noise values for the simulations have been set according to experimental tests conducted on the 5-coils fg prototype, with an excitation current of 1 a rms for each transmitting coil, thus obtaining noise values 𝜎acq= 20 nv and 𝜎𝐼 = 0.07 ma, as in [19]. matlab fmincon function is used to minimize the cost functions thus optimizing the fg configuration, by setting the boundaries of the transmitting coils positions within a box volume of 300 mm × 300 mm × 70 mm. a planar grid of 500 mm × 500 mm is considered at a height from the fg of z = 650 mm, and with a step of 50 mm along xand y-axis, for a total of 121 positions. at each position, the sensor coil is aligned along the x-, y-, and z-axis, respectively, therefore 𝑁𝑝 = 121 × 3 = 363 sensor poses are considered. 4.2. evaluation of position tracking accuracy to compare different fg configurations, we define a test volume consisting of a grid of 𝑁𝑝 points: a volume of 500 mm × 500 mm × 600 mm is considered (from z = 50 mm to z = 650 mm), with a step of 50 mm along xand y-axis, and 100 mm along z-axis, for a total of 847 positions of the sensor coil. at each position, the sensor coil is aligned along the x-, y-, and z-axis, respectively, therefore 𝑁𝑝 = 847 × 3 = 2541 sensor poses are considered. for each point, we evaluate the position error: 𝑒𝑗 = √(�̂�𝑗 − 𝑥𝑗 ) 2 + (�̂�𝑗 − 𝑦𝑗 ) 2 + (�̂�𝑗 − 𝑧𝑗 ) 2 , (17) where 𝑥𝑗 , 𝑦𝑗 , 𝑧𝑗 denote the actual sensor position at the j-th grid point, whereas �̂�𝑗 , �̂�𝑗, �̂�𝑗 denote the estimated position by applying a reconstruction algorithm based on the dipole model approximation, as we have also done in [16], [18], [19]. in a real scenario, the position reconstruction algorithm will use the sensor pose estimation of the previous point as starting guess for the estimation of the following one. to simulate a real scenario, a position and orientation perturbation are added to the starting guess pose: for each cartesian coordinate, values form uniform distribution 𝑈(−√3 mm, √3 mm) are added, whereas for polar angle and the azimuthal angle in spherical coordinates we consider 𝑈(−1 °, 1 °). the levenberg–marquardt algorithm is used to solve the position reconstruction problem. for the details, see [19]. 4.3. results and discussions three different fg configurations are compared, as shown in figure 7: two of them are composed of eight transmitting coils, whose arrangement in space has been optimized by employing f1 and f2, whereas the other one represents the 5-coils fg prototype developed in [17] and it has been designed to minimize the mutual inductances still maintaining a high induced voltage in the test volume. as reported in [12], eight transmitter coils are commonly applied for real systems; this number of coils has been found elsewhere to be the minimum number of coplanar coils required to continuously track a sensor coil [13]. hence, eight transmitting coils are considered in this work for a direct comparison with [12]; all transmitting coils are identical in their electrical and geometrical parameters, and are powered with sinusoidal currents of 1 a rms. in contrast to [12], where the authors considered the same frequency of 1 khz for all transmitting coils, here we consider different frequencies for each coil, with a frequency gap of 1 khz according to the values used for first fg prototype, as shown in table 2; the optimization procedure will take into account this difference. the magnetic field produced by the 8-coils fgs is in compliance with ieee standards for safety levels [26], [27] beyond 10 cm from the fgs (i.e., z ≥ 10 cm in the fg reference system). table 1 shows the position error metrics related to the three different fg configurations. it can be noted that the optimized configurations lead to more accurate position tracking with respect to the configuration with five coils. moreover, optimizing with respect to 𝐹2 leads to better results than optimizing with table 1. position error metrics related to three different fg configurations: a) 5-coils compact fg, b) 8-coils fg optimized with f1, c) 8-coils fg optimized with f2. position error (mm) fg configuration (a) (b) (c) mean 0.80 0.77 0.42 std 0.92 0.82 0.59 max 9.44 5.96 2.70 figure 7. three different fg configurations under test: a) a 5-coils fg representing the prototype developed in [17]; b) an 8-coils fg optimized by applying 𝐹1 as done in [12]; c) an 8-coils fg optimized by applying 𝐹2. same scale is applied to all subfigures. acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 7 respect to 𝐹1. note also that both configurations fall within the defined volume, and configuration (b) is more compact than configuration (c), thus resulting in a smaller size of the fg, which is an aspect to consider during system design. figure 8 shows the value of the mean induced voltage (i.e., 1/𝐹1, for better visualization) and of the cost function 𝐹2 for each grid point. only points with index > 363 are considered, i.e., z ≥ 350 mm, since for z ≤ 250 mm the position error does not present significant difference between the different configurations. as expected, the maximum mean induced voltage is achieved for configuration (b), nevertheless, the best position tracking accuracy is obtained with configuration (c), which presents the lowest value of cost function 𝐹2. configuration (a) and (b) presents similar values of 𝐹2, but the mean induced voltage of configuration (a) is much lower than configuration (b): this is probably responsible for the reduced accuracy of the 5-coils fg. it should be noted that both the mean induced voltage and the cost function 𝐹2 assume an oscillating trend, which resemble the behavior of the pseudoinverse of the jacobian matrix shown in section 3, thus it is in accordance with previous results. overall, the obtained position tracking accuracy is within the requirements of many surgical applications [8] and can be furtherly enhanced by improving the position reconstruction algorithm. 5. conclusions commercial emtss for surgical navigation present a limited tracking volume, mainly due to the reduced sensitivity of the small magnetic sensors with the distance from the fg. studies are currently being conducted to optimize the fg configuration of emtss for surgical navigation, since it affects tracking accuracy, hence an optimized configuration can increase tracking accuracy in regions far from the fg. in this paper, we discussed the influence of magnetic field gradients on tracking accuracy. therefore, we optimized the configuration of the fg by employing a metric based on the maximization of the magnetic field gradients, obtaining better performance than the case where the amplitude of the magnetic field is maximized (as done in literature [12]). overall, position tracking accuracy is within the requirements of many surgical applications. the spatial gradient of the magnetic field produced by the fg assumes an important role in the design of the fg itself, and it paves the way toward the exploitation of new metrics to achieve higher accuracy in regions far from the fg, thus increasing the tracking volume which is the main limitation of current commercial systems. references [1] g. fiori, g. bocchetta, s. 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of magnetic field noise on sensor position estimation in surgical em tracking, 2021 ieee int. symposium on medical measurements and applications (memea), lausanne, switzerland, 23-25 june 2021, pp. 1–6. doi: 10.1109/memea52024.2021.9478723 [20] o. shafrir, e. paperno, a. plotkin, magnetic tracking with a flat transmitter. lambert academic publishing, 2010. online [accessed 18 june 2023]. https://www.lappublishing.com/catalog/details/store/ru/book/978-3-83834436-2/magnetic-tracking-with-a-flat-transmitter [21] o. talcoth, g. risting, t. rylander, convex optimization of measurement allocation for magnetic tracking systems, optim. eng. 18(4) (2017), pp. 849–871. doi: 10.1007/s11081-016-9342-1 [22] g. andria, f. attivissimo, a. di nisio, a. m. l. lanzolla, m. a. ragolia, assessment of position repeatability error in an electromagnetic tracking system for surgical navigation, sensors 20(4) (2020), pp. 961–961. doi: 10.3390/s20040961 [23] v. pasku, a. de angelis, g. de angelis, a. moschitta, p. carbone, magnetic field analysis for 3-d positioning applications, ieee trans. instrum. meas. 66(5) (2017), pp. 935–943. doi: 10.1109/tim.2017.2682738 [24] f. santoni, a. de angelis, i. skog, a. moschitta, p. carbone, calibration and characterization of a magnetic positioning system using a robotic arm, ieee trans. instrum. meas. 68(5) (2019), pp. 1494–1502. doi: 10.1109/tim.2018.2885590 [25] m. a. ragolia, f. attivissimo, a. d. nisio, a. maria lucia lanzolla, evaluation of position rms error from magnetic field gradient for surgical em tracking systems, 2020 ieee international instrumentation and measurement technology conference (i2mtc), dubrovnik, croatia, 25-28 may 2020, pp. 1–6. doi: 10.1109/i2mtc43012.2020.9128837 [26] ieee and i. s. c. c. 28, ieee standard for safety levels with respect to human exposure to electromagnetic fields, 0-3 khz, (2019). doi: 10.1109/ieeestd.2019.8859679 [27] ieee standards, ieee standard for safety levels with respect to human exposure to radio 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file://///fs.isb.pad.ptb.de/home/imeko/acta%20imeko%20papers/vol%2012,%20no%202%20(2023)/1_pre-formatted%20copies/10.1109/jsen.2020.3042647 https://doi.org/10.1109/memea52024.2021.9478723 https://www.lap-publishing.com/catalog/details/store/ru/book/978-3-8383-4436-2/magnetic-tracking-with-a-flat-transmitter https://www.lap-publishing.com/catalog/details/store/ru/book/978-3-8383-4436-2/magnetic-tracking-with-a-flat-transmitter https://www.lap-publishing.com/catalog/details/store/ru/book/978-3-8383-4436-2/magnetic-tracking-with-a-flat-transmitter https://doi.org/10.1007/s11081-016-9342-1 https://doi.org/10.3390/s20040961 https://doi.org/10.1109/tim.2017.2682738 https://doi.org/10.1109/tim.2018.2885590 https://doi.org/10.1109/i2mtc43012.2020.9128837 https://doi.org/10.1109/ieeestd.2019.8859679 https://doi.org/10.1109/ieeestd.2006.99501 https://doi.org/10.1109/memea49120.2020.9137161 https://doi.org/10.1201/9780203026786 microsoft word article 7 161-1297-1-le.docx acta imeko  february 2015, volume 4, number 1, 35 – 43  www.imeko.org    acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 35  power system of the guanay ii auv  ivan masmitjà, julián gonzález, gerard masmitjà, spartacus gomáriz, joaquín del‐río‐fernández   sarti reserch group, electronics dept., universitat politècnica de catalunya (upc), rambla exposició 24, 08800, vilanova i la geltrú.  barcelona. spain +(34) 938 967 200. www.cdsarti.org      section: research paper   keywords: state of charge; batteries; auv; wireless connection; ni‐cd  citation: ivan masmitjà, julián gonzález, gerard masmitjà, spartacus gomáriz, joaquín del‐río‐fernández, power system of the guanay ii auv, acta imeko,  vol. 4, no. 1, article 7, february 2015, identifier: imeko‐acta‐04 (2015)‐01‐07  editor: paolo carbone, university of perugia   received december 12 th , 2013; in final form november 8 th , 2014; published february 2015  copyright: © 2014 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: this work was supported by the spanish ministry of economy and competitiveness under the research project: “sistemas inalambricos para la  extension de observatories submarinos” (ctm2010‐15459)  corresponding author: ivan masmitjà, e‐mail: ivan.masmitja@upc.edu    1. introduction  the study of the sea and oceans has become increasingly important among fishers and biologists especially because marine species change their behavior depending on environmental variables such as temperature, salinity and ph, among others [1]. for these oceanographic observations different tools are used. aerospace technologies can focus on the study of the oceans at a global level but are unable to carry out detailed observations in a specific sector or depth. oceanographic ships, on the other hand, can solve this problem but the actual planning and mission deployment required to obtain a satisfactory special-time resolution data are very expensive. as a solution to these needs, these tools are migrating towards the creation of autonomous marine vehicles. the autonomous underwater vehicle (auv) guanay ii [2] [4] is a vehicle developed by the sarti group of the technical university of catalonia (with the co-financing of imedea-csic) with the objective to provide a platform for measuring oceanographic variables, such as temperature and salinity of the water column, with a high simultaneously spatial and temporal resolution. figure 1 shows the vehicle. the knowledge of the charge status of the batteries in an autonomous vehicle is an important factor to ensure security of the vehicle and mission. this work presents a measurement system and energy figure  1. autonomous underwater vehicle guanay ii auv.  abstract  guanay  ii  is  an  autonomous  underwater  vehicle  (auv)  designed  to  perform  measurements  in  a  water  column.  in  this  paper  the  aspects of the vehicle’s power system are presented with particular focus on the power elements and the state of charge of the  batteries. the system performs both measurement and monitoring tasks and also controls the state of charge (soc) of the batteries. it  allows simultaneous charging of all batteries from outside the vehicle and has a wireless connection/disconnection mode. guanay ii  uses  a  nicd  battery  and  for  this  reason  the  current  integration  as  a  soc  methodology  has  been  selected.  moreover,  it  has  been  validated that it is possible to obtain instant consumption from the soc circuit. finally, laboratory and vehicle navigation tests have  been performed to validate the correct operation of the systems and the reliability of the measured data.  acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 36  management for the guanay ii vehicle. initially different solutions for electrically measuring the batteries charge state are analyzed, and the methodology of current integration is chosen [5] due to the nature of the batteries used in the guanay ii. subsequently, the construction of a prototype and the experimental tests performed both in laboratory and field missions in order to validate the correct operation of the device is presented. finally, this device is complemented with a design of a battery charger system, accessible from outside the vehicle, which allows simultaneous charging of all battery packs, and also a wireless connection-disconnection of the battery. 2. power system of guanay ii auv  the guanay ii’s power system can be divided into three parts: propulsion system; communication and control system; and battery system. below, each of these elements are briefly described. the propulsion systems consist of a series of propellers for longitudinal and directional movement and a motorpiston set to modify the buoyancy of the vehicle to perform dives. the principal characteristics of the main engine, for longitudinal movement, are: 24 v of direct current (dc) supply; 300w of output power; and can be controlled via rs232 communication. two engines are located on the rear side of the vehicle to control its direction. the engines have 24 v of power supply, maximum power output of 110 w each and are controlled by an rs232 communication port. finally, the pistonengine set comprises a motor of 24 vdc and a piston that can move up to 1.5 litres. this piston can either take in or eject seawater. with this action the buoyancy of the vehicle can change and therefore the dives can be controlled. these systems consume most of the guanay ii auv power. the second part of guanay ii vehicle is the control and communication system. the main control system of the vehicle is an embedded computer located inside. this computer is responsible for controlling and managing the various elements of the vehicle (sensors, actuators, motors, communications, etc.) also, by using a radio link and a wifi network the vehicle can be remotely controlled from a base station. finally, an energy storage system has been mentioned. this vehicle has autonomy of around 4 hours thanks to the nickel-cadmium (ni-cd) battery pack of 24 v and 21 ah. this pack consists of a subset of 12 vdc and 7 ah batteries arranged in two series and three in parallel configuration (2s3p). with these batteries and the average consumption of electronic components of the vehicle, the autonomy under normal conditions can be established. figure 2 shows the development of the vehicle in 3d where the three parts of the power system can be seen (1, communication and control system. 2, propulsion system. 3, battery system). finally, in table 1 the values of the theoretical consumption of various devices of the vehicle can be seen. these values are specified in the technical specifications of the devices. as seen in table 1, the engines consume most (90%) of the vehicle’s energy. the other electronic systems consume the remaining 10%. given these data and the total capacity of the batteries, the autonomy of the vehicle can be known. if the engines run at full power, a maximum of less than 1.5 h has been obtained. however, various factors have to be taken into account. first, the side engines correct the direction of the vehicle, but are never on for long time. with regard to the main engine, a compromise between speed and autonomy of the vehicle is necessary. finally, the engine piston set is only activated to make the dives. in experimental results, it has been observed that the vehicle has a range of about 4 hours under normal conditions. 3. measurement system for battery state of  charge  the state of charge (soc) is mathematically defined as in the following equation (1). nom t nom ahdttiahtsoc /))(()( 0  (1) table 1.   theoretical consumptions of the guanay ii.  maximum values v  a  w gps 5  240 m  2 pc104 5  0.9  9.9 compass 5  <20 m  0.1 radio‐modem 12  1.5  18 engine driver 5  31 m  0.15 main thruster 24  16  300 lateral thruster 24  4.25  110 piston‐engine 24  3  50 total 25.94  490.15 figure 2. this figure shows the development of the vehicle in 3d with the  three parts of the power system (1, communication and control system. 2, propulsion system. 3, battery system).   acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 37  where i(t) is the current extracted from the battery (which is assumed positive while discharging the battery), and ahnom is the nominal battery capacity. equation (1) assumes that the current integration starts at soc(t) = 100% at t = 0. several methods of estimating the soc of a battery have been used. some are specific to particular cell chemistries. this work is subject to the use of ni-cd batteries used in the guanay ii. 3.1. methods for soc estimation  for such batteries there are different methods to measure the charge, such as the measurement of voltage, impedance and current integration. a. voltage based soc estimation uses the voltage of the battery cell as the basis for calculating the remaining capacity [6]. results can vary widely depending on discharge rate and temperature and compensation for these factors must be provided to achieve reasonable accuracy. figure 3 shows the relationship between the voltage and the capacity at different discharge rates. b. internal impedance measurements can also be used to determine the soc. however these are not widely used because of difficulties in measuring the impedance while the cells are active as well as difficulties in interpreting the data and very complex calculations are required [7] and [8]. c. current based soc estimation, known as coulomb counting [5] [11]. in coulometric measurements however, the amount of capacity taken out or put into a battery is measured in terms of ampere-hour or in %. the coulomb counting approach basically implements equation (1) to evaluate the soc. it uses a more general definition, defined as can be seen in (2).  t m nom dtti ah soctsoc 0 )( 1 )0()( (2) where soc(0) is the starting value of soc and im(t) is the measured current. charge accumulation techniques where soc is determined by monitoring battery charge and discharge current are impractical in the long term due to accumulation of errors. for this reason this technique makes it impractical to be used by itself. a monitoring technique combining the opencircuit voltage under no-load condition, and coulometric measurements under constant load has been implemented in [13] on a microcomputerbased circuit. on the other hand, correction factors are required for different discharge rates and ambient temperatures [12]. 3.2. gas gauge ic for power‐assist applications  current based soc estimation, known as coulomb counting [5], which is currently selected, calculates the state of charge by measuring the instantaneous current of the battery and integrating in time both in the process of charging and discharging. the reason for using this method is because it obtains a good correlation of the measurement data and also because it can be implemented quickly because of the already existing specific integrated circuit (ic). this allows us to rapidly develop a prototype that can be used to estimate the remaining charge left in the battery in guanay ii and to perform some field tests. in this implementation the bq2013 ic (a gas gauge ic for powerassist applications) from texas instruments company, specifically designed for ni-cd batteries [9] has been used. figure 4 shows a block diagram of the soc estimator where the batteries, motors (load) and charger can be observed. the bq2013 constantly monitors the current flowing through the batteries and also measures the voltage and the internal temperature to compensate for various factors. finally, the communication to the soc estimator is through a serial communication, allowing the user to read and write some parameters of the configuration and read the value of the charge of the batteries with the microcontroller. figure  4. block diagram of the soc estimator.  figure  3. relationship between the voltage and the capacity at different  discharge rate. source: saft batteries [10].  acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 38  3.3. prototype  the prototype developed is shown in figure 5, where the communication ports with the pc-104 and leds for visual indication can be seen. this ic includes a simple single-pin serial data interface with command-based protocol (hdq plus return). for this reason a rs-232 to hdq interface has been developed to communicate with both gas gauge and pc-104 systems [9]. 3.4. the soc measurement method  as explained in section 3.1 there are some drawbacks in using a simple coulomb counting soc. these drawbacks include the noise, temperature effects, discharge rates and how to estimate the initial soc value. the algorithm used should include these effects to get a reliable reading of the soc. the bq2013 implements compensations to minimize these drawbacks. the operational overview diagram in figure 6 illustrates the operation algorithm of the ic. as can be seen in figure 6, the ic compensates charge current for charge rate and temperature. discharge current is load compensated and allows automatically adjustment for self-discharge. the main counter, nominal available capacity (nac), represents the available battery capacity at any given time. battery charging increments the nac register, while battery discharging and self-discharge decrease the nac register and increment the dcr (discharge count register). the dcr and lmd (last measured discharge) registers are used to update and adjust the initial soc value. texas instruments provides equation (3) to obtain the value of the internal register nac (in mvh) of the bq2013 ic, which monitors the voltage drop across a resistor connected in series between the battery and ground. scaletorsenseresis mahacitybatterycapmvhnac   )( )()( (3) where senseresistor = 0.01 ω and scale = 640 (predefined). we can see the relation between the nac register and the soc in mah. as seen in (3) the nac value depends on the capacity of the battery and is determined by charging and discharging. therefore, the consumption can be calculated from expressions (4) and (3): )( )( )( ht ahacitybatterycap anconsumptio    . (4) finally, by correcting scale factors, we can obtain the instant consumption using equation (5): 01.01000640 3600 )( )( )(      st mvhnac anconsumptio , (5) where 3600 is the time factor correction; 640 is scale factor determined by the ic; 1000 is the ampere scale factor correction; and 0.01 is the sense resistor in ω.   figure  6. scheme of charging system and battery connection.  figure  5 . state of charge measuring prototype. a) top view b) bottom view.  acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 39  4. charging system and battery connection  in this section the implementation of charging system and battery connection are described. these two systems can improve the operability of the vehicle and increase safety and reliability. figure 7 shows a schematic model of this system. it shows the three battery packs connected in parallel. the switch (sw) between batteries and the auv electronics is radio-controlled and allows the user to turn on and off the vehicle. finally, an external charger can be connected using a two pin connector to charge the batteries. 4.1. charging system  the energy of the guanay ii comes from the 2s3p battery packs that can provide 24 vdc and 21 ah. however, a problem arises with this configuration when the batteries have to be charged. many manufacturers do not recommend charging two battery packs in parallel because this can cause damage especially at high charge rates. until now, types of chargers that can charge a single pack only were used, but the vehicle must be opened every time. in order to maximize the mission time a system was designed for simultaneous parallel charging of all battery packs through a single external connector. the location of this single point for charging means the it is not necessary to disassemble the vehicle’s mechanical and electronic systems and so the charging operation is faster. figure 8 shows the connector used for charging the batteries. this connector is located in one of the covers of the sealed cylinder. it is manufactured by subconn company. specifically it is a circular 4-pin capable of supporting up to 600 v and 10 a at pressures up to 1400 bars. after discussions with the manufacturer, it has been decided to use a low rate constant voltage charge applied to all the 2s3p batteries to prevent damage. a constant charge around 0.2 c is used. by keeping the charge current low enough so that the battery does not generate any heat, this method performs charging without using any control. the calculation formula for semi-constant-voltage charge system is as follows (6):  ch c n kv v   , (6) where: vch is the output voltage of the dc power supply for charge; vc is the single-cell battery voltage (1.45 v/cell: average battery voltage during charge at 20º c, 0.1 c); n is the number of cells used; (k) is the stabilizing constant and must be selected in accordance with the purpose of the device in which the battery pack is used. the value of the above-mentioned stabilizing constant (k) must be selected carefully and after discussions with the manufacturer, using a value of 1.25 has been decided upon. using equation (3) a charging voltage of 29 v has been obtained. moreover, the current has been limited to 5 a to prevent any overcurrent. charge rate is often denoted as c or c-rate and signifies a charge or discharge rate equal to the capacity of a battery in one hour. equation (7) shows the relation between the capacity of battery in ah and the current in a. ( ) (1/ ) ( )c apacityofbattery ah c h c urrent a´ = (7) 4.2. battery connection  in order to have control and easy access to the battery connection/disconnection to electronics and propulsion of the vehicle, a wireless device that acts as a switch has been incorporated. figure 7 shows a schematic model of this system. the user can turn off/on the switch of the vehicle using the remote controller. this operation enables the connection or disconnection of the batteries while the auv is in the water, obtaining greater security in the vehicle. a hir6-433 rf am 433 mhz receiver/decoder from rfsolutions has been used for this purpose. these types of modules provide a very low power receiver, combined with a flash programmable controller, supplied pre-programmed to operate with the keeloq transmitter encoder. the range from such a system can be up to 50 meters line of sight (los). finally, a new antenna with epoxy resin has been designed to allocate the hir6-433 and provides the necessary protection in the water and in the depth. besides hosting this module, the antenna is also used to install a new wifi communication module and gps antenna. 5. experimental results  initially both systems, the connection/disconnection of the battery and its charging and soc estimation have been tested in the laboratory, where the proper functioning of all the systems designed was verified.   figure  7. scheme of charging system and battery connection.    figure  8. subconn connector for charging in one of the covers of the sealed cylinder of guanay ii auv.  acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 40    figure  9. shows the two laboratory tests. in 1a charge testing of all packs of batteries in parallel can be seen. in 2b discharge testing and a comparison  between sensed current from the multimeter and sensed current from the bq2013 can be observed.    1.a) block scheme of the battery charge test. three battery packs, three digital multimeters, one voltage source and one laptop to monitor the variables. all  of them are connected through a gpib port.  1.b) shows the test bench where the battery charging has been tested.  1.c) the current of the three packs (red, green and purple) of batteries during a low rate charging in parallel.   2.a) block scheme of the battery discharge test. five resistors of 18 ohms in parallel as a load have been used. the three measurements (from the ammeter,  from the bq2013 and theoretical) have been compared using a laptop.  2.b) test bench that has been used to check and adjust the soc sensor of the battery. for this laboratory experiment a bank of five loads in parallel has been used.  each load of 18 ohms introduces a 433 ma of consumption current.  2.c) vehicle current measurement error using the soc circuit (x‐axis is the total current of the vehicle).     (1.a) (2.a) (1.b) (2.b) (2.c) (1.c) acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 41  one of the most critical parts is the parallel charging of battery packs, in particular the individual currents of each pack. laboratory tests have been conducted to validate the correct performance of the charging. in this test a power supply and three digital multimeters to monitor individual currents of each pack during charging has been used (see figure 9 point 1.a, block scheme). figure 9 in point 1.b shows the test bench where the battery charge has been tested. for this purpose, three ammeters and one power supply connected to a computer by gpib have been used. this computer is responsible for controlling and saving instrument data. figure 9 in point 1.c shows the current of the three packs of batteries during charging in parallel mode. in this picture it can be seen that after 10 hours of charge the current drops to 0.15 a and the battery is practically charged. also, the difference of the current charge in one of the packs can be observed. this is one reason why the high rate current charge is not recommended in order to prevent damages. in figure 9 point 1.c the rate of current used to charge the batteries can also be seen. according to the equation (7) a maximum current rate of 0.17 c is obtained during the first hour of charge, from which this rate decreases to less than 0.05 c. on the other hand, as seen in equation (5), the current consumption of the vehicle can be calculated using the soc circuit. a test and a calibration have been conducted to validate the system, using a digital multimeter and five 18 ohms resistors as a load (see figure 9 point 2.a, block scheme). figure 9 in point 2.b shows the test bench that has been used to check and adjust the soc sensor of the battery. for this laboratory experiment a bank of five loads in parallel has been used. each load of 18 ohms introduces 433 ma of consumption current. figure 9 point 2.c shows the vehicle’s current measurement error using the soc circuit bq2013. the error varies according to the total current of the vehicle with less than ±50 ma. therefore, using the nac register a sufficiently accurate method to estimate the power consumption from guanay ii has been obtained. afterwards, these systems have been validated in various vehicle navigation tests. for example, one of these tests was performed at the canal olímpic de catalunya in castelldefels (the olympic channel located in castelldefels) as shown in figure 10. figure 11 shows different trajectories performed by the vehicle during the field test. figure 16 shows the instantaneous battery consumption due to the action of the propulsion engine and two direction motors of the vehicle during a path on the water. the first three graphs show the action of propellers in %   figure  11. different trajectories performed by the vehicle during the field test.    figure  12.  percentage of power to the engine requested vs. current drawn.  (propulsion propeller).    figure  10. guanay ii auv in the field test.    figure  13.  percentage of power to the engine requested vs. current drawn.  (direction propeller).  acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 42  (up to one). in blue the action of the main propeller is shown and in red and green the action of side propellers is shown. the fourth graph shows the instantaneous battery consumption (black line) and the mean of this (dashed line). one can observe the relationship between consumption and the action of the engines. also, the average consumption from the remaining electronic elements (pc104, radio modem, gps, etc.) can be seen. this value may be contrasted with the consumption values of table 1. the difference may come due to the time of use of the radio modem or pc104. in any case, this can be a field to investigate in future work. from the experimental results shown in figure 14, the relationship between the percentage of power to the engine requested and the current drawn has been made. figure 13 shows this relationship for the left direction propeller and figure 12 for the propulsion propeller. the results show that, at low power, the engine will not draw power and therefore will not work. this is because a minimum amount of power is needed to run the motors. for the direction propellers more than 50% of power has been required and for the propulsion propeller more than 20%. however, more field test points are needed to obtain a more accurate relationship. these tests also confirmed that disconnecting the batteries when the vehicle is still in the water provides security at its landing ground. 6. conclusions  a system for measurement and for energy management for an underwater vehicle have been designed. the system monitors the state of charge of the batteries and the instantaneous consumption of the thrusters, information that the control mission needs to optimize and facilitate navigation. a single external access connector in the vehicle allows all batteries to be charged and a radio frequency control switch is used to connect and disconnect the vehicle’s power supply even in water. the navigation tests performed confirm that the system operates correctly and the data measured is reliable. acknowledgement  this work was supported by the spanish ministry of economy and competitiveness under the research project: “sistemas inalambricos para la extension de observatorios submarinos” (ctm2010-15459).  references  [1] j. timothy pennington and f. p. chavez, “seasonal fluctuations of temperature, salinity, nitrate, chlorophyll and primary production at station h3/m1 over 1989-1996 in monterey bay, california” deep sea research part ii:   figure  14. consumption of the vehicle versus power of the propellers.  acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 43  topical studies in oceanography, vol. 47, no.5, pp. 947-973, 2000. [2] s. gomáriz, j. gonzález, a. arbos, i. masmitja, g. masmitja, j. prat. "design and construction of the guanay-ii autonomous underwater vehicle". oceans 2011 ieee/oes. santander, spain. june 2011. [3] s. gomáriz, j. prat, a. arbos, o. pallares, and c. viñolo, “autonomous vehicle development for vertical submarine observation” in international workshop on marine technology, (vilanova i la geltrú, spain), november 2009. [4] s. gomáriz, j. prat, p. gayà, and j. del río, “development of low-cost autonomous oceanographic observation vehicle”, in oceans’09 mts/ieee, (brement, germany), may 2009. [5] f.codecà, s. m. savaresi, v. manzoni. “the mix estimation algorithm for battery state-of-charge estimator – analysis of the sensitivity to measurement errors” in join 48th ieee conference on decision and control and 28th chinese control conference shangahai, p.r. china, december 1618, 2009. [6] m. gonzlaez, m.a. perrez, j.c. viera, c. carballo, a. garrido. “a new, reliable and easily implemented nicd/nimh battery state estimation method”. instrumentation and measurement technology conference, 1999. imtc/99. proceedings of the 16th ieee. page(s): 1260 – 1264 vol.2. [7] n. kato, k. yamamoto. “estimation of the capacity of nickel-cadmium batteries by measuring impedance using electrolyte-deficient battery characteristics” telecommunications energy conference, 1995. intelec ’95. 10.1109/intlec.1995.499046 . [8] i. damlund. “analysis and interpretation of acmeasurement on batteries used to assess state-of-health and capacity-condition” telecommunications energy conference, 1995. intelec ’95. 10.1109/intlec.1995.499055. [9] texas instruments. “hdq communication basics”. application report. slua408a – december 2006 – revised september 2008. [10] saft batteries. http://www.saftbatteries.com/technologies_nickel_nicd_ 293/language/en-us/default.aspx. may 2013 [11] y. çadirci, y. özkazanç. “microcontroller-based on-line state-of-charge estimator for sealed lead-acid batteries” journal of power sources 129 (2004) 330-342 [12] chyuan-yow tseng, chiu-feng lin. “estimation of the state-of-charge of lead-acid batteries used in electric scooters” journal of power sources 147 (2005) 282-287. [13] j.h. aylor, a. thieme, b.w. johnson. “a battery state-ofcharge indicator for electric wheelchairs”. ieee trans. ind. electron. 39 (5) (1992) 398-409. a learning model for battery lifetime prediction of lora sen-sors in additive manufacturing acta imeko issn: 2221-870x march 2023, volume 12, number 1, 1 10 acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 1 a learning model for battery lifetime prediction of lora sensors in additive manufacturing alberto morato1, tommaso fedullo2,3, stefano vitturi1, luigi rovati2, federico tramarin2 1 national research council of italy, ieiit-cnr, padova, italy 2 department of engineering ''enzo ferrari'', university of modena and reggio emilia, modena, italy 3 department of management and engineering, university of padova, italy section: research paper keywords: lora; lorawan; iiot; battery lifetime; machine learning citation: alberto morato, tommaso fedullo, stefano vitturi, luigi rovati, federico tramarin, a learning model for battery lifetime prediction of lora sensors in additive manufacturing, acta imeko, vol. 12, no. 1, article 24, march 2023, identifier: imeko-acta-12 (2023)-01-24 section editor: francesco lamonaca, university of calabria, italy received november 17, 2022; in final form february 15, 2023; published march 2023 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: federico tramarin, e-mail: federico.tramarin@unimore.it 1. introduction today, novel and intelligent measurement systems are increasingly developed thanks to the internet of things (iot) paradigm [1]. moreover, the combination of iot with the industry 4.0 [2], [3] paradigm (often referred to as industrial iot), introduces a number of unprecedented challenges to measurement capabilities [4], with the ever-increasing need to collect reliable yet accurate data from mobile, battery-powered nodes over potentially large areas. in this scenario, one of the enabling technologies of industry 4.0 is additive manufacturing (am). basically, am makes it possible to create 3d objects, such as prototypes of possible products designed with cad tools, without the burdens usually imposed by traditional production systems in terms of work organization, delivery times and material utilization. this has a tremendous impact on manufacturing processes, making them more efficient, timely, scalable, and customizable [5], [6]. the benefits resulting from the introduction of am also place unprecedented demands on sensor systems and related measurement techniques. in fact, there are several am applications, such as those based on powder-bed processes, where suitable sensor systems need to be developed to collect data during the manufacturing process of potentially large objects, e.g., for early detection of defects and anomalies, and for abstract today, an innovative leap for wireless sensor networks, leading to the realization of novel and intelligent industrial measurement systems, is represented by the requirements arising from the industry 4.0 and industrial internet of things (iiot) paradigms. in fact, unprecedented challenges to measurement capabilities are being faced, with the ever-increasing need to collect reliable yet accurate data from mobile, battery-powered nodes over potentially large areas. therefore, optimizing energy consumption and predicting battery life are key issues that need to be accurately addressed in such iot-based measurement systems. this is the case for the additive manufacturing application considered in this work, where smart battery-powered sensors embedded in manufactured artifacts need to reliably transmit their measured data to better control production and final use, despite being physically inaccessible. a low power wide area network (lpwan), and in particular lorawan (long range wan), represents a promising solution to ensure sensor connectivity in the aforementioned scenario, being optimized to minimize energy consumption while guaranteeing long-range operation and lowcost deployment. in the presented application, lora equipped sensors are embedded in artifacts to monitor a set of meaningful parameters throughout their lifetime. in this context, once the sensors are embedded, they are inaccessible, and their only power source is the originally installed battery. therefore, in this paper, the battery lifetime prediction and estimation problems are thoroughly investigated. for this purpose, an innovative model based on an artificial neural network (ann) is proposed, developed starting from the discharge curve of lithium-thionyl chloride batteries used in the additive manufacturing application. the results of experimental campaigns carried out on real sensors were compared with those of the model and used to tune it appropriately. the results obtained are encouraging and pave the way for interesting future developments. mailto:federico.tramarin@unimore.it acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 2 process qualification [7]. this is the case addressed in this paper, where a large powder-bed 3d printer is capable of producing different types of artifacts using mixtures of powders, derived from recycled materials or natural components such as sand, water and chemical-free reagents [8]. a peculiarity of the system is that the artifacts are permanently equipped with sensors. these sensors are embedded in the manufactured objects at the beginning of the production phase and cannot be physically accessed. the sensors are needed to provide measurements of appropriate variables, such as temperature and humidity, which are used for two main purposes: i) during artifact production, as mentioned above, they provide feedback that allows on-line tuning of the 3d printing process and assist in defect detection [9]; ii) during the artifacts' lifetime, when they are in their final positions, sensor data is collected to perform off-line analysis as well as to monitor environmental conditions. a system with these specifications has been developed as part of an italian regional project called admin-4d (additive manufacturing & industry 4.0 as innovation driver). of course, to achieve the above goals, the sensors must also be able to communicate and send the measured data to the correct destination(s). for this purpose, technologies such as low power wide area networks (lpwans) represent a promising solution to ensure sensor connectivity, as they are optimized to minimize energy consumption while ensuring longrange operation and low-cost deployment. in the application addressed in this paper, one of the most representative implementations was chosen, namely lorawan (long range wan) [10], [11]. it is worth noting that the application of lorawan in the industrial scenario has been extensively analyzed [12], [13], proving its effectiveness, possibly after a suitable protocol optimization. therefore, the targeted am application poses several challenges to the sensor network system, and three main issues must be evaluated for the efficient collection of measurement data: i) the effective acquisition of readings from sensors embedded in artifacts; ii) the transmission range of the sensors; iii) the lifetime of the batteries used by the sensors. we have already investigated both the actual transmission capability of the embedded sensors and the covered distances in our previous work [14]. the results are satisfactory. it was found that transmission ranges of several tens of meters could be effectively achieved with a low packet loss rate and under different environmental conditions, in line with the requirements. in this paper, we deal extensively with the latter issue, namely battery life. obviously, this is a critical aspect for the whole project, since once an artifact has been produced, its embedded sensors can no longer be accessed, nor can its batteries be replaced/recharged. consequently, once the lifetime of the batteries has expired, measurements from the sensors will no longer be transmitted. it should also be noted that battery lifetime prediction is of paramount importance in several other iiot-based applications, such as cooperative robotics [15], [16]. battery lifetime prediction is closely related to the battery discharge model. unfortunately, the definition of an analytical discharge model, although theoretically possible, is difficult to achieve from the data typically provided by the manufacturers. therefore, in this paper we present a novel approach to battery discharge modeling based on a hybrid scheme where a static model and a machine learning (ml) one coexist. in particular, we use an artificial neural network (ann), which is necessary to overcome the nonlinearity of battery capacity as a function of temperature, and which is trained and validated using data derived from the battery datasheet. this ann model is then used in conjunction with a closed-form static battery model (e.g., an accurately adapted version of the matlab model) and finally introduced in the context of the am application described. the ultimate goal is to develop a model that can accurately predict the battery life of sensors under various operating conditions, including changes in temperature, power consumption, and other variables. the model should also be able to account for any nonlinearities that may occur, such as the effect of extreme temperatures on battery performance. experimental campaigns were conducted to compare the predictions of the models with real battery data. the models were then configured to emulate typical operating conditions to achieve realistic battery life predictions. as a result, it will be possible to more accurately predict sensor lifetime and optimize communication parameters to ensure maximum battery life. the paper is organized as follows. section 2 gives a brief description of the targeted am application and of the admin4d project and outlines the requirements for sensor data acquisition. section 3 discusses some relevant related works and provides an overview of the contributions provided by this paper. section 4 introduces the adopted sensors and reports considerations about battery selection and characterization. section 5 describes the adopted dynamic battery discharge model and its tuning, whereas section 6 presents the developed datadriven discharge model. section 7 then presents the outcomes of experimental assessments and compares them with the model results. section 8 presents battery life predictions in the operational context of admin-4d. finally, section 9 concludes the paper with some considerations for future developments. 2. the admin-4d project the structure of the admin-4d project is shown in figure 1. its operation is characterized by two distinct phases, namely production and final deployment. the production phase, in which an artifact is created, takes a variable amount of time, depending mainly on the size of the artifact itself. typical production times can be tens of hours. during this phase, sensors are embedded in the artifact and immediately begin transmitting data that is collected by the 3d printer's automation system and used for online feedback. as discussed in the previous section, the system uses a sensor network based on lorawan. specifically, a lorawan gateway (gw) device is connected to the 3d printer's automation system via the admin-4d intranet and provides connectivity to the embedded figure 1. automation system of the 3d printer acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 3 sensors that act as lorawan end devices (eds). after production, the final deployment phase begins, the artifacts are placed in their final locations and the lorawan gateway device is positioned within range of the sensors and connected to the internet. this allows sensor data to be transmitted to a remote cloud for offline analysis. for the sake of clarity, the two types of sensor data transmission shown in figure 1 (referred to as “sensor data for on-line feedback” and “sensor data for off-line analysis”, respectively) will never occur simultaneously. in fact, during the production phase of an artifact, only the intranet connection is used, while during the final deployment phase, only the internet communication is active. battery lifetime of embedded sensors is strongly influenced by the periodicity of their transmission as well as the amount of data transmitted, which directly affects the time required for each transmission. in fact, transmission is the operating situation where the lora interface of the sensors has the higher power consumption. in the context of the intended application, the transmission time depends on the operational phase (production or final deployment), while the amount of data transmitted remains the same throughout the life of the artifacts. obviously, during the production of an artifact, short transmission times are required (relative to the time needed to produce the artifact), since sensor data is used for on-line feedback. conversely, in the final deployment phase, there is no need for strict timing and periods can be relaxed. table 1 shows the typical values of periods and amount of data exchanged used in the following battery life analysis. 3. related works and contribution some interesting contributions dealing with the power consumption aspects of lpwans are available in the scientific literature. in [17], the authors provide a comprehensive assessment of the power consumption of commercially available modules by defining theoretical models based on real data obtained from experimental sessions. in [18], the energy consumption of a solid waste management system is addressed in a simulated scenario that also allows the estimation of battery life. similarly, the authors of [19] present a self-optimizing wireless water level monitoring system that can improve battery life based on operating conditions. energy consumption is also a major constraint for the authors of [20], where a lora-based localization system is addressed. in [21], a battery lifetime analysis is proposed. interestingly, it refers to lora sensors deployed in manholes, which are difficult to access for maintenance purposes. nevertheless, an effective a priori estimation of the battery lifetime is crucial in the am application targeted in this manuscript, and this can only be achieved with accurate battery discharge models. in this direction, models based on machine learning (ml) techniques have already been successfully applied, since they allow to easily take into account the non-linearities that often occur in this context. for example, in [22], an ml-based system was used to detect the end of battery life. furthermore, in [23]-[27], ml techniques are used to build models for the state of charge, discharge curves, and lifetime prediction of lithium batteries. unfortunately, the models proposed in these works cannot be considered in admin-4d because the artifacts produced would be expected to operate even under potentially extreme weather conditions. actually, these models were developed for generic lithium batteries, but for the intended application, heavy lithium batteries were chosen that present very different characteristics. this highlights the importance of defining a more comprehensive battery discharge model, capable of capturing the effect of battery performance under different usage conditions, such as temperature variations or different configurations of sensor communication parameters. such a model would be an essential tool for the design of systems using battery-powered sensors, as it would allow the lifetime of the sensors to be estimated based on the conditions in which they will be used, and therefore decide which communication parameters and sensor suite to use to ensure maximum battery life. it is important to note that battery discharge curves are generally obtained using static parameters such as temperature and discharge current. in reality, however, sensors often operate under dynamic conditions where these parameters can vary continuously. therefore, it is necessary to use battery discharge models that take these variables into account to obtain more accurate results. moreover, given the multiple nonlinearities of the batteries under consideration, the ability to simulate the discharge process dynamically and under different conditions allows to predict in advance possible problems in the on-board sensors, such as those due to voltage and temperature variations, allowing improvements and optimizations of the measurement system before the final deployment. 4. sensors and batteries selection in consideration of the peculiarities of the am application described in the previous sections, the board hosting sensors and communication interfaces, which must be embedded within the manufactured artifact, should also present suitable mechanical and waterproof properties. we identified a good candidate in the tinovi pm-io-5-sm device [28], which can host different type of sensors and is already equipped with a lora interface, hence acting as a lorawan end device in the targeted lorawan sensor network scenario. in the adopted configuration, the tinovi system provide both humidity and temperature measurements (the latter in the range [-20, 70] °c with an uncertainty of 0.6 °c) transmits information about the battery state of charge with a good resolution. another important consideration is that the lora interface of such sensors can be configured using over-the-air activation (otaa) mode to activate the end device. in fact, artifacts are typically moved from the production site to the final deployment site, so the sensors need to be able to join different lorawan networks during their lifetime. considering that the sensor becomes inaccessible after production, the sensor must use otaa so that the device address and session key required to join a network are dynamically assigned. otaa also allows network parameters (e.g., measurement’s transmission period) to be changed remotely and dynamically. the tinovi sensors can be powered by different types of batteries. we have considered those listed in table 2, where they are compared for energy density and temperature range. in fact, the specific am printing process requires the batteries to work in a wide temperature range. for this reason, we chose lithium table 1. transmission periods and data payloads. phase period data amount per sensor production ≤ 300 s 20 bytes final deployment ≥ 3600 s 20 bytes acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 4 thionyl chloride (lisocl2) batteries. in fact, they are specifically designed to operate over a wide [-60, +85] °c temperature range, which makes them suitable for military and medical applications, locking mechanism or metering applications that require long life without the need to replace the battery [29]–[30]. it should be noted that admin-4d also requires a sufficient energy density to keep the size of the batteries as small as possible and to ensure that they can be easily inserted into the artifacts. for this reason, the widespread and cheap sla batteries are not considered in this study. furthermore, the energy density of the selected lisocl2 batteries is considered sufficient for the application. 4.1. modelling of lithium thionyl chloride batteries in a first stage, saft ls 17500 lisocl2 batteries were tested as they are widely available off-the-shelf [31]. their main characteristics are summarized in table 3. unfortunately, the battery manufacturer does not provide a public dataset for the different measurements provided in the battery datasheet. for this reason, we have derived our data from the graphs contained in the datasheet using a graphical data extraction method. the available data consists of i) voltage profiles, ii) the correlation between plateau voltage and current drawn at different temperatures and, finally, iii) the relationship between capacity and current drawn at different temperatures. the results of this extraction are represented with solid lines in figure 2, figure 3, and figure 4. considering the high resolution of the available images, the accuracy with which the curves are reproduced here is rather good. indeed, it is also possible to provide an estimate of the uncertainty associated to the data points. in fact, each curve has been originally sampled with 25 data points, and for figure 2 we have a maximum uncertainty of 2.3 mv, while for figure 3 it is 4.5 mv. for the data points related to capacity in figure 4 the uncertainty is 23 ma h. as can be seen in figure 2, the discharging profiles strongly depend on the current drawn, i.e., the current absorbed by the battery. the longest lifetime is 2523 hours, obtained with constant 1.3 ma current. it can also be seen that the level of the plateau voltage decreases gradually at higher current levels. this effect is due to the high nonlinear internal resistance, estimated to be 13.6 ω on average. this voltage drop is not a negligible effect, as the drop below the minimum operating voltage of the sensor can affect its actual functionality. the voltage variations are also evident considering discharge curves at different temperatures, as reported in figure 3. as can be seen, for a given current, there are considerably different plateau voltage values, depending on the working temperature. this behaviour may represent an issue in several applications (the admin-4d project is one of them, actually) where the batteries used to feed the sensors are exposed to highly variable climatic conditions. temperature and current drawn also impact on the total available battery capacity. from figure 4, it is clear that these table 2. comparison of different battery chemistries. chemistry energy density (w h l−1) temperature range (°c) sealed lead acid (sla) 70 -40 / +60 alkaline (𝑍𝑛 − 𝑀𝑛𝑂2) 340 -20 / +70 lithium cobalt oxide (𝐿𝑖𝐶𝑜𝑂2) 560 -20 / +60 lithium manganese nickel (𝐿𝑖𝑁𝑖𝑀𝑛𝐶𝑜𝑂2) 580 -20 / +60 lithium thionyl chloride (𝐿𝑖𝑆𝑂𝐶𝑙2) 350 -60 / +85 table 3. saft ls 17500 battery specification. description specification rechargeable no nominal voltage 3.60 v nominal capacity 3600 ma h nominal cut-off voltage 3.3 v operating temperature -60 °c to 85 °c figure 2. typical discharge profile at 20 °c for the saft ls 17500 battery. extracted from [26]. figure 3. dependence of the plateau voltage from current drawn at different temperatures. extracted from [26]. figure 4. dependence of the total available capacity from current drawn at different temperatures. extracted from [26]. acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 5 types of cells are susceptible to the peukert’s effect [32], which lowers down the total available battery capacity depending on the current draw and temperature, with the consequent reduction of the battery lifetime. it can be concluded that the design of an accurate battery lifetime model is a challenging task that requires the consideration of several different aspects. admittedly, accurate models can be developed by exploiting the equations describing the chemical interaction between the anode and cathode. such models allow to obtain a very high accuracy, since they depend on the intrinsic parameters of the battery. unfortunately, they are difficult to define and tune for commercially available cells, since the information available for such devices is typically insufficient. therefore, to address this challenge, in this work we have adopted a hybrid approach, where a suitably tuned generic battery dynamic model is used in conjunction with a data-driven approach based on artificial neural networks. in the following section, we will discuss the tuning of the first dynamic model, while the ann approach will be discussed in section 7. 5. tuning of the generic battery dynamic model several models for generic purpose lithium-ion based batteries have been proposed in the scientific literature. a widely used one is discussed in [33] and its implementation is currently found in matlab simscape under the name generic battery dynamic model, hereafter referred to as gbdm. this model actually describes the behaviour of a generic rechargeable battery using a combination of three stages. first, a (short) exponential voltage drop occurs suddenly when the battery is fully charged and starts to deliver energy; this is followed by a so-called nominal region (a plateau region) where the energy is delivered from the battery with a quasi-constant voltage level until the voltage starts to drop below the nominal battery voltage level. the third section is related to the sudden loss of energy, where the battery is discharged, and the voltage drops abruptly. the gdbm is implemented by an equivalent feedback system, whose parameters can be tuned to adequately model different battery typologies and their discharge characteristics. in this regard, we considered the parameters provided by the battery manufacturer, in particular the discharge profiles specific to the saft ls 17500 lisocl2 batteries shown in the previous figure 2 to figure 4. from the discharge profile at the nominal current of 1.3 ma, we were able to extract the required model parameters that are reported in table 4. the tuning process of the gdbm model resulted in the discharge curves shown in figure 5, that have been subsequently derived for different current values (solid lines) and compared with those reported in the battery datasheet (dotted lines). as can be observed in the figure, the model discharge profiles (and thus the lifetime estimate) are in good agreement with those declared by the manufacturer for current values of 1.3, 3 and 8 ma. for higher currents, the experimental and model trends are completely different to the point that the model estimate of the discharge profile at 120 ma is completely inconsistent with the experimental one (in fact, it is not even visible in the plot). the above voltage profiles can be compared from a numerical point of view by looking at both the average deviation between the experimental and model curves and the predicted battery life. the former can be evaluated by the root mean square error (rmse), calculated as the root of the average squared deviations between the model predicted value and the expected voltage level of the battery, as extracted from the manufacturer's discharge curve for a given current and after a given time. table 5 reports the obtained values, where it can be observed that the rmse is relatively low for low currents and consistently increases as the current increases. analogously, the same trend can be observed in the lifetime estimation, although in a less obvious way, because the estimated lifetime is only consistent with the nominal current of 1.3 ma used for the tuning process. the obtained results are not surprising since, as already pointed out, the lithium chloride thionyl batteries present a considerable internal resistance that varies non-linearly with the current draw. also, such batteries are affected by the peukert’s effect, and the resistance is almost constant at the nominal current and tends to decrease at higher currents. unfortunately, this model does not consider these effects since, as a matter of fact, it keeps the resistance constant for every current draw, so that the peukert’s effect is not considered at all. table 4. parameters of the gdbm model, for the characterization of the discharge profile. parameter value nominal voltage 3.6 v maximum capacity 3.6 a h fully charged voltage 3.67 v nominal discharge current 1.3 ma internal resistance 13.6 ω nominal ambient temperature 20 °c second ambient temperature −40 °c maximum capacity at −40 °c 2.744 a h initial discharge voltage at −40 °c 3.12 v figure 5. comparison between the discharge profiles at 20 °c extracted from the datasheet (dotted lines) and derived from the tuned gbdm model (solid lines), for 5 different current values. table 5. comparison between the discharge profile (at 20 °c) extracted from the datasheet and the gbdm model. current (ma) lifetime (h) rmse in voltage profile datasheet estimated error (%) (v) 1.3 2523.88 2523.92 0.00 2.97 · 10-4 3.0 1140.39 1093.70 4.09 1.03 · 10-3 8.0 425.76 410.14 3.67 1.47 · 10-3 33.0 90.38 99.43 10.02 1.18 · 10-1 120.0 17.20 27.34 58.95 9.73 · 10-1 acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 6 as discussed earlier, temperature is another source of complexity and nonlinearity that the gdbm takes into account, although it cannot reproduce the same behaviour as the lithium chloride thionyl batteries. this is visually illustrated in figure 6, which shows the discharge profile obtained at a constant discharge current of 1.3 ma for different temperatures. in the figure, five different temperatures are considered, and the discharge curves obtained from the gdbm are shown with solid lines, while the manufacturer's curves are shown with dotted lines. the graph clearly indicates that the match between model and datasheet is rather satisfactory only for temperatures of 20 °c and −40 °c, that are those used for tuning the model, as reported in table 4. conversely, for other temperatures a significant discrepancy can be observed, for example, at 55 °c and 70 °c. looking at figure 6, it can also be observed that both the plateau voltage and total capacity depend on the temperature in a strongly non-linear way. moreover, in the leftmost part of the solid gdbm curves, the exponential variation of the voltage is evident (notice that it is linear since the x-axis is in logarithmic scale). this behaviour is less evident only for t = 20 °c in both figure 5 and figure 6, since the model is tuned around that temperature, confirming the temperature dependence. the results obtained from the tuned gdbm model suggest that it is able to provide a satisfactory description of the main features of the discharge behaviour for the adopted lisocl2 batteries but limited to the cases close to the operating points selected for the model calibration. in the next section, a proposal to overcome this limitation is presented, based on a data-driven approach to be used in conjunction with the gdbm approach. 6. a data-driven discharge model models based on a data-driven approach may prove advantageous to address the negative effects due to nonlinearities that are experienced with models based on battery parameters, such as the gdbm discussed in the previous section [34]. in particular, an approach that has already proven its effectiveness [35] relies on the use of artificial neural networks (anns) to profitably approximate the nonlinear functions related to the battery discharge behaviour with the desired accuracy. therefore, to address the issues of the dynamic battery model that have been highlighted in the previous section, in particular related to the nonlinear dependence with the temperature and the internal resistance, we exploited a suitable ann with the aim of increase the estimation accuracy of the lifetime duration of lisocl2 batteries. in particular, we implemented a multiple-layer perceptron neural network (mlpnn) since it demonstrated to be promising in solving similar problems [36]. the structure of the mlpnn is shown in figure 7. in the input layer, the input parameters are state of charge (soc), temperature (t) and current drawn (c). two hidden layers, each with 64 neurons, realize the non-linear transformation from the input layer. finally, the output layer provides battery voltage (v) and lifetime estimation (t). the structure of the mlpnn was chosen with an embedded system implementation in mind, so that it can be deployed directly in the sensor, which can change the period and transmission parameters based on the estimated duration. in particular, the minimum number of neurons and hidden layers was chosen to solve the estimation problem without causing overfitting problems, while ensuring low memory consumption and computation time. the training and validation phases of the applied ann clearly require a consistent and sufficiently large dataset. unfortunately, as mentioned in section 4.1, the battery manufacturer does not provide a public dataset for the different measurements provided in the battery datasheet, which led us to use a graphical extraction procedure. in order to obtain an adequate amount of data to train and then identify complex relationships between inputs and outputs, we needed to apply a data augmentation technique to artificially increase the size of the dataset from the initial 25 points. to do this, we used a spline curve to fit the original raw data points while minimizing the rmse. given the low uncertainty associated with the extracted data, and the fact that the rmse in this fit was kept extremely low, the uncertainty associated with the data inferred from the spline can be considered consistent with those revealed in section 4.1. the obtained augmented dataset consists of 250000 unique entries, which were split between training and validation in a ratio of 80%-20%. other meaningful hyperparameters are shown in table 6. as shown in table 6, the number of training epochs was set to 150. this value was determined using the early stopping technique to avoid overfitting. the trends of training and validation loss are shown in figure 8. as can be seen from the figure, both training and validation loss decrease with the number of epochs and almost reach zero. after training the figure 6. comparison between the discharge profile extracted from the datasheet (dotted lines) and the gbdm model (solid lines) at different temperatures. all the curves are obtained considering a constant discharge current of 1.3 ma. figure 7. structure of the adopted mlpnn. acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 7 model, its behaviour was compared with the experimental data obtained from the manufacturer. the results of modelling with the mlpnn are shown in figure 9, where the discharge profiles obtained for different currents are reported. table 7 provides more detailed statistics. as can be seen, the results obtained are better than those obtained with the gbdm model. in fact, although the percentage error of the lifetime estimation at 1.3 ma is higher than that of the gbdm model, the lifetime estimation error is limited to 2% over the whole discharge current range. the accuracy of the model on the test data set is also confirmed by the rmse in the voltage profile, which is definitely very limited. while for the gbdm model both the lifetime estimation error and the mse increase moving away from the profile where the tuning was performed, the mlpnn keeps both metrics bounded and rather stable over the different profiles. the advantage of using such a model seems clear. in fact, it is able to better generalize the estimation problem, giving a higher accuracy in a wide range of operating conditions. these considerations can be further confirmed by the discharge profiles obtained for a constant current of 1.3 ma, but for different temperatures. this comparison is shown in figure 10, whereas some statistical details are reported in table 8. even in this case, the discharge profiles as well as the lifetime estimations agree with the experimental ones with a very good accuracy, and an estimation error under 6% in the worst case. 7. experimental assessment of the proposed discharge models in order to evaluate the effectiveness of the proposed models, we first performed some experimental tests on the tinovi smart sensor alone. the goal is to verify whether the proposed models, tuned on the data provided by the manufacturer, are able to provide a consistent prediction of the actual battery behaviour in the final system. to increase the reproducibility of the experiments, the experimental sessions were conducted in a room with a controlled temperature of 20°c, with the tinovi device (namely, the lorawan ed) placed 1 m away from the lorawan gateway. the adopted tinovi pm-io-5-sm smart sensor is configured to transmit in each lorawan frame an internal reading of the battery voltage, whose value is encoded in binary format to map the range from 2.8 v to the maximum value of 4.2 v reached during charging. the resolution of the voltage reading is 1%, which correspond to 14 mv. during these tests, the sensor is configured to transmit a packet with a period of five (5) minutes, containing the measured variables (mostly, temperature and humidity) and the information about the battery status. it should be noted that the sensor can be in two different states, active and sleep: the former is the state in which the sensor is either sending or receiving data from its probes, while the latter is the idle state in which the sensor does not perform any action. the main table 6. parameters for the characterization of the discharge profile in the mlpnn model. parameter value activation function tanh loss function mse optimizer adam learning rate 0.0001 (non-scheduled) epoch 150 batch size 512 table 7. comparison between the discharge profile at 20 °c extracted from the validation dataset and the mlpnn model. current (ma) lifetime (h) rmse in voltage profile datasheet estimated error (%) (v) 1.3 2523.88 2568.19 1.76 3.10 · 10-4 3.0 1140.39 1122.50 1.56 7.41 · 10-5 8.0 425.76 425.52 0.06 9.33 · 10-5 33.0 90.38 89.21 1.29 2.57 · 10-5 120.0 17.20 16.94 1.51 1.19 · 10-5 table 8. comparison between the discharge profile at constant discharge current 1.3 ma for different temperatures. temperature (°c) lifetime (h) rmse in voltage profile datasheet estimated error (%) (v) -40.0 1666.21 1760.29 5.65 3.17 · 10-4 -20.0 2110.56 2233.70 5.83 3.44 · 10-4 20.0 2523.66 2568.19 1.76 3.10 · 10-4 55.0 1883.47 1944.04 3.22 3.33 · 10-4 70.0 1688.57 1711.55 1.36 3.31 · 10-4 figure 8. training and validation loss. figure 9. comparison between the discharge profile at 20 °c on the test dataset and the one generated by the mlpnn. markers represent the test data while solid lines represent the data generated by the model. acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 8 parameters used to configure the tinovi smart sensor during these experimental sessions are shown in table 9. the average current consumption of the sensor, as shown in the table, is 2.6 ma. we have therefore used the two proposed models to generate the corresponding discharge curves for this specific current and at a constant temperature of 20°c. the estimates of the voltage profiles were then compared with the results of the real voltage measurements taken by the sensor. the results are shown in figure 11 and table 10, respectively. the comparison highlights that both models produce similar discharge profiles, and in a rather good accordance with each other. in particular, the mlpnn is able to better approximate the nominal (plateau) voltage phase and is very close to the actual measured final fast discharge phase. conversely, the gbdm is more precise at modelling the starting point and the initial part of the discharge phase, where the voltage starts to drop below the nominal value. definitely, mlpnn provides a higher accuracy in the prediction of both the voltage values and the total battery lifetime, as can be seen from table 10. in fact, the gbdm model tends to overestimate the plateau and to slightly underestimate the lifetime of the battery. another aspect that affects the power consumption of the sensor is the spreading factor (sf) adopted by the lora modules. in fact, the use of a higher sf (at the same transmit power, of course) allows a more robust transmission, but at the cost of longer transmission times, which consequently result in a higher power consumption of the whole sensor. for example, in the case of the tinovi pm-io-5-sm, the transmission of 20 bytes with sf12 increases the time in the active state of the ed from 5 s to 6 s (remember that the packet airtime with sf12 is 1810 ms). as a result, the average current required increases from 2.6 ma to 3.1 ma, with a corresponding decrease in battery life. based on the outcomes of the previous experiment, the mplnn model can be profitably exploited to estimate the battery lifetime both at different temperatures and for a higher sf. the results of this analysis are shown in table 11, which confirms a decrease of about 16-17% in battery life when moving from sf = 7 to sf = 12. 8. battery life estimation in the admin-4d application context in this section, the developed battery models have been used to estimate the lifetime in the real application context, i.e., considering the whole 3d printing process of an artifact (production phase) and its subsequent positioning at the final site (deployment phase), where sensor data is transmitted to the remote cloud. it should be noted that the production of a real table 9. parameters adopted in the experimental setup. parameter value transmission frequency 868 mhz spreading factor 7 bandwidth 125 khz code rate 4/5 payload length 20 sleep time 300 s wake-up + acquisition + transmission time ∼ 5 s sleep current consumption 0.15 ma active current consumption 150 ma average current consumption 2.6 ma module operating temperature range −20 °c – +70 °c voltage operating range 2.5 v – 6 v output power 14 dbm figure 10. comparison between the discharge profiles from the validation dataset and the ones generated by the mlpnn, at different temperatures. markers represent the validation data while line represent the data generated by the model. all the curves are obtained considering a constant discharge current of 1.3 ma. table 10. evaluation statistics relevant to the curves in figure 11. lifetime (h) lifetime error (%) rmse (v) experimental 1323.94 mlpnn 1322.32 0.12 2.33 · 10-4 gbdm 1302.38 1.63 1.06 · 10-3 figure 11. comparison between the discharge profiles generated by the mlpnn and the gdbm models, with respect to the experimentally measured one. controlled environment, t = 20 °c, sensor only. table 11. battery lifetime estimation using the mlpnn model at different temperatures for sf = 7 and sf = 12. temperature (°c) lifetime (h) sf = 7 -20 1126.90 20 1323.94 70 938.56 sf = 12 -20 928.30 20 1112.94 70 784.43 acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 9 artifact is a process that takes 72 h. in this phase, the sleep time of the lorawan eds (which corresponds to the period of sensor data transmission) was set to 300 s, in accordance with the values given in table 1. subsequently, in the final deployment phase, the sleep time was increased to 3600 s in order to maximize battery life while maintaining the required periodicity of measurement data for monitoring purposes. with these values for the sleep period duration, the resulting average current consumption is about 2.6 ma in the production phase and 0.35 ma in the final deployment phase. the two phases are clearly very different from each other, both in terms of power consumption and temperature range. moreover, the production phase is characterized by strong temperature variations, as experimentally observed, which range from 20 °c to 70 °c. from the analysis carried out in the previous sections, none of the models considered is able to satisfactorily emulate both phases. indeed, in the production phase, the gbdm model appears unsuitable due to the high temperature dynamics, while the mlpnn shows a good behaviour with respect to this type of operating conditions. conversely, in the deployment phase, which is practically static, the gbdm is more effective, taking into account the fact that the mlpnn provides inconsistent estimates, since this model has not been trained for the low current that characterizes such a phase. as a consequence, we adopted a hybrid approach to model the discharge curve in a real application context. specifically, the mlpnn model is used for the production phase and the gbdm for the deployment one. figure 12 shows the resulting voltage profile obtained with this hybrid approach. as can be seen, there is a slight voltage increase in the initial part (see the inner rectangle showing the curve from 0 to 100 hours) due to the increase in temperature. this is followed by the expected voltage plateau and finally by the rapid voltage drop. note that the glitch at t = 72 hours is due to the switch between the two models. it represents the initial rapid exponential part of the discharge curve typical of the gbdm model. taking into account the assessment of the tuned models made in the previous sections, the results obtained in this last analysis allowed to estimate the battery lifetime, for the final application conditions, to be up to 9104 hours. this corresponds to more than one year during which the sensor is able to send useful monitoring data. in fact, this value meets the requirements of the admin-4d project. 9. conclusions and future directions of research this paper presents an additive manufacturing experiment in which battery-powered sensors are embedded in the manufactured artifacts. since the sensors are inaccessible after manufacturing, the acquisition of their measurements is totally conditioned by the lifetime of the batteries. therefore, we investigated the possibility of modelling the discharge curve of the batteries in order to estimate their lifetime, taking into account the challenging operating conditions imposed by the targeted application. for this purpose, an innovative model based on an artificial neural network (ann), specifically a mlpnn, has been proposed and developed starting from the discharge curve of the adopted lithium-thionyl chloride batteries. the model has been used in conjunction with a popular one, namely the gbdm, realizing a hybrid approach that proved effective for the battery lifetime estimation. both models have been calibrated using information from the battery datasheet and validated using measurements obtained from actual sensors. the admin-4d project has been completed and some prototype artifacts have already been produced and positioned at the partners' sites where they transmit sensor data as described for the final deployment phase. this scenario paves the way for some interesting future activities. the first, related to the followup of the project, is represented by the off-line analyses that will provide data to better tune the upcoming artifact production. second, and more focused on the topics of this paper, the mlpnn model needs to be further refined to become effective also for low currents. in this respect, the data coming from the currently installed artifacts will prove particularly useful, since they can be used to perform a 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doris potočnik 1,3 , marijan nečemer 2 , darja mazej 1 , radojko jaćimović 1  and nives ogrinc 1,3  1  department of environmental science, jožef stefan institute, jamova 39, 1000 ljubljana, slovenia  2  department of low and medium energy physics, jožef stefan institute, jamova 39, 1000 ljubljana, slovenia  3  jožef stefan international postgraduate school, jamova 39, 1000 ljubljana, slovenia      section: research paper   keywords: milk; edxrf; k0‐inaa; icp‐ms; lda; geographical origin  citation: doris potočnik, marijan nečemer, darja mazej, radojko jaćimović and nives ogrinc, multi‐elemental composition of slovenian milk: analytical  approach and geographical origin determination, acta imeko, vol. 5, no. 1, article 5, april 2016, identifier: imeko‐acta‐05 (2016)‐01‐05  section editor: claudia zoani, italian national agency for new technologies, energy and sustainable economic development affiliation, rome, italy  received september 6, 2015; in final form december 20, 2015, year; published april 2016  copyright: © 2016 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: project funding by iaea under contract no. 17897  corresponding author: nives ogrinc, e‐mail: nives.ogrinc@ijs.si    1. introduction  milk is an important everyday nutrition, since it is a source for many key nutrients including proteins, energy, many essential minerals and vitamins, factors that all contributed to an increased consumption in recent years. milk contains more than 20 different micro and macro elements. among them are elements such as cu, co, se, zn, mn, mo, i and fe that are essential for human health in certain concentrations. these elements have a beneficial effect on animals and humans, since they are important for normal functioning of metabolism, growth and development and any concentration discrepancies of these elements may cause disturbance in organisms [1], [2]. concentration of micro and macro elements in milk is not constant, but is influenced by genetics of individual animal species, stage of lactation, farm management (nutritional regime) and environment factors such as season, locality of the farm, nature of soil, fertilizer application and industrial activities [3]. industrial and other anthropogenic activities could also influence the distribution and levels of elements, mainly toxic elements such as cd, ni, co, pb, cr, hg in milk, since they are abstract  the main objective  in multi‐elemental analysis  in food has traditionally been and still  is, to ensure food quality and safety. three  different methods were investigated in the present study to obtain the elemental content in milk samples: energy dispersive x‐ray  fluorescence spectrometry (edxrf), k0‐instrumental nuclear activation analysis (k0‐inaa) and the  inductively coupled plasma mass  spectrometry (icp‐ms). results indicate that the obtained data with different methods are in good agreement with certified reference  materials. precision was found to be satisfactory with relative standard deviations always between 1 and 10 %, except for se, as, cd  and  pb.  the  concentrations  of  these  elements  were  close  to  the  detection  limit  and  thus,  precision  higher  than  10  %.  an  intercomparison  exercise  between  edxrf  and  k0‐inaa  showed  satisfactory  agreement  and  only  two  samples  exceeded  95  %  confidence interval for rb and zn with lower data obtained by k0‐inaa. it was found that edxrf was the cheapest, simplest and most  environmental friendly method for analysis of multi‐elemental composition (p, s, cl, k, ca, zn, br, rb, sr) in milk samples, while for the  determination of mn, fe, cu, se content and possible identification of pollutants such as as, cd and pb icp‐ms it was a method of  choice due to its excellent sensitivity and accuracy. these two methods were also used to determine the multi‐elemental composition  in slovenian raw cow milk from different geographical regions: alpine, mediterranean, dinaric and pannonian  in december  2013.  linear discriminant analysis (lda) was used to explore multi‐elemental analysis of milk samples to obtain classification according to  geographical regions. regional discrimination was most successful taking into account ca, s, p, k, and cl with prediction ability of 66.7  %.  acta imeko | www.imeko.org  april 2016 | volume 5 | number 1 | 16  accumulated through the food chain [1], [4], [5]. they can easily accumulate in plants grown on polluted soils and thus cause toxic effects in cattle and consequently also in humans who consume “contaminated” milk [4], [6]. for this reason, it is necessary to control the concentration of micro and macro elements/toxic elements in consumed food. further, multi-element analysis has been successfully used for the determination of the geographical origin of different foodstuff such as wine [7], [8] onion [9]-[11], tea [12] and tomatoes [13]. in relation to milk and dairy products, most of the studies have been performed on cheese for descriptive purposes [14] and for origin authentication purposes [15]. sacco et al. [16] used multi-elemental analysis in combination with stable isotope analysis for differentiation between southern italy and foreign milk. trace elements in milk and dairy product have been used to investigate and to provide information about correlation between animals fodder, time of year, elemental conditions. it was found that season has more influence on mineral concentration than regions [15]. several different techniques are available for simultaneous multi-element chemical analysis including: energy dispersion xray fluorescence spectrometry (edxrf), neutron activation (k0-instrumental nuclear activation analysis; k0-inaa), inductively coupled plasma atomic emission spectroscopy (icpaes) and inductively coupled plasma mass spectrometry (icpms) [1]. each of these techniques has the capacity to provide a chemical profile or chemical fingerprinting, which can be used to characterize the material in question and to compare it with other samples or reference materials. among inductively coupled plasma systems icp-aes and icp-ms have been widely used for the analysis of minor and trace elements in food, since they provide satisfactory result and allow performing simultaneous-sequential determination of multiple elements. the advantages of these techniques are short time of analysis and low limit of detection [4], [17]. on the other hand, edxrf as a highly sensitive, fast and cheap technique offers the possibility of performing direct multi-element analysis of solid samples over a wide dynamic range. this technique is especially useful in food authentication and traceability studies where there is a requirement for a database of genuine samples to which the sample can be compared to establish its authenticity. for effective data processing, the large number of independently measured parameters is needed including multielemental composition [18]-[20]. another nondestructive multielement technique is k0-inaa. the main disadvantages of this method are long turn-around time, is labour intensive and a nuclear reactor or other source of activating particles is required. nevertheless, providing a different approach, this method is often an important asset in studies regarding the analysis of standard reference materials [21]-[23] and as an additional technique in evaluation of the accuracy of analytical results [24]. thus, the main objectives of this paper are: (1) to verify the methods for elemental composition in milk samples including edxrf, k0-inaa and icp-ms; and (2) to differentiate the milk according to geographical region in slovenia based on elemental composition. sample preparation and the analysis procedure for each of the above mentioned analytical techniques are described and a comparison of analytical and other parameters such as uncertainty, accuracy, limits of detection (lod), cost of analysis per sample, instrumental cost etc. is critically evaluated. 2. materials and methods  2.1. sampling  samples of the whole milk were provided by four slovenian diary producers: ljubljanske mlekarne d.d., pomurske mlekarne d.d., mlekarna planika d.o.o. and mlekarna celeia d.o.o. in december 2013 covering different geographical regions (mediterranean, pannonia, dinaric and alpine) in slovenia. the samples were stored at -20oc before analysis. all together 40 samples were obtained for multi-elemental analysis. 2.2. energy dispersive x‐ray fluorescence spectrometry (edxrf)  multielement determination of macro (p, s, cl, k, ca) and micro elemental (zn, br, rb, sr) content was nondestructively performed by energy dispersive x-ray fluorescence spectrometry (edxrf). the analysis was performed on freezedried samples. the pellets were prepared from 0.5 to 1.0 g of powder sample material by a pellet die and hydraulic press. for excitation, the disc radioisotope excitation source of fe-55 (25 mci) and cd-109 (20 mci) from eckert and ziegler were used. the emitted fluorescence radiation was measured by en energy dispersive x-ray spectrometer constituted of a si(li) detector (canaberra), a spectroscopy amplifier (canberra m2024), adc (canberra m8075) and pc based mca (s-100 canberra). the spectrometer was equipped with a vacuum chamber (fe-55) for the measurement of light elements p-cl, while the energy resolution of the spectrometer was 175 ev at 5.9 kev. the analysis of complex x-ray spectra was performed by the axil spectral analysis program [24], [25]. for the evaluated uncertainty of this procedure, it is required to include the statistical uncertainty of measured intensities and the uncertainty of the mathematical fitting procedure. for this purpose, quantification was then performed utilizing qaes (quantitative analysis of environmental samples) software, developed in our laboratory [24], [25]. the estimated uncertainty of the analysis was around 5 % to 10 %. rather high total estimated uncertainty is mainly due to contributions of matrix correction and geometry calibration procedures, which include errors of tabulated fundamental parameters, and contributions of spectrum acquisition and analysis. in the case of edxrf, the limits of detection calculated from the signal to background ratio [26] on the realistic sample nist 1549 for p, s, cl, k, ca, zn, br, rb and sr were 400, 200, 100, 20, 15, 4, 2, 1, 1 µg/g of dry milk sample, respectively. 2.3. k0‐inaa measurements  for k0-inaa measurements aliquot of the freeze-dry sample (0.10 to 0.20 g) was weighed into a polyethylene ampoule (spronk system, the netherlands). for the determination of long-lived radionuclides the sample was irradiated together with the standard al-au (0.1 %) for 12 hours. irradiation was carried out in a reactor triga mark ii. after irradiation the sample was kept in a polyethylene ampoule and gamma activity of radionuclides induced in the sample after 4, 9-15, and 24-33 days of cooling was measured. all measurements were performed on hpge detector (40 and 45 % yield). for the evaluation of gamma spectra we used the program hyperlab 2002, and element content was calculated with the program kayzero for windows. limits of detection for br, ca, k, rb, zn calculated as three times the standard deviations of the blank sample, are 1, 500, 500, 5, 5 µg/g milk sample, respectively. acta imeko | www.imeko.org  april 2016 | volume 5 | number 1 | 17  the uncertainty level for elemental determination by k0inaa was 610 %, in the case of lower elemental content such as se and mn the uncertainty is higher, around 20 %. 2.4. icp‐ms measurements  for multi-elemental analysis of milk samples with icp-ms two procedures of sample preparation were used: a) microwave digestion: about 1 ml of milk sample (0.15 g of lyophilized sample) was weighed into quartz tubes. 1 ml 65 % hno3, (merck, germany, suprapur) and 1 ml 30 % h2o2, (merck, germany, suprapur) were added and samples were subjected to closed vessel microwave digestion (microwave system ethos 1, milestone sn 130471) at max. power of 1500 w: ramp to 130°c 10 min, ramp to 200 °c 10 min, hold 20 min, cooling 20 min. then the samples were equilibrated to room temperature. the solution was quantitatively transferred into 10 ml polyethylene graduated tubes and filled to the mark with mili-q water. before determination by icp-ms, samples were diluted five times. external calibration was made with icp multi element standard solution xvi certipur (merck). b) dilution (adapted from barany [27]): an aliquot of 1 ml of milk sample (0.15 g of lyophilized sample) was diluted five times with alkaline (merck, suprapur) solution containing triton x-100 (sigma aldrich, sigmaultra) and ethylenediaminetetraacetic acid disodium salt dehydrate (edta, fisher scientific, analytical reagent grade). for calibration the standard addition procedure was utilised. measurements of prepared solutions were made by an octapole reaction system inductively coupled plasma mass spectrometer (7500ce, agilent) equipped with an asx-510 autosampler (cetac). instrumental conditions: babington nebulizer, scott-type spray chamber, spray chamber temperature 5 °c, plasma gas flow rate 15 l/min, carrier gas flow rate 0.8 l/min, make-up gas flow rate 0.1 l/min, sample solution uptake flow rate 1 ml/min, rf power 1500 w, reaction cell gas helium 4 ml/min, isotopes monitored 55mn, 56fe, 63cu, 66zn, 75as, 78se, 111cd, 114cd, 206pb, 207pb, 208pb, isotopes monitored as internal standard added to all solutions: 45sc, 69ga, 89y, 157gd. tuning of the instrument was made daily using a solution containing li, mg, y, ce, tl and co. concentrations of cd, pb and as in milk samples are usually very low <0.5 ng/g and thus, insufficient detection limits were obtained with microwave digestion. therefore preparation with simple dilution of milk samples was introduced. limits of detection for cd, pb, as, se, mn, cu, zn and fe, calculated as three times the standard deviations of the blank sample, were 0.1, 0.2, 0.05, 2, 1, 6, 35 and 30 ng/g milk sample, respectively. estimated combined uncertainty of results was 5 % for cu, zn, fe, 10 % for se, mn and 15 % for as, cd, pb. 2.5. quality control  the accuracy of results was checked as follows: a) analysis of the certified reference materials: whole milk powder nist 8435, non-fat milk powder nist 1549 (both national institute of standard and technology) and skim milk powder bcr 150 (ec-jrc-irmm), ermbd150 and erm-bd151 (ec-jrc-irmm); b) comparison with independent methods: edxrf and k0inaa; c) for as there are no reference materials, therefore, comparison was made by an independent method radiochemical neutron activation analysis [28], [29]: result for internal quality control milk sample by k0-rnaa 0.28 ± 0.02 ng/g and by icp-ms 0.36 ± 0.05 ng/g; d) participation in interlaboratory comparison scheme fapas (food and environmental research agency, sand hutton,york, vb). 2.6. statistical evaluation   statistical calculations and multivariate analysis were carried out using xlstat software package (addinsoft, new york, usa) basic statistics included mean values (median and arithmetic mean), standard deviation (s.d.), minimum and maximum. multivariate analysis involved linear discriminant analysis (lda). 3. results and discussion  3.1. results on certified reference materials   results of the validation of the accuracy of multi-elemental analysis performed by icp-ms with certified reference materials (nist 1549, nist 843) are collected in table 1, while results of the validation of the accuracy with certified reference material erm-bd 150 and erm-bd 151 with all three methods are collected in table 2. results indicate that the obtained data with different methods are in good agreement with certified reference materials. precision was also found to be satisfactory with relative standard deviations (rsds) always between 1 and 10 %. only in four cases (of se, as, cd and pb), for which the table 1. validation of the accuracy of icp‐ms multi‐elemental procedures proposed (nist 1549, nist 8435 and bcr 150).      nist 1549   nist 8435   bcr 150  element  certified   value  found   value  certified   value  found   value  certified  value  found  value    dry weight mg/kg  mn  0.26 ± 0.06  0.25 ± 0.03  0.17 ± 0.05  0.19 ± 0.02  ‐  ‐  fe  1.78 ± 0.10  1.71 ± 0.09  1.80 ± 1.10  2.0 ± 0.1  11.8 ± 0.6  9.79 ± 0.48  cu  0.70 ± 0.10  0.67 ± 0.03  0.46 ± 0.10  0.40 ± 0.02  2.23 ± 0.08  1.95 ± 0.1  zn  46.1 ± 2.2  39.3 ± 2.0  28.0 ± 3.1  23.8 ± 1.2  ‐  ‐  se  0.11 ± 0.1  0.11 ± 0.1  0.131 ± 0.14  0.128 ± 0.01  ‐  ‐  cd  0.0005 ± 0.0002  0.0005 ± 00001  ‐  ‐  0.0218 ± 0.0014  0.0203 ± 0.0031  pb  0.019 ± 0.003  0.023 ± 0.004  ‐  ‐  1.00 ± 0.04  0.84 ± 0.13  acta imeko | www.imeko.org  april 2016 | volume 5 | number 1 | 18  measured concentrations were very close to the detection limit, precision was worse and higher than 10 %. 3.2. comparison of elemental composition between edxrf and  k0‐inaa methods  comparison of results of milk samples obtained by edxrf and k0-inaa is presented in table 3. it was found that only two samples exceeded 95 % confidence interval for rb and zn with lower data obtained by k0-inaa (bold values in table 3). the possible reason for this discrepancy could be in the inhomogeneity of distribution of these two elements in the two aliquotes of the same sample analysed by edxrf and k0inaa. 3.3. results of the interlaboratory comparison scheme fapas  we participated in the interlaboratory comparison scheme fapas in 2012, 2013 and 2014 for milk samples in powdered form with all analytes present at low natural levels. results are collected in table 4. the results indicated good agreement between measured and assigned values. with these independent assessment competence of our laboratory to analyse low levels of as, cd and pb was demonstrated. common characteristic of all three analytical techniques applied edxrf, k0-inaa and icp-ms is their multi-element capability. preparation of samples was simple and nondestructive in the case of edxrf and k0-inaa, meanwhile icp-ms required decomposition of samples. it is obvious that the most sensitive method applied in this study was icp-ms with lods in the range of few tens of ng/g. the sensitivity of edxrf and k0-inaa is comparable to each other, according to obtained estimated uncertainty (5-10 %) and lods for the analysed elements in the range from hundred to few µg/g. this means that lods of icp-ms were approximately two orders of magnitude lower compared to edxrf and k0-inaa. on the other hand, the determination of elements such as p, s, cl by icp-ms was impossible due to the fact that the ions formed by the icp discharge are typically positive ions, m+ or m+², therefore, elements that prefer to form negative ions (such as cl, i, f, etc.) are very difficult or impossible to determine by icp-ms. on the other hand, edxrf enables analysis of very important macro elements (p, s, and cl) in milk samples, meanwhile k0-inaa as an alternative analytical tool allows the determination of only cl. determination of p was impossible due to the production of pure negatron emitter 32p after activation. in the case of s, poor activation of 37s (short half life t1/2 = 5.05 min) resulted in high lods (3000-5000 µg/g). since the s content found in milk was in the concentration range of 2000-3000 µg/g, it was evident that determination of s by k0inaa was omitted due to its insufficient sensitivity. the advantage of k0-inaa in relation to edxrf is the determination of other important macronutrients mg and na. the determination of mg and na by edxrf was impossible due to insufficient instrumental sensitivity in concentration range from one to few thousand µg/g found in real milk samples. another disadvantage of the k0-inaa and edxrf techniques was their inability to perform the analysis of toxic elements such as as, pb and cd in the concentration range found in milk (table 2). further, considering the cost per sample, its multi-elemental capability, simple non-destructive sample preparation, the minimum number of steps in the measurement and quantification procedure, edxrf was undoubtedly the cheapest, simplest and most environmental friendly method among the applied analytical techniques and the most suitable for analysis of multi-elemental composition (p, s, cl, k, ca, zn, br, rb, sr) in milk samples. however, for the analysis of elemental content of mn, fe, cu, se and possible identification of pollutants such as as, cd and pb the icp-ms method was found as a method of choice due to its excellent sensitivity and accuracy. these two methods were also used for the determination of multi-elemental content in authentic slovenian raw cow milk. 3.4. multi‐elemental composition of slovenian milk  the elemental content of slovenian raw cow milk from different geographical regions collected in december 2013 is presented in table 5. the elemental composition is as follows k > ca > p, cl > s > zn > rb > br > fe > sr > cu> mn > table 2. determination of element concentrations in mg/kg in reference materials by icp‐ms, edxrf and k0‐inaa (erm‐bd 150 and erm‐bd 151).     erm‐bd 150  erm‐bd 151   element  certified value  found value  by edxrf  found value  by k0‐inaa  found value  by icp‐ms  certified  value  found value  by edxrf  found value  by k0‐inaa  found value  by icp‐ms  ca  13900 ± 800  13000 ± 1000  13487 ± 1013 ‐ 13900 ± 700  13200 ± 1100  13309 ± 1018  ‐ mg  1260 ± 100  ‐  1266 ± 117 ‐ 1260 ± 70  ‐  1268 ± 112  ‐ mn  0.289 ± 0.018  ‐  0.313 ± 0.101 ‐ 0.29 ± 0.03  ‐  0.306 ± 0.072  ‐ p  11000 ± 600  9100 ± 900  ‐  ‐ 11000 ± 600  9600 ± 900  ‐  ‐ k  17000 ± 700  16500 ± 1300  17665 ± 1263 ‐ 17000 ± 800  16200 ± 1300  17447 ± 1246  ‐ se  0.188 ± 0.014  ‐  0.201 ± 0.053 0.227 ± 0.022 0.19 ± 0.04  ‐  0.188 ± 0.046  0.224± 0.022 na  4180 ± 190  ‐  4397 ± 313 ‐ 4190 ± 230  ‐  4348 ± 308  ‐ sr  ‐  3.71 ± 0.3  ‐  ‐ ‐  4.10 ± 0.3  ‐  ‐ zn  44.8 ± 2.0  44.3 ± 3.5  46.7 ± 3.7  42.2 ± 2.1  44.9 ± 2.3  43.0 ± 3.4  46.1 ± 4.0  44.3 ± 3.2  cu  1.08 ± 0.06  ‐  ‐  1.05 ± 0.05 5.00 ± 0.23  ‐  ‐  5.24 ±0.33 cl  9700 ± 2000  9600 ± 900  10143 ± 743 ‐ 9800 ± 1200  9000 ± 900  10077 ± 737  ‐ cd  0.0114 ± 0.0029  ‐  ‐  0.0122 ± 0.002 0.106 ± 0.013  ‐  ‐  0.106 ±0.008 pb  0.019 ± 0.004  ‐  ‐  0.019 ± 0.003* 0.207 ± 0.014  ‐  ‐  0.206 ± 0.02 s  ‐  3300 ± 300  ‐  ‐ ‐  3300 ± 300  ‐  ‐ br  ‐  13.2 ± 1.0  ‐  ‐ ‐  13.6 ± 1.0  ‐  ‐ rb  ‐  17.7 ± 1.4  ‐  ‐ ‐  17.0 ± 1.4  ‐  ‐ acta imeko | www.imeko.org  april 2016 | volume 5 | number 1 | 19  se > pb > as > cd and is consistent with literature data. the content of the elements, such as cd, cu, fe, mn, ni, pb, sr and zn is influenced by feed and environmental conditions [30]. the concentrations of toxic elements such as as, cd and pb, are low and do not represent a threat to human health. multi-elemental composition of milk was used for possible discrimination among four slovenian geographical regions. results of statistical evaluation by lda are presented in figure 1. partial separation between groups was obtained, where alpine and pannonian groups were well separated, while dinaric and mediterranean groups were overlapping. the later two groups are close to each other and thus insufficiently separation was obtained; however their discrimination tendency is promising. the most important factors affecting the geographical origin were ca, s, p, k, and cl. in a cross validation test 66.7 % of the samples are classified correctly. the highest rate of classification was for the alpine and dinaric samples (78.6 % and 71.4 %, respectively). it is expected that more precise and efficient separation between four geographical regions could be obtained by the involvement of more analysed parameters such as stable isotope values of milk samples. table  3.  comparison  between  k0‐inaa  and  edxrf  methods  for  selected  macro‐ and  micro‐elements  in  milk  samples.  samples  exceeding  95 %  confidence interval of comparison are marked bold.     k0‐inaa  edxrf  sample  no.  br  ca  k  rb  zn  br  ca  k  rb  zn  dry weight mg/kg  1  14.0  7970  10969  12.1  28.8  13.9  8710  11200  13.2  30.4  2  11.4  8210  10595  22.1  18.9  11.7  9530  10600  23.3  21.5  3  9.04  7520  8706  6.85  23.3  8.87  7430  8210  7.98  25.2  4  7.71  8680  11800  12.6  31.7  7.85  9210  11600  14.0  33.3  5  7.77  8200  11110  10.3  29.2  8.38  8720  11600  8.85  30.9  6  8.32  8350  11300  12.9  28.5  6.97  8760  11200  14.2  29.9  7  8.10  7670  10390  11.9  27.3  8.38  8740  10600  12.5  28.7  8  7.59  8610  10880  11.5  26.7  8.27  10000  12100  12.9  29.8  9  8.34  8590  11140  16.6  26.6  7.74  9000  11300  19.3  29.6  10  11.4  8120  10980  9.33  29.2  11.4  8810  11300  8.3  30.6  11  8.22  8550  11332  14.0  29.0  9.74  8950  11900  15.0  31.8  12  14.6  8410  11170  12.5  28.6  13.2  9680  12600  11.9  29.6  13  9.12  8420  11230  20.3  27.7  7.93  9660  12400  20.3  29.8  14  16.0  6040  8595  11.1  23.0  15.9  7150  8610  11.4  23.0  15  10.9  7020  9898  22.3  23.6  14.3  6150  8070  28.1  30.9  16  14.1  9470  12251  14.1  29.8  14.0  9930  12600  13.3  30.1  17  16.6  9000  12635  22.8  31.2  17.4  9630  12900  23.6  29.6  18  9.44  6070  8113  9.72  20.7  12.5  6360  8320  15.4  26.0  19  13.7  7340  10140  15.7  25.6  13.8  8700  10700  16.5  26.1  20  27.5  7660  10332  14.8  23.2  28.2  8510  10900  14.9  26.1  table 4. results of the metallic contaminants in milk powder in interlaboratory schemes fapas in 2012, 2013 and 2014.      2012: fapas 07172  2013: fapas 07190  2014: fapas  element  assigned value  found   value  assigned  value  found  value  assigned  value  found  value    µg/kg  as  56.4 ± 12.4  45.2 ± 6.1  49.1 ± 10.8 53.5 ± 6.0 58.1 ± 12.8  53.8 ± 1.6 cd  18.59 ± 4.09  19.9 ± 5.8  16.2 ± 3.56 15.8 ± 1.0 12.2 ± 2.69  12.5 ± 0.8 pb  66.17 ± 14.6  51.8 ± 3.3  50.8 ± 11.2 47.8 ± 3.4 47.5 ± 10.5  49.3 ± 3.0   figure  1.  discrimination  between  geographical  regions  using  significant  elemental parameters. function 1 represents 72.43 % of variability, while  function 2 represents 19.91 % of variability.   label: a‐alpine, d‐dinaric, m‐ mediterranean, p – pannonian.  acta imeko | www.imeko.org  april 2016 | volume 5 | number 1 | 20  4. conclusions  there is a need for reliable milk element concentration data to provide information about nutritional uptake and at the same time to provide information about the excess amounts of trace and toxic elements. in addition, multi-elemental composition provides important information on the authenticity and geographical origin of food including milk and dairy products. this paper provides some interesting comparisons between three different techniques (edxrf, k0-inaa and icp-ms) in determination of multi-elemental composition in milk samples. quality assurance proved entirely satisfactory for all involved measurements and an intercomparison exercise between edxrf and k0-inaa showed satisfactory agreement. the simple, fast, and inexpensive edxrf method in combination with icp-ms, which was found the most appropriate technique for the analysis of elemental content of mn, fe, cu, se and toxic elements such as as, cd and pb, were further used for multi-elemental analysis of slovenian raw cow milk samples. multi-elemental content in milk samples was combined further in a linear discriminant model to discriminate milk according to geographical origin. only partial separation was possible using elemental content with overall predicting ability of 66.7 %. it is expected that if elemental data are used in conjunction with other characteristic chemical indices such as isotope analysis, a more holistic and accurate picture in relation to geographical region separation could be created. these data represent a basis for a database of authentic samples of milk in slovenia, which could be incorporated into a traceability system. acknowledgement  the research represents part of the era chair iso-food project and was partially supported by the iaea project under contract no. 17897 entitled “the use of stable isotopes and elemental composition for determination of authenticity and geographical origin of milk and dairy products” as part of crp d5.20.38 “accessible technologies for the verification of origin of dairy products as an example control system to enhance global trade and food safety”. references  [1] n. khan, i. s. jeong, i. m. hwang, j. s. kim, s. h. choi, e. y. nho, j. y. choi, k. s. park, k. s. kim, analysis of minor and trace elements in milk and yogurts by inductively coupled plasma-mass spectrometry (icp-ms), food chem. 147 (2014) pp. 220 – 224. 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[26] p. kump, some considerations on the definition of the limit of detection in x-ray fluorescence spectrometry, spectrochim. acta part b 52 (1997) 405 – 408. [27] e. barany, i. a. bergdal., a. schutz, s. skerfving, a. oskarsson, inductively coupled plasma mass spectrometry for direct multielement analysis of diluted human blood and serum, j. anal. atom. spectrom. 12 (1997) pp. 1005 – 1009. [28] a. r. byrne, low level simultaneous determination of as and sb in standard reference materials using radiochemical neutron activation analysis with isotopic 77as and 125sb tracers. fresenius z. anal. chem. 326 (1987) pp. 733 – 735. [29] a.r. byrne, a. vakselj, rapid neutron activation analysis of arsenic in a wide range of samples by solvent extraction of the iodide, croat. chem. acta 46 (1974) pp. 225 – 235. [30] y. j. tu, x. y. han, z. r. xu, y.z. wang, w. f. li, effect of cadmium in feed on organs and meat colour of growing pigs, vet. res. commun. 31 (2007) pp. 621 – 630. acta imeko  issn: 2221‐870x  june 2015, volume 4, number 2, 18‐22    acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 18  geometric part of uncertainties in the calculation constant of  the primary four electrode conductivity cell  alexander mikhal 1 , zygmunt warsza 2   1  institute of electrodynamics national academy of science, av. peremogy 56, 03680 kiev, ukraine   2  industrial research institute of automation and measurements piap, al. jerozolimskie 202, 02‐486 warszawa poland      section: research paper  keywords: conductivity; primary cell; geometric errors; standard deviation  citation: alexander mikhal, zygmunt warsza, geometric part of uncertainties in the calculation constant of the primary four electrode conductivity cell,  acta imeko, acta imeko, vol. 4, no. 2, article 4, june 2015, identifier: imeko‐acta‐04 (2015)‐02‐04  editor: paolo carbone, university of perugia, italy  received april 30, 2014; in final form july 13, 2014; published june  2015  copyright: © 2015 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: this work was supported by the presidium of the national academy of sciences of ukraine.  corresponding author: alexander mikhal, e‐mail: a_mikhal@ukr.net    1. introduction  in the recent years, the leading economically developed countries have established national standards of electrolytic conductivity (ес), the basis of which uses the “absolute (direct)” method of reproduction of a physical quantity [1]. world practice knows a lot of ways to implement this method; however, the principle of its operation is almost the same [2-4]. the “absolute (direct)” method is based on the measurement of the liquid column resistance and on the calculation of the ec by the known length of the column cross-sectional area. the basic components of the ec standard of ukraine are a four-electrode cell with the calculated constant, a special conductivity ac bridge, a thermostat for temperature control at 25ºс and a precision digital temperature meter. the ec is determined in accordance with the expression: 25(1 )k gk t   (1) where g is the conductivity of the liquid column obtained as a measurement result of the conductivity bridge; k is the cell constant obtained as a calculation result based on the actual geometric dimensions of the cell; α is the temperature coefficient for conductivity of potassium chloride solution; δt25 is the temperature deviation from 25ºс obtained as a measurement result using the digital thermometer. the uncertainty (here referred to as standard uncertainty type b only) of the national standard will be influenced by many factors. most significant errors are the ones in calculating the primary cell constant. error components being determined by the geometric parameters of the liquid column have the greatest influence. let us consider these error components on the example of the four-electrode conductivity cell which is used in the ec standard of ukraine [4]-[6]. the basis of the conductivity cell is the sensor element, a sketch of which is shown in figure 1. the sensor element is a tube with internal diameter d. the inside of the tube is filled with an electrolyte solution. typically, this is a solution of potassium chloride. the tube is used to fix the geometry of the liquid conductor and consists of three parts. the central part of the tube 1 has length l, two side abstract  the paper presents the construction of a primary four electrode conductivity cell with calculated constant for the ukrainian primary  standard of electrolytic conductivity (ec). the equations for calculating the cell constant and the error budget for calculating uncer‐ tainty are presented. the components of the budget are: errors due to the non‐uniformity of the force lines of the electric field; errors  due to the accuracy of measurement standards and measuring  instruments for determining  length and diameter of the tube; and  errors due to manufacturing techniques of tubes and their assemblage. the article considers in detail the errors due to the non‐ideal  profile of the central part of the tube. two methods to reduce the standard deviation are given: the method of linear interpolation for  compensation of the concave form which occurs along the axis of the tube and the method of equivalent triangles to compensate for  deviations from a circle that occur across the axis of the tube. acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 19  portions 2 have equal lengths l. the ends of the central portion of tube 1 are coated with circular potential electrodes 4. their width corresponds to the tube wall thickness. two discs 3 are fixed at the edges of the tube. the inner surface of the discs is coated with a metallic film 5 which performs the function of current electrodes. discs 3 have central holes 6 with diameter d which serve for filling with the liquid. the inner disc surface has the form of a cone with angle α. such configuration is intended to facilitate the removal of air bubbles when filling the cell with the liquid. the tube and the discs are made of quartz glass which has good insulating properties, temporal stability and a minimum coefficient of thermal expansion. platinum is used as the metal for the electrodes as it has a minimum polarization effect for most electrolytes. the cell in its four points a, b, c and d is connected to the bridge which ensures the measurement of conductivity of the liquid column. the cell constant is determined by calculating the ratio of the length of the tube 1 to the cross-section area. however, this definition is true for an idealized object of measurements with uniform distribution of current flow lines. distortion of such lines will be due to: the presence of holes for solution filling; the form of current electrodes and the presence of potential electrodes; and the non-ideal profile of the inner quartz tube 1, figure 1. therefore, the calculation of the constant will show errors. 2. budget of uncertainty cells  the error budget for the constant calculation of the cell, shown in figure 1, can be written as [7]: [ , , ( , )]k st cal geom tec peu f      (2) where: δst is an error due to the accuracy of measurement standards and measuring instruments that determine the length and diameter of the tube; δcal is an error due to the deviation of the calculation model for the cell constant in real conditions relative to the idealized model; δgeom is an error in assessment of geometrical dimensions. we do not focus our attention on the method of processing errors (function f), which is used in calculating the uncertainty. this is regulated by international guidelines [8]. each error in eq. (2) has in its turn a number of components. minimizing of the error δst is limited by the level of metrological assurance for measurements of tube length and diameter. it is defined by the metrological characteristics of the standards and instruments for length measurement. error δcal has two origins and two components accordingly: an error due to the alternating current measurement and an error due to the discontinuity of an electric field in the cell because of its finite dimensions and design features. the geometric error δgeom also has two components: δtec is an error due to manufacturing technology for the tube sections and their assemblage; δpe is an error due to the presence of the potential electrodes. the latter component of the error depends on the finite thickness of the potential electrodes and on changing position of a singular point of the potential electrodes upon assemblage. this component is related to the calculation of the electric field inside the cell. it will be considered in other papers. in this paper, we examine the component of error δtec. this error is due to the deviation of the actual profile of the inner surface of tube 1 (figure 1) from the ideal profile of the tube. the latter one is presented as a rectangle along the longitudinal section and as a circle in cross-section. it should be mentioned that the cost of tube production from a monolithic quartz crystal is extremely high. as a rule, tubes are manufactured from work pieces (preform) which undergo precision machining. if precision machining of the inner surface is too deep, the mechanical resistance of the tube will reduce significantly. tubes of less than 1 mm in thickness will crack (fracture) under elastic forces (adhesive polymerization, temperature differences). therefore, grinding of the work piece inner profile should be of minimum depth. on the other side, the work piece inner surface can have wedge-like cracks which are in parallel to the axis of the work piece. these cracks are due to manufacturing techniques of the work piece production and depend on the quality of nozzles through which the work piece is pulled. therefore, due to the lack of deep machining of tubes, we can observe deviations from a circle in cross-section along the entire profile. the second reason of a non-ideal profile may be the precession of the grinding tool. during processing, quality control of the tube is practically impossible. after final grinding, the tube profile may differ from the ideal rectangle. 3. error due to manufacturing  to determine the actual profile of the tube, its diameter and length should be measured according to the following algorithm. we performed р measurements of the tube length l in different directions. these directions were distributed uniformly around the circumference. conventionally, uniformly along the length, the tube is divided into m sections. to define the diameter, n measurements are made in the cross-section of each part of the tube in different directions. as a result, we obtain n×m measurements of the tube diameter and р values of the tube length. the constant can be determined from the measurement results through averageing the values of diameter dav and tube length lav. 2 4 av av l k d  (3) in modern technologies for processing quartz glass it is much easier to manufacture a tube with stable length value than a tube with stable internal diameter. from the experimental data we observe distortions of the internal profile of two types. the figure 1. construction of the primary conductivity cell.   acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 20  first one is a deviation of the tube profile from a rectangle along the longitudinal section due to precession of the grinding tool. the second one is a deviation of the tube profile from a circle in cross-section due to the presence of wedge-like cracks on the inner surface of work pieces. the geometric dimensions of the actually manufactured tube can be measured with much higher accuracy than the error of profile. parameter measurements are performed with a device with an lsb of 0.1 μm. random components of the device are negligible. the actual tube profile deviates from the ideal rectangle by more than 10 lsb of this device. therefore, the following expression can be taken as a metrological model for the constant calculation: 2 2 0 (1 ), 2k k d lk k        (4) where: k0 is the true value of the constant which is determined by the actual profile of the tube inner surface; δk is a systematic relative error of the constant calculation; k, d and l are standard deviations of the mean cell constant, diameter and length of the tube, respectively. therefore, an error due to manufacturing δtec has two components: a systematic δk and a random k. hereinafter, under such deviation we understand the standard deviation of the mean (sdm) of the diameter. the average diameter and the sdm of the diameter in each i-th section of the tube are determined from the expressions: , 1 11 m nm iji av i ji av dd d m m n       (5a) 2 1 1 ( ) ( 1) n id ij iav j d d n n       (5b) where: di,av is the mean diameter in the i-th section of the tube and dij is the diameter in the j-th direction and in the i-th section of the tube. the sdm value from the m section (cut) can be expressed as: 2 1 1 ( 1) m d id im m       (5с) 3.1. error in the longitudinal section  the measuring results for the mean diameter of one of the sections of the tubes for n=8 and m=10 are shown in figure 2. these data indicate that the average diameters in sections 1 and 6 differ by almost 20 μm. in general, the profile of the tube internal section can be expressed through an arbitrary function d(x). the measuring results for average diameters along the length of the tube (figure 2) show that this dependence has clearly a deterministic character. the discrete nature of the data allows us to use linear interpolation for the function d(x). the results are shown in the following formula: 2 2 00 0 4 4 ( ) ( ) ixl m i dx dx k d x ax b          (6) where: a and b are linear interpolation coefficients; xi the length of the region between the i-th and i +1 section. the polynomial coefficients are expressed as: 1i i i d d a x    (6a) ib d (6b) after simple transformations, the expression for calculating the constant with the proposed correction takes the form: 1 , 1, 4 m i i i av i av x k d d      (7) we can use equation 3 and 5, but then we get an increase in the standard deviation. let us show in one figure the graphs for the sdm of diameter measuring results with and without the deterministic component by using linear interpolation (figure 3). as it can be seen, the sdm of the diameter measurement results, taking into account the deterministic component, is close to almost one value and has a very small scatter of results compared with the case where the deterministic component is ignored and the average diameter value is calculated according to eq. (3). as a result of this correction, the change of the sdm along the cross-section is reduced by 10 to 15 times. the sdm of the diameter with correction, calculated according to eq. (5с), is less than 0.0004 mm. the same parameter without correction is 3 times larger. when calculating the random component of an error (eq. 4), we obtain an error reduction by two. it should be noted that such correction method shifts the average value of the diameter. thus, the constants calculated by eqs. (3) and (7) for tube 1 in figure 1 differ by 0.027 %. this is the rate by which systematic relative errors δk differ (4) when calculating the cell constant with and without correction. 3.2. error in the cross‐section  let us consider another tube for which the values of the mean radius in each of the sections are grouped along a virtually horizontal line. however, in each individual section the surface profile differs from a circle. example of the function for the deviation from the mean diav (figure represented by circle 0.5d3,av = 4.569 mm) in one of the sections with m = 3 is shown in figure 4. in all ten sections with m = 1 to 10 we observe triangular run outs along the lines 3 to 11 and 6 to 14 (figure 4). such   figure 2. influence of linear interpolation on the level of sdm (σid).   10,15 10,155 10,16 10,165 10,17 10,175 1 2 3 4 5 6 7 8 9 10 d i, a v m m section number, m figure 3. profile along the axis of the tube.  acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 21  character of profile distortions allows using a method of the equivalent triangles. this method involves the assessment of the effective area sief of the tube section and subsequent diameter corrections. the algorithm for calculation is to replace the diameter values in section 3 and 6 with the values of the mean diameter dav, then to use standard formula for calculating the basic mean diameter dbias and the basic area of the tube section sbias: 2( ) 4bias biass d (8) next, we calculate the correction value in each section separately. it is represented as areas of triangles: 2i i is c h (9) сi is the base of the assumed triangle in direction 3,11 or 6,14 (figure 4); hi is the height of the assumed triangle in the direction 3,11 or 6,14 (figure 4). we take into account the influence of the deterministic component by forming the effective area of each section which is expressed as: 3,6 3,6 2ief bias i bias i i i i s s s s ch        (10) according to eq. (11) we calculate the corrected dicor which is put in eqs. (2,3) instead of diav: 4icor iefd s  (11) the difference between sdm values for the measurement result of diameter diav before correction and sdm values for the mean diameter dicor after correction is shown in figure 5: as can be seen in this figure, the sdm with the correction of the deterministic component is 2.5 to 3 times less than without correction. the use of an algorithm of effective areas shifts the mean diameter value. the constants calculated by eqs. (5) and (11) differ by 0.015 %. just as in the previous case, this value represents the difference of systematic errors (3, 4) for the cell constant calculation. 4. conclusions  the above methods were used to calculate the corrections to the cell constant. for the primary ес standard of ukraine, several cell designs were made. the primary cell is shown in figure 6. correctness, sufficiency and adequacy of the all selected models for correction constant cells is confirmed by international comparisons p22, p47, k36, which involved primary standard of ukraine (laboratory ukrcsm). 14 laboratories of the leading nmi of the world participated in international comparisons ccqm-k36: usa (nist), germany (ptb), israel (inpl), slovakia (smu), denmark (dfm), and others. to all participants samples of potassium chloride solutions with a nominal electrolytic conductivity 0.5 s/m and 5 ms/m were sent. each laboratory using equation (1) and own metrological features establishes the exact value ec klab and uncertainty u(klab) conductivity samples. the rules of key comparison specify that a key comparison reference value must be derived against which the participants’ results are compared, and a degree of equivalence of each laboratory must be inferred. in [6] the model of reference value kref was presented. the degree of equivalence is given as:   figure 4. real profile (blue) across the axis of the tube for section m=3 and  0.5di,av=4.569 mm (red).     figure 6. the cells for the primary standard.   figure 7. the results of international comparisons ccqm‐к36.а [6].   0 0,0002 0,0004 0,0006 1 2 3 4 5 6 7 8 9 10d e v ia ti o n , m m s ection number m diav dicor figure 5. sdm (σid) without correction diav and with correction dicor. acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 22   /  lab refd sm m k k (12) the results of the degree of equivalence with uncertainty on the international comparisons ccqm-k36 are shown in figure 7. the proposed methods of correction constants cells (method of linear interpolation and method of effective areas) allowed us to solve two problems [7]. firstly, the value obtained by the ukrcsm klab, practically coincides with the value kref [6]. secondly, by minimizing the random component of the error (eq. 4, 5c), a minimum value of uncertainty u(klab) was obtained. acknowledgement  the authors express their gratitude to prof. m.n. surdu, as supervisor of researches [4, 5] and v.g. gavrilkin, as a scientist keeper of primary standards [5, 6], head of the department of metrological provision of physical and chemical quantitative measurement (lab. ukrcsm, kiev). references  [1] wu y.c., a dc method for the absolute determination of conductivities of the primary standard kcl solution from 0°c to 50°c, j. res. nati. inst. stand. tecnol., 1994, 99, №3, 241-246. [2] maґriaґssy m., pratt k.w., spitzer p., major applications of electrochemical techniques at national metrology institutes, metrologia 46 (2009) 199–213 [3] shreiner r.h., pratt k.w., standard reference materials: primary standards and standard reference materials for electrolytic conductivity, nist special publication 260-142, 2004 ed. [4] brinkmann f. and etc., general paper: primary methods for the measurement of electrolytic conductivity, accred qual assur. – 2003. – 8:346 – 353 doi 10.1007/s00769-003-0645-5. [5] gavrilkin v.g. and etc., state primary standard unit of electrical conductivity of liquids, ukrainian metrology journal, 2006, №3, p. 47-51. (ukr.) [6] jensen h d 2006 final report of key comparison ccqm-k36. 15 august 2006, available online at http://kcdb.bipm.org/appendixb/appbresults/ccqmk36/ccqm-k36_final_report.pdf [7] miкhal a.a., warsza z.l. influence of geometric uncertainties on the accuracy of calculated constant of the primary conductivity cell. 11th international symposium on measurement and quality control. imeko, poland, cracow-kielce, 11-13 september 2013, p.8 [8] guide to the expression of uncertainty in measurement, isbn 92-67-10188-9, 1st ed., international organization for standardization, geneva, switzerland, 1993. acta imeko  december 2014, volume 3, number 4, 17 – 25  www.imeko.org  acta imeko | www.imeko.org  december 2014 | volume 3 | number 4 | 17  thermal modeling and characterization for designing reliable  power converters for lhc power supplies  massimo lazzaroni 1,2 , mauro citterio 2 , stefano latorre 2 , agostino lanza 3 , paolo cova 4,3 , nicola  delmonte 4,3 , francesco giuliani 4   1  dipartimento di fisica, università degli studi di milano, via celoria, 16, 20133 milano, italy  2  infn milano, via g. celoria, 16, 20133 milano, italy  3  infn pavia, via a. bassi, 6, 27100 pavia, italy  4  dipartimento di ingegneria dell’informazione, university of parma, viale g.p. usberti 181/a, 43124 parma, italy  section: research paper  keywords: measurement; reliability; rams; hostile environment; lhc; atlas  citation: massimo lazzaroni, mauro citterio, stefano latorre, agostino lanza, paolo cova, nicola delmonte, francesco giuliani, thermal modeling and  characterization for designing reliable power converters for lhc power supplies, acta imeko, vol. 3, no. 4, article 5, december 2014, identifier: imeko‐acta‐ 03 (2014)‐04‐05  editor: paolo carbone, university of perugia   received october 11 th , 2013; in final form october 11 th , 2013; published december 2014  copyright: © 2014 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: this work was supported by apollo collaboration of infn, italy  corresponding author: massimo lazzaroni, e‐mail: massimo.lazzaroni@unimi.it  1. introduction the large hadron collider (lhc) is, nowadays, the world's largest and highest-energy particle operative accelerator, built by the european organization for nuclear research (cern). founded in 1954, the cern laboratory sits astride the franco-swiss border near geneva [1]. the lhc can be used in order to verify the predictions of different theories of particle physics and high-energy physics with particular attention to prove the existence of the higgs boson but also the large family of new particles predicted by super-symmetric theories. the aforementioned collider extends the frontiers of particle physics thanks to its high energy and luminosity [2]. it is important to underline that inside the collider high interaction rates, radiation doses, particle multiplicities and energies are present. all this pushes the features of the used electronic instrumentations and measurements equipment at new standards that are necessary to consider during the design phase. this is particularly true for the detector atlas (a toroidal lhc apparatus) and for every device involved in the experiments [1], [2]. in fact, electronic converters employed in power supplies for the lhc atlas experiments operate in a hostile environment, due to the simultaneous presence of radiation and high static magnetic field, which impose severe design constraints. moreover, abstract power supplies for lhc experiments (atlas) require dc‐dc power converters able to work in very hostile environments. the apollo  collaboration, funded by the italian istituto nazionale di fisica nucleare (infn), aims to study dedicated topologies and to design, build  and test demonstrators, developing the needed technology for the industrialization phase.  besides the presence of radiation and magnetic fields, thermal specifications are particularly stringent in the working environment. in  order  to  have  the  wanted  features  in  terms  of  reliability  and  availability  during  the  experimental  activity,  these  power  electronics  circuits must be cooled by specifically designed water heat sinks, and an accurate thermal design is mandatory in order to guarantee  safe and reliable operation. moreover, an optimized thermal design allows to have a maintenance strategy in compliance with the  requirements of the experiments.  in this paper thermal characterization is used for tuning a coupled thermo‐fluid‐dynamic 3d numerical model, for both the water heat  sink and the whole system. based on this model an optimized water heat sink was designed and fabricated. thermal characterization of  the power converter demonstrator in different operating conditions shows good agreement with simulation results.  acta imeko | www.imeko.org  december 2014 | volume 3 | number 4 | 18  the power supply has to be quasi-adiabatic, due to the proximity of very sensitive detection circuitry, and this requirement implies water cooling and very stringent thermal constraints [3], [4]. thermal management represents the key issue in designing these converters and accurate thermal modelling is mandatory to maximize the rams performances (reliability, availability, maintainability and safety) [5], [6]. in the present work accurate thermal modelling of the water heat sink is addressed, in order to design an optimized water path for a specific converter which will be possibly adopted in the next atlas power supplies. the phase-2 upgrade of the lhc is planned on 2022 long shutdown. the detectors will be upgraded or substituted, and new ones will be installed to improve the performances and sustain the higher rates and backgrounds at the new design luminosity: 51034 cm-2s-1. this increase of radiation background will cause the accumulation of a total ionizing dose (tid) up to 10 kgy in silicon, and fluences up to 21013 protons/cm2 and 81013 neutrons/cm2. both the new scenario and the increased power demand of the detectors, lead to re-design the power distribution system. a description of the developed 1.5 kw dc-dc converter can be found in [7] and a detailed thermal model of it, with identification of the main heating component is reported in [8]. the proposed power distribution system (figure 1) and the implemented main converter (mc) (figure 2), will be briefly described in section 2 whereas in section 3 a theory of reliability will be given. for further details see [7]. for ad-hoc water heat sink development, a fluid-dynamic model of the water path has to be coupled with thermal models of both liquid and solid domains of the heat sink. this is not an obvious task, since some parameters are known with large uncertainty and the models have to be tuned experimentally. these aspects will be discussed in section 4 in which the procedure followed for setting and tuning such models on the basis of thermal characterization of a known heat sink prototype will be described. moreover the thermal behaviour of the whole converter mounted on the heat sink has to be simulated. due to the complexity of the converter structure, which is made by many heating components, an established simplifying procedure has been followed to obtain an effective reduced converter thermal model [9]. on the basis of the numerical thermo-fluid-dynamic model the specific water heat sink was designed and manufactured, as described in section 5. section 6 shows the good agreement between thermal characterization of the whole system and simulation results. discussions concerning simulation results and the results obtained by means of a deep measurement activities will be given in section 7. finally, conclusions are given in section 8. 2. the power distribution and the main converter  an isolated dc-dc mc has to supply an intermediate medium voltage bus able to distribute the voltage to the electronics installed on the detectors. at the board level, point of load (pol) converters should be implemented on front-end boards (febs) for precise voltage adaptation and regulation as depicted in figure 1 [10]. the mc is designed starting by a modular approach in order to improve the overall reliability of the system and provide the requested redundancy as well described in the following section iii. the transient-resonant topology   figure 1. the proposed power supply distribution for atlas lar calorimeter  [10].     figure 2. the implemented main converter.     figure 3. the adopted switch‐in‐line topology.  acta imeko | www.imeko.org  december 2014 | volume 3 | number 4 | 19  adopted is based upon a switch-in-line-converter (silc) and the basic schematic is drawn in figure 3. the typical features for this topology can be summarized as: possible multiple outputs, low switch voltage stress, soft switching operation, first order dynamic, decreasing the sensitivity of power devices to ionizing radiations, limiting the overall power losses and, finally, the generated electromagnetic interference (emi) in compliance with lhc requirements [7]. as far as the galvanic isolation is concerned it is guaranteed by a dedicated planar transformer ad hoc designed. the new planar transformer has overall reduced dimensions and minimizes the iron losses. the use of both non-commercial magnetic material for the transformer core and the adoption of auxiliary windings in order to face the external magnetic field has been also investigated. in particular, cores in kool m� has been used [11]. in fact, kool m� powder cores, made with a ferrous alloy powder, have low losses at elevated temperatures. magnetic kool m� cores are made from a 85% of iron, 9% of silicon, and 6% of aluminum alloy powder in order to obtain low losses at elevated frequencies. the typical advantages of kool m cores are: high saturation (1.05 t), lower core loss than powdered iron, moderate cost, low magnetostriction, very high curie temperature, stable performance with temperature, variety of available shapes. the very interesting features of this material is that it is suitable to be used in the switching regulator where the most important and critical parameter of the switching regulator inductor is the ability to provide inductance, or permeability, also under dc bias. figure 4 depicts the proposed planar transformer. 3. an introduction on reliability  all the rams requirements are relevant to this case. in fact, reliability is defined as the ability of an item to perform a required function under given conditions for a given time interval, and, quantitatively, assuming that the item is capable of carrying out its required function at the beginning of a specific time interval. reliability is the probability of failure-free performance over the specified timeframe, under specified conditions [5]. moreover, availability is the ability to perform a required function under given conditions at a given time instant or over a given time interval, assuming that the required external resources are provided [5]. maintainability is the ability of an item under given condition of use, to be retained in, or restored to, a state in which it can perform a required function, when maintenance is performed under given conditions and using stated procedures and resources [5]. finally, it is important to underline that also in this kind of applications, safety (qualitatively defined as the absence of catastrophic consequences on the users and the environment in case of malfunction) is mandatory. it is very important, at this point, to highlight that for these devices maintenance must be correctly scheduled in order to be in compliance with the experimental activity: some types of maintenance can be, in fact, planned only during the shutdown periods. at this aim, the maximization of the parameters connected to the reliability, availability and safety must be taken into account. for example, some kinds of redundancies are mandatory, as described in the following. diagnostic-on-board tools can be, obviously, very useful in these kinds of applications. in fact, only appropriate diagnostic allows addressing the problem of maintenance between two different policies. in the first policy, called corrective maintenance (cm), the figure 4. the proposed planar transformer [7]. dimensions in mm.  (a)      (b)  figure 5. the functional configuration 2oo3 when: modules without failures (a), and with a module in fault state (b).   acta imeko | www.imeko.org  december 2014 | volume 3 | number 4 | 20  maintenance is performed after a system failure. in the second one, called preventive maintenance (pm), maintenance is performed before a system failure [12], [13]. in order to follow this second approach, policy diagnostic tools are necessary as aforementioned. not least, the thermal behaviour of the system has to be kept under control, in order to prevent components degradation. an accurate thermal modelling may be very useful not only for thermal sizing, but also to identify the most critical components and find the most appropriate placement of thermal sensors for diagnostics [12]-[17]. as far as the redundancy is concerned the applied approach is based on the functional configuration k out of n. redundancy is particularly useful in experiments where very high dependability features are mandatory. in particular, this is true if high reliability, availability and safety of equipment are requested. parallelism of items (devices or systems) in the reliability block diagram (rbd) does not mean automatically and necessarily parallel in the hardware block diagram. in the used configuration the redundant elements are subject to a lower load (load sharing) until one of the elements fails as a functional configuration denoted as warm redundancy (wr). in fact, in the final configuration three 1.5 kw main converter modules work together in order to deliver a power of 3 kw. the load is shared and each module delivers 1 kw during safe operations. if a module fails the remaining two modules share the full load (in fact, each module is able to operate up to 1.5 kw). this way to operate is known as the k out of n functional configuration that, in the present case, results in a 2oo3 configuration. in this particular type of redundancy operation is ensured if at least k (2) number of elements out of a total of n (3) elements are functioning normally. in figure 5 the designed situation has been drawn. many examples of such fault tolerant systems can be found: airplane with a multi-engine propulsion system, cockpit with a multi display system, etc. in order to evaluate the reliability of this configuration the binomial distribution is used. furthermore, we fix that the generic element of the system can assume only two conditions: correct functioning and failure. indicating with  r t the reliability of the generic element and with  sr t the reliability of the system, the reliability of the systems can be evaluated as [5]:                   1 n i n i s i k n r t r t r t i (1) where       ! !( )! n n i i n i . (2) assuming a constant failure rate                  1 n i n i t t s i k n r t e e i (3) the mean time to failure of the system can be immediately calculated as                0 (1 ) n n i t t n i s ii k mttf e e dt . (4) from the previous equations, it is interesting to recall that for  1k this configuration coincides with the parallel configuration, while for k n with the series configuration. 4. validation of thermo‐fluid‐dynamic fe model  a known water heat sink, made by milled aluminium, was used as a reference for thermo-fluid-dynamic 3d numerical model tuning. the thermal performances of this cooling device were characterized at various liquid flow rates and compared with simulation results in the same operating conditions. 4.1. experimental set‐up  in the application the converter must be thermally insulated from the surrounding electronic detection circuitry, operating in quasi-adiabatic condition, then almost all the heat dissipated by electronics must be collected by the heat sink and extracted by the water. for this reason a test bench was built, in which all the surfaces of the heat sink, except the back side, used for measurements, were thermally insulated by a teflon (a)    (b)  figure 6. 3d model of experimental set‐up for numerical model tuning (a)  and schematic of the test bench (b).   acta imeko | www.imeko.org  december 2014 | volume 3 | number 4 | 21  polystyrene box, as illustrated in figure 6 (a). in addition, figure 6 (b) shows a schematic view of the measurement setup. three power resistors provide heat input to the heat sink. power was regulated by a couple of tti cpx400a power supplies. measured parameters include inlet and outlet cooling water temperatures and the heat sink surface thermal map, obtained by two dedicated thermocouples in the connection ducts and an infrared camera (flir a325), respectively. in addition, three thermocouples are inserted in the thermal insulation box to monitor the resistor temperatures and prevent overheating. all the thermocouples were k-type. tests were carried out by changing inlet water flow rate and electric power in the resistors. for each value of flow rate and heating power, data were collected after steady-state conditions were achieved. the steady-state conditions were assumed to be reached when the change of the maximum temperature reading was less than 0.2 °c within 25 min. 4.2. numerical model tuning  figure 7 (a) shows the geometry of the reference model and the water velocity field inside. we used and compared two commercial software packages: comsol multiphysics and ansys-fluent. the detailed flow field and heat transfer inside the heat sink were investigated by the computational fluid dynamics (cfd) method, which combines the governing equations for the fluid flow with the heat convection in fluid and the heat transfer in solid equations [18]. the relatively small inlet flow rate of the studied case (around 1 l/min) suggests water laminar flow. this preliminary hypothesis was made using the reynolds number and was confirmed by simulation results. figure 7 (b) shows some of the parameters set in the model (due to the symmetry, only half of the structure was modelled). the uniform heat flux on the resistors’ contact surface is equivalent to their total dissipated power (560 w). the lateral surfaces were set as adiabatic, while on the top surface natural horizontal air convection has been considered. figure 8 shows an example of simulation results (comsol) for a dissipated power of 560 w and a delivery (a)        (b)  figure 7. geometry of the reference water heat sink (a) and fe model, with  indication of boundary conditions (b).  (a)    (b)    (c)  figure  8.  (a)  steady  state  simulated  thermal  map  (comsol)  for  the  reference heat sink (half structure) at operating conditions of figure 7 (b);  (b)  ir  temperature  map;  (c)  measured  and  simulated  (ansys)  heat  sink  surface temperature along the black horizontal lines of figure 8 (b).  22.5 23.5 24.5 25.5 26.5 27.5 0 50 100 150 200 250 t em p er at u re [ °c ] position [mm] ir measurement ansys-fluent simulation acta imeko | www.imeko.org  december 2014 | volume 3 | number 4 | 22  of 1.3 l/min. maximum temperature was around 30 °c on the heat sink and 60 °c on the resistors. the temperatures measured by the two thermocouples at the coolant inlet and outlet, and the temperature across the top surface estimated by infrared thermography were used for model validation. as fitting parameter the heat convection coefficient h at the air-exposed boundary were set. this parameter can be estimated by a rule of thumb, but it is very hard to evaluate its actual value because it depends on many others parameters and/or operating conditions, so, to obtain good results from simulations, we tested different values to find the one that allows obtaining the best fit. here, h was set to around 7 w/(m2k), a typical value for natural air convection. by comparing the simulated thermal map of figure 8 (a) with figure 8 (b), which reports the corresponding ir measurement, a good agreement appears. this can be better appreciated in figure 8 (c), which shows the comparison between temperatures along the symmetry line on the heat sink top surface. the matching is also good for water temperatures at the inlet (both 18.8 °c) and outlet (measurement: 24.8 °c, ansys simulation: 25.1 °c). 5. optimized heat sink design  the developed model was exploited to design a specific water heat sink, able to comply with constraints, which were: inlet and outlet on the same short side; thickness 15 mm, water path diameter 5 mm; flow rate 0.63 l/min, maximum pressure drop 350 mbar;  18 cinlett ; maximum  25 coutlett . the heat generation in the converter is not uniform, but as a first approximation, it can be divided in four main regions where heating takes place: the planar transformer region, the primary and the secondary regions and the auxiliary power supply region [6]. in order to easily evaluate the performance of different water paths we simulated the heat sink alone, by modelling the inward heat flux from the power electronics as four different uniform heat fluxes, as shown in figure 11 for a delivered power  1.5 kwoutp . with these boundary conditions different topologies were examined in order to enlarge as much as possible the heat exchanging surface area between solid and fluid in the region were the input heat is higher [19]. the chosen design is illustrated in figure 10 (a). finally, figure 10 (b) shows the thermal map simulated with this structure at the surface in contact with the power converter delivering 1.5 kw. 6. full converter modeling and characterization  the power converter was modelled by the simplified blocks method [8], [9] and the thermal behaviour of the assembly made by the converter mounted on the designed heat sink was simulated. the heat sink was fabricated by milling an aluminum plate and the aluminum cover plate brazed on it. the converter was mounted on the heat sink ensuring optimal thermal contact by a thin layer of thermal paste, and uniform pressure was guaranteed by screws. the whole assembly was tested at different operating conditions by a test bench able to fix and measure all the figure 9. the 2‐c heat sink.    (a)                                                                                                                          (b)  figure  10.  water  velocity  field  in  the  designed  heat  sink  (a)  and  thermal  map  simulated  at  the  surface  in  contact  with  the  power  converter  operating  at pout = 1.5 kw (b).    figure 11. boundary conditions on the heat sink surface in contact with the  converter, in case of pout = 1.5 kw.  acta imeko | www.imeko.org  december 2014 | volume 3 | number 4 | 23  operational parameters. in particular: a. differential inlet/outlet water temperature by means of a thermometer based on k-type thermocouples (0.1 °c of resolution) at the input and at the output of the heat sink. the minimum differential measured temperature was 1.7 °c and the maximum differential measured temperature was 2.7 °c. the mean value during the experimental activity was about 2.2 °c. both the water temperature measured during the tests and the differential inlet/outlet water temperature are in compliance with the constraints above reported:  18 cinlett ; maximum  25 coutlett . b. differential inlet/outlet water pressure by means of two pressure sensors type pse560-02 from smc installed near the input and the output of the heat sink. the pressure is measured using an interface type pse200-ma4c from smc (+/0.5% f.s.). the differential inlet/outlet water pressure during the experimental activity was stable at 210 mbar. this value is in compliance with the specification aforementioned. in fact, a maximum pressure drop of 350 mbar is allowed. c. water flux by a water digital flux-meter type pf3w704-f03-btn-m from smc. the nominal range of the adopted sensor is 0.5 4 l/m (+/3% f.s.). during the experimental activity the reading of the water flux was stable at a mean value of 0.6 l/m. also in this case, the measured value complies with constraints concerning a water flow rate 0.63 l/min, as aforementioned. d. temperature map by a flir a325 infrared thermocamera. all the reflecting surfaces were painted black, in order to obtain an almost homogeneous emission coefficient. e. temperature of the most heating components by means of k-type thermocouples and pc-driven switching unit. figure 12 shows, as an example, the comparison between simulation and measurement in the case of the converter delivering  1.2 kwoutp . in figure 13 the experimental setup has been depicted. the positions of the thermocouples are depicted in figure 14 whereas in table 1 the function of every thermocouple is finally described.   (a)                                                                                                                   (b)  figure 12. thermal map of the power  converter  mounted  on  the  designed heat sink at operating conditions: pout = 1.2 kw, tinlet = 18 °c, tamb = 21 °c. (a)  simulation; (b) ir measurement.   figure 13.the experimental setup.    figure 14. thermocouples position inside the mc.  acta imeko | www.imeko.org  december 2014 | volume 3 | number 4 | 24  7. discussion  complex systems operating inside the lhc must comply with very stringent rams requirements. one of the key issues is the thermal behaviour of electronic converters in power supplies. an accurate thermal modelling, accounting for conduction, air convection and fluid-dynamic heat transport, drives to the knowledge of the most critical components and conditions in the system, allowing the design of an optimized water heat-sink. moreover, we asserted the importance of performing preventive diagnostics. this is true even at the design stage. at this time the identification of the most appropriate positions for temperature sensors able to detect component degradation at the early stage is mandatory. in this way, with the support of the developed thermal model, it is possible to identify and localize thermal and/or thermomechanical degradation modes, and it is possible to take proper corrective actions and even to substitute power modules which show component degradation, when maintenance interruptions are scheduled. it is important to highlight that predictive diagnostics can also put out design errors or weaknesses of the design. for example, the planar transformer proved to be the weakest point only after the thermo-graphic analysis and simulation of the complete main converter. in fact, in the design phase with electronic simulation tools (such as spice) it is not always possible to put in full light the aspects of thermal dissipation. finally, it is important to highlight that predictive diagnostics can therefore lead to design reviews with significant savings in the prototyping phase. 8. conclusions  after a brief description of an isolated dc-dc mc able to supply an intermediate “medium” voltage on a distribution bus for atlas (lar), a coupled thermo-fluiddynamic 3d model of a water heat sink for power electronic converters has been developed and tuned by thermal characterization of a known prototype. the model was used for designing a suitable solution for a specific complex dc-dc converter for application in lhc future experiments, with very stringent thermal constraints. the heat sink was fabricated and thermal measurements performed on the converter mounted on it show a good agreement with thermo-fluid-dynamic simulations of the whole assembly. the proposed cold plate can be utilized with the mc in order to obtain the wanted delivered power. considerations about dependability of the proposed mc have been presented. in particular, the reliability of the adopted solution has been discussed. finally, both diagnostic aspects and maintenance strategies in compliance with the experimental activity have been discussed. acknowledgement  the authors would like to thank the other members of the apollo collaboration, founded by italian istituto nazionale di fisica nucleare (infn), for their fruitful support [20]. references  [1] http://home.web.cern.ch. [2] g. aad et al., the atlas experiment at the cern lhc, journ. of instrumentation 3 (2008) s08003. [3] j. bohm, j. stastny, v. vacek, cooling performance test of the sct lv&hv power supply rack, atl-indet-pub2006-004, nov. 2005. [4] p. tenti, g. spiazzi, s. buso, m. riva, p. maranesi, f. belloni, p. cova, r. menozzi, n. delmonte, m. bernardoni, f. iannuzzo, g. busatto, a. porzio, f. velardi, a. lanza, m. citterio, c. meroni, power supply distribution system for calorimeters at the lhc beyond the nominal luminosity, journ. of instrumentation 6 (2011) p06006. [5] m. lazzaroni, l. cristaldi, l. peretto, p. rinaldi and m. catelani, reliability engineering: basic concepts and applications in ict, springer-verlag, berlin heidelberg, 2011, isbn 978-3-642-20982-6, e-isbn 978-3-642-20983-3. [6] iso 9000:2005, quality management systems fundamentals and vocabulary. [7] m. alderighi, m. citterio, m. riva, s. latorre, a. costabeber, a. paccagnella, f. sichirollo, g. spiazzi, m. stellini, p. tenti, p. cova, n. delmonte, a. lanza, m. bernardoni, r. menozzi, s. baccaro, f. iannuzzo, a. sanseverino, g. busatto, v. de luca, f. velardi, power converters for future lhc experiments, journ. of instrumentation 7 (2012) c06012. [8] p. cova, n. delmonte, thermal modeling and design of power converters with tight thermal constraints, microelectronics reliability 52 (2012) pp. 2391-2396. [9] p. cova, n. delmonte, r. menozzi, thermal characterization and modeling of power hybrid converters, microelectronics reliability, 46 (2006) pp. 1760-1765. [10] s. baccaro, g. busatto, m. citterio, p. cova, n. delmonte, f. iannuzzo, a. lanza, m. riva, a. sanseverino, g. spiazzi, reliability oriented design of power supplies for high energy physics applications, microelectronics reliability 52 (2012) pp.2465–2470. [11] http://mag-inc.com/products/powder-cores/kool-mu/learnmore-kool-mu. [12] l. cristaldi, m. faifer, m. rossi, s. toscani, m. lazzaroni, “condition based maintenance through electrical signature analysis: a case study”, i2mtc international instrumentation and measurement technology conference, may 3-6, 2010, austin, texas, usa, pp. 1169 – 1174. [13] l. cristaldi, m. faifer, m. lazzaroni, s. toscani, an inverter-fed induction motor diagnostic tool based on timedomain current analysis, ieee trans. on instrumentation table 1. thermocouples position.  thermocouple  position  1  toroidal inductor  2  mosfet drain flange at primary  3  planar transformer core (inner point)  4  planar transformer windings (inner point)  5  isotop diode flange for heat‐sink at secondary  6  ambient (not shown in figure 14)    acta imeko | www.imeko.org  december 2014 | volume 3 | number 4 | 25  and measurement 58 (2009) pp. 1454-1461. [14] l. cristaldi, m. lazzaroni, a. monti, f. ponci, f.e. zocchi, “a genetic algorithm for fault identification in electrical drives: a comparison with neuro-fuzzy computation”, imtc instrumentation and measurement technology conference, may 18-20, 2004, como, italy, pp. 1454-1459. [15] l. cristaldi, m. lazzaroni, a. monti, f. ponci, a neurofuzzy application for ac motor drives monitoring system, ieee trans. on instrumentation and measurement 53 (2004) pp. 1020-1027. [16] a. azzini, l. cristaldi, m. lazzaroni, a. monti, f. ponci, a.g.b. tettamanzi, “incipient fault diagnosis in electrical drives by tuned neural networks”, imtc instrumentation and measurement technology conference, april 24-27, 2006, sorrento, italy, pp. 1284-1289. [17] l. cristaldi, m. faifer, m. lazzaroni, s. toscani, “a vi based tool for inverter fed induction motor diagnostic”, imtc instrumentation and measurement technology conference, may 12-15, 2008, victoria, canada, pp. 15601565. [18] j.h. ferziger and m. perìc, computational methods for fluid dynamics, springer, 2002, isbn-10: 3540420746, isbn-13: 978-3540420743. [19] p. cova, n. delmonte, f. giuliani, m. citterio, s. latorre, m. lazzaroni, a. lanza, thermal optimization of water heat sink for power converters with tight thermal constraints, microelectronics reliability 53 (2013) pp. 17601765. [20] http://www2.pv.infn.it/~servel/apollo/index.html. template for an acta imeko event paper acta imeko issn: 2221-870x december 2016, volume 5, number 4, 88-99 acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 88 comparison of the performance of a microwave-based and an nmr-based biomaterial moisture measurement instrument petri österberg1,2, martti heinonen3, maija ojanen-saloranta3, anssi mäkynen1 1university of oulu / cemis-oulu, kehrämöntie 7, fi-87400 kajaani, finland 2kajaani university of applied sciences, kuntokatu 5,, fi-87101, kajaani, finland 3vtt technical research centre of finland ltd, centre for metrology mikes, tekniikantie 1, 02150 espoo, finland section: research paper keywords: moisture measurement; bioenergy; microwave; nmr; loss-on-drying citation: petri österberg, martti heinonen, maija ojanen-saloranta, anssi mäkynen, comparison of the performance of a microwave based and an nmrbased biomaterial moisture measurement instrument, acta imeko, vol. 5, no. 4, article 14, december 2016, identifier: imeko-acta-05 (2016)-04-14 section editor: paul regtien, measurement science consultancy, the netherlands received may 31, 2016; in final form november 15, 2016; published december 2016 copyright: © 2016 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: this work was funded by the research grant from the european metrology research programme (emrp-reg). the emrp is jointly funded by the emrp participating countries within euramet and the european union corresponding author: petri österberg, e-mail: petri.osterberg@measurepolis.fi 1. introduction the loss-on-drying (lod) method is conventional and is still the only standardized method of measuring the moisture content of biomaterial load. to determine the moisture content of the load, a sample of at least 300 g is taken from the biomaterial load of a truck or trailer according to an appropriate sampling standard [1]. the sample is milled to reduce the maximum particle size to meet the requirements of a sample preparation standard [2]. finally, the moisture content of the biomaterial sample is determined according to the standardized lod method [3], that is, by weighing the sample and then drying it for several hours in an oven at a temperature of 105 ± 2 °c and performing final weighing after the moisture of the sample has been fully removed (i.e. after the mass change of the sample is below the detection limit given in the standard). however, the drying time should not exceed 24 hours. after the final weighing, the moisture content of the sample is calculated as the ratio of the mass change to the mass of the original sample. the lod procedure is too slow for efficient process control, but if not in all countries, at least in several countries it is still the only standardized moisture measurement method for the moisture-content determination of biomaterial delivery lots and is therefore used mostly in invoicing. in a typical case at a power plant, the lod-based moisture content estimate for the biomaterial load is available from two days after delivery. to overcome the problem of the slowness of the lod method, novel rapid moisture measurement methods have challenged the conventional oven drying method. in this research, we studied two of these new methods, namely the nuclear magnetic resonance (nmr)-based moisturemeasurement solution developed by valmet ltd, called the mr abstract this article compares the performance of an nmr-based and a microwave-based moisture measurement instrument designed for biomaterials. the conventional moisture measurement method, loss-on-drying, serves as a reference measurement for both instruments. six different biomaterials at three moisture content levels were measured with the microwave instrument and five biomaterials were measured with the nmr instrument. after instrument calibrations, the difference and variation of the measurement results for parallel samples and the repeatability of measurement results obtained by the nmr and microwave instruments were estimated. reasonable agreement between the measurement methods was achieved. acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 89 moisture analyzer, and the microwave-based moisture measurement solution developed by senfit ltd, called the bma desktop. the calibration of the nmr instrument is very simple: it is done with a water container, first empty and then filled, whereas the microwave instrument calibration is more complicated: it must be calibrated separately for each biomaterial section with the reference lod method. however, the microwave instrument is more rapid than the nmr instrument: a single measurement takes only ten seconds, whereas an nmr measurement takes two minutes. still both are very rapid in comparison with the conventional method: the lod-based moisture content estimate takes at least 16 hours. however, new research results are urgently needed to demonstrate the performance of novel instruments designed for delivering moisture data for biomaterial invoicing. this article is constructed using the research and data published and presented at two scientific conferences [4], [5]. the article describes a comparative study of two novel alternative instruments designed to mostly or fully replace the use of the conventional oven drying method. the benefits and drawbacks of the nmr and microwave instruments and their suitability for power plants are compared and discussed. during our research, swedish and canadian researchers published an article [6] in which they described the testing of the metso (now renamed as valmet) mr moisture instrument with forest-based biomaterial samples with different moisture contents in both countries. in comparison with our measurements, the measurement difference from the oven drying was smaller in fridh and eliasson’s results in sweden but larger in volpé’s results in canada, and the repeatability of the measurements was also worse in the research results obtained in both countries. in the article [6], the researchers used four to five moisture classes per material, whereas in this article three moisture classes but more sample materials and more samples per material and per moisture class were measured. one of the swedish authors of the previous publication, fridh, also published a working report on the nmr instrument for skogforsk at the same time [7]. in comparison with reference oven drying, the difference and standard deviation given by fridh’s results were slightly smaller than in this article. in addition to these, some publications of the moisture measurement instrument prototypes using nmr technology are available [8], [9]. only a few publications about commercial biomaterial moisture measurement instruments based on microwave technology could be found during the research. two theses written in finnish described the performance of corresponding senfit bma desktop microwave instruments [10], [11], but they researched the performance with selected biomaterials in a restricted moisture range, namely within the range of moisture contents at delivery. the research described in this article is part of a european project, metefnet [12], which concentrates on the creation of unambiguous si (système international d’unités i.e. the international system of units) traceability chains for measurements of moisture in solid materials. section 2 describes the solid biofuel sample materials, sampling, and sample preparation procedures used during the research. in section 3 we will present the research instruments. instrument calibrations are described in section 4. section 5 describes how the moisture measurement process with all the three measurement methods – the nmr-instrument, the microwave instrument, and the reference lod method – is performed. in sections 6 and 7, measurement results are presented for the nmr instrument and the microwave instrument respectively. the discussion in section 8 compares the performance of the measurement instruments. finally, we conclude the article and the future plans are discussed. 2. biomaterial samples and sample preparation the biomaterial samples for the research were collected from two locations: from a nearby sawmill and from lorries transporting solid biofuel loads to a local power plant. six different biofuel sections presented in figure 1 were chosen: 1. sawdust from a sawmill 2. bark waste from a sawmill 3. chipped pruning residues 4. chipped small-sized trees (mixed with sawdust) 5. crushed and chipped stumps 6. milled peat however, milled peat samples were not measured with the nmr instrument but only with the microwave instrument. this was due to the possible ferromagnetic components in peat and is discussed more detailed at the end of section 3: ‘research instruments’. two measurement sessions were arranged, but the microwave instrument was available only for the first session. the nmr instrument was available for both sessions. for the first measurement session in october 2014, bark waste and pruning residue samples were stored for several months in a pile before measurement so that the original greenish colour of bark samples changed fully to grey/brown and the green needles in pruning residue samples dried and dropped away. for the second measurement session in may 2015, bark waste samples and pruning residue samples were fresh and collected only some hours after debarking of the logs and delimbing of the trees. for both measurement sessions, sawdust samples were fresh and stumps were stored for several months in a roadside storage area before processing and measurement. chipped small-sized trees were not delivered to the power plant during the second measurement session and this sample material could be measured only during the first measurement session. about 50 l of each selected biofuel section was collected. each biofuel section was taken from a single location of the same truckload or storage pile to ensure the similarity of parallel samples for the research. this was the opposite of the normal sampling procedure, in which a biofuel sample is collected from several locations of the load or pile to ensure that the sample represents the whole load or pile. all biofuel sections were measured at three different moisture levels. thus the 50 l section samples were divided into three parts. the first part was measured at the moisture content present at delivery. the second part was spread in a shallow empty pool for drying at room temperature for three to five days depending on the initial moisture and the drying speed of each biomaterial. the third part of the biofuel sample was placed in a water tub and additional water was inserted at the beginning to increase the moisture content of the sample. the amount of water added was determined according to the moisture content at delivery of each biofuel section. the sample was stored for two days in the water tub with a lid. after the two-day moistening period, the free water was removed from the moisturized biomaterial. altogether, there were 15 different biofuel sample sets for the nmr instrument research and 18 acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 90 sets for the microwave instrument research, that is, five biofuel sections at three moisture-content levels for the nmr instrument and six biofuel sections at three moisture-content levels for the microwave instrument. samples were milled to meet the particle size requirements of the sampling standard for the oven drying method [2] (the particle size must be smaller than 31 mm × 31 mm × 31 mm for oven drying). before a measurement session, each biomaterial sample of ca. 15 kg at three moisture levels was mixed separately in a cement mixer to obtain homogenous sample material. homogenized sample material was divided into ~1 kg subsamples in plastic bags. the plastic bags were closed so that as little air as possible remained in them. before the moisture content measurement, the biomaterial subsamples were stored for one night at room temperature to obtain the constant temperature and moisture content of the biomaterial samples inside the plastic bags. the number of samples was from 6 to 20 pieces for each biomaterial and moisture level per instrument, depending on the biomaterial and moisture content (6 to 13 samples for the microwave instrument and 8 to 20 samples for the nmr instrument). altogether the moisture contents of 398 biomaterial samples were measured in this study with the nmr moisture measurement instrument and 191 samples with the microwave one. corresponding lod reference measurements were also made for the samples (589 lod measurements). additionally, some of the samples were measured several times to test the repeatability of the nmr and microwave instruments. the volume of biomass samples for the nmr instrument was always 0.8 l, but the sample weight varied between 161.4 and 492.1 g, whereas the sample weight for the microwave instrument was always 400 ± 1 g, but the sample volume varied according to the material and moisture content. 3. research instruments four ovens were used as the reference moisture measurement instruments to dry out the biomaterial samples during the measurement session. all the ovens were made by termaks [13], and they included two ts4115 models and two newer models, ts8056 and ts8136. a single oven can simultaneously dry 6 to 18 biofuel samples of about 400 g depending on the oven model and measures of sample containers. precisa ep 2220m and sartorius cp4202s scales were used in the weighing of the lod samples. two valmet mr moisture analyzers [14] were used as the nmr-based instrument in this study (see the left part of figure 2). in the first measurement session, an earlier version of the same instrument was used; it was called the metso mr moisture analyzer prior to the company’s demerger. the moisture content measurement range of the instrument is 0–90 %. the instrument measures biomaterial samples in the 0.8 l measurement container seen in the image on the right side of figure 2. the container is inserted in the measurement chamber and the measurement of the moisture content of a sample takes two minutes. the measurement chamber is located inside a magnet, and thus samples having ferromagnetic components may cause problems during the measurement. for example, peat samples may contain ferromagnetic components and thus the instrument’s manufacturer does not recommend their use as measurement samples. senfit bma desktop [15] was used as the microwave-based instrument in this study (see the left part of figure 3). the moisture content measurement range of the instrument is 0–70 %. the instrument was tuned for a 400 ± 5 g biomaterial sample placed in a plate-shaped measurement bowl seen on the right side of figure 3. the bowl is inserted in the measurement chamber and the moisture measurement of the sample takes figure 1. solid biofuel samples from upper left corner: sawdust, non-fresh bark waste, non-fresh pruning residue, chipped small-sized trees, crushed and chipped stumps, and milled peat. acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 91 about ten seconds. the particle size of the sample should be no more than 31 mm × 31 mm × 31 mm; thus all the samples were milled with a senfit sample mill [16] to obtain the appropriate particle size. 4. instrument calibrations before the start of the measurement session, the drying ovens were tested and calibrated against a calibrated thermometer to ensure that they met the requirements of the standard [3]. the temperature must stay between 103 and 107 °c during the drying time, that is, at least 16 hours. the maximum drying time is 24 hours according to the standard. the calibration of the nmr moisture measurement instrument is very simple. one 0.8 l sample container was filled with water, closed with a lid, and stored for a day in a measurement room so that the water temperature equalled the temperature of the sample stored in the measurement room. another sample container was left empty. every morning before the actual measurements, these two containers were measured to calibrate the nmr instrument. the calibration water in the container was not changed during the measurement session. the calibration procedure took about five minutes. careful calibration of the microwave moisture measurement instrument is essential to achieve reliable measurement results. the calibration of the microwave instrument is done with calibration samples whose moisture content has been determined using the lod method. the closer the properties of the calibration material are to the properties of the sample material, the better the measurement results that are achieved. the calibration of the microwave instrument should be supplier-specific for every biomaterial section and should be done prior to the biomaterial moisture measurements. although the measurement range of the microwave instrument is a moisture content of 0 to 70 %, the instrument must be calibrated for the moisture content ranges of 0–15 % and 15– 70 % separately for all material sections. however, solid biofuels drier than 15 % are very rare at combustion plants. the measurement range in this research was planned to be 15–70 % and the microwave instrument was calibrated for this range. the calibration was carried out by using calibration samples taken from the same truck load and location as the measurement samples. two calibration samples were picked out from each sample material set and moisture level after milling (to meet the particle size requirements of the standard) and mixing the biomaterial. thus the calibration curve was derived from six calibration points for each biomaterial (two samples at three moisture levels). we also tested a calibration case that is weaker, but still possible in real life at power plants. according to the instructions given by the instrument manufacturer personnel, two samples of each material were dried or moistened to four moisture levels so that there were eight calibration points for each biomaterial section to create a calibration curve for the microwave instrument. unlike in the previous calibration, the calibration samples were collected one to three weeks before carrying out the measurement procedure on the research samples. thus in this case the calibration samples were taken at least from different loads, but likely from a different forest stand or roadside storage area too. for five out of the six materials, the driest calibration samples had moisture contents slightly below 15 % (the final moisture content was between 9 and 12 %) and thus the calibration curve was slightly distorted at the drier end. additionally, two of the six biomaterial suppliers (suppliers for chipped small sized trees and milled peat) changed between the collection of the calibration samples and the measurement session. these factors probably decreased the measurement accuracy considerably, in spite of the fact that the measurement samples were visually the same as the calibration samples. 5. moisture measurements during the research, two moisture measurement sessions were arranged. the first measurement session was performed in autumn 2014 and the second in spring 2015. the nmr instrument was available for both sessions and the microwave instrument was available for the first measurement session. additionally, all the samples were measured with the lod reference method. the nmr instrument was changed for the second measurement session, but it was similar to the instrument used in the first measurement session. before the measurements, the biomaterial samples of ca. 1 kg were stored overnight in plastic bags at room temperature to obtain a constant temperature and moisture content inside the plastic figure 2: valmet mr moisture analyzer. left: nmr moisture measurement instrument; right: measurement container of the instrument with sawdust sample. acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 92 bag. the measurement procedure of a single sample started by shaking a plastic bag to mix the sample and the bag was emptied onto a clean table. the sample was divided into two parts. the first part was a sample of 400 g ± 1 g, which was weighed and placed in the measurement bowl of the microwave instrument seen on the right side of figure 3. the microwave moisture measurement takes about ten seconds and the instrument saves the measurement data. the instrument measures the sample temperature using an ir-sensor and tunes the microwave moisture measurement result based on the temperature of the sample. the second sample part is placed in the 0.8 l nmr instrument sample container seen on the righthand side of figure 2. the nmr instrument weighs the sample, measures the moisture content of the sample in two minutes, shows the result on the screen, and saves the measurement data. after the microwave or nmr moisture measurement, the biomaterial sample was placed in a metal container, weighed, and inserted in the oven for the reference moisture measurement with the lod method. the drying time was chosen as 23–24 hours, ensuring at least 16 hours heating at 105 °c to meet the requirements of the standard [3]. due to the long drying time and large number of samples, several ovens were used to achieve an efficient operation. after the drying period, the sample was weighed again. the obtained mass loss represents the vaporized moisture from the sample. finally, the moisture content estimate for the sample was calculated by dividing the mass loss by the initial sample mass. the repeatability of the microwave and nmr instruments’ results was tested by repeating the measurement of the same sample five times. the moisture contents of 18 (with the microwave instrument) and 15 (with the nmr instrument) different biomaterial samples (six and five biomaterial sections, respectively, at three moisture levels) were measured five times each. the sample was removed and placed back in the measurement chamber of the instrument between separate measurements. the time gap between consecutive nmr measurements was at least five minutes to avoid warming of the samples inside the nmr instrument. the time gap between consecutive microwave moisture measurements was shorter, because sample warming is negligible in the microwave instrument. the operating power of the microwave moisture measurement instrument is about 10 mw and thus very small in comparison with, for example, a microwave oven. the sample container of the microwave instrument was turned by about 30° between consecutive measurements. the position of the sample container inside the nmr instrument was random. altogether 923 separate measurements were made during this research. the measurements include the nmr and microwave instrument performance measurements, the repeatability measurements, and the corresponding lod reference measurements. 6. measurements results of nmr instrument 6.1. common nmr instrument results for all biomaterial samples as stated in the introduction, most of the measurement results and the data published in section 6 were included in the conference proceedings [5]. the comparative analysis of the two instruments based on the previously published data is the additional information provided in this article. the representation of the research results has also been improved. for five chosen biomaterials, the average difference between all the moisture measurement results determined with the nmr instrument and the oven drying method is 1.0 ± 3.8 %mc (percentage points of moisture content) when the uncertainty is given at the 95 % confidence level (k = 2). thus the nmr instrument overestimated the moisture content of the sample by about 1 %mc in comparison with oven drying. the moisture range for five different forest-based biomaterials varied from 14.3 to 68.6 %. on average, the standard deviation of moisture measurements for parallel samples was 0.9 %mc for the nmr measurement instrument and 0.4 %mc for the oven drying method, when considering all biomaterials at three moisture levels. thus, the variation of the measurement results for parallel samples of single material is smaller with the lod method. the repeatability tests for the nmr instrument showed that the standard deviation for the measurement repeated five times on single samples taken from all five sample materials at three moisture levels was 0.5 %mc on average. repeated measurement results seemed to deviate randomly. the material figure 3. left: senfit bma desktop, the microwave moisture measurement instrument; right: plate-shaped measurement bowl of the instrument with a crushed and milled stump sample divided into three parts. acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 93 specific and moisture-level-specific repeatability tests are described in section 6.3. obviously, the five repeated measurements on the single sample cannot be done for the oven drying method, because the sample changes (dries) during the first measurement. 6.2. biomaterial-specific measurement results for the nmr instrument at three moisture levels this section summarizes the biomaterial-specific and moisture-level-specific nmr moisture measurement results for all the biomaterials compared with the reference lod measurements. figure 4 shows the performance of the nmr measurement instrument for sawdust, bark waste, and pruning residue samples in comparison with the lod method by showing all the measurements with red crosses (the first nmr instrument and measurement session) and blue circles (the second nmr instrument and measurement session). the x-axis represents the difference in moisture measurements between the nmr instrument and the reference lod method, while the y-axis represents the moisture content of the sample measured with the reference lod method. sawdust is typically a quite homogenous material and the uncertainty was the second smallest for the nmr instrument after measurements of crushed and milled stump samples (nmr – lod differences of stump samples can be seen on the left-hand side of figure 5). here, the mean difference in moisture content between the oven-drying method and the nmr instrument was 1.6 ± 2.1 %mc for sawdust when a 95 % confidence level was applied (k = 2). in this case, the measurement results made with the second instrument during the spring 2015 measurement session were very good and were closer to the reference lod results than the measurement results achieved by the first measurement session and instrument. the middle image of figure 4 shows the performance results of the nmr instrument with bark waste samples. for the first measurement session, chipped bark samples were collected from the unloading and feeding station of the combustion process of a power plant. this means that bark material was stored in the pile for some weeks before the measurement so that the greenish colour of the sample material vanished and the solid biofuel became acceptable for combustion at the power plant. for the second measurement session, bark samples were fresh and greenish and were collected some hours after debarking the logs. despite that, the results of both measurement sessions are quite similar, as seen in the middle image of figure 4. for chipped bark samples, the deviation of the nmr instrument results from the reference lod method results was 3.0 ± 2.4 %mc on average when a confidence level of 95 % was applied. thus bark waste was the biofuel whose moisture content was most overestimated by the nmr instrument. the right-hand image in figure 4 shows the performance results of the nmr instrument with pruning residue samples. for the pruning residue samples, the mean deviation between all the nmr moisture measurement readings and the reference lod method readings was –0.9 ± 4.1 %mc when a 95 % confidence level was applied. thus the pruning residue was the only biofuel whose moisture content was underestimated by the nmr instrument. however, when observing the right-hand image of figure 4, it seems that the nmr instrument underestimates the moisture content mostly with dry samples. pruning residue is very inhomogeneous material and that may be the reason why the standard deviation and the uncertainty, respectively, were quite large and nearly doubled in comparison with the other four measured forest-based biomasses. in particular, the deviation in moisture content of the moistened pruning residue samples measured in the first measurement session was large on the right side of figure 4. the largest single difference between the nmr instrument and the reference lod method measurement results is no less than ~6 figure 4. the differences of all measurement pairs between the nmr measurement and the reference lod method are presented for sawdust (left-hand image), bark waste (middle image), and pruning residue (right-hand image). the x-axis represents the nmr – lod difference in moisture content while the yaxis represents the moisture content of the sample according to the reference measurement. if the circle or cross falls on the dashed line, no deviation between the nmr and reference instrument measurements exists. the corresponding images of the remaining biomaterials are presented in figure 5. acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 94 %mc in the right-hand image of figure 4. typically, the reason for such outliers was incorrect weighing (either manual weighing during the lod method or automatic nmr weighing), but the reason for this outlier could not be found and therefore this is included in the analysis. for the first measurement session, the pruning residue sample material was collected from a truckload at the unloading station of the power plant, and before that the samples were stored for several months in a pile in a forest roadside storage area to get rid of the needles in order to increase the fuel energy content and decrease the harmful corrosive components of the solid biofuel. most needles dried and dropped away during the storage period and transportation. for the second measurement session, the sample material was collected from the forest stand during logging, and thus the fresh and green pruning residue sample material with needles was used in this measurement session. however, a considerable difference in measurement results was not obtained. in both cases the moisture content was underestimated with dry samples and overestimated with wet samples in comparison with the lod measurement and this explains the higher standard deviation. the freshness of the pruning residue samples of the first and the second measurement sessions can also be noticed in the right-hand image in figure 4: the moisture content at delivery is about 27 % in the first measurement session (i.e. the most middle cluster of the three clusters of measurement results marked with red crosses) while the moisture content at delivery in the second measurement session is about 50 % (i.e. the middle-most cluster of the three clusters of measurement results marked with blue circles). the right-hand image in figure 5 shows the deviation between the nmr instrument and the reference lod method measurement pairs for chipped small-sized trees. the sample material of chipped small-sized trees was available only for the first measurement session and the deviation of the nmr measurement results in comparison with the reference lod method was 0.7 ± 2.6 %mc on average when a 95 % confidence level was applied. during the collection of chipped small-sized tree material samples, we noticed that the chosen biomaterial was not pure but was mixed with sawdust, which may affect the results in comparison with pure material. the measurement results here, indeed, were quite close to the sawdust results, although the material seemed totally different, when observing the left-hand images in the upper and lower corners of figure 1. the crushed and chipped stump material contained more soil than the other four solid biofuel sample materials, and thus weaker nmr measurement results were expected. however, both differences in comparison with the reference lod method and the 2 × sigma based measurement error were the smallest among the five tested biomaterials, being 0.1 ± 1.8 %mc. the right-hand image in figure 5 shows that the measurement differences from the reference lod method results and their variation are small and the results here are beautifully concentrated quite close to the zero line. biomaterials are typically very inhomogeneous and the variation of moisture measurement readings between parallel samples was notable even for sawdust. the standard deviation of moisture readings of parallel samples obtained by the oven drying method varied from 0.0 %mc to the poorest case of 1.4 %mc depending on the sample material and moisture level. the mean of these standard deviations was 0.4 %mc. correspondingly, the standard deviation of moisture readings of parallel samples obtained by the nmr moisture measurement instrument varied from 0.4 %mc to the poorest case of 2.1 %mc depending on the sample material and the moisture level. the mean of the standard deviations of nmr measurements was 0.9 %mc. a clear dependence between the moisture content of the solid biomaterials and the standard deviation of the parallel measurements could not be observed, even though the dependence between the moisture content of the biomaterial and nmr instrument repeatability for the very figure 5. the differences of all measurement pairs between the nmr measurement and the reference lod method are presented for chipped small-sized trees in the left-hand image and for crushed and chipped stumps in the right-hand image. the x-axis represents the nmr – lod difference in moisture content and the y-axis represents the moisture content of the sample according to the reference measurement. if the circle or cross falls on the dashed line, no deviation between the nmr and the reference instrument measurements exists. acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 95 same sample is obvious, as described later in section 6.3. four of the 388 measurement pairs were omitted from the analyses due to incorrect sample weighing by the nmr instrument. the scale of the nmr instrument showed the masses of those four samples to be tens of grams different from the real masses. in addition to weighing by the nmr instrument, the sample must be weighed manually before oven drying, and the result must be quite close to the nmr weighing result. also, the masses of parallel samples should be rather close to each other, because the sample container must always be completely full. if considerable deviation between parallel samples or between nmr and manual weighing is observed, it can be supposed that there is a weighing error. two of 388 measurement pairs were omitted due to incorrect handling of samples during lod measurements. the milled peat sample material was also available during the measurements, but it was measured only with the microwave instrument. the nmr instrument manufacturer does not recommend the use of peat samples due to the fact that the measurement is based on a homogenous magnetic field. namely, some peat materials from certain geological areas may have ferromagnetic components, which may distort the magnetic field inside the nmr instrument and further distort the moisture measurement readings. 6.3. repeatability measurements of the nmr-based moisture measurement instrument table 1 summarizes the material-specific and moisture-levelspecific repeatability tests of the nmr instrument for single samples. one sample of each sample class was chosen, the nmr moisture measurement was repeated five times on the sample, and the average moisture and standard deviation of the five measurements were calculated. table 1 shows that the standard deviation of the repeated measurement results varied from 0.2 to 1.1 %mc, depending on the biomaterial. the average of all standard deviations of single samples for different materials and moisture levels was 0.5 % and the higher standard deviation values were achieved with drier samples. in section 6.1, we showed that the standard deviation with parallel samples (not exactly the same) was 0.9 %mc on average; thus roughly half of the variation of the nmr instrument results can be explained by the variation of the biomaterial and another part can be explained by the instrument properties. the repeatability test values from table 1 are plotted on a graph (see figure 6, right). the graph clearly shows that the standard deviation of consecutive measurements of a single sample increases when the sample is drier. thus repeatability is better with moister samples. however, this dependence vanishes when variation due to the material properties is added, as mentioned at the end of section 6.2. the repeatability test results for the small-sized tree samples in the spring 2015 measurement session are missing from table 1, because this solid biofuel material was not available during the second measurement session. 7. measurement results of microwave instrument 7.1. common microwave instrument results for all biomaterial samples similarly to the nmr research data and measurement results, most of the measurement results and the data obtained with the microwave instrument were included in another conference proceedings [4]. the representation of the research results of the microwave instrument has also been improved and the measurement results are discussed from the viewpoint of the differences between the instruments. the measurements made with the microwave-based moisture measurement instrument were similar to those made with the nmr moisture measurement instrument in section 6. however, the samples were measured less, because the microwave instrument was available only for the first measurement session. contrary to the nmr measurements, the peat samples were measured with the microwave instrument. for six chosen biomaterials, the average difference between all the moisture measurement results determined with the microwave instrument and the oven drying method is 0.0 ± 1.8 %mc when the uncertainty is given at the 95 % confidence level table 1. the repeatability test results of the nmr measurement instrument for the five solid biomaterials at three moisture levels. figure 6. the graph presents the values from table 1 graphically, and the dependence between repeatability and moisture content can be clearly seen. moisture moisture std moisture std class on avg. /% /% mc on avg. /% /% mc dried 26.2 0.7 14.7 1.1 normal 54.7 0.2 54.9 0.4 moistened 64.4 0.2 68.4 0.3 dried 26.4 0.7 21.3 0.9 normal 50.9 0.2 50.4 0.7 moistened 64.7 0.2 59.6 0.2 dried 15.9 1.1 18.8 1.0 normal 27.1 0.9 51.8 0.2 moistened 58.1 0.4 64.0 0.2 dried 17.2 0.7 normal 49.9 0.7 moistened 64.2 0.2 dried 15.9 0.5 11.1 0.4 normal 35.4 0.5 27.7 0.5 moistened 50.3 0.3 58.7 0.9 autumn 2014 spring 2015 smal l -si zed tree crushed stump sawdust bark waste pruni ng resi dues acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 96 (k = 2). this can be considered a very good result, since the moisture range for five different forest-based biomaterials varied from 14.3 to 68.6 %. however, one should keep in mind here that calibration was carried out by the reference lod method and the microwave instrument was tuned according to the oven-drying measurement results. also, perfect calibration samples taken from the same truck loads or piles as the measurement samples were used. on average, the standard deviation of each biomaterial in the three moisture ranges was 0.7 %mc for the microwave measurement instrument and 0.4 %mc for the oven-drying method. thus, the variation of the measurement results for parallel samples of a single material is smaller to that of the lod method. to demonstrate the significance of the good representativeness of the actual measurement samples provided by the calibration samples, the microwave instrument was also used with a calibration carried out using calibration samples collected one to three weeks earlier than the measurement samples from another biomaterial load supplied from a different location. in addition to that, the driest calibration samples had moisture contents slightly below 15 % and the suppliers of two biomaterials (milled peat and chipped small sized trees) changed between the collection of calibration and measurement samples. despite the supplier change, the peat samples and chipped small-sized tree samples seemed visually similar to the calibration samples. however, such imperfect collection of calibration samples may be possible at power plants. the difference in the moisture measurement results between the oven-drying method and the microwave instrument was in this case 0.1 ± 5.2 %mc on average when the incomplete microwave instrument calibration was applied. this result indicates that without optimal representativeness of the calibration samples, the variation in the moisture readings may be large, while the mean value may stay close to the correct value. the repeatability tests for the microwave instrument showed that the standard deviation for the measurement repeated five times on single samples taken from all six sample materials at three moisture levels was 0.3 %mc on average. the materialspecific and moisture-level-specific repeatability tests are described in section 7.3. obviously, the five repeated measurements on the single sample cannot be done for the oven-drying method, because the sample changes (dries) during the first measurement. 7.2. biomaterial-specific measurement results for the microwave instrument at three moisture levels this section summarizes the biomaterial-specific and moisture-level-specific microwave moisture measurement results for all the biomaterials. the left-hand image in figure 7 shows the performance of the microwave measurement instrument for sawdust samples. sawdust is typically quite a homogenous material, but surprisingly the largest dispersion of the microwave and lod differences among the six chosen biomaterials was found for sawdust samples: the mean difference in moisture content between the oven-drying method and the microwave instrument was –0.1 ± 2.7 %mc for sawdust when a 95 % confidence level was applied (k = 2). when considering not only different biomaterials but also moisture levels, the difference and standard deviation were largest for dry sawdust samples among all materials and all moisture levels, being –1.5 ± 2.6 %mc, and thus the microwave instrument underestimated the moisture content of dry samples, as can be seen in the lefthand image in figure 7. the measurement results for chipped bark waste and chipped pruning residues are quite similar, although the materials are quite different among forest-based biomasses. the results are presented in the middle and right-hand images in figure 7. the differences between the microwave moisture figure 7. the differences of all measurement pairs between the microwave measurement and the reference lod method are presented for sawdust (lefthand image), bark waste (middle image), and pruning residue (right-hand image). the x-axis represents the microwave – lod difference in moisture content and the y-axis represents the moisture content of the sample according to the reference measurement. if the circle or cross falls on the dashed line, no deviation between the microwave and reference instrument measurements exists. the corresponding images of remaining biomaterials are presented in figure 8. acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 97 measurements and lod measurements are quite small in terms of the mean value and standard deviation, being 0.2 ± 1.4% mc for bark waste samples and 0.4 ± 1.4%mc for pruning residue samples, when a 95 % confidence level was applied (k = 2). for chipped small-sized trees, the measurement results are presented in the left-hand image of figure 8. the difference between the microwave and lod moisture measurement methods was 0.3 ± 1.9 %mc with a 95 % confidence level. the standard deviation was the second poorest after pure sawdust samples, but it can be recalled from section 2 that chipped small-sized tree material was mixed with sawdust and this may have affected the results. the best measurement results for the microwave instrument were achieved with chipped and crushed stump samples. this can also be seen in the middle image of figure 8, in which all the crosses indicating a measurement difference between the microwave instrument and the lod method are located close to the dashed zero line. the difference between the microwave instrument measurements and the reference lod method was – 0.2 ± 1.1 %mc when a 95 % confidence level was applied (k = 2), so due to the small variation, that mild underestimation can be easily observed in the middle image of figure 8. the last tested biomaterial was milled peat and these samples were measured only with the microwave instrument and the reference lod method, but not with the nmr instrument. the difference between the moisture measurement results achieved with the microwave instrument and the reference lod method was 0.1 ± 1.5 with a 95 % confidence level. as seen in the right-hand image of figure 8, the variation of the measurement results for milled peat is quite high for wet samples, but the measurement results for dried peat samples and the peat samples having the moisture content present at delivery are very close to the zero line and thus match the reference lod method results well. 7.3. repeatability measurements of the microwave-based moisture measurement instrument table 2 summarizes the material-specific and moisture-levelspecific repeatability tests of the microwave instrument for single samples. one sample of each sample class was chosen, the microwave moisture measurement was repeated five times on the sample, and the average moisture and standard deviation of the five measurements were calculated. table 2 shows that the standard deviation of the repeated measurement results varied between 0.1 and 1.1 %mc, depending on the biomaterial. the average of all standard deviations of single samples for different materials and moisture levels was 0.3 %. the highest standard deviation values were achieved with chipped smallsized tree samples and milled peat samples. in section 7.1, we showed that the standard deviation with parallel samples (not exactly the same) was 0.7 %mc on average; thus roughly more than half of the variation of the microwave instrument results can be explained by the variation of the biomaterial and the rest can be explained by the instrument properties. the repeatability test values from table 2 are plotted on a graph (see figure 9, table 2. the repeatability test results of the microwave measurement instrument for the six solid biomaterials at three moisture levels. the repeatability test results of the measurement session in autumn 2014 are presented. figure 8. the differences of all measurement pairs between the microwave measurement and the reference lod method are presented for chipped smallsized trees (left-hand image), crushed and chipped stumps (middle image), and milled peat (right-hand image). the x-axis represents the microwave – lod difference in moisture content and the y-axis represents the moisture content of the sample according to the reference measurement. if the circle or cross falls on the dashed line, no deviation between the microwave and reference instrument measurements exists. moisture moisture std moisture std class on avg. /% /% mc on avg. /% /% mc dried 23.4 0.1 16.5 0.9 normal 55.3 0.1 53.6 0.7 moistened 66.4 0.2 68.7 0.5 dried 23.7 0.2 15.6 0.1 normal 52.2 0.1 36.2 0.1 moistened 69.4 0.1 54.8 0.1 dried 17.5 0.1 25.3 1.1 normal 32.8 0.1 50.5 0.1 moistened 71.2 0.1 61.3 0.5 autumn 2014 autumn 2014 sawdust bark waste pruning residues crushed stump small-sized tree milled peat acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 98 right). a clear dependence between the standard deviation of consecutive measurements of a single sample and the moisture content of the sample cannot be addressed as was done with the nmr instrument, but a dependence between the material type and standard deviation seems to be evident. 8. discussion when the two measurement instruments – the senfit bma desktop microwave instrument and the valmet mr moisture nmr instrument – were compared, the senfit bma achieved slightly better results in terms of both precision and accuracy (0.0 ± 1.8 vs. 1.0 ± 3.8 %mc for all samples on average). however, the good measurement results for the microwave instrument are highly dependent on the calibration procedure. in particular, similarity between the calibration material and the sample material is important for the microwave instrument: if a deviation occurs between the calibration and sample material properties, the measurement accuracy worsens rapidly and the uncertainty may easily be tripled. the calibration process of the nmr instrument is much more rapid and simpler than for the microwave instrument. the nmr instrument is calibrated only with an empty and water-filled sample container, and the same calibration works for all sample materials. thus, another moisture measurement method (e.g. the lod) is not needed for the calibration. the nmr instrument calibration takes about five minutes daily. in comparison, the microwave instrument must be calibrated against another moisture measurement method (typically lod) separately for each biomaterial and each biomaterial supplier and the calibration process takes at least one day. an appropriate calibration line must always be loaded from memory when the biomaterial type or supplier changes. thus another (most commonly the lod) moisture measurement method cannot be neglected with the senfit bma desktop, because it is still needed for the calibration of the microwave instrument. the precision of the microwave instrument is always quite good due to the fact that calibration is carried out according to the reference lod method measurements. thus the microwave instrument is tuned towards the lod reference measurement results during the calibration process. the measurement range of the nmr instrument is larger than that with the microwave instrument (0–90 vs. 0–70 %). both the instruments have their own restrictions: the samples for the microwave instrument should be milled to meet the particle size requirements, similarly to the lod method. the maximum particle size for the nmr instrument samples is not restricted. the samples for the nmr instrument must not include ferromagnetic components and this may occur with, for example, some peat samples. also, the nmr instrument sample must include at least 20 g of water for a reliable measurement result. this typically corresponds to a moisture content of 5–10 % depending of the mass of the 0.8 l biomaterial sample. the moisture measurement results showed that the total variation between the moisture content values of parallel samples was slightly larger for the nmr instrument than for the microwave instrument (the standard deviation was on average 0.9 %mc vs. 0.7%mc). additionally, the repeatability tests of the very same sample showed that on average the uncertainty caused by the instrument was also slightly larger for the nmr instrument than for the microwave instrument (the standard deviation was on average 0.5 %mc vs. 0.3 %mc). the uncertainty caused by the nmr instrument increases when the samples are drier, but this does not occur with the microwave instrument measurements. when observing the average standard deviation of the measurements of parallel samples and average standard deviation of the repeated measurements of the very same sample, as given above, it can be noticed that 0.4 %mc of the total variation explains the variation caused by material properties for both instruments. this was also the average standard deviation of moisture content measurement of parallel samples measured with the lod method. based on this result, it can be concluded that the total variation of the lod method is almost fully explained by material variation. thus, we obtained the result that the variation caused by the lod method itself is close to zero, although we cannot carry out the repeatability test for the lod method by drying the same sample several times. the variation of the properties between different biomaterials affected the performance of both moisture measurement instruments and the lod method as well, which was expected. there is no given measurement accuracy interval for the standardized lod method in [3], due to the high variability of the properties of different solid biofuels. however, for the solid biofuels having a particle size smaller than 1 mm, a deviation smaller than 0.2 % is accepted for parallel samples [17]. the particle size of all solid biofuels in this research excluding milled peat was larger than 1 mm. the authors held discussions with the key personnel of an enterprise that measures the daily quality of supplied biomasses for a few finnish power plants with the conventional lod method. according to them, the standard deviation of 0.5 % for moisture measurement results (i.e. ±1.0 %-point uncertainty with a 95 % confidence level) is considered as a good result for parallel samples measured with the reference lod method. they also tested the novel instruments discussed in the article. in industrial use, the commonly accepted deviation between moisture readings of an excellent moisture measurement instrument and the reference lod method is ±2.0 %-point with a 95 % confidence level. the uncertainty interval of the microwave instrument, 0.0 ± 1.8 %, is hardly within the commonly accepted limits for an excellent instrument on average, but the performance varies according to the biomass material: the microwave instrument figure 9: the graph on the right presents the values from the table on the left graphically, and the clear dependence between repeatability and moisture content cannot be observed. acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 99 satisfies the limits for five materials out of six. the standard deviation of the measurements was outside the range only for sawdust samples. the offset of the microwave instrument measurements was very small due to the fact that this instrument must be calibrated against the reference lod method. the uncertainty interval of the nmr instrument, 1.0 ± 3.8 %, was not inside the acceptable uncertainty limits for an excellent instrument on average. among the five materials researched, the measurement uncertainty of crushed and milled stump material was inside these limits even with the offset. for most materials, the offset was larger in comparison with the microwave instrument. the standard deviation of the measurement results of the nmr instrument was close to the acceptable limits for the three sample materials but not for pruning residue. if the offset is tuned, good enough measurement uncertainty is achieved with the nmr instrument. 9. conclusions and future work as a conclusion, both the nmr-technology-based valmet mr and the microwave-technology-based senfit bma desktop moisture measurement instruments might be reasonable alternatives to replace (nmr) or reduce (microwave) the number of the lod-based measurements, when rapid measurements are needed. however, it seems that neither of them can beat the lod method in terms of accuracy and precision, but good enough results can be achieved for biomaterial invoicing with the microwave instrument, and the nmr instrument achieves results that are close to the acceptable limits as well. in future, we will test the performance of our reference lod method. a few parallel samples have already been measured with an enhanced lod method [18]. the enhanced oven drying system is equipped with a cold trap to collect water and other vocs (volatile organic compounds) from vaporized gases during drying. tentatively, a small but non-significant effect of vocs in the moisture measurement results has been observed but this will be reported in more detail in future work. acknowledgements the research project is funded by a research grant from the european metrology research programme (emrp-reg). the emrp is jointly funded by the emrp participating countries within euramet and the european union. in addition to emrp, the authors would also like to thank vtt mikeskajaani for providing its infrastructure for sample preparation and measurements, senfit ltd, valmet ltd, and the university of oulu for loaning the measurement instruments, the combustion plant kainuun voima ltd for supplying the sample material for the research, and finally prometec ltd for loaning its sample mill and sample mixer as well as for providing information and assistance with practical work on biomaterial measurements at the power plant. references [1] en-ts 14778:2011 solid biofuels. sampling. 2011. [2] en 14780:2011 solid biofuels. sample preparation. 2011. [3] cen-ts 14774-2 solid biofuels – determination of moisture content – oven dry method – part 2: total moisture – simplified method. 2004. [4] österberg p., heinonen m., ojanen-saloranta m. and mäkynen a, “the comparison of a microwave based bioenergy moisture measurement instrument against the loss-on-drying method”, xxi imeko world congress “measurement in research and industry” 30 august – 4 september, 2015, prague, czech republic. 6 p. [5] österberg p., heinonen m., ojanen-saloranta m. and mäkynen a., “comparison of an nmr-based bioenergy moisture measurement instrument against the loss-on-drying method”, isema 2016, 11th international conference on electromagnetic wave interaction with water and moist substances, 23–27 may 2016, florence, italy. 11 p. [6] fridh l., volpé s., and eliasson l. “an accurate and fast method for moisture content determination”, int. j. for. eng. 25 (2014), no. 3, pp. 222-208. issn 1494-2119. [7] fridh l., “evaluation of metso mr moisture analyser”, work report number 839-2014 of skogforsk. 2014. [8] järvinen t., “rapid and accurate biofuel moisture content gauging using magnetic resonance measurement technology”, vtt technology. espoo 2013. [9] rytkönen t. kiinteiden biopolttoaineiden lämpöarvon ja kosteuden määrittäminen nmr – menetelmällä [the determination of calorific value and moisture content of solid biofuels using nmr – technology]. master’s thesis. university of eastern finland. 2012, 60 p (in finnish). [10] toivakainen j. voimalaitoksen leijukattilassa käytettävän metsäpolttoaineen laadunmääritys [determining the quality of forest biofuel which is used in the power plant’s fluidized bed boiler]. diploma thesis. tampere university of technology. june 2014 (in finnish). [11] hautakorpi j. bma-kosteusanalysaattorin hyödynnettävyys biopolttoainejakeiden ja hakkeiden kosteusmäärityksissä [usability of the bma moisture analyzer in measuring moisture content of biofuel materials and wood chips]. bachelor’s thesis. tampere university of applied sciences. june 2013 (in finnish). [12] metefnet. webpage of emrp-project. [online] available: http://metef.net/. 17 february 2016. [13] termaks ltd. [online] available: www.termaks.com. 22 february 2016. [14] valmet ltd. product webpage. [online] available: www.valmet.com/products/automation/analyzers-andmeasurements/analyzers/mr-moisture/. 22 february 2016. [15] senfit ltd. product webpage. [online] available: http://www.senfit.com/products/biomass-sample-mill.html. 22 february 2016. [16] senfit ltd. product webpage. [online] available: www.senfit.com/products/biomass-moisture/bma.html. 22 february 2016. [17] cen-ts 14774-3 solid biofuels – determination of moisture content – oven dry method – part 3: moisture in general analysis sample. 2004. [18] ojanen m., sairanen h., riski k., kajastie h. and heinonen m., “moisture measurement setup for wood based materials”, ncsli measure j. meas. sci., 9 (2014), no. 4, pp. 56–60. http://metef.net/ http://www.termaks.com/ http://www.valmet.com/products/automation/analyzers-and-measurements/analyzers/mr-moisture/ http://www.valmet.com/products/automation/analyzers-and-measurements/analyzers/mr-moisture/ http://www.senfit.com/products/biomass-sample-mill.html http://www.senfit.com/products/biomass-moisture/bma.html comparison of the performance of a microwave-based and an nmr-based biomaterial moisture measurement instrument evaluating chemometric strategies and machine learning approaches for a miniaturized near-infrared spectrometer in plastic waste classification acta imeko issn: 2221-870x june 2023, volume 12, number 2, 1 7 acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 1 evaluating chemometric strategies and machine learning approaches for a miniaturized near-infrared spectrometer in plastic waste classification claudio marchesi1 , monika rani1, stefania federici1, matteo lancini2, laura e. depero1 1 department of mechanical and industrial engineering, university of brescia & udr instm of brescia, via branze 38, 25123 brescia, italy 2 department of medical and surgical specialties, radiological sciences, and public health, university of brescia, viale europa 11, 25123 brescia, italy section: research paper keywords: plastic waste sorting; near-infrared spectroscopy (nirs); circular economy; chemometrics; machine learning citation: claudio marchesi, monika rani, stefania federici, matteo lancini, laura e. depero, evaluating chemometric strategies and machine learning approaches for a miniaturized near-infrared spectrometer in plastic waste classification, acta imeko, vol. 12, no. 2, article 40, june 2023, identifier: imekoacta-12 (2023)-02-40 section editor: leonardo iannucci, politecnico di torino, italy received march 31, 2023; in final form june 19, 2023; published june 2023 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: the research was funded by the project pon “r&i” 2014-2020: sirimap—sistemi di rilevamento dell’inquinamento marino da plastiche e successivo recupero-riciclo (no. ars01_01183, cup d86c18000520008) and partially based upon work from cost action ca20101 plastics monitoring detection remediation recovery priority, supported by cost (european cooperation in science and technology), www.cost.eu. corresponding authors: stefania federici, e-mail: stefania.federici@unibs.it; matteo lancini, e-mail: matteo.lancini@unibs.it 1. introduction in the perspective of whole-system economic sustainability, the enormous volume of urban plastic waste and the constant increase in human plastic consumption require a high level of waste valorisation. by the numbers, global plastic production reached 367 million tons in 2021, with europe accounting for 16 % of the total [1]. 9 % of plastic was recycled, 12 % was incinerated, and 79 % ended up in landfills or natural compartments [2]. the recycling of polymer waste has significant environmental advantages owing to the replacement of primary manufacturing, and waste sorting optimization plays a critical role in the development of the recycling process [3], [4]. recycling is a technique for plastic product end-of-life waste management [5]. basically, two types of recycling processes can be distinguished: mechanical and chemical processes [3], [6]. in both, sorting is the most critical stage in the recycling process, and this is true regardless of how effective the recycling program is [3], [4]. the use of automated sorting equipment makes the process more efficient [7]. usually, these devices rely on vibrational spectroscopic techniques [8]-[11], and camera abstract optimizing the sorting of plastic waste plays a crucial role in improving the recycling process. in this contribution, we report on a comparative study of multiple machine learning and chemometric approaches to categorize a data set derived from the analysis of plastic waste performed with a handheld spectrometer working in the near-infrared (nir) spectral range. conducting a cost-effective nir study requires identifying appropriate techniques to improve commodity identification and categorization. chemometric techniques, such as principal component analysis (pca) and partial least squares discriminant analysis (pls da), and machine learning techniques such as supportvector machines (svm), fine tree, bagged tree, and ensemble learning were compared. various pretreatments were tested on the collected nir spectra. in particular, standard normal variate (snv) and savitzky-golay derivatives as signal pre-processing tools were compared with feature selection techniques such as multiple gaussian curve fit based on radial basis functions (rbf). furthermore, results were combined into a single predictor by using a likelihood-based aggregation formula. predictive performances of the tested models were compared in terms of classification parameters such as non-error rate (ner) and sensitivity (sn) with the analysis of the confusion matrices, giving a broad overview and a rational means for the selection of the approach in the analysis of nir data for plastic waste sorting. mailto:stefania.federici@unibs.it mailto:matteo.lancini@unibs.it acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 2 systems for the polymer identification of clear and coloured products [5], [12]. other techniques are based on ultraviolet (uv) spectroscopy [13], [14], x-ray [15], and hyperspectral imaging [16]-[18]. over the years, this strategy has increased the purity of the output plastic, achieving a high percentage of recyclates in the production of secondary materials. however, these systems reach their limits with mixed plastics that require additional sorting elsewhere and can affect the quality of the recyclate if not appropriately allocated. a positive cost-benefit analysis is only possible if the separated polymer fractions have a high purity grade and satisfy the market demand for high-quality recyclates. therefore, post-consumer recycling consists of many essential steps: collection, sorting, cleaning, size reduction and separation, and/or compatibilization to reduce polymer contamination [5]. in this scenario, the prospect of combining a well-established polymer identification technology with a small, portable, lowcost, real-time spectrometer for local and intermittent semiautomatic sorting is highly desirable, accompanied by robust data analysis [19], [20]. in recent years, chemometric analysis of nondestructive spectroscopic data has been widely investigated as an automated method for improving plastic sorting systems [21][24]. this improvement has been driven by the need to reduce the environmental impact [25]. recently, machine learning has attracted considerable attention in plastic waste recognition using spectroscopic techniques [26]-[32]. in this study, we compared machine learning and chemometric techniques for classifying plastic waste data acquired with a portable near-infrared (nir) spectrometer (see figure 1 for the scheme of the work). comparisons were made between chemometric approaches, principal component analysis (pca) and partial least squares – discriminant analysis (pls-da), and machine learning techniques, support-vector machines (svm), fine tree, bagged tree, and ensemble learning. a comparison was also made in terms of pre-processing: traditional techniques, such as standard normal variate (snv) and savitzky-golay derivatives were examined in contrast to feature reduction techniques, such as multiple gaussian curve fit based on radial basis functions (rbf). the predictive performances of the tested models were compared in terms of classification parameters, such as nonerror rate (ner) and sensitivity (sn) with the analysis of confusion matrices, providing a comprehensive overview and a rational means of selecting the approach for the analysis of nir data for plastic waste sorting. 2. materials and methods 2.1. samples collection the first batch of plastic samples was collected in the selection division of the montello spa recovery and recycling plant (bergamo, italy), which accepts post-consumer plastic in the form of municipal waste for recycling [20]. subsequently, the dataset was expanded to include new samples from municipal waste collected before ending up in landfills. a total of 325 samples from a variety of polymer classes were used in this study. specifically, the products studied were: 75 samples of poly(ethylene terephthalate) (pet), 100 samples of polyethylene (pe), 75 samples of polypropylene (pp), and 75 samples of poly(styrene) (ps). the assortment included bottles, containers, and packaging of various sizes, shapes, and colours. 2.2. nir analysis plastic samples were analysed using the micronir on-site spectrometer (viavi solutions inc., ca, united states) in reflectance mode without pre-treatment of the samples. the instrument is a palm-sized, portable spectrometer weighing approximately 250 g and measuring less than 200 mm in length and 50 mm in diameter. the instrument is equipped with a linear variable filter (lvf), coupled to a linear detector array, which operates in the wavelength range 950-1650 nm. control settings for spectral data acquisition were set to 10 milliseconds integration time and 50 scans, resulting in a short measurement time of 0.25 seconds. a point-and-shoot technique was used to perform 5 replicates for each sample to reduce the effects caused by sample non-uniformity. a total of 1625 spectra were acquired, and acquisition was performed using micronirtm pro v3.0 software (viavi solutions inc., ca, united states). 2.3. spectral pre-processing and chemometrics pre-processing nir spectral data has become an essential aspect of chemometric modelling. the goal is to eliminate physical events from the spectra to improve subsequent multivariate regression, classification model, or exploratory analysis [33]. in this study, the spectra were retrieved in a single matrix of 1625 × 125 (samples × wavenumbers) and preprocessing was applied using the savitzky-golay second derivative method with seven data points and a second order polynomial followed by standard normal variate (snv). the second derivative was applied to correct the drift effect [34], [35] figure 1. scheme of the work. acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 3 in the nir spectra, while snv corrects the baseline shift [36]. snv was calculated as follows [36]: 𝑋corr = 𝑋org − 𝑎0 𝑎1 (1) where 𝑋corr is the spectrum corrected, 𝑋org is the raw spectrum collected by the instrument, 𝑎0 is the value of the mean of the spectrum to be corrected, and 𝑎1 is the standard deviation. in addition, normalization was performed by mean centering. different chemometric methods were used for the correct evaluation of the data of all analysed samples. pca was initially applied as an exploratory analysis to investigate the data structure and was performed on 1625 nir spectra from all polymer classes. then, pls-da was applied as a supervised pattern recognition tool to separate the different commodities. prior to using pls-da, data were split into a training set and a test set using a matlab proprietary function. the process was repeated 500 times, generating a different training and test set each time (75 % of the samples belonged to the training set and 25 % to the test set). all chemometric analyses were performed with matlab 2021b (the mathworks, inc, natick, ma, usa) using the pls-toolbox (eigenvector research, inc. manson, washington, usa). 2.4. machine learning and pre-processing various machine learning algorithms were applied for classification purposes; svm, fine tree, ensemble learning, and bagged tree. in addition, a likelihood-based aggregation procedure (here called combo) was used to integrate the data into a single predictor, and the same procedure was applied with a monte carlo method (mcm) to make a perturbation on raw data, to improve the generalization performance. the chosen hyperparameters are the following: for fine tree gini's diversity index (gdi) was used as split criterion with 100 maximum number of splits; svm was performed with a linear kernel function with kernel scale equal to 3. lastly, ensemble learning was performed with the bagged tree method with 30 cycles of learning. to test the reliability of the system, 200 random extractions were performed for splitting the training and testing set. again, 75 % of the samples were used for training and the rest for testing. machine learning methods were performed on three different datasets: the raw data collected as specified in the previous paragraph (2.2), data reduced using the gaussian rbf curve fit [37], and a dataset obtained combining raw and pre-processed data. each curve of the dataset has been fitted using a combination of 12 gaussian functions and a linear interpolation with a seconddegree function, thus reducing the dataset dimension to 12 rbf centres and 12 sigma values. the procedure is as follows: 1. the second order derivative is computed and fed to find detection algorithm for the initial guesses of the rbf centres (here the matlab function “findpeaks” was used with a limitation of 12 peaks maximum and excluding the first and last 20 samples of the spectrum). 2. a linear regression with a second-degree equation is used to remove offset and second-order trends. 3. the rbf centres are used as initial guess to an optimization procedure based on a sequential quadratic programming constrained minimization function [38]. the cost function 𝜀 used is reported in (2) where 𝐴𝑖 is the frequency of the 𝑖-th sample, 𝑦𝑖 is its raw values, and 𝜇𝑗 , 𝜎𝑗 , and 𝐴𝑗 are the centre, sigma and amplitude of the 𝑗-th rbf function respectively. 𝜀 = ∑ ∑ 𝜀𝑖,𝑗 𝑗𝑖 𝜀𝑖,𝑗 = { 0, 𝜎𝑗 < 0 𝐴𝑗 𝑒 − (𝑓𝑖−𝜇𝑗) 2 2 𝜎𝑗 2 , 𝜎𝑗 ≥ 0 (2) 4. the centres and sigmas found are collected as features of the new dataset. the condition posed in (2) on the positive value allows to reduce dynamically the number of rbf functions actually used, while the interpolation removes trends that could hide peaks. a third dataset combining the two previous dataset (raw and rbf gaussian fit) is also created simply joining the two tables. all calculations were performed using matlab and statistics toolbox release 2021b (the mathworks, inc, natick, ma, usa). automation of the procedure was implemented using matlab functions created in-house. in figure 2 the data analysis approach starting from raw data is reported, both for chemometrics and machine learning modelling. 3. results ann discussion 3.1. nir spectra the main advantage of nir spectroscopy is that it is a fastresponse analytical technique capable of collecting spectra without prior processing and predicting physical and chemical properties from a single spectrum [39]. the absorption bands in the nir region are caused by overtones and/or combination bands of primarily carbon-hydrogen vibrations and oxygenhydrogen vibrations. correct band assignment is difficult since it may be caused by various combinations of fundamental vibrations. also, overtone vibrations are highly overlapping [40]. representative nir reflectance spectra of the four polymers (pe, pet, pp, and ps) are shown in figure 3. figure 2. data analysis approach. acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 4 the main absorbance band for pet was found at 1660 nm, which is related to the 1st overtone of c-h stretching [41], with other two peaks at about 1130 nm and 1415 nm. for pe the peak around 1211 nm is related to 2nd overtone of methylene c-h group, while the peak at about 1217 nm is related to the c-h stretch [42]. peaks at 1391 nm and 1168 nm, correspond, respectively, to c-h combination band and 2nd overtone of ch2 symmetric stretch. regarding pp, the 2nd overtone of the asymmetric methyl c-h stretch is around 1193 nm, while the asymmetric methylene c-h stretch occurs at about 1211 nm [43]. the two peaks at 1391 nm and 1397 nm are related to methyl and methylene (c-h) combination. lastly, for ps the peak at 1205 nm corresponds to the 2nd overtone of the aromatic c-h stretch; the stretching vibrational mode of c-h which occurs around 1639 nm, and the 1st overtone of aromatic c-h stretch overlaps with c-h combination band, which occurs at about 1391 nm [42]. to allow comparison between the raw spectra and the same spectra after applying the savitzky-golay 2nd derivative and snv, figure 4 shows the representative spectra of the four commodities after pre-processing. 3.2. principal component analysis the pca calculation was performed after the pre-processing described above for the entire spectral range. for data structure analysis, pca is a useful chemometric method. the goal of pca is to extract the information stored in many variables into a smaller number of variables, called principal components [44]. figure 5 shows the score plot of the first two components (73.88 % of the total explained variability), in which a clear separation between the polymer classes can be seen. along pc1 pet is distinguished from the other commodities. pet samples show very negative score values, while the other samples show positive score values. on the other hand, along pc2, ps is clearly separated from the other plastics. a clear separation between pp and pe can be noticed in the score plot of pc1 vs pc3 in figure 6, where pc3 accounts for 15.83 % of the total information and explains the difference of pp from the other class of polymers. 3.3. partial least squares discriminant analysis following the exploratory pca analysis, a supervised classification technique was used to distinguish the different plastic groups. in pls-da, a classification objective is added to figure 3. representative near-infrared (nir) spectra of the four classes of polymers. figure 4. nir spectra of the four classes of the polymers after the typical preprocessing for chemometric analysis: savitzky-golay 2nd derivative and standard normal variate. figure 5. results of pca performed with spectral data of different commodities. the score plot of pc1 vs pc2 is presented. figure 6. results of pca performed with spectral data of different commodities. the score plot of pc1 vs pc3 is presented. acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 5 the pls regression technique. the response variable is categorical and reflects the class to which the statistical units belong. pls-da returns the prediction as a vector with values between 0 and 1 and a length equal to the number of classes in the predictor variables [45], [46]. each time pls-da was performed, the parameters such as ner and sensitivity were calculated in fitting, in cross-validation (cv), and for the test set. the cross-validation procedure was based on venetian blind approach with 5 groups. cv was also used to determine the optimal number of latent variables (lvs) for each pls-da model. figure 7 shows all sensitivities for each class, calculated for training set, cv, and for test set. the values are close to 1, indicating a very high classification performance. moreover, the results are very balanced between training, cv, and test set; therefore, overfitting is completely avoided, and the model can be considered reliable and stable. table 1 shows the ner defined as mean class sensitivity [47], calculated for all the training set, cross-validation, and test set. overall, 99 % of the samples were correctly classified for each of the 500 iterations. 3.4. machine learning due to the complexity and the large number of results, for the machine learning analysis the classification parameters are presented only for the test set. figure 8 shows the ner of the classes for each computed model and for each treatment of the data. it is noticeable that the models run on raw data have the worst performances. the ner ranges from 0.74 (fine tree) to 0.9 (svm), indicating a high variability in the results. for raw data only svm can be considered as a satisfactory model for pattern recognition. lower variability in the results is observed for pretreated data and for a mixture of pre-treated and raw data, where the ner ranges from 0.96 to 0.99 and from 0.96 to 0.98, respectively. thus, there is no difference in the results between pre-processed data and the combination of raw and pre-treated data. these results confirm that feature reduction based on the gaussian curve with rbf gives high performances for pattern recognition in machine learning analysis. in general, the model performance is comparable between machine learning and multivariate analysis methods. after random extraction of training and test data repeated 500 and 200 times for chemometrics and machine learning, respectively, the ner calculated for the test set is above 0.95 for both methods. however, the use of chemometrics reduces the computational time, compared to the computationally intensive machine learning algorithms. 4. conclusion this paper included a side-by-side comparison between conventional chemometric methods and machine learning algorithms for the classification of a dataset obtained from the study of plastic waste with a portable near-infrared (nir) spectrometer. multivariate methods such as principal component analysis (pca) and partial least squares discriminant analysis (pls da) were investigated, as well as machine learning methods such as support vector machines (svm), fine tree, bagged tree and ensemble learning. results were also compared in terms of data processing: signal preprocessing tools, snv, and savitzky-golay derivatives were compared with feature reduction approaches such as multiple gaussian curve fit based on radial basis functions (rbf). in addition, the machine learning algorithms were run on raw data, pre-processed data, and the combination of the two approaches. the results from pls-da showed very high performances for pattern recognition; in fact, the ner for the training set, in cv, and for the test set are all equal to 0.99. in contrast, for machine learning, the ner for raw data ranges from 0.74 for fine tree to 0.90 for svm, indicating high variability in the results. the results for the pre-processed data show lower variability with ner value ranging from 0.96 to 0.99, which is also valid for the combination of raw data and pre-processed data. this confirms that rbf-based variable reduction is the most crucial point to improve classification performances. contrarily to some results found in the literature regarding the pre-treatment of data having a negative effect in accuracy using chemometrics [48], the pretreatment of data is generally an improvement in the detection accuracy using machine learning techniques. we can conclude that the multivariate and machine learning approaches produce comparable results in terms of model performance. the ner estimated for the test set is above 0.95 for both chemometrics and machine learning after randomly extracting the training and figure 7. pls-da model. class sensitivities (sn) calculated for training set, in cross-validation and for test set. table 1. pls-da model. non-error rate calculated for training set, in cv and for test set. ner training 0.99 cv 0.99 test 0.99 figure 8. machine learning. comparison of the non-error rate (ner) calculated from the confusion matrices for each model. results are presented for raw data, pre-treated data, and the combination of raw and pre-treated data. acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 6 test data and repeating them 500 and 200 times, respectively. on the other hand, chemometrics is characterised by a 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    section: research paper  keywords:  eddy currents; remote field testing; tube inspection; amr sensor; gmr sensor  citation: dario j. l. pasadas, a. lopes ribeiro, helena g. ramos, tiago j. rocha, defect detection in stainless steel tubes with amr and gmr sensors using  remote field eddy current inspection, acta imeko, vol. 4, no. 2, article 11, june 2015, identifier: imeko‐acta‐04 (5)‐02 ‐11  editor: paolo carbone, university of perugia, italy  received november 26, 2014; in final form march 9, 2015; published june 2015  copyright: © 2015 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: instituto de telecomunicações project evaltubes and projects: uid/eea/50008/2013, sfrh/bd/81856/2011 and sfrh/bd/81857/2011 of the  portuguese science and technology foundation (fct)  corresponding author: dario j. l. pasadas, e‐mail: dpasadas@lx.it.pt    1. introduction  the remote field eddy current testing (rfec) is currently applied in non-destructive evaluation (nde) of metallic tubes [1]-[3]. this testing method is a special case of eddy current testing (ect) currently used to evaluate the metal thickness [4], [5] and to detect defects in the material [5], [6]. the rfec testing is an electromagnetic technique that allows the inspection of discontinuities in the metallic materials under test. remote field probes have an equal sensitivity to internal and external tube defects and the phase shift is directly proportional to wall metal loss. detection, localization and characterization [7], [8] of defects are the three basic activities in the ect field. the achievement of these three inspection actions continues to be in study by several researchers to improve the current systems used in industrial maintenance. the development of computers with good performance, simulation tools, advanced signal processing and new electromagnetic sensors are attracting many scientists to work in the nde fields. the remote field eddy current technique requires one coil excited with a time-varying current to produce a magnetic field that penetrates the tube wall under test. the excitation can be sinusoidal [9] or pulsed [10]. in this article the excitation current is sinusoidal with constant amplitude. the magnetic field produced by the excitation coil induces currents in the tube wall. these currents are called eddy currents. the field diffusion along the tube wall is the base of the through-wall indirect technique [11]. analysing a material with a defect, the eddy current flow changes around the defect and the magnetic field produced by it also changes. this magnetic field perturbation is measured with a magnetic detector in order to evaluate the defect features. nowadays, small pick-up coils are the most used as magnetic detectors. recently other magnetic detectors with improved characteristics were introduced, such as hall effect sensors [12], [13] and magneto-resistor sensors like anisotropic magnetoabstract  the  purpose  of  this  paper  is  to  compare  the  performance  of  the  giant  magneto‐resistor  (gmr)  and  anisotropic  magneto‐resistor  (amr)  sensors  for  remote  field  eddy  current  testing  in  stainless  steel  tubes.  two  remote  field  eddy  current  probes  were  built  to  compare detection and characterization capabilities in standard defects like longitudinal and transverse defects. both probes include a  coil to produce a sinusoidal magnetic field that penetrates the tube wall. each probe includes a detector with gmr and amr sensors,  where each sensor has four magneto‐resistive elements configured in a wheatstone bridge. each sensor needs be biased differently to  operate in the high sensitivity linear mode.  the description of the measurement system used to detect defects is presented in this  paper. for the choice of the detector’s optimal position, numerical simulations and experimental measurements were performed. for  comparison of these sensors in defect detection using remote field eddy current testing, experimental measurements were performed  under the same conditions. the results are presented and discussed in this paper. acta imeko | www.imeko.org  june 2015 | volume 4 | number 2  | 63  resistors (amr) [14], [15] or giant magneto-resistors (gmr) [16], [17]. table 1 summarizes the specifications of the magnetic sensors above mentioned. in this paper, special attention has been given to the amr and gmr sensors to provide advantages over coil based magnetic sensors. the directional characteristics, high sensitivity with linear response and large bandwidth (dc to 1 mhz) provided by these sensors make them excellent candidates for defect detection applications in steel tubes. 2.  characteristics of the tube sample under test   the inspected material is a tube sample of austenitic stainless steel (aisi 304) with internal diameter of 26 mm and external diameter of 28 mm. the magnetic permeability of this material is equal to 70 4 10    h/m and the electric conductivity is equal to 1.4  ms/m. this type of steel is non-ferromagnetic and is used in several industries due to its good resistance to corrosion. some typical standard defects like longitudinal and transverse cracks were made into the tube sample in order to test the defect detection with amr and gmr sensors. figure 1 presents the two types of defects analysed in this paper. 3. experimental setup  the experimental setup used in this work is illustrated in figure 2. a single axis positioning system is used to move the probe along the inner tube wall with steps of 0.5 mm. this positioning system is controlled through an rs232 interface by the personal computer. a data acquisition board (pxi-6251) included in a pxi system from national instruments (ni), measures the output voltage (from gmr or amr sensors), amplified by an instrumentation amplifier (ina118) with 40 db of gain, and the voltage across the current sampling resistor rs equal to 0.22 ω. this resistor rs is used to monitor the amplitude and phase of the excitation current. the data acquisition board has 16-bit of resolution and a maximum sampling rate of 1.25 ms/s per channel. the excitation coil current was sinusoidal and generated by a fluke5700a calibrator controlled through the gpib interface. the pxi system was controlled through the rs232 interface by a personal computer running matlab program. at each step of the probe movement, the sensor sinusoidal signal was acquired during 100 periods of the excitation current at the maximum sampling frequency of the board. for each measurement point, a three-parameter sine-fitting algorithm was used to extract the parameters of the sinusoidal sensor output and the excitation current. the amplitude of the sensor signal and the phase difference between the sensor signal and the excitation current are the parameters estimated in this work. the excitation frequency was 5 khz. it takes 20 ms to acquire the signal at each point of the scan, but it is necessary to wait 0.5 s at each point to stabilize the probe. 3.1. amr sensor  the amr sensor used in this work is the hmc 1021z from honeywell. this sensor has one single sensing axis with high sensitivity (1 mv/v/gauss) and a field range of 6 gauss. the sensing axis is oriented along the longitudinal x-axis of the tube. the hmc 1021z sensor is configured in a wheatstone bridge with four magneto-resistive elements. the power supply applied to the bridge was 12 v. a set/reset drive circuit providing pulses of electrical current was used to force the sensor to operate in the high sensitivity and linear mode. this pulse of current was made before each data measurement. 3.2. gmr sensor  the gmr sensor is the aa002-02 from non volatile electronics. this sensor includes four magneto-resistive elements with gmr technology configured in a wheatstone bridge, where two gmr elements are shielded, working as passive resistors and the other two are gmr sensing elements with resistance values changing linearly with the variation of the magnetic field. the bridge was powered by a power supply equal to 12 v. due to the output characteristic of the gmr sensor, a small permanent magnet was placed close to the sensor to polarise it in the high sensitivity and linear mode. table 1. magnetic sensors characteristics.  type of magnetic sensor   sensitivity to  magnetic field range  frequency range   coil  d dt    1 nt to more than 10 t  3 khz to more than 5 mhz hall  b    1 mt to 10 t dc to 10 khz  amr  h    10   t to 1 t dc to 1 mhz  gmr  h    10   t to 10 t dc to 1 mhz  figure 1. representation of the experimental setup.    figure 2. representation of the standard defects tested in this work.  acta imeko | www.imeko.org  june 2015 | volume 4 | number 2  | 64  4. choice of the optimal position for the detector  the design of the probe for remote eddy current inspection is very important for detection optimization. first of all, it is necessary to determine the optimum distance between the magnetic sensor and the excitation coil. for this purpose, a finite element commercial program was used to study the remote field eddy current phenomenon along the inner wall of the stainless steel tube aisi 304. the simulation was made applying an excitation current to the coil with 150 ma at 5 khz. the coil was placed in a fixed position into the tube as depicted in figure 3. the magnetic field intensity along the inner tube wall was obtained and is presented in figure 4. the operating frequency was chosen considering the tube thickness and the relation to the standard depth of penetration. at this frequency the magnetic field diffuses easily to the outside of the stainless steel tube with thickness equal to 2 mm. when the rfec technique is applied, three operating zones exist. observing figure 4 with the measured signal along the inner wall, the three operating zones are visible and are called direct zone (0 mm < x < 0.3 mm), transition zone (0.3 mm < x < 5 mm) and remote zone(x > 5 mm). as the direct zone is close to the excitation coil, the magnetic field produced by the eddy currents is dominated by the strong influence of the magnetic field produced by the excitation coil which doesn’t allow the detection of the defect. the remote zone is the region where the magnetic field is dominated by the field produced by the eddy currents and the field intensity decreases exponentially. when a defect is present in the tube wall, the perturbation of the magnetic field produced by the deviation of the eddy currents from the defect can be sensed by a magnetic sensor placed in that zone. a transition zone is visible in figure 4 that corresponds to the region where the magnetic field produced by the eddy currents begins to overlap the excitation magnetic field and creates a minimum (x = 1 mm) in the total magnetic field amplitude due to their opposing phases. experimental results were obtained fixing the excitation coil into the tube and moving the amr and gmr sensors along the inner wall in order to experimentally visualize the rfec phenomena in a stainless steel aisi 304, validate the model, and to choose the optimal position of the sensor. figure 5 depicts the magnetic field intensity along the inner tube wall, obtained experimentally with the amr and gmr sensors. the direct and transition zones are not visible due to the size of the package of the sensor that does not allow measuring the magnetic field close to the excitation coil. however, it is possible to observe the final part of the transition zone that corresponds to the maximum amplitude obtained in figure 5. the information given in figure 5 shows that the distance between the excitation coil and the detector must be greater than 3 mm to ensure that the measured field is inside the remote zone. the magnetic field difference between the amr and gmr output sensors may originate from one factor. this difference is due to the different distances between the detectors and the tube wall. the gmr paths are closer to the tube wall due to its smaller size. 5. experimental results in defect detection  two probes were mounted and tested for defect detection in the stainless steel tubes. in one probe the magnetic sensor is used to detect longitudinal defects in the tube wall and the other one transverse effects. figure 6 shows photographs of both probes. to ensure that the detectors are in the remote zone, the distance between the excitation coil and the detectors was chosen 15 mm. figure  3.  illustration  of  the  experimental  geometry  including  the  sensor’s  path to obtain simulation results.  figure 4. simulated magnetic field obtained with the finite element model.    figure  5.  magnetic  field  line  along  the  tube  wall  obtained  with  the  experimental setup.  figure  6.  photographs  of  the  two  mounted  probes:  (a)  amr  probe  with  1021z sensor from hmc; (b) gmr probe with aa002‐02 sensor from nve.  acta imeko | www.imeko.org  june 2015 | volume 4 | number 2  | 65  for testing purposes a longitudinal defect and a transverse defect both 10 mm in length, were scanned using the experimental setup with each probe. the first experimental test was made moving the amr probe along the inner tube wall in the sample that has a longitudinal defect. a sinusoidal current with amplitude equal to 150 ma was applied to the excitation coil. the second test was scanning the same defect with the gmr probe. the output amplitudes of both sensors are depicted in figure 7. the phase difference between the excitation current and each detector signal was also measured and depicted in figure 8. it should be noted that the sensing axis of both sensors were directed along the longitudinal direction, close to the inner tube wall. observing figures 7 and 8, an amplitude perturbation and phase perturbation is visible when the probes passed close the longitudinal defect. the perturbation occurred between the positions x=45 mm and x=55 mm and corresponds to the defect position when the detector passed the defect. the defect zone is denoted by two orange lines. observing the output amplitudes of the sensors depicted in figure 7, a perturbation is present but it is not possible to conclude anything about the geometry of the defect. also, the amplitude signals contain noise due to lift-off effect (distance between sensor and the tube wall). observing the phase perturbation depicted in figure 8, the defect zone is clearly identified by a negative phase peak, matching approximately the real length of the longitudinal defect. the unexpected perturbation represented in figure 7 and figure 8 between x=20 mm and x=40 mm is caused by the passage of the excitation coil under the defect, changing the eddy current amplitudes. the same experimental test was made for both probes applying a sinusoidal current with amplitude 50 ma to the excitation coil. figures 9 and 10 show the amplitude perturbation and phase perturbation when the probes passed close the defect. in figure 9 the attenuation of the amplitude perturbation in both output signals is visible. however, in figure 10 it is seen that the phase perturbation continues to be present between x=45 mm and x=55 mm with approximately the same variation type shown in figure 8 for different amplitudes. this shows that the phase difference between the excitation current and the detector signal can be useful to detect and characterize defects with approximately the same information, but different excitation current amplitudes. the same comparison was made analysing the transverse defect. the results for the phase perturbation applying a sinusoidal amplitude current of 150 ma and 50 ma are depicted in figure 11 and figure 12, respectively. the defect position is marked with an orange line. figures 11 and 12 show that the defect position is clearly identified by a negative peak for each probe. the unexpected perturbation presented when the longitudinal defect was scanned is not present when the excitations coil crosses the defect position. this is due to the circumferential nature of the excitation current, the eddy current direction being parallel to the defect and the sensing axis of the detectors. it is also visible that the phase perturbation obtained figure  9.  output  amplitudes  (in  tesla  units)  obtained  with  the  amr  and  gmr  sensors  for  detection  of  a  longitudinal  defect,  applying  a  sinusoidal  current with amplitude 50 ma to the excitation coil.  figure 10. phase difference between the excitation current and each sensor  signal  for  defect  detection  of  a  longitudinal  defect  applying  a  sinusoidal  current with amplitude 50 ma to the excitation coil.  figure  7.  output  amplitudes  (in  tesla  units)  obtained  with  the  amr  and gmr sensors for detection of a  longitudinal defect, applying a   sinusoidal current with amplitude 150 ma to the excitation coil.  figure 8. phase difference between the excitation current and each sensor signal  for  defect  detection  of  a  longitudinal  defect  applying  a  sinusoidal current with amplitude 150 ma to the excitation coil.  acta imeko | www.imeko.org  june 2015 | volume 4 | number 2  | 66  between the excitation current and each sensor signal is maintained when a sinusoidal amplitude current of 150 ma or 50 ma is applied to the excitation coil. 6. conclusions  the probes presented in this paper proved that amr and gmr sensors as detectors seem to be good alternatives to conventional remote field eddy current probes that use coils as detectors. the main difference between these magneto-resistive sensors and coils is the possibility to use low frequency tests that allow an easy magnetic field penetration into thick wall tubes. when coils are used it is usually necessary to increase the frequency of operation to obtain good signal to noise ratios. experimental results proved that these sensors are able to detect defects in stainless steel tubes using remote field eddy current testing. the directional characteristics and high sensitivity with linear response over a large bandwidth (dc to 5 mhz) provided by these sensors can be useful to detect subsurface defects at lower frequencies, where coil detectors can’t be used. the results clearly show that the gmr sensor is more sensitive to the magnetic field than the amr sensor. the results also show that the phase perturbation contains more clear information about defect presence when compared to the amplitude perturbation. furthermore, the phase perturbation remained unchanged for both 150 and 50 ma of excitation current, while the amplitude perturbation decreased with a decrement of the excitation current. this means that when the excitation current amplitude decreases, the amplitude output signal of both sensors becomes difficult to measure, however the phase still contains useful information caused by the defect presence. note that the chosen distance between the excitation coil and the detector is important to ensure that the measured field perturbation of the eddy currents is strong enough to detect defects with minimal signal attenuation. as future work we consider testing these sensors on stainless steel tubes with larger thicknesses and using lower frequencies in order to increase the penetration depth of eddy currents, and to apply this method to other industrial applications. another important conclusion is related to the accuracy of the measurement of the geometrical sizes of the defects. taken into consideration the phase measurements it is possible to conclude that we always got uncertainties under 5 mm for longitudinal defects. this quantity refers to the positions of the defect edges. for transverse circular defects the uncertainty is much lower, especially if the defect is perfectly circularly oriented. acknowledgement  this work was developed under the instituto de telecomunicações project evaltubes and supported by the projects: uid/eea/50008/2013, sfrh/bd/81856/2011 and sfrh/bd/81857/2011 of the portuguese science and technology foundation (fct). this support is gratefully acknowledged. references  [1] y-j kim, s-s lee, “eddy current probes of inclined coils for increased detectability of circumferential cracks in tubing”, ndt & e international, july, 2012, vol. 49, pp. 77-82. [2] d. h. hur, m. s. choi, d. h. lee, s. j. kim, j. h. han, “a case study on detection and sizing of defects in steam generator tubes using eddy current testing”, nuclear engineering and design, january, 2010, vol. 240, no. 1, pp. 204-208. [3] j.bruce nestleroth, richard j. davis, “application of eddy currents induced by permanent magnets for pipeline inspection”, ndt & e international, january, 2007, vo. 40, no. 1, pp. 77-84. [4] helena g. ramos, tiago rocha, jakub král, dario pasadas, artur l. ribeiro, “an svm approach with electromagnetic methods to assess metal plate thickness”, measurement, august, 2014, vol. 54, pp. 201-206. [5] tianlu chen, gui yun tian, ali sophian, pei wen que, “feature extraction and selection for defect classification of pulsed eddy current ndt”, ndt & e international, september, 2008, vol. 41, no. 6, pp. 467-476. [6] d. pasadas, t. rocha, h. g. ramos, a. l. ribeiro, “evaluation of portable ect instruments with positioning capability”, measurement, january, 2012, vol. 45, no. 1, pp. 393-404. [7] d. kim, l. udpa, s. udpa, “remote field eddy current testing for detection of stress corrosion cracks in gas transmission pipelines”, materials letters, june, 2004, vol. 58, no. 15, pp. 2101-2104. [8] dario pasadas, tiago rocha, artur l. ribeiro, helena g. ramos, “ect characterization of a linear defect from multiple angle measurements”, 19th symposium imeko tc 4 symposium and 17th iwadc workshop advances in instrumentation and sensors interoperability, barcelona, spain, july 18-19, 2013, pp. 218-221. [9] a. l. ribeiro, h. g. ramos, j. gonçalves; "thickness measurement of a nonmagnetic metallic plate using harmonic eddy current excitation and a gmr sensor", proc imeko tc4 symp., natal, brazil, , september, 2011, pp. 1-6. figure 7. phase difference between the excitation current and each sensor signal  for  defect  detection  of  a  transversal  defect  applying  sinusoidal current with amplitude 150 ma to the excitation coil.  figure 8. phase difference between the excitation current and each sensor signal  for  defect  detection  of  a  transversal  defect  applying  a  sinusoidal current with amplitude 50 ma to the excitation coil.  acta imeko | www.imeko.org  june 2015 | volume 4 | number 2  | 67  [10] b. yang, x. li, “pulsed remote field technique used for nondestructive inspection of ferromagnetic tube”, ndt&e international, january, 2013, vol. 53, pp. 47-52. [11] d. l. atherton, s. sullivan, “the remote-field through-wall electromagnetic inspection technique for pressure tubes”, materials evaluation, december, 1986, vol. 44, no. 13, pp. 1544–1550. [12] y. he, f. luo, m. pan, f. weng, x. hu, j. gao, b. lui, “pulse eddy current technique for defect detection in aircraft riveted structures”, ndt&e international, , mar. , 2010, vol. 43, no. 2, pp. 176-181. [13] k. kosmas, c. sargentis, d. tsamakis, e. hristoforou, “nondestructive evaluation of magnetic metallic materials using hall sensors”, sensors and actuators a: physical, 2005, vol. 161, no 1-2, pp. 359-362. [14] d.j. pasadas, a. lopes ribeiro, t. j. rocha, h. g. ramos, “remote field eddy current ndt in tubes using anisotropic magneto-resistors”, proc. 13th imeko tc10 workshop on technical diagnostics advanced measurement tools in technical diagnostics for systems reliability and safety, warsaw, poland, june 26-27, 2014, pp. 60-64. [15] a. jander, c. smith, and r. shneider, “multi-frequency ect with amr sensor”, ndt & e international, september, 2011, vol. 44, no. 5, pp. 438-441. [16] l. kufrin, o. postolache, a. l. ribeiro, h. g. ramos, "image analysis for crack detection", proc ieee international instrumentation and technology conf., austin, united states, may, 2010, vol. 1, pp. 1096 1100. [17] j.h. espina-hernández, e. ramírez-pacheco, f. caleyo, j. a. pérez-benitez, j.m. hallen, “rapid estimation of artificial nearside crack dimensions in aluminium using a gmr-based eddy current sensor”, ndt & e international, october, 2012, vol. 51, pp. 94 – 100. microsoft word 36-186-1-pb.doc acta imeko  july 2012, volume 1, number 1, 4  www.imeko.org    acta imeko | www.imeko.org  july 2012 | volume 1 | number 1 | 4  editorial: the revival of acta imeko   paul p.l. regtien   measurement science consultancy, julia culpstraat 66, 7558jb hengelo, the netherlands    keywords: journal; imeko   citation: paul p.l. regtien, editorial, acta imeko, vol. 1, no. 1, article 4, july 2012, identifier: imeko‐acta‐01(2012)‐01‐04  editor: paul regtien, measurement science consultancy, the netherlands  copyright: © 2012 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  corresponding author: paul p.l. regtien, e‐mail: paul@regtien.net    the first issue of the official publication of imeko was published in 1958, the year that imeko was founded. the title of this issue was simply “acta imeko 1958”, with subtitle “proceedings of the international measurements conference” printed in 5 languages on the second page (figure 1). the publication was issued by the hungarian sci. soc. for measurement & automation. that issue contains the papers presented at the first imeko world congress, and since then is organized every three years. during many years, the contributions to imeko world congresses were published in acta imeko. later, the proceedings of the world congresses were issued by the organizers as proceedings, and the name acta imeko disappeared from these printed versions. in the last decades, the printed proceedings were gradually replaced by proceedings on cdrom’s, and more recently on memory sticks. the proceedings of the tc-events followed the same development. selected papers from world congresses and tc events were published in a newly established imeko journal “measurement” which first issue came out in 1983. this journal evolved gradually into an international journal published by elsevier, with scientific articles not only from imeko events but from scientists all over the world working in the realm of measurement and instrumentation. the restricted number of issues per year prevents publication of the many qualified contributions to imeko workshops, symposia and congresses. this was a major reason for the general council of imeko to decide in 2009 to publish an e-journal. its main goal is the enhancement of academic activities of imeko and a wider dissemination of scientific output from tc events. starting a new journal is not just a matter of pushing a button. i am much indebted to the persons who assisted with the preparatory work. special thanks go to dirk röske who is supervising the electronic platform underpinning this journal, pedro ramos who acted as the first guest editor and in this capacity had to brake fresh ground, and francisco alegria who designed the journal’s template and act as the layout editor for all articles in this issue. without their help it was not possible to create this first electronic version of acta imeko as a modern successor of the original printed versions. hopefully this first special issue will soon be followed by many others presenting selected papers from future imeko events to the world-wide community of measurement scientists and engineers.   figure 1. image of the second page of the first issue of acta imeko, 1958.  microsoft word 247-1780-2-le-rev acta imeko  issn: 2221‐870x  september 2015, volume 4, number 3, 47 ‐ 52    acta imeko | www.imeko.org  september 2015 | volume 4 | number 3 | 47  broadband corbino spectroscopy and stripline resonators to  study the microwave properties of superconductors  marc scheffler 1 , m. maximilian felger 1 , markus thiemann 1 , daniel hafner 1 , katrin schlegel 1 ,   martin dressel 1 , konstantin s. ilin 2 , michael siegel 2 , silvia seiro 3 , christoph geibel 3 , frank steglich 3   1  1. physikalisches institut, universität stuttgart, pfaffenwaldring 57, 70569 stuttgart, germany  2  institut für mikro‐ und nanoelektronische systeme (ims), kit, hertzstraße 16, 76187 karlsruhe, germany  3  max planck institute for chemical physics of solids, nöthnitzer straße 40, 01187 dresden, germany      section: research paper   keywords: superconductivity; microwaves; optical response; microwave spectroscopy  citation: marc scheffler, m. maximilian felger, markus thiemann, daniel hafner, katrin schlegel, martin dressel, konstantin s. ilin, michael siegel, silvia  seiro, christoph geibel, frank steglich, broadband corbino spectroscopy and stripline resonators to study the microwave properties of superconductors,  acta imeko, vol. 4, no. 3, article 8, september 2015, identifier: imeko‐acta‐04 (2015)‐03‐08   editor: paolo carbone, university of perugia, italy  received february 11, 2015; in final form april 9, 2015; published september 2015  copyright: © 2015 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: this work was supported by deutsche forschungsgemeinschaft (dfg)  corresponding author: marc scheffler, e‐mail: scheffl@pi1.physik‐uni‐stuttgart.de    1. introduction  superconductivity belongs to the most intensively studied phenomena of solid state physics. this research is driven both by the interest to gain fundamental understanding of electronic properties of solids and the aim to establish new applications of superconductors or to improve the existing ones. both fields require experimental techniques to measure the characteristic material properties that characterize a superconductor. while certain “static” approaches like measuring the dc resistivity, the dc magnetization, or the specific heat are routine approaches in the material development and characterization of superconductors, electrodynamics experiments are much less common. in this contribution, we first motivate why microwave studies are particularly helpful in superconductivity research and then we present two complementary techniques, the broadband corbino approach and the multi-frequency stripline resonators, that we have implemented successfully to study different types of superconductors using microwaves. 1.1. optical response of superconductors  though microwave experiments are technically performed in a very different manner compared to conventional optics, the actual outcome is rather similar: one probes the electrodynamics of the sample under study. therefore, we briefly review the optical response of superconductors with a special focus on microwave frequencies [1], [2]. the crucial energy scale of the optical response of a superconductor is the superconducting energy gap  that in the case of a weak-coupling bcs superconductor is related to the critical temperature tc via 2 = 3.53 kb tc. with typical tc of the order of a few k, the energy gap is found in the range of abstract  superconducting materials are of great interest both for the fundamental understanding of electrons in solids as well as for a range of  different applications. studying superconductors with microwaves offers a direct experimental access to the electrodynamic response  of these materials, which in turn can reveal fundamental material properties such as the superconducting penetration depth. here we  describe two different techniques to study superconductors at microwave frequencies: the broadband corbino approach can cover  frequencies from the mhz range up to 50 ghz continuously but is limited to thin‐film samples whereas the stripline resonators are  sensitive  enough  to  study  low‐loss  single  crystals  but  reveal  data  only  at  a  set  of  roughly  equidistant  resonant  frequencies.  we  document the applicability of these two techniques with data taken on an ultrathin tan film and a single crystal of the heavy‐fermion  superconductor cecu2si2, respectively.  acta imeko | www.imeko.org  september 2015 | volume 4 | number 3 | 48  µev to mev, i.e. up to thz frequencies. this means that at microwave frequencies one operates well within the superconducting energy gap and therefore probes the response of the superconducting ground state of the material. at these frequencies, two types of charge carriers contribute to the electrodynamics: firstly, there are the cooper pairs that constitute the superconducting condensate, the fundamental manifestation of superconductivity. secondly, there are thermally excited quasiparticles that are lossy (i.e. ohmic with respect to conduction) like conventional metallic electrons, but that do not show up in dc resistance measurements because they are short-circuited by the loss-less cooper pairs. microwave measurements probe both kinds of electrons: the quasiparticles determine the real part 1 of the complex conductivity  = 1 + i 2 whereas the cooper pairs dominate the imaginary part 2. in particular, 2 exhibits a 1/ frequency dependence (= 2f ), which can be explained via kramerskronig transformation of the ()-peak of the cooper pairs in 1. the magnitude of the 1/ dependence directly depends on the cooper pair density, which in turn is related to the superconducting penetration depth. 1.2. general aspects of optical and microwave measurements  optical measurements are contact free and usually only demand a sample significantly bigger than the wavelength, i.e. typically of size of order mm and with at least one flat surface. compared to other spectroscopic techniques, these requirements can often be met rather easily. however, in the case of conducting samples, the crucial question is whether the sensitivity of a particular optical technique is sufficient to resolve the features of interest. in particular for superconductors, this is a stringent restriction as the optical reflectivity of a bulk superconductor is very close to unity and therefore is extremely difficult to study even with state-of-theart optical spectrometers of the relevant spectral range, i.e. typically at thz and/or infrared frequencies. at microwave frequencies, the situation is in principle even more difficult, as the lower frequency of the employed radiation directly means reflectivity yet closer to unity. however, because of the rather long wavelength of microwave radiation, typically of order cm, the employed experimental techniques do not follow optical routines, but are intrinsically different ones where the sample as well as the guides for the microwave radiation are smaller than the wave-length. i.e. although the fundamental physics of the microwave response of superconductors follows the theory of “optics of superconductors”, the experiments are rather different than a conventional optics approach. 2. microwave measurements on superconductors  a very well established and very versatile technique to study the microwave properties of conducting materials is cavity resonators [3]. here, a three-dimensional conducting cavity has dimensions of order of the wavelength, and the sample under study acts as a small perturbation. the major disadvantages of this technique are that it is usually operated only at a single frequency (i.e. one does not gain any spectral information) and that it is difficult to obtain quantitative information due to the in general poorly defined geometry (whereas relative information, e.g. as a function of temperature, is obtained easily). therefore, different new experimental techniques have been devised. these can be roughly divided into two classes: the first group comprises truly broadband techniques that cover the full frequency dependence. here two approaches have been applied to the study of superconductors, namely the corbino approach [4], [5] and the bolometric approach [6]. the main difference in their capabilities is that the corbino approach reveals phase information (giving amplitude and phase of the response) but has a rather poor sensitivity, whereas the bolometric approach is very sensitive but does not reveal phase information. the second group is based on resonators that can be operated at several frequencies. while our approach below is based on onedimensional transmission line structures, namely superconducting stripline resonators [7], [8], a threedimensional resonator with a dielectric resonator in its core has recently also been established successfully [9]. 3. broadband corbino spectroscopy  in corbino spectroscopy, the flat sample terminates the open end of a coaxial cable (schematically shown in figure 1). in this way, the sample reflects the microwave signal traveling in the coaxial cable, and the reflection coefficient s11, which can conveniently be measured with a network analyzer, then directly yields the impedance zl of the sample via the following relation (with z0 = 50  the characteristic impedance of the coaxial cable): 11 11 0 1 1 s s zz l    (1) while this approach is commonly used at room temperature and in particular in the study of dielectrics [10], it is much more difficult for the case of superconductors. this is due to the elaborate calibration procedure that has to be applied to correct for the strongly temperature-dependent transmission properties of the coaxial cable [4], [5]. different groups have come up with different schemes how this challenge can be met most successfully. it turns out that the best approach can differ drastically depending on the particular goal. in contrast to groups that concentrate on detailed studies of a single sample (or a selected few) [11], we have focused on a procedure that allows a high throughput combined with rather low temperatures. presently, we operate a corbino spectrometer with one cooldown per day, base temperature of 1.1 k (in a 4he cryostat), frequency range 45 mhz to 50 ghz, and two samples measured during each cooldown. with this rate, we can easily study extended sets of samples. current examples are nbn and tan thin films as a function of deposition parameters. ultrathin films of these materials with thicknesses about 5 nm or even smaller are interesting both for applications in superconducting singlephoton detectors [12], [13] as well as for understanding of the so-called superconductor-to-insulator transition (sit) [14], [15]. the typical critical temperatures of these materials are of order 5 k to 10 k, and as a result the energy gap is located in the figure 1. scheme of a corbino spectrometer. the network analyzer (nwa)  measures the reflection coefficient of the microwave signal that travels in the coaxial cable and is reflected by the sample.  acta imeko | www.imeko.org  september 2015 | volume 4 | number 3 | 49  thz frequency range, much higher than accessible with corbino spectroscopy. however, for such ultrathin films, thz transmission measurements are easily possible and can directly determine the energy gap, whereas these studies usually have difficulties to resolve quasiparticle dynamics at the lowfrequency limit of their spectral range [1], [16]. therefore, the combination of corbino spectroscopy at ghz frequencies and thz transmission measurements is an ideal match to study the complete charge dynamics in superconductors, if the materials are available as very thin films. if a conductive thin sample is studied in corbino spectroscopy and if the microwave fields and current density are uniform through the sample thickness (which usually is the case for conductive thin films of thickness up to several tens of nm), the complex conductivity of the sample can directly be calculated from the sample impedance zl as follows: lzd aa   2 )/ln( 12 (2) here d is the thickness of the superconducting thin film and a1 and a2 are the inner and outer diameters of the corbino disk, respectively [4], [5]. this corbino disk is created by deposition of concentric conductive contacts (e.g. made of gold) onto the superconducting film, and it defines the geometry of the sample impedance zl that is probed. the thin-film samples are usually deposited onto dielectric substrates, and typically these substrates do not affect corbino microwave measurements much in the frequency range below 20 ghz [17], [18]. in figure 2 we show typical microwave spectra, both real and imaginary parts of the conductivity, for a tan film of thickness 5 nm that was sputtered onto a sapphire substrate. for temperatures above the critical temperature tc = 8.9 k, the real part 1 is frequency independent and the imaginary part 2 basically vanishes, as expected for a conventional metal at ghz frequencies. below tc, 1 rises strongly upon cooling. our frequency range is well below the energy gap, and therefore 1 is not suppressed yet compared to the normal-state value even for the lowest temperature of 1.2 k [19]. the detailed frequency dependence reflects the density of states in the superconducting state and can phenomenologically be fitted with the drude formula over a substantial frequency range [19]. also 2 increases strongly upon cooling below tc, and its characteristic 1/f frequency dependence gives direct access to the cooper pair density and the penetration depth [1], [19]. corbino spectroscopy at 4he temperatures is thus well suited to study thin films of superconductors with tc of 1.5 k and higher. considering that for many spectroscopic studies one desires the photon energy in a similar range as the thermal energy, we have also implemented a corbino setup at 3he temperatures [20]. here the requirements for the calibration of the microwave coaxial cable in principle are even more stringent, but since in our laboratory we have a 4he corbino spectrometer well established, we limit our 3he spectrometer to the temperature range 0.45 k to 2.0 k, and we can use the overlap in temperature with the 4he setup for additional calibration purposes. all our studies mentioned so far, as well as corbino experiments on superconductors by other groups [11], [19]-[24], are limited to thin-film samples (thickness of a few tens of nm or less). the reason is the limited sensitivity of corbino spectroscopy [5]. but this is no restriction if the superconductor of interest can be prepared as thin films with the same material properties as the more commonly studied bulk samples or if the particular properties of interest only become manifest in thin films, like the sit. for highly disordered superconductors close to the sit, the conductivity in the superconducting state at frequencies below 2 has recently gained substantial attention because deviations from the canonical predictions of bcs theory might occur [25]-[28]. 4. superconducting stripline resonators  many unconventional superconductors, which are the topic of present research, are not available as thin film samples and as such not accessible with microwave corbino spectroscopy. many of these are ternary, quaternary, or even more complex intermetallics, and growth of high-quality thin films is extremely demanding [29]-[31]. furthermore, tc of unconventional superconductors is often suppressed if the mean free path of the conduction electrons is reduced, and then the small thickness of thin films can be an intrinsic limitation to superconductivity in these materials. to still be able to study these materials with microwaves as a function of frequency, we have developed a resonant technique for bulk samples that can be operated at numerous, roughly equidistant frequencies. the general idea is that in a transmission line structure based on a stripline (also called triplate), as shown as cross section in figure 3, one of the ground planes, which surround the signalcarrying center strip, can be formed by the conducting bulk material of interest. to be sensitive to low-loss samples, the stripline is created as a one-dimensional transmission line resonator, which has a fundamental frequency typically between 1 ghz and 3 ghz (depending on sample size) and which can easily be excited at numerous harmonics, i.e. one obtains a set of data at roughly equidistant frequencies, spanning a range up to approximately 15 ghz at present. particular care has to be taken in the choice of the center conductor material: here, the density of the microwave currents is an order of magnitude larger than in the ground planes, and therefore the overall figure 2. microwave conductivity spectra of a tan thin film, measured with a corbino spectrometer.  acta imeko | www.imeko.org  september 2015 | volume 4 | number 3 | 50  losses of the stripline first of all depend on the center conductor. if we want to study a superconductor as ground plane, this means that the center conductor has to be fabricated from a thin film of an even “better” superconductor, i.e. one with even lower microwave losses at low temperatures. in general, such very low losses can be achieved if the resonator is operated at temperatures well below tc, i.e. materials with a high tc are desirable for the center conductor. however, the materials should be conventional s-wave superconductors, as superconductors with nodes have much higher losses at low temperatures due to the larger amount of thermally excited quasiparticles. this restriction excludes the high-tc cuprate superconductors, which are d-wave, from this application. therefore, we fabricate our stripline resonators either from nb or pb thin films [8], [32]. while we originally implemented this technique to study unconventional metals, such as heavy fermions with their peculiar microwave properties [33]-[39], it turned out that this technique is also well capable to study bulk superconductors if their tc is lower than that of the stripline material [8]. after test measurements on the conventional elemental superconductors ta and sn [8], we now apply it to the unconventional heavyfermion superconductor cecu2si2 [40]. while cecu2si2 was long believed to be a d-wave superconductor [41], recent studies instead suggest that it is a two-gap s-wave superconductor [42]. this question is of particular interest as it is related to the possible pairing mechanism for superconductivity, which is thought to be of magnetic nature in cecu2si2 [43]. one possibility to address whether a superconductor has an order parameter with nodes or not is measure the temperature dependence of its penetration depth. this can be done with microwaves. so far, no microwave studies on cecu2si2 have been reported. one difficulty here is the rather low tc of around 0.6 k. while this temperature can be reached with 3he cryostats, a detailed study of the penetration depth requires a base temperature of order tc/10 or lower, i.e. one has to employ a 3he/4he dilution refrigerator. here, another advantage of our stripline probe comes into play: its physical dimensions are much smaller than the fundamental wavelength, as this onedimensional resonator can be shaped as a meander. as a result, the typical mounting of resonator and sample amounts to just a few cm3, a volume that can easily be incorporated in a commercial dilution refrigerator. furthermore, the microwave signals are guided from the room-temperature electronics to the cryogenic probe and back via coaxial cables, which is much more generic than waveguides that are often used for threedimensional cavity resonators. therefore, the stripline resonator is a widely applicable, very sensitive technique to study the microwave response of superconductors. in contrast to many other resonator approaches, here it is rather simple to obtain data at several different frequencies, i.e. spectral information. while for addressing the penetration depth, which in this range basically is frequency independent, a single resonator frequency might be sufficient, there are other questions of interest, such as the quasiparticle dynamics, where data as a function of frequency are crucially required. these frequency dependences are usually broad, and therefore the rather restricted spectral resolution of order 1 ghz is sufficient, whereas the truly broadband corbino approach has a much better frequency resolution that can also reveal narrow spectral structures. as a demonstration for mk measurements using a stripline resonator, in figure 4 we show data obtained on a cecu2si2 single crystal for two resonator modes, at frequencies 4.36 ghz and 6.52 ghz, measured during the same temperature sweep. when cooled below tc, the surface resistance rs decreases strongly due to the reduction of microwave losses. at the same time, the resonator frequency f0 increases because of the decreasing penetration depth. above tc, in the metallic heavyfermion state, rs is also temperature dependent, even for these low temperatures below 1 k. this indicates that the electronic conduction is governed by intrinsic scattering mechanisms (in contrast to temperature-independent impurity scattering), and it shows promise for future studies of the microwave response of heavy fermions in the normal state of cecu2si2, an interesting field of study on its own [33]-[38]. comparing the two resonator frequencies reveals that the superconducting penetration depth well below tc does not depend on frequency, whereas rs is noticeably enhanced for the higher frequency, both below and above tc, as expected. our stripline approach thus can reveal the frequencydependent microwave response of a superconductor. in the present case of cecu2si2, where a 3he/4he dilution refrigerator was employed, measurements down to 30 mk were performed. thus a wide range of unconventional low-tc superconductors can be studied with this technique. recently, another microwave approach, namely a multimode dielectric resonator [9], was used to study the heavy-fermion superconductor cecoin5 [44]. these two techniques have rather complementary figure  4.  temperature  dependence  of  stripline  resonator  frequency  f0 (normalized  to  the  frequency  at  the  lowest  temperature)  and  sample  surface  resistance  rs  measured  on  a  cecu2si2  single  crystal  sample.  both  quantities are shown for two different resonator modes.  figure 3. schematic cross section of a stripline as used in resonator studies. the  signal‐carrying  center  strip  (thickness  t  =  1  µm,  width  w  =  45  µm)  is  sandwiched between two ground planes at distance h = 127 µm (thickness of dielectric spacers, made of sapphire, with dielectric constant r). center  strip and lower ground plane are made of a low‐loss superconductor (pb or nb),  whereas  the  bulk  superconducting  sample  of  interest  acts  as  upper ground  plane  of  the  stripline.  the  microwave  fields  enter  the superconductors within the superconducting penetration depth  (indicated  as shaded regions).   acta imeko | www.imeko.org  september 2015 | volume 4 | number 3 | 51  advantages: our stripline approach offers a wide and flexible range of numerous ghz frequencies, it can be used to resolve anisotropy in the microwave response, and due to its compact design it can conveniently be installed into a wide range of dilution refrigerators (thus achieving even lower temperatures than those demonstrated in [44]), whereas the multimode dielectric resonator can be operated in high magnetic field, which is not possible for our stripline approach based on the superconductors pb or nb. 5. conclusions  we have discussed why microwave measurements on superconductors are powerful tools to understand their electrodynamics and to obtain detailed information about fundamental characteristics of superconductivity, such as the temperature-dependent penetration depth. in recent years, several groups have developed new techniques for cryogenic microwave spectroscopy. while the corbino approach is particularly popular and offers the possibility to cover a very large frequency range completely, it is limited to the study of very thin films. as an alternative, we have established a rather different technique, which are stripline resonators where the bulk sample of interest is one ground plane of the stripline. with these two approaches available, we can cover a very large range of superconducting materials and address their microwave properties both for fundamental research as well as for materials optimization for applications. acknowledgement  we thank g. untereiner for assistance 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[44] c.j.s. truncik, w.a. huttema, p.j. turner, s. özcan, n.c. murphy, p.r. carrière, e. thewalt, k.j. morse, a.j. koenig, j.l. sarrao, d.m. broun, nodal quasiparticle dynamics in the heavy fermion superconductor cecoin5 revealed by precision microwave spectroscopy, nat. commun. 4 (2013) article 2477. microsoft word article 15 171-1324-1-le.doc acta imeko  february 2015, volume 4, number 1, 97 – 104  www.imeko.org    acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 97  set up and characterization of a measuring system for pq  measurements in a mv site with pv generation  gabriella crotti, domenico giordano  istituto nazionale di ricerca metrologica, strada delle cacce, 91, 10135 torino, italy       section: research paper   keywords: non‐conventional transformers, pq measurements, mv substation, pv plants  citation: gabriella crotti, domenico giordano, set up and characterization of a measuring system for pq measurements in a mv site with pv generation,  acta imeko, vol. 4, no. 1, article 15, february 2015, identifier: imeko‐acta‐04 (2015)‐01‐15  editor: paolo carbone, university of perugia   received january 15 st , 2014; in final form november 21 st , 2014; published february 2015  copyright: © 2014 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: the research leading to the results here described is part of the european metrology research program (emrp), which is jointly funded by the  emrp participating countries within euramet and the european union.  corresponding author: domenico giordano, e‐mail: d.giordano@inrim.it    1. introduction  the measurement of the quality of the electrical supply is today gaining more and more attention. this is due to the need of accurate evaluation of the power quality for the assessment of the compliance to the limits indicated for public distribution networks, the identification and location of disturbances and the verification of the impact of newly connected distributed sources [1]-[5]. as to the power quality (pq) measurement systems, attention is generally mainly focused on the performances of the measuring instruments. test procedures, expected performances and uncertainties are extensively prescribed for the measuring instruments only in the relevant standards [6], [7]. when dealing with pq measurements in medium and low voltage grids, a reduction of current and voltage level is needed. to this end, the instrument transformers already available in the measurement site are generally used. however, their use for the measurement of pq events cannot be considered completely satisfactory and their contribution to the overall uncertainty can be significant [8]. as an alternative, non-conventional transformers are more and more considered, because of their better performances regarding linearity and frequency response and their low level power output [9]-[13]. prescriptions for the measurement uncertainties associated to transducers used in pq measurements are given in [15]. according to this, the percentage error has to be within 1% of the measured value for the first and second harmonic and within 5% for the higher order ones. with reference to the required performances, the features of different types of current and voltage sensors are analysed in [8]. to estimate and reduce the uncertainty of the pq parameters, the characteristics of the adopted transducers and their performance in relation to the expected measurement conditions including the circuit sensors arrangement and the surrounding environment should be known. to this abstract  the  features  of  a  power  quality  measurement  system  which  includes  both  voltage  and  current  transducers  and  a  self‐developed  measuring instrument are described. the system is intended for measurements in substations at medium voltage level. a rogowski  coil and a voltage divider are the used transducers, whereas the measuring instrument is based on a reconfigurable i/o‐fpga system  with embedded software. attention is focused on the procedures adopted for the characterization of the voltage and current sensors,  which  are  carried  out  taking  into  account  the  expected  on‐site  measurement  conditions.  an  example  of  use  of  the  systems  for  measurement of some pq parameters  in a private substation which connects an  industrial  load and two photo‐voltaic generation  plants to the public mv voltage network is presented, together with the associated voltage and current uncertainty budget. by making  use of the results obtained in the characterization phase, the current and voltage measurement uncertainty can be reduced.  acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 98  end, an extended and focused transducer characterisation is needed. the paper deals with a system for pq measurement at medium voltage (mv) up to 24 kv rms, which makes use of a voltage divider and three rogowski coils (rc) and is completed by a measuring instrument based on a national instrument compact reconfigurable i/o programmable automatic controller platform. the system is intended for indoor use in a mv site with photo-voltaic (pv) generation. the use of a clamp-on and flexible coil for the current measurement enables its positioning in harsh and narrow sites, where measurement conditions quite far from the laboratory ones are met, without need of power circuit disconnection and/or outage. however, the same features, flexibility and presence of a coil turn gap, make the sensor more sensitive to perturbing effects, such as those due to the position of the primary or other current-carrying conductors, and less accurate with respect to rigid, closed coils, with expected measurement uncertainty of the order of one percent or higher [17]. as to the voltage transducer, a solid insulated voltage divider is selected, to exploit its linearity and frequency performances. for this kind of transducer, the effects due to the divider capacitive coupling with close metallic elements at ground or floating potential and the environmental conditions have to be considered and controlled. attention is therefore focused on the characterisation procedure of the sensors that, if carefully performed under conditions that reproduce the experimental ones, can allow the decrease of the on-site measurement uncertainty and enables their use in compliance with the given accuracy limits [8], [15]. application of the system to pq measurements in a private mv/lv substation that supplies an industrial load and is connected both to the public supply network and to photo-voltaic (pv) generation plants is eventually shown and discussed. 2. the measurement system  the scheme of the pq measurement system is shown in figure 1. the system includes a voltage divider and three commercial flexible rogowski coil (rc) with the associated integrator as transducers, a ni compact reconfigurable i/o based measuring instrument and the connection to a gps receiver for time synchronisation [16]. in the following, a description is given of the measuring system components and of the relevant characterisation tests carried out to estimate the uncertainty contributions to be considered in the on-site use of the current and voltage sensors. 2.1. the current transducer   the on-site operating conditions are generally quite different from those taking place in the laboratory when calibrating the system components. they can significantly affect the performances of the sensor as a function of its construction characteristics. in particular for clamp-on rcs, the path of the primary and return conductors, the presence of external magnetic field sources and the coil nonorthogonality with respect to the primary conductor can lead to a considerable variation of the coil mutual inductance, whose amount depends on the sensor characteristics and construction solutions [18]. taking into account the expected measurement location (measurement in a mv/lv substation on the mv side) and the range and type of currents to be measured (from hundreds of milliamperes to several tens of amperes), commercial flexible and clamp-on shielded rogowski coils, equipped with a three channel integration system, have been selected. the rated characteristic of the rcs and associate integrator are summarised in table 1. to investigate the sensitivity of the used coil to influencing and perturbing factors and predict their effect on the expected measurement accuracy, an extensive characterisation of the rc with the associated integrator is carried out in the laboratory, by reproducing configurations similar to those expected on-site. characterisation of the current sensor includes: i) calibration at power frequency and determination of the rc transformation ratio; ii) linearity over the expected measurement range; iii) frequency response determination; iv) evaluation of return conductor proximity effects and perturbing source presence; v) estimation of the temperature effect. a scheme of the supply and measuring circuit for the calibration at power frequency and the linearity tests is shown in figure 2a. the current is generated by a clarke-hess 8100 transconductance amplifier (0.2 ma to 100 a, dc to 100 khz) supplied by a stable waveform generator (fluke 5500). the current value is obtained by measuring the voltage figure 1. pq measuring system scheme.   table 1 – rc coil and integrator rated characteristics.  type   rogowski coil with integrator  rc length  400 mm  rc section   44 mm 2   integrator selectable output ranges  20  a/v,  100  a/v,  ...  10  ka/v  (overrange 1000%)  integrator temperature coefficient   < 0.01%/°c  rc+integrator temperature coefficient   0.04%/°c  bandwidth of the rc+integrator  1 hz to 100 khz  rc + integrator rated accuracy  1%  acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 99  across a calibrated non-inductive shunt (tinsley 0.05  standard resistor). the reference and rc signals have been acquired by two synchronised agilent 3458a multimeters, exploiting their digitising features, with a sampling frequency varying from 1 khz to 1 mhz depending on the supply frequency. the measurements are carried out by centring the coil around the primary conductor by a rigid, but easily moveable non-conductive support to limit the uncertainty contribution due to coil positioning. both the shape of the primary conductor and its diameter are chosen close to those of the mv cables (figure 2b). linearity is investigated at 50 hz by generating currents in the range 0.1 a to 100 a with the return conductor 30 mm distant from the coil loop and opposite with respect to the coil gap. the characterization is carried out for the range 20 a/1 v, exploiting the overrange capabilities of the selected coils (table i). the relative deviations of the measured rc ratio from the rated one are found within 610-4 over the investigated current range. a systematic increase of +1×10-3 is measured when the acquisition and measurement system is included in the measurement chain. a relative decrease of about 2×10-4 a for currents lower than 1 a occurs due to the lower values of the digitiser input voltage. regarding the frequency investigation, the impedance magnitude and phase of the coil, without the integrator, have been first measured by an agilent 4294a impedance analyser. figure 3 compares the measured behaviour with that computed by considering a simple rcl coil model, whose parameter values are indicated in the graph. the resonance frequency is around 300 khz. the frequency behaviour of the rc with integrator and connection cable is then measured in the same circuit used for the linearity evaluation. current is measured in this case by a guildline 7320 ac shunt, whose ac/dc difference is within 200 ppm at 100 khz. exploiting the use of stable and repetitive signals, evaluation of the rc error is carried out with the agilent digitisers configured in dcv mode [14] from few hertz up to hundreds of hertz and in time equivalent sampling mode (also named subsampling mode) [14] for higher frequencies. since first experimentation of the system is planned on a site where a wide harmonic spectrum can be expected (pv generation, with inverter switching frequency around 20 khz), measurements are carried out up to some tens of kilohertz. figures 5a and 5b show the rc transformation ratio, normalised to the rated one, and the phase error measured over the range 20 hz ÷ 40 khz with the return conductor 35 mm distant. limit values given in [15] for the 1st to the 50th harmonics for the ratio and phase errors are shown for comparison by the red continuous lines. the dashed lines are limit values extrapolated to the lower and higher harmonic ranges, where no indication is presently given. measurements are then repeated by varying the minimum distance d of the return conductor from the coil loop from 30 mm to 200 mm. rc ratios normalised to the rated value are shown in figure 6, for two distances d of the return conductor (conf. i) 30 mm and ii) 200 mm) with the coil gap external to the current loop as shown in figure 2b. the behaviour measured with the gap internal, when distance d is 200 mm (conf. iii), is also shown. a quite constant deviation of about 3.2×10-3 is found between conf. i) and conf. iii). the influence of external magnetic field sources is then investigated by measuring the coil stray signal induced by a 400 a current flowing in a conductor external to the coil loop, placed at increasing distances d. the induced stray signal is 2 mv for d=100 mm and reduces to 0.5 mv when d is 600 mm. finally the response of the coil in the worst positioning conditions with respect to the primary conductor, that is a)  b)   c)    figure 2.   a)  linearity,  proximity  and  frequency  test  circuit;  b)  rc  coil  experimental  arrangement  with  the  coil  centered  on  the  primary  conductor; c) rc coil experimental arrangement with the coil hung from  the primary conductor.   figure 3. measured magnitude and phase of the rc impedance.  acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 100  with the coil directly suspended on it as shown in figure 2c, is also determined. the relative deviation of the rc ratio from the value measured with the centred coil increases up to 2%, when the gap is close to the conductor. 2.2. the voltage transducer   the voltage sensor is a resistive divider with rated scale factor sf 10000, 1 m rated load, rated primary voltage um=24 kv, withstand voltage 50 kv, rated accuracy class 0.5. the divider is polyurethanic resin insulated and is equipped with a 10 m coaxial cable. dielectric tests (power frequency withstand voltage, impulse test voltage, partial discharge test) are first carried out and successfully passed by the divider. characterisation tests performed include: i) sf and phase errors determination at 50 hz up to 24 kv/3; ii) proximity effects analysis at 50 hz; iii) frequency response; iv) temperature dependence. the divider sf is measured at 50 hz by comparison with an inrim standard measurement voltage transformer (uncertainty 50 ppm and 50 µrad for their ratio and phase errors respectively). from 5 kv to 12 kv a relative decrease of 110-3 of the sf value and an increase in the phase error up to 1 mrad are detected, with respect to the error measured at 5 kv. the characterisation at mv voltage is repeated by inserting the divider inside a grounded metallic box with side 80 cm (figure 6). the stray capacitive couplings between the high voltage elements and the surrounding metallic surfaces do not significantly modify the divider sf, but lead to an increase of the phase error of 3 mrad. a systematic correction for the phase angle can then be considered in case of use of the divider inside the grounded box. the frequency response is investigated at low voltage (600 v) from dc up to 30 khz. the divider ratio error complies with the standard requirements up to 1.4 khz. at increasing frequency the divider response amplifies the harmonic amplitude. this effect is exploited to better detect the low amplitude high frequency components. the measured value of the harmonic amplitudes and the thd are then evaluated by introducing the frequency corrections determined in the calibration phase. as an alternative, the correction of the divider frequency behaviour according to the method described in [19], can allow optimisation of its response. the variation of the sf in the range -5 °c to 40 °c is measured at low voltage (600 v). figure 7 shows the time evolution of the sf variation from the 23 °c value, with the divider inside a climatic chamber when the chamber temperature is set to 40°c and -5 °c. the sf deviation reached at thermal balance is +1.7‰ and -1.2% respectively. the mean temperature coefficient is, thus, 100 ppm/°c. several hours are needed to reach a steady sf value, due to the amount of mass of the insulated transducer. 2.3. the measuring instrument   the transducer voltage and current signals are digitized and pq parameters are estimated and stored by a ni compact reconfigurable i/o (ni compact rio) programmable automatic controller platform, which includes a crio embedded real-time controller with a 4 slots reconfigurable chassis, equipped with a userprogrammable fpga. two 10 v a/d cards (16 bit 1 msample/s/ch and 24 bit 50 ksample/s/ch), a gps synchronisation module and two sd cards for pq parameters recording complete the assembly. the ni compact rio platform and the acquisition and elaboration strategy are shown in figure 8. the quantities of interest: frequency, rms values, harmonics and thds are evaluated, in real time according to [6] and [7]. in particular, the algorithm for the frequency estimation, over a time interval of 10 s [6], and the current and voltage rms values are implemented on the fpga, while the thd and the harmonic content estimation algorithm is implemented in a real time operating figure 4.   measured rc plus integrator normalized ratio and phase errors  vs frequency.  figure 5.   normalized rc plus integrator ratio for different distances d of  the return conductor and coil gap position.  acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 101  system. this approach gives a real time estimation of the power quality parameters. different time aggregation starting from 3 s can be user selected, as a function of the measurement approach. presently, four files are stored in the sd cards: one contains the time stamp and the corresponding frequency value, the second stores the time stamp, the voltage and current thd, the rms values of one voltage and three currents and corresponding first harmonics. the last two files contain the time stamp and harmonic contents for one voltage and one current phase, respectively. 2.4. uncertainty budget  the uncertainty budget for the rms current values from 3 a to 40 a is shown in table 2. it is evaluated taking into account the calibration uncertainty of the assigned rc coil ratio and associated measurement instrument (acquisition and evaluation system), its linearity, the effect of the other mv conductors and that of the plant lv conductors. two estimates are presented: in the first case, a more conservative evaluation is given (i), whilst the second one (ii) is performed considering correction factors and uncertainties relevant to the specific measurement conditions, evaluated taking into account the results obtained in the characterisation carried out as previously detailed. the resulting uncertainty (95% coverage level) reduces from 2% to 4×10-3. as to the rms voltage, and taking into account the contributions respectively due to mv divider linearity from 5 kv to 12 kv and the temperature variation in the range (2310) °c, the measuring system relative uncertainty (95% coverage probability) is estimated to be within 2×10-3. 3. on‐site measurement  the developed system has been deployed in a power quality measurement campaign carried out in a private mv/lv 15 kv/400 v, 70 a private substation. the substation is connected to the public medium voltage distribution network and supplies a factory equipped with two pv generation plants, respectively rated 200 kw and 800 kw. the private network can absorb up to 800 kw from the distribution network operator (dno) and supply up to 1 mva. voltage and current are measured on the mv line connecting the 800 kva step-up transformer to the dno mv network, by making use of the measurement system equipped with the characterised voltage and current transducers and the measuring system previously described. in the following, the measurement arrangement, with specific reference to the solutions adopted for connection of the voltage and current transducers is described. 0 1 2 3 4 6 7 8 9 -1.2 -0.8 -0.4 0.0 0.4 0.8 1.2 1.6 2.0 s f d e vi a tio n ( ‰ ) elapsed time (h) measured sf deviation: 40 °c measured sf deviation: -5 °c fitted sf deviation: 40 °c fitted sf deviation: -5 °c   figure 7.   time evolution of the divider sf relative variation from 23 °c.     figure 6.   voltage  calibration  with  the  divider  inside  a  metallic  grounded  box.   table 2 – uncertainty budget (rms current from 3 a to 40 a). quantity/influence factor  relative  standard  uncertainty (10‐3)  conf. (i)   relative standard   uncertainty (10‐3)  conf. (ii)  rc and acquisition system  ratio   0.25  0.25  linearity   1.5  1.0  return  conductor,  gap  position  variation  and  orthogonality  of  the  sensor  6.5  30 mm<d<350 mm,  0.8   (30  mm<d<35  mm, gap  external,  sensor centered)   perturbing  magnetic  field  sources  7   (d>100 mm)   1.2   (d>600 mm)  ambient temperature   2.3        ta=(23 10) °c  1.15        ta=(23 5) °c  stability and repeatability  2  0.6  expanded rel. uncertainty  (95% coverage probability) 20  4,0    acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 102  3.1. the measurement arrangement   the connection of the sensors to mv level is a crucial point. as regards the voltage sensor, the connection point is made inside the metal enclosed air-insulated switchgear unit on the mv line connecting the 800 kw step-up transformer to the dno network. taking into account the dimensions of the cubicle, the electrical clearances to be respected between the introduced mv and lv components and conductors and the surrounding energized, earthed or floating elements, the divider is located outside the unit. to avoid any casual contact with live conductor the divider is placed inside the same grounded metallic box used in the calibration phase (figure 7) and the mv connection is made by means of a mv cable with suitable terminations. firm connections to the mv busbars inside the unit are established by means of bolted connections. the divider lo connector is earthed. in order to avoid dangerous overvoltages at the measuring instrument input, a surge arrester is connected between the lv divider output terminal and earth. regarding the current sensors, since their positioning on the bare power conductors inside the switchgear unit is not possible, the rcs are centred on the mv insulated cable between the mv transformer of the 800 kw pv plant and the switchgear unit by a suitable insulating support. attention is paid to locate the coil opening gap opposite to the other conductors: keeping it as far as possible from the other mv conductors. systematic effects of the other phase conductors placed in close proximity were quantified in the laboratory during the coil characterization phase, as described in the previous section. measurement of the stray signals due to the currents possibly flowing in the mv cable grounded shields was carried out with the pv plant disconnected. the influence of the stray currents was estimated to be within 5 part per thousand and a few percent for the lower current values. this component is then taken into account in the figure 8. ni compact rio and acquisition architecture. a) weather conditions: cloudy b) figure 9.   current measurement on two consecutive days  in a late autumn  day.   figure 10.  current waveforms normalized to the peak values detected at 11  a.m.,  when  the  current  thd  is  about  2%  a)  and  at  5  p.m.  when  thd  is  around 8% b). a) b) figure 11. current, voltage (rms and thd) and global irradiance measured  on two consecutive days in a late spring day. acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 103  uncertainty budget for the on-site current measurement. since the produced current is related to the solar conditions and in particular to the irradiance, a passive pyranometer is installed close to the solar panels [20]. the pyranometer output voltage is digitized by the measurement instrument simultaneously to the acquired current signal. 4. results and discussion  voltage and current measurement are performed under different weather conditions during different seasons. the current and voltage signals are digitised with 50 khz sampling frequency and the frequency, rms, thd, harmonic parameters are evaluated. a preliminary current analysis, carried out on the current waveforms with higher sampling frequencies (up to 500 khz), does not detect any harmonic of the 18 khz fundamental one. a quite significant variation in the maximum rms currents is found depending on the weather conditions, as shown in figure 9. to give evidence on the waveform distortion which dramatically depends on the generated current amplitude, thus on the weather conditions, figure 10 is introduced. it shows one period of the injected currents, normalized to their peak values, recorded at 11:00 a.m. during a sunny day when the related thd is about 2% and the rms is about 16 a and at 5 p.m. when the thd reaches 8 – 10 % while the rms falls down to 2 a (see figure 9). the measured rms current and irradiance show quite the same behaviour (figure 11a). as expected, the current thd significantly increases for values lower than some amperes. when the rms current decreases from 8 a (measured irradiance 0.3 w /m2) to 3 a (measured irradiance 0.2 w /m2), the corresponding current thd increases from 2.4% to 8.5%. however, the corresponding measured voltage behaviour remains quite unchanged (thd = 0.6%/0.7%). this can be explained considering the low value of the ratio of the pv plant power to the short-circuit power at the point of common coupling. the simultaneously measured voltage is shown in figure 11b. the rms voltage values slightly increase during the weather conditions: sunny evening/night hours. the harmonic analysis detects the presence of two significant components (the 5th and the 7th). analysis of currents recorded during one week period shows, as expected, a marked dependence on the weather conditions (fig 12). as regards the voltage, the measured behaviour again indicates an increase of the voltage during the nights. maximum variations of the rms voltage are found within 3%. the slight voltage increase (about 1%) during the night is more evident if the mean rms voltages measured in five working days in the day-time hours (7-19) are compared with those registered during the night (19-7). on the contrary, this voltage increase is far less evident or nor detected during the weekends. an example of the performances of the measuring system as regards the voltage harmonic analysis is shown in figure 13, where the daily behaviour of the three most significant harmonics, normalised to the fundamental one, are shown. harmonics voltage variations down to some hundreds of ppm of the fundamental one are well evidenced by the system. the uncertainty associated with the harmonic measurement is estimated to be within 0.25% of the power frequency component.     figure 12.  current and irradiance measurements over one week. figure 13.  daily  behavior  of  5th,  7th  and  11th  voltage  harmonics  detected  on  a  working  day  (amplitude  normalized  to  the  50  hz  component). acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 104  5. conclusions  pq measurement accuracy in mv and hv plants can significantly depend on the performances of the used current and voltage transducers in relation to the actual measurement conditions. the measurement uncertainty can be estimated and reduced by carrying out a careful characterisation of the used transducers. characterisation procedures under conditions as close as possible to the onsite ones have been experimented on a current and voltage measurement system which includes a clamp-on and flexible rc current sensor, whose expected on-site accuracy is of the order of some percent, and an accuracy class 0.5 voltage divider. by taking into account the results obtained in the characterisation phase the estimated uncertainty of the current and voltage measurements carried out on-site by the developed systems are reduced to 6 and 2 part per thousand, respectively. acknowledgement  the research leading to the results here described is part of the european metrology research program (emrp), which is jointly funded by the emrp participating countries within euramet and the european union. moreover, the authors acknowledge the ftm (fabbrica trasformatori di misura) factory for its valuable contribution in the development of the voltage divider and cos.pel which made the mv station of the mondovì (italy) plant available for the pq measurements. references  [1] en 50160, voltage characteristics of electricity supplied by public distribution networks, 2011-05. [2] p. kadurek, j. f. g. cobben , and w. l. kling “smart mv/lv transformer for future grids”, speedam 2010, international symposium on power electronics, electrical drives, automation and motion. [3] n. r. browne, t. j. browne, s. elphick, “monitoring intelligent distribution power systems – a power quality plan”, harmonics and quality of power (ichqp), 2010 14th international conference on, pp.1-7. [4] s. elphick, v. smith, v. gosbell, r. barr, “australian long term power quality survey project update”. [5] mindykowski, “fundamentals of electrical power quality assessment” xvii imeko world congress, 22-27 june 2003 dubrovnik, croatia, pp. 552-557. [6] iec 61000 “emc part 4-30: power quality measurement methods”, 2009. [7] iec 61000 “emc part 4-7: “general guide on harmonics and interharmonics measurements and instrumentation for power supply systems and equipment connected thereto”, 2009. [8] iec/tr 61869-103 “instrument transformers the use of instrument transformers for power quality measurements”, 2012. [9] a. cataliotti, d. di cara, e. emanuel, s. nuccio., “current transformers effects on the measurement of harmonic active power in lv and mv networks”, ieee trans. on pd, vol. 26, no. 1, pp. 360-368, 2011. [10] d. gallo, c. landi, m. luiso, e. fiorucci, g. bucci, f. ciancetta, “realization and characterization of an electronic instrument transducer for mv networks with fiber optic insulation”, wseas transactions on power systems, issue 1, volume 8, january 2013. [11] n. locci, c. muscas, s. sulis, “experimental comparison of mv voltage transducers for power quality applications”, i2mtc 2009 international instrumentation and measurement technology conference, singapore, 5-7 may 2009. [12] m. faifer, s, toscani, r. ottoboni, “electronic combined transformer for power-quality measurements in highvoltage systems”, ieee trans. on instrum. and meas., vol. 60, no. 6, june 2011. [13] s. kucuksari, g. g. karady “experimental comparison of conventional and optical current transformers, ieee trans. on power delivery, vol. 25, no. 4, october 2010, pp. 2455-2463. [14] agilent technologies 3458a multimeters user’s guide, http://cp.literature.agilent.com/litweb/pdf/03458-90014.pdf [15] iec 60044-8 instrument transformers part 8: electronic current transformers, 2004. [16] delle femine, a.; gallo, d.; landi, c.; luiso, m., "powerquality monitoring instrument with fpga transducer compensation," instrumentation and measurement, ieee transactions on , vol.58, no.9, pp.3149,3158, sept. 2009. [17] j.d. ramboz, “machinable rogowski coils, design and calibraton”, ieee trans. on instrum. and meas., vol. 45, no. 6, april 1996. [18] m. chiampi, g. crotti, a. morando “evaluation of flexible rogowski coil performances in power frequency applications”, ieee trans. on im, vol. 60, no. 3, pp. 854-862, 2011. [19] g. crotti, d. gallo, d. giordano, c. landi, m. luiso “a real-time compensation method for mv voltage transducer for power quality analysis”, proc. of 19th imeko tc 4 symposium and 17th iwadc workshop advances in instrumentation and sensors interoperability, july 18-19, 2013, barcelona, spain. [20] m patsalides, d. evagorou, g. makrides, z. achillides, g. e. georghiou, a. stavrou, v. efthimiou, b. w. schmitt, j. h. werner “the effect of solar irradiance on the power quality behaviour of grid connected photovoltaic systems”. microsoft word article 9 197-1088-1-pb.docx acta imeko may 2014, volume 3, number 1, 38 – 40 www.imeko.org acta imeko | www.imeko.org may 2014 | volume 3 | number 1 | 38 structured instrument design p.h. sydenham school of electronic engineering measurement and instrumentation systems, south australian institute of technology, adelaide south australia section: research paper keywords: measurement theory, value, error, uncertainty citation: p.h. sydenham, structured instrument design, acta imeko, vol. 3, no. 1, article 9, may 2014, identifier: imeko-acta-03 (2014)-01-09 editor: luca mari, università carlo cattaneo received may 1st, 2014; in final form may 1st, 2014; published may 2014 copyright: © 2014 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited 1. need for more fundamental approach the need for measuring instruments arises in application domains. an instrument forms the link to observed processes for gathering knowledge about that process. application domain persons are those who know best about the knowledge sought but often do not have sufficient background to efficiently design instrument systems. experienced instrument designers, in contrast, have the design expertise but need to become familiar with the application domain knowledge to provide worthwhile support. this mismatch of knowledge requirement has resulted in haphazard improvement in instrument design and application. there has been too much reinvention and too little systematic building upon work of others. the overall approach to the design of instruments, and related electronics, has never been streamlined since its sophistication began around the 1920’s. ref. [1] provides an historical overview of the developments. study of past general catalogue style texts on measuring technology show that whilst the form of implementation is changing the basic fundamentals are relatively static. the need for improved methodology and practice is increasing as measuring system performance and the cost and time to produce them come under closer scrutiny. it is now becoming clear that this inherited, uncoordinated, methodology can be improved by integrating the core knowledge of what has been learned over this past half century. available computing technology and software techniques, the mature state of recorded knowledge about measurement systems theory and practice sets the scene for revising the approach to the development and application of measuring systems. this paper provides an overview of work of the measurement and instrumentation systems centre misc in this regard placing that contribution in perspective with other related work. 2. sensory interfaces to the physical world there has been much identified and well supported activity for fundamental research into the design and application of the human to computer interface, hci. in contrast it has been most difficult to gain respectability for counterpart work into the physical world to computer interface pwci – the interface that provides people and hence, also computer systems, with knowledge about the world in which we exist. the pwci is a sensory interface [2] established with a properly designed and installed measurement system. a measuring instrument is equally as much an information machine, as is a computer. this concept has been cursorily developed by finkelstein, another “fundamental issues” contributor [3, 4], and was a prime realisation of the historical study [1], being reflected in its title. measurement systems are the “cinderella” of the information technology movement. the research programme of misc concentrates on development of universally applicable sensory interfacing this is a reissue of a paper which appeared in acta imeko 1988, proceedings of the 11th triennial world congress of the international measurement confederation (imeko), “instrumentation for the 21st century”, 16-21.10.1988, houston, pp. 271–275. the overall system design of measuring instrumentation is currently carried out by an intuitive process. using a “required plus available” knowledge approach, implemented in computers, the user’s domain can be interfaced to measurement engineering expertise providing an effective cae tool. progress in this new approach, that promises to dramatically reduce design time, place system generation in the hands of application domain users, and bring about more unity in measurement scholarship, is reviewed. acta imeko | www.imeko.org may 2014 | volume 3 | number 1 | 39 concepts. it is strengthened by working in close co-operation with the loughborough university of technology, computer human interface centre, lutchi and the cad of instruments group of the city university, london. research activities into the pwci and the hci must be integrated to ensure that humans, computers and the physical world each are interfaced with respect to the common entity all need, that is, knowledge. this concept has been expanded in [5]. 3. key stages in instrument design design or selection of a suitable instrument system has identifiable serial steps. each step should progressively better interface the necessary application domain knowledge to the instrument design expertise base as neither knows enough of the other at the various points of that process. the instrument design task is, fortunately, not as diverse as its manifestation and varied forms have made it appear. the process of working from a “knowledge needed” statement through to installed hardware is not over complex once the necessary information and procedural path are clarified and adopted in a systematic, constrained and pragmatic manner. the design of measurement systems should begin firmly embedded in terms of the knowledge needed from the sensory interface to be created by the designer who is most likely to be a user. a strategy to assist applications persons develop rigorous operational and technical design specifications has been developed [6] and it is being implemented as software under the name, measurement interface design system, minds. this step helps set up the vital metrological requirement in terms of measurands, their key metrology parameter values, influence variables and how the several measurands’ information is to be combined to form the “many to fewer” mapping process needed in the sensory interface. a means to convert the output of minds into an engineering specification document, or to directly write a specification where it is already known what measurement system measurand is needed, has been developed as software called specriter [7]. this stage calls for more knowledge about non-metrological aspects of a real instrument examples being, packaging, environment, testing, power supplies, cooling, and mass. having developed a sound specification it can be used to either purchase a ready-made item, have it designed and manufactured or use it as basic knowledge for the next step of the misc designer system which provides core sensor design shells. the purchase of proprietary items is not being studied in the misc programme because of the existence of an extensive project that already exists at the warren springs laboratory, uk. their thesac centre is developing an on-line, dial-up, knowledge based, service whereby users can rapidly learn which proprietary sensors come close to their need. although that programme is targeted at the process industry the over 70 measurands catered for are applicable to other applications. more adequate understanding of the taxonomy of information systems, formed as instruments and electronic systems, is now becoming essential. this has become a key necessity in a misc venture [8] investigating automatic electronic system design software that can be used by applications persons not having electronic training. before that process can be automated a classification of electronic building blocks formed in terms of a knowledge and information role was needed. at present designers use heuristic knowledge, in an art form manner, to decide which block to use and where. the names and concepts used go back to the 1930’s, well before information technology was recognised as a major entity. at the sensor design shell level a key taxonomy consideration now is which shells to develop and whether there exists a generic building block approach that can be used in a hierarchical manner. to build a shell for every measurand known would appear to be the wrong way to proceed. the misc project currently has length, pressure and temperature shells in development along with other foregoing shells exploring how these shells can cover their use for measurement of other variables. the shells provide extensive engineering documentation to support manufacture. having the appropriate sensor systems and knowledge about how their signals should be combined is still not the end of the applications domain users task. installation, commissioning and operation also require assistance. the misc programme framework also has projects in place to develop these aspects by incorporating heuristic and formal knowledge into software and hardware. these will tell the user how to install the sensors, automatically test and set them up (with some help from the user), and then automatically run the system with regular operational health checks. the overall programme aims to take sensory system design into a new era providing do-it-yourself capability [9]. counterpart work by others touches on some facets of the commissioning and use step. distributed sensory systems are currently a more widespread research interest, the work of brignell, university of southampton, being an example. smart sensors are studied by several of the silicon sensor develop teams. misc has the work of haskard to call upon [10]. at this early stage it is already clear that the caeinst software system now needs extension to cater for systems of sensors. work in 1988 covers setting up interfaces between caeinst designed sensors and manufacturing and scientific computers and of a data logger arrangement. a review of the project has recently been prepared that gives more detail of individual projects [11]. this, now several year long experience, permits some general conclusions to be given about cad of instruments. 4. using computers to assist design knowledge based systems allied to traditional formal knowledge are essential to develop application user driven designer systems. to provide cad tools based only in formal description modelling is to throw away the powerful rules of thumb that are used so effectively in practice to get fast, adequate results. the place for rigorous modelling is in assisting in-depth development needed to squeeze more out of a given design form, such as in a major product line situation or where the mathematical approach is superior to the heuristic way, for example to calculate the thickness of a pressure gauge diaphragm. the exemplary formal transducer modelling work of the city university, london, has been supported by manufacturers wishing to improve their products. misc has incorporated a acta imeko | www.imeko.org may 2014 | volume 3 | number 1 | 40 simple version of their mediem modelling software into its temperature sensor shell [12] to calculate thermal parameters and response times of the thermowell protected sensor. much of sensory system design needs are not for enhanced modelling elegance but are in the more subjective aspects such as, selection of appropriate device, management of the design process, awareness of standards, terminology and other semantic fields for such aspects as costing and quality. although the best computational methodology for implementing the above concept is not yet clear, the benefits of using the software based design approach are already emerging. once developed the method becomes very low cost to use and to maintain. the necessary knowledge is not so fast moving as might be thought. basic considerations will remain the same for many years. the speed at which a user can be in possession of a good sensory system design, including full manufacturing detail, will reduce from months to days. the limiting factor for a “oneoff” task now appears to be the time to manufacture the system, not to develop its design. work on computer integrated manufacture will improve that aspect. it is also becoming clear that the knowledge bases needed for this field are reasonably small – a few hundred words and a hundred or so rules will often be sufficient to design a major aspect of a need. the computing task should not blow out to one concerning such breadth as parsing of english language in its totality. 5. effort needed, problems of being pioneers as already alluded to above, gaining acceptance that fundamental development in measurement science and instrument engineering is both possible and worthwhile is not easy. as this is a radical change in both method and attitude it needs resourcing support of centralised governmental kind. it is too early to expect commercial support and its nature, being of advantage to many persons in many places yet not in large identified groups, makes support from users unlikely until it is ready to use. at what stage this concept will be taken up by others is not clear. misc has difficulty in citing the contribution of others to the overall systems approach because there appears to be little other work to report. to assist software dissemination, and hopefully gain constructive criticism and faster development, misc software is being made available to others interested in making and a contribution to the method. momentum will gradually develop and using this opportunity it is hoped more people will work together to move to the new methods. that will be hard decision for many as those who enter this field at this early stage will face lack of appreciation by others who cannot see that the current ways can be bettered. 6. horizons opened up once instrument cae in use to complete this review it is worth considering where the approach leads to once established. given that a series of, knowledge based, questions and answers can yield the full design of a sensory interface the idea can be extended to ask how many of those questions can the sensory interface resolve for itself given a sufficiently large and appropriate knowledge base and sensory system of its own. this concept may sound like a science-fiction scenario but already worthwhile applications appear possible. a misc project is developing a listening sensory interface that will work out the heuristic rules of a sound enabling recognition of indicated sounds that it should, in future, discern. these rules, currently used manually (but in future possibly used by the automatic electronic system designer), have been used to build a simple analogue/digital circuit that provides the sensory mapping needed by a rule based, rather than, formal processing, method. thus the concept of using information machines to generate the full manufacturable design of other information machines now appears to be viable. this theme is soon to be explored in more general manner using a sensory head having hearing and vision sensors. there the aim is to reach the highest possible programming level, that is, spoken conversation between the operator and the inspection head as the latter sits in its workplace. self repairing and adaptive sensory systems will also become closer because sensory systems will be able to decide if their own sensor and electronics design needs have changed and switch parts as needed without operator help. such visionary concepts are not new but sensory interface development based in heuristic and formal knowledge embedded in computers does provide the closest yet chance of building economically viable systems. references [1] p.h. sydenham, measuring instruments: tools of knowledge and control, peregrinus, stevenage, 1979. [2] p.h. sydenham, computer aided engineering of measuring instrument systems, cae jnl., 4 (1987) pp.117–123. [3] l. finkelstein, instrument science series: introductory article, j. phys. e. :sci. instrum., 10 (1977), pp.566–572. [4] l. finkelstein, instrumentation, control and information technology, j.phys.e.:sci.instrum., 19 (1986), pp.246–247. [5] p.h. sydenham, computer aided engineering of sensing interfaces, abstract. proc. conf, eurosensors, cambridge, sep. 1987. [6] p.h. sydenham, structured understanding of the measurement process, measurement, 3 (1985), pp. 115–120 and pp. 161–168. [7] s. cook, automatic generation of measuring instrument specifications, measurement, 6 (1988), pp.155-160. [8] l.c. jain, p.h. sydenham, caenic—user-characterised, electronic systems development software, cae jnl., 5 (1988), pp.200-205. [9] p.h. sydenham, a. skinner, r.w. beijer, do it yourself measurement and control cae package, jnl. meas. contr., 21 (1988), pp.69-75. [10] m.r. haskard, general purpose intelligent sensors, microelectronics jnl., 17 (1986), pp.9–14. [11] p.h. sydenham, cae of instrument and electronic systems proc. conf. “control ‘88”, christchurch, new zealand, may, 1988. [12] p.h. sydenham, n.a. morgan, m. anfiteatro, thermocouple temperature sensors cad tool, cae jnl., 5 (1988), pp.206-210. template for an acta imeko event paper acta imeko issn: 2221-870x december 2017, volume 6, number 4, 113-120 acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 113 a root cause analysis and a risk evaluation of pv balance of systems failures loredana cristaldi, mohamed khalil, payam soulatiantork deib, politecnico di milano, milano, italy section: research paper keywords: reliability assessment; root cause analysis of failures; markov process; photovoltaic systems; balance of system; fmeca citation: loredana cristaldi, mohamed khalil, payam soulatiantork, a root cause analysis and a risk evaluation of pv balance of systems failures, acta imeko, vol. 6, no. 4, article 18, december 2017, identifier: imeko-acta-06 (2017)-04-18 section editor: lorenzo ciani, university of florence, italy received september 28, 2016; in final form october 19, 2017; published december 2017 copyright: © 2017 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: mohamed khalil, mohamedmahmoud.khalil@polimi.it 1. introduction a balance of system (bos) comprises all the non-module components of photovoltaic (pv) power plants. failures of bos components are the major reason behind the presence of non-producing modules in pv field. ten years survey [1] was carried out by sandia national laboratories on 35 pv systems, and results showed that failure of bos components such as switches, fuses, dc contactors and surge arrestors were responsible for 54 % of the non-producing modules that were found, around 10,000 non-working modules. the layout of the pv system varies according to the architecture design; it can be a singleinverter system where all the strings are connected to a central inverter: a string-inverter system where each string has its own inverter, or a multi-inverter system where the pv field is divided into groups of strings connected to an inverter. accordingly, bos varies in design according to the layout of the pv systems. the most optimized bos whose components are the basic for any design is presented in figure 1. the failure of any of its components contributes significantly in the failures of pv system. it is worth mentioning that protection equipment is excluded since the utility switchgear is sufficient for the protection purposes of an optimum bos. it is worth noting that the utility switchgear is full of protection relays and tripping equipment that trip immediately the pv plant in case of any power faults. figure 1. bos components. abstract the photovoltaic (pv) system is divided mainly into two subsystems; pv modules and a balance of system (bos) subsystems. this work shows two approaches for a reliability analysis on the subsystem level of abos: failure mode effects criticality analysis (fmeca) and a markov process. fmeca concerns the root causes of failures and introduces prioritization numbers to highlight critical components of a bos. meanwhile, a markov process is a reliability methodology that aims to predict the probability of success and failure of a bos. in this way, a markov process is a supportive tool for helping decision-makers to judge the criticality of failures associated with the operation of pv systems. results show that the pv inverter contributes significantly to the failures of a bos. accordingly, further investigations are conducted on a pv inverter to prioritize the maintenance activities by determining the risk priority number of its component failures through quantitative ca. the novelty of the proposed methodologies stems from analyzing the roots of failure causes of bos components and estimating the probability of failure of these components in order to improve the early development of a bos, enhance maintenance management, and satisfy the demanding reliability by electric utilities. acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 114 in literature, most of the studies focus on pv modules reliability evaluation and only very limited publications consider the reliability of bos. among these publications, a qualitative reliability analysis is presented in [2] using fault tree analysis and other efforts in [3] investigated the reliability of both pv modules and bos using petri's networks in order to estimate the lifetime, reliability, and availability. in general, the first step towards enhancing a system’s reliability is to detect the root causes of systems’ failures. in this respect, failure mode effect criticality analysis (fmeca), a well-known methodology, is used in order to analyze the failure causes of systems. it focuses mainly on identifying the possible failure causes. in addition, it is one among several methods used for risk assessment and management by selecting the most proper maintenance strategies to enhance the system performance. a recent research [4] applied fmeca on a pv system designed by brookhaven national laboratory and results show that inverter and ground system of the pv system have the highest risk priority number (rpn). it provides a strong investigation of the fmeca on all pv system components; however [4] considered a specific design and did not list recommended actions to limit the failure causes, and the potential failure modes were listed without highlighting the contribution of each system component in the failure of the whole pv system. on the other hand, the fmeca methodology, in this work, is limited with more details to bos only whose components are more optimized. the failure causes of each component are studied in detail, and recommended actions are listed. in addition, a prioritization number is assigned to each failure, based on iec-60182, to improve the maintenance activities. moreover, a further reliability investigation is carried out by a markov process to be a supportive tool along with fmeca in case of any confusion on judging the priority number by maintenance management. more details on the confusion of judging rpn is available in [5]. both techniques illustrate results that can be utilized during the design phase in order to reduce the major field problems and improve the reliability of the systems. also, they can be used during the operation phase by improving the maintenance management and reducing the failure probability of occurrence. this paper is organized as follows; section 2 provides a general overview on the failure causes of bos components; section 3 presents fmeca results; section 4 shows the markov process conducted on bos; and section 5 focuses only on criticality analysis (ca) of the pv inverter. finally, section 6 includes the conclusions. 2. balance of system failure causes mapping the failure causes is the first step towards the reliability analysis for determining the underlying failures and enhancing failure prediction methods. figure 2 shows a schematic diagram of optimized bos that consists of the necessary components needed to be installed on a pv string. in this section, the failure cause of each bos’s component is described in detail. 2.1. ac and dc cables ac and dc cables represent the veins of pv systems and their failures result in partial or complete shutting down of the pv plant. in addition, cable problems at the module levels can lead to a severe mismatch for other modules cabled in the same parallel block [6] and result in drop of the output power. loose cables by poor workmanship due to excess torque and pressure during installation, undersized cables, overvoltage and over current, insufficient protection are the main causes for pv cabling. 2.2. bypass diode bypass diodes are usually supplied inside the module junction box or manufactured only inside pv modules for sophisticated module types only [7]. a study was carried out in [8] on 1272 modules showed that 47 % of the modules have defective bypass diodes and 3 % of the defective bypass diode caused burn mark on the modules. generally, the main function of a bypass diode is to allow the current to pass around the shaded or cracked cells and thereby reduces the power losses within the module itself. hence, hot spots will be avoided and a long lifetime of the system will be guaranteed [9]. bypass diodes have a junction temperature reaching up to 150-200 °c and they possess a significant self-heating [10], however the main reason of their failures is the applicable thermal stress during their operation because they are not exposed to sufficient air flow for cooling. 2.3. string fuse depending on the necessary capacity of a pv system, there might be several strings which are connected in parallel for higher currents and more power. only pv systems that have at least three strings require a fuse to be placed on each string. pv systems which have less than three strings will not generate sufficient fault current and do not present a safety hazard [11]. in general, a fuse can be considered as a conductor with a relatively low melting temperature surrounded by a dielectric insulator. string fuses have different failure modes that can be summarized into false operation, design factor, cracks of dielectric packaging and shift in fuse resistance; the resistance is increased during the normal operation, or it becomes relatively low during tripping [12],[13]. moreover, the fatigue factor contributes to the aforementioned failure modes; string fuses are subjected to wear out since switching on and off would heat up and cool down the fuses. consequently, fuse fatigue is developed by time. 2.4. ac and dc isolation switches ieee std c37.100-1992 [14] defines the isolating switch as a mechanical switching device used for changing the connection in a circuit or for isolating a circuit or equipment from the power source. in pv systems, the installation of a dc switch on each string is necessary for maintenance purposes of strings in order to avoid shutting down the inverter and consequently disconnecting the whole strings. on the ac side, the cable connecting the inverter to the grid is usually dimensioned to carry currents higher than the maximum current which the inverter can deliver. so a figure 2. bos of pv string. acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 115 protection against overload is not necessary and a circuit breaker at the utility switchgear is sufficient to protect against faults from the grid. however, an ac switch is still necessary and should be installed for maintenance purposes of the inverter [15]. the most common failure mode of isolating switches is a failure in the mechanical mechanism; thus the switch fails to open or close, and contacts will carbonize; that results in local temperature rise and reduction of contact quality. 2.5. inverter in a grid-connected pv plant, the inverter represents an expensive and complex key component. a typical three-phase pv inverter includes: igbt power modules, cooling fans, control software and dc link capacitors implemented on printed circuit boards (pcbs) in addition to ac & dc contactors. an igbt power module fails as a result of thermal runaway [16], ceramic substrate to base plate solder fatigue [17], partial discharge [18], and fwd if short circuited [19]. ac and dc contactors fail to open or close due to design defects, mechanical locks, failure of the tripping coil, arcs and overheating that cause degradation of the electric contacts. solder fractures and cracks are the main failure causes of pcbs and result in overheating and gradual resistance increase of the solder joints [20]. the control software fails in case of improper design, absence of health monitoring facility and incapability to adapt the change in electrical and environmental parameters. 2.6. surge arrestors surge arrestors are designed to isolate the pv circuit from the ground during normal voltage operation and to connect to ground when the voltage of the line exceeds the threshold value. in pv systems, they are installed to provide a complete protection against lightning and induced over-voltages. on the dc side, a surge protection device is always placed on the supply side of the inverter’s isolating device in order to provide a complete protection when the isolating device is opened. in service, surge arrestors are exposed to frequent lightening that results in excessive overheating and leads to degradation of its characteristics. also, moisture ingress can find its way inside the surge arrestor in case of sealing defects and contribute in dielectric degradation. 3. failure mode effect criticality analysis fmeca consists of two separate parts, the failure mode and effects analysis (fmea) and the criticality analysis (ca). fmea includes a list of possible equipment failure modes, reason of these failures, local and final effects that refer to the impact of each failure on the system element and the whole system respectively, and the alternative recommended corrective actions to avoid each failure [21]. on the other hand, criticality analysis plans and focuses the maintenance activities according to a set of priorities by giving failures with the highest risk the highest priority. 3.1. fmea on bos the analysis starts with gathering information on the functions and failures of bos components. the impact of each failure cause for each component is investigated on the component itself and the pv modules and strings, stated in the local effect. afterwards, the impact of each component failure, on the whole pv system, is stated in final effect. finally, the most proper recommendations are given to reduce the failure of each bos component. the working of fmea on bos is listed in table 2. 3.2. ca on bos the criticality is a manner to quantify how much attention is necessary to pay about determined component failure or event; this is carried out either through qualitative means based on experience and field background or quantitative means if previous failure data are available. currently, field data are not available for bos, therefore, a qualitative ca is the most relevant means to evaluate ca. this is managed by assigning each failure mode to a risk priority number (rpn), defined by rpn = o × s × d, where s represents a scale for the failure severity and the risks behind the failure occurrence, o denotes the probability of failure mode occurrence, and d means detection, and represents the possibility to recognize the failure before the system or the customers are affected. for expectation purposes of components’ failures, an iec evaluation criterion is selected as shown in table 1. in the ca evaluation, the occurrence is evaluated in accordance to the failure rate of the bos components stated in table 3; the severity is based on the expected interruption of power and possible damages to pv modules, and detection considers the fault detection tools and equipment in the field. the evaluated rpn is presented in figure 3. it is depicted from figure 3 that the inverter has the highest rpn because of the complexity of its components. the bypass diode follows the inverter since its rate of occurrence is quite high compared to the rest of other components; its failure results in burned marks on pv modules and reduction of table 1. iec-60182 evaluation criteria for occurrence, severity, detection. occurrence ( o) severity ( s) detection ( d) ranking failure is unlikely no discernible effect almost certain 1 low: relatively few failures very minor very high 2 minor high 3 moderate: occasional failures very low moderately high 4 low moderate 5 moderate low 6 high: repeated failures high very low 7 very high remote 8 very high: failure is almost unavoidable hazardous with warning very remote 9 table 3. component adopted failure rates. component failure rate bypass diode 0.027 f/year [26] dc switch 0.0018 f/year [27] ac wire 0.00011 f/year. [28] dc wire 0.00042 f/year. [28 ] ac switch 0.0003 f/year [27] string fuse 0.00017 f/yr [26] photovoltaic inverter 0.125 f/year [25] acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 116 power. in the field, failure detection of the bypass diode is done by infrared cameras and signal transmitter devices. both the inverter and bypass diode are the key elements for a safe system operation; therefore, it is recommended to conduct the aforementioned detection procedures of the bypass diodes along with the routine maintenance of the pv inverter in time. on the other hand, the surge arrestor has the lowest rpn, since it is very rarely when it fails in short circuit mode, and it is not opening the circuit in case of open circuit mode. 4. markov analysis on bos a markov process is a sequences of random variables in which the future variable is determined by the present variable and independent on the way in which the present state arose from its predecessors. the analysis looks at a sequence of events and analyzes the tendency of one event to be followed by another [22]. this tendency is the probability evaluation of transition from one state to another until the system has reached the final state. thus, a markov process is defined by a process {p(t), t≥0} with state space x={0,1,2,3,….r} and stationary transition probabilities: 𝑃ij= 𝑃𝑃(𝑝(𝑡) = 𝑗│ 𝑝(0) = 𝑖) 𝑓𝑓𝑃 𝑎𝑎𝑎 𝑖, 𝑗 ∈ 𝑋 (1) where p(t) is a random variable denotes the state and belongs to state space x. the rate of change from one state to another is estimated based on the transient analysis point of view, through kolmogorov forward equations, table 2. fmeca on bos. su bs ys te m outage mode possible outage cause local effect final effect compensating provision against failure s o d rpn bo s ac cables thermal expansion and contraction. loose cables. undersized cables. overvoltage and over current. slow output power degradation and increased power losses shutdown of one or more pv strings. arcs and fire risk minimizing electrical cables wiring, proper design, sufficient protection, using cable ducts, routine visual inspection 3 2 2 12 dc cables 3 3 2 18 bypass diodes thermal stress, insufficient cooling. over voltages and high currents. insufficient rating hot spots and burn marks on pv module bypass diode is open circuited: no change in output power. bypass diode short circuited: significant drop of power. proper design, installing surge arrestors 5 6 2 60 string fuses false operation. improper design, cracks of dielectric packaging. shift in fuse resistance, thermal wear out. in closed circuit mode: slow output degradation and increase of power losses. in open circuit mode: isolation of the one or more strings significant reduction of output power proper design, installing surge arrestors. regular visual inspection 3 2 3 18 dc isolating switches mechanical mechanism failure, improper design, carbonized contacts increase in contact resistance and power losses partial or complete shutdown of the pv system enhance periodic maintenance and proper inspection of operating mechanism 4 5 2 40 ac isolating switches 4 3 2 24 surge arrestors excessive overheating sealing defects and environmental contamination characteristics degradation. leakage current increases and dielectric integrity fails to discharge over voltages. partial discharge arching, induced over voltages and lightning strikes on pv equipment regular testing ( leakage current and meggar) visual inspection to avoid dust accumulation and sealing defects. maintaining and ensuring proper grounding systems. 2 2 3 12 figure 3. rpn of bos components. acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 117 d𝑝(𝑡) d𝑡 = 𝑝(𝑡). 𝐀 (2) and � p(𝑡) 𝑛 𝑗=0 = 1 (3) where d𝑝(𝑡) d𝑡 is a vector that represents the state probability p(t) at time t, and a is a matrix of transition probability between states. as the number of possible states are finite, (3) is necessary because the probabilities of all states at any time t should equal 1 and the system can be in one and only one of these states. in case of zero repair rates, i.e. poisson birth-death process, (2) can be rewritten as d𝑝𝑖(𝑡) d𝑡 = – ∑ λ𝑖,𝑗 𝑖≠𝑗 𝑃𝑖(𝑡) + ∑ λ𝑗,𝑖 𝑖≠𝑗 𝑃𝑗(𝑡) (4) although a markov process, from the theoretical viewpoint, is flexible and versatile, special precautions are necessary to deal about the difficulties of practical applications. the main problem is that the number of system states and possible transitions increases rapidly with the number of events in the system [23]. therefore, assumptions become a necessity. the usual assumptions considered by current standards and references, i.e. iec-61165 [23], iec 61508[24], and [22], can be summarized as follows: i) failure and repair rate are constant, ii) failure and repair events are independent, iii) the transition probability from one state to another state occurs within a very small time interval, iv) only one event occurs at the same time. in systems modelling without repairs, iec-61165 [23] considered three possible states for the system: the up, degraded and absorbing states. the up state represents a system free of any failure. the degraded state is related to a system state whose performance meets the warranty limits although its operation is associated with failures. absorbing states are the final states for the system when it falls. in the markov process, states are absorbing if they are once reached by the system; the system will remain there forever. the term bos is a very general term since it includes all the non pv module components and it depends as well on the design of the pv system, whether it is a central inverter, a string-inverter system or a multi inverter pv system. therefore, the reliability analysis is carried out on the bos components of the pv string shown in figure 2. it is assumed that the surge arrestor never fails in short circuit mode and it is not opening the circuit in case of failure; therefore it will be excluded from the markov process analysis. the major problem that always appears on any reliability study concerning the pv system is the lack of pv components failure information and absence of reliability, therefore the components failure rate of the bos are gathered from literature [26]-[28] in table 3. it is worth to mention that the inverter failure rate is calculated by considering one failure in 8 years [25] so the failure rate is 0.125 failure/year. according to the string configuration shown in figure 2, once any component fails the whole pv string fails; therefore, each components is assumed to have two states: up and down. all the possible scenarios for the failures of the string bos components are listed in table 4. accordingly, the state transition diagram is illustrated in figure 4. consequently, the state equations of a string bos can be estimated from (4) as follows: 𝑃0∗(𝑡) = −(𝜆1 + 𝜆2 + 𝜆3 + 𝜆4 + 𝜆5 + 𝜆6 + 𝜆7)𝑃0(𝑡) (5) from the definition, reliability is the probability to perform its required function without any failures, under given conditions and for a stated period of time. therefore, the string bos reliability is equal to the probability of state 0, 𝑃0(𝑡). hence 𝑅(𝑡) is presented in figure 5. based on figure 5, the mttf of a string bos is around 6 years which is close to the mttf of the inverter. in order to highlight the impact of the inverter on the reliability of the string bos, figure 6 is given. the mttf of the string bos without the inverter is around 33 years. figure 5. reliability of string bos. table 4. system states. state scenario 0 all components work 1 bypass diode fails 2 dc switch fails 3 ac wire fails 4 dc wire fails 5 ac switch fails 6 string fuse fails 7 photovoltaic inverter fails λ1 λ2 λ3 λ4 λ5 λ6 λ7 figure 4. state transition diagram of string bos failures. acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 118 5. criticality analysis of pv inverter our past work in [29] considers the pv inverter (pvi) to be just a component among other bos components. however, results of ca on bos, presented in figure 3, show that pvi contributes significantly in the failures of bos. meanwhile, the main concern of the current studies is quite limited to reliability estimations of pvi and reliability improvements of current pvi [30]-[34]. therefore, performing a ca on pvi components is very crucial to implement the best preventive measures, based on the prioritization of failure modes for pvi components, in order to limit and avoid the outage modes of pvi and reduce the risk number of pvi in the ca of bos. the availability of failure data plays a critical role in determining whether the ca will be qualitative, based on previous failure information or quantitative, based on past experience in the field. both approaches, however, require an identification of the system components as a first step before carrying out the ca. a typical three phase pvi includes: igbt power modules, cooling fans, control software and dc link capacitors implemented on printed circuit boards (pcbs) in addition to ac & dc contactors. a quantitative ca approach is followed up for pvi based on a survey carried out by sunedison company that operates more than 600 pv systems in four continents with 1500 in-service inverters from 16 vendors and more than 2.2 million pv modules from 35 manufacturers [35]. the surveyed period is during interval january 2010 and march 2012 for 350 systems. the most significant failure modes of pvi inverters in the sunedison survey are shown in table 5. inverter tickets are issued whenever there is a reported failure with pvi. accordingly, both the percentage of tickets and kwh lost reflects the occurrence (o) and severity (s) of ca respectively. in regards to occurrence, failures modes which have percentage of tickets greater than 30 % are considered very high and almost unavoidable failures, percentage of tickets in between 10 % to 20 % and 20 to 30 % are assigned to occasional and high repeated failures, respectively. meanwhile, a failure mode which has a ticket percentage less than 10 % represents failures with low occurrence. the severity of the failure mode is evaluated considering the performance and safety issues resulting from the occurrence of each failure mode. severity is considered low or minor if the percentage of lost kwh is less than 10 %. it is assigned to moderate, high, and very high rankings when the percentage of lost kwh reaches 10 %-20 %, 20 %-30 % and more than 30 %, respectively. the detection (d) is measured according to the probability of identifying the failure before the system is affected. this can be carried out through the different field indicators. for instance, the alarming system, visual inspection, and comparing measured parameters of monitoring systems to reference values. ca is conducted based on both table 2 and table 5, and rpn are evaluated in table 6. in the detection evaluation, it is worth mentioning that igbt and pcb are assigned to very low and low detection possibilities, respectively, according to iec criteria, since both are hard to be detected in the field and several studies are currently available to enhance their detection methods. on the other hand, control software has the highest detection possibility; because the changes in generated electricity can be easily observed with a strong power and energy metering. moreover, the protection scheme is tested frequently in the site during the scheduled maintenance activities. results of ca in table 6 show that pcb has the highest rpn compared to the rest of the components. although it has a low rate of occurrence, its severity is the highest, because the highest percentage of kwh in sunedison is assigned to its failures. in addition, it has a high detection ranking since the failure detection possibilities of pcbs in the field are very limited. table 5 frequency of tickets and associated energy loss for each pvi failure mode. specific failure area percentage of tickets percentage of kwh lost control software 28 % 15 % pcb board 13 % 22 % ac contactors 12 % 13 % dc contactor 4 % 1 % fans 6 % 5 % igbt modules 6 % 5 % capacitors 3 % 7 % table 6. ca of pvi. component failure s o d rpn igbt power module 3 3 7 63 dc link capacitor 3 2 5 30 ac/dc contactors 6 5 5 150 cooling fans 4 3 4 48 pcb 7 4 6 168 control software 6 7 3 126 figure 6. comparison of string bos reliability w and w/o inverter. acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 119 meanwhile, ac/dc contactors recorded the second highest rpn because of their high assigned severity value in sunedison survey. improper operation of the contactor subjects the human’s life to danger and contributes to fault propagation in the grid. in the field, ac/dc contactors, generally, have moderate detection facility during routine protection tests only. control software has the third highest detection possibility. although the failure detection of control software can be easily detected in the field. the igbt power module has a quite moderate rpn. the igbt power module has the highest detection ranking since it is generally quite difficult in the field to detect its failure. however, both their recorded failures occurrence and the percentage of losses kwh in the sunedison survey are moderate compared to other failures. consequently, the igbt order is the fourth in the priority. both cooling fans and dc link capacitors have the lowest rpn. in the sunedison survey their failures have insignificant impact on power interruption, and they have low failure frequency of occurrence as well. in addition, both cooling fans and dc link capacitors have high possibilities to be detected in the field. for instance the ipc-9591 standard [36] specified six symptoms for cooling fans’ failures in the field: shaft rotational speed reduction, current consumption increase, loud noise, incorrect or erratic operation of electronic interface, visible cracking of the structure, and visible leakage of lubricants. on the other side, the dc link capacitor symptoms of failures can be detected through rf noise during operation and dissipation factor tests during maintenances besides esr monitoring during its operation. 6. conclusions the root failure causes of pv string bos are studied in detail through a fmea approach, and a qualitative ca was conducted in order to prioritize these failure causes, to enhance bos maintenance activities and decision-making. ca shows that the pv inverter has a high rpn compared to other failure causes and this result was supported by a markov process. in the markov analysis, the mttf of string bos is significantly low, around six years, due to the high failure rate of the inverter. the estimated mttf of string bos excluding the inverter impact is around 33 years; 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/hrv (za stvaranje adobe pdf dokumenata najpogodnijih za visokokvalitetni ispis prije tiskanja koristite ove postavke. stvoreni pdf dokumenti mogu se otvoriti acrobat i adobe reader 5.0 i kasnijim verzijama.) /hun 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can be opened with acrobat and adobe reader 5.0 and later.) >> /namespace [ (adobe) (common) (1.0) ] /othernamespaces [ << /asreaderspreads false /cropimagestoframes true /errorcontrol /warnandcontinue /flattenerignorespreadoverrides false /includeguidesgrids false /includenonprinting false /includeslug false /namespace [ (adobe) (indesign) (4.0) ] /omitplacedbitmaps false /omitplacedeps false /omitplacedpdf false /simulateoverprint /legacy >> << /addbleedmarks false /addcolorbars false /addcropmarks false /addpageinfo false /addregmarks false /convertcolors /converttocmyk /destinationprofilename () /destinationprofileselector /documentcmyk /downsample16bitimages true /flattenerpreset << /presetselector /mediumresolution >> /formelements false /generatestructure false /includebookmarks false /includehyperlinks false /includeinteractive false /includelayers false /includeprofiles false /multimediahandling /useobjectsettings /namespace [ (adobe) (creativesuite) (2.0) ] /pdfxoutputintentprofileselector /documentcmyk /preserveediting true /untaggedcmykhandling /leaveuntagged /untaggedrgbhandling /usedocumentprofile /usedocumentbleed false >> ] >> setdistillerparams << /hwresolution [2400 2400] /pagesize [612.000 792.000] >> setpagedevice acta imeko, title acta imeko issn: 2221-870x december 2016, volume 5, number 4, 2-3 acta imeko | www.imeko.org december 2016 | volume 5 | number 4| 2 introduction to the special section of the 14th imeko tc10 workshop on technical diagnostics lorenzo ciani, marcantonio catelani department of information engineering, university of florence, italy section: editorial citation: lorenzo ciani, marcantonio catelani, introduction to the special section of the 14th imeko tc10 workshop on technical diagnostics, acta imeko, vol. 5, no. 4, article 1, december 2016, identifier: imeko-acta-05 (2016)-04-01 section editor: paul regtien, measurement science consultancy, the netherlands received december 22, 2016; in final form december 22, 2016; published december 2016 copyright: © 2016 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: lorenzo ciani, email: lorenzo.ciani@unifi.it dear reader, in the first part of this issue, acta imeko publishes ten papers that were originally presented at the 14th imeko tc10 workshop on technical diagnostics in milan (italy) and that are here presented in their extended versions. technical diagnostics is taking in the years an increasingly important role. behind this, the fact that in high-tech industry and in different fields of application it is mandatory to fulfil the requirements related to diagnostics, reliability, maintainability and logistic support as well as risk and safety assessment. the capability to monitor and diagnose a component, a system, an equipment or an industrial plant – in general term, an item – with the aim to verify its functions represents the starting point for more complex rams (reliability, availability, maintainability and safety) evaluations and assessment. the 14th imeko tc10 workshop was held at the politecnico di milano, italy, on june 27-28, 2016. the preparation of the technical program was particularly challenging since 95 abstracts were received from all over the world and the final program hosted 39 oral and 42 poster papers scheduled over two days. the first paper by giulio d’emilia, david di gasbarro and emanuela natale describes a methodology for continuous checking of the settings of a low-cost vision system for automatic geometrical measurement of welding embedded on components of complicated shape. the method aims to check the holding of optimal measuring conditions by using a machine learning approach for the vision system: based on a such methodology single images can be used to check the settings, therefore allowing a continuous and on-line monitoring of the optical measuring system capabilities. the second paper by christian schlegel, holger kahmann and rolf kumme aims to propose a solution for a traceable torque calibration of nacelles by using transfer torque transducers which are calibrated in special standard calibration machines which are traced to the si. in this context, a calibration of up to 1.1 mn·m of such a transducer was realized. the third paper is the result of an international collaboration among researchers from national research university of information technologies, mechanics and optics, saintpetersburg (russia), ilmenau university of technology, ilmenau, (germany) and society for production engineering and development (germany). this research showed the successful results of using a computer vision approach for roughness assessment of a metal surface with help of different texture features extracted from 2d images. such method of non-contact quality control gives a possibility to detect parts with certain defects in a fast and reliable way and can increase the outcome of good products. the fourth paper in this issue is another international collaboration between politecnico di milano (italy), national research council cnr-ifn, milan (italy), bauman moscow state technical university (bmstu)centre for photonics and infrared technology, moscow, (russia) and swansea university, swansea, (wales-uk). the paper deals with the development and characterization of a novel fiber-pumped single-mode yb,er:glass microchip laser. single-frequency output power up to 8 mw was observed with a laser slope efficiency of ∼10 % and a laser threshold as low as 30 mw. acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 3 the fifth contribution by philippe chiquet et al. proposes to investigate the effect of short pulsed program/erase signals on the functioning of flash memory transistors. a novel experimental setup used to replace standard electric signals with short pulses is described and measurement results showing the benefits of programming and erasing non-volatile memories with short pulses are presented. the sixth paper by alberto lavatelli, emanuele zappa investigates and discusses what are the main limitations of vision based modal analysis in comparison with the classic transducer based application, with particular focus on isolating the main sources of uncertainty in dynamic contexts. the experiments demonstrated that the frequency response measurement bias is present also in real situations, with good statistical significance. the seventh paper by zsolt jános viharos et al. introduces a methodology to define production trend classes and also the results to serve with trend prognosis in a given manufacturing situation. the proposed solution is applicable to realize production control inside the tolerance limits to proactively avoid the production process going outside from the given upper and lower tolerance limits. the proposed approach was developed and validated by using real data. the eighth paper by micaela caserza magro et al. presents a roadmap towards the condition based maintenance of a fleet of railway vehicles. the paper associates to each maintenance policy its benefits and its requirements in terms of technological infrastructure and operating costs. bombardier transportation italy, that supports this research, started this roadmap a few years ago, for moving from a reactive maintenance policy to a proactive policy. the ninth contribution by tommaso addabbo et al. takes into consideration the most common suggested standards used in designing radar based railway level crossing surveillance systems and introduces new general concepts which demystify the use of such standards in actual applications. this paper illustrates the roadmap to be followed in general when designing such monitoring systems, to minimize the risk due to object misdetection occurring on barrier closure when exploiting radar technology. finally, the tenth paper in this issue is authored by mohamed khalil, christian laurano, giacomo leone and michele zanoni analyses the impact of the outages occurred to the italian overhead transmission lines (ohtls) from 2008 to 2015 is carried out. a new simple and effective reliability index, namely the severity factor, is introduced with the aim to drive the prioritization of the failure causes and the enhancement of the maintenance activities. the proposed methodology, thanks to the introduction of the severity factor, is a useful and effective tool for the identification of the transmission network criticalities and the enhancement of the related maintenance activities. let me take this occasion to remind everyone the next 15th imeko tc10 workshop on technical diagnostics – “technical diagnostics in cyber-physical era” that will be held in budapest, hungary, on june 5-6, 2017 and i am pleased to invite you to attend the conference. the workshop is a forum for advancing knowledge and exchange ideas on methods, principles, instruments and tools, standards and industrial applications on technical diagnostics as well as their diffusion across the scientific community. participants have an excellent opportunity to meet top specialists from industry and academia all over the world and to enhance their international co-operation. the program will feature industry leading keynote speakers and selected presentations. all details about imeko tc10 workshop are available on the conference website: http://www.imekotc10-2017.sztaki.hu/ lorenzo ciani guest editor marcantonio catelani imeko tc10, chair http://www.imekotc10-2017.sztaki.hu/ introduction to the special section of the 14th imeko tc10 workshop on technical diagnostics template for an acta imeko event paper acta imeko issn: 2221-870x july 2017, volume 6, number 2, 4-7 acta imeko | www.imeko.org july 2017 | volume 6 | number 2 | 4 realization of an si traceable small force of 10 to 100 micronewton using an electrostatic measuring system jile jiang, gang hu, zhimin zhang force and torque laboratory, division of mechanics and acoustics, nationai institute of metrology, china section: research paper keywords: small force; electrostatic; capacitance gradient citation: jile jiang, gang hu, zhimin zhang, realization of an si traceable small force of 10 to 100 micro-newton using an electrostatic measuring system, acta imeko, vol. 6, no. 2, article 2, july 2017, identifier: imeko-acta-06 (2017)-02-02 section editor: min-seok kim, research institute of standards and science, korea received march 18, 2016; in final form february 4, 2017; published april 2017 copyright: © 2017 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: this work was supported by the national key technology research and development program of the ministry of science and technology of china, no.2011bak15b06 corresponding author: jile jiang, e-mail: jiangjl@nim.ac.cn 1. introduction the invention of atomic force microscopy (afm) has had a great impact on the development of nanotechnology [1]. this technology has the capability of characterizing material properties such as the surface morphology and interaction potentials [2], [3]. the reliable measurement of the force based on an afm tip depends on the accuracy of the spring constant. the determination of the spring constant is essential for the absolute measurement of forces below 1 mn. many methods have been proposed to estimate the spring constant of the afm tip, but these methods are not si traceable and lack accuracy [4]–[9]. high-accuracy measurements of si traceable small forces in the range 1–100 μn have been realized by several nmis recently [10]-[15]. most of the standard devices are based on the electrostatic force, and recently a small forcemeasuring system based on it has been established. most parts of the measuring system have been completed and the entire project will be continued under further arrangement at nim, china. 2. principles 2.1. methodology a standard measuring system for small forces in the range 10–100 μn has been realized using an electrostatic force. the structure of the entire measuring system is shown schematically in figure 1. the measuring system was placed in a vacuum chamber and the chamber placed on an optical table supported by six legs. the level of the chamber was adjusted with an air bubble level. the optical table featured a mass block to increase the inertia of the entire system. the inner diameter of the vacuum chamber was 700 mm. because the vibration from the pump may cause instability of the inner electrode, the measurements were operated under normal air pressure. abstract a small force of (10–100) μn traceable to the international system of units (si) has been realized using an electrostatic measuring system at the national institute of metrology, china. the key component of the measuring system is a pair of coaxial cylindrical electrodes. the inner electrode is suspended with the support of a self-balanced flexure hinge, while the outer electrode is attached to a piezoelectric moving stage. the stiffness of the self-balanced flexure hinge was also designed so as to be both sufficiently stable and sensitive to the small force applied to the inner electrode. two sets of cameras were used to capture the shape of the electrodes and to obtain a better coaxial arrangement of the inner and outer electrodes. with the help of a capacitance bridge and a piezoelectric moving stage, the relative standard uncertainty of the capacitance gradient does not exceed 0.04%. associated with a laser interferometer and a dc voltage power source, the feedback system that controls the position of the inner electrode is responsible for the generation of a force of 10–100 μn. the standard uncertainty associated with the force of 100 μn does not exceed 0.1 %. acta imeko | www.imeko.org july 2017 | volume 6 | number 2 | 5 the key component of the measuring system is the electrostatic force generator, a pair of coaxial cylindrical electrodes, shown in figure 1. the inner electrode is mounted on the self-balanced hinge, while the outer electrode is mounted on the moving stage. the self-balanced hinge is made of copper and its spring constant is 11.679 n/m. the inner electrode can be moved along the z axis, nominally parallel to the direction of gravity. the degree of overlap between the two cylinders is measured by a heterodyne laser interferometer. the surface of the bottom inside the inner electrode is smooth enough to reflect the laser beam. a mirror with an al and mgs2 coating was mounted on the outer electrode to reflect the reference laser beam. in order to obtain the proper reflection of the laser beam from two cylinders and a proper coaxial alignment, four main steps were followed. the outer electrode was first adjusted with the air bubble level to ensure that the axial alignment was in the direction of gravity. the laser beam was then tuned to obtain the proper reflection from the outer electrode. the inner electrode was adjusted with the help of an x-y plane moving stage and x-y rotation stages to obtain the desirable alignment to the outer electrode. the alignment is observed by using the shape of the two electrodes. two cameras were used to capture the shape of the electrodes in the x and y direction. the centric distance of the electrodes should be less than 1 μm and the angle between the axes of the electrodes should be less than 1 mrad. although a self-balanced hinge is introduced to the system, an unexpected movement of the inner electrode can still be found, as shown in figure 2. this movement was unpredictable. it measured less than 5 μm in 8 hours when the operators were in the room and less than 1 μm in 80 hours without operators. the creep may have been initiated by temperature variation, because the thermal expansion coefficients of the materials comprising the supporting stages of the inner and outer electrodes are different. comparison between the force generated by the coaxial cylinders and a standard weight (1 mg, 2 mg, 5 mg, and 10 mg, provided by mettler toledo) was carried out, following the sequence shown in figure 3. the comparison was achieved by controlling the voltage on the electrodes to maintain a fixed position of the two cylinders. a proportional integral-derivative (pid) method was applied to the system using a computer program. once the weight was loaded on the inner electrode, the displacement was measured by the interferometer. then, after the weight was unloaded, an electrostatic force was generated to move the inner electrode to its original position. 2.2. capacitance gradient measurement the force generated by the system is determined by two factors: the voltage between the two electrodes and the capacitance gradient along the z direction, described as 21 . 2e dc f u dz = (1) it is very important to obtain dc/dz with high accuracy. when measuring the capacitance gradient, the inner electrode is considered to be static. the outer electrode was driven vertically by a piezoelectric stage. the total nominal displacement between the electrodes was 100 μm. in each test, dc/dz was measured when the displacement between two electrodes was −50, 0, and +50 μm. the outer electrode was held in each position for 30 s. the capacitance was measured by a sensitive bridge (andeen-hagerling model ah2700a, cleveland, oh, usa), while the displacement was measured by the interferometer. because the reference mirror is fixed to the outer electrode, the displacement from the interferometer equals the displacement between the inner and outer electrode. the creep effect can be disregarded here. as shown in figure 4, dc/dz is calculated from the linear fit. typical measurement results are shown in figure 4. as can be seen in the expanded part in figure 4a, the nonlinearity of the capacitance versus displacement in 2 nm may be the cause of the difference and the higher standard deviation. 3. comparison to deadweight the comparisons to deadweights of 1 mg, 2 mg, 5 mg, and 10 mg were carried out after the capacitance gradient was figure 1. schematic of the electrostatic measuring system. figure 2. creep of the inner electrodes. figure 3. process of comparison between deadweight and force. cantilever holder assembly outer electrode adjustment assembly 0 0.5 1 1.5 2 2.5 3 3.5 x 10 5 -2 0 2 4 6 time(s) d is pl ac em en t( µ m ) 20.52±0.13oc without operator 21.98oc v ol ta ge ul uh w ei gh t r el at iv e po si ti on load unload null position time acta imeko | www.imeko.org july 2017 | volume 6 | number 2 | 6 measured. the comparison procedure is shown in figure 3. the voltage was held for approximately 200 s once the deadweight was loaded and unloaded. data acquisition was applied all the time, but after 100 s the value of the voltage can be considered stable. nominally, the room temperature was 21.0±0.5 oc. the typical response of the voltage and displacement of the inner electrode is shown in figure 5, in which the deadweight artefact was 10 mg. comparison between deadweights of 1 mg, 2 mg, 5 mg, and 10 mg is shown in table 1. the relative standard deviation of the electrostatic force fe became smaller when the larger deadweight was loaded. 4. uncertainty considering (1) and assuming all the factors are uncorrelated, the uncertainty of the measuring force is 2 2 2 2( ) ( ) ( ) ( ) ( ), ( )( ) f dc f u f u u u dc dz u dz 2∂ ∂= + ∂∂ (2) where the capacitance gradient is obtained by the following method: 1 2 1 ( )( ) . ( ) n i i i n i i z z c c dc dz z z = = − − = − ∑ ∑ (3) ci is the capacitance value corresponding to the relative displacement of the inner electrode, zi. the average values of ci and zi are c and z , respectively. it can be verified that 1 1 2 2 2 2 2 2 1 1 1 1 2 ( 2 ) ( ) ( ) n n i i i i i i i i in n n n i i i i i i i i i dc c z c z c cdz dz z z dz z z z z z = = = = = = ∂ = − − + ∂ ∑ ∑ ∑ ∑ ∑ ∑ (4) and 2 1 .ii in i i i dc zdz dc dc c z = ∂ = ∂ ∑ (5) the uncertainty associated from a linear fit by the least square method, to determine the capacitance gradient, can be omitted. the relative standard uncertainty of the force generated by the system is expressed as 2 2 2 2 2 ( ) ( ) ( ) ( ) ( ) 4 ( ).r r r r r u f f u r u c u z u d u u= + + + + (6) ( )ru r is caused mainly by the repeatability of dc/dz, associated with vibrations from the air flow and the unexpected vibration from the base. the actual displacements of the inner and outer electrodes were measured by the laser interferometer, so ur(z) should be considered. ur(u) is caused by voltage noise from the figure 5. variations of the voltage and displacement of the inner electrodes when a deadweight of 10 mg was applied. a b c d figure 4. measurement of the capacitance gradient. a: capacitance values at 0 μm and ±50 μm. drifts of the capacitance and displacement are 0.0013 pf and 2.5 nm, respectively. b: variation of dc/dz vs time (mean=1.019077 pf/mm, σ=3.0×10-4 pf/mm, σ/ n =6.7×10−5 pf/mm). c: capacitance gradient of 35 sets of six points at 0, 10, 20, 30, 40, and 50 μm (mean=1.018901 pf/mm, σ=1.5×10−3 pf/mm, σ/ n =1.8×10−4 pf/mm), 21.75±0.12 oc. d: capacitance gradient of 10 sets of nine points at 0, 12.5, 25, 37.5, 50, 62. 5, 75, 87.5, and 100 μm (mean=1.019012 pf/mm, σ=3×10−4 pf/mm, σ/ n =6.2×10−5 pf/mm), 21.54±0.11 oc. 0 400 800 1200 1600 2000 2400 2800 3200 3600 4000 4400 400 600 800 1000 1200 v ol ta ge ( v ) time(s) 0 400 800 1200 1600 2000 2400 2800 3200 3600 4000 4400 -20 -18 -16 -14 -12 -10 -8 -6 -4 -2 0 d is ta nc e ( µ m ) voltage distance 0 0.05 0.1 14 14.05 14.1 14.15 displacement(µm) c ap ac ita nc e( pf ) 0.0485 0.049 0.0495 0.05 14.0616 14.0618 14.062 14.0622 14.0624 14.0626 14.0628 displacement(µm) c ap ac ita nc e( pf ) 21.40 21.45 21.50 21.55 21.60 1.0184 1.0186 1.0188 1.0190 1.0192 1.0194 1.0196 1.0198 0 500 1000 1500 2000 te m pe ra tu re (o c ) dc /d z( pf /m m ) time (s) forward reverse mean temperature 1.012 1.014 1.016 1.018 1.02 1.022 1.024 0 2000 4000 6000 8000 10000 12000 dc /d z( pf /m m ) time (s) forwa rd reverse mea n 1.01840 1.01860 1.01880 1.01900 1.01920 1.01940 1.01960 1.01980 0 500 1000 1500 2000 2500 dc /d z( pf /m m ) time (s) forwa rd reverse mea n acta imeko | www.imeko.org july 2017 | volume 6 | number 2 | 7 dc power source. this is related to the actual force generated. noise in the measurement of the capacitance, ur(c), and the displacement error of the piezoelectric moving stage, ur(d), are also included. according to the calibration results of the ah2700a capacitance bridge, the dc power source, the laser interferometer, and the piezoelectric stage, the relative standard uncertainties ur(c), ur(u), ur(z), and ur(d) are 0.5×10−6, 2.5×10−5, 1×10−8, and 1×10−4, respectively. according to the results shown in figure 4, u(r)=1.8×10−4 pf/mm, and the relative standard uncertainty of the force generated by the system is 2.69×10−4. according to the results of the comparison shown in table 1, the relative difference between the electrostatic force and the gravity of the deadweight is larger than 2.69×10−4. in general, the following aspects should be considered: 1) the test mass is placed close to the inner electrode. the shapes of these test masses are different, which will affect the actual capacitance measurement since the mass is lifted by the stainless-steel wire and attached directly to the self-balanced hinge. 2) the fork used to lift the test mass is made of stainless steel and is isolated from the ground. considering that the outer electrode is grounded and the inner electrode is applied to a high voltage, the test mass can be electrically charged and attracted by the inner electrode. this could be studied by reversing the polarity of the electrodes. the possible primary improvement of the system should be realized by electrically isolating the inner and outer electrodes from the other parts, especially the test mass. 5. conclusions forces of (10–100) μn traceable to the international system of units (si) were realized by a measuring system based on electrostatic force. primary analysis shows a relative standard uncertainty of 2.69×10−4. the deviation between deadweight and the electrostatic force was less than 0.6 %. acknowledgements this study was supported by “research on measurement standards and reference materials based on micro-nano technology” (no. 2011bak15b00) and by “research on key technologies in establishment and traceability of microgram mass standards and 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[7] burnham n a, chen x, hodges c s et al., “comparison of calibration methods for atomic-force microscopy cantilevers”, nanotechnology, 14(2003) pp. 1-6. [8] sader j e, chon jwm, mulvaney p. “calibration of rectangular atomic force microscope cantilevers”. rev. sci. instrum., 70(1995) pp. 3967-3969. [9] ohler b. “cantilever spring constant calibration using laser doppler vibrometry”. rev. sci. instrum., 78(2007) pp. 063701063701-5. [10] pratt j r, newell d b, kramar j a. “a flexure balance with adjustable restoring torque for nanonewton force measurement. joint international conference imeko tc3/tc5/tc20, celle, ge: vdi berichte, 2002, 1685 pp. 77-822 [11] pratt j r, smith d t, kramar j a, “microforce and instrumented indentation research at the national institute of sandards and technology, gaithersburg, md[r]: charlotte, nc: nist, 2003. [12] christopher w j, richard k l. “review of low force transfer artefact technologies, report eng 5 [r]. middlesex: npl, 2008. [13] richard k l, simon o. 2004. “design of a bi-directional electrostatic actuator for realising nanonewton to micronewton forces, report depc-em 001[r]. middlesex: npl, 2004. [14] chen sj, pan s s. “a force measurement system based on an electrostatic sensing and actuating technique for calibrating force in a micronewton range with a resolution of nanonewton scale” . meas. sci. technol., 22(2011) pp 045104-045104-8. [15] vladimir n. “facility and methods for the measurement of micro and nano forces in the range below 10-5 n with a resolution of 10-12n (development concept)”. meas. sci. technol., 2007, 18(2007) pp. 360-366. table 1. comparison between deadweights of 1, 2, 5, and 10 mg. gravity of deadweight g (µn, with expanded uncertainty, k=2) electrostatic force fe (µn) relative standard deviation of fe, σ (fe g)/ g (%) 9.806±0.002 9.8008 0.0112 -0.055 19.609±0.002 19.595 0.0143 -0.071 49.009±0.002 48.746 0.0025 -0.54 98.021±0.002 97.455 0.00074 -0.58 realization of an si traceable small force of 10 to 100 micro-newton using an electrostatic measuring system template for an acta imeko event paper acta imeko issn: 2221-870x december 2016, volume 5, number 4, 19-23 acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 19 the application of texture features to quality control of metal surfaces konstantin trambitckii1,2, katharina anding2, galina polte2, daniel garten3, victor musalimov1, petr kuritcyn2 1national research university of information technologies, mechanics and optics, av. kronverksky 49, 197101 saint-petersburg, russia 2ilmenau university of technology, gustav-kirchhoff-platz 2, 98693 ilmenau, germany 3society for production engineering and development (gfe e.v.), naeherstiller strasse 10, 98639 schmalkalden, germany section: research paper keywords: image processing; texture features; surface quality citation: konstantin trambitckii, katharina anding, galina polte, daniel garten, victor musalimov, petr kuritcyn, the application of texture features to quality control of metal surfaces, acta imeko, vol. 5, no. 4, article 4, december 2016, identifier: imeko-acta-05 (2016)-04-04 section editor: lorenzo ciani, university of florence, italy received september 22, 2016; in final form november 7, 2016; published december 2016 copyright: © 2016 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: the research project, which forms the basis of this paper, was supported by the thuringian ministry of economy, employment and technology (tmwat) with means from the european social fund (esf). the author konstantin trambitckii was supported by the michail lomonosov grant corresponding author: konstantin trambitckii, e-mail: konstantin.trambitckii@tu-ilmenau.de 1. introduction a surfaces quality control is an important step during the production process of metal parts. it is necessary in order to check whether surface quality meets the requirements. the quality control gives the possibility to detect parts with certain defects and can help to increase the outcome and the reliability of good products. a surface roughness assessment is one of the main ways to control the quality of a surface. there are two main groups of roughness assessment methods: contact and non-contact measurements. non-contact measurements in comparison with contact measurements have the advantage that a measured surface remains undamaged after the end of the process. nowadays optical industrial cameras are widely used in the non-contact quality assessment of metal surfaces. the measurement speed of this group of surface assessment methods is higher in comparison with contact methods. if the surface of a measured part has a curved shape and the camera lens has a narrow depth of field (because of the short focal length), some regions of the image, obtained by the camera, can be out-of-focus. these regions have less information about the surface, because fewer details can be obtained from them. trambitckii et al. used different texture features to segment and remove such regions [1]. when the out-of-focus regions were removed, further surface quality analysis can be performed. authors of different papers proposed various methods of optical surface quality control. the image texture analysis was already used in the field of quality assessment of metal surfaces. laws [2] and haralick [3] descriptors are used for detection of various defects of metal surfaces. laws developed an approach, based on the calculation of texture energy abstract quality assessment is an important step in production processes of metal parts. this step is required in order to check whether surface quality meets the requirements. progress in the field of computing technologies and computer vision gives the possibility of visual surface quality control with industrial cameras and image processing methods. authors of different papers proposed various texture feature algorithms which are suitable for different fields of image processing. in this research 27 texture features were calculated for surface images taken in different lighting conditions. correlation coefficients between these 2d texture features and 11 roughness 3d parameters were calculated. a strong correlation between 2d features and 3d parameters occurred for images captured under ring light conditions. acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 20 parameters. convolution masks are used to filter the source image. then the texture energy descriptors are computed by applying non-linear window operations to the filtered images. finally, all the descriptors are combined to achieve rotation invariance of the resulting feature. to evaluate haralick’s descriptors, firstly the grey level co-occurrence matrix (glcm) is calculated for an image. values of the glcm are calculated considering a spatial location of neighbourhood pixels. glcms can be created for different directions and for different distances taking into account locations of neighbourhood pixels. after that step for each glcm haralick’s descriptors (energy, contrast, correlation, etc.) are calculated. alegre et al. used these both groups of descriptors as input vectors for k-nn classifiers to divide stainless steel surfaces into two classes based on their roughness quality [4]. alves et al. used haralick’s descriptors to define roughness of the surface and used multilayer perceptron (artificial neural network) to classify these surfaces into three classes [5]. another way to calculate descriptors for classification is the wavelet transform. alegre et al. [6] proposed to apply the haar transform to decompose original surface images. as the next step, haralick’s features were calculated for these decompositions. this method showed a reliable classification of surfaces based on their finish quality. quality assessment can be based on the fourier transform as well. tsai et al. used the two-dimensional fourier transform to classify cast surfaces with different roughness into several groups [7]. naïve bayes and neural network classifiers were implemented to solve this task. the negative side of an application of the fourier transform is computational complexity of the method. another group of texture features is represented by focus features. these features are successfully used in autofocus systems of cameras, microscopes, etc. authors of several papers developed and suggested different focus features. the focus features are essential for the autofocus algorithms, giving numerical values to a whole image or to different regions. the focus features give the maximum response to the sharpest image from a set of images. the higher the contrast of the specific region, the higher the value of the focus feature will be. it is possible to use these features for in-focus surface images as the descriptors of surface roughness. the different surface quality will influence the surface visual appearance on the image. these surfaces will have different contrast and intensity variations, which can be described with the help of the focus texture features. nayar [8] developed a feature, which is based on the laplacian operator. this feature was called as the sum modified laplacian (sml). the sml can be used as a descriptor for sharpness of specific regions. pech et al. [9] suggested to use variance of the laplacian operator convoluted with an input image. thelen et al. [10] created a modification of nayar’s sml feature, additionally taking into account the intensity information from diagonal pixels. due to the increased amount of points used for calculation, this improved feature showed more reliable results on noisy images. in this paper the focus is on the quality control of metal surfaces using various texture features. for this goal a correlation between different texture features and roughness parameters was calculated. it gives the possibility to find the most correlated pairs, suitable for non-contact visual surface quality assessment. the algorithms were tested on surfaces produced by a milling and drilling operation. chapter 2 gives detailed information about the data acquisition system for both 2d and 3d surface images. chapter 3 and 4 contain descriptions of some texture features and roughness parameters, used in this research. there is an explanation of the correlation calculation algorithm in chapter 5. in chapter 6 the main results of this work are presented. chapter 7 contains brief conclusions, which summarize the performed research. 2. data acquisition in this research, metal parts with cone shaped surfaces were used. following the main concept of this experiment, processed test surfaces of metal parts were marked, as shown in figure 1. that gives a possibility to find the same regions in both 2d images and 3d surface data. in our research the size of the region of interest is around 1×1 mm2. 3d roughness information of metal surfaces was obtained with the alicona 3d infinite focus g4 measurement system. a lens with the magnification of 20× was used. the lateral resolution (along x and y-axis) of the measurement system with 20× lens is 2.93 µm, the vertical resolution (along the z-axis) is around 100 nm. 2d images were obtained with an optical camera which was built-in in the same system. in the optical measurements of metal surfaces light plays an important role because of the complex reflectance characteristics of such surfaces. in this research, different light sources were used. the light through the lens system of the alicona scanner was used, as well as a ring light in different modes. the main advantage of the ring light is rotation invariance of shadows on surface images relatively to the lighting source. a sample image of the surface with the ring light source, used in this research, is shown in figure 2. the observed workpiece area is a counter sink. the cutting speed of the tool, used to produce these drill hole, varied from 175 to 185 m/min. figure 1. marked regions in both 2d image (top) and 3d surface (bottom). acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 21 3. 2d features extraction 27 different texture feature maps were calculated for each image of the metal surfaces. the most correlated features are described in the present chapter. algorithms were written in matlab. 3.1. thresholded gradient the thresholded absolute gradient is calculated using the following equation [11]: ,),(),1( 1 1 ∑∑ = = −+= m i n j grat jiijiif (1) while ν>−+ ),(),1( jiijii , where m and n are the numbers of horizontal and vertical pixels of the image, i(i,j) is the grey level intensity of pixel (i,j), and v is the gradient threshold. 3.2. absolute central moment shirvaikar proposed to calculate the absolute central moment of an image. that texture feature showed robust work on the test images. it has peak values when the region of interest has high contrast. the absolute central moment facm is defined by [12]: ,)( 1 0 ∑ − = ⋅−= l k acm khkf µ (2) where l is the number of grey levels in the image, μ is an average value of grey levels of the image, and h(k) is the value of the histogram h for the k-th grey level. 3.3. spatial frequency frequencies for rows and columns are defined by [13] ∑∑ = = −+ ⋅ = m i n j jiijii nm rfreq 1 1 2),(),1( 1 (3) and .),()1,( 1 1 1 2∑∑ = = −+ ⋅ = m i n j jiijii nm cfreq (4) thus, the spatial frequency is defined as .)()( 22 cfreqrfreqfsfrq += (5) 3.4. laplacian based features as described in the introduction, nayar suggested to use the sml feature, based on the laplacian operator. the modified laplacian feature is calculated by using the following equation [8]: ,),(),(),(2 ),(),(),(2 1 1 stjiistjiijii jstiijstiijiif m i n j ml +−−− ++−−−= ∑∑ = = (6) where st is a distance between pixels, used to compute the feature. thelen et al. improved this feature by taking into account the intensity information from diagonal pixels [10]. it is defined by .),(),(),(2 2 1 ),(),(),(2 2 1 ),(),(),(2 ),(),(),(2 1 1 stjstiistjstiijii stjstiistjstiijii stjiistjiijii jstiijstiijiif m i n j xml −+−+−− +++−−−− ++−−− +−−−−= ∑∑ = = (7) the improved feature showed the more robust results for noisy images. 3.5. brenner’s gradient brenner et al. proposed to calculate the sum of squares of the second derivatives, which is larger than a threshold value v [14]: ( ) ,),(),2( 1 1 2∑∑ = = −+= m i n j bren jiijiif (8) while ( ) ν>−+ 2),(),2( jiijii . 4. 3d roughness parameters the surface quality can be estimated using roughness parameters established in international standards [15]. in this research the following iso roughness parameters were used: sa (arithmetical mean deviation of the assessed surface), sq (root mean square deviation of the surface), ssk (skewness of the surface), sku (kurtosis of the surface), sv (maximum valley height of the surface), sp (maximum peak height of the surface), sz (maximum height of the surface, i.e. the difference between the highest peak and the deepest valley), s10z (10 point maximum height), sdq (root mean square surface slope) and sdr (developed interfacial area ratio). along with the iso parameters listed above another roughness parameter from another source was used – ssc (mean summit curvature) [16]. all these parameters were calculated for marked areas of the 3d surface, which correspond to the same areas of the 2d images. 5. correlation evaluation for this research 6 sample surfaces were taken and marked. these marks can be visible both on the images obtained with a 2d industrial camera and on the surfaces obtained with the alicona system, as shown in figure 1. that gives the figure 2. sample 2d image of metal surface. acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 22 opportunity to calculate different texture features and roughness parameters for the corresponding regions of the surface. as the first step, regions of interest were set for every obtained 2d image and 3d surface. these regions of interest correspond to the marked areas of surfaces. then marked areas were divided into subregions. the subregions for the 2d images and the 3d surfaces have different sizes because of their different original dimensions. the marked areas of the 2d images were divided into the subregions of 35×35 pixels, the marked areas of the 3d surfaces were divided into the subregions of 140×140 points. these region sizes result in the equal sizes for the corresponding arrays. it is an important condition for further estimation of correlation. in the next step, the texture features were calculated for each subregion of the 2d images. for every image, every single texture feature results in an array of size corresponding to the number of subregions. roughness parameters for the 3d surfaces were calculated the same way. after the texture features and roughness parameter arrays were calculated, correlation coefficients between every pair of the parameters were estimated. having two data arrays of similar sizes gives the possibility to find the correlation coefficient, which shows statistical relationships between these data values. if the coefficient is equal to 0, that means that two values are linearly independent. the closer the coefficient to -1 or 1, the stronger the correlation between these two values. in other words, the closer the coefficient to these values, the stronger the linear dependency they have. for the correlation coefficient an interpretation of the brosius criteria [17] was used. these criteria are listed in table 1. this interpretation of the correlation coefficients gives an easier explanation whether values have weak or strong correlations. 6. results as the first step, a set of texture features was tested against two sets of roughness parameters, calculated for raw surfaces and calculated for surfaces with a removed curvature shape. the correlation for the second set was very weak (correlation coefficients are below 0.39), and in the next research only the raw surfaces data were used. images under ring light conditions gave interesting results. the texture features calculated for these images showed strong (up to 0.85) correlation between the roughness parameters. the correlation for a ring light environment is stronger than for light through lens conditions, see the box plot in figure 3. the correlation coefficients between 27 texture features and 11 roughness parameters were calculated. it results into an array of 297 pair-wise correlation coefficients. for all the coefficients absolute values were taken, as some of them have negative values. then all pairs were sorted from the highest to the lowest values of the correlation coefficient. this information was used to draw a plot, which shows the distribution of the correlation coefficients for all 297 pairs, see figure 4. the x-axis represents the absolute correlation coefficient value. the y-axis is an index of pair, sorted by the correlation coefficient value in descending order. the performed studies showed that for our conditions the most correlated roughness parameters are sa, sq, sv, sp, s10z, ssc, sdq and sdr. the most correlated texture features are the absolute central moment, the thresholded gradient and the spatial frequency. an example of the most correlated pair is shown in figure 5. an average value of the correlation coefficient between the average central moment and the average roughness (sa) is equal to 0.85. linear regression (the table 1. correlation coefficients interpretation. absolute value of coefficients interpretation 0 no correlation 0 < r < 0.2 very weak 0.2 < r < 0.4 weak 0.4 < r < 0.6 medium 0.6 < r < 0.8 strong 0.8 < r < 1 very strong 1 perfect figure 3. box plot for ring light and light through lens conditions. figure 4. distribution of correlation coefficients. acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 23 red line in figure 5) was performed to get the relationship between these two values. 7. conclusions this research showed the successful results of using a computer vision approach for roughness assessment of a metal surface with help of different texture features extracted from 2d images. such method of non-contact quality control gives a possibility to detect parts with certain defects in a fast and reliable way and can increase the outcome of good products. overall, 27 texture features were calculated for surface images taken in different lighting environments. correlation coefficients between these texture features and 11 roughness parameters were calculated. strong correlations between features and parameters showed up for images under ring light conditions. future work will analyse surface images taken with other industrial cameras and different lens systems. the results showed that surface defects, which are reflected in a deviation of roughness parameters, can be detected with fast operating low-cost 2d cameras using 2d texture analysis instead of using time-consuming and expensive 3d measurement devices. acknowledgement the research project, which forms the basis of this paper, was supported by the thuringian ministry of economy, employment and technology (tmwat) with means from the european social fund (esf). the author konstantin trambitckii was supported by the michail lomonosov grant. the responsibility for the content of this paper lies with the authors. references [1] k. trambitckii, k. anding, g. polte, d. garten, “elimination of out-of-focus regions for surface analysis in 2-d colour images”, proc. of ilmenau scientific colloquium, 2014, vol. 58. [2] k. laws, “texture energy measures”, image understanding workshop, darpa, 1979. [3] r. m. haralick, k. shanmugam, i. dinstein, “textural features for image classification”, ieee trans. on systems, man and cybernetics, vol. smc-3, no. 6, 1973, pp. 610-621. [4] e. alegre, j. barreiro, m. castejón, s. suarez, “computer vision and classification techniques on the surface finish control in machining processes”, lecture notes in computer science, vol. 5112, pp. 1101-1110. [5] m.l. alves, e. clua, f.r. leta, “evaluation of surface roughness standards applying haralick parameters and artificial neural networks”, ieee trans. on systems, signals and image processing, april 2012, pp. 452-455. [6] p. morala-argueello, j. barreiro, e. alegre, s. suarez, v. gonzalez-castro, “qualitive surface roughness evaluation using haralick features and wavelet transform”, annals of daaam & proceedings, january 2009, pp. 1241-1242. [7] d.m. tsai, c.f. tseng, “surface roughness classification for castings”, pattern recognition, vol. 32, no. 3, march 1999, pp. 389–405. [8] s.k. nayar, tech. report: shape from focus. 1989. [9] j. pech, g. cristobal, j. chamorro, j. fernandez, diatom autofocusing in brightfield microscopy: a comparative study. 2000. [10] a.thelen, s. frey, s. hirsch, p. hering, interpolation improvements in shape-from-focus for holographic reconstructions with regard to focus operators, neighborhood-size, and height value. 2009 [11] a. santos, c. ortiz de solórzano, j.j. vaquero, j.m. peña, n. malpica, f.del pozo, “evaluation of autofocus functions in molecular cytogenetic analysis”, journal of microscopy, december 1997, vol. 188, no. 3, pp. 264–272. [12] m.v. shirvaikar, “an optimal measure for camera focus and exposure”, proc. of the thirty-sixth southeastern symposium on system theory, 2004, pp. 472-475. [13] a.m. eskicioglu, p.s. fisher, “image quality measures and their performance”, ieee transactions on communications, 1995, vol. 43, no. 12. [14] j.f. brenner, b.s. dew, j.b. horton, j.b. king, p.w. neirath, w.d. sellers, “an automated microscope for cytologic research,” j. histochem. cytochem, vol. 24, pp. 100-111, 1971. [15] iso 25178: geometric product specifications (gps) – surface texture: areal. [16] k.j. stout, p.j. sullivan, w.p. dong, e. mainsah, n. luo, t. mathia, h. zahouani, “the development of methods for the characterization of roughness on three dimensions”, publication no. eur 15178 en of the commission of the european communities, luxembourg, 1994. [17] f. brosius, “spss 8“, international thomson publishing, 1998, p. 503. figure 5. a graph showing the relationship between the average surface roughness (sa) and the absolute central moment (facm). the application of texture features to quality control of metal surfaces microsoft word 34-178-1-pb.doc acta imeko  july 2012, volume 1, number 1, 2  www.imeko.org    acta imeko | www.imeko.org  july 2012 | volume 1 | number 1 | 2  a message from the president of imeko  dae‐im kang  kriss ‐ korea research institute of standards and science, 267 gajeong‐ro, yuseong‐gu, daejeon 305‐340, republic of korea      keywords: imeko; acta; president; message; global metrology community  citation: dae‐im kang, a message from the president of imeko, acta imeko, vol. 1, no. 1, article 2, july 2012, identifier: imeko‐acta‐01(2012)‐01‐02  editor: paul regtien, measurement science consultancy, the netherlands  copyright: © 2012 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  corresponding author: dae‐im kang, e‐mail: dikang@kriss.re.kr    colleagues and readers from all over the world, it is a pleasure and a privilege for me to extend my heartfelt congratulations on the first publication of acta. since its establishment in 1958, imeko has made substantial contributions to the advancement of measurement technology, which is indispensable for developing industry, science and technology and enhancing the quality of human life. as the president of imeko, i have great confidence in and respect for the confederation for such proactive contributions, which i am sure will ever continue in close partnership with all of its member organizations. we live in a world of ever-increasing interdependency and thus of shared global challenges – the challenges of sustainable economic growth, of improving health care for all our people, of food safety and energy security, of clean accessible water, of the environmental conservation, etc. new technologies and discoveries will be essential to addressing the challenges our interconnected world is facing. the science of measurement and its applications serve as one of the most important tools that can deliver workable solutions to the global agenda. therefore, this is a particularly vital moment for academia, researchers, and engineers to work more closely together by sharing ideas and vision on metrology than ever before. i would like to take this opportunity to encourage all of the members of imeko and outstanding experts in metrology to take a leading role to enable enduring advancement of measurement science and technology ranging from fundamental to the cutting-edge areas. all in these efforts, i am confident that acta will be of great assistance by being a fresh new arena in timely sharing a lot of insightful research achievements in metrology that is essential to the growth of the imeko and the global metrology community as well. on behalf of imeko, i would like to thank prof. paul p. l. regtien, vice president for publications and all my colleagues at imeko who have done their best to make the birth of acta possible. last but not least, i hope that acta will also be instrumental to keep the global metrology community working in great harmony in days to come. once again, i would like to express my sincere congratulations and best wishes for the everlasting success of acta. here is a message from the president of imeko, to colleagues and readers of acta imeko from all over the world, celebrating the  publication of the first issue of the journal.  microsoft word article 18 176-1310-1-le.docx acta imeko  february 2015, volume 4, number 1, 121 – 127  www.imeko.org    acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 121  machine vision systems for on line quality monitoring in  industrial applications  giuseppe di leo, consolatina liguori, alfredo paolillo, antonio pietrosanto  department of industrial engineering, university of salerno, via giovanni paolo ii, 132 fisciano (sa), italy       section: research paper   keywords: machine vision, non‐contact measurement, image processing, gloss, color image, stereo vision  citation: g. dileo, c. liguori, a. paolillo, a. pietrosanto, machine vision systems for on line quality monitoring in industrial applications, acta imeko, vol. 4,  no. 1, article 18, february 2015, identifier: imeko‐acta‐04 (2015)‐01‐18  editor: paolo carbone, university of perugia   received january 15 th , 2014; in final form november 17 th , 2014; published february 2015  copyright: © 2014 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: (none reported)  corresponding author: giuseppe di leo, e‐mail: gdileo@unisa.it    1. introduction  thanks to new technologies that today allow to implement automatic nonintrusive solutions at convenient costs, on line inspection systems are going to be widespread also in areas driven by off-line quality control requirements. dimensions and external finish of products can be today monitored on-line at costs that can be recovered after few months of production waste reduction. a number of inspection techniques have been thought to be implemented on-line and the choice among them must be made on the basis of the product nature. non-contact measurements are strongly recommended in many applications, e.g. to avoid load effects when deformable objects are to be inspected. machine vision methods have provided the efficient and reliable techniques for the design of image-based non-contact measurement systems [1]. in some industrial fields, such as the production of either electronic components and boards or mechanical parts, automated inspection using machine vision has already had a relevant number of applications. this is not the case of the production of rubber profiles, due to the complexity of the production process and to the flexibility and the dark color of the product. in this field the authors realized an image-based measurement system for inspection tasks in a plant producing rubber profiles for the automotive industry [2][4]. a stereo vision system was designed to make the on-line contour extraction, the 3d reconstruction of the transversal section of rubber profiles, and the measurement of the main dimensional parameters. it is based on the registration of each observed profile onto a reference profile. a suitable software tool let users specify the dimensional lengths to be measured. in this way, users have been able to maintain the software up to date for new models of profiles and new measurements [2]. nevertheless the measurement system still keeps some limits: i) the robustness of the contour extraction algorithm gets low when a metal reinforcement is included in the profile section; ii) the software does not provide statistical information on the product quality; iii) no inspection is made on the external surface of the profile. in this paper which is an extended version of [3] the authors, after a brief recall of the overall structure of the extrusion line, describe the solutions they propose to abstract  this paper is concerned with the topic of non‐contact measurements for industrial applications. it presents the case of vision‐based  systems realized by the authors expressly for the measurement of geometric and/or chromatic parameters of rubber profiles for the  automotive  industry. after a brief description of the extrusion process, a stereo vision system for the on‐line measurement of the  dimensional  characteristics  of  the  profile  transversal  section  is  first  described  in  all  its  main  components  and  features,  then  two  modules for the inspection of the finish of profile surfaces are presented. acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 122  overcome the aforementioned limits of the contactless measurement system. 2. the extrusion line  at the site of battipaglia, italy, the cooper standard automotive s.p.a. manufactures rubber profiles for car window and door sealings. the production cycle is divided into several basic steps (see figure 1). the extrusion essentially compresses the pasty material at high pressures through a matrix which reproduces the interior and exterior shape of the piece. some profiles, the so-called armed profiles, have a metal insert with stiffening function. the extruded rubber flows through ovens for the vulcanization which causes a change in the molecular structure of the polymer resulting in an increase of the elasticity and tensile strength and in an improvement of other physical and chemical properties. subsequently, the product passes to the phase of electrostatic flocking which gives the rubber a velvety appearance, obtained by first sprinkling an adhesive solution over the surface and then uniformly distributing fibers over it by means of an electrostatic field. after a cooling step, those products for which it is required undergo a brushing step. this phase has a predominantly aesthetic purpose so as to give it a matt appearance to part of the outer surface. the step of cutting to a given length ends the in-line processing, followed by the collection of the products in the cases for the transport. 3. the measurement system  the measurement station for the verification of the geometrical dimensions of the profile has been placed at a side of the second conveyor belt used for the collection of the finished product (figure 2): it is constituted by two cameras, an illuminator, a photocell and a data acquisition system. the cameras are suitably positioned to frame the cross section of the profile with different angles of view. the on-line operation of the measurement station can be viewed as a sequence of modules, detailed in the following and shown in figure 3. the main module is the on-line procedure, which is executed each time a new profile goes into the visual field of the two cameras. both (left and right) images feed the 2d procedure which outputs the edge points of the two contours. then, starting from the contours of the two 2d images, a 3d reconstruction [5]-[6] of the section of the profile is run. in order to determine the position in the space of the profile points of the profiles from the pair of images, taken by the two cameras, it is necessary to know the parameters of the intrinsic and extrinsic cameras, obtained by means of a calibration procedure [7]. the measurements are carried out on the three-dimensional reconstruction by a third module of the developed application, called the profile editor. 3.1.  two‐dimensional image processing  since the measurements are made on the 3-d reconstruction of the 2-d contours, the localization of the cross-section contour plays a key role in the measurement procedure. the developed image processing algorithms apply a sequence of known image processing techniques: a histogram enhancement routine normalizes the grey level distribution in the image, a region growing stage locates all the pixels belonging to the profile section (the foreground region), and a contour tracking algorithm locates the contour as the border of the foreground region. for the profiles with metal reinforcement, the tuning of parameters such as integration time, lens aperture and amplifier gain of the camera becomes critical as they have a direct influence on the brightness of the captured image. if a value of these parameters is found so that the rubber part of the front section is represented with a sufficiently high grey level, the metal sheet, which has a much higher reflectance, saturates (i.e. reaches the maximum grey level) and generates glares, smearing or blooming broadening the appearance of the lit final product vulcanization surface finish • flocking • painting cuttingextrusion plate compound figure 1. stages for the rubber manufacturing.  1st conveyor belt 2nd conveyor belt profile right camera left camera illuminator camera support photoelectric cell figure  2. the hardware architecture of the measurement station.  acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 123  part and affecting the measurement. in order to obtain high-contrast and to avoid these undesirable phenomena, the dynamic range of the image on which measurements have to be done must be expanded. two techniques were considered: the high dynamic range (hdr) imaging [8] and exposure fusion (ef) [9], which, starting from a sequence of images of the same scene taken with different exposures, reconstruct an image with a higher dynamic range. the hdr technique aims to reconstruct the radiance map of the scene: from the sequence of images at different exposures, the characteristic function (crf, camera response function) which relates the radiance of the scene to the brightness of individual pixels is first reconstructed. then by inverting this function the radiance map of the scene can be achieved. the hdr processing, however, imposes a significant computational load that is not compatible with the requirements of on-line control. furthermore, the images obtained with this technique tend to create undesirable halos around bright areas of the image, and then the presence of reflections on the metal part would cause a broadening of the contours. the ef technique addresses some of the limitations of hdr: the final image is composed by taking the individual pixels from images of the sequence on the basis of a score, according to specific figures of merit associated to the individual pixels of each image. by this way, two goals can be achieved: i) the execution of the algorithm is much faster, because there is no need to sort the map of radiance again; ii) the effect of halos that was observed in hdr processed images is eliminated, thus allowing a more accurate contour extraction. figures 4a) and b) show the images obtained with the two techniques mentioned above and the related measures. in both cases the rubber part is measured correctly, while the measurements of the metal part have a maximum error of 21% compared to the reference for the image obtained by means of hdr, and 6% in the case of images obtained via ef, thus demonstrating the effectiveness of the ef technique in preserving the outlines of the brighter areas. 3.2. three‐dimensional reconstruction  the two 2-d contours determined in the previous step are processed in order to obtain a 3-d reconstruction of the contours of the leading cross-section. the procedure can be divided into two phases: i) the search for matching stereo pairs, namely the pair of image points in the left and right images generated by the same real point, and ii) the calculation of the 3-d coordinates of the profile contour points as a function of the two stereo pairs. the search for stereo pairs is performed by exploiting the epipolar constraint [6]: given a point on one image (e.g. the left one), the corresponding point on the other (e.g. the right) image lies on a line, the so-called epipolar line, whose localization can be determined from the calibration parameters. the point on the right image which corresponds to a given point on the left image can be searched for within the intersections between its epipolar line and the profile contours. once the calibration parameters of the two cameras and the pixel coordinates of two image points are figure 3. the software architecture.    a)    b)  figure 4. a) hdr image with overlaying measurement indicators; b) enfused  image with overlaying measurement indicators.    2d processing profile and measurement database stereo matching 2d-3d profile fitting plane calibration parameters measurement procedure list of measurements left image right image acquisition 3d transversal section reconstruction off-line calibration off-line profile editor measurement extraction profile and measurement database comparison pass / fail quality control 2-d proc 2-d proc acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 124  known, a triangulation algorithm [5] allows to obtain the measurement of the three absolute coordinates of each point of the contour. in order to simplify the subsequent extraction of measurements from the 3d contour, a fitting plane is determined as the least square plane approximating the points of the cross section of the profile, and then the measurements are performed on the 2-d projection of the contour on this plane. 3.3. measurements  a procedure of measurement valid for all the profiles and products has been implemented and set up. the procedure can also be extended to future production, since the list and the description of the measurements can be easily tailored for a new model of profile. the dimensions to be measured must be only once specified by the user on the coordinate system of the reference profile contour. then, the measurement procedure consists of two main steps: i) each new observed 3d contour of the profile is superimposed ("registered") to the reference contour of the profile; ii); the measurements specified on the reference are made on the observed contour. the reference profile library management is entrusted to another application module, called profile editor. this module allows the user to select and view the reference profile of a section, to specify the measurements to be made on the profile. the measurements are defined by primitives, each of which requires the user to define two points on the reference contour; these couples of points must be placed in the coordinate system of the reference contour. the different types of measurement primitives that can be applied to profiles are: the “gauge”, distance between the two intersections of a specified segment (p, q) and the observed profile; the so-called “tip-to-tip”, distance between the two points of the profile observed that best correspond, in terms of proximity and curvature, to two specified points on the reference profile (p, q). it is also possible to project the measured segment onto a straight line ("axis") determined by two points belonging to the contour (r and s of figure 5a) and specified using the profile editor. for each selected measurement the nominal value can be stored in the data base and compared on line with the measured one. in figure 5b) a screen output of the software is shown. 3.4. calibration  in order to determine the spatial position of the profile contour points the three-dimensional reconstruction algorithm requires that the intrinsic and extrinsic parameters of the cameras are known. goal of the calibration procedure is the determination of the intrinsic and extrinsic parameters of the cameras. it must be repeated whenever one of the configuration parameters of the system (position or angle of the cameras, distance, focus) are expected to have changed. the authors’ proposal is based on the approach of zhang [7], which requires the two cameras to observe a different planar target, whose geometry in 3-d space is known with very good precision, shown at a few different unknown poses. a sandblasted metal plate containing a pattern of 6×5 square holes (120 squares corners) is adopted as the target, due to its good accuracy (~10 μm). the knowledge of the corner positions in several pairs of images, at least five, and of the target geometry in mm, allows specific calculations based on the maximum likelihood criterion in order to determine the intrinsic and extrinsic parameters of the cameras. 3.5. quality checks, report and metrological characterization  each measurement result is compared with the specification limits in order to assess its compliance with design tolerances and the software reports the outcome of the dimensional test with a “pass/fail” indication. the presentation of the results includes, for each piece, the superposition of the observed and the reference profiles, the table of last results, the time chart of the results and some statistics. some report pages can be recalled to view the chart of the quantities of interest. the user’s queries can include: the line, the section, date and time of beginning and ending of observation, and the quantity of interest. summary reports of different types can be generated: i) basic, ii) advanced, iii) failure chart, iv) production line, v) summary. the metrological characterization was carried out on the system in use at cooper standard automotive plant in battipaglia, italy. tests have been performed to assess: (i) the influence of the relative position of the cameras, (ii) the systematic errors of the system, and (iii) the repeatability of the measurement results. choosing the optimal position of the cameras and after performing a correction of systematic errors, the standard uncertainty [10] of all types of measurements was contained in 0.2 mm. the processing time of the proposed measurement system, figure 5. a) measurement primitives; b) example of use of the profile editor.  acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 125  at the typical extrusion speed of the line (3 pieces / min), allows to monitor the full 100% of the produced pieces. 4. a system for painting quality control  the coating process consists in depositing a transparent silicone paint on the exterior surface of the profile. the coating has both an aesthetic and a functional purpose: to confer a shiny effect and to make a waterproof seal. the coating system in use at the plant is produced by kremlin, consists of four air spray guns, and is managed by a plc, which allows to set for each gun both the pressure of the paint and of the air. the system for the regulation of the pressure is based on the use of electronic controllers, which allow to obtain a great precision in regulating the value of air flow and paint. this results in an excellent finish quality with a very high degree of pulverization, and in a very low thickness of the painting (from 10μm to 30�m). nevertheless, temporary or permanent gun failures may result in parts not painted that must be detected and discarded. since the paint is transparent, in order to allow visual verification by the operator, it is mixed with an additive (uvitex ob by ciba) which emits in the visible when illuminated with ultraviolet (uv) radiation. in the previous system, a uv lamp illuminated the profile, a cctv system grabbed images of that profile and showed them on a monitor, where a human operator had to periodically check whether the profile image had the required brightness. the vision system designed by the authors replaces the human operator in the control of paint. it has been designed to estimate the amount of paint deposited on the surface of the profile in order to detect possible faults on the air spray guns. the vision system, schematically shown in fig 6a), is positioned immediately after the spray booth and uses two camera-illuminator pairs in order to monitor the whole external surface that must be painted. the two 640 × 480 color cameras are featured with ring type illuminators having a wavelength of 365 nm. the system acquires at regular time intervals two images of the profile surface and then processes them to measure the amount of paint and compare it with a predefined threshold. the image processing is based on the measurement of the amount of blue color present in each acquired image, since it is proportional to the amount of additive and then of paint. in particular, the algorithm identifies a sub-image centred onto the profile contour, and calculates an index m proportional to the average brightness of the blue component extracted from the acquired sub-image. the profile contour is located by searching the first and last edge along vertical lines with a differential operator. the control panel (fig. 6b) contains the processing unit, the monitor for display, the stack lights, acoustic signalling devices and the power supply plugs. if the measured value is out of tolerance, an alarm is generated. a chart is also shown on the operator panel with the history of the last 300 measurements. in this way, the operator can periodically monitor the measurements and anticipate unwanted trends. 4.1. assessment of fault detection capability  tests have been carried out in order to verify the functionality of the system and the compliance to production needs. faults on each gun and multiple faults are simulated obstructing the guns and the indices measured on the images of the two cameras are recorded. for each kind of fault, the fault state is maintained for about 30 seconds and the variability range of the indices is recorded. this whole procedure is repeated 3 times. said p1, p2, p3 and p4 the status of the 4 guns and m1 and m2 the two indices measured on the images from the two cameras, table 1 reports the minimum and maximum observed value during the tests for the main fault cases (single or multiple) and for unfaulty conditions. as can be seen, the sensitivity to the spray gun fault is different for the two parameters (m1, m2), due to the positioning of the four air spray guns and the two cameras, as shown in fig. 6a. moreover, different absolute values happen to be measured in unfaulty condition, since different types of profiles have different shapes and then respond to the illumination differently. analyzing the results, the unfaulty condition appears clearly separate from the single faults, that are the most difficult to detect. the diagnostic capability is acceptable since it is possible to identify, at least, on which side of the profile there is one or more guns not working correctly. p4 p1 p2 camera 1 (a) paint spray guns illuminator camera 2 p3 (b) figure 6. a) painting quality measurement system; b) software user  interface.  acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 126  other tests are carried out in order to verify the system sensitivity to partial obstruction of a paint spray gun. in this case the system detects the fault only when the obstruction is relevant. finally, tests with a complete obstruction of a single gun for a short time period are performed. the promptness of the detection is related to the measurement rate; the time required to process a couple of images is about 100 ms. since the maximum line speed is 20 m/min, and the field of view is about 10 cm, each measurement covers a length of about 3 cm. then, the system is able to detect the lack of paint for length at least greater than 6 cm. these characteristics fully respond to specific requests of the factory quality control system. 5. on‐line gloss meter  one of the final stages of the production line is the brushing, where a rotating brush can be moved vertically in order to abrade with a different intensity the surface of the profile and then to give a different matt appearance to the surface. also this stage has a predominantly esthetical purpose. in order to verify the obtained matt appearance, the degree of gloss (expressed in gloss unit, gu) was traditionally measured off-line on some samples using a commercial gloss-meter. these measurements are then compared with the gloss acceptance range [1.5 3.5 gu]. the gloss measurement exploits the phenomenon of specular reflection of a surface illuminated by a light beam. the gloss-meter projects a non-polarized white light on the sample surface, at a predetermined angle of incidence and it measures the intensity of the reflected light with a sensor located in a specular position with respect to the light source. according to the standard [11], the angles of incidence for the gloss measurement are either 20°, 60° or 85° depending on the gloss value. the off-line measurements show several drawbacks: they require a relevant time, they cannot be carried out on all the pieces, and specialized operators are required. the research activity has been aimed at achieving an online non-contact gloss-meter. the approach suggested by the standard [11] cannot be precisely followed. the incidence and reflection angles of light rays cannot be accurately controlled online, since the rubber profile can have small movements and its surface is generally not flat. the proposed technique uses a white led illuminator ring (advanced illuminator rl127) as the light source, and a digital smart camera (ni 1772, 640×480 pixels, 8 bit, 110 fps) as the sensing device [12]. the measurement station is positioned immediately after the brushing booth (fig. 7a). the smart camera is positioned on top of the profile and acquires an image of a portion of the profile. the processing steps applied onto the acquired image are the following: i) the position of the profile within the image is determined by edge detection; ii) a sub-image of 30 × 20 pixels is located at the centre of the detected profile; iii) the average brightness of the sub-image is evaluated. the measured value can be related to the gloss value through a linear relationship that is experimentally estimated with a calibration procedure described in section 5.1. the measured gloss is compared with a tolerance value defined by the product specifications. if the measured value is out of tolerance, an alarm is generated. the operator panel (fig. 7b) shows the acquired image, the overlay of the sub-image rectangle, the measured gloss value and a chart with the history of the last 300 measurements. table 1. test results for faulty and unfaulty cases.  p1 p2 p3 p4 m1 m2 ok ok ok ok 80-90 65-80 fault ok ok ok 40-65 50-75 ok fault ok ok 40-60 50-80 ok ok fault ok 80-90 20-30 ok ok ok fault 75-90 25-40 fault fault ok ok 30-45 50-80 ok ok fault fault 70-85 15-25 fault fault fault fault 10-20 10-20 illuminator conveyor belt gasket profile brushing  machinesmart camera vision system (a) (b) figure 7. a) gloss measurement system; b) software user interface.  g = 0,113 l ‐ 7,988 r² = 0,978 0,0 1,0 2,0 3,0 4,0 75 80 85 90 95 100 105 l g [g u ] figure 8. the results of calibration.  acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 127  5.1. calibration procedure  a calibration procedure is required in order to estimate the relationship between the average brightness of the subimage and the gloss value. as a reference instrument, an erichsen pico-glossmaster 500 is used, since it is the instrument currently in use at the quality laboratory of the plant for offline measurements. it has a measurement range of [0, 100] gu and an accuracy equal to 0.2 gu. the following steps of the calibration procedure are repeated for n different brush positions: the brush is set in a fixed position. the average brightness of the sub-image is measured for 30 seconds. mean (l) and standard deviation (�l) of the averages are evaluated. while the profile is being measured the starting and final point are marked on its surface. the marked profile length is measured offline with the reference instrument, by taking 30 uniformly spaced measurements. mean (g) and standard deviation (σg) of these measurements are evaluated. a linear fit is applied on the pairs (gi, li) in order to estimate the calibration curve. figure 8 shows the calibration points, the obtained curve, its analytical formulation and the determination factor. being r2 equal to 0.978, the linear approximation can be considered acceptable. the standard deviation �y of the output of the linear fit model can be estimated with the following relationship: 1 1 where is the value obtained with the linear approximation at the abscissa li .for almost all the calibration points the standard deviation �l is practically the same in all points and nearly equal to 4. the standard deviation �y is equal to 0.2 gu. the uncertainty [10] of final gloss measurements can be estimated with the following relationship: ∙ where s = 0.113 is the slope of fitting line, and resulted to be equal to ug = 0.5 gu. as far as the measurement rate is concerned, since the system is able to process about 4 images per second and the maximum speed of the line is equal to 20 m / min, the system is in able to perform a measurement every 8 cm. the obtained metrological characteristics meet the requirements of the quality control system. 6. conclusions  a vision based system for the on-line measurement and control of dimensional and esthetical parameters of rubber profiles has been designed and characterized by the authors. the measurement system has demonstrated to be efficient and to fully meet the specifications of the industrial quality production control system. moreover, this on-line noncontact solution has caused a drastic reduction of production waste, and then a decrease of costs. the systems presented in this paper are going to be connected to the same fieldbus plant network. in this way a supervising application, which is under development, will allow to monitor the status of the entire production line, to save results into a database, to manage report printing, to coordinate the feedback actions. references  [1] te-hsiu sun, chun-chieh tseng, min-sheng chen, “electric contacts inspection using machine vision” image and vision computing, volume 28, issue 6, june 2010, pages 890-901. [2] r. anchini, g. di leo, c. liguori, a. paolillo, “metrological characterization of a vision-based measurement system for the online inspection of automotive rubber profile”, ieee transactions on i&m, vol. 58, no. 1, jan. 2009, pp. 4-13. [3] g. di leo, c. liguori, a. paolillo, a. pietrosanto, “contactless measurements for on line quality monitoring in rubber extrusion processes”, 19th symposium imeko tc 4 symposium and 17th iwadc workshop advances in instrumentation and sensors interoperability july 18-19, 2013, barcelona, spain pp. 197-202. [4] g. di leo, c. liguori, a. paolillo, "covariance propagation for the uncertainty estimation in stereo vision", ieee transactions on instrumentation and measurement, vol. 60, issue 5, doi 10.1109/tim.2011.2113070, accepted for publication. [5] r. i. hartley and p. sturm, triangulation, comput. vision image understand., 68-2 (1997-11), 146-157. [6] hartley, zisserman, "multiple view geometry in computer vision", 2nd ed, cambridge university press. [7] z. zhang, “a flexible new technique for camera calibration”, ieee transactions on pattern analysis and machine intelligence, 22(11):1330-1334, 2000. [8] e. reinhard, g. ward, p. debevec, w. heidrich et al., “high dynamic range imaging: acquisition, display and imagebased lighting”, 2nd ed., 2010, morgan kaufmann. [9] t. mertens, j. kautz and f. van reeth, “exposure fusion”, pacific graphics 2007. [10] jcgm 100:2008, "evaluation of measurement data guide to the expression of uncertainty in measurement". [11] iso 2813:1994, “paints and varnishes determination of specular gloss of non-metallic paint films at 20 degrees, 60 degrees and 85 degrees”. [12] t. aida, s. hayasaki, t. sakai, “high speeding of the glossmeter for curved surfaces using ccd line sensor”, conference on precision electromagnetic measurements, 7-10 june 1988, pp. 359-360. microsoft word 405-2987-1-le-rev acta imeko  issn: 2221‐870x  july 2017, volume 6 number 2, 65‐69    acta imeko | www.imeko.org  july 2017 | volume 6 | number 2 | 65  reduction of the effect of floor vibrations in a checkweigher  using an electromagnetic force balance system  yuji yamakawa 1 , takanori yamazaki 2   1 the university of tokyo, hongo 7‐3‐1, bunkyo‐ku, tokyo, japan  2 tokyo denki university, hatoyama, saitama, japan      section: research paper   keywords: mass measurement; electromagnetic force balance system; dynamic behavior; floor vibration  citation: yuji yamakawa, takanori yamazaki, reduction of the effect of floor vibrations in a checkweigher using an electromagnetic force balance system,  acta imeko, vol 6, no. 2, article 11, july 2017, identifier: imeko‐acta‐06 (2017)‐02‐11  section editor: min‐seok kim, research institute of standards and science, korea  received june 30, 2016; in final form february 2, 2017; published july 2017  copyright: © 2017 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  corresponding author: yuji yamakawa, e‐mail: yuji_yamakawa@ipc.i.u‐tokyo.ac.jp    1. introduction  recently, some robots have been introduced in various situations such as fa (factory automation), inspection, food industry and logistics industry etc. by applying the robots to such cases, automation of task realization has been improved effectively. in such situation, a high-speed and good-accuracy mass measurement is also strongly required for accurately sorting products and improving a working time. for example, the continuous mass measurement for 300 products per minute is required. in general, two types of mass measurement systems of the checkweigher exist, such as a load cell type [1]-[4] and an electromagnetic force compensation (emfc) type. in this research, we adopt the emfc type in order to achieve a highspeed and good-accuracy mass measurement. until now, we have proposed a simple dynamic model of the checkweigher with emfc, and we confirmed the validity of the proposed model for the step responses [5]. in this paper, we explore effects of floor vibrations, which are considered to have one dynamic input, in the system with emfc [6], [7]. floor vibration is one of the disturbances to the system and may occur by human walking and surrounding machine motion etc. the effect of floor vibration is an important problem to be solved in order to apply the system to an actual environment and situation and to achieve highaccuracy mass measurements. in this paper, our goal is to duplicate the effect of floor vibration with the proposed dynamic model and to propose a simple reduction method for the effect of the floor vibration. in case that an emfc system is used in a realistic situation, it is extremely important to consider the effect of floor vibration induced to the system by motions of surrounding machines and human walking. the emfc system is a feedback type, and is more susceptible to floor vibrations than the feedforward type (the load cell type). therefore, we propose a model that can estimate the effect of the floor vibration and a simple reduction method based on the model for reducing the error of mass measurement due to floor vibration. finally, we verify the effectiveness of the proposed method through experiments in several situations. 2. electromagnetic force balance system  figure 1 shows an overall scheme of the checkweigher. in this figure only the measuring conveyor is shown; the feed conveyor is located to the left of the measuring conveyor and the sorter is located to the right. the products are moved by the feed conveyor, the product mass is measured by the abstract  the objective of this paper is to propose a simple dynamic model of the system and a reduction method for effect of floor vibration.  the  dynamics  of  the system  with  electro‐magnetic  force  compensation  is  approximated  by a mass‐spring‐damper  system,  and  an  equation of motion is also derived. then, a comparison of a simulation result with a realistic response is carried out. finally, effects of  floor vibration are explored with the model, and the effectiveness of the proposed reduction method is confirmed.  acta imeko | www.imeko.org  july 2017 | volume 6 | number 2 | 66  measuring conveyor and the product is removed by a sorter outside of the measurable range. the mass measurement system consists of a weighing platform, the roberval mechanism, the lever-linked roberval mechanism, a counter weight, the electromagnetic force actuator to control the lever displacement and a displacement sensor to measure the lever displacement. by applying the roberval mechanism to the measurement mechanism, the mass of the product can be measured regardless the location of the product on the weighing platform. the mass of the product is estimated from the current of the electromagnetic force actuator to control the lever displacement. the procedure of mass measuring is as follows: 1. when the product with mass m is put on the measurement system, this causes a displacement of the roberval mechanism. 2. the displacement of the roberval mechanism is magnified by the lever. 3. the magnified displacement (the lever displacement) can be measured by the displacement sensor. 4. the current is controlled so that the displacement of the lever is maintained at zero. 5. the mass is estimated based on the current. since the control in the procedure is performed based on the lever displacement, the error of the mass measurement may be generated due to the displacement by the floor vibration. 3. derivation of the dynamic model  figures 2 and 3 show the basic mechanism and structure, and the physical model of the measurement system, respectively. the equation of motion about mass m can be written as follows: flmgkxxcxlmmm l  )( 2 , (1) where m is the mass of the roberval mechanism, m is the mass of the product to be measured, c is a damping coefficient, k is a spring constant, g is the acceleration of gravity, l (= 20 m/m) is the lever ratio, and x is the displacement of mass m. in addition, ml is the mass of the lever, xl is the displacement of mass ml, and f (= bli, where b is the magnetic flux density, l the length of the coil, and i the current) is the electromagnetic force input to control the position xl of mass ml. from (1), the natural frequency f can be calculated as 22 1 lmmm k f l   . (2) from step responses in an open-loop system, the natural frequency appeared to be 5 hz. thus, the spring constant k can be obtained based on (2) as follows: k  (2 f )2  (m  m ml l 2 )  89.2 103 . (3) in addition, the damping coefficient c can be adjusted so as to match the convergence rate in the responses of the lever. from (1), the roberval displacement x in the steady-state in the open-loop system (f = 0) can be described by: x  mg k . (4) further, the lever displacement xl in the steady state, which is the roberval displacement magnified by the lever, can be obtained by the following equation: xl  lx . (5) simulations of the mass measurement system can be executed using (1)-(5). figure 4 shows a block diagram of the mass measurement system. the effectiveness of the dynamic model (1)-(5) can be verified by comparing the output of the lever displacement sensor (xl). 4. model validation  here, the validity of the proposed dynamic model (1)-(5) is confirmed based on step responses in open-loop and closedloop systems [5]. 4.1. open‐loop system  the validity of the proposed model was first examined by comparing the simulation and experimental results in the openloop system. figure 5(a) depicts the experimental and simulation results for m = 0.01, 0.02, 0.05 and 0.1 kg. the red, blue, green and black lines depict the results for m = 0.01, 0.02, 0.05 and 0.1 kg, respectively. also, the dotted and solid lines indicate the experimental and simulation results, respectively. the start-up operation is the time when the product of the mass m is put on the measurement system. at time 0.2 s, the product of the mass m is removed. at the same time, the lever displacement xl is measured by the displacement sensor. in figure 5(a), the figure 2. basic mechanism and structure.  figure 3. physical model.  figure 1. overall mass measurement system.  weighing platform counter weightroberval mechanism lever m m k spring c damper x xl=lx f= bli ml 1:l xb product mass of roberval mass of lever electromagnetic actuator floor vibration acta imeko | www.imeko.org  july 2017 | volume 6 | number 2 | 67  vertical axis is the output of the displacement sensor about the lever displacement. it can be seen from this figure that the responses of the simulations were in excellent agreement with the experimental results. thus, the validity of the proposed model was confirmed in the open-loop system. 4.2. closed‐loop system  this section explains the experimental and simulation results with emfc. the electromagnetic force was controlled by a proportional-integral-derivative (pid) control algorithm. taking the actual circuit of the d action into account, the ideal d action could not be implemented. thus, we implemented the approximated d action instead of the ideal d action. namely, the transfer function of the pid controller c(s) is described by c(s)  kp  ki s  kdns kdds1 , (6) where kp is the proportional gain, ki is the integral gain, and kdn and kdd are the numerator and denominator coefficients of the differential gains, respectively. the control voltage uv(s) (laplace transform of uv(t)) can be adjusted as follows: uv (s)  c(s)e(s) , (7) where e(s) is the laplace transform of e(t), and e(t) (= xlr − xl) is the error between the reference xlr (=0) and the output of the lever displacement sensor xl. the pid control can be performed by an fpga (field programmable gate array) every 0.1 ms. figure 5(b) depicts the experimental and simulation results for m = 0.01, 0.02, 0.05 and 0.1 kg. as in figure 5(a), the red, blue, green and black lines depict the results for m = 0.01, 0.02, 0.05 and 0.1 kg, respectively. the dotted and solid lines indicate the experimental and simulation results, respectively. the simulation and experimental conditions are the same as the open-loop system.   figure 4. block diagram of the measurement system including the reduction method for floor vibration.        figure 5. comparisons between experimental results and simulation results.  acta imeko | www.imeko.org  july 2017 | volume 6 | number 2 | 68  responses such as the convergence time, the rise time and the settling time were almost the same for the simulation and experimental results. thus, the validity of the proposed dynamic model with emfc was confirmed. however, the peak value of the simulation result for m = 0.1 kg was different from that of the experimental result. we assume that the reasons for the modelling error were the frictional force in small displacements and the magnification mechanism of the lever. 5. floor vibration and reduction method  in this section we explore effects of floor vibrations. floor vibration is one of the system disturbances to be considered. first we compare experimental results with simulation results for floor vibration and confirm the validity of the model about the floor vibration. then we propose a new method for decreasing the effect of the floor vibration to achieve a high accuracy mass measurement. figure 4 shows the block diagram of the overall system in mass measurement including the vibration input. the upper and lower block diagrams show a reduction method for floor vibration and the proposed dynamic model of the system, respectively. in experiments, the floor vibrations are acted on the system by using a vibration exciter. in simulations, floor vibration inputs are applied to the system as the environmental disturbance, as shown in figure 4. a displacement of the floor vibration in the simulation can be given by tatx bbb sin)(  , (8) where xb(t) is the displacement of the floor vibration, ab [mm] is the amplitude of the floor vibration, ωb [rad/s] is the angular frequency of the floor vibration, and t is time. therefore, an acceleration ab(t) of the floor vibration can be described as follows: tata bbbb  sin)( 2 . (9) floor vibration input to the system can be represented by the product of the acceleration ab(t) and the mass mb of the part of the roverbal mechanism. the input of the floor vibration is added to the right side in (1) as mbab. thus (1) can be rewritten as bbl amflmgkxxcxlmmm  )( 2 . (10) the following five conditions about the floor vibrations for experiments and simulations are chosen; (a) ab = 1 mm, fb = 10 hz, (b) ab = 1 mm, fb = 15 hz (c) ab = 0.75 mm, fb = 15 hz, (d) ab = 1.25 mm, fb = 5 hz (e) ab = 1.25 mm, fb = 10 hz 5.1. effect of floor vibration  first we compare experimental results with simulation results about the effects of the floor vibration. then we will confirm the validity of the proposed model for several floor vibrations. the figures on the left hand side in figure 6 show experimental results (red lines) and simulation results (blue lines) about floor vibrations. from these results, we found that almost the same responses between the simulations and the experiments are obtained. thus we confirmed the validity of the proposed dynamic model for floor vibrations. however, differences between the simulation result and the experimental result in the condition as shown in figures 6(b) and 6(c) occured. the reason is that the mass value in the experiment becomes smaller due to mechanical limits of the lever. as a result, the error of the mass measurement due to the floor vibration will be generated as shown in figure 6. thus we propose a reduction method for the effect of the floor vibration in the next section. 5.2. method for decreasing floor vibration effects  since the same responses for floor vibrations can be obtained using the proposed model, we can compensate the effect of floor vibration in terms of mass measurement by a simple subtraction. namely, we compensate the error using the following equation, simesti mmm ˆ , (11) where m̂ , mesti and msim mean the compensated mass value, the mass value obtained from the system and the mass value obtained by the simulation with the proposed model, respectively. since a high-speed mass measurement is required, it is desirable that the processing time is short and the response after the processing does not become slower. therefore, the proposed method is considered to be appropriate to our objective. the figures on the right hand side in figure 6 show the compensated mass values for floor vibrations under the same conditions as before. it can be seen from these results that the errors of the mass measurements by the floor vibration can be effectively decreased more than half of the original errors. actual compensation methods for mass measurement errors due to the floor vibrations can be considered as follows: “an acceleration of the floor vibration is measured using an acceleration sensor. the measured acceleration is used as the input of the floor vibration in the proposed model. then the reduction method for the floor vibration in the mass measurement is performed.” here we discussed the effectiveness of the proposed method through simulations with the experimental data. in the future, we will implement the proposed method to the actual mass measurement system in order to verify the validity. 6. conclusions  in this paper, we proposed a dynamic model of the mass measurement system with emfc. we examined the validity of the proposed model for the open-loop and the closed-loop systems. comparing the experimental results with the simulation result, the same responses can be obtained. furthermore, we discussed the effect of the floor vibrations to the mass measurement system, and we proposed a simple reduction method for decreasing the effect of the floor vibration. finally, we evaluated the proposed method by experiments in various situations. consequently, the mass measurement error for the floor vibration can be reduced effectively by at least 50 % with the proposed method. in the future, we plan to perform the experiment for lower frequencies of the floor vibration. moreover, we will implement the proposed method to the realistic system and explore the effectiveness of the proposed method in an actual environment. references  [1] t. ono, “basic point of dynamic mass measurement”, proc. sice, pp. 43-44, 1999. [2] t. ono, “dynamic weighing of mass”, instrumentation and automation 12, no. 2, pp. 35, 1984. acta imeko | www.imeko.org  july 2017 | volume 6 | number 2 | 69  [3] w.g. lee et al., “development of speed and accuracy for mass measurements in checkweighers and conveyor belt scales”, proc. ismfm, pp. 23-28, 1994. [4] k. kameoka et al., “signal processing for checkweigher”, proc. apmf, pp. 122-128, 1996. [5] y. yamakawa and t. yamazaki, “mathematical model of checkweigher with electromagnetic force balance system”, acta imeko, vol.3, no.2, pp.9-13, 2014. [6] y. yamakawa and t. yamazaki, “modeling and control for checkweigher on floor vibration”, xxi imeko world congress, tc3-o-079, 2015. [7] y. yamakawa and t. yamazaki, “modeling of floor vibration effect for high-speed mass measurement system”, 2015 asiapacific symposium on measurement of mass, force and torque (apmf2015), pp.161-166, 2015.   figure 6. comparisons between experimental results and simulation results of the effects of floor vibrations.  template for an acta imeko event paper acta imeko issn: 2221-870x april 2017, volume 6, number 1, 27-32 acta imeko | www.imeko.org april 2017 | volume 6 | number 1 | 27 dependence of ultrasonic contact impedance hardness on young’s modulus of elasticity of creep-resistant steels michal junek, jiří janovec, petr ducháček ctu in prague, faculty of mechanical engineering, department of materials engineering, praha 2, karlovo náměstí 13, czech republic section: research paper keywords: ultrasound hardness tester; young’s modulus of elasticity; calibration; creep-resistant steel; residual operation life citation: michal junek, jiří janovec, petr ducháček, dependence of ultrasonic contact impedance hardness on young’s modulus of elasticity of creepresistant steels, acta imeko, vol. 6, no. 1, article 5, april 2017, identifier: imeko-acta-06 (2017)-01-05 section editor: konrad jędrzejewski, warsaw university of technology, poland received march 2, 2016; in final form march 17, 2016; published april 2017 copyright: © 2017 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: this work was supported by the ministry of education, youth and sport of the czech republic within the project no. lo1207 of the program npu1 corresponding author: m. junek, e-mail: michal.junek@fs.cvut.cz 1. introduction at present a trend of developing hardness measurements by portable hardness testers exists, allowing "non-destructive" hardness measurements of large construction parts without any need of specimen separation for stationary hardness testers. four physically distinct methods used are: uci ultrasonic method, dynamic rebound method, optical method tiv (through indenter viewing) and electrical resistance method (handy esatest). in the following text, the focus was aimed only on the uci method. one of the many possibilities of using portable uci hardness testers is the hardness measurement in assemblies (steam boilers, steam pipe-lines) functioning in energy units, which operate in creep conditions (temperature approx. 550 °c and a pressure approx. 15 n.mm-2). long-term operation (2.5×105 hours) in these conditions leads to degradation of the material accompanied by a decrease of the hardness values. uci hardness measurement method should thus serve to detect thresholds hardness values that indicate the poor mechanical properties of the material, the risk of failure and the necessity of a more thorough examination of the experimental material state (microstructure evaluation, small punch samples preparation). 2. the uci method of hardness measurement an uci probe consists of a vickers diamond tip attached to the end of a metal rod, which is excited to perform a longitudinal oscillation with a frequency of 70 khz, by piezoelectric transducers (figure 1). t is the piezoelectric transducer, r the piezoelectric receiver, o the oscillating rod, v the indenter (for example vickers diamond) and m is the test material. figure 1. uci probe schematics [1]. abstract the paper deals with the hardness measurements by mobile uci hardness testers as a means of determining the residual operation life of power unit components. it aims to answer questions regarding the level of dependence of uci hardness on young´s modulus of creep-resistant steels and determining the conditions of a uci hardness tester calibration. the experimental part describes comparative measurements of hardness values obtained using stationary hardness testers and uci hardness testers. acta imeko | www.imeko.org april 2017 | volume 6 | number 1 | 28 the uci method does not require the diagonals of the test indentation to be measured which is necessary for the vickers hardness determining. in this method, the shift of an ultrasonic frequency of the oscillating indenter is electronically related with the area of the indentation and thus resulting in the final hardness value. the deeper the diamond indenter penetrates, the larger is the indentation area, the larger the frequency shift of the diamond tip and the lower the resulting hardness, see (1), (2) and figure 2 [2]. ∆𝑓 = 𝑓(𝐸𝑒𝑒𝑒 ∙ 𝐴) , (1) 𝐻𝐻 = 𝐹 𝐴� , (2) where δf is the frequency shift, eeff the effective modulus of elasticity (contains the elastic constants of both the indenter and the test piece), a is the area of indentation, hv the vickers hardness value and f is the test load. equation (1) implies that a frequency shift depends on the effective modulus of elasticity as well. therefore, young’s modulus of elasticity in tension must be considered using the uci method in practice. the equipment must be calibrated when determining the hardness of different materials with different hardness values. but the question is: to what extent does the uci hardness depend on the modulus of elasticity? [2] 2.1. calibration the astm a1038 – 05 standard [1] states that a uci hardness tester usually has been calibrated on non-alloyed and low-alloyed steel, that is, certified hardness reference blocks with young’s modulus of elasticity equal to 210 000 n.mm-2. because unalloyed or low-alloy steels have a similar young’s modulus of elasticity, accurate results are obtained with the standard calibration. in many cases, the difference in young’s modulus of medium-alloy and high-alloy steels is so insignificant that the error created falls within the allowable tolerances of the part. but the question is: what is considered as a similar young’s modulus of elasticity? hardness reference blocks are needed for calibration to other materials with a different young’s modulus of elasticity. this paper should answer the question of calibration uci hardness testers used for hardness measurement of components functioning in power units. 2.2. comparison of non-destructive methods of hardness measurement several methods for non-destructive hardness measurements are used in practice. table 1 shows various methods for nondestructive measuring of hardness using portable hardness testers. furthermore, a comparison of the areas of applications of each portable hardness testers is given. from this comparison, the uci method seems to be best for hardness measurements of large construction parts functioning in energy units (steam pipe-lines, steam boilers and welds of these parts). 3. degradation mechanisms of high pressure (hp) steam pipe-lines the best resistance against operating in conditions of creep damage shows crmov low-alloy steels in the normalized or heat-treated state (13crmo4-5, 14mov6-3, 10crmo9-10). in case of higher operating parameters, it is 9-12 % cr martensitic steel x10crmovnb9-1 (p91) and x10crwmovnb9-2 (p92) steel. the main factors affecting the lifetime of hp steam pipelines is a combination of their material properties and operating conditions. the main degradation mechanisms therefore include material degradation in the form of structural changes caused by coagulation of carbide particles and the nucleation and formation of cavities due to creep damage. this table 1. recommended applications for hardness measurement by portable hardness testers [2]. applications dynamic rebound method uci method tiv method handy esatest solid (big) parts * * o o coarse-grained materials * x x x steel and aluminium cast alloys * o o o haz with welds x * * * tubes: wall thickness > 20 mm * * * * tubes: wall thickness < 20 mm x * * * inhomogeneous surfaces o x x x sheet metal, coils x o * * thin layers x o * * hard to get at positions x * x * coarse surfaces * x x o finally machined surfaces o * * * electrically conductive materials * * * x dusty environments * o o x explanatory notes to table: x not recommended o sometimes suitable (in the case of elimination conditions having adverse impacts on measurement results) * especially well-suited figure 2. frequency shift of an ultrasonic contact impedance (uci) probe as a function of hardness [3]. acta imeko | www.imeko.org april 2017 | volume 6 | number 1 | 29 degradation is accompanied by a reduction of hardness. [4] the structural changes are temperature and time dependent processes which could lead to a decrease in both short-term characteristics (yield strength, ultimate strength, hardness, and fracture toughness) and long-term characteristics (creep strength, creep deformation, plasticity). in case of low-alloy creep-resistant steels an area of predominant hardening and an area of predominant softening exists. hardening of the material (up to 1000 hours of operation) in crmov steels is caused by additional precipitation of vanadium carbides, thereby increasing the number of new dispersed particles, reducing their average size, increasing their volume fraction and decreasing their interparticle distance. on the other hand, during material softening coarsening of dispersed particles via diffusion processes occurs, resulting in an increase of their average size, volume fraction decrease and increase of the interparticle distance [4]. stages of degradation of creep resistant, low-alloy steel of chemical composition of 0.5 % cr 0.5 % mo 0.25 % v (14mov6-3) subjected to creep exposure are in figure 3. stage 0 corresponds to the initial state with a ferritic-bainitic structure at the beginning of creep exposure (figure 3 stage 0). the first stage of structural changes is characterized by a moderate decomposition of bainite. that is accompanied by coagulation of m3c carbides in these areas and further precipitation of m23c6 carbides along the ferritic grains boundaries. at the same time, very fine mc carbides precipitate within the ferrite grains (figure 3 stage 1). the next stage is characterized by significant decomposition of bainite and coagulation of m3c carbides into relatively large carbide particles at the grain boundaries. the m23c6 carbides precipitate on the boundaries of ferritic grains and form chains. simultaneously, fine mc carbides are observed within ferritic grains (figure 3 stage 2). final structural changes result in a ferritic matrix containing mc and m6c carbides inside ferrite grains and large m23c6 carbides precipitated along the grain boundaries (figure 3 stage 3). depending on the operating conditions, the material may contain also other types of carbides, e.g. m7c3 carbides. after such a degradation of the material microstructure and further creep exposure creep cavities are formed [5]. 3.1. surface decarburization surface decarburization takes place most frequently on the outer surface of steel components and is accompanied by rapid reduction of carbon content on the surface due to diffusion caused by high temperatures. a decarburized layer has a lower hardness than the material below the layer due to reduced carbon content. the carbon content gradient in the decarburized layer increases with the distance from the outer surface, see figure 4 and figure 5. therefore, the hardness values measured after removal of the decarburized layer are higher. since the hardness is measured on the outer surface components, it is imperative to remove this layer in order to achieve relevant results. the decarburized layer has a thickness usually of up to 1.0 mm. 4. experiments the experimental part was focused on determining uci hardness dependence on young’s modulus of elasticity in tension via comparative measurements of hardness values obtained by the classic hv10 vickers method and values obtained using the uci hardness tester. deviation of the mean values measured by the uci hardness tester and by the stationary (laboratory) hardness tester were evaluated. 4.1. used measurement equipment comparative measurements as a means of determining the uci hardness on young’s modulus of elasticity were performed in accord with the čsn en iso 6507-1 standard using a calibrated stationary hardness tester by the vickers method with a load of hv10. as a representative of portable hardness testers the uci hardness tester krautkrämer mic20 and krautkrämer mic10 with uci probe mic 2010 of load of 98 n was selected. figure 3. stages of microstructure degradation after creep exposure [5]. figure 4. surface decarburization of hp steam pipe-line of fossil fuel power plants. figure 5. dependence of rockwell hardness on the carbon content [6]. acta imeko | www.imeko.org april 2017 | volume 6 | number 1 | 30 4.2. experimental materials materials with different young’s modulus e were selected for comparative measurements. values of young’s modulus e were verified using data from literature. the mean values without standard deviations are shown in table 2. the experimental determination of the young’s modulus values was not carried out due to lack of time. the experimental samples were removed from the steam pipe-lines or boiler tube in different states of degradation. in the case of steels x20crmov12-1 and x5crnicunb17-4-4, it was possible to get the samples from the rotor blades. table 3 shows a description of individual samples. 4.3. preparation of the measurement site hardness of the materials described above was measured always on the outer surface (approximately 0.5 to 1.0 mm was grinded off in order to remove the decarburized layer), and across the tube wall thickness. in case of steels of a tensile modulus of 200 n.mm-2 hardness was measured only in the cross-section of the blade lock. the surface was prepared using a metallographic grinder with sandpaper grit of 400, thereby achieving a surface roughness ra of 0.07 – 0.12 µm (measured by a surface roughness tester hommelwerke lv-5e). the selected number of indentations on every surface was 10. 4.4. measurement process on the prepared surfaces with an approximate area of 10 mm2 at first the hardness was measured according to čsn en iso 6507-1 using a stationary hardness tester (vickers method) with a load of hv10. subsequently, uci hardness tester was used for hardness measurement in close vicinity (approximately 5 mm) of these indentations. during uci tester measurements the following finding was observed. the probe mic 2010 was firstly used for measuring the initial state hardness (without a fixture providing perpendicular positioning). hardness values measured in this way showed significant deviations and unreal values. therefore, it was decided to use the probe with a fixture providing perpendicular positioning to solve the problem. it implies that when measuring the hardness, it is necessary to keep the probe perpendicular to the surface, which can be achieved by installing a fixture or by a properly trained and experienced person performing the measurements. 4.5. measurements results all hardness measurement results are summarized in table 3 to table 10. the tables contain the average hardness values from 10 measurements for each sample measured on the surface and throughout the wall thickness by the laboratory hardness tester and by uci hardness testers. the tables further include standard deviations (std) of measured values and the deviation of the average value of the uci hardness from average values measured by the stationary hardness tester (lab). a) steels with e = 218 000 n .mm -2 in case of steels with the same young’s modulus 218 000 n.mm-2 the measurements show that the values of the deviations of both uci hardness testers (krautkrämer mic20 and mic10) vary in the same trend. the average value of the deviations is approximately -21 hv (see tables 4 and 5). b) steels with e = 210 000 n.mm-2 in case of 14mo6-3 steel and t23, t24 steels with the same young’s modulus 210 000 n.mm-2 it was observed that the table 4. steel x10crmovnb9-1; samples p21 and p6. x10crmovnb9-1; sample p21 (ø 324 x 28 mm); initial state after heat treatment measured on surface measured through the wall thickness lab mic 20 mic 10 lab mic 20 mic 10 hv10 uci hv10 uci hv10 hv10 uci hv10 uci hv10 avg. 201 187 186 215 199 195 std ± 5 ± 6 ± 9 ± 5 ± 11 ± 10 deviation 14 15 16 20 x10crmovnb9-1; sample p6 (ø 270 x 25 mm); laboratory ageing at 600 °c/10 000 hours avg. 228 208 206 238 218 217 std ± 5 ± 5 ± 5 ± 4 ± 6 ± 6 deviation 20 22 20 21 table 3. selected samples of materials and their state of operating (laboratory) degradation. material sample state of degradation x10crmovnb9-1 p6 – lab. ageing at 600 °c/10 000 hours p21 – initial state after heat treatment x10crwmovnb9-2 bt3 – lab. ageing at 650 °c/20 000 hours t33 – lab. ageing at 650 °c/5 033 hours 14mov6-3 k1 – degraded at 525 °c/240 000 hours epr – degraded at 560 °c/261 800 hours epc – degraded at 540 °c/240 066 hours t23 v23 – initial state after heat treatment d23 – lab. ageing at 650 °c/5 033 hours t24 v24 – initial state after heat treatment d24 – lab. ageing at 650 °c/5 033 hours x20crmov12-1 l1 – unknown x5crnicunb17-4-4 l2 – unknown steel super 304h 2030 – dissolving annealing 1150 °c/2 min table 2. selected materials and their young’s modulus values. materials e [n.mm-2] x10crmovnb9-1 (p91) x10crwmovnb9-2 (p92) 218 000 14mov6-3 t23 t24 210 000 x20crmov12-1 x5crnicunb17-4-4 200 000 steel super 304h 193 000 table 5. steel x10crwmovnb9-2; samples bt3 and t33. x10crwmovnb9-2; sample bt3 (ø 350 x 39 mm); laboratory ageing at 650 °c/20 000 hours measured on surface measured through the wall thickness lab mic 20 mic 10 lab mic 20 mic 10 hv10 uci hv10 uci hv10 hv10 uci hv10 uci hv10 avg. 226 211 207 232 205 211 std ± 2 ± 9 ± 9 ± 3 ± 5 ± 7 deviation 15 19 27 21 x10crwmovnb9-2; sample t33 (ø 528 x 94 mm); laboratory ageing at 650 °c/5 033 hours avg. 223 204 198 230 205 216 std ± 3 ± 12 ± 6 ± 3 ± 7 ± 6 deviation 19 25 25 14 acta imeko | www.imeko.org april 2017 | volume 6 | number 1 | 31 values of the deviations of both uci hardness testers (krautkrämer mic20 and mic10) vary in the inverse trend. the average value of the deviations is approximately -13 hv for 14mo6-3 steel and 13 hv for t23, t24 steels (see table 6 table 8). c) steels with e = 200 000 n.mm-2 the deviation values measured in steels with young’s modulus of 200 000 n.mm-2 of both uci hardness testers (krautkrämer mic20 and mic10) are similar. the average value of the deviations is approximately -3 hv (see tables 9 and 10). d) steels with e = 193 000 n .mm -2 in case of austenitic steel super 304h with young’s modulus 193 000 n.mm-2 it was observed that the measured values show large measurement errors. the standard deviation of the value measured by the uci hardness tester is up to 59 hv. the measurement of hardness of austenitic steels is generally a problem due to their strengthening. the dynamic rebound method (brinell method) was not carried out because specimens, on which the measurement was to be performed, did not reach the required minimum weight of 5 kg and minimum wall thickness 20 mm. the average value of the deviations is very different (see tables 11). 5. conclusion the original idea of practice that hardness measurement by mobile uci hardness testers is independent of the values of young’s modulus has proven not to be entirely correct. the experimental results show the following (see table 12). the larger is young’s modulus the larger is the negative deviation of the measured hardness. the explanation of this dependence is table 6. steel 14mov6-3; samples pk1, epr and epc. 14mov6-3; sample k1 (ø 273 x 26 mm); degraded at 525 °c/240 000 hours measured on surface measured through the wall thickness lab mic 20 mic 10 lab mic 20 mic 10 hv10 uci hv10 uci hv10 hv10 uci hv10 uci hv10 avg. 201 191 187 214 202 202 std ± 3 ± 7 ± 7 ± 5 ± 9 ± 14 deviation 10 14 12 12 14mov6-3; sample epr (ø 245 x 36 mm); degraded at 560 °c/261 800 hours avg. 151 137 133 150 138 133 std ± 1 ± 11 ± 7 ± 2 ± 8 ± 8 deviation 14 18 12 17 14mov6-3; sample epc (ø 324 x 48 mm); degraded at 540 °c/240 066 hours avg. 128 121 119 133 117 119 std ± 4 ± 2 ± 7 ± 3 ± 5 ± 5 deviation -7 -9 -16 -14 table 7. steel t23; samples v23 and d23. t23; sample v23 (ø 38 x 5.6 mm); initial state after heat treatment measured on surface measured through the wall thickness lab mic 20 mic 10 lab mic 20 mic 10 hv10 uci hv10 uci hv10 hv10 uci hv10 uci hv10 avg. 205 222 214 201 221 210 std ± 4 ± 11 ± 6 ± 1 ± 6 ± 6 deviation 17 9 20 9 t23; sample d23 (ø 38 x 5.6 mm); laboratory ageing at 650 °c/5 033 hours avg. 157 169 167 166 176 169 std ± 2 ± 8 ± 8 ± 3 ± 9 ± 9 deviation 12 10 10 3 table 8. steel t24; samples v24 and d24. t24; sample v24 (ø 38 x 5.6 mm); initial state after heat treatment measured on surface measured through the wall thickness lab mic 20 mic 10 lab mic 20 mic 10 hv10 uci hv10 uci hv10 hv10 uci hv10 uci hv10 avg. 229 250 239 228 243 231 std ± 2 ± 12 ± 7 ± 5 ± 6 ± 11 deviation 21 10 15 3 t24; sample d24 (ø 38 x 5.6 mm); laboratory ageing at 650 °c/5 033 hours avg. 175 186 192 176 199 190 std ± 2 ± 12 ± 8 ± 4 ± 11 ± 11 deviation 11 17 23 14 table 11. steel super 304h; sample 2030. super 304h; sample 2030 (ø 38 x 6.3 mm); dissolving annealing 1150 °c/2 min measured on surface measured through the wall thickness lab mic 20 mic 10 lab mic 20 mic 10 hv10 uci hv10 uci hv10 hv10 uci hv10 uci hv10 avg. 181 366 176 227 std ± 2 ± 59 ± 4 ± 32 deviation 185 49 table 10. steel x5crnicunb17-4-4; sample l2. x5crnicunb17-4-4; sample l2 (rotor blades); unknown state measured through the cross-section measured through the cross-section lab mic 20 mic 10 lab mic 20 mic 10 hv10 uci hv10 uci hv10 hv10 uci hv10 uci hv10 avg. 503 502 500 508 509 504 std ± 7 ± 8 ± 7 ± 3 ± 10 ± 6 deviation 1 3 1 4 table 9. steel x20crmov12-1; sample l1. x20crmov12-1; sample l1 (rotor blades); unknown state measured through the cross-section measured through the cross-section lab mic 20 mic 10 lab mic 20 mic 10 hv10 uci hv10 uci hv10 hv10 uci hv10 uci hv10 avg. 352 348 347 361 357 355 std ± 5 ± 11 ± 13 ± 5 ± 7 ± 7 deviation 4 5 4 6 acta imeko | www.imeko.org april 2017 | volume 6 | number 1 | 32 evident from the figure 2, which shows that the greater the frequency shift the smaller the measured hardness. therefore, we can say that steel with a higher modulus of elasticity has a greater frequency shift. therefore, we measured a lower hardness for steels with a higher young’s modulus. the measured results showed the existing dependence of uci hardness on young’s modulus. although the dependence is very small, it is necessary to consider it and to perform the proposed calibration of uci hardness testers using suitable calibration plates. the measured results are summarized in table 12. from the results it is possible to state that with increasing young’s modulus of elasticity e the value of the negative deviation of hardness values measured by the uci hardness tester increases in comparison with the values measured using a stationary (laboratory) tester. the dependence can be considered as linear. however, that doesn´t apply in the case of t23 and t24 steels with young’s modulus of 210 000 n.mm-2 where the deviation of uci hardness varies in the opposite direction. the general validity of these conclusions has not been confirmed in one case. t23 and t24 steels showed the deviation trend opposite to the deviation measured in 16mov63 steel. the average value of the deviations is approximately -13 hv for 14mo6-3 steel and 13 hv for t23, t24 steels. finding the reasons for this difference in deviations values is not yet completed. a possible cause could be a different wall thickness, because boiler tubes from these materials have a wall thickness of 5.6 mm and steam pipe-lines from other materials have wall thicknesses from 20 up to 80 mm. the observation of the influence of different wall thicknesses (steam and boiler tubes), the influence of microstructure and verification of the real young’s modulus will be subjects for the further research. considering the above measured results, we propose the following groups of calibration plates: 1) calibration plates for 9 % cr martensitic steel – made from p91 (p92) steel: e = 218 000 n.mm-2 2) calibration plates for low alloy crmov steels – made from 16mov6-3 steel: e = 210 000 n.mm-2 3) calibration plates for 2% cr steels – made from t23 (t24) steel: e = 210 000 n.mm-2 4) calibration plates for steels for rotor blades – made from x20crmov12-1 steel: e = 200 000 n.mm-2 it will be necessary to determine the hardness of these calibration plates at the lower and upper limit of practically measured values of hv hardness in operating conditions. the rebound method was not carried out because specimens, on which the measurement was to be performed, did not reach the required minimum weight of 5 kg. measured values would be thus misleading. these results are used in normative technical documentation for requirements for hardness measurements by portable hardness testers in czech classic power plants. acknowledgement this work was supported by the ministry of education, youth and sport of the czech republic within the project no. lo1207 of the programme npu1. references [1] astm a1038 – 05. “standard practice for portable hardness testing by the ultrasonic contact impedance method”. west conshohocken: astm international, 2005. [2] s. frank, “mobile hardness testing application guide for hardness testers”. general electric company [online]. 2005 [cit. 2015-02-13]. available from: https://www.gemeasurement.com/sites/gemc.dev/files/hardnes s_testing_application_guide_english_0.pdf. [3] k. herrmann, “hardness testing: principles and applications”. materials park, ohio: asm international, 2011. isbn 978-161503-832-9. [4] m. junek, “posouzení životnosti vt parovodů v podmínkách creepového poškození”. diploma thesis. praha: 2014. ctu in prague, faculty of mechanical engineering, department of material engineering. p. 87. [5] pa. philadelphia, “power plant life management and performance improvement” woodhead pub. ed. john e. oakey, 2011, 684 p. isbn 978-184-5697-266. [6] slide share. slide share [online]. 2013 [cit. 2015-08-29]. available from: http://image.slidesharecdn.com/f4-131118222248phpapp02/95/hardening-14-638.jpg?cb=1384813955. table 12. a summary of the results of the average deviations. materials e [n.mm-2] average deviations hv10 x10crmovnb9-1 (p91) x10crwmovnb9-2 (p92) 218 000 -21 14mov6-3 210 000 -13 t23 t24 210 000 13 x20crmov12-1 x5crnicunb17-4-4 200 000 -3 https://www.gemeasurement.com/sites/gemc.dev/files/hardness_testing_application_guide_english_0.pdf https://www.gemeasurement.com/sites/gemc.dev/files/hardness_testing_application_guide_english_0.pdf http://image.slidesharecdn.com/f4-131118222248-phpapp02/95/hardening-14-638.jpg?cb=1384813955 http://image.slidesharecdn.com/f4-131118222248-phpapp02/95/hardening-14-638.jpg?cb=1384813955 dependence of ultrasonic contact impedance hardness on young’s modulus of elasticity of creep-resistant steels microsoft word article 3 154-1288-1-le.docx acta imeko  february 2015, volume 4, number 1, 5 – 10  www.imeko.org    acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 5  a novel intelligent instrument for the detection and  monitoring of thrust reverse noise at airports  c. asensio 1 , m. ruiz 1 , m. recuero 1 , g. moschioni 2 , m. tarabini 2   1  universidad politécnica de madrid (i2a2), 28031 madrid, etsi topografía – ctra. de valencia km 7, spain  2  politecnico di milano (dipartimento di meccanica), 23900 lecco, via d'oggiono 18, italy      section: research paper   keywords: airport noise; thrust reverse; aircraft; noise regulations, detection, classification  citation: c. asensio, m. ruiz, m. recuero, g. moschioni, m. tarabini, a novel intelligent instrument for the detection and monitoring of thrust reverse noise  at airports, acta imeko, vol. 4, no. 1, article 3, february 2015, identifier: imeko‐acta‐04 (2015)‐01‐03  editor: paolo carbone, university of perugia   received october 24 th , 2013; in final form april 21 th , 2014; published february 2015  copyright: © 2014 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: this work was supported by i2a2 research group of the universidad politécnica de madrid  corresponding author: césar asensio, e‐mail: casensio@i2a2.upm.es    1. introduction  noise is a major reason for concern regarding environmental protection around airports. among all the noise sources, the activation of the thrust reverser to slow down the aircraft after landing is well known by airport authorities as a major cause of acoustic impact (and also emissions), annoyance and complaints in the vicinity of airports. although the weight of this noise source on the overall long-term average noise assessment can be moderate (for instance in terms of day-evening-night and night-time figure 1. examples of audio signals of a landing with thrust reverse activation (highlighted). abstract  many airports all over the world have established restrictions for the use of thrust reverse for slowing down aircraft after landings, especially during the  night period, as a way of reducing noise impact and the number of complaints in the vicinity of airports. this is the case of madrid airport, where the  universidad  politécnica  de  madrid,  in  collaboration  with  aena,  and  the  politecnico  di  milano  have  been  researching,  and  developing  intelligent  instruments to improve the detection and classification of thrust reverse noise among other noise sources present in the airport. based on a traditional  approach,  the  thrust  reverse  noise  detection  tool  detects  two  consecutive  sound  events,  and  applies  pattern  recognition  techniques  for  the  classification of each of them (such as landing and thrust reverse). a second improvement refers to the use of a microphone array linked to a noise‐ monitoring unit, which enables tracking the direction of arrival of the sound, thus  improving the classification rates. by taking the  latter,  it  is also  possible  to  track  the  aircraft  location  along  the  runway,  which  enables  sound  pressure  measurements  to  be  transformed  into  sound  power  level  estimations. although the novel instrument can still be optimized and customized, the results have shown quite good classification rates (over 90%).  acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 6  noise indicators for strategic noise mapping purposes), especially in those airports where runways are used either for landings and take-offs, the reverse thrust can be quite disturbing, as a rapid change of engine power from idle to reverse occurs causing a sudden burst of noise [1-4], as shown in figure 1. therefore, many airports have established restrictions for the use of thrust reversal after landing, especially during the night period, as a way of reducing the noise impact of airport operations on the community in circumstances where it is critical (for instance paris-orly, london-heathrow, o’hare-chicago [5,6]). the traditional approach for identifying thrust reverse noise is based on the detection of two consecutive sound events, by applying thresholds to the sound level measurements acquired by the airport’s noise monitoring units. each event is detected if the noise level is over the threshold for longer than a pre-established time period. but this technique has proved not to work properly in many cases, as there are many factors affecting the strength, separation and duration of both sound events: the aircraft model and the type of thrust reverser, the weather conditions, the company procedures, the aircraft’s final destination, the pilot’s behaviour… make it difficult to perform the detection task by just using a traditional threshold-based approach (figure 1). the poor performance of the traditional methods, in practice, disables the sanctioning procedures, reducing the efficacy of regulations in fighting noise impact, and originating misalignment between environmental and time efficiency policies. extended and updated information regarding this specific noise source will help to keep and customize efficient thrust reverse restrictions where necessary, while removing them in airports where they are not useful (thrust reverse has many benefits regarding safety, time efficiency and maintenance cost). 2. methodology  the methodology described in this paper is also based on a threshold detection of two consecutive sound events (ev1, for landing noise, and ev2, for thrust reverse noise). but, unlike the traditional approach, which uses the overall sound level, in this case, the detection is improved by the application of filters, acoustic modelling and signal processing practices. furthermore, two cardioid microphones (referred to as left and right, l-r microphones) acquire the sound signal, making the system more robust against sounds arriving from a direction where landings are not expected. a specific filter is applied in each of the detectors, so that the sound level is analysed for customized bandwidths. furthermore, a transformation is applied to improve the detection of the second event (ev2), so that sound power instead of sound pressure can be analysed. if both detectors are successful, ev1 and ev2 will be classified using statistical pattern recognition techniques. in order to complete the identification of a thrust reverse sound, both sound events must be detected and properly classified. figure 2 shows the main system block diagram and figure 3 shows a scheme on the field locations. 2.1. the ev1 detector  the first stage in the trend (thrust reverse noise detector) system is the detection of a landing sound event (ev1). a microphone acquires the sound signal, and the running sound pressure level (lp), is calculated and used for the detection of this first sound event applying time and level thresholds. as high frequency sounds are quite   figure 2. thrust reverse noise detection system block diagram.  figure 3. field measurements scheme.  figure 4. example of ev1 detection.  figure 5.  ev2 detector block diagram.  acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 7  unusual in environmental noise sources, a high frequency band pass filter (5.0 to 5.2 khz) is applied before the lp measurement, to improve the detection rates by reducing the false positives rates (lp, filtered). figure 4 shows an example of the performance of the ev1 detector. 2.2. the ev2 detector  when a landing (ev1) is detected, the system starts searching for ev2 (figure 5). the ev2 detector consists of two microphones (mic1 and mic2) forming an array that is used for tracking the aircraft’s position along the runway, as follows. the delay between the signals captured in each microphone changes, depending on the relative position of the noise source (the aircraft) and the array. the delay is calculated using a cross-correlation method in the frequency domain [7]. this time delay of arrival allows estimating the direction of arrival of the sound [8], which is used to estimate the distance (r) between the aircraft and the sensors (see equation 1, where d is the distance from the array to the runway, c is the speed of sound, x12 the distance between the two microphones in the array, and is the delay obtained in the measurement). figure 6 shows a simplified scheme of parameters involved in the source location tracking process. ∙ δ (1) unlike sound pressure level (spl), the distance from the source to the receiver does not affect sound power level; therefore, when the thrust reverser is activated, the sound power emitted increases suddenly, making it easier to detect. taking advantage of this phenomenon, and using an approach similar to the one presented by the authors in [9], the estimation of the sound power level has been carried out using a simplified inverse sound propagation model based on iso 9613 [10], and shown in equation 2. 20log (2) where lw(t) is the sound power level (db), lp(t) the sound pressure level (db), r(t) the distance from the source to the receiver (m), α is a coefficient describing the atmospheric attenuation of sound with the dista nce (db/km), and a is a constant that counts for all other factors. using this transformation every thrust reverse sound event is enhanced, making its dynamic range higher (see figure 7), thereby improving the performance of a threshold detector. 2.3. ev1 detection improvement  during landings, the aircraft arrives from the south (in this test case) and, as the distance decreases (figure 3), the sound pressure level increases, triggering ev1 in time t0. at that time, the aircraft is at the left of the trend system, and during ev1 the aircraft will change its position to the right. therefore, during ev1 the array will detect a positive delay at the beginning that becomes negative in t1, which is the time when the aircraft is in front of the array axis (see figure 8). in the event that a negative zero crossing is not perceived within ev1, the event will be rejected (this is a constraint in this measurement setup). in the event that t1 is detected, the system starts searching ev2. this is a way to reduce the ev1’s false positive detection rates, as any sound event is rejected if its source was not strictly figure 6. aircraft location tracking.  figure 8. time delay during a landing.  figure 7. ev2 enhancement for detection. figure 9. performance of ev1 detector for early thrust reverse activation. acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 8  moving from left to right during the detection duration. 2.4. ev2 detection improvement  the ev2 detection must be done once the landing sound event is finished, which should fit to the end of ev1. however, by just using a threshold-based ev1 detector, it is not possible to tune the ev1 threshold in a very accurate way for all of the cases (figure 9). in view of those cases having “early” thrust reverse activation, the ev1 ending time cannot be a reference and there must be extra constraints to start searching for ev2. a little time after t1 (or at t1) the estimated sound power level must start decreasing (as the aircraft is moving away slowing down). this is the time we have called t2. only after t2, thrust reverse can be detected. this will prevent the system form false positives caused by aircraft directivity. after t2, if thrust reverse is activated, lw(t) will increase suddenly at a high rate, determining t3. t2 and t3 are calculated from analysing the slope trends in the sound power level time history (by just establishing some thresholds for these constraints, see figure 10). this is a way to prevent false positives caused by distant sound events in the airport (run-ups, taxi…), as thrust reverse usually produces strong level increments. 2.5. events classification  after detecting two consecutive sound events (ev1 and ev2), the system classifies them independently, in order to reduce the false positives identification rates. this process is carried out through the application of statistical pattern recognition techniques. as the classification stage is performed after an autonomous previous detection of two sound events, it was decided to use two independent classification stages for ev1 and ev2. as the characteristics of the target sounds are completely different (landings for the ev1 detector, and thrust reverse for the ev2 detector), using specific classification algorithms could improve the performance of each of the classifiers. a feature extraction process is carried out for both events, describing the dynamic and frequency characteristics of the sounds, and also the position of the aircraft for each of the events. mel frequency cepstral coefficients (mfcc) have shown a good performance in sound recognition applications [11-15], so the first 20 coefficients were selected. two new features were selected to describe the evolution of the time delay between microphones during ev1 and ev2, which is highly correlated to the location and movement of the aircraft during both events. the training and testing of the system was developed with matlab-prtools. the recognition process starts with a table 1. ev1 detection rates prior to classification.  detection rates ev1 detected ev1 not detected error (%) landings 398 1 0.0 take-offs 0 252 0.0 table 2. ev2 detection rates prior to classification.  detection rates ev2 detected ev2 not detected error (%) landings with thrust reverse 315 14 4.0 landings without thrust reverse 20 63 24.0 table 3. overall identification results.  event type detection error rates (%) classification error rates (%) overall error rates (%) landing with thrust reverse 4.0 4.9 8.5 landing without thrust reverse 24.0 40.0 9.6 figure 10. ev2 enhancement for detection.  figure 11. thrust reverse noise identification.  acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 9  principal components analysis (pca), used to decorrelate the data. afterwards, different algorithms were tested for each of the classifiers independently, so that those two showing the best performance were selected: a k-nearest neighbour [16] was used for thrust reverse events. this is a non-parametric classifier. it calculates the distance between the object to be classified and those in the training set, and assigns the class that is most repeated among the k nearest neighbours (k=5). a parzen classifier [17] was used for landing events. this is a non-parametric classifier. during the training, the classifier makes a non-parametric estimation of the probability density functions of each class, which are used for the classification of new objects. the identification of thrust reverse activation is positive if the first event is classified as landing and the second one is classified as thrust reverse noise (see figure 11). 3. results  some tests were carried out at madrid-barajas airport. the recordings were manually edited and labelled, creating a sound events database, consisting of 315 landings with thrust reverse activation and 83 without it. tables 1 and 2 show the performance of the detectors. the ev1 detector has shown a great performance: only landings are detected as ev1s, and almost every landing is detected. on the other hand, ev2 detector performance is lower, as sources other than thrust reverse are incorrectly detected as ev2. an adjustment with a higher threshold would remove many of these false positives, which are generated by aircraft taxiing or run-ups at a long distance, but, on the other hand, many thrust reverse events could also be missed. as no prior information was available regarding the number of these low level thrust reverse events, a conservative scenario was considered. however, after the classification stage, the operation point of the detectors can be optimized, and the overall identification rates will be improved. the tests showed an error rate lower than 10% (table 3), which can still be optimized, on the one hand, with a proper customization of the sensors and the measurement setup, and on the other, with the use of the prior probability of thrust reverse occurrence, during the training stage of the classifier. 4. conclusions  the variability in the duration, strength and separation of the landing and thrust reverse sound events cannot be properly implemented by just using thresholds on the overall running sound pressure level measurements. this is the reason why traditional threshold detectors are not effective for identifying thrust reverse noise at airports. the novelty in this paper derives from: a) the improvement in the detection through the application of signal processing to track the aircraft position and transform sound pressure into sound power levels; b) the application of pattern recognition techniques for the classification of previously detected sound events in the process of identification thrust reverser being activated. obviously, aircraft will have generated most of the noise events inside an airport, making it difficult to recognize each of them, as the different sound classes are not neatly separated. tracking the location of the aircraft along the runway optimizes the performance of both the detection and the classification stages, reducing the error rates below 10%. the sound power level estimation has shown a great performance for the detection of thrust reverse sound events, but it involves taking extra precautions, as not only the sound level is important, but also the cross-correlation between microphones (for instance, the protection of microphones against environmental conditions must be improved). it should be noted that the location of the trend system must be carefully selected. the location must be as far as possible from the braking area in order to enhance the separation between sound events. but it must be close enough to capture the thrust reverse sound. the array parameters and the distance from the array to the runway must be chosen so that the operating angle of the instrument covers the typical landing and braking areas. the described methodology can be implemented easily and reliably in an instrument, by simply applying common hardware and software resources to provide real time results regarding the activation of thrust reverse. these results, linked to those provided by noise monitoring units can be used, for instance, to complete any sanctioning procedure at an airport, or to analyse the effect or the need for restrictions regarding thrust reverser use. the methodology has been valid and effective for the measurements carried out at madrid-barajas airport [18,19]. acknowledgements  the authors are very grateful to sea milano and aena for showing their support for this project. the authors are grateful to the spanish “ministerio de educación, cultura y deportes” for the financial support provided to facilitate the corresponding author’s research stay at the politecnico di milano in italy.  references  [1] b.m. dunkin, a.a. atchley, k.k. hodgdon, directivity and spectral characteristics of aircraft noise during landing operations, journal of the acoustical society of america. 121, 2007, p. 3112. [2] b.m. dunkin, directivity and spectral source noise characterization of commercial aircraft during landing with thrust reverser engagement, 2008. [3] k.k. hodgdon, a.a. atchley, r.j. bernhard, low frequency noise study, partner-coe-2007-001 (2007). acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 10  [4] r.m. gutiérrez, a.a. atchley, k.k. hodgdon, characterization of aircraft noise during thrust reverser engagement, journal of the acoustical society of america. 118, 2005, p. 1852. [5] boeing, airports with noise and emissions restrictions, 2012. [6] c.c. rice, c.m. walton, restricting the use of reverse thrust as an emissions reduction strategy. report nºswutc/03/167231-1 2, report no. swutc/03/167231-1 2, 2001. [7] d. hertz, time delay estimation by combining efficient algorithms and generalized cross-correlation methods, acoustics, speech and signal processing, ieee transactions on. 34, 1986, pp. 1-7. [8] j.m. valin, f. michaud, j. rouat, d. létoumeau, robust sound source localization using a microphone array on a mobile robot, proceedings international conference on intelligent robots and systems, 2003. [9] c. asensio, i. pavón, m. ruiz, r. pagan, m. recuero, estimation of directivity and sound power levels emitted by aircrafts during taxiing, for outdoor noise prediction purpose, appl. acoust. 68, 2007, pp. 1263-1279. [10] iso, attenuation of sound during propagation outdoors. part 2: general method of calculation. iso 9613-2:199, 1996. [11] c. asensio, m. ruiz, m. recuero, real-time aircraft noise likeness detector, appl. acoust. 71, 2010, pp. 539-545. [12] a. rabaoui, z. lachiri, n. ellouze, automatic environmental noise recognition, industrial technology, 2004. ieee icit '04. 2004 ieee international conference on., vol. 3, 2004, pp. 1670-1675. [13] a. rabaoui, h. kadri, n. ellouze, new approaches based on one-class svms for impulsive sounds recognition tasks, machine learning for signal processing, 2008. mlsp 2008. ieee workshop on., 2008, pp. 285-290. [14] m. cowling, r. sitte, comparison of techniques for environmental sound recognition, pattern recognition letters, 24, 2003, pp. 2895-2907. [15] m. tabacchi, c. asensio, i. pavón, m. recuero, j. mir, m.c. artal, a statistical pattern recognition approach for the classification of cooking stages. the boiling water case, appl. acoust. 74, 2013, pp. 1022-1032. [16] t.m. cover, p.e. hart, nearest neghbor pattern classification, ieee transactions on information theory, 13, 1967. [17] a.k. jain, m.d. ramaswami, classifier design with parzen windows, pattern recognition and artificial intelligence, 1988, pp. 221-228. [18] c. asensio, g. moschioni, m. ruiz, m. tarabini, m. recuero, implementation of a thrust reverse noise detection system for airports, transportation research part d: transport and environment, 19, 2013, pp. 42-47. [19] c. asensio, aportaciones a los sistemas de discriminación de fuentes sonoras en al medida de ruido en aeropuertos, 2012. microsoft word 410-2443-1-le-rev acta imeko  issn: 2221‐870x  september 2016, volume 5, number 2, 2‐3    acta imeko | www.imeko.org  september 2016 | volume 5 | number 2 | 2  introduction to the special section of the 1st international  metroarcheo conference 2015  sabrina grassini 1 , alfonso santoriello 2   1 department of applied science and technology, politecnico di torino, italy  2 dipartimento di scienze del patrimonio culturale, università di salerno, italy    section: editorial   citation: sabrina grassini, alfonso santoriello, introduction to the special section of the 1st international metroarcheo conference 2015, acta imeko, vol. 5,  no. 2, article 1, september 2016, identifier: imeko‐acta‐05 (2016)‐02‐01  section editors: sabrina grassini, politecnico di torino, italy; alfonso santoriello, università di salerno, italy  received august 23, 2016; in final form august 23, 2016; published september 2016  copyright: © 2016 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  corresponding author: sabrina grassini, email: sabrina.grassini@polito.it; alfonso santoriello, email: asantori@unisa.it      dear reader, this special issue of acta imeko contains a selection of papers presented at the 1st international conference on metrology for archaeology – metroarcheo2015. this conference, which took place in benevento, italy, on october, 22-23 2015, was a real and concrete opportunity for gathering together science and measurement with the humanistic world, specifically archaeology, with the final aim of encouraging discussion and exchange of knowledge and expertise between worlds still considered, often wrongly, distant and different one from the other. archaeology, and more generally the cultural heritage field, in recent decades, has undergone so important and profound changes. for over two centuries, archaeological studies have been mainly focused on collecting information and knowledge on the artistic and monumental production of the ancient world. today it appears clear that the horizons are greatly expanded, and now encompass renewed complex of interactions between the humanistic world and the so-called “exact sciences”. new integrated systems able to provide answers to historiographical questions and face real problems in research, protection, monitoring and exploitation of cultural heritage and archaeological sites by providing useful contributions also in terms of urban and regional development planning, have been developed. the cooperation among experts of different disciplines often results in a multidisciplinary approach in which the individual disciplines and their instruments, work together, but without a unique methodological plan, with the final result of multiplying the problems, increasing the number of documentary series and juxtapose the obtained data. vice versa, this cooperation should result in a transdisciplinary vision, i.e. comparison among disciplines, and in the creation of new data that should be the splice among the same disciplines. transdisciplinarity is not just a question of terminology, it presupposes an open rationality which overcomes the excessive formalism. from the methodological ad conceptual point of view, this transdisciplinary approach really contributes to increase the interest toward archaeology and cultural heritage, by extending their objectives and enriching their technical and instrumental approaches. knowing how to read a heritage context taking advantage of the know-how of different disciplines allows one to have access to knowledge and methods, to propose a global and poliedrical view for the study of anthropogenic phenomena, and to develop tailored strategies for conservation, safeguard and management of tangible and intangible cultural heritage and landscapes. multiplicity and flexibility in the developed approaches also significantly contribute to explain the traces of mankind and their relation with the environment. moreover, it should be pointed out that metrology, the science of measurements, includes all aspects both theoretical and practical with reference to measurements, whatever their uncertainty are, and in whatever fields of science or technology they occur. consequently, the valorisation, characterisation and preservation of cultural heritage is deeply related to metrological issues for the collection, interpretation and acta imeko | www.imeko.org  september 2016 | volume 5 | number 2 | 3  validation of data collected with the different analytical, physical-chemical, and mechanical techniques, digital technologies, new ict tools, etc. metroarcheo2015 was particularly satisfactory in the methodological and conceptual aspect above-described. this special issue collects ten papers which give a complete view of the different possibilities offered by such integrated analytical and measurement approaches and well represent the worldwide attendance to the conference. the first two papers move from the dating to the "quantification" of time in absolute terms (desachy), to the different methods and statistical analysis systems (maspero et al.). the third and fourth papers highlight examples of contributions of archaeometry. the papers show how, through the analysis of stable isotopes and their comparison with archaeological records and other documentary series, it is possible to obtain consistent results in determining diets and health conditions in large territories and in sites of particular relevance in the central and southern italy (bondioli et al.) and in the magna graecia (ricci et al.). another paper describes the importance of geoarchaeological survey and geophysical data managed by gis, which gives impetus to studies of the environmental contexts and biosphere, overcoming the limits of traditional archaeology, which generally only takes into consideration the anthropogenic contest (cozzolino et al.). the paper of ullo et al. shows how remote sensing, in the frame of the sentinel-2 mission (2015), allows following the transformations occurred over time, particularly in vegetation, caused both by natural phenomena and anthropogenic causes. eventually, the last four contributions deal with several issues and fundamental aspects of protection, such as preventive archaeology and safeguard, monitoring and enhancement of cultural and archaeological heritage in urban areas or in specific sites. one of the discussed cases is devoted to the radical changes of the city of naples (italy) due to the construction of the underground line, which allows to put in evidence the extraordinary stratigraphy of the old city. in this contest the use and the optimization of 3d technology surveying, as well as the necessity of improving its accuracy and usability, are fundamental for collecting reliable data (fregonese et al.). 3d technologies and photogrammetric solutions are discussed in the paper of limongiello et al. for the documentation and survey of narrow spaces in the "villa di giulia felice" in pompeii (italy) for the relief and representation of the monuments with the final aim of monitoring their long-time conservation state. another example of the potentialities of three-dimensional techniques for collecting information and reading important details on monuments is provided by the research conducted on the important harkhuf tomb in egypt, inscribed with hieroglyphic texts. the paper of angelini et al. discusses the methodological approach, the technique and the results obtained thanks to the development of a mathematical model, which provides important information for the safeguard of this important witness of mankind culture. eventually, the last paper deals with the study of urban archaeological sites and geophysical prospecting in urban areas, discussing the case studies of the palatine hill and the st. john lateran basilica in rome (piro et al.) highlighting how these non invasive approaches are particularly suitable especially in the presence of archaeological structures built on natural soils. our hope is that these contributions are able to highlight the importance of the synergy among the different heritage actors and disciplines, which provides important tools and measuring approaching for the monitoring, preservation, management and safeguard of cultural heritage assets. last but not least, it is our sincere hope that this special issue gives the reader new points of interest for metrology and measurement science applied to cultural heritage taking also into account their fundamental role in our society. hope you will have an interesting reading! sabrina and alfonso acta imeko  issn: 2221‐870x  june 2015, volume 4, number 2, 85‐89    acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 85  two application examples of rfid sensor systems‐ identification and diagnosis of concrete components and  monitoring of dangerous goods transports  matthias bartholmai, markus stoppel, sergej petrov, stefan hohendorf and thomas goedecke  federal institute for materials research and testing, unter den eichen 87, 12205 berlin, germany      section: research paper   keywords: rfid sensors systems; diagnosis of concrete components; monitoring of dangerous goods  citation: matthias bartholmai, markus stoppel, sergej petrov, stefan hohendorf and thomas goedecke, two application examples of rfid sensor systems‐ identification  and  diagnosis  of  concrete  components  and  monitoring  of  dangerous  goods  transports,  acta  imeko,  vol. 4,  no.  2,  article  15,  june 2015,  identifier: imeko‐acta‐04 (5)‐02 ‐15  editor: paolo carbone, university of perugia, italy  received october 13, 2014; in final form november 24, 2014; published june 2015  copyright: © 2015 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding:  section 2. of this paper is part of the study (fe no. 29.0322/2013/bast) conducted on behalf of the federal ministry of transport and digital  infrastructure, represented by the federal highway research institute (bast)  corresponding author: matthias bartholmai, e‐mail: matthias.bartholmai@bam.de    1. introduction  the combination of radio-frequency identification (rfid) tags with different types of sensors offers excellent potential for applications with regard to identification, diagnosis, and monitoring. this should be demonstrated by means of two examples of actual developments carried out by the federal institute for materials research and testing (bam). the identification and diagnosis of concrete components is a major task in the maintenance of critical infrastructure, for instance concrete bridges with heavy traffic volume. a feasibility study investigates the application of rfid sensor systems for this task. the second example reviews the transportation of dangerous goods. using modern technologies enables promising possibilities to reduce accidents and to avoid nonconformity with transportation regulations. project results demonstrate an innovative technical solution for monitoring of dangerous goods transports with rfid sensor systems. a literature and web research was carried out to determine the state of the art and state of research in the field of embedded rfid-based sensors. the vast majority of rfid manufacturers and suppliers develop and distribute components that are used in logistics or in trade. because of the huge demand, rfid electronics are very inexpensive. rfid tags combined with passive (powered by the induced electromagnetic field) or active (battery powered) sensors are a niche product that only few manufacturers have in their portfolio. nevertheless, in several areas of application a growing demand of rfid sensor systems is developing [1], [2], [3], [4]. 2. identification and diagnosis of concrete  components  infrastructure is subject to continuous ageing. this has given life cycle management of infrastructure an important role. therefore, an increasing demand for reliable inspection and monitoring tools is noticeable. the prediction of the service life of a new structure at the design stage or the diagnosis and the evaluation of the residual service life of existing structures is a key aspect of concrete structure management. life cycle analysis and risk evaluation methods can be beneficially used to assess existing structures: actions on structures, inspectionabstract  the  combination  of  rfid  tags and  energy efficient  sensors  offers  promising potential  for  identification,  diagnosis,  and monitoring  applications  −  particularly  when  it  comes  to  objects,  which  require  continuous  observation  and  which  are  difficult  to  access  with  conventional tools. this paper presents two examples as an outlook for rfid sensor systems in embedded structures and in mobile  applications.  acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 86  oriented design and construction, characteristics of components and structures, life cycle costs, risk analysis and the environmental performance of a structure over its lifespan are important factors which have to be considered when the remaining service life of an infrastructure building is in question. if the possibility of efficient inspections during construction, operation, and maintenance has already been considered during the design phase, one prerequisite for reliable assessment is given. assessment of the safety of engineering works must be conducted by examining all aspects of their behaviour and all possibilities for failure which can be manifested. analysing potential critical situations of structures in europe is performed by identifying so-called limit states [1]. a limit state is defined as a condition beyond which a structure is no longer able to satisfy the provisions for which it was designed. however, it has to be distinguished between ultimate and serviceability limit states. primarily, the reactive maintenance approach has been implemented in europe to manage the road infrastructure network with respect to deterioration. this approach may be valid for well-managed structures without deficits and exposition to unchanging loads. however, this approach is unsatisfactory to structures containing structural deficits and subjected to increasing loads, modifications or widening measures for which they are not designed. therefore, a paradigm change from reactive to proactive infrastructure management is required and non-destructive inspection methods play a key role here. a reliable prognosis of the condition and behaviour of a structure is an important basis for an effective service life management. in order to determine the most economical moment for repair measures in the lifetime of a structure, the knowledge about the deterioration process at exposed regions as well as a detailed knowledge about the current condition of the whole structure is essential [6]. embedded sensors are meeting the demands for gathering more information from the inside of a structure. wired sensors are meanwhile well established but may have disadvantages in case of a subsequent installation in existing structures. wireless sensors which can be embedded in concrete provide an attractive alternative solution. this study summarizes the activities of a research project which dealt with the suitability of rfid sensor tags for concrete bridge structures. figure 1 displays the concept of the overall system for identification and diagnosis of concrete bridge elements and the interaction between the main system components. rfid is the wireless non-contact use of radio-frequency electromagnetic fields to transfer data (figure 2) for automatically identifying and tracking tags attached to objects and is well developed and known in commercial use. upcoming   figure 1. concept of the overall system for  identification and diagnosis of concrete bridge elements and  interaction between the main system  components.  figure  2.  scheme  of  the  interaction  between  reader  coil  and  embedded  sensor in concrete. acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 87  developments in sensor based tags for monitoring purposes in logistic and transport can be modified and used in civil engineering. this technology is by all means promising to be used within a monitoring system enabling a condition assessment for concrete bridge structures. the aim of the presented project, which was commissioned by the german federal highway research institute (bast), was to investigate modular rfid sensor systems with respect to their basic suitability for the use in concrete (bridge) structures. the selected rfid tags have a sensor interface to be connected to sensors with low power consumption. the measured values of the embedded sensors can then be read wirelessly. after a literature and product research, the project focused on basic investigations, such as: • selection of suitable sensors for humidity, temperature and corrosion activity [7], [8], which are the most relevant parameters for the long term structural integrity of concrete structures; • application of the sensors to reinforced concrete structures in order to perform an accurate and reliable diagnosis; • determination of the maximum distance (cover), at which an rfid sensor tag can still be discovered and delivers correct data to gain knowledge about the possibilities and limits of structure integration; • comparison between high frequency (hf) and ultra-high frequency (uhf) radio communication for performance evaluation using structure integrated rfid tags; • investigation in laboratory and real world application to approach the application relevant scenarios step by step. for this purpose, prototypes or rfid sensor tags where developed, assembled, encapsulated (figure 3), and embedded in concrete specimen with size of about 30×20×5 cm3 (figure 4). measurements in climate chambers were carried out and showed useful results. figure 5 shows the results of humidity measurements in a climate chamber comparing the encapsulated and in concrete embedded rfid sensor tag (red) with a not embedded tag (black). relative humidity was applied between 10 and 90 % in steps of 20 %. while the signal sequence of the not embedded tag follows immediately the applied humidity settings, the sequence of the embedded tag is delayed caused by the transport processes through the concrete. the delay increases with increasing humidity indicating slower transport characteristics, presumably due to approximating saturation. however, measurements were successfully performed with the embedded specimen, with coherent results over the full measurement range.  range measurements were carried out to determine the maximum distance of rfid data communication through concrete. the results show that an effective range of up to 170 mm is reachable with the developed rfid uhf system in the undisturbed case. in addition, the developed rfid sensors were successfully embedded in a reinforced concrete slab with properties of a typical bridge superstructure. the laboratory experiments have confirmed the basic functional efficiency and reliability of the developed sensor system. further investigations will be carried out on bridge components under real application conditions. the demand for embedded sensors in civil engineering is increasing. for further developments, other measuring principles as a supplement to or a combination with the developed system are being investigated. 3. monitoring of dangerous goods transports  the project “sensor-enabled rfid tags for safeguard of dangerous goods” with acronym sigrid investigates and evaluates possibilities to improve safety and security of dangerous goods transports through the use of the latest rfid technology [9]. this technology can be used to greatly enhance the transparency of the supply chain and aid logistics companies figure 3. encapsulated rfid sensor tag with reinforcement.  figure 4. preparation of concrete specimen with embedded rfid sensor tag. figure 5. humidity measurements, comparison between embedded and not embedded rfid sensor tags.  acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 88  in complying with regulations. in the context of sigrid custom rfid sensor tags (figure 6) were developed to monitor dangerous goods during transport and help to prevent hazards by allowing timely countermeasures. this requires the combination of communication technology and sensor functionality with low power consumption and small design. to achieve long battery-life, the use of very energy efficient sensors is mandatory. other desirable properties of the sensors include high accuracy, long lifetime, and short response time. for gas sensors a high selectivity is also very important. currently four types of sensors are integrated in the rfid tag, which are a combined humidity and temperature sensor, gas sensors for carbon monoxide (co) and oxygen (o2), and a tilt sensor. other interesting sensor options that might be tested in future include sensors for detecting the filling level and sensors for monitoring the operation of equipment that is built into the container like a stirring unit. the integrated sensors enable the system for recognizing and evaluating of different scenarios. adequate gas sensors indicate an emission from the containments via measured concentrations. if a possible gas release from the transported substance cannot be detected because of lacking the proper sensor, the o2-sensor can indicate a leakage through decreasing oxygen values. for numerous dangerous goods a maximal transport temperature is defined to prevent any chemical reaction. temperatures can be measured and compared periodically to substance specific values. if that value or a tolerance is exceeded an alarm or countermeasure can be activated. the tilt sensor can be triggered on heavy vibrations or tilting of the containment. in case of a dangerous good accident the available information about the type, amount, and condition of the dangerous goods can be used to accurately inform the relief forces. unavailable or inaccurate information represents a significant problem. this often leads to a delay of the rescue operation, because relief forces must be aware of the involved substances and their condition to effectively protect themselves against them. within the scope of the project, an rfid tag was developed, that allows connecting with different types of sensors. this rfid tag combines the advantages of semi-active (only sensors are battery supplied) and active tags (sensors and radio communication are battery supplied). on one side this tag is compatible to the iso 18000 respectively epc-gen2 standards, on the other side this tag has also the ability to communicate via the widely adopted wireless lan standard wi-fi. because the tag is woken up the same way as battery-less passive tags and for that reason does not need to power-up a receivermodule, battery-lifetimes of more than half a year are possible just as with semi active tags. after the tag is woken up, the wlan module is activated and allows very fast data transmission, that otherwise would only be achievable with active tags. this greater transmission speed makes the tag suitable as storage device for much larger amounts of data, than the ones that are normally possible with rfid tags. the possibility to store great amounts of data in combination with a long battery lifetime makes this tag ideal for use as a data logger. logging intervals can be configured individually for every sensor. the tag has also an open interface which allows an easy integration of different kinds of sensors. sensor tags, data communication, and software are combined to an interactive solution, which can tackle various scenarios during dangerous goods transports. the underlying information is provided by a data base with expert knowledge, in this case the bam dangerous goods database "gefahrgut" [10]. figure 7 displays the interaction between the main system components during transport. figure 7. concept of the overall system for monitoring of dangerous goods transports and interaction between the main system components.    figure 6. prototype of the sensor enabled rfid tag.  acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 89  the focal point of the vehicle equipment is the on-board unit (obu), which consists of a ruggedized industry pc that is specially designed for use in a truck. the main functions of the obu include acquisition of position data via gps, routing, generation of transport documents, data communication via the mobile phone network, monitoring of the load with sensors and surveillance cameras as well as wlan connectivity. it is either possible to read the sensors of the semi-active transponders or sensors that are permanently installed in the loading area. the obu constantly monitors the measurements to ensure, that they are in the allowable range. if that is not the case, an alarm is automatically triggered. current status messages are transmitted to the centralized database, that has also the cargo manifest stored. in case of need, the obu should supply the relief forces with all required information via wlan. but if the obu gets destroyed during an accident, all information is still available through the centralized database. 4. conclusions  several applications require identification, diagnosis, or monitoring or a combination of them. often, general conditions, like mobility or embedding, favour wireless solutions. the presented examples demonstrate, that radio based communication and small-sized, low-energy demanding sensor technologies offer the basis for innovative solutions in this regards. subsequent tasks like sensor data fusion and automated analysis and evaluation processes offer additional benefits. acknowledgement  the contribution of further and former colleagues of bam, namely werner daum, alexander pettelkau, mahin farahbakhsh, david damm, and others, is gratefully acknowledged. section 2. of this paper is part of the study (fe no. 29.0322/2013/bast) conducted on behalf of the federal ministry of transport and digital infrastructure, represented by the federal highway research institute (bast). the responsibility for the content lies solely with the authors. references  [1] i. zalbide, e. d'entremont, a. jimenez, h. solar, a. beriain, r. berenguer, “battery-free wireless sensors for industrial applications based on uhf rfid technology”, proceedings of ieee sensors, 2014, pp. 1499-1502. [2] g. schirripa spagnolo, l. cozzella, m. caciotta, r. colasanti, g. ferrari, “analogue fingerprinting for painting authentication”, proceedings of 20th imeko tc-4 international symposium measurement of electrical quantities, 2014, pp. 297-402. [3] m. merenda, i. farris, c. felini, l. militano, s. c. spinella, f. g. della corte, a. iera, “performance assessment of an enhanced rfid sensor tag for long-run sensing applications”, proceedings of ieee sensors, 2014, pp. 738-741. [4] s. kim, t. le, m. tentzeris, a. harrabi, a. collado, a. georgiadis, “an rfid-enabled inkjet-printed soil moisture sensor on paper for "smart" agricultural applications”, proceedings of ieee sensors, 2014, pp. 1507-1510. [5] d. diamantidis, “probabilistic assessment of existing structures”, rilem publications s.a.r.l, 2001. [6] j. h. kurz, h. rieder, m. stoppel, a. taffe, “control and data acquisition of automated multi-sensor systems in civil engineering”, proceedings of non-destructive testing in civil engineering, ndt-ce, paris, laboratoire central des ponts et chaussées (lcpc), 2009. [7] c. dauberschmidt, c. sodeikat, “innovative zustandserfassung durch korrosionsmonitoring und ultraschallmessungen“, tiefbau, vol. 4, 2008 (in german). [8] s. lee, “development of corrosion sensors for monitoring steelcorroding agents in reinforced concrete structures”, materials and corrosion, 2003. [9] t. goedecke, a. pettelkau, s. hohendorf, d. damm, m. bartholmai, and m. farahbakhsh, “securing of dangerous goods transports by rfid tags with sensor-functionality and integrated database “gefahrgut” information (sigrid)”, proceedings of the 17th iapri world conference on packaging, 2010, pp. 639-642. [10] url: http://www.dgg.bam.de acta imeko  issn: 2221‐870x  september 2015, volume 4, number 3, 1    acta imeko | www.imeko.org  september 2015 | volume 4 | number 3 |  1  journal contacts    about the journal    acta imeko is an e-journal reporting on the contributions on the state and progress of the science and technology of measurement. the articles are based on presentations presented at imeko workshops, symposia and congresses. the journal is published by imeko, the international measurement confederation. the issn, the international identifier for serials, is 2221870x.      editor‐in‐chief  paolo carbone, italy    honorary editor‐in‐chief  paul p.l. regtien, netherlands    editorial board      leopoldo angrisani, italy filippo attivissimo, italy eulalia balestieri, italy eric benoit, france catalin damian, romania pasquale daponte, italy luigi ferrigno, italy alistair forbes, united kingdom fernando janeiro, portugal konrad jedrzejewski, poland andy knott, united kingdom fabio leccese, italy rosario morello, italy helena geirinhas ramos, portugal pedro ramos, portugal sergio rapuano, italy dirk röske, germany alexandru salceanu, romania constantin sarmasanu, romania lorenzo scalise, italy emiliano schena, italy enrico silva, italy marco tarabini, italy susanne toepfer, germany rainer tutsch, germany ian veldman, south africa luca de vito, italy     about imeko    the international measurement confederation, imeko, is an international federation of actually 39 national member organisations individually concerned with the advancement of measurement technology. its fundamental objectives are the promotion of international interchange of scientific and technical information in the field of measurement, and the enhancement of international co-operation among scientists and engineers from research and industry.     addresses    principal contact  prof. paolo carbone university of perugia dept. electr. inform. engineering (diei) via g.duranti, 93, 06125 perugia, italy email: paolo.carbone@unipg.it    acta imeko  prof. paul p. l. regtien measurement science consultancy (msc) julia culpstraat 66, 7558 jb hengelo (ov), the netherlands email: paul@regtien.net    support contact  dr. dirk röske physikalisch-technische bundesanstalt (ptb) bundesallee 100, 38116 braunschweig, germany email: dirk.roeske@ptb.de     acta imeko  december 2014, volume 3, number 4, 1  www.imeko.org    acta imeko | www.imeko.org  december 2014 | volume 3 | number 4 | 1  journal contacts    about the journal  acta imeko is an e-journal reporting on the contributions on the state and progress of the science and technology of measurement. the articles are based on presentations presented at imeko workshops, symposia and congresses. the journal is published by imeko, the international measurement confederation. the issn, the international identifier for serials, is 2221-870x. editor‐in‐chief  paolo carbone, italy honorary editor‐in‐chief  paul p.l. regtien, netherlands editorial board francisco alegria, portugal leopoldo angrisani, italy filippo attivissimo, italy eulalia balestieri, italy eric benoit, france catalin damian, romania pasquale daponte, italy luigi ferrigno, italy alistair forbes, united kingdom fernando janeiro, portugal konrad jedrzejewski, poland andy knott, united kingdom fabio leccese, italy helena geirinhas ramos, portugal pedro ramos, portugal sergio rapuano, italy dirk röske, germany alexandru salceanu, romania constantin sarmasanu, romania lorenzo scalise, italy emiliano schena, italy enrico silva, italy marco tarabini, italy susanne toepfer, germany rainer tutsch, germany luca de vito, italy about imeko  the international measurement confederation, imeko, is an international federation of actually 39 national member organisations individually concerned with the advancement of measurement technology. its fundamental objectives are the promotion of international interchange of scientific and technical information in the field of measurement, and the enhancement of international co-operation among scientists and engineers from research and industry. addresses  principal contact prof. paolo carbone university of perugia dept. electr. inform. engineering (diei) via g.duranti, 93, 06125 perugia, italy email: paolo.carbone@unipg.it acta imeko prof. paul p. l. regtien measurement science consultancy (msc) julia culpstraat 66, 7558 jb hengelo (ov), the netherlands email: paul@regtien.net support contact dr. dirk röske physikalisch-technische bundesanstalt (ptb) bundesallee 100, 38116 braunschweig, germany email: dirk.roeske@ptb.de article 1 contacts microsoft word 4-prima pagina acta imeko  issn: 2221‐870x  june 2015, volume 4, number 2, 23‐31    acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 23  uncertainty restrictions of transfer click‐torque wrenches  andreas brüge  physikalisch‐technische bundesanstalt (ptb), bundesallee 100 , 38116 braunschweig, germany      section: research paper   keywords: torque; click‐torque wrench; iso 6789; transfer  citation: andreas brüge, uncertainty restrictions of transfer click‐torque wrenches, acta imeko, vol. 4, no. 2, article 5, june 2015, identifier: imeko‐acta‐04  (2015)‐02‐05  editor: paolo carbone, university of perugia, italy  received april 21, 2015; in final form june 8, 2015; published june 2015  copyright: © 2015 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  corresponding author: andreas brüge, e‐mail: andreas.bruege@ptb.de    1. introduction  click-torque wrenches (ctws) are setting torque tools of the type ii according to iso 6789 [1], which are wrenches with a release mechanism to limit the transferable torque. in particular this definition excludes screwdrivers or wrenches with flexion bars. they are to be calibrated according to iso 6789, where conformity limits are required which amount to ±4 % and ±6 % respectively, for the relative deviation of the releasing value. nevertheless, ctws intended as transfer standards for the traceability of the calibration facilities concerned should comply with much stricter requirements. they should correspond to the best measurement capabilities (bmcs) of laboratories accredited for ctw calibration in the german accreditation body (dakks). these bmcs cover the range from 0.2 % to 1 % (figure 1). a key comparison of the deutscher kalibrierdienst (dkd) based on procedures according to the iso 6789 using ctws demonstrated the insufficiency of the combination of the ctws with these procedures for transfer purposes. therefore, nowadays only indicating transfer torque wrenches can deliver the traceability of the static torque calibration in the concerning facilities according to the bmc values of calibration laboratories. but specific dynamic requests to the facilities destined for ctw calibration cannot be verified by indicating torque wrenches. therefore the use of transfer ctws is necessary particularly during assessments to complement the traceability of laboratories working in the field of iso 6789. in order to submit proposals for the selection of suitable types of ctws and for measurement procedures which help overcome known restrictions of the available types, this work surveys important sources of measurement uncertainty of different types of ctws. figure 1. distribution of  listed bmcs  in the dakks for click‐torque wrench  calibration according to iso 6789. abstract  for  different  types  of  click‐torque  wrenches,  measurement  uncertainty  contributions  of  typical  calibration  conditions  were  investigated. procedures which differ from those given in iso 6789 were suggested for transfer measurements at calibration facilities  for click‐torque wrenches in order to improve their reproducibility. examinations were carried out for influences like lever length, rise  time, temperature and humidity.  acta imeko | www.imeko.org   june 2015| volume 4 | number 2| 24 2. characterisation of ctws  2.1. calibration facility  the measurements for this paper were performed at the 2kn·m calibration facility of the physikalisch-technische bundesanstalt (ptb) in braunschweig. this facility generates torque loads continuously using an electric motor and a gearbox. the detection of the load was conducted with raute reference torque transducers of type tt1 and an hbm dmcplus transient recorder. fitted with a dv55 carrier frequency amplifier module, this recorder is able to detect releasing signals (figure 2) under the following conditions: carrier frequency 4.8 khz resolution (24 ... 15) bit sampling rate (150 ... 9600) hz. altogether, the expanded relative contribution of the reference measuring chain results in approximately 8·10-4. 2.2. measurements  calibrations of ctws according to iso 6789 are mainly meant to achieve the deviation of the actual release torque a from the value which is set at the ctw. for this purpose, repeated releases are to be performed five times each at 20 %, 60 % and 100 % of the nominal torque. hence inherent properties of the ctw, like running-in and repeatability, should be part of the result. in contrast the intention of the measurements in this work is to figure out the ctw’s coefficients of disturbing quantities and other contributions to their measurement uncertainty. furthermore, procedures are questioned in order to minimize the impact of disturbing quantities and to maximize the reproducibility during a transfer measurement in a calibration laboratory. these procedures are discussed later in this paper. in the following, brief descriptions of the measurement procedures used for determining considerable influences on the calibration of ctws are given. 2.3. release torque a    the calibration facility performs a constantly increasing load until the ctw releases and the measured torque falls sharply. the peak value a of the recorded signal of the reference measuring chain is calculated, taking into account the relative noises of the release curve by detecting the span ba (figure 3), which is a specific value of the ctws between 2·10-5 and 2·10-4 and of the reference measurement chain’s zero signal (typically 3.5·10-5). together with the relative uncertainties of the signal recording and of the sensitivity of the reference transducer, plus the influences of the reference transducer creeping and of the zero signal drift, the combined relative uncertainty of the determination of the release torque a amounts to about 4·10-4. 2.4. relative repeatability srpt  as a mechanical mechanism, a ctw is affected by the history of the load, temperature and humidity. every ctw in this investigation was stored under laboratory conditions for one day at least before use. furthermore, preliminary series of releases were executed. with this preliminary series of up to 120 releases, several purposes are to be served. first, the ctws’ lubrication after long idleness should be recovered and, second, the maximum release rate should be achieved. if releases succeed too fast, thermal effects in the release mechanism provoke a drift of the release torque value. furthermore, the relative standard deviation of the values of a provides the relative repeatability srpt of the mean value ̅ of the release torque (figure 4). rpt ∑ ̅ 1 (1) achieving the mean value of the release torque is a specific task of a transfer measurement. usually measurements with a ctw according to iso 6789 are never given as an average, because every single release in the calibration has to fulfil the demands of the standard. figure 3. determination of the release torque a from the torque signal of a  releasing click‐torque wrench over time considering the contribution ba of  the signal noise to the measurement uncertainty.   figure  4. devolution of the release torque a of a ctw during consecutive  releases.  the  running‐in  reaches  to  value  no.  14  (shaded  area)  and  the  relative  standard  deviation  of  the  rest  of  the  data  delivers  a  relative repeatability srpt of 7.6∙10 ‐4 . figure 2. torque signal of a releasing click‐torque wrench over time.   acta imeko | www.imeko.org   june 2015| volume 4 | number 2| 25 due to friction and resolution effects within the release mechanism, even without thermal effects the relative shortterm repeatability of consecutive releases can exceed 10-3. series of 30 repeated release events at least were performed and averaged for the determination of each value of ̅ in order to overcome this instability of the ctws. this is important, as a high value of the repeatability relative to the other coefficients is to be taken into account at the uncertainty estimations of them. 2.5. relative reproducibility srpr  the relative reproducibility srpr is defined as the relative difference between two independent measurements of ̅ with one ctw. manufacturers of ctws recommend readjusting them to a minimum value after each use to avoid mechanical drifting during long-term storage due to the spring tension inside the release mechanism. nevertheless, keeping up the tension of the ctw over the days of investigation turned out to be advantageous. the drift that manufacturers warn against could not be detected during extensive tests of the specimens. therefore, avoiding the readjustment of the ctws between measurements yields the benefit of a smaller value of srpr by eliminating the resolution uncertainty (figure 5). the relative resolution of the ctw, which amounts from 2·10-4 to 3·10-3 for the specimens used in this work, is one of the main uncertainty contributions for calibrations according to iso 6789. it originates from the uncertainty of the adjusting scale due to the width of lines and due to mechanical clearance in the adjusting mechanism, even if these are latching. as usual, calibrations at the beginning and at the end of a transfer can deliver a reliable value of srpr. in this work, these calibrations were simulated by repeated remounting of the ctw while the adjustment was kept. 2.6. relative coefficient of lever length cl  the distance between the pivot of the square drive and the support at the handhold of a ctw affects the amount of a supposedly by a cross force effect in the pivot. furthermore, deviations of a correlative to the lever length could be caused by an eccentric female square drive in the calibration facility [2]. this has been avoided by careful alignment of the facility axis with an eccentricity of less than 0.05 mm. so it is possible to detect just such an eccentricity in the calibration facility under test during a transfer calibration using the ctw, by gaining increased values of c l there. to find the value of c l, two additional series of 30 releases each are performed at a lever length 10 mm shorter respectively longer than the normal length. the relative coefficient of lever length thus can be found with ̅ (2) where ̅ is the average of peak signals of the calibration facility gained at the preliminary series mentioned above, and s is the averaged peak signal with the longer or the shorter lever respectively, which length is given by l. though the determination of c l usually has to be performed at minimum level of torque adjustment, for the purposes of transfer measurements this should be done preferably at nominal torque adjustment in this context. in this way, no readjustment of the ctw takes place and the uncertainty contribution of adjustment reproducibility can be avoided. 2.7. relative coefficient of temperature ct  the peak value of a ctw can be influenced by its temperature by way of altering the lubrication viscosity, by way of dimensional changes in the release mechanism or, if a strain gauge is integrated, by temperature dependence of the gauge sensitivity [3]. one possibility to get an estimation of ct is to track the relaxation of a heated ctw from 40 °c by repeated series of 30 release events each to the normal laboratory temperature of 21 °c. the ctw can be heated inside a climate cabinet overnight and then it will have to be moved quickly into the calibration facility, which is assisted by the quick and easy mounting of the ctw with a square drive. at the beginning of the experiment, the repetition rate of the series should be high. because of the downtime of measurements during the transport of the ctw from the climate cabinet into the calibration facility, the initial status of the ctw is only available by an extrapolation. the relaxated state, in contrast, is to be measured simply by waiting some hours until the stability of occasional measurements is equal to the repeatability of the ctw obtained at the preliminary measurement. this method is equivalent to the method which was proposed for simplified measurements of the relative humidity coefficient [4]. with this set-up, ct can be obtained by ̅ (3) where ̅ and s are defined analogical to (2), but in the case of a temperature step cab lab between a climate cabinet and the laboratory. in contrast to ctw calibration according to iso 6789, transfer measurements can be reduced to very close limits of temperature, because these measurements are only reasonable in laboratories with low uncertainty budget and thereby with an efficient temperature control. thus, the contribution of ct in a transfer measurement usually remains small. 2.8. relative coefficient of humidity cf  like temperature, humidity can affect the lubrication of the mechanism or the sensitivity of a gauge within a ctw [5]. in a similar way as described above, the humidity of a ctw can be changed in a climate cabinet and its relaxation to the laboratory level can be observed. then c f is given by figure  5.  amount  of  ctws’  relative  resolution,  given  mainly  by  adjusting  uncertainty.   acta imeko | www.imeko.org   june 2015| volume 4 | number 2| 26 ̅ (4) with ̅ and s defined analogical to (2), but in the case of a humidity step between a climate cabinet and the laboratory. strict humidity control is challenging and expensive even for ambitious laboratories. therefore, humidity deviation between ptb and a laboratory under test is usually greater than 5 %rh, often greater than 10 %rh. consequently, the impact of c f on the uncertainty budget potentially could be higher than that of other coefficients discussed in this paper. 2.9. relative coefficient of rise time ctb  the rise time tb of a ctw release event is defined as the time between 80 % load and the release load which defines the 100 % load. the dedicated relative coefficient ctb can be obtained by ̅ , , (5) with ̅ and s defined analogical to (2), but in the case of a step in the rise time , , . measurement of the rise time needs correction if the ctw under test exhibits a high reading error. then the release torque is far away from the expected nominal value and the starting point of the rise time has to be shifted to 80 % of the actual release torque. in the most cases, shifting to an earlier time is necessary, which is possible if the loading curve was recorded completely as given in this work. if a calibration facility delivers only the peak value of the curve, correction of the rise time value is impossible. 2.10. relative coefficient of square drive influence cv  the quality of the square drive can influence the result of a ctw calibration by reactive forces and moments which are possible if angularity, planarity and dimensional accuracy are inadequate. the usual procedure to determine cv is to perform five series of 10 measurements with orientation alterations of the square drive by 90° between them. during this investigation only a short version of this procedure was performed with 30 release events at 0° and 90° position of the square drive each, because of the great time need of such measurements. then, a coefficient could be obtained using ° ° ̅ (6) with ̅ and s defined analogical to (2), but in the case of the alteration of the square drive position from 0° to 90°. of course, transfer measurements should be performed with a fixed position of the square drive in both calibration facilities involved, but the determination of cv does not become dispensable. assuming the female square drive of the ptb facility to be well aligned, an increased value of cv in the facility under test would indicate some of the mechanical problems described above at the female square drive of the latter. in this sense, cv is due to the properties of the calibration facility in the first order. therefore, the contribution of cv to the measurement uncertainty of the ctw should be taken into account if cv is increased in the laboratory under test. 2.11. relative coefficient of the ratchet influence crch  if the ctw comes with a ratchet, eccentricity of the rotating part of it is an important source of deviation, as in the case of eccentricity of the machine’s axis, mentioned in section 2.6. about c l. because the ratchet is not a part of the calibration facility under test, in a transfer measurement this influence should be eliminated by using a fixed or labelled position of the ratchet in both facilities. since ratchets often come with high angular resolution and are not usually fixable, an alteration of the ratchet position could take place unnoticed during each handling. thus, the effect of the ratchet position alteration was determined by ̅ (7) with ̅ and s defined analogical to (2), but in the case of the alteration in the ratchet position by ± 1 cog. because of the sinusoidal character of the eccentricity this measurement was undertaken at a ratchet position of 0° and of 90°. the maximum value of crch was used. 2.12. uncertainty of coefficients  the uncertainties of the coefficient determinations described in this paragraph are dominated by that of s, which is given mainly by the repeatability srpt of the ctw. therefore, the contribution of the coefficients to the uncertainty is at least in the range of srpt multiplied by a sensitivity coefficient given by the amount of alteration of the influence quantity in question (see section 4.4). this underlines the importance of small repeatabilities for the value of a ctw for transfer measurements. the use of at least 30 release events for the determination of each value of ̅ reduces the influence of the release instability by more than a factor of 5. 3. specimens  in order to obtain an overview of possible properties of ctws, some different types of them were investigated in the manner described in the chapter above (table 1). most of them are of the common mechanical release type. these wrenches are equipped with a release mechanism consisting of an instable crank, which can be pre-stressed by a spring and thereby be adjusted for a certain release torque. when the torque load exceeds the adjusted value, the instable crank turns over and the torque load at the square drive falls abruptly. while this mechanism is integrated into the body of the mechanical ctw, a buckling ctw is constructed to fold table 1. ctws used in this investigation.   no.  type  nom.  torque  in  n∙m  release type  1 m210 210  mechanical 2 s.305 da 350  mechanical 3 no 18 700  buckling  4 714/10 100  electromechanical 5 5122 ct 180  mechanical 6 1800 ql 180  mechanical 7 721nf/80 800  mechanical 8 typ d 760  mechanical acta imeko | www.imeko.org   june 2015| volume 4 | number 2| 27 entirely at the half-way point of the lever. this design is outdated and nowadays in use merely for screwing in workshops. one exponent of this type is used in the survey to have a look at the lower end of the state-of-the-art. an electromechanical ctw includes a strain gauge to measure the torque load. when this measure exceeds a digitally preset limit, an electromagnetic force unlocks the square drive for release. this design is quite new, but nevertheless it raises expectations for solving some of the problems which are known about mechanical ctws. 4. results  4.1. sampling rate  to obtain a ctw with a well-defined and repeatable release value, the act of releasing has to be as short as possible. the signal in time therefore tends to be discontinuous, which implies high requirements for the sampling rate of the amplifier which detects the release event. comparisons of release measurements with different sampling rates exhibit relative deviations of the peak value up to 1.3·10-3 (figure 6). using a signal of a release event measured with a sampling rate of 9.6 khz, a simulation of signal integrations with time periods corresponding to lower sampling rates was calculated. this simulation curve shows a qualitative shape similar to that of the measurements. according to that, the curve should change from a high inclination at lower sampling rates to an approximately constant part for rates higher than 2 khz. therefore it is advisable for a comparison of two calibration facilities either to use a sampling rate of more than 2 khz or to agree on a certain value of the sampling rate, if only lower rates are available. 4.2. rise time  in daily workshop use a ctw is loaded with torque by hand and as quickly as the worker is able to. thus, the typical rise time of torque is shorter than one second. to accommodate this fact, in iso 6789 values of tb are required to be within 0.5 s and 4 s. the higher value is far beyond the practical value for manual loading and is to be seen as a concession to the deficient speed of calibration machines. the mechanical parts of ctws are subject to inertia effects and to friction. both depend on the velocity of the moving parts. in this way, rise time can affect the release torque value in ctws. electromechanical ctws additionally have to face the runtime error of the release bolt and the influences of the sensing element resolution and of the internal sampling rate. in this spirit, a rise time of 0.5 s is unreasonably short for transfer measurements. in this connection, the limits of iso 6789 are too broad for practical application and too close for the requirements of transfer measurements. the situation becomes worse if the calibration facility under test lacks a precise rise time gauging. then the transfer measurement not only has to test the release torque measurement capability of the calibration machine under test. besides this, the procedure should provide information about the rise time of the calibration machine in question. measurements with different rise times show a relative deviation of the release torque of some 10-3 per second at rise times shorter than 4 s, in agreement with the consideration above (figure 7). in the range between 4 s and 5 s the dependence of the release torque on the rise time is minimal. thus, for the purpose of transfer, measurements should be performed slowly with a rise time of 5 s. as manufacturers are obliged to design their calibration machines according to iso 6789, the machines are often unable to run so slowly. in this case, the two comparing laboratories should agree upon rise time, which poses the problem of measuring the rise time exactly. 4.3. selection of a transfer ctw  in order to rate the ability of the ctws for transfer purposes, a table of the coefficients of the ctws in the survey is given (table 2). figure 6. relative deviation of peak value, depending on the sampling rate  of  the  amplifier.  the  dashed  curve  is  the  result  of  a  simulation  which performs signal integrations with corresponding time periods at a measured release signal gained at a sampling rate of 9.6 khz.   table 2. absolute values of relative coefficients in 10 ‐6  of ctws listed in table 1. some data were not measured due to time constraints; the referring  coefficients were treated as equalling zero. the deviation of specimen no. 3 is due to friction‐induced drift and was treated as a systematic contribution.  rel. coefficient  in 10 ‐6 1  2  3  4  5  6  7  8  c l in mm ‐1  2548  92  no data 326  394  636  330  37  c  t in k ‐1  1041  351  no data 507 903  510  1008  181  c  f in (%rh) ‐1  1154  560  no data 20  268  77  no data  229  c tb in s ‐1  8153  6029  no data 2293  5749  1259  4537  3084  c v   no data  4071  no data 220  7453  586  151  8041  crch (one cog)   6787  1249  no data 305  5783  2316  2635  no ratchet  srpr  2748  217  no data 143  3419  674  989  5184  srpt  580  234  27842  96  139  524  646  334  acta imeko | www.imeko.org   june 2015| volume 4 | number 2| 28 this survey is neither representative of the ctws available on the market nor could a conclusion be drawn about the properties of a ctw type, this would require tests at a higher number of ctws of a certain type. the survey shall give examples of which parameters are important. better than a competition of coefficients, an analysis of their uncertainty impacts give evidence of the suitability of a ctw for transfer calibrations (table 3, figure 8). the contributions in table 3 indicate the priority for compensating the uncertainty sources of a ctw or for its improvement in order to qualify it for transfer measurements. the most influential contribution to the measurement uncertainty to be considered at the selection of a transfer ctw is the repeatability srpt. further parameters not included in table 2 are the object of a qualitative appraisal. the shape of a release curve should feature a well-defined peak value with a monotone rising and a fast break off, kickback-free for at least one second. the specific running-in of the release value from the initial increase to stable amounts has to be observed at preliminary tests and is to be considered at the calibrations by omitting the referring data points. moreover, it has to be ensured by tests that keeping the ctw in tension during a transfer calibration would not increase repeatability uncertainty due to mechanical drift as mentioned in section 2.5. 4.4. uncertainty budgets  the combined relative uncertainty of the reference torque measurement with a ctw wref consists of the contributions of the reproducibility wrpr (9), of the release torque detection wa (10) and of the influence of the coefficients wx (11): ref rpr a x (8) rpr rpr 12 2 rpt (9) a 2 zero nn nn a 12 (10) the contribution of the reproducibility is given by the measured amount of srpr and twice the uncertainty of this measurement which is dominated by the repeatability srpt. the contribution wa consists in addition to the span ba of the relative uncertainty of the national standard facility wnn, the relative long-term stability of the facility snn and twice the relative stability of the zero signal of the standard facility. table 3. relative standard measurement uncertainties w in % of ctws listed in table 1 due to examined quantities introduced in the text for a transfer  measurement  between  a  national  standard  machine  and  a  calibration  laboratory.  the  contributions  of  the  specific  coefficients  listed  in  table 2  are  calculated  using  the  calibration  conditions  listed  in  table 4  both  for  a  standard  machine  and  for  a  calibration  laboratory.  the  resulting  combined  expanded relative uncertainty wref is given both under the assumptions given in table 4 and neglecting the influence of a ratchet. the contributions,  except for wrch, are graded in reference to their amount by colour. the highest contributions are printed in red, the second highest in orange.  uncertainty in %  1  2  3  4  5  6  7  8  wa  0.042  0.041  0.044  0.041  0.044  0.043  0.042  0.041  wrpr  0.161  0.048  0.557  0.020  0.142  0.108  0.135  0.222  w l    0.026  0.001  0.007  0.003  0.004  0.007  0.004  0.001  wt   0.063  0.021  0.041  0.030  0.054  0.031  0.061  0.012  wf   0.180  0.087  0.053  0.004  0.042  0.016  0.012  0.036  wtb   0.122  0.089  0.100  0.034  0.085  0.026  0.071  0.047  wrch    0.041  0.119  0.197  0.009  0.215  0.041  0.046  no ratchet            wref , k=2  0.57  0.37  1.29  0.13  0.57  0.26  0.35  0.47  wref, k=2   (no ratchet)  0.56  0.28  1.12  0.13  0.37  0.25  0.34  0.47  figure 8. relative standard uncertainties of the  investigated contributions calculated according to (9), (10) and (11).   figure 7.  peak value of a ctw depending on the torque load rise time.  acta imeko | www.imeko.org   june 2015| volume 4 | number 2| 29 the coefficients cx (x stands for one of the indices l, t, f, tb and rch introduced in chapter 2), stability contributions srpr and srpt, uncertainty of calibration conditions ux (table 4), and distribution functions were processed to relative standard uncertainty contributions wx under the condition that the sampling rate is chosen adequately and cv is equal in both laboratories. because the complete analysis according to the guide to the expression of uncertainty in measurement [6] is disproportionately extensive, a simplified approximation was used: nn x lab 12 2 rpt ∆ (11) while the uncertainty of the coefficient’s detection given mainly by the repeatability srpt cannot be neglected, the contribution of a specific coefficient cx has to be extended by the xth fraction of srpt. here, x is the step of the calibration condition quantity, in units of this quantity, performed during the detection of the coefficient. the contribution of the coefficient cx is derived from a span, thus a rectangular distribution is to be taken into account. in contrast, the repeatability srpt originates from an averaging, thus a normal distribution is to be assumed. because cx is achieved by a difference, the extension due to the repeatability is to be taken into account twice. to obtain the actual contribution of the coefficients, the expanded specific coefficients have to be multiplied by the uncertainty of the corresponding calibration conditions x, which are given in table 4 for the reference calibration with the national standard machine (nn) and for a typical accredited calibration laboratory (lab). thus the relative standard uncertainty x includes the impact of the ctw properties at both calibration facilities involved. the uncertainty contribution of the stability wrpr is calculated equivalent to the second part of (11) from the reproducibility srpr with a rectangular distribution and twice the repeatability srpt with a normal distribution. as an overall result, the expanded relative combined uncertainty wref (k=2) is given for each ctw in figure 9. this uncertainty implies the contributions due to ctw properties in combination both with the calibration conditions in the national standard machine (tn-nn) and with those in a calibration machine of a laboratory under test (tn-lab) as well as a contribution from the national standard machine (nn). if a transfer calibration should confirm the bmc of a calibration machine under test, the en value (12) must not exceed 1: | | ̅ ̅ ref lab 1 (12) the en value compares the difference between two measurements of the release torque ̅ with the quadratic sum of uncertainties referring to these calibrations. in this work, the transfer ctw is to be understood as a part of the reference calibration, thus the referring expanded uncertainty uref corresponds to the relative expanded uncertainty wref given in figure 9: ref nn tn-nn tn-lab (13) the examination of uref is important since (12) implies that even at equality of the two comparison measurements ( ̅ ̅ , the smallest bmc the comparison could verify is lab ref. therefore, a ctw for transfer application without restriction in accredited laboratories according to figure 1 should feature a value of wref smaller than 0.2 %. only specimen no. 4 meets this requirement and is able to be used in transfer measurements for all laboratories. the majority of bmc values in figure 1 is greater than or equal to 0.5 %. transfer to these laboratories should be possible also with ctw no. 6 with a sufficient uncertainty buffer. the best results were achieved with ctw no. 4 which is of the electromechanical type. apparently the higher effort in engineering is reflected in a very low dependency of the release torque on calibration conditions. in table 3, the contributions are graded in reference to their amount by colour. the highest contributions are printed in red, the second highest in orange. excluded is wrch, because this contribution could be avoided by using no ratchet but rather a fixed square drive. for ctw no. 4, the contribution of wa, which is mainly the uncertainty of the standard calibration facility, is the highest, the contribution of wtb is the second highest. a further improvement of this ctw should start with an enhancement of the internal sampling rate, which should reduce the influence of the rise time. for the other ctws, an improvement of this aspect is only to be expected by re-designing the releasing parts – quite a large effort which does not meet the actual goals of the manufacturers. but many of the ctws could be improved fundamentally in this field. most of the ctws would benefit also from an improvement of the relative repeatability srpt. this is indicated in table 2 by the amount of the repeatability of a ctw in comparison to the other influences. srpt should be in the range figure 9. combined expanded relative uncertainty wref for the investigated  specimens  in  comparison  to  the  smallest  bmc  of  the  dakks‐accredited  laboratories (yellow bar).  table  4.  uncertainty  of  calibration  conditions  ux  expressed  as  a  half‐span  value  used  for  the  calculation  of  an  uncertainty  budget  in  a  transfer  measurement  (table 3).  given  are  measured  values  of  the  ptb  standard  calibration  machine  (nn)  and  assumed  values  for  typical  accredited  calibration laboratories (lab).  condition  index x  ux (nn)  ux (lab) lever length  l  0.25 mm  0.25 mm temperature  t  0.5 k  2 k  humidity  f  2 %rh  5 %rh rise time  tb  0.1 s  0.5  s ratchet position  rch  0 cog  1 cog acta imeko | www.imeko.org   june 2015| volume 4 | number 2| 30 of the smallest contribution in this compilation. many of the tested ctws cannot fulfil this requirement. ctw no. 3 exhibits friction-induced drift during the preliminary measurements. this drift is to be understood as a systematic contribution and therefore has to be taken into account by absolute value. thus, this contribution on its own exceeds the requirements of transfer application and hence no more measurements were performed with this ctw. coefficients which were not measured are marked in table 2 with “no data”. these are counted as zero in order to get a lower estimation of wref at the end. nevertheless, the uncertainty contributions wx of these conditions are greater than zero in this analysis because of the contribution of srpt in (11). 4.5. procedure for transfer measurements   the measurements of the survey show that the procedure of iso 6789 (figure 10) is not adequate for transfer measurements with ctws. readjusting uncertainty of the ctws can be avoided when the adjustment of the ctw is not changed during the measurements. this adjustment should be made at the nominal value of the ctw in order to use the best possible signal-tonoise ratio. extensive tests with ctws stored several days under the tension of this adjustment did not show a drift of the release torque. furthermore, the ctw with the best overall uncertainty is of the electromechanical type and hence is free of mechanical tension due to adjustment. the transfer measurement should consist of series of at least 30 release events to reduce the repeatability uncertainty (figure 11). the load rise time should be fixed to a certain value, preferably at 5 s. furthermore, the laboratories should agree on a fixed value of the sampling rate, at best more than 2 khz. the position of the square drive and of the ratchet should be the same in both laboratories. additional series after alteration of the square drive position by 90° should be performed at least for two positions. the length of the lever, the temperature and the humidity should be controlled and documented. 5. conclusions  the capability of ctws for use as transfer transducers depends not only on their technical specifications. restricted specifications could be compensated if adapted procedures are employed which can largely differ from those according to iso 6789. the objective of these procedures should be an optimization of the reproducibility of the ctws’ response. a suggested procedure designates the measurements required in iso 6789 at 20 % and 60 % of the nominal value, but calls for at least 30 repeated measurements instead of 5. parameters like rise time, positions of square drive and ratchet, sample rate, temperature, humidity and lever length are to be agreed upon and reproduced in the laboratories within narrow limits. the survey delivers the following results: 1. the use of transfer ctws for dakks assessments is possible under special conditions and with adequate types of ctws for the laboratories accredited by the dakks. therefore, the development both of specific measurement procedures and of selection criteria for ctws, which was carried out to complement the traceability of the laboratories working in the field of iso 6789, should be supplemented by further improvement of the ctws, especially of their repeatability and rise time sensitivity. 2. the detected coefficients of the used specimens vary over a wide range of amounts. the selection of a ctw for transfer application therefore requires the analysis of the combined measurement uncertainty budget in the setup of the intended transfer measurement. 3. a new design of ctw with an electromechanical release mechanism yields the best results among the tested specimens. here further improvements could be possible with higher internal sampling rates. acknowledgment  thanks go to sebastian kaletka, who carefully performed hundreds of measurements for this investigation. references  [1] iso 6789:2003-10, “assembly tools for screws and nuts hand torque tools requirements and test methods for design conformance testing, quality conformance testing and recalibration procedure”, international organization for standardization, geneva, switzerland. [2] brüge a., “influence of eccentric mounting on the calibration of torque measuring devices”, proceedings of the 15th imeko tc3 conference, october 7-11, 1996, madrid, spain, pp. 255259. [3] sanponpute, t.; arksonnarong n., “temperature and humidity dependence on stability of torque measuring devices”, imeko 22nd tc3, 15th tc5 and 3rd tc22 international conferences, february 3-5, 2014, cape town, republic of south africa figure 10. loading schedule according to iso 6789.   figure 11. proposed loading schedule for transfer measurement with ctw.  acta imeko | www.imeko.org   june 2015| volume 4 | number 2| 31 url: http://www.imeko.org/publications/tc3-2014/imekotc3-2014-006.pdf [4] brüge a., “influence of humidity on torque transducers – estimation methods for calibration laboratories”, xx imeko world congress, metrology for green growth, 9 – 14 sept. 2012, busan, republic of korea. url:http://www.imeko.org/publications/wc-012/imekowc-2012-tc3-o30.pdf [5] röske, d.; mauersberger, d., ”on the stability of measuring devices for torque key comparisons”, imeko xviii world congress and iv brazilian congress of metrology, "metrology for a sustainable development", rio de janeiro, 17-22, september, 2006, brazil, on cd-rom, file name: 00181.pdf [6] bipm, “evaluation of measurement data – guide to the expression of uncertainty in measurement”, jcgm 100:2008 url:http://www.bipm.org/utils/common/documents/jcgm/j cgm_100_2008_e.pdf acta imeko  issn: 2221‐870x  june 2015, volume 4, number 2, 45‐51    acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 45  investigations for the model‐based dynamic calibration of  force transducers by using shock excitation  michael kobusch 1 , sascha eichstädt 2 , leonard klaus 1 , thomas bruns 1   1  physikalisch‐technische bundesanstalt (ptb), bundesallee 100, 38116 braunschweig, germany  2  physikalisch‐technische bundesanstalt (ptb), abbestr. 2‐12, 10587 berlin, germany      section: research paper   keywords: dynamic calibration; shock force; dynamic modelling; parameter identification  citation: michael kobusch, sascha eichstädt, leonard klaus, thomas bruns, investigations for the model‐based dynamic calibration of force transducers by  using shock excitation, acta imeko, vol. 4, no. 2, article 8, june 2015, identifier: imeko‐acta‐04 (2015)‐02‐08  editor: paolo carbone, university of perugia, italy  received september 29, 2014; in final form february 13, 2015; published june 2015  copyright: © 2015 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.  funding: this work is part of the joint research project ind09 ”traceable dynamic measurement of mechanical quantities” of the european metrology  research programme (emrp). the emrp is jointly funded by the emrp participating countries within euramet and the european union.  corresponding author: michael kobusch, e‐mail: michael.kobusch@ptb.de    1. introduction  dynamic force measurements are widely used in many industrial areas, and the increasing demands on measurement accuracy have set new metrological challenges. but still, traceability for dynamic measurements is solely based on static calibrations, and documentary standards or commonly accepted guidelines for dynamic measurements do not exist. for this reason, the establishment of traceable measurements under dynamic conditions is a highly relevant topic. its importance is emphasized by a current european metrology research programme (emrp) joint research project dedicated to the traceable dynamic measurement of mechanical quantities [1]. in this context, the general approach of a model-based calibration methodology will be followed in which the dynamic behaviour of the force transducer in a given mechanical calibration set-up is described by an appropriate model consisting of a series arrangement of spring-massdamper elements. the characteristic dynamic model parameters of the force transducer – i.e. values describing its distribution of mass, stiffness and damping – have to be determined. by fitting modelled and measured shock force data, the parameters of interest may be identified from the dynamic measurements. considering the fact that the measurement data may show modal oscillations of the mechanical set-up of various sources, which were found in previous experimental studies supported by finite element simulations [2], the development and selection of adequate methods and procedures to analyse the shock force data is of great importance. the general purpose of a model-based calibration is the determination of the characteristic parameters which define the force transducer’s dynamic measurement behaviour in a given dynamic application. the parametric model of the transducer could in principle be employed as part of a larger parametric model of the particular measurement application, for instance of a shock calibration device or a uni-axial testing machine. the approach of the model-based calibration will furthermore allow the calculation of realistic measurement uncertainties in abstract  within the scope of the joint research project emrp ind09 “traceable dynamic measurements of mechanical quantities”, numerous  measurements were performed at ptb’s 20 kn primary shock force calibration device to investigate and validate the approach of a  model‐based  dynamic  calibration  of  force  transducers  by  using  shock  excitations.  the  tests  included  several  strain  gauge  force  transducers  of  greatly  differing  structural  design,  size,  weight  and  mechanical  coupling.  by  looking  at  a  few  examples,  some  investigated physical models of the measurement set‐up and a developed data analysis procedure for parameter identification based  on measured shock data are presented and discussed. the models reproduce the dynamic response  including the observed modal  oscillations  of  various  origins  that  limit  the  usable  measurement  bandwidth.  moreover,  these  modal  oscillations  may  have  an  important role for the parameter identification process, which is further discussed. this paper is an extended version of the original  contribution to the imeko 2014 conference in cape town, south africa. acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 46  dynamic force measurements, which are basically not known at the moment. the remainder of this paper is outlined as follows. section 2 presents experimental results from shock force calibration tests performed with different force transducers. section 3 then discusses the mathematical modelling and the parameter identification procedure. finally, the last two sections give an outlook to future research and conclude the presented work. 2. experimental tests  within the scope of the above-mentioned emrp project, several strain gauge force transducers of greatly differing size, weight, mechanical design and adaptation were investigated at ptb’s 20 kn primary shock force calibration device. 2.1. shock calibration device  figure 1 shows the calibration device and illustrates its working principle. two cube-shaped mass bodies (mb1, mb2) each of 10 kg and the force transducer under test are brought to collision. the traceability of the shock force is realized by the determination of mass (from weighing) and acceleration (by means of laser vibrometers). further information about this device is given in [3]. a recent modification of its measurement geometry now allows on-axis vibrometer measurements similar to those obtained with a larger 250 kn calibration device [4]. as this geometry is not susceptible to parasitic rotational vibrations possibly excited by the impact [5], the quality of the timedependent acceleration signals and has been improved. 2.2. force transducers  the tested force transducers are equipped with a threaded bolt with a spherical end face, and can measure compression as well as tension forces. this choice also allows calibrations with sinusoidal forces for comparing the different dynamic results. figure 2 shows two investigated force transducers and their adaptation to the mass body of the calibration device. the small hbm u9b / 1 kn has a mass of about 63 g and uses a measuring body with a flexing diaphragm applied with strain gauges that ends in the upper threaded bolt. this diaphragm, which is considered as the measuring spring, divides the mechanical structure of the force transducer into two parts. the effective mass of the upper part (head mass, see section 3.1) is less than 3 g. the manufacturer specifies a fundamental resonance of 24 khz. in contrast, the large interface 1610 / 2.2 kn is a shear beam strain gauge transducer of more than 1.5 kg (including the connector) and a diameter of 105 mm. this transducer hardly fits into the squared air bearing of 108 mm clearance and thus clearly marks the maximum size for the 20 kn shock force calibration device. former tests with an even larger force transducer of similar design (interface 1032 / 225 kn) performed at the 250 kn shock calibration device showed that the shock response can be predominately influenced by a comparably low coupling resonance [2], [6], which denotes the vibration of the transducer against its fixation on the reacting mass body. regarding the small hbm u9b with its thin threaded bolt that connects to the base adapter, similar behaviour might also be expected. 2.3. shock force measurements   typical examples of measured shock force signals are shown in figure 3. the small hbm u9b / 1 kn measured a very smooth pulse without post-impact signal ringing. in contrast, the much heavier interface 1610 / 2.2 kn responded with a shorter pulse with superposed oscillations and prominent signal ringing. the plots demonstrate the possible variety of responses in the time domain. different types of transducers with their specific mechanical adaptations can respond quite differently. the tests used a hard metallic shock contact in order to achieve short pulses that are capable of exciting the transducer’s mechanical resonances, i.e. no pulse-shaping material was applied between the impact surfaces. under these conditions, the impacting mass body of 10 kg generated shock pulses of a duration of about one millisecond. figure 1. the 20 kn shock force calibration device at ptb:     schematic diagram of the working principle (top), photographs (bottom) of the  device  with  visualized  beams  of  the  laser  interferometers,  the  inset  shows the beam deflection at the spring‐driven acceleration mechanism.   figure 2. mounted strain gauge force transducers:     hbm u9b / 1 kn (left), interface 1610 / 2.2 kn (right).  figure 3. measured shock force pulses:     force  transducer  hbm  u9b / 1 kn  (left),  interface  1610 / 2.2 kn  (right),  all  signals low‐pass filtered at 20 khz. acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 47  supplementary measurement data that might be beneficial for the subsequent parameter identification could be obtained by applying an additional load button (see figure 5). such a modification of the mechanical impact configuration alters the transducer’s dynamic response as the increased mass at the force introduction bolt lowers the transducer’s fundamental resonance frequency, which basically is a function of the transducer’s head mass and its elastic coupling (see section 3.1). to achieve well-defined, reproducible testing conditions, the threaded connections of all mechanical couplings were fastened with a defined torque in each case. typical shock measurements obtained with two different transducers (hbm u9b / 1 kn, interface 1610 / 2.2 kn) are presented as examples in the following paragraphs. the plots show the three acquired measurement signals, which are the accelerations and of both mass bodies derived by differentiation of the vibrometer signals, as well as the transducer output signal force . the first example in figure 4 obtained with the hbm u9b / 1 kn demonstrates that the acceleration signals can experience strong shock-excited noise which probably originates from modal oscillations of the cube-shaped mass bodies. these high-frequency signal components might have to be filtered correctly for the subsequent parameter identification process. using a low-pass filter of 20 khz cut-off frequency, which is already below the expected fundamental resonance of this transducer, the noise is still very strong. the acceleration of the impacting mass body exhibits an almost undamped vibration with a superposed beat signal. similar behaviour is shown by the signal of the reacting mass body with the mounted transducer, but the damping is stronger. in general, the repeatability of the observed high-frequency oscillation pattern is excellent, which is demonstrated by three repetitive measurements (figure 4 insets). the signal responses to excitations of similar shock intensity are nearly indistinguishable in the time domain. the effect of an additional load button was investigated with the hbm u9b / 1 kn in order to shift the transducer’s fundamental resonance frequency below the above mentioned noise components of the experimental set-up. figure 5 shows the mounted force transducer with two different load buttons fixed at its threaded rod, and figure 6 illustrates the measured spectral content of the post-impact signal ringing for the respective configurations. without an additional head mass, the transducer exhibits an oscillation below 30 khz that probably is its fundamental resonance. this peak apparently shifts towards figure 4. shock‐excited ringing of the acceleration signals:     force transducer hbm u9b / 1 kn with load button of 7.0 g, insets show 3 repetitive measurements (span 2 ms), all signals low‐pass filtered at 20 khz. figure 5. hbm u9b / 1 kn with two load buttons with spherical end faces,  increased head mass of 3.1 g (left) and 7.0 g (right).  figure 6. spectral analysis of the signal ringing for the hbm u9b / 1 kn with  different load buttons. figure  7.  shock  signals  in  the  time  domain  and  frequency  domain  (signal  ringing  only)  obtained  with  the  force  transducer  interface  1610 / 2.2 kn,  time signals low‐pass filtered at 20 khz.  acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 48  lower frequencies with the increasing mass of a load button. in addition, a second strong resonance that only slightly depends on the load appears at about 9 khz. this resonance could be identified as a second axial vibration mode due to the mass and elasticity of the transducer housing [7], which is possibly further influenced by a non-rigid mechanical coupling at the base. the last example in figure 7 was performed with the interface 1610 / 2.2 kn. both force and acceleration signals show a strong vibration at 3.7 khz, and the spectral analysis reveals a weaker component at 6.7 khz. with similar amplitudes of the dominant component in both signals, this behaviour actually differs from the previously mentioned tests with the larger transducer model, which may indicate different sources. looking at the quite different shock responses obtained with the transducers presented as examples, the identification of the transducer’s dynamic parameters from shock measurements may require various analysis tools to provide satisfying results. to elaborate these mathematical methods and analysis procedures for the parameter determination from the experimental data, hundreds of shock force measurements were performed. the large number of tests will further provide an assessment of the reliability of the proposed parameter identification methods. the first trials on the parameter identification (see section 3.3) gave consistent results for the force transducer interface 1610 / 2.2 kn which responded with strong signal ringing, cf. figure 3. on the other hand, it showed that the smooth shock pulses measured with the hbm u9b / 1 kn apparently contain insufficient information to be able to identify the parameters that describe the transducer’s dynamic behaviour. tests with additional head mass obtained data better suited for the parameter identification process, but it seems that this transducer is more complex and its model description has to be refined. for this reason, the following sections will focus on the first transducer (interface 1610 /2.2 kn), and a dedicated paper will cover the second one (hbm u9b / 1 kn) in the near future presenting further research. 3. modelling and procedures for parameter  identification  3.1. model of the force transducer  apart from the various influences of a coupled mechanical environment that have to be taken into account to correctly understand a dynamic measurement, the force transducer itself exhibits a dynamic behaviour which is mainly affected by inertia forces due to the motion of its internal mass structure. to describe this behaviour, the force transducer is mathematically modelled by a spring-mass-damper system (figure 8) consisting of two concentrated masses (base mass , head mass ) which are connected by a linear spring element (stiffness , damping ). the body motions are described by the linear displacement coordinates and , and the forces acting at both sides are denoted by the input force and the reaction force . the output signal of the force transducer is assumed to be proportional to the elongation of the measuring spring. such a mechanical system responds with a characteristic resonance when subjected to a dynamic excitation. considering a transducer rigidly mounted at its base and neglecting damping, the fundamental resonance frequency given in equation (1) is a function of and . ⁄ (1) this parameter is often used to judge the dynamic suitability of a force transducer. it is mentioned in the german directive vdi/vde 2638 [8] and values are specified in some force transducer data sheets. however, to understand the dynamic measurement behaviour in an application, this single parameter is not sufficient, but rather the four model parameters , , and have to be known. 3.2. model of the calibration device  by expanding the basic model of the force transducer, the mechanical system of the calibration device with a mounted force transducer is modelled by a one-dimensional multi-body system, which consists of a linear series arrangement of lumped masses coupled by visco-elastic springs. models of different degrees of freedom using 3, 4 and 5 model masses were investigated (figure 9) in order to account for a possible non-rigid coupling at the transducer base as previous shock tests have shown that this mechanical coupling may significantly influence the dynamic measurement behaviour. the three models basically differ in their description of the mechanical adaptation of the transducer to the reacting mass body. the 3-mass model assumes a rigid fixation, whereas the models with 4 and 5 masses describe an elastic coupling. for the 4-mass model, the adapter mass is split and allocated to the neighbouring masses depending on the specific mechanical set-up. in each case, the coupling of an optional load button is assumed to be rigid. the assumption of a rigid coupling of has to be revised figure 8. basic model of a force transducer.  figure 9. models of the shock force calibration device: 3‐mass model (top),  4‐mass model (middle), 5‐mass model (bottom).  acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 49  if the resulting coupling resonance between and becomes important. this is the case for a considerably large mass and/or a small coupling stiffness which will result in a low resonance frequency that cannot be further neglected. dynamic force calibrations with sine excitations [9] typically apply large load masses (= ) and the corresponding model descriptions consider an elastic coupling, i.e. a spring element is introduced between and . however, the model description of the shock force calibration device may have to be modified accordingly if necessary. the dynamic behaviour of the model components is expressed by a system of linear ordinary differential equations (ode) derived from the equilibrium of forces at each mass element. the ode system has the form (2) where , and are the motion vectors (displacement, velocity, acceleration), and , , and denote the matrices for mass, damping and stiffness, and is the load vector. the mass matrix is diagonal while the damping and stiffness matrices are tri-diagonal with non-zero elements on the main diagonal and the upper and lower sub-diagonal. as an example, the components of the 5-mass model are given in the following equations. 0 0 0 0 0 0 0 0 0 0 0 0 (3) 0 0 0 0 0 0 (4) 0 0 0 0 0 0 (5) , , , (6) ,0,0,0 (7) the ode system of the 5-mass model consists of only four equations, as the shock contact, i.e. the coupling between the impacting mass and the force transducer (model mass ) is not expressed by another elastic coupling element but rather by the measured shock force . differing from the basic model of the force transducer (figure 8), the models of the shock calibration device substitute the reaction force with the corresponding inertia force calculated by the ode system, where the motion of the reaction mass body mb2 denotes the input quantity. some of the model parameters, such as the mass values of the two mass bodies and the adaptation parts, can be measured before the actual calibration. the remaining parameters, in particular those of the force transducer, have to be inferred from the measured dynamic calibration data or from cad data. 3.3. parameter identification  measurement data gained from the calibration experiment are the displacements and of both mass bodies and the corresponding force transducer output signal . using numerical differentiation by means of finite differences together with low-pass filtering for noise attenuation, the acceleration data ( , ) of the mass bodies is calculated. the measured acceleration of the impacting mass body is then used to calculate the system input force . in order to account for the system change during the measurement interval due to the transient shock contact, we employed a time window to suppress post-impact signal components. for the fit, the transducer output signal and the acceleration of the reacting mass body at the time instants ,…, are considered: , with 0… (8) assuming normally distributed measurement noise, we carry out a maximum-likelihood estimation of all parameters by means of non-linear least squares in the time domain. therefore, a numerical integration method for the differential equations (2) is employed together with the nelder-mead nonlinear simplex method for optimization [10, 11]. the benefit of this optimization method is that it does not require the calculation of derivatives. the optimization merit function is ‖ , , ‖ (9) where ‖ ⋅ ‖ is the euclidean norm, the vector of the sought stiffness and damping parameters, the measured data and the ode initial values. in this approach, the mass values of all model components are assumed to be known. the design function ∙ denotes the numerical integration of the ode (2) corresponding to the considered system model and the calculation of the resulting system output data. for the 5-mass model ∙ is calculated as , , x / (10) with the two ode trajectories denoting the force signal as a function of the elongation of the measuring spring, and the acceleration of the reacting mass body mb2 calculated from (2). as the ode integration is part of the evaluation of the function (10), the calculation of derivatives is a cumbersome analytical and numerical task. very high precision in the ode integration would be required to obtain sufficiently precise finite differences for a derivative-based numerical optimization. for this reason, the derivative-free nelder-mead simplex method was applied for parameter estimation. results of the parameter identification for the transducer interface 1610 / 2.2 kn are presented in the following. the attenuation of noise in the calculation of acceleration from displacement data was carried out by a 4th order butterworth low-pass filter of 12 khz cut-off frequency. using the model parameters estimated by each model, figure 10 displays the calculated trajectories of the two elements (force , acceleration ) of the function ∙ and compares them with the corresponding measurement data. for more clarity, the fit residuals are additionally plotted. regarding the 4-mass model, the adapter mass was fully allocated to the reacting mass body as the adapter is more rigidly coupled to the reacting mass than to the transducer comparing the different mounting conditions (contact area, thread size, mounting torque). it is seen that the 3-mass model yields considerably large deviations, in particular in acceleration . on the other hand, the 4-mass and 5-mass models both show much smaller deviations which demonstrates that the coupling of the acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 50  transducer to the reacting mass body cannot be assumed to be completely rigid for the considered mechanical set-up. as both models achieve a nearly identical fit quality, the chosen 4mass model suffices to describe the system’s behaviour. according to figure 7, there are just two components in the frequency range of interest, which corresponds to such a model. this behaviour is further proved by a short-time dft analysis of the measured ringing in acceleration after the main pulse (figure 11). table 1 summarizes the identified model parameters for the particular data set (as in figures 10, 11) using models with 3, 4, and 5 masses. for neglected damping, the corresponding resonance frequencies (see table 2) are calculated from the eigenvalues of the characteristic system matrix of (2) as eig (11) it is worth noting that the stiffness of the coupling is ten times greater than that of the transducer’s measuring spring. all three models reproduce the lowest resonance at about 3.7 khz well. figure 10. comparison of  modelled and measured  shock response signals for  the  interface  1610 / 2.2 kn:  force  signal    (top)  and  corresponding  fit residuals ∆ , acceleration   of the reacting mass body and corresponding fit residuals ∆  (bottom).  figure 11. short‐time dft of the ringing  in the measured acceleration  ,  shock  calibration  of  the  interface  1610 / 2.2 kn,  the  diagram  displays  the colour‐coded acceleration amplitude over time.  table 2. calculated resonance frequencies of the different models. resonant  frequency  unit  3‐mass  model  4‐mass  model  5‐mass  model    khz  3.687  3.687  3.687  ,   khz ‐  6.695  6.694 ,   khz ‐  ‐  35.62 table 1. identified model parameters of the interface 1610 / 2.2 kn using  different models, analysis of one shock pulse.  parameter unit  3‐mass  model  4‐mass  model  5‐mass  model    10 6  n/m  166  187  186    kg/s 109  154  154     10 6 n/m ‐  1743  2867   kg/s ‐  539  154     10 6 n/m ‐  ‐  4680   kg/s ‐  ‐  1.12 figure  12.  identified  model  parameters  of  the  interface  1610 / 2.2 kn  obtained with the 4‐mass model, analysis of 27 shock pulses.  acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 51  the additional resonance of the 5-mass model falls well outside the considered frequency range, which further confirms that the modelling of the coupling stiffness at both sides of the adapter (cf. figure 9) does not show any benefit. using the 4-mass model, a first analysis of 27 repetitive shock measurements of about 2 kn peak value gives promising results. the identified parameter values of the force transducer interface 1610 /2.2 kn (spring stiffness , damping ) and its coupling ( , ) are plotted in figure 12. the relative standard deviations are quite small, in particular those of the stiffness parameters. the larger variance of the damping parameters is probably due to their comparably small influence on the dynamic behaviour of the shock force calibration device, and to the fact that the parameter identification process based on the available measured data is therefore less sensitive. 4. outlook  at the moment it is far too early to specify associated uncertainties for the identified parameters due to numerous factors that may have an influence on the parameter identification process, e.g. the fitting methods, data length, weighting, windowing and filtering. in addition, the remaining deviations observed in the time domain still need some explanation. in any case, the investigated models cannot reproduce the additional spectral peaks at higher frequencies (cf. figure 7). in the end, all these topics have to be covered in future research. the parameter identification presented as an example was obtained with shock pulses of considerably strong post-impact signal ringing. some first trials on the identification of smooth pulses of almost no ringing (cf. figure 3 left) indicate that the excited modal oscillations presumably carry the crucial information to unambiguously identify the model parameters of a multi-body system having more than one degree of freedom. these findings as well as future investigations on this topic will be presented in a dedicated publication. the analysis of various measurements with an identical force transducer under modified measurement conditions (e.g. pulse intensities, mechanical adaptations, various mounting conditions) will eventually allow a verification of the suitability of the mechanical model and the data analysis methods applied. in particular, it will demonstrate the influence of various disturbances, e.g. from high-frequency modal oscillations [2] not explained by the chosen model, on the estimation of the model parameters of interest. the result of these investigations will then be incorporated into an extended system model if necessary. 5. conclusions  this paper presented new research on the model-based dynamic calibration of force transducers by using shock excitations. by analysing experimental shock measurements obtained with force transducers of different structural design, size, weight and mechanical coupling, the suitability of the mathematical models and the applied methods for the estimation of the transducer’s model parameters were described and discussed. a first parameter identification applied to shock data obtained with a force transducer that responds with strong shock-excited ringing gave satisfying results. future research will investigate the numerous influences on the parameter identification process, as well as the application of the proposed methods to other types of force transducers, e.g. those that respond without strong signal ringing. it will also focus on the comparison of results from sinusoidal excitation experiments and, finally, on the evaluation of the measurement uncertainties for parameter identification. references  [1] c. bartoli et al., “dynamic calibration of force, torque and pressure sensors”, proc. of imeko tc3, tc5 and tc22 int. conf., cape town, south africa, 2014. [2] m. kobusch, l. klaus, t. bruns, “model-based analysis of the dynamic behaviour of a 250 kn shock force calibration device”, xx imeko world congress, busan, 2012, republic of korea. [3] m. kobusch, a. link, a. buss, t. bruns, “comparison of shock and sine force calibration methods”, proc. of imeko tc3 & tc16 & tc22 int. conf., merida, mexico, 2007. [4] m. kobusch, t. bruns, l. klaus, m. müller, “the 250 kn primary shock force calibration device at ptb”, measurement: 46 (2012), 5, 1757 – 1761. [5] m. kobusch, t. bruns, “uncertainty contributions of the impact force machine at ptb”, proc. of the xviii imeko world congress, rio de janeiro, brazil, 2006. [6] m. kobusch, “influence of mounting torque on the stiffness and damping parameters of the dynamic model of a 250 kn shock force calibration device”, 7th workshop on analysis of dynamic measurements, paris, france, 2012. [7] m. kobusch, s. eichstädt, l. klaus, t. bruns, “analysis of shock force measurements for the model-based dynamic calibration”, 8th workshop on analysis of dyn. meas., turin, italy, 2014. [8] vdi-richtlinie vdi/vde/dkd 2638, kenngrößen für kraftaufnehmer ‒ begriffe und definitionen, beuth, 2006. [9] c. schlegel et al., “traceable periodic force measurement”, metrologia, vol. 49, 224–235, 2012. [10] j. nocedal, s. j. wright, numerical optimization, springer series in operations research, 1999. [11] s. eichstädt, c. elster, “reliable uncertainty evaluation for ode parameter estimation – a comparison”, j. phys. 490, 1, 2014. microsoft word 27-141-1-pb.doc acta imeko  july 2012, volume 1, number 1, 77‐84  www.imeko.org    acta imeko | www.imeko.org  july 2012 | volume 1 | number 1 | 77  design of an efficient mobile measurement system for urban  pollution monitoring  andrea bernieri, domenico capriglione, luigi ferrigno, marco laracca  diei, università di cassino e del lazio meridionale, via g. di biasio 43, 03043, cassino (fr), italy    keywords: pollution monitoring; mobile sensor networks; wireless sensor network; distributed measurement systems.  citation: andrea bernieri, domenico capriglione, luigi ferrigno, marco laracca, design of an efficient mobile measurement system for urban pollution  monitoring, acta imeko, vol. 1, no. 1, article 15, july 2012, identifier: imeko‐acta‐01(2012)‐01‐15  editor: pedro ramos, instituto de telecomunicações and instituto superior técnico/universidade técnica de lisboa, portugal  received january 16 th , 2012; in final form may 28 th , 2012; published july 2012  copyright: © 2012 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: this work was supported by the emilia romagna region (italy), call prriitt 2008, measure 3.1, action a and 3dinformaticatm ltd.  corresponding author: luigi ferrigno, e‐mail: ferrigno@unicas.it  1. introduction  the timely and reliable knowledge of environmental pollution in urban areas has become, in recent years, one of the most critical issues for local public authorities, even for the not negligible impact with regard to political and social choices to be made. in particular, many local authorities wish (or must) to offer continuous services about the determination of the pollution levels, especially when they exceed limits that are considered unsafe or that are regulated by local laws [1]. in this scenario, the ability to have data timely, reliable, and well spread over the territory on which base suitable management policies for the urban community, can enable to develop several actions for safeguarding the health of the population, for optimizing the resource, and for controlling the industrial and anthropogenic emissions. the typical set-up adopted to retrieve information on air quality is based on fixed measurement units, positioned in suitable points of the area under test. these measurement units are characterized by good metrological performance somewhat counterbalanced by high costs, weights, dimensions, large measurement times, and power consumption requirements. values of the pollutants in different and neighboring areas are then obtained by applying suitable predicting mathematical models. the position of these units must be appropriately designed to enable a reliable extension of the information collected throughout the range of interest. however, in cases where the hilly terrain or urban structure does not allow an extension of information to wide areas of territory, additional monitoring stations should be employed. typically, this circumstance leads to a meaningful increase of the monitoring cost that sometimes becomes prohibitive for the interested authority. if there is a need for accurate measurements in areas where fixed stations are not installed, some innovative solutions, present in the literature, suggest the use of mobile (i.e. portable) measurement systems that, indeed, re-propose the use of fixed in recent years, the pollution monitoring in urban areas has become one of the most critical issues for local public authorities, which  wish  (or  must)  verify  that  pollution  levels  not  exceed  limits  considered  unsafe  or  that  are  regulated  by  local  laws.  generally,  the  pollution monitoring is performed by using measurement stations located in few points of the region of interest since these stations  are generally characterized by high costs, weights, and dimensions. then, the pollution levels over the remaining area are predicted by  means  of  suitable  interpolation  models.  due  to  the  great  variety  of  urban  scenarios,  it  becomes  very  difficult  to  obtain  reliable  pollution  levels  in  area  in  which  the  measurements  has  not  been  directly  taken  but  only  predicted.  consequently,  the  pollution  monitoring can suffer of a lack of reliable information indispensable for the actuation of proper environmental management policies.  in this framework, this paper proposes a mobile measurement system for the real time monitoring of environmental pollutions over  urban areas. the proposed approach is based on the use of a set of vehicles, typically employed for public transportation inside the  urban area, equipped with the proposed mobile measurement system allowing it to measure, store, and transmit the acquired data to  a remote supervisor unit somewhere on the path followed by the vehicles. particular attention has been paid on the definition of the  metrological characteristics of the measurement devices with the aims of complying with the applicable european directives accuracy  requirements and of selecting a suitable trade‐off between accuracy and cost. the experimental measurement campaign performed  on a suitable urban scenario has confirmed the goodness of the proposed system. acta imeko | www.imeko.org  july 2012 | volume 1 | number 1 | 78  units made transportable through their installation on trucks or cars [2]-[4]. thus, measurement campaigns can be implemented in wider parts of the territory, but the simultaneity of the data recorded is closely related to the total number of systems used for surveying. in this context, it thus becomes of interest to have mobile (i.e. moving) analysis systems that measures the environmental pollution in wider parts of a territory according to defined paths of the vehicles on which they are installed. in this way, the cost of urban pollution monitoring can be substantially reduced avoiding the use of many more expensive fixed or transportable monitoring units. by adopting a suitable data management technique, it is then possible to perform the integration of the information in "clusters" of data. this approach makes possible, with a small number of devices, to achieve full coverage, accurate, and timely provision of pollutions in wide areas. to these aims, some measurement architectures have been presented in the literature [5]-[7]. however, they does not fully consider the issue of the reliability of data collected from the standpoint of measurement uncertainty and of the maintenance of the metrological characteristics of the units during the whole operative cycle. then some troubles about the reliability of such solutions arise. thanks to previous experience in the field of wireless measurement systems and sensing devices [8], wireless sensor networks [9] and uncertainty estimation [10], in this paper the authors propose an effective mobile measurement system for the real time monitoring of environmental pollutions over urban areas [11]. key features of the proposed system, thought to be located on public urban ground transportation vehicles, are the significant low cost, the high portability of the measurement units, the autonomous power supply, and the measurement strategy able to minimize the measurement uncertainty. to these aims, suitable interpolation techniques aggregating data acquired over the spatial and time domains can be applied. in the following, starting from requirements of directives and laws operating in the european countries, the metrological specifications for the sensing devices are defined. then, the architecture of the proposed measurement system is presented, together with some details about the measurement unit, the considered sensors, and the software architecture. finally, to validate the proposed system, a measurement campaign has been performed on a suitable urban area. 2. european regulation for the air pollutant  this section summarizes the legal regulations and limit values for the air pollution parameters which have been considered to drive the selection of the transducers and the design of the measurement system. in europe, european directive number 30 of the april 22-th 1999 related to environmental air quality limit values regulates the presence of air pollutions [12]. the considered quantities are the sulphur dioxide (so2), nitrogen dioxide (no2), nitrogen oxides (no), particulate matter (pm) and lead. in 2000/69/ec the ambient air quality limit values for benzene (c6h6) and carbon monoxide (co) are given [13]. these directives are then to be acknowledged by the state trough suitable decrees. as an example, in italy the italian ministerial decree number 60 of the 02/04/2002 has acknowledged these directives. typically, standards and laws express the limits of the pollutants to which the human body can be exposed in milligram on cubic meter (mg/m3) or microgram on cubic meter (g/m3), and impose limitations about the period of exposure. table 1 summarizes these limits for the main air pollutants, and reports them also in parts per million (ppm) or parts per billion (ppb), quantities often used in the transducer specifications. the limits reported in the table are generally referred to a mean value in a considered exposure period; on the contrary, alert thresholds are usually given in terms of maximum number of events in which the specific pollutant may exceed a corresponding limit, in a defined time period. in the table, for the unit conversion, a reference pressure equal to 101325 pa and a reference temperature equal to 298.16 k were considered. 3. the measurement system   the general architecture of the proposed system is sketched in figure 1. it is based on two main classes of devices: mobile measurement unit (mmu): designed to be hosted on board of vehicles, as urban service buses or service vehicles of the local authority. central supervisor system (css): designed to be located in a fixed position and devoted to remotely access to measurement data. the mmus, whose architecture and components are described in detail in section 4, perform the acquisition, the collection, and the transmission of the measured data (environmental quantities and geo-localization information) to the css. table 1. pollutant limits according to the european directives. pollutant  limit [mg/m 3 ]  limit [ppm]  exposure period  co  10  8.7  daily  no2  0.2  0.11  hourly  0.03  0.016  annual  0.4  0.21  alert threshold  so2  0.35  0.13  hourly  0.125  0.048  daily  0.02  0.008  annual  0.5  0.19  alert threshold  o3  0.18  0.084  hourly  c6h6  0.005  0.144  daily  wifi connection mobile  measurementunit bus pole wired connection umts/gprs/gsm connection wifi connection mobile  measurementunit bus pole wired connection centralsupervisor system figure 1. general architecture of the measurement system.  acta imeko | www.imeko.org  july 2012 | volume 1 | number 1 | 79  the css is a workstation equipped with the necessary devices and software for retrieving, storing, and viewing measurement data collected by mmus as well as for managing the data elaboration. in particular, using appropriate software modules suitably developed, the css ensures:  the management of involved mmus by means of a graphical i/o user interface for enabling/disabling, control, configuring and verifying the mmu operation;  the representation of each mmu position and its measurement results on a digital cartography;  the storage of the data collected in time series for their statistical analysis over time;  the dynamic integration of data collected from different mmus, with reference to mathematical models of pollution diffusion in the monitored area, and data fusion techniques to correlate data relating to the same location and detected by different mmus;  the availability of the data processed on the internet for a free use or a controlled use by the operators enabled. 4. the mobile measurement unit  the core of the proposed system is the mobile measurement unit (mmu). it is designed and realized according to a modular architecture, in which various sensors can be combined as required by the specific needs of the measurement purposes. the sensor assembly is also designed to be easily removed from the rest of the unit, in order to perform the check and periodic maintenance necessary to ensure proper traceability and accuracy of the measurements. with reference to figure 2, the mmu is composed by four main sub-modules: (i) the probe head, (ii) the sensing, conditioning and acquiring devices, (iii) the processing and memory unit, and (iv) the data communication modules. figure 3 shows a picture of the realized mmu. 4.1. the probe head  a schematic of this device is reported in figure 4. it is composed by a package in which the sensing elements are suitably placed, and by a controlled air aspiration system that is able to guarantee the desired air flux whatever the vehicle velocity is. in particular, the sampling air port is placed in the opposite direction with respect to the vehicle direction, and the air flow is fixed by the aspiration system. inside the probe head, some fins able to impose a suitable air velocity for each considered sensing element are also placed. this choice allows the reliability of the mobile measurements to be improved. 4.2. the sensing, conditioning, and acquiring devices  the sensing and conditioning circuits are developed to measure the following quantities: air temperature, air humidity, and the concentrations of: carbon monoxide (co), nitrogen dioxide (no2), sulphur dioxide (so2), ozone (o3), and benzene (c6h6). in the future version of the measurement unit p r o b e h e a d nox sox co c6h6 o3 gps data  concentrator c c c c c temperature, humidity,  velocity gps sensing conditioning and acquiring devices temperature humidity inertial platform wifi tx/rx umts/gprs/gsm bus pole hand held unit central supervisor system data communication modules odometer figure 2. architecture of the proposed mobile measurement unit.  figure 3. the realized mobile measurement unit (mmu).  acta imeko | www.imeko.org  july 2012 | volume 1 | number 1 | 80  also the fine particles (pmx) and other quantities of chemical interest will be considered. the sensors are selected mainly on the basis of range, accuracy, and measurement rate, according to requirements imposed by the european directives and limits showed in the table 1. in particular, for the pollutant quantities, kanomaxtm aeroqual high spec sm50 sensors are used [14]. table 2 shows the main metrological characteristics of the selected sensors. all sensors have an internal resistance used to bring the conversion head to a temperature that avoids any influence from external environment factors, such as atmospheric temperature and humidity, at the time of the readings. moreover, they have the current output, typically with a load resistance ranging from 100 to 200 ω, with the aim of falling in the voltage input range 0-5v of the analog to digital converter. a suitable on-board kanomax aeroqual adc with 0-5v input range / 12 bit resolution is adopted; in addition this converter also has an output interface based on rs232/rs485 standards. 4.3. the processing and memory unit  it is composed by a microprocessor and data storage hardware. the advantech ark 1388 industrial pc was used for the purpose. it is characterized by small dimensions and consumptions and by high robustness and good i/o capabilities [15]. the microprocessor manages the local unit and controls all the measurement and data acquisition tasks. the data are geo-referenced by means of a high-resolution gps, embedded on the industrial pc, together with an odometer corrsys-datron l-400 and an inertial platform landmark gladiator lmrk10, which allow retrieving the geographic position even in areas not covered by gps signal. the system also provides the local data storage necessary to save measured data between two successive data transmissions to css. 4.4. the data communication modules  the mmu has been equipped with suitable wireless mobile communication modules. in particular, wifitm tx/rx system and umts/gprs/gsm modem, both embedded on the industrial pc, are used. in this way, the measured data can be transmitted to the css in two alternative ways: a) by means of short-range wifi connections installed at the predefined locations (e.g. the bus poles); in this case the connection between the bus pole and the css is performed by a wired lan connection; b) by means of long-range umts/gprs/ gsm wireless connection. the solution a) is a reliable and free of charge connection that guarantees high data rates but, due to the short range of the communication link, it is usable only at the predetermined locations (bus poles). even if the measurement data are collected in real time by mmus, they are retrievable (by the css) only off-line, i.e. when the vehicle is close to the bus pole. consequently, a time delay occurs in the management of the pollution data and of the measurement system, even if it can be usually acceptable for pollution monitoring purposes. the solution b) performs an on-line communication on long-range distance (if there is an adequate cellular network coverage), but it is generally characterized by a low data rate and not free of charge connection. then, it is used only if the data must be on-line transmitted (e.g. alarms) or if the solution a) is temporarily out-of-order. 5. the software architecture  the measurement and control software has been developed in labviewtm environment. the software is based on a clientserver architecture and it is constituted by different modules running on different devices. in particular, two main software units can be identified: i) the program running on the mmu (which operates as “measurement server”), and ii) the program running on the external remote devices (hand-held units and bus pole units which act as clients). figure 5 shows the general architecture of the developed software, also considering the wifi connection of the mmu at the bus pole. as for the measurement server, all functions (data acquisition from sensors and external devices, storing and transmission) have been implemented into the firmware of the embedded pc on the mmu. once the device has been powered on, they go in a standby mode waiting for configuration (by means of a control software running on the hand held devices) and/or for download of the storage data (by means of a download software running on the bus poles). after the configuration phase, the whole operating is completely automatic. the firmware acquires the data from transducers and from the devices devoted to the geo localization (gps, odometer, and inertial platform). these data are stored on the internal memory unit (as suitable files). in addition, the firmware manages the connection and the communication with the client devices. during the acquisition phase, a suitable software module allows the on-line upload of the stored data on the client devices. as for the clients, two different software have been developed. the first one is the control software (usually running on the hand held units) that allows the connection to the mmu and, depending on the mmu’s status, sends the configuration and/or start command (if the mmu is in standby mode) and, when required, the stop command (if the mmu is in acquisition mode). the second one is the download software (running on suitable unit placed on bus poles) that continuously scans the wifi network to connect with the mmu, and then downloads all data previously stored in the mmu internal memory. it manages the data download in both the mmu standby and acquisition modes. 6. strategies for pollution data evaluation  to provide a satisfying characterization of the environmental pollution, it is possible to adopt several approaches for the data sampling and interpolating over a regional scale [16], [17]. generally, a regional distribution of the polluting can be achieved by means of either predictive models (which are characterized by a good spatial coverage and poor accuracy) or careful measurements suitably post-processed with line drawing line drawing sampling port sampling port direction co c3h3 sox nox o3 measuring cell air flow air flow air flow air flow controller rate aspiration pump figure 4. sketch of the realized probe head.  acta imeko | www.imeko.org  july 2012 | volume 1 | number 1 | 81  interpolation methodologies. in the literature, we can find a number of interpolation methods developed for different applications [17], [18]. starting from punctual data set, their aim is to obtain spatial polluting concentration fields. these methods can be classified in two main categories, namely deterministic and geo-statistical. the main difference between these methods is that the former are only based on punctual measurements, whereas the latter exploit the data statistical spatial structure providing also the estimate of the variance on the interpolated values. more specifically, the methods belonging to these categories are: (a) deterministic: classic interpolation methods such as voronoi diagrams, thiessen polygons, delaunay triangles, inverse distance weighting (idw), radial basis functions (rbf), bier. (b) geo-statistical: interpolation methods, based on the knowledge of the statistical structure of the aleatory field of the point of interest, such as the optimum interpolation, the kriging (in the various existing versions), the geo-statistical multivariate model. in the following, a deterministic method, namely the voronoi diagram, and a geo-statistical method, namely the ordinary kriging, were used to present the achieved results obtained by means of the proposed measurement system. 7. experimental results  in order to evaluate the performance of the proposed measurement system, a number of measurement sessions were carried out. for the sake of brevity, only the results related to the co, no2, and o3 will be reported in the following. the measurement sessions have the following aims: hand held unit mobile measurement unit bus pole unit start configuration data upload in standby mode configuration? no yes start acquisition data upload in acquisition mode data acquisition gps sensors standby waiting for configuration & data management speed control speed limit < speed limit data storage memory unit stop? noyes > stop command local stop start search mmu for connection connection? no yes connected? no yes mmu status standby send configuration command selection acquisition end send stop command stop acquisition discon nection start search mmu for connection download? no yes connected? no yes mmu status standby download in standby mode acquisition end download acquisition mode end   figure 5. the architecture of the developed software.  table 2. main metrological characteristics of the selected sensors (ldl: lower detection limit). measured  quantity  calibrated  range [ppm]  maximum  exposure [ppm]  ldl [ppm]  accuracy  resolution  [ppm]  response   time [s]  operational range  temperature [k]  rh [%]  co  0 to 100  200  0.5  +/‐ 5ppm  0.1  <150  273 to 313  5 to 95  no2  0‐0.2  0.5  0.001  +/‐ 0.01 ppm (0 to 0.1 ppm) +/‐ 10% (0.1 to 0.2 ppm)  0.001  <180  273 to 313  30 to 70  so2  0 to 10  20  0.2  +/‐ 0.5 ppm  0.01  <60  253 to 313  5 to 95%  o3  0 to 0.5  1  0.001  +/‐ 0.008 ppm (0 to 0.1 ppm) +/‐ 10% (0.1 to 0.5 ppm)  0.001  <60  268 to 313  5 to 95  c6h6  10 to 1000  n/a  10  10 ppm  0.1  <20  268 to 313  5 to 95  acta imeko | www.imeko.org  july 2012 | volume 1 | number 1 | 82  (i) of validating the mmu performance by means of a comparison between the results obtained from the mmu and those obtained from a calibrated traditional measurement instrumentation, positioned at the same location; to this aim, a dasibi 2108 chemiluminescent nitrogen oxide analyzer, a dasibi 2003 ozone analyzer, and a ml9830b carbon monoxide analyzer were used as reference instruments; (ii) of evaluating the distribution of pollutants in an extended area, by means of a great number of measurements acquired during predefined paths performed by using a service vehicle on which the mmu was installed; the measurements were then elaborated by means of suitable procedures, running on the css, which perform the elaboration of voronoi diagrams and the kriging variograms. to simplify this preliminary evaluation phase, the on-line data transmission from the mmu and the css was not performed, and the data collected during the considered paths were locally stored on the mmu and then downloaded to css at the end of each path. the measurements were collected on suitable paths in the urban area of the city of bologna, italy, using a commercial vehicle. for the considered pollutants, figure 6 shows the results obtained in the (i) tests, performed during a period of about 60 hours. the results show a good agreement between the reference values and those obtained by means of the mmu, with a mean deviation always less than 10%. these results are in agreement with the metrological performances of the employed sensors. figure 7 and figure 8 show the considered pollutant distribution in the urban area using voronoi diagrams and kriging variograms, respectively. in both figures, the circles indicate the punctual measurement data acquired by means of the mmu and geo-referenced using the google map of the city. the results showed are computed as mean of seven different paths, performed in a period of one week of stable weather conditions (absence of wind and of rain). also the single-day and the single-path results are available, but they do not show substantial differences. in the figures 7 and 8, the red triangles indicate the positions of two traditional fixed measurement units. it is evident that the use of the proposed mobile measurement system is capable to highlight some urban areas in which the pollutant concentration is very high. using the traditional approach, in these areas the pollution information may be neglected or obtained by means of prediction models which could not assure the same accuracy of the punctual measurements. 8. conclusions  the paper proposes an efficient architecture for mobile systems involved in urban pollution monitoring. it is constituted by two main classes of devices: a mobile measurement unit (mmu) and remote nodes which in turn can be used both for control the mmu (and then the measurement settings) and download data collected by the mmu. the mobile measurement unit is made to be installed on public vehicles and is able to perform pollution monitoring during the usual vehicle service. the proposed measurement system is thought to provide the collected data to a suitable central supervision system which controls many measurement units and use the acquired data to perform a spatial pollution evaluation in the area of interest (as an example by means of suitable data fusion and/or interpolation techniques). the modular architecture of the proposed measurement system assures good feasibility and efficiency. in addition, the described solution is very attractive because allows exploiting the spatial and time capillarity of a typical public transportation system to achieve in an easy way a high amount of data (from the spatial and time points of view) useful also for improving the prediction models. with reference to the employed data elaboration techniques (voronoi diagrams and ordinary kriging variograms), the achieved results show a good agreement thanks to the high number of the punctual measurements carried out. however, further experimental activity has to be addressed in order to identify the best interpolation strategy related to the number and distribution of measured data and to the accuracy to be achieved. moreover, the use of many mobile measurement systems installed on different vehicles, which perform the same or different urban paths, can improve the whole measurement uncertainty using suitable data fusion techniques. this is the goal of the future research development, when many mobile measurement systems will be available. 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 13/12/10 18.00 14/12/10 0.00 14/12/10 6.00 14/12/10 12.00 14/12/10 18.00 15/12/10  0.00 15/12/10 6.00 15/12/10  12.00 15/12/10 18.00 16/12/10 0.00 16/12/10 6.00 16/12/10 12.00 mmu reference co [ppm]    0 10 20 30 40 50 60 13/12/10 18.00 14/12/10 0.00 14/12/10 6.00 14/12/10 12.00 14/12/10 18.00 15/12/10 0.00  15/12/10  6.00  15/12/10 12.00 15/12/10  18.00 16/12/10 0.00 16/12/10 6.00 16/12/10 12.00 mmu reference no2 [ppb]    0 5 10 15 20 25 30 35 13/12/10 18.00 14/12/10 0.00 14/12/10 6.00 14/12/10 12.00 14/12/10 18.00 15/12/10  0.00 15/12/10  6.00 15/12/10 12.00 15/12/10  18.00 16/12/10 0.00 16/12/10 6.00 16/12/10 12.00 o3 [ppb]  mmu reference figure 6. mmu validation test results.  acta imeko | www.imeko.org  july 2012 | volume 1 | number 1 | 83  references  [1] *** “health aspects of air pollution with particulate matter, ozone and nitrogen dioxide”, report on a who working group, bonn, germany, 2003. [2] k. d. zoysa and c. keppitiyagama, “busnet – a sensor network built over a public transport system”, proceedings of the 4th european conference on wireless sensor networks, 2007. [3] d. t. n. r. group, “dtn reference implementation v2.3.0.” 2006. [online]. http://www.dtnrg.org/docs/ code/dtn2 [4] f. gil-castineira, f. gonzalez-castano, r. duro, and f. lopezpena, “urban pollution monitoring throught opportunistic mobile sensor networks based on public transport”, proceedings of the ieee international conference on computational intelligence for measurement systems and applications, 2008. [5] weber, c., hirsch, j., perron, g., kleinpeter, j., ranchin, t., ung, a. and wald, l., “urban morphology, remote sensing and pollutants distribution: an application to the city of strasbourg, france”, international union of air pollution prevention and environmental protection associations (iuappa) symposium and korean society for atmospheric environment (kosae) symposium, 12th world clean air & environment congress, greening the new millennium, 26 – 31 august 2001, seoul, korea. [6] g. varela, a. paz-lopez, r. j. duro, f. lopez-pena1, f. j. gonzalez-castano, “an integrated system for urban pollution monitoring through a public transportation based opportunistic mobile sensor network”, ieee international workshop on figure 8. kriging variograms of the considered pollutants (the red triangles indicate the position of traditional fixed measurement systems).  figure 7. voronoi diagrams of the considered pollutants (the red triangles indicate the position of traditional fixed measurement systems).  acta imeko | www.imeko.org  july 2012 | volume 1 | number 1 | 84  intelligent data acquisition and advanced computing systems: technology and applications, 21-23 september 2009, rende (cosenza), italy. [7] v. cavalho,j.g. lopes, h.g. ramos, f. correa alegria, “citywide mobile air quality measurement system”, proceedings of ieee sensors 2009 conference, christchurch (nzl), 25-28 oct. 2009, pp. 546-551. [8] g. betta, d. capriglione, l. ferrigno, g. miele, “influence of wifi computer interfaces on measurement apparatuses”, ieee transactions on instrumentation and measurement. vol. 59, issue 12, dec 2010, pp. 3244-3252. [9] ferrigno l., paciello v.,pietrosanto a., "low-cost visual sensor node for bluetooth-based measurement networks," ieee transactions on instrumentation and measurement, vol. 55, no.2, april 2006, pp. 521527. [10] g. betta, d. capriglione, l. ferrigno, g. miele, “experimental investigation of the electromagnetic interference of zigbee transmitters on measurement instruments”, ieee transactions on instrumentation and measurement. vol.57, issue 10, oct 2008, pp. 2118-2127. [11] a. bernieri, d. capriglione, l. ferrigno, m. laracca, “a mobile measurement system for urban pollution monitoring”, proceedings of the xiii tc-4 imeko symposium and ix semetro, natal, brazil, september 2011. [12] directive 1999/30/ec of 22 april 1999 relating to limit values for sulphur dioxide, nitrogen dioxide and oxides of nitrogen, particulate matter and lead in ambient air. [13] directive 2000/69/ec of the european parliament and of the council of 16 november 2000 relating to limit values for benzene and carbon monoxide in ambient air. [14] [online] http://www.kanomaxusa.com/iaq/sensor/gas_sensor_head.html. [15] [online] http://www.advantech.com. [16] b. denby, v. garcia, f. holland, c. hogrefe, “integration of air quality modeling and monitoring for enhanced health exposure assessment”, special edition of em magazine “monitoring and modeling needs in the 21st century”. [17] b. denby, j. horálek, s.e. walker, k. eben, and j. fiala, “interpolation and assimilation methods for european scale air quality assessment and mapping, part i: review and recommendations, etc/acc technical paper 2005/7. [18] j. horálek, p. kurfürst, b. denby, p. de smet, f. de leeuw, m. brabec, j. fiala, “ interpolation and assimilation methods for european scale air quality assessment and mapping, part ii: development and testing new methodologies”, etc/acc technical paper 2005/7. microsoft word article 2 editorial.docx acta imeko december 2013, volume 2, number 2, 2 www.imeko.org acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 2 editorial paul regtien measurement science consultancy (msc), julia culpstraat 66, 7558 jb hengelo (ov), the netherlands section: editorial citation: paul regtien, editorial, acta imeko, vol. 2, no. 2, article 2, december 2013, identifier: imeko-acta-02 (2013)-02-02 editor: paolo carbone, university of perugia copyright: © 2013 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: paul regtien, e-mail: paul@regtien.net this issue of acta imeko comprises the second series of articles based on papers presented at the xx imeko world congress, busan, republic of korea, 9-12 september, 2012. this time acta imeko includes the extended and updated versions of papers from tc2 (photonics), tc11 (metrological infrastructures), tc18 (measurement of human functions), tc21 (mathematical tools for measurements) and tc22 (vibration measurement), illustrating the wide range of facets encompassing the field of metrology. the issue starts with five research papers on mathematical tools (tc21), followed by four papers about vibration measurements (tc22), two papers on photonics (tc2) and one article on measuring human functions (tc18). an essential aspect of metrology is the international cooperation with respect to standardization and calibration. acta imeko has introduced a section on technical notes, offering a position for contributions on metrological infrastructures and other important aspects of metrology, next to the section with regular research papers. the two technical notes in this issue are about the development of metrological infrastructures (tc11). we like to thank all those who have contributed to this issue: authors, reviewers, layout editors and journal managers. special thanks go to the section editors bernhard zagar (tc2), mladen borsic (tc11), yasuharu koike (tc18), alistair forbes and franco pavese (tc21) and gustavo ripper (tc22). we hope you enjoy this issue. microsoft word 288-2053-3-le-rev acta imeko  issn: 2221‐870x  april 2016, volume 5, number 1, 10‐14    acta imeko | www.imeko.org  april 2016 | volume 5 | number 1 | 10  analysis of cadmium and lead in honey: direct‐ determination by graphite furnace atomic absorption  spectrometry  andrea colabucci, anna chiara turco, maria ciprotti, marco di gregorio, angela sorbo, laura ciaralli   european union reference laboratory for chemical elements in food of animal origin (eurl‐cefao), department of food safety and  veterinary public health, italian national institute of health, v.le regina elena, 299‐ 00161 rome, italy       section: research paper   keywords: method validation; chemical elements; honey; atomic absorption  citation: andrea colabucci, anna chiara turco, maria ciprotti, marco di gregorio, angela sorbo, laura ciaralli, analysis of cadmium and lead in honey:  direct‐determination by graphite furnace atomic absorption spectrometry, acta imeko, vol. 5, no. 1, article 4, april 2016, identifier: imeko‐acta‐05  (2016)‐01‐04  section editor: claudia zoani, italian national agency for new technologies, energy and sustainable economic development affiliation, rome, italy  received july 31, 2015; in final form december 22, 2015; published april 2016  copyright: © 2016 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: this work has been carried out with the financial support of the sanco 2005/food safety/003‐residues program of the european commission.  the contents of this manuscript are the sole responsibility of the authors and can in no way be taken to reflect the views of the european commission.  corresponding author:  andrea colabucci, e‐mail: andrea.colabucci@iss.it    1. introduction  according to the task listed in article 32 of reg. 882/2004 [1] the eurl-cefao is responsible of the annual evaluation for group b3c (chemical elements) of the national residue control plans, that the european union member states (eu mss) have to perform in accordance with the requirements of directive 23/96/ec [2]. in general terms, these plans foresee the analyses of different food products in order to monitor the presence of unauthorised substances, residues of veterinary medicinal products and chemical contaminants that may represent a risk for public health. as for chemical elements, the eu mss usually choose to investigate those for which maximum levels (mls) are set in the relevant eu legislation (cr 1881/2006) [3] but other element/matrix combinations can be selected according to each ms control strategy. following this last approach the number of eu mss that include the analysis of chemical elements in honey had increased in 2012. in particular, according to the relevant eurl-cefao evaluation, cadmium and lead in honey were considered of interest by 26 eu mss although no ml for chemical elements in this matrix was set in eu legislation at that time. however, values at which samples are abstract  one of the emerging issues regarding the analysis of inorganic contaminants in food is the determination of cadmium (cd) and lead  (pb)  in honey. in fact, this kind of analysis is foreseen in most of the national residue control plans (nrcps) that european union  member states (eu mss) have to perform according to directive 96/23.  with  the  aim  to  provide  technical  support  to  the  eu  national  reference  laboratories  (nrls),  an  easy‐to‐use  method  on  direct  determination  (dd)  of  cd  and  pb  in  honey  by  graphite  furnace  atomic  absorption  spectrometry  (gf‐aas)  after  dissolution  in  an  aqueous mixture of 2.5 % hno3 (v/v) and 12.5 % h2o2 (v/v) was in‐house validated and distributed to the nrls network. analyses  were carried out on a batch prepared for the feasibility study of the eurl‐cefao 19th pt on honey. the validation levels (20 and 100  μg/kg for cd and pb, respectively) were chosen similar to those that would have been proposed for the pt.  the method proved its efficacy in terms of analytical performance; it appears to be low cost and time saving compared to a microwave  sample preparation.  in addition,  it also decreases the environmental  impact, being the amount of acid and reagents considerably  reduced.  acta imeko | www.imeko.org  april 2016 | volume 5 | number 1 | 11  rejected being non-compliant, namely action levels (als), were set for cd and pb in 14 eu countries. the other eleven mss that analyzed honey did not set or gave any indication on the levels considered. the als were spread all over the european union ranging from presence (few µg/kg) up to hundreds of µg/kg. according to the efsa report 2012 [4], the presence of lead represented the most important cause for sample rejection. furthermore, as the differences in rules adopted by the member states may hinder the functioning of the common market, the possibility of setting an harmonised maximum level for lead in honey was discussed at eu level and a ml was set in cr 1005/2015 [5], amending cr 1881/2006 [3], to be adopted as of 1 january 2016. as a consequence of this picture, the availability of analytical methods suitable to quantify chemical elements in honey becomes a key point for both the eu national reference laboratories and laboratories dealing with official controls. the lack of certified reference materials (crms) [6] as well as the scarcity of specific proficiency tests [7] make it difficult to assess the trueness and the performance of these methods. according to its tasks [1], the eurl-cefao organized in 2013 an interlaboratory comparison for its network on the determination of cadmium and lead mass fraction in honey making especially efforts to prepare an adequate material for this proficiency test [8]. in fact, from a physical point of view, honey is a high-viscosity liquid foodstuff containing a range of important nutritional complementary elements including a complex mixture of carbohydrates [9]. the difficulties in obtaining an homogeneous and stable material have never led to the effective production of a reference material certified for chemical elements [10], [11]. therefore, this proficiency test was particularly valuable because of the production of a material suitable to check the nrls capability in dealing with this matrix. as the basal content of cadmium and lead was found negligible in the honey used as start material, it was spiked with these chemical elements so as to obtain a mass fractions of ~ 0.02 mg/kg and ~ 0.10 mg/kg for cd and pb, respectively. these values were selected on the basis of a rough estimate of the als mean value but were also considered adequate to make the exercise as profitable as possible. furthermore, the eurl-cefao developed, validated and distributed among the nrls “ad-hoc” in-house methods to quantify cd and pb in honey so as to fulfil another’s key task, namely providing national reference laboratories with details of analytical methods. the validation was carried out with the two analytical techniques used in the network (icp-ms and gf-aas) around the values of mass fraction foreseen for the pt. this activity was of particular interest due to the intrinsic difficulty in the analysis of a high sugary matrix such as honey is. in particular for gf-aas two different sample preparation approaches were investigated: the first one was based on the microwave sample digestion (mw gf-aas) while the second one, consisting in a direct determination (dd gf-aas), was chosen on the basis of information gathered from a survey of the literature [9], [12]. the method validation was planned and conducted according to the eurl-cefao internal procedure based on the eurachem guide [13]. this paper describes the full “in house” method validation of the direct determination of pb and cd in honey by gf-aas and its efficacy and capabilities especially considering its lowcost and time saving peculiarities. 2. materials and instrumentations  all the steps of the material preparation were performed in the eurl-cefao facilities. the different types of honey were purchased at the retail store. in particular ~ 15 kg of the wildflower one were used for the screening, the production of the preliminary batch and the final pt items. moreover, other types of honey (chestnut, flowers, honeymoon, eucalyptus and melon) having different fluidity where bought to test the method and for screening purposes. all chemical reagents were at least of suprapure grade: hno3 67-69 % (v/v) (romil, cambridge, uk) and h2o2 30 % (v/v) (romil) to dissolve or digest the samples. the spiking solution and the calibrants were prepared from cd and pb elemental stock solutions of high purity grade at 1000 µg/ml in 2 % hno3 (volume fraction) (high-purity standards, charleston, usa directly traceable to srm 3100 series spectroscopic standard solutions) and diluted in high purity deionised water (resistivity 18 mω cm) (zeneer up 900 water purification system, human corporation, seoul, korea) up to the final concentrations. tfmtm polytetrafluoroethylene vessels and microwave ovens (ethos 1 and ethos-900, milestone, sorisole, italy) were used for the microwave assisted acid digestions. the mass fractions of the elements were determined by graphite furnace atomic absorption spectrometry with zeeman effect background correction (z-gf-aas aanalyst 800, perkinelmer, waltham, usa) using thga graphite tubes and electrodeless discharge lamps (cd and pb, perkinelmer). matrix modifiers were prepared starting from magnesium nitrate (mg(no3)2·(6h2o), suprapur merck, darmstadt, germany), ammonium dihydrogen phosphate (nh4(h2po4) suprapur merck) and palladium nitrate (pd(no3)2, perkinelmer sol. 1 % in 15 % hno3). an inductively coupled plasma mass spectrometer (icpms, elan drc ii, perkin elmer, waltham, usa) with cyclonic spray chamber and meinhard quartz concentric nebulizer (meinhard, golden, co, usa) was employed for screening and comparison purposes. minitab 15 statistical software was used for statistical scopes. the following equipments were used to prepare the preliminary batch and the final sample for the 19th eurlcefao pt: an ols200 shaking water bath (grant instruments, cambridge, uk) to warm up the jars in order to enhance the fluidity of honeys and to pasteurize the final samples; polypropylene beakers for food use (5 l) to collect the sample before homogenization; two technical balances (mettler-toledo, greifensee, switzerland 4200s and 16001l) and an analytical balance (mettler-toledo at 261 dr); a t 25 ultra-turrax high-speed homogenizer (ika-werke gmbh & co. kg, germany) equipped with a stainless steel anchor stirrer rod for liquids characterized by medium/high viscosity; a granite machine (fbm l (bras international spa, italy) for higher quantity of highly viscous liquids equipped with a dispenser. acta imeko | www.imeko.org  april 2016 | volume 5 | number 1 | 12  3. method validation  3.1. general scheme  a preliminary study on the possible interferences on the determination of cd and pb was carried out especially evaluating the occurrence of a matrix effect. linearity of the calibration curve used for quantification purposes, limit of detection (lod) quantification (loq), repeatability [14], intermediate precision and trueness were the parameters assessed during the validation process. in particular, the repeatability, lod and loq were calculated according to what stated cr 333/2007 [15], even if it is mandatory only for element/matrix combinations for which mls are set in cr 1881/2006 [3] and following amendments. 3.2. instrumental set‐up and preliminary studies  ashing and atomization curves were performed to allow the eurl to set the best temperatures in the furnace programmes: cd 700 °c1550 °c and for pb 600 °c1700 °c. the instrumental conditions are reported in table 1. these investigations, as well as the analysis foreseen for the validation, were performed using spare parts (e.g. graphite tubes) at about their half-life to better reproduce the typical working conditions of the laboratory. moreover, it is to be underlined, that even if a carbonaceous deposit may remain in the graphite tube, its working life is not sensibly reduced. the high matrix effect, observed for both elements, lead to analyze samples using the matrix matched calibration approach. this choice was also due to the necessity to overcome the problems raised from the different density between standard aqueous solutions and the sample resulting in an underestimation of both element mass fractions evidenced in screening experiments (ca. 20 %). moreover, matrix modifier concentrations were more diluted compared to those used when analyzing microwave digested samples and prepared in triton x 0.05 % to enhance the fluidity of the whole injected solutions. the calibration curves were constructed using 5 point (0 and base excluded) to cover all the concentrations of interest (0.25 – 2 and 2  20 µg/kg for cd and pb, respectively), diluting one part of the aqueous standard solutions in one part of basal honey (1:1). the curves were found linear (minimum correlation coefficient 0.9995) in the investigated range. these investigations, as well as the analysis foreseen for the validation, were performed using spare parts (e.g. graphite tubes) at about their half-life to better reproduce the typical working conditions of the laboratory. moreover, it is to be underlined, that even if a carbonaceous deposit may remain in the graphite tube, its working life is not sensibly reduced. 3.3. limits of detections and limits of quantification  lods and loqs were assessed according to the indications set in cr 333/2007 [15]. twenty-one samples from five different types of honey were analyzed. in particular, four samples of honeymoon, chestnut, eucalyptus, flowers each, and five of the wildflowers honey were spiked with a small amount of the analytes (0.1 µg/kg and 2 µg/kg for cd and pb respectively) so as to have a quantifiable analytical signal. the samples were then analysed for cd and pb content and the means and the standard deviations of each group were determined. the sample means were statistically compared by using the anova test and no significant differences were found considering an alpha of 0.05 (p-value = 0.173 and 0.582 for cd and pb, respectively) so the pooled standard deviation was used to calculate lod and loq for both cd and pb (figure 1). in particular, the pooled standard deviation was multiplied by 3 to derive the lods (0.8 µg/kg and 13 µg/kg for cd and pb, respectively) and by 10 to derive the loqs (2.7 µg/kg and 42 µg/kg for cd and pb respectively). the obtained values for these parameters fulfilled the internal requirements established by the eurl. 3.4. assessment of repeatability and intermediate precision    the method performance under repeatability condition [14] was evaluated analyzing a set of 10 samples (from the preliminary batch) versus two different calibration curves in the same day. as no suitable reference material (rm) with certified values of the analytes of interest was available, the mean values of the results (n=20) was compared with those obtained analysing the material both with gf-aas and icp-ms after microwave assisted sample digestion (mw). the use of the “surrogate” recovery (spiking of the sample with a known amount of the analytes before the digestion) was table 1: instrumental conditions for cd and pb.  parameter  cd  pb  ashing t (°c)  700  600  atomization t (°c)  1550 1700 volume injected (µl)  20  20 matrix modifier  nh4h2po4 0.06 g/l in  0.01% triton x  nh4h2po4 0.06 g/l +  mg(no3)2 0,05 g/l   in triton x 0.01%  calibration points  (µg/l)  0.25, 0.5, 1.0, 1.5, 2.0  2.5, 5, 10, 15, 20  calibration curve  method of addition  calibrate  method of addition  calibrate  integration mode  peak area  peak area  number of replicates  2  2  minimum sample  dilution  1:1  1:1  wavelength (nm)  228.8  283.3  background correction  zeeman effect  zeeman effect  lamp  electrodeless  discharge lamp  electrodeless  discharge lamp  lamp current (ma)  230  440      figure 1. test one‐way anova for pooled standard deviations.  acta imeko | www.imeko.org  april 2016 | volume 5 | number 1 | 13  discarded as considered of lesser significance. in fact, the “surrogate” recovery is effective when samples are quantified versus an external calibration (e.g. to verify if matrix effect occurs). in this method, samples were quantified versus a calibration curve prepared on spiked honey so that any interference found in the calibration step would probably similarly affect the quantification of the samples. the mean values of mass fraction calculated from the 20 results showed a good agreement among the different procedures and analytical techniques applied for both cd and pb as summarised in table 2. this accordance obtained in the different sample preparation/analytical technique combinations as well as the foreseen mass fractions, given by the sum of the incurred and the spiked mass fractions, were considered good evidences to judge the methods as “fit for the purpose”. a further proof that can also be considered is the good recoveries obtained (96 % for cd and 102 % for pb) on the material used for the eurl-cefao 17th pt on infant formula analyzed by icp-ms during the repeatability assessment. this matrix was chosen considering its similarity honey in nutritional components, as reported in the nist triangle for food equivalence. the values of the relative standard deviations were compared with the horratr value [16]. the criterion to be < 1, prescribed in the eurl-cefao procedure, was met. moreover it was found that, using dd gf-aas, lower standard deviations were observed compared to those obtained when analysing the samples after microwave assisted acid digestion; this effect was probably due to the minimum handling during sample preparation for analysis. the intermediate precision of the method [14] was evaluated as described: six independent aliquots of the material used to assess the repeatability underwent the afore-mentioned sample treatment in two different periods of time (april 2014 and july 2014) and then were analysed for the content of cd and pb. the results coming from each set of analysis (n=12) were combined with the first six results obtained during the repeatability assessment to calculate the standard deviation used to derive the parameters obtained in intermediate precision. as prescribed in the internal procedure, they were collected in a period of time longer than 5 months. in particular, the intermediate precision conditions were simulated by changing the operator in the second run, while another atomic absorption spectrometer (same model and brand) was employed by the first user in the third analytical run. 3.5. certainty estimate  the estimate of bias, that is the systematic measurement error, is a crucial parameter to assess the uncertainty of a measurement method. the analytical result given by a laboratory needs a “true” value to be compared with and this can be done using a certified reference material. as previously reported, no crm for chemical elements in honey was available on the market. for this reason accuracy was evaluated versus the assigned value of the eurl-cefao 19th pt on honey, obtained by consensus from the participants (national reference laboratories of the european union) according to iso 13528 [17]. this approach was chosen bearing in mind that the low value of uncertainty of the material, that does not take into account all the parameters considered when dealing with a crm or a rm [18], [19], could lead to an underestimation of the method uncertainty. for this reason the contributions of the non-significant bias and instrumental drift were included in the uncertainty budget. it is also to be underlined that the consensus values were, anyway, obtained from the results of outstanding laboratories (eu nrls). moreover the assigned values (0.0199 mg/kg and 0.102 mg/kg for cd and pb, respectively) were in full agreement with the grand mean of the results obtained by the eurl-cefao (0.0201 mg/kg and 0.106 mg/kg for cd and pb, respectively) when assessing the sufficient homogeneity of the material [8] according to the international harmonized protocol [20]. according to the internal procedure based on the eurachem guide for quantifying uncertainty [21], two sets of 4 samples each were analysed together with the sets for the intermediate precision assessment and a third set in september 2014 to cover a time interval longer than 5 months. the mean of the values obtained for cd and pb for the twelve samples was considered. a test for the significance of the recovery was performed using the student test. the tcalc from (1) was compared with 2-tailed critical values for n-1 (i.e. 11) degrees of freedom at 95 % confidence (tcrit = 2.20) using the average recovery (rec) and its standard uncertainty (u ): (1) the tcalc both for cd (1.71) and for pb (0.97) were lower than the tcrit = 2.2. even if the comparison between tcalc and tcrit showed that the bias was not significant, the contribution of the bias was however included in the uncertainty budget [22]. the combined uncertainty was estimated taking into account the figures obtained in intermediate precision, bias and instrumental drift according to the following equation: (2) sr and cr are the standard deviation and the mean value of the measurements in intermediate precision conditions, and m is the number of replicates analyzed in routine analysis (three in the eurl-cefao procedure). the drift (continuous change in an indication, related neither to a change in the quantity being measured nor to a change of any recognized influence quantity of the instrument [14] was also included. the uncertainty of the bias was estimated taking into account the figures obtained in intermediate precision condition (mean value, bias, standard deviation): (3) the maximum uncertainties associated, using 2 as a coverage factor [15], were 10 % and 12 % for cd and pb respectively, complying with the maximum allowable value set in cr 333/2007 [15]. the figures of merit are reported in table 3. table  2:  results  obtained  under  repeatability  conditions  using  gf‐aas  dd, gf‐aas mw and icp‐ms mw .   cd  gf‐aas dd  gf‐aas mw  icp‐ms mw meanr (µg/kg)  20.9  21.1  21.5  sr (µg/kg)  0.3  0.5  0.6 rsdr (%)  1.4  2.4  2.6  r (µg/kg)  0.8  1.4  1.7 pb  gf‐aas dd  gf‐aas mw  icp‐ms mw  meanr (µg/kg)  115  112  113  sr (µg/kg)  2  4  3  rsdr (%)  1.7  3.6  2.7 r (µg/kg)  6  11  8  tcalc |1 rec| urec uc c x sr √m cr 2 ubias cbias 2 driftmax √m 2     bias mrm 2 srm mrm √m 2 ubias crm 2   acta imeko | www.imeko.org  april 2016 | volume 5 | number 1 | 14  4.  conclusions  method validation was carried out on an in-house prepared honey sample with ad-hoc mass fractions of cd and pb. the long standing experience as producer of pt test items permitted the eurl-cefao to obtain a suitable material, whose homogeneity and stability for the time necessary for the validation, were verified according to internal procedures based on iso/iec 13528. validation levels were chosen taking into account a rough mean of the action levels set by the eu mss in their 2012 national residue control plans for these element/matrix combinations. this choice has proved to be of particular relevance considering the recent introduction of a maximum level for lead (cr 1005/2015 amending cr 1881/2006) in this matrix at a value consistent with the mass fraction used in the eurl method validation. the preliminary results produced applying the direct determination method were in good agreement with those obtained by icp-ms and gf-aas after mw sample digestion. the minimum sample handling also allowed the dd gf-aas method to provide high quality data either under repeatability or intermediate precision conditions (table 1). moreover, notwithstanding the lack of a proper certified reference material, it was possible to verify the capability of this method, in terms of accuracy, versus leftover test items material employed for the eurl-cefao 19th pt on honey. as reported in table 3, the good recovery for both the elements (103.5 % and 102.9 % for cd and pb, respectively) versus the consensus values produced by outstanding laboratories (nrls of the eu mss) is a further proof of the appropriateness of the method performance. this outcome demonstrates the suitability of the method for the direct determination of cd and pb in honey by graphite furnace atomic absorption spectrometry in terms of capability and precision. its main strong point is the minimum sample handling resulting in a very low cost, time saving and more “eco-friendly” process. acknowledgement  authors thank guendalina fornari luswergh for editorial assistance on the manuscript. references  [1] regulation (ec) no 882/2004 of the european parliament and of the council of 29 april 2004 on official controls performed to ensure the verification of compliance with feed and food law, animal health and animal welfare rules. oj l 165: 1–141. [2] council directive 96/23/ec of 29 april 1996 on measures to monitor certain substances and residues thereof in live animals and animal products and repealing directives 85/358/eec and 86/469/eec and decisions 89/187/eec and 91/664/eec. oj l 125:10–32. [3] commission regulation (ec) no 1881/2006 of 19 december 2006 setting maximum levels for certain contaminants in foodstuffs. oj l 364:5–24. [4] european food safety authority, 2014. report for 2012 on the results from the monitoring of veterinary medicinal product residues and other substances in live animals and animal products. efsa supporting publication 2014:en-540. 65 pp. [5] commission regulation (eu) no 2015/1005 of 25 june 2015 amending regulation (ec) no 1881/2006 as regards maximum levels of lead in certain foodstuffs. oj l 161:9-13. [6] http://www.comar.bam.de/en [7] https://www.eptis.bam.de/en/index.htm [8] l. ciaralli, a. c. turco, m. ciprotti, a. colabucci, m. di gregorio, a. sorbo. honey as a material for proficiency testing accred. qual. assur., doi 10.1007/s00769-015-1119-2. [9] m.d. ioannidou, g.a. zachariadis, a.n. anthemidis, j.a. stratis. direct determination of toxic trace metals in honey and sugars using inductively coupled plasma atomic emission spectrometry, talanta (2005); 65(1): 92-97. [10] s. caroli, g. forte, m. alessandrelli, r. cresti, m. spagnoli, s. d'ilio, j. pauwels, g.n. kramer. a pilot study for the production of a certified reference material for trace elements in honey, microchemical journal (2000); 67(1–3): 227-233. [11] g .forte, s. d'ilio, s. caroli. honey as a candidate reference material for trace elements, journal of aoac international (2001); 84(6): 1972-1975. [12] p. viñas, i. lópez-garcía , m. lanzón, m. hernández-córdoba. direct determination of lead, cadmium, zinc, and copper in honey by electrothermal atomic absorption spectrometry using hydrogen peroxide as a matrix modifier, j. agric. food chem. (1997); 45(10): 3952–3956. [13] eurachem guide: the fitness for purpose of analytical methods. a laboratory guide to method validation and related topics, 1st edition, 1998. [14] international vocabulary of metrology – basic and general concepts and associated terms (vim), 3rd edition, 2012. [15] commission regulation (ec) no 333/2007 of 28 march 2007 laying down the methods of sampling and analysis for the official control of the levels of lead, cadmium, mercury, inorganic tin, 3mcpd and benzo(a)pyrene in foodstuffs. oj l 88:29–38. [16] w. horwitz, r. albert. the horwitz ratio (horrat): a useful index of method performance with respect to precision. j. aoac int., (2006); 89(4): 1095-1109. [17] iso/iec 13528 (2005) statistical methods for use in proficiency testing by interlaboratory comparisons. international organization for standardisation, geneva, switzerland. [18] iso/iec 34 (2009) general requirements for the competence of reference material producers. international organization for standardisation, geneva, switzerland. [19] iso/iec 35 (2006)reference materials-general and statistical principles for certification. international organization for standardisation, geneva, switzerland. [20] m. thompson, s.l.r. ellison, r. wood, international harmonized protocol for the proficiency testing of analytical chemistry laboratories. pure appl. chem. (2006); 78(1): 145-196. [21] eurachem/citac guide cg 4 “quantifying uncertainty in analytical measurement, third edition, 2012. [22] b. magnusson, s. l. r. ellison, treatment of uncorrected measurement bias in uncertainty estimation for chemical measurements analytical and bioanalytical chemistry (2008); 390(1): 201-213. table 3: figures of merit of the method.  parameter  cd  pb  validation level (µg/kg)  20  100  sr (µg/kg)  0.3  0.002  rsdr (%)  1.4  1.7  r (µg/kg)  0.8  0.006  sr (µg/kg)  0.2  0.004  rsdr (%)  1.0  3.5  r (µg/kg)  0.6  0.011  lod (µg/kg)  0.8  13  loq (µg/kg)  2.7  42  recovery (%)  103.5  102.9  u (µg/kg)  1  6  u (%)    5  6  u (µg/kg)  2  12  u  (%)  10  12  microsoft word article 2 editorial 189-1056-2-ga.docx acta imeko may 2014, volume 3, number 1, 2 – 3 www.imeko.org acta imeko | www.imeko.org may 2014 | volume 3 | number 1 | 2 introductory notes for the acta imeko special issue on “historical papers on fundamentals of measurement” l. mari school of industrial engineering, università carlo cattaneo – liuc, c.so matteotti, 22 21053 castellanza (va) – italy section: editorial keywords: measurement theory, uncertainty, history citation: l. mari, introductory notes for the acta imeko special issue on “historical papers on fundamentals of measurement”, acta imeko, vol. 3, no. 1, article 2, may 2014, identifier: imeko-acta-03 (2014)-01-02 editor: paolo carbone, university of perugia received may 1st, 2014; in final form may 1st, 2014; published may 2014 copyright: © 2014 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: l. mari, e-mail: lmari@liuc.it the term “acta imeko” was originally used to designate the proceedings of the imeko world congresses, and explicitly the first twelve congresses from budapest, hungary, 1958, to beijing, china, 1991, held in a period of about thirty crucial years for our scientific, technological, social, and political history. this special issue of the (new) acta imeko journal offers the reader a small selection of papers on fundamentals aspects of measurement, that were originally published in those proceedings. it is an opportunity, first of all, to pay a tribute to the many distinguished colleagues that excellently contributed to the development of measurement science in the context of imeko, but also to celebrate imeko in its history and unique role in the promotion of international sharing of knowledge and collaboration among scientists. despite the common feeling that we live in an epoch of extraordinary, and extraordinarily rapid, changes, the papers that have been reissued here mainly maintain their scientific significance, for the outstanding visions and analyses that propose, while their most temporally situated positions are often of great interest as lively witnesses of what the state and the perspectives of measurement science were a few decades ago. a chronological study on the (original) acta imeko might be conducted, for example, on the changes induced on measurement by the progressive diffusion of digital devices and numerical methods, and the corresponding formation of the concept of measuring systems as information machines. unmodified significance of the basic proposals and historical interest of the foreseen perspectives: these have been the guiding principles in the selection of the papers for the present special issue, all of them sharing an interest in contributing to the development of a well-structured, systemic interpretation of the fundamentals of measurement (the selection has been limited by the unfortunate fact that not all volumes of the proceedings were available in the selection stage). in the present social and economic situation more and more emphasizing the role and the importance of measurement processes, identifying the pivotal elements of a culture of measurement becomes a critical task for metrology, the “science of measurement and its application” (as the international vocabulary of metrology defines it), possibly formalized in the development of a body of knowledge on measurement science. the diachronic perspective offered by these papers might give some contributions in highlighting continuities and discontinuities in fundamental concept formation in the last fifty years of measurement-related scientific and technical history. the reader might be interested to know that the current default choice of english as the only language for the published papers is relatively recent in the context of the imeko community: in the first fifteen years of acta imeko, multiple options were available as for the language of papers (and the proceedings of the first two congresses, budapest, hungary, 1958 and 1961, include texts in five languages, english, french, german, hungarian, and russian!). on the other hand, only english written papers have been selected for this special issue. the reissued papers are identical in contents to the originals (having thus accepted lack of homogeneity in the layout, abstract and keywords, sometimes absent), and they have only imeko dedicates this special issue to 2014 world metrology day, that celebrates the signature of the metre convention on may 20th 1875 by representatives of seventeen nations acta imeko | www.imeko.org may 2014 | volume 3 | number 1 | 3 been edited to make them better readable within web pages and to fix the few material errors that have been found. the order in which they are presented is chronological. the role of measurement in the development of industrial automation, by s.s. carlisle [acta imeko 1967, proceedings of the 4th international measurement congress, 1967, warsaw, vol. 1, pp. 37–50], is far-sighted paper that sets a frame on the role of measurement and control as critical tools to support the automation of manufacturing processes. in this perspective the author discusses three main requirements for measurement, i.e., to identify where automation can be most profitably used; to investigate individual process behaviors and hence to formulate process control strategies; and finally to perform quality control of products. also with the support of several practical examples, with interesting mentions to the growing importance of digital computer and pattern recognition techniques, the author proposes a systemic vision, in which measurement is convincingly intended as a communication enabler among the different subjects involved in industrial innovation processes: control engineers, process and product engineers, managers, and end users. fundamental concepts of measurement, by l. finkelstein [acta imeko 1973, proceedings of the 6th congress of the international measurement confederation, “measurement and instrumentation”, 17-23.6.1973, dresden, vol. 1, pp. 11–27], proposes a clear, synthetic presentation of some basics of measurement science. while in the last forty years many things have changed in the technological aspects of measurement (so that, in particular, what the author calls “associative measurement” is much more widespread than one might suppose from the short mention that is devoted here), the conceptual framework that follows is an excellent entry point for the reader interested in the epistemological and logical foundations of measurement. the measurement chain and validation of experimental measurements, by r.j. moffat [acta imeko 1973, proceedings of the 6th congress of the international measurement confederation, “measurement and instrumentation”, 17-23.6.1973, dresden, vol. 1, pp. 45–53], witnesses the sophisticated discussion that, well before the publication of the guide to the expression of uncertainty in measurement (gum), was active in the measurement science community around the subject of error and uncertainty, and its consequences on the structure of the measuring process and the way it is performed. some of the ideas presented here, and particularly the hypothesis that what the gum calls the standard uncertainty admits multiple methods of computation depending on the aim of the process, are still delicate, open issues today. problems in theory of measurement today, by l. gonella [acta imeko 1979, proceedings of the 8th imeko congress of the international measurement confederation, “measurement for progress in science and technology”, 21-27.5.1979, moskow, vol. 1, pp. 103–110], challenges, in an informed and convincing way, some basic presuppositions of the “classical” theory of measurement, particularly highlighting the biases induced by an often acritical adoption of the geometric paradigm, and consequently of real numbers, in measurement. theoretical, physical and metrological problems of further development of measurement techniques and instrumentation in science and technology, by d. hofmann and yu. v. tarbeyev [acta imeko 1979, proceedings of the 8th imeko congress of the international measurement confederation, “measurement for progress in science and technology”, 21-27.5.1979, moskow, vol. 3, pp. 607–626], lists a number of issues of measurement science and technology, thus interestingly witnessing their state in the second half of the 20th century. measurement and meaningfulness, by j. van brakel [acta imeko 1985, proceedings of the 10th world congress of the international measurement confederation, “new measurement technology to serve mankind”, 22-26.4.1985, prague, pp. 319– 333], is a learned paper on a key concept in measurement, i.e., the meaningfulness of statements reporting measurement results, whose connections, among the others, to scale invariance, conditions for derived measurement, dimensional analysis, and definition of measurement standards are clearly presented and discussed also by means of several examples. structured instrument design, by p.h. sydenham [acta imeko 1988, proceedings of the 11th triennial world congress of the international measurement confederation (imeko), “instrumentation for the 21st century”, 16-21.10.1988, houston, pp. 271–275], proposes an overview on the delicate subject of the possibility of automatic design and configuration of measuring systems (“cad of instruments”, or “instrument cae”), in a well-conceived scenario of measuring systems intended as an important class of information machines. despite in those early days of computer-aided processes the position, sometimes made into a research plan, was not unusual that the creative activity of development can be fully automatized (as in the case of the efforts towards universal software code generators), the author is well aware of the importance not only of formal algorithms, but also of heuristics, “rules of thumb”, and even subjective judgement, critical source of complexity for the purported target. impact of modern instrumentation on the system of basic concepts in metrology, by j.m. jaworski, j. bek, and a. j. fiok [acta imeko 1988, proceedings of the proceedings of the 11th triennial world congress of the international measurement confederation (imeko), “instrumentation for the 21st century”, 16-21.10.1988, houston, pp. 125–134], offers several analytical arguments on the practical importance of rethinking the basics of measurement science as a concept system, in response to the critical increase of complexity of measurement problems in both spatial and temporal dimensions, i.e., relating to multivariate, dynamic quantities. the authors significantly emphasize the role of models and of pragmatics, including the costs of designing and performing the process. intelligent instrumentation: a quality challenge, by h.j. kohoutek [acta imeko 1988, proceedings of the proceedings of the 11th triennial world congress of the international measurement confederation (imeko), “instrumentation for the 21st century”, 16-21.10.1988, houston, pp. 337–345], presents a systematic analysis of the consequences of the adoption of artificial intelligence techniques in measuring systems and the related open issues. the author emphasizes that the growing importance of software components offers new opportunities for the development of flexible, “intelligent” instrumentation, but at the same time poses new problems in the metrological assurance of their behavior. our hope is that, together with the pride and the curiosity that this small sample can produce on imeko and its history, the reader can find here new reasons of interest for measurement science and its fundamental role for our society. microsoft word 20-142-1-pb.doc acta imeko  july 2012, volume 1, number 1, 49‐58  www.imeko.org    acta imeko | www.imeko.org  july 2012 | volume 1 | number 1 | 49  a two‐stage, guarded inductive voltage divider with small  ratio errors for coaxial bridge applications  gregory a. kyriazis 1 , j. angel moreno 2 , jürgen melcher 3   1  instituto nacional de metrologia, qualidade e tecnologia, av. nossa senhora das graças, 50, 25250‐020, duque de caxias, rj, brazil  2  centro nacional de metrología, km 4.5 carretera a los cués, municipio del marqués, cp 76900, querétaro, mexico  3  physikalisch‐technische bundesanstalt, bundesallee 100, 38116 braunschweig, germany    keywords: capacitance; coaxial bridges; inductive voltage dividers; traceability chain; calibration; metrology  citation: gregory a. kyriazis, j. angel moreno, jürgen melcher, a two‐stage, guarded inductive voltage divider with small ratio errors for coaxial bridge  applications, acta imeko, vol. 1, no. 1, article 11, july 2012, identifier: imeko‐acta‐01(2012)‐01‐11  editor: pedro ramos, instituto de telecomunicações and instituto superior técnico/universidade técnica de lisboa, portugal  received january 9 th , 2012; in final form may 17 th , 2012; published july 2012  copyright: © 2012 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: this work was supported by instituto nacional de metrologia, qualidade e tecnologia (inmetro), brazil  corresponding author: gregory a. kyriazis, e‐mail: gakyriazis@inmetro.gov.br  1. introduction  the traceability chain to derive the capacitance unit from the quantized hall resistance comprises a few coaxial bridges: a 1:1 ratio bridge, a 10:1 ratio bridge, a quadrature bridge and an inductive voltage divider (ivd) calibration bridge. in fact only three bridges are necessary as a 1:1 ratio bridge is easily changed to a 10:1 ratio bridge by a cable rearrangement. all these bridges can use the same isolation transformer and main ivd [1]-[3]. the main ivd to be used in coaxial ratio bridges [4] should be constructed to provide an overall bridge uncertainty at 1:1 and 10:1 ratios (at 1 khz and 1.592 khz) of a few parts in 108. this requires the use of special guarding and two-stage techniques typically adopted in the 10-100 khz range [5]. a similar ivd had been constructed previously during the development of inmetro´s two terminal-pair coaxial capacitance bridge [6][7]. this bridge has been in operation since 2005. the old ivd design details and calibration results had been reported in [8]. the new ivd discussed here was completely constructed at inmetro and presents much smaller ratio errors than the previous one. this work benefitted from the technical expertise of centro nacional de metrología (cenam) and physikalishtechnische bundesanstalt (ptb). the coaxial capacitance bridge construction details are reviewed in section 2. the corresponding measurement model and the cable effects are respectively analyzed in sections 3 and 4. section 5 presents the construction details of the new ivd and the design changes that we believe were responsible for the results obtained. the ivd calibration is discussed in section 6. the old ivd was replaced by the new one in the coaxial capacitance bridge and the overall bridge uncertainty was reevaluated as reported in section 7. the conclusions are drawn in section 8. 2. two terminal‐pair coaxial capacitance bridge  the bridge design is originally from ptb and is similar to that adopted by cenam [9]. both inmetro and cenam participated successfully with similar bridges in an international comparison on capacitance (sim.em-k4, sim.em-s4 and sim.em-s3 [10]) among several national metrology institutes from the inter-american metrology system (sim). the capacitance bridge uses a two terminal-pair coaxial design [4]. one way of looking at a coaxial bridge is to see it as two superposed networks (figure 1). the first of these consists of straightforward meshes of components and the interconnecting wires between them. the second network comprises the shields of the components and the outer, coaxial shield of the connecting cables. the configurations of the two the traceability chain to derive the capacitance unit from the quantum hall resistance comprises some coaxial bridges. these bridges  employ  a  main  two‐stage  inductive  voltage  divider  to  provide  the  voltage  ratio  needed.  one  such  divider  has  been  recently  constructed and calibrated at inmetro. the design techniques responsible for the small ratio errors of the device and the calibration  method employed are both detailed. the new divider was  installed  in  inmetro´s two terminal‐pair coaxial capacitance bridge with  significant improvements in the bridge resolution and accuracy. acta imeko | www.imeko.org  july 2012 | volume 1 | number 1 | 50  networks are identical and by providing every mesh with an equalizing device, the current in the outer shield is constrained to be equal in magnitude and shifted 180° to the current in the components and central conductors. the current in any cable as a whole is zero and no external magnetic field is created. the second network of shields and cable outer conductors has low impedance and it is all at nearly the same potential, so that there is no significant external electric field. this construction has the further advantage that such networks do not respond to fields from external sources. otherwise, the bridge balance conditions could be affected. current equalization is achieved by threading a coaxial cable through a high permeability (typically supermalloy) toroidal core so that core and cable act as a 1:1 transformer [11][12]. usually, the impedance of the outer conductor is several orders of magnitude smaller than the impedance of the inner conductor (which includes impedance standards, transformers, etc.). a current equalizer has therefore no direct effect on the current flowing along the inner conductor; it just gives rise to an equal current flowing along the outer conductor which is supplied from the same source as the inner conductor. to achieve equal return currents in the whole bridge network, every independent mesh of the bridge network must be provided with one current equalizer in a way that every bridge component is connected to the central ground point of the bridge with just one path without current equalizer. this central point is connected to earth ground. a simplified scheme of a two terminal-pair coaxial bridge for comparing two standard capacitors of the same nominal value (cn and cx) is shown in figure 1. cx is the capacitor under calibration and cn is the reference standard. the current equalizers are also shown in the figure. the main ivd operates with a voltage u across the taps. its actual ratio deviates from the nominal 1:1 ratio by a small complex amount. further, the two capacitances are not ideal and do not have precisely the same value. the complex voltage ud detected at the node between the two capacitances is therefore not null. the real part of this voltage is then compensated by injecting an adjustable in-phase current. this is done by applying an adjustable voltage u/ to capacitor c. likewise, the imaginary part is compensated by injecting an adjustable quadrature current. this is done by applying an adjustable voltage u/ to conductance g. when the detector voltage is nullified in this way, we obtain the in-phase balance equation    x n 1 2 1 2 c c c c             , (1) where c  is the parasitic capacitance (not shown in the figure) that shunts the conductance g. the dividing factors 1 and 1 are obtained by balancing the bridge with the capacitors cn and cx positioned as shown in figure 1, and 2 and 2 are obtained by rearranging the cables so that cn and cx are interchanged. the complex ratio error of the main ivd is cancelled by this technique. the constructive details of the bridge built at inmetro are discussed in the sequel. the detailed scheme of the capacitance bridge is shown in figure 2. the current equalizers are shown as black rectangles figure 2. detailed scheme of the coaxial capacitance bridge (original design from ptb).  figure  1.  simplified  scheme  of  the  two  terminal‐pair  coaxial  capacitance bridge.  acta imeko | www.imeko.org  july 2012 | volume 1 | number 1 | 51  (for clarity, we preferred not to draw the coaxial cable shields). the two terminal-pair coaxial ratio bridge operates at 1 khz and 1.592 khz and compares decadic capacitors in the range from 10 pf to 1 nf at 1:1, 10:1 and 1:10 ratios. the scheme of the 1:1 ratio bridge is shown in figure 2. a 1:1 ratio bridge is easily changed to a 10:1 ratio bridge by a simple cable rearrangement (the corresponding scheme of the 10:1 ratio bridge is shown in [6]). the bridge comprises an isolation transformer, a two-stage main ivd, a voltage predivider, a t-network box, a switch box, a short-circuit box, and a wagner current transformer. the following commercial equipment complement the bridge: an ultra-pure sinusoidal oscillator, a wideband power amplifier, a low-noise preamplifier, a lock-in amplifier, a 1 pf fused-silica standard capacitor and two coaxial, double, six-decade ivds. a photo of the capacitance bridge is shown in figure 3. the isolation transformer is used to isolate the bridge from the mains. the two-stage main ivd is constructed to provide an uncertainty in 1:1 and 10:1 ratios of a few parts in 108. both the isolation transformer and the main ivd have a flexible design that allows their use in other bridges of the aforementioned traceability chain. the specific design and construction details of these devices were published in [13][8]. the power amplifier is cascaded to the sinusoidal oscillator to apply the voltage required to the isolation transformer primary taps, so that up to u = 200 v is available across the +5 and –5 secondary taps (worst case when two 10 pf capacitors are compared for maintenance of the unit). the voltage u depends on the bridge ratio and the standard capacitor value (see table 2 in the next section). table 1 lists the values of voltage and frequency which are recommended for the whole chain from 10 pf to 1 nf. the voltages applied to the standards decrease by a factor of 10 for each 10:1 ratio step in the chain avoiding corrections due to voltage dependence of the standards (that of the ivd ratio error is negligible). the transition in frequency from 1 khz to 1.592 khz is necessary for the traceability chain to derive the capacitance unit from the quantum hall resistance. it is made with the standard which has the lowest frequency dependence, namely, the 1 nf capacitor. the voltage choice ultimately means that the current flowing through the standards being compared is always kept at approximately 6.3 a at 1 khz (or 10 a at 1.592 khz). the main balance voltage is derived from the voltage across the +1 and 0 taps of the isolation transformer secondary winding and reduced by the 10:1 voltage predivider. the resulting voltages at the 0.1 taps of the voltage predivider are applied to variable impedances (1 pf fused silica capacitor and a t-network box cascaded to coaxial, double, six-decade ivds), thus generating the in-phase and quadrature main balance currents, respectively. the t-network box is built with metalfilm resistors. the short-circuit box is a multiport connector in which the inners are brought together at a point, as are the outers. a suitable construction is illustrated in [4] where the inners are soldered symmetrically to a disc. the short-circuit box receives the currents flowing through the standards being compared and the in-phase and quadrature main balance currents. it has also figure 3. inmetro´s two terminal‐pair coaxial capacitance bridge.  table 1. voltage applied to standard capacitors.   standard  (pf)  frequency  (hz)  voltage  (v)  10  1000  100  100  1000  10  1000  1000 and 1592  1  acta imeko | www.imeko.org  july 2012 | volume 1 | number 1 | 52  an output port for voltage monitoring. the main balance is adjusted by monitoring the voltage at the short-circuit box while changing the impedances connected to the 0.1 taps of the voltage predivider. the low-noise preamplifier is cascaded to the lock-in amplifier to monitor the balance voltage. with the preamplifier gain set to 103, we have been able to balance the bridge so that the lock-in amplifier readings are within 20 v. the isolation transformer has a separate secondary winding for the wagner balance [13]. this balance is done first (the battery-operated switch box is used to select the balances to be monitored). we have verified that this balance is crucial to the accuracy of the main balance. the voltage across the 1 w taps of this separate winding is applied to coaxial, double, sixdecade ivds with selectable fixed impedances and the generated current is injected in the isolation transformer zero tap connection to ground through an 100:1 injection transformer (installed inside the isolation transformer). this changes the potential between the zero tap and ground. the current in tap terminal 5 of the main ivd is changed accordingly. the wagner balance is adjusted by monitoring the current in tap terminal 5 with the 1:100 wagner current transformer while changing the settings of the wagner balance ivds. we have been able to balance the wagner network so that the lock-in amplifier readings are within 5 v. 3. measurement model  the simplified scheme of the capacitance bridge is shown in figure 4. the main ivd operates with a voltage u across the 0 and 10 taps for the 1:1 ratio or across the 0 and 11 taps for the 10:1 (or 1:10) ratio (see figure 2). the voltage value depends on the bridge ratio and the standard capacitor value (table 2). the nominal ratio of the main ivd is d = 1/2 for the 1:1 ratio bridge and d = 1/11 for the 10:1 (and 1:10) ratio bridge. the value of 1/ ratio depends on the bridge ratio and the voltage predivider ratios (table 3). for instance, for a 1:1 ratio bridge, the isolation transformer ratio at its +1 secondary tap is 1/10. the voltage predivider ratio at its +0.1 tap (typically used) is 1/10. the value of 1/ ratio in this case is therefore 1/100. the resulting dividing factors of the bridge main balance are  = (a  0.5)/0.5 and  = (b  0.5)/0.5, where a and b are the settings of each coaxial six-decade main balance ivd. cn (and gn) is the known capacitance (and conductance) of the standard capacitor, and cx (and gx) is the unknown capacitance (and conductance) of the capacitor under calibration. c is the 1 pf fused-silica standard capacitor. g and c  are the conductance and parasitic capacitance, respectively, of the t-network box (with connecting cables), measured with a commercial capacitance bridge. the ratio error is here expressed as a fraction of unit. in this case, the ivd ratio is expressed as a sum of the nominal ratio d and the complex ratio error  = k + jk (k and k are respectively the in-phase and quadrature components), that is 0.1 0 1.1 0u u d d k jk        . (2) the main balance is obtained when n x b m 0   i i i i . (3) the balance equation for the 1:1 ratio is given by (1), which is reproduced here for the reader convenience,    x n 1 2 1 2 c c c c             . (1) the dividing factors 1 and 1 are obtained by balancing the bridge with the capacitors positioned as shown in figure 4, and 2 and 2 are obtained by rearranging the cables so that the capacitors are in the reversed position. as mentioned before the complex ratio error of the main ivd is cancelled by this technique. however, it is interesting for quality control purposes to compute the in-phase and quadrature components of the ratio error of the main ivd for the 1:1 ratio, that is       n x n 1 2 1 2 1 2 1 4 4 c c k c c g k c                       (4) where  is the angular frequency. the corresponding balance equation for the 10:1 ratio is  x n 1 1 gtc c t        , (5a) table 2. total applied voltage u.   standard  (pf)  bridge  ratio  voltage  (v)  10  1:1  200  10  10:1  110  100  1:1  20  100  10:1  11  1000  1:1  2  table 3. value of .   predivider ratio  bridge  ratio   10:1  1:1  100  10:1  10:1  110  1:1  1:1  10  1:1  10:1  11  figure 4. simplified scheme of the capacitance bridge.  acta imeko | www.imeko.org  july 2012 | volume 1 | number 1 | 53  where       n n n n tan 1 1 1 g c c k c c k t d k c c t c c t k g t t c                                               (5b) where tan n is the loss tangent of the standard capacitor. here it is neither possible nor necessary to use the aforementioned cabling reversal technique. one needs however to know accurately the in-phase ratio error of the main ivd for the nominal ratio d = 1/11. thus, the main ivd needs to be calibrated at this ratio (see section 6). the physical arrangement of the bridge is the same for both the 10:1 and 1:10 bridge ratios. the only change is the reversed position of cn and cx. the balance equation for the 1:10 ratio is  x n n ntg 1 1 1 1 k g c tc c c c t t                   . (6) 4. cable correction  the values of cx obtained in (1), (5) and (6) include the cable parasitic contributions. we must correct the results for the cable errors. a simplified scheme of the capacitance bridge with the connecting cables modelled as delta networks is shown in figure 5. cable losses can be neglected for capacitance bridges as high quality cables are used. l1 and c2 are, respectively, the inductance and half the capacitance of the cable labelled ‘(7)’ in figure 2. l3 and c6 are, respectively, the inductance and half the capacitance of the cable labelled ‘(8)’. the contribution of c1 (and c5) is not taken into account since these capacitances are in parallel with the source. the contribution of c4 (and c8) is negligible when the bridge is balanced. l2 and c3 are, respectively, the inductance and one-fourth the capacitance of the cable labelled ‘(9)’ in figure 2. l4 and c7 are, respectively, the inductance and onefourth the capacitance of the cable labelled ‘(10)’. chg and clg are the parasitic capacitances to ground of the high and low inputs of the standard capacitor cn, respectively. hgc  and lgc  are the capacitances to ground of the high and low inputs of the capacitor under calibration cx, respectively. the cable relative error for the standard capacitor is, approximately,    tn nrn n hg n lg n 2 1 2 2 3 c c l c c l c c c           ,(7) where ctn is the total capacitance of the standard capacitor (capacitor plus cables), c2hg = c2 + chg and c3lg = c3 + clg. the cable relative error for the capacitor under calibration is, approximately,    tx xrx x hg x lg x 2 3 6 4 7 c c l c c l c c c            ,(8) where ctx is the total capacitance of the capacitor under calibration (capacitor plus cables), hg6 6 hgc c c   and lg7 7 lgc c c   . for the 1:1 ratio, the measurement result should be corrected for the total relative error, that is, r = rn rx. for the 10:1 ratio, we regard both the standard capacitor and its associated cable as a whole standard. the value of the standard capacitor is therefore inserted in the spreadsheet as  n rn1c  , and the measurement result is corrected for the cable error of the capacitor under calibration, that is,  x rx1c  . 5. ivd design and construction  a schematic diagram showing the arrangement of cores, windings and guarding of the main ivd is shown in figure 6. the ivd operates with up to 200 v (at 1 khz) across the 0 figure 5. simplified scheme of the capacitance bridge with cable modeling.  figure 6. schematic diagram showing the arrangement of cores, windings,  and guarding (the grounding conductors are not shown for clarity).  acta imeko | www.imeko.org  july 2012 | volume 1 | number 1 | 54  and m10 taps. therefore the first stage core comprises two plastic-encased supermalloy toroidal cores with 76.2 mm inner dia. x 101.6 mm outer dia. x 25.4 mm height and 0.0254 mm tape thickness (magnetics 01500441f) placed one on top of the other. (note: the previous design described in [8] used aluminium-encased cores). a uniform one-layer 220-turn (with 0.57 mm dia. magnet wire) magnetizing winding covers the entire first stage core. this winding has three main taps for external connection whose leads are coloured for easy identification, namely, 0 (red), m10 (blue) and m11 (black), which are equivalent to ratios 0, 1.0 and 1.1, respectively. the guard source for the divider winding is obtained by tapping the magnetizing winding at appropriate points (yellow). the guard source taps are equally spaced around the core (arranged in a star configuration – see figure 7). the guard voltages are 0.05 v, 0.15 v, 0.25 v, … , 1.05 v (assuming 1 v input). the magnetizing winding is covered with kapton tape (an insulating tape with good electrical, mechanical and thermal properties) and enclosed in a soldered toroidal shield made of 0.1 mm copper foil (figure 7). care should be taken in the shield overlaps to avoid the shorted turn. the shield is set at mid-tap potential, i.e. connected to the magnetizing winding tap corresponding to 0.55 ratio (see figure 6). the cross-sectional area of the second stage core was chosen so that it has a magnetic permeance of about 1/3 that of the first stage core. therefore two plastic-encased supermalloy toroidal cores with dimensions 76.2 mm inner dia. x 95.3 mm outer dia. x 9.5 mm height and 0.0254 mm tape thickness (magnetics 01500671f) were chosen. one core is placed on top of the shielded magnetizing winding and the other below it (figure 8). this symmetric arrangement was chosen to minimize leakage fields. (note: both the first and the second stage cores had been enclosed together within the toroidal shield in the design described in [8]). the ivd ratio winding consists of a 20-turn rope of 11 coaxial cables wound around the core assembly whose ends are connected in series to create a 220-turn winding (figure 9). (note: here lies another difference from the previous design: a rope arrangement of the cables had not been employed in [8]). the tapped terminals 1 ... 10 are brought out from these interconnections (see figure 6). the centre conductor is tapped at ten points, which are equivalent to ratios 0.1, 0.2, 0.3 ... 1. the outer shield of the cable is cut (and insulated) at each tap. in this type of construction each section has the same resistance and is equally well coupled to all other sections, thus balancing the mutual and leakage impedances. before arranging the cables in a rope, a coloured wire is soldered as accurately as possible to the middle of the outer conductor of each coaxial cable (figure 10). the whole assembly is then insulated with glass-fiber tape and the separate wires are connected to the corresponding magnetizing winding taps (figure 11). coloured wires are used to ensure that each wire is being connected to the correct guard tap. we confirmed that mistakes here are common and always result in large ivd ratio errors. this guarding method is an attempt to equalize the admittances between each half of each guard and nearby figure 7. shielded magnetizing winding with guard source taps.  figure 9. ratio winding (rope arrangement).  figure 8. shielded magnetizing winding with second stage cores (a second stage core is located below the assembly and cannot be seen).  figure 10. a wire soldered to the middle of the outer conductor.  acta imeko | www.imeko.org  july 2012 | volume 1 | number 1 | 55  conductors. the coaxial cable is gore gsc 6591 (conductor size: 19 x 0.127 mm awg 24 (19/36), 0.24 mm2; conductor material: cuag, 78.5 m/m; dielectric diameter: 0.84 mm; dielectric material: ptfe; screen details: braided screen from cuag awg 38 (1); jacket material: 0.15-mm ptfe; nominal diameter: 1.6 mm.). the whole assembly is then insulated with kapton tape, isolated from mechanical vibrations (with 25 mm extruded polystyrene slabs fixed by two opposing insulating boards kept firm with four aluminium rods) and enclosed in a metal box with holes on its top panel for later penetration of the coaxial output sockets (figure 12). the method of bringing out the taps to the coaxial connectors requires consideration if the highest possible accuracy is to be attained. the coaxial output sockets (bpo connectors) are fixed to a rigid insulating board placed above the ivd assembly (figure 13). they are insulated from the metal box in figure 14. stout conductors (1.3 mm dia. magnet wire) are taken from the socket outers for the ivd ratio winding, routing them close to the short tap connections to a point well within the volume of the box where they are joined together (see figure 12). this point is grounded to the metal box through an output socket. the electrical resistance between each socket outer and the metal box was measured to be less than 0.015 . the same arrangement for bringing out the taps to the coaxial connectors is adopted for the ivd magnetizing winding leads. the joint point of the stout conductors is also grounded to the metal box through another output socket. the resistance between each socket outer and the metal box was measured to be less than 0.009 . the bpo connectors for the magnetizing winding are the three ones located on the right side of figure 13. see [4] for more details on this grounding arrangement. the metal box is made of 1.5 mm chromium-coated carbon steel and the box inner surface is covered with 0.79 mm mumetal sheets. the box panel has the following outputs: (a) divider taps: 0 (twofold), 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 (twofold), and 11 (twofold), (b) magnetizing winding taps: 0, m10 and m11, and (c) two independent ground connections (both the magnetizing winding and the ratio winding stout conductor joint points were always grounded to the metal box in the ivd calibration and capacitance bridge measurements – see figure 2 for grounding connections). the final assembly of the main ivd is shown in figure 14. the short-circuiting plugs of the two independent ground connections are not shown in the figure. 6. ivd calibration  as remarked in section 2, one needs to know accurately the in-phase ratio error of the main ivd for the nominal ratio d = 1/11. the ratio error of the new main ivd was therefore calibrated at inmetro. the old ivd was replaced by the new one in the coaxial capacitance bridge and the bridge was used to figure 14. main ivd final assembly. figure 12. the whole assembly is isolated from mechanical vibrations.  figure 13. coaxial output sockets (bpo connectors).  figure 11. separate wires connected to the magnetizing winding taps.  acta imeko | www.imeko.org  july 2012 | volume 1 | number 1 | 56  compare 10 pf and 100 pf fused-silica standard capacitors which had been previously calibrated by the bureau international des poids et mesures (bipm) with a relative uncertainty of 4 parts in 108. with this method one can only measure the in-phase component of the complex ratio error. the calibration was performed at 110 v (across the 0 and m11 taps) and at both 1 khz and 1.592 khz. the dividing factors  and  in (5) can be expressed as , r r                 (9) where  and  are the values, r and r are the resolutions and and  are the random deviations of the  and readings, respectively. neglecting the contribution from k” in (5), and rearranging terms, the ivd ratio can be expressed as n n x c a t c c    , (10) where  1a c c     . (11) the values of cn and cx in (10) include their drifts with time and the cable parasitic contributions, that is     n n rn n x x rx x 1 1 , c c d c c d         (12) where nc and xc are the capacitance values reported in the calibration certificate, rn and rx are the relative corrections of the cables connected to each capacitor according to (7) and (8), and dn and dx are the capacitor drifts in the period between the date the capacitor calibration certificates were issued by bipm and the date of the ivd calibration. we regard both the standard capacitor and its associated cable as a whole standard. the value of cn is therefore inserted in the spreadsheet as cn(1 + rn) and the value of cx is inserted in the spreadsheet as cx(1  rx). since the main uncertainty contributions to the cable error are associated with the parasitic inductances and capacitances of the capacitor and cable, we may neglect the uncertainty contribution associated with the capacitor values and write n n n n x x x x , c c c d c c c d         (13) where cn and cx are the absolute corrections of the cable errors (in f), that is    n n n hg n lg2 1 2 2 3c c l c c l c c       , (14a)    x x x hg x lg2 3 6 4 7c c l c c l c c         . (14b) here cn = 10 pf and cx = 100 pf, and it is assumed that their contributions to the uncertainty associated with cn and cx are negligible. the in-phase component of the main ivd ratio error is estimated from k t d   . (15) by taking into account that the variables nc and xc are strongly correlated as the capacitors were calibrated at bipm with the same system, that is    n x n x, 0 , 0,u c c u c c   (16) the squared standard uncertainty associated with that estimate is then [14]                            x n n x n x n x n x x n n x n x 2 2 2 2 4 2 2 4 2 2 4 1 2 , c a u k u t u c c c c a u c c c u a c c c a c a u c c c c                    (17) where                                             n n n n x x x x 2 n n n hg 2 n n lg 2 x x x hg 2 x x lg 2 2 2 2 2 2 2 2 2 2 2 2 2 1 2 2 2 2 2 2 3 2 2 2 2 2 3 6 2 2 2 2 4 7 2 2 2 2 2 2 2 2 2 2 2 2 4 2 ( ) ( ) ( ) ( ) 1 1 u c u c u c u d u c u c u c u d u c c u l c u c c u l c u c u c c u l c u c c u l c u c u a c u u c c u u c c c u u                                                                     n x n x n x 2 2 2 2 2 2 2 , , u u r u u u u r u u c c u c c u c u c                  (18) since a correlation  n x, 1c c  is assumed here. the above uncertainty contributions were evaluated according to [14]. the in-phase 10:1 ratio errors k and their expanded uncertainties u(k), evaluated respectively with (15) and (17), are listed in table 4. table 5 lists the in-phase and quadrature 1:1 ratio errors evaluated from (4). the old ivd whose construction was reported in [8] was calibrated again using the same method described here. the reader should compare the results listed in tables 6 and 7 with those for the new ivd. the ratio errors of the new ivd are indeed much smaller than those of the old one. acta imeko | www.imeko.org  july 2012 | volume 1 | number 1 | 57  7. capacitance bridge measurement uncertainty  the new ivd was installed in the two terminal-pair coaxial capacitance bridge and the uncertainty obtained in capacitance calibration was assessed again here. this bridge has been used for calibrating stable standard capacitors at inmetro since its construction in 2005. an experiment was made to evaluate the consistency of the measurement results obtained with this bridge. consider three 10 pf fused-silica standard capacitors labelled here for convenience as a, b, and c. capacitor a is traceable to bipm. firstly, capacitor b was calibrated by comparing it with known capacitor a. secondly, capacitor c was similarly calibrated and the (computed) difference between b and c values was recorded. capacitor b (assuming it to be unknown) was then calibrated by comparing it with the (now) known capacitor c and the (measured) difference between b and c values was also recorded. all measurement results were corrected for cable errors. the computed and measured differences between b and c values differed by 710 at 1.592 khz when the new ivd was installed in the bridge. contrast this with the 6.710 figure obtained when the old ivd was installed in the bridge (or even with the 1.510 figure reported earlier in [6]). this is a bridge systematic error which is detected when comparing several standards of same nominal value for consistency in the results. this error contributes to the overall uncertainty of the bridge at 1:1 ratio. so, this uncertainty contribution was reduced by one order of magnitude as a result of the reduced ratio errors presented by the new ivd. table 8 shows the uncertainty budget for the calibration at 1:1 ratio and at 1.592 khz of a stable 10 pf fused-silica standard capacitor (cx) from a similar capacitor (cn) traceable to bipm. the combined relative standard uncertainty associated with the difference between the capacitance values of the standards being compared (cxcn) is less than one part in 108. the uncertainty contribution due to the capacitance bridge is therefore negligible compared to other contributions such as table 4. in‐phase 10:1 ratio errors (new ivd).  frequency (hz)  k´  u(k´)  1000  3 × 10 9   23 × 10 9   1592  12 × 10 9   23 × 10 9   table 5. in‐phase and quadrature 1:1 ratio errors (new ivd).   frequency (hz)  k´  k”  1592  23 × 10 9   9 × 10 9   table 6. in‐phase 10:1 ratio errors (old ivd).  frequency (hz)  k´  u(k´)  1000  124 × 109  23 × 109  1592  164 × 109  23 × 109  table 7. in‐phase and quadrature 1:1 ratio errors (old ivd).  frequency (hz)  k´  k”  1592  280 × 109  23 × 109  table 8. uncertainty budget at 1:1 ratio (10 pf – 1.592 khz).  quantity  standard  uncertainty  sensitivity  coefficient  eval.  type  cn  (1)   4.0×10 7  pf  1  b   3.16×106  1.00×102 pf  a   7.48×107  8.00×106 pf  a  c  4×10 8  pf  6.56×10 5   b  c  0.0008 pf  8.00×107  b   0.1  6.63×107 pf  b  r (2)   1×10 8  pf  1  b  cxcn (3)   7×10 8  pf  1  comb.  error  (4)   7×10 9  pf  1  b  cx  (5)   7×10 8  pf  1  comb.  rk‐90  (6)   1.00×10 6  pf  1  b  biannual drift  (7)   1.00×10 6  pf  1  a  cx  (8)   1.5×10 6  pf    comb.  (1) relative combined standard uncertainty reported in the bipm calibration certificate for cn (a 10 pf capacitor). (2) uncertainty contribution associated with the correction for the cable errors. (3) combined standard uncertainty associated with the difference between the capacitances of the standards being compared (see text). (4) systematic error that is detected when comparing several standards for consistency in the results (see text). (5) combined standard uncertainty associated with cx without taking into account the uncertainty contributions associated with rk-90 and the reference standard biannual drift. cx is a 10 pf capacitor. (6) standard uncertainty associated with the recommended value of rk-90. (7) biannual drift evaluated by fitting a straight line to data reported in bipm calibration certificates in the last six years. (8) combined standard uncertainty associated with cx by taking into account all known uncertainty contributions. table 9. uncertainty budget at 10:1 ratio (100 pf – 1.592 khz).   quantity  standard uncertainty  sensitivity  coefficient  eval.  type  cn  (1)   4.0×10 7  pf  10  b   1.5×106  1.00×101 pf  a  k´  1.2×108  1.21×103 pf  b  r (2)   1×10 8  pf  1  b  cx  (3)   1.51×10 5  pf  1  comb.  rk‐90  (4)   1.00×10 5  pf  1  b  biannual drift  (5)   1.00×10 5  pf  1  a  cx  (6)   2.1×10 5  pf    comb.  (1) relative combined standard uncertainty reported in the bipm calibration certificate for cn (a 10 pf capacitor). (2) uncertainty contribution associated with the correction for the cable errors. (3) combined standard uncertainty associated with cx without taking into account the uncertainty contributions associated with rk-90 and the reference standard biannual drift. cx is a 100 pf capacitor. (4) standard uncertainty associated with the recommended value of rk-90. (5) biannual drift evaluated by fitting a straight line to data reported in bipm calibration certificates in the last six years. (6) combined standard uncertainty associated with cx by taking into account all known uncertainty contributions. acta imeko | www.imeko.org  july 2012 | volume 1 | number 1 | 58  the relative uncertainty reported in the bipm certificate, the reference standard drift, and the uncertainty associated with the recommended value of rk-90 (von klitzing constant). the major uncertainty contributions are now the 1/ ratio and the stability of the 1 2) readings. table 9 shows the uncertainty budget for the calibration at 10:1 ratio and at 1.592 khz of a stable 100 pf fused-silica standard capacitor (cx) from a similar 10 pf capacitor (cn) traceable to bipm. the combined relative standard uncertainty associated with the calibration result (without the uncertainty contributions associated with rk-90 and with the reference standard drift) is 1.510. the major contribution to the overall bridge uncertainty at the 10:1 ratio is that associated with the in-phase component of the ivd ratio error. as mentioned in the introduction, the main ivd to be used in coaxial ratio bridges should be constructed to provide an overall bridge uncertainty at 1:1 and 10:1 ratios of a few parts in 108. in order to achieve this for the 10:1 ratio it is necessary to reduce the uncertainty associated with the in-phase component of the ivd ratio error by one order of magnitude. this demands the construction of a special system for calibrating the ivd 10:1 ratio with an uncertainty of a few parts in 109. note: quantities not listed in tables 8 and 9 were found to have negligible uncertainty contributions. 8. conclusions  the constructional details of the new main two-stage ivd for coaxial bridge applications, recently built at inmetro, were presented along with the design changes implemented. the design changes are: (a) plastic-encased cores are used as first stage cores, (b) only the magnetizing winding and the first stage cores are copper shielded and (c) a rope arrangement is employed for the coaxial cables in the ratio winding. the method used to calibrate the ivd ratio error was also described in detail. the in-phase 10:1 ratio error was determined from known values of two stable decadic standard capacitors. it was confirmed that the ratio errors of the new ivd are much smaller than those reported previously for another ivd. upon installing the new ivd in inmetro´s coaxial capacitance bridge, the major contributions to the overall uncertainty of the 1:1 ratio bridge are those associated with the predivider ratio and with the stability of the readings. the major contribution at 10:1 ratio is that associated with the in-phase component of the ivd ratio error. a special system for calibrating the ivd 10:1 ratio with an uncertainty of a few parts in 109 is required if one needs to achieve an overall uncertainty of parts in 108 with the 10:1 ratio bridge. acknowledgement  g. k. thanks l. m. ogino and e. afonso for providing the resources for this project. g. k. also wishes to thank all inmetro/sengi personnel and the late a. etchebehere for the construction of the ivd metal box. he also thanks r. p. miloski for soldering the ivd coaxial cables and r. t. b. vasconcellos for previous collaboration in the construction and initial operation of the capacitance bridge. references  [1] j.melcher, j.schurr, “modular system for the calibration of capacitance standards based on the quantum hall effect”, proc. of v semetro, 2002, apr. 9-12, rio de janeiro, brazil. [2] j.melcher, j.schurr, k.pierz et al., “the european acqhe project: modular system for the calibration of capacitance standards based on the quantum hall effect”, ieee trans. instrum. meas., apr. 2003, 52, no.2, pp. 563-568. [3] physikalisch-technische bundesanstalt (ptb), j.schurr, j.melcher, “modular system for the calibration of capacitance standards based on the quantum hall effect,” workshop and project documentation, january 2002, brauschweig, germany. available in cd-rom. [4] b.p.kibble, g.h.rayner, coaxial ac bridges, adam hilger ltd., 1984. [5] d.homan, t.zapf, “two stage, guarded inductive voltage divider for use at 100 khz”, isa trans., 1970, 9, no. 3, pp. 201-209. [6] g.a.kyriazis, r.t.b.vasconcellos, l.m.ogino, j.melcher, j.a.moreno, “design and construction of a two terminal-pair coaxial capacitance bridge”, proc. of vi semetro, 2005, sept. 21-23, rio de janeiro, brazil, pp. 57-62. [7] g.a.kyriazis, r.t.b.vasconcellos, l.m.ogino, j.melcher, j.a.moreno, “a two terminal-pair coaxial capacitance bridge constructed at inmetro”, cpem digest, 2006, july 9-14, turin, italy, pp. 522-523. [8] g.a.kyriazis, j.melcher, j.a.moreno, “a two-stage, guarded inductive voltage divider for use in coaxial bridges employed in the derivation of the capacitance unit from quantized hall resistance”, cpem digest, 2004, june 27 – july 2, london, uk, pp. 131-132. [9] j.a.moreno, r.hanke, “maintenance and development of the capacitance unit at cenam”, cpem digest, 2000, may 14-19, sydney, australia, pp. 218-219. [10] a.koffman, n.f.zhang, y.wang, s.shields, b.wood, k.kochav, j.a.moreno, h.sanchez, b.i.castro, m.cazabat, l.m.ogino, g.a.kyriazis, r.t.b.vasconcellos, d.slomovitz, d.izquierdo, c.faverio, "sim.em-k4 capacitance comparison summary", cpem digest, 2012, july 1-6, washington, dc, usa, pp. 398-399. [11] g.a.kyriazis, r.t.b.vasconcellos, “unequalized currents in two terminal-pair coaxial capacitance bridges”, proc. xviii imeko world congress, 2006, sept. 17-22, rio de janeiro, brazil. [12] j.schurr, j.melcher, “unequalized currents in coaxial ac bridges”, ieee trans. instrum. meas., jun. 2004, 53, no. 3, pp. 807-811. [13] g.a.kyriazis, l.m.ogino, e.afonso, j.a.neves, “an isolation transformer for coaxial bridges used in the derivation of the capacitance unit from the ac quantized hall resistance”, mapan jmsi, jul.-sept. 2003, 18, no. 3, pp. 137-144. [14] bipm, iec, ifcc, ilac, iso, iupac, iupap and oiml, evaluation of measurement data – guide to the expression of uncertainty in measurement, gum 1995 with minor corrections, joint committee for guides in metrology, jcgm 100, 2008, http://www.bipm.org/utils/common/documents/ jcgm/jcgm_100_2008_e.pdf (accessed 29.10.11). acta imeko  december 2014, volume 3, number 4, 26 – 31  www.imeko.org  acta imeko | www.imeko.org  december 2014 | volume 3 | number 4 | 26  how geometric misalignments can affect the accuracy of  measurements by a novel configuration of self‐tracking ldv  giuseppe dinardo, laura fabbiano, gaetano vacca  dept. of mechanics, mathematics and management (dmmm) politecnico di bari, university, bari, italy  section: research paper   keywords: self‐tracking ldv, accuracy analysis, vibration, rotating object, geometric misalignment  citation: g. dinardo, l. fabbiano, g. vacca, how geometric misalignments can affect the accuracy of measurements by a novel configuration of self‐ tracking ldv, acta imeko, vol. 3, no. 4, article 6, december 2014, identifier: imeko‐acta‐03 (2014)‐04‐06  editor: paolo carbone, university of perugia   received october 10 th , 2013; in final form march 24 th , 2014; published december 2014  copyright: © 2014 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: (none reported)  corresponding author: laura fabbiano  1. introduction an accurate evaluation of dynamic stresses due to vibrations affecting rotating machines components is crucial for both design and diagnostics interests. the accurate evaluation and determination of the dynamic stresses due to vibrations is indeed of considerable importance in the design, testing and running of turbomachinery, typically, as the measurements of these stresses become critical in the case of moving components, especially rotating ones. reason of that is the difficulty in providing a suitable set-up capable of acquiring data from sensors placed in the same environment and transferring them to the processing unit. in order to obtain more precise measurements, some innovative nonintrusive techniques (based on the use of appropriately installed noncontact sensors) have been developed. nowadays, in industrial ambit, optical or electromagnetic sensors are used to characterize the vibrational state of the rotating parts of the machines, ensuring nonintrusive measurements [1]. one of the most suitable techniques is based on the use of ldv technology: a laser beam, characterized by spatial and temporal coherence, impacts on a vibrating surface; then, reflected light wave impinging the laser receiver undergoes a frequency shift (doppler effect) allowing the evaluation of the vibrational velocity of the object or structure under examination. the embedded difficulties in the ldv technique for the measurement of moving and vibrating organs, as turbine blades, consist in the apparent impossibility of tracking the object, from which the data required to assess the nature and the entity of vibrations have to be extracted. this is why both industrial and academic researchers focus their efforts to develop new tracking systems based on laser technology. there is a vast literature on this subject suggesting different techniques for tracking moving objects by adding optics to the laser equipment, [2], [3], [4]. more specifically, there are several solutions involving the adoption of a scanning laser doppler vibrometer [5], [6], [7]. nevertheless, the analysis of the errors in the measurement of vibrations by means of these configurations remains very abstract aim of the paper is to evaluate the reliability of a novel configuration of a laser doppler vibrometer (ldv) for the measurement of  vibrations affecting rotating components of machines. in the paper an analysis of errors due to the possible static misalignments of the  mirrors utilized in an experimental apparatus has been carried out. this instrument was set up for the estimation of the out of plane  vibrations of moving (rotating) objects, in order to give a better characterization of the self‐tracking technique employed with the use  of  a  1d  single  point  ldv  and  its  measurements.  the  accuracies  of  the  measures  are  mostly  linked  to  the  interaction  between  environment and instrumentation. acta imeko | www.imeko.org  december 2014 | volume 3 | number 4 | 27  complex and little explored. one of those techniques has been proposed by the authors in [8], by using a 1d single point ldv apparatus. additional equipment made by optical mirrors installed in an appropriate way is required, in order to appropriately perform the tracking of a given point lying on the rotating structure to be analysed. other authors have analysed the geometric errors due to misalignments in the optical core system only of laser tracking configurations [9], [10], [11]. this paper deals with the study of some errors caused by the employment of additional external optics to better evaluate the relative uncertainty of the measurement. so, a qualitative analysis of uncertainties of such a configuration has been performed. these considerations are related to the static misalignments which may exist between the laser beam, the rotor axis moving the test beam and the axis of the equipment supporting the mirrors. in this context, other possible misalignments, the so called dynamic ones due to dynamic effects induced by the movement of the object itself, are not taken into consideration. 2. experimental apparatus  the experimental configuration of reference for the present analysis is the one presented in [8] and here succinctly reported in figure 1 and in figure 3 in the essential components. figure 1 shows the mutual position between ldv (pdv 100 polytec) and the fold mirrors structure. the fold mirrors’ support is free to rotate around its axis so that their angular position can vary. figure 2 shows the configuration of the experimental apparatus. in figure 3 the laser beam paths are simulated for easy understanding. the fold mirrors and the vertex-mirror (rod mirror) placed on the static support and the rotor respectively are all 45° mirrors (figure 4). 3. misalignment errors analysis  by the ldv technique (in the self-tracking configuration) the measure of vibrations is possible at every angular velocity in principle. nevertheless, the system can be particularly sensitive to possible static misalignments among the laser beam, the vertex mirror axis, the axis of the fold mirrors support and the rotor axis. it is immediately evident how those static misalignments cause a displacement of the measurement point on the rotating blade. as the purpose of the arranged set-up is to measure the out-of-plane vibration of a pre-set blade point while that is rotating, a perfect alignment between each pair of the setup components is required. if the alignment is not accomplished in all the four angular positions of the blade, the measurement point (where the laser spot hits) changes and thus the measurement relative to the four angular positions are not representative of the vibration of the same blade point under analysis. figure 1. mutual position between ldv and fold mirrors structure.   figure 2. experimental apparatus  figure 3. laser beam paths.  figure 4. technical image of the right angle mirror (fold mirror) [12] and of  the 45° rod mirror (vertex mirror) embedded in the rotor [13].  acta imeko | www.imeko.org  december 2014 | volume 3 | number 4 | 28  the measurement errors are mainly due to the fact that those misalignments provoke laser beam reflections not orthogonal to the incident surface. thus, the vibration sensed is not the desired out-of-plane one and this circumstance causes additional spectral components which are superimposed to the expected measurement spectrum, causing the presence of harmonic components non representative of the original vibration velocity signal. these complications, then, are exacerbated by dynamic effects due to the revolution, to dynamic imbalances and so on. although the contact absence between the blade and the sensor is an advantage (with no persistent and systematic loading effects which would influence the dynamic behaviour of the rotating structure), another source of uncertainty has to be mentioned: the vibrometer would also sense the vibration component due to the rigid body motion of the rotating structure (linked to the structural imperfections of the governor), which is superimposed to the effective and real vibration motion of the rotating object due to the fluid dynamic phenomena affecting it. other sources of uncertainty are certainly due to imperfections and misalignments of the optical components included into the core of the ldv head and to the output analogue signal manipulation. 4. ldv errors analysis   the description of the frequency shift due to the doppler effect, which provides the measure sensed by the laser beam incident onto diffusive and reflective surfaces, has well been discussed in [14] and here synthetically proposed again for clarity. 4.1. diffusive surface  several tests on diffusive surfaces have been performed in order to evaluate the amount of the errors affecting the measurements of the vibration velocity when misalignments between laser beam and surface under analysis are forced. these impositions have the main purpose of simulating misalignments which could occur in practice and due to improper installations of the components of the experimental set-up. by theoretical considerations, these misalignments influence the frequency shift affecting the laser beam (once it is refracted by the incident surface), by a specific and fixed amount. thus, these static misalignments (due to imprecise installation of the components of the experimental arrangement) can be treated as systematic errors. the frequency shift of the laser beam reflected by a diffusive surface is expressed by the well-known equation: ∆ ≅ (1) where, is the wavelength of the laser beam and is the component of the vibration velocity vector (time dependent) along the direction of the incident laser beam. from equation (1), it is clear that misalignments or improper installation of the components of the experimental arrangement, influence the vibration velocity of surface under analysis. an analysis and characterization of the errors in measuring the blade vibration velocity due to the cited effects has already been carried out in [14]. stressing the rotating blade by a simple periodic vibration in different directions (the angles between the laser beam and the vibration velocity vector of the diffusive surface are forced), it has been shown how the induced vibrations affect its motion. the measured velocity is the component of the vibration velocity vector along the laser beam direction. when the impressed vibration velocity onto the blade is directed along the direction of the incident laser beam, the velocity of vibration is directly given by equation (1). in the cases where the angle between the velocity vector and the laser beam is less than 90°, a vibrational velocity is measured with same frequency as the first case but with reduced amplitude, in proportion to increasing angle. if that angle is right 90°, no component of the velocity vector along the laser beam direction is detected, except for noise signal. so the output returns indeed a not null signal, though very small, because of the noise floor, speckle noise and so many other causes, mainly given by interfering phenomena. by this analysis on diffusive surfaces, it has been possible to infer the following deductions: • only vibrations with a velocity component along the laser beam direction give a significant output signal; • vibrations orthogonal to the laser beam produce an output which is characterized only by noise, speckle noise and other interfering effects of difficult characterization. 4.2. reflecting surface  since the experimental set-up includes optical components (stationary fold mirror system and the rotating vertex mirror), an analysis of how reflection surfaces influence the frequency shift (due to doppler effect) experienced by the laser beam is required. the fold mirrors do not induce a frequency shift, since they are stationary. on the contrary, a moving mirror induces a frequency shift expressed by the formula, [14]: ∆ ∙ ∙ (2) where is the velocity vector of the laser spot point on the figure 5 ‐ general configuration for induced and reflected laser beams.  acta imeko | www.imeko.org  december 2014 | volume 3 | number 4 | 29  mirror surface, is the surface direction and is the direction of the reflected beam, figure 5. in the arrangement of the self-tracking ldv, the only component giving a frequency shift is the vertex mirror, whose reflecting surface is inclined of 45°. it is installed on the centre of the hub supporting the blades. all the points on the reflective surface have the same angular velocity (equal to the angular velocity of the rotor). in addition each point on the reflective surface has a velocity vector which is tangential to the circular trajectory described by each of them. the point on the reflecting surface illuminated by the light spot has a velocity vector which results to be constant in time (if the angular velocity of the rotor is constant and the laser beam is stationary. this is the case here, since the fold mirror reflecting the incident laser beam is stationary). since the reflecting surface is inclined, its normal vector will not be stationary but will rotate at the same angular velocity of the rotor and will describe a cone or a truncated cone, depending on the particular point chosen on the reflecting surface. these considerations have to be taken into account in order to quantify the frequency shift induced by the vertex mirror. the plane containing the incident laser beam onto the vertex mirror and the reflected rotates at the same angular velocity of the rotor and is parallel to the rotation axis. the projection of the point of the reflective surface illuminated by the laser spot and the surface of both incident and reflected laser beams vary with a sinusoidal law as the following equation states: (3) where, is the velocity vector of the point illuminated by the laser spot, its projection on the rotating surface and the angle between the velocity vector, , and the surface. since the incident laser beam lies on a non-stationary point, it is affected by a frequency shift. in order to avoid the frequency shift affecting the laser beam incident on the vertex mirror, a perfect alignment between laser beam, the vertex mirror and the rotor axis is required. in this way the point illuminated by the laser spot coincides with the centre of rotation of the entire system and thus, it remains stationary. no frequency shifting is then generated if the alignment is guaranteed. 5. further considerations on static misalignments  in this paragraph, further considerations on static misalignments between laser beam and rotor and fold mirrors supporter are developed. it is clear that these static misalignments are due to improper installation of the equipment of the described setup, leading to evaluation of vibrations of multiple points on rotating beam as far as these static misalignments cause a slip between the laser beam reflected by fold mirrors (and impinging the plane where the rotating test beam lies on). two types of static misalignments are going to be described in this paper: so called parallel shift misalignments and angular misalignments. the first ones occur whenever the axes of the considered components are perfectly parallel shifted and there are no angular misalignments between them. the second ones contemplate the existence of angular misalignment between the axes. figure 6 represents the ideal case where a perfect alignment between laser beam, vertex mirror axis and fold mirror structure is achieved. figure 7 also shows the geometric distances taking place between the most considerable points in the setup. it can be noted that a parallel shift between laser beam and vertex mirror axis results in a displacement of the figure 6. perfect alignment between laser beam, vertex mirror axis and fold mirror structure  figure 7. parallel shift misalignment   figure 8. parallel shift between vertex mirror axis and fold mirrors support  acta imeko | www.imeko.org  december 2014 | volume 3 | number 4 | 30  detected point on the test blade (by the laser), by an amount equal to the parallel misalignment between the laser beam and the vertex mirror axis. if we would assume that the fold mirror structure given by a cylindrical mirror with internal reflective surface inclined by 45’, the laser beam reflected by this ideal mirror and impinging the test blade plane would be a cylinder with an axis characterized by a displacement with respect to the rotor axis equal to the amount of the parallel shift. the maximum error in tracking the effective impinging point of the laser beam onto the rotating blade is /2. a larger error would cause a complete missing of the incidence of the laser beam onto the vertex mirror. analogous considerations could be given if we would consider a parallel shift between fold mirrors support and vertex mirror axis or a parallel shift between the former and the laser beam itself, as shown in figure 8 the overall parallel displacement between the measuring point and the one occurring in the case of perfect alignment is given by the algebraic sum of the partial shifts occurring between the couples laser beam – vertex mirror axis and between the latter and the fold mirror support axis. another source of geometric uncertainty needs to be evaluated. as explained above, if the fold mirror would be the internal surface (inclined by 45’) of a cone trunk, the reflected beam successively impinging the blade plane, would be a cylinder with parallel axis and shifted to the rotor axis. actually the fold mirror complex is formed by four mirrors with plane reflective surface. the resulting reflected beam is thus discontinuous (occurring whenever the rotating laser beam reflected by the vertex mirror impinges each fold mirror) and track segments instead of arches (because of the planarity of the fold mirror surfaces). but if the length of each fold mirror tends to zero, these segments would tend to portions of arc. this approximation would allow us to state that at each of the four angular positions (where the fold mirrors are installed), the laser point lying on the test blade would track a fixed point, but (due to parallel misalignment between laser beam – vertex mirror axis – fold mirror support axis), the point tracked at each angular position would change by an amount equal to the overall displacement between the formerly mentioned axes. now angular misalignments are going to be discussed. the following picture (figure 10) portraits these types of misalignment. in this case, the actual measuring point is displaced by an amount proportional to tan(), with  the angular misalignment between rotor axis and laser beam and proportional to the distance between the impinging point of the laser beam on the fold mirror and the rotor plane. also in this case, if the fold mirror would be the internal surface (inclined by 45’) of a cone trunk, the reflected beam successively impinging the blade plane would be a cylinder with axes inclined of ± respect to the vertex mirror axis. since the fold mirror complex is formed by four angularly displaced mirrors with plane reflective surfaces, the reflected beams consist of a tracked segment on the blade plane, tracking more than one single point. this would be another source of error to be neglected in the case of infinitesimal fold mirror surface length. figure 9 shows an angular misalignment between the fold mirror system axis and that of the rotor. also in this case the same consideration as above can be made. the overall shift of the measuring point (with respect to the ideal one lying on the same point of the blade during its rotation) is given by the algebraic sum of the simpler shifts due to the stepwise angular misalignments between the several parts of the laser tracking system. the analyzed errors due to static misalignments imply an uncertainty in the effective tracked point on the rotating test beam. one would expect that at each reflection performed by each fold mirror, the laser beam tracks the figure 9. angular misalignment between fold mirrors structure axis and that of the vertex mirror.  figure 10. angular misalignment between laser beam and rotor axis  acta imeko | www.imeko.org  december 2014 | volume 3 | number 4 | 31  same point (although a relative uncertainty in which point is going to be tracked still persists). actually due to a parallel shift (occurring in the case of parallel misalignments) and to an angular shift (occurring in the case of angular misalignments) of the portions (taking place at the reflection locations at each fold mirror) of the hypothetical cylindrical surface (if the length of reflective surface of fold mirror would tend to zero), the point tracked by the laser beam on the rotating test beam isn’t the same. 6. conclusions  the considerations made in the paper are effective in the case of only static misalignments. if the proper alignment among laser beam, rotor axis, vertex mirror axis and fold mirrors support axis could be guaranteed, the laser beam generated by the ldv head would experience a frequency shift only due to out-of-plane vibrations of the plane blade under analysis due only to fluid-dynamics and rigid body motion phenomena. as it is unlikely to avoid such kind of imperfections, it is necessary to estimate at least the maximum error involved by them. then, given the systematic nature of such kind of errors, indeed non-perfectly evaluable, their estimation could be incorporated in the global accuracy of the equipment by means of their maximum values valuation. the rotation of the blades and other dynamic phenomena, such as dynamic unbalances of the rotor, might introduce dynamic misalignments which affect the overall frequency shift experienced by the laser beam. these so-called dynamic phenomena introduce random effects, which aren’t predictable and highly difficult to quantify. another phenomenon which theoretically could influence the measurements by the self-tracking ldv arrangement and its efficacy is the speckle noise. this phenomenon occurs when a relative motion is present between the laser beam spot and the surface under test. the phenomenon, due to the micro reflections and refractions locally distributed on the surface due to its roughness, essentially results in a noise floor which is superimposed on the vibration velocity signal and as such it can be opportunely filtered. by comparing the ldv technique with other likewise sophisticated techniques such as the interferometry, the former minimizes indeed such kind of error. references  [1] i. ye zablotskiy, a. yu korostelev, l. b. sviblov, “contactless measuring of vibrations in the rotor blades of turbines”, technical translation fdt-ht-23, 1974, pp. 673-74. [2] w. k kulczyk, q. v. davis, “laser doppler instrument for measurement of vibration of moving turbine blade”, proceedings of the institution of electrical engineers, vol. 120, n. 9, 1973. [3] m. zielinski, g. ziller, “noncontact vibration measurements on compressor rotor blades”, meas. sci. technol., 11, 2000, pp. 847-856. [4] r. a. lomenzo, a. j. barker, a. l. wicks, “laser vibrometry system for rotating bladed disks” in proc. of the 18th imac, 1999, pp. 277-282. [5] h. dietzhausen, k. bendel, n. scelles, “tracking scanning laser doppler vibrometers: extending laservibrometry to arbitrarily moving objects” in proc.of imac xxi, 2003. [6] p. castellini, n. paone, “development of the tracking laser vibrometer: performance and uncertainty analysis”, review of scientific instruments, vol, 71, 2000, pp. 4639–4647 [7] m. tirabassi, s. j. rothberg, “scanning ldv using wedge prisms” optics and laser in engineering, 47 (3-4), 2009, pp. 454-460. [8] g. dinardo, l. fabbiano, g. vacca “influences of geometric configuration on the analysis of uncertainties affecting a novel configuration of self-tracking ldv” in proc of 12th imeko tc10, florence, italy, 2013, pp. 195-201. [9] p. l. teoh, b. shirinzadeh, c. w. foong, g. alici, «the measurement uncertainties in the laser interferometry-based sensing and tracking technique», measurement, vol. 32, n. 2, 2002, pp. 135–150. [10] k. umetsu, r. furutnani, s. osawa, t. takatsuji, t. kurosawa, “geometric calibration of a coordinate measuring machine using a laser tracking system”, measurement science and technology, 16(12), 2005, pp. 24662472. [11] s. ibaraki, t. hata, a. matsubara, “a new formulation of laser step-diagonal measurement—two-dimensional case”, precision engineering, vol. 33, n. 1, 2009, pp. 56–64. [12] http://www.edmundoptics.com/optics/opticalmirrors/flat-mirrors/right-angle-mirrors/1930. [13] http://www.edmundoptics.com/optics/prisms/specialtyprisms/45-degree-rod-lenses-mirrors/1413. [14] r. a. lomenzo, “static misalignment effects in a selftracking laser vibrometry system for rotating bladed disks”, dissertation, blacksburg, usa, 1998. editorial to selected papers from the 2022 imeko tc11&tc24 joint conference acta imeko issn: 2221-870x june 2023, volume 12, number 2, 1 2 acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 1 editorial to selected papers from the 2022 imeko tc11&tc24 joint conference marija cundeva-blajer1, leonardo iannucci2 1 ss. cyril and methodius university in skopje (ukim), faculty of electrical engineering and information technologies (feeit), st. rugjer boskovik no. 18, 1000 skopje, republic of north macedonia 2 politecnico di torino, department of applied science and technlogy, corso duca degli abruzzi 24, 10129 torino, italy section: editorial citation: marija cundeva-blajer, leonardo iannucci, editorial to selected papers from the 2022 imeko tc11&tc24 joint conference, acta imeko, vol. 12, no. 2, article 4, june 2023, identifier: imeko-acta-12 (2023)-02-04 received june 26, 2023; in final form june 30, 2023; published june 2023 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: leonardo iannucci, e-mail: leonardo.iannucci@polito.it dear readers, this special issue collects the extended version of some of the contributions presented at the 2022 imeko tc11&tc24 joint conference, held in dubrovnik (croatia) from the 17th to the 19th of october 2022. this international conference gathered experts both from industry and academia, covering different topics from the field of ‘measurement in testing, inspection and certification’ (imeko tc11) and ‘chemical measurements’ (imeko tc24). considering the wide interdisciplinarity of the two technical committees, many topics and metrological issues were addressed by the conference participants. in the following, the published papers will be individually presented. in the technical note ‘statements of conformity provided by laboratories’, a. čop [1] examines the approach presented in ilac g8 and other pertinent literature about decision rules for statements of conformity to specifications or standards. this issue is of paramount importance for laboratories, which need to correctly take into account measurement uncertainty in order to properly reduce the risk of incorrect decisions. the paper ‘evaluating chemometric strategies and machine learning approaches for a miniaturized near-infrared spectrometer in plastic waste classification’ by c. marchesi et al. [2] reports on a comparative study of different computational tools to categorize a data set derived from near infra-red measurements. the results showed a very good performance of the multivariate methods (such as principal components analysis), which outperformed the investigated machine learning tools. these findings are of great interest for the applications related to plastic waste sorting and polymers recycling. the contribution “metrology infrastructure for radon metrology at the environmental level” by a. röttger et. al. [3], presents results in the areas of novel 226ra standard sources with continuous controlled 222rn emanation rate, radon chambers aimed to create a reference radon atmosphere and a reference field for radon flux monitoring. the new infrastructure is capable of filling this gap in traceability. the achieved results make new calibration services, far beyond the state of art, possible. the contribution “decision-making on establishment of recalibration intervals of testing, inspection or certification measurement equipment by data science” by m. cundevablajer [4], is related to issues on decisions in conformity assessment, especially in testing, inspection, and certification (tic). the paper conducts a survey of options for deployment of data science in the tic decision making processes, based on conclusions with complementary usage of empirical “measurements” and the “data science”, through a case study for determining the instrument calibration frequency, presenting a model to establish recalibration interval with reduced risk in the final decision delivery over the next re-calibration moment. during the second large hadron collider long shutdown, the liquid argon calorimeter of the atlas experiment at cern has been upgraded with a new trigger readout electronics which provides digital information with higher granularity to the atlas trigger system as presented in “power distribution board: quality control, quality assurance and testing” by a. carbone et al. [5]. in particular the new lar trigger digitizer boards process and digitize the "super cells" (group of readout calorimeters cells) and send the processed data to the back-end electronics. the power distribution board is a mezzanine board that provides the power distribution to the ltdb. the paper ‘developments of interlaboratory comparisons on pressure measurements in the philippines’ by m. n. i. salazar [6] presents the results from interlaboratory comparisons in pressure measurement in the philippines. the initiative derived from an increasing demand in proving the competency of local calibration laboratories and it was successfully implemented, performing two interlaboratory comparisons 6 years apart. mailto:leonardo.iannucci@polito.it acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 2 an important case study is then reported in the article ‘the experience gained from implementing an iso 56000-based innovation management system’, by t. gueorguiev [7]. the author presents the adoption of the iso 56000 series of standards at the university of ruse, in bulgaria. as a result, significant improvement in the innovation management system was achieved, and the main initiatives are outlined in the paper. we hope you will enjoy your reading. marija cundeva-blajer leonardo iannucci guest editors for the special issue references [1] a. čop, statements of conformity provided by laboratories, acta imeko, vol. 12 (2023) no.2, pp. 1-3. doi: 10.21014/acta_imeko.v12i2.1468 [2] claudio marchesi, monika rani, stefania federici, matteo lancini, laura e. depero, evaluating chemometric strategies and machine learning approaches for a miniaturized near-infrared spectrometer in plastic waste classification, acta imeko, vol. 12 (2023) no.2, pp. 1-7. doi: 10.21014/acta_imeko.v12i2.1531 [3] annette röttger, stefan röttger, daniel rábago, luis quindós, katarzyna woloszczuk, maciej norenberg, ileana radulescu, aurelian luca, metrology infrastructure for radon metrology at the environmental level, acta imeko, vol. 12 (2023) no.2, pp. 1-7. doi: 10.21014/acta_imeko.v12i2.1440 [4] m. cundeva-blajer decision-making on establishment of recalibration intervals of testing, inspection or certification measurement equipment by data science, acta imeko, vol. 12 (2023) no.2, pp. 1-8. doi: 10.21014/acta_imeko.v12i2.1465 [5] antonio carbone, francesco tartarelli, massimo lazzaroni, stefano latorre power distribution board: quality control, quality assurance and testing, acta imeko, vol. 12 (2023) no.2, pp. 1-4. doi: 10.21014/acta_imeko.v12i2.1486 [6] mary ness i. salazar, developments of interlaboratory comparisons on pressure measurements in the philippines, acta imeko, vol. 12 (2023) no.2, pp. 1-6. doi: 10.21014/acta_imeko.v12i2.1448 [7] tzvetelin gueorguiev, the experience gained from implementing an iso 56000-based innovation management system, acta imeko, vol. 12 (2023) no.2, pp. 1-6. doi: 10.21014/acta_imeko.v12i2.1461 https://doi.org/10.21014/acta_imeko.v12i2.1468 https://doi.org/10.21014/acta_imeko.v12i2.1531 https://doi.org/10.21014/acta_imeko.v12i2.1440 https://doi.org/10.21014/acta_imeko.v12i2.1465 https://doi.org/10.21014/acta_imeko.v12i2.1486 https://doi.org/10.21014/acta_imeko.v12i2.1448 https://doi.org/10.21014/acta_imeko.v12i2.1461 template for an acta imeko event paper acta imeko issn: 2221-870x december 2016, volume 5, number 4, 29-36 acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 29 enhancement of flash memory endurance using short pulsed program/erase signals philippe chiquet1, jérémy postel-pellerin1, célia tuninetti2, sarra souiki-figuigui1, pascal masson2 1aix-marseille universite, im2np-cnrs umr 7334, rue frederic joliot-curie, 13451 marseille, france 2nice sophia-antipolis university epoc, 06410 biot, france section: research paper keywords: non-volatile memory; flash; reliability; advanced electrical characterization citation: philippe chiquet, jérémy postel-pellerin, célia tuninetti, sarra souiki-figuigui, pascal masson, enhancement of flash memory endurance using short pulsed program/erase signals, acta imeko, vol. 5, no. 4, article 6, december 2016, identifier: imeko-acta-05 (2016)-04-06 section editor: lorenzo ciani, university of florence, italy received september 26, 2016; in final form november 6, 2016; published december 2016 copyright: © 2016 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: philippe chiquet, e-mail: philippe.chiquet@im2np.fr 1. introduction over the last few years, flash memory cells, which can nowadays be found in common products such as usb flash drives and smart cards, have become the dominant non-volatile memory devices in the semiconductor industry. flash memory is built around a floating gate transistor, a mos transistor to which an extra tri-layer stack oxide “oxide/nitride/oxide” (ono) and a top electrode (control gate) have been added, as shown in figure 1. information is stored in the device as an electric charge located in the floating gate which, thanks to the various oxide layers surrounding it, retains this charge when the power supply is removed, thus assuring the non-volatile behaviour of the flash transistor [1]. according to the value qfg of this charge, two distinct logical states ‘0’ and ‘1’ can be differentiated, and the threshold voltage vt of the transistor can be calculated according to equation (1): pp fg tt c q vv −= 0 , (1) where vt0 is the natural threshold voltage of the cell and cpp the control gate / ono / floating gate capacitance [2]. the transition from one state to the other is obtained when electric signals are applied to some of the transistor’s terminals, inducing the injection (or removal) of electrons into (or from) the floating gate through the tunnel oxide. in the remainder of this paper, the logical states corresponding to the highest and lowest threshold voltage values of the floating gate transistor will be called the “programmed” and “erased” states figure 1. schematic (left) and electric (right) representation of a flash floating-gate transistor. abstract the present paper proposes to investigate the effect of short pulsed program/erase signals on the functioning of flash memory transistors. usually, electrical operations related to said devices involve the application of single long pulses to various terminals of the transistor to induce various tunneling effects allowing the variation of the floating gate charge. according to the literature, the oxide degradation occurring after a number of electrical operations, leading to loss of performance and reliability, can be reduced by replacing dc stress by ac stress or by reducing the time spent under polarization by the mos-based devices. after a brief presentation of the functioning of the flash memory transistors tested in this work, the experimental setup used to replace standard electric signals with short pulses will be described. electrical results showing the benefits of programming and erasing non-volatile memories with short pulses will then be presented. acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 30 respectively. programming is usually achieved through channel hot electron (che) injection in flash memories, while erasing is generally obtained thanks to fowler-nordheim (fn) injection [3]. due to this functioning, the tunnel oxide layer, which can be degraded due to repeated electrical operations, is critical to the reliability of the memory devices and various solutions can be considered in order to improve their overall reliability [4]. the present work will focus on the use of short-pulsed signals, which consists in replacing long standard signals by a series of pulses with short plateau widths, as previous studies show that mos-based devices are less impacted by ac stress than dc stress [5]-[11], or by shorter pulses of higher amplitude [12]. 2. generation of short-pulsed signals 2.1. experimental setup in order to investigate the endurance of the tested flash memory cells under short pulses, a complete setup has been developed with the help of a keysight semiconductor device parameter analyzer b1500a [13] equipped with two wgfmus (waveform generator fast measurement unit b1530a), two spgus (semiconductor pulse generator unit) and four smus (source monitor unit), as described in figure 2. the smus are used to measure the ids(vgs) characteristics necessary to the reading (threshold voltage extraction at a given drain current value) of the cell state while the spgu enables the definition of pulses for the successive programming/erasing operations. an agilent 16440a selector switches between the smu and spgu during the endurance test. the additional wgfmu in the setup, connected via rsu (remote-sense and switch unit) enables the definition of arbitrary waveforms and the dynamic measurement of the drain current, which can be useful when evaluating the consumption of the programming operation during the endurance test [14], [15]. 2.2. definition of the applied signals standard (std) flash che signals consist in simultaneous control gate and drain pulses of amplitude vpp equal to +9 v (during 5 µs) and +4.2 v (during 1 µs) respectively, while standard fn erasing is obtained by applying a single 500 µs pulse of amplitude vpp equal to −18 v on the control gate. the first approach here, which will be developed in section 3, consists in replacing standard programming and/or erasing signals by trains of short pulses. these new signals have been chosen so as the “programmed” and “erased” threshold voltages measured for a fresh flash device are close to the threshold voltage values obtained when standard signals are used. it has been experimentally observed that such a condition is met when the total plateau time spent on short pulses is equal to the duration of the standard signal. in order to obtain comparable threshold values using series of short pulses, the standard drain signal in che programming and the standard control gate signal in fn erasing have respectively been transformed into a series of 20 pulses of 50 ns plateau time and a series of 10,000 pulses of 50 ns plateau time. in each case, the signals created with the experimental setup described above have been checked with the help of an oscilloscope as seen in figures 3 to 5. the second approach, which will be the topic of section 4, is much simpler on an experimental level and consists in replacing standard signals with single pulses of higher amplitude and shorter plateau time. figure 3. oscilloscope observation of the control gate (yellow) and drain (blue) signals for a short-pulsed che programming of the flash memory cells. figure 4. close-up observation of signals observed in figure 3. figure 5. oscilloscope observation of the control gate signal of a flash floating gate transistor during a fn erase operation. figure 2. experimental setup used to perform endurance tests with current consumption measurements. acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 31 3. electrical results: pulse trains 3.1. qualitative results and total window closure measurements carried out on numerous devices yielded electrical results highlighting the effect of pulsed program/erase operations on flash memory endurance. endurance of memory cells is mainly characterized by the evolution of their programming window, defined as the difference between the threshold voltages of the “programmed” and “erased” states, over numerous program/erase electrical operations. in order to quantify the gradual programming window closure linked to device degradation, a criterion had to be chosen. in the present work, the total window closure δvt after k program/erase cycles, defined by equations (2) and (3) has been retained: erase t prog tt vvv ∆−∆=∆ , (2) )1()()( ,,, eraseprogt eraseprog t eraseprog t vkvkv −=∆ . (3) according to its definition, δvt is calculated as the sum of two negative values and represents the combined shifts of the “programmed” and “erased” threshold voltages. this total window closure is plotted in figure 6 as a function of the number of program/erase cycles up to 106 cycles. it can be seen that both the short (20×50 ns)/std and std/short (10000×50 ns) cyclings provide a decrease in programming window closure of about 0.5 v compared to the std/std cycling. in order to understand the effect of the pulses’ parameters on this observed endurance improvement, measurements involving different plateau amplitudes and duty cycles have been carried out. 3.2. effect of pulse amplitude electrical results presented in figure 7 compare the programming window closure of the std/short (10000×50 ns) and std/std cyclings for vpp values of −17 v, −18 v and −19 v. while a difference in programming window closure between the two cycling modes can be observed for the first two vpp values after 104 cycles, erasing at vpp = −19 v with short pulses instead of the standard signal makes no difference. this could be explained by the fact that at a given duty cycle, the benefits of replacing dc stress with ac stress become noticeable only if the plateau time of the pulses is reduced as much as possible as the stressing field is increased [11]. other studies have also shown that such results cannot be achieved at ambient temperature when plateau times of the applied pulses become long enough [5]. 3.3. effect of duty cycle in order to achieve lower device degradation, allowing oxide relaxation during electrical stress is a common solution [5], [16]. it has been achieved in this study by increasing the delay between the short pulses constituting the signals applied to the control gate and the drain of the transistor. the duty cycle, which is defined as the ratio of the plateau duration tpl to the period value t as shown in figure 8, is thus decreased. electrical results obtained after short/short cycling (vpp = −18 v) for duty cycles equal to 0.1 and 0.01 have been compared to experimental data obtained for std/std cycling in figure 9. it can be observed that decreasing the duty cycle allows to reduce the programming window closure. using experimental data from figure 9 allows to plot δvt as a function of the duty cycle as shown in figure 10. the total window closure is shown to have a logarithmic behaviour with respect to duty cycle. 3.4. measurement of device consumption energy consumption of non-volatile memory devices has been of recent interest as experimental possibilities offered with the coming of new parameter analyzers in recent years [14], [15]. it can be defined according to the following equation (4): figure 7. programming window closure up to 106 program/erase (std/std and std/short) cycles for various erasing vpp values. each measurement point has been averaged over ten flash memory cells. figure 6. programming window closure up to 106 program/erase (std/std, short/std and std/short) cycles. each measurement point has been averaged over ten flash memory cells. figure 8. schematic representation of a period of the electrical signals applied to the control gate (vgc) or drain (vd) of the memory transistor. the duty cycle is calculated as the ratio of the plateau duration tpl to the period value t. acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 32 ∫= dtive dsdsc , (4) where vds and ids are the drain-source voltage and the drain current of the transistor, respectively. results of time-resolved measurements of drain current performed during a programming operation before and after electrical cycling (std/std and short/std), which can be observed in figures 11 and 12, show that device consumption typically increases with device degradation. figure 13 allows a comparison between the respective consumptions of a fresh flash device during std and short programming operations. taking (4) into consideration, it is pretty obvious that device consumption is higher in the latter case, as the integral of the red curve (short) is clearly higher than that of the black one (std). it can be easily explained as in this case, the pulses constituting the short programming signal are characterized by rise and fall times of the same order of magnitude as the plateau times. these rise and fall times, which do not exist during a std programming signal, make up an important part of the short signal and thus explain this consumption increase. 3.5. reliability of flash memory cells under various cycling modes the statistical behaviour of flash cell populations subjected to std/std, short/short (duty cycle equal to 0.1), and std/short (vpp = −17 v) cyclings has been investigated. flash memory cells remain functional as long as their programming window remains greater than a certain value, below which the figure 10. total window closure as a function of duty cycle obtained from data of figure 9 at 105 and 106 cycles. figure 11. time-resolved measurement of the drain current ids during a standard programming signal before (circles) and after (squares) std/std program/erase cycling (106 cycles). figure 12. time-resolved measurement of the drain current ids during a short (20x50 ns) programming signal before (circles) and after (squares) short/std program/erase cycling (106 cycles). figure 13. current consumption measurements during std (black) and short (red) programming operations performed on a fresh flash device. figure 9. programming window closure up to 106 program/erase cycles obtained for various duty cycle values. each measurement point has been averaged over ten flash memory cells. acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 33 two logical states cannot be distinguished anymore. in that case, the threshold voltage value obtained when reading such a cell would not allow determining if it is “programmed” or “erased”. for each cycling mode, the cells were considered failed when the programming window went below 2 v. generally speaking, failure of electronic systems or components over time has often been described using a weibull distribution [17], [18]. assuming such a distribution in the present case, the ratio of failed cells after k program/erase cycles f(k) can be written according to equation (5): β τ )( 01)( kk ekf − − −= , (5) where k0, τ and β are three fitting parameters of the distribution. weibull plots of experimental data obtained from the three cell populations are presented in figure 14. it can be seen that the failure of the tested flash memory cells can be well described by a weibull distribution. the fitting of experimental data of the three cell populations is realized using the parameter values reported in table 1. as expected, cycling flash cells with signals that are less degrading induces a shift of the failed cell distribution towards higher numbers of cycles. the failure rate [19]-[21] λ(k) can be expressed as a function of the number of cells still functional after k cycles n(k) according to equation (6): dk kdn kn k )( )( 1 )( −=λ . (6) the “bathtub” shape of the failure rate, calculated from experimental data of the three cell populations and shown in figure 15, is typical for such statistical distributions [18], [22]. 4. electrical results: single short erase pulse while the last section investigates the consequences of changing standard program/erase signals into series of short pulses, the idea is here to find the electrical characteristics of a short single pulse that can be used to replace a given standard signal during device operation. the study presented in this section has been limited to erasing pulses. 4.1. choice of the erase pulse characteristics in order to achieve better electrical performance by speeding up the erasing process while targeting a similar vt erase value, the new pulse will be characterized by a shorter plateau time and a higher amplitude (i.e. tpl < 500 µs and |vpp| > 18 v). under these conditions, the necessary plateau time value for a given pulse amplitude can be determined by studying the erasing kinetics of the tested flash devices, according to the method described in figure 16. successive short pulses are applied to the control gate of a programmed cell in order to obtain a progressive shift of its threshold voltage for a given vpp value. the evolution of the threshold voltage can be monitored figure 15. calculated failure rate of flash memory cells subjected to std/std, short/short (dc = 0.1), and std/short (vpp = -17 v) cyclings. figure 16. experimental method to investigate erasing kinetics of the tested flash cells. table 1. best weibull parameters allowing the fitting of experimental data for memory cells subjected to std/std, short/short (dc = 0.1), and std/short (vpp = -17 v) cyclings. cycling condition k0 τ β std/std 6.11×104 9.29×104 1.02 short/short dc = 0.1 1.56×105 2.86×105 1.10 std/short vpp=-17 v 5.78×104 1.58×105 1.01 figure 14. weibull plot obtained for flash memory cells subjected to std/std, short/short (dc = 0.1), and std/short (vpp = -17 v) cyclings. the lines displayed in that figure correspond to the fitting of experimental data using weibull parameters found in table table 1. acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 34 through reading operations performed between these individual pulses. electrical results for vpp values ranging from −18 v to −23 v and from −24 v to −31 v are displayed on figures 17 and 18, respectively, where the threshold voltage evolution is plotted as a function of the erasing time, defined in this experiment as the total plateau time spent on the various pulses depicted in figure 16. the particular erasing time needed to reach the targeted vt erase value (equal to 2.9 v as measured after an erasing operation using a standard signal) can be extracted for each vpp value as the intersection point between the corresponding experimental curve and the dotted line in both figures. these results show that for appropriate amplitudes (typically |vpp| > 26 v), the erasing pulse’s plateau time could be shorter than 50 ns to reach a vt erase value of 2.9 v. assuming rise/fall times of 100 ns and tpl = 50 ns, the standard erasing operation (lasting around 500 µs) could be sped up by a factor 2000, all the while retaining the typically low fowler-nordheim current consumption. the use of shorter and higher-amplitude erasing pulses could thus be considered for various applications where higher voltages are tolerated and higher device speed is of primordial importance. 4.2. total window closure in order to test the endurance of flash cells subject to these new erasing pulses, electrical program/erase cyclings (with standard programming operations) have been performed for vpp values ranging from −18 v to −28 v. results presented in figure 19 show that the total window closure of the flash cells is decreased when compared to the std/std case (vpp = −18 v). it can however be noticed that the flash devices’ endurance is optimal for an intermediate vpp value which seems to be around −20 v. device degradation, monitored through programming window closure, should be more important as the amplitude of the erasing pulse increases (see figure 7 for example), but that phenomenon is compensated by the resulting decrease of the plateau time. to further explain the existence of such an optimal amplitude value, tcad simulations could be considered in order to monitor the temporal evolution of the tunnel oxide electric field in each case, and in particular, to try to estimate the impact of the erasing pulse rise time, which should limit the oxide electric field at higher vpp values. endurance tests have also been performed for vpp values up to −40 v, but due to technical limitations of the parameter analyzer used for this study (the minimum value that can be set for tpl being 10 ns), the approach presented at the beginning of this section had to be modified. for erasing pulses of amplitude equal to −30 v, −35 v and −40 v plateau time had to be set to 10 ns, and as a consequence the obtained threshold voltages (reported in table 2) are well below the standard value (i.e. 2.9 v). electrical results presented in figure 20 show that window closure is generally smaller than the one obtained with standard signals, except for vpp = −40 v. even in the latter case where the window closure is close to the standard one, it should not be as critical for device operation as the initial programming window is much larger to begin with. figure 19. programming window closure up to 106 program/erase cycles for various erasing vpp values (from -18 v to -28 v). each measurement point has been averaged over ten flash memory cells. table 2. “erased” vt values obtained for tpl = 10 ns. amplitude threshold voltage -30 v 2 v -35 v -3 v -40 v -6.8 v figure 17. erasing kinetics for -23 v < vpp < -18 v. “erased” threshold voltage measured after a standard erasing operation is added for comparison (dotted line). figure 18. erasing kinetics for -31 v < vpp < -24 v. “erased” threshold voltage measured after a standard erasing operation is added for comparison (dotted line). acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 35 5. conclusions the impact of the use of short pulses on the endurance of flash memories during the che programming and fn erasing operations has been investigated in this paper. the proposed experimental setup helped produce two main types of signals (trains of short pulses or single short pulses of high amplitude) that could potentially be used for specific industrial applications. replacing standard program/erase signals by trains of short pulses with plateau times as low as 50 ns allowed the observation of a decrease in programming window closure during program/erase cycling. the impact on this closure of parameters such as amplitude or duty cycle of the signals applied to the terminals of the memory transistors has been investigated. as the results presented in section 3 show, the endurance improvement obtained by replacing standard control gate and drain signals by series of short pulses can only occur at a cost as the overall signal durations and device consumption have been shown to increase. this trade-off could prove useful in specific application fields (i.e. automotive and avionic industries) where reliability of the memory devices is the most important factor. moreover, automotive and avionic applications typically require memory devices to operate within a much larger temperature range (typically −40 °c to 150 °c). as some other authors have shown, decreasing the duty cycle value yields better electrical results at higher temperatures [5], making the present results potentially even more interesting for such applications. replacing standard (erasing) signals by single shorter pulses of higher amplitude seems to be another recipe to produce a noticeable enhancement of the endurance of flash memory devices, as the programming window closure has also been shown to decrease in this case. moreover, current consumption should remain quite low, and the erasing speed can potentially be greatly improved. on the downside, these signals should only be used for applications in which higher voltages can be tolerated. as a perspective, many other types of measurements could be considered to complete the study presented here. firstly, both approaches showcased respectively in sections 3 and 4 of this paper could probably be combined to maximize the endurance enhancement of flash devices (i.e. train of short pulses characterized by |vpp| > 18 v). moreover, measurements presented in this work could be reproduced using different plateau times, duty cycles, rise/fall times to be able to understand the effect of these parameters on device degradation and to optimize them. finally, complementary measurements could be performed on test mos capacitors to investigate the effect of short-pulsed electrical stress in terms of interface trap density [16] and stress induced leakage current (silc), which are physical manifestations of oxide degradation that can lead to program/erase window closure in non-volatile memory devices. acknowledgement authors wish to thank jean-luc ogier (stmicroelectronics, rousset) for his technical support. references [1] w.brown, j.brewer, “nonvolatile semiconductor memory technology: a comprehensive guide to understanding and to using nvsm devices”, ieee press, 1998. [2] p. pavan, r. bez, p. olivo, e. zanoni, “flash memory cells an overview”, proc. of the ieee 85 (8) (1999), pp. 1248-1271. [3] r.laffont, r.bouchakour, o.pizzuto, j.-m.mirabel, “a 0.18 um flash source side erasing improvement”, proceedings of nvmts (2004), pp. 105-109. [4] j.postel-pellerin, g.micolau, p.chiquet, j.melkonian, g.just, d.boyer, c.ginoux, “setting up of a floating gate test bench in a low noise environment to measure very low tunnelling currents”, acta imeko, vol.4, no.3, 2015, pp. 36-41. [5] b.rebuffat, j-l.ogier, p.masson, m.mantelli, r.laffont, “relaxation effect on cycling on nor flash memories”, proc. of ieee international conference on electron devices and solid-state circuits (edssc), 2015, pp. 613-616. [6] a.cester, a.paccagnella, g.ghidini, “stress induced leakage current under pulsed voltage stress”, solid-state electronics, vol.46, 2002, pp. 399-405. [7] m.nafria, j.sune, d.yelamos, x.aymerich, “high field dynamic stress of thin sio2 films”, microelectron. reliab., vol.35, no.3, 1995, pp. 539-553. [8] d.caputo, r.feruglio, f.irrera, b.ricco “effect of pulsed stress on leakage current in mos capacitors for non-volatile memory applications”, proc. of ieee european solid-state device research conference (essderc), 2002, pp. 567-570. [9] f.irrera, b. ricco, “pulsed tunnel programming of nonvolatile memories”, ieee trans. on electron devices, vol.50, no.12, december 2003, pp. 2474-2480. [10] a.chimenton, f.irrera, p.olivo, “improving performance and reliability of nor-flash arrays by using pulsed operation”, microelectronics reliability, vol.46, 2006, pp. 1478-1481. [11] f.irrera, t.fristachi, d.caputo, b.ricco, “optimising flash memory tunnel programming”, microelectronics engineering, vol.72, 2004, pp. 405-410. [12] p.canet, r.bouchakour, j.razafindramora, f.lalande, j.m.mirabel, “very fast eeprom erasing study”, proc. of ieee european solid-state circuit conference (esscirc), 2002, pp. 683-686. [13] keysight technologies, keysight b1500a data sheet, february 2016. [14] v.della marca, j.postel-pellerin, g.just, p.canet, j-l.ogier, “impact of endurance degradation on the programming efficiency and the energy consumption of nor flash memories”, microelectron. reliab., vol.54, no.9-10, 2014, pp. 2262-2265. figure 20. programming window closure up to 106 program/erase cycles for various erasing vpp values (from -30 v to -40 v, with standard -18 v results for comparison). each measurement point has been averaged over ten flash memory cells. acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 36 [15] v.della marca, t.wakrim, j.postel-pellerin, “advanced experimental setup for reliability and current consumption measurements of flash non-volatile memories”, proc. of the 20th imeko tc4 international symposium, 2014, pp. 10361040. [16] b.rebuffat, p.masson, j-l.ogier, m.mantelli, r.laffont, “effect of ac stress on oxide tddb and trapped charge in interface states” proc. of international symposium on integrated circuits (isic), 2014, pp. 416-419. [17] american department of defense, military handbook – reliability prediction of electronic equipment (mil-hdbk-217f), 1991. [18] u.gurel, m.cakmakci, “impact of reliability on warranty: a study of application in a large size company of electronics industry”, measurement, vol. 46, 2014, pp. 1297-1310. [19] m.catelani, l.ciani, s.rossin, m.venzi, “failure rates sensitivity analysis using monte carlo simulation”, proc. of the 13th imeko tc10 workshop on technical diagnostics, 2014, pp. 195-200. [20] l.ciani, m. catelani, “a fault tolerant architecture to avoid the effects of single event upset (seu) in avionics applications”, measurement, vol. 54, 2014, pp. 256-263. [21] m.venzi, s.rossin, c.michelassi, c.accillaro, m.catellani, l.ciani, « improved fbd and rbd generation for system reliability assessment”, proc. of the 12th imeko tc10 workshop on technical diagnostics, 2013, pp. 266-270. [22] c.calaiselvan, l.rao, “accelerated life testing of nano ceramic capacitors and capacitor test boards using non-parametric method”, measurement, vol. 88, 2016, pp. 58-65. enhancement of flash memory endurance using short pulsed program/erase signals acta imeko, title acta imeko issn: 2221-870x december 2016, volume 5, number 4, 80 acta imeko | www.imeko.org december 2016 | volume 5 | number 4| 80 introduction to selected papers from the xxi imeko world congress 2015 paolo carbone university of perugia, italy section: editorial citation: paolo carbone, editorial to selected papers from the xxi imeko world congress 2015, acta imeko, vol. 5, no. 4, article 12, december 2016, identifier: imeko-acta-05 (2016)-04-12 section editor: paul regtien, the netherlands received december 26, 2016; in final form december 26, 2016; published december 2016 copyright: © 2016 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: paolo carbone, email: paolo.carbone@unipg.it dear reader, the last papers in this issue were presented at the 2015 imeko world congress 2015 in their original version and then updated and extended for being published in this journal after the usual review process. the first paper by maik rosenberger et al. addresses the subject of measuring geometric parameters using ratiometric cameras. the procedure proposed by the authors follows a standard published by the european machine vision association and presents experimental results aimed at discussing the applicability characteristics of such standard. the final paper is authored by petri österberg et al. and addresses the topic of measurements of moisture in biomaterials using microwaves and nmr. it describes in full detail the procedure followed in both cases and presents experimental results that also include data from the standard loss-on-drying method that apparently remains the most accurate and precise among the three approaches. have a fruitful reading of the fourth issue of acta imeko in 2016! introduction to selected papers from the xxi imeko world congress 2015 microsoft word 251-1770-1-le-rev2 acta imeko  issn: 2221‐870x  september 2015, volume 4, number 3, 36 ‐ 41    acta imeko | www.imeko.org  september 2015 | volume 4 | number 3 | 36  setting up of a floating‐gate test bench in a low noise  environment to measure very low tunneling currents  jeremy postel‐pellerin 1 , gilles micolau 2 , philippe chiquet 1 , jeanne melkonian 1 , guillaume just 1,3 ,  daniel boyer 4 , cyril ginoux 2   1  aix‐marseille university, im2np‐cnrs, umr 7334, imt technopole de chateau‐gombert, 13451 marseille, france  2  avignon university, emmah‐inra, umr 1114, 33 rue louis pasteur, 84000 avignon, france   3  stmicroelectronics, zi de rousset bp 2, 13106 rousset cedex, france  4  laboratoire sous‐terrain bas bruit, lsbb‐cnrs, ums 3538, la grande combe, 84400 rustrel, france       section: research paper   keywords: flash memory; reliability; floating‐gate technique; leakage current; low noise   citation: jeremy postel‐pellerin, gilles micolau, philippe chiquet, jeanne melkonian, guillaume just, daniel boyer, cyril ginoux, setting up of a floating‐gate  test bench in a low noise environment to measure very low tunneling currents, acta imeko, vol. 4, no. 3, article 6, september 2015, identifier: imeko‐acta‐ 04 (2015)‐03‐06  editor: paolo carbone, university of perugia, italy  received february 14, 2015; in final form april 17, 2015; published september 2015  copyright: © 2015 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  corresponding author: jeremy postel‐pellerin, e‐mail: jeremy.postel‐pellerin@im2np.fr    1. introduction  the floating-gate technique (fgt) has been developed for many years [1], [2] in order to evaluate very low leakage currents through dielectrics. these currents are not accessible through direct electric characterization, even with high-performance analyzers [3], [4]. the principle of the technique lies in evaluating a slow temporal evolution of the electric charge of a mos capacitance by an indirect measurement. the floatinggate device is a micro-electronic integrated test device. it is embedded in the scribe lines of a wafer and has to be probed under needles. in the context of non volatile memories (nvm) reliability studies [5]-[9], the time range of one experiment is typically between several weeks and several months. to maintain the electrical contacts assured by needles over this long time, this technique requires protecting the probed device from external perturbations (principally mechanical vibrations and thermal variations). in a classical laboratory or industrial environment, it is difficult to set such a dedicated prober. for instance, this technique has been benched in the im2np characterization room [10] in a context of nvm flash type reliability studies. on working days, the electrical contact of the needle can be hold, in best cases, over several hours and during winter holidays (empty laboratory), over a maximum of ten days. the development of this technique in a classical environment has been proposed in previous studies [11]. it requires a self-sustained prober, with compressed air network, in a thermalized artificial environment. this solution is technically and economically heavy (price of the hardware, space consuming, thermal control). if not impossible, it is very difficult to be developed. thus, the main idea of our experimental platform lies in placing it in an underground room, naturally insulated from external perturbations. in the south of france near apt, the very low noise underground laboratory, called lsbb ("laboratoire sous-terrain à bas bruit") [12], offers such facilities. it consists in a set of horizontal galleries dug in the bedrock of big mountain, bordering the south albion plateau. it was dedicated to be the abstract  we propose and develop a complete solution to evaluate very low leakage currents in non‐volatile memories, based on the floating‐ gate technique. we intend to use very basic tools (power supply, multimeter,...) but with a very good current resolution. the aim of  this work is to show the feasibility of such measurements and the ability to reach current levels lower than the ones obtained by any  direct measurement, even from high‐performance devices. the key node is that the experiment is led in a very particular low‐noise  environment (underground laboratory) allowing to keep the electrical contacts on the device under test as long as possible. we have  demonstrated the feasibility of this approach and obtained a very promising 10 ‐17 a current level in less than two weeks.   acta imeko | www.imeko.org  september 2015 | volume 4 | number 3 | 37  former cockpit shooting #1 of the french nuclear deterrent force, from 1973 to 1998. rooms and galleries are shielded from electromagnetic waves, by concrete and massive slabs of steel. the depth varies between ground level down to 525 meters underneath the top of big mountain. the first peculiarity of this underground laboratory (compared to others) lies in its location in a natural national park, far from big communication ways (no highway, no railroad). the second appreciable peculiarity is its size: there are a lot of rooms of different sizes, each of them offering classical facilities (electrical and informatic network) for a scientific environment. the lsbb has been used by the national institute of universe (insu) as a hydrogeological, geophysical (net of seismographs) and astronomical (muons detectors) observatory for a few years. it also allows to test microelectronic devices in a nonradiative environment. for our purpose, this laboratory is a very good environment since it naturally prevents from external perturbations (electromagnetic, mechanical, thermal variations) without any additional devices. it is important to notice that this laboratory is easy to access. moreover the occupation fee for a single room is cheaper than a self-sustained marble table. because the fgt only requires very basic electrical measurements (applying biases and acquiring currents), the technical hardware is standard and the probe station is a light one (35 kg). the entire experimental platform occupies around 2 square meters. this paper contains three major parts of development and a conclusion. first a background concerning nvm and fgt measurement is exposed, next the details of the experimental setup are presented, and finally the first results are shown and limitations of the measurements are discussed. 2. backgrounds  2.1. non‐volatile memories  the most widespread solution to enable semiconductor memories to be non-volatile that is to say able to keep information without any power supply is to use mos transistors whose threshold voltage is shifted by a charge stored in an isolated gate above the channel [5]. the most common technology consists in adding a second gate between the gate and the channel of a classical mos transistor. flash memories are based on this principle. this second gate, made of conductor or semiconductor materials, can isolate charges to make the transistor threshold voltage variable. most of the time, charges are injected through a dielectric, in general a thin silicon dioxide (sio2) layer, placed between the transistor channel and this second gate, as presented in figure 1 (left). lastly, the two gates of this "transistor" are also separated by a dielectric, most commonly a tri-layer stack oxide "oxide/nitride/oxide" (ono). thus, the nvm elementary cell can be seen as a classical mos transistor in series with a capacitor cpp (figure 1 right). the common electrode is called "floating-gate" (fg) because its electrical potential cannot be leaded by an external contact. it can store a charge while the top electrode of the capacitor becomes the "control-gate" (cg) of the cell. barrier transparency in the tunnel oxide, which can be electrically modeled by a current source i, allows the injection of charges in the floating gate, shifting the mos transistor threshold voltage vt according to (1), thus defining two different logical states: (1) where vt0 is the natural threshold voltage of the cell, qfg the charge amount in the floating-gate and cpp the capacitance between the control-gate and the floating-gate. a total quantity of elementary charges (electrons or holes) qfg is injected into the fg using an adequate set of biases, depending on technology [5]-[9]. these charges must be stored for a long time in the fg but they can escape from it through a damaged dielectric. according to (1), if charges are lost, the stored information will also be lost, since the two logical states cannot be distinguished anymore. a better understanding of physical phenomena responsible for the charge leakage, generally related to tunneling currents, is crucial to improve the whole quality of memory cells. this leakage current is not directly measurable in the nvm device. the quality of this dielectric is a key for high reliability devices. that’s why it is important to develop powerful methods allowing precise electrical characterizations of this dielectric. the main difficulty is to reach the very low current levels, responsible for such leakage, because they are not accessible through direct measurements, even with highperformance analyzers [3], [4]. thus, some indirect measurement techniques have to be used. the floating-gate technique is one of the most widely used methods. 2.2. floating‐gate technique (fgt)  the fgt is based on the use of a mos capacitor and a mos transistor in parallel [1], [2] as presented in figure 2. it consists in a high-voltage transistor whose gate is common with the top electrode of a large tunnel capacitor, denoted ctun. this common gate plays the role of the floating-gate in a memory cell but is directly accessible to apply biases. the tunnel capacitor represents the injection zone of the memory cell. indeed it has exactly the same process conditions as the injection region (oxide thickness, doping conditions…). nevertheless, its area is much larger than that of the memory cell. since the area ratio between the capacitor and the transistor is very high (around 1200:1) and the hv oxide is twice as thick as the tunnel oxide, the observed leakage is almost exclusively due to the tunnel capacitor. the fgt is based on the voltage measurement of the initially charged gate of the mos capacitor, which is then disconnected from the external circuit during the experiment. figure 2. floating‐gate test structure.  figure  1.  schematic  of  a  flash‐type  nvm  (left)  and  its  electrical  scheme (right).  acta imeko | www.imeko.org  september 2015 | volume 4 | number 3 | 38  the measurement of this slowly decreasing gate voltage is performed indirectly through the measurement of the drain current of the transistor sharing its gate with the capacitor. this transistor "converts" the charge of the capacitor, and thus the gate voltage, in a measurable drain current. the temporal variation of this drain current is directly linked to the variation of the gate voltage and thus to its gate charge which is the same as the capacitor charge. figure 3 depicts the full methodology to obtain the leakage current ileak as a function of the floatinggate voltage vfg. the drain current acquisition over time can be directly linked to a floating-gate voltage variation thanks to a preliminary ids(vfg) measurement, illustrated in figure 4. the floating-gate voltage is the image of the floating-gate charge qfg. it is extracted from the ctun(vfg) characteristics, presented in figure 5, using (2): (2) the leakage current ileak is then the variation of this floatinggate charge qfg over time (3): (3) the acquisition of the drift of the drain current over very long time ranges is performed, keeping electrical contacts on the transistor (except on the floating-gate) during the whole experiment. as already detailed in the introduction, the main difficulty is to keep these electrical contacts for months: the longer the measurement, the lower the extracted current. the improvement presented here consists in developing a simple but very sensitive test bench. the low noise environment ensures the robustness against external perturbations. 3. low noise environment   the experimental platform has been installed at the end of the lsbb gallery, where the perturbations are supposed to be the smallest. 3.1. proposed test bench  the test bench is composed of a classical probe station with 5 manipulators and needles. these manipulators are directly screwed on the probe station instead of being fixed by vacuum because the air pump would create undesired vibrations. the needles are then connected using standard coaxial cables to the power supply and the multimeter. an agilent e3631a triple output dc power supply has been chosen because of its very good stability over time (0.1 % + 5mv after 12 months) [13]. its cost is relatively limited and its use is widely spread, making the development of the remote controlling program easier. concerning the drain current acquisition, the measured level is around 200 μa and a good resolution is needed to measure very low variations of this drain current. the tektronix dmm4050 digital multimeter with a 6.5 digit and 100 pa resolution has been chosen [14]. its cost is also relatively limited and it is easily programmable with any classical remote controlling software. the probe station is placed on a wood and iron workbench with an anti-vibration mat to absorb potential external perturbations. the power supply and the multimeter are placed on a separate support, also to reduce parasitic vibrations. the global test bench is shown in figure 6. to avoid as much as possible any movement around the test bench, a fully remote controlled experiment is required. figure  5.  preliminary  ctun(vfg)  measurement  on  the  capacitor  from  the  floating‐gate test structure.  figure  4.  preliminary  ids(vfg)  measurement  on  the  transistor  from  the floating‐gate test structure.  figure  3.  the  five  key  points  of  floating‐gate  technique  methodology  to  extract leakage current.  figure 6. picture of the proposed test bench, embedded  in the  low‐noise  environment.  acta imeko | www.imeko.org  september 2015 | volume 4 | number 3 | 39  3.2. remote controlling the experiment  labview from national instruments has been chosen as a remote controlling software, which enables the use of the gpib interface available on the devices to control them. a first program has been developed to perform the initial ids(vfg) characteristics presented in figure 4. the floating-gate of the transistor (and the capacitor) is then charged at the initial bias. the floating-gate needle is then lifted to let the capacitor slowly discharge through the dielectric. the drain current acquisition (three current measurements, repeated each minute), controlled by a second program, is performed. data are stored in a text file and saved regularly to be exported to an external computer located out of the lownoise environment. 4. preliminary measurement results  4.1. raw measurements  the first raw temporal acquisition of the drain current is shown in figure 7. noise currents appear randomly at different days. the relative rms value is around 5 % of the mean drain current value (around 225 μa). it seems that there is an electrical external perturbation responsible for that. at this time we are not able to clearly explain the origin of this noise, but the ways to explore are linked to the hardware used in the setup experiment (pc, usb/gpib adaptor…). it is probably an electrical phenomenon because during this first acquisition, there was a significant earthquake in barcelonnette (small town in the "alpes de haute-provence") located 50 km from the lsbb. this earthquake occurred on 07/04/2014 at 21h17min (local time), with a 5.1 magnitude on the richter’ scale [15]. all the seismographs in the lsbb have detected the earthquake, including the nearest of our platform (located at a few meters), but measurements have not been perturbed as can be seen in the zoom of the measurements presented in figure 8. that is to say the mechanical contacts between the points and the electric pads of the tested device seem to be reliable for high frequency perturbations. the high frequency noise observed in measurements is visible because when noise occurs, the three consecutive measurements at the same minute are different. that is to say the current noise is at higher frequency than the time resolution measurement of the dmm4050 multimeter. to further explore the reason of this noise on the drain current acquisition, the spectrum of this noise current could be studied. nevertheless, this noise current has been removed by increasing the number of successive acquisitions while drastically reducing the number of measurements over time (six successive acquisitions every hour instead of three successive acquisitions every minute previously) and with a higher integration time (2 seconds instead of some tens of milliseconds previously). the improvement can be observed in figure 9. 4.2. estimation of the global sensitivity of the measurement  protocol  figure 10 presents the results of measured drain currents (ids), on a discharged capacitor, for around 400 measurements, each separated by 20 minutes. three drain polarizations (vds = 0.1 v ; vds = 0.3 v and vds = 0.5 v) have been investigated to eventually detect a sensitivity due to the drain polarization. the statistical means and standard deviation of the drain current extracted from figure 10 are reported in table 1. these results are coherent with the ones obtained when the multimeter dmm4050 is not connected (empty measurements) figure 7. raw drain current acquisition. each vertical line denotes a change of day (midnight), from 03/04/2014 to 16/04/2014. the initial gate voltage of the capacitor is 7v here and the drain‐source bias is vds = 0.1 v.  figure 9.  improved raw current measurements, using the new acquisition  protocol.  figure 10. statistical global current sensibility estimation.  figure  8.  raw  drain  current  acquisition  during  the  earthquake  day  07/04/2014. (same legend than figure 7).  acta imeko | www.imeko.org  september 2015 | volume 4 | number 3 | 40  to any device and the ones given by the constructor [14]. the lowest significant variation of the current can be estimated around 1 na while the lowest significant current measurable can be evaluated around 8 na. for the following process of extraction of the leakage current, those values are not critical. however it will be important to take them into account at the end of the retention time for very low values of ids. indeed, the derivation process of the charge is impacted by the precision in the measurements. 4.3. low leakage current extraction   from non-noisy measurements presented in figure 9 and the preliminary characteristics, the total electrical charge in the floating gate is extracted according to the fgt methodology (figure 3). the numerical derivative (or a local fitting) gives the leakage current assuming that the numerical variations taken into account are greater than the stds given in table 1. it is clearly the case for the level of currents involved in this section. then, it is also possible to plot this leakage current versus the floating-gate voltage as presented in figure 11. we can first notice in figure 11 the continuity between the two methods (direct measurements and fgt extraction) at high current levels around 6.5 v. for low levels, the best result obtained with a classical agilent hp4156 (with a long integration time and force/sense cables) is around 10-15 a. however, using the fgt, in less than two weeks two additional decades have been reached with a level around 10-17 a. to our knowledge, keeping electrical contacts over such a long time in a classical laboratory has needed very heavy equipment with extracted current levels over 10-17 a after a maximum of 45 days [11]. moreover, the resolution obtained for the leakage current can be statistically estimated. a first evaluation is done thanks to the three successive acquisitions performed during the experiment. considering these three measurements as three independent acquisitions, the difference between the maximum value and the minimum value of these three independent extractions of the leakage current is plotted in figure 12.   we notice that the resolution of the extraction improves drastically during the experiment. indeed, for relatively high current level extraction the resolution is around 10-14 a but when extracting lower current levels, the resolution decreases down to around 10-19 a. it is one of the main known advantages of the floating-gate technique to have a resolution improving with time. this is a promising preliminary result, awaiting better results from further measurements which can be performed over months in the proposed specific environment. 5. conclusion and perspectives  following the very classical protocol of the floating-gate technique, the feasibility of an approach with a very simple test bench, embedded in a very low-noise environment has been proven. in less than two weeks a leakage current lower than the minimum directly measurable one with any classical tool has been extracted. the first improvement performed on the proposed experimental platform has been to find the reasons of the electrical noise and to reduce it by modifying the acquisition methodology. the natural perspective of this work is the continuation of the drain current acquisition in a long-time range. it also requires a reliable and reproducible process to extract leakage current from the charge versus time evolution. this experimental work should also be completed by a theoretical and numerical approach that would allow to theoretically and numerically model the loss of charges through the oxide. the developed fgt can also be reproduced on oxides, previously damaged with conditions close to those used in nvm products, to study the reliability of such memory cells. references  [1] b. fishbein, d. krakauer, b. doyle, “measurement of very low tunneling current density in sio2, using the floating-gate technique”, ieee electron device letters, vol. 12, no. 12, 1991, pp. 713-715. [2] f.h. gaensslen, j.m. aitken, “sensitive technique for measuring small mos gate currents”, ieee electron device letters, vol. 1, no. 11, 1980, pp. 231-233. [3] agilent4156c precision semiconductor parameter analyzer user’s guide vol. 2, 6th edition, march 2008. [4] agilent b1500a semiconductor device analyzer data sheet, october 2013. [5] w. brown, j. brewer, “nonvolatile semiconductor memory technology: a comprehensive guide to understanding and to using nvsm devices”, ieee press, 1998. figure 12. evaluation of the floating‐gate technique resolution using three  independent acquisitions during a single experiment.  table  1.  statistical  mean  and  standard  deviation  (std)  of  the  measurements presented in figure 10.  vds (v)  0.1  0.3  0.5  mean (na)  6.54  7.34  7.62  std (na)  1.1  0.97  0.95  figure  11.  comparison  between  direct  measurement  and  floating‐gate  technique extraction.  acta imeko | www.imeko.org  september 2015 | volume 4 | number 3 | 41  [6] p. canet, r. bouchakour, n. harabech, p. boivin, j.-m. mirabel, “eeprom programming study-time and degradation aspects”, proc. of iscas, 2001, pp. 846-849. [7] r.laffont, r.bouchakour, o.pizzuto, j.-m.mirabel, “a 0.18um flash source side erasing improvement”, proc. of nvmts, 2004, pp. 105-109. [8] p. canet, r. bouchakour, n. harabech, p. boivin, j.-m. mirabel, c. plossu, “study of signal programming to improve eeprom cells reliability”, proc. of mwscas, 2000, pp. 11441147. [9] p. canet, r. bouchakour, j. razafindramora, f. lalande, j.m. mirabel, “very fast eeprom erasing study”, proc. of esscirc, 2002, pp. 683-686. [10] p. chiquet, “etude et modélisation des courants tunnel: application aux mémoires non-volatiles”, phd thesis, aixmarseille université (france), 2012. [11] s. burignat, “mécanismes de transport, courants de fuite ultrafaibles et rétention dans les mémoires non volatiles à grille flottante”, phd thesis, insa lyon (france), 2004. [12] http://www.lsbb.eu/index.php/en/ [13] agilent e3631a triple output dc power supply service guide 12th edition, agilent technologies, april 21, 2014. [14] tektronix dmm4050 digital multimeter users manual, 0770361-00, october 05, 2009. [15] url: http://www.lsbb.eu/index.php/fr/ct-menu-item-243/ctmenu-item-244/ct-menu-item-248 template for an acta imeko event paper acta imeko issn: 2221-870x december 2017, volume 6, number 4, 95-99 acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 95 assessment of mass comparator sensitivity christian müller-schöll mettler-toledo gmbh, im langacher 44, 8606 greifensee, switzerland section: research paper keywords: mass calibration; comparator sensitivity; uncertainty; mass comparator; oiml r111 citation: christian müller-schöll, assessment of mass comparator sensitivity, acta imeko, vol. 6, no. 4, article 15, december 2017, identifier: imeko-acta06 (2017)-04-15 editor: paolo carbone, university of perugia, italy received january 29, 2016; in final form november 18, 2017; published december 2017 copyright: © 2017 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: mettler-toledo gmbh, greifensee, switzerland corresponding author: christian müller-schöll, e-mail: christian.mueller-schoell@mt.com 1. introduction when calibrating weight pieces, the difference between the unknown weight and the reference weight is calculated from the indications of the mass comparator display (or the data interface signal). this difference is then used to calculate the conventional mass of the unknown weight. for common industrial and laboratory applications, weight pieces are calibrated in conventional mass [1] and electronic balances and mass comparators also indicate in this conventional unit. the term sensitivity is defined in [2] as “quotient of the change in an indication of a measuring system and the corresponding change in a value of a quantity being measured” [2, 4.12]. according to this definition the general equation for sensitivity of a mass comparator is 𝑆 = ∆𝐼 ∆𝑚𝑐 (1) with change in indication ∆𝐼 and conventional mass of a weight ∆𝑚𝑐 (and not the reciprocal value as is sometimes used, e.g. in [3] and [7]). the general assumption is that for electronic balances and mass comparators, the difference ∆𝐼 of the indications is equal to the difference in conventional mass ∆𝑚𝑐. this corresponds to a sensitivity equal to 1. however, there are sources that suggest that this is not always the case. in earlier days, it was apparently usual that comparators with an optical scale did not necessarily indicate correct mass differences. therefore, [3, sop4] and [1, c.4.1.2], for example, describe weighing cycles for mass calibrations including a sensitivity weight (s), e.g. of the form “a – b – b+(s) – a+(s)”. with these cycles, the sensitivity is determined in every single weighing cycle to convert the scale divisions into a mass difference. in section 2 we will review existing literature on this subject. sections 3 and 4 deal with uncertainty questions. in sections 5 through 7 we derive a procedure for assessing comparator sensitivity based on optimized uncertainty. section 8 presents a verification through experimental data. the conclusions finally summarize the findings. 2. review of the literature 2.1. testing procedures for sensitivity according to some sources in the literature, it is not necessary to determine sensitivity in every weighing cycle for modern electronic comparators: • "a sensitivity weight is not required if the electronic mass comparator that is used has been tested (with supporting data available) to determine that the balance has sufficient accuracy…" [3, gmp 14]. the uncertainty of this assumption shall be included as an uncorrected systematic error in the uncertainty budget and acceptable limits are "2 %" [3, sop8]. abstract for the calibration of weight pieces, the evaluation of comparator sensitivity and its associated uncertainty component is essential. yet, there is not much documented guidance on how to assess these values. this paper proposes a procedure for assessing, evaluating and optimizing mass comparator sensitivity. acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 96 • kochsiek et al. mention in the german edition of their book “massebestimmung” [4] (translated to english: “mass determination”) that for a “frequently adjusted electromagnetically compensated balance” sensitivity 𝑆 shall be assumed to be 1 and an uncertainty in 𝑆 shall be assumed to be less than 5 𝐸 − 04 or may even be neglected, especially when using auxiliary weights …”.[4]. however, it remains unclear how to adjust the comparator so that it fulfils the condition of being “adjusted”, nor are there any suggestions as to the accuracy and thus uncertainty of the adjustment procedures, nor as to how often an adjustment must take place to fulfil the requirement of being “frequently adjusted”. it is interesting to note that the cited numerical value (5 𝐸 − 04) is omitted in the (later) english edition of the same book [5]. • chapter c.6.4.2 of [1] requires that an uncertainty component for sensitivity be included in the budget when calibrating weights, but gives no specific guidance on how to assess the value of this component and especially on how to select a proper sensitivity test weight regarding its size and its calibration quality. in general, there are two ways to treat sensitivity: one is to assume ideal sensitivity and set a limit for the deviation of the sensitivity from "1" and test and verify the sensitivity against that limit. this limit value is considered in the uncertainty budget as an uncorrected error. the other one is to evaluate a value for sensitivity, correct all balance readings with this value, calculate the uncertainty of this correction and have this contribute to the uncertainty budget (which results in a smaller uncertainty but requires a higher calculation effort). since mass calibrations involve a significant amount of calculations, it is common practice today to use at least software spreadsheets or even dedicated software for the calculations. if this is the case, the sensitivity value can easily be incorporated as a correction in the calculation of the mass differences and its uncertainty will then contribute to the uncertainty calculation. in some commercially available calibration software (e.g. scalesnet, mclink), a numerical value of sensitivity is determined in a separate test for each comparator. but for its uncertainty, the uncertainty value given in [4] is frequently used without alteration. where the mass difference between the calibrated weight and the reference weight becomes large, this might have significant influence on the final combined calibration uncertainty of the weight piece under test. one result of this paper is to answer the question whether this worst case estimation of a sensitivity uncertainty of 5 𝐸 − 04 is justified. 2.2. test weights for sensitivity testing there seems to be common understanding that sensitivity of mass comparators is tested with a “small” weight: reference [3, sop 2] mentions a “small weight”, [3, sop 34] mentions a maximum of 0.5 % of balance capacity, while [3, gmp 10] mentions a maximum of 1 % of balance capacity. it remains unclear if "balance capacity" means the full load value or the electrical weighing range in this context. no standard method for the selection of the sensitivity test weight appears to be available. 3. assessment of sensitivity the vim [2] defines adjustment of a measuring system as “set of operations carried out on a measuring system so that it provides prescribed indications corresponding to given values of a quantity to be measured”. today’s mass comparators with electromagnetic force compensation are equipped with provisions for self-adjustment. these consist of one (or more) internal weight piece(s) and an algorithm which can be time and/or temperature controlled or manually triggered. furthermore, external mass calibration software allows for an additional “adjustment” of the reading by applying a sensitivity factor in the processing of the value that was read from the comparator output. in this light, we consider the calibration software a part of the “measuring system” together with the comparator (see figure 1), and the application of a sensitivity value is considered an adjustment of this system. the sensitivity factor applied by the software is gained from the following test procedure: the comparator is loaded with a pre-load (between zero and nominal load) which brings the comparator into a typical working range and working state. then a test is carried out using an “abba” cycle which starts with the mentioned pre-load (“a”), then a calibrated sensitivity test weight with conventional mass ∆𝑚𝑐,𝑆 is added (“b”), this step is repeated (“b”) and finally the test weight is removed (“a”). from the calibration value of the test weight and the calculated, buoyancy-corrected difference of comparator indication during the test ∆𝐼𝑆, the sensitivity value 𝑆 is calculated according to (1). (subscripts upper case "𝑆" are used in this paper to indicate the context of a sensitivity test). once this sensitivity value is determined, it must be used for any further processing of a reading of that comparator. we assume here that the self-adjustment procedure of the comparator is run about once a day, so that any climate-induced changes (air density, temperature) and their effect on the comparator sensitivity are negligible, this means that the sensitivity of the comparator stays “the same” over time (but will not necessarily be 1 exactly). 4. uncertainty of sensitivity 4.1. general considerations given the procedure above and (1) for the calculation of sensitivity, we find the following sources of uncertainty when determining s from a sensitivity test: • readability of the comparator 𝑢𝑟𝑟𝑟𝑟 • repeatability of the comparator 𝑢𝑟𝑟𝑟𝑟𝑟𝑟 • calibration uncertainty of the sensitivity test weight 𝑢𝑤𝑟𝑤𝑤ℎ𝑟 figure 1. the measuring system. measuring system: delivers processed value software (external of comparator): processes indication value with a factor comparator: delivers indication value acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 97 the uncertainty in 𝑆 can thus easily be derived as: 𝑢𝑆2 = � 1 ∆𝑚𝑐,𝑆 × 𝑢𝑟𝑟𝑟𝑟� 2 + � 1 ∆𝑚𝑐,𝑆 × 𝑢𝑟𝑟𝑟𝑟𝑟𝑟� 2 + + � ∆𝐼𝑆 ∆𝑚𝑐,𝑆 2 × 𝑢𝑤𝑟𝑤𝑤ℎ𝑟� 2 (2) with the components 𝑢𝑟𝑟𝑟𝑟 = 𝑑 2 × 1 √3 for an abba cycle (derived from the calculation of an abba difference) and with comparator indication readability 𝑑, 𝑢𝑟𝑟𝑟𝑟𝑟𝑟 = 𝑠 √𝑛 with the repeatability standard deviation 𝑠 of the comparator and the number of cycles of the sensitivity test 𝑛 and 𝑢𝑤𝑟𝑤𝑤ℎ𝑟 = 𝑈𝑐𝑟𝑐 𝑘 with coverage factor 𝑘, taken from the calibration certificate of the sensitivity test weight. for simplification of the uncertainty calculation (only) we set ∆𝐼𝑆 = ∆𝑚𝑐 so that (2) becomes for a sensitivity test: 𝑢𝑆2 = � 1 ∆𝑚𝑐,𝑆 × 𝑟 2×√3 � 2 + � 1 ∆𝑚𝑐,𝑆 × 𝑠 �𝑛𝑆 � 2 + � 1 ∆𝑚𝑐,𝑆 × 𝑈𝑐𝑐𝑐 2 � 2 (3) please note that since 𝑆 is a relative number, its uncertainty 𝑢𝑆2 is also a relative number while its uncertainty contributors are in mass units. 4.2. uncertainty of conventional mass – connection to oiml r111 oiml r111 [1] requires an uncertainty contribution 𝑢𝑠 (note lower case “s” in the subscript) to be estimated that, as a component of the balance uncertainty 𝑢𝑏𝑟, accounts for the uncertainty of the sensitivity in the calculation of conventional mass. in our case, where a factor 𝑆 is used to calculate the conventional mass difference according to (1), this uncertainty component of conventional mass is (with sensitivities 𝑆 close to 1): 𝑢𝑠2 = � ∆𝐼 𝑆2 × u𝑆� 2 ≈ [∆𝐼 × u𝑆]2 (4) note that 𝑢𝑠2 (with lower case subscript s) is taken from [1] while the last term (with upper case s) is our concept. 5. initial choice of a sensitivity test weight as has been mentioned above, there is only little literature available on mass comparator sensitivity. the publication of r. davis [6] appears to have been written in the light of comparators with optical scales and a sensitivity assessment in every mass calibration cycle and therefore provides only a rough direction for today’s questions. lee shih mean’s paper [7] focuses on a very special problem (the calibration of stainless steel against pt-ir standards and evaluating true mass) and thus has its own reasons for the choice of the weight size. all sources, however, agree in the general idea that the weight should be “small” and in the magnitudes of the weighing differences that will be obtained. the reason is probably, that this procedure tries to approximate an ideal differential sensitivity 𝜕𝐼 𝜕𝑚𝑐 from the test with finite values ∆𝐼𝑆 ∆𝑚𝑐,𝑆 thus avoiding any influence of non-linearities in the characteristic curve of the comparator. as a first practical assumption, we chose calibrated test weights with a nominal value of about 100 times the readability 𝑑 of the comparator, but not smaller than the smallest oimlweight which is 1 mg. the weights are made of stainless steel to avoid any complications arising from buoyancy effects. as test objects we chose the manual comparators in the mass calibration laboratory of mettler toledo, accredited as scs 0032. (table 1). we further assume for simplicity reasons that the sensitivity test weights were calibrated with an uncertainty (k=2) of one third of the mpe of class e1 according to [1]. thirdly, we used "datasheet repeatablities" for our calculations and 𝑛𝑆 = 1 repetitions for the sensitivity test. for each mass comparator used in our laboratory, we calculated the uncertainty of the sensitivity as given in (3). this revealed some unexpected results. figure 2 shows the sensitivity uncertainties (k=1) for each comparator type listed in table 1. we note the following important findings: • although we applied the same basic idea for the choice of the sensitivity test weight, the differences of the sensitivity uncertainties between the comparators were of about 3 magnitudes, ranging from 6 e-04 to 2 e-02. • the maximum uncertainty value observed (2 e-02) was significantly higher than the simplified value of 5 e-04 from the literature although our procedure uses a correction. we conclude that a general assumption of an uncertainty of comparator sensitivity of 5 e-04 (as can be found in the literature) is not justified. 6. variation and optimization of parameters 6.1. optimization to reach an uncertainty of 5 e-04 further investigation of the uncertainty budget of the sensitivity revealed that in most cases (and especially in the case table 1. mass comparators, datasheet repeatabilities and readabilities and test weights nominal values of “initial choice”. c om pa ra to r x p 64 00 3l x p 10 00 3s x p 50 03 s x p 20 04 s a x 10 05 a x 10 6 x p 6u 𝑑 (mg) 5 1 1 0.1 0.01 0.001 0.0001 𝑠 (mg) 8 1 0.8 0.1 0.02 0.003 0.0003 5 𝑚𝑐,𝑆 (mg) 500 100 100 10 1 1 1 figure 2. sensitivity uncertainties based on initial choice of the sensitivity test weight (k=1). acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 98 of the high values identified above), the dominant contributor to the uncertainty budget was the influence of repeatability (which is the second component in (3)). the equation suggests that this contributor could be reduced by an increase in the number of weighing cycles used in the adjustment. however, this has little effect since it is not practical to use more than about 5 abba cycles. (for this publication, we will continue to use datasheet repeatabilities. however, it is obvious that using individually determined repeatabilities (which are usually smaller) will have a significant, improving impact on the uncertainty.) re-visiting equation (3), we find that the nominal value of the chosen sensitivity test weight influences all three uncertainty contributors. this opens the door to optimizing the sensitivity uncertainty by adjusting number of cycles and nominal values of the test weights. increasing the nominal weight value will lead to smaller sensitivity uncertainties. a massive increase would, however, violate the principle of “small” sensitivity test weights (as explained above in section 2.2), so we prefer to keep nominal values small. additionally, we will only use nominal values that are specified in [1]. with these restrictions, we iteratively increased the nominal values in the sensitivity test weight with the aim of reducing uncertainties of all sensitivities to approximately 5 e-04 or less (which is the literature value). in order to keep the procedures easy to understand for all laboratory personnel, we fixed the number of weighing cycles for the sensitivity test to 𝑛𝑆 = 3 abba cycles for all types of comparators. the new values are shown in figure 3. please note the difference in y-axis scale compared to figure 2. these results were achieved using the sensitivity test weights for the comparator models as shown in table 2. the nominal values of these weights are below the maximum value of 0.5 % of balance capacity as stipulated in [3], so this choice does not violate the concept of a "small" weight. 6.2. further optimization to reach smaller uncertainties by increasing the number of cycles and by increasing the nominal values of the weight pieces, it is possible to reach values for 𝑢𝑠 of e.g 1 e-04. however, with an attempt to reach 1 e-05 (still using datasheet repeatabilities), the necessary test weights approach the "0.5 % of capacity limit" (see above) and thus are no longer considered "small". 6.3. variation of sensitivity test weight accuracy except for microbalance comparators, the calibration uncertainty of the sensitivity test weight 𝑈𝑐𝑟𝑐 has little influence on the uncertainty in 𝑆. so using weights calibrated in e2 quality instead of e1 weights is possible without major disadvantage. 7. general cookbook procedure for assessing optimized comparator sensitivity and its uncertainty the following procedure for assessing sensitivity uncertainty can be derived from the considerations above: 1. set a maximum acceptable value for sensitivity uncertainty (e.g. 5e-04 or 1e-05). 2. set a number of abba cycles for the sensitivity test, use (3) and iteratively increase the nominal weight value until the above condition is fulfilled for the comparator concerned. 3. execute sensitivity test and apply the value found for 𝑆 to all future readings. 4. use the value of 𝑢𝑠 for the uncertainty estimation of mass calibrations according to [1]. 8. experimental verification in order to verify the theoretical ideas outlined above, we have done several tests. one series of tests was done on an xp2004s comparator balance. we have tested different weight sizes and numbers of test cycles. the results are found in figure 4: the diagram shows 3 series of sensitivity values and their calculated uncertainties. the first series was done with a 100 mg sensitivity test weight and one cycle, while the second series was done with a 500 mg weight and three cycles. the third series was done with a 1 g test weight and three cycles. the picture confirms the theoretical considerations: • as could be expected, the variability of the values gained with higher uncertainty is bigger. the larger the test weights and the number of cycles, the smaller the variability. • most data points are consistent to each other: an en-test shows that most data points are consistent with every other one, especially the points gained with more than one cycle (i.e. 𝑛𝑆 > 1 ) show excellent agreement with each other. figure 3. sensitivity uncertainties with optimized procedure (k=1). table 2. mass comparators and test weight nominal values for optimized sensitivity. c om pa ra to r x p 64 00 3l x p 10 00 3s x p 50 03 s x p 20 04 s a x 10 05 a x 10 6 x p 6u 𝑚𝑐 (mg) 10000 2000 1000 200 50 5 1 figure 4. three series of sensitivity test points together with calculated uncertainties (k=2). nominal values and number of cycles for each series: 100 mg (n=1), 500 mg (n=3), 1 g (n=3). acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 99 9. conclusions the sensitivity of mass comparators and its associated uncertainty are both values with important significance in the field of weight piece calibration. the literature does not provide much guidance neither on procedures to be used for assessing sensitivity and its uncertainty nor on the selection of suitable weights for sensitivity testing. we have presented a procedure for assessing and the mathematics for evaluating sensitivity and its associated uncertainty. by means of iterative application, a weight for a sensitivity test can be selected so that relative sensitivity uncertainties 𝑢𝑠 of e.g. 1 e-04 are achieved with the prerequisite that a correction for sensitivity is applied. references [1] oiml: “oiml r111-1:2004: weights of classes e1, e2, f1, f2, m1, m1–2, m2, m2–3 and m3 part 1: metrological and technical requirements”: www.oiml.org. [2] jcgm 200: 2012: “international vocabulary of metrology – basic and general concepts and associated terms (vim) 3rd edition”: www.bipm.org. [3] nist: ir 6969: www.nist.gov. [4] kochsiek m, glaeser m, massebestimmung, vch, weinheim, 1997 [5] kochsiek m, glaeser m (eds.). comprehensive mass metrology, wiley-vch, berlin, 2000. [6] r. davis: “note on the choice of a sensitivity weight in precision weighing”, journal of research of the national bureau of standards, volume 92, number 3, pp 239-242, may-june 1987. [7] s. m. lee, r. davis, l. k. lim: “calibration of a 1 kg stainless steel standard with respect to a 1 kg pt-ir prototype: a survey of corrections and their uncertainties”, asia-pacific symposium on mass, force and torque (apmf 2007), oct 24-25 2007. certain commercial equipment, instruments, or materials are identified in this paper in order to adequately describe the experimental procedure. such identification does not imply recommendation or endorsement by the author, nor does it imply that the materials or equipment identified are the only or best available for the purpose. assessment of mass comparator sensitivity << /ascii85encodepages false /allowtransparency false /autopositionepsfiles true /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /error /compatibilitylevel 1.4 /compressobjects /tags /compresspages true 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can be opened with acrobat and adobe reader 5.0 and later.) >> /namespace [ (adobe) (common) (1.0) ] /othernamespaces [ << /asreaderspreads false /cropimagestoframes true /errorcontrol /warnandcontinue /flattenerignorespreadoverrides false /includeguidesgrids false /includenonprinting false /includeslug false /namespace [ (adobe) (indesign) (4.0) ] /omitplacedbitmaps false /omitplacedeps false /omitplacedpdf false /simulateoverprint /legacy >> << /addbleedmarks false /addcolorbars false /addcropmarks false /addpageinfo false /addregmarks false /convertcolors /converttocmyk /destinationprofilename () /destinationprofileselector /documentcmyk /downsample16bitimages true /flattenerpreset << /presetselector /mediumresolution >> /formelements false /generatestructure false /includebookmarks false /includehyperlinks false /includeinteractive false /includelayers false /includeprofiles false /multimediahandling /useobjectsettings /namespace [ (adobe) (creativesuite) (2.0) ] /pdfxoutputintentprofileselector /documentcmyk /preserveediting true /untaggedcmykhandling /leaveuntagged /untaggedrgbhandling /usedocumentprofile /usedocumentbleed false >> ] >> setdistillerparams << /hwresolution [2400 2400] /pagesize [612.000 792.000] >> setpagedevice microsoft word article 7 193-1072-1-pb.docx acta imeko may 2014, volume 3, number 1, 23 – 31 www.imeko.org acta imeko | www.imeko.org may 2014 | volume 3 | number 1 | 23 theoretical, physical and metrological problems of further development of measurement techniques and instrumentation in science and technology d. hofmann, y.v. tarbeyev friedrich -schilleruniversity, 32 ernst-thälmannring, 69 jena, gdr section: research paper keywords: metrology, measurement technology, measurement theory, standards, fundamental physical constants, uniformity of measurements citation: d. hofmann, y.v. tarbeyev, theoretical, physical and metrological problems of further development of measurement techniques and instrumentation in science and technology, acta imeko, vol. 3, no. 1, article 7, may 2014, identifier: imeko-acta-03 (2014)-01-07 editor: luca mari, università carlo cattaneo received may 1st, 2014; in final form may 1st, 2014; published may 2014 copyright: © 2014 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited 1. introduction the conversion of science into a major productive force is inevitably accompanied with growing interest in increasing measurement accuracy. many objective studies of reality are reduced to measurements. at the same time the results of scientific research serve as a basis for the further development of this reality. experimenters in science and practical engineers are interested in higher measurement accuracy, in technical improvement of measuring instrumentation and in higher reliability of measurements. these aspects make up the object of metrology. the task of meeting the urgent requirements of science and technology is the motive force for the progress of metrology and improvement of systems of units and standards which in the long run should provide for an optimum metrological assurance. this capacious concept includes not only the establishment of standards per se but also the development of most efficient methods of dissemination of the values of units and metrological supervision of the correctness of measurements as well as the development of up-to-date measuring transducers, fast automatic measuring systems that require minimum metrological supervision [1]. metrology is a science, whose scope of knowledge is extremely polytechnical. it should embrace a wide range of electromagnetic, optical, mechanical, physical and chemical, nuclear and many other phenomena and incorporate an immense number of measuring problems that cover measurement transformations, estimation of measurement results and uncertainties, realization of the units of physical quantities and their dissemination. therefore, it is not difficult to foresee that the task of establishing metrology as an independent science enjoying full rights and having its own large domain of specially arranged knowledge, theorems, methods of investigation, is complicated. the practical requirements and continuous growth of the role of measurements urgently demand that the progress of metrology must be accelerated. the aim of the present paper is to show the directions and ways of the further development of metrology. as examples you will become acquainted with some actual problems of modern metrology and measurement theory. 2. common trends in the development of research and application in the field of metrology and measurement technology in the 20th century natural, technical and social sciences have become a decisive productive force. as a result of such this is a reissue of a paper which appeared in acta imeko 1979, proceedings of the 8th imeko congress of the international measurement confederation, “measurement for progress in science and technology”, 21-27.5.1979, moskow, vol. 3, pp. 607–626. common interest of both metrologists and representatives of science and technology in constant improvement of measurements as well as general trends in the development of research in the field of metrology, measurement technology and instrumentation at the present-day stage are shown. problems of general metrology, of improving systems of units and standards (“natural” standards in particular) are considered in detail. acta imeko | www.imeko.org may 2014 | volume 3 | number 1 | 24 sharp increase in the practical role of sciences, more and more technical subjects go over from empirically descriptive representation of the observed facts to generalizing theoretical considerations and concepts. in doing so, the peculiar features of the modern progress in scientific research are: – a sharp increase in the use of systems approach to all problems – an increase in the scope and intensity of fundamental research in the total volume of research work. the above-mentioned statements are not only valid for metrology, but are also especially characteristic of the present stage in its development. it is determined by substantial qualitative and quantitative changes. the most prominent quantitative changes are: 1. an increase in the number of measurements and in the number of fields in which measurements are employed. practically, there are no technical fields where measurements are not made; the measurement expenses reach 50 to 60% of the total production expenses in the most advanced industries (figure 1) [2]. 2. measurements provide indispensable technical key information for evaluating and controlling specialized cooperative production. they are carried out on a large scale and provide the decision criteria for acceptance or refusal of labor having been realized in masses. 3. natural, technical and social sciences operate an instrument technique possessing industrial dimensions and demanding universal knowledge as well as a concerning behaviour of the scientists (in the fields of nucleonics, microelectronics, space research, radiology, electron-scan microscopy, psychological behavior research). planned experiments are intended to prove the correctness of theoretical considerations. 4. measurements have two main functions:  an increased observability of preferred technical-physicalchemical states or procedures, widely exceeding the natural limit being set to the sensing organs of the measuring persons.  the objectivation of observations made by measuring persons. it is achieved by comparing measured quantities with known and settled standards. qualitatively new requirements concerning measurement theory are additionally caused by the following facts: 5. the accuracy of measurements required both in scientific experiments and in industry is more and more approaching the level of standard measurements. 6. due to the high sensitivity of measuring instruments the environmental conditions and external influencing factors are producing ever-increasing effects on the measurement results. 7. sophisticated measuring systems whose calibration by traditional methods presents big problems are becoming widespread. 8. greater sophistication of measuring instrumentation in respect of both the operating principle and the design features makes still higher demands of its operators. thus, today in measurements, measurement technology and instrumentation take place those qualitative and quantitative changes which not only require the proper investigation of individual metrological problems but also inevitably result in an urgent need for the development of a general theory of metrology and, first of all, a general (unified) theory of measurements. the development of a general measurement theory is complicated by the following state of affairs: 1. measurement science has a long history and therefore a mighty tradition. in our days this tradition is still aimed at a further differentiation. 2. measuring has two different definitions:  measuring in a narrow sense, that is experimental comparing of a measured quantity to a known comparison quantity of the same kind (quality) which had been determined to be the unit of measurement. the measurement of physical quantities and the use of ratio scales are typical procedures. (figure 2) [3]. figure 1. acquisition of measurement information in production. acta imeko | www.imeko.org may 2014 | volume 3 | number 1 | 25  measuring in a wide sense is the coordination of numbers with states or procedures, following some rule. scaling technical and no technical matters without special knowledge and using low value scales are typical procedures (figure 2) [3]. at measuring in a narrow sense the subjective influence of the measuring person can little by little be diminished. this does not apply to measuring in a wide sense. at measuring you will observe disadvantages and inaccuracies if it is not considered  that each quantity is always a quantity of a certain quality  that the coordination of numbers with objects is also possible without knowing their quality  that formal operating with numbers for any data you like (measured values or non—measured values) is possible and  that mathematical models cannot express themselves if they represent correct facts or arbitrary phenomena [3, p. 37]. 3. historically we are just beginning to provide generalized knowledge in the fields of measurement engineering and measurement theory. the example of mass measurement is to prove that. already 2500 b.c. the equal-armed balance had been graphically represented in a pyramid in gizeh in ancient egypt. about 300 b.c. aristotle, euclid and archimedes provided the theory for this balance. electromechanical balances, with strain gages and radiometric balances are developments of the 20th century [4, 5]. 4. the special language of measurement engineering comprises a great number of terms. numerous terms have not yet been well defined. synonyms and polysems are frequently used [5]. classifications and teaching conceptions in measurement engineering have the following points of view:  measured quantities (length, force, temperature measurement engineering)  measuring devices (strain-gage, oscillograph and digital measurement engineering)  measurement principles (isotope, ultrasonic, infrared measurement engineering)  field of measurement (production, process, laboratory measurement engineering). 3. problems of the generalized measurement theory topical tasks of measurement theory are 1. to increase the uniformity of measurements 2. to assure correct measurements 3. to get valid models of measurements and its aims. working with models has methodical advantages. in contrast to the originals (measured signals, measuring systems and measurement processes) models are simpler, cheaper, more easily described, can be transformed in time and space, varied, limited and optimized. as a rule, modelling is associated with a certain simplification of the original. for example, a certain body is characterized by the following properties: length, surface roughness, volume, mass, density, temperature, colour, etc. usually, only selected parameters of the object are considered exactly. physical (homolog) models transform the scale of the original only or just simplify the original; the measured quantities of the original and model coincide. mathematical (analog) models describe similar (analog) processes in different fields of knowledge; the measured quantities of the original and model do not coincide. models consist of a certain number of elements and relations (links) between them. a great advantage is that in gaining knowledge and building up a theory the originals of different nature can produce similar images that allow a uniform treatment and interpretation. in any case the knowledge that was gained using a model in the image field should be confirmed in practice for the actual object. otherwise the value of this knowledge is subject to argument. the measurement theory deals mainly with well-defined or poorly-defined models of behaviour [6] expressed as algorithms. these models describe:  measured signals and measuring systems in terms of mathematical algorithms,  measurement processes in terms of heuristic algorithms. figure 2. classification of scales and their properties after [3]. acta imeko | www.imeko.org may 2014 | volume 3 | number 1 | 26 specific features of mathematical algorithms are determinism, finiteness, universality and guarantee of solution [7]. heuristic algorithms are not associated with such strict requirements and still they guarantee only a solution probability 0<p(l)<1 which tends to 1 [8]. both mathematical and heuristic algorithms take the shape of measurement rules or definitions. one should bear in mind that mathematical relationships obtained by a deductive reasoning cannot be used for scientific substantiation of metrological characteristics if they do not agree with the empirical knowledge (figure 3) [9-11]. there are no good or bad models. a model can be considered a good one if it meets the practical requirements and is in good agreement with empirical data. there is always the danger of:  using too simple or too complicated models;  solving partial problems only for which certain models are known;  ascribing greater significance to those stages of processes that are described by mathematical algorithms than to those that can for the time being be described in terms of heuristic algorithms only. here are some typical examples of this sort:  in the preference of random errors rather than systematic ones in the measurement error analysis;  in the description of a measurement process the preference of an estimate to the detriment of the measurement process itself, with the measurement preparation being almost totally ignored;  the preference of the normal distribution even when considering events whose distribution differs from the normal one;  the preference of incompletely formulated measurement problems (research problems) rather than completely formulated measurement problems (repetition problems);  the neglecting of ergonomic aspects when developing new measuring instruments or processes;  the neglecting of economic consequences as a result of incorrect measurements or the absence of measurements. operations (actions, decisions) aimed at an optimum solution of measurement problems are called the measurement strategy. as a rule, the measurement strategy consists of numerous mathematical and heuristic algorithms. an algorithm transforms the input quantities into output quantities in accordance with a system of rules. the use of algorithms is inevitable for automation of measurement processes. the measurement strategy should cover the preparation, realization and evaluation of measurements (figure 4) [2]. when one looks up in metrological literature for possible ways of solving measurement problems, one discovers a totally unsatisfactory situation. first of all, there is no clarity as to the necessary contents of measurement problems with guaranteed solution. the same applies to the choice of measuring instrumentation and the construction of measuring systems [12]. mathematization of the measurement theory is, first of all, of great importance for microprocessor application. neugebauer is of the opinion [13, p.44] that mathematics originated from metrological problems. in any case, afterwards it developed by a deductive method independently of metrological and other technical problems and at present it has in part surpassed the limits of practical problems. a sharply increasing interest in mathematization of measurement processes results in a greater interest for publications on the axiomatic foundation of measurements. we are speaking here about literature devoted to the algorithmic figure 3. characteristic functions of dynamics properties of measurement systems. acta imeko | www.imeko.org may 2014 | volume 3 | number 1 | 27 theory of measures and measurement theory [14-17]. the algorithmic theory of measures deals with the determination of the contents of geometrical structures or point sets. it is especially important for integral or probability calculations. in integral calculations the use of a measure instead of the contents results in the lebesgue integral (expansion of the riemann integral). when estimating the probability, one random event is interpreted as a particular quantity xai of the set of all elementary events and the measure p(xai) is considered to be the probability of the event xai. in mathematics the point sets having the contents are called measurable. the prerequisite is the possibility of generating a set m of natural numbers and their resolution. this is true, for example, for the set of all even numbers, fibonacci numbers and prime numbers which can be produced recursively. the algorithmic theory of measurements proceeds from the fact that non-numerical algorithms also can be reduced to recursive functions and recursive sets of natural numbers if the class k of non-numerical input and output quantities can be coded using natural numbers. for this purpose it is necessary to have:  an algorithm that controls the coding;  an algorithm that checks whether a certain natural number is the image of a non-numerical object from k;  an algorithm that reconstructs the object from its image;  an algorithm that encodes the non-numerical class k as a whole. numerous publications on algorithmic theory of measurements are devoted to these problems. mainly, they deal with consistent classifications and establishment of scales. there is a certain process which is determined by specific laws of the development of deductive theories and which is still to be investigated by metrologists. on the other hand, it was found that any sufficiently powerful theory cannot be resolved. here are the natural limits of the axiomatic method. 4. measurement theory as an educational factor the measurement theory concentrates the knowledge about the status of measurement technology and instrumentation, reduces the immense quantity of metrological phenomena to a limited number of fundamental methods, principles and means and makes for the understanding of measurements. solving measurement problems in a systematic and planned manner with the help of the measurement theory one can optimize their solutions, limit possible incorrect actions and spending and predict future requirements (figure 5) [18]. until now the educational programs and those of refresher courses were hardware-oriented. however, automation of measurements and modern technology can be introduced only if specialists will also be software-oriented. one must understand not only instruments but processes as well. the software-oriented thinking is not restricted by the writing of computational programs and includes the following problems:  choice of appropriate concepts and definitions;  target-oriented exact formulation of measurement problems;  discrimination between hardware-oriented and softwareoriented functions;  discrimination between central and decentral functions;  clearing-up of the relationships between the system operator and the measurement system, between measurement systems as well as between operators;  automation of measurement processes after consideration of economic criteria, of the costs associated with measurements that were or were not carried out, with the measuring instrumentation that is figure 4. components and functions of automatic measuring devices. acta imeko | www.imeko.org may 2014 | volume 3 | number 1 | 28 already available or is to be purchased and with the training of the personnel. the more productive is the measurement theory, the easier are solutions and the smaller is the educational expenditure. however, the ever-increasing training costs should not be underestimated. it is through education and training that the results of scientific research add to the efficiency of production (figure 6). 5. problem of improving systems of units the theory of building up a system of units, which could adequately meet complicated requirements of science and practice has become one of those few branches of general metrology, that was successfully developed and logically completed in 1954–1960 by accepting the unquestionably progressive international system of units (si). however, some old unresolved problems have still remained in this field and new ones that require immediate solution have appeared since. it can be explained by the fact, that the number of independent and derived measurands is constantly increasing. now the international system of units already consists of 135 units of physical quantities and the number of parameters, measured in science and technology, can reach 700 in 1980 and increase up to 2000 in the nearest future, according to the existing forecast. such a quantitative growth causes difficulties from the point of view of si, whose aim is to establish simple relations between existing units, and suggests the necessity of introducing some alterations. thus, at present we are faced with the problem of reviewing the definition of the metre as the old definition doesn’t allow to improve the accuracy of realization of the unit. till nowadays the definition of the unit of temperature (using the thermodynamic scale) is in a certain contradiction with its realization (using ipts). the problem of reviewing the definitions of most important units in the field of electricity and magnetism is an urgent one. a number of fundamental questions of the theory of systems of units remains unsolved as well. so during several years a discussion about the status of angular units is underway. some metrologists consider it necessary to speak out in favour of introducing the unit of plane angle into the number of basic units and placing the solid angle unit among derived units. all these questions demand additional theoretical study and broad discussion by the scientific community. 6. problem of improving basic standards and establishing a system of interrelated standards the problem of improving standards is closely related to the problem of improving the systems of units, though this relation is not a straightforward one. at present the change from standards, forming a certain set to a system of interrelated standards, based on stable physical phenomena and fundamental physical constants should be considered the main trend in this field. undoubtedly first of all we must think about improving the standards of basic units. but it is not obligatory that a system of basic standards should coincide with the set of basic units. the system of standards of electrical units, where the main standards are the standard of the e.m.f. unit – the volt (and not the ampere) – and the standard of the farad can serve as an example of such a non-antagonistic difference. we are going to show below that such a structure of the system of standards is not in contradiction with the definition of the ampere as a basic unit of si. from the point of view of accuracy the best system would be such a system of standards, in which all the basic standards were “countable”. at present the standard of the second (based on the counting of the number of periods of 137cs radiation) and the metre (based on the counting of the number of wavelengths corresponding to the orange line in 86kr radiation) are designed in such a way. the prospect of a changeover to the definition of the mass unit, according to which the kilogram figure 5. working steps for computer-supported coordinate measurement, after [18]. acta imeko | www.imeko.org may 2014 | volume 3 | number 1 | 29 will represent the mass of countable number of atoms of a certain element, most probably the 28si isotope. the fact that there will be no less of accuracy in transferring the value of the unit into the field of molecular and atomic masses (though the accuracy of realization will be comparable with the accuracy of the existing standard or even lower) can be considered one of the main advantages of such a definition of kilogram. figure 6. development of measurement devices, working fields and educational requirements. acta imeko | www.imeko.org may 2014 | volume 3 | number 1 | 30 in principle it’s also possible to design the standard of one of the electrical units on the basis of counting the number of elementary charges. however in spite of the fact, that modern technology allows to realize such counting the realization of a standard on such a basis is a matter of the future. after the establishment of basic standards of a system on the basis of counting the set of values of fundamental physical constants (both dimensional and dimensionless) can be measured. the obtained values of constants should be mathematically processed in order to determine most reliable adjusted values and then the standards of derived units should be established on their basis. the system of such standards will be self-consistent and will not contradict the system of units. the principle of building up such a system of standards as illustrated by the realization of the e.m.f. unit – the volt using the josephson effect and the fundamental physical constants is given in [19] and besides will be considered in detail in a special report to the section “metrological assurance”. the aim of the creation of such a system is not only to establish a more objective internal interrelation and self-consistency of the standards of units of physical quantities, but also to improve the accuracy of all standards of the system. the necessity of radical rearrangement of the system of standards (primary and secondary ones) for another basic unit – the kelvin – through the transfer to the thermodynamic temperature scale from the international practical temperature scale is quite evident. it is due to the fact, the absolute temperature values are needed for fundamental physical research, as absolute thermodynamic temperatures come into physical equations. the problems associated with this go beyond the bounds of metrology per se. thus, a further improvement of the gas thermometer aimed at practical realization of the thermodynamic temperature scale demands the development of the theory and methods of molecular dynamics, the theory of the virial equation of the state of real gases and more accurate determination of appropriate virial coefficients for pure gases and of the universal gas constant. the improvement of the accuracy of the determination of the gas constant and the avogadro number in its turn allows to have a more precise value for the boltzmann constant and the second constant in the planck radiation law, which is essential for building a high temperature thermodynamic scale and for many thermodynamic calculations. on the other hand, the improvement of these constants allows to go over to a new method of realization of another basic unit – the candela – and to improve the whole system of standards in the field of photometry in a wide spectral range using the “black body” radiation and to effect the change-over to the energy photometry. 7. problem of developing “natural” standards and using fundamental physical phenomena this problem, that arose at the moment when it was felt that the international uniformity of measurements, should be ensured, reflects the natural and therefore constant tendency among metrologists to use the most stable natural phenomena for the realization of units of physical quantities, that is to establish reproducible standards. 100 years of intensive investigations of metrologists in this direction have resulted in the establishment of natural standards of the above mentioned length unit and time unit. in recent years the standards of magnetic induction (for high fields), angular velocity, etc. whose operating principles are based on relatively easily reproducible natural phenomena. at present “natural” standards are being developed in the following fields: of thermometry (the use of the phenomenon of the nuclear quadrupole resonance), of voltage measurements (the effects of josephson, stark and pockels), of radiation energy (the monochromaticity of atomic and nuclear transitions), etc. the following two common trends can be rather clearly seen in all these studies:  a wide use of quantum effects which even resulted in the coining of the term “quantum metrology”;  the use of fundamental physical constants that are most stable realizations of natural laws as references in standards. in this connection, the problem of a more accurate determination and adjustment of fundamental physical constants is in itself becoming very important. it was shown above what is the role of this problem in the establishment of an interrelated system of standards. if one also takes into account the fact that the accuracy of physical constants characterizes the depth of our notions of physical phenomena and laws interrelating them, then it can be said with confidence that everybody – from physicists and metrologists to practical engineers – is interested in the improvement of accuracy of fundamental physical constants. in this respect, the work done at vniim has shown that, from the point of view of the progress of metrology, in the first place it is necessary to improve the accuracy of the constants involved in the realization of the volt using the josephson effect. as to the other trend in the improvement of standards, i.e. the trend of a wide use of quantum effects and phenomena that are not substantially influenced by environmental factors, the use of the superconductivity phenomenon is the most characteristic feature of the present stage. this phenomenon has attracted a special attention of metrologists after it was shown that superconductivity presents a unique manifestation of the existence on a macroscopic scale of such quantum mechanical phenomena as the meissner effect (of ideal diamagnetism), the phenomenon of quantizing the magnetic flux in superconductors with multiple links, a whole class of contact phenomena united by the common name “josephson effects” as well as the phenomenon of quantum interference of currents in superconductors which is related to the latter effects. the ideal diamagnetism of superconductors provides a unique opportunity for the development of magnetic suspensions and bearings that will undoubtedly enhance the improvement of high-precision measuring instrumentation in the field of mechanical measurements where the total absence of friction is of exclusive importance. the main (classical) property of superconductors (the absence of dissipative losses, i.e. the ideal conductivity) together with the meissner effect (ideal diamagnetism) have long ago led to the idea of constructing superconducting magnets with high values of induction and highly uniform magnetic fields that practically do not consume any energy. at present 7.5 t magnets have been produced with a field inhomogeneity of less than 2×10–7 in a 0.5 cm3 volume (england). such a magnet made a resolution of approximately 5×10–9 possible. the work on the development of new materials having high superconducting transition temperatures and stable structures that is underway in the whole world allows to expect the emergence of magnets with field inductions up to 20 to 30 t. various quantum-mechanical properties of superconductors acta imeko | www.imeko.org may 2014 | volume 3 | number 1 | 31 (from ideal diamagnetism to the josephson effects) open the prospects (and are already used) for the development of superconducting gravimeters for relative measurements of gravity that have an extremely high sensitivity. the theoretical limit of one of the designs of such gravimeters proposed by nbs [20] is ~10–14 g. a dilatometer based on the principle of superconducting cavity resonator and having a sensitivity of ~10–14 m was built at the institute of physical problems of the ussr academy of sciences. the gravimeter which is an embodiment of this principle makes it possible to obtain a sensitivity of ~10–10 g for averaging time of ~1 minute. apparently, widest prospects for the use of unique properties of superconductors are open in the fields of electrical and magnetic measurements. already at present josephson volt standards are used in the majority of metrological laboratories of the world for the maintenance of the volt value with an accuracy ~ (3-5)×10—8. this brings about, in particular, an important task of developing a transportable transfer standard for the e.m.f. unit in which all achievements of physics and low-temperature technology should be used. numerous applications will be found for superconducting quantum magnetometers using the phenomenon of quantum interference of currents and josephson effects. the sensitivity of the transducers of these magnetometers (squids) may be close to the fundamental limit whose value can be estimated as   2/1 08 2/1 x hz 10 minf φ δ δφ  (1) where xδφ is the minimum detected change in the magnetic flux; fδ is the frequency bandwidth of the signal; 0φ is the magnetic flux quantum equal to 2×10–15 wb. these magnetometers are beyond competition in respect of their sensitivity and accuracy as compared with all other types of magnetometers in the field of measurement and stabilization of low and medium magnetic fields. of no less importance is the fact that together with other superconducting devices, such as current comparators, magnetic flux transformers, super conducting screens, these magnetometers provide great opportunities for increasing the sensitivity and accuracy of a wide class of electrical measuring devices. in particular, they make feasible measurements of voltages of the order of 10–12 to l0–15 v comparisons of d.c. currents with a relative uncertainty of 10–8 to 10–10, inductance comparisons with a sensitivity of the order of 10–14 h, attenuation measurements at radio frequencies with a relative uncertainty of 10–3 and in a number of other measurement fields requiring high accuracy. we should also note that among the promising fields are applications of superconducting cavities for frequency stabilization of microwave oscillators (at present relative frequency instabilities of 1.2×10–12 for one hour and 1.2×10–13 for 10 seconds have been achieved) and the use of non-linear properties of josephson junctions for multiplication of frequencies in the 3-cm band up to the far infrared part of the spectrum which is of great importance for bridging a gap between the frequencies of cesium clocks and lasers. 8. conclusions even this brief review of individual problems, that are most important for the development of metrology and measurement technology and instrumentation, shows that metrology (including the general theory of measurements) is a rapidly developing field of scientific knowledge, i.e. a science whose progress will undoubtedly enhance further progress in various branches of industry and scientific research and whose actual formation can be promoted by concerted efforts of metrologists in collaboration with scientists and practical workers in 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[2] d. hofmann, handbuch messtechnik und qualitätssicherung (handbook measurement engineering and quality assurance) berlin: veb verlag technik 1979. [3] w. gutjahr, die messung psychischer eigenschaften (measurement of psychological properties). berlin: veb deutscher verlag der wissenschaften 1971. [4] p.s. sawelski, die masse und ihre messung (mass and its measurement). moscow: mir, leipzig: veb fachbuchverlag 1977. [5] g. hofmann, effektivitätssteigerung durch monosemierung der fachsprache der messtechnik (improving efficiency by using monosemantic term in the terminology of measurement engineering). feingertetechnik 28(1979) h. 5, s. 216–218. [6] m. peschel, modellbildung für signale und systeme (models for signals and systems). berlin: veb verlag technik 1978. [7] n.a. krinickij, algoritmy vokrug nas (algorithms around us). moscow: nauka 1977. [8] d. hofmann, heuristische algorithmen zur rationalisierung von messprozessen (heuristic algorithms for the rationalization of measurement processes). messen – steuern – regeln 14(1971) h.5, s. 172–174. [9] f.h. lange, methoden der messstochastik (methods of stochastical measurements). berlin: akademie verlag 1978. [10] g.i. kavalerov, mandelstam, s.m.: vvedenie v informacionnuyu teoriyu izmerenii (introduction in measurement-information theory). moscow: energija 1974. [11] l. finkelstein, instrument science. introductory article. journal of physics e: scientific instruments 10 (1977) s. 566–572. [12] d. bosman, instrument science. systematic design of instrumentation systems. journal of physics e: scientific instruments 11(1978) s. 97–105. [13] a.p. stachov, vvedenie v algoritmicheskuyu teoryu izmerenija (introduction in algorithmic measurement theory). moscow: sovetskoye radio 1977. [14] p.r. halmos, measure theory. new york, heidelberg, berlin: springer verlag 1974. [15] j. pfanzagl, v. baumann, h. huber, theory of measurement, 2nd. ed. würzburg, wien: physica verlag 1971. [16] l. finkeistein, the relation between the formal theory of measurement and pattern recognition, in: practical measurement for improving efficiency. acta imeko 1976. budapest: akademiai kiado 1977. [17] y. morita, a. kobayashi, h. iida, m. ishigami, of classification and industrial measurement, in: practical measurement for improving efficiency. imeko vii. preprint vol. 3, 1976. [18] w. lotze, m.w. hartmann, maus (2d) programmsystem zur berechnung von mass, form und lage punktweise gemessener ebener werkstücke. nutzerbeschreibung (maus (2 d) programmsystem for the calculation of measure, shape and position pointlike measured plane objects). dresden: technical university 1977. [19] y.v. tarbeev, izvestija an sssr. ser. energetika i transport, 1978, n. 3. [20] r.p. giffard, j.c. gallop, b.w. petley, progress in quantum electronics, 1978, vol. 4, p.4. 404 not found microsoft word 378-2426-1-le-rev acta imeko  issn: 2221‐870x  november 2016, volume 5, number 3, 9‐15    acta imeko | www.imeko.org  november 2016 | volume 5 | number 3 |9  application of optimization methods for improved electrical  metrology  marija cundeva‐blajer  ss. cyril and methodius university, faculty of electrical engineering and information technologies, ul. ruger boskovic br. 18, pobox 574,  1000 skopje, r. macedonia        section: research paper   keywords: optimization; genetic algorithm; electrical measurements; instrument transformer; resistance standard  citation: marija cundeva‐blajer, application of optimization methods for improved electrical metrology, acta imeko, vol. 5, no. 3, article 3, november 2016,  identifier: imeko‐acta‐05 (2016)‐03‐03  section editor: franco pavese, italy  received april 18, 2016; in final form april18, 2016; published november 2016  copyright: © 2016 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  corresponding author: marija cundeva‐blajer, e‐mail: mcundeva@feit.ukim.edu.mk    1. introduction  in metrology among other problems two of them represent general challenges [1]: 1. reduction of errors and uncertainties during the design process of measurement instrumentation and devices; 2. prediction of the time-behaviour of measurement instrumentation or standards, i.e. estimation of the optimal recalibration interval. construction of functions based on experimental data is a widespread and important procedure in measurement problems (e.g. fitting of calibration curves for measuring instruments, or deduction of equations for indirect measurement methods) [2]. in these cases the quality of the empirical function is essential for ensuring the integral quality of measurement. the empirical function must be sufficiently precise for practical purposes and its estimates of the accuracy characteristics must be reliable. the traditional method for fitting of functions is the classical least squares (ls) method. the ls estimates are optimal only under strict conditions upon errors of data, that is, within a classic regression model. so, when it does not hold in practice, some other methods are required, so-called confluent methods. however, a general confluent model is not appropriate for practice either, since neither ls nor other data processing methods can provide statistically consistent estimates for functions. therefore, the key problem is to construct proper extensions of regression models, which allow obtaining consistent estimates for functions [2]. the proper model should include additional information on data or data errors, which makes it possible to construct consistent estimates. confluent estimates should be constructed in accordance with the type of the extended model. however, the most important problem for measurements is to study the accuracy characteristics of these estimates, taking into account both random and systematic components of data errors. in [2], several ways for expansion of regression models are outlined, and associated groups of estimates are studied. the accuracy properties of estimates are considered, which may be compared with the corresponding characteristics of classical ls estimates. this is a heuristic approach, but the optimisation does not work in this case. the main criterion for deriving confluent methods is the consistency as an asymptotic property of estimates. however, confluent estimates are used in the cases of small or modest volumes of data. therefore, the accuracy characteristics of various confluent estimates should be also compared with the classical ls fitting under practical conditions [2]. abstract  in electrical quantities metrology numerous examples of stochastic processes exist and a need for optimal solutions is posed. here  stochastic genetic algorithm optimization is used for solving two typical metrological problems: 1) minimization of the metrological  parameters (final accuracy) in the design process of instruments and 2) predicting metrological reference standard’s time‐drift, i.e. re‐ calibration interval. the first case is the optimal metrological design of a combined instrument transformer and the second case is the  analysis of resistance standard time‐drift.  acta imeko | www.imeko.org  november 2016 | volume 5 | number 3 |10  in electrical measurements numerous examples of stochastic processes exist, like the electrical quantities standards timebehaviour, [1], [3], [4], [5], and [6]. in recent years the application of stochastic optimization tools for the design of electrical devices (including electrical instrumentation) is evident [10]-[13]. in electrical power distribution grids, state estimation is the main part of the supervisory control and data acquisition (scada) system. the aim is to determine an estimate of the system state based on a network model and the measurements available. the network state consists of voltage bus magnitudes and angles, line flows, loads, transformer taps, active and reactive power, power injections and generator outputs. in a typical state estimation scenario, the measurement data includes active and reactive power flows, power injections and voltage magnitudes at the bus. in [3] a quasi-dynamic state estimation method, based on a disturbance observer is proposed. its aim is to reconstruct electrical power (active and reactive) from available measurements. on the distribution level, where a high number of nodes is accompanied by a small number of measurements, the typical approach is to replace missing data with so-called pseudo-measurements. however, in this way the state estimation quality depends on the reliability of the pseudomeasurements. that means that in order to improve the accuracy and reliability of the state estimation result, improved pseudo-measurement models are needed. in [3] stochastic models to obtain reliable pseudo-measurements and to improve state estimation results are investigated. the autoregressive moving average models (arma) to model the error of pseudomeasurements are used. the employed disturbance observer state estimation method is then able to improve the quality of the pseudo-measurements based on assumed arma process coefficients. the optimal measurement theory arose as a branch of the inverse problems theory [4]. the mathematical justification of the algorithm for numerical restoration of the signal distorted by inertia and resonances in measuring transducers is presented in [4]. one of the directions for the development of this theory is mathematical justification of restoration of stochastic signals, in particular ”white noise”. design applications generally aim to minimise manufacturing or material costs subject to performance criteria, which is an inverse problem requiring optimisation techniques [7]. what mathematical physics denotes as inverse problems is the class of problems which are fundamental in measurement theory and practice [8]. the main objective of such problems is to develop procedures for acquiring information on object and phenomena, accompanied by decreasing the distortion caused by measuring instruments [8]. in [7] the implementation of levenburg-marquardt, sequential quadratic programming, nelder-mead, simulated annealing and practical swarm optimisation for metrology and design with continuous models is reported. in [10]-[13] the genetic algorithm (ga) as stochastic optimization method is applied for optimal design. calibration consists of two stages [15]. in stage 1 a relation is established between values provided by measurement standards and corresponding instrument response values. in stage 2 this relation is used to obtain measurement results from further response values. polynomials of various degrees, determined by least squares, are extensively used as empirical calibration functions in metrology [9]. in this contribution an original computer program with embedded genetic algorithm developed at the ss. cyril and methodius university in skopje [6] is used for solving two metrological problems: a) metrological optimal design of a combined instrument transformer; b) predictions of resistance standards time drift for the determination of the optimal recalibration period, [16]. 2. description of genetic algorithm program for  metrological optimization  on one side the concept of a genetic algorithm is very simple as displayed in figure 1, but on the other side it is a very powerful and robust computational tool. the ga implemented in the original computer program developed at the ss. cyril and methodius university in skopje figure  1.  genetic  algorithm  for  solving  problems  of  stochastic  nature  in  metrology of electrical quantities.  acta imeko | www.imeko.org  november 2016 | volume 5 | number 3 |11  [10] starts with the initial units (chromosomes) stochastic generation. each chromosome is composed of the same number of genes. it is the initial population (generation). the size of the population is between 20 and 100. iteration in the ga is the cycle of estimation, selection and reproduction of the population. each unit represents the possible optimal solution. a possible solution for each unit is estimated according to the following criterion: the value of the goal function and each solution is assigned an adequacy measure. according to the comparison of all adequate measures of the units, a decision is made which one of them will be allowed to form the next generation and with which probability in the step of selection. the selection strategy is the “virtual roulette wheel”. all population units are potentially capable for reproduction. however, whether the unit will be selected depends on the value of the goal function (a sorted ranking list is formed). finally, the virtual roulette wheel is formed according to the assigned number in the list. conditionally, most of the “positions” in the wheel are assigned to the best units, and the worst units have no “positions”. the selected units from the “virtual roulette wheel” are copied into a new generation ready for generic operation cross-over and mutation. the elitism function is embedded, enabling automatic coping (cloning) of the best unit (according to the goal function value). thus, the danger of losing the best unit during the cloning process is avoided. in the next step reproduction between pairs is performed, through simple copying of the units in the next generation. this is a process until the moment when cross-over and mutation happen according to the previously defined probability. the selection of units exposed to the process of cross-over is random. they are grouped and the spot of division of the genes from the two parents is random again. through the cross-over an exchange and recombination of the genetic material is achieved which forms a new chromosome with genes from the two parents. the mutation is a change of a unit in the chromosome. the mutation is a very important factor in the evolution, because it introduces new genes in the process. this additionally increases the diversity of the units in the population. pairs of chromosomes (parents) are exposed to the process of cross-over and the mutated units are returned in the population of the new generation. the cycle of estimation, selection and reproduction continues until a predefined acceptable behaviour level is achieved. if the termination condition is the predefined generation number, then the best unit in the last generation is the optimum. the population units (chromosomes) in each next generation are more similar to each other with convergence towards specific part of the search space. 3. case study no. 1 ‐ ga optimal design of an  instrument transformer  instrument transformers are of great metrological significance for electrical power measurements, especially in the field of legal metrology and trade of electrical energy. the accurate metrological analysis of a 20 kv combined instrument transformer (cit) (voltage transformation ratio v3/ 100:v 3/ 20000 and current transformation ratio 100 a: 5 a) is possible only by numerical calculation of the magnetic field distribution in the 3d domain [11]. the following four metrological parameters are relevant to the cit:  voltage measurement core (vmc) voltage error pu,  vmc phase displacement error δu,  current measurement core (cmc) current error pi,  cmc phase displacement error δi. the cit must comply with the standard iec 61869-4 [14]. it comprises two transformer systems in complex electromagnetic coupling:  a current measurement system with a regime close to a short circuit in the secondary winding and  a voltage measurement system with a regime of almost unloaded transformer, i.e. open-circuit regime. both measurement systems are in a common housing with strong electromagnetic influence which significantly contributes to the metrological properties of the whole device. the geometry of the cit is displayed in figure 2. the goal during the optimal design process of the cit is the minimisation of its metrological parameters pu, δu, pi and δi, as well as the vmc voltage error pu0,25 at 0.25 of vmc rated load, according to the specifications of the iec standard [10] as discussed and with initial results given in [11], [12]. also, the difference of the absolute values of the vmc voltage error at rated load and at 0.25 of the rated load should be as minimal as possible. the minimization requirements of all the metrological parameters could be contradictory. so, the process of metrological optimal design has to be performed by posing a single objective function comprising them all. this is an example of a multi objective optimisation problem. the optimization design of the 20 kv cit, the case study in this paper, is made through variation of eleven input constructional and geometrical variables of the cit. the program ga maximizes the objective function, therefore, the reciprocities of the errors are considered. the objective function can be defined with a conservative approach from a metrological point of view the certain measurement errors expressed through the absolute values are the metrological objective function: iuuuu opt ppppp f         1 1 1 1 1 1 1 1 25,025,0 1 (1) as combination of the vmc voltage error pu at rated load, vmc voltage error pu0,25 at 0.25 of the rated load and cmc current error pi at rated load. the phase displacement errors are constraints during the optimization process in order to satisfy the achieved figure. 2. geometry of the instrument transformer  1. vmc iron core  2. cmc iron core  3., 4. vmc windings  5., 6. cmc windings.  acta imeko | www.imeko.org  november 2016 | volume 5 | number 3 |12  accuracy class of both measurement cores. the cit is a highly non-linear electromagnetic system, with mutual electromagnetic influence of both cores which determines the metrological performance of the device. the ga optimal design is coupled to the results of the fem-3d analysis of the magnetic field distribution in the cit as in [11], [12]. some of the initial results of the optimal design process with the goal function as defined in (1) are given in [11], [12]. according to the guide for expressing the uncertainty in measurements (gum) [15], a statistical approach can be applied:   2225,02225,0 2 1 1 iuuuu opt ppppp f   (2) in the objective functions (1) and (2) the phase displacement error u andi are constraints (a control criterion of the maximal allowed values of the phase displacement errors for certain cit accuracy classes in the ga algorithm cit mathematical model is embedded, as in [14]). the cit optimal design starts with the parameters derived by classical analytical design of the cit based on the t-equivalent circuit as a cit model given in [11], [12] by coupling with fem-3d modelling and calculation of the main electromagnetic characteristics. the chromosome comprises 11 genes (input variables-geometrical, constructional and electrical parameters). the mapping range of the eleven ga input variables is defined according to the previously derived results by fem-3d and the analytical transformer design, some most characteristic, are given in table 1. the optimal solution is derived by the following genetic parameters: cross-over probability 0.65, mutation probability 0.03, population size 16, maximal number of generations 30000. in figures 3 to 5, the changes throughout the 30000 generations in the ga optimisation process of the current densities in the cit windings, as input variables (genes) are comparatively displayed for both objective functions: conservative approach fopt1 and statistical approach fopt2. the main input variables in the calculation of the cit metrological parameters are the equivalent t-circuit parameters, i.e. the winding’s resistances and the leakage reactances. in figures 6 to 12, changes throughout the 30000 generations in the ga optimisation process of the cit equivalent t-circuit parameters are comparatively displayed for both objective functions: conservative approach fopt1 and statistical approach fopt2. in table 2 some of the optimal results (metrological parameters, geometrical, constructional and electromagnetic) derived by both the goal functions (1)-conservative approach and (2)-statistical approach, are compared. figure. 3.  genetic algorithm changes through the generations of the vmc primary winding current density.    figure. 4.  genetic algorithm changes through the generations of the cmc  secondary winding current density.    figure. 5.  genetic algorithm changes through the generations of the vmc secondary winding current density.  table 1. input variables for cit optimal design (mapping range). input variable  minimum maximum  initial value  (analytical)  vmc primary winding  number of turns   23584  24000  24000  vmc primary winding  current density [a/mm 2 ]  1.5  3.0  2.04  vmc secondary winding  current density [a/mm 2 ]  2.0  3.0  2.61  vmc magnetic core outside  length [mm]  183  193  185  cmc secondary winding  number of turns   115  125  120  cmc primary winding  current density [a/mm 2 ]  1.0  1.6  1.36  cmc secondary winding  current density [a/mm 2 ]  2.0  3.0  2.55  cmc magnetic core outside  length [mm]  136  162  142  acta imeko | www.imeko.org  november 2016 | volume 5 | number 3 |13  figure. 6.  genetic algorithm changes through the generations of the vmc  primary winding resistance per turn.      figure. 7.  genetic algorithm changes through the generations of the vmc secondary winding resistance per turn.      figure. 8.  genetic algorithm changes through the generations of the cmc  secondary winding resistance per turn.      figure. 9.  genetic algorithm changes through the generations of the vmc primary winding leakage reactance per turn.  figure. 10.  genetic algorithm changes through the generations of the vmc  secondary winding leakage reactance per turn.    figure. 11.  genetic algorithm changes through the generations of the cmc  primary winding leakage reactance per turn.      figure. 12.  genetic algorithm changes through the generations of the cmc  secondary winding leakage reactance per turn.    table  2.  comparison  of  the  cit  metrological,  electromagnetic  and  constructive  parameters  derived  by  goal  function  with  conservative  and  statistical approach.  ga derived cit  parameter  conservative  approach  statistical  approach  pu [%]  ‐0.795  ‐0.625  pu0,25 [%]  0.795  0.855  pi [%]  0.0  10 ‐7   δu [min]  ‐69.43  ‐61.62  δi [min]  ‐10.45  2.30  bmu [t]  0.424  0.336  bmi [t]  0.539  0.165  mfe [kg]  35.18  46.58  mcu [kg]  12.02  13.57  acta imeko | www.imeko.org  november 2016 | volume 5 | number 3 |14  4. case study no. 2 ‐ ga for prediction of resistance  standards time drift  the estimation of the optimal moment for calibration of the standards is a challenge for most of the metrological laboratories [16]. the decisions are made based on the empirical experience of the laboratory staff, the behaviour history and the conditions of the standard [16]. the trend lines are usually extrapolated to predict the time-behaviour of the standards by using the classical least squares method, as in [5]. however, these models do not take into account the random nature of the calibration results and uncertainties. in this contribution a new model of a time-drift trend line of resistance standard derived by using the lagrange polynomial (lp), coupled with genetic algorithm optimization, is presented. the time points of the performed calibrations and the gained values of standard resistance are input data. the final model of the trend line using five input experimental values can be reduced to a polynomial of fourth order: r(t)=a0t 4 + a1t 3 + a2t 2 + a3t + a4 (3) where t is the time, r is the resistance and ai, i=0,...,4 are the lp coefficients, depending on the values of the resistance at five calibration moments. the lp model is derived on the basis of the experimental data of calibrations of 1  standard in time intervals from 1996 to 2005 year as in table 3. the goal function of the optimization process by the genetic algorithm (ga) is the minimum difference between the theoretical resistance value, derived by the lp, and the experimental value of the resistance at certain time moment (2007-04-20). the original ga program maximizes the goal function and the following goal function fopt is given in the optimization process: exp661 1 rr f opt   (4) where r6=r2007 is the resistance of the standard derived by the lp, determined by ga, at the time moment 20. 04. 2007 and r6exp is the resistance of the standard derived by experimental calibration on 2007-04-20. a verification of the gained model with known experimental data is done (for the time point 2010-06-14). the verification criteria k is: exp77 exp77 rr ss rr k    (5) where r7=r2010 is the resistance of the standard derived by the lp, determined by ga, at the time moment 2010-06-14 and r7exp is the resistance of the standard derived by experimental calibration on 2007-06-14, sr7 and sr7exp are the respective uncertainties. the optimal solution is for the following genetic parameters: cross-over probability 0.65, mutation probability 0.03, population size 10, maximal number of generations 10000. the resistance standard values in the five time points derived from the process of genetic algorithm optimization are given in table 4. the derived ga results after 10000 generations for the lagrange polynomial coefficients are given in table 5. the value of the 1 ω standard resistance in 2007 derived by the lp coupled with the ga optimization process is r2007 =0.999992349 ω. the verification of the method is done through comparison of the theoretical standard resistance in 2010, with the experimentally derived value through calibration. the theoretical value of the standard resistance on 2010-06-14, gained by the lp trend line and through the ga process, is 0.9999916844, with difference of 7.7710-7 , which is the same order of uncertainty as the input variable with highest uncertainty r1996. the experimental input values of the 1 ω resistance standard are compared in figure 13 to the resistance values in different time points derived by the lagrange polynomial coupled to the genetic algorithm optimization process. the uncertainties of the theoretical values are compared to the measurement uncertainties of the experimental data and overlapping of the both uncertainties interval exists. the overlapping is satisfactory for the point in 2010, which is a verification point of the methodology. table 4. resistance standard values at five time points derived from the ga  optimization process.  r1996 []  r1998 []  r2000 []  r2003 []  r2005 []  0.99999245  0.99999242  0.9999923  0.9999923  0.99999235  table 5. the ga output data for the lp model.   lp coefficient  ga derived value  a0 [ω/month 4 ]  0.00000000000  a1 [ω/month 3 ]  0.00000000000  a2 [ω/month 2 ]  ‐0.00000000025  a3 [ω/month]  0.00000000378  a4 [ω]  0.999992438  figure.  13.    comparison  of  the  ga  derived  lp  time‐trend  line  and  the  experimentally derived time‐drift of 1 ω resistance standard.  table 3. the experimental ga input data for the lp model.   date  r []  uncert. of r []  1996‐09‐26  0.999992300  ±510‐7  1998‐10‐23  0.999992450  ±310‐8  2000‐12‐28  0.999992330  ±610‐9  2003‐04‐08  0.999992306  ±610‐9  2005‐05‐09  0.999992353  ±110‐8  2007‐04‐20  0.999992349  ±110‐8  2010‐06‐14  0.999992461  ±110‐8  acta imeko | www.imeko.org  november 2016 | volume 5 | number 3 |15  5. conclusions  in this paper the stochastic optimization method-genetic algorithm is applied for gaining a solution of two different, but challenging problems in metrology of electrical quantities: 1. reduction of errors and uncertainties during the design process of an instrument transformer; 2. prediction of the time-drift of a resistance standard. one original and universal ga software solution is used for two problems of diverse nature. the derived results are satisfactory and verified. it can be concluded that lots of options exist, i.e. the genetic algorithm further to be applied for gaining other optimal solutions in different metrology areas and problems of stochastic nature. references  [1] m. cundeva-blajer, “optimization methods in metrology of electrical quantities”, proc. of 21st imeko world congress, aug. 30-sept. 4, 2015, prague, czech republic, pp. 1903-1908. 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[12] m. cundeva-blajer, l. arsov “influence of the objective function in the process of optimal design of an electromagnetic measurement system”, int. journal of applied electromagnetics and mechanics vol. 39, ios press, amsterdam, 2012, pp. 705-710. [13] d. filipovic-grcic, m. pljak, z. stih, “optimization of the paperoil insulation of high voltage inverse current transformers using genetic algorithm”, proc. of 13th int. symp. on electromagnetic fields in el. eng. isef’07, prague, (paper id 065 on a cd rom), 2007. [14] iec 61869-4, edition 1.0, 2013-11: instrument transformers, part 4: additional requirements for combined transformers, geneva, 2013 [15] expression of the uncertainty of measurements in calibration, (gum), jcgm 100 with member organizations (bipm, iec, ilac, oiml, ....), 1999. [16] ilac-g24:2007 / oiml d 10:2007 (e) guidelines for the determination of calibration intervals of measuring instruments, 2007. beamforming-based algorithms for recovering information from fetal ecg sensors acta imeko issn: 2221-870x june 2023, volume 12, number 2, 1 8 acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 1 beamforming-based algorithms for recovering information from fetal electrocardiographic sensors john p. djungha okitadiowo1, a. lay-ekuakille1, t. isernia2, v. bhateja3, s. p. singh4 1 dieis, university mediterranea of reggio calabria, 89124 reggio calabria, italy 2 department of innovation engineering of salento, lecce 73100, italy 3 department of electrical and computer engineering, srmcem, 226028 luckow (up), india 4 department of eletronics and communication engineering, netaji subhas university of technology, 110078 new delhi, india section: research paper keywords: beamforming; array of sensors; ecg; fetal signals; adaptive filtering; music technique; apnea; applied mathematics citation: john p. djungha okitadiowo, a. lay-ekuakille, t. isernia, v. bhateja, s.p. singh, beamforming-based algorithms for recovering information from fetal ecg sensors, acta imeko, vol. 12, no. 2, article 20, june 2023, identifier: imeko-acta-12 (2023)-02-20 section editor: laura fabbiano, politecnico di bari, italy received february 1, 2023; in final form april 19, 2023; published june 2023 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: john p. djungha okitadiowo, e-mail: johnpdjungha@unicr.it 1. introduction eminent researchers in the scientific world are in the process of or have studied the separation or extraction of signals contained in an original envelope. the medicine applied in the field of paediatrics, more precisely in the branch of cardiology, encounters certain difficulties, to extract the fetal component of the electrocardiography (ecg) signal from the mother’s ecg signal to acknowledge problems of congenital diseases of the fetal/infant; in order to allow doctors to treat this as soon as possible, and to prevent the infant from being born with deformities or other problems that may complicate his/her life in the period of growth. the objective of this paper is to separate, by extracting from the raw ecg of the mother, the ecg of the fetal by preserving information content using beamforming [1], [2], [3], [4], [5]. currently, there are many methods for processing ecg signals, among which the transient electromagnetic method, short-time fourier transform, de-shape short-time fourier transform, and nonlocal median become the preferred method of researchers in ecg signal processing due to its fast signal extraction and decomposition, high efficiency, and great depth of exploration [6], [7], [8], [9]. the authors used the support vector machine to classify the electroencephalography signals of focal and non-focal electroencephalography using entropies [10]. experimental results showed that the proposed abstract we deal with the extraction of the fetal electrocardiography (ecg) signal from the raw ecg signals of the mother by the beamforming based algorithms. the foetal ecg sensors bring out signals containing information from the pregnant mother and the infant. detailed and separate signals are already provided by the foetal ecg instruments; but for some specific studies related to the infant conditions, it is necessary to improve the quality of the signal with a dedicated processing. in this paper, four techniques, with some enhancements, are proposed to perform the processing; we have applied the following techniques: least mean square (lms) with adaptive noise cancellation technique, discrete wavelet transform (dwt)-based technique, empirical wavelet transform (ewt) technique, and multiple signal classification (music). the lms and the music pertain to beamforming approach. the techniques were used to decompose and identify the different elements constituting the source signal (mother's signal) and noise cancellation by multivariate empirical mode decomposition (memd) technique. the signal was adaptively decomposed by lms, dwt and music according to optimised parameters to extract some hidden components of the source signal, such as the foetal features, qrs, heartbeat etc. the results have showed that lms, with enhancements, is more effective in identifying and removing useless noise. the techniques were applied to the ecg signal of a 30-year-old healthy pregnant woman, which allowed to verify their applicability. the present research leads to the below main contributions among others: separation of the ecg signal of the foetus from the mother, highlighting the functional state of the foetal heart rhythm (heart rate and heartbeat,) and this can show us if the foetal ecg has malfunctions. mailto:johnpdjungha@unicr.it acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 2 approach distinguishes between seizure and non-seizure electroencephalography signals using performance measures such as sensitivity, accuracy, specificity, score, and matthew correlation coefficient (mcc) to study the single-channel ecg to reconstruct the waveform of the respiratory signal, using the discrete wavelet transform (dwt) [11]. the author applied multivariate empirical mode decomposition (memd) technique to separate maternal ecg from the abdomen ecg, and wavelet-based technique is employed to find fetal r-peaks. the performance was analysed by calculating cross correlation between the true and detected fetal heart rate signals [12]. the author applied an adaptive noise cancellation technique followed by the stationary wavelet transform (swt) in extracting fetal ecg from composite signal using identification of systems (daisy) and physionet database. also, the fetal heart rate and heart rate variability have been determined to identify fetal suffering [13]. in relation to all the techniques previously used, we have applied beamforming with a combination of its two techniques (lms and music) to perform decomposition to accurately extract the fetal electrocardiogram (fecg) components from the mother's ecg: the techniques of decomposition (music multiple signal classification, memd multivariate empirical mode decomposition, lms least mean square, and ewt empirical wavelet transform) it is obvious that the application of this combination has allowed to obtain one of the best results on the processing of the ecg the result obtained that we had implemented in this study is to have applied some enhanced techniques for the decomposition and extraction of the hidden components in the source signal. 2. electrocardiogram the data quantity in the medical field is rapidly increasing, thus obliging scientists to find effective and adequate techniques and means for their rapid treatments to save human lives. from this need a series of methods have emerged, however electrocardiogram (ecg) occupies the best place in the field of measuring cardiac activities. ecg is a test that studies the functioning of the heart by measuring its electrical activity. with each heartbeat, an electrical impulse (or 'wave') passes through the heart. this wave causes the heart muscle to contract so that it expels blood from the heart. an ecg measures and records the electrical activity that passes through the heart. a doctor can determine whether the electrical activity seen is normal or irregular. an ecg may be recommended if one has arrhythmia, chest pain or palpitations. abnormal ecg results can be used to detect various heart problems [13]. the ecg is used to detect arrhythmias that can lead to blood clots; that is to detect heart problems such as recent or ongoing heart attacks, arrhythmias (irregular heartbeats), blockages in the coronary arteries, damaged areas of the heart muscle (caused by a previous heart attack), enlargement of the heart and inflammation of the heart wall (pericarditis). in the case of a pregnant woman, it is mandatory to monitor the heart rate of the fetal to take appropriate precautions for treatment before and after birth if the baby has heart problems. 2.1. heartbeat modelling heartbeat modelling is an essential step to automatically identify characteristic waves. the aim is to find a mathematical representation, as simple and compact as possible, of the shape of each wave constituting the heartbeat. thus, the most natural representation of waves is to describe the signal by its amplitude at each instant. thus, a vector in a space whose dimension would be equal to a few hundred. the identification of the characteristic waves of the cardiac rhythm is carried out in two stages: segmentation and labelling. segmentation corresponds to the division of the cardiac rhythm into zones likely to contain each cardiac wave. labelling corresponds to the attribution of a medical label (p, q, r, s or t) to each of the zones defined during segmentation. the signal to be decomposed is therefore an isolated heartbeat. for example, if we consider the signal 𝑆 of a beat to be modelled (figure 1) sampled at 500 hz, it is composed of 342 points. the signal 𝑆0, used for the decomposition, is the vector composed of the signal 𝑆 preceded by 85 zeros and followed by 84 zeros, which brings the dimension of this vector to 512, or 29 ∙ 𝑆0 is therefore also a vector 𝑆0 of the 512-dimensional space whose 𝑖-th coordinate in the canonical base is the value of the signal at point 𝑖. in all that follows, the vectors of this space are noted in bold type. the first step in the decomposition is the construction of the wavelet basis. if 𝑆0 is the signal, to be decomposed, of length 𝑁𝑝 (the number of points), the basis consists of 𝑁𝑝 orthogonal wavelets 𝑆 𝐼, all of which are deduced from the "mother" wavelet by translations and dilations. let 𝜑 be the mother wavelet; the basis is constructed as follows: 𝐵 = {𝜑(2𝑚 × ±𝑛), 𝑛 ∈ [1. . 2𝑚−2], 𝑚 ∈ [1. . log2( 𝑁𝑝)]}. (1) in (1) it is shown that, 𝑚 and 𝑛 are respectively the expansion and position coefficients of each wavelet, and 𝑁𝑝the length of the signal to be modelled wavelets, and 𝑁𝑝is the length of the signal. the 𝑁𝑝 − 𝑝 basic functions are denoted {𝜑1}𝑖=[1..𝑁𝑝−1] in the following. such a library is shown in figure 2; wavelets figure 1. the orthogonal wavelet transforms. figure 2. wavelet family used for the decomposition of the 𝑆0 signal. acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 3 (here coiflets) that have the same expansion (constant 𝑚) are represented on the same line. the modelling of the signal is computationally inexpensive due to the orthogonality property of wavelets mentioned above. once the basis is constructed, the decomposition of the signal 𝑆0 is to apply to the vector 𝑆0 the matrix of passage from the canonical basis to the in other terms, to calculate the coordinates of the 𝑆0 vector in wavelet basis wavelet basis: 𝑆0 = ∑ ⟨𝑆0|𝜑𝑖 ⟩𝜑𝑖 𝑁𝑝−1 𝑖=1 . (2) also, in (2) where ⟨𝑆0|𝜑𝑖 ⟩ represents the 𝑖-th coordinate of the signal in the wavelet base. thus, if we decide to choose 𝑁 < 𝑁𝑝 − 1 wavelets to model the signal 𝑆0, the best 𝑌 model will be obtained with the 𝑁 wavelets having the largest scalar product in value. the model 𝑌 demonstrated in [15] will be obtained with the wavelets having the largest absolute scalar product with the signal 𝑌(𝑡) = ∑ ⟨𝑆|𝜑𝑖 ⟩𝜑𝑖 (𝑡) 𝑖={𝐴} , (3) where 𝐴 represents the indices of the 𝑁 largest absolute scalar products between the 𝜑𝑖 and 𝑆0, the mean square modelling error is then written in (4) as 𝐽 = 1 𝑁𝑝 ∑((𝑆(𝑖) − 𝑌(𝑖))2 𝑁𝑝 𝑗=1 . (4) the results of the decomposition the example of an 𝑁 = 10 wavelet model of the previous beat is shown below in figure 3. 3. signal post processing techniques broadly speaking, we have two procedures for extracting fetal ecg signal (fecg). the first is to attach an electrode directly to the fetal scalp (head), but this makes the mother too uncomfortable. the last and second one, for the one we used, consists in the extraction of the fecg using electrodes placed on the mother (figure 4) to mean that here we will have in a single envelope carrying the signal both ecgs, which will carry out the objective of our study to carry out the extraction of the fecg from the ecg signal of the mother (mecg). the importance of this method is that it avoids any contact between the fetal and any external energy. it is very important to point out that the ecg technique is a painless, non-invasive procedure, which means that nothing is injected into the body. the technique is carried out by means of electrodes (figure 4 and figure 5), usually between 12 and 15, which are attached to various parts of the body such as the arm, leg, and chest, in a particular way its electrodes are attached with small suction cups or adhesive patches, which have sensors that detect the electrical activity of the heart. an ecg normally takes between 5 and 10 minutes [14]. for the adaptive filter unit, the two main parts (blocks) are: first digital filter and last the adaptive algorithm; the filtering is performed by the digital filter and the adaptive algorithm performs a weight adjustment; the operation is carried out automatically, see figure 6, and see also the adaptive filter with explanation: 𝑑(𝑛) desired signal, 𝑦(𝑛) output of a digital filter driven by input signal 𝑥(𝑛), and the error signal 𝑒(𝑛) is the difference between 𝑑(𝑛) and 𝑦(𝑛). a predetermined cost function that is related to 𝑒(𝑛) that the adaptive algorithm can minimize by updating the weights of the filters [15]. the adaptive filter is implemented with a different number of structures and realization: the calculation, the complexity and the level of performance are decided by the choice of the structure and the number of iterations. in its adaptive implementation: speed of learning or convergence, computational complexity figure 3. the best 𝑌 model with n = 10 coiflets for the signal s0 is shown on the left (a). the decomposition is the weighted sum of the 10 wavelets shown on the right (b). figure 4. acquisition of the maternal and fetal ecg signal, a1 … a4: abdominal leads, v0: reference electrode, n: active ground. figure 5. ecg signal collection in a pregnant mother. figure 6. diagram of adaptative filter. acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 4 numerical precision and the stability of the algorithm are of the fundamental questions. lms is a simple and stable algorithm but, its speed is slow. in the lms algorithm, the weight update is written in the form below. the relationships of the filter algorithm are given in (5) and (6). 𝑦(𝑛) = 𝑤 t(𝑛 − 1) 𝑢(𝑛) (5) 𝑒(𝑛) = 𝑑(𝑛) − 𝑦(𝑛) (6) 𝑤(𝑛) = 𝛼 𝑤(𝑛 − 1) + 𝑓(𝑢(𝑛), 𝑒(𝑛), 𝑢). (7) lms filters allow us to find the filter coefficients, minimizing the difference between the desired signal and the error signal, which formulas are described in (8) to (10) 𝑓(𝑢(𝑛), 𝑒(𝑛), 𝑢) = 𝑢(𝑛) 𝑢∗(𝑛) (8) 𝑤(𝑖 − 1) + 𝑢 𝑒(𝑖) 𝑥(𝑖) (9) where 𝑒(𝑖) is the error signal, 𝑥(𝑖) is the input, 𝑢(𝑛) is step size parameter, 𝑢∗(𝑛) is the estimated filter; interpreted as the estimation of the filter coefficients after 𝑛 samples, and 𝑤(𝑖) is the weight function. the parameter 𝑢 satisfies the condition 0 < 𝑢 < 1 𝜆max . (10) figure 7 shows detection and extraction of the fetal heartbeat from the mother's heartbeat (measured signal) by adaptive filtering technique; figure 8 shows the cancellation and error detection also noise contained in the signal to output reference signal with memd. 4. proposed approach we have proposed four joint approaches, including wavelet transformation technique. the wavelet technique is essential in clinical studies to detect the different signals that make up a raw one; the wavelet transformation is a recent technique in noninvasive electrocardiology. details of the ecg signal are highlighted in time and frequency resolution using wavelet transformation [16]. during our study, we have opted for the wavelet technique to extract up to the minute information of the abdominal signal and knowing that this algorithm remains complex. the goodness of this set of algorithms is the following: the best representation of the signal missed by one member could necessarily be represented by another, to have the fecg (fetal ecg) signal. we have tried to apply the wavelet (db10), with the regularity of the wavelet is db10 (1.25) which is a little higher than that of db4 (2.90). fecg signal is extracted from the original signal using a two-level wavelet transform. wavelet transformation technique uses the convolution of the wavelet function 𝜓(𝑡) with the signal 𝑥(𝑡) is the wavelet transform. in this technique, the dyadic orthonormal discrete wavelets will be put together to scaling functions 𝜑(𝑡), the coefficient approximation is produced by convolving the scaling function and the signal. the dwt is written in eq. (11), [17]. to reconstruct the original signal, we have selected a wavelet base 𝜓𝑚,𝑛 (𝑡) of coefficient approximation, at the scale and in the location as represented by the 𝑚, 𝑛, is described in (11) and (12) 𝑆𝑚,𝑛 = ∫ 𝑥(𝑡) 𝜑𝑚,𝑛(𝑡) d𝑡 ∞ −∞ (11) 𝑥0(𝑡) = 𝑥𝑀 (𝑡) + ∑ 𝑑𝑚 (𝑡) 𝑀 𝑚=1 . (12) the least mean square is a search algorithm in which a simplification of the gradient vector computation is made possible by appropriately modifying the objective function. the lms algorithm, as well as others related to it, is widely used in various applications of adaptive filtering due to its computational simplicity. the convergence characteristics of the lms algorithm are studied in order to establish a range for the convergence factor that will guarantee stability; (difference between the desired and the actual signal), as shown from (13), up to (16). 𝑊𝑘 (𝑛 + 1) = 𝑊𝑘 (𝑛) − 𝜇 ∇𝐽 , (13) where, ∇ is the gradient of mse 𝐽, 𝜇 is the step size, 𝑊𝑘 (𝑛) is tap value of kth tap nth iteration. the step size can be variable or constant. in lms algorithm, it is a constant positive number whose value ranges from 0 < 𝜇 < 2 𝑌max , (14) where 𝑌max is maximum eigen value of r. if 𝜇 exceeds the limit, then the trajectory of 𝑊𝑘 becomes unstable. in (15) and (16) of steepest descend algorithm [18], [19]. now lsm algorithm estimates the gradient as: 𝑊𝑘 (𝑛 + 1) = 𝑊𝑘 (𝑛) + 𝜇 𝐸[𝑒(𝑛) ∗ 𝑋(𝑛)] (15) 𝑊𝑘 (𝑛 + 1) = 𝑊𝑘 (𝑛) + 𝑢 ∗𝑒𝑛 × 𝑋𝑛. (16) with this algorithm, calculating the value of the next catch becomes easier. updating the kth value requires only 1 multiplication and 1 addition. therefore, for a filter of order p+1, p+1 multipliers and adders are needed. an adder is needed to find 𝑒(𝑛) and a multiplier for 𝑢∗ 𝑒(𝑛) [20], [21]. finally, p adders and p+1 multipliers are needed to find the output 𝑦(𝑛). figure 7. fetal ecg detection/extraction using the adaptative filtering technique. figure 8. adaptative noise cancellations. acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 5 thus, a total of 2 p+3 multipliers and 2 p+2 adders are needed. flowchart of lms algorithm, the flowchart of is based on the following three equations: 𝑦(𝑛) = 𝑤(𝑛)∗ 𝑥(𝑛) (17) 𝑤(𝑛 + 1) = 𝑤(𝑛) + 𝜇∗ 𝑒(𝑛)∗ ∙ 𝑥(𝑛). (18) equation (17) calculates the output of filter by multiplying input with the filter weights, (6) calculates the error between desired signal and output. the equation (18) is of filter weight adaptation. it denotes the lms algorithm. the weights are calculated by changing the previous weight and the converging factor. 𝜇∗ is the complex conjugate of the vector of input samples. we also used the music technique, the literature on the music algorithm is described in [20]. pre-processing in the field of signal processing, it is obviously necessary to prepare the signal before processing; to improve the quality of the signal and the operation is called pre-processing. the objective is to remove all kinds of impurities and unwanted elements from the signal as shown in figure 9. we have used the memd technique to pre-process the signal, which has allowed us to detect and remove the noise as shown in figure 9. segmentation the segmentation of components of the mother's ecg signal allows to point out them , at this stage also the components of our fetal ecg signal (peak, qrs, frequency, heartbeat, apnea peak etc.) will appear automatically, by means of the music algorithm (figure 10 and figure 11). the use of the algorithms saves time in the interpretation of the ecg signal and especially for a pregnant woman, which brings the fetal signal inside [21]. post-processing an ecg is a test that looks at how the heart works by measuring its electrical activity. with each heartbeat, an electrical impulse or wave passes through the heart. this wave causes the heart muscle to contract so that it expels blood from the heart, as discussed in section ii. here we are not dealing with an ordinary ecg signal, but with an ecg signal from a pregnant woman, so by taking this ecg automatically we have a double ecg in its components (mother and fetus). the objective of this study is to separate all the components of the source signal to highlight the ecg of the fetal as well as its internal components, as shown by the different results. this is of paramount importance to provide information to the cardiologist or paediatrician to diagnose cardiological condition of the mother and much more so of the fetal, and to prepare for the different eventualities if the fetal heart has abnormalities. the algorithms allow to clearly separate the components of the two signals in terms of frequency, wave peaks, power spectrum, decimations, interpolations. 5. results ecg, by its nature, is a non-invasive technique that provides electrical signals of the heart activity. in this paper, ecg signal processing with appropriate techniques (music, memd, lms, ewt) have brought a significant improvement in the interpretation of an ecg signal, especially in the interpretation of an ecg signal from a pregnant woman. the detection of the different components of the signal, the most innovative improvement is the separation the components of two ecg signals (mother and fetus), which were initially in one ecg signal, the mother's as we have mentioned at the beginning of our work, that is the framework in which we have worked, the results figure 12–figure 20 show how the two ecg signals are separated. in order to help the cardiologist in saving time in the interpretation of the ecg signal of a pregnant woman, which will allow him/her to have precise information on the cardiac activities of the fetus, to see if the heart of the fetus is in good health, on the contrary, to prepare an appropriate therapy immediately, because the algorithms will highlight the problems detected so the cardiologist can give a precise diagnosis. figure 9. fetal ecg detection/extraction using ewt technique. figure 10. detection and labelling of apnea components. figure 11. decimated and interpolated tremor suppression (left), interpolated and decimated and noise cancellation signal (right). acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 6 the plot of the figure 13 shows a overlapping of the two signals (power spectrum of the ecg signal of (the red) mother and that of the fetal in blue); the plot indicates that the fetal heart is still dependent on that of its mother, since the peaks of the two signals are at the same rate with just a small difference because that of the fetal lives on the blood of its mother. this leads us to confirm that the algorithm has worked. figure 15 shows a separation of the signals which were contained in a single envelope of origin or source (maternal ecg signal) with the different r peaks of each signal; the present result is obtained thanks to the multiple signal classification algorithm (music), which allows to classify, to separate and to provide figure 12. abdominal ecg after suppressed power line interference noise at 50 hz and abdominal ecg after removing interference from the power line, line based on the wandering and trembling. figure 13. power spectrum of simulated mother ecg signal and power spectrum extracted fetal ecg signal. figure 14. power spectrum overlapping of the mother's ecg signal and her fetal component. figure 15. fetal qrs detection from mother's ecg signal. figure 16. power spectrum overlapping of the monther’s apnea and her fetal signal. figure 17. power spectrum overlapping of the mother’s apnea and her fetal signal on the left, detection of maternal, fetus and airway pressure apnea peaks on the right. figure 18. a comparison of the snr of the ewt and lms algorithm. acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 7 unbiased estimates of the number of muniscule signals and also to discriminate the signals from the noise which was on board during transmission, recording or during its diffusion in different transmission channels and, sometimes with certain details as detected, see figure 19. we have decided, given the objectives of the paper to introduce decimation and interpolation for a further interpretation. decimation and interpolation are two operations that affect the timescale of a signal. for discrete signals, time scaling corresponds to increasing or decreasing the length of the signal. however, we must pay attention to how these operations are carried out. decimation is the time compression of a signal. this corresponds to speeding up a signal or reducing its sampling rate. suppose we have a signal 𝑥[𝑛] which corresponds to a continuous signal 𝑥(𝑡) samples at 𝑡𝑠 intervals. the signal 𝑦[𝑛] is then equivalent to the compressed signal which is sampled at and contains the samples 𝑥[0], 𝑥[2], 𝑥[4], . .. . 𝑦[𝑛] can also be obtained directly if the signal 𝑥(𝑡) is sampled at intervals of 2 𝑡𝑠. decimating by a factor of n is equivalent to keeping all n samples of a signal. this can lead to information loss. figure 10 details the procedure for detecting the components of the apnoea signal and labelling them with the music algorithm, on a normal class of apnoea including hypopnoea which are shown here in bright colours: red, yellow, and green, as well as other lines labelled in this way, with this legend: af: airflow, ae: abdominal effort, te: thoracic effort st: stage label. another needed operation is the interpolation. it is the stretch over time. this corresponds to slowing down the signal or increasing the sampling rate. suppose we have a signal 𝑥[𝑛] which corresponds to a continuous signal 𝑥(𝑡) sampled at intervals 𝑡𝑠. the signal 𝑦[𝑛] = 𝑥 [ 𝑛 2 ] is then equivalent to the signal 𝑥(𝑡) which is sampled at intervals of 𝑥(𝑡) (or at a rate of 𝑆 = 2 𝑡𝑠 ). we therefore have twice as many samples: we have stretched the signal (8) in both directions. however, how do you calculate the value of the new samples? three popular techniques exist for doing the interpolation: 1. zero interpolation: implies that each new sample is zero, 2. step interpolation: we take the previous value for the new sample, and 3. linear interpolation: the values on each side of the new sample are averaged, the figure: interpolation shows an example of three types of interpolation. figure 11 is the original signal. the samples in red are the new samples. decimation is the reverse of interpolation, but the inverse is not necessarily true. when we apply the decimation, we lose some information; so we are not able to reproduce the original signal correctly. as an example, we use the sequence. {1,2,6,4,8}  first, we apply the step interpolation to this sequence, and we get: {1,1,2,2,6,6,4,4,8,8}  however, if we inverse the order of operations, we will not find the original sequence. first, we apply the decimation on the sequence 𝑥[𝑛], and then we apply the step interpolation: {1,1,6,6,8,8}  which is not the same sequence as the original. 6. conclusions currently, considerable progress has been made with the advent of the beamforming technique. beamforming, also called spatial filtering, or channel formation, is a signal processing technique used in antenna and sensor networks for the transmission or directional reception of signals. four approaches are used (lms, music, memd, ewt) as shown in figure 20 during different experiments. the objective of this study is to decompose, visualize and highlight internal components of the ecg signal of a pregnant woman, as we are seeing in figure 11 – figure 20, the various techniques made it possible to carry out a normal decomposition and separation of the internal components of the initial signal (mother's ecg). referring to the previous paragraph, the goal is to save time for health professionals (cardiologist) in the interpretation of an ecg signal, especially the ecg of a pregnant woman including that of the fetus. the approach used in this paper remains mostly filtering to separate components of a signal by their frequencies and on their channels; the beamforming approach (adaptive filtering) is reliable in the processing of ecg signals; each component of the signal source has been filtered by its appropriate frequency. figure 19. ewt flowchart. figure 20. a comparison of the snr of the music, memd, lms and ewt algorithms for ecg (a) and apneas (b) of the mother and the fetus. acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 8 references [1] a. k. rahmati, s.k. setarehdan, b.n. araabi, a pca/ica based fetal ecg extraction from mother abdominal recordings by means of a novel data-driven approach to fetal ecg quality assessment. j biomed phys eng. 2017 mar 1;7(1):37-50. pmid: 28451578; pmcid: pmc5401132. 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https://doi.org/10.3390/s22082843 https://doi.org/10.1016/j.sigpro.2021.108344 https://doi.org/10.1016/s0167-6393(02)00087 microsoft word 259-1771-1-le-rev acta imeko  issn: 2221‐870x  september 2015, volume 4, number 3, 23 ‐ 29    acta imeko | www.imeko.org  september 2015 | volume 4 | number 3|23  time coordination of standalone measurement instruments  by synchronized triggering  francesco lamonaca, domenico luca carnì, domenico grimaldi  department of computer science, modeling, electronics and system science, university of calabria, rende (cs), italy        section: research paper   keywords: synchronization; embedded hardware; standalone measurement instrument; distributed measurement system  citation: francesco lamonaca, domenico luca carnì, domenico grimaldi, time coordination of standalone measurement instruments by synchronized  triggering, acta imeko, vol. 4, no. 3, article 4, september 2015, identifier: imeko‐acta‐04 (2015)‐03‐04  editor: paolo carbone, university of perugia, italy  received february 16, 2015; in final form may 25, 2015; published september 2015  copyright: © 2015 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: this work was partially supported by the italian ministry of economic development by means of the riditt: ”italian network for innovation and  technology transfer to enterprises”, under the project “di.tr.im.mis diffusione e trasferimento di tecnologie ad imprese nel settore delle misure”.  corresponding author: domenico luca carnì, e‐mail: dlcarni@dimes.unical.it    1. introduction  in [1] the hardware interface (hi) architecture was proposed to bring the common sense of the time to the standalone measurement instruments (mis) [2], [3] in operative scenarios in which no stable network is available. the time coordination of the operations of mis is difficult when: (i) the network topology does not provide a continuous, reliable and stable link, wired or wireless, to the whole network [4]-[15], (ii) the installation of new wired or wireless links or repeaters would be expensive or not physically or technically possible [16]-[19], and (iii) a time constraint requires both fast and efficient solutions [20]-[22]. in these cases, no network services are available for the standalone mis and synchronization services based on packet exchange as the std. ieee1588 cannot be used [23]. the hi architecture includes a counter driven by a clock, and a programmable interface controller (pic). the his are synchronized by enabling the hi counters at the same time. after that, the his are moved and connected to the standalone mis that can be in different places where no networking is available. the time coordination of the measurement operations is achieved by triggering the mis in synchronized modality. the software and hardware architecture of the hi are designed so that (i) the random effects of the operating system are avoided, and (ii) the polling cycle to check the trigger condition is reduced [24]-[26]. the result is that the synchronization accuracy between two triggers sent to the standalone mis is one clock period. in the paper, the mathematical model of the contribution of each hardware and software component to the synchronization delay is developed. the analysis of the mathematical model highlights that to improve the synchronization accuracy two abstract  a hardware interface (hi) to synchronize the operations of standalone measurement instruments (mis) in the absence of networking  has been proposed  in the recent  literature. the synchronization accuracy achieved  is one period of the clock equipping the hi. to  improve the synchronization accuracy two solutions can be argued on the basis of the mathematical model of the delay between his.  the first involves increasing the clock frequency; the second concerns the compensation of the phase delay between hi clocks. in this  paper  the second  solution  is  adopted  in  order  to:  (i)  reduce  the  energy  consumption,  and  (ii)  not  increase  the  complexity  of  the  hardware architecture. the phase delay compensation is obtained by introducing a programmable delay line after the hi clocks. the  phase  delay  evaluation  and  the  successive  tuning  of  the  delay  line  are  performed  in  the  synchronization  phase  of  the  his.  once  synchronized, each hi is moved to the standalone mi to trigger it according to the common sense of time. during the execution of the  measurement  procedure,  networking  is  not  necessary.  experimental  tests  validate  the  correct  operation  of  the  upgraded  hi  architecture and indicate that the achievable synchronization accuracy is a low percentage of the hi clock period.  acta imeko | www.imeko.org  september 2015 | volume 4 | number 3|24  solutions can be argued: (i) increasing the clock frequency, and (ii) reducing the phase delay between clocks. the first solution involves problems in the design of the hi [27], and increases the power consumption [28]. the second solution is more advantageous than the first and it is developed in this paper. to this purpose, the hardware architecture of the hi is upgraded by adding the programmable delay line after the clock block. this delay line permits realignment of the clock signals of the his once their phase delays are previously evaluated. the phase delay evaluation and the successive tuning of the delay line are performed in the synchronization phase of the his. the first step of the synchronization phase concerns the evaluation and compensation of the clock delays by tuning the programmable delay line. the second step concerns the synchronized start of the counter block, as in [1]. during the synchronization, communication between the his is required. during the triggering of the mis communication among his is not required and the synchronized measurement procedure can be performed also in the absence of networking. the paper is organized as follows: in section 2 the hardware architecture and the performed logical operations of the upgraded hi are presented; in section 3 the mathematical model of the synchronization delay is analysed; in section 4 the experimental results are discussed; finally, conclusions are drawn in section 5. 2. operations performed by the upgraded hi  the upgraded hi compensates the delay between the trigger signals sent to the mis, as in [1], as well as the clock phase delays, to further reduce the synchronization delay. the upgraded hi is composed by five blocks (figure 1): (i) clock block: drives the programmable interface controller (pic) and the counter block; (ii) programmable delay line: compensates the phase delay between two hi clocks; (iii) 8-bit counter block: feeds the pic with synchronized impulses; (iv) pic: generates the synchronized trigger signal on the pin rb1 for the mi. moreover, it tunes the programmable delay line in the synchronization phase on the basis of the information sent by the master pc; (v) board rabbit rcm4400w: operates as wi-fi interface between the hi and the master pc. it is used only for the initial synchronization, and after that is disconnected from the hi. the synchronization between two his is executed by the starter block (figure 1) and the master pc on the basis of the procedure described in subsection 2.1. 2.1. hi synchronization procedure  the synchronization procedure is organized in two successive steps. the first step requires the connection among the master pc, two his and the starter block, as shown in figure 1. the master pc commands: (i) each hi to pull down the pin trok in order to inhibit the start count block, (ii) the starter block to measure the phase delay tph between the clocks of hi#1 and hi#2 connected to the pins clk1 and clk2 of the phase delay measurement block (pdmb). the pdmb is equipped by an internal counter with period tclk_starter= 5 ns equal to 1/20 of the hi clock period that is 100 ns. therefore, the clock frequency of the starter block is 200 mhz. the mean value of 100 measures of the clock phase delay is sent to the master pc by the universal serial bus connection, and, successively, by the wi-fi interface to the his for tuning the programmable delay line. experimental tests indicate that after the synchronization, due to the hardware and software architecture of the hi and the starter block, the phase delay is lower than the resolution of the starter block (5 ns). in particular, by using a digital storage oscilloscope with resolution 0.1 ns, the measured clock phase delay is 2.2 ns with standard deviation 0.9 ns. increasing the number of measurements does not provide any advantage because the mean value does not significantly change and the standard deviation is less than the resolution of the starter block. the tuning procedure ends if:  _max ,ph pdl clk startert t t  (1) where tpdl is the resolution of the programmable delay line. this condition ensures that the tuning procedure ends in the case the measured phase delay cannot be further evaluated, if _ph clk startert t  , or compensated, if ph pdlt t  . once the phase delay is compensated, the second step of the synchronization procedure starts. as in [1], the master pc sends the following trigger conditions to the two his:  nitp number of impulses that the pic must count before triggering the mi,  nm number of repetitive measures to be executed by the mi,  m number of impulses between two consecutive measures. in order to execute synchronized measurement operations it is necessary that nitp and nm are identical for both the his. once this information is received, the hi pulls up the pin trok. when both the pin trok of hi#1 and hi#2 are high, the pin ps of the start count block is pulled up and the counter block of each hi starts. the signal on pin ps is sent to the his in parallel connection, guaranteeing that the delay between the start of the counter blocks is determined only by their hardware characteristics. once the pics receive the impulse, the his can be disconnected from the starter block and connected to the standalone mis. pht   figure  1.  block  scheme  of  the  connection  among  upgraded  his,  starter block, and master pc in the synchronization phase.  acta imeko | www.imeko.org  september 2015 | volume 4 | number 3|25  the wi-fi connection is involved in the synchronization process of the hi only for the transmission of the trigger generation parameters (nitp, nm, m) and the measured phase delay for the tuning of the programmable delay line. therefore, the wi-fi interface does not influence the synchronization accuracy. as a consequence, any boards implementing the conversion wi-fi-serial communication can be used. 2.2. triggering procedure  the time coordination among the operations executed by the mis is obtained by the synchronized trigger signals sent by the his. at the beginning, each hi set nitp and after that generates the trigger signal. successively, all the his start the count simultaneously. the program set up in the pic counts the counter impulses and checks whether the actual count is equal to the set-up value. these operations are performed before the successive impulse occurs. this reduces the effect of the polling to check the trigger condition. figure 2 shows the block diagram of the operation performed by the pic: 1) the internal variable i is incremented; 2) if i = nitp, then hi sends the trigger signal by generating an impulse signal in the rb1 line, and the new values are updated nitp = nitp+m, nm = nm-1; 3) else go to step 1; 4) if nm = 0, the triggering procedure ends. 3. synchronization delay analysis  the synchronization delay t between two triggers is determined by the hardware architecture of the two his and the time needed to execute their operation (figure 3). it is [1]:     1 1 1 2 2 2 ; c c p p p c c p p p c p t n t n t e n t n t e o o           (2) where:  tc1 and tc2 are the clock periods of hi#1 and hi#2, respectively;  tp1 and tp2 are the clock periods of pics equipping hi#1 and hi#2, respectively;  nc is the counting number of hi#1 and hi#2, equal to nitp2 n where n is the number of bit of the counter;  np is the number of elementary operations to execute the triggering procedure in the pic;  oc is the start offset time between the counting on hi#1 and hi#2. it depends on tph and the time instant t1 and t2 (figure 4) corresponding to the opening of the counter figure 3. unified modelling language diagram of the operations executed  by the hi components involved in the synchronized triggering of the mis. a) b) figure 4. delay between the opening of the counter gate and the start of counting. a) t1 and t2  occur before the raising of the two clock signals, b) t1 and t2 occur between the raising of the two clock signals.   figure 2. block diagram of the performed operation by each hi for the synchronized generation of the trigger signal.  hi#1 v t hi#2 t start countingv clock clock tc tc oc= tphtph t1 t2 start counting acta imeko | www.imeko.org  september 2015 | volume 4 | number 3|26  gate of hi#1 and hi#2, respectively;  op is the start offset time between two pics;  ep1 and ep2 are the time intervals required by the hardware of the two pics to pull up the pin rb1 and, then, trigger the mi. owing to the synchronized start of the counting, op has no influence in the evaluation of t. by considering the vector x made up of: nc, np, tc1, tc2, tp1, tp2, ep1, ep2, oc, the variation of the synchronization time delay (1) evaluated according to [29] is:             1 2 1 2 1 2 22 2 2 2 2 2 c c p p c p p t c t c t p tx x x p t o e e x n n n n                  (3) where:  tc1 and tc2 are the variation range of the values of tc1 and tc2, respectively,  tp1 and tp2 are the variation range of the values of tp1 and tp2, respectively,  ep1 and ep2 are the variation range of the values of ep1 and ep2, respectively.  oc is the variation range of the values of oc. since the two his have the same hardware, it is: 1 2 1 2 1 2 1 2, , c c c p p p c c c p p p t t t t t t t t t t t t              . (4) moreover, absence of conditional jumps and correlation between parameters can be assumed. in fact, to check the trigger condition in the software set up in the pic there are no conditional jumps and the parameters in (2) are related to independent devices. the variation range of tc and tp is according to the propagation delay of the hardware components and can be evaluated by their datasheet. the variation range oc depends on tph. in particular two cases can occur:  if t1 and t2 are before the raising edge of the clock signals (figure 4a), oc =tph. indeed, the counters of hi#1 and hi#2 are sensitive to the raising edges of the clocks that have lower delay;  otherwise oc = tctph (figure 4b). the gate of the hi#1 counter is opened before the occurrence of the raising edge of the hi#1 clock, the gate of hi#2 counter is opened after the occurrence of the raising edge of the hi#2 clock. consequently, the hi#2 counter will be sensitive to the successive raising edge of the hi#2 clock. since tph varies in the range [0, tc], oc varies in the range [0, tc], also. decreasing tph, the probability that oc=tph increases (figure 4a). as a consequence, oc value is decreased. the programmable delay line permits the reduction of the variation range of tph according to its resolution. as a consequence of the parallel connection among starter block and the two his, |t2t1| is of some ps and then negligible with respect to tph. in fact, |t2t1|is determined by the propagation delay in the two cable lengths of (0.150±0.001) m connecting the pin ps of the starter block with the start input pin of the his, and the difference between times to change the gate states. owing to the propagation speed of the signal, the difference between t1 and t2 is about 5 ps. the difference between the times to change the gate states is in the order of 10 ps. therefore, the difference between t1 and t2 is almost 15 ps, negligible with respect to tc = 100 ns. it can be assumed that t1≈t2=t*. due to periodicity of the clock signal the probability that t* is included in the time interval tc is 1. the corresponding probability density function is: 1 ( ) c f t t  . (5) consequently, the probability p that t* occurs in the time interval tph is [30]: 0 ( 0 * ) ( ) p ht p h p h c p t f t d t t t t       p . (6) by taking into consideration that at the end of the synchronization procedure tph<5ns and tc=100ns, it results in p<5 %. therefore, by using the programmable delay line, both oc and oc can be reduced to the order of ns. 4. experimental tests  experimental tests are developed in order to validate the correctness of the operations performed by the upgraded hi and to evaluate the time delay between two trigger signals. the hi is equipped with a thermal compensated clock with period 100 ns. the measurement of the time delay between the trigger signals is executed by connecting the pins rb1 of the hi#1 and hi#2 to the input channels ch#1 and ch#2, respectively, of the digital storage oscilloscope (dso) tektronix tds7154b, as shown in figure 5. the sampling frequency of the dso is set to 10gs/s, to ensure a sampling period equal to 0.1 ns. the master pc is connected by a gpib interface bus to the dso, and sends the trigger conditions to the two his by wi-fi connection. in the experimental tests the trigger conditions are the same for both his. the time delay between the triggers is evaluated by means of the number of samples occurring between the crossing of the established threshold by the two input signals. in particular, denoting i and j the indexes corresponding to the samples crossing the threshold, and dt the sampling period of dso, the time delay is:  t i j dt   . (7)   figure  5.  experimental  setup  to  evaluate  the  delay  between  the  trigger signals generated by two his.  acta imeko | www.imeko.org  september 2015 | volume 4 | number 3|27  in the first set of experiments the following parameters are set: nitp = 351562, nm =4000, andm = 39062. with this setup, the first trigger is generated after nitp*28*100 ns = 9 s from the end of the his synchronization. this time is enough to disconnect the his from the starter block and to connect them to the mis. the successive 4000 triggers are generated eachm * 28 * 100 ns = 1 s. figure 6a) shows the trend of t versus the acquisition time, figure 6b) its histogram in the case the synchronization of the two his is performed at the beginning and the programmable delay line is not used. in this case t is characterized by the mean value = 62.3 ns and standard deviation = 0.4 ns. similarly, figures 6c) and 6d) show the trend of t and its histogram evaluated by repeating the synchronization procedure. in this case t is characterized by =86.1 ns and = 0.4 ns. figure 6 shows that, without the programmable delay line, the mean value of the delay between the hi triggers depends on the synchronization procedure and does not change during the observation time. in the two different experiments the widths of the variation ranges are comparable. figure 7a) shows the trend versus the acquisition time of t, figure 7b) its histogram in the case the synchronization of the two his is performed at the beginning and the programmable delay line is used. in this case t is characterized by = 5.8 ns and = 0.4 ns. similarly, figures 7c) and d) show the trend of t and its histogram evaluated by repeating the synchronization procedure. in this case t is characterized by = 7.3 ns and = 0.4 ns. these results confirm the effectiveness of the proposed synchronization procedure to compensate for tph. the results of repeated experiments highlight: (i) the proper operation of the upgraded starter block makes that the delayed start of the pic does not influence the synchronization accuracy and (ii) with the upgraded hi architecture, the delay between the trigger signals is a low percentage of the clock period of hi. 5. conclusions  the synchronization accuracy of the hardware interfaces (his) bringing the common sense of the time to standalone mis [1] is improved. the hi architecture is upgraded by adding the programmable delay line after the clock in order to reduce the phase delay that is the main source degrading the synchronization accuracy. the synchronization procedure is also upgraded to provide the measurement of the phase delay that must be compensated by adjusting the delay line. the experimental results indicate that, by considering the actual realization of the upgraded hi, the accuracy obtained is a low percentage of the hi clock period, whereas with the previous configuration it is one clock period. the on-going activity is devoted to developing a hi able to work in scenarios with or without networking. when the network is not available, the hi works as described in the paper. when the network is available, the hi will implement a synchronization procedure based on packet exchange.  6. acknowledgement  this work was partially supported by the italian ministry of economic development by means of the riditt: ”italian network for innovation and technology transfer to enterprises”, under the project “di.tr.im.mis diffusione e trasferimento di tecnologie ad imprese nel settore delle misure”. references  [1] f. lamonaca, d. l. carnì, d. grimaldi, "new hardware interface to synchronize stand-alone measurement instruments," proc. of: 20th imeko tc4 international symposium and 18th international workshop on adc modelling and testing a) b) c) d) figure 6. trends of t versus the acquisition time a), and their histogram b),  in the case where the synchronization of the two his  is performed at the beginning  and  the  programmable  delay  line  is  not  used.  c)  and  d)  are evaluated by repeating the synchronization procedure.  0 500 1000 1500 2000 2500 3000 3500 4000 60 61 62 63 64 time [s] s y n ch ro n iz at io n t im e d el ay [ n s] 61 62 63 64 0 200 400 600 800 1000 1200 synchronization time delay [ns] h is to g ra m 0 500 1000 1500 2000 2500 3000 3500 4000 84 85 86 87 88 time [s] s y n ch ro n iz at io n t im e d el ay [ n s] 84 85 86 87 88 0 200 400 600 800 1000 1200 1400 synchronization time delay [ns] h is to g ra m acta imeko | www.imeko.org  september 2015 | volume 4 | number 3|28  research on electric and electronic measurement for the economic upturn, benevento, italy, 2014. 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[11] l. kebin, m. qiang, l. haoxiang, c. zhichao, l. yunhao, "endto-end delay measurement in wireless sensor networks without synchronization", in proc. of ieee 10th international conference on mobile ad-hoc and sensor systems (mass), 2013, pp. 583-591. [12] p. kieu-ha, t. huy, j. tiete, l. tran, k. steenhaut, "lowoverhead time synchronization for schedule-based multi-channel wireless sensor networks", in proc. of 19th ieee workshop on local & metropolitan area networks (lanman), , 2013, pp. 1-6. [13] j.-t. guo, m.-y. hou, y.-x. wang, g.-f. zhu, g.-b. zou, c.-h. zhai, x.-h. liu, n. zhang, "application of ieee1588 in time synchronizing system of smart distribution grid", in proc. of 12th iet international conference on developments in power system protection (dpsp 2014), 2014, pp. 1-4. [14] d. kashyap , t. k. jain, "comparative analysis of time synchronization schemes for wireless sensor networks", in proc. of 2014 international conference on signal propagation and computer technology (icspct), 2014, pp. 106-109. [15] a. puhm, m. kramer, p. moosbrugger, m. horauer, "problems and solutions for refitting a sensor network with ieee1588 clock synchronisation", in proc. of ieee emerging technology and factory automation (etfa), 2014, pp. 1-7. [16] f. lamonaca, g. pizzuti, a. arcuri, "environmental parameters in the national gallery of cosenza: a case of study," proc. of: 4th imeko tc19 symposium on environmental instrumentation and measurements, lecce, italy, 2013. [17] m. l. vincent, f. kuester, t. e. levy, "opendig: in-field data recording for archaeology and cultural heritage", in proc. of digital heritage international congress (digitalheritage), 2013, pp. 539542. [18] d. l. carnì, d. grimaldi, f. lamonaca, "improved synchronizing procedure of pdas to delivery the common sense of the time to stand alone measurement instrument", in proc. of 19th imeko world congress 2009, 2009, pp. 1335-1340. [19] f. lamonaca, g. pizzuti, n. arcuri, a. m. palermo, r. morello, "monitoring of environmental parameters and pollution by fungal spores in the national gallery of cosenza: a case of study", measurement, vol. 47, pp. 1001-1007, 2014. [20] c. h. cuadra, n. shimoi, t. nishida, m. saijo, "estimation of dynamic properties of traditional wooden structures using new bolt sensor", in proc. of 13th international conference on control, automation and systems (iccas), 2013, pp. 1593-1598. [21] l. gang, s. j. qin, z. donghua, "a new method of dynamic latent-variable modeling for process monitoring", ieee transactions on industrial electronics, vol. 61, pp. 6438-6445, 2014. [22] m. lydon, s. e. taylor, d. robinson, p. callender, c. doherty, s. k. t. grattan, e. j. o'brien, "development of a bridge weigh-in-motion sensor: performance comparison using fiber optic and electric resistance strain sensor systems", ieee sensors journal, vol. 14, pp. 4284-4296, 2014. [23] "ieee standard for a precision clock synchronization protocol for networked measurement and control systems", ieee std 1588-2008 (revision of ieee std 1588-2002), pp. c1-269, 2008. a) b) c) d) figure 7. trends of t versus the acquisition time a), and their histogram b), in the case where the synchronization of the two his is performed at the  beginning  and  the  programmable  delay  line  is  used.  c)  and  d)  are evaluated by repeating the synchronization procedure.  0 500 1000 1500 2000 2500 3000 3500 4000 4 5 6 7 8 time [s] s y n ch ro n iz at io n t im e d el ay [ n s] 4 5 6 7 0 200 400 600 800 1000 1200 synchronization time delay [ns] h is to g ra m 0 500 1000 1500 2000 2500 3000 3500 4000 5 6 7 8 9 time [s] s y nc h ro n iz at io n t im e de la y [ ns ] 6 7 8 9 0 200 400 600 800 1000 1200 synchronization time delay [ns] h is to g ra m acta imeko | www.imeko.org  september 2015 | volume 4 | number 3|29  [24] f. lamonaca , d. grimaldi, "trigger realignment by networking synchronized embedded hardware", ieee transaction on instrumentation and measurement, vol. 62, pp. 38-49, 2013. [25] f. lamonaca, a. gasparri, e. garone, d. grimaldi, "clock synchronization in wireless sensor network with selective convergence rate for event driven measurement applications", ieee transactions on instrumentation and measurement, vol. 63, pp. 2279-2287, 2014. [26] f. lamonaca, d. grimaldi, r. morello, "dynamic scheduling of trigger command for sub-μs alignment accuracy in distributed measurement system", ieee transaction on instrumentation and measurement, vol. 63, pp. 1795 1803, 2014. [27] electromagnetic compatibility of integrated circuits: springer science+business media, 2006. [28] a. y. feinstein, m. a. thornton, f. kocan, "system-on-chip power consumption refinement and analysis", in proc. of 2007 ieee dallas/cas workshop on system-on-chip (soc): design, applications, integration, and software, dcas-07, 2007, pp. 81-84. [29] i. o. f. standardization, guide to the expression of uncertainty in measurement (gum), 1998. [30] m. h. degroot, m. j. schervish, x. fang, l. lu, d. li, probability and statistics vol. 2: addison-wesley reading, ma, 1986. microsoft word 385-2422-2-le-rev2 acta imeko  issn: 2221‐870x  november 2016, volume 5, number 3, 47‐54    acta imeko | www.imeko.org  november 2016 | volume 5 | number 3| 47  signal and zero padding to improve parameters estimation of  sinusoidal signals in the frequency domain  dušan agrež 1 , damir ilić 2 , janko drnovšek 1   1 university of ljubljana, faculty of electrical engineering, trzaska 25, 1000 ljubljana, slovenia  2 university of zagreb, faculty of electrical engineering and computing, unska 3, 10000 zagreb, croatia       section: research paper   keywords: sampling process; sampling by averaging; signal padding; zero padding; estimation of parameters; interpolated dft  citation: dušan agrež, damir ilić, janko drnovšek, signal and zero padding to improve parameters estimation of sinusoidal signals in the frequency domain,  acta imeko, vol. 5, no. 3, article 8, november 2016, identifier: imeko‐acta‐05 (2016)‐03‐08  section editor: konrad jedrzejewski, warsaw university of technology, poland  received may 13, 2016; in final form july 11, 2016; published november 2016  copyright: © 2016 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  corresponding author: dušan agrež, e‐mail: dusan.agrez@fe.uni‐lj.si    1. introduction  in the last decade, considerable research has been carried out on the analysis of efficient methods capable of accurately estimating parameters of the frequency components of interest [1], [2]. in many cases the problem of evaluating the spectral performance of a given periodic signal reduces to the estimation of parameters of each spectral component (frequency, amplitude, and phase). parameter’s estimations of periodic signals mostly base on sampling and acquiring digital values of samples by analog-to-digital converters. in this procedure values of sampling points are results of averaging in the aperture time – measurement time. this averaging (or integration) gives reduction of noise but causes systematic errors in estimations of the signal parameters [3]–[5]. in this paper, algorithms for estimation of parameters by signal and zero padding first and then interpolation in the frequency domain are presented. a sampling process can be modelled with four signals and their frequency spectra in the time and the frequency domain: measured signal )()( f fgtg  (figure 1), impulse response of the sampling channel )()( f fhth  (figure 2), sampling function )()( f fsts  (figure 3), and the window function of the measurement interval )()( f fwtw  (figure 4) where f stands for the fourier transformation from time domain to frequency domain and vice versa. maxf maxft  tg f  fg f figure 1. measured signal.  figure 2. impulse response of the sampling channel.  0 f t  th f  fh 0 abstract  this paper presents the procedure to  improve the estimation of the basic sinusoidal signal parameters (frequency, amplitude, and  phase, respectively) in the case of signal sampling by averaging in the aperture time. prior to estimation in the frequency domain by  the interpolated dft algorithms the sampled signal is padded with the signal average values in the aperture times and zeroes in the  rest of the sampling interval. we can increase padding points and a number of the signal cycles in the whole measurement interval  and with this nearing the errors to the  level as with estimation of the signal without average sampling even the sampling nyquist  condition is not fulfill.  acta imeko | www.imeko.org  november 2016 | volume 5 | number 3| 48  the measured signal )(tg is always filtered by the impulse response of the input front circuit of the measurement channel represented by )(th f in figure 5 and after that sampled by the real finit duration sampling pulses represented by )(th convolved on the sampling function )(ts (figure 3: typically time uniformly distributed) and finally only a finite number of samples is taken into consideration represented by the window function )(tw (figure 4) to get a filtered, sampled and windowed signal )(* , tg wf (figure 5). considering that the equivalent of multiplication in the time domain is convolution in the frequency domain      d)()()()( 21 f 21 fggtgtg and vice versa )()(d)()( 21 f 21 fgfgtgg      [6] the sampling procedure can be modelled as follows (figure 5): it is evident that the spectrum of the sampled signal starts to change with modification of the sampling pulses )()( f fhth  (figures 6 and 7). 2. padding and estimation of parameters  sampling by the frequency ss 1 tf  of the periodic band limited signal  tg composed of m components can be expressed as        1 0 ss 2sin m m mmm tnfatnw  with mf , ma and m as frequency, amplitude, and phase of a particular component, respectively. in the estimation procedure one has to take into account that values of samples (figure 8: line d) are representatives in the aperture time apt or typically average values of the signal in the aperture integration interval (figure 8: line c). for a demonstration of sampling, in figure 8 three periods of the one component sine signal are presented with a duty cycle of sampling 4.0sap  ttd and with sampling ratio of 6.1ss  ttffr mm . with sampling representatives – average values – we lose some information of the signal and especially in the cases where the aperture time is so long that the signal changes significantly  th  ts  tg  thf    tg f  ts    tg f  tw  tg wf, t s2t tst0s2t st s2t tst0s2t st t 1 mt figure 6. signals in the sampling process.  sf s2fs2f sf f0  fs  fh  fgf  fg wf,  fgf  fs  fg  fhf  fw     maxf maxf f nyqf 0 fsf s2fs2f sf sf s2fs2f sf f0 figure 7. spectra of signals in the sampling process.   1  1 1  1  1 f t )()( stntts n      st s2t0s2t st f  s1t  s1t s1t  s1t  s1t   )(1 ss     n t n f t fs  s1 t s2 t0s2 t s1t figure 3. sampling function in the time and the frequency domain.     tg 0 m1 t m1 t 1 mt t f  fw f  tw mt figure 4. window function of the measurement interval.  f  tg  thf   ts  th   tg f  ts    tg f  tw  tg wf ,  fg f  fg wf ,  fg f  fs   fg  fh f  fs  fh  fw     figure 5. procedure of sampling in the time and the frequency domain.  acta imeko | www.imeko.org  november 2016 | volume 5 | number 3| 49  in this interval and where the sampling nyquist condition is not fulfilled ( 2s  mffr ). one possibility to overcome these problems is to add zeroes (known as zero-padding technique [7]) and average values in the aperture times with the duty ratio sap ttd  (figure 8: line c). the aperture time is positioned symmetrically around the known sampling instants. by increasing the number of samples and acquired signal cycles it is possible to detect signals below the nyquist condition (figure 9: the whole acquired signal contains around 50 cycles of the sinusoid 50s  ntfff mmm on 00020n points). an undersampled sine wave still appears as a sampled sine wave but at a lower frequency mfkff  s ( ...,2,1k ) or expressed by the relative frequency mk   s where s is the relative sampling frequency. with padding points we apparently increase the sampling frequency in relation to the signal frequency. to estimate parameters of the time-dependent signals containing any periodicity, it is preferable to use a transformation of the signal in the frequency domain. the discrete fourier transformation (dft) of the windowed signal    kgkw  on n sampled points at the spectral line i is given by:          m m mmm mm iwiwaig 0 j-j ee 2 j   , (1) where mmmm iff   is the frequency component related to the base frequency resolution sm 11 tntf  and consists of an integer part and the non-coherent sampling displacement term 5.05.0  m . a finite measurement time is a source of dynamic errors, which are shown as leakage parts of the measurement window spectrum convolved on the spectrum of the measured-sampled signal (figure 9). the long-range leakage contributions can be reduced in more ways: by increasing the measurement time   meas 1 tf , by using windows with a faster reduction of the side lobes (like the rife-vincent windows class i rv1, etc. [8]), or by using the multi-point interpolated dft algorithms. dedicated algorithms are needed to obtain the correct parameters of the sinusoidal components in the signal. parameters of the measurement component can be estimated by means of interpolation 9. from a comparative study 10 it can be concluded that the key for estimating the three basic parameters is in determining the position of the measurement component mmm i between the dft coefficients )( mig and )1( mig surrounding the component m . in estimations, the well-known expressions for the three-point estimations for frequency (2), amplitude (3), and phase (4) were used. the three-point dft interpolation gives optimal results owing to: symmetry around the local peak amplitude dft coefficient; equal suppression of leakage coming from both sides; equal minimal error curves as with one-, fiveand multi-point interpolations. only the order p of the windows has to be changed using rv1 windows [9].            121 11 13    mmm mm m igigig igig p (2)             121 πsin π !22 2 2 1 1 22 2 3        mmm p l m m m p m igigig l p a     (3) the single phase can be estimated with the arguments of the three largest local dft coefficients   mi igm arg [9], [11]:     2 π 3 2 6 141 11 3     mimiimm a mmm   . (4) 3. evaluation of the estimations  3.1. systematic behaviour  we can estimate three basic parameters of a particular component (true or apparent on figure 9) by the three-point interpolations since we need only the local largest dft coefficients. the estimation errors were compared for the frequency (2), amplitude (3), and phase (4) estimations using the hann window ( 1p ). the absolute errors of the frequency estimation trueest.)(  e and the phase estimation trueest.)(  e , and the absolute relative errors of the amplitude estimation 1)( trueest.  aaae are first checked for one sine component in the signal with a double scan, varying specific sampling parameters and the phase of the signal 0 1 1   maxgtg t a b apt st d c mt st figure 8. signals of sampling by averaging in the aperture time: a – original  signal,  b  –  truncated  signal  in  the  aperture  intervals,  c  –  average  signal values  in  the  aperture  integration  interval,  d  –  average  value representatives of the sampled signal in the aperture integration interval.    g 50 100 150 200 10-6 10-4 10-2 1 s s2 s3 a b c d c d m ms   ms   figure 9. spectra of signals from figure 8: a – original signal, b – truncated  signal  in  the  aperture  intervals,  c  –  signal  with  average  values  in  the  aperture integration interval, d – signal with average values in the aperture integration interval with sinc correction.  acta imeko | www.imeko.org  november 2016 | volume 5 | number 3| 50  at particular relative frequencies because the long-range leakages are frequencyand phase-dependent (figures 10 to 23: 1ma , and 2π2π   , 18π ). first, the sampling ratio mffr s was changed to find intervals where the interpolation algorithm can be used (figures 10 to 19). the absolute maximum error values (from 19 iterations changing phase) at a given sampling ratio are compared using the duty ratio of 4.0sap  ttd . in figure 10, we can see that it is possible to estimate the frequency of one component even better than if we have a complete signal also when the sampling ratio mffr s is in a interval between 1 and 2 (the sampling condition is not fulfilled). the vicinities around the integer values 1 and 2 where we cannot estimate parameters as in the case of the original signal depend on the number of signal cycles in the whole measurement interval mt . the largest value  gives a better frequency resolution and borders come closer to integer values 1 and 2 (figures 10, 11, and 12). the width of the error estimation main-lobe around integer values depends on the position of the investigated component and interspacing between neighbouring components if we have enough sampling points in one period (400 points are used in simulations from analysis in figures 22 and 23). if we have 50 cycles in the measurement interval (figure 10) we get a basic bin resolution 501 and this resolution gives error main-lobe borders 15.185.0  r and 3.27.1  r around integers where the frequency estimation errors increase due to leakage influence on the investigated component m from its replicas mk  s 2,1k (figures 9 and 12). it can be also noticed that there are error peaks which number increases below 32r due to replicas mk  s with higher values of ..,4,3k (figure 12). decreasing the bin resolution to 201 increases unusable intervals to 2.18.0  r and 5.25.1  r (figure 11). going in opposite direction by increasing the bin resolution to 1001 (figure 12) reduces unusable intervals to 1.19.0  r and 15.285.1  r , and the frequency can be accurately estimated also in the interval of sampling ratio 85.11.1 s  mff what is below the sampling condition. in the case of the amplitude estimation (figures 13 and 15) we need to correct the estimated amplitude or the complete amplitude dft spectrum (figure 9, line d) by the well-known sinc correction    mm ttttk apapsinc-corr πsinπ  [3]. we see the same behaviour of the errors in the cases of amplitude and phase estimations as with the frequency estimation (figures 13 to 16). a bin resolution of 501 reduces unusable intervals 15.185.0  r and 2.28.1  r for the phase estimation as for frequency estimation (figure 14) and even better for the amplitude estimation (figure 13: unusable intervals 1.19.0  r and 3.27.1  r ). a resolution of 1001 further reduces unusable intervals to 08.192.0  r and 15.285.1  r (figures 15 and 16). mffs 1 1.5 2 2.5 10-5 10-3 10-1 a b 10 3 1k2k3k...  maxe figure 12. absolute maximum values of errors of the frequency estimation  in relation to the sampling ratio: a – original signal, b – signal with average  values;  k40n ,  hz320hz10s f ,  100m .  mffs  aemax 1 1.5 2 2.5 10-5 10-3 10-1 a b 10 3 figure  13.  absolute  maximum  values  of  relative  errors  of  the  amplitude estimation  in relation to the sampling ratio: a – original signal, b – signal  with average values;  k20n ,  hz160hz5s f ,  50m . mffs  maxe 1 1.5 2 2.5 10-5 10-3 10-1 a b 10 3 figure 10. absolute maximum values of errors of the frequency estimation in relation to the sampling ratio: a – original signal, b – signal with average values;  k20n ,  hz160hz5s f ,  50m .  mffs  maxe 1 1.5 2 2.5 10-5 10-3 10-1 a b 10 3 figure 11. absolute maximum values of errors of the frequency estimation in relation to the sampling ratio: a – original signal, b – signal with average values;   k8n ,  hz64hz2s f ,  20m .  acta imeko | www.imeko.org  november 2016 | volume 5 | number 3| 51  algorithms were analysed also by the multi-component signal. the second harmonic component as the closest and the most disturbing component was added with amplitude 1012 aa  ( 1.0thd ) and phase 02  , other parameters of simulations are the same as for figure 8. we get the most disturbing replicas of the second harmonic at positions 5.1r and 3 due to the leakage effect mfkff 2s  ( 2,1k ) (figures 17 to 19). simulation results show that the proposed estimation procedures give very good results when the sampling condition is fulfilled, but beyond the nyquist frequency the thd has to be low ( 01.0 ). if we change the duty ratio sap ttd  in the sampling interval (changing the aperture time at a fixed sampling frequency) the estimation errors do not change very much in comparison to the estimation of the original signal if we correct the estimation by a sinc correction [12]. in figures 20 and 21 the duty ratio was changed almost in the whole possible interval 998.0001.0 d at 6.1s  mffr with 400p n mffs   degmax e 1 1.5 2 2.5 10-2 1 a b 102 3 figure 14. absolute maximum values of errors of the phase estimation  in relation to the sampling ratio: a –  original signal, b – signal with average values;  k20n ,  hz160hz5s f ,  50m .    mffs  aemax 1 1.5 2 2.5 10-5 10-3 10-1 a b 10 3 figure  15.  absolute  maximum  values  of  relative  errors  of  the  amplitude estimation  in relation to the sampling ratio: a – original signal, b – signal  with average values;  k40n ,  hz320hz10s f ,  100m .    mffs   degmax e 1 1.5 2 2.5 10-2 1 a b 102 3   figure 16. absolute maximum values of errors of the phase estimation  in relation to the sampling ratio: a –  original signal, b – signal with average values;  k40n ,  hz320hz10s f ,  100m . mffs 1 1.5 2 2.5 10-5 10-3 10-1 a b 10 3 mfk ;1 mfk 2;1mfk ;2 mfk 2;2  maxe figure 17. absolute maximum values of errors of the frequency estimation in relation to the sampling ratio: a – original signal, b – signal with average  values;  k20n ,  hz160hz5s f ,  50m .    mffs  aemax 1 1.5 2 2.5 10-5 10-3 10-1 a b 10 3 figure  18.  absolute  maximum  values  of  relative  errors  of  the  amplitude estimation  in relation to the sampling ratio: a – original signal, b – signal  with average values;  k20n ,  hz160hz5s f ,  50m .    mffs   degmax e 1 1.5 2 2.5 10-2 1 a b 102 3 figure 19. absolute maximum values of errors of the phase estimation  in relation to the sampling ratio: a –  original signal, b – signal with average  values;  k20n ,  hz160hz5s f ,  50m .  acta imeko | www.imeko.org  november 2016 | volume 5 | number 3| 52  samples in one period (other parameters were the same as in figure 8). the estimation also much depends on the number of points in the period pn and in the whole measurement interval. padding with more points (average signal values and zero values) will improve estimations since interpolation equations (2), (3), and (4) are derived for large number of points 1n [9]. in figures 22 and 23, the number of points in the period pn was changed from 10p n to more than 512p n (other parameters were the same as in figure 8). we can see that the frequency estimation does not differ significantly if we have reduced the information of the signal (figure 22: curves a and b) and both estimations errors decrease with increasing number of points. the amplitude estimation much more depends on the number of points (figure 23) but after having more than 128p n points per period the estimation errors drop to the level as with the estimation of the original signal without averaging in the aperture time. 3.2. noise propagation  the price for the effective leakage reduction is in the increase of the estimation uncertainties related to the unbiased cramér-rao bounds 13 fixed by the signal-to-noise-ratio for a particular component  22 2 tmm asnr  corrupted by a white noise with standard uncertainty t 14. in figures 24 and 26, there are standard uncertainties of the frequency, amplitude, and phase estimations related to the cr bounds, respectively.    crb, 113  nsnr , (5) aa nsnr crb, 11   , (6)   crb, 11 2  nsnr . (7) moving away from integers of the relative frequency, what is the case when the sampling ratio is below 2, the standard uncertainties increase in relation to the minimal attainable values (figure 24 for the frequency estimation, figure 25 for the amplitude estimation, and figure 26 for the phase estimation) but these changes can be neglected. in the case of amplitude estimation the standard deviations even decrease if the frequency  is estimated first. 1 2 3   crb, 6 1 5432 figure 24. standard uncertainty of the three‐point displacement estimation  (2) related to the cr bound (5).  figure 20. absolute maximum values of errors of the frequency estimation in  relation  to  the  duty  ratio:  a  –  original  signal,  b  –  signal  with  average values in the aperture interval.    sap tt   5max 10ae 0 0.1 0.5 2.2 2.6 3.0 a b 3.4 1 figure  21.  absolute  maximum  values  of  relative  errors  of  the  amplitude estimation in relation to the duty ratio: a – original signal, b – signal with  average values in the aperture integration interval with sinc correction.  figure 22. absolute maximum values of errors of the frequency estimation in  relation  to  the  number  of  sampling  points  in  the  period:  a  –  original  signal, b – signal with average values in the aperture interval.    pn  aemax 100 200 300 400 10-5 10-3 10-1 a b 5000 figure  23.  absolute  maximum  values  of  relative  errors  of  the  amplitude  estimation in relation to the number of sampling points in the period: a – original  signal,  b  –  signal  with  average  values  in  the  aperture  integration  interval with sinc correction.  sap tt   5max 10e 0 0.1 0.5 1 3 5 a b 1 pn  maxe 20 40 60 80 10-4 10-3 a b 100 0 120 acta imeko | www.imeko.org  november 2016 | volume 5 | number 3| 53  4. experimental results  to demonstrate the proposed algorithms in reducing both estimation errors (phase and noise contributions at different frequencies) in a real measurement environment we use a digitizing voltmeter to acquire signal, agilent 3458a [15] and a stable voltage generator, keysight 33500b [16] to generate a nominal sine voltage with amplitude v1n, ua ( %04.0thd ) and changing frequency. in opposite to simulations, the sampling frequency was set at khz10s f μs100s  t and the signal frequency was changed from khz100m f down to khz125.3m f to achieve the same sampling ratio 2.31.0ms  ffr . the fixed aperture time μs40ap t determined the duty cycle of sampling 4.0sap  ttd and with 80s.p. n acquired sampling points the measurement time was ms8ss.p.m  tnt with a frequency resolution of hz1251 m  tf . as in the simulations, 400padd n padding points (for signal and zero padding) was used around the acquired points, and altogether k32padds.p.  nnn was used in the interpolation dft algorithms. each sampling sequence of 80 points was synchronized using the ‘sync out’ terminal of the generator and the external trigger input of the voltmeter. maximal estimation errors are shown in figures 27 to 29, where reference values were those set on the generator ( 1a , khz125.3khz100 f and 2π2π   , 18π ) and with 50 trials at each frequency and phase. the estimation error behaviours are very close to those in the simulations. it can be noticed that systematic error contributions behaves as expected and a very small second harmonic component is presented in the measurement system. the frequency estimation (figure 27) can be compared with the simulation results from figure 10 except the noise floor is higher and at the level of 4max 102)( e even in the interval below the sampling condition 8.115.1 s  mff . the results of the 6.1 4.1 2.1 1 4 51 2 3 6  b a aa crb, figure 25. standard uncertainty of the amplitude three‐point estimation (3) related to the crb (6); a –   is estimated, b –   is known.    0 3 1 4 2   crb,m 53 61 2 4 a b  figure 26. ratios of the uncertainties of the phase three‐point estimation (4) related the crb (7); a –   is estimated, b –   is known.  mffs 10-2  maxe 1 1.5 2 2.5 10-4 10-3 10-1 1 3 figure 27. absolute maximum values of errors of the frequency estimation in  relation  to  the  sampling  ratio;  80s.p. n , k32n ,  khz10s f ,  25800mm  ff .    mffs 10-2  aemax 1 1.5 2 2.5 10-4 10-3 10-1 1 3 figure 28. absolute maximum values of the relative errors of the amplitude estimation  in  relation  to  the  sampling  ratio;  80s.p. n , k32n ,  khz10s f ,  25800mm  ff .    mffs   degmax e 1 1.5 2 2.5 10-1 1 101 3 figure 29. absolute maximum values of errors of the phase estimation  in relation  to  the  sampling  ratio;  80s.p. n ,  k32n ,  khz10s f ,  25800mm  ff .  acta imeko | www.imeko.org  november 2016 | volume 5 | number 3| 54  amplitude (figure 28) and the phase estimations (figure 29) are worse in comparison to the simulation results (figures 13 and 14) due to inaccurate values at the output of the generator and inaccuracy of the sampling voltmeter, but the systematic contributions of errorss confirm the expected behaviour like with the frequency estimation. 5. conclusions  the paper proposes algorithms for the estimation of basic sinusoidal parameters (frequency, amplitude, and phase of the frequency component), when the acquired sampling points do not fulfil the sampling condition and some signal information is lost due to signal averaging in the aperture time. in the proposed procedure, the empty space between successive real sampling points is virtually padded by average values of the real sampling point in the interval of knowing aperture and zeroes in the rest of the sampling interval. this procedure with suitable large acquired cycles in the whole measurement interval improves the frequency resolution, and the interpolated dft estimation algorithms can be adopted for particular frequency components. in many cases the number of sampling points is limited but by performing the algorithms on a computer we can increase padding points and with this nearing the errors to the level as with estimation on the theoretically original signal without averaging in the aperture time and with the number of points equal to all padding points. simulation and experimental results show that the parameters’ estimations are possible also even in the interval below the sampling condition. references  [1] g. d’antona, a. ferrero, digital signal processing for measurement systems. theory and applications, springer science, 2006. [2] d. agrež, "dynamics of frequency estimation in the frequency domain", ieee trans. instr. meas. 56 (2007) pp. 2111-2118. [3] r. l. swerlein, "a 10 ppm accurate digital ac measurement algorithm", proc. ncsl workshop symp., 1991, pp. 17–36. [4] g. a. kyriazis, "extension of swerlein’s algorithm for ac voltage measurement in the frequency domain", ieee trans. instr. meas. 52 (2003) pp. 367-370. [5] h. e. van den brom, e. houtzager, s. verhoeckx, q. e. v. n. martina, g. rietveld, "influence of sampling voltmeter parameters on rms measurements of josephson stepwiseapproximated sine waves", ieee trans. instr. meas. 58 (2009) pp. 3806-3812. [6] w. mcc. siebert, circuits, signals and systems, the mit press, mcgraw-hill, cambridge, new york, 1986, pp. 497-502. [7] g. leus, m. moonen, "semi-blind channel estimation for block transmissions with non-zero padding", records of conf. on signals, systems and comp., 2001, pp. 762-766. [8] f. j. harris: "on the use of windows for harmonic analysis with the discrete fourier transform", proc. ieee 66 (1978) pp. 51-83. [9] j. štremfelj, d. agrež, "nonparametric estimation of power quantities in the frequency domain using rife-vincent windows", ieee trans. instr. meas. 62 (2013) pp. 2171-2184. [10] j. schoukens, r. pintelon, h. van hamme, "the interpolated fast fourier transform: a comparative study", ieee trans. instr. meas. 41 (1992) pp. 226-232. [11] t. j. stewart, "spectral leakage errors when using an agilent 3458a to measure phase at mains power frequencies", proc. ncsl workshop symp. 1991, pp. 17–36. [12] d. agrež, d. ilić, j. drnovšek, "estimation of periodic signal parameters by signal and zero padding", in proc. xxi imeko world congress/2015, prague, czech republic, 2015, tc4: 474479. [13] a. moschitta, p. carbone, "cramer-rao lower bound for parametric estimation of quantized sinewaves", proc. ieee imtc/2004, como, italy, 2004, pp. 1724–1729. [14] widrow b, kollar i, quantization noise, cambridge univ. press, 2008. [15] 3458a multimeter user's guide, ed. 5, 1988-2012, 2011, agilent technologies. [16] 33500b series waveform generators – data sheet, 59910692en, 2015, keysight technologies. microsoft word 310-2521-1-le-rev acta imeko  issn: 2221‐870x  november 2016, volume 5, number 3, 64‐69    acta imeko | www.imeko.org  november 2016 | volume 5 | number 3 | 64  multi‐capacity load cell prototype  seif m. osman 1 , rolf kumme 2 , hany m. el‐hakeem 1 , frank löffler 2 , ebtisam h. hasan 1 , ragaie m.  rashad 3 , fawzaia kouta 3   1 national institute for standards (nis), giza, egypt  2 physikalisch‐technische bundesanstalt, bundesallee 100, 38116 braunschweig, germany  3 faculty of engineering‐cairo university, giza, egypt       section: research paper   keywords:  force measurements; transducer; conceptual design; proposed design  citation: seif. m. osman, rolf.kumme, hany. m. el‐hakeem, frank. löffler, ebtisam h. hasan, ragaie.m.rashad, fawzaia kouta, multi‐capacity load cell  prototype, acta imeko, vol. 5, no. 3, article 10, november 2016, identifier: imeko‐acta‐05 (2016)‐03‐10  section editor: paul regtien, the netherlands  received january 7, 2016; in final form august 12, 2016; published november 2016  copyright: © 2016 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  corresponding author: seifelnasr m. osman, e‐mail: seifelnasr_nis@yahoo.com    1. introduction  a force measurement system is made up of a transducer and associated instruments. the most common commercial force transducer is based on an electrical principle (the load cell). different force transducers are usually used by national and accredited laboratories to calibrate force generated systems. the main part in the load cell is the elastic element on which a wheatstone bridge circuit is formed [1]. the stiffness of the elastic element is a governing factor in determining the load cell capacity. the concept of a force transducer with different capacities (multi-capacities, changeable-capacities) was recently introduced by nis and ptb to overcome the additional costs of requiring several force transducers. 2. conceptual design  building a multi-capacity load cell requires different values of stiffness. the multi-capacity load cell was introduced based on increasing the stiffness (k) for each range [2]. proposing a multi-capacity load cell with three different capacities requires a concept to offer three values of stiffness, one for each capacity [3]. the three different values could be offered through using three different elastic elements, one for each range, or combining elastic elements together to form three different stiffness values. the combined stiffness resulting from adding elastic elements to each other in parallel is the sum of the individual stiffnesses. equation (1) shows the combined stiffness (kp) resulting from coupling three elastic elements in parallel [4]. 321 kkkk p  (1) where kp is the resulted stiffness from coupling elastic elements in parallel, k1, k2, k3 are the stiffness values of three elastic elements combined together in parallel. in the current research work, for the first capacity an elastic element nominated for the first working range is used (see figure 1). for the second capacity a new element is introduced instantaneously with the first element to withstand the applied load together (see figure 1), and for the third capacity another new element is used instantaneously with the first and the second elements to withstand the load together (see figure 1). abstract  this  article  illustrates  an  advanced  approach  in  force  measurement  standards.  it  gives  a  spot  on  the  design,  manufacturing  and  evaluation for a prototype force transducer with multi‐capacity. this prototype has three adjustable capacities (5 kn, 10 kn and 15 kn)  and  works  in  compression  mode.  the  introduced  design  offers  a  comparative  load  cell  looking  forward  to  replaceing  three  force  transducers  with  the  same  capacities  (5  kn,  10  kn  and  15  kn)  which  are  commercially  available.  experimental  results  reveal  satisfactory agreements with that calculated with an analytical method and simulation results using finite element techniques. the  detailed metrological characteristics of this multi‐capacity load cell will be published later.  acta imeko | www.imeko.org  november 2016 | volume 5 | number 3 | 65  3. proposed design  the concept of the proposed design is adding elastic elements in parallel before applying the loads. figure 2 shows the parts forming the proposed design. mainly, the proposed design is based on using a base which has a protruded cylinder, a main rod, protruded cylinder, rotating cap and a set of strain gauges. the set of strain gauges are bound on the main element forming a wheatstone bridge circuit. the main element and the concentric cylinder are assembled on the base which has a protruded built-in cylinder. the cylinders have protrusions (see figure 2). the load is applied using a universal rotating cap enclosed to the load cell which is designed to rotate relatively to the load cell body. the universal cap has three pre-determined marks (see figure 3). each mark is nominated for a range. there are also three other marks on the load cell top cover. each is nominated also for a specific range. these marks indicate three positions: position 1: for applying the load on the main element; position 2: for applying the load on the main element and the cylinder; position 3: for applying the load on the main element, cylinder and base with the protruded cylinder. the load cell capacity is determined by rotating the cap until the required capacity mark is in line with the counterpart mark which is on the load cell top cover, which means that the right protrusions of both the cylinders and the cap face each other. 4. prototypes  different prototypes were manufactured and evaluated at nis and ptb. the preliminary checks carried out on the manufactured prototypes show that the conceptual design was applicable, the range selection mechanism was satisfactory and works successfully but the results reveal some criticism which was taken into consideration during developing the design of this prototype at ptb in order to manufacture an accurate and precise multi-capacity load cell. 5. final prototype  the final prototype was proposed after developing various tests. it is characterized by some new features related to the design and reflected on the load cell dimensions and weight to manufacture a comparative load cell (see figure 4). based on ptb past experience [5], the four main parts (main element, figure  1.  schematic  of  the  concept  of  adding  elastic  elements  to  form  a three‐range multi‐capacity load cell.   figure 2. proposed prototype.  figure 3.  universal rotating cap lower side.  figure 4. load cell overall dimensions.   dimensions in mm protrusions base with protruded cylinder first cylinder main element loading plate side cover rotating cap top cover loading ball second capacity + third capacity + + first capacity acta imeko | www.imeko.org  november 2016 | volume 5 | number 3 | 66  cylinder, the base with the protruded cylinder and the rotating cap) were manufactured from din 1.6580 (30crnimo8, бy≈1000 mpa). din 1.4301 (x5crni18-10, бy≈190 mpa) was used to manufacture the rest of the load cell as it has good corrosion resistance. during the design phase; a comprehensive stress analysis using a finite element analysis program (abaqus fe program version 6.5-1) was carried out to develop, optimize and check the efficiency of adding elements. a compressive test load of 15 kn was used to verify the effect of adding elements in parallel. three models were evaluated and each model simulates one capacity of the three capacities. first model: composed of the main element only; second model: composed of the main element and the cylinder with protrusions; third model: composed of the main element, the cylinder with protrusions and the base with the protruded cylinder. the 15 kn compressive load was applied on the three models. results show that the stress on the main element (first capacity) decreases by adding the new element (first cylindersecond capacity) from the hypothetical value 164 mpa to 115.5 mpa (i.e. 30 % decrease) and after adding the last element (the base with the protruded cylinder-third capacity) decreases more to be 100 mpa (i.e. 40 % decrease) with a difference equal to 64 mpa from the first range. figures 5 to 7 show the results of the finite element analysis. 6. manufacturing  the machining process was carried out in ptb`s scientific instrumentation department which is equipped with high accurate machines. after manufacturing the parts, the main element, the cylinder and the base with protruded cylinder were assembled together in what is known as main parts assembly (see figure 8). low tolerances (approximately 10 µm flatness) were required at the top of the main parts assembly (see figure 8). this is to ensure that the main element and the protrusions are on the same plan. this machining tolerance was achieved by a grinding process. grinding is the final machining process that was applied to the top surface of the main parts assembly after the strain gauges were adhered to the main element and protected. two bi-axial strain gauges 1-xg11-3/350 manufactured by hbm with a 3 mm gauge length and a 350 ohm gauge resistance were adhered on the main element (one on each side) to form the wheatstone circuit [6]. 7. measurements  two series of measurements were carried out to evaluate the manufactured prototype using the ptb 20 kn deadweight machine. first, test measurements were carried out on the manufactured load cell to practically study the effect of adding elements. two loads (3 kn and 5 kn) were applied on the three ranges. table 1 shows the response of the manufactured figure 7. results of fe stress due to 15 kn load on 3 rd  range.   figure 5.  results of fe stress due to 15 kn load on 1st range.    figure 6. results of fe stress due to 15 kn load on 2 nd  range.  figure 8.  main parts assembly.  main element first cylinder second cylinder third capacity 100 mpa second capacity 115.5 mpa first capacity 164 mpa acta imeko | www.imeko.org  november 2016 | volume 5 | number 3 | 67  transducer under the loads. results prove the efficiency of adding elements to increase the stiffness. in the next step the outputs of the three capacities were measured under loads up to the maximum capacity of each range to evaluate the efficiency of adding elements. these measurements were repeated during three different days. eeach day the load cell was removed from the machine and placed again with different orientation with respect to the loading axis in order to randomize the measuring conditions. table 2 and figure 9 represent the average response of the three ranges for the first prototype under loads up to the maximum capacity for each range. it was not possible to apply 15 kn load on the third capacity due to the loading schemes of the ptb 20 kn deadweight machine which increases loads by steps of 2 kn beginning from a 10 kn load. figure 9 shows that the values of the response decrease by adding a new elastic element which indicates that the range selection mechanism is satisfactory and works. 8. comparison between f.e.a. and s.g. results  a comprehensive stress analysis using a finite element analysis program (abaqus fe program version 6.5-1) was carried out during the design stage. table 3 compares values of calculated stresses and deflections at maximum capacities (5, 10 and 15 kn) based on actual measured responses (table 2) and the deduced responses using finite element analysis (section 5). the first and the second rows of table 3 show the stress and the deflection of the main sensing element, concluded from the finite element analysis. the indicated values are for the maximum capacities. the third row of the table shows the actual response of the main sensing element presented in mv/v. the fourth and the fifth rows of the table show the calculated deflection and stress based on the actual response (third row). they were calculated by applying equations (2), (3) and (4) [7] which relate the wheatstone bridge output to the induced deflections taking into consideration that in the manufactured prototype a full wheatstone bridge circuit (four strain gauges) was used. assuming an ideal case for strain distribution (ε1= ε3, ε2= ε4 and ε1= 0.3 ε2) and taking into consideration that the gauge factor (k) equals 2 and the gauge length (l) equals 30 mm.  4321 4   k v v e a (2) 165.0 k v v e a  (3) 1ll  (4) where va is the wheatstone bridge output voltage, ve wheatstone bridge input voltage, ε1, ε2, ε3, ε4, the strain induced in the strain gauges under load, k is the gauge factor of the strain gauge and δl is the deflection. figures (10) and (11) illustrate the calculated values and the deduced ones for deflections and stresses, respectively. the difference between the stress and calculated deflections based on the measured response and that concluded from finite element analysis stated in table 3. it is worth mentioning that, in the ideal case, the stresses, strains and deflections of the main sensing element remain the same for each capacity and their values are equal to that of the first capacity. 0 0,1 0,2 0,3 0,4 0,5 0,6 0 5 10 15 load (kn) a v e ra g e  r e sp o n se  ( m v / v ) 5kn 10kn 15kn figure  9.    representative  graph  for  the  average  response  for  the  three  capacities at maximum load. table 1. response under loads.  load  response (mv/v)  kn  first range  second range  third range 3  0.221479  0.135401  0.103403 5  0.363655  0.232232  0.185130 table 2. response of the three capacities up to maximum capacity.  load  response(mv/v)  kn  first range  second range  third range 0.5  0.037586  ‐‐‐‐  ‐‐‐‐ 1  0.074674  0.039140  ‐‐‐‐ 1.5  0.111562  ‐‐‐‐  0.043274 2  0.148373  0.087196  ‐‐‐‐ 2.5  0.185084  ‐‐‐‐  ‐‐‐‐ 3  0.221479  0.135401  0.103403 3.5  0.257326  ‐‐‐‐  ‐‐‐‐ 4  0.292858  0.183655  ‐‐‐‐ 4.5  0.328269  ‐‐‐‐  ‐‐‐‐ 5  0.363655  0.232232  0.185130 6  ‐‐‐‐  0.281025  0.226545 7  ‐‐‐‐  0.330136  0.268401 8  ‐‐‐‐  0.379518  0.310590 9  ‐‐‐‐  0.429235  0.353143 10  ‐‐‐‐  0.478793  0.395926 12  ‐‐‐‐  ‐‐‐‐  0.482352 14  ‐‐‐‐  ‐‐‐‐  0.569091 table 3. strain, stress and deflection deduced from f.e.a. and s.g. results.       first range ( 5 kn) second range (10 kn)  third range (15 kn) 1  deduced  using  f.e.a  deflection (mm)  0.0074 0.0098 0.0112 2  stress (mpa)  54.98 77.23 100.42 3  measured  response (mv/v)  0.363655 0.478793 0.60974 4  calculated  deflection (mm)  0.0083 0.011 0.014 5  stress (mpa)  58.56 77.73 98.93 acta imeko | www.imeko.org  november 2016 | volume 5 | number 3 | 68  9. opinions and interpretations  9.1. wheatstone bridge circuit  the used wheatstone bridge circuit was composed of four active strain gauges. gauging was carried out through this work by the simplest method while in expert companies strain gauges bonding (curing, adhesive, uniform adhesive layer, gauging presser, etc) are carried out in more professional methods. in addition, more resistors and strain gauges could be introduced to the simple wheatstone bridge circuit resulting in an improved, complicated and more reliable wheatstone bridge circuit. the complementary strain gauges will work on adjusting the zero signals, compensate temperature effect and improve the load cell linearity. 9.2. elastic element  the manufactured prototype was designed based on a simple column loading principle. the main reason to design and manufacture the main element as a column with rectangular cross section was to facilitate the machining process. a rectangular cross section was used as this offers a larger surface area on which the strain gauges could be easily fixed, but it makes the elastic element not symmetric and this may cause high effect induced by the rotation with respect to the loading axis. 9.3. load distribution  in the manufactured prototype and according to the conceptual design, the load is distributed through the rotating cap to the required elastic elements. force interaction between contacts of the rotating cap and those of the main element, the first cylinder and the second cylinder has a big influence on the multi-capacity load cell. more investigations are required for better contact profile in order to improve the efficiency of the load transmission which will be reflected on the metrological characteristics. 9.4. uncertainty  a new source of uncertainty resulting from the effect of cap rotation will be introduced. it needs more research to be defined and estimated correctly as it may be affected by the manufacturer (depending on precise machining), by the user (depending on the user experience) or a combination of both. 10. conclusion  in this article a prototype of multi-capacity load cell was designed, manufactured and evaluated as a facility in the force measurement field. the manufactured prototype works in compression mode with three-changeable capacities 5 kn, 10 kn and 15 kn (see figures 12 and 13). it can replace three ordinary one capacity load cells which are commercially available. the pillar of the design concept increases the stiffness of the sensing element as the capacity is being changed. the required capacity can be chosen by rotating a special designed rotating cap. this cap allows loads to be distributed among several elastic elements; one of these elastic elements is the main sensing element on which the strain gauges are bonded to form a wheatstone bridge circuit. performance evaluation results showed that – in case of accurate machining and finishing  the design concept is effective. figure 12 . manufactured multi‐capacity load cell with loading plate.  0.004 0.006 0.008 0.01 0.012 0.014 0.016 1st range 2nd range 3rd range d e fl e c ti o n ( m m ) ranges f.e.a. calculated figure 10.  illustration for calculated and f.e.a. deflections.  figure 13. manufactured multi‐capacity load cell without loading plate.  40 50 60 70 80 90 100 110 1st range 2nd range 3rd range s tr e s s ( m p a ) ranges f.e.a. calculated figure 11. illustration for calculated and f.e.a. stress.  acta imeko | www.imeko.org  november 2016 | volume 5 | number 3 | 69  11. future work  the manufactured multi-capacity load cell will be calibrated according to the international standard iso 376-11 to investigate its metrological characteristics [8]. the main idea of the multi-capacity approach by incorporating elastic elements with only one sensing element could be generalized and investigated in other areas in the field of force measurements. also it would be more valuable to apply this concept in manufacturing a multi-capacity load cell with a range difference increase by a power of ten, for instance 5 kn, 50 kn and 500 kn. references  [1] dan mihai stefanescu, alexandru stefanescu.”criteria for choosing the elastic elements of force transducers” proceedings of the 17th international conference imeko, pp. 134-140, istanbul, turkey, sept.2006. [2] seif. m. osman, ebtisam h. hasan, h. m. el-hakeem, r. m. rashad, f. kouta “conceptual design of multi-capacity load cell” proceedings of 16th international congress of metrology 7-10 october 2013 paris, france. http://dx.doi.org/10.1051/metrology/201303002 [3] seif. m. osman, ebtisam h. hasan, h. m. el-hakeem, r. m. rashad, f. kouta “multi-capacity load cell concept” sensors & transducers journal, vol.178-179, issue 9, pp.229-233, september 2014. [4] shigley’s mechanical engineering design,” mechanical engineering”, mcgraw-hill, eights edition 2006, isbn:0-39076487-6. [5] f.tegtmeier, m. peters “multicomponent sensor for stress analysis in buildings” proceedings of the joint international conference imeko tc3/tc5/tc20 on force, mass, torque, hardness and civil engineering metrology in the age of globalization, vdi-berichte: 1685, celle, germany, 2002. [6] hbm catalogue “strain gauges and accessories” www.hbm.com. [7] karl hoffmann, 2011 “applying the wheastone bridge circuit”, www.hbm.com [8] international standard iso376:2011 “metallic materials – calibration of force-proving instruments used for the verification of uniaxial testing machines” fourth edition, june 2011. random noise test of analog-to-digital converters acta imeko issn: 2221-870x june 2023, volume 12, number 2, 1 8 acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 1 random noise test of analog-to-digital converters francisco alegria1 1 instituto de telecomunicações and instituto superior técnico/universidade de lisboa, av. rovisco pais 1, 1049-001 lisbon,portugal section: research paper keywords: analog to digital converter; testing; noise; estimation; error analysis; random; applied mathematics citation: francisco alegria, random noise test of analog-to-digital converters, acta imeko, vol. 12, no. 2, article 27, june 2023, identifier: imeko-acta12 (2023)-02-27 section editor: laura fabbiano, politecnico di bari, italy received march 11, 2023; in final form may 7, 2023; published june 2023 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: francisco alegria, e-mail: francisco.alegria@tecnico.ulisboa.pt 1. introduction the standard deviation of the random noise present in an analog to digital converter (adc) is an important parameter used to describe the performance of adcs and to choose the proper adc to use in a given application. the knowledge of the noise standard deviation in a test setup is also needed when performing other adc tests, namely the standard static test [1], [2] and the standard histogram test [3]-[11]; the small waves histogram test [12]-[16]; the sinefitting test [17]-[23] and the jitter test; [24]-[26] for the determination of the error; and precision of the adc parameters estimated with them. in some cases, random noise is used, with great advantage, as a stimulus signal [27]. the issue of quantifying the uncertainty of the estimates of adc characteristics does not concern only the tests recommended in the ieee 1057 standard but are common to all adc tests, like the ones in the iec 62008 standard [28]-[29], and the ieee 1241_2000 standard [30], for example. in [31] an analysis off the precision of the estimates obtained with this test has been carried out. there, an expression for the minimum number of samples required to guarantee a certain bound on the uncertainty of the results, was presented. this is important in order to minimize the duration of the test because the exact number of samples required can be calculated. the presence of phase noise in the stimulus signal generator as well as jitter in the sampling instants also affect electronic systems in general and are important to quantify [1], [31] [34]. the test, as described in [1], section 8.6.2, consists in synchronously acquiring two sets of a certain number of samples (𝑀). the noise standard deviation (𝜎) is then estimated from the root mean square of the difference between the output codes of those two sets (mse – mean square error). if the noise standard deviation is high enough, a null input voltage is sufficient to perform the test, if not, a triangular stimulus signal should be used. in section 3, the error in the estimation of the random noise standard deviation due to the use of the heuristically derived estimator recommended in the ieee standard is analyzed. in section 4 we show how to use a sinusoidal stimulus signal instead of a triangular shaped one. a study about the influence of the stimulus signal amplitude and offset on the estimation of the random noise standard deviation is then presented. this will allow the derivation of an expression for the minimum value of stimulus signal amplitude that guarantees an upper bound on the random noise estimator bias. an expression that allows the exact computation of the random noise standard deviation when necessary is going to be presented. this approach is computationally intensive and not appropriate for hand computation but can be easily implemented in a computer if necessary. 2. dc stimulus signal random additive noise in adcs, as described in [1], is a nondeterministic fluctuation of the adc output and is described by abstract the random noise test of analog to digital converters recommended by the ieee 1057 standard for digitizing waveform recorders is studied. the heuristically derived expression presented for the estimation of the random noise standard deviation is experimentally validated. the standard suggests a triangular stimulus signal. here we will show how to use a sinusoidal stimulus signal to carry out the test. the influence of stimulus signal offset and amplitude on the estimation error is also analyzed and an expression is presented to compute the minimum amplitude value that guarantees an upper bound on the estimator bias. mailto:francisco.alegria@tecnico.ulisboa.pt acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 2 its frequency spectrum and statistical properties. it is usually considered that the noise present is white (flat frequency spectrum), with a stationary probability density function. also, it considered that the noise is additive and independent of the stimulus signal. due to the presence of random noise at the adc input, the output code (𝑘) can be considered a discrete random variable which can assume any value between 0 and 2𝑛𝑏 − 1 for a 𝑛𝑏-bit adc. when the additive noise standard deviation is higher than the adc ideal code bin width (𝑄) the suggested method in [1] is to short circuit the adc input and acquire two sets of samples (𝑘𝑎𝑗 and 𝑘𝑏𝑗) and subtract the codes obtained. this eliminates fixed errors of the adc but preserves the random nature of the output codes. considering that the additive noise present has a null mean (a non-null value would be accounted as an offset error), the average of the output codes will also be null. therefore, the mean square error (𝑚𝑠𝑒) is just: 𝑚𝑠𝑒 = 1 𝑀 ∑(𝑘𝑎𝑗 − 𝑘𝑏𝑗) 2 𝑀−1 𝑗=0 . (1) the mean square error obtained is twice the variance of each set since they are independent of each other. according to [20] the expected value of the mean square error determined by (1) is twice the variance of the output codes: 𝐸{𝑚𝑠𝑒} = 𝜎𝑘𝑎 2 + 𝜎𝑘𝑏 2 = 2𝜎𝑘 2 . (2) taken this into account, and considering that the variance of the output codes is equal to that of the additive noise, the estimated variance of the latter is just 𝜎𝑟 2̂ = 𝑚𝑠𝑒 2 , (3) where 𝜎𝑟 is the normalized noise standard deviation 𝜎𝑟 = 𝜎/𝑄. however, this only works when the additive noise standard deviation is higher than the adc ideal code bin width as we will show next. it is chosen here to use normalized voltages, expressed in lsb (least significant bits) units, by dividing the voltages with the ideal adc code bin width 𝑄. the normalized stimulus signal voltage is represented by 𝑦 and the normalized random noise voltage is represented by 𝑟. the normalized sampled voltage at instant 𝑡𝑗 is thus given by 𝑢(𝑡𝑗) = 𝑦(𝑡𝑗) + 𝑟(𝑡𝑗) . (4) considering that the normalized additive noise has a null mean and a standard deviation represented by 𝜎𝑟, we have for the sampled voltage, which is also a random variable: 𝜇𝑢 = 𝑦, 𝜎𝑢 = 𝜎𝑟 . (5) being the additive noise normally distributed, the sampled voltage probability density function is 𝑓𝑢(𝑢|𝑦) = 1 √2 π 𝜎𝑟 ⋅ 𝑒 − (𝑢−𝑦)2 2 𝜎𝑟 2 (6) and its distribution function is 𝐹𝑢(𝑈|𝑦) = ∫ 𝑓𝑢(𝑢) ⋅ d𝑢 𝑈 −∞ = 1 √2 π 𝜎𝑟 ⋅ ∫ 𝑒 − (𝑢−𝑦)2 2⋅𝜎𝑟 2 ⋅ d𝑢 𝑈 −∞ = 1 2 + 1 2 ⋅ erf( 𝑈 − 𝑦 √2 ⋅ 𝜎𝑟 ) . (7) due to the subtraction of the codes obtained with the two sample sets in (1), we can consider the adc as having an ideal behavior since any fixed errors were eliminated by the subtraction and random errors can be considered as being present in the stimulus signal input. the probability 𝑝𝑘 of a sample having output code 𝑘 is equal to the probability of the sampled voltage being equal to or lower than the transition voltage 𝑇[𝑘 + 1] and equal to or greater than transition voltage 𝑇[𝑘] (for the middle codes): 𝑝𝑘 = 𝑃{𝑈[𝑘] ≤ 𝑢 ≤ 𝑈[𝑘 + 1]} , 𝑘 = 1, . . . ,2 𝑛𝑏 − 2 (8) where we used the normalized transition voltage 𝑝0 = 𝑈[𝑘] = 𝑇[𝑘]/𝑄. additionally, for the first code we have 𝑝0 = 𝑃{𝑢 ≤ 𝑈[1]} (9) and for the last code we have 𝑝2𝑛𝑏−1 = 𝑃{𝑈[2 𝑛𝑏 − 1] ≤ 𝑢} . (10) the probability 𝑝𝑘 can thus be expressed with the help of the sampled voltage distribution function: 𝑝𝑘(𝑦) = { 𝐹𝑢(𝑈[1]|𝑦) , 𝑘 = 0 𝐹𝑢(𝑈[𝑘 + 1]|𝑦) − 𝐹𝑢(𝑈[𝑘]|𝑦), 𝑘 = 1,2, . . . ,2 𝑛𝑏 − 2 1 − 𝐹𝑢(𝑈[2 𝑛𝑏 − 1]|𝑦) , 𝑘 = 2𝑛𝑏 − 1 . (11) in figure 1, the noise probability function and the probability of occurring code 𝑘 (shaded area) is represented. the mean, second moment and variance of the output codes are, by definition [20]: 𝜇𝑘|𝑦 = ∑ 𝑘 ⋅ 𝑝𝑘(𝑦) 2𝑛𝑏−1 𝑘=0 , 𝑚2𝑘|𝑦 = ∑ 𝑘 2 ⋅ 𝑝𝑘(𝑦) 2𝑛𝑏−1 𝑘=0 𝜎𝑘|𝑦 2 = 𝑚2𝑘|𝑦 − 𝜇𝑘|𝑦 2 . (12) in figure 2 the standard deviation of 𝑘 as a function of the noise standard deviation for two different values of stimulus signal is represented. it was calculated using (7), (11) and (12). for values of noise standard deviation much greater than the ideal code bin width (𝜎𝑟 ≫ 1), the standard deviation of the u[k+1]u[k] u f u 0 figure 1. representation of the noise probability function and the probability of occurring code k (shaded area). acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 3 output codes is the same as the standard deviation of noise (not clearly seen in figure 2 because the horizontal axis only goes to 1 lsb). this corresponds to the case 𝑄 → 0. in these circumstances the discrete variable 𝑘 becomes a continuous variable whose probability density function tends to the probability density function of noise. in this case the test can be used to estimate the noise standard deviation: 𝜎𝑟 → ∞ ⇒ 𝜎𝑘|𝑦 → 𝜎𝑟 . (13) in the opposite situation, when the noise standard deviation is lower than 1 lsb (𝜎𝑟 < 1), it can be observed in figure 2 that 𝜎𝑘 is not equal to 𝜎𝑟 anymore, becoming even independent of 𝜎𝑟 when it goes to 0 (the curve becomes horizontal). this makes it impossible to estimate the noise standard deviation from the output code’s standard deviation. with the goal of justifying the analytical expression presented in [1] for the relation between 𝜎𝑟 and 𝜎𝑘 we are going to analyze the case 𝜎𝑟 ≪ 1. in figure 3 it can be seen the case where only two output codes, 𝑘 − 1 and 𝑘, have a significant probability of occurring for any value 𝑦 of stimulus signal. the index of the transition voltage closest to 𝑦 determines the value of 𝑘 and consequently which two codes occur for that particular value of 𝑦. since the normalized transition voltages are integer values (considering an adc transfer function with no true zero), the difference 𝑈[𝑘] − 𝑦 will always be between -0.5 and 0.5: 𝑈[𝑘] − 𝑦 = 𝑦 − ⌊𝑦 + 0.5⌋ , (14) where ⌊𝑥⌋ represents the highest integer lower than 𝑥. the probability that the output code of a sample is 𝑘 − 1 is 𝑝𝑘−1(𝑦) = ∫ 𝑓𝑢(𝑢|𝑦) ⋅ d𝑢 𝑈[𝑘] 𝑈[𝑘−1] . (15) because we are considering the case where the probability density function of the sampled voltage is negligible for voltages lower than 𝑈[𝑘 − 1], we can write 𝑝𝑘−1(𝑦) = ∫ 𝑓𝑢(𝑢|𝑦) ⋅ 𝑑𝑢 𝑈[𝑘] −∞ = 𝐹𝑢(𝑈[𝑘]|𝑦) . (16) the probability of occurring code 𝑘 is obviously 𝑝𝑘(𝑦) = 1 − 𝑝𝑘−1(𝑦) = 1 − 𝐹𝑢(𝑈[𝑘]|𝑦) (17) since we considered that only codes 𝑘 and 𝑘 − 1 could occur. the mean of the output codes is thus 𝜇𝑘|𝑦 = (𝑘 − 1) ⋅ 𝑝𝑘−1(𝑦) + 𝑘 ⋅ 𝑝𝑘(𝑦) (18) and the variance 𝜎𝑘|𝑦 2 = (𝑘 − 1)2 ⋅ 𝑝𝑘−1(𝑦) + 𝑘 2 ⋅ 𝑝𝑘(𝑦) − 𝜇𝑘|𝑦 2 = 𝑝𝑘−1(𝑦) − 𝑝𝑘−1 2 (𝑦) . (19) from (7) and (16) this variance can be written as 𝜎𝑘|𝑦 2 = [ 1 2 + 1 2 ⋅ erf( 𝑈[𝑘] − 𝑦 √2 ⋅ 𝜎𝑟 )] ⋅ [ 1 2 − 1 2 ⋅ erf( 𝑈[𝑘] − 𝑦 √2 ⋅ 𝜎𝑟 )], 𝜎𝑟 ≪ 1 , (20) which can be rewritten as 𝜎𝑘|𝑦 2 = 1 4 − 1 4 ⋅ erf2 ( 𝑈[𝑘] − 𝑦 √2 ⋅ 𝜎𝑟 ) , 𝜎𝑟 ≪ 1 . (21) this conditional variance is represented in figure 4 as a function of the dc input voltage. note that this function is periodic because the transition voltage 𝑈[𝑘] in [21] is the closest to 𝑦, and has a period of 1 because the voltage 𝑦 is normalized to the ideal code bin width (𝑄). to justify the values of code standard deviation in the absence of noise (𝜎𝑟 → 0) consider the following two situations. first the dc input voltage is equal to one of the transition voltages (𝑈[𝑘]), as represented in figure 5a. half of the represented curve is to the left of the transition voltage and the other to the right. the probability of having code 𝑘 − 1 is equal to the probability of having code 𝑘 (50%) so the variance is 1/4 and the standard deviation is 1/2 as seen by the dashed line in figure 2 for 𝜎𝑟 ≪ 1. in figure 5b, the case where figure 2. representation of the code variance as a function of the additive noise standard deviation. the dashed line represents the situation where the input dc voltage is exactly equal to one of the adc transition voltages and the solid line represents the case where the dc input voltage is exactly between two consecutive transition voltages. u[k+1]u[k] u f u u[k−1] p k-1 y p k figure 3. representation of the probability density function of noise. figure 4. representation of the conditional variance of the output codes as a function of the dc input voltage. acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 4 the dc input voltage is exactly between two consecutive transition voltages is represented. when the noise standard deviation goes to 0, the probability of having code 𝑘 is 1, so the standard deviation is 0 (solid line in figure 2). for intermediate cases the curve is between the two lines in figure 2. in conclusion, the use of a dc input signal is not appropriate when estimating small values of additive noise because the output codes of the acquired samples do not vary enough to make their standard deviation directly related to the additive noise standard deviation, as stated in [1]. 3. triangular stimulus signal to overcome the problem referred in the previous section, the ieee standard [1] suggests the use of a triangular stimulus signal that spans several adc codes (about 10). due to the subtraction of the codes obtained from the two sets of samples, the mean square error is still twice that of the additive noise if the sets are acquired synchronously in order for the samples of both sets to have the same phase. the variance of the output codes can be calculated from the amplitude distribution, 𝑓𝑦, of the stimulus signal [20]. 𝜎𝑘 2 = ∫ 𝜎𝑘|𝑦 2 (𝑦) ⋅ 𝑓𝑦(𝑦) d𝑦 ∞ −∞ . (22) for a triangular stimulus signal, with an amplitude 𝐴 and an offset 𝐶, normalized by the ideal code bin width (𝐴𝑄 = 𝐴/𝑄 and 𝐶𝑄 = 𝐶/𝑄) the amplitude distribution is 𝑓𝑦(𝑦) = { 1 2 𝐴𝑄 , |𝑦 − 𝐶𝑄| < 𝐴𝑄 0 , otherwise . (23) the output codes variance is thus 𝜎𝑘 2 = 1 2 𝐴𝑄 ∫ 𝜎𝑘|𝑦 2 (𝑦) d𝑦 𝐶𝑄+𝐴𝑄 𝐶𝑄−𝐴𝑄 . (24) inserting (21) leads to 𝜎𝑘 2 = 1 2 𝐴𝑄 ∫ [ 1 4 − 1 4 ⋅ erf2 ( 𝑈[𝑘] − 𝑦 √2 ⋅ 𝜎𝑟 )]d𝑦 𝐶𝑄+𝐴𝑄 𝐶𝑄−𝐴𝑄 , 𝜎𝑟 ≪ 1 . (25) the corresponding standard deviation is represented in figure 6 by a solid line. equation (24) states that the variance 𝜎𝑘 2 is the average of the curve in figure 4, which is the representation of (21), in an interval with length 2𝐴𝑄 centered at𝐶𝑄. 3.1. low noise approximation the integral in (24) does not have a closed form. we can, however, obtain an approximated expression. to achieve that we look into the case where we have a small value of noise standard deviation. in this situation the individual bell-shaped curves in figure 4 do not overlap each other. furthermore, when 𝐴𝑄 goes to  the result of (24) is the same as integrating the curve in figure 4 from −1 to 1. in this case the result in the value of (24) is independent on the triangular wave offset 𝐶. mathematically this statement can be expressed as 𝜎𝑘 2 = 1 2𝐴𝑄 ∫ 𝜎𝑘|𝑦 2 (𝑦) d𝑦 𝐶𝑄+𝐴𝑄 𝐶𝑄−𝐴𝑄 | 𝐴𝑄→∞ = 1 2 ∫𝜎𝑘|𝑦 2 (𝑦) d𝑦 1 −1 . (26) using (21) we can write lim 𝐴𝑄→∞ 𝜎𝑘 2 = √2 ⋅ 𝜎𝑟 4 ∫[1 − erf2(𝑥)]𝑑𝑥 ∞ −∞ , 𝜎𝑟 ≪ 1 . (27) this integral can be numerically calculated leading to a value of √8 π⁄ . the variance is thus lim 𝜎𝑟→0 𝐴𝑄→∞ 𝜎𝑘 2 = 1 √π 𝜎𝑟 . (28) this approximation is represented in figure 6 by the dasheddot line. it can be seen that, in fact, it approximates the solid thin line for small values of 𝜎𝑟. 3.2. high noise approximation we will look now into the case of a noise standard deviation much higher than the ideal code bin width(𝜎𝑟 → ∞). when carrying out the test with a dc stimulus signal with normalized voltage 𝑦, the variance of 𝑘 conditional to 𝑦 does not depend on the value of 𝑦 and is equal to the random noise variance as stated in (13). in the case of a triangular stimulus signal this leads, from (24), to lim 𝜎𝑟→∞ 𝜎𝑘 2 = 𝜎𝑟 2 . (29) u[k+1]u[k] u u[k−1] f u p k-1 =0.5 p k =0.5 u[k+1]u[k] u f u p k =1 a) b) figure 5. representation of the probability density function of noise and the probability of occurring code k − 1 and code k. figure 6. representation of the code standard deviation as a function of the additive noise standard deviation (solid line) for a triangular stimulus signal with amplitude q/2. the dashed and the dot-dashed lines represent the approximation for small and large additive noise standard deviation given by (28) and (29) respectively. the circles represent the uncertainty of the codes standard deviation obtained experimentally. acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 5 this situation is represented by the dotted line in figure 6. the goal of the random noise test is to be able to estimate the random noise standard deviation, 𝜎�̂� from the measured mean square error (𝑚𝑠𝑒). 3.3. ieee 1057 noise estimator as we have demonstrated in the previous sections, the expected value of the mean square error is a function of the random noise standard deviation. combining (28) and (29) with (2), leads to lim 𝜎𝑟→0 𝐴𝑄→∞ 𝜇mse = 2 √𝜋 𝜎𝑟 lim 𝜎𝑟→∞ 𝜇mse = 𝜎𝑟 2 . (30) from these approximations the ieee standard [1] presents a heuristically derived expression for the random noise standard deviation estimation: 𝜎�̂� = [(√ 𝑚𝑠𝑒 2 ) −𝑅 + (√π 𝑚𝑠𝑒 2 ) −𝑅 ] − 1 𝑅 , 𝑅 = 4. (31) note that the variables 𝜎 and 𝑚𝑠𝑒 used in [1] are expressed in volt and squared volt respectively while the variables 𝜎𝑟 and 𝑚𝑠𝑒 used here are dimensionless. the error of the additive noise standard deviation estimation, represented in figure 7, is given by 𝑒𝜎�̂� = 𝜎�̂� − 𝜎𝑟 . (32) it can be seen that the error is always smaller, in absolute value, than 0.022 lsb. when 𝜎𝑟 → ∞ the error goes to 0. the factor 4 used in (31) is very close to the value of 3.8203 which leads to the optimum approximation for an expression of the kind of (31) and corresponds to the position of the minimum in figure 8. we have, so far, showed how the expression for the estimation of the random noise standard deviation from the measured mean square error recommended in the ieee 105794 standard was derived. the remaining of this paper we will go a step further by showing how this test can be carried out with a sinusoidal stimulus signal as well. we will also focus on the influence that the stimulus signal amplitude and offset have on the test results. as we saw in this section, the expression for the random noise estimator was heuristically derived from approximations for which an infinite stimulus signal amplitude is assumed. furthermore, in the ieee standard a value of 5 lsb for the stimulus signal amplitude is recommended without any justification. in test conditions where the stimulus signal generator has low accuracy on the wave amplitude and has offset error or the adc under test has high resolution, it is difficult to have a stimulus signal amplitude exactly equal to 5 lsb. it is thus important to know how different values of stimulus signal amplitude and offset affect the bias of the estimated random noise standard deviation. this will be carried out here for both types of stimulus signal shapes – triangular and sinusoidal. as it will be shown, the higher the amplitude the lower the bias. the goal will be to have an expression that will tell us what should be the minimum value to use in order to guarantee an upper bound on the estimator bias. 4. sinusoidal stimulus signal although the ieee 1057 standard recommends the use of a triangular wave signal for the cases where the random noise to be measured is small compared to the quantization step, this test can also be carried out with a sine wave as we will show here. since sine wave generators are more common than triangular wave ones, this will benefit those wanting to measure small values of random noise but without having to look for a triangular wave generator. the variance of the output codes can be calculated the same way as in the case of triangular wave stimulus, using (22), but with a different amplitude distribution, 𝑓𝑦. for a sine wave, one has [8]: 𝑓𝑦(𝑦) = { 1 π√𝐴𝑄 2 − (𝑦 − 𝐶𝑄) 2 , |𝑦 − 𝐶𝑄| < 𝐴𝑄 0 , otherwise . (33) inputting this into (22) leads to 𝜎𝑘 2 = ∫ 𝜎𝑘|𝑦 2 (𝑦) 1 𝜋√𝐴𝑄 2 − (𝑦 − 𝐶𝑄) 2 d𝑦 𝐶𝑄+𝐴𝑄 𝐶𝑄−𝐴𝑄 . (34) the strategy used in the case of the triangular stimulus signal was to derive two expressions for the asymptotic cases of high and low random noise standard deviation and then heuristically construct an expression for the estimator. as it turns out, those two asymptotic expressions are the same regardless of the stimulus signal shape. for large values of random noise we concluded earlier that the variance of the output codes is independent of the dc stimulus signal value. using eq. (13) in (34) leads to figure 7. representation of the estimation error of the additive noise standard deviation, obtained with (31), as a function of the actual standard deviation. figure 8. representation of the estimation error of the additive noise standard deviation, obtained with (31), as a function of the parameter r. acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 6 𝜎𝑘 2|𝜎𝑟→∞ = 𝜎𝑟 2 ⋅ ∫ 1 π√𝐴𝑄 2 − (𝑦 − 𝐶𝑄) 2 d𝑦 𝐶𝑄+𝐴𝑄 𝐶𝑄−𝐴𝑄 = 𝜎𝑟 2 (35) which is the same as the one obtained for the triangular shaped stimulus signal (eq. (29)). for small values of random noise standard deviation, we made an extra approximation that was to consider an infinite stimulus signal amplitude. the adc input range will thus only cover the middle of the stimulus signal range. there the sinusoidal shape can be approximated by a straight line which makes it similar to a triangular wave and thus will lead to the same approximation for the output codes variance: 𝜎𝑘 2|𝜎𝑟→0 𝐴𝑄→∞ = 1 √π 𝜎𝑟 . (36) it is thus shown that the same heuristically derived expression for the triangular stimulus case, eq. (31), can be employed when using a sinusoidal stimulus signal. 5. experimental validation a triangular stimulus signal, with amplitude 39.216 mv and frequency 10 hz, was applied to a 12-bit data acquisition board (keithley das 1601) in the 10 v range and two sets of 10000 samples were acquired at a sampling frequency of 100 khz. only the 8 most significant bits were used so that the adc would present an almost ideal behavior. this was validated by testing the data acquisition board (using all 12 bits) with the standard histogram test and obtaining an integral non-linearity (inl) and differential non-linearity (dnl) lower than 0.5 lsb. a stanford ds360 function generator was used to generate the additive noise and a wavetek 9100 calibrator was used to generate the triangular wave. the data acquisition board used possesses 16 single-ended or 8 differential inputs. it was one of those differential inputs that was used (ch0). the stanford ds360 function generator was connected between pins 37 (“ch0 hi in”) and 19 (“ll gnd”) and the wavetek 9100 calibrator was connected between pins 18 (“ch0 low in”) and 19 (“ll gnd”). this way the gaussian noise and the triangular wave were added (actually, subtracted) internally. the results are represented in figure 6 were a good agreement with the theoretical curve, represented by the thin solid line is observed. 6. stimulus signal amplitude in the previous sections we addressed the problem of determining an estimator for the random noise standard deviation when using a triangular and a sinusoidal stimulus signal. as observed in equation (24), the code standard deviation depends on the stimulus signal amplitude. this can be verified in figure 9 where two cases are plotted, one for 𝐴𝑄 = 0.5 (dotted line) and another for 𝐴𝑄 = 0.75 (solid line). the difference is highest for values of noise standard deviation smaller than 0.5 lsb. that dependence can be also observed in figure 10 for two different values of triangular wave offset. the case of null amplitude (𝐴𝑄 = 0) corresponds to the dc input case represented in figure 2. the higher the triangular wave amplitude the less influence it has on the output code standard deviation. whenever the amplitude 𝐴 is a multiple of 𝑄/2, that is, 2𝐴𝑄 is integer, the code standard deviation has the same value. when performing the test to estimate the random noise standard deviation, a value of triangular wave amplitude has to be chosen. this choice will influence the estimated value, however, observing figure 10, there are some values that seem more advantageous than others, namely the odd multiples of 𝑄/2 (𝐴𝑄 = 0.5,1.5,2.5,…) because the slope of the curve in those points is smaller making the estimate less sensible to errors in the triangular wave amplitude that in practical conditions are sure to exist. also, in that case, the output code variance becomes practically independent of the stimulus signal offset. the higher the triangular wave amplitude, the higher will be the additive noise introduced by the function generator (because of its different ranges with higher amplification) making very high values of amplitude not advisable. the ieee standard [1] suggests the use of 𝐴𝑄 = 5 without explaining the reason. we hope the work presented here sheds some light on the subject. as seen here, a marginally better choice would be, for instance, 𝐴𝑄 = 5.5 since the slope of the curve in that point is smaller than at 𝐴𝑄 = 5 and consequently the test would be less affected by eventual inaccuracies in the signal amplitude. figure 9. representation of the code standard deviation as a function of the additive noise standard deviation for a triangular stimulus signal with amplitude q/2 (dotted line) and an amplitude 0.75q (solid line). figure 10. representation of the code standard deviation as a function of the triangular wave amplitude divided by the ideal code bin width. the solid line represents the situation of an offset equal to an adc transition voltage and the dotted line a situation where the offset value is exactly between two consecutive transition voltages. the dotted line represents the value taken by the code standard deviation when the triangular amplitude is q/2. a normalized value of noise standard deviation of 0.1 (one tenth of the ideal adc code bin width) was used. 0 0,1 0,2 0,3 0,4 0,5 0 0,5 1 1,5 2 2,5 3 3,5 4 4,5 5 a q (lsb) sk (lsb) acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 7 7. conclusions in this paper, the test method proposed by the ieee 1057 standard [1] to estimate the random noise standard deviation was analyzed in detail. the expression presented there to calculate the additive noise standard deviation from the adc output codes mean square error is explained here from a different perspective. the influence of both triangular wave amplitude and offset on the estimation error was studied. in figure 11, the maximum estimation error for different values of triangular stimulus signal amplitude and offset is represented. it can be seen that for small values of amplitude the maximum error depends strongly on the triangular wave offset. the actual error on estimating the noise standard deviation when using the expression presented in [1] is calculated and it was concluded that it was smaller than 2.2% of the ideal adc code bin width (0.022 lsb). the choice of triangular stimulus amplitude is studied and a justification for the value proposed in [1] is given. we suggest here that in the case were a triangular or sinusoidal wave generator is not available the test be carried out with a dc stimulus signal, with any value inside the adc range, and that the noise standard deviation be estimated as being equal to the code root mean square error, divided by 2. if the value obtained is smaller than q/2 the value used as the noise standard deviation estimative should be q/2. of course, the standard deviation could be smaller, but, as shown here, it could not be higher. in summary, we propose, just for the case where a triangular or sine wave generator cannot be used, the following expression for the estimation: 𝜎�̂� = min( 1 2 ,√ 𝑚𝑠𝑒 2 ) . (37) experimental results were presented that validate the approach taken. references [1] ieee standard for digitizing waveform recorders, ieee std 1057-2007 (revision of ieee 1057-1994) (full-use), vol., no., pp.1-168, 18 april 2008: doi: 10.1109/ieeestd.2008.8291822 [2] a. j. ginés, e. j. peralías, a. rueda, an adaptive bist for inl estimation of adcs without histogram evaluation, in proceedings 2010 ieee 16th international mixed-signals, sensors and systems test workshop (ims3tw), la grande motte, france, 07-09 june 2010, pp. 1-6. doi: 10.1109/ims3tw.2010.5502997 [3] l. michaeli, fast dynamic methods of the systematic error autocorrection, proceedings of 5th imeko tc-4 international symposium, vienna, april 1992, 1992, pp. 247 249. 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[18] d. belega, d. petri, statistical performance of the effectivenumber-of-bit estimators provided by the sine-fitting algorithms, ieee transactions on instrumentation and measurement 62(3) (2013), pp. 633-640. doi: 10.1109/tim.2012.2218679 [19] f. c. alegria, a. moschitta, p. carbone, a. m. da cruz serra and d. petri, effective adc linearity testing using sinewaves, ieee transactions on circuits and systems i: regular papers 52(7) (2005), pp. 1267-1275. doi: 10.1109/tcsi.2005.851393 [20] b. renczes, i. kollár, t. dabóczi, efficient implementation of least squares sine fitting algorithms, ieee transactions on instrumentation and measurement 65(12) (2016), pp. 2717-2724. doi: 10.1109/tim.2016.2600998 [21] f. c. alegria, p. girao, v. haasz, a. c. serra, performance of data acquisition systems from the user's point of view, ieee transactions on instrumentation and measurement 53(4) (2004), pp. 907-914. doi: 10.1109/tim.2004.830757 [22] f. alegria, p. silva girão, v. haasz, a. serra, performance of data acquisition systems from the user's point of view, ieee instrumentation and measurement technology conference, 940-945, vail, colorado, usa, pp. 940-945, 20-22, may 2003. doi: 10.1109/imtc.2003.1207891 [23] f. c. alegria, a. c. serra, uncertainty of the estimates of sine wave fitting of digital data in the presence of additive noise, 2006 ieee instrumentation and measurement technology conference proceedings, sorrento, italy, 24-27 april 2006, pp. 1643-1647. doi: 10.1109/imtc.2006.328187 [24] j. m. janik, d. bloyet, b. guyot, measurement of timing jitter contributions in a dynamic test setup for a/d converters, ieee transactions on instrumentation and measurement 50(3), pp. 786-791. doi: 10.1109/19.930455 [25] j. m. janik, d. bloyet, timing uncertainties of a/d converters: theoretical study and experiments, ieee transactions on instrumentation and measurement 53(2) (2004), pp. 561-565. doi: 10.1109/tim.2003.820461 [26] g. chiorboli, m. fontanili, cross-correlation noise measurements in a/d converters, ieee transactions on instrumentation and measurement 48(6) (1999), pp. 1282-1286. doi: 10.1109/19.816149 [27] j. holub, j. vedral, stochastic testing of adc—step-gauss method, computer standards & interfaces 26(3) (2004), pp. 251-257. doi: 10.1016/j.csi.2003.09.001 [28] f. c. alegria, a. c. serra, gaussian jitter-induced bias of sine wave amplitude estimation using three-parameter sine fitting, ieee transactions on instrumentation and measurement 59(9) (2010), pp. 2328-2333. doi: 10.1109/tim.2009.2034576 [29] iec, performance characteristics and calibration methods for digital data acquisition systems and relevant software – iec 62008:2005, july 2005. [30] ieee standard for terminology and test methods for analogto-digital converters, ieee std 1241-2000, pp.1-98, 22 june 2001. doi: 10.1109/ieeestd.2001.92771 [31] m. emmenegger, f. jenni, r. kunzi, h. jaeckle, s. schnabel, analysis and calibration of a high precision ad converter, 2007 european conference on power electronics and applications, aalborg, denmark, 02-05 september 2007, pp. 1-10 doi: 10.1109/epe.2007.4417276 [32] l. xu, d. chen, a low cost jitter estimation and adc spectral testing method, 2015 ieee international symposium on circuits and systems (iscas), lisbon, portugal, 24-27 may 2015, pp. 2277-2280. doi: 10.1109/iscas.2015.7169137 [33] v. pálfi, i. kollár, efficient execution of adc test with sine fitting with verification of excitation signal parameter settings, 2012 ieee international instrumentation and measurement technology conference proceedings, graz, austria, 13-16 may 2012, pp. 2662-2667. doi: 10.1109/i2mtc.2012.6229502 [34] d. belega, d. petri, analog-to-digital converter dynamic testing by linearized four-parameter sine-fit algorithm, 2021 ieee 6th international forum on research and technology for society and industry (rtsi), naples, italy, 06-09 september 2021, pp. 502506. doi: 10.1109/rtsi50628.2021.9597328 https://doi.org/10.1109/tim.2009.2013674 https://doi.org/10.1109/tim.2012.2218679 https://doi.org/10.1109/tcsi.2005.851393 https://doi.org/10.1109/tim.2016.2600998 https://doi.org/10.1109/tim.2004.830757 https://doi.org/10.1109/imtc.2003.1207891 https://doi.org/10.1109/imtc.2006.328187 https://doi.org/10.1109/19.930455 https://doi.org/10.1109/tim.2003.820461 https://doi.org/10.1109/19.816149 https://doi.org/10.1016/j.csi.2003.09.001 https://doi.org/10.1109/tim.2009.2034576 https://doi.org/10.1109/ieeestd.2001.92771 https://doi.org/10.1109/epe.2007.4417276 https://doi.org/10.1109/iscas.2015.7169137 https://doi.org/10.1109/i2mtc.2012.6229502 https://doi.org/10.1109/rtsi50628.2021.9597328 extraction of a floor plan from a points cloud: some metrological considerations acta imeko issn: 2221-870x june 2023, volume 12, number 2, 1 9 acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 1 extraction of a floor plan from a points cloud: some metrological considerations giulio d’emilia1, luciano chiominto1, antonella gaspari2, stefano marsella3, marcello marzoli3, emanuela natale1 1 diiie, università degli studi dell’aquila, via gronchi 18, 67100 l’aquila, italy 2 dmmm, politecnico di bari, via orabona 4, 70125 bari, italy 3 dipartimento dei vigili del fuoco del soccorso pubblico e della difesa civile, ministero dell’interno, 00184 rome, italy section: research paper keywords: complex buildings; evacuation plan; laser scanner; data synthesis; causes of variability; measurement uncertainty citation: giulio d’emilia, luciano chiominto, antonella gaspari, stefano marsella, marcello marzoli, emanuela natale, extraction of a floor plan from a points cloud: some metrological considerations, acta imeko, vol. 12, no. 2, article 33, june 2023, identifier: imeko-acta-12 (2023)-02-33 section editor: alfredo cigada, politecnico di milano, italy, andrea scorza, università degli studi roma tre, italy, roberto montanini, università degli studi di messina, italy received december 6, 2022; in final form april 22, 2023; published june 2023 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: antonella gaspari, e-mail: antonella.gaspari@poliba.it 1. introduction many fields of application require and benefit from approaches able to combine the information coming from several data sources, of both theoretical and experimental nature (i.e., deriving from analytical/design models and from measurement systems, respectively). in this context, data fusion and data validation techniques represent a core activity in the wider field of big data management, dealing with a large variety of data types. graphical synthesis and visualization represent interesting areas of investigation, mainly due to the remarkable size of the acquired and stored data, and the need of adequate procedure for the correct interpretation of the outcomes, to support and exploit data reliability, mostly when they are used as an input for possible next steps of data processing. at least 4 macro-areas of interest can be envisaged, to potentially benefit of enhanced techniques for assuring measurements accuracy: • air/water/up to space navigation systems, in the aeronautical/ aerospace sector [1], • military systems [2], [3], • industrial cyber-physical systems and sustainable mobility solutions [4] • cultural heritage and safety systems for civil infrastructures and buildings to the best of the authors’ knowledge, limitations regarding the accuracy of the existing solutions remain, thus the evaluation of the effects induced by both procedural and data processing aspects appear helpful in overcoming the related issues. in this abstract the design of evacuation plans for safety purposes is based on the graphical synthesis and visualization of data derived from highperformance measurement systems. the data post-processing requires some suitable procedures including many steps, to obtain the information needed. these are related to the global use of the measurement system, to the real measurement operating conditions, to the complexity of the building, to the simplifying hypotheses, to the data management. as a preliminary step, this work outlines a procedure for extracting a floor plan from a points cloud acquired by means of a laser scanner. the test case is a baroque cathedral, being a large structure and presenting some elements of irregular geometry. some critical passages have been examined, presenting similar widths but different characteristics i.e., flat and regular geometries of the elements on the walls, in some cases, statues, columns and adornments, in some others. the data management plays a key-role in the correct interpretation of the outcomes, having a great impact on the results reliability. the obtained estimated variability of the passage width is in the order of few tenth centimeters. this is strictly related to the specific operating procedure. how this affects simulation software for route optimization of emergency paths will be the object of further investigations. mailto:antonella.gaspari@poliba.it acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 2 work, attention is paid to an application referred to safety operations, to support rescue procedures based on the use of 3d models of buildings and civil infrastructures. keeping into account only the last two fields, in the last years, 3d models of cities (or parts of city, or even of single buildings) have become valuable for several purposes beyond visualization, and are used in many domains, as [5]-[11]: estimation of the propagation of noise in an urban environment, urban planning, facility management, estimating solar irradiation, evaluating seismic damage, planning emergency response. a typical 3d model can be derived from various acquisition techniques, as, for instance, photogrammetry and laser scanning [12], [13], unmanned aerial vehicle (uav) tilt photography [14]-[16], synthetic aperture radar (sar) techniques [17]. compared to other systems, the laser scanner technique appears to be a suitable measurement system when the reconstruction requires extreme detail and the acquisition conditions are critical, such as in emergency situations. these reflections are due to laser scanner characteristics of resolution, accuracy, ease of use, relative low cost, low acquisition and processing time, possibility to measure at safe distance [18]. the internal/external mapping of building, based on points cloud is a very diffused and studied application, for many purposes (rapid automatic drawing of building mapping for construction, restoring, maintenance purposes, safety requirements, support to rescue in emergency condition) and very recent studies could be found. different instruments have been used, as, lidar laser scanning or time of flight terrestrial laser scanner. manual processing of the points cloud is often used to process data, but some problems arise, especially for buildings of large dimensions as in the case of historical building, like big churches, amphitheaters, and large monuments: • remarkable errors due to human effect in tracing the profiles and the connection lines in the maps, • huge number of measuring points, with the need of setting procedures for big data analysis • lack of reproducibility of results. to prevent the above problems, the recent works typically present procedures for automatic drawing and mapping. many solutions have been proposed for automatic drawing, based on point fitting, outlier removing and tracing of the wall, also including the use of artificial intelligence, (e.g., convolutional neural networks, used to both recognize perimetral elements like walls and individuate specific elements like windows and/or doors). also, accuracy of results is discussed with reference to different parameters (iot, the door of windows number, the area ratio with respect to the manual experimental data); even though modern buildings are studied, some difficulties are highlighted when the geometry of the area is not so regular, presenting niches, sharp variations of direction or of the angle between the walls. the evaluation of the effects induced by both procedural and data processing aspects appear helpful in overcoming the related issues when safety of people is a key requirement. among all the applications, the procedures for supporting rescue in emergency conditions play a relevant role. these aim at tracing the escaping tracks and maps and/or checking the available areas for safe evacuation of people. in a previous application [19] the authors faced the above problem with reference to the definition of an evacuation plan of a baroque cathedral located in italy, which is characterized by having a complex structure and where hundreds of people could be present at the same time. to this purpose, a preliminary mapping of the monument has been carried out by using a terrestrial laser scanning to obtain the cloud points. all the objects between the ground and 2 meters of height should be included in the floor plan, to consider possible obstacles to the evacuation, e.g., tall people, or rescue objects. according to this criterion, reducing the available surface for escaping is important especially in historical buildings, like churches, where there are numerous architectural elements on the walls or on the pillars. in fact, these are generally not contained in floor plans. however, since the floor plan has been drawn using acquisition of real objects, some geometrical inconsistencies are presents: • the walls are not always straight, so some lines in the floor plan are inclined in the range of a few degrees • curved or rounded elements like columns, corners or spiral staircases are not perfect. it is difficult to represent these parts in the floor plan according to their actual shape. with reference to the final accuracy of data, some procedural aspects, which make the resulting floor plan strongly depending on the quality of the acquired points cloud, are equally important. acta the factors that influence the most the generated points cloud are: • presence of obstacles for the laser beam. this results in a loss of information and the created cloud has voids and holes. a) b) figure 1. obstacles during the acquisition phase: a) shadowing effects due to a church bench; b) presence of people during the acquisition. acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 3 • since the church considered as test case is open to the public, the presence of people in the acquisition is inevitable. they create the same shadowing effects to the laser beam as the obstacles. • effect of the positioning of the laser scanner with respect to the target point, especially when the angle positioning is not favorable • presence of reflective objects like windows or glass doors because this kind of surfaces distort the reflected laser beam. figure 1 reports some errors during the acquisition phase like a church bench on the left or people on the right. based on these considerations, assessing the uncertainty of the data provided for the applications the points cloud are used is very difficult and the examples given in literature where the reference geometry is different from each other do not allow to define a standardized procedure, which could be very useful to realize reliable and trustworthy procedures. a standard procedure, encompassing a whole uncertainty evaluation is a long-term goal. nevertheless, a step-by-step approach could be useful to analyze the main components of the uncertainty budget, where a higher attention should be put to better describe the real situation. in this work, with reference to a previously obtained map of a church located in italy, two situations are studied, being similar with respect to a general evaluation, (i.e., showing same minimum width of the passage and characteristics of the surrounding areas), but different with respect to the characteristics of the passage: flat walls and regular geometries of the elements in the former case; statues, altars and/or high/bas reliefs in the latter case. having an idea of the variability of results at different heights of the same section, is expected to help to complete the information about the eventual dangerousness of the passage itself. with reference to the main sources of uncertainty that can be recognized in the selected test-case, the entity of the induced effects of these causes of variability on the parameters used to extract a floor plant from a points cloud is quantitively evaluated making use of the canonical techniques for measurement uncertainty evaluation. the emerging metrological considerations referred to the extraction of a floor plant from a graphical information provided by the advanced measurement systems are expected to be easily transferred also to other similar applications. in section 2 with reference to the selected test case (2.1), the methodology followed to evaluate quantitatively the effects of the variability in the determination of the needed quantity will be explained (2.2); it is worth noting that the definition of the parameter of interest is not a trivial task. in section 3, the outcomes will be analyzed and discussed. conclusions and future works end the paper 2. materials and methods 2.1. test case peculiarities the analysis of the most influencing uncertainty causes is referred to a complex structure where thousands of people have access daily, and hundreds could be present at the same time. the building is a baroque cathedral located in italy. one of the direct implications of the metrological characterization of the method is the enhancement of algorithms devoted to indoor localization and positioning for emergency situations based on context-aware software requiring, among other, a detailed knowledge of buildings floor plans [20]-[23]. in this work, a leica rtc360 3d laser scanner, and the related leica software suite for the points cloud analysis, were used. the laser scanner technique has some strength points when used for the design of an evacuation plan or for safety purpose: (1) it allows an acquisition of the real dimensions of a building; (2) all the objects between the ground floor and an arbitrary height could be included in the floor plan. this is very important especially in historic buildings like churches where there are numerous architectural elements on the walls or on the pillars. these are generally not contained in floor plans, with a high level of detail. the cathedral has a floor area of more than 15000 m2 with the central aisle longer than 186 m. for these reasons, acquisitions in different positions are required to scan completely the internal area. in this case, the laser scanner has been in different position according to the complexity of the surroundings. the distance between different acquisition setups ranges from 10 m to 36 m. the distances have been chosen according to the complexity of the surrounding areas. smaller values have been used when there are more architectural elements to have a better reconstruction. greater distances between acquisitions setups have been used in areas with simpler shape to reduce the amount of data. when possible, the instrument has been placed halfway between two facing walls. for all the acquisition, the laser scanner has been placed on a tripod with a height of 1.6 m. each acquisition has been joined with the subsequent using the leica software register360 field. to have a better alignment of different acquisitions, the instrument has a visual inertial system (vis) that tracks the movement of the unit relative to the previous scan position. since the site has not been closed to the public during the acquisition campaign, the function “double scan” of the laser scanner has been used for each setup. this allows to eliminate all the objects and visitors that have moved between one scan and the other. the points cloud of the whole building analyzed in this work is generated using 318 acquisitions setups and the resulting cloud is composed of 10414.4 million (10414448131) of points. a spherical photo is also acquired for each setup, and it could be useful for a visual inspection to better interpret the points cloud and extract the floor plans. in fact, especially in complex structures, it could happen that the laser beam could not reach some zones. to avoid this problem, the setups’ locations must be carefully planned to scan all the zones. it must be pointed out that procedural and interpretative aspects, connected to synthesis and to the post-processing of the obtained data, have a great impact on the reliability of the results. the simplification of the procedures is needed as more as the buildings have a remarkable extension and complex shapes. to ensure the reliability of the measurement data, the calibration of the measurement systems is a necessary preliminary step [24]-[27]. the leica rtc360 is a high-speed and high-accuracy 3d laser scanner, which was tested in a previous work of the authors under field conditions [28]. the measurement precision under repeatability conditions which has been estimated, and which is in line with what is declared by the manufacturer, is of the order of 1 mm. the resolution is of 3 mm at 10 m distance. therefore, such laser scanner performances appear adequate for the use of the instrument in the described application of interest, that is the design of an evacuation plan from a complex structure [29]. in addition, the good practices have been correctly carried out, according to the field procedures for testing geodetic and surveying instruments. it is worth noting the simplified test procedure provided in [30] makes joint use of two measurement acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 4 stations, besides considering other main specific contribution to the standard deviation, which is not always feasible. moreover, the operators could perform the set-up of the instrumentation as end-users and with no particular attention to the aspects of metrological interest. regarding the usage of a terrestrial laser scanner to design an evacuation plan, the results will describe the main problems concerning the extraction of a floor plan from a points cloud. the different acquisitions have been joined and cleaned using leica cyclone register 360 [31]. the points cloud has been processed using autocad 2023 and the plugin leica cloudworx for autocad; this has been used to extract from the points cloud a series of layers at different heights, parallel to the floor of the building, up to two meters. in facts, in safety applications, all the obstacles between the floor and two meters of height must be considered. switching to the top view of the cloud, the points on this plane has been used for drawing the floor plan. figure 2 shows an example of the process from the points cloud to the floor plan figure 2. the comparison aims at highlighting the peculiar shape of the scanned profiles, that could mislead the interpretation of the extracted floor plan itself, given on the one hand, the top view of the sliced points cloud considering a 2meter-high layer (figure 2a), and manually interpolating the profile of the scanned objects, considering the maximum extension on the floor level, on the other hand (figure 2b). 2.2. methodology for quantitative evaluation to provide quantitative evidence of the minimum area available for designing evacuation plans, the effects of the following main aspects were considered: 2.2.1. number of critical regions on the horizontal plane (on x-y plane) seven passages are considered critical (on an arbitrary basis) for escape purposes. this criterion will require further analyses, using an iterative approach with the goal of covering the entire building. 2.2.2. number of layers on vertical axis (z-axis) on a height of 2.00 m, a total of 8 horizontal layers are analyzed at 0.25 m each from the next one. this choice is expected to be sensitive to shady areas that could not be captured by the system. 2.2.3. definition of the parameter of interest for each layer, two quantities have been identified: • distance, 𝑑𝑖𝑗, between the walls of the aisles suitable for escape, evaluated on each critical region, i. the variability is evaluated considering 10 up to 15 widths, along the direction of the passage, j. in some cases, corresponding to 0.30 m to 0.50 m of passage depth. the standard deviation of a normal distribution is used to estimate the related variability. • variation of the construction points positions of the pillar perimeter, δk, as the height increases, evaluated considering the difference between the layer that envelops the maximum perimeter minus the minimum one, with respect to a reference layer along the z-axis. the standard deviation of the differences assuming a uniform probability distribution is used to estimate the related variability. 2.2.4. identification of the reference for the parameter evaluation the parameters defined in 2.2.3 both require the choice of a reference for their estimation. therefore, it is essential identifying the horizontal and vertical references, as it follows: • reference for the z-axis direction: the variability can be studied using a different reference plane, for the identification of the ground floor, which determines the slicing layers. this also affects δk, which can refer to one of the available layers, e.g., middle layer (1 m). • reference for the x-y plane: the choice is driven by the processing opportunity offered by the software to operate manually to the selection of the quantity of interest. some options could be the reference system of the cad software, the median lines of aisles / naves of the church or a basic geometrical shape enveloping the architectonical element analyzed (e.g., a simple rectangle representing a pillar). 2.2.5. effect of the irregularities of the contours for each layer, the profiles of the scanned objects are based on the manual interpolation of the available points clouds, traced with closed broken lines. this is a task that can be automated with different techniques and by means of different software [32]-[34]; nevertheless, this aspect is a matter of investigation for metrological characterization purposes. a) b) figure 2. top view of the sliced points cloud. a) detail of a 2-meter-high section of the points cloud. b) floor plan (in green) drawn considering the maximum dimensions on the floor. acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 5 2.2.6. number of points cloud reduced configurations to study accessibility can also determine further variability to the final measurements, from the point of view of the number of points considered in the calculations, compared to the reference configuration, which includes all the monitoring points [35]. 3. results for the evaluation of the width of the aisle (𝑑𝑖𝑗), seven different passages have been considered; for each passage, 10 up to 15 widths have been manually recorded, for each of the 8 layers. the passages have been selected as for the characteristics highlighted in the following bullet list; an example of the examined situations is depicted in figure 3. • the regularity of the opposite walls, meaning that no substantial anomaly is present, in terms of permanent obstacles, like columns, sculptures nor adornments, which may obstruct the passage itself; namely these are indicated as rr1 and rr2. “r” stands for “reference”: as a matter of fact, the expected variability within the layer and as the layer changes, is at its lowest here, and for this reason this can be thought as a reference width between the two walls. rr1 and rr2 can be assumed having similar absolute values, being in the same area. • the presence of some irregularities at only one of the walls, with reference to the distance, 𝑑𝑖𝑗, is evaluated. these are recalled as cc1 and cc2, where the letter “c” stands for “columns”. again, in this case the expected absolute values for cc1 and cc2 are of the same magnitude, being the walls close to each other, and the columns of very similar shapes. the variability within the layer is evaluated considering 15 widths, as depicted in figure 2. the passage indicated as cc10 should be considered as a sub-class of possible passages where both opposite walls show the presence of columns; the distance, 𝑑𝑖𝑗, in this case is evaluated just on the minimum space available, considering only 10 widths per passage. • the presence of some irregularities on both opposite walls, with reference to the width, 𝑑𝑖𝑗. namely, ws and ss refer to the cases wall-statue and statue-statue, respectively. figure 4a) shows an example of the sliced area, in the x-y plane, where 10 distances have been acquired (cc10). looking at the obtained distances (figure 4b), it can be easily recognized figure 3. example of three passages analysed: white 15 straight lines represent the direction for dij evaluation. the coloured contours are referred to the floor plans extracted at different layers. a) b) figure 4. analysis of the passage defined by two opposite columns, evaluated using 10 distances (column-column 10 points, cc10): a) sketch of the drawing; b) 3-dimensional representation of the obtained widths, at each layer height, at each measuring point. acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 6 an increasing value of 𝑑𝑖𝑗, due the peculiar shape of the columns. the uncertainty on the definition of the quantity to be measured can lead to the presence of some outliers, especially on the external edges (e.g., measuring point 10). the same effects can be noticed, looking at the graphs from figure 5 to figure 7, summarizing the results obtained in the three areas examined. the variability within the layer is indicated through error bars in the graphs, considering the standard deviation of 15 widths. looking at figure 5, it can be noted a very low variability of the regular passage, up to 1.50 m. this confirms that this passage could be taken as a reference, at least in a part. the minimum variability of the method is in the order of 0.10 %. at the highest layers, however, it does not exceed 4.5 %. on the other hand, it is very difficult to find very regular walls up to 2 m of height in the cathedral. figure 6 refers to a more complex situation that is, when some disturbing elements can be found on the passage. the maximum variability occurs with reference to the 1.75 m section, being nearly 5 %. the variability among the layers is higher than rr1and rr2, being in the order of 2-3 %. it must be pointed out that the focus of the analysis, in this region, has been referred to the portion of the passage, affected by statues, without considering other types of irregularities. figure 5. width of the passages rr1 and rr2 i.e., regular opposite walls. the error bars consider the standard deviation of 10 widths. figure 6. width of the passages ws, i.e., regular wall, opposite to a statue, and ss, i.e. when both sides of the passage present a statue. the error bars consider the standard deviation of 15 widths. figure 7. width of the passages cc1 and cc2 i.e., both offering a passage delimited by columns, on both sides. error bars consider the standard deviation of 15 widths. 10,50 12,50 14,50 16,50 0,25 0,50 0,75 1,00 1,25 1,50 1,75 2,00 d is ta n c e , d ij (m ) layer (m) rr1 rr2 2,00 4,00 6,00 8,00 0,25 0,50 0,75 1,00 1,25 1,50 1,75 2,00 d is ta n c e , d ij (m ) layer (m) ws ss 5,00 7,00 9,00 11,00 0,25 0,50 0,75 1,00 1,25 1,50 1,75 2,00 d is ta n c e , d ij (m ) layer (m) cc1 cc2 acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 7 in fact, looking at graph of figure 7, here in the two regions a negligible difference is confirmed between the average values obtained for the distances, while the variability within the layers increases significantly (in the order of 20 %). the variability among the layers is in the order of 6 %. this is mainly due to the uncertainty of the measurand, which cannot disregard the edge effects linked to variations in the plan section of the identified architectural elements. these effects are highlighted in figure 8. in figure 8 the results obtained with reference to δk are reported. figure 8 shows the points considered and reports the obtained behavior of the absolute variation of the construction points positions of the pillar perimeter, δk, averaged on the 7 available layers. δk reaches values up to 60 cm. it is worth noting that changing the height of the layer and the type of the architectural elements crossed, the reconstruction perimeter changes layer by layer, as well. the error bars represent the standard deviation of the differences assuming a uniform probability distribution. this choice guarantees a conservative evaluation of the associated variability, which resulted in the order of 20 cm. the values of δk and its variability suggest a careful choice of the layer to be considered for the extraction of the floor plan. table 1 summarizes the mean of the widths and the standard deviation among layers, for each case considered (values in meters). concerning the number of the points cloud, this work laid the groundwork for further analyses linked to the effects of having available a smaller size of the stored data. the original points cloud has been created using more than 10000 million points. the cloud has been subsampled at 5000 million points (50 % of the original) and then in 2000 million points (20 % of the original). figure 9 illustrates an example of the same section of a pillar in the different points clouds. in the bottom there is the original point, moving to the top section there is the subsampled cloud in 2000 million points. it can be noted the number of points decreases so the borders of the element under analysis becomes less distinguishable but still visible. using the manual floor plan extraction procedure explained in section 2, the subsampling of the points cloud should not introduce further variability. this result is true especially in the change of direction for elements having a right-angle shape. in case of an automatic floor plan extraction algorithm the number of points should be taken into consideration for the variability of the result. this will be the object of further analyses, in future work. 3.1. discussion the obtained results show that the extraction of a floor plan from a points cloud requires careful examination of the causes of variability of the experimental data. the variability is not attributable to the measurement system, and it is mainly due to: • the adopted procedure for the floor plan mapping, due to procedural and interpretative aspects with a great impact on the reliability of the results (e.g., the cloud does not fully cover the whole areas due to obstacles, the selection of the slicing plane, …). • the simplifying assumptions that could be made, especially for buildings with complex shape or great extension (reducing the number of measuring points, subsampling of the point cloud, …). • local peculiarities of the structure (approximations of architectural elements, irregularities in both the floor and walls, …). a) b) figure 8. analysis of a pillar perimeter evaluated using 23 construction points positions; a) scheme accentuating the irregular profile of the pillar; b) behaviour of the mean values of the construction point positions δk, along the pillar perimeter; the error bars represent the variability of δk evaluated on the seven layers. figure 9. example of the same profile (portion), obtained by subsampling at 50 % (5000 million points) and 20 % (2000 million points) of the original (10000 million points). table 1. summary of the passage widths for each case considered. values are reported in meters. r1 r2 ws ss cc 1 cc 2 cc10 mean value 12.68 12.47 4.95 4.63 7.73 8.06 6.69 standard deviation 0.002 0.01 0.15 0.06 0.45 0.28 0.31 0,00 0,20 0,40 0,60 0,80 1 4 7 10 13 16 19 22  k ( m ) construction points positions of the pillar perimeter acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 8 in synthesis, the results highlight the following main aspects: • the width of the passage both depends on the height at which the floor plan is extracted, and the shape of the specific elements, which are present in the passage itself. • the variability both among layers and within layers are correlated with each other. • using a single value for the width of the passage, the relative variability reaches 20 %, since different heights and depths are considered. • in case of very regular geometry of the walls of the passage, very precise measurements could be made, with an accuracy in the order of a few centimeters. 4. conclusions in the first step of the work, the metrological characteristics of using a laser scanner have been evaluated, as a preliminary study for the development of simplified procedures for the design of an evacuation plan for complex and large structures. the whole measurement procedure was considered, starting from an infield calibration procedure. previous results show that the laser scanner is suitable for the application of interest, if the accuracy in determining the single monitoring points is considered. however, the synthesis and data post-processing require that a procedure including many further steps is defined, to obtain information suitable to design an evacuation plan, which are related to the global use of the laser scanner, to the real operating conditions of measurement, to the complexity of the building, to the simplifying hypotheses, to the data management. as a preliminary step, to investigate some of these aspects, a procedure for drawing a floor plan from a points cloud has been outlined and applied to a real case, being a cathedral. some critical regions of the church have been examined, which are similar with respect to a general evaluation, having the same minimum width of the passage and characteristics of the surrounding areas. these are different with respect to the characteristics of the passage, flat walls, and regular geometries of the elements in the former case, statues, altars and/or high/bas reliefs in the latter. the encountered problems during the application of the procedure mainly refer to the following aspects: the integration of data processing software, big data management, complexity of the structure, quality of the acquired points clouds. 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https://leica-geosystems.com/it-it/products/laser-scanners/scanners/leica-rtc360 https://doi.org/10.3390/buildings12111817 https://doi.org/10.3390/su13020456 https://doi.org/10.3390/buildings12091408 https://doi.org/10.1109/metrolivenv54405.2022.9826963 microsoft word article 1 contacts.doc acta imeko december 2013, volume 2, number 2, 1 www.imeko.org acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 1 journal contacts about the journal acta imeko is an e-journal reporting on the contributions on the state and progress of the science and technology of measurement. the articles are based on presentations presented at imeko workshops, symposia and congresses. the journal is published by imeko, the international measurement confederation. the issn, the international identifier for serials, is 2221-870x. editor-in-chief paolo carbone, italy honorary editor-in-chief paul p.l. regtien, netherlands editorial board francisco alegria, portugal leopoldo angrisani, italy filippo attivissimo, italy eulalia balestieri, italy eric benoit, france pasquale daponte, italy luigi ferrigno, italy alistair forbes, united kingdom konrad jedrzejewski, poland andy knott, united kingdom fabio leccese, italy sergio rapuano, italy dirk röske, germany lorenzo scalise, italy emiliano schena, italy enrico silva, italy marco tarabini, italy susanne toepfer, germany luca de vito, italy about imeko the international measurement confederation, imeko, is an international federation of actually 39 national member organisations individually concerned with the advancement of measurement technology. its fundamental objectives are the promotion of international interchange of scientific and technical information in the field of measurement, and the enhancement of international co-operation among scientists and engineers from research and industry. addresses principal contact prof. paolo carbone university of perugia dept. electr. inform. engineering (diei) via g.duranti, 93, 06125 perugia italy email: paolo.carbone@unipg.it acta imeko prof. paul p. l. regtien measurement science consultancy (msc) julia culpstraat 66, 7558 jb hengelo (ov) the netherlands email: paul@regtien.net support contact dr. dirk röske physikalisch-technische bundesanstalt (ptb) bundesallee 100, 38116 braunschweig germany email: dirk.roeske@ptb.de acta imeko, title acta imeko issn: 2221-870x april 2017, volume 6, number 1, 1 acta imeko | www.imeko.org april 2017 | volume 6 | number 1 | 1 journal contacts about the journal acta imeko is an e-journal reporting on the contributions on the state and progress of the science and technology of measurement. the articles are mainly based on presentations presented at imeko workshops, symposia and congresses. the journal is published by imeko, the international measurement confederation. the issn, the international identifier for serials, is 2221-870x. editor-in-chief paolo carbone, italy honorary editor-in-chief paul p.l. regtien, netherlands editorial board section editors (vol. 4 and 5) leopoldo angrisani, italy filippo attivissimo, italy eulalia balestieri, italy eric benoit, france catalin damian, romania lorenzo ciani, italy pasquale daponte, italy luca de vito, italy luigi ferrigno, italy edoardo fiorucci, italy alistair forbes, united kingdom helena geirinhas ramos, portugal sabrina grassini, italy fernando janeiro, portugal konrad jedrzejewski, poland andy knott, united kingdom massimo lazzaroni, italy fabio leccese, italy rosario morello, italy michele norgia, italy pedro miguel pinto ramos, portugal sergio rapuano, italy dirk röske, germany alexandru salceanu, romania constantin sarmasanu, romania lorenzo scalise, italy emiliano schena, italy enrico silva, italy marco tarabini, italy eric benoit, france marcantonio catelani, italy paolo carbone, italy lorenzo ciani, italy spartacus gomariz, spain sabrina grassini, italy konrad jędrzejewski, poland min-seok kim, korea franco pavese, italy serge rapuano, italy paul regtien, the netherlands alexandru salceanu, romania jan saliga, slovakia krzysztof stepien, poland marco tarabini, italy ian veldman, south africa claudia zoani, italy final editing rosario morello, university mediterranea of reggio calabria, italy about imeko the international measurement confederation, imeko, is an international federation of actually 42 national member organisations individually concerned with the advancement of measurement technology. its fundamental objectives are the promotion of international interchange of scientific and technical information in the field of measurement, and the enhancement of international co-operation among scientists and engineers from research and industry. addresses principal contact prof. paolo carbone university of perugia dept. electr. inform. engineering (diei) via g.duranti, 93, 06125 perugia, italy email: paolo.carbone@unipg.it acta imeko prof. paul p. l. regtien measurement science consultancy (msc) julia culpstraat 66, 7558 jb hengelo (ov), the netherlands email: paul@regtien.net support contact dr. dirk röske physikalisch-technische bundesanstalt (ptb) bundesallee 100, 38116 braunschweig, germany email: dirk.roeske@ptb.de journal contacts microsoft word 0 acta imeko  issn: 2221‐870x  september 2016, volume 5, number 2, 1    acta imeko | www.imeko.org  september 2016 | volume 5 | number 2 |  1  journal contacts    about the journal  acta imeko is an e-journal reporting on the contributions on the state and progress of the science and technology of measurement. the articles are mainly based on presentations presented at imeko workshops, symposia and congresses. the journal is published by imeko, the international measurement confederation. the issn, the international identifier for serials, is 2221-870x.      editor‐in‐chief  paolo carbone, italy  honorary editor‐in‐chief  paul p.l. regtien, netherlands    editorial board      leopoldo angrisani, italy filippo attivissimo, italy eulalia balestieri, italy eric benoit, france catalin damian, romania pasquale daponte, italy luigi ferrigno, italy edoardo fiorucci, italy alistair forbes, united kingdom sabrina grassini, italy fernando janeiro, portugal konrad jedrzejewski, poland andy knott, united kingdom fabio leccese, italy david macii, italy rosario morello, italy michele norgia, italy helena geirinhas ramos, portugal pedro ramos, portugal sergio rapuano, italy dirk röske, germany alexandru salceanu, romania constantin sarmasanu, romania diego scaccabarozzi, italy lorenzo scalise, italy emiliano schena, italy enrico silva, italy marco tarabini, italy susanne toepfer, germany rainer tutsch, germany ian veldman, south africa luca de vito, italy final editing    rosario morello, university mediterranea of reggio calabria, italy   about imeko  the international measurement confederation, imeko, is an international federation of actually 39 national member organisations individually concerned with the advancement of measurement technology. its fundamental objectives are the promotion of international interchange of scientific and technical information in the field of measurement, and the enhancement of international co-operation among scientists and engineers from research and industry.       addresses  principal contact  prof. paolo carbone university of perugia dept. electr. inform. engineering (diei) via g.duranti, 93, 06125 perugia, italy email: paolo.carbone@unipg.it    acta imeko  prof. paul p. l. regtien measurement science consultancy (msc) julia culpstraat 66, 7558 jb hengelo (ov), the netherlands email: paul@regtien.net    support contact  dr. dirk röske physikalisch-technische bundesanstalt (ptb) bundesallee 100, 38116 braunschweig, germany email: dirk.roeske@ptb.de microsoft word 329-2077-1-le-rev acta imeko  issn: 2221‐870x  april 2016, volume 5, number 1, 3‐4    acta imeko | www.imeko.org  april 2016 | volume 5 | number 1 | 3  introduction to the special section of the 1st international  imekofoods conference 2014  claudia zoani  italian national agency for new technologies, energy and sustainable economic development affiliation, rome, italy     section: editorial   citation: claudia zoani, introduction to the special section of the imekofoods, acta imeko, vol. 5, no. 1, article 2, april 2016, identifier: imeko‐acta‐5  (2016)‐1‐02  section editor: claudia zoani, italian national agency for new technologies, energy and sustainable economic development affiliation, rome, italy  received march 5, 2016; in final form march 5, 2016; published april 2016  copyright: © 2016 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  corresponding author: claudia zoani, email: claudia.zoani@enea.it      dear reader, this first issue in 2016 of acta imeko contains a special section dedicated to a selection of papers related to the 1st international conference imekofoods “metrology promoting objective and measurable food quality and safety”. the conference, which took place in rome (italy) on 12nd 15th october 2014, was organised by enea (the italian national agency for new technologies, energy and sustainable economic development) and the imeko tc 23 “metrology in food and nutrition” with the general objective to promote the discussion, the scientific debate and the encounter between the different realities that revolve around the "world of measures", promoting the harmonization and integration and addressing the "world of research" to the emerging needs of the civil society and the productive sectors. this issue contains 7 papers representing the extended versions and advancements of papers presented at the 1st imekofoods conference and selected at the end of the conference by an ad hoc committee. the first paper describes an lc-tandem mass spectrometry method developed by researchers from the institute of sciences of food production of the italian national research council (ispa-cnr, italy) as a screening tool for multiple detection of allergenic ingredients in complex foods, with an application and a follow-up for sensitive multiplex detection of five allergenic ingredients in processed, complex food matrices. the second contribution describes the full “in house” method validation performed by the european union reference laboratory for chemical elements in food of animal origin (eu rl-cefao) for the direct determination of pb and cd in honey by graphite furnace atomic absorption spectroscopy (gf-aas) and its efficacy and capabilities especially considering its low-cost and time saving peculiarities, carried out with the aim to provide technical support to the network of the eu national reference laboratories (nrls). the third paper presents a work in cooperation among different institutes of the jožef stefan institute of ljubljana (slovenia) on analytical approaches based on the definition of the multi-elemental composition of food products for the geographical origin determination, especially focused on the characterisation of slovenian milk by three different methods: energy dispersive x-ray fluorescence spectrometry (edxrf), k0-instrumental nuclear activation analysis (k0inaa) and the inductively coupled plasma mass spectrometry (icp-ms). the fourth paper is authored by researchers from the university of primoska and the university of ljubjiana (slovenia) and illustrates a study carried out with the support of the eu founded project uelije – aimed to determine the variations in the levels of phenolic compounds and sensory properties during the storage of virgin olive oil. the fifth contribution, authored by researchers of the university of genoa, covers the topic of spectroscopic fingerprinting techniques for food characterization describing how non-selective signals can be used for obtaining useful information about a food and, as an example, shows the acta imeko | www.imeko.org  april 2016 | volume 5 | number 1 | 4  construction of a reliable quantitative model for the detection of addition of barley to coffee using nir spectroscopy and chemometrics. the sixth contribution, authored by researchers from the university of florence, gives an introduction on biosensor devices with a focus on label free and real-time affinity–based sensing, providing some applications for different classes of analytes of interest in food analysis like pesticides, gmos and pathogens using optical and gravimetric sensors. the last paper, authored by researchers of the chemical sciences and technologies department of the university of rome "tor vergata", presents some examples of rapid, simple and cost effective screening methods that can be realized by the use of screen printed electrodes (spes) coupled with portable and cheap instrumentation for the monitoring of food quality & safety, together with some applications to real samples for the determination of arsenic, pesticides and toxins aimed to demonstrate their effectiveness. i would like to thank all those who contributed to this issue and made possible its realisation: starting from the authors, to the reviewers and the layout editors, up to the journal managers. hoping you will have a fruitful and interesting reading! microsoft word 266-1863-1-le-rev2 acta imeko  issn: 2221‐870x  december 2015, volume 4, number 4, 9‐15    acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 | 9  positioning uncertainty of the tricept type parallel kinematic  structure  eva kureková 1 , milada omachelová 1 , martin halaj 2 , ilja martišovitš 3  1  faculty of mechanical engineering, slovak university of technology, bratislava, slovakia  2  bratislava, slovakia  3  microstep spol. s.r.o., bratislava, slovakia      section: research paper   keywords: parallel kinematic structures; tricept; positioning accuracy; coordinates uncertainty   citation: eva kureková, milada omachelová, martin halaj, ilja martišovitš, positioning uncertainty of the tricept type parallel kinematic structure, acta  imeko, vol. 4, no. 4, article 4, december 2015, identifier: imeko‐acta‐04 (2015)‐04‐04  section editor: franco pavese, torino, italy  received april 2, 2015; in final form may 19, 2015; published december 2015  copyright: © 2015 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  corresponding author: eva kureková, e‐mail: eva.kurekova@stuba.sk    1. introduction  parallel kinematic structures (pks) represent a nonconventional way for the arrangement of movement elements, compared to the widely used serial kinematic structures. they employ parallel arranged movement elements (telescopic rods, arms) that have one end located at a base frame and the second end connected to a movable platform. the tricept belongs among the most known pks [1]-[4]. it is a fixed platform connected to a movable platform via three driving telescopic rods and a non-driven central rod (figure 1). the central rod is connected to a movable platform by a solid linkage, while it can move axially against the fixed platform (rotation of the central rod is prevented). the effector is usually connected to a movable platform, carrying the tools or technological heads. a servomotor located at the end of each telescopic rod enables extension of the rod by a ball screw and nuts. the skeleton together with a primary platform create a single kinematic element [1]-[9]. figure 1. parallel kinematic structure of the tricept type.  abstract  the  paper  discusses  theoretical  aspects  that  arose  during  the  determination  of  the  theoretical  positioning  accuracy  of  parallel  kinematic  structures  (pks)  with  special  regard  to  the  tricept  type  pks.  apart  from  the  conventional  serial  structures  that  employ  translational or rotational movement (or a combination) of  individual driving axes. parallel structures comprise a set of telescopic  driving rods that are joined together via a solid platform. due to this fact, the mathematical model describing the relationship between  the  driving  actions  of  the  telescopic  rods  and  the  resulting  coordinates  of  the  desired  effector’s  endpoint  is  rather  complex.  to  determine  the  theoretically  achievable  positioning  accuracy  of  the  endpoint,  authors  investigated  the  theoretical  influence  of  geometrical  imperfections of the machine design and employed the  law of uncertainties propagation. the aim was to  investigate  theoretically  the  achievable  positioning  accuracy  of  the  machine,  prior  to  the  final  design  solutions,  thus  helping  the  designer  to  optimize the machine’s design.  acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 | 10  2.  reaching the desired position of the end  effector  to control the position of the tricept end effector, one must necessarily know the relation between the extensions of the individual telescopic rods and the position of the effector’s endpoint. a mathematical model of this relation is described in [7] and [8]. the positioning of the effector’s endpoint is carried out by rotating the movable platform about the axes x and y together with its shifting along the z axis. as the movement of the end point generates an irregular workspace, possible singularities and collision points must be investigated [10]. let qq be the reference point, tightly connected with the movable platform (i.e. static against point p’), whose position will be investigated. to reach the defined position of point q in the workspace, the movable platform has to turn around the x and y axes and shift along the z axis. we obtain the cartesian positions q =[qx, qy, qz] of point qq (relative to the "static" coordinate system bound with the static platform (relative to point p) by applying the next three transformations in the following order (on [qx, qy, qz]): 1. translation along the z axis (so coordinates of point qq relative to "static" coordinate system are changed to [qx, qy, qz + ze3]); 2. rotation about the x axis by the angle  (represented by orthogonal matrix ox()); 3. rotation about the y axis by the angle  (represented by orthogonal matrix oy()); where (in general) angles ,  and shift z are non-zero. after applying these three transformations we obtain the cartesian coordinates q = [qx, qy, qz] of point qq in the "static" coordinate system, relative to point p (where ,  and z are arbitrary), as a function of qx, qy, qz, ,  and z. the matrix notation of such a transformation is q = oy()  ox()  (q + ze3) that can be itemized as                                           zq q q q q q z y x z y x . cossin0 sincos0 001 . cos0sin 010 sin0cos     .(1) movement of any reference point qq with constant coordinates q[qx, qy, qz] to a new point q =[qx, qy, qz] is represented by a change of the parameters ,  and z. the required changes in the lengths of the individual telescopic rods, necessary for the implementation of such movement, are derived in [7]. let a0 be the change in length of rod aa’, the change in length of rod bb’ be a1 and the change in length of rod cc’ be a-1 (see figure 2):                       sin. 2 1 .sin 2 3 sin. 2 1 4 3 .coscos. 4 1 22221 rzzrrrzrra (2)   cos.sin.cos.22220 zrrzrra  (3)                       sin. 2 1 .sin 2 3 sin. 2 1 4 3 .coscos. 4 1 22221 rzzrrrzrra (4) where free parameters ,  and z must meet the following conditions: 22222 yxzyxz qqqqqsqz  ; 22 222 .. sin zx xxzxzx qq qqqqkqq    ; 22 222 .. cos zx zxzxxx qq qqqqkqq    ; 2222 22222222 .... sin xzyx xzxyyxzyxy qqqq qqqqkqqqqqsq    ; 2222 22222222 ... cos xzyx yyxzxxyzyx qqqq qqqqqqqqqqsk    ; constants k and s are equal to 1 in this case. 3. influences that affect the reaching of the  desired position   the positioning accuracy of any manufacturing device represents the closeness between the actual reached position of the end effector and a programmed position, specified by the control system. for pks, the effector’s endpoint is the point at the end of the central rod, respectively it is the precisely defined point on the tool or technology head [5], [6]. in our case, the point p' is considered. based on a theoretical analysis, one can summarize three types of errors affecting the reaching of the desired position by the pks effector. the geometrical errors arise due to inaccuracies in manufacturing, inaccurate relative position of individual elements or due to wear of the joints. the stiffness errors originate from elasticity of joints between individual elements as well as from flexures caused by the own weights of individual elements or by an external load. their magnitudes depend on the actual position of the effector. the thermal errors arise from thermal stress and dilatation of elements due to heat generated by internal or external sources, e.g. motors, bearings, etc. [11]-[18]. to document the complexity of the uncertainty calculation of reaching the desired position of the point q, let us summarize the list of geometrical parameters that contribute to the overall uncertainty of reaching the desired position: figure 2. schematic representation of telescopic rods, joints and platforms. acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 | 11  a) distance of joints from the centre of the fixed platform, i.e. the distances ap, bp, cp (position of joints approximated by a circle with radius r); b) relative positions of joints at the fixed platform, i.e. the distance of points ca, cb, ba; c) distance of joints against the centre of the movable platform, i.e. the distances a’p’, b’p’, c’p’ (position of joints approximated by a circle with radius r); d) distance of joints at the movable platform, ie. the distances c’a’, c’b’, b’a’; e) distance of the fixed platform and the movable one at the central rod, i.e. the distance pp’; f) if the effector is mounted, the distance between the effector’s endpoint and the point of effector fixation at the movable platform p’ (it is actually a determination of the vector q[qx, qy, qz]); g) lengths of individual telescopic rods, i.e. the distances aa’, bb’, cc’. 4. methodology for determination of the desired  position   if the device designer knows the theoretically achievable positioning accuracy, he has an important opportunity to influence critical pieces of equipment in the process of the construction work. an uncertainty balance will help to identify the most significant influences on the theoretically achievable positioning accuracy of the effector, which opens up the possibility of corrective interventions into the structure. only geometrical influences on the overall uncertainty are considered in the first phase, as shown in the further text. for the sake of simplicity we do not consider the covariances among the individual parameters. let a little change of the endpoint position be given as the product of the jacobian of the tangential displacement in the direction of motion and a dimensionless vector of rotations and displacement. this is described by the following equation:                                                          dz d d z qqq z qqq z qqq dq dq dq zzz yyy xxx z y x      . . (5) the matrix in (5) is denoted as m33. we obtain its elements later by partial derivation of (1). a marginal change of the vector of rotations and translation from (4) depends on a limit change of lengths of the telescopic rods and radii r and r, as given by the following equation:                                                                                dr dr da da da r z r z a z a z a z rraaa rraaa dz d d 1 1 0 110 110 110 .     . (6) the matrix in (6) is designated as m35. the telescopic rod lengths from (2) to (4) will be used for the calculation of its elements. their shapes are rather complicated for partial derivation, so that they will be adapted. since the lengths of the rods cannot be negative, we can square them and find their appropriate linear combinations to get the simplest relations equivalent to (2) to (4). three equations can be obtained in this way 0cos..cos.. 3 222 2 1 2 1 2 0     rrrrzrr aaa (7) 0sin.sin..sin..2 3 2 1 2 1      rrzr aa , (8) 0cos..cos..sin.cos..2 3 2 21 2 1 2 0     rrrrzr aaa . (9) let us denote the left sides of (7) to (9) as functions l1, l2, l3 that depend on parameters a0, a1, a-1, , , z, r, r. we will consider the movement of point q in time t that will be marginally close to 0 and parameters a0, a1, a-1, , , z, r, r will depend on time t as well. if partial derivation of the left sides of (7) to (9) is carried out, similar to obtaining derivatives of the implicit function (parameters , , z are derived from parameters a0, a1, a-1, r, r), the following equation is found: 0wwwmw   1553155333 (10) where                                                        sincos2)sincoscos2()sin2(sin sin2cos.sin)sin2(cos 2sinsin 333 222 111 33 rrzrzrr rrrrzr zrrrr z lll z lll z lll w                                                                                )sin2(coscos)coscos( 3 2 3 2 3 4 )sin.2(sinsinsin 3 2 3 2 0 )cos(cos2)cos(cos2 3 2 3 2 3 2 110 11 110 33 1 3 1 3 0 3 22 1 2 1 2 0 2 11 1 1 1 1 0 1 53    zrrr aaa rzr aa rrrr aaa r l r l a l a l a l r l r l a l a l a l r l r l a l a l a l w                   )0( )0( )0( )0( )0( 1 1 0 15 r r a a a      w . the relation (10) can be transformed to   0wwmw   15535333 (11) matrix m35 is calculated from (11): 53 1 3353     wwm (12) let us return to the expression of matrix m33 from (5). when multiplying (1) from left by matrix )(tyo , we get: acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 | 12  )).(().().().( 3zeqoooqo xy t y t y   (13) and subsequently 0)).(().( 3  zeqoqo x t y  (14) after multiplying of (3) we get 0sincos   zxx qqq (15) 0sin)(cos.   zqqq zyy (16) 0sin.sin.cos.cos).(   xyzz qqqzq . (17) if we denote the left sides of (15) to (17) as h1, h2, h3 and we carry out their partial derivatives, we get the following matrices:                                                     cos0sin 010 sin0cos 333 222 111 33 zyx zyx zyx q h q h q h q h q h q h q h q h q h f (18) and                                                          cossincossin)(cos sin0sincos)( 0cossin0 333 222 111 33 zxzy yz zx qqzqq qzq qq z hhh z hhh z hhh g .(19) analogically to m35, we can calculate the matrix m33 [8] as the product of matrices 133  xf and 33g . subsequently we get the matrix of sensitivity coefficients as the product of matrices m33 and m35: 533353 1 3333 1 3353 )()( xxxxxx mmwwgfa    . (20) matrix 53a in (20) is used for the calculation of estimates of uncertainties of indirectly measured parameters. the covariance matrix of those estimates is uy = a·ux·at , (21) where matrix ux in the form                  2 ,,,,, , 2 ,,,, ,, 2 ,,, ,,, 2 ,, ,,,, 2 , 5545352515 5444342414 5343332313 5242322212 5141312111 xxxxxxxxxx xxxxxxxxxx xxxxxxxxxx xxxxxxxxxx xxxxxxxxxx x uuuuu uuuuu uuuuu uuuuu uuuuu u (22) is a known covariance matrix of the random vector x = (x1, x2, x3, x4, x5) = (a0, a1, a-1, r, r), where ix u is the standard uncertainty of the estimate xi of quantity xi, i = 1, 2, … 5, jix u , is the covariance between estimates xi and xj, i = 1, 2, .., 5, j = 1, 2, .., 5. the position uncertainty of any point q in the workspace can be calculated, if the matrix ux is known. let us briefly introduce the principle how to estimate the matrix uy. the matrix a35(qx, qy, qz) represents a functional relationship of position of the reference point q, i.e. the position of the tricept. as we know the numerical values of qx, qy, qz, we can quantify the matrix of the partial derivatives of a35. let ux be a known constant symmetric matrix of 55 type, and uy be an unknown symmetric matrix of 33 type that we want to determine and is given by (21). it is clear that the matrix uy is correlated with the position of the point q[qx, qy, qz]. we want to find the intervals for values of the matrix uy, when considering that qx, qy, qz may take any value, depending on how the reference point q moves in some regular subspace (let it be a cube for purposes of this estimate, see figure 5) of the overall workspace. to analyze the impact of individual components of the known matrix ux on elements of the matrix uy, we employ the linearity of this dependence (for fixed q). we will just consider only the base symmetrical matrices ux such that there is always only one element equal to 1 (for diagonal elements; outside the diagonal we consider also symmetrically associated elements). this element or these elements are multiplied by the respective weighting coefficient for the particular matrix ux. if we fix the angles  and , a virtual beam arises in the cube, along which the reference point will move. when the reference point q moves along the beam, we want numerically determine the intervals for the values of elements of the matrix uy. each element value of the matrix uy (since it is a symmetrical matrix, 6 different elements are considered) can be precisely expressed using a formula that is represented by the sum of 6 square roots of polynomials of the variable z divided by other polynomials also dependent on z (their shape is too extensive for the length of this paper). the derivative of this formula to z can be algebraically adjusted (it shall be multiplied by analogous expressions, where we change only marks of generated roots, thus, removing roots and a multiplied derivative, obtained in this way, simplifies the polynomial). it is true that any stationary point of the original expression is also the root of this polynomial at the same time. it is sufficient to evaluate the expression for a particular beam only in the roots of this polynomial (if they overlap the workspace) and also in the endpoints of the beam, defined by the workspace borders. among them we find the minimum and maximum, which will form the search interval for the selected element of matrix uy, for fixed angles  and  and a base matrix ux. resulting intervals that represent the impact of elements of the matrix ux in the overall workspace can be obtained by uniting intervals for all permissible values of  and . to do so, a sufficiently fine division of the workspace, with a properly chosen step only in a two-dimensional area that is created by projection of the workspace on z (along rays) into the variables  and , must be considered. the impact of each element of the matrix ux can be displayed using a three-dimensional function (see figures 3 to 10). a search estimate of the matrix uy is obtained as a matrix of ordered pairs (minimum and maximum impacts of components of the matrix ux). acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 | 13  figure 3. influence of element ux[1,2] on minimum value of the matrix uy [1,1].  figure 4. influence of element ux[1,4] on maximum value of the matrix uy [1,1].  figure  5.  influence  of  element  ux[4,4]  on  maximum  value  of the matrix uy [1,1].  figure 6. influence of element ux[4,5] on minimum value of the matrix uy [1,1]. figure 7. influence of element  ux [5,5] on maximum value of the matrix  uy [1,1].  figure 8. influence of element  ux [4,4] on minimum value of the matrix  uy [1,2]. figure 9. influence of element  ux [4,4] on minimum value of the matrix  uy [2,2].  figure 10. influence of element  ux [5,5] on minimum value of the matrix  uy [3,3]. acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 | 14  they represent the range of values of elements of the matrix uy. estimates of 15 base matrices ux can be linearly combined and can thus provide an estimate for any general (non-base) symmetric matrix ux. using the software system mathematica, we created a program to search the entire workspace (or its subset thereof) and to estimate the matrix uy. to do so, the matrix ux must be specified and the required division of the workspace must be selected. for example for                              2 2 2 2 2 3/01.00000 03/01.0000 003/005.000 0003/005.00 00003/005.0 xu (23) as well as for the reference point qq, identical to point p’ (when q = [qx, qy, qz] = [0,0,0]), we found the estimate of uy valid for all possible positions of the point q over the regular space (a cube) that fully fits into the workspace:                      0.000013810×7.339410×7.2188 10×7.33940.000037010×7.0896 10×7.218810×7.08960.0000368 0.000003810×7.3394-10×7.326210×7.3394-0.000018510×7.089610×7.3262-10×7.0896-0.0000185 -6-6 -6-6 -6-6 -6-6 -6-6 -6-6 yu (24) the diagonal of the covariance matrix uy contains estimates of uncertainties squares 2 xq u , 2 yq u 2 zq u , valid over the whole considered cube (see figure 11 and table 1). 5. conclusion   this paper analyzed various issues related to the control of structures with parallel kinematics, especially that relating to the positioning accuracy. the function describing the lengthening and shortening of the individual telescopic drives and the desired setpoint is non-linear. because of this, the equations cannot be partially derived, making the uncertainty analysis unfeasible. in order to overcome this difficulty, the employment of an approach using infinite geometrical changes in the parameters is suggested. the marginal values for uncertainties were calculated here, suggesting that the achievable positioning accuracy is not constant for all setpoints within the workspace of the tricept device. further research will be focused on finishing the tricept prototype, in which the presented analysis will provide orientation for the designers to assess end effector accuracy. the investigation of the contributing sources of uncertainties and a practical verification of the results of this article remains a further challenge. acknowledgement  this paper was created within the research grant supported by the scientific grant agency of the ministry for education, grant number vega 1/0604/15. references  [1] pkm tricept, http://www.pkmtricept.com. [2] j.l. olazagoitia, s. wyatt, “new pkm tricept t9000 and its application to flexible manufacturing at aerospace industry”, in: sae technical paper, 2007. [3] g. pritschow, “parallel kinematic machines (pkm) limitations and new solutions”, annals of the cirp, 2000, 49 (1), pp. 275280. [4] j.p. merlet, parallel robots, springer series: solid mechanics and its applications, 2006, isbn 978-94-010-9587-7 [5] i. onderová, l. kolláth, “testing and verification of selected technological parameters of the pks”, proc. of 15th international carpathian control conference ieee, may 2830, 2014, velké karlovice, czech republic, pp. 398-402. [6] ľ. kolláth, m. halaj, e. kureková, “positioning accuracy of nonconventional production machines”, proc. of 19th imeko world congress, sept. 6-11, 2009, lisbon, portugal, pp. 20992102. [7] m. omachelová, i. martišovitš, e. kureková, ľ. kolláth, “analytical expression of the lengths of tricept telescopic rods ejection”, proc. of 37th international conference instruments and control, 2013, ostrava, czech republic, pp. 45-52. [8] m. omachelová, e. kureková, m. halaj, i. martišovitš, “theoretical aspects of control of the tricept type parallel kinematic structure”, proc. of 15th international carpathian control conference ieee, may 28-30, 2014, velké karlovice, czech republic, pp. 393-397. [9] s. besnard, w. khalil, “identifiable parameters for parallel robots kinematic calibration” ieee international conference on robotics and automation, 2001, pp. 2859–2865 vol (3). [10] ľ. kolláth, i. martišovitš, m. omachelová, “solving the problems of workspace in parallel kinematic structure”, annals of the faculty of engineering hunedoara, issn 1584 – 2673, 2014, pp. 15-17. [11] w. knapp, “measurement uncertainty and machine tool testing”, cirp annals – manufacturing technology, 2002, 51 (1), pp. 459-462. [12] h. schwenke, et al., “geometric error measurement and compensation of machines an update”, cirp annals manufacturing technology, 2008, 57 (2), pp. 660-675. [13] s. sartori, g.x. zhang, „geometric error measurement and compensation of machines”. cirp annals manufacturing technology, 19954, 4 (2), pp. 599–609. figure 11. scheme of the cube that represents the biggest regular object in  the workspace.  table 1. considered parameters and their uncertainties.  para‐  meter  value / mm  permissible   deviation / mm  probability  distribution  uncertainty /  mm  a0  568 to 858  0.005  rectangular  0.0029 a1  568 to 858  0.005  rectangular  0.0029 a‐1  568 to 858  0.005  rectangular  0.0029 r  330  0.010  rectangular  0.0058 r  140  0.010  rectangular  0.0058 acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 | 15  [14] a. weckenmann, “the accuracy of coordinate measuring machines”, proc. of 9th imeko world congress, 1982, may 24-28, west-berlin, pp. 266-275. [15] a. balsamo, a. meda, “geometrical error compensation of coordinate measuring systems”, nanotechnology and precision engineering, 2006, issn 1672-6030, pp.83-91. [16] m. kušnerová, j. valíček, m. harničárová, t. hryniewicz, k. rokosz, z. palková, v. václavík, m. řepka, m. bendová, “a proposal for simplifying the method of evaluation of uncertainties in measurement result”, measurement science review, 2013, issn 1335 – 8871, 13 (1), pp. 1-6. [17] k. ostrowska, a. gaska, j. sladek, „determining the uncertainty of measurement with the use of a virtual coordinate measuring arm” the international journal of advanced manufacturing technology, issn 1433-3015, 2014, 71 (1-4), pp.529-537. [18] s. aguado, j. santolaria, d. samper, j.j. “aguilar, influence of measurement noise and laser arrangement on measurement uncertainty of laser tracker multilateration in machine tool volumetric verification” precision engineering, elsevier, issn 01416359, 2013, (4), pp. 929-943. electroencephalographic-based wearable instrumentation to monitor the executive functions during gait: a feasibility study acta imeko issn: 2221-870x june 2023, volume 12, number 2, 1 6 acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 1 electroencephalographic-based wearable instrumentation to monitor the executive functions during gait: a feasibility study pasquale arpaia1,2, renato cuocolo3, paolo de blasiis4, anna della calce1, allegra fullin3,4, ludovica gargiulo1, luigi maffei1, nicola moccaldi1,5 1 department of electrical engineering and information technology (dieti), university of naples federico ii, naples, italy 2 interdepartmental center for research on management and innovation in healthcare (cirmis), university of naples federico ii, naples, italy 3 department of advanced biomedical sciences, university of naples ''federico ii'', via sergio pansini 5, 80131 naples, italy 4 department of mental and physical health and preventive medicine, section of human anatomy, university of campania "luigi vanvitelli", 80138 naples, italy 5 department of engineering for innovation, university of salento, lecce, italy section: research paper keywords: executive function monitoring; wearables; brain computer interface; dual task; electroencephalographic features citation: pasquale arpaia, renato cuocolo, paolo de blasiis, anna della calce, allegra fullin, ludovica gargiulo, luigi maffei, nicola moccaldi, electroencephalographic-based wearable instrumentation to monitor the executive functions during gait: a feasibility study, acta imeko, vol. 12, no. 2, article 10, june 2023, identifier: imeko-acta-12 (2023)-02-10 section editor: francesco lamonaca, university of calabria, italy received january 27, 2023; in final form march 2, 2023; published june 2023 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: pasquale arpaia, e-mail: pasquale.arpaia@unina.it 1. introduction the performance of most motor tasks requires the use of cognitive resources [1]. cognitive resources, also called executive functions (efs), are a set of neurocognitive processes necessary for the organization, planning and regulation of daily life actions [2]. according to diamond et. al [3], the basic efs are working memory, i.e., the ability to keep in mind information, inhibition, i.e., the ability to interrupt a motor and/or cognitive response no longer appropriate to the circumstances, and the cognitive flexibility, i.e., the ability to adapt to rapidly varying circumstances. the more demanding an activity, the greater the cognitive demand. humans are involved in the simultaneous performing of several activities most of the time. it is common for humans to walk and talk on their mobile phones at the same time, or play sports and listen to music. according to the attentional capacity theory, humans have limited cognitive capacity, so when the performed activities require cognitive resources that exceed those available, there is a decrease in performance quality [4]. moreover, if a task is too demanding and the required brain resources exceed a certain threshold, cognitive processing of further tasks may be precluded [5]. therefore, understanding how cognitive resources are allocated during tasks is crucial and abstract a feasibility study on electroencephalographic monitoring of executive functions during dual (motor and cognitive) task execution is presented. electroencephalographic (eeg) signals are acquired by means of a wearable device with few channels and dry electrodes. the light weight and wireless device allow for walking in a natural way. the most significant eeg features are investigated to classify different levels of activation for two fundamental executive functions (ef) both in sitting and walking conditions. power spectral density in the gamma band resulted in the most relevant feature in discriminating low and high levels of inhibition. power spectral density in the beta and gamma bands resulted the most discriminating the level of activation of working memory. the study poses the basis for (i) monitoring the activation levels of ef during gait allowing loss prevention in the elderly and (ii) specific cognitive rehabilitation aimed at the most relevant executive functions during walking. mailto:pasquale.arpaia@unina.it acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 2 allows improving the user-task interaction by adapting the task to the cognitive state [6]. the cost of performing a dual task is greater in the elderly and in people with neurodegenerative conditions; therefore, monitoring how cognitive resources are allocated in a dual task context is particularly useful. in these people, cognitive resources and the ability to allocate them properly are limited. therefore, monitoring a condition of cognitive resources fatigue makes it possible both to predict risks (i.e., risk of falling), and to set up rehabilitation interventions aimed to strengthen the cognitive resources and their correct allocation [4]. neurophysiological measures are particularly suitable for monitoring cognitive resource fatigue in a dual-task context as they provide direct and continuous information on the subject’s brain state. among them, electroencephalographic (eeg)-based methods are becoming increasingly important due to their high temporal resolution and good real-time performance [7]. moreover, wearable devices are becoming more and more reliable in signal acquisition [8], [9], also allowing monitoring of eeg outside laboratories and clinical settings [10], [11], [12], [13]. several bio-markers linked to the activation and fatigue of the basic executive functions are proposed in the literature. in particular, eeg power spectral density (psd) have been extensively studied [14], [15] in fz, cz, pz, fz, c3 [16], [17], [18], [19] but the link between activation and fatigue of a particular executive function and the psd is not yet unambiguously defined. this study investigates the neural correlates of executive functions during a dual task execution. the use of a highly wearable device minimizes the interferences on the execution of the ambulation in a spontaneous way. in particular, the aims are (i) eeg-based identification of working memory and inhibition activation during walking, and (ii) the investigation of the most informative eeg features about activation levels of a specific executive function. 2. material and methods 2.1. experimental sample five healthy males voluntarily chose to participate in this study. the following inclusion criteria were: age (20, 30), bmi < 25 kg/m², absence of pain, no surgeries in the last 6 months, no muscle-skeletal injuries in the last 3 months, no skeletal dysmorphism and no cognitive impairment. they were all righthanded and able to understand the purpose of this study. all the volunteers were informed in detail about the objectives of the project. subjects authorized inclusion in the study by signing the informed consent form. in accordance to the declaration of helsinki, ethical approval was obtained from the ethics committee of psychological research of university of naples federico ii. 2.2. hardware and software eeg data acquisition were acquired by the abmedica helmate system. it is a wireless class ii a device used for eeg signal measurements [20]. it consists of an ultralight foam helmet capable of performing eeg-signal acquisition by means of dry electrodes. helmate provides 10 dry electrodes arranged according to the international 10/20 positioning system: fp1, fp2, fz, cz, c5, c6, o1, o2, afz (ref), and fpz (ground). a custom software was realized to provide the cognitive tasks and monitor and store the eeg signal [18]. quantitative evaluation of walking was performed using an optoelectronic system composed of eight smart-d cameras (bts bioengineering, italy) set at a frequency of 100 hz and two force platforms (bts bioengineering-milano, italy). helen hayes m.m. markers set protocol was used for 3d-stereophotogrammetric analysis [21]. 2.3. experimental protocol participants entered a very quiet room and were explained the experimental protocol. after the subject sat down on a comfortable chair, the eeg-acquisition system was set up. the subject held a wireless controller with his right hand to perform cognitive tasks. the wireless controller was used instead of the vocal response to minimize muscular artifacts. cognitive tasks employed for this study were: • go-no go task: participants were required to either respond (i.e., pressing designated key) or inhibit a response (not pressing designated key) depending on whether a go stimulus or a no-go stimulus was presented. the difficulty of the task increased by reducing the time between stimuli. the task predominantly activates inhibition. • n-back task: participants were presented a sequence of stimuli (i.e., a sequence of letters) one-by-one. for each stimulus, they needed to decide if the current stimulus was the same as the one presented n trials before. the load factor n varied according to the difficulty of the task. the task predominantly activates working memory. task stimuli were provided acoustically to allow the subject to perform the task in motion. the study was divided into ten experimental trials for a total duration of approximately one hour. the recording of the eeg signal was carried out during the following conditions: • while the subject was seated and relaxed, preventing head movements. (1 minute); • while sitting and performing go-no go task at the first level of difficulty (5 minutes); • while sitting and performing go-no go task at the second level of difficulty (5 minutes); • while sitting and performing n-back task at the first level of difficulty (1 minute); • while sitting and performing n-back task at the second level of difficulty (1 minute); • while walking naturally (1 minute); • while walking and performing go-no go task at the first level of difficulty (5 minutes); • while walking and performing go-no go task at the second level of difficulty (5 minutes); • while walking and performing n-back task at the first level of difficulty (1 minute). while walking and performing n-back task at the second level of difficulty (1 minute). the kinematic patterns were assessed for each subject, while walking barefoot on a 6 m walkway at a self-selected normal paced speed. at the end of the experiment, the subjects were asked to fill in two questionnaires: • system usability scale (sus) to assess the usability of a system. it consists of a 10-item questionnaire with five response options, ranging from strongly agree to strongly disagree; the nasa task load index (nasa-tlx) for the workload assessment. it consists of a 6-item questionnaire, rated within a 100-points range. the workload is rated across six dimensions (mental demand, physical demand, temporal demand, effort, acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 3 performance, frustration level) to determine an overall workload rating. 2.4. eeg data processing in the pre-processing phase, data were filtered by a fourth order bandpass butterworth filter [0.5 45] hz and the artifact subspace reconstruction (asr) [22] procedure was applied with a cut off equal to 15 to remove artifacts. the asr is a component-based artifact removal method suitable for removing transient or large-amplitude artifacts. the asr decomposes a signal into components, then automatically identifies a threshold, based on the distribution of the signal variance; finally, it rejects the noisy components above the threshold and reconstructs the signal by considering the remaining components. then eeg tracks are divided into 1–s epochs and psd in alpha ([8-13] hz), theta ([4-8] hz), beta ([13-20] hz), gamma ([30-45] hz) and delta ([1-4] hz) bands is computed. in the features selection phase, the sequential feature selection (sfs) [23] is applied in order to identify the most significant features to discriminate among different conditions. the support vector machine is the classifier adopted within the sfs. in particular, the selection of the most significant features is carried out for the following cases: (i) sitting baseline and walking baseline, (ii) sitting baseline and sitting easy n-back execution, (iii) sitting baseline and sitting difficult n-back execution, (iv) sitting easy n-back execution and sitting difficult nback execution, (v) sitting baseline and sitting easy go-no go execution, (vi) sitting baseline and sitting difficult go-no go execution, (vii) sitting easy go-no go execution and sitting difficult go-no go execution, (viii) walking baseline and walking easy n-back execution, (ix) walking baseline and walking difficult n-back execution, (x) walking easy n-back execution and walking difficult n-back execution, (xi) walking baseline and sitting easy go-no go execution, (xii) walking baseline and walking difficult go-no go execution, and (xiii) walking easy go-no go execution and walking difficult go-no go execution. finally, the acquired data were divided into 1-s epochs, organized in the form [epoch x channel x band] and were given in input to the sfs. 3. results the sfs results are reported in the colour-map of figure 1, figure 2, figure 3, figure 4 and figure 5 for each comparison. for each binary problem, channels are coloured if a band resulted the most discriminating for at least 3 out of 5 subjects. motor performances did not statistically differ between conditions. this could be due to the small sample size. also on cognitive performances, no statistical relevant differences emerged. however, results (table 1) confirmed a high level of engagement in performing the required tasks. the overall mean accuracies for discriminating difficulty levels were (63.7 ± 7.8) % and (63.9 ± 6.3) % for go-no go and n-back, respectively. 4. discussion according to the literature, psd in gamma and beta band increases in all cognitive processes requiring attention [24], [25] and activation of mnestic processes [26], [27]. in figure 1, results on the most discriminating features between a condition of motor inactivity and one of motor activity are shown. psd in the gamma band turns out to be the most discriminating feature in 3 out of 5 subjects in fp1, fp2, fz, c3, c4, cz and o2. motor activity requires activation of the areas responsible for walking and movement. an increase in power spectral density in the fast bands (beta and gamma) in the motor area and in the prefrontal area is expected when a subject is involved in a motor activity. therefore, the results are consistent with expectations. in figure 2, the betaand gamma-band psds best discriminate the resting condition from that of cognitive activation due to the performance of the n-back. these results are consistent with expectations since the nback is a concentration and memory exercise and the beta band is linked to attention [25], concentration and motor programming [28], [29] while the gamma band is related to memory processes [26], [27]. the n-back also requires acoustic attention, therefore, some form of activation is expected in the brodmann area presiding over acoustic signal processing. in particular, an increase in the fast bands (beta and gamma) resulted at c3 and c4, the available channels nearest to the temporal area, responsible for processing acoustic information. in the occipital area, the psd in alpha band emerges. the alpha rhythm is considered a thalamus-cortical or cortico-cortical interaction rhythm [30] and it is likely that a decrease in the alpha band occurs as a result of the cognitive engagement in demanding tasks [31]. as far as the two difficult level of go-no go task execution and its comparisons with the sitting baseline is concerned (figure 3), the most significant feature is the psd in the gamma band. this result is also consistent with the previous statements. figure 1. sequential feature selector color-map in comparing sitting baseline and walking baseline. table 1. cognitive task performances. task condition percentage of correct answers (%) easy go-no go sitting walking 99 ± 1 100 ± 0 difficult go-no go sitting walking 99 ±2 97±2 easy n-back sitting walking 98 ± 2 97±3 difficult n-back sitting walking 94 ±2 92 ± 6 acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 4 in the case of the comparison between the baseline acquired during walking and the n-back and the n-back tasks at two level of difficulties performed in motion (figure 4), the discriminability between the various conditions seems to be related also to the delta and theta bands as well as alpha, gamma and beta. the delta-band psd variation is probably due to motion artefacts, and frontal theta-band psd variation could also be due to eye movements. therefore, as far as memory is concerned, no discriminative elements emerge during walking and the function is not stressed enough to stand out against the background noise. the noise conditions between the two experiments were statistically comparable after a t-test application (p-value = 0.27). finally, as far as the comparison between the two level of difficulties of gono go performed in walking and the walking baseline comparison is concerned, the psd in gamma and beta bands in the frontal and motor area are the most discriminative features, consistently with what emerges when the subject is seated. however, the experiment is not pure from executive function overlaps that are not the subject of this study, because each proposed task does not only stimulate memory or inhibition. in fact, the acoustic channel is also employed by the proposed tasks, and thus there is a co-activation of neuronal systems of various areas that communicate with each other, e.g. the brodmann area that processes the acoustic signal. in the case of walking, it can be concluded that the activation of memory is not discriminable while that of the inhibition function emerges more clearly. due to limitations of the eeg device, some areas, such as temporal areas, responsible for mnestic processing, are not available. in a future study, psd in beta2 band ([21-30] hz) will be focused on, as it is the one mainly related to mnemonic and attentional processes. in the future, it could be interesting to exploit more wearable sensors for monitoring also gait [32] and/or further biosignals [33], [34]. 5. conclusions the neural correlates of executive functions were investigated during dual task execution. the use of a highly wearable device guaranteed the execution of the ambulation in a spontaneous way. different degrees of involvement for working memory and inhibition were classified by means of different machine learning algorithms. a sensitivity analysis revealed the eeg features maximizing the classification accuracy. power spectral density in the gamma band is the most relevant feature in discriminating between low and high levels of inhibition. an equally high level of discrimination between high and low levels of activation of working memory is not reached. this study is a first step towards the implementation of a fall prevention system and represents a contribution to a more targeted definition of cognitive rehabilitation interventions. further studies will be carried out by using devices with more electrodes also in the temporal areas to better monitor mnestic processes and by focusing on beta2 band. acknowledgement this work was carried out as part of the “intense” project, which was financially supported by the italian ministry of economical development, (mise). the authors thanks andrea pollastro for the software implementation. references [1] h.-j. huang, v. s. mercer, dual-task methodology: applications in studies of cognitive and motor performance in adults and children, pediatric physical therapy 13(3) (2001), pp. 133-140. online [accessed 20 february 2023] https://journals.lww.com/pedpt/_layouts/15/oaks.journals/do wnloadpdf.aspx?trckng_src_pg=articleviewer&an=00001577200113030-00005 [2] c. l. carlson, m. mann, attention-deficit/hyperactivity disorder, predominantly inattentive subtype, child and adolescent psychiatric clinics of north america 9(3) (2000), pp. 499–510. doi: 10.1016/s1056-4993(18)30103-2 [3] a. diamond, executive functions, annual review of psychology 64 (2013), pp. 135–168. 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and validation of an e-textile based wearable system for remote health monitoring, acta imeko 10(2) (2021), pp. 220-229. doi: 10.21014/acta_imeko.v10i2.912 [34] p. arpaia, a. cataldo, s. criscuolo, e. de benedetto, a. masciullo, r. schiavoni, assessment and scientific progresses in the analysis of olfactory evoked potentials, bioengineering 9.6 (2022): 252. doi: 10.3390/bioengineering9060252 https://doi.org/10.1073/pnas.1913092116 http://dx.doi.org/10.1093/acprof:oso/9780195050387.003.0010 https://doi.org/10.21014/acta_imeko.v7i4.575 https://doi.org/10.21014/acta_imeko.v10i2.912 https://doi.org/10.3390/bioengineering9060252 microsoft word 301-2190-1-le-rev2 acta imeko  issn: 2221‐870x  april 2016, volume 5, number 1, 51‐58     acta imeko | www.imeko.org  april 2016 | volume 5 | number 1 | 51  magnetic fields influence on sensors with electrical output  under sinusoidal excitations  nieves medina 1 , jesús de vicente 2 , jorge robles 1  1  centro español de metrología, calle alfar 2, tres cantos, 28760 madrid, spain  2  escuela técnica superior de ingenieros industriales, calle de jose gutierrez abascal 2, 28006 madrid, spain      section: research paper   keywords: magnetic effects; shaker; dynamic sensor  citation:  nieves  medina,  jesús  de  vicente,  jorge  robles,  magnetic  fields  influence  on  sensors  with  electrical  output  under  sinusoidal  excitations,  acta  imeko, vol. 5, no.1, article 10, april 2016, identifier: imeko‐acta‐05 (2016)‐01‐10  editor: paolo carbone, university of perugia, italy; section editor: marco tarabini, politecnico di milano, italy  received december 17, 2015; in final form march 11, 2016; published april 2016  copyright:  ©  2016  imeko.  this  is  an  open‐access  article  distributed  under  the  terms  of  the  creative  commons  attribution  3.0  license,  which  permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: this work was supported by the european research metrology program.  corresponding author: nieves medina, e‐mail: mnmedina@cem.minetur.es      1. introduction  this paper describes the magnetic effects studied at cem in their realization of a primary standard for dynamic force calibration using sinusoidal excitations of force transducers. this work was part of a project called “traceable dynamic measurement of mechanical quantities” financed by the european union under the european research metrology program [1]. this standard is based on the direct definition of force as mass times acceleration. the transducer is loaded with different calibrated masses and different accelerations are generated by a vibration shaker system. the acceleration is measured by a laser vibrometer traceable to the unit of length (laser wavelength). the laser vibrometer (polytec clv 2534) is placed over the shaker (lds 726 with power amplifier pa 2000, which can work from 5 hz up to 2400 hz) by means of a special table designed for this purpose, see figure 1 for reference. being a fully dynamic measurement it requires a multichannel data acquisition system in real time. a ni pxi 1033 module with a 4462 card (24 bits, 204.8 ks/s) has been used. the implemented software, which is programmed in labview, samples the signals separately with a speed of 40 ks/s and applies the sine approximation method in order to figure  1.  overview  of  the  standard  for  dynamic  force  calibration  using  sinusoidal excitations.   abstract  this paper describes the magnetic effects studied at cem in their realization of a primary standard for dynamic force calibration using  sinusoidal  excitations  of  force  transducers,  although  they  can  also  affect  any  sensor  with  an  electrical  output  mounted  on  an  electrodynamic shaker. in this study the electromagnetic behaviour for the interaction between sensor and shaker or a similar source  of magnetic fields is explained and a solution to minimise this interaction is also included.   acta imeko | www.imeko.org  april 2016 | volume 5 | number 1 | 52  determine the signals amplitudes and phases in real time. the sensor is characterised by its dynamic sensitivity, which is the ratio of its electrical output signal of the force transducer and the acting dynamic force. the sensitivity phase shift is determined as the phase difference between the sensor output and the laser vibrometer output. the required masses for generating the forces on the sensors have been manufactured and calibrated to determine their mass and their corresponding uncertainty. the masses have nominal values 347 g, 1 kg, 2 kg, 7.3 kg and 12.3 kg. the three smaller masses are screwed to the sensor under calibration; the bigger ones are connected under pressure by means of a special adaptor. depending on the sensor to be calibrated, special adaptors may be required in order to screw the masses to the sensor or the sensor to the shaker. different corrections and influence factors have to be taken into account for this standard. references [2] and [3] provide complete information about this development. 2.  description  the parameter that characterises the force transducer is the sensitivity s, which is defined as the ratio of the electrical output signal of the transducer to the acting dynamic force. its module can be determined as   amm v u s   i , (1) where u is the output of the conditioning amplifier, v is the amplification factor of the conditioning amplifier, m is the mass for loading the transducer, mi is the internal mass of the transducer that contributes as a load and a is the acceleration measured by the laser vibrometer. the sensitivity phase is determined as the phase difference between the sensor output and the laser vibrometer output. the possible effect on the phase of the conditioning amplifier is considered negligible. this work arises from the study of the behaviour of the sensor sensitivity at low frequencies. in principle, the lower the frequency the behaviour should be increasingly closer to the static behaviour, that is, the sensor sensitivity must remain constant. this sensitivity behaviour is as expected for heavy loads, but not for small loads. in fact, the smaller the load, the sensitivity variation increases. in this work the sensitivity behaviour has been studied in the range from 5 hz to 200 hz. this study was conducted for different excitation accelerations according to the possibilities of the vibration shaker and for several sensors with different size and working principle, resistive or piezoelectric. figure 2 to figure 6 are examples of this kind of behaviour for different sensors with two different working principles. there is no effect on the phase shift for the piezoelectric sensor, so no plot is shown. the expanded uncertainty estimation is 0.5 % for the sensitivity modulus divided by the sensitivity modulus for 200 hz and 0.5º for the sensitivity phase shift. the results of this study can be summarised as follows. the first issue that has to be remarked is the fact that these effects are not dependent on the acceleration. they do not depend on either its magnitude or the chosen reference to measure it, because the same results were obtained using the laser vibrometer or a reference accelerometer. they only appear for low frequencies, typically less than 40 hz. there is a case, the hbm u9b sensor, which is a resistive sensor, where this effect does not occur. the only difference that distinguishes it from the other tested resistive sensors is its small size, so it is clear that size may have an influence. these effects increase as the load decreases. in fact it is more important when the sensor is not loaded. 0.80 0.85 0.90 0.95 1.00 1.05 0 20 40 60 80 100 120 140 160 180 200 frequency /hz s e n s it iv it y m o d u lu s / s e n s it iv it y m o d u lu s f o r 2 0 0 h z without load 0.35 kg 1 kg figure  2.  plot  showing  the  sensitivity  modulus  divided  by  the  sensitivity  modulus  for  200  hz  versus  excitation  frequency  for  the  interface  1610  sensor (resistive sensor) for the cases: without load, 0.35 kg and 1 kg.    0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 0 20 40 60 80 100 120 140 160 180 200 frequency /hz s e n s it iv it y m o d u lu s / s e n s it iv it y m o d u lu s f o r 2 0 0 h z without load 0, 35 kg 1 kg figure  3.  plot  showing  the  sensitivity  modulus  divided  by  the  sensitivity  modulus  for  200  hz  versus  excitation  frequency  for  the  hbm  u2b  sensor  (resistive sensor) for the cases: without load, 0.35 kg and 1 kg.    0.95 1.00 1.05 1.10 1.15 1.20 1.25 1.30 1.35 1.40 1.45 0 20 40 60 80 100 120 140 160 180 200 frequency /hz s e n s it iv it y m o d u lu s / s e n s it iv it y m o d u lu s f o r 2 0 0 h z without load 0.35 kg 1 kg figure  4.  plot  showing  the  sensitivity  modulus  divided  by  the  sensitivity  modulus  for  200  hz  versus  excitation  frequency  for  the  kistler  9175b  sensor (piezoelectric sensor) for the cases: without load, 0.35 kg and 1 kg.    -5 0 5 10 15 20 25 30 35 0 20 40 60 80 100 120 140 160 180 200 frequency /hz s e n s it iv it y p h a s e s h if t /º without load 0.35 kg 1 kg figure 5. plot showing the sensitivity phase shift versus excitation frequency  for  the  interface  1610  sensor  (resistive  sensor)  for  the  cases:  without  load, 0.35 kg and 1 kg.   acta imeko | www.imeko.org  april 2016 | volume 5 | number 1 | 53  the fact that these effects are not dependent on the acceleration and how it is measured, decrease with excitation frequency and load and decrease with the sensor size, indicates that they cannot be dynamic effects such as rocking motion or resonances. it is therefore thought that an interaction between sensor and vibration shaker may be a possible explanation for these effects. the operation principle of the electrodynamic shaker (the armature moves because of the lorentz force) makes magnetic fields presence necessary for its operation, so a kind of magnetic effect is thought to be a good candidate for a possible explanation. 3. understanding the effects  as a first step the magnetic field in contact with the centre of the shaker armature (where the sensor is connected) was measured obtaining a value of 2.3 mt. at this same position but 15 cm higher the magnetic field is less than 0.5 mt. these fields are relatively small and are within the specifications of the shaker itself. in order to check whether the observed effects are actually caused by magnetic fields, a large magnetic field generated by a large permanent magnet was put close to the sensor. the intention is to magnify these effects by the presence of the magnet. as a first attempt the magnet was near the sensor and the sensor was connected to the shaker. sensor tests were then performed in the range from 5 hz to 200 hz with the sensor moving and the magnet hanging in a stationary position. these tests were carried out at several distances, different accelerations and always with the sensors unloaded, so that their output could only be influenced by magnetic effects. as a result no different effects from the ones that had already been observed could be seen. in the second attempt (figure 7) it was decided to reverse the sensor-magnet configuration and connect the magnet to the shaker, so it moves with it, and leave the sensor hanging to remain static. with this new configuration the effects presented from figure 8 to figure 18 were obtained. these results were completely unexpected because an unloaded sensor in a static position should have a constant output (zero output) that should not depend on other external factors. on the contrary, these results show a clear dependency on excitation frequency: for the resistive sensors as the inverse of the frequency (figure 12, figure 13 and figure 14) and for the piezoelectric sensor as the inverse of the frequency squared (figure 15). for the hbm u9b sensor this effect is much smaller and there is only a clear influence for frequencies less than 10 hz (figure 14). it was also obtained that for frequencies more than 200 hz no effect was observed. according to figure 16, figure 17 and figure 18 it can be deduced that the sensor’s output phase shift is -90° for resistive sensors and 0° for piezoelectric sensors ( 180º, due to the indeterminacy of the arc tangent function used for calculating the phase). the effect on the phase shift for the hbm u9b sensor is negligible, so no plot is shown. -10 0 10 20 30 40 50 60 0 20 40 60 80 100 120 140 160 180 200 frequency /hz s e n s it iv it y p h a s e s h if t/ º without load 0,35 kg 1 kg figure 6. plot showing the sensitivity phase shift versus excitation frequency for  the  hbm  u2b  sensor  (resistive  sensor)  for  the  cases:  without  load, 0.35 kg and 1 kg. sensor magnet armature sensor magnet armature   figure  7.  photograph  showing  the  sensor‐magnet  configuration  in  the  second attempt: the sensor is hanging, without any acting force on it, and the  magnet  is  moving  sinusoidally  versus  time  as  it  is  attached  to  the shaker.  0 10 20 30 40 50 60 0 20 40 60 80 100 120 140 160 180 200 frequency /hz s e n s o r o u tp u t m o d u lu s / 1 05 v figure. 8. plot showing sensor output modulus versus excitation frequency  for interface 1610 sensor (resistive sensor).  0 20 40 60 80 100 120 0 20 40 60 80 100 120 140 160 180 200 frequency /hz s e n s o r o u tp u t m o d u lu s /1 05 v figure 9. plot showing sensor output modulus versus excitation frequency for hbm u2b sensor (resistive sensor).  0 2 4 6 8 10 12 14 16 18 0 20 40 60 80 100 120 140 160 180 200 frequency /hz s e n s o r o u tp u t m o d u lu s / 1 05 v figure 10. plot showing sensor output modulus versus excitation frequency  for hbm u9b sensor (resistive sensor).  acta imeko | www.imeko.org  april 2016 | volume 5 | number 1 | 54  these measurements are more reliable at low frequencies, where the results are closer to constant values; but generally these constant values can be extrapolated to the entire measurement range, as their measurement errors are related to the fact that the sensor’s output amplitude will be much lower as the excitation frequency increases. the conclusion from these experiments is that very similar effects as the ones previously observed were obtained, but magnified. this is due to the high intensity of the magnetic field generated by the moving magnet; but the most important achievement has been to prove that these effects were caused by a magnetic field whose intensity close to the sensor varies sinusoidally with time. in this case the distance between the moving magnet and the sensor varies sinusoidally with time and, therefore, the magnetic field intensity generated close to the sensor varies accordingly.  4. theoretical justification  the operating principle of the electrodynamic shaker, in order to achieve a sinusoidal motion of its armature, is based in the lorentz force law. a coil with n turns with length l and a current i passing through, which varies sinusoidally with 0 10 20 30 40 50 60 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 frequency-1/hz-1 s e n s o r o u tp u t m o d u lu s /1 0 -5 v figure  12.  plot  showing  sensor  output  modulus  versus  the  inverse  of  the frequency for interface 1610 sensor (resistive sensor).    0 20 40 60 80 100 120 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 frequency-1/hz-1 s e n s o r o u tp u t m o d u lu s /1 0 -5 v figure  13.  plot  showing  sensor  output  modulus  versus  the  inverse  of  the frequency for hbm u2b sensor (resistive sensor).    0 2 4 6 8 10 12 14 16 18 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 frequency-1/hz-1 s e n s o r o u tp u t m o d u lu s / 1 0 -5 v figure  14.  plot  showing  sensor  output  modulus  versus  the  inverse  of  the frequency for hbm u9b sensor (resistive sensor).    0 1 2 3 4 5 6 7 8 9 10 0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 frequency-2/hz-2 s e n s o r o u tp u t m o d u lu s / 1 04 p c figure  15.  plot  showing  sensor  output  modulus  versus  the  inverse  of  the frequency squared for kistler 9175b sensor (piezoelectric sensor).  -120 -100 -80 -60 -40 -20 0 0 20 40 60 80 100 120 140 160 180 200 frequency /hz s e n s o r o u tp u t p h a s e s h if t /º figure 16. plot showing sensor output phase shift versus the frequency for interface 1610 sensor (resistive sensor).    -130 -120 -110 -100 -90 -80 -70 -60 -50 0 20 40 60 80 100 120 140 160 180 200 frequency /hz s e n s o r o u tp u t p a h s e s h if t /º figure 17. plot showing sensor output phase shift versus the frequency for hbm u2b sensor (resistive sensor).    -20 0 20 40 60 80 100 120 0 10 20 30 40 50 60 70 80 90 100 frequency /hz s e n s o r o u tp u t p h a s e s h if t /º figure 18. plot showing sensor output phase shift versus the frequency for  kistler 9175b sensor (piezoelectric sensor).  0 1 2 3 4 5 6 7 8 9 10 0 20 40 60 80 100 120 140 160 180 200 frequency /hz s e n s o r o u tp u t m o d u lu s / 1 04 p c figure 11. plot showing sensor output modulus versus excitation frequency for kistler 9175b sensor (piezoelectric sensor).  acta imeko | www.imeko.org  april 2016 | volume 5 | number 1 | 55  excitation frequency , is attached to the shaker armature. this coil is immersed within a static magnetic field with magnetic flux density b. this is necessary so that the armature can move according to the lorentz force law: bif  tt nl  jj ee . (2) the magnetic flux density b is generated by another coil with a direct current passing through it. there is another coil called "degauss" coil that counteracts the effects of b in the environment. these magnetic fields are static and have no effect on the measurement. the frequency dependent magnetic field that could explain these effects comes from the current i(), which passes through the coil. this magnetic field b'() will vary with the excitation frequency and comes from the application of biot and savart law [4]:    2 j0j de π4 e r ni rtt ul b   , (3) where r is the distance between the coil and the place where the magnetic flux density is evaluated and l is the length of each turn. using cylindrical coordinates (r, z,) i will only have one component i, so the magnetic field b’ can only have two components b’z and b’r. the effect of this sinusoidal magnetic field produces a current density j'() at the same time, which also varies sinusoidally, see figure 19. if the medium that generates this output can be assumed as isotropic and homogeneous as a first approximation, maxwell equations can be applied as follows [5], where the medium is considered to have permeability  and conductivity  and the fields variation with time is sinusoidal with frequency , jb bj    j . (4) the solution for this system of equations is rather complex. the current density j’ will only have one component, j’, as b’ has two components, b’z and b’r. this solution is given by (5),         rzkzrj 21121 1jjjexp ,   (5) where j1 is the bessel function for first kind, and k and  are constants. in order to obtain this solution it has been taken into account that, as long as the sensor is far from the source of the electromagnetic field (either z or r increase) the induced current density should become zero. it is also required that  > 1 in order to avoid divergent solutions. as a consequence, the induced current will have one single component along the  direction and its module will be given as the solution for (6),   dsji  . (6) the solution for the previous integral is given by the following expression (7),            1-j 1jjjexp 210 21 rzk i   . (7) this solution applies to each surface element of the medium that generates the sensor output. it is obvious then that the induced current will depend on this medium size. in order to study the modulus behaviour of the induced current i as a function of the excitation frequency , the function y(x) has been studied. it is defined according to (8):     x x xy x 21 0je 21    . (8) the plots for this function versus x and 1/x are given in figure 20 and figure 21, respectively. for figure 20 it is clear that the same behaviour is observed as in figure 8, figure 9, figure 10 and figure 11. for figure 21 it is also clear that the observed behaviour is the same as in figure 12, figure 13, figure 14 and figure 15. firstly the function value is zero but as 1/x increases (or x decreases) there is a clear linear dependency. as a consequence and for practical purposes, for excitation frequencies where an effect is observed, the dependency in (9) is expected,  j 1 i . (9) y(x) x y(x) x figure 20. plot of y(x) versus x.      y(x) 1/x y(x) 1/x figure 21. plot of y(x) versus 1/x.  r z i b’ i’ r z i b’ i’   figure 19. schematic diagram showing the current i that passes through a turn of the shaker coil, the magnetic field lines for b’ and the subsequent induced current i’ in the sensor.  acta imeko | www.imeko.org  april 2016 | volume 5 | number 1 | 56  the result shown in (9) is very important and it is indeed what explains the observed effects. the induced current modulus is proportional to the inverse of the excitation frequency and its phase shift is -90º. as it applies to each surface element of the medium that generates the sensor output, it also explains why this effect depends on its size. most force sensors comprise an elastic element that deflects with the force and a sensing element fixed to the elastic element, which is deformed as the elastic element deflects. the sensing element in the case of a resistive sensor consists of four strain gauges in a wheatstone bridge configuration. it measures the deformation (strain) as a change in electrical resistance, which is a measure of the strain and hence the applied force. the strain gauges and their corresponding electrical connections are electrically isolated from the elastic element and they are the media that generates the sensor output, so the output will depend directly on their size. in most cases resistive sensors with the same type but different capacity only differ on the size of the elastic element, so the expected effect will be the same for any of them. the result shown in (9) is clearly what was obtained when the magnet was connected to the shaker and the sensor was hanging freely over it. although, when only the shaker is considered, the generated magnetic field is smaller and the sensor sensitivity dependency with the inverse of the excitation frequency may not be so clear. on the other hand, the strain gauges that generate the sensor output may not fulfil homogeneous and isotropic conditions in full. all of the previous statements make sense for resistive sensors. since their impedance is basically resistive, their output will be directly proportional to the induced current. in the case of piezoelectric sensors, the observed output depends on the inverse of the frequency squared. this case is also justified because these sensors direct outputs are not currents but charges. in general, the charge q that is generated by a sinusoidal current i with amplitude i0 is given by (10):   j e j0 i dtidtiq t   . (10) this result indicates that the charge generated by the current induced by the sinusoidal magnetic field is itself inversely proportional to frequency and with a -90º phase shift. moreover, it must be pointed out that, in the current use of the sensor with load and no influence of non static magnetic fields, currents are never generated and only charges which are proportional to the force are generated. as a consequence, this dependency with frequency and this -90º additional phase shift are never observed. another way to express this fact is to say that the piezoelectric impedance is basically capacitive (z = 1/jc). therefore, if the current induced by the non static magnetic effects depends on the inverse of the excitation frequency with a -90º additional phase shift, the output voltage induced by this effect in a piezoelectric sensor will show a dependency as the inverse of the excitation frequency squared and a 180° total phase shift (or 0° due to the indeterminacy of the arc tangent function used for determining the phase shift). the sensing element in the case of piezoelectric sensors is a piezoelectric material. piezoelectric materials have very low conductivity, so it is very unlikely these materials may generate the electrical output under study. however there will be elements such as electrical connections and plates, which will be in touch with the piezoelectric material, and are made of conductive metals or alloys. these elements will be the media where the current will be induced. 5. minimizing the effects  as a consequence of the dependency with the distance to the source of the electromagnetic fields provided by (7) and in order to minimise these magnetic effects, a special coupler has been used to increase the connection distance between sensor and shaker armature with excellent results (figure 22). as a general result, a sufficient increase in the distance between the sensor and the shaker armature will avoid the problem. in order to determine this sufficient increase expression (11) is evaluated. this expression has been directly obtained from (7).         1221 1 2 jexp ll lzi lzi       . (11) in principle the required increase in the distance l2 –l1 can be easily deduced from (11).  is an unknown constant, which has to satisfy  >1. in order to obtain this sufficient increase in the distance, the worst case can be assumed, that is   1. the worst case can also be assumed for the permeability . as a consequence the assumed relative permeability will be r  1. the only cases where it will be very different from 1 are the cases of ferromagnetic materials, where r >> 1 but, as the worst case is assumed, the previous assumption is justified. the main problem is to determine the conductivity, as no information is easily found about the sensor materials and the conductivity of metal alloys can differ from 7  107 s/m to 6  105 s/m. obviously the excitation frequency  to be considered will be the lowest of the shaker working range. in order to determine an appropriate length for the coupler the following considerations have been taken into account. the lowest excitation frequency to be considered is 5 hz, as it is the limit for the shaker. there was no information available about the materials that could be part of the sensing element for the sensors under study, so some assumptions were made. in the case of the resistive sensors constantan (45 % ni, 55 % cu) with 2  106 s/m could be assumed as being a typical material for strain gauges. in the case of the piezoelectric sensor the sensor load coupler sensor load coupler   figure 22. photograph showing the measurement configuration when the coupler is used to minimise the magnetic effects.    acta imeko | www.imeko.org  april 2016 | volume 5 | number 1 | 57  assumed material was steel, so an approximate conductivity of 1.3  106 s/m can be assumed as the worst case. this value is lower than the one assumed for the resistive sensors, so it will be assumed as the worst case value. figure 23 shows the decrease of these effects versus the increase in the distance to the source of the magnetic fields as a result of (11). for an increase in the distance of 10 cm the decrease in the effect is 98 %. the coupler used in this work is 17 cm high. for this increase in the distance a 99.9 % decrease in the magnetic effects is expected. this result agrees completely with the experimental results shown in figure 24 to figure 28. they show the effects on sensitivity modulus and phase shift when using this coupler compared to not using it in the case of no load, where the influence of a non static magnetic field is maximum. this coupler, however, may have the disadvantage of magnifying the effects of transversal acceleration and resonances, which increase with excitation frequency. as a consequence, the coupler should be used only at low frequencies and it is recommended the sensor to be coupled directly to the exciter at higher frequencies. 6. conclusions  in this paper the magnetic effects caused by a non static magnetic field on sensors with electrical output have been described and fully explained. the sensors under study have been piezoelectric and resistive force sensors and the non static magnetic field has been generated by an electrodynamic vibration shaker. it has been discovered that these effects are more important the lower the excitation frequency and the sensor load are, but they usually increase with the sensor size. the magnitude of the magnetic effects discovered for force sensors in this study is very important. in the current use of accelerometers with electrodynamic shakers some similar behaviour could be expected, but it may not be so important because the accelerometer size is generally smaller as well as the current that passes through the shaker armature coil used in accelerometer calibrations. on the other hand, a sufficient vertical distance between sensor and armature, which could be achieved increasing the coupler length, assures that these effects could be negligible independently of the sensor size. 0.95 1.00 1.05 1.10 1.15 1.20 1.25 1.30 1.35 1.40 1.45 0 20 40 60 80 100 120 140 160 180 200 frequency /hz s e n s it iv it y m o d u lu s / s e n s it iv it y m o d u lu s f o r 2 0 0 h z without coupler with coupler figure  26.  plot  showing  the  sensitivity  modulus  divided  by  the  sensitivity  modulus  for  200  hz  versus  frequency  for  the  kistler  9175b  sensor  (piezoelectric) without load for the cases: with or without coupler.  -5 0 5 10 15 20 25 30 35 0 20 40 60 80 100 120 140 160 180 200 frequency /hz s e n s it iv it y p h a s e s h if t /º without coupler with coupler figure 27. plot showing the sensitivity phase shift versus frequency for the  interface  1610  sensor  (resistive)  without  load  for  the  cases:  with  or  without coupler.  -10 0 10 20 30 40 50 60 0 20 40 60 80 100 120 140 160 180 200 frequency /hz s e n s it iv it y p h a s e s h if t /º without coupler with coupler figure 28. plot showing the sensitivity phase shift versus frequency for the  hbm  u2b  sensor  (resistive)  without  load  for  the  cases:  with  or  without  coupler.  0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 distance /cm figure  23.  plot  showing  the  decrease  of  the  magnetic  effects  versus  the increase  in  the  distance  to  the  source  of  the  electromagnetic  fields  as  a function of expression (11).  0.80 0.85 0.90 0.95 1.00 1.05 0 20 40 60 80 100 120 140 160 180 200 frequency /hz s e n s it iv it y m o d u lu s / s e n s it iv it y m o d u lu s f o r 2 0 0 h z without coupler with coupler figure  24.  plot  showing  the  sensitivity  modulus  divided  by  the  sensitivity  modulus  for  200  hz  versus  frequency  for  the  interface  1610  sensor (resistive) without load for the cases: with or without coupler.  0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 0 20 40 60 80 100 120 140 160 180 200 frequency /hz s e n s it iv it y m o d u lu s / s e n s it iv it y m o d u lu s f o r 2 0 0 h z without coupler with coupler figure  25.  plot  showing  the  sensitivity  modulus  divided  by  the  sensitivity modulus  for  200  hz  versus  frequency  for  the  hbm  u2b  sensor  (resistive) without load for the cases: with or without coupler.   acta imeko | www.imeko.org  april 2016 | volume 5 | number 1 | 58  acknowledgement  the authors would like to thank eusebio rubio and jesús prieto from bruël & kjaer ibérica (spain) for their help and support. references  [1] c. bartoli et al. “traceable dynamic measurement of mechanical quantities: objectives and first results of this european project”, xx imeko world congress, pattaya, thailand, 2012, https://www.imeko.org/publications/wc-2012/imeko-wc2012-tc21-o7.pdf. [2] n. medina, j. robles, j. de vicente, “realization of sinusoidal forces at cem”, imeko 22nd tc3, 15th tc5 and 3rd tc22 international conferences, cape town, south africa, february 2014, https://www.imeko.org/publications/tc22-2014/imeko-tc3tc22-2014-001.pdf. [3] n. medina, j. de vicente, “force sensor characterization under sinusoidal excitations”, sensors 2014, 14(10), p 18454-18473. [4] w. r. smythe, static and dynamic electricity, mcgraw-hill, new york, 1950. [5] z. popovic, et al, introductory electromagnetics, 1ª ed. prentice hall, 2000. microsoft word 23-153-1-ga.doc acta imeko  july 2012, volume 1, number 1, 59‐64  www.imeko.org    acta imeko | www.imeko.org  july 2012 | volume 1 | number 1 | 59  low distortion signal generator based on direct digital  synthesis for adc characterization  walter f. adad, ricardo j. iuzzolino  instituto nacional de tecnología industrial, av. general paz 5445, buenos aires, argentina     keywords: direct digital synthesizer, frequency synthesizers, direct digital synthesis, adc characterization.  citation: walter f. adad, ricardo j. iuzzolino, low distortion signal generator based on direct digital synthesis for adc characterization, acta imeko, vol. 1,  no. 1, article 12, july 2012, identifier: imeko‐acta‐01(2012)‐01‐12  editor: pedro ramos, instituto de telecomunicações and instituto superior técnico/universidade técnica de lisboa, portugal  received january 10 th , 2012; in final form july 10 th , 2012; published july 2012  copyright: © 2012 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: this work was supported by instituto nacional de tecnología industrial  corresponding author: walter f. adad, e‐mail: adad@inti.gob.ar  1. introduction  analog-to-digital converters (adc) and digital-to-analog converters (dac) have to be characterized in static and dynamic regime [1]. static characterization can be made using a solid state dc voltage standard or a josephson system. while, for dynamic characterization, it is necessary to excite the adc with different types of signals waveforms, amplitudes and frequencies. to attack these problems we have designed an arbitrary function generator based on direct digital synthesis (dds). this technique has some advantages in comparison with others techniques, such as phase-locked loop (pll). the dds technique has higher frequency resolution, usually generators based on pll have a limited frequency resolution in the order of 1:106, although some advanced pll achieve much higher resolutions, while dds technique can achieve values of frequency resolution in the order of 1:1014. additionally, the total harmonic distortion (thd) of pll generators typically has values of -40 db in comparison with a thd better than -70 db achievable by dds devices. moreover, in dds devices the stability in frequency depends on the reference external oscillator, multiple devices can be synchronized, facility of great importance in multi-tone applications. finally, the dds devices are software programmable and easy to use. this function generator can be used in other applications such as impedance measurements (as described in [2]) and any measurement schemes which require alternating signals as stimulus. 2. theory of operations of direct digital synthesis  dds technique consists in digital processing to generate signals at different frequencies and phases selectable by software, from a reference clock. as the dds technique consists in dividing the reference clock frequency from a tuning word selectable by software, the relationship between the tuning word, the clock reference, the number of bits of the dds and the desired output signal frequency is given by [3] 2 clock o n f f m t     , (1) where δt is the duration of a dds time step (1/fclock) and δφ is the phase angle changing in one time interval δt. considering that the tuning word (m) is the amount by which the phase accumulator increments on each dds time step and that 2n is the capacity of the phase accumulator, then δφ = m / 2n (with n equal to the number of bits of the phase accumulator). combining these results gives the frequency of the output sine wave (fo). replacing m = 1 in equation (1) gives the frequency resolution of the dds devices as 2 clock r n f f  . (2) this paper presents a low distortion signal generator with a frequency range from 0 to 10 khz using the direct digital synthesis (dds)  method  for  adc  characterization.  the  results  show  that  the  maximum  distortion  in  the  whole  frequency  range  is  ‐80.37 db,  the  frequency  resolution  is  1.421 nhz  (with  a  48‐bits  dds  chip),  the  stability  in  frequency  is  25 μhz/hz  and  the  amplitude  stability  is  13 μv/v.  acta imeko | www.imeko.org  july 2012 | volume 1 | number 1 | 60  for example, with 48 bits dds and a reference clock of 400 khz it is possible to obtain a frequency resolution of 32.52 nhz. a simplified block diagram of dds device is depicted in figure 1. the block diagram consists of four blocks: a reference clock, a phase accumulator, a look-up table and a d/a converter. the phase accumulator sums at each clock pulse the tuning word. thus, its output is a digital ramp (binary code), as shown in figure 1. the look-up table converts the phase accumulator output to a digital sinusoidal waveform. because of limitations in the number of bits of the look-up table, the high resolution output of the phase accumulator (32 or 48 bits) is truncated. this truncation introduces spurious components in the output signal spectrum that must be filtered in order to obtain low distortion. as is explained in [3], the main spurious component due to phase accumulator truncation is situated at    ( )mod , 2 2 n w clock spur n w f f m    (3) where fspur is the frequency of the main spurious component due to truncation in the phase accumulator, mod() is the modulus operator, n is the resolution of the phase accumulator and w is the number of bits entering the look-up table. finally, the d/a converter transforms the digital sinusoidal waveform into an analogue signal. the nonlinearities of the d/a converter are the major source of harmonic distortion. 3. system description  figure 2 shows a simplified block diagram of the developed generator. internal clock and external clock blocks in the diagram represent the reference clock. the clock selector allows choosing between the internal clock (a quartz crystal) and an external clock. this clock selector is controlled by a microcontroller. since the dds technique does not introduce frequency instability to the system, the stability in frequency is dominated by the internal or external clock. the clock provider block delivers the clock signal to the system and also provides an external output for synchronization with other systems. the system generates single-tone, multi-tone, triangular, saw-tooth and square waveform signals. to generate dual tone signals, two ad9852 devices are used, while to generate singletone signals, one of them can be used. since the ad9852 are only capable of generating sinusoidal and square waveforms, a third dds was added, the ad9834, to generate triangular and saw-tooth waveforms. to generate the triangular waveform, the ad9834 internally bypasses the look-up table and directly connects the phase accumulator output to the dac. figure 2. simplified block diagram of the developed generator. figure 1. simplified block diagram of a dds device.  figure 3. simplified block diagram of the sawtooth waveform generation.  acta imeko | www.imeko.org  july 2012 | volume 1 | number 1 | 61  on the other hand, to generate the saw-tooth waveform, as suggested in [4], pulses generated by the ad9852 switch between the phase of the two triangular waveforms produced by the ad9834. figure 3 illustrates this situation. as it has been described in last section the digital output signal must be filtered. therefore, butterworth low pass passive filters of third order with a cut-off frequency of 20 khz were placed at the output of the ad9852. as in [5], they were implemented with passive components so that the only noise source is the thermal noise introduced by resistors. the butterworth topology was used because of the flatness in the passband. to generate different amplitudes of the output signals, low distortions programmable gain amplifier (pga) were employed, the ad8250. the gain can be set to 1, 2, 5 or 10. next to the low distortion amplifiers, lowpass filters were placed (in the case of sinusoidal signal) to remove spurious frequencies generated by the pga. they have the same topologies of the first one, but with a cut-off frequency of 100 khz. finally there are an adder and a multiplier circuit to generate the dual tone signals. ad734an is used as multiplier because of its low distortion. 4. sources of distortion  in order to evaluate the dds behaviour the main distortion sources were analyzed, which are:  phase accumulator truncation.  internal d/a converter.  interference between tracks in the pcb design. to study the capabilities of the system a simulation program was developed. simulation results have shown that the spurious frequency due to phase accumulator truncation were over the bandwidth of the system (10 khz), and can be filtered properly with the filters mentioned in the previous section. these reconstruction filters, also reduce the harmonic distortion caused by the d/a converter. to reduce the quantization noise effects introduced by the d/a converter, different measurements were performed in the system. some of these measurements are showed in section 5.1 and they consist of varying the relationship between the dds output frequency and the reference clock frequency. the pcb was designed to reduce interference between tracks, as suggested in [6] and [7]. the following issues were taken into account:  ground plane inclusion. a two layers pcb was designed. the bottom layer as a ground plane. with its inclusion the thd has been reduced in 10 db.  capacitive crosstalk reduction. to reduce the interference between tracks, parallel tracks were avoided, keeping them as short as possible.  dds’s ad9852 synchronization. the generation of dual-tone signals employs two ad9852 devices synchronized. to accomplish that, the tracks from the reference clock to these devices were routed keeping both the same length.  reference clock low impedance return path. tracks crosses behind the track of the reference clock (bottom layer) were avoided in other to prevent the spread of the return current throughout the circuit. figure 4 shows the pcb design of the two-tone sinusoidal generator. 5. results  to verify the system performance, different tests were carried out. most of these tests were realised in order to achieve the target thd. 5.1. spectral analysis without reconstruction filters  in order to evaluate the thd of the system, a simulation program was developed. it implements the four blocks shown in figure 1. figure 5, shows the dds spectrum obtained by simulation figure 4. pcb design. acta imeko | www.imeko.org  july 2012 | volume 1 | number 1 | 62  at the output of the d/a converter. a spurious free dynamic range, sfdr = -51.67 dbc was achieved.  then, the output of the system without the reconstruction filter was measured using the setup showed in figure 6. in this scheme, the dds was programmed through a microcontroller connected to a pc to generate a frequency of 1 khz. the output signal of the dds was acquired with an acquisition system developed in the laboratory which has a thd = -100 db. the measured output spectrum is depicted in figure 7, where a sfdr = -57.17 dbc can be observed. these results illustrate the necessity of including reconstruction filters to obtain a low distorted signal. it is important to point out that the sfdr achieved is due to a reconstruction filter was not applied in order to evaluate the dds performance. 5.2. spectral analysis after the application of the reconstruction  filter  using the measurement setup showed in figure 6, but adding the reconstruction filter described in previous section at the output of the dds, a thd of -80.37 db was obtained by measurement while a thd of -80.62 db was obtained by simulation. it is important to mention that in this case the sfdr is equal to the thd because the amplitudes of the harmonics three to six is depreciable with respect to the strongest spurious frequency (situated in the second harmonic). the results are summarized in table 1 and showed in figure 8. as a conclusion, the introduction of the reconstruction filter reduced considerably the thd, eliminating harmonics above 7 khz and reducing the amplitude of the first five. 5.3. influence of reference clock frequency on the output signal  spectrum  to study the behaviour of the system, the thd of the dds was calculated at different frequencies of the reference clock using the measurement setup showed in figure 9. as in figure 6, the dds was programmed through the microcontroller connected to a pc to generate a determinate frequency (1 khz and 0.227 v). using a programmable signal generator, the reference clock frequency of the dds was varied in order to select the best frequency for it. the filtered output signal of the dds was acquired with the same acquisition system as in the previous test. table 2 and figure 10 show the thd obtained by measurement at different relationships between the frequencies of the reference clock and the dds output signal. these results also confirmed the dac sfdr specification of the ad9852 device increasing the sfdr as the reference clock frequency and output frequency ratio change [8]. as a conclusion, a thd of -80 db (objective of this work) was achieved when the relationship between frequencies of the reference clock and the dds output signal was set to 80 and 100. figure 5. dds output spectrum obtained by simulation.  figure 7. dds output spectrum obtained by measurement.  figure 6. measurement setup employed to evaluate the output spectrum of the system without the reconstruction filters.  figure  8.  dds  output  spectrum  obtained  by  measurement  after  the  application of the reconstruction filter.  acta imeko | www.imeko.org  july 2012 | volume 1 | number 1 | 63  this behaviour is taken into account when selecting the clock frequency, and for that reason we use as upper limit to select the clock frequency 400 khz. as indicated previously with this range is possible to filter the output signal of the dds by a 3rd order low pass filter. 5.4. influence of set the dds output frequency to an exact  submultiple of the reference clock frequency  using the measurement setup shown in figure 9, the dds was programmed to generate an output frequency which is multiple of the reference clock. after that, the dds was programmed to generate an output frequency which is not multiple of the reference clock. the results are shown in table 3. they show a difference of 0.2 db in the thd when the output signal frequency was not an integer multiple of the reference clock frequency. 5.5. internal frequency multiplier  the ad9852 has an internal frequency multiplier with which is possible to increase the frequency of the device internal clock. to evaluate the influence of this frequency multiplier on the thd, from the measurement setup showed in figure 9, the dds was programmed to generate the same output frequency using different multiplier settings. then the thd was obtained. the results are depicted in table 4. it is important to point out that the dds internal frequency was the same at both cases (400 khz). as can be seen in table 4, no changes were observed in the thd at different values of the dds internal multiplier, therefore there is not variation of the thd as a function of the dds internal multiplier. 5.6. frequency and amplitude stability  in order to measure the frequency and amplitude stability of the signal generator designed, the dds was programmed through a microcontroller to generate 62.5 hz sinusoidal wave (0.227 v) using the internal clock of the system. to analyse the frequency stability, the output signal of the generator was connected to a time interval counter, which was programmed to collect data through a pc during ten hours. to measure the amplitude stability, the output of the generator was measured employing a high accuracy multimeter. the measurement results of the frequency and amplitude stability are shown in table 5. figure 11 depicts the allan deviation, σy(τ), of the dds output signal frequency when the internal clock was used. the standard deviation is a statistic tool employed to quantify the dispersion of a number of samples. the variance is a numeric measure of the deviation of these samples with respect to the mean value. however, the classic variance only can be calculated from stationary data. it implies that the data is independent of time. when data is not stationary (such as in frequency and amplitude measurement), classic variance does not converge and another tool is required. so, the allan figure 9. measurement setup.   76.5 77.0 77.5 78.0 78.5 79.0 79.5 80.0 80.5 81.0 8 0 1 2 0 0 2 3 2 0 3 4 4 0 4 5 6 0 5 6 8 0 6 8 0 0 7 9 2 0 9 0 4 0 1 0 1 6 0 11 2 8 0 1 2 4 0 0 1 3 5 2 0 1 4 6 4 0 1 5 7 6 0 1 6 8 8 0 1 8 0 0 0 1 9 1 2 0 |t h d ( d b )| frequency of the reference clock / dds output frequency figure 10. thd as a function of the frequency of the reference clock.  table 1. maximum total harmonic distortion of the system.  thd simulated  thd measured  ‐80.62 db  ‐80.37 db  table 2. thd at different frefclk, f0 ratios.   dds output  frequency  clock reference  frequency  0 refclkf f   thd  1 khz  20 mhz  20000  ‐77.87 db  1 khz  100 khz  100  ‐80.37 db  1 khz  80 khz  80  ‐80.57 db  table 3. thd as function of dds output frequency.  dds output frequency  clock reference output  frequency  thd  1000 hz  80 khz  ‐80.57 db  1001.457 hz  80 khz  ‐80.77 db  table 4. influence of multiplier in the thd.  dds output  frequency  clock reference  frequency  multiplier  thd  1 khz  80 khz  x5  ‐80.16 db  1 khz  100 khz  x4  ‐80.16 db  table 5. frequency and amplitude stability of the dds output.  frequency stability  amplitude stability  25 μhz/hz  13 μv/v    acta imeko | www.imeko.org  july 2012 | volume 1 | number 1 | 64  variance was used because it converges for different types of noise present in electronic circuit [9]. figure 11 shows that an averaging time in the range of 20 to 16000 s can be employed because of the predominant noise in the whole range is white noise. it is important to note that the dds technique does not introduce frequency instability to the system, so it is the frequency stability of the reference clock. figure 12 shows the allan deviation of the output signal amplitude. in this case, to achieve the optimal σy(τ), an averaging time (τ) of 15.25 s has to be used because this τ is the minimum σy(τ) in the range of white noise. 6. conclusion  we designed a low distortion (-80.37 db) arbitrary function generator of a resolution equal to 1.41 nhz and frequency stability of 25 μhz/hz. the total harmonic distortion reached by the proposed design is suitable for the application, but one possible modification can be performed in order to improve this feature. it consists in the use of an external digital to analogue converter (dac) with more bits and low distortion at the output of ad9852. it is important to note that this device can be programmed to employ an external or internal dac. the proposed signal generator was designed to generate multi-tone signals in a relatively simple way. for example, to generate four tones, as the device can be synchronized to an external clock, adding a second board and connecting an external clock to both pcbs, the system will be capable of generating four tones. in comparison with commercial signal generators most of them can generate two tone signals. during these research other dds commercial chip was analyzed, the ad9959. this device has four dds cores. the advantage of employing it is that four tone signals can be generated with one pcb (in comparison with the system presented in this paper). despite this, the design with this device was discarded because it was analysed that while a good pcb design could ensure a feature of – 100 db sfdr, the limitation is given by the isolation between channels of the device specified as better than 65 db [10]. other disadvantages are lower frequency resolution (32 bits in comparison with 48 bits of the ad9852) and internal dac of 10-bits instead of 12-bits converters of the implemented design. other important conclusion obtained from the results is that as the direct digital synthesis technique is based on digital signal processing it does not introduce instability to the frequency. therefore, the only contribution to the frequency uncertainty is the relative uncertainty of the reference crystal. as the system has the possibility to use an external reference clock, the achieved stability can be improved. this function generator may be used in electrical metrology and in schemes of measurements that require ac signals as a stimulus, for example, in time and frequency domain, where a cesium clock can be used to synchronize the system proposed (so the frequency stability of the system is the frequency stability of the cesium) and the dds can be programmed to generate different frequencies to test frequency meters. references  [1] ieee, tech. rep., “ieee std 1241.2001, ieee standard for terminology and test methods for analog-to-digital converters,” 2001. [2] fu, y.; li, z.; zhang, z.; sun, j.; chen, l.; , "dds sources for precise measurement", precision electromagnetic measurements (cpem), 2010 conference on , vol., no., pp.402-403, 13-18 june 2010. [3] analog devices, “a technical tutorial on digital signal synthesis”, 1999. [4] collins, n. & limerick, “dds devices produces sawtooth waveform”, 2003. [5] lacanette, k., application note, an-779, “a basic introduction to filters – active, passive, and switched capacitor ni”, 1991. [6] montrose, m. i. “printed circuit board design techniques for emc compliance”, 2000. [7] brandon, d. “synchronizing multiple ad9852 dds-based synthesizers”, 2003. [8] analog devices, “cmos 300 msps complete dds ad9852”, 2002-2007. [9] lópez, j. m., “la varianza de allan en la metrología del tiempo y la frecuencia”, 2005. [10] analog devices, “4-channel, 500 msps dds with 10-bit dacs ad9959”, 2005-2008. figure 11. allan deviation of the dds frequency output signal.  figure 12. allan deviation of the dds amplitude output signal. acta imeko, title acta imeko issn: 2221-870x september 2017, volume 6, number 3, 29-34 acta imeko | www.imeko.org september 2017 | volume 6 | number 3 | 29 3d modelling and measurements of historical violins piercarlo dondi1, luca lombardi2, marco malagodi1,3, maurizio licchelli4 1university of pavia, arvedi laboratory of non-invasive diagnostics, via bell'aspa 3, 26100 cremona, italy 2university of pavia, department of electrical, computer and biomedical engineering, via ferrata 5, 27100 pavia, italy 3university of pavia, department of musicology and cultural heritage, corso garibaldi 178, 26100, cremona, italy 4university of pavia, department of chemistry, via taramelli 12, 27100, pavia, italy section: research paper keywords: 3d scan; 3d measurement; historical violins citation: piercarlo dondi, luca lombardi, marco malagodi, maurizio licchelli, 3d modelling and measurements of historical violins, acta imeko, vol. 6, no. 3, article 5, september 2017, identifier: imeko-acta-06 (2017)-03-05 section editor: sabrina grassini, politecnico di torino, italy received march 8, 2017; in final form may 25, 2017; published september 2017 copyright: © 2017 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: this work was supported by fondazione cariplo [research project “opificio del suono”], regione lombardia [por ob. 2 fse 2007/2013 research project n° 61385772 “viva musica”] and fondazione arvedi buschini of cremona corresponding author: piercarlo dondi, e-mail: piercarlo.dondi@unipv.it 1. introduction studying the morphology of a violin is a complex but important task. many kinds of information can be retrieved and measured, such as the external dimensions, the internal thicknesses or even the impact of varnishes on the acoustic [1]. these data are particularly valuable in the case of historical violins, not only for researchers, but also for violin makers, that can use them as reference for the making of new instruments. due to the importance of these data, accurate measures are always needed. in this work, we focus on the measurement of the external surfaces of the violins, a task that, up to now, is only performed by the manual use of a calliper. even if this procedure is standard with modern instruments and can be repeated several times, it can be performed only occasionally on historical ones. this is not only due to the restricted access but also to the need of limiting the direct contact of the calliper to its bare essential, in order to not ruin the ancient varnishes or even (in the worst scenario) chip the wood. the varnishes could represent a limit in the application of mechanical measurements because of their historical importance, brittleness and thickness [2]. moreover, damages on the varnishes could be a key factor of the material degradations, especially biological ones, with a consequent requirement of a restoration work [3], [4]. a faithful 3d model of a violin can be very helpful in this field [5]. the 3d model allows to easily repeat measures several times without risks and also reach areas nearly inaccessible on the original instrument without opening it (e.g. the highness of the belly). however, the digitalization of a historical violin is a difficult process: the complex morphology and the presence of a great number of small details require a very high accuracy and degrees of freedom during the scanning phase [6]; moreover, the highly reflective varnishes complicates the use of some models of laser scanners or other scanning technologies, such as photogrammetry [7]. abstract measuring historical violins provides crucial information about the morphology of the instruments, useful both for researchers and violin makers. generally, these measures are taken manually using a calliper, but they can be repeated only occasionally due to both the restricted access to these precious instruments and the need of avoiding accidental damages to the wood or to the varnishes. in this work, we describe and assess the accuracy of a protocol for the acquisition and creation of high quality 3d models of violins, suitable for taking accurate measurements. six historical violins of 17th – 18th centuries, kept in “museo del violino” in cremona (italy), were used as test set. the quality of the final outcomes was checked comparing measures taken on the 3d meshes with the correspondent ones taken by calliper on the original instruments. finally, a comparison between the sound board of the instruments were performed. acta imeko | www.imeko.org september 2017 | volume 6 | number 3 | 30 in one of our previous works, presented at metroarcheo2016 [8], we discussed a protocol for the creation of accurate 3d models of historical violins using a laser scanner, and we tested it on the famous antonio stradivari “cremonese” (1715). this article extends that work, the main improvements involve: i) the acquisition of other five important instruments of the 17th – 18th centuries held in “museo del violino” of cremona (italy); ii) a more complete assessment of the accuracy of the models; iii) a set of comparisons between the shapes of the instruments. the paper is organized as follows: section 2 describes the acquisition and the modelling protocol; section 3 discusses the measurement problem and the tests performed; section 4 draws the conclusions. 2. acquisition and 3d modeling the digitalization of historical artworks is becoming a standard practice in many museums and archaeological sites [9]. different imaging technologies are available (laser scanner, photogrammetry, stereo-cameras and so on). the final choice depends on the characteristics of the object to acquire [10]. these different techniques may be merged to improve the quality of the reconstructed models. as an example both photogrammetry and 3d scanning were used to create a model of the donatello's “maddalena” [11]. mathys et al. presented a comparison among the different techniques, including also computer tomography; as a conclusion, the authors suggest using laser scanners when only external surfaces are needed and computer tomography (ct) if it is necessary to acquire internal structures [12]. also, the possibility to use low cost devices, such as the kinect, has been demonstrated for the reconstruction of excavation sites [13]. as previously stated, the acquisition of historical violins is particularly critical due to their complex morphology and the reflectivity of the varnishes. for these reasons, the digitalization of those instruments is generally obtained by a ct-scan. this is surely the most precise existing scanning technology; it can acquire both external and internal surfaces of an instrument and provides important information, such as the thickness of the ribs or the presence of internal damages and repairs [14]. recently, the smithsonian institution adopted the ct-scan for the study of seven stradivari violins and proposed a detailed protocol for their acquisition and modelling. they also demonstrated the usability of such models for measurement tasks [15]. the use of laser scanners is instead very limited, since they can acquire solely the external surfaces. only some notable cases are described in literature, such as the strad 3d project [7] (however, that uses in combination also ct-scan data) or the analysis of elastic deformation performed on guarneri del gesù’s “cannone” (1734) [16]. nevertheless, some advantages of this technology with respect to ct-scans must not be neglected: it is more affordable; it only needs a single operator to perform the acquisition instead of a team of experts; and it can be used directly in the museum without the need to move the instrument. furthermore, in our field of interest the limitation to the external surface is not critical, since most of the important measures involve the outside of the violin. figure 1 provides an overview of our procedure. we shall now describe in details the various steps. 2.1. scanning even if an optimal solution does not exist, some characteristics are crucial in the choice of the scanner to adopt: i) a high degree of freedom, to reach all the surfaces of the instrument; ii) a high accuracy, to acquire as many details as possible. for these reasons, we chose a “rs3 integrated scanner” (a linear laser scanner with a stated accuracy of 30 µm) mounted on a mobile arm with 7 degrees of freedom (romer absolute arm 7-axis "si") both produced by hexagon metrology. as scanning software, we used the polyworks suite provided with the scanner. since wood instruments are sensible to abrupt variations in humidity and temperature that can alter their morphology [17], the working laboratory was always maintained at the same climatic condition of the showcases of the museum, a temperature of 20° and a humidity of 50 %. before the scanning, all the mobile parts of the violin that can interfere with the acquisition are removed: the tailpiece, the bridge and the chin rest because they hide part of the belly; the pegs because they partially hide the outer wall and the pegbox floor of the head; the strings because they can alter the light beam of the scanner. their loss is acceptable, they are not strictly relevant for measurements and generally they are not the original ones (nearly all the mobile parts of historical violins have been substituted during the centuries). each instrument was acquired in four steps, generating many partial scans that will be aligned and connected during the elaboration phase: left side of the body, right side of the body, front side of the head, back side of the head. two supports in plexiglas, a vertical one (figure 2a) and a horizontal one (figure 2b), were designed to place the instrument in the optimal positions for reaching as much detail as possible. the vertical support was used for the body (better for reaching the internal edges of the f-hole and the part of the top plate under the fingerboard with the laser); the horizontal one for the head (better for reaching the pegbox floor, the peg holes and the throat of the scroll). clamps covered with gum were used to maintain a stable position of the violins during the scanning figure 1. overall schema of the proposed procedure of acquisition, 3d modelling and measurement of a violin. figure 2. support used for the scanning process: (a) vertical; (b) horizontal. acta imeko | www.imeko.org september 2017 | volume 6 | number 3 | 31 process without damaging the varnishes. the acquisition takes around six hours of work to be completed. 2.2. correction and validation after the acquisition of the raw data, the resulting cloud of points is cleaned removing redundant or badly acquired scans. in this step, particular attention must be paid to the head of the violin, because overlapping scans are very frequent due to its small dimension and to the presence of a lot of details that requires multiple runs. then, the cleaned cloud of points is triangularized and converted in a raw 3d mesh. the refinement of the raw 3d mesh was performed by two open source 3d modelling software packages, meshlab and blender. the number of corrections varies from instrument to instrument; it generally depends on the state of the surface. in particular, the presence of abrupt changes in colour or of very dark areas can interfere with the laser and produce errors or loss parts. despite these considerations there are some areas of the instruments that are always problematic to reach with the scanner: the internal part of the head, the throat of the scroll, the internal edges of edge holes. high resolution images, previously acquired with a nikon d4 camera equipped with a 50 mm objective [18], [19], were used as reference for the reconstruction of these areas. the procedure is iterative, each correction is validated using the images and the supervision of the curator of the museum, and repeated until the level of accuracy is considered acceptable. this refinement step may take one to two weeks, depending on the quality of the scans. it is important to highlight that, even if the adopted laser scanner has a stated accuracy of 30 µm, it was designed for objects and surfaces more regular and less detailed than a violin. therefore, the high number of scans needed for acquiring the entire surface can frequently produce an oversampling, even after the cleaning step. for obtaining more suitable models that can be used efficiently without high expensive hardware, we introduced a decimation step (quadric edge collapse decimation) during the correction phase. this simplification is focused only in those areas that present a clear oversampling, but of course, a decimation can alter the morphology of the object and then the accuracy of the measures. the following measurement tests were designed to prove that also these simplified 3d models are suitable for our measuring purposes. 3. measurements six historical violins were digitalized using the defined protocol: five made by antonio stradivari (“clisbee” (1669), “hellier” (1679), “joachim-ma (1714)”, “cremonese (1715)” and “vesuvius” (1727)) and one made by guarneri del gesù (“principe doria” (1734)). this is an exhaustive test set, since each instrument has a peculiar history and was undergone to different restoration processes during the centuries that altered surfaces in different ways. thus, even with a small test set, we could assess the quality of our acquisition method in heterogeneous conditions. 3.1. calliper vs 3d taking geometrical measures on a violin is a complex task. the rounded shapes and the imperfect symmetry makes it difficult to obtain accurate and reproducible values. moreover, the definition of the areas to be measured is not always univocal and can depend on subjective choices. to assess the accuracy of the acquired 3d models, we asked the curator of the museum to take a series of linear measures with a digital calliper (with a stated accuracy of 20 µm) on the original instruments. we subsequently replied the same measures (always with the supervision of the curator) on the corresponding 3d models using freecad, an open source cad software. twenty-five metrics, typically used by violin makers and distributed on the entire surface of the instrument, were considered. table 1 shows the absolute and relative difference between calliper and 3d measures for each of the six violins. as can be seen, the quality of the 3d models, even if simplified, is high. in all the cases the overall average difference between calliper and 3d goes from 0.11 mm to 0.14 mm. it is interesting to analyse separately the various “macro areas” of the instrument (the back plate, the table, the ribs, the f-holes and the scroll), to check if there are different levels of accuracy among them, due to inaccuracies in acquisition or to alterations generated by the elaboration of the meshes. the graphic in figure 3 summarizes the average absolute difference for these zones and shows that no area is constantly more accurate than the others, with the only exception of the ribs. this behaviour proves that the accuracy of the model is quite uniform over the whole surface; the oscillations are compatible with random uncertainties due to the manual use of the calliper on a rounded surface. instead, measures on the ribs are always very stable since they are easier to take. the accuracy of the model is sufficient for most of the cases and for the needs of violin makers. however, if we want to perform high accuracy comparisons between instruments we need to check if an average difference of 0.14 mm is critical or not. for performing this verification, we computed for all the metrics the difference in terms of percentage (d) between each measurement taken with the calliper (mc) and the corresponding measurement taken from the 3d model (m3d), as defined in (1). − = 3c d c m m d m . (1) the graph reporting the mean percentage differences (figure 4) shows clearer trends if compared to the graph reporting the mean absolute differences (figure 3). the four parts that compose the body (back, table, f-holes and ribs) have always low average values that go from 0.05 % to 0.28 %. on the contrary, measurements taken on the scroll show a greater variability (from 0.43 % to 0.81 %). in particular, the scroll volute top width is very problematic, as can be seen in table 1. as for the ribs, also for the scroll the areas to measure are small, however, the heights of the ribs are very easy to measure (the surface is perfectly straight in those areas), whereas the scroll is very rounded, and therefore it is more complex to find figure 3. mean absolute difference (in mm) between 3d and calliper measures taken on the five main macro areas of the violins. acta imeko | www.imeko.org september 2017 | volume 6 | number 3 | 32 its correct reference points (both with calliper and with cad software). therefore, the uncertainty on the scroll is greater than on the ribs (see the two corresponding curves in figure 3). we can conclude that, even if measurements taken from the 3d models of the scroll are enough accurate for violin making purposes, it is better to not use them for making comparisons between violins; in this scenario it is more convenient to use only the body of the instruments. 3.2. comparison among violins the acquired stradivarius violins are representative of the entire working period of the famous violin maker. comparison of the bodies, and in particular of the soundboards, can give useful information about the evolution of stradivari’s construction techniques. in fact, antonio stradivari, during his long life, significantly modified his construction style, with variations in the volumes and sizes of the violins. for instance, one of the most important developments in stradivari’s work over the years was to flatten and straighten the arch, removing most of the recurve at the perimeter, and thereby providing a more responsive and powerful tone; with those modifications, the distances between the f-holes on the top changed, too [20]. most of stradivari’s violins from the ‘golden period’ (after 1700s) show a fair symmetry between front and back, with generally less than a single millimetre discrepancy in height, and usually having the greater measurement on the front [21]. we extracted from the 3d models the external profile of the five soundboards and of the corresponding f-holes. the overlapping of these profiles allows to perform a fast and accurate comparison. different measurements are taken again considering always the same metrics stated before. figure 5a shows the comparison between “clisbee” and “hellier”. even if only 10 years passed between the construction of the two instruments the differences are remarkable. “clisbee” is clearly smaller than “hellier”: the length of the soundboard is 5.6 mm shorter, the max higher width (the maximum width of the upper half of the soundboard) is 13.9 mm narrower, the max lower width (the maximum width of the lower half of the soundboard) 16.5 mm, and the min width at the c-bouts (the width of the narrowest part of the soundboard) 9.3 mm. on the contrary, the dimensions of f-holes are not so different, only the lobes distance is lower in “clisbee”, due to its reduced width. this discrepancy can be explained considering that “clisbee” is one of the first works of antonio stradivari, thus his style was still table 1. absolute and percentage differences between measures taken by digital calliper on the violin and by cad software on 3d model. metric clisbee (1669) hellier (1679) joachim-ma (1714) cremonese (1715) vesuvius (1727) principe doria (1734) mm % mm % mm % mm % mm % mm % back length 0.03 0.01 0.12 0.03 0.07 0.02 0.00 0.00 0.08 0.02 0.05 0.02 back max higher width 0.03 0.02 0.16 0.09 0.32 0.19 0.12 0.06 0.18 0.11 0.03 0.02 back min width c-bouts 0.10 0.10 0.20 0.17 0.07 0.06 0.19 0.18 0.11 0.10 0.22 0.21 back max lower width 0.25 0.13 0.26 0.12 0.35 0.17 0.08 0.05 0.04 0.02 0.15 0.07 table length 0.11 0.03 0.25 0.07 0.05 0.01 0.04 0.00 0.19 0.05 0.18 0.05 table max higher length 0.25 0.16 0.26 0.15 0.30 0.18 0.04 0.00 0.00 0.00 0.02 0.01 table min width c-bouts 0.26 0.26 0.28 0.25 0.23 0.22 0.29 0.27 0.08 0.07 0.19 0.17 table max lower width 0.18 0.09 0.14 0.07 0.46 0.23 0.05 0.05 0.19 0.09 0.14 0.07 ribs height lower bouts 0.05 0.16 0.20 0.62 0.09 0.28 0.03 0.00 0.05 0.17 0.00 0.00 ribs height lower corners 0.17 0.58 0.01 0.03 0.04 0.13 0.03 0.00 0.28 0.92 0.01 0.04 ribs height upper corners 0.01 0.04 0.10 0.30 0.05 0.15 0.17 0.63 0.05 0.16 0.14 0.49 ribs height upper bouts 0.10 0.35 0.01 0.02 0.01 0.04 0.15 0.33 0.01 0.03 0.13 0.46 diapason 0.14 0.07 0.02 0.01 0.03 0.02 0.07 0.05 0.09 0.05 0.29 0.15 f-holes upper lobes distance 0.10 0.29 0.07 0.19 0.12 0.31 0.12 0.23 0.15 0.35 0.16 0.41 f-hole notches distance 0.27 0.40 0.04 0.06 0.23 0.32 0.29 0.40 0.20 0.27 0.27 0.38 f-holes lower lobes distance 0.17 0.17 0.20 0.19 0.15 0.14 0.20 0.18 0.11 0.10 0.02 0.02 f-hole right length 0.19 0.26 0.23 0.31 0.02 0.02 0.01 0.00 0.37 0.50 0.18 0.25 f-hole left length 0.07 0.09 0.04 0.05 0.01 0.01 0.09 0.13 0.07 0.09 0.06 0.08 scroll length 0.13 0.13 0.03 0.02 0.02 0.02 0.02 0.00 0.17 0.16 0.07 0.06 scroll eyes width 0.13 0.32 0.16 0.39 0.14 0.35 0.00 0.00 0.05 0.11 0.19 0.50 scroll neck heel 0.05 0.20 0.20 0.76 0.06 0.24 0.12 0.39 0.18 0.69 0.15 0.56 scroll volute height 0.05 0.10 0.16 0.32 0.17 0.35 0.26 0.61 0.44 0.87 0.00 0.01 scroll first turn height 0.24 0.64 0.12 0.30 0.02 0.04 0.14 0.26 0.09 0.22 0.09 0.23 scroll volute top width 0.22 2.11 0.03 0.29 0.27 2.45 0.07 0.85 0.22 2.13 0.44 3.96 scroll max length over nut 0.06 0.26 0.23 0.91 0.27 1.10 0.08 0.40 0.00 0.01 0.09 0.37 average 0.13 0.28 0.14 0.23 0.14 0.28 0.11 0.20 0.14 0.29 0.13 0.34 figure 4. mean percentage difference between 3d and calliper measures taken on the five main macro areas of the violins. acta imeko | www.imeko.org september 2017 | volume 6 | number 3 | 33 similar to that of his master, nicola amati. in fact, the situation significantly changes comparing “hellier” with “cremonese”. in this case, the shape of the two sound boards became quite similar, although the “cremonese” was made 36 years later than the “hellier” (figure 5b). the total length is the same, the max higher width and the min width at the c-bouts are comparable, too. the only remarkable difference is in the max lower width, where “hellier” is 3 mm wider. the dimensions of f-holes are comparable, but there is a difference in the position, since in the “cremonese” they are placed around 2.5 mm lower. from this moment, the construction style remains quite stable. dimensions of “vesuvius”, built 12 years later than “cremonese”, are very similar (figure 5c). the differences are very limited, less than 1 mm, with the only notable exception of the min width at the c-bouts: that in “vesuvius” in 2.6 mm narrower that in “cremonese”. finally, as a final test, we compared the “vesuvius”, the last stradivarius violin of our set, with “principe doria”, a violin made only 7 year later by another violin maker, guarneri del gesù. this test is important since it can give information about difference in construction styles of makers who lived in the same historical period. the data retrieved are very interesting and the differences are remarkable (see figure 4d). “principe doria” is 7.4 mm shorter than “vesuvius”, a length close to that of “clisbee”, built in the previous century. on the contrary, the other dimensions are comparable: there are 0.5 mm of difference between max lower widths of the two tables and 0.2 mm between the two min widths at the c-bouts. only max lower width shows again a significant difference, since the “principe doria” is 1.5 mm narrower than “vesuvius”. regarding the f-holes, the dimensions are quite the same, with few tenths of millimetres of difference. their vertical position is different, but this is clearly due to the lower length of the “principe doria”. 4. conclusions in this works we described a protocol for the acquisition of 3d models of historical violins, suitable for taking accurate measurements. the proposed approach was used to digitalize six historical violins kept in “museo del violino” in cremona, five made by antonio stradivari and one by guarneri del gesù. the accuracy of the obtained 3d models was assessed comparing measurements taken by a calliper on the original instruments and measures taken from cad software on the 3d meshes. the overall accuracy was very high, showing an average discrepancy between calliper and 3d of around 0.14 mm. comparisons performed among the six instruments proved to be useful to understand the evolution of the construction style of stradivari and to underline differences between his work and that of guarneri del gesù. future developments involve the acquisition of instruments of other violin makers of the same historical period to perform further comparisons and a more in depth analysis of the 3d models that considers the morphology of their entire surface. acknowledgement we would like to thank “fondazione museo del violino antonio stradivari”, association “friends of stradivari” and “distretto culturale di cremona” for collaboration. a special acknowledgment to the curator of the museum fausto cacciatori. references [1] f. setragno, m. zanoni, f. antonacci, a. sarti, m. malagodi, t. rovetta, c. invernizzi, feature-based analysis of the impact of ground coat and varnish on violin tone qualities, acta acustica 103 (1) (2017), pp. 80-93. 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[14] s. a. sirr, j. r. waddle, use of ct in detection of internal damage and repair and determination of authenticity in high-quality bowed stringed instruments, radiographics 19 (3) (1999) pp. 639-646. [15] b. frohlich, g. sturm, j. hinton, e. frohlich, the secrets of the stradivari string instruments. a non-destructive study of music instruments from the smithsonian institution, the library of congress, and private collections. a pilot study of seven violins made by antonio stradivari in cremona, italy, between 1677 and 1709, the materialise group, leuven, belgium and materialise, usa, ann arbor, mi, usa, washington, dc (march 2009). [16] m. fioravanti, g. goli, b. carlson, structural assessment and measurement of the elastic deformation of historical violins: the case study of the guarneri “del gesù” violin (1743) known as the ‘cannone’, journal of cultural heritage 13 (2) (2012) pp. 145153. [17] g. goli, m. fioravanti, s. busoni, b. carlson, p. mazzanti, measurement and modelling of mass and dimensional variations of historic violins subjected to thermo-hygrometric variations: the case study of the guarneri “del gesù” violin (1743) known as the “cannone”, journal of cultural heritage, 13 (3) supplement (2012) pp. s154–s160 [18] p. dondi, c. invernizzi, m. licchelli, l. lombardi, m. malagodi, t. rovetta, semi-automatic system for uv images analysis of historical musical instruments, proc. of spie 9527, optics for arts, architecture, and archaeology v, june 21, 2015, munich, germany, pp. 95270h-1 95270h–9. [19] p. dondi, l. lombardi, c. invernizzi, t. rovetta, m. malagodi, and m. licchelli. 2017. automatic analysis of uv-induced fluorescence imagery of historical violins. journal on computing and cultural heritage 10 (2) (2017) pp. 12:1 – 12:13. [20] h. k. goodkind, violin iconography of antonio stradivari, new york, larchmont, 1972, isbn 9780960049813. [21] s. f. sacconi, i ‘segreti’ di stradivari, libreria del convegno, cremona, 1972. 3d modelling and measurements of historical violins microsoft word article 11 165-1308-1-le.docx acta imeko  february 2015, volume 4, number 1, 68 – 76  www.imeko.org    acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 68  design and implementation of novel fpga based time‐ interleaved variable centre‐frequency digital  modulators  işıl kalafat kızılkaya, mohammed al‐janabi, izzet kale  university of westminster, 115 new cavendish street, w1w 6uw, london, uk      section: research paper   keywords: sigma‐ delta modulator; time‐interleaved; d/a conversion; tonality; vhdl; fpga  citation: işıl kalafat kızılkaya, mohammed al‐janabi, izzet kale, design and implementation of novel fpga based time‐interleaved variable centre‐frequency  digital  modulators, acta imeko, vol. 4, no. 1, article 11, february 2015, identifier: imeko‐acta‐04 (2015)‐01‐11  editor: paolo carbone, university of perugia   received december 16 th , 2013; in final form april 28 th , 2014; published february 2015  copyright: © 2014 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: (none reported)  corresponding author: işıl kalafat kızılkaya, e‐mail: i.kalafat_kizilkaya@my.westminster.ac.uk  1. introduction  sigma-delta (σ-∆) modulators have been used in many applications such as in data converters [1] and frequency synthesizers [2]. they employ relatively simple hardware in combination with oversampling and noise-shaping to acquire high resolution [3], [4]. however, conventional discrete-time and digital single-stage  modulators are mostly limited to narrowband signal applications due to their high oversampling requirements [3-6]. on the other hand, emerging technologies such as gsm, gnss, cdma, dect have accelerated the need for designers to implement σ-∆ modulators that have extended bandwidths while also maintaining relatively relaxed oversampling ratios osrs [1], [2], [7]. the time-interleaving (ti) technique overcomes this limitation by using m inter-connected  modulators working in parallel, where the processing speed of the modulator can be reduced by m times. a further advantage of the ti approach is that it offers an elegant means to increase the signal bandwidth for both a/d and d/a applications without the need to use faster or higher-order  modulators [5-7]. there are several behaviouraland circuit-level ti  modulator topologies reported in the open literature but to the best knowledge of the authors, all these topologies have concentrated on lowpass (lp) and mid-band resonator based  modulators [5-9]. in recent studies, analog multi-path ti variable-centre frequency  modulators were designed, analysed, evaluated and compared using ideal and non-ideal behavioural-level models [10], [11]. in [12], the concepts proposed on discrete-time ti variable-centre frequency  modulators were extended to digital sigma-delta modulators (ddsms), whose noise transfer functions (ntfs) employed butterworth, chebyshev, inversechebyshev and elliptical filters. the first objective of this paper is to implement these designed behavioural-level ddsms on the xilinx® spartantm-3 development kit. the second objective is to evaluate and compare the abstract  novel,  multi‐path,  time‐interleaved  digital  sigma‐delta  modulators  that  can  operate  at  any  arbitrary  frequency  from  dc  to  nyquist  are  designed,  analysed and synthesized in this study. dual‐ and quadruple‐path fourth‐order butterworth, chebyshev, inverse chebyshev and elliptical based digital  sigma‐delta modulators, which offer designers the flexibility of specifying the centre‐frequency, pass‐band/stop‐band attenuation as well as the signal  bandwidth are presented. these topologies are compared in terms of their signal‐to‐noise ratios, hardware complexity, stability, tonality and sensitivity  to  non‐idealities.  detailed  simulations  performed  at  the  behavioural‐level  in  matlab  are  compared  with  the  experimental  results  of  the  fpga  implementation of the designed modulators. the signal‐to‐noise ratios between the simulated and empirical results are shown to be different by not  more than 3‐5 dbs.  furthermore, this paper presents the mathematical modelling and evaluation of the tones caused by the finite wordlengths of  these digital multi‐path sigma‐delta modulators when excited by sinusoidal input signals.   acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 69  performance of these ddsms with their behavioural level counterparts, therefore further validating the design process published in [12]. a major challenge associated with the design of highresolution single-bit quantizer ddsms is that they are highly tonal [13-17]. however, it is a well-known phenomenon that single-bit  d/a converters require significantly less hardware when compared to their multibit counterparts, as the latter need extra dynamic element matching (dem) circuitry [3], [18]. digital mash  modulators, whose constituent  modulators employ single-bit quantizers, overcome this problem by reducing the quantization bits step-by-step in each block, therefore substantially alleviating the occurrence of spectral tones as well as ensuring modulator stability [16], [17], [19]. in [12], the tonal behaviour caused by the finite wordlengths of digital variable centre-frequency single-bit based  modulators when excited by sinusoidal input signals was mathematically modelled. as this is an extension of the reported work in [12], the mathematical model is discussed covering the tonal behaviour of the ti topologies. 2. ti variable‐centre frequency ddsm design  the ti  modulator design starts with the conventional singlepath (sp)  modulator prototype, which involves two main steps: the synthesis of the ntf followed by the mapping of the ntf to an appropriate topology [4]. the generalized transfer function of an -order bandstop (bs) ntf is given in (1) which can be obtained for a given design by specifying the filter type (i.e. butterworth, chebyshev, etc.), centre frequency, bandwidth as well as the attenuation parameters. ∏ ⋯ — (1) in [12], a 4th-order error-feedback (ef) topology was chosen for the d/a modulators as this topology does not cause any signal corruption for its unity-gain signal transfer function (stf). the final design step involves assigning an appropriate loop-filter topology. the time delay and accumulate (tda) topology, commonly known as the direct form-1 iir filter topology, shown in figure 2, is preferred for the loop-filter as this topology uses smaller feedback and feedforward coefficients. note that in this study, fixedpoint arithmetic is used where these implementations are likely to result in overflow due to the recursive nature of the iir filters. also instead of accumulators, delayers are used as the main building blocks to contain the internal signals, therefore preventing overflow and hence resulting in fewer internal data paths. table 1 summarizes the designed filter specifications and table 2 shows their corresponding coefficients. as proposed in [12], the node-equation method is applied to convert the designed sp  modulator to its npath ti counterpart. it is an easy-to-apply technique and results in fewer components when compared to the polyphase decomposition method [6], [7]. the main idea of the node equation method is to write the node equations of the sp topology in the time-domain and individually convert these equations in a time-interleaved manner to construct the corresponding n-path topology. figure 3 depicts the 2-path ti topology of the sp  modulator given in figures 1, 2. moreover, a 4-path topology using this developed technique was designed, modelled and evaluated as will be reported in section 4.   x z  y z    h z r z  s z figure 1. block diagram of the ef topology.  1z   in h z   out h z  1z 1z 1z        1l 2l 3l 4l 1k 2k 3k 4k figure 2. the tda topology.  1z   in h z   out h z  1z 1z 1z        1l 2l 3l 4l 1k 2k 3k 4k figure 3. designed 2‐path ti topology. figure 4. set‐up environment.  acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 70  table 1. designed filter specifications.   design specs. butterworth chebyshev inv. chebyshev elliptical centre-frequency 0.2 0.2 0.2 0.2 bandwidth 0.02 0.004 0.02 0.02 passband ripple 1 db 1 db stopband ripple 60 db 80 db   table 2. feedback and feedforward coefficient.  butterworth fractional 0.0549 -0.2099 0.1557 -0.1628 -1.1836 2.1736 -1.0828 0.8372 sd.15-bit 1799 -6878 5103 -5335 -38785 71223 -35481 27433 chebyshev fractional 0.0406 -0.1548 0.1133 -0.1175 -1.1980 2.2287 -1.1252 0.8825 sd.15-bit 1329 -5072 3713 -3850 -39255 73030 -36870 28918 inverse chebyshev fractional 0.2323 -0.8750 0.5650 -0.5381 -1.0037 1.5067 -0.6711 0.4619 sd.15-bit 7613 -28672 18513 -17634 -32890 49370 -21991 15134 elliptical fractional 0.0406 -0.1548 0.1133 -0.1175 -1.1980 2.2287 -1.1252 0.8825 sd.15-bit 1329 -5072 3713 -3850 -39255 73030 -36871 28918   table 3. allocated fpga sources.   design specs. butterworth chebyshev inv. chebyshev elliptical sp # slices used # luts # mult. 18x18 158 40,960~1% 289 40,960~1% 8 40 20% 158 40,960~1% 281 40,960~1% 8 40 20% 162 40,960~1% 294 40,960~1% 8 40 20% 158 40,960~1% 282 40,960~1% 8 40 20% 2-path # slices used # luts # mult. 18x18 174 40,960~1% 435 40,960~1% 16 40 40% 174 40,960~1% 435 40,960~1% 16 40 20% 179 40,960~1% 458 40,960~1% 16 40 20% 174 40,960~1% 435 40,960~1% 16 40 40% 4-path # slices used # luts # mult. 18x18 196 40,960~1% 747 40,960~1% 32 40 80% 196 40,960~1% 747 40,960~1% 32 40 80% 201 40,960~1% 794 40,960~1% 32 40 80% 196 40,960~1% 747 40,960~1% 32 40 80% acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 71  3. fpga implementation  the designed 4th-order sp, 2-path and 4-path d/a  modulators are implemented in vhdl and synthesized on the xilinx® spartantm-3 development kit. as explained in section 2, fixed-point arithmetic is chosen and a 16-bit single sinusoidal input signal is applied to the system using look-up tables. note that the 16-bit signal is composed of a 1-bit sign and 15-bits for the fractional component, hence any internal path with a bit number bigger than 16 has an integer part. in table 3 this arithmetic is symbolized as sd.15-bit where s represents the sign bit, d represents the decimal bits and the 15-bit represents the fractional part of the number. in other words, the total bit number can be calculated as s+d+15. for instance: to represent a value of 1.6537, 15-bits for the fractional part, 1-bit for the decimal part and 1-bit for the sign are needed resulting in a 17-bit fixed point representation. the binary value is ‘01101001110101100’ which equates to an integer number of 54190 that can easily be calculated by multiplying 1.6537 with 215. the output data is taken via an rs232 connection and processed in the matlab environment including the decimation filtering. figure 4 shows the block diagram of the designed set-up. there are two universal clocks on xilinx® spartantm-3 development kit, 66 mhz and 100 mhz. in this study, it is preferred to clock the modulators at 66 mhz. this results in a clock frequency of 33 mhz and 16.5 mhz for the 2path and 4-path modulators’ individual paths respectively. since a normalized input frequency of 0.2 is needed, a sinusoid with a frequency of 26.4 mhz is sampled at 66 mhz and sample values are restored in a look-up table. the tonality behaviour of d/a  modulators will be discussed in section 4. it should be pointed out that dithering will be used to alleviate the tonal behaviour of the designed  modulators. a 16-bit fibonacci linear feedback shift register (lfsr) is built for its relatively white output spectrum. the first 14-bits are taken as output to lower the applied noise power. as depicted in figure 6, -70 db of white noise is obtained for normalized frequencies above 0.055. in figure 5, the output spectrums of the implemented  modulators are illustrated for different filter types. the occurrence of the tones observed for the elliptical based ddsm topology will be explained in section 4. the allocated fpga sources are summarized in table 3. it should be pointed out that the number of slices given in a) butterworth, sp    b) chebyshev, 2‐path    c) inv. chebyshev, 4‐path    d) elliptical, 4‐path  figure 5. output spectrums of the fpga implementations. figure 6. output spectrum of the fibonacci lfsr.  acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 72  table 3 includes the number of flip-flops and latches used. since delayers are chosen as the main building blocks to confine the internal path number within the minimum range, the adding blocks of the integrators are already eliminated. the number of multipliers can be easily calculated for a ti topology using its corresponding sp prototype as , where is the number of multipliers used in the sp prototype and is the path number of the ti topology. however, all other components are realized using the slices and luts of the fpga, including the upsamplers, downsamplers, adders, input lut, reset circuitry and rs232 circuitry. therefore, it is hard to estimate number of slices and luts used for a ti topology using its corresponding sp prototype. 4. tonality  the quantization of a sinusoid creates tones, whose amplitudes and frequencies can be mathematically determined. this applies to variable-centre frequency  modulators when excited by sinusoidal inputs. extra care must be taken especially in the case of digital  modulators as these tones are caused both by the quantizer and finite wordlengths. these tones can be modelled using the sawtooth quantization error model [20]. however, since the quantization error is highly input signal dependent, the actual power of these tones may become extremely difficult to predict. the first step of the sawtooth quantization noise model is to use the additive noise model. figure 7 depicts the additive noise approximation of the quantizer [4]. the second step is to define this additive quantization error as an input signal dependent sawtooth function. to have better understanding of this theory, the simulated quantization error of a sinusoid for a 1-bit quantizer is shown in figure 8. as seen, the quantization error of a sinusoid is a sawtooth signal with a frequency of 2 multiplied by a sinusoidal signal with a frequency of , where is the input signal frequency. note that for an m-level quantizer, the frequency of the sawtooth signal will be 2 as expressed in (2), where q is the quantization step size and is the input amplitude. also, ε and ε are the input amplitude dependent errors that are assumed to have a white distribution. however, in this case they are assumed to be negligible and are approximated to zero. 2 (2) ∑ 1  (3) 2 ∑ 1  (4) an ideal sawtooth wave function can be written as the sum of sinusoids with integer multiples of the fundamental frequency which is 2 in this case (3). hence, the sawtooth signal is multiplied by a cosine (4), the process of calculating the quantization error signal is similar to double-sided am modulation. it is well known that, if a sinusoidal signal with a frequency of is am-modulated by another sinusoid with a frequency of , the resulting tones will be at , , . since 2 ,4 ,6 … and , the resulting tones will be at … 6 1 , 4 1 , 2 1 , 2 , 2 1 , 4 1 , 6 1 …]. in order to explain it clearly, an example of the sawtooth quantization error model is given. it is assumed that a sinusoid with a normalized frequency of 0.15 (5) is applied to a 1-bit quantizer. this results in a quantization error of (6). for the ideal case, the quantization error is not input amplitude dependent. 2 0.15 (5) 2 0.15 ∑ 1  . (6) figure 9a shows the spectrum of the sawtooth with a fundamental frequency of 0.3 and figure 9b depicts its am a) quantization block diagram                  b) linear model of the quantization  figure 7. additive white noise block diagram.  a )  0.9  b) 0.2  figure 8. quantization error of a sinosoid for a 1‐bit quantizer.  acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 73  modulation by a cosine with a frequency of 0.15. it should be remembered that tones beyond 0.5 are folded back and added to the already existing tones within the range of [0, 0.5] as their frequency is mapped to f f 0.5 . the operator x represents the largest integer less than or equal to x and f is the normalized frequency of the signal. finally, figure 9c shows the output of the quantizer that is equal to the sum of the quantization error shown in (6) and the input signal given in (5). the only difference between figure 9b and figure 9c is the amplitude increase of the input frequency tone. the sawtooth quantization error model can be summed up as follows: 1. the expected high tones in a  modulator resulting from the quantization of a sinusoid, not the limit cycle tones, can be calculated in terms of their frequency and amplitude. 2. these tones can be whitened by dithering, especially for multi-level quantizers due to their smaller q resulting in lower amplitude tones when compared to the 1-bit quantizer. however, at some particular frequencies such as 0.25, 0.125, 0.375…etc dithering may not work sufficiently for all the tones. this is because harmonics of the sawtooth signal are mapped and added to each other at the same frequencies resulting in higher amplitude tones. on the other hand, variable centre frequency  modulators exhibit more but smaller amplitude tones that can be reduced sufficiently when dithering is employed. this makes the proposed topologies more attractive compared with their mid-band counterparts. 3. input frequencies, whose values are irrational, result in a higher number of tones since the harmonics of the sawtooth signal are not folded back to the same frequencies. therefore, the resulting tones are expected to have lower amplitudes. on the other hand, it is apparent from (3) that the amplitude series of the sawtooth harmonics is divergent. besides, irrational frequency tones are mapped close to each other and may not be sufficiently suppressed within the signal-band, thus resulting in significant snr reduction. 4. the white noise approximation still applies as can easily be seen from the noise floor of figure 9. this noise floor is caused by and errors. needless to say that in a  modulator, both the quantization noise and the tones will be accumulated and shaped. 5. as already explained, both the quantizer and the finite wordlengths of the d/a  modulators cause the sawtooth signal tones. interestingly, the tones created by the finite wordlength effects are whitened during the accumulation process of the loop-filter since their amplitudes are very small for a 16-bit or higher wordlength quantization. however, for shorter wordlengths, they are not whitened sufficiently well and are subsequently processed by the 1-bit quantizer. since these resulting tones’ amplitudes are not input amplitude dependent, these finite wordlength based tones create extra tonality, which may result in  modulator instability. moreover, using ef modulator topologies, as already explained, helps to diminish the finite wordlength based tones since the already whitened/accumulated feedback signal is extracted from the finite wordlength input sinusoid. a) sawtooth signal’s harmonics   b) the resulting quantization error  c) output of the quantizer  figure 9. 1‐bit quantizer under sinusoidal excitation.  acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 74  4.1. quantization tones in ti  modulators the sawtooth quantization error is caused both by the quantizer and the finite wordlengths of the ddsms as mentioned earlier. hence, both sp and ti ddsms are excited by a quantized sinusoid. the quantization tones are also given as input to these modulators and may affect the stability and the snrs especially for the ti ddsms. the ti concept depends on the idea of the perfect cancellation of the signal images created by the downsampling process [5-7]. however, the quantization tones’ images and the input signal images produced by downsampling cannot be cancelled perfectly due to the finite wordlength of the internal paths. this situation can be analogous to the path mismatches that characterize discrete-time a/d  modulators [20-23]. the input normalized signal frequency is 0.2 thus causing quantization tones at frequencies: 0.1, 0.3, 0.4 and 0.5. figure 10 depicts some of the fpga implementation output spectrums of the designed ddsms. it is apparent that the power of the quantization error tones depends on the topology coefficients as well as the path number of the ddsm. unfortunately,  modulators are non-linear therefore making it hard to mathematically model the effects of the coefficients and the path number on the amplitude of these tones. consequently, the tones seen in figure 10a are quantization tones whilst the tones seen at random frequencies in figure 10b, figure 10c and figure 10d are caused by the limit cycle oscillations.   a) butterworth, 2‐path      b) butterworth, 4‐path    c) elliptical, 2‐path    d) elliptical, 4‐path  figure 10  output spectrums of the fpga implementation.  a) butterworth, behavioural model  b) butterworth, fpga implementation  figure 11. snr plots for the butterworth filter.  -80 -70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 input amplitude in dbs s n r in d b s sp 2-path 4-path acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 75  5.    simulations and experimental results  the snr curves of the sp, 2-path and 4-path ddsms for an osr of 64 are shown in figure 11-figure 14 both for the behavioural-level models and fpga implementations. the effective osr of the sp is 64, whilst it is 128 for the 2path and 256 for the 4-path topologies. extensive behavioural-level simulations for various centre frequencies and bandwidths demonstrate that the inverse chebyshev and elliptical filters provide superior snr values compared to their butterworth and chebyshev counterparts. this is attributed to the evenly distributed zeros and steeper transition bands of the former two which result in lower quantiztion noise power over the signal bandwidth. furthermore, inverse chebyshev filters can be designed to have normalized bandwidths up to 0.001 (asuming nyquist to be at 0.5) without causing the  modulator to become unstable. on the other hand, the highest snr difference occurs for the chebyshev filter. this is due to the chosen design specifications and resulting coefficients of the loop-filter. therfore it is intended to further extend the work for different frequencies and different design specifications in order to have a better understanding of this discrepancy. for the remaining filter types, an snr decrement of 3-5 db is observed when implemented on fpga due to the circuit imperfections and clock jittering effects. 6. conclusions  novel variable centre frequency dualand quadruple ti  modulators, which employ an assortment of filter types and topologies, were designed, modelled and analysed. the proposed ti  modulators are well suited for a wide range of applications as they offer designers and practitioners the flexibility of defining the centrefrequency, bandwidth as well as the pass-band and stopband parameters. the topology complexity, snrs, stability and tonality of the aforementioned multi-path topologies were evaluated and compared with each other and against their single-path counterparts. the designed topologies were implemented and synthesized on xilinx® spartantm-3 development kit using fixed-point arithmetic. circuit outputs were taken via rs232 connection provided on the fpga board and evaluated using the matlab routines developed by the authors. these routines included the decimation process as well. the experiments undertaken by the authors further validated the design methodology presented in the paper. furthermore, the implemented topology outputs also confirmed the proposed mathematical model of the quantization tones of these digital multi-path  modulators, when excited by sinusoidal input signals. a) chebyshev, behavioural model  b) chebyshev, fpga implementation  figure 12. snr plots for the chebyshev filter.  a) inv. chebyshev, behavioural model b) inv. chebyshev, fpga implementation  figure 13. snr plots for the inv. chebyshev filter.  acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 76  references  [1] ostrovskyy, p.; gustat, h.; ortmanns, m.; christoph scheytt, j., "a 5-gb/s 2.1–2.2-ghz bandpass δ-σ modulator for switch-mode power amplifier," microwave theory and techniques, 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[7] r. khoini-poorfard, l. lim, and d. johns, “timeinterleaved oversampling a/d converters: theory and practice,” circuits and systems ii: analog and digital signal processing, ieee transactions on, vol. 44, no. 8, pp. 634 –645, aug 1997. [8] x. wu, v. a. chouliaras, j. l. nunez, and r. m. goodall, “a novel control system processor and its vlsi implementation,” ieee trans. very large scale integr. (vlsi) syst., vol. 16, no. 3, pp. 217–228, mar. 2008. [9] m. kwon, j. lee, and g. han, “a time-interleaved recursive loop band-pass delta-sigma modulator for a digital if cdma receiver,” circuits and systems ii: express briefs, ieee transactions on, vol. 52, no. 7, pp. 389 – 393, july 2005. [10] kizilkaya, i.k.; al-janabi, m.; kale, i., "design and evaluation of time-interleaved variable center-frequency sigma-delta modulators," digital signal processing (dsp), 2013 18th international conference on , vol., no., pp.1,6, 1-3 july 2013. [11] kizilkaya, i.k.; janabi, m.-a.; kalei, i., "novel timeinterleaved variable-center frequency δ-σ modulators-design, analysis and critical evaluation," instrumentation and measurement technology conference (i2mtc), 2013 ieee international , vol., no., pp.480,483, 6-9 may 2013. [12] kizilkaya, i.k.; al-janabi, m.; kale, i., “novel timeinterleaved variable centre-frequency, single-bit a/d and d/a sigma-delta modulator topologies,” imeko, 2013 19th international conference on , , vol., no., pp.599,604, july 2013. [13] i. galton, “one-bit dithering in delta-sigma modulator-based d/a conversion,” in proc. ieee int. symp, circuits syst., 1993, pp. 1310–1313. [14] k. hosseini and m. p. kennedy, “maximum sequence length mash digital delta-sigma modulators,” ieee trans. circuits syst. i, reg. papers, vol. 54, no. 12, pp. 2628–2638, dec. 2007. [15] karema, t.; ritoniemi, t.; tenhunen, h., "intermodulation in sigma-delta d/a converters," circuits and systems, 1991., ieee international sympoisum on , vol., no., pp.1625,1628 vol.3, 11-14 jun 1991. [16] fitzgibbon, b.; kennedy, m.p.; maloberti, f., "hardware reduction in digital delta-sigma modulators via bussplitting and error masking—part ii: non-constant input," circuits and systems i: regular papers, ieee transactions on , vol.59, no.9, pp.1980,1991, sept. 2012. [17] fitzgibbon, b.; kennedy, m.p.; maloberti, f., "hardware reduction in digital delta-sigma modulators via bussplitting and error masking—part i: constant input,"circuits and systems i: regular papers, ieee transactions on , vol.58, no.9, pp.2137,2148, sept. 2011. [18] chen, a.j.; yong-ping xu, "multibit delta-sigma modulator with noise-shaping dynamic element matching," circuits and systems i: regular papers, ieee transactions on, vol.56, no.6, pp.1125,1133, june 2009. [19] bornoosh, b.; afzali-kusha, a.; dehghani, r.; mehrara, m.; atarodi, s.m.; nourani, m., "reduced complexity 1-bit high-order digital delta-sigma modulator for low-voltage fractional-n frequency synthesis applications," circuits, devices and systems, iee proceedings , vol.152, no.5, pp.471,477, 7 oct. 2005. [20] abuelma'atti, m.t., "spectrum of a nonlinearly quantised equal-amplitude dual-tone signal," circuits, devices and systems, iee proceedings , vol.148, no.1, pp.11,18, feb 2001. [21] t. salo, s. lindfors, and k. halonen, “a double-sampling scresonator for low voltage bandpass delta; sigma;modulators,” circuits and systems ii: analog and digital signal processing, ieee transactions on, vol. 49, no. 12, pp. 737 – 747, dec 2002. [22] hwi-ming wang; tai-haur kuo, "the design of high-order bandpass sigma-delta modulators using low-spread singlestage structure," circuits and systems ii: express briefs, ieee transactions on , vol.51, no.4, pp.202,208, april 2004. [23] reekmans, s.; rombouts, p.; weyten, l., "mismatch insensitive double-sampling quadrature bandpass δ-σ modulation," circuits and systems i: regular papers, ieee transactions on , vol.54, no.12, pp.2599,2607, dec. 2007.   a) inv. elliptical, behavioural model   b) elliptical, fpga implementation  figure 14, snr plots for the inv. elliptical filter.  template for an acta imeko event paper acta imeko issn: 2221-870x july 2017, volume 6, number 2, 8-12 acta imeko | www.imeko.org july 2017 | volume 6 | number 2 |8 research on volume determination of mass standards with two acoustic measuring chambers man hong hu, jian wang, yue zhang, chang qing cai, rui lin zhong, jing an ding, and xiao ping ren division of mechanics and acoustic, national institute of metrology, no.18, bei san huan dong lu, chaoyang dist, beijing, p.r.china section: research paper keywords: mass standard; volume measurement; acoustic method citation: man hong hu, jian wang, yue zhang, chang qing cai, rui lin zhong, jing an ding, and xiao ping ren, research on volume determination of mass standards with two acoustic measuring chambers, acta imeko, vol. 6, no. 2, article 3, july 2017, identifier: imeko-acta-06 (2017)-02-03 section editor: min-seok kim, research institute of standards and science, korea received march 19, 2016; in final form october 22, 2016; published july 2017 copyright: © 2017 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: this work was supported by natural science foundation of china (nsfc) (grant no. 51205379), p.r. china corresponding author: hu manhong, e-mail: hmh@nim.ac.cn 1. introduction for the mass measurement of a weight, the abba cycle on a mass comparator is used, in which a is the reference (standard) mass weight and b is the test mass weight. the mass comparison is usually carried out in air, based on the difference of the gravitational force caused by the standard or test weight on the mass comparator. the air bouncy can contribute a big uncertainty especially in the high accuracy mass measurement such as the prototype level or e1 class level. thus the mass standard’s volume needs to be precisely determined for the air buoyancy correction [1]. there are many measurement technologies such as the hydrostatic method, the dimension measuring method and the acoustic method. although the measuring accuracy cannot be on the same level as the hydrostatic method (usually with relative combined uncertainty as low as 1×10-6 (k=1)), the acoustic method is a promising volume measuring method because there is no contact with mass standards during the whole volume measuring procedure, especially for the weights with non-regular shape and 3d curved surfaces. both the standard weights and test weights don’t need to be immersed in any liquid. m. ueki et al. firstly developed an acoustic measuring system to determine the volume of mass weights ranging from 1 g to 10 kg at the national metrology institute of japan (nmij) [2][6]. for the weights with nominal value ranging from 100 g to 10 kg, a relative uncertainty of 1×10-3 (k=2) is achieved. for weights ranging from 1 g to 100 g, the measuring combined standard uncertainty is below 0.0021 cm3 [2]-[5]. an acoustic volume measuring system has also been designed at the national institute of metrology china (nim) to extend the measurement range of the nominal value of mass weights up to 20 kg [7]. however, since the air inside the chamber does not change perfectly adiabatically (necessary to get a high measuring accuracy), the ratio of shape and volume of the reference weight needs to be similar to that of the test weight. otherwise a large non-linearity measurement error will be introduced to the measuring process. to investigate the non-linearity contribution in the acoustic measuring method, an acoustic measuring system with two measuring chambers is designed by the national institute of metrology china. the volumes of mass standards ranging from abstract the acoustic volume measuring method is a promising non-contact method for volume determination of mass weights. to improve the measuring accuracy of volume determination with the non-contact acoustic method, an acoustic measuring system with two measuring chambers is newly designed to compensate for the non-linearity measuring errors. when reducing the remaining air in the measuring chamber, the measuring accuracy can be greatly reduced. the volumes of mass standards ranging from 100 g to 5 kg are tested to evaluate the non-linearity errors of the volume measurement. the relative uncertainties of the acoustic volume determination are below 7.0 ×10-4 (k=2). acta imeko | www.imeko.org july 2017 | volume 6 | number 2 |9 200 g to 5 kg are tested to evaluate the non-linearity errors of the acoustic measuring process. 2. experimental apparatus and measuring procedure 2.1. experimental apparatus the acoustic method is based on gas compressibility laws. assuming the gas changes adiabatically, the air pressure, p, has a constant relation with the volume of air, v, as expressed in (1): p v consγ× = (1) here, γ is the ratio of the specific heats, which is 1.40 at atmospheric pressure and room temperature. the newly designed measuring apparatus made of aluminum alloy with two measuring chambers is shown in figure 1. a sinusoidal drive signal from a signal generator is applied to a loudspeaker between the two measuring chambers. this will alternately generate a compression wave with inverse phase in the left chamber and the right chamber. two sound pressure sensors (also called microphones) are used to separately measure the pressure changes, that is, ∆p1 in the left chamber and ∆p2 in the right chamber, respectively, as shown in (2) and (3), where p0 is the air pressure in the chamber. the output signals from two microphones, e1 and e2, are converted into digital signals and sent to a computer for sound pressure calculation. the resulting sound pressure ∆px is measured. the ratio of the pressures rn can be calculated as ∆p1/∆p2 [3]. 010 1 v v p p ∆ = ∆ γ (2) 020 2 v v p p ∆ = ∆ γ (3) 2.2. measurement results when measuring the volume with the acoustic method, it is assumed that air changes adiabatically in the two measurement chambers [2]-[7]. however, air near the surface of the test weight or reference weight and the wall of the containers changes isothermally [3]. thus, during the measurement, the actual displaced volume in the chamber by the test weight,vt0 and the reference weight, vr0 can be expressed with (4) and (5). 0t t tv v ds= − (4) rrr dsvv −=0 (5) where st and sr are the surface area of the test weight and reference weight, and d is the thickness of the air isothermal layer [5]. the effect of surface area to the volume measurement is not considered firstly, and it means that vt0≈vt, and vr0≈vr. based on the measuring sequence in figure 2, (6)−(8) can be derived. thus, the volume of the test weight, vt, can be calculated with (9). this equation is used to evaluate the nonlinearity error caused by the effect of the weight’s surface [3]. r t vv vv p p r − − = ∆ ∆ = 01 02 2 1 1 (6) t r vv vv p p r − − = ∆ ∆ = 01 02 2 1 2 (7) 01 02 2 1 3 v vvv p p r rt −− = ∆ ∆ = (8) 2 3 1 1 3 2 ( )(1 ) ( )(1 )t r r r r v v r r r − + = × − + (9) 3. measurement results and uncertainty analysis 3.1. measuring results according to (9), the amplitude ratio r is the key parameter for the acoustic volume measurement. as the driving signal to the loudspeaker showed in figure 3, the amplitude and frequency of the sinusoidal signal should be carefully chosen to achieve the best measurement of sound pressure and the amplitude ratio r. figure 4 shows the relationship between r and the amplitude and frequency of the sinusoidal signal using two measuring chambers. for each amplitude and frequency, 100 samples of sound pressure are acquired. it can be seen that to obtain a repeatability of r better than 1×10-4, the amplitude of the sinusoidal drive signal should be between 1.6 v and 1.9 v, a) b) c) figure 2. the schematic of procedures (a, b, c) of the measuring process , in which 1: bottom of measuring chamber; 2: side walls of measuring chamber; 3: left measuring chamber; 4: sound pressure sensor 1; 5: connecting tube; 6: sound pressure sensor 2; 7: right measuring chamber; 8: separating wall; 9: loudspeaker; 10: reference weight; 11:test weight. a) b) figure 1. schematic and pictures of the new measuring apparatus with two measuring chambers. 4 9 8 5 v1=v01-vr △p2 △p1 6 v2=v02-vt vr vt 10 11 4 9 8 5 v2=v02-vr △p2△p1 6 v1=v01-vt vrvt 3 7 11 101 2 4 9 8 5 v2=v02-vr-vt △p2△p1 6 v01 vr vt acta imeko | www.imeko.org july 2017 | volume 6 | number 2 |10 and the frequency should be 43 hz. the parameters used for the mass volume measurement are shown in table 1. according to (9), as shown in figure 5 to evaluate the nonlinearity errors, a nominal mass weight of 100 g is used as the reference weight, and nominal mass weights ranging from 200 g to 5 kg are measured with the 3-step method showed in figure 2. the non-linearity error is expressed as the deviation of the measurement value from the acoustic method to the volume measuring value from the hydrostatic method. with the same reference weight, the non-linearity error of the measured volume of the test weight increases with its nominal value. as also shown in figure 6, a test weight with a nominal value of 2 kg is used as the test weight, and weights ranging from 100 g to 1 kg are used as the reference weight. the nonlinearity error of the measured volume of 2 kg weight decreases when the nominal value of the reference weight increases. based on the analysis in figure 5 and figure 6, it can be concluded that the larger the difference between the volume, surface or the ratio of volume and surface of the reference weight and the test weight, the higher the non-linearity of the acoustic measuring method can be introduced. if the effect of the weight surface can be compensated, the accuracy of the acoustic volume method can be improved significantly. to evaluate the effect of the weight surface and to improve the volume measuring accuracy, certain artefacts were put into both measuring containers separately as shown in figure 7(a) to reduce the volume of air remained in the two containers. the tests weights ranging from 100 g to 5 kg are measured with a 1 kg reference weight. the non-linearity errors were evaluated to investigate the effects of weights’ surface. the non-linearity error was defined as the deviation between the volume measured by the acoustic method and the volume measured by the hydrostatic method. the results are shown in figure 7(c). from 100 g to 5 kg, the non-linearity error shows no significant difference when the surface ratio between the test weight and reference weight changes. 3.2. uncertainty analysis according to (9), describing the calculation of the volume, there are four main uncertainty contribution factors which are the reference weight’s volume, r1, r2 and r3. the uncertainty budget of the volume of the 200 g weight using the 100 g weight as reference weight is shown in table 2. with the same uncertainty evaluation method, the volume uncertainty evaluation results of weights ranging from 200 g to 5 kg are shown in table 3, and the relative extended uncertainties show no significant differences for the big contribution of the non-linearity error during the volume measuring process. the volume uncertainties of the 2 kg weight using different reference weights are shown in table 4. the figure 3. schematic of the connection of the signal generator to the loudspeaker. 39 40 41 42 43 44 45 46 47 48 49 50 51 0.000 0.001 0.002 0.003 0.004 0.005 r m ax r m in rmax rmin f (hz) r 0.924 0.925 0.926 0.927 0.928 the average of r the sinusoidal signal voltage :0.6 v 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 0.0000 0.0005 0.0010 0.0015 0.0020 0.0025 0.0030 r m ax r m in sinusoidal signal voltage (v) rmax rmin r 0.9240 0.9245 0.9250 0.9255 0.9260 0.9265 0.9270 0.9275 0.9280the sinusoidal signal frequency:43 hz the average of r a) b) figure 4. relationship between r and the amplifier or frequency of the sinusoidal signal. 100 g 200 g 500 g 1000 g 2000 g reference weight test weight 0 200 400 600 800 1000 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 test weight: 2 kg n on -li ne ar ity e rr or (c m 3) nominal value of reference weight (g) non-linearity error a) b) figure 6. the reference weights ranging from 100 g to 1 kg and test weight of 2 kg (a) and the measurement results (b). a) loud speakerleft container right container artefacts for reducing the air in the chambers reference weighttest weight 1 kg reference weight test weights 0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 0.000 0.005 0.010 0.015 0.020 0.025 0.030 n on -li ne ar ity e rr or (c m 3 ) nominal value (g) non-linearity error b) c) figure 7. artefacts in the chamber (a) to reduce the remaining air in the chambers, the picture of 1 kg reference weight and test weights ranging from 100 g to 5 kg (b) and the measuring results (c). table 1. parameters used for the volume measurement. parameters configuration of the sinusoidal signal gain of left chamber (db) 20 gain of right chamber (db) 20 sinusoidal signal frequency (hz) 43 sinusoidal signal voltage (v) 1.6 ~ 1.9 100 g 200 g 500 g 1000 g 2000 g reference weight test weight 0 1000 2000 3000 4000 5000 0 5 10 15 20 25 30 35 40 reference weight: 100 g n on -li ne ar ity e rr or (c m 3 ) nominal value of test weight (g) non-linearity error a) b) figure 5. the reference weight of 100 g and test weights ranging from 200 g to 5 kg (a) and the measuring results (b). sinusoidal signal acta imeko | www.imeko.org july 2017 | volume 6 | number 2 |11 relative expanded uncertainty decreases with the reference weight’s nominal value, and this indicates that the non-linearity errors decrease in the same trend. however, when the volume of air in both measuring chambers is reduced as shown in figure 7(a), the measuring accuracy improved immediately, as shown in table 5. all the relative expanded uncertainties are below 7.8×10-4(k=2). the volume of 5 kg weight is 500 times that of the 100 g weight. however there is no big difference between the volume measuring accuracy of the test weights with different nominal values ranging from 100 g to 5 kg using the same 2 kg weight as the reference weight. 4. conclusions to investigate the non-linearity contribution in the acoustic measuring method, an acoustic measuring system with two measuring chambers is newly designed. the optimization of measurement parameters was investigated by analyzing the relationship between r and the amplitude or frequency of sinusoidal signal. the volumes of mass standards ranging from 100 g to 5 kg are tested to evaluate the non-linearity errors of the acoustic measuring method. when the air in the container is not adjusted, a poor volume measuring accuracy is achieved, the shape or surface/volume ratio will greatly influence the acoustic volume measuring accuracy. the residual air in the measuring chambers has a big influence on the acoustic measuring accuracy. when the volume of air in the measuring chamber is reduced to a proper volume by using the new designed experimental setup, the measuring accuracy improved immediately, and the shape or surface/volume ratio is no longer the main uncertainty contribution during the volume determination using the acoustic method. acknowledgement this research is supported in part by natural science foundation of china (nsfc) (grant no. 51205379), in part by the national science and technology support program under grant 2011bak15b06, in part by the nim jbs scientific project 2015 and in part by national key research and development program for national quality infrastructure under grant 2016yff0200103. references [1] weights of classes e1, e2, f1, f2, m1, m1-2, m2, m2-3, and m3, oiml regulation r111, edition 2004 (e) [2] m. ueki, t. kobata, s. mizushima, y. nezu, a. ooiwa and y. ishii, “application of an acoustic volumeter to standard weights”, 15th imeko world congress, osaka, 1999, pp. 213220. [3] t. kobata, m. ueki, a. ooiwa and y. ishii, “mesurement of the volume of weights using an acoustic volumeter and the reliability of such measurement”, metrologia, 41(2004) pp. s75-s83. [4] m. ueki, t. kobata, k. ueda and a. ooiwa, “automated volume measurement for weights using acoustic volumeter”, table 5. uncertainty budget of test weights using 1 kg weight as reference weight after reducing the air in the acoustic chambers. sources test weights 5 kg 2 kg 1 kg 500 g 200 g 100 g volume of reference weight(cm3) 0.015 0.006 0.003 0.002 0.001 0.0003 r1(cm 3) 0.068 0.028 0.017 0.009 0.005 0.002 r2(cm 3) 0.014 0.008 0.005 0.004 0.002 0.001 r3(cm 3) 0.084 0.041 0.026 0.012 0.006 0.003 combined uncertainty(cm3) 0.110 0.051 0.032 0.016 0.008 0.004 expanded uncertainty(cm3) 0.220 0.102 0.063 0.032 0.017 0.008 relative expanded uncertainty( k=2, ×10-2) 0.036 0.041 0.050 0.052 0.068 0.064 table 2. uncertainty budget of the volume of 200 g weight using the 100 g weight as reference weight. sources uncertainty sensitivity coefficient uncertainty contribution volume of reference weight 0.0005 cm3 2 0.0010 cm3 r1 0.00001 11152 cm 3 0.11 cm3 r2 0.00001 5545 cm 3 0.06 cm3 r3 0.00001 16672 cm 3 0.17 cm3 combined uncertainty 0.21 cm3 expanded uncertainty ( k=2) 0.42 cm3 relative expanded uncertainty (×10-2, k=2) 1.7 table 3. uncertainty budget of test weights using the 100 g weight as reference weight. sources test weights 5 kg 2 kg 1 kg 500 g 200 g volume of reference weight(cm3) 0.025 0.010 0.0050 0.0025 0.0010 r1(cm 3) 3.14 1.2 0.53 0.27 0.11 r2(cm 3) 0.06 0.06 0.05 0.05 0.06 r3(cm 3) 3.20 1.17 0.59 0.32 0.17 combined uncertainty(cm3) 4.49 1.68 0.80 0.42 0.21 expanded uncertainty(cm3) 8.97 3.36 1.60 0.84 0.42 relative expanded uncertainty( k=2, ×10-2) 1.4 1.4 1.3 1.3 1.7 table 4. uncertainty budget of the volume 2 kg weight using different reference weights. sources reference weights 1 kg 500 g 200 g 100 g volume of reference weight(cm3) 0.006 0.008 0.010 0.010 r1(cm 3) 0.11 0.22 0.55 1.18 r2(cm 3) 0.06 0.06 0.06 0.06 r3(cm 3) 0.16 0.27 0.60 1.17 combined uncertainty(cm3) 0.21 0.36 0.9 1.7 expanded uncertainty(cm3) 0.41 0.71 1.8 3.4 relative expanded uncertainty( k=2, ×10-2) 0.4 1.2 7.2 27 acta imeko | www.imeko.org july 2017 | volume 6 | number 2 |12 20th conference on measurement of force, mass and torque, mexico, 2007. [5] m. ueki, t. kobata, k. ueda and a. ooiwa, “measurements of the volume of weights from 1 g to 50 g using an acoustic volumeter”, sice annual conference, okayama, japan, 2005, pp. 1742-1747. [6] i. torigoc and y. ishii, “acoustic bridge volumeter”, trans.soc.instrum.controleng, e-1(2001) pp. 164-170. [7] m.h. hu, j. wang, y. zhang, c.q. cai and x. l. wang, “experimental research on volume measurement method of weights based on acoustic principle”, chinese journal of scientific instrument, 33(2012) pp. 2337-2342. acta imeko, title acta imeko issn: 2221-870x july 2017, volume 6, number 2, 1 acta imeko | www.imeko.org july 2017 | volume 6 | number 2 | 1 journal contacts about the journal acta imeko is an e-journal reporting on the contributions on the state and progress of the science and technology of measurement. the articles are mainly based on presentations presented at imeko workshops, symposia and congresses. the journal is published by imeko, the international measurement confederation. the issn, the international identifier for serials, is 2221-870x. editor-in-chief paolo carbone, italy honorary editor-in-chief paul p.l. regtien, netherlands editorial board section editors (vol. 4, 5 and 6) leopoldo angrisani, italy filippo attivissimo, italy eulalia balestieri, italy eric benoit, france catalin damian, romania lorenzo ciani, italy pasquale daponte, italy luca de vito, italy luigi ferrigno, italy edoardo fiorucci, italy alistair forbes, united kingdom helena geirinhas ramos, portugal sabrina grassini, italy fernando janeiro, portugal konrad jedrzejewski, poland andy knott, united kingdom massimo lazzaroni, italy fabio leccese, italy rosario morello, italy michele norgia, italy pedro miguel pinto ramos, portugal sergio rapuano, italy dirk röske, germany alexandru salceanu, romania constantin sarmasanu, romania lorenzo scalise, italy emiliano schena, italy enrico silva, italy marco tarabini, italy eric benoit, france marcantonio catelani, italy paolo carbone, italy lorenzo ciani, italy spartacus gomariz, spain sabrina grassini, italy konrad jędrzejewski, poland min-seok kim, korea franco pavese, italy serge rapuano, italy paul regtien, the netherlands janaina m rodrigues, brazil alexandru salceanu, romania jan saliga, slovakia krzysztof stepien, poland marco tarabini, italy ian veldman, south africa claudia zoani, italy final editing rosario morello, university mediterranea of reggio calabria, italy about imeko the international measurement confederation, imeko, is an international federation of actually 42 national member organisations individually concerned with the advancement of measurement technology. its fundamental objectives are the promotion of international interchange of scientific and technical information in the field of measurement, and the enhancement of international co-operation among scientists and engineers from research and industry. addresses principal contact prof. paolo carbone university of perugia dept. electr. inform. engineering (diei) via g.duranti, 93, 06125 perugia, italy email: paolo.carbone@unipg.it acta imeko prof. paul p. l. regtien measurement science consultancy (msc) julia culpstraat 66, 7558 jb hengelo (ov), the netherlands email: paul@regtien.net support contact dr. dirk röske physikalisch-technische bundesanstalt (ptb) bundesallee 100, 38116 braunschweig, germany email: dirk.roeske@ptb.de journal contacts microsoft word 372-2382-1-le-rev acta imeko  issn: 2221‐870x  september 2016, volume 5, number 2, 64‐70    acta imeko | www.imeko.org  september 2016 | volume 5 | number 2 | 64  testing gopro for 3d model reconstruction in narrow spaces  fausta fiorillo 1 , marco limongiello 1 , belén jiménez fernández‐palacios 2 1 diciv‐departament of civil engineering, university of salerno, via giovanni paolo ii, 132, 84084, fisciano, italy  2 department of cartographic and terrain engineering, university of salamanca, hornos caleros, 50, 05003, avila, spain      section: research paper  keywords: photogrammetry; action camera; low‐cost systems; archaeology  citation: fausta fiorillo, marco limongiello, belén jiménez fernández‐palacios, testing gopro for 3d model reconstruction in narrow spaces, acta imeko,  vol. 5, no. 2, article 9, september 2016, identifier: imeko‐acta‐05 (2016)‐02‐09  section editors: sabrina grassini, politecnico di torino, italy; alfonso santoriello, università di salerno, italy  received: march 25, 2016; in final form july 5, 2016; published september 2016  copyright: © 2016 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  corresponding author: marco limongiello, e‐mail: mlimongiello@unisa.it     1. introduction  in the archaeological field, 2d representation is more prevalent and used compared to 3d restitutions (especially for technical use). in fact, realization of a photoplan is often required to collect chromatic and metric information in a single file. however, due to intrinsic (e.g. non-planar surfaces) and/or extrinsic (e.g. reduced distance to the object) conditions, it is not always possible to produce them. the use of multi-image photogrammetry for close-range surveys is an alternative solution, especially in the cultural heritage field and for complex acquisition conditions [1], [2]. in recent years there have been important developments in close-range photogrammetry thanks to various well known factors, such as: (i) the application of computer vision algorithms that allows automatic camera calibration and calculation of exterior orientation parameters; (ii) the improvement of photogrammetric software; (iii) and the increase of the quality of low cost digital cameras (able to provide high resolution images). the software packages developed, combining computer vision image-matching algorithms and photogrammetry principles, allow to obtain accurate 3d reconstructions and to export detailed orthoimages from the textured 3d models, with the implementation of a well-defined workflow. the estimation of geometric camera characteristics (focal length, centre of projection of the image, radial lens distortion coefficients) can be automatically performed within the software processing workflow. at the same time, the mathematical advances provide more flexibility to use a fisheye camera as well for photogrammetry applications. the fisheye lens has a wide coverage and helps to solve the problems related to narrow spaces and complex conditions for camera stations. the wide field of view can reduce the number of shots to cover the entire scene and the time of the acquisition phase; the disadvantages are the extreme distortions of the images [3], [4]. this paper presents the result of a test performed with a gopro hero 3 black (figure 1) used to survey an external wall surface in a very narrow space. the final aim is to obtain a detailed orthophoto from the feature-based 3d reconstruction of the wall surface. the photo processing was developed with different photogrammetric software (agisoft photoscan 1.2.3, pix4dmapper 1.4.46 and 3df zephyr aerial 2.3061l), analysing the accuracy of the final 3d models. 2. data acquisition  the selected area for the test is a perimetric wall, on the east side of “villa di giulia felice” (figure 2) in the archaeological abstract  the main objective of this paper  is to analyse the potential as well as the  limitations of an action camera (gopro hero 3 black)  in  photogrammetric applications for architectural cultural heritage reconstruction. the investigations were carried out in a site of notable  historical interest, “villa di giulia felice” in pompeii. in order to estimate the work pipeline, the time‐consuming processing and the  output product accuracy using fisheye camera  images, three commercial  image processing software packages were tested: agisoft  photoscan, pix4dmapper and 3df zephyr aerial. several comparisons among the final 3d models produced have been developed and  the results achieved. despite the problems found related to  lens distortion and the small distance from the camera to the object  (average distance ~80 cm), the test provided good results in terms of accuracy (average error [2‐3.5] cm) and reliability.  acta imeko | www.imeko.org  september 2016 | volume 5 | number 2 | 65  site of pompeii, italy. the villa, located near porta sarno, was explored first between 1755 and 1757 and then in 1953. the survey campaign of “villa di giulia felice” is part of the “great pompeii project”, whose aims are the protection, conservation, maintenance, and restoration of the site. an orthophoto of the entire wall was required for archaeological studies. the use of a terrestrial laser scanner (tls) was not considered due to the very close space, a corridor with the maximum distance between the two walls of 1.1 m. given the camera to object distance of 0.3 – 1.1 m and the length and height of the wall ~20 m and ~4 m respectively (figure 3), the wide-angle optic is an advantage in the acquisition phase of the images. indeed, this type of lens increases the field of view and thus decreases the number of shots to carry out. therefore, the project involved the use of a gopro hero 3 black with a nominal focal length of 3 mm and a pixel size of 0.00155 mm (sensor size of 6.2×4.65 mm). there are several image capture modes of the gopro camera. in this research, only the “wide” mode is utilized with the following specifications: image size 12 mp (4000×3000 pixel); vertical fov 94.4° and horizontal fov 122.6°. the advantages due to the use of gopro are its small size and low weight of the camera body (73 g for the black edition), which facilitates the photographic acquisition in narrow spaces. on the other hand, the disadvantage is that the wide-angle lenses are more exposed to distortions (especially at the extremes of the frame [5], [6]), which generate a distorting effect known as “bowl-effect” [7]. due to this factor, the use of fisheye camera is less common for photogrammetric purposes, leading to a loss in output precision [8]. in the present case study, the strong distortion effects are accentuated owing to the very small distances to the object. however, thanks to the implementation of a specific camera calibration algorithm to fit ultra-wide lens distortions, some commercial photogrammetric software is able to process photographs captured with wide-angle lenses (without the use of external software to create undistorted images). the acquisition project of the images included the use of a telescopic pole to obtain the photographs for the entire height of the wall. we have acquired 263 images in 11 horizontal strips, with overlapping of ~80 % (average baseline: 40 cm, minimum distance: 30 cm, maximum distance: 110 cm, figure 4) and parallel and horizontal axis camera. according to the sensor resolution and the acquisition distance, the medium gsd (ground sample distance) is 0.5 mm on the front area. using a total station (leica tcr 705 with nominal accuracy of 5 mm), 14 natural reference points uniformly distributed on the perimetric wall were measured (about one point per 5 m2, figure 5). the coordinates of control points were set in the gauss-boaga reference system. the natural control points were chosen as easily recognizable features on the images. in this way the measured reference coordinates were marked on the photos and used as ground control points (gcps) to scale and figure 1: size of gopro hero 3 black.  figure  4:  camera  to  wall  surveyed  distances:  on  the  x‐axis  there  is  the  distance from the object (m) and  on the ordinate the relative number of  photos.  figure 2: the plant of “villa di giulia felice” and the investigated perimetric wall (red).  figure 3: the corridor and perimetric wall surveyed of “villa di giulia felice”. acta imeko | www.imeko.org  september 2016 | volume 5 | number 2 | 66  georeference correctly the 3d reconstruction and to estimate the accuracy of the final model. all of these points were also used to have a better control on the error dispersion over the entire surveyed wall [9], [10]. 3. data processing  in order to evaluate the work pipeline, the time-consuming processing and the output products accuracy using action camera images, three commercial multi-image processing software packages were tested: agisoft photoscan 1.2.3 (founded in 2006), pix4dmapper 2.0.104 and 3df zephyr aerial (both founded in 2011). all software is focused on computer vision technology using image-matching algorithms that combine the automation of computer vision techniques with the photogrammetric principles. for each one, the whole process of 3d reconstruction, from the image orientation to the textured 3d model creation up to the orthophoto extraction, can be automated, with minimal external human intervention (e.g. input parameters setting, check/control point input). the common steps in the standard workflow are: 1) calculation of internal and external orientation parameters (with automated detection of key point and tie points) and creation of sparse cloud; 2) extraction of a dense cloud; 3) construction of a polygonal model; 4) texture mapping and 5) orthophoto production. furthermore, according to the increasing use of action cameras, the software is able to support a fisheye lens, implementing a specific camera model to fit ultra-wide lens distortions. as a first step, the estimation of the cameras' orientations allows alignment of the photographs and the reconstruction of sparse unscaled point clouds. the georeferencing process allows a linear transformation of the model using three parameters for translation, three for rotation and one for scaling. a least three reference points (gcps) must be known to calculate the transformation parameters. it is also possible optimize the estimated point cloud and internal and external camera orientation parameters using the gcps. the optimization procedure allows not only to achieve higher accuracy in calculating camera parameters but also to correct possible distortion (e.g. the bowl-effect). during the optimization process, the sum of re-projection errors and reference coordinate misalignment errors is minimized. the re-projection error concerns the difference between the control points position defined on the original image and their estimated position during optimization process. for that, the images were processed in a bundle adjustment, incorporating metric information in the form of gcps, which were manually inserted assigning the coordinates measured with the leica tcr 705 total station. in order to develop a coherent comparison among the results obtained from different programs, some precautions have been taken. in particular, (i) in the three chosen programs the gcps were detected on the same images; (ii) the whole process was performed with the same computer (a work station i7-4790, 3.60 ghz, ram 32,0, nvidia geforce gtx 970 2 gb) and in addition, (iii) the most similar settings were selected in the corresponding steps. the software gives us a summary report with information on the whole process, such as the setting of the input data, the final calibration parameters of the camera, the re-projection error on the control points, etc. in agisoft photoscan, the data processing is based on 5 consecutive steps: 1) align photos (internal/external camera orientation and sparse cloud creation), 2) build dense cloud, 3) build mesh, 4) build texture, 5) and export orthophoto. in the first step, full resolution images were used and the default parameter configuration was set with 40.000 key points limit and 4.000 tie points limit; all the cameras were aligned (263/263). the camera calibration parameters were also optimized using gcps. the dense matching was built in “high quality”; this setting involves a reduction factor of the image of 1/4 (two times by each side). the dense point cloud has more than 19 million of points and it does not show an information gap. the flowchart of pix4dmapper is based on 3 steps: 1) initial processing (internal/external camera orientation and sparse cloud creation), 2) point cloud and mesh (dense point cloud and textured polygonal model creation) and 3) dsm (digital surface model), orthomosaic and index. in this last phase it is possible to generate the orthophoto, according to a resolution requested by the user. using a default configuration template for the parameters setting, the software cannot align all cameras. the number of automatically detected key points was augmented to 15.000 and at the same time, the number of pairs for each image was augmented (setting the maximum value 12). with this configuration setting, the software aligns all images. gcps were used also to adjust the camera calibration and estimate a mean rms (root mean square) error of 0.017 m. in the second step, for the construction of the dense cloud, the same settings of agisoft photoscan were used, i.e. an image scale factor of 1/4 (half resolution on each side). the final point cloud has more than 24 million points. the data processing performed in 3df zephyr aerial is subdivided in 5 consecutive steps: 1) camera orientation and figure 5: topographic points (gcps).  figure 6: orthophoto and top view in agisoft photoscan.  figure 7: orthophoto and top view in pix4dmapper.    figure 8: orthophoto and top view in 3df zephyr aerial.  acta imeko | www.imeko.org  september 2016 | volume 5 | number 2 | 67  sparse point cloud generation (structure from motion); 2) dense point cloud generation (multi view stereo); 3) mesh extraction; 4) texture extraction and 5) ortophoto extraction. in order to use a similar processing setting, the used parameters were: a limit of 15,000 key points for the image in the first step; and an image scale factor of 1/4 in the second step. the software aligns 257/263 images and the dense point cloud has more than 2 million points. table 1 schematises the workflow steps of all software. table 2 shows the summary of data processing parameters. figures 6 to 8 show the final orthophotos obtained with agisoft photoscan, pix4dmapper and 3df zephyr aerial, respectively. it is possible to note that: (i) there is no visible bowl-effect (on the top view the wall profile is linear and has not curvature); (ii) the 3df zephyr aerial orthoimage (figure 8) has problems and missing parts on the upper area of the wall. a first consideration involves the time-consumption for the processing data. the duration of the elaborations with this latest software has been greater than the other two. instead, agisoft photoscan and pix4dmapper completed the camera alignment and matching process in comparable times (in total about 6 hours: 3 for the internal/external calibration and 3 for the construction of dense point cloud). 4. output comparisons  a first analysis was carried out on the spatial reconstruction of the position of each camera (external orientation). in the report released by the software, we can find (i) the coordinates (x, y, z) of the centre of each aligned frame and (ii) the rotations of the axes (respectively for the x, y, z axes). for each combination of software pairs (agisoft photoscan vs pix4d, agisoft photoscan vs 3df zephyr, pix4d vs 3df zephyr), the difference between the xyz coordinates and the  angles of corresponding photos were calculated. table 3 reports the average value, the standard deviation and the maximum value of the calculated difference. it can be noted that the major deviations are obtained in comparisons with 3df zephyr aerial. instead, the camera position and spatial orientation for each photo, found by agisoft and pix4d during the alignment step, are very similar. in fact, the comparison between agisoft photoscan and pix4dmapper has minimal residues (in the order of cm) for the xyz coordinates and differences lower than 1° for the rotations. a second comparison was conducted analysing the reprojection errors on the gcps calculated by each software package. in table 4 for each gcp the corresponding reprojection error on the xyz coordinates and their vector module (error 3d) is reported. the points have been inserted in the same images for each software package and by the same operator, in order to avoid more uncertainties in the calculation. agisoft photoscan presents the greatest deviations values (the average error is 3.5 cm and the standard deviation is 4 cm), while pix4d has the lowest errors (the average error is 2.2 cm and the standard deviation is 2.6 cm). we can also note that the points c06 and c10 show a very large deviation in every software package (about 7 cm and 10 cm respectively). probably they are due to errors in the topographical survey. a third analysis has foreseen in the statistical comparison of the georeferenced point clouds, removing the other points of the reconstructed 3d scene that do not belong to the wall investigated. a statistical analysis of the wall surface can provide an estimate of the point cloud noise and the deviations between the produced surfaces in the studied software. in figures 9 to 11 on the left column the comparison between pairs of the georeferenced point cloud obtained in each software package is shown (made by cloud compare). the deviation between the corresponding points of two point clouds is represented in false colour: the maximum deviations (red points) are present on the top wall and in the edges (e.g. doors, windows and roof). the point-to-point comparison results were plotted in the diagrams on the right column of figures 9 to 11. the graphics have on x-axis the deviation values (m) and on y-axis the corresponding number of points. the agisoft photoscan vs pix4d point clouds comparison presents the lowest deviations whereas pix4d vs 3df zephyr point clouds comparison presents the maximum deviation. according to the conclusion about the external camera parameter of table 3, the 3df zephyr aerial model presents greater deviations from the other. the statistical analysis of such deviations shows that they agree with a weibull type distribution, represented on the graphic with the grey continuous curve. the correspondence between the distribution and frequency histograms obtained is in fact remarkably significant (especially for comparison between agisoft vs pix4d). in the weibull distribution, the highest frequencies of the curves represent the deviation (on the x-axis) associated to the maximum number of compared point (on the y-axis). it is possible to observe that the highest frequencies of the distribution correspond to a deviation value table 3: comparisons among external camera calibration parameters.    x(cm)  y(cm)  z(cm)     a g is o ft  v s   p ix 4 d   mean  0.76  1.26  0.62  0.23  0.11  0.22  st. dev.  0.34  0.61  0.54  0.77  0.21  0.69  max   1.50  2.46  2.17  0.57  0.24  0.54  a g is o ft  v s   3 d f  z e p h y r  mean  18.15  10.88  21.03  0.22  0.06  0.21  st. dev.  11.18  7.51  3.73  0.14  0.06  0.14  max  48.26  37.95  44.82  9.60  2.86  9.42  p ix 4 d  v s    3 d f  z e p h y r  mean  17.66  11.29  21.61  0.17  0.11  0.17  st. dev.  11.09  7.78  3.45  0.81  0.21  0.72  max   48.85  37.14  44.08  10.96  2.82  9.74  table 1: workflow steps.  agisoft photoscan  pix4dmapper  3df zephyr  step 1  align photos  initial processing  sparse point cloud  step 2  build dense cloud  point cloud &  mesh  dense point cloud  step 3  build mesh  dsm, orthomosaic  and index  mesh  step 4  build texture  /  textured mesh   step 5  export orthophoto  /  orthophoto   table 2: output software comparison.     agisoft photoscan  pix4dmapper  3df zephyr   aligned cameras  263/263  263/263  258/263  tie point extracted  230.508  460.922  378.435  dense cloud   19.507.347  24.817.288  2.580.530  acta imeko | www.imeko.org  september 2016 | volume 5 | number 2 | 68  of about: 1.5 mm for the agisoft vs pix4d comparison, 3 mm for the agisoft vs 3df zephyr comparison and 2 mm for the pix4d vs 3df zephyr comparison. it means that the comparison between agisoft photoscan and pix4dmapper has a distribution that highlights how the two generated models are very similar, having a 1.5 mm deviation for most of the compared points of the model. furthermore, from graphics of the weibull distribution, it is possible to observe the presence of a higher noise in comparison between agisoft photoscan and pix4dmapper with 3df zephyr aerial. in figure 12 the weibull curves of the three comparisons have been represented on the same scale. the diagram underlines that the distribution peak corresponds to the lowest value of deviation (“absolute distance” on the x-axis) for agisoft vs pix4d comparison (orange curve) and to the highest value for agisoft vs 3df zephyr (blue curve). a further statistical analysis was performed using analysis of pearson’s correlation. the pearson product-moment correlation coefficient (or pearson correlation coefficient, for short) is a measure of the strength of a linear association between two variables and is denoted by “r”. basically, a pearson product-moment correlation attempts to draw a line of best fit through the data of two variable. the pearson correlation coefficient, “r”, can take a range of values from +1 to -1. a value of 0 indicates that there exists no linear relationship between the two variables. a value greater than 0 indicates a positive association; that is, as the value of one variable increases, so does the value of the other variable while a value less than 0 indicates a negative association. the standard method that statisticians use to measure the ‘significance’ of their empirical analyses is the p-value. a low pvalue (such as 0.05) is taken as evidence that the null hypothesis can be rejected (ho: ρ=0, where the correlation coefficient ρ is the ratio between the covariance between the two variables and the product of their standard deviations). a low p-value implies that the parameter is highly significant. figure 13 shows that the p-value max does not exceed in many cases (23/35) a p-value <5 %, does therefore there is a good correlation between the errors calculated in the various software. 5. conclusion  although there are wide-angle lenses with less distortions, we have chosen the gopro camera for its size and lightness and its control through a wi-fi connection, which has facilitated and simplified the photographic acquisitions. it was possible to use a simple and economic pole in order to take photos at different height. despite the narrow space, with a low-cost equipment, a contained number of pictures and easy operations, the horizontal and vertical coverage of the wall was obtained. the action camera represents a low-cost instrument that can have many applications, therefore it seemed interesting to verify the behaviour and reliability for photogrammetric applications even in extreme situations such as in this case study. in a previous step of this research [11], an external program was used to correct the distortion of each image before to import them in the photogrammetric software. within the nacional research project prin 2010-2011 “architectural perspective: digital preservation, content access and analytics (research unit of salerno), the initial studies and analysis were reported. these first researches have demonstrated that an external pre-filtering of the images (for example with the “lens correction” tool of adobe photoshop) could be useful when the photogrammetric software has no internal algorithms to correct fisheye lens distortions. in fact, during these early studies the first problem found is that pix4dmapper automatically read the parameters of the camera and automatically process the images by calculating, again, the distortions of wide-angle lenses. the results are models with evident deformation. instead, agisoft photoscan allows the table 4: gcp errors in tested software.  gcps name  error x (cm)  error y (cm)  error z (cm)  error 3d (cm)  agisoft  pix4d   zephyr  agisoft  pix4d  zephyr  agisoft  pix4d  zephyr  agisoft  pix4d  zephyr  c01  1.91  ‐0.60  ‐1.04  ‐2.17  0.01  0.25  ‐1.06  ‐0.10  ‐0.39  3.08  0.61  1.14  c02  0.73  0.10  1.57  ‐1.28  ‐0.30  ‐0.05  ‐1.16  0.60  ‐0.37  1.88  0.68  1.61  c03  1.25  ‐0.10  ‐0.17  ‐1.33  ‐0.30  ‐0.02  ‐0.87  0.60  ‐0.08  2.02  0.68  0.19  c04  ‐0.03  0.70  0.74  ‐0.30  ‐0.80  ‐0.26  ‐0.51  1.00  0.15  0.59  1.46  0.80  c05  ‐7.27  ‐0.10  2.58  11.57  0.10  1.01  ‐0.56  1.00  0.80  13.67  1.01  2.89  c06  ‐0.60  1.00  1.64  ‐0.68  ‐0.50  0.16  7.16  ‐6.00  ‐7.21  7.22  6.10  7.40  c07  ‐0.22  0.60  0.95  ‐0.38  ‐0.60  ‐0.18  0.27  0.10  0.00  0.52  0.85  0.97  c08  0.97  ‐0.30  0.05  ‐1.92  1.20  1.50  1.05  0.60  0.65  2.39  1.37  1.64  c09  0.72  0.30  0.09  ‐1.82  0.90  0.99  0.47  0.50  ‐0.15  2.02  1.07  1.00  c10  3.38  ‐2.80  ‐2.81  ‐5.73  5.10  5.89  ‐7.57  7.70  8.44  10.08  9.65  10.67  c11  0.18  0.30  0.26  0.34  ‐0.80  ‐0.47  0.47  ‐0.40  0.04  0.61  0.94  0.53  c12  ‐1.36  1.30  0.91  2.21  ‐2.20  ‐2.27  1.32  ‐1.20  0.22  2.91  2.82  2.46  c13  0.23  ‐1.20  ‐0.23  ‐0.02  ‐0.40  0.15  0.55  ‐0.70  0.01  0.60  1.45  0.27  c14  0.11  0.01  1.30  1.51  ‐1.70  0.05  0.45  ‐0.50  ‐0.40  1.58  1.77  1.36  mean (cm)  1.35  0.67  1.02  2.23  1.07  0.95  1.68  1.50  1.35  3.51  2.18  2.35  st.dev. (cm)  2.38  1.02  1.32  3.82  1.71  1.78  2.99  2.78  3.10  3.99  2.58  3.00    acta imeko | www.imeko.org  september 2016 | volume 5 | number 2 | 69  figure  12:  weibull  distribution  of  absolute  distances  (on  the  x‐axis  the  deviation  values  and  on  the  y‐axis  the  corresponding  number  of  compared points).  figure 13: pearson correlation coefficient (r) and p‐value (p‐val).      figure 9: the comparison between the photogrammetric point clouds of agisoft photoscan and pix4dmapper (on the left) and the corresponding graphic with weibull distribution (on the right). on the x‐axis the deviation values (m) and on the y‐axis the corresponding number of compared points of the two  models.          figure 10: the comparison between the photogrammetric point clouds of agisoft photoscan and 3df zephyr aerial (on the  left) and the corresponding  graphic with weibull distribution (on the right). on the x‐axis the deviation values (m) and on the y‐axis the corresponding number of compared points of the two models.         figure 11: the comparison between the photogrammetric point clouds of pix4dmapper and 3df zephyr aerial (on the left) and the corresponding graphic  with weibull distribution (on the right). on the x‐axis the deviation values (m) and on the y‐axis the corresponding number of compared points of the two  models.  acta imeko | www.imeko.org  september 2016 | volume 5 | number 2 | 70  elaboration of images previously corrected from the distortions with external software, but also in this case the model has nonlinear deformations. in conclusion, we have better results using the algorithm implemented in the photogrammetric software. the developed analysis enables us to consider the accuracy achieved acceptable using a fisheye camera for photogrammetry applications (max average error of 3.5 cm table 4). the new versions of the photogrammetry software that allow using directly fisheye images, guarantee better results and certainly simplify the elaboration. the comparison between the tested programs shows that the results can be considered comparable: the maximum deviation is between agisoft vs 3df zephyr point cloud and it is about 3 mm at the highest frequency of the weibull distribution. the conducted test allows us to deduce that the low cost sensors of action camera can be considered a useful tool for the survey of cultural heritage [12]. the main advantages of this technology are the cost of the equipment and the easy handling of the camera. the acquisitions with a fisheye camera are very useful to speed up and simplify the survey above all for very close range acquisition. finally, the survey approach tested in this case study proves to be efficient and successful. references  [1] f. fiorillo, b. jiménez fernández-palacios, r. remondino, s. barba, “3d surveying and modelling of the archaeological area of paestum, italy”, var virtual archaeology review, vol. 4, no. 8 (2013), pp. 55-60. [2] f. fassi, l. fregonese, s. ackermann, v. de troia, “comparison between laser scanning and automated 3d modelling techniques to reconstruct complex and extensive cultural heritage areas”, the international archives of the photogrammetry, remote sensing and spatial information sciences, vol. xl-5/w1 (2013), pp. 73-80. [3] c. strecha, r. zoller, s. rutishauser, b. brot, k. schneider-zapp, v.chovancova, m. krull, l. glassey, “quality assessment of 3d reconstruction using fisheye and perspective sensors”, isprs annals of the photogrammetry, remote sensing and spatial information sciences, vol. ii-3/w4 (2015), pp. 215-222. [4] j. covas, v. ferreira, l. mateus, “3d reconstruction with fisheye images strategies to survey complex heritage buildings”, proc. of digital heritage international congress, vol. 1, (2015), pp. 123-126. [5] c. balletti, f. guerra, v. tsioukas, p. vernier, “calibration of action cameras for photogrammetric purposes”, sensor, vol. 14, no. 9 (2014), pp. 17471-17490. [6] j. kim, m. pyeon, y. eo, i. jang, “an experiment of threedimensional point clouds using gopro”, international scholarly and scientific research & innovation, vol. 8, no. 1 (2014), pp. 82-85. [7] m. bolognesi, a. furini, v. russo, a. pellegrini, p. russo, “testing the low-cost rpas potential in 3d cultural heritage reconstruction”, the international archives of the photogrammetry, remote sensing and spatial information sciences, vol. xl-5/w4 (2015) pp. 29-235. [8] f. he, a. habib, “target-based and feature-based calibration of low-cost digital cameras with large field-of-view”, proceedings of the asprs 2015 annual conference, may 4-8 2015, tampa, florida (2015), pp. 25-32. [9] y. ninsalam, j rekittke, “landscape architectural foot soldier operations”, sustainable cities and society, september (2015), pp. 158-167. [10] a. guarnieri, a. vettore, s. el-hakim, l. gonzo, “digital photogrammetry and laser scanningin cultural heritage survey”, the international archives of the photogrammetry, remote sensing and spatial information sciences, volume xxxv part b5 (2004), pp. 154-158. [11] m. limongiello, b. jiménez fernández-palacios, “action camera for metric archaeological documentation in narrow spaces”, poster at the 1st international conference on metrology for archaeology (2014). [12] j. garcía fernández, a. alvaro tordesillas, s. barba, “an approach to 3d digital modeling of surfaces with poor texture by range imaging techniques. ‘shape from stereo’ vs. ‘shape from silhouette’ in digitizing jorge oteiza’s sculptures”. international archives of the photogrammetry, remote sensing and spatial information sciences, vol. xl-5/w4 (2015), pp. 2529. template for an acta imeko event paper acta imeko issn: 2221-870x july 2017, volume 6, number 2, 70-74 acta imeko | www.imeko.org july 2017 | volume 6 | number 2 | 70 progress on vacuum-to-air mass calibration system using magnetic suspension to disseminate the planck-constant realized kilogram eric c. benck, corey stambaugh, edward c. mulhern, patrick j. abbott, zeina j. kubarych mass and force group, national institute of standards and technology, 100 bureau dr., ms 8221, gaithersburg, md 20899, united states section: research paper keywords: magnetic suspension; mass metrology; planck constant; revised si; si units; watt balance citation: eric c. benck, corey stambaugh, edward c. mulhern, patrick j. abbott, zeina j. kubarych, progress on vacuum-to-air mass calibration system using magnetic suspension to disseminate the planck-constant realized kilogram, acta imeko, vol. 6, no. 2, article 12, july 2017, identifier: imeko-acta-06 (2017)02-12 section editor: min-seok kim, research institute of standards and science, korea received june 30, 2016; in final form october 14, 2017; published july 2017 copyright: © 2017 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: eric c. benck, e-mail: eric.benck@nist.gov 1. introduction the kilogram is the only remaining base unit in the international system of units (si) that is still defined by an artefact, the international prototype kilogram (ipk). the ipk is made of a platinum-iridium alloy and is maintained at the international bureau of weights and measures (bipm) in sevres, france. therefore the unit of mass can only be realized at the bipm, and must be disseminated to the rest of the world through a chain of comparison calibrations by the world’s national metrology institutes (nmis). the nmis maintain traceability to ipk through periodic comparisons of their 1 kg standard(s), which in most cases are also made of the same platinum-iridium alloy, with the working standards of the bipm. the nmis then realize a mass scale from approximately 1 mg up to several thousand kilograms through multiple and sub-multiple calibrations involving their 1 kg standard(s) and working standard artefacts. redefinition of the kilogram based on planck’s constant will take place in 2018 [1], [2]. this will complete the redefinition of base units in the si from artefacts to fundamental constants. when this happens both the watt balance and the international avogadro coordination (iac) project will realize the kilogram in a vacuum environment [3], [4]. but most industry and research laboratories will continue to perform mass metrology in air, so at some point the unit of mass realized in vacuum must be transferred to masses in air. consequently, methods of transferring the vacuum realization to atmospheric pressure will have to be developed [5]. currently there are two approaches to establish traceability from the unit of mass realized in vacuum to the mass unit in air: sorption artefacts and a mass comparator utilizing magnetic coupling between air and vacuum [6]. the sorption artefact technique is based on accurately modelling the adsorption process of water and other contaminants from air. the mass of artefacts with similar mass but different surface areas are compared both in air and in vacuum. these measurements are then used to determine the adsorption coefficient of water and other contaminants from air abstract the kilogram is the unit of mass in the international system of units (si) and has been defined as the mass of the international prototype kilogram (ipk) since 1889. in the future, a new definition of the kilogram will be realized by fixing the value of the planck constant. the new definition of the unit of mass will occur in a vacuum environment by necessity, so the national institute of standards and technology (nist) is developing a mass calibration system in which a kilogram artefact in air can be directly compared with a kilogram realized in a vacuum environment. this apparatus uses magnetic suspension to couple the kilogram in air to a high accuracy mass balance in vacuum. acta imeko | www.imeko.org july 2017 | volume 6 | number 2 | 71 per unit area. this is then used to calculate a correction factor which is simply added to a mass brought from vacuum into air. measurement uncertainties on the order of several micrograms can be obtained with this technique [7]. the simplicity of simply adding a fixed mass correction value to a mass as it is removed from vacuum is a great advantage. but this is an indirect correction technique which depends on the accuracy of the adsorption model. the adsorption process is very sensitive to the surface state of the masses. it has been shown that adsorption coefficients depend on the artefact material, cleanliness, and surface roughness. other factors such as oxide layer thickness and composition may also play an important role. unsurprisingly, there is significant variations in values of reported sorption values [8], [9]. great care is required to insure the surface state of masses remain identical to those involved in the adsorption modelling measurements. the magnetic suspension technique, on the other hand, involves a direct comparison of a mass in vacuum with a mass in air. no assumptions based on the adsorption modelling need to be made. figure 1 shows a schematic of the magnetic suspension mass comparator apparatus. in the upper chamber is a commercial mass comparator kept in vacuum on which masses in the upper chamber can be compared. a second weighing pan and carriage are magnetically suspended in the lower, air-filled chamber. this second weighing pan is connected to the mass comparator load cell by magnetic fields which penetrate through the vacuum-to-air boundary flange. this enables direct comparisons to be made between masses in vacuum in the upper chamber with masses in air in the lower chamber. one challenge of the magnetic suspension technique is that in addition to the 1 kg mass, the mass comparator load cell must also support the upper and lower magnet assemblies, the air weighing pan and the associated support structures. consequently, the mass comparator must be designed to handle a load significantly larger than 1 kg. for the system described in this paper, the mass comparator has a maximum load of 10 kg and a resolution of 10 µg. it is the goal of this project to achieve an uncertainty of 20 µg for a 1 kg mass. with an anticipated uncertainty of the watt balance at nist of about 20 µg, the total uncertainty would then be on the order of 30 µg (k=1). this is acceptable for dissemination purposes and not much of a change from what nist currently can do for customers (≈20 µg, k=1). in section 2 of this paper we will discuss our method for calibrating a mass in air using a calibrated mass in vacuum with the msmc. in section 3 we discuss how a mass will be transported under vacuum between the watt balance and the msmc and other facilities requiring a vacuum. 2. nist method for transferring vacuum mass calibrations to air 2.1. vacuum chamber and mass comparator the apparatus is housed in two separate aluminium chambers mounted above one another as shown in figures 1 to 3. the upper chamber will house the mass comparator and will be kept under vacuum while the lower chamber will normally be kept at atmospheric pressure. these chambers are constructed out of non-ferromagnetic aluminium so that stray magnetic fields from the magnetic suspension system will not figure 2. illustration of magnetic suspension mass comparator facility with vacuum door opened. figure 1. schematic of the magnetic levitation principle used in the magnetic suspension mass comparator. the all-aluminium vessel has an upper chamber containing a vacuum compatible mass comparator and a lower chamber for the mass in air to be compared with the upper masses in vacuum. figure 3. magnetic suspension mass comparator. a.) upper vacuum chamber, b.) lower (atmospheric) chamber, c.) raised access platforms. acta imeko | www.imeko.org july 2017 | volume 6 | number 2 | 72 couple to the walls and interfere with the mass measurements. the chambers are rectangular in shape with the upper and lower chambers being 88 cm × 67 cm × 111 cm and 77 cm × 67 cm × 80 cm, respectively. large rectangular doors provide easy access to components in the vacuum chamber when necessary. although a maximum pressure of 0.1 pa would be suitable for removing buoyancy corrections, other factors such as the adsorption of water on to the masses require a lower target pressure on the order of (10-3 to 10-4) pa [10]. a magnetically levitated turbo pump produces an effective pumping rate of 134 l/s on the upper vacuum chamber. the ports and windows are sealed with fluoropolymer elastomer orings. the large door seals have a differentially-pumped dual oring design [11]. although normally filled with air, the lower chamber can also be evacuated or filled with an inert gas. figure 3 shows the assembled vacuum chambers housing the msmc. two raised platforms on opposite sides of the apparatus provide access to the upper vacuum chamber. the chambers are mounted on a concrete pillar underneath the floor in order to vibrationally isolate the apparatus from the rest of the room, including the raised access platforms. the laboratory has a specified temperature and humidity stability to within ±0.01 °c and ±1 % respectively. the high precision commercial mass comparator (10 kg capacity with 0.010 mg resolution) is mounted inside the upper vacuum chamber. it includes a mass turntable which can hold up to 3 separate masses under vacuum for comparisons. in addition to the magnetically suspended weighing pan, the lower chamber also contains another mass turntable which can hold up to 4 masses in air for comparisons. 2.2. suspension system two samarium cobalt (smco) permanent disk (3.81 cm diameter) magnets with a residual field of 1.16 t provide the necessary magnetic field to suspend both the carriage and mass in air. according to earnshaw’s theorem it is not possible to stably suspend an object with only ferrimagnets [12]. therefore, dynamic control is required and is provided by a solenoid (75 ω) wound around the upper magnet [13]. a proportionalintegral-derivative (pid) control algorithm is used to stabilize the suspended mass [14]. the pid control system operates with a field programmable gate array (fpga) that can operate independently of the computer control system. the position of the suspended mass is first determined using a hall probe mounted on the vacuum-to-air boundary flange between the two magnets. this provides the feedback signal to the pid circuit to stabilize the suspended mass by modifying the current sent to the solenoid. mass comparisons with an “a-b-b-a” cycle are performed between masses in the upper vacuum chamber and the lower air chamber. during all measurements the carriage in the lower chamber is suspended, even when there is no mass on this pan. in this way, the mass of the lower weighing pan and associated suspension and support structures will cancel out of the comparison between the vacuum and air masses. in order to insure that the weighing pan is not dropped, it is not suspended when masses are transferred on/off the weighing pans. 2.3. gravitational field gradient one significant difference between the magnetic suspension measurements and a typical mass comparison measurement is that the two masses in the suspension system are being compared at different heights. the local vertical gravitational gradient is therefore used to correct the value of gn used in the mass comparison calculations. the gravitational gradient in the room which will house the magnetic suspension apparatus was measured with a portable commercial relative gravimeter. the gradient was measured both before and after the installation of vacuum chambers and access platforms [15]. the final results are shown in figure 4. the masses in the msmc will be separated vertically by (1.082±0.001) m and located at the corresponding points a and c in the figure. the resulting gravitational gradient is -2.74×10-6 s-2 and would result in a correction on the order of 300 µg. 3. vacuum mass transport 3.1. mass transport vehicle nist will realize the new kilogram with the nist-4 watt balance [16]. the watt balance and the magnetic suspension system are located in different laboratories at nist and it is necessary to transport masses in vacuum between the two apparatus. this will be accomplished with a mass transport vehicle (mtv) which is essentially a mobile vacuum chamber (see figure 5). it is built mainly out of copper-gasket-sealed, stainless steel, vacuum components. it is not necessary to use non-ferromagnetic components since the mtv will not be attached to the magnetic suspension apparatus during figure 4. variation of gravity vs. height in the magnetic suspension laboratory. the air and vacuum masses will be located at the positions corresponding to data points a and c respectively. uncertainty bars represent 1σ of the measurements. figure 5. mass transport vehicle. acta imeko | www.imeko.org july 2017 | volume 6 | number 2 | 73 measurements. only the gate valve has o-ring seals. the mtv connects to the other vacuum chambers with an iso band clamp so that it can easily be connected and disconnected. a hot cathode ion gauge is used to monitor the vacuum and an ion pump is used to maintain the vacuum (<1×10-3 pa) during transport. the ion pump can be battery operated to aid in keeping the pressure low during transport. both the ion gauge and the ion pump are oriented in a way there is no direct line of sight between them and the mass to prevent possible ion sputtering onto the mass. figure 6 shows a cutaway view of the interior of the mtv. masses being transported will sit on a slotted block in the center of the vehicle. circular indents designed to match several different types of masses keep the mass from sliding sideways during transport. three triangular wedges made from polyether ether ketone (peek) support the top of the mass to prevent it from tipping over. all other surfaces which could come into contact with the mass are coated with a “white bronze” tri-alloy (cu-sn-zn) to prevent material from being transferred to the mass. the slotted block and triangular wedges can be independently raised and lowered so that the mass can be placed on a transfer fork from the load lock which enters the mtv through the gate valve. the transfer fork when loaded with 1 kg and fully inserted into the mtv may sag by approximately 1 cm. in order to vertically and horizontally align the transfer fork with the slotted block, a peek ramp supports the fork at the proper height. 3.2. load lock system masses are transferred from the mtv into the magnetic suspension upper vacuum chamber through a load lock which maintains the mass under vacuum during the transfer process. the load lock is built using a 6 way cross as shown in figure 7. the front flange of the 6 way cross is an iso band clamp to which the mtv will be connected. once the mtv has been attached, the load lock is evacuated with an independent turbo pump station. once a suitable (≈10-3 pa) vacuum is obtained in the load lock, the gate valves on the mtv and the load lock can be opened. mass fork 1 in the back of the chamber, which is attached to a long linear vacuum translator, is moved into the mtv to transport the mass back to the center of the load lock. the transfer platform which is composed of an array of rectangular posts can then be moved upward to pick the mass off the mass fork. mass fork 2, perpendicular to mass fork 1, is then moved into a position underneath the mass and between the transfer platform posts. the transfer platform can then lower the mass onto fork 2. this mass fork then transfers the mass onto the circular mass turntable of the mass comparator. the tines of mass fork 2 have different lengths so that the mating slots in the circular mass turntable for different mass positions will not intersect. mass fork 2 is finally retracted and the gate valves closed. after venting the load lock, the mtv can be detached and removed before the mass comparison measurements are initiated. as with the mtv, all surfaces in the load lock which come into contact with the mass are coated with white bronze. 4. conclusion the mass and force group of nist has built a magnetic suspension system which will enable the direct comparison of a mass in air with a mass in vacuum using the same mass comparator. this is accomplished by using a magnetic suspension technique to couple a mass in the lower chamber (atmospheric pressure) to a mass comparator in the upper chamber (vacuum). the system is currently undergoing stability testing and evaluation of systematic effects. this system will become part of the nist mise en pratique, the set of instructions for realizing and disseminating the kilogram, after redefinition. a mass transfer vehicle and load lock system will enable masses to be transported under vacuum from the nist watt balance to the magnetic suspension apparatus and to other facilities operating under vacuum. references [1] “on a new definition of the kilogram,” recommendation g 1 of the consultative committee for mass and related quantities submitted to the international committee for weights and measures (2013), http://www.bipm.org/cc/ccm/allowed/14/31a_recommend ation_ccm_g1(2013).pdf [2] p. richard, j. ullrich, “joint ccm and ccu roadmap towards the redefinition of the si in 2018”, http://www.bipm.org/utils/common/pdf/siroadmap.pdf#page=3. [3] i. robinson and s. schlamminger, “the watt or kibble balance: a technique for implementing the new si definition of the unit of mass”, metrologia, 53 (2016) pp. a46-a74. [4] k. fujii, h. bettin, p. becker, e. massa, o. rienitz, a. pramann, a. nicolaus, n. kuramoto, i. busch, m. borys, “realization of the kilogram by the xrcd method”, metrologia, 53 (2016) pp. a19-a45. [5] m. stock, “watt balance experiments for the determination of the planck constant and the redefinition of the kilogram”, metrologia, 50 (2013) pp. r1-r16. [6] s. davidson, j. berry, p. abbott, k. marti, r. green, a. malengo, l. nielsen, “air-vacuum transfer; establishing traceability to the new kilogram”, metrologia, 53 (2016) pp. a95a113. [7] “measurements report: air-vacuum characterization of the 1 kg cylindrical platinum-iridium stack c18”, october 7, 2014, bipm, pavillon de breteuil, f-92312 sèvres dedex. figure 6. illustration of the mtv interior. figure 7. illustration of a typical load lock. http://www.bipm.org/cc/ccm/allowed/14/31a_recommendation_ccm_g1(2013).pdf http://www.bipm.org/cc/ccm/allowed/14/31a_recommendation_ccm_g1(2013).pdf http://www.bipm.org/utils/common/pdf/si-roadmap.pdf#page=3 http://www.bipm.org/utils/common/pdf/si-roadmap.pdf#page=3 acta imeko | www.imeko.org july 2017 | volume 6 | number 2 | 74 [8] k marti, p. fuchs, s. russi, “traceability of mass ii: a study of procedures and materials”, metrologia, 52 (2015), pp. 89-103. [9] z. jabbour, p. abbott, e. williams, r. liu, v. lee, “linking air and vacuum mass measurements by magnetic levitation”, metrologia, 46 (2009) pp. 339-344. [10] p. abbott, z. jabour, “vacuum technology considerations for mass metrology”, j. res. natl. inst. stand. technol., 116 (2011) pp. 689-702. [11] a. roth, vacuum technology, elsevier science, amsterdam, 1990, isbn 0 444 88010 0, pp 416-419. [12] s. earnshaw, “on the nature of the molecular forces which regulate the constitution of the luminferous ether”, trans. of the cambridge philosophical society, 7 (1842) pp. 97-112. [13] g. marsden, “levitation!”, nuts & volts, 24 (2003) pp. 58-61. [14] c. stambaugh, “the control system for the magnetic suspension mass comparator system for vacuum-to-air mass dissemination”, acta imeko, vol. 6, no 2, 2017. [15] e. mulhern, corey stambaugh, “characterization of the nist magnetic suspension mass comparator apparatus and facility”, accepted for publication in ncsl international measure, 2017. [16] d. haddad, f. seifert, l.chao, a. cao, g. sineriz, j. pratt, d. newell, s. schlamminger, “first measurements of the flux integral with the nist-4 watt balance”, ieee trans. instr. meas., 64 (2015), pp. 1642. progress on vacuum-to-air mass calibration system using magnetic suspension to disseminate the planck-constant realized kilogram microsoft word article 8 198-1092-1-pb.docx acta imeko may 2014, volume 3, number 1, 32 – 37 www.imeko.org acta imeko | www.imeko.org may 2014 | volume 3 | number 1 | 32 measurement and meaningfulness j. van brakel state university of utrecht, physics laboratory. foundations of physics, 3508 ta utrecht, the netherlands section: research paper keywords: measurement theory, uncertainty citation: j. van brakel, measurement and meaningfulness, acta imeko, vol. 3, no. 1, article 8, may 2014, identifier: imeko-acta-03 (2014)-01-08 editor: luca mari, università carlo cattaneo received may 1st, 2014; in final form may 1st, 2014; published may 2014 copyright: © 2014 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited 1. introduction consider statements in which reference is made to measured (quantified) properties of empirical systems, for example:  the width of this piece of paper is approximately four prekik (1)  the viscosity of this fluidised bed is 600 cp (2)  the average pore size of this brick is 10 m (3)  the poverty in the south of this country has increased drastically (4) assuming that we know what is referred to by ‘this’ in (1)– (4), can we specify criteria on the basis of which it can be decided whether such a statement is meaningful and what precisely its meaning is? the question of meaningfulness has been discussed in some detail in literature on the foundations of measurement (see references in [1], [2] and [3]). this discussion is related to and intertwined with discussions on the notions of scale invariance and dimensional invariance. in this paper no contribution to this mathematical theory of meaningfulness will be made. instead the problem area “measurement and meaningfulness” will be placed in a wider perspective by investigating what conceptual problems arise if we simply raise the question what the meaning is of a statement involving measurements. 2. does the measurement represent the concept? a measurement always has a purpose; this purpose is part of the context in which the measurement is carried out [4]. relative to this purpose the measurement has a validity and an accuracy (the terms ‘validity’ and ‘accuracy’ are used here in the sense that is common in the literature on psychometrics, e.g., the “validity of a test”). a measurement result is valid to the extent that it measures what it is meant to measure; it is accurate (and stable) to the extent that it measures what it measures accurately. if the sole purpose of a measurement is to predict something (for example, a test to predict the performance of students at a particular examination), its validity is easy to assess (the test is a good one, is valid, if it predicts well; whether it predicts well is easy to assess). as long as we can make good predictions, we may not bother about what we are measuring. however, usually we do bother. either, because we are concerned about the predictive validity of the measurement in other contexts. or, because we explicitly require the measurement to measure something. this something is a concept (width, viscosity, pore size, poverty). in most cases we will say that the concept denotes properties of things in the real world. the examples (1)–(4) are chosen to draw attention to a number of different ways in which we may have to analyse the validity of a measurement statement: this is a reissue of a paper which appeared in acta imeko 1985, proceedings of the 10th world congress of the international measurement confederation, “new measurement technology to serve mankind”, 22-26.4.1985, prague, pp. 319–333. the question raised in this paper is: what is the meaning of a statement in which reference is made to measurements? measuring a property of a system produces numbers. the question can be made more specific by considering the following aspects of these numbers: validity and purpose of numbers to be obtained; accuracy and stability of numbers obtained; possibility of obtaining numbers; meaningfulness of relations between numbers obtained; relation to numbers representing other properties; what is arbitrary and what not in what the numbers denote; in short: what do the numbers obtained in measurement signify. acta imeko | www.imeko.org may 2014 | volume 3 | number 1 | 33 (a) the validity of (4) will depend on the normative, including political, motivations that are operative in specifying what we want the concept poverty to mean. for example: if we decide (should we?) to explain poverty in terms of income, do we mean income in money only or also other sorts of “income” (goods, services from other people)? further, what do we require of the procedure by which the income data are processed into a measure of poverty? for example, it seems sensible to require that a transfer of income from a rich to a poor person (all other things remaining equal) should always reduce the measure of poverty – but until recently few poverty measures in actual use met that criterion [5]. (b) on first view it might seem that it is crystal clear what the concept of length is. however, what do we mean if we talk about the size of pores in a brick and we express this size on a length-scale. numerous instruments and techniques are available which produce numbers which are said to represent the pore size (or the pore size distribution). but if we would investigate what information precisely is extracted from the brick by these instruments, it turns out that they all measure something different. if we would make photographs of cross sections through the brick, it is perfectly clear what we mean by referring to the distance between point a and point b (the length ab). it is also more or less clear what the different “porosimetric” instruments measure, but it is completely unclear what we mean by “pore size”. hence, the question of the validity of the instruments can only be assessed relative to whatever the measurement results are to be used for. (c) the viscosity of a fluid bed is measured with the same type of instrument that is used to measure the viscosity of viscous liquids; data obtained are very reproducible. so there seems to be no problem here. but a fluidised bed is a mass of solid particles (for example sand) supported by a flowing fluid such that the particles do not form a fixed bed, but are in constant movement relative to one another. if we measure the so called viscosity of such s fluid-particles system, is this really the same sort of concept we are using when referring to the viscosity of liquids? i hope the above examples give some idea of the way the meaning of a measurement statement depends on the context in which it is produced. the meaning of a measurement statement is related to the meaning of the concept it intends to represent and all uncertainties and vagueness inherent in a concept are not eliminated by having a procedure that produces “accurate” numbers in a reproducible way. i won’t discuss the relation of the concepts of accuracy and reproducibility to the meaning of measurement statements in any detail. for the moment, we can more or less circumvent these aspects by considering only the meaning of statements concerning one measurement, which is expressed on a scale chosen in such a way that there is no reasonable doubt about the exactness of the measurement. the following explanation of the meaning of (1) should illustrate this. as a unit of length i introduce the prekik. the distance between the two vertical bars in (5) is equal to one prekik. length measurements in prekiks are possible using the following instruction: in order to measure the distance between two points, these two points have to be connected by a straight line and then it has to be counted how many times the standard of one prekik fits on that line (6) many foundational problems are hidden in (6). for the moment (6) should be complemented with the following elucidations: (i) it may be practical to make a number of copies of the standard or, using the standard, to construct a measuring rod on which there are little marks, the distance between subsequent marks being always one prekik. in this case we will say that we use different instruments, but all these instruments correspond to one measurement procedure, which is formulated in (6). (ii) we do not consider parts of one prekik. (they don’t exist so to say.) then a distance in prekiks, for example (1), is exact if the distance is very large compared with the inaccuracies involved in the operational procedures referred to in (6) and (i) above. (there are some problems suppressed here, but we won’t bother.) (iii) we assume that the length of the standard does not change during transport. whether this assumption is a convention of length measurements or whether this is a property of the empirical world is a subject of discussion in the foundations of relativity theory (see [6] and references given there). 3. when is a measurement possible? in the previous section the basic assumption was made that we measure something. before we start measuring we should have an idea what concept we intend to quantify through measuring. such a quantification of a concept through measuring is only possible if the concept has certain properties. let us assume that measurement is the representation of empirical structures on numerical structures (for the representational theory of measurement see for example [7] or [8]). because the objects in the empirical structure are ordered in certain ways with respect to one of their properties, it is possible to represent this property in the numerical structure by assigning numbers to the objects in the empirical structure: the order in the empirical structure is mapped into the numerical structure. what does this mean? consider the concept “mass” as an example: (a) there is a concept “mass”, the meaning of which is partly explained by telling what we mean if we say ba m (7) read: object b is heavier than object a. an operational definition of m refers for example to the use of a balance and tells us how to go about assessing whether in a particular case (7) is true or not. the explanation of what we mean by m is part of the meaning of any measurement statement about the mass of an object. if we know what we mean by m it is easy to define m , m , m , and m . note that it is irrelevant what instrument we use (asking somebody whether (7) is true or not in any particular case also counts as an instrument), except that we would like m to have certain properties. for example, it is advantageous if we can establish empirically that m is acta imeko | www.imeko.org may 2014 | volume 3 | number 1 | 34 connected and transitive. if this is so m generates a weak order in the set of objects, am, which have the property mass. (b) the meaning of the concept of mass is further explained by referring to the fact that an operation of physical concatenation (= adding together), m , can be applied to elements of am. just as in the case of m it is attractive if m has certain properties. for example it is advantageous if it has the properties of being associative and monotoneous. whether this is so, is again a matter of empirical investigation. (c) if m and m exist and have the properties mentioned, then it is usually possible to construct a standard sequence. in particular this is possible if the following holds for any e and a in am and any integer k, there exists an integer n such that ne m ka (8) in which the meaning of ne is explained by 1e = e and (n + 1)e m (ne m e) (9) and similarly for ka. it is not obvious in what way we should consider (8) a condition which has to be fulfilled empirically. also there are writers who object to using such “archimedean axioms” in the first place (see [9] and [10], a review of [7]). however, i pass that over. if something like (8) holds and m and m have the properties as mentioned, we can construct a standard sequence for mass, which allows us to assess the relative mass of objects as follows. let us choose for e the standard kilogram in paris. first we make many copies of e, so that we are never at a loss to construct an ne. assume further that we want to determine the mass of president reagan. for simplicity we choose k = 1 and then we find the smallest value of a for which ne m am m (n – 1)e (10) where am represents reagan with respect to his mass. equation (10) fixes the mass of reagan relative to that of the standard kilogram in paris. ideally, we would like to be able to find ne m am (11) this could be so by accident, but usually it is not the case. however, we can approach an equivalence by choosing a large value of k (for which purpose we have to make k copies of reagan, at least k copies of reagan’s mass). of course, this is not the way we actually determine the mass of an object. nevertheless, this is part of what it means if we say that a property like mass can be measured. (d) note that until now, we have done nothing more than counting the number of identical copies, manipulating objects and assessing whether m or m applies. because all this is possible, we can measure mass on a scale φ , which is, in this case, an additive function from am to the real numbers, and, by convention, φ (e) = 1. many questions can be raised in connection with this account; i mention only two clusters of questions: (i) operational definitions of m and m have been given referring to a balance. however, we cannot put the earth or an electron on a balance. how is the mass of these objects related to the standard kilogram in paris? similarly, on a strictly operationalistic point of view, the ten or so different methods to measure stellar distances, would all measure different properties or concepts. but this is not what we want, because most of us would believe that there is just one distance between two stars. (ii) i said, if m and m have certain properties and (8) holds, then mass can be measured on an additive scale φ . but what do we do if we find an exception, for example, in a particular case we observe that m is not transitive or m is not associative. why are we so sure this must be due to inaccuracies in our instruments or our observations? (in fact, in modern physics mass is not “additive” and [11] has proposed to drop mass as a basic unit and to replace it by the “quantum of action”) i have used mass as an example, because this is by far the simplest case: for length there are alternative interpretations for l and we have to account for the three dimensionality of space (or four dimensionality of space-time); for time the definition of t and the choice of a unit is more problematic; for force we have to take into account the direction; velocity has an upper and a lower bound (at least that is what we think, do we?), which presents special problems; and so on (for references to literature on some of these questions see [4]). but always, it is part of the meaning of a measurement statement, what the interpretation is of  and  for the particular concept (or why there is no  , if there is no such operation). 4. meaningfulness of statements involving a comparison of measurements on an abstract level we can say: meaningful numerical statements are those that remain invariant under permissible changes in the representation. what does this mean? consider the following measurement statements: the average radius of the earth is 6371 km (12) the average radius of the earth is 3960 mi (13) a quantity such as mass or length can be measured on different scales. because of the arbitrary choice of the standard e, any scale φ is invariant with respect to transformations of the sort φ=φ ' (14) where  is a real number. we encounter such an  if we transform (12) into (13) or if we transform prekiks into meters. in that sense the change of (12) to (13) (or vice versa) is meaningful. but there is another question: statement (12) is concerned with the average radius. the number 6371 is the result of manipulations with other numbers. what is the meaning of such an average? in the first place there is the question of validity discussed in section 2: which distances have to be averaged? answer (say): connect every point on the surface of the earth with its centre. questions: where is the centre? what sort of concept does this average represent? secondly, if we have agreed on the measurement data that have to be averaged, are we allowed to do that? this question can be given an exact answer in the representational theory of acta imeko | www.imeko.org may 2014 | volume 3 | number 1 | 35 measurement as follows (for this technical concept of meaningfulness see [1], [12], [13], and references given there; the concept was introduced in a systematic way for the first time by [14]). a statement in which something is said about the relation between two averages (or any other manipulation of measurement data) is meaningful, if the truth-value of this statement does not change if we choose another scale to express the original data. for example, with φ a scale for length, consider:    i m 1i i n 1i b m 1 a n 1  == φ>φ (15) if we substitute first 'φ and then (14) in (15) we obtain:    i m 1i i n 1i b m 1 a n 1 φ>φ ==   (16) this is equivalent to (15); hence (15) is meaningful. another example of a meaningful statement for length is:    bna φ=φ (17) with n an integer. although this may seem trivial, please note that for temperature measured on an interval scale (such as °c or °f), (16) is meaningful, but not (17). for wind force measured on the scale of beaufort or hardness measured on mohs scale, neither (16) nor (17) is meaningful. all physical quantities which are embedded in general theories are measured on ratio scales and usually we are completely familiar with what are meaningful statements and what not. we know we can compare averages and we see at once that       2cba φ=φ+φ (18) is not meaningful. however, in the social sciences, in psychology, and in the technical sciences there are many isolated quantities, which often are not measured on ratio scales or where there are other hidden traps. consider for example a price index defined as: 1 i 0 i 0 i 0 i pq/pqp = (19) here 0iq is a chosen consumption pattern (say 10 kg potatoes, 2 eggs, 0.01 television set, and so on) and 0ip and 1 ip are the prices at times t0 and t1 respectively. in its generality p is not meaningful, because whether p decreases or increases depends on the choice of 0iq (more detailed examples in [15]). 5. derived measurement and dimensional invariance why do we see at once that (18) is not meaningful? this is because (18) is not dimensionally homogeneous. various explanations or theories have been proposed to explain why numerical laws should be dimensionally homogeneous (see [7], pp. 504–512, [16] and [17]). the explanation that has been worked out in most detail is the explanation according to which the requirement of dimensional invariance follows directly from the concept of meaningfulness defined in the previous section and the properties of the qualitative representations of the numerical laws ([2]; see also references in [1] and [3]). (here “qualitative” means: representing the laws without using scales.) there is no room to discuss the subject of dimensional invariance in detail here. however, one aspect i would like to draw attention to. it has been argued in the literature on this subject that because of these criteria of invariance or meaningfulness, the possible forms of a law are extremely limited (for the first time by [3]; most recently by [18]; invariance requirements have also been invoked to solve paradoxes in the a priori estimation of equal probabilities, like the bertrand paradox: see [19] and references given there). for example, we do not only know that cme 2= (20) is not a law of nature (because it is e = mc2). we know more, it is argued. we know for certain that (20) cannot be a law of nature. this fact has been of use for a long time in the practice of dimensional analysis. however, this “setting limitations on the possible forms of the law” should not be exaggerated, because the criterion only holds if we are certain that we know all the variables that enter into the law. but if we still have to discover the law, how can we be so certain that we know all the variables that enter into the law? moreover, if one of the variables is dimensionless, or two or more (but not all) of the variables can be combined into a dimensionless group, then the dimensionless part can occur in the law as the argument of any mathematical function. in the applied sciences, laws often have “empirical” forms. an example is ncl dl du = (21) in which c and n are constants, and du is the energy required to effect a small change dl in the size of a unit of material in a size reduction process (as in a mill). the value of n is different for different materials. in practice values of 1.5 or 2 are often used for n. it is obvious that in this case the requirement of dimensional invariance sets very few limitations on the form of the law, because c is only defined by its occurrence in this law and absorbs whatever dimension is generated by the value of n. the fact that the experimentally determined value of n is not a whole number is no problem, because experimentally we can at best determine the value of rational numbers (as distinct from real numbers) and if n is a fraction, we can always eliminate the denominator. moreover, because c is not further interpreted,we can make all terms dimensionless by dividing u and l by a reference value which has the same dimensions. in a way this is what happens in kinetic laws such as the law of radioactive decay:  texpn/n 0 λ=  (22) the decay constant is the inverse of a reference value for t. equation (22) makes sense, because we have a particular interpretation for λ . but if we would not have that interpretation, any functional relationship between n/n0 and λ t would be allowed. acta imeko | www.imeko.org may 2014 | volume 3 | number 1 | 36 another example of a “pseudo-law” is: β= ts  (23) where s is the tension (at constant strain), t is time and  and β are constants for a particular material. the so called anomalous character of (23) has generated a whole discussion in the literature (this discussion has been generated by a peculiar presentation to the royal society in london [20]; see also [17], pp. 45–46, and [7], p.464), in which doubts have been raised concerning the newtonian definition for equality of time intervals and the parameter  has been ascribed the dimensions “antilog (log l log t)”. however just as in (22) the constant of proportionality has no meaning independent of the equation in which it occurs. hence, we might as well write instead of (23): re ps /t += τ (24) which solves all problems of awkward dimensions (if the fit of (24) is not as good as that of (23) we add one more exponential term.) it may be of interest to note that (23) is of some historical interest, if it is read as an equation describing the falling distance of an object as a function of time (in the seventeenth century various laws were proposed to describe “galilei’s law”. in that context chr. huygens was probably the first to argue that certain laws could be excluded a priori). 6. standards of measurement consider the following statements: 1 m = 1000 mm (25) 1 m = 19.2 prekik (26) 1 m = the length of the standard meter in paris (27) if we would analyse the meaning of each of (25) – (27) we would see that they are three quite different kinds of statements. i won’t go into the “philosophical” details of this here, except for drawing attention to one particular peculiarity of standards of measurement in the contribution they make to the meaning of a measurement statement. i have said that the meaning of an object having a mass of 10 kg can be partly explained by saying that the mass of this object is equivalent ( m , not =) to the physical concatenation of 10 standard kilograms each of which is equivalent in mass to the standard kilogram kept in sèvres near paris. the meaning of the measurement statement is so to say explained by pointing to a certain relation the object of the measurement statement bears to the standard. now assume that the building in sèvres, including the standard kilogram which is kept there, is completely destroyed in a fire. does this mean that, when this happens, instantaneously all statements giving measurement results in kg become meaningless? presumably not, but what then is the meaning of these statements? 7. concluding remarks in the previous sections i have only touched upon some aspects that have to be considered when we pose the question what the meaning is of a measurement statement. i have emphasised what i think are important aspects and i have used up a lot of space giving examples from the practice of measurement. in a further systematic development of the points raised here, it will be necessary, i think, to do this within the terminological framework of philosophical theories of meaning. this would include (i) analysis of the meaning of measurement statements using the fregean concepts of sense and reference (which in the context of measurement might be called “operational” and “factual” meaning), (ii) taking into account recent philosophical discussions concerning the question whether there are statements which are a priori and contingent or statements which are a posteriori and necessary (as this may shed light on the distinction between what is fact and what is convention in measurement) and (iii) an analysis of the different sorts of identity or equivalence that are presupposed in measurement statements. (these three aspects are all interrelated in a theory of meaning). on the applied side, a further development of the points raised in this paper would also require analysis of a number of “cases”, to make clear which quantities are measured on what scales. for example, most scientists would probably say that density is measured on a ratio scale, this is not self-evident and we may well have to conclude that density is measured on an interval scale [16]. and to give a last example, in the design of grinding mills for coal the hardgrove grindability index is used to predict the energy consumption of the mill for a given specification of the size reduction to be achieved. this index can be interpreted to be some sort of measure of the material constant c in equation (21). but if we analyse a large set of data for this index, it is dubious whether we can even say that the grindability is measured on an ordinal scale: different samples from the same coal-seam have different grindabilities. there is no correlation between the grindability of a coal and its rank, or its composition. also, there is no correlation between the grindability as determined in the standard mill used to determine the hardgrove index and the grindability as determined in another standard mill (for example bond’s index, in which case n in eq. (21) is 1.5 instead of 2). furthermore there is no relation between the grindability and the modulus of elasticity of coal or the hardness of coal determined by some standard test (for example the brinell test). so what do we mean if we say that the grindability has been determined to be, say, 60? no doubt some of the considerations involved in a study concerning the meaning of measurement statements as suggested above, will be rather abstract or philosophical, but i belief such a study could make an important contribution to a better understanding of what measurement is by giving a conceptual analysis of the meaning of measurement statements. references [1] d.h. krantz et al, foundations of measurement, vol. 2 (to be published, typescript consulted dated may 1979). [2] r.d. luce, dimensionally invariant numerical laws correspond to meaningful qualitative relations, phil. sci., 45 (1978) pp.1–16. [3] r.d. luce, on the possible psychophysical laws, psychol. rev., 66 (1959) pp.81–95. [4] j. van brakel, norms and facts in measurement, measurement, 2 (1984), pp.45–51. [5] a. sen, poverty: an ordinal approach to measurement, econometrica, 44 (1976), pp. 219–231. [6] w. balzer, a. kamlah, geometry by ropes and rods, erkenntnis, 15 (1980), pp. 245–267. [7] d.h. krantz et al, foundations of measurement, vol.1, new york, 1971. [8] f.s. roberts, measurement theory, reading (mass.), 1979. acta imeko | www.imeko.org may 2014 | volume 3 | number 1 | 37 [9] j.o. ramsay, algebraic representation in the physical and behavioral sciences, synthese, 33 (1976), pp. 419–453. [10] j.o. ramsay, review of foundations of measurement, psychometrika, 40 (1975), pp. 257–262. [11] e.j. post, physical dimensions and covariance, found. physics, 12 (1982), pp. 169–195. [12] l. narens, theory and decision, 13 (1981), pp.1–70. [13] f.s. roberts, on luce’s theory of meaningfulness, phil. sci, 47 (1980), pp.424–433. [14] j. pfanzagl, theory of measurement, würzburg, 1968. [15] d.k. osborne, unified theory of derived measurement, synthese, 33 (1976), pp.455–481. [16] d.k. osborne, on dimensional invariance, quality and quantity, 12 (1978), pp.75–89. [17] j. palacios, dimensional analysis, london, 1964. [18] j.c. falmagne, l. narens, synthese, scales and meaningfulness of quantitative laws, 55 (1983), pp.287–325. [19] p. milne, a note on scale invariance, brit. jour. phil. sci., 34 (1983), pp.49–55. [20] g.w.s. blair et al, limitations of the newtonian time scale in relation to non-equilibrium rheological states and a theory of quasi-properties, proc. of the royal society of london, series a-mathematical and physical sciences, 189 (1947), pp.69-87. a case study on providing fair and metrologically traceable data sets acta imeko issn: 2221-870x march 2023, volume 12, number 1, 1 6 acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 1 a case study on providing fair and metrologically traceable data sets tanja dorst1, maximilian gruber2, anupam p. vedurmudi2, daniel hutzschenreuter3, sascha eichstädt2, andreas schütze1 1 lab for measurement technology, saarland university, campus a5 1, 66123 saarbrücken, germany 2 physikalisch-technische bundesanstalt, abbestraße 2-12, 10587 berlin, germany 3 physikalisch-technische bundesanstalt, bundesallee 100, 38116 braunschweig, germany section: research paper keywords: data set; fair digital objects; traceability; digital si; research data management citation: tanja dorst, maximilian gruber, anupam p. vedurmudi, daniel hutzschenreuter, sascha eichstädt, andreas schütze, a case study on providing fair and metrologically traceable data sets, acta imeko, vol. 12, no. 1, article 5, march 2023, identifier: imeko-acta-12 (2023)-01-05 section editor: daniel hutzschenreuter, ptb, germany received november 17, 2022; in final form february 14, 2023; published march 2023 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: research for this paper has received funding within the projects 17ind12 met4fof and 17ind02 smartcom from the empir program co-financed by the participating states and from the european union’s horizon 2020 research and innovation program and from the federal ministry of education and research (bmbf) project famous (01is18078). corresponding author: tanja dorst, e-mail: t.dorst@lmt.uni-saarland.de 1. introduction in 2016, the fair principles (and their 15 subprinciples) which provide guidelines to improve the findability, accessibility, interoperability, and reusability of digital resources such as code, data sets, research objects, and workflows, were published [1]. providing data in a fair way contributes to the data curation process, which allows to maintain a high value of data over extended periods of time [2]. given the widespread and increasing advancement of digitalization, the main focus of the guideline is on machinereadability, i.e., the ability of computers to find, access, (inter)operate with, and reuse data with minimal or no human intervention. the fair principles only describe attributes and behaviors; however, they do not provide strict rules to achieve the desired fairness for digital resources, i.e., they only serve as a framework for the sustainability of data. fair and metrologically sound data are a basis for the exchange of digital measurement values in research and industry [3]. thousands of petabytes of data collected every year cannot be used to their full potential as the data are often not fair-compliant [4]. apart from the pure fair framework, the interoperability of measurement data requires an unambiguous machine-readable provision of its metrological properties. essential metadata are units of measurement, types of physical quantities, and, where appropriate, traceability to measurement standards in the form of measurement uncertainty provided by calibration. in this contribution, an existing data set of a lifetime test of an electromechanical cylinder (emc) is restructured and extended with metadata in a manner that conforms to the fair principles and addresses metrological properties by application of the d-si metadata model [5]. 2. use case the used test bed (see figure 1) consists of an emc as device under test (dut) and a pneumatic cylinder, which simulates an abstract in recent years, data science and engineering have faced many challenges concerning the increasing amount of data. in order to ensure findability, accessibility, interoperability, and reusability (fairness) of digital resources, digital objects as a synthesis of data and metadata with persistent and unique identifiers should be used. in this context, the fair data principles formulate requirements that research data and, ideally, also industrial data should fulfill to make full use of them, particularly when machine learning or other datadriven methods are under consideration. in this contribution, the process of providing scientific data of an industrial testbed in a traceable and fair manner is documented as an example. mailto:t.dorst@lmt.uni-saarland.de acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 2 axial load on the dut [6]. typically, more than 500,000 working cycles of duration 2.8 s are executed, where each consists of a forward stroke, a waiting time, and a return stroke. data are recorded with the help of two different data acquisition systems (daq). the zema daq acquires data from eleven different sensors during each cycle. three motor current sensors, one microphone, three accelerometers (at the plain bearing, the ball bearing, and the piston rod), and four process sensors (axial force, velocity, pneumatic pressure, and active current of the emc motor) with different sampling rates are used in the test bed. in addition, the smartup unit (suu) (see figure 1) records global navigation satellite system (gnss) timestamped data continuously with three micro-electro-mechanical systems (mems) sensors [7]: a 9-axis inertial measurement unit, a 3-axis accelerometer (for comparison reasons [8]), and a combined pressure and temperature sensor. the only link between both data acquisition systems is a trigger signal of the zema daq, indicating the start of a working cycle. this trigger signal is recorded with the suu to enable the alignment of both data sets. the aligned data set used in this publication was acquired during a lifetime test of an emc executed in april 2021, which lasted approx. 16.5 days. the data format of this aligned data set is suitable for the use of an automated toolbox for statistical machine learning [9]. it consists of numerical measurement values which can be associated to a corresponding unit of the “système international d’unités” (si) [10]. 3. achieving fair data the creation of a fair data set for the given use case can be broken down into different aspects, each with a specific contribution to one or more of the fair principles. this is also summarized by the blue (contributes) or light grey (does not contribute) symbols next to the headings of the following sections. the justification of this summary is given by the detailed evaluation provided in table 1 and further discussed in section 4. 3.1. open and known formats the use of common, well-described, and open (source) data formats provides the foundation of a fair data set [11]. because the selected use case consists of large amounts of numerical data, the hierarchical data format version 5 (hdf5) is chosen. it supports the structuring of numerical data in a filesystem-like hierarchy and allows the addition of annotations for an alignment of metadata with a description of the numerical data to every node in this hierarchy. the annotation possibilities of hdf5 are utilized by storing relatively short strings in the data-interchange format javascript object notation (json), which represent dictionary entries of key-value pairs corresponding to the specific node. the structure of the corresponding hdf5 file is shown in figure 2. it integrates two main parts corresponding to two different measurement systems, each consisting of groups representing sensors and/or physical quantities. as the zema daq is based on sensors, each measuring only a single physical quantity, no further hdf5 group is needed in this case. in contrast, the suu sensors each measure more than one physical quantity. thus, a further hdf5 group is inserted, which has the name of the corresponding sensor (bma_280 in this example). open and known formats do not contribute to the findability of the data set, as this choice only concerns the structure of the data, but not the “external visibility”. 3.2. semantic descriptors semantically expressive concepts used to describe the metadata are an important step towards not only machinereadable but machine-interpretable data. to our knowledge, no single metadata scheme provides all the necessary concepts required for the present use case. therefore, different ontologies and knowledge representations are used to describe specific aspects of the data set: figure 1. hdf5 file (grey) structure containing groups (green), data sets (orange) and associated metadata (blue). figure 2. smartup unit (small picture) used together with the zema daq for data acquisition of the testbed for an emc life time test (big picture). acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 3 • digital system of units (d-si) [5], • dublin core (dc) [12], • quantity, unit, dimension and type (qudt) [13], • resource description framework (rdf) [14], and • sensor, observation, sample, and actuator (sosa) [15]. by using existing and semantically enriched knowledge representations as a basis, researchers, organizations, institutions, machines, or algorithms are enabled to understand the data set without the expert knowledge of the initial creator. once the meaning of the data is clear, the selection of applications for analysis and processing can become more informed. semantic descriptors do not contribute to the findability of the data set, as they are only used inside the data set to describe the different data, hence not contributing to the “external visibility”. 3.3. top level metadata one of the most important steps preceding the (re)use of appropriate data is to find them. with the help of machinereadable top level metadata, both humans and computers can easily find the digital resource. the top level metadata are split into four groups with several subproperties. where appropriate, identifiers from the dublin core ontology [12] are used and specified by the dc: prefix. in the group “project”, general information is stored about the project in which the data set was generated. the creators of the data set are listed in the group “person”. in the “publication” group, the doi and the license can be found among other information. to assign the data set to a unique emc test, the “experiment” group provides information about the specific test. the following list is not complete but provides suggestions for the properties of the four different groups as used in the emc data set: • project: fulltitle, acronym, websitelink, fundingsource, fundingadministrator, acknowledgementtext, funding programme, fundingnumber • person: dc:author, e-mail, affiliation • publication: dc:identifier, dc:license, dc:title, dc:type, dc:description, dc:subject, dc:sizeorduration, dc:issued, dc:bibliographiccitation • experiment: date, dut, identifier, label top level metadata in json are added, as shown in listing 1. in this contribution, only the most important top level metadata are shown. the complete list of all top level metadata for this emc data set is included in the published data set itself [16]. top level metadata is primarily used to find and categorize the data set on a rough level. 3.4. publication in general, several aspects need to be considered: 1) choosing a suitable publishing license and an online access repository; 2) making the data set findable under a persistent identifier, and 3) providing example code to access and reuse the data set. 3.4.1. publishing license the selection of a suitable publishing license enables the reuse of existing data not only by the creators but also by other researchers. in general, the license defines who can reuse the data for which purposes and how the usage should be reported. guidance for such a decision is given, e.g., in [17]. for the emc data set, a creative commons attribution 4.0 international (cc-by4.0) license is chosen to maximize reusability, as it places no restrictions on any entities using the data as long as the original creators are credited. 3.4.2. online access repository the finalized data set is published at the online service zenodo [18], which specializes in open science. other platforms with a similar scope also exist, e.g., the “open access repository of the physikalisch-technische bundesanstalt” (ptb-oar) [19]. only publication in suitable archives with searchable metadata ensures that others are able to find and access the created data set. 3.4.3. persistent identifier to make the data set addressable under a globally unique and persistent identifier, a digital object identifier (doi) is generated within the zenodo service [18]. doi is a standard (iso 26324:2012) widely used in the scientific community to resolve an identifier to the current storage (web)site [20]. even if the data is no more available online, the doi still resolves to some information about the data set and its repository. 3.4.4. example access code alongside the publication at an online service, basic scripts for the emc data set in python and matlab® are provided to facilitate the file opening. 3.5. quantity, unit and sensor metadata at its core, the data set provides numerical measurement data from different sensors. to foster a clear understanding of the numerical data, it is required to add metadata with suitable machine-readable descriptions of the underlying quantities, units, and sensors. each group of actual measurement data (the "leaffolders" of figure 1 is associated with its own specific metadata. in listing 2, the json representation of this metadata is shown for two exemplary quantities of this data set. according to the recommendations from the d-si metadata model, si:unit introduces a mandatory machine-readable definition of the unit of measurement based on the si system. elements from the qudt ontology add information and semantics describing the measured data (e.g., qudt:value), and listing 1: json code for the most important top level metadata. "project": { 1 "fulltitle": "metrology for the factory of the future", 2 "funding programme": "empir", 3 "fundingnumber": "17ind12" 4 }, 5 "person": { 6 "dc:author": ["tanja dorst", "maximilian gruber", "anupam 7 prasad vedurmudi"], 8 "e-mail": ["t.dorst@zema.de", "maximilian.gruber@ptb.de", 9 "anupam.vedurmudi@ptb.de"], 10 "affiliation": ["zema ggmbh", "physikalisch-technische 11 bundesanstalt", "physikalisch-technische bundesanstalt"] 12 }, 13 "publication": { 14 "dc:identifier": "10.5281/zenodo.5185953", 15 "dc:license": "creative commons attribution 4.0 16 international (cc-by-4.0)", 17 "dc:title": "sensor data set of one electromechanical 18 cylinder at zema testbed (zema daq and smart-up unit)", 19 "dc:subject": ["measurement uncertainty", "sensor 20 network", "mems"], 21 "dc:sizeorduration": "24 sensors, 4776 cycles and 2000 22 datapoints each" 23 }, 24 "experiment": { 25 "date": "2021-03-29/2021-04-15", 26 "dut": "festo esbf cylinder", 27 "identifier": "axis11" 28 } 29 acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 4 the type of associated measurement uncertainty (qudt:standarduncertainty). sosa provides a relation to the sensors creating the data (sosa:madebysensor). quantity, unit, and sensor metadata do not contribute to the findability and accessibility of the data set, as this choice only concerns the “internal visibility”. 3.6. traceable measurement data metrological traceability is defined as the relation of a measurement value to a reference through a chain of calibrations [21]. in practice, this is achieved by quantifying the measurement uncertainty of this measurement value. the actual numerical measurement data for a single quantity are provided by two multidimensional arrays of equal dimensions. one array qudt:value stores the measured values in the unit specified by the metadata. another array qudt:standarduncertainty stores the standard uncertainty of the measured value in the same unit. the uncertainty information can be derived from datasheets or from calibration measurements. both arrays are named in the same way as their corresponding metadata entries by design. the number of rows is the number of cycles, the number of columns is the number of data points, and multiple pages (up to three) correspond to different directions (x, y, and z). traceable measurement data do not contribute to the findability of the data set, as this choice only concerns the measurement data, but not the “external visibility”. 4. discussion of the fairness level the data set is evaluated according to the fair data maturity model [22]. all indicators belonging to the four main fair data principles are qualitatively rated from 0 (not considered) to 4 (fully implemented) as proposed by [23], and the outcome is shown in figure 3. a more detailed mapping of specific subindicators to the subsections of section 3 is provided in table 1. the evaluation of all indicators already shows a good compliance listing 2. json code for the metadata of the sound pressure sensor g.r.a.s.46 be of the zema daq and the 3-axis accelerometer bma 280 of the suu. "/zema_daq/sound_pressure": { 1 "sosa:madebysensor": "g.r.a.s.46be", 2 "rdf:type": "qudt:quantity", 3 "si:unit": "\\pascal", 4 "qudt:hasquantitykind": "qudt:soundpressure", 5 "qudt:value": { 6 "si:label": "sound pressure", 7 "misc": { 8 "raw_data": false, 9 "comment": "converted from adc values based on 10 appropriate conversion." 11 }, 12 }, 13 "qudt:standarduncertainty": { 14 "si:label": "sound pressure uncertainty" 15 } 16 }, 17 "/ptb_suu/bma_280/acceleration": { 18 "sosa:madebysensor": "bma 280", 19 "rdf:type": "qudt:quantity", 20 "si:unit": "\\metre\\second\\tothe{-2}", 21 "qudt:hasquantitykind": ["qudt:acceleration", 22 "qudt:acceleration", "qudt:acceleration"], 23 "qudt:value": { 24 "si:label": ["x acceleration", "y acceleration", "z 25 acceleration"] 26 }, 27 "qudt:standarduncertainty": { 28 "si:label": ["x acceleration uncertainty", "y 29 acceleration uncertainty", "z acceleration 30 uncertainty"] 31 }, 32 "misc": {"interpolation_scheme": "cubic"} 33 } 34 figure 3. detailed rating of the sub-indicators of the fair data maturity model. acta imeko | www.imeko.org march 2023 | volume 12 | number 1 | 5 between the data set and the fair data principles. however, certain aspects need further improvement. also, it should be considered that even if the maximum value for all indicators is reached, it is not guaranteed to represent meaningful data. 5. conclusions in this contribution, the extension of an existing data set for an emc lifetime test with metadata according to the fair principles has been demonstrated. the data set, including all metadata, can be downloaded at zenodo (https://zenodo.org/ record/5185953) in the hdf5 file format. future research should further investigate the semantic model of the metadata structure. moreover, the feasibility of an automated evaluation of the fair data maturity model is another topic of interest. acknowledgement research for this paper has received funding within the projects 17ind12 met4fof and 17ind02 smartcom from the empir program co-financed by the participating states and from the european union’s horizon 2020 research and innovation program and from the federal ministry of education and research (bmbf) project famous (01is18078). references [1] m. d. wilkinson, m. dumontier, i. j. aalbersberg, g. appleton, m. axton (+48 more authors), the fair guiding principles for scientific data management and stewardship, scientific data 3(1) (2016) art. no. 160018. doi: 10.1038/sdata.2016.18 [2] s. a. thomas, f. brochu, a framework for traceble storage and curation of measurement data, measurement: sensors 18 (2021) table 1. fair data maturity model indicators evaluated by the subsections of section 3 in this paper. a checkmark indicates that the specific subindicator is fulfilled by a specific aspect. the different aspects identified as relevant for the data set are explained in section 3. open and known formats semantic descriptors top level metadata publication quantity, unit and sensor metadata 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https://doi.org/10.1515/libri-2020-0018 http://www.bipm.org/utils/common/documents/jcgm/jcgm_200_2012.pdf http://www.bipm.org/utils/common/documents/jcgm/jcgm_200_2012.pdf http://doi.org/10.5334/dsj-2020-041 https://doi.org/10.15497/rda00050 template for an acta imeko event paper acta imeko issn: 2221-870x december 2016, volume 5, number 4, 49-55 acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 49 production trend identification and forecast for shop-floor business intelligence zsolt jános viharos1,3, jenő csanaki2, jános nacsa1, márton edelényi2, csaba péntek2, krisztián balázs kis1, ádám fodor1, jános csempesz1 1fraunhofer project center for production management and informatics, institute for computer science and control, hungarian academy of scineces, kende u. 13-17., 1111, budapest, hungary 2opel szentgotthárd ltd., füzesi út 15., 9970, szentgotthárd, hungary 3kecskemét college, izsáki út 10, 6000, kecskemét, hungary section: research paper keywords: production trend identification and forecast; business intelligence; quality control charts and patterns citation: zsolt jános viharos, jenő csanaki, jános nacsa, márton edelényi, csaba péntek, krisztián balázs kis, ádám fodor, jános csempesz: production trend identification and forecast for shop-floor business intelligence, acta imeko, vol. 5, no. 4, article 8, december 2016, identifier: imeko-acta-05 (2016)-04-08 section editor: lorenzo ciani, university of florence, italy received september 22, 2016; in final form december 22, 2016; published december 2016 copyright: © 2016 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: this work has been supported by the grants of the highly industrialized region on the west part of hungary with limited r&d capacity: research and development programs related to strengthening the strategic future oriented industries manufacturing technologies and products of regional competences carried out in comprehensive collaboration, vksz_12-1-2013-0038. and also it has been supported also by the hungarian scientific research fund (otka), grant no. 113038 corresponding author: dr. zsolt jános viharos, e-mail: viharos.zsolt@sztaki.mta.hu 1. introduction the paper introduces a methodology to define production trend classes and also the results to serve with a trend prognosis in a given manufacturing situation. the overview on production trend forecast methods in the next paragraph concludes that the identification and forecast of production trends are key issues on the shop-floor of manufacturing plants; moreover, many artificial intelligence techniques are applied in this field [1][2]. trend types can be formulated to define a classification model for the prognosis; control chart pattern (ccp) is the mostly used keyword for these classes [3], [4], however the definition of the existence or absence of a trend situation is not specified in the literature, so the current paper proposes a novel methodology for that issue described in section 4. section 3 gives an overview of the usually applied production databases and emphasizes the typical difficulty of their connections and integration. the integration of quality, manufacturing execution, alarm handling and machine log information and the availability of developed machine learning techniques allowed building up production trend identification learning models as reported in the fifths section. the sixth section describes the markov process and time series analysis of the collected production data. abstract the paper introduces a methodology to define production trend classes and also the results to serve with trend prognosis in a given manufacturing situation. the prognosis is valid for one, selected production measure (e.g. a quality dimension of one product, like diameters, angles, surface roughness, pressure, basis position, etc.) but the applied model takes into account the past values of many other, related production data collected typically on the shop-floor, too. consequently, it is useful in batch or (customized) mass production environments. the proposed solution is applicable to realize production control inside the tolerance limits to proactively avoid the production process going outside from the given upper and lower tolerance limits. the solution was developed and validated on real data collected on the shop-floor; the paper also summarizes the validated application results of the proposed methodology. acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 50 conclusions on all the results, acknowledgement and referred literature close the paper. 2. production trend forecast methods production forecasting has received significant attention in the last decades [5][6][7]. there are many approaches for trend identification and forecasting in time domain to ensure with the required level of production quantity and quality. this is one of the important assignments of the industry 4.0, direction, too [8], that is mentioned in usa as industrial internet or in more general as cyber-physical production systems (cppss) [9][10]. el-midany et al. used anns to recognize a set of sub-classes of multivariate abnormal patterns [11] in machining of a crank case as one of the main components of a compressor. they used a simulated and a real world data set as well; furthermore they can identify the responsible variable(s) on the occurrence of the abnormal pattern. ranaee and ebrahimzadeh used a hybrid intelligent method [3] to recognize whether a process runs in its planned mode or it has unnatural patterns. this method includes three modules: a feature extraction module, a multi-class svm-based classifier module (mcsvm) and an optimization module using a genetic algorithm. they tested the algorithm on synthetically generated control charts. control chart patterns (ccps) with different levels of noise were analysed by lavangnananda and khamchai [4]. they implemented and compared three different classifiers: decision tree, ann, and the self-adjusting association rules generator (sarg) for process ccps that were generated by predefined equations of garh (generalized autoregressive conditional heteroskedasticity) model for x ̅ chart. pelegrina et al. used different blind source separation (bss) methods in the task of unmixing concurrent control charts to achieve high classification rates [12]. gutierrez and pham presented a new scheme to generate training patterns for ml algorithms: support vector machine (svm) and probabilistic neural network (pnn) [13]. yang et al. proposed a hybrid approach that integrates extreme-point symmetric mode decomposition (esmd) with extreme learning machine (elm) to identify typical concurrent ccps [14]. motorcu and güllü constructed x-r control charts for each production line on the data obtained from the shop-floor to provide high quality production by eliminating key problems: undesirable tolerance limits, poor surface finish or circularity of spheroidal cast iron parts during machining [15]. huybrechts et al. applied standardization, trend modelling, and an autoregressive moving average (arma) model to determine short-term correlation between subsequent measurements. the out-of-control observations can be determined precisely with the dijkstra model and cumulative sum chart of the corrected residuals between the measured and predicted values. milk yield data from two automatic milking system (ams) farms and one farm with a conventional milking system were used for the case study [16]. köksal et al. in 2011 reviewed the quality management related applications of various data mining techniques in manufacturing industry published between 1997-2007 [1]. they grouped the quality related assignments into four groups: product/process quality description, predicting quality, classification of quality, and parameter optimization. they proved the increasing importance of such research and application techniques and their relevance in industry. their analysis on the literature also indicated that data mining applications were mostly encountered in the metal, computer and electronic products manufacturing industries, and relatively less observed in plastics, glass, paper, food processing and chemical manufacturing industries. the importance of integrating production and quality data was highlighted in their paper, too. applications involving classification of quality were not as many as those in the predicting quality category. viharos and monostori presented an approach, already in 1997 [17] for optimization of process chains by artificial neural networks and genetic algorithms using quality control charts. it was shown that the control of “internal” parameters (temporal parameters along the production chain) is a necessity, by this way, early decisions can be made whether to continue the production of a given part or not. also continuous optimization of the production system is possible using the proposed solution. concerning the applied techniques, the most prevalent approaches are based on statistical methods, such as autoregression, moving average and their combinations: autoregressive integrated moving average model (arima) [18] with use of linear regression analysis, quasi-linear autoregressive model [19] or markov chain models (mcm) [20]. these methods based on historical production or time series data for modelling and prediction. another approach has appeared with the evolution of artificial intelligence, such us modelling with artificial neural networks (ann), support vector machines (svm) or nearest neighbour approaches based on pattern sequence similarity [21]. there are several curve-fitting methods in this field for small sample data, such as genetic algorithms [22]. the use of artificial neural networks combined with statistical methods to compensate drawbacks of the separate approaches in trend forecasting leads to better classification and approximation results. a mixed, physical model integrating real process measurements was presented by r. paggi et. al. for computing process uncertainties beyond their prognosis values [23]. various physical modelling techniques, like finite element methods, analytical equations can represent the known dependencies. francesco et. al. [24] used effective measurements derived from the conformity tests to improve the accuracy of the remaining useful life (rul) evaluation. 3. manufacturing assigment & information sources many different it systems are running on the shop-floor, all of them have their main functionality they are supporting. in a business intelligence approach these data can be linked together to result more knowledge about the details of the production system. without the appropriate linkage a significant part of this knowledge is hidden from the operators and the production engineers. vogel-heuser et al. reported that the appropriate integration of it systems in production environment is still a challenge for the industry, since “on the software side, a typical problem is the consistency between interfaces of components both on the syntactical as well as semantic levels” [23], so, the continuous maintenance of overlapping information is a key question. figure 1 shows the systems related to the published analysis, as a typical example of shop-floor it environment. physically, information is inherited from different sensors and controllers (plcs and cncs), measuring machines or acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 51 sometimes the operators; the figure represents the logical sources that are relevant in a business intelligence point of view: the machines, the operators and the workpieces. one of the main scopes of the paper is to collect and link all the information about individual workpieces, and to detect possible trends and relations in the production based on the measurement series of the workpieces. the quality control systems check the geometry and other features of the workpieces. measuring machines process the products independently and the correlations among the measure sets of different machines are typically unknown. it is often simple to link the given measured data to the workpiece, e.g. using a serial code, the time of the measure. one of the main difficulties is the frequency of the measures, because in the most cases only every ‘nth’ are measured, according to a certain sampling frequency. the manufacturing execution system (mes, product tracking in figure 1) logs all the operations and the production time of the individual workpieces. this is the main information source to link the manufacturing machines/operations to the individual workpieces. on the basic level the production monitoring system provides measurement information with the related timestamps about the status of the manufacturing machines and the possible reasons when they are not in production. other reports are mainly created by the human operators (e.g. daily working reports, special test results). during etl process, an important feature of a business intelligence system is to correct inconsistency among manually recorded data. the machine logs are typically not stored in the factory central it systems and the data they have are deleted after a certain time period. on the other hand many machines have internal measuring systems, moreover, they store internally the measured data and e.g. the machining parameter corrections performed on the basis of these measurement. to solve this synchronization/mapping problem an algorithm was developed to find the best fitting delta time between the two independent time series provided by the two independent it systems (machine logs and mes). the appropriately connected data set can be used as an integrated data source of the comprehensive shop-floor business intelligence system. 4. production trend definition the preliminary results of the shop-floor business intelligence applications allowed analysing/reporting special manufacturing related problems using connected data sets. the overview on production trend forecasting proved the importance of identifying and estimating the production trends already before the process is going outside the tolerance limits, e.g. to ensure the requested process capability and stability indexes. the review concluded and also the experiences are mirroring that it is far not an easy task to define and formulate production trends. a decision is needed about a time sequence (time series) of some measurement points whether they form a trend or not. some aspects of this decision is e.g. what trend length is expected/required, what is the minimum and maximum value, what is the minimum frequency of the measurement points, in what direction the trend is going, what form it has (e.g. linear, exponential), where it starts, etc. even so if a production process that can be described with equations, or by any other form of knowledge representation method, the answers for these questions are not easy, moreover it is also dependent on the engineering aspects of the given manufacturing assignment. in a majority of industrial cases: • the plant engineers do not have a fix definition what time sequence they consider as trend, • usually, there exists a hypotheses that there are trends in the processes, • many and various ideas arise how to use trend forecasts if they are recognized. consequently, the identification and forecast of “any” trends have significant manufacturing potentials and benefits. the next illustration gives a novel methodology to automatically determine “what is a production trend?”. in the next analysis, a critical product feature (e.g. a measure) was selected that is manufactured in an operation step in the linear production line. it was assumed that the measurements at the end of line (eol) show significant correlations of the circumstances of this operation. the prepared, linked database allowed working with the eol values ordered by the machining sequence according to the time of the second operation; consequently, in general, with this approach it is possible to analyse an operation through the measured data collected at another operation. this is an important advantage of the presented approach, e.g. analyses based on varying, available data frequencies and measurement time points are possible. four different time-window periods were selected for the examination but results of one, having the highest industrial relevance (according to the engineers’ opinions working in the analysed production plant) is presented in the paper. figure 2 shows one example where the average and the deviation on a certain time-window of the individual figure 1. data sources on the shop-floor. figure 2. measurement points, averages (lines’ vertical positions) and deviation values (lines’ thickness) in the investigated data set. quality measurements daily reports product monitoring workpiecescncs, plcs machine logs test records product tracking workpiecesoperatorscncs, plcs acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 52 measurements are also visualized. the figure shows also that sudden average jumps can be recognized in the given production process. based on these visual investigations a systematic algorithm was defined to explore trends in the given data sets. average values and also linear regression and average jumps were sought after. the average jumps were defined at a given time point of one workpiece machining as difference between the average of its previous values and the average of its following values for a certain time window. average deviation on these time windows was also calculated. it was clear that the trend exploration is not possible during the time periods when the production is running with moderate speed, consequently, only those periods were accepted (presented and used) when the production (data) reached a certain, minimum frequency level. figure 3 shows all the average jump levels up or down (vertical axis) and their calculated deviations (horizontal axis) in the analysed (long) production horizon. this representation (jumps in the averages versus distribution in the data) has motivated the definition of the trend cases. trend means a special sequence of data in time and in the value domain together, otherwise no trend is given. figure 4 shows typical production trends on the left side, while the same production data are shown on the right side, but with random mixing the time points of the same measured data (so no trend is given). this implication (figure 4) leads to the approach taking the measured real production values that are pairs of values & time points from the shop-floor but the mixing of their time points (“fully”) randomly results such a measurement series that have the same distribution than the original production sequences but for sure all time domain trends are eliminated. the preparation of such a “trendless” dataset and its representation in the same method as in figure 4, together with the original production dataset having trends formulates clearly those nonoverlapping zones where production trends are given (figure 5). the data points of the original production trend are on the left side of the figure, representing much lower distribution than the mixed, trendless data points on the right side, while the trend jumps are similar or slightly larger in the original production measurements. this experience indicates the existence of real trends since: • the trend “sizes” (jump levels, vertically) are slightly larger, moreover • the distribution of the related measurement values are significantly smaller (horizontally) than in case of the mixed dataset and the two areas are mainly separated with only slightly overlapping zones. a range could be specified between the two datasets, defining the border between trend and not trend situations (figure 6). the experienced near to ellipsoidal characteristics of the trendless point zone implicated fitting an ellipsoid around these values (around the black values in figure 6). principal component analysis with e.g. 95 % confidence interval can be figure 3. jump level (vertical axis) and deviation (horizontal axis) values in the investigated data set. figure 4. the same measured production/workpiece values (real case) with their identical distributions, having trends in the left and without any trend in the right (horizontal axis: time, vertical axis: production quality measure values). figure 5. data sets with trends (left size) and without trends (right side). acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 53 applied to determine an exact border between the two datasets (figure 6). an interesting experience arose that the largest principal vector was almost parallel to the horizontal axis and almost crossing at the zero vertical value, but not exactly. the jump level is a statistical characteristic of the analysed production time series and the classical statistical deviation, too. however this differentiation of trend and not trend cases can be based on many other calculated statistical parameters, e.g. the 3rd or 4th momentum with the assumption that the point set of these two classes will be separated. it is a challenging topic of further research. having an exact border between the trend and not trend cases allows defining a classification problem for trend identification (figure 7). when the distribution values of the production points are horizontally above the middle point of the ellipsoid they are considered as no trend situations. consequently, three classes can be formulated: • trend up: production measures of the analysed workpieces in the considered interval form a trend up class through the points in figure 7 above the zero vertical axis and horizontally less than the ellipsoid centre when they are outside the identified ellipsoid. • trend down: these situations can be defined similarly to the trend up cases but the values are below the zero horizontal axis. • no trend: points inside the ellipsoid and also that have a distribution higher than the centre of the identified ellipsoid. each point in figure 7 represents a trend class and can be ordered to one individual workpiece, consequently, for each workpiece a trend class can be ordered representing the trend in its future. consequently, it is a classification assignment where trend classes define the outputs while past values of many other, related measurements collected typically on the shopfloor form the available input information. naturally, the relationship between past production measurements and the trend classes is unknown, consequently, it is impossible to form a closed form, or any equation based model for this task. however given measurements at each workpiece (even with sampling) lead to the possibility to use any learning model based on available shop-floor data. the probably non-linear and clearly multidimensional relationship among input and output data indicated the application of an artificial neural network model for the formulated classification assignment [26], where the model is able to forecast the trend in form of trend class specification at each manufactured and measured workpiece. 5. production trend forecast results the previously defined trend class specification can be applied dynamically to the production data resulting in a large labelled dataset which can be used for model building and testing. the precision of the forecast model can be measured by the amount of data points correctly labelled by the model which can be expressed as a percentage of the whole dataset (figure 8). figure 8 shows the classification results of the applied forecast model by measuring the percentage of data points falling into the nine possible recognition categories. as there are three real and three prognosis classes (f: trend up, l: trend down, n: no trend) the number of different model recognition evaluation cases is nine (real classes are formed from real measured values, prognosis classes are the neural model estimates). on the diagram the blue, middle columns show the amount of correct recognition and remaining, red columns show the incorrect recognitions. one can see that the ratio of correct trend classification is around 60-70 %. figure 9 shows the trend sections on a small part of the figure 8. classification results: target represents the real measurements while output means the model classification decision. figure 7. identified fields and borders of trend classes (f: trend goes upwards, l: trend goes downwards, n: no trend is given). figure 6. the exact border between trend and trendless production acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 54 whole year time series. the upper points are production measurements and its moving average while on the point series below the black (on the upper side) denotes the correctly diagnosed f (trend up) trends; blue (on the lower side) denotes the correctly diagnosed l (trend down) trends, red (mainly in the middle) denotes any form of misclassification and yellow indicates that no reliable forecast is available. it can be seen that the incorrectly diagnosed data points are concentrated in the middle, transition area e.g. when changing from upward trend to downward trend. it is natural in time series forecast; so, results are applicable in the given plant. consequently, it is proven that the production trends can be recognized with fair accuracy with the proposed approach. as the classes are connected to the individual elements of a time series (manufactured workpieces) the recognition categories can be arranged in continuous trend sections (figure 9). 6. conclusions the paper presents a methodology to define production trend classes and also the results to serve with trend prognosis in a given manufacturing situation. the solution may be useful in batch or (customized) mass production environments because it continuously collects and analyses data from the shop-floor sensors and may be applicable to realize production control inside the tolerance limits to proactively avoid the production process going outside from the given upper and lower tolerance limits. the review of the production trend forecast methods concluded and also the experiences are mirroring that it is far not an easy task to define and formulate production trend classes. the developed solution for solving this issue collects shop-floor data and based on the concrete manufacturing values it is able to define three trend classes: i) trend goes up, ii) trend goes down and iii) no trend is given. this decision can be an automatized process, consequently, for this decision no prescriptions are needed from the plant engineers or shop-floor operators. however, the resulted classes can be modified by them when any hypothesis or experiences are given in this field. the described method for production trend prognosis considers past values from any related (e.g. physically previous) operations e.g. measurements, alarms, etc. using historical data the applied artificial neural network model determines the prognosis at each produced workpiece. this model is built up also on shop-floor data of the process analysed, consequently, it is valid for that concrete process under the given, prompt situations. furthermore, autoregressive and markov process analysis highlighted the most relevant operations along the whole production line. the solution was developed and validated on real data collected on the shop-floor; the paper summarizes the validated application results of the proposed methodology, too. acknowledgement work presented here has been supported by the grants of the highly industrialised region in western hungary with limited r&d capacity: “strengthening of the regional research competencies related to future-oriented manufacturing technologies and products of strategic industries by a research and development program carried out in comprehensive collaboration”, under grant no. vksz_12-1-2013-0038. references [1] g. köksal, i. batmaz, m. c. testik, “a review of data mining applications for quality improvement in manufacturing industry”, expert systems with applications, 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[26] zs. j. viharos, “ai based, automatic production system decomposition”, 5th international conference on digital enterprise technology, det2008, october 22-24, 2008, nantes, france, paper nr. 94. production trend identification and forecast for shop-floor business intelligence acta imeko, title acta imeko issn: 2221-870x december 2017, volume 6, number 4, 105-112 acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 105 effects of baffle reflection and shadow in an integrating sphere on the total luminous flux measurement of a linearlyshaped lamp kamol wasapinyokul1,2, santhad chuwongin1,2, ajchara charoensook2 1college of advanced manufacturing innovation, king mongkut’s institute of technology ladkrabang, 1, soi chalongkrung 1, ladkrabang, bangkok, 10520, thailand 2national institute of metrology (thailand), 3/4-5, moo 3, klong 5, klong luang, pathumthani, 12120, thailand section: research paper keywords: integrating sphere; total luminous flux; linearly-shaped lamp; baffle; spatial correction factor citation: kamol wasapinyokul, santhad chuwongin, ajchara charoensook, effects of baffle reflection and shadow in an integrating sphere on the total luminous flux measurement of a linearly-shaped lamp, acta imeko, vol. 6, no. 4, article 17, december 2017, identifier: imeko-acta-06 (2017)-04-17 section editor: marco tarabini, politecnico di milano, italy received february 1, 2017; in final form july 24, 2017; published december 2017 copyright: © 2017 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: kamol wasapinyokul, e-mail: kamol.wa@kmitl.ac.th 1. introduction the integrating sphere substitution method is a method to realise the total luminous flux of a light source [1]-[5]. the method generally comprises an integrating sphere, a standard light source with a known total luminous flux value, and a test light source, the flux of which is to be measured. the integrating sphere is a hollow sphere, the inner wall of which is coated with a highly reflective material. once there is a light source inside, due to the high reflective spherical-shaped wall, multiple reflections of the luminous flux occurs. this leads to the illuminance of the light due to the light source at every point on the sphere wall to be theoretically equal to each other. a photo-detector, installed somewhere on the sphere inner wall, then detects and measures this uniform illuminance in a form of an electronic signal. in order to measure the total luminous flux of the test source, both the standard and test sources would be, one at a time, installed at the centre of the sphere. the electronic signals corresponding to the illuminance of each lamp, detected by the photo-detector, are consequently collected. as the total luminous flux of the standard source is known, such value for the test source can be calculated, hence the name substitution method. accuracy of the measured total luminous flux value is generally a topic of interest in the researches [6]-[9]. for the integrating sphere substitution method, a source of the inaccuracy is the spatial non-uniformity of the illuminance on the sphere wall [9], [10]. such non-uniformity is partly attributed to the occurrence of a crucial component in the sphere called baffle. the responsibility of a baffle can be explained as follows: as the photo-detector is to detect the abstract the measured total luminous flux of a linearly-shaped lamp by using the integrating sphere substitution method against a standard spherical lamp can be deviated from accurate by the presence of a baffle in the sphere. the baffle introduced two main effects on the sphere response: the baffle reflection, or high-signal region, and the baffle shadow, or low-signal region. once the baffle condition changed, the two effects changed, causing the measured value to change differently regarding the lamp alignment. in a perpendicular alignment, increasing the baffle length increased the measured flux value. this was due to the dramatic increase in the magnitude of the signal in the baffle reflection behind the baffle. in a coaxial alignment, on the contrary, increasing the baffle length resulted in the decrease of the measured flux value. this was due to the increase in the area of the baffle shadow on the opposite hemisphere from the baffle, which increased at a higher ratio than that of the baffle reflection. in both alignments, the measurement uncertainty increased with the baffle length due to the increasing magnitude of the signal fluctuations. the trends were similar for all linearlyshaped test lamps with different lengths and diameters. acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 106 uniform illuminance due to multiple reflections of the flux from the sphere wall, it must not detect the direct flux from the light source. thus a thin opaque plate called baffle is needed to be installed between the light source and the photo-detector; its duty is to shield the direct flux from the light source not to incident directly on the photo-detector. the baffle is a necessary component of an integrating sphere; however, it is alien to the plain sphere wall. the uniformity of illuminance is thus destroyed, leading to the inaccurately measured total luminous flux of the test source. this inaccuracy in the measured value is enhanced when the luminous intensity distributions of the standard and test sources become increasingly different. an apparent example of such case is when the standard light source is a typical spherical incandescent lamp while the test light source is of linear shape, such as a linear fluorescent lamp or a linear led lamp. moreover, the alignment of a linearly-shaped lamp in an integrating sphere can be varied, leading to different baffle conditions, hence the different non-uniformity characteristics of the sphere response. a number of methods have been reported to reduce the spatial non-uniformity of the sphere caused by the baffle. these include increasing the number of photo-detectors on the sphere wall [9], [10] and improving the response of the detector [11]. these methods, however, require either modification of the sphere or changes of the sphere components, both of which may not be economically or practically suitable for an existing sphere. some reports studied the effects of baffle on the sphere uniformity. it has been reported that the baffle characteristics, such as its shape and reflectance, could increase and decrease the measured total luminous flux value through its reflection and shadow, respectively [7], [12], [13]. this method could be used to reduce the spatial non-uniformity of the sphere. however, the reports have studied only in the cases where the standard and test light sources were not much different in shape. there is still a lack of study on the effects of baffle in an integrating sphere on the measurement of the total luminous flux of a linearly-shaped lamp by using a typical spherical standard lamp, even though such measurements have been practiced generally due to the extensive use worldwide of the linearly-shaped lamps. this paper reports the effects of baffle in an integrating sphere on the total luminous flux measurement of a linearlyshaped lamp at two different lamp alignments: perpendicular and coaxial alignments. two baffle effects – baffle reflection and baffle shadow – have been varied; their concurrent effects were then studied through the trends of changes of a correction factor called spatial correction factor and its relative measurement uncertainty. the results from this study could be used to optimise the baffle condition and hence improving the sphere uniformity. it could consequently be further developed to be an economic and practical method for any existing sphere to improve the accuracy the measured total luminous flux, by the integrating sphere substitution method, of a linearly-shaped lamp. 2. experimental the integrating sphere used in this work was a 2-m-diameter ul2000 integrating sphere from lmt lichtmesstechnik gmbh, at the centre of which a lamp was to be installed. the sphere inner wall was coated with lmt php80 paint with the percentage reflectance of 80 % ± 1.5 %. the right hemisphere was movable for sphere access. at the centre on the left hemisphere wall a photo-detector was placed, in front of which was a baffle. the detector was from lmt and had a field of view of 2π, with the spectral correction factor '1f of 0.5 % and the luminous responsivity of 18.96 na/lx. these characteristics, nevertheless, had no effect on the trend of changes of the measured total luminous flux value when the baffle changed, which was a main purpose of this study. figure 1 shows the schematic diagram of the sphere with the photo-detector and the baffle. each position on the sphere inner wall was denoted angularly by two angles – vertical angle θ and horizontal angle φ, where the reference position of θ = 0 was at the bottom of the sphere and φ = 0 at the hemisphere front-interface. both angles were measured from their corresponding reference positions in a counter-clockwise direction as shown in figure 1. the experiment was started where the spatial response distribution function, k, of the sphere was determined. this parameter is the electronic signal obtained by the photodetector when a light source illuminates at a specific position of the sphere wall; hence it is a function of the sphere wall position, i.e. k(θ,φ). the parameter was obtained by installing a sweeping led at the centre of the sphere as shown in figure 1. the sweeping led set was from lmt lichtmesstechnik gmbh and comprised of a led and two stepping motors, namely θ-motor and φ-motor. the led had an aperture of 33 mm diameter and emitted typical cool-white light, the spectrum of which can be found elsewhere [14]. due to the aperture, the led would directionally shine the light onto a specific angular position on the sphere wall. with the two stepping motors, the led could be rotated to shine on every position on the sphere wall. to start the scanning procedure, firstly, the led was arranged to illuminate at the beginning position, i.e. the bottom point of the sphere; (θ,φ) = (0,0). the led was biased at 3.5 v by using a power generator, providing a driven current of 0.17 figure 1. front-viewed schematic diagram of the sphere with the sweeping led set to obtain the spatial response distribution function, k(θ,φ), at a specific angular position. acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 107 a. prior to the sweeping, the led was left biased at this condition for two hours before the scanning was executed in order for the led light to be stable. after two hours, the scanning started; the response signal k(0,0), detected by the photo-detector at this beginning position, was collected by a computer program. subsequently the θ-motor rotated the angle θ by increasing at a 5° interval, and thus the led illuminated the new positions. at each position, the response signals, k(5°,0), k(10°,0), k(15°,0), and so on, were collected, until the illuminated position was the top of the sphere and k(180°,0) was collected. then the θ-motor rotated the led back to (0,0) while the φ-motor rotated the angle φ for a step of 5° around the vertical axis to be at (0,5°). with φ at 5°, the previous procedure was repeated where the led was rotated horizontally; the response signals k(0,5°), k(5°,5°), k(10°,5°), and so on, until k(180°,5°), were collected. the led was then rotated back to the position that θ = 0 and new φ increased by 5°. such procedure was repeated until φ = 360°, where (180°,360°) was the last illuminated position that the response signal was collected, i.e. the whole inner wall of sphere was swept and k(θ,φ) for every angle θ and angle φ was obtained. this set of data was the k profile of the sphere for a given sphere condition. for example, a different baffle would provide a different k profile. apart from obtaining k from the sphere, the luminous intensity distribution, i, of a standard spherical lamp and a linearly-shaped lamp were also acquired. this parameter is the luminous intensity, in the unit of candela, at each angular position around an illuminating lamp. thus the parameter is a function of angular position on the sphere wall; i.e. i(θ,φ). the luminous intensity distribution at each angular position of the standard lamp, is(θ,φ), and of the test lamp, it(θ,φ), were obtained by using a mirror-type gonio-photometer from photometric solutions international, psi. the operating principle of the gonio-photometer can be found elsewhere [15]. briefly, each lamp was operated on the gonio-photometer while a mirror rotated around the lamp and reflected light to a photometer to collect its luminous intensity at a 5° interval for both θ and φ angles, similarly to the steps of the k(θ,φ). once completed, the is(θ,φ) and it(θ,φ) profiles were obtained. in this study, the standard lamp was a polaron lf200 spherical incandescent lamp, and the test lamps were five linear fluorescent lamps, denoted as lamps a, b, c, d, and e. all test lamps had been preliminarily burnt for 300 hours prior to the experiment to ensure their emission stability during the experiment. all test lamps had different diameter, length, and colour correlated temperature (cct) as shown in table 1. the objectives of using such test lamps with different characteristics were to investigate whether the diameter, length, and cct of the test lamps would affect the effect of baffle in the sphere. figure 2 shows is(90°,φ) and it(90°,φ) profiles of the standard lamp and five linearly-shaped test lamps, respectively, at θ = 90°, which was the horizontally cross-sectional plane at the middle of the sphere. for the standard lamp, is(90°,φ) looked roughly uniform for all angles φ, corresponding to the spherical shape of the lamp. this was much different from those of linearly-shaped test lamps. for the test lamps, a pole of the lamp was set at the position φ = 0. for all test lamps, it(90°,0) and it(90°,180°) were zero as they were at the poles of the lamp. the values gradually increased to maximum at it(90°,90°) and it(90°,270°) as such positions were at the middles of the lamp. the it(90°,φ) of all lamps were of similar shape but with different magnitudes due to their different lengths; the lamps a, b, and e, provided expectably higher i due to their longer lengths and hence higher power outputs. k(θ,φ), is(θ,φ), and it(θ,φ) of each angular position (θ,φ) were further used to calculate their normalised values, ( )* ,k θ φ , ( )* ,si θ φ , and ( ) * , t i θ φ , respectively, as follows [3]. ( ) ( ) ( ) π π φ θ π θ φ θ φ θ φ θ θ φ ∗ = = = ∫ ∫ 2 0 0 4 , , , , sin k k k d d (1) ( ) ( ) ( ) π π φ θ θ φ θ φ θ φ θ θ φ ∗ = = = ∫ ∫ 2 0 0 , , , , sin s s s i i i d d (2) ( ) ( ) ( ) π π φ θ θ φ θ φ θ φ θ θ φ ∗ = = = ∫ ∫ 2 0 0 , , . , sin t t t i i i d d (3) subsequently, ( )* ,k θ φ , ( )* ,si θ φ , and ( ) * , t i θ φ were used to calculate the spatial correction factors of the standard lamp, scfs, and of the test lamp, scft, as follows [3]: ( ) ( ) π π φ θ θ φ θ φ θ θ φ∗ = = = ∫ ∫ 2 ∗ 0 0 1 , , , sin s s scf i k d d (4) figure 2. luminous intensity distributions, is(90°,φ) and it(90°,φ), of the standard lamp and five linearly-shaped test lamps, respectively, at the plane θ = 90°. table 1. details of the linearly-shaped test lamps. lamp diameter (mm) length (mm) cct (k) color a b c d e 26 26 26 26 16 1175 1175 560 560 1130 6120 2820 6120 3865 6120 daylight warm white daylight cool white daylight acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 108 ( ) ( ) π π φ θ θ φ θ φ θ θ φ∗ = = = ∫ ∫ 2 ∗ 0 0 1 . , , sin t t scf i k d d (5) consequently, the final spatial correction factor of the measuring system, scf, was calculated as the ratio of scft and scff as follows: = .t s scf scf scf (6) this scf is a factor indicating how different the measured total luminous flux value is from the accurate value when the luminous intensity distributions of the standard and test lamps are different. due to the normalised nature of both i* and k*, the value of scf technically falls around one. the value of exactly one indicates that the measured total luminous flux is equal to the accurate value. if scf is less than one, the measured value is higher than the accurate value, and vice versa. however, k can be affected by the scanning angular width and the aperture of the scanning led [12], thus the absolute value of scf calculated in this study may deviate if those parameters are varied. as a purpose of this study is to investigate the trend of change of the measured total flux value, the attention was then focused only on the trend of scf regarding the change in the baffle conditions; i.e. if the change induced the increase in scf, the measured total luminous flux value was then decreased, and vice versa. a source that causes the scf to deviate from one is the presence of the baffle. this is because the baffle creates two non-uniform characteristics on the sphere wall [12], [16]: (1) the baffle reflection, or the high-signal region, on the sphere wall around the detector behind the baffle, and (2) the baffle shadow, or the low-signal region, on the sphere wall opposite to the detector. in this report, these effects on the total luminous flux measurement of a linearly-shaped lamp were examined in three studies as follows: a. effects of the baffle reflection on the total luminous flux measurement of a linearly-shaped lamp with perpendicular alignment (i.e. the lamp was aligned perpendicularly to the line from the photo-detector as shown in figure 3a). b. effects of baffle reflection and baffle shadow on the total luminous flux measurement of a linearly-shaped lamp with perpendicular alignment. c. effects of baffle reflection and baffle shadow on the total luminous flux measurement of a linearly-shaped lamp with coaxial alignment (i.e. the lamp was aligned along with the line from the photo-detector, as shown in figure 3b). the perpendicular and coaxial alignments were chosen for study as they are the two common installing configurations of a linearly-shaped lamp inside an integrating sphere. in each study, the effects of baffle reflection and baffle shadow were varied by changing the baffle size (in studies a., b. and c.) and position (in study a.). then the corresponding scf was evaluated. in all studies, the baffles were painted with lmt php80 paint at both front and back sides. thus their reflectances were controlled to be at 80 % ± 1.5 %, similar to that of the sphere wall, during the whole experiment. the trend of change in the measurement uncertainty of scf was also examined. in each study, the measurement uncertainty of scf was calculated through the monte-carlo simulation [17][19]. this was processed by the factor being calculated repeatedly for 10,000 times. in each time, the parameters k(θ,φ), is(θ,φ), is(θ,φ), and θ, were randomly varied within their uncertainty ranges, and scf was calculated by using (1) (6). consequently, a total of 10,000 different scf values were obtained. due to the random nature of the calculated parameters, the 10,000 scf values were normally distributed. however, with such a large number of scf, its uncertainties estimated by using the normal distribution assumption were very small. thus, in the following results, each uncertainty was estimated under the rectangular distribution to provide a larger value and to ensure that the uncertainty was covered in the range. this change in assumption had no effect on the trend of change in uncertainty, which was a purpose of this study. regarding the rectangular distribution of scf values, the absolute and relative uncertainties of scf were calculated by using the maximum scf, scfmax; minimum scf, scfmin; and mean scf, scfmean, of the 10,000 scf values, as follows: − = max minabsolute uncertainty of 12 scf scf scf , (7) = × mean abs. unc. of relative uncertainty of 100% scf scf scf . (8) all the relative uncertainties reported in this study were with a coverage factor of k = 1, corresponding to the level of confidence of 57.74 % regarding the rectangular distribution [20]. 3. results and discussions figure 4 shows a typical normalised spatial response distribution function, k*, of the left and right hemispheres of the sphere used in this experiment obtained by using a circular figure 4. a 3-dimensional plot of a typical set of k*(θ,φ) of (a) left and (b) right hemispheres of the integrating sphere with a circular baffle; the horizontal axis, y, and the vertical axis, z, are the distances from the sphere centre. figure 3. top-viewed schematic diagram of a linearly-shaped test lamp in the sphere with (a) perpendicular and (b) coaxial alignments. acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 109 baffle. the baffle had a diameter of 120 mm, or 12 % of the sphere radius, and was located at 250 mm, or 25 % of the sphere radius, from the photo-detector. the result indicated that k* was not uniform all over the sphere. two distinct characteristics could be observed: (1) the high-signal region at the centre of the left hemisphere where the photo-detector and the baffle were located, and (2) the lowsignal region at the centre of the plain right hemisphere. both anomalies are of circular shape as a result of the circular baffle. both characteristics were originated from the presence of the baffle [12]. the high-signal region at the left hemisphere was due to the baffle reflection; the light from the lamp was incident at the sphere wall around the baffle; it then incidented and reflected from the back surface of the baffle to the photodetector, hence the signal at that area higher than those in the surroundings. the low-signal region at the right hemisphere was due to the shadow of the baffle seen by the photo-detector. this is represented in figure 5 which shows the relation between k∗(90°,φ) and φ, i.e. at the plane with θ = 90°, the horizontally cross-sectional plane at the middle of the sphere. these two characteristics were expected to affect scf differently and were examined in the three following subsections. 3.1. effects of the baffle reflection on the total luminous flux measurement of a linearly-shaped lamp with perpendicular alignment the effect of the baffle reflection, i.e. the high-signal region, on scf was studied by the reflection being varied while the baffle shadow was controlled to have a constant size, and the corresponding scf was calculated. the experiment was carried out in a perpendicular alignment due to the space limitation in the sphere and the effect enhancement. the baffle was in a racetrack shape as shown in figure 6, with the width, w, fixed at 120 mm, and the length, l, varied. to vary the baffle reflection but not the baffle shadow, the baffle was positioned at different distances, d, from the photodetector. when d increased, the baffle length, l, increased in the ratio that the baffle shadow on the opposite hemisphere at every baffle position was of the same size. the values d of 106 mm, 154 mm, 202 mm, and 250 mm, therefore corresponded to the value l of 220 mm, 320 mm, 420 mm, and 520 mm, respectively. at each position d, the scf was calculated. figure 7 shows the schematic diagram of the sphere and baffle when the baffle was at the closest position to the photo-detector. figure 8 shows the effect of baffle distance, d, on scf and its corresponding relative uncertainty. the relations of all five test lamps are of similar trend and show that, once the baffle was further away from the photo-detector, scf increased, indicating the decrease in the measured total luminous flux value. as the baffle shadow (low-signal region) was the same for all baffle distances, the reason of this behaviour was mainly due to the baffle reflection (high-signal region) generated around the figure 7. top-viewed schematic diagram of the sphere with a perpendicular lamp alignment with the baffle position and size changed such that the baffle reflection varied but the baffle shadow remained the same. solid line of the baffle shows its closest position to the photo-detector. figure 6. sketch of the racetrack shaped baffle with width w and length l. figure 8. relations between the baffle distance, d, and scf and its corresponding relative uncertainty, with a coverage factor of k = 1, of linearly-shaped test lamps with perpendicular alignment. figure 5. relation between k*(θ,φ) and φ, at θ = 90°, i.e. the horizontally cross-sectional plane at the middle of the sphere. 0 100 200 300 0.990 0.993 0.996 0.999 sc f baffle distance, d (mm) lamp a lamp b lamp c lamp d lamp e 0.00 0.02 0.04 0.06 r elative uncertainty (% ) 0 50 100 150 200 250 300 350 0.9 1.0 1.1 1.2 baffle shadow hemisphere border k * φ ( o ) baffle reflection acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 110 photo-detector. when the baffle became further away from the detector, light reflecting from the back of the baffle had decreasing magnitudes. this then led to the decrease in height of the signal at such area and the area size, resulting in the decreasing trend of the total luminous flux value. the relative uncertainty of scf on this effect decreased with the increase in the baffle distance. this was expected to be a result from the fluctuation of the high signal in the reflection region. once the baffle was further away from the detector, the signals were less fluctuated, leading to the narrower range of the signal and hence lower uncertainty. 3.2. effects of the baffle reflection and baffle shadow on the total luminous flux measurement of a linearly-shaped lamp with perpendicular alignment here the baffle was in a racetrack shape with the width, w, fixed at 120 mm, and the length, l, varied from 120 mm to 520 mm. its position d was fixed at 250 mm from the photodetector. as the baffle position was fixed but its size changed, both the baffle reflection and shadow would be consequently changed simultaneously. figure 9 shows the relations between the length, l, of the baffle and scf of the system and its corresponding relative uncertainty. the relations of all test lamps exhibited a similar trend where scf decreased while its relative uncertainty increased with the increase in baffle length l. this indicates that, for this perpendicular arrangement, once the baffle became longer, the measured total luminous flux would increase. this trend of scf was a combined effect of both baffle reflection and baffle shadow. once the baffle length increased, both effects would be enhanced as both areas increased. the increases in baffle reflection and baffle shadow caused the increase and decrease, respectively, of the measured total luminous flux [12]. however, the increases in both effects were of different ratios where, in this lamp alignment, the effect of baffle reflection dominated. this was because once the baffle length increased, its edges became closer to the sphere wall around the photo-detector. this caused the magnitude of the high signal due to the baffle reflection to increase dramatically with the baffle length, while the magnitude of the low signal did not change. consequently the effect of baffle reflection dominated that of the baffle shadow, resulting in the higher measured total luminous flux and the lower scf. the relative uncertainty of scf in this lamp alignment was increased with the baffle length, indicating that, once the baffle length increased, the signal was more fluctuated. this was expected to be a result from the fluctuated signals in the baffle reflection region which became more fluctuated when the baffle was longer and closer to the sphere wall. 3.3. effects of the baffle reflection and baffle shadow on the total luminous flux measurement of a linearly-shaped lamp with coaxial alignment in this study, the lamp alignment was in a coaxial configuration as shown in figure 3b, while the baffle was in a racetrack shape with width, w, of 120 mm, and the varied length, l, from 120 mm to 520 mm. in each l, scf was calculated. figure 10 shows the relations between the baffle length, l, and scf and its corresponding relative uncertainty. the relations show that the increase in baffle length caused scf increased, indicating the decreasing measured total luminous flux. this is a combination effect of both baffle reflection and baffle shadow, where both increased with the increase in baffle length. however, the decrease in the measured total luminous flux value indicated that the increase in baffle shadow was more pronounced. this can be explained by using the areas of both regions as shown in figure 11. when the baffle length figure 10. relations between the baffle length, l, and scf and its corresponding relative uncertainty, with a coverage factor of k = 1, of linearly-shaped test lamps with coaxial alignment. figure 11. schematic diagram of the sphere showing the areas of baffle reflection and baffle shadow when the baffle was short (black), and long (grey). figure 9. relations between the baffle length, l, and scf and its corresponding relative uncertainty, with a coverage factor of k = 1, of linearly-shaped test lamps with perpendicular alignment. 0 100 200 300 400 500 600 0.994 0.996 0.998 1.000 1.002 r elative uncertainty (% ) sc f baffle length, l (mm) lamp a lamp b lamp c lamp d lamp e 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0 100 200 300 400 500 600 0.994 0.996 0.998 1.000 1.002 1.004 r elative uncertainty (% ) sc f baffle length, l (mm) lamp a lamp b lamp c lamp d lamp e 0.000 0.005 0.010 0.015 acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 111 increased, the areas of both shadow and reflection regions are both increased. however, as the shadow was further away from the photo-detector than the reflection, the size of the former increased at the higher ratio than the latter. hence its effect dominated, resulting in the decrease in the measured total luminous flux value with the increase in baffle length. effects of the increase in area of baffle shadow caused the decrease in the measured total luminous flux value only in the coaxial alignment, not in the perpendicular alignment. this was because in the former, the increasing area of the baffle shadow was the area perpendicular to the middle of the lamp, which was the area with high luminous intensity distribution i(θ,φ), while the reflection region was the area with low i(θ,φ) from the lamp pole [21]. thus, the shadow effect was more pronounced in the coaxial alignment. in the perpendicular alignment, the baffle reflection region was the area with high i(θ,φ). thus its effect was more pronounced as explained in the previous section. the relative uncertainty of scf in this coaxial alignment increased with baffle length. this was because the increases in both baffle shadow and baffle reflection regions induced more fluctuated signals. it can be observed that the uncertainty of scf from this coaxial alignment was around 10 times higher than that in the perpendicular alignment obtained from the same baffle size. this indicated the poorer measurement performance of this alignment than that of the perpendicular one. 4. conclusions baffle is a crucial component in an integrating sphere which is used in the measurement of the total luminous flux of a linearly-shaped light source. however, the presence of a baffle in front of the photo-detector in the sphere created two nonuniform characteristics of the sphere response on the sphere wall – (1) the baffle reflection, or the high signal region, around the detector behind the baffle, and (2) the baffle shadow, or the low signal region, on the opposite hemisphere. by using the spatial correction factor, these effects were found to differently cause the deviation of the value of measured total luminous flux from accurate. the increase in the baffle reflection caused the measured total luminous flux value to increase. on the contrary, the increase in the baffle shadow caused the value to decrease. the increase in size of a baffle increased both effects. however, the dominant effect of the two depended on the lamp alignment. in the perpendicular lamp alignment, the effect of baffle reflection dominated that of the baffle shadow, resulting in the increase in the measured total luminous flux value, i.e. decrease in scf. this was because, when the baffle length increased, its edge was closer to the sphere wall and hence the height of the signal in the reflection region increased more dramatically compared to the baffle shadow. in the coaxial lamp alignment, on the contrary, the effect of baffle shadow dominated. this resulted in the decrease in the measured total luminous flux value, i.e. increase in scf, when the baffle length increased. this was due to the increasing size of the shadow which was at a higher ratio than the increasing size of the reflection. another reason was that the increasing shadow area was the area with high luminous intensity of the linearly-shaped lamp, and hence it was more effective than the increasing reflection area, which was the area with very low luminous intensity. the increase in both baffle reflection and shadow also affected the uncertainty of the scf. once the area of either region increased, the signals were more fluctuated, resulting in the increase in the measurement uncertainty. it was observed that the relative uncertainty of scf in the perpendicular alignment was 10 times lower than that in the coaxial alignment. this indicated that the former alignment could be used to improve the measurement uncertainty. moreover, such alignment also provides more space for adjusting the baffle conditions to improve the measurement accuracy. these trends on the measured total luminous flux value and its measurement relative uncertainty were similar regardless of the difference in the length and diameter of the linearly-shaped test lamps. this indicated that the results from this study could be applied to other linear shape lamps with different lengths and diameters from those used in the study. as the presence of a baffle can affect the accuracy of the measured total luminous flux, its conditions including size and distance need to be carefully designed to provide the measured total luminous flux as accurately as possible. this paper focused mainly on the trend of change in the measured total fluxed value. nevertheless, with further studies, the results could be developed thus that the baffle adjustment could be an economic and practical method to improve the accuracy the measured total luminous flux, by the integrating sphere substitution method, of a linearly-shaped lamp. acknowledgement kw would like to thank rojana leecharoen and soontorn chanyawadee, nimt, for the help in sphere scanning programming; and thayathip thongtan, nimt, and siridech boonsang, kmitl, for technical discussions. references [1] c. decusatis, handbook of applied photometry, springer, 1997. isbn 978-1-56396-416-9. [2] y. ohno, new method for realizing a luminous flux scale using an integrating sphere with an external source, j. of the illuminating engineering society, 24(1) (1995), pp. 106-115. [3] y. ohno, realization of nist 1995 luminous flux scale using integrating sphere method, j. of the illuminating engineering society, 25(1) (1996), pp. 13-22. [4] f. samedov, m. durak, realization of luminous flux unit lumen at national metrology institute of turkey (ume), optica applicata, 34 (2004), pp. 265-274. [5] m.-q. liu, x.-l. zhou, w.-y. li, et al., study on methodology of led’s luminous flux measurement with integrating sphere, j. of physics d: applied physics, 41-14 (2008), pp. 144012. [6] t. poikonen, t. pulli, a. vaskuri, et al., luminous efficacy measurement of solid-state lamps, metrologia, 49 (2) (2012), pp. s135-s140. [7] k. ohkubo, m. saito, k. hirokawa, et al., comparison of total luminous flux for integrating sphere photometers, j. of the illuminating engineering institute of japan (shomei gakkai shi), 98(8a) (2014) pp. 362-368. [8] t. donsberg, t. pulli, t. poikonen, et al., new source and detector technology for the realisation of photometric units, metrologia, 51(6) (2014), pp. s276-s281 [9] s. park, d.-h. lee, s.-n. park, six-port integrating sphere photometer with uniform spatial response, applied optics, 50(15) (2011) pp. 2220-2227. [10] s. park, d.-h. lee, s.-n. park, experimental validation of the six-port design for a highly uniform integrating sphere photometer, applied optics, 52(29) (2013), pp. 7178-7185. [11] k. ohkubo, s. mishima, approximate cosine response detector head for integrating sphere photometers, j. of the illuminating acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 112 engineering institute of japan (shomei gakkai shi), 96(11) (2012) pp. 761-766. [12] k. wasapinyokul, r. leecharoen, s. chanyawadee, et al., effects of integrating sphere conditions on the spatial response distribution function in the total luminous flux measurement, xx imeko world congress, busan, republic of korea, 2012. [13] f. lin, t. li, d. yin, et al., research on effects of baffle position in an integrating sphere on the total luminous flux measurement, proceedings of spie 9684 (2016), pp. 96841c. [14] v. k, khanna, fundamentals of solid-state lighting: leds, oleds, and their applications in illumination and displays, crc press, new york. (2014), pp. 230. [15] t. yoshizawa, handbook of optical metrology: principles and applications, second edition, crc press, new york (2015), pp. 56. [16] y. ohno, detector-based luminous flux calibration using the absolute integrating-sphere method, metrologia, 35 (1998), pp. 473-478. [17] c. e. papadopoulos, h. yeung, uncertainty estimation and monte carlo simulation method, flow measurement and instrumentation, 12 (2001), pp. 291-298. [18] i. farrance, r. frenkel, uncertainty in measurement: a review of monte carlo simulation using microsoft excel for the calculation of uncertainties through functional relationships, including uncertainties in empirically derived constants, the clinical biochemist reviews, 35(1) (2014), pp. 37-61. [19] k. godo, t. saito, h. shitomi, t. zama, i. saito, development of a total luminous flux measurement facility for leds at the national metrology institute of japan, 9th international conference on new developments and applications in optical radiometry, davos, switzerland, (2005). [20] joint committee for guides in metrology (jcgm), evaluation of measurement data – guide to the expression of uncertainty in measurement, international bureau of weights and measures (bipm), (2008), pp. 70 [21] y. ohno and r. o. daubach, integrating sphere simulation on spatial nonuniformity errors in luminous flux measurement, j. of the illuminating engineering society, 30(1) (2001), pp. 105-115. effects of baffle reflection and shadow in an integrating sphere on the total luminous flux measurement of a linearly-shaped lamp << /ascii85encodepages false /allowtransparency false /autopositionepsfiles true /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /error /compatibilitylevel 1.4 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false 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/hrv (za stvaranje adobe pdf dokumenata najpogodnijih za visokokvalitetni ispis prije tiskanja koristite ove postavke. stvoreni pdf dokumenti mogu se otvoriti acrobat i adobe reader 5.0 i kasnijim verzijama.) /hun 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can be opened with acrobat and adobe reader 5.0 and later.) >> /namespace [ (adobe) (common) (1.0) ] /othernamespaces [ << /asreaderspreads false /cropimagestoframes true /errorcontrol /warnandcontinue /flattenerignorespreadoverrides false /includeguidesgrids false /includenonprinting false /includeslug false /namespace [ (adobe) (indesign) (4.0) ] /omitplacedbitmaps false /omitplacedeps false /omitplacedpdf false /simulateoverprint /legacy >> << /addbleedmarks false /addcolorbars false /addcropmarks false /addpageinfo false /addregmarks false /convertcolors /converttocmyk /destinationprofilename () /destinationprofileselector /documentcmyk /downsample16bitimages true /flattenerpreset << /presetselector /mediumresolution >> /formelements false /generatestructure false /includebookmarks false /includehyperlinks false /includeinteractive false /includelayers false /includeprofiles false /multimediahandling /useobjectsettings /namespace [ (adobe) (creativesuite) (2.0) ] /pdfxoutputintentprofileselector /documentcmyk /preserveediting true /untaggedcmykhandling /leaveuntagged /untaggedrgbhandling /usedocumentprofile /usedocumentbleed false >> ] >> setdistillerparams << /hwresolution [2400 2400] /pagesize [612.000 792.000] >> setpagedevice microsoft word 248-1782-1-le-rev2 acta imeko  issn: 2221‐870x  september 2015, volume 4, number 3, 72 ‐ 79    acta imeko | www.imeko.org  september 2015 | volume 4 | number 3 | 72  some thoughts on quality models: evolution and  perspectives  luigi buglione  gufpi‐isma (gruppo utenti function point italia – italian software metrics association), rome, italy        section: research paper   keywords: software quality; quality models; non‐functional requirements; fpa; snap; iso 25010; gqm   citation: luigi buglione, some thoughts on quality models: evolution and perspectives, acta imeko, vol. 4, no. 3, article 12, september 2015, identifier:  imeko‐acta‐04 (2015)‐03‐12  editor: paolo carbone, university of perugia, italy  received february 12, 2015; in final form june 22, 2015; published september 2015  copyright: © 2015 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  corresponding author: luigi buglione, e‐mail: luigi.buglione@gufpi‐isma.org     1. introduction  ‘quality’ is a risky and misleading term because including so many meanings and attributes – even if often seen simply as ‘defectability’ within a single word that often in assessments and evaluations it besides in the ‘qualitative’ side more than be extended also in the ‘quantitative’ one, finding proper measures for quantifying it. thus, questions such as ‘which is the value for quality? how to measure quality?’ are typical also in the software engineering community. it can be quite easy to count something but less to evaluate its quality side, because difficult to express the core question (“what does it mean quality”?). tom demarco said that “you cannot control what you cannot measure” [31]. but coming one step back, it is also true that “you cannot measure what you cannot define”. coming one step back again, “you cannot define what you don’t know”. thus, it is a knowledge problem and the priority is to move from a common, shared definition. reading these three statements in the opposite order, (1) if you know something, you’re able to properly describe it and share such definition with others; (2) if you are able to share definitions, it will be easier to quantify such ‘thing’ in the same way (looking at metrology, two measurers should vary very few counting/evaluating the same ‘thing’ → repeatability); (3) if you are able to measure something in a proper way, understanding what attribute(s) you’re measuring, you can have information and should be sufficiently aware for taking decisions. just a short example for better expressing the need and value when having (or not) a clear and not ambiguous definition: asking what is a loc (line of code), possible answers could be: (a) a physical statement; (b) a logical statement; and both could be complemented (c) with or (d) without commented lines. thus, counting locs for a software system, numbers could vary a lot just applying slightly different definitions1. another short example with function points (fp): 1 according to jones [13], there could be variability till 500 % between extremes. abstract  ‘quality’ is an evolving concept, quite difficult to be defined, whatever the application domain observed. as tom demarco said, it’s  true that “you cannot control what you cannot measure”. but coming back, it’s also true that “you cannot measure what you cannot  define” and again “you cannot define what you don’t know”. thus, moving from a common, shared definition is the priority for any  activity and creates also measures from any measurement and benchmarking activity. along the years, several ‘quality models’ (qm)  have been produced: the scope for non‐functional attributes (part of the ‘quality’ definition, also according to iso) is enlarging. this  paper discusses the evolution of the quality concept (broader than the one referred to the solely ‘product’, e.g. service quality)  in  order to observe which perspectives can be drawn up for the next years.    acta imeko | www.imeko.org  september 2015 | volume 4 | number 3 | 73  the ifpug method till v4.2 formally included the so-called vaf (value adjustment factor), expressing 14 non-functional attributes ‘adjusting’ the initial functional size value. thus, afp (adjusted fp) formula included also vaf, while ufp (unadjusted fp) not. but what should it mean the solely fp acronym? which should be the right number of function points to count and declare for such activity? as in figure 1, since any ‘thing’ to be evaluated is a mix of quantity and quality and each side has different parameters for being evaluated (in terms of productivity, costs and so on), it is fundamental to deeply analyse the ‘quality’ side – that has been right now the less explored (also because more complex) part of the ‘yin-yang’ representation. the paper is organized as follows: section 2 will propose a short history of quality models (qm) from mid ‘70s on. section 3 will discuss the stakeholders’ issue: the inclusion (or not) for an attribute in a qm could be also due to the viewpoint faced and the stakeholders included (or not) in the analysis. moving from the historical perspective shown, section 4 will propose perspectives about how qm are evolving and should still evolve for properly catching the value for software quality during next years. 2. a short history of quality models (qm)  our core question is: what is quality? ‘quality’ is a multifacet term because it is an aggregator for multiple attributes. if you should express why you’ve appreciated a certain food, you would start to list a series of ‘attributes’ such as: flavour, taste, way to be presented, freshness of ingredients, the quality/price ratio, etc. next step would be their quantification, trying to find a shared way to ‘count’ them. that’s the application of the well-known goal-questionmetric (gqm) paradigm [20]. the same happened (and still happens) in software engineering with quality models (qm). if the ‘quantity’ side expresses the functionalities (what the software product – not the software project! is asked to do), the ‘quality’ side should express the non-functionalities (how those functions should work for satisfying its users-clients). thus a qm can be defined as a shared list of 2 another way to express the same concept is using a coin: quality and quantity are the two faces of a coin. it is not possible to obtain a comprehensive evaluation not dealing with both faces. but each one has its own properties (attributes) and measures. attributes/characteristics that an entity of interest (eoi) can own, expressing its non-functional side (‘how’). a qm can be articulated in one or more tiers: in the second case, there will be a hierarchy of attributes with high-level and low-level attributes. for ‘completing’ a qm, typically a further tier is added with measures that help in quantifying a certain attribute. moving also from other similar studies [24]-[28], now a list of more known qm will be presented, trying to stress their peculiarities for catching useful elements for improving the next generation of qms. 2.1. fcm (factor‐criteria‐model)  this is the first qm, produced in the mid ‘70s within the air navy [1]. as shown in figure 2, it contained 11 factors (the first layer-tier) and 23 criteria (the second layer). each factor was linked to two or more criteria. of course, as in any qm, each element needs to have a clear definition with unambiguous statements. factors were classified into three moments in time along the software life cycle (slc): product operation, product revision, product transition. 2.2. boehm quality mode  one year later, boehm proposed his own qm, with 7 highlevel characteristics (1st level) and 12 primitive characteristics (2nd level) [2] (figure 3). also here a high-level char could be linked to 2+ primitive characteristics. introduced the ‘utility’ concept, splitting the ‘as-is utility’ and the ‘maintainability’ for software products. 2.3. the fft test  moving from such early qms, iso decided – after the realising of the first 9001 version in 1986 – to release its own qm [3]. the model, as shown in figure 4, included 6 figure 1. quantity and quality – a ‘yin‐yang’ representation 2 .  figure 2. factor‐criteria‐model.   acta imeko | www.imeko.org  september 2015 | volume 4 | number 3 | 74  characteristics and 18 sub-characteristics. here each high-level characteristic is subdivided in a more refined list, with nocrossed links. ieee 1061-1992 replied the content of iso 9126:1991, including such list of ‘attributes’ in the appendix a. in 1998, ieee 1061-1998 deleted such list, considering an open list of values and not a closed list of attributes. 2.4. iso 9126‐1:2001  after 10 years, iso refined its view on quality and proposed the new version for the 9126 qm [4] (figure 5). it was introduced the concept of different viewpoints by different stakeholders: internal, external and quality in use viewpoints. here the first two ones, with 6 characteristics and 26 subcharacteristics. each low-level characteristic was linked with 1+ process(es) from the iso/iec 12207 process model for any related process improvement activity. a second model included the quality in-use view (figure 6), with the four additional characteristics. 2.5. iso 25010:2011  after 10 year more, iso revised again its view on quality and evolved 9126 into the square (software product quality requirements and evaluation) 25000 series with the new 2501x block of standards [5] (figure 7). iso 25010:2011 now includes 8 characteristics and 38 subcharacteristics (figure 8). refined some characteristics (e.g. usability is stressing more the accessibility issue than before) and introduced others as security. also the quality in-use part evolved, including now five characteristics, maintaining from the previous version effectiveness and satisfaction and adding efficiency, freedom from risk and context coverage, defining also a lower-level. 2.6. other qms and nfr‐related approaches  after observing the evolution of the iso view on qms, this section will briefly introduce and list other possible qm produced in the software engineering arena from the ‘90s on: • furps(+): furps is the acronym for a software product quality taxonomy – as well as iso 9126 by grady & caswell [8] and refined with more attributes into furps+ [9]. furps stands for functionality (to be split into: feature set, capabilities, generality, security), usability (human factors, aesthetics, consistency, documentation), reliability (frequency/severity of failure, recoverability, predictability, accuracy, mean time to failure), performance (speed, figure 7. iso 25010:2011 – quality model.   figure 6. iso 9126‐1:2001 –quality in‐use view.   figure 8. iso 25010:2011 – quality in‐use.   figure 5. iso 9126‐1:2001 – external/internal quality view.   figure 3. boehm’s quality mode.   figure 4. iso 9126:1991.   acta imeko | www.imeko.org  september 2015 | volume 4 | number 3 | 75  efficiency, resource consumption, throughput, response time), supportability (testability, extensibility, adaptability, maintainability, compatibility, configurability, serviceability, installability, localizability, portability). the “+” addition represents an aid for remembering concerns such as: design requirements, implementation requirements, interface requirements and physical requirements. • ecss-e-10a + iso 21351:2005: ecss (european cooperation for space standardization) is an initiative established to develop a coherent, single set of user-friendly standards for use in all european space activities. among the several standards produced, ‘technical requirements’ are diffusely treated. ecss-e-10a [6] was used for creating iso 21351:2005 [7]. • ifpug vaf: from albrecht’s initial study [29], passing for a single revision in 1983 [30] till ifpug cpm v4.2, the fpa method proposed a ‘value adjustment factor’ (vaf) based on 14 non-functional attributes (gsc – general system characteristics), mostly referred to the software product, some others to the software project entity. the 14 gsc are: data communication, distributed data processing; performance; heavily used configuration; transaction rate; online data entry; end-user efficiency; online update; complex processing; reusability; installation ease; operational ease; multiple sites; facilitate change. the aim of vaf was to ‘adjust’ the product functional size by a series of quality attributes for ‘optimizing’ the statistical relationship in historical series of adjusted product functional size vs project effort. in 1998 iso decided to keep of such element from any fsm (functional size measurement) method, because not proportional to the product functional side, stating that non-functional requirements (nfr) must be evaluated apart from fur in a different way. in the current ifpug cpm v4.3 such list has been maintained in appendix c [11]. • ifpug snap: more recently, ifpug proposed a new separate methodology from fpa named snap (software non-functional assessment process). from the analysis of product nfr, the method calculates the number of snap points (sp). the current v2.2 [12] includes 14 sub-categories grouped into 4 categories. as in fpa, each sub-category has 2+ complexity parameters for deriving for each scu (snap counting unit) the associated number of sp. here the list of categories and sub-categories that could be used also as a qm, not considering the sp calculation algorithm: data operations (data entry validation; logical & mathematical operations; data formatting; internal data movements; delivering added value to users by data configuration); interface design (ui changes; help methods; multiple input methods; multiple output methods); technical environment (multiple platform; database technology; batch processing system); architecture (component based sw dev (cbsd); multiple input/output interface). table 1 summarizes the different qms and approaches discusses above. 3. possible criteria for evaluating a qm  analyzing the proposed qm it is possible to derive the following considerations in order to understand the value to be provided by a qm: • stakeholders – as stressed in well-recognized management guides such as pmbok [17] or itil [18], it is fundamental to understand from the beginning which are the right stakeholders to involve for creating a good qm. for instance, users are fundamental but often have been considered only for providing final feedback (customer/user satisfaction), not for driving assessment criteria. remember that a customer (the business) is not necessarily the user, but could be separate people. remember also to involve those secondary stakeholders (e.g. foreign tourists could be useful for describing how to improve a mobile touristic app for a certain city providing a different viewpoint than a citizen from that city). • grouping criteria – quality represents the ‘how’ a product should be realized according to initial requirements. thus, several criteria should be considered. for instance (a) time: a lifecycle view should be included and/or linked to a qm (e.g. iso 9126-1:2001 inserted the related process(es) from iso 12207 and target audience for any sub-characteristic). it could be useful for improving the product during its lifetime for maintainability purposes. it could be applied at the beginning of a project, for planning the expected quality level and/or at the end of a project, for controlling the actual quality levels; (b) viewpoint/stakeholder positioning: internal, external and quality in-use viewpoints, as proposed by iso from 2001 with 9126-1 and now with the 25010 standards; (c) viewpoint/context-content: the wider the list of attributes and sub-attributes, the more comprehensive the analysis of a product by its qm. as in the balanced scorecard (bsc) approach [19], it would be desirable to have at least 4-5 perspectives (e.g. time, cost, risk, quality, ethics, etc.) against which grouping quality attributes; (d) measurable entity: a qm should take into account one, single entity of interest (eoi). for instance, qm presented before are about the software product. 4. eam: taxonomize a qm   the presented qm can have structurally a two (or three)level structure, but they are all about (software) products. what table 1 – list of main qm – a short summary    name year author(s)  no.chars/sub‐ chars  notes fcm 1977 mccall et  al.  11  factors  (3  groups),  23  criteria  n:m  relations  boehm qm 1978 boehm 3  main  chars,  6  second‐lev  chars,  15 third‐lev chars  ‐‐‐ vaf 1979 albrecht  10  gscs  (general  systems chars)  adj  factor  to fpa  vaf 1983 albrecht‐ gaffney  14 gscs  adj  factor  to fpa  furps(+) 1987/92 grady‐ caswell  5  chars,  28  sub‐ chars  ‐‐‐ iso 9126 1991 iso 6  chars,  21  sub‐ chars  ‐‐‐ iso 9126‐1 2001 iso 6  chars,  27  sub‐ chars  +  4  quality‐ in‐use chars  2  models  (int‐ext  quality;  quality  in  use)  iso 21351 2005 iso 16  funct.  and  tech req.  iso 25010 2011 iso 8  chars,  31  sub‐ chars  +  5  quality‐ in‐use  chars  and  11 sub‐chars  2  models  (int‐ext  quality;  quality  in  use)  snap 2011 ifpug 14  method acta imeko | www.imeko.org  september 2015 | volume 4 | number 3 | 76  about a qm for a different entity? one of the main problems in benchmarking activities is that often comparisons are done between not homogeneous entities and attributes. a simple example from the software industry can be the so-called ‘backfiring’, that’s a linear conversion between locs and function points (fp). but a line of code (loc) expresses only the length of the source code for a software product: two programmers could generate the same amount of functionalities from the user viewpoint but producing different numbers of locs, according to their experience, programming style, if the loc definition for that organization includes (or not) commented and blank lines etc. fp born in the late ‘70s just to overcome such contractual problem, willing to express a neutral amount of functionalities for a software system provided by a provider to a customer. but also in this case there is often a confusion for many ict professionals about what a fp is sizing: a fp – whatever the fpa variant expresses the functional size for a software product, thus not the size for a whole software ‘project’. simple demonstration: the so-called bfc (base functional components) in a fsm (functional size measurement) method are all about the ‘product’, not the ‘project’ managing it. again, if a project activity included into a gantt chart does not vary (directly or indirectly) the number of fp, it means that such activity cannot be seen as related to a fur (functional user requirement), that’s the needed input to calculate fp. eam (entity-attribute-measure) [15] is an effective way to taxonomize measures trying to depict a three-layered table for allowing understanding if your own set of entities of interest (eoi) has been properly analyzed through a palette of quality attributes. table 2 presents the previous example using loc and fp. it easily explains why loc and fp cannot be ‘backfired’ or that mccabe’s cyclomatic complexity index v(g) expressing the complexity for a chunk of software – is something different from the ‘functionality’. thus, a more complex software cannot be measured and sized by fp but by v(g) or other similar measures, and so on. eam could be a complement way to run a gqm (goal-question-metric) [20] and bpm [22] analysis in order to check if we are measuring the ‘right things’ and ‘things right’, also because one of the main reasons why organizations measure a few is their perception of a higher cost than expected within the project budget. but often the risks are to measure too much (over-measuring), too few (under-measuring) or the wrong phenomenon (bad-measuring) [22]. eam would like to be a way to do that. looking at the ‘project’ entity, figure 9 shows a proposal by pmi (www.pmi.org) for a list of 14 attributes (or characteristics) for a generic ‘project’ [21], but it is not shared e.g. with the other worldwide project management association such as ipma (www.ipma.ch) or apmg (www.apmg-international.com). again, looking to processes, it is sufficient to take a look to the different attributes proposed in the cmmi constellations (gp generic practices) and in spice (iso/iec 15504, the socalled pa process attributes) for evaluating and rating a process using the n/p/l/f (not/partially/largely/fully achieved) ordinal rating scale. thus, the match between a typical qm architecture and eam can help an organization to clearly link operational measures in a project with the related entities and attributes (eventually two-layered) for the adopted qm, returning which could be the (sub)characteristic(s) with missing data/information to work on. 5. bmp: balancing multiple perspectives in qm  after understanding the quality level adopting the current version of a qm, next step could be to check and improve it over time. as in life, one fundamental criterion for being successful is to properly balance elements, possibly finding the most correct alchemy. it is not a simple task, but it implies a deeper knowledge about the way an organization works and the leverages to be used for achieving better results. bmp (balancing multiple perspectives) [22], figure 10, is a technique helping organizations in analyzing whether they are properly balanced against a set of criteria or not: are you managing your projects only by time and cost measures or including also quality, risk and/or ethics? each ‘perspective’ (in this example: time, cost, quality, risk) could represent the trigger for a specific qm. and each ‘specific’ qm could be deployed till the third level adopting the eam approach presented before (measures – here shown as ‘m’ followed by the initial of the perspective (t, c, q, r) and an ordinal number), in order to check the level of completeness for each ‘leaf’ of this tree. such approach, as described in [22], would help to drive an organization in obtaining more informative value from the same set of measures yet adopted, analyzing them in groups of two, or three or four, using a rca (root-cause analysis) approach. figure 9. project attributes [21].   figure 10. bmp approach [22] – perspectives and measures.   table 2 – entity (e), attributes (a), measures (m) [15].   e – entity  (software) product  (software) product a  –  attribute   code length    functionality  m  ‐  measure  lines  of  code  (loc)  function  point  (fp)  acta imeko | www.imeko.org  september 2015 | volume 4 | number 3 | 77  an example: the number of software defects before the project will be delivered or a certain number of loc (or fp) could be good or bad, according to the thresholds established for each single measure. but correlating them – without gathering necessarily new measures, thus adding costs to the project – could give the project back additional information, e.g. measuring the ‘defect density’ for that chunk of software. and so on. thus, in this context bmp perspectives could be interpreted as: • at a higher level, as entity of interest (eoi) – in this case the ‘bubbles’ could be represented by e.g. the five entities described in the star taxonomy (organization, project, resources, process, product) [23] • at a lower level, as qm attributes – in this case the balancing would be, within an eoi, among the different attributes (and related sub-attributes) till the measure layer. figure 11 proposes a graphical view of what previously discussed, merging the typical qm structure with eam, the two possible applications of the bmp technique. the project historical database (phd) represents the knowledge base where all the information should be stored for being shared along the organization. for instance, itil proposes a ‘rule of thumb’ when dealing with how many kpi per each goal to formalize and use, proposing 2-3 kpi per goal and 2-3 base measures per each kpi. measurement should be more and more an opportunity more than a cost: thus, organizations and projects could start to calculate also a rom (return on measurement) as a variant of the traditional roi, considering also the benefits and not only the cost from a proper measurement & analysis (ma) process, as stated e.g. from cmmi or iso 9001 as a process to be deployed as soon as possible. 6. a short example  in order to show an application of the concepts above presented, a short example follows, presenting before an eam table summarizing the involved elements and later a gqm-like tree, depicting three levels (goal, kpi, measure). such measurement plan has been produced taking into account four measurable entities (project, resources, process, product), planning one (or more) kpis per each measurable entity and two (or more) measures per each kpi, as shown in table 3. and now the gqm-like tree for the four (4) goals: o g1. improve projects’ estimation capability ( entity: project)  kpi.01 – improve the ‘historicization’ of project data  m.01 – no. of total projects managed per quarter by business unit  m.02 – no. of projects uploading ‘project closing’ data per quarter  m.03 – avg/median estimated % effort for (pmbok) ‘project closure’ phase  m.04 avg/median actual % effort for (pmbok) ‘project closure’ phase o g2. improve the requirement elicitation process ( entity: resources)  kpi.02 – optimize the project teaming process  m.05 – avg/median capability level per project role (periodically monitored)  m.06 –avg/median seniority ratio per project role by that functional domain  m.07 avg/median cost per project role (by m/d)  m.08 – avg/median no. of changes in the team (per project) o g3. improve the requirement elicitation process ( entity: process)  kpi.03 – minimize the number of implicit requirements  m.09 – analysts avg/median capability level (periodically monitored)  m.10 – analysts avg/median seniority ratio by that functional domain  m.11 % of scope creep by m/d (economical)  kpi.04 – optimize the fp/sp counting capability  m.12 – avg/median % of scope creep per project by a certified fsm-people  m.13 avg/median % of scope creep per project by a certified nfsm-people o g4. improve software quality ( entity: product)  kpi.05 reduce the number of defects per delivery  m.14 no. of defects fur-related  m.15 no. of defects nfrrelated  m.16 no. of defects project-related  kpi.06 reduce the number of not granular (product) urs  m.17 – no. of (product) furs not expressed at the ‘elementary process’ level  m.18 – no. of (product) nfrs not expressed at the ‘elementary process’ level gathering periodically those measures allows a project manager to better control a project with a close balanced monitoring by four eois, not only one and not only looking at time and costs. in that way it can be easier eventually to detect where root-causes can be placed for being removed quickly, lowering the total cost of ownership (tco) for that project (not necessarily once at a time, but 2 or more concurrently contributing to a certain final effect). a high number of defects could be due to a real low quality of that software, but also to a low coverage of test cases vs user requirements and that low coverage could be due (second step back in this example rootcause analysis) to a reduced budget for the analysis+testing phases or because of unexperienced test specialists, not including the right number of tests or – if the coverage level would be ok the proper test cases for verifying such part of the software. thus, a balanced measurement plan can easily drive to better solutions, not necessarily spending more than in the ‘traditional way’. 7. quality models and the next decade  looking at the content of the presented qm against the period they were produced, it is possible to list a series of thoughts in order to designing qm for the next decade: • content: a number of product attributes in a qm is useful for better describing and evaluating a product, but as figure 11. merging eam, bmp, star taxonomy and qm.   table 3 – eam tables (split in two blocks).   entity project resources process attribute estimation capability teaming capability req. elicitation capability measure m.01~ m.04 m.05~ m.08 m.09~ m.11 entity process product product attribute req. elicitation capability defectability req. granularity measure m.09~ m.11 m.14~ m.16 m.17~ m.18 acta imeko | www.imeko.org  september 2015 | volume 4 | number 3 | 78  usual – the right number of attributes is in the middle (not too many, not too few). product observation is fundamental for listing what is needed and it could change along time. for instance, smartphones have created a different way to describe and define ‘operability’ and/or ‘usability’ against mobile software produced just 3-4 years ago because of the ‘touching’ interaction on the screen. again, sustainability can be a new product quality attribute to consider for new systems/software [14] because of a ‘greener’ perspective on software. • usage: qm can be used not only for a ‘retrospective’ evaluation but also in early slc phases as simple checklists for understanding the level of completeness for a product design, moving from a ‘wishing list’. another way to use qm is for estimation purposes: since qm express nfr, estimators can use needed nfr-related (quality) measures to be included (at least 2+ ones) as independent proxies in estimation models, allowing the reduction of mre (mean relative error) figures as much as possible, saving project resources and improving the overall project value for its stakeholders (e.g. iso 9126 parts 23-4 define a plenty of measures to be read and applied). • perspectives/viewpoint: a stakeholders’ analysis is needed for understanding if the proper number of viewpoints in included (or not) in a qm. if too few perspectives have been included when designing a qm, feedbacks could be lower than expected at the delivery stage. a more comprehensive design can reduce maintenance costs along the product expected lifetime. • measurement: another aspect to consider is the measurement issue, that’s the lower level in a multi-tier model as a qm is. people less skilled in measurement typically affirm that not anything can be measured. but, as in the introduction, if you are able to describe an entity of interest, you’ll be also able to measure it (e.g. using the gqm approach). iso 15939 [10] refined the gqm paradigm proposing mim (measurement information model) template that could be a good way to start defining how to monitor & control a non-functional (quality) attribute for a product. the suggestion is to follow a revised version of the well-known 5w+h approach (who, why, what, when, where, how), adding a second ‘h’ (how much), that could represent ‘targets – thresholds’ for checking the processin levels for that measure. • entity of interest (eoi): even if such qm born in the software engineering arena, they could be redesign or sometimes simply applied to a different eoi. looking at the iso 25010 quality model, it can be applied with very small changes for evaluating a service, not necessarily an ict one. imagine needing to evaluate a service desk service, as defined by itil or another it service management (itsm) best practice. you should take into account its reliability, continuity, security or ‘reusability’ of some components of a service. of course some definitions could need to be slightly modified, but also the ‘quality in use’ part could be applied as is. sometimes we act during time consolidating habits and concepts, but maintenance is a service, thus itil, cmm-svc or other itsm guides could be applied not only looking to business continuity service or something similar, but also for planning and managing a software maintenance service. by the way, some organizations can apply cmmi-dev practices and processes also for maintenance projects, but cmmi-svc would have a better fit. 8. conclusions and next steps  quality models (qm) represent a good way in software engineering for evaluating software products from their initial concept till their realization and in-use stage. non-functional requirements (nfr) are composed from quality and technical requirements; thus quality is one the two sides, maybe the more complex to analyze. since quality is a multifaceted concept, it is very difficult to find a complete and stable definition for it: quality definition can evolve along time related to newer ways users could request, of course influenced by technology (e.g. smartphones, cloud computing, etc.). qm can help sharing the view on products and be used both in a qualitative (checklists) and quantitative way (measuring low-level attributes with one or more related measures). this decade will consolidate some new technology paradigm and will propose new ones: the important thing will be to observe more interesting trends for proposing evolutions and integrations of new, emerging facets for quality more than creating new models at all. again, even if trivial, we need to clearly define which the entity to be analyzed (product, process, project, organization and resources) in order to avoid effort/cost estimation issues [15]. but – as said before – such qms can be applied to any eoi, thus also to services, processes and projects. evolution, not revolution, can be the right way to understand more about the ‘how’ realize better software systems and ict services. references  [1] mccall j.a., richards p.k. & walters g.f., factors in software quality, voll. i, ii, iii: final tech. report, radc-tr-77-369, rome air development center, air force system command, griffiss air force base, ny, 1977 [2] boehm b.w., brown j.r., kaspar h., lipow h., macleod g.j. & merritt m., characteristics of software quality, elsevier north-holland, 1978 [3] iso/iec, is 9126:1991 information technology software product evaluation – quality characteristics and guidelines for their use [4] iso/iec, is 9126-1:2001 software engineering -product quality -part 1: quality model [5] iso/iec, is 25010:2011 systems and software engineering - systems and software quality requirements and evaluation (square) -system and software quality models [6] ecss, space engineering – system engineering: part 6. functional and technical specifications, european cooperation for space standardization, ecss-e-10 part 6a rev.1, october 31 2005, url: www.ecss.nl [7] iso, 21351:2005 space systems -functional and technical specifications [8] grady r. & caswell d., software metrics: establishing a company-wide program, prentice hall, 1987, isbn 0138218447. [9] eeles p., capturing architectural requirements, ieee developerworks, 2005, url: http://goo.gl/7ptm2z [10] iso/iec 15939:2007 systems and software engineering - measurement process [11] ifpug, function points counting practices manual (release 4.3.1), international function point user group, westerville, ohio, january 2010, url: www.ifpug.org [12] ifpug, snap (software non-functional assessment process) apm v2.2, june 2014, url: www.ifpug.org [13] jones c., applied software measurement: assuring productivity and quality, 2/e, mcgraw-hill, 1996 [14] lami g., buglione l., measuring software sustainability from a process-centric perspective, proceedings of iwsm-mensura acta imeko | www.imeko.org  september 2015 | volume 4 | number 3 | 79  2012, 22th int. workshop on software measurement and 7th int. conference on software process and product measurement, assisi (italy), october 17-19 2012, pp.53-39 [15] buglione l., ebert c., estimation, encyclopaedia of software engineering, taylor & francis publisher, june 2012, isbn: 9781-4200-5977-9 [16] iso, is 21351:2005, space systems – functional and technical specifications, may 19, 2005, url: www.iso.ch [17] pmi, project management body of knowledge (pmbok), 5th ed., 2013, www.pmi.org [18] axelos, itil v3 – it infrastructure library, refresh 2011, 2014, www.itil-officialsite.com [19] kaplan r., norton d., the balanced scorecard: translating strategy into action, harvard business school press, 1996, isbn 0875846513 [20] basili v.b., caldiera g. & rombach h.d., the goal question metric approach, encyclopedia of software engineering. wiley 1994, url: http://goo.gl/uu5jjc [21] turner r.j., huemann m., anbari f.t., bredillet c.n., perspectives on projects, routledge, 2010, isbn 978-0-41599374-6 [22] buglione l. & abran a, multidimensional project management tracking & control related measurement issues, proceedings of smef 2005, software measurement european forum, 16-18 march 2005, rome (italy), pp. 205-214, url: http://goo.gl/2drl5j [23] buglione l. & abran a., iceberg: a different look at software project management, iwsm2002 in "software measurement and estimation", proceedings of the 12th international workshop on software measurement (iwsm2002), october 7-9, 2002, magdeburg (germany), shaker verlag, isbn 3-8322-0765-1, pp. 153-167, url: http://goo.gl/t5y0hl [24] jamwal r.s., jamwal d., padhad., comparative analysis of different software quality models, proceedings of the 3rd national conference; indiacom-2009, computing for nation development, february 26 – 27, 2009, url: http://goo.gl/kdvkyl [25] al-badareen a.b., selamat m.h., jabar m.a., din j., turaev s., software quality models: a comparative study, in software engineering and computer systems, part i: second international conference, icsecs 2011, kuantan, malaysia, june 27-29, 2011. proceedings, pp. 46-55 [26] suman, wadhwa m., a comparative study of software quality models, international journal of computer science and information technologies, vol. 5 (4) , 2014, pp.5634-5638, url: http://goo.gl/rtsyqe [27] sharma k., comparison of various software quality models, proc. of the intl. conf. on recent trends in computing and communication engineering -rtcce 2013, pp.48-51 [28] sanjay kumar dubey, ghosh s., rana a., comparison of software quality models: an analytical approach, int. journal of emerging technology and advanced engineering, vol. 2, no.2, feb 2012 [29] albrecht a., measuring application development productivity, proceedings of the ibm applications development symp., monterey, ca (usa), oct.14-17, 1979, url: http://goo.gl/f0rn26 [30] albrecht a. & gaffney j.e., software functions, source lines of code, and development effort prediction: a software science validation, ieee transactions on software engineering, vol. 9, no. 6, nov. 1983; url: http://goo.gl/qyikl9. [31] demarco t., controlling software projects: management, measurement & estimation, yourdon press, 1982. acta imeko, title acta imeko issn: 2221-870x september 2017, volume 6, number 3, 94-101 acta imeko | www.imeko.org september 2017 | volume 6 | number 3 | 94 interdisciplinary study of 13th century silver coins of the juchid (based on the materials of the burundukovsky hoard, tatarstan, russia) rezida khramchenkova1,2, eugenia shaykhutdinova1-3, aleksey bugarchev1, bulat gareev2, airat sitdikov1,2 1archaeology institute named after a. kh. khalikov of the tatarstan academy of sciences, butlerova str. 30, 420012 kazan, russian federation 2kazan (volga region) federal university, kremlyevskaya str. 18, 420008 kazan, russian federation 3kazan national research technical university named after a.n.tupolev kai, k.marx str. 10, 420111 kazan, russian federation section: research paper keywords: archaeological silver coins; golden horde; volga bulgaria; 13th century; chemical composition; structure; embossing technology; x-ray fluorescence analysis; emission spectral analysis; scanning electron microscopy; optical microscopy citation: rezida khramchenkova, eugenia shaykhutdinova, aleksey bugarchev, bulat gareev, airat sitdikov, interdisciplinary study of 13th century silver coins of the juchid (based on the materials of the burundukovsky hoard, tatarstan, russia), acta imeko, vol. 6, no. 3, article 15, september 2017, identifier: imeko-acta-06 (2017)-03-15 section editor: sabrina grassini, politecnico di torino, italy received march 15, 2017; in final form march 15, 2017; published september 2017 copyright: © 2017 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: the works were performed with the support of rhsf grant no. 16-11-16015 “research numismatic material of the juchids state using interdisciplinary methods of arheotehnology and archaeological materials”, russian federation corresponding author: eugenia shaykhutdinova, e-mail: eugen.shaykhutdinova@gmail.com 1. introduction in may of 1987 a hoard was discovered at the edge of a forest near burunduki village in the tatarstan republic (figure 1). it was inside a red clay ceramic vessel with a silver cover [1]. the hoard consisted of 906 coins: with the name of an-nasir li-din allah (2 pcs.), mongke (609 pcs.), ariq boke (291 pcs.), and unattributable (4 pcs). all coins were minted at a mint in the the capital of volga bolgaria – the town of bolgar. the hoard was deposited in the first half of the 1260s. it is presently deposited in the museum of archaeology under the academy of sciences of the tatarstan republic (kazan, russia). since its discovery the hoard has only been studied fragmentarily using classical techniques of numismatics: attribution according to the stamp and metrology. in order to study the dynamics of economic development and handicraft traditions of this area in the 13th century, it is very important to restore the formulation and technology of coin minting. a complex approach with the use of interdisciplinary research methods allows to establish a pattern of the quality variation of silver coins in the course of time on the basis of objective analytical data. abstract this work represents an interdisciplinary study of archaeological silver coins discovered in the territory of burunduki village located in the tatarstan republic (russia). the coins were minted in the 13th century in bolgar (volga bolgaria) during the rule of the juchid dynasty (the golden horde). the silver-containing numismatic material was studied using three analytical methods: x-ray fluorescence analysis, emission spectroscopy and scanning electron and optical microscopy. surfaces of coin fractures were studied in order to determine the inner structure. the results of this study demonstrated that fracture mapping corresponds to physical processes occurring during pressure processing. as a result, local material was clearly classified into two groups in terms of the chemical composition and manufacturing technique. acta imeko | www.imeko.org september 2017 | volume 6 | number 3 | 95 presently, there are several methods of research of chemical composition and structure of materials. a classical method of research of chemical composition of coins is the emissionspectral method (esa), which most accurately determines the composition of materials, including microcomponents [3], [4]. a disadvantage of this analysis method is the destruction of 2030 mg of the sample during analysis and the infeasibility of elemental mapping. another popular non-destructive method of numismatic material research is the x-ray fluorescence analysis (xrf). this method allows to analyze the surface layer of coins. esa and xrf do not allow to determine the inner structure [3]-[9]. however, xrf falls into the category of non-destructive analytical technologies, and can provide a rather accurate distribution pattern of chemical elements across the lateral area of the coin during the research of sample fractures. the experience of using scanning electron microscopy (sem) [5] demonstrated good results during the research of the inner structure of coins, and can provide layer-by-layer research of chemical composition, but the accuracy of its impurity analysis is rather low. the method is also destructive, which is related to the small volume of the working chamber and the requirement of studying a fresh fracture in order to obtain an adequate pattern. double-pulse laser atomic-emission spectroscopy described in [6] which is used for layer-by-layer determination of chemical composition also requires damaging of the coin. a disadvantage of this method lies in the unknown location of the structural layers of a coin, which results in that the layers are randomly determined. besides, an inevitable evaporation of crater walls blurrs the pattern of each subsequent layer. various types of cleaning and special preparation of coins, as described in [10], [11], may provide blurring of the pattern of elemental distribution inside the coin. it is clearly demonstrated in [5], where a "blurred" lateral fracture leads to a several percent increase of silver content in the centre of the coin. 2. numismatic materials the composition of the hoard allows to conclude that by the moment of its deposition in the early 1260s the monetary circulation of the bolgar region still contained dinars: the name of a deceased baghdad caliph an-nasir li-din-allah (died in 622 a.h./1225 a.d.) [1]. the dinars were minted at a bolgar mint in the mid-1240s (a more accurate dating is not possible, as there is no minting date on the coins). two-thirds of the hoard are silver coins with the name of mongke khan (son of tului, grandson of genghis khan) who began to rule the mongol empire in 1251. since that period in the course of almost 10 years bolgar wilayah (province)minted coins bearing his name and tamga (stamp) [12]-[14]. in the very beginning of the 1250s minted coins had a mandatory weight of 1.20 g. after several weight reductions in the course of almost 10 years the mandatory weight amounted to 0.71 g. figure 1. burunduki village with respect to bolgar, kazan and moscow (the map was compiled using an information system for archaeological research registration "archgis" (tatarstan, russia [2]). acta imeko | www.imeko.org september 2017 | volume 6 | number 3 | 96 the majority of coins have a weight of 0.71±0.02 g (10.6 %). besides, other two weight ranges are observed: 0.98±0.02 g and 0.89±0.01 g. the average weight is 0.80 g. coins weighing 0.62– 0.80 g constitute a half (51.4 %) of all mongke coins from this hoard. minting was carried out in accordance with several "mandatory norms". after mongke's death the throne was supposed to be rightfully taken by ariq boke, another son of tului. however, the third son of tului by the name of kublai did not acknowledge the rights of his older brother ariq boke and also went after the hereditary throne of genghis khan. a war between brothers began in 1260 [12]-[14]. in the middle volga region far away from central mongolia the mints of bolgar’s land began to issue coins with the name of the rightful heir ariq boke. the maximum weight range is 0.79±0.02 g (21 %). the second weight range is 0.70±0.02 g. the average weight is 0.76 g. coins weighing 0.73-0.85 g constitute over half (65.2 %) of all ariq boke coins from the hoard. notably, the weight range of mongke coins amounting to 0.71 g is 0.08 g lower than that of the ariq boke coins, amounting to 0.79 g. this is most probably accounted for by the loss of weight in the process of circulation due to a long circulation period of mongke coins compared to those of ariq boke. there were notable technical specifications given to mints for coin issuance. early coin issues (with the name of the deceased baghdad caliph an-nasir li-din-allah) were carried out in accordance with an el-marco system. however, when the throne was taken by mongke khan the situation changed – the mint began to issue coins in accordance with a "mandatory" norm, i.e. with identical weight for each allowable weight standard. subsequently, this production characteristic never changed. notably, the el-marco system was used by numerous mints of the mongol empire in the early period of establishment of proprietary silver and gold minting. a peculiar feature of the circulation of such coins is that their value was determined by weighing, and in retail trade it led to chipping of coins if their buyer did not have a coin with a corresponding weight. the inconveniences of circulation of such coins are evident, and therefore in a certain while effectively all mints of the mongol empire (including the ones in the bolgar wilayah) made a transition to issuing coins with a uniform "mandatory" weight (within specified ranges). as it is known, “kublai who prevailed over ariq boke in the power struggle spared his brother's life for about two years, letting him make use of the former rights and privleges, althoug keeping him under special supervision” [12]. ariq boke died in 664 a.h. (october 13th, 1265 – october 1st, 1266). the minting of coins with his name most probably continued in the region through 1266. after ariq boke's death power in the ulus of jochi was taken over by mengu-timur. he refused to obey to kublai considering him an usurper, and proclaimed the establishment of his own state which is presently referred to as the golden horde. it took place in 665 a.h./1266-1267 a.d. since that period a new tamga of mengu-timur (frequently referred to as the "batu house tamga") appeared on silver and copper coins of khwarezm, bolgar, crimea, jand, sarai and other regions of the newly established state [12]-[14]. 3. chemical composition the present work is dedicated to a study of 7 silver monetary artefacts from the burundukovsky hoard. the analytical selection includes two coins minted by each of the rulers: 2 coins emitted by an-nisir (figure 2a, b), 2 coins emitted by mongke khan (figure 2c, d), and 2 coins minted by ariq boke (figure 2e, f). a coin-shaped silver coin discovered in the same hoard was used as a reference standard (figure 2g). the technique of silver-containing material sampling for coin research providing adequate results is described in paper [5]. the proposed procedure of sample preparation for analysis provided correlation between the results obtained using various research methods (esa, xrf, sem). a system of stage-by-stage studying of samples aimed at the reduction of research costs and the optimization of the correctness of analytical data was implemented within the scope of this work. all 7 coins were studied using om and xrf, chemicals of 5 coins were determined using esa, and three coins were analyzed using sem. the research of coins with the optical microscopy technique was carried out in the institute of archaeology of the tatarstan academy of sciences using a levenhuk dtx 90 digital microscope. it provides a 10 to 300-fold magnification and is equipped with a 5-megapixel camera. xrf was conducted by a tornado x-ray phase analysis spectrometer manufactured by bruker in kazan (volga region) federal university (kazan, russia). sem was performed using an auriga crossbeam digital microscope and an inca xmax energy-dispersive spectrometer at kazan national research technical university, named after a.n. tupolev (kazan, russia). the research techniques are specified in [15]. the esa technique is as follows: 10-15 mg of the sample are incinerated until complete evaporation from a carbon electrode crater in an ac arc. the exposure time for highly volatile elements is 30 s at a current of 8 а, after which the spectrum is overlapped and the sample is further incinerated at a current of 18 а. the spectrum obtained using a dfs-458 diffraction spectrograph was registered on pfs-03 photographic plates. the working lattice of the spectrograph is no.3 with 1800 pcs./mm. the obtained spectrograms were formed on a mf-2 spectrophotometer. the researched samples were analyzed using a classical method of 3 reference standards represented by jewellery silver grades 875 and 925, and a silver coin grade 500 of 1927. state reference standards of bronze alloys were used in order to assess lead, tin, zinc and other impurities. chemical composition data is given in table 1. the results of xrf and sem analyses were recalculated for a "pure" composition with no consideration of silicon, calcium and aluminium. despite certain differences in the elemental content, the results of the analyses provide rather comparable data. significant variations in terms of concentration can be accounted for by two factors. the first one is accounted for by the characteristic features of the methods: xrf and sem only analyze the surface layer, whereas esa provides an overall composition of the entire sample. surface polishing during research of silver coins leads to distortion of chemical composition data [5]. however, the relief surface of fractures also introduces a measurement error upon addition of data across the area. sem energy-dispersive analysis failed to provide determination of gold content in the samples, discovered by еsa and sem. lead sensitivity for sem was limited to 1 %. irregularity in the distribution of elements in the matrix may constitute the second factor. figure 3 shows photographs of a fracture obtained using an optical microscope. areas with different compositions are visible in the fractures, which signifies a non-uniform distribution of elements in the coin matrix. acta imeko | www.imeko.org september 2017 | volume 6 | number 3 | 97 figure 2. analyzed coins of the juchid dynasty. table 1. chemical composition of silver coins obtained using three independent methods (weight, %). № method ag as au bi co cu fe mn ni pb sn sb zn 1 esa 96.13 0.0056 0.15 0.024 0.0008 1.51 0.79 0.004 0.0004 1.36 0.0008 0.006 0.009 xrf 96.03 0.6 2.92 0.37 sem 95.06 3.48 0.16 1.3 2 esa 95.33 0.0067 0.28 0.057 0.0006 2.38 0.13 0.002 0.0006 1.77 0.02 0.007 0.005 xrf 95.16 0.86 3.16 0.18 0.63 3 esa 82.43 0.001 4.67 0.4 0.0008 10.16 0.11 0.008 0.0002 2.09 0.0016 0.007 0.005 xrf 93.88 0.54 4.99 0.52 sem 94.98 5.02 4 xrf 98.37 0.2 0.98 0.16 5 esa 85.95 0.0047 2.96 0.17 0.0009 8.66 0.11 0.005 0.0003 2.12 0.0049 0.005 0.003 xrf 93.78 0.46 4.95 0.79 6 esa 95.57 0.0008 0.44 0.099 0.0004 3.22 0.021 0.006 0.0008 0.61 0.0008 0.006 0.004 xrf 93.6 0.55 5.18 0.66 sem 94.06 5.94 7 xrf 94.9 0.32 3.35 0.06 0.77 acta imeko | www.imeko.org september 2017 | volume 6 | number 3 | 98 research of correlation dependencies between the elements allows to make a conclusion regarding the raw material used during coin minting. figure 4 shows diagrams of relative concentration values (with respect to the average value). as seen from the diagrams, coins of various rulers are characterized by certain chemical compositions. diagram "a" (figure 4a) demonstrated a correlation between gold, copper and bismuth, whereas the other group is formed by lead and zinc. diagram "b" (figure 4b) for four coins demonstrates a direct dependence between arsenic, tin and nickel, with the exception of tin in coin no.1 and nickel in coin no.6. a linear dependence between lead, cobalt and stibium is observed in all coins except coin no.6. despite the presence of dependencies, practically all identical regularities are characteristic for coins no.1 and no.2; no.3, no.4 and no.5, which signifies the use of certain sources of the gold, copper and lead components during the reign of different rulers.. coin no.6 is distinguished by its special composition. an evidence of the existence of correlation between copper and gold content in coins no.1 and no.3 is provided by xrf elemental distribution maps (figure 5a, b). for coins no.5 and a b figure 4. diagrams of relative concentrations: а – au, bi, cu, pb, zn; b – ag, as, sn, ni, co, pb, sb. figure 3. photographs of sem optical microscopy of coin fractures: a no.1, b no.3, с no.6. a b c d figure 5. elemental distribution across coin fractures: a – coin no.1 cu and au; b – coin no.3 cu and au; c – coin no.5 cu, au and pb; d – coin no.6 cu, au and pb. acta imeko | www.imeko.org september 2017 | volume 6 | number 3 | 99 no.6 elemental mapping confirms the identical location of copper, gold and lead (figure 5c, d), proving a special source of raw material from which ariq boke's coins were minted. 4. structure and technology below is the description of the peculiar features of chemical composition, structure and technology of coin manufacturing. coin no.1 (an-nasir, bolgar, 13th c.) was crafted from silver of a high grade. it features a three-layer structure with very thin outer layers (approximately 60 µm). the thickness of the coin varies from 255 to 325 µm depending on the die pattern. the round shape of coin no.1 signifies that its blank features a similar shape (figure 2a). the edge of the coin still contains sshaped grooves (figure 6) characteristic for free pressing between two parallel surfaces. copper concentration in the outer layer is significantly less than in the inner layer. colour mapping demonstrates the absence of lead components in the outer layers. the distribution of this element across the area of the inner layer is uniform (figure 5a). the metal matrix is composed of silver with trace impurities of pb-cu-ag (figure 7, table 2). the structure of the coin clearly demonstrated recrystallization processes as a result of minting (figure 3c). a correlation is noticeable between copper and gold content, which signifies a certain source of raw material. the inner layer features traces of cold hammering indicating the use of high pressure in coin minting. coin no.2 (an-nasir, bolgar, 13th c.) was crafted from silver of a high grade. the distribution of elements across the area of the fracture is uniform, with the exception of a thin surface layer in which the content of impurities is lower than that inside the coin. no clear correlation between the elements is observed. inside the coin, likewise coin no.1, there was a highly deformed layer with a recrystallized structure. the thickness of the inner layer is approximately 360 µm, and outer layers 20 to 45 µm. the minting technology of coin no.2 matches that of coin no.1. coin no.3 (mongke khan, bolgar, 13th c.) was crafted from silver of a high grade. the coin is very thin (approximately 240 µm) and has a clear laminar structure, which may signify extension of the silver base of the coin material. presumably, coin no.3 was crafted using an extended silver wire. this is confirmed by the extended oval shape with a characteristic thickening and a trace of cutting from a continuous extended blank (figure 2c). the thin outer layer (30 to 45 µm) does not contain lead. uneven distribution of lead and gold (figure 5b) in the inner layer may signify an insufficient melting temperature during crafting of the blank, as a result of which these elements did not dissolve in the alloy matrix. a colour photograph clearly indicated the presence of a yellow outer layer (figure 3b) which was not registered during elemental mapping. according to esa results, this coin contains 4.67 % of gold, and according to xrf results 0.54 %. the structure of the coin bears traces of strong recrystallization processes accompanied by cold hammering in the middle of the central layer (figure 8). lamination of the outer layers is observed. coin no.4 (mongke khan, bolgar, 13 c.) is also characterized by a high grade of silver. the plasticity of coin material did not allow to obtain a brittle fracture, so the pattern of the distribution of elements did not reveal any characteristic table 2. chemical composition of trace impurities, coin no.1. element weight % element weight % c k 6.08 c k 9.14 o k 14.05 o k 2.01 cu k 23.59 al k 0.35 ag l 18.25 cu k 2.05 pb m 38.03 ag l 49.39 pb m 37.06 figure 6. photographs of coin no.1. figure 7. structure of coin no.1 with trace impurities. figure 8. structure of coin no.3. acta imeko | www.imeko.org september 2017 | volume 6 | number 3 | 100 features in the structure and distribution of chemical elements. however, it is highly probable that there is a gold overlap likewise coin no.3. thickness of the coin is about 210 µm, and width of the fracture is 45 µm. the coin has an oval shape, but is not elongated as no.3. however, traces of cutting have preserved on one of its sides. coin no.5 (ariq bokeа, bolgar, 13th c.) has an oval shape and has been crafted using silver of a high grade. the distribution of elements across the mapping area is uniform. like the case with coin no.3 and no.4, a certain intensiveness of gold content is observed (figure 5c). the thickness of the coin is about 190 µm, and the width of the fracture is 75 µm. the external shape of the coin indicates that it was made of a plate, and angles in the form of a flare have preserved in the form of welding burrs along the edge of the coin (figure 2e). no traces of cutting have preserved. one can assume that it was minted using a flattened wire. the three-layer coin no.6 (ariq bokeа, bolgar, 13th c.) has a high silver content and an oval shape. the distribution of elements across the scanning area is uniform. the inner layer contains inclusions of lead-copper conglomerates. gold, like in coin no.1, correlated with copper, excluding the outer layers (figure 5d) in which higher concentrations of au are observed. the coin is approximately 290-310 µm thick depending on the die pattern, and the thickness of the outer layers is 45 µm. the coin has an oval shape like coin no.5, and a slight trace of cutting has remained on one of its sides. presumably, a black in the form of a wire was used during minting. despite the fact that in the optical microscopy image the areas marked with a yellow colour are clearly visible (figure 3c), the distribution of copper and silver correlates with each other (figure 5d). therefore, this color was obtained due to the presence of gold in the sample, and a concentration of 0.45 % was recorded by xrf and esa methods. analytical data obtained for the coin demonstrated that the total spectrum along the fracture does not register lead, while in the central part the concentration of this element is significant, amounting to 1.5 %. microparticles of a lead-silver and silvercopper-nickel composition are registered in the structure of the metal matrix (table 3, figure 9). the results of the sem analysis of chemical composition of trace impurities provided information on the feedstock used in minting of the central part of the coin. during minting, copper-nickel and lead-silver additives were included in the alloy, as a result of which the silver alloy grade decreased, although the weight index remained approximately the same as for pure silver. thus, the technology of coin minting is similar to the technology used in crafting of the 10th-11th century coins of the samanid dynasty [15]. coin-shaped pendant no.7 is made of high-grade silver. pendant no.7 has a round shape, the die was unilateral, and a single hole was punched therein (figure 2g). no traces of cutting are observed, and it is probable that the blank was circular in shape along the longitudinal section. the artefact has a three-layer structure, similar to coin no.6. unlike the first coins no.1 and no.2, copper and gold contained in the pendent feature an inverse correlation, and the distribution of silver and gold on colour maps is the same. in outer layers, the concentration of these elements is higher. this circumstance indicated a source of silver different from the first coins. coin thickness is about 270 µm, and outer layers 60 µm. structural changes are observed under the influence of a high pressure applied during minting. this is confirmed by xrf results. the distorted structure contains grains of silver of high grade with copper dissolved therein, lead-silver and lead-copper inclusions. all coins and coin-shaped pendants have a faint coating of a gold colour (figure 10). 5. conclusions research of numismatic material involves a number of challenges. the major one in this case is the determination of a "golden mean" between the cost of ongoing research and the quality of the results, which allows to adequately assess the pattern of structural changes. in this study, it was possible to maintain the effectiveness of the developed method [5], [16] by means of a successive parallel study of the mass coin material. a study of microphotographs and maps of the distribution of elements obtained using xrf and sem methods demonstrated that coins from this hoard have a strongly deformed structure with traces of etching, and a possible gold coating. all coins are made of high-grade silver over 90 %. research of alloys carried out on the basis of three basic alloying elements: copper, lead and gold demonstrated that all table 3. chemical composition of inclusins in the metal matrix of coin no.6. element / inclusion 1 spectrum 3, weight % element / inclusion 2 spectrum 4, weight % c k 5.58 c k 12.26 o k 13.77 al k 2.25 cu k 1.61 ni k 4.31 ag l 24.56 cu k 10.96 pb m 54.49 ag l 70.23 figure 9. microphotographs with analytical points of the structure of coin №6: a lead-silver trace impurity, b silver-copper inclusion. figure 10. gold coating on the coin-shaped pendant no.7. acta imeko | www.imeko.org september 2017 | volume 6 | number 3 | 101 researched coins from the burundukovsky hoard were made using the same formulation. however, coins of different rulers are made of different raw materials, as evidenced by the varying correlation dependencies of copper, gold, lead and nickel impurities. in general, despite the insufficient resolution of images in the case of fine-grained crystallization of metals, xrf proved to be an adequate and inexpensive method of coin fracturing. sem confirmed the results of the xrf analysis. all coins feature obvious traces of metal processing using pressure and etching. the difference in the chemical composition of xrf, esa and esa, observed for some coins, can be explained both by the characteristic features of analytical technologies and the uneven distribution of elements in the coin matrix. the structure of coins can be subdivided into two main groups: group 1. coins have a strongly deformed structure with a subtle or evident cold hammering of the inner layer. this indicates the application of a strong pressure, as a result of which the metal layers in the centre were compacted so much that the material recoiled and recrystallized. friction between the layers provoked a diffusion process. movement of the layers of metal occurred through the centre of the coin along the longitudinal axis. this group includes coins no.1, no.2 and no.7. group 2. coins made of a wire with a direct hammer blow, which may indicate a possible rush during minting. this circumstance can indicate a high mobility of mints. such coins have an elongated oval shape with traces of an inaccurate cutting, and their thickness often radically varies, whereas holes are observed in certain samples. this group includes coins no.3, no.4, no.5 and no.6. coin no.4 made from a pre-flattened wire, appears more accurate in shape. coin no.5, made of a plate. it is likely that wire was also used as the initial blank, but in this case there was an intended preparation of the plate with uniform thickness. for two coins and a single coin pendant, the original blank was a a metal blank with a regular circumferential shape: no.1, no.2 and no.7. all coins feature obvious traces of pressure treatment and have been subjected to active etching. references [1] g. fedorov-davydov, “the hoard of coins from the 13th century volga bulgaria”, problems of eurasian archeology, moscow, 1991, pp. 181-192. [2] g. sayfutdinova. cartographic representation of archeological monuments on the territory of the republic of tatarstan and selected regions of russia, all-russian scientific conference. international year of maps in russia: combining space and time, мoscow. 2016, p. 257. [3] e. begovatov, v. lebedev, r.khramchenkova, chemical composition of silver coins of the 10th century of i semenovsky settlement (republic of tatarstan), the volga river region archaeology, №3(5), 2013, pp. 169-174. [4] e. begovatov, v. lebedev, r. khramchenkova, coin-like cast pendants of volga bulgaria, numismatics of the golden horde, iss. 2, pp. 163174 [5] r.khramchenkova, e.begovatov, e.shaykhutdinova, a.sitdikov, “archaeometry reseach of silver coins the 10th century from volga bulgaria (premilinary results)”, the volga river region archaeology, № 3 (13), 2015, pp. 176-188. [6] e.s. voropai, k.f. ermalitskaya, v.m. sidorovich, a.n. plavinsky, "analysis of silver coins by double-pulse laser-atomic emission spectroscopy", bsu, series 1: physics. mathematics. computer science, №1, 2013, pp. 11-16. [7] v. kantarelou, f. j. ager, d. eugenidou, f. chaves, a. andreou, e. kontou, n. katsikosta, m.a. respaldiza, p. serafin, d. sokaras, c. zarkadas, k. polikreti, a.g. karydas, “x-ray fluorescence analytical criteria to assess the fineness of ancient silver coins: application on ptolemaic coinage”, spectrochimica acta b, vol. 66, iss. 9-10, 2011, pp. 681-690. [8] b.b. tripathy, tapash r. rautray, a.c. rautray, v. vijayan, “elemental analysis of silver coins by pixe technique”, applied radiation and isotopes, vol. 68, iss. 3, 2010, pp. 454-458. [9] a. pitarch, i. queralt, a. alvarez-perez, “analysis of catalonian silver coins from the spanish war of independence period (1808–1814) by energy dispersive x-ray fluorescence”, nuclear instruments and methods in physics: beam interactions with materials and atoms, vol. 269, iss. 3, 2011, рр. 308-312. [10] e.g. ioanid, a. ioanid, d.e. rusu, f. doroftei, “surface investigation of some medieval silver coins cleaned in highfrequency cold plasma”, journal of cultural heritage, vol. 12, iss. 2, 2011, pp. 220–226. [11] l. beck, e. alloin, c. berthier, s. réveillon, v. costa, “silver surface enrichment controlled by simultaneous rbs for reliable pixe analysis of ancient coins”, nuclear instruments and methods in physics, vol. 266, iss. 10, 2008, pp. 2320-2324. [12] dalai ch. mongolia in the xiii-xiv centuries. мoscow: "nauka", 1983. 232 p. [13] singatullina a.z. consolidated index of published treasures / fedorov-davydov ga money business of the golden horde. moscow: paleograph, 2003. 352 p. [14] fren, h.m. coins of khans ulus dzhuchiev or golden horde with coins of different mohammedan dynasties in addition. st. petersburg. 1832. 80 p. [15] shaykhutdinova e., khramchenkova r., mukhametshin j., bugarchev a., sitdikov a. investigation of numismatic material from the juchid state (the golden horde) of the 13th century (based on the materials of burundukovsky hoard, tatarstan, russia). proceedings of imeko international conference on metrology for archaeology and cultural heritage (metroarchaeo 2016). october 19 21, 2016. torino, italy. pp. 197-201. [16] khramchenkova r., begovatov e., shaykhutdinova e., safina i., sitdikov a., zinnatullin a. research of 10th century silver coins from semenovskoe settlement (volga bolgaria). benevento, italy. october 21 23, 2015. pp. 203-207. interdisciplinary study of 13th century silver coins of the juchid (based on the materials of the burundukovsky hoard, tatarstan, russia) acta imeko  issn: 2221‐870x  june 2015, volume 4, number 2, 68‐71    acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 68  modelling of linear test bench for short distance  measurements  lauryna šiaudinytė   institute of geodesy, vilnius gediminas technical university, saulėtekio al. 11, lt‐10223, vilnius, lithuania        section: research paper   keywords: calibration bench; laser interferometer; linear measurements  citation: lauryna šiaudinytė, modelling of linear test bench for short distance measurements, acta imeko, vol. 4, no. 2, article 12, june 2015, identifier:  imeko‐acta‐4 (2015)‐02‐12  editor: paolo carbone, university of perugia, italy  received october 15, 2014; in final form november 27, 2014; published june 2015  copyright: © 2015 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: this work was supported by the european social fund under the global grant measure  corresponding author: lauryna šiaudinytė, e‐mail: lauryna.siaudinyte@vgtu.lt    1. introduction  the main standard dealing with total stations testing is iso 17123. however, this standard focuses on field testing procedures. although a total station is mainly intended for outdoor measurements, indoor calibration in controlled environmental conditions provides the best way to determine systematic errors. however, testing devices under laboratory conditions usually come with new issues of which lack of space is typically one of the most important. this brings to the need of test bench development for the calibration of geodetic distance measuring equipment within relatively short distances. in many calibration laboratories there are special benches designed for tape calibration which could be adjusted for electronic distance measuring equipment calibration. section 2 reveals applied instrumentation for indoor distance measurements of various calibration laboratories. proposed possibilities for the development of a short distance linear calibration bench using the existing basis of special supports for cyclic error determination are presented further in this paper. the new calibration bench of aluminium construction is cost saving and lightweight compared to stainless steel. within 13 meters of the bench it is possible to determine systematic errors and therefore to increase accuracy. automation of the carriage will reduce the measurement uncertainty caused by human interaction. 2. distance measuring instrumentation  preservation of the primary length standard the meter is the main role of length metrology. it also has to ensure the infrastructure needed for dimensional and positional measurements traceability to the meter. the meter is defined as the length of the path traveled by light in vacuum during the time interval of 1/299 792 458 of a second. an interferometer is an instrument providing standard distance measurements. there are a few main types of laser interferometers used in laboratory measurements – homodyne and heterodyne laser interferometers are the most commonly used. homodyne interferometers are based on the interference of two beams (one split beam) of the same frequency. the optics of homodyne interferometers is similar to michelson interferometers, however it produces a lower signal due to a high noise ratio compared to the heterodyne interferometer. heterodyne interferometers are based on two beams with different frequencies (low and high), different polarization mixed with each other and in non-linear combination creating two new frequencies (heterodynes). the double frequency abstract  to perform the calibration of total stations under  laboratory conditions,  lack of space  is a common  issue. the paper presents the  analysis of applied solutions for short distance measurements and assumptions of the construction development of a linear test bench  for cyclic error determination. various aspects of improvement are explained in the paper, and the preliminary design of a linear test  bench being developed at vilnius gediminas technical university is presented.   acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 69  interferometers measure the relative displacement of two reflectors by splitting the beam. then two beams are directed to different retro-reflectors and the resulting signals are returned to a photodetector [7]. the working principle of the laser interferometer is shown in figure 1. heterodyne interferometers are very sensitive and usually more accurate than homodyne interferometers. there are environmental factors such as temperature, pressure and humidity influencing linear measurements. the most important factor of length measurement accuracy is temperature. the sensitivity of interferometric systems to varying temperature can significantly influence the measurement results because of linear expansion of materials affecting the stability of the structural geometry. precision measurement systems are usually placed in a controlled environment with a constant temperature of 20.0 °c ± 0.2 °c. to meet such requirements the interferometric system should be placed in a special chamber with many parameter measuring sensors to maintain an effective thermal and pressure isolation. these sensors as well as temperature fluctuation can bring in the uncertainty in such precise length measurements [3]. another important uncertainty source in dimensional metrology is the abbe error which occurs due to the difference between the axis of the measurement object and the axis of the measuring device. this error can also be caused by the motion of the measurement object [6]. 2.1. applied solutions for indoor calibration  in the national metrology institute of germany (physikalisch-technische bundesanstalt) there is a 50 m long geodetic base which is also a comparator established for the calibration of length measuring instruments where the principle of michelson’s interferometer is used as a length standard. the carriage is mounted on the rails fixed along the 50 m base. the stationary part of an interferometer is placed on a stable base in front of the rails. the laser beam is split into two paths generated by a reflector consisting of three mirrors arranged on the surface of a cube, and fixed to the moving carriage. the zero point of the interferometer in respect of the rangefinder is determined by means of a gauge block. the measurement error of the distance measuring device can be determined by comparison with the distance measured by an interferometer and a calibrated instrument [10]. in kriss the 50 m long bench was developed for the calibration of surveying tapes. the main components of this system are the interferometer and the carriage moved by the servo motor attached to it along steel rods and controlled by a computer through wireless communication. the interferometer axis is set in the center between the rails where the tape is placed. there is an optical system with the microscope installed in the moving carriage to detect the graduation of the tape. the distance measured by the interferometer is compared to the distance determined by the tape being calibrated [4]. 50 m long benches are very common in metrology institutes, however, such distance may cause additional uncertainty. it is a challenging task to align all the components and install the precision rail system while avoiding inclination and geometric alterations due to thermal expansion or compression. therefore, the finish centre for metrology and accreditation developed a shorter – 30 m long distance calibration bench. for stability this bench was built (constructed) two floors under the ground. concrete pillars are placed at every 2.8 m and a 31.5 m monolithic concrete beam on top of them. there are two separate bases established in both ends of the bench for the reference equipment and instruments under calibration. the rails are mounted on additional supports and the carriage moving along the rail is equipped with laser interferometer optics for both distance and angle measurements as well as a microscope with ccd camera for linear position detection. this interferometric bench is suitable for a laser tracker, a total station and tape calibration. the expanded uncertainty (k = 2) of the length scale is 2.6·10-6 m for a 30 m displacement [9]. although smaller benches can fit laboratory premises better, there is a need to calibrate total stations in longer distances because they are designed to be used outdoors by measuring longer stretches. to extend measuring distances in the laboratory a special mirror system can be used. to double or triple the measuring distance a zigzag path can be created for the beam to travel by using mirrors mounted on certain points and rotated at a certain angle. such method allows avoiding reflector mounting outside the laboratory [5]. due to the space issue at vilnius gediminas technical university it was decided to create a 13 m long linear bench for short distance measurements and cyclic periodic error determination. 3. the linear bench  the linear bench under development at the institute of geodesy of the vilnius gediminas technical university is suitable for the calibration of geodetic electro-optical equipment. the main instrument used in survey is a total station. this instrument is exceptional because of its ability to measure horizontal and vertical angles as well as distances simultaneously. an electronic distance meter (edm) is embedded in the total station for the distance measurements. this instrument is typically based on the phase difference measuring principle for distance measurements. the measuring signal modulated on the carrier wave in the emitter travels a distance d from the total station a to reflector b and back to the instrument receiver. in the receiver the difference between transmitted and received signals are measured and compared. point a and point a' shown in figure 2 are the same however the returned wave has been opened out. in an edm the double distance length (2d) is measured by a difference in phase angles of two sinusoidal waves (transmitted and received). this distance can be expressed in terms of wavelength of the measuring unit [8]: mmnd  2 (1) where λm is the wavelength of the measuring unit, n is an integer number of wavelengths travelled by the wave, δλm is a fraction of the wavelength travelled by the wave. figure 1. working principle of a laser interferometer.   acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 70  the fraction of the wavelength travelled by the wave (δλm,), when the phase angle of the transmitted wave is φ1 and the phase angle of the received wave is φ2, is expressed as follows:   mm    2 12  (2) deterioration of the inner system components over time can influence the measurement results. therefore, instruments have to be tested regularly to determine short periodic or cyclic errors. the cyclic error is sinusoidal with a wavelength equal to the unit length of the instrument. 3.1.  the former stand for cyclic error determination  a 15 m geodetic baseline for edm short distance calibration at the vilnius gediminas technical university was established in collaboration with the finish geodetic institute in 2001. in total 16 stands were attached to the grooved profile which was fixed to a stabilized wall. one stand was established especially for edm mounting and the other 15 for reflector mounting. all the stands were aligned, levelled and established at a pitch of 1 m. the basement location of this laboratory led to a stable construction of this bench. after establishment this bench was calibrated by the finish geodetic institute. a wild distomat di 2002 and a wild theomat t2002 tachometer along with a wild gph1ap reflector were calibrated at the nummela standard baseline and used for distance measurements. in total 70 lengths were measured performing repetitive measurements from stands 0, 2, 5, 7, 8. calibration results are shown in table 1. these indoor facilities can be used for all electronic distance measuring equipment. the edm of laser scanners are also calibrated using this cyclic error determination baseline. the target is placed on the established stands and scanned. each scanned target consists of a cloud of approx. 39 000 points which are used to determine the coordinates of the target centres. then the measured distances are compared with the calibrated distances between the cyclic error determination baseline stands [1]. 3.2. the linear bench under development  distance measurement equipment has long been tested and controlled by implementing non automated measurements and measuring objects at certain well-known distances. at the institute of geodesy such principle has been used by implementing multiple fixed indoor stands for short distance measurements and cyclic error correction, as well as several fixed outdoor pillars were used for long distance measurement control. nonetheless, implementation of such instrumentation requires a lot of human interaction in the process which slows down the process considerably and introduces multiple operator related errors. additionally, since modern electronic measurement equipment (distance meters, total stations etc.) can determine an enormous amount of measurement data with very high resolution, the implementation of fixed pillars for error determination is not fully correct. the distances between fixed stands might include possible systematic errors which are left uncovered by such a calibration. the issues like the calibration step under laboratory conditions are neither covered in the international standards (i.e. iso). implementation of automated measurement and control of the process would allow to increase both the speed and the accuracy of the measuring process. however, automated systems are available mostly for short distances. although the mentioned automation brings in lots of benefits to the measurement process, it still remains costly, complex and not always affordable for some organizations. therefore, the institute of geodesy is developing a linear test bench consisting of standard components and cost effective aluminum profiles. the test bench implements the distance meters’ cyclic correction stand composed of multiple firmly mounted supports as the base for the linear movement rail. the rail itself is a standard aluminum profile made for steel roller bearing linear guides to roll the mounted moving car. implementation of aluminum profiles ensures a lower total mass relevant due to implementation of inherited components of the base. however, some inaccuracies due to the lower stiffness are unavoidable. the profile rail is connected supports (with a step of 2 meters) and precisely levelled. preliminary design of the linear calibration bench and its components (a – moving carriage, b – rails, c – supports, d – reflector for total station measurements, e – corner cube fixed to the moving carriage to create the path for the split laser beam, l distance between the firmly mounted supports, 1 meter) is shown in figure 3. the moving carriage of the bench is an aluminum body plate moving on precise ball bearings. a motorized linear air bearing would be the main component ensuring a smooth motion of the carriage. therefore, vertical and horizontal straightness as well as rotational motion errors have to be determined before using the carriage for the calibration of length measuring instruments [2]. implementation of aluminum here is preferable due to the decrease of moving mass its inert influence on the rail profile. figure 2. principle of phase shift measurement.   table 1. the results of distance measurements between the stands.  stand  number  measurement result  y±u95% (k=2)  0‐1    1000.2 mm ± 0.5 mm  0‐2    2000.6 mm ± 0.2 mm  0‐3    3001.4 mm ± 0.3 mm  0‐4    4001.4 mm ± 0.4 mm  0‐5    4999.2 mm ± 0.3 mm  0‐6    5999.2 mm ± 0.5 mm  0‐7    7000.8 mm ± 0.3 mm  0‐8    7997.9 mm ± 0.3 mm  0‐9    9000.8 mm ± 0.5 mm  0‐10  10001.0 mm ± 0.5 mm  0‐11  10999.2 mm ± 0.5 mm  0‐12  12000.4 mm ± 0.5 mm  0‐13  12998.9 mm ± 0.5 mm  0‐14  14000.4 mm ± 0.5 mm  0‐15  15005.1 mm ± 0.5 mm  a' b b a a nλm ∆λm dd φ1=0° φ2=90 acta imeko | www.imeko.org  june 2015 | volume 4 | number 2 | 71  the movement of the car is automated by step motors (with coarse distance determination) and the car can be positioned at practically any linear position within the distance of the bench. the precise position of the car is determined by a stationary fixed renishaw interferometer with its mirror mounted on the moving car. for the uncertainty evaluation it is important to consider the following aspects. since the aluminum profile is supported at a pitch of 1 m, the deflection of the profile is 0.03 mm in every stretch with a bending stress of 0.74 n/mm2 because of both carriage and reflector weights, which is up to 10 kg. emd measurements are mostly affected by environmental conditions which can be minimized by performing measurements in a laboratory. however, stabilizing the temperature and atmospheric correction is obligatory according to iso 17123:5 because temperature, pressure and humidity affect the velocity of the signal. it is also very important to perform slope and zero point corrections. zero point errors are related to electrical delays, the differences between the electronic and mechanical center of the instrument. the scale error which is caused by internal frequency errors also has an impact on measurement results. the abbe error is one of the main error sources in these linear measurements because of nonlinearity of the carriage base plate and the rails. to sum up, the alignment of reflector and total station, abbe error and the motion of the carriage are the main uncertainty sources. so far the research showed that an expanded uncertainty of 3.0·10-6 m for 13 m displacement can be achieved, however, the system needs to be stabilized and further multiple measurements in various displacements will be provided with mentioned uncertainty components in further research. 4. conclusions  the assumptions of using fixed cyclic error determination stands as a base for the development of a linear calibration bench with an automated moving carriage are presented in this paper. the advantages of this proposed bench are the lightweight of the used materials as well as a cost saving solution. however, it is more complicated to precisely control the environment where the calibration is performed. the main purpose of this linear bench is to reduce human interaction in the process and to determine cyclic and other distance measurement errors by performing measurements at a desired distance within the length of the bench. acknowledgement  this research is funded by the european social fund under the global grant measure (project no. vp1-3.1-šmm-07-k01-102). references  [1] u. antanavičiūtė, r. obuchovski, e. paršeliūnas, d. popovas, d. šlikas, “some issues regarding the calibration of the terrestrial laser scanner leica scanstation c10”, in: geodesy and cartography, vol. 39(3), 2013, pp. 138-143. [2] w. gao, y. arai, a. shibuya, s. kiyono, c. h. park, “measurement of multi-degree-of-freedom error motions of a precision linear air-bearing stage”, in: precision engineering, vol. 30, issue 1, 2006, pp. 96-103. [3] r. hamid, d. sendogdu, c. erdogan, “the temperature stabilization and temperature measurement of a kösters interferometer”, in: measurement science and technology, 2005. vol. 16, p. 2201-2207. [4] j.w. kim, d. y. jeong, m. s. kim, c. s. kang, j. a. kim, t. b. eom, “a 50 m laser interferometer for automatic calibration of surveying tapes using wireless communication” in: measurement science and technology, 2008, vol 19. [5] r. khalil, “new compact method for laboratory testing edm instruments”, proc. of fig working week 2005 and 8th international conference on the global spatial data infrastructure, april 16-21, 2005, cairo, egypt, pp. 1-8. [6] r. köning, j. flügge, h. bosse, “a method for the in situ determination of abbe errors and their correction”, in: measurement science and technology, 2007, vol 18, p. 476-481. [7] r. paschotta, “optical heterodyne detection”, in: rp photonics consulting gmbh papers, 2012. [8] w. f. price, j. uren. laser surveying. london: van nostrand reinhold (1989), isbn 0747600236, 256p. [9] j. unkuri, a. rantanen, j. manninen, v.p. esala, a. lassila, “interferometric 30 m bench for calibrations of 1d scales and optical distance measuring instruments”, measurement science and technology vol. 23, 2012, pp. 1-8. [10] m. wedde, k. meiners-hagen, a. abou-zeid, “interferometrische kalibrierung von strichmaßen, laserinterferometern und entfernungsmessgeräten: die geodätische basis der ptb”, in: ptb-mitteilungen, vol 120, 2010. figure 3. preliminary design of the linear bench.   microsoft word 397-2950-1-le-rev acta imeko  issn: 2221‐870x  july 2017, volume 6, number 2, 32‐41    acta imeko | www.imeko.org  july 2017 | volume 6 | number 2 | 32  confirmation of calibration capability of low‐nominal  capacity reference torque wrenches at nmij  atsuhiro nishino, koji ogushi and kazunaga ueda  national metrology institute of japan, aist, tsukuba central 3, 1‐1‐1 umezono, tsukuba, ibaraki, 305‐8563, japan      section: research paper   keywords: torque; low‐nominal capacity; torque wrench; reference torque wrench; comparison  citation: atsuhiro nishino, koji ogushi and kazunaga ueda, confirmation of calibration capability of low‐nominal capacity reference torque wrenches at  nmij, acta imeko, vol. 6, no. 2, article 6, july 2017, identifier: imeko‐acta‐06 (2017)‐02‐06  section editor: min‐seok kim, research institute of standards and science, korea  received june 23, 2016; in final form may 6, 2017; published july 2017  copyright: © 2017 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  corresponding author: atsuhiro nishino, e‐mail: a.nishino@aist.go.jp    1. introduction  there are two routes in the torque traceability system in japan, one for “pure torque” loading without any parasitic components and the other for “torque wrench” loading with an inevitable transverse force and bending moment. figure 1 shows the si traceability system of torque used in japan [1], which was proposed by the national metrology institute of japan (nmij) in cooperation with the japanese industry. the reference standards for pure torque and torque wrench in firstgrade accredited laboratories are torque measuring devices (tmds) and reference torque wrenches (rtws), respectively. the torque standard for rtws has been disseminated in the range from 5 n·m to 5 kn·m using two dead weight torque standard machines (dwtsms) at nmij with rated capacities of 1 kn·m (1-kn·m-dwtsm) and 20 kn·m (20-kn·mdwtsm). to properly tighten screws, high-accuracy hand torque wrenches are used in precision mechanical equipment, medical devices, and other similar applications. thus, there remains a strong demand to expand the lower end of the range of the torque standard for rtws. in a previous study, we designed adaptations that allow torque transducers with a low nominal capacity to be mounted on the 10-n·m-dwtsm to expand the torque standard range for rtws, and we demonstrated that rtws could be calibrated using this modified 10-n·mdwtsm [2], [3]. the 10-n·m-dwtsm has been compared with the 1 n·m and 1 kn·m tsms developed at the physikalisch-technische bundesanstalt (ptb, germany) by calibrating low-nominal capacity tmds in the range of 0.1 n·m to 10 n·m, and the torque realized by the 10-n·m-dwtsm at nmij was found to be equivalent to that realized by the two tsms at ptb [4]. however, the rtw calibration capability of the 10-n·m-dwtsm has not been compared with those of other tsms. to confirm the rtw calibration capability of the 10-n·m-dwtsm, it was compared with the existing 1-kn·mdwtsm at nmij by calibrating low-nominal capacity rtws. abstract  the calibration procedure for a reference torque wrench (rtw), which is a torque transducer in the form of a torque wrench, is based  on the jmif‐016 (ttsg‐w102) technical guideline in japan. the mounting position of an rtw is set to three directions (0°, 120°, and  240°; case 1) when it is calibrated using dead weight torque standard machines (dwtsms) with rated capacities of 1 kn∙m (1‐kn∙m‐ dwtsm) and 20 kn∙m (20‐kn∙m‐dwtsm). however, the mounting position is set to three different directions (0°, 90° and 270°; case 2)  when the 10‐n∙m‐dwtsm is used. in this study, the effect of the difference of the rtw mounting position on the calibration results  was investigated. next, to confirm the equivalence of the rtw calibration capabilities of the 10‐n∙m‐ and 1‐kn∙m‐dwtsms, they were  compared  using  two  low‐nominal  capacity  rtws.  the  results  obtained  in  cases  1  and  2  showed  good  agreement  within  the  uncertainties. the recommended rtw calibration procedure is based on case 1. but, if an rtw cannot be mounted in the 120° and  240° directions because of the space problem, case 2 can be adopted as the rtw calibration procedure. the rtw calibration capability  of the 10‐n∙m‐dwtsm was found to be equivalent to that of the 1‐kn∙m‐dwtsm in the range of 5 n∙m to 10 n∙m.    acta imeko | www.imeko.org  july 2017 | volume 6 | number 2 | 33  the 1-kn·m-dwtsm was part of a cipm key comparison (ccm.t-k1) [5] and was shown to yield results equivalent to those obtained using tsms developed by other national metrology institutes. in addition, ogushi et al. [6] conducted an inter-laboratory comparison of rtw calibrations in which the 1-kn·m-dwtsm at nmij and the 2 kn·m torque calibration machine at ptb were compared in the range of 10 n·m to 100 n·m under their respective procedures and demonstrated the equivalence of the rtw calibrations performed by nmij and ptb [6]. moreover, the rtw calibration capability of the 20kn·m-dwtsm at nmij was shown to be equivalent to that of the 1-kn·m-dwtsm at nmij [7]. in addition, an interlaboratory comparison of rtw calibration was conducted between nmij and ptb in the range from 200 n·m to 2 kn·m, and agreement of the rtw calibration results of the two nmis was confirmed for calibration steps above 600 n·m [8]. the calibration procedure of an rtw was based on the jmif-016 (ttsg-w102) technical guideline established by nmij in cooperation with the japanese industry [9]. the method of changing the mounting position of the transducer was outlined in the following manner: 5.5.3 the torque transducer should be calibrated in at least three directions (0°, 120°, and 240°) by changing the torque transducer direction itself or the mechanical coupling adapter direction. alternatively, if the torque transducer has the square drive, it would be useful, from the metrological point of view, to be calibrated in four directions (0°, 90°, 180°, and 270°) [9]. thus, there are two methods of changing the rtw mounting position: one involving “changing the sensing body direction together with the lever” and the other involving “changing the detachable square drive direction without changing the sensing body direction.” except in special circumstances, the former has been recommended by nmij because changing the rotational mounting position at only mechanical parts including the square drive is not recognized as truly changing the direction of the sensing element. this method requires a sufficient amount of space around the tsm. the 1-kn·mand 20-kn·m-dwtsms have sufficient space to change the torque transducer direction itself in the 120° and 240° directions. however, it is not possible to mount the torque transducer on the 10-n·m-dwtsm in the 120° or 240° direction because there is not sufficient space under the measurement axis in this compactly designed machine. the rtw must be mounted in the 0°, 90°, and 270° directions when it is calibrated by the 10n·m-dwtsm. thus, it was necessary to investigate whether the difference in the combination of lever angles affected the calibration results before the 10-n·mand 1-kn·m-dwtsms were compared. in this study, the effect of the difference between the two abovementioned sets of rtw mounting positions on the calibration results was first investigated. despite the difference between the mounting positions for the 10-n·mand 1-kn·mdwtsms, an intra-laboratory comparison between the 10n·mand 1-kn·m-dwtsms was then conducted regarding rtw calibrations in the torque range of 5 n·m to 10 n·m. finally, the capability of the 10-n·m-dwtsm in the calibration of low-nominal capacity rtws at nmij was then confirmed. 2. experimental conditions  2.1. difference of the combination of lever angles when changing  the rtw mounting position  first, the effect of the difference between the mounting positions of rtws on the calibration results was investigated. because there is not sufficient space around the 10-n·m figure 1. si traceability system of torque in japan [1].  acta imeko | www.imeko.org  july 2017 | volume 6 | number 2 | 34  dwtsm to change the rtw mounting position at intervals of 120°, this experiment was conducted using the 1-kn·mand 20-kn·m-dwtsms. figures 2 and 3 show schematics of how the mounting position was changed by changing the sensing body direction to the 0°, 120°, and 240° directions (case 1) and the 0°, 90°, and 270° directions (case 2), respectively. in these cases, the lever direction was also changed to align with the sensing body direction. case 1 was used for the daily calibrations of the 1-kn·mand 20-kn·m-dwtsms at nmij, whereas case 2 was used for the 10-n·m-dwtsm. to estimate the effect of these different sets of rtw mounting positions on the calibration results, three rtws were calibrated using these different cases. table 1 gives the experimental conditions. the mounting positions of the rtws were changed to the sets of directions in cases 1 and 2. in this experiment, torque transducers with rated capacities of 50 n·m (ts50) and 1 kn·m (tts/1000nm) were calibrated using the 1-kn·m-dwtsm, and a torque transducer with a rated capacity of 5 kn·m (ts5000r) was calibrated using the 20kn·m-dwtsm. the direction of the torque transducers was changed as shown in figures 2 and 3. the characteristics of these transducers have already been evaluated and were found to be similar in the clockwise (cw) and counterclockwise (ccw) directions. thus, they were calibrated in only one direction in this study. table 2 gives the calibration ranges and calibration and measurement capabilities (cmcs) of the torque standard machines (tsms) at nmij [10]–[12]. figure 4 shows a photograph of the ts5000r mounted on the 20-kn·mdwtsm. the back plate, rod, square hole adapter flange, and counterbalance weight were used to mount the rtw on the 20kn·m-dwtsm. similarly, figure 5 shows a photograph of the ts50 mounted on the 1-kn·m-dwtsm. the rtw is clamped by the square hole adapter. then, the rtw mounting position was changed by rotating the rtw and the square hole adapter in a body. thus, the square drive was kept attached to the square hole adapter all the time during the calibrations. the details of the procedure have been described in [1]. the installation procedure of the counterbalance weights is determined to each rtw based on its weight and shape. figure 6 shows a schematic of the loading cycle conducted in accordance with the jmif-016 (ttsg-w102) guideline [9]. eight torque steps (10 %, 20 %, 30 %, 40 %, 50 %, 60 %, 80 %, and 100 % of the maximum torque of each torque transducer) were applied. the lever length l was changed in the final mounting position. lmax and lmin are the average and minimum lever lengths, respectively. the dmp40s2 (hbm gmbh, germany) was used as the indicator/amplifier. a 0.1 hz bessel filter was employed as a low-pass filter. 2.2. intra‐laboratory comparison  an intra-laboratory comparison of the 10-n·mand 1kn·m-dwtsms was conducted to confirm the capability of figure  2.  schematic  showing  how  the  mounting  position  was  changed  by changing the lever direction in case 1.   figure  3.  schematic  showing  how  the  mounting  position  was  changed  by changing the lever direction in case 2.   figure 4. photograph of the ts5000r mounted on the 20‐kn∙m‐dwtsm.   figure 5. photograph of the ts50 mounted on the 1‐kn∙m‐dwtsm.   0 directiono 120 directiono 240 directiono moment arm side lever cable counter balance weight 0 directiono 90 directiono 270 directiono moment arm side lever cable counter balance weight acta imeko | www.imeko.org  july 2017 | volume 6 | number 2 | 35  the 10-n·m-dwtsm in the calibration of low-nominal capacity rtws at nmij. 2.2.1 reference torque wrenches  figure 7 shows the high-accuracy torque transducers dmts/10nm (gtm gmbh, germany) and tp-5n-1109 (showa measuring instruments co., ltd., japan), which are components of the traveling standards with rated capacities of 10 n·m and 5 n·m. they were connected to an indicator/ amplifier (dmp40s2) with specific cables to form rtws. table 3 gives the specifications of the two types of torque transducers and the experimental conditions. the square drive of each torque transducer is 6.3 mm. lmax and lmin of each torque transducer are 200 mm and 140 mm, respectively. a 0.1 hz bessel filter was employed as a low-pass filter. various characteristics of the rtw, such as its temperature and humidity characteristics, and the short-term drift have already been revealed. although the 10-n·mand 1-kn·m-dwtsms were set up in different buildings, the environmental conditions, such as the temperature and humidity, were nearly equivalent in each room. thus, the temperature and humidity dependence of the rtws was not considered. the short-term drift was considered by taking measurements in the following order: the 10-n·m-dwtsm (pre-calibration), the 1-kn·mdwtsm, and the 10-n·m-dwtsm (post-calibration). 2.2.2 torque standard machines  figures 8 and 9 show schematics of the 10-n·mand 1kn·m-dwtsms, respectively, and table 4 gives the characteristics of the two tsms. the lever deadweight system was adopted in each tsm. the relative expanded uncertainty of torque realized by the 10-n·m-dwtsm was 6.6 × 10–5 in the 0.1 n·m to 10 n·m torque range [12]. to realize a low-nominal capacity torque standard, a binary mass stack exchange system was adopted as the weight loading system of the 10-n·m figure 6. schematic of the loading cycle conducted in accordance with the  jmif‐016 guideline.   figure 7. photograph of tp‐5n‐1109 and dmts/10nm.   table 3. specifications of low‐nominal capacity torque transducers and experimental conditions.  type  rated capacity / n·m  square drive / mm lmax / mm lmin / mm changing the mounting position dmts/10nm  10  6.3  200  140  square drive direction  tp‐5n‐1109  5  torque transducer direction itself table 1. experimental conditions.  torque  transducers  capacity /  n·m  tsm  direction  lever length / mm lmax / lmin  changing the mounting  position  ts50  50  1‐kn∙m‐dwtsm ccw 250 / 200 case 1 *)  / case 2 **)   tts/1000nm  1 000  1‐kn∙m‐dwtsm cw 1 000 / 700 case 1 *)  / case 2 **)   ts5000r  5 000  20‐kn∙m‐dwtsm ccw 1 750 / 1 400 case 1 *)  / case 2 **)   *) case 1: 0°, 120°, 240°  **) case 2: 0°, 90°, 270°    table 2. calibration ranges and cmcs of tsms at nmij.  tsm tmd calibration range  (cmc)  rtw  calibration range  (cmc)  10‐n∙m‐dwtsm  0.1 n∙m – 10 n∙m (1.0 × 10 ‐4 )  0.1 n∙m – 10 n∙m  (3.0 × 10 ‐4 )   1‐kn∙m‐dwtsm  2 n∙m – 20 n∙m (1.0 × 10 ‐4 )   5 n∙m – 1 kn∙m  (5.0 × 10 ‐5 )  2 n∙m – 20 n∙m  (3.0 × 10 ‐4 )   5 n∙m – 1 kn∙m  (7.0 × 10 ‐5 )  20‐kn∙m‐dwtsm 200 n∙m – 20 kn∙m (7.0 × 10 ‐5 )  200 n∙m – 5 kn∙m  (1.0 × 10 ‐4 )   case 1 and 2: 0° position (1st series) l = lmax max zero preloading 1st cycle 2nd cycle 1st cycle max zero preloading case 1: 120° and 240° position case 2: 90° and 270° position (2nd and 3rd series) l = lmax 1st cycle max zero preloading case 1: 240° position case 2: 270° position (4th series) l = lmin acta imeko | www.imeko.org  july 2017 | volume 6 | number 2 | 36  dwtsm. in accordance with oiml r111, weights were prepared in a series from 10 g to 1 kg. the aerostatic bearing was adopted as a fulcrum supporting the moment arm to minimize rotational friction. the moment arms are made of low-thermal expansion alloy (super invar). two types of square hole adapters with different apical parts were prepared. one is an insertion-type adapter, which only allows the insertion of the square drive of a torque transducer, and the other is a tightening-type adapter, which is tightened with screws. figure 10 shows a photograph of the square hole adapters and a flange of the 10-n·m-dwtsm. figure 11 shows the rod component of the 10-n·m-dwtsm, which acts as the counter loading point. an aerostatic shaft with a collar was adopted to reduce any parasitic forces [3]. the relative expanded uncertainty of the torque realized by the 1-kn·m-dwtsm was 4.9 × 10–5 in the 5 n·m to 1 kn·m torque range [10]. linkage weights were adopted as the weights for this dwtsm. the moment arms of the 1-kn·m-dwtsm are made of austenitic stainless steel. to correct the moment arm length, the temperature of the moment arms was monitored by platinum thermometers. the square hole adapter of the 1-kn·m-dwtsm is the tightening-type adapter, and a solid shaft was adopted as the counter loading point. 2.2.3 calibration procedure  the calibration procedure and conditions were based on the jmif-016 (ttsg-w102) guideline [9]. the mounting position of the dmts/10nm was changed by changing the direction of the detachable square drive, as indicated in table 3, because the counterbalance weights could not be attached to it. note that, without these weights, the moment arm of any dwtsm could not be balanced, leading to disablement of the dwtsm for the figure 8. schematic of the 10‐n∙m‐dwtsm.   table 4. characteristics of the 10‐n∙m‐ and 1‐kn∙m‐dwtsms.    10‐n·m‐dwtsm 1‐kn·m‐dwtsm  calibration range  0.1 n∙m – 10 n∙m 5 n∙m – 1 kn∙m  wtsm (k = 2)  6.6 × 10 ‐5 4.9 × 10 ‐5 weights  weights according to oiml r111 linkage weights  fulcrum  aerostatic bearing material of moment arm  super invar austenitic stainless steel  square hole adapter tightening type or insertion type tightening type  rod  aerostatic shaft with a collar solid shaft figure 10. photograph of square hole adapters and a flange of the 10‐n∙m‐ dwtsm.       figure 11. photograph of the rod component of the 10‐n∙m‐dwtsm, which  acts as the counter loading point.  figure 9. schematic of the 1‐kn∙m‐dwtsm.   (6) torque transducer (7) installation part of the torque transducer (3) arm part (4) weight loading part (2) bearing part for fulcrum (aerostatic bearing ) (5) bearing part for counter torque (1) pedestal part (6) torque transducer (7) installation part of the torque transducer (3) arm part (4) weight loading part (2) bearing part for fulcrum (aerostatic bearing ) (5) bearing part for counter torque (1) pedestal part acta imeko | www.imeko.org  july 2017 | volume 6 | number 2 | 37  rtw calibrations. in contrast, because the counterbalance weights could be mounted on the tp-5n-1109, its mounting position was changed by changing the sensing body direction, as described in section 2.1. figures 12, 13, and 14 show schematics of the loading timetable and the rotational mounting positions of the dmts/10nm and tp-5n-1109, respectively. the mounting position of the dmts/10nm was set to four directions (0°, 90°, 180°, and 270°) by changing the mounting position of its square drive. this allowed the mounting position of the torque transducer to be maintained in the 0° direction. two torque calibration steps (50 % and 100 % of the maximum torque) were applied. the lever length was changed from lmax = 200 mm to lmin = 140 mm by moving the rod as the counter loading point in the final mounting position. the mounting position of the tp-5n-1109 was set to three directions (0°, 90°, and 270°). the counterbalance weights were installed in the tp5n-1109 when it was mounted in the 90° and 270° directions, as shown in figure 14. one torque step (100 % of the maximum torque) was applied. as with the dmts/10nm, the lever length was changed from lmax = 200 mm to lmin = 140 mm in the final mounting position. figures 15 and 16 show photographs of the dmts/10nm and tp-5n-1109, respectively, mounted on a tsm. an initial torque of 1 % or 2 % of the maximum torque of each rtw was applied by placing a small weight on the end of the moment arm of each tsm. the rtws were calibrated separately in the cw and ccw directions. 2.2.4 environmental conditions and measurement dates  table 5 gives the environmental conditions that must be satisfied when conducting a calibration in nmij. the present comparison was conducted at a temperature of 23 °c ± 1 °c. the environmental conditions were nearly equivalent in the laboratories of the 10-n·mand 1-kn·m-dwtsms. table 6 gives the measurement dates of the intra-laboratory comparison. this comparison was conducted in the spring to achieve a stable room temperature of 23 °c. 3. results and discussion  3.1. difference of the calibration results between case 1 and case  2  the arithmetic averages of the measurement results at the three mounting positions (0°, 120°, and 240° for case 1 and 0°, 90°, and 240° for case 2) were obtained for each torque step, where n = case1, case2 indicates the set of mounting positions and i is the calibration step. the uncertainties wn,i were evaluated in accordance with jcg209s11 [13], which is a guideline for evaluating the uncertainty of calibrations of tmds and rtws that was issued by the international accreditation japan at the national institute of testing and evaluation ins , figure 12. schematic of  loading timetable for  intra‐laboratory comparison between the 10‐n∙m‐ and 1‐kn∙m‐dwtsms.       figure  13.  schematic  of  how  the  mounting  position  was  changed  by changing the square drive direction for the dmts/10nm.    figure  14.  schematic  of  how  the  mounting  position  was  changed  by changing the lever direction for the tp‐5n‐1109.  table 5. environmental conditions for calibrations in nmij. environmental  conditions  minimum  maximum  acceptable  variations  temperature 18 °c 28 °c  < 1 k relative humidity 20 % 70 %  < 10 % atmospheric  pressure  990 hpa  1040 hpa  < 30 hpa      table 6. measurement dates of the intra‐laboratory comparison.  date tsm  may 29 – 30, 2014 10‐n∙m‐dwtsm (pre‐calibration) june 3, 2014 1‐kn∙m‐dwtsm  june 4 – 5, 2014 10‐n∙m‐dwtsm (post‐calibration) 0°position, l = lmax. tp-5n-1109: 90° and 270° position, l = lmax. max zero max zero preloading 1st 2nd 1st 270° position, l = lmin. max zero 1st dmts/10nm: 90°, 180° and 270° position, l = lmax. 200 mm 140 mm 0o 90o 180o 270o 0 directiono 90 directiono 270 directiono moment arm side lever cable counter balance weight 140 mm 200 mm acta imeko | www.imeko.org  july 2017 | volume 6 | number 2 | 38  (iajapan/nite), an accreditation body for calibration and testing laboratories in japan. the relative deviation rdi is expressed as , (1) where , is given by . (2) tables 7, 8, and 9 give the calibration results for the ts50, tts/1000nm, and ts5000r, respectively. absolute values of the measurement results obtained in case 2 were larger than those in case 1 under most of the considered calibration conditions, and the magnitudes of the uncertainties wn,i in case 1 were slightly larger than or almost equal to those in case 2. we presume that the reason for this is due to the larger mounting position interval in case 1 than that in case 2, resulting in slightly larger reproducibility in different mounting positions. the en number is expressed as , , . (3) en numbers were less than one at all torque steps as shown tables 7, 8 and 9. thus, the agreement of the rtw calibration results of the two procedures was confirmed. in principle, the calibration procedure for an rtw is based on case 1. however, it was demonstrated that the calibration procedure based on case 2 can yield results consistent with those obtained using the procedure based on case 1. 3.2.  intra‐laboratory comparison  the comparison results for the dmts/10nm with the torque changed at intervals of +5 n·m, +10 n·m, and –5 n·m are shown in figures 17, 18, and 19, respectively. the arithmetic averages of the measurement results at mounting positions of 0°, 90°, 180°, and 270° were obtained for each torque step, where h = a_pre, a_post, and b indicates the preand post-calibration results obtained using the 10n·m-dwtsm and the results obtained using the 1-kn·mdwtsm, respectively. rd is expressed as , (4) where _ , is given by . (5) in figures 17, 18, and 19, zero on the vertical axis is the average of the preand post-calibration results obtained using the 10-n·m-dwtsm. the bars represent the relative expanded uncertainty wh of the measurement results, which is given by , (6) where k is the coverage factor, wtsm_h is the relative combined standard uncertainty of torque realized by the tsm, and wtra_h,i i iin i s ss rd ave, case1, ,  2 case2, case1, ave, ii i ss s   ihs , i ii i s ss rd a_ave, a_ave, b,  2 a_post, a_pre, a_ave, ii i ss s   2 ,tra_ 2 tsm_,, ihhihih wwkwkw  (a)  (b) figure 15. photograph of torque transducers mounted on the 10‐n∙m‐dwtsm. (a) dmts/10nm (0° mounting direction). (b) tp‐5n‐1109 (270° mounting  direction).      (a)    (b)  figure 16. photograph of torque transducers mounted on the 1‐kn∙m‐dwtsm. (a) dmts/10nm (0° mounting direction). (b) tp‐5n‐1109 (270° mounting  direction).  acta imeko | www.imeko.org  july 2017 | volume 6 | number 2 | 39  is the relative combined standard uncertainty ascribable to the traveling standards. wtra_h,i is given by , (7) where wrot_h, wrep_h, wlvr_h, and wres_h are the relative standard uncertainties due to the reproducibility when changing the mounting position, the repeatability at a constant mounting position, the reproducibility when changing the lever length, and the resolution of the traveling standard, respectively. the measurement results showed good agreement with each other within the uncertainties, as shown in figures 17, 18, and 19. similarly, the comparison results for the tp-5n-1109 with the torque changed at intervals of +5 n·m are shown in figure 20. the results for the tp-5n-1109 also showed good agreement within the uncertainties. table 10 shows the comparison results for the dmts/10nm. the relative expanded uncertainty, wa, is given by the following equation, which includes the influence of short-term drift: , (8) 2 drf, 2 res_a_pre, 2 lvr_a_pre, 2 rep_a_pre, 2 rot_a_pre, tra_a, iiiiii wwwwww  , (9) where the subscript a indicates the results measured using the 10-n·m-dwtsm and wdrf is the relative standard uncertainty due to the short-term drift of the dmts/10nm. wdrf is evaluated from the preand post-calibration results obtained using the 10-n·m-dwtsm. in this table, en numbers were less than one at all torque steps. therefore, the equivalence of the two calibration methods involving changing the square drive direction of the dmts/10nm using the 10n·mand 1-kn·m-dwtsms was confirmed. similarly, the comparison results for the tp-5n-1109 given in table 11 demonstrate that en numbers were less than one when the torque was changed at intervals of ±5 n·m. thus, the measurement results of the calibration method involving changing the tp-5n-1109 direction using the 10-n·mdwtsm were shown to be equivalent to those obtained using the 1-kn·m-dwtsm. 2 ,res_ 2 ,lvr_ 2 ,rep_ 2 ,rot_ ,tra_ ihihihihih wwwww  2 tra_a, 2 tsm_a a, a, iii wwkwkw  table 7. calibration results for the ts50. torque / n·m  case 1  case 2  rd  en  is ,case1   / (mv/v)  w / %  is ,case2   / (mv/v)  w / %  ‐5 ‐10  ‐15  ‐20  ‐25  ‐30  ‐40  ‐50  ‐0.171168  ‐0.342361  ‐0.513570  ‐0.684799  ‐0.856056  ‐1.027342  ‐1.369798  ‐1.712257  0.15 0.11  0.082  0.063  0.051  0.044  0.037  0.028  ‐0.171176 ‐0.342371  ‐0.513593  ‐0.684839  ‐0.856118  ‐1.027420  ‐1.369926  ‐1.712424  0.13 0.10  0.071  0.054  0.043  0.037  0.030  0.020  ‐0.000046  ‐0.000029  ‐0.000046  ‐0.000058  ‐0.000073  ‐0.000077  ‐0.000094  ‐0.000098  ‐0.02  ‐0.02  ‐0.04  ‐0.07  ‐0.11  ‐0.13  ‐0.20  ‐0.28      table 8. calibration results for the tts/1000nm.  torque / n·m  case 1  case 2  rd  en  is ,case1   / (mv/v)  w / %  is ,case2   / (mv/v)  w / %  100 200  300  400  500  600  800  1000  0.158576  0.316898  0.475182  0.633468  0.791752  0.950021  1.266550  1.583038  0.27 0.17  0.11  0.067  0.048  0.042  0.042  0.018  0.158594 0.316925  0.475205  0.633493  0.791782  0.950056  1.266588  1.583076  0.27 0.17  0.10  0.065  0.046  0.041  0.043  0.019  0.000115  0.000083  0.000049  0.000039  0.000038  0.000037  0.000030  0.000024    0.03  0.03  0.03  0.04  0.06  0.06  0.05  0.09      table 9. calibration results for the ts5000r.  torque / n·m  case 1  case 2 rd  en  is ,case1   / (mv/v)  w / %  is ,case2   / (mv/v)  w / %  ‐500 ‐1000  ‐1500  ‐2000  ‐2500  ‐3000  ‐4000  ‐5000  ‐0.139539  ‐0.279084  ‐0.418627  ‐0.558171  ‐0.697712  ‐0.837247  ‐1.116304  ‐1.395363  0.011 0.010  0.010  0.010  0.010  0.010  0.010  0.010  ‐0.139539 ‐0.279088  ‐0.418637  ‐0.558186  ‐0.697733  ‐0.837273  ‐1.116343  ‐1.395406  0.012 0.010  0.010  0.010  0.010  0.010  0.010  0.010  0.000004  ‐0.000014  ‐0.000024  ‐0.000027  ‐0.000030  ‐0.000032  ‐0.000035  ‐0.000031   0.02  ‐0.10  ‐0.17  ‐0.19  ‐0.21  ‐0.22  ‐0.24  ‐0.22  acta imeko | www.imeko.org  july 2017 | volume 6 | number 2 | 40  (a) (b)  figure 17. results for the dmts/10nm with torque changed at intervals of +5 n∙m in the (a) cw and (b) ccw directions.    (a) (b)  figure 18. results for the dmts/10nm with torque changed at intervals of +10 n∙m in the (a) cw and (b) ccw directions.    (a) (b)  figure 19. results for the dmts/10nm with torque changed at intervals of –5 n∙m in the (a) cw and (b) ccw directions.    (a)  (b)  figure 20. results for the tp‐5n‐1109 with torque changed at intervals of +5 n∙m in the (a) cw and (b) ccw directions.  -500 -250 0 250 500 r e la tiv e d e v ia tio n / 1 0 -6 pre-cal. 10-n·m-dwtsm 1-kn·m-dwtsm post-cal. 10-n·m-dwtsm calibration result (mean value) uncertainty bar = wh -500 -250 0 250 500 r e la tiv e d e v ia tio n / 1 0 -6 pre-cal. 10-n·m-dwtsm 1-kn·m-dwtsm post-cal. 10-n·m-dwtsm calibration result (mean value) uncertainty bar = wh -500 -250 0 250 500 r e la tiv e d e v ia tio n / 1 0 -6 pre-cal. 10-n·m-dwtsm 1-kn·m-dwtsm post-cal. 10-n·m-dwtsm calibration result (mean value) uncertainty bar = wh -500 -250 0 250 500 r e la tiv e d e v ia tio n / 1 0 -6 pre-cal. 10-n·m-dwtsm 1-kn·m-dwtsm post-cal. 10-n·m-dwtsm calibration result (mean value) uncertainty bar = wh -500 -250 0 250 500 r e la tiv e d e v ia tio n / 1 0 -6 pre-cal. 10-n·m-dwtsm 1-kn·m-dwtsm post-cal. 10-n·m-dwtsm calibration result (mean value) uncertainty bar = wh -500 -250 0 250 500 r e la tiv e d e v ia ti o n / 1 0 -6 pre-cal. 10-n·m-dwtsm 1-kn·m-dwtsm post-cal. 10-n·m-dwtsm calibration result (mean value) uncertainty bar = wh -1500 -1000 -500 0 500 1000 1500 r e la tiv e d e v ia ti o n / 1 0 -6 pre-cal. 10-n·m-dwtsm 1-kn·m-dwtsm post-cal. 10-n·m-dwtsm calibration result (mean value) uncertainty bar = wh -1500 -1000 -500 0 500 1000 1500 r e la tiv e d e v ia ti o n / 1 0 -6 pre-cal. 10-n·m-dwtsm 1-kn·m-dwtsm post-cal. 10-n·m-dwtsm calibration result (mean value) uncertainty bar = wh acta imeko | www.imeko.org  july 2017 | volume 6 | number 2 | 41  4. conclusion  the rtw calibration procedure adopted by nmij is based on the jmif-016 (ttsg-w102) technical guideline. the mounting position of an rtw was set to three directions (0°, 120°, and 240°; case 1) when it was calibrated using the 1kn·mand 20-kn·m-dwtsms. however, the mounting position was set to three different directions (0°, 90° and 270°; case 2) when an rtw was calibrated using the 10-n·mdwtsm. in this study, the effect of these differences in the rtw mounting positions on the calibration results was first investigated. second, to confirm the equivalence of the capabilities of the 10-n·mand 1-kn·m-dwtsms in the calibration of low-nominal capacity rtws, an intra-laboratory comparison between the two tsms was conducted using two high-accuracy rtws. the following results were obtained. (1) the calibration results obtained using the two sets of mounting position (cases 1 and 2) showed good agreement with each other within the uncertainties. thus, although case 1 is recommended in principle, case 2 can also be adopted. (2) absolute values of the calibration results obtained in case 2 were slightly larger than those in case 1 under almost all calibration conditions. (3) the uncertainties obtained in case 1 were nearly equal to or slightly larger than those obtained in case 2. (4) the rtw calibration capabilities of the 10-n·mand 1kn·m-dwtsms were found to be equivalent. references  [1] k. ogushi, a. nishino, k. maeda and k. ueda, “range expanded expansion of the reference torque wrench calibration service to 5 kn m at nmij”, measurement 45 (2012) 1200–1209. [2] a. nishino, k. ogushi and k. ueda, “development of a new small-rated-capacity reference torque wrench”, proc. apmf 2013, fm0003, taipei, taiwan, nov. 2013. [3] a. nishino, k. ogushi and k. ueda, “calibration of reference torque wrenches using a 10n m deadweight torque standard machine”, measurement 61 (2015) 1–8. [4] a. nishino, k. ogushi, k. ueda, d. röske and d. mauersberger, “bilateral comparisons of measurement capabilities for the calibration of low nominal capacity torque measuring devices between nmij and ptb in the range from 0.1 n·m to 10 n·m using different procedures”, measurement 68 (2015) 32–41. [5] d. röske, “final report on the torque key comparison ccm.t-k1 measurand torque: 0 n·m, 500 n·m, 1000 n·m”, 2009, internet: http://www.bipm.org/utils/common/pdf/final_reports/m/tk1/ccm.t-k1.pdf. [6] k. ogushi, t. ota, k. ueda, d. peschel and a. brüge, “interlaboratory comparison of reference torque wrench calibration between nmij and ptb”, proc. xviii imeko world congress, cd-rom, rio de janeiro, brazil, sept. 2006. [7] k. ogushi, a. nishino, k. maeda and k. ueda, “intra-laboratory comparison between calibration capabilities of reference torque wrench using two deadweight torque standard machines” (in japanese), proc. 27th sensing forum of sice, pp. 243–248, kiryu, japan, sept. 2010. [8] k. ogushi, a. nishino, k. maeda, k. ueda, a. brüg and d. röske, “inter-laboratory comparison of reference torque wrench calibration between nmij and ptb at the range from 200 n·m to 2 kn·m”, proc. of xx imeko world congress, idtc3-o-32, busan, republic of korea, 2012. [9] torque traceability study group ed., jmif-016: guideline for the calibration laboratory of reference torque wrenches (in japanese), japan measuring instrument federation, 2004. [10] k. ohgushi, t. ota and k. ueda, “load dependency of the moment-arm length in the torque standard machine”, proc. xvii imeko world congress, pp. 383–388, dubrovnik, croatia, june, 2003. [11] k. ohgushi, t. ota and k. ueda, “uncertainty evaluation of the 20 kn·m dead weight torque standard machine”, proc. the 19th int. conf. on force, mass & torque imeko tc3, cdrom, cairo, egypt, feb. 2005. [12] a. nishino, k. ogushi and k. ueda, “uncertainty evaluation of a 10n·m dead weight torque standard machine and comparison with a 1kn·m dead weight torque standard machine”, measurement 49 (2014) 77–90. [13] jcg209s11-05, jcss guideline on uncertainty estimation – torque meters and reference torque wrenches, international accreditation japan, national institute of testing and evaluation, 2012 (in japanese). table 10. comparison results for the dmts/10nm.  direction  t / n·m  inc. / dec  10‐n·m‐dwtsm 1‐kn·m‐dwtsm wdrf *)   rd *)   en sa_ave  / (mv/v)  wa *)   sb  / (mv/v)  wb *)     ‐5  inc.  ‐1.000400  300 ‐1.000431 85 1.9  30  0.10 cw  ‐10  ‐‐‐‐‐  ‐2.000867  300 ‐2.000914 102 0.9  24  0.07   ‐5  dec.  ‐1.000343  300 ‐1.000422 103 1.9  79  0.25 cw  5  inc.  1.000426  300 1.000437 104 3.2  12  0.04   10  ‐‐‐‐‐  2.000932  300 2.000986 122 1.5  27  0.08   5  dec.  1.000405  300 1.000488 124 3.3  83  0.26      *) relative values × 10 ‐6      table 11. comparison results for the tp‐5n‐1109.  direction  t / n·m  inc. / dec  10‐n·m‐dwtsm 1‐kn·m‐dwtsm wdrf *)   rd *)   en sa_ave  / (mv/v)  wa *)   sb  / (mv/v)  wb *)   ccw  ‐5  ‐‐‐‐‐  ‐1.155700  491 ‐1.156213 500 11 443  0.63 cw  5  ‐‐‐‐‐  1.155670  572   1.156193 539 0.2  453  0.58      *) relative values × 10 ‐6 acta imeko, title acta imeko issn: 2221-870x september 2017, volume 6, number 3, 82-86 acta imeko | www.imeko.org september 2017 | volume 6 | number 3 | 82 photons and electrons for the study of a white veil covering some walls in prehistoric caves rémy chapoulie1, léna bassel1, guilhem mauran1, catherine ferrier2, alain queffelec2, delphine lacanette3, philippe malaurent3, bruno bousquet4, vincent motto-ros5, florian trichard5, frédéric pelascini6, vincent rodriguez7 1iramat-crp2a, umr cnrs 5060, université bordeaux montaigne, maison de l’archéologie, domaine universitaire, esplanade des antilles, 33607 pessac cedex, france 2pacea, umr cnrs 5199, université de bordeaux, bâtiment b8, allée geoffroy saint hilaire, cs 50023, 33615 pessac cedex, france 3i2m, umr 5295 cnrs, université de bordeaux, 33607 pessac, france 4celia, umr cnrs 5107, université de bordeaux, 351 cours de la libération, 33405 talence cedex, france 5institut lumière matière, umr 5306, université de lyon 1 – cnrs, université de lyon, 69622 villeurbanne cedex, france 6critt matériaux alsace, 19 rue de st junien, c. s. 80023, 67305 schiltigheim, france 7ism, umr cnrs 5255, université de bordeaux, 351 cours de la libération, 33405 talence cedex, france section: research paper keywords: white veil; archaeometry; taphonomy; prehistoric caves citation: rémy chapoulie, léna bassel, guilhem mauran, catherine ferrier, alain queffelec, delphine lacanette, philippe malaurent, bruno bousquet, vincent motto-ros, florian trichard, frédéric pelascini, vincent rodriguez, photons and electrons for the study of a white veil covering some walls in prehistoric caves, acta imeko, vol. 6, no. 3, article 13, september 2017, identifier: imeko-acta-06 (2017)-03-13 section editor: sabrina grassini, politecnico di torino, italy received may 30, 2017; in final form august 3, 2017; published september 2017 copyright: © 2017 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: this work was supported by the région nouvelle-aquitaine, the cluster of excellence lascarbx laboratoire des sciences archéologiques de bordeaux (anr-10-labx-52), the cnrs, and the universities of bordeaux montaigne and bordeaux corresponding author: rémy chapoulie, e-mail: chapoulie@u-bordeaux-montaigne.fr 1. introduction in the south west of france, the vézère valley represents a famous region of prehistoric caves. this region is indeed rich with ornate caves; either painted or engraved, and represents therefore an area where rock art cave conservation is an important issue. curators are faced with different kinds of calcitic coatings on the walls that make the underlying rock art disappear. for instance this is the case at rouffignac cave in dordogne (see figure 1 left). in the present study, we focused on two types of alterations characterized by two types of facies. the first one is the so-called calcitic moonmilk, described by the presence of thin needle fiber calcite, which is frequently abstract our research deals with the evolution of wall surfaces in prehistoric caves. the focus of this paper is dedicated to the physico-chemical characterization of a white concretion partially covering some walls in caves. in one of the caves, a non-ornate one which became a laboratory-cave (the leye cave at marquay, dordogne, france) located in the vézère valley, a set of physical methods has been proposed and tested on the first samples taken: sem-edxs, cathodoluminescence and laser-based techniques such as raman spectroscopy and libs. thus two different facies mainly composed of calcium carbonate crystals have been determined. the identification of theses crystalline phases is the first step of an ambitious research project that plans to understand the development of unexpected layers on the cave walls of the famous ornate caves listed as part of the unesco cultural heritage sites. this first set of data provides good insight to the physico-chemical composition and structure of the involved materials. future works will be dedicated to bring knowledge about the facies chronology, the climatic conditions of environment (temperature, co2 rate and air velocities) over a long period. acta imeko | www.imeko.org september 2017 | volume 6 | number 3 | 83 found in caves and has already been extensively studied [1]-[4]. the second one, referred as coralloid, presents a pop-corn like aspect [2]. in order to investigate the mechanisms responsible for the genesis and development of such concretions, a cave without any archaeological relevance and containing both moonmilk and coralloid types of concretions, has been selected since 2008 along the vézère valley area to become a laboratorycave [5]. the leye cave is thus considered to be representative of the ornate cavities from the area from both morphological and geological aspects. this paper shows the first data (observations and analyses) obtained from the first samples taken out from this laboratorycave, that enabled to determine the two facies and their chemical composition and structure. the methods used were chosen in this aim: optical microscope for observing the moonmilk which seems rather homogeneous at first sight, scanning electron microscopy for the identification of calcite needles in the moonmilk at micron scale, and the methods for chemical composition (x ray fluorescence, raman and laser induced breakdown spectroscopy) mainly for the coralloid which were analyzed at the surface and in cross-section. also used is the cathodoluminescence method to know about the presence of impurities and point defects in the coralloid structure. 2. methods and data all methods did not show significant results on the two facies. so only the relevant data are shown here. 2.1. optical and electronic methods for observation while the coralloids were rather easy to recognize by sight (see figure 4), optical microscopy was required to reveal the presence of needles in moonmilk. the mobile optical microscope used was a vcr-800 hirox type with maximum magnification of 400, led lighting and ccd camera. figure 1 shows the instrument at rouffignac cave and some calcite needles picture taken at combarelles cave also located in dordogne, france. sem-edxs was used to provide not only images but also analytical data. samples have been observed by electron microscopy in various modes using a jeol jsm-6460lv linked to an oxford instrument x max energy dispersive xray spectrometer (edxs). two samples from the leye cave were gold-coated and observed in high vacuum. these samples were chosen due to their high diversity of elements of biological origin and of needle fiber calcite shape [6]. macroscopic differences can be seen between the samples from the laboratory-cave but no clear trend about microscopic differences on the structure of the biominerals [1] can be highlighted. all the different kinds of needles were observed on the biomineral samples. each sample presented a large variability in the needles’ morphology. figure 2 presents the different habits, while figure 3 presents the variability of the needle fiber calcite and of the epitaxial needles from different samples. cathodoluminescence imaging was only performed on the cross-section of a coralloid sample (figure 4); it was of no use figure 1. left: optical microscope at rouffignac cave lighting a moonmilk layer. right: calcite needles seen at magnification 160 (scale bar shows 100 microns) at combarelles cave figure 3. sem-secondary electron image mode showing the variability of the needle fiber calcite and of the epitaxial needles. figure 4. cathodoluminescence imaging of a coralloid. left: white-light imaging of the cross-section. right: cathodoluminescence imaging of the same cross-section with a dark blue layer made of calcite (spot 1), a light blue for the coralloid (spots 2, 3, 5), and an orange emission from the laminae fronts (spots 4 and 6). spot 5 is also analyzed by raman spectroscopy (see figure 9). figure 2. (a) to (f) sem-secondary electron image mode of some carbonated habits. (a) broken needle fiber calcite lying on a corroded carbonate mineral. (b) whiskers chain (white arrow) linked by epitaxial needles (red arrows). (c) typical needle-fiber calcite with serrated edges (white arrows), epitaxial nodule (red arrows). (d) typical paired needle fiber calcite without serrated edges (white arrows), needle-fiber calcite with serrated edges (red arrow). (e) nanofiber and small bacteria. (f) calciumrich spheroids and epitaxial needles. acta imeko | www.imeko.org september 2017 | volume 6 | number 3 | 84 on the moonmilk sample which could not be prepared as crosssection. the experiment was carried out with a luminoscope system producing an electron beam accelerated by 10 kv, wide enough to observe the luminescence emission on a large area (10×15 mm). with this method, the luminescent centers are revealed under the electron beam impact [7]. blue luminescence is to be linked to the presence of calcite carbonate defect at co3 group. here (figure 4 right) a dark blue layer can be seen covering the bedrock. a lighter and more intense blue comes out of the main part of the coralloid while the smaller orange areas reveal the presence of low concentrations in manganese preferentially located at the laminae fronts. 2.2. photonic methods and results two portable systems were used. a pxrf (x-sort from ametek-spectro) showed the obvious presence of calcium as major and the presence of iron and strontium in low amounts for both moonmilk and coralloids (figure 5). a noticeable difference appears for sr which is between 4 to 10 times more abundant in coralloids. non-contact measurements were also tested and it was proved that a 4 mm distance from the wall was still performant whenever a tripod was set, which is relevant for cave art studies. with a portable libs equipment (easylibs from ivea solution) the chemical elements detected for the moonmilk and the coralloids were: mg, si, fe, al, sr and ca. it is noticeable that mg and sr intensities were much higher in coralloids than in moonmilk. so far it should be also noticed that strontium and magnesium were considered as elements of major interest in the composition of speleothems [8]-[10]. actually, the presence of strontium is directly linked to the growth rate of speleothems and was consequently used as an indicator of the growth speed [8]. strontium is also probably more favorable to the orthorhombic aragonite structure than to the rhomboedric calcite structure when precipitation occurs [8] while magnesium is considered to be a good indicator of hydrology in the caves and to be responsible for the inhibition of calcite growth [8] [11]. this direct libs analysis (8 cm from the wall surfaces with our equipment) gave access to this list of elements detected on the surface of the coralloid. a specific attention was paid to the coralloids. figure 6 shows some of them spread as white concretions on the cave walls. the sampling area is clearly visible in this image and part of the extracted coralloid is shown in figure 6b. analyzing the sample along a cross-section was necessary to enhance our understanding of genesis and growth. thus, seventeen libs measurements were performed with the portable instrument along a transect axis aligned with the growth direction, as described in figure 7. the spacing between two libs analyses was fixed to 0.5 mm and the location of the transect was arbitrarily chosen. from figure 7, three different layers can be clearly seen: the underlying bedrock, a brownish layer that we will call internal layer and an external layer appearing whitish, which is the final surface layer of the wall. thus, this in-depth profiling allowed highlighting the variation of the libs signals related to strontium and magnesium. these first plibs data are better illustrated in the mapping picture (figure 8). indeed this cross-section was additionally scanned by benchtop libs (figure 8). the instrument used to provide the libs images has been described in [12] and [13] (laser source was a nd:yag emitting 8 ns pulses at 1064 nm with a 100 hz repetition rate; laser energy was reduced at typically 1.5 mj; laser pulses were focused onto the sample by a 15× magnification objective resulting in a crater size close to 8 µm). the spatial distributions of mg and sr were found to be rather similar in the external layer (figure 8a and 8c). the intensity ratio between the external and the internal layer is close to 3 for mg (figure 8a) and close to 7 for sr (figure 8c). it was demonstrated that the presence of sr and mg in the cave waters was mainly due to bedrock dissolution [14]. our data are in line with this statement since on all the images presented in figure 8, the underlying host rock which is calcareous in the laboratory cave contains all the elements detected in the external layer. soil leakage could also have played a role in the presence of sr as revealed by isotopic data [15], [16]. even though, it is remarkable that the internal layer seems to be more depleted in the elements of interest. this could be the result of migration of the elements from the base of the coralloid to the external limit in contact with air. indeed, this internal layer appears to be made of calcite crystals. figure 6. coralloid sample from laboratory-cave (after [17]). a) picture of a cave wall from the laboratory-cave covered by coralloids showing the sampling area; b) zoom in on the coralloid extracted from the wall. figure 7. cross-section of coralloid and location of the transect along which 17 libs measurements were performed (black circles). three phases were identified as external layer (1), internal layer (2) and underlying host rock (3) (after [17]). figure 5. typical xrf spectra from moonmilk and coralloid showing the presence of ca, fe and sr. acta imeko | www.imeko.org september 2017 | volume 6 | number 3 | 85 regarding the spatial distribution of the libs signal of si (figure 8b), it particularly reflects the presence of a series of laminations linked to higher libs signal of si. the fact that laminations were observed is not surprising since it is consistent with the supposed formation process by successive precipitation events at the interface with the ambient air of the cave. the high contrast in libs signal of si observed at the interface between the two layers suggests the presence of a detrital clay interface richer in si than the calcium carbonate around it [17]. raman spectroscopy was also carried out with an xplora confocal raman microscope from horiba scientific (objective 20×, na=0.32), using a 785 nm laser excitation in order to minimize luminescence drawbacks. it enabled to distinguish the stable phases of calcium carbonates: aragonite and calcite. specific peaks were identified and a mapping (figure 9) was performed. figure 9 corresponds to the area of spot 5 in figure 4 right; it shows the localization of aragonite surrounded by crystals of calcite. 3. conclusions the major achievements of the research presented in this manuscript are the identification of the two facies of calcium carbonates found in this cave, a laboratory-cave which is quite similar to the famous caves of the vézère valley in dordogne, france. the moonmilk concretions were identified with sem thanks to their morphology. it is noticeable to mention that optical microscopy can show their presence at high magnification because of their needle structure. for coralloids, both calcite and aragonite were formally identified thanks to raman spectroscopy in their classical habitus : massive for calcite and fibrous for aragonite. their location can thus be studied by means of raman cartography. the chemical composition of coralloids was defined by libs. the libs and cl methods confirmed the laminae structure. this structure indicates many phases of interactions between the surface of the wall and the ambient air. these methods bring us important data that should be crossed with others such as environmental and chronological ones (in progress) to reach out our main objective of understanding the origin and growth kinetics of these calcitic facies. acknowledgements this work was supported by the région nouvelle-aquitaine, the french state managed by the french national research agency within the framework of the cluster of excellence laboratoire des sciences archéologiques de bordeaux (anr-10-labx-52), the cnrs, and the universities of bordeaux montaigne and bordeaux. special thanks to mister delbos and his family for allowing us to go and work in the laboratory-cave at marquay, dordogne, france. references [1] e. p. v. k. e. verrecchia, (1994). needle-fiber calcite: a critical review and a proposed classification. j of sediment res 64(3). [2] c. a. hill, and p. forti (1997). cave minerals of the world, national speleological society. [3] a. borsato, s. frisia, b. jones, k. van der borg (2000). calcite moonmilk: crystal morphology and environment of formation in caves in the italian alps. j sed res 70(5), pp. 1179-1190. [4] s. sanchez-moral, m. c. portillo, i. janices, s. cuezva, a. fernández-cortés, j. c. cañaveras and j. m. gonzalez (2012). the role of microorganisms in the formation of calcitic moonmilk deposits and speleothems in altamira cave. geomorphology 139-140, pp. 285-292. [5] d. lacanette, d. large, c. ferrier, n. aujoulat, f. bastian, a. denis, v. jurado, b. kervazo, s. konik, r. lastennet, p. malaurent and c. saiz-jimenez (2013). a laboratory cave for the study of wall degradation in rock art caves: an implementation in the vézère area. j archaeol sci 40(2), pp. 894-903. figure 8. elemental images of a coralloid sample by libs (after [17]). a) mg image at 285.21 nm; b) si image at 288.16 nm; c) sr image at 407.77 nm. the intensity signal displayed corresponds to the raw emission signal after background subtraction. intensity threshold calibration bars display the minimum value in black corresponding to the background level and the maximum value in white. figure 9. raman mapping (~300×300 µm2) using the ratio of 210 cm-1 aragonite peak / 280 cm-1 calcite peak. spot diameter is ca 2.5 µm. red, pink acta imeko | www.imeko.org september 2017 | volume 6 | number 3 | 86 [6] j.c. cañaveras, s. sanchez-moral, v. soler, and c. saiz-jimenez (2001). microorganims and microbially induced fabrics in cave walls. geomicrobiology journal, 18, pp. 223–240. [7] s. cazenave, r. chapoulie , g. villeneuve (2003). cathodoluminescence of synthetic and natural calcite: the effects of manganese and iron on orange emission. mineralogy and petrology 78, no 3, pp. 243-253. [8] i. j. fairchild, a. baker, a. borsato, s. frisia, r. w. hinton, f. mcdermott, a. f. tooth (2001). annual to sub-annual resolution of multiple trace-element trends in speleothems (english). j geol soc london 158(5), pp. 831-841. [9] a. borsato, s. frisia, i. j. fairchild, a. somogyi and j. susini (2007). trace element distribution in annual stalagmite laminae mapped by micrometer-resolution x-ray fluorescence: implications for incorporation of environmentally significant species. geochim cosmochim ac 71(6), pp. 1494-1512. [10] d. j. sinclair, j. l. banner, f. w. taylor, j. partin, j. jenson, j. mylroie, e. goddard, t. quinn, j. jocson and b. miklavič (2012). magnesium and strontium systematics in tropical speleothems from the western pacific. chem geol 294-295, pp. 1-17. [11] w. white (2012). speleothem microstructure/speleothem ontogeny: a review of western contributions. int j of speleol 41(2), pp. 329-358. [12] v. motto-ros, e. negre, f. pelascini, g. panczer and j. yu (2014). precise alignment of the collection fiber assisted by realtime plasma imaging in laser-induced breakdown spectroscopy. spectrochim acta b 92, pp. 60-69. [13] l. sancey, v. motto-ros, b. busser, s. kotb, j. m. benoit, a. piednoir, f. lux, o. tillement, g. panczer and j. yu (2014). laser spectrometry for multi-elemental imaging of biological tissues. sci rep 4: 6065 (7p). [14] i. j. fairchild, c. l. smith, a. baker, l. fuller, c. spötl, d. mattey, f. mcdermott and e.i.m.f (2006). modification and preservation of environmental signals in speleothems. earth sci rev 75(1-4) pp. 105-153. [15] i. j. fairchild, p. c. treble (2009). trace elements in speleothems as recorders of environmental change. quaternary sci rev 28(56) pp. 449-468. [16] c. bourdin, e. douville, d. genty (2011). alkaline-earth metal and rare-earth element incorporation control by ionic radius and growth rate on a stalagmite from the chauvet cave, southeastern france. chem geol 290(1-2) pp. 1-11. [17] l. bassel, v. motto-ros, f. trichard, f. pelascini, f. ammari, r. chapoulie, c. ferrier, d. lacanette and b. bousquet (2016). laser-induced breakdown spectroscopy for elemental characterization of calcitic alterations on cave walls. environ. sci. pollut. res. 1-8; doi 10.1007/s11356-016-7468-5. photons and electrons for the study of a white veil covering some walls in prehistoric caves template for an acta imeko event paper acta imeko issn: 2221-870x april 2017, volume 6, number 1, 33-42 acta imeko | www.imeko.org april 2017 | volume 6 | number 1 | 33 metrological characterization of 3d biometric face recognition systems in actual operating conditions giovanni betta1, domenico capriglione3, mariella corvino1, alberto lavatelli2, consolatina liguori3, paolo sommella3, emanuele zappa2 1diei, university of cassino and of southern lazio cassino (fr), italy 2department of mechanical engineering, politecnico di milano, via la masa, 1, milano, italy 3diin, university of salerno, via giovanni paolo ii, 132, fisciano (sa), italy section: research paper keywords: face recognition; measurement uncertainty; 3d features; stereo vision; image classification citation: giovanni betta, domenico capriglione, mariella corvino, alberto lavatelli, consolatina liguori, paolo sommella, emanuele zappa, metrological characterization of 3d biometric face recognition systems in actual operating conditions, acta imeko, vol. 6, no. 1, article 6, april 2017, identifier: imekoacta-06 (2017)-01-06 section editor: paul regtien, the netherlands received may 31, 2016; in final form february 2, 2017; published april 2017 copyright: © 2017 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: the research project partially described in this paper has been partially funded by a prin grant by the italian ministry of education, university and research (miur) corresponding author: giovanni betta, email: betta@unicas.it 1. introduction facial recognition (fr) systems are well-known computer vision applications whose aim is to identify (or verify) a person upon the measurement of some selected facial features [1]. moreover, fr systems exploit digital images and photogrammetry to provide contact-less identification [2], [3]. those systems spread widely due to an increasing demand in the fields of security assurance and automatic identity verification [4] [6]. in fact, it is possible to find applications showing good performances and good maturity [7] when operating under constrained conditions, so that the person to be identified lies in a controlled environment and faces the camera(s) with a specified orientation. however, the extension of fr systems to the generic unconstrained environment (variable lighting, variable orientation of the person toward the camera, moving person…) is still problematic [8], [9], since abstract nowadays, face recognition systems are going to widespread in many fields of application, from automatic user login for financial activities and access to restricted areas, to surveillance for improving security in airports and railway stations, to cite a few. in such scenarios, the architectures based on stereo vision and 3d reconstruction of the face are going to assume a predominant role because they can generally assure a better reliability than solutions based on a single camera (which make use of a single image instead of a couple of images). to realize such systems, different architectures can be considered by varying the positioning of the pair of cameras with respect to the face of the subject to be identified, as well as both kind and resolution of camera considered. these parameters can affect the correct decision rate of the system in classifying the input face, especially in presence of image uncertainty. in this paper, several 3d architectures differing in camera specifications and geometrical positioning of the camera pair (with respect to the input face) are realized and compared. the detection of facial features in the images is made by adopting a popular method based on the active appearance model (aam) algorithm. 3d position of facial features is then obtained by means of stereo triangulation. the performance of the realized systems has been compared in terms of sensitivity to the quantities of influence and related uncertainty, and of typical indexes for the analysis of classification systems. main results of such comparison show that the best performance can be reached by reducing the distance between cameras and subject to be identified and by minimizing the horizontal angle between the plane containing the camera pair axis and the face to be identified. acta imeko | www.imeko.org april 2017 | volume 6 | number 1 | 34 person identification occurs with high uncertainty and low repeatability, which is the main problem in the forensic context. in order to tackle this critical issue, the research activity focused on the analysis of human perception with the aim of describing mathematically the processes that lead to the identification of a person [10] and then to implement in fr algorithms able to operate in uncontrolled environments [11]. the main principle of these algorithms is to focus on the analysis of a set of biometric features rather than on the analysis of the full 3d shape of a face. this is done by processing the image(s) of a person with suitable feature matching and extraction algorithms prior to stereo triangulation. the most of the biometric fr systems proposed in literature are based on the analysis of the appearance of frontal and nonfrontal faces acquired in different instants, without metrological issues [2], [7], [12]. therefore, it is not possible for those systems to deal with pose variation or the natural variation of face expression. conversely, the authors proposed a system that provides a fusion between the point-cloud 3d reconstruction and the pure image feature analysis by using a stereoscopic vision rig [13], [14]. the stereoscopic setup acquires synchronously from several different calibrated cameras the images of a person’s face, so that it is possible to triangulate the feature points and express the biometric characteristics in the 3d space using couples of cameras grabbing the face from different orientation. using different couples of synchronized stereo cameras, it is possible to evaluate the effect of the pose variability without the interaction of face expression variation, since images are acquired in synchrony from stereo pairs having different orientations towards the face. in any case, the process of image formation is yet complex and stochastic, so that the generic image of the face is affected by an uncertainty contribution that propagates throughout all the processing stages. in addition, the final classification result is uncertain, up to a point that it may be impossible to find a perfect matching between images of the same person acquired in different moments [11]. hence, there is a clear risk in accepting the generic classification results without a proper analysis of the uncertainty [15], [16]. in this context, the scientific literature proposes a good variety of approaches in order to limit or reduce the risk of an incorrect classification by the formulation of a priori uncertainty models [2], [8], [17] [20]. in this way, given a set of uncertainty sources, it is possible to solve the problem in a bayesan framework so that the most probable matching can be computed. however, these approaches have a limit since they can deal only with the supposed uncertainty sources, whereas the actual image can be characterized by other sources of uncertainty, which could be not included in the analysis model. this situation brings to a general overestimation or underestimation of uncertainty. nonetheless, uncertainty propagation is not performed in the way advised by the isogum standard. in addition, the evaluation of the performance of fr systems is still an open problem. typically, this process relies on the estimation of the recognition reliability from a given database of biometric images. furthermore, recognition reliability is judged with the help of synthetic indexes that express the probability of having a false positive or a false negative [16], [21], [22]. if perfectly run, this procedure is able to estimate the performance only for a given set of faces, thus it is hard to estimate the inference of such a result in the generic case. oppositely, the authors proposed in previous papers a metrological approach to the problem of characterizing fr systems [14], [23]. this has been done by analyzing the physical relations between the uncertainty in the final classification result and the uncertainty of input influence quantities [24]. the experimental activity demonstrated that the performance of these kinds of systems depends on several aspects from image acquisition to the classification procedure through the biometrical algorithm. the quantification of these uncertainty contributions lead the authors to formulate an original classification method which was able to improve the classification performance with respect to traditional scorebased approaches [25], [26]. even if the authors were able to propose an accurate metrology based method to evaluate the performances of fr systems, there are still open question in the design phase: what is the optimal layout of new fr systems? does the face recognition algorithm interfere with the selection of the optimal vision rig? in literature, different approaches have been presented [27] [29] to optimize the generic vision system, but only little is known on how to choose an architecture for a specific problem. in this scenario, the aim of this work is to compare, from a metrological point of view, different architectures of a 3d system for face recognition. in particular, starting from the approaches proposed in [23][26] for evaluating the confidence level of the classifier output, a detailed study of the influence on the metrological performance of different camera configurations (in terms of different position with respect to the subject), different kind of cameras (in terms of sensor and resolution), and different poses (in terms of subject face rotation with respect to the couple of cameras) is reported in this paper. the comparison is based upon the analysis of uncertainty in both feature extraction and the classification process. this study is claimed to provide useful indications for system designers, whose task in practical applications is to select the best compromise between the system performances (in terms of correct classification rate), geometrical constraints (i.e. vision rig arrangement) and hardware constraints (i.e. cost of optics or acquisition hardware). in the following, after a brief recall of the developed biometric system, the description of the processing for face recognition and the comparison of the metrological performance of different measurement configurations are shown. 2. the biometric system 2.1. system for the images acquisition as mentioned before, the image database was created with a peculiar multiple stereovision rig. the setup is described schematically in figure 1 and consists of an aluminum frame carrying a stereovision matrix composed by 3 stereo pairs having 3 different nominal orientations toward the face: 0°, 5° and 10°. the cameras of the first row of the matrix have an attitude of −22.5° towards the observer, while the second row cameras have an attitude of +22.5° towards the observer. this choice leads to a relative angle (incidence) of 45° between each camera of the pair, which is a good compromise between accuracy in depth measurement (which increases with increasing incidence [30]) and avoiding the risk of view occlusion (which, conversely, is more probable with higher incidence of the stereo pair [31]). in addition, a seventh camera has been positioned at mid-height (cam 7 in figure 1b), acta imeko | www.imeko.org april 2017 | volume 6 | number 1 | 35 between the two rows with zero orientation and zero attitude, to collect passport-like photos that can be used for the validation of 2d recognition techniques. the configuration of the vision rig allows the field of view size to be approximately 300~400 mm, that means face registration can be performed with the subject standing about 1000 mm away from the cameras. stereoscopic 3d point reconstruction is obtained by coupling the view of each top camera with the corresponding bottom camera. the cameras have the characteristics listed in table 1 and are all equipped with 25 mm focal length fixed optics. camera calibration is performed pair-by-pair with the well-known zhang method [32] by means of the camera calibration toolbox from j.y. bouget. image acquisition is performed with 3 ieee1394 pci adapters (for cameras 1 to 6) and a gigabit ethernet adapter (for camera 7). the synchronization of acquisition between the 7 cameras is provided by a common rising edge trigger source. exposure time and aperture are regulated in order to have similar luminance with similar depth of focus. with the help of a labview virtual instrument, the images of all cameras are simultaneously recorded in bitmap format. the control system acquires sequences of images with a user-defined interval (default set 5 s), in order to obtain multiple images of the same person in about the same position but with small variation of face expression that naturally happens between each capture. 2.2. biometric algorithm the biometric algorithm considered is composed by two main steps: a) segmentation of the biometric features in the images acquired by the upper and lower camera with the help of an active appearance model (aam) algorithm [33]. b) triangulation of the 2d points of the biometric features to retrieve a 3d mask representation of the biometric features. the main phase, a), is the segmentation of 2d features out of a generic image of a human face. the scientific literature proposes several ways to perform this task and a good review can be found in [1] and [2]. the authors chose the aam algorithm due to its effectiveness in matching both the shape and appearance of a given pattern [34]. the shape itself is defined as a 2d points set describing the geometry of a target body [35]. the aam algorithm is able to deal with shape modification since it uses a dynamic model of shapes in the form of principal component analysis (pca), so that any given shape 𝑥𝑆 can be expressed in the form: 𝑥𝑆 = �̅� + [𝜙] ⋅ 𝑏 , (1) where �̅� represents the average shape, [𝜙] represents the matrix of principal components [𝜙1|𝜙2| … |𝜙n] that express deformation modes and 𝑏 a vector of real numbers that set the model deformable shape parameters. the appearance is defined as the texture (a map of level of gray or color) of a portion of the target. the aam algorithm deals with appearance variation with appearance pca models that arrange all pixel intensity variations of the images around the mean shape. the pca formulation gives for the general appearance vector 𝑔𝑎: 𝑔𝑎 = �̅� + �𝜙𝑔� ⋅ 𝑏𝑔 , (2) where �̅� represents the average appearance, �𝜙g� represents the matrix of principal components �𝜙g,1�𝜙g,2� … �𝜙g,n] which express luminance modes and 𝑏g a vector of real numbers that set the models variable luminance parameters. shape and appearance models are based on the analysis of training images. so, once the model formulation is set, it is necessary to submit a sample of images where facial features have been manually annotated in order to define the principal components [𝜙] and �𝜙g�. the annotation process consists in tracing univocal landmarks that describe the most important facial traits on each image. in this work, the biometric description involves 58 landmarks that define the shape of jaw, mouth, nose, eyes, and eyebrows. the choice of the 58 points to be matched agrees with methodologies documented in other studies [36], [37], in order to be able to compare data. once the aam is trained, it is able to find biometric landmarks in the generic image of one’s face. given the high difference in attitude, it has been necessary to train a model for the upper camera and a model for the lower camera. the a) b) c) figure 1. design scheme of the vision rig used for fr purposes: a) upper view b) front view c) lateral view. table 1. characteristics of the vision equipment. camera id model sensor 1, 2 avt – pike f-145b 1388x1038 px color ccd progressive 3, 4, 5, 6 avt – marlin f-131b 1280x1024 px cmos sensor 7 ids – gige ui-5490se-m 3840x2748 px cmos sensor person 0° pair 5° pair 10° pair cam 1,7,2 cam 3,4 cam 5,6 cam 1 avt pike cam 3 avt marlin cam 5 avt marlin cam 7 ids ui cam 2 avt pike cam 4 avt marlin cam 6 avt marlin -22.5° attitude 22.5° attitude 0° attitude person cam 1,3,5 cam 2,4,6 cam 7 acta imeko | www.imeko.org april 2017 | volume 6 | number 1 | 36 software implementation is provided by an application programming interface dedicated to aam provided by [38], [39]. subsequently, for the generic face it is possible to match a set of points in the stereo pair and then to triangulate them with the epipolar constraint [28]. eventually a 3d mask of a face is recorded. figure 2 schematizes the main phases for the realization of the 3d mask. 2.3. the identity database the image acquisition system described before is then ready to be tested. the first step consists in the acquisition of a suitable database of facial images. the database was built by acquiring images of 117 volunteers, using the vision rig described in section 2.1. for each of the 117 individual three series of images have been recorded: one set with subject’s face oriented towards the point in the middle between cam1 and cam2 (0° neck rotation); one set with 10° neck rotation on the right hand side; one set with 20° neck rotation on the right hand side. neck rotation is controlled by asking the person to look toward a point that has been fixed at the wanted orientation with respect to the 0° neck rotation. in this sense, the values of neck rotation used in this work are to be considered as approximate values, with the only aim to get indications on the variation of the recognition accuracy with the neck rotation. each set contains 9 pictures of the same subject in the same position, acquired with a rate of 1 images every 5 s. in this way it is possible to record natural minor changes in head position and in facial expression and let the aam model cover them. on the whole, 162 images for every person have been acquired. the position of the person with respect to the vision rig, as well as the relative distance between cameras and face, has not been controlled. this was chosen on purpose, since the aim of this work is to evaluate the performance of the system in operating conditions: modern identity verification systems, in fact, do not require constraining the person to a rigid frame and most of them work in completely unconstrained environments [3]. as previously underlined, aam models should be trained properly upon a set of images, which is supposed to sample correctly the actual variability of face appearance. therefore, as the number of facial images used for the training increases, the aam model is able to cover a broader range of shape and texture variations. however, when the number of images used for training grows, the handling of such a great size of information becomes hard, the weight of random effects over deterministic phenomena becomes higher and the final reliability of face recognition becomes lower [13]. a good compromise has been found building a model using 200 images from 50 people belonging to the database of 117 individuals. note that images from all the couples of cameras and with the 3 different head rotation (0°, 10° and 20°) were used to create the model. it is not convenient to train the aam model with the same images that will be used in personal identification (i.e. to test the accuracy of the results). because of this reason, images used for aam model training were not included in the database used for the evaluation of the performances of the fr. 3. processing for face recognition 3.1. evaluation of scores in order to perform identity verification, the 3d mask of a person is compared with each of the 3d masks included in an identity database. the comparison outputs a value named score, which is computed by means of the weighted sum of squared differences between a collection of masks stored in an identity database. therefore, for the i-th mask stored in the database the i-th score, si is given by: 𝑆𝑖 = ∑ (𝑊𝑘∗(𝑉𝑘,𝑖−𝑉𝑘,𝑟𝑟𝑟) 2)𝑛𝑘=1 𝑛 , (3) where, vk,i are the coordinates of the k-th point for the i-th individual, wk is the weight of the k-th point of the mask. then, the score 𝑆𝑖 represents the sum of squared discrepancy between the 3d coordinates of the mask to be recognized and the corresponding coordinates of each mask in the database. prior to the evaluation of point-to-point distances (𝑉𝑘,𝑖 − 𝑉𝑘,𝑟𝑟𝑟), a roto-translation is computed in order to move the coordinate frame of one mask to be onto the coordinate frame of the other mask with a rigid motion. the roto-translation allows the compensation of differences in position and orientation of the subject with respect to the stereoscopic system in acquisition. the analysis of the variance of the identification parameters calculated on identifying the same person at different neck rotation conditions generates the weights 𝑊𝑘. the procedure for weight estimation is: images of the same individual are submitted to fr, for the 𝑘-th landmark the variance 𝑉𝑉𝑟𝑘 of figure 2. the schematic of biometric algorithm with featured detected on stereoscopic images and the obtained 3d mask. stereo acquisition segmentation biometric features in 3d space aam triangulation acta imeko | www.imeko.org april 2017 | volume 6 | number 1 | 37 the 3d coordinates is estimated (𝑉𝑉𝑟𝑘 is an estimate of identification uncertainty), the weight for the 𝑘-th landmark is defined as the inverse of 𝑉𝑉𝑟𝑘 [13]. 3.2. sources of score variability and bias as previously described, the evaluation of si (by means of (3)), results from a suitable sequence of operations performed starting on the input images. in particular, they are: i. 2d image acquisition; ii. 2d images processing by means of aam algorithm; iii. triangulation; iv. roto-translation. each one of these operations could introduce uncertainty and/or bias on the si estimate [24], [40]. as for step i, the acquisition conditions, in terms of luminance, lens defocus, presence of motion on the image itself (due for example to vibration of the system during the image acquisition or to the subject movement), and of face pose angle can modify the images with respect to ones contained in the identity database, thus changing the positions of the landmarks on 2d pair of images, and consequently on the resulting 3d mask. as an example, figure 3 shows such effects for the case of luminance, motion blur and lens defocus: the interpolation of the landmarks significantly changes as the motion blur or lens defocus is present, whereas it is weakly influenced by luminance. as a consequence, it is expected that si will be affected by such quantities of influence, in particular by motion blur and lens defocus. as an example, figure 4 reports the scores evaluated at different levels of lens defocus and when the subject #1 of the identity database and the pair cam1-2 are considered. for a sake of clarity, the scores evaluated considering only the first seven subjects (of the identity database) have been reported. as you can see, if the level of lens defocus increases, the value of the score, s1, increases, whereas the distances between the classes (subjects of the identity database) decrease, thus making worse the system performance in terms of correct decision rate. similar trends have been observed when other input subjects or other pairs of cameras (3-4 and 5-6) are considered. then, the image uncertainty propagates in the next step (step ii). with reference to the aam algorithm, the uncertainty in the building of the shape model and the appearance model [20] determines the accuracy of feature localization. since the weights, wk, of (3) are defined in the training phase on the basis of a number of a pair of images for each of all subjects, an intrinsic score variability should be considered to take into account such aspect. as an example, figure 5 reports, for a subject of the database, the nine images used for the training of the aam, together with the scores. the first image on the left in figure 5 is used as a reference. as for step iii, the triangulation is realized by means of the method proposed by zhang [27] and a residual error is mainly due to the non-ideality of the calibration phase. as for step iv, the roto-translation is made by means of an iterative procedure and the residual error is mainly linked to the noise on the 3d features of the masks. 3.3. uncertainty modelling for score the effects of triangulation (step iii) and roto-translation (step iv) are here modelled by means of residual systematic score greater than zero also for the correct class, whilst the measurement uncertainty introduces variability on the score of the correct class that strictly depends on the image acquisition condition (step i) and on the accuracy of aam (step ii). therefore, in the following, all the systematic effects are directly included in the measured score, whilst only the measurement uncertainty (due to the acquisition process and to the accuracy of aam) is considered. as for the uncertainty component related to step i, a statistical approach [26] is considered to estimate the uncertainty model corresponding to each quantity of influence. in particular, as for the quantities of influence luminosity, lens defocus and motion blur, new couples of artificial images are used to achieve further images characterized by the desired values of the quantities of influence. the new couples of images were generated by applying suitable digital filtering on the reference images (i.e. the ones contained in the identity database, hereinafter denoted as database a). for each pair of cameras, several values of variation (with respect to the reference image contained in the identity database) have been considered in order to generate the new images (see table 2). the variations were applied to luminosity (11 different values), lens defocus (7 different values) and motion blur (11 different figure 4. effects of the lens defocus on the scores. figure 3. effects of the acquisition conditions (the red arrows point the areas mainly affected by the quantity of influence): a) luminance, b) motion blur, c) lens defocus. a) b) c) acta imeko | www.imeko.org april 2017 | volume 6 | number 1 | 38 values). as for the image luminosity, the values considered are referred to a 8-bit grey scale, whereas, as for lens defocus and motion blur the values considered are referred to the standard deviation (in pixels) of the gaussian filter employed. on the contrary, as for the pose face angle, for each subject, the images acquired at different neck rotation (see section 2.3) were considered, obtaining the image types summarized in table 2. in this way new databases (one for each pair of cameras, 1-2, 3-4, and 5-6, hereinafter denoted as database b) of images designed for the uncertainty estimation are achieved. then, for each considered condition, the procedure adopted for uncertainty estimation for each pair of cameras is the following: for each subject of these new databases, the 3d features are evaluated applying the aam algorithm and the triangulation; the scores with respect to the recorded data in the identity database of the subject itself is estimated; on the scores obtained for all the subjects, a statistical analysis is made, and the uncertainty is estimated according to the iso-gum, by using the simple model described in the following equation. (𝑢𝑆)𝑙 = � �µ𝑆 2�𝑙 3 + (𝜎𝑆 2)𝑙 , (4) where, (us)l is the contribution due to the l-th quantity of influence on the score uncertainty, (µs)l and (σs)l are respectively the mean and the sample standard deviation of the measured scores related to the database b. then, for each quantity of influence, the identification of the relationship between (us)l and the value of l can be based on a large experimental analysis and on the fitting of simple models onto experimental data [26]. as for the uncertainty related to step ii, (us)aam, we pose: (𝑢𝑆)𝐴𝐴𝐴 = � �µ𝑆 2�𝐴𝐴𝐴 3 + (𝜎𝑆 2)𝐴𝐴𝐴 , (5) where (µs)aam and (σs)aam are respectively the mean and the sample standard deviation of all of the measured scores achieved for each subject when the nine images (related to the database a) are considered. as for the overall uncertainty on the scores, us, all of the quantities of influence are considered uncorrelated with the other ones, then the combined uncertainty on the score is evaluated as: 𝑢𝑆 = �∑ (𝑢𝑆)𝑙 2𝐴 𝑙=1 + (𝑢𝑆)𝐴𝐴𝐴 2 , (6) where m is the number of the considered quantities of influence. applying these models, the uncertainty of the scores is evaluated. the hypothesis made of uncorrelated quantities has been also verified through experiments. 3.4. classification procedure starting from the scores and the related uncertainty, us, estimated as described in the previous sections, the classification procedure provides the subject identification and the corresponding related confidence level (cl). the output of such a procedure is a classification list in which all the possible identified subjects are included with their cl. to these aims, at first, the probability that the input subject belongs to each j-th class, pj , is evaluated; then, on the basis of the obtained probability, the classification list is created with a selection of the probable classes; and finally, the confidence level of each class in the list is evaluated. in more detail, the decision algorithm (see figure 6) is composed by three steps [23], [25]: step a: for each subject, j , present in the database, the measured score, sj, is used together with the corresponding uncertainty, us, in order to evaluate the probability, pj, that the input subject is the subject j. in particular, pj represents the probability that the score of the j-th subject is equal to zero given a measured value (sj), considering the score as a random variable (see figure 6), whose standard deviation is equal to us [25]; b) figure 5. definition of wk: a) images acquired by camera 1 for subject #1, b) scores evolution (the first image of the subject is used as reference). table 2. values of variations of the quantities of influence with respect to the reference images of the identity database. quantity of influence values units luminosity [-30, -10, -3, 0, +3, +10, +30] grey levels lens defocus (σfilter) [0, 3, 5, 7, 10, 15, 20] pixels motion blur (σfilter) [0, 3, 9, 15, 17, 19, 23, 27, 30, 50, 70] pixels pose angle [0, 10, 20] degree 1 2 3 4 5 6 7 8 9 0 0.05 0.1 0.15 0.2 poses s co re s acta imeko | www.imeko.org april 2017 | volume 6 | number 1 | 39 step b: using all the estimated probabilities the classification list is created including only the subjects whose probability is greater than a threshold, th; step c: the confidence level, clj, of the subjects in the classification list is evaluated as clj=pj/k where k is equal to the sum of all the pj greater than th. as for the selection of th, it should be chosen as best trade-off between sensitivity and selectivity required by the specific application. indeed, it is expected that the higher is th, the higher is the selectivity and the lower is the sensitivity of the system. therefore, as an example, on the basis of the requirements of the application in terms of true acceptance rate (tar evaluated as true positive/all positive) and of the false acceptance rate (far evaluated as false positive/all negative) the value of th could be selected [26]. 4. performance comparison in this section, the comparison of the three considered system architectures (i.e. cam1-2, cam3-4, cam5-6) is reported. in particular, at first, the comparison of all uncertainty components involved in (4) and (5) is made. then, the recognition performance of the systems is presented by comparing suitable figures of merit, typically adopted for the analysis of classification systems [40]. 4.1. score uncertainty estimation in this section the uncertainty components related to the acquired 2d images (luminosity, lens defocus, motion blur and pose angle) and to the accuracy of aam are reported for each pair of cameras. in particular, figure 7 shows the value of (us)l calculated by means of (4) and table 3 reports the value of (us)aam estimated by means of (5). looking at figure 7 and table 3, some considerations can be drawn: for each pair of cameras, the image luminosity is the quantity of influence corresponding to the lowest uncertainty value ((us)lum), whereas the pose angle represents the quantity responsible of the highest uncertainty values ((us)angle). except for the pose angle, for each quantity of influence and whatever be its deviation from the corresponding value on the reference image, the lowest value of uncertainty is achieved with the pair cam1_2, whereas the pair cam5_6 shows always the highest value of uncertainty. these results are mainly due to the smaller distance (from the subject to be identified) and orientation of the pair cam1_2 with respect to the other pairs (see figure 1). as for the pose angle, pair cam1_2 shows the worst performance. these results are due to the different orientation of camera pairs from the subject to be identified. in particular, the pair cam1_2 is the most well aligned with the person to be identified, thus it results as the more sensitive to the pose angle. the uncertainty due to the aam algorithm, (us)aam, is comparable with the uncertainty due to the luminosity of the acquired images (us)lum, while it is lower than the a) b) c) d) figure 7. (us)l versus the quantities of influence: a) luminosity, b) lens defocus, c) motion blur, d) pose angle. the classification list is composed only by the subjects with a probability, pj, greater than a threshold, th. step a 𝐾 = ∑𝑃𝑗; 𝐶𝐶𝑗 = 𝑃𝑗 𝐾 ith subject with cli jth subject with clj mth subject with clm step b step c figure 6. main steps of the classification procedure. -30 -20 -10 0 10 20 30 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 luminance (grey levels) (u s ) l u m cam1-2 cam3-4 cam5-6 0 2 4 6 8 10 12 14 16 18 20 0 0.05 0.1 0.15 0.2 0.25 σfilter (pixels) (u s ) s f g cam1-2 cam3-4 cam5-6 0 10 20 30 40 50 60 70 0 0.05 0.1 0.15 0.2 0.25 σfilter (pixels) (u s ) m o t cam1-2 cam3-4 cam5-6 0 2 4 6 8 10 12 14 16 18 20 0 0.1 0.2 0.3 0.4 0.5 angle [°] (u s ) a n g le cam1-2 cam3-4 cam5-6 acta imeko | www.imeko.org april 2017 | volume 6 | number 1 | 40 uncertainty due to the other quantities of influence. 4.2. classification performance in order to evaluate and compare the overall classification performance of the three system architectures, the following figures of merit have been considered. correct classification (cc): the right class is identified with the highest cl value; abstention (ab): the right class is in the classification list but there are some other classes with the same cl, thus the system does not provide any decision; missed classification (mc): the classification list is empty; wrong classification (wc): either the correct class is not in the classification list or it is present without the highest value of cl. these performance indexes have been evaluated by considering the following values of th: 0.3, 0.5, 0.7. in this way, it is possible to compare the three system architectures for different degrees of selectivity and sensitivity. the recognition procedure has been run by considering the images of database b. table 4 compares the considered figures of merit for different values of th and camera pairs. as expected, whatever the pair of cameras, the value of th affects the values of the figures of merit. in particular: the lower is th, the lower is the mc percentage and the higher are the ab and wc percentages; the best performance in terms of cc is observed for the considered intermediate value of th. in addition, for a given value of th: even if pairs cam1-2 and cam3-4 are based on different hardware (see table 1), they show similar performance in terms of all the considered figures of merit. therefore, it seems that the type of sensor weakly influence the performance of the system. pair cam5-6 shows the best performance in terms of cc and mc only when th=0.7, whereas, it seems the worst solution for th=0.3 and th=0.5. therefore, since the pairs cam3-4 and cam5-6 share the same hardware (see table 1), the different performance can be imputable to their distances from the input subject (in the case of cam5-6 such a distance is the largest one). consequently, depending on the performance constraints required by the application or the trade-off to be satisfied, a designer/user can select the most suitable solution in terms of both arrangement of pair of cameras and value of th. 5. conclusions the paper has compared the metrological performance of different architectures for face recognition based on 3d features. the study has been conducted by considering a popular algorithm, the aam, for facial feature detection and then stereo triangulation for 3d position measurement and by considering the classification procedure proposed by authors in previous papers. the work took into account the main causes of uncertainty generally affecting the performance of face recognition systems with a direct impact on the reliability of the final decision in the classification stage in terms of correct classification, abstention, missed classification and wrong classification percentages. in particular, the following main results can be drawn: for the considered sensor resolutions the kind of hardware does not influence the metrological and the classification performance of the system; the distance between the subject (to be recognized) and the pair of cameras generally affects the metrological performance of the system. in particular, the pair cam1_2 shows slightly better performance in terms of uncertainty on the score due to the variation of image luminosity, lens defocus and motion blur (see figures 7a-7c); the horizontal angle between the plane containing the camera pair axis and the face to be identified, generally affects the metrological performance of the system in terms of uncertainty on the score (see figure 7d). such phenomenon is more evident for the pair cam1_2 because it is the pair most well aligned with the person to be identified, thus it is more sensitive to the pose; as for the classification performance, the best performance is shown by pairs cam1_2 and cam3_4 if low values of th are considered (i.e. highest selectivity and the lowest sensitivity), whereas the pair cam5_6 shows best performance if high values of th are considered. such results confirm that the distance between cameras and the subject affects the performance, and that a designer could also prefer the pair cam5_6 if the specific application constraints require the lowest selectivity and the highest sensitivity (i.e. a high value of th). the quantification of such kind of information could be very useful also for system designers and/or users, which, in practical applications, have often to select the best trade-off between camera arrangements and system performance. future developments will investigate the possibility of exploiting more pairs of camera at a time with the aim of fusing the information extracted, looking for improving the system classification performance. table 3. values of (us)aam for the pairs of cameras. cam1_2 cam3_4 cam5_6 (µs)aam 0.140 0.139 0.17 (σs)aam 0.006 0.005 0.02 (us)aam 0.080 0.080 0.10 table 4. performance comparison of the pairs of cameras for different th. th figure of merit cam1_2 cam3_4 cam5_6 0.3 cc [%] 84.7 82.3 70.9 ab [%] 3.3 3.6 3.4 mc [%] 1.0 2.3 2.3 wc [%] 11.0 11.8 23.3 0.5 cc [%] 88.0 85.7 87.5 ab [%] 1.1 1.1 1.6 mc [%] 7.6 10.0 6.2 wc [%] 3.3 3.1 4.6 0.7 cc [%] 77.7 76.8 84.2 ab [%] 0.2 0.5 0.7 mc [%] 21.8 22.3 14.4 wc [%] 0.3 0.3 0.7 acta imeko | www.imeko.org april 2017 | volume 6 | number 1 | 41 references [1] s. li e a. jain, handbook of face recognition, springer london, 2011. [2] d. yadav e a. bhattacharya, “face identification methodologies in videos”, communication technologies (gcct), 2015 global conference on, 2015. [3] a. danelakis, t. theoharis e i. pratikakis, “a survey on facial expression recognition in 3d video sequences,” multimedia tools and applications, vol. 74, n. 15, pp. 5577-5615, 2015. [4] d. white, p. jonathon phillips, c. hahn, m. hill e a. o’toole, “perceptual expertise in forensic facial image comparison,” proceedings of the royal society b: biological sciences, vol. 282, n. 1814, 2015. [5] z. sufyanu, f. mohamad e a. ben-musa, “a proposed integrated human recognition for security reassurance,” american journal of applied sciences, vol. 12, n. 2, pp. 155-165, 2015. [6] y. park, g. joung, y. song, n.-s. yun e j. kim, “a study on the safety management system of a passenger ship using biometrics,” journal of nanoelectronics and optoelectronics, vol. 11, n. 2, pp. 194-197, 2016. 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[16] bundesamt für sicherheit in der informationstechnik, “study: an investigation into the performance of facial recognition systems relative to their planned use in photo identification documents – biop i” public final report. available on line: http://www.bsi.bund.de. [17] m. de-la-torre, e. granger, r. sabourin e d. gorodnichy, “an adaptive ensemble-based system for face recognition in person re-identification,” machine vision and applications, vol. 26, n. 6, pp. 741-773, 2015. [18] a. punnappurath, a. rajagopalan, s. taheri, r. chellappa e g. seetharaman, “face recognition across non-uniform motion blur, illumination, and pose,” ieee transactions on image processing, vol. 24, n. 7, pp. 2067-2082, 2015. [19] y.-t. chou, s.-m. huang e j.-f. yang, “class-specific kernel linear regression classification for face recognition under lowresolution and illumination variation conditions,” eurasip journal on advances in signal processing, vol. 2016, n. 1, pp. 19, 2016. [20] a. fathi, p. alirezazadeh e f. abdali-mohammadi, “a new global-gabor-zernike feature descriptor and its application to face recognition,” journal of visual communication and image representation, vol. 38, pp. 65-72, 2016. [21] s. a. rizvi, p. phillips e h. moon, “verification protocol and statistical performance analysis for face recognition algorithms,” 1998. [22] p. wang, q. ji e j. wayman jr., “modeling and predicting face recognition system performance based on analysis of similarity scores,” ieee transactions on pattern analysis and machine intelligence, vol. 29, n. 4, pp. 665-670, 2007. [23] g. betta, d. capriglione, m. gasparetto, e. zappa, c. liguori e a. paolillo, “managing the uncertainty for face classification with 3d features,” in proceedings of 2014 ieee international instrumentation and measurement technology conference (i2mtc 2014), p. 412-417, montevideo, uruguay, 12-15 may, 2014. [24] g. betta, d. capriglione, m. corvino, m. gasparetto, e. zappa, c. liguori e a. paolillo, “metrological performance comparison of biometric system architectures for 3d face recognition,” xxi imeko world congress “measurement in research and industry”, september 4, 2015, prague, czech republic. [25] g. betta, d. capriglione, m. corvino, c. liguori e a. paolillo, “a proposal for the management of the measurement uncertainty in classification and recognition problems,” ieee transactions on instrumentation and measurement, vol. 64, n. 2, pp. 392-402, 2015. [26] g. betta, d. capriglione, m. gasparetto, e. zappa, c. liguori e a. paolillo, “face recognition based on 3d features: management of the measurement uncertainty for improving the classification,” measurement: journal of the international measurement confederation, vol. 70, pp. 169-178, 2015. [27] s. zhang, handbook of 3d machine vision: optical metrology and imaging, crc press, 2013. 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[34] g. edwards, t. cootes e c. taylor, “advances in active appearance models,” proceedings of the 1999 7th ieee international conference on computer vision (iccv'99), 1999, kerkyra, greece. [35] g. edwards, c. taylor e t. cootes, “interpreting face images using active appearance models,” 3rd ieee international conference on automatic face and gesture recognition, fg 1998, nara, japan. [36] e. zappa, p. mazzoleni e y. hai, “stereoscopy based 3d face recognition system,” 10th international conference on computational science 2010, iccs 2010; amsterdam, netherlands. acta imeko | www.imeko.org april 2017 | volume 6 | number 1 | 42 [37] e. zappa e p. mazzoleni, “reliability of personal identification base on optical 3d measurement of a few facial landmarks,” 10th international conference on computational science 2010, iccs 2010; amsterdam, netherlands. [38] m. stegmann, “the aam-api: an open source active appearance model implementation,” medical image computing and computer-assisted intervention, miccai 2003 6th international conference proceedings; montreal, canada, 2003. [39] m. stegmann, b. ersbøll e r. larsen, “fame a flexible appearance modeling environment,” ieee transactions on medical imaging, vol. 22, n. 10, pp. 1319-1331, 2003. [40] f. tortorella, an optimal reject rule for binary classifiers, lecture notes in computer science (including subseries lecture notes in artificial intelligence and lecture notes in bioinformatics), vol. 1876 lncs, 2014, pp. 611–620. metrological characterization of 3d biometric face recognition systems in actual operating conditions introductory notes for the acta imeko special issue on the xxx italian national congress on mechanical and thermal measurements acta imeko issn: 2221-870x june 2023, volume 12, number 2, 1 2 acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 1 introductory notes for the acta imeko special issue on the xxx italian national congress on mechanical and thermal measurements alfredo cigada1, roberto montanini2, andrea scorza3 1 dipartimento di meccanica, politecnico di milano, via la masa 120156 milano, italy 2 dipartimento di ingegneria, università degli studi di messina, c.da di dio 98166 villaggio sant’ agata, messina, italy 3 dipartimento di ingegneria industriale elettronica e meccanica, università degli studi roma tre, via vito volterra 62 00146 roma, italy section: editorial citation: alfredo cigada, roberto montanini, andrea scorza, introductory notes for the acta imeko special issue on the xxx italian national congress on mechanical and thermal measurements, acta imeko, vol. 12, no. 2, article 3, june 2023, identifier: imeko-acta-12 (2023)-02-03 received june 28, 2023; in final form june 30, 2023; published june 2023 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: francesco lamonaca, e-mail: editorinchief.actaimeko@hunmeko.org dear readers, the scientific knowledge and technological progress are based on the evaluation of physical quantities, therefore it should be not surprising that research activities focused on measurement methods and systems, as well as the culture of measurements, are matter of actual interest and consideration for the scientific and technical community. in particular, among the most significant research topics, mechanical and thermal measurements must be considered a science on its own and list a very wide range of applications concerning mechanical and thermal quantities: from the design and development of sensors to the characterization of measurement methods and systems, from processing of the measurement signal to the management and quality assessment of the measurement data, from sensor networks to applications in mechanical and thermal systems, materials and data science, automation and home automation, iot, transport, environment and cultural heritage, avionics, instrumentation for diagnosis and healthcare, safety, space instrumentation. the heterogeneity and specificity of all the abovementioned aspects find a lively and fruitful field of comparison and discussion in the yearly italian meeting of “forum nazionale delle misure” where new ideas and trends in the measurement science and technology are proposed together with the colleagues manly dealing with the electric and electronic measurements. in 2022 the forum was organized as in-person meeting and took place in september 2022 in brescia, italy. as usual, some joint meetings between scientists from the mechanical and thermal measurement and the electrical and electronic measurement academic groups have taken up as part of the workshop, providing inspiring discussions and feedback on the shared topics thanks to the different viewpoints and cultural background. moreover more specific topics have been covered in separate sessions, where in depth debates within the two groups have been carried out. this special issue collects a selection of 7 papers presented during the three days of the “forum”: the authors have been asked to review their work and prepare an extended version, fit for the publication on acta imeko. the heterogeneity of the topics covered in the selected works is coherent with the transversality and cultural richness of the mechanical and thermal measurements field, providing a lot of food for thought to the reader. in the paper ‘development and verification of self-sensing structures printed in additive manufacturing: a preliminary study’, a. quattrocchi and r. montanini [1] deal with the development of a demonstrative self-sensing structure, which was obtained by embedding a fbg sensor during the 3d stereolithographic (sla) printing process. they report the strategies developed in order to ensure a correct adhesion of the fbg sensor embedded into the structure and the experimental tests used for validating the structural response of the self-sensing specimen. the paper ‘development and metrological characterization of cementbased elements with self-sensing capabilities for structural health monitoring purposes’ by g. cosoli et al. [2] provides the results of the metrological characterization of different types of mortar with self-sensing capabilities, embedding sensing electrodes for electrical impedance measurement, thus proving their piezoresistive ability. the results were compared to standard measurements performed with traditional strain gages. alizzio et al. [3] of the research group of the mediterranean university of reggio calabria presented a paper entitled ‘iot enviromental quality monitoring in smart buildings in presence of measurement uncertainty: a decision making approach’, dealing with the project and implementation of a smart monitoring system, which acquires and analyses data collected by a network of distributed mailto:editorinchief.actaimeko@hunmeko.org acta imeko | www.imeko.org june 2023 | volume 12 | number 2 | 2 sensors and, through the setup and calibration of two decision making algorithms, activates forced ventilation if the level of comfort is below the desired threshold, taking also into account how measurement uncertainty affects the decision taken. the paper, ‘experimental analysis on the exploding wire process for nanopowder production: influence of initial energy and exploding atmosphere’ by caposciutti et al. [4], deals with exploding wire technique in nanoparticles production, exploring the effect of the input energy and the medium in which the explosion takes place on the final dimensional distribution of nanopowders. utilizing a proper power supply and an exploding system developed on purpose, they apply up to 10 ga/m² to copper wire samples immersed, in air or water, and collect the products for morphological analysis, performing a shape and dimensional characterization, based on a statistical analysis of the obtained particles. the paper entitled ‘extraction of a floor plan from a points cloud: some metrological considerations‘ by d’emilia et al. [5] addresses the procedure for extracting a floor plan from a points cloud acquired by means of a laser scanner, during the design of evacuation plans for safety purposes in a baroque cathedral. in the work some critical passages have been examined, presenting similar widths but different characteristics i.e., flat and regular geometries of the elements on the walls, in some cases, statues, columns and adornments, in some others. a completely different field of application concerns the paper ‘a principal component analysis to detect cancer cell line aggressiveness’, that reports some very last results obtained by the research group led by prof. rizzuto at the university of roma la sapienza [6]. the paper aims at providing the use of principal component analysis (pca) as a new post-processing method for the detection of breast and bone cancer cell lines cultured in vitro using a microwave biosensor. the results showed that the four analyzed cancer cell lines exhibited peculiar dielectric properties when compared to each other and to the growth medium, confirming that pca could be employed as an alternative methodology to analyze microwave characterization of cancer cell lines. in the end, the paper ‘sample volume length and registration accuracy assessment in quality controls of pw doppler diagnostic systems: a comparative study’, written by the research unit of the university of roma tre, led by prof. sciuto [7], deals with the performance assessment of clinical doppler ultrasounds, focusing on a comparative investigation based on a novel test parameter for the automatic analysis of faults in sample volume length and range gate registration accuracy in pulsed wave (pw) systems. the test parameter was assessed through an automatic method that postprocesses pw spectrogram images acquired at six sample volume depths with respect to the vessel radius. tests were repeated for three brand-new ultrasound diagnostic systems, equipped with convex and phased array probes, in two different working conditions. in the paper ‘monte carlo human identification refinement using joints uncertainty’, m. de cecco, a. luchetti, and m. tavernini [8] present a new method to re-identify the same individual among different people using rgb-d data. the process extracts a colour-based descriptor and a local feature descriptor through a monte carlo-based algorithm taking into account the uncertainty of human joints and, applied to each descriptor, refines the similarity match against a spatiotemporal database that updates over time. the robustness of the method makes it suitable also for industrial applications. we gratefully acknowledge all the authors who have contributed to this special issue, as well as all the reviewers. a special thanks goes to prof. francesco lamonaca, editor in chief of acta imeko, for his enthusiastic and precious help that supported this special issue. we are flattered to have helped our national research group in its growth thanks to this special issue and we look forward to a wider and qualified participation in the next editions of our forum. alfredo cigada, roberto montanini and andrea scorza, guest editors references [1] antonino quattrocchi, roberto montanini, development and verification of self-sensing structures printed in additive manufacturing: a preliminary study, acta imeko, vol. 12, no. 2, 2023, pp.1-7. doi: 10.21014/acta_imeko.v12i2.1431 [2] gloria cosoli, alessandra mobili, elisa blasi, francesca tittarelli, milena martarelli, gian marco revel, development and metrological characterization of cement-based elements with selfsensing capabilities for structural health monitoring purposes, acta imeko, vol. 12, no. 2, 2023, pp.1-12. doi: 10.21014/acta_imeko.v12i2.1420 [3] damiano alizzio, claudio de capua, gaetano fulco, mariacarla lugarà, valentina palco, filippo ruffa, iot enviromental quality monitoring in smart buildings in presence of measurement uncertainty: a decision making approach, acta imeko, vol. 12, no. 2, 2023, pp.1-8. doi: 10.21014/acta_imeko.v12i2.1421 [4] gianluca caposciutti, bernardo tellini, paola saccomandi, alfredo cigada, experimental analysis on the exploding wire process for nanopowder production: influence of initial energy and exploding atmosphere, acta imeko, vol. 12, no. 2, 2023, pp.1-9. doi: 10.21014/acta_imeko.v12i2.1426 [5] giulio d’emilia, luciano chiominto, antonella gaspari, stefano marsella, marcello marzoli, emanuela natale, extraction of a floor plan from a points cloud: some metrological considerations, acta imeko, vol. 12, no. 2, 2023, pp.1-9. doi: 10.21014/acta_imeko.v12i2.1427 [6] livio d’alvia, serena carraro, barbara peruzzi, enrica urciuoli, ludovica apa, emanuele rizzuto, a principal component analysis to detect cancer cell line aggressiveness, acta imeko, vol. 12, no. 2, 2023, pp.1-7. doi: 10.21014/acta_imeko.v12i2.1136 [7] giorgia fiori, gabriele bocchetta, silvia conforto, salvatore a. sciuto, andrea scorza, sample volume length and registration accuracy assessment in quality controls of pw doppler diagnostic systems: a comparative study, acta imeko, vol. 12, no. 2, 2023, pp.1-6. doi: 10.21014/acta_imeko.v12i2.1425 [8] mariolino de cecco, alessandro luchetti, mattia tavernini, monte carlo human identification refinement using joints uncertainty, acta imeko, vol. 12, no. 2, 2023, pp.1-11. doi: 10.21014/acta_imeko.v12i2.1423 https://doi.org/10.21014/acta_imeko.v12i2.1431 https://doi.org/10.21014/acta_imeko.v12i2.1420 https://doi.org/10.21014/acta_imeko.v12i2.1421 https://doi.org/10.21014/acta_imeko.v12i2.1426 https://doi.org/10.21014/acta_imeko.v12i2.1427 https://doi.org/10.21014/acta_imeko.v12i2.1136 https://doi.org/10.21014/acta_imeko.v12i2.1425 https://doi.org/10.21014/acta_imeko.v12i2.1423 acta imeko, title acta imeko issn: 2221-870x december 2016, volume 5, number 4, 1 acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 1 journal contacts about the journal acta imeko is an e-journal reporting on the contributions on the state and progress of the science and technology of measurement. the articles are mainly based on presentations presented at imeko workshops, symposia and congresses. the journal is published by imeko, the international measurement confederation. the issn, the international identifier for serials, is 2221-870x. editor-in-chief paolo carbone, italy honorary editor-in-chief paul p.l. regtien, netherlands editorial board section editors (vol. 4 and 5) leopoldo angrisani, italy filippo attivissimo, italy eulalia balestieri, italy eric benoit, france catalin damian, romania lorenzo ciani, italy pasquale daponte, italy luca de vito, italy luigi ferrigno, italy edoardo fiorucci, italy alistair forbes, united kingdom helena geirinhas ramos, portugal sabrina grassini, italy fernando janeiro, portugal konrad jedrzejewski, poland andy knott, united kingdom massimo lazzaroni, italy fabio leccese, italy rosario morello, italy michele norgia, italy pedro miguel pinto ramos, portugal sergio rapuano, italy dirk röske, germany alexandru salceanu, romania constantin sarmasanu, romania lorenzo scalise, italy emiliano schena, italy enrico silva, italy marco tarabini, italy eric benoit, france marcantonio catelani, italy paolo carbone, italy lorenzo ciani, italy spartacus gomariz, spain sabrina grassini, italy konrad jędrzejewski, poland min-seok kim, korea franco pavese, italy serge rapuano, italy paul regtien, the netherlands alexandru salceanu, romania jan saliga, slovakia krzysztof stepien, poland marco tarabini, italy ian veldman, south africa claudia zoani, italy final editing rosario morello, university mediterranea of reggio calabria, italy about imeko the international measurement confederation, imeko, is an international federation of actually 40 national member organisations individually concerned with the advancement of measurement technology. its fundamental objectives are the promotion of international interchange of scientific and technical information in the field of measurement, and the enhancement of international co-operation among scientists and engineers from research and industry. addresses principal contact prof. paolo carbone university of perugia dept. electr. inform. engineering (diei) via g.duranti, 93, 06125 perugia, italy email: paolo.carbone@unipg.it acta imeko prof. paul p. l. regtien measurement science consultancy (msc) julia culpstraat 66, 7558 jb hengelo (ov), the netherlands email: paul@regtien.net support contact dr. dirk röske physikalisch-technische bundesanstalt (ptb) bundesallee 100, 38116 braunschweig, germany email: dirk.roeske@ptb.de journal contacts microsoft word 294-2485-1-le-rev2 acta imeko  issn: 2221‐870x  november 2016, volume 5, number 3, 24‐31    acta imeko | www.imeko.org  november 2016 | volume 5 | number 3 | 24  neural network approach to reduce dynamic measurement  errors  andrei s. volosnikov, aleksandr l. shestakov  south ural state university, lenin prospekt 76, 454080 chelyabinsk, russian federation      section: research paper   keywords: dynamic measurement errors; neural network model; inverse sensor model; recovery of sensor input signal; dynamic measurement data  processing  citation: andrei s. volosnikov, aleksandr l. shestakov, neural network approach to reduce dynamic measurement errors, acta imeko, vol. 5, no. 3, article 5,  november 2016, identifier: imeko‐acta‐05 (2016)‐03‐05  section editor: franco pavese, italy  received october 7, 2015; in final form august 10, 2016; published november 2016  copyright: © 2016 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  corresponding author: andrei s. volosnikov, e‐mail: volosnikovas@susu.ru    1. introduction  the output of a measuring system may suffer from dynamic errors. an input signal recovery approach is presented that reduces these dynamic errors [1]. this problem is ill-posed and requires the numerical solution to the convolution integral equation [2], [3]. however, the analysis of measuring systems can be made in terms of the control theory [4], as well as of the theory of automatic control systems sensitivity [5], [6]. the automatic control theory approach effectively improves the dynamic measurement accuracy [7]-[9]. along with it, the artificial neural network (ann) approach to designing the dynamic models of measuring systems and algorithms for the data processing of dynamic measurements is a way of intelligent measuring systems development. recent publications reflect researchers’ interest in the matter of dynamic measurement error reduction. in [10] the double channel correction method is proposed. this method is developed for improving the dynamic properties of methane sensors on the basis of two measurement transducers. the method assumes that they are described by identical first-order models with different time constants. reference [11] deals with the dynamic compensation of a sensor approach based on an ann infinite impulse response filter. this filter represents the inverse model of the sensor that recovers the sensor input signal. in the present paper the ann inverse sensor model is also considered. however, this model is based on the ann finite response filter that ensures stability of the inverse sensor model. moreover, the proposed approach without reference to the order of the sensor model represents its inverse model as a special designed filter and a set of identical ann sections that implement only the inverse model of the first-order system. furthermore, the criterion of training sets formation and their length definition on the stage of the ann model adjustment is proposed. in section 2 of the paper the ann representation of the direct model of a sensor is discussed. on the basis of this representation in section 3 the ann inverse model of the sensor in three different representations is proposed. in section 4 results of experimental data processing are given in order to validate the efficiency of the proposed approach. finally, in the concluding section the major results are summarized. 2. neural network direct model of a sensor  suppose a primary measuring transducer (sensor) is described by the transfer function (tf) as follows: abstract  the neural network inverse model of a sensor with filtration of the sequentially recovered signal is considered. this model effectively  reduces the dynamic measurement errors due to deep mathematical processing of measurement data. the result of the experimental  data processing of a dynamic temperature measurement validates the efficiency of the proposed neural network approach to reduce  dynamic measurement errors.  acta imeko | www.imeko.org  november 2016 | volume 5 | number 3 | 25                       n lj j l j jjj m ki i k i iii s ptptpt ptptpt k pu py pw 1 1 1 11 22 1 1 2 1 22 22 2 0 112 112   , (1) where u and y are the sensor input and output signals respectively; jt1 and it2 are time constants; j1 and i2 are damping coefficients; mi ...,,2,1 , nj ...,,2,1 ( nm ); 0k is the static gain; p is the complex number frequency. the continuous tf(1) of the sensor can be represented in the following way:       0111 01 1 1 ... ... apapap bpbpbpb pu py pw n n n m m m m s        , (2) where  022 ,, ktbb iiii  for mi ...,,2,1 are coefficients depending on the parameters of (1) numerator and the static gain;  jjjj taa 11 , for nj ...,,2,1 are coefficients depending on the parameters of (1) denominator. the discrete analogue of the continuous tf(2) in general can be represented as follows:       nn n n s zzz zzz zu zy zw        ...1 ... 2 2 1 1 2 2 1 10 , (3) where )(zu and )(zy are the z-transformation of the sensor input and output signals, respectively;  taabb nmii ,,...,,,..., 100   for ni ...,,1,0 and  taabb nmjj ,,...,,,..., 100  for nj ...,,2,1 are coefficients depending on the parameters of (2) and the sampling period t . the difference equation corresponding to the discrete tf(3) of the sensor is as follows:         n j j n i i jkuikyky 01  , (4) where  ku and  ky are samples of the sensor input and output signals, respectively, at discrete times tktk  for ...,2,1,0k . the relationship between the output and input of the discrete sensor model is described by the following recurrence equation derived from the previous one:         n j j n i i jkuikyky 01  . (5) the parameters of the discrete model (3) can be determined on the basis of the ann direct sensor model shown in figure 1. this model is the recurrent ann [12] consisting of a single neuron with the linear activation function  netfa and the zero bias. the structure of this model is fully consistent with the recurrence equation (5). the recurrence equation that determines the relationship between input and output of the ann direct sensor model is as follows:           n j j n i ia jkuwikyvnetnetfky 01 ** , (6) where  ku and  ky* are samples of the sensor input signal and the output signal of the ann direct sensor model, respectively, at discrete times tktk  for ...,2,1,0k ; jw for nj ...,,1,0 and iv for ni ...,,2,1 are adjustable coefficients (weights) of the ann direct sensor model. by means of an appropriate procedure of the input and target training sets formation, that reflects the relationship between the input and output of the discrete sensor model (3), the weights of the ann direct sensor model can be adjusted during the training process so that the samples of the ann sensor model output will be equal to the respective samples of the continuous sensor model (1) output for a given level of the accuracy. the indicated possibility follows from the linearity and the conformity of the discrete and the ann direct models of the sensor. indeed, if    kyky * for ...,2,1,0k , then from (5) and (6) the following equality can be obtained:              n j j n i i n j j n i i jkuwikyv jkuiky 01 01  . (7) provided the sensor input is nonzero, the last equality becomes the identity only when jj w for nj ...,,1,0 and ii v for ni ...,,2,1 . the functional block diagram of the ann direct sensor model training is shown in figure 2. the procedure of the ann direct sensor model training (i.e. its weights adjustment) consists in minimization of the training error represented as the aggregate standard deviation for all 1n samples of the input training set )(khu between the target )(khy and actual )( * kh y outputs of the ann direct sensor model: figure 1. block diagram of the ann direct sensor model. acta imeko | www.imeko.org  november 2016 | volume 5 | number 3 | 26         n k yy khkh n e 0 2*1 . (8) 3. neural network inverse model of a sensor  the approach to the ann direct sensor model creation discussed in the previous section can be used for the solution to the problem of the sensor (1) dynamic measurement error reduction. then, this problem is formulated as the problem of the dynamically distorted sensor input signal recovery on the basis of the respective samples of its output signal. considering the indicated formulation it is necessary on the basis of the ann direct sensor model and the functional block diagram of its training to create the ann inverse sensor model and, similarly, the functional block diagram of its training. this ann inverse sensor model should provide the recovery of the dynamically distorted sensor input signal, i.e. implement the inverse relationship between the sensor input and output. 3.1. general representation  the discrete model (3) of the sensor is considered to obtain the structure of the ann inverse sensor model. this tf is cast to the inverse form as follows:           n n n n nn nn n n n n n n n n s zzz zzz zzz zzz zzz zzz zzz zzz zy zu zu zy zw                                                            ...1 ... ...1 ... 1 ... ...1 ...1 ... 2 2 1 1 2 2 1 10 0 2 0 21 0 1 0 2 0 21 0 1 0 2 2 1 10 2 2 1 1 1 2 2 1 1 2 2 1 10 1 1 (9) where 0 0 1    , 0   ii  , 0   ii  for ni ...,,2,1 . the difference equation corresponding to the inverse discrete tf(9) of the sensor is as follows:         n j j n i i jkyikuku 01  , (10) where  ku and  ky are samples of the sensor input and output signals, respectively, at discrete times tktk  for ...,2,1,0k . the relationship between the input and output of the inverse discrete sensor model is as the following recurrence equation derived from the previous one:         n j j n i i jkyikuku 01  . (11) the structure of (11) for the inverse discrete sensor model is the same as the structure of (5) for the direct discrete sensor model. therefore the structure of the ann inverse sensor model will also be the same as the structure of the ann direct sensor model. the block diagram of the ann inverse sensor model is shown in figure 3. this model is the recurrent ann consisting of a single neuron with the linear activation function  netfa and zero bias. the structure of this model is fully consistent with the recurrence equation (11). the recurrence equation that determines the relationship between the input and output of the ann inverse sensor model is as follows:           n j j n i ia jkywikuvnetnetfku 01 ** , (12) where  ky and  ku* are samples of the sensor output signal and the output signal of the ann inverse sensor model, respectively, at discrete times tktk  for ...,2,1,0k ; jw for nj ...,,1,0 and iv for ni ...,,2,1 are weights of the ann inverse sensor model. the criterion for the ann inverse sensor model training as in the case of the ann direct sensor model training is the minimum of the training error represented as the standard deviation between the target and actual outputs of the ann inverse sensor model. figure 2. functional block diagram of the ann direct sensor model training. acta imeko | www.imeko.org  november 2016 | volume 5 | number 3 | 27  obviously, both in the criterion and in the functional block diagram of the ann inverse sensor model training it is necessary to swap the input and target training sets towards the criterion and the functional block diagram of the ann direct sensor model training. this is the implementation of the inverse learning approach [13], applied to designing the dynamic control systems, in designing the dynamic measuring systems. 3.2. cascade representation  in order to avoid possible problems with the stability of the inverse model discussed, the cascade representation of the ann inverse sensor model in the form of firstand secondorder sections is proposed. this approach is based on the representation of the direct sensor model (1) in the following form of r second-order and s first-order cascades with real coefficients:         s j j r i is pwpwpw 1 1 1 2 , (13) where the discrete tf of the second-order cascade in general is as follows   2 2 1 1 2 2 1 10 2 1      zz zz zw ii iii i   (14) and the discrete tf of the first-order cascade in general is as follows:   1 1 1 10 1 1      z z zw j jj j   . (15) inverse tfs of cascades  pw i2 and  pw j1 derived analogically to (9) are as follows:   2 2 1 1 2 2 1 101 2 1       zz zz zw ii iii i   , (16)   1 1 1 101 1 1       z z zw j jj j   . (17) the functional block diagram of the ann inverse model of the sensor in the cascade representation is shown in figure 4 (where ][2 ic are sections implementing the inverse model (16) of second-order cascades  pw i2 for rj ...,,2,1 and ][1 jc are sections implementing the inverse model (17) of first-order cascades  pw j1 for si ...,,2,1 ). thus, the structure of the cascade ann inverse model of the sensor corresponds to the structure of the sensor in the cascade representation. therefore, each section is the ann inverse sensor model of the respective cascade in the structure of the sensor represented by (13). the block diagram of the ann second-order section ][2 ic is shown in figure 5 and the block diagram of the ann firstorder section ][1 jc is shown in figure 6. 3.3. sequential representation  the recovery of the input signal of the sensor (1) is implemented by its measured output signal processing on the basis of the ann inverse sensor model. this model is represented as the sequential connection of the correcting filter and identical first-order sections [14]. every such section is the ann inverse model of a first-order lti system with the following tf:   1 1 1 1   pt pw . (18) the value of the time constant 1t in (18) is set equal to such a value among the time constants jt1 in (1), that provides the proximity of the step responses of systems represented by (1) and (18). the tf )( pwcf of the correcting filter is the inverse tf of the sensor, which is supplemented with a certain number mnq  of tfs(18) to ensure the stability of the inverse model. the tf )( pwcf of the correcting filter is as follows: figure 4. functional block diagram of the ann inverse model of the sensor  in the cascade representation.  figure  3.  block  diagram  of  the  ann  direct  sensor  model  in  the  general representation.  acta imeko | www.imeko.org  november 2016 | volume 5 | number 3 | 28                    mn m ki i k i iii n lj j l j jjj q s q scf pt ptptpt ptptpt k pt pwpwpwpw                     1 1 112 112 1 1 1 1 1 2 1 22 22 2 1 1 1 11 22 1 0 1 1 1 1   . (19) the functional block diagram of the ann inverse sensor model in the sequential representation is shown in figure , where the first-order section  11 tc is the ann inverse model of the first-order lti system described by (18). generally speaking, the direct tf(18) can be represented in the discrete form (15) and the inverse one in the form (17). therefore, the structure of the section  11 tc , that implements the inverse tf(18), can be represented, in its turn, as it is shown in figure 7. however, taking into account that the order of the tf(18) numerator is always equal to zero unlike the order of the tf(15) numerator, the discrete analogue of the tf(18) can be represented as follows:   1 1 0 1 1   z zw   . (20) the inverse model of the system (20) is described by the following tf:   1 10 1 0 1 0 0 1 1 1 1 1 01 1 1 1 1                  zz z z zw         (21) where 0 0 1    , 0 1 1     . the block diagram of the ann section  11 tc that implements the inverse model (21) is shown in figure 8, where 0w and 1w are weights of the ann section  11 tc . it should be noted that the structure above represents an ann finite impulse response filter. the basic advantage of this structure is its guaranteed stability. therefore, it ensures stability of the ann inverse model of the sensor as a whole and provides stable the sensor input signal recovery. 3.4. noise cancellation approach  in practice, the recovery of the dynamically distorted input signal of the sensor on the basis of its ann inverse model is accompanied by the significant increase of the additive noise at the sensor output, as well as the internal noise of the ann inverse sensor model. for the correct recovery of the input signal of the sensor it is expedient to extend the ann inverse sensor model, taking into account the presence of the additive noise at the sensor output. this extension can be implemented as the additional low-pass filtration of the recovered signal by means of an increase of the order of the sequential sections  11 tc in the structure of the ann inverse sensor model. the block diagram of the ann d -order section  1tcd with the filtration of the recovered signal is shown in figure 9. the discrete tf of the section  1tcd is as follows: figure 7. functional block diagram of the ann inverse model of the sensor in the sequential representation.  figure 8. block diagram of the ann section c1[t1].  figure 5. block diagram of the ann second‐order section c2[i].  figure 6. block diagram of the ann first‐order section c1[j].  acta imeko | www.imeko.org  november 2016 | volume 5 | number 3 | 29    ddcd zwzwwzw   ...110 , (22) where dwwww ,...,,, 210 weights of the section. the cancellation of additive noise amplified during the sensor input signal recovery is implemented due to the presence of the internal filter in the structure of the ann section  1tcd . the discrete tf of the internal filter in the structure of the ann section  1tcd is described by the following equation:      zwzwzw cdfd 1 , (23) where  zw1 is the discrete analogue (20) of (18). it should be noted again that the structure of the section  1tcd and hence its internal filter  zwfd represents an ann finite impulse response filter. therefore, it ensures stability of the ann inverse model of the sensor as a whole and simultaneously provides stable sensor input signal recovery and filtration. 3.5. training procedure and training sets composition  the block diagram of the ann section  1tcd training is shown in figure 10. the procedure of the ann section training (i.e. its weights dwwww ,...,,, 210 adjustment) consists in the minimization of the training error represented as the aggregate standard deviation for all 1n samples of the input training set  kh y between the target  khu and actual  khu* outputs of the ann section  1tcd :        n k uu khkh n e 0 2*1 . (24) a criterion for the training sets composition is proposed. this criterion allows to evaluate the training sets length 1n . in order to implement the filtration ability the sinusoidal smoothing of the ann section  1tcd step response was applied. the filter (23) bandwidth regulation is achieved by the section order d adjusting. on this basis, the samples of the target training set are composed according to the following equation (where t is the sampling period and nk ...,,1,0 ):                          nkd dk d kt khu ,1 0, 2 22 sin1 )(  . (25) then, the input training set should be composed from the step response samples of (18) as follows:      11 /exp1/exp1 ttkttkh ky  , (26) where tktk  are discrete times for nk ...,,1,0 . suppose, provided 1 , starting with time tnth  all the successive samples of the tf(18) step response lie within the following range:   1h . (27) then, the training sets can be composed in accordance with (25) and (26). the sensor step response as the source signal for the input training set composition underlies the definition of the required length of training sets  kh y and  khu . this consequence follows from the analysis of (24) for the ann section  1tcd training in terms of the problem considered. the limiting value of the training error, where the length of the training sets approaches infinity, is as follows: figure 10. block diagram of the ann section cd[t1] training.  figure 9. block diagram of the ann section cd[t1].  acta imeko | www.imeko.org  november 2016 | volume 5 | number 3 | 30        2 00 2 0 * 2100 1 1 lim ...,,,,limlim                             d i i n k d i uiu n d nn wikhwkh n wwwweee (28) therefore, under conditions defined by (27) the error of the ann section  1tcd training will lie within the following range:          202 2 0 2 2 0 0 0 2 0 0 2 0 111 11 1 11 1 1                                                ew w n w n w n d j i n k d i i n k d i i n k d i ihhe . (29) thus, (29) shows the direct relationship between the deviation of the training error from its limiting value and the training sets length 1n . 4. results of experimental data processing  an algorithm for the recovery of dynamically distorted signals on the basis of the proposed ann inverse sensor model was developed. in order to validate experimentally the efficiency of the model and the algorithm, a dynamic measurement of temperature was made. the step response of the thermoelectric transducer (thermocouple) «metran-281» by heating it from 0 °с to 800 °с was obtained. the result of the experimental data processing at d = 66 in the form of the plots of the thermocouple measured output  ty and the thermocouple recovered input  tu* is shown in figure 11. the obtained result shows that the dynamic measurement error after the correction decreased by 40 % in comparison with the initial state without any additional correction. in addition, the time of the dynamic temperature measurement decreased from st = 306 s to dt = 60 s, that is more than 5 times. these evaluations validate the efficiency of the proposed ann approach to dynamic measurement error reduction. 5. conclusions  the ann approach to the recovery of dynamically distorted signals effectively reduces the dynamic measurement errors caused by the inertia of the sensor and the additive noise at its output. the considered ann inverse model of a sensor with filtration of sequentially recovered signal effectively improves the sensor dynamic behaviour due to deep mathematical processing of measurement data. a criterion for the training sets composition for the ann inverse model adjustment is proposed. this criterion helps to evaluate the length of training sets on the basis of the required training error. the results of the experimental data processing validate the considerable improvement of the sensor dynamic behaviour due to the application of the proposed ann approach to the dynamic measurement of the temperature. the future work direction is to develop the dynamic measurement error estimator on the basis of the ann approach. it is expected that the additional information obtained from the error estimator will optimize the adjustment procedure of the ann inverse model. references  [1] v.a. granovskii, “models and methods of dynamic measurements: results presented by st. petersburg metrologists, in: advanced mathematical and computational tools in metrology and testing x. f. pavese, w. bremser, a. chunovkina, n. fischer, a.b. forbes (editors). world scientific publishing company, singapore, 2015, isbn 978-9-81-467861-2, pp. 29–37. [2] a.n. tikhonov, v.y. arsenin, solution of ill-posed problems, v.h. winston & sons, washington, 1977, isbn 978-0-47099124-4. [3] yu.p. petrov, v.s. sizikov, well-posed, ill-posed, and intermediate problems with applications (inverse and ill-posed problems), vsp international science publishers, leiden, 2005, isbn 978-9-06-764432-7. [4] s. engelberg, a mathematical introduction to control theory, imperial college press, london, 2005, isbn 978-1-86-094570-0. [5] e. rosenwasser, r. yusupov, sensitivity of automatic control systems, crc press, boca raton, 2000, isbn 978-0-84-9322938. [6] m. eslami, theory of sensitivity in dynamic systems: an introduction, springer-verlag, berlin, 1994, isbn 978-3-66201634-3. [7] a.l. shestakov, dynamic error correction method, ieee transactions on instrumentation and measurement, 45 (1996) pp. 250–255. [8] a.l. shestakov, theory approach of automatic control in dynamic measurements: monograph, susu publishing, chelyabinsk, 2013, isbn 978-5-69-604468-2 (in russian). [9] a.l. shestakov, “dynamic measurements based on automatic control theory approach”, in: advanced mathematical and computational tools in metrology and testing x. f. pavese, w. bremser, a. chunovkina, n. fischer, a.b. forbes (editors). world scientific publishing company, singapore, 2015, isbn 978-9-81-467861-2, pp. 66–77. [10] r. bogacz, b. krupanek, “double channel dynamic error correction of methane sensor”, proc. of 21st imeko world congress, aug. 30 – sept. 4, 2015, prague, czech republic, pp. 1764-1767. [11] z. zhang, y. li, “correction of signal distortion caused by sensor’s dynamic characteristic”, proc. of 21st imeko world congress, aug. 30 – sept. 4, 2015, prague, czech republic, pp. 1024-1029. [12] s. haykin, neural networks: a comprehensive foundation (2nd edition), prentice hall, new jersey, 1999, isbn 978-0-13273350-2. figure 11. result of experimental data processing.  acta imeko | www.imeko.org  november 2016 | volume 5 | number 3 | 31  [13] j.-s.r. jang, c.-t. sun, e. mizutani, neuro-fuzzy and soft computing: a computational approach to learning and machine intelligence, prentice hall, new jersey, 1997, isbn 978-0-13261066-7. [14] a.s. volosnikov, a.l. shestakov, “dynamic measurements error correction on the basis of neural network inverse model of a sensor”, proc. of 21st imeko world congress, aug. 30 – sept. 4, 2015, prague, czech republic, pp. 2082-2087. microsoft word 0 acta imeko  issn: 2221‐870x  april 2016, volume 5, number 1, 1    acta imeko | www.imeko.org  april 2016 | volume 5 | number 1 |  1  journal contacts    about the journal  acta imeko is an e-journal reporting on the contributions on the state and progress of the science and technology of measurement. the articles are mainly based on presentations presented at imeko workshops, symposia and congresses. the journal is published by imeko, the international measurement confederation. the issn, the international identifier for serials, is 2221-870x.      editor‐in‐chief  paolo carbone, italy  honorary editor‐in‐chief  paul p.l. regtien, netherlands    editorial board      leopoldo angrisani, italy filippo attivissimo, italy eulalia balestieri, italy eric benoit, france catalin damian, romania pasquale daponte, italy luigi ferrigno, italy edoardo fiorucci, italy alistair forbes, united kingdom sabrina grassini, italy fernando janeiro, portugal konrad jedrzejewski, poland andy knott, united kingdom fabio leccese, italy david macii, italy rosario morello, italy michele norgia, italy helena geirinhas ramos, portugal pedro ramos, portugal sergio rapuano, italy dirk röske, germany alexandru salceanu, romania constantin sarmasanu, romania diego scaccabarozzi, italy lorenzo scalise, italy emiliano schena, italy enrico silva, italy marco tarabini, italy susanne toepfer, germany rainer tutsch, germany ian veldman, south africa luca de vito, italy final editing    rosario morello, university mediterranea of reggio calabria, italy   about imeko  the international measurement confederation, imeko, is an international federation of actually 39 national member organisations individually concerned with the advancement of measurement technology. its fundamental objectives are the promotion of international interchange of scientific and technical information in the field of measurement, and the enhancement of international co-operation among scientists and engineers from research and industry.       addresses  principal contact  prof. paolo carbone university of perugia dept. electr. inform. engineering (diei) via g.duranti, 93, 06125 perugia, italy email: paolo.carbone@unipg.it    acta imeko  prof. paul p. l. regtien measurement science consultancy (msc) julia culpstraat 66, 7558 jb hengelo (ov), the netherlands email: paul@regtien.net    support contact  dr. dirk röske physikalisch-technische bundesanstalt (ptb) bundesallee 100, 38116 braunschweig, germany email: dirk.roeske@ptb.de microsoft word article 4 155-1289-1-le.docx acta imeko  february 2015, volume 4, number 1, 11 – 18  www.imeko.org    acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 11  utilizing arbitrary waveform generators to produce noise  with imposed spectral characteristics   l. angrisani, m. d’apuzzo, m. d’arco, e. napoli, a. strollo   dipartimento di ingegneria elettrica e tecnologie dell’informazione, università di napoli federico ii, via claudio 21, 80125 napoli, italy      section: research paper   keywords: colored noise, filter design, digital‐to‐analog conversion, spectral spurs, welch periodogram.  citation: l. angrisani, m. d’apuzzo, m. d’arco, e. napoli, a. strollo, utilizing arbitrary waveform generators to produce noise with imposed spectral  characteristics, acta imeko, vol. 4, no. 1, article 4, february 2015, identifier: imeko‐acta‐04 (2015)‐01‐04  editor: paolo carbone, university of perugia   received november 12 th , 2013; in final form april 14 th , 2014; published february 2015  copyright: © 2014 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: this work was supported by the science and technology division of the university of naples ‘federico ii’ through the program faro 2012.  corresponding author: mauro d’arco, e‐mail: darco@unina.it    1. introduction  noise has been extensively studied during the twentieth century. the first model that has been proposed to describe its randomness is the white noise model, which is, at present, widely used for analysing the behaviour of electronic systems. sources that produce analogue white noise are also available on the market, and dedicated instruments that analyse the effects of the noise on the behaviour of complete electronic systems, but also single devices and components, are currently realized and marketed [1]-[3]. noise sources are regularly employed in applications such as: the measurement of the frequency response of linear systems, the measurement of noise figures, the functional test of microwave and optoelectronic links. for instance, very fast full frequency response measurements can be gained using thermal noise, which is characterized by a flat spectrum that agilely spans whichever band of interest. the use of noise sources in conjunction with a calibrated instrument allows the measurement of noise figures of delicate equipment, such as mixers or receiver front ends. radar and digital communication links are intermittently verified by means of built-in noise sources that are activated during off-times [4]; alternatively, the noise can be superimposed to signals including modulated data to perform measurements of the bit error rate (ber) curves also during on-times. the performance of optoelectronic systems that transmit data through fiber cables is also tested using noise sources in conjunction with phase modulators and oscillators to produce jitter in test signals [5]-[6]. for advanced encryption applications, robust random occurrences of digital codes can be produced by sampling a voltage generated by a real noise source: these codes require much more efforts to be cracked by hackers abstract  the reliability and performance of many systems mostly depend of their noise rejection capability. the behavior of systems that are  prone to noise is therefore investigated since the early production stage, as well as routinely during periodical maintenance actions.  actually,  there  are  very  few  noise  sources  available  on  the  market,  the  majority  of  which  is  capable  of  generating  only  noise  characterized by a uniform power spectral density in a limited bandwidth; generators capable of producing colored noise are instead  very rare. in theory, arbitrary waveform generators could be deployed and suitably programmed to serve as noise sources. but, the  technicians  charged  of  test  and  measurement  assignments  will  barely  find  application  notes  that  support  them  in  stepping  from  theory to practice.  this  work  presents  tutorial  notes  related  to  the  simulation  of  colored  noise,  and  proposes  an  analog  generator  that  exploits  an  arbitrary waveform generator as noise source. the proposed generator is capable of producing noise signals characterized by arbitrary  power spectral densities.  acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 12  with respect to the pseudorandom codes produced by calculus schemes [7]. finally, noise sources play an interesting role also in the enhancement of analogue-todigital converters performance [8] and in data encryption applications [9]. in fact, noise permits to enhance the linearity and the dynamic range of high-speed digital-toanalogue converters by means of dithering, which consists in adjoining noise characterized by spectral contents residing outside the band of interest [10]-[12]. in order to straightforwardly characterize noise and its effects common scalar noise metrics such as noise factor (nf), equivalent noise temperature (te), noise power (pn), yfact factor, and excess noise ratio (enr) are utilized in research and industrial applications. all these metrics are not independent form each other; for instance the equivalent noise temperature te is a function of the noise factor nf, as well as the excess noise ratio combines the yfact factor and noise factor nf. besides the aforementioned metrics, an exhaustive characterization of noise can be gained by measuring the power spectral density (psd). the psd of any random process, such as noise, is defined as the fourier transform of its autocorrelation function, anyway in practice, it is usually gained through more straightforward approaches. in particular, it is measured by means of spectrum analysers or fft-based instruments, which provide the fourier transform of a portion of the noise. it is worth noting that many natural phenomena as well as many mechanisms that take place in modern systems produce noise that exhibits some different characteristics with respect to white noise, which is commonly referred to as coloured noise [13]. for many mechanisms that produce coloured noise, and especially for the most recently discovered ones, practical descriptive models are unavailable. also, the classical measurement approaches to characterize noise features are criticized. the calculation of scalar metrics such as the mean square value of the coloured noise could be not realistic as it happens in the presence of flicker or random walk noise [14]. nonetheless, fft-based approaches can be adopted only in the presence of white noise whereas produce biased results when applied to coloured noise. actually, the use of fft analysers to measure noise spectral features is critical because they suffer from spectral leakage, which can be controlled only in the presence of repetitive signals, i.e. signals characterized by a discrete number of sinusoidal components. in these cases, coherent sampling or, alternatively, windowing can be used to counteract spectral leakage and obtain correct estimations of the magnitude of each component. specifically, to get rid of spectral leakage, coherent sampling requires the acquisition and processing of a record of points including an exact number of cycles of the input signal; windowing instead provides the necessary tolerance for accurately measuring the magnitude of spectral components by reducing the broadband leakage at the expense of the narrowband one. unfortunately, in the presence of noise, there are no means to counteract spectral leakage, because neither coherent sampling condition exists, nor windowing would be effective: any trade-offs between broadband and narrowband spectral leakage would not compensate bias effects. at the state of art, the most advanced solution to evaluate the spectral features of noise signals relies on the use of welch’s averaged, modified periodogram method. the welch method is an fft-based approach that involves averaging the results attained for partially overlapped segments of the input signal; averaging should mitigate bias effects and improve measurement accuracy and repeatability. all the aforementioned issues related to applications, metrics, models and measurement techniques make plain why the use of coloured noise appears in theoretical approaches whereas, due to the superior complexity of a coloured noise generator, it is not common in experimental tests [15]-[16]. the paper shows how synthesizing digital coloured noise and, successively, how generating it into analogue form by exploiting the potentialities of arbitrary waveform generators [17]. the reader is thus provided with practical guidelines for the design of a programmable noise generator, which allows the user to choose the psd of the noise he wants to produce. the proposed approach can be utilized anytime the equipment under test has to be stimulated by a signal with power residing in assigned frequency intervals and characterized by an arbitrary power spectral density. the main applications consist of: noise sensitivity analysis, characterization of sensors and actuators, and identification of linear and time invariant systems. 2. noise with imposed characteristics  different techniques to synthesize discrete-time coloured noise sequences have been proposed in the recent past. the proposed one shares with other existing techniques the basic approach of deriving coloured noise according to a general and well-known technique, namely, by filtering white noise with a linear and time invariant filter. the underlying theory is well known: the power spectral density of the output noise is shaped by the frequency response of the adopted filter. for input noise characterized by a uniform power spectral density, the psd of the output noise coincides with the squared modulus of the filter frequency response. therefore, to obtain a discrete random signal y(n) with a pre-assigned real power spectral density syy(), the input white noise w(n) is chosen as to have a unitary power density sww(), and the frequency response h() = |h()|e-j() of the adopted filter as to satisfy:     yysh  (1) specifically, the proposed generator relies on the use of a filter with finite-impulse response. the user describes the shape of the frequency response of the filter, h(), through a frequency sampling approach. in other terms, the user designs the filter specifying the number of coefficients of the filter, the bandwidth b in which he wants to confine the acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 13  noise, and two short arrays of equal length, namely, a frequency array, freq, and a magnitude array, mag. the frequency array contains in ascending order frequency values within (0, 1) that are normalized values with respect to the bandwidth b. the magnitude array expresses the gain of the filter at the normalized frequency values given in freq. the short arrays typically contain a small number of elements, much less than the number of coefficients that the user may require to match precision goals. they serve to describe in a schematic way the desired psd. the shape of the frequency response of the filter is then calculated through linear interpolation between the gain values defined in the magnitude array for a number of normalized frequencies equal to one half of the selected filter length. the phase response () of the filter cannot be arbitrarily chosen but has to satisfy the constraints of physical realization. as a rule, a linear phase response that introduces a suitable amount of delay can assure the physical realization. among the different possibilities the delay can conveniently be chosen to avoid the use of complex arithmetic for the synthesis of the noise. the procedure to synthesize the discrete-time noise with an imposed power spectral density can be definitively summarized as: • selection of the number of coefficients, 2l, of the shaping filter; • choice of the bandwidth b that the psd of the noise has to occupy; • definition of the frequency and magnitude arrays, freq and mag, that specify the psd of the target noise within the chosen bandwidth; • calculation through interpolation of the l-sequence of the values, {|h(0)|, |h(1/l)|, |h(2/l)|, ... |h(l/2l)|}, i.e. the values for the normalized frequency values, , within the interval (0, 1/2); • multiplication by -1 of the samples of the aforementioned l-sequence corresponding to odd normalized frequencies, |h(1/l)|, |h(3/l)|, |h(5/l)|, …; this operation confers to the filter the linear phase response:    ljj 2 (2) • specification of h() in terms of the 2l sequence {h(0), h(1/l), h(2/l), ... h(2l 1/2l)} which gives the frequency response for normalized frequency values  within the interval (0, 1). to this end the available l sequence is prolonged through mirroring, which preserves the required hermitian symmetry of the frequency response: samples at normalized frequencies (l + i)/2l must have the complex conjugate values of those at normalized frequencies (l i)/2l, i ranging from 1 to l 1. • calculation of the impulse response of the filter, h(n), through the inverse discrete fourier transformation of the aforementioned 2l-sequence. as commentary, it is worth noting that the proposed filter is characterized by a frequency response expressed by coefficients which are all real numbers: no complex values are processed. the filter, being a finite impulse response filter, is inherently stable, thus the output never diverges. the causality of the filter is granted at the mere expense of an output delay: the first output sample is available after l input samples. the sample rate related to the obtained noise signal is exactly twice its bandwidth. as an example, figure 1 shows portions of both a white noise and a coloured noise obtained filtering the former by figure 3. power spectral density of the discrete noise estimated by means of the welch periodogram. the logarithmic scale makes the triangular function  appear as a bell‐shaped curve. the residual noise  in the digital frequency  intervals (0, 0.30) and (0.70, 1), in which it is expected to be negligible, is at least 90 db below the maximum power density level at 10 db.  figure 1. graphics of the white noise (dotted line) and coloured noise (solid  line) obtained by means of the shaping filter.  figure 2. frequency response in amplitude of the filter adopted to achieve  coloured  noise  by  filtering  white  noise.  the  filter  is  defined  by  the  short  digital  frequency  array  freq  =  [0  0.30  0.50  0.70  1]  and  magnitude  array mag = [0 0 1 0 0]; the  length of the filter  is 2l = 512. the filter assigns a  triangular shape to the power spectral density of the noise; the power of  the  noise  should  be  zero  in  the  digital  frequency  intervals  (0,  0.30)  and (0.70, 1) and reaches a maximum value at 0.50.  acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 14  means of a shaping filter. the designed filter has the frequency response in amplitude given in figure 2. it is defined by the short digital frequency array freq = [0 0.3 0.5 0.7 1] and magnitude array mag = [0 0 1 0 0], and gives to the psd of the noise a triangular shape, characterized by symmetrical transition bands in the intervals (0.3, 0.5) and (0.5, 0.7), and stop bands in the intervals (0, 0.3) and (0.7, 1). figure 3 shows the psd of the discrete noise on a logarithmic scale as estimated by means of the welch periodogram. note that the logarithmic scale makes the triangular psd appear as a bell-shaped curve. the power level of the residual noise in the digital frequency intervals (0, 0.3) and (0.7, 1), in which it is expected to be negligible, is at least 90 db below the maximum power density level, which is equal to 10 db, when the length of the filter is 2l = 512 taps. the design of the filter could be optimized by windowing the obtained impulse response with an opportune window. there are many types of window such as: bartlett, blackman, chebyshev, gaussian, hamming, hanning, that could be thought of. anyway, it is not convenient to perform optimization at this level, because when the discrete sequence is played in order to produce the analogue noise signal, the features of the synthesized noise are unavoidably modified, as it is illustrated in the next section. 3. generating noise into analogue form  in order to realize a programmable noise generator, i.e. a noise generator that allows the user to specify the psd of the output noise, an arbitrary waveform generator can be exploited. as well known, arbitrary generators read from a waveform memory a stream of digital data that is given in input to a digital-to-analogue converter (dac). the dac operates as a clocked zero-order-hold circuit, hence the analogue noise signal is expected to be as a piece-constant waveform (in other terms the generator keeps constant any voltage level for a time interval equal to the duration of the clock period, and at the occurrence of the clock pulse performs a step change of the output voltage) [18]-[19]. the piece-constant nature of the analogue signal should not be jeopardized since it involves some undesired effect. in particular, aliases of the spectral contents of the analogue output are expected to appear in the frequency intervals adjacent to the band of interest and centred at multiple values of twice the selected bandwidth b. to counteract the undesired effects of aliasing, which are inherent to the use of a zero-order hold circuit, the use of smoothing analogue filters is regularly deployed. all arbitrary waveform generators are equipped with one or, in the case of more expensive models, a few internal low-pass filters, which can be selected according to the particular waveform to be played. when the noise is filtered the step changes in the output voltage are smoothed and exhibit a finite rise time. the transient that characterizes the voltage change depends both on the type and the parameters of the filter. anyway, the number of available filters inside the generator cannot satisfy every need: often, the smoothing effect can be poor or so significant to adversely affect the power spectrum of the signal. to gain satisfying performance, the user should deploy an external filter specifically designed to match the features of the signal that he intends to play. this approach can, however, be very expensive. on the other hand, the use of hand-made filters manufactured in laboratory through printed-circuit-boards connection of discrete components can lead to poor results. alternatively, to enhance the integrity of the noise signal, the piece-constant waveform should be played imposing a generation frequency much greater than the related nyquist frequency. this approach represents a simple and promising solution that also simplifies the work of the low pass filter in smoothing the abrupt voltage steps. the available filter becomes more effective because its intervention is now addressed to a kind of refinement [20]. in order to increase the generation frequency without modifying the power spectral density of the signal, appropriate interpolation algorithms are deployed. the interpolation should grant uniform gain within the bandwidth in which the noise is assigned, and a fast decaying frequency response in the transition bandwidth. nonetheless, the computational burden of the interpolation should be thoroughly limited in order to grant real-time processing execution in the generation chain. to this end the discrete noise sequence is first up-sampled, i.e. prolonged by inserting a given number of zeroes between each couple of samples; the number of zeroes inserted between adjacent samples is equal to fup-1, being fup the upsampling factor. the prolonged sequence is hence convolved with an opportune interpolating window in order to obtain the interpolated version of the noise. the choice of the type and length of the window should satisfy a trade-off between the accuracy of the spectral features of the noise and the cost required to the processing task. the figure  4.  digital  noise  signal  as  it  should  appear  when  played  at  full  bandwidth  by  an  arbitrary  waveform  generator  (dotted  line)  and  up‐ sampled and interpolated version (solid line); an up‐sampling factor equal  to  10  has  been  utilized.  simulations  have  been  performed  utilizing  100  points  for  each  voltage  level,  thus  the  dotted  line  is  a  piecewise  line characterized  by  100  points/piece,  while  the  continuous  line  is  characterized by 10 points/piece.  acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 15  time necessary for interpolation depends on the processing capabilities of the processor inside the generator, the length of the interpolation window, and the required generation frequency. for the sake of clarity a further example of noise is presented in figure 4, which shows the time domain evolution of both a piece-constant noise (dotted line) and of its version obtained after up-sampling by a factor fup = 10 and interpolation with a chebyshev window (solid line). the noise is characterized by the imposed psd shown in figure 5, that has been defined in terms of a quartic polynomial function in the digital frequency interval (0.20, 0.80) and a uniform noise floor with level 100 times smaller than the peak power density in (0, 0.20) and (0.80, 1). in order to highlight the benefits that should be expected when up-sampling and interpolation processing are performed in figure 6, the simulated power spectral densities curves of both versions of noise are given: the dotted line is the psd of the piece-constant noise, the solid line is the noise up-sampled by fup = 10 and interpolated with a chebyshev window of length equal to 4 fup. the performance of the interpolation window increases with the length of the window; on the other side a longer interpolation window is more demanding in terms of computational capabilities and can be not compatible with real-time processing constraints. note that interpolation has an inherent low-pass filter behaviour, thus it also introduces some attenuation within the bandwidth b. the aforementioned attenuation can be tolerated whenever the signal does not involve relevant spectral contributions next to the upper limit of the selected band. the work of the analogue output filter should further improve the coherence between the expected output power spectrum and the reproducible one. 4. experimental set‐up  in order to confirm the results of the simulations shown in the previous section an experimental set-up has been arranged and some experiments have been carried out. the experimental set-up, illustrated in figure 7, has included an rf vector signal generator (agilent e4438c esg), equipped with a 14 bit digital to analogue converter (dac) capable of functioning with a clock rate up to 100 mhz, and a spectrum analyser (hewlett packard 8594e). the vector signal generator can play rf signals that combine i and q baseband modulating signals, which can be created by means of the internal 14 bit dac. to be exact the i and q components of the modulated rf signal can be arbitrary defined by the user. the i and q data points have to be created in a binary format, that is signed 2’complement 2-byte integer values in the range (-32767, +32767), big endian byte order. then the i and q data points have to be interleaved in a single array in order to create a data waveform that has to be successively downloaded to the local memory of the generator via ftp using the internal generator web server. in the experimental tests, only the baseband section of the vector generator has been exploited. in particular, the synthesized noise has been assigned to the i component and a zero q component has been inserted to complete the data waveform. for the sake of clarity, the matlab® script adopted to prepare the data that are finally downloaded into the memory of the vector generator is given in appendix a. the analogue version of the i component played by the internal dac has been picked-up at the auxiliary i output connector. (it is worth noting, however, that for applications that require noise residing in a radio-frequency band according to given patterns, it is sufficient to figure  5.  power  spectral  density  of  a  noise  signal  defined  in  the  digital  frequency interval (0.19, 0.81) by the polynomial y(x) =  100(x 2 ‐x+0.16) (x 2 ‐ x+0.24)+0.1; in the complementary intervals, i.e. (0, 0.19) and (0.81, 1), the power  spectral  density  level  is  100  times  smaller  than  the  peak  power density.  figure 7. set‐up arranged to carry out the experiments.  figure. 6. portion of the spectrum of the noise signal (dotted line) and of its up‐sampled  and  interpolated  version  obtained  with  fup = 10,  and  a  chebychev  window  of  length  4 fup  (solid  line);  in  the  generation  the expected spectrum is replicated at a pace equal to the adopted clock rate fck. this undesired distortion effect  is effectively mitigated by up‐sampling  and interpolation.  acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 16  synthesize the baseband version of the noise and use a single sideband (ssb) modulation approach to transpose it at rf frequencies). the first experiments shown hereafter are related to the digital sequence describing the coloured noise characterized by a triangular psd already discussed in section 2. for the purposes of the test a very long sequence made up of 5 242 880 waveform points has been adopted. the noise has been spread in a bandwidth b = 5 mhz, and hence played with a generation frequency equal to 10 mhz. successively, also the noise signal obtained after up-sampling by fup = 10 and interpolation with a chebyshev window of length equal to 4 fup has been downloaded and played, this time using a generation frequency equal to 100 mhz. no auxiliary analogue filters have been exploited to smooth the effects due to the inherent dac functioning. both signals have been picked up at the baseband i output connector and measured by means of the spectrum analyser. figure 8a shows the power spectrum of the two version of the noise in the frequency range (100 khz, 5 mhz); figure 8b shows instead their power spectrum in a wider frequency range, namely (100 khz, 20 mhz). the benefits assured by the proposed approach and already highlighted through simulations are proved also by the experimental tests, that confirm the significant reduction of the power spectral leakage outside the band of interest, obtained without any intervention of external low-pass filters. in addition, further test results are given figure 9a and 9b, which report the power spectrum of the noise characterized by the quartic polynomial power spectral density, already discussed in section 3. 5. conclusions  the paper has presented an approach to generate noise with imposed spectral characteristics by means of an arbitrary waveform generator. the proposed approach requires a first stage devoted to the synthesis of the noise in a digital form, in which the output noise is achieved filtering a digital white noise signal. the spectrum of the output noise is shaped by means of a filter suitably designed in order to accomplish the specified spectral features, given in terms of occupied bandwidth and power levels in the selected frequency band. at the reproduction stage it has been taken into account that (i) the reproduction of any digital signal in analogue form by the dac internal to arbitrary waveform generators introduces distortion that has to be removed, (ii) for several applications the use of the general purpose filters available on the arbitrary generator leads to poor results, figure  9.  power  spectral  density  of  the  noise,  characterized  by  a  quartic polynomial  power  spectral  density,  produced  by  the  dac  internal  to  the  adopted generator. the dashed line is the noise played using a generation  frequency equal to 10 mhz, the solid line instead is the noise obtained after upsampling by fup = 10,  interpolation with a chebyshev window of  length  equal to 4 fup, and played using a generation frequency equal to 100 mhz. in  a)  the  analysed  frequency  interval  ranges  from  100 khz  up  to  5 mhz,  while in b) from 100 khz up to 20 mhz in order to highlight the benefits due  to the proposed generation strategy in reducing the alias contribution due to the inherent functioning of the dac. once more the weak spectral line at  10  mhz  that  characterizes  the  dashed  trace  witnesses  the  adoption  of  a  generation frequency equal to 10 mhz.  figure 8. power spectral density of the noise, characterized by a triangular  power  spectral  density,  produced  by  the  dac  internal  to  the  adopted  generator. the dashed line is the noise played using a generation frequency  equal  to  10  mhz,  the  solid  line  instead  is  the  noise  obtained  after  up‐ sampling  by  fup = 10,  interpolation  with  a  chebyshev  window  of  length  equal to 4 fup, and played using a generation frequency equal to 100 mhz. in  a)  the  analysed  frequency  interval  ranges  from  100 khz  up  to  5 mhz,  while in b) from 100 khz up to 20 mhz in order to highlight the benefits due  to the proposed generation strategy in reducing the alias contribution due  to  the  inherent  functioning  of  the  dac.  note  the  weak  spectral  line  at  10 mhz that characterizes the dashed trace: it witnesses the adoption of a  generation frequency equal to 10 mhz for that signal.  acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 17  (iii) the use of external low-pass filters reveals to be an expensive solution. hence, an additional processing stage has been demanded to the processor internal to the generator in order to upsample and interpolate the digital noise before its reproduction. thanks to this additional processing stage it has been shown that the action of the general purpose lowpass filter internal to the generator is relieved. it is worth noting that the strength of the proposed approach consists in the distribution between the two stages of the computation needed to suitably synthesize the noise. actually, the two stages are characterized by different demands in terms of processing rapidity. the first stage is less demanding and allows the consideration of digital filters with adequate length and related heavy computational burden to shape the spectrum of the noise. at the second stage the stream of sample values addressed to the dac is increased by a factor equal to the up-sampling factor fup and convolved with an interpolating window; in order to grant real time functioning a minor computational burden is a vital requirement for the processing at this second stage. the proposed experimental setup has demonstrated the effectiveness of the proposed approach even if it does not work in streaming and is limited in terms of the length of the generated sequence. future developments will be oriented to the design of a complete coloured noise generation circuit, without limits on the length of the generated sequence, implemented on field programmable gate arrays, a low cost technology that has been demonstrated ideal for the implementation of fast and efficient circuits, [21]-[23]. appendix a  matlab® script adopted to prepare the data that are downloaded into the memory of the vector generator. %load digital waveform  iwave=load('c:\users\mauro\wave.txt');  points=length(iwave);  qwave=zeros(1,points);  waveform(1:2:2*points) = iwave;  waveform(2:2:2*points) = qwave;  % normalize the data between ±1 and scale to use full range of the dac  waveform = round(waveform * (32767 / max(abs(waveform))));   %  preserve  the  bit  pattern  but  convert  waveform  to  unsigned  short  integers  waveform = uint16(mod(65536 + waveform,65536));  % if on a pc swap the bytes to big endian  if strcmp( computer, 'pcwin64' )      waveform = bitor(bitshift(waveform,‐8),bitshift(waveform,8));  end  % save the data to a file  filename = 'c:\users\mauro\formatted_wave.txt' ;  % open a file to write data  [fid, message] = fopen(filename,'w');   if fid == ‐1 error('cannot open file'); end  fwrite(fid,waveform,'uint16');  fclose(fid);  references  [1] dong-u lee, wayne luk, john d. villasenor, peter y.k. cheung “a gaussian noise generator for hardware based simulations,” ieee transactions on computers, vol. 53, no. 12, dec. 2004, pp. 1523-1534. [2] dong-u lee, wayne luk, john d. villasenor, guanglie zang, philip h. w. leong, “a hardware gaussian noise generator using the wallace method,” ieee transactions on very large scale integration (vlsi), vol. 13, no. 8, august 2005, pp. 911-920. [3] c. imperiale, “programmable uniform-gaussian, quasiwhite noise generator and data acquisition-processing system for electrophysiology,” measurement, vol.20, n.3, march 1997, pp.149-163. [4] p. robbins, “using noise for rf receiver built-in test applications,” microwave journal, february 2004. [5] l. angrisani, m. d’apuzzo, m. d’arco, “modelling timing jitter effects in digital to analog converters,” ieee transactions on instrumentation and measurement, vol.58, n.2, february 2009, pp.330-336. [6] l. de vito, s. rapuano, m. villanacci, “prototype of an automatic digital modulation classifier embedded in a realtime spectrum analyzer,” ieee transactions on instrumentation and measurement, vol.59, october 2010, pp.2639–2651. 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[17] l. angrisani, m. d’arco, e. napoli, “programmable power spectral density noise source,” 19th imeko tc 4 symposium and 17th iwadc workshop advances in acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 18  instrumentation and sensors interoperability july 18-19, 2013, barcelona, spain. [18] m. d’apuzzo, m. d’arco, l. liccardo, m. vadursi, “modelling dac output waveforms,” ieee transactions on instrumentation and measurement, vol.59, n.11, november 2010, pp.2854-2862. [19] l. angrisani, m. d’apuzzo, m. d’arco, “fast transformation for dac parameter identification,” ieee transactions on instrumentation and measurement, vol.55, december 2006, pp.2007-2013. [20] domenico luca carnì, luca de vito, sergio rapuano, “a portable instrument for automatic detection and classification of telecommunication signals,” measurement, vol.45, no.2, february 2012, pp.182-189. 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[23] m. d'arco, m. genovese, e. napoli, m. vadursi, "design and implementation of a preprocessing circuit for bandpass signals acquisition," ieee transactions on instrumentation and measurement, 2014, vol 63, no.2, pp.287-294. template for an acta imeko event paper acta imeko issn: 2221-870x december 2016, volume 5, number 4, 37-48 acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 37 uncertainty in vision based modal analysis: probabilistic studies and experimental validation alberto lavatelli, emanuele zappa department of mechanical engineering, politecnico di milano, via la masa 1 20156 milano section: research paper keywords: uncertainty analysis; photogrammetry; modal analysis; vision systems; vibrations; citation: alberto lavatelli, emanuele zappa, uncertainty in vision based modal analysis: probabilistic studies and experimental validation, acta imeko, vol. 5, no. 4, article 7, december 2016, identifier: imeko-acta-05 (2016)-04-07 section editor: lorenzo ciani, university of florence, italy received october 3, 2016; in final form november 22, 2016; published december 2016 copyright: © 2016 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: alberto lavatelli, e-mail: alberto.lavatelli@polimi.it 1. introduction in the relevant field of structural health monitoring (shm) it is possible to highlight several possible applications of vision based monitoring techniques. in particular, the possibility to measure the displacement of several monitored points without directly instrumenting those pushes the interest toward the application of image based vibration measurement systems. even if vision based shm systems have been proposed [1][5], the application of those techniques is still limited. one main reason is the difficulty found in the management of uncertainty. in fact, the process of image formation when dealing with moving targets, variable lighting and optical non-linearity is hardly predictable. in some circumstances (such as indoor tests of highly regular mechanical systems) it may be possible to describe the whole image formation process, so that all the parameters of the optical transfer function are known. nonetheless, the effect of motion blur, lighting variations and optical aberration on the final measurement accuracy is different with respect to each possible image analysis technique. therefore, also the uncertainty connected to the extraction of metrological information from a set of images is hard to predict. furthermore, the dynamic behaviour of actual structures is itself uncertain: given a certain input, the frequency response in terms of displacement and acceleration is variable by stochastic means [6]-[9]. given those premises, the aim of this paper is to find a robust solution to approach the problem of uncertainty estimation when monitoring a vibrating mechanical system with a vision based measurement system. due to complexity of the system under analysis the solution sees the application of a probabilistic framework. this choice follows the recommendation of the iso gum standard [10], where a detailed chapter is dedicated to the analysis of uncertainty by abstract this paper analyses the issues related with the application of photogrammetric methods in the context of modal analysis. in this field of mechanical testing, the vibrations of the measurand are monitored. the possibility to measure displacement of several monitored points without directly instrumenting those, pushes the interest toward the application of an image based measurement system. however, the process of acquiring a moving target with a camera is complex due to the motion blur phenomenon. the effects of this particular phenomenon on uncertainty generation and propagation are discussed in parallel with monte carlo simulation and experimental approach. the case study here proposed focuses on the estimation of the main modal parameters (first two resonant frequencies and amplitudes) of a structure with the help of 2d blob analysis, with particular focus in comparing the performances of photogrammetry with the ones of classic motion transducers. the results of both the approaches confirm that the choice of using photogrammetry as an input for modal analysis should be made carefully, since the actual behavior of a structure can be distorted. nonetheless, the results show that particular care should be made in the choice of structural excitation when using vision based techniques. acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 38 means of monte carlo techniques [11]. the solution found is then tested with an experiment designed on purpose. eventually, this paper proposes both a probabilistic and experimental analysis of uncertainty. on one hand, this choice is justified by the interest in giving an experimental confirmation of the findings proposed by the monte carlo simulation. on the other hand, this procedure tries to provide a more complete view on the uncertainty analysis problem. in fact, despite any possible partial knowledge of uncertainty sources, the process of uncertainty propagation and estimation may itself be under discussion, since it relies on the model chosen to represent uncertainty itself. from a statistical point of view, uncertainty can be derived from a bayesian or frequentist approach. monte carlo methods in particular, belongs to the first family, since the results returned are conditional probabilities [12]. the experimental approaches, conversely, are hard to be said as bayesian, since the observation of the phenomenon is mediated by an instrument who has its own stochastic behaviour. furthermore, the occurrences of measurements are computed upon the recorded output of a sensor. fortunately, for what concerns most of the engineering testing (such as in most wind-tunnel tests, for example) it is possible to say that the choice between a bayesian or frequentist model will not introduce large variation in the significance of the numbers computed using them [13]. nonetheless, from a theoretical point of view, several recent works propose a substantial unification of the two approaches [14]-[16]. in this sense, the contemporary presence of a monte carlo analysis and an experimental analysis of uncertainty accords with the previous unification statement. 2. camera based measurement operating principle the practice of extracting metrological information from an acquired image is well established through the years and it is generally named as photogrammetry. more in details, photogrammetry is a contact-less measurement approach that uses a series of recorded images to identify coordinates of points, patterns, and features in the images or to detect the boundaries of objects [17]. a series of images enables the operation of tracking the motion of a given measurand, hence vibration monitoring can be carried out by means of vision based techniques. a single camera is able to measure a planar motion or 3d motion of known points on rigid bodies [18], [19], while camera pairs are necessary to monitor 3d motion of generic objects. without loss of generality, photogrammetry is performed on digital images. the formation of a digital image converts the appearance of a 3d object into a discretized 2d planar projection of the same object. this procedure involves a cascade of optoelectronic phenomena that is known as the image formation process. although complex, image formation has been modelled successfully by several scientists [20], [21]. in the context of computer vision application for mechanical measurements, the most widely accepted solution is the one proposed by zhang [22], who proposed also an efficient way to calibrate cameras and recover the image formation model parameters. as for every calibration procedure, uncertainty exists and involves several aspects of photogrammetry. considering static images, the main sources of uncertainty are due to optical aberration, image blur, non-uniform luminance and limited image resolution, whose presence generates uncertainty in the estimation of the intrinsic and extrinsic parameters of a camera system. the fore mentioned sources do not appear on calibration only, since they appear also on the generic image of the measurand. when recording moving objects other problems gather with the fore mentioned ones, in particular camera synchronization and motion blur [17]. 3. monte carlo simulation the approach proposed by the authors sees the application of monte carlo methodology [11], [12] in order to simulate the whole measurement process. this choice is claimed to be the most accurate when estimating uncertainty of complex measurement systems [23]-[25]. firstly, an image based measurement technique to evaluate is selected. then it is necessary to select a structure to be monitored and recover a dynamic response model as a set of laplace domain transfer functions (lti model). the variability of structural response is represented by sampling modal parameters (mainly natural frequencies and damping ratios) from suitable probability density functions (pdfs). then it is necessary to sample a structural excitation time history 𝐹(𝑡), by means of parametric analytic functions 𝐹0(𝑡, 𝑝1, 𝑝2, … , 𝑝𝑛) having the 𝑝𝑛 parameters of (1) sampled from appropriate pdfs. it is also possible to add some process noise 𝑁𝑝(𝑡). 𝐹(𝑡) = 𝐹0(𝑡, 𝑝1, 𝑝2, … , 𝑝𝑛) + 𝑁𝑝(𝑡) . (1) the next step sees the simulation of mechanical response by inputting structural excitation into the lti model. consequently, an ideal displacement time history 𝑥𝐼𝐼(𝑡) is retrieved. then, by using a displacement transducer model, it is possible to calculate the measured displacement 𝑥𝑇𝑇(𝑡) from 𝑥𝐼𝐼(𝑡). at the same time the ideal displacement time history is used to simulate the image formation process. in particular, it is possible to take into account motion blur, optical aberration and lighting changes using a proper image formation model that will be further discussed. once the images of vibrating target are recovered, it is possible to send them to the processing stage, where the vision based method is used to recover a vision based displacement signal 𝑥𝑉𝐼𝑉(𝑡). furthermore, starting from 𝐹(𝑡), the excitation measurement 𝐹�(𝑡) is calculated from an appropriate transducer model. up to this stage of the algorithm, two displacement signals of the structure are available: one coming from usual transducers 𝑥𝑇𝑇(𝑡), another from vision system 𝑥𝑉𝐼𝑉(𝑡) and the excitation signal 𝐹�(𝑡) . therefore, it is possible to send them to a transfer function estimation algorithm in order to estimate the modal parameters of interest. the previous sequence of operations provides a single realization of the value of monitored modal parameters for a given structure in case of traditional transducers and vision based measurement systems. by running those operations sequentially for a high number of times, we get the monte carlo loop described in figure 1. 3.1. simulating image formation the process of image formation is quite complex and it should be simulated by providing some simplifying assumptions. in particular, the rigid motion of the structure is simulated by phase shifting the fourier transform of the image of the target in equilibrium position [26]. simultaneously motion blur is generated using the averaging algorithm [27]. this transformation is known as the epsa (exact phase shift and averaging) algorithm. acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 39 then it is possible to add contrast stretch or amplification with operations on the image histogram. optical aberration may be modelled by resampling the image with a quadratic or cubic radial aberration model. other contributes (optical blur, noise..) can be modelled with appropriate image filters. 4. numerical study: analyzing the effects of motion blur on frf estimation a simple case study is proposed in order to evaluate the effects of motion blur on frequency response function (frf) estimation and validate the probabilistic architecture here proposed [28]. for the purpose of this paper, a linear time independent (lti) model of the flexural behaviour of a stadium grandstand is simulated. the parameters were extracted from [7] to provide a realistic example. the model represents the vertical displacement response of a platform tip for a vertical force for the first three natural frequencies. it can be expressed as a parallel of three mono-modal transfer functions in the laplace domain 𝐺𝑖(𝑠) in (2). � 𝐺𝑖(𝑠) = 1 𝑚�𝑖𝑠2+2𝑚�𝑖ℎ𝑖𝜔𝑖𝑠+𝑚�𝑖𝜔𝑖 2 𝜔𝑖 = 2𝜋𝑓𝑖 . (2) modal masses 𝑚�𝑖 are fixed, while the natural frequencies 𝑓𝑖 and damping ratios ℎ𝑖 are random variables. the stochastic nature of 𝑓𝑖 is represented by normal pdfs, while the variability of damping ratios is expressed by a random uniform variable. the pdfs for each variable are listed in table 1. the assumptions on probability distributions of modal parameters come from the analysis of a set of data of the authors' personal experience as well as from a review of documented experimental cases found in scientific literature [29], [30]. the modal testing system simulated in this paper is a simple decay test. the excitation force is transduced by a simulated load cell having a signal-to-noise ratio (snr) equal to 5. the displacement 𝑥𝑇𝑇(𝑡) is transduced by a simulated tof laser having a snr equal to 10. in this case the authors considered to simulate correctly calibrated instruments, for which the transducers noise can be modeled as zero-mean gaussian. this assumption accords with the type a uncertainty model of a calibrated transducer [10]. structure excitation is modeled with (3), where 𝛼 is a random uniform variable with mean 5.6 and standard deviation 0.0115. this choice provides a smoothed impulse, so (3) reproduces the impulse smoothing behavior of hydraulic actuators [31]. excitation peak 𝐹0 is 1000±50 n uniformly distributed. process noise 𝑁𝑝(𝑡) for a mechanical actuating device can be modelled as zero mean gaussian [32] with standard deviation equal to 100 n. the standard deviation of process noise accords with the actually available actuation accuracy for 1-10 kn size hydraulic force actuators [33]. 𝐹(𝑡 − 𝑡0) = 𝐹0 𝑒⋅𝛼 ⋅ (𝑡 − 𝑡0)𝑒−𝛼⋅ (𝑡−𝑡0) + 𝑁𝑝(𝑡) . (3) for the purpose of this paper, all contributions to uncertainty related to instrument calibration are neglected. both the signals are sampled at 50 hz and processed using the wellknown welch periodogram method in order to estimate the structural frf. the estimated frf is calculated with a frequency resolution of 0.01 hz. in order to avoid windowing problems, the excitation starts at 𝑡0=33 s. in this case study, the modal parameters monitored are the first two resonant frequencies 𝑓1, 𝑓2 and their relative resonant amplitudes 𝑎1, 𝑎2 (which are the amplitude of estimated frf at the estimated resonant frequencies). resonant frequencies are identified by peak picking the absolute value of the imaginary part of frf. since the structure’s displacement is monitored at the same time by a classic transducer and by a vision technique, it is possible to compare the estimation of the fore mentioned modal parameters provided by the vision system with the reference one provided by the tof laser. 4.1. imaging technique and investigation path for what concerns the case study here presented, vibrations are monitored with a simple 2d blob detection method. this method has been chosen due to its simplicity and straightforward implementation inside a monte carlo loop. nonetheless blob analysis is a widespread and reliable methodology. for sure it is possible to simulate more modern and sophisticated methods (i.e. pattern matching, digital image correlation), however the effects of motion blur are expected to behave similarly: motion blur is able to degrade all the table 1. pdfs of structural response variability. variable pdf mean std. dev. 𝑓1 normal 2.0 0.1 ℎ1 uniform 0.06 0.0014 𝑓2 normal 2.5 0.05 ℎ2 uniform 0.03 0.0003 𝑓3 normal 3.16 0.2 ℎ3 uniform 0.15 0.0029 figure 1. monte carlo loop description: single realization of the monitored structure. sample excitation sample modal parameters lti model system dynamics simulation mechanical transfer function ideal response signal simulate image formation image processing methodology vision based displacement signal exposure time ligthing optical aberration sample noise excitation transducer model excitation signal displacement transducer model transducer displacement signal sample noise frf estimate frf estimate vision estimated modal parameters transducer estimated modal parameters monte carlo loop probability density functions of transducer estimated modal parameter probability density functions of vision estimated modal parameter acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 40 geometrical features nested in an image, so that all the existing segmentation methods suffer from the presence of motion blur [34], [35]. the simulated blob consists in a bright circle on a black background. the luminance profile of the blob is flat with edges smoothed by a gaussian blur. blob detection is carried out by a fixed threshold, then the centroid of the bright region is computed. for each sample of structure displacement, the reference blob image is proportionally translated. for the purpose of this paper, the amount of translation is directly proportional to 𝑥𝐼𝐼(𝑡) by means of a simple scaling factor equal to 20 𝑝𝑝 𝑚𝑚 . up to now, an image of the tracked blob without motion blur is available for each sample of the actual displacement of structure 𝑥𝐼𝐼(𝑡) (in fact the frames are virtually shot with infinitesimal exposure time as in figure 2a). as said previously, the aim of this work is to investigate the effect of motion blur. this is done by averaging the rigidly translated blob images up to a specified exposure time [27]. in this way the actual physics of motion blur formation is simulated: during exposure time the blob travels through the recorded scene. the final result of this procedure is shown in figure 2b where it is possible to see how motion blur smears shapes by convoluting marker motion with the acquisition window. in this case study nine levels of exposure times are tested: 40, 60, 80, 100, 120, 140, 160 and 180 ms. the specified path corresponds to exposure to period ratios 𝐸2𝑃𝑃 [36] spanning from 0.04 up to 0.4. the testing path here described investigates motion blur starting from the typical condition of a performing experimental setup (where 𝐸2𝑃𝑃 spans in the range 0.01-0.08 [36]) ending to high motion blur situations (where exposure is about 40 % of the vibration period). 𝐸2𝑃𝑃 has been demonstrated to be one of the most relevant parameters to estimate the effect of motion blur on uncertainty of vibration imaging. in fact, when the exposure to period ratio is close to zero, images are recorded in quasi-static, conversely high values will generate visible motion blur. eventually 𝐸2𝑃𝑃 has been proven a valuable index to evaluate the performance of a vision rig applied in vibration monitoring tasks [36]. higher exposure configurations have not been tested due to the obvious impossibility to measure due to aliasing. 4.2. monte carlo loop and probabilistic analysis for each value of exposure time, the monte carlo analysis saw the realization of 1000 structure excitation and modal parameter estimations. each realization saw the simulation of 100 s of system dynamics, for a total amount of 27 h, 46 min and 40 s of simulated response for each value of exposure time. at the end of the simulation, for each value of frequency it was possible to retrieve 1000 realizations of frf for both vision and transducer, consequently 1000 realizations of modal parameters. from this set of values, it was possible to compute the posterior pdfs and their relative cumulative density functions (cdfs). then it was possible to estimate, for each frequency value, the mean value of frf and 90 % confidence bounds from the posterior cdfs. to sum up, as a final output, for each value of exposure time tested in this case study and for both vision and transducer, it was possible to retrieve: estimated frf with mean and posterior confidence bounds, pdfs of estimated resonant frequencies and pdfs of estimated resonant amplitudes. eventually, uncertainty is not treated by postulating a particular uncertainty model, but it is computed “as is” from the simulation data. 4.3. discussion of simulation results: motion blur as a particular kind of filter once the system has been simulated for all the values of exposure time, it is possible to analyse data of frf estimation and modal parameters in order to draw some conclusions. in figure 3 it is possible to see the overall effects on frf estimation. the vision-estimated response appears smaller in magnitude and more damped in average. the confidence bounds are similar in shape with respect to the transducerbased estimation. by looking at the diagrams of figure 4, it is possible to state that using a vision system instead of a transducer has no effect on the identification of natural frequencies, since both the measurement systems display identical identification statistics. or, at least, no effect is recognizable until exposure time is lower than the frequency aliasing threshold. conversely, the process of resonant amplitude identification is affected by the use of vision techniques. the statistical analysis highlights that the presence of a finite exposure time leads to an underestimation of 𝑎1, 𝑎2. as a general trend, the higher the exposure time, the lower is the identified resonant amplitude. in figure 5, the statistics of the resonant amplitude identification process are plotted for each value of exposure time. as a result, a graph displaying the standard deviation of figure 3. frf gain diagram for both monitoring techniques with 90 % confidence bounds (exposure time 140 and 60 ms). a) b) figure 2. tracked blob at peak of excitation. on the left the ideal image of marker, on the right motion blur simulation (exposure time 140 𝑚𝑠). acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 41 identified amplitude 𝜎𝐴 as a function of the mean value of identified amplitude 𝜇𝐴 is shown. as exposure time and motion blur increase, not only the mean value of resonant amplitude decreases, but also the standard deviation of resonant amplitudes decreases. this result is itself interesting: • as a first effect, motion blur acts as a moving average filter. this is an expected phenomenon and it explains why the mean value of resonant amplitude is shifted to a lower value with respect to a classic transducer. as any mobile average filter [37] it decreases the useful bandwidth of transfer function estimation with a sinc kernel. in particular motion blur produces a clipping of the vibration amplitude at the peak of the sinusoidal motion, as demonstrated in [27]; • a second effect of motion blur is to decrease the standard deviation of resonant amplitude estimation. this phenomenon might lead to the incorrect and risky conclusion that the uncertainty on amplitude estimation is reduced. a physical interpretation of this phenomenon is not found in literature. however, in authors’ opinion, the phenomenon of clipping at the inversion point restrains the space of observable states of the system. in other words, as motion blur increases, it is possible to have a set of system configurations that can produce the same measured transfer function due to the filtering effect of the convolution phenomenon. to sum up, using vision based measurement techniques to monitor vibrations may produce a relevant bias in the evaluation of physical properties of mechanical systems. in the case here discussed, the apparent stiffness is lower and less variable with respect to the ideal case. this phenomenon is triggered by motion blur: the higher is 𝐸2𝑃𝑃, the lower the estimated resonant amplitude. for the system discussed in this case study, the 𝐸2𝑃𝑃 limit for accurate frf estimation is about 0.12. higher values of exposure will produce estimation errors higher than 10 % of the nominal values. in figure 6 the width of frf uncertainty band is plotted as a function of frequency. in this case the presence of a fictitious figure 6. uncertainty of frf amplitude estimation (with 90% level of confidence) for different values of exposure time. figure 5. statistics of identification of resonant amplitudes for the vision based system as motion blur increases. figure 4. posterior pdfs of modal parameters identification for exposure time equal to 140 ms. acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 42 decrease of uncertainty is detectable in all the process of frf estimation. as a consequence, the estimation of modal parameters in a single run appears more reliable. this behavior is potentially dangerous in the field of model based shm, where the value of snr and the stability of modal parameters are often used to judge the accordance of the experimental behaviour of a structure with the one predicted by a model. 4.4. evaluating the uncertainty contributions of the measurement method one last analysis available within the proposed uncertainty estimation framework is the evaluation of the uncertainty generated by the modal analysis method. this is done by erasing the uncertainties of the structural response so that the standard deviation of the parameters listed in table 1 is identically equal to 0. in this way, an ideal noise-free transducer should provide a perfect identification of modal parameters, which should be estimated without uncertainty. conversely all transducer models used in the monte carlo simulation take into account disturbances such as electrical noise for analog sensors and motion blur for cameras. nonetheless, the structural excitation model takes into account process noise. as a consequence, the measurement method itself introduces uncertainty in the estimation of modal parameters, even in the case of an ideal structure [7]. to sum up, it is possible to provide an estimation of measurement uncertainty for the generic modal parameter. in table 2 the results of modal parameter estimation at various levels of exposure are listed. table 2 reports for the first two modes the following quantities: the value of exposure time 𝑡𝑠ℎ, the mean value of resonant amplitude 𝜇𝐴, the standard deviation of resonant amplitude 𝜎𝐴, the mean value of resonant frequency 𝜇𝐹 and the standard deviation of resonant frequency 𝜎𝐹. for what concerns the method used in this case study, the results confirm that motion blur is able to alter the output of modal analysis by introducing measurement bias and changing the apparent measurement uncertainty on frf amplitude estimation. a further representation of this phenomenon can be found in figure 7 where the plot of 𝜎𝐴 as a function of 𝜇𝐴 is displayed. for what concerns the estimation of natural frequencies, it is possible to state that the vision based methodology here discussed is not sensitive to motion blur. in fact, for both the modes, 𝜇𝐹 and 𝜎𝐹 are almost constant in value as exposure changes. conversely it is possible to see a clear trend for 𝜇𝐴 and 𝜎𝐴, since both monotonically decrease in value as motion blur increases. at low level of exposure, the decreasing slope is quite slow, conversely at high level of exposure the decreasing slope is steep, resulting in a non-linear behaviour. the results also confirm the disguising behaviour of motion blur. at low 𝐸2𝑃𝑃 values the presence of this contribution is indeed negligible for a good estimation of modal parameters. however, the 𝐸2𝑃𝑃 depends not only on the exposure time, but also on the period of vibration. hence each natural frequency is characterized by a different value of 𝐸2𝑃𝑃. in conclusion, the estimation of uncertainty in this context is not only related to the method itself, but also to the dynamics of the oscillating system under analysis. 5. experimental activity the probabilistic method proposed previously is claimed to give a glimpse into the critical aspects of the application of vision based measurement techniques for modal analysis purposes. in order to verify the goodness of the probabilistic approach, this paper reports the results of an experimental activity designed for model verification purposes. 5.1. design of the experiment the aim of the experimental activity is to test the conclusions discussed in the previous part of the paper. to do so, the basic idea is to replicate the scenario simulated in section 4 with a real mechanical system. more in details, the concept of the experiment is the following: • pick a mechanical assembly; • equip the assembly with a vibration exciting device; • install vibration monitoring transducers on the mechanical assembly; • select a vision rig in order to perform vision based vibration measurement; • then, for each value of exposure time, run modal testing several times; • after that, analyse data and retrieve the statistical distribution of the modal parameters of interest. for sure, the selection of the total amount of modal tests run for a single value of exposure time is crucial to the statistical significance of the experiment. nonetheless, time limitations make it impossible to replicate exactly the testing path described in the monte carlo simulation presented in section 4. in fact, in that simulation frf was estimated with a frequency resolution of 0.01 hz, which meant about 28 hours of simulated acquisition for each value of exposure time. considering the fact that 8 levels of exposure time have been tested, the replication of the experiment would require to figure 7. trends of apparent uncertainty reduction due to motion blur for the vision method analysed. table 2. uncertainty of the vision based modal parameter estimation method for a set of exposure time values. mode 1 mode 2 [ms] [mm kn-1] [hz] [mm kn-1] [hz] 𝑡𝑠ℎ 𝜇𝐴 𝜎𝐴 𝜇𝐹 𝜎𝐹 𝜇𝐴 𝜎𝐴 𝜇𝐹 𝜎𝐹 40 4.44 0.32 2.01 0.01 2.78 0.24 2.50 0.01 60 4.36 0.31 2.01 0.01 2.69 0.22 2.50 0.01 80 4.23 0.31 2.00 0.01 2.57 0.21 2.50 0.01 100 4.07 0.28 2.01 0.01 2.42 0.20 2.51 0.01 120 3.90 0.27 2.00 0.01 2.25 0.18 2.50 0.01 140 3.67 0.25 2.01 0.01 2.05 0.16 2.50 0.01 160 3.45 0.24 2.01 0.01 1.83 0.14 2.51 0.01 180 3.21 0.22 2.00 0.01 1.67 0.10 2.50 0.01 acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 43 acquire 224 hours of data, coming from the selected mechanical systems. in order to reduce the time needed for the experiment, two solutions have been evaluated: to reduce the number of modal testing for each value of exposure time or to reduce the frequency resolution of frf estimation. the first one was discarded due to the risk of losing statistical significance. the second one, conversely, has been selected because the quality of frf estimation wouldn’t suffer as long as the natural dynamic of the system is more than 50 times the frequency resolution (this means that the natural frequency of the mechanical assembly must be above 5 hz). consequently, the authors chose to run validation on a purposely designed mechanical system, so that modal tests could be run on 10 seconds time histories. in this way it has been possible to run modal testing 1000 times for each value of exposure time. in addition, only for 3 levels of exposure time (2 ms, 4 ms and 6 ms), so that the total amount of acquisition time is limited to 8 hours and 20 minutes. 5.2. design of the mechanical assembly the main goal was to find a simple and reproducible mechanical system, so that all the sources of uncertainties in system behaviour would be under control. furthermore, the system has to be easily accessible for instrumentation. from what concerns the dynamic characteristic, the first two natural frequencies should be contained in the interval 5-50 hz: the lower limit is discussed in the previous subsection, while the upper one is chosen in order to avoid aliasing, since the camera’s framerate is limited to 120 fps. to keep the assembly as simple as possible, the choice was to mount a cantilever rectangular section beam on an electromagnetic shaker, so that the encastre is actuated vertically with motion 𝑥𝐹(𝑡) by the shaker rod (as shown in figure 8). the beam material is aluminium and its section is 4 mm thick and 30 mm wide. the cantilever length is 600 mm, which locates the first natural frequency at 9.1 hz and the second one at 57 hz. this last value is obviously not compatible with the constraints formulated above. to reduce the value of the second natural frequency, it was decided to put a 0.23 kg mass on the beam, around the antinode of the second mode shape. in fact, around that point the lagrangian component of acceleration is much higher for the second mode than for the first one. in order to select the correct position of the mass, a fem simulation has been run. in figure 9 it is possible to see that the optimal point was found between 220 and 240 mm away from the beam encastre, where the frequency of the second mode is minimum. with this last choice, the fem predicted values for the natural frequencies are 𝑓1 ≈ 7.1 hz and 𝑓2 ≈ 32.4 hz, which let us explore the effects of motion blur at the values of 𝐸2𝑃𝑃 listed in table 3. 5.3. actuating device and instrumentation description the selection of the actuating device and the instrumentation is crucial for modal analysis purposes. for what concerns the choice of electromagnetic shakers, it is necessary to ensure the necessary frequency range for the encastre forcing. the choice was then made for a lds v408 actuator having a useful actuating range of 5-9000 hz with an operative maximum sine force peak of 98 n. the maximum peak to peak displacement is 17.6 mm, which is enough to accommodate the minimum desired amplitude of 3 mm peak to peak. the shaker is powered by a lds pa500l lock-in amplifier, with a 500 w power rating. the actuation signal is synthetized with a national instruments (ni) 9263 analog output module (±10 v, 16 bit, 10 khz bandwidth) for what concerns the transducer setup, vibrations are then monitored with two pcb 333b30 icp accelerometers, one positioned on the encastre, one on the beam tip. their nominal sensitivity is 100 mv/g, with a ±50 g measurement range and 0.00015 g rms resolution. signals are acquired with ni 9234 analog input module (±5 v, 24 bit, 51 ks/s, embedded antialiasing filter). acceleration signals are sampled at 2048 hz. vision measurements are carried out with the help of the well-known blob analysis technique. a painted blob is positioned on the beam tip (figure 10) and its motion is monitored frontally by a single camera. the camera used in the experiment is an avt marlin f131b (focal length of 12 mm according to iso 517) mounted on an aluminium tripod. the performances of the electro-optical setup have been evaluated from the 0.10sfr response level according to the iso 12233 standard [38], showing a sampling efficiency rating 𝐸𝑠 of 91 %. images have been transferred to the pc using the ieee 1394 bus and stored in ieee 12234-compliant tiff files. after calibration, the value retrieved for the image scaling factor is 0.1266 mm/px. images are sampled at 120 fps. figure 9. optimization of the mass position by means of fem simulation. table 3. values of e2pr investigated during the experimental activity. exposure time [ms] mode 1 e2pr mode 2 e2pr 2 0.014 0.065 4 0.028 0.129 6 0.042 0.194 figure 8. view of the mechanical testing assembly: a) electromagnetic shaker b) cantilever aluminium beam c) accelerometers d) blob analysis target e) additional mass. 0 100 200 300 400 500 600 mass position [mm] 0 10 20 30 40 50 60 fr eq ue nc y [h z] mode 1 mode 2 a b c d e 𝒙𝑭(𝒕) 𝒙𝑭(𝒕) acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 44 5.4. data acquisition and processing the experimental setup described above has been made with the objective to compare the transducer-based frf estimation with the vision based one. in this case the frf input is the acceleration at the encastre, while the output is the acceleration of the beam tip. when dealing with signals coming from the accelerometers, frf estimation is done by applying the well-known h1 estimator [39] on each single modal test acquisition. for what concerns signal windowing, the choice was to use the hanning window. the reason of this choice is to use a common general purpose window for white noise excitation [40]. when changing the window function, in fact, the shape of frf around peaks is modified depending on the damping coefficient of each mode [39]. in any case, the peak deformation due to changes in the window function acts equally on the vision estimated frf and on the accelerometer estimated frf (and this has been tested on the data by comparing the results of hanning window against the flat top window). consequently, the conclusions about the effects of motion blur on modal analysis are computed without loss of generality. then the estimation of the first two resonance frequencies 𝑓1, 𝑓2 is carried out by peak picking the imaginary part of the h1 estimator. resonant amplitudes 𝑎1, 𝑎2 are then retrieved by selecting the value of magnitude of h1 estimator in coincidence of the natural frequencies. the processing of vision data, conversely, is not straightforward as in the case of accelerometers. the first step sees the segmentation of the image using a simple fixed threshold. then the center of gravity of the tracked blob is calculated. so, the position of the barycenter with respect to the static position (in px) is available for each frame acquired. displacement is then calculated by applying the mm/px scaling to the previous data. now we have a signal 𝑥𝑣𝑖𝑠(𝑡) that represents the beam tip displacement time history, sampled at 120 hz. this measure has to be further processed, since the excitation signal is an acceleration sampled at 2048 hz. the first step is to oversample the vision displacement signal using fourier interpolation, so that the informative content is not altered. in this way a displacement signal with the same sampling rate of the acceleration signal is available. now to match displacement with acceleration, the spectrum of 𝑥𝑣𝑖𝑠(𝑡) is calculated and then multiplied by −𝜔2. in the end, it is possible to apply the h1 estimator and estimate the modal parameters also for the vision system. another important issue to be solved at the acquisition stage is the synchronization of image acquisition with analog acquisition. this result is achieved by driving camera acquisition and the adc conversion of the accelerometer signal with two different clocks that share a common time base. in this way the analog acquisition rate and the camera sampling frequency are integer multiples of a common fast time-base, hence it is possible to carry out all digital signal processing steps that require re-sampling and re-phasing in a correct and consistent way. for each run of modal testing values for natural frequencies and resonant amplitudes are estimated by the accelerometers monitoring system and by the vision monitoring systems. at the end of the test 1000 modal tests are recorded for each value of exposure time, hence 1000 estimation of modal parameters of interest. as a consequence, process statistics (mean, std. deviation, pdfs) are calculated for the 𝑓1, 𝑓2, 𝑎1, 𝑎2 quantities estimated during the experiment. so that it is possible to further compare the performance of the vision system against the accelerometer-based one. 5.5. experimental results analysis in this section the results of the experimental activity are presented with a particular focus on investigating how the application of the vision based measurement method may affect the estimation of modal parameters of a structure. in figure 11 the frf estimation over 1000 time histories of 10 s for the acceleration transducer and vision system for the 3 levels of exposure is presented. as first comment, it is important to say that, in average, the frf estimated by the vision system is similar and close to the one of accelerometers. this is the same as predicted by the monte carlo simulation. it confirms the validity of approaching vibration monitoring with vision based measurements, since the mechanical behaviour of the structure is resembled correctly in large part. however, by zooming the peaks of the mechanical frf, it is possible to see clearly the action of exposure time, since the frf peaks are lowered as the motion blur increases. the trend is similar to the one predicted by the monte carlo simulation in figure 3, hence the probabilistic approach was able to predict the rising of this trend. furthermore, it is possible to see that the peak lowering and flattening phenomenon is more evident for the second mode, where the e2pr is higher. in particular, the pace with the frf peak goes down with exposure time is faster for the second mode with respect to the first. once again this feature has been highlighted also in the monte carlo case study. in order to quantify this phenomenon two dimensionless indexes are formulated: • 𝜒𝜇 which represents the ratio between the average resonant amplitude estimated by the vision system and the one estimated by the accelerometers; • 𝜒𝜎 which represents the ratio between the standard deviation of resonant amplitude estimated by the vision system and the one estimated by the accelerometers. those indexes have been selected because they indicate clearly the phenomenon of reduction of both the mean value and the standard deviation of the resonant amplitude with motion blur as described by the simulation in figure 4. in fact, by looking at table 4 it is possible to see that both the indexes display a decreasing trend with motion blur. for both the figure 10. the test-stand as mounted in the laboratory. the white blob is mounted on the beam tip and is frontally recorded by the camera. acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 45 modes their value is below 1. the blur decreasing trend is more evident for mode 2, as expected by the higher e2pr. another feature to highlight is the apparent higher accuracy (𝜒𝜇 ≈ 100 %) of estimating amplitude of mode 2 with respect to mode 1 at low exposure. this phenomenon is connected with geometrical errors due to the rotation of the beam tip, which, for a given value of displacement, is higher for mode 1 than for mode 2. this is a “static” bias, due to the 2d blob analysis: it happens every time an out-of-plane occurs, since the 3d motion of a point is projected on the 2d surface of the image sensor. for what concerns identification statistics, the analysis process is the same as for the output of the monte carlo simulation. in fact, for each value of exposure time 1000 estimations of natural frequencies 𝑓1, 𝑓2 and resonant amplitudes 𝑎1, 𝑎2 are available, hence it is possible to calculate the pdfs for each modal parameter. in figure 12 the identification statistics for the tests at 𝑡𝑠ℎ=4 ms are presented as an example. starting from the natural frequency estimation, data behave according to the predictions of the monte carlo study, since, the relative pdfs are quite similar. for mode 1 this is particularly true, while for mode 2 a small shift between the two curves is noticeable. this may seem not according with figure 11, where the peak position seems the same as for the accelerometer. this happen because the estimation of natural frequencies is made by peak picking the imaginary part of the h1 estimator: so, if we consider small phase errors that may happen during experimental modal analysis, this may not be exactly coincident with the magnitude peak of the h1 estimator. the uncertainty of estimation is almost similar. the estimation of resonant amplitudes, conversely, is more complex in experimental cases, compared to what happens in the monte carlo simulation. to be more precise, the pdfs are far for the gaussian hypothesis but in any case, it is possible to see that the probability density peak of the vision system estimation is slightly left shifted. this feature, once again, accords with the monte carlo simulation where this phenomenon is more evident because higher values of e2pr have been investigated. 5.6. managing the structural excitation during tests vision systems are based on the detection of image motion, hence they are “displacement-based” sensors. in fact, even in the case of image velocimetry techniques, the metrological input is nested in the alteration of the pixel position of the feature detectable in the image. as a consequence, signal-tonoise ratio (snr) is higher when displacement is high. in straight words, regardless the measurement technique, it is hard to monitor vibrations characterized by very small displacement with vision based instruments. however, when a structure is excited in a way that a high displacement is generated, nonlinear dynamic phenomena may occur easily. this is an obvious drawback of vision based measurement systems in comparison with modern accelerometers. for instance, if we consider recent structural testing grade accelerometers (such as the ones using in this figure 11. comparison between fem simulated frf, accelerometer based frf and vision based frf for different values of exposure time. the curve plotted represents the frf averaged over 1000 runs. table 4. uncertainty of the vision based modal parameter estimation method for a set of exposure time values. mode 1 mode 2 𝑡𝑠ℎ [ms] 𝜒𝜇 𝜒𝜎 𝜒𝜇 𝜒𝜎 2 95.5 % 94.6 % 100.0 % 100.1 % 4 95.2 % 94.4 % 97.7 % 96.4 % 6 94.9 % 93.7 % 94.3 % 94.3 % 5 10 15 20 25 30 35 40 45 50 frequency [hz] 10 0 10 1 10 2 fr f a m pl itu de [] fem transd. t sh = 2 ms t sh = 4 ms t sh = 6 ms 6.8 6.9 7 7.1 frequency [hz] 60 65 70 fr f a m pl itu de [] transd. t sh = 2 ms t sh = 4 ms t sh = 6 ms 32 32.5 33 33.5 frequency [hz] 25 30 35 40 45 fr f a m pl itu de [] transd. t sh = 2 ms t sh = 4 ms t sh = 6 ms acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 46 paper) it is common to have a measurement resolution in the 10-4 g rms region or below. as a consequence, modal tests can be run efficiently even with 10-2 g rms excitation, provided that a high quality dac system is used. in practice, it means that accelerometer based modal analysis can easily be accomplished in the full linear dynamic region (small displacements). in principle, when dealing with linear dynamics, modal tests can be run with any kind of excitation (as long as the excitation spectrum covers all the frequency band of interest and the structure motion develops in the linear region) so that, regardless the kind of excitation, the frf estimation output should not change. when the dynamic is not linear, conversely, frf estimation is very sensitive to changes in structural excitations. during the experimental activity, at first, it was decided to use the same excitation used in the monte carlo simulation. that excitation signal is composed by two terms: an impulse and an additive process noise. the impulsive part, in particular, generated a beam tip displacement of more than 6 mm peak-to peak, which is far away from the small displacement/small rotation region. as a consequence, the statistical distribution reflected the presence of the two terms: as can be seen in figure 13, the distribution of parameters is not plain. for the mode 2 in particular, it is possible to highlight the presence of a peak when the amplitude values are around 270. in order to correct this issue, it was chosen to excite the structure with random uniform band limited white noise. in this way the distribution of modal parameters got back to a monomodal situation. this phenomenon is interesting and it is a clear warning that when operating vision based modal analysis the choice of excitation is crucial to detect correctly the dynamics of the structure. another interesting nonlinearity related to excitation in vision based modal analysis is due to motion blur. in fact, motion blur itself is generated by the target moving during exposure. as demonstrated in previous works [27], [35], [36], this phenomenon is driven by the peaks in the instantaneous speed of the measurand. so, if we compare two possible excitation signals, the one that develops higher velocity peaks in the measurand is more prone to produce an under-estimation of resonant amplitude in the h1 estimator (when using vision based measurement system). to sum up, all the issues discussed in this subsection suggest that the path throughout an extensive development of vision based modal analysis should discuss the problem of finding an figure 13. statistical distribution of resonant amplitudes when exciting the structure with a two component (impulse + white noise) signal. figure 12. pdfs of experimental modal parameters identification for exposure time equal to 4 ms. 0 100 200 300 400 500 600 700 amplitude [-] 0 2 4 6 8 pd f [] 10 -3 mode 1 mode 2 6 6.5 7 7.5 8 8.5 frequency [hz] 0 1 2 3 4 pd f [1 /h z] mode 1 identification transd. vision 31 32 33 34 frequency [hz] 0 1 2 3 pd f [1 /h z] mode 2 identification transd. vision 0 200 400 600 amplitude [-] 0 0.002 0.004 0.006 0.008 0.01 pd f [] transd. vision 0 200 400 600 amplitude [-] 0 0.005 0.01 0.015 0.02 pd f [] transd. vision acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 47 optimal structural excitation. more in details, the problem is defined by counteracting constraints: displacement should be high so that the snr is high, but at the same time it should be low to avoid non-linearity. furthermore, for a correct amplitude estimation, structure excitation should minimize motion blur by controlling the maximum instantaneous speed of the target(s). 6. conclusions the presented research work investigated and discussed what are the main limitations of vision based modal analysis in comparison with the classic transducer based application, with particular focus on isolating the main sources of uncertainty in dynamic contexts. the first part of the research sees the simulation of the measurement process with the help of a monte carlo framework. it was found that vision systems may alter the measured behaviour of a structure: in brief, the estimation of resonant amplitude is biased and the bias is higher as the exposure time grows. consequently, an experimental activity has been run in order to test the validity of the findings retrieved with the monte carlo simulation. the experiments demonstrated that the frequency response measurement bias is present also in real situations, with good statistical significance. nonetheless, the experiment here proposed suggests that the development of vision techniques in the context of modal analysis should discuss the optimization of structural excitation in order to reduce uncertainty. references [1] zhou, h.f., dou, h.y., qin, l.z., chen, y., ni, y.q., ko, j.m., “a review of full-scale structural testing of wind turbine blades” (2014), renewable and sustainable energy reviews, 33, pp. 177187. [2] ellenberg, a., branco, l., krick, a., bartoli, i., kontsos, a., “use of unmanned aerial vehicle for quantitative infrastructure evaluation” (2015), journal of infrastructure systems, 21 (3), art. no. 04014054. [3] winkler, j., hendy, c.r., waterfall, p., “improved structural health monitoring strategies for better management of civil infrastructure systems” (2015), multi-span large bridges proceedings of the international conference on multi-span large bridges, 2015, pp. 855-862. [4] desai, n., “small-strain measurement in bridge connections using the digital image correlation (dic) technique” (2016), proceedings of spie the international society for optical engineering, 9805, art. no. 980530. [5] busca, g., cigada, a., mazzoleni, p., zappa, e. “vibration monitoring of multiple bridge points by means of a unique vision-based measuring system” experimental mechanics (2014), 54 (2), pp. 255-271. [6] s. adhikari, m. friswell, k. lonkar, and a. sarkar, “experimental case studies for uncertainty quantification in structural dynamics,” probabilistic engineering mechanics, vol. 24, no. 4, pp. 473 – 492, 2009. [7] a. cattaneo, s. manzoni, and m. vanali, “measurement uncertainty in operational modal analysis of a civil structure,” in proceedings of isma 2010 international conference on noise and vibration engineering, including usd 2010, pp. 5103–5115, 2010. [8] m. shinozuka, “structural response variability”, journal of engineering mechanics, vol. 113, no. 6, pp. 825–842, 1987. [9] c. bucher and m. shinozuka, “structural response variability ii”, journal of engineering mechanics, vol. 114, no. 12, pp. 2035– 2054, 1989. [10] iso/iec guide 98-3:2008 guide to the expression of uncertainty in measurement (gum:1995). [11] iso/iec guide 98-3/suppl.1:2008 propagation of distributions using a monte carlo method. [12] e. zio, “the monte carlo simulation method for system reliability and risk analysis”, springer series in reliability engineering, springer science & business media, 2012. [13] hugh w. coleman and w. g. steele. “engineering application of experimental uncertainty analysis”, aiaa journal, vol. 33, no. 10 (1995), pp. 1888-1896. [14] bayarri, m. j., and j. o. berger. “the interplay of bayesian and frequentist analysis”, statistical science, vol. 19, no. 1, 2004, pp. 58–80. [15] guyon, i., saffari, a., dror, g., cawley, g., model selection: beyond the bayesian/frequentist divide, (2010) journal of machine learning research, 11, pp. 61-87. [16] a. kardara, c. bucher, and m. shinozuka, “structural response variability iii”, journal of engineering mechanics, vol. 115, no. 8, pp. 1726–1747, 1989. [17] j. baqersad, p. poozesh, c. niezrecki, p. avitabile, “photogrammetry and optical methods in structural dynamics – a review”, mechanical systems and signal processing, 2016, issn 0888-3270. [18] haralick, b.m., lee, cn., ottenberg, k. et al., “review and analysis of solutions of the three point perspective pose estimation problem”, int j comput vision (1994) 13: 331. [19] d. cremers, b. rosenhahn, alan l. yuille, frank r. schmidt, “statistical and geometrical approaches to visual motion analysis: international dagstuhl seminar, dagstuhl castle”, july 13-18, 2008, revised papers, springer berlin heidelberg, 2009. [20] o. faugeras, “three-dimensional computer vision: a geometric viewpoint”, mit press, 1993. [21] r. hartley, a. zisserman, multiple view geometry in computer vision. cambridge university press, 2004. [22] z. zhang, “a flexible new technique for camera calibration”, ieee transactions on pattern analysis and machine intelligence, vol. 22, no. 11, pp. 1330-1334, nov 2000. [23] r. mullen and r. muhanna, “probability bounds for the system response of non-linear structures”, in safety, reliability, risk and life-cycle performance of structures and infrastructures proceedings of the 11th international conference on structural safety and reliability, icossar 2013, pp. 499–502, 2013. [24] m. damgaard, l. andersen, l. ibsen, h. toft, and j. srensen, “a probabilistic analysis of the dynamic response of monopile foundations: soil variability and its consequences”, probabilistic engineering mechanics, vol. 41, pp. 46–59, 2015. [25] s. mukhopadhyay, r. betti, and h. lus, “structural identification and uncertainty quantification in operational modal analysis with minimal instrumentation”, in safety, reliability, risk and lifecycle performance of structures and infrastructures proceedings of the 11th international conference on structural safety and reliability, icossar 2013, pp. 1879–1884, 2013. [26] p. reu, “experimental and numerical methods for exact subpixel shifting”, experimental mechanics, vol. 51, no. 4, pp. 443–452, 2011. [27] e. zappa, a. matinmanesh, and p. mazzoleni, “evaluation and improvement of digital image correlation uncertainty in dynamic conditions”, optics and lasers in engineering, vol. 59, pp. 82– 92, 2014. [28] q. huang, p. gardoni, and s. hurlebaus, “assessment of modal parameters considering measurement and modelling errors”, smart structures and systems, vol. 15, no. 3, pp. 717–733, 2015. [29] i. behmanesh and b. moaveni, “accounting for environmental variability, modeling errors, and parameter estimation uncertainties in structural identification”, journal of sound and vibration, vol. 374, pp. 92 – 110, 2016. [30] e. reynders, k. maes, g. lombaert, and g. d. roeck, “uncertainty quantification in operational modal analysis with stochastic subspace identification: validation and applications”, acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 48 mechanical systems and signal processing, vol. 6667, pp. 13 – 30, 2016. [31] darby, a. p., blakeborough, a. and williams, m. s. (2001), “improved control algorithm for real-time substructure testing”, earthquake engng. struct. dyn., 30: 431–448. [32] m. compare and e. zio, “predictive maintenance by risk sensitive particle filtering”, in ieee transactions on reliability, vol. 63, no. 1, pp. 134-143, march 2014. [33] alleyne a, liu r., “on the limitations of force tracking control for hydraulic servosystems”, asme. j. dyn. sys., meas., control. 1999;121(2):184-190. [34] y. wang, p. lava, p. reu, and d. debruyne, “theoretical analysis on the measurement errors of local 2d dic: part i temporal and spatial uncertainty quantification of displacement measurements”, strain, 2015. [35] a. lavatelli and e. zappa, “modeling uncertainty for a vision system applied to vibration measurements”, in ieee transactions on instrumentation and measurement, vol. 65, no. 8, pp. 1818-1826, aug. 2016. [36] g. busca, g. ghislanzoni, and e. zappa, “indexes for performance evaluation of cameras applied to dynamic measurements”, measurement: journal of the international measurement confederation, vol. 51, no. 1, pp. 182–196, 2014. [37] g. d’antona and a. ferrero, “digital signal processing for measurement systems: theory and applications”, information technology: transmission, processing and storage, springer us, 2006. [38] iso 12233:2014 photography -electronic still picture imaging resolution and spatial frequency responses. [39] d.j. ewins, “modal testing: theory, practice, and application”, 2nd edition, research studies press, 2000. [40] gade, s. and herlufsen, h., “use of weighting functions in dft/fft analysis”, brüel & kjær technical reviews nos. 3 & 4, 1987. uncertainty in vision based modal analysis: probabilistic studies and experimental validation microsoft word 273-1906-1-le-rev2 acta imeko  issn: 2221‐870x  december 2015, volume 4, number 4, 30‐37    acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 | 30  type a evaluation of measurement uncertainty when the  sample size is not predetermined  robin willink   16 rochdale avenue, glendowie, auckland 1071, new zealand    section: research paper   keywords: bias; propagation of errors; propagation of uncertainty; stopping rule; unbiased estimation  citation: robin willink, type a evaluation of measurement uncertainty when the sample size is not predetermined, acta imeko, vol. 4, no. 4, article 8,  december 2015, identifier: imeko‐acta‐04 (2015)‐04‐08  section editor: franco pavese, torino, italy  received april 26, 2015; in final form november 7, 2015; published december 2015  copyright: ©2015 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  corresponding author: robin willink, e‐mail: robin.willink@gmail.com    1. introduction  the assessment of the variance of a component of measurement error by statistical means is known as type a evaluation of measurement uncertainty. the principal form of type a evaluation discussed in the guide to the expression of uncertainty in measurement, (the gum) [1], [2], relates to the estimation of a quantity by the averaging of individual results regarded as having been independently drawn from a normal distribution with mean equal to the quantity of interest. the analysis there does not explicitly acknowledge that it assumes the sample size to be determined without reference to the data, as if it had been fixed before the analysis. yet many laboratory experiments involve sample sizes that are not known in advance. typically, an experimental scientist will continue to make observations until he or she is satisfied with the results, so that the final sample size is affected by whether the growing dataset makes sense and appears adequate for the experimenter's purpose. it is therefore important to study whether the accuracy of the gum's process of type a evaluation is affected by such practice. there are several relevant results from the theory of classical statistics. let be the random variable for the th value drawn independently from the normal distribution with unknown mean and unknown variance . define ≡ and ≡ . so and are the random variables for the sample mean and sample variance when an independent sample of fixed size n is taken. the results of fundamental importance are italicized in the following statements. 1. the distribution of is normal with mean and variance / , (and these expressions for the mean and variance of are correct even if the distribution is not normal). abstract  the archetypal procedure  in type a evaluation of measurement uncertainty  involves making n observations of the same quantity,  taking the sample figure s 2  to be an unbiased estimate of the underlying variance and quoting the figure s/√n as the relevant standard  uncertainty. although this procedure is theoretically valid when the sample size n is fixed, it is not necessarily valid when n is chosen in  response to the growing dataset. in fact, when the experimenter makes observations until a certain level of uncertainty in the mean is  reached, the bias in the estimation of the variance can be as much as ‐45 %. likewise, the usual nominal 95 % confidence interval can  have  a  level  of  confidence  as  low  as  88  %.  this  issue  is  discussed  and  techniques  are  suggested  so  that  type  a  evaluation  of  uncertainty  becomes  as  accurate  as  is  implied.  the  ‘objective  bayesian’  approach  to  this  issue  is  discussed  and  an  associated  unacceptable phenomenon is identified.    acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 | 31  2. the expected value of is , (and this is the case even if the distribution is not normal). so is called an ‘unbiased estimator’ of . consequently, the realization or outcome of , which is a number denoted , is called an unbiased estimate of . 3. the sample-mean random variable is independent of every function ,⋯ for which ,⋯ ,⋯ for every constant [3]. 4. the random variable / /√ has the tdistribution with 1 degrees of freedom. so, with , denoting the p-quantile of the t-distribution with degrees of freedom, the random interval ≡ , . √ , , . √ has probability 0.95 of covering whatever the values of and . therefore, this random interval is a ‘95 % confidence interval’ for . the corresponding numerical interval ̅ , . √ , ̅ , . √ can be called a ‘realized 95 % confidence interval for ’ or an ‘evaluated 95 % confidence interval for ’. (regrettably, there are two slightly different definitions of a confidence interval that can be found in authoritative encyclopedias of statistics. following the international dictionary of statistics [4], we take the term confidence interval to refer to the estimator, not the estimate. therefore the confidence interval itself is and the adjective ‘realized’ or ‘evaluated’ is added when referring to the numerical interval that is the resulting estimate of . in this, we differ with the encyclopedia of statistical sciences [5], which takes the confidence interval itself to be the numerical interval.) result 1 tells us that the standard uncertainty to be associated with the observed sample mean ̅ should be an estimate of /√ , which is the standard deviation of the measurement error. result 2 is important because many measurement processes involve combining estimates of several ‘input quantities’, and the law of propagation of uncertainty involves summing the estimates of the variances of the errors. the ensuing estimate of the variance of the total error will be unbiased if the component estimates are unbiased, (notwithstanding the effect of approximating the measurement equation by a locally linear function). result 3 implies that is independent of any statistic that provides no information about . in particular, it implies the well-known result that and are independent. result 4 is important because it implies that 95 % of the ‘95 % confidence intervals’ evaluated in a long series of problems will enclose the unknown actual values of the measurands. this gives a practical and unambiguous meaning to the output of an uncertainty analysis. now consider a situation where the sample size is not fixed but is somehow chosen by the experimenter in response to the data. the sample size is now the numerical value taken by a discrete random variable . the point estimators of and become ≡ and ≡ , and the confidence interval for (which is the interval estimator of ) becomes ≡ , . √ , , . √ . with the sample size being random, the estimator does not necessarily behave in the same way as , so we cannot say that the expected value of is equal to . similarly, the random intervals and are different entities, so while it is known from theory that has probability 0.95 of covering , the probability that will cover at different values of and is not known. to be more specific, we do not know the minimum probability over all possible values of and , which is the ‘confidence coefficient’ or ‘confidence level’ of . as shall be seen, the expected value of and the confidence coefficient of depend on the rule that the experimenter uses to decide when to stop making observations, which is called the ‘stopping rule’. we will find that, if the stopping rule is particularly unfavourable, can be underestimated by approximately 45 % on average and the confidence coefficient of can be reduced to approximately 88 %. with the sample size being random, we must also recognise that the variance of the measurement error, which is equal to the variance of , is no longer a constant. however, our interest lies in estimating not the unconditional variance of the measurement error but the variance conditional on the observed sample size, i.e. conditional on the event . we cannot take it for granted that this conditional variance is equal to / , so we cannot assume that the conditional standard deviation of the measurement error is given by /√ . more importantly, we cannot even take it for granted that is an unbiased estimator of and, therefore, we cannot assume that the sample mean is the preferred estimate of the quantity measured. (the estimator would not be unbiased, for example, if our rule was to stop sampling when the sample mean reached a certain level.) thus there are some basic questions to answer about before we examine the behaviours of and . 1.1. the conditional distribution of the measurement error  we have just hinted at the fact that the distribution of the sample mean on the condition that the sample size is could conceivably differ from the normal distribution with mean and variance / . if so, using the sample mean as the estimate of the quantity measured and taking our estimate of /√ as the standard uncertainty might be misguided. however, as now shown, there is no cause for alarm if our stopping rule depends only on the spread of the data and not on the location of the data with respect to any origin. we would like our stopping rule to be such that the distribution of on the condition that is the normal distribution with mean and variance / . result 3 above implies that, for a fixed sample size , the sample mean random variable is independent of every function of the variables that gives us no information about location. so if the decision to stop is made according to the value of some such function then the conditional distribution of at any point is   acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 | 32  unaffected by whether sampling stopped at that point or not; i.e., knowing that our stopping criterion will be first met by a sample of size would give us no extra information about the mean of those observations. it follows that the distribution of on the condition that is equal to the normal distribution with mean and variance / , as required. the only stopping rules we discuss in the rest of this paper meet this condition, and so our principal task remains the unbiased estimation of , from which we obtain a (conditional) unbiased estimate of / simply by dividing by . we would then seek an estimate of this to use as the square of the standard uncertainty, ̅ , for potential use in the ‘law of propagation of error’. consequently, our performance measure of relative bias in / retains its relevance. 1.2. sequential statistics and stopping rules  the idea that the sample size depends in some way on the data is the key idea of sequential statistics. perhaps the best example of a situation involving sequential statistics is where the efficacies of two treatments are to be compared in patients recruited to a medical trial. it is unethical to subject people to a treatment known to be sub-optimal, so the trial should be stopped as soon as the identity of the preferred treatment becomes clear. yet the overall procedure must maintain its statistical rigour for the comparison to be scientifically acceptable. this requires the development of a formal stopping rule that preserves the statistical integrity of the conclusion. in our context of type a evaluation, such integrity relates to the conditional distribution of the estimator of , (which we have just discussed), to bias in the estimator of and to the probability that the procedure generates an interval enclosing , which is an event that can be called ‘success’. whatever stopping rule is put into place, our estimate of should not be negatively biased and the minimum probability of success should be at least 95 %. in addition to the idea of having a fixed sample size, there are at least two broad possibilities for the way in which an experimenter might decide to cease making observations: 1. stop when the result is deemed to be sufficiently accurate, i.e. when the measurement uncertainty is sufficiently low; 2. stop when the data suggests that the experimental procedure is behaving as expected. as written, these two ideas are somewhat vague. we need to be more specific when describing a stopping rule because if there is no formal criterion by which an experimenter decides to finish sampling then there is no well-defined procedure whose statistical properties can be determined or claimed, either using theory or simulation. so let us now describe the basic situation in mathematical terms. observations of a single quantity are made one by one, the results being , ,⋯, , and after obtaining the figures ̅ ≡ and ≡ ̅ are calculated. the process is stopped when the growing dataset of , ̅ and values satisfies some well-defined condition, one possibility being a condition that reflects a minimum required level of measurement accuracy and another possibility being a condition that relates to the general appearance of the dataset. the sample size , which is the outcome of a random variable n, is the final value of . 2. various stopping rules and their effects  let us now examine the effects of different procedures on the aspects of performance described, which are (i) the extent of any bias in the estimation of and (ii) the extent of any reduction in the probability that the confidence interval contains . we focus on the idea of stopping when the measurement is deemed sufficiently accurate for the experimenter’s purposes, which in essence is when the figure /√ is sufficiently small. other criteria for stopping are discussed only briefly. 2.1. stopping at a predetermined level of accuracy  consider the notion of making observations only until a predetermined level of accuracy is reached. when the quantity that is measured is just one input to a broader measurement, (which is the primary context of the gum), this implies stopping when the corresponding figure of standard uncertainty, /√ , first drops beneath a predetermined maximum value . in contrast, when the measurement stands alone, it implies stopping as soon as , . /√ , where is a predetermined maximum half-width for the 95 % uncertainty interval. these are different stopping criteria, and we will indicate them by g and h respectively. the performances of these procedures will also depend on any initial sample size. with being unknown, the smallest possible sample size is two. however, we can also envisage a situation in which the experimenter is never prepared to use a sample of less than three, or maybe a larger number still. the rule of starting with a sample of size two and stopping as soon as /√ can be called g 2, , while the rule of starting with a sample of size three and stopping as soon as , . /√ can be called h 3, . the general forms of these rules are: g , : stop if and /√ h , : stop if and , . /√ . in this paper, always indicates a predetermined minimum sample size. the solid lines in figure 1 show the relative bias of for various stopping rules of the g kind. the bias is a function of / alone: the results were obtained by simulation where, without loss of generality, was set to unity. for each rule, 105 replications were conducted at each value of / indicated by the markers. figure 1 shows that if we use g 2, and the true unknown value of is approximately 2.5 then there is a negative bias of 45 %, i.e. the expected value of is as low as 0.55 . if there were many uncertainty components and each was affected in this way then the result would be the quotation of an uncertainty interval that was 26 % shorter than the appropriate interval. the solid lines in figure 2 show the probability that encloses for stopping rules of the h kind. the probability is a function of / alone. for each rule, 105 replications were conducted at each value of / indicated by the markers.   acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 | 33  figure 2 exhibits the same general pattern as figure 1, but the scale differs in the x-direction because needs to be larger than for comparable results. the success rate of with h 2, can be as low as 88 %, and this occurs when 2 . these results should be of some concern to any scientist whose practice is to measure and average the results until the uncertainty is ‘small enough’. that practice entails stopping at a point where the sample estimate of variance tends to be smaller than average at that sample size, and the validity of the statistical inference becomes affected. performance levels can be improved, but are not able to be fully corrected, by using larger values of the initial sample size, . restoring performance levels   performance can be restored somewhat by involving an ‘effective sample size’ smaller than the actual sample size. accordingly, the dashed lines in figures 1 and 2 show the results obtained when using the figure / 2 as the estimator of instead of . so the effective sample size for the estimation of the variance is deemed to be 2 not . the two corresponding rules are: g* , : stop if and /√ 2 h* , : stop if and , . /√ 2 . after applying either of the rules, the sample size is equal to , the standard uncertainty to be quoted with ̅ as an estimate of the quantity measured is ̅ /√ 2 and the realized 95 % confidence interval to be quoted for is ̅ , . √ 2 , ̅ , . √ 2 . this is the outcome of the confidence interval ∗ ≡ , . √ 2 , , . √ 2 . (in accordance with usual relationship between the sample size and the number of degrees of freedom, the method uses the multiplier , . instead of , . .) the dashed lines in figures 1 and 2 show the performances of g* , and h* , . the minimum possible sample size is three with a g* rule and four with an h* rule. using g* 3, in place of g 3, reduces the worst negative bias from -31 % to -8 % and using h* 4, instead of h 4, increases the confidence coefficient from 91½ % to 94½ %. the conservatism at low values of / and / is of little consequence; at these points in the parameter space, the absolute bias and the width of the confidence interval will be small. (the relative bias at / → 0 approaches 100 2/ 2 %, so the dashed line for g* 3, on figure 1 approaches the y-axis at the value 200 %.) the rule g* 4, has a worst negative bias of 5½ % and attains a confidence coefficient of 94½ %, (which is not shown graphically). these levels of performance might be regarded as acceptable, so it is suggested that the rule g* 4, be used when measuring for a prescribed level of accuracy in this way. thus, the minimum sample size suggested is four. conditional confidence and conditional bias  the confidence coefficient is the minimum probability that the procedure generates an interval containing . we have seen that the nominal 95 % estimation procedure involving h 4, has confidence coefficient 88 %, which seems too low for the label ‘95 % uncertainty interval’ to be acceptable. but perhaps we should be more interested in the minimum probability conditional on the observed value of the random variable , i.e. conditional on the event . in other words, perhaps we should prefer to generate 95 % ‘conditional confidence intervals’, where the relevant probability statement holds on the condition that takes its observed value. that is, if a random interval , , satisfies pr , , ∋ | 0.95 then , , on the condition that is a ‘95 % conditional confidence interval for ’. in this way, we would be making more use of the available information, and potentially generating a narrower uncertainty interval. the availability of a conditional-confidence procedure seems to depend on the availability of an ancillary statistic [6], which is a statistic whose distribution does not depend on the value of the parameter being studied. with stopping rule g , , for example, the variable is not ancillary when estimating because its distribution depends on / . further analysis of these ideas of ancillarity and conditional inference is outside the scope of this paper. 2.2. a two‐stage procedure for a predetermined standard  uncertainty  we now give two methods where the total sample of size is formed from an initial sample of predetermined size and a figure  1.  bias  of    (solid  lines)  and  / 2   (dashed  lines)  in  the estimation of   using stopping rules g ,  and g* ,  respectively.   figure  2.  probability  that  intervals  (solid  lines)  and  ∗   (dashed  lines) enclose  using stopping rules h ,  and h* ,  respectively.    acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 | 34  second sample of a size chosen using the data in the initial sample. so . the first method is found in the following paragraphs and the second method is given in section 2.3. both methods rely on the fact that the sample variance obtained after a fixed number of observations is an unbiased estimate of . therefore, on the condition that the total sample size is , the figure / is an unbiased estimate of the figure sought, / . suppose that the requirement of section 1.1 is met. also, as in section 2.1, suppose that there is a predetermined figure that would be a maximum acceptable value for the standard uncertainty of the measurement. set ∗ / . then, notwithstanding the fact that ∗ will not be a whole number, we would set ∗ and the figure of standard uncertainty would be . in practice we would set equal to the value of ∗ rounded up. thus, an acceptable procedure is: 1. make n observations, calculate and / ; 2. if 0 then make n more observations, calculate the overall mean x and quote ̅ . if n 0 then calculate x from the first sample and set 1 /√ 1 . this procedure is analogous to stein's fixed-width confidence interval [7], which has been an influential technique in sequential statistics. stein showed that a procedure for realizing a 95 % confidence interval of a fixed width 2 for the mean of a normal distribution is as follows: 1. make observations, calculate and , . / ; 2. if 0 then make more observations and calculate the overall mean ̅. if 0 then calculate x from the first sample; 3. quote the interval ̅ , ̅ as a realized 95 % confidence interval for . the drawback of these procedures is that neither makes use of the information about the variance contained in the second sample. 2.3. a two‐stage procedure with the standard uncertainty chosen  after the first stage  so far we have considered the situation where the minimum required level of precision represented by or is known beforehand. that is somewhat unrealistic, for the experimenter is potentially committing himself or herself to making a huge number of observations depending on the ratio of / or / with unknown beforehand. so now we envisage that an acceptable value for the standard uncertainty is identified after taking the first sample and that the corresponding size of the second sample is then calculated. the sample variance obtained after a fixed number of observations is an unbiased estimate of based on 1 degrees of freedom. thus, on the condition that the overall sample has size and the requirement of section 1.1 is met, / is an unbiased estimate of the conditional variance of the overall sample mean, which is / . therefore, a valid procedure is: 1. make observations and calculate ; 2. without paying attention to the mean of the first sample, choose n so that /√ is an acceptable figure of standard uncertainty; 3. make more observations, calculate the overall mean ̅ and define ̅ /√ . this procedure can be improved upon if the experimenter accepts the possibility of the corresponding standard uncertainty being a little larger than the figure chosen. no matter how is chosen, the quantity stwo 2 = 1 1 , which is the sample variance of the second sample, is an unbiased estimate of based on 1 degrees of freedom. therefore the pooled estimate of variance, pool 1 1 two 2 is an unbiased estimate of based on 2 degrees of freedom, and so has a smaller parent variance than . thus, provided that the choice is made beforehand, the third step above can be replaced by: 3. make more observations, calculate the overall mean ̅ and define ̅ pool/√ . of the methods described so far, the method of this section is, perhaps, the method of choice for practical experimentation. it provides the experimenter with some control over both the final standard uncertainty and the final sample size. 2.4. other stopping rules  in the course of this work a number of other stopping rules were examined, almost all of which appeared to lead to statistical exactness, as would be hoped for. the basic requirement for exactness, when sampling from a normal distribution, is that the decision to stop is not influenced by the observed values of the sample mean and sample variance. a method for preventing this, albeit one that most experimentalists would find unacceptable, would be as follows. let and be constants whose values are chosen randomly and are not revealed until sampling has stopped. (we require 0.) instead of observing , ,⋯ the experimenter observes , ,⋯. with and being completely unknown, the observations give no information about the sample mean and sample variance (of the recoverable values) and hence no information about and . therefore, no stopping rule can affect the validity of the statistical inference, and so the performance levels of the type a evaluation procedure will not be affected. perhaps the value of that somewhat impractical idea is greatest when it is used in a thought process. if the experimenter can honestly say that they would have stopped taking observations at the same point if they had been observing instead of for all choices of and then the rule by which sampling was ceased does not introduce error. last, we consider the idea of measuring until forced to stop simply because of time constraints. in this case, the sample size is not influenced by any of the data, and so this stopping rule does not introduce error.   acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 | 35  3. bayesian analysis  currently, there is some interest in the use of bayesian statistics for the evaluation of measurement uncertainty. indeed, the revision of the gum is being carried out in line with some of the principles of bayesian statistics [8], these principles being associated with ‘objective-bayesian’ methods. so we now discuss a bayesian understanding of the issue described in this paper. the output of a bayesian statistical analysis depends on the prior distributions for the unknown parameters and the likelihood function for the data observed. it does not depend on the stopping if the stopping rule is ‘noninformative’, which means that the value taken by provides no information about the parameter being estimated beyond what is contained in the prior distributions and the likelihood function at that point [9], [10], [11]. this is a weak condition, and all the stopping rules discussed in this paper would be noninformative in a bayesian analysis. thus the bayesian paradigm does not distinguish between the reliability of the output obtained with a fixed sample size and the output generated by one of the stopping rules we have examined. this presents the objective-bayesian statistician with a number of related problems when considering type a evaluation of uncertainty, as we now demonstrate. these problems can be added to a growing list of difficulties associated with the use of bayesian ideas in the evaluation of measurement uncertainty [12]-[17]. 3.1. an inconsistency and an invalid procedure   our measurement problem involves random sampling from a normal distribution with mean and unknown variance . it is well known that when the standard prior distributions are used in an objective-bayesian analysis of this problem, the posterior distribution of after observing a sample of size is the distribution of ̅ /√ where is a random variable with the t-distribution with 1 degrees of freedom. (this is also the distribution advocated to describe in supplement 1 to the gum [18].) the conclusion would be drawn that ‘there is 95 % probability that lies in the interval ̅ , . / √ , ̅ , . /√ ’, which is seen to be the same interval realized by . this would be the case in an objectivebayesian analysis under any of the stopping rules we have discussed. therefore, when the rule is to take a fixed number of observations, the bayesian analysis generates the same interval as the classical methodology, which is known to be successful on 95 % of occasions. yet when using rule h 2, , for example, the analysis is successful on less than 95 % of occasions, especially when the reference set of problems involves small values of / ; see figure 2. so the results of two procedures with different success rates are, in effect, always regarded by the objective-bayesian statistician as being equally reliable! 3.2. an unacceptable principle   the idea that bayesian inference does not depend on the typical stopping rule opens up the hypothetical possibility of abuse. for simplicity, suppose that is known exactly. after making observations ,⋯, the bayesian statistician using the flat prior distribution for (and the person adhering to supplement 1 to the gum [18]) will assign to the normal distribution with mean ̅ and standard deviation √ . whatever the value of ̅ and however the observation process was terminated, the person will feel able to state ‘pr ∈ ̅ . √ , ̅ . √ 0.95’. if a probability of 0.05 is used as a threshold of plausibility, the person will consider all values outside that interval to be implausible as values for . suppose the person is prepared to make up to 10 observations and, for some reason, wishes to reach the conclusion that a certain value is not a plausible value for . simulations show that they will be able to do so with probability at least 0.195 even if is equal to the true value of . that is, if is the random interval ∑ ⁄ 1.96 √⁄ ,∑ ⁄ 1.96 √⁄ then pr ∌ or ⋯ or ∌ 0.195 for all and, in particular, for . if the person is prepared to make up to 30 observations then the figure becomes 0.29. worse behaviour is admitted by moving to large, albeit astronomical, sample sizes, as an informal argument now shows. after observations the objective bayesian statistician will calculate a symmetric 95 % credible interval for to be ̅ . √ , ̅ . √ . the (frequentist) probability that the interval calculated after observations will exclude is 0.05. now let be a number so much greater than that the effect of the first results on the mean of the first results is negligible. then, in effect, the probability that the interval calculated after observations will exclude is 0.05 independently of whether the interval after observations excludes . thus, the probability that one or both of these two intervals will exclude is 1 0.95 0.1. now let be a number so much greater than that the effect of the first results on the mean of the first results is negligible, and so on. if there are stages then the probability that at least one of the intervals will exclude is 1 0.95 , which can be made arbitrarily close to 1 by making sufficiently large. moreover, this argument holds even if we carry out the analysis with 99 % intervals, or with any other threshold for implausibility. thus, by measuring enough times, (albeit an astronomical number of times), the objective-bayesian statistician can always conclude that the true value of is implausible as a value for . evidently, this is also the case for any other value that they wish to rule out. 3.3. poverty of objective bayesian statistics   the set , ,⋯ is a sample path of a stochastic process , ,⋯ . under the assumption of independent sampling from a shared distribution with mean and standard deviation , the related stochastic process ≡ ∑ ⁄ obeys the law of the iterated logarithm, which states that lim → sup √2 with probability one. therefore, lim → sup √⁄ ∞. this implies that lim → sup √⁄ ∞ where ≡ ∑ ⁄ is the stochastic process of the sample mean. and, by symmetry, lim → inf √⁄ ∞. thus, by   acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 | 36  sampling long enough, we can make the quantity ̅ √⁄ pass through any value we choose, and in particular we can reach values and such that the intervals ̅ , ̅ and ̅ , ̅ are disjoint for any finite predetermined value of the coverage factor , say 1.96. if the distribution is normal then process is a brownian motion, which has a self-similarity property. this means we can repeat the process ad infinitum to obtain new pairs , with this property. importantly, we can choose arbitrarily and then wait until a suitable arises. this phenomenon permits a whimsical but correct analysis of how profit can be guaranteed when betting against an objective-bayesian statistician who trusts the figures of probability they calculate and behaves accordingly. a bayesian acting as a ‘rational agent’ will always accept a bet in which they view the expected value of their profit as positive and will reject a bet in which they view the expected value of their profit as negative. in the current problem of estimating the mean of a normal distribution, we take a sample of arbitrary size ( ) and offer the person 1 cent against 10 cents that lies outside the 95 % credible interval they calculate for it. they will necessarily accept, believing their expected profit to be 0.95 1 0.05 10 0.45 cents. we then continue sampling until the corresponding 95 % credible interval is disjoint from the original one, which as has been shown must happen at some point ( ), and we present the same offer with the new interval. again, the person must accept if they are to be acting in a way consistent with their philosophy. then we point out that they must be incorrect in at least one of the wagers, and we generously concede the other. our profit is 9 cents. this process can be repeated until matters of morality and compassion dictate. there is nothing in the bayesian theory to invalidate this behaviour by the objective-bayesian statistician. it is an example of the incoherence admitted by using an ‘improper’ prior distribution in a bayesian analysis. the same result applies in the case of normal sampling with unknown because the sample standard deviation is a consistent estimator of . 4. type a evaluation of the variance in the  technique  this paper has addressed type a evaluation of measurement uncertainty when all the data originates in the measurement at hand. accordingly, is deemed unknown a priori and is estimated only from the data obtained in the measurement. in this situation we are primarily concerned with obtaining an unbiased estimate of ⁄ where is the final sample size. the standard uncertainty to be quoted is then √⁄ , where is some figure that in effect acts as an estimate of . usually . however, there are many situations where has been estimated beforehand in order to ‘calibrate’ or ‘characterize’ a measurement process and to establish ‘the standard uncertainty of the measurement technique, ’. this is to be contrasted with ‘the standard uncertainty in the particular measurement result √⁄ ’. for example, the gum describes a situation in which the variability of one aspect of the procedure was estimated beforehand from 25 observations, so that the corresponding estimate of variance was based on 24 degrees of freedom [1], [2]. in the earlier measurement, the objective will have been to obtain an unbiased and sufficiently accurate estimate of , (which is a quantity that does not vary with ), rather than an unbiased and sufficiently small estimate of ⁄ . if this estimate of is subsequently used in the measurement at hand then the experimenter knows in advance how many observations must be taken to obtain a sufficiently accurate result. so the analysis given in section 2 is not necessary and statistical exactness is maintained provided that the behaviour of the experimenter is not affected by the changing value of the sample mean. 5. conclusion  the thought processes by which experimentalists decide that enough observations have been taken will be many and varied and they might not be describable in rigorous form. however, one idea is that observations are made until the corresponding component of standard uncertainty associated with repeatability or reproducibility is, apparently, smaller than some predetermined level. figures 1 and 2 show that if such a ‘stopping rule’ is applied in gaussian sampling and if this figure of standard uncertainty is obtained from the observations at hand then the effect is to invalidate the statement of uncertainty. without appropriate modification, the procedure gives, on average, too small an estimate of the measurement variance, the worst possible bias being -45 %. similarly, the associated nominal 95 % confidence interval can have a confidence coefficient as low as 88 %. a simple modification described in section 2.1 involving the use of an effective sample size two less than the actual sample size restores the performance to an acceptable level. an appropriate related stopping rule is g* 4, . perhaps it is more realistic to envisage that the final number of observations made is chosen after observing the data from an initial set. accordingly, section 2.2 and section 2.3 give twostage procedures that do not compromise the validity of the type a evaluation. the method of section 2.3 seems preferable because it gives some control over both the sample size and the standard uncertainty of measurement. one finding of this paper is that making observations until the corresponding standard uncertainty ‘looks small enough’ does not lead to a valid statistical procedure and hence does not lead to valid type a evaluation of measurement uncertainty. an advocate of bayesian statistical methods would disagree with that statement, but unacceptable results appear when the analysis is carried out using the standard objective-bayesian prior distributions. this further calls into question the wisdom of adopting bayesian ideas in the supplements to the gum and in the revision of the gum. the material presented in this paper relates to type a evaluation of uncertainty where the variability of the measurement technique is assessed during the measurement at hand. it does not relate to the situation where the estimate of the variance was obtained beforehand, as when the technique is calibrated. references  [1] guide to the expression of uncertainty in measurement, clause 4.2, (geneva: international organization for standardization) 1995.   acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 | 37  [2] jcgm 100:2008 evaluation of measurement data guide to the expression of uncertainty in measurement, clause 4.2, http://www.bipm.org/en/publications/guides/ [3] j.k. patel, c.b. read, handbook of the normal distribution, section 5.2.3, marcel dekker, new york, 1982. [4] j. pfanzagl, estimation: confidence intervals and regions, international encyclopedia of statistics (eds.) w.h. kruskal, j.m. tanur, the free press, macmillan, 1978, pp. 259-267. [5] g.k. robinson, confidence intervals and regions, encyclopedia of statistical sciences, vol. 2, (eds.) s. kotz, n.l. johnson, c.b. read, wiley, 1982, pp. 120--127. [6] j. kiefer, conditional inference, encyclopedia of statistical sciences, vol. 2, (eds.) s. kotz, n.l. johnson, c.b. read, wiley, 1982, pp. 103-109. [7] c. stein, a two sample test for a linear hypothesis whose power is independent of the variance, ann. math. statist. 16 (1945) 243258. [8] w. bich, revision of the ‘guide to the expression of uncertainty in measurement’. why and how. metrologia 51 (2014) s155-s158. [9] j.o. berger, statistical decision theory and bayesian analysis, section 7.7.5, 2nd ed., springer, new york, 1985. [10] a. o'hagan, kendall's advanced theory of statistics, volume 2b, section 5.19, bayesian inference, edward arnold, 1994. [11] p.h. garthwaite, i.t. jolliffe and b. jones, statistical inference, section 7.6, 2nd ed., oxford university press, 2002. [12] b.d. hall, evaluating methods of calculating measurement uncertainty, metrologia 45 (2008) l5-l8. [13] r. willink, difficulties arising from the representation of the measurand by a probability distribution, measurement science and technology 21 (2010) 015110. [14] r. willink, on the validity of methods of uncertainty evaluation, metrologia 47 (2010), pp. 80-89. [15] r. willink, measurement of small quantities: further observations on bayesian methodology, accreditation and quality assurance 15 (2010) 521-527. [16] r. willink, measurement uncertainty and probability, cambridge university press, 2013. [17] r. willink, what can we learn from the gum of 1995?, measurement (in press). [18] jcgm 101:2008 evaluation of measurement data supplement 1 to the ‘‘guide to the expression of uncertainty in measurement’’ propagation of distributions using a monte carlo method, clauses 6.4.9 and 6.4.7, http://www.bipm.org/en/publications/guides/   microsoft word 281-1911-1-le-rev3 acta imeko  issn: 2221‐870x  december 2015, volume 4, number 4, 48‐56    acta imeko | www.imeko.org  december 2015| volume 4 | number 4 | 48  measurement of human movement under metrological  controlled conditions  francesco crenna, giovanni battista rossi, alice palazzo  university of genova, dime/mec, via all’opera pia 15a, i‐16145 genova, italy      section: research paper   keywords: complex measurement systems; kinematic measurements; biomechanics  citation: francesco crenna, giovanni battista rossi, alice palazzo, measurement of human movement under metrological controlled conditions, acta  imeko, vol. 4, no. 4, article 10, december 2015, identifier: imeko‐acta‐04 (2015)‐04‐10  editor:  paolo carbone, university of perugia, italy   received june 8, 2015; in final form december 3, 2015; published december 2015  copyright: © 2015 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: this research was partially funded through unige research programs pra 2013‐2014.  corresponding author: francesco crenna, e‐mail: francesco.crenna@unige.it    1. introduction  in biomechanical literature, generally speaking, the quality and reliability of measurement results is often taken as granted [1], [2]. vendors of biomechanical instrumentation often refer to clinical studies, which give little metrological attention to reliability of measurement data [3]. yet, a deeper investigation in motion generation and control, shows that measurement uncertainty plays a critical role [4]-[9]. for example, in the experimental study of human movement is it essential to be able to distinguish between inter-subjective variability, which is often the object of the investigation, and the dispersion due to measurement noise, also accounting for the indeterminacy due to measurement uncertainty related to systematic effects [4]. the study of human movement requires the measurement of both kinematics quantities, such as linear and angular displacement, velocity and acceleration of relevant body points and segments [5], and external forces that characterise the interaction of persons with the environment in which motion takes place. internal driving forces produced by muscles may instead be indirectly measured starting from direct motion and external-forces measurement through the solution of an “inverse dynamic problem”. in this latter case, the reduction of the uncertainty of initial direct measurement is particularly critical, due to uncertainty propagation effects. therefore, with the aim of contributing to improve the quality of measurements in experimental biomechanics, we present the design, realisation and testing of a complex measurement system, including some kinds of sensory redundancy, in order to ensure the reliability of the results and their metrological characterisation, in terms of uncertainty evaluation. the system is based primarily on a video system operating with markers, together with inertial sensors and force platforms [7], [9]. lower limbs kinematic may be obtained from two independent methods, so it is possible to provide verification by results comparison from the two methods [10]. the position abstract  the  study  and modeling  of  human  movement  requires  accurate  measurement  of  forces  and  of  kinematic  quantities,  with  proper  control and monitoring of measurement uncertainty. the metrological aspects of such experimentation are often overlooked in the  biomechanical literature. in contrast, we present here a complete measurement system that combines a double force platform with  vision and inertial sensors. thanks to some degree of instrumental redundancy a cross‐validation of different measuring channels is  possible. therefore the system permits the characterization of a person’s movement, enabling, at the same time, the verification of  the acquired experimental data. details are provided about the system and about the measurement procedure, in order to allow the  reproducibility  of  the  experiment,  if  needed,  in  other  laboratories.  then,  as  a  test  case,  the  experimental  study  of  hopping  is  considered. three male subjects performed the tests on several sessions. experimental results are discussed, with special focus on  kinematic quantities, and system performance is discussed, by properly accounting for uncertainty issues.  acta imeko | www.imeko.org  december 2015| volume 4 | number 4 | 49  of various points of interest in the sagittal plane can be obtained directly from video, or they can be also evaluated through the use of inertial data, measured on each body segment, and applied to the model of the limbs [10]-[12]. as test case we exploit the gesture of hopping, since it is frequently used in literature to describe inferior limbs behaviour [13]-[18]. the paper is organised as follows. in section 2 we present a detailed description of the measuring system, outlining sensory redundancy and synchronisation issues. then, after a brief presentation and motivation of the selected test case (in place hopping) (section 3), in section 4 we present the calibration and testing procedure, highlighting critical aspects, and providing enough details for the procedure to be reproduced and compared in other laboratories. then, in section 5 we present experimental results and provide an in depth discussion of uncertainty issues. 2. measurement system  a scheme of the general measurement setup is presented in figure 1. the overall system consists of a combination of three subsystems, which were originally autonomous, and independent of each other: force platform, video and inertial measurement systems. in order to guarantee the same high reliability level of each subsystem as regards both data recording and timing, the overall setup synchronises them as communicating, but still autonomous systems. such solution offers more reliability, at the cost of a more complex management of the set up. let us now discuss how all this works. 2.1. markers and video system  the video system is based on a black and white camera and a set of markers placed along the subject’s leg in specific anatomical landmarks. two cameras have been considered and their performances have been evaluated. the first solution, more commonly used in our laboratory at present, consists in a basler camera, connected to a computer via a standard interface ieee 1394. the alternative solution is based on a dalsa falcon camera that has a higher resolution and a higher frame rate. the second option presents lower measurement uncertainty, mainly due to higher resolution, and a higher frame rate. a comparison of the two systems is reported in table 1. a 12 mm focal length optics has been selected to have a complete view of the subject during hopping, from ground to head and shoulders. we have also considered the movement required in the test case. in this configuration the viewing angle in the vertical direction is about 30°. the video system is completed with a set of active markers consisting of high intensity, 5 mm diameter, white led (2300 mcd @ 3.6 v and 20 ma), which are patched over the subject’s skin in the selected anatomical landmarks, facing the camera line of sight. in general a proper number of markers are used to identify specifically the movement of the most interesting landmarks, according to the biomechanical model in use. for example, if interested in lower limb movements, hip, knee and ankle movements are fundamental. other markers might be placed on foot and/or along leg segments [6], [12], [13], [15]. three markers are sufficient for the measurement of legsegments orientation and relative angles, at the ankle, and knee [10]. anyway, the system is flexible and allows the processing of a variable number of markers, according to the test requirements. camera image acquisition parameters are settled to obtain a black image with white spots produced by the high intensity led, placed on anatomical landmarks. the video acquisition system saves an avi file at 50 hz frame rate, composed by bitmap raw images without any compression to maintain the original image resolution. the file is further processed to obtain marker positions in the image plane. image processing is carried out off line: white spots are located in each image frame by frame and their positions are obtained, as their white level weighted centres. therefore, even if the spot diameter on the image of a 5 mm led diameter is larger than 2 pixels, marker position is determined with a sub-pixel resolution. position on image plane is then transferred on the sagittal plane considering sensitivity obtained through a previous calibration, under the hypothesis that subject’s movements are in a plane parallel to the image plane. such hypothesis has to be verified through experimental results. 2.2. inertial measurement system  the subject under test is equipped also with a set of inertial sensors (inertial measurement unit, imu), model mtw by xsens technology. each unit includes tri-axial accelerometer, gyroscope and magnetometer, and is wirelessly connected to a master base station. some relevant technical features are reported in table 2. imu signals are internally pre-processed by the original xsens software, yielding the orientation of each sensor in the three dimensional space. sensor boxes (of dimensions 34.5 × 57.8 × 14.5 mm (w × l × h)) are placed on the subject’s body. for the monitoring of lower limb movement, they are placed on foot, tibiae, thigh and usually on torso too, by elastic velcro® strips with sensor figure 1. schematic of the overall measurement system  table 1. video systems comparison.    basler a601f  dalsa falcon  resolution [pixel]  640 x 480  1400 x 1024  pixel size [m]  9.9 x 9.9  7.4 x 7.4  sensor dimension [mm]  12.7  12.7  max. frame rate [hz]  60  100  data transmission  ieee 1394  camera link  spatial resolution (at 1.2 m  distance camera‐subject)  [mm/pixel]  2.94  1.34  acta imeko | www.imeko.org  december 2015| volume 4 | number 4 | 50  supports [12]. each unit has a local coordinate reference system (lcs) and the system measures its angular position with reference to a fixed world reference system (wcs) to the master station. each unit is previously reset in order to have both ylcs and zlcs axes coincident with the wcs ones and oriented along the same direction. ywcs and zwcs are oriented along subject’s frontal direction and vertically [10], [11], [19]. acceleration data are also available to the user for any further investigation. the inertial measurement system is autonomous and with four imu sensors wirelessly connected to the control station, the system operates at 75 hz sampling frequency. 2.3. force measuring system  the force measuring system consists of two p6000d platforms by bts engineering with their own data acquisition and processing system; some technical characteristics are given in table 3. each platform is equipped with four tri-axial force sensors and is able to measure the overall forces exchanged by subject with ground during movement (ground reaction force, grf) and the corresponding centre of pressure, cop. the cop provides the position on the ground plane (or platform plane in this case) where the force transmission between the subject and the ground takes place. the position of the cop, as compared with that of the centre of mass (com), is a key quantity for studying, e.g., the subject’s vertical stabilisation [20]-[21]. platforms are initialised before every test in order to take into account their zero deviation; zero drift has been verified on a time span much longer than a test session and it resulted to be negligible. platforms operate at 1 khz sampling frequency. during tests it is possible to load each platform with one foot or with both feet, in order to study postural or force transversal asymmetries. the former situation results with cop and force data relative to each leg, while the latter measures the complete subject’s body without left-right leg distinction. 2.4. measurement system synchronisation  the video acquisition is autonomous and assumes the role of master in the synchronisation of the overall measurement system. it sends to imu control station and to the force platforms a start and stop trigger to control data acquisition. the imu control station, which performs at the lowest sampling rate among all the equipment, sends a trigger signal to the video master with the timing of each inertial measurement on rising edges, useful during off line processing to eventually resynchronise all measurements. 3. a test case: hopping  in order to validate the proposed measurement system, we have considered a “reference” gesture, hopping in place, for which many experimental results have been reported in the literature, therefore providing a sort of validation reference [13]-[18], [20], [22], [23]. this gesture is on the one hand informative, in that it provides a good example of lower limb movement, on the other it may be satisfactorily described in a single plane, which constitutes a noteworthy simplification in motion analysis that allowed us to focus on metrological aspects. experimentation repeatability and reproducibility has been enhanced by controlling subject’s hopping rate through a metronome. in fact, in previous studies a preferred hopping rate was identified, which seems to be quite independent of subject’s physical characteristics and may therefore be kept, without interfering excessively on subject’s attitude [10], [20], [13], [17]. currently available studies on hopping mainly consider the evaluation of the moments required, at the limbs joints, for performing the gesture. for example, they investigate the adjustment of the lower limbs behaviour according to the required performance [24]-[25], or due to the geometric nature of the ground, such as inclined or uneven, or to ground's dynamic nature, such as rigid or compliant [13], [18], [22]. on the other hand, a deep investigation of the displacement of the com and cop seems to be missing [17]. in contrast, these are important biomechanical variables, for investigating the motion control required to maintain the (dynamic) equilibrium in a fairly constant position along the horizontal, anterior-posterior axis, whilst cyclically moving along the vertical axis. these latter quantities will be therefore addressed in some detail. 4. experimental procedure  as a part of the present metrological approach, a detailed measurement procedure, involving both instrumental setup and subject training, has been developed, with a special attention to ensure the highest achievable reproducibility. 4.1. video system calibration  the video system is set up to have the image plane parallel to subject’s sagittal plane. before every test session, the camera distance from subject is adjusted to obtain a whole body image starting from the lower edge of the force platform, and considering some extra space to accommodate the hopping height. the overall height in the frame is about 1.9 m. in the first phase optics aperture and camera parameters are adjusted to have a good image quality. in such conditions the video system is calibrated using a vertical reference with marks every 100 mm. the reference covers the middle part of the image for a height of about 1.2 m. the reference is located at the same distance from the camera of the instrumented right hand side of the subject. the image is processed with a matlab® script to identify the ticks and a linear regression is carried out to obtain video system sensitivity. results are pretty stable and reproducible: table 2. inertial sensors characteristics    xsens mtw  dynamic range    200 rad/s ‐ 160 m/s 2 static accuracy (roll‐pitch)  0.5 deg  static accuracy (yaw)  1 deg  angular resolution  0.05 deg  internal sampling rate  1800 hz  bandwidth   >120 hz  wireless sampling rate (up to 6 sensors)  75 hz  table 3. force measurement system    bts p6000d   x ‐  y sensor range  up to ±2000 n z sensor range  up to 2000 n sensitivity/resolution  16 bit over selected range sensitivity deviation on plate surface  <1.0 % full scale output hysteresis  <0.2% full scale output acta imeko | www.imeko.org  december 2015| volume 4 | number 4 | 51  generally speaking, after linear regression on 12 references (about 1.2 m) we obtained a sensitivity of about 2.9 mm/pixel, with a standard deviation of residuals less than 0.3 pixels and a very good linear behaviour along the image height. reproducibility due to different measurement setup is very good, since sensitivity variations are usually less than 0.2 %. thanks to the high quality of the optics and camera ccd sensor, a more detailed calibration procedure, following for example dlt or zhang methods [26]-[27], has shown both negligible distortion correction and negligible anisotropies for this application. besides that, the calibration image gives the reference point for the zeroing of video readings in both vertical and sagittal direction, so that the video results will be strictly related to the force platform readings as schematised in figure 2. for this purpose, the calibration reference is positioned in the horizontal direction on the force platform left edge: from a calibration image it is possible to measure video cop zero position obtaining the common point between video and force sensors. 4.2. imu  imu sensors require an initial set up procedure. they are placed horizontally on the floor, with local z axis aligned along the frontal axis near the test site; after activation, they rest quiet for at least one minute to warm up and stabilise the readings, and then readings are zeroed in this position. a static recording of the four sensors is carried out before installing them on the subject. 4.3. subject set up  as discussed in section 3, the test case requires the detailed measurement of com together with leg and foot movements. for this reason seven anatomical landmarks are selected for marker placement, as presented in table 4. they allow the measurement of both absolute and relative angles at the joints, for the four ‘rigid’ segments considered: foot, lower leg, thigh and torso with arms and head. the same segments are instrumented with imu sensors [12], [15], [21]. a running suit with a full set of markers sewn in proper position is available, but in order to obtain a more accurate marker positioning on each subject, we found that sticking of each marker on the skin is far better and it avoids suit movements on the skin. once markers are placed on subject’s right side facing the camera, imu sensors are placed on each segment by using velcro elastic straps. force platforms are zeroed and a static acquisition of subject standing quiet is carried out before proceeding with hopping tests. 4.4. tests   once subject is properly instrumented, force platforms are zeroed and camera parameters are set to obtain black images with white markers. we are ready now for a test session. the hopping test is performed with a reference sound giving 2.2 hz pace. after a short training, each subject is able to reproduce the hopping pace: test starts when the subject feels a good accordance with the pace, and it lasts for at least 10-15 s (about 20-30 events). then the subject rests for some time, while data are checked. an experimental session consists of several tests performed both naturally, i.e., without taking care of height performance except for pace, and forcing the hopping to obtain a higher height, while maintaining the pace [24]. test session ends, as it was started, with a static recording. after the test, all the measurement data are verified and if they present inconsistencies, such as misalignments in still measurements, rhythm anomalies and so on, the test is discarded. 4.5. data processing  the processing requires some subsystem specific preprocessing to obtain properly synchronised and scaled data. video files are processed in lab-view®, identifying the positions of white marker centres in the dark image and then verifying marker tracking by vertical alignment from foot to shoulder [28]. marker positions are saved in a file for further processing. imu signals are processed by xsens software package, mtw manager, to obtain angle data. force platform and cop data are exported as separate files for further processing. further processing is carried out in matlab® and consist of: synchronisation of signals from the three subsystems considering in particular the imu measurement trigger. resampling of all the data at 200 hz, to obtain a common time reference. restitution of markers position in millimetres, by considering sensitivity obtained during calibration. evaluation of segment absolute and relative angles from video and imu sensors. determination of the ground and aerial hopping phases by using a threshold on the vertical force signal. evaluation of quantities at touch down and their excursions at ground. data verification. data presentation and output. table 4. markers and imu positions.  imu position  imu number  marker position  marker led  number  foot tip  1  foot back 1 metatarsus  2  heel  3  tibiae 2 ankle  4  knee  5  femur 3 hip  6  lower back 4 shoulder  7  figure  2.  calibration  setup.  zi  is  the  front  to  rear  direction  on  the  image plane;  zs  is  the  same  direction  but  measured  by  force  platforms.  the common origin is given by the calibration image.  acta imeko | www.imeko.org  december 2015| volume 4 | number 4 | 52  5. experimental results  the focus of our experimentation is to validate the proposed measurement system. for this purpose we present here some results obtained from three male subjects (24-50 years old, 1.701.80 m, 64-70 kg) hopping at natural and maximum height. to validate the proposed measurement systems we compare quantitative results from one subject, with data available in literature, and we propose self-consistency tests and an evaluation of the measurement uncertainty. figures 3 and 4 present time histories of vertical ground reaction force (grf) and of the markers position. in figure 3 it is possible to identify hopping ground and aerial phases, as obtained by thresholding the grf. note that the force is normalised according to subject weight, in order to be able to compare different subjects. in figure 4 it is possible to appreciate hopping repeatability and the small movement of the subject in the ante-posterior direction during the performance. figure 5 presents joints angles together with ground phase indication: again good repeatability is evident. it is possible to make a comparison with literature values considering the ground phase. we can consider values for some quantities of interest at touchdown and their variations during the ground phase, as presented in table 5 for a 65 kg, 1.72 m, male subject, together with some reference values from the literature [13]. mean values show a rather good agreement with literature considering that results from this study refers to repeated executions of the test by a specific subject while in literature a rather large set of subjects was considered, with balanced gender and age composition. we are instead mainly interested in validating the procedure, rather than in obtaining statistics for a population of subjects, and we therefore focus on a single subject. however it is worth noting that absolute angles at touchdown depend on initial marker alignment on the subject's limb. this is particularly critical since it requires very accurate marker sticking on anatomical landmarks. if some of them can be identified on the skin surface with good accuracy, (malleolus, knee) for others this is rather difficult (metatarsus, hip and shoulder in alignment with hip). this results in a bias in touchdown angle evaluation and systematic uncertainties in relative ones, as will be discussed in section 5.3. this is evident from values in table 5, where it appears that the hip angle at touchdown and relative angle at the ankle presenting the largest deviation from literature. figure 5. relative angles at  lower  limb  joints obtained by marker positions measured by the video system figure  3.  ground  reaction  force  components  and  contact  signal  as measured by force platform and normalized on subject weight.     figure 4. marker trajectories in the sagittal plane.  table  5.  leg quantities  during  ground  phase.  literature  reference  values  from [13].  quantity  experiment  ϭ  literature  ϭ  hopping rate {hz]  2.2  1.8 %    time spent at ground [ms] 297  7.5  308  8  ground reaction force [n]  1860  103  1740  87  touchdown angles [deg]        hip  180  1.5  171  3.7  knee  154  3  150  4.81  ankle  126  1.9  125  3.09  excursion at ground [deg]          hip  12  1.9  10  1.3  knee  26  1.7  23  2.3  ankle  42  4.2  32  1.0  16.5 17 17.5 18 18.5 19 19.5 20 20.5 21 90 100 110 120 130 140 150 160 170 180 190 relative angles time [s] a n g le [ d e g ] knee ankle hip contact 14 16 18 20 22 24 -5 0 5 10 15 20 25 30 35 ground reaction force time [s] n or m al is e d g rf [ n /k g ] x y z contact acta imeko | www.imeko.org  december 2015| volume 4 | number 4 | 53  5.1. details on the ground phase  moving now to another aspect of this study, we consider the displacements of whole body centre of mass (com) and of centre of pressure (cop). cop and com positions may be measured directly through the markers data or evaluated considering the com of each body segment and its position in the space, together with anthropometric data. this second procedure may use of marker video data, or orientation data from the inertial sensors. figure 6 presents a time history for the frontal movement in the sagittal plane of hip and metatarsus markers, cop and the whole body com. the ground phase is evident since no cop data are available during flight. figure 7 presents a detail for a single ground phase. during the ground phase we can consider that, since the com is located just above the hip marker, and the torso is a bit inclined and not fully upright, the com position is a bit displaced ahead from the hip. from a dynamical point of view the cop at touchdown tends to the back and returns in the front of the foot, while the com presents a smoother behaviour in accordance with cop. in addition to the points previously described, the instantaneous centre of rotation (cor) plays an important role for subject stability. generally it is assumed to be coincident with foot metacarpus, identified by a specific marker, yet it might be interesting to measure its position. a possibility is to assume the foot top as a rigid surface and considering velocity vectors measured at malleolus and metacarpus markers. since a foot top can be considered as a rigid element, at every instant the two straight lines by the extremes of the rigid element, corresponding to the velocity vectors, intersect in the instantaneous centre of rotation. such procedure presented some computational criticalities probably due to the composition of rotational and translational movements, so for the moment we will take the metacarpus position as foot centre of rotation. as shown in figure 6, cor, or metatarsus, during the ground phase moves from an anterior position backward and then returns ahead before take-off, following the movement of the foot. it is possible to analyse anterior-posterior force also, as presented in figure 8. here anterior-posterior force component follows the cop movement to maintain equilibrium during contact and hopping stability as shown for a set of successive hopping presenting some anterior posterior com movement which is compensated by the force component in the same direction. as regards left to right force component, this is not of interest in this case since we are not studying gesture symmetry, but we assume the same behaviour for the two legs. this approach seems to be reasonable for a healthy subject hopping in a stable position as shown in figure 9.   figure 8. antero‐posterior ground reaction force and cop position  figure 6. antero‐posterior movements of cop, com as measured at the hip, com evaluated from segments positions and metatarsus considered as foot rotation center     figure 9. normalised antero‐posterior and medio‐lateral grf components.  figure 7. detail of anterior‐posterior ground reaction force and cop position during a single ground phase.  -100 -50 0 50 100 150 antero-posterior force and cop time [s] a n te ro -p o s te ri o r f o rc e [ n ] 28 28.5 29 29.5 30 30.5 0 50 100 150 200 250 c o p d is p la ce m e n t [ m m ] fz cop z 5 5.5 6 6.5 7 -100 -80 -60 -40 -20 0 20 time [s] a n te ro -p o s te ri o r p o s iti o n [ m m ] antero-posterior movements cop metatarsus hip cm body 21 22 23 24 25 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 normalised antero-posterior and medio-lateral grf time [s] n o rm al is e d g r f [ n /k g ] x z contact 5.8 5.9 6 6.1 -90 -80 -70 -60 -50 -40 -30 -20 -10 0 time [s] a n te ro -p o s te ri o r p o s iti o n [ m m ] antero-posterior movements: contact detail cop metatarsus hip cm body acta imeko | www.imeko.org  december 2015| volume 4 | number 4 | 54  5.2. data verification  as discussed in section 2, the present measurement system architecture offers several data verification possibilities. we can introduce for example angle measurements from video and imu systems, as presented in the following figure 10. angles from the video system are computed by trigonometric formulae applied to marker positions measurements, while the imu system measures angles directly [23]. in general we obtained a good agreement between the two measurement systems: the mean differences between the angles measured at -ankle, -knee and -hip are respectively 1.7, 1.9 and 1.6 degrees with a relative standard deviation about 0.81, 3.7, and 4.3 %. as an example we can consider the hip angle. figure 10 shows the limited differences between the two independent measurement methods. nevertheless it is worth noting that small differences might be due to a misalignment between the sagittal plane, assumed coincident with the plane of the image, and the imu coordinate system, due for example to imu sensor placement or to its movement during hopping. for this reason this effect is different on each angle and it varies from a minimum of ± 3 deg up to ±5 deg for the foot, when the imu is near the force platform. another verification possibility regards the length of body segments, which we are assuming as rigid. figure 11 presents the femur and tibiae lengths as a function of time during the hopping. segments lengths are evaluated considering the markers distance between malleolus and knee (tibiae) and between knee and hip (femur). they are not rigorously constant, probably because of both small marker misplacements from the expected landmark, and of markers movement from their reference positions during performance. since markers are stitched on the skin to avoid suit movements, the effect is probably due to skin itself that moves during hopping performance in respect to segment rigid frame or bone. this is a typical problem in biomechanics [6], but in this case we had the possibility to directly quantify these effects. both the rather small variation in segment length, which is less than 1.5 % for tibiae, the worst case, and the good agreement on the angles computed from video and inertial data, give confidence about measurement system reliability. 5.3. uncertainty evaluation  let us now discuss the main uncertainty sources, as identified in the experimentation here presented. systematic uncertainty contributions are mainly due to misalignment of the video measuring system and imu with subject. another important factor is marker placement [2], [29]. angular misalignments produce a second order effect that is negligible if the camera is properly oriented during setup. in fact subject’s rotation around the vertical axis during hopping are not common and in general they require test interruption to recover equilibrium. through geometrical considerations it is possible to estimate the effect of such a rotation on position and angle measurements. for a rotation around vertical axes of 5 deg, which is the maximum we expect during performance, we obtained a 0.7 % deviation on all position measurements. it has no effect on angle measurements, since they are based on relative positions. angle measurements might be affected, as already introduced in section 5.2, by marker positioning. we can distinguish two situations when dealing with absolute or relative angles. for absolute angles we can distinguish between markers moving or misplaced along the segment or we can hypothesize movements along the camera horizontal and vertical axis. in the former case it produces no effect. in the latter, it is possible to evaluate a deviation of about 1 deg for the experienced length variations. for relative angles we have to consider the correlation between marker movements. let’s focus on knee angle and markers of tibiae and femur. in the case presented in figure 11, we have a linear correlation coefficient of about 0.8, indicating that most probably the knee marker was placed a little bit forward in respect to the knee centre of rotation. a geometrical evaluation of the corresponding knee angle mean deviation is lower than 1 deg, variable with the angle itself. of course other marker movements might contribute, but the very limited segment length variation and the good correspondence between video and imu angles, indicate a proper marker and imu placement. nevertheless, ankle angle presents a nearly constant difference, for video and imu measurements, with a systematic reduction for ankle angle video measurement, as compared with imu. these phenomena might be explained geometrically, considering metatarsus marker and its placement. if the position of this marker is slightly moved toward foot tip, this will introduce a corresponding reduction in ankle angle measurements. it is possible to estimate this effect geometrically, obtaining, for a displacement of about 3 cm, an   figure 10. hip angle measurements verification: comparison of the readings obtained by markers plus video and by imu.    figure 11. validation of marker position measurement by video: femur and  tibiae lengths  18.6 18.8 19 19.2 19.4 19.6 19.8 20 20.2 20.4 20.6 165 170 175 180 185 time[s] a n g le s [ d e g ] hip absolute angles imu video 16.5 17 17.5 18 18.5 19 19.5 20 20.5 21 400 420 440 460 480 500 520 540 560 femur and tibiae length time[s] le n g th [ m m ] tibiae femur acta imeko | www.imeko.org  december 2015| volume 4 | number 4 | 55  angle measurement reduction of about 20 %, that corresponds to the difference experimentally verified between video and imu. such a large marker displacement is realistic if we consider that estimating metacarpus position form foot skin or from outside a shoe is not easy in general, and in particular for non-physician personnel. another possible source of uncertainty is subject to camera distance. deviations of this distance from the standard calibration distance affect video sensitivity and consequently marker position measurements. during tests subject position is independently monitored by the cop readings, so that it is possible to evaluate this uncertainty contribution experimentally. distance variations are less than 10 % and the same relative contribution affects marker position measurements. note that such effect does not contribute to angle uncertainty since it determines a proportional horizontal and vertical variation, without affecting angles. to sum up, if a proper set up of the system is carried out, main uncertainty contributions are due to marker placement, which might heavily affect measurements. on the other hand data verification with the imu sensor eventually makes this problem evident, giving the opportunity to compensate data or to correct marker position on the subject. as regards random contributions of course they are dominated by repeatability during a repeated task, due to subject ability to keep the correct pace, and by reproducibility when considering different subjects or test sessions carried out during different days. 6. conclusions  the measurement of human movement has been considered in a metrological perspective. a complex and partially redundant measurement system has been developed and thoroughly tested. the system will be used, in the future, for studying various gestures, including on site high jump or spinning, walking and running. as validation test case we have considered hopping since it presents several measurement issues and measurement data are available in literature. furthermore we think that in order to understand properly hopping dynamics, a better characterisation of the contact conditions is required, especially if stability issues have to be studied. in the approach here presented, a combination of marker, video and inertial sensors enable results verification and internal validation. validation and verification methods are presented in the paper together with uncertainty evaluation and a discussion on main uncertainty contributions and their control. the main issue in this kind of measurement is connected to marker positioning. the proposed approach gives the operator the possibility to verify results, pointing out experimental tests, potential problems and successively enables measurement uncertainty evaluation. imu sensors demonstrate high flexibility and simple setup on the subject; they are a good choice in combination with video systems, even if they still present a rather large uncertainty due to the effect of the surrounding magnetic environment. acknowledgement  this research activity has been partially funded through university of genova research projects pra 2013-2014. references  [1] losier k h, eriksson a, leg stiffness measures depend on computational method, j. biomechanics, 47 (2014), pp. 115-121. 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[29] g.b. rossi, “measurement and probability”, springer, dordrecht, (2014), isbn 978-94-017-8825-0. microsoft word 496-3688-1-le acta imeko  issn: 2221‐870x  march 2018, volume 7, number 1, 80‐85    acta imeko | www.imeko.org  march 2018 | volume 7 | number 1 | 80  development and characterisation of a low pressure transfer  standard in the range 1 pa to 10 kpa  frédéric boineau, sébastien huret , pierre otal   , mark plimmer     laboratoire commun de métrologie lne‐lcm, 1 rue gaston boissier, f‐75015 paris        section: research paper   keywords: pressure; vacuum; transfer standard; resonant silicon gauge; capacitance diaphragm gauge  citation: frédéric boineau, sébastien huret, pierre otal, mark plimmer, development and characterisation of a low pressure transfer standard in the range   1 pa to 10 kpa, acta imeko, vol. 7, no. 1, article 16, march 2018, identifier: imeko‐acta‐7 (2018)‐01‐16  section editor: jorge torres‐guzman, cenam – centro nacional de metrologia, santiago de querétaro, mexico  received july 24, 2017; in final form december 17, 2017; published march 2018  copyright: © 2018 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: this work was supported by the european metrology programme for innovation and research (empir). empir is jointly funded by the empir  participating countries within euramet and the european union  corresponding author: frédéric boineau, e‐mail: frederic.boineau@lne.fr    1. introduction  we have developed a low pressure transfer standard in the pressure range from 1 pa to 10 kpa in both absolute and gauge modes, in the frame of the empir project 14ind06 “industrial standards in the intermediate pressure-to-vacuum range”. the objective for this transfer standard is to get an uncertainty contribution (k = 1) in relative value, of the order of 1×10-4 so as to use this standard in comparisons between calibration services on a national level. for pressures lower than 1 kpa, capacitance diaphragm gauges (cdgs) are usually employed as transfer instruments, but they suffer from a relative mean-term instability of a few 10-4 which can dramatically increase after transportation. in the upper pressure range, between 1 kpa and 10 kpa, the best quartz reference pressure transducers (q-rpt) or resonant silicon gauges (rsg) we have used provide a stability of 0.5 pa over their whole range, too high to meet our objective. in the recent key comparison ccm.p-k4.2012, in the same range [1], the pilot laboratory has developed a transfer standard based upon a cdg 100 pa and a special 10 kpa rsg. over the course of the comparison, this latter, with a resolution of 0.01 pa, showed a stability between a few ppm at 10 kpa to about 1×10-4 at 100 pa, allowing one to rescale the cdg which could consequently provide a quite low uncertainty contribution as a transfer standard between 1 and 100 pa. this method is also commonly used for very low pressure in the vacuum range, where the pressure given by an ionisation gauge is normalised by a comparison measurement with an associated spinning rotor gauge [2]. in the work described in this paper the rescaling procedure is applied in a slightly different way. we have used a 130 kpa rsg which is poorly stable if we observe its calibration history for a single pressure point. however, the used instrument has a good long-term stability of the correction slope and a nice linearity between 5 and 10 kpa. by performing, in absolute mode, a slope comparison in this range with a cdg 13 kpa full-scale, it is then possible to rescale the cdg signal which in turn is used to normalise a cdg 1.3 kpa full-scale, between 500 pa and 1kpa. abstract  we describe a transfer standard for low absolute and gauge pressure in the range 1 pa to 10 kpa. this transfer standard is composed  of three differential capacitance diaphragm gauges (cdgs) of full‐scale 130 pa, 1.3 kpa and 13 kpa respectively and one absolute  130 kpa  resonant  silicon  gauge  (rsg).  the  objective  for  the  relative  uncertainty  contribution  (k=1)  of  this  standard  during  a  comparison is a few tens of ppm at 10 kpa to a few hundred ppm at 1 pa. it relies on a good long‐term stability of the calibration slope  of the rsg used, between 5 kpa and 10 kpa, disseminated to cdgs in absolute mode and subsequently in gauge mode. the methods to  assess such uncertainty and the preliminary characterization of the transfer standard are presented.  acta imeko | www.imeko.org  march 2018 | volume 7 | number 1 | 81  the latter finally allows one to rescale a cdg 130 pa full-scale, between 50 pa and 100 pa. rescaled cdgs can afterwards be applied in the relative mode. the paper describes first the preliminary observations we have made about the metrological performances of our pressure standards. thereafter, the method for rescaling the cdgs is presented and the experimental setup is detailed. from the performance of the gauges and the characterisation of the transfer standard, the uncertainty is assessed. 2. metrological performances of the standard  pressure gauges  2.1. resonant silicon gauge 130 kpa  a resonant silicon gauge (druck type dpi1421) was acquired for the daily calibrations in the pressure range 10 to 130 kpa, in the vacuum department. from the successive calibrations of the rsg with a pressure balance, this rsg was found to drift mainly in offset (figure 1). the figure 2 shows the scattered drift of the correction 1  identification of commercially available instruments in this paper does  not imply recommendation or endorsement.  slope (determined by means of a simple linear least squares line). the drift is estimated to be (-1.0 ± 6.6) ppm per year. as the nominal range of the sensor is 3.5 to 130 kpa, we then decided to characterise it also in the range 5 to 10 kpa. the calibration with the force-balanced piston gauge (fluke fpg 8601) of the lne-lcm2 has shown a nice linearity of the rsg in this range (figure 3). so far, only three calibrations were performed in this range. the maximum drift for the correction slope was found equal to 14.5 ppm, compatible with observations of figure 2. the rsg can then be applied to check and possibly rescale the calibration function of our working standard cdg 10 kpa used to calibrate customer’s gauges. 2.2. capacitance diaphragm gauges  relative and absolute capacitance diaphragm gauges from the manufacturer mks (full scale 130 pa, 1.3 kpa and 13 kpa) are currently used as secondary and working standards at lnelcm. the use of the analogue output u (0-10 v), rather than the digital one, is generally preferred to enhance the gauge resolution. thus the calibration function is expressed with an equation of the form: , (1) where u0 is the output signal of the gauge when a zero pressure is applied (which in absolute mode corresponds to a pressure lower than one tenth of the gauge resolution) and is a fourth degree (at the maximum) polynomial function, to which the takaishi-sensui thermal transpiration correction [3] is applied to calculate the reference pressure . in absolute mode, the polynomial function and the gauge temperature are determined from a calibration by means of the fpg8601, between 1 pa up to 13 kpa. we fit the calibration data corrected with the thermal transpiration correction in which we assign a temperature to the gauge by successive approximations. to get a better estimate of this temperature, fifteen calibration points are performed below 50 pa, where the thermal transpiration correction is significant. 2  sub‐division of lne dealing with primary metrology in mass, pressure,  temperature,  radiometry  and  spectrophotometry,  and  dimensional  metrology.  figure 1. several rsg calibrations by means of a pressure balance.  it highlights the stability of the correction slope over time.  figure  2.  relative  drift  in  the  slope  correction  coefficient  of  the  rsg between the current calibration and the previous one.  figure 3. linearity deviation from a regression line of the rsg, stated from  three successive calibrations.  each  calibration  consists  of  three  runs  performed  by  increasing  and  decreasing  pressure  steps:  linearity  deviations  also  include  hysteresis  effects.  ‐20 ‐15 ‐10 ‐5 0 5 10 15 20 25 0 200 400 600 800 1 000 1 200 1 400 d e v ia ti o n  ( p a ) pressure (hpa) history of the rsg 04/2007 02/2008 11/2009 02/2010 06/2011 12/2015 ‐20 ‐15 ‐10 ‐5 0 5 10 15 × 1 0 ‐6 date relative drift of the rsg correction slope ‐0.20 ‐0.15 ‐0.10 ‐0.05 0.00 0.05 0.10 0.15 0.20 40 60 80 100 li n e a r  d e v ia ti o n  ( p a ) pressure (hpa) linearity errors of the rsg calibration 1 calibration 2 calibration 3 acta imeko | www.imeko.org  march 2018 | volume 7 | number 1 | 82  from the numerous performed calibrations of cdgs, it was stated that, on the mean term, the shape of a calibration curve does not change much, as one can see at a glance in figure 4; consequently, it is possible to estimate the new calibration function by a linear correction of the former one, . let us denote by the function determined by calculation. and are the calibration functions obtained from the cdg successive calibrations. we have: . (2) the correction factor is the slope coefficient of the least squares line that is estimating the reference pressure as a function of , in the range between 40 % and 80 % of the full scale of the cdg3. this is the method used to rescale a cdg. from the example of figure 4, with a quite large drift of deviation of the cdg (of about 1.2×10-3 in relative value), the aforementioned method was applied and the difference is plotted in figure 5 as a function of the 3  a  large part of the cdgs used at lne‐lcm exhibits a significant non  linearity between 80% and  100% of the full scale, and  is not used  in this  range.  pressure. on this same graph, the residuals of the cdg calibration curve, i.e. the difference between and the reference pressure given by the standard fpg8601, are plotted. as one can see in figure 5, the residuals and the deviation between the rescaled pressure and the modelled pressure are of the same order of magnitude and lower than 4.0×10-5 in relative value. 3. experimental setup  the metrological features of the instruments, described in § 2, make possible the rescaling of three cdgs of respective full scale 13 kpa, 1.3 kpa and 130 pa, starting with a calibration of the cdg 13 kpa with the rsg between 5 kpa and 10 kpa. when rescaled, the cdg 13 kpa is used to rescale the cdg 1.3 kpa and applying the same method the cdg 130 pa is rescaled. the experimental setup of the transfer standard is described in figure 6. the cdgs from the manufacturer mks instruments are the relative pressure transducers 698. they are used as absolute cdgs by means of an ion pump ip which maintains a stable vacuum on their reference port. each cdg is connected to a 3channel multiplexer 274, which allows to thermostat the transducers around 45 °c, itself connected to the electronics 670. the analog (0-10 v) pressure reading is made via a digital multimeter agilent type 34401 linked to the 670. the rsg is the druck dpi142 silicon gauge, isolated with the valve vrsg as long as the calibrated pressure is lower than 5 kpa. figure 7 shows a general view of the transfer standard with the plate where the transducers, vessels and ion pump, the module with displays, pump controller and the transportation box are placed. 4. procedure to use the transfer standard  to obtain the lowest uncertainty contribution of the transfer standard, it is necessary to rescale the cdgs at each calibration cycle, in absolute pressure mode. in others words, in addition to the calibration pressure points of the comparison protocol, some common measurements have to be performed between figure 4. plot of the deviation of two calibration functions of a cdg 13 kpa full  scale,  in  absolute  pressure  mode.  the  deviation  is  the  difference between the cdg calibration function and a linear function  ∙ , where   is an arbitrary coefficient.  figure  5.  difference  between  the  calibration  function  of  a  cdg  10kpa obtained in one case by modelling the calibration data  , and in the other case  by  applying  a  correction  factor    on  the  former  calibration  function  .      is  the  slope  coefficient  of  the  least  squares  line  that  is estimating the reference pressure  as a function of the cdg pressure modelled with the function   , in the range between 40 % and 80 % of the  full  scale  of  the  cdg.  this  difference  is  plotted  together  with  the residuals of the model:  .  figure 6. setup for the transfer standard.   cdg100, cdg1k, cdg10k capacitance diaphragm gauges mks type 698 of respective  full  range  130 pa,  1.3 kpa  and  13 kpa;  rsg:  resonant  silicon  gauge  druck  type  dpi142  (3.5‐130 kpa);  ip:  ion  pump;  frm:  combined pirani‐penning  manometer;  vm:  isolation  valve  of  the  transfer  standard;  vrsg: isolation valve of the rsg vb: bypass valve; vip: isolation valve of the ion pump; vfv: valve used to connect a fore vacuum pump with ultimate pressure suitable for the ion pump.  ‐10 ‐8 ‐6 ‐4 ‐2 0 2 4 6 0 2000 4000 6000 8000 10000 d e v ia ti o n  ( p a )  pressure (pa) deviation between two successive  cdg 13 kpa calibration functions current calibration former calibration ‐0.25 ‐0.20 ‐0.15 ‐0.10 ‐0.05 0.00 0.05 0.10 0.15 0.20 0.25 0 2 4 6 8 10 12 (p a ) pressure (kpa) rescaling vs modelling for a cdg 10 kpa ip vrsg vip vfv vb frm cdg 10k cdg 1k cdg 100 rsg vm acta imeko | www.imeko.org  march 2018 | volume 7 | number 1 | 83  each couple of gauges: rsg-cdg10k, cdg10k-cdg1k, cdg1k-cdg100, to rescale each cdg according to the method described in § 2.2. the common measurements points are defined in table 1. the initial calibration functions of the different manometers are , , and for rsg, cdg10k, cdg1k and cdg100 respectively. applies on the reading in pressure value prsg of the rsg, as the other functions apply on the analog output of the cdgs corrected with the corresponding zero value (see § 2.2). when the transfer standard is used, the actual calibration functions of cdgs, , and are determined after the post-processing of the measurements data common to different gauges. is plotted as a function of for the four corresponding pressure levels of table 1 and the linear rescaling coefficient for the cdg10k is determined by means of a least squares regression. the linear rescaling coefficient for the cdg1k, , is determined in a similar way by plotting ; as a function of ; and finally the linear rescaling coefficient for the cdg100, , is determined from the plotting ; as a function of ; . it is implied that the thermal transpiration correction is applied to each cdg signal. as the cdgs are rescaled from the dissemination of the stable correction in slope of the rsg, this procedure allows the correction of the drift of the transfer standard for each participant in the comparison, who performs a calibration in absolute pressure between 1 pa and 10 kpa. once the calibration of the transfer standard in absolute mode has been performed, one can use it in gauge mode (the reference ports of cdgs are put under atmospheric pressure). it will be then assumed that the mean rescaling coefficient of each cdg determined in absolute mode is available, with a supplementary source of uncertainty to be taken into account for gauge mode (§ 6). 5. characterisation of the transfer standard  5.1. rescaling of cdgs in absolute mode  the transfer standard was connected to a vacuum chamber and some calibration points were performed by increasing pressure levels (including the additional points of table 1), after the zero of each cdg has been recorded. five calibration cycles were achieved. the data were processed in order to determine at each calibration cycle the rescaling coefficients , and (§ 4). the corresponding experimental standard deviation of the linear regressions , and are shown together with the coefficients in table 2. except for the first value of , most of the standard deviations lie below 5×10-5. relative drifts in the successive coefficients (compared with the previous determined one) are plotted in figure 8, in which the uncertainty bars denote single standard deviations. cdg1k exhibits a poorer stability in between cycles, up to 3×10-4, compared with the other cdgs. this confirms the necessity to perform the additional calibration points at each cycle to keep the relative standard uncertainty below the objective of 1×10-4. 5.2. temperature coefficient of the rsg  as the overall performance of the transfer standard is based table 1. additional calibration points during a comparison, used to rescale  the cdgs.    cdg1k  cdg10k  rsg  cdg100  50 pa, 70 pa,  90 pa, 100 pa      cdg1k    500 pa, 700 pa,  900 pa, 1 kpa    cdg10k      5 kpa, 7 kpa,  9 kpa, 10 kpa  figure  8.  relative  drifts  of  cdgs  correction  coefficients  determined  by  means of the rescaling, at each cycle. the whole calibration was performed over five days.  uncertainty bars correspond to the standard deviation of the coefficient.  figure 7. general view of the transfer standard with its transportation box.  ‐2.0e‐4 ‐1.5e‐4 ‐1.0e‐4 ‐5.0e‐5 0.0e+0 5.0e‐5 1.0e‐4 1.5e‐4 2.0e‐4 1 2 3 4 5 calibration cycle number relative drift of the rescaling coefficient from the  previous cylce cdg10k cdg1k cdg100 acta imeko | www.imeko.org  march 2018 | volume 7 | number 1 | 84  on the correction slope of the rsg, it is important to check to what extent it is affected by temperature. furthermore, an intercomparison is planned, in the frame of the project empir 14ind06, between four institutes which have different reference temperatures for calibrations (20 °c or 23 °c). to determine the temperature coefficient, the rsg was placed in a climatic chamber successively at 20 °c, 15 °c, 25 °c and back to 20 °c and was compared with a similar calibrated rsg which was left at the ambient temperature of 20 °c. the variation in the correction slope of the transfer standard rsg was studied as a function of temperature. the temperature coefficient was determined to be (-5.5×10-7 ± 3.8×10-7) k-1. for a difference of only 3 k, the temperature effect never exceeds 2×10-6 in relative value and so can be neglected. 5.3. cdgs in gauge mode  in gauge mode, each cdg has its calibration function determined from a calibration with fpg8601. it is assumed that the rescaling coefficients established in absolute mode, also apply in gauge mode, provided the cdgs are calibrated exactly at the same time in both modes. in practise, this means cdgs have to be consequently calibrated in gauge and absolute modes within a short period of time (two weeks). 6. uncertainty budget  an uncertainty budget of the contribution of the transfer standard is established from the metrological features of the rsg and cdgs (§ 2) and the characterisation of the standard (§ 5). table 3 presents this budget for the absolute mode. the uncertainty u10k(p) of the cdg10k depends on the uncertainty of the correction slope of the rsg (calibrated with the fpg8601), its drift over time (figure 2) and the linearity error (figure 3). the uncertainty of the rescaling coefficient was estimated from the standard deviation esd10k to be roughly 5×10-5×p and it is assumed that it also includes the linearity error of the rsg. according to figure 5, rescaling errors and modelling errors of the cdg10k are of the same order of magnitude. in the uncertainty budget, only the latter are taken into account and added to the combined uncertainty (2×u10k). uncertainty for cdg1k, u1k(p), is based on the uncertainty of u10k(p) (without taking into account the modelling errors of cdg10k) and the uncertainty of the rescale coefficient σ1k, also estimated to be roughly 5×10-5×p; the modelling errors are added linearly to the calculated combined uncertainty (2×u1k). the uncertainty for cdg100, u1k(p), is based on the uncertainty of u1k(p) (without taking into account the modelling errors of cdg1k), the uncertainty of the rescale coefficient esd100, also estimated to be roughly 5×10-5×p and the uncertainty from the ambient temperature disseminated by the thermal transpiration function (an uncertainty of 1 k associated to a rectangular distribution is considered); the modelling errors are then added linearly to the calculated combined uncertainty (2×u100). the relative standard uncertainty of the transfer standard is finally plotted figure 9. as one can see on the graph, it lies between 1×10-4 and 5×10-5 in the range from 100 pa to 10 kpa and rises up to 3×10-3 when the pressure falls to 1 pa. for uncertainty assessment in gauge pressure, we have considered an additional contribution due to the short-term table 2. rescaling coefficients of the cdgs and corresponding standard deviations.         cycle 1  0.999 607  1.3 × 10 ‐5   0.999 901 1.5 × 10 ‐4 0.999 719  1.7 × 10 ‐5 cycle 2  0.999 654  3.4 × 10 ‐5   0.999 827 2.9 × 10 ‐5 0.999 727  1.9 × 10 ‐5 cycle 3  0.999 674  3.7 × 10 ‐5   0.999 689 1.5 × 10 ‐5 0.999 738  4.6 × 10 ‐5 cycle 4  0.999 661  5.7 × 10 ‐5   0.999 643 4.3 × 10 ‐5 0.999 737  7.9 × 10 ‐5 cycle 5  0.999 657  3.0 × 10 ‐5   0.999 720 3.1 × 10 ‐5 0.999 693  2.7 × 10 ‐5 figure 9. relative uncertainty (k=1) of the transfer standard as a function of  absolute pressure.  table 3. uncertainty budget of the transfer standard in absolute pressure. uncertainty component  cdg10k  cdg1k cdg100 calibration  1.5×10 ‐5 ×p (fpg8601) 5.3×10 ‐5 ×p ( u10k) 7.3×10 ‐5 ×p ( u1k) rsg slope stability  6.6×10 ‐6 ×p ‐ ‐  rescale coefficient  5.0×10 ‐5 ×p 5.0×10 ‐5 ×p 5.0×10 ‐5 ×p ambient temperature at ± 1k  (thermal transpiration effect)  negligible  negligible  8.1×10 ‐5 ×p + 0.0017 pa  combined uncertainty  u10k = 5.3×10 ‐5 ×p u1k = 7.3×10 ‐5 ×p u1k = 1.2×10 ‐4 ×p + 0.0017 pa modelling errors  2.4×10 ‐6 ×p + 0.013 pa 9.9×10 ‐6 ×p + 0.0039 pa 0.0024 pa expanded uncertainty (k = 2)  u10k = 1.1×10 ‐4 ×p + 0.013 pa u1k = 1.5×10 ‐4 ×p + 0.0039 pa u100 = 2.4×10 ‐4 ×p + 0.0059 pa 1.e‐5 1.e‐4 1.e‐3 1.e‐2 1 10 100 1000 10000 pressure (pa) relative standard uncertainty of the transfer  standard in absolute pressure cdg100 cdg1k cdg10k acta imeko | www.imeko.org  march 2018 | volume 7 | number 1 | 85  drift of cdgs from the characterisation in absolute mode (figure 8), as the cdgs cannot be rescaled at each calibration cycle. this contribution was estimated to be 1×10-4×p. the expanded uncertainty (k = 2), given in table 4, is about twice as large as that in absolute pressure, except in the range 1 to 100 pa where cdg100 is not affected by the thermal transpiration effect. the uncertainty contributions of the transfer standard in absolute and gauge pressure are plotted together on the graph of figure 10. 7. conclusion  a pressure transfer standard between 1 pa and 10 kpa has been characterised in both absolute and gauge modes. it is based on the stability of the linear correction slope of a resonant silicon gauge (rsg) in the range 5 to 10 kpa, in absolute mode. with a stepwise procedure, this allows one to rescale three capacitance diaphragm gauges (cdgs) of respective full scale 13 kpa, 1.3 kpa and 130 pa, provided the transducers are compared pairwise at four pressure points. the uncertainty contribution of the transfer standard was estimated to be lower than 1×10-4 in the range 100 pa to 10 kpa, which is at least ten times better than the usual performance of a capacitance diaphragm gauge; it rises to 3×10-3 when the pressure falls to 1 pa, due to the thermal transpiration effect on the cdg 130 pa. when used in gauge mode, shortly after the rescaling procedure, the relative uncertainty contribution is about twice higher than that in absolute pressure between 100 pa and 10 kpa, but slightly better in the range 1 pa to 100 pa, where the thermal transpiration effect does not apply. a comparison between four laboratories will be held in 2017, in the frame of the empir project 14ind06 with this new transfer standard. references  [1] j. ricker et al., ‘final report on the key comparison ccm.pk4.2012 in absolute pressure from 1 pa to 10 kpa’, metrologia, vol. 54, no. 1a, p. 07002, 2017. [2] d. a. olson, p. j. abbott, k. jousten, f. j. redgrave, p. mohan, and s. s. hong, ‘final report of key comparison ccm.p-k3: absolute pressure measurements in gas from 3 × 10−6 pa to 9 × 10−4 pa’, metrologia, vol. 47, no. 1a, p. 07004, jan. 2010. [3] t. takaishi and y. sensui, ‘thermal transpiration effect of hydrogen, rare gases and methane’, trans. faraday soc., vol. 59, no. 0, p. 2503−2514, 1963. figure 10. relative uncertainty (k=1) of the transfer standard as a function of pressure in both absolute and gauge modes.  table 4. uncertainty budget of the transfer standard in gauge pressure.   cdg10k cdg1k cdg100 expanded uncertainty (k = 2)  u10k = 2.3×10 ‐4 ×p + 0.013 pa u1k = 3.3×10 ‐4 ×p + 0.039 pa u100 = 3.9×10 ‐4 ×p + 0.0024 pa   1.e‐5 1.e‐4 1.e‐3 1.e‐2 1 10 100 1000 10000 pressure (pa) relative standard uncertainty of the transfer  standard in absolute and gauge pressure absolute pressure gauge pressure microsoft word 352-2363-1-le-rev acta imeko  issn: 2221‐870x  september 2016, volume 5, number 2, 44‐54    acta imeko | www.imeko.org  september 2016 | volume 5 | number 2 | 44  contribution of sentinel‐2 data for applications in vegetation  monitoring  pia addabbo 1 , mariano focareta 2 , salvo marcuccio 3 , claudio votto 3 , silvia l. ullo 4  1 università telematica giustino fortunato, via raffaele delcogliano 12, 82100 benevento, italy  2 mediterranean agency for remote sensing and environmental control (marsec), via perlingieri 1, 82100 benevento, italy  3 università di pisa, aerospace section, dept. of civil and industrial engineering, via girolamo caruso 8, 56122 pisa, italy  4 università degli studi del sannio, engineering department, piazza roma 21, 82100 benevento, italy portugal      section: research paper  keywords: remote sensing; sentinel‐1; sentinel‐2; landsat‐8; vegetation monitoring  citation: pia addabbo, mariano focareta, salvo marcuccio, claudio votto, silvia l. ullo, contribution of sentinel‐2 data for applications in vegetation  monitoring, acta imeko, vol. 5, no. 2, article 7, september 2016, identifier: imeko‐acta‐05 (2016)‐02‐07  section editors: sabrina grassini, politecnico di torino, italy; alfonso santoriello, università di salerno, italy  received march 20, 2016; in final form june 22, 2016; published september 2016  copyright: © 2016 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  corresponding author: silvia l. ullo, e‐mail: ullo@unisannio.it    1. introduction  in the past, capturing images of our planet earth from space has fascinated people, and it still does. in these days images of the earth are continuously detected and acquired and scientific communities use them to better understand and improve the management of the land itself and its environment [1]. these images allow us to see the world through a wide frame, to witness large scale phenomena, with an accuracy and completeness that human efforts on the ground could barely reach [2]. this becomes extremely beneficial for acquiring data of areas in the world too remote or otherwise inaccessible. the applications range in a wide circle of study subjects, such as geology (i.e. topography or production of dems, digital elevation models of the terrains), agriculture (i.e. classification of crops, soil moisture), forestry (i.e. tree biomass, height, species, plantation, deforestation, or forest monitoring and fire detection), cities (i.e. urban structure and density, control for the detection of unauthorized urban construction), natural disaster management (i.e. earthquake monitoring, identification of landslides, damage monitoring and classification), abstract  with the entry into operation of the sentinel‐2 mission in june 2015, a new land monitoring constellation of twin satellites has been  added to the copernicus project from esa and new insights have been derived through the combination of sentinel‐2 data with other  optical/multispectral data, and with other data from satellites belonging to the same copernicus  project.  to this end, the objective of  this paper has been to present new added‐value tools first through the integration of different satellite platforms: data from nasa  landsat‐8 and esa sentinel‐1 have been used and combined, and furthermore through the comparison of satellite data all from the  same  copernicus  project:  data  from  sentinel‐1  and  sentinel‐2  have  been  jointly  processed  and  compared.  although  data  from  optical/multispectral sensors, as those of landsat‐8 and sentinel‐2, and data from sar on board of sentinel‐1,  are very different,  their  combination provides useful and interesting results. the integration and combination of these data can find useful application in many  fields from oceans to waterways, from land surfaces to fossil deposits, from vegetation to forest areas. in this works authors have  focused  their  interest  in  green  areas  and  vegetation  monitoring  applications,  by  choosing  as  case  of  interest  the  royal  palace  of  caserta  and  its  gardens.  the  idea  has  started  from  the  increasing  interest  in  monitoring  the  cultural  heritage  monuments  and  in  particular  the surrounding vegetation with the green areas and the parks inside. satellite images can put into evidence boundaries  modifications, the vegetation state, their possible degradation, and other phenomena such as changes in the territories due both to  natural and to anthropogenic causes. data combination from different sources as above specified gives a good number of indexes very  useful to analyze the vegetation state and its health in a very deep way. many of these indexes have been calculated and discussed for  investigation.  acta imeko | www.imeko.org  september 2016 | volume 5 | number 2 | 45  oceanography (i.e. waves, wind, ship detection), man-caused disasters management (oil spills and monitoring of patches), to name a few [3]. in very recent years scientists have realized how remote sensing can find a new application by providing accurate, inexpensive, and timely information related to artistic heritage, either natural (parks, landscapes, etc.) or cultural (monuments, archaeological sites, and so on) [4]. the action of man, pollution, corrosion due to rain, cold, harsh weather conditions, solar radiation and thermal stress, can damage the artistic heritage. in addition, operations like excavations and corings have appeared to be risky, expensive and invasive techniques. on the other side, remote sensing presents many advantages: a global vision of the artistic and cultural sites, included the surrounding area; a periodic monitoring of the events, with quite fast intervention in case anomalies or dangers are detected; the identification of certain sites in the world, that present characteristics so special to be classified and associated, for instance, as part of world heritage; besides the possibility to locate buried, not yet excavated sites. another consideration is that environmental changes and their actions are very slow and the resulting damages appear only after a very long time. remote sensing can help also in this sense, because of its ability to discriminate even small changes. we have underlined in our previous work [5] the peculiarity of remote sensing in providing information not only on the cultural heritage but also on the surrounding vegetation and parks inside. this aspect is very recent, because more extensive research has been performed in the field of artefacts and manufacts, less instead in the field of parks and green areas related to them. to this end, the objective of this paper is to present new added-value tools by the integration of different satellite platforms: data from nasa landsat-8 and esa sentinel-1 have been used and combined; and also by the comparison of data from satellites belonging to the same platform, as sentinel1 and sentinel-2. we would like also to highlight that in our previous work as presented in [5] the choice of landsat-8 satellite, to use in combination with sentinel-1, was not made randomly but it was a carefully reasoned choice. in fact at the time of our initial research, sentinel-2 data were not available yet and the idea to use lansat-8 and sentinel-1 data together was based on two main considerations: the possibility to increase the available information for land and vegetation monitoring thanks to such different data sources, and the opportunity to anticipate results and discussion that could be resumed and confirmed, in a certain manner, as soon as sentinel-2 would have been available. in this work our study has focused mainly on green areas in order to analyse and monitoring the health state and the different types of vegetation. yet, obviously it can be extended to different applications, considering also that we have already carried out a wide research on the use of combined data from sentinel-1 and landsat-8 on other different fields ([5], [6]), by achieving very interesting results that now can be also improved with the availability of sentinel-2 data. the paper is organized as follows: in section 2, landsat-8, sentinel-1 and sentinel-2 main characteristics are presented and some anticipations on our work given; in section 3 the area chosen for the analysis is described and the characteristics of data from landsat-8, sentinel-1 and sentinel-2 are discussed for the area of interest; in section 4 results are provided, through some combinations of different landsat-8 and sentinel-2 data bands, while sentinel-1 signals are processed and discussed. in the final sections, some important indexes are calculated and compared. results are presented and some conclusions derived. 2. landsat‐8,  sentinel‐1 and sentinel‐2  2.1. landsat 8  the landsat-8 (official name: ldcm landsat data continuity mission) is the last of the historic landsat series designed by nasa, one of the first missions of earth observation and by far the most long-lived with decades of experience from the first satellite in 1972. launched in february 2013, just a month later the satellite became operational providing shots with a time of 16 days. the improved image quality, compared to the previous landsat-7, the introduction of new spectral channels and the availability of constant acquisitions make landsat-8 a useful tool for monitoring and analysing the territory. it mounts on board the optical sensor oli (operational land imager) and the thermal sensor tirs (thermal infrared sensor). thanks to their different spectral bands (11 in total) and to their characteristics, it is possible to get satellite maps consolidated as real colour maps, infrared maps, change detection, vegetation indexes, multiband fusion with the panchromatic channel. in addition with the availability of two new bands of the thermal sensor it is possible to obtain maps of temperature with limited noise pollution and production of maps of gravity on land routes from fires. moreover, the ir (infrared) band 9 allows clouds monitoring in the atmospheric field, while the new deep blue band (band 1) is ideal for the study of coastal zones and phenomena related to aerosol pollution. the mission landsat-8 is guaranteed up to 2018 and beyond, thus ensuring stability and comparison in analysis for many decades, with the coaching in the coming years of comparable esa sentinel missions. more detailed information can be found in [7]. 2.2. sentinel 1  sentinel-1 constitutes the first series of operational satellites responding to the earth observation needs of the eu-esa global monitoring for environment and security (gmes) programme. its design has been driven by the need for continuity of ers/envisat class data provision ([8]-[10]). the two-satellite sentinel-1 constellation carries a synthetic aperture radar (sar) instrument at c-band and has been designed to address medium and high resolution applications. the sar operates at a center frequency of 5.405 ghz and has the ability to scan the area with double polarization, that is with support for hh-hv and vv-vh simultaneous coand crosspolarization receiving channels. sentinel-1a was expected to be launched in 2013 and its sister sentinel-1b 18 months later. actually, the launch of the first satellite, sentinel-1a, took place on april the 3rd in 2014, and the launch of the second is scheduled to occur by 2016. four operational modes are supported in terms of resolution and swath width: strip map mode (smm); interferometric wide swath (iws); extra-wide swath mode (ewsm) and wave-mode (wm). see [11]-[13] for more details. a sentinel-1 toolbox is distributed with free license by esa and is available freely downloaded from [12]. in figure 1, a screenshot from the sentinel-1 toolbox is shown, with the different processing that can be launched, as an example. in particular, for the polarimetric elaboration the available polsarpro package has been used. it is a universal program for acta imeko | www.imeko.org  september 2016 | volume 5 | number 2 | 46  the management of satellite sar data, a valuable tool used for any satellite platform. the drawback is that it has many modules and their functionality is wide. therefore, to get to know all products this software could generate for analysing and processing the sentinel-1 sar data, has been a daunting task. another issue has been that sar data in order to be compared with other data need to be georeferenced. in effect, both the sentinel-1 toolbox and the polsarpro software have the georeferencing function. the problem with polsarpro was that by dealing only with bursts couldn’t process the whole image. therefore we made the necessary coordinate transformation through a specific qgis program module. after that, a classification algorithm was used. among all classification algorithms, we used the unsupervised wishart provided by polsarpro. more precisely, on the basis of a sar image, we generated a wishart supervised classification after an automatic pre-setting with a wishart unsupervised. see [11][13] for more details. as shown in 4.2, different algorithms may be used for sar image classification. the first technique introduced at this end was the cloude’s decomposition, based on the target coherency matrix and on the decomposition theorem. from cloude’s decomposition, pauli and wishart classifications have been then derived. interested readers can refer to [14]-[18] for more details. 2.3. sentinel 2  the sentinel-2 mission is a land monitoring constellation of two satellites that provide high resolution optical imagery and provide continuity for the current spot and landsat missions. it provides a global coverage of the earth's land surface every 10 days with one satellite and every 5 days with the second satellite, making the data of great use in on-going studies. sentinel-2a was launched on the 23rd of june 2015 and the first images were available for download in december 2015. sentinel-2b launch is planned for mid-2016. sentinel-2a (as sentinel-2b) is equipped with the state-of-the-art msi (multispectral imager) instrument, that offers high-resolution optical images. similarly to landsat-8, sentinel-2 is provided on board with an optical/multispectral high-resolution sensor, that however operates on 13 different bands of which 4 have a resolution of 10 meters, 6 a resolution of 20 meters and 3 bands a resolution of 60 meters. therefore, it provides data on the reflectance of the land surface for many different wavelengths, as landsat-8, but for some of them the resolution is much better, until 10 meters versus the 30 meters of landsat-8. also in this case, by suitably combining the obtained data, it is possible to get numerous diagnostic and investigative results with geo-referenced information on a local, regional, national and international scale. esa distributes free appropriate software for all sentinel satellite data, as already pointed out in the subsection 2.2. the snap (sentinel application platform) is currently available and is programmed for the simultaneous use of data acquired by sentinel-1, sentinel-2 and sentinel-3 satellites, even if the last has been orbited only recently, and its data are not available yet. having a single software for processing sar and optical / multispectral data turns out to be a huge advantage for their combined use and makes data processing more efficient. figure 2 shows a screenshot of the snap application, where on the left the command window of the software is open; an acquisition of sentinel-1 is presented in the middle, where the image (one of three major swaths) is shown as it appears, with the single bursts visible, when the snap package is used. here the processing has been realized at the l1 slc (single look complex) level, without multilook, therefore it appears “stretched”. on the right, an acquisition of sentinel-2 is shown, in particular an rgb true colour image, obtained by combining the three spectral bands of the visible (band 4, band 3 and band 2). figure 2 shows how through snap, you can work simultaneously on satellite data of sentinel-1 and sentinel-2. in this way, the combined use turns out to be much more practical and simultaneous. figure 3 again shows the snap software command window together with some pictures of different details of the royal palace of caserta. the first detail on the left represents the figure 2. a screenshot of snap (sentinel application platform) for sentinel.  figure 3. a screenshot of other images obtained with the snap application.    figure 1. a window of sentinel‐1 toolbox ‐ version 1.1.0.  acta imeko | www.imeko.org  september 2016 | volume 5 | number 2 | 47  acquisition of sentinel-1 (a scanned image in iw mode, with vv polarization, not georeferenced); in the middle and on the right there are two rgb images of sentinel-2, obtained through the combination of the spectral bands in true colours (bands 4, 3 and 2) and in false colours (bands 8, 4 and 3) respectively. it is important to underline that the improvements obtainable now that sentinel-2 data are available are related not only to the fact that sentinel-1 and sentinel-2 satellites are part of the same space mission, and a common platform can be used for processing, but also because sentinel-2 presents more innovative and advanced features and instrumentation with respect to other satellites that mount on board optical/multispectral sensors as landsat-8 for instance. in figure 4 the spatial resolution is plotted as a function of wavelength for the different sentinel-2 spectral bands, by putting into evidence how different bands find suitable applications in different fields of investigation. as already underlined sentinel-2 is able to acquire images through 13 spectral bands. through the main bands (the visible and infrared) it acquires images with a spatial resolution up to 10 meters, differently from landsat-8 which reaches only a 30 meter resolution. sentinel-2 is also able to acquire images with a spatial resolution of 20 meters through a series of spectral bands in the near infrared (band 5, band 6, band 7 and band 8a). these bands are very useful especially for the study of agriculture and vegetation. through their use in fact, it is possible to generate a series of indexes, for instance related to the presence of chlorophyll (which has a very strong response in the infrared), useful for the discrimination and classification of the type of covering present on a territory. moreover sentinel-2 offers three specific bands for the study of cirrus, water vapor and aerosols, whose corresponding images have a resolution of 60 meters. more information can be found at [19]. 3. area of interest and selected data  the chosen test site was the province of caserta and included the city of caserta and the surrounding area for an extension of about 300 square kilometres. the site has been selected by taking into account a number of particular characteristics that could help the combination process between sar and optical/multispectral data. in particular, we found a little accentuated morphology that could facilitate the observations within the sar scope, well-defined alternating between urban and predominantly agricultural areas, mountainous areas distinguishable as the different ground covers. these conditions typical of some areas of the apennines can be optimal for defining parameters and methods of analysis that can then be extended to other areas of interest. given the wide coverage area of single acquisitions by landsat8, sentinel-1 and sentinel-2 satellites, a reduced selected area has been taken into consideration. for instance for sentinel-1 a single 90×20 km size burst has been selected, the burst n.9 of the second swath, and the corresponding areas have been chosen from sentinel-2 and landsat-8 images. the covered area is still broad enough to encompass various types of territory: mountain, hilly, urbanized areas, cultivated fields, hedges and wooded grasslands. to conduct our study we have used landsat-8 data downloaded from [20] and sentinel-1 and sentinel-2 data downloaded from [21]. in table 1 all the characteristics for the selected data are summarized. in figure 5 we can see how the image from sentinel-1 (related only to burst n.9) overlaid a portion of landsat-8 image. it should be said that the snap software might also be used to process the landsat-8 data, but since they are distributed as tiff images, the qgis software is better suited for this purpose. we can observe that the two regions are not completely overlapping since at the left bottom corner there is a small portion not covered. this however has not been a problem because the territory of our interest is all within the selected area. moreover, we will concentrate our analysis and finalize our conclusions to a portion of the whole area, where there is the royal palace of caserta [22], an artistic and cultural site of big interest. in figure 6 an aerial photo of the site is shown. table 1. characteristics for landsat8, sentinel‐1 and sentinel‐2. characteristic   landsat‐8   sentinel‐1                      sentinel‐2  acquisition date dec. 26, 2015   dec. 25, 2015                dec.25,2015 product type  l1t  l1slc                              l1c                   acquisition mode  oli/tirs  ascendant/iws             msi information  all bands  amplitude and phase  all bands figure 6. aerial photo of the royal palace of  caserta.   figure  5.  overlay  of  sentinel‐1  image  (only  burst  n.9)  and  georeferenced  landsat‐8  image.   figure 4. spatial resolution for the different sentinel‐2 spectral bands, [19].  acta imeko | www.imeko.org  september 2016 | volume 5 | number 2 | 48  we can see in figure 6 that the royal palace is adorned with a vast grassland in front and behind the main squared building. our choice has felt on this site right because of the presence and the huge extension of parks and fountains that have appeared to be suitable to the aims of our study. in the following figures 7 and 8, rgb true colour images from sentinel-2 and landsat-8 are shown. the difference in the details between the two images is immediately visible, due to the better spatial resolution that sentinel-2 can reach. to better appreciate the quality of images reached through sentinel-2 data, thanks to its high resolution, another image of the same area is shown in figure 9, where a rgb image of the royal palace of caserta is presented in false colours. 4. results for the selected area  4.1. area analysis with landsat‐8 bands combination  the ndvi  index.  landsat-8 offers eleven bands, some with the oli sensor, some with tirs, and band5 with both. by their combination different images can be plot and several conclusions derived. for sake of simplicity, we will refer to the different combinations just with the corresponding band numbers. in figure 10 many qgis images are presented in sequence. the band combination number appears in the bottom corner on the left. from the area of interest a portion including the royal palace of caserta has been selected. in effect, the royal palace shape appears almost in the middle of each image, on the right side. the different bands have been combined in the three channel rgb (red, green, blue), so as to obtain different unique images in false or true colours, that bring into showing properties of the territory. for instance, the 543 combination corresponds to the image in rgb where in red is the band 5, in green the band 4 and in blue the band 3. combination 543 is the standard image in "false colours". the vegetation appears in shades of red, urban areas are cyan and soil varies from dark to light brown. conifers appear dark red for hardwoods. this combination is useful for studies of vegetation, monitoring of the drainage, terrain models and control of different stages of crop growth. in general, deep shades of red indicate wide leaf and / or healthy vegetation, while the light red tones mean grasslands or sparsely vegetated areas. urban areas appear in blue. we can see that the royal palace appears in blue and all the parks behind and in front of the main central building appear red; it is also possible to see the wooded area in a deep shade of red, surrounding the park on the top. combination 432 is the image in "true colours". the ground features appear in colours similar to their natural look. healthy vegetation is green, unhealthy vegetation is brown and yellow; the streets are grey and shorelines are white. sparsely vegetated or disposed areas are not as easily detected as   figure 7. sentinel‐2 rgb true colour image (10 m of resolution).   figure 9. a sentinel‐2 rgb false colour image of the royal palace of caserta (10 m of resolution).   figure 8. landsat‐8 rgb true colour image (30 m of resolution).   figure  10.  combinations  of  landsat‐8  bands‐qgis  images  and  the  ndvi  index on the left bottom corner.   acta imeko | www.imeko.org  september 2016 | volume 5 | number 2 | 49  in 562 or 543 combinations. moreover also the types of vegetation are not easily distinguishable as in the 562 combination. shallow waters are not as well distinguished from the soil as in 764 combination. about the royal palace, the parks appear in a green colour and the building is light brown. the presence of tanks and fountains is here more evident than in 543. combination 753 provides a "natural-like" yield. the healthy vegetation appears bright green, grasslands appear green, pink areas represent sterile soil. the orange and brown are for sparsely vegetated areas. dried vegetation appears orange and the water of blue colour. sand, soil and minerals are highlighted in a multitude of colours. this band combination provides amazing pictures for the desert regions. it is useful for geological studies, agricultural and wetlands. urban areas appear in shades of magenta. grasslands appear bright green and points of green light inside the city indicate a grassy cover, i.e. parks, cemeteries or golf courses. here the parks of the royal palace are very evident with a very bright green colour. in combination 562 healthy vegetation appears in shades of red, brown, orange and yellow. the deep waters appear to be very dark. soils can be green and brown; urban characteristics are white, cyan and grey; bright blue areas represent land recently working and reddish areas show new growth of vegetation or grassland probably scattered. for studies of vegetation, the sensitivity to detect various stages of plants growth or stress is increased. in this combination it is possible to differentiate the yellow parks of the royal palace from the wooded area surrounding them, that appears very red testifying a wooded area. also the combination 764 provides a "natural-like" yield and also penetrates the weather particles, smoke and haze. the vegetation appears in shades of dark green and bright green during the growth season. urban characteristics are white, grey, cyan or purple; sand, soil and minerals appear in a variety of colours. water sources are well highlighted in the image. flooded areas appear blue or very dark black, much better than the 432 combination where the non-deep flooded regions appear grey and are difficult to distinguish. combination 654 as the 562 provides the user with a large amount of information and contrasts. healthy vegetation is bright green and soils are mauve. while the 753 combination contains geological information, the 654 contains information of a more agricultural nature. this combination is useful for the study of the vegetation and is widely used in the fields of wood management and pest infestation. combination 765 can be used to study the characteristics of soil weaving and moisture. the vegetation appears blue so if green is preferred this combination should be replaced. this band combination is also useful for geological studies. combination 642 showcases the topographical features of the land, while replacing the band 7 to 6, then with the combination 742, the differences between the types of rocks can be displayed. in figure 10 the last image on the left bottom corner refers to the ndvi index. it was generated by combining the near infrared and the visible bands for landsat-8, in order to obtain information on the type of vegetation present on the scene. the index is given by the following equation (1): ndv i = (b5 − b4)/(b5 + b4) . (1) the ndvi index gives all the information about the status of the crop which are mainly related to the density of biomass produced. for immediate display a scale of colour representation has been chosen, which combines the minimal values to deep blue and the maximum value to deep red. thus the blue colour indicates areas where vegetation is virtually absent, as buildings, infrastructure and bare lands. very light blue colours indicate areas where the vegetation is mostly absent or dry. red colours indicate areas with a certain type of vegetation, that can be poor (very light red) or thick (deep red). where red is deep, it indicates that the vigour of the vegetation is very high, so it can indicate a plantation in growth or a healthy grassy. thus the information that is derived from the ndvi index concerns mainly the presence of vegetation cover and its state. 4.2. area analysis with sentinel 1. the ctest1 index  after its launch, sentinel-1a has shown all its great potentiality in terrain observation and high resolution retrievals for getting information on land surface and soil moisture ([8], [23]). a sentinel-1 sar image of the area of interest, after a wishart supervised classification, and the image of the same area, processed with the algorithm of pauli are presented in figure 11 on the left and on the right, respectively. it may be noticed that when wishart is applied, the urbanized area is represented by the red colour while the natural areas are green-yellow. infrastructures such as roads and railways, and rivers are shown in blue. furthermore, also the forested areas or in general surfaces with a mainly volumetric geometry are represented in blue. when the same area is processed with the algorithm of pauli, the image appears a bit different. the polarimetric interferometry module has been used through a pauli decomposition. the pauli coherent decomposition provides the interpretation of a full polarimetric slc (single look complex) data set in terms of elementary scattering mechanism. see [13] and [24] for a full description of the algorithm overview. through a rgb combination according to the rule of pauli, the sar image presents the urbanized area in a light blue colour, whitish for more reflective infrastructures. the land and cultivated fields appear with a colour that ranges from light brown to dark blue, depending on their surface geometry. mainly flat lands (eg. gravel) are represented by a dark brown. the waterways and the rivers are represented by the black colour. mountain areas with the side facing the incident direction of the radar appear very bright (white) and with the side in the shade appear very dark (brown almost black). similarly to landsat-8, instead of ndvi we have calculated the ctest1 index for sentinel-1. this index has been obtained by data of sentinel-1 according to (2): ctest1 = (svv − svh )/(svv + svh ) . (2) figure 11. sentinel‐1 sar images after supervised wishart classification (left) and after pauli rule application (right).   acta imeko | www.imeko.org  september 2016 | volume 5 | number 2 | 50  this equation represents the amount of signal received back by the radar respectively in the two modes: vertical and horizontal polarization. the term svv is representative of the acquisition mode vv, while the term svh is representative of the vh acquisition mode. the term svv refers to the acquisition mode vv, when a vertical polarization is used both in the transmitter and in the receiver, while the term svh refers to the acquisition mode vh, when the signal is transmitted with a vertical polarization and the receiver is settled on a horizontal polarization signal acquisition. the same coloured scale used for the ndvi, has been used also in this case, to facilitate image comparison. the values range from blue colour (minimum) to red (maximum). related essentially to the geometric characteristics of the surfaces, the signal will be reflected with a horizontal polarization, a vertical polarization, or both. when the ctest1 index is minimum (blue colour), the vh mechanism reflection is predominant if compared to the vv, that means that the horizontal polarization component in the received signal is very high, while the vertical polarization component is very low or absent. therefore, depending on how the signal is received after been reflected from a target, it is possible to understand the type of geometry that has changed the signal polarization. since the signal is transmitted by the sar with vertical polarization, the geometry of a target able of returning a signal of this type, must be mainly vertical or close up. on the contrary when horizontal polarization is very high in the received signal, this means that the surface was practically flat. as regarding the vegetated areas, targets associated to this type of geometry are "high" covers, such as vineyards, wheat crops, crops of corn, reeds, etc., in the first case (red colour for the ctest1 index), while areas where blue is predominant indicate that the radar signal has not encountered obstacles that have reflected back the vertically polarized component, but a horizontally polarized signal comes back. such targets have a geometry predominantly "flat"; they are surfaces which dissipate most of the signal sent from the satellite. the targets associated to this type of response are "low" coverages, such as grassy fields, joke grounds, young plantations, and so on. 4.3. area analysis with sentinel 2.  the ndvi index.  similarly to the case presented in 4.1, the ndvi index can be calculated also for sentinel-2, by obtaining information about the state of the vegetation in the area of interest. the only difference is that for sentinel-2 the near infrared band is the band 8. the formula for ndvi index in this case is the following, given by (3): ndv i = (b8 − b4)/(b8 + b4) . (3) thus the two indexes present the same visible green band, the band 4, for both, while they differ in the near infrared band (b5 for landsat-8 and b8 for sentinel-2). in figures 12 and 13, the two indexes are shown referring to the area around the park of the royal palace. the more precise definition of the details given by sentinel-2 is clearly evident, thanks to its better resolution that makes the ndvi index very useful also for the study of very small areas. in figure 14 a detail of the park (approximately 600 × 300 meters) has been taken inside the royal palace for comparison between sentinel-2 and landsat-8 satellites. to see how different their acquisitions are, the ndvi index for both has been calculated and compared for the selected area shown in figure 14, where vegetation is predominant, see figure 15. it is possible to appreciate the big difference between the two indexes due to different resolutions for sentinel-2 and landsat-8 that result in different pixel dimensions. 4.4. statistics of ndvi index  for sentinel 2 and landsat‐8  figure 16 shows the histograms for the distribution of the analyzed pixels for the ndvi indexes in the two cases. the   figure 12. sentinel‐2 ndvi index.    figure 13. landsat‐8 ndvi index.   figure 14. a detail of the park inside the royal palace of caserta.   figure 15. ndvi for sentinel‐2 (left) and landsat‐8 (right) on the selected  very vegetated area.   acta imeko | www.imeko.org  september 2016 | volume 5 | number 2 | 51  snap application allows to process data and calculate also the statistics for specific indexes and parameters of interest. figure 16 shows the screenshot of this elaboration. thanks to the higher resolution, in the same area ndvi index for sentinel-2 samples 3015 pixels with respect to the 345 pixels sampled by the ndvi index of landsat-8. if we look at the statistics, the analysis shows a standard deviation in the acquisition of landsat-8 (σ = 0.0785) lower if compared to that of sentinel-2 (σ = 0.1262), which apparently would seem to show a minor deviation from the mean value for landsat-8, and therefore a more accurate analysis. in effect, by comparing the variation coefficients, or the dispersion indexes, which allow to compare measurements of phenomena relating to different units of measurement, it emerges that for the acquisition of landsat-8 there is a greater variability (coefvariation = 0.68) and therefore a greater dispersion with respect to the acquisition of sentinel-2 (coefvariation = 0.41). moreover, the maximum value that can be measured by sentinel-2 appears to be more than twice of that measured by landsat-8. all this demonstrates the improved efficiency of sentinel-2 when compared to landsat-8. another very important advantage that can be gained by using sentinel-2 data, is given by the possibility of using the images captured through the spectral bands in the near infrared, which can generate a series of indexes for the vegetation analysis that cannot be obtained through landsat-8 data. refer to [25] for more information about all the possible indexes that can be calculated. 5. indexes calculation and results comparison   to define and classify the type of coverage present on the ground and show the results of our analysis, we have compared the information obtained from the ndvi index with those obtained from the ctest1. we have further restricted the area of interest by taking into consideration just a part of the zone around the royal palace of caserta. as shown in figure 17 the ndvi index on the left identifies the central building with a deep blue colour, while grassland appears in red. the index gives also information about the vigour of the vegetation, that is high (brilliant red), but does not provide other information for instance on the type of coverage. through the index ctest1, represented on the right of figure 17, the building is identified with a light pink colour and the ground grass with a deep blue colour. this indicates that from the building the signal is sent back to the radar through a double bounce, thus the horizontal and vertical components of the signal are almost equal, typical response of buildings; while the response on grasslands highlights how the signal sent back to the radar disperses almost completely and the satellite can capture back only a horizontal component sent from the reflection on the ground. this is clearly indicative of the fact that the target has a predominantly flat geometry. if we put together this information to that given by ndvi (vegetation of high vigour) we can conclude that the place corresponds to a healthy grassy field, just as the photo of the royal palace shown in figure 6 confirms. data comparison and combination show that only putting together the results obtained from these indexes the final information is complete and helpful. moreover, we can observe that the thin strip of water formed by the tanks in the garden of the palace appears blue both with the ndvi index and with the ctest1. this means that the area is without vegetation and flat, i.e. a water mirror. in practice, the ndvi index can tell us how vegetation is: dry or healthy; and the ctest1 index tells more about geometry: flat or rugged profiles, tall or small targets. in the case of the royal palace the two indexes together say: "grasslands healthy, well maintained". the availability now of sentinel-2 data with respect to our previous work [5], allows to extend our analysis and to obtain more interesting results. as already mentioned, in addition to acquiring images through the main band in the near infrared (band 8), sentinel-2 has also other spectral bands in the near infrared through which acquire other images. figure 18 shows the position of these spectral bands for sentinel-2 according to their centre wavelengths, together with the reflectance response of healthy (green) vegetation, dry vegetation and soil as a function of the wavelength itself [26]. many studies have been conducted to investigate the relationship between the biological status of plants and their spectral responses since chlorophyll (chl), as well as a number of other pigments, are strong absorbers of light in certain welldefined wavelengths, specifically in the blue and the red spectral regions. pinar and curran [27], and filella and peñuelas [28] found that the red edge region is sensitive to chl content and n-status. these studies revealed that if chl concentration increases, the typical slope in the red edge spectral region figure 16. statistics for sentinel‐2 (top) and landsat‐8 (bottom) ndvi index.  figure  17.  comparison  between  landsat‐8  ndvi  and  sentinel‐1  ctest1  indexes.   acta imeko | www.imeko.org  september 2016 | volume 5 | number 2 | 52  shifts towards the near-infrared (nir). since the red edge band is sensitive to chl status and leaf and canopy structure, it is expected that this band would contribute to the characterization of different plant cover types [29]. for example, figures 19 and 20 show the images obtained respectively for the green chlorophyll index (cl_green) and the red edge chlorophill index (cl_red edge) of the royal palace park. they are two of the most important indexes known in the literature for vegetation monitoring [30]. they are defined by (4) and (5): cl 1 , (4) cl 1 , (5) where spectral band 3, band 5 and band 7 of sentinel-2 are used for calculations. the cl_green is an index that shows and distinguishes green areas in the territory. the cl_red edge is an index of the amount of chlorophyll present in the analysed territory. both images were represented in false colours where the minimum values correspond to a brownish almost yellow colour that tends to a bright green for the maximum values. if we furtherly restrict the area under analysis inside the park of the royal palace of caserta, as shown in figure 21, which refers to a 250 × 300 meter surface, we can calculate the indexes over a completely vegetated area for a specific analysis. in order to compare the data of sentinel-2 and landsat-8, even if the images have different resolutions, we have used the main bands in the near infrared, which are comparable. the indexes expressed in (6) and (7) have been calculated: reci_cl _2 1 (sentinel-2) , (6) cl _landsat 8 1 (landsat-8) . (7) in (6) and (7) the band 8 and the band 5 are respectively the infrared band of sentinel-2 and landsat-8, while band 3 is for both the spectral band in the visible green. in figure 22 the statistics for the indexes calculated for sentinel-2 and landsat-8 with (6) and (7) are represented. the snap application has been used. the results show a clear difference in the maximum values measured by the two satellites (3.07 with sentinel-2) and (0.99 with landsat-8). analysing the same area through the index cl_green (green chlorophyll index), here renamed reci_cl _green, defined by the (8): reci_cl 1 (8) and obtainable only for sentinel-2, the results turn out to be better than the index reci_cl _green_2 of (6). in fact, thanks to the fact that the band 7 of sentinel-2 is much narrower (20 nm) compared to the band 8 (115 nm) used in the index reci_cl _green_2, the measurement is much more sensitive, and obtained values have higher highs (max = 3.166) as shown in the figure 23. figure  18.  reflectance  of  green,  dry  vegetation  and  soil  with  sentinel‐2  bands visualization [26].   figure 20. sentinel‐2 cl‐red edge index.   figure  21.  a  250x300  meter  area  selected  inside  the  park  of  the  royal  palace of caserta.   figure 19. sentinel‐2 cl‐green index.   acta imeko | www.imeko.org  september 2016 | volume 5 | number 2 | 53  if a similar index, here named cl _green_3, is generated for sentinel-2 by using the spectral band 8a in place of the band 7, the (8) turns into (9): cl _3 1 . (9) as reported in the statistics of figure 24, the measured maximum values are even higher (max = 3.743). the band 8a in fact, is a band in a near infrared region that is not affected by water vapour and that appears to be sensitive to iron oxides present in plants or in the soil, unlike the other bands. see [31] for more details. all the above results demonstrate how the use of sentinel-2 data greatly enhances the information that can be obtained for the study of vegetation. moreover since sentinel-2 indexes provide the highest values, they are better to discriminate the types of vegetation, because more marked differences are given. at the beginning of this section we have discussed the comparison between landsat-8 ndvi and sentinel-1 ctest1 indexes by showing how the ndvi index can tell us how vegetation is: dry or healthy; and the ctest1 how geometry is. thanks to sentinel-2 data availability a possible comparison can be done also between its ndvi index and sentinel-1 ctest1, as shown in figure 25. the advantage is given by the sentinel-2 resolution that allows to appreciate very small details, as already pointed out. moreover all the new indexes, specific for vegetation analysis, can provide additional information on the type and status of vegetation by reaching to classify the coverage in a more appropriate way. our study based on different combinations of landsat-8 and sentinel-2 bands, and on sentinel-1 sar images, through the calculation of so many different indexes such as, the ndvi and the ctest1 indexes, the green chlorophyll index, the red edge chlorophyll index, and so on, has surely offered new tools for vegetation monitoring through the use of remote sensing techniques. 6. conclusions  with the entry into operation of the sentinel-2 mission in june 2015, a new land monitoring constellation of twin satellites has been added to copernicus project from esa and new insights have been derived through the combination of sentinel-2 data with other optical/multispectral data, and with other data from satellites belonging to the same copernicus project. to this end, the objective of this paper has been to present new added-value tools first through the integration of different satellite platforms: data from nasa landsat-8 and esa sentinel-1 have been used and combined, and furthermore through the comparison of satellite data from the same copernicus project: data from sentinel-1 and sentinel-2 have been jointly processed and compared. moreover data from figure 23. statistics of reci_cl _green index for sentinel‐2.   figure 25. sentinel‐1 ctest1 (top) and sentinel‐2 ndvi (bottom).   figure 22. statistics for sentinel‐2 (top) and landsat‐8 (bottom) chlorophyll indexes.   figure 24. statistics of cl _green_3 for sentinel‐2.   acta imeko | www.imeko.org  september 2016 | volume 5 | number 2 | 54  different optical/multispectral sensors, as those of landsat-8 and sentinel-2, and data from sar on board of sentinel-1, have been all jointly analysed, compared and discussed by providing useful and interesting results. as future works it would be interesting to design a suitable “data fusion” algorithm to combine data and obtain information automatically on specific issues of interest. at last, with respect to our previous research, the availability now of sentinel-2 data has allowed very good improvements in terms of vegetation monitoring and definition, as obtained results have demonstrated. references  [1] lillesand, t. and kiefer r.w. and chipman j. “remote sensing and image interpretation”, wiley and sons, 2014, 7th edition. [2] elachi, c. and vanzyl, j. “introduction to the physics and techniques of remote sensing”, wiley and sons, 2006, 2th edition. [3] schott, j. r. “ remote sensing. the image chain approach”, 2007, oxford university press. [4] masini, n. and soldovieri, f. “integrated non-invasive sensing techniques and geophysical methods for the study and conservation of architectural, archaeological and artistic heritage”, journal of geophysics and engineering, vol.8, 2011. [5] m. focareta, s. marcuccio, c. votto, s.l. ullo, “combination of landsat-8 and sentinel-1 data for the characterization of a site of interest. a case study: the royal palace of caserta”, 1rst international conference on metrology for archaeology, benevento, italy, october 22-23, 2015. [6] p.addabbo, m.focareta, s. marcuccio, c. votto, s.l. ullo, “land cover classification and monitoring through multisensory image and data combination”, submitted to igarss’16, beijing, china, july 10-15, 2016. [7] http://landsat.gsfc.nasa.gov/landsat8 [8] f. de zan,f. and monti guarnieri, a.m. “topsar: terrain observation by progressive scans”, ieee transactions on geoscience and remote sensing, vol.44, issue 9, sept.2006, pp. 2352-2360. [9] attema, e. “mission requirements document for the european radar observatory sentinel-1”, es-rs-esa-sy-0007, issue 1.4, 11 july 2005. [10] snoeij, p. and attema, e. and davidson, m. and duesmann, b. and floury, n. and levrini, g. and rommen, b. and rosich, b. “the sentinel-1 radar mission: status and performance, ieee international radar conference-surveillance for a safer world, 12-16 oct. 2009, bordeaux, pp. 1-6 [11] snoeij, p. and attema, e. and davidson, m. and duesmann, b. and floury, n. and levrini, g. and rommen, b. and rosich, b. “the sentinel-1 radar mission: status and performance”, proceedings of radar conference surveillance for a safer world, oct. 2009, pp. 1-6. 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[19] https://directory.eoportal.org/web/eoportal/satellitemissions/c-missions/copernicus-sentinel-2 [20] http://earthexplorer.usgs.gov/ [21] https://scihub.esa.int/ [22] http://www.realcasadiborbone.it/en/historydocuments/palaces-residences/the-royal-palace-of-caserta/ [23] hornacek, m. and wagner, w. and sabel, d. and hong-linh truong and snoeij, p. and hahmann, t. and diedrich, e. and doubkova, m. “potential for high resolution systematic global surface soil moisture retrieval via change detection using sentinel-1”, ieee journal of selected topics in applied earth observations and remote sensing, vol.5, n.4, aug. 2012, pp. 1303-1311. [24] https://sentinel.esa.int/web/sentinel/sentinel-1-sar-wiki//wiki/sentinel one/algorithm+overview [25] j.g.p.w. clevers, anatoly gitelson, publication/283419755, “using the red-edge bands on sentinel-2 for retrieving canopy chlophyll and nitrogen content”, january 2012. http://congrexprojects.com/docs/12c04_docs2/poster1_18_cle vers.pdf [26] http://www.tankonyvtar.hu/en/tartalom/tamop425/0032_terin formatika/ch04s04.html [27] pinar, a. and curran, p.j., “grass chlorophyll and the reflectance red edge”, international journal of remote sensing, 17, s. 351-357, (1996) [28] filella i. and peñuelas, j., “the red edge position and shape as indicators of plant chlorophyll content, biomass and hydric status”, international journal of remote sensing 15, s.14591470, (1994). [29] weichelt, h. and rosso, p. and marx, a., and reigber, s. and others, “the rapideye red edge band”, white paper, http://www.blackbridge.com/rapideye/upload/red_edge_whit e_paper.pdf). [30] ilina b. kamenova, “opportunities of hyperspectral vegetation indices to assess nitrogen and chlorophyll content in crops”, technical reportbulgarian academy of sciences, space research and technology institute department of remote sensing and gis, sofia, bulgaria. http://csebr.cz/scerin2014/presentations/29_kamenova.pdf [31] sentinel-2, user handbook, esa standard document, issue 1, rev 2, july 24, 2015. acta imeko, title acta imeko issn: 2221-870x december 2017, volume 6, number 4, 31-36 acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 31 high-precision amplifiers for strain gauge based transducers first time realized in compact size andré schäfer hottinger balwin messtechnik gmbh (hbm), im tiefen see 45, 64293 darmstadt, germany keywords: high-precision amplifiers; strain gauges; traceability; force; torque; pressure citation: andré schäfer, high-precision amplifiers for strain gauge based transducers -first time realized in compact size, acta imeko, vol. 6, no. 4, article 5, december 2017, identifier: imeko-acta-06 (2017)-04-05 editor: alexandru salceanu, technical university of iasi, romania received may 5, 2017; in final form july 4, 2017; published december 2017 copyright: © 2017 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: dr.-ing. andré schäfer, e-mail: andre.schaefer@hbm.com 1. introduction strain gauge precision transducers are always based on a wheatstone bridge. by the help of strain gauges (s.g.) arranged in a wheatstone bridge, mechanical quantities such as force, pressure, torque generating strain can be converted into a variation of resistance and later to a voltage (figure 1). this wheatstone bridge is necessary, as the pure change of resistance is far too small. however the unbalance of the wheatstone bridge, comparable to a double voltage divider, allows an output signal of nominal 2 mv/v excitation voltage ub. the excellent read-out of the signal benefits from two main principles used in the instrument, which should be briefly explained here as they are the basis for the next sections. in the elastic deformation range of materials the methods of calculating the material stresses from the measured strains are based on hooke’s law. in its simplest form for uniaxial stress state hooke’s law is [1], [2]: 𝜎 = 𝐸 ∙ 𝜀 (1) with: σ = material stress ε= modulus of elasticity of the material ε = strain. the variation of resistance is defined by ∆𝑅 𝑅0 = 𝑘 ∙ 𝜀⁄ (2) with: ∆r = change of resistance r0 =basic resistance ε = strain k = gauge factor. this bridge can be extended by useful outer circuitry to “trim” the transducer. for instance zero-point, linearity as well as the temperature compensation of zero point (tc zero) and span (tc span) [3], [4] can be matched (figure 2). the bridge can be supplied by dc or better ac with a frequency (fc) as “carrier”. now we have to distinguish between the frequency of the carrier and those of the measuring signal. the mechanical quantity changes with the signal frequency (fm). the transducer is supplied with an alternating excitation voltage figure 1. schematic view of the strain gauge amplifier. abstract in cases where accuracy matters, and in metrology of mechanical quantities such as force, torque or pressure this is definitely the case, are widely realized by transducers, which are based on strain gauges. to exploit the potential of the widely used high-precision static measurements the demands on precision amplifiers steadily grow. this paper gives exact details to further progress of the precision amplifiers used and how compact they can be realized nowadays. acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 32 and works as a modulator. in this way, an interfering signal (ac or dc) can be separated from the measurement signal of the same frequency by the carrier frequency method. precision measurements are carried out preferably at fc equal 225 hz and voltage conditions of 2 mv/v. the frequency of 225 hz has been chosen by many national metrology institutes (nmi´s) around the world, as it is no multiple of the disturbing 50 hz or 60 hz power supply understanding that it is only allowing a signal bandwidth of approx. 50 hz, so really static measurement, but on the other hand with very high resolution. this very high resolution is first priority, as it is the basis for the reachable accuracy. at the same time it allows a bandwidth suitable for measurement processing of static and quasi-static signals. another reason is that it keeps the influence of noise low. in figure 3 the spectra of the used carrier frequency principle can be seen. in the graph on the left side the dotted line 6 describes the noise spectrum. with the existing 1/f noise function (see no. 6 in figure 3) the choice of 225 hz ensures, that one is definitely in the low noise, so above the area of significant noise, which occurs close to 0 hz. 2. basics and relevance of the topic traceability of mechanical quantities is relevant to many fields of industry. to name an example, torque measurement is essential for power as well as efficiency measurement and thus to the degree of carbon monoxide pollution and nitrogen oxides in the air. the same is true for force transducers, which in combination with actuators, are components for all kind of test stands [5]. another example is the use of pressure transducers as altimeter in aircrafts, so in an in-flight application. national flight authorities often carry out inter-comparisons using hbm calibration equipment such as the dmp precision amplifier series. in calculating uncertainty of the whole measuring chain using hbm precision amplifiers you will notice that the overall uncertainty of the measuring chain is often required to be very close or at least in the range of the measurement uncertainty of the transducer itself. to safeguard critical applications the quality and performance of any kind of calibration equipment is getting more and more important [6], [7], [8]. due to its worldwide need metrological applications are the most important application fields of precision measurement. in metrological applications one uses the term "calibration pyramid", expressing that the precision of the measurement should correspond to the level of calibration needed. hbm has taken part in numerous euramet driven metrology projects, in various research fields such as measurement uncertainties for measurements of very large force, torque and pressure values, so mn forces, mnm torque values as well as pressures in the gpa range. another subject we deal with is the challenge of dynamic calibration and here again mainly on mechanical quantities, all to be understood as an especially pioneering field of basic research in the field of metrology of mechanical quantities [9], [10]. 3. new compact size modules this chapter will explain how it was possible to develop such a compact and still very accurate, new type of precision amplifier for strain gauge based transducers. despite we have been able to build amplifiers “on the physical limit” down to an accuracy of 5 ppm as early as 1981 (birth date of dmp series), for a better traceability of mechanical quantities in industrial applications; there is also the requirement of building up measuring chains of a total uncertainty of “only” 100 ppm. this total uncertainty should not be evenly distributed to transducer and amplifier, but the requirement to the amplifier is significantly higher, so we put 25 ppm as our goal. this idea was set by hbm already in the early 1980ies, and created some series of models with “...38” in their name, such as the digital comparators dk 38 introduced in 1982, dk 39 introduced in 1985, mgcplusml38 introduced in 1995 and mgcplusml38b introduced in 2005. this way hbm was creating its own class of legendary precision amplifiers. during the years the seemingly constant demand for an accuracy of 25 ppm has actually become an increasingly higher requirement, as the requirements of the electromagnetic compatibility over all the time steadily increased. of course miniaturization and compact size per channel is a requirement of many calibration applications. in most calibration applications two channels have to be compared (“actio = reactio”), e.g. a reference force transducer and a specimen pressed on each other in a deadweight or hydraulic force calibration machine. thus a new amplifier module should feature two independent (simultaneously measuring) amplifiers, each of the figure 3. amplifier input and demodulator frequencies by using carrier frequency principle. figure 2. internal wiring of a precision force transducer. acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 33 channels offering an accuracy class of 0.0025 (25 ppm). this is realized by using a 24-bit a/d converter (sigma-delta adc) for each of the amplifier channels. this new module (figure 4) should be called quantum mx 238b belonging to the quantumx family and referring to the above mentioned famous models with “…38” in their name. it is based on the principle of carrier frequency (cf principle), what has several advantages to be discussed here. the advantages are mainly in the insensitivity to interference and thus in a higher quality of measurement. in the result the use of a carrier frequency allows a much better signal to noise ratio. hbm, as described in the previous section, also for the reason of good traceability back to the national standard, is using a carrier frequency of 225 hz for this new amplifier as well [5]. hereby the transducer is fed by a sinus-wave voltage and works therefore like a modulator. details of this can be reviewed in [6], [7], [8]. a further interesting feature is the automatic channel parameterization by teds (transducer electronic data sheet), a technology that allows you to plug in sensors and get started, as a self-recognising process. the characteristics of a transducer are stored inside as an electronic data sheet. the amplifier can import these data from the transducer. it then converts it automatically into the right settings and gets on with measuring straight away, in the correct units, with no further effort on the customer side. it basically allows you to just plug in and get started and significantly simplifies the handling of sensors. generally, teds can be mounted in the sensor housing or in the plugs of the transducer. when teds is mounted in the plug, it should stack with the transducer, preferably with a plug mounting from hbm [10], [11]. for a schematic block diagram of mx 238b, see figure 6. it is belonging to the hbm quantumx family, an amplifier family that allows you to process nearly any kind of signal, for instance with the mx840b module which supports as many as 15 different transducer technologies. module mx410b is also a suitable amplifier for dynamic calibration as it can be seen in [12], [13]. with the introduction of this module quantumx offers the ability to process and display multiple signals of very different accuracy-bandwidth combinations at the same time. this is one essential advantage of the fact, that mx238b is member of a big family of modules, the hbm main product family quantumx. the existing diversity of modules reflects the wide range of measurement tasks, such as needed on test rigs, to perform efficiently with quantumx by covering many different measurement principles and aspects. now the user can choose from a number of modules, namely quantumx mx238b, quantumx mx430b, quantumx mx840b, and quantumx mx410b in fine steps of very different accuracy-bandwidth combinations. based on this a real differentiation with respect to the specific measuring task can be done. all these modules can be connected by ieee 1394b firewire and synonymous measurement (in the range of 1 µs) is supported by ptp (precision time protocol) v2. first time in hbm history we were using a hermetically sealed resistive divider. this component is already very good on its own. however we decided to still implement active temperature compensation. this additional active temperature compensation is further reducing the already very low temperature coefficient to close up the gap to inductive dividers. this advantage comes with the advantage of miniaturization. now hbm can offer such a dualchannel module at the same time as compact as this is necessary for a realization in the quantumx family. despite of or even because of using a hermetically sealed resistive divider as a reference, one of the outstanding features of the new amplifier is its independence from temperature for both of the relevant values, so concerning the temperature drift of the zero point and of the span. for details see table 1. these results are astonishing, as no other series of precision amplifiers, available on the market, can offer temperature stability in the zero point as well as in the span like this. note these values refer to the change with 10 kelvin. in practical laboratory use there should be no temperature dependency at all. by its active temperature compensation quantum mx238b shows again better than mgcplus ml38b. figure 6. schematic view of module quantum mx 238b. figure 4. the new amplifier module for any kind of strain gauge transducers named quantum mx 238b, realized in compact size. figure 5. hbm high precision amplifiers are based on carrier frequency principle (cf). acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 34 the idea of background calibration, first used in dmp41 ([14]-[16]), also has been implemented into the quantum mx238b. as can be seen in figure 7, the short-term stability of the quantum mx238b has been measured. figure 7 illustrates a measurement plot of a 2 mv/v signal over 35 hours, where a background-calibration was triggered every minute. during the measurement many such background calibrations are performed. other user cases have shown that this method has no negative or disturbing influences. the adjustments of the background calibration cycles are in the sub-ppm range. only the constant thermal noise of the sensor and mainly the voltage noise of the amplifier are recognizable. there is not any interference visible in the noise floor. thus this demonstrates that it is now possible to calibrate during measurements without any interruption. deactivation of the background-calibration does not improve the accuracy, but it will even worsen the accuracy. the user does no longer have to worry about the active control of the previous auto-calibration to get no interruption when measuring. possibly we will be able to improve this value again after some time, as we will monitor long term stability the same way, as we have done for a measuring chain consisting out of bn100 and dmp39 for approx. 35 years. details for this are given in the next section. 4. factors for the reliability of the measurment in this chapter the following topics, as they are considered important factors of reliability of the measurement, should be discussed. these are: a) traceability b) long term stability c) referencing the measurement to an international standard d) ensuring reliable measurement. first of all traceability is a must. primary calibration is the highest level, carried out by the national metrology institute. thus metrological applications realize chains of traceability linking measurements made in practice back to reference standards. such standards are realized by hbm reference transducers and highest precision instruments. this core concept in metrology metrological traceability is supported by the outstanding performance of precise products. the main advantages of using strain gauge based technology are, that measurement chains offer an extremely low measurement uncertainty at very good long term stability. this leads to another factor of measurement reliability. it is the stability and beside of the data of short-term stability data gathered and described in the previous section, also for longer periods of time the new quantumx mx238b measuring amplifier shows, as far as this can be evaluated shortly after its introduction, very good results. as the short term stability is excellent we suppose long-term stability also to be very good, however for this just introduced device, so far, there are no measurements available. first we have specified < 0.0015%/a with “auto calibration >>on<<” to be on the safe side and will follow up this over the next years (see table 2). since the introduction of the dmp device series in 1981 we continuously observed the long-term stability of the measuring chain consisting of “dmp39 and bn100“(on the basis of inductances we built our own) and can state that over now more than 35 years its span variation for +2 mv/v and -2 mv/v signals was every time smaller than the accuracy class of 5 ppm (see following figure 8) [17]. for electrical calibration of these bridge standards measuring amplifiers are used, which have to be traceable to international standards. as a matter of fact, the standards of hbm and of “physikalisch-technische bundesanstalt”, the german national metrology institute, are not the same. the reason for this lies in history. hbm needed its own >>normal<< (mv/ v) to produce first precision amplifiers such as dk37 and dk 38. at that time there was no existing traceability. then, at the time around 1980, the ptb has set up a normal mv/v, however these bridge standards showed an offset. table 1.comparison of temperature drift of the different amplifiers. precision device dmp41 mgcplusml38b quantum mx238b type of voltage divider inductive resistive temperature drift zero < 2 ppm/10 k typ. < 1 ppm/10 k < 10 ppm/10 k < 5 ppm/10 k temperature drift span < 5 ppm/10 k typ. < 2 ppm/10 k < 20 ppm/10 k < 10 ppm/10 k figure 7. short-term stability of quantum mx238b: measuring with activated background-calibration (2.5 mv/v @ 5.0 v rms). table 2. comparison of different amplifiers in terms of their technical data. comparison choice of precision instrument device type dmp41 ml38b mx238b initial accuracy 5 ppm 25 ppm 25 ppm short term drift < 5 ppm/24h typ. < 2 ppm/24h < 20 ppm/24h < 10 ppm/24h long term stability/a < 5 ppm/a typ. < 2 ppm/a < 25 ppm/a < 15 ppm/a linearity < 5 ppm typ. < 2 ppm < 20 ppm < 10 ppm selectable bandwidth (-3db) 0.04…40 hz 0.03…10 hz 0.01…50 hz -0,8 -0,6 -0,4 -0,2 0,0 0,2 0,4 0,6 0,8 0,0 5,0 10,0 15,0 20,0 25,0 30,0 35,0 d ev ia ti o n in p p m time in h acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 35 the standard of ptb is technically constructed differently than the normal in hbm; however, these values can be “translated” into each other. in relation to the values acquired by ptb the end values of all hbm devices with a carrier frequency of 225 hz, for reasons of continuity, are given 10 ppm lower than the nominal values. there are publications on the details [18], [19]. mx238b demands the connection of the transducers in a six-wire-connection-circuitry (with two additional sensing lines), as for this class of accuracy the resistance of cable lengths in the range of only cm already counts. subsequently the device offers also detailed error detection with wire break detection for sensor lines, feeder lines and test leads. switchable shunt resistors of 100 ω allow an artificial bridge detuning. in a 350 ω wheatstone bridge, so the standard for the strain gauge pickup principle, there is an upset of 0.886 mv/v at 2 mv/v measuring range, i.e. a fairly significant detuning of approximately 40 %. regarding a more reliable measurement it can be said that the built-in galvanic isolation ensures avoiding ground loops and therefore the amplifier appears much more robust. also meeting the high emc requirements according to en 61326 for field strengths of 10 v/m are a further advantage of the device. 5. conclusions in general the market shows a request that the measured signals can be processed more accurately. in this paper it has been shown that there is substantial progress regarding compact size precision amplifiers for strain gauge based transducers such as force, torque and pressure reference transducers used in metrology. in fact today also amplifiers based on resistive dividers can perform most tasks with sufficient accuracy. for many decades this was not the case. due to intensive research, hbm can now offer such a dual-channel module at the same time as compact as this is necessary for a realization in the block structure the quantumx family. it is quite certain, that this previously unknown compactness will open up new fields of integration into applications. the product family quantumx now offers the ability to process and display multiple signals of very different accuracy– bandwidth combinations at the same time. thus the decisive progress realized with the quantumxmx238b is, that now the cooperation with all the other measurement modules of the product family quantumx with ptpv2 (precision time protocol) timing, allows data processing absolutely simultaneously. thus the isochronous bus system (firewire) allows synchronous processing with many other physical quantities. the precision measurement module mx238b thus combines the advantages of combination with all quantumxmembers with the possibility of taking measurements with an accuracy class 0.0025 (25 ppm). this is achieved by the use of the carrier frequency principle and an applied carrier frequency of 225 hz. for the first time hbm offers its patented background calibration of this precision class. this ensures that the measurement of the two integrated amplifiers can be continued without interruption during the measurement, so even during the internal calibration. furthermore, the transducer can be recognized with the socalled transducer electronic data sheet (“teds”) and the individual properties of the transducer can be considered and easily identification of read out. the measurement results have been showing the very good time as well as temperature stability of resistive dividers. nevertheless one has to say that the requirements for a top performance, so amplifiers of highest precision for primary calibration purposes, built to operate on the physical limit (such as our dmp-series), further relies on inductive dividers (voltage ratio derived from the number of windings of coils), as they remain still more accurate than any resistive dividers can be. references [1] hoffmann, k.; an introduction to measurementsusing strain gauges, hottinger baldwin messtechnikgmbh, darmstadt, germany, 1989, pp. 145. [2] kreuzer, m.; high-precision measuring technique for strain gauge transducers, internalpublication of hottinger baldwin mestechnik gmbh, darmstadt, germany, 1999. [3] kleckers, t., graef, m.; high capacity reference transducer for tensile forces¸ imeko 22nd, tc3, 12th tc5 and 3rd tc22 international conferences, 2014, cape town, south africa. [4] kleckers, t, precise radial symmetric shear beam force transfer transducer for compression force; xxi imeko world congress, sept. 2015, prague, czech republic. [5] schäfer, a.; force, strain and pressure transducers based on foil type strain gauges, proceedings of eurosensors; xxii conference, dresden, germany, october 2008. [6] schäfer, a. new possibilities with a high pressure measuring chain consisting of reference pressure transducer p3 topclassblueline and dmp 41 high-precision instrument, ptb report th6, brunswick, germany, april 2016. [7] haller, m. viel, w.; schäfer, a.; enhancement of the measurement characteristics of pressuretransducers up to 15000 bar through monolithicmeasuring design and foil type strain gauges, proceedings of xix imeko world congress lisbon, portugal, 2009. [8] schäfer, a.; et al.; a new type of transducer for accurate and dynamic pressure measurement up to 15000 bar using foil type strain gauges, xvii imeko world congress 2003, dubrovnik, croatia, 2003. [9] eichstädt. s., esward, t. j., schäfer, a. on the necessity of dynamic calibration for improved traceability of mechanical quantities, xxi imeko world congress, sept. 2015, prague, czech republic. [10] schäfer, a.; dynamic measurements as an emerging field in dynamic metrology; ptbmitteilungen125(2015) volume 2. [11] ieee 1451.4 standard for smart transducers, issued by ieee 1451.4 standard working group, 1451.2wg, 2004. [12] schäfer, a., challenges in dynamic torque and force measurement with special regard to industrial demands, bipm figure 8. long term stability of the established calibration method of a measuring chain consisting out of bn100 and dmp39 (+/-). acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 36 workshop on challenges in dynamic measurements. [13] schäfer, a., development results for static and dynamic torque measurement, proceedings of torque symposium of “zentrumfürkonstruktionswerkstoffe“, mpa (state material testing institute), darmstadt, germany, 2014. [14] schäck, m., underestimated impact of measuring cables on high-precision carrier frequency amplifier results and compensation methods therefore, xxi imeko world congress, sept. 2015, prague, czech republic. [15] schäfer, a.; kitzing, h.; dmp41a new chapter of ultraprecision instrument for strain gauge transducers, xx imeko world congress, september 2012, busan, republic of korea. [16] schäfer, a.; the ultra-precision instrument dmp41-first experience & appropriate filter settings, imeko 22nd tc3, 12th tc5 and 3rdtc22 conference, february, 2014, south africa. [17] schäck, m.; high-precision measurement of strain gauge transducers at the physical limit without any calibration interruptions; imeko 22nd tc3, 12th tc5 and 3rd tc22 international conferences, february, 2014, south africa. [18] vollmert, r.; ramm, g.; realization, maintenance and dissemination of the measurand>>ac voltage ratio in mv/v<< for strain gauge measurements , proceedings of the 17th international conference of imeko tc3, istanbul, turkey, 17-21, september, 2001. [19] ramm, g. ; investigation of bridge standards for use in straingauge measurements, reports in applied measurementram, vol. 6 (1990), no. 1, pp. 26-30. high-precision amplifiers for strain gauge based transducers -first time realized in compact size << /ascii85encodepages false /allowtransparency false /autopositionepsfiles true /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /error /compatibilitylevel 1.4 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.0000 /colorconversionstrategy /cmyk /dothumbnails false /embedallfonts true /embedopentype false /parseiccprofilesincomments true /embedjoboptions true /dscreportinglevel 0 /emitdscwarnings false /endpage -1 /imagememory 1048576 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/hrv (za stvaranje adobe pdf dokumenata najpogodnijih za visokokvalitetni ispis prije tiskanja koristite ove postavke. stvoreni pdf dokumenti mogu se otvoriti acrobat i adobe reader 5.0 i kasnijim verzijama.) /hun 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/pdfxoutputintentprofileselector /documentcmyk /preserveediting true /untaggedcmykhandling /leaveuntagged /untaggedrgbhandling /usedocumentprofile /usedocumentbleed false >> ] >> setdistillerparams << /hwresolution [2400 2400] /pagesize [612.000 792.000] >> setpagedevice microsoft word 254-1803-1-le-rev2 acta imeko  issn: 2221‐870x  september 2015, volume 4, number 3, 30 ‐ 35    acta imeko | www.imeko.org  september 2015 | volume 4 | number 3 | 30  hybrid time synchronization for underwater sensor  networks   oriol pallarés 1 , pierre‐jean bouvet 2 , joaquín del río 1   1  sarti research group[electronics dept.]. universitat politècnica de catalunya (upc).  rambla de l’exposició 24, 08800, vilanova i la geltrú,  barcelona, spain  2  underwater acoustics lab isen brest, c.s. 42807, 29228 brest cedex 2 – france      section: research paper   keywords: uwsn; time synchronization; ofdm; acoustic communication  citation: oriol pallarés, pierre‐jean bouvet, joaquín del río, hybrid time synchronization for underwater sensor networks, acta imeko, vol. 4, no. 3, article  5, september 2015, identifier: imeko‐acta‐04 (2015)‐03‐05  editor: paolo carbone, university of perugia, italy  received february 14, 2015; in final form april 23, 2015; published september 2015  copyright: © 2015 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: this work is supported by the projects nexos and fixo3 from the european union’s seventh programme under grants agreement no 614102 and  no 312463 respectively.  corresponding author: oriol pallarés, e‐mail: oriol.pallares@upc.edu    1. introduction  underwater acoustics has recently drawn attention from industry and research in the way that many processes can be interconnected and share valuable information. in order to perform this information exchange and create collaborative tasks, time synchronization is one of the main fields to study in underwater sensor networks (uwsn) for distributed applications such as target tracking, data acquisition systems or acoustic beam-forming. since a gps signal cannot reach below 1 meter of water column due to the high attenuation of the electromagnetic waves over the water channel, it is necessary to provide time synchronization to sensors through acoustic communication. many cabled and terrestrial wireless synchronization protocols are capable of providing high precision time offset estimation. they are widely used in many systems which need high precision time synchronization and do not have access to a gps reference. the best known protocol and most common in our daily lives is the network time protocol (ntp), which synchronizes our computers over ethernet. another cabled synchronization system that is used in industrial applications, majorly due to its high synchronization precision is the precision time protocol (ptp) ieee 1588 standard. this protocol has been ported by [1] to a terrestrial wireless network with timing accuracy below a few microseconds. although these synchronization protocols can provide high timing accuracy in terrestrial networks such as ethernet and wsns these protocols cannot be used for the uwsn because they do not face some of the main problems of the underwater channel [2]. actually, there are few protocols capable of providing time correction to underwater sensors [3], [4], but most of them do not take into account all the underwater physical layer communication challenges such as doppler effect correction, drift between clocks or propagation time variability, which become significant with high latency communication caused by low celerity of sound in water channel. in this work, we present an hybrid synchronization system mixing linear frequency modulation (lfm) for precise time arrival detection and orthogonal frequency division multiplexing (ofdm) communication with channel impulse abstract  time  synchronization  is  a  critical  part  of  distributed  applications  over  a  sensor  network.  in  this  work  we  investigate  time  synchronization problems over uwsn deployed in shallow water, taking into account all main communication challenges of the water  channel and observing its behavior in simulation and real tests.  a hybrid frame was proposed based on time synchronization using  both lfm and ofdm communication with channel impulse response equalization. simulation results show how hybrid synchronization  outperforms existing synchronization protocols and how these results are affected in real underwater tests.  acta imeko | www.imeko.org  september 2015 | volume 4 | number 3 | 31  response (cir) equalization in order to correct drift, doppler velocity and detect propagation time variations between message exchanges. this hybrid synchronization yields similar simulation results to [5], but in our case, the study of synchronization is carried out both over simulation and real water experiments whereas results of [5] are limited to simulation. this result will be compared to the ones given by time synchronization for high latency acoustic networks (tshl) and timing-sync protocol for sensor networks (tpsn) underwater synchronization protocols which are the ones most used in actual underwater networks. the paper will be divided in four main sections. first, in section 2 the challenges faced in uwsn synchronization are presented. before describing the model design, two of the main underwater synchronization systems and their weaknesses are presented in subsection 2.4. then, section 3 presents the system model used to perform simulation. the water tests for verifying time error accuracy of the system, is detailed in section 4. conclusions and future works are discussed in section 5. 2. physical layer challenges in high latency  networks  when simulating underwater communications, cabled synchronization algorithms techniques can easily be applied, but in underwater real tests, conditions do not fit exactly simulation model, these non-idealities issues affect system performance and rigorous study of such effects is required in order to achieve synchronization errors below 100 microseconds. at the starting point, we took the synchronization algorithm basic message exchange, represented in figure 1, and modified this protocol as we went deeper in this section. this synchronization message exchange base, is used in most cabled and wireless message exchange based protocols. in order to compute offset between nodes a and b, four time stamps were necessary (t1, t2, t3 and t4). these timestamps provide an estimate of the propagation time between nodes, leading to a precise synchronization in case of ideal propagation channel as is described in equation (1) for the offset β and equation (2) for the propagation time τ possible. t2 and t3 are enclosed in the message in order to make them available at the slave side (node a): (1) , (2) where propagation time is computed as the average of propagation time from a to b (denoted ) and propagation time from b to a (denoted ). this algorithm makes the assumption that both times are equal to . we also observed the dependence between time stamping precision and offset estimation. the way how this assumption is affected by non-idealities conditions is described in the following. first, subsection 2.1 studies the error of each time stamp for the incoming and outgoing messages and how this affects the main algorithm. the time stamping issue is handled also in cabled synchronization algorithms, so we will discuss how this uncertainty is solved for these networks and how it can be applied to uwsn. then in sections 2.2 and 2.3 the main problems added by the water channel are introduced, which may be negligible in cabled networks but not in high latency ones such as underwater sensor grids. 2.1. time stamping  considering a non-deterministic time stamping for the message input or output, equations (1) and (2) are reformulated as follows: (3) (4) where 1, 2, 3 and 4 stand for the time stamp uncertainty in both incoming and outgoing messages. this uncertainty used to be around hundreds of microseconds for software time stamps and some nanoseconds for hardware time stamps such as the one presented in ptp. hardware time stamping has higher precision because the messages are referenced to a time base in the medium access controller (mac), avoiding in this way unpredictability introduced by operating system (os) or medium access algorithms, which can be about hundreds of microseconds. hence, it is necessary to port the mechanisms used in cabled time synchronization algorithms for hardware time stamping to the underwater communication algorithms in order to get a similar performance. a common workaround for solving time stamping ambiguity is to use a preamble before each frame, which can be recognized by the mac, making possible the time reference acquisition by hardware before any processing of the frame. 2.2. propagation time variation  regardless of time stamping issue, propagation time variations between and will be noticed. this alteration may be due to different communication paths in cabled communications. analogously, it happens with a multi-path channel such as the underwater one, with the further difficulty added by moving nodes in a communication with high latency, which increases propagation time variation ( ). developing (1) and (2) with 1 2 leads to: δτ (5) , (6) where the real clock offset is estimated by with an error equal to . 2.3. clock drift and doppler effect  besides clock offset between two nodes, there is another timing factor that must be taken into account when synchronizing two clocks. this is the drift between clocks, figure 1. sender‐receiver synchronization workflow.   acta imeko | www.imeko.org  september 2015 | volume 4 | number 3 | 32  which is the variation between a clock frequency compared to an ideal one, given in parts per million. this factor gets importance as the network has bigger latency. the reason of this assumption lies in the amount of time that the protocol needs to compute the offset. while synchronization offset is computed, one clock is drifting. therefore, by the end of the computation, we will have the initial offset to be compensated, plus the drift of this second clock. in a small latency network, time offset estimation is computed fast enough for neglecting the error introduced by the drift, as happens in the ntp protocol. but in high latency networks, a few seconds of propagation time will result in an error caused by drift equal to hundreds of microseconds at the end of the offset estimation protocol. in literature, most of the synchronization protocols manage the time stamping issue, some of them also offer solutions for propagation time variations, but it is quite difficult to find studies of the drift compensation if we do not go deep in high precision synchronization protocols, such as ieee-1588 (ptp) or in underwater networks tsmu scheme and tshl protocol. the main issue of estimating clock drift in non-cabled networks resides in the mobility of the nodes. when a node has a velocity relative to the medium it is affected by the doppler effect, consequently, it suffers from sampling base time variations. as described in [6], this sampling time variation is also produced by drift, which increases the difficulty to estimate drift and doppler velocity separately. equations (7) and (8) describe how and are modified, introducing clock drift (ɵ) affecting only one of the clocks: ɵ ɵ ɵ (7) ɵ ɵ ɵ . (8) table 1 presents a description of all symbols used for the algorithm development. 2.4. related work  despite the growing interest, during the last years, in deploying uwsns, there is only a few literature highlighting underwater wireless high precision synchronization algorithms. tshl is designed for high latency acoustic networks. it splits time synchronization in two phases. in the first phase, it estimates the clock drift based on a centralized timebase, and in the second phase, it determines the offset by a message exchange protocol. in order to perform clock drift estimation, a linear regression is computed over several beacon messages sent within a known time. the time deviation of the message arrival at the receiver side is assumed to be caused by clock drift. the main problem of this assumption is that, instead of having all the nodes completely quiet, multi-path and water currents will affect the message propagation time between nodes, causing a wrong measurement of clock drift. both ptp and tsmu protocols estimate drift jointly with doppler effect, and then by analysing doppler scaling, the method presented in an article from the underwater acoustics lab of connecticut’s university is capable of determining which part is due to doppler velocity and which is due to clock drift, as described in [5]. in this way, they are capable of improving tshl or tpsn performance by introducing doppler movement to the offset and propagation time equations [6]. however, ptp is designed for cabled networks, so it cannot work directly with high propagation times, and tsmu is a scheme which had not been tested in real tests where the complexity of doppler scaling estimation increases due to multi-path noise. 3. hybrid time syncrhonization model  in this section we present an alternative time synchronization algorithm that can overcome the main underwater channel problems. for this first approach, doppler scaling effects are not taken into account. thus, main issues discussed in this paper are the frame time stamping, the propagation time variations detection, and how to feedback these parameters to the system. output framing detection or time stamps, can be performed by most of the commercial daq systems, being capable to trigger the dac output at a certain time with a hw pulse. this means that the output time stamping has a deterministic and negligible error. time stamping of an incoming message is performed by main communication algorithms in order to detect where the raw data is placed inside of an acquisition window. in this way, the frame can be sent to a dsp to demodulate the desired data. then, these algorithms can be used to detect the exact arrival time. this is done by performing a coarse estimation in order to window a 'pilot' signal [7], and then analyse the channel impulse response (cir) of this pilot for detecting the first sample which arrived, with 1 sample of error. consequently, this system will reach a maximum frame detection precision equal to 1/bw where bw denotes the signal bandwidth. as a result, larger bw will result in a better timing accuracy. this could lead us to increase bw to improve timing precision. however, an increase of bw results of a decrease of the robustness against communication channel due to extended inter symbol interference (isi). hence, it is necessary to find a trade-off between this robustness and synchronization accuracy. the workaround proposed in this paper is to use an lfm (chirp signal) in conjunction with cir frame arrival detection. this will allow us to keep a low bw for communication and increase it in the lfm. thus lfm is used to detect the reception of frames by performing correlation between the received signal and the expected chirp. then, the computation of center of gravity improves the accuracy of the correlation peak leading to a precise time stamping. finally, signal detection is performed, without the necessity of using a threshold, by using schmidl & cox algorithm [7] which provides in addition an estimation of the carrier frequency offset which modulates the main signal and the lfm. table 1. notation summary.  symbol  description    estimated clock offset    real clock offset 1, 2  propagation delay of a‐b and b‐a      1 2  1, 2, 3, 4  time stamp uncertainty  ɵ  doppler scaling   bidirectional propagation time    initial time    user defined time between messages acta imeko | www.imeko.org  september 2015 | volume 4 | number 3 | 33  we can send the global time stamp in the acoustic ofdm message and detect the frame arrival time with the lfm. 4. results  in this section, the results with hybrid synchronization using lfm and cir are discussed, in order to perform time stamping and estimate and by adding both 1 and 2 without the indeterministic time errors given by the low bw time stamping procedure. 4.1. workbench description  in order to perform real underwater tests, we used a water 'test tank' of dimensions 150 x 40 x 40 cm, with 1 meter separation between hydrophones. since this study is not a matter of communication robustness but time synchronization and detection, we can extrapolate the results obtained in this test tank to shallow water sea communication in terms of timing accuracy. in order to send data from a computer to the water tank, we used the national instruments crio-9103 with an acquisition module (ni-9201) and a signal generator (ni-9263). this device consists of an fpga and a real time controller, with the capacity to interconnect directly i/o modules to the fpga, and control them in the labview programming environment. the main reason for using this system is the ease of programming the data input/output and function triggering. this module is in charge of sending and receiving information to the hydrophones, and to trigger clock readings each time a signal is generated or an acquisition starts. in order to link the computer and the water channel, the crio controller implements a tcp/ip communication in order to communicate the data between the fpga and the computer. the fpga has a hardware description consisting of four different loops, two of them running a clock and the other two for acquisition and generation respectively. these loops are triggered pair wise, one trigger for the clock and the generation module and the second trigger for the other clock and acquisition device. this hardware description allowed us to implement the desired hw time stamp for synchronization. when the signal is sent from a laptop to the crio it is saved in a memory register. once the signal is completely received, an 'i/o' flag is set, causing a trigger in the fpga generation gear together with a clock reading for the signal output time stamp. simultaneously, the acquisition engine is triggered with its corresponding clock reading for an input signal time stamp. after a user defined acquisition time elapse, given by the maximum propagation time plus message duration, the 'i/o' flag is cleared. then the controller sends back all the time stamps information to the laptop, plus the received acoustic raw data. this hardware application makes it possible to perform a message exchange based synchronization protocol in a laptop, providing both time stamps, the generation and acquisition ones, besides information of the raw data enclosing a global time reference. to connect the ni-9201 (adc)/ni-9263 (dac) and the hydrophones, a power and charge amplifier board has been designed for boosting both the generator power and the received signal. the power amplifier module consists of two operational amplifier stages in order to boost the current given by the ni-9263 generation module and at the same time adapt the impedance of the dac to the hydrophone one. in this way, the amount of power loss before sending the signal to the water channel is reduced. likewise, the charge amplifier consists of a two stage low noise amplifier with hydrophone adapted impedance, so that we sent an amplified signal with rejected noise to the ni-9201 acquisition module. the module used for real tests is represented in figure 2. we also used a t/r switch for a bidirectional communication using only one hydrophone. with this, we obtained a robust and convenient system to perform underwater acoustic communication. this module was designed following the dimensions of a compact rio module and using the same mounting to attach this system to the crio-9103. figure 3 represents the final design used for underwater communication and synchronization as described above. a block diagram of the communication workflow is presented in figure 4. for both simulations and real 'test tank' tests, a master clock and a slave clock were used. for the evaluation of the algorithms a slave clock generated from the master clock was used, in order to know the delay between clocks a priori. as described in figure 1, we defined in a matlab® script t1 as base time ( ), t2 is given by t1 plus the propagation time from slave to master ( ) plus an offset and drift between clocks. t3 equals t2 plus a user defined time between messages ( ), and finally t4 equals t1 plus both propagation times (∆ ) and the user defined time between messages. figure 2. power and charge amplifier front view.   figure 3. crio module plus power and charge amplifier board.   acta imeko | www.imeko.org  september 2015 | volume 4 | number 3 | 34  equations from (9) to (12) describe how time stamps are defined in the script in order to simulate two different clocks. 1 ε1 (9) 2 1 (10) 3 2 (11) 4 1 ∆ (12) with this definition we were able to simulate time stamp uncertainty, clock drift and offset in the script, and verify the proper detection of our algorithms for each time synchronization uncertainties. 4.2. main results  here, we present results given by simulations, which are compared to tpsn and tshl, and also real tests demonstrating hybrid-sync performance. comparisons with tpsn and tshl were made by implementing a tpsn-like (tpsn-l) and tshl-like (tshl-l) protocol in matlab® simulations. by this nomenclature, we mean that the simulation captured the essence of both protocols as described in [8]. however, for this work, an uncertainty due to propagation time variations was included, which leads this protocols to bigger uncertainty in synchronization precision. figure 5 shows the results of a simulation comparing tpsnl, tshl-l and hybrid synchronization performance. for this test a propagation time variation with standard deviation of 15 parts per pillion (ppm) relative to the distance between nodes was used. due to the fact it is possible to estimate propagation times with higher precision, it is possible to compensate ∆ /2, resulting in the hybrid synchronization outperforming actual system time offset and delay estimation accuracy. then, in figure 6 the results of an underwater test performed in the 'test tank', described in subsection 4.1 are depicted. this verifies the proper performance of the synchronization algorithm over snr variations. therefore, the mean estimation accuracy of the synchronization algorithm with an accuracy below 2 microseconds, and the variance, due to propagation time variations, below 100 microseconds for a low snr, i.e. 5 db, can be observed 5. conclusions and future work  in this article, we presented a new approach for a frame time stamp, which is a relevant part of frame based synchronization algorithms. our study leads to a constant synchronization accuracy below 10 microseconds, independent of distance between nodes, by improving the frame time stamping for computing propagation times from master to slave nodes and vice-versa. so, we can conclude that our algorithm overcomes the studied synchronization algorithms found in literature. despite the achieved good results, we must take into account the assumption done during all our study of an absence of any doppler effect, neither in simulation nor real tests. in order to compare the hybrid time synchronization accuracy versus the recent studies of time synchronization [9], doppler scaling should be added to our tests. research is actually on-going, by the authors, to compare several doppler scale estimation algorithm performance, in order to achieve the best performance in hybrid time synchronization algorithm. acknowledgement   this work is supported by the projects nexos and fixo3 from the european union’s seventh programme under grants agreement no 614102 and no 312463 respectively. the authors would also like to thank national instruments spain for their support providing us the necessary hardware used in this study. figure 5. quiet shallow water synchronization.   figure 4. frame detection workflow diagram for real test.   figure 6. snr vs synchronization performance.   acta imeko | www.imeko.org  september 2015 | volume 4 | number 3 | 35  references  [1] toma, d., río fernandez, j. d., manuel lázaro, a., gomáriz castro, s., & carreras pons, n., “precision timing in tdma based wireless sensor network through ieee 1588 standard.” 19th imeko tc-4, 2013 pp.683-686. [2] akyildiz, i. f., pompili, d., & melodia, t. “challenges for efficient communication in underwater acoustic sensor networks.” acm sigbed review, 2004, 1(2), pp. 3-8. [3] chirdchoo, n., soh, w. s., & chua, k. c. “mu-sync: a time synchronization protocol for underwater mobile networks.” in proceedings of the third acm international workshop on underwater networks (pp. 35-42). acm (2008, september). [4] lu, f., mirza, d., & schurgers, c. “d-sync: doppler-based time synchronization for mobile underwater sensor networks.” in proceedings of the fifth acm international workshop on underwater networks (p. 3). acm (2010, september). [5] liu, j., wang, z., peng, z., zuba, m., cui, j. h., & zhou, s “tsmu: a time synchronization scheme for mobile underwater sensor networks.” in globecom, pp. 1-6. ieee, 2011. [6] stojanovic, m., freitag, l. “integrated doppler tracking and efficient resampling for phase coherent acoustic communication”. internet draft may 2007, available: http://www.mit.edu/~millitsa/resources/pdfs/joedop.pdf [7] schmidl, t.m., & cox, d.c. “robust frequency and timing synchronization for ofdm” communications, ieee transactions on 45.12 (1997), pp. 1613-1621. [8] syed, a.a., & heidemann, j.s. “time synchronization for high latency acoustic networks." in infocom (2006, april). [9] liu, j., wang, z., zuba, m., peng, z., cui, j. h., & zhou, s. "da-sync: a doppler-assisted time-synchronization scheme for mobile underwater sensor networks", ieee transactions on mobile computing 3 (2014), pp. 582-595. acta imeko, title acta imeko issn: 2221-870x june 2018, volume 7, number 2, 61-64 acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 61 a novel technique based on the law of comparative judgment for quality-related problems fiorenzo franceschini, domenico maisano politecnico di torino (dept. of management and production engineering), corso duca degli abruzzi 24, 10129, turin, italy section: research paper keywords: quality engineering/management; law of comparative judgment; scaling; preference ordering; ratio scale citation: fiorenzo franceschini, domenico maisano, a novel technique based on the law of comparative judgment for quality-related problems, acta imeko, vol. 7, no. 2, article 11, june 2018, identifier: imeko-acta-07 (2018)-02-11 section editor: franco pavese, italy received december 5, 2017; in final form april 10, 2018; published june 2018 copyright: © 2018 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: domenico maisano, e-mail: domenico.maisano@polito.it 1. introduction a common problem in the field of quality engineering/management is that in which a set of judges express their individual (subjective) judgments on a specific attribute of some objects and they have to be fused into a collective one [1], [2], [15]. possible examples can be: (i) the fusion of individual customer expectations on a set of product requirements, or (ii) the fusion of judgments by reliability and maintenance engineers on the severity of a set of possible process failures. the scientific literature encompasses a plurality of fusion techniques, which differ from each other for (at least) three features [4]: (1) the response mode for collecting the (input) judgments, e.g., expressed in the form of preference orderings, pairedcomparison relationships, ratings, rankings, etc.; (2) the underlying principle of the fusion technique, e.g., heuristic, mathematical/statistical, or fuzzy models; (3) the type of (output) collective judgment, e.g., expressed in the form of rankings or ordinal/interval/ratio scale values. the simplest fusion technique is probably that in which judges evaluate the objects using an ordinal response scale with a predetermined number of levels – e.g., five: very low, low, intermediate, high, and very high degree of the attribute. then, for each object, the resulting scale levels are converted into conventional scores (e.g., 1, 2, 3, 4, 5 or -2, -1, 0, 1, 2, for fivelevel scales) and aggregated through central-tendency indicators. in the field of voting theory, we recall the method by borda [3], while in the field of multicriteria analysis decision-making, we recall the analytic hierarchy process (ahp) [6]. for an exhaustive discussion of the existing techniques, we refer the reader to the vast literature and extensive reviews [1], [4]. regardless of the peculiarities of the individual fusion techniques, a key element for their success is the simplicity of the response mode. generally speaking, response modes that are relatively simple and understandable are more likely to be accepted, and the relevant data collection process is more likely to be accurate and reliable [3], [5]. for example, various studies show that comparative judgments of objects (e.g., “oi is more preferred than oj”) are generally easier than judgments in absolute terms (e.g., “the degree of the attribute of oi is low/high”) [5], [8]. as to the typology of collective judgments, we note that they are often treated as if they were defined on a ratio scale, even when they actually are not; e.g., rankings or ordinal-scale values of the objects are often improperly “promoted”, in the moment in which they are combined with other indicators through abstract a relatively diffused problem in the quality engineering/management field is that in which a set of judges express their individual (subjective) judgments on a specific attribute of some objects, and these judgments have to be fused into a collective one. the goal of this paper is to develop a new technique that combines the thurstone’s law of comparative judgment with an ad hoc response mode based on preference orderings. this technique allows to express the collective judgment of the objects on a ratio scale and is applicable to a variety of practical contexts. acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 62 weighted sums, geometric averages, or – more generally – statistics permissible to ratio-scale values [9], [10]. these promotions are potentially dangerous, as they can lead to significant distortions [3]. this article focuses on the thurstone’s law of comparative judgment (lcj) [11], i.e., a consolidated model that, starting from judgments expressed in the form of paired-comparison (ordinal) relationships of a set of objects, allows constructing an interval scaling of these objects. the lcj is a milestone of psychometry and has stimulated the construction of numerous other models [7]. despite its elegance and relative computational simplicity, the lcj is not very popular in the quality engineering/management field, probably because of two major limitations: (1) the response mode based on paired comparisons is inevitably tedious, especially when the number of objects tends to be high; (2) the output is defined on an interval scale, which is not as powerful as a ratio scale. the goal of this contribution is to develop a new fusion technique, which can be easily applied to typical problems in the quality engineering/management field. the proposed technique will be based on the combination of the canonical lcj model with an ad hoc response mode based on preference orderings. apart from the regular objects (o1, o2, …, on), these orderings will also include two anchor objects, to univocally identify the zero point and a conventional unit of the output scale. in this way, the output scale will (reasonably) be considered as a ratio one. 2. thurstone’s lcj thurstone [11] postulated the existence of a psychological continuum, that is to say an abstract and unknown unidimensional scale, in which objects are positioned depending on the degree of a certain attribute – i.e., a specific feature of the objects, which evokes a subjective response in each judge. the position of one object is directly proportional to the degree of the attribute. according to thurstone, the position of a generic i-th object (oi) is distributed normally: oi ~ n(µi, σi2), where µi and σi2 are the unknown mean value and variance of that object’s attribute. this distribution has been postulated to reflect the intrinsic variation between judges in positioning the objects in the continuum. in addition, the objects’ variances are all equal (σi2 = σj2 = … = σ2), and the intercorrelations (in the form of pearson coefficients ρij) between pairs of objects (oi, oj) are all equal too ( j,i,ij ∀= ρρ ). the application of the lcj is based on five steps: (1) a set of (m) judges (j1, j2, …, jm) formulate their preferences for each object (oi) versus every other object (oj), considering all possible nc 2 = n·(n–1)/2 pairs, n being the total number of objects. preferences are expressed through relations of strict preference (e.g., “o1 > o2” or “o1 < o2”) or indifference (e.g., “o1 ~ o2”) [10]. results are then aggregated into a frequency matrix (f). precisely, for each judge who prefers oi to oj, the general element fij ∈ [0, m] is incremented by one unit; if two objects are considered indifferent (i.e., “oi ~ oj”), fij and fji are both conventionally incremented by 0.5. in mathematical terms: b.af ij ⋅+= 50 (1) where “| |” is the cardinality operator that corresponds to the number of elements of a set, and the two sets ⊆a {jk: “oi > oj”} and ⊆b {jk: “oi ~ oj”}. of course, the complementarity relationship fij = m – fji holds. (2) next, the fij values are transformed into pij values, through the relationship: mfp ijij = (2) where pij represents the observed proportion of times that oi was chosen over oj. the pij values are aggregated into a proportion matrix (p). (3) next, pij values are transformed into zij values, through the relationship: zij = f-1(1 – pij), (3) f() being the cumulative distribution function of the standard normal distribution. the element zij represents a unit normal deviate, which will be positive for all values of (1 – pij) over 0.50 and negative for all values of (1 – pij) under 0.50. in general, objects are judged differently by judges. however, if all judges express the same preference for each outcome, the model is no more viable (pij values of 1.00 and 0.00 would correspond to zij values of ±∞ ). a simplified approach for tackling this problem is to associate values of pij ≤ 0.023 with zij = f-1(1 – 0.023) = 1.995 and values of pij ≥ 0.977 with zij = f-1(1 – 0.977) = -1.995. more sophisticated solutions to deal with this issue have been proposed [14]. (4) next, the zij values related to the possible paired comparisons are reported into a matrix z. the element zij is reported in the i-th row and j-th column. the relationship zji = -zij holds, being unit normal deviates related to complementary cumulative probabilities (i.e., pji = (1 – pij)). (5) the scaling can be performed by (i) summing the values into each column of the matrix z and (ii) dividing these sums by n. it can be demonstrated that the result obtained for each j-th column (xj) corresponds to the unknown average value (µj) of the object’s attribute, up to a positive scale factor and an additive constant [12], [13]: ( ) 21 kkn/zx jj ijj +⋅== ∑ µ . in other words, the lcj results into an interval scaling, i.e., objects are defined on a cardinal scale (x) with arbitrary zero point and unit [9], [10]. 3. description of the new technique 3.1. introduction of preference orderings a significant limitation of the canonical lcj is that the direct formulation of paired-comparison (ordinal) judgments can be tedious and complex to manage, since much repetitious information is required from judges. alternatively, judges can directly formulate preference orderings that are then turned into paired-comparison data. precisely, each judge constructs a linear preference ordering, i.e., a chain of hierarchical levels containing the objects of interest, linked by arrows depicting the strict preference (“>”) relationship (see the example in figure 1). two or more objects in the same hierarchical levels are linked by the indifference (“~”) relationship. we note that this acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 63 response mode forces judges to be transitive (e.g., if “o1 > o2” and “o2 > o3”, then “o1 > o3”). even though the formulation of preference orderings may sometimes be less practical then the use of ordinal response scales (e.g., in the case of telephone or street interviews), the fact remains that it is less prone to the following problems: (i) ordinal scales tend to be used subjectively, as there is no absolute reference shared by all judges, and (ii) the number of categories in the ordinal scale (e.g., a five-level scale) may conflict with the real discriminatory power of judges. 3.2. zm anchoring of the thurstone’s scaling another limitation of the canonical lcj is that the resulting (interval) scale is not “anchored” with respect to the (unknown) psychological continuum, in which objects are positioned, due to the arbitrary zero point and unit. this limitation makes the results of different scaling processes difficult to compare. we have developed a new anchoring technique, denominated “zm”, based on the application of the lcj, including two anchor objects among the regular ones (o1, o2, etc.): z, i.e., an anchor object corresponding to the absence of the attribute of interest, and m, i.e., an anchor object corresponding to the maximum-imaginable degree of the attribute, consistently with the current technological and socio-economic development. likewise, the regular objects, z and m are assumed to project a normal distribution on the psychological continuum, with unknown mean value and unknown variance, equal to that of the other objects (see sect. 2). each judge has to formulate a preference ordering of the objects, with two important requirements: (1) z should be positioned at the bottom of the preference ordering. in the case the attribute of another object is judged to be absent, that object will be considered indifferent to z. (2) m should be positioned at the top of the preference orderings. in the case the attribute of another object is judged to be the maximum-imaginable, that object will be considered indifferent to m. next, the thurstone’s scaling is performed and the resulting (interval) scale is transformed into a new one, defined in the conventional range [0, 100], through the following linear transformation: zm zj j zm zjj xx xx y xx xxy − − ⋅=⇒ − − = − − 100 0100 0 (4) where xz and xm are the scale values of z and m, resulting from the lcj; xj is the scale value of a generic j-th object, resulting from the lcj; yj is the relevant transformed scale value in the conventional range [0, 100]. the introduction of z and m allows to anchor the lcj scale (x) into a new scale (y) with a conventional unit and a zero point (which corresponds to the absence of the attribute); it is therefore reasonable to consider y as a ratio scale. we remark that setting the value of m to 100 is a normalization that makes the scale unit comparable with that ones obtained from other lcj processes. in the example in figure 2, five judges (j1 to j5) have to compare four products (o1 to o4, i.e., the objects of interest), based on their simplicity of use (i.e., the attribute of interest). precisely, each judge formulates a preference ordering of the four (regular) objects and two anchor objects (z and m). the introduction of z and m increases the information content of preference orderings. for example, the information that the degree of an attribute is zero or the maximumimaginable one is richer than the information that it is just lower or higher than the remaining ones. the price to pay for this information enrichment is the increased effort of judges, who should also consider the two dummy/anchor objects, envisaging their “absolute” meaning. the preference orderings are then translated into pairedcomparison relationships and the lcj is applied (see figure 3). we have verified that the new anchoring technique provides results in line with those obtained from other existing techniques, such as that suggested by torgerson’s [14]. 4. conclusions this paper proposed a new fusion technique that combines the canonical lcj model with an ad hoc response mode based on preference orderings. apart from the regular objects, these orderings will also include the two anchor objects z and m. this allows representing the objects on a conventional ratio scale, included between 0 to 100, without any conceptually prohibited promotion. in addition, the response mode based on preference orderings is relatively practical and user-friendly. a limitation of the proposed technique is that the introduction of the two anchor objects (z and m) requires an additional o1 o2 o3 o4 1st hierarchical level 2nd hierarchical level 3rd hierarchical level (o3 ~ o4) > o1 > o2 resulting (linear) preference ordering (analytic form): tags related to the individual objects paired-comparison relationships: o1 o2 o3 o4 o1 > < < o2 < < o3 ~ o4 figure 1. practical technique to support the construction of preference orderings using tags and (indirect) determination of paired-comparison relationships. j1 j2 j3 j5 o3 o2 z o1 o4 m o1 o2 o3 m o4 z o2 o1 o3 o4 m z o3 o2 o4 m o1 z m z o3 o4 o2 o1 j4 figure 2. example of preference orderings formulated by five judges (i.e., j1 to j5), including four regular objects (o1to o4) and two anchor objects (z and m). acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 64 effort for judges. regarding the future, we plan to simplify the response mode, e.g., assuming that judges formulate preference orderings that include only a few most/least preferred objects [5], and to apply the proposed technique to real-life problems in the quality engineering/management field. references [1] k. coaley, an introduction to psychological assessment and psychometrics, 2nd edn., sage, london, 2014, isbn 9781847874795. [2] j.c. krynicki, introduction to soft metrology, proc. of xviii imeko world congress, sep. 17-22, 2006, rio de janeiro, brazil. [3] f. franceschini, m. galetto, m., d. maisano, management by measurement: designing key indicators and performance measurement systems, springer, berlin, 2007, isbn 978-3-54073212-9. [4] r.f. devellis, scale development: theory and applications, 4th edn., sage, london, 2016, isbn 978-1412980449. [5] a.w. harzing, j. baldueza, ... and y.k. liang, rating versus ranking: what is the best way to reduce response and language bias in cross-national research?. international business review, 18 (2009) pp. 417-432. [6] t.l. saaty, decision making with the analytic hierarchy process. international journal of services sciences, 1(2008): 83-98. [7] d. andrich, relationships between the thurstone and rasch approaches to item scaling. applied psychological measurement, 2 (1978) pp. 451-462. [8] g. moors, i. vriens, j.p. gelissen, j.k. vermunt, two of a kind. similarities between ranking and rating data in measuring values. survey research methods, 10 (2016) pp. 15-33. [9] f.s. roberts, measurement theory: with applications to decision making, utility, and the social sciences, in: encyclopedia of mathematics and its applications, vol. 7, addison-wesley, reading, ma, 1979, isbn 978-0521102438. [10] s.s. stevens, on the theory of scales of measurement. science, 103 (1946) p. 2684. [11] l.l. thurstone, a law of comparative judgments. psychological review, 34 (1927) p. 273. [12] f. franceschini, d. maisano, prioritization of qfd customer requirements based on the law of comparative judgments. quality engineering, 27 (2015) pp. 437-449. [13] l.l. thurstone, l.v. jones, the rational origin for measuring subjective values. journal of the american statistical association, 52 (1957) pp. 458-471. [14] w.s. torgerson, theory and methods of scaling, wiley, oxford, england, 1958, isbn 978-0471879459. [15] j.j. dahlgaard, g.k. khanji, k. kristensen, fundamentals of total quality management, routledge, london, 2008, isbn 9780748772933. o1 o2 o3 o4 z m o1 2.5 3.5 0.0 0.5 5.0 0.0 o2 1.5 2.5 0.0 0.0 4.5 0.0 o3 5.0 5.0 2.5 3.0 5.0 0.5 o4 4.5 5.0 2.0 2.5 5.0 1.0 z 0.0 0.5 0.0 0.0 2.5 0.0 m 5 5 4.5 4 5 2.5 (b) matrix f (c) matrix p (d) matrix z fij denotes the number of times that oi is preferred to oj; pij denotes the proportion of times that oi is preferred to oj; zij = f-1(1 – pij); notes: o1 o2 o3 o4 z m o1 0.50 0.70 0.00* 0.10 1.00* 0.00* o2 0.30 0.50 0.00* 0.00* 0.90 0.00* o3 1.00* 1.00* 0.50 0.60 1.00* 0.10 o4 0.90 1.00* 0.40 0.50 1.00* 0.20 z 0.00* 0.10 0.00* 0.00* 0.50 0.00* m 1.00* 1.00* 0.90 0.80 1.00* 0.50 o1 o2 o3 o4 z m o1 0.00 -0.52 2.00 1.28 -2.00 2.00 o2 0.52 0.00 2.00 2.00 -1.28 2.00 o3 -2.00 -2.00 0.00 -0.25 -2.00 1.28 o4 -1.28 -2.00 0.25 0.00 -2.00 0.84 z 2.00 1.28 2.00 2.00 0.00 2.00 m -2.00 -2.00 -1.28 -0.84 -2.00 0.00 σj -2.75 -5.23 4.96 4.18 -9.26 8.11 xj = σj / n -0.46 -0.87 0.83 0.70 -1.54 1.35 yj [0,100] 37.5 23.2 81.9 77.4 0 100 z is an anchor object denoting the zero preference level; m is an anchor object denoting the maximum-imaginable preference level; n=6 is the total number of objects, including z and m; (*)values of pij ≤ 0.023 and ≥ 0.977 have been conventionally associated with zij = 1.995 and -1.995 respectively; xj is the (interval) scale value of the j-th object, resulting from the lcj; yj is the xj value transformed in the conventional range [0, 100] (transformation in eq. 4). (a) judgements, paired-comparison relationships and fij, pij, zij indicators paired comparison j1 j2 j3 j4 j5 fij pij zij (o1, o2) > > ~ > < 3.5 0.70 -0.524 (o1, o3) < < < < < 0 0.00* 1.995 (o1, o4) < < < ~ < 0.5 0.10 1.282 (o2, o3) < < < < < 0 0.00* 1.995 (o2, o4) < < < < < 0 0.00* 1.995 (o3, o4) ~ ~ < > > 3 0.60 -0.253 (o1, z) > > > > > 5 1.00* -1.995 (o2, z) > > > ~ > 4.5 0.90 -1.282 (o3, z) > > > > > 5 1.00* -1.995 (o4, z) > > > > > 5 1.00* -1.995 (o1, m) < < < < < 0 0.00* 1.995 (o2, m) < < < < < 0 0.00* 1.995 (o3, m) < ~ < < < 0.5 0.10 1.282 (o4, m) < ~ ~ < < 1 1.00* -1.995 0 20 40 60 80 100 o2 o1 o4 o3 decreasing degree of the attribute increasing degree of the attribute y (e) graphical representation of the scaling figure 3. example of lcj application to the preference orderings in figure 2: (a) paired-comparison relationships, (b) matrix f, (c) matrix p, (d) matrix z and resulting scaling, and (e) graphical representation of the scaling. a novel technique based on the law of comparative judgment for quality-related problems template for an acta imeko event paper acta imeko issn: 2221-870x december 2016, volume 5, number 4, 24-28 acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 24 yb,er:glass microlaser at 1.5 µm for optical fibre sensing: development, characterization and noise reduction alexey pniov1, andrey zhirnov1, dmitriy shelestov1, konstantin stepanov1, evgeny nesterov1, valery karasik1, paolo laporta2, gianluca galzerano3, stefano taccheo4, luigi piroddi5, michele norgia5, alessandro pesatori5, cesare svelto5 1bauman moscow state technical university (bmstu)centre for photonics and infrared technology, 2-ya baumanskaya ul. 5, 105005 moscow, russian federation 2politecnico di milano (polimi) – dfis-polimi, piazza leonardo da vinci 33, 20133 milan, italy 3national research council cnr-ifn, piazza leonardo da vinci 33, 20133 milan, italy 4swansea university, singleton park, sa2 8pp swansea, wales, uk 5politecnico di milano (polimi) – deib-polimi, via ponzio 34/5, 20133 milan, italy section: research paper keywords: optical fiber sensing; erbium; solid-state laser; amplitude noise; stabilization citation: alexey pniov, andrey zhirnov, dmitriy shelestov, konstantin stepanov, evgeniy nesterov, valery karassik, paolo laporta, gianluca galzerano, stefano taccheo, luigi piroddi, michele norgia, alessandro pesatori, cesare svelto, yb,er:glass microlaser at 1.5 µm for optical fibre sensing: development, characterization and noise reduction, acta imeko, vol. 5, no. 4, article 5, december 2016, identifier: imeko-acta-05 (2016)-04-05 section editor: lorenzo ciani, university of florence, italy received september 26, 2016; in final form november 4, 2016; published december 2016 copyright: © 2016 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: this work was supported by the international cooperation agreement between bmstu and polimi and by grant agreement № 14.577.21.0224 (unique identifier rfmefi57716x0224) corresponding author: cesare svelto, e-mail: cesare.svelto@polimi.it 1. introduction single-frequency low-noise laser sources operating around 1.5 µm wavelength [1]-[3] find several applications in optical fibre telecommunications, eye safe measurements, optical fibre sensors, remote monitoring and diagnostic over distributed measurement areas, precision measurements, etc.. the quality of analogue optical systems [4] is significantly affected by the instability of the laser source in terms of its amplitude and phase/frequency noise [5]. in particular, for coherent otdr (c-otdr) [6], [7], a very low phase noise [8] in the time interval relevant to the measurement is extremely important as well as low laser intensity noise [9], [10] in the measurement bandwidth. the aim of this work is to develop and characterize novel compact and single-frequency solid state lasers, operating in the spectral region from 1.53 µm to 1.56 µm, with very low noise characteristics and suitable for fibre measurements, in particular for c-otdr remote-sensing applications. within the framework of an international cooperation between bmstu and polimi, a fibre-pumped evolution of the yb,er:glass microchip laser [11] was recently developed and characterized, as discussed in the following. compared to previous research on the yb,er:glass microchip laser [12], this is the first time that the monolithic glass chip is end-pumped by perfectly circular, single-transverseand single-longitudinal-mode, 976 nm pump radiation from a fibre-coupled dfb pump laser diode. this abstract a fiber-pumped single-frequency microchip erbium laser was developed and characterized with the aim of using it in coherent optical time domain reflectometry (otdr) measurements and sensing. the laser is pumped by a fiber-coupled 976 nm laser diode and provides 8 mw tem00 single-frequency output power at 1.54 µm wavelength, suitable for efficient coupling to optical fibers. the amplitude and phase noise of this 200 thz oscillator were experimentally investigated and a relative intensity noise (rin) control loop was developed providing 27 db rin peak reduction at the relaxation oscillation frequency of 800 khz. acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 25 allows the best mode-matching with the erbium tem00 laser mode within the yb,er:glass microchip and less pump-induced amplitude and phase/frequency noise of the erbium laser. the laser intensity noise and wavelength stability were measured for the first time since not previously available for the microchip laser in the literature. also, active amplitude noise suppression for the microchip laser was never achieved before and now this monolithic 1.5 µm source reached a rin level at or below -100 db/hz at any fourier frequencies. section 2 describes the microlaser system and its properties. section 3 provides the laser basic characterization in terms of output power and single mode operation. in section 4 results of the experimental characterization regarding amplitude and frequency noise are presented. section 5 discloses recent achievements in active amplitude noise reduction by an optoelectronic control loop providing significant noise suppression at the laser relaxation oscillation frequency. in the conclusion, after summarizing the main results, future perspectives and next steps of this work are discussed. 2. the laser system the erbium microchip laser set-up is shown in figure 1. the laser active medium [11] is a phosphate glass, doped with er3+ ions at 1.5×1020 ions/cm3 and co-doped with yb3+ ions at 2×1021 ions/cm3, to provide for short-length pump absorption at 976 nm and allow efficient energy transfer to the lasing erbium ions. the laser emits at 1.5 µm wavelength in a quasi-three level laser scheme [13] and the active medium is cut as a thin-disk microchip with a few millimetres diameter and approximately 200 µm thickness. the monolithic microchip structure [12], obtained with multi-dielectric coating on the input and output surfaces of the chip, allows having an optical resonator coincident with the active medium itself. this short cavity length gives wide longitudinal mode separation and hence allows for single mode oscillation at the erbium peak emission wavelength. the two facets of the laser disk are multidielectric coated to provide for the required dichroic reflectivities at both pump and laser wavelengths. the mirror at the input facet, in fact, is coated for high transmission (tp>95 %) at the pump wavelength of 976 nm and for broadband ultra-high reflectivity (r1>99.9 %) at around 1.54 µm. the output mirror, instead, provides for a high reflectivity at the pump wavelength (rp>90 % at 976 nm) and laser output coupling of 2 % (r2=98 % broadband at 1.54 μm). laser material gain and coupling losses were calculated to support emission at the erbium peak emission wavelength so to reduce adjacent-mode competition. the pump source is a fibre coupled ingaas semiconductor laser coupled to a single mode optical fibre, with available output power up to 250 mw at the temperature tuned central wavelength of 976 nm. out of the fibre, the pump beam is passed through a simple telescopic system made of two lenses (see figure 1) to provide for a pump beam waist diameter of ∼50 μm within the active medium. in this way, efficient mode matching with the tem00 fundamental laser mode as well as double-pass pumping along the microchip thickness is achieved. such microlaser can demonstrate good slope efficiency (in the order of 10 %) and low pump threshold (∼30 mw to 50 mw). the proposed microlaser setup requires minimum number of adjustments and once produced is quite easy to repeat, promising for significant cost reduction in industrial manufacturing of such laser sources. considering the reduced number of laser components and their costs (pump ld, two lenses, and the microchip active medium), the final cost in a mass production will be limited by the fibre coupled pump diode, which however is an already well developed commercial product. the final goal is to achieve a complete microlaser cost in the order of 500 usd, for a single-frequency and low-noise solid-state compact laser operating at 1.5 μm wavelength. 3. microlaser characterization the output power from the erbium microlaser was measured using a calibrated optical power meter, with ingaas detector, commercially available from ophir (mod. vega). a silicon plate long-wave-pass filter was placed before the power meter, in order to remove the residual component of the pump radiation exiting the microlaser cavity from the output mirror (with transmission 10 % at 976 nm and residual pump power levels in the range of a few milliwatts). in this way a correct, or even underestimated, measurement of the erbium laser optical power was taken as a function of the input pump power. the measured points are shown in figure 2 and from these one can get a very low laser threshold pth=32.7 mw and a differential slope efficiency η=9.7 %. single transverse mode operation was verified by the transverse profile and divergence of the laser beam (resulting in a pure tem00 gaussian mode with quality factor m2<1.1). single longitudinal mode operation was observed on the optical spectrum of the laser radiation (see figure 3), showing a single peak at λ=1534.14 nm with two adjacent longitudinal modes at −60 db from the oscillating single mode power level. the longitudinal mode spacing of 3.2 nm is consistent with the microchip thickness of ∼240 µm. raising the incident pump power above 120 mw, additional longitudinal modes can start figure 2. measured input output laser behaviour with a linear regression of the data points above threshold showing the laser threshold and slope efficiency in single-mode operation. figure 1. set-up of fiber-pumped yb,er:glass microchip laser. 0 2 4 6 8 10 0 20 40 60 80 100 120 pump power (mw) la se r p o w er ( m w ) laser output λl = 1.54 µm pump laser diode λp = 976 nm lenses yb,er:glass microchip sm pump fiber mirrors pump laser diode λp = 976 nm lenses yb,er:glass microchip sm pump fiber mirrors acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 26 lasing with significant power and for now the maximum single mode output power is below 10 mw. the power level, however, is not a critical issue since a few milliwatts are more than enough for most opticalfibre applications. 4. amplitude and frequency noise measurements the amplitude noise (relative intensity noise, rin) of the pump diode is shown in figure 4. the measured noise level, relative to the average optical power, was at −75 db/hz from dc up to 100 khz with a roll off at −20 db/decade for higher fourier frequencies. some residual technical noise at the power supply frequency (50 hz and harmonics) is observable in the low-frequency part of the noise spectrum, but peak values are quite low and they could be eliminated by battery supplying the ld current driver. due to the high divergence of the pump beam at the fibre tip (na=0.22) which implies negligible back reflections from the lenses and due to the high transmission at pump wavelength of the first microchip facet, the effects of back reflections into the ld are small and can be made negligible by slightly tilting the flat input facet of the active medium. therefore, no optical isolation in the pump path is used, saving additional and costly components. the amplitude noise of the erbium microchip laser was characterized in terms of its rin in the spectral region from dc to 10 mhz (see figure 5), since this is the typical frequency band where fibre sensors and c-otdr systems perform their measurements. the relaxation oscillation peak occurs at a relatively high frequency, of about 2 mhz, due to the short laser cavity. the measured peak value is at −70 db/hz, relative to the carrier power, and outside of the spectral region of relaxation oscillations, rin is below −110 db/hz. form this amplitude noise figure, it is clear that to improve the amplitude stability of this microlaser it is primarily important to reduce the rin peak level, being this peak (40 db above the floor level) the main contribution to the laser amplitude noise. frequency and wavelength stability of the microlaser was characterized by a precision wavelength meter (ångstrom wavelength meter, model wsu2), recording wavelength deviations over time. figure 6 plots the laser wavelength values over a time interval of 10 minutes, with a sampling period of 110 ms. the average laser wavelength was λ=1534.8576 nm with a standard deviation σλ=0.06 pm over the observed 5 500 samples taken in the 10 minutes time interval. the relative wavelength or frequency stability was σλ/λ=σν/ν=5×10-8 i.e. 50 parts per billion. this result was achieved working in standard laboratory environment with no active temperature stabilization of the laser set up neither of the laboratory. on longer time records the shortand medium-term frequency stability remains the same whereas long-term frequency drifts become observable. form this frequency noise figure, it is clear that the monolithic microchip laser is inherently shortand medium-term frequency stable and only to improve its longterm frequency stability temperature control/stabilization becomes important. for c-otdr measurements, the interrogation time of the optical fibre is below a few seconds and the temperature effects are negligible, over such time intervals. therefore, no need for laser or ambient temperature stabilization is needed. 5. amplitude noise reduction considering the significant amplitude noise at around the relaxation oscillation frequency (see figure 5), an optoelectronic figure 3. measured optical spectrum of the erbium microchip laser. figure 5. rin spectral density for the yb-er:glass single-mode laser. figure 4. rin spectral density for the pump laser. figure 6. laser wavelength stability over 10 minutes. -100 -90 -80 -70 -60 -50 -40 -30 -20 1529 1531 1533 1535 1537 1539 wavelength (nm) p o w er d en si ty ( d b m /n m ) -130 -120 -110 -100 -90 -80 -70 1e+0 1e+1 1e+2 1e+3 1e+4 1e+5 1e+6 1e+7 frequency (hz) ri n (d b/ h z) -110 -100 -90 -80 -70 1e+1 1e+2 1e+3 1e+4 1e+5 1e+6 1e+7 frequency (hz) r in ( db /h z) 1 hz 201 hz 30 hz 3030 hz 200 hz 10.2 khz 9 khz 509 khz 400 khz 1.4 mhz 1534.8572 1534.8574 1534.8576 1534.8578 1534.8580 0 1 2 3 4 5 6 7 8 9 10 time (min) w av el en gt h ( n m ) acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 27 control loop was designed in order to reduce the rin peak. experiments of amplitude noise suppression in diode-pumped solid-state lasers in the near-infrared region were performed by different groups using nd:yag [14]-[16], nd:ylf [17], yb,er:glass [18]-[20], and ho,tm:yag [21] active media. in all cases a fraction of the laser output power is photodetected and converted into an electrical signal whose fluctuations are negatively fed back to the pump diode driver thus reducing the solid-state laser original amplitude fluctuation. one significant difference between our yb,er:glass microchip laser and other microlasers is the much shorter optical length of the resonator, being ∼300 µm for our microchip and some 10 mm to 30 mm in the other experiments. this shorter laser cavity length implies a rin peak located at higher fourier frequencies [22], where typically it is more difficult to achieve efficient amplitude noise suppression. in the experiments found in the literature, the rin peak frequency could range from 50 khz to 500 khz, whereas in our microchip laser it can be as high as 2 mhz at high pump power levels and operating the laser well above threshold condition. in order to simplify the stabilization loop it can be useful to reduce the rin peak frequency. the experimental setup for the amplitude-noise-reduction control loop, used for the first time with the erbium microchip laser, is depicted in figure 7. a small fraction of the nir laser radiation is detected with a fast (125 mhz bandwidth) and highgain (40 v/ma) commercial low-noise ingaas/pin photoreceiver (new-focus, mod. 1811-fs). the photodiode has an optical-to-electrical responsivity of ∼1 a/w at 1.54 µm wavelength and keeps linear operation up to 100 µw of input optical power incident on the 0.3 mm diameter semiconductor surface. therefore, the maximum input dc optical power is 0.1 mw corresponding to an output voltage of ∼4 v in linear operation. after the photoreceiver, the electrical signal is ac-amplified by a commercial low-noise amplifier (femto, mod. hva-10m-60b), with selectable gain of 40/60 db over a 10 mhz −3 db bandwidth. the amplifier output is fed to a custom capacitive band-pass filter in order to achieve adjustable correct phase margins at both 0 db crossing points of the control loop gain curve, on the left and on the right of the rin peak frequency. after the filter, a variable attenuator provides fine tuning of the overall loop gain in order to maximize the amplitude noise suppression without exciting spurious oscillations due to improper combination of loop gain and phase margin [23]. using the described optoelectronic control loop, a significant reduction of the laser amplitude noise was achieved, as shown in figure 8. the figure shows different traces recorded with an electrical spectrum analyser (esa) at the voltage amplifier output. the recorded power traces were normalized to the dc electrical power (dc voltage squared and divided by the esa 50 ω input impedance) and divided by the esa resolution bandwidth of 3 khz, providing the rin spectral densities in the figure. during these experiments, as compared to the ones of figure 5, the rin peak was located at a reduced ∼0.8 mhz as obtained by active medium tilting and some lowering of the optical pump power level. by changing the electronic loop gain, the rin peak value could be lowered from the original level of −70 db/hz down to approximately −100 db/hz level, with a rin maximum suppression of 27 db. intermediate rin levels were achieved, as shown in figure 8, by using different values of the loop gain. as mentioned, the rin peak of figure 8 is located at lower fourier frequency then the one shown in figure 5, namely at about 0.8 mhz instead of 2 mhz. this happens because in the stabilization experiments, performed a few months after the laser characterization experiments, the laser gain and pump power level abovethreshold were both reduced, thus lowering the relaxation oscillation frequency (frequency of the rin peak). in fact, the microchip was slightly tilted with respect to the incident pump power direction to reduce backreflections into the pump diode. in addition, the pump power level was reduced to ensure stable single longitudinal and transverse oscillation of the erbium laser. both of these actions reduce the rin peak frequency providing for a simpler (at lower frequencies) control loop and a reduced amplitude noise at frequencies in the range from 1 to 5 mhz (since the rin peak happens before) where the c-otdr detection is working. 6. conclusions a novel fiber-pumped single-mode yb,er:glass microchip laser was developed and characterized. single-frequency output power up to 8 mw was observed with a laser slope efficiency of ∼10 % and a laser threshold as low as 30 mw. the free-running laser amplitude noise was observed being dominated by the relaxation oscillation phenomenon resulting in a rin peak of −70 db/hz at around 0.8 mhz. by an optoelectronic control loop acting on the pump ld the rin peak was successfully reduced by 27 db and down to the constant laser amplitude noise level at approximately −100 db/hz in the 0.6÷1 mhz spectral region. the developed low-noise 1.5-µm microlaser showed great potentials for precision, eye-safe or fibre-coupled, optical measurements. stable nir radiation is in fact needed for a number of optical monitoring techniques also to assess reliability of power plants and large-size structures. in figure 8. rin spectral densities (higher traces) at different control loop gains. lowest trace is the electronic noise floor of the measurement system. figure 7. experimental block diagram of the control loop for the microchip laser intensity noise suppression. -130 -120 -110 -100 -90 -80 -70 0 200 400 600 800 1000 frequency (khz) ri n (d b/ h z) acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 28 particular, this laser was designed for use in c-otdr systems aimed at remote optical fibre sensing applications on large-area/length systems such as pipelines, plants, dams, security perimeters, etc.. further optimization of the laser source is in progress and its use in practical phase-sensitive otdr measurements [24] will be the future activity. acknowledgement the authors are grateful to the rector politecnico di milano  namely professor giovanni azzone  and to the rector of the bauman moscow state technical university  namely professor anatoly alexandrovich aleksandrov  for encouraging and continuously supporting this fruitful scientific cooperation. specific gratitude regarding the experimental activity goes to different students and researchers of the bauman centre for photonics and infrared technology, who helped in the laboratory over many phases of the experiments. in particular, we would like to thank alina borisova and anton khaperskiy for their useful assistance during the optical measurements and collection of the experimental data. this work was also economically supported under the grant agreement № 14.577.21.0224 (unique identifier rfmefi57716x0224) with rf education and science ministry. references [1] k.t.v. grattan, t. sun, “fiber optic sensor technology: an overview”, sensors and actuators a: physical 82 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[2] t.l. boyd, d. klemer, p.a. leilabady, j. noriega, m. pessot, “a 1.55-um solid-state laser source for dwdm applications”, j. lightwave technol. 17 (1999) pp. 1904-1908. [3] s. taccheo, p. laporta, c. svelto, “widely tunable singlefrequency erbium–ytterbium phosphate glass laser”, appl. phys. lett. 68 (1996) pp. 2621-2623. [4] y. koyamada, m. imahama, k. kubota, k. hogari, “fiber-optic distributed strain and temperature sensing with very high measurand resolution over long range using coherent otdr”, j. lightw. technol. 27 (2009) pp. 1142-1146. [5] r. paschotta, h. r. telle, u. keller, “noise of solid state lasers”, solid-state lasers and applications (ed. a. sennaroglu), crc press, boca raton fl, chapter 12 (2007) pp. 473–510. [6] y. horiuchi, k. mochizuki, h. wakabayashi, “novel coherent heterodyne optical time domain reflectometry for fault localization of optical amplifier submarine cable system”, ieee photon technol. lett. 2 (1990) pp. 291-293. [7] m. imahama, y. koyamada, k. hogari, “restorability of rayleigh backscatter traces measured by coherent otdr with precisely frequency-controlled light source”, ieice trans. commun. e91.b-4 (2008) pp. 1243-1246. [8] s. camatel, v. ferrero, “narrow linewidth cw laser phase noise characterization methods for coherent transmission system applications” j. lightw. technol. 26 (2008) pp. 3048-3055. [9] d.e. mccumber, “intensity fluctuations in the output of cw laser oscillators. i”, phys. rev. 141 (1966) pp. 306–322. [10] j. rollins, d. ottaway, m. zucker, r. weiss, r. abbott, “solidstate laser intensity stabilization at the 10-8 level”, opt. lett. 29 (2004) pp. 1876-1878. [11] s. taccheo, p. laporta, s. longhi, o. svelto, c. svelto, “diode pumped bulk erbium ytterbium lasers”, appl. phys. b 63 (1996) pp. 425-436. [12] p. laporta, s. taccheo, s. longhi, o. svelto, c. svelto, “erbium ytterbium microlasers: optical properties and lasing characteristics”, opt. materials 11 (1999) pp. 425-436. [13] p. laporta, s. taccheo, s. longhi, o. svelto, g. sacchi, “diodepumped microchip er-yb:glass laser”, opt. lett. 18 (1993) pp. 1232-1234. [14] c.c. harb, m.b. gray, h-a. bachor, r. schilling, p. rottengatter, i. freitag, h. welling, “suppression of the intensity noise in a diode-pumped neodymium: yag nonplanar ring laser”, ieee journ. quantum electron 30 (1994) pp. 2907-2913. [15] c.c. harb, t.c. ralph, e.h. huntington, d.e. mcclelland, h-a. bachor, i. freitag, “intensity-noise dependence of nd:yag lasers on their diode-laser pump source”, j. opt. soc. am. b 11 (1997) pp. 2936-2945. [16] b.c. buchler, e.h. huntington, c.c. harb, t.c. ralph, “feedback control of laser intensity noise”, phys. rev. a 57 (1998) pp. 1286-1294. [17] j. zhang, h. ma, c. xie, k. peng, “suppression of intensity noise of a laser-diode-pumped single-frequency nd:yvo4 laser by optoelectronic control”, appl. opt. 42 (2003) pp. 1068-1074. [18] s. taccheo, g. de geronimo, p. laporta, o. svelto, “intensity noise reduction in a single-frequency ytterbium-codoped erbium laser” optics lett. 21 (1996), pp. 1747-1749. [19] g. de geronimo, s. taccheo, p. laporta, “optoelectronic feedback control for intensity noise suppression in a codoped erbium-ytterbium glass laser”, electron. lett. 33 (1997) pp. 13361337. [20] s. taccheo, p. laporta, o. svelto, g. de geronimo, “theoretical and experimental analysis of intensity noise in a codoped erbium–ytterbium glass laser”, appl. phys. b: lasers and optics 66 (1998) pp. 19-26. [21] c. svelto, s. taccheo, m. marano, g. sorbello, p. laporta, “optoelectronic feedback loop for relaxation oscillation intensity noise suppression in tm-ho:yag laser”, electron lett. 36 (2000) pp. 1623-1624. [22] o. svelto, principles of lasers, springer, 5th ed., 2010, isbn 978-1-4419-1301-2, pp. 313-317. [23] k.j. aström, r.m. murray, feedback systems, princeton university press, 2008, isbn 978-0-691-13576-2. [24] a.b. pniov, a.a. zhirnov, k.v. stephanov, e.t. nesterov, d.a. shelestov, v.e. karasik, “mathematical analysis of marine pipeline leakage monitoring system based on coherent otdr with improved sensor length and sampling frequency”, j. phys.: conf. ser. 584 (2015) pp. 425 436. yb,er:glass microlaser at 1.5 µm for optical fibre sensing: development, characterization and noise reduction matrix reference materials development for food safety application in philippine products acta imeko issn: 2221-870x september 2020, volume 9, number 3, 75 78 acta imeko | www.imeko.org september 2020 | volume 9 | number 3 | 75 matrix reference materials development for food safety applications in philippine products benilda ebarvia1, sharlene cabanilla1, aaron dacuya1, alleni tongson1, cyril cortez1, kim christopher aganda1, natividad mamplata1 1 industrial technology development institute, department of science and technology (itdi-dost), taguig city, philippines section: research paper keywords: reference material; food contaminant; histamine; benzoic acid; food safety citation: benilda ebarvia, sharlene cabanilla, aaron dacuya, alleni tongson, cyril cortez, kim christopher aganda, natividad mamplata, matrix reference materials development for food safety application in philippine products, acta imeko, vol. 9, no. 3, article 12, september 2020, identifier: imeko-acta09 (2020)-03-12 section editor: claudia zoani, enea, italy received december 29, 2017; in final form july 15, 2020; published september 2020 funding: this work was supported by department of science and technology (dost) and the philippine council for industry, energy and emerging technology for research and development (pcieerd), philippines corresponding author: benilda s. ebarvia, e-mail: bsebarvia@yahoo.com 1. introduction the need to ensure comparable and traceable chemical measurements is important for a country engaging in global trade. this need requires the establishment of a national chemical measurement organisation tasked with improving the capability for accurate measurements throughout the country. its sub-tasks could include the provision of certified reference materials (crms) and general reference materials (rms), the conduct of proficiency testing (pt) schemes and the accreditation for iso/iec 17025. the organisation could guarantee the quality of chemical measurements performed within the country and increase the confidence in such measurements abroad. as part of a vision to establish such an organisational infrastructure for chemical metrology in the philippines, the industrial technology development institute (itdi) – department of science and technology (dost) pioneered the development of rms to support the needs of chemical testing laboratories in the country. food safety issues were prioritised in this activity since food chemicals could affect the health of consumers and the level of some substances in food has been a reason for detention of some philippine export products abroad. this study focused on the development of rms for measuring histamine in canned tuna and benzoic acid in mango juice. these two chemicals are vital criteria for export food products as other countries have based their import approval decisions on the levels of these chemicals. further, the monitoring of these two chemicals are essential for quality control and food safety in general. crms for benzoic acid have been developed for orange juice [1], soy sauce [2] and quasi-drug drinks [3]; however, currently, there are no philippine rms or pt schemes available for benzoic acid in mango juice, nor are there rms or pt schemes for histamine levels in canned tuna available. crms for histamine in fish products have not been reported. included in this study are homogeneity and stability studies, rm characterisation, and the process for conducting pt schemes for these substances. abstract the philippine industrial technology development institute (itdi) – department of science and technology pioneered the development of reference materials (rms) as part of its vision to establish metrological traceability and to support the rm needs of chemical testing laboratories in the country. food safety of philippine products were prioritised in this activity as these could affect the health of consumers as well as trade issues. the focus of this study was on contaminants like histamine and benzoic acid in canned tuna and mango juice, respectively. the materials were prepared and characterised, and they were found to be homogeneous and stable for at least six months. higher order methods like the isotope dilution technique, coupled with liquid chromatography–triple quadrupole mass spectrometry, were used to assign reference values to the rms, and the results were cross-checked using gravimetric techniques for high-performance liquid chromatography analysis. certified rms purchased from metrology institutes were also used to further support traceability of measurements. these steps resulted in rms for histamine and benzoic acid in philippine products, and itdi disseminated the measurement traceability to local laboratories through proficiency testing schemes organised and conducted for the analytes. mailto:bsebarvia@yahoo.com acta imeko | www.imeko.org september 2020 | volume 9 | number 3 | 76 2. experimental 2.1. chemicals and materials the calibration standards for benzoic acid were srm 350b benzoic acid, 99.9994% (national institute of standards and technology (nist, usa) and hrm-1002a benzoic acid (health science authority (hsa), singapore). the crms for benzoic acid were hrm-1001a benzoic acid in orange juice (hsa) and isotopically labelled analogue benzoic acid ring-d5 (cambridge isotope laboratory, cil). mass spectrometer (ms)grade solvents and high-performance liquid chromatography (hplc)-grade solvents were used. for histamine, the calibration standards were histamine standard (sigma aldrich, singapore) and histamine dihydrochloride (national institute of metrology china). isotopically labelled analogue histamine dihydrochlorided4 (cil) was used as an internal standard. brij-35, mercaptoethanol, o-phthaldialdehyde (opa), and 1-octanesulfonic acid sodium salt used in the hplc analysis were all obtained from sigma aldrich. 2.2. measurement of benzoic acid and histamine by hplc for benzoic acid analysis, the hplc (shimadzu prominence lc-20a with uv vis detector, set at 234 nm) with waters c18 sunfire (3.5 μm, ø 4.6 mm × 150 mm) column was used following a validated, in-house-developed method [4]. for histamine analysis, hplc with waters c18 sunfire column and fluorescence detector (340 nm excitation and 445 nm emission) was utilised after post-column derivatisation with opa [5]. 2.3. preparation of reference material the rm for benzoic acid in mango juice was prepared using locally made commercial mango juice drinks, all manufactured on the same date. the juice was mixed, centrifuged, filtered, spiked with benzoic acid and pasteurised. the hot juice was poured into 100-ml amber bottles, which were then sealed, labelled, shrink wrapped and stored at 4 °c. the rm for histamine was prepared from canned tuna which was homogenised until a paste-like consistency was obtained. the tuna sample was spiked with a set amount of histamine and it was thoroughly mixed. the spiked tuna sample (80 mg) was packed into a 100-ml amber bottle with screw cap. the packed samples were properly labelled and sterilised at 115.6 ºc for about 3 hours and stored at 4 °c after cooling. 2.4. assessment of homogeneity and stability the assessment of homogeneity was carried out after 10 bottles from the batch were selected by stratified random sampling. two subsamples were taken from each bottle for measurement. statistical evaluation was based on the iupac (international union of pure and applied chemistry) harmonised protocol [6], and the stability of rms was also considered. a short-term stability test using isochronous design was carried out for three weeks at three temperatures: 4 °c, 30 °c and a simulated transport temperature of about 40 °c. two randomly selected bottles were transferred from the control temperature to each temperature setting. after the study period, all bottles were collected and analysed under repeatability conditions. for long term stability, three bottles were analysed at designated intervals: one, three and six months. the statistical evaluation was based on iso guide 35 [7]. 2.5. value assignment of reference material liquid chromatography-isotope dilution mass spectrometry (lc-idms) with gravimetric sample preparation was used for value assignment of the prepared rms. the mass fraction of the measurand was calculated based on the equation below [8]: 𝐶 = 𝑚is−sp,sample ∙ 𝑚std ∙ 𝐴𝑅sample ∙ 𝐶stock,std 𝑚sample ∙ 𝐴𝑅std ∙ 𝑚is−sp,std (1) where c is the concentration of the analyte in the sample (mg/kg), mis-sp, sample is the mass of isotopically labelled analyte solution spiked into the sample (g), mstd is the mass of the analyte standard solution added into the calibration blend solution (g), arsample is the observed response ratio of the of the analyte/isotope for the sample solution, cstock, std is the concentration of the analyte standard solution (mg/kg), msample is the mass of sample taken for analysis (g), arstd is the observed response ratio of the of the analyte/isotope in the calibration blend solution and mis-sp, std is the mass of isotopically labelled analyte solution added into the calibration blend solution (g). liquid chromatography–triple quadrupole mass spectrometry (agilent 1290 lc system) coupled to a 6460 lc/ms triple quadrupole (agilent technologies, inc.) with agilent zorbax eclipse plus c18 column (3.5 µm, ø 4.6 mm × 100 mm) were employed for the benzoic acid. the sample was diluted with a mobile phase solution after the addition of the benzoic acid ringd5 solution. for histamine, the separation was achieved by using the agilent poroshell 120 hilic column (2.7 µm, ø 2.1 mm × 100 mm). sample extraction was based on agilent application note [9]. histamine dihydrochloride-d4 was used as the internal standard. 3. results and discussion 3.1. method validation the hplc and lc-idms methods for benzoic acid and histamine with gravimetric sample preparation were validated, and the results are tabulated in table 1 and table 2, respectively. the use of crm hrm-1001a benzoic acid in orange juice for benzoic acid analysis established the metrological traceability to the international system of units (si). the results are within the criteria set by aoac for repeatability, trueness (% recovery) and linearity for the concentration range of 5-100 mg/kg [10]. the lod and loq obtained were lower than the ranges of sample. table 1. summary of validation results obtained by hplc method for the matrix rms. parameter benzoic acid (n = 10) histamine (n = 10) repeatability, % rsd 1.08 2.1 trueness, % recovery 99.3 101.6 linearity, r 0.9999 0.9997 lod, mg/kg 0.0161 0.0537 loq, mg/kg 0.0537 0.0503 table 2. summary of validation results obtained by lc-idms. parameter benzoic acid (n = 10) histamine (n = 10) repeatability, % rsd 1.21 1.35 accuracy, % recovery 100.5 104.7 linearity, r 0.997 0.997 lod, mg/kg 0.0292 0.0014 loq, mg/kg 0.0974 0.0046 acta imeko | www.imeko.org september 2020 | volume 9 | number 3 | 77 3.2. homogeneity and stability the rms prepared was found to be sufficiently homogeneous for benzoic acid and histamine studies. the results of the statistical evaluation for the homogeneity test of the benzoic acid and histamine rms are shown in table 3 and table 4, respectively. trend analyses for both rms used for shortterm stability evaluation confirms that no significant instability of the materials were observed during transport at 4 °c, 30 °c and 40 °c for a period of three weeks. the long-term storage of the rms at 4 °c was determined to be about 6 months for benzoic acid and 12 months for histamine. figure 1 shows the plot of the long-term stability test for histamine in canned tuna. 3.3. reference value assignment the reference values assigned for benzoic acid in mango juice and histamine in canned tuna listed in table 5 are the mean values of duplicate measurements from the three bottles from the batch, analysed by the lc-idms method. as part of quality assurance, the acceptable percentage recovery of the crm hrm-1001a and control materials was achieved for benzoic acid and histamine, respectively. for benzoic acid, ms was operated in electrospray ionisation in negative mode. benzoic acid and its isotope analogue were detected by selectively monitoring the collision-induced dissociation channels of [m-h]→ [m-co2h]at m/z 121.1 → 77.2 and m/z 126.1 → 82.1, respectively. for histamine, ms was operated in electrospray ionisation in positive mode. histamine and its isotope analogue were detected by selectively monitoring the collision-induced dissociation channels of [m+h]+ → [m+h-nh3]+ → [m+h-ch2ch2nh2]+ at m/z 112.1 → 95.1 → 68.2 and m/z 116.1 → 99.1.→ 85.2, respectively. 3.4. measurement of uncertainty the expanded uncertainties of the reference values of the rms produced presented in table 5 were estimated based on the iso gum approach [11]. the highest contribution to the expanded uncertainty of the reference values of rm benzoic acid (94.6%) was obtained from the peak area ratio of sample-isotope blend, while homogeneity gave the highest contribution (97.67%) for rm histamine in canned tuna. the remainder of the uncertainty sources included masses of samples, standards and solutions, and purity of the calibration standard. 3.5. participation in international supplementary comparison in 2014, in support of the reference value assignment capability, the standards and testing division (std) of itdi the designated institute for metrology in chemistry in the country participated in a supplementary comparison, apmp.qm-s8: determination of mass fraction of benzoic acid, methyl paraben and n-butyl paraben in soy sauce [2]. this was coordinated by the health sciences authority of singapore under the auspices of the organic analysis working group (oawg) of the comité consultatif pour la quantité de matière (ccqm). in this comparison, itdi-std demonstrated its competence for benzoic acid analysis in soy sauce. 3.6. conduct of proficiency test (pt) scheme the rms developed in this study were also used for accuracybased pt schemes, organised to assess the performance of local testing laboratories based on iso/iec 17043 [12]. the reference value obtained by a candidate higher order method was used as the assigned value for the pt scheme. the z-score was based on the reference values of the rms. government and private testing laboratories, research institutes and the academe participated in the pt schemes by using routine methods of analysis. in the pt round for benzoic acid in mango juice (micpt-14-01), most of the participants used hplc, as shown in figure 2. nine (82 %) out of 11 laboratories obtained satisfactory performance, and two (18 %) obtained unsatisfactory results. the unsatisfactory results were observed to be due to computation error and overestimation of results due to matrix effects. furthermore, two different analytical techniques based on fluorometry and elisa were used by twelve participants for the pt round for histamine (micpt-14-04). however, only 50 % satisfactory performance was achieved. results for this pt round figure 1. long-term stability for histamine in canned tuna. table 3. summary of statistical analysis for homogeneity of benzoic acid in mango juice. criteria critical value obtained result remarks cochran’s test, c 0.602 0.403 passed test for analytical precision of the method < 0.5 0.125 passed test for sufficient homogeneity 417.75 0.437 passed test for adequate homogeneity 14.25 0.661 passed table 4. summary of statistical analysis for homogeneity of histamine in canned tuna. criteria critical value obtained result remarks cochran’s test, c 0.602 0.356 passed test for analytical precision of the method < 0.5 0.473 passed test for sufficient homogeneity 42.451 33.395 passed test for adequate homogeneity 0.602 0.356 passed table 5. assigned value for the developed reference material. reference material mass fraction (mg/kg) n assigned value uncertainty (k = 2) benzoic acid in mango juice 774 42 6 histamine in canned tuna 148 14 6 acta imeko | www.imeko.org september 2020 | volume 9 | number 3 | 78 are displayed in figure 3, with reference to the sample preparation used prior to fluorometric detection. under estimation of results were observed from four laboratories using the aoac 977.13 method. the relative standard deviation of the participating laboratories with satisfactory performance is approximately 9%. different parameters for the extraction techniques and the need for validation of the method were identified problems in this pt scheme. 4. conclusions in this work, the capability to developed rms for benzoic acid in mango juice and histamine in canned tuna, the provision of reference value to rms using a candidate higher order method (lc-idms), and the conduct of pt schemes were demonstrated in the country through itdi-std. the prepared rms were homogeneous and stable and were made available for the first time to local chemical testing laboratories. through the pt exercises conducted using these rms, an assessment for better performance by local laboratories for benzoic acid than for histamine analysis was observed. likewise, this served as a means for our institute to disseminate the traceability of measurement to si standards. the continuous development and use of rms, pt schemes and method validation by the institute, along with local laboratories participating in pt schemes, are regarded as essential to further improving laboratory performance in the country and facilitating fair trade in food globally. acknowledgement funding support for dost and the philippine council for industry, energy and emerging technology for research and development (pcieerd) is gratefully acknowledged. the authors are also thankful for the histamine standards provided by the national institute for metrology, china. references [1] l. j. de carvalhjo, e. c. pires do rego, b. c. garrido, quantification of benzoic acid in beverages: the evaluation and validation of direct measurement techniques using mass spectrometry, anal. methods 2016, 8, 2955-2960. [2] t. l. teo, t. lu, e. m. gui, p. s. cheow, p. giannikopoulou, e. kakoulides, e. lampi, s. choi, y. yip, p. chan, s. hui, w. wollinger, l. carvalho, b. garrido, e. c. p. rego, s. ahn, b. kim, x. li, z. guo, d. styarini, y. aristiawan, d. p. ramadhaningtyas, n. aryana, b. s. ebarvia, a. dacuaya, a. tongson, k. chr. aganda, th. j. fortune, p. tangtrirat, th. mungmeechai, a. c. gören, s. gündüz, h. yilmaz, apmp.qm-s8: determination of mass fraction of benzoic acid, methyl paraben and n-butyl paraben in soy sauce, metrologia 54(1a):08028-08028. doi: https://doi.org/10.1088/0026-1394/54/1a/08028 [3] i. lee, s. ahn, b. kim, e. hwang, y. s. kim, analysis of benzoic in quasi-drug drink using isotope dilution liquid chromatography mass spectrometry, bull. korean chem. soc., 29 (2008), pp. 21252128. [4] organic chemistry section std-itdi (tm-ocs-202) unpublished in-house technical procedure for benzoic acid in fruit juice using hplc with gravimetric sample preparation, 2015. [5] f. b. custodio, e. tavares, m. b. a. gloria, extraction of bioactive amines from grated parmesan cheese using acid, alkaline and organic solvents, journal of food composition and analysis, 20 (2007), pp. 280-288. [6] m. thompson, s. r. ellison, r. wood, the international harmonized protocol for the proficiency testing of analytical chemistry laboratories, pure appl. chem., vol. 78. no.1, pp. 145-196, 2006. doi: https://doi.org/10.1351/pac200678010145 [7] iso guide 35:2006 reference materials – general and statistical principles for certification, (2006). [8] h. shin, b. kim, j. lee, e. hwang, development of an isotopedilution liquid chromatography/mass spectrometric method for the accurate determination of acetaminophen in tablets, bull. korean chem. soc. 2010, vol. 31, no. 12. doi: https://doi.org/10.5012/bkcs.2010.31.12.3663 [9] n. byrd, quick, easy and reliable detection of histamine in food using the agilent 6490 triple quadrupole lc/ms with jet stream technology, agilent application note. 5991-1286en, 2013. [10] official methods of analysis of aoac international. aoac international, gaithersburg, md, usa, 2012. 19th ed., vol. 2. [11] joint committee guides in metrology, jcgm 100:2008, evaluation of measurement data – guide to expression of uncertainty in measurements (iso gum). 1st ed. [12] iso/iec 17043: 2010 conformity assessment – general requirements for proficiency testing, international standards organisation, switzerland. • figure 2. participant results for micpt-14-01: benzoic acid in mango juice using capillary electrophoresis and high-performance liquid chromatography with uv detector. figure 3. participant results for micpt-14-04: histamine in canned tuna using fluorometer detection with different sample preparation. https://doi.org/10.1088/0026-1394/54/1a/08028 https://doi.org/10.1351/pac200678010145 https://doi.org/10.5012/bkcs.2010.31.12.3663 template for an acta imeko event paper acta imeko issn: 2221-870x april 2017, volume 6, number 1, 43-49 acta imeko | www.imeko.org april 2017 | volume 6 | number 1 | 43 an overview of commercially available teslameters for applications in modern science and industry dragana popovic renella, sasa spasic, sasa dimitrijevic, marjan blagojevic, radivoje s popovic senis ag, neuhofstrasse 5a, 6340 baar, switzerland section: research paper keywords: teslameter; gaussmeter; magnetic measurement; hall probe citation: dragana popovic renella, sasa spasic, sasa dimitrijevic, marjan blagojevic, radivoje s popovic, an overview of commercially available teslameters for applications in modern science and industry, acta imeko, vol. 6, no. 1, article 7, april 2017, identifier: imeko-acta-06 (2017)-01-07 section editor: paul regtien, the netherlands received january 18, 2016; in final form march 2, 2017; published april 2017 copyright: © 2017 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: dragana popovic renella, e-mail: info@senis.ch 1. introduction a teslameter, also called gaussmeter, is an instrument for measuring magnetic flux density. usually, a teslameter consists of a magnetic field probe and an electronic module. the probe converts the measured magnetic field into an electrical signal. most teslameter probes used in modern science and industry are based on the hall-effect [1] the probe contains a halleffect device [2]. the electronic module supports the operation of the probe, processes the signal of the probe, and provides the interface with the user. modern teslameters are digital, meaning that the signal processing includes analogue-to-digital conversion and digital signal conditioning, and the user interface includes a digital display. a digital teslameter with a magnetic field probe based on a hall-effect device is often called − in short – a hall teslameter. hall teslameters are used for measuring magnetic flux densities in the magnetic field range from a few µt to about 30 t, a frequency range from dc to several tens of khz, and a temperature range from a few degrees k to about 200 °c (although most of commercially available teslameters are intended for a much narrower temperature range around room temperature). the best published characteristics of modern hall teslameters include a magnetic resolution as high as 0.1 ppm and a magnetic field measurement accuracy of up to 50 ppm of the measurement range. however, the measurement accuracy of a teslameter is usually strongly deteriorated by temperature variations, at ac measurement conditions, and by nonhomogeneity of the measured magnetic fields. accurate abstract the hall-effect based teslameters (also called gaussmeters) are the mostly applied instruments for measuring dc and ac magnetic flux densities in modern science and industry. this paper gives an overview of commercially available teslameters at the high-end performance level. the teslameters have been evaluated by following characteristics that are published by suppliers: probe dimensions, magnetic field sensitive volume, accuracy, magnetic resolution, measurement range, frequency bandwidth, temperature coefficient sensitivity, and price/performance ratio. the teslameter that best matches the measurement needs in various application fields incorporates a 3-axis integrated hall probe, analog electronics based on the spinning-current technique, an analog-to-digital converter, an embedded computer, and a touchscreen display. the 3-axis hall probe is a single silicon chip integrating both horizontal and vertical hall magnetic sensors and a temperature sensor. the spinning-current eliminates most of the hall probe offset, low-frequency noise, and the planar hall voltage. the errors due to the hall sensor non-linearity and the variations in the probe and electronics temperatures are eliminated by a calibration procedure. the errors due to the angular imperfections of the hall probe are eliminated by a calibration of the sensitivity tensor of the probe. this teslameter can measure magnetic field vectors from about 100 nt to 30 t, with a spatial resolution of 100 µm, magnetic resolution ±2 ppm of the range, accuracy 0.002 % of the range, a temperature coefficient less than 5 ppm/°c, and angular errors less than 0.1°. acta imeko | www.imeko.org april 2017 | volume 6 | number 1 | 44 measurement of inhomogeneous magnetic fields is steadily getting more important and challenging because of the long trend of miniaturization of technical systems. for example, this is so in the field of mapping of small magnets used in conjunction with magnetic sensors [3]–[5], or in the measurement of the magnetic field of the undulators of electron light sources [6]. in chapter 2 we briefly introduce some teslameter applications in modern science and industry in order to demonstrate the importance of certain characteristics of the teslameters. the measurement accuracy of nonhomogeneous magnetic fields is additionally deteriorated if the following conditions are not sufficiently met [7], [8]: small sensitive volume of the probe, which allows for high spatial resolution of the magnetic measurement; measurement of all three components of a magnetic field simultaneously at the same spot; small overall dimensions of the probe, so that it can be inserted into a small space; accurate spatial positioning of the probe with respect to a coordinate system; accurate angular positioning of the probe with respect to a coordinate system; parallelism of the sensitivity vector(s) of the probe with the axes of the coordinate system; no planar hall effect. each magnetic measurement task requires a specific combination of the characteristics of a teslameter. unfortunately, it is often not easy to select the most appropriate teslameter for a given application. the difficulties stem from the fact that vendors sometimes apply different terminology for the same characteristics, do not publish precise conditions of the validity of the quoted data, or simply neglect to publish some important characteristics. in spite of these difficulties, in this paper, we will try to compare the published characteristics of the commercially available teslameters. the global teslameter market is currently served by about 50 vendors with more than 100 types of teslameters [9]. we will concentrate in this study on the highend segment of the market, with the highest-performance teslameters for a magnetic measurement range from about 1 μt to 30 t at about room temperature. since the high-performance teslameters are commonly available as desktop versions only, most of hand-held teslameters were not evaluated in the study. out of the reviewed tesalemeters, 9 models are functioning with 3-axis hall probes, and four of them incorporate the integrated hall probes on a single chip. the other 3-axis hall probes are hybrid probes that consist of 3 discrete hall sensors positioned in the way that each of them measures one component of the magnetic field. the comparison of the teslameters is organized in the following way: in chapter 3 we first compile and compare the best published values of the most important characteristics; then, based on these data, in chapter 4 we try to identify the teslameter type with the best combination of the characteristics, which best meets the needs for accurate magnetic measurements in modern science and industry, as introduced in chapter 2. 2. teslameter applications in modern science and industry in order to better understand why some characteristics of the teslameters are important, we briefly present in this chapter some applications in modern science and industry and their requirements for high performance teslameters. 2.1. teslameter application in superconductive magnets one application in industry is the use of teslameters for the fast measuring of magnetic fields during the on-site calibration process of superconductive magnets. figure 1 shows a senis low noise & high resolution teslameter that is used for high accuracy measurements of magnetic fields up to 20 t in superconductive magnets of bruker biospin ag [10]. for this application, the characteristics of the teslameter must include very high dc magnetic resolution (low-frequency noise), high stability (low temperature coefficient of offset and sensitivity < 5 ppm/°c)), high measurement accuracy better than 0.003 % and high magnetic field range. magnetic resolution and stability should be correlated. any improvement in one of these parameters should be followed by an improvement in the other one. 2.2. teslameter application in undulator systems an important teslameter application in modern science is the measurement of the magnetic field in undulators of the electron light sources such as swissfel at the paul scherrer institut [10]. in addition to the standard requirements, such as high accuracy and resolution and high stability, some additional features for good teslameters are required, and particularly for their hall probes. these are twoor three-axis probes, small overall dimensions of the probe (10×10×1.4 mm3 (3-axis)), as shown in figure 2, a flat probe that is positioned in the gap of the undulator, small and compact magnetic field sensitive figure 1. low noise & high resolution teslameter used during the calibration process of superconductive magnets [10]. figure 2. flat senis 3-axis hall probe used for magnetic measurements in electron light sources at psi [10]. acta imeko | www.imeko.org april 2017 | volume 6 | number 1 | 45 volume of the hall probe and no planar hall effect. the similar application of the magnetic field mapping utilizing the high performance teslameters can be found in other national research laboratories throughout the world in the field of accelerators, light sources, synchrotron radiations, etc. 2.3. teslameter application for magnetic field mapping around large coils still another modern application requires the magnetic measurement around very large coils for current centerline determination of the superconductors, as for the iter project, see figure 3. here it is important to achieve an accurate positioning of the probe with respect to a coordinate system of the measured coil (0.1 mm accuracy), measuring all three components of a magnetic field simultaneously at the same spot and small angular error of the sensitivity vectors of the probe (0.1°). 2.4. teslameter application for magnetic field mapping around small magnets modern magnetic position and angle sensors consist of a combination of a permanent magnet and a magnetic field sensor, and they are widely used in a broad range of applications involving precision motion and control in automotive industry and sensing in industrial and consumer products. the characteristics of such magnets that are parts of positioning and angle magnetic sensors must be carefully controlled. this is usually done with a magnetic field mapper that combines a precise motion control unit and a high precision teslameter. the senis magnetic field mapper measures all three components of the magnetic field at precisely defined points around permanent magnets (from 1 m to 50 cm) with the magnetic field measurement range from 100 μt to 3t. among others, bomatec ag, figure 4, utilizes the senis magnetic field mapping system for quality control of permanent magnets [10]. the university of buffalo [11] performs model validation and material/device characterization using the senis magnetic field mapper, which enables the measurement of 3d magnetic field distribution for arbitrary material/component geometries with 10 µm spatial resolution, see figure 5. 3. comparison of teslameters characteristics in each of the following tables we list three to five teslameters with the following best characteristics: 1) probe dimensions, size of the magnetic field sensitive volume, and probe angular errors; 2) magnetic resolution; 3) magnetic field measurement accuracy; 4) measurement range; 5) frequency bandwidth and 6) temperature coefficient of sensitivity. 3.1. probe dimensions, magnetic field sensitive volume and probe angular error for measuring non-homogenous magnetic fields the following characteristics are crucial: small overall dimension of the probe, measuring all three components of the magnetic field at the same spot (i.e. small magnetic field sensitive volume (mfsv) of the probe); and small angular errors of the probe (orthogonality error of the hall devices within the probe). in table 1 the three teslameters that best meet the required needs are presented. many vendors of teslameters do not quote exact dimensions of the magnetic sensitive volume nor the angular error of their hall probes. the senis integrated 3-axis hall probe is described in [16]. 3.2. comparison of the magnetic field measurement accuracy the magnetic field measurement accuracy of the teslameter is defined as the maximum difference between the actual measured magnetic flux density and that given by the teslameter. usually, the accuracy is expressed in percentage of the measurement range. in table 2 we list five teslameters with the best magnetic field measurement accuracy. 3.3. comparison of the magnetic resolution resolution of the teslameter is the smallest detectable change of the magnetic flux density that can be revealed by the teslameter. the resolution is limited by the noise and depends on the frequency band of interest. vendors usually quote both figure 5. senis 3d magnetic field mapper for model validation and material/device characterization at university of buffalo, ny [11]. figure 4. bomatec ag utilizes the senis magnetic field mapping system for quality control of permanent magnets. figure 3. senis low-noise & high-accuracy magnetic field measurement system is used for the iter project. iter shall demonstrate the scientific and technological feasibility of fusion energy on an industrial scale [10]. acta imeko | www.imeko.org april 2017 | volume 6 | number 1 | 46 dc resolution and ac resolution, but they rarely specify the related bandwidth. though, one vendor [12] expresses dc resolution by quoting “offset fluctuation & drift” over the period of time 10 s (or 100 s) with the sampling rate 20/s which corresponds to a frequency range from 0.1 hz (or 0.01 hz) to 10 hz. table 3 gives a comparison of the five teslameters with the best magnetic resolution. 3.4. comparison of the measurement range and frequency bandwidth hall-effect-based teslameters are applied for measuring dc and ac magnetic flux densities in the range from about 1 μt to about 30 t. the frequency bandwidth characterizes how well a magnetometer tracks rapid changes in magnetic field. some vendors do not specify whether or not there is inductive pickup in the connections of their hall probes at the upper part of the broad frequency range they quote, which makes the comparison difficult. table 4 gives a comparison of the measurement ranges and frequency bandwidth of the five best teslameters. 3.5. comparison of the temperature coefficient of sensitivity the thermal stability of the teslameter is the dependence of the measurement on temperature. it is given as a temperature coefficient of sensitivity in ppm per degree celsius. table 5 gives a comparison of the temperature coefficient sensitivity of the five best teslameters. 3.6. summary of the teslameters performances and prices by comparing the teslameters by the above characteristics we found that there are two providers offering 3-axis hall probes with very small magnetic field sensitive volume and small probe angular errors. two providers guarantee very high dc-accuracy of the measurement range. three providers offer high magnetic resolution for their hall-effect based 3-axis teslameters. the temperature coefficient of sensitivity is better than 30 ppm in three cases. last but not least is the price/performance ratio of the evaluated teslameters. whereas 1-axis high-end teslameters range from 3 k$-6 k$, the 2-axis are between 5 k$-10 k$, and the 3-axis teslameters are in the price segment of 14 k$-20 k$. 4. the teslameter with the best combination of performances in four out of five tables, the teslameter of senis [12] appears at the top of the list. in summary, this new teslameter can measure magnetic field vectors of a magnitude from about 1 µt to 30 t, with an angular error less than 0.1°, spatial resolution 100 µm, magnetic resolution ±2 ppm of the range, with an accuracy of 0.002 % of the range, and a temperature coefficient less than 5 ppm/°c. we present below some details of this teslameter [21]. 4.1. novel integrated three-axis hall probe accurate measurement of highly non-homogeneous magnetic fields requires a small and compact sensitive volume of the hall probe and measuring all three components of a magnetic field simultaneously at the same spot. an early version table 4. comparison of the measurement ranges and frequency bandwidth. published measurement range published frequency bandwidth (per axis) provider 1 μt-30 t dc-75 khz senis [12] 1 μt-30 t dc-50 khz fw bell [20] 1 μt-30 t dc-20 khz lake shore [18] 0.3 t-3 t dc-3 khz group3 [19] 20 mt-2 t dc-1 khz projekt elektronik [17] table 5. comparison of the temperature coefficient sensitivity. published temperature coefficient sensitivity provider 3 ppm projekt elektronik [17] 5 ppm senis [12] 10 ppm group3 [19] 30 ppm lake shore [18] 200 ppm fw bell [20] table 3. comparison of the magnetic resolution. published magnetic resolution provider 0.1 μt senis [12] 0.1 μt group3 [19] 0.1 μt projekt elektronik [17] 0.1 mt note: for the range of 30 μt, a resolution of 0.1 nt available fw bell [20] 0.1 mt note: for the range of 3.5 μt, a resolution of 2 nt available lake shore [18] table 1. comparison of probe dimensions, magnetic field sensitive volume and angular errors of the probe. published probe characteristics provider • fully integrated 3-axis hall probe; • various and customizable dimensions (width x thickness), e.g.: probe c: 4.0 mm x 0.9 mm probe k: 2.0 mm x 0.5 mm thinnest probe: 0.25 mm • mfsv distance from the tip of the probe: 0.15 mm • mfsv: 0.1 x 0.1 x 0.1 mm3 • orthogonality error (calibrated): 0.1° senis ag [12] • hybrid 3-axis hall probe; • probe dimensions (wxth): 5 mm x 2 mm • fsv distance from the tip of the probe: 3 mm • mfsv: 0.1 x 0.1 x 0.1 mm3 • orthogonality error (calibrated) coliy [13] • applies senis hall probes, among others [14] metrolab [15] table 2. comparison of the magnetic field measurement accuracy. published magnetic field accuracy dc-accuracy of range provider 0.002% integrated 3-axis hall probe senis [12] 0.005 % single channel 0.5 % 3-channel projekt elektronik [17] 0.05 % single channel 0.1 % 3-channel lake shore [18] 0.01 % single channel only group3 [19] 0.1 % 3-channel fw bell [20] acta imeko | www.imeko.org april 2017 | volume 6 | number 1 | 47 of the senis integrated 3-axis hall probe [16] fulfilled well these requirements, but the residual flicker noise of this probe limited the measurement accuracy to about 0.1 %. senis recently developed a new integrated 3-axis hall probe with much better performance, figure 6. the new senis 3-axis hall probe incorporates several horizontal and vertical hall devices, monolithically integrated on a single silicon cmos chip. both horizontal and vertical hall devices feature high magnetic sensitivity (higher than 0.04 v/vt) and low flicker noise (the corner frequency lower than 2 khz) [22]. for each measurement axis, eight equal parallelconnected hall devices are used. this allows reducing the noise equivalent magnetic field spectral density of the thermal noise of each group of hall devices to about 40 nt/√hz. all 24 hall devices occupy an area of only 100 µm × 100 µm. a temperature sensor, which is also integrated on the hall probe chip, enables an efficient temperature compensation of the influences of the probe temperature. the electronic module drives the hall devices in the probe according to an optimized spinning-current method. the spinning-current method is well described in appendix a of [16]: the hall device is connected to a biasing source and an output circuit via a group of switches. the switches are turned on and off so that the diagonally situated terminals of the hall device are periodically commutated and alternatively used as the current (input) and the sense (output) contacts. thus, the bias current virtually rotates in the device for 90° back and forth (therefore, the term “spinning current”). the hall voltage rotates with the biasing current and appears at the output with a sign that depends only on the orientation of the magnetic field b. if the magnetic field does not vary much during a switching period, then the hall voltage is quasi-dc. on the other hand, the offset voltage at the output has an opposite sign during each phase of the spinning current. therefore, the offset voltage appears as an ac signal with the switching frequency and can be filtered out from the output voltage. the resulting modulation-demodulation process of the hall voltages eliminates the offset and the flicker noise of both the hall devices and the amplifiers of the hall signal; and it also eliminates the planar hall voltage. after analog-to-digital conversion of the “clean” and amplified hall signals with up to 22 significant bits, the hall signals are further processed by an embedded computer. the digital processing includes filtering, linearization, and corrections of temperature dependence. the cancellation of the temperature influence is based on a novel calibration procedure that takes into account both the temperature of the probe and the temperature of the electronics module. the calibration process also includes the precise measurement of the nine components of the sensitivity tensor of the probe [23], which allows the elimination of the errors that might be caused by the angular tolerances of the hall probe. the interface with the user is provided by a touch-screen display. 4.2. latest measurement results of the novel teslameter figure 7. histogram of the noise-equivalent magnetic field of the 3mh5 senis’ teslameter with the low-noise hybrid hall probe, in the magnetic measurement range 2 t and a frequency range from 0.1 hz to 10 hz. the calculated standard deviation is 1.2 µt and the six sigma peak-to-peak noise is 7.2 µt, which corresponds to the width of the histogram. at dc measurements (integration time 1 s) the teslameter has a peak-to-peak resolution of about 1 ppm of the measurement range. figure 8. illustrating offset stability of the 3mh5 senis’ teslameter. solid red line: offset, left scale; dashed blue line: probe temperature, right scale. the figure shows that, at room temperature, the offset fluctuations are not correlated with temperature. figure 6. senis 3d hall probe. spacial resolution:100 µm x 100 µm the probe is packaged in alumina ceramics. probe dimensions (wxthxl) in mm: 6.0x1.6x14.5. figure 9. the measurement error of the 3mh5 senis’ teslameter in the measurement range 2 t. if the room temperature varies within +/-3 °c around 22 °c, the measurement error is within +/-15 ppm (parts per million) of the measured value. acta imeko | www.imeko.org april 2017 | volume 6 | number 1 | 48 first measurement results of the novel teslameter have been published [7], [8]. here we present the latest characterization results regarding magnetic resolution, measurement accuracy, temperature compensation and orthogonality error – see figures 7 – 10 and table 6. 5. conclusions we compared the published performance of the best, mostly 3-axis, hall-effect teslameters that are commercially available on the global market. the comparison was based on the following characteristics: probe dimensions, magnetic field sensitive volume, probe angular errors, magnetic resolution, magnetic field measurement accuracy, magnetic measurement range, frequency bandwidth, and temperature coefficient of sensitivity. the novel senis 3-axis teslameter with integrated hall probe seems to match best the demanding measurement needs in various applications in modern science and industry. references [1] e. h. hall, “on a new action of magnetism on a permanent electric current”, am. j. sci. series 3, 20 161-186, 1880 [2] r.s. popovic, “hall effect devices”, 2nd ed., iop publishing bristol and philadelphia, 2004. [3] m. blagojevic, n. markovic, and r. s. popovic, “testing the homogeneity of magnets for rotary position sensors”, intermag ieee international magnetics conference may 48, 2014, dresden, germany. [4] d. popovic renella and s. spasic, “high-accuracy 3-axis teslameter and its application in the 3d mapping of magnetic fields”, uk magnetic society international event on magnetic materials and applications, oct. 28-29, 2014, lucerne, switzerland. [5] s. spasic, i.j. walker, m. blagojevic, n. markovic, “integrating magnetic field mapping and coordinate measurement”, magnetics 2015 conference, jan. 21-22, 2015, orlando fl, usa [6] s. sanfilippo, “hall probes: physics and application to magnetometry”, psi switzerland, 2011. [7] d. popovic renella, s. dimitrijevic, s. spasic and r.s. popovic, “high-accuracy teslameter with thin three-axis hall probe”, proceedings of 20th imeko tc4 international symposium and 18th international workshop on adc modelling and testing research on electric and electronic measurement for the economic upturn, benevento, italy, september 15-17, 2014. [8] d. popovic renella, s. dimitrijevic, s. spasic, r.s. popovic, “high-accuracy teslameter with thin high-resolution three-axis hall probe”, measurement, elsevier, issn: 0263-2241, in press, available online 15 september 2015. [9] s. spasic, z.mitrovic, “teslameter-market analysis”, unpublished, senis internal document, february 2015. [10] teslameters in customer applications: http://www.senis.ch/products/applications/references [11] http://www.cbe.buffalo.edu/people/full_time/e_furlanires_cm mapp.php [12] www.senis.ch [13] www.coliy.com [14] ph. keller, “recent advances in hall magnetometers”, magnetics magazine 2015: http://www.metrolab.com/wpcontent/uploads/2015/07/thm1176_tech_note-published-inmagnetics-technology-international.pdf [15] www.metrolab.com [16] d. popovic, s. dimitrijevic, m. blagojevic, p. kejik, e. schurig, and r. popovic, “three-axis teslameter with integrated hall probe”, ieee trans. on instrum. and meas., vol. 56, no. 4, august 2007 [17] www.projekt-elektronik.com [18] www.lakeshore.com [19] www.group3technology.com [20] www.fwbell.com [21] d.r. popovic renella, r.s. popovic, “hall magnetic transducers with nmr-like resolution”, immw 16 – international magnetic measurement workshop, bad zurzach, switzerland, 26-29 october 2009. figure 10. illustrating the correction of the angular errors of a 3-axis integrated hall probe [23]. in order to demonstrate the efficiency of the calibration method, the probe was rotated with respect to the z-axis for about 15°. left: the “raw” sensitivity matrix (before calibration); middle: the inversed “raw” sensitivity matrix; right: the corrected sensitivity matrix, which is an almost identity matrix, with negligible off-diagonal terms. the related angular errors are shown in table 6. table 6. angular errors of the 3-axis integrated hall probe with the sensitivity matrices shown in figure 5. notation: rx roll of x sensor; tx tilt of x sensor; py pitch of y sensor; ry roll of y sensor; pz pitch of z sensor; tz tilt of z sensor. in spite of the deliberately produced big initial error rx and ry , after the calibration of the probe, all angular errors are reduced to less than 0.1°. rx tx py ry pz tz before calibration 14.85° 1.45° 1.16° -14.79° -1.54° -1.38° after calibration 0.017° -0.005° 0.006° -0.059° 0.018° 0.008° http://www.senis.ch/products/applications/references http://www.cbe.buffalo.edu/people/full_time/e_furlanires_cmmapp.php http://www.cbe.buffalo.edu/people/full_time/e_furlanires_cmmapp.php http://www.senis.ch/ http://www.coliy.com/ http://www.metrolab.com/wp-content/uploads/2015/07/thm1176_tech_note-published-in-magnetics-technology-international.pdf http://www.metrolab.com/wp-content/uploads/2015/07/thm1176_tech_note-published-in-magnetics-technology-international.pdf http://www.metrolab.com/wp-content/uploads/2015/07/thm1176_tech_note-published-in-magnetics-technology-international.pdf http://www.metrolab.com/ http://www.projekt-elektronik.com/ http://www.lakeshore.com/ http://www.group3technology.com/ http://www.fwbell.com/ acta imeko | www.imeko.org april 2017 | volume 6 | number 1 | 49 [22] r.s. popovic, “high resolution hall magnetic sensors”, miel may 12-14, 2014, belgrade, serbia. [23] s. spasic, m. blagojevic, n. markovic, z. mitrovic2, l. popovic and r.s. popovic, “mapping permanent magnets with high angular accuracy”, ama sensor conference, nuremberg, 19 21 may 2015. an overview of commercially available teslameters for applications in modern science and industry table 5. comparison of the temperature coefficient sensitivity. microsoft word 25-144-1-pb.doc acta imeko  july 2012, volume 1, number 1, 70‐76  www.imeko.org    acta imeko | www.imeko.org  july 2012 | volume 1 | number 1 | 70  an electronic approach to homogenize the impedance phase  characteristics of heterogeneous gmi sensors  eduardo c. silva 1,2 , luiz a.p. gusmão 1 , carlos r.h. barbosa 2 , elisabeth c. monteiro 2   1  department of electrical engineering, pontifícia universidade católica do rio de janeiro, rua marquês de são vicente 225, 22451‐900 rio  de janeiro, brazil  2  postgraduate program in metrology, pontifícia universidade católica do rio de janeiro, rua marquês de são vicente 225, 22451‐900 rio  de janeiro, brazil    keywords: homogenization; impedance phase; giant magnetoimpedance; magnetic sensor  citation: eduardo c. silva, luiz a.p. gusmão, carlos r.h. barbosa, elisabeth c. monteiro, an electronic approach to homogenize the impedance phase  characteristics of heterogeneous gmi sensors, acta imeko, vol. 1, no. 1, article 14, july 2012, identifier: imeko‐acta‐01(2012)‐01‐14  editor: pedro ramos, instituto de telecomunicações and instituto superior técnico/universidade técnica de lisboa, portugal  received january 10 th , 2012; in final form june 5 th , 2012; published july 2012  copyright: © 2012 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: this work was supported by the brazilian funding agencies cnpq and finep  corresponding author: eduardo c. silva, e‐mail: edusilva@ele.puc‐rio.br  1. introduction  the giant magnetoimpedance effect (gmi) began to be intensively studied in the 1990s. it was observed a huge impedance variation when a sample of a ferromagnetic amorphous material, excited by an alternating current, was submitted to an external magnetic field [1-19]. the gmi phenomenon is induced by the application of an alternating current (i) along the length of amorphous ferromagnetic samples, which are submitted to an external magnetic field (h) [1-6]. the impedance of gmi samples changes as a function of h, as shown in figure 1, and h can be inferred by measuring the difference of potential (v) between the extremities of the gmi sample. the complex impedance zsens(h) of the gmi samples can be electrically modelled as a resistor rsens(h) in series with an inductor lsens(h), as defined by (1). ( ) ( ) ( )sens sens sensz h r h jwl h  , (1) where ( ) ( ) cos ( )sens sens sensr h z h h and (2) ( ) ( ) sin ( )sens sens senswl h z h h . (3) where θsens is the impedance phase of the gmi samples. over the last years, the research group of the laboratories of sensors and instrumentation (lsi) and biometrology (labiomet) of puc-rio have worked on the development of transducers based on the gmi effect [10-19]. recent researches, conducted by the lsi and labiomet team allowed observing that phase-based gmi transducers are much more sensitive than the usual magnitude-based gmi transducers [14-19]. besides, a newly developed electronic circuit capable of improving by about a hundred times the impedance phase sensitivity of the phase-based gmi transducers [17] allowed to envision the application of the gmi magnetometers for the figure 1. typical measurement of the gmi effect: the difference of potential  (v)  between  the  extremities  of  a  gmi  sample,  submitted  to  an  excitation  current (i), as a function of the magnetic field (h).  over  the  last  decade,  giant  magnetoimpedance  (gmi)  sensors  have  been  studied  as  a  new  possibility  for  measuring  ultra‐weak  magnetic fields. to achieve that goal, the employment and optimization of gradiometric configurations are a key factor to reduce the  magnetic interference. the performance of gmi gradiometers is strongly related to the homogeneity of the impedance characteristics  of  the  gmi  samples.  this  work  presents  a  method  and  corresponding  electronic  implementation  to  homogenize  the  phase  characteristics of heterogeneous gmi samples, aiming at improving the performance of phase‐based gmi magnetometers.  acta imeko | www.imeko.org  july 2012 | volume 1 | number 1 | 71  measurement of ultra-weak magnetic fields, such as biomagnetic fields [5, 7-9, 20-21]. by definition, biomagnetic fields are magnetic fields generated by living organisms or by magnetic markers placed on these organisms. the intensities of the magnetic flux densities of biomagnetic fields are comprised between 1 ft and 1 nt, and their frequencies are in the range of dc to 1 khz [20-21]. among all the magnetometers (fluxgates, hall effect, magnetoresistance, etc.), the superconducting quantum interference devices (squid) are the most sensitive, and the only ones capable of satisfactorily performing biomagnetic measurements. however, squid systems need cryogenic cooling, presenting the drawback of high cost of fabrication, installation and operation [5-6, 20-21]. this drawback of the squid systems hinders the broad dissemination of an important non-invasive diagnostic tool based on biomagnetic field measurements [5, 20-21]. high sensitivity magnetic transducers, capable of measuring ultra-weak magnetic fields, have their resolutions limited by spurious magnetic interference due to diverse sources (vehicles, elevators, earth's magnetic field, etc.) which, in a typical laboratory environment, is in the order of 10 nt in the neighbourhood of 1 hz [5, 20-21]. thus, in principle, no matter how sensitive a transducer is, it cannot detect magnetic fields below the noise/interference level. to solve that issue, there are two usual approaches: magnetic shielding and gradiometric configurations. in spite of its high attenuation of the magnetic noise and interference, magnetic shields are very expensive, especially when it is necessary to filter low frequency fields, which significantly affect the measurements of biomagnetic fields, because it requires the use of materials with high magnetic permeability at low frequencies, like μ-metal alloys [20-21]. on the other hand, the gradiometric configuration approach, which consists in the use of spatial filters, is a much cheaper alternative, and also effective in most applications. since the magnetic field of a current dipole decreases with the cube of the distance r between this source and the point of measurement, the gradient field detection using two sensor elements separated by a distance d, significantly weakens the contribution of magnetic interference sources located at r >> d in relation to the contribution of the closest signal source at r << d [20-21]. the performance of gradiometric configurations is highly dependent of the sensors homogeneity [20-21]. in this work, aiming at the future implementation of gmi gradiometers, the heterogeneity of impedance phase characteristics of gmi samples is experimentally analysed and a method to homogenize the gmi sensors behaviour is developed. it is also proposed and discussed the electronic implementation of the homogenization method. 2. impedance characterization  the gmi sensors analyzed in this work are ribbon-shaped samples of co70fe5si15b10, produced by the single-roller meltspinning technique. these samples show a particular case of the gmi effect, called longitudinal magnetoimpedance (lmi). lmi samples are much more sensitive to the component of the magnetic field parallel to its length. the sensitivity of the gmi samples is typically affected by some parameters such as amplitude, frequency and dc level of excitation current, dimensions (length, width, thickness) of the gmi samples; biasing magnetic field (generated by an external source to ensure that the sensor operates in its most sensitive range), among others. therefore, the influence of these parameters under the gmi effect was experimentally investigated in order to ascertain which is the set of parameters that generates the optimum sensitivities [1-4]. thus, the behaviour of the impedance magnitude |zsens(h)| and phase θsens(h) of the ribbon-shaped gmi sensors, as a function of the external magnetic field h, was analyzed under different conditions. it was studied the dependency in relation to the excitation currents of the samples for dc levels between 0 ma and 100 ma, and frequencies between 75 khz and 30 mhz. also, it was verified that the amplitude of the ac component of the excitation current does not significantly affect the impedance behaviour, so it was kept in 15 ma. the influence of the length of the gmi sensors was also investigated. the measurements were performed with four different lengths: 1 cm, 3 cm, 5 cm and 15 cm [14-19]. the analysed gmi samples were put in the centre of a helmholtz coil, as shown in figure 2, in order to be excited by a magnetic field longitudinal to the direction of the current that flows through them. also, the sample–coil set was positioned in a way to guarantee that the direction of the earth’s magnetic field was transversal to the ribbons-shaped gmi samples. thus, its influence in the measurements has been minimized, because the gmi sensors used are of the lmi type. the variation of the magnetic field generated by the helmholtz pair was obtained by a dc current source according to (oe) 2.877 ( a )h i  . (4) the impedance measurements of the gmi samples, analyzed during the characterization studies, were performed by the preciion lcr meter agilent 4285a, which was also responsible for both ac and dc excitation of the samples. the uncertainty of the impedance phase measurements of the samples uθ is directly related to the impedance phase uncertainty of the lcr meter, which is indicated in its operational manual, in degrees, as 180 100 eau      , (5) where ae is defined as (%) ( )e n c ta a a k    , (6) where an is the component of the uncertainty due to the equipment intrinsic characteristics, ac is the cable length factor and kt is the temperature factor. the temperature factor kt is equal to one in the range of 18 oc to 28 oc. the measurements were always performed figure  2.  block  diagram  of  the  system  used  in  the  experimental  characterization of the ribbon‐shaped gmi samples.  acta imeko | www.imeko.org  july 2012 | volume 1 | number 1 | 72  within this temperature range, then it can be admitted that kt = 1. on the other hand, for impedance magnitudes below 5 kω, ac is given by (%) 15 m c f a  , (7) where fm is the frequency, in mhz, used to excite the sample. then, knowing that all of the experimental measurements of the impedance magnitudes of the gmi sensors returned values between 10 mω and 100 ω, the parameter an is defined as 2 2 100 (%) % 3% 0.02% 0.1% 30 30 m m n m f f a n z                    ,(8) where |zm| is the absolute value of the measured impedance in ohms and n2 is a frequency-dependent factor which can be equal to 0.15 – frequencies between 75 khz and 3 mhz – or to 0.38 – frequencies above 3 mhz. the standard uncertainty of the magnetic field (uh) generated by the helmholtz pair is dependent of the standard uncertainty of the dc current source (icel, ps-4000), which is equal to ±3.25 ma. then, supposing, by simplicity, that the geometric configuration of the helmholtz coils is satisfactorily close to the one considered on the theoretical model and knowing that the relation between the current and the magnetic field generated by the helmholtz pair is given by (4), uh is expressed as (oe) 2.877 ( a ) 9.35 moeh iu u     . (9) thus, the expanded uncertainty uh, for a confidence level of 95.45%, is 2 18.70 moeh hu u    . (10) the oersted (oe) is the unit of magnetic field in the cgs system, which is broadly used in the giant magnetoimpedance literature. the si unit (international system of units) of magnetic field is ampere per meter (am−1), which is related to the oersted by 3 111 oe 10 am 4   . (11) 3. phase homogenization method  assuming two heterogeneous gmi sensor elements (a and b), with different behaviours of the impedance phase as a function of the magnetic field (h), this section describes a method that allows the homogenization of the impedance phase behaviour of such sensors in a given range of magnetic fields (h1 to h2). the sensor b should be inserted in an "homogenizer circuit", shown in figure 3, in order to generate an equivalent impedance zb such that the phase of zb in h1 and h2 (θ1b and θ2b) becomes equal to the phase of the sensor a in these same points (θ1a and θ2a). regions should be chosen preferably where there is a linear, or almost linear, relationship between the phase of the gmi sensors and the magnetic field, so that the homogenization procedure can be optimal. when the sensor b is connected to the "homogenizer circuit" (figure 3), it is created an equivalent impedance zb, which phase characteristics behaves similarly to the ones of sensor a, within a given magnetic field range. the block reactance shown in figure 3 may be replaced by an inductance (ladj) or capacitance (cadj) as required, and the block resistance may represent a positive or negative resistance depending on the case. it is emphasized that despite the difficulties to implement a negative resistance using passive elements, it is possible to reproduce its behaviour by means of active electronic structures called gic’s (generalized immittance converters) [22]. among the possible implementations of gic’s the one that uses two coupled opamp’s, as shown in figure 4a, is considered the best, due to the fact that it is very stable and highly tolerant to nonideal properties of the opamp’s, particularly its finite gain and limited bandwidth [22]. one can infer that a gic can be used to simulate a negative resistance by adequately choosing the impedances zi that compose the gic structure. these circuits are called fdnr’s (frequency-dependent negative-resistance), because the generated negative resistances are dependent of the frequency. among the implementations of fdnr’s, the most recommended is shown in figure 4b [22]. the input equivalent impedance (zgic) of the gic shown in figure 4a, assuming ideal operational amplifiers, is given by 2 4 6 3 5 .gic z z z z z z  (12) and the equivalent impedance (zfdnr) of the fdnr shown in figure 4b is given by 3 44 2 2 2 6 3 5 2 6 5 1 . r r fdnr fdnr r z z w c c r r w c c r      (13) by considering a reactance xadj (inductive or capacitive) and a resistance radj (positive or negative), which will be placed in series with the sensor b, in order to homogenize the phase figure 4. (a) generalized immittance converters  (gic) and (b) frequency‐ dependent negative‐resistance (fdnr).  figure 3. block diagram of the “homogenizer circuit”.  acta imeko | www.imeko.org  july 2012 | volume 1 | number 1 | 73  characteristics one should attend 11 1 1 1 1 1 1 1 1 22 2 2 2 2 2 2 2 2 ( ) ( ) ( ) ( ). b adja a b a b adj a b adj a b adj b adja a b a b adj a b adj a b adj x xx r r r x r r r x x x xx r r r x r r r x x                           (14) by solving the system of equations presented in (14), it is highlighted that xadj may yield either a positive value (meaning that xadj is an inductance ladj) or a negative value (meaning that xadj is a capacitance cadj), leading to adj adj x l w  or 1 adj adj c wx   . (15) 4. results and discussion  figure 5 shows the phase characteristics of two gmi ribbon-shaped sensor elements (a and b), both of them 3 cm long and submitted to the same excitation current dc level of 80 ma, 15 ma of amplitude and 100 khz of frequency. all of the impedance phase values shown in this section have their respective measurement uncertainties uθ, obtained by applying (5), equal or smaller than ± 1 o. in turn, figures 6 and 7 represent, respectively, the dependence of the resistance (rsens) and inductance (lsens), as a function of magnetic field (h), for the same gmi sensors a and b whose phase characteristics are presented in figure 5. sensors a and b are clearly heterogeneous, then it is aimed to make the phase behaviour of sensor b similar to the phase behaviour of sensor a, in the region between h1 = 0.3 oe and h2 = 1.0 oe. thus, the homogenization can be accomplished by applying the experimental data of rsens and lsens, obtained from sensors a and b, on the system of equations defined in (14). the following parameters of interest were used: r1a = 0.9237 ω, l1a = 566.7 nh, r2a = 0.9819 ω, l2a = 753.1 nh, r1b = 1.0670 ω, l1b = 425.6 nh, r2b = 1.0990 ω, l2b = 541.2 nh. with these parameters it is possible to solve the system given by (14), obtaining 0.4737 1 0.0387 41.12 μf adj adj adj adj r x c wx              . (16) figure 8 compares the phase characteristics of sensor a with the ones of the equivalent impedance zb, generated by the insertion of sensor b in the electronic circuit of figure 3, using the reactance and resistance values given by (16). on the other hand, figure 9 shows the point-to-point error, in degrees, between the phase curves, shown in figure 8, as a function of the magnetic field. the analysis in figure 9 is presented for the region of magnetic fields where the homogenization was applied (0.3 oe to 1.0 oe). by observing figures 8 and 9 it is possible to note that the proposed procedure of homogenization was fairly successful, generating satisfactorily alike phase characteristics. figure  8.  comparison  between  the  impedance  phase  characteristics  of  sensor  a  and  sensor  b,  with  sensor  b  connected  to  the  “homogenizer  circuit”.  figure 5. impedance phase characteristics, of heterogeneous gmi samples (a and b), as a function of the external magnetic field.  figure  6.  characteristics  of  the  resistive  component  of  the  impedance,  of heterogeneous  gmi  samples  (a  and  b),  as  a  function  of  the  external  magnetic field.  figure  7.  characteristics  of  the  reactive  (inductive)  component  of  the  impedance, of heterogeneous gmi samples (a and b), as a function of the  external magnetic field.  acta imeko | www.imeko.org  july 2012 | volume 1 | number 1 | 74  comparisons between the resistive components of sensors a, b, and b connected to the “homogenizer circuit” and between their inductive components are shown, respectively, in figures 10 and 11. at first notice, it could be expected that inserting the sensor b in the “homogenizer circuit” would lead to having their resistive and inductive components similar to those presented by sensor a. however, by observing figures 10 and 11, it is noticed that this did not happen. actually, that did not happen because the aim of the homogenization process is having as close as possible (homogenize) the phase characteristics of heterogeneous sensors, in a defined magnetic field region, which does not necessarily implies in also homogenizing the characteristics of the resistive and reactive components of the sensors. 4.1. sensitivity analysis  aiming at knowing the influence of the adjustment parameters (radj and cadj) in the homogenization results, a sensitivity analysis was performed. this analysis consists in changing only one parameter (input) at a time, while all other parameters are assumed as constants, and observe how it affects the results (output). first, it was analysed the effect of variations of ±10% around the theoretical value obtained for cadj – equation (16). the results are presented in figure 12. it is highlighted that 10% was selected because this is a usual tolerance value for ceramic capacitors. it is also important to note that, in practice, it is difficult to precisely set capacitance values of the order of tenths of μf, then it is desirable that these changes in cadj do not affect significantly the homogenization results. second, it was analyzed the effect of variations of ±10% around the theoretical value obtained for radj – equation (16). these results are shown in figure 13. once the value of radj is set by properly adjusting the internal impedances of an fdnr structure equation (13) it is relatively easy to guarantee the correct adjustment of that parameter. despite the frequency being not an adjustment parameter, it can affect the homogenization once the negative resistance implemented by fdnr's depends on the frequency equation (13) and the ideal value for cadj also depends on the frequency equation (15). thus, a sensitivity analysis was performed, varying the frequency ±10% around its theoretical value (100 khz), in order to observe the influence of that parameter in the homogenization, and the results are presented in figure  11.  comparison  between  the  inductive  components  of  the  gmi  sensors:  a, b and b connected to the “homogenizer circuit”.  figure 9. error in the homogenized region (0.3 oe to 1.0 oe) between the phase curves of sensor a and sensor b, when the last one is connected to the “homogenizer circuit”.  figure  10.  comparison  between  the  resistive  components  of  the  gmi sensors:  a , b and b connected to the “homogenizer circuit”.  figure  12.  influence  of  cadj  in  the  homogenization  curves:  (a)  impedance  phase characteristics and (b) error in the homogenized region.  acta imeko | www.imeko.org  july 2012 | volume 1 | number 1 | 75  figure 14. it should be mentioned that sinusoidal oscillators of 100 khz with uncertainties much smaller than ±10% are very common. figure 12 allows observing that cadj does not significantly affects the homogenization, even for variations of the order of 10% around its theoretical value (41.12 μf), which is extremely desirable. on the other hand, radj has a much larger influence on the results, as shown in figure 13, pointing that radj should be precisely settled. this fact is somewhat preferable because, in practice, as mentioned previously, it is easier to adjust radj than cadj. about the frequency, the results shown in figure 14 lead to conclude that it affects the results in a nonlinear way. then, the sinusoidal oscillator must be carefully chosen to minimize these undesirable effects. 4.2. alternative homogenization approach  it is highlighted that, in the region of interest (0.3 oe to 1.0 oe), sensor a has a higher sensitivity than sensor b. the applied method aimed at making the phase behaviour of sensor b similar to the one presented by sensor a, so it was concluded that one should use a capacitor and a negative resistance in series with sensor b. however, if the purpose was the opposite, i.e. making the phase behaviour of sensor a similar to the one presented by b, it should be used a positive resistance and an inductor in series with sensor a. more specifically, in that case, solving (14) leads to 0.762 , 0.06641 105.7 nh. adj adj adj adj r x x l w           (17) this alternative approach presents homogenization results as good as the ones obtained with the original method. in practical terms, this approach is simpler than the original one, because it does not need a negative resistance. however, it reduces the sensitivity of the homogenized sample, which is a drawback. thus, it should be used only when sensitivity is not a critical factor. 5. conclusions  this paper presents a new method capable of homogenizing the impedance phase characteristics of heterogeneous gmi sensors. it is also described an electronic circuit, called “homogenizer circuit”, which can be used on the implementation of the proposed method. the results obtained demonstrate the effectiveness of the developed homogenization method, as shown in figures 8 and 9, allowing that gmi sensors with different phase behaviours, after being inserted in the developed “homogenizer circuit”, evolve to almost homogeneous phase behaviours. within the homogenized region, the difference between the phase characteristics of the sensors was always lower than 0.15o. in future work, a gradiometric configuration will be implemented using the developed “homogenizer circuit”, aiming at improving the quality of the gradiometric readings and, consequently, enhancing the performance of phase-based gmi magnetic transducers. acknowledgement  we thank the brazilian funding agencies cnpq and finep, for their financial support, and prof. fernando luis de araújo machado (department of physics / ufpe) for the gmi samples provided. figure  13.  influence  of  radj  in  the  homogenization  curves:  (a)  impedance phase characteristics and (b) error in the homogenized region.  figure  14.  influence  of  the  frequency  f  in  the  homogenization  curves:  (a)  impedance phase characteristics and (b) error in the homogenized region.  acta imeko | www.imeko.org  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[22] l.t.bruton, rc-active circuits: theory and design, prentice hall international, london, 1980, isbn 978-0-137-53467-8. automatic fall monitoring using floor vibration acta imeko issn: 2221-870x march 2019, volume 8, number 1, 40 47 acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 40 automatic fall monitoring using floor vibration marco tarabini1, filip gocanin1, bortolino saggin1, diego scaccabarozzi1, marco bocciolone1 1 politecnico di milano, department of mechanical engineering, via la masa 1, milan, italy section: research paper keywords: vibration, fall monitoring, measurements, assisted living, active ageing citation: marco tarabini, filip gocanin, bortolino saggin, diego scaccabarozzi, marco bocciolone, automatic fall monitoring using floor vibration, acta imeko, vol. 8, no. 1, article 7, march 2019, identifier: imeko-acta-08 (2019)-01-07 section editor: ronaldo dias, inmetro, brazil received october 01, 2017; in final form january 15, 2019; published march 2019 copyright: © 2019 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was supported by a grant from regione lombardia (‘avviso pubblico per la realizzazione di progetti di ricerca industriale e sviluppo sperimentale nel settore delle smart cities and communities por-fesr 2007-2013 asse 1 linea di intervento 1.1.1.1. azione e’). corresponding author: marco tarabini, e-mail: marco.tarabini@polimi.it 1. introduction the continuous improvement of living standards has created new challenges for humanity: the average extended human lifespan is one of the major burdens on today’s healthcare systems. estimations show that by 2050, we could expect the number of elderly persons living in their own homes but requiring assistance to triple [1]. falls are one of the most hazardous scenarios [1], [2]. approximately 33 % (range: 15 % to 44.9 %) of elderly community-dwelling persons in the usa are older than 65 years, and up to 60 % of nursing home residents fall each year [3]-[5]. falls (loss of balance resulting in coming to rest on the floor) are the one of the major causes of injuries resulting in death and of injury-related hospitalization among senior citizens. different works have focused on the possibility of detecting a fall using portable systems [5]-[24]. the majority of the methods rely on wearable sensors [7]-[10] and vision techniques [11]-[18]. the main limitation of the wearable systems is that these methods are not effective when the sensor is not worn, and elders show a certain hostility versus new technologies and often forget to wear or recharge the sensor. vision systems might fail in the presence of occlusions between the camera and the subject; consequently, the experimental setup might be complex if it is necessary to cover the entire house surface. the use of floor vibration as a diagnosis for the presence or fall of people inside a room has already been considered in the literature [19][19]. alwan et al. [19] observed that, as human activities cause measurable vibrations on the floor [25], it is possible to detect human falls by monitoring vibration patterns. the above hypothesis entails that the vibration signature of the floor differs from that of common daily life activities and from that of falling objects. the authors used piezoelectric accelerometers preloaded against the ground using a mass and spring system. the system includes battery-powered pre-processing electronics that start from the measured vibration, which produce a binary fall signal. we performed the tests in a controlled laboratory environment and concluded that it is possible to detect falls by using vibration abstract this work investigates the possibility of monitoring the activity and the falls of people in dwellings using three or more accelerometers fixed on the ground. the main difference between the proposed method and existing ones is the use of acceleration to estimate the impact force by using the apparent mass of the floor; the latter is experimentally identified in each room in which the tests were performed using the heel drop test. the study has two parts: 1. the apparent masses of different dwellings’ floors have been measured. 2. the ground reaction force is studied using a purposely designed force platform with a surface of approximately 2 m × 1 m. the force platform allowed the measurement of the forces generated by the falls of 21 subjects, of a crash test dummy (falling in front or rear direction from seated and standing position, with or without the interposition of objects on the trajectory), and of common objects (e.g. dishes, water bottles, books). the impact location is estimated by triangulation, using a wavelet algorithm derived from the existent literature. the results show the possibility of identifying the presence of subjects inside the room and the fall of subjects in the majority of dwellings. we conclude that the proposed method allows a clear distinction between the fall of subjects and objects, given that the difference in terms of force (which is estimated from the floor’s apparent mass and from the measured acceleration) is at least of one order of magnitude. mailto:marco.tarabini@polimi.it acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 41 patterns. the detection range was around 4.5 m on concrete slab floors. we consider that the results depend on the floor dynamics, which can be measured with the instrumented heel drop test proposed by blakeborough and williams [25], but no further details are reported in the paper. in the standard heel drop test, a subject with mass standing on the balls of his feet, with talons at approximately 8 cm from the ground, suddenly relaxes and lets the heels fall on the ground. litvak et al. [20] and ziegel et al. [21] proposed an automatic fall detection system based on floor vibration and acoustic noise. a pattern recognition algorithm was used to discriminate between falls and spurious events. tests for human falls were simulated using a mannequin and were only performed in a laboratory environment; to date, no study has compared the force of dummies and of young subjects. furthermore, the forces generated by older adults may be different from those of young subjects because of the elderly persons’ limited muscular force and their reduced mental alertness. the works of alwan, allen, and ziegel are highly relevant in the field, but they did not analyse vibration transmissibility in residential buildings, nor did they consider that falls against objects or from seated positions may result in very low acceleration levels. in this work, we outline our study for the detection of people’s falls using an estimation of ground reaction force (grf). the estimation is performed by measuring floor vibration by accelerometers attached to the floor and by characterising the floor’s apparent mass and the vibration transmissibility before starting the measurements. preliminary tests for the discrimination between falls of objects and subjects were performed by using a purposely designed force platform. in addition to the validation of the method, this paper investigates: • the transmissibility of vibrations in different residential floors that have not yet been studied. given that a fall generally occurs at variable distances from the sensors, it is necessary to identify the vibration transmissibility in order to assess building-dependent modifications of the signal; • the force generated by people fall is studied for the frontal and rear falls of young healthy subjects [27]-[29]; the paper is structured as follows: the proposed method is described in section 2. section 3 describes the experiments performed in the laboratory (falls of a crash test dummy, athletes, and objects), in dwellings (identification of vibration transmissibility), and in real conditions. results are discussed in section 4, and the study’s conclusions are presented in section 5. 2. materials and methods the detection of subjects’ falls using the floor vibration measured on the ground is influenced by three phenomena: the vibration is modified by the floor transmissibility, the force generated by the impact is unknown, and there may be other events leading to vibration signals that might be similar to that deriving from a person’s fall. the proposed approach is presented in figure 1: the vibration transmissibility through the different grounds and the force generated from the different impacts are studied separately in order to be able to predict the vibration generated by different falls on the different floors. in other words, with the separate characterization of the force generated by an impact on an infinitely rigid floor and of the vibration transmissibility through the floor, it will be possible to understand a variety of combinations between impact locations, fall types, and floor characteristics. the estimated force signals will be used in future works as training phases for the algorithms for event detection, similar to [30]-[32]. proposed method the dependence between the fall impact force and the measured floor vibration is constrained by the ground mechanical impedance, i.e. the ratio between the force and the velocity, or by the ground apparent mass, i.e. the ratio between the force and the acceleration [33]-[36]. if the behaviour of the ground is linear, both the apparent mass and the impedance can be estimated with impact tests similar to those described in [26]. the ground apparent mass (am) can therefore be estimated at low frequencies with the heel drop test by knowing the force generated by the heel (f) and the acceleration (a) measured at position j when the heel drop test is performed at location i. 𝐴𝑀𝑖,𝑗(𝑓) = 𝐹𝑖(𝑓) 𝑎𝑗(𝑓) (1) given that this quantity is constant (if the ground behaviour is linear), if the subject falls close to location i, it is possible to estimate the force generated by the fall by multiplying the apparent mass of the ground ami,j(f) times the measured acceleration during the real fall at position j. 𝐹𝑖,fall(𝑓) = 𝐴𝑀𝑖,𝑗(𝑓) ∙ 𝑎𝑗,fall(𝑓) (2) with this method, it is possible to classify the fall events using the force instead of the acceleration as used in all the existing methods required by existing studies. the proposed approach requires knowledge of the impact location, which in this case, was identified using the wavelet method described in [37]. the method requires positioning at least three triaxial accelerometers on the ground in order to detect the plate longitudinal waves. the continuous wavelet transform, using the gabor function as mother wavelet as suggested in [37], was used to identify the time of arrival of the waves to the different sensors. at each frequency, the time difference between the first arrivals to two sensors is related to the so-called wave group velocity through the known distance between two sensors. the group velocity cg(f) was therefore computed as the ratio between the distance l and the difference between the times of arrival t1(f) and t2(f) computed from the wavelet scalogram at frequency f. 𝐶𝑔(𝑓) = 𝐿 𝑡2(𝑓)−𝑡1(𝑓) (3) in this study, the time of arrival was evaluated by analysing only the fastest propagation mode identified by the scalogram maxima. figure 1. schematic of the proposed method. ground impact force vibration transmissibilty vibration at the impact location impact location algorithms for the event classification experiments in real conditions acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 42 once the location of the impact was identified, the force time history could finally be computed using the inverse fourier transform of equation (2). ground transmissibility the impact on the ground generates compressive and flexural waves that propagate through the floor from the impact position to the sensors’ location. the subjects’ fall can be identified by observing different features of the signal (in either the time or frequency domain) and by using information about the impact location (to discard, for instance, shocks occurring in specific positions). the first step for the feasibility study is the identification of the floor vibration transmissibility in residential buildings given that if the transmissibility tends to zero, it is not possible to also measure the vibration at small distances from the impact location. therefore, we decided to analyse a group of houses with different characteristics in order to evaluate the possibility of locating the transducers in positions that are not critical for the daily activities of people. since the vibration transmissibility depends on the mechanical and geometrical characteristics of the base and of the floor, experiments were performed in different conditions. for the purposes of this study, four iepe accelerometers model bruel&kjaer 4508 b, with nominal sensitivity of 10 mv/(m/s2) measured the vibration at the positions indicated in figure 2. three accelerometers (indicated by green circles in figure 2) were fixed at positions that depended on the room dimensions (longer room side a, shorter room side b). another accelerometer was moved close to the impact location. the vibration signals were sampled using a national instruments ni 9234 data acquisition board. the sampling frequency was 2048 hz. the stimulus was given by the force generated in a heel drop test. the subject that performed these tests also performed the same tests on the instrumented platform described in section 2.3 (20 tests performed in repeatability conditions), in order to obtain the average excitation force together with its variability (the standard uncertainty was 13 % in our tests). the vibration transmissibility and the ground apparent mass were measured in 40 rooms, with surfaces between 2 m² and 50 m², with different floor materials (wood, stones, tiles). the experimental results have been summarised by averaging the vibration transmissibility of different buildings. the latter was measured using the h1 estimator of the frequency response function, by averaging the results of five tests (lasting 2 s with a pre-trigger of 0.3 s). the ordinary coherence function has been computed as well. measurement of the ground reaction force given that the proposed method is based on the computation of the grf, we have designed a force platform with a surface large enough to perform fall tests. the platform was built with a sandwich honeycomb panel (2.5 m × 1.25 m) supported by four piezoelectric load cells pcb 211b. the sandwich thickness is 100 mm, with a sheet thickness of 1 mm and a honeycomb thickness of 50 µm. the upper sandwich layer was covered by 5 mm-thick compensated wood in order to protect the surface from the localised impacts generated by the dummy. the theoretical computations pointed out a resonance frequency of approximately 85 hz. the dynamic behaviour of the force platform has been experimentally verified with an impact hammer, and we found that the frequency pass-band (± 3 db) was 40 hz. the first natural frequency of the unloaded plate was 63 hz. furthermore, in this case, data was pre-triggered so that the first impact of the object on the platform occurred after 0.3 s. three groups of simulations were performed: • fall simulations performed by subjects falling forward and backward with complete fall arrest: tests were performed on the force platform by 21 healthy young subjects with a height between 1.65 m and 2.00 m and a body mass between 45 kg and 95 kg (average 72 kg). subjects were instructed on how to perform the simulation to avoid any injuries. tests were performed in accordance with the ethical guidelines of politecnico di milano. the tests differed because of: • direction of fall: front fall (f) or rear fall (r) • subject body mass (in kg) • subject height (in m) • hip height (in m) • a fall simulation performed by a dummy (humanetics pedestrian dummy, hybrid iii 50th percentile, mass 104 kg) was used to identify the force generated by different fall configurations. simulations were performed with and without limited fall arrest according to the following configurations: • type of fall: rear, front, or side • pre-fall posture: standing or sitting • height of the hips before fall: 0.5, 0.6, 0.7, or 0.8 m • limbs arrest posture: no arrest, one arm arrest, two arms arrest, and elbow arrest • distance between the feet before the fall: 0.2 or 0.3 m • description of fall trajectory: free trajectory or fall over the objects • falls of common objects of different weights, sizes, and shapes from different heights: • objects: plastic bottle, glass, glass bottle, dish, pot or box • object weight • height of the fall: 0.7 or 1.4 m • number of falling objects figure 2. position of the accelerometers and of the impact in the transmissibility tests. acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 43 during the experiments, both the acceleration and the grfs were recorded. data was summarised using basic descriptive statistics. the ratio between the average mass of the subjects that performed the test and the mass of the dummy is 0.7, and the grf should also be in a similar proportion if the fall configuration is similar. not all the possible combinations between factors were included given that parts of them were meaningless: for instance, it is almost impossible in a frontal fall to have an elbow impact. only realistic fall configurations were reproduced and analysed. method validation the proposed method for the localisation of the fall using wavelets and for the computation of the force is validated in two steps. the first validation was performed on the force platform: the expected vibration of the sensor was computed using equation (2) using the location of the impact (estimated using the wavelet triangulation) and the force measured by the load cells. the expected vibration has been compared with the acceleration measured at different locations. the difference between the estimated and the measured acceleration of all the tests was used in order to assess the method’s reliability. in the second step, an embedded system, based on mems accelerometers and a raspberry pi board, was installed in a residential building, and the vibration was monitored inside a small bath and in the bedroom. the rooms were characterised using the method described in section 2.1. the tests aimed for the validation of the architecture and for detecting common activities of daily living. 3. results method validation the method was validated by comparing the acceleration measured on the force platform, predicted using the measured grf and the apparent mass of the force platform itself. figure 3 shows an example of the comparison in one of the tests performed by with the crash test dummy. the results, apart from a tonal component at the resonant frequency of the plate was not reduced by the digital filtration of data, show maximum errors that are lower than 20 %. due to system linearity, the error is the same for the estimation of acceleration during the method validation (by finding the grf) or for the estimation of the force during the real tests (by measuring the acceleration and the floor characteristics). the average error on the amplitude of the first acceleration peak is 23 %; the average was computed on the entire data set. the error is due to difficulties in the identification of the location of the impact, which in approximately 50 % of cases resulted in the choice of a point close to the one at which the impact occurred. in addition, the apparent mass of the plate was measured without the mass of the dummy/subject, thus leading to a biased compensation of the frf. this aspect was confirmed by the spectral analysis of the error, which was dominated by components at frequencies between 55 hz and 63 hz. these values are close to the first natural frequency of the platform and vary between different tests depending on the position of the subject/object. the average error (figure 4) decreased to 16 % when the accelerations were computed for the falls of the subjects and the crash test dummy (i.e. excluding the objects, for which the measured forces were characterised by a poor snr). in these conditions, the error of magnitude of the largest acceleration/force peak decreased to 14 %; the average error, apart from the force peak, was lower than 5%. furthermore, in this case, the spectral analysis evidenced the dominance of the frequency components close to the first resonance of the platform. ground transmissibility the average vibration transmissibility measured in 40 rooms is shown in figure 5(a), and the average coherence between the input and output position (asterisks and circles in figure 2) is shown in figure 5(b). plots include the effect of the different room sizes, of the floor mechanical characteristics, and of the different positions of the impact and of the sensors. the plots show that the modulus of the vibration transmissibility is, on average, lower than 1 in the band between 0 hz and 150 hz. the average transmissibility has a minimum below 15 hz, but the lower value (0.3) does not prevent measurements in that region. the coherence is, on average, larger: between 20 hz and 50 hz. this interval represents that in which the siso system approximation is more reasonable. at lower frequencies, the effect of non-measured inputs (such as the vibration of the building induced by natural agents or by traffic) might be relevant. above 50 hz, the lack of energy in the stimulus might lead to a low signal-to-noise ratio. further analyses evidenced that the vibration transmissibility and the coherence function depend on the floor type and on the room size: the transmissibility measured on the wooden floors is, on average, lower than that on the tiles and stones, especially at high figure 3. force signal (upper plot), measured vibration (black line), expected vibrations (red line) (middle plot), and the test error (lower plot). figure 4. average difference between the measured and the predicted acceleration generated by the falls of the dummy and subjects. 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 0 5000 10000 15000 time [s] f a ll g r f [ n ] 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 -20 0 20 time [s] a c c e le ra ti o n [m /s 2 ] 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 0 50 100 time [s] e rr o r [% ] 0 0.5 1 1.5 2 0 10 20 30 time [s] a v e ra g e e rr o r [% ] acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 44 frequencies. as expected, the first resonance frequency depends on the room size, while the effect of the furniture is negligible in comparison with the effects of floor type and room size. three examples of the effects of the room properties on the coherence and transmissibility are shown in figure 6, figure 7, and figure 8. figure 6 shows that the transmissibility of the vibration with the parquet is smaller, on average, than the vibration transmissibility when the floor is covered with tiles. the average transmissibility between 1 hz and 150 hz with the tiles was 0.54; the value dropped to 0.26 when the floor was covered by the parquet. a similar effect was noticed for the room size, where the vibration transmissibility of small rooms (0.55 between 1 hz and 150 hz) was, on average, larger than that of large rooms (0.38 in the same frequency range). the coherence between the stimulus (force) and the response (acceleration), shown in figure 7, was independent from the room size, between 15 hz and 50 hz (0.87, 0.89, and 0.87 for large, medium and small rooms respectively). at higher frequencies (100 hz to 150 hz), the coherence measured in large rooms (0.61) was smaller than that of medium (0.70) and small rooms (0.79) because of the lower signal-to-noise ratio when the accelerometer was far from the impact location. the effect of the ratio between the room dimensions was small on both the transmissibility and coherence (figure 8). the average coherence at frequencies between 15 hz and 50 hz was 0.88 independently from the ratio between the room dimensions a and b. between 100 hz and 150 hz, the average coherence was 0.76 for rooms with a side ratio that was lower than 2 and 0.84 for rooms with a ratio higher than 2. impact force the average grf measured during the fall of the dummy, of the healthy subjects, and of the objects is shown in figure 9. the results evidence that the force generated by the dummy is, on average, much larger than that generated by objects and healthy subjects. this is partially due to the larger mass of the humanetics dummy in comparison with the average subjects’ masses. the ratio between maximum force generated (on average) by the fall of a dummy (close to 10 kn) and the maximum force generated by the fall of a subject (approximately 1 kn) is 10, i.e. much larger than the ratio between the masses (1.4). this large difference can be explained by two factors: the first is that the dummy does not have any conscious reaction to the fall. this aspect is typical of elders, who usually do not protect themselves using their arms. the second factor is the difference between the biomechanical characteristics of the crash test dummy and the subjects. this factor is expected to be limited given that the dummy is designed to mimic the behaviour of subjects exposed to impulsive accelerations. a comparison between the average grfs and the average accelerations generated by the falls of objects, subjects, and dummies is shown in figure 10. the frequency content of the fall of subjects and objects is usually different, as shown by the wavelet transforms in figure 11. the plot shows that the impact of hard objects (b) creates vibrations at higher frequencies (over 70 hz) while the bandwidth of acceleration generated by the impact of a soft object (paper box, a) is lower than 35 hz. as a comparison, the spectrum measured during the fall of the dummy is reported in figure 11(c). figure 5. average transmissibility of the room in which the tests were carried out (a) average coherence of the transmissibility tests; (b) solid line: test average; dotted line: maxima and minima measured during the tests. figure 6. influence of floor covers on signal transmissibility: floors covered by tiles (solid line) and by parquet (dashed line). figure 7. influence of room size on signal coherence: small-sized rooms (solid line), middle-sized rooms (dashed line), and large-sized rooms (dotted line). figure 8. influence of room dimensions ratio a/b on measured signal coherence: if ratio a/b is higher than 2 (solid line), if ratio a/b is lower than 2 (dashed line). 0.0 0.2 0.4 0.6 0.8 1.0 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 f r f m o d u lu s [( m /s ²) /( m /s ²) ] frequency [hz] 0 0.2 0.4 0.6 0.8 1 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 c o h e re n c e frequency [hz] a b 0.0 0.5 1.0 1.5 2.0 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 f r f m o d u lu s [( m /s ²) /( m /s ²) ] frequency [hz] 0 0.2 0.4 0.6 0.8 1 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 c o h e re n c e frequency [hz] 0 0.2 0.4 0.6 0.8 1 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 c o h e re n c e frequency [hz] acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 45 the possibility of distinguishing between the falls of objects and people is confirmed by the wavelet transforms of the average grf measured during all the tests (shown in figure 12). different to the falls of objects, the grf has relevant components that are also at low frequencies. the dominating frequency components of observed during the falls of objects is close to 15 hz, while the dominating frequency components of falls generated by subjects are typically below 5 hz. another characteristic that allows for distinguishing the falls of objects from those of subjects is the number of impacts. the falls of both subjects (b) and the dummy (c) usually generate two spectral peaks (the impact of two body parts, such as knees and hands or elbows and head) while falls of objects usually generate a single spectral peak. preliminary experimental results the last step in the research is the installation of the embedded measurement system in a residential building. to date, there have been no falls, and consequently, it has not been possible to validate the fall detection system or to obtain real fall data. nevertheless, the results of this study have shown that the system detects activities of daily living, showing vibration levels much larger than floor noise during the day. 4. discussion the results presented in this paper evidence the compatibility between the frequency of fall-generated excitations and the frequency region in which the vibration transmissibility is high. the vibration transmissibility suggests that it is possible to measure the vibration at any position of the room, independent of the impact location; therefore, theoretically, one accelerometer is enough for detecting the presence of people in the room or their fall. the simultaneous use of at least three transducers, however, allows for identifying the location of the fall and consequently for the estimation of the grf that has generated the vibration. this parameter is, in principle, more reliable than the vibration generated by the fall, which depends on both the event that has generated the fall and on the distance between the impact location and the transducer. figure 9. time histories of the grf measured during (a) the dummy falls; (b) object falls; and (c) healthy subject falls. solid lines: averages, dotted lines: maximum and minimum. figure 10. comparison between the average grf and acceleration signals obtained during dummy, subject, and object falls. figure 11. wavelet transforms of acceleration signals generated by falls of (a) a soft object, (b) a hard object, and (c) the crash test dummy. figure 12. wavelet transforms of the average of the (a) objects’ grf; (b) subjects’ grf; and (c) dummy’s grf. 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 -0.5 0 0.5 1 1.5 2 2.5 x 10 4 time [s] f o rc e [ n ] dummy average grf grf min&max 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 -20 -10 0 10 20 30 40 time [s] a c c e le ra ti o n [ m /s 2 ] dummy average grf grf min&max 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 -500 0 500 1000 1500 2000 time [s] f o rc e [ n ] object average grf grf min&max 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 -10 -5 0 5 10 time [s] a c c e le ra ti o n [ m /s 2 ] object average grf grf min&max 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 -2000 0 2000 4000 6000 8000 10000 time [s] f o rc e [ n ] subject average grf grf min&max 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 -10 -5 0 5 10 15 20 time [s] a c c e le ra ti o n [ m /s 2 ] subject average grf grf min&max a b c 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 -5000 0 5000 10000 15000 time [s] f o rc e [ n ] object average grf subject average grf dummy average grf 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 -10 -5 0 5 10 15 time [s] a c c e le ra ti o n [ m /s 2 ] object average acc. subject average acc. dummy average acc. time [s] a p p ro x im a te f re q u e n c y [ h z ] 0 0.5 1 1.5 10 20 30 40 50 60 70 time [s] a p p ro x im a te f re q u e n c y [ h z ] 0 0.5 1 1.5 10 20 30 40 50 60 70 time [s] a p p ro x im a te f re q u e n c y [ h z ] 0 0.5 1 1.5 10 20 30 40 50 60 70a b c time [s] a p p ro x im a te f re q u e n c y [ h z ] 0 0.5 1 1.5 5 10 15 20 25 30 35 40 time [s] a p p ro x im a te f re q u e n c y [ h z ] 0 0.5 1 1.5 5 10 15 20 25 30 35 40 time [s] a p p ro x im a te f re q u e n c y [ h z ] 0 0.5 1 1.5 5 10 15 20 25 30 35 40 a b c acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 46 the uncertainty of the grf is limited by the heel drop test used for the identification of the ground apparent mass. from this perspective, we have already performed tests by dropping a silicone ball from a constant height; the results are certainly repeatable and will be presented in future studies. the force generated by falls of subjects is larger than that generated by falls of objects, and the identification of a threshold is, in principle, easier than the identification of the threshold for the acceleration level, which depends both on the impact magnitude and on the position of the fall with respect to the accelerometer. the identification of thresholds has not been included in this work and deserves attention in forthcoming studies, in which we will try to use the techniques proposed in [23][32], using the force instead of the acceleration as the main feature for the recognition. the results show that the use of three accelerometers allow for estimating the location of the fall; however, in approximately 50 % of laboratory falls, the impact location was identified at a point that was adjacent to the actual one (the error being lower than 1 m). the optimal transducer location will be studied in forthcoming studies, but thanks to the low cost of the mems accelerometers, it seems reasonable to put more than three sensors in each room in order to obtain a more reliable impact position estimation. the main limitation of this work is surely the lack of experimental validation in real conditions. during the scheduled experimental activities in the residential buildings for elders, there were no falls, and consequently, it was not possible to verify the proposed method. nevertheless, the results of the tests performed in laboratory conditions were promising, and we expect to obtain more data in future experimental sessions. 5. conclusion in this paper, we have described a method for the detection of human falls in dwellings using grf as a main parameter for the event classification. the method is based on the identification of the floor’s apparent mass using the heel drop test and on the computation of the impact location using at least three triaxial accelerometers. the method was validated on a purposely-designed force platform. fall simulation tests were performed with a crash test dummy and young healthy subjects. the force generated by falls of objects was also measured as a term of comparison. given that there is an order of magnitude between the grf generated by the falls of objects and subjects, the parameter should be more reliable than the acceleration. tests were also performed to identify the apparent mass and vibration transmissibility of common residential buildings. the results evidence that the transmissibility is larger than 0.3 up to 150 hz, thus showing that the vibration generated by the fall (which has a bandwidth close to 40 hz) can be transmitted inside the room and can therefore be measured by the different accelerometers. preliminary tests were not useful for the method validation, given that there were no real falls during the measurements. future developments in this area will include a new series of tests in residential buildings, a more reliable method for the measurement of the ground apparent mass, and the simultaneous usage of more accelerometers in order to improve localisation accuracy. acknowledgements the authors wish to thank all partners in the project (www.smarta-project.it), specifically: datamed s.r.l., flextronics design s.r.l., argonet s.r.l., software team s.r.l., electron, dipartimento di informatica unimi, dipartimento di design of politecnico di milano and codebri. this work was supported by a financial grant from regione lombardia (‘avviso pubblico per la realizzazione di progetti di ricerca industriale e sviluppo sperimentale nel settore delle smart cities and communities por-fesr 2007-2013 asse 1 linea di intervento 1.1.1.1. azione e’). references [1] m. e. tinetti, c. s. williams, the effect of falls and fall injuries on functioning in community-dwelling older persons, the journal of gerontology a, biological sciences and medical sciences 53 (1998), p. m112-9. 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[33] b. saggin, d. scaccabarozzi, m. tarabini, optimized design of suspension systems for hand–arm transmitted vibration reduction, journal of sound and vibration 331 (2012) pp. 2671-2684. doi: https://doi.org/10.1016/j.jsv.2012.01.024. [34] m. tarabini, b. saggin, d. scaccabarozzi, g. moschioni, hand-arm mechanical impedance in presence of unknown vibration direction, international journal of industrial ergonomics 43 (2013) pp. 52-61. [35] p. gardonio, s. elliott, driving point and transfer mobility matrices for thin plates excited in flexure, nasa (1998). [36] p. gardonio, m. j. brennan, mobility and impedance methods in structural dynamics, in: f. fahy, j. walker (eds.), advanced applications in acoustics, noise and vibration, spoon press, london, 2004. [37] l. gaul, s. hurlebaus, identification of the impact location on a plate using wavelets, mechanical systems and signal processing 12 (1998) pp. 783-795. https://doi.org/10.1260/0957-4565.45.6.13 https://doi.org/10.1016/j.trd.2012.12.003 https://doi.org/10.1016/j.apacoust.2014.03.013 https://doi.org/10.1016/j.jsv.2012.01.024 acta imeko, title acta imeko issn: 2221-870x march 2018, volume 7, number 1, 5-12 acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 5 wireless sensor network for temperatures estimation in an asynchronous machine using a kalman filter yi huang, clemens gühmann technische universität berlin, chair of electronic measurement and diagnostic technology, sekr. en13 einsteinufer 17, d-10589 berlin, germany section: research paper keywords: kalman filter; temperature estimation; asynchronous machine; thermal losses; wireless sensor network citation: yi huang, clemens gühmann, wireless sensor network for temperatures estimation in an asynchronous machine using a kalman filter, acta imeko, vol. 7, no. 1, article 3, march 2018, identifier: imeko-acta-07 (2018)-01-03 section editor: lorenzo ciani, university of florence, italy received september 24, 2017; in final form november 29, 2017; published march 2018 copyright: © 2018 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: this work was supported by chair of electronic measurement and diagnostic technology, technische universitäte berlin corresponding author: yi huang, e-mail: yi.huang@campus.tu-berlin.de 1. introduction the wsns consist of sensor nodes including power module, data processing module, wireless module and other peripherals for untethered communication. many applications such as the monitoring of the environment, the industry and the medical care are used based on wsn. the signals acquired and preprocessed by a wireless sensor node will be transmitted to the host sensor node and be processed there. this project is focused on the monitoring algorithm implementation in a wsn. because of the restrictions of the sensor nodes, such as low cost, low power, weak computation power and small memory size, many challenges have to be solved by the implementation. the temperature monitoring system of an asynchronous machine was examined for the implementation of the algorithm on wsn. due to the characteristics, such as low cost, robustness and low maintenance, asynchronous machines are widely used in industrial and electrical vehicles. the thermal behavior of an asynchronous machine largely determines the maximum lifetime, the ability of over-load and the accuracy in highperformance controller [1]. the high temperature, which is exceeding the service limit, may result in insulation deterioration as well as rotor faults [2]. as a result, the temperature monitoring of the stator winding, the rotor cage and the stator core, can be used to initiate certain reactions in order to circumvent thermal fault and predictive monitoring. it can protect the machine, extend the life span and contribute to the high performance of the machine [2]. generally, three methods can be employed for the temperature monitoring. a direct way is that the temperature can be measured by using mounted sensor. it is possible to fix a sensor on the surface of the stator core and inside the stator abstract a 4th-order kalman filter (kf) algorithm is developed based on the thermal model of an asynchronous machine. the thermal parameters are identified and kf is implemented in a wireless sensor network (wsn) to estimate the temperatures of the stator windings, the rotor cage, and the stator core of an asynchronous machine. the rotor speed, coolant air temperature, and the effective current and voltage are acquired by a wtim (wireless transducer interface module) separately and transmitted to a ncap (network capable application processor) where the kf algorithm is implemented. losses of the stator windings and the rotor cage are copper losses, and the stator core losses are iron losses. the losses of the stator windings, the rotor cage and the stator core are calculated from the measurements and are processed with the coolant air temperature by kf. as the resistance varies with temperature, the estimated temperature of the stator windings is used for compensating the rising of resistance. simulations and experiments on the test bench were performed prior to the kf algorithm is implemented on a wireless sensor node. the real-time temperature estimator on wsn is independent from control algorithm and can work under any load condition with very high accuracy. acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 6 winding. the temperature of the rotor cage can be acquired using wsn, but the high cost and the instability of the measurement system make it unsuitable for industrial usage [21]. the second way is that the temperatures of the stator winding and the rotor cage can be calculated by identifying the resistance, because it is a temperature dependent variable. a sensor-less internal temperature monitoring method for induction motor was introduced [1]. the author in [5] proposes a method to estimate the rotor and stator temperatures using extended kalman filter, when satisfying the assumption that there is no temperature rising for the coolant air temperature. the authors in [3] present an approach to estimate temperatures based on a thermal model. thermal analysis based on lumped-parameter thermal network, finite-element analysis, and computational fluid dynamics were considered. the design of a state-of-the-art rotor temperature monitoring system for contact measurement was proposed in [4]. building the thermal model of an electric machine is a complex multi-disciplinary problem, and it must evaluate the main internal losses of the machine, also [6]. the most frequently estimation techniques rely on a calculation of the resistance in steady state [7], [8]. the estimation of the stator resistance and rotor resistance has been presented in [5] and [9], but neither of them estimates the resistance simultaneously. this paper describes the implementation of a temperature monitoring system in a wsn using a kalman filter. the temperatures are estimated directly based on the thermal model of the asynchronous machine. the state-space of the system is presented in section ii. objected-modelling and parameter identification are introduced in section iii. section iv illustrates the kf algorithm and the tuning of the covariance matrix. in section v, the model-in-the-loop (mil) method is used for simulation and kf algorithm verification in simulink and the experiment results are described. section vi discusses the implementation of the kf on the wireless sensor node. finally, the conclusions are given in the last section. 2. the thermal model of the asynchronous machine heat transfer in the machine can be simplified as heat conduction in the solid parts and convection in the boundary layer of flowing medium. however, radiation is not considered in this paper. the thermal network is analogous to an electrical network, as temperature to voltage, power to current, thermal resistance to electrical resistance and thermal capacitance to electrical capacitance. by summarizing the structure of the machine, the thermal network is shown in figure 1. 2.1. the state-space equations a lumped-parameter thermal circuit is built based on the losses balance theory and the simplified thermal model in figure 1, the state-space can be defined as following equations: 𝑑𝑇𝑠𝑠 𝑑𝑑 = −𝑅𝑠𝑠𝑇𝑠𝑠 𝐶𝑠𝑠 + 𝑅𝑠𝑠𝑇𝑠𝑠 𝐶𝑠𝑠 + 𝑃𝑠𝑠 𝐶𝑠𝑠 (1) 𝑑𝑇𝑟𝑠 𝑑𝑑 = −𝑅𝑟𝑠𝑇𝑟𝑠 𝐶𝑟𝑠 + 𝑅𝑟𝑠𝑇𝑠𝑠 𝐶𝑟𝑠 + 𝑃𝑟𝑠 𝐶𝑟𝑠 (2) 𝑑𝑇𝑠𝑠 𝑑𝑑 = −𝑅𝑠𝑠𝑇𝑠𝑠 𝐶𝑠𝑠 + 𝑅𝑟𝑠𝑇𝑟𝑠 𝐶𝑠𝑠 + 𝑅𝑠𝑠𝑇𝑠 𝐶𝑠𝑠 + (𝑅𝑠𝑠+𝑅𝑟𝑠+𝑅𝑠𝑠)𝑇𝑠𝑠 𝐶𝑠𝑠 + 𝑃𝑠𝑠 𝐶𝑠𝑠 (3) where 𝑇𝑠𝑠, 𝑇𝑟𝑠, 𝑇𝑠𝑠, 𝑇𝑠 indicate the temperature above ambient of the stator winding, the rotor cage, the stator core and the coolant air respectively. 𝑅𝑠𝑠, 𝑅𝑟𝑠, 𝑅𝑠𝑠 are the thermal resistance of the stator winding, the rotor cage and the stator core. 𝐶𝑠𝑠, 𝐶𝑟𝑠, 𝐶𝑠𝑠 indicate the thermal capacitances of the stator wingding, the rotor cage and the stator core. 𝑃𝑠𝑠, 𝑃𝑟𝑠, 𝑃𝑠𝑠 are the power losses of the stator winding, the rotor cage and the stator core. in the simplified thermal model, 𝑃𝑠𝑠 , 𝑃𝑟𝑠 are ohmic losses, and 𝑃𝑠𝑠 is the frequency-dependent iron loss, 𝑅𝑠 and 𝑅𝑟 are resistances in ohm, between any two-line terminals, 𝜔𝑚 is the mechanical speed of the rotor in rad/s, 𝑘𝑖𝑟𝑖𝑖 is the iron loss constant which is given in table 3. the losses can be represented as: 𝑃𝑠𝑠(𝑑) = 𝐼𝐿2(𝑑)𝑅𝑠(𝑇𝑠𝑠(𝑑)) (4) 𝑃𝑠𝑠(𝑑) = 𝑘𝑖𝑟𝑖𝑖𝜔𝑚2 (5) as the current of the rotor is not available to be measured or to be estimated using simple method, the rotor cage losses can be calculated indirectly which is defined in ieee power engineering society's publication [12]. 𝑃𝑟𝑠(𝑑) = (𝑃𝑖𝑖(𝑑) − 𝑃𝑠𝑠(𝑑) − 𝑃𝑠𝑠(𝑑))𝑠(𝑑) (6) 𝑃𝑖𝑖(𝑑) = 3𝑈𝐿(𝑑)𝐼𝐿(𝑑) cos ∅ (7) 𝑠(𝑑) = 𝜔𝑠−𝜔𝑟 (𝑑) 𝜔𝑠 (8) where 𝑃𝑖𝑖 is the input power of the machine, 𝑈𝐿 and 𝐼𝐿 are the line voltage and the line current, respectively. 𝜔𝑠 is the synchronous speed, 𝜔𝑟 is the rotor speed, 𝑠 is the slip of the machine and 𝑐𝑐𝑠 ∅ is the power factor. the temperatures of the stator winding and rotor cage increase largely, which leads 40 % rising of the reference resistances. as a result, the temperature dependent characteristic of the resistance is used to compensate the ignored increasing resistance. all in all, the stator winding losses can be calculated much more accurately. 𝑅𝑠 can be replaced by the following equation: 𝑅𝑠(𝑑) = 𝑅𝑠𝑅𝑠𝑠(𝑑)(1 + 𝛼𝑠𝑇𝑠𝑠(𝑑)) (9) where 𝑅𝑠𝑅𝑠𝑠 is the stator winding resistance at the reference temperature. the 𝛼𝑠 is the copper temperature coefficient of psw pscprc csw crc csc rsw rrc rsc coolant air tsw trc tsc figure 1. thermal network of the asynchronous machine. acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 7 stator winding with the value of 0.004041. 2.2. definition of the system the state-space equations of the system can be acquired by calculating the losses 𝑃𝑠𝑠 , 𝑃𝑟𝑠 and 𝑃𝑠𝑠 which are defined in equations (4) (6), and importing them into equations (1) (3). by summarizing the previous equations, the system can be rewritten as a 4th-order linear continuous time-invariant system in the state space model form: 𝒙′(𝑑) = 𝐴𝒙(𝑑) + 𝐵𝒖(𝑑) (10) 𝒛(𝑑) = 𝐶𝒙(𝑑) + 𝐷𝒖(𝑑) (11) where: 𝒙 = [𝑇𝑠𝑠, 𝑇𝑟𝑠, 𝑇𝑠𝑠, 𝑇𝑠]𝑇 (12) 𝒛 = 𝑇𝑠 (13) 𝒖 = [𝑃𝑠𝑠, 𝑃𝑟𝑠, 𝑃𝑠𝑠, 0]𝑇 (14) 𝐴 = ⎣ ⎢ ⎢ ⎢ ⎢ ⎡ −𝑅𝑠𝑠 𝐶𝑠𝑠 0 𝑅𝑠𝑠 𝐶𝑠𝑠 0 0 −𝑅𝑟𝑠 𝐶𝑟𝑠 𝑅𝑟𝑠 𝐶𝑟𝑠 0 𝑅𝑠𝑠 𝐶𝑠𝑠 𝑅𝑟𝑠 𝐶𝑠𝑠 −(𝑅𝑠𝑠+𝑅𝑟𝑠+𝑅𝑠𝑠) 𝐶𝑠𝑠 𝑅𝑠𝑠 𝐶𝑠𝑠 0 0 0 0 ⎦ ⎥ ⎥ ⎥ ⎥ ⎤ (15) 𝐵 = ⎣ ⎢ ⎢ ⎢ ⎢ ⎡ 1 𝐶𝑠𝑠 0 0 0 0 1 𝐶𝑟𝑠 0 0 0 0 1 𝐶𝑠𝑠 0 0 0 0 0⎦ ⎥ ⎥ ⎥ ⎥ ⎤ (16) 𝐶 = 1 (17) 𝐷 = 0 (18) in the state equations, 𝒙(𝒕) is the state vector, 𝒖(𝒕) is the control vector. the matrices 𝑨 and 𝑩 are constant coefficient which are the system transition matrix and the input matrix respectively. in the measurement equation, 𝑪 is the output matrix, which is a constant in this system. 𝑫 is the feedthrough matrix, which is zero. the coolant air temperature 𝑻𝒄 is considered a constant parameter due to the slow variation with time. 3. objected-oriented modelling and parameter identification model-based algorithm development method firstly relies on the physical model of the system, which can be used for the validation of the proposed temperature estimation algorithm. the physical model is developed to simulate the characteristics of the asynchronous machine, including the mechanical, electrical and thermodynamic characteristics. 3.1. modelling of an asynchronous machine with a simplified thermal model the physical model consists of two parts in modelica® [13]. one is the squirrel cage asynchronous machine with six types of losses which is provided by the modelica standard library, and the other is the simplified thermal model. the electronic and magnetic behaviours as well as six parts of the machine losses can be simulated with the asynchronous machine model. figure 2 shows the simplified thermal model, which is based on the figure 1. the heat sources of the machine are from the red thermal port, where imports the losses of stator winding 𝑷𝒔𝒔, the rotor cage 𝑷𝒓𝒄, the stator core 𝑷𝒔𝒄, the rotor core, friction losses and stray load losses into the thermal circuit [11]. there is no core in the squirrel cage rotor, so the losses of the rotor core are zero. the friction losses and the stray load losses, which are not contributed to the thermal transfer, are dissipated to the environment. the mentioned three parts are represented as “rotorcore”, “fiction” and “strayload” in figure 2. the components named in “cstatorwinding”, “crotorcage”, “cstatorcore” are capacitors respectively. the components named in “gstatorwinding”, “grotorcage”, “gstatorcore” are the reciprocal of resistances respectively. the blue components below are the air cooling system. power losses are injected into the thermal model by enabling an internal thermal port of the machine model. all the losses can be imported to the thermal network. the complex simulation model in figure 3 is built in dymola [14], where the dynamic behaviour of the system can be simulated. the temperatures of stator windings, the rotor cage and the stator core can be simulated by the thermal model. 3.2. parameter identification of the model the parameters of the test bench are identified in [15], [16]. the parameters of the asynchronous machine are listed in table 3 in appendix. the thermal resistances can be calculated directly figure 3. asynchronous machine and simplified thermal model. figure 2. simplified thermal model. acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 8 based on the state-space equations (1) (2) (3). full-load test are performed to identify the thermal resistances. after the heat exchanges among ambient and machine’s components are stable, the derivatives of temperature with respect to time will be zero. by measuring the end temperature of 𝑇𝑠𝑠 , 𝑇𝑟𝑠 , 𝑇𝑠𝑠 and 𝑇𝑠 and calculating of power losses in machine, the resistances can be derived from the equations (1) (2) (3). table 4 lists the thermal resistances. the thermal capacitances cannot be computed directly from the equations (1) (2) (3). however, it is possible to utilize optimization methods to determine the best-fit thermal capacitances using genopt, an optimization program for the minimization of a cost function. the detailed description of the capacitance identification can be referred to a bachelor thesis. table 5 lists the capacitances. 3.3. validation of the model the model must be at first validated for the simulation. the literature [19] provides several methods to prove the validity of the presented load models, which are all relied on the mechanical output power. generally, the torque-speed (or torque-slip) characteristics of a three-phase asynchronous machine can also indicate the motor performance at any rotor operating speed according to [20]. therefore, four different characteristic curves of the measurement and the simulation results are shown in figure 4 under various load torque. the first three characteristic curves in figure 4 show good coincidence of the simulated curve and measured one. nevertheless, it is also to be noticed, that simulated input current is a little bit higher than the measured one, which may lead to excessive temperature rising in the thermal model. thermal model plays an important role in temperature monitoring. combined together with the asynchronous machine model, the validation of the thermal model is performed. all the machine and thermal model related parameters are set to be the identified parameters based on the test bench. two experiments are tested for the model validation. one is the continuous full-load s1 and the other is the intermittent load s6 which aims to confirm that the thermal model could also represent the temperature of asynchronous machine under other load conditions. the temperatures of the machine on the test bench are measured and compared with the simulated temperatures in the model. the comparisons indicate that the physical model can be used for model-based algorithm development, which will be introduced in section 5. 4. kalman filter algorithm kalman filter is a set of mathematical equations that provides an efficient computational (recursive) means to estimate the state of a process, in a way that minimizes the mean of the squared error [3]. in general, both the process noise and the measurement noise should be taken into account in the system model and measurement model. 𝑥(𝑘 + 1) = 𝑓(𝑥(𝑘), 𝑢(𝑘)) + 𝜔(𝑘) (19) 𝑧(𝑘) = ℎ𝑥(𝑘) + 𝑣(𝑘) (20) it is assumed that the process noise 𝜔𝑘and the measurement noise 𝑣𝑘 are independent of each other, random white gaussian noise with zero mean. their variance can be described by covariance matrices 𝑄 and 𝑅 respectively. they can be defined as: 𝐸[𝜔𝑘𝜔𝑘 𝑇] = 𝑄 (21) 𝐸[𝑣𝑘𝑣𝑘 𝑇] = 𝑅 (22) where 𝑄 is a 4×4 positive semi-defined constant matrix and 𝑅 is constant. 4.1. the prediction stage of kalman filter kalman filter estimates a process by using a feedback control: the filter estimates the process state at some time and then obtains feedback in the form of (noisy) measurements [3]. as such, the equations of kalman filter can be divided into two groups: prediction equations and correction equations. the equations of prediction stage shown in (23) and (24) are responsible for projecting forward (in time) the current state and error covariance estimates to obtain a priori estimates for the next time step. equation (23) is used for updating the state vector from previous sampling time 𝒌 − 𝟏to current time 𝒌. the equation (24) is state of updating error covariance matrix. 𝒙�𝒌 − = 𝑨𝒅𝒙�𝒌−𝟏 − + 𝑩𝒅𝒖𝒌−𝟏 (23) 𝑷�𝒌 − = 𝑨𝒅𝑷𝒌−𝟏𝑨𝒅 𝑻 + 𝑸 (24) the model (10) (11) is a time-continuous system which cannot be processed by computer. euler's approximation is used to discrete the model, so that the sampled data can be used in the kf algorithm. the discrete of the system can be defined as equation (25) 𝒙(𝒌)−𝒙(𝒌−𝟏) 𝝉 = 𝑨𝒙(𝒌 − 𝟏) + 𝑩𝒖(𝒌 − 𝟏) (25) by simplifying the equation (25), a new equation can be expressed as: 𝑥(𝑘) = (1 + 𝜏𝐴)𝑥(𝑘 − 1) + 𝜏𝐵𝑢(𝑘 − 1) (26) as 𝐴 and 𝐵 are the matrix, the time-discrete model is 𝑥(𝑘) = 𝐴𝑑𝑥(𝑘 − 1) + 𝐵𝑑𝑢(𝑘 − 1) (27) where 𝐴𝑑 = 𝐸 + 𝜏𝐴 and 𝐵𝑑 = 𝜏𝐵, 𝐸 is 4×4 unit matrix, 𝐶𝑑 is equal to 𝐶, 𝜏 is the sampling time. ad = ⎣ ⎢ ⎢ ⎢ ⎢ ⎡1 − rswτ csw 0 rswτ csw 0 0 1 − rrcτ crc rrcτ crc 0 rswτ csc rrcτ csc 1 − (rsw+rrc+rsc)τ csc rscτ csc 0 0 0 1 ⎦ ⎥ ⎥ ⎥ ⎥ ⎤ (28) figure 4. thermal network of the asynchronous machine. 0 1000 2000 3000 4000 2 4 6 8 mechanical power-current pmech/w cu rre nt /a simulated measured 0 1000 2000 3000 4000 1400 1450 1500 speed-mechanical power pmech/w sp ee d/ rp m simulated measured 0 1000 2000 3000 4000 0 0.5 1 mechanical power-efficiency pmech/w po w er e ffi ci en cy simulated measured 1400 1450 1500 0 10 20 30 speed-torque speed/rpm to rq ue /n .m simulated measured acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 9 bd = ⎣ ⎢ ⎢ ⎢ ⎢ ⎡ τ csw 0 0 0 0 τ crc 0 0 0 0 τ csc 0 0 0 0 0⎦ ⎥ ⎥ ⎥ ⎥ ⎤ (29) 4.2. the correction stage of kf the equations of correction stage are responsible for the feedback-i.e. for incorporating a new measurement into a priori estimation to obtain an improved a posteriori estimation [3]. 𝐾𝑘 = 𝑃𝑘 −𝐻𝑘 𝑇(𝐻𝑘𝑃𝑘 −𝐻𝑘 𝑇 + 𝑅)−1 (30) 𝑥�𝑘 = 𝑥�𝑘 − + 𝐾𝑘(𝑧𝑘 − 𝐻𝑘𝑥�𝑘 −) (31) 𝑃𝑘 = (𝐼 − 𝐾𝑘𝐻𝑘)𝑃𝑘 − (32) where 𝐾𝑘 is kalman gain and is the measurement matrix 𝐻𝑘 is a row vector [0,0,0,1]. 5. model in the loop (mil) simulation and test the asynchronous machine model, connected with the simplified thermal model in dymola is compiled as a dymolablock in simulink. the dymolablock and the kf algorithm block are simulated in simulink. 5.1. the simulation model in simulink the combined model in dymola is compiled as a dymolablock in simulink. the configuration and implementation of the dymola model in simulink is described in detail by garron fish [19]. kf algorithm is implemented as an s-function, and connected with the dymolablock in matlab/simulink. in the complete simulation model, threephase stator currents are the measurement. three-phase voltages and the coolant air temperature are the control vectors. all of them can be exported from dymolablock as the input of kf. the estimated temperatures from kf can be compared with the temperature calculated by the simplified thermal model. the whole simulation model in simulink is shown in figure 5. 5.2. the mil simulation of kf algorithm the temperatures of the stator wingding, the rotor cage and the stator core can be estimated in any operating condition. for all the experiments, fixed simulation step is set to 0.1 ms. the sampling time, which is set by rate transition block, is 1 s. the initial error covariance matrix, process noise covariance matrix, and measurement covariance matrix are obtained by trial and error method: 𝑷(𝟎) = 𝒅𝒅𝒅𝒅[𝟐𝟎 𝟐𝟎 𝟐𝟎 𝟐𝟎] 𝑸 = 𝒅𝒅𝒅𝒅[𝟎. 𝟎𝟎𝟏 𝟎. 𝟎𝟎𝟏 𝟎. 𝟎𝟎𝟏 𝟎. 𝟏] 𝑹 = 𝟎. 𝟏 from the state-space equations (1) (3), losses of the thermal model largely determine the accuracy of the estimated temperatures. in order to prove the assumption, we proposed three different methods to get losses when the whole system is running. it is expected that the deviations between the simulated and estimated temperatures would be less than 1 k. the first method is to export losses directly from the dymolablock to kalman filter. the second is to calculate the losses based on the equations (4) (8), where 𝑅𝑠 is taken as a constant. the third method is to calculate the losses as the same as in the second method, but the difference is that for every simulation step, the estimated temperature of stator winding 𝑇𝑠𝑠 would be sent back to losses calculation block to compensate the ignored stator winding resistance rising defined in equation (9). for every method, two experiments are performed. one is the continuous full-load s1, which means that the machine runs at the rated condition until the temperatures of three-parts reach stable. the other experiment is called s6, which is an intermittent load with six minutes noload followed by four minutes 140 % full-load. 5.2.1. the estimation results by exported losses from dymola. losses of the stator windings 𝑷𝒔𝒔, rotor cage 𝑷𝒓𝒄 and stator core 𝑷𝒔𝒄 are exported from dymolablock and sent to kf algorithm as the inputs. under s1 condition, the maximal deviation of stator winding is 0.15 k, the rotor cage is 0.01 k and the stator core is 0.03 k. under s6 condition, the maximal deviation of stator winding is 0.26 k, the rotor cage is 0.08 k, and the stator core is 0.02 k. 5.2.2. the estimation results by calculating losses without temperature compensation. in the experiment, the resistance of the stator winding is a constant. however in the real situation, the resistance increases as the temperature rises. that is the reason why the estimated temperatures are much lower than the simulated temperatures. under the condition s1 and s6, the maximal deviation of stator winding, the rotor cage and the stator core are 9.5 k, 4.8 k and 5.2 k respectively. 5.2.3. the estimation results by calculating losses with temperature compensation. the maximal deviation of the stator winding is 0.2 k, of the rotor cage 0.35 k and of the stator core is 0.18 k. under an intermittent load s6, the maximal error of the stator winding, the rotor cage and the stator core are 0.55 k, 0.7 k and 0.3 k. 5.3. the experiment results based on the test bench after the validation of the kalman filter algorithm in simulink, experiments on the test bench are performed. all the related parameters are listed in table 3, table 4 and table 5 respectively from the dissertations [15] [16]. the temperatures of the stator winding, the stator core and the coolant air are measured by data acquisition card ni pci-6023e. the temperature of rotor cage is measured by wireless sensor network. figure 5. modelling and simulation in simulink. acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 10 all acquired three-phase currents and voltages are processed by the kf algorithm off-line. the comparisons between the measured and estimated temperatures are shown in figure 6 and figure 7. all the temperatures can be estimated accurately under s1 with the maximum error of 1.5 k, and under s6 with the maximum error of 4.5 k from the rotor cage. 6. the implementation of the temperature estimatione system on wireless sensor networ 6.1. overview of the system wireless sensor network was implemented based on ieee1451.0 [20] and ieee1451.5 [21] standard by using contiki os. the communication between ncap and wtim is through 6lowpan [20]. the wireless sensor node is a preon32 module from virtenio gmbh. it contains 32-bit arm microcontroller with 64 kb sram, 2.4 ghz wireless transceiver that is compliant to ieee 802.15.4 standard. two 12-bit analog-to-digital converters (adc) with maximum sampling rate of 1 m samples/s are provided [22]. the line current and voltage, rotor speed and coolant air temperature from three other wtims are transmitted to ncap, where the kalman filter is implemented using fixed point. figure 7 shows the structure of the entire temperature estimation system on wsn. 6.2. implementation of data acquisition system in wtims hall sensors were mounted on the conditioning board with low-pass filter to process analog three-phase currents and voltages. fir filter was implemented in a 32-bit arm microcontroller for the digital filtering using contiki os. the sampling rate is 2000 hz. however, by using etimer, the effective value could be calculated every 1 second with the block size 40 samples/block. rotary encoder, mounted to the end of the machine shaft, was connected to a conditioning board with another sensor node. the numbers of the pulse are counted and transferred into the real rotor speed in rad/s. the coolant air temperature is measured by pt1000 and transformed to the physical unit in k. when the wtims are powered on, the processes are started automatically. data are acquired by wtims and transmitted to ncap every 1 second when wtims receive the starttrigger command. 6.3. implementation of kf algorithm in ncap normally, the ncap is implemented for data acquisition from different wtims wirelessly. however, in this application, the pre-processed data from different wtims are acquired and processed as the inputs of kf algorithm. the operation of ncap can be summarized as follows: as soon as ncap is powered on, the main process started automatically. the timdiscovery command is called in a condition loop to discover the specified wtim until all the three wtims are found. after discovering the wtims, starttrigger command is broadcasted to trigger all the wtims for periodically data acquisition and transmission. then ncap waits for the data from different wtims. the received messages will be stored in the buffers which are identified by the id of wtim. for every step, data from three wtims will be passed to kf algorithm and be processed. temperatures of the stator winding, the rotor cage and the stator core were estimated on-line with the fixed-point arithmetic, which can largely improve the calculation speed of the algorithm. 7. the experiments two experiments are performed on the test bench using wsn temperature estimation system. the structure of the whole system is shown in figure 9. tim1 (transducer interface module) is used to acquire effective value of stator current and voltage, tim2 is for coolant air temperature, tim3 is for the rotor speed. the kf algorithm, which is implemented in figure 7. comparison of off-line estimation under s6. figure 6 . comparison of off-line estimation under s1. figure 8. structure of the temperature estimation system on wsn. 0 1000 2000 3000 4000 5000 6000 7000 0 50 time [s] st at or w in gd in g te m pe ra tu re [° c ] measurement estimation 0 1000 2000 3000 4000 5000 6000 7000 0 50 100 time [s] ro to r c ag e te m pe ra tu re [° c ] measurement estimation 0 1000 2000 3000 4000 5000 6000 7000 0 50 time [s] st at or c or e te m pe ra tu re [° c ] measurement estimation 0 1000 2000 3000 4000 5000 6000 7000 0 50 100 time [s] st at or w in gd in g te m pe ra tu re [ °c ] 0 1000 2000 3000 4000 5000 6000 7000 0 50 100 time [s] ro to r ca ge te m pe ra tu re [ °c ] 0 1000 2000 3000 4000 5000 6000 7000 0 50 time [s] st at or c or e te m pe ra tu re [ °c ] measurement estimation measurement estimation measurement estimation acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 11 ncap, will estimate the temperatures when receiving the input data from tims. the comparisons of the estimated and measured temperatures are shown in figure 10 and figure 11, table 1 and table 2. one is continuous full-load test s1 and another is intermittent-load s6. under s1 condition, the maximum deviation is 3.5 k from rotor cage and under s6, the maximal deviation is 3.5 k from rotor cage. 8. conclusions this paper proposed a method to estimate the temperatures of the stator winding, the rotor cage and the stator core of an asynchronous machine using kalman filter. a 4th-order kalman filter was implemented in matlab and both the simulation experiment and the test bench experiment are performed well under s1 and s6 condition. the algorithm was implemented in a ncap and three other distributed wtims were as the data acquisition systems. compared to the floatingpoint implementation, the fixed-point arithmetic has been shown to provide the same signal quality at only about onetenth of computation time. the experiments prove that the implementation is suitable for real-time temperatures monitoring on a resource limited wireless sensor node. the implemented kf algorithm is independent from the control strategy and the running conditions of the machine. that means no matter what the rotor speed is, and what the mechanical load is, as long as there are currents through the stator winding, the temperature can be estimated correctly. 9. appendix figure 11 . comparison of measured and estimated temperatures under s6. table 1. the error and nrmse of the estimated temperatures under s1. variables maximum error nrmse stator winding temperature 2.3 °c 3.2 % rotor cage temperature 3.5 °c 2.08 % stator core temperature 2 °c 2.71 % table 2. the error and nrmse of the estimated temperatures under s6. variables maximum error nrmse stator winding temperature 3.5 °c 2.69 % rotor cage temperature 3.5 °c 2.45 % stator core temperature 1.5 °c 1.36 % table 3. parameters of the asynchronous machine. parameters symbols values nominal output pm 3 kw nominal voltage v 380 v nominal frequency f 50 hz nominal torque t 20 n.m connection delta pole pair pn 2 stator resistance rs 1.9693 ω rotor resistance rr 1.8081 ω main reactance xm 52.025 ω stator leakage reactance xs 2.02 ω rotor leakage reactance xr 2.02 ω rotor's moment of inertia jr 0.01654 kg.m 2 iron loss constant kiron 0.00664 w/(rad/s) 2 figure 10. . comparison of measured and estimated temperatures under s1. rotary encoder dc motor torque/speed (hbm t30fm) inverter current /voltage current /voltage current /voltage ni pci-6023e tsw trc measurement box dc-powersupply (sm6020) ni usb-6009 tsc tc tim1 tim2 ncap rs232 usb usb tim3 pci tim4 asynchronous machine sw: stator windings rc: rotor cage sc: stator core c: coolant air figure 9. structure of test bench. 0 1000 2000 3000 4000 5000 6000 7000 0 50 time [s] st at or w in gd in g te m pe ra tu re [° c ] measurement estimation 0 1000 2000 3000 4000 5000 6000 7000 0 50 100 time [s] ro to r c ag e te m pe ra tu re [° c ] measurement estimation 0 1000 2000 3000 4000 5000 6000 7000 0 50 time [s] st at or c or e te m pe ra tu re [° c ] measurement estimation 0 1000 2000 3000 4000 5000 6000 7000 0 50 100 time [s] st at or w in gd in g te m pe ra tu re [ °c ] measurement estimation 0 1000 2000 3000 4000 5000 6000 7000 0 50 100 time [s] ro to r ca ge te m pe ra tu re [ °c ] measurement estimation 0 1000 2000 3000 4000 5000 6000 7000 0 50 time [s] st at or c or e te m pe ra tu re [ °c ] measurement estimation acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 12 acknowledgement i would like to express my appreciation to my colleagues, especially dr. jürgen funck, who built the basic wsn and helped me a lot. i would also thank ms. wenjun zhu for the thermal parameters identification. references [1] m. o. sonnaillon, g. bisheimer, c. d. angelo, and g. o. garca, “online sensorless induction motor temperature monitoring,” ieee transactions on energy conversion, vol. 25, no. 2, pp. 273–280, june 2010. [2] r. beguenane and m. e. h. benbouzid, “induction motors thermal monitoring by means of rotor resistance identification,” ieee transactions on energy conversion, vol. 14, no. 3, pp. 566–570, sep 1999. [3] g. welch and g. bishop, “an introduction to the kalman filter,” chapel hill, nc, usa, tech. rep., 1995. [4] m. ganchev, b. kubicek, and h. kappeler, “rotor temperature monitoring system,” in electrical machines (icem), 2010 xix international conference on, sept 2010, pp. 1–5. [5] g. m. b. s. ozsoy, e.e., “simultaneous rotor and stator resistance estimation of squirrel cage induction machine with a single extended kalman filter,” turk. j. elec. eng. & comp. sic., 2010. [6] y. du, t. g. habetler, and r. g. harley, pp. 229–233, april. [7] z. gao, t. g. habetler, and r. g. harley, “an online adaptive stator winding temperature estimator based on a hybrid thermal model for induction machines,” in ieee international conference on electric machines and drives, 2005., may 2005, pp. 754–761. [8] z. gao, t. g. habetler, r. g. harley, and r. s. colby, “a novel online rotor temperature estimator for induction machines based on a cascading motor parameter estimation scheme,” in diagnostics for electric machines, power electronics and drives, 2005. sdemped 2005. 5th ieee international symposium on, sept 2005, pp. 1–6. [9] c. kral, t. g. habetler, r. g. harley, f. pirker, g. pascoli, h. oberguggenberger, and c. j. m. fenz, “rotor temperature estimation of squirrel-cage induction motors by means of a combined scheme of parameter estimation and a thermal equivalent model,” ieee transactions [10] a. haumer, c. kral, v. vukovic, a. david, c. hettfleisch, and a. huzsvar, “a parametrization scheme for high performance thermal models of electric machines using modelica,” in mathmod vienna, 2012. [11] d. zeng, advances in computer science and engineering. springer publishing company, incorporated, 2012. [12] anton haumer, christian kral, hansjörg kapeller, thomas bäuml, and johannes v gragger. the advancedmachines library: loss models for electric machines. in the 7 international modelica conference, como, italy, pages 847–854. linköping university electronic press, october 2009. [13] “modelica language document,” 2017. [online]. available: https://www.modelica.org/documents [14] “dymal”, [online]: https://en.wikipedia.org/wiki/dymola [15] a. haumer, c. kral, h. kapeller, t. buml, and j. v. gragger, “the advanced machines library: loss models for electric machines,” in proceedings of the 7th modelica conference, 2009, pp. 847–854. [16] a. h. c. kral, “modelica libraries for dc machines, three phase and polyphase machines,” 4th international modelica conference, pp. 549–558, march 2005. [17] florian. döhring, “parameteridentifikation einer asynchronmaschine am prüfstand” diploma thesis, tu-berlin, july 2009. [18] wenjun. zhu, “parameter identification for the thermal model of an asynchronous machine”, bachelor thesis, tu-berlin, august 2016. [19] g. fish, “dymola-simulink interface,” 2011. [online]. available: http://www.claytex.com/blog/ dymola-simulink-interface/ [20] “ieee standard for a smart transducer interface for sensors and actuators common functions, communication protocols, and transducer electronic data sheet (teds) formats,” ieee std 1451.0-2007, pp. 1–335, sept 2007. [21] “ieee standard for a smart transducer interface for sensors and actuators wireless communication protocols and transducer electronic data sheet (teds) formats,” ieee std 1451.5-2007, pp. c1–236, oct 2007. [22] a. bock, d. liu, j. funck, a. giedymin, c. gühmann, “wireless sensor for temperature and flux measurements in an axial flux machine”, ama conference 2015-sensor 2015 and irs2 2015. [23] s. ben brahim, r. bouallegue, j. david, t. h. vuong, and m. david, “a wireless on-line temperature monitoring system for rotating electrical machine,” wireless personal communications, pp. 1–21, 2016. table 4. thermal capacitances of the simplified model. parameters symbols values stator winding capacitance csw 3000 j/w rotor cage capacitance crc 1366 j/w stator core capacitance csc 7000 j/w table 5. thermal resistances of the simplified model. parameters symbols values stator winding resistance 𝑅𝑠𝑠 13.8 k/w rotor cage resistance 𝑅𝑟𝑠 3.52 k/w stator core resistance 𝑅𝑠𝑠 15.3 k/w https://en.wikipedia.org/wiki/dymola wireless sensor network for temperatures estimation in an asynchronous machine using a kalman filter << /ascii85encodepages false /allowtransparency false /autopositionepsfiles true /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) 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can be opened with acrobat and adobe reader 5.0 and later.) >> /namespace [ (adobe) (common) (1.0) ] /othernamespaces [ << /asreaderspreads false /cropimagestoframes true /errorcontrol /warnandcontinue /flattenerignorespreadoverrides false /includeguidesgrids false /includenonprinting false /includeslug false /namespace [ (adobe) (indesign) (4.0) ] /omitplacedbitmaps false /omitplacedeps false /omitplacedpdf false /simulateoverprint /legacy >> << /addbleedmarks false /addcolorbars false /addcropmarks false /addpageinfo false /addregmarks false /convertcolors /converttocmyk /destinationprofilename () /destinationprofileselector /documentcmyk /downsample16bitimages true /flattenerpreset << /presetselector /mediumresolution >> /formelements false /generatestructure false /includebookmarks false /includehyperlinks false /includeinteractive false /includelayers false /includeprofiles false /multimediahandling /useobjectsettings /namespace [ (adobe) (creativesuite) (2.0) ] /pdfxoutputintentprofileselector /documentcmyk /preserveediting true /untaggedcmykhandling /leaveuntagged /untaggedrgbhandling /usedocumentprofile /usedocumentbleed false >> ] >> setdistillerparams << /hwresolution [2400 2400] /pagesize [612.000 792.000] >> setpagedevice influences of shaker armature dynamics on periodic force measurement acta imeko issn: 2221-870x september 2019, volume 8, number 3, 42 – 47 acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 42 influences of shaker armature dynamics on periodic force measurement saher r. hassan1,2, ch. schlegel1, r. kumme1 1 physikalisch-technische bundesanstalt, bundesallee 100, 38116 braunschweig, germany 2 national institute for standards (nis), giza, egypt section: research paper keywords: dynamic force; periodic force calibration; shaker armature. citation: saher r. hassan, ch. schlegel, r. kumme, influences of shaker armature dynamics on periodic force measurement, acta imeko, vol. 8, no. 3, article 8, september 2019, identifier: imeko-acta-8 (2019)-3-8 editor: dušan agrež, university of ljubljana, slovenia received february 22, 2018; in final form july 19, 2019; published september 2019 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: saher r. hassan, e-mail: saher.r.hasan.ext@ptb.de 1. introduction the measurement of dynamic force is widely used in many industrial applications, such as material testing, production processes, and vehicle dynamics. therefore, a number of national metrology institutes have developed periodic force calibration facilities [1]-[3]. the dynamic behaviour of a periodic calibration is affected by three main factors: the internal structure of the force transducer; the mechanical coupling between the transducer and the shaker armature on one side and between the transducer and the top mass on the other side; and the dynamic behaviour of the shaker armature. the former factor introduces a periodic input excitation force to the base of the force transducer. this force is directly connected to the shaker armature’s dynamic parameters. the main goal of the calibration process is to estimate the dynamic parameters of the force transducers, such as stiffness and damping, in addition to their dynamic sensitivity. the stiffness 𝑘𝑓 and damping 𝑏𝑓 of the force transducer can be estimated according to equation (1) [4], which describes the force transmissibility function. where �̈�𝑓 and �̈�𝑠 are the measured acceleration values of the load mass attached to the force transducer and the shaker armature, respectively. 𝑚𝑡, 𝑘𝑓, and 𝑏𝑓 are the top mass, stiffness, and damping coefficient of the force transducer, respectively. these parameters can be estimated only if the acceleration is measured at two points: on the top of the load mass and at the base of the force transducer. one of the main limitations of this approach is that the measurement of the acceleration at the base of the force transducer is actually performed on the shaker armature and not directly under the force transducer. this actual acceleration measurement is affected by the acceleration distribution over the shaker armature. so far, research has tended to focus either on the modal analysis of the measurement setup with the assumption of a lumped mass shaker armature [5] or on the force transducer only without the shaker and the load mass [6]. to investigate the dynamic behaviour of the complete mechanical structure of a periodic force measurement setup, a finite element (fe) model has been developed and verified to carry out the modal and harmonic analyses. the model takes the complete mechanical structure of the setup into consideration, including the force transducer, load mass, and shaker armature as in a real application. �̈�𝑓 �̈�𝑠 = √ 1 + ( 𝑏𝑓 𝑘𝑓 ∙ 𝜔) 2 (1 − 𝑚𝑡 𝑘𝑓 ∙ 𝜔2) 2 + ( 𝑏𝑓 𝑘𝑓 ∙ 𝜔) 2 (1) abstract in this article, we investigate the influence of the shaker armature’s mechanical structure on periodic force measurement, with particular emphasis on measurement uncertainty. in order to perform modal and harmonic analyses of the measurement setup, a finite element model was iteratively developed. the model was validated by measurements with a scanning vibrometer. the results show that the dynamic behaviour of the shaker armature should be taken into consideration before carrying out a periodic force measurement. mailto:saher.r.hasan.ext@ptb.de acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 43 2. methodology a flow diagram of the procedure that has been followed to undertake this research is illustrated in figure 3, which was first published in [7]. a photograph of the mechanical portion of the experimental arrangement is provided on the left-hand side of figure 1, while a 3d model of the mechanical structure is shown on the righthand side of the same figure. more details about the measurement setup can be found in [8]. a 100 mm high hollow aluminium bar was used as a force transducer. the bar’s outer diameter is 26 mm, with a wall thickness of 2 mm. the bar was mounted onto the shaker armature using a mechanical thread adapter. a 80 mm diameter and a 97 mm high brass cylinder with a mass value of about 4 kg was used as a load mass. this load mass has a threaded hole in the centre at one end, which is attached to the bar using a mechanical thread adapter. the vertical acceleration was measured at 69 points. these points are distributed over the entire top surface of the load mass, as shown in figure 1. pseudorandom excitation was applied to the shaker with a predetermined frequency range and amplitude in order to obtain the frequency response of the whole mechanical structure. as the acceleration distribution is only measured on top of the load mass by measuring the vertical acceleration, only the modes that have longitudinal displacement/acceleration components can be detected. an fe model has been developed to perform the modal and harmonic analyses of the complete mechanical structure of the periodic force measurement setup. the mechanical structure includes the force transducer, load mass, and shaker armature. the mechanical properties of the three parts were taken from tabulated data. all threaded connections were assumed to be rigid. since the estimation of the damping coefficient is not a focus of this study, the fe model assumes the absence of damping; therefore, it is predicted that a steep frequency response curve and relatively high resonance amplitudes will occur. to perform modal analyses for all possible vibration modes, fixed boundary conditions were provided at the base of the shaker armature, without any other boundary conditions on the rest of the structure. harmonic analysis was performed by adding harmonic excitation signals to the base of the shaker armature. the frequency response of the averaged acceleration on the top surface of the load mass was visualised in order to simulate the real measurement setup as realistically as possible and obtain comparable results. a comparison between the measured and simulated longitudinal resonances of the shaker armature has been used to verify the fe model proposed. ansys software was used to perform the fe calculations. figure 1. photograph of a periodic measurement setup (left) and the 3d model (right). 13 22 31 40 49 6 14 23 32 41 50 58 1 7 15 24 33 42 51 59 65 2 8 16 25 34 43 52 60 66 3 9 17 26 35 44 53 61 67 4 10 18 27 36 45 54 62 68 5 11 19 28 37 46 55 63 69 12 20 29 38 47 56 64 21 30 39 48 57 figure 2. distribution of the scan points on the load mass. 69 points are uniformly distributed over the load mass surface. the scanning vibrometer measures the vertical acceleration of each point. results measurement setup simulation measurement v e ri fi c a ti o n modal analysis harmonic analysis uncertainty analysis mass f t shaker arnature modal & harmonic analysis (no damping) pseudorandom excitation scanning vibrometer 69 scan points different vibration modes frequency response of vertical acceleration acceleration distribution over load mass l o n g it u d in al m o d e s o f sh ak e r ar m at u re figure 3. a flow diagram showing the methodology of the work. acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 44 3. results and discussion the results are introduced in this section in the following order: verification of the fe model, modal analysis, and harmonic analysis. verification of fe model figure 4 (top) represents the longitudinal mode of the shaker armature using the fe model. the simulated resonance frequency of the shaker armature is 2481.3 hz, and it deviates about 1.3% from the measured resonance frequency. figure 4 (bottom) shows the measured frequency response using pseudorandom excitation in the range of 0-10 khz. as can be seen in figure 4 (bottom), the measured resonance frequency is 2450 hz. modal analysis of the measurement setup figure 5 shows the spatial displacement for the different dynamic modes of the simulated setup in the range of 0-2,600 hz. these modes can be categorised into tilting, torsion, and longitudinal modes. in titling modes, one element of the structure (e.g. load mass) tilts relative to the other two elements. the first and second modes show that the load mass tilts relative to the force transducer. the 4th, 5th, 8th, and 9th modes represent the tilting of both the load mass and the force transducer relative to each other. the tilting of the shaker armature is represented in the 10th and 11th modes. in torsion modes, there is a difference in the spatial displacement over the radial direction of either the load mass (as shown in the 3rd mode) or the shaker armature (as shown in the 6th mode). figure 4. the fe modal analysis of the shaker armature with the longitudinal resonance (top) and the measured frequency response (bottom). mode #1: 76.08 hz mode #2: 76.427 hz mode #3: 258.14 hz mode #4: 782.39 hz mode #5: 783.55 hz mode #6: 877.4 hz mode #7: 928.42 hz mode #8: 1068.9 hz mode #9: 1072.4 hz mode #10: 2413.9 hz mode #11: 2414.4 hz mode #12: 2540.1 hz figure 5. fe modal analysis of the complete measurement setup including force transducer and load mass mounted on the shaker armature. the analysis contains 12 modes in a frequency range of 0-2600 hz. the colour code gives the displacement values whereby red indicates high displacement and blue indicates zero displacement. note the exaggeration of the displacement values. acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 45 vertical displacement/acceleration components are assumed to be close to zero in the torsion modes. two longitudinal modes (main system resonances) have been noted. the first resonance, which corresponds to the force transducer, appears in the 7th mode, and the second one, which corresponds to the shaker armature, appears in the 12th mode. figure 6 shows the vertical acceleration distribution over the load mass at certain frequencies (written below each figure). these frequencies were selected where certain modes can obviously be observed. the acceleration values are normalised to the maximum positive acceleration value at each frequency status. there is a significant dispersion of the vertical acceleration distribution of the tilting modes rather than longitudinal modes and normal operating frequencies; thereby, a good agreement has been achieved between the measured and the simulated results. harmonic analysis figure 7 shows the measured and simulated frequency resonance of the complete structure. the results show a good agreement between the simulated and measured resonance frequencies of the system, with a deviation of about 2.3 %. figure 7 (bottom) reveals that there has been a sharp drop in the fe model amplitudes far from the resonances caused by the assumption of an absence of damping in the fe model. the consideration of damping in the fe model might not significantly improve the results, as the aim of the study is to detect the position of different vibration modes to prevent 288 hz: torsion mode 456 hz: tilting mode 731 hz: tilting mode 787 hz: tilting mode 950 hz: longitudinal mode 1069 hz: tilting mode 2488 hz: longitudinal mode 294 hz: normal frequency 0 10 20 30 40 50 60 70 -1.0 -0.5 0.0 0.5 1.0 288 hz 456 hz 731 hz 787 hz 950 hz 1069 hz 2488 hz 294 hzn o rm a li z e d a c c e le ra ti o n point number (--) figure 6. normalised measured vertical acceleration distribution on the load mass of different frequencies (top), showing high variations for tilting modes (bottom). crosses represent tilting modes, while circles and squares represent the other modes. the values of acceleration are normalised to the maximum positive values in each frequency status. 950 2481.25 500 1000 1500 2000 2500 3000 0.0 0.2 0.4 0.6 0.8 1.0 frequency (hz) n o rm a li z e d a m p li tu d e 928.39 2547.3 500 1000 1500 2000 2500 3000 0.0 0.2 0.4 0.6 0.8 1.0 frequency (hz) n o rm a li z e d a m p li tu d e figure 7. harmonic analysis in the range of 0-3,000 hz of the measurement setup using the fe model due to harmonic excitation in the longitudinal direction (bottom) and the measured acceleration resonance over the load mass using pseudorandom excitation (top). acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 46 these modes during the calibration of dynamic force transducers. another reason is that the materials of the system components are metals, which have a light damping coefficient. compared with the second resonance, the first resonance (which represents the fundamental frequency of the force transducer) exerts higher amplitudes stemming from the measurement/visualisation of the frequency response over the load mass. it is almost certain that the second resonance (which represents the shaker’s resonance) amplitude would be higher than that of the first resonance if the frequency response were measured/visualised on the top surface of the shaker armature. uncertainty analysis it is obviously predicted that the calibration of force transducers within the tilting modes will lead to higher contributions of type a uncertainty, and hence, the expanded uncertainty will significantly increase. it is well known that the main uncertainty contribution in the periodic force calibration of force transducers comes from the acceleration measurement of the load mass [9]. figure 8 shows the relative standard uncertainty of the measured acceleration on the top surface of the load mass at the same selected frequencies as in section 3.3. it should be noted that the uncertainty given above does not include the contribution influenced by the measuring equipment. a typical calibration of a dynamic force transducer is shown in figure 9. the calibration has been performed according to the procedure outlined in [1] using two different load masses, 2 and kg, with the corresponding expanded uncertainty of the individual sensitivity points. the results show the effect of the tilting modes on the resulting uncertainties. in addition, the variation in the acceleration distribution at the top of the load mass increases after resonance (around 1 khz) of the applied load masses due to the increasing effect of the rocking modes within higher frequencies. 4. conclusions in this article, the mechanical influences of the shaker armature on the periodic force measurement have been thoroughly investigated. because the investigations have been performed through the establishment of a measurement setup and a new fe model proposal, the following conclusions can be drawn: a. a new numerical model of the complete mechanical structure was developed. b. a good agreement between the simulated and measured results has been achieved. a deviation of only about 1.3 % between the measured and simulated results has been observed. c. the reasons for the small deviation in the simulated results may be due to using tabulated rather than measured material properties, the assumptions of boundary conditions, or the assumption of rigid connections. d. based on the given results, the tilting modes of the complete mechanical structure introduce a significant impact on the acceleration distribution over the load mass and hence on the expanded uncertainty. e. the accuracy of the fe model can be enhanced through the inclusion of the damping properties. acknowledgement the authors gratefully acknowledge support from the braunschweig international graduate school of metrology b-igsm. the authors would also like to thank their colleague holger kahmann from ptb for his support in fem using ansys software. references [1] c. schlegel, g. kieckenap, b. glöckner, a. buß, r. kumme, traceable periodic force calibration, metrologia 49, 3 (2012) pp. 224-235. [2] n. medina, j. de vicente, force sensor characterization under sinusoidal excitations, sensors (switzerland), 14, 10 (2014) pp. 18454-18473. online [accessed 20190912]: https://www.mdpi.com/1424-8220/14/10/18454/pdf [3] n. vlajic, a. chijioke, traceable dynamic calibration of force transducers by primary means, metrologia, 53, 4 (2016) pp. s136s148. [4] r. kumme, investigation of a primary method for a dynamic calibration of force measuring instruments: a contribution to reduce the measuring uncertainty, doctoral thesis, in german, ptb, 1996. [5] c. schlegel, g. kiekenap, h. kahmann, r. kumme, mechanical influences in sinusoidal force measurement, acta imeko, 4, 2 (2015), pp. 57-61. online [accessed 20190912]: http://dx.doi.org/10.21014/acta_imeko.v4i2.206 0 500 1000 1500 2000 2500 0.1 1 10 100 0.5 0.8 10.0 81.9 24.4 0.2 56.7 1.0 r e l. s t. u n c e rt a in ty ( % ) frequency (hz) rel. st. uncertainty figure 8. the relative standard uncertainty of the measured acceleration on the top of the load mass. the uncertainty was obtained by averaging 69 acceleration measuring points. in addition, note the log scale of the y-axis. figure 9. typical dynamic sensitivity of a dynamic force transducer using two different load masses, 2 and kg, with the corresponding expanded uncertainty of the individual sensitivity points. the results show the effect of the tilting modes on the resulting uncertainties. 6,5e-05 8e-05 10 100 1000 s e n si ti v it y ( m v /v /n ) frequency (hz) 2 kg 4 kg tilting modes 6.5e-05 https://www.mdpi.com/1424-8220/14/10/18454/pdf http://dx.doi.org/10.21014/acta_imeko.v4i2.206 acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 47 [6] m. kobusch, s. eichstädt, challenges of the model-based dynamic calibration of force transducers – a case study, proc. of the xxist imeko world congress, prague, czech republic, 30 august 4 september 2015. online [accessed 20190912]: https://www.imeko.org/publications/wc-2015/imeko-wc2015-tc3-053.pdf [7] s. r. hassan, c. schlegel, r. kumme, mechanical influences of the shaker armature on periodic force measurement, proc. of the imeko 23rd tc3, 13th tc5 and 4th tc22 conferences, helsinki, finland, 30 may 1 june 2017. online [accessed 20190912]: https://www.imeko.org/publications/tc3-2017/imeko-tc32017-021.pdf [8] c. schlegel, g. kieckenap, r. kumme, application of a scanning vibrometer for the periodic calibration of force transducers, proc. of the xxth imeko world congress, busan, republic of korea, 9 12 september 2012. online [accessed 20190912]: https://www.imeko.org/publications/wc-2012/imeko-wc2012-tc3-o8.pdf [9] ch. schlegel, g. kiekenap, r. kumme, uncertainty contributions in sinusoidal force measurement, proc. of the imeko 22nd tc3, 12th tc5 and 3rd tc 22 international conferences, cape town, south africa, 3 6 february 2014. online [accessed 20190912]: https://www.imeko.org/publications/tc3-2014/imeko-tc32014-014.pdf https://www.imeko.org/publications/wc-2015/imeko-wc-2015-tc3-053.pdf https://www.imeko.org/publications/wc-2015/imeko-wc-2015-tc3-053.pdf https://www.imeko.org/publications/wc-2015/imeko-wc-2015-tc3-053.pdf https://www.imeko.org/publications/wc-2015/imeko-wc-2015-tc3-053.pdf https://www.imeko.org/publications/tc3-2017/imeko-tc3-2017-021.pdf https://www.imeko.org/publications/tc3-2017/imeko-tc3-2017-021.pdf https://www.imeko.org/publications/tc3-2017/imeko-tc3-2017-021.pdf https://www.imeko.org/publications/tc3-2017/imeko-tc3-2017-021.pdf https://www.imeko.org/publications/wc-2012/imeko-wc-2012-tc3-o8.pdf https://www.imeko.org/publications/wc-2012/imeko-wc-2012-tc3-o8.pdf https://www.imeko.org/publications/wc-2012/imeko-wc-2012-tc3-o8.pdf https://www.imeko.org/publications/wc-2012/imeko-wc-2012-tc3-o8.pdf https://www.imeko.org/publications/wc-2012/imeko-wc-2012-tc3-o8.pdf https://www.imeko.org/publications/wc-2012/imeko-wc-2012-tc3-o8.pdf https://www.imeko.org/publications/tc3-2014/imeko-tc3-2014-014.pdf https://www.imeko.org/publications/tc3-2014/imeko-tc3-2014-014.pdf https://www.imeko.org/publications/tc3-2014/imeko-tc3-2014-014.pdf https://www.imeko.org/publications/tc3-2014/imeko-tc3-2014-014.pdf https://www.imeko.org/publications/tc3-2014/imeko-tc3-2014-014.pdf https://www.imeko.org/publications/tc3-2014/imeko-tc3-2014-014.pdf microsoft word 318-2478-1-le-1-rev acta imeko  issn: 2221‐870x  november 2016, volume 5, number 3, 55‐63    acta imeko | www.imeko.org  november 2016 | volume 5 | number 3 | 55  model parameter identification from measurement data as a  prerequisite for dynamic torque calibration –   measurement results and validation  leonard klaus  physikalisch‐technische bundesanstalt, bundesallee 100, 38116 braunschweig, germany      section: research paper   keywords: mechanical modelling; model‐based calibration; linear and time invariant system  citation: leonard klaus, model parameter identification from measurement data as a prerequisite for dynamic torque calibration – measurement results  and validation, acta imeko, vol. 5, no. 3, article 9, november 2016, identifier: imeko‐acta‐05 (2016)‐03‐09  editor: paolo carbone, university of perugia, italy  received february 5, 2016; in final form august 5, 2016; published november 2016  copyright: © 2016 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: this work was part of the joint research project ind09 traceable dynamic measurement of mechanical quantities of the european metrology  research programme (emrp). the emrp is jointly funded by the emrp participating countries within euramet and the european union  corresponding author: leonard klaus, e‐mail: leonard.klaus@ptb.de    1. introduction  several applications with dynamic torque excitation require traceable measurement. at present, only standards and procedures for the static calibration of torque transducers exist. static calibration is an insufficient base for an analysis of dynamic measurements in terms of measurement uncertainties and influences from dynamic signal components. to be able to describe the dynamic influences of a torque transducer on a measurement set-up and vice versa, a corresponding calibration is mandatory [1]. therefore, a measuring device and procedures for a dynamic characterisation of torque transducers were developed in the context of a joint european research project [2]. this paper is a substantially extended version of a contribution to the xxi imeko world congress 2015 [2] and includes additional measurement results and analyses for the validation of the correctness of the identified parameters. the focus of this contribution is to demonstrate that a model parameter identification technique can be applied to identify parameters of a torque transducer from measurement data. for this purpose, parameter estimation results are validated by means of independent measurements and additional measurements with a modified set-up. a measurement uncertainty evaluation for the identified parameters will be covered in a separate publication, as it would be significantly beyond the scope of this article. 2. dynamic torque measuring device  the measurement principle of the dynamic torque measuring device (depicted in figure 1) is based on newton's second law. the product of a known static mass moment of inertia of a body and a measured time-dependent angular acceleration equals the time-dependent torque: abstract  the dynamic calibration of torque transducers requires the modelling of the measuring device and of the transducer under test. the  transducer's dynamic properties are described by means of model parameters, which are going to be identified from measurement  data. to be able to do so, two transfer functions are calculated. in this paper, the transfer functions and the procedure for the model  parameter identification are presented. results of a parameter identification of a torque transducer are also given, and the validity of  the  identified  parameters  is  analysed  by  comparing  the  results  with  independent  measurements.  the  successful  parameter  identification is a prerequisite for a model‐based dynamic calibration of torque transducers. acta imeko | www.imeko.org  november 2016 | volume 5 | number 3 | 56  ⋅ . (1) the measuring device consists of a rotatable vertical shaft assembly, on which all essential components are arranged in series. at the bottom, a rotational exciter generates a forced excitation by means of sinusoidal oscillations. the torque transducer under test (device under test, dut) is arranged on top of the exciter between two coupling elements. these couplings are designed to be both torsionally stiff and compliant for parasitic bending moments and axial loads simultaneously. at the top, the arrangement of the dut and the couplings is followed by an angular grating disk for the angular acceleration measurement and an air bearing to prevent axial loads acting on the components of the drive shaft. the angle position at the top ( ) is measured by means of a laser doppler vibrometer and the radial grating disk. the angular acceleration at the bottom ( is measured by an angular accelerometer embedded in the rotor of the rotational exciter. 3. modelling  the dynamic behaviour of the transducer under test is described by a linear time-invariant (lti) model. it is based on the mechanical design of typical strain gauge torque transducers. the model consists of two rigid mass moment of inertia elements (mmoi) which are connected by a massless torsional spring and damper in parallel. a sole modelling of the transducer under test is not sufficient, because of the fact that the dynamic behaviour of torque transducers can be influenced by the coupled components. torque transducers are always coupled to their mechanical environment at both sides which causes a bidirectional influence on the dynamic behaviour of the torque transducer and the coupled mechanical components (which are always arranged in some type of drive assembly). to be able to identify the model parameters of the dut, the modelling was expanded from the model of the transducer to a model of the whole measuring device, which is the mechanical environment in the case of the calibration. the model again consists of mass moment of inertia elements, torsional springs and dampers, assuming the lti behaviour of the measuring device. it is based on the mechanical design of the components of the drive shaft. the representation of the model components and the corresponding components of the measuring device are given in figure 2. this model can be described by a set of inhomogeneous ordinary differential equations (odes). with the mass moment of inertia matrix , the torsional stiffness matrix and the damping matrix there follows ⋅ ⋅ ⋅ . (2) the angle vector describes the angle excitations at different positions in the model; its derivatives and represent the angular velocity and angular acceleration, respectively. the load vector describes the excitation of the rotational exciter. based on this equation system, the model parameter identification will be carried out. the excitation signals chosen are monofrequent sinusoids. with these harmonic waveforms, not all necessary angle position, angular velocity and angular acceleration data has to be derived independently or by numerical differentiation / integration but can be calculated as follows: ⋅ e , ⋅ e , (3) ⋅ e , where √ 1 denotes the imaginary number. measurements are not possible at all angle positions given in (2) and depicted in figure 2. technically, measurements at the positions and cannot be carried out, because sufficiently precise measurement components would require elaborating adjustments after each mounting and dismounting of the transducer under test. to gather the information necessary for parameter identification, the output signal of the transducer is used as an indicator for the difference in the torsion angle above and below the transducer. this assumption is valid, figure 1. dynamic torque measuring device with the different components arranged vertically on top of the exciter.   figure 2. dynamic torque measuring device (left) and corresponding model  representation (right) of the measuring device (blue) and the dut (orange).  acta imeko | www.imeko.org  november 2016 | volume 5 | number 3 | 57  because it is assumed that the output signal of the transducer is proportional to its torsion δ giving ∝ δ . (4) during calibration, three measurement signals are acquired simultaneously: the angle position at the top, the angular acceleration at the bottom, and the voltage output of the transducer under test. these signals will be processed to determine the magnitude and phase of each harmonic and monofrequent signal by means of a sine fit. 4. known and unknown model parameters  a prerequisite for the model parameter identification of the unknown parameters of the transducer under test is a sufficiently low number of unknown model parameters of the system. to this end, the necessary parameters of the measuring device were determined in advance. the properties of the measuring device will not change for different transducers, and therefore needed to be determined only once. three auxiliary measurement set-ups for the measurement of the mass moment of inertia, torsional stiffness and rotational damping were developed [3]-[5] and the corresponding properties of the measuring device's components were determined. the only model parameters to remain unknown prior to the model parameter identification are the parameters of the transducer under test. 5. model parameter estimation  the model parameter identification is carried out based on transfer functions. generally, a transfer function i describes the relation between input i and output i of a system as follows i . (5) a model parameter estimation based on such a transfer function is carried out by approximating a best set of parameters of the model function to the measured data. this set of parameters is characterised by giving the lowest deviations of model assumptions and measurement data. however, due to measurement uncertainty deviations, which will always occur, an exact measurement of the different input quantities i , i will not be possible. the measured data will always be disturbed randomly due to measurement errors. the measurement data i , i is therefore modelled as multivariate random numbers with estimated state of knowledge probability density functions (pdf). figure 3 depicts the relation of the measured input and output quantities and a model transfer function for the general example from (5). applying this general approach to the dynamic torque calibration, two complex transfer functions of the system are calculated based on the three available signals acquired during calibration measurements. one transfer function i describes the dynamics of the top part of the measuring device giving i ⋅ (6) with the (still unknown) proportionality factor linking the voltage output of the transducer to its torsion (cf. equation (4)). the same applies to the bottom part of the measuring device giving the transfer function: i ⋅ . (7) the two transfer functions are illustrated in figure 4. the underlying ode system gives the corresponding equations for both transfer functions. each transfer function contains the unknown parameters of the transducer under test. the parameters , , , , can be derived from , while contains all the wanted parameters: , , , , and . the derivation of the model parameters from the ode system and the relations with the two transfer functions is thoroughly described in [6]. the model parameter identification is carried out based on both of the two transfer functions. the more information is taken into account for the model parameter estimation, the more reliable the outcome can be. based on the chosen estimator, knowledge about the input quantities, the output quantities, and about the parameters to be identified prior to the estimation (so-called a priori knowledge) can be considered. three typical estimators are compared regarding their demand of knowledge of the different quantities in table 1. a ‘ ’ indicates necessary information about the distribution of a quantity; a ‘ ’ indicates that it is not necessary to know the distribution. figure  4.  the  two  calculated  transfer  functions  based  on  the  acquired  measurands.   figure  3.  schematic  illustration  of  a  transfer  function  and  its  relation  to input and output.   acta imeko | www.imeko.org  november 2016 | volume 5 | number 3 | 58  the classical frequentist approach to a model parameter estimation takes no uncertainty contributions in terms of pdfs of input quantities into account. therefore, the estimated set of parameters consists of the estimated values, but not of information about their distribution. the least squares (ls) estimator and the maximum likelihood estimator are frequentist estimation approaches. the least squares estimator minimises the squared sum of residuals of the measured transfer function data , , , and of a model function . the data of the two transfer functions is merged in a data vector , i , , i , , , i for each of data points, the same applies to the model function which consists of the model representations of the two mentioned transfer functions i , , and i , , accordingly, giving the least squares estimator: arg min ∑ , i i , , . (8) the estimation result is denoted with a hat ⋅ . in the case of the model parameter estimation of torque transducers, the model function consists of a vector of parameters of the measuring device , which is known, and the vector of unknown parameters of the transducer under test , which is estimated. no weighting of the different values is applied with this estimator. the likelihood function ℓ for a vector of samples , ,…, with the sample’s pdf , for the estimation of the parameters is the joint pdf of all samples giving ℓ ∣ ∣ ∏ ∣∣ . (9) assuming normally distributed and independent input quantities for the dynamic torque application described (i.e. for all of the measurement channels merged in the data vector and therefore for both, input and output quantities of the measured transfer functions); the likelihood function becomes ℓ , i ∝ ∏ e , , , . (10) with the measurement data vector , at the corresponding angular frequency of each data point, the corresponding measurement uncertainty vector , and the model function . the likelihood function is maximised for the parameter estimation. the estimated parameters are given by argmax ℓ , i . (11) the maximum likelihood estimator with a likelihood function as given in (10) can be reduced to a weighted least squares (wls) estimator [7]. then, the parameter estimation can be carried out as follows argmin ∑ , , , , (12) applying one of the widely available wls estimators. for the given application, the induced errors are based on the different measurement quantities and need to be quantified by a measurement uncertainty evaluation. based on this analysis, a correct weighting of the input measurement channels can be carried out. the uncertainties of the estimated parameters cannot be calculated directly for the frequentist approaches. the calculation of lower uncertainty limits or of confidence intervals is not applicable for the dynamic torque calibration, because the influences of the uncertainties of the parameters of the measurement 1 and influences due to mounting and dismounting of the dut are invisible for the estimator. therefore, the uncertainty evaluation of the estimated parameters will be carried out according to the recommendations of the guide to the expression of uncertainty in measurement [8] (gum) and its supplements 1 [9] and 2 [10], respectively. a monte carlo simulation with all input pdfs will be carried out to evaluate the uncertainty of the estimated parameters. the estimation of the uncertainties will not be described in this contribution, but will be part of a dedicated publication. differently from the frequentist approach, a parameter estimation based on bayes’ statistics assumes all parameters to be uncertain. based on bayes’ theorem, the a posteriori pdf follows from the a priori pdf, the evidence's pdf and the likelihood (cf.(9)) giving a posteriori ⋅ . (13) for the parameter estimation of torque transducers, the latter equation leads to , ∣ ∣ i ℓ , . (14) it becomes obvious that uncertainties are inherent in a parameter estimation by means of a bayes’ estimator, including uncertainty contributions of the parameters of the measuring device , the contributions of the uncertainty of the measurement data, and contributions of prior knowledge of the parameters to be estimated . the prior distribution of the parameters to be estimated does not need to be known exactly prior to the estimation (although this is mentioned accordingly in literature [7]); instead reasonable initial pdfs should be available. despite all of its advantages, the bayes’ estimator is still rarely used for parameter estimation of mechanical systems. it requires much more effort than ls approaches to be implemented, because it has to be developed individually for each application, whereas for the least-squares-based approaches, an application of the widely available ls algorithms is possible. this effort is the reason why this paper focuses on the least squares approaches in a first step. however, it is planned to develop a bayes’ estimator for the application to dynamic measurements of mechanical quantities in the future. 6. implementation of the model parameter  estimation  the parameter estimation based on the measurement data is implemented in mathworks matlab and in the open source scientific computing software gnu octave. the parameter estimation uses a maximum likelihood estimator. this estimator is implemented as a weighted least squares estimator assuming normally distributed and independent input quantities (cf. equation (12)). table 1. requirements of different estimators.   knowledge about →  estimator ↓  distribution of  input quantities  distribution of the  (unknown)  parameters  bayes’ estimator      maximum likelihood  estimator      least squares estimator    acta imeko | www.imeko.org  november 2016 | volume 5 | number 3 | 59  both transfer functions (see equations (6), (7)) are used jointly for the parameter estimation, i.e. one set of parameters is estimated based on the two measured transfer functions. due to the fact that the inverse transfer function equations are more handy than the non-inverted equations (cf. [6]), the implementation of the parameter estimation is based on i and i . the weighting of the different data points of each transfer function is based on two conditions: 1. each transfer function is weighted inversely proportional to its measurement uncertainty. this measurement uncertainty is based on the uncertainty contributions of the incorporated measurement channels. the uncertainty of each transfer function is calculated by means of a quadratic summation of the two different uncertainty contributions of the input quantities for the transfer function calculation giving . these uncertainty contributions were estimated for magnitude and phase of the transfer functions. 2. the uncertainty of each measurement value is taken into account by evaluating the covariance-variance-matrix as outcome of each sine approximation applied to the time series data. the variances for the approximated parameters (the diagonal elements of the covariance-variance-matrix) are normalised and inverted for the weighting. the weights for each data point of each transfer function were calculated by multiplying the two mentioned weighting factors correspondingly for the real ( ) and imaginary part ( ). the equations representing the model parameters within the transfer functions are nonlinear in their parameters. therefore, only iterative algorithms for nonlinear regression were applicable. to avoid complex numbers in the results for the model parameters, it was necessary to constrain the algorithm to real numbers in the parameter vector. to this end, one set of approximated parameters was calculated by means of the real ( ) and imaginary parts ( ) of the two complex inverse transfer functions giving i , i , i , i . the resulting model function follows as , , ⋅ , , ⋅ , , ⋅ , , ⋅ , , , (15) with the assignment vectors , , , and the frequency vector . all vectors have the length 4 and are summarised in the matrix giving , , , , , (16) 1 0 0 0 ⋮ 1 0 0 0 1 0 0 0 0 1 0 0 ⋮ 0 1 0 0 0 1 0 0 0 0 1 0 ⋮ 0 0 1 0 0 0 1 0 0 0 0 1 ⋮ 0 0 0 1 0 0 0 1 . (17) the data vector follows as , , ⋮ , , , , ⋮ , , , , ⋮ , , , , ⋮ , , . (18) the weights for each data point are summarised in the weighting vector accordingly, giving , ⋮ , , ⋮ , , ⋮ , , ⋮ , . (19) the cost function , , , , ⋅ (20) corresponds directly with (12) and is minimised for the parameter estimation. 7. measurement results  the analysis of the feasibility of the parameter estimation was carried out with one hbm t5 10 n·m shaft type torque transducer (depicted in figure 5). the chosen transducer is a passive strain gauge transducer, the bridge signals are transmitted by means of slip rings. the signal conditioning electronics, data acquisition systems, and filters have been calibrated dynamically prior to the measurements. the electrical influences of these components of the measurement chain have been compensated for [11]. figure 5. torque transducer hbm t5 – nominal torque 10 n∙m.   acta imeko | www.imeko.org  november 2016 | volume 5 | number 3 | 60  for the measurement, sinusoidal excitations in a frequency range of about 10 hz to 1 khz can be applied. it is advantageous for the model parameter identification to include frequencies which are as high as possible, because the sensitivity to parameter changes increases with higher frequencies [6]. however, the upper frequency limit for the measurement is not only limited by the capabilities of the rotational exciter, but by the dynamic behaviour of the whole shaft assembly. beyond the (first) resonance frequency of the shaft assembly, the components arranged above the dominant spring element will be dynamically decoupled. the higher the excitation frequency is with respect to this resonance frequency, and the lower the damping of the system, the stronger the decoupling is. in practice, the angular acceleration at the top of the measuring device ( , cf. figure 1) will become too small and too disturbed to be reasonably measured for frequencies far beyond the resonance frequency. for the investigated transducer hbm t5 10 n·m, the upper frequency limit for the measurements was reached somewhat above 300 hz. the dynamic torque magnitude generated by the dynamic torque calibration device is relatively small at present and is in the range of about 0.5 n·m to 1 n·m. figure 6 shows the magnitude and phase responses of the complex transfer functions of the top and of the bottom of the measuring device i and i for both measurement data and estimation. the measurement results show that the deviations from the estimated values of the top transfer function increase for frequencies higher than the resonance frequency of the system. this is caused by the mentioned increasing decoupling of the top part of the shaft assembly for frequencies beyond the resonance frequency. the results of the model parameter estimation show a fairly good agreement of the measurement data and the outcome of the regression, which proves the validity of the model assumptions for the transducer and for the measuring device. the real and imaginary parts of the two inverse transfer functions are given in figure 7. these are the transfer functions, with which the parameter estimation was carried out. the real parts of the two transfer functions show very good agreement of measured values and fit, however, the imaginary parts reveal discrepancies between observed values and the regression data. as a next step, the correctness of the identified parameters was assessed. agreement between measurement and parameter estimation results does not automatically yield parameters that correspond with the transducer’s real properties. this may happen if the model assumption and the mechanical system do not agree, but somehow a fit is possible, e.g. if some of the parameters of the measuring device are not correct or the transducer’s mechanical design is not sufficiently represented by the model. therefore, a validation of the parameter estimation results was carried out by means of independent measurements and additional available information. this validation – as described in the next paragraphs – is an intermediate step towards a validation incorporating measurement uncertainties of all available sources. the agreement of measurement data and the results of the parameter identification will be analysed in terms of the uncertainties of both measurement data and model parameters in a subsequent publication. due to the fact that the parameters of the measuring device – which were determined with assigned measurement uncertainties – are also incorporated in the transfer function equations, a comparison of identified parameters and measurement data considering only the measurement uncertainties of the measurement data is not sufficient. 8. validation by means of independent  measurements  the identified model parameters of the investigated transducer hbm t5 are given in table 2. the parameters identified describe the mechanical properties of the transducer, which can be found – to some extent – in technical data sheets. however, this information is not reliable, because neither can one ascertain how these specifications are derived, nor are any uncertainties for the given specifications available. nevertheless, the data sheet information of the hbm t5 transducer [12] was compared with all other results. additionally, some parameters of the investigated transducer were determined by independent static measurements. these measurements were carried out with the auxiliary measurement figure 7. measurement data of the hbm t5 transducer (blue) and fit result  (red) for real ( ) and imaginary ( ) parts of  i  (top) and  i (bottom).   figure 6. measurement data of the hbm t5 transducer (blue) and fit result (red) for  i  (top) and  i  (bottom) in magnitude and phase.   acta imeko | www.imeko.org  november 2016 | volume 5 | number 3 | 61  set-ups already used for the determination of the mechanical properties of the measuring device [3]. while the torsional stiffness was measured without any modifications of the transducer, the friction of the slip rings and of the ball bearings made a measurement of the mass moment of inertia of the transducer’s rotor by means of a pendulum set-up infeasible. instead, a bare rotor shaft without bearings of a similar transducer (same type and same torque capacity) was used for that purpose. it was not possible to validate the damping coefficient by independent measurements, but due to the known low damping of transducers and the large resonance rise experienced (cf. figure 6), the influence of the damping can be assumed to be low and therefore to be less important for the dynamic behaviour of the transducer. the results of the measurements and of the manufacturer’s specifications are given in table 3. table 2 and table 3 show that the data sheet specifications of the torsional stiffness agree very well with the experimental results from the auxiliary measurement, but the results from the mass moment of inertia deviate significantly. two issues exist for the validation of the results of the mass moment of inertia parameters of the transducer: 1. only the overall mass moment of inertia can be determined by independent measurements. the top mass moment of inertia was roughly estimated by means of the geometry of the rotor and with an assumed generic density of steel of 7.8g cm⁄ giving ~ 6 kg ⋅ mm . this number is so small that even with the set-up dedicated for the determination of mass moment of inertia, the resulting measurement uncertainty would be in the same magnitude as the measured quantity. 2. the identified mass moment of inertia parameter of the bottom part of the transducer b is significantly larger than expected. this deviation is most likely caused by the friction of the slip rings, which are arranged below the sensing element of the transducer. 9. validation by means of a variation of the  properties of the device under test  to overcome these issues, an alternative validation approach was applied. the existing drive shaft assembly was modified with two different additional mass moment of inertia elements, which could be added to . the design of these mass bodies was chosen in such a way that a rigid connection minimises influences due to the mounting of the adapter. the mass moment of inertia of the mass bodies was measured independently in the same way as for the rotor of the torque transducer. one mass body is based on a modified coupling element (depicted in figure 8); it has a mass moment of inertia of 878.86 kg ⋅ mm 2 5 ⋅ 10 . furthermore one mass body is an aluminium ring (depicted in figure 9) with a mass moment of inertia of 295.64 kg ⋅ mm 2 1 ⋅ 10 . both mass bodies were used for validation measurements simulating a transducer with a correspondingly larger . the added mass moment of inertia in conjunction with the unchanged torsional stiffness of the torque transducer under test leads to lower resonance frequencies, and therefore to a decreased upper frequency limit for the dynamic measurements. the measurement results for the mass moment of inertia element using the coupling element ( 900 kg ⋅ mm ) are depicted in figure 10, and the results for the aluminium ring ( 300 kg ⋅ mm ) are shown in figure 11. the resulting magnitude and phase responses of the parameters identified agree well with the corresponding measurement result. the parameters identified are given in table 4. for both measurements with additional mass bodies, the parameters identified agree very well with the expectations. the mass moment of inertia of the top agrees excellently with the table 2. results of the parameter identification.        measurement 1  4 ⋅ 10 kg ⋅ mm 603n ⋅ m rad⁄ 3 ⋅ 10 n ⋅ m ⋅ s rad⁄ 446 kg ⋅ mm   measurement 2  8 ⋅ 10 kg ⋅ mm 607 n ⋅ m rad⁄ 1 ⋅ 10 n ⋅ m ⋅ s rad⁄ 272 kg ⋅ mm   figure 8. torque transducer hbm t5 (left) with the mass moment of inertia  element  900 kg ⋅ mm  (right).   figure  9.  moment  of  inertia  element  300 kg ⋅ mm   mounted  on  a  clamping nut.  table 3. manufacturer’s specifications and  independent measurement  results for mass moment of inertia and torsional stiffness with assigned  expanded relative measurement uncertainties u .        specifications   from data sheet  [12]  41 kg ⋅ mm   640 n ⋅ m rad⁄ measurement  results   38.6 kg ⋅ mm 2 9.0 %  638.29 n ⋅ m rad⁄ 2 0.1 % acta imeko | www.imeko.org  november 2016 | volume 5 | number 3 | 62  added mass moments of inertia. the torsional stiffness parameter matches the independent measurements even better than with the measurements of the transducer alone. is in a range similar to that of the measurements without additional mass bodies due to the influences from the friction of the slip rings. however, the reduced excitation frequency range causes higher uncertainties of the identified parameters, which will be described in a later publication. 10. summary and outlook  a model-based approach enables the identification of the dynamic properties of torque transducers from measurement data. the model is linear and time-invariant and consists of known model properties of the measuring device and unknown model properties of the transducer under test. the parameter identification is carried out using the acquired measurement data which was corrected for influences from signal conditioning electronics and the data acquisition system. the parameters are estimated by means of a maximum likelihood estimator. three signals are acquired during the calibration measurements. based on this data, two complex transfer functions are calculated. a nonlinear regression is carried out based on the two transfer functions to estimate a set of common parameters. these parameters describe the dynamic behaviour of the torque transducers under test. the measurement results of a first transducer show good agreement to the model assumptions. remaining discrepancies in the imaginary parts of the two transfer functions require further investigation. the parameter estimation based on the measurement data gives reasonable results. a validation of the identified parameters was carried out by means of independent measurements, as well as with a modified measurement set-up. the validation results agree very well with the identified parameters of the transducer and therefore confirm the results of the parameter estimation. the described validation is an intermediate step towards model-based dynamic torque calibration. the estimation of the uncertainties of the measurement and of the parameter identification, as well as the comparison of these results with independent measurements, is beyond the scope of this paper and will be covered by a dedicated publication in future. acknowlegdement the author would like to thank the company hbm – in particular dr andré schäfer – for the items provided on loan (sometimes at short notice), which reduced the time needed for the validation of the parameter estimation results. references  [1] l. klaus, th. bruns and m. kobusch, “dynamic torque calibration – necessity and outline of a model-based approach”, proc. of 5th international competition best young metrologist of coomet 2013, pp. 61–64, braunschweig, germany, june 2013, doi: 10.7795/810.20140929. [2] c. bartoli et al., “traceable dynamic measurement of mechanical quantities: objectives and first results of this european project”, international journal of metrology and quality engineering, vol. 3 (3), pp. 127–137, may 2013, doi: 10.1051/ijmqe/2012020. [3] l. klaus, “identification of model parameters of a partially unknown linear mechanical system from measurement data”, proc. of xxi imeko world congress, prague, czech republic, sep. 2015, http://www.imeko.org/publications/wc-2015/ imeko-wc-2015-tc3-050.pdf. [4] l. klaus, th. bruns and m. kobusch, “modelling of a dynamic torque calibration device and determination of model figure  10.  measurement  data  of  the  hbm  t5  transducer  with  additional mass moment of  inertia element  900 kg ⋅ mm  (blue) and fit result (red) for  i  (top) and  i  (bottom) in magnitude and phase.   figure 11. measurement data of hbm t5 with additional mass moment of inertia element  300 kg ⋅ mm  (blue) and fit result (red) for  i (top) and  i  (bottom) in magnitude and phase.   table 4. results of the parameter identification with additional mass moment of inertia elements.       hbm  t5  with  mass  body  900 kg ⋅ mm   945 kg ⋅ mm   637 n ⋅ m rad⁄   2 ⋅ 10 n ⋅ m ⋅ s rad⁄   874 kg ⋅ mm   hbm  t5  with  mass  body  300 kg ⋅ mm   311 kg ⋅ mm   635 n ⋅ m rad⁄   9 ⋅ 10 n ⋅ m ⋅ s rad⁄   239 kg ⋅ mm   acta imeko | www.imeko.org  november 2016 | volume 5 | number 3 | 63  parameters”, acta imeko, vol. 3 (2), pp. 14–18, june 2014, doi: 10.21014/acta_imeko.v3i2.79. [5] l. klaus and m. kobusch, “experimental method for the noncontact measurement of rotational damping”, proc. of joint imeko international tc3, tc5 and tc22 conference 2014, cape town, south africa, february 2014, http://www.imeko.org/publications/tc22-2014/imeko-tc3tc22-2014-003.pdf. [6] l. klaus, b. arendacká, m. kobusch and th. bruns, “dynamic torque calibration by means of model parameter identification”, acta imeko, vol. 4 (2), pp. 39–44, june 2015, doi: 10.21014/acta_imeko.v4i2.211. [7] j. schoukens, r. pintelon, identification of linear systems – a practical guideline, pergamon press, oxford, 1991. [8] bureau international des poids et mesures, evaluation of measurement data — guide to the expression of uncertainty in measurement = évaluation des données de mesure — guide pour l’expression de l’incertitude de mesure, 2008. [9] bureau international des poids et mesures, evaluation of measurement data — supplement 1 to the “guide to the expression of uncertainty in measurement” — propagation of distributions using a monte carlo method = évaluation des données de mesure — supplément 1 du “guide pour l’expression de l’incertitude de mesure” — propagation de distributions par une méthode de monte carlo, 2008. [10] bureau international des poids et mesures, evaluation of measurement data — supplement 2 to the “guide to the expression of uncertainty in measurement” — extension to any number of output quantities = évaluation des données de mesure — supplément 2 du “guide pour l’expression de l’incertitude de mesure” — extension à un nombre quelconque degrandeurs de sortie, 2011. [11] l. klaus, th. bruns and h. volkers, “calibration of bridge-, chargeand voltage amplifiers for dynamic measurement applications”, metrologia, vol. 52(1), pp. 72–81, february 2015, doi: 10.1088/0026-1394/52/1/72. [12] hottinger baldwin messtechnik gmbh, “t5 torque transducer data sheet”, version b0071-1.2, 2004. microsoft word article 7 tc4 paper 3.docx acta imeko  august 2013, volume 2, number 1, 16 – 20  www.imeko.org    acta imeko | www.imeko.org  august 2013 | volume 2 | number 1 | 16  development of a spin‐exchange relaxation free  magnetometer with a compact heating system  hyun joon lee 1,2 , kiwoong kim 2* , seong‐joo lee 2 , chan‐seok kang 2 , kwon kyu yu 2 , yong‐ho lee 2 , and  han seb moon 1   1 department of physics, busan national university, busan 609‐735, korea  2 brain and congnition measurement lab., korea research institute of standards and science (kriss), daejoen, korea      keywords: atomic vapour; magnetometer; serf;resistive heating system  citation: hyun joon lee, kiwoong kim, seong‐joo lee, chan‐seok kang, kwon kyu yu, yong‐ho lee, and han seb moon, “development of a spin‐exchange  relaxation free magnetometer with a compact heating system”, acta imeko, vol. 2, no. 1, article 7, august 2013, identifier: imeko‐acta‐02(2013)‐01‐07  editors: paolo carbone, university of perugia, italy; ján šaliga, technical university of košice, slovakia; dušan agrež, university of ljubljana, slovenia  received january 10 th , 2013; in final form july 1 st , 2013; published august 2013  copyright: © 2013 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: none reported  corresponding author: kiwoong kim, e‐mail: kwkim@kriss.re.kr    1. introduction  the atomic coherence between the ground states generated by the interaction of laser light with atoms has been applied to interesting topics such as an atomic magnetometer, frequency standards, light storage, quantum memory, and quantum cryptography [1-10]. detection of a magnetic field with an atomic magnetometer is performed by monitoring the spin precession due to the external field as observing the optical rotation signal. the principal mechanism of an atomic magnetometer can be described by the classical faraday rotation. magneto-optical rotation is an optical effect that makes the polarization plane of linearly-polarized light rotated during its propagation through a medium placed in the magnetic field. when the magnetic field is applied along the light propagation direction, we can see this magneto-optical rotation known as the faraday’s effect. the linearly-polarized light can be decomposed into left (σ-) and right (σ+) circularlypolarized light. in the presence of a longitudinal magnetic field, the zeeman sublevels are shifted and two circular polarized light beams are resonant on the transitions with different resonance frequencies, satisfying the selection rules. the zeeman splitting leads to different dispersion of two circularlypolarized components of the linearly-polarized light. consequently, the phase difference between two components drives the rotation of its polarization plane [8]. by using this phenomenon, budker et al have achieved the nmoe (nonlinear magneto-optic effects) signal with an effective resonance width of γ≈ 2π x1.3 hz [9]. in recent reports, the most sensitive atomic magnetometer is the spin-exchange relaxation free (serf) magnetometer [11]. in the serf regime, relaxation due to spin-exchange collisions is eliminated where the spin-exchange rate is much greater than the rate of larmor precession. in ref. [10] with a serf magnetometer, operating potassium (k) vapour cell at 200 oc, a sensitivity of 0.54 ft/hz1/2 was achieved. the serf magnetometer is operated with a low magnetic field and a high atomic density for a rapid spin-exchange rate. for the high atomic density, most atomic magnetometers with abstract  atomic  magnetometers  based  on  spin‐exchange  relaxation  free  (serf)  regime  typically  operate  with  high  optical  density  and  low  magnetic fields. a serf magnetometer with a compact heating system for compact and efficient temperature control is presented. a  resistive heating system with high frequency alternating current was used to achieve high optical density. the current frequency was  carefully  chosen  to  avoid  resonance  with  the  atomic  magnetometer.  this  resulted  in  a  compact  heating  system  that  also  allows  independent operation of multiple heating systems. an optical rotation signal was obtainedat200 o cand width was about 3 ntwith  pump  laser  power  of  55  mw.  bandwidth  of  the  magnetometer  with  the  compact  heater  was  40  hz.  consequently,  the  compact  resistive heating system proved to be a good alternative to the air‐flow heating system. additionally, residual field dependent signal‐ to‐noise yielded a noise level of 50 ft/hz 1/2  at 10 hz.  acta imeko | www.imeko.org  august 2013 | volume 2 | number 1 | 17  serf regime have used an air flow oven that requires bulky items such as a double-walled oven that perfectly surrounds the cell for preventing air leakage and perturbation of the optical path. moreover, an air flow system not only responses sensitively to external vibrations but also has a complex valve system to operate with an independent control on multiple parts in the heating system. in this paper, we present a serf regime magnetometer with a compact heating system using modulated current. further, we experimentally and theoretically investigate the rapid spinexchange effect in a low magnetic field. to attain a better understanding of the residual field effects, we analytically calculated the optical rotation spectra using density matrix equations assuming the spin-temperature distribution to be in the ground state of the d1 line of k. the influence of the residual fields on the resonance signal was observed for different magnetic fields, and the bandwidth as a function of the frequency of the external field was measured to estimate the influence of the resistive heating system. 2. theoretical model  the density matrix theory was used for the description of the electron spin polarization. the time evolution of the density matrix  well describes the interaction of spins with magnetic field. the density matrix equation is as follows [12, 13]:        , 21 41,2           ss ss r tti h d dt d sdse (1) where ρss  4/ is the purely nuclear part of the density matrix and h is the hamiltonian related to the magnetic moment interaction with the magnetic field, sbsi  bshf gah  , where hfa is the hyperfine constant, sg is the electron’s g-factor, b the bohr magneton, i the nuclear angular momentum, and s the spin angular momentum. here r is the optical pumping rate, sdt is the relaxation time due to spin-destruction collisions, and s is the optical pumping vector, giving the direction and the degree of circular polarization of the light. we first neglect the diffusion term for a magnetic field b in parallel to the direction of optical pumping s. under the condition that the total angular momentum (f) including the nuclear spin (i) and the number of the k atoms are conserved, we can simplify the density matrix as a spin-temperature distribution by using the expectation-maximum algorithm. deduced results show that the spin evolution in the ground state can be described by bloch equations for electron spin polarization s/sp  [11, 14, 15]: ))(( )( 12 prbs rg pq d dt d  ppsbpp p  , (2) where )(pq is the nuclear slowing down factor, which depends on the hyperfine sublevel distribution of ensembles of k atoms, and pr is the rate of depolarization due to the probe beam. for the explanation of the serf regime in the steady-state, )(pq depends on the degree of the partial polarization. 3. experimental setup  in order to investigate the optical rotation under serf regime in the d1 line of k atomic vapour, the experimental apparatus shown in fig. 1 is used. the experiment was performed with a glass cell containing k vapour, 600-torr he buffer gas to reduce the rate that atoms in the cell diffuse towards its wall, and 15-torr n2 to improve optical pumping by quenching. the cell was placed inside a three-layer set of the cylindrical mu-metal chamber. the cell consisted of two parts, a main cell and a stem, respectively. the main cell was roughly cubic, with size about 2.5 cm, and was connected to a stem acting as a reservoir. the stem was composed of a specific glass that prevents k vapour from depositing on the surface of the cell. the cell was heated up to 200 oc by the resistive heating method. the resistive heater was operated by the 25 khz modulated current source. fig. 2(a) and (b) show an overall diagram and circuit diagram of the heating system, respectively. the sine wave generated by the xr-2206 oscillator with external frequency adjustment was multiplied with the output voltage from a pid temperature controller after passing through the high pass filter. then the modulated voltage was amplified by the crown ce1000 amplifier and converted to the modulated current source. the oscillation frequency f is determined by the external timing capacitor c, between pin 5 and 6 of xr-2206, and by the timing resistor r, connected to pin 7 of the xr-2206. its oscillation frequency is given by 1/rc and can be adjusted by varying either r or c. in our experimental setup, two independent heaters were used to avoid deposition of the vapour. the temperature differences between the main cell and stem were maintained automatically, most k atoms could stay in the stem at room temperature. the density of the k vapour was about 1.4×1015 cm-3. the resistive heater was insulated by the insulation panel. the coil system in fig. 1 consisted of 16 square coils and a helmholtz coil that eliminates the residual field or generates arbitrary homogeneous fields and gradient fields. when a magnetic field was applied to the k vapour cell perpendicularly to the pump light propagation, serf condition was satisfied at near zero magnetic field. figure 1. setup of the serf magnetometer. circularly polarized light tuned  to  the  d1  line,  propagating  in  the  z‐direction  produces  ground  state  orientation  in  the  z‐direction.  optical  rotation  of  the  probe  beam  is  detected  using  a  polarimetric  method.  (pbs:  polarizing  beam  splitter,  pd: photo diode, λ/2: half wave plate, λ/4: quarter wave plate).  acta imeko | www.imeko.org  august 2013 | volume 2 | number 1 | 18  the distributed feedback (dfb) laser was used in the experiment. the optical pumping was accomplished by circularly polarized laser light propagating in the z-direction, exactly tuned to the center of the k d1 line by monitoring the shape of the signal. the pump beam power was amplified from 15 mw to 150 mw by a tapered amplifier. the wavelength of the pump laser was monitored by a wavemeter and stabilized on the k d1 line. to prevent optical feedback we used an optical isolator. we adjusted the laser power by using a polarization beam splitter (pbs) and a half wave plate (hwp). the linearly-polarized probe beam, propagating in the xdirection, was detuned from the k d1 line about several nanometres. the probe beam was generated by a single mode dfb laser and monitored by a fabry-perot interferometer. after passing the hwp and the k vapour cell, the laser beam goes through an analyzer which divides the laser beam into two orthogonal directions. the difference between the signals of the photodiodes pd1 and pd2 was measured. the vapour cell was inside a three-layer μ-metal shield that can minimize the effects of the external magnetic field including the earth’s magnetic field. after degaussing, the residual fields inside the shield were of the order of 0.1 to 0.2 nt in the x and z-direction and 4 to 5 nt in the y-direction. 4. experimental results  the sensitivity of the magnetometer depends on the signalto-noise ratio (s/n) of the zeeman resonance signal and the linewidth of the resonance. when the optical pumping rate is less than or similar to the spin destruction rate, the linewidth of the resonance is proportional to the transverse spin relaxation time. in this case the sensitivity δb can be written mathematically by )/( ns b b   , (3) where ∆b is the line width of the magnetic resonance [13]. the sensitivity is commonly regarded as the slope of the curve near resonance. to optimize the magnetometer sensitivity the peakto-peak amplitude and width of the optical rotation signal in the near zero field resonance were observed, for pump powers ranging from 15 mw to 90 mw. the width was linearly increased according to the pump power (fig. 3(a)), while the amplitude of the signal appears roughly saturated (fig. 3(b)). in this case the maximum slope (∂φ/∂b) occurs at 55 mw, as shown in fig. 3(c). since the slopes near the resonance provide the relative intrinsic sensitivity of the magnetometer, the system is most sensitive when the pump power is 55 mw. we observed the resonance signals. the dispersive curve shown in fig. 4 is a typical optical rotation signal as a function of the magnetic field obtained in the atomic system. the spectral width is about 3 nt when the pump beam power was 55 mw. the experimental results show a good agreement with the expression described by eq. (2). the analytic results were (a) (b) (c) figure 3. (a) half width at half maximum and (b) peak‐to‐peak amplitude of  the zero field resonance were observed to optimize the magnetometer. (c)  the magnetometer sensitivity is proportional to the slope of the dispersion  curves near resonance. the magnetometer was optimized at 55 mw.  (a) (b) figure 2.  (a) the schematic diagram of the resistive heating system and (b) the circuit diagram of the high frequency alternating current generator. the xr‐2206 generates a sinusoidal waveform voltage which passes through the high  pass  filter  with  cut‐off  frequency  1.5  khz.  the  sinusoidal  wave  is  multiplied with the carrier input signal (pin 6 of mpy634) supplied by a pid  controller.      acta imeko | www.imeko.org  august 2013 | volume 2 | number 1 | 19  obtained as a function of by for all other fields zeroed. the experimental result shows a same linewidth compared to analytic results but an asymmetric curve in the positive magnetic field region. the downward peak was larger than the upward peak of the curve. the analytic calculation shows that this result was caused by the residual magnetic fields in the x and z-direction in the same sign. to investigate the effects of residual fields in the x and zdirection in the mu-metal chamber, we applied a bias magnetic field in the x and z-direction. fig. 5 shows the optical rotation signals for several different values of the bias magnetic field. the signal response related to the direction change of the magnetic field is influenced by the magnetic field in the zdirection. in fig. 5(a) and (b), the zero-field resonance signal varied asymmetrically according to the magnetic field in the x and z-direction. at increasing magnetic field, the peak-to-peak amplitude of the signal decreased. at the negative magnetic field region the signal change is remarkable. this result means that there is a negative detuned magnetic field in the z-direction. in other words, spin precession transients cannot decay completely before a measurement of the signal is made. the magnetic field dependent signals are the results of the redistribtution of the ground-state populations caused by the magnetic field in the direction of the laser’s propagation. the atomic magnetic momentum is arranged to the z-axis, the direction of the pump beam’s propagation. however, when there are weak residual fields along the xand z-direction, the atomic magnetic momentum is changed around the zero-field. to estimate the magnetometer performance, the frequency response and noise level were measured. to calibrate the bandwidth, we apply a small oscillating field with 30 nt at several frequencies in the y-direction. the bandwidth of the magnetometer is about 40 hz as shown in fig. 6(a). the result shows that the resistive heating system did not affect the magnetometer because the modulation frequency of the heating system was 24 khz. we evaluated the performance of the magnetometer by monitoring the noise level at the output of the balanced polarimeter using a spectrum analyzer. to calibrate the magnetometer, we applied a small oscillating field of 7.7 hz. the resulting spectrum is shown in fig. 6(b). the powers of the pump and the probe lights were 55 mw and 3 mw, repectively, and the y-component of the magnetic field was zeroed. the sensitivity was about 50 ft/hz1/2 at 10 hz. 5. conclusions  we have demonstrated the operation of a magnetometer under serf regime with a compact heating system. the optical rotation spectra were estimated in the d1 line of k atoms with (a) (b) figure  6.  (a)  the  frequency  response  and  (b)  the  noise  spectrum  of  the  atomic magnetometer approaching to the serf regime.   (a) (b) figure 5. the effect of dc magnetic fields (a)  in the z‐direction and (b) x‐ direction.   figure 4. a normalized zero‐field resonance spectrum. the horizontal axis in  the  figure  shows  the  magnetic  field  in  y‐direction  when  the  other components  of  the  magnetic  field  were  almost  nulled.  the  solid  line represents the measurements and the circles represent the values from the expression given by eq. (2).  acta imeko | www.imeko.org  august 2013 | volume 2 | number 1 | 20  orthogonal pumping and probing light beams with different power of the pump laser. we have obtained a magneto-optical rotation feature with a linewidth of 3 nt. the achieved sensitivity based on optical rotation measurements in the experiment was 50 ft/hz1/2. this value is an order worse than in [10], which is partially explained by the presence of a weak residual field in the magnetic shields. in the serf regime, the rate of larmor precession depends on the absolute dc magnetic fields. we have confirmed that the resistive heating system with high frequency alternating current did not affect the atomic system since the frequency was outside the bandwidth of the magnetometer. two independent heating systems prevent the atomic vapour from being deposited on the cell window. however, low frequency noise brought into the heating system through the external power line might affect the performance of the magnetometer. therefore, with adequate measures to prevent inflow of low frequency noise, the resistive heating system could be a practical alternative to air flow heaters. references  [1] j. vanier, “atomic clocks based on coherent population trapping”, appl. phys. b, vol. 81, no. 4, 2005, pp. 421-442. [2] m.d. lukin, “trapping and manipulating photon states in atomic ensembles”, rev. mod. phys. vol. 75, 2003, pp. 457-472. [3] c. liu, z. dutton, c.h. behroozi, l.v. hau, “observation of coherent optical information storage in an atomic medium using halted light pulses”, nature, vol. 409, 2001, pp. 490-493. [4] m. bajcsy, a.s. zibrov, m.d. lukin, “stationary pulses of light in an atomic medium”, nature, vol. 426, 2003, pp. 638-641. [5] c. ottaviani, d. vitali, m. artoni, f. cataliotti, p. tombesi, “polarization qubit phase gate in driven atomic media”, phys. rev. lett. vol. 90, 2003, pp. 197902-197905. [6] m. fleischhauer, m.o. scully, “quantum sensitivity limits of an optical magnetometer based on atomic phase coherence”, phys. rev. a, vol. 49, 1994, pp. 1973-1986. [7] d. budker1, d.f. kimball, s.m. rochester, v.v. yashchuk, m. zolotorev, “sensitive magnetometry based on nonlinear magneto-optical rotation”, phys. rev. a, vol. 62, 2000, pp. 043403-043409. [8] l. lee, h.s. moon, j.b. kim, “the characteristics of nonlinear magneto-optical effects based on coherence population trapping in the d1 line of rb atom”, hankook kwanghak hoeji, vol. 17, no. 1, 2006, pp. 1-6. [9] d. budker, v. yashchuk, m. zolotorev, “nonlinear magnetooptic effects with ultranarrow widths”, phys. rev. lett. vol. 81, no. 26, 1998, pp. 5788-5791. [10] i.k. kominis, t.w. kornack, j.c. allred, m.v. romalis, “a subfemtotesla multichannel atomic magnetometer”, nature, vol. 422, 2003, pp. 596-599. [11] j.c. allred, r.n. lyman, t.w. kornack, m.v. romalis, “highsensitivity atomic magnetometer unaffected by spin-exchange relaxation”, phys. rev. lett., vol. 89, 2002, pp. 130801-130804. [12] w. happer, a.c. tam, “effect of rapid spin exchange on the magnetic-resonance spectrum of alkali vapors”, phys. rev. a, vol. 16, 1977, pp. 1877–1891. [13] s. appelt, a. ben-amar baranga, c.j. erickson, m.v. romalis, a.r. young, w. happer, “theory of spin-exchange optical pumping of 3he and 129xe”, phys. rev. a, vol. 58, 1998, pp. 1412–1439. [14] s.j. seltzer, m.v. romalis, “unshielded three-axis vector operation of a spin-exchange-relaxation-free atomic magnetometer”, appl. phys. lett., vol. 85, 2004, pp. 4804-4806. [15] m.p. ledbetter, i.m. savukov, v.m. acosta, d. budker, m.v. romalis, “spin-exchange-relaxation-free magnetometry with cs vapour”, phys. rev. a, vol. 77, 2008, pp. 033408-033414. microsoft word 389-3013-1-le-rev acta imeko  issn: 2221‐870x  july 2017, volume 6, number 2, 13‐20    acta imeko | www.imeko.org  july 2017 | volume 6 | number 2 | 13  investigation of a small force standard with the mass based  method  gang hu 1 , jile jiang 1 ,  zhimin zhang 1 ,  yue zhang 1 ,  uwe brand 2 , min‐seok kim 3   1 national institute of metrology, no.18, bei san huan dong lu, 100029 chao yang district, beijing, p.r.china  2 physikalisch‐technische bundesanstalt (ptb), 38116 braunschweig, germany  3 korea research institute of standards and science (kriss), daejeon, republic of korea    section: research paper   keywords: small force; electromagnetic compensation balance; nano‐positioning stage; cantilevers; atomic force microscope  citation: gang hu, jile jiang, zhimin zhang, yue zhang, uwe brand, min‐seok kim, investigation of a small force standard with the mass based method, acta  imeko, vol. 6, no. 2, article 4, july 2017, identifier: imeko‐acta‐06 (2017)‐02‐04  section editor: min‐seok kim, research institute of standards and science, korea  received may 27, 2017; in final form may 27, 2017; published july 2017  copyright: © 2017 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: this work was supported by the ministry of science and technology of china, china  corresponding author: gang hu, e‐mail: hugang@nim.ac.cn    1. introduction  there is a great demand for small force measurements in a large variety of scientific research fields such as material science, micromechanics, physics, biology or medicine. as an important measurement tool, mems (micro-electro-mechanical system)based micro-force transducers are widely used in micro-robot systems, micromanipulation and micro-assembly systems, micro tribology research, biomechanical research, tiny implantable surgery, single cell manipulation and scanning probe microscopy (spm) [1]. thin films are the most widely used materials in components of mems and microelectronics, and is the basis of devices of microelectronic and moems (microoptoelectromechanical) systems [2]. accurate measurements and traceability of small forces are required in determining the mechanical properties of thin films. in the research on antigen-antibody adhesion and dna molecules tensile strength, the small force between cells or molecules ─ minimum order of magnitude of up to pn ─ has an impact on complex biological processes [3]. atomic force microscopy and instrumented indentation are two techniques that are widely used in material science and nanoand biomechanics [4]. spring constants of the cantilevers in atomic force microscopes (afm) and load scale factors (lsf) of capacitance sensors in nano-indentation instruments are calibrated based on different principles and methods. but most of them are not traceable to si units. as a result, mechanical properties established from different manufacturers and devices often do not agree with each other. for example, young’s modulus measured by afm using different cantilevers can differ by orders of magnitude [4]. these inconsistencies are hindering a successful commercialization of nanoand biotechnology. abstract  a small force standard with the mass based method  is developed.  it  is composed of an electromagnetic compensation balance, a  nano‐positioning  stage  and  a  displacement  and  pitch  angle  adjustment  unit.  the  small  force  standard  is  used  to  measure  spring  constants of cantilevers  in atomic force microscope (afm) and forces of small force transducers  in related applications. since the  balance and the nano‐positioning stage are traced to mass and length standards, respectively, the small force standard is si traceable.  the  principle  and  structure  of  the  small  force  standard  and  its  main  components  are  discussed  in  detail.  the  spring  constants  of  several  cantilevers  used  in  afm  were  calibrated  by  the  small  force  standard.  the  calibration  results  are  demonstrated  and  uncertainties  are  evaluated.  for  verifying  metrological  characteristics  of  the  small  force  standard,  a  preliminary  international  comparison between nim, ptb and kriss was carried out. the comparison results were  in good agreement and  indicate that the  calibration capabilities of the participating small force standards were equivalent within their reported uncertainties.  acta imeko | www.imeko.org  july 2017 | volume 6 | number 2 | 14  for solving the problem, some nmis launched small force research projects and developed si traceable standards over the last decade. generally, there are two kinds of small force standards in the micro-newton to nano-newton range: the first one is based on the mass method, such as the micro-force measuring devices at ptb [5], and nfc (the nano force calibrator) at kriss [6]; the second one is based on the electrostatic force realization principle, such as the efb (electrostatic force balance) at nist [7], the primary low force balance at npl [8], and the force measurement system at cms [9]. the establishment of these si traceable standards significantly improves the reliability of small force measurement results. the first inter-laboratory comparison was carried out among kriss, nist, npl and ptb from 2008 to 2010 [10]. the comparison results were in good agreement, suggesting that the calibration capabilities of the participating small force facilities were equivalent within their reported uncertainties. nim launched a small force metrology project in 2011 and developed two small force standards with afore-mentioned two principles [11]. in section 2 the principle and structure of the small force standard with the mass based method are described. in the next two sections spring constant measurements of afm cantilevers and uncertainty evaluation are presented. in section 5 a preliminary international comparison between nim, ptb and kriss is introduced and its results are demonstrated. finally, in the concluding section the structure and experimental results of the small force standard are summarized. 2. the principle and structure of the small force  standard  the small force standard is composed of an electromagnetic compensation balance, a nano-positioning stage, and a displacement and pitch angle adjustment unit. its structure and photo are shown in figure 1. the cantilever to be calibrated is mounted on the nano-positioning stage and can move in vertical direction, driven by the stage. as soon as the cantilever is in contact with the weighing pan of the balance, it has an elastic deformation and applies an external force to the balance at same time. the external force is balanced by the electromagnetic force generated by the balance. the displacement of the nano-positioning stage is set up and controlled by a controller. at each displacement set point, the displacement of the stage, the output signals of the balance and cantilever are acquired simultaneously. generally, there are two kinds of small force sensors: passive and active types [5]. the cantilever is a commonly used passive sensor. the applied force can be converted into a deflection. the spring constant of the cantilever k is calculated by the following equation: d fk  （1） where f is the applied force, d is the deflection of the cantilever, and k is the spring constant of the cantilever. the active sensor, which integrates strain gauges and a bridge circuit, converts the applied force into a voltage. the force sensitivity of sensor sf is given by uδ fsf = （2） where f is the applied force and △u is the output voltage change of the sensor. the electromagnetic force of the balance is traceable to the mass standards. the displacement of the nano-positioning stage is measured by a capacitance sensor in the stage. the output voltage of the sensor can be measured by an electrical measuring instrument. the displacement sensor and the electrical measuring instrument can be traced to length and electrical standards, respectively. in these ways, si traceability of k and sf are realized. the electromagnetic compensation balance is a key component in the small force standard. the xp6u type balance made by mettler-toledo is adopted. its maximum capacity is 6.1 g and the readability is 0.1 μg. correspondingly, the maximum force is about 60 mn，and the force resolution is about 1 nn. the schematic diagram is shown in figure 2. the weighing pan is connected to a compensation coil through a coupling element and a lever mechanism. the compensation coil is put in a permanent magnet. when a force f is applied to the weighing pan, lever and coil move together in vertical direction. their displacements are measured by an embedded displacement sensor. the current in the coil increases through an automatic compensation circuit. therefore, an electromagnetic force is generated in the coil. it is equal to the applied force in value and opposite in direction, and restores the weighing pan to its original position. the current in the coil is proportional to the applied force f. the other key component is the nano-positioning stage. the p-621.1cd type piezo linear stage made by pi is adopted. its closed-loop travel is 100 μm, the closed-loop resolution is 0.4 nm, the closed-loop linearity error is 0.02 %. a high stiffness is achieved with the fea-optimized design of the frictionless flexure elements, which assure excellent guiding accuracy and dynamics. as a result, its pitch/yaw is within ±3 μrad. pi's proprietary capacitive sensor is used. it is free of friction and 1 electromagnetic compensation balance  2 weighting pan  3 cantilever   4 nano‐positioning stage  5 displacement and pitch angle adjustment unit  figure 1.  the structure and photo of nim’s small force standard with the  mass based method. 1 weighing pan   2 lever mechanism  3 coupling element    4 pan carrier   5 parallel guiding system   6 compensation coil  7 permanent magnet   8 displacement sensor   9 automatic  compensation circuit  figure 2. the schematic diagram of electromagnetic compensation balance. acta imeko | www.imeko.org  july 2017 | volume 6 | number 2 | 15  hysteresis and has a very high level of linearity. the displacement and pitch angle adjustment units, which are composed of a three-dimensional linear slide and a twodimensional tilting stage, are used to adjust displacements and pitch angles of the cantilever in 5 degrees of freedom. besides, a microscopic observation unit is applied to monitor the position and contact state of the cantilever and weighing pan. for minimizing the influence of vibration and air flow, the standard is put on a vibration isolation desk, and shielded by an organic glass enclosure to ensure a stable environmental condition. 3. the spring constant measurements and  correction  3.1. the spring constant measurements  the spring constants of several cantilevers used in afm were calibrated by the small force standard. the main specifications of the cantilevers are listed in table 1. two pieces of cantilevers for each type, which are numbered no.1 and no.2, respectively, were measured. for clfc, there are 3 cantilevers with different lengths. the longest cantilever (length 400 μm) was used in our experiments. a silicon plate is fixed on the weighing pan of the electromagnetic compensation balance for the measurement of cantilevers with tip (nsg03 、 csg10). a conical diamond probe tip with 1 μm tip radius and 90º cone angle is applied to measure the tipless cantilevers (clfc). the silicon plate and conical diamond probe tip are shown in figure 3. the measurement position for nsg03、csg10 is approximately at the centre of the cantilever tip. for clfc, it is approximately at the end of the cantilever beam. the measurement procedure is conducted as follows: 1. the cantilever is mounted on the small force standard. adjust the displacement and pitch angle adjustment unit, make the cantilever to the proper position above the weighing pan and perpendicular to the balance axis (tilted angle is zero). 2. drive the nano-positioning stage and make the cantilever move downward in vertical direction. as soon as a force is applied to the cantilever, the indication of the balance has a significant increase. this position is determined as contact point. 3. the displacement control of the nano-positioning stage is adopted. the nano-positioning stage with cantilever moves downward step by step in vertical direction. the cantilever is loaded up to maximum deflection with 6 uniformly distributed points. then it is unloaded down to the original deflection with the same 6 points. there is 30 s waiting time at each displacement set point. once the output of the balance is stable, the indicated mass of the balance and displacement of the nano-positioning stage are acquired simultaneously. the whole experiment includes 10 cycles. the force and displacement graphs are shown in figure 4. the x-coordinate indicates absolute displacements of the nanopositioning stage. the y-coordinate indicates corresponding forces calculated from the output of the balance. the red circles represent measurement points of the loading process and the blue crosses represent measurement points of the unloading process. the spring constants of the cantilevers are derived from a linear regression equation based on force-displacement data of loading and unloading. the small force standard was operated under ambient conditions. the experiments were carried out in the range 21.0 °c to 22.0 °c.  (a) the silicon plate                       (b) conical diamond probe tip  figure 3.  the silicon plate and probe tip for different cantilevers. table 1.  the main specifications of the cantilevers used in our experiments. type  dimensional parameters (μm)  spring constant (n/m)  notes  nsg03    length：130  width：35  thickness：1.2  nominal value：0.5‐2.2  typical value： 1.1  with tip  csg10    length：250  width：35  thickness：1  nominal value：0.03‐0.2  typical value： 0.1  with tip  clfc    length：400  width：29  thickness：2  （longest cantilever）  nominal value：0.112  no tip  acta imeko | www.imeko.org  july 2017 | volume 6 | number 2 | 16  3.2. the correction of the measurement results  the cantilever, balance and stage are connected in series in the process of spring constant measurements. considering the stiffness of the measurement chain, which includes the balance, the stage, and the displacement and pitch angle adjustment unit, the measurement results of the spring constants should be corrected. the stiffness of the measurement chain was measured, while the cantilever was not mounted on the stage. the conical diamond probe tip, shown in figure 3(b), was mounted on the weighing pan of the balance. the measurement procedure was conducted as shown in figure 5. the displacement interval is 5 nm, and the waiting time is 30 s at each displacement set point. the whole measurement has 10 cycles. in calculating the deflection of the measurement chain, the deformation of the si surface created by the diamond probe tip should be taken into account. the deformation h is estimated by [12] 3/13/2 3/23/2 4 3 re f h r         (3) with 2 2 2 1 2 1 111 eeer      (4) where r is the radius of the diamond probe tip, f is the applied force, er is the reduced young’s modulus, e1 , e2 are the young’s modulus of diamond and silicon, υ1 , υ2 are poisson’s ratios of diamond and silicon, respectively. the deflection of the measurement chain is corrected by subtracting the deformation of the si surface h from the measured deflection. if a 100 μn force is applied, there will be approximately 6.1 nm deformation according to (3). the stiffness of the measurement chain is derived from a linear regression equation based on force-deflection data of loading and unloading. the measurement results are shown in figure 6. the stiffness of the measurement chain ki was determined as 1099.1 n/m with a relative standard uncertainty u(ki) 6.3 %. the measurement result of the spring constant km is corrected by the following equation [10], [13]: mi mi c kk kk k   (5) where kc is the corrected spring constant. the measurement results and the corrected results of the spring constants are summarized in table2. 4. the uncertainty evaluation  the uncertainty evaluation is based on the iso/iec guide to the expression of uncertainty in measurement. the           figure 4.   the force and displacement graphs of the spring constant measurements.  acta imeko | www.imeko.org  july 2017 | volume 6 | number 2 | 17  uncertainty due to repeatability is classified as type a evaluation. the other uncertainty budgets are estimated as type b evaluation. 4.1. the uncertainty evaluation of the force measurement  for estimating the uncertainty due to the force measurement, the balance used in our small force standard was calibrated. in addition, the following factors, such as acceleration of gravity, ambient temperature change, measurement procedure, and the direction of the applied force, should be taken into consideration. two sets of standard weights with high accuracy were used for calibrating the balance. the measurement range runs from 50 μg to 50 mg with 10 calibration points ─ corresponding force range is approximately from 500 nn to 500 μn. standard weights of 1 mg, 2 mg, 5 mg, 10 mg, 20 mg, and 50 mg with class e1 were used. standard weights of 50 μg, 100 μg, 200 μg, 500 μg, made of aluminium alloy, were adopted and traced to the mass standard through an automatic mass comparator [14]. the calibration procedure of the balance refers to the chinese verification regulation for electronic balances [15]. generally, as the balance indication gets larger, the repeatability（absolute value）becomes larger. therefore, the standard deviation of 10 times measurement results in 50 mg（ absolute value ） is considered as the uncertainty due to repeatability. the uncertainties due to standard weights are derived from their calibration results by the mass standard with higher class. the uncertainty due to the scale interval of the balance is estimated according to the guideline [16]. since the position of the applied force is always in the centre of the weighing pan, the uncertainty due to eccentric load is negligible. the standard uncertainty due to the balance calibration uf1 is determined by the root-squared sum of these uncertainty budgets. the measured acceleration of gravity in the laboratory where the standard is built up amounts: g = 9.801264562 m/s2, uf2 = 3.0×10-7. the uncertainty due to ambient temperature changes uf3, assuming a rectangular distribution, is calculated by 32 δc f3 tt u  (6) where the ambient temperature change in the laboratory △t is less than 1 °c(±0.5 °c) and the temperature coefficient of the balance tc is 0.0001 %/°c. as a result, uf3 = 2.9×10-7. when the standard is used for calibration of the cantilever, the loading and unloading procedures are similar to those of the balance calibration. in this case, creep or hysteresis of the balance has a similar influence on the measurement results. the uncertainty due to differences of measurement procedures uf4 is negligible. mounting of the cantilever may cause inconsistency of the directions between the applied force and gravity. the uncertainty due to this factor uf5, assuming a projection distribution, is calculated as follows [17]:  table 2.  the measurement results and the corrected results of the spring constants. type/no.  the measurement results of spring constants km  (n/m)  the corrected results of spring constants kc (n/m)  nsg03/ no.1  1.241 n/m  1.242 n/m   nsg03/ no.2  1.165 n/m  1.166 n/m   csg10/ no.1  0.183 n/m  0.183 n/m   csg10/ no.2  0.187 n/m  0.187 n/m   clfc/ no.1  0.111 n/m  0.111 n/m   clfc/ no.2  0.101 n/m  0.101 n/m   0 500 1000 1500 2000 2500 3000 3500 27.28 27.3 27.32 27.34 time(sec) d is p la c e m e n t( u m ) 0 500 1000 1500 2000 2500 3000 3500 80 100 120 140 time(sec) fo rc e (u n ) (a)  displacement graph                                                                                            (b)  force graph  figure 5.   the displacement and force graph of the stiffness of the measurement chain.      figure 6.  the measurement results of the stiffness of the measurement chain.  acta imeko | www.imeko.org  july 2017 | volume 6 | number 2 | 18  10 )cos1(3 f5  u (7) where  is the angle between the directions of the applied force and gravity, and generally is less than 1°. therefore, uf5 = 4.6×10-5. the standard uncertainty budgets of the force measurement in each calibration point are summarized in table 3. the relative combined standard uncertainty of the force measurement u(f) is calculated with the following equation:   22222 f5f4f3f2f1 uuuuufu  (8) it is demonstrated in table 3 that the force measurement uncertainty of the standard is better than 7.0×10-3 in the range of 500 nn – 500 μn. 4.2. the uncertainty evaluation of the spring constant  measurement   the uncertainty due to the repeatability u1 is estimated by the standard deviation of the average of 10 measurement results. for the type b evaluation, the following uncertainty contributions, such as force and displacement measurements, non-linearity of calibration curves, orientation of the cantilevers, location accuracy of the measurement point and the stiffness of the measurement chain should be taken into account. the uncertainty due to the force measurement is estimated in 4.1. at smaller forces the uncertainty is larger. the uncertainty at the lower limit of the whole force range is considered as the uncertainty of the force measurement u2. the uncertainty due to the displacement measurement is estimated from the calibration results of the nano-positioning stage. the displacements of the nano-positioning stage were measured by a laser interferometer (zygo zmi 4004) over 100 μm travel range with 5 μm step. the standard uncertainty of the measurement results was 68.2 nm. a conservative estimation of the relative uncertainty due to the displacement measurement u3 is determined as 1.36×10-2. because the spring constant of the cantilever is derived from a linear regression equation of force-displacement data, the residuals of the linear regression are calculated and considered as the uncertainty due to the non-linearity of the calibration curves u4. the orientation of the cantilevers affects the measurement results. this factor is analyzed and discussed in the literature [6] and [13]. assuming the maximum tilted angle is 1°, the relative uncertainty due to the orientation of the cantilever u5 is 3.8×10-3. according to euler’s equation, the stiffness of the cantilever k is related to young’s modulus and the cantilever dimensions [18]. therefore, the location accuracy of the measurement point should be considered for tipless cantilevers. for cantilevers with tip (the measurement point is at the tip of the cantilever), the uncertainty due to location accuracy is negligible. in our standard, the location accuracy of the measurement point in the direction of the beam is estimated as ±3 µm. assuming a rectangular distribution, the relative standard uncertainty due to location accuracy u6 can be given by [13]        l l u  36 (9) where δl is the maximum position deviation from the ideal location. in the case of the clfc, u6 is calculated to be 1.3× 10-2 using δl=3 μm, l=400 μm . the stiffness of the measurement chain kb was determined to be 1099.1 n/m. assuming a rectangular distribution, the relative standard uncertainty due to the stiffness of measurement chain u7 is given by [13] b b mb m k k kk k u    3 1 7 (10) where δkb=120 n/m is the maximum deviation of the stiffness of the measurement chain kb. the combined standard uncertainty u(kc) is determined using: 2 7 2 6 2 5 2 4 2 3 2 2 2 1)( uuuuuuuku c  (11) table 3.  standard uncertainty budgets of the force measurement. standard uncertainty budgets  the measured forces  (μn)  0.5  1  2  5  10  20  50  100  200  500  balance  calibration     uf1  measurement  repeatability  6.2×10 ‐3   3.1×10 ‐3   1.6×10 ‐3   6.2×10 ‐4   3.1×10 ‐4   1.6×10 ‐4   6.2×10 ‐5   3.1×10 ‐5   1.6×10 ‐5   6.2×10 ‐6   standard  weights   1.8×10 ‐3   1.2×10 ‐3   1.1×10 ‐3   6.7×10 ‐4   8.8×10 ‐4   4.4×10 ‐4   1.8×10 ‐4   8.8×10 ‐5   4.4×10 ‐5   2.3×10 ‐5   scale interval  of the balance  8.6×10 ‐4   4.3×10 ‐4   2.2×10 ‐4   8.6×10 ‐5   4.3×10 ‐5   2.2×10 ‐5   8.6×10 ‐6   4.3×10 ‐6   2.2×10 ‐6   8.6×10 ‐7   eccentric load  negligible  the acceleration of gravity  uf2  3.0×10 ‐7   ambient temperature change  uf3  2.9×10 ‐7   difference of measurement  procedures uf4  negligible  inconsistency of the  directions between applied  force and gravity uf5  4.6×10 ‐5   relative combined standard  uncertainty u(f)  6.5×10 ‐3   3.4×10 ‐3   1.9×10 ‐3   9.2×10 ‐4   9.3×10 ‐4   4.7×10 ‐4   1.9×10 ‐4   1.0×10 ‐4   6.5×10 ‐5   5.2×10 ‐5   acta imeko | www.imeko.org  july 2017 | volume 6 | number 2 | 19  standard uncertainty budgets, the combined standard uncertainty and the standard uncertainty of the corrected spring constants are summarized in table 4. 5. preliminary international comparison  for verifying the metrological characteristics of the small force standard, a preliminary international comparison between nim, ptb and kriss was carried out in 2015. the small standards in ptb and kriss, which are based on the electromagnetic compensation balance and a nano-positioning stage, were applied for this comparison [5], [6]. three types of commercial cantilevers for afm, which is mentioned in section 3.1, were used in this comparison. there was no identical measurement procedure in this comparison. the common aspects in the comparison are as follows: 1. the cantilevers were installed on the nano-positioning stage or the weighing pan of the balance in such a way that the plane of the cantilevers is perpendicular to the balance axis (tilted angle is zero). 2. the measurement range of deflections and the forces were roughly the same in different laboratories. uncertainty of measurement results is evaluated by each laboratory individually. the deviations of measurement results between nim and ptb/kriss δ are calculated using: ptb/krissnim xx  (12) where xnim, xptb/kriss are the measurement results at nim and ptb/kriss, respectively. the degree of agreement of the measurement results between nim and ptb/kriss is expressed by en. the ratio en is given by 2 ptb/kriss 2 nim uu e n    (13) where unim, uptb/kriss are the expanded uncertainties of the measurement results of nim and ptb/kriss, respectively. the comparison results are shown in table 5. this table shows that the absolute value of en is less than 1, the calibration capabilities of small force standards are equivalent within their reported uncertainties. 6. conclusions  a small force standard with the mass based method is developed. it is composed of an electromagnetic compensation balance, a nano-positioning stage, and a displacement and pitch angle adjustment unit. the balance was calibrated by standard weights with high accuracy. a nano-positioning stage with good guiding accuracy is adopted and minimizes the influence of inconsistency of the directions between applied force and gravity. the displacement and pitch angle adjustment unit is used to adjust the displacement and pitch angle of the cantilever and improves the positioning accuracy. the spring constants of several cantilevers used in afm were calibrated by the small force standard and uncertainties were evaluated. table 5.  comparison results.  type/no.  measurement results and expanded uncertainties（k=2）  en  nim  ptb kriss nim‐ptb  nim‐kriss nsg03/ no.1  1.242 n/m  ±0.044n/m  1.256m ±0.054n/m  1.282m ±0.028n/m  ‐0.20  ‐0.76  nsg03/ no.2  1.166 n/m  ±0.042n/m  1.182 n/m ±0.050n/m  1.196 n/m ±0.028n/m  ‐0.25  ‐0.60  csg10/ no.1  0.183 n/m  ±0.006n/m  0.185/m ±0.008n/m  0.186/m ±0.004n/m  ‐0.28  ‐0.47  csg10/ no.2  0.187 n/m  ±0.006n/m  0.193/m ±0.008n/m  0.191/m ±0.004n/m  ‐0.53  ‐0.51  clfc/ no.1  0.111 n/m  ±0.004n/m  0.109 n/m ±0.004n/m  0.107 n/m ±0.002n/m   0.39   0.85  clfc/ no.2  0.101 n/m  ±0.004n/m  0.100 n/m ±0.004n/m  0.100 n/m ±0.002n/m   0.09   0.20   table4.  standard uncertainty budgets of the spring constant measurement. standard uncertainty budgets  the relative standard uncertainty  nsg03 no.1  nsg03 no.2  csg10 no.1  csg10  no.2  clfc  no.1  clfc no.2  repeatability   u1 1.36×10 ‐3   1.90×10 ‐3   5.91×10 ‐4   5.79×10 ‐4   4.84×10 ‐4   3.13×10 ‐4   force measurement   u2  3.40×10 ‐3   3.40×10 ‐3   3.40×10 ‐3   3.40×10 ‐3   3.40×10 ‐3   3.40×10 ‐3      displacement measurement u3  1.36×10 ‐2   1.36×10 ‐2   1.36×10 ‐2   1.36×10 ‐2   1.36×10 ‐2   1.36×10 ‐2   non‐linearity of calibration curve u4  1.01×10 ‐2   1.08×10 ‐2   6.43×10 ‐3   7.43×10 ‐3   2.11×10 ‐3   1.73×10 ‐3   orientation of the cantilever  u5  3.80×10 ‐3   3.80×10 ‐3   3.80×10 ‐3   3.80×10 ‐3   3.80×10 ‐3   3.80×10 ‐3   location accuracy u6  negligible  negligible  negligible  negligible  1.30×10 ‐2   1.30×10 ‐2   stiffness of measurement chain u7  7.12×10 ‐5   6.69×10 ‐5   1.05×10 ‐5   1.07×10 ‐5   6.36×10 ‐6   5.79×10 ‐6   relative combined standard uncertainty of   the corrected spring constants  u(kc)  1.77×10 ‐2   1.82×10 ‐2   1.59×10 ‐2   1.63×10 ‐2   1.96×10 ‐2   1.96×10 ‐2   acta imeko | www.imeko.org  july 2017 | volume 6 | number 2 | 20  a preliminary international comparison between nim, ptb and kriss was carried out. the comparison results are in good agreement and indicate that the calibration capabilities of small force standards are equivalent within their reported uncertainties. it seems that there are systematic deviations in measurements of nsg03, csg10 and clfc, respectively. further investigation and analysis on the sources of these systematic deviations will be made. besides, the relative movement of the cantilever and other unknown phenomena at the contact point remains to investigate. structural improvement and optimization of nim’s small force standard will be made. further metrological characteristics experiments and international comparisons with nmis will be carried out. better transfer standards should be applied, a more rigorous technical protocol should be discussed and determined for further comparisons. acknowledgement  this work was supported by the national key technology research and development program of the ministry of science and technology of china under the project “establishment of microgram mass standards and micro-nanonewton force standards and research on key technologies in traceability”, project number: 2011bak15b06. references  [1] lu xiaoguang, “study on microforce sensor based on piezoelectric thin film”, master's degree thesis, dalian university of technology，2007. [2] mei tao， ， ，kong deyi zhang peiqiang wu xiaoping, “mechanical properties and scaling effects in micro-electromechanical systems”, journal of mechanical strength，23 (2001) pp. 373-379. [3] willemsen o h, snel m m e, cambi a, et al., “biomolecular interactions measured by atomic force microscopy”, biophys， （ ）79 2000 pp.3267-3281. [4] min-seok kim, jon r. pratt, “si traceability: current status and future trends for forces below 10 micronewtons”, measurement, 43 (2010) pp.169-182. [5] ingo behrens, lutz doering and erwin peiner, “piezoresistive cantilever as portable micro force calibration standard”, journal of micromechanics and microengineering, 13 (2003) pp. s171s177. [6] min-seok kim, jae-hyuk choi, yon-kyu park and jong-ho kim, “atomic force microscope cantilever calibration device for quantified force metrology at microor nano-scale regime: the nano force calibrator (nfc)”, metrologia, 43 (2006) pp. 389-395. [7] jon r. pratt, john a kramar, david b newell and douglas t smith, “review of si traceable force metrology for instrumented indentation and atomic force microscopy”, measurement science and technology, 16 (2005) pp. 2129-2137. [8] leach r, chetwynd d, blunt l, et al. “recent advances in traceable nanoscale dimension and force metrology in the uk”, measurement science and technology, 17 (2006) pp. 467-476. [9] sheng-jui chen, sheau-shi pan, yu-shan yeh and yi-ching lin, “measurement of cantilever spring constant using an electrostatic sensing and actuating force measurement system”, measurement science and technology, 25 (2014) pp.1-10. [10] min-seok kim, jon r. pratt，uwe brand and christopher w jones, “report on the first international comparison of small force facilities: a pilot study at the micronewton level”, metrologia, 49 (2012) pp.70-81. [11] hu gang, song le, meng feng, zhang wei, zhang zhimin, zhang yue and zheng yelong, “research and development of small force standards at nim” measurement of mass, force and torque (apmf 2013) international journal of modern physics: conference series, 24 (2013) pp.1360020-1-9. [12] t. chudoba, n. schwarzer, f. richter, “determination of elastic properties of thin films by indentation measurements with a spherical indenter”, surface and coatings technology, 127 (2000) pp.9-17. [13] min-seok kim, jae-hyuk choi, jong-ho kim and yon-kyu park, “si traceable determination of spring constants of various atomic force microscope cantilevers with a small uncertainty of 1%”, measurement science and technology, 18 (2007), pp.3351-3358. [14] jian wang, peter fuchs, stefan russi, et al, “uncertainty evaluation for system of weighing equations for the determination of microgram weights”, ieee transactions on instrumentation and measurements, 64（ ）8 (2015) pp. 22722279. [15] verification regulation for electronic balance (jjg1036-2008), 2008. [16] european co-operation for accreditation (ea) laboratory committee, “ea-10/18 ea guidelines on the calibration of non-automatic weighing instruments”, 2005. [17] li qingzhong，li yuhong, “guideline to uncertainty evaluation of force, torque and hardness measurements”, china metrology publishing house, beijing, 2003, isbn 7-5026-1758-2/tb ·649. [18] m.-s. kim, y.-k. park and j._h. kim “millimeter-scale piezoresistive cantilevers for accurate force measurements at the nano-newton level”, mapan-journal of metrology society of india, 28(4) (2013) pp.251-257.   template for an acta imeko event paper acta imeko issn: 2221-870x december 2016, volume 5, number 4, 56-63 acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 56 cbm for a fleet of railway vehicles: infrastructure and algorithms micaela caserza magro1, paolo pinceti1, marco antonelli2, enrico de paola2, pierluigi firpo2, enrico marino2 1university of genoa, dept. diten, via all’opera pia 11a, 16145, genova, italy 2bombardier transportation italy spa, via tecnomasio, 17047, savona, italy section: research paper keywords: railway vehicles; condition based maintenance (cbm); key maintenance indicators; condition monitoring; industrial communication citation: micaela caserza magro, paolo pinceti, marco antonelli, enrico de paola, pierluigi firpo, enrico marino, cbm for a fleet of railway vehicles: infrastructure and algorithms, acta imeko, vol. 5, no. 4, article 9, december 2016, identifier: imeko-acta-05 (2016)-04-09 section editor: lorenzo ciani, university of florence, italy received september 7, 2016; in final form november 9, 2016; published december 2016 copyright: © 2016 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: this work was supported by regione liguria, project (po cro fse 2007-2013 asse iv) corresponding author: paolo pinceti, e-mail: paolo.pinceti@unige.it 1. introduction optimizing the maintenance for a fleet of machineries or vehicles means to guarantee a high quality of service (qos) with a minimum number of interventions. the concept of qos varies for each specific application. for railway vehicles, the passengers that want to have reliable trains on time measure the qos. for machinery, qos means almost often to work without interruptions and with a constant quality of the production. in all cases, the qos is related to the availability that is the percentage of time a system is in normal operating conditions. an optimized maintenance strategy coordinates the scheduled stops with the maintenance interventions, e.g. railway vehicles must stop for safety reasons a fixed number of times per year or after a given number of kilometres according to national regulations and laws. maintenance policies are discussed in [1]-[9]. condition based maintenance (cbm) and predictive maintenance (pm) seem to be the most effective policies, since their goal is to start a maintenance intervention only when it is necessary. to do this, it is mandatory to monitor the operation of a vehicle (or a machinery, or a plant) to find symptoms of incoming failures (“fix it before it fails”). for this purpose, various metrics and sensor-based methods can be used to measure and monitor continuously the condition of the abstract data acquisition and communication technologies give the possibility of receiving and storing a huge amount of data from machinery and plants in operation. from these data it is possible to create a set of key maintenance indicators (kmi) useful for optimizing the maintenance policy. raw data from the field are to be processed and filtered for obtaining effective kmis to use in algorithms aimed at discovering anomalies or abnormal operation of one or more machineries or plants. this paper presents a roadmap towards the condition based maintenance of a fleet of railway vehicles. the paper associates to each maintenance policy its benefits and its requirements in terms of technological infrastructure and operating costs. bombardier transportation italy started this roadmap a few years ago, for moving from a reactive maintenance policy to a proactive policy. increasing the effectiveness of maintenance implies the sensorization of the machines and the creation of a network for funneling information from the machineries to the central maintenance room. a company must find an equilibrium point between complexity and expected benefits. results achieved by means of a specifically developed tool for data analysis applied to some sub-systems of the vehicles are presented. acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 57 equipment. to do this, we introduced the concept of key maintenance indicator (kmi) that may represent: a simple counter of variables of the vehicle (e.g. distance travelled, hours of operation, the number of times a door opened and closed, etc.; a calculated variable that uses different parameters of the vehicle (e.g. number of occurrence of “x” when “y” was on, etc.); measures from sensors (e.g. oil analysis, vibration analysis, etc.) specifically installed for maintenance purposes. the kmis are the foundation for the construction of algorithms aimed at monitoring the status of the vehicle and at deciding maintenance interventions [11], [12]. collecting and processing the raw data that will become kmis require a complex technological infrastructure described into details in [10]. railway vehicles require two sub-systems: “on-board system” (the train): that produces and collects raw data and sends them to the; “off-board system”: collects data from all the vehicles and implement the algorithms for cbm. this paper shortly describes the on-board and off-board infrastructures, and focuses on the procedures for the data analysis and for the discovery of rules and metrics useful for cbm; in other words, the “conditions” to active a maintenance intervention. a joint research team of bombardier transportation italy and of the university of genoa, dept. diten, developed a tool for the data filtering, sorting, and analysis that was applied on the historical database of about 5000 vehicles to find rules for deciding the maintenance interventions. the paper presents some results of this analysis for three different sub-systems. 2. the technological infrastructure 2.1. the on-board infrastructure railway vehicles are composed by a set of independent subsystems, each one equipped with a dedicated train control & management system (tcms) that collects data both for control purposes and for diagnosis. all the tcmss communicate via the multifunction vehicle bus (mvb), standardized in iec 61375-3. tcmss transmit the process data for the vehicle control with a sampling time of 1 second. when a tcms detects an anomaly, a diagnostic data set (dds) is generated. the on board database server (odbs) stores both process and diagnostic data. all data are transmitted to the offboard servers with their time-stamp every two hours. in case a proper transmission channel is not available, data are stored and transmitted when a connection is available. it is possible to implement some simple kmis (mainly counters) directly in the tcmss. 2.2. the off-board infrastructure the off-board infrastructure consists of a powerful, redundant server that collects the data from all the odbss of the fleet. a software package, called maintenance software package, makes data from the trains available to all the users. the maintenance staff should: monitor the fleet; detect and prevent faults and anomalies; define the maintenance work schedule. the maintenance staff is coordinated with the various regional operation rooms. 3. maintenance policies as explained in [10], with cbm an equipment is maintained only when it needs maintenance, so no unnecessary intervention is carried out. an effective cbm requires reliable data from the trains and powerful algorithms for their analysis. effectiveness of maintenance evolves together with the evolution of the technological framework of the company. for sake of simplicity, we split this evolution into four steps: reactive maintenance: it includes cyclic interventions and overhauls during the stops at the depot. no train-ground data transfer is required; remote maintenance: the maintenance staff can access the odbs for monitoring the status of each vehicle. alarms are received real-time; reactive maintenance: the maintenance staff uses rules and algorithms for monitoring the trains and communicates with the on-board personnel in case of detection of anomalies. maintenance schedule is dynamically updated; condition based maintenance: kmis and algorithms are used for calculating the residual life of components. a cost/benefit analysis is necessary for deciding to switch from one step to a higher one. 4. data analysis for cbm techniques of data mining are useful for analysing the large amount of data from the trains stored in the centralized database. data mining means to extract useful information hidden in the data and to present them in the more simple and usable ways. data mining methods use statistical approach and different mathematical techniques suitable for managing data and database to look for correlations. essentially, a database for cbm purposes contains in the rows different objects in our case different vehicles of the same fleet and in the columns the properties and the parameters describing the behaviour of the object (see figure 1). a typical locomotive has about 2000 operating parameters, split between about 10 major subsystems. a subset of these parameters may give useful information for maintenance purposes. the case histories at pos.5 show some examples of maintenance-related parameters for specific subsystems. considering such a database structure, it is possible to define three possible approaches to the data analysis: horizontal: analyses the same property of different figure 1. example of cbm oriented database. acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 58 vehicles; vertical: analyses different properties of the same vehicle; mixed: it is a mix of the previous approaches, i.e. different properties of different vehicles or of each single vehicle. each analysis is useful for achieving specific results, as described in the following paragraphs. for each analysis, it is important to define the kmis of interest. 4.1. horizontal analysis horizontal analysis allows checking and comparing the behaviour of a single vehicle/apparatus/subsystem with the other elements of the fleet. therefore, the analysis may identify the vehicles with abnormal behaviours in the fleet. kmis for comparative horizontal analysis should highlight the overall status of the machine, like the total number of stops, the average consumption, etc. the first and easiest horizontal analysis is the calculation of average values (xavg) and variances (σ). machines with parameters outside the average value of the fleet with a band of, for example, ±σ or ±2σ become candidates for a more accurate and precise analysis. considering random failures, we can assume a gaussian distribution of the samples. the level of confidence for setting the alarm threshold for each parameter is set according to the sample distribution. considering the example in figure 2 over 10 vehicles, it is apparent to understand that machine 4 and 9 show an abnormal behaviour. 4.2. vertical analysis vertical analysis considers the behaviour of a vehicle/subsystem over the time. thus, this analysis shows the upcoming of abnormal behaviour or degradation of a parameter. different approaches exist for the vertical analysis: trend analysis: is the study of the behaviour of a kmi over the time that may identify a deviation from the expected trend (see figure 3). the alarm threshold is set on the difference between the current value and the linear regression of the historical data; anomalies analysis: with a long-term monitoring, it may identify anomalies in the behaviour of a kmi, like sudden changes or abnormal operations; statistical analysis: it may identify abnormal deviations in the historical behaviour of a kmi; signature analysis of a device: the signature of a device is the set of values that significant parameters of a machine or a system have in normal operating conditions or during the execution of a normal operation. deviations from the signature indicates abnormal operations. figure 4 shows an example of signature for a pneumatic actuator using the profile of the output pressure during an open/close cycle. 4.3. mixed analysis the mixed analysis may consider different parameters of the same machine/system or also different parameters of different machines. the main scope of the mixed analysis is to identify correlations between different parameters or events. the basic techniques used for the mixed analysis are: regression: it is necessary to find a formula that expresses the relation between different selected variables; correlation: an index quantifies the correlation between two or more variable. several indexes exist for measuring the degree of correlation of different variables. one of the most used index is the “pearson correlation index”. the research of correlation indexes requires the comprehension of the physical relations between the variables (to avoid incongruous results). figure 3. example of trend analysis. variance = 1 σ variance = 2 σ figure 2. example of variance analysis. figure 4. example of signature for a pneumatic actuator. 8 9 10 11 1 2 3 4 5 6 7 8 9 10 8 9 10 11 12 1 2 3 4 5 6 7 8 9 10 acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 59 5. a case history: bombardier transportation italy 5.1. a short history this paper describes the evolution of maintenance for two very popular locomotives: model e483 and model e186 that run in almost all the european countries. both vehicles today have a technological structure that allows the implementation of remote maintenance. process and maintenance data are collected on-board, and are fully accessible by the maintenance staff. both locomotives started about ten years ago from a status of simple reactive maintenance, and the implementation of on-board and off-board infrastructures was therefore necessary to achieve remote maintenance. today all vehicles are equipped with a diagnostic controller that collects all the diagnostic data and sends them to an ethernet gateway that communicates with the off-board system via gprs. similarly, the controller of each sub-system on board collects and transmits all the process data, including environmental data like position, speed, ambient temperature, etc. the control centre in vado ligure downloads these two databases through the gprs modem and makes data available to maintenance staff through the web portal mybtfleet. for safety reasons, national rules or laws make a maintenance stop for each vehicle mandatory at fixed schedule (e.g. in italy every 6 months). through cbm techniques the maintenance operators can define the list of maintenance activities (not safety related) to schedule at a given stop. 5.2. actions towards condition based maintenance for increasing the level from remote maintenance to cbm, it was necessary to implement a system for the graphical display of the diagnostic data stored in the database to give to the maintenance staff an immediate view of the evolution of every parameter. a specific tool for the intelligent data sorting and visualization was created with matlab. figure 6 shows an example of graphical output for digital variables. maintenance engineers may use this tool for soting variables using: a defined period of time, the id code of the variables, the sub-system, the status (e.g. show the variables that were “off” in the period from x to y). the tool can also calculate simple kmis like: counters (in a defined period of time), conditioned counters (e.g. operating time with variable x “on” and variable “y” greater than z), statistics for each signal or group of signals (number of positive or negative transitions, time on/off, etc.). more complex techniques of data mining are possible, but the size of the database suggests prudence. the monitored vehicles are about 150, and the considered parameters are about 500 for each vehicle, with a sampling time of 1 second (data transmission is only by exceptions). the next three examples refer to the bombardier fleet of vehicles, and represent: figure 5. example of correlation between different variables. figure 6. sample display of the visualization tool. acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 60 a horizontal analysis for the identification of cbm rules for the main circuit breakers, a vertical analysis based on the signature for the air compressor on board, a complex analysis of multiple measurements of the wheels consumption for determining their expected end-of-live. the manufacturers of the mcbs consider an average number of operation equal to 1000 after two year, and of 4000 after 8 years. figure 7 and figure 8 show that the main circuit breaker of all the locomotives have a much lower number of operations (both ac and dc). 5.3. rules for cbm: main circuit breaker the main circuit breaker (mcb) connects the locomotive to the supply line, and it is one of the most critical components. the mcb may be either dc (code “ir”) or ac (code “ip”). the mcb is an electro-mechanical equipment enduring both mechanical and electrical stresses. we used the historical database of the fleet for analysing all the failures of the mcbs looking for kmis related with the failures. this phase of the study uses the knowledge of the equipment. in other words, we pre-selected a subset of data according to the knowledge we have of the construction and operation of circuit breakers to reduce the variables of the problem. the following kmis proved to be useful: kmi#1: counter of normal opening/closing operations of the mcb (with no current); kmi#2: counter of mcb trips (the mcb opens the short circuit current); condition#1: train speed > 3 km/h (the train is not in a station); condition#2: status of the pantograph (open/close). we carried out a mixed analysis in search of correlations on the entire fleet of locomotors e186 for the year 2013 that is 55 vehicles for about 11 months (a locomotive remains in the depot for about 1 month a year). the two kmis are further split into two classes using condition#1. when the pantograph is open (condition#2), the counter is not increased. the average value of these kmis per month, together with their standard deviation was calculated to find out vehicles with abnormal values. through the study of the maintenance record of these vehicles, we invented the following heuristic rule: if (kmi#1 average per month > 3.5) and (kmi#2 average per month > 3.5) then (maintenance is required) no vehicle with smaller kmis experienced any failure of the circuit breaker. on the other hand, 33 out of 100 circuit breakers with higher kmis did not experienced failures. the proposed heuristic rule gives the following results: number of maintenance interventions per year: . with remote maintenance 55 . with cbm rules 15 of which 10 necessary and 5 unnecessary. the application of the rule on the fleet of vehicles for one year of operation reduces the number of unnecessary interventions from 45 to 5 and does not cause any missing maintenance for the mcbs that really need it. 5.4. rules for cbm: air compressor the air compressor in a locomotive is an essential part of the braking system. the compressor has two output sections that supply the main braking circuit at a rated pressure of 10 bar, and the primary circuit at 5 bar. pressure losses happen when the brakes operate and for the normal leakages of the circuits. the compressor is controlled in on/off mode by a pressure switch with a hysteresis of 0.05 bar around the rated value. typically, manufacturers of compressors for railway applications suggest a complete revision after 12,000 working hours (cyclic maintenance). to have a more accurate maintenance indicator the “signature” of the compressor was defined. when the train runs in normal coasting between two stations, the leakages of the circuits cause the pressure losses. the compressor compensates these losses and maintains the pressure in the range of 4.95-5.05 bar. the horizontal analysis of data shows that the recharging time during the coasting phase for a new compressor lays between 60 s and 70 s. longer periods, in a reasonable percentage, should be an indicator of an overcoming problem in the compressor, mainly due to malfunctioning of the bearings. the following quantities are sampled every second: date and time speed pressure in the main pipes pressure in the principal pipes running kilometres data were collected for a period of two months on the fleet of 186 locomotors in the netherlands. we identified the “signature” of sound compressors and used it as a benchmark for finding out compressors with abnormal operation. the term “signature” refers to a set of measurement or quantities that identify the behaviour of a given component. the proposed kmis for the compressors considers: the overall average duration of a “on” period for the compressor, the average duration of a “on” period for the compressor during coasting, figure 7. number of ip-mcb operations per year. figure 8. number of ir-mcb operations per year. acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 61 the load cycle of the compressor when the train is running (percentage of time “on” compared with the total running time). figure 9 shows a typical profile of the air pressure during normal operation of a train. pressure (blue line) increases when the compressor is running (red line), and it decreases when the compressor is off. coasting is detected considering the train speed (light blue line); when it is constant the train is coasting, and pressure losses are only caused by the pipes leakages. pressure variation during braking phases depends on the action of brakes, and are not related with the compressor status. when the three kmis that compose the signature are listed in a table like in table 1, some anomalies can be detected. as table 1 shows, compressors work during the coasting phase an average time of about 70 s. a longer time and an increase of this time over different periods may identify an upcoming problem of the compressor. on the other hand, table 1 shows that the average value of the working percentage of the compressor is around 25 %. again, values higher than the average may indicate that a failure or a malfunctioning of the compressor is approaching. for this analysis, we consider the average value and the standard deviation σ to identify the elements or vehicles that show an abnormal behaviour. considering the normal distribution and the standard deviation, table 1 points out two vehicles that have a very abnormal behaviour: locomotive 186113 and 186144. the main pipe recharging time, during the coasting phase, is a good indicator of the compressor performance. it is plain that the performances of e186113 and e184144 have a completely different behaviour from the other vehicles of the fleet, and an accurate check of these compressors during the next maintenance stop is scheduled. as all the compressor manufacturers indicate, another important parameter for maintenance purposes is the total working hours of the compressor. typically, a complete overhaul is recommended after 12,000 working hours. considering the average daily mileage and the effective percentage of working hours, it is possible to estimate the total worked hours for each compressor, and to evaluate when it will reach the target of 12,000 hours. table 2 shows the estimated end-of-life calculated for the compressors under analysis. with the present usage rate, the table 1. comparison of kmis for different locomotors. table 2. estimated remaining life of the compressor. figure 9. typical profile of pressure and compressor operations. #vehicle avg time on avg time on coasting avg % operation # samples 186111 90.83 74.48 25.30 % 6 186112 122.91 73.98 30.05 % 4 186113 213.59 126.45 48.46 % 6 186114 73.81 64.47 18.58 % 4 186115 75.49 59.74 16.98 % 14 186116 68.27 54.39 16.13 % 4 186117 88.11 71.74 25.36 % 8 186118 75.79 60.09 22.77 % 8 186119 76.03 60.17 18.64 % 9 186120 77.69 64.45 22.42 % 10 186121 90.03 75.18 26.03 % 6 186122 89.81 67.35 26.89 % 3 186144 121.95 92.87 32.81 % 5 186236 76.05 63.22 21.81 % 5 # vehicle daily working hours est. remaining life [years] 186111 4.44 7.41 186112 4.54 7.25 186113 9.08 3.62 186114 3.38 9.72 186115 3.07 10.72 186116 1.90 17.27 186117 4.58 7.18 186118 3.79 8.68 186119 3.12 10.54 186120 4.23 7.77 186121 4.16 7.89 186122 4.30 7.64 186144 5.25 6.26 186236 3.39 9.70 acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 62 compressor of vehicle 186113 will reach the target working hours in 3.5 years, so it has to be the first to be overhauled. 5.5. rules for cbm: wheel profile monitoring when a locomotive enters/exits the depot, the automatic vehicle inspection system (avis) collects data using laser, thermal and optical imaging technologies. data are stored and analysed and, when necessary, avis generates alarms and automatic maintenance orders (see figure 10). the two main parts of avis are: vems (vehicle equipment measuring system) that includes all the sensors, lasers, and cameras that generate the raw measurement data; orbiflo: the bombardier software platform that analyses the data for assessing the health status of the vehicle and for implementing rules for maintenance. figure 11 shows vems, a modular system that contains the following modules: axle end temperature monitoring system; it measures the dimensions and temperatures of axes; brake pad monitoring system: it measures the thickness of every brake pad, calculates brake pad wear rate and predicts when replacement is due; brake disc monitoring system: it measures the brake disk thickness, the disk profile and its maximum wear depth; wheel profile monitoring system: it measures the wheel profile and assess condition in comparison to several key markers (flange height, thread hollow, etc.); pantograph wear monitoring system: it checks the carbon strip profile, its maximum wear depth, and localized chip size; wheel damage monitoring system (wdms): it measures flat spots on wheels and determines when a wheel must be re-profiled by means of thresholds on the various measures; visual image capture system: through laser scanning and optical imaging, it captures many data points of the train exterior, permitting verification of any deviation from the vehicle profile or previous vehicle condition. it is possible to measure the car height, coupler height, to detect missing or displaced elements, open equipment boxes, foreign bodies, etc. it also detects and assesses damper leakage conditions and vehicle contamination (oil leakage, impact damage, graffiti, etc.). the study that we carried out uses the data of the wheels of a british fleet composed of 302 trains measured and collected by avis. measurements on each vehicle include 16 wheels; 8 right wheels (rhs) and 8 left wheels (lhs). the analysis considers the following wheel parameters (see figure 12): diameter of the wheel; flange height; flange thickness; difference between the right wheel’s diameter and the left one; tread hollowness. to facilitate the data analysis, the data management system of figure 13 was created. a server runs osisoft pi analysis software composed by pi interface, pi data archive. the users are client of the pi server, and run pi processbook and pi datalink for visualizing the data, together with matlab for implementing the researched maintenance algorithms. the combined use of pi processbook for the dynamical graphic representation of the variables of more wheels simultaneously (figure 14), together with specific analysis developed in matlab environment, made it possible to find out a formula for predicting the lifespan of the wheels: 𝐿𝐿𝐿𝐿𝐿𝐿𝐿𝐿 = 𝑇𝑇 365 ∗ 𝐷𝑚𝑚𝑚 −𝐷𝑚𝑚𝑚 − �𝑇𝑇 ∗𝐷𝑑𝑑𝑑𝑑� 365 � �𝑇𝑇 ∗𝐷𝑑𝑑𝑑𝑑� 365 � − 𝛥𝐷 . (1) with conventional preventive maintenance policy, wheels figure 10. the avis block diagram. figure 11. vehicle equipment measuring system (vems). figure 12. wheel profile. figure 13. data management system with osisoft. acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 63 are re-profiled every 200 or 300 days, according to the region the vehicle works in. with a policy of cbm, the analysis showed that it is possible to calculate the remaining useful lifespan of a wheel starting from the values collected by the wdms. interpolating the available values, it is possible to predict when a wheel will run out of tolerance (see figure 15). this algorithm may reduce the maintenance interventions for the vehicle wheels of about 10 %. its application just started few months ago, and sufficient data are not yet available for evaluating its effectiveness. 6. conclusions maintenance is one of the higher costs for a company that controls a fleet of vehicles. on the other hand, maintenance has an important effect on the quality of service of the same company. an intelligent maintenance policy has positive effects in terms of both costs and qos. in the paper we propose a roadmap that may lead a company from a cyclic maintenance policy (either costly or ineffective) to condition based maintenance (maintenance only when required). increasing the effectiveness of the maintenance requires a technological infrastructure for on-board data collection and transmission to a ground maintenance centre where data are stored and analysed. raw data from the vehicles are to be processed and grouped into key maintenance indicators (kmi) that become the basic elements to use for maintenance-oriented algorithms. the identification of kmis for a device or sub-system is based on two milestones: the technical knowledge of the construction and operation of the device or sub-system to find-out a set of parameters that may be useful for maintenance purposes, the analysis of the historical database for cross-correlating the candidate kmis and the behaviour of the device or subsystem. the paper shows three examples of this analysis: the kmis and the rule for deciding when the main circuit breaker of locomotive has to be maintained; the kmis and the rule for understanding when the air compressor of the locomotive need an intervention. a rule for calculating the expected end of live is also presented; a real-time data analysis structure for the calculation of the wheel consumption and the prediction of the necessity of their re-profiling. these rules are implemented on line, and are used for sending automatic maintenance warnings to the maintenance staff. references [1] a. h. c. tsang. “strategic dimensions of maintenance management”, journal of quality in maintenance engineering, vol. 8, no. 1, pp. 7-39, (2002) [2] w. wang, a. h. christer. “a survey of maintenance policies of deteriorating systems”, european journal of operational research, vol. 139, pp. 469-489, (2002) [3] m. bevilacqua, m. braglia. “the analytic hierarchy process applied to maintenance strategy selection”, reliability engineering & system safety, vol. 70, pp. 71-83, (2000) [4] x. zhou, l. xi, j. lee. “reliability-centered predictive maintenance scheduling for a continuously monitored system subject to degradation”, reliability engineering & system, vol. 92, no. 4, pp. 530-534, (2007) [5] l. mann jr., a. saxena, g. m. knapp. “statistical-based or condition-based preventive maintenance”, journal of quality in maintenance engineering, vol. 1, no. 1, pp. 46-59, (1995) [6] m. bevilacqua, m. braglia. “the analytic hierarchy process applied to maintenance strategy selection”, reliability engineering & system safety, vol. 70, pp. 71-83, (2000) [7] t.farinha, i.fonseca, a.simoes, m.barbosa, j.viegas, “new ways for terology through predictive maintenance in an environmental prospective”, wsea transactions on circuits and systems, issue 7, vol.7, july 2008, pp.630-647 [8] r. dekker, p. a. scarf. “on the impact of optimization models in maintenance decision making: the state of the art”, reliability engineering & system safety, vol. 60, pp. 111-119, (1998) [9] m. bohlin, m. forsgren, a. holst, b. levin, m. aronsson, r. steinert, “reducing vehicle maintenance using condition monitoring and dynamic planning”, railway condition monitoring, 2008 4th iet international conference on, pp. 1-6, (2008) [10] p. pinceti, m. caserza magro, e. de paola, p. firpo, “a technological infrastructure for implementing a policy of condition based maintenance for a fleet of railway vehicles”, 3rd int. conference on circuits, systems, communications, computers and applications (cscca ’14), florence (it), 22-24 november 2014 [11] g. del gobbo, m. giovannuzzi, m. romairone, p. masini, s. rizzo, f. romano, m. romeo, “the telediagnostica system for trenitalia e464 and e405 fleet”, ingegneria ferroviaria, vol. 2, pp. 137-160, (2012) [12] i. zolotova, t. lojka, “online data stream mining in distributed sensor network”, wsea transactions on circuits and systems, vol.13, 2014, pp.412-421. figure 14. top: diameter, flange height and flange thickness trends of the same wheel; bottom: diameter trends of two different wheels. figure 15. flange thickness (red line), its linear regression (blue line) and its limits (green and pink lines). cbm for a fleet of railway vehicles: infrastructure and algorithms acta imeko, title acta imeko issn: 2221-870x june 2018, volume 7, number 2, 45-49 acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 45 using a benthic lander to explore and monitor vulnerable ecosystems in the mediterranean sea gianfranco d’onghia, francesca capezzuto, roberto carlucci, angela carluccio, porzia maiorano, michele panza, pasquale ricci, letizia sion, angelo tursi department of biology, university of bari aldo moro, via e. orabona 4, 70125 bari – conisma local research unit section: research paper keywords: journal; template; imeko; microsoft word citation: gianfranco d’onghia, francesca capezzuto, roberto carlucci, angela carluccio, porzia maiorano, michele panza, pasquale ricci, letizia sion, angelo tursi, using a benthic lander to explore and monitor vulnerable ecosystems in the mediterranean sea, acta imeko, vol. 7, no. 2, article 8, june 2018, identifier: imeko-acta-07 (2018)-02-08 section editor: fabio leccese, università degli studi di roma tre, italy received january 18, 2018; in final form march 23, 2018; published june 2018 copyright: © 2018 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: gianfranco d’onghia, e-mail: gianfranco.donghia@uniba.it 1. introduction several systems exist to take photographs and videos of organisms and habitats in deep waters, ranging from submersible to remotely operated vehicle (rov), from landers to underwater platforms settled on the bottom [1]-[6]. in addition to fotoor videocameras, all these systems can be equipped with ctd, current meters, sediment traps, baitrelease and other devices. although all these systems suffer from the fact that quantification and even identification of species can be doubtful, their low impact has become fundamental in the study of sensitive and vulnerable habitats, where grabs, sledges and fishing gears would damage the benthic fauna and habitats unacceptably [4], [5], [7], [8]. moreover, video systems are innovatory since they allow to collect data on environmental parameters, seafloor features, habitat and associated fauna, even in less accessible habitats, such as seamounts and canyons [7]-[10]. the main applications of a baited lander used to explore and monitor marine ecosystems in the mediterranean sea, with indications of advantages and limitations, are reported in this paper. 2. lander technical features the ecology team of the department of biology has developed the memo (marine environment monitoring system) baited lander, as part of the eu_7fp coralfish project, with the aim of investigating the seafloor characteristics, biological diversity and environmental variables in the deep-sea ecosystems. the memo lander consists of a stainless steel frame (ø 2.15 m; h 1.65 m); 2 video cameras (hd multi seacam) with 2 white led lights, an electronic compass, inclinometer and altimeter; a multiparametric probe (sea-bird electronics seacat profiler 19 plus) for the measurement of pressure, temperature, conductivity, oxygen, ph and turbidity; a doppler current meter (nortek aquadopp 2000); 4 deep-sea batteries (12 v, 80 ah); an acoustic modem (teledyne benthos atm-900 series); an electronic control unit (communication abstract the memo (marine environment monitoring system) baited lander is used to explore and monitor marine ecosystems in the mediterranean sea. memo is equipped with 2 video cameras, a multiparametric probe and a current meter, fully operative down to 1000 m in depth for 24 consecutive hours. since 2010, the ecology team of the department of biology of the university of bari (italy) deployed the memo lander in some sensitive and vulnerable deep-sea habitats of the mediterranean sea, as part of national and international research projects. data on environmental parameters (depth, salinity, temperature and current) and related to benthopelagic species, their small-scale distribution, size and behaviour, have been recorded. memo has been also deployed in fragile and structurally complex habitats, such as coralligenous and cold-water coral habitats, in order to monitor physico-chemical and biological variables as part of the eu marine strategy framework directive. some main applications of the memo lander are reported here, with indications of relative advantages and limitations. acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 46 technology, ltd) capable of managing the entire system (figure 1). on the seabed memo is linked by a zinc-coated steel cable to buoys which keep the cable under tension (back up buoys) and then to a surface floating signalling buoy. the system is lowered to the seabed by winch and the surface buoy remains connected for recovery. continuous connection is maintained via the acoustic modem with an on board pc software platform, making images and sensor data available on the vessel. the memo lander can work down to 1000 m in depth for 24 consecutive hours and can be baited. by means of an ad hoc methodological approach of image analysis, the videos recorded by both video-cameras during each deployment are analysed using the adobe premier pro software and information on abundance and behaviour of bentho-pelagic species, including those of fishery interest, is collected. moreover, sizes of recorded specimen are measured from images using image j 1.46q, a java-based public domain program. 3. main applications the memo lander has been mostly deployed in some sensitive and vulnerable deep-sea habitats of the mediterranean sea, such as the santa maria di leuca (sml) cold-water coral (cwc) province (northern ionian sea) and the bari canyon (bc) (southern adriatic sea), as part of national and international research [9]-[16] (table 1). data on the characteristics of substrate together with depth (m), salinity (psu), temperature (°c) and current (m/s) were recorded. data on the species composition and abundance as well as their small scale-distribution and behaviour were also recorded. in order to estimate the species abundance, both the time of first arrival (1st arrival time), i.e. the time after touchdown in which a given species first arrives, and the maximum number of individuals in a single frame (max n) were also recorded by species [17]. a contribution to the knowledge of the deep-sea megafauna diversity in the sml cwc province and bc was provided in [9], [11]-[16]. the first in situ documentation of the crab paromola cuvieri behaviour interacting with other deep-sea species in the mediterranean sea was reported [11]. this crab was recorded at depths between 547 and 648 m in the sml cwc province. all the specimens carried a sponge on their exoskeleton using the pereiopods. they were distinguishable by the size and shape of the carried sponge. both passive covering behaviour and active behaviour were observed. in fact, some specimens of p. cuvieri were observed to use the sponge to push away oncoming possible competitors, such as the teleost fish blackbelly rosefish (helicolenus dactylopterus), or to discourage the attacks of other scavengers and/or predators, such as the bluntnose sixgill shark (hexanchus griseus) [11] (figure 2). while investigating the benthopelagic fauna of the sml cwc province, a total of 20 benthopelagic species (1 cephalopod, 6 decapod crustaceans, 5 chondrichthyes and 8 osteichthyes) were identified at depths between 547 and 790 m [12]. the fish blackspot seabream (pagellus bogaraveo) was exclusively observed in the coral habitat. the fishes european conger (conger conger) (figure 3) and h. dactylopterus were the most abundant species in this type of habitat [12]. a positive significant relationship between the species abundance and the current speed was detected in the investigated habitats [16]. the occurrence and behaviour of the sharks the gulper (centrophorus granulosus), the kitefin (dalatias licha), the velvet belly (etmopterus spinax) and the bluntnose sixgill (hexanchus griseus) in the sml cwc province were also recorded [13]. during four deployments of memo carried out in the bc, at depths of 443-788 m, a total of 12 benthopelagic fish species were identified [9], [14]. the blackspot seabream was the most observed fish species. groups of up to 40 individuals of p. bogaraveo were attracted to the bait and were observed even in single frames. in particular, individuals were both exploring the figure 2. digital frames showing the active behaviour of paromola cuvieri and its interaction with helicolenus dactylopterus (above) and hexanchus griseus (below) recorded by memo lander in the sml cwc province. figure 1. the memo lander equipment. acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 47 area and feeding actively on the bait (figure 4). a gregarious behaviour was observed. detection of p. bogaraveo individuals increased significantly when the current velocity decreased. the european conger was recorded at each deployment. clear scavenger behaviour was also observed in this teleost fish and in the shark e. spinax. the shark species c. granulosus and h. griseus were also recorded but, although attracted by the bait, they were never seen feeding on it. other fish species, harvested on fishing grounds, such as merluccius merluccius, h. dactylopterus and polyprion americanus (figure 5), were also recorded in bc. since 2015, the memo lander has also been used as part of the monitoring activity foreseen by the eu marine strategy framework directive (msfd) to detect changes in the sensitive and vulnerable habitats, not only in cwc sites and canyons but also in coralligenous and maerl habitats. for example, during 2016 memo was deployed off vieste and off pontine islands to observe coralligenous and maerl habitats, respectively. during two deployments in coralligenous habitats, environmental parameters were recorded and a total of 10 species (2 decapod crustaceans and 8 teleost fishes) were identified (figure 6). in maerl habitat off pontine islands a total of 6 taxa (2 crustaceans, 1 echinoderm and 3 teleost fishes) were identified (figure 7) (table 1). 4. advantages and limitations a benthic lander like memo, equipped with video cameras together with ctd, current meters and other devices have a great potential in monitoring changes in the ecosystems [5], [17]-[19]. it is highly suitable for recording physico-chemical parameters on the bottom, observing local bottom features and counting fish and invertebrates, providing a new insight on species small-scale distribution, behaviour and interactions. a benthic lander can be deployed in structurally complex habitats, such as cwc communities, canyons and seamounts. it is relatively non-destructive and less invasive, thus reducing the impact of the investigation on sensitive and fragile habitats. in general, a benthic lander needs a small sea time requirement for a long observation. its deployment and recovery is also possible from smaller vessels [5], [9], [16], [18], [20]. however, several difficulties and limitations should be taken into consideration during analysis of mobile benthopelagic fauna [5], [18]. one of the key difficulties in dealing with deep waters is low faunal density, which requires a very large sampling effort in order to have valid estimates. low densities figure 3. digital frame of conger conger recorded by memo lander in the sml cwc province. figure 4. digital frame of pagellus bogaraveo recorded by memo lander in the bari canyon. figure 5. digital frame of polyprion americanus recorded by memo lander in the bari canyon. figure 6. digital frames of pagellus erythrinus, serranus cabrilla, spicara flexuosa, (above) and dardanus arrosor, pagellus erythrinus, serranus cabrilla (below) recorded by memo lander in coralligenous habitat off vieste. acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 48 make statistical comparison between areas difficult because of the high variance in replicate counts. in addition, identifying invertebrates and fishes to species level only from video can also be challenging in areas with a high species diversity. although landers produce little disturbance when collecting data, they will only attract scavenging invertebrates and fishes, so are highly selective. precise positioning of the lander can also prove difficult. due to the complex topography of the cwc and canyon habitats, free fallings landers may sometimes miss their target or land directly on the corals causing damage and increasing the risk of entanglement. other disadvantages consist of equipment cost and relatively chance of its loss, many assumptions in the abundance estimates as well as in the weight of the animals, which cannot be determined directly, requiring the adoption of the length-weight relationships [20]. 5. conclusions the applications of the memo lander reported in this paper represent in situ documentation, with a very low impact, of the environmental parameters and fauna in sensitive and vulnerable habitats. they have provided new insights into megafauna diversity in the sml cwc province and bc, small scale distribution and behaviour of invertebrates and fishes and linkage between species and habitats. since these habitats show a high biodiversity and can act as refuge sites for valuable species [4], [14], they often are impacted by fishing activities and other anthropogenic injuries [8]. in addition to the use of a benthic lander, a rov can be a good complementary tool with the lander to investigate deepsea habitats since it explores a larger area, recording the sessile fauna (corals, sponges) distributed on the seafloor. however, the rov has limitations mainly related to noise and light created by the gear typology that may either scare away or attract invertebrates and fishes causing under or over estimations of their true abundances [3], [4], [6], [21]. acknowledgements this paper benefited from data recorded during the eu_7fp coralfish project, obama_prin and ritmare projects funded by the italian ministry of university and research (miur), marine strategy framework directive monitoring project funded by the italian ministry of the environment (mattm). the authors wish to thank daniela potenza for setting up the figures and the table. references [1] i.g. priede, p.m. bagley, j.d. armstrong, k.l. smith, n.r. merrett, “direct measurement of active dispersal of food-falls by deep-sea demersal fishes”, nature, vol. 351, 1991, pp. 647-649. [2] p.b. adams, j.l. butler, t.e. laiding, k.a. dahlin, w.w. wakefield, “population estimates of pacific coast groundfishes from video transects and swept-area trawls”, fishery bulletin, vol. 93, 1995, pp. 446-455. [3] v.m. trenkel, p. lorance, s. mahevas, “do visual transects provide true population density estimates for deepwater fish?”, ices journal of marine science, vol. 61(7), 2004, pp. 1050-1056. [4] m.j. costello, m. mccrea, a. freiwald, t. lundälv, l. jonsson, b.j. bett, t.c.e. van weering, h. de haas, j.m. roberts, d. allen, “role of cold-water lophelia pertusa coral reefs as fish habitat in the ne atlantic”, in: freiwald a., roberts j.m. (eds), cold-water corals and ecosystems. springer-verlag berlin heildelberg, 2005, pp. 771-805. [5] d.m. bailey, n.j. king, i.g. priede, “cameras and carcasses: historical and current methods for using artificial food falls to study deep-water animals”, mar. ecol. prog. ser., vol. 350, 2007, pp. 175-178. [6] a.w. stoner, c.h. ryer, s.j. parker, p.j. auster, w.w. wakefield, “evaluating the role of fish behavior in surveys conducted with underwater vehicles”, can. j. fish aquat. sci., vol. 65, 2008, pp. 1230-1243. [7] f.m. porteiro, j.n. gomes-pereira, c.k. pham, f. tempera, r.s. santos, “distribution and habitat association of benthic fish on the condor seamount (ne atlantic, azores) from in situ observations”, deep-sea res part ii, vol. 98, 2013, pp. 114-128. [8] a. cau, a. alvito, d. moccia, s. canese, a. pusceddu, r. cannas, m. angiolillo, m.c. follesa, “submarine canyons along the upper sardinian slope (central western mediterranean) as table 1. list of habitats explored by means of the memo lander in different surveys, years and areas with indication of the investigated depth range and total number (n) of the bentho-pelagic species video recorded. survey year area habitat depth (m) n of species coralfish, obama, msfd 2010, 2011, 2015 sml, tricase coral 547-797 24 ritmare, msfd 2012, 2015, 2016 bari and nora canyons canyon 442-788 27 coralfish, obama, msfd 2010, 2011, 2015 sml, gallipoli, monopoli mud 485-790 20 msfd 2015, 2016 vieste, egadi islands coralligenous 55-96 16 msfd 2016 pontine islands, egadi islands maerl/rhodoliths 63-87 7 figure 7. digital frames of maerl habitat as seen by the upper camera (above) and muraena helena feeding on bait (below) recorded by memo lander off pontine islands. acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 49 repositories for derelict fishing gears”, marine pollution bulletin, vol. 123, (1-2), 2017, pp. 357-364. [9] g. d’onghia, f. capezzuto, a. carluccio, r. carlucci, a. giove, f. mastrototaro, m. panza, l. sion, a. tursi, p. maiorano, “exploring composition and behaviour of fish fauna by in situ observations in the bari canyon (southern adriatic sea, central mediterranean)”, marine ecology, vol. 36, 2015, pp. 541-556. [10] a. cau, m.c. follesa, d. moccia, a. bellodi, a. mulas, m. bo, s. canese, m. angiolillo, r. cannas, “leiopathes glaberrima millennial forest from sw sardinia as nursery ground for the small spotted catshark scyliorhinus canicula”, aquatic conservation: marine and freshwater ecosystems, vol. 27, (3), 2017, pp. 731-735. [11] f. capezzuto, p. maiorano, m. panza, a. indennidate, l. sion, g. d’onghia, “occurrence and behaviour of paromola cuvieri (crustacea, decapoda) in the santa maria di leuca cold-water coral community (mediterranean sea)”, deep-sea research i, vol. 59, 2012, pp. 1-7. [12] p. maiorano, l. sion, f. capezzuto, r. carlucci, f. mastrototaro, m. panza, a. tursi, g. d’onghia, “exploring deep-sea benthopelagic fauna using a baited lander in the santa maria di leuca cold-water coral province”, rapp. comm. int. mer médit., vol. 40, 2013, pp. 719. [13] l. sion, f. capezzuto, r. carlucci, a. carluccio, a. indennidate, p. maiorano, g. d'onghia, “behaviour of deep-sea sharks observed in the santa maria di leuca cold-water coral province”, rapp. comm. int. mer médit., vol. 40, 2013, pp. 494. [14] g. d’onghia, f. capezzuto, f. cardone, r. carlucci, a. carluccio, g. chimienti, g. corriero, c. longo, p. maiorano, f. mastrototaro, p. panetta, a. rosso, r. sanfilippo, l. sion, a. tursi, “macroand megafauna recorded in the submarine bari canyon (southern adriatic sea, mediterranean sea) using different tools”, mediterranean marine science, vol. 16, no.1, 2015b, pp. 180-196. [15] t.d. linley, m. lavaleye, p. maiorano, m. bergman, f. capezzuto, n. cousins, g. d’onghia, g. duineveld, m. shields, l. sion, a. tursi, i.g. priede, “interaction of cold-water corals with fish diversity and density across differing european regions: data from three baited lander systems”, deep sea res. ii., vol. 145, 2017, pp. 8-21. [16] p. maiorano, f. capezzuto, c. calculli, r. carlucci, a. carluccio, l. cornacchia, m. panza, a. pollice, l. sion, a. tursi, g. d’onghia, “in situ observations of the benthopelagic fauna in the santa maria di leuca cold-water coral province (central mediterranean)”, plos one, submitted. [17] n.j. king, i.g. priede, “coryphaenoides armatus, the byssal grenadier: global distribution, abundance, and ecology ad determined by baited cameras”, am. fish. soc. symp., vol. 63, 2008, pp. 139-161. [18] g. d’onghia, j.h. fosså, “regional comparisons of different fish assemblages recorded from analysis of in situ video”, euproject coralfish, deliverable d50-report, 2012, pp. 166. [19] j.m. roberts, o.c. peppe, l.a. dodds, d.j. mercer, w.t. thomson, j.d. gage, d.t. meldrum, “monitoring environmental variability around cold-water coral reefs: the use of a benthic photolander and the potential of seafloor observatories”, in: freiwald a., roberts j.m. (eds), cold-water corals and ecosystems. springer-verlag berlin heildelberg, 2005, pp. 483-502. [20] a.j. jamieson, d.m. bailey, h.j. wagner, p.m. bagley, i.g. priede, “behavioural responses to structures on the seafloor by the deep-sea fish coryphaenoides armatus: implications for the use of baited landers”, deep-sea res. i, vol. 53, 2006, pp. 11571166. [21] c.h. ryer, a.w. stoner, p.j. iseri, m.l. spencer, “effects of simulated underwater vehicle lighting on fish behavior”, mar. ecol. prog. ser., vol. 391, 2009, pp. 97-10. microsoft word 302-1955-1-le acta imeko  issn: 2221‐870x  december 2015, volume 4, number 4, 1    acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 |  1  journal contacts    about the journal    acta imeko is an e-journal reporting on the contributions on the state and progress of the science and technology of measurement. the articles are mainly based on presentations presented at imeko workshops, symposia and congresses. the journal is published by imeko, the international measurement confederation. the issn, the international identifier for serials, is 2221-870x.      editor‐in‐chief  paolo carbone, italy    honorary editor‐in‐chief  paul p.l. regtien, netherlands    editorial board      leopoldo angrisani, italy filippo attivissimo, italy eulalia balestieri, italy eric benoit, france catalin damian, romania pasquale daponte, italy luigi ferrigno, italy edoardo fiorucci, italy alistair forbes, united kingdom sabrina grassini, italy fernando janeiro, portugal konrad jedrzejewski, poland andy knott, united kingdom fabio leccese, italy david macii, italy rosario morello, italy michele norgia, italy helena geirinhas ramos, portugal pedro ramos, portugal sergio rapuano, italy dirk röske, germany alexandru salceanu, romania constantin sarmasanu, romania diego scaccabarozzi, italy lorenzo scalise, italy emiliano schena, italy enrico silva, italy marco tarabini, italy susanne toepfer, germany rainer tutsch, germany ian veldman, south africa luca de vito, italy       about imeko    the international measurement confederation, imeko, is an international federation of actually 39 national member organisations individually concerned with the advancement of measurement technology. its fundamental objectives are the promotion of international interchange of scientific and technical information in the field of measurement, and the enhancement of international co-operation among scientists and engineers from research and industry.     addresses    principal contact  prof. paolo carbone university of perugia dept. electr. inform. engineering (diei) via g.duranti, 93, 06125 perugia, italy email: paolo.carbone@unipg.it    acta imeko  prof. paul p. l. regtien measurement science consultancy (msc) julia culpstraat 66, 7558 jb hengelo (ov), the netherlands email: paul@regtien.net    support contact  dr. dirk röske physikalisch-technische bundesanstalt (ptb) bundesallee 100, 38116 braunschweig, germany email: dirk.roeske@ptb.de  a standard for rotatory power measurement acta imeko issn: 2221-870x september 2019, volume 8, number 3, 48 – 58 acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 48 a standard for rotatory power measurement andreas brüge1, harald pfeiffer2 1 physikalisch-technische bundesanstalt (ptb), bundesallee 100, 38116 braunschweig, germany 2 physikalisch-technische bundesanstalt (ptb), abbestraße 2 – 12, 10587 berlin, germany section: research paper keywords: power measurement, ergometer, standard, medical devices, uncertainty citation: andreas brüge, harald pfeiffer, a standard for rotatory power measurement, acta imeko, vol. 8, no. 3, article 9, september 2019, identifier: imeko-acta-08 (2019)-03-09 editor: min-seok kim, kriss, republic of korea received march 1, 2018; in final form july 1, 2019; published september 2019 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: andreas brüge, e-mail: andreas.bruege@ptb.de 1. introduction 1.1. retrospection in the 1960s, the first ergometer test benches were based on lever-mass systems, whose measurement uncertainty of about 2 % was suitable for the performance of the then friction-braked ergometers [1]. with the advent of non-contact braking methods, it became possible to raise ergometer monitoring to a new level. for this purpose, manufacturers submitted a proposal for a rotational calibration device together with a demand for traceability [2]. in addition, the physikalisch-technische bundesanstalt (ptb) set the minimum requirements for ergometers [3] to ensure this traceability. the increased requirements can be fulfilled on the test equipment side using torque transducers, which, in a simple case, work as reaction transducers without rotation. for the highest requirements, they are located in the drive train as rotation transducers. the qualitative analysis of the measurement uncertainty influences on such rotating transducers undertaken by wegener and andrea [4] as well as experimental approaches [5] show that the transfer of the findings on non-rotating torque transducers to rotating transducers is a key element in the traceability of systems for the measurement of rotatory power. although ergometer calibrations are always performed in stationary states of constant revolutions, older literature occasionally speaks of ‘dynamic power measurement’ [6]. to avoid confusion with the measurements that are actually carried out dynamically, like in test benches of the automotive industry, recent work refers exclusively to ‘rotatory power measurement’. 1.2. ergometry today in germany, the medical assessment of the physical capabilities of patients is mainly based on measurements carried out with so-called ‘foot crank ergometers’. these ergometers are considered as medical devices with a measuring function and must, in accordance with the medical devices act (in german: medizinproduktegesetz – mpg) [7] and the subordinate documents medical products operating ordinance (in german: medizinproduktebetreiberverordnung – mpbetreibv) [8] and the guideline on metrological checks of medical devices with a measuring function (in german: leitfaden zu messtechnischen kontrollen von medizinprodukten mit messfunktion – lmkm) [9], be traceably linked to national standards at regular intervals. the task of the ptb is thus to make a national standard available which is suited to ensure the traceability of rotatory power measurements and complies with the requirements of the traceability chain from the national standard down to the ergometers. as a further development of a testing facility developed at ptb [10], ‘dyncal 100’ has been operated as such a standard at ptb since 2004. its metrological properties are described in this article. in the following sections, ‘standard’ is used to refer to this specific national standard for rotatory power. when other standards are mentioned, their full designation is always used. abstract in germany, the realisation and dissemination of the unit of rotatory power is a legal task of the national metrology institute aimed at ensuring the safe operation of ergometers for patients. at the physikalisch-technische bundesanstalt, this task is carried out by means of a standard that can be operated both as a regulated brake and as a regulated drive, so that the effective torque and the revolution speed can be measured traceably. in this article, technical details and uncertainty calculations about this standard are given. mailto:andreas.bruege@ptb.de acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 49 all symbols, abbreviations, and designations used in this paper are collected in alphabetical order in the glossary at the end of this article. 2. requirements placed on a national standard 2.1. the traceability chain in accordance with ‘mpbetreibv’, medical ergometers must, at regular intervals, undergo a so-called ‘metrological check’. thereby, comparisons with a working standard (testing device) must be performed within the working range of the ergometer (figure 1), which spans the range of the permitted values for revolution speed and power. the testing device thereby drives the crankshaft of the ergometer with a defined revolution speed, whereas the ergometer keeps a specified power constant by means of a regulated braking facility. from the values of the revolution speed and the torque measured by the testing device at the crankshaft, the power input of the ergometer is determined and compared with the specified power set at the ergometer. thereby, the deviation of the indication of the power measurement of the testing device may not exceed a third of the deviation of the indication of the ergometer under test (‘onethird requirement’). the testing devices, in turn, are traced back by means of reference standards that are operated in calibration laboratories and are directly linked to the national standard at ptb. the national standard must thus exhibit measurement uncertainties, which, in the course of the dissemination via the three subordinated levels described above, maintain a sufficient margin to the measurement uncertainties of the ergometer. 2.2. measurement uncertainty limits because ergometers in germany are mostly conceived in accordance with the standard din vde 0750-238 [11], the requirement that this standard places on ergometers can be considered as the minimum requirement. according to the din vde standard, the deviation of the power indication may not exceed 5 % or 3 w, depending on which of the values is greater. the deviation of the revolution speed indication may not exceed 2 min-1. numerous manufacturers, however, specify a maximum deviation of the indication of 3 % for their ergometers, such that this value is used as a guideline for testing devices. in accordance with the abovementioned ‘one-third requirement’ laid down in mpbetreibv, the testing device must, in this case, exhibit deviations of the indication smaller than 1 % or 1 w for the power, and smaller than 0.7 min-1 for the revolution speed. if the factor 3 is also applied to the other levels of traceability, the national standard should be able to comply with deviations of the indication of approximately 0.1 % or 0.1 w for the power, and 0.05 % for the revolution speed in the working range. with the coverage being k = 2, there is a requirement for relative, expanded measurement uncertainties of 0.2 % – or 0.2 w for the power – and 0.1 % for the revolution speed, respectively. 2.3. further requirements in the traceability chain described above, ergometers act – as a matter of principle – as brakes, since this is their actual mode of operation when a patient feeds in power via the foot crank. as a consequence, the testing devices must be designed as drives, and the reference standards of the next highest level must, in turn, operate as brakes. to be able to directly link both the figure 1. working range of a foot crank ergometer in accordance with din vde 0750-238. in this range, the ergometer must be able to keep the input power constant, corresponding to the power value set, within the measurement uncertainty and regardless of the revolution speed. figure 2. working range for the revolution speed of the national standard for rotatory power in the braking mode. in this range, the standard must be able to keep the input power constant, corresponding to the power value set, within the measurement uncertainty and regardless of the revolution speed that has been fed in. figure 3. working range for the torque of the national standard for rotatory power in the braking mode. acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 50 reference standards and the testing devices to the national standard, the national standard must be suited to both modes of operation. furthermore, the national standard should be composed of modules so that it can be disassembled into transportable units for in-situ calibrations. in accordance with the din vde standard, the working range of medical ergometers spans a range of powers from 25 w to 400 w and a range of revolution speeds from 40 min-1 to 100 min-1. in this range, the ergometer must be able to regulate the set power in a sufficiently stable manner, even if the revolution speed is fluctuating. for special applications, such as in sports medicine, there are ergometers which have a considerably extended working range. the national standard should therefore have a working range with powers from 5 w to 1,000 w and revolution speeds from 5 min-1 to 150 min-1 (figure 2). combinations of high revolution with low power lead to low torque values. values smaller than 0.5 n·m are not typical for medical applications and difficult to handle in breaking mode. therefore, the specification in figure 2 is restricted in the upper left corner. at the lower end of the revolution speed working range, the national standard is in the torque limitation (figure 2, black line). here, the minimum attainable revolution speed is determined by the nominal torque limit of 75 n·m (figure 3, green line). compliance with this limit is ensured within the control oscillations via an adjustable safety clutch and an electronic overload cut-off. 3. description of the standard the national standard for rotatory power measurement [12] consists of the measurement module, a motor control with a drive unit, and an amplifier (figure 4). the measurement module contains the sensors used to obtain the measuring signals for the torque, the revolution speed, and the temperature of the torque sensor. an external probe delivers the temperature of the ambient air. the motor control complies with the electric requirements of the measuring facility to generate the revolution speed and the power at the driveshaft constantly enough according to the settings determined by the operator. to this end, the control unit contains microcontrollers, a power regulator, a load resistor, and safety circuits. the measuring facility is driven by a brushless ac servomotor with an incremental encoder providing 4,096 pulses per revolution. the driveshaft is connected with the measurement module via a single-stepped, under-driven (at a 10:1 ratio) planetary gear train, a flexible coupling, and an adjustable safety clutch (figure 5). the measurement module includes a torque measuring flange with a nominal torque of 100 n·m. the frequency-modulated torque signals of this flange can be read out contact-free. furthermore, the module comprises a slit coupler, a transmittedlight encoding disc with 360 divisions per revolution, and a noncontact infrared temperature sensor. on the driving side of the measuring module, an adapter that is suited to the task can be clamped centrically using a hydraulic chuck. usually, the connection between the swivelling axes of an ergometer and of a testing device is established during the metrological check via a hexagonal telescopic bar. for that reason, an adapter with a hexagon head was chosen also for establishing the traceability between the national standard and the reference standard. for other tasks, such as monitoring measurements with a surveillance lever, a flat shaft end is needed as an output adapter. figure 4. national standard for rotatory power measurement. figure 5. measuring facility of the national standard for rotatory power. in the configuration shown, a balance beam is located at the torque measuring drive shaft for monitoring measurement purposes. figure 6. mounting of the separate measurement module into a national torque standard machine of ptb for static torque calibration. acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 51 all components of the measuring facility are mounted onto a frame in such a way that they can easily be disassembled for transport when in-situ measurements have to be carried out. furthermore, this opens up the possibility of mounting the measurement module separately into a torque standard machine in order to establish the traceability of the torque flange (figure 6). by means of the three levelling stands, the frame can be adapted to the different installation heights of the calibration objects, and horizontal positioning of the measuring axis can be set. 4. measuring principles 4.1. measurement of the revolution speed the revolution speed n in min-1 is measured optically by means of an encoding disc that is mounted on the driveshaft in the measurement module. this disc supplies 360 pulses per revolution, i.e. a square-wave signal sn is generated with the frequency 𝑓n = 360 60 s min 𝑛 (1) with a revolution speed of 120 min-1, fn yields 720 hz. a counter, zn, which is integrated in the amplifier, is armed by the rising edge of the revolution speed signal sn (figure 7, t1). with the next rising edge of the reference pulse sref with the frequency fref = 32 mhz, the counter reading is transmitted to a memory (figure 7, t2). the next rising edge of sref starts the counter with the value ‘1’ (figure 7, t3). from then on, the rising edges of the counting pulse szn with the frequency fzn = 8 mhz are counted and archived in the counting result p(zn). after a period of the revolution speed signal sn, the counter is, in turn, armed by the rising edge without influencing the running count (figure 7, t4). hereupon, the next rising edge of the reference pulse causes the current counter reading to be transmitted to the memory (figure 7, t5). the uncertainty of this triggering amounts to a maximum of one reference pulse at the beginning and one at the end of the counting process, as well as to a maximum of one counting pulse during the counting. the total uncertainty of the period interval measurement thus amounts to 𝑢pd = 2 32 mhz + 1 8 mhz = 1.875 ∙10−7 s (2) the largest relative uncertainty of the period interval measurement 𝑤pd is obtained at the highest revolution speed of 150 min-1 (fn = 900 hz). the relative uncertainty of an instantaneous period interval measurement thus amounts to 𝑤pd,e = 1.875 ∙10 −7 s ∙ 900 hz = 1.688 ∙10−4 (3) whereby the uncertainty of the reference pulse was neglected. for an averaging depth m of six periods and the assumption of a normal distribution, the uncertainty contribution of the averaged period interval measurement amounts to 𝑤pd,m6 = 𝑤pd,e √6 ∙360 = 3.6 ∙10−6 . (4) based on each measured value for the period interval, the controller computes the instantaneous revolution speed ne with a resolution of 32 bits, which is then, with a resolution of 0.005 min-1, sent to the computer for further analysis. this takes place 360 times per revolution. the highest relative contribution of the revolution speed resolution to the measurement uncertainty 𝑤rn of the revolution speed mean value lies at n = 5 min-1 and amounts, for an averaging depth m of six periods, to 𝑤rn6 = 0.005 min−1 5 min−1 ∙ √6 ∙360 = 2.2 ∙10−5 (5) the errors of the slots due to the limited resolution of their lithographic manufacturing of 1·10-3 mm led to a relative error of the slit edges of 1.15·10-3 and, together with the number of pulses per six revolutions (2160), to a mean relative jitter error wej of 2.47·10-5. the analysis of the relative contribution to the mean value of the period interval and of the revolution speed merely serves as orientation regarding the required relative measurement uncertainty for the revolution speed measurement. with 1·10-3, the required relative uncertainty is larger by a factor of more than 40 than the contribution of (5). in the case of the standard, averaging starts at the level of the instantaneous powers. the estimation above shows, however, that the resolution of the revolution speed values is sufficient for fulfilling the requirements placed on the standard. the instantaneous revolution speed ne in min-1 is obtained from the counting result p(zn) and the counting pulse frequency fzn and yields: 𝑛e = 60 ∙𝑓zn 360 ∙ 𝑝(zn) (6) 4.2. torque measurement 4.2.1. rotatory torque measurement in this way, a rising edge of the revolution speed pulse sn arms the torque counter zp, which determines the period interval of the torque signal in periods of the counting pulse szm (figure 8, t1). the following rising edge of the torque signal sm starts this counter (figure 8, t2). after the revolution speed counter zn (figure 8, t3) has stopped, the next rising edge of the torque signal sm ends the counting process of zp (figure 8, t4). at the same time as zp, the counter zm, which counts the number of periods of the torque signal sm that lie within the gate time of zn and zp, is triggered. figure 7. triggering of the counting process for the measurement of the revolution speed (according to [12]). acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 52 since arming the counter takes place at the same time as the rising edge of the revolution speed signal, the beginning and end of the counting processes for the revolution speed and the torque differ from each other by a maximum of one period interval of the torque signal. thus, the uncertainty is obtained from the difference between the gate time focuses for the two measurements as: 𝑢tz = 1 𝑓m . (7) depending on the applied torque, due to the frequency modulation of the torque signal, the value of utz oscillates between 2·10-4 s at the 100 n·m left-hand torque and 7·10-5 s at the 100 n·m right-hand torque. this still does not make the actual influence of this uncertainty in the result of the power determination accessible, because it depends on the dynamic evolution of the revolution speed and of the torque within the gate time of the counters. however, one can conclude that variations of the revolution speed and of the torque with frequencies below 500 hz – i.e. one tenth of the smallest occurring frequency of the torque signal – can still be reliably resolved by the measuring system. this is 200 times the upper revolution speed limit and is – also in view of the mass inertias which are involved in the rotation – sufficient for imaging the dynamic system. the instantaneous frequency of the torque signal fme, which is assigned to the current torque, is obtained from the counting results p(zp) and p(zm) as well as from the counting pulse fzm, and it yields: 𝑓me = 𝑝(zm) 𝑝(zp) 𝑓zm . (8) the counting uncertainty of p(zp) has the most unfavourable influence at the minimum frequency of the torque signal and at the maximum revolution speed i.e. at fm = 5 khz and n = 150 min-1. then, the frequency ratio between fm and fn only amounts to 5.55. when the phase angle is unfavourable, only four full periods of sm (5 khz) then fit into the gate time shown in figure 8. in this case, there is a counting error of one pulse of p(zp). on the other hand, the mean counting result amounts to 25,600 pulses (four periods of 5 khz need 0.8 s of time; in this time, 0.8 s · 32 mhz pulses are counted). the relative contribution of the counting error of p(zp) to the measurement uncertainty of the instantaneous torque measurement thus amounts to 𝑤zpe = 1 25600 = 3.9 ·10−5 . (9) at an averaging depth of six revolution periods, this results in a relative uncertainty contribution of 𝑤zpm = 3.9 ·10−5 √6 ∙360 = 8 ·10−7 . (10) the controller computes the instantaneous torque from the values of fme, taking the zero-point signal of the torque sensor of 10 khz and its sensitivity of 5 khz/100 n·m into account, according to 𝑀e = 𝑓me −10 khz 5 khz 100 n∙m (11) with a resolution of 1/325 n·m. at the most unfavourable torque value of the working range of 0.48 n·m (p = 5w, n = 100 min-1), this resolution corresponds to a relative uncertainty contribution of 6.4·10-3. by averaging over six revolution periods, the relative contribution is reduced to wrmm = 1.4·10-4. in contrast to speed measurements, a jitter error here does not also result from mechanical manufacturing tolerances; rather, it exclusively comes from electrical signal distortions, for which an upper estimate was found by equating them with wej. 4.2.2. static torque measurement in certain cases, it makes sense to also measure the torque applied to the sensor without rotation. this is, for example, the case if a weighing beam is mounted on the sensor output to establish the traceability of the torque sensor with direct-load masses (figure 5). the counters zp and zm can then be armed directly by the rising edges of the torque signal sm. the control surface of the standard allows such measurements with averaging depths m of 128 or 512 periods of sm. in accordance with equations (8) and (11), static torque values ms are calculated from the counting results. these are not instantaneous values as in the case of a rotation; rather, they are values that are averaged over a period of time that is obtained from the mean torqueequivalent frequency of sm and the averaging depth. analogous to equation (2), the triggering error of the torque counting which has to be taken into account yields, in the most unfavourable case of fm = 15 khz and m = 128, a contribution to the uncertainty of the static torque of wzsmm = 3·10-6. analogous to the previous section, the resolution, with m = 128, has a relative uncertainty contribution of wrs = 5.7·10-4. for m = 512, uncertainty contributions are yielded, which amount to wzsmm = 1.4·10-6 and wrs = 2.8·10-4. 4.2.3. corrections prior to further use, systematic errors must be eliminated from the uncorrected torque values m (me and mm, respectively). when in operation, the standard for rotatory power generates a considerable amount of heat. despite the thermal decoupling between the motor module and the sensor module, this leads to temperature increases of a few k during calibration. typical relative temperature coefficients of the torque sensor used lie around 1·10-5 / k. this allows for the contribution of usual temperature increases of up to 5 k to the relative measurement uncertainty to be estimated as wtm = 1.4 · 10-5 and the correction measures to be discarded. the temperature coefficient of the torque sensor zero signal usually lies clearly higher than that of the sensitivity, such that a correction must be performed. this is achieved by loosening the figure 8. triggering of the counting process for the measurement of the torque (according to [12]). acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 53 mechanical connection to the calibration object at appropriate locations during the calibration in order to determine the current idle torque m0,rot with the sensor output open and at medium revolution speed. this value is then used to correct the raw values m according to 𝑀korr1 = (𝑀 − 𝑀0,rot)𝐸korr (12) thereby applying a correction value ekorr for the drift of the signal processing of the torque sensor. the correction value ekorr is determined by means of intermediate electric tests using the internal calibration signal of the torque sensor. the characteristic curves of the torque sensors include nonlinearities that may lead to relative errors of typically a few 10-4. while tracing back the torque sensor used in the standard, these linearity errors are documented as a third-order polynomial for the right-hand torque branch and one for the left-hand torque branch of the characteristic curve, and the linearity errors are archived in the control program of the standard. when correcting the linearity errors, this polynomial is applied to each torque value: 𝑀korr2 = 𝑎1𝑀korr1 + 𝑎2𝑀korr1 2 + 𝑎3𝑀korr1 3 , (13) with the cubic coefficients ax (x = {1, 2, 3}) for the left-hand as well as for the right-hand torque. the hysteresis of the torque sensor is not included in the cubic polynomial and leads to a relative contribution of whym = 4.3 · 10-4 to the measurement uncertainty of the torque. 4.3. power determination when the din vde standard is operated, variations in the revolution speed occur due to instabilities in the feedback control of both the standard and the calibration object. in combination with the inertia torques of the rotating masses, this results in elastic vibrations, which either reduce or add to the system's idle power. the instantaneous values of the revolution speed and of the torque are obtained during periods of time that are short compared to the oscillation periods of the idle power. they enable the calculation of instantaneous values of the power, which are able to temporally resolve the idle powers. in this way, the measured values of the active power of the rotating system are available, which are necessary for assessing the control behaviour of the calibration objects. the instantaneous rotatory power pe is defined as the product of the instantaneous values of the torque me and of the angular velocity  e. based on the values of ne and mkorr2, which are provided by the controller, the control program computes 360 individual values of the instantaneous power per revolution 𝑃e = 𝜔e𝑀korr2 = 2𝜋 60 𝑛e 𝑀korr2 (14) which are made available as a timing diagram for the analysis of the control behaviour and are saved in a database. for the total work in the averaging interval, which – with equations (11), (12) and (13) – yields 𝐴m = ∑ 2𝜋 360 𝑀korr2,𝑖 𝑚∙360 𝑖=1 (15) and with the duration of this interval, which is yielded with equation (6), 𝑡m = ∑ 60 360 1 𝑛e,𝑖 𝑚∙360 𝑖=1 (16) the averaged power is calculated for an averaging depth m 𝑃m = 𝐴m 𝑡m (17) which can be selected in the control program. since significant components of the power variations correlate with the revolution speed of the measuring axis [10], these power variations can, for the most part, be averaged out by averaging over the complete revolutions of the measuring axes. in practical application, an averaging depth of m = 6 is typically used. this averaging depth is a compromise between a low measurement uncertainty and an appropriate time expenditure. in the case of traceability measurements, much larger averaging depths are usually selected in order to reduce, for example, the influence of the resolution of the signal processing on the measurement uncertainty. 5. regulation depending on the purpose of use, the standard must be able to work either as a brake or as a drive. in speed-regulated operation, the operator can pre-select a revolution speed that is maintained as stable by the standard – within the limits of the control parameters – against the variable braking torque of the calibration object. in this case, the standard operates in driving mode. in braking mode, the standard maintains a pre-defined torque or a pre-defined power by varying the braking torque against the variable revolution speed of the calibration object. due to the masses involved, both modes of operation can only realise the settling times in the range of a few seconds. this agrees well with the intention of the application, since fast variations in the revolution speed or in the torque of the ergometers must categorically be prevented for the patient's safety. observance of appropriate waiting times after a change in the parameters before starting a measurement sequence is therefore an integral part of the calibration rules. 6. traceability of the national standard since the realisation of the rotatory power measurement by means of the national standard presented here is not directly based on absolute quantities, the composed quantity must be traced back via the respective national standards of the individual quantities i.e. along five different paths (figure 9). the relative measurement uncertainties that are to be assigned to these paths are taken from the ishikawa diagram as shown in section 7 (figure 11) and from the indications given in table 1 and table 2. with the aid of transfer standards and auxiliary devices, the partial quantities ‘revolution speed’ and ‘torque’ can be traced back both statically and rotatorily. hereby, ‘statically’ means a measurement in which the measuring axis of the standard does not perform a rotation, whereas ‘rotatorily’ designates measurements where the measuring axis of the standard rotates. as described in section 4.2, the metrological difference between these two cases resides in the type of triggering. furthermore, static measurements can elucidate the properties of the amplifier separately to the interference values of the drive and of the acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 54 sensors, while the rotatory measurements shed light on the behaviour of the system as a whole. dynamic traceability can be attained without the rotation of the measuring axis. rather, it is attained by feeding simulated revolution speed signals into the amplifier, exploiting the triggering that is intended for rotatory operation. 6.1. traceability of the revolution speed the revolution speed is traced back to the national standard of time by means of a counter, which serves as a transfer standard (tf) for the reference frequency fr of 10 mhz. the measuring amplifier is equipped with an input and an output for the revolution speed signal sn by means of which external ttl signals can be fed into the measured value acquisition unit, and the signals from the encoded disk can be tapped without influencing their measurement in the amplifier. 6.1.1. dynamic traceability of the revolution speed with the aid of a function generator, a ttl signal is applied to the revolution speed input of the amplifier. the frequencies of this signal correspond to those provided by the encoded disk in the working range of the standard (figure 9, path 5). the revolution speed indication of the standard is compared with the fed-in frequency, which is determined by means of the abovementioned counter. the simultaneity of the two measurements is ensured by external triggering of both the amplifier and the counter. 6.1.2. rotatory traceability of the revolution speed here, instead of comparing the measurements of external signals generated by a function generator, the signals of the encoded disk are used. for this purpose, the measuring axis of the standard is caused to rotate without load. the internal triggering signal of the slit coupler is used to synchronise the amplifier and the counter (figure 9, path 4). these measurements can also be carried out with tolerable effort as intermediate tests of the revolution speed measurement. 6.2. torque traceability 6.2.1. static torque traceability the measuring module is decoupled from the rest of the standard and is mounted into the measuring axis of ptb's national standard for static torque, the 1-kn·m-dm standard facility (figure 6). there, the torque sensor can be calibrated statically i.e. as a temporally stable load in accordance with din 51309 [13] (figure 9, path 1). as a result of this calibration, three coefficients are calculated for the cubic correction of the linearity error of the torque sensor for left-hand as well as for right-hand torque. these coefficients are then archived in the control program of the standard. a second, subsequent calibration using these updated coefficients provides the torque indication errors of the standard, which are included in the relative measurement uncertainty of the static traceability wrms. a static electric stability measurement of the torque is carried out in the sensor module when ekorr is determined by means of an internal calibration signal. since here, only relative changes compared to a reference value are analysed, this is not a traceability process, and it is therefore not included in figure 9. 6.2.2. rotatory torque traceability since there is no national standard for rotatory torque traceability, traceability of this quantity can only be established via special investigations (figure 9, path 3). a ptb report [14] demonstrates that the relative influence wrmr of the angular velocity on the sensitivity of the standard's torque sensor from 8 n·m to 100 n·m is smaller than 5.1·10-4 at revolution speeds up to 600 min-1. the facility that was developed for this investigation (the rtd, figure 10) consists of a machine frame in which the torque sensor to be tested is mounted between two drives which, during rotation, apply a torque to the sensor via phase-shifted operation. this torque can then be measured as a reaction torque, using a reference transducer installed at the bottom of the machine. influences that can directly be attributed to the rotation (e.g. due to centripetal forces) can thus be estimated with wrmr in the measurement uncertainty analysis. other effects of the rotation that can increase the uncertainty of the static traceability of the figure 9. traceability of the national standard for rotatory power measurement to the national standards for time and static torque. there are five different traceability paths. figure 10. rotational torque device (rtd). facility for the determination of the influence of rotation on the sensitivity of the reference torque transducer of the national standard [14]. acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 55 torque measurement (such as friction and vibrations) can be eliminated in the first approximation by zero correction (12). 6.2.3. dynamic torque traceability ttl signals are applied to the inputs of the amplifier using a function generator. these signals simulate both the revolution speed output and the torque output of the sensor module. the torque indication of the standard is compared with the torque that is equivalent to the simulated frequency fed in, which is, in turn, measured by the transfer counter (figure 9, path 5). this allows the triggering and the data processing in the controller to be traced back independently of the mechanical generation of torques in the drive. 6.2.4. static intermediate testing of the torque measurement static torques can be applied to the rotation-protected measuring module using a facility consisting of a balance beam and load masses (figure 5). at ptb's torque calibration facilities, this device can be linked with the national standard for static torque (figure 9, path 2). in this way, stability tests of the torque measurement can be carried out between the regular link-ups of the static torque according to figure 9, path 1, without having to remove the measuring module. 7. measurement uncertainties when determining the measurement uncertainty for the rotatory power measurement by means of the national standard, the contributions of the counters involved in the measurement; the static and of the rotatory traceability of the torque; and various application-related quantities must be taken into account (figure 11). among the latter, the systematic indication errors of the measuring instruments are of particular importance. some of the contributions must be taken into account several times. the transfer counter enters, for example, into the rotatory traceability of the revolution speed and into the dynamic traceability of the torque. furthermore, the contributions of the counter zp to the torque determination enter both into the dynamic traceability and into the determination of ekorr and m0,rot. the relative combined measurement uncertainty for the determination of the rotatory power consists of the relative measurement uncertainties for the revolution speed and for the torque, each of which is, in turn, sub-divided and can thus be traced back to the relative standard measurement uncertainties indicated in table 1 as numerical values for the upper estimations of the contributions to the best measurement capability. hence, these are estimations in case an ideal calibration object is used in which, for instance, no feedback of the control oscillations to the standard need to be taken into account. the model function of the uncertainty budget is given according to equation (14) by equation (18): 𝑃e = 2𝜋 60 𝑛 𝑀 (1 − ∆𝑛)(1− ∆𝑀) (18) with ∆𝑛 = ∆𝑍n + ∆𝑍tf,n + ∆𝑛appl (19) and ∆𝑀 = ∆𝑀tr +∆𝑍p + ∆𝑀appl + ∆𝑍tf,m . (20) furthermore, taking into account parameters x, which correspond to the contributions in table 1 and are named according to the indices of these contributions, the following equations describe the compositions of n and m: ∆𝑍n = ∆𝑥dz + ∆𝑥tn+∆𝑥rn6+∆𝑥rn60+∆𝑥pd,m6 +∆𝑥pd,m60 (21) ∆𝑍tf,n = 2∆𝑥tn + 2∆𝑥dz +∆𝑥ttz +∆𝑥rz+∆𝑥tz (22) ∆𝑛appl = ∆𝑥anr+∆𝑥atz+∆𝑥ej (23) table 1. relative standard measurement uncertainties as contributions to the relative total measurement uncertainty. the index indicates the percentage of contributions to the sum of squared uncertainties, which are weighted with their respective multiple occurrences in the combined uncertainty. contribution probability distribution value index in % wrmr normal 2.55 ·10-4 38.42 wrms normal 1.70 ·10-4 17.08 whym rectangle 1.60 ·10-4 15.13 wfm rectangle 1.44 ·10-4 12.31 wrmm6 (0.48 n·m) rectangle 1.38 ·10-4 11.24 wrmm60 (0.48 n·m) rectangle 4.36 ·10-5 3.37 wanr (150 min-1) rectangle 4.02 ·10-5 0.96 wej normal 2.47 ·10-5 0.72 wr n6 (150 min-1) normal 2.20 ·10-5 0.29 wamd normal 1.90 ·10-5 0.21 wtm rectangle 1.44 ·10-5 0.12 wdm rectangle 1.00 ·10-5 0.12 wtn rectangle 3.00 ·10-6 0.02 wpd,m6 (150 min-1) rectangle 3.63 ·10-6 0.00 wttz rectangle 1.75 ·10-6 0.00 wpd,m60 (150 min-1) rectangle 1.15 ·10-6 0.00 wzpm6 normal 8.40 ·10-7 0.00 wdz rectangle 2.89 ·10-7 0.00 wzpm60 normal 2.66 ·10-7 0.00 watz rectangle 2.89 ·10-7 0.00 wr n60 (150 min-1) normal 2.27 ·10-7 0.00 wrz rectangle 1.00 ·10-7 0.00 wtz normal 1.00 ·10-8 0.00 figure 11. ishikawa diagram for the contributions to the measurement uncertainty of the rotatory power in ptb's national standard. acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 56 ∆𝑀tr = ∆𝑥rmm = ∆𝑥rms+∆𝑥rmr (24) ∆𝑍p = ∆𝑥zpm6 +∆𝑥zpm60+∆𝑥dz+∆𝑥rmm6+∆𝑥rmm60 (25) ∆𝑀appl = ∆𝑥amd + ∆𝑥ej+2∆𝑥zpm60+∆𝑥dz+2∆𝑥rmm60 +2∆𝑥dm+∆𝑥fm+∆𝑥tm+∆𝑥mhy (26) ∆𝑍tf,m = 2∆𝑥dz + ∆𝑥rz+∆𝑥ttz+∆𝑥tz . (27) the relative combined measurement uncertainty for the revolution speed wn results according to equation (19) from the contributions of the transfer counter ztf, from the contributions of the revolution speed counter zn in the standard, and from the contributions of the mechanical components nappl in the measuring module. hereby, wn yields, with the values from table 1 and the symbols from table 2: 𝑤n = |𝑤anr + 𝑤atz| +( 3 ∙𝑤dz 2 + 𝑤pd,m6 2 +𝑤pd,m60 2 + 𝑤rn6 2 +𝑤rn60 2 + 3∙𝑤tn 2 + 𝑤ttz 2 +𝑤rz 2 + 𝑤tz 2 +𝑤ej 2 ) 1/2 = 7.4 ·10−5 . (28) in terms of the contributions for the long-term drift wdz and for the temperature dependence wtn, it is assumed that they are the same for both counters. therefore, the respective measurement uncertainties enter twice for the traceability and once for the application. the relative indication errors of the two counters 𝑤anr and 𝑤atz are systematic components and contribute to the uncertainty. the prevailing contributions are the relative revolution indication error of the standard wanr, the uncertainty of the period interval measurement wpd,m, and the resolution of the revolution speed values of the standard wr n. wpd,m and wr n are applied as a function of the selected revolution speed and of the averaging depth m. the values used here are the values at n = 150 min-1, which lead to the maximum value of wn. when the standard is used for calibrations, m = 6 is selected in order to keep the time effort and the thermal load as low as possible. here, the contributions wpm,d6 and wr n6 must be applied. in the case of traceability measurements, longer averaging makes sense (e.g. m = 60 at n = 150 min-1), whose contributions are the relative uncertainties wpd,m60 and wr n60. in this context, the dynamic traceability of the revolution speed (figure 9, path 5) has the character of an additional test of the processing of revolution speed signals and is not necessary for the complete link-up of the revolution speed measurement. it is therefore not taken into account in this measurement uncertainty budget. since the counters are not in contact with the drive unit, the influence of the rise in temperature caused by the measuring operation is negligible. the relative combined measurement uncertainty for the mechanical link-up of the torque wrmm is obtained from the relative measurement uncertainty for the static torque traceability wrms and from the relative measurement uncertainty for the rotatory torque traceability wrmr with the values from table 1 and the symbols from table 2 and yields 𝑤rmm = √𝑤rms 2 +𝑤rmr 2 = 3.1 ·10−4 . (29) the relative combined uncertainty of the measurement of the torque wm yields – with (29), with the values from table 1 and with the symbols from table 2 – 𝑤m = |𝑤amd + 𝑤atz| + ( 𝑤rmm 2 + 𝑤zpm6 2 + 3 ·𝑤zpm60 2 + 𝑤rz 2 + 𝑤ttz 2 +𝑤rmm6 2 + 3 ·𝑤rmm60 2 + 4 ·𝑤dz 2 + 2 ·𝑤dm 2 + 𝑤ej 2 + 𝑤fm 2 + 𝑤tm 2 + 𝑤tz 2 + 𝑤hym 2 ) 1/2 = 4.3 ·10−4 . (30) the relative indication errors of the acquisition of the torque signal wamd and of the transfer counter watz enter as systematic error contributions to the measurement uncertainty, analogous to equation (28). the other components of the transfer counter are included as contributions to the dynamic traceability of the torque measurement (figure 9, path 5). the uncertainties of the counter zp, of the standard wzpm, and of its resolution are taken into account, analogous to equation (28) with m = 6 and m = 60, respectively. thereby, the components are included with m = 60 for the link-up itself, for the determination of ekorr and for the determination of m0,rot, i.e. three times. therefore, wrmm60 and wdz are also applied three times; in the case of wdz, factor 4 appears because the drift of the counter zp is also estimated with the value of wdz. components from the static intermediate testing of the torque measurement (figure 9, path 5) do not enter, because these measurements only serve as a stability test of the torque measurement. with equations (14), (28), and (30) and the coverage factor k = 2, the best measurement capability of the rotatory power measurement wp yields 𝑊p = 2√𝑤n 2 + 𝑤m 2 = 8.7 ·10−4 (31) and the best measurement capabilities (k = 2) for the revolution speed wn and for the rotatory torque wm amount to 𝑊n = 1.5 · 10 −4 (32) and 𝑊m = 8.5 ·10 −4 . (33) the requirements that are placed on the national standard and are described in section 2.2 are thus met. to confirm the coverage factor k = 2 in this measurement uncertainty budget, the effective degrees of freedom of the relative combined measurement uncertainty were determined according to welch-satterthwaite [15] to eff = 1315 and applied to the student-t distribution. acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 57 table 2. part 1. symbols used in the text. symbol unit description am ws work in the averaging interval ax 1 cubic coefficients for the correction of the linearity error of the torque sensor (x = {1, 2, 3}) ekorr 1 correction factor for the sensitivity drift of the torque sensor fm hz frequency of the torque signal fme hz instantaneous value of the frequency of the torque signal fn hz frequency of the revolution speed signal fr hz reference frequency of the transfer counter ztf, 10 mhz fref hz frequency of the reference pulse fzm hz frequency of the counting pulse for the torque measurement, 32 mhz fzn hz frequency of the counting pulse for the revolution speed measurement, 8 mhz i 1 continuous index k 1 coverage factor m 1 averaging depth, only full number of revolution periods of the measuring axis m n·m torque m0,rot n·m idle torque me n·m instantaneous torque mkorr1 n·m torque value after correction of zero drift and sensitivity drift mkorr2 n·m torque value after correction of the linearity error mm n·m averaged torque ms n·m statically measured torque n min-1 revolution speed ne min-1 instantaneous revolution speed p 1 counting result pe w instantaneous power pm w averaged power t s time tm s duration of the averaging interval upd s mu of the period interval of the revolution speed signal utz s mu of the difference between the gate time of the focuses of the revolution speed counting and that of the torque counting wamd 1 indication error of the torque measurement, dynamic, counter zp wanr 1 relative indication error of the revolution speed measurement, rotatory, counter zn watz 1 relative indication error of the transfer counter ztf wdm 1 relative mu due to drift of the torque sensor wdz 1 relative mu due to long-term drift of the counters wej 1 relative jitter error of the encoded disc wfm 1 relative mu due to humidity of the torque sensor table 2. part 2. symbols used in the text. symbol unit description whym 1 relative mu due to hysteresis of the torque sensor wm 1 relative combined mu of the torque measurement wp 1 relative expanded total mu of the rotatory power measurement wpd 1 relative mu of the revolution speed due to triggering wpd,e 1 relative mu of the revolution speed due to triggering, instantaneous value wpd,m 1 relative mu of the revolution speed due to triggering (period interval), averaged value wpd,m6 1 relative mu of the revolution speed due to triggering, averaged value from 6 periods wpd,m60 1 relative mu of the revolution speed due to triggering, averaged value from 60 periods wrmm 1 relative mu of the rotatory/dynamic torque measurement due to the resolution of the data transmission wrmm 1 relative combined mu of the mechanical traceability of the torque wrmm6 1 relative mu of the dynamic torque measurement due to the resolution of the data transmission for m = 6 wrmm60 1 relative mu of the dynamic torque measurement due to the resolution of the data transmission for m = 60 wrmr 1 relative mu of the rotatory traceability of the torque sensor wrms 1 relative mu of the static traceability of the torque sensor wr n 1 relative mu of the revolution speed measurement due to the resolution of the data transmission wr n6 1 relative mu of the revolution speed measurement due to the resolution of the data transmission for m = 6 wr n60 1 relative mu of the revolution speed measurement due to the resolution of the data transmission for m = 60 wrs 1 relative mu of the static torque measurement due to the resolution of the data transmission wrz 1 relative mu of the transfer counter due to the resolution wtm 1 relative mu of the torque sensor due to temperature wtn 1 relative mu of the counters due to temperature wttz 1 relative mu of the transfer counter due to triggering wtz 1 relative mu of the transfer counter wzpe 1 relative mu of the counting for the torque measurement, instantaneous value wzpm 1 relative mu of the counting for the torque measurement, averaged value wzpm6 1 relative mu of the counting for the torque measurement, averaged value from 6 periods wzpm60 1 relative mu of the counting for the torque measurement, averaged value from 60 periods wzsmm 1 relative mu of the counting for the static torque measurement, averaged value e 1 instantaneous angular velocity xy influence quantity for uncertainty calculation, y: index of standard uncertainties wy wm 1 relative expanded total mu of the rotatory torque measurement wn 1 relative combined mu of the revolution speed measurement wn 1 relative expanded total mu of the revolution speed measurement acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 58 8. summary the traceability of medical ergometers to national standards, which is stipulated by law in germany, is ensured by a standard for rotatory power measurement that is operated at ptb. this standard is, in turn, linked up with ptb's national standards for time and static torque. in order to ensure sufficient measurement uncertainty budgets for the traceability chain from the national laboratory down to the medical ergometers at all levels involved, the best measurement capabilities of 0.2 % for the power in the range from 5 w to 1000 w and of 0.05 % for the revolution speed in the range from 5 min-1 to 150 min-1 are stipulated. the analysis of the measurement uncertainties that have to be taken into account for the traceability and the operation of the standard for rotatory power shows that these requirements are being met and that the standard described is therefore suitable for fulfilling this legal task. references [1] public health service publication no. 1000 – series 2 – no. 21, calibration of two bicycles ergometers used by the health examination survey, washington, d.c., 1967 [2] w. sontopski, kalibrierung und qualitätssicherung bei elektronischen ergometern (calibration and constant quality for electronic ergometers), in: standardisierung, kalibrierung und methodik in der ergometrie (calibration, standardizing and methodology in ergometry), h. mellerowicz, i. w. franz (editors). perimed, erlangen, 1982, pp. 26-33. [3] e. cramer, performance requirements for bicycle ergometers, in: progress in ergometry: quality control and test criteria, proc. of the fifth international seminar on ergometry, springer, 1984, pp. 170-173. [4] g. wegener, j. andrea, measurement uncertainty of torque measurements with rotating torque transducers in power test stands, measurement 40, 7-8 (2007) pp. 803-810. [5] a. brüge, influence of rotation on rotary torque transducers calibrated without rotation, proc. of the xiv imeko world congress, tampere, finland, 1-6 june 1997, vol. iii, pp. 79-84 [6] d. woods, i. withers, the dynamic calibration of cycle ergometers; international journal of sports medicine, 15, 4 (1994) pp. 168-171. [7] gesetz über medizinprodukte (medical devices act – mpg), official gazette i, aug. (1994) p. 1963, amendment: official gazette i, jul (2017) p. 2757 [in german]. online [accessed 16 september 2019]: http://www.gesetze-im-internet.de/mpg/index.html [8] verordnung über das errichten, betreiben und anwenden von medizinprodukten (medical products – operating ordinance – mpbetreibv), official gazette i, jun. (1998) p. 1762, amendment: operating ordinance 53, jul. (2017) p. 2842 [in german]. online [accessed 16 september 2019]: http://www.gesetze-im-internet.de/mpbetreibv/index.html [9] s. mieke, t. schade (editors), leitfaden zu messtechnischen kontrollen von medizinprodukten mit messfunktion (guide on the metrological verification of medical devices with a measuring function lmkm) ausgabe 3.0 teil 1 und 2, ptb-bericht mm-11, physikalisch-technische bundesanstalt, braunschweig, 12/2016, issn 0721-0906, isbn 978-3-95606-296-4, [in german] [10] r. drahn, h. pfeiffer, w. riedel, h.-j. thiemich, j. tilgner, prüfeinrichtung für medizinische tretkurbelergometer (testing device for medical pedal crank ergometers), ptb-mitteilungen 105, braunschweig 6/95, [in german]. [11] din vde 0750-238:2002-10, medizinische elektrische geräte – teil 238: besondere festlegungen für die sicherheit von kurbelergometern (medical electrical equipment – part 238: particular requirements for the safety of crank ergometers), beuth verlag gmbh, berlin 2002, [in german]. [12] betriebshandbuch, beschreibung für dyncal100 – eine normalmesseinrichtung zur bestimmung rotatorischer leistungen (description for dyncal100 a standard measuring device for the determination of rotational powers), hbs gmbh, rudolstadt, 2003, [in german]. [13] din 51309:2005-12, werkstoffprüfmaschinen – kalibrierung von drehmomentmessgeräten für statische drehmomente (materials testing machines – calibration of static torque measuring devices), din deutsches institut für normung e.v., beuth-verlag gmbh, berlin, 2005-12, [in german]. [14] d. peschel, ptb-bericht (ptb report), ptb-1.22_96-024, braunschweig, 1996, [in german]. [15] bipm, jcgm 100:2008: evaluation of measurement data – guide to the expression of uncertainty in measurement. online [accessed 16 september 2019]: http://www.bipm.org/utils/common/documents/jcgm/jcgm _100_2008_e.pdf table 3. abbreviations and designations used in the text. symbol description bgbl. federal law gazette lmkm guideline on metrological checks of medical devices with a measuring function mpbetreibv medical products operating ordinance mpg medical devices act mu measurement uncertainty nn national standard ptb physikalisch-technische bundesanstalt rtd rotational torque device tf transfer standard sm torque signal sn revolution speed signal sref reference pulse szm counting pulse for the torque measurement szn counting pulse for the revolution speed measurement zm counter for the number of periods of the torque signal within a gate time zn counter for the period interval of the revolution speed signal in periods of the counting pulse zp counter for the period interval of the torque signal in periods of the counting pulse ztf counter transfer standard http://www.gesetze-im-internet.de/mpg/index.html http://www.gesetze-im-internet.de/mpbetreibv/index.html http://www.bipm.org/utils/common/documents/jcgm/jcgm_100_2008_e.pdf http://www.bipm.org/utils/common/documents/jcgm/jcgm_100_2008_e.pdf acta imeko, title acta imeko issn: 2221-870x june 2018, volume 7, number 2, 24-31 acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 24 isme activity on the use of autonomous surface and underwater vehicles for acoustic surveys at sea gianluca antonelli1, filippo arrichiello1, andrea caiti2, giuseppe casalino3, daniela de palma4, giovanni indiveri4, matteo razzanelli2, lorenzo pollini2, enrico simetti3 1isme-node university of cassino and southern lazio, italy 2isme-node university of pisa, italy 3isme-node university of genova, italy 4isme-node university of salento, italy section: research paper keywords: wimust citation: gianluca antonelli, filippo arrichiello, andrea caiti, giuseppe casalino, daniela de palma, giovanni indiveri, matteo razzanelli, lorenzo pollini, enrico simetti, isme activity on the use of autonomous surface and underwater vehicles for acoustic surveys at sea, acta imeko, vol. 7, no. 2, article 5, june 2018, identifier: imeko-acta-07 (2018)-02-05 section editor: fabio leccese, università degli studi di roma tre, italy received january 15, 2018; in final form march 22, 2018; published june 2018 copyright: © 2018 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: this work has received funding from the european union’s horizon 2020 research and innovation programme under the grant agreement no. 645141 (wimust project) corresponding author: filippo arrichiello, e-mail: f.arrichiello@unicas.it 1. introduction isme, the interuniversity center on integrated systems for the marine environment, is an italian institution that involves a set of universities (named as isme-nodes) that actively work in the field of marine robotics and acoustic systems. isme benefits from the research infrastructure of the participating departments (laboratories, workshops, etc.), and it has the purpose of exploiting synergies and collaborative efforts among the participants within an agile and flexible organizational structure. in the recent years, isme has been involved in several national and european projects concerning acoustics and marine robotics, as e.g. [1], [2]. the full list of projects can be found on the internet portal: http://www.isme.unige.it among its recent activities, isme has been involved in different research projects concerning the development and the use of autonomous surface crafts (ascs) and autonomous underwater vehicles (auvs) for a number of applications including environmental monitoring and seismic surveys. in particular, isme has coordinated the h2020 research and innovation action funded by the european commission (work programme 2014 2015, leitict, 5. leadership in enabling and industrial technologies information and communication technologies) named widely scalable mobile underwater sonar technology (wimust), 2015-2018. http://www.wimust.eu/ the main objective of the wimust project was the design, development, and testing of a robotic system composed of cooperating ascs and auvs to perform acoustic based geotechnical surveying and geophysical exploration [3]-[5]. to this purpose, the wimust project involved four academic partners and five industrial partners with specific expertise in the field of marine robotics, acoustic systems, and geophysical and geotechnical surveys: isme, instituto superior técnico abstract the paper presents an overview of the recent and ongoing research activities of the italian interuniversity center on integrated systems for the marine environment (isme) in the field of geotechnical seismic surveying. such activities, performed in the framework of the h2020 european project wimust, include the development of technologies and algorithms for autonomous surface crafts and autonomous underwater vehicles to perform geotechnical seismic surveying by means of a team of robots towing streamers equipped with acoustic sensors. http://www.isme.unige.it/ http://www.wimust.eu/ acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 25 (ist), centro de investigação tecnológica do algarve (cintal), university of hertfordshire (uh), graal tech s.r.l. (gt), evologics (el), cgg, geo marine survey systems (geo), and geo surveys (gs). acoustic seismic surveying methods allow the exploration of the sea bottom and sea bottom subsurface for applications that span from the geophysical domain (e.g. oil & gas) to the geotechnical one (e.g. civil and commercial applications, underwater constructions). the seismic acoustic survey is traditionally performed using a surface vessel carrying one or more powerful acoustic sources (e.g. sparkers or airguns), and towing a number of streamers equipped with several hydrophones to record acoustic signals. sparkers generate seismic waves by intermittently releasing electric pulses that produce low frequency sound waves. such acoustic waves travel towards the sea floor and are reflected back to the hydrophones. the time delay with which waves return to the surface, registered with the hydrophones, provides valuable information about the properties of the ocean sub-bottom. the wimust project aims at expanding and improving the functionalities of current cooperative marine robotic systems by effectively enabling a distributed acoustic array technology for seismic surveying. indeed, the wimust system is composed of two ascs carrying a sparker each, and of a fleet of auvs towing streamers equipped with hydrophones to acquire subbottom profiling acoustic data. the hydrophones on the streamers towed by the auvs can be envisioned as a 3d acoustic array that, by means of the autonomous and coordinated motion among ascs and auvs, can have an adaptive variable geometry. indeed, by actively controlling the geometry of the robot formation, it becomes possible to change the shape of the acoustic array antenna, according to the needs of the considered application (see figure 1). the resulting operational flexibility holds potential advantages, as it allows improving the seabed and sub-bottom resolution and obtaining sidelobe rejection at almost any frequency and for any plane. the availability of the proposed system, other than possibly improving the quality of the acquired data, also greatly facilitates the operations at sea, thanks to the lack of physical ties between a surface ship and the acquisition equipment. on the other side, the lack of the towed streamers from the surface vessel introduces high complexity due to the need of ad-hoc cooperative navigation and control architecture for the team of ascs and auvs, as well as the establishment of an underwater acoustic communication network, where all the nodes need to be synchronized [6]. moreover, the hydrophones’ positions, needed for acoustic data processing, cannot be easily computed as in the case of surface surveys since the auvs while diving cannot rely on gps. 2. isme activities in the wimust project among the overall activities of the wimust project, the contributions of isme (nodes of lecce, genova, pisa and cassino) can be identified in the following activities. 2.1. system integration isme was responsible for the overall system integration of the wimust project that involves software and hardware integration of the different components of the system, i.e. the different type of autonomous vehicles (delfim catamaran from ist, ulisse catamaran from isme, 4 medusa auvs [7] from ist, 4 folaga auvs from isme and graaltech), the acoustic communication devices, the streamers with hydrophones and the sparkers. thus, the system integration deals with issues as: mechatronic integration of the sensing payloads and the communication devices on board the auvs; software integration of the modules concerning group navigation and coordination; software integration of communication protocols and strategies to manage two acoustic communication networks (medium frequency for acoustic localization and high frequency for seismic data quality control), and a wi-fi network for the communication on the sea surface; hardware and software integration of the sparkers with the asc catamarans. the overall system has been validated through experimental tests at sea (three integration weeks have been already performed in sines, portugal in november 2016 and july 2017, and in lisbon in december 2017). for the final experiments, performed in january 2018, the autonomous vehicles constituting the wimust system have performed cooperative guidance, navigation, and control by implementing the solutions and methods developed for the project purposes and to acquire geoseismic acoustic data of a specific area. the overall wimust control architecture allowed the vehicles to successfully perform maintain formation while concurrently performing operational tasks related to the level of individual power supply, intra-vehicle distance, and quality of service of vehicle-to-vehicle communication. to manage the fleet of auvs and ascs composing the wimust system, a range based localization strategy and a formation control algorithm have been developed by ist to make the system navigate in preassigned formation (see figure 2). in this context, isme activities has been focused on the development of low level motion control strategies for the ulisse catamaran and for the folaga auvs towing streamers. 2.2. development of an autonomous catamaran the isme-node genova has designed and realized the ulisse platform, i.e. the second generation of autonomous figure 1. sketch of the wimust system. figure 2. computer animation of a wimust mission towing streamers ulisse isme autonomous catamaran. acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 26 surface vehicles, following a previous design created for a private company in 2015. ulisse is a 3 m long and 1.8 m wide catamaran, built in fiberglass (see. figures 3-5). the two hulls are fixed together thanks to two carbon fiber bars, which pass through the two hulls below the deck line, and are fixed on both the interior and exterior of the catamaran. the catamaran has been designed capable to transport payloads up to 200 kg. the catamaran is propelled by two commercial torqeedo cruise 2r electric motors, of 2 kw electrical power and about 50% propulsive efficiency. the motor control electronics is connected to the ulisse control electronics through a rs485 bus line. the motors can run in reverse, producing around 50% of the thrust w.r.t. going forward. this allows the asc to be controlled in differential mode, although some scaling needs to be taken into account to handle the different forward/reverse thrust properly. the vehicle is controlled by a pcm-3362 single board computer, equipped with a 1.67 ghz intel® atom™ n450 processor. while the computational power is moderate, it is however sufficient for running path following and low level control algorithms needed for the wimust survey. both hulls have four internal compartments. the first one, accessible through a waterproof hatch, hosts the control electronics and the battery modules (around 3.2 kwh energy each). the starboard side one also hosts the pcm single board computer, the inertial sensors (compass, accelerometers and gyroscopes), as well as the batteries). the second and third compartments, one near the bow and one near the stern, host two ballast tanks, which are controlled by pumps and electrovalves contained in the fourth compartment. the ballast system can be used to adjust the nominal roll and pitch of the vehicle once a certain payload has been installed. the vehicle has been designed with the possibility of maximum re-configurability. in fact, along both hulls there is an aluminum bosch rexroth profile, which can be used to fix custom decks hosting the necessary payload for the mission at hand. motors cables go through a connector box that is placed on the stern side of both hulls. these boxes are connected together, to ensure that all the electronics is connected to the control computer that is installed in the starboard hull. furthermore, an ethernet cable connects the control computer to an ubiquiti 5 ghz access point, located on the roll-bar. a further ethernet connector is available to connect a payload to the internal network. finally, both connector boxes host keys to turn on and off the system, a connector to recharge the battery and a spare power connection that can be used to power up payloads, with about 25a@24v power available. communication with the base station or with other surface vessels is granted by a standard 5 ghz communication system, which proved to be sufficient for the distances that were so far tested (around 300 m). the first engineering tests have shown that the vehicle was capable of a speed of around 4 m/s (about 8 knots) when the propellers were commanded at about 75%. it is therefore estimated that the maximum achievable speed is around 10 knots. furthermore, the two motors allow for a differential control of the catamaran and thus the vessel can turn on the spot. the maximum rotational speed on the spot is around 25 deg/s. in the wimust project, ulisse carries one of the two acoustic sparkers. the sparker itself has been mounted with a clamp system, exploiting the two carbon fiber bars, in the middle part of the vehicle, in such a way to be at least 30 cm below the water level. the sparker can be seen in figure 4. the sparker is powered by a power supply that charges high-voltage capacitors (4 kv) necessary to create the spark. two dedicated decks have been manufactured for the wimust project. one, installed near the stern under the rollbar hosting the antennas, was dedicated to hosting the sparker's power supply. the second one, installed near the bow, hosts the movable pole to lower down the modem's transducers, an additional waterproof case hosting the modems electronics, and two honda eu20i portable power generators, necessary to produce the 220 v for the sparker's power supply. still in figure 4, the two modems transducers (one for seismic data communication and one for acoustic localization) can be seen near the vehicle’s bow. the bow deck also hosts an additional waterproof case, containing the modem electronics and an ethernet switch, figure 3. ulisse with no payload mounted. figure 4. ulisse configuration for the wimust system: the acoustic sparker and the two acoustic modem transducers can be seen in the middle part and front part respectively. figure 5. ulisse isme autonomous catamaran. acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 27 which connects the two modems and the sparker triggering electronics to the onboard network and to the outgoing 5 ghz connection. the catamaran is capable of autonomously navigating exploiting information from onboard sensors, as gps, imu, compass, and of the information exchanged with other network nodes using wi-fi for surface nodes and acoustic modems for underwater nodes. the motion control software of the ulisse catamaran has been enhanced with a control mode, in the framework of ros (robot operating system) [8], composed of a heading controller and a surge controller. figures 6-10 show the results of a mission execution where the asc was commanded to follow a trajectory specifically designed for the survey. in particular, figure 6 shows the asc position as measured from gps signal. figures 7-8 respectively show the asc actual/desired heading and the error of the heading control algorithm (a pd controller). figures 9-10 respectively show the asc actual/desired surge and the error of the surge control algorithm (a pi controller). 2.3. folaga auv motion control the folagas are torpedo-like auvs developed from graal tech srl. the vehicle is actuated by 8 waterjet pumps and 1 propeller. the propeller provides trust along the surge direction, while 8 pumps, 4 in the front and 4 at the rear, generate forces and torques on the vehicle that allow lateral and vertical translation, and pitch and yaw rotation. the internal battery pack is mounted on a motorized frame that allows to change the longitudinal position of the auv’s center of mass. the folaga auv hull has a modular structure that allows adding one or more segments containing payload instrumentations. within the wimust a specific payload module has been realized containing a single board computer, two acoustic modems, the electronic boards for signal conditioning and data storage of seismic data and the mechanical interface for the streamer, the array of hydrophones that constitutes the main mission payload. the streamer, about 10 meters long and towed by the auv, collects seismic signals reflected by the sea bottom in order to analyze its geophysical structure; figure 11 shows 4 folagas with their streamers laid on the dock. the folaga auv has a built in mission control system that allows performing basic operations. early tests in lisbon and sines (2016) highlighted that the existing folaga mission control system could not be used for implementation of a complete wimust mission where precise trajectory tracking and coordination with other vehicles is necessary. thus, a new mission control system was overlaid to the existing one in order figure 9. autonomous navigation mission with ulisse catamaran: reference and actual surge. figure 10. autonomous navigation mission with ulisse catamaran: error of the surge control. figure 6. autonomous navigation mission with ulisse catamaran: effective trajectory reconstructed from gps data. figure 7. autonomous navigation mission with ulisse catamaran: reference and actual heading. figure 8. autonomous navigation mission with ulisse catamaran: error of the heading control. acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 28 to implement the necessary functionalities. the new control system was implemented in the additional single board computer (in the payload segment) using the robot operating system (ros). the use of ros, as middleware for control system implementation, revealed successful in particular with respect to the ease of integration of the control modules that were developed by the different partners of the project: the structure of the control system is shown in figure 12. the new folaga mission control system is composed by several different guidance, navigation, and control modules. reference depth for the mission is set prior to mission beginning, but can be changed during the mission using a specific acoustic message that the vehicle receives trough the acoustic modems; this allows reconfiguration of the mission formation during its execution. the depth controller is a fuzzypid control system that blends two different control approaches: when the vehicle velocity is low, vertical pumps are used to control depth (with the vehicle almost leveled). when velocity is higher instead, the vehicle pitch angle is used as control variable to increase or decrease depth; this latter is the control mode that is used mostly throughout the mission. the adoption of this technique was necessary since the velocity of the vehicles during the mission may vary, and the effectiveness of depth control using the pitch angle is poor when velocity is low. the fuzzy blending strategy was tuned with field tests. velocity and heading controllers are of pid-type, and the control allocation block, specific for the pumps geometry, takes care of assigning appropriate pump and propeller commands in order to achieve the desired thrust and control forces and moments. the navigation system used when the vehicle is at the surface fuses vehicle odometry and gps measurements using a kalman filter (kf). a different navigation system, developed by ist, is used when underwater. the underwater navigation system is composed of a kalman filter and a maximum likelihood estimator (mle), developed by ist. these two, together, use measurements of ranges between the vehicle and a set of anchors to estimate the vehicle position when underwater. the anchors are equipped with acoustic modems that ping periodically sending current anchor position and time. ping times are synchronized using the gps pulse-per-second (pps) signal; estimation of distance is performed by measuring the time of flight of acoustic signals and using chip-scale atomic clocks (csac) inside the modems allows keeping a common time base with minimal drifts during the entire duration of the mission. the vehicles after reception of the pings from the anchors, calculate their distance from them and use this information to update the position estimate in the kalman filter. at least two anchors are needed to compute a position fix; in the wimust architecture, surface vehicles (the two catamarans usually) act as anchors. thanks to the adoption of ros as middleware, the kf and mle components were implemented as two ros nodes that run unmodified on the folagas and the medusas, even if the two auvs differ both in hardware and software. the mle node requires an estimate of vehicle velocity as input; since no dvl is present on the folagas, a static mapping between propeller rpms and vehicle velocity was created through extensive tests performed in la spezia (italy). the guidance system is implemented by the tracking controller ros node, developed by ist and university of hertfordshire (uh). the tracking controller computes feasible reference velocity and heading angle for the auvs, and the catamarans in order to guide the vehicle from their initial positions toward the mission starting point prior to the start of the survey, and to remain in a fixed relative position with respect to the formation leader during the survey. in addition, an observer [9] is employed to estimate sea current in order to achieve a very precise formation tracking. again, thanks to the adoption of ros, a complete virtual environment was created where entire missions were simulated prior to sea trials in order to verify correctness of the integration of the various software components. then, sea trials, initially in la spezia, later in lisbon and sines, were performed to field test the integration and to tune the tracking controller parameters to the folaga dynamics. sea trials in la spezia, lisbon and sines were performed to field test the integration and to assess the folaga vehicle capability to remain in formation while underwater. figures 13-15 illustrate the results of a 2 hours experimental test involving, a virtual ship and 3 actual vehicles. the virtual ship, simulated by a pc in ground control station, follows a racetrack like trajectory several times and acts as formation leader. two folagas, vehicles f_57 and f_55 in the figure, were configured to stay in formation with the virtual ship, and to remain at the surface. the two surface folagas were also configured to act as localization anchors, that is their acoustic modems were set up to ping periodically and send auv position estimated using the onboard surface navigation system. the third folaga, f_54 was configured as underwater vehicle, that is it used range measurements to f_57 and f_55 (obtained from the acoustic modem) to compute its position while navigating underwater. in particular, figure 13 shows the figure 11. folaga auvs towing streamers with hydrophones. figure 12.1 block diagram of the folaga guidance, navigation and control system. acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 29 trajectory of the formation leader (the virtual ship), and of the surface folagas as shown in the ground control station console screen developed by gt. the two large apparent deviations from the desired trajectory of f_55 and f_57 are actually artifacts part of the initial vehicle maneuvers needed to reach formation with the ship starting from their initial positions. figure 14 shows the trajectory of folaga f_54; part of the mission was conducted at the surface: f_54 started from its initial position, joined the formation and conducted half of the racetrack at the surface before receiving the command to dive. then the vehicle dives to 5 meters depth, and starts using the ist navigation system output (ekf trajectory) as feedback for the tracking controller. the output of the navigation system used at the surface, which relies on gps, drifts soon after the vehicle dives. figure 15 shows desired and measured depth during the mission, the auv pitch angle, and the ranges to the anchor nodes. 2.4. hydrophones’ localization algorithm for a proper post-processing of the wimust acquired acoustic data, there is the need to know the hydrophones’ positioning during the navigation; however, the absence of a localization infrastructure requires the development of a strategy to estimate the hydrophones’ positioning from the available set of information. to the purpose, the isme-node cassino developed a localization strategy for the auv-streamer system that, beyond using the data from navigation sensors of the auv, as gyroscope, compass, inertial measurement unit, usbl (as in [10]), it also makes use of information gathered from acoustic signals generated for the seismic survey. indeed, considering that the sparkers and auvs are equipped with acoustic modems with atomic clocks that allow precise synchronization, it is possible to use the direct time of arrival of the acoustic signals from the sparker to the hydrophones to compute range measurements. thus, in the proposed localization strategy, range measurements from sparker to each hydrophone (see figure 16) are collected and fused with auv on board navigation sensors to estimate the overall system state using an extended kalman filter. to the purpose, the dynamic model of the auv-streamer system (see figures 17-18) has been firstly derived considering figure 13. results of an experimental test with two folaga auvs navigating at the surface and following a virtual ship leader. figure 14. results of an experimental test with a folaga estimating its position while diving using range measurements from two surface nodes. figure 15. results of an experimental test with a folaga: depth, pitch and acoustic ranges measurements. figure 16. sketch of the localization problem. figure 17. picture of medusa auvs towing a streamer with hydrophones. acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 30 the auv as a fully-actuated rigid body, and the streamer as a serial chain of rigid links with spherical unactuated joints in correspondence of each hydrophone as [11]. the underwater dynamics of each rigid body has been modelled following the approach in [12]. in particular, the parameters of the dynamic model of the auv have been defined on the basis of the folaga characteristics, i.e. a 2mlength torpedo shaped auv; the streamer, instead, has been modeled as a set of almost neutrally buoyant thin cylindrical elements connected via unactuated spherical joints. nominal hydrodynamic terms and drag coefficients have been chosen accordingly to the effective geometry of the system, and on the basis of the common velocity regimes of the wimust system (i.e. cruise velocity about 1 m/s). given the large dimension of the overall system state, the direct dynamic functions, needed for both numerical simulations of the system dynamics and for the computation of the prediction terms of an extended kalman filter, have been developed referring to the newtoneuler recursive procedure used for industrial manipulators [13]. the overall localization approach has been extensively tested in numerical simulations in matlab environment, considering the case of single and multiple sparkers, and considering the auv equipped with different set of navigation sensors. moreover, the localization algorithm has been tested using data gathered from the experimental tests performed for the wimust project purposes (sines, portugal, nov. 2016). a preliminary study on the feasibility of the localization strategy of the auv-streamer system considering the availability of range measurement from sparkers to hydrophones has been investigated in [14]; however, that study was based on the use of an instantaneous set of sensor data, assuming the system as static. we then extend such work by developing the system dynamic model (in [15]), and by deriving the ekf equations and performing numerical validations also integrating data from experimental tests. the acoustic signals collected with the hydrophones in the experimental campaigns were stored in the format of seg-y files, i.e. the standard format for storing geophysical data. specifically, the seg-y file contains a set of traces for each hydrophone, where a trace represents the trend of the acoustic signal in a time slot of predefined length and time synchronized with the acoustic signal generated by the sparker. by post processing the seg-y file, beyond studying the sea subsurface from the signal reflections, it was also possible to extract the range measurements from the sparkers to hydrophones by computing the direct time of arrival of the acoustic signals (i.e. the time to reach the hydrophone without reflections on the sea bottom). figure 19 illustrates an example of the range data extracted from the seg-y file and filtered out of a set of outliers following a threshold base approach. such data have been used, together with the auv’s navigation sensor measurements, to perform a realistic simulation of the hydrophones’ localization approach performance. in that tests, the auv was commanded to navigate on the surface, so gps readings were available and used as desired reference trajectory for a simulated auv, where a basic pi controller has been used as a low-level dynamic control law. figure 20 shows the actual and the estimated position of the auv and of the hydrophones while the auv followed the desired path. the norm of the positioning estimation error for each hydrophone is plotted in figure 21. from the figure it can figure 18. modelling of auv-streamer system. figure 19. range measurements from the sparker to the hydrophones collected from geoseismic acoustic data. figure 20. plot of the desired trajectory (red), and few snapshots showing. the auv and hydrophones real and estimated positions. figure 21. plot of the hydrophones estimation errors. acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 31 be noticed that, as can be expected, the estimation error decreases from the large initial error value, it keeps relatively low values with oscillations due to the process and measurement noise, and it keeps highest values for the farthest hydrophone from the auv. 3. conclusions in this paper we presented an overview of the activities of the isme interuniversity center performed for the h2020 wimust project (under grant agreement n. 645141), and concerning the development of a team of marine robots to perform acoustic seismic survey. in particular, the isme activity has focused on the complete design of an autonomous catamaran to carry a sparker, on the development of control strategies for a the folaga autonomous underwater vehicles, on the coordination of the system integration activities of the project, and on the localization of hydrophone positioning using range measurement from seismic signals. references [1] casalino, g., allotta, b., antonelli, g., caiti, a., conte, g., indiveri, g., melchiorri, c., simetti, e., “isme research trends: marine robotics for emergencies at sea”. in oceans 2016shanghai (pp. 1-5). ieee, 2016 [2] di lillo, p. simetti, e., de palma, d., cataldi, e., indiveri, g., antonelli g., casalino, g., “advanced rov autonomy for efficient remote control in the dexrov project”. marine technology society journal, 50(4), 67-80, 2016 [3] al-khatib, h., antonelli, g., caffaz, a., caiti, a., casalino, g., de jong, i.b., duarte, h., indiveri, g., jesus, s., kebkal, k., pascoal, a., and polani, d. (2015b). “navigation, guidance and control of underwater vehicles within the widely scalable mobile underwater sonar technology project: an overview”. in 4th ifac workshop on navigation, guidance and control of underwater vehicles ngcuv 2015 dedicated to the memory of professor geoff. roberts, volume 48, 189-193 [4] al-khatib, h., antonelli, g., caffaz, a., caiti, a., casalino, g., de jong, i.b., duarte, h., indiveri, g., jesus, s., kebkal, k., pascoal, a., and polani, d. (2015a). “the widely scalable mobile underwater sonar technology (wimust) project: an overview.” in proceedings of mts/ieee oceans '15, 1-5. genova, italy. [5] antonelli, g., arrichiello, f., caffaz, a., caiti, a., casalino, g., volpi, n.c., de jong, i.b., de palma, d., duarte, h., gomes, j.p., grimsdale, j., indiveri, g., jesus, s., kebkal, k., kelholt, e., pascoal, a., polani, d., pollini, l., simetti, e., and turetta, a.. “widely scalable mobile underwater sonar technology: an overview of the h2020 wimust project.” marine technology society journal (research initiatives in europe: cooperation for blue growth), 50(4). guest editors: andrea caiti, giuseppe casalino and andrea trucco., vol. 50,n.4, pp. 42--53, 2016 [6] valls, oriol pallares, pierre-jean bouvet, and joaquín del río. "hybrid time synchronization for underwater sensor networks." acta imeko 4.3 (2015): 30-35. [7] abreu, p., botelho, j., gois, p., pascoal, a., ribeiro, j., ribeiro, m., rufino, m., sebastiao, l., and silva, h. (2016). the medusa class of autonomous marine vehicles and their role in eu projects. in proc. oceans 2016 mts/ieee shanghai, china, 10-13 april. [8] quigley, morgan, et al. "ros: an open-source robot operating rossystem." icra workshop on open source software. vol. 3. no. 3.2. 2009. [9] aicardi, m., casalino, g., indiveri, g., aguiar, a., encarnaçao, p., & pascoal, a. (2001, june). a planar path following controller for underactuated marine vehicles. in ninth ieee mediterranean conference on control and automation, dubrovnik, croatia. [10] f. arrichiello, h. k. heidarsson, and g. s. sukhatme, “opportunistic localization of underwater robots using drifters and boats,” in 2012 ieee international conference on robotics and automation, may 2012, pp. 5307–5314 . [11] j. e. cochran, m. innocenti, t. s. no, and a. thukral, “dynamics and control of maneuverable towed flight vehicles,” journal of guidance, control, and dynamics, vol. 15, no. 5, pp. 1245–1252, sep 1992. [12] t. fossen, marine control systems: guidance, navigation and control of ships, rigs and underwater vehicles. trondheim, norway: marine cybernetics, 2002 [13] b. siciliano, l. sciavicco, l. villani, and g. oriolo, robotics: modelling,planning and control. springer, 2009. [14] f. arrichiello, g. antonelli, and e. kelholt, “shape estimate of a streamer of hydrophones towed by an autonomous underwater vehicle,” ifac-papersonline, vol. 49, no. 23, pp. 181 – 186, 2016. [15] f. arrichiello, s. sarkar, s. chiaverini, and g. antonelli, “dynamic modelling of a streamer of hydrophones towed with an autonomous underwater vehicle,” modelling and simulation for autonomous systems. mesas 2017. lecture notes in computer science, vol 10756. springer, cham, pp 179-192, 2018. isme activity on the use of autonomous surface and underwater vehicles for acoustic surveys at sea microsoft word article 20 204 template advertisement_hbm.docx acta imeko  february 2015, volume 4, number 1, 135  www.imeko.org    acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 135  section: advertisement  acta imeko, title acta imeko issn: 2221-870x september 2017, volume 6, number 3, 67-70 acta imeko | www.imeko.org september 2017 | volume 6 | number 3 | 67 application of gas chromatography coupled with mass spectroscopy (gc/ms) to the analysis of archaeological ceramic amphorae belonging to the carthaginian fleet that was defeated in the egadi battle (241 b.c.) rosa pitonzo1, francesco armetta1,2, maria luisa saladino2, eugenio caponetti1,2, francesca oliveri3, sebastiano tusa3 1centro grandi apparecchiature-aten center, university of palermo, via f. marini 14, i-90128 palermo, italy 2stebicef department, university of palermo, viale delle scienze pad.17, palermo i-90128, italy 3soprintendenza del mare, regione siciliana, via lungarini 9, i-90133 palermo, italy section: research paper keywords: underwater ceramic amphorae; gs-ms; egadi battle; pine resin citation: rosa pitonzo, francesco armetta, maria luisa saladino, eugenio caponetti, francesca oliveri, sebastiano tusa, application of gas chromatography coupled with mass spectroscopy (gc/ms) to the analysis of archaeological ceramic amphorae belonging to the carthaginian fleet that was defeated in the egadi battle (241 b.c.), acta imeko, vol. 6, no. 3, article 10, september 2017, identifier: imeko-acta-06 (2017)-03-10 editor: sabrina grassini, politecnico di torino, italy received march 12, 2017; in final form june 1, 2017; published september 2017 copyright: © 2017 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: this work is part of the project “development and application of innovative materials and processes for the diagnosis and restoration of cultural heritage delias” pon03pe_00214_2 (programma operativo nazionale ricerca e competitività 2007-2013) corresponding author: eugenio caponetti, e-mail: eugenio.caponetti@unipa.it 1. introduction though remarkable underwater sites have been discovered in many parts of the world, the mediterranean area is rightly recognized as one of the most fruitful locations for deep-water archaeology, due to the significant number of ancient shipwrecks discovered. most of these are found intact, often with the contents in their original position, making it possible to understand the origin of the ship and to reconstruct the development of shipbuilding traditions [1]. the final battle of the first punic war between rome and carthage, the battle of the egadi islands, took place in 241 b.c. and saw the victory of the romans. finds of multiple bronze warship rams, helmets, and amphorae, destined for a carthaginian garrison on sicily, confirm the naval battle general location and define its landscape. in cargo ships, amphorae are known to have been used to carry wine, olive oil, spices and fish products, as well as several other liquid or semi-liquid goods [2]. therefore, these objects are extremely precious for deducing agricultural and foodrelated practices, on the basis of direct archaeological evidence of residues found inside (relatively infrequent), or more frequently the traces found in these relics [3], [4]. amphorae, in their many types and guises, were traditional package material throughout antiquity, producing a treasure of archaeological data about the rates of production, associated regional products, artisanal production organization, the different actors involved in production as well as exchange, amplitudes of distribution patterns and past networking, with a clear potential of sustaining historically inspired enquiries. compared to the archaeological prevalence of amphorae, abstract the aim of this preliminary work was to identify characteristic compounds in 7 underwater marine ceramic amphorae sherds dating from the period of the battle of the egadi islands that decided the end of the first punic war (241 b.c.) by gas chromatography coupled with mass spectroscopy (gc/ms). acta imeko | www.imeko.org september 2017 | volume 6 | number 3 | 68 ancient sources on their use and content are rare and not always straightforward to interpret [5]. archaeological data, together with chemical analysis of pottery remains, can provide information on the diet of a region’s inhabitants during a chosen period of time. organic residues recovered from pottery sherds can be used as fingerprints of the food materials once inside those containers. the use of gas chromatography/mass spectrometry (gc/ms) in the analysis of organic residues and the use of biomarkers to identify the original source materials, which formed those residues, are well-established techniques in archaeological science. the aim of this work was to identify the characteristic compounds in 7 underwater marine ceramic amphorae sherds, dating from the period of the battle of the egadi islands. samples of underwater marine ceramic amphorae are shown in figure 1. among 308 amphorae that were identified during the survey till 2013 (some of them were rescued to take samples for analysis), 278 are greco-italic (type mgs v-vi), while 33 are punic (type: “ovoid maltese”). since 5th century b.c. maltese islands produced amphorae having a typical shape, deriving from a punic tradition. these amphorae were ovoid in shape, without neck, with ear shaped handle placed on the upper part of the body and thin ribbon rim. these amphorae had a wide diffusion in malta and their production would have continued until the roman period. clay fabric is orange-red with a cream coloured slip. outside malta these amphorae are present in central and western mediterranean. some of them are found in camarina, lilibeo, ibiza, cartage and pantelleria in iv – ii cent. bc layers. 2. methodology sample extraction all samples were analysed at the research mass spectrometry laboratory of “centro grandi apparecchiature aten center” of the university of palermo. ceramic samples were analysed to identify the amphorae contents and organic coatings, and lipid extraction used to test for presence of lipids of either plant or animal origin that might suggest lipid-rich commodities (such as olive oil or garum), which may have been transported in the vessels. the analysed samples belong to greco-italic amphorae (type mgs vi) dated to the 3rd century bc. these amphorae have a rim with triangular section, cylindrical neck, elongated body and pointed base. the sherds were first surface-cleaned with a gentle stream of inert nitrogen gas before the ceramic sample for analysis was crushed in a mortar. no mechanical ablation procedure was used that could remove the compounds of interest. however, two samples were analysed in parallel, one before and one after the cleaning, in order to verify that the used cleaning procedure does not affect the result of analysis. data (not showed in this paper) are the same for both samples. all reagents and analytical standards were acquired from sigma aldrich. the lipid extraction technique for ceramic samples was adapted from that used by evershed et al [6]: five grams of a sample was weighed; the lipids were extracted with 30 ml of chloroform and methanol (2:1 v/v) for 3 h, t=40 °c; the extraction solvent was then evaporated to dryness under a stream of nitrogen gas; the amount of dry extract obtained was around 100 mg/g sherd; samples were stored at −20 °c until analysis gc/ms. immediately prior to analysis, dry lipid extract was derivatized to obtain fatty acid methyl esters (fames): dry lipid extract was dissolved in 50 ml of toluene; subsequently 100 ml of methanolic potassium hydroxide (5 %) was added. after 5 min stirring the reaction was stopped by addition of 200 ml of bidistilled water; fames were extracted with 1 ml of hexane, followed by solvent evaporation at reduced pressure at room temperature. dried lipid extract was dissolved in 500 µl of hexane for gc/ms analysis. for samples that appeared too concentrated to inject in splitless mode, analysis was first conducted in split mode (10:1) in order to access the quantity of organic constituents before being run in splitless mode. analytical conditions samples were thus analysed by gc/ms using a trace™ 1310 gc (thermo scientific) gas chromatograph equipped with a phenomenex zebron zb-5ms column (30 m length x 0.25 mm id x 0.25 μm film thickness, 5 % diphenyl – 95 % dimethylpolysiloxane stationary phase). the mass spectrometer figure 1. samples of underwater marine ceramic amphorae belonging to the carthaginian fleet that was defeated in the egadi battle (241 b.c.). acta imeko | www.imeko.org september 2017 | volume 6 | number 3 | 69 was a thermo scientific isq single quadrupole mass spectrometer, operated in electron ionization mode (70 ev). the scan range was m/z 35-650. the gas chromatograph conditions were as follows: inlet temperature 270 °c, flow rate 1.0 ml/min, transfer line temperature 265 °c. helium was used as the carrier gas. the temperature program for the gc oven was a 40−140 °c at 10 °c/min, 140−340 °c at 15 °c/min, with a 5 minutes isothermal hold at 340 °c. injections were made by a thermo scientific autosampler ai/as 1310, triplus™ rsh and sample injection volume was 1 μl in splitless mode. 3. results and discussion gc/ms analysis allowed the identification, among other compounds, of fatty acids (oleic, palmitic and stearic acids), steroids (stigmastanol) and resin sealants. these data suggest that pinaceae resins were used as the amphorae sealants and that vegetable fat residues were present in all the analysed amphorae. information regarding the making of the pitch sealants can be inferred from the chemical profile of the samples. identification of marker compounds was achieved by comparison of retention times and mass spectra against authentic standards as well as comparison against the nist11 mass spectral database (table 1). typical chromatograms are shown in figures 2 and 3. the most abundant compound was the oleic acid, which is one of the main constituents of olive oil, followed by the stearic and palmitic acids. the presence of these fatty acids, together with other acids such as lauric, palmitoleic, linoleic, behenic and lignoceric, shows the presence of fat, but does not allow us to attribute an animal or vegetable origin. stigmastanol, present in significant amounts in the analysed sherds, indicates presence of a vegetable fat. this compound originates from degradation processes occurred over the years, after reduction of the ß-sitosterol that is the precursor and the typical marker of vegetable oils. dehydroabietic acid and its oxidation products are typical components of oxidised resin that exudes from plants of the pinaceae family. further confirmation derived from the identification of heating markers such as retene and 8isopropyl-1,3-dimethyl phenantrene [7], [8]. the amount of methyl dehydroabietate, observed in all samples, is significantly higher with respect to dehydroabietic acid. this compound is indicative of the pitch production method: it is formed by heating resin-bearing wood, where the wood is burned to simultaneously extract resin from the source wood and convert it to pitch [7], [8]. the lipid extracts analysed contain large amounts of organic substances, which allow us to hypothesize the presence of strongly degraded oil, presumably olive oil, beeswax and heat treated pine resin. beeswax is confirmed by the presence of free fatty acids from c22:0 to c30:0 with prevalence of the lignoceric acid and high levels of palmitic acid (table 1). one of the most plausible hypothesis is the use of beeswax as waterproofing or sealant [9]. the presence of the stigmastanol together with oleic acid, in all samples, suggests that vegetable fat residue are present in the amphorae. only the high level of stearic acid may suggest the presence of animal fats. however, the absence of cholesterol and other animal sterols suggests that it may be a contamination, rather than an indication of the amphora content [9]. the chromatograms show several diterpenoid acids such as dehydroabietic acid and 7-oxodehydroabietic acid, together with polycyclic aromatic hydrocarbons, such as retene and dehydrogenated derivatives, that are markers for pine resin in aged materials [7], [10]. 4. final considerations the possibility to easily understand the contents of ancient container that, due to the long period of permanence in the sea, have disappeared, could be of great help for the archaeologists. although we generally know that normally each amphorae type could be connected to a specified product such as oil, wine, grains, fish sauces (garum and other similar products), frequently amphorae were reused for different purposes and to contain different varieties of goods. it will be very important for archaeologists and historians to get the chance to understand the real contents of amphorae of an ancient wreck because it will be possible to frame the ancient sea routes and, consequently, understand the real economic dynamics either in a synchronic and in diachronic perspective. we need also to keep in mind that this information is very attractive for a wider set of people, such as the visitors of museums. therefore, this methodology could have a wide application either in the field of scientific research and in the area of museography. figure 2. total ion chromatogram of the lipid extraction of ceramic sample 1. figure 3. total ion chromatogram of the lipid extraction of ceramic sample 5. acta imeko | www.imeko.org september 2017 | volume 6 | number 3 | 70 this work is part of the project “development and application of innovative materials and processes for the diagnosis and restoration of cultural heritage delias” pon03pe_00214_2 (programma operativo nazionale ricerca e competitività 2007-2013). references [1] s. tusa. research. protection and evaluation of sicilian and mediterranean marine cultural heritage. conserv. sci. cult. herit. 9 (2009). pp.79-112. [2] d. twede. commercial amphoras: the earliest consumer packages? j. macromark. 22 (2002) pp. 98-108. [3] p.e. mcgovern. ancient wine: the search for the origins of wine culture. princeton university press. princeton. jones. 2003. 1986. [4] m.c. hansson. b.p. foley. ancient dna fragments inside classical greek amphoras reveal cargo of 2400-year-old shipwreck. j. archaeol. sci. 35 (2008) pp.1169-1176. [5] k. romanus. j. baeten. j. poblome. s. accardo. p. degryse. p. jacobs. d. de vos. m. waelkens. wine and olive oil permeation in pitched and non-pitched ceramics: relation with results from archaeological amphorae from sagalassos. turkey. journal of archaeological science 36 (2009) pp. 900–909. [6] r.p. evershed. c. heron. l.j. goad. analysis of organic residues of archaeological origin by high-temperature gas chromatography and gas chromatography-mass spectrometry. analyst 15 (1990) pp. 1339-1342. [7] m.p. colombini. g. giachi. f. modugno. p. pallecchi. e. ribechini. the characterization of paints and waterproofing materials from the shipwrecks found at the archaeological site of the etruscan and roman harbour of pisa (italy)”. archaeometry 45(4) (2003) pp. 659–674. [8] m.a. pollard. c. heron. archaeological chemistry. cambridge: the royal society of chemistry.1996. [9] f. bordignon. m. botto. m. positano. g. trojsi. mediterranea. quaderni annuali dell'istituto di studi sul mediterraneo antico (già «quaderni di archeologia etrusco-italica»). rivista a cura del consiglio nazionale delle ricerche. fabrizio serra editore. pisa – roma. volume ii. 2005. [10] i. jerković. z. marijanović. m. gugić. m. roje. chemical profile of the organic residue from ancient amphorae found in the adriatic sea determined by direct gc and gc-ms analysis. molecules. 16 (2011) pp. 7936-7948. table 1. percentages relative areas of the marker compounds identified in 7 underwater marine ceramic amphorae samples belonging to the carthaginian fleet that was defeated in the egadi battle (241 b.c.). marker compounds sample 1 sample 2 sample 3 sample 6 sample 4 rec 58 sample 5 rec 60 sample 7 rec 57 %area %area %area %area %area %area %area lauric acid, methyl ester 0.27 0.02 0.03 0.20 0.14 0.11 azelaic acid. dimethyl ester 0.20 0.01 myristic acid. methyl ester 1.04 0.74 0.77 1.31 1.10 1.32 1.75 palmitoleic acid. methyl ester 3.64 3.99 3.48 4.63 7.18 3.81 4.61 palmitic acid. methyl ester 22.01 17.70 10.76 19.50 23.68 27.90 25.11 linoleic acid. methyl ester 3.48 4.32 1.29 2.00 7.22 3.54 3.69 oleic acid. methyl ester 26.64 34.30 23.52 24.70 34.34 34.02 34.25 stearic acid. methyl ester 20.90 9.48 7.43 14.11 20.65 19.27 18.29 retene (methyl isopropyl phenanthrene) 3.83 9.29 23.09 4.50 methyl arachidate 1.05 1.32 1.17 1.05 2.86 0.75 1.16 8-isopropyl-1.3-dimethyl phenantrene 1.43 3.11 4.7 2.36 dehydroabietic acid. methyl ester 7.17 7.66 13.09 12.59 behenic acid. methyl ester 1.36 2.62 2.01 7-oxodehydroabietic acid. methyl ester 4.56 6.01 7.13 6.27 lignoceric acid. methyl ester 0.87 0.54 0.72 1.81 0.95 2.04 2.56 β-sitosterol 1.33 stigmastanol 1.60 1.51 2.8 3.60 2.01 4.58 6.45 microsoft word 569-4534-2-le.docx acta imeko issn: 2221-870x december 2018, volume 7, number 4, 9-14 acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 9 proposal of a super-symmetric microcalorimeter for single-step realization of an rf primary power standard emil vremera1, luciano brunetti2 1 “gheorghe asachi” technical university of iasi, faculty of electrical engineering, bld. dimitrie mangeron 21-23, 700050, iasi, romania 2 istituto nazionale di ricerca metrologica – inrim, strada delle cacce 91, 10135 torino, italia section: research paper keywords: rf power standard; coaxial microcalorimeter; super-symmetric inset, thermoelectric sensor, insulating rf-transmission line citation: emil vremera, luciano brunetti, proposal of a super-symmetric microcalorimeter for single-step realization of an rf primary power standard, acta imeko, vol. 7, no. 4, article 3, december 2018, identifier: imeko-acta-07 (2018)-04-03 editor: vilmos pálfi, budapest university of technology and economics, hungary received march 23, 2018; in final form october 2, 2018; published december 2018 copyright: © 2018 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: emil vremera, e-mail: evremera@tuiasi.ro 1. introduction a microcalorimeter is a measurement system that is specifically used for determining the effective efficiency of the power sensors at small power levels (1-10 mw). such a system is operative in the most important national metrology institutes (nmis), which implement different architectures with the aims of obtaining a high level of accuracy, wide bandwidth, and limited measurement time. basically, this system may be seen either on a thermostatic water bath or an active thermostatic shield, which creates a highly stable thermal environment for single or, more often, twin thermal loads. figure 1 shows a scheme of a typical twin structure in coaxial transmission lines. the manner in which this system works has been widely discussed in the relevant literature by other researchers [1-3] and by us [4-6]. however, independent of the technical solution adopted in each laboratory, the effective efficiency of the power sensor used as the calorimetric load can be expressed as the rational function of sensor mount temperatures. these ones are measured with a differential thermometer in two states of thermal equilibrium, obtained by the supply (or not) of the measurement channel with radio-frequency (rf) power. the critical points of the microcalorimeters are: the temperature stability of the thermostatic shields, the degree of symmetry of the twin inset (if this configuration is used), and most importantly, the dismounting/mounting operations abstract the realization of rf primary power standards by means of the microcalorimeter technique requires a long measurement time due to the necessity of performing two completely distinct steps: measurement of the power standard and system calibration. such a procedure is detrimental to overall measurement accuracy; therefore, in this paper, a new improved design of a dual-channel broadband twin microcalorimeter is proposed. it is based on modified hardware and new, more suitable algorithms, which allows for the selfcalibration of the system without the dismounting/mounting operations between the measurement cycles. the paper has some key results: a significantly shorter measurement time, maintenance of (at least) the usual microcalorimeter accuracy, an improvement in thermal noise rejection, perfect correlation of the measured data series, and direct calculus by a simple formula. figure 1. scheme of a classic coaxial microcalorimeter based on the twin architecture. acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 10 requested by the microcalorimeter calibration step. this latter operation has an effect on non-coherent datasets, acquired during the microcalorimeter calibration and in the power sensor measurements. another disadvantage of such microcalorimeters is that they run for a long time due to the high time constant and the large number of test frequencies that are usually requested. 2. actual stage some nmis, e.g., physikalisch technische bundesanstalt (ptb), istituto nazionale di ricerca metrologica (inrim), and the korean research institute of standards and science (kriss), implement the rf power standard mainly by using thermoelectric power sensors. other nmis, such as the national institute of standards & technology in the us (nist) and the national institute of advanced industrial science and technology in japan (aist), prefer bolometric power sensors. in any case, progress in this field is possible if suitable choices are made for each of the following technical options: twin loads or a single load; compensations or corrections; direct results or postponed calculations; mid-term or long-term measurements; power-switching (or not) between channels in the twin architecture; thermal conduction or thermal insulation; an inner thermal shunt or high inner thermal resistance; thermocouples or bolometers as thermal loads. unfortunately, not all of the previously listed points have been thoroughly considered by the scientific community until now, especially those concerning the dry twin-microcalorimeter for thermoelectric loads. 3. single-step measurement proposal in order to measure the power sensors’ effective efficiency (which depends on frequency) with the microcalorimeter, two different measurement steps are necessary. in the first step, the power sensor under test is fed alternately with the rf power and the reference power, respectively, and a raw effective efficiency of the power sensor is calculated [4].the second step is necessary to make corrections to the contribution of the microcalorimeter feeding lines. unfortunately, several problems arise in the calibration step, including microcalorimeter dismount/mount operations, long-term thermal equilibrium, inherent uncontrollable changes in the thermal paths, differences between the power levels adjusted for each step, and very low measured values [1-5]. the power-switching method, proposed in [6] by us, allows us to obtain the final result without corrections or microcalorimeter dismounting/mounting operations, but the involved quantities are, however, obtained in different measurement phases and by changing the instrumentation setup. this contributes to the generation of non-coherent datasets and, therefore, possibly undetected measurement errors. only with a new design of the feeding line complex can the classic twin microcalorimeter [4-5] allow single-step measurements and, therefore, direct results for the power sensors’ effective efficiency. the technical solution that we propose involves duplicating the classic twin-line inset [4] according to the scheme in figure 2 [7] in order to obtain the needed super-symmetric inset. such a proposal was initially imagined as a solution to the intrinsic failure of the classic twin structure in creating a reference channel whose thermal behavior towards the external environment should be the same as the measurement channel. because this does not happen in practice due to the small but unavoidable differences in the thermal paths, rejection of the thermal noise of the external environment is not yet satisfactory in terms of obtaining the best measurement accuracy. however, the new architecture, which we have named the super-symmetric microcalorimeter, has turned out to be useful in realizing the measurement process for the effective efficiency parameter in a single step. this proposal requires changes to the microcalorimeter design, the instrumentation setup, and measurement algorithms. a) b) figure 2. a) scheme of a cross-section of the new coaxial microcalorimeter inset at the base of power sensor input connector, where thermometers measure the temperature differences; b) schematic vertical cross-section of the new microcalorimeter. acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 11 4. new twin-load microcalorimeter at each of channels a and b in the classic twin configuration [5], also shown in figure 1, an additional transmission line is coupled so that both a and b have their own thermal reference channel. every additional thermal reference channel is terminated with a dummy load, which, from the electromagnetic point of view, acts as a broadband low loss short circuit. figure 2 shows two schematics of the axial and transverse sections of the new coaxial microcalorimeter. the channels a and b are fed, alternately, by the rf power and by the reference power, whereas the new reference channels are fed only with the rf power. this scheme must compensate for rf losses in the power sensors’ feeding lines and further reduce the effects of the external environment on the measures. in this manner, we can obtain a virtual isothermal reference plane at the sensors’ input connectors level as in the case of a lossless ideal feeding line. in practice, each measurement channel must be electrically balanced against its own reference channel to compensate for any imperfections in the device realizations. a simple balance circuit may be introduced like that which is shown in figure 3. 4.1. description of system components the most important components of a microcalorimeter are described in detail as follows: a) the feeding lines are surely the most critical component of the system because they introduce the dominant error term in the whole measurement process. the general criteria for a suitable design for the feeding lines include that it must match both thermal and electrical behaviors very well and it must have good and adjustable thermal insulation, an inner thermal shunt present at the reference plane level, and a small reflection coefficient at the test load input port. a new profiled machinable air-gap insulating line, as shown in figure 4, is proposed in order to fulfill these goals. the device is different than that which is described in [8] and incorporates further improvements. concerning the effect of a 1 mm air gap, we point out that an improvement of about 50 % of the thermal insulation [9, 10] is obtained and does not have notable changes in the s-parameter values in the entire working bandwidth for a 2.92 mm coaxial line, as table 1 shows. a particular feature of this new insulation line is that, apart from the 45 ° angle of the air gap cross-section and four direct current (dc) thin wall small bridges, there is also an internal thermal shunt [11, 14]. b) the dummy loads are also important. they must be good “imitations” of the power sensors that are undergoing calibration in terms of their mechanical size and thermal properties [1]. a dummy load must be fitted with a metrological-grade rf connector and behaves as a short circuit as much as possible. the sensors’ output wires must also be replicated and electrically connected in the same manner for reproducing the same thermal paths and bridges as the real sensors in the test case. c) the microcalorimeter thermal sensing devices consist of two differential thermopiles with two balanced asymmetric outputs of the same polarity. they directly measure the thermal load response, both absolute and relative to its thermal reference load, for each channel, as shown in figure 2 and figure 3. figure 3. scheme used to eliminate the electrical dissymmetry between measurement and reference channels. a) b) figure 4. a) profiled machinable air-gap insulating line. b) improvements in the thermal isolation (comsol multiphysics). to = temperature at the thermal reference plane; ts = temperature at the input power cross-section; rth = thermal resistance; wag = with air gap; woag = without air gap; inner-line thermal shunt, present [11]. acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 12 d) due to the low power level used in such measurements, normally recommended as around 1 to 3 mw, the thermostatic case must provide a microcalorimeter kernel temperature stability of only a few mk. 4.2. instrumentation setup and the measurement process as figure 5 below shows, the instrumentation setup consists of a thermostat implemented with a nested structure of active and passive thermal shields, two rf generators, two audio frequency (af) current generators, two dc millivoltmeters, and two dc nanovoltmeters. basically, this setup duplicates (if these instruments are not dual-channel models) the number of the instruments that are normally requested for running the classic coaxial microcalorimeter [1], [2], [4]. a switching box allows for supplying both the loads and dummy loads with correct rf/af power levels according to the sequences requested for both the microcalorimeter calibration and power sensor calibration. the dedicated computer controls the temperature inside the thermostat by means of peltier elements, whereas a second computer performs the operations and the controls requested during all the measurement processes. the measurement procedure starts when the microcalorimeter thermal loads are in thermal equilibrium, a status detected by the measured temperatures. the first phase consists of the characterization of the sensors pairing in three conditions: the same af reference power (ref), the same sensor output voltage, and the same sensor temperatures (last calculated or measured at the connector reference plane). this data allows for the calculation of three ratios that are necessary for the next measurement and computing phases. in the second phase, channel a is fed with an rf power level that produces the same microcalorimeter thermocouple output voltage as the af ref simultaneously with a second rf source that is of the same frequency but with half the power level and is applied to the thermal reference line of channel a. this operation allows for extraction from the temperature response of the feeding line contribution due to its transmission losses. finally, the effective efficiency is found using the ratio between the af and rf thermopile output voltages for each test frequency. another algorithm can be applied in order to achieve thermopile balance. it can eliminate thermopile nonlinearity, but the operation does increase measurement time. in [15], this main parameter of the thermoelectric power sensors, the effective efficiency, is obtained by a simple formula that uses the ratio between temperature values: sc e ee e 11 2 − =η , (1) where e1, e1sc, and e2 are the thermopile responses in three situations realized in two steps: with the rf test power at the sensor input (e1), with the reference af power (e2), and with half of the rf test power. the sensor input is in short-circuit condition (e1sc). with the new proposed two-channel microcalorimeter, all these three quantities are available at the same time; moreover, no changes in the thermodynamic equilibrium of the system are provoked. the effective efficiency can be directly computed with simple formulas, like equations (3) or (4) below, which are equivalent and basically have the same form. this effect is related to the introduction of the additional thermal reference lines. the formulae contain the thermopiles’ output voltages ratio or the power sensors’ output voltages ratio as well as the ratio of the two coefficients that define the pairing of the sensors and the balance in the feeding paths: ab ab a ab a a k k e e p p s erfdasa eerefdbsb cterfsain refsain eff − − − =− = ==η 1, , . | | | | and (3) ab ab rfdasadbsb rfab a k k e e s eerefb eerefb eff − − −− = = =η 1, , | | , (4) where: eff η is the desired quantity, [1], [4], [5]; sain p is the input power dissipated in the power sensor that is being tested, [1], [4]; dasae − , dbsbe − are the thermopiles’ differential output voltages; ae , be are the thermoelectric power sensors’ output voltages; abk −1 defines the balance in the insulating lines in the thermal transmission coefficients [1], [5], [6], and [11]; and absk − refers to the pairing of the power sensors in the power conversion sensitivity [9]. these two coefficients should be considered as being unitary as a consequence of two actions: power sensor pairing, ba ee = for the same ref, and a balance in the thermopiles’ differential output voltages, dasae − = dbsbe − , by applying a similar technique to that which is presented in figure 3. both equations (3) and (4), therefore become ideal: a ab a erfdasa eerefdbsb eff e e , , | | − =− =η and (5) table 1. scattering parameter trend versus frequency for the insulating section with and without air gap; inner line thermal shunt was present [11]. parameter value type of il frequency s21 0.9900 wag 1ghz s21 0.9901 wag 20ghz s21 0.9875 wag 40ghz s21 0.9885 woag 40ghz s11 0.0297 wag 1ghz s11 0.0186 wag 20ghz s11 0.0686 wag 40ghz s11 0.0687 woag 40ghz lumped ports situation s21 1.0028 wag 1ghz s21 1.0022 wag 20ghz s21 0.9992 wag 40ghz s21 0.9960 woag 40ghz s11 0.0205 wag 1ghz s11 0.0190 wag 20ghz s11 0.0860 wag 40ghz s11 0.0836 woag 40ghz z11 51.62 wag 1ghz z11 53.91 wag 20ghz z11 47.29 wag 40ghz z11 57.43 woag 40ghz acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 13 rfdasadbsb rfab a eerefb eerefb eff e e −− = = =η , , | | . (6) however, only (3) or (4) are considered in the uncertainty budget estimation. in this manner, we can take into account the imbalances and the imperfect pairing, regardless of the nonlinearity of the thermopiles [19]. if this nonlinearity affects the compensation of the line losses, the changes that may arise due to this effect must be estimated [20] and cancelled by adjusting the summation network, as shown in figure 3. a simplified evaluation of the overall relative uncertainty leads to the next formulae: ( ) ( ) ( ) ( )2222 1% kkee uuuuu sdbsbdasa +++= −−η and (7) ( ) ( ) ( ) ( )2222 1% kkee uuuuu sba +++= η . (8) if we suppose that the four relative terms in equation (7) and equation (6) are under 0.1 %, an overall uncertainty of 0.2 % is expected. another issue concerning the correctness of the algorithm measurements is represented by the correlated power leveling of both rf generators. because the short circuit in the thermal reference is not perfect (гsc ≠ 1), the power ratio of the rf power inputs must be: ( )( ) ( )2 21 2 21 22 21 111 spisspi asad ilscil −=γ+− −− db spi pi scil il as ad 3 1 1 2 21 2 −> γ+ =⇒ − − , (9) where: asil pi − and adil pi − are the incident powers at the insulated lines’ input for the sensor that is being tested and its thermal reference, respectively; and s21 is the paired lines’ forward transmission coefficient. if equation (9) is used in the rf power correlation, a degree of imbalance exists, however, due to the limits of the leveling loops. the leveling of rf power that is better than 0.02 db and a power separation coefficient k that is smaller than 0.1 lead to 0.05 % uncertainty due to the imperfect compensation of the insulating line losses. this value does not have the effect of changing the estimated overall uncertainty. at the end of the measurements according to the frequencies list, the whole process must be reiterated according to the computing requirements for calculating uncertainty and the number of degrees of freedom. a successful finish, proven by the same behaviors in the first phase of the measurement, allow for the immediate start of the measurements for the channel b sensor. the equations are, of course, similar to equation pairs (3) and (5) or (4) and (6). figure 5. scheme of the proposed instrumentation setup acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 14 the measurement time required for two paired rf power sensors and for the same number of degrees of freedom is half of the measurement time that is used in the actual microcalorimeter for a single sensor, which is, at least, 18 hours/six repeated measurements at every frequency test point. 5. conclusion the doubling of the channels in the classic twin microcalorimeter seems to be a better way of solving the problems still inherent in this architecture. such a microcalorimeter and the new measurement methods and algorithms that can be applied solve the problems mentioned in the other research papers that have been dedicated to thermoelectric rf power standards [4-6, 11, 16-19]. the complications arising due to the realization of the true dualchannel rf microcalorimeter are mitigated by calibrating two paired sensors in a single measurement step and by involving coherently acquired data in the computation. in comparison with the methods described in the relevant literature until now [1-6, 8, 11, 15-21], this proposal offers key advantages: a much shorter measurement time, calculus that is based on two completely correlated datasets, and feeding line losses in each measurement moment. references [1] a. e. fantom, radiofrequency and microwave power measurement, peter peregrinus ltd, london, 1990, isbn 086341-120-7. [2] f. r. clague, microcalorimeter for 7 mm coaxial transmission line, nist technical note 1358, 1993. [3] e. f. glenn, microwave circuit theory and foundations of microwave metrology, peter peregrinus ltd., london 1992, isbn 0-86341-287-4. [4] l. brunetti, e. vremera, a new microcalorimeter for measurements in 3.5-mm coaxial line, ieee transactions on instrumentation and measurement 52(2) (2003) pp. 320-323. [5] l. brunetti, l. oberto, m. sellone, e. vremera, latest determination of a coaxial microcalorimeter calibration factor, measurement science and technology 22 (2011) pp. 025101-1025101-6. [6] e. vremera, l. brunetti, l. oberto, m. sellone, power sensor calibration by implementing true-twin microcalorimeter, ieee trans. instr. meas. 60(7) (2011) pp. 2335-2340. [7] e. vremera, l. brunetti, “primary rf-power standard realization in a single-step measurement process”, proc. of 22nd imeko tc4 int. symposium and 20th int. workshop on adc modelling and testing, sept. 14-15, 2017, iasi, romania, pp. 424-428. [8] j. y. kwon, d. j. lee, adiabatic design for a coaxial transmission line, ieee transactions on instrumentation and measurement 63(7) (2014) pp. 1760-1768. [9] g. c. m. meijer, a.w. herwaarden (editors), thermal sensors, inst. of physics publishing, bristol, 1994, isbn 0-7503-0220-8. [10] t. hatakeyama, f. x. quinn (editors), thermal analysis: fundamentals and applications to polymer science, john wiley & sons ltd., london 1999, isbn 978-0-471-98362-0. [11] e. vremera, l. brunetti, design of 2.92 mm coaxial adiabatic lines for quasi-ideal twin microcalorimeter, ieee transactions on instrumentation and measurement 61(6) (2012) pp. 1692-1702. [12] c. b. carter, m. g. norton, ceramic materials: science and engineering, springer, new york, 2007, isbn 978-0-387-462714. [13] j. ma, h. h. hng, high thermal conductivity ceramic layered system substrates for microelectronic applications, journal of materials science: materials in electronics 13 (2002) pp. 461-464. [14] s. l. shindé, j. goela (editors), high thermal conductivity materials, springer, new york 2006, isbn 978-0-387-22021-5. [15] l. brunetti, l. oberto, e. vremera, thermoelectric sensors as microcalorimeter load, ieee transactions on instrumentation and measurement 56(6) (2007) pp. 2220-2224. [16] e. vremera, l. brunetti, l. oberto, m. sellone, alternative procedures in realizing of the high frequency power standards with microcalorimeter and thermoelectric power sensors, measurement 42(2) (2009) pp. 269-276. [17] j. y. kwon, t. w. kang, j. h. kim, j. s. kang, development of a 3.5 mm coaxial microcalorimeter for rf and microwave power standards at kriss, ieee transactions on instrumentation and measurement 60(7) (2011) pp. 2609-2614. [18] l. brunetti, l. oberto, m. sellone, e. vremera, comparison between thermoelectric and bolometric microwave power sensors, ieee transactions on instrumentation and measurement 62(6) (2013) pp. 1710-1715. [19] l. brunetti, e. monticone, thermopile linearity errors in the hfcalorimeter method, measurement 8(4) (1990) pp. 146-152. [20] j. y. kwon, t. w. kang, n. w. kang, v-band waveguide microcalorimeter for millimeter-wave power standards, ieee trans. instr. meas. 66(6) (2017) pp. 1598-1604. [21] l. oberto, l. brunetti, rf/mw power standard realization without unitary efficiency assumption at dc/lf, ieee trans. instr. meas. 67(4) (2018) pp. 925-929 microsoft word 267-1843-1-le-rev2 acta imeko  issn: 2221‐870x  december 2015, volume 4, number 4, 38‐47    acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 | 38  key comparisons: the chance for discrepant results and some  consequences  franco pavese  torino, italy        section: research paper   keywords: key comparisons; bipm kcdb; systematic effects; corrections  citation: franco pavese, key comparisons: the chance for discrepant results and some consequences, acta imeko, vol. 4, no. 4, article 9, december 2015,  identifier: imeko‐acta‐04 (2015)‐04‐09  section editor: franco pavese, torino, italy   received april 7, 2015; in final form august 17, 2015; published december 2015  copyright: © 2015 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  corresponding author: franco pavese, e‐mail: frpavese@gmail.com    1. introduction  the only means to verify the existence of significant systematic effects in the measurement results (in short results) of a laboratory is to perform comparisons of results of several (at least two) laboratories, in the assumption of lack of strong reasons for correlation of their results. the mutual recognition arrangement (mra) [1] requires performing such an exercise, called “key comparison” (kc), as the main method to obtain statistical evidence of these effects. there are several categories of the kcs, the top rank being the cipm kcs. other types are the “regional”, “bi-(multi-)lateral”, “supplementary”, etc. a database (kcdb) of the results of these exercises is maintained at bipm [2], which represents a unique and precious repository for understanding the state-of-the-art of each specific metrological field, and for the detection of significant systematic effects. in the mra, the latter is called “systematic undetected differences” (sud) between participants, requiring the application of a specific procedure in respect to the declaration of the calibration and measurement capabilities (cmc), [1]. in general, the sets of kcdb results are also extremely valuable to assess the frequency of occurrence of significant systematic effects, or the lack of such occurrence. this is “prior information” (not necessarily in the bayesian sense) for the next exercises, in the statistical treatment of experimental data, namely of metrological ones. this issue should be taken in due account irrespective to the use of a frequentist, bayesian or of any other approach, namely in the assessment of the quality of a measurement standard. in this work an analysis of a large number of the kcs included in the kcdb is performed, as described in sections 2 to 4. then the paper discusses the reasons for inconsistent results in sections 5 and 6, and proposes some remedies in section 7, before the conclusions are drawn. an appendix acts as a digest useful to have at hand a summary of the current definitions of the main terms used in the paper. 2. a statistics from the bipm kcdb  the analysis was performed on all the cipm (master) kcs—some already including the relevant supplementary ones—included in the kcdb, with final results available at the date of november 15, 2012. they amounted to 339 of the total abstract  the paper reports an analysis of the results of complete sets of key comparisons available on the bipm repository (kcdb), which shows  that the majority of them show some discrepant results. the main reason of this fact is due to unknown or ill‐estimated systematic  effects, and indicates that the usual method of ‘correcting’ for known systematic effects is inefficient. the paper then discusses some  of the possible reasons and an alternative approach, starting from a proposed revision of the meaning of systematic effect/error.   acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 | 39  819 of all different types then included in the kcdb. they include all 8 areas in which the quantities are subdivided at the bipm among the consultative committees (cc): auv, ccl, ccm, cct, em, pr, qm, ri. the analysis concerns all the ‘results’, meaning all the specific comparison tables in the final result files (they may be one or dozens, depending on each kc) non-consistency is defined here as the non-overlap of the pairs of uncertainty intervals (provided for k = 2 in the kcdb). the number of occurrences of non-overlap of the results of each participant with respect to the key comparison reference value (kcrv, account being taken of its uncertainty) was recorded. the number of occurrences of non-overlap of the results of pairs of participants was separately recorded. 3. analysis of the kcdb results  overall, the number of cipm kcs with no inconsistencies is about one fourth only of the total. the number of cipm kcs with pair inconsistencies is more than half (59 %). the anomalies are differently distributed for the kcrv non-overlap and for the pairs non-overlap. the statistics may be different for some metrological fields. table 1 reports a synthesis of the data collected. the corresponding typical overall mixture [3]–[5] (or pooled [6]) distributions of several cct kcs is shown in figure 1, together table 1. cipm kcs from the kcdb.  kcdb  all ok  kcrv  no‐overlap  x  pairs  no‐overlap  marginal  no‐overlap  y  all ccs a  (339) b   90  50  140 (41 %)  161 (41 %)  38  199 (59 %)  auv  (9)  4  4  8 (89 %)  0  1  1 (11 %)  ccl   (10)  2  1  3 (30 %)  6  1  7 (70 %)  ccm (39)  10  6  16 (41 %)  20  3  23 (59 %)  cct     (7)  1  1  2 (29 %)  5  0  5 (71 %)  em   (45)  22  3  25 (53 %)  10  10  20 (47 %)  pr       (8)  0  3  3 (38 %)  5  0  5 (62 %)  qm (162)  33  26  59 (36 %)  85  18  103 (64 %)  ri     (59)  18  6  24 (41 %)  30  5  35 (59 %)  z  27 %  73 %  a fields (including derived quantities): auv: acoustics, ccl: length, ccm: mass; cct: temperature; em: electromagnetics; pr: ; qm: quantity of substance; ri: ionising radiation. in parentheses the total master kcs of that field. b at november 15, 2012: grand total of all types 819. x proportion of kcs without participant-pairs non-overlap, but showing some non-overlap to the kcrv. y proportion of kcs with participant-pairs non-overlap (kcrv non-overlap not considered). z proportion of kcs with some non-overlap. figure 1. mixture distributions for 13 fixed points in 3 cipm cct‐kcs (temperature), from 7‐20 local pdfs each (after [6]).    acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 | 40  with the distributions of three common location parameters. figure 2 shows a similar situation for a ccqm kc [7]. a more limited exercise of the same type was published in [8]. 4. discussion of the results of the analysis  the literature data on ways to deal with this type of result inconsistencies is very wide; the reader can consult, e.g., [9][17]. some of the proposed methods consist of criteria and procedures for modifying the results of the inter-comparison, or to find a subset of the overall results that is consistent and on which a representative reference value is then estimated. however, while these may be reasonable solutions for generic inter-comparisons, that way-out is not allowed for the results of a mra kc [1], which is a non-hierarchical intercomparison designed mainly to obtain a degree of equivalence between the participants,1 not their equivalence (called metrological compatibility in [18]). in fact, all originallycommunicated results must be reported in the kc final report, and a participant can only possibly withdraw (but its participation remains recorded), or, should a so-called systematic unresolved difference (sud) occur, can correct the uncertainty associated to its result (not the values), for the purpose of his application to include them into its cmcs. in all instances, the validation of the laboratory results requires considering inter-comparisons as a mandatory step for the assessment of the quality of measurement standards. on the other hand, the kc exercise in itself should be intended to check for the default, though top, accuracy of the measurement standards i.e. it is a realisation under repeatability conditions, not an exceptional realisation similarly to a “proficiency test” [19], but without being hierarchical. 1 it can be with respect to the kcrv, computed by using all the results, or as a difference between pairs of participants. an inter-comparison is a distinct step with respect to intralaboratory information, consisting in the addition of interlaboratory information gained by means of that exercise. it should therefore be included and discussed in the international guidelines concerning the evaluation of the uncertainty of the results, like the gum, [20] or in the relevant iso standards. [21], [22] 5. systematic effects/errors in common data‐ treatment approaches  inconsistencies resulting in inter-comparisons may arise, in general, from: • errors in the measurement of the value of influence quantities, responsible of systematic effects; • under-evaluation of the associated uncertainties, often due misevaluated systematic effects; • omission of considering some influence quantities, namely some of those responsible of systematic effects (model imperfection: epistemic problem). the meaning of ‘systematic effect’ is assigned in both, the “uncertainty approach” (gum [20]), in respect to the meaning of “input quantity” and “correction” and, the error approach (e.g., [18], [21]), where it is called “systematic error”, in respect to the meaning of “bias” and “correction”. it is necessary to understand first how these terms are defined in order to discuss their pros and cons.2,3 2 the fact reported in the gum, note to clause 3.3.3 that “a ‘random’ component of uncertainty in one measurement may become a ‘systematic’ component of uncertainty in another measurement in which the result of the first measurement is used as an input datum” is not a justification for avoiding to introduce the systematic effects. see later on. figure 2. results of a ccqm kc, showing on the right side the individual results and the mixture distribution (after [7]).    acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 | 41  5.1. uncertainty approach  according to the gum approach, regarding the (mathematical) modelling of the measurement, clause (4.1) tells: “in most cases, a measurand y is not measured directly, but is determined from n other quantities x1, x2, ..., xn through a functional relationship f: y = f(x1, x2, ..., xn) ” (gum, clause 4.1.1) (1) and, “the input quantities x1, x2, ..., xn upon which the output quantity y depends may themselves be viewed as measurands and may themselves depend on other quantities, including corrections and correction factors for systematic effects” … “the function f as it appears in this guide is to be interpreted in this broader context, in particular as that function which contains every quantity, including all corrections and correction factors [the systematic effects] that can contribute a significant component of uncertainty to the measurement result” (gum, clause 4.1.2). however, after corrections are recognised in the gum as part of the measurement model, in clause (3.2.3) it is also indicated: “it is assumed that, after correction, the expectation or expected value of the error arising from a systematic effect is zero”: this clause mandates corrections deserving a preliminary step where the initial model is changed, as shown in the following simple example [25]: model after gum 4.1.2: y = (x1 + c1) + (x2 · c2) , (2) where x1 and x2 are two ‘input quantities’, c1 and c2 are a “correction” and a “correction factor” to x1 and x2, respectively; model after gum 3.2.3: y ≈ (x1* + c1*) + [x2*· (1 + c2*)2], (3) where x1* = x1 + e(c1); x2* = x2 · e(c2); c1* = c1 – e(c1); c2* = c2 – e(c2), where, in general, e(c1) ≠ 0 and e(c2) ≠ 0. according to the gum, the new quantities c1* and c2* have zero expectation: e(c1*) = 0 and e(c2*) = 0. expression (3) is the actual gum model subject to the uncertainty analysis. the corrections cn (uncertain, according to gum clause 3.3.1 “the result of a measurement after correction for recognized systematic effects is still only an estimate of the value of the measurand because of the uncertainty arising from random effects and from imperfect correction of the result for systematic effects.”) 4 are “compensations”, as said in vim3 (2.53), “for an estimated systematic effect”. 3 see the appendix to this paper. note that the gum terminology is based on the vim2 [36], obviously not on the subsequent vim3. 4 in gum clause 3.2.3 a note states: “the uncertainty of a correction applied to a measurement result to compensate for a systematic effect is not the systematic error, often termed bias, in the measurement result due to the effect as it is sometimes called. it is instead a measure of the uncertainty of the result due to incomplete knowledge of the required value of the correction. the error arising from imperfect compensation of a systematic effect cannot be exactly known. the terms ‘error’ and ‘uncertainty’ should be used properly and care taken to actually, they are compensations for ‘deviations’ of measured from a ‘reference condition’ of specified quantities,5 due to perturbations caused by other influence quantities producing off-set conditions, called systematic effects (the term ‘bias’ is not used in the gum).6 in short, the reference condition can be defined as the one where all the perturbations caused by other influence quantities are null (thus no corrections). 5.2. error approach  in the error approach, the ‘deviations’ caused by the systematic effects are customarily called “measurement bias” (in short ‘bias’, see also footnote 4), bn, and corrections are taken as cn = – bn (but see also footnote 7 later on). in the vim3 (2.18) “measurement bias” is defined “estimate of a systematic measurement error” (emphasis added), where the “systematic measurement error” (in short ‘systematic error’) is defined in (2.17) “component of measurement error that in replicate measurements remains constant or varies in a predictable manner”, and with note 3 to (2.17) indicating “systematic measurement error equals measurement error minus random measurement error”. in turn, the “measurement error” (in short error) is defined in vim3 (2.16) “measured quantity value minus a reference quantity value” and “random measurement error” (in short ‘random error’) (2.19) “component of measurement error that in replicate measurements varies in an unpredictable manner”. thus, the error is referred to each single observation of the set, while the distinction between systematic and random can arise only from “replicated measurements”. consequently, according to these definitions, the n-th variable representing the population of the errors i,n, en, has the systematic error as expectation e(en), and the one that comes from the distribution of the errors as random error. the bias, as defined in (2.18), can be an “estimate” of an error only if the difference between the “measured quantity value” and the “reference quantity value” cannot be known distinguish between them” (emphases added). it is true—see section 5.2—but actually the knowledge is always incomplete. 5 the gum jcgm 104:2009 (gum supplement 4) [28], issued years after [20], admits in clause 3.11 that “correction terms should be included in the model when the conditions of measurement are not exactly as stipulated. these terms correspond to systematic error values” (emphasis added). thus a “stipulated” condition is introduced, looking consistent with the term used here, ‘reference condition’, but the need is confirmed that, “given an estimate of a correction term, the relevant quantity should be corrected by this estimate”. notice that this definition of “influence quantity” is not the vim3 one (2.52) “quantity that, in a direct measurement, does not affect the quantity that is actually measured, but affects the relation between the indication and the measurement result” (emphasis added), but the gum one, clause b.2.10 “quantity that is not the measurand but that affects the result of the measurement”. 6 by definition, once the influence quantities have been identified in each specific case, no other quantity outside the set of the input quantities plus the quantities responsible for systematic effects/errors can significantly influence the value of the measurand.   acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 | 42  exactly. however, the first is the measured value. the second, being a “quantity value used as a basis for comparison with values of quantities of the same kind” (5.18), must be estimated only in the case (a) of the note 2 to (5.18): “a reference quantity value can be a true quantity value of a measurand …”, while in the case (b) “…is usually provided …”, i.e. is an exact value— though it might have an associated uncertainty. finally, bias is here relative to “a reference quantity value”, and, as indicated in note 2 to (2.17), “a correction can be applied to compensate for a known systematic measurement error” (emphasis added). as defined, the term bias looks not a variable since it is referred to each realisation, as the term error is. in the iso 3534:2006 (3.3.2) [21], “bias” is defined “difference between the expectation of a test result or measurement result and a true value”. “error of result” is defined in (3.4.4) “test result or measurement result minus the true value”, and is made of two components: (3.4.6) the “random error of result” is the “component of the error of result which, in the course of a number of test results or measurement results, for the same characteristic or quantity, varies in an unpredictable manner” with “note it is not possible to correct for random error”; (3.4.7) the “systematic error of result” is the “component of the error of result which, in the course of a number of test results or measurement results, for the same characteristic or quantity, remains constant or varies in a predictable manner”. finally, “correction” is defined in (3.1.16) “action taken to eliminate a detected nonconformity” with “note 1 a correction can be made in conjunction with corrective action” and “note 2 a correction can be, for example, reworked or regraded”, the definitions of the terms in italics being in iso 9000:2005, 3.6.6. in turn, “nonconformity” is defined (3.1.11) “non-fulfilment of a requirement” and “corrective action” is defined (3.1.15) “action to eliminate the cause of a detected nonconformity or other undesirable situation”. comparing the term “bias” (3.3.2) with the term “error” (3.4.4), the only difference is that the first is the expectation of the second, but in the second there is a single “true value” while in the first “a true value” is indicated. however, note 1 to (3.3.2) states “bias is the total systematic error as contrasted to random error”: its meaning can be ambiguous. in fact, in (3.4.2) “measurement result” is defined “value of a quantity obtained by carrying out a specified measurement procedure”, where “observed value” in (3.2.8) is “obtained value of a quantity or characteristic”, which can be assimilated to “value” from note 2 to (3.2.8) “observed values may be combined to form a test result or measurement result”. therefore, the measurement result is, in general, a set of observations, thus the term bias is not assigned to each single observation, being this consistent with the use of the term ‘expectation’ of the variable (‘observed differences between test or measurement values and a true value’). on the contrary, note 1 to (3.3.2) does not indicate the same concept, unless the systematic error is defined as the expectation of the error and the random error is defined as the variability of the error. 5.3. meaning of bias due to systematic effects in this paper  there are two basic issues about the differences in the meaning of the term bias in iso 3534:2006 that need be noted with respect to the vim3 relevant set of definitions. the first issue is that the term bias looks not to be assigned to a variable in the vim3, while it is in the iso 3534:2006. in addition, the latter specifies that it concerns the total effect of the systematic errors, so that the use of the plural ‘biases’ may be unnecessary. the second issue is that bias in the iso 3534:2006 is the deviation from “a true value”, though note 3 to (3.3.2) tells “in practice, the accepted reference value is substituted for the true value”, being the latter unknown, while in the vim3 it is the deviation form a “reference quantity value”. if the aim of the measurement is not to hunt for the true value, but to provide a value of the measurand according to the best estimated model of it and of the measurement system, the meaning of bias in the second issue is the same in both vim3 and iso 3534:2006 with respect to a reference value. concerning the first issue, it is more difficult to understand if the two definitions are equivalent, because of the ambiguity in the vim3. in this paper, the term “bias” is assumed to be represented by random variables, bn, one for each of the relevant quantities involved in the model. see the appendix to this paper. in addition, the set of influence quantities, according to the model set in each specific “design of experiment”, in the presence of bias is subdivided, as shown, e.g., in section 5.1, into two sets: input quantities and quantities requiring a correction, the latter coincident with those responsible of systematic effects/errors. as said in section 5.1, the ‘deviations’ are between the measured values and a ‘reference condition’ for each of the quantities responsible for a bias component. nonzero bias means that the reference condition is evaluated not having been met (because a deviation is actually observed); or, that an unexpected deviation from a ‘standard condition’ occurs, so that the measurement results are systematically ‘off centre’ in the variations of their values. finally, it should be clear that the term “systematic” should not be taken as ‘fixed’: at best one can use the expected value of a distribution of its values. 6. analysis of the reasons of failure of the prior‐ correction approach  the historical practice of requiring a prior correction for the systematic effects/errors has been shown to fail in the majority of cases of the cipm kc results—also representative of other similar exercises. that practice does not ensure sufficient confidence in taking effectively into account the systematic effects. the reason of failure looks residing in an insufficient understanding of two basic features of a reliable process of evaluation of measurement data: (a) the overall, single-step nature of the analysis required by the ‘design of experiment’, forming the within-laboratory knowledge framework; (b) the intrinsic need to consider the inter-comparison of independent results as an integral final step in the betweenlaboratory subsequent framework of the process, before a statement about result consistency (metrological compatibility [18]) can be assessed with sufficient confidence/belief. the feature (a) will not be discussed here in details and is deferred to a subsequent study [29]. it seems sufficient to recall here that the process of identifying the influence quantities is not a two-step one, the first concerning what in the gum are called the “input quantities” and the second concerning the quantities responsible for the systematic effects calling for corrections. in addition, it is a fact that no measured or evaluated value of a quantity, including corrections, can be   acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 | 43  exempt from ignorance as to the actual location of the true value, or from uncertainty arising from the dispersion of the measured data, nor can any evaluation be considered exact, except by convention, irrespective to the process bringing to it (either type a or type b). therefore, in a typical model no difference in principle should exist between the influence quantities called “input quantities” xi and those responsible of “systematic effects”, called biases bn in the error approach, and said to require prior correction in both approaches. [23]-[27] each of them can in fact be expressed as follows (a) xi = e(xi) + xi*, (b) bn = e(bn) + bn*, (4) perfectly equivalent with each other, where the starred quantities have zero mean and take into account the variability of the data—the normally non-zero mean (the ‘quantity value’) has been ‘extracted’ from the non-starred quantities. in (a) e(xi) is the estimate of the quantity value, in (b) e(bn) is the estimate of the systematic component of the effect/error, i.e. of the difference between the value of the measured quantity and a reference value; in (a) xi* is what is normally indicated as ei, the random error, in (b) bn* is what is indicated in the standards and guidelines as the random component of the effect/error. the bias indicates the deviations of the measured values from a set of reference values forming what was called hereinbefore a ‘reference condition’. for example, for a single additive bias bi, xi = xi + bi (the symbol of ‘standard state’  is borrowed here, for analogy, from physical chemistry to indicate the reference condition for which e(bi) =: 0), and xi + bi = xi – ci. 7 7. a consequence  the resulting model, rather than (1), should be: y = f(w1, w2, …, wm, …, wm), (5) where wm are either the x1, …, xi and the c1, …, cn, with i = 1…i, n = 1…n, m = i + n: the wm include the ‘correction’ terms, carrying in also an uncertainty component.8 the only characteristics of bias is functional (e.g., in a causeeffect diagram): it applies to an ‘input quantity’, or to another bias term. the practical difference between correction cn and input quantity xi basically is: (i) for an “input quantity”, xi, the measurement results always involve one or more type a components; (ii) for a correction term, cn, the evaluation may involve only type b components. for any random variable, the expectation of the measured or estimated dispersion of the data does not allow, within each set 7 notice that, in general, e(ci) is intended for ‘correction’. 8 whether should the model include the corrections or the systematic effects is something worth to ponder about. it depends on the purpose of the model. the gum one looks not to be the ‘measurement model’ in the sense of a model constructed prior the measurements are performed—which should use the (physical) influence quantities (xi and bn in the notation of this paper)—but the posterior model of the measurement outcomes—using the measured (or evaluated) influence quantities mi, and cn. [29] of measurements, to estimate with sufficient confidence that no bias exists due to missed or ill-evaluated systematic effects. therefore, a model like (5) only eliminates the confusion that can arise from an incorrect use of the corrections, but does not alleviate the problem of assessing the consistency of independent measurements. that assessment, which can be called the between-laboratory traceability, is a basic need in metrology, as no isolated measurement can ensure it—remaining in practice useless. according to the above feature (b), only inter-comparisons can increase the confidence in the correctness of the assessment. [30]–[33] on the contrary, as said at the beginning of section 4, most of the published effort has been devoted to find methods to deal with inconsistent data, giving for granted the separate treatment of the systematic effects, though their definitions were not univocal, as shown in section 5. 8. conclusions  the analysis of the kcdb indicated that the method of the prior correction of systematic effects fails utterly in the majority of cases.9 the kcdb prior information—that the kcs outcomes are most likely to reveal non-consistent data—cannot be omitted from the measurement model, nor one can assume with sufficient confidence that “after correction, the expectation or expected value of the error arising from a systematic effect is zero”. a different more robust way should be applied to the treatment of the measurement results, irrespective to the approach used for the treatment (frequentist, bayesian or else; error or uncertainty). all ‘corrections’ should be considered what they actually are: imprecise compensations of bias, a type of ‘input quantity’ to be included in the model as any other influence quantity and treated as such. the (true) value of a ‘correction’ remains as unknowable as that of any ‘input quantity’, and as that of the measurand itself. incidentally, the estimated value of a correction can be set to zero10 (randomised) to mean ignorance about its value (and sign), but it always carries an uncertainty taking also this fact in due consideration. the rational for that is the fact that the bias originates from systematic effects (at least the non-epistemic ones) and should be treated as a random variable of the same nature of those of the variables modelling the ‘input quantities’, as shown in (4). a separate step for applying some corrections (meaning by using e(cn)) is not forbidden, but in principle is unnecessary and prone to possible confusion. the common characterisation of the systematic effects in sentences like, e.g., “the ‘systematic’ sources are such parameters associated with calibration standards, reference data, 9 in [35] a systematic effect is said to “constitute an element for which treatment conventional statistics fails utterly”, where ‘conventional’ stands for ‘non-bayesian’. author’s opinion is that this paper has shown that the statement might not be true. 10 this fact indicates that, differently from the ‘input quantities’, the values of the influence quantities responsible for the systematic effects are not used, only their differences with respect to the reference state.   acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 | 44  bias corrections, etc., whose effects are usually common and not altered under repeatability conditions” [34] implies that no uncertainty is associated to the effects. this is not true: also type b evaluations (in gum language) are based, directly or indirectly, explicitly or implicitly, on experimental data or on subjective judgement, both uncertain. each of the estimates of the influence quantities appearing in the model is affected by their own uncertainty. 11 an alternative way has been suggested in this paper, the treatment of an overall (single) model including as a single one the two sets of influence quantities in which the overall set is traditionally subdivided: the input quantities and the bias quantities (the latter traditionally suggested to require prior correction, a practice that should be abandoned). the model can be broken into sub-models only for a matter of convenience in computation, but only in very simple cases without basic implications for the overall treatment. a separate step for applying some corrections (meaning by using e(cn)) is not forbidden, but is in principle unnecessary. as a final step, the result assessment process must always be integrated with inter-comparisons. that need does not concern only metrological studies, but also studies in other fields, like testing (e.g., assessment of reference materials), see [37]. appendix  meaning of some critical terms used in this paper with respect to their definitions in the vim3 [2], gum [1] and iso 3534-2:2006 [9] there are persisting difficulties in terminology arising from the fact that the same concept can be found defined differently in different international guidelines and standards. this fact is partly due because the current versions of these documents, some regulatory some informative, date back to different times. for the ones taken in consideration here, the vim3 [2] basically dates 2008, the gum [1] dates 1998 (so the terms are defined in it according to the vim2 [3], issued in 1998), and the iso 3534-2 dates 2006. as a consequence, not necessarily all the relevant definitions are consistent with each other. it is not the aim of this appendix to discuss the issue in full, but only to compare in brief the above documents with the meaning that the author have chosen to assign in this paper to certain critical terms. however, no term is used in this paper with a meaning not conforming at least one of the existing guidelines or standards. the appendix is not in the alphabetical order of the terms because the illustration may require interrelating them logically. 11 in gum appendix e, the clause e.3.5 states “in the traditional terminology, the third term on the right-hand side of equation (e.6) is called a ‘random’ contribution to the estimated variance u2(z) because it normally decreases as the number of observations n increases, while the first two terms are called ‘systematic’ contributions because they do not depend on n”. it is correct for n observations of another quantity, but an increasing number of observations, concurring to estimate the expectation of a quantity responsible for a systematic effect, concurs in decreasing its uncertainty. measurement method according to the vim3 term 2.5, one can have “direct measurement methods, and indirect measurement methods”. this paper only refers to indirect methods. actually, in the vast majority of cases and for sufficiently high accuracy a direct method is only an ideal approximation, since, due to the corrections,12 the number of influence quantities is higher than that strictly required for the direct method. influence quantity in this paper, we include in this term all quantities whose effect has a significant influence on the numerical value of the “measurement result” (vim3 term 2.9), according to its “target uncertainty level” (vim3 term 2.34), and irrespective to the fact that the numerical values are provided by measurement, or are obtained with methods requiring type b uncertainty evaluations. for the reasoning in this paper, we do not consider appropriate the vim3 term 2.52 definition “quantity that, in a direct measurement, does not affect the quantity that is actually measured, but affects the relation between the indication and the measurement result” (emphasis added, see measurement method). the meaning in this paper is basically the one in the gum term b.2.10 “quantity that is not the measurand but that affects the result of the measurement”, taken from the vim2. thus, this set of quantities is broader than that of the input quantities (see section 2). the iso 3534-2:2006 does not define this term. measurement result this paper uses the vim3 2.9 definition “set of quantity values being attributed to a measurand together with any other available relevant information”. the iso 3534-2:2006 term 3.4.2 definition, “value of a quantity obtained by carrying out a specified measurement procedure”, is different, and the gum considers the measurement result basically as single valued (e.g., clause 4.1.5). measurement model in this paper, it is the model including all the influence quantities. the vim3 term 2.48 definition looks similar, but does not have exactly the same meaning: “mathematical relation among all quantities known to be involved in a measurement”. clearly this type of model depicts indirect measurements. the gum clause 3.1.6 tells: “the mathematical model of the measurement that transforms the set of repeated observations into the measurement result is of critical importance because, in addition to the observations, it generally includes various influence quantities that are inexactly known. this lack of knowledge contributes to the uncertainty of the measurement result, as do the variations of the repeated observations and any uncertainty associated with the mathematical model itself”. clauses 4.1.1–4.1.3 depict the same model of the vim3, but with different definitions for its components (see input quantity, correction). 12 as in vim3, the bold type is used here for other terms illustrated in the appendix.   acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 | 45  measurement function in this paper, it is the functional relationship implementing the measurement model. in the vim3 term 2.49, it is a “function of quantities, the value of which, when calculated using known quantity values for the input quantities in a measurement model, is a measured quantity value of the output quantity in the measurement model”. “note 1 if a measurement model h(y, x1, …, xn) = 0 can explicitly be written as y = f(x1, …, xn), where y is the output quantity in the measurement model, the function f is the measurement function. more generally, f may symbolize an algorithm, yielding for input quantity values x1, …, xn a corresponding unique output quantity value y = f(x1, …, xn)”. it is basically the same in the gum. input quantity this paper uses the term in the meaning of the gum, as expressed in clause 4.1.2: “the input quantities x1, x2, …, xn upon which the output quantity y depends may themselves be viewed as measurands and may themselves depend on other quantities, including corrections and correction factors for systematic effects, thereby leading to a complicated functional relationship f that may never be written down explicitly. further, f may be determined experimentally or exist only as an algorithm that must be evaluated numerically. the function f as it appears in this guide is to be interpreted in this broader context, in particular as that function which contains every quantity, including all corrections and correction factors that can contribute a significant component of uncertainty to the measurement result. thus, if data indicate that f does not model the measurement to the degree imposed by the required accuracy of the measurement result, additional input quantities must be included in f to eliminate the inadequacy. this may require introducing an input quantity to reflect incomplete knowledge of a phenomenon that affects the measurand” (emphasis added). the vim3 term 2.50 definition is “quantity that must be measured, or a quantity, the value of which can be otherwise obtained, in order to calculate a measured quantity value of a measurand”, with “note 2 indications, corrections and influence quantities can be input quantities in a measurement model”. the different meaning here of influence quantity is evident, so is the restricted meaning of “indication” (term 4.1). see the term correction for its relationship to input quantity. the iso 3534-2:2006 does not define this term. correction, correction factor correction is the key term analysed and discussed in this paper, where it is treated as a random variable, according to the gum clause 4.1.2 “the input quantities x1, x2, …, xn upon which the output quantity y depends may themselves be viewed as measurands and may themselves depend on other quantities, including corrections and correction factors for systematic effects”. thus in this paper the quantities responsible for corrections are not included among the input quantities, while they are among the influence quantities (but, e.g., gum clause 4.1.3 “…corrections for influence quantities…”) and expressing systematic effects (e.g., gum clause e.1.1 “correction for a systematic effect”). apparently “correction” is only for an additive correction term and “correction factor” only for a scale-factor correction term. in the vim3 term 2.53 is defined as “compensation for an estimated systematic effect. note 1 see gum:1995, 3.2.3, for an explanation of ‘systematic effect’. note 2 the compensation can take different forms, such as an addend or a factor, or can be deduced from a table” (emphasis added). in the vim3 term 2.50 input quantity saying “note 2 indications, corrections and influence quantities can be input quantities in a measurement model”, does not exactly match the gum definition. in the vim3 the term correction only comes in the following terms: 2.3 measurand, 2.17 systematic measurement error, 2.26 measurement uncertainty, 2.39 calibration, 2.50 input quantity, 2.53 correction and 5.10 intrinsic measurement standard. the iso 3534-2:2006 term 3.1.16 definition is “action taken to eliminate a detected nonconformity”, where the term “nonconformity” (3.1.11) definition is “non-fulfilment of a requirement”. none of the above documents indicates what exactly the “action” or “compensation” means. as to uncertainty of the correction, the gum clause 3.2.3 note states: “the uncertainty of a correction applied to a measurement result to compensate for a systematic effect is not the systematic error, often termed bias, in the measurement result due to the effect as it is sometimes called. it is instead a measure of the uncertainty of the result due to incomplete knowledge of the required value of the correction. the error arising from imperfect compensation of a systematic effect cannot be exactly known.” (emphasis added). as to the accuracy of the correction, the gum clause 3.2.3 states: “it is assumed that, after correction, the expectation or expected value of the error arising from a systematic effect is zero”. the vim term 2.17 note 2 states “systematic measurement error, and its causes, can be known or unknown. a correction can be applied to compensate for a known systematic measurement error.” (emphasis added), apparently different from gum. measurement error in this paper this term has the meaning defined in the iso 3534-2:2006 term 3.4.4 “test result or measurement result minus the true value”. the vim3 term 2.16 definition is “measured quantity value minus a reference quantity value”, different from the iso 35342:2006 term 3.4.4., irrespective to the fact that the latter also tells “note 1 in practice, the accepted reference value is substituted for the true value”. gum, not adopting the term, tells in clause 3.2.1. “in general, a measurement has imperfections that give rise to an error in the measurement result. traditionally, an error is viewed as having two components, namely, a random component and a systematic component. note error is an idealized concept and errors cannot be known exactly” (emphasis added). systematic effect, systematic error, bias in this paper, the term systematic effect is used with the same meaning it has in the gum clause e.3.4 (after equation e.4): “here α is a constant “systematic” offset or shift common to each observation, and β is a common scale factor. the offset and the scale factor, although fixed during the course of the observations, are assumed to be characterized by a priori probability distributions, with α and β the best estimates of the expectations of these distributions”. thus these effects are   acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 | 46  random variables. systematic errors are caused by systematic effects. also relevant here is the gum clause e.1 in full. in this paper, the term bias is used with the same meaning it has in the iso 3534-2:2006 term 3.3.2 “difference between the expectation of a test result or measurement result and a true value. note 1 bias is the total systematic error as contrasted to random error. there may be one or more systematic error components contributing to the bias. a larger systematic difference from the true value is reflected by a larger bias value. note 3 in practice, the accepted reference value is substituted for the true value”. however, note 3 is applied. the vim3 term 2.18 definition of measurement bias is “estimate of a systematic measurement error” and term 2.17 definition of the latter is “component of measurement error that in replicate measurements remains constant or varies in a predictable manner. note 1 a reference quantity value for a systematic measurement error is a true quantity value, or a measured quantity value of a measurement standard of negligible measurement uncertainty, or a conventional quantity value. note 2 systematic measurement error, and its causes, can be known or unknown. a correction can be applied to compensate for a known systematic measurement error. note 3 systematic measurement error equals measurement error minus random measurement error”. see the text above for a discussion about the meaning of clause 2.18. in the gum clause 3.2.3 note the specification “…systematic error, often termed bias …” indicates that the term bias is not recommended, being relative to an error, and is not used. standard state, reference condition, nominal value this paper introduces the concept of standard state in section 6 and of reference condition in subsection 5.1, as a condition where known significant systematic effects are absent. (not simply null) bias, as an out-of-reference condition, is introduced in subsection 5.2. the gum clause 5.1.5 introduces a nominal value: “if equation (1) [the measurement model] for the measurand y is expanded about nominal values xi,0 of the input quantities xi, then, to first order (which is usually an adequate approximation), y = y0 + c1δ1 + c2δ2 +… + cnδn, where y0 = f (x1,0, x2,0, …, xn,0), ci = (∂f/∂xi) evaluated at xi = xi,0, and δi = xi − xi,0. thus, for the purposes of an analysis of uncertainty, a measurand is usually approximated by a linear function of its variables by transforming its input quantities from xi to δ”. however, except for the above operational illustration, in the gum the meaning of nominal value is not defined. the δi are not labelled corrections. in the vim3 term 4.6 nominal quantity value definition is “rounded or approximate value of a characterizing quantity of a measuring instrument or measuring system that provides guidance for its appropriate use” and looks having a meaning different from the previous. references  [1] cipm, mutual recognition of national measurement standards and of calibration and measurement certificates issued by national metrology institutes (mra). (1999) bureau international des poids et mesures, sèvres. http://www.bipm.org [2] http://kcdb.bipm.org [3] p. ciarlini, g. regoliosi and f. pavese: “a bootstrap algorithm for mixture models and interval data in inter-comparisons”, proceedings a4a4 conference, huddersfield, uk, 2002, pp. 138-145. [4] p. ciarlini, m.g. cox, f. pavese and g. regoliosi: “the use of a mixture of probability distributions in temperature interlaboratory comparisons”, metrologia, 41 (2004) pp. 116-121. [5] b. toman, bayesian approach to assessing uncertainty and calculating a reference value in key comparison experiments, j. res. natl. inst. stand. technol. 110 (2005) pp. 605-612 [6] a.g. steele, k.d. hill, r.j. douglas, data pooling and key comparison reference values, metrologia 39 (2002) pp. 269–277 [7] d.l. duewer, a comparison of location estimators for interlaboratory data contaminated with value and uncertainty outliers, accred. qual. assur. 13 (2008) pp. 193–216 [8] a. hornikova, w.f. guthrie, a survey of design, analysis, and reporting of results in key comparisons, xviii imeko world congress, september, 17–22, 2006, rio de janeiro, brazil [9] k. beissner, on a measure of consistency in comparison measurements: ii. using effective degrees of freedom, metrologia 40 (2003) pp. 31-35 [10] m.g. cox, a.b. forbes, j.l. flowers, p.m. harris, least squares adjustment in the the presence of discrepant data, in advanced mathematical and computational tools in metrology vi” (p. ciarlini, m.g. cox, f.pavese, g.b. rossi, eds."), vol.6, series on advances in mathematics for applied sciences vol.66, world scientific, singapore, 2004, pp. 37-51. isbn: 981-238-904-0 [11] a.g. steele, r.j. douglas, consistency measures for peer-to-peer comparisons, xviii imeko world congress, september, 17 – 22, 2006, rio de janeiro, brazil [12] m.g. cox, the evaluation of key comparison data: determining the largest consistent subset, metrologia 44 (2007) pp. 187-200 [13] a.g. chunovkina, c. elster, i. lira, w. wöger, analysis of key comparison data and laboratory biases, metrologia 45 (2008) pp. 211-216 [14] r.n. kacker, a.b. forbes, r. kessel k.-d. sommer, bayesian posterior predictive p-value of statistical consistency in interlaboratory evaluations, metrologia 45 (2008) pp. 512–523 [15] a.g. chunovkina, n. zviagin, n. burmistrova, evaluation of inconsistent data of key comparisons of measurement standards, acta imeko 2012 tc21 [16] c. elster, b. toman, analysis of key comparison data: critical assessment of elements of current practice with suggested improvements, metrologia 50 (2013) pp. 549–555 [17] r. willink, models for the treatment of apparently inconsistent data, in "advanced mathematical and computational tools in metrology and testing x” (f.pavese, w. bremser, a.g. chunovkina, n. fischer, a.b. forbes, eds.), vol.10, series on advances in mathematics for applied sciences vol 86, world scientific, singapore, 2015, pp. 78-89 [18] international vocabulary of metrology – basic and general concepts and associated terms 3rd edition (vim3) 2008 (electronic version jcgm 200:2012 from the bipm/jcgm is available at http://www.bipm.org/utils/common/documents/ jcgm/jcgm_200_2012.pdf ) [19] iso 13528:2005 and iso 17034:2010, international organization for standardization (geneva, switzerland) 2005 and 2010 [20] guide to the expression of uncertainty in measurement (gum) 2nd edn 1995 (bipm, iec, ifcc, ilac, iso, iupac, iupap, and oiml) isbn 92-67-10188-9 (electronic version jcgm 100:2008 from the bipm/jcgm is available at http://www.bipm.org/utils/common/documents/jcgm/jcgm _100_2008_e.pdf ) [21] iso 3534-2:2006, statistics—vocabulary and symbol —part 2: applied statistics, international organization for standardization (geneva, switzerland) 2006 [22] iso 5725, 1994 2nd edn accuracy (trueness and precision) of measurement methods and results, international organization for standardization, geneva, switzerland. [23] f. pavese, about the treatment of systematic effects in metrology”, measurement 42 (2009) pp. 1459-1462 [24] f. pavese, on the difference of meanings of “zero correction”: zero value vs no correction, and of the associated uncertainties, in “advanced mathematical and computational tools in   acta imeko | www.imeko.org  december 2015 | volume 4 | number 4 | 47  metrology and testing ix” (f.pavese, m. bär, j.-r. filtz, a. b. forbes, l. pendrill, k. shirono eds.), vol. 9, series on advances in mathematics for applied sciences vol. 84 world scientific, singapore, 2012 pp. 297-309 [25] f. pavese, why should correction values be better known than the measurand true value?, journal of physics: 459 (2013) 012036 doi:10.1088/1742-6596/459/1/012036 [26] f. pavese, why the distinction between corrections and input quantities, st. petersburg seminar 2012 [27] f. pavese, corrections and input quantities in gum-compliant models, xx imeko world congress, metrology for green growth, september 9−14, 2012, busan, republic of korea [28] evaluation of measurement data — an introduction to the “guide to the expression of uncertainty in measurement” and related documents, 2009 (electronic version jcgm 104:2009 from the bipm/jcgm is available at http://www.bipm.org/ utils/common/documents/jcgm/jcgm_104_2009_e.pdf) [29] f. pavese, a note on the classification in random errors and systematic errors, in preparation. [30] f. pavese, on some consequences of the different nature of withinand between-laboratory data, metrologia 46 (2009) pp. l29-l32 [31] f. pavese, dependence of the treatment of systematic error in interlaboratory comparisons on different classes of standards, acqual 15 (2010) pp. 305-315 [32] f. pavese, on hierarchical vs. non-hierarchical comparisons in metrology and testing, int. j. metrol. qual. eng. 1 (2010) pp. 7– 10 [33] f. pavese, mathematical and statistical tools in metrological measurement, 2013, chapter in physical methods, instruments and measurements, [ed. unesco-eolss joint committee], in encyclopœdia of life support systems (eolss), developed under the auspices of the unesco, eolss publishers, oxford, uk, http://www.eolss.net [34] in jung kim, byungjoo kim, euijin hwang, an approach for the uncertainty evaluation of the overall result from replications of measurement: separately combining individual uncertainty components according to their ‘systematic’ and ‘random’ effects, bull. korean chem. soc. 35 (2014) pp. 1057– 1060 [35] i. lira, w. wöger, comparison between the conventional and bayesian approaches to evaluate measurement data, metrologia 43 (2006) pp. s249–s259 [36] international vocabulary of basic and general terms in metrology (vim2), second edition, 1993, international organization for standardization (geneva, switzerland) [37] d.l. duewer, h. gasca-aragon, k.a. lippa, b. toman, experimental design and data evaluation considerations for comparisons of reference materials, accred. qual. assur. 17 (2012) pp. 567–588. microsoft word 33-168-1-ga.doc acta imeko  july 2012, volume 1, number 1, 5‐6  www.imeko.org    acta imeko | www.imeko.org  july 2012 | volume 1 | number 1 | 5  introduction to the acta imeko issue dedicated to selected  papers presented in the 18 th  imeko tc4 symposium  pedro m. ramos  instituto de telecomunicações and instituto superior técnico/universidade técnica de lisboa, av. rovisco pais 1, 1049‐001 lisbon,portugal      keywords: imeko tc4 symposium; measurement of electrical quantities; natal; brazil  citation: pedro m. ramos, introduction to the acta imeko issue dedicated to selected papers presented in the 18th imeko tc4 symposium, acta imeko,  vol. 1, no. 1, article 5, july 2012, identifier: imeko‐acta‐01(2012)‐01‐05  editor: paul regtien, measurement science consultancy, the netherlands  copyright: © 2012 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  corresponding author: pedro m. ramos, e‐mail: pedro.m.ramos@ist.utl.pt    1. introduction  the articles presented in this issue were selected by the imeko tc4 board from the papers presented at the 18th imeko tc4 symposium which took place in natal brazil in september 2011. the authors were invited in late 2011 to submit extended and updated versions of their papers presented in the symposium. all of the authors accepted this invitation and papers begun arriving into the new acta imeko online submission system in early 2012 upon which a large number of reviewers quickly begun their task of assessing the papers and submitting their recommendations. the final list of 11 published papers in this issue is a good representation of the high quality papers presented in the symposium. overall, 27 reviewers were involved in the review process and helped shape the final manuscripts with constructive in-depth reviews that pushed the authors to further improve the papers and their work. 2. about tc4  the main objective of the imeko tc4 (measurement of electrical quantities) is emphasizing both theoretical and practical aspects of research in the field of electrical and electronic measurements. tc4, was officially established in 1984 and since then it has organized at least one symposium each year and has actively co-operated in the success of the imeko world congresses, which are held every three years. the activities of tc4 allowed more and more researchers to be familiar with the development and use of electrical and electronic instruments for measuring, monitoring and recording electrical signals. the current members of tc4 are:  l. michaeli (slovak republik), chairman  j. mindykowski (poland), dep. chairman  d. dallet (france), secretary  m. savino (italy), honorary chairman  p. daponte (italy) , past chairman  a. serra (portugal), past chairman  m. sedláček (czech republic), past chairman  d. agrež (slovenia)  c. eugène (belgium)  u. pogliano (italy)  v. didenko (russia)  e. kayafas (greece)  t. özkan (turkey)  v. haasz (czech republic)  g. rocha (brazil)  s. fakhry (egypt)  i. kale (u.k.)  p. ramos (portugal)  o. kanoun (germany)  i. kollar (hungary)  j. halttunen (finland)  a. salceanu (romania)  p. händel (sweden)  y. tesyk (ukraine)  l. van biesen (belgium)  r. land (estonia)  j. šaliga (slovak republic) this  manuscript  is  a  brief  introduction  to  the  acta  imeko  issue  dedicated  to  selected  papers  presented  in  the  18 th   imeko  tc4  symposium which took place in natal, brazil in september 2011 together with ix semetro. acta imeko | www.imeko.org  july 2012 | volume 1 | number 1 | 6   r. pallás-areny (spain)  i. kodjabashev (bulgaria)  v. vujicic (serbia)  g. kreiseler (germany)  jae kap jung (korea)  d. ilić (croatia)  w. xiaofei (china)  m. creţu (romania)  s. g. semenchinsky (russia) 3. the articles  in [1], the authors present an image based system implementing a well-known diagnostic method for the automatic detection of melanomas as support to clinicians. the proposed method, based on a statistical approach, is validated with a large set of real melanoma images. impedance circuit identification through spectroscopy is the subject of [2] where the authors use gene expression programming and genetic algorithms for automatic impedance circuit identification. multiple experimental and simulation results are presented. the characterization of power quality transient phenomena of dc railway traction supply is presented in [3]. a range of transients typical of dc railway systems are considered and classified according to their time/frequency characteristics. the optimal settings for wavelet analysis, which are almost always referred to ac distribution networks in industrial systems, are adjusted for a dc railway system. in [4], a novel high-speed multichannel impedance measuring system in presented. the impedance determination is based on the measurement of amplitudes at two distinct frequencies. the imaging capability of the proposed multichannel system was demonstrated using a planar array sensor. the characterization of the isdb-tb critical spectrum mask is addressed in [5], where the authors discuss different test procedures using either a spectrum analyzer or a dedicated digital tv analyzer to measure the isdb-t transmission mask. the construction details of a new main two-stage inductive voltage divider for coaxial bridge applications, recently built at inmetro, is presented in [6]. the design techniques responsible for the small ratio errors of the device and the calibration method employed are detailed. in [7] the authors describe the development of a low distortion signal generator with a frequency range from 0 to 10 khz using the direct digital synthesis (dds) method for adc characterization. the development of a new primary standard for the ac-dc current transfer at inmetro, based on planar multijunction thermal converters is described in [8] with detailed uncertainty analysis. an electronic approach to homogenize the impedance phase characteristics of heterogeneous gmi sensors is the subject of [9]. the authors present experimental results that demonstrate the effectiveness of the developed homogenization method. in [10], a mobile measurement system for real time monitoring of environmental pollutions over urban areas is proposed. the approach is based on the use of a set of vehicles, typically employed for public transportation inside urban areas, which are equipped with the developed mobile measurement system. this allows these vehicles to measure, store, and transmit the acquired data to a remote supervisor unit. a current transformer intended for high precision currentto-voltage transducers was proposed in [11]. it uses a two-stage technique plus an electronic device that cancels the magnetic flux in the compensating core. 4. conclusions  it was a great pleasure to act as guest editor for this groundbreaking issue of acta imeko. the effort behind this first issue goes beyond its principal outcome which is the manuscripts and all the work of the authors and reviewers. this first issue was very important to test and fine-tune the online submission system and gain important know-how for the next issues. personally, i must thank the authors, the reviewers and specially paul regtien and dirk röske for their support and help with the submission system. references  [1] v. vita, g. leo, g. fabbrocini, c. liguori, a. paolillo, p. sommella, statistical techniques applied to the automatic diagnosis of dermoscopic images, acta imeko, vol. 1 (2012), no. 1, pp. 7-18. [2] f. janeiro, p. ramos, gene expression programming and genetic algorithms in impedance circuit identification, acta imeko, vol. 1 (2012), no. 1, pp. 19-25. [3] a. mariscotti, characterization of power quality transient phenomena of dc railway traction supply, acta imeko, vol. 1 (2012), no. 1, pp. 26-35. [4] m. silva, e. santos, t. vendruscolo, high-speed multichannel impedance measuring system, acta imeko, vol. 1 (2012), no. 1, pp. 36-41. [5] p. césare, m. tenorio, characterization of the isdb-tb critical spectrum mask, acta imeko, vol. 1 (2012), no. 1, pp. 42-48. [6] g. kyriazis, j. moreno, j. melcher, a two-stage, guarded inductive voltage divider with small ratio errors for coaxial bridge applications, acta imeko, vol. 1 (2012), no. 1, pp. 49-58. [7] w. adad, r. iuzzolino, low distortion signal generator based on direct digital synthesis for adc characterization, acta imeko, vol. 1 (2012), no. 1, pp. 59-64. [8] m. klonz, r. afonso, r. m. souza, r. landim, new generation of ac-dc current transfer standards at inmetro, acta imeko, vol. 1 (2012), no. 1, pp. 65-69. [9] e. silva, l. gusmão, c. barbosa, e. monteiro, an electronic approach to homogenize the impedance phase characteristics of heterogeneous gmi sensors, acta imeko, vol. 1 (2012), no. 1, pp. 70-76. [10] a. bernieri, d. capriglione, l. ferrigno, m. laracca, design of an efficient mobile measurement system for urban pollution monitoring, acta imeko, vol. 1 (2012), no. 1, pp. 77-84. [11] d. slomovitz, l. trigo, c. faverio, two-stage current transformer with electronic compensation, acta imeko, vol. 1 (2012), no. 1, pp. 85-88. microsoft word article 12 167-1353-1-le.docx acta imeko  february 2015, volume 4, number 1, 77 – 81  www.imeko.org    acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 77  architecture of the multi‐tap‐delay‐line time‐interval  measurement module implemented in fpga device  marek zieliński, maciej gurski, dariusz chaberski  institute of physics, faculty of physics, astronomy and informatics, nicolaus copernicus university, grudziadzka 5, 87‐100 torun, poland      section: research paper   keywords: time‐interval measurement, tapped delay lines  citation: marek zieliński, maciej gurski, dariusz chaberski, architecture of the multi‐tap‐delay‐line time‐interval measurement module implemented in  fpga device, acta imeko, vol. 4, no. 1, article 12, february 2015, identifier: imeko‐acta‐04 (2015)‐01‐12  editor: paolo carbone, university of perugia   received december 17 th , 2013; in final form november 26 th , 2014; published february 2015  copyright: © 2014 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: (none reported)  corresponding author:  m. gurski, e‐mail: gural@fizyka.umk.pl    1. introduction  high resolution time-interval measurement systems (tims) are widely applied in many system. for example they can be applied in quantum cryptography experiments, in the characterization of clock phase fluctuations, in lifetime measurements of the excited atomic states, in ultrasonic flow-meters or monitoring systems of time-offlight mass spectrometers [1-8]. limiting the total number of time-interval measurements and increasing at the same time the system resolution, the effective duty cycle and the total power consumption could be decreased [9]. the system saves energy in two ways. firstly, measurements are realised in parallel, so the measuring time is shorter and the energy consumption is lower. the second way of energy consumption depends on a selective module turning-on. this procedure is especially profitable because every module that can be turned-off (no power delivered) or can go into stand-by mode (no standard clock provided) when it is not used elongates the working-time with the same battery set. some areas of a field programmable gate array (fpga) device can be turned off. the system is designed in such a way that the system parts that are used selectively in time are implemented into those fpga areas that can be turned off without any influence on the whole system operation. for example the calibration module can be removed (turned off) after calibration is done, so the power consumption can be decreased, which extends battery lifetime. the measurement memory must be turned on as long as the measurement series has not been read by the computer even when no write process is realised. without these procedures the fpga consumes 4.4 w of power and with the procedures described above the energy consumption lowers to 0.65 w. the measurement system presented in this paper enables not only an increasing system resolution but also a significantly decreasing uncertainty of the single time-interval measurement. abstract  this  paper  describes  the  architecture  of  a  multi‐tap‐delay‐line  (mtdl)  time‐interval  measurement  module  of  high  resolution  implemented in a single fpga device. the new architecture of the measurement module enables to collect sixteen time‐stamps during  a single measuring cycle. it means that the measured time‐interval can be precisely interpolated from the collection of the sixteen  time‐stamps after each measuring cycle. such architecture of the measurement module leads directly to an increased resolution, to a  limited total measurement time and a decreased duty cycle of the measurement instrument. acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 78  2. architecture of the tims  tims as a virtual instrument consists of a hardware unit, flexible software and a computer. the block diagram of the new tims architecture is shown in figure 1. the system is implemented using the xilinx virtex 4 fpga structure and uses chip-outside-located 64 mb ddr of ram for data collection. the main unit of the system is a 32 bit soft processor – microblaze. other parts of the system are implemented as microprocessor peripherals. ise and edk programming tools have been used for the implementation of the virtual microprocessor (microblaze), that has been programmed in c language and its peripherals. the implemented system consists of a group of sixteen two-hundred-element multi-tap delay lines with coupled registers, sixteen code converters, four clock cycles counters, blocks of inside memory (bram), an interface and a control unit. a carry chain of clbs (configurable logical block) is used to tapped delay lines and register implementation (figure 2). it is possible to change the placement of the elements and connections to change the line delay. using this method, the characteristics of the delay-line can be shaped. data transferred to the personal computer is worked out by the python-script-language that allows, in a simple way, to obtain a very efficient, multiprocessor programming environment. these scripts also generate corrected vhdl (very high speed integrated circuits hardware description language) and ucf (user's constraints file) source files, so the process of designing the measuring module is fully automated. mainly the location of tdls and the order of their output is corrected. the system can be calibrated without the need of reprogramming. the calibration module can be removed (turned off) after calibration is completed, so the power consumption can be decreased, which extends battery life-time. first the taps must be sorted, because the time-of-clock (clk) net is not monotonic (figure 3). the values read from the fpga editor do not contain information about the fluctuations of the delays in the fpga structure. for this reason, the system can read all tdl registers in raw mode, bypassing the code converter and storing them in bram. it is not possible to read all the sixteen tdl states at the same time (there are not enough brams for this operation implementation on the chip that was used). there is a 16-to1 multiplexer to switch between tdl's registers. another signal multiplexer can connect either to the divided by n system clock (100 mhz) or an external pulse signal. the raw waveform of the recorded system clock is triggered by the divided by n built-in clock and then stored in bram. data from bram are sent to the computer. figure 1. the block diagram of tims.  figure 2. two taps of tdl made of carry‐chain with registers before tuning.  figure 3. example of   “clk” net delay  in fpga device for 128 slices – read  from the fpga editor.  acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 79  the calibration program on the pc computer is seeking anomalies (bubbles) in the waveform (table 1). the anomalies can be single, double and so on. reordered zeros or ones usually appear at the beginning and at the end of every sequences obtained for the same values. the program uses a special correlation procedure to obtain the right order of all taps implemented in the fpga structure. the next procedure generates vhdl code of the sorted tdl. this procedure is repeated for every tdl. after compilation the corrected tdls are implemented in the fpga. quadrupling of the clock cycle counter ensures a sufficient set-up time during incrementation and the possibility of asynchronous read-out of data from one of the counters. the most proper counter is being chosen on the basis of the information obtained from a particular tdl. these data are used for computing all sixteen timestamps from all tdls. each measured time stamp consists of a main part, which is taken from one of the clock cycle counters, and a residual part, which is taken from one of the tdl-registers and then compressed to binary-natural-code. the theoretical precision of the tdl is 25 ps. there is a great need to implement the code converter, which converts from thermometric to binary-natural-code. the main reason to use the code converter is limitation of the data memory size and speed of data transfer. single time-interval measurements can generate more than four hundred bytes of data. this value is similar in size to the capacity of the single bram on the chip. the code converter reduces about hundred-fold data for storage. requirements for a code converter are to reach a compromise between speed and space occupancy on the fpga chip. to achieve this in any converter 16-to-4 bits conversions in parallel mode were implemented. finally, the chosen code-converter requires only two brams for one tdl in comparison to more than six without code converter. the ddr sdram 64 mb of capacity is used for data acquisition. in case when there are more than five hundred time stamps in one series and the intensity of measurement of time-intervals is too fast (more that 100 k samples per second) data are sent to the pc indirectly through the ddr sdram, which allows obtaining about four thousand times more space for measurements. 3. principle of time‐interval measurement  the main task of the measuring system is registration and collection of time-stamps. the time-stamp is combined from two parts. the first one expresses the total number of standardclock periods that have appeared from measuring module initialization it can be called standard-clock period counter. this measuring stage is realized by four mutuallycomplementing counters-register pair. the second part, that is realized with the use of tapped-delay-lines, expresses the standard-clock phase when the event occurred. the value of the second part is proportional to the index of the bit which is set and is followed by bits that are cleared. the measured time-interval δtm is calculated by the difference of two time-stamps. the precision of tims in practice depends on the precision of interpolators, that measure residual time intervals and accumulated jitter of the standard clock [9]. if multi-tap delay lines are used in the design of the interpolator then the value of the single segment delay τ and its standard deviation determines the precision of the time-interval (δtm) measurement. if the interpolator consists of n delay lines of relatively high resolution, then during a single measuring cycle it is possible to obtain n different results of time-intervals δtm [10, 11]. such solution leads directly to an increase of precision of the time-interval table  1.  the  exemplary  section  of  waveform  before  sorting  a)  and  after  sorting  b)  the  taps  97,  98  and  100  are  accordingly  shifted  into  place  and  now their taps numbers are respectively 100, 97, 98.    figure 4. the idea of multiple tdls measurement.  acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 80  measurement and reduces the number of measurement cycles. this method significantly reduces the power consumption for battery-powered systems, because the number of measuring cycles is generally significantly decreased. knowing delay line characteristics such as dnl (differential nonlinearity) and inl (integral nonlinearity) and using the quantization-and-nonlinearity-minimization (qnm) method, it is possible to obtain a two-sampledifference histogram, increased system resolution and decreased uncertainty of the time-interval measurement [12]. in the presented system with 16 tdls which consists of 200 taps (for each tdl), the expected accuracy should be better than 15 ps for a single-shot measurement. 4. experimental results   a series of time-interval measurements was performed to verify the measurement system. for the test, a single section of coaxial cable was used as a delay element as is shown in figure 5.  figures 6 and 7 show time-stamp histograms obtained for start and stop pulses. one should notice that the surfaces of both diagrams are equal to the number of tdls. figure 8 shows the time-interval histogram obtained as a difference between start and stop measurements. it is worth being noticed that the uncertainty of the time-interval does not increase significantly in comparison to the source timestamp uncertainties. 5. conclusions  designed time-interval measuring system allowed to obtain a higher precision of measurements thanks to simultaneous measurements by 16 tdls at the same time. measurements for every hit are realized in parallel, so the measuring time is not increased, and the energy consumption is reduced because the measuring module can go periodically in stand-by mode for a longer time. the obtained time-interval histograms are characterized by a low uncertainty in comparison to time-stamps uncertainties even though tdls characteristics are very nonlinear, because the qnm method takes into account the characteristics of the tdls. the design process of the measuring module has been fully automated, which is especially useful for tdls implementation. the tdl output wires are automatically rotated to obtain proper thermometric codes that allows the implementation of tdls with a precision equal to one carry element delay. figure 5. the experimental measurement system.  figure 7. time‐stamp histogram obtained for stop pulse.    figure 6. time‐stamp histogram obtained for start pulse.    figure 8. time‐interval histogram.  acta imeko | www.imeko.org  february 2015 | volume 4 | number 1 | 81  references  [1] m. zieliński, m. kowalski, r. frankowski, d. chaberski, s. grzelak and l. wydźgowski, accumulated jitter measurement of standard clock oscillators, metrology and measurement systems, vol. xvi, no. 2, pp. 259 – 266, 2009. [2] w. wasilewski, p. kolenderski, r. frankowski, spectral density matrix of a single photon measured, phys. rev. lett. vol. 99, no. 12, p. 123601, 2007. [3] stephan henzler. time-to-digital converters, volume 29 of springer series in advanced microelectronics. springer publishing company, incorporated, 1st edition, 2010. [4] guansheng li, y.m. tousi, a. hassibi, and e. afshari. delay-line-based analog-to-digital converters. circuits and systems ii: express briefs, ieee transactions on, 56(6):464– 468, june 2009. [5] m. zielinski, d. chaberski, m. kowalski, and s. frankowski, r. grzelak. high-resolution timeinterval measuring system implemented in single fpga device. measurement, 35(3):311–317, april 2004. [6] claudio favi and edoardo charbon. a 17ps time-to-digital converter implemented in 65nm fpga technology. in proceedings of the acm/sigda international symposium on field programmable gate arrays, fpga ’09, pages 113– 120, new york, ny, usa, february 2009. acm. [7] jian song, qi an, and shubin liu. a high-resolution time-todigital converter implemented in fieldprogrammable-gatearrays. nuclear science, ieee transactions on, 53(1):236– 241, february 2006. [8] c. hervé, j. cerrai, and t. le caër. high resolution time-todigital converter (tdc) implemented in field programmable gate array (fpga) with compensated process voltage and temperature (pvt) variations. nuclear instruments and methods in physics research a, 682:16–25, august 2012. [9] m. zieliński, review of single-stage time-interval measurement modules implemented in fpga devices, metrology and measurement systems, vol xvi, no.4, pp 641647, 2010. [10] d. chaberski, m. zieliński, s. grzelak, the new method of calculation sum and difference histogram for quantized data, measurement vol. 42, no. 9, pp. 1388 – 1394, 2009. [11] r. szplet, z. jachna, p. kwiatkowski, and k. różyc. a 2.9 ps equivalent resolution interpolating time counter based on multiple independent coding lines. measurement science and technology, 24(3):1–15, february 2013. [12] r. szplet, z. jachna, and j. kalisz. a flash time-todigital converter with two independent time coding lines. in international workshop on adc modelling, testing and data converter analysis and design and ieee; adc forum; orvieto, italy, pages 138–143, july 2011. microsoft word 380-2234-1-le acta imeko  issn: 2221‐870x  april 2016, volume 5, number 1, 2    acta imeko | www.imeko.org  april 2016 | volume 5 | number 1 | 2  editorial  paolo carbone  university of perugia, italy     section: editorial   citation: paolo carbone, editorial, acta imeko, vol. 5, no. 1, article 1, april 2016, identifier: imeko‐acta‐05 (2016)‐01‐01  editor: paolo carbone, university of perugia, italy  received april 25, 2016; in final form april 26, 2016; published april 2016  copyright: © 2016 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  corresponding author: paolo carbone, email: paolo.carbone@unipg.it      dear reader, this first issue in 2016 of acta imeko contains a set of interesting papers associated to imekofoods 2014, the first imeko event on metrology in the area of food and nutrition (see the following editorial by the guest editor claudia zoani). two other papers from the 2015 imeko world congress and two technical notes complete this issue. n. medina, j. de vicente and j. robles author the first paper originating from the imeko world congress. this paper deals with magnetic effects in the realization of a primary standard for dynamic force calibration. the effect of magnetic fields on the sensitivity of the force transducer was characterized and measured using various configurations and detailed experimental setups. practical solutions were suggested to minimize the influence of these fields on the measured data. h. kjikawa and t. kobata author the second paper originating from the 2015 world congress. it deals with the calibration of a pressure gauge. a new method is proposed that is expected to be precise and low-cost. the paper contains both theory and experimental results. two technical notes conclude this issue. the first one is authored by d.m.toma et al. it deals with marine observations of sea temperature, level of acidification and noise pollution. it describes the process of quality assurance needed to guarantee the validity of data, so that they have the quality required for the purpose justifying their collection. a. maina, i. veldman and h. ploug author the last technical note. it reports the results obtained in a trilateral comparison regarding vibration. three laboratories were involved: the kenya bureau of standards, the national metrology institute of south africa and brüel & kjær calibration laboratory. results of the comparison are precisely detailed, along with the characteristics of the procedure adopted to measure and manage the transfer standard. i wish you a fruitful reading of this first issue of acta imeko in 2016! template for an acta imeko event paper acta imeko issn: 2221-870x july 2017, volume 6, number 2, 2-3 acta imeko | www.imeko.org july 2017 | volume 6 | number 2 | 2 introduction to the special section of the asia-pacific symposium on measurement of mass, force and torque (apmf) 2015 min-seok kim korea research institute of standrads and science, 267 gajeong-ro, yuseong-gu, daejeon 34113, republic of korea section: editorial citation: min-seok kim, introduction to the special section of the asia-pacific symposium on measurement of mass, force and torque (apmf) 2015, acta imeko, vol. 6, no. 2, article 1, july 2017, identifier: imeko-acta-06 (2017)-02-01 section editor: paul regtien, measurement science consultancy, the netherlands received june 15, 2017; in final form june 15, 2017; published july 2017 copyright: © 2017 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: min-seok kim, e-mail: minsk@kriss.re.kr dear readers, the asia-pacific symposium on measurement of mass, force and torque (abbreviated as apmf) is a biannual symposium held in the asia-pacific region (mostly in china, japan and korea). the apmf was firstly held in 1992 just as a small joint symposium between china and japan (at that time, it was called as international symposium on measurement of force and mass, abbreviated as ismfm) to exchange technical information and expertise in the area of mass, force and torque measurement. over the last two decades, however, it has developed into a most active symposium in the asia-pacific region, where almost 100 experts get together to share up-todate information and discuss current issues in mass, force and torque measurement. imeko technical committee number 3 (tc-3) on “measurement of force, mass and torque” has sponsored the apmf symposium with a partnership since the first joint conference between imeko tc-3 and apmf was held in 1998 in daejeon, korea. the recent apmf symposium was held in seoul, korea in october 2015. it was a successful symposium contributed by almost 90 participants from 10 countries presenting 35 scientific papers. in this special issue, you can find 12 articles selected by the international program committee of the apmf, which is considered to be worth of publishing in acta imeko journal after peer reviewing. the papers included in the special issue show recent progress of the research on the measurements in the field of mass, force and torque in the asia-pacific region. among 12 articles, the number of papers whose topic belongs to mass, force and torque is 5, 3 and 4, respectively. more in detail, in the topic of mass metrology, two papers from nist, authored by eric and corey for each, present recent results on the development of a mass calibration system using a magnetic suspension balance for vacuum-to-air dissemination of mass scale in response to the upcoming change in the definition of the kilogram. the volume of mass artefacts must be known accurately for determination of their density. manhong hu presents a nim volume measurement system of mass standards using an acoustic method. yuji yamakawa introduced a modelling of the floor vibration effect, which could contribute to the improvement of a high-speed mass measurement system, usually used to control processes in the factories. interesting and valuable information about the long history of mass comparators over 19 years is presented by masasaki ueki. this work results from his dedicated long-year service in the field of mass metrology at nmij/aist, japan. in the field of force metrology, you can find recent activities and results on small force metrology of nim, china (hu gang and jile jiang). nim started small force research lately compared to other national metrology institutes (nist, ptb, kriss, npl and cms/itri), but it seems that nim is catching up rapidly the former researches. hu gang reports the inter-laboratory comparison results among nim, ptb and kriss, which show the degree of equivalence in determination of spring constants of micro-cantilevers used in atomic force microscopy while jile jiang introduces the first results of the nim’s electrostatic force system. on the other hand, a practical approach for estimating pressure distribution during the metal press casting process is introduced by ryosuke tasaki through acta imeko | www.imeko.org july 2017 | volume 6 | number 2 | 3 a measurement of molten metal’s reaction force. it is very difficult to measure pressure directly inside casting molds due to their temperature, so the proposed technique could provide a practical solution to know the pressure distribution inside the casting molds. torque metrology is a fast-growing field in asia. over the last 10 years, several nmis in asia newly established torque standards in response to increasing demands from industries. this issue includes 4 articles in this topic. zhimin zang introduces a recently developed small torque standard machine with 1 n⋅m-capacity. nmij is one of key players of torque metrology in asia. koji ogushi reports inter-comparison results of torque standards between nmij and kriss in the range from 50 n⋅m to 2000 n⋅m. nmij and kriss firstly conducted a bilateral comparison in 2004. after the first comparison, the bilateral comparison was conducted again in 2010 to confirm the improvement of uncertainty of the 1 kn⋅m deadweight nmij torque machine. the 2 kn⋅m torque range was selected to check the stability of the 20 kn⋅m deadweight torque machine in nmij. atsuhiro nishino presented the results of intra-laboratory comparisons of reference torque wrenches in japan. reference torque wrenches are popularly used for calibration of torque wrench testers, calibrators, and torque wrenches. this paper can give you information about the calibration capabilities of reference torque wrenches. finally, in a paper presented by tassanai sanponpute, nimt introduces torque transducers with an additional (the second) strain gauge bridge attached on the second spring body whose stiffness is at least 2 times higher than that of the first spring body where the first bridge is attached to produce a measurement signal. the second bridge produces a check signal to be used for monitoring the stability of torque transducers. through a series of overloading tests, he demonstrated the function ratio defined by a ratio of outputs of the measurement signal and the check signal could be a useful factor to check the stability. i would like to give my sincere thanks to all contributors, editors, authors and reviewers, who make this issue possible and hope that you will enjoy reading this special section. introduction to the special section of the asia-pacific symposium on measurement of mass, force and torque (apmf) 2015 gait monitoring on instrumented walkers: a proposal based on force and inertial sensors acta imeko issn: 2221-870x december 2018, volume 7, number 4, 33 41 acta imeko | www.imeko.org december2018 | volume 7 | number 4 | 33 application of force and inertial sensors to monitor gait on legacy walkers vítor viegas1,2, j. m. dias pereira1,2, octavian postolache1,3, pedro silva girão1,4 1 instituto de telecomunicações, 1049-001 lisboa, portugal 2 estsetúbal, 3cdp2t, instituto politécnico de setúbal, 2910-761 setúbal, portugal 3 iscte – instituto universitário de lisboa, 1600-077 lisboa, portugal 4 instituto superior técnico, universidade de lisboa, 1049-001 lisboa, portugal section: research paper keywords: force measurement; load cell; imu; walker assistive device; gait analysis citation: vítor viegas, j. m. dias pereira, octavian postolache, pedro silva girão, gait monitoring on instrumented walkers: a proposal based on force and inertial sensors, acta imeko, vol. 7, no. 4, article 7, december 2018, identifier: imeko-acta-07 (2018)-04-07 editor: vilmos pálfi, budapest university of technology and economics, hungary received march 28, 2018; in final form june 22, 2018; published december 2018 copyright: © 2018 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: this work was supported by fundação para a ciência e tecnologia, project ptdc/dtp-des/6776/2014. corresponding author: vítor viegas, e-mail: vitor.viegas@estsetubal.ips.pt 1. introduction assistive walking devices play an important role in extending the autonomy of elderly people and in preventing accidental falls. this is especially important in modern societies, where the population is aging fast: in north america, 25 % of the population is estimated to be over the age of 65 years by the year 2025 [1-2]; and in the eu, by the year 2060, the life expectancies of women and men are anticipated to be around 89 and 84.5 years, respectively [3]. assistive walking devices are also key in recovering the mobility of people affected by locomotion disabilities (due to amputation, injuries to the spine, muscular dystrophies, or other causes). the use of walking aids is, in general, positive because it improves the user’s confidence levels during long-term rehabilitation therapies. a study published by vogt et al. [4] showed that the use of rollators (walkers with wheels) does not interfere with rehabilitation outcomes and, in some cases, may be highly useful in reducing rehabilitation times. it is consensual that the proper use of mobility aiding devices contributes to the sustainability of healthcare systems [5-6], but the very same devices, if used inappropriately, can be harmful and cause serious injuries [7-8]. the safety margin could be improved if users were educated on how to use the equipment properly, and if artificial intelligence was incorporated on the devices to detect and avoid risky situations. in many rehabilitation processes, the evaluation of the user’s balance and motor coordination is usually based on subjective data acquired through human observations. the decisions of the physiotherapist could be improved if they were based on objective metrics obtained from sensed data. abstract walker assistive devices play an important role in extending the autonomy of elderly people and in recovering the mobility of people affected by locomotion disabilities. the next generation of walkers are hoped to include embedded sensors and data processing capabilities that will allow for the extraction of objective metrics (about gait and body posture) to assist the work of physiotherapists and to enable the self-control nature of walker usage. the paper presents the andante, our latest proposal of a smart walker intended to monitor and analyze gait in real time. the system makes use of e-textile electrodes to sense the heart rate of the user, load cells to measure the forces applied on the walker legs, and an inertial measurement unit to sense motion and orientation. these signals are sampled locally and transferred over a bluetooth link to a remote host, where they are processed in real time. data processing includes the detection, classification, and characterization of the steps. a rich set of parameters is presented for each step, including estimates of balance and motor coordination, step length and azimuth, and lift of the walker frame. this information can be used by physiotherapist to objectively assess the physical condition of the user and tune rehabilitation therapy if needed. the proposed solution can be easily integrated into any commercial walker without any loss of functionality. mailto:vitor.viegas@estsetubal.ips.pt acta imeko | www.imeko.org december2018 | volume 7 | number 4 | 34 to address these problems, several instrumented walkers have been proposed to monitor and analyze gait [9-10]. load measurements are usually done by placing force sensors on the walker hand grips or on the walker legs [11-15]. kinematic parameters related with human gait are often measured using accelerometers or inertial motion units (imus) attached to the walker frame. in this case, long-term drifts and persistent dc components must be filtered out so as not to affect the time integration of the signals [16]. solutions based on radar measurements are also considered in the relevant literature [17-18], and they are successful concerning the identification of gait patterns. however, they are not particularly efficient in obtaining accurate measurements about walking distance measurements. ultrasounds can also be used for obtaining distance measurements [19], but they are not directional and are affected by environmental factors such as temperature and humidity. several works [20-21] have also considered the usage of image processing techniques to monitor gait. the main advantages of these methods are related to their non-invasiveness, but they have strong limitations concerning camera positioning, and it is not possible, at all, to acquire data concerning force intensities. our team has been working on smart walkers over recent years [22-25]. we have tried a broad set of sensors, including force sensing resistors and load cells to measure force, and lidar and microwave doppler radar to measure distance and velocity. our work has two main purposes: first, to monitor (and eventually correct) the usage of assistive gait devices and second, to extract objective gait metrics to assist physiotherapists in their work. in this paper, we present the andante, our latest proposal for a smart walker and the evolution of the prototype presented in [25]. it includes e-textile electrodes for capturing the heart rate of the user, load cells to measure the forces applied on the walker legs, an imu to sense motion and orientation, and software to bind all the above together. care was taken to preserve the native functionality of the device and to reduce the upgrade costs. the paper is organized as follows: section ii defines the geometry and metrics used in the paper; section iii describes the implemented measurement system; section iv presents experimental results; and section v draws conclusions from the results. 2. preliminary definitions a cartesian coordinate system is established to represent the walker frame, and two indicators are defined to characterize user’s gait. 2.1. balance the coordinate system illustrated in figure 1 is used to study the forces applied on the walker frame. the walker legs are numbered from 1 to 4 (as quadrants) and the y-axis points to the forward direction. according to this arrangement, the centre of forces (cof) is given by: 𝐶𝑂𝐹𝑥 = 𝑊12(𝐹1−𝐹2)+𝑊43(𝐹4−𝐹3) 2(𝐹1+𝐹2+𝐹3+𝐹4) (1) 𝐶𝑂𝐹𝑦 = 𝐿(𝐹1−𝐹4)+𝐿(𝐹2−𝐹3) 2(𝐹1+𝐹2+𝐹3+𝐹4) , (2) where fk represents the force applied on the kth leg, w12, and w43 represents the distance between the front and rear legs, corresponding roughly to the mean walker width w. l represents the distance between the front and rear legs, corresponding to the walker length. in the present case, we use a ayudas dinámicas, model ad230 walker [26], with the dimensions w12 = 51 cm, w43 = 53 cm, l = 45 cm, and weight = 2.5 kg. the relative deviation of the cof in relation to the geometrical centre of the walker polygon is a measure of imbalance. conversely, the complement to one is an indicator of balance b: 𝐵(%) = 100 × (1 − √(𝐶𝑂𝐹𝑥) 2+(𝐶𝑂𝐹𝑦) 2 √(𝑊/2)2+(𝐿/2)2 × 𝛼) (3) alpha () is a weighting factor that gives significance to b only when the user is loading the walker. it takes values from 0 to 1 depending on whether the walker is resting or is fully loaded with the user weight, as follows: 𝛼 = 𝐹1+𝐹2+𝐹3+𝐹4 𝐹𝑈 = 𝐹𝑇 𝐹𝑈 , (4) where ft represents the total force applied on the walker legs, and fu represents the user weight. 𝑷𝟏 = (+ 𝑾𝟏𝟐 𝟐 ;+ 𝑳 𝟐 ) 𝑷𝟐 = (− 𝑾𝟏𝟐 𝟐 ;+ 𝑳 𝟐 ) 𝑷𝟑 = (− 𝑾𝟒𝟑 𝟐 ;− 𝑳 𝟐 ) 𝑷𝟒 = (+ 𝑾𝟒𝟑 𝟐 ;− 𝑳 𝟐 ) figure 1. coordinate system of the walker frame: (a) instrumented walker (b) cartesian plane, dimensions, and coordinates. y x w43 p1p2 p3 p4 w12 l acta imeko | www.imeko.org december2018 | volume 7 | number 4 | 35 2.2. motor coordination motor coordination refers to the correct sequence of movements needed to complete a step. consider the state machine illustrated in figure 2, where the states represent the gait phases, gps, and the continuous arrows represent normal transitions between states [27-28]. if the user completes a step passing through all the gait phases, the step is classified as good; otherwise is classified as bad. the number of good steps g in the last n is an indicator of motor coordination mc: 𝑀𝐶(%) = 100 × 𝐺 𝑁 . (5) 3. measurement system the measurement system includes a wireless data acquisition module, an imu, a heart rate sensor, four load cells, and software to bind all the above together. 3.1. data acquisition data acquisition was performed by a shimmer3 module from shimmer sensing [29], including an embedded imu, four external analog inputs (ai1-ai4) and a bluetooth link. wiring was passed through the tubular structure of the walker to connect the sensors to the analog inputs. samples from all sensors (imu and external) were acquired at a rate of 51.2 s/s and sent wirelessly to a remote host. this rate is well beyond the expected nyquist frequency for physical movements, but not too high that it could overload the transmission and processing chain. the value itself is pre-defined in the shimmer3 firmware. 3.2. inertial sensors the embedded imu is used to sense motion and orientation. it has nine degrees of freedom (9dof) given by a 3-axis accelerometer (model kxrb5-2042, kionix), a 3-axis gyroscope (model mpu-9150, invensense), and a 3-axis magnetometer (model lsm303dlhc, stmicroelectronics). the imu is placed horizontally, with its coordinate system aligned to that of the walker (shown in figure 1). this way, the acceleration across the y axis gives motion in the forward backward direction, and the acceleration across the z axis, subtracted by 1 g, gives motion in the vertical direction. the shimmer3 module is also able to know the orientation by fusing the data provided by the imu in a so-called “gradient descent algorithm [30].” the algorithm gives orientation in absolute enu coordinates (local east-north-up) with a delay of a few tenths of a second. the imu must be calibrated in advance to cancel offset errors and tune gain matrices. the calibration procedure [31] is executed once, in situ, and the results are saved into a nonvolatile memory. 3.3. heart rate sensor heart rate is sensed by a pair of dry e-textile electrodes placed over the handgrips of the walker, tied with a tiny velcro strip. they are removable, washable, and seem like ordinary fabric, thus not causing undue stress to the user. the ecg signal is conditioned by an analog front end based on the ad8232. the signal passes through a two-pole band-pass filter, with cut-off frequencies at 7 hz and 24 hz, followed by an amplifier with 1000 gain. the narrow-band filter eliminates motion artefacts and line noise, but distorts the ecg waveform significantly, making it suitable only for heart rate detection. the output signal is connected to the first analog input, ai1, of the data acquisition module. 3.4. force sensors the forces applied on the walker legs are sensed by four load cells. each cell is attached to the extremity of a leg, using a dedicated plastic adapter grown on a low-cost 3d printer, as shown in figure 3a. the cell contains four strain gauges fixed to a small bending beam (55.3 x 12.7 x 12.7 mm) that supports 20 kgf. other characteristics include: nominal output sensitivity = 1 mv/v, non-linearity = 0.05 % fs (full-scale) and hysteresis = 0.05 % fs. bending beam cells were preferred to inline/axial cells because they are much cheaper. some engineering had to be undertaken to fit the four load cells into the three analog inputs available: • referring to the auxiliary axes p and q, passing over the load cell pairs 1 & 3 and 2 & 4 respectively, as shown in figure 3b, the pair (p, q) is not perfectly orthogonal because the mean walker width w is not equal to the walker length l. nevertheless, the pair is almost orthogonal, with a rotation angle that is near to +45 ° relative to (x, y). • each load cell is supplied at 3 v and conditioned by an instrumentation amplifier with 100 gain. the voltages are combined in pairs, subtracted from each other (1-3 and 2-4), amplified by 4 and biased around 1.5 v, as shown in figure 3c. the result is a voltage that varies linearly between 0.3 v and 2.7 v for forces applied along the axes p and q, between -d/2 and +d/2, where 𝐷 = √𝐿2 + 𝑊2 is the walker diagonal. the cof coordinates over the auxiliary axes are given as: 𝐶𝑂𝐹𝑝 = ( 𝐷 2.4 ) × (𝑉𝐴i2 − 1.5) (6) 𝐶𝑂𝐹𝑞 = ( 𝐷 2.4 ) × (𝑉ai3 − 1.5) (7) figure 2. gait phases (gps) during a walker step (the polygon delimits the support area). gp0: the walker is resting on the floor; gp1: the walker is flying; gp2: the walker is on the floor waiting for the injured foot to move forward; gp3: the injured foot is moving forward; gp4: waiting for the healthy foot to move forward; gp5: the healthy foot is moving forward. acta imeko | www.imeko.org december2018 | volume 7 | number 4 | 36 where vaix represents the voltage acquired at analog input 2 or 3, in volt. • the conversion from (p, q) to (x, y) coordinates is undertaken by applying a rotation matrix with  = -45 °: [ 𝐶𝑂𝐹𝑥 𝐶𝑂𝐹𝑦 ] = [ cos𝜃 sin𝜃 −sin𝜃 cos𝜃 ][ 𝐶𝑂𝐹𝑝 𝐶𝑂𝐹𝑞 ] (8) • additionally, the signals from all load cells are summed together and injected in the analog input ai4 to get the total load applied on the walker. the resulting voltage varies linearly between 0 and 1.2 v for a total load between 0 and 80 kgf, thus giving: 𝐹𝑇 = ( 80 1.2 ) × 𝑉𝐴𝐼4 (9) playing with the walker symmetry, together with some basic geometry, we were able to fit everything into just three analog inputs. 3.5. software the samples arriving at the host are handled by the andante application, which acts as the interface for the physiotherapist. the application was developed using visual studio 2012 and makes use of shimmer c# api [32] to interact with the shimmer3 module. the andante application records its state on a relational database designed in microsoft access©. the database makes it possible to see how certain factors evolve over time (e.g., querying the performance of a given user during several therapy sessions) and to trace actions (e.g., querying which therapist assisted a given user in a given session). the database is composed in the form of five relevant tables, as shown in figure 4 and described below: • physiotherapists: a list of physiotherapists registered in the application. each physiotherapist has a unique id associated with their name. • users: a list of users registered in the application. each user is described according to their id, name, age, weight, and injury (left or right limb). • walkers: a list of four-legged instrumented walkers registered in the application. each walker is characterized by its id, brand, model, serial number, the width between the front legs (w12), the width between the rear legs (w43), the distance between the front and rear legs (l), and offset errors. • sessions: a list of physiotherapy sessions overseen by the application. each session is characterized by its id, physiotherapist, user receiving treatment, walker in use, timestamp, and location. • steps: the list of steps detected during a given physiotherapy session. each step is characterized by its id, gait metrics computed by the measurement system, and free comments added by the physiotherapist. the information in the tables of physiotherapists, users, and walkers must be edited in advance, outside of the andante application, using microsoft access© tools, for instance. the other tables are updated by the andante application in runtime. a) b) c) figure 3. load cells attached to the walker legs: a) photo b) auxiliary axes c) signal conditioning. figure 4. main window of the andante application. y x lc1lc2 lc3 lc4 pq - cof cofy cofx d/2 1.5v ai4 100 + ai2 ai1ecg front end e-textile electrodes (c) (a) power supply +3 v lcx: load cell x; blocks based on: a) ad 8232; b) ad623; c) mcp6272. 100 (b) (b) + 4 + + (b) lc1 lc3 1.5v 100 ai3 100 (b) (b) + 4 + + (b) lc2 lc4 acta imeko | www.imeko.org december2018 | volume 7 | number 4 | 37 the main window of the andante application has five buttons that give access to the following options (see figure 5): • login: selecting this option opens a dialog box into which the physiotherapist enters the data needed to create a new physiotherapy session (the physiotherapist in charge, user receiving treatment, and walker in use). • connect: selecting this option establishes a bluetooth connection with the instrumented walker. • calibrate sensors: a dialog box is opened to extract the offset errors of the measurement system. with the walker resting on the floor, with no external force applied, the physiotherapist is instructed to register the cof coordinates and the weight of the walker frame. these residues are stored in the database in the record corresponding to the walker, columns cofx0, cofy0, and f0. later, during data streaming, these residues are subtracted from equations (1) and (2) and from the numerator of equation (4). • stream data: a child window is opened, and the physiotherapist can monitor the usage of the walker (see section 3.5.1). • disconnect: the bluetooth connection with the instrumented walker is closed. 3.5.1. stream data child window the stream data child window opens a high-speed communication link to transfer data from the instrumented walker to the host (the link is unidirectional). the window launches two threads: a high-priority thread that stores the fastarriving samples into a buffer and a low-priority thread that runs every 200 ms to process the buffered data and update the user interface. the buffer contains a variable number of samples, from 10 to a few more, depending on the computer load. an update rate of 5 frames/s is enough to give the application good responsiveness and fluidity. the buffered samples are processed sequentially in the same order in which they arrived. the processing includes the following main tasks: 1. computation of the heart rate from the signal connected to ai1. a simple schmitt-trigger algorithm, with fixed thresholds at 0.5 v and 1 v, is executed to detect the heart beats. the time between beats is used to compute the heart rate. 2. computation of cofx and cofy from the signals connected to ai2 and ai3. this is done by solving equations (6), (7), and (8) in that order. 3. computation of the balance indicator b considering the signal connected to ai4. this is done by solving equations (9), (4), and (3) in that order. 4. detection of good steps by running the state machine presented in figure 6. 5. computation of the motor coordination indicator mc by solving equation (5). the state machine illustrated in figure 6 is a key element of the instrumented walker because it classifies steps as either good or bad. states 0 to 5 represent the gait phases (as in figure 2) and state 10 represents a waiting state to rearm the machine. transitions between states (tfrom,to) are triggered by sensor measurements as follows: • t0,1 is enabled if ft < weight_th. this transition occurs when the total force measured by the load cells falls below a given threshold (below the walker weight). in other words, it occurs at the beginning of a new step when the walker is lifted in the air. • t1,2 is enabled if ft > weight_th. this transition occurs when the walker touches the floor again. • t2,3 is enabled if cofx > right_th or cofx < left_th depending on the injury (left or right limb, respectively). this transaction occurs when the user moves forward the injured limb and applies force on the opposite side. the injured limb is the first to move forward. the type of disability must be defined in advance to determine the threshold value. • t3,4 is enabled if cofx < right_th or cofx > left_th depending on the injury (left or right limb, respectively). this occurs when the user alleviates the force previously applied on the walker, making cofx return to the origin. figure 5. main window of the andante application. figure 6. state machine used to classify steps as good or bad. 0 t0,1 1 t1 ,2 2 t2 ,3 3 t3 ,4 4 t4,5 5 t5,0 t2,0t4,0 10 t10,0 t4,10 t2,10 acta imeko | www.imeko.org december2018 | volume 7 | number 4 | 38 • t4,5 is enabled if cofx < left_th or cofx > right_th depending on the injury (left or right limb, respectively). this transaction occurs when the user moves forward the healthy limb and applies force on the opposite side. the healthy limb shall be the last to move forward. • t5,0 is enabled if cofx > left_th or cofx < right_th depending on the injury (left or right limb, respectively). this transaction occurs when cofx returns to the origin. • t2,0 and t4,0 are enabled if cofx moves to the wrong side, i.e., if the user moves the wrong limb forward. the machine returns to state 0 and waits for a new step. • t2,10 and t4,10 are enabled if ft < weight_th. this transition occurs if the user lifts the walker before finishing the step. in other words, it occurs if the user aborts the step. • t10,0 is enabled if ft > weight_th. this transition occurs when the walker touches the floor again. the machine returns to state 0 and waits for a new step. a step is marked as good if the machine passes through all the states from 0 to 5. a step is marked as bad if the user aborts it or moves forward the wrong limb. the threshold weight_th is equal to half of the weight of the walker frame. the thresholds right_th and left_th are dynamic and computed as follows: { 𝐿𝐸𝐹𝑇_𝑇𝐻(0) = − 𝑊 6 𝐿𝐸𝐹𝑇_𝑇𝐻(𝑘) = 𝑚𝑖𝑛𝐶𝑂𝐹𝑥(𝑘−1) 2 if (𝐿𝐸𝐹𝑇_𝑇𝐻(𝑘) > −50 mm) 𝐿𝐸𝐹𝑇_𝑇𝐻(𝑘) = −50 mm (10) { 𝑅𝐼𝐺𝐻𝑇_𝑇𝐻(0) = + 𝑊 6 𝑅𝐼𝐺𝐻𝑇_𝑇𝐻(𝑘) = 𝑚𝑎𝑥𝐶𝑂𝐹𝑥(𝑘−1) 2 if (𝑅𝐼𝐺𝐻𝑇_𝑇𝐻(𝑘) < +50 mm) 𝑅𝐼𝐺𝐻𝑇_𝑇𝐻(𝑘) = +50 mm (11) where k represents the kth good step, and min/maxcofx(k-1) represents the minimum/maximum value reached by cofx during the previous (k 1) good step. the processed data is shown in the graphical interface shown in figure 7, which contains four panels: • panel 1: the instantaneous value of the heart rate. this is useful for estimating the effort and stress felt by the user. • panel 2: the cof is computed, and the result is presented as a cross, moving over an xy graph. when the user loads his left side, the cross moves toward negative values of x; otherwise, the cross moves toward the positive values of x. the same applies to the front/back direction over the y axis, much like a joystick. a vertical slider gives the instantaneous value of balance b. • panel 3: a set of six leds is turned on sequentially while the state machine moves forward from state 0 to state 5. if the user passes through all the gait phases successfully, all the leds light up and the step is marked as good. if the user violates any gait phase, the machine is reset to state 0 and the step is marked as bad. the number of good and bad steps is registered. a vertical slider gives the instantaneous value of the motor coordination mc. • panel 4: a list of all steps detected. for each step, the following information is provided: step#: index of the step. timestamp: date and time of the step. heart rate: last value of heart rate measured during the step. balance: minimum value of balance b measured during the step. motor coordination: value of motor coordination mc considering the current step. quality: classification of the step (good or bad). lift: lift of the walker frame while moving forward, in millimeters. the lift is estimated by double integrating the acceleration across the z axis, subtracted by 1 g = 9.81 m s-2. prior to integration, the acceleration samples, collected only during state 1, are smoothed to remove high-frequency noise and are subtracted from the baseline to remove drift. figure 7. graphical interface of the stream data child window. acta imeko | www.imeko.org december2018 | volume 7 | number 4 | 39 step length: length of the step, in millimeters. the distance is estimated by double integrating the acceleration across the y axis. prior to integration, the acceleration samples, collected only during state 1, are bandpass filtered, as described above. azimuth: azimuth of the walker frame, in degrees. the values 0°, 90°, 180°, and 270° correspond to north (magnetic), east, south, and west, respectively. failure: details the quality of the step according to the following enumeration: none, step aborted, injured foot failed to move forward, or healthy foot failed to move forward. the physiotherapist can also add free comments to each step. the list can be saved to the database (into the steps table) by clicking on a button. 4. experimental results the experimental tests were undertaken using a standard four-leg walker, ayudas dinámicas model ad230 [26], with dimensions w12 = 51 cm, w43 = 53 cm, l = 45 cm, and weight = 2.5 kg. the first test was intended to evaluate the accuracy of the cof measurements. the four walker legs were placed over four digital scales, model spider 1s-60s from mettler toledo, and load was applied on the walker frame to move the cof to the 2nd quadrant. the cof coordinates were computed knowing the geometry of the walker, and the weights were indicated by the scales (with a resolution of 20 gf). the cof coordinates measured by the andante application were compared against the cof coordinates computed from the scales (the scales serve as a reference). the experiment was repeated, rearranging the load to move the cof to the 4th quadrant. table 1 shows the cof coordinates measured by both systems (andante and scales) in both arrangements (2nd and 4th quadrants). the distances to the origin r are also shown, as well as the corresponding relative errors . the values obtained, below 10 %, are acceptable considering that the load cells are low cost and their alignment is not perfectly orthogonal (as explained in section 3.4). the second test was undertaken in order to understand how the cof swings to the right and to the left during a good step. figure 8 shows precisely that, together with the value of balance b. both are related, with the balance falling whenever the cof moves away from the origin. the curves are explained as follows: • state 0 was omitted because it is a waiting state. • state 1 begins when the walker is lifted in the air and ends when the walker touches the floor again. • state 2 is a waiting state that ends when the user moves the injured limb forward (making cofx cross left_th). • during state 3, the user loads his left side to compensate for the lack of support on the injured limb. cofx then returns to the origin and the machine moves to the next state. • state 4 is another waiting state that ends when the user moves the healthy limb forward (making cofx cross right_th). • during state 5, the user transfers part of his weight to the right. the load applied during state 5 is lower than that applied on state 3 because the healthy limb moves more easily than the injured limb. the step ends when both feet stay sideby-side, making cofx return to the origin and restart the state machine. the third test was undertaken with the help of a user with impaired gait caused by an injury in the right lower limb. the user was instructed to follow an imaginary straight line, first in an early stage of the therapy figure 9a) and then some weeks later in a later stage of the therapy (figure 9b). the xy graphs cover 12 steps, with each step represented by a line segment. the length and angle of the segment represent the traveled distance and azimuth of the walker. the user moves from the origin (0, 0) to the periphery. table 2 summarizes the metrics obtained for both experiments. the averages and standard deviations cover the 12 steps, except for the motor coordination mc, which considers only the final value. the user followed a trajectory that was almost linear in both experiments, with mean squared errors equal to 9.3 cm and 2.2 cm, in the early and final stages of the therapy, respectively. in fact, all metrics show that the physical condition of the user has improved: balance and motor coordination increased, the total table 1. comparison between the andante and a reference system regarding the computation of cof coordinates. experiment cof coordinates distance to origin r andante scales andante scales  2nd quadrant (-122; 78) mm (-126; 96) mm 145 mm 158 mm 8.2 % 4th quadrant (180; -44) mm (194; -45) mm 185 mm 199 mm 7.0 % figure 8. synchronized plots of cofx and balance (b) during a good step. the numbered circles have correspondence with the states of figure 6. table 2. gait metrics acquired while the user was following an imaginary straight line. experiment quantity sub-quantity early stage final stage balance b (%) average 49 78 mc (%) final 60 90 walker lift (cm) average 1.3 6.5 std 1.2 1.5 step length (cm) average 21.2 33.6 std 9.4 8.9 total travelled distance (m) ----2.54 4.03 0 10 20 30 40 50 60 70 80 90 100 -250 -200 -150 -100 -50 0 50 100 150 0 0,5 1 1,5 2 2,5 3 3,5 4 4,5 5 b a la n ce ( % ) c o fx ( m m ) time (s) cofx balance 1 2 3 4 5 acta imeko | www.imeko.org december2018 | volume 7 | number 4 | 40 distance traveled also increased, steps became longer and more regular, and the walker began to be handled more easily (as indicated by higher lifts). 5. conclusion the experimental results show that the andante is able to detect and classify steps on a standard four-legged walker. each step is characterized by a rich set of parameters, including estimates of balance and motor coordination, step length and azimuth, and lift of the walker frame. this record of information can be used by a physiotherapist to objectively assess the physical condition of the user. if these records are saved during multiple sessions, the physiotherapist can look at them, understand how the rehabilitation therapy is progressing over time, and take corrective actions if needed. acknowledgement the work was supported by instituto de telecomunicações and fundação para a ciência e tecnologia, projects ptdc/dtpdes/1661/2012 and ptdc/dtp-des/6776/ 2014. references [1] c. a. werner. the older population: 2010. 2010 census briefs, u.s. census bureau. https://www.census.gov/ prod/cen2010/briefs/c2010br-09.pdf (february 2018) [2] federal interagency forum on aging-related statistics. older americans 2008: key indicators of well-being. 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[3] european commission. the demographic future of europe – from challenge to opportunity. http://europa.eu/ legislation_summaries/employment_and_social_policy/situation _in_europe/c10160_en.htm (february 2018). [4] l. vogt, k. lucki, m. bach, w. banzer. rollator use and functional outcome of geriatric rehabilitation. journal of rehabilitation research & development 47(2) (2010), pp. 151-156. [5] s. m. bradley, c. r. hernandez. geriatric assistive devices. american family physician journal 15, 84(4) (2011) pp. 405-411. [6] s. r. faruqui, t. jaeblon. ambulatory assistive devices in orthopaedics: uses and modifications. journal of the american academy of orthopaedic surgeons 18(1) (2010) pp. 41-50. [7] h. bateni, b. e. maki. assistive devices for balance and mobility: benefits, demands, and adverse consequences”. archives of physical medicine and rehabilitation 86(1) (2005) pp. 134-145. [8] j. a. stevens, k. thomas, l. teh, al greenspan, unintentional fall injuries associated with walkers and canes in older adults treated in u.s. emergency departments. journal of the american geriatrics society 57(8) (2009) pp. 1464-1469. [9] joel a. delisa, gait analysis in the science of rehabilitation. rehabilitation research and development service, department of veterans affairs, 1998. [10] a. muro-de-la-herran, b. garcia-zapirain, a. mendez-zorrilla. gait analysis methods: an overview of wearable and non-wearable systems, highlighting clinical applications. sensors review 14(2) (2014) pp. 3362-3394. [11] r. a. bachschmidt, g. f. harris, g. g. simoneau. walker-assisted gait in rehabilitation: a study of biomechanics and instrumentation. ieee transactions on neural systems and rehabilitation engineering 9(1) (2002) pp. 96-105. 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[16] i.-min lee, e. j. shiroma, using accelerometers to measure physical activity in large-scale epidemiological studies: issues and challenges. british journal of sports medicine 48(3) (2014) pp. 197-201. [17] o. postolache, p. silva girão, g. postolache, g. mendes. “cardiorespiratory and daily activity monitor based on fmcw doppler radar embedded in a wheelchair”, proc. of 33rd annual international conference of the ieee engineering in medicine and biology society, sept. 2011, boston, ma, u.s.a, pp. 19171920. [18] g. postolache, p. silva girão, o. postolache. “applying smartphone apps to drive greater patient engagement and personalized physiotherapy”, proc. of ieee international symposium on medical measurements and applications (memea), jun. 2014, lisbon, portugal, pp. 1-6. (a) (b) figure 9. user following a linear trajectory: a) in the early stage of the therapy; b) in a later stage of the therapy. https://www.census.gov/prod/cen2010/briefs/c2010br-09.pdf https://www.census.gov/prod/cen2010/briefs/c2010br-09.pdf https://agingstats.gov/docs/pastreports/2008/oa2008.pdf http://europa.eu/legislation_summaries/employment_and_social_policy/situation_in_europe/c10160_en.htm http://europa.eu/legislation_summaries/employment_and_social_policy/situation_in_europe/c10160_en.htm http://europa.eu/legislation_summaries/employment_and_social_policy/situation_in_europe/c10160_en.htm acta imeko | www.imeko.org december2018 | volume 7 | number 4 | 41 [19] a. ekimov, j. m. sabatier. human motion analyses using footstep ultrasound and doppler ultrasound. journal of the acoustic society of america 123(6) (2008) pp. 149-154. [20] x. cai, g. han, j. wang, “assessment of gait parameters during unconstrained straight-line walking using single camera”, proc. of 7th ieee international conference on awareness science and technology (icast), sept. 2015, qinhuangdao, china, pp. 72-77. [21] x. cai, g. han, x. song, j. wang. single-camera-based method for step length symmetry measurement in unconstrained elderly home monitoring. ieee transactions on biomedical engineering 64(11) (2017) pp. 2618-2627. [22] o. postolache, p. girão, j. m. dias pereira, g. postolache. “smart walker for pervasive healthcare”, proc. of international conference on sensing technology (icst) vol. 1, dec. 2011, palmerston north, new zealand, pp. 1-5. [23] o. postolache, j. m. dias pereira, v. viegas, p. silva girão, “gait rehabilitation assessment based on microwave doppler radar embedded in walkers”, proc. of ieee international symposium on medical measurement and applications (memea), may 7-9, 2015, torino, italy, pp. 208-213. [24] j. m. dias pereira, v. viegas, o. postolache, p. silva girão, “combining distance and force measurements to monitor the usage of walker assistive devices”, proc. of ieee international instrumentation and measurement technology conference (i2mtc), may 22-25, 2017, torino, italy, pp. 1-6. [25] v. viegas, j. m. dias pereira, o. postolache, p. silva girão, “application of force and inertial sensors to monitor the usage of walker assistive devices”, proc. of 22nd imeko tc4 international symposium, sept. 2017, iasi, romania, pp. 189-194. [26] ayudas mecánicas, manual de usuário ad230ad/ ad233ad/ ad232. https://www.ayudasdinamicas.com/caminador-deincorporacion.pdf (february 2018). [27] d. a. winter, biomechanics of normal and pathological gait: implications for understanding human locomotor control. journal of motor behavior 21(4) (1989) pp. 337-355. [28] e. marder, r. l. calabrese, principles of rhythmic motor pattern generation. physiological reviews 76(3) (1996) pp. 687-717. [29] shimmer. shimmer3 imu unit. http://www.shimmersensing.com/products/shimmer3-imusensor (december 2018). [30] s. o. h. madgwick, a. j. l. harrison, r. vaidyanathan, “estimation of imu and marg orientation using a gradient descent algorithm”, proc. of the ieee international conference on rehabilitation robotics, 29 june 1 july 2011, zurich, switzerland, pp. 1-7. [31] shimmer, 9dof calibration application user manual rev. 2.5a, 2014. [32] shimmer. shimmercapture/shimmer c# api. http://www.shimmersensing.com/products/shimmercapture (december 2018). https://www.ayudasdinamicas.com/caminador-de-incorporacion.pdf https://www.ayudasdinamicas.com/caminador-de-incorporacion.pdf http://www.shimmersensing.com/products/shimmer3-imu-sensor http://www.shimmersensing.com/products/shimmer3-imu-sensor http://www.shimmersensing.com/products/shimmercapture microsoft word 407-2746-1-le-rev acta imeko  issn: 2221‐870x  july 2017, volume 6, number 2, 75‐79    acta imeko | www.imeko.org  july 2017 | volume 6 | number 2 | 75  the control system for the magnetic suspension comparator  system for vacuum‐to‐air mass dissemination  corey stambaugh   mass and force group, national institute of standards and technology, gaithersburg, md u.s.a 20899, u.s.a.      section: research paper   keywords: si; magnetic suspension; pid; fpga; mass metrology  citation: corey stambaugh, the control system for the magnetic suspension comparator system for vacuum‐to‐air mass dissemination, acta imeko, vol. 6,  no. 2, article 13, july 2017, identifier: imeko‐acta‐06 (2017)‐02‐13  section editor: min‐seok kim, research institute of standards and science, korea   received june 30, 2016; in final form october 20, 2016; published july 2017  copyright: © 2017 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  corresponding author: corey stambaugh, e‐mail: corey.stambaugh@nist.gov    1. introduction  the redefinition of the kilogram will cut the tie to a physical artifact by using, for example, the watt balance to define the kilogram in terms of planck's constant [1]. to realize this definition, the magnitude of planck's constant will be fixed [2]. overall, mass dissemination will remain the same after redefinition and mass comparators and artifacts will still be used. however, in the new definition the kilogram will be realized in vacuum, unlike the international prototype kilogram which is kept in air. below the national metrology level, mass calibration will still be carried out in air. as such, a reliable method is required for accounting for mass changes from sorption effects when moving from vacuum-to-air. to date, the primary method for vacuum-to-air transfer involves developing a model for the sorption effects [3]. in this procedure, the mass comparison is carried out in vacuum and then the test mass is brought to air. at that time, a correction based on the sorption model would be added to the calibrated mass value. in this indirect way, mass calibration can be performed between vacuum and air. at the national institute of standards and technology (nist) we are implementing an alternative, direct approach [4]. here, the standard artifact is kept in vacuum while the test mass is kept in air. the mass comparison, performed using a single, precision comparator, is carried out by coupling an upper assembly (mass comparator and mass pan in vacuum) to a lower assembly (mass pan in air) through magnetic suspension [5], [6]. through this direct vacuum-to-air comparison the need to adjust measurements post hoc to account for mass changes resulting from sorption effects is eliminated. to aid in characterizing and improving the overall performance of the magnetic suspension mass comparator system (msmc) for vacuum-to-air mass dissemination, a proof of concept (poc) system was built. this testing bed allows us to focus on just the magnetic suspension system (mss). the mss can further be broken down into several key components: (1) the actuator, (2) the sensor, and (3) the control loop. in this paper we will focus on the control loop and its implementation using a field programmable gate array (fpga). abstract  high  precision  mass  comparisons  are  typically  performed  such  that  both  masses  are  measured  under  nearly  identical  conditions.  however,  there  are  situations  where  an  artefact  needs  to  be  calibrated  against  a  standard  kept  under  different  conditions.  for  example, the new realization of the kilogram will be carried out in vacuum but the majority of calibration services are performed at  atmospheric pressure. at the national institute of standards and technology, we have developed an apparatus to perform direct mass  comparison under such a condition using magnetic suspension. the stability of the magnetically suspended portion of the system plays  a large role in the overall uncertainty of a measurement. here, the control system for the magnetic suspension and the improvements  that have been made to improve stability are described.  acta imeko | www.imeko.org  july 2017 | volume 6 | number 2 | 76  2. principles  2.1. vacuum‐to‐air mass dissemination   before delving deeply into the control system, a brief description of the operating principle for vacuum-to-air mass dissemination and for the magnetic suspension is warranted. for any standard mass comparison, an unknown mass is compared to a known reference mass of nominally the same mass. in the case of mass comparators, this is done by first weighing one of the two masses on a mass comparator. the result is recorded and then the other mass is weighed using the same comparator. the difference in the two readings is used to determine the actual mass of the unknown with respect to the known reference mass. vacuum-to-air mass dissemination works the same, in principle, except one of the two masses is kept in vacuum while the other is kept in air. as shown in figure 1, the mass comparator is placed in an upper chamber, and the upper magnetic assembly (uma) is hung from it. below the upper chamber is a second chamber, which houses the lower magnetic assembly (lma). to couple the lma to the mass comparator in the upper chamber, magnetic suspension is utilized. when the magnetic suspension is engaged, the suspended lma is coupled to the uma and the mass comparator. thus, its mass can effectively be removed from the measurement. the measurement sequence is now straightforward. place the known mass on the uma and ensure the lma is suspended. take a reading. remove the mass from the uma and turn off the suspension. load the unknown mass on to the lma and re-engage the suspension. the difference between this reading and the prior one is the difference between the masses less corrections for air buoyancy and the gravitational difference resulting from the masses being placed at different heights. for the proposed vacuum-to-air comparisons the upper chamber would be held at vacuum, while the lower is at atmospheric pressure. 2.2. magnetic suspension   in the above measurement, magnetic suspension plays the crucial role of coupling the suspended lma to the uma and subsequently the mass comparator. the basic principle behind the suspension is as follows [5]. attached to the lower and upper mass pans are two permanent magnets. these provide the majority of the necessary lifting force. additionally, an electromagnetic coil is wound around the upper magnet. a hall sensor is placed between the two permanent magnets and used to monitor the separation between the suspended lma and the uma. the hall sensor provides a proportional voltage which is fed to the control system. the control system outputs a voltage that provides the necessary current to the coil to counteract the motion of the suspended lma, thus keeping it suspended. 3. control system  the implementation of a control loop to sustain suspension of the lma is paramount to achieving stability in the mass readings. to aid in understanding the control system, we construct the closed-loop system as shown in figure 2. by calculating or measuring the values of the different components, a complete model can be developed. this model can then be simulated using tools like mathworks simulink. different parameters for the proportional-integrator-derivative (pid) controller can then be tested for stability. the results can then be verified in the poc. the accurate realization of the poc in the model is vital for several reasons: (1) it provides a means to assess which parameters provide the most stable response, (2) it allows for an assessment of our knowledge of the magnetic suspension system, (3) it provides a convenient viewpoint to identify the major sources of instability, and (4) it helps ensure that when the final system is built we do not waste time determining the stable operation point. in this section, we describe each component in the closed-loop, state how their values were determined and compare the model’s operation to actual suspension in the poc. 3.1. closed‐loop  to the right of the controller block in figure 2 is the i/v block. this block converts the applied voltage to a current and consists of an amplifier and the electrical side of the electromagnetic coil. the amplifier takes the controller output figure 2. schematic of the closed‐loop control. sp, the setpoint, is chosen to be the magnetic field value (converted to voltage) measured by the probe when  the relation   holds. the pid controller is implemented on a fpga chip to eliminate latency and time jitter.   figure 1. illustration of essential components of proof of concept used for testing  magnetic  suspension.  the  poc  is  housed  in  a  single  box,  here  an upper and lower chamber is shown to illustrate how the system would be  segregated  in an actual system. the diagram on the  lower  left shows the main two forces acting on the suspended lower magnetic assembly (lma). acta imeko | www.imeko.org  july 2017 | volume 6 | number 2 | 77  and amplifies by a gain, a. for simplicity, we take the coil to be an rl filter. the resistance, r, is the resistance of the coil wire and the leads. the inductance of the coil is . their values are 45 ω and 160 mh. all together they form a low-pass filter with gain a, whose transfer function is , (1) the rl filter has a cut-off frequency of 45 hz. the current now works, as described by the plant, to produce a force that will act upon the lma. the plant operation is determined by solving the equation of motion, assuming negligible damping, for the lma, under the influence of gravity, , and the total magnetic field, , : , . (2) here, , is the sum of the force generated by the permanent magnets at a separation and the force generated by the electromagnet with a current and is the total mass of the lma (this can be with or without the test mass). for small motion (2) can be linearized by expanding , : . (3) the parameters and represent the change in force with respect to for the permanent magnet and for the coil. by choosing such that we can take the laplace transform of (3) to get where and are and , respectively. therefore, the transfer function of the plant is . (4) the values of the parameters and are approximately 3.0 n/a and 1.6 10 n/m, respectively. both values were determined through finite element modelling (fem) simulations and experiment which will be discussed in more detail in a future publication. the motion of the lma changes the separation distance between the lma and the uma, which changes the magnetic field between them. a hall sensor, placed between the two magnets, is used to monitor the changing magnetic field. the block / represents the conversion between the changes in separation to the change in the magnetic field as measured at the position of the hall sensor and has a value of about 30 t/m. now, it is important to recognize that there is another contributing factor to the magnetic field change; namely the magnetic field generated by the electromagnetic coil, denoted as / in figure 2. its value is approximately 0.026 t/a. both values depend on , here 11.5 mm, and the probe position relative to the uma (~6 mm). in subsection 3.2, the importance of the probe position is examined more closely. these two magnetic field changes are summed to provide the total magnetic field measured at the hall sensor. the hall sensor has a sensitivity of 5v t⁄ and provides some low-pass filtering at 5 khz. finally, an additional low-pass filter (~300 hz) is used before the signal is passed to the pid controller. the pid transfer function for closed-loop control is 1 / . (5) here , , and are the proportional, integral, and derivative gain terms and n is the filtering coefficient. the filtering coefficient helps smooth the derivative term to avoid sudden changes in the response. the closed-loop transfer function is then , (6) where is the open-loop response composed of all the blocks in figure 2 except for the controller. as will be discussed in section 4, the pid is implemented on a digital system so a discrete version of is employed. however, given that are control loop is run at 100 khz and the frequency response of is 100 hz, we can effectively treat the system as continuous. 3.2. simulation  to test the closed-loop operation and to verify that the pid parameters provide a stable control loop, we converted the closed-loop system described above into a simulink model. in this program the known values for the open-loop system as described in the previous section are entered. the simulation can be used to determine test pid parameters for stable operation. also, different test functions can be applied to determine how stable the loop is in the presence of noise and other external perturbations. in figure 3 we show results from a test run where we compare the system response of our model (orange, solid lines) to that of the actual poc (blue, dashed lines). identical pid parameters were used for both systems: p=50, i=1.5, d=0.5, and n=10. the white noise injected into the model is chosen so that the standard deviation of the controller output in the steady-state for both the model and the poc are approximately the same. we applied a step change, equivalent to a change in b-field of 0.2 mt (0.001 v), at t = 0 s and measured the output at the three monitor points in figure 2: control out [v], z [m], and b-field [v]. the agreement in both magnitude and temporal response for the b-field and controller out is excellent. the z response, which was measured independently using an interferometer, shows good agreement as well. the mass readings were taken using a mass comparator with a resolution of 0.1 mg. the reading before and after the disturbance is 0.1 mg apart; such a change is expected for the comparator over the time scale shown and is not a result of the figure 3. comparison of step response of simulink model (orange, solid) and  real magnetic suspension on poc (blue, dashed). the bottom graph shows the deviations of the measured value of the mass.  acta imeko | www.imeko.org  july 2017 | volume 6 | number 2 | 78  magnetic suspension. the probe position between the two magnets has an overall effect on the stability of the feedback loop. to investigate this effect, we utilized the simulation of the closed-loop response as a function of the proportional gain and the distance along the zaxis of the probe from the upper magnet. as the probe is moved, increasing the distance between the probe and upper magnet, the values of / and / change; this in turn alters the dynamics of the closed-loop response. simulations were run holding i and d fixed. the standard deviation from t=75 s to t=90 s was then computed. a contour plot of the result, where log10 was taken of the computed standard deviation, is shown in figure 4. the y-axis is the probe position and the x-axis is the gain, p. the yellow patches are regions where the simulation did not finish due to instabilities. a probe position of approximately 6 mm, which is roughly half way between the two magnets, provides a robust region where a stable response is found for a wide range of gains. the simulation also indicates that moving the probe closer to the upper magnet leads to a larger instability; a fact that we have observed experimentally. the instability results because ( / / / decreases as the probe approaches the uma. the / , which does not provide information on the motion of the suspended object, tends to overwhelm / in this case. 4. implementation of the control system  in the previous section, the model for the magnetic suspension was described and results from an actual magnetic suspension test were compared to it. here we describe how the pid controller was realized in the poc. the implementation of the control loop marks a major upgrade in our magnetic suspension comparator system. the prior realization utilized a standard desktop computer and was implemented using labview1. while suspension was achieved, the level of stability was insufficient to meet our needs. standard computer operations like a mouse click would occasionally lead to jumps in the feedback control. the primary reason for this lies in the time jitter and latency introduced by the operating system. for example, our initial control loop was set to update at a rate of 30 khz, however, while the actual achieved rate peaked at this frequency it deviated around this point by several kilohertz, as shown in figure 5. additionally, occasional delays occurred in the loop that led to peaks at several lower frequencies. these problems occurred because the control loop was sharing resources with the non-real time operating system. to completely eliminate this issue, we migrated the control loop to a field programmable gate array (fpga) using labview fpga, the red line shows the control loop rate (100 khz) when running on the fpga. an fpga allows for programmatically hard-wiring the code through the reconfiguring of gate arrays on an integrated circuit. because the fpga uses dedicated hardware to process the control logic, no operating system is involved nor is overhead incurred; this provides real-time determinism. additionally, the logic gates can be set independently, yet synchronized to the same master clock. in this way, true parallelism can be realized by allowing multiple control loops to be run simultaneously. again, since there is no overhead the control loops update rate is fixed and time jitter and latency occur at levels below the inverse of the loop rate. furthermore, digital filters and real-time monitoring can be added. the fpga is directly wired to analog-to-digital (adc) and digitalto-analog convertors (dac) and data stream exists between the fpga and host computer. this allows real-time monitoring and adjustment of the pid parameters. the trade-offs when using fpga include increased complexity in programming, limited physical space on-chip for implementing code, and limited hardware for both adc and dac. since our execution rate is 100 khz and the master clock of the fpga is 40 mhz we have plenty of time to accomplish all required tasks within a single cycle of the control loop. while limited space on the fpga constrains the size of the overall code, we found that we are still able to fit 3 controls loops, multiple filters, and several ‘fpga to host’ transfer lines on a single chip; this far exceeds our base needs. the hardware issue, while currently not a limitation, could be an impediment in the future. the issue lies in the limited resolution of the hardware available for the adc and dac on the fpga. from our experience, the fpga has greatly improved the performance of the magnetic suspension. as shown in figure 3, the system is stable and sudden changes in setpoint do not cause the magnetic suspension to fail. interestingly, the best evidence of its benefit comes from direct interaction with the system. before implementing the fpga, manually positioning the suspended mass at a point where stable suspension could occur was difficult. fluctuations induced by imprecise positioning of the lma and unsteadiness of the operator’s hands when placing the lma into the proper position for figure 4. contour plot of standard deviation of b‐field closed loop response from t=75 s to t=90 s. the color mapping is log10.  figure 5. histogram of frequency distribution of loop‐rate for control system  implemented on desktop computer (grey bars) and fpga (red  line at 100  khz).  acta imeko | www.imeko.org  july 2017 | volume 6 | number 2 | 79  magnet suspension induced large field changes to which, when coupled with the non-deterministic nature of the non-fpga control loop, quickly lead to instability. such instability has been effectively eliminated by the new fpga based control system. 5. conclusion  in this paper we described a method for performing direct mass comparison of two masses where one is kept under vacuum and the other in air. of course the exact environment is of little consequence, but rather the ability to couple the gravitational force acting on the mass in one chamber to the mass comparator in the other is of great utility. this work is primarily focused on dissemination of the standard kilogram in light of the new realization of the kilogram which will occur under vacuum. a further application would include cross checking sorption studies that aim to model the mass added when changing environments. the goal here was to analyze the model used for the control system, compare it to the magnetic suspension observed in the poc and discuss how moving to an fpga system provides increased stability. the strong agreement in figure 3 indicates that we are now properly modelling the system; an advancement over previous work [7]. the incorporation of the fpga was a key step in acquiring this stable system as it eliminated the time-jitter and latency in the previous system. with this work, we are well posed to continue improving the magnetic suspension system stability for use in vacuum-to-air mass dissemination. acknowledgement  the author would like to acknowledge valuable conversations with n. vlajic. certain commercial equipment, instruments, or materials are identified in this paper in order to specify the experimental procedure adequately. such identification is not intended to imply recommendation or endorsement by the national institute of standards and technology, nor is it intended to imply that the materials or equipment identified are necessarily the best available for the purpose. references  [1] d. b. newell, “a more fundamental international system of units”, physics today 67, 7 (2014), pp. 35-41. [2] comptes rendus de la 24e cgpm (2011), 2013, p.532. [3] a. picard, h. fang, “methods to determine water vapour sorption on mass standards”, metrologia 41, 333-9 (2004). [4] z. j. jabbour, p. abbott, e. williams, r. liu and v. lee, “linking air and vacuum mass measurement by magnetic levitation”, metrologia, 46, 3 (2009), pp. 339-344. [5] n. a. shirazee and a. basak, “electropermanent suspension system for acquiring large air-gaps to suspend loads”, ieee trans. mag., 31, 6 (1995), pp. 4193-4195. [6] clark, john w., “an electronic analytical balance”, rev., sci. instr. 18 (12), (1947) 915-918. [7] p. abbott, r. c. dove, e. c. benck, and z. j. kubarych, “progress on a vacuum-to-air mass calibration system using magnetic suspension to disseminate the planck constant realized kilogram”, proc. of imeko 22nd tc3, 12th tc5 and 3rd tc22 international conferences, feb. 3-5, 2014, cape town, rsa. acta imeko, title acta imeko issn: 2221-870x march 2018, volume 7, number 1, 68-71 acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 68 mathematical analysis of the effect of the constant temperature assumption in the leak test of ptb testing instructions, volume 25 oscar yazit salah garcía, carlos eduardo garcía sánchez grupo de investigación en fluidos y energía, centro de desarrollo tecnológico del gas, parque tecnológico guatiguará (km 2 vía refugio), piedecuesta, colombia section: research paper keywords: leak flow rate; modelling; temperature; ptb 25 citation: oscar yazit salah garcía, carlos eduardo garcía sánchez, mathematical analysis of the effect of the constant temperature assumption in the leak test of ptb testing instructions, volume 25, acta imeko, vol. 7, no. 1, article 13, march 2018, identifier: imeko-acta-07 (2018)-01-13 section editor: jorge torres-guzman, cenam – centro nacional de metrologia, santiago de querétaro, mexico received july 13,2017; in final form november 18, 2017; published march 2018 copyright: © 2018 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: carlos eduardo garcía sánchez, e-mail: cgarcia@cdtdegas.com 1. introduction "ptb testing instructions, volume 25: gas meters – test rigs with critical nozzles" (ptb 25) is a document that contains physical fundamentals, mathematical models, and design and operation parameters for sonic nozzle test rigs used for calibration of gas meters [1]. ptb 25 is a very complete text for design, construction and operation of calibration benches with critical nozzles, intended to function with air at pressures close to the atmospheric pressure. for this reason, ptb 25 is used as a reference document by many manufacturers and operators of nozzle test rigs for calibration. in the section which is dedicated to the use of the nozzle test rig, it is included an assay which is intended to evaluate the tightness of the test section of the rig. this assay will be referred as the “leak test” in the rest of this work. it is assumed in ptb 25 that the leak test is an isothermal process, and this has a significant impact over the resulting equations to evaluate the leak test results. the goal of the present work is to analyse if the mathematical model of the leak test used in ptb 25, which assumes constant temperature, is adequate for the evaluation of the tightness of a nozzle test rig. in section 2 the modelling of the leak test process and how the assumption of isothermal process converts the model in the one shown in ptb 25 are showed, and after that the simulations done in the present work are described. in section 3 the results of the simulations are presented and discussed. finally, in section 4 the conclusions of the work are summarized. abstract "ptb testing instructions, volume 25: gas meters – test rigs with critical nozzles" (ptb 25) includes a detailed description of a test to evaluate the tightness of the rig (leak test). the mathematical modeling of this test assumes that temperature remains constant, and it is established a maximum allowable change of 0.1 k during the test. ptb 25 defines a maximum value of the leak flow rate as criteria for approval of the test. in this work, the effect of the assumption of constant temperature over the leak test result was analyzed. different processes in which the real leak flow rate was zero and there was a change in the temperature of the system equal to the maximum permissible (according to ptb 25) were simulated, and the leak flow rate calculated by ptb 25’s model was determined. the assumption of constant temperature does not simplifies drastically the mathematical model. it is concluded that ptb 25’s maximum allowable temperature change is too high for most practical situations, leading to errors that exceeds the maximum allowable leakage rate. the factors “test time” and “enclosed volume” have a high effect over the magnitude of the error given by ptb 25’s leak flow rate model. acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 69 2. methods and models the first step in the leak test consists in generating a pressure differential between the interior and the exterior of the nozzle test rig. the next step is to close the bench valves upstream and downstream the test section, so that the region of interest of the bench is a closed system (i.e. it does not allow transfer of mass). if the rig does not have sufficient airtightness, there will be an air flow from the outside of the rig to the inside (or vice versa, depending on the pressure differential). after a period of stabilization, the initial values of pressure and temperature are registered. subsequently, a certain test time is allowed, and test’s final pressure and temperature are registered. with the recorded data and the test time, it is possible to quantify the leak of substance and to get the conclusions of the test. figure 1 shows a schematic diagram of the process for the leak test of nozzle test rigs. section 2.1 shows the mathematical model for the described process. section 2.2 presents the constant temperature simplification used in ptb 25, and shows the resulting mathematical model. section 2.3 describes the case studies used in the present work to investigate the effect of ptb 25’s assumption over the leak test. 2.1. modelling of the leak test in the following, the situation in which internal pressure is lower than external pressure is considered. units are as follow: mass terms are in kg, volume 𝑉 is in m3, temperature terms are in k, and 𝑅’ is in pa*m3*kg-1*k-1. mass balance of the leak test process (shown in figure 1) leads to equation (1) and (2), where 𝑚𝑖 is the initial mass of gas in the system, 𝑚𝑓 is the mass of gas in the system at the end of the process, and 𝑚𝑙𝑙𝑙𝑙 represents the mass entering the system during the process, that is, equals the mass of the leak. [2]. 𝑚𝑓 = 𝑚𝑖 + 𝑚𝑙𝑙𝑙𝑙 (1) 𝑚𝑙𝑙𝑙𝑙 = 𝑚𝑓 − 𝑚𝑖 (2) pressure in the system is not high, so the ideal gas model can be considered valid. substituting the aforementioned model in equation (2), the equation (3) is obtained. 𝑚𝑙𝑙𝑙𝑙 = 𝑃𝑓𝑉 𝑅𝑓 ′𝑇𝑓 − 𝑃𝑖𝑉 𝑅𝑖 ′𝑇𝑖 (3) in equation (3), 𝑅′ is the specific gas constant of the gas or mixture of gases that are enclosed in the control volume. 𝑉 represents the control volume (enclosed volume), and it is constant (the leak test is an isochoric process). if the gas at the interior of the control volume do not change its chemical composition, the specific gas constant will be the same at the start and at the end of the process, so the equation (4) can be written. 𝑚𝑙𝑙𝑙𝑙 = 𝑉 𝑅′ � 𝑃𝑓 𝑇𝑓 − 𝑃𝑖 𝑇𝑖 � (4) expressing the leak volume in the final test conditions, equation (5) is obtained. 𝑃𝑓𝑉𝑙𝑙𝑙𝑙 𝑅′𝑇𝑓 = 𝑉 𝑅′ � 𝑃𝑓 𝑇𝑓 − 𝑃𝑖 𝑇𝑖 � (5) writing the leak volume in term of the mean leak flow rate, 𝑉𝑙𝑙𝑙𝑙 = 𝑄𝑙𝑙𝑙𝑙∆𝑡, where ∆𝑡 is the duration of the leak test and 𝑄𝑙𝑙𝑙𝑙 is the average flow rate of the leak. substituting in equation (5), the following equations are developed: 𝑃𝑓𝑄𝑙𝑙𝑙𝑙∆𝑡 𝑇𝑓 = 𝑉 � 𝑃𝑓 𝑇𝑓 − 𝑃𝑖 𝑇𝑖 � (6) 𝑄𝑙𝑙𝑙𝑙 = 𝑉𝑇𝑓 𝑃𝑓∆𝑡 � 𝑃𝑓 𝑇𝑓 − 𝑃𝑖 𝑇𝑖 � (7) 𝑄𝑙𝑙𝑙𝑙 = 𝑉 ∆𝑡 �1 − 𝑃𝑖𝑇𝑓 𝑇𝑖𝑃𝑓 � (8) if the ideal gas model is valid and assuming temperature homogeneity at the interior of the nozzle test rig, equation (8) represents correctly the mean leak flow rate during the leak test. 2.2. modelling of the leak test in ptb 25 the leak test model used in ptb 25 to obtain the leak flow rate is based in the mathematical process showed in section 2.1, but additionally it is assumed that the temperature at the interior of the nozzle test rig does not change throughout the leak test. with the aforementioned assumption, equation (8) is simplified into equations (9) and (10). equation (10) corresponds to ptb 25’s equation (45) [1]. 𝑄𝑙𝑙𝑙𝑙,𝑃𝑇𝑃25 = 𝑉𝑇 𝑃𝑓∆𝑡 � 𝑃𝑓−𝑃𝑖 𝑇 � (9) 𝑄𝑙𝑙𝑙𝑙,𝑃𝑇𝑃25 = 𝑉 ∆𝑡 �∆𝑃 𝑃𝑓 � (10) ∆𝑃 is the difference between the final and initial pressure in the leak test. in ptb 25, equation (10) is used in conjunction with a maximum allowable value of leak to give the conclusion of the test. equation (10) is correct for isothermal leak tests, modeling the gas mixture as an ideal gas and assuming temperature homogeneity. in the practice, it is almost impossible to guarantee a perfect stability of temperature. for this reason, ptb 25 establishes a non-zero maximum permissible change in the temperature, being equal to 0.1 k. however, temperature has a high effect over the volume of gases, and also the leak flow rates to be identified have very low values. in the present study, it was therefore decided to simulate several different conditions to determine whether the maximum allowable temperature change of ptb 25 ensures the validity of the equation (10), or whether it is necessary to use equation (8) for the quantification of leaks. ptb 25 establishes an upper limit over the leak value, taken from another volume of ptb testing instructions (volume 4) [1]. this upper limit serves as acceptance criterion, so if the leak rate is lower than the maximum permissible leak rate during the leak test, then the rig is considered airtight. this upper limit is 0.3 % of the minimum flow rate for diaphragm gas meters, and 0.1 % for other kinds of meters. so the maximum permissible leak flow rate for g10 diaphragm meters during the leak test is 8.333 x 10-8 m3/s; for g16, 1.333 x 10-7 m3/s; and for g25, 2.083 x 10-7 m3/s. on the other hand, a non-diaphragm gas meter with a minimum flow rate of 0.10 m3/h would have a maximum leak rate of 2.78 x 10-8 m3/s. figure 1. scheme of the leak test process. acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 70 2.3. case studies as case studies were used situations in which the rig is airtight (that is, there is no leak), but there is a change of temperature equal to the maximum permitted change according to ptb 25 (that is, 𝑇𝑓 = 𝑇𝑖 + 𝑇𝑚𝑚𝑚 or 𝑇𝑓 = 𝑇𝑖 − 𝑇𝑚𝑚𝑚, where 𝑇𝑚𝑚𝑚 is 0.1 k). so in all of the simulated situations, actual 𝑄𝑙𝑙𝑙𝑙 (equation (8)) is zero, but 𝑄𝑙𝑙𝑙𝑙,𝑃𝑇𝑃25 (equation (10)) is erroneously not zero. if there is no leak, the mass balance of the control volume leads to the following relationship between initial and final pressure of the process: 𝑃𝑖𝑉 𝑛𝑖𝑅𝑇𝑖 = 𝑃𝑓𝑉 𝑛𝑓𝑅𝑇𝑓 → 𝑃𝑖 𝑇𝑖 = 𝑃𝑓 𝑇𝑓 → 𝑃𝑓 = 𝑃𝑖 � 𝑇𝑓 𝑇𝑖 � (11) in this way, the restrictions imposed over the simulation (no leak, change of temperature of 0.1 k or -0.1 k) set the value of the final temperature and pressure, and there are four input variables (𝑉, ∆𝑡, 𝑇𝑖 and 𝑃𝑖). it was decided to study four levels of every input variable, and test all the possible combinations between the levels of the variables. this study was done two times: the first one with a temperature change of -0.1 k, and after that with 0.1 k. thus, there were 256 simulated cases with temperature decrease, and 256 with temperature increase during the leak test. in all cases, 𝑃𝑖 was lower than the external pressure, so the direction of the leak (if it existed) would be inward of the enclosed volume. as levels for the input variables were chosen the following sets of values: 𝑉 = {0.025, 0.050, 0.100, 0.150} in m3; ∆𝑡 = {60, 120, 240, 360} in s; 𝑇𝑖 = {280, 290, 300, 310} in k; and 𝑃𝑖 = {70 000, 80 000, 90 000, 100 000} in pa. those levels represents common values in actual nozzle test rigs. 3. results in section 3.1 the results of the case studies with temperature decrease during the leak test are presented. section 3.2 shows the results of case studies in which an increment of temperature was simulated. figures 2 to 6 presents the results. the figures include lines showing the maximum leak flow rates (𝑄𝑙𝑙𝑙𝑙,𝑚𝑙𝑚), according to ptb 25, in three different situations. the blue line represents 𝑄𝑙𝑙𝑙𝑙,𝑚𝑙𝑚 for a rig used to calibrate g10 diaphragm gas meters; the orange one, 𝑄𝑙𝑙𝑙𝑙,𝑚𝑙𝑚 for a calibration bench of g16 diaphragm meters; and the green one, 𝑄𝑙𝑙𝑙𝑙,𝑚𝑙𝑚 assigned to a calibration rig for gas meters which are not of diaphragm type and have a minimum flow rate of 0.1 m3/h. 3.1. decreasing temperature figure 2 presents 𝑄𝑙𝑙𝑙𝑙,𝑃𝑇𝑃25 for the case studies with temperature reduction; in the abscissa the enclosed volume is shown. in all the cases in which there was simulated a decrease of the temperature equal to the maximum change allowed for ptb 25, there was a negative error in the calculation of the leak flow rate. in other words, ptb 25’s model calculates a negative leak (which is not possible) in the case studies in which 𝑇𝑓 = 𝑇𝑖 − 0.1 and there is not leak. this means that a reduction of temperature, even within the limits established in ptb 25, would contribute to conceal the presence of a leak in the control volume when using the mathematical model for leak tests presented in ptb 25. it is also important to notice that the magnitude of the negative leak flow rates is not negligible compared with the allowable leak flow rate. 3.2. increasing temperature in the case studies with increasing temperature, many values of 𝑄𝑙𝑙𝑙𝑙,𝑃𝑇𝑃25 exceed the maximum allowable leak rate, leading to wrong conclusions about the airtightness of the rig. the effect of every one of the four studied factors is analysed. results are presented using pairwise scatter plots, with 𝑄𝑙𝑙𝑙𝑙,𝑃𝑇𝑃25 in the ordinate in all cases, and lines showing maximum permissible leak flow rates, as described at the start of section 3. figure 3 presents the effect of the enclosed volume on the error of ptb 25’s leak test model. it is evident that an increase in the enclosed volume raises the error in ptb 25’s leak model, and that the effect of the former variable over the latter is very large. the enclosed volume, for the case of critical nozzle test rigs, is a property of the rig which depends mainly of the internal volume of the gas meters that are calibrated in the test rig (which is associated with the kind of gas meter, which in turn affects 𝑄𝑙𝑙𝑙𝑙,𝑚𝑙𝑚), and of the quantity of gas meters that can be calibrated simultaneously. in almost all of the studied cases, 𝑄𝑙𝑙𝑙𝑙,𝑃𝑇𝑃25 exceeded 𝑄𝑙𝑙𝑙𝑙,𝑚𝑙𝑚,𝑜𝑡ℎ𝑙𝑒. figure 2. calculated leak flow rates using ptb 25’s model in the simulated cases (leak flow rate is zero) with decreasing temperature in the process (𝑇𝑓 = 𝑇𝑖 − 0.1) versus enclosed volume. the lines show the maximum allowable leak flow rate (according to ptb 25) for calibration rigs for three kinds of meters (details in the text). figure 3. leak flow rates calculated using ptb 25’s model in cases with temperature increase (𝑇𝑓 = 𝑇𝑖 + 0.1), versus enclosed volume. the lines present the maximum allowable leak flow rate according to the meters that are calibrated in the test rig. acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 71 𝑄𝑙𝑙𝑙𝑙,𝑃𝑇𝑃25 was larger tan 𝑄𝑙𝑙𝑙𝑙,𝑚𝑙𝑚,𝐺10 for enclosed volumes higher than 0.1 m3, while for smaller enclosed volumes obtaining a correct or incorrect conclusion about the tightness depends on the time elapsed of the leak test. a similar reasoning can be applied to 𝑄𝑙𝑙𝑙𝑙,𝑚𝑙𝑚,𝐺16, although with a little more success regarding the correct conclusion of the leak test. the effect of the duration of the leak test on ptb 25’s error in the calculation of the leak flow rate is presented in figure 4. an increase in the test time causes a reduction in the error committed by ptb 25’s leak flow rate model, so an increase large enough of the test time would make it possible to guarantee that ptb 25’s leak test equation yields the correct conclusion. however, it is important to remember that the simulated cases conform to ptb 25’s restriction with respect to the maximum permissible change in temperature (0.1 k). in practice, a longer leak test makes it more difficult to maintain the desired stability in temperature, and additionally a larger duration of the test is undesirable for users of the calibration rigs. figure 5 illustrates the relationship between the calculated leak flow rate according to ptb 25 and the initial temperature of the leak test. figure 6 shows 𝑄𝑙𝑙𝑙𝑙,𝑃𝑇𝑃25 versus the initial pressure of the enclosed volume in the leak test. in these last two cases, the effect of the studied factors (𝑇𝑖 and 𝑃𝑖) on the error of ptb 25’s leak flow rate equation is negligible, so neither of these variables could be modified to lessen the error of the equation. 4. conclusions in the mathematical modelling of the leak test in ptb 25 it is assumed that the temperature is constant, but the same document allows a temperature change during the experimental run of the test. this causes that the equation used to determine the leak flow rate of the calibration rig has a significant error, in many situations so large that it conduces to erroneous conclusions about the tightness of the rig. the magnitude of the error committed by ptb 25’s leak test equation depends on the enclosed volume of the rig and the duration of the leak test. on the other hand, values of initial pressure and temperature do not affect significantly the error of ptb 25 equation. the correct modelling of the process of the leak test for sonic nozzle test rigs operating at low pressures leads to the equation (8) of this work. ptb 25 adds the assumption that the process is isothermal, and this leads to equation (10) of the present paper. a comparison of the equations allows to conclude that the difference in complexity is negligible. additionally, both models require the same measurements; although in equation (10) the initial and final temperatures of the test do not appear, they must also be measured in this case to determine compliance with the restriction on the maximum permissible change in temperature. for these reasons and considering the errors that can be made in estimating the leak flow rate using equation (10), it is possible to conclude that it is preferable to use equation (8) to determine the leak flow rate of a calibration rig in the leak test. references [1] g. wendt, h. dietrich, b. jarosch, r. joest, b. nath, f. frössl, m. ruwe, "ptb testing instructions, volume 25: gas meters – test rigs with critical nozzles", physikalisch-technische bundesanstalt, berlin, 2000, issn 0341-7964. [2] g. v. reklaitis. "introduction to material and energy balances", john wiley and sons, 1983, isbn 978-0-471-04131-3. figure 5. calculated leak flow rates using ptb 25’s model in the case studies with temperature increase (𝑇𝑓 = 𝑇𝑖 + 0.1), versus initial temperature in the leak test. the lines indicate the maximum allowable leak flow rate in three different cases. figure 4. calculated leak flow rates using ptb 25’s model in the cases with temperature increase (𝑇𝑓 = 𝑇𝑖 + 0.1), versus leak test duration. the lines indicate the maximum allowable leak flow rate in three different cases. figure 6. leak flow rates calculated using ptb 25’s model in leak tests with temperature increase (𝑇𝑓 = 𝑇𝑖 + 0.1), versus initial pressure of the control volume. the lines indicate the maximum allowable leak flow rate according to the meters that are calibrated in the test rig. << /ascii85encodepages false /allowtransparency false /autopositionepsfiles true /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) 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can be opened with acrobat and adobe reader 5.0 and later.) >> /namespace [ (adobe) (common) (1.0) ] /othernamespaces [ << /asreaderspreads false /cropimagestoframes true /errorcontrol /warnandcontinue /flattenerignorespreadoverrides false /includeguidesgrids false /includenonprinting false /includeslug false /namespace [ (adobe) (indesign) (4.0) ] /omitplacedbitmaps false /omitplacedeps false /omitplacedpdf false /simulateoverprint /legacy >> << /addbleedmarks false /addcolorbars false /addcropmarks false /addpageinfo false /addregmarks false /convertcolors /converttocmyk /destinationprofilename () /destinationprofileselector /documentcmyk /downsample16bitimages true /flattenerpreset << /presetselector /mediumresolution >> /formelements false /generatestructure false /includebookmarks false /includehyperlinks false /includeinteractive false /includelayers false /includeprofiles false /multimediahandling /useobjectsettings /namespace [ (adobe) (creativesuite) (2.0) ] /pdfxoutputintentprofileselector /documentcmyk /preserveediting true /untaggedcmykhandling /leaveuntagged /untaggedrgbhandling /usedocumentprofile /usedocumentbleed false >> ] >> setdistillerparams << /hwresolution [2400 2400] /pagesize [612.000 792.000] >> setpagedevice acta imeko, title acta imeko issn: 2221-870x june 2018, volume 7, number 2, 91-95 acta imeko | www.imeko.org june 2018 | volume 7 | number 2| 91 ethanol content determination in hard liquor drinks, beers, and wines, using a catalytic fuel cell. comparison with other two conventional enzymatic biosensors: correlation and statistical data m. tomassetti, r. angeloni, m. castrucci, e. martini, l. campanella department of chemistry, university of rome ”la sapienza”, rome, italy section: research paper keywords: analysis by catalytic fuel cell; ethanol in beverages and hard liquors; statistical data; correlation with other biosensors citation: m. tomassetti, r. angeloni, m. castrucci, e. martini, l. campanella, ethanol content determination in hard liquor drinks, beers, and wines, using a catalytic fuel cell. comparison with other two conventional enzymatic biosensors: correlation and statistical data, acta imeko, vol. 7, no. 2, article 16, june 2018, identifier: imeko-acta-07 (2018)-02-16 section editor: claudia zoani, italian national agency for new technologies, energy and sustainable economic development affiliation, rome, italy received february 1, 2017; in final form may 21, 2018; published june 2018 copyright: © 2018 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: this work was funded by university of rome “la sapienza”, center “protezione dell’ambiente e dei beni culturali (ciabc)” and “istituto per lo studio dei materiali nanostrutturati (ismn)” of cnr corresponding author: mauro tomassetti, e-mail: mauro.tomassetti@uniroma1.it 1. introduction our team recently investigated the feasibility of using an adapted small catalytic ‘fuel cell’ (see figure 1), originally constructed for the purpose of obtaining energy from methanol or ethanol, for analytical purposes [1], [2], based on the following reaction: c2h5oh + 3h2o 12h+ + 12 e+ 2co2. the goal was to see whether this kind of device can also be used for ethanol determination in real samples. actually, there is no lack of analytical methods for the determination of ethanol in alcoholic beverages, such as chromatographic [3]-[5], colorimetric [6], [7], nir [8], [9], ir [10] and titrimetric [11] methods. however, the possibility of using direct methanol (or ethanol) catalytic fuel cell (dmfc) for analytical purposes has not been sufficiently investigated, so this aspect has been studied by our research group in recent times [1], [2]. 2. methods to this end water-alcohol solutions containing increasing percentages of ethanol were added to the cell, recording the current increase that occurs between two electrodes of the cell, figure 1. dmfc device functioning layout. abstract a catalytic fuel cell has been employed to check ethanol content in several samples of commercial wines, beers and hard liquor drinks. two other conventional biosensors, based on catalase or alcohol oxidase enzyme, were also used to the same purpose on the same samples. data obtained by the three sensor methods have been compared and correlated. lastly t-test and f-test were applied. acta imeko | www.imeko.org june 2018 | volume 7 | number 2| 92 working under potentiostatic control. lastly, once the current achieved the steady state, after each hydroalcoholic addition, calibration curves for ethanol were obtained. owing to the good analytical results obtained operating on standard ethanol solutions, the fuel cell was then used to check the ethanol content in several wine, beer and hard liquor commercial drink samples. results obtained analyzing red and white wines or beers are also compared with those obtained using two conventional different enzyme electrodes that have been fabricated by immobilizing either alcohol oxidase, or catalase, in kappacarrageenan gel layer overlapped to an amperometric gaseous diffusion clark type oxygen electrode. the variation of the oxygen concentration in the aqueous solution, due to the enzymatic reactions, was measured at a constant applied potential of –650 mv. in the case of the catalase electrode [12], [13], the measurement was performed by adding hydrogen peroxide to a buffer solution which was diffused through the dialysis membrane towards the enzymatic layer where the reaction catalysed by the catalase enzyme took place: h2o2 1/2 o2 + h2o. since this reaction led to the production of oxygen, the concentration of the latter in the measurement solution increased. this increase triggered an increase in the cathodic current, which increased from the original value to a new value corresponding to a new stationary state. at this stage, further additions (equal to 20 µl) of a standard solution of ethanol were made; after each addition a catalyzed reaction of the following type occurred: ch3ch2oh + h2o2 ch3cho + 2 h2o. the second reaction removed part of the h2o2 substrate from the first reaction, which was slowed down; this slowdown was accompanied by a decrease in the level of oxygen produced in the solution during the first reaction; that was evidenced by the decrease in the measured cathodic current which attained a new stationary state after each addition of alcohol solution. the current variation was read off after each alcohol addition [12], [13]. when the biosensor used alcohol oxidase, the operating procedure was much simpler [12] as it consisted of directly making successive additions of the standard ethanol solution to a buffer solution in which the measurement was being performed. after each addition a reaction catalyzed by the alcohol oxidase enzyme of the following type took place: ch3ch2oh +o2 ch3cho +h2o2. the reaction led to the oxygen present in the solution being consumed with a consequent decrease in the cathodic current, which was measured until a new stationary state was reached. also, in this case, the current variation was read off after each addition. all the experiments were carried out in a reaction cell thermostated at 23°c containing 15 ml of 0.05 mol l-1 phosphate buffer solution. 3. results the effect of ph on the response of the conventional biosensors, investigated in detail, showed that the best ph was 7.5 for the catalase electrode and 8.0 for the alcohol oxidase electrode, respectively. the catalase biosensor displayed a much greater sensitivity to ethanol than, for instance, methanol, unlike the alcohol oxidase biosensor. the catalase biosensor also displayed a stability and a life-time higher than that of the alcohol oxidase biosensor, as well as a limit of detection (lod) at least one decade lower. it also displayed a better repeatability and reproducibility for ethanol standard solutions. on the other hand, the alcohol oxidase biosensor was found to be more sensitive to methanol than to ethanol, but allowed the test to be carried out slightly faster than with the catalase device. the method based on the catalytic fuel cell is very simple, inexpensive, and its lifetime is also very long, in practice longer than three months. the only drawback of the catalytic fuel cell was its long response time, but its lifetime was very long if compared to those of the two conventional enzyme electrodes. a comparison of the main analytical results [1] for the three used devices is done in the table 1, for the convenience of the reader. a comparison between the results obtained applying the catalytic fuel cell and two conventional amperometric enzymatic sensors was also performed by studying the correlation of results obtained by applying the three methods for ethanol content determination in several commercial wines and beers (see figures 2-4). table 1. table1. main analytical results. method linearity range(m) lod(m) life time (days) analysis time (min) catalase biosensor 2.0x10-6 – 2.0x10-5 0.4 x 10-6 ≈30 ≈34 alcohol oxidase biosensor 9.2x10-6 – 3.4x10-4 3.6 x 10-6 ≈7 ≈25 fuel cell (sc) potentiostatic format at oap 1.0x10-3 – 4.0x10-2 8.0 x 10-4 >90 ≈55 figure 2. correlation between results found by fuel cell and catalase biosensor. figure 3. correlation between results found by fuel cell and alcohol oxidase biosensor. acta imeko | www.imeko.org june 2018 | volume 7 | number 2| 93 it can be observed that the alcoholic content of the beer samples is about three times lower than that of wine samples. however, the correlations found between fuel sensor data and the data obtained using the two biosensor methods are sufficiently good and better than those observed between the two enzyme biosensor methods. in addition, the correlation of data from each method and the nominal values given by beverages producer firms (figures 5, 6 and 7) was also evaluated and found satisfactory. also in this case the best correlation was found when the fuel cell was used. lastly, also t-test was applied (table 2). moreover, a statistical evaluation of variance by f-test was performed. see table 3. as reported in figure 8, the comparison between the results figure 4. correlation between results found by catalase biosensor and alcohol oxidase biosensor. figure 7. correlation between nominal values and results found by alcohol oxidase biosensor. table 2. paired t-test, two sided, ν=5, (p=95%) applied to values obtained by three methods for beers and wines samples vs nominal values. by catalase biosensor by alcohol oxidase biosensor by fuel cell t-experimental -0.8881 1.421 0.8476 t-critical 2.571 2.571 2.571 |t-exp.| < t-cr. |t-exp.| < t-cr. |t-exp.| < t-cr. results of t-test not significant not significant not significant table3. f-test: comparison among precisions, two sided (p=95%). result of the f-test samples alcohol oxidase vs. catalase biosensor fuel cell vs. catalase biosensor fuel cell vs. alcohol oxidase biosensor b.n.1 n.s. s. s. b.n.2 n.s. n.s. n.s. b.n.3 n.s. s. s. w.n.1 n.s. n.s. n.s. w.n.2 n.s. n.s. n.s. w.n.3 n.s. n.s. n.s. w.n.4 n.s. n.s. n.s. n.s.= not significant; s.= significant; b.= beer; w.= wine figure 5. correlation between nominal values and results found by fuel cell. figure 6. correlation between nominal values and results found by catalase biosensor. figure 8. agreement between nominal values and results found by fuel cell for the hard liquor drinks. acta imeko | www.imeko.org june 2018 | volume 7 | number 2| 94 concerning the ethanol content, determined in several hard liquor drinks using the catalytic fuel cell, and the nominal values given by producers exhibits a good agreement. lastly, in table 4 all the found results concerning the ethanol content in hard liquor drinks and the applied t-test are displayed. 4. conclusions as previously mentioned in the introduction, there is no lack of classic analytical methods, including biosensoristic methods (see for instance ref. [14]), for ethanol determination in alcoholic beverages. however, the main objective of this research was to test a method based on a dmfc device, used so far especially for energy production purposes and to propose this alternative device also for analytical purposes. the advantages can be identified above all in the extremely low cost of this fuel cell based sensor, with respect to both other analytical instrumental methods [3]-[10] and conventional enzymatic biosensors for ethanol [10]-[16]. in comparison with conventional biosensors, it is sufficient to pay attention to the long lifetime of our proposed device, which does not require special precautions to be used for long periods. for completeness of the research, the main performances of this device were analytically compared with those of two conventional enzyme biosensors, even if based on different enzymatic reactions. as previously mentioned, this kind of biosensors is already extensively reported in the literature [14][16]. so those two, developed and used by us (based for instance on a different type of enzyme, but on the same enzyme immobilization method) and well tested in our previous works (see ref. [12] and [13]), showed on “average” a performance not very different from other conventional enzymatic biosensors of the same type. the results demonstrated a good correlation between the three different methods as well as with the nominal values. the f-test evidenced that the repeatability of the three methods is usually of the same order. the analysis of the obtained data highlights that the catalytic fuel cell may be considered a suitable analytical device to check ethanol content in commercial beverages and hard liquor drinks at low cost and in a simple way. the comparison with two other conventional enzymatic biosensor methods confirms this assertion. it is also interesting, for instance, to observe that the best correlation between experimentally determined and nominal values (see r2) was found only in the case of the catalytic fuel cell device. acknowledgement this work was funded by university of rome “la sapienza”, center “protezione dell’ambiente e dei beni culturali (ciabc)” and “istituto per lo studio dei materiali nanostrutturati (ismn)” of cnr. references [1] m. tomassetti, r. angeloni, g. merola, m. castrucci, l. campanella, catalytic fuel cell used as an analytical tool for methanol and ethanol determination. application to ethanol determination in alcoholic beverages, electrochim. acta 191 (2016) pp. 1001–1009. [2] m. tomassetti, g. merola, r. angeloni, s. marchiandi, l. campanella, further development on dmfc device used for analytical purpose: real applications in the pharmaceutical field and possible in biological fluids, anal. bioanal. chem. (in press). [3] m.l. wang, y.m. choong, n.w. su, m.h. lee, a rapid method for determination of ethanol in alcoholic beverages using capillary gas chromatography, j. food drug anal. 11(2) (2003), pp. 133-140 [4] d. naviglio, r. ramano, g. attanasio, rapid determination of ethanol content in spirits and in beer by high resolution gas chromatography, industrie delle bevande 30(172) (2001), pp. 113-115. [5] s.k. brill, m.s. wagner, alcohol determination in beverages using polar capillary gas chromatography-mass spectroscopy and an acetonitrile internal standard, concordia college j. anal. chem., 3 (2012), pp. 6-12. [6] s.v. sumbhate, s. nayak, d. goupale, a. tiwari, r.s. jadon, colorimetric method for the estimation of ethanol in alcoholicdrinks, j. anal. techn.,1(1) (2012), pp. 1-6. [7] p. pinyoua, n. youngvises, j. jakmuneea, flow injection colorimetric method using acidic ceric nitrate as reagent for determination of ethanol, talanta 84 (2011), pp. 745-751. [8] p. tipparat, s. lapanantnoppakhun, j. jakmunee, k. grudpan, determination of ethanol in liquor by near-infrared spectrophotometry with flow injection, talanta 53 (2001), pp. 1199-1204. [9] d.w. lachenmeier, r. godelmann, m. steiner, b. ansay, j. table 4. potentiostatic measurements of ethanol in hard alcoholic drinks using the catalytic dmfc device. sample type nominal value as (v/v%) nominal value as (m) (a) found ethanol (m) mean (m) (b) sd (m) δ%= [(b-a)/a]% two sided t-test: p=95%, ν=3-1=2 run n. i run n. ii run n. iii texper. tcritic results of t-test myrtle liquor 32% 0.0549 0.0507 0.0574 0.0561 0.0547 ±0.0036 -0.30 -0.080 4.303 n.s. berries grappa 30% 0.0515 0.0556 0.0577 0.0512 0.0548 ±0.0033 +6.55 1.768 4.303 n.s. bitters 27% 0.0463 0.0433 0.0494 0.0412 0.0446 ±0.0042 -3.67 -0.694 4.303 n.s. rum 37.5% 0.0322 0.0319 0.03280 0.0333 0.0326 ±0.0007 +1.52 1.208 4.303 n.s. whisky 40% 0.0343 0.0308 0.0300 0.0340 0.0316 ±0.0021 -7.92 -2.247 4.303 n.s. dry gin 40% 0.0343 0.0338 0.0359 0.0332 0.0343 ±0.0014 +0.02 0.009 4.303 n.s. old brandy 38% 0.0326 0.0384 0.0354 0.0322 0.0353 ±0.0031 +8.41 1.536 4.303 n.s. acta imeko | www.imeko.org june 2018 | volume 7 | number 2| 95 weigel, g. krieg, rapid and mobile determination of alcoholic strength in wine, beer and spirits using a flow-through infrared sensor, chem. cent. j. 4(5) (2010), pp. 1-10. [10] d.w. lachenmeier, rapid quality control of spirit drinks and beer using multivariate data analysis of fourier transform infrared spectra, food chem. 101 (2007), pp. 825-832 [11] outreach college of science university of canterbury private bag 4800 christchurch new zealand: determination of ethanol by redox titration with potassium dichromate. www.outreach.canterbury.ac.nz (accessed march 12, 2014) [12] r. angeloni, m. tomassetti, m. castrucci, l. campanella, ethanol determination in alcoholic beverages using two different amperometric enzyme sensors, cur. anal. chem. 11 (2015) pp. 56-67. [13] l. campanella, g. spuri capesciotti, t. gatta, m. tomassetti, an innovative organic phase enzyme electrode (opee) for the determination of ethanol in leadless petrols, sens. and actuators b 147 (2010) pp. 78-86. [14] t.b. goriushkina, a.p. soldatkin, s.v. dzyadevych, application of amperometric enzyme biosensors for wine and must analysis, procedia chemistry 1 (2009), pp. 277-280. [15] y. chen, k. y. chen, a.c.c. tseung, an electrochemical alcohol sensor based on a co-electrodeposited pt |wo3 electrode, j. electroanal. chem. 471 (1999), pp. 151–155. [16] c. calas-blanchard, m. cortina-puig, l. barthelmebs, t. noguer, electrochemical biosensors for the determination of the antioxidant capacity in foods and beverages based on reactive oxygen species, curr. anal. chem. 8(4) (2012), pp. 428-435. ethanol content determination in hard liquor drinks, beers, and wines, using a catalytic fuel cell. comparison with other two conventional enzymatic biosensors: correlation and statistical data acta imeko  september 2014, volume 3, number 3, 43 – 49  www.imeko.org    acta imeko | www.imeko.org  september 2014 | volume 3 | number 3 | 43  multi tapped delay line time‐interval measurement system  implemented in a programmable structure  marek zieliński, dariusz chaberski, maciej gurski, marcin kowalski  institute of physics, nicolaus copernicus university, grudziądzka 5/7, 87‐100 toruń, poland  section: research paper   keywords: time‐interval measurement, measurement systems, fpga applications, qnm method.  citation: marek zieliński, dariusz chaberski, maciej gurski, marcin kowalski, multi tapped delay line time interval measurement system implemented in  programmable structure, acta imeko, vol. 3, no. 3, article 10, september 2014, identifier: imeko‐acta‐03 (2014)‐03‐10  editor: paolo carbone, university of perugia   received february 4 th , 2013; in final form april 22 th , 2014; published september 2014  copyright: © 2014 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: this work was supported by the polish national science centre (ncn ) grant no n n505 484540  corresponding author: marek zieliński, e‐mail: marziel@fizyka.umk.pl    1.  introduction  during the last several years many time-interval measurement methods and systems were described and discussed in different scientific journals and conference proceedings [1 4]. usually, high resolution tims (timeinterval measurement system) should be characterized by a resolution better than 1 ns, and fortunately, can be realized by modern fpga (field programmable gate array) devices [5-6]. high resolution tims are widely applied. examples of tims with single-stage interpolation implemented in cmos fpga devices are systems for life-time of the excited atomic states measurement, systems for clock characterization, systems for quantum cryptography experiments, systems for ultrasonic flow-meters or monitoring systems of time-of-flight mass spectrometer [7, 8]. of course, it should be noticed that systems implemented in fpga devices require relatively much energy. limiting the total time of measurements by parallelization leads to shortening of the duty cycle and limits the total power consumption [4]. the measurement system presented in this paper enables the increase of the system resolution that leads to higher precision of the single time-interval measurement. 2.  principle of time‐interval measurement   the main task of the measuring system is the registration and collection of time-stamps corresponding to incoming pulses, which appeared at the “start” and “stop” inputs. the measured time-interval is calculated by using a pair of time-stamps. the system can work in simple start / stop mode figure 1. principle of time‐interval measurement.   abstract   this paper describes a time‐interval measurement system with  increased resolution using multiple taped delay  lines.  in this time‐ interval measurement system, sixteen time‐stamps are registered during a single measuring cycle (one shot). it means that the value  of the measured time‐interval can be  interpolated with higher resolution without  increasing the number of measurements or the  interpolation time. limiting the total measurement time reduces the energy consumption which is particularly important in battery  powered systems.   acta imeko | www.imeko.org  september 2014 | volume 3 | number 3 | 44  or in multi-stop mode. most of the high resolution tims use the measurement method shown in   |  49 [1, 4]. in this method the measured time-interval is divided into three parts, which are separately measured: 1) δtp – the time interval between rising edges of the “stop” pulse and the nearest period of the clock signal, 2) δtk – the time interval between rising edges of the “start” pulse and the nearest clock period, 3) δtn – the integer number n of standard clock periods (δtn = nt0) which appeared between “start” and “stop” pulses. the result of time-interval measurement will be given as: 0m n p k p kt t t t t t nt            (1) usually, integer number of clock cycles is counted by two counters of which the first one is incremented by the clock rising edge and the second one is incremented by the clock falling edge. in some cases the number of standard clock period counters can be increased to obtain correct information about δtn for each tapped delay line (tdl). precision of tims in practice depends on the resolution of interpolators (type of interpolator and its implementation technology), that measure residual time intervals δtp, δtk and standard clock stability (accumulated jitter). in case of small ranges of measured time-intervals (10 ns – 10 μs), for example: measurement of small flow or time-of-flight in mass spectrometry, precision does not depend on accumulated jitter [4]. if tdl is used in the design of the interpolator then the value of the single segment delay τ and its deviation (nonlinearity) determine the precision of the time-interval δtm measurement. if the interpolator consists of n tdls, then during the single measurement cycle it is possible to obtain n different results of the δtm measurement. such a solution leads straight to an increase of the time-interval measurement resolution. the knowledge of tdls characteristics such as dnl (differential nonlinearity) and inl (integral nonlinearity) and use of the quantization-and-nonlinearity-minimization (qnm) method allow to obtain a two-sample (“start” and “stop”) difference histogram with higher resolution [9]. 3. architecture of the tims   tims as a virtual instrument consists of a hardware unit, flexible software and a computer. the block diagram of the new tims architecture is shown in figure 2. the system consists of a group of sixteen tdls (sixty-four delay elements each) with independent registers, code converter, four clock cycles counters, block of memory, interface and control unit. such tims can be implemented in a single spartan or virtex fpga device. in case the incoming pulses have different amplitudes, two constant fraction discriminators should be applied accordingly to the both “start” and “stop” inputs [10]. application of the constant fraction discriminators decreases the pulse position error and increases the precision of the time-interval measurement. multiplying of the clock cycles counter ensures sufficient set-up time during incrementation and the data can be asynchronously read-out from one of the counters and can extend the information about the measurand. the code converter enables data conversion from the pseudo-thermometric code to the natural binary code. in the presented measurement system a single code converter is applied. such solution significantly decreases fpga resources used for implementation, decreases power consumption and increases slightly the total conversion time. figure 2. the block diagram of tims.  figure 3. tims implementation in virtex‐4 device.     figure 4. single delay‐line element.  acta imeko | www.imeko.org  september 2014 | volume 3 | number 3 | 45  each measurand consists of a main part, which is taken from one of the clock cycles counters and a residual part, which is taken from one of the delay-line registers through the data converter. carry chains of clbs (configurable logical block) are used to tdls and register implementation as shown in figure 4. when using the carry chains for the delay elements implementation, an average resolution for single tdl of approximately 50 ps can be obtained. each tdl is placed in a single fpga column of the clb. selection of appropriate delay elements is forced by the ucf (user constrains file). calibration of the delay-line elements is possible but not easy. firstly, the place of the delay-line can be changed by the choice of the proper clb column. secondly, in each clb the proper one of two slices can also be chosen. delay of individual elements can be increased by increasing the input capacity, which is possible by connecting additional elements in the fpga structure. 4. multi‐tapped delay lines unit  even if the calibration process of the tdls runs very precisely which is not always possible, the characteristics of the tdls will be different. it means that the time-bins in different lines have a different width. the widths of bins for sixteen tdls obtained using the statistical method [5, 6] are shown in figure 5. however the characteristics of the lines are not identical, the results of the time-interval measurements have a gaussian statistic, as is shown in figure 6. assuming that residual time-intervals δtp, δtk are interpolated by sixteen independent delay lines and the obtained measurements are not correlated it is possible to improve the system uncertainty by a factor of four. in practice, the interpolators are correlated and further improvement of the measurement uncertainty is possible because of the direct increase in resolution [9]. 5. time‐interval histogram calculation  the simplest and the most effective way of time-interval histogram calculation when the multi-tapped-delay-line (mtdl) is being applied is the qnm method [9]. when the tims consists of n tdls then for each time-interval, the time-interval histogram is updated n times. when all tdl characteristics are uncorrelated, the time-interval histogram can be calculated even with n times greater resolution (smaller binsize). for each time-interval, two time-stamps are being generated by each tdl. the contribution that is added to the timeinterval histogram equals the modified convolution of the timestamp probability-density-functions (pdf). the argument of the first function has been changed in sign [9]. let the i -th time-stamp be registered in the ikp , -th quantization step, the j -th time-stamp be registered in the jkp , -th quantization step and  tnk , be the pdf of the n -th (all tdl considered) quantization step (figure 7). in this case, the value that should be added to the time-interval histogram in the range  21, tt for the k -th tdl equals       2 1 1 2 k,p k,pk,i k, j t k,i, j e e t ρ t , t σ σ dτ    . (2) when the measurement module consists of n tdls then for each time-interval, the time-interval histogram in the range figure 5. comparison of the line delay characteristics.  figure 6. the time‐intervals histogram.  figure 7. two exemplary convolutions obtained as the result of time interval  measurement  between  the  i ‐th  and  j ‐th  pulses  registered  by  the  k ‐th  and  l ‐th  tapped  delay  lines;  the  quantization  steps  a)  differ  in  width  ( jkik pkpk ee ,, ,,  ), b) equal to each other ( jlil plpl ee ,, ,,  ).  acta imeko | www.imeko.org  september 2014 | volume 3 | number 3 | 46   21, tt is updated by the sum of all contributions, so     1 , 1 2 , , 1 2 0 , , n i j k i j k t t t t      . (3) figure 7 shows the calculation of contributions generated by the k -th and l -th tdls. the two quantization steps in the k th tdl into which the time-stamp has been registered are different in width ( jkik pkpk ee ,, ,,  ), so the pdf of this contribution is trapezoidal (fig. 6a). the contribution that is generated by the l -th tdl is isometric triangular (figure 7b) because the widths of two time-stamps, into which the timeinterval has been registered in this tdl, are equal ( jlil plpl ee ,, ,,  ). the contributions introduced by these two tdls differ because of tdl characteristics lack correlation. the area of the rising pattern represents the contribution that is added by the k -th tdl in the range  21, tt to the resultant time-interval histogram (figure 7a) and analogically the area of the falling pattern is the contribution that is added by the l -th tdl. application of the qnm method allows to obtain a timeinterval histogram where the position of the time peak is determined with greater certainty (figure 8b) than in the timeinterval histogram shown in figure 6. figure 8a shows timeinterval histograms obtained from four randomly chosen tdls. the time unit was chosen to be equal to average the value of the quantization step, and the average value of the peak position equals to 0 because the information about the number of periods was removed. 6. time‐interval measurement  a series of time-interval measurements was performed to verify the new measurement system. for the test, a single section of a coaxial cable was used as a delay element as shown in figure 9. the results of time-interval measurements obtained during the test are shown in figure 10. certainty of time-interval determination increases when the number of tdls increases. uncertainty of time-interval determination during a single measuring cycle is about 50 ps when the tims contains only a single tdl but significantly decreases to several picoseconds when the tims contains sixteen tdls. 7. error analysis  the time-to-digital converter, in the system described above, consists of sixteen independent tapped delay lines. the delay of a single segment for each line can be calculated by a statistical method [1]. if we mark the total number of measurements made in a series by n, the number of data included in the i-th segment by ni,j and the j-th line, then the delay of a single segment for the jth line can be written as: a) b) figure 8. time‐interval histogram after qnm correction; a) contribution  histograms obtained separately from four randomly chosen tdls, b) the  resultant time‐interval histogram.  figure 9. verification of the measurement system.  figure 10. probability density functions of measured time‐intervals versus  the number of the tdl.  acta imeko | www.imeko.org  september 2014 | volume 3 | number 3 | 47      , , 0 , 0, 1, ..., 1 , 0, 1, ..., 1 i j i j n q t i m n j k      (4) where t0 is the system clock period, m is the number of time channels, and k is the number of tapped delay lines. if all segment delays (within a single tapped delay line) are summed up, and divided by the number of tdl segments, then it is possible to calculate the average segment delay: 1 , 0 1 m i jj i q q m     . (5) by subtracting the width of the i-th segment from the average value, the differential nonlinearity can be calculated as: , ,i j i j jdnl q q  . (6) summing up the particular deviation from the average segment width, the integral nonlinearity errors are given (for each line) by: 1 , , 0 m i j i j i inl dnl     . (7) the integral nonlinearity determines how large an error during the measurement of time-interval, using this module, will be committed. for a single delay line, a time-interval measure is described by equation [11]:   0j j j jt n m q nt     , (8) where mj and nj are the delay segment numbers, for “start” and “stop” pulses respectively, n the total number of clock cycles and t0 the standard clock period. the integral nonlinearity error is associated with each delay segment. therefore the uncertainty, associated with the timeto-digital converter characteristic in a single time-interval measurement can be described by:    ,2 max , 0, , 1jt n jinl n m    . (9) because of the finite size of the quantization step to obtain the total uncertainty, the quantization error must be taken into account. assuming that the constant time-interval t is measured, the two time intervals tnt  11 and tnt  12 are obtained, where  is a time-interval measurement module segment delay (resolution). introducing a new factor 1 /c n n and defining it as the probability that the measured time-interval t will have a length 1t , and defining  2tq as the probability that the measured time-interval t will have a length 2t , the following relation applies [1]:  1p t c , (10)  2 1q t c  . (11) the time interval can be described by:    1 0 2 0t p t t q t t     . (12) the standard deviation associated with quantization effect is given by:    1 2q p t q t   . (13) in this way, the maximal uncertainty associated with the quantization effect is defined as: 2 q    . (14) labview environment was used to prepare the simulation, and results are shown in figure 11. the total uncertainty for the j-th tapped delay line is described by the relation: 2 2 j j jall t q     . (15) when the single time-interval is measured by sixteen tapped delay lines, sixteen different time-intervals are registered as a result of this measurement. each of them has different uncertainty jall  . because each time interval is measured with different uncertainty, the average time interval measured by sixteen tapped delay lines is described by equation: 2 0 2 0 1 j j k j j all k j all t t          . (16) in fact, it is a weighted average value with weights: 2 1 j j all w   . (17) two estimators of variance can be calculated, the internal variance: 2 int 2 0 1 1 j k j all     (18) and the external variance: figure 11. quantization error as a function of channel width.  acta imeko | www.imeko.org  september 2014 | volume 3 | number 3 | 48  2 2 2 int 0 j k j ext j all t t k               . (19) internal and external variances, during the experimental process, can be different. when this occurs, then the larger of them is chosen. the total uncertainty for the j-th tapped delay line is described by the relation:  2 2 2intmax , .extt    (20) the uncertainty of a single time interval measurement was verified using a real time interval measurement system. results of these investigations are shown in table 1. 8. experimental results  this section contains measurement module characteristics, time-stamp histograms for “start” and “stop” and timeinterval histograms of exemplary time-interval measurements. figure 12 shows dnls of four chosen tdls. it is clearly visible that although the character of all dnls is similar they are not fully correlated, so each shot appearance time can be determined more precisely than the average resolution of a single tdl. the lack of this correlation can also be confirmed by inl characteristics (figure 14). it is not significant and it is not fault that all chosen inl characteristics are positive, actually almost all other inls are positive, too. the varying components differ enough to be treated as random errors which contribute to average time interval error reduction. figure 13 can be helpful in explanation of the inls character. one can observe that first delay segment widths are slightly greater than the average value of all segment widths and that additionally average value is being lowered by the last segment widths. all tdls inls (figure 14) for the last segment approach zero – the inl error is being reset periodically, so the inl error has only a local (standard-clock period) range. it does not matter that almost all time-stamps are overstated. the time-intervals are being calculated as the differences of two consecutive time-stamps, so these overstatements are reduced and do not play any role. figure 15 and figure 16 show time-stamp histograms obtained for “start” and “stop” pulses. it should be noticed that the area of both diagrams is equal to the number of tdls. figure 17 shows the time-interval histogram obtained as a difference between “start” and “stop” measurements. it is worth being noticed that the uncertainty of the timetable  1.  a  single  time  interval  measurement  uncertainty  using  a  single  jall   and sixteen delay lines t .  j  psjall j  psjall  pst 0 174.1 8 109.0 30.9 1 149.1 9 152.1 2 137.2 10 129.6 3 143.2 11 100.5 4 107.9 12 117.5 5 109.8 13 101.9 6 130.7 14 104.0 7 128.1 15 175.8 figure 12. differential non‐linearity diagrams obtained for the 0th, 1st, 2nd  and the last tapped‐delay‐line.    figure 13. channel widths obtained for the 0 th , 1 st , 2 nd  and the last tapped‐ delay‐line.  figure 14.  integral nonlinearity diagrams  obtained for the 0 th ,  1 st ,  2 nd  and  the last tapped‐delay‐line.  acta imeko | www.imeko.org  september 2014 | volume 3 | number 3 | 49  interval did not increase significantly in comparison to source time-stamp uncertainties. this advantage is achieved mainly by inl reduction, mentioned earlier in this section, while the time-interval is being calculated. 9. summary and conclusions  implementation of the time-interval measuring module in the fpga devices allows to greatly increase the scale of system integration. moreover, using the multi-tap delay lines technique enables to significantly increase the system resolution. application of the qnm method and implementation of the tapped delay lines using the carry path may result in a system resolution of picoseconds. such a resolution can significantly improve the quality of time-of-flight mass spectrometers, ultrasonic flow-meters and life-time measuring systems. we also want to point out that application of the programmable logical devices, especially the fpga devices, increases the flexibility and reliability of the measuring system. acknowledgments  this work was supported by the polish national science centre (ncn ) grant no n n505 484540. references  [1] j. kalisz, “review of methods for time interval measurements with picosecond resolution”, metrologia, no. 41, pp 17-32, 2004. [2] w. zhou, x. ou, h. zhou, b. wang, x. yang, “a time interval measurement technique based on time – space relationship processing”, ieee int. freq. contr. symp. and exposition, pp. 260-266, june 2006. [3] j. song, q. an, and s. liu, “a high-resolution time-to-digital converter implemented in field-programmable-gate-arrays”, ieee trans. on nucl. sci., vol. 53, no. 1, pp. 236-241, feb. 2006. [4] m. zieliński, “review of single-stage time-interval measurement modules implemented in fpga devices”, metrology and measurement systems, vol xvi, no.4, pp 641-647, 2010. [5] jinhong wang, shubin liu, qi shen, hao li, qi an, “a fully fledged tdc implemented in field-programmable gate arrays", ieee transactions on nuclear science, vol.57, no.2, pp.4460-450, april 2010. [6] m. zieliński, d. chaberski, m. kowalski, r. frankowski and s. grzelak, „high-resolution time-interval measuring system implemented in single fpga device”, measurement, vol 35, no. 3, pp. 311 – 317, 2004. [7] m. daigneault, j.p. david, "a novel 10 ps resolution tdc architecture implemented in a 130nm process fpga", 8th ieee international newcas conference (newcas), pp.281,284, 2023 june 2010. [8] w. wasilewski, p. kolenderski, r. frankowski, “spectral density matrix of a single photon measured”, phys. rev. lett. vol. 99, no. 12, p. 123601, 2007. [9] m. zieliński, d. chaberski, s. grzelak, „the new method of calculation sum and difference histogram for quantized data”, measurement vol. 42, no. 9, pp. 1388 – 1394, 2009. [10] m. zieliński, d. chaberski, s. grzelak, r. frankowski and m. kowalski, “high-resolution real-time multichannel scaler implemented in single fpga device”, xvii imeko world congress, dubrovnik, croatia 2003. figure 15. time‐stamp histogram obtained for “start” pulse.  figure 16. time‐stamp histogram obtained for “stop” pulse.  figure 17. time‐interval histogram.  acta imeko, title acta imeko issn: 2221-870x september 2017, volume 6, number 3, 71-75 acta imeko | www.imeko.org september 2017 | volume 6 | number 3 | 71 the x-ray diffraction beamline mcx at elettra: a case study of non-destructive analysis on stained glass jasper r. plaisier1, luca nodari2, lara gigli1, elena paz rebollo san miguel2, renzo bertoncello3, andrea lausi1 1elettra-sincrotrone trieste s.c.p.a., strada statale 14 km 163,5 in area science park, 34149 basovizza, trieste, italy 2istituto di chimica della materia condensata e di tecnologie per l’energia cnr– icmate, area della ricerca di padova, corso stati uniti 4, 35127 padova, italy 3dipartimento di scienze chimiche, università degli studi di padova, via marzolo 1, 35131 padova, italy section: research paper keywords: synchrotron radiation; x-ray diffraction; grisaille; stained glass citation: jasper r. plaisier, luca nodari, lara gigli, elena paz rebollo san miguel, renzo bertoncello, andrea lausi, the x-ray diffraction beamline mcx at elettra: a case study of non-destructive analysis on stained glass, acta imeko, vol. 6, no. 3, article 11, september 2017, identifier: imeko-acta-06 (2017)-0311 section editor: sabrina grassini, politecnico di torino, italy received march 15, 2017; in final form july 29, 2017; published september 2017 copyright: © 2017 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: jasper r. plaisier, e-mail: jasper.plaisier@elettra.eu 1. introduction the beamline material characterization by x-ray diffraction (mcx) is the general purpose powder diffraction beamline at the elettra synchrotron source in trieste, italy, one of currently four diffraction beamlines at elettra [1]. the beamline is designed to host a wide range of experiments, which cover many scientific fields with standard applications such as phase identification and structure determination using the rietveld method or microstructure determination by line profile analysis. examples can be found in references [2]-[5]. also in situ diffraction experiments designed for following reaction mechanisms or kinetics can be performed (see e.g. [6], [7]). one of the strengths of mcx is the well-defined narrow instrumental profile, which allows accurate identification of phases in complex mixtures, which is often the case for objects or fragments that are being studied in the field of cultural heritage. the large space that is available at the sample position makes it possible to host a range of custom sample holders to study a wide variety of such objects. here, a non-destructive study performed at the mcx beamline on stained grisaille glass is presented. since the end of the xix century the study of glass corrosion and its weathering products was considered of great interest for cultural heritage science. in the last decades the interest of the scientific community was mainly focused on the characterization of glass corrosion products, their formation processes, and on the development of new materials and strategies for the protection of the glass surface. an interesting abstract the mcx beamline at the synchrotron elettra is the general purpose diffraction beamline that is well suited for non-destructive and innovative x-ray diffraction (xrd) experiments in the field of cultural heritage. the experimental station houses a large number of instruments facilitating a range of different types of analysis. recently, a comprehensive study of the alteration products in grisaille paints was performed at the beamline. this type of analysis is very important to understand the complex processes involved in the deterioration of this type of glass decoration. an exhaustive characterization of these products and so a full understanding of the mechanism of their formation may lead to the development of new protective materials for conservation and restoration. xrd experiments at the mcx beamline allowed us to recognize the alteration products on the grisailles surface and to propose a mechanism for the formation of alteration patina. here we present the beamline, its instrumentation and its capabilities by showing an example of the study on grisaille paints. acta imeko | www.imeko.org september 2017 | volume 6 | number 3 | 72 and breaking research field, still nowadays almost unexplored, regards the investigations on stained glass windows alteration, especially on those painted using the grisaille method. grisaille can be defined as a painting mixture formed by a finely powdered pigment (mainly transition metal oxides) and glass with a low melting point (lead rich glass) forming a coloured layer applied on the glass surface. the adherence to the glass is being guaranteed by a low temperature firing of the grisaille without melting the glass support. from a chemicalphysical point of view, the obtained material is a system in which the crystalline phases (the pigments) are embedded in an amorphous phase (the low melting glass) which acts both as a dispersing medium and as an agent fixing the pigments onto the substrate (the stained glass window). the interaction of this surface with the environment (i.e. the effect of pollutant agents) promotes the formation of alteration crusts involving both the grisaille paint layer and the glass window surface. the basilica di san giovanni e paolo, located in venice, italy, has been decorated with windows painted with the grisaille method. the construction of the edifice had been completed around 1430, but the stained glass windows have been placed at the end of the fifteenth century. the long period of exposure to the environment, especially an aggressive environment in the immediate vicinity of seawater, makes these windows excellent case studies for investigating the effect of the environment over a prolonged period of time on these windows. in this study three fragments taken from the large windows of the basilica were analyzed at mcx. the present paper presents the experimental setup and developments of the mcx beamline and illustrates its capabilities by the characterization of the alteration products of ancient grisaille paint layers using non-destructive synchrotron x-ray diffraction. in section 2 we will discuss the setup of the beam line and the possibilities at the experimental station are being outlined. section 3 gives the experimental details of the study performed on three glass fragments painted by the grisaille method. in section 4 the results of this study are being presented and discussed in detail. 2. the beamline 2.1. the source the storage ring of elettra sincrotrone trieste operates at two different energies: 2.0 gev with a ring current of 310 ma and 2.4 gev with a current of 160 ma. mcx is installed on a bending magnet x-ray source. when the ring is operating at an energy of 2 gev its critical energy is 3.2 kev, at e = 2.4 gev the critical energy is 5.5 kev. the source provides a broad energy spectrum with usable photons of energies as high as 25 kev 2.2. the optics the optical setup of the beamline consists of two mirrors and a monochromator. the first cylindrical mirror is pt coated and collimates the beam on the monochromator. the second optical element is a fixed exit monochromator equipped with two si(111) crystals. the second crystal is mounted on a bending mechanism for focusing in the sagittal plane. the second pt-coated mirror is placed downstream the monochromator and can be used flat or can be bent, with a radius of 6 km for focusing in the longitudinal direction. the overall optical layout works in a strictly 1:1 configuration, with the monochromator at 18 m from the source and the focus (sample position), and the mirrors are positioned symmetrically around it. the optical setup of the beamline produces an x-ray beam with energy between 6 and 21 kev corresponding to a wavelength between 2.0 and 0.6 å. the beam spot at the experiment can be varied from point focus (0.3 × 0.3 mm2), to line focus (5 × 1 mm2). the flux at the sample is ~1011 photons per second. the characteristics of the beamline are summarized in table 1. the effect of the optical elements on the diffraction line profile is discussed in detail in [8]. 2.3. the experimental station the experimental setup is based on a 4-circle huber diffractometer, equipped with a high-count rate fast scintillator detector. the 2θ arm can be equipped with a pair of slits or an analyser crystal for improved angular resolution. in a standard diffraction measurement, data is collected in flat plate (reflection mode) or capillary mode (transmission mode). for the latter mode a cryo-stream or a hot air blower may be installed so the samples may be cooled to 100 k or heated to 1273 k. as an alternative, a multi-channel analyser can be installed. this can be used to eliminate the background signal resulting from the fluorescence of the sample in a diffraction experiment. it also allows measuring x-ray fluorescence from the illuminated spot on the sample. this way chemical element analysis may be performed on the same spot where the diffraction is being measured. for measurements in reflection mode (often the case for objects rather than powders as for example. in cultural heritage research) a laser interferometer is available for accurate sample positioning. the large space at the sample position in combination with the sample positioning system allows measurements of large objects such as shown in figure 1. as an alternative detection setup a marccd camera can be installed on the 2θ arm, which can be used both for measurements in transmission mode as well as in reflection mode. as this allows measuring of a large range of the complete diffraction pattern at once, diffraction mapping of objects can be performed in relatively short times. it can also be employed to follow dynamical processes as the exposure time is reduced to 10-60 seconds. an independent experimental setup consists in a furnace that allows to measure powders under controlled conditions to table 1 characteristics of the beamline mcx with the storage ring of elettra working at 2.0 gev. the corresponding values for 2.4 gev are given in brackets. the source x-rays at the sample critical energy 3.2 kev (5.5 kev) energy range 6-21 kev width 0.139 mm ( 0.197 mm) photon flux 10 11 ph/s height 0.028 mm (0.030 mm) vertical spot size 0.3-1 mm vertical divergence 0.009 mrad (0.009 mrad) horizontal spot size 0.3-5 mm horizontal divergence 2.0 mrad acta imeko | www.imeko.org september 2017 | volume 6 | number 3 | 73 temperatures up to 1273 k. a special capillary holder has been developed to allow gases to flow through the sample while heating. a translating image plate makes it possible to follow the changes in the diffraction pattern during the heating process. this setup, described in detail in [9], can be used for instance to simulate aging processes in pigments by heating the materials under investigation under a well-defined gas flow in order to speed up the degradation processes that normally occur over a much larger time span. the beamline is completely controlled by a computer program with a user friendly graphical interface based on the cyclops suite [10]. the program is written in python using pyqt for constructing the gui. new equipment can be easily integrated through its plugin architecture. 3. experimental three glass fragments from the basilica di san giovanni e paolo in venice were selected to study the alteration processes. the original location in the large windows is shown in figure 2. figure 3 shows the fragments, ssgp1, ssgp2 and ssgp3. in table 2 the colours characteristic of the fragments are listed for the glass, the grisaille and the patina. the glass fragments were measured in grazing incidence geometry. the incidence angle was kept fixed during the measurement at ~2°. the x-ray wavelength was set to 1.319 å. diffraction patterns were collected in the 5-60° 2θ range, with a step size of 0.01°. the exposure time was one second per point. the samples were aligned carefully to measure the characteristic areas of the fragments where grisaille paint or patina was located. the diffraction patterns were subsequently compared with the diffraction patterns of the materials present in the pdf4 database to analyse the crystalline phases present in the illuminated area. as a control xrf measurements were performed on three points of fragment ssgp1 (glass, grisaille and patina) using an artax μxrf portable spectrometer from bruker axs. the spectrometer is equipped with a 50 kv mo excitation tube, a peltier-cooled silicon drift detector with an energy resolution of 150 ev at 5.9 kev and 10 × 103 cps. the experimental set-up involves a tube voltage and anode current of 50 kev and 0.7 ma and an acquisition time of 180 s. the incident beam was collimated to a diameter of 1 mm by using a pinhole nozzle. the measurements were performed by fluxing he between sample and detector, in order to increase the detector sensitivity towards the light elements. 4. results and discussion a typical diffraction pattern is shown in figure 4. clearly, the signal to noise ratio is low with a large number of diffraction peaks. the maximum count rate obtained for this measurement was ~400 counts per second. this is a clear indication that the amount of crystalline material is very small and that, therefore, it is necessary to use synchrotron radiation figure 2. windows of the basilica di san giovanni e paolo from which the fragments were selected. the red circle shows the location from which the fragments originate (ssgp1 and ssgp2 left, ssgp3 right). figure 1. the diffractometer, equipped with the laser interferometer, hosting an object relevant for cultural heritage for x-ray diffraction analysis. figure 3. fragments, ssgp1, ssgp2 and ssgp3 from the basilica di san giovanni e paolo. table 2. characteristic colours of the glass, grisaille and patina regions of the glass fragments. fragment glass grisaille patina ssgp1 green dark brown ssgp2 green brown white ssgp3 bright yellow blue white acta imeko | www.imeko.org september 2017 | volume 6 | number 3 | 74 to study these fragments in order to get some information out of the data. the peaks in the diffraction pattern of the grisaille part of fragment ssgp1 were confronted with the patterns in the pdf4 database. most of the peaks could be attributed to coal2o4 (jc-pdf card 01-082-2422), laurionite (pbcl(oh))(jc-pdf card 04-012-3672) and anglesite (pbso4) (jc-pdf card 04-008-8386). the remaining peaks were too few and too small to assign to other materials. laboratory xrf analysis on three points of the same sample (glass, grisaille and patina) confirmed a significantly higher concentration of both lead and cobalt on the grisaille layer. phase identification was performed for all measured patterns. in many cases patterns showed no peaks at all and the sample was moved until a signal was detected. for the patina part of ssgp1 no diffraction signal was found. as x-ray diffraction only detects crystalline materials it can be concluded that the material of the patina of ssgp1 is amorphous. the crystalline compounds that were detected on the grisaille and the patina parts of the fragments are listed in table 3. the xrd patterns collected show similar results for all fragments with regards to the quality of the measurements. a low signal to noise ratio was found in all cases, when crystalline material was found at all. in fragment ssgp2 an original pigment was found (pb2sb2o7) on the grisaille and alteration products of original pigments, such as feo(oh) and feso4(oh)(h2o)2. the brown colour of the grisaille of this fragment can be explained by the mixture of yellow lead(ii) antimonate darkened by the firing process or in a mixture with brown iron containing salts. in fact two iron containing salts were found on the patina of this fragment. the fact that no iron containing compounds were found on the grisaille may be explained by the nature of the measurements, where some spots are chosen on an inhomogeneous material such as a grisaille layer, where no crystalline iron salts were present. the presence of feo(oh) and feso4(oh)(h2o)2 does suggest that iron containing pigments were used in the grisaille of this fragment. in fragment ssgp1 the dark grisaille shows the presence of coal2o4 together with lead sulfate and lead hydroxychloride, pb(oh)cl. the color of cobalt aluminate, a deep blue pigment, which turned to dark, may be due to candle soot deposition or to the overfiring of the glass during the fixing process. the presence of coal2o4, which is a pigment first synthesized in the xviii century [11], evidences a substitution/restoration of the original xv century stained glass windows. the blue color of the grisaille of fragment ssgp2 is in agreement with the presence of the same pigment. furthermore, the analysis performed on the white patina, reveals also the presence of sulfates, caso4, pbso4. similar results were obtained for the other fragments, in which sulfates are the predominant compounds found on the alteration patina. this result is not surprising as sulfur is present in large quantities in seawater. because of the location of the basilica di san giovanni e paolo in venice there has obviously been contact with seawater in the form of aerosols or other. the presence of phosphates such as found on the patina of ssgp3 is less obvious and has probably a biological origin. this may be due to the fume from burning wax candles. fragment ssgp2 shows the presence of aluminum containing compounds both on the grisaille and on the patina. the origin of these compounds is not clear, although it has been observed in similar studies on stained windows [12]-[14]. schalm et al. [14] in a study of 19th century grisaille paint layers identified the presence of ca and al rich particles inside pores in the grisaille suggesting that their origin maybe from burned organic materials used as a painting agent. on the basis of these evidences, a deterioration mechanism can be proposed. an aggressive environment with large thermal variation, typical of the venice lagoon, can promote microcracking on the grisailles surface. these micro-breaks, together with the natural roughness of the grisailles layer, may favor the condensation processes on the surface. in this way, the formation of a deterioration-induced porous system can act as a series of micro-reactors for leaching phenomena with the subsequent salt precipitation. similar results have been reported recently on spanish 17th century grisailles [15]. heavy weathering of the samples was figure 4. diffraction pattern of a part of the grisaille of fragment ssgp1. the positions of the diffraction peaks of the spinel coal2o4 (green), laurionite pbcl(oh) (black) and anglesite pbso4 (red) are indicated. table 3. identified compounds on the grisaille and patina regions of the fragments. fragment grisaille patina ssgp1 coal2o4 pbso4 pb(oh)cl amorphous ssgp2 pb2sb2o7 pbso4 caso4·(h2o)2 caal2si2o8 feo(oh) feso4(oh)·(h2o)2 pbso4 caso4·(h2o)2 al2si2o5(oh)4 ssgp3 coal2o4 pbso4 capo3(oh)·(h2o)2 sio2 pbs pbso4 caco3 capo3(oh)·(h2o)2 acta imeko | www.imeko.org september 2017 | volume 6 | number 3 | 75 reported and as a result of the reaction of pb with the environment, many lead salts, including phosphates and sulfates, have precipitated. a glassy layer with low glass content is observed in the reaction glass/grisaille layer. the investigation presented here does not provide any depth information regarding the identified compounds. in order to get insight in possible layered structures in a further investigation into these samples depth information will be obtained by varying the incidence angle of the diffraction experiment. 5. conclusions the experimental set-up of the mcx beamline at elettra is described in this paper along with the possibility for nondestructive synchrotron x-ray diffraction experiments, especially in the field of cultural heritage research. the characteristics of the beamline and the large space available at the experimental setup allow the study of large objects without the need to remove parts for analysis. the beamline also allows for other type of experiments with the aid of additional equipment such as a furnace, which can be used for example for simulating alteration processes. the capabilities have been illustrated by the characterization of alteration products on grisaille glass originating from large windows of the basilica di san giovanni e paolo. with the aid of diffraction a large number of degradation products and original pigments could be identified, which allowed the proposition of a deterioration mechanism. the results presented here show that the bending magnet beamline for x-ray diffraction, mcx, offers excellent opportunities for analysis in the field of cultural heritage. acknowledgement the authors wish to acknowledge giulio zerauschek for his continuous technical support in maintaining and upgrading the mcx beamline. mcx takes part in the project echo: elettra cultural heritage office. references [1] a. lausi, m. polentarutti m, s. onesti, j.r. plaisier, e. busetto, g. bais, l. barba, a. cassetta, g. campi, d. lamba, a. pifferi, s.c. mande, d.d. sarma, s.m. sharma, g. paolucci , status of the crystallography beamlines at elettra, the european physical journal plus 130 3 (2015) pp 1-8. [2] c. artini, m. pani, m.m. carnasciali,m.t. buscaglia, j.r. plaisier, g.a. costa, structural features of smand gd-doped ceria studied by synchrotron x-ray diffraction and -raman spectroscopy, inorganic chemistry 54 (2015) pp. 4126-4137. [3] i m. abdellatief., a. lausi., j.r. plaisier, p. scardi, influence of lattice defects on the grain growth kinetics of nanocrystalline fluorite, metallurgical and materials transactions a: physical metallurgy and materials science 45 (2014) pp. 123-128. [4] l. rebuffi, a. troian, r, ciancio, e. carlino, a. amimi, a, leonardi, p. scardi, on the reliability of powder diffraction line profile analysis of plastically deformed nanocrystalline systems, scientific reports 6 (2016) pp. 20712. [5] l.e. fuentes-cobas, l. pardo, m.e. montero-cabrera, j.r. plaisier, a. garcia, j.k. brebã, e. mercadelli, c. galassi, the 0.96(bi0.5na0.5)tio3 0.04batio3 crystal structure: a high-q, high-counting statistics synchrotron diffraction analysis, crystal research and technology 49 (2014) pp. 190-194. [6] m. merlini, f. sapelli, p. fumagalli, g.d. gatta, p. lotti, s. tumiati, m. aabdellatief, a. lausi, j.r. plaisier, m. hanfland, w. crichton, j. chantel, j. guignard, c. meneghini, a. pavese, s. poli, high-temperature and high-pressure behavior of carbonates in the ternary diagram caco3-mgco3-feco3, american mineralogist 101 (2016) pp. 1423-1430. [7] r. arletti, l. gigli, f. di renzo, s. quartieri, evidence for the formation of stable co2 hydrates in zeolite nay: structural characterization by synchrotron x-ray powder diffraction, microporous and mesoporous materials 228 (2016) pp. 248-255. [8] l. rebuffi., j.r. plaisier, m. abdellatief, a. lausi, p. scardi, mcx: a synchrotron radiation beamline for x-ray diffraction line profile analysis, zeitschrift fur anorganische und allgemeine chemie 640 (2014) pp. 3100-3106. [9] p. riello, a. lausi , j. macleod, j.r. plaisier, g. zerauschek, p. fornasiero, in situ reaction furnace for real-time xrd studies, journal of synchrotron radiation 20 (2013) pp. 194-196. [10] j.r. plaisier, l. jiang, j.p. abrahams, cyclops: new modular software suite for cryo-em, journal of structural biology 157 (2007) pp. 19-27. [11] l.j. thenard, considerations generales sur les couleurs, suivies d’un procede pour preparer une couleur bleue aussi belle que l’outremer, journal des mines, 86 (1803) pp. 128-136. [12] z.z cilova, i kucerova, m knizova, t trojek, corrosion damage and chemical composition of czech stained glass from 13th to 15th century, glass technology: european journal of glass science and technology a 56 (2015) pp. 153-162. [13] r. prochazke, natural corrosion of uranium-colored historical glasses, journal of non-crystalline solids 353 (2007) pp. 20522056. [14] o. schalm, k. janssens, j. caen, characterization of the main causes of deterioration of grisaille paint layers in the 19th century stained-glass windows by j.-b. capronnier, spectrocimica acta b 58 (2003) pp. 589-607. [15] t. pradell, g. molina, s. murcia r. ibã¡nez, c. liu, j. molera, a.j. shortland, materials, techniques, and conservation of historic stained glass grisailles, international journal of applied glass science 7 (2016) pp. 41-58. the x-ray diffraction beamline mcx at elettra: a case study of non-destructive analysis on stained glass measuring the shape: performance evaluation of a photogrammetry improvement applied to the neanderthal skull saccopastore 1 acta imeko issn: 2221-870x october 2018, volume 7, number 3, 79 85 acta imeko | www.imeko.org month year | volume a | number b | 79 measuring the shape: performance evaluation of a photogrammetry improvement applied to the neanderthal skull saccopastore 1 costantino buzi1, ileana micarelli2, antonio profico1, jacopo conti3, roberto grassetti4, walter cristiano5, fabio di vincenzo4, mary anne tafuri1, giorgio manzi1 1 dipartimento di biologia ambientale, sapienza università di roma, rome, italy 2 dipartimento di scienze dell’antichità, sapienza università di roma, rome, italy 3 dipartimento di scienze della terra, sapienza università di roma, rome, italy 4 museo di antropologia “giuseppe sergi”, rome, italy 5 dipartimento di scienze biologiche, geologiche e ambientali – bigea, università di bologna, bologna , italy section: research paper keywords: 3d-imaging; digital acquisition; geometric morphometrics; paleoanthropology; virtual anthropology citation: costantino buzi, ileana micarelli, antonio profico, jacopo conti, roberto grassetti, walter cristiano, fabio di vincenzo, mary anne tafuri, giorgio manzi, measuring the shape: performance evaluation of a photogrammetry improvement applied to the neanderthal skull saccopastore 1, acta imeko, vol. 7, no. 3, article 13, october 2018, identifier: imeko-acta-07 (2018)-03-13 section editor: sabrina grassini, politecnico di torino, italy received april 20, 2018; in final form september 21, 2018; published october 2018 copyright: © 2018 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: costantino buzi, costantino.buzi@uniroma1.it 1. introduction since digital technologies started to be applied to specimens of paleontological or archaeological interest, there has been a great improvement, from both quantitative and qualitative point of views, of the information extractable from a single object of study [1]. this has happened mainly thanks to the development of virtual models of the actual specimens, which allow many researchers around the world to access unique remains or artifacts with no risk of damages and less difficulties in general [2], [3], [4]. digital x-ray based acquisitions allow to observe the specimens with minimum or no handling at all, up to highly detailed level, also in the internal portions not accessible in the actual object [5], [6], [7], [8]. apart from this scientific major achievement, a related advantage for the scientific community is the increase of the data-sharing policy for such virtual specimens, which is topped by the availability of online databases (e.g. nespos, morphosource, digimorph, morphomuseum, kupri). in palaeoanthropology, the most important and renowned specimens are indeed of inestimable value and thus their access is more difficult. thus, the diffusion of virtual anthropology protocols and techniques facilitated the spreading of the research on human fossils and the production of a new type of casts coming from the digital specimens via rapid prototyping (the more and more common 3d printing) [9], [10]. notwithstanding the diffusion of such techniques also for purpose as study, displaying or dissemination [11], [12], many of abstract several digital technologies are nowadays developed and applied to the study of the human fossil record. here, we present a low-cost hardware implementation of the digital acquisition via photogrammetry, applied to a specimen of paleoanthropological interest: the neanderthal skull saccopastore 1. such implementation has the purpose to semi-automatize the procedures of digital acquisition, by the introduction of an automatically rotating platform users can easily build on their own with minimum costs. we provide all the technical specifications, mostly based on the arduino uno™ microcontroller technology, and evaluate the performance and the resolution of the acquisition by comparing it with the ct-scan of the same specimen through the calculation of their shape differences. in our opinion, the replication of the automatic rotating platform, described in this work, may contribute to the improvement of the digital acquisition processes and may represent, in addition, a useful and affordable tool for both research and dissemination. mailto:costantino.buzi@uniroma1.it acta imeko | www.imeko.org month year | volume a | number b | 80 the actual objects remain nevertheless of limited availability to subjects interested to them, while a good quality cast can be of considerable cost. to overcome such difficulties there has been an increase in the use of acquisition and printing procedures less demanding from the perspectives of both costs and needed equipment [13]. the most commonly used is photogrammetry, which relies mainly on the performance of one or more series of photos (arranged in chunks) of the object in different norms [14]. the chunks are subsequently assembled via a dedicate software (e.g. agisoft photoscan) to eventually obtain a virtual volume (mesh) reporting the visible details and texture of the object. 1.1. digital acquisition virtual anthropology is based on the study of the digital acquisition of human remains. the methods commonly used to digitally acquire an object are computerized tomography (ct), laser scanning and photogrammetry. among the three, the ct scan is the acquisition procedure which returns both the inner and outer structures of the object, while both laser scan and photogrammetry only return its external topology, including colours and texture [15]. ct scan works by x rays, coming from a rotating source, passing through the object to be scanned at the centre of the rotation axis, to reach a series of sensor located on the opposite side of the source. the different densities of the object attenuate the x radiation and the output of the acquisition is a series of cross-sections of the object (slices) spaced out by a distance (inter-slice) which determines the final resolution. the slices are actually 2d images, useful for the medical purposes of the common ct scans. to obtain a 3d output, thus a polygonal mesh, the 2d images have to be processed, by applying the “marching cubes” algorithm, after the definition of a value of radio density (expressed as hounsfield unit) [2], [16], [17]. to be carried out, a ct acquisition requires the availability of professional medical equipment and space. alternatively, a proper medical structure or research institution where the specimen to be scanned has to be carried, with consequent expenses for transportation and insurance, is needed. the laser scanner acquisition relies on the triangulation built by a structured light beam intercepting the surface of the specimen and being detected by a sensor put at a known distance from the source [18], [19]. laser scanner technology can return very highly detailed reproductions of the external surface of the objects: the quality of the acquisition depends on the hardware of the devices used, directly proportional to their cost, often expensive. no inner structures or details can be detected by this type of acquisition technology. photogrammetry is the less expensive acquisition procedure and is based on the assemblage of one or more series of images of the object, in its different views [20]. the images are usually photos taken by an operator and commonly grouped in different chunks, one for each norm. a neutral background and an optimal lighting, constant for the entire photo session, contribute significantly to a high-quality final output. the photos, exported to a dedicated software (e.g. agisoft photoscan), are aligned to obtain a sparse point cloud and subsequently a dense point cloud and a 3d mesh is built through a triangulation algorithm [21]. the information on texture and colour is comprised in the final reconstruction, while a metric reference is needed, as photogrammetry, unlike ct and laser scans, does not return the object at the original scale [22], [23]. not including the costs of a photographic equipment, the only added cost is that of a commercial software, as agisoft, for the processing of the images, although several open source alternatives are available (e.g. insight3dng, regard3d, meshrecon and others). even though photogrammetry is a low-cost acquisition procedure, lacks a similarly cheap automation and can be significantly time consuming both from the point of view of the photoshoot, to be performed by an operator, and of the subsequent processing of images. in addition, there is no instant feedback on the output. 1.2. purpose the main purpose of this work, apart from presenting a hardware implementation we designed, is to evaluate an automated procedure of photogrammetry and demonstrate that the resulting output can substitute that of a more expensive and demanding procedure, as the ct scan, whenever the latter is not possible. moreover, we demonstrate through a morphological analysis the feasibility of a good photogrammetry acquisition for research purposes as well as dissemination and education ones. the hardware improvement we present in this paper is designed to overcome the need of manually performing the photoshoot, to stabilize the process, allowing the operator, at the same time, to perform other tasks such as the import of photos, the preparation of other specimens or the check for the alignment of previous photo sessions. 1.3. summary in the next section, we first point at the chosen case study, the renowned neanderthal skull saccopastore 1 from rome, italy; subsequently we present the technical specifications of the micro-ct scan used as comparison and the equipment used for the photogrammetry, with a focus on the hardware and the rotating platform; lastly, we point at the methods used for the sampling and the analysis. in the last section, we present both the digital reconstructions of saccopastore 1 and discuss the results of the analysis. 2. materials and methods 2.1. saccopastore 1 saccopastore 1 (scp1) is a nearly complete cranium coming from a neanderthal individual of remarkable antiquity, with a debated dating estimated to be between 100 130 ka [24], and 250 ka [25]. the specimen, lacking the mandible and bearing some taphonomic damages, as the missing maxillary processes and part of dentition, probably comes from a female individual and appears nonetheless well preserved [26], despite some damages occurred at the moment of the discovery (especially to the whole supraorbital torus and the vault) in april 1929. the cranium, after its discovery in a gravel quarry in the rome’s countryside, today corresponding with the present city’s suburbs, was brought by the quarry’s owner to the anthropologist sergio sergi, who recognized and described its neanderthal features, such as the mid-facial prognathism and the general shape of the cranial vault [27]. the skull is today preserved at the ‘g. sergi’ anthropology museum of sapienza university of rome. acta imeko | www.imeko.org month year | volume a | number b | 81 2.2. acquisition of the specimen scp1 has been ct scanned using a tomoscan auep (philips), with an interslice distance equals to 1 mm scans. data were exported as dicom files, with a matrix of 512×512 pixels. scp1 was scanned at 75 ma and 140 kv (scantime = 4 s), with a fov of 250 mm and a consequent pixel size of 0.49 mm [28] (figure 1). the close-range photogrammetry of the same specimen was performed at the ‘g. sergi’ anthropology museum of sapienza university of rome, by using a reflex camera canon eos 700d, with a canon zoom lens 18-55 mm, set to autofocus with iso on 100, f-stop at f/10 and exposing time to ½ sec. the photos have been exported as .jpg files with 5184×3456 pixels resolution at 72 dpi. the camera was set up on a manfrotto tripod at a distance of about 45 cm from the specimen, located in a havox lightbox 60×60×60 cm. four chunks were performed: the first was carried out by positioning scp1 on the frankfurt horizontal, right in front of the camera; the same position was kept for the second one, but positioning the camera 30° above the horizontal, and subsequently, scp1 was positioned upside down, resting on its bregmatic area, for the third chunk; eventually, for the fourth chunk, scp1 was positioned on its occipital squama. each chunk consisted of 50 photos, separated by 240 angular steps (more on it in section 2.3). the four chunks thus acquired were processed by using the software agisoft photoscan™ (v. 1.2.6.2834). first, agisoft allowed to remove the background from the images by using a mask; then, by using the metadata associated with the exchangeable image file format (exif) [29] of the photos, the program extrapolated from the data the position and orientation figure 2. digital model of scp1, acquired via photogrammetry. figure 1. digital model of scp1, acquired via computerized tomography. figure 3. scheme of the rotating platform system; the single modules are reported. 1: microcontroller arduino uno™ r3; 2: 4 relay module connected with the step motor; 3: 2 relay module connected with the camera; 4: matrix board; 5: lc display 1602; 6: keypad. acta imeko | www.imeko.org month year | volume a | number b | 82 of the camera for each photo and built a first 3d point cloud. subsequently, the program built a dense point cloud by estimating the depth information for each shot. these steps were performed for each chunk and eventually all of them were aligned and combined by the software in a single dense cloud. the resulting mesh can be exported either in .ply, .obj or .wrl format, among others (figure 2). the mesh obtained via photogrammetry was scaled to the real object size (cast of scp1) by using the nasion-inion distance as reference. 2.3. the rotating platform the system we designed is based on the arduino uno™ technology, a microcontroller board based on the atmega328p microchip [30], [31], powered it with a ac-to-dc adapter (7.5 12v; 2a). the microcontroller allows to synchronize the rotation of the platform with the focus and the photo shoot, via a step motor; among the further functions available, it is possible to link a beeper to signal the end of the shooting session. our system consists of the following modules: microcontroller arduino uno™ r3, a lc display 1602 (16 characters, 2 rows), a 4 relay module for operating the step motor, a 2 relay module for operating camera focus and shooting, a 4x4 keypad and a matrix board for the connections between modules; the latter is needed for the distribution of the voltage to the different modules from the microcontroller. the detailed scheme of the system is reported in figure 3. the microcontroller has been programmed via arduino software™ 1.8.5; we provide the code in the supplementary information. the system is run by entering the number of steps to take between each shoot, as indicated by the display. the number of photos decided by the operator is in relation with the number of steps needed by the step motor to perform a complete rotation: the more steps the platform can perform, the more precise is the system. the platform we used comes from a broken laser scanner and requires 12000 steps to perform a complete rotation. a calculation (1) is needed to obtain the requested number of steps for each shoot: steps for a complete rotation number of photo shoots (1) thus, for performing a set of 50 photos, the system is programmed by entering first the number of steps required (e.g. 12000/50 = 240); subsequently, the ‘a’ key (figure 2) is pressed to accept the entry and the number of shoots requested (e.g. 50) is entered, followed by the ‘#’ key to run the program. to stop and reset the program, if needed, the key to enter is the ‘*’ (asterisk). 2.4. sampling as stated before, we used a ct-scan of scp1 to evaluate the precision of the model built via photogrammetry. for the comparison between the two digital reconstructions of scp1 we used a landmark-based, geometric morphometrics approach [32], [33]. geometric morphometrics allows a quantitative analysis of the shape of the objects of study by using sets of homologous points, called landmarks [34]; the sets, or landmarks configurations, are eventually compared by excluding the effects of their size, orientation and position in the space by the generalized procrustes analysis (gpa) [35]. the chosen landmark set for the comparation is represented in figure 4. once the landmark configurations have been sampled by the 3d imaging software thermo scientific amira™ (version 5.4.5) on the two 3d models, their mesh distance [36] was evaluated by an analysis performed in r statistical environment [37], package ‘morpho’ [38]. the mesh distance calculates the distance from the reference to the target 3d models, by projecting each vertex of the reference mesh (obtained by photogrammetry) to the target mesh (obtained by ct-scan). figure 4. landmark configuration used for the shape analysis. 3. results the same landmark configuration was defined and acquired on both models of scp1. the definition of a homologous landmark configuration allowed to obtain a rotation matrix (via gpa, [39]) in order to align the two 3d models. the procrustes distance between the two landmark configurations is equal to 53.83 mm, while the euclidean distance amounts to 27.48 mm. the landmark displacement is ranged from 0.43 to 3.08 mm with a mean displacement equal to 1.6 mm. the mesh distance measured and mapped the vertex displacement from the reference mesh (scp1 via photogrammetry) to the target mesh (scp1 via ct-scan). the mean vertex displacement is equal to 0.047 mm and the first and the third quantiles amount to -0.789 and 1.243 mm respectively. acta imeko | www.imeko.org month year | volume a | number b | 83 the results are represented in figure 5, reporting the vertex displacement between the two models. in figure 6, the mesh distance between the two models is graphically displayed. figure 5. boxplot of the vertex displacement (mm) obtained after the mesh distance computing. 4. conclusions in recent years, several methods aimed at the digital acquisition were developed leading to an increase of technologies applied to the study of the human fossil record and cultural heritage. the use of the photogrammetry led to an increase in the sharing of human remains, by hosting them into virtual museums such as nespos (www.nespos.org), morphosource (morphosource.org) and africanfossils (africanfossils.org). the 3d models acquired by digital methods can be used both for research and dissemination purposes. this contribution presented the photogrammetric surveying and 3d modelling of the neanderthal skull of saccopastore 1, one of the most important and complete neanderthal skulls ever discovered, characterized by a challenging texture, a complex shape/geometry and difficult optical properties (e.g. reflectivity). we described in detail how to build, set up and use a rotating platform connected to an arduino uno™ microcontroller to speed-up and automatize the process of photogrammetry, as well as the procedures to correctly launch it. the model obtained by our acquisition resulted complete and detailed, with the anatomical traits fully recognizable and feasible for the definition of landmarks and/or linear measurements. in contrast to other expensive or more demanding approaches, the method of close-range photogrammetry of scp1, here presented, can be evaluated as a cutting edge effective, low-cost and automated technique. the reproducibility of the model was evaluated by comparison with a ct scan of the same specimen. the anatomical distance both in terms of landmark and vertex displacement is low, indicating an almost full overlap of the two 3d models. the only structures presenting appreciable differences between the values of vertex distance resulted to be ‘grey’ areas, not easy to acquire. the resulting topological artifacts are easy to fix by using specialized software (e.g., geomagic studio and zbrush). alternatively, the speeding up of the whole process allows to possibly include preliminary test sessions for making adjustments on aspects as the camera setting. in fact, a crucial point of the process is based on finding the perfect setting of the reflex. by setting the lowest value of iso (100) the quality of the images we obtained still showed a very fine grain. moreover, the f-stop at f/10 determined a good balance between the ratio of the system's focal length and the diameter of the entrance of the light for the set into the lightbox. these two settings guaranteed the high-quality of the photos. the use of the automatic rotating platform as shown in this work could represent an improvement highly facilitating research and dissemination for the cost-effectiveness of both hardware and software and fast and reproducible protocols for the digitalization of human remains. figure 6. on the left, the overlapped models of scp1 (white: ct-scan; yellow: photogrammetry), aligned after gpa. on the right, the mesh distance performed between the two 3d models. acta imeko | www.imeko.org month year | volume a | number b | 84 acknowledgement we thank the “g. sergi” museum of anthropology of sapienza university of rome for making available the used materials. we want to thank, in addition, the committee of the international workshop on metrology for archaeology and cultural heritage (metroarcheo). references [1] a. profico, l. bellucci, c. buzi, f. di vincenzo, i. micarelli, f. strani, m. a. tafuri, and g. manzi, virtual anthropology and its application in cultural heritage studies, studies in conservation, sep. 2018. 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[39] f. j. rohlf, d. slice, extensions of the procrustes method for the optimal superimposition of landmarks, syst. zool., vol. 39, no. 1, p. 40, mar. 1990. investigation of a deadweight force standard machine with a 1 mn – 10 n range acta imeko issn: 2221-870x september 2019, volume 8, number 3, 3 – 9 acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 3 investigation of a deadweight force standard machine with a 1 mn – 10 n range gang hu1, jile jiang1, tao li2, zhimin zhang1, honglei ji2, lijun zhuang2 1 national institute of metrology (nim), beijing, p.r. china, bei san huan dong lu no.18, chao yang district, 100029, beijing, p.r. china 2 shanghai marin equipment research institute (smeri), shanghai, p.r. china section: research paper keywords: small force; deadweight force standard machine; air bearing; balance mechanism citation: gang hu, jile jiang, tao li, zhimin zhang, honglei ji, lijun zhuang, investigation of a deadweight force standard machine with a 1 mn – 10 n range, acta imeko, vol. 8, no. 3, article 2, september 2019, identifier: imeko-acta-08 (2019)-03-02 editor: jeerasak pitakarnnop, nimt, thailand received july 13, 2018; in final form may 29, 2019; published september 2019 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was supported by general administration of quality supervision, inspection and quarantine of the people's republic of china. corresponding author: gang hu, e-mail: hugang@nim.ac.cn 1. introduction in recent years, the demands of force measurements beyond common ranges are growing. for small force measurements in the range of millinewtons to newtons, there are more and more applications in biology, pharmaceutics, advanced materials, hardness metrology, precision manufacturing, and more. for example, small force transducers in the pharmaceutical industry and small force testing machines for solder in integrated circuit package industry should be measured and calibrated. small forces from millinewton to newton are required to be traced in instrumented indentation test for hardness and materials parameters [1]. for realising small force, some national metrology institutes (nmis) have developed small force standard machines, with minimum forces at newton or sub-newton range. for instance, a 200 n deadweight force standard machine was established by the physikalisch-technische bundesanstalt (ptb) in germany. the force range of this machine is from 0.5 n to 200 n [2]. the korea research institute of standards and science (kriss) developed a 22 n deadweight force standard machine, whose minimum force is 0.5 n [3]. however, the minimum force value of china’s calibration and measurement capabilities (cmcs) published in the bipm key comparison database is 10 n at present [2]. traceable force standard machines below 10 n with high accuracy were found to be lacking. as a result, higher accuracy force measurements in this range cannot be achieved. this issue has an adverse influence on relative fields and industries. concerning the structure and design of deadweight force and torque standard machines [4]-[6], a new 10 n deadweight force standard machine has been developed by nim [7],[8]. its minimum force is extended to 1 mn. this machine consists of deadweights; an automatic loading and unloading system; a load frame; a deadweight hanging system; a balance beam; and a control system. the structure and special design of the mechanical system and electronic control system are described in the next two sections. in section 4, the performance experiments and results are demonstrated. uncertainty evaluation is introduced in section 5. finally, in the concluding section, the major features and characteristics of this machine are summarised. abstract a 10 n small force standard machine with air-bearing support has been developed by nim. differing from traditional deadweight force standard machines, this machine is equipped with a balance beam supported by air bearing. the weights of the load frame and deadweight hanging system are balanced, and the initial force is extended to 1 mn. furthermore, there are innovative designs in the load frame and tension force joint. performance experiments such as sensitivity, repeatability, and rotation effect are carried out. the uncertainty of this machine is evaluated as 1.0 × 10-4 (k = 2) in the range of 1 mn – 0.5 n, 4.0 × 10-5 (k = 2) in the range of 0.5 n – 10 n. the structure of this force standard machine and its key components are introduced in detail. the performance experimental results and uncertainty evaluation are demonstrated in this paper. mailto:hugang@nim.ac.cn acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 4 2. the structure and main components of the mechanical system figure 1 shows the structure of the 10 n deadweight force standard machine. differing from traditional deadweight force standard machines, the aerostatic bearing technique is applied to support the balance beam, and innovative designs and special structures in the load frame and tension force joint are used. 2.1. balance beam with aerostatic bearing support for mitigating the weights of load frame and deadweight hanging system, a balance beam made by titanium alloy is used in this machine. its structure is shown in figure 2. the end of the beam is a built-in counterweight balance mechanism. by adjusting the position of the counterweights, the weights of the load frame and deadweight hanging system are balanced. there is a fine adjustment mechanism in the middle of the beam. it is used for the fine-adjusting centroid of the balance beam. the position of the beam is measured by two laser displacement sensors, which are placed on two sides of the beam symmetrically. finally, the beam is adjusted in a state of neutral equilibrium. a self-developed aerostatic bearing is used for the support of the beam. compared to knife-edge support, air bearing support has smaller friction and only a rotational degree of freedom. therefore, standard uncertainty caused by support is minimised. furthermore, it has the advantage of the measurement and control of the position of the balance beam and the accurate positioning of deadweights. an air supply system includes an oil-injected air compressor, air dryer, filters, and an air tank. clean and dry gas with stable pressure is provided by the air supply system and is injected into the bearing surface through the throttle. the gap between the rotor and stator is no larger than 0.01 mm. when it works, there is a very thin pressure film between the rotor and stator. the pressure film can resist the external impact force and make the rotor suspend in the centre of the stator. a clamping mechanism is composed of two sets of motion module, driven by servo-motors on both sides of the balance beam. as soon as the mechanism clamps the beam, the displacement of the beam is restricted within ± 0.2 mm. as a result, the beam and air bearing support are protected from unexpected impact or force. there is a suspending mechanism for the load frame and deadweight hanging system at the end of the balance beam. it has two perpendicularly mounted thin steel strips. this mechanism makes the load frame and deadweight hanging system suspend freely and prevents the influence of additional forces and moments. 2.2. load frame a compact four-column structure is used as the load frame. figure 3 shows its structure. it consists of four hollow pillars made by titanium alloy; upper and lower beams; and a base plate. i-type beams are used for reducing the weights. there is a base plate in the middle of load frame. the upper space of the load frame is applied to the compression force measurement, and the lower space is used for tension force measurement. the position of the base plate can be adjusted through a linear motion module. the motion module driven by a servo-motor can move vertically along the linear guide. in this way, the positioning accuracy of the base plate is 0.02 mm. 2.3. a knife-edge universal joint for tension force measurement in contrast to the traditional tension force joint, a knife-edge universal joint is used. figure 4 shows its structure. it is composed of a cross head, two knife edges, a universal joint seat, and a universal joint head. the universal joint seat is connected to one knife edge, and the universal joint head is connected to the other knife edge. the two knife edges are set up perpendicularly. as a result, there is only line contact between two edges, and the friction is very small. in addition, this universal joint reduces the standard uncertainty due to the non-coaxiality of the tension force application. 2.4. other components and units two automatic loading and unloading systems, which are composed of weight supports and three-dimensional movement figure 1. a 10 n deadweight force standard machine. figure 3. load frame. figure 2. balance beam with aerostatic bearing support. acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 5 units, are adopted for loading and unloading deadweights. there are ten layers and five weight supports distributed evenly on each layer. deadweights are placed on the weight supports. three-dimensional movement units include rotating, horizontal moving, and vertical moving mechanisms. there are ten layers in the deadweight hanging system. to prevent it from shaking, a clamping mechanism clamps the hanging system during the loading and unloading processes. as soon as the deadweights are placed on the hanging system, the clamping mechanism releases. therefore, forces are applied to force the transducer freely in a vertical direction. deadweights are divided into ten groups, with the following combinations: 1 mn × 10, 2 mn × 10, 5 mn × 10, 10 mn × 10, 20 mn × 10, 50 mn × 10, 100 mn × 10, 200 mn × 10, 500 mn × 10, and 1 n × 10. the deadweights of 10 mn 1 n are made of stainless steel, deadweights of 1 mn 5 mn are made of titanium alloy. this machine is mounted on a marble platform, which is placed on a concrete base block isolated from the floor of the laboratory. there is a plexiglass cover outside of this machine for protecting airflow disturbance and dust. 3. the electronic and control system the electronic and control system consists of a pc, a programmable logical controller (plc) and self-developed software. figure 5 shows the electronic and control system. there are three control modules: deadweight automatic loading and unloading; balance beam position control; and measurement process control. because the fit clearance of the positioning between deadweights and the racks of the hanging system is less than 0.1 mm, the precise positioning of the deadweights is crucial in the loading and unloading process. in the control module of deadweight automatic loading and unloading, absolute coordinate positioning control technology is adopted to ensure the accurate positioning of the deadweights. when starting origin restoration, the positions of the deadweights, which are put on weight supports, are initialised. the deadweights that are to be selected are rotated to the position immediately facing the hanging system by the rotating mechanism. the horizontal moving mechanism transfers the deadweights into the hanging system. while the vertical moving mechanism moves downward, the deadweights are applied to the racks of the hanging system step by step. unloading is completed in a reverse process. as long as the deadweights are unloaded, they can be restored to their initial positions without any deviation. the balance beam mechanism balances the weights of the load frame and deadweight hanging system and is in a state of neutral equilibrium. during the force measurement process, the deformation of the force transducer that is to be calibrated causes the balance beam to deviate from its original position. displacement feedback control is realised by the control module of the balance beam position. according to the design requirements of the control system, the horizontality of the balance beam should not be larger than ±0.1 mm/m and overshoot should be prevented. as soon as the balance beam deviates from its equilibrium position, the change of position is measured by the laser displacement sensors, and the position of base plate is adjusted by the linear motion module. as a result, the balance beam is restored to a position of equilibrium. under these conditions, the weights of the load frame and deadweight hanging system are compensated entirely, and there are no lateral forces applied to the force transducer that require calibration. in the measurement process control module, there are three operation modes: standard mode (fully automatic operation), single point mode (semi-automatic operation), and manual operation mode. in standard mode, the measurement parameters, such as measurement range; force points; preload numbers; measurement cycle numbers; and waiting and reading times are set by the user interface. the whole measurement process: loading; unloading; and data acquisition and processing are completed automatically. in single point mode, one-time loading or unloading operation can be realised. this mode is applied in a one-step operation or by commissioning the machine. there are four operation programs in the manual operation mode: origin restoration; range replacement; selection and deadweight groups adjustment; and clamp and release adjustment. they are used for the adjustment and setting of the main components of the machine. a self-developed software is applied in the electronic and control system. the software of the pc receives a user’s instructions through the interface; sends corresponding commands to the plc system; and realises specific operation functions. the plc software implements positioning, speed, and timing controls for the servo-motors, sensors, and actuators. the output of the force transducer that requires calibration are acquired by the pc, restored, and analysed by database software. figure 4. a knife-edge universal joint for tension force measurement. figure 5. the electronic and control system. acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 6 4. peformance experiments performance experiments for the 10 n deadweight force standard machine were carried out. the force transducers used in the experiments were calibrated and found to comply with the requirements of iso 376: 2011(e) for class 00. 4.1. sensitivity experiments sensitivity is a main parameter for evaluating the magnitude of friction of the balance beam. when the beam was adjusted in a state of neutral equilibrium, a sensitivity experiment without loading was carried out. this experiment is shown in figure 6. in the case of an unloading condition, a wire with very small mass was applied on one side of the balance beam. the position change of the balance beam was observed at the same time. it was found that the minimum mass of the wire, which caused a significant position change, was only 10 μg. the arm length of the balance beam is 250 mm. therefore, the friction torque of the balance beam was found to be less than 0.03 μn∙m. a sensitivity experiment with a load was carried out when a force was applied to a force transducer. as soon as the output of the transducer was stable, a small standard weight was loaded on the end of the beam. the output of the transducer was monitored. sensitivity is calculated as follows: % m mδ s 100×= 1 1 1 (1) where △m1 is the minimum mass of the standard weight, which caused a significant change of output of the transducer, and m1 is the nominal mass corresponding to the applied force. the sensitivity experiment results with different forces are summarised in table 1. 4.2. repeatability experiments the repeatability experiments were conducted in force ranges of 1 n – 10 n, 0.5 n – 5 n, and 0.2 n – 2 n. for 1 n – 10 n, a gtm ktn-z/d 10 n force transducer was used. for 0.5 n – 5 n and 0.2 n – 2 n, a gtm ktn-z/d 5 n force transducer was used. the experiment procedure was as follows: preload three times. the first two times preload the maximum force directly. in last one, preload step by step. then, load from 10% to 100% of the nominal force step by step, three times. a 60-second interval between different force steps was used. the repeatability is calculated as: x xx r minmax = (2) where xmax, xmin, and x are the maximum, minimum, and average values of the outputs of the transducer respectively. the results of the repeatability experiments are shown in figure 7. it is clear that the repeatability is less than 4.0 × 10-5. 4.3. rotation effect experiments rotation effect experiments were carried out as follows: preloadings and measurement series are taken four times and three times respectively at 0º position. then, preload once and measure once at other positions. the measurement sequence is given in figure 8. the force steps in this experiment are 5 n and 10 n. the readings should be taken four minutes after a change in every force step. gtm ktn-z/d 10 n and hbm s2m 10 n force transducers were used. the rotation effect is characterised by reproducibility. it is given as 𝑅𝑟𝑒𝑝𝑟𝑜 = √ ∑ (𝑥𝑖−�̅�𝑖) 29 𝑖=1 9×(9−1) �̅�𝑖 (3) where 𝑥𝑖 is a deflection of the force transducer at 0º, 90º, 180º, 270º, … �̅�𝑖 is mean deflection. table 1. sensitivity experiment results with different forces. force (n) nominal mass of standard deadweight (g) minimum mass of weight (mg) sensitivity 10 1020.42916 5 4.90 × 10-6 1 102.042916 2 1.96 × 10-5 0.5 51.021458 1 1.96 × 10-5 figure 6. sensitivity experiment without loading. figure 7. the results of repeatability experiments. figure 8. the measurement sequence. force step in % time in min 50 100 0 4 8 12 16 20 40 80 100 60 0° three pre-loadings 0° 90° pre-loading three meas. cycles ... pre loading meas. cycle ... acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 7 the experiment results are shown in table 2. the results demonstrate that the reproducibility is less than 4.0 × 10-5. 5. uncertainty evaluation the force realised by the deadweight force standard machine can be calculated as:         −= w a ρ ρ mgf 1 (4) where m is the mass of the deadweights, g is the local gravitational acceleration, ρ a is the density of air, and ρ w is the density of the deadweights. from the mathematical model, the following standard uncertainty components should be considered: 1) mass calibration of the deadweight 2) local gravitational acceleration 3) the density of air 4) the density of the deadweights furthermore, the structure and mounting factors of this machine are also uncertainty contributions. they include the following components: 5) the friction in the balance beam 6) the swing of the deadweights 7) plate inclination (for compression force only) 8) non-coaxiality (for tension force only) 5.1. standard uncertainty of mass calibration of deadweight the nominal mass of the deadweights are calculated according to equation (4). the maximum permissible errors of the deadweights are in the range of ±1×10-5 of its nominal value. the deadweights were calibrated by the mass laboratory of nim. the deadweights, the measured mass of which are within permissible errors, are selected and put on the machine. the nominal mass and measurement uncertainties of the deadweights are demonstrated in table 3. 5.2. standard uncertainty of local gravitational acceleration the gravitational acceleration at the location where this machine is built up was measured as 9.80126037 m/s2 by the gravity laboratory of nim. considering the influence of the earth tide, the standard uncertainty of the measurement result ug is 3.00 × 10-7. 5.3. standard uncertainty of the density of air through observations and statistics for many years, the average value of the density of air is 1.2 kg/m3 in our laboratory, and its variation does not exceed 0.04 kg/m3. assuming a uniform probability distribution, the standard uncertainty of the density of air uρ a can be given as: 2 10921 213 040 ×= × = . . . u aρ (5) 5.4. standard uncertainty of the density of the deadweights the density of the deadweights was measured by the mass laboratory of nim. for deadweights made of stainless steel, the density is 8,045 kg/m3 with an expanded uncertainty of 140 kg/m3(k=2). for the deadweights made of titanium alloy, the density is 4469 kg/m3, with an expanded uncertainty of 140 kg/m3(k=2). therefore, the standard uncertainties of density of the deadweights uρw are 8.70 × 10 -3 and 1.57 × 10-2 for deadweights made of stainless steel and titanium alloy respectively. 5.5. standard uncertainty of friction in the balance beam the friction of the air bearing support in the balance beam is the primary uncertainty contribution of this machine. for evaluating this uncertainty budget, the sensitivity of the balance beam was tested. the sensitivity experiment results are demonstrated in the final section of this article. according to conservative estimates, the standard uncertainties caused by friction in the balance beam ufr are 1.96 × 10-5 in the range of 0.5 n – 10 n and 4.90 × 10-5 in the range of 1 mn – 0.5 n respectively. 5.6. standard uncertainty of the swing of deadweights during the loading process, the swing of the deadweights may slightly influence the direction of the applied force. for this machine, there is almost no swing of the deadweights due to the use of the clamping mechanism in the deadweight table 2. rotation effect experiment results. degrees gtm ktn-z/d 10 n hbm s2m 10 n 5 n 10 n 5 n 10 n deflections (mv/v) deflections (mv/v) 0 1.001652 2.002996 1.000316 2.000226 90 1.001716 2.003168 1.000316 2.000252 180 1.001699 2.003182 1.000304 2.000216 270 1.001682 2.003088 1.000356 2.000228 360 1.001677 2.003059 1.000303 2.000215 450 1.001707 2.003160 1.000305 2.000248 540 1.001694 2.003184 1.000302 2.000222 630 1.001704 2.003128 1.000353 2.000241 720 1.001649 2.002991 1.000317 2.000235 mean deflection (mv/v) 1.001687 2.003106 1.000319 2.000231 reproducibility 8.0 × 10-6 1.3 × 10-5 7.0 × 10-6 2.3 × 10-6 table 3. the nominal mass and measurement uncertainties of the deadweights. force in n nominal mass in g relative standard uncertainty material of deadweights 1 102.042916 8.82 × 10-8 stainless steel 0.5 51.021458 9.80 × 10-8 0.2 20.408583 1.96 × 10-7 0.1 10.2042916 2.94 × 10-7 0.05 5.1021458 4.90 × 10-7 0.02 2.0408583 9.80 × 10-7 0.01 1.0204292 1.47 × 10-6 0.005 0.5102755 2.74 × 10-6 titanium alloy 0.002 0.2041102 5.88 × 10-6 0.001 0.1020551 7.84 × 10-6 acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 8 hanging system. the standard uncertainty of the swing of the deadweights us is estimated as less than 1.00 × 10-6. 5.7. standard uncertainty of plate inclination (for compression force only) due to limitations of manufacturing and assembly techniques, the base plate is slightly inclined. this results in the disalignment of the axis between the applied force and force transducer that require calibration. the level degree of the machine was adjusted and measured within 0.02 mm/m. as a result, the standard uncertainty of the plate inclination ui can be estimated as: 𝑢𝑖 = 1 − cos 𝛼 = 1 − cos (arctan ( 0.02 1000 )) = 2.00 × 10−10 . (6) 5.8. standard uncertainty of non-coaxiality (for tension force only) due to limitations in manufacturing and assembly techniques, there is a slight misalignment of the axis between the upper and lower tension force joints. the misalignment angle β was estimated less than 0.03˚. therefore, the standard uncertainty of the non-coaxiality for the tension force ua can be given as: 𝑢𝑎 = 1 − cos 𝛽 = 1 − cos(0.03°) = 1.37 × 10 −7 . (7) the standard uncertainty budgets of this machine are summarised in table 4. 5.9. combined standard uncertainty and expanded uncertainty the relative combined standard uncertainties are calculated for compression or tension forces as equation (8) or equation (9) respectively: 22222 a 2222 +++)-( )+(++= wa isfrawρρgmrc uuuρρ/ρuuuuu (8) 22222 a 2222 +++)-( )+(++= wa asfrawρρgmrt uuuρρ/ρuuuuu (9) the relative expanded uncertainties for compression or tension forces are given as: rcrc uu ×= 2 (10) rtrt uu ×= 2 (11) the combined standard uncertainty and expanded uncertainty of the 10 n deadweight force standard machine are calculated according to equations (8) to (11), and their results are summarised in table 5. 6. conclusions a 10 n deadweight force standard machine with air bearing support has been developed by nim. its force range is from 1 mn to 10 n. in contrast to traditional deadweight force standard machines, a balance beam is applied to compensate the weights of load frame and a deadweight hanging system. the aerostatic bearing technique is applied to support the balance beam, and innovative structures in load frame and tension force joint are used. performance experiments for this machine were carried out. the friction torque of the balance beam was evaluated as being less than 0.03 μn∙m. concerning 0.5 n to 10 n, the sensitivity is no larger than 1.96 × 10-5, and repeatability is less than 4.0 × 10-5. the reproducibility, which is caused by the rotation effect, is less than 4.0 × 10-5 at 5 n and 10 n. the standard uncertainties of the main influence factors are analysed, and the expanded uncertainty is estimated as 1.0 × 10-4 (k = 2) in 1 mn – 0.5 n, 4.0 × 10-5 (k = 2) in 0.5 n – 10 n, respectively. further investigation and experiments will be conducted. comparisons with other deadweight force standard machines will be carried out. acknowledgement this project was supported by the dissemination of value of a quantity and quality safety project of the general administration of quality supervision, inspection, and quarantine of the people’s republic of china ― ‘the research and establishment of (1 mn – 10 n) force standard machine’, project number: 2015qk225/22-alc1503. table 4. standard uncertainty budgets. sources of standard uncertainty distribution coverage factor relative uncertainty mass calibration of deadweights um,s / um,t uniform 1.47 × 10-6 (for stainless steel) 7.84 × 10-6 (for titanium alloy) gravity acceleration ug / / 3.00 × 10-7 density of air uρa uniform 1.92 × 10-2 density of the deadweights uρw / 2 8.70 × 10-3 (for stainless steel) 1.57 × 10-2 (for titanium alloy) friction in balance beam ufr / / 4.90 × 10-5 (in 1 mn 0.5 n) 1.96 × 10-5 (in 0.5 n 10 n) swing of deadweights us / / 1.00 × 10-6 plate inclination (for compression force) ui / / 2.00 × 10-10 non-coaxiality (for tension force) ua / / 1.37 × 10-7 table 5. the combined standard uncertainty and expanded uncertainty of 10 n deadweight force standard machine. force ranges the combined standard uncertainty the expanded uncertainty (k = 2) 1 mn 0.5 n 5.0 × 10-5 1.0 × 10-4 0.5 n 10 n 2.0 × 10-5 4.0 × 10-5 note: the uncertainty evaluation results are available for compression force and tension force. 3 3 acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 9 references [1] iso 14577-2:2015, metallic materials – instrumented indentation test for hardness and materials parameters – part 2: verification and calibration of testing machines. [2] bipm website, calibration and measurement capabilities. online [accessed 20190912]: http://kcdb.bipm.org/appendixc/ [3] y.-k. park, h.-k. song, h.-y. lee, m.-s. kim, small-capacity deadweight force standard machine compensating loading frame, measurement 45 (2012) pp. 68-73. [4] zh. zhang, y. zhang, t. li, h. ji, the design of 1mn∙m~1n∙m torque standard machine at nim, measurement of mass, force and torque (apmf 2013), international journal of modern physics: conference series 24, (2013) pp. 1360024-1~7. [5] zh. zhang, t. li, y. zhang, h. ji, f. meng, research on the moment-arm system of 1mn∙m~1n∙m torque standard machine, acta metrologica sinica 37, 2 (2016) pp. 151-154. [6] zh. zhang, y. zhang, h. zhou, x. zhang, k. wu, g. hu, a fullautomatic 1 mn deadweight force standard machine, acta metrologica sinica 29, 1 (2008) pp. 65-68. [7] h. gang, j. jiang, t. li, zh. zhang, h. ji, l. zhuang, a new 10n deadweight force standard machine, proc. of imeko tc3, tc5, tc22 joint conferences, helsinki, finland, 30 may – 1 june, 2017. online [accessed 20190912]: https://www.imeko.org/publications/tc3-2017/imeko-tc32017-009.pdf [8] h. gang, l. zhuang, zh. zhang, t. li, j. jiang, a new deadweight small force standard machine based on air bearing support, measurement technique 10 (2017) pp. 7-11. http://kcdb.bipm.org/appendixc/ http://kcdb.bipm.org/appendixc/ https://www.imeko.org/publications/tc3-2017/imeko-tc3-2017-009.pdf https://www.imeko.org/publications/tc3-2017/imeko-tc3-2017-009.pdf https://www.imeko.org/publications/tc3-2017/imeko-tc3-2017-009.pdf https://www.imeko.org/publications/tc3-2017/imeko-tc3-2017-009.pdf https://www.imeko.org/publications/tc3-2017/imeko-tc3-2017-009.pdf https://www.imeko.org/publications/tc3-2017/imeko-tc3-2017-009.pdf template for an acta imeko event paper acta imeko issn: 2221-870x december 2016, volume 5, number 4, 64-72 acta imeko | www.imeko.org december 2016 | volume 5 | number 4| 64 on the safety design of radar based railway level crossing surveillance systems tommaso addabbo, ada fort, cristian della giovampaola, marco mugnaini, alberto toccafondi, valerio vignoli department of information engineering and mathematical science, university of siena, via roma 56, siena, italy section: research paper keywords: availability studies; reliability studies; safety; radar; railway level crossing citation: tommaso addabbo, ada fort, cristian della giovampaola, marco mugnaini, alberto toccafondi, valerio vignoli, on the safety design of radar based railway level crossing surveillance systems, acta imeko, vol. 5, no. 4, article 10, december 2016, identifier: imeko-acta-05 (2016)-04-10 section editor: lorenzo ciani, university of florence, italy received september 23, 2016; in final form october 28, 2016; published december 2016 copyright: © 2016 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: marco mugnaini, e-mail: marco.mugnaini@unisi.it 1. introduction the design of any system interacting with the signalling infrastructure of railways implies the integration of safety requirements. the case of surveillance systems for level crossings (lc) is a clear example of how a system can be deeply affected by such an issue. different techniques can be exploited for the design of an lc monitoring system, nevertheless the advantages with respect to the drawbacks of each possible technical solution should be weighted towards the safety constraints before actual implementation. in particular highly performing solutions may result in unfeasible designs when considering safety requirements. usually the approaches followed for safe system design derive from the standard in [1] and on minor modifications of previous designs for similar applications, as in [2]-[4]. according to these guidelines, software, hardware and communication issues are taken into consideration in granting the needed safety targets. what happens in practice is that designers when following the applicable standards often do not include important problems, which cannot specifically be related to these three main categories, but rather to the physical realization of the system and above all to its interactions with the environment. this is due to a lack in the preliminary problem analysis [1]-[4]. some authors, tried to approach the problem just in terms of system performance as [5], [6] neglecting to design the system in order to meet the minimum safety requirements. others as [7]-[9] used the standard approach to safety design trying to exploit reliability block diagram methods or fault tree analysis resulting in a partial problem overview. of course, this kind of approach is important and is a fundamental aspect of a safety design but, in many cases, it is not sufficient: the safety abstract recent accidents experienced at railway level crossings are pushing researchers to design surveillance systems able to grant safety of passengers and structural integrity of trains at level crossings. the challenge is represented by granting at the same time an appropriate reliability, availability and maintainability degree despite the high safety requirements imposed by the application. the approach proposed in this paper takes into consideration the most common suggested standards used in designing this kind of systems and introduces new general concepts which demystify the use of such standards in actual applications. this paper illustrates the roadmap to be followed in general when designing level crossing monitoring systems, to minimize the risk due to object misdetection occurring on barrier closure when exploiting radar technology. a specific surveillance system based on fmcw radar technology was used as a case study, but the approach presented in this paper has a general validity acta imeko | www.imeko.org december 2016 | volume 5 | number 4| 65 assessment for a complex system needs in addition a deep application scenario analysis. speaking of monitoring or surveillance system design, in general, a mixture of different aspects as described in [10]-[20] should be considered, depending heavily on the specific technology used for all the components, with a special attention to the sensor technology which tremendously affects the overall system safety integrity. in this paper the problem of designing a monitoring system for railway lcs taking into account all the safety requirements is discussed. in particular a roadmap on the needed steps is provided to meet the highest safety degree required by commonly used standards [1-4]. the approach followed in this paper, is to consider the safety allocation and requirements as pre-established values which have to be satisfied according to the selected lc application. alternatively, in what follows, the allocation phase outcomes are considered as problem given inputs. the allocation analysis and the failure modes analysis are therefore not performed. the requirements in terms of safety integrity level and tolerable hazard rate are taken as given values. the paper is organized as follows: in section 2 the general safety theory is presented and a generalized approach to apply it to railway monitoring and interfacing system is provided. a general description of the most used standards is supplied as well. in section 3 a case study is presented, based on radar technology, by using this specific example it is possible to discuss how all safety aspects, also those not explicitly mentioned in the standards, impact on the system design. finally, the conclusions are presented. 2. theory of safety design of railway monitoring systems this section aims at the definition of the approach which should be followed when designing a complex system, such as an lc monitoring system, satisfying a pre-established safety level. in particular for railway level crossing systems (lcs) sil4 is the requirement to be met. the problem is usually approached through a top-down analysis. several tools can be used and merged at this level as suggested in [8], [9]. usually to act effectively on a complex problem, decomposition in hardware, software and communication infrastructures is followed, which implies the division of the system into subsystems interconnected through a diagram representation as proposed in figure 1, where the proposed blocks identify both hardware and software components interacting through the communication infrastructure to generate a composite fail safe system. actually the highest safety degree which can be claimed by these structures considered into continuous operating mode [1][4] depends on the maturity of the hardware technology and its specific design, on system software-hardware integration and diagnostic coverage design and on other factors too. it has to be underlined, however that, in many complex applications such as the one considered in this paper for the detection of obstacles in railway lcs, there are some aspects going far beyond the simple involvement of both hardware and software, as it will be discussed later. starting from the system decomposition, to address safety study, the roadmap shown in figure 2 is followed. the first step addresses the system conceptual design through a preliminary hazard analysis (pha). then the safety requirements identified for the specific application jointly with the design requirements are used to build the final system specification meant in terms of both functional and rams requirements. once the preliminary design is completed a detailed low level (system and subsystems) hazard analysis (ha) is performed and the countermeasures for the specific application identified and managed as detailed safety requirements for subsequent safety allocation. after the safety analysis is conducted and the safety case is managed, then the final system is designed and verified and retrofitted according to the evaluation phase outcomes. in the validation phase both the tolerable hazard risk (thr) or probability of failure on demand (pfd) in continuous applications are evaluated and compared with the safety levels allocated in the previous steps [1]-[4]. the key point of the proposed scheme (in figure 2) is that addressing hardware and software and their mutual integration may allow, in principle, to achieve the safety targets and cover the identified safety needs requested by each designed function. nevertheless, in practice, there are a multitude of cases where the system design is “application dependent” needing to address specific physics or scenario assumptions. the avoidance of such approach may result in critical and poor design where just a partial risk mitigation takes place. in what follows a sample case study of a monitoring system for lcs based on radar technology is treated in order to show how apparently, by considering only the hardware safety the pre-established sil targets seems to be achieved, but the figure 2. roadmap to be followed during the design phase of a sil 4 safety related application. figure 1. diagram of the sub-systems composing an lcs. acta imeko | www.imeko.org december 2016 | volume 5 | number 4| 66 scenario or the physical interaction with the environment may actually influence the safety of the overall system. hence neglecting these aspects leads to an unsafe system design and to wrong conclusions. modifications of the initially proposed architecture are also discussed as possible solutions to cope with safety requirements which cannot alternatively be met once issues linked to the specific scenario are introduced. in this paper, in particular, considerations on missed detection probability and on under threshold objects presence in the scenario, may result in jeopardizing exogenous contributions to the achievement of safety target. 3. case study: radar monitoring system for a raylway lcs several technologies can be exploited to implement a monitoring or surveillance system for lc structures. each one has its own peculiarity in terms of sensitivity and detection capabilities and of course some pitfalls. in this manuscript the authors decided to use, as a case study, a specific short-range radar architecture exploiting frequency modulated continuous wave (fmcw) technology, which has proved to be a promising solution due to its simple working principle. the operating frequency 24 ghz ism-band (250 mhz) was chosen for this application. 3.1. hardware and software structures the general hardware architecture is the one depicted in figure 3. the radar sensor is a linear-fmcw system, equipped with a single transmitter and two receivers with the receiving antennas placed at the distance d. signals received by the receiving antennas are processed (demodulated by i/q system), filtered, sampled, acquired and post-processed. the transmitting antenna radiates an electromagnetic wave with frequency sweeping linearly from a low frequency (fmin) up to a maximum one (fmax) in the time interval (tc). the wave reflected by the target is received by the two receiving antennas and the transmitted signal is used to down-convert the received signals to baseband. each beat signal gives the frequency difference between the transmitted and the reflected waves, which in turn is proportional to the distance r between each antenna and the detected object. figure 4 illustrates the periodic saw-wave function (upper plots) which is the modulating signal of the fm transmitted (blue), the frequency of received signal (red) and the beat frequency (lower plot) vs. time. with reference to figure 4, r can be determined using equation (1). f b tcc r s c ∆== 22 00τ (1) where bs=(fmax-fmin) is the sweep bandwidth and ∆f is the measured frequency difference. the relative motion of the object with respect to the radar sensor can be detected since it causes a doppler frequency shift of the received signal frequency. using two receiving channels allows to estimate also the angular position of an object considering it constrained on a plane, exploiting spatial diversity and combining the two measured distances. summarizing, with the sensor and monitoring system in figure 3, range, angle position and, if needed, relative velocity can be estimated through proper (complex) processing algorithms on the demodulated and sampled radar signals [12]. a processing/controller unit has the task of controlling the sensor operations, of processing acquired signals and finally of comparing the estimated received signal power with a proper threshold value in order to detect the presence of an obstacle. once the angular position of the target and the distance have been determined the data is transmitted on a physical support (which can be either traditional copper wiring or optical fiber) to another processing unit for the data post-processing and final decision on the occupancy of the level crossing. following the guidelines in the standards, the safety performance of the described system depends on the failures of the hardware (antennas, conditioning electronics, acquisition electronics, control and processing unit), of the software components (control of the hardware operations, data processing, data managing) and of the data transmission systems. 3.2. safe conceptual design, testing and validation for shortness, in this section, the discussion is limited to the hardware and the data transmission system proposed for this specific application, whereas the aspects concerning the safety of software components will not be treated. for the evaluation of the system thr, using the safety analysis performed during the system design, the safety function figure 3. fmcw structure with one transmitter and two receivers. the presence of two receivers enables to detect both target distance and angular position. figure 4. above: frequencies of the transmitted and received signals as a function of time. below: frequency difference. acta imeko | www.imeko.org december 2016 | volume 5 | number 4| 67 involving the highest complexity in terms of dangerous failure rates, lower diagnostic coverage and highest number of critical components is evaluated. usually such approach should not be followed due to the fact that on safety matters 100 % coverage should be followed for safety function evaluation. nevertheless depending on the designer and assessor experience it is reasonably correct to take into considerations the functions that more than others may be involved in granting the system safety and integrity due to the peculiar system definition and operating conditions. the main function of this system is to change the status of the signaling system from the safe state (red light), which is adopted in all the situations, to the unsafe state (green light) if and only if the system has not detected any object in the lcs specific area once the barriers are closed within a specified scanning time (max. 7s). the corruption of this function can lead to the most critical outcomes in terms of severity. such function is performed by the following items: • radar sub-system • data processing and management • i/o interface board as mentioned in the previous sections the required system sil for railway signaling applications is 4. this safety level can be hardly reached by a single item specifically designed for a new application as described also in the iec61508 standard. the use of multiple items can therefore be exploited to grant the required safety level and to increase the detection capabilities. the corresponding functional block diagram is the one depicted in figure 5. in such system the radar sub-system is represented at least with two radars but it can be set up with any devices number. the data processing and management is defined as 2oo2 main board [1]-[4] due to the fact that the architecture has to be “composite fail-safe”. the board is equipped with two processors [10] which are devoted to the evaluation of each radar signal content enabling the system to drive the i/o interface, if and only if, there is no reasonable doubt to have correctly scanned the area. the i/o output board has at least two outputs driving the energized sides of an actuator in order to send a signal only in case both the cpu on the main processing board give consensus to the change of the system from safe state (red light for train passage) to actuated state (green light to train passage). such architecture allows in case of failure of one unit (cpu or radar or i/o output) to interrupt the communication avoiding the inappropriate driving of the signaling system. in figure 6 the corresponding reliability block diagram (rbd) is derived from the system mission definition referring to the blocks of figure 5. the overall system is treated as a reliability series due to the fact that the failure of any item contributing to the safety function coverage implies the missing of the safety requirements. once the working environment has been defined (for this application is 40 °c and ground fixed), the failure rates can be derived exploiting the mil-hdbk-217f database, while the repair rates have been assessed on the basis of similar applications, referring to the specific system hardware. for the sil evaluation (thr according to the needed standard [1]-[4]) an homogeneous first order memoryless markovian model has been implemented in the case of two radar systems. the number of states necessary to evaluate the system safety integrity level has been reduced to a minimum of three in this specific scenario. this simplified representation can be successfully exploited for the problem because any markov model can be reduced if the proper aggregation procedure on failure rates is considered. moreover this kind of scheme results in a conservative diagram with respect to different and more complex ones. in figure 7 the obtained markov model (figure 6) is represented, in which the state w represents the system working condition, dd corresponds to the state in which the system has experienced a detected dangerous failure, which does not affect the system safety function, finally du represents a undetected dangerous failure with consequent loss of safety coverage. the diagnostic coverage of the system has been considered to be 99 % due to a set of software self-tests performing hardware checks. the aggregated failure rates and repair rates, obtained from the analysis of the overall system as mentioned above, and needed to describe the transitions among the three states, are summarized in table 1. figure 6. reliability representation (rbd) of the proposed detection system based on radar sensing technology (case of two radar systems). figure 5. functional representation of the proposed detection system based on radar sensing technology (case of two radar systems). figure 7. markovian homogeneous diagram of the system proposed in figure 6. acta imeko | www.imeko.org december 2016 | volume 5 | number 4| 68 starting from the failure rates (λxx, xx=du, dd) and repair rate (µ), the transition matrix t can be evaluated substituting the proper detected and undetected dangerous failure and repair rates into the corresponding diagram symbols of figure 7 obtaining (2): 𝑇 = � 1 − (𝑎 + 𝑏)∆𝑡 𝑎∆𝑡 𝑏∆𝑡 𝜇∆𝑡 1 − 𝜇∆𝑡 0 0 0 1 � , (2) where: 𝑎 = 2𝜆𝐷𝐷1 + 𝜆𝐷𝐷2 + 𝜆𝐷𝐷3 𝑏 = 2𝜆𝐷𝐷1 + 𝜆𝐷𝐷2 + 𝜆𝐷𝐷3 the matrix defined in (2) relates the state probabilities at time t with the state probabilities after a certain discrete time increment ∆t as per (3) � 𝑃𝑊(𝑡 + ∆𝑡) 𝑃𝐷𝐷(𝑡 + ∆𝑡) 𝑃𝐷𝐷(𝑡 + ∆𝑡) � = [𝑇]′� 𝑃𝑊(𝑡) 𝑃𝐷𝐷(𝑡) 𝑃𝐷𝐷(𝑡) � . (3) the differential equations describing the system dynamics can be derived from (3) as shown in (4). lim∆𝑡→0 ⎩ ⎪ ⎨ ⎪ ⎧ 𝑃𝑊(𝑡+∆𝑡)−𝑃𝑊(𝑡) ∆𝑡 = −(𝑎 + 𝑏)𝑃𝑊(𝑡) + 𝜇𝑃𝐷𝐷(𝑡) 𝑃𝐷𝐷(𝑡+∆𝑡)−𝑃𝐷𝐷(𝑡) ∆𝑡 = 𝑎𝑃𝑊(𝑡) − 𝜇𝑃𝐷𝐷(𝑡) 𝑃𝐷𝐷(𝑡+∆𝑡)−𝑃𝐷𝐷(𝑡) ∆𝑡 = 𝑏𝑃𝑊(𝑡) ⎭ ⎪ ⎬ ⎪ ⎫ (4) using the values of table 1, it is possible to evaluate the probability of being in the state of du within one hour. such probability for continuous operating system claiming a sil 4 should be lower than 10-8 (thr <10-8 1/hr). the case study exploiting two fmcw radars hardware implementation satisfies the requirements needed for signaling railway systems as can be seen in figure 8. anyway the configuration selected for the two radars is a functional 1oo2 from a safety standpoint, to take into account that the two radars are observing the same scene from different viewpoints. nevertheless the resulting model corresponding to the markov diagram of figure 7 is a conservative representation of the proposed configuration and can be therefore considered as representative of a worst case condition. the behavior of pdu confirms that such probability within the first hour is never larger than what is required by the standards [1]-[4]. looking the plots in figure 8 it is evident that, within the considered time interval, the probability reaches a value close to the limit set by the standard. this can appear as a dangerous situation since apparently there is a little safety margin. nevertheless it should be reminded that the threshold values suggested by the standards include a sufficient safety margin, and, according to [1]-[4] remain sufficient to accept the proposed design. in any case the results shown in figure 8 are obtained considering a specific sensor redundancy and scenario, but the approach used is generally valid. it is possible to add other sensors increasing the margin with respect to the standard probability threshold. 3.3. safety issues related to the sensing system and application scenario after having evaluated the impact on safety design of the hardware implementation it is important to analyze and take into account the influence of the sensing system devoted to the obstacle detection with reference to the case study. in a radar sensor, the detection of an object in a monitored area is based on the measurement of the power reflected back to the receiving antenna by the object, when illuminated by an electromagnetic wave from the transmitting antenna. the radar detection process involves making decisions based on a set of “return power” measurements. obviously, due to the presence of noise in the received signals, an error-free decision is not always possible, so on analogy to what happens when studying the hardware or software implementation, to evaluate the sensing system safety level, the problem is treated in a statistic framework. let’s define the following quantities: 1. pfa (false alarm probability): it is intended as the probability to declare an object present when it is not actually present. 2. pd (detection probability): it is the probability to declare an object present when it is actually present on the scene. the probability of declaring an object absent when it is actually present on the scene of interest is defined as the probability of missed detection pmd and can be derived from the previous definition as pmd=1-pd. radar systems, even if providing excellent performances, cannot grant 100 % object detection probability as well as a false alarm rate equal to zero. in the present work authors tried to combine the probability of missed detection (pmd) with the standard sil evaluation in order to assume that the composed sil (silc) level is matched, if both the pmd and the hardware sil are within the required probability window of a specific safety allocated target (for this example sil4 of pmd=pfd<10-4 for on demand mode operating systems). in the studied case, it is possible to derive the probability of missed detection of the proposed radar on the basis of some simple and generic assumptions. table 1. dangerous detectable and undetectable failure rates of the subsystem components. 𝛌𝐃𝐃[g/h] 𝛌𝐃𝐃[g/h] radar 2.2853e-007 2.3084e-009 2oo2 main board 2.7423e-007 2.7700e-009 i/o 1.7395e-007 1.7570e-009 figure 8. probability dynamic behavior of the proposed homogenous markovian system. 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 0.5 1 x 10 -8 time [h] p d u ( s ta te ) 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 10 -2e-007 10 0 time [h] p w ( s ta te ) 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 0.5 1 x 10 -6 time [h] p d d ( s ta te ) acta imeko | www.imeko.org december 2016 | volume 5 | number 4| 69 usually when radars are involved the neyman-pearson criteria is used for such determination. to minimize the pfa and maximize pd at the same time can result in some cases in an unfeasible problem. the two above defined probabilities for the threshold based decision algorithm can be related by means of the so called roc (receiving operating characteristic) curves. each curve is the plot of pd(t) versus pfa(t), when varying t (threshold level) at a given signal to noise ratio at the output of the radar system, snrout. if we assume that the noise is gaussian distributed such roc curves [22], [23] for a coherent receiver and for a single pulse can be expressed by (5): [ ]22 2 1 1 /snr)p(erfcerfcp outfad −= − (5) where erfc() is the complement of the error function moreover, in case of free space propagation, the receiver input power, pr in, can be written as: ( ) ( ) 43 2 4 r ggp p rttinr π σλ = (6) where pt is the transmitted power, gt and gr are the gain of the transmitting and receiving antennas, λ is the wavelength of the transmitted wave and σ is the target radar cross-section. it can be seen that the power received from the target depends linearly on the radar cross section, which results in a measure of the detectability of the target; this parameter depends on the object size, orientation, shape and surface characteristics. note that the detection system is aimed at detecting only dangerous objects, named hereafter targets to be detected (ttd), i.e. static targets large enough to cause an accident when the train passes (such as vehicles) correspondingly there will be a minimum radar cross section to be taken into account in the design. objects with a smaller radar cross section (e.g. small animals), have not to be detected, in order to avoid useless stops of the train traffic reducing the overall railway network availability. an estimation of the noise due to the receiver architecture can be obtained using the characterization data in table 2 obtained, for the used radar, by measurements in an anechoic environment. using data of table 2, such power is given by the following equation: 56.77)( [mw] 10753.1 8 −= ⋅= − dbmp p in r in r (7) if the receiver is characterized by a noise figure fr, the receiver output snrout ratio can be evaluated exploiting (8) nr in r out pf p snr =1 (8) where pn is the input noise power. the noise due to the receiver can be therefore estimated according to eq.(8). again exploiting data of table ii it is possible to compute the noise introduced by the receiver as (9): 56.117)( 1075.1 15 −= ⋅= − mnr nr dbpf pf . (9) once found this value from the characterization measurements, (8) can be used to find a theoretical estimate of the receiver snrout when varying the distance radar-target or the radar cross section. in particular if the maximum distance from the radar to an object in the lc area is considered as the distance r, and if the cross-area of the smallest dangerous obstacle is considered, the minimum output signal to noise ratio, (db) min,outsnr related to the presence of a ttd in the scene can be found. this minimum signal to noise ratio value allows for selecting the threshold for the detection of an object in the scene. in table 3 data concerning the four limit situations to be considered in the case study are reported: two possible maximum distances (for two possible areas of lcs) and two minimum radar cross sections (selected as the cross-area of the smallest ttd). the false alarm probability, which has no direct impact on the safety level, can be a priori established by the system end user. in this case the authors assumed that the system should declare only once per year that an object is on the scene when actually it is not present. supposing a working time of 5000 hours per year this corresponds to a pfa= 2x10-4 (fixing the threshold of 12 db). the plot in figure 9 shows the detection probability, pd, for the considered radar, obtained from the roc curves (at different snrout) that relate pfa and pd, keeping pfa= 2x10-4. as far as safety level is concerned, instead, due to the fact that the desired sil level is 4, the pd should fulfill the sil 4 requirements (pd =0.9999). exploiting the curve of figure 9 it is possible to determine that such value is reasonable if the receiver has a snr=17db which corresponds to a detectable object with a minimum radar cross section of 0.63 m2 within 25 m distance, in ideal conditions. however in a real scenario, the detection system capability depends also on the presence of clutter (non-gaussian noise), which degrades the snr. this problem is usually mitigated by clutter cancellation techniques, such as integration over multiple scans [23]. in the present case two radars are used to monitor the same scene from different positions so the overall probability of missed detection is the product of the two probability, counterbalancing the effect of clutter. numerical evaluation shows that the target limits are respected. even if the probability of missed detection fulfills the requirements for a sil4 system, there is another issue that has table 3 data corresponding to different conditions: two minimum σ and two maximum distances for the target. σmin (m 2) (m) maxr (dbm) inmin,rp (db) in min,outsnr 0.31 30 -105.5 12 0.31 25 -102.3 15.2 0.63 30 -102.5 15 0.63 25 -99.3 18.2 table 2 characterization data for the radar sensor. parameter value measured signal to noise ratio (snr) 40 db measurement distance, r 8 m target radar cross section, σ 1 m2 transmission power eirp (ptgt) 20 dbm receiver antenna gain, gr 9,6 db acta imeko | www.imeko.org december 2016 | volume 5 | number 4| 70 to be considered to grant the desired safety level. in fact the detection system is designed to detect only dangerous objects, ttds, on the other hand objects with a smaller σ (e.g. small animals), have not to be detected. nevertheless a non-detectable object placed on the scene may cause safety problems when in the optical path between the sensor and a ttd. in this case, in fact, the under threshold target (utt) can mask the ttd due to signal attenuation. therefore even if this kind of scenario is not described in the applied standard it can represent an hazard whose effect has to be modeled and quantified in order to be included in the final calculation of the missed detection probability. in the following, the authors assessed the probability of a missed detection due to the presence of an utt masking the ttd in a worst case framework, based on the spatial resolution characteristics of the case study radar detection system. in analyzing the problem we consider the lc area split by a mesh defined considering the range and angular resolutions of the radar system, ∆θ and ∆ρ, (see figure 10), so that the scene is covered by a grid of resolution cells that can be referenced by an angular index, θ, and range one, ρ, being impossible to distinguish two targets which are located inside the same cell. assuming that the utt occupies a single radar resolution cell, to perform the calculation in the worst case it was assumed that also the ttd occupies a single cell irrespective of its distance from the radar sensor. this is clearly a pessimistic scenario because a vehicle radar cross section will occupy more than a single cell. note that fixing the cell number occupied by an object irrespective of the object position in the scene, is indeed a simplification, because the resolution cell has a variable area. in any case considering the furthest cells area when fixing the number of cells occupied by the targets gives a worst case scenario. in the lc monitoring system a dedicated reference object is placed in front of each radar sensor on the opposite side of the monitored area, which has always to be detected and allows also for taking into account environmental parameter variations (temperature compensation, rain etc…). it is assumed that the unwanted object can never lay on the optical path connecting the radar to its reference object because in this case the absence of the detection will bring the system in a safe state. considering the grid made up of nθ angular division, and nρ range division, we assume a uniform probability for an object (both ttd and utt) to be in a certain cell (ρ, θ) given by pρ,𝜃 = 1 nθ𝑁𝜌 . (7) for one radar the probability of missing the detection of a ttd can be evaluated as follows: 𝑃𝑀𝐷 = 𝑃𝑁𝐷𝑃{𝑇𝑇𝑇 𝑖𝑖 𝑡ℎ𝑒 𝑠𝑠𝑒𝑖𝑒}, (8) where pnd is the probability the object is in the scene (in the lcs area) but it is not detected due to the presence of an utt. to find pnd we consider the following equation: 𝑃𝑁𝐷 = ∑ 𝑃′𝜌,𝜃𝜌,𝜃 𝑃{𝑇𝑇𝑇 𝑖𝑖 𝑠𝑒𝑐𝑐 𝜌, 𝜃 }, (9) 𝑃′𝜌,𝜃 = 𝑃{𝑈𝑇𝑇 𝑖𝑖 𝑡ℎ𝑒 𝑜𝑜𝑡𝑖𝑠𝑎𝑐 𝑜𝑎𝑡ℎ|𝑇𝑇𝑇 𝑖𝑖 𝑠𝑒𝑐𝑐 𝜌, 𝜃}. the conditional probability 𝑃𝜌,𝜃 ′ can be easily evaluated considering that it corresponds to: 𝑃′𝜌,𝜃 = 𝑃 {𝑈𝑇𝑇 𝑖𝑖 𝜃, 𝑅 < 𝜌} = ∑ 𝑜𝜃,𝑅𝑅<𝜌 (10) where r represents the distance of the utt. in figure 11 the value of 𝑃′𝜌,𝜃 is color coded, obviously it increases with the distance of the ttd from the radar, in this figure the optical path to the reference object is assigned a 𝑃′𝜌,𝜃 equal to zero. note that 𝑃′𝜌,𝜃 is always smaller than 1/nθ. so with this very simple approach we can write: 𝑃𝑁𝐷 = ∑ ∑ 𝑃′𝜌,𝜃 1 𝑁𝜌𝑁𝜃 𝜃𝜌 = ∑ ∑ 𝑜𝜃,𝑅𝑅<𝜌𝜌 1 𝑁𝜌 = 1 𝑁𝜃𝑁𝜌 2 ( 𝑁𝜌�𝑁𝜌−1� 2 − 1 ) < 1 𝑁𝜃 (11) where we considered all the events ‘the ddt is not detected but in the scene mutually' exclusive events. these very simple results give a rule of thumb, which is obvious: by increasing the angular resolution the missed detection probability decreases. actually this remains true only if the radar cross section of the utt allows it to remain in a figure 9. probability of detection of a target under the assumption pfa= 2x10-4. figure 10. cell distribution of a single radar sensor on a 6x6 m2 area (lcs area) considering 2° of angular resolution and 60 cm range resolution for the selected bandwidth of 250 mhz. radar position (ρ,θ) 0 nθ 0 nρ reference object ttd acta imeko | www.imeko.org december 2016 | volume 5 | number 4| 71 resolution cell, otherwise the probability pnd depends on the angular aperture determined by the cross section. the designer can evaluate how the sensor redundancy mitigates this type of missed detection. in fact by using more than one sensor the missed detection due to a single utt is avoided since the lines of sight from the ttd to the radar systems are different. in case of n radar systems the missed detection can be caused only by the simultaneous presence of n utts obscuring the three lines of sight. such a situation can be described by the joint probability of n independent conditioned events, each described by the same 𝑃′𝜌,𝜃. in the analyzed case pnd is close to 10-4 for the scenario with two radars whereas three radars pnd give a value close to 10-6 which is a satisfactory result according to pfd requirements. in the last case, the overall probability of failure on demand (pfd), which corresponds to the probability of missed detection due to the presence of three utts in the scene (pmd) is given by the product of the estimated pnd and p{ttd in the scene}. this latter quantity is an a priory value obtained from the analysis of the historical data for train. to grant the performance of a sil 4 system it has to be smaller than 1/10. the conditional probability obtained with three different radars placed at the three corners of the lcs area is shown in figure 12. in the more realistic case a ttd occupies more than a resolution cell, while the utt occupies 1 cell, therefore a modified approach can be followed. assuming that the ttd occupies kxk cells, if the value of 𝑃′𝜌,𝜃 is evaluated using (10) again, but considering an angular resolution ∆θ’=k∆θ and a range resolution ∆ρ’=k∆ρ. therefore the following inequality holds: 𝑃𝜌,𝜃 ′ = ∑ 𝑜𝜃,𝑅𝑅<𝜌 < 1 𝑁𝜃 ′ = 𝐾 𝑁𝜃 (12) note that this probability is not anymore the probability of having the ddt in the position (ρ,θ) masked, but the probability that if the ddt is in (ρ,θ) than a udt (occupying only 1/k2 of the cell) is in the cells on the line from the ddt to the radar. so the conditional probability of having the ddt in (ρ,θ) masked is the probability of having at the same time at least k udts in the same cell (ρ,θ) aligned so to mask the object. hence we have: 𝑃𝜌,𝜃 𝐾 = 𝑃{𝑇𝑇𝑇 𝑚𝑎𝑠𝑚𝑒𝑚| 𝑇𝑇𝑇 𝑖𝑖 (𝜌, 𝜃)} = = (𝑃𝜌,𝜃 ′ 𝐾) (𝐾−1)! 𝑘𝑘−1 < � 𝐾 𝑁𝜃 � 𝐾 . (13) so if k < nθ we have a reduced conditional probability, as expected. the probability pnd is obtained in the same way as before (considering ∆θ’ and ∆ρ’). in the case taken into account, by choosing a value k = 2 (and three radar) the value of pnd is far smaller than 10-8, so in this case irrespective of the actual value of the a priori probability of having k×n udts on the scene the sil 4 requirements are fulfilled. note that k=2 is a really cautious value for the radar cross section of any dangerous object (vehicle). 4. conclusions in this paper the authors presented a case study to analyze the problem of designing a safe monitoring system for level crossing using radar technology. in the present manuscript a strategy to manage the problem was proposed and an extensive evaluation of a possible configuration satisfying sil 4 requirement was illustrated. in the paper, analyzing a case study concerning a railway crossing monitoring system based on fmcw radar technology, a specific configuration has been suggested in order to satisfy the safety constraints provided by the standard. the analysis of this special case allowed for highlighting how a safety study based on the analysis of component failure only, is not sufficient, and can be in a certain way misleading if the overall scenario is not taken into consideration. in particular it has been shown that it is important to analyze and take into account the influence of the sensing system and its interaction with the environment. in the studied case, for instance, it has been shown how the probability of missed detection of a figure 11. 𝑃′𝜌,𝜃 represented as a function of the position in grid. the value of the probability is color coded. the dark line represents the forbidden area where the utt is masking the reference object with a consequent safe state determination. figure 12. conditional probability 𝑃′𝜌,𝜃 given by three utts. acta imeko | www.imeko.org december 2016 | volume 5 | number 4| 72 dangerous obstacle due both to the radar performance and to the presence on the scene of small undetectable obstacles have to be modeled and evaluated in order to obtain a sil4 monitoring system. moreover it has been shown that a safety design has to find a good tradeoff among functional requirements, overall system complexity, and safety parameters. references [1] iec 61508 functional safety of electrical/electronic/programmable electronic safety-related systems (e/e/pe, or e/e/pes). [2] en 50126 railway applications the specification and demonstration of reliability, availability, maintainability and safety (rams). 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[20] addabbo, t., fort, a., mugnaini, m., vignoli, v. “gas turbine sensor loop availability driven fall back strategies design” (2015) conference record ieee instrumentation and measurement technology conference, 2015-july, art. no. 7151343, pp. 641645. [21] stove, a.g.; linear fmcw radar techniques (radar and signal processing, iee proceedings f, volume 139,issue 5, oct. 1992, pp.343-350). [22] richards, m.a. fundamental of radar signal processing, 2nd ed.; mcgraw-hill: new york, ny, usa, 2014. 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october 2018 | volume 7 | number 3 | 42 advice for archaeological survey with recent technologies andrea angelini1, damiano portarena2 1 institute for the technologies applied to cultural heritage, cnr-itabc, monterotondo, italy 2 university of rome tor vergata, rome, italy section: research paper keywords: archaeological survey; laser scanner, photomodeling, drones, surface reconstruction, texture mapping citation: andrea angelini, damiano portarena, advice for archaeological survey with recent technologies, acta imeko, vol. 7, no. 3, article 8, october 2018, identifier: imeko-acta-07 (2018)-03-08 section editor: egidio de benedetto, university of salento, italy received april 13, 2018; in final form september 21, 2018; published october 2018 copyright: © 2018 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: the methodological work is part of the research activity carried out in the cnr. corresponding author: andrea angelini, andrea.angelini@itabc.cnr.it 1. general introduction since the use of direct methods, the survey activity has always represented the base for studying and analyzing ancient monuments. above all in archaeology, nowadays, it is necessary to deal with new technologies. the recent survey techniques are characterized by user-friendly instruments able to collect large amounts of data. important for data processing, the survey activity must be a valid support to the archaeological investigation and not the aim of a research project. for this reason, it is necessary to make some considerations on the appropriate use of these techniques, adjusting the new technologies to the historical-archaeological needs. different errors can be added together and influence the final results, so some aspects need to be clarified. the archaeologist should be a professional figure able to move on different application fields and skilled above all in survey acquisition, data processing, synthesis of the results and 3d/2d representation. the aim of the paper is to highlight the measurement errors that occur during a survey, considering the recent and innovative techniques adopted in archaeology. at the same time the authors provided, where possible, some solutions and suggestions for reducing the errors and checking the general quality of the work [1]. the main survey techniques used in archaeology and architecture are focused mainly on range-data (laser scanning) and image-based systems (photomodeling). based on completely different geometric principles the two techniques present similar results (point clouds) [2] [3]. the two methods are based on automated processes that do not allow an appropriate data control, like in a “black box” model. different output can be extracted from point clouds in order to improve the results. graphic documentation represents the main instrument for analyzing and studying an archaeological subject and, generally, it is the base for the virtual reconstruction process. abstract despite the technologies are now part of the archaeological discipline, they must not divert attention from archaeological issues and should be aimed at solving different historical and methodological questions, such the phases of a monument or innovative representation methods. the paper focuses the attention mainly on the use of range-data and image-based systems applied to the archaeological heritage, in order to highlight differences between the techniques and the related errors. some considerations are necessary in order to find possible solutions for improving the quality and the accuracy of a survey project, considering the recent and innovative techniques adopted in archaeology. at the same time the authors provided, where possible, some solutions and suggestions for reducing the errors and checking the general quality of the work. different experimentations have been made on some case studies that show how to manage technologies trying to reduce as much as possible the errors in the different phases of the survey pipeline. a specific part has been dedicated to the photogrammetric process from drones compared to traditional acquisitions, usually performed with aluminium poles, and the common errors in the representation of the archaeological excavations. mailto:andrea.angelini@itabc.cnr.it acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 43 some considerations are necessary for evaluating common errors generated during an archaeological survey, in order to find possible solutions and improving the quality and the final accuracy of a project. the paper focuses the attention mainly on the use of laser scanner and digital photogrammetric systems, but it evidences also different issues associated to the processing phase. the latter has been analyzed through the use of integrated software and focused mainly in the surface reconstruction and texture mapping process for the generation of reality-based virtual models. some experimentations were performed on different case studies in order to highlight differences between the techniques and the related errors. a specific part was also dedicated to the use of drones (unmanned aerial vehicles, uavs) in archaeology compared to traditional acquisitions, usually made with aluminium poles, and the errors in the representation of the archaeological excavations. 2. the measurements errors in the survey the aim of a survey project and the choice of the representation scale influence directly the parameters for an appropriate acquisition phase. in addition to the instrumental error declared by manufacturers, the main element to consider is the error allowed in 2d representation (besides the graphic error). it gives us information about the value beneath which it makes no sense to take measurements. this value was established in relation to the human sight (0.3 mm), so that in the 1:50 representation scale, the maximum allowed error is 1.5 cm while, for instance, in the 1:100 representation scale the allowed error is 3 cm [4]. although the resolution and accuracy of the instruments are very high [5] [6], main errors occur both during the acquisition phase and the processing of the point clouds (registration of the scans), also considering the high number of acquisitions necessary for accomplishing a survey. the integration of different survey techniques allows to have complete and heterogeneous data in the same reference system but usually it tends to decrease the accuracy of an entire project. still today survey methods in archaeology and architecture are based on data integration between laser scanner and topographic systems. with the aid of a total station, traverses are performed in order to register the scans of a survey project and to decrease the spread error of each scan of the laser. the entire scan project is registered through the use of planar targets (more recently spherical targets for automatic registration), the ground control points (gcps). more precisely, each single scan of the laser presents the same characteristics of the main nodes of a total station, due to the same method of acquisition (polar). in topography each node of a traverse suffers from a spread error that must be compensated with known algorithms (angular and distance compensation) [7]. the approach with the gcps allows to align the scans in the reference system, checking the registration error of the final survey. the mse (mean squared error) is the average error of the scans based on the gcps used in the registration phase. the choice of adequate points and their arrangement represent the main quality parameters for the registration step. since the 90s a series of algorithms for the correct mutual position of the “scans” has been implemented. such algorithms are known as iterative closest points (icps), able to move a point cloud over another point cloud (c2c registration) on the basis of thousands of points (from 5.000 to 20.000 and above), thus reducing the final overlapping error (figure 1) [8] [9]. the algorithm needs a pre-arranged combination of two scans and oriented points in order to be satisfied. this approach allowed to modify the procedure during the acquisition phase. at present, it is more important to have a good overlap (60-80%) between the scans in order to satisfy the requirements of the icp algorithms. the icp registration is able to reduce the spread error down to a few millimetres, effectively excluding a topographical survey. one of the risks is that the acquisition of gcps with a total station could increase the registration value due to systematic errors included both in the acquisition on the field and during the processing step. once the clouds have been registered, figure 1. two and more point clouds can be aligned according to icps algorithms in reliable way. figure 2. dense cloud generated from photomodeling system with a 50 mm optic lens and 5 gcps identified on the numerical model. acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 44 usually the bundle adjustment algorithm is applied to distribute the error on the entire scan project. below some considerations on icps registration method: icps algorithms are based on statistical methods for the recognition of the points between two clouds. although the results seem to be accurate, the points control is not available. the target method (gcps) is based on limited number of points but each error can be controlled any time from the user; despite icps algorithms are very useful for data integration, they are not able to satisfy all the conditions. some archaeological subjects need the topography to correct the mutual position of the scans, considering their extension and complexity. the entire fortress wall of an ancient settlement represents an example where the topography cannot be completely excluded. in order to avoid that the walls do not close (assuming the beginning and the end of a closed ideal figure) it is necessary to define a verifiable traverse and specific target for referencing the entire scan project. in non-urban areas the authors suggest the definition of the main traverse with a differential gps system (dgps) instead of a total station, used eventually only for the recognition of the target. dgps allows to collect information of each node of the traverse with the same instrumental error, influenced only by satellites visibility and disposition, thus avoiding further corrections (±1 cm over the entire investigated area. the system used in real time mode is able to maintain the same accuracy up to 35 km) [10, 11]. by orienting the total station on 3 nodes of the dgps traverse it is possible to reference (and georeference) each new job of the scan project. similar issues occur also in the photomodeling process. the literature in this field is very wide [12, 13], so only some considerations will be presented in relation to the archaeological survey. different characteristics and procedural errors can sum up together (figure 2): the size of the camera sensor (full-frame, aps-c, the real sensor dimension expressed in mm varies according to the device); the type of lens used (fixed or zoom) and its characteristics expressed by mtf graphic (resolution expressed in lines/mm and contrast); the maximum resolution of the camera (mp); the calibration parameters calculated from different software (internal and external orientation usually based on mathematical models and fixed chessboards); the appropriate data acquisition of the subject (parallel and/or convergent acquisition); the reconstruction algorithm used by the software for generating the dense cloud; the projection error in the scaling transformation. a test was performed on the facchino’s fountain in rome with two different photogrammetric software applications; agisoft photoscan and 3d zephyr. the same images and gcps (5 points) were used. the first evaluation concerned the projecting error of the two numerical models. the mse of photoscan cloud is 3.5 mm, while the zephyr cloud is 3.1 mm. this gap is mainly due to the choice of the points on the numerical models, characterized by different resolution. although the error is quite the same, differences can be noticed in the dense cloud reconstruction. measuring the distance between the two numerical models, in a threshold of 5 mm, it is possible to highlight how the reconstruction algorithms determine also the final result (cloudcompare, figure 3). the histogram shows differences between the two point clouds generated by the corresponding software. many discontinuities are evidenced along the entire area, both on the sculpture and the architectural part (figure 4). editing some settings (i.e. alignment and depth filtering) the position of the points in the virtual space can be improved, modifying also the final result. photomodeling systems generate not scaled numerical models. two different approaches are possible for scaling the point clouds: direct measurements on the subject. this approach according to the authors is less accurate because it is based on direct distance between two points of the subject; the measurements depends also on the dimension of the subject and from the possibility to reach inaccessible parts; point coordinates. this system is based on the acquisition of specific points. even though at least 3 points are enough to scale an object in virtual space, the scaling process needs almost 510 points well distributed on the cloud to reduce the projection error of the 3d model. in archaeological context the integration between range-data and image-based systems is very useful for recognizing inaccessible parts and filling shadow areas produced from the laser scanner. often point clouds generated from figure 3. the distance between the two point clouds highlighted different discontinuities in the dense-cloud reconstruction process. figure 4. the histogram shows the 8 classes where points are distributed. only a small part of them is included in the minimum distance between 0 and 0.6 mm. acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 45 photomodeling systems have a higher resolution than those of the laser scanner. this difference in resolution could make the surface reconstruction and the representation difficult. different tools offer the possibility to sub-sampling the clouds at the same resolution for a homogeneous result. 3. remotely piloted aircraft system a particular attention has been dedicated to the remotely piloted aircraft system (rpas; drone) and the related digital photogrammetric acquisition. a complete research paper, published by grussenmayer, evidenced interesting ideas and best practices for a correct approach both in the acquisition and processing phases with multi-copter rotary-wing type [14]. the paper presents also important information regarding the point clouds reconstruction and camera calibration. drones allow to fly in restricted areas and are very useful for the archaeological survey and photo interpretation, considering their capability to visualize from a different point of view archaeological environment (top view); most of them are equipped with a camera and a gps antenna for controlling the flight. usually convergent data acquisition is suggested for satisfying dense-cloud reconstruction algorithms and to better calculate the extrinsic parameters of the camera. however, two are the main issues that influence the flight and the shot: the life of the batteries that currently have an autonomy of 20 minutes per flight; differences in image-texture homogeneity for each mission due to different radiometric values. usually photogrammetric software packages employ exif information for georeferencing the numerical model of the camera in the virtual space. the gps antenna is not a differential system and its accuracy is between 1 m and 10 m, depending from different factors. despite the result seems to be accurate the integrated gps increases the final projection error. except where gps data is corrected by a reference system, the model must be referenced with the aid of a total station or another gnss system. a test was performed at 10 m high with 6 targets on the ground, measured both with a total station and directly on the numerical model generated by the photogrammetric software (figure 5). the starting measured distance between the target differs from the reality of about 0.60 m. by orienting the numerical model in the software with local coordinates (in the reference options) the problem can be partially solved, and the error decreased down to 0.045 m. another experimentation was performed with the same images without gps data. this is possible disabling the reference accuracy in the alignment process or editing the exif file, removing gps data. in the first case the external parameters of the cameras are calculated without gps and the model is scaled. the second option generated an un-scaled dense cloud with a final error of 0.019 m (improved with a total station). for the quick test a self-calibration method was used but in general a full calibration is advisable for avoiding distortions and improving the general accuracy of the project [15]. an important issue concerns the choice of the right height for taking photo. the resolution of the camera sensor today allows to generate ortho-rectified images with high details for any representation scale. however, in order to plan correctly a flight, it is important to establish the approximate height of the drone in relation to the representation scale. this is important to avoid excessive data and long processing time. the gsd (ground sampling distance) and the representation scale are the two important factors for choosing the drone height: the gsd represents the relationship between the sensor width of the camera (millimetres), the focal length (millimetres), the image width (pixel) and the flight height (meters); the representation scale generally used in archaeology for a survey is 1:100, 1:50 and 1:20 for details. gsd was theorized for a dji phantom 4. the drone has a width sensor of 6.17 mm, a focal length of 3.61 mm and an image width of 4000 pixel. at a height of 10 m the gsd is 0.43 cm/pixel, a value more accurate than the one required. by defining the gsd in relation to the supposed representation scale, it is possible to provide the approximate height for the drone flight. supposing a linear dimension of the pixel of 1.5 cm, the drone should fly at 35 m height (presuming that the resolution of the result cannot be lower than the error allowed in the 1:50 representation scale). certainly, this represents only a theoretical approach because different factors can influence the choice of the height. for instance, the extension of the area can determine the best choice above all when the excavation is not particularly extensive. the gsd calculated on ortho-rectified images is an average value that depends also on the geomorphology of the ground. the flight should be done maintaining always the same height from the ground. the right height of the drone allows to acquire a limited number of images for a large territory, optimizing the energy of the batteries. a further consideration concerns the relationship between the rectified image and 2d representation, normally executed by drawing the main archaeological features and details. the archaeologist should evidence a number of details that satisfy the chosen representation scale (otherwise it is useless to have high-resolution ortho-photos). the drawing is the only method figure 5. ortho-rectified image of a flat surface improved with total station measurements. acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 46 based on critic approach and able to give information useful for the historical reconstruction of a monument. recently an innovative method for the representation of rupestrian architecture was experimented and adopted for the archaeological representation [16]. the method is based on the use of the “contour lines” for describing objectively all forms not attributable to a defined geometry, such the rock-hewn architecture. this representation method can be adopted also for archaeological excavations where structures and layers were altered by the time. the system allows to define the edges and the outline that must be represented in the chosen representation scale, modifying the number of the sections on the numerical models with specific settings. 3.1. carbon pole vs drone a specific experimentation was tested for comparing the acquisition and processing steps between a camera mounted on a carbon pole and the built-in camera of the drone on the same archaeological area (figure 6). the comparison was planned in order to evaluate the impact of a proper exploitation of different technologies in order to reduce errors. for the test a carbon pole was used with a maximum extension of 6 m (fanotec carbon pole, figure 7). this configuration is the only able to support a reflex in inclined position due to the weight of the camera. the camera used was a canon 5d mark ii with a 28 mm optic lens. another configuration is available with an extension of 9 m, but no reflex cameras can be mounted. the comparison was executed with a semi-professional rotary-wing platform (dji phantom 4) with a built-in camera (fc330). an archaeological excavation was investigated for the experimentation, characterized by different features like ancient structures and mosaics but also some obstacles such trees and a little building for an overall area of 500 m2. the experimentations aimed mainly at: defining an acquisition procedure repeatable also in other contexts; defining some aspects concerning both the acquisition and the processing phases; understanding which survey techniques are suitable in archaeological excavations considering also the issues related to the environment. figure 6. three numerical models of an archaeological excavation. the test was performed for highlighting differences between laser scanner data and digital photogrammetry from a drone and a carbon pole. figure 7. fanotec carbon pole used for testing the gsd of a reflex camera at different heights. acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 47 as known photomodeling generates non-metric numerical models that need to be scaled with measurements or coordinates. for the test 8 scans were also acquired and matched together in the same reference system. in this way the choice of the points for scaling the clouds in the processing step has been possible. information regarding the two acquisitions is reported below: the acquisition with the drone was performed in gps mode at 78 m from the ground (the height normally is referred to the starting point). a parallel/convergent acquisition was performed for the entire area even if some problems occurred near the trees. due to the weak gps signal the drone was switched in manual mode (atti mode) with high difficulties in the management of the aircraft, subject also to the wind. for that reason, a part of the excavation was acquired at different height and with tilted camera for avoiding the closeness with the trees. the average gsd of the images is 0.30 cm/pixel at 7 m and the theoretical area covered for each image is 108 m2; in the acquisition step the carbon pole was inclined at 6065 degrees to avoid the shadows and to visualize only the archaeological area. for shooting images, a sampling of one photo each 3 steps was used. it would be possible to mount a tablet on the pole to visualize the survey area; however, the light radiation would make it difficult and inconvenient. the acquisition method was organized for avoiding footprints on the ground and shadows, considering the ease of moving the earth during the excavation. the starting point depended from the position of the sun. parallel acquisitions were performed at a height of 5.20 m. this value was supposed and calculated exploiting the following trigonometric formula: h = l sin β where h is the height of the camera, l is the length of the pole (6 m), sin β is the inclination of the pole (60°). at that height the average gsd of the images is 0.12 cm/pixel, but the covered area is only 28 m2 for each image. a total of 70 images was acquired over the entire area. some difficulties were noticed in the management of the carbon pole and its weight, beyond the difficulties of moving on the excavation, depended mainly from the morphology of the area. the processing step was performed with photoscan and jrc reconstructor. the laser scanner clouds were registered together for achieving local coordinates of the points for the scaling process. for the alignment and dense cloud reconstruction of the two groups same parameters were set (high alignment and medium dense cloud). two different clouds were generated, one from the carbon pole (a) and another from the drone (b) without gps data. from the laser scanner 6 points were identified for scaling the photogrammetric models. the model from the carbon pole (a) registered an error of 7 mm on the ground, the second model without the gps (b) registered an error of 8 mm. the comparison made in cloudcompare evidenced a good overlapping between the models a and b with little mismatching in the reconstruction process. the results of the experimentation can be summarized as follow: the models were generated with different number of points; in particular, model (a) is composed by a number of points 3 times higher than the model (b). after the scaling process both the models are enough accurate for the archaeological representation. little differences are not evident in the 1:50 representation scale; model (b) covered an area greater than model (a); the drone is simple to use and can cover large area in about 20 minutes (thanks to the long-life batteries). the main problems regarded the obstacles and the built-in camera quality that is not comparable with a reflex camera; the carbon pole is a very suitable instrument for the photomodeling acquisition on the excavation. by using a wide lens, such the 28 mm, it is possible to acquire a lot of data in very short time, exploiting the properties and the quality of a reflex camera. no permission from the civil aviation are necessary for using a pole in an excavation. the major difficulty regarded the final weight of the device used for long time. the resolution and the quality of the results are evident in the final step even if the processing time has taken more time; the problem of the image homogeneity was influenced by acquisition time; with the carbon pole the acquisition needs 25 minutes while the multi-copter employed only 10 minutes for the entire area. the test was performed during a cloudy day for avoiding high contrast on the ground, but sometime the acquisition stopped due to the direct light radiation. figure 8. orthomosaics of the archaeological area; the first image, from the drone, has a resolution of 2.5 mm/pixel while the second image, from the carbon pole camera, has a resolution of 1.1 mm/pixel. acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 48 the last comparison regarded the quality of the orthomosaics that represent the base for the digitalization in the archaeological representation. differences are present above all for the details (mosaics, figure 8) even if the representation scale allows to use indifferently both the results. 4. the surface reconstruction once point clouds have been correctly filtered (noise reduction and pre-processing filters), the surface reconstruction process becomes necessary to achieve the final 3d representation. surface reconstruction must be considered as a further interpolation of the data. this approximation of the surfaces depends mainly on the algorithms used for the reconstruction but also from the critical approach to the subject. to obtain good results, data synthesis and critical approach must be performed from the archaeologists, defining which are the important parts and the representative points of the 3d model. in this step points are filtered for the representation of the subject with different level of details [17]. the process is executed through the meshing techniques. this transformation is necessary to reduce the point clouds, to create connections among the points, to measure the connected and continued surfaces and to apply different maps from the 2d images (texture mapping process). different mesh generators for unstructured clouds exist, even if the most common are mainly based on the studies made by owen in 1998 [18]. the most used method for generating triangular meshes is that known as delaunay criterion. this is therefore not just a real algorithm, but a selection criterion associated to an algorithm, which subsequently generates the triangular surfaces [19]. the meshing technique is an automated procedure that does not allow the full control of the interpolation, even if some of the algorithms present editable values (i.e. number of triangles, ratio, accuracy, size). independently from the algorithm used, the surface reconstruction aims at generating light numerical models with high details. the main question concerns the choice of the number of triangles, a value difficult to manage in the reality. the issue is how many triangles are necessary for a detailed surface reconstruction, considering that in the representation discipline we are not used to thinking about triangles as units of measure. most of the software applications are able to perform different algorithms for automatic processes with reliable results. the problem is given by the target of the research and the expected results. the possibilities for organizing a surface reconstruction, independently from the algorithm used, are four: a low-res numerical model, poor of details but easy to manage in the virtual space; a high-res numerical model, high in details but difficult to manage with the memory of the computer, characterized by the entire point cloud; a multi-resolution model based on automated algorithms, able to recognize and fit discontinuities on the model (commonly used); a multi-resolution model based on “semantic” subdivision of the point cloud, applying suitable meshing parameters for each architectonic or sculptural element (figure 9). a point cloud can be divided in different entities such as the main walls, the columns, the capitals and altars, separating the general geometry of the structure from the architectural details. for instance, the main walls need low resolution while the columns, the capitals and the altar need a large number of triangles [20]. the result should be a light numerical model with a good number of details (semi-automatic process). the surface model can be also improved through: the optimization methods, such as the smoothing algorithms that maintain the connectivity but rearrange the nodes of the triangles; the cleanup algorithms that maintain the position of the nodes but change their connectivity. the latter improves the general quality of the mesh and their organization but contemporary generates different interpolations that alter original data. other procedures are used to improve the final result: the repair of the mesh that is required when the algorithmic operation is not completely successful, so the model can have holes, or topological problems (i.e. self intersections); the decimation filter that uses a series of algorithms to simplify the model and generate multi-resolution models; the densification or refinement processes to increase the detail of the mesh. the algorithms for the densification processes (such as edge bisection, point insertion, templates) are countless. the archaeological representation scale is able to hide differences between the position of the points and the related figure 9. examples of different surface reconstructions based on controlled multi-resolution methods. acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 49 triangle. sometimes this problem is completely irrelevant for the representation, considering the large amount of data. the suggestion is to systematize the models according to a precise organization of the data decided from the user a-priori and considering the result expected for the research. 5. the texture mapping process the surface reconstruction is also finalized to the texture mapping process. the aim is to map high resolution images on unstructured surface model in order to generate a reality-based virtual model. also in this case it is necessary to take images at high resolution and with a quality optic lens. usually a 28 mm lens is a good compromise between shooting range and radial distortions. the shape and the details of the archaeological subject influence the entire shot. the most common software packages use similar systems for performing texture mapping process, however a test has been conducted with the aid of jrc reconstructor, for the closeness to the descriptive geometry issues. the pipeline can be summarized in the two following steps: the full camera calibration; the image projection process on the numerical model (mesh). the full camera calibration is performed between the image and the point cloud. the accuracy of this operation depends on the optic lens used, the number of homologous points identified and their geometric arrangement, besides the different resolution of the point clouds and images (figure 10). recognizable homologous points can be described with a different number of pixels (for instance, the eye of a holy character). when the resolution of the images differs far (greater/lower) from that of the numerical model used by the laser, the projection error increases. despite 11 points are necessary for the full camera calibration, the authors suggest more than 20 points per image, considering the complexity of an investigated area and the level of detail (lod). the mathematical models are also able to rectify wide lenses such as a 14 mm, but it requires the use of more than 25 points per image (increasing the minimum sensibility requested for solving parameters). to determine the external parameters of an image that represents an architecture, the suggestion is to choose the correspondences on the representative points of the architecture, facilitating the image projection process. a particular difficulty occurs during the recognition of the homologous points. the choice of the 3d points is done on the equirectangular projection of the point cloud (or directly on the 3d model). in this projection, only the central area is useful for the identification of the points, while in the polar areas it is impossible to see details due to their deformation. the possible solutions of the problem are two: the transformation of equirectangular projection into cubic projection, with the support of specific software for the recognition of the correspondences; the acquisition step made in different way (tilting the camera) considering also the poles deformation above all in the indoor spaces. the authors experimented also an innovative procedure with the use of “virtual scan” tool by gexcel [21, 22]. the tool is able to collect virtual scan of a subject in virtual space by using different projection systems such as orthographic, cylindrical, spherical and perspective cameras in any virtual position, established by the user. for instance, by setting a spherical camera, the tool allows to register a new equirectangular grid (360 × 180), composed by the same points without altering their position (graphics card assembled on the computer can influence the resolution and reduce the processing time). a perspective camera has been set with the projection centre directed to the roof and with a wide fov (field of view, 120°). a new structured scan has been collected for camera calibration process without any deformation in the 2d structured image. the full calibration was repeated for all the images and for each calibration a new projector was created. the origin of the projector coincides with the coordinate of virtual projection centre of the related image (x0, y0, z0). by turning it on and off (loading the image) it is possible to view the projected image on the model and to evaluate the accuracy of the process, especially on the edges and corners of the subject (figure 11). figure 10. the different resolution between laser scanner and the related image can generate a high projection error in the full calibration process. figure 11. the texture mapping process increases the quality of the result of an architecture even if the main projection errors can occur in the edges and corners of the numerical model. acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 50 for that reason, it is important to have a light model with high details for evidencing mismatching between the projected images and the structure of the numerical model. by lighting on multiple projectors some errors can be noticed, above all comparing the main features of the images. in order to reduce the mismatching, the images in the overlapping areas can be cut or, where possible, the same points used in the full calibration step can be detected. alternatively, the tool allows to set the boundaries and orientation of the image to be projected on the model. in this way the points recognition is facilitated during the identification process, exploiting the entire image for solving the internal parameters [23]. once the error (pixel) allows to visualize the homogeneous distribution of the image projections (no splitting images), texture mapping process can be performed directly on the mesh model. usually a multi-blending algorithm is used in order to correct the difference in colour tone. the result is a realitybased virtual model able to describe the complexity of an archaeological subject from different points of view. 6. conclusion some final considerations are necessary for fixing the main objectives of the paper. the authors aimed at giving a general view on the survey issues in archaeology, evidencing two important elements: the importance of the new technologies as support to archaeological research; the innovative investigation methods and the risk to produce new errors in the entire survey process. in the last 20 years, surveying activity has changed in the archeological excavations, so that today it is very important to be aware on the use of these techniques. compared to the past the acquisition and processing phases are more complex, considering the amount of data and the operations necessary to accomplish an accurate result. these technologies require more knowledge borrowed from different disciplines and application fields such as the descriptive geometry, the computer graphics, the drawing, the photography, the theory of the errors and many others. all these elements have become for the archaeologists innovative tools that, if well used, are able to generate new information useful for the investigation and study of an archaeological subject. on the other hand, the technologies must not divert attention from archaeological issues and should be aimed at solving different historical and methodological questions, such the phases of a monument or innovative representation methods. a numerical model represents only the base for further processes. a reality-based virtual model can be managed in different ways in order to create new forms of representation difficult to reproduce with the traditional methods. the test presented in the paper showed how to manage technologies in order to reduce as much as possible the errors in the different phases of the survey pipeline. 7. acknowledgements the paper is the result of the collaboration between the authors. in particular, andrea angelini developed the paragraphs 1, 2 and 5, damiano portarena the paragraphs 3, 4 and 6. 8. references [1] a. angelini, d. portarena, some considerations on measurement errors in archaeological survey, in imeko international conference on metrology for archaeology and cultural heritage lecce, italy, october 23-25, 2017, pp. 541-546. 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[13] f. remondino, m. g. spera, e. nocerino, f. menna, f. nex, state of the art in high density image matching, the photogrammetric record, vol. 29, issue 146, june 2014, pp. 144–166. [14] a. murtiyoso, p. grussenmeyer, documentation of heritage buildings using close-range uav images: dense matching issues, comparison and case studies, the photogrammetric record, 32(159), 2017, pp. 206-229. [15] t. schenk, introduction to photogrammetry. department of civil and environmental engineering and geodetic science, the ohio state university, usa, 2005. [16] m. carpiceci, c. inglese, laser scanning and automated photogrammetry for the knowledge and the representation of the architecture cave in cappadocia: sahinefendi and the open air museum in goreme, in caa 2014: 21st century archaeology concepts, methods and tool. proceedings of the 42nd annual conference on computer applications and quantitative methods in archaeology, paris, france, 2015, pp. 87-94. [17] c. bianchini, documentation of mediterranean ancient theatres. athena’s activities in mérida, gangemi editore, roma, 2012, isbn 978-88-492-2524-2. [18] s. j. owen, a survey of unstructured mesh generation technology, proc. of 7th international meshing roundtable, oct. 26-28, 1998, dearborn, michigan, usa, pp. 239-267. [19] r. migliari, geometria descrittiva. tecniche e applicazioni, vol. ii, 2009, isbn 978-88-251-7330-7. [20] m. carpiceci, m. russo, a. angelini, the digital model of the s. zenone chapel inside santa prassede in rome. a case study for the morphological analysis, in 23rd international conference on acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 51 virtual systems and multimedia (vsmm2017): through the looking glass back to the future of virtual reality, dublin, 2017, pp. 238-246. [21] m. sgrenzaroli, g. p. m. vassena, tecniche di rilevamento tridimensionale tramite laser scanner. volume 1 – introduzione generale, starrylink editrice, brescia, 2007, isbn 978-88-8972073-8. [22] a. angelini, g. capriotti vittozzi, m. baldi, the high official harkhuf and the inscriptions of his tomb in aswan (egypt). an integrated methodological approach, in s. grassini, a. santoriello, (eds) the e-journal of the international measurement confederation (imeko) acta imeko, vol. 5 (2016) 2, pp. 71-79. [23] s. dequal, un concetto innovativo: l'immagine solida. bollettino sifet, 1/2003, pp. 67-76. template for an acta imeko event paper acta imeko issn: 2221-870x july 2017, volume 6, number 2, 81-88 acta imeko | www.imeko.org july 2017 | volume 6 | number 2 | 81 a preliminary approach to study the behavior of human fingertip at contact via experimental test and numerical model maria laura d’angelo1, ferdinando cannella1, mariapaola d’imperio1, matteo bianchi1,2 1department of advanced robotics, istituto italiano di tecnologia, via morego 30, 16163 genova, italy 2research center “e.piaggio”, school of engineering, university of pisa, 56126 pisa, italy section: research paper keywords: biomechanics; pressure sensors; pressure distribution; contact area; finite element model; fingertip at contact citation: maria laura d’angelo, ferdinando cannella, mariapaola d’imperio, matteo bianchi, a preliminary approach to study the behavior of human fingertip at contact via experimental test and numerical model, acta imeko, vol. 6, no. 2, article 15, july 2017, identifier: imeko-acta-06 (2017)-02-15 section editor: paul regtien, measurement science consultancy, the netherlands received june 5, 2016; in final form april 29, 2017; published july 2017 copyright: © 2017 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: european community funded project wearhap (no. 601165) and by the erc advanced grant no. 221166 softhands corresponding author: maria laura d’angelo, e-mail: marialaura.dangelo@iit.it 1. introduction the investigation of the mechanisms determining human tactile perception represents a fundamental topic in haptics (i.e. the science and the technology of touch) community. indeed, if the comprehension of the underpinning human perceptual mechanisms is clearly the main goal of many neuroscientific studies, it could also lead to a correct development of tactile devices and haptic systems, as they are intended to convey controllable and effective stimuli. what is noticeable is that the mandatory step to properly develop such investigation is the clear understanding of mechanical properties of soft tissues and, more specifically, the tissues of the main “organ of touch”, i.e., human hand and its fingers. indeed, tactile stimuli are mainly mechanical as the inputs eliciting the response of tactile receptors [1]. in literature, we can find different approaches for in vivo acquisition of mechanical measurements related to human fingerpad (see e.g. [2], [3] and [4]) via different systems, such as digital camerabased techniques to measure skin deformation [5], magnetic resonance (mr) images taken before and during compressional loading of finger tissues [6] or non-intrusive suction instruments [7], among the others. previous studies have determined the mechanical behaviour of human fingerpad under quasi-static and cycling loading, using indenters of various shapes and sizes [8]. furthermore, at neurophysiological level, other works focused on the relationship between fingerpad mechanoreceptors response and different static and dynamic mechanical load stimuli [1], [4] and [9]. looking at non-invasive methods for fingerpad pressure measurements (at the contact between the fingertip and the indenters) [10], they leverage upon different techniques, e.g. pressure sensitive films to determine the peak of force and the pressure centroid during the grasping of objects [11], or flat abstract how human fingertip deforms during the interaction with the environment represents a fundamental action that shapes our perception of external world. in this work, we present the proof of concept of an experimental in vivo set up that enables to characterize the mechanical behavior of human fingertip, in terms of contact area, force and a preliminary estimation of pressure contour, while it is put in contact against a flat rigid surface. experimental outcomes are then compared with the output of a 3d finite element model (fem) of the human fingerpad, built upon existing validated models. the good agreement between numerical and experimental data suggests the correctness of our procedure for measurement acquisitions and finger modeling. furthermore, we will also discuss how our experimental data can be profitably used to estimate strain limiting deformation models for tactile rendering, while the here reported 3d fe model has also been profitably employed to investigate hypotheses on human tactile perception. acta imeko | www.imeko.org july 2017 | volume 6 | number 2 | 82 capacitive array sensors to determine the whole pressure distribution on the human fingerpad [12]. however, to the best of our knowledge none of these aforementioned studies have characterized fingertip behaviour at contact through experimental measurement including contact area, force and pressure distribution, simultaneously, as we report in this work. these in vivo measurements are then commonly used to develop finite element models (fem) of the human finger, which represent useful tools adopted by the haptic research community to investigate some properties that are hardly measurable. in practice, a fem of human fingerpad indentation response could be verified against the in vivo biomechanical data. many numerical models have been proposed in literature, which differ for dimensionality, accuracy and mechanical response tailored on different tasks (see e.g. [3], [4], [14], [15] and [16] among the most significant). however, despite individual differences, all these fems need to be first validated, or, in other words, to be tested in terms of the matching between numerical and experimental results for a large variety of mechanical conditions and parameters. for these reasons, an exhaustive investigation of the mechanical characteristic response of the fingerpad and the acquisition of in vivo data is usually strongly required. in particular, the usage of matrices-array that are able to map force and position over a continuous area or thin and flexible single zone force sensing resistor (frs), are really common for those situations where we have high pressure and wide contact area as biomedical applications. for example, palm, plantar and car seat pressure distribution measurements [17], [18] and [19] (e.g. by novel (st. paul, mn), tekscan (boston, ma) and sensitronics (bow, wa)) are used to investigate grasping, walking and seating forces. the effectiveness of these sensors relies on their low stiffness and high sensitivity. in this paper, we propose an innovative experimental test-rig that can measure in vivo finger deformation, contact and pressure area, while a flat rigid plate is pushed against the fingertip in a controllable fashion. contact area is obtained through vision-based techniques, while pressure contact area information is acquired through a pressure film sensor in paperboard material with 0.05 mn·m stiffness and 10 kpa of threshold [22]. this pressure sensor, made by a film of microscopic pigmented particles adhered to the donor substrate, can adapt to the high deformation of the fingertip (whose compliance is around 0.1 mn·m) [20], [21] that undergoes during the contact of the fingerpad with the external flat rigid surface. in this way, the microscopic particles were attracted to the chemically surface-treated receiver sheet, only when the pressure sensor threshold is exceeded (higher than 10 kpa). our aim is to present a proof of concept of an integrated acquisition system, which allows to study the fingerpad mechanical properties under a specific condition. these experimental measures are then compared with the numerical outcomes of a fe model of human fingertip that we built upon existing validated models and data in literature (gerling et al. [15] and wu et al. [16]). the experimental data we collected have been also applied to correctly compute deformations and contact forces for haptic rendering [23]. furthermore, the model we propose in this work, which exhibits a good agreement with our experimental data, has been used for the investigation of human touch perceptual mechanisms [24]. this paper is divided in two parts: the first one describes the experimental set-up of the test-rig for the measurement of contact area, pressure area (i.e. the area with a pressure above the sensor threshold that is equal to 10 kpa) and contact forces (using a 6 dof force/torque sensor) during experiments of finger indentation. the contact area can be obtained with a high-resolution camera placed beneath the flat rigid plate, using a luminosity threshold-based approach. on the contrary, pressure area is defined as the footprint area over the sensing film. in the second part, we discuss the comparison of our experimental measurements and numerical outputs provided by our fe model of human fingertip built upon literature. finally, we show how the here reported experimental data can be profitably employed to numerically investigate some perceptual hypotheses of human touch perception and how the 3d fe model can be also used for the estimation of parameters for haptic rendering applications. 2. materials and methods during the experimental acquisitions, we indented the subject’s fingertip with a flat surface using a linear actuator, and we measured fingertip area size, deformation and pressure distribution. contact area measurements were carried out in two different ways: i) the contact area was obtained through high resolution video-camera (see details in the next section), ii) the area of the pressure distribution was obtained from the pressure fingertip print on the pressure sensing film. both types of measurements were used for the comparison with a 3d finite element (fe) model of human fingerpad. to perform the experiments, we built the experimental test rig shown in figure 1 and described in the next section. 2.1. test-rig instrumentation the test-rig is a one degree of freedom (dof) device that permits to move a flat rigid surface towards the fingertip and to deform/indent it in a controllable fashion. the displacement is given by a linear dc actuator (faulhaber lm2070-040-01 plus mclm3003 controller) with 9.2 n of constant force, max acceleration of 94 m/s2, 200 µm of accuracy and 60 µm of repeatability. the contact area is recorded by a video-camera (sony hdrcx505ve 12mp) with 1500 x 1120 effective pixels per video frame and 50 fps. the one direction movement is guaranteed by the slider (standa 7t125z-10), which has an angular deviation < 200 µrad. the perpendicular force [n], figure 1. test-rig instrumentation system. acta imeko | www.imeko.org july 2017 | volume 6 | number 2 | 83 acquired during each experimental test, is recorded using the 6 dof force/torque sensor (ati 6 dof nano17) with 0.00625 n of resolution for force and 0.03125 nmm for torque. using this 6 dof sensor we were sure to take under control also the undesired force components that were not perpendicular. the force sensor is rigidly connected to the flat rigid surface (indenter), which is moved against the fingerpad. pressure is measured with a film pressure pressurex-micro green (sensor products inc. pmg1), with 10-400 kpa pressure range and 45 µm spatial resolution. the pressurex-micro green sensor is extremely thin and flexible (0.2 mm), thus conforming to curved surfaces [22]. it is simple to use and it is ideal for noninvasive applications. the procedure that we used during the experiments was: i) the sensor was correctly positioned between the human fingerpad and the flat rigid surface; ii) the flat surface was moved in a controllable fashion against the fingerpad; iii) the film pressure sensor was removed and the pressure distribution profile, which arose from the contact between the two media, revealed. moreover, since the human fingerpad measurements in terms of force [n], contact and pressure area [mm2] are very influenced by the fingertip orientation into the fingerholder, we employed a micro goniometer (standa 7g174-30) with reading resolution of 0.1° in the measurement setup to guarantee uniform and repeatable experimental conditions. the measurement scheme used during the experimental tests is depicted in figure 2 and figure 3. 3. experimental test procedure during each experimental test, we positioned the fingertip in the finger-holder and moved the flat plexiglas surface toward the fingerpad, as shown in figure 3. to prevent losing contact during the experiment, the finger was placed within a custommade finger-holder and fixed to it on the top of the nail and was oriented at 15° w.r.t. the flat surface. the fingerpad was free of callus and subject (right-handed female, age 28) gave her informed consent to the experimental test. we performed three experimental tests indenting the fingertip at three different levels of indentation (1 mm, 2 mm and 3 mm), while the force sensor and the video-camera recorded the force values and contact area, respectively. the linear actuator can control both the indentation velocity and displacement of the flat indenter. three different displacement levels were considered (1 mm, 2 mm and 3 mm) and only one single velocity (2 mm/s), as a preliminary test case. we tested the participant’s fingertip under the different experimental conditions the same day, in the morning, when the fingertip skin was not affected by any mechanical stress or temperature variation due to the daily duties. some results of the images captured by video-camera are displayed in figure 4, where four snapshots show the different contact areas per each indentation displacement (0, 1 mm, 2 mm and 3 mm, respectively). once the high-resolution video-camera images were acquired, the fingertip contact area was measured based on an automated algorithm, built in matlab, which will be discussed in the next section. 3.1. contact area values and force in order to not saturate the fingertip mechanoreceptors response, the finger displacement was along a vertically downward direction with a displacement from 0 to 3 mm [8]. moreover, since we wanted to estimate only the elastic properties of the fingerpad skin, minimizing the effect of viscosity, we selected a very low velocity for the indenter (2 mm/s). the contact area was recorded, when the fingertip skin was completely stuck to the plexiglas surface. since the dimension of the subject’s fingerpad who carried out the experimental tests was 15.5 mm x 12.8 mm, we performed a calibration setup positioning the high-resolution video-camera at 100 mm of distance from the human fingerpad and we chose a video-camera resolution equal to 1500 x 1120 pixels. in this way, we obtained that 1 pixel corresponds to a square with a dimension of 0.0393 mm (0.0015 mm2). using the figure 2. measurement scheme during experimental tests. figure 3. positioning of the fingertip in the finger-holder; the flat surface was moving against the fingerpad. figure 4. contact area at different indentation values: a) 0 mm, b) 1 mm, c) 2 mm d) 3 mm. acta imeko | www.imeko.org july 2017 | volume 6 | number 2 | 84 aforementioned pixel dimension, the imaging post-processing was performed on images with 400 x 350 pixels. once we collected all fingertip indentation images, the post-processing consists of three main steps: 1) for each rgb image, we generated a grey scale image and then we visualized the image pixel distribution, through a pixel intensity histogram. 2) in order to focus only on the subject’s fingertip contact area, we chose a threshold value: we assigned to the pixels whose grey intensity value was above the threshold (value = 50), the intensity level of 0 (black) and hence they were discarded. in this way only the fingertip contact area pixels were considered for the image post-processing phase [28]. 3) the value of the contact area, between the fingertip and the flat window, can be obtained by summing all the pixels whose intensity was different from 0 (black). as previously mentioned, based on the camera and lens characteristics, one pixel has an area of 0.0015 mm2. to determine the threshold (discussed at point 2), we performed preliminary measurements, where we used a five cent coin as ground truth and placed it over the flat rigid surface; while the high-resolution camera acquired the images, the area of contact was estimated using our algorithm in matlab. we found that a threshold of 50 for the intensity ensured a good agreement between the estimated contact area and the geometric one (error less than 2 %). in this manner, we obtained all contact area values, which are reported in table 1. mean areas and standard deviation, between the three experimental tests (per each indentation value), are also reported in table 1. the mean values are obtained as the average of the results across the three tests we performed for each displacement interval. the force values recorded during these tests are reported in table 2, together with their peak force values. 3.2. pressure distribution area values the pressure sensor was made by a film of microscopic pigmented particles adhered to the donor substrate, which are attracted to the chemically surface-treated receiver sheet. the chemical compatibility between the receiver sheet and the donor sheet causes intermolecular diffusion that creates an interpenetrating network between the pigment donor and the receiver coating. upon separation, micro-shear forces compel the particles to dislodge from the pigmented donor substrate, leaving an impression [22]. the film pressure sensor was inserted between the fingerpad and the flat moving surface (see figure 2) and changed for each measurement. the pressure sensor full scale is sufficient for our application. however, considering the film pressure sensor threshold (10 kpa) and resolution (15 kpa), we cannot expect an output with more than two levels of pressure (lower and greater than 10 kpa), since the maximum expected value is around 2530 kpa [25]. note that our goal is to provide a proof of concept of the experimental test-rig, which enables to acquire, simultaneously and for the first time, displacement, force, contact and pressure area. the area with a pressure above the threshold is named “pressure area”. results of pressure area, obtained during the experimental tests from the pressure sensor film, for each displacement interval, are shown in figure 5. from left to right (1 mm to 3 mm), it is possible to observe the pigmented particle (red particles) that adhered to the donor substrate, during the fingertip indentation against the flat plexiglas surface. since the pressure sensor threshold is 10 kpa and the resolution is 15 kpa, we have highlighted only the pressure area at 2 mm and 3 mm of indentation level, with a dashed white line; while the pressure value at 1 mm of indentation was less than 10 kpa. pressure area values, obtained from the pressure sensor film, were manually measured. at the end of each experimental test, the post-processing procedure was the following one: 1) each film pressure sensor was scanned using a normal scanner and the image post-processing was performed in matlab; 2) per each fingertip pressure area, the length and the width at each indentation level (1 mm, 2 mm and 3 mm) was measured using a ruler; 3) finally the pressure area was obtained and calculated considering the fingerprint on the film pressure sensor as an ideal ellipse. pressure area results, with average and standard deviation, are summarized and shown in table 3. some preliminary results, related to both contact area and pressure area, obtained from the post-processing phase, are table 1. contact areas value [mm2] of the indentation with related average and standard deviation (std). experimental area values [mm2 ] indentation 1 mm 2 mm 3 mm test 1 92.82 148.8 147.2 test 2 70.07 117.8 129.6 test 3 99.19 106.6 121.6 average 87.36 124.4 132.8 std 15.31 21.86 13.10 table 2. contact force values [n] of the indentation with related average and standard deviation (std). experimental force values [n] indentation 1 mm 2 mm 3 mm test 1 0.80 1.01 1.72 test 2 0.75 1.05 1.76 test 3 0.78 1.02 1.64 average 0.78 1.03 1.71 std 0.03 0.02 0.06 figure 5. pressure contact area with different indentation: 1 mm, 2 mm and 3 mm, obtained from the pressure sensor film. acta imeko | www.imeko.org july 2017 | volume 6 | number 2 | 85 shown in figure 6. from top to bottom, it is possible to notice the subject’s finger and the contact area images, the latter achieved via the high-resolution camera. it is also possible to observe the pressure area (yellow dotted line). 4. finite element (fe) model the numerical model is a fully parametrized finite element (fe) model that replicates the fingertip shape of right-handed subject (female, age 28), who carried out experimental tests. 4.1. geometry and materials the geometry of the fingertip was determined by the physical one of the fingertip of the subject. we built a model in ansys [26], in order to simulate the shape and structure of a human distal phalanx. the fingertip was assumed to be composed of two skin layers (epidermis and dermis), subcutaneous tissue, bone and nail as shown in figure 7. the fingertip materials used in the model were the same of gerling et al. [15] and wu et al. [16]. in particular, we modelled epidermis, bone and nail as linear materials, while dermis and subcutaneous tissue were modelled as mooney-rivlin hyperelastic material [15]. the mechanical properties of these materials are reported in table 4 and table 5, respectively. moreover, the viscoelastic behaviour of the subcutaneous tissue is described in (1), where w represents the mechanical strain energy, 𝐶𝑖𝑖 are invariant related to the distortional response while 𝐷 is a constant linked to the volumetric response, 𝐼1 and 𝐼2 respectively are the first two deformation invariants. 𝑊 = σ𝑖σ𝑖𝐶𝑖𝑖 (𝐼1 − 3)3(𝐼2 − 3)𝑖 + 𝐷(𝐽 − 1)2 . (1) the fe model of fingertip has 7736 solid brick 8 node elements, with 21714 dof as shown in figure 8. the numerical fingertip model was indented using an inclined plane of 15° w.r.t the finger longitudinal axis. the parameters of the flat rigid surface, in terms of young modulus and poisson ratio were 71.3 gpa and 0.17, respectively. the displacement of the plane is perpendicular to the plane axis, as in the test-rig. fixed constrain was set on the nail. the contact was established between the moving plate and the bottom part of the finger. 4.2. numerical simulations the simulations were carried out moving the flat surface against the fingertip as in the experimental tests, adopting a quasi-static analysis. in figure 9, some images of the simulations are reported. from left to right, it is possible to note an increasing of the indentation and the contact area. the contours display the pressure level between the plane and the skin. the fingertip pressure areas are manually measured from the numerical output obtained by ansys software [26], as shown in figure 9. in other words, per each level of indentation: (1 mm, 2 mm and 3 mm) we manually measured table 3. pressure area values of the indentation with related average and standard deviation. experimental pressure area [mm2] indentation 1 mm 2 mm 3 mm test 1 40.3 87.8 104.1 test 2 37.8 77.0 82.0 test 3 49.0 61.6 79.7 average 42.3 75.4 88.6 std 5.88 13.1 13.4 figure 6. (top) contact area (dotted cyan line) measured through high resolution camera. (middle) pressure area (dotted yellow line) measured with film pressure sensor. (bottom) comparison between contact and pressure area. from left to right: 0 mm (no contact), 1 mm, 2 mm, 3 mm indentation. figure 7. finger 3d cad geometry. table 4. mechanical properties of the linear elastic materials. material young modulus [mpa] poisson ratio epidermis 2.0 0.30 bone 17.0 0.30 nail 170.0 0.30 plate 71300 0.17 table 5. mechanical properties for the mooney-rivlin model materials. material c10 c01 c20 c11 c02 d1 dermis 2430 5420 239000 262000 74700 13.3 subcutaneous tissue 300 671 29800 32700 9330 106.5 figure 8. human fingerpad finite element (fe) model. acta imeko | www.imeko.org july 2017 | volume 6 | number 2 | 86 the length and the width of each pressure area (e.g. < 10 kpa, > 10 kpa and > 20 kpa) and then, as for the contact area, it was approximated to an ellipse. finally, the numerical pressure area, obtained from the numerical simulations were compared to the ones obtained from the pressure sensor film, already reported in table 3. 5. results the results, in terms of force, contact and pressure area, obtained from experimental tests, are then compared with the numerical outputs of a 3d fe model of the finger, which we built in ansys [26]. our fe fingertip model was used to find information on pressure distribution within fingerpad tissues that cannot be achieved experimentally. the fe numerical outputs of force [n], contact and pressure area [mm2] were reported and then compared with the experimental ones, as shown in table 6. per each level of indentation (1 mm, 2 mm and 3 mm) and, per each parameter acquired, the percentage difference (accuracy) between the numerical output and the experimental value was obtained. as table 6 shows, all the percentage values are less than 20 %. 5.1. tactile flow for softness perception the concept of tactile flow, which mathematically describes the rate of expansion of isostrain volumes in human fingerpad, was introduced in [27] to provide a computational model for the explanation of some haptic perceptual illusions as well as for the mechanisms of human softness perception [27]. however, despite many psychophysical evidences support the tactile flow model, a thorough numerical validation still lacks. the 3d fe model proposed in this work has been used to simulate the interaction between a fingerpad and a surface to verify if different levels of softness could be discriminated based on the rate of isostrain volume expansion in the .fingerpad. the results of our analysis reveals that it is possible to discriminate between two materials with different softness. both force-indentation and force-area plots are considered, as shown in figure 10. as it is possible to notice, the curve for contact with the silicone material (red curve) shows lower forces, as one would expect, compared to the steel one (blue curve). moreover, the force-area curve for silicone shows a higher casr (contact area spread rate) with respect to the force-area curve for steel, which is coherent with [27]. 5.2. model estimation: haptic rendering the experimental data we collected have been then used to estimate the deformation model to reproduce the behaviour of finger skin for haptic rendering [23]. in particular, in order to reproduce the behaviour observed in the experimental measurements the experimental setup has been replicated together with the simulation of the response of a virtual finger being compressed by a flat surface. we aim for the simulated contact forces to be as similar as possible to the experimental ones. moreover, we also replicate the boundary conditions of the experimental setup by positioning the finger in the same configuration and fixing the bottom nodes. during simulation, contact forces between the plane and the finger are computed using penalty forces with a stiffness value large enough to guarantee small penetrations. figure 11 shows the finger model figure 10. force-area curves for the finger in contact with steel and silicone plate. figure 11. the images show two simulation steps of the finger in contact with the indenter surface during the estimation procedure. on top, in the undeformed configuration, and on the bottom, fingerpad deformed with a contact area of 150 mm2. left: full render of the finger and the indenter; middle: triangle mesh of the finger used for contact handling; right: tetrahedral mesh of the finger used for deformation. figure 9. numerical pressure distribution results with different indentation: a) 0 mm, b) 1 mm, c) 2 mm, d) 3 mm. the cyan area is comparable to the contact area of the experimental results as well as the yellow area to the pressure area. table 6. force, contact and pressure area values comparison at three different levels of indentation (1 mm, 2 mm and 3 mm). accuracy percentage value is reported as a comparison between the experimental and numerical output. experimental and numerical values comparison indentation 1 mm 2 mm 3 mm experimental contact area [mm2] 87.3 ± 15.31 124 ± 21.86 133 ±13.10 numerical contact area [mm2] 78.2 111 129 accuracy [%] 10.4 10.4 3.00 experimental pressure area [mm2] 42.4 ± 5.88 75.4 ± 13.1 88.6 ± 13.4 numerical pressure area [mm2] 53.3 61.4 80.1 accuracy [%] 20.4 18.5 9.6 experimental force [n] 0.78 ± 0.03 1.03 ± 0.02 1.71 ± 0.06 numerical force [n] 0.65 1.10 1.90 accuracy [%] 16.6 6.63 10.0 acta imeko | www.imeko.org july 2017 | volume 6 | number 2 | 87 [23] in its undeformed state and once deformed. figure 12 shows the contact area vs. contact force evaluation plot. the purely linear model in [23] averages the behaviour for small and large contact areas, but it appears too stiff for small contact areas and too soft as the contact area grows. we can also see how a nonlinear model separates from its linear component when constraints are activated around 100 mm2 contact area, and how the difference becomes larger as the contact area approaches 200 mm2. moreover, results demonstrate that an efficient and robust nonlinear model of skin mechanics can be estimated from controlled force and area experimental measurements. also in this case, higher accuracy would require a thorough investigation material properties, such as viscosity and friction, as well as material heterogeneity. 6. discussion and conclusions in this paper a preliminary integrated characterization of the human fingertip properties in terms of force [n], contact area [mm2], contact pressure [mm2], and indentation [mm], is presented. using an innovative and noninvasive test rig device, we performed three experimental tests indenting the fingertip at three different levels of indentation (1 mm, 2 mm and 3 mm), while, only one single velocity value (2 mm/s), as a preliminary test case, was considered. the results, obtained from the experimental tests, are then compared with numerical outputs of a 3d finite model, which was built on previous 3d model in literature. per each level of indentation (1mm, 2mm and 3 mm) and, per each parameter acquired, the percentage difference (accuracy) between the numerical output and the experimental value was obtained. moreover, the here proposed fe model has been also used to numerically investigate the perceptual mechanism of human softness perception. results, although preliminary, are encouraging and motivate us to proceed towards a more accurate, exhaustive and effective mechanical characterization of finger characteristics, with more subjects and more experimental test parameters, such as different levels of indentation and velocity. finally, we show how our experimental data can be used for the estimation of the nonlinear deformation model for tactile rendering. acknowledgement this work was supported in part by grants from the eu fp7 (project no. 601165 wearhap) and the european research council (erc advanced grant no. 291166 softhands). references [1] johnson k. o., the roles and functions of cutaneous mechanoreceptors, (2001) curr. opin. neurobiol., vol. 11, no. 4 pp. 455-461. [2] fruhstorfer h., abel u., garthe c. d., and knuttel a., (2000) thickness of the stratum corneum of the volar fingertips. [3] wu, j. z., dong, r. g., rakheja, s., schopper, a. w., smutz, w. p. (2004). a structural fingertip model for simulating of the biomechanics of tactile sensation. medical engineering physics, 26, 2, 165-175. [4] maeno, t., kobayashi, k., yamazaki, n. (1998). relationship between the structure of human finger tissue and the location of tactile receptors. jsme international journal series c, 41, 1, 94-100. [5] levesque, v. (2004). measurement of skin deformation using fingerprint feature tracking. ottawa: national library of canada bibliothèque nationale du canada. [6] bowden, a. e., rabbitt, r. d., weiss, j. a., (1999). stress and strain in the human distal phalanx under indentation. 1. [7] giavazzi, s., ganatea, m. f., s ̌uštaric ̌, p., rodic ̌, t. (2010). inverse determination of viscoelastic properties of human fingertip skin. [8] gulati, r. j., and m. a. srinivasan. (1995). human fingerpad under indentation i: static and dynamic force response. asme -publicationsbed. 29: 261. [9] srinivasan, m. a., lamotte, r. h. (1991). encoding of shape in the responses of cutaneous mechanoreceptors. information processing in the somatosensory system: proceedings of an international symposium at the wenner-gren center, 59-69. [10] peine, w. j., howe r. d. (1998). do humans sense finger deformation or distributed pressure to detect lumps in soft tissue?. in r.j.furness, ed., proc. of the asme dynamic system and control division, asme international mechanical engineering congress and exposition, anaheim, nov. 19-20, 64, 273-278. [11] lee, j. w., rim, k. (1991). measurement of finger joint angles and maximum finger forces during cylinder grip activity. journal of biomedical engineering, 13, 2, 152-162. [12] pawluk, d. t. v., howe, r. d. (1999). dynamic contact of the human fingerpad against a flat surface. journal of biomechanical engineering, 121, 6, 605-611. [13] nakamura, t., kimura, f., & yamamoto, a. (2013). a photoelastic tactile sensor to measure contact pressure distributions on object surfaces. journal of robotics and mechatronics, 25, 2, 355-363. [14] dandekar, k., raju, b. i., srinivasan, m. a. (2003). 3-d finiteelement models of human and monkey fingertips to investigate the mechanics of tactile sense. journal of biomechanical engineering, 125, 5, 682-691. [15] gerling, g. j., rivest, i. i., lesniak, d. r., scanlon, j. r., lingtian, w. j. r. (2014). validating a population model of tactile mechanotransduction of slowly adapting type i afferents at levels of skin mechanics, single-unit response and psychophysics. ieee transactions on haptics, 7, 2, 216-228. [16] wu, j. z., krajnak, k., welcome, d. e., dong, r. g. (2008). three-dimensional finite element simulations of the dynamic response of a fingertip to vibration. journal of biomechanical engineering, 130(5) : 054501. [17] abdul h. a. r., aladin z., begg r. k., & y. wahab. (2012). foot plantar pressure measurement system: a review. molecular diversity preservation international. [18] yong, f., yunjian, g., quanjun, s., (2011) international conference on information and automation (icia). a human figure 12. evaluation of contact area vs. contact force for the estimated linear and energy-limiting models, as well as for the linear component. acta imeko | www.imeko.org july 2017 | volume 6 | number 2 | 88 identification method based on dynamic plantar pressure distribution. 329-332. [19] http://www.sensorprod.com/automotive-pressure-mapping.php [20] shima, k., tamura, y., tsuji, t., kandori, a., yokoe, m., sakoda, s. (2009). estimation of human finger tapping forces based on a fingerpad-stiffness model. conference proceedings : annual international conference of the ieee engineering in medicine and biology society, 2663-2667. [21] bending stiffness of paper and paperboard, t 535 om-96 the technological association of the pulp and paper industry. [22] pressurex micro green (http://www.sensorprod.com/pressurex_micro.php) [23] e. miguel, m. l. d'angelo, f. cannella, m. bianchi, m. memeo, a. bicchi, d.g. caldwell and m. a. otaduy. characterization of nonlinear fingerpad mechanics for tactile rendering. world haptics conference (whc), 2015 ieee, evanston, il, 2015, pp. 63-68. [24] battaglia, e., bianchi, m., d'angelo, m. l., d'imperio, m., cannella, f., scilingo, e. p., & bicchi, a. (2015). a finite element model of tactile flow for softness perception. conference proceedings: annual international conference of the ieee engineering in medicine and biology society, 2015, 24302433. [25] pawluk, d. t., howe, r. d. (1999). dynamic contact of the human fingerpad against a flat surface. journal of biomechanical engineering, 121, 6, 605-11. [26] ansys academic teaching mechanical, release 14.5. [27] a. bicchi, d. e. de rossi, and e. p. scilingo, (2000) “the role of the contact area spread rate in haptic discrimination of softness,” ieee trans. on robotics and automation, vol. 16, no. 5, pp. 496–504. [28] m. bianchi and a. serio, “design and characterization of a fabric-based softness display,” ieee trans. haptics, vol. 8, no. 2, pp. 152–163, apr.–jun. 2015. http://www.sensorprod.com/pressurex_micro.php a preliminary approach to study the behavior of human fingertip at contact via experimental test and numerical model acta imeko, title acta imeko issn: 2221-870x september 2017, volume 6, number 3, 4-11 acta imeko | www.imeko.org september 2017 | volume 6 | number 3 | 4 the cultural heritage open laboratory system (chelabs). unfolding a cultural heritage-driven development in science and technology paola calicchia1, luca pitolli2, paolo salonia3 1institute of acoustics and sensors “o. m. corbino”/national research council, via fosso del cavaliere 100, 00133 rome, italy 2institute of complex systems/national research council, via dei taurini 19, 00185 rome, italy 3institute for technologies applied to cultural heritage/national research council, via salaria km. 29,300 c.p. 10, 00015 monterotondo st. (rome), italy – icomos it executive board section: research paper keywords: cultural heritage territorial system; monuments and sites safeguard; multidisciplinary innovation dynamics; open access&sharing citation: paola calicchia, luca pitolli, paolo salonia, the cultural heritage open laboratory system (chelabs). unfolding a cultural heritage-driven development in science and technology, acta imeko, vol. 6, no. 3, article 2, september 2017, identifier: imeko-acta-06 (2017)-04-02 section editor: sabrina grassini, politecnico di torino, italy received march 14, 2017; in final form march 14, 2017; published september 2017 copyright: © 2017 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: this work was supported by national research council within the project sm@rtinfra – sshch (miur md n. 973 of 25.11.2013) corresponding author: paola calicchia, e-mail: paola.calicchia@idasc.cnr.it 1. introduction the present work fundamentally wishes to conjugate the basic safeguard principles, consolidated both in the theoretical speculations and in the practices, with the identification of innovative technologies and their sustainable development. while the effective preventive conservation represents the objective towards which this integration is oriented, the proposed approach deals with the building of a system that connects the heritage assets, the conservation operators, the research facilities, and the local communities. on one side, the first relevant assumption of the existence of this kind of dialogue can be found in the conservation discipline expressed in the venice charter in 1964 [1] (following which the international council of monuments and sites – icomos was established [2]) stating, in art. 2, that “the conservation and restoration of monuments must have recourse to all the sciences and techniques which can contribute to the study and safeguarding of the architectural heritage”. thus, it introduces the concept of the diffusion of the scientific and technical knowledge into the safeguard practice for those heritage assets that, due to their intrinsic value, represent the world heritage. the venice charter opened the way to a number of abstract a territory, with its distinctive cultural heritage ecosystem effectively linked to the technological capabilities, represents a co-evolved system that contributes to the socio-economic growth and to a knowledge-based society. this work illustrates a new model of territorial system recently launched, the cultural heritage open laboratory system (chelabs), that aims at the construction of a favourable context addressing the challenge of a ch-driven development in science and technology. the concept, the inspiring principles, the vision, the activities and the implementation steps of this system are discussed, in addition to some key factors that trigger innovation and sustain knowledge building dynamics useful for shaping the future chelabs environment. the cultural asset is at the centre of this system, where a number of representative heritage sites and monuments are the nodes of a scalable and distributed laboratory, based on the open access and sharing culture. the current and bottom-up phase of implementation of the system is described: on a web-based platform, the heritage science community is invited to participate to an interactive survey. the expected results will highlight the problems, perceived by the experts community, demanding for new knowledge and also the technological trends that could unfold effective innovation dynamics. acta imeko | www.imeko.org september 2017 | volume 6 | number 3 | 5 important recommendations for the specific cultural heritage conservation domain, proposed by public and private organizations, among which icomos as unesco advisory body. from these documents it is possible to get some meaningful indications to which the project, presented in this paper, was inspired. mainly, we wish to refer to the nara document on authenticity (1994) [3], to the faro convention (2005) by the council of europe on the value of cultural heritage for society [4], to the hangzhou declaration (2013) that sets the culture at the center of the policies for the sustainable development [5], leading to the unesco agenda 2030 and the sustainable development goals (sdgs) [6]. mentioning the final document of the icomos general assembly in florence (2014), we can recall the importance of the so called community-driven practices, and the capacity building, as well as the recommendations for an informed use of the innovative technologies (florence declaration heritage and landscape as human values) [7]. finally, the premise of the guidelines to the annex of the unesco convention on the protection of the underwater cultural heritage (2001) [8] suggests a paradigm shift in the motivation of the onsite preservation, built on the awareness that “heritage sites are not an inexhaustible resource. [...] the future holds unimaginable advances in technology, which may lead to far more innovative methods of trace analysis that could profitably be used in archaeology. even more importantly, research questions develop step-bystep, building on prior knowledge and understanding. in order to address research questions that will arise in the future as a result of this creative scientific process, it is essential that at least a selection of sites remains untouched and researchable.” adopting this perspective not only for underwater heritage, the heritage sites are also perceived as a space whose onsite preservation has a value for the experimentation and for the scientific and technological innovation, with the awareness that the scientific knowledge creation process will profitably contribute to the future preservation practices. considering now the other element of the initial dialogue related to the scientific facilities, the networks and the research infrastructures, specifically those of the hard science applied to the heritage domain, it is worth noted the vision of the esfri strategy, as part of the most advanced policy in building systems [9]. firstly the charisma eu project, and presently the iperion-ch eu project have strongly pushed the culture of the open access facilities among this community. this recalls the well known experience of all the large-scale facilities that, in few decades, have advanced the knowledge in many research fields and brought a positive growth on the hosting territory. furthermore, a modern environmental planning and management approach for cultural heritage sites was developed in the cherplan project, within the see program [10]. this approach provided a strong base for a synergy between conservation and socio-economic growth of the territory (principles of agenda unesco 2030). concerning long-term actions addressing innovation in the cultural heritage safeguard, the experience of few decades of hard science applications in this field suggests the need of both the following aspects: i) an insight into single innovative technologies and their opportunities; ii) a better understanding of the favourable context and the collective dynamics characterizing the behaviour of the heritage community with respect to innovations. in our vision, the innovation dynamics based on knowledge creation is an essentially collective process, occurring inside a community acting as a group or, more precisely, as a complex adaptive system. the environment and the quality of the interaction within the community may effectively orient this process. we refer here to some basic concepts derived from the science of complex systems. specifically, for this task, we refer to the studies conducted on the dynamics inside the highperformance working teams [11], extensively applying the concept of working team to larger groups of scholars focused on the same topic. furthermore, attention is paid to a rather new research field studying the dynamics of creativity and innovations [12], [13], for a better understanding of the behaviour of both individuals and communities exposed to innovations. this work presents the vision statement of the cultural heritage open laboratory system (chelabs), and describes its fundamental characteristics for addressing innovation in the heritage domain. this is a new model of territorial system, still in its initial phase of development, where a number of representative heritage sites and monuments are the interconnected nodes of a scalable and distributed laboratory, based on the open access and sharing culture. the novelty of the chelabs system resides in the combination of the open access policy, for strengthening innovation potential in the heritage science, with the recognition that the cultural asset, grounded in the territory, offers the scenario for a new territorial growth. the concept of the chelabs system is presented in section 2, and its subsections, including a description of: the inspiring principles in 2.1; the vision in 2.2; the potential activities in 2.3; the implementation steps in 2.4. successively, the current phase consisting in the implementation of the chelabs platform, is described in section 3, and its subsections, encompassing the description of: the community in 3.1; the survey in 3.2; the interactive map in 3.3. finally, some general aspects are summarized in the conclusions, in section 4. 2. the chelabs system each element belonging to the cultural heritage is usually exposed to a particular risk, is characterized by a specific decay process evolution, requires defined risk management plans and conservation actions. although economic barriers may complicate the adoption of good practices, in many cases the most critical issues can be faced through the application of known tools, fully validated methodologies and well assessed approaches. in other cases effective tools and solutions to these critical issues may not be already known, or mature enough to be available on site at the proper time to avoid impairs to those assets that need to be preserved. thus unsolved problems can be identified and challenging issues characterize these heritage sites. actually, they are the most interesting and adequate sites within the chelabs vision because they definitely lay at the frontier of what is already known, while demanding for new knowledge. these cases show an added value: they offer an attracting context able to solicit the experts community to move a step forward, while effective solutions are not yet apparent. the chelabs approach recognizes in these contexts the opportunity to foster the scientific and technological knowledge, contributing to their attractiveness, though they need to gather other relevant key factors for triggering innovation in the cultural heritage domain, as discussed later on. acta imeko | www.imeko.org september 2017 | volume 6 | number 3 | 6 2.1. inspiring principles in many fields of science as in everyday life the comprehension of the dynamics of innovation is essential for facing challenges and exploring new solutions to unsolved problems. in the domain of social psychological science, also related to the economic science, many studies investigated the distinctive characters of the high performance working teams. high performance working teams, when observed during meetings and analysed in terms of cross-correlations of speech acts, were found to be characterized by chaotic dynamics [11]. chaotic dynamics is related to the behaviour of dynamical systems that are highly sensitive to initial conditions. so that very little differences in initial conditions lead to very different evolutions, making impossible any long-term prediction. beyond unpredictability the positive aspect is that, chaotic dynamics is linked to the optimal learning landscape and to adaptation (i.e. the ability to respond flexibly to a changing environment), as well as to a high degree of connectivity of the team. other studies investigated the role of leadership facilitating exploratory behaviours among the members of a team [14], [15]. in this framework, the leadership plays a fundamental role in shaping the processes within the team by cultivating openness, effective group interaction, exchange of ideas and a non-evaluative context during this exchange. it is also evidenced the need for the members of the group to effectively process and integrate the shared information, as well as the importance to have the opportunity or motivation to build on the ideas of others [15]. in the chelabs vision these concepts could be extended to a community of experts, acting as a group within the shared context of an open heritage site: tools, platforms, policies and a proper management enhancing the connectivity, and encouraging the knowledge sharing and integration among the community, are key factors of a favourable environment. the heritage science domain has an added value in the multidisciplinary. within the chelabs vision, each subject of the community has an offer and a need. any need, evidenced by one subject, can be satisfied by the offer brought by another subject. any participant can share his offer and explicit his need in a peer process, which can be here identified with the term reciprocity. amplifying the scope of the concept cited above, we dare say that each subject may experience the importance of having the opportunity or the motivation to explore on the base of the need/offer of others. the cited research [15] also recalls some threats that reduce the positive dynamics, bringing the groups to converge towards the direction of low performers. one of these factors is the attitude to focus on information which the members have in common, instead of taking advantage of the unique expertise of each member and benefit of such diversity. this threat is very frequent inside multidisciplinary contexts, such as in the heritage science domain, encompassing terminology and scientific background among humanities and hard science. in our vision, a fundamental key factor in the multidisciplinary environments is the attitude to explore the diversity, preventing the confinement into what is in common. the free access to datasets and previous knowledge, the experimentation facing unsolved problems but also the meeting of constraints, and the compliance to stringent practical requirements push the whole community towards innovative solutions. besides innovative events are usually correlated to, or triggered by, a previous novelty or innovation. in the domain of the science of complex systems, recent projects investigate the dynamics of creativity, novelties and innovation. mathematical models are purposely analysed for mapping and predicting the occurrence of innovative events within an environment, and the correlation between successive events, in order to understand which is the optimal landscape for unfold creativity and innovation [12]. the research actually encompasses both social, biological and technological systems. these studies are based on the interesting kauffman’s theoretical concept of the so called adjacent possible. loreto et al. [13] describes this notion as follows “originally introduced in the framework of biology, the adjacent possible metaphor include all those things, ideas, linguistic structures, concepts, molecules, genomes, technological artifacts, etc., that are one step away from what actually exists, and hence can arise from incremental modifications and/or recombination of existing material. [...] the strange and beautiful truth about the adjacent possible is that its boundaries grow as one explores them. the very definition of adjacent possible encodes the dichotomy between the actual and the possible [...] the actual realization of a given phenomenon and the space of possibilities still unexplored”. furthermore, the cited authors suggest that this correlation of innovative events is realized by exploration of a space – physical, conceptual, technological or biological – that enlarges itself as a new region is reached. from this perspective we can think that this space needs to be populated with existing materials, and in order to be populated (things and people) this space needs to be shared, where the accessibility and the connectivity between existing things and people is sustained. in the chelabs vision a shared space, where the exposure of the experts community to the unsolved problems and to the challenging issues is supported, becomes a space of possibilities where any new piece of knowledge opens up the opportunity to reach other regions of knowledge still unexplored. the studies on these topics are still young but the expected outcomes regard some general rules for the identification of the determinants of creativity, and of the schemes for modelling the dynamics of innovations. furthermore great expectation is in finding, within different contexts, the key factors that enhance creativity, as well as the optimal environment for it. in our vision, the above mentioned space is inside or around the object of heritage interest. specifically the heritage asset, when exposed to its characteristic decay process, is considered the observable system that can be monitored and analysed in its natural evolution. when top-level expertise and state of the art technologies are gathered in this space, a new scenario will be opened: the cultural asset becomes the space where methodological and technological innovation systematically occur. we also suggest that in the heritage domain, where a great number of methodologies and technologies have been introduced in few decades, it becomes important to have a sort of scalable ability to see in depth to single new technologies and to see extensively to the dynamics of the community integrating these innovations. it is fundamental to observe how the multidisciplinary community of the heritage science behaves in its distinctive environment; under this perspective, next to the heritage asset, the community itself becomes the other observable system to study. the main categories and the related key factors, introduced and described in this section, are synthesized in the scheme shown in figure 1. acta imeko | www.imeko.org september 2017 | volume 6 | number 3 | 7 2.2. vision according to the previous consideration, in the chelabs vision sites and monuments, historical centres and collections are at the centre of the system, becoming the places of a dynamic integration among people, different expertise and capabilities. this process is possible by implementing the access policies, the management, the most advanced technologies, and the activities encompassing research, training and dissemination actions within the patrimony itself. as shown in figure 2, all these aspects are defined during the implementation phase (on the left of the scheme) and are useful to configure the services to the external users, and to accomplish their activities during the operative phase (on the right of the scheme). in the cultural heritage domain, any relevant project includes a pilot site or a number of case studies, in order to conduct field experimentation of innovative systems or of new approaches in conservation science. of course the on-site experimentation is accessible only to the project partnership. this practice guaranties that the planned activity can be properly accomplished following defined workflows; nevertheless it can be observed that this kind of context can also confine the potential for a more extensive knowledge integration. once a heritage site is equipped with the most advanced technological instrumentation for monitoring and diagnostics, a suitable policy regulating the access of external experts can amplify the scope and scale of the knowledge building process. in the chelabs system, the implementation of a site access policy is envisioned to open a heritage site to external users, among the experts community, who wish to carry out their own research activity. this well-known approach, commonly adopted in many world-class facilities and open access laboratories (ol), is regulated through periodical calls for proposal and shows the advantage of widening the variety of expertise gathered up for studying the same topics. nowadays open access to information resources and datasets (od) is one of the most important key factors for innovation, and generally recognized as a global trend. the free access to od is sustained by many international organizations, among which unesco stating “while open information resources are main part of the knowledge generation, dissemination and growth cycle, open data will help in new interpretations, trend predictions and diverse and innovative applications of data” [16]. while in the past the data were held confidential for producing profit, today the awareness is growing to recognize the od as a tool for producing new knowledge. thus a paradigm shift is occurring today, moving from the enhancement of goods production towards the advancement of knowledge. the implementation of the od policy represents a fundamental element for the chelabs system, also considered as an empowerment of the site access policy. while inducing the users to be engaged into a collective process, the availability of od helps the integration of heterogeneous pieces of knowledge more than their sum. one key factor for building creative contexts is supporting the sharing attitude of a community. this attitude shall be addressed in the policies and may encompass different aspects: i) sharing the place of the heritage sites, where different expertise may convene and work on a same problem; ii) sharing the state of the art technologies contributes to the capacity building process; iii) sharing knowledge helps the integration of information and the creation of new knowledge, of new solutions to unsolved problems, of new products or services; iv) sharing scientific and traditional knowledge by integrating the experts community and the local community. in this way the sharing attitude is strictly related to the concept of sustainability, including equal opportunities in the scientific advancement and in the sustainable development. beyond the access policies, for a heritage site to become a node of the chelabs system it is essential to create a local team for the site management (local unit), to make the most advanced instrumentations available onsite for continuous monitoring and periodic tests, as well as an ict platform. the local units have the responsibility to define and regulate a list of possible activities that can be proposed and realized in that specific site, as well as a defined period for the open access policy according to the needs for conservation actions (also in terms of enhancement and avoiding the interruption in the enjoyment of goods). when a number of sites, configured in such a way, are linked together they constitute a distributed laboratory, with a general management involved into a harmonization process. the envisioned chelabs structure, as shown in figure 3, accounts for a coordinating team, a sites management composed of the leaders of the local units, and these last involved in the management of each single site. an advisory board is appointed to give support for defining the goals and the policies, the harmonization of the sites, the best practices exchange, the standardization of methods, the quality assurance, and the validation of shared data. the system of sites, with their figure 1. scheme recalling the inspiring principles of the chelabs system: basic assumptions, main categories and relevant key factors. figure 2. configuration of a site of heritage interest when integrated into the chelabs system. acta imeko | www.imeko.org september 2017 | volume 6 | number 3 | 8 distinctive character, constitutes an open access area accessible, for on-site investigations, to the local units and to the external users who propose their activities. this system is a scalable and harmonized network involving different players in a multidisciplinary context, stimulating those dynamics that trigger events favourable to creativity and innovation. 2.3. potential activities on a chelabs site, the local unit together with the organization, responsible for the safeguard of the heritage site, define the allowed activities and the requirements useful to regulate the accessibility. the users can access: the heritage asset, the state of the art instrumentations, the information and datasets. according to the above regulation, the possible activities can be organized as follows: knowledge: research on the heritage asset using the instrumentations available onsite; diagnostics/monitoring: research on the risk/decay kinetics using the instrumentations available onsite; technology: research on the instrumentations brought by the users, or available onsite, used to study the heritage asset; comparison: research on the users’ instrumentation, applied to the heritage asset, by means of a comparative analysis using the instrumentations available onsite; validation: validation of innovative technologies or analytical methods developed by the users, employed on the heritage asset, by integrating the results of the onsite experimentation with the information from previous knowledge and datasets about the heritage asset; prototype testing: a prototype of an innovative device or instrumental system becomes part of the instrumentations available onsite, in order to be tested by a great number of users and obtain information on its applicability and potential improvements; training: professional courses including theory and practice, focused on the study of the heritage asset or on the instrumentations available onsite. beyond this variety of activities that can be realized on a single site, more articulated actions can be easily accomplished in the chelabs system as a system of sites. for instance: practices exchange: if two or more sites of the system present similar elements, a successful practice developed in one of them can be easily verified on the other ones by simple internal bureaucratic procedures. this possibility accelerates the process for the verification and the adoption of good practices, according to the standards established by the authoritative organisations. applicability: if a user needs to undertake an applicability assessment of an innovative method/device by testing it on different artifacts, he can propose to apply the same experimental method on different heritage assets available on different sites. for instance an innovative diagnostics tool, after laboratory validation, may need to be verified on different types of paintings. the chelabs system can guarantee the accessibility to a suitable variety of paintings in a suitable period of time, allowing the user to easily accomplish this task. this possibility accelerates the time-to-market of new important technologies with a solid background of experimental tests, and their adoption within the conservation practices. protocol-standard: as for the good practice exchange, also protocols and new standards can be implemented on a knowledge base, built by integrating the results of the experimentation on different sites. this possibility helps the harmonization and the standardization processes. all the above mentioned activities involve many aspects, relevant for a solid approach based on those key principles belonging to the metrology framework. for instance, the validation of methods, the use of comparison, the calibration of the instruments available onsite, the use of reference materials, and finally the implementation of standards. 2.4. implementation steps what happens when a site of significant heritage interest becomes a node of the chelabs system? let us analyze a practical example where the implementation of the chelabs system is accomplished in an archaeological area. let us assume that this area is located in the proximity of a lagoon, and is characterized by seismic hazard related to the geomorphology of the soil and by flooding. furthermore, we can also assume that structural decay processes affect the built heritage located in that area. this site may present a number of challenges in the comprehension of the geophysical characteristics of the territory, correlated to the accurate knowledge of the structural decay evolution in the buildings, and in the risk management; all these challenging issues require the development of innovative approaches as well as technologies for effective and preventive conservation actions. chelabs integrates the site into the system, through the following phases, synthetically shown in figure 4: phase 1 – candidature of the site the organization responsible for the safeguard of the archaeological area candidates the site to the chelabs system, and agrees to adopt the proper policies for the accessibility of the site and the data regarding the site. the management identifies the specific requirements in order to regulate the possible users activities, such as: allowed activities – research studies; validation; comparison; training camp; dissemination events; annual availability – maximum six months per year; requirements – compatibility with visitors access and enjoyment; additional spaces for storing external users’ equipments – available onsite; duration of the open access policy – five years. phase 2 – evaluation of the candidature the chelabs advisory board evaluates the application analyzing the representativeness of the site with respect to the risk/decay specificity, and identifies challenging issues that are figure 3. structure of the chelabs system. acta imeko | www.imeko.org september 2017 | volume 6 | number 3 | 9 strategic elements for the accomplishment of the chelabs objectives. all the practical aspects related to the authorization are preventively faced and solved at this stage. successively, in order to prepare the site for the full operability, the equipment of the site is accomplished with the state of the art instrumentations which are now available onsite, such as: devices for specific studies – multichannel seismic antennas; systems for geophysical measurements of the subsoil; systems for structural tests for the static deformation parameters definition in buildings. devices for constant monitoring – seismic sensors in borehole; wifi accelerometer arrays for the high-dynamics monitoring of the buildings; 3d laser scanner; systems for interferometric data. data accessibility – open access to archives; data sharing. site accessibility services. management – local unit creation. all these elements contribute to the configuration of the site that, now, is ready to become a node of the chelabs system. phase 3 – call for proposals calls for proposals are periodically launched to collect, evaluate and schedule the future users activities. phase 4 – performing the scheduled activities proposed by the users. the site achieves full operability, as shown in figure 5, hosting the external users activity, such as: project 1 – integration of interferometric data, seismic data, and deformation data. project 2 – digital modelling of monuments by 3d laser scanner and sfm systems (structure from motion). project 3 – dynamic analysis of structural decay processes with respect to the maximum deformation of the soil. project 4 – nonlinear dynamics analysis of the soil. although a relevant effort should be directed to guarantee the most suitable instrumental equipment onsite, the effectiveness of the model does not depend on the state of the art technologies that, in a specific period of time, are available onsite. this means that the implementation of the system shall be continuously renewed with innovative technologies. the added value of the chelabs approach consists in building the more favourable environment that naturally attracts the major experts working on the most challenging issues, in a specific field. furthermore this system presents a flexibility that, in the long-term, facilitates the achievement of new solutions not necessarily predictable and planned in the implementation phase, thus taking into account that unpredictability characterizing the dynamics of the complex systems evolution. 3. the chelabs platform today the foundation of the chelabs system is set by means of a web-based platform, where the heritage experts together with the entire community are invited to participate to an interactive survey. the principal aims of this survey is to collect and correlate the knowledge needs with the expertise and offer from the other participants, as well as with new and original ideas from the citizens within a vision of public engagement. the participation also gives the opportunity to operate through specific functionalities on an interactive map. since this initial phase, the web-based platform of the chelabs system tends to represent that shared space facilitating the connectivity among the community, and the exchange of ideas. figure 6 shows a scheme of the platform structure with its fundamental elements: the community, the survey, and the interactive map. 3.1. the community the potential participants to the survey come from all the public and private organizations and experts that operate in the cultural heritage domain, as well as the citizens. this wide community has been articulated into six different profiles: 1) territorial organizations, owners, superintendence; 2) conservation operators, heritage scientists; 3) technology operators; 4) schools, high education institutions, universities, research institutes; 5) funding agencies; 6) citizens. suitable dissemination actions for reaching a statistically significant sample will be undertaken. 3.2. the survey the survey is structured in six questionnaires, that are composed of a section dedicated to gather specific profile figure 4. fundamental steps constituting the preparatory phase, aiming at the integration of a heritage site into the chelabs system. figure 5. example of the users activities performed during the full operative phase of a chelabs site. acta imeko | www.imeko.org september 2017 | volume 6 | number 3 | 10 information, and a common section dedicated to the acquisition of information useful for building need/offer correlations and matching issues between unsolved problems and potential solutions. for each profile the main interest is focused on few questions related to the needs for knowledge advancement and the needs for technological development, indicated by the participants on the base of their professional experience. the information regarding these needs are transferred to the interactive map, and linked to the heritage sites suggested by the participants, in order to become visible to the whole community. the contribution from the five profiles, gathering the professionals connected in some ways to the heritage domain, is expected to highlight the current perceived limits demanding for new knowledge, and also the technological trends that could unfold new solutions. on the other hand the sixth profile, gathering the citizens with no specific expertise, has been purposely taken into account for two main reasons: i) mapping the perception of the heritage value, as individuals and as community, together with a sense of belonging; ii) triggering new ideas on expected learning experiences inside a cultural space. this information, provided by this profile, will solicit the experts to move towards a community-driven development. 3.3. the interactive map the participants can operate on an interactive map, and manifest their interest for a heritage site, considered particularly significant for fostering the research. as described above, some information are directly transferred into the map as a contextualization of what is expressed in the questionnaires. the participants can also add other notes to the dossier of a site according to its specificity; in particular they are invited to play, as in a sort of game, by choosing among two options. one option gives the possibility to briefly express thoughts on critical issues/unsolved problems; the other option gives the possibility to express opportunities/solutions. thus, starting on a very practical base the participants may virtually meet in a heritage site, share some information and exchange ideas, being involved in a creative discussion. if properly conducted, it is expected that, at least on the most interesting sites, these discussions will evolve according to the chaotically complex dynamics that characterizes the high performance teams, as treated in subsection 2.1. 4. conclusions chelabs is intended to become a sort of distributed laboratory, attracting competencies and generating excellence, built on the open access and sharing culture. today, the bottom-up character of the participated survey is inspired by these same principles, inviting the heritage science community together with the citizens to configure a new scenario that will guide the chelabs system in its future operating phase. this paper describes the concept and the current phase of the project, that is moving towards the successive steps of implementation. in the future, the project aims at the integration of a heritage site into the chelabs system opening the operative phase. some efforts have been spent to identify possible pilot sites, and some actions are presently at study. for this task a fundamental step, not deeply discussed in this work, shall regard the configuration of an appropriate partnership including all relevant stakeholders that will be engaged in the management, services and activities on each specific site. acknowledgement the initial phase of the chelabs project has been funded within the project sm@rtinfra – sshch (c.n.r. – dsu; miur md n. 973 of 25.11.2013). the authors wish to acknowledge sara de simone for her help in the implementation of the survey, massimiliano moscatelli and his research group at the institute of environmental geology and geoengineering (igag cnr) for their relevant help in the definition of the example reported in section 2.4, and vittorio loreto (full professor at the university of rome “sapienza”) for his insight into the science of complex systems for a better understanding of the dynamics of innovation. finally the authors are grateful to the associate partners (enterprises and research institutes) that are supporting the chelabs initiatives since these first steps. references [1] international charter for the conservation and restoration of monuments and sites (the venice charter). https://www.icomos.org/charters/venice_e.pdf, 1964 (accessed 06.03.17). [2] icomos statutes. https://www.icomos.org/en/abouticomos/mission-and-vision/statutes-and-policies, 1965 (accessed 06.03.17). [3] nara document on authenticity. https://www.icomos.org/charters/nara-e.pdf, 1994 (accessed 06.03.17). [4] faro convention. http://www.coe.int/en/web/culture-andheritage/faro-convention, 2005 (accessed 06.03.17). figure 6. scheme of the web-based platform of the chelabs project, with the main topics regarding the community, the survey, the interactive map. https://www.icomos.org/charters/venice_e.pdf https://www.icomos.org/en/about-icomos/mission-and-vision/statutes-and-policies https://www.icomos.org/en/about-icomos/mission-and-vision/statutes-and-policies https://www.icomos.org/charters/nara-e.pdf http://www.coe.int/en/web/culture-and-heritage/faro-convention http://www.coe.int/en/web/culture-and-heritage/faro-convention acta imeko | www.imeko.org september 2017 | volume 6 | number 3 | 11 [5] hangzhou declaration. placing culture at the heart of sustainable development policies. http://www.unesco.org/fileadmin/multimedia/hq/clt/i mages/finalhangzhoudeclaration20130517.pdf, 2013 (accessed 06.03.17). [6] unesco and sustainable development goals. http://en.unesco.org/sdgs, 2015 (accessed 06.03.17). [7] the florence declaration on heritage and landscape as human values. https://www.icomos.org/images/documents/secretariat/2 015/ga_2014_results/ga2014_symposium_florencedeclarati on_en_final_20150318.pdf, 2014 (accessed 06.03.17). [8] manual for activities directed at underwater cultural heritage. guidelines to the annex of the unesco 2001 convention. pp. 22-23 (2013), isbn 978-92-3-001122-2. http://www.unesco.org/culture/en/underwater/pdf/uchmanual.pdf (accessed 06.03.17). [9] esfri, “european strategy report on research infrastructures: roadmap 2016”, isbn 978-0-9574402-4-1, mar 2016. [10] enhancement of cultural heritage through environmental planning and management: cherplan (see/0041/4.3/x). [11] m. losada, “the complex dynamics of high performance teams”, mathematical and computational modelling 30 (1999) pp.179192, 10.1016/s0895-7177(99)00189-2. [12] v. loreto, “unfolding the dynamics of creativity, novelties and innovation”, white paper of the kreyon project, 2015. [13] v. loreto, v.d.p. servedio, s.h. strogatz and f. tria “dynamics on expanding spaces: modeling the emergence of novelties”, in: creativity and universality in language. m. degli espositi, e.g. altmann, f. pachet (editors). springer international publishing, 2016, isbn 978-3-319-24403-7, pp.59-83. [14] o. caridi-zahavi, a. carmeli, and o. arazy, “the influence of ceos' visionary innovation leadership on the performance of high-technology ventures: the mediating roles of connectivity and knowledge integration”, journal of product innovation management 33, iss.3 (2016) pp.356-376, 10.1111/jpim.12275. [15] a. carmeli, p.b. paulus, “ceo ideational facilitation leadership and team creativity: the mediating role of knowledge sharing”, the journal of creative behavior 49, iss.1 (2014) pp. 53-75, 10.1002/jocb.59. [16] unesco “open access for researchers – module 2 concepts of openness and open access”, 2015, isbn 978-92-3-100079-9. http://www.unesco.org/fileadmin/multimedia/hq/clt/images/finalhangzhoudeclaration20130517.pdf http://www.unesco.org/fileadmin/multimedia/hq/clt/images/finalhangzhoudeclaration20130517.pdf http://en.unesco.org/sdgs https://www.icomos.org/images/documents/secretariat/2015/ga_2014_results/ga2014_symposium_florencedeclaration_en_final_20150318.pdf https://www.icomos.org/images/documents/secretariat/2015/ga_2014_results/ga2014_symposium_florencedeclaration_en_final_20150318.pdf https://www.icomos.org/images/documents/secretariat/2015/ga_2014_results/ga2014_symposium_florencedeclaration_en_final_20150318.pdf http://www.unesco.org/culture/en/underwater/pdf/uch-manual.pdf http://www.unesco.org/culture/en/underwater/pdf/uch-manual.pdf the cultural heritage open laboratory system (chelabs). unfolding a cultural heritage-driven development in science and technology setup for the dynamic calibration of bridge amplifiers from dc up to 10 khz acta imeko issn: 2221-870x march 2019, volume 8, number 1, 19 – 24 acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 19 setup for the dynamic calibration of bridge amplifiers from dc up to 10 khz leonard klaus, m. florian beug, thomas bruns physikalisch-technische bundesanstalt (ptb), bundesallee 100, 38116 braunschweig, germany section: research paper keywords: dynamic bridge amplifier calibration; traceability; dynamic measurement; dynamic bridge standard citation: leonard klaus, m. florian beug, thomas bruns, setup for the dynamic calibration of bridge amplifiers from dc up to 10 khz, acta imeko, vol. 8, no. 1, article 4, march 2019, identifier: imeko-acta-08 (2019)-01-04 section editor: maija ojanen-saloranta, vtt technical research centre of finland, national metrology institute mikes, finland received august 15, 2018; in final form october 10, 2018; published march 2019 copyright: © 2019 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: leonard klaus, leonard.klaus@ptb.de 1. introduction mechanical quantities, such as acceleration, force, torque, and pressure, are often measured with transducers, based on strain measurement by means of strain gauges, or sensor elements, based on the piezoresistive effect. these sensing elements change their resistance proportionally to the strain (strain gauge) or the compression/tension (piezoresistive element). however, both transducer principles require signal conditioning by bridge amplifiers. in many applications, these mechanical quantities change rapidly over time, thus requiring a dynamic calibration in order to be traceable. until now, bridge amplifiers – as well as the corresponding transducers – have almost exclusively been calibrated statically. to overcome this problem, a joint european project [1], [2] researched the necessary procedures, measuring devices, and mathematical tools to establish dynamic calibration. for signalconditioning electronics (including all types of measuring amplifiers), it is known that deviations between static and dynamic behaviour exist [3]. these deviations may influence dynamic measurements significantly. therefore, one outcome of this project was a newly developed dynamic bridge standard [4] that enables bridge amplifier calibration from a static regime (dc) up to frequencies of 10 khz. this bridge standard is incorporated in the calibration setup described here. this paper is a revised and extended version of [5]. figure 1: wheatstone bridge circuit with four resistive sensing elements 𝑅1 … 𝑅4, excitation voltage 𝑈exc, and bridge output voltage 𝑈br. abstract measurements of mechanical quantities are often carried out with transducers with a bridge output. the output signals are conditioned using bridge amplifiers. if dynamically changing quantities are going to be measured traceably, the bridge amplifier must be calibrated dynamically. this paper describes a dynamic bridge amplifier calibration setup based on the new ptb dynamic bridge standard. the calibration is carried out by the synchronous sampling of the bridge amplifier output voltage and a reference signal provided by the calibrated dynamic bridge standard. the dynamic bridge standard enables calibrations in a frequency range from dc (static calibration) up to 10 khz. an overview of the different measurement uncertainty contributions is given, and the first measurement results show good agreement with a previously established measurement setup. mailto:leonard.klaus@ptb.de acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 20 2. dynamic bridge standard bridge amplifiers are always connected to some type of wheatstone bridge circuit, as depicted in figure 1. the resistive sensing elements (e.g. the strain gauges or piezoresistive sensor elements) are connected as a quarter bridge (one sensing element of variable resistance), half bridge (two sensing elements), or full bridge (four sensing elements) in order to maximise the output signal of the bridge. what all configurations of a wheatstone bridge have in common is that the output voltage level 𝑈br depends on the bridge excitation voltage 𝑈exc (in this case a dc voltage) featuring a ratiometric output, which is typically given in mv v⁄ . the excitation voltage is supplied by the bridge amplifier. the output of the bridge amplifier is therefore proportional to the ratio of the bridge output voltage to the excitation voltage. as a result, the bridge output voltage 𝑈br has no connection to ground. in the bridge amplifier calibration, this bridge voltage 𝑈br must be ratiometrically provided by the dynamic bridge standard based on the excitation voltage, undertaken in the form of a calibrated voltage ratio and phase. the dynamic bridge standard working principle is based on two multiplying digital-to-analogue converters (mdacs), which generate output voltages proportional to their reference voltage without dependency to ground. this reference voltage is chosen as the bridge excitation voltage ±𝑈exc . a schematic diagram of the dynamic bridge standard components can be seen in figure 2. subsequent to the mdac outputs, a resistive 1/400 voltage divider supplies the small output voltages as required for the bridge amplifier calibration. the load of a strain gauge transducer is simulated by a load resistance of 350 ω. the output voltage of the two mdacs is used as reference for the phase calibration. for this purpose, it is conditioned by buffer amplifiers to avoid any influence on the mdac output and is fed to connectors placed on the front panel. the waveforms generated by the mdacs, which can be either static or time-dependent (arbitrary or sinusoidal), are programmed by using an optical computer link. 3. implementation of a calibration setup for the calibration of a bridge amplifier, not only is a bridge standard required, but additional data acquisition hardware, a proper data analysis, and a measurement uncertainty evaluation are also required. the calibration setup incorporates the dynamic bridge standard described as well as a data acquisition system featuring two synchronised sampled data channels. the connection of the different components, including the device under test (dut), is depicted in figure 3. 1 commercial instruments are identified in this paper only to adequately specify the experimental setup. such identification does not imply recommendation by ptb, nor does it imply that the equipment identified is necessarily the best available for the purpose. the dynamic behaviour of an amplifier can be described by its frequency dependent complex transfer function 𝐻 (i𝜔), with its input 𝑋(i𝜔) and its output 𝑌(i𝜔) giving 𝐻(i𝜔) = 𝑌(i𝜔) 𝑋(i𝜔) . (1) more commonly, it is given as a magnitude response 𝐴(𝜔) and phase response 𝜑(𝜔), giving 𝐴(𝜔) = 𝐴𝑌 (𝜔) 𝐴𝑋(𝜔) , (2) 𝐴𝜑(𝜔) = 𝜑𝑌 (𝜔) − 𝜑𝑋 (𝜔) , (3) with the magnitudes 𝐴𝑋, 𝐴𝑌, and phase angles 𝜑𝑋 , 𝜑𝑌 of the input and output. based on the magnitude and phase responses, the associated complex transfer function can be derived and vice versa [3]. in the case of the calibration of a bridge amplifier, the amplifier’s magnitude excitation at the input 𝐴𝑋 is well known, because the dynamic bridge standard is calibrated. for the determination of the magnitude response, only the output of the amplifier 𝐴𝑌 needs to be analysed. this could be done by means of a calibrated sampling system or even a calibrated ac voltmeter. the phase response determination requires the phase measurement between the output of the bridge amplifier 𝜑y(𝜔) and the dynamic bridge standard’s reference signal, since this signal has a calibrated phase relation to the input of the bridge amplifier 𝜑x(𝜔). 4. data acquisition in our setup (figure 4), we simultaneously sample the bridge amplifier output and the dynamic bridge standard reference signal by means of two synchronous sampling data acquisition channels. the hardware used for the data acquisition is a pxie system with a high-resolution digitiser card from national instruments (pxi-59221), which has a flexible resolution of 18 bits to 24 bits. with sampling rates applicable to the calibration of bridge amplifiers (𝑓s ≪500 khz), the resolution is 24 bits. the pxi-5922 digitiser card was thoroughly analysed in terms of its dynamic properties, precision, and metrological suitability [6], [7]. additionally, automated calibration procedures exist for this acquisition card at ptb (section 6), as it is widely used in the realization of acceleration working group. the oscillators of the different systems, namely the oscillator of the dynamic bridge standard and of the data acquisition system, are synchronised in order to avoid spectral leakage. the dynamic bridge standard is equipped with an optical clock input. the pxie system is equipped with a timing and synchronisation figure 2. schematic overview of the dynamic bridge standard. figure 3. components of the dynamic bridge amplifier calibration setup. acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 21 card (pxie-6672), which features a stable oscillator (tcx0). its one-year stability is in the range of a few 106 [8]. the 20 mhz reference frequency for the dynamic bridge standard is then generated by direct digital synthesis (dds) based on the 10 mhz tcx0 oscillator. the electrical frequency output provided by the pxie card is converted to an optical link as required by the bridge standard. dynamic bridge excitation signals are generated by two mdacs with a sampling frequency that is a fraction of the reference frequency. based on this sampling frequency, the sinusoidal waveforms can be generated based on a finite number of data points per oscillation period, which are then repeatedly output. the sampling frequencies and the acquisition time of the data acquisition were chosen based on two criteria: 1) the sampling rate should be multiples or integer fractions of the bridge standard’s sampling rate. generally, it would not be favourable to sample with a higher sampling rate than the signal generation rate. however, the chosen digitiser card has a minimum sampling rate of 50 ks/s, requiring a down-sampling of the data after acquisition. this decimation process is carried out by calculating an averaged decimated waveform from the original data. 2) the sampling rate was chosen in order for us to achieve a finite number of samples for the selected number of oscillations requiring acquisition. the corresponding acquisition time should include an integer number of periods of the excitation frequency. the sampling frequency of the chosen digitiser cannot be chosen arbitrarily. it is chosen as an even integer divider of the sampling frequency of 120 ms/s for the bitstream input of the incorporated delta-sigma analogue-to-digital converters (adcs) and must be in a range of 50 ks/s and 15 ms/s. 5. data analysis the acquired and synchronously sampled waveforms of the two input channels make up the input for the data analysis. one data channel is the reference output of the dynamic bridge standard, and therefore contains a signal with a known relation to the input of the transducer, while the second data channel contains the voltage output of the device under test. the transformation of the time series data into the frequency domain is carried out by means of digital fourier transform (dft). because of the synchronised generation and acquisition frequencies, spectral leakage can be avoided. the sampling of full periods of the sinusoidal waveform allows us to go without any windowing prior to the dft. based on the outcomes of the two dfts and the calibration results of the dynamic bridge standard, the magnitude and phase responses of the dut at each excitation frequency can be calculated as described in section 3. the magnitude response is derived only based on the output of the device under test, as the input magnitude is known from the bridge standard’s calibration data. the phase response (i.e. the frequency-dependent phase delay) requires input from both measurement channels, as the absolute phase position cannot be derived from calibration data. for each frequency point, repeated measurements are carried out (the number of repetitions can be set), and the mean and the standard deviation are calculated, giving information about the stability of the input-output relation of the dut for each calibration frequency. 6. measurement uncertainty evaluation the measurement uncertainty is evaluated according to the guide to the expression of uncertainty in measurement (gum) [9] and supplement 1 thereto (gum s1) [10] by means of monte carlo simulations. gum distinguishes between two types of measurement uncertainty contributions, both of which are considered for the measurement uncertainty estimation: 1) type a uncertainty contributions: these can be determined by the statistical analysis of measurements. 2) type b uncertainty contributions: these include all contributions that cannot be estimated by statistical analysis. they may include datasheet content, results from calibrations (of, for example, components of the setup), operator know-how, records from previous measurements, or other prior knowledge. the measurement is modelled by means of mathematical model function 𝑓 propagating all the input quantities 𝑋1 … 𝑋𝑛 to the measurement result 𝑌, giving 𝑌 = 𝑓(𝑋1, 𝑋2, … , 𝑋𝑛) . (4) each input quantity can be described mathematically by its distribution, which leads to estimations of the value of the input quantity and its uncertainty. depending on the sources of information about the input quantity and its properties, the corresponding distributions (gaussian distributed, rectangularly distributed, etc.) are assigned. independent calibrations of the components of the calibration setups are incorporated in the uncertainty estimation as sub-models, i.e. the distributions of the calibration results are assigned to the according input quantity 𝑋. this is the case for the two components detailed below. 6.1. calibration of the dynamic bridge standard the bridge standard was calibrated in advance by calibrating the resistive voltage divider, the mdacs, and the signal conditioners of the outputs. the expanded measurement uncertainties (u(k=2)) of this calibration are 0.03 % (magnitude) and 0.05° (phase angle) for frequencies up to 1 khz and 0.05 % (magnitude) and 0.1° (phase angle) for frequencies above 1 khz and up to 10 khz. figure 4. dynamic bridge amplifier calibration setup, consisting of the bridge amplifier under test (top), the dynamic bridge standard (middle), and the data acquisition system (bottom). acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 22 there are systematic effects, which are in the same order as the expanded measurement uncertainty for the magnitude transfer function and less than 0.2° for the phase angle at 10 khz. 6.2. calibration of the digitiser channels the two digitiser input channels were calibrated dynamically using a voltage calibrator. in typical conditioning amplifier calibration setups, it is not necessary to calibrate the data acquisition hardware so thoroughly if the input and output are acquired simultaneously and a second measurement with swapped input channels is carried out. in this case, possible influences due to the different input channel characteristics can be corrected. however, in the case of the dynamic bridge standard, this procedure is not applicable: as the magnitude response is only derived from the bridge standards output (section 3), all influences due to the digitalisation need to be determined. the calibration of the digitiser channels was carried out using a fluke 5700a voltage calibrator (which was itself calibrated dynamically beforehand). for the calibration, both input channels were connected to the voltage calibrator’s outputs, and the voltages of known magnitude and frequency were generated. from the acquired signal, the root mean square (rms) value was derived for each frequency point of the calibration. the digitiser card shows a characteristic transfer function, which could be compensated for as it is only dependent on the sampling rate [6], [7]. figure 5 shows the calibration results for a sampling rate of 50 ks/s (the same rate used for the calibration measurements). the stable local oscillator of the pxie timing and synchronisation card was calibrated traceably to ptb’s frequency standards by means of a distributed frequency signal. the different measurement uncertainty contributions are depicted in figure 6, and detailed information about the different type b contributions, including their distributions, is given in table 1. 6.3. statistical analysis of the measurements the type a uncertainty contributions are also analysed. the stability of the measurement setup and of the different duts are assessed by carrying out many repeated measurements in a short time (referred to as ‘repeatability’ in figure 6) and by repeating the calibration at larger intervals, including the disconnection of all cables and turning the calibration devices and devices under test off between measurements (referred to as ‘reproducibility’). a complete evaluation of the type a uncertainty contributions requires extensive measurements with different duts and numerous repetitions, which have not yet been carried out. 6.4. correction of systematic effects both the signal acquisition card and the dynamic bridge standard show a frequency-dependent change in their transfer function. gum recommends a correction of all systematic effects. however, if necessary, it is possible to treat those systematic effects as uncertainty contributions [11]. both the magnitude-related systematic contributions are in the same range as the random measurement uncertainty component of the calibration of the dynamic bridge standard. therefore, in the first step, these are not corrected; rather, they are included in the measurement uncertainty estimation as input quantities. in future, these could be corrected to reduce the resulting measurement uncertainty for the bridge amplifier calibration. figure 5: calibration results for a pxi-5922 digitiser card. the relative deviations are given in blue, and the assigned measurement uncertainty u(k=2) is red. the sampling rate was 50 ks/s, and the input voltage span ±5 v. figure 6. ishikawa diagram of the measurement uncertainty contributions for a dynamic bridge amplifier calibration. table 1. type b measurement uncertainty contributions of the calibration setup. input quantity uncertainty contribution distribution dynamic bridge standard (magnitude) 𝑢rel ≤ 2.5 × 10 −4 gaussian dynamic bridge standard (phase) 𝑢 ≤ 0.05° gaussian dynamic bridge standard systematic (mag.) 𝑢rel ≤ 5 × 10 −4 gaussian frequency standard 𝑢rel = 1 × 10 −10 gaussian stability tcx0 (1 year) 𝑢rel = 1 × 10 −6 rectangular temperature stab. tcx0 (0 °c – 55 °c) 𝑢rel = 2 × 10 −6 rectangular skew 𝑢 = 5 × 10−10 s rectangular cal. pxi-5922 (magnitude) 𝑢rel ≤ 2.5 × 10 −6 gaussian pxi-5922 systematic deviations (magnitude) 𝑢rel = 6 × 10 −4 gaussian pxi-5922 dc accuracy (magnitude) 𝑢rel = 5 × 10 −4 and 𝑢 = 1 × 10−5 v rectangular rectangular acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 23 6.5. application of the monte carlo simulation the measurement uncertainty evaluation is carried out by means of a monte carlo simulation according to gum s1. for this purpose, repeated simulations of the measurement are carried out. the different input quantities are modelled according to their input distributions. the resultant distribution of the output describes the measurement results, including uncertainty. 7. first measurement results to obtain a first impression of the performance of this new setup, measurements are carried out with a bridge amplifier under test, which had previously been dynamically calibrated. the previous calibration had been carried out with an earlier prototype of the dynamic bridge standard in conjunction with a sampling voltmeter [12]. the results of the magnitude and phase responses are given in figure 7 and figure 8, respectively. the chosen bridge amplifier shows only small deviations (in the range of a few 10-3 for the magnitude response) from the ideal behaviour in the chosen frequency range. the measurement results agree very well both in terms of magnitude and phase, despite that the individual frequency responses of the corresponding bridge standards have not yet been compensated for. the differences between the results of the two measurements are even slightly smaller than the largest single type b measurement uncertainty contribution. 8. conclusion this paper describes a dynamic bridge amplifier calibration setup based on the new ptb dynamic bridge standard. the output of the bridge standard and the dut is sampled synchronously by a high-precision digitiser. a synchronisation of the oscillators of both the data acquisition system and the bridge standard avoids spectral leakage. the new calibration setup enables the dynamic calibration of bridge amplifiers according to the measurement conditions required by iso 4965-2 [13], in which a dynamic signal on a constant bias is stipulated. the first measurement results of the new setup agree very well with measurements previously carried out with a prototype of the dynamic bridge standard. references [1] c. bartoli et al., traceable dynamic measurement of mechanical quantities: objectives and first results of this european project, international journal of metrology and quality engineering 3, 3 (2013) pp. 127-135, doi: 10.1051/ijmqe/2012020. [2] c. bartoli et al., dynamic calibration of force, torque, and pressure sensors, in proc. of the joint imeko international tc3, tc5 and tc22 conference 2014, cape town, south africa, 2014, https://www.imeko.org/publications/tc22-2014/imeko-tc3tc22-2014-007.pdf. [3] l. klaus, t. bruns, h. volkers, calibration of bridge-, charge-, and voltage amplifiers for dynamic measurement applications, metrologia 52, 1 (2015) pp. 72-81, doi: 10.1088/0026-1394/52/1/72. [4] m. f. beug, h. moser, g. ramm, dynamic bridge standard for strain gauge bridge amplifier calibration, in proc. of the 2012 conference on precision electromagnetic measurements (cpem), 2012, washington dc, usa, pp. 568-69, doi: 10.1109/cpem.2012.6251056. [5] l. klaus, m.f. beug, t. bruns, a new calibration set-up for the dynamic calibration of bridge amplifiers from dc up to 10 khz, in proc. of the 23rd tc3, 13th tc5 and 4th tc22 imeko international conference, helsinki, finland, 2017, https://www.imeko.org/publications/tc3-2017/imeko-tc32017-011.pdf. [6] g. rietvield et al., characterization of a wideband digitizer for power measurements up to 1 mhz, ieee transactions on instrumentation and measurement 60, 7 (2011) pp. 2195-2201, doi: 10.1109/tim.2011.2117330. [7] f. overney et al., characterization of metrological grade analogto-digital converters using a programmable josephson voltage standard, ieee transactions on instrumentation and measurement 60, 7 (2011) pp. 2172-2177, doi: 10.1109/tim.2011.2113950. [8] national instruments corporation, ni pxie-6672 user manual, 2010. [9] bureau international des poids et mesures, et al., evaluation of measurement data – guide to the expression of uncertainty in measurement, 2008. [10] bureau international des poids et mesures, et al., evaluation of measurement data – supplement 1 to the guide to the expression of uncertainty in measurement – propagation of distributions using a monte carlo method, 2008. figure 7. magnitude response calibration results of a bridge amplifier using a prototype of the bridge standard (blue) and of the actual setup (red). figure 8: phase response calibration results of a bridge amplifier using a prototype of the dynamic bridge standard (blue) and of the actual setup (red). http://dx.doi.org/10.1051/ijmqe/2012020 https://www.imeko.org/publications/tc22-2014/imeko-tc3-tc22-2014-007.pdf https://www.imeko.org/publications/tc22-2014/imeko-tc3-tc22-2014-007.pdf http://dx.doi.org/10.1088/0026-1394/52/1/72 http://dx.doi.org/10.1109/cpem.2012.6251056 https://www.imeko.org/publications/tc3-2017/imeko-tc3-2017-011.pdf https://www.imeko.org/publications/tc3-2017/imeko-tc3-2017-011.pdf http://dx.doi.org/10.1109/tim.2011.2117330 http://dx.doi.org/10.1109/tim.2011.2113950 acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 24 [11] i. h. lira, w. wöger, evaluation of the uncertainty associated with a measurement result not corrected for systematic effects, in measurement science and technology 9(6) (1998) p. 1010, doi: 10.1088/0957-0233/9/6/019. [12] l. klaus, entwicklung eines primären verfahrens zur kalibrierung von drehmomentaufnehmern mit dynamischer anregung, ptb report ma-93, 2016, pp. 59-61, pp. 146-50, doi: 10.7795/110.20160714. [13] iso/tc 164/sc 5, iso 4965-2:2012, metallic materials – dynamic force calibration for uniaxial fatigue testing – part 2: dynamic calibration device (dcd) instrumentation, international organization for standardization, 2012. http://dx.doi.org/10.1088/0957-0233/9/6/019 http://dx.doi.org/10.7795/110.20160714 a rams analysis for a precision scale-up configuration of the ‘smart street’ pilot site: an industry 4.0 case study acta imeko issn: 2221-870x june 2019, volume 8, number 2, 3 11 acta imeko | www.imeko.org june 2019 | volume 8 | number 2 | 3 a rams analysis for a precision scale-up configuration of the ‘smart street’ pilot site: an industry 4.0 case study enrico petritoli1, fabio leccese1, martina botticelli2, stefano pizzuti3, francesco pieroni3 1 science department, università degli studi ‘roma tre’ via della vasca navale n°84, 00146, rome, italy 2 dipartimento di ingegneria dell'informazione università politecnica delle marche, via brecce bianche, 60131, ancona, italy 3 enea italian national agency for new technologies, energy and sustainable economic development, via anguillarese 301, rome, italy section: research paper keywords: smart street; rams; reliability; prediction; fmeca; cots citation: enrico petritoli, fabio leccese, martina botticelli, stefano pizzuti, francesco pieroni, a rams analysis for a precision scale-up configuration of the ‘smart street’ pilot site: an industry 4.0 case study, acta imeko, vol. 8, no. 2, article 2, june 2019, identifier: imeko-acta-08 (2019)-02-02 section editor: emiliano sisinni, university of brescia, italy received may 31, 2018; in final form june 13, 2019; published june 2019 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: fabio leccese, e-mail: fabio.leccese@uniroma3.it 1. introduction the system called smart street is a public streetlighting line managed in an automatic, programmable way. its general architecture is shown in figure 1. the first part of this document analyses the smart street pilot site system located at the enea centre in casaccia (rome) from a qualitative point of view. the second part examines the scalability of the system to cover 5,000 streetlamps and some final considerations about architectural configurations. this prediction approach, its trade-offs, and a study of the possible configurations are presented based on the military handbook mil-hdbk-217, ‘reliability prediction of electronic equipment’ [1]-[4], appropriately modified to take account of new physical factors inherent in cots components. a failure mode, effects, and criticality analysis (fmeca) is also performed herein. 1.1. the smart street system study in our study, a reliability analysis of the system is performed. then, in order to preserve or increase the operating time in optimal conditions [5]-[7], the identification of some critical components (at the subsystem or component level) enables the implementation of an inventory stock in order to evaluate the cost effort in terms of time and labour related to maintenance [8]-[11]. the second analysis is a fmeca study [12]-[15], which, through logical sequences of operation, aims to mitigate the severity of the effects of a possible failure at the subsystem or component level [16]-[20]. a synthesis of the two previous methodologies gives us a sort of ‘x-ray plate’ of the system, highlighting the weak parts or subsystems. all this data is useful for evaluating the logistic efforts in terms of stock, intervention times, and possible maintenance. figure 1. the general architecture of the smart street pilot site system. abstract in order to develop a scale-up for a ‘smart street’ pilot site, we perform a rams analysis: the combination of reliability analysis and fmeca helps to identify the best configuration to scale up systems without incurring in marginally reliable configurations, allowing refining an industry 4.0 case study. mailto:fabio.leccese@uniroma3.it acta imeko | www.imeko.org june 2019 | volume 8 | number 2 | 4 2. the reliability analysis: methods and results reliability predictions are an important tool for making design trade-off decisions and estimating the reliability of the design. they are often used for making initial product support decisions (such as how many spares are required to support the systems). inaccurate predictions can lead to overly conservative designs and/or the procurement of excessive spare parts, resulting in added life cycle cost [21]. 2.1. the failure probability the failure of a component or system is an event whose occurrence can be examined in two different ways: a. a casual event, whose causes remain substantially unknown and whose frequency is empirically observed; b. the results of the chemical-physical degradation processes that evolve during the operation. in the first case, the probability of failure is considered as the ratio between the occurrences of the observed failures and all executed tests [22]-[25]. the failure probability is calculated as a function of the observed failures, represented by the ratio: 𝜆(𝑡) = − 𝑑𝑅(𝑡) 𝑑𝑡 𝑅(𝑡) ; (1) therefore, the expression of the reliability is: 𝑅(𝑡) = 𝑒 ∫ 𝜆 (𝑡)𝑑𝑡 ∞ 0 . (2) the advantage is that the reliability of each item is easily studied and can be easily applied [26]. the disadvantage is that no information is supplied about the failure mode and therefore about the prognostic for each single item [27]-[28]. in the second case, the probability of failure is realised as the measure of uncertainty of the predictions; therefore, reliability can be calculated as a function of stress-strength relations, including the uncertainty in which the processes can be realised. in this case, a great amount of data and/or studies are needed [29]. we perform the reliability first by using the mil-hdbk-217 handbook method, but the results are mitigated by factors that take into account degradation processes and environmental factors [30]-[35]. 2.2. the mil-hdbk-217 the mil-hdbk-217 is the military handbook for the reliability prediction of electric and electronic equipment. it was developed in 1961 and has been frequently updated, with the most recent update in 1995 (see table 2). the purpose of milhdbk-217 is to establish consistent and uniform methods for estimating the inherent reliability of equipment and systems. by inherent, we mean the reliability of a mature and confident design [37]-[40]. mil-hdbk-217 is a worldwide standard for performing reliability predictions. it includes a series of failure models based on empirical studies. these models virtually cover all electrical/electronic parts that are based on empirical studies. it also estimates 14 separate operational environments, such as ground fixed, airborne inhabited, etc. [41]. 2.3. the reasons for success of the mil-hdbk-217 the first handbook on reliability prediction, the ‘reliability stress analysis for electronic equipment’ or tr-1100, was released by the radio corporation of america (rca), one of the most important manufacturers of electronic components (especially vacuum tubes). the handbook presents some important mathematical models for estimating electronic component failure rates and was the predecessor of what would be become the standard and a mandatory requirement for reliability prediction in the decades to come, mil-hdbk-217 [4]. the first mathematical approach used in mil-hdbk-217 is to estimate the failure rate by fitting a line through the field failure data. soon after its introduction, all reliability predictions were based on this handbook, and all other sources of failure rates, such as those from independent experiments, gradually disappeared. the failure to use these other sources (see tab. 1) was due to a very important fact: mil-hdbk-217 was usually cited as a contractual document, preventing contractors from using other models or handbooks. in this sense, mil-hdbk217 is strongly linked to the practical and active aspects of the industrial world and is part of the industry 4.0 because the companies involved in the design and installation of efficient lamps managed by a remote control draw on the latest and most innovative technologies of the internet of things (iot) by means of a monitoring and/or control and feedback process fully integrated into the iot itself. 2.4. two prediction methods in mil-hdbk-217, there are two important prediction approaches: the part stress technique and the parts count technique. as the names imply, the part stress method requires knowledge of the stress levels of each component to determine their failure rates, while the parts count technique assumes average stress levels as a means of providing an early design estimate of the failure rates. throughout this article, we use only this second method for the analysis [42]. in mil-hdbk-217, reliability is usually expressed in failures in time (fit): the number of failures that can be expected in one billion (109) device hours of operations. typical factors used in determining a part’s failure rate include a temperature factor (πt), power factor (πp), power stress factor (πs), quality factor (πq), and environmental factor (πe) in addition to the base failure rate (λb) method [43]-[44]. for example, the failure rate model for a resistor is as follows: 𝜆𝑔 = 𝜆𝑏 ∙ 𝜋𝑇 ∙ 𝜋𝑃 ∙ 𝜋𝑆 ∙ 𝜋𝑄 ∙ 𝜋𝐸 . (3) ideally, the parts count technique is applied early in the design phase to determine that the predicted reliability is in the same ‘ball park’ as the reliability requirements. as more detailed design information becomes available, such as detailed circuit schematics, the predictions should be refined to reflect actual applied component stress levels [45]. table 1. overview of reliability manuals. name application last issue mil-hdbk-217 mil/commercial 1995 bellcore / telcordia telecom 2006 rdf 2000 telecom 2000 sae rel. pred. meth. automotive 1987 ntt procedure telecom 1985 siemens sn29500 siemens prop. 1999 china 299b chinese mil 1998 prism mil/commercial 2000 acta imeko | www.imeko.org june 2019 | volume 8 | number 2 | 5 this necessitates switching to the more detailed part stress reliability prediction methodology, which can result in significantly more labour hours for circuit analysis to compute the actual stress levels for each part application. in some cases, the parts count results are used as the final mean-time-betweenfailure (mtbf) estimator, despite that the results can be conservative because of the default stress levels assumed in the methodology. this situation sometimes leads to an excess of spare components [46]. in our analysis, we use the parts count method. it gives more conservative results than the part stress method, and it assumes the typical operating conditions of part complexity, environmental temperature, general electrical conditions, and operation conditions (called reference conditions). the failure rate is calculated according to the parts count method, 𝜆𝑇𝑂𝑇 = ∑ 𝑁𝑖 ∙ (𝜆𝑔 ∙ 𝜋𝑄 )𝑖 𝑛 𝑖=1 , (4) where: λg is the failure rate of the generic part under environmental conditions ni is the quantity of the part i th πq is the quality factor i is the number of part as explained above, the mil-hdbk-217 parts count method provides physically inconsistent data, so it is necessary to overcome the traditional approach to reliability analysis based on the failure rate or hazard rate, which is currently mature and has been consolidated in the international technical literature [47]-[53]. as mentioned, due to the extremely conservative feature of the parts count method, we have added the πcf corrective factor, so the final version of equation (4) is now 𝜆𝑇𝑂𝑇 = ∑ 𝑁𝑖 ∙ (𝜆𝑔 ∙ 𝜋𝑄 ∙ 𝜋𝑐𝑓 )𝑖 𝑛 𝑖=1 . (5) 2.5. cots part and quality the quality factor and the corrective factor are critical for establishing the reliability of the system. smart street is composed entirely of commercial off-the-shelf (cots) electronic components. the approach to establishing the quality of the component cannot therefore be the canonical one of milhdbk-217 and therefore requires other considerations about the evolution of the failure and the environment [54]. the term ‘cots part’ or ‘component’ is defined by the society of automotive engineers (sae) as ‘an electronic component developed by a supplier for multiple customers, whose design and configuration are controlled by the supplier’s or an industry specification.’ the use of cots parts needs a special technical approach due to a lack of confidence in traceability, long-term performance, and new concerns affecting such parts, including compliance, counterfeiting, etc. [55]. a complete reliability analysis of these parts that properly evaluates risk and countermeasures is dependent on sufficient technological knowledge from prior usage, lessons learned, datasheets, modelling techniques, and manufacturer’s information [56]. specific studies should be made to evaluate the quality factor, with the primary concerns being: − part lifecycle and obsolescence monitoring, particularly for standard product lines; − use of historical performance data, long-term performance, and repeatable results; − lessons learned concerning a particular part type, part number, or manufacturer; − the part’s specific temperature constraints; − lot homogeneity; − procurement technique i.e. a controlled manufacturer’s specification; and − counterfeit avoidance. 2.6. the corrective factor πcf the πcf takes into account several parameters, including the technological factor, procedural factor, stress factor, and temperature factor [57] (see figure 2). the complete expression is 𝜋𝑐𝑓 = 𝜋𝑡𝑒𝑐ℎ ∙ 𝜋𝑝𝑟𝑜𝑐 ∙ 𝜋𝑡𝑒𝑚𝑝 ∙ 𝜋𝑠𝑡𝑟𝑒𝑠𝑠 . (6) in specific detail, we have: 𝜋𝑡𝑒𝑐ℎ technological factor: mil-hdbk-217 has so far remained behind regarding the technological capabilities of the individual electronic components since at the time of the last issue, it did not even imagine the rapid development of many manufacturing techniques and circuit integrations [58]. 𝜋𝑝𝑟𝑜𝑐 procedural factor: the general philosophy of the handbook is to examine ‘all the possible types of electronic components in all possible conditions in all imaginable environments’ [59]. for this reason, mil-hdbk-217 is highly rigid and schematic. in other words, to work well, the method needs to ‘freeze’ or rather to give a picture of the current electronic technology [60]; to study all the details; and then, after careful evaluation and testing, to deliver the reliability calculation algorithm. 𝜋𝑡𝑒𝑚𝑝 ∙ 𝜋𝑠𝑡𝑟𝑒𝑠𝑠 stress and temperature factor: in its broadest sense, to define the electric and thermal stress of the component, the environment (and other local stress) in which it is placed is not sufficient to establish its reliability. figure 2. corrective factor πcf composition. πcf πstress πtemp πproc πtech acta imeko | www.imeko.org june 2019 | volume 8 | number 2 | 6 moreover, it is important the type and the components from which, the part in question is surrounded. the πtech technological factor is composed of three other factors, 𝜋𝑡𝑒𝑐ℎ = 𝐹𝑐𝑜𝑚𝑝𝑙𝑒𝑥 + 𝐹𝑐𝑜𝑛𝑓𝑖𝑑𝑒𝑛𝑐𝑒 + 𝐹ℎ𝑜𝑚𝑜𝑔 , (7) where: fcomplex – complexity factor: this parameter takes into account the class of the technological complexity of the component or subsystem. fconfidence – confidence factor: this parameter takes into account the duration of the production period of the component or subsystem and its length of service. fhomog – homogeneity factor: this parameter takes into account the differences in production and the level of difference between the different production lots. the πproc procedural factor is composed of three other factors, 𝜋𝑝𝑟𝑜𝑐 = 𝐹ℎ𝑢𝑚 + 𝐹𝑚𝑎𝑛𝑢𝑓𝑎𝑐𝑡 , (8) where: fhum – human factor: this parameter takes into account any human-caused errors or imperfections of assembly. fmanufact – manufacturer factor: this parameter takes into account any errors or imperfections due to secondary processing techniques. the πproc temperature factor is expressed as: 𝜋𝑡𝑒𝑚𝑝 = ∫ 𝑓𝑇𝑑𝑒𝑔𝑟 𝑇𝑡 𝑇0 (𝑇)𝑑𝑇 . (9) the function ftdegr expresses the degradation mode of the component or the subsystem that evolves from the temperature t0 up to the final temperature tt, recording the trend and all the possible variations. the πstress stress factor is expressed as: 𝜋𝑠𝑡𝑟𝑒𝑠𝑠 = ∫ 𝑓𝑆𝑡𝑟𝑒𝑠𝑠 𝑡 𝑡0 (𝑡)𝑑𝑡 . (10) the function ftdegr expresses the way in which the component or the subsystem is stressed or how much and how its operation moves from the ideal mode and approaches its critical limit [62]. the total corrective factor is now expressed as: 𝜋𝑐𝑓 = (𝐹𝑐𝑜𝑚𝑝𝑙𝑒𝑥 + 𝐹𝑐𝑜𝑛𝑓𝑖𝑑𝑒𝑛𝑐𝑒 + 𝐹ℎ𝑜𝑚𝑜𝑔 ) ∙ (𝐹ℎ𝑢𝑚 + 𝐹𝑚𝑎𝑛𝑢𝑓𝑎𝑐𝑡 ) ∙ [ ∫ 𝑓𝑇𝑑𝑒𝑔𝑟 𝑇𝑡 𝑇0 (𝑇)𝑑𝑇] ∙ [ ∫ 𝑓𝑆𝑡𝑟𝑒𝑠𝑠 𝑡 𝑡0 (𝑡)𝑑𝑡] . (11) note that some elements of the function are not linear and in any case are not related to each other over time [63]. from a physical point of view, we can see the mathematical function as if many degradation processes act on the various components of the system in a completely independent way [64]. 2.7. uncertainty and component degradation in this section, starting from the component degradation model, we define the limits and uncertainty criteria for determining the optimal value that meets or exceeds the maximum life threshold (expected life). we evaluate the level of degradation for the ith specimen for a time �̂�∈[0, ∞) in a cycle of single use ψi = {ψi,1, … , ψi,q }, q ∈ ℕ, then a set of random specimen variables ωi = {ωi,1, … , ωi,v }, v ∈ ℕ that defines the health (degradation status) of the part, following a normal (laplace–gauss) distribution. now, we define the expected useful life (reliability) as the probability that the degradation at time 𝑇𝜂 reaches the theoretical failure threshold 𝜂 before time �̂� is: 𝑃𝑟 {𝑇𝜂 < �̂�} = 𝑃𝑟 {𝛨(�̂� ; 𝛹𝑖 , 𝛺𝑖 ) > 𝜂}, ∀𝑖 ∈ 𝐼 . (12) we consider a component with the following degradation path (which has been chosen because it is typical of such components): 𝛨𝑖 (𝑡, 𝛹𝑖 , 𝛺𝑖 ) = 𝜓𝑖,1 + 𝜔𝑖,1 ∙ �̂� 𝜓𝑖,2 , (13) where 𝛹𝑖 = {𝜓𝑖,1, 𝜓𝑖,2} and 𝛺𝑖 = {𝜔𝑖,1}, then: 𝑃𝑟 {𝑇𝜂 < �̂�} = 𝑃𝑟 {𝜓𝑖,1 + 𝜔𝑖,1 ∙ �̂� 𝜓𝑖,2 > 𝜂} (14) 𝑃𝑟 {𝑇𝜂 < �̂�} = 𝑃𝑟 {𝜔𝑖,1 > 𝜒 − 𝜓𝑖,1 �̂� 𝜓𝑖,2 } . (15) for a random specimen health variable, 𝜔𝑖,1 ≥ 0, we evaluate the cumulative density function 𝐹𝜔𝑖,1 : 𝑃𝑟 {𝑇𝜂 < �̂�} = 1 − 𝐹𝜔𝑖,1 ( 𝜂 − 𝜓𝑖,1 �̂� 𝜓𝑖,2 ) . (16) we define the control time points as a time interval in which the performances of the component are checked (tested). obviously, the component cannot be monitored continuously throughout the degradation process. rather, it is checked at regular intervals. now, we evaluate the probability according to which, between the two control time points (n − 1)τ and nτ, the expected life el is reached: 𝑃𝑟 {𝛨𝑖 ((𝑛 − 1)𝜏; 𝛹𝑖 , 𝛺𝑖 )} ≤ 𝐸𝐿 < 𝛨𝑖 (𝑛𝜏; 𝛹𝑖 , 𝛺𝑖 ), ∀𝑛 ∈ ℕ . (17) so we define 𝑇𝐸𝐿 as: 𝑃𝑟 {(𝑛 − 1)𝜏} ≤ 𝑇𝐸𝐿 < 𝑛𝜏, ∀𝑛 ∈ ℕ . (18) the real failure threshold hi is reached before time point nτ only if it has satisfied the following condition: 𝑃𝑟 {𝛨𝑖 (𝑛𝜏; 𝛹𝑖 , 𝛺𝑖 ) > 𝜂𝑖 } = 𝑃𝑟 {𝑇𝐻𝑖 < 𝑛𝜏}, 𝑖 ∈ 𝐼. (19) moreover, assuming the degradation path is monotonic (typical of this kind complex systems), we have 𝐸𝐿 < hi and 𝑇𝐸𝐿 ≤ 𝑇𝐻 . the probability that a failure happens at the specific time nτ after the degradation level of the ith component has reached the failure limit (𝑛 − 1) ∙ 𝜏 ≤ 𝐸𝐿 < 𝑛𝜏 is: acta imeko | www.imeko.org june 2019 | volume 8 | number 2 | 7 𝑃𝑟 {𝑃𝑀 𝑎𝑡 𝑛𝜏} = 𝑃𝑟 {𝑇𝐻𝑖 > 𝑛𝜏, (𝑛 − 1)𝜏 ≤ 𝑇𝐸𝐿 < 𝑛𝜏}. (20) so, we define the uncertainty in the reliability assessment of the component uλ as: 𝑈𝜆 = 𝐻𝑖 − 𝐸𝐿 . (21) 2.8. the baseline reliability analysis the baseline pilot system is presented in detail in table 2. it consists of five street lamps (each complete with led arrays, drivers, and an ip node), one of which is equipped with a camera [65]-[71]. information on the health status of the street lamp, its consumption, and the camera data are sent to a hub, then the information is converted for transmission in the telephone network through two media converters. in the end, the information are sent to a multiswitch that sends them to users who request them [72]-[73]. the failure analysis of the baseline system returned a mtbftot equal to 5,483 hours, which is equivalent to about 20 months of operation (with the lamps being active 10 hours per day) [74]. the most critical component is the led driver, which alone gives around 50% of all fits of the system. it is the part that is most prone to failure and therefore the most likely candidate for an appropriate stock escort [75]. the analysis resulted in a value of mtbfcamera equal to 4,300 hours, equivalent to about six months of uninterrupted operation. this seemingly low value is due to the fact that the camera is still prototypal, and updates, both software and hardware, are constantly made to it. a critical component is certainly the hub which, in the face of a comforting mtbfhub of 86,200 hours and in the case of catastrophic failure, should cause complete shutdown to all the sections of the lamp network [76]-[81]. 3. fmeca: method and results 3.1. criteria the fmeca is a methodology designed to identify the potential failure modes for a product or process, to assess the risks associated with those failure modes, to rank the issues in terms of importance, and to identify and carry out corrective actions to address the most serious concerns [82]. in the fmeca, the entire system is examined on a functional level, and the effects of failures and malfunctions on the performance of the equipment are determined. the interface circuits of the various subsystems, on the other hand, are analysed at the component level [83]. for each failure mode, the effect, cause, and ways of remedying the failure are studied, with particular attention to any recommendations for changing the project or prevention methods and the frequency at which the failure mode is observed (if possible) [84]-[88]. these results are used for identifying ‘singular’ faults (single point failure [spf]), critical elements, errors, and underestimations of the project and for checking, as far as possible, that all critical failure modes can be fully tested at the subsystem level during assembly [89]-[91]. in table 3, we can see an extract of the fmeca tables. we have selected all the occurrences of grade 1 faults (the most serious), i.e. those in which the failure of one system (or unit) can spread to other systems (or adjacent units). the table, for the sake of brevity, does not show all the columns of the fmeca. rather, only the most interesting and coherent ones are presented. 3.2. fmeca assumptions in the subsequent fmeca, the following presumptions are made: − the total failure of the examined function is presumed; − only one failure mode is considered at a time (i.e. no correlated failures are taken into consideration, except as recommendations); − the fmeca is performed at the function and unit levels, with the exception of interfaces that are analysed at the component level (wherever possible and opportune); and − the probability of failure, which is the subject of a specific analysis, is not considered. 3.3. results the presumed failure mode reported in the fmeca tables is reported as a function of the worst fault i.e. the loss of the function of the main unit [92]. the smart street system has been examined in the form of a pilot system with a number of led lamps reduced to five, one of which is equipped with a video camera. a possible increase or decrease in the number of the lamps without a camera does not significantly affect the philosophy of the approach to the reliability study of the overall system [93]. note that, despite no part of the system is redundant, the system is quite resilient and not “damage sensitive”. in fact, not table 2. reliability analysis summary (baseline). description λp in f/(106 hrs) σ q.ty λp tot in f/(106 hrs) mtbf in hrs ethernet (multiswitch side) 0.08 0.05728 1 0.08 12 048 192.8 media converter (multiswitch side) 4.78 0.01047 1 4.78 209 205.0 telephone cable 0.07 0.00788 1 0.07 15 384 615.4 media converter (hub side) 4.78 0.34634 1 4.78 209 205.0 ethernet (hub side) 0.08 0.06739 1 0.08 12 048 192.8 hub 11.60 0.00911 1 11.60 86 206.9 camera 232.55 0.00084 1 232.55 4 300.2 ip nx8390 5.41 0.01278 5 27.05 36 968.6 led driver le414 108.75 0.56069 5 543.77 1 839.0 led lamp 57.00 0.08015 5 285.00 3 508.8 total reliability: λtot in f/(10 6 hrs) 1109,77 (equivalent to) mtbf (rtotal) in hrs 901,09 table 3. the smart street system: ‘level 1’ criticalities (fmeca extract). n° it e m / s u b -i te m f a il u re m o d e o b se rv a b le s y m p to m s p re v e n ti o n a n d c o m p e n sa ti o n m e th o d s c ri ti ca li ty r e co m m e n d a ti o n s a n d c o m m e n ts 1.1.4 concentrator major failure (short circuit) offline concentrator and electricity line safety devices engaged. impossible to access individual nodes. none 1 the failure could also spread on the line to other units. 1.2.2 concentrator major failure (short circuit) concentrator off. the fault also propagates to the other parts of the concentrator. power line safety devices engaged. impossible to access individual nodes. none 1 the failure could also spread on the line to other units. acta imeko | www.imeko.org june 2019 | volume 8 | number 2 | 8 only can it withstand the lowering of light intensity, but the loss of operation of more than one led, strip, or even a whole streetlight is borne without reaching a minimum brightness that would prevent a correct view of the road [94]. for the avoidance of doubt, although all streetlight systems are strictly the same (except the one with the camera) and have all the same functions, the system cannot be considered as a parallel reliability system [95]-[97]. 4. scalability of the system scalability is the capability of a system or network to handle a growing amount of work or its potential to be enlarged to accommodate that growth [98]. our system is a classic case of a system that lends itself very well to scalability. now, we examine the strategies necessary to perform the scale-up of the system. we estimate that up to 5,000 street lamps are needed for a small city [99]. the two configurations that we will be using in the trade-off (table 4) are quite different. in the first case, we use a single hub to connect all the groups ‘ipnx8390 + led driver + led lamp’ and to manage the single street lamp. in the second case, we use one hub for every five groups of street lamps. in both of the configurations, we observe how the reliability varies with the increase in the scale in terms of the number of streetlights [100]. the first line gives us the number of lamps considered in the scale-up; and the second and third return the mtbf value of the two configurations under consideration. the last two columns illustrate the δmtbf between the first single-hub and multi-hub configurations in both absolute and percentage terms. we can immediately see that after 100 lamps (figure 3), the δmtbf factor varies very little, so the reliability of the two architectures seems to be equal. this is a consideration that is incorrect in substance. note that after 200 lamps, the difference between the two systems remains absolutely negligible before significant changes. from a fmeca point of view, you can see that all the lamps in the first configuration depend on only a single hub [101]. even if the simple evaluation of the parts count method tells us the opposite, we immediately note that the availability of the system is seriously compromised because a hub fail causes the switch off all the lamps. in the second configuration instead, we would have a much more modest loss of the only group of five lamps [102]. this is evidenced by the fact that the hub of the first configuration is a ‘single point failure’ of a critical type, which therefore entails the disastrous loss of all the functionality of the system. 5. conclusions the combination of a reliability analysis and fmeca has allowed us to manage the scale-up of the systems without needing to choose apparently reliable configurations that instead prove to be extremely vulnerable from the point of view of operational availability. the reliability analysis was performed using the parts count analysis of the mil-hdbk-217, appropriately modified to take into account several parameters (including the technological, procedural, stress, and temperature factors), in order to obtain the most realistic results for the study of the cots components of the smart street system. the fmeca helped us to identify the ‘single point failure’ of a configuration that proved to be suitable. in any case, one cannot ignore the “reliability analysis” in identifying the weakest and most vulnerable points. in our study, the fmeca pointed out which parts of our system need to do more stock, regardless of its reliability value. the study of the scalability of the system shows that some critical parts (i.e. the hub) impose a ‘single point failure’ that could affect the functionality of the system itself. even at the cost of raising the figure of overall reliability, it is necessary to multiply these parts in order to remove this point of singularity. references [1] u.s. department of defense, ‘reliability prediction of electronic equipment’ tech. rep., mil-hdbk-217f/2, 1991. [2] l.gullo, ‘the revitalization of mil-hdbk-217’ ieee reliability society 2008 annual technology report. [3] e. de francesco, r. de francesco and e. petritoli, "obsolescence of the mil-hdbk-217: a critical review," 2017 ieee international workshop on metrology for aerospace (metroaerospace), padua, 2017, pp. 282-286. doi: 10.1109/metroaerospace.2017.7999581. [4] u.s. department of defense, mil-1388-2b logistics support analysis record, table 4. configurations trade-offs. # of led lamps mtbf in hrs δmtbf δmtbf in % single hub multi-hub 5 901.1 901.1 0.0000 0.0000 10 452.5 454.9 2.3882 0.0053 15 302.1 304.3 2.1329 0.0071 20 226.8 228.6 1.8039 0.0080 25 181.5 183.0 1.5415 0.0085 50 90.8 91.7 0.8696 0.0096 100 45.4 45.9 0.4596 0.0101 150 30.3 30.6 0.3119 0.0103 200 22.7 23.0 0.2360 0.0104 250 18.2 18.4 0.1898 0.0104 500 9.1 9.2 0.0959 0.0105 1000 4.5 4.6 0.0482 0.0106 1500 3.0 3.1 0.0322 0.0106 2000 2.3 2.3 0.0242 0.0106 2500 1.8 1.8 0.0193 0.0106 5000 0.9 0.9 0.0097 0.0106 figure 3. number of led lamps vs. δmtbf [%]. acta imeko | www.imeko.org june 2019 | volume 8 | number 2 | 9 [online] available: http://ld.hq.nasa.gov/docs/milstd-13882b_dod_requirements.pdf [access date: 09.05.2016]. [5] esa standard document, ‘new reliability prediction methodology aimed at space applications’, appendix 1 to ao/18811/16/nl/ps, 12.09.2016. 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[102] d. l. carnì, d. grimaldi, f. lamonaca, l. martirano and g. parise, "a smart control to operate the lighting system in the road tunnels," 2013 ieee 7th international conference on intelligent data acquisition and advanced computing systems (idaacs), berlin, 2013, pp. 786-790. doi: 10.1109/idaacs.2013.6663033 measuring the modal parameters of a cultural heritage tower by using strong-motion signals acta imeko issn: 2221-870x october 2018, volume 7, number 3, 86 94 acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 86 measuring the modal parameters of a cultural heritage tower by using strong-motion signals mariella diaferio1, dora foti1, nicola ivan giannoccaro2, salvador ivorra3 1 depertment of sciences of civil engineering and architetture, politecnico di bari, bari, italy 2 department of innovation engineering, università del salento, lecce, italy 3 department of civil engineering, university of alicante, alicante,spain section: research paper keywords: operational modal analysis (oma); forced vibrations of historical buildings; strong motion modal parameters identification; vibrodyne excitation citation: mariella diaferio, dora foti, nicola ivan giannoccaro, salvador ivorra, measuring the modal parameters of a cultural heritage tower by using strong-motion signals, acta imeko, vol. 7, no. 3, article 14, october 2018, identifier: imeko-acta-07 (2018)-03-14 section editor: sabrina grassini, politecnico di torino, italy received april 27, 2018; in final form september 13, 2018; published october 2018 copyright: © 2018 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: this work was supported by structural monitoring of artistic and historical building testimonies (s.m.art. buil.t.) project of the european territorial cooperation programme greece-italy 2007-2013 and by prin-miur 2010 research project entitled “dynamics, stability and control of flexible structures”. corresponding author: mariella diaferio, mariella.diaferio@poliba.it 1. introduction historical buildings represent a large part of the cultural heritage of italy. among them towers represent the part that is more vulnerable to dynamic forces – and not only – due to their slenderness. therefore, in the last decades, attention has been drawn to these structures and to their protection and safeguard. the design of interventions to reduce their vulnerability requires a deep knowledge of these structures for which, unfortunately, there is a lack of information because of their historical nature. moreover, to assess their dynamic behaviour, non-destructive tests should be performed because of their historical character. the tests are carried out recording data from a series of accelerometers installed in specific points of the structure and then operational modal analysis (oma) techniques [1]–[12] are applied in order to dynamically characterize the structure. in the case of slender structures, such as towers, this type of investigation can give good results [4]–[6], because their vibrations are easily and clearly recordable even for low force intensity such as environmental loads. in the present study, however, the tower considered for the analysis has a squat profile; its vibrations, therefore, are difficult to be recorded by the usual adopted accelerometers and equipment, and other kind of forcing must be applied. in fact, in these cases, the signals to be recorded are so low that could be confused with the noise which inevitably is present during the acquisitions. in order to guarantee a good ratio between the signal and the noise, forced vibrations are better to be utilized to get reliable information on the dynamic characteristics of the structure. sometimes the input could be obtained from the swinging of the bells when the bells are present and working. in abstract this paper presents the dynamic experimental campaign carried out on a stocky masonry clock tower situated in the swabian castle of trani (italy). the main objective of this paper is, after estimating the main frequencies and vibration modes of the considered structure, defining the transmission of vibrations along the height of the tower by varying the forced frequency at the base. at this aim, short acceleration records have been acquired simultaneously in 20 points of the tower at different levels, due to a series of sinusoidal forced vibrations applied at the base by using a pneumatic shaker device specifically designed for the tests. the proposed procedure permits to extract for each monitored point the amplitude of the sinusoidal component related to the excitation frequency and the phase shift due to the structure damping. the results of the proposed procedure are compared with the results of a classical operational modal analysis in environmental conditions in order to demonstrate that the short forced tests permit to classify the typology of the structural mode shapes. mailto:mariella.diaferio@poliba.it acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 87 other cases, the use of a shaker mechanical device, usually called vibrodyne, could be considered, if it does not compromise the structure of the ancient monument. examples of squat structures or forced excitation can be found in [10–13]. a vibrodyne is able to impress a known signal to the structure; therefore, it is possible to produce different unidirectional harmonic loads with a sweeping frequency depending on the mass of the shaker. usually, a vibrodyne consists of moving/rotating masses by an electric motor. a method for forced vibration testing of structures is proposed in [14]. the method can identify a linear shaker input motion, which produces a structural response like that of the building subjected at the base by a seismic force, and pre-correct the input motion to account for control–structure interaction effects. a vibrodyne was also utilized to test the civic tower of rieti (italy), which was equipped with a tuned mass damper (tmd) system placed on the roof of the building [15-16]. it is utilized to experimentally assess the behaviour of composite structures [17]. in order to get the dynamic response of the squat clock tower of the castle of trani (same characteristics of the structural analysis in [18]) an appropriately realized electrohydraulic shaker device (vibrodyne) has been used to generate forced oscillation on the tower. in this paper the equipment and the experimental setup that has been used for in-situ dynamic identification tests on the squat clock tower of trani’s castle (in italy) are described and an extensive analysis about the effects of the shaking device on the vibrations of the tower is presented completing the preliminary analysis shown in [7]. the performed analysis may be considered very particular and innovative in this field for the opportune post processing elaboration which may underlined the relevant characteristics of structural transmission damping ratio at the different excitation frequencies. the comparison of the extracted transmitted amplitudes and phase shift in the monitored points with the identified mode shapes at different frequencies validates the proposed strategy that has the advantage of giving precise structural transmissivity information at different frequencies. 2. geometrical description the clock tower here considered is part of the swabian castle of trani (south-east of italy). more in detail it is located on the eastern façade, on the main entrance of the castle [19]. the latter was built on the edge of town and the height of the towers allowed to guard the entrance of the port and the access roads to the village. the castle, originally had a simple and functional quadrangular enhanced layout with four square towers of the same height. the castle, with the succession of dynasties, the first angioin, then the aragonese, was always, to this day, owned by the government, except for a brief period (1385-1419), when it was assigned to captain alberico da barbiano. in the sixteenth century, with the advent of the first weapons, the castle was adapted to the new defensive techniques, in response to widespread demand for refortification of the mediterranean coast, under threat from the turks. these operations involved the rise of the southern front, least favourite course and overlooking towards the open country, and the construction of two bastions at opposite edges to the southwest (a spear) and northeast (square), thus ensuring a complete fire coverage around the perimeter of the fortress. the castle has played its military role continuously, except for the years 1958-1677, when it hosted the "holy royal audience" in the province of bari, until, in the nineteenth century, it was transformed into rent provincial destination maintained until 1974. the clock tower of the castle, in limestone, was added to the main entrance on the west side in the nineteenth century, just when ferdinand ii duke of bourbon converted it in the provincial central prison. in 1844, in fact, it came up the need to equip the prison with a clock to adjust the internal life of the prison and in 1848 the new clock tower was built in order to make the sound of the clock reaching the whole castle. with this change of use of the entire castle underwent many changes, but fortunately the work of the nineteenth century were only superimposed to the old structures. the new clock tower, in fact, was built on the old walls of the fortress. in 1976 it was delivered to the superintendence for architectural, artistic and historical heritage of apulia. from 1979 to 1998 the entire castle was then restored and almost all the additions of the period of the prison were demolished. at present it can be visited as a historical monument and the clock tower maintains its use like a clock. the clock tower has an overall height equal to 7.20 m from its main door. the tower has a square plan transversal section, with the inner side of about 2.05 m, constant along the total height, while the outer side is variable along the height because of the presence of a trani stone covering. in fact, the outer side is 4.25 m at the base and 3.55 m at the intermediate height of 4.00 m and, from this level up to the top, it keeps constant. at the intermediate height of 4.60 m there is also a toroidal stringcourse. the tower façades are characterized by openings arranged on two different levels: (i) at the lower level, there is the doorway on the west side, while on the other sides, there are archivolted windows with a width of about 1.40 m and an average height of 60 cm. (figure 1b). (ii) at the upper level, there are archivolted windows smaller than the previous ones, with a width of 85 cm and a height of 55 cm, arranged on all sides except for the east one, which is blind to host the large central clock. (figure 1a). on the eastern facade, differently by the other ones, the masonry keeps for about 2.25 m, with a gabled portal with an arched opening in the middle where a bell is located. figure 1. clock tower. trani’s swabian castle: (a) east view; (b) west view; (c) south view. 3. the experimental test to evaluate the tower main frequencies and its experimental mode shapes, the tower has been instrumented with 23 high sensitivity seismic accelerometers icp pcb 393b31 with a sensitivity of about 10 v/g. all the accelerometers were (a) (b) (c) acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 88 connected by flexible wired cables to a ni data acquisition system, the latter connected to a laptop with a specific acquisition software positioned at the base of the tower. (a) (b) figure 2. experimental setup: (a) positions and directions of acquisition of the 23 accelerometers; (b) tower’s 3d view. the accelerometers were placed at four different levels on the four lateral sides of the tower: 8 accelerometers at the four corners of the floor over the clock, 6 accelerometers at three corners at the intermediate level, 6 at the three corners at the lower level as part of the clock tower, and 1 accelerometer at the basis as a reference sensor (figure 2a). appropriate rectangular blocks, where the accelerometers were inserted with screws, were used for ensuring the orthogonality of each couple of accelerometers installed in the same point. it is important to note the points a, b, c and d, aligned on the left part of the tower and placed at different levels (figure 2a) and the points e, f and g aligned with a, b and c on the opposite side of the front of the tower. two accelerometers were placed on the superior arch for monitoring the oscillation of the upper part, probably the most significant local modes for stability analysis. 3.1. environmental tests results four consecutive acquisitions of environmental vibrations (called test 1, test 2, test 3 and test 4) were carried out by recordings of 15 min each with a sampling frequency of 1024 hz. the specific artemis software [20] was used for the extraction of the modal parameters. two different oma methods were used for each analysis: the enhanced frequency domain decomposition (efdd), which operates in the frequency domain, and the stochastic subspace identification (ssi) using unweighted principal components (upc), which operates in the time domain. in this analysis, the ssi method has demonstrated to be able to better estimate the frequencies when it works at 1024 hz. figure 3 shows the ssi diagram of the first environmental test and the frequencies of the tower clearly identified. the identified frequencies (in hz) for the 4 tests conducted under environmental actions are summarized in table 1. it is evident the good repeatability of the identified frequencies for all the environmental tests; moreover, similar results were obtained also adopting the efdd technique. so, nevertheless the squat profile of the analyzed tower, the power of the recent oma techniques and the sensitivity of the available sensors permit to identify with a good confidence the frequencies and the corresponding mode shapes. the same confidence is not possible for the evaluation of the damping ratios; for this reason, the structure has been forced with a specific electrohydraulic shaker (vibrodyne) designed for exciting the structure from the basis. the mode shapes corresponding to the identified frequencies are related to bending modes (along y and x axis indicated in figure 2, respectively) for the first two frequencies; the third frequency is related to a torsional mode, the fourth frequency to a second bending mode (y axis), the fifth frequency to a bending-torsional mode, the sixth one to a torsional mode. figure 3. ssi diagram for the test 1. 3.2. forced tests results as aforementioned, other tests have been performed by installing a vibrodyne at the main entrance of the tower (figure 4). the vibrodyne accumulator was charged by an electric motor. the vibrodyne was installed between two frames to anchor it to the base of the tower by means of four screws (figure 4 b). the forces produced by the vibrodyne were characterized by a chosen frequency and a fixed amplitude. (a) (b) figure 4. a) transportation of the vibrodyne; b) vibrodyne installed at the main entrance of the clock tower. table 1. identified frequencies for the 4 environmental tests using the ssi technique. frequency order test 1 hz test 2 hz test 3 hz test 4 hz 1st 7.52 7.5 7.48 7.53 2nd 10.32 10.36 10.34 10.31 3rd 13.28 13.33 13.35 13.51 4th 15.93 15.98 15.79 16.09 5th 18.08 18.34 6th 21.85 21.83 21.83 22.08 acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 89 a preliminary test was carried out considering an excitation produced by a 200 kg moving mass having the same amplitude and changing the frequency from 1 hz to 15 hz, with a step of 2 hz modified about every 2 min. figure 5 shows the temporal acceleration values during the tests in points a, b, c and d placed at different levels as indicated in figure 2. the final 80 s of acquisition have been done switching off the diesel motor, only with the pressure accumulated in the accumulator, to evaluate the influence of the motor on the recorded oscillations; it is evident that there is a brusque diminution of the oscillations. figure 5. accelerations in the positions a, b, c and d during the forced tests, the final 80 s are acquired after switching off the motor. from figure 5, it is evident that the shaker device amplifies the oscillations in all the monitored points of the tower. considering the last 80 s of figure 5 when the pump motor is off, the results clearly show that the dominant effect of the oscillations is due to the pump motor. this preliminary analysis convinced to use the shaker device with only the accumulator power (motor switched off), in order to excite the structure avoiding the vibrations of the motor. the preliminary analysis was very useful for arranging further tests without doubts about the possibility of acquiring data only related to the shaker device forcing action and not influenced by the pump motor effects. short tests were carried out using only the accumulator energy as forcing action. however, the accumulator autonomy was very short and also depending by the frequency; for a frequency of 3 hz the accumulator had 110 s of autonomy, but it decreased to about 50 s for 9 hz, to about 25 s for 18 hz and to only 15 s of autonomy for 20 hz. 3.3. filtering procedure in order to analyze directly the effect of each forced frequency at the base with each response at each position on the tower an innovative strategy is here introduced to use the forced vibration signals in a more specific way. the approach is based on considering a digital filtering of the acquired signals; in detail, a classical windowed linear-phase fir digital filter has been designed [21]. the filter is normalized so that the magnitude response of the filter at the center frequency of the passband wn = [w1 w2] is 0 db. the fir filter is a non-recursive one because the output signal is a function only of the input signal u. the response of such a filter to a generic input signal is a finite sequence of m+1 samples, where m is said the filter order. in order to generalize the response to any input signal, the system function h(z) of the fir filter has to be evaluated. it is the z-transform of the impulse response that is the output when the kronecker delta function δ is given as input. the system function h(z) is calculated in (1) where a0 and bi (i=0, …, m) are constant coefficients, with a0, bm ≠ 0.              m i i i m i i zb a ikbz a khzzh 0000 1 )( 1 )()(  (1) the aim of filtering the acquired signal was achieved by the calculation of the coefficients bi (i=0,…,m) through the fir1 function in matlab [22]. a two element vector [wl wh] was passed to this function of the signal processing toolbox in such a way as to specify the band of normalized frequencies for bandpass configuration. this is a hamming-window based, linear-phase filter with normalized cut-off frequency. the lower and upper bound filters were chosen as in equation (2). different values for the parameter ε were firstly settled for testing the filter capabilities and subsequently the value 0.05 hz was fixed to obtain the desired bandwidth. 2 l s f w f        , 2 h s f w f        (2) 3.4. application of the filtering process to the forced vibrations data the application of digital filters to experimental accelerometers data in environmental conditions have been recently tested by the authors [23], [24], [25] in [23] the postprocessing has permitted to synchronize different wireless accelerometers by analysing the phase of the signals; in [24] it was demonstrated that, for environmental data, a digital fir filter is not able to separate an accurate sinusoidal signal for the effect of close components. in this paper, the digital technique is applied to forced vibrations data, considering the band-pass central frequency coincident with the shaker device excitation frequency. the digital post-processing has been successfully applied to all the acquired accelerometers data. in the following the data referred to four acquisition points a, b, c, d along the y direction will be analysed. the filtered signal and its frequency spectrum in comparison with the original signal and its spectrum are shown for the case of excitation at 9 hz for points a, b, c, d, and, for point a, also for the case of 16 hz and 18 hz excitation in figures 6-9. it is evident that the digital filter is able to eliminate the external components to the forcing frequency and that the filtered signal is almost sinusoidal with a very low fluctuation regarding its amplitude, considering a transitory time of 1 s. the same effect has been carried out for all the acquired accelerometers data and for all the considered excitation frequency. figure 6. filtering effect for points a, b, c and d, excitation frequency at 9 hz. acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 90 for all the considered filtered sinusoidal signals it is possible to calculate automatically the amplitude as the average value of half of the peak to peak value all over the full acquisition period, and also an initial phase angle may be calculated for estimating the delay respect to the initial evaluation time of the first maximum. figure 7. filtering effect for point a, excitation frequency at 16 hz. figure 8. filtering effect for point a excitation frequency at 18 hz. figure 9. zoom of the filtering effect for point a (y direction), excitation frequency at 18 hz. in order to compare the effect of the digital filter with the environmental situation, in figure 10 also the case of environmental excitation is shown with reference to the same point a (y direction) and considering the same digital filter centered at the first estimated frequency (7.5 hz). this demonstrates the necessity of the shaker device effect that allows to analyse the data as really forced vibration data, using parameters such as the amplitude and the phase angle also for testing the dynamic behavior of the structure. 4. analysis of the filtered forced vibration data the parameters obtained by means of the described filtering procedure may be analysed for extracting further interesting information about the modal parameters which characterize the structure but also about the transmissibility of the forced oscillations at different levels and from different sites of the structure. in the present research, 7 significant points have been considered, already introduced in paragraph 3, and constituting two vertical lines placed at the left part (points a, b, c and d) and at the right part (point e, f, g) at the same level of the tower. for each of the aforementioned seven points both the accelerometers (along direction indicated with x and y, see figure 2) have been considered in such a way to have an idea of the spatial behaviour of the structure. figure 10. filtering effect for point a, environmental conditions. 4.1. excitation frequency at 9 hz the peak to peak amplitude for the seven considered points along the y direction is represented in figure 11 by considering the different height of the consider points with respect to the ground (where the vibrodyne is installed). the trend is crescent in an almost quadratic way and the interesting thing is that the values of the filtered peak to peak transmitted vibrations are very close in both the two sides of the front part of the tower. moreover, the corresponding phase values, evaluated respect to the phase of the accelerometer in point a (y direction) and shown in figure 12 clearly show that all the filtered signal are perfectly in phase with a very low phase shift on the two considered lines of points. the trend carried out in figures 11 and 12 is typically associated with a first flexural behavior; effectively, considering the environmental test results shown in paragraph 3, there is a flexural mode directed along y axis around 7.5 hz. the sinusoidal force at 9 hz has been able to excite the flexural mode and the proposed procedure permits to detect the flexural behavior with an acquisition of around one minute. figure 11. excitation frequency of 9 hz: peak amplitude of the analyzed points, y direction. acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 91 figure 12. excitation frequency of 9 hz: phase shift of the analyzed points, y direction figure 13. excitation frequency of 9 hz: peak amplitude of the analyzed points, x direction figure 14. excitation frequency of 9 hz: phase shift of the analyzed points, x direction 4.2. excitation frequency at 16 hz analysing by means of the proposed procedure the forced data with excitation frequency of 16 hz (corresponding as shown in paragraph 3 at about the value of the second flexural mode along y axis), the peak amplitude and the phase shift along y axis shown in figures 15 and 16 have been obtained. it is evident that the situation is changed with respect to the previous case; the peak amplitude has not an increasing behavior (with respect to the height) but it has a minimum point at about the center of the tower for both the lines constituted by the points a-b-c-d and e-f-g. this behavior is in accordance with the second flexural mode shape. moreover, there is an increasing phase shift which arrives at about 170° between the extreme point a and d and 160° between e and g confirming the plausibility of a second flexural mode along y. figure 15. excitation frequency of 16 hz: peak amplitude of the analyzed points, y direction figure 16. excitation frequency of 16 hz: phase shift of the analyzed points, y direction a further confirmation may be obtained by evaluating the peak amplitude (figure17) and phase shift (figure18) along x axis, where, confirming a peak decreasing around half of the height, there is not anymore a phase shift of around 180° between extreme points. moreover, considering that the second mode (flexural along x), has been estimated around 10.5 hz, the same analysis has been conducted for the accelerometers placed along the x direction (points a-b-c and e-f-g) and the results are shown in figures 13 and 14. it is evident that the peak amplitudes transmitted have the same shape of the y direction (lower value) and that there is a constant phase between the points a-b-c and e-f-g. figure 17. excitation frequency of 16 hz : peak amplitude of the analyzed points, x direction acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 92 figure 18. excitation frequency of 16 hz: phase shift of the analyzed points, x direction 4.3. excitation frequency at 18 and 20 hz the excitation at 18 and 20 hz are referred, as identified in paragraph 3, mainly to bending-torsional and torsional modes of the structure. the results along y axis for peak amplitude and phase shift are shown in figures 19-20 (excitation at 18 hz) and figures 21-22 (excitation 20 hz). the results of figures 19-22 are completely different from the results with excitation frequency at 9 hz and also are quite different from the ones for excitation frequency at 16 hz. it is immediate to recognize for the excitation frequency at 20 hz a very different behavior (both in phase and in peak amplitude) between the two different sides of the tower which is justifiable with a heavy torsional effect excited at that frequency (as it appears from the results in paragraph 3). figure 19. excitation frequency of 18 hz: peak amplitude of the analyzed points, y direction figure 20. excitation frequency of 18 hz: phase shift of the analyzed points, y direction figure 21. excitation frequency of 20 hz: peak amplitude of the analyzed points, y direction figure 22. excitation frequency of 20 hz: phase shift of the analyzed points, y direction moreover, at 18 hz, there is a slight different behavior about the phase between the two sides while the peak amplitudes are quite comparable between the two sides. this is also justifiable because the mode at 18 hz has been classified as a flexuraltorsional mode, so it has some characteristics of a flexural mode but also a phase shift difference at the opposite sides. in order to complete the presentation of the analysis, in figures 23-26 are depicted the results along the x direction for the considered excitation frequencies of 18 and 20 hz. the last figures confirm the torsional effect evident for the excitation at 20 hz and the flexural-torsional effect of the excitation at 18 hz. figure 23. excitation frequency of 18 hz: peak amplitude of the analyzed points, x direction acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 93 figure 24. excitation frequency of 18 hz: phase shift of the analyzed points, x direction figure 25. excitation frequency of 20 hz: peak amplitude of the analyzed points, x direction. figure 26. excitation frequency of 20 hz: phase shift of the analyzed points, x direction it is important to remark that all the considerations carried out in this paragraph permit to characterize the modes of the instrumented tower by only analysing very short forced tests with respect to the long acquisitions in environmental conditions which often, for structure not streamline, do not permit any dynamic analysis. moreover, the proposed procedure permits to individuate with accuracy the transmitted vibrational effect on the different monitored points of the structure. this information can also be used for validating models of the structure with information related to the damping ratio effect between the monitored points of the structure at the different forcing frequencies. 5. conclusions the paper discusses the dynamic experimental tests performed on the clock tower of the swabian castle of trani and the subsequent dynamic identification. the tests have been conducted acquiring the vibrations induced by environmental actions and also by installing an electro-hydraulic shaker device at the base of the tower. the analysis of the environmental vibrations performed in the framework of the oma has allowed the identification of the natural frequencies. the main novelty here proposed is related to an innovative strategy for analyzing the forced vibrations with quick tests during which the structure was excited by a vibrodyne placed on its ground. the short tests were carried out by driving the vibrodyne with different frequencies and the strategy here proposed permits to insulate, by means of opportune digital band-pass filters, the sinusoidal component at the excitation frequencies in all the monitored points. the sinusoidal component may be used for determining, at the different frequencies, the transmitted amplitude and the phase shift between the different points of the structure. this information may confirm, as demonstrated in this case, the modal shapes of the structure, and also allows to tune opportune models by ensuring the relevant characteristics of structural transmission damping ratio. acknowledgement the authors gratefully acknowledge the funding by structural monitoring of artistic and historical building testimonies (s.m.art. buil.t.) project of the european territorial cooperation programme greece-italy 2007-2013 and by prin-miur 2010 research project entitled “dynamics, stability and control of flexible structures”.f. paparella and the staff of the “laboratorio m. salvati” of the polytechnic of bari are gratefully acknowledged for their help during the tests references [1] c. gentile, a. saisi, ambient vibration testing of historic masonry towers for structural identification and damage assessment, constr. build. mater., 2007, 21, 6, pp. °1311–1321. [2] c. gentile, a. saisi, a. cabboi, structural identification of a masonry tower based on operational modal analysis, int. j. archit. herit., 2014, 9, 2, pp. 98–110. [3] s. ivorra, f. j. pallarés, dynamic investigations on a masonry bell tower, eng. struct., 2006, 28, 5 pp 660-667. [4] d. foti, m. diaferio, n. i. giannoccaro, s. ivorra, structural identification and numerical models for slender historical structures. handbook of research on seismic assessment and rehabilitation of historic structures, 2015, pp. 674-703. [5] d. bru, s. ivorra, f. j. baeza, r. reynau, d. foti, oma dynamic identification of a masonry chimney with severe cracking condition, in 6th international operational modal analysis conference, iomac 2015. [6] l. f. ramos, l. marques, p.b. lourenco, g. de roeck, a. campos-costa, j. roque, monitoring historical masonry structures with operational modal analysis: two case studies, mechanical systems and signal processing, 2010, 24, pp. 12911305. [7] m. diaferio, n. i. giannoccaro, d. foti, s. ivorra, identification of the modal properties of an historic masonry clock tower, in sahc2014 –2014, 9th international conference on structural analysis of historical constructions. [8] a. tomaszewska, c. szymczak, identification of the vistula mounting tower model using measured modal data, engineering structures, 2012, 42, pp. 342-348. acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 94 [9] m. diaferio, d. foti, c. gentile, n.i. giannoccaro, a. saisi, dynamic testing of a historical slender building using accelerometers and radar, prooceedings of 6th international operational modal analysis conference, iomac 2015; abba hotel gijon; spain; 12-14 may 2015. 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[14] e. yu, d. h. whang, j. p. conte, j. p. stewart, j. w. wallace, forced vibration testing of buildings using the linear shaker seismic simulation (lsss) testing method. earthq, eng. struct. dyn. 2005; 34, pp. 737–761. [15] e. m., tronci, d. pietrosanti, g. cordisco, m. de angelis, vibration analysis of the civic tower in rieti, procedia engineering, 2017, 199 pp. 2268–2273. [16] f. fabbrocino, i. farina, m. modano, loading noise effects on the system identification of composite structures by dynamic tests with vibrodyne, 2017, composites part b 115 pp. 376-383. [17] a. barbieri, a borri, m. corradi, a. di tommaso, dynamic behaviour of masonry vaults repaired with frp: experimental analysis, in: proc. of the 6th international conference of the british masonry society, uk; 2002. pp. 7-16. 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[24] n. i. giannoccaro, l. spedicato, d. foti, a digital analysis of the experimental accelerometers data used for buildings dynamical identification, in eesms 2016 2016 ieee workshop on environmental, energy, and structural monitoring systems, proceedings, 2016. [25] m. diaferio, d. foti, n. i. giannoccaro, s. ivorra, experimental methodology for measuring the structural dynamic transmission damping of a cultural heritage tower, proc. of imeko, international conference on metrology for archeology and cultural heritage, lecce, 23-25 october 2017, pp. 1-6, 2017. template for an acta imeko event paper acta imeko issn: 2221-870x july 2017, volume 6, number 2, 42-49 acta imeko | www.imeko.org july 2017 | volume 6 | number 2 | 42 usage history of three mass comparators in the past 20 years masaaki ueki, jianxin sun and kazunaga ueda national metrology institute of japan/aist, central 3, 1-1-1 umezono, tsukuba, ibaraki 305-8563, japan section: research paper keywords: mass comparator; mass measurement; weight calibration; reliability citation: masaaki ueki, jianxin sun and kazunaga ueda, usage history of three mass comparators in the past 20 years, acta imeko, vol. 6, no. 2, article 7, july 2017, identifier: imeko-acta-06 (2017)-02-07 section editor: min-seok kim, research institute of standards and science, korea received june 28, 2016; in final form september 28, 2016; published july 2017 copyright: : © 2017 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: masaaki ueki, e-mail: m.ueki@aist.go.jp 1. introduction in general, the reliability of mass measurements can be confirmed by referring to the mass values of calibrated weights. as a national metrology institute, the national metrology institute of japan (nmij), advanced industrial science and technology (aist), has established and maintained national mass standards ranging from 1 mg to 5000 kg based on the japanese copy of the prototype kilogram (no.6). these mass standards are disseminated to various users of mass metrology in japan. nmij/aist offers two kinds of weight calibration services [1]. one of the services, named “jcss calibration”, is a specific calibration service of the japan calibration service system (jcss), and the other is a service for verification standard weights for legal metrology. the jcss calibration services, which are regularly offered to the jcss accredited weight calibration laboratories, are available for weights of classes e1 and e2 as specified in oiml r111 [2]. these weight calibration services are controlled by a quality management system of nmij/aist based on iso/iec 17025 [3]. three mass comparators for the weight calibrations ranging from 10 g to 10 kg have been maintained as part of an important calibration facility since march 1996 [4], [5]. in the past 20 years, the most challenging situation for the three mass comparators was the occurrence of a huge earthquake on friday 11 march 2011. this earthquake was later named “the 2011 off the pacific coast of tohoku earthquake” and had a magnitude of 9.0 on the richter scale. peak accelerations of 3.3 m s-2 in the horizontal direction and 2.5 m s-2 in the vertical direction were observed in tsukuba city, where nmij is located. fortunately, the main quake caused almost no damage to the sets of standard weights and mass comparators used for weight calibrations at nmij, because the mass comparators were not in operation at that time. after waiting for the decay of aftershocks and reconfirming the mass values of the sets of standard weights, calibration services resumed six months after the 3.11 earthquake while carefully monitoring the mass comparators in operation. in spite of this precautious, an earthquake aftershock occurred on 3 december 2011, causing serious damage to a weight exchanger for the one of the three mass comparators requiring replacement of the whole exchanger. although minor troubles of unknown origin have occurred a few times per year, all three mass comparators have been generally kept in good condition. nmij has continuously improved the accuracy and efficiency of weight abstract since march 1996, three mass comparators have been used for mass calibrations of class e1 and e2 weights ranging from 10 g to 10 kg at national metrology institute of japan (nmij). these mass comparators are each equipped with four-position weight exchangers and carry out fully-automated mass comparison with relative sensitivities from 10-9 to 10-7. the mass comparisons among four weights, including repeated measurements, take a measuring time of 8 h or longer, so a series of mass comparisons can be made only once a day. over about 20 years, at least 1700 mass comparison series for each of the three mass comparators and more than 5700 series in total have been performed. this paper describes the usage history of the mass comparators during the period from 1996 to 2016, along with some measures taken to maintain and improve their reliability. acta imeko | www.imeko.org july 2017 | volume 6 | number 2 | 43 calibrations. in the following sections, an outline of 20 years of history using the three mass comparators, a combined comparison measurement procedure, and estimation of the reliability of mass comparisons performed by fully automated systems will be presented. 2. mass comparison procedure 2.1. principle of mass calibration in mass determination for weights, a mass comparator of the electronic force compensation type with a single weighing pan is used. a test weight is compared in air with a reference weight by the substitution weighing method. at equilibrium conditions, the mass mb of the test weight is expressed by the following equations: (1) (2) where di is the difference between indicated values of the comparator, v is the volume of the weight, ρa is the air density, ma is the mass of the reference weight, cb-a is the correction for the acceleration due to gravity, g and γ are the acceleration due to gravity and its vertical gradient, z is the distance from the bottom surface of the weight to its centre of gravity, and the reference and test weights are indicated by suffixes a and b, respectively. in the laboratory, γ = -3.07×10-6 s-2, and g = 9.79948981 m s-2. 2.2. weighing cycle a-b-a a weighing between a reference weight a and a test weight b, which is adopted by the nmij mass laboratory and known as the a-b-a cycle [2], consists of nine successive weighings in the order of a1, b1, a2, b2, a3, b3, a4, b4 and a5. the mean value dib-a of the mass difference between the weights is calculated by the following equations: (3) (4) where ibi and iai are the indicated values of the comparator. when the indicated values change linearly, the influence of the change can be compensated for, and the mass difference between the two weights is measured accurately. 2.3. combined comparison among four weights in order to realize calibrations with high reliability using a mass comparator equipped with a four-position weight exchanger, all comparisons for six combinations of two weights from among the four weights are carried out in the course of the measurements. a result is determined using the least squares method involving six comparisons among four weights. the unknown mass of the test weight is calculated from the solution of the matrix. the comparison pattern among the four weights which the authors adopt is shown in figure 1, and table 1 shows the weighing scheme for nominally equal mass comparisons of the reference weight a, the check weight c, and two test weights b and d. in table 1, “-1”, “0” and “+1” represent the factors indicating the constitute weight at a given comparison mass. here, the equations showing the conditions for weight comparison are rewritten as a matrix. the mass differences xi (i=1, 2··· 6) given by equation (5) indicate the values of the mass comparator including corrections for the air buoyancy and the acceleration due to gravity. (5) masses mb, mc and md of the test weights are calculated as follows from the mass differences xi obtained as the results of the series of mass comparisons shown in table 1. (6) the standard deviation s46 of the mass comparisons among the four weights using the six combinations is calculated as follows: figure 1. mass comparison pattern among four weights. table 1. weighing design matrix for comparison among four weights. weight a b c d no. reference te s t che ck te s t 1 -1 +1 0 0 x 1 2 -1 0 +1 0 x 2 3 -1 0 0 +1 x 3 4 0 -1 +1 0 x 4 5 0 -1 0 +1 x 5 6 0 0 -1 +1 x 6 res ul t ( ) aa-baaba-bb mcvvdim ++−+= ρ ( ) abaa-b mzzg c −= γ ( ) ( ) ( ) ( ) ( ) ( ) ∑ = = −+− = −+− = −+− = 3 13 1 2 2 2 a-b 5444 3 3332 2 2111 1 , , i d d d d idi iaibiaib iaibiaib iaibiaib ( ) ( ) ( ) ( ) ( ) ( ) c-dacdcd6b-dabdbd5 b-cabcbc4a-daadad3 a-caacac2a-baabab1 , ,, ,, cvvdixcvvdix cvvdixcvvdix cvvdixcvvdix +−+=+−+= +−+=+−+= +−+=+−+= −− −− −− ρρ ρρ ρρ ( ) ( ) ( ) a65321d a64321c a54321b 2 4 1 2 4 1 2 4 1 , , mxxxxxm mxxxxxm mxxxxxm +++++= +−+++= +−−++= acta imeko | www.imeko.org july 2017 | volume 6 | number 2 | 44 (7) (8) 3. three mass comparators the specifications of the three mass comparators, mc100g, mc1kg and mc10kg, are summarized in table 2. three mass comparators are generally used for weight calibrations in the mass range from 10 g to 10 kg. each of the mass comparators is equipped with an automatic exchange mechanism for four weights. the mass comparator is connected to a personal computer through an rs-232c serial port, and is operated using control software provided by the manufacturer. the mass comparisons among four weights are performed fully automatically for specified weight combinations and with a specified number of repetitions being controlled by the personal computer, with a relative sensitivity of 10-9 to 10-7. 3.1. maintenance of the three mass comparators repetition of the combined comparisons among four weights take a measuring time of 8 h or longer, so only one series of these mass comparisons can be made in a day. the results file for one series is stored in a personal computer and is given the code name. the symbols "a", "b", and "c" are used to identify mc100g, mc1kg and mc10kg, respectively. the history of these measurements is summarized in a record sheet, and statistical analysis is applied to check the results. table 3 shows an example of the record sheet for mc1kg. at least 1700 result files for each of the three mass comparators and more than 5700 files in total have been backed up to a host computer over the past 20 years. the quality of the calibration results using the three mass comparators is controlled in the following ways. 1) the mass comparison of test weights is performed using the weighing cycle a-b-a specified in oiml r111. 2) the calibration result is evaluated as satisfactory when: i) the standard deviation s46 obtained from the measurements of mass differences is smaller than 1/10 of the expanded uncertainty required for a class e1 weight. ii) the difference between the previous calibrated value and the new value of the check weight is smaller than half of the combined standard uncertainty of the reference weight. 3.2. calibration room building 3-8 was constructed in the aist tsukuba central area, and operations started in march, 1998. this building houses laboratories for the establishment of the national standards, and provides calibration services for mass, length, temperature, vacuum, and standard materials. rooms with floor areas of about 150 m2 in the building are assigned to weight calibrations, and the weight calibration facility previously operated in an old building was moved to the new room. each of the mass comparators is mounted on a granite monolith, which is in turn mounted on a vibration-isolation concrete block separated from the main structure of the building. room b103 is located in the basement to reduce the disturbing influence of heat and vibration from outside. the room is airconditioned year-round, and temperature and relative humidity are controlled to within ±0.2 °c in the temperature range from 22 °c to 24 °c and within ±4 % in the relative humidity range from 40 % to 60 %. to realize a dust-free ambient environment, the room meets the specifications of class 7 clean rooms except that the air draft velocity is 60 cm s-1 or less. the absolute pressure, temperature and relative humidity of air are measured using a set of instruments to evaluate the ambient conditions and the density of air in the rooms. the indications of these monitoring instruments are collected on a personal computer through gpib interfaces, and the density of air in the rooms is calculated using the cipm international air density equation. these calculations are made at 10 min intervals over 24 h, and all results are stored on computer hard disks. figure 2 shows an example of the measured results for temperature, relative humidity, atmospheric pressure and calculated air density of the room in january 2000. since temperature fluctuation is usually less than ±0.2 °c, the air density changes depending mainly on atmospheric pressure, and air density varies in the range from 1.1721 kg m-3 to 1.2080 kg m-3 in january 2000. 3.3. influence of room temperature fluctuations on mc10kg as mentioned above, the air-conditioning system for the building was installed in 1998. around 2013, the system began to encounter frequent mechanical trouble due to aging over the table 3. record sheet of measurement history for mc1kg with an example. table 2. specific features of the mass comparators. pos.1 pos.2 pos.3 pos.4 b_1901 07-jun-15 0:00 nmij 1000 a0 t5 a9 t6 0.0005 b_1902 08-jun-15 0:01 nmij 1000 a0 t5 a9 t6 0.0015 b_1903 09-jun-15 0:00 nmij 1000 a0 t7 a9 t8 0.0019 05:44 intensity 3, rather strong earthquake b_1904 10-jun-15 0:01 nmij 1000 a0 t7 a9 t8 0.0007 compared 4 weights s 46 /mg remarkcode date start time castomer n.m. /g type mc100g mc1kg mc10kg me a s ura nd 10 g to 100 g 200 g to 1 kg 2 kg to 10 kg mass comparator manufacturer mettler-toledo mettler-toledo mettler-toledo model at106h at1006 at10005 capacity 111 g 1011 g 10.011 kg readability 1 μg 1 μg 10 μg repeatability*1 1.5 μg 2 μg 20 μg *1: manufacturer specification ( ) ( ) ( ) ( ) ( ) ( ) ( ) 2 1 2 46 cd66bd55 bc44ad33 ac22ab11 6 136 1 , ,, ,,         = − −−=−−= −−=−−= −−=−−= ∑= i mmxmmx mmxmmx mmxmmx i s ε εε εε εε acta imeko | www.imeko.org july 2017 | volume 6 | number 2 | 45 course of 15 years after installation. the temperature of air flowing into the calibration room could vary suddenly, and it became difficult to keep the room temperature sufficiently constant. in order to maintain weight calibration services even before the air-conditioning system was renovated, measures against sudden temperature changes had to be taken to assure the reliability of the mass measurements in such an environment. using mc10kg which is the most sensitive to its surrounding conditions, the temperatures of the mass comparator and its surroundings were measured in detail, and a draft shield to cover the mass comparator was newly designed and its effect was tested. figure 3 shows a photograph of the mc10kg used to investigate the influence of room temperature fluctuations on mass measurements. through air-inlets situated in the ceiling, air from the air-conditioning system flows into the room with a flow velocity of about 0.6 m s-1. mc10kg is divided roughly into four parts, namely a weighing part for detecting weight loads, a weighing house for storing measured weights, a weightexchanging mechanism and a motor-driving part for weightexchanging. the motor part generates the most heat, operating intermittently at 40 w of electric power consumption, the next most heat-generating component is the weighing part, whose electric circuits consume 15 w. the heat generation in the weighing house and the weight-exchanging mechanism is negligibly small. the temperatures of mc10kg and ambient air have been measured by platinum resistance thermometers shown in figure 4. these thermometer elements are of the four-wire type with a nominal resistance of 100 ω and a nominal current of 1 ma, and are of two kinds of shapes. the dimensions of mc10kg are 718 mm in width, 890 mm in height and 315 mm in depth, and the wire cables of the thermometer elements are about 2 m in length. for accurate measurements of temperatures at the points of the elements, four-wire resistance measurements are adopted, and measurement errors arising from temperature distributions along the cables are compensated for. twenty elements, five elements for the main body of mc10kg and 15 for the surrounding air, are distributed in space. these elements have short time constants of several seconds because of their small size and can quickly respond to temperature variations. a digital multi-meter with a built-in scanner makes 20-channel resistance measurements and monitors temperatures with a readability of 0.01 °c. these temperature measurement systems are calibrated against a reference platinum resistance thermometer certified with an expanded uncertainty of 0.01 °c by the japan electric meters inspection corporation. the digital multi-meter is controlled by a personal computer (pc), and all data for corrected results of 20-channel temperature measurements are recorded on the pc. in the first stage of the experiments, the temperature measurements at the 20 points are made at 10 min intervals. air temperatures around mc10kg are sufficiently stable to within ±0.2 °c in most cases, but sudden large variations of up to about 2 °c in 10 min are occasionally observed. even after monitoring the temperature for days, no patterns can be found in either the arising frequency of these unstable sudden temperature variations or in the time of day at which they occur. to reduce the influences of these large and irregular temperature variations of up to 2 °c, a draft shield was newly designed to cover mc10kg. in the past, the authors tried to cover mc10kg with an air-tight glass wind shield having dimensions of 1 m in width, 1 m in height and 1 m in depth, but the standard deviation of the mass comparison measurement results could not be improved, this was mainly due to heat generation in the motor part of the weight exchanger. in the present work, a new draft shield was designed with a larger height of 1.5 m, as shown in figure 3, and with an figure 2. results of measurements of the temperature, relative humidity, atmospheric pressure and air density of the weight calibration room b103. a) frames made of aluminium alloy b) anti-static bubble wrap figure 3. mc10kg and its new draft shield. a) two kinds of thermometer elements b) personal computer, scanner and display figure 4. temperature measurement system. 100 102 104 106 1.16 1.18 1.20 1.22 pr es su re / k pa air density [ min:1.1721 kg m-3 max:1.2080 kg m-3 ] a ir de ns ity / kg m -3 air density pressure 43 53 63 22.6 22.9 23.2 1 11 21 31 h um id ity / % te m pe ra tu re / ° c date /day [ jan. 2000 ] temp. humidity 1 m 1 m 1.5 m 10 mm for air for body surface acta imeko | www.imeko.org july 2017 | volume 6 | number 2 | 46 opening at the upper part of its side walls with the aim of alleviating heat flow. the frames of the shield are made of aluminium alloy, and the side and top walls are wrapped with anti-static bubble wrapping sheets to prevent any problems caused by magnetic force and dust affecting precision mass measurements. to verify the effect of the new draft shield, mass comparisons of 10-kg weights were carried out by means of the weighing cycle a-b-a with a series of continuous measurements lasting about 7 h. in the weighing cycle a-b-a, two 10-kg weights, a and b, are alternatively measured at even time intervals, and the mass difference ∆m is then calculated using equations (3) and (4). as an example, measurement results for the mass differences before and after the use of the draft shield are shown in figure 5, as well as indication values of mc10kg and the measurement results for the temperatures of the weighing house and ambient air. the mass difference ∆mi in the figures is the mean value over three repeated a-b-a weighing cycles and is given as a deviation from the average of 18 observed values, ∆mmean. error bars give the standard deviation of ∆mi. as shown in a) of figure. 5, when the draft shield is not used, the temperatures in the weighing house are affected by sudden temperature variations of the calibration room, and become unstable, resulting in large indication changes. consequently, the observed mass differences are also dispersed with a standard deviation of 0.116 mg, which does not satisfy the specifications of mc10kg. on the other hand, when the draft shield is used, the temperatures in the weighing house are quite stable, as shown in b), and the mass differences can be calculated with a considerably improved value of standard deviation of 0.012 mg. in 2014, the air-conditioning system of building 3-8 was renovated and it became possible once again to maintain the environmental conditions of the calibration rooms sufficiently well. thereafter, mass comparisons using all the three mass comparators have yielded satisfactory results. 4. performance of the three mass comparators to examine the performance of the three mass comparators, the repeatability, reproducibility and effect of weight exchanges were evaluated experimentally from the results of mass comparisons. 4.1. repeatability as described above, a series of mass comparisons among four weights was made only once a day, since each series takes 8 h or longer. from march 1996 to february 2016, at least 1700 results files for each of the three mass comparators and more than 5700 files in total have been accumulated. figure 6 presents histograms showing the distributions of the standard deviation s46 of the mass comparisons for different nominal masses. more than 1700 results for mc100g and, 2000 results for mc1kg have been compiled to produce these histograms. in a) results for mc100g b) results for mc1kg figure 6. histograms showing the distributions of the standard deviation s46 for mass comparison measurements. a) the case without using the draft shield b) the case using the draft shield figure 5. measurement results for the mass difference and the temperatures of mc10kg. 0 100 200 300 fr eq ue nc y standard deviation / μg 100 g 200 g 500 g 1 kg 0.0 1.0 2.0 3.0 4.0 0 100 200 300 fr eq ue nc y standard deviation / μg 10 g 20 g 50 g 100 g 0.0 1.0 2.0 3.0 4.0 -0.3 0.0 0.3 ∆m i− ∆m m ea n /m g -2.0 0.0 2.0 in di ca tio n ch an ge / m g weight a weight b 21 22 23 24 8:00 10:00 12:00 14:00 16:00 18:00 a ir t em pe ra tu re /º c time /hh:mm weighing house around mc10kg 21 22 23 24 8:00 10:00 12:00 14:00 16:00 18:00 a ir t em pe ra tu re /º c time /hh:mm weighing house outside shield -2.0 0.0 2.0 in di ca tio n ch an ge / m g weight a weight b -0.3 0.0 0.3 ∆m i − ∆m m ea n /m g acta imeko | www.imeko.org july 2017 | volume 6 | number 2 | 47 the figures, the histogram distribution is uneven in the results for mc1kg while a tendency of decreasing frequency to the right is observed for mc100g. the averages of the standard deviation in all mass ranges are 0.8 μg for mc100g and 0.9 μg for mc1kg. although there is no clear tendency in the distributions of the standard deviation of the mass comparison results obtained by using either mc100g or mc1kg, all of the standard deviations are sufficiently small and are nearly equivalent to the readabilities of the mass comparators. therefore, it was confirmed that the mass comparisons using these two comparators were made successfully with good repeatability. 4.2. reproducibility in the usual cases of weight calibrations at nmij, mass comparisons are made between one reference weight, one check weight and two test weights, as described in section 2.3. the performance of mass comparators can be monitored by the reproducibility of the calibration results of this check weight. these mass comparisons by the combined method are judged in correctness from the compatibility between the new results and previous calibration results. to discuss the reproducibility, calibrations were made for the check weights of 100 g and 50 g using mc100g and for the 2-kg check weight using mc10kg in april 2015. results of the calibrations are shown respectively in figure 7 a) and b). similarly, figure 7 c) gives four series of calibration results for the 1-kg check weight using mc1kg during the period from 1997 to 2015, although time intervals between the four series were not always regular. the ordinate in each figure gives the deviation from the average of the calibration results, and the abscissa gives the file number or measurement number of the calibrations. error bars for each result show the standard deviations of the three repeated measurements. mean values of these standard deviations, s(∆), are also shown in the figures. as shown, the mean values, s(∆), were 0.2 μg for mc100g, 0.6 μg for mc1kg and 7 μg for mc10kg. all of these values are better than those in the manufacturer’s specifications. as clearly seen in figure 7 a) to c), no significant tendency is found in the calibration results of the check weights, and it proves that the weight calibrations were made with dispersions in the level of readability for each comparator. it takes a longer time for a series of measurements using the combined comparison method: about three times as long as for conventional one-to-one comparison measurements. it has been, however, confirmed that the calibrations of weights can be realized while maintaining high reliability by monitoring the adequacy of mass difference measurements. 4.3. influence of repetitive weight exchanges at nmij, a group of the stainless steel weights of 1 kg with different characteristics has been maintained to keep the reference value of 1 kg with a high accuracy [6]. in other words, the mutual relations among the mass values of a number of weights with different specifications are monitored, and possible changes in the masses are dealt with adequately. for this purpose, a series of mass comparison measurements has been performed in the period from april, 2015 to june, 2015. to ensure that numerous repetitions of weight exchanges cause little or no influence on the masses of weights, changes in the masses of 1-kg weights after a single weighing them more than 2400 times using the mass comparators were measured. six stainless steel 1kg weights marked a0, a9, a1, a2, a4 and a5, were selected for this experiment. austenitic stainless steel used for these weights has good characteristics as standard weights material. in its manufacturing process, in-vacuum melting was conducted twice so as to minimize absorbed gases such as oxygen, nitrogen and hydrogen, and the process involved sufficient forging, rolling and heat treatment to achieve good homogeneity with minimum segregation and small crystal particle sizes. each of these weights has a surface area of 138.6 cm2 and is of cylindrical shape with dimensions of 54 mm in diameter and 54 mm in height. the hardness and surface roughness of the weights were measured before the experiment as 160 or higher in brinell hardness and 0.08 µm or less in terms of surface roughness rz. this confirms that the weights satisfy the technical requirements applied to standard weights used at nmij. figure 7. reproducibility of calibration results for check weights using the three mass comparators. -0.010 -0.005 0.000 0.005 0 20 40 60 80 ∆m i − ∆m m ea n /m g measurement number -0.004 -0.002 0.000 0.002 1560 1570 1580 1590 1600 1610 ∆m i − ∆m m ea n /m g file number 100 g 50 g -0.08 -0.04 0.00 0.04 0.08 1772 1782 1792 1802 1812 1822 ∆m i− ∆m m ea n /m g file number a) results for mc100g at the 100-g and 50-g level b) results for mc10kg at the 2-kg level c) results for mc1kg at the 1-kg level s(∆):0.2 μg s(∆):0.6 μg 1997 2000 2010 2015 s(∆):7 μg acta imeko | www.imeko.org july 2017 | volume 6 | number 2 | 48 table 4 shows the calibration history of the six 1-kg weights. in calibration no.01, mass values of the weights a0 and a9 were evaluated by comparison with a reference weight s1_1, which is traceable to the international prototype of the kilogram. the latest internal calibration of this reference weight was made just after the “bipm extraordinary calibrations” in 2014. thereafter, weights a0 and a9 were used respectively as the reference weight and check weight for calibrations of 35 pieces of 1-kg weights. in one of the series of calibrations, calibration no.02-1, two weights a1 and a2 were calibrated against the reference weight a0 by the combined comparison method, and weights a4 and a5 were similarly calibrated as calibration no. 02-2. after calibration no.02-2, the two weights a4 and a5 were kept in a storage box to minimize their mass changes for about three months from 27 april 2015 to 25 june 2015. the storage box’s outer walls were coated with conductive paint to prevent any static electrification, and a clean atmosphere with low-humidity was realized in the box by using hepa filters and a built-in electro-dehumidifier. in the storage box, the weights are placed on cotton-fibre wipers spread on stainless steel shelves. in calibration no.03, 31 pieces of 2-kg weights were calibrated using mc10kg and referring to the weights a1 and a2, where weight a1 was stacked on weight a2 to realize a combined weight of 2 kg. therefore, weight a2 was exchanged more than 2400 times with the load doubled. as a final step, four weights, a1, a2, a4 and a5 were calibrated again during the period from 23 june to 28 june. calibration results of the four weights a4, a5, a1 and a2 are summarized in figure 8. the ordinate in the figures give deviations between the two calibration results for the respective weights to show the tendencies of mass changes, and the abscissa indicates the calibration number. error bars for each result show the expanded uncertainty of the mass comparisons. as seen in figure 8 a), no significant change was found in the calibration results of weights a4 and a5, proving that the mass value of weight a0 is quite stable in spite of more than 2840 repeated weighings during the period from 22 april to 28 june. moreover, figure 8 b) shows that no significant change in the mass of weight a2 was observed, even after more than 2400 repeated weighings at the 2-kg level. figure 9 shows micrographs of the bottom surface of weight a2 after used in weighing more than 2400 times at 2-kg level. the micrographs were taken by means of a digital microscope with a magnification from 25 to 175. as seen in figure 9 a), areas are classified into “original area” and “contact area”. the original area has only a few slight scratches, as shown in figure 9 b), and seems to maintain its initial surface structure. on the other hand, some marks were observed in the contact area, as given in figure 9 c). the marks might have been made by contact with the weighing pan of mc10kg, not by regular wear table4. calibration history of six stainless steel 1-kg weights a0, a9, a1, a2, a4 and a5. a) after weighing by mc1kg at 1 kg level b) after weighing by mc10kg at 2 kg level (no.03) figure 8. mass changes of the 1-kg weights after weighing over 2400 times or more using fully automated mass comparators. here, the calibration procedures for no.02-1, 2, 3, 4, and no.03 are shown in table 4. reference w eight check w eight test w eights comparator number of w eighings 01 17 apr. 2015 ----s1_1 ----a0, a9 prototype balance ----02 22 apr. 2015 to 28 june 2015 b_1861 to b_1920 a0 a9 35 w eights of 1 kg mc1kg over 3100 times 02-1 22 apr. 2015 to 24 apr. 2015 b_1861 to b_1863 a0 a9 a1, a2 mc1kg over 130 times 02-2 25 apr. 2015 to 27 apr. 2015 b_1864 to b_1866 a0 a9 a4, a5 mc1kg over 130 times 03 28 apr. 2015 to 22 june 2015 c_1773 to c_1821 a1, a2 (combination 2 kg) a1 (2 kg) 31 w eights of 2 kg mc10kg over 2400 times 02-3 23 june 2015 to 25 june 2015 b_1915 to b_1917 a0 a9 a1, a2 mc1kg over 130 times 02-4 26 june 2015 to 28 june 2015 b_1918 to b_1920 a0 a9 a4, a5 mc1kg over 130 times no. date file no. calibration procedure -0.006 -0.003 0.000 0.003 0.006 m as s ch an ge / m g calibration number a4 a5 no.02-4no.02-2 -0.006 -0.003 0.000 0.003 0.006 m as s ch an ge / m g calibration number a1 (upper part) a2 (lower part) no.02-3no.02-1 exchanging a0 over 2840 times exchanging over 2400 times acta imeko | www.imeko.org july 2017 | volume 6 | number 2 | 49 but rather due to specific friction events. therefore, the contact marks do not seem to pose a serious problem. it can be concluded that the influence of more than 2840 repeated weighings at the 1-kg level and more than 2400 at 2-kg level is negligibly small with regard to the mass values of the six stainless steel 1-kg weights. 5. summary for mass calibration of class e1 and e2 weights, the three mass comparators mc100g, mc1kg and mc10kg were installed in march 1996, and have been maintained into 2016 as part of an important calibration facility which will continue into the future. each of the mass comparators is equipped with a fourposition weight exchanger, and performs fully automatically combined comparisons between four weights. until now, numerous series of mass comparisons including at least 1700 series for each comparator and more than 5700 series in total, have been performed in the mass range from 10 g to 10 kg. for mc10kg with a relative sensitivity of 10-9, the influence of irregular changes of ambient air temperatures on mass measurements was studied using platinum resistance thermometers with 20 fast-response measuring elements with a of high resolution of 0.01 ºc. temperature was measured simultaneously at five locations on the main body of mc10kg and at 15 locations in the surrounding air. a new draft shield made of aluminium alloy frames and anti-static bubble wrapping sheets was prepared aiming to mitigate the influence of room-temperature fluctuations. as a result, even in unstable environments such as with sudden room temperature changes of up to 2 ºc occurring irregularly, mass difference measurements of 10 kg weights were improved from 0.116 mg to 0.012 mg in terms of their standard deviations. the performance of each of the three mass comparators has been assured by monitoring the reproducibility of the results of mass comparisons continually. the reproducibility of calibration results for the check weights has been kept at satisfactory levels of 0.2 μg for mc100g, 0.6 μg for mc1kg and 7 μg for mc10kg. it was confirmed that the influence of 2840 repeated weighings at the 1-kg level or 2400 at the 2-kg level is negligibly small with regard to the mass values of the six stainless steel 1kg weights used as a part of the national mass standards system. it can be concluded that the three mass comparators have been maintained with sufficient performance over the 20 years since their inception in 1996. references [1] home page, https://www.nmij.jp/english/service/p/jcss/. [2] oiml: “oiml r111-1:2004: weights of classes e1, e2, f1, f2, m1, m1–2, m2, m2–3 and m3 part 1: metrological and technical requirements”: https://www.oiml.org/en. [3] iso/iec 17025:2005 “general requirements for the competence of testing and calibration laboratories”: http://www.iso.org/iso/catalogue_detail.htm?csnumber=39883. [4] m. ueki, y. nezu, a. ooiwa,“new facility for weight calibration service at nmij”, proc. of 14th imeko world congress, june 16, 1997, tampere, finland, vol. iii, pp. 59-64. [5] m. ueki, y. nezu, s. mizushima, a. ooiwa, “weight calibration by mass comparators with automatic weight exchanging mechanism”, proc. of 16th imeko tc3/apmf ’98, sept. 14-18, 1998, taejon, republic of korea, pp. 220-226. [6] m. ueki, s. mizushima, j. sun, k. ueda, “mass stability of 1 kg stainless steel weights in the past 13 years”, proc. of sice annual conference 2008, aug. 20-22, 2008, tokyo, japan, pp. 1020-1023. figure 9. the outer periphery region of the bottom surface of weight a2 after weighing over 2400 times using mc10kg at the 2-kg level. a) outer periphery region (×25) b) original area (×175) c) contact area (×175) original area contact area https://www.nmij.jp/english/service/p/jcss/ https://www.oiml.org/en http://www.iso.org/iso/catalogue_detail.htm?csnumber=39883 usage history of three mass comparators in the past 20 years reconfigurable measuring system for the automatic detection of bacterial growth in a specimen processing platform acta imeko issn: 2221-870x june 2019, volume 8, number 2, 35 44 acta imeko | www.imeko.org june 2019| volume 8 | number 2 | 35 reconfigurable measuring system for the automatic detection of bacterial growth in a specimen processing platform paolo bellitti1, michele bona1, michela borghetti1, emilio sardini1, mauro serpelloni1, stefania fontana2 1 università degli studi di brescia, via branze 38, 25123 brescia, italy 2 copan italia s.p.a., via f. perotti 10, 25125 brescia, italy section: research paper keywords: bacterial growth detection; wasplab® platform; impedance; petri dish; industry 4.0 citation: paolo bellitti, michele bona, michela borghetti, emilio sardini, mauro serpelloni, stefania fontana, reconfigurable measuring system for the automatic detection of bacterial growth in a specimen processing platform, acta imeko, vol. 8, no. 2, article 6, june 2019, identifier: imeko-acta-08 (2019)02-06 section editor: alessandro depari, university of brescia, italy received july 20, 2018; in final form june 07, 2019; published june 2019 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was supported by the italian ministry for education, university and research, as part of the adaptive manufacturing project (ctn01_00163_216730), which was promoted by the italian ‘cluster tecnologico nazionale fabbrica intelligente’. corresponding author: michele bona, email: m.bona002@unibs.it 1. introduction worldwide bacterial infections are a major concern for human health according to reports and recommendations from the world health organization [1]-[2]. emerging pathogens, together with antimicrobial resistances spreading within bacteria species, challenge microbiology laboratories to achieve rapid and efficient detection of bacterial presence in biological samples. in the past few decades, research activities in academia and industry have led to the development of techniques permitting the detection of bacterial growth in a biological sample much earlier than by traditional colony count methods [3]-[4]. in this way, positive specimens are recognised faster and can be further analysed, allowing the eventual provision of appropriate therapy for patients. such techniques include fluorescence spectroscopy [5], impedance microbiology [6], surface plasmon resonance [7], isothermal microcalorimetry [8], and flow cytometry [9]. at the same time, increasing laboratory automation enables the automated management of specimen handling, plating, and incubation, reducing the processing time and improving the quality of results [10]. the combination of such techniques with automation has resulted in the appearance on the market of systems aiming to meet the abovementioned goals. for instance, both the bactec™ (becton, dickinson, and company) [11][12] and the bact/alert (biomérieux sa) [12]-[14], in their different versions, monitor the change in fluorescence caused by abstract the detection of bacterial infections in biological samples represents a fundamental task in human health preservation. this paper describes a measuring system that aims to detect bacterial growth in petri dishes while they are inside the incubator of the wasplab® specimen processing platform (copan italia s.p.a.). in fact, the interaction between the system and the wasplab® enhances the analytical process. the proposed system monitors bacterial growth by measuring the impedance related to the examined petri dishes. furthermore, it can be easily reconfigured according to user requirements. finally, it has some features that enhance its connectivity with the wasplab® and other devices, developing a structure that is compliant with the industry 4.0 model. we assess the system performance through different tests, in which we monitored the growth of s. aureus atcc 6538, starting from concentrations in the order of 4.5.108 cfu/ml, 4.5.107 cfu/ml, and 4.5.106 cfu/ml. the results show the system’s correct operation both in an ad hoc setup and while working with the wasplab®. furthermore, the connectivity between the system and other devices allows for a user to be informed about the analyses that are on course and to act on the system at any time and even from remote locations. mailto:m.bona002@unibs.it acta imeko | www.imeko.org june 2019| volume 8 | number 2 | 36 a co2 increase as a by-product of bacterial activity. the rabit (don whitley scientific ltd.) [15] and the bactrac (sy-lab geräte gmbh) [16] measure electric impedance, whose variation is related to pathogen growth. the biacore system (ge healthcare) monitors an optical signal that is dependent on bacterial presence [17]. the thermal activity monitor (ta instruments) detects heat produced by the chemical reactions associated with the growth [8]. within automated solutions, the walk-away specimen processor™ wasp® from copan italia s.p.a. is a benchmark for the automatic and efficient elaboration of clinical samples, improving the quality of microbiological analyses and laboratory reports [18]. the wasp® is integrated in a more complex specimen processing platform called wasplab® [19]. this platform combines the wasp® with co2 and non-co2 incubators, an image acquisition station for each incubator, and a digital imaging interface. it monitors bacterial growth by taking images of incubated petri dishes, which contain biological samples for analysis. in this way, a considerable number of highquality plate images is produced, with positive repercussions also for species identification and antibiotic susceptibility tests [20]. as well as the most advanced manufacturing systems, the wasplab® is intended to achieve higher and higher throughput quality standards. in addition, a market that is in constant evolution requires systems to adapt promptly to its increasingly fast dynamics. one example is the increasing number of requests for system and/or product customisation and reconfiguration to meet end-user requirements [21]. this is especially true in the industry 4.0 age, when systems are conceived to be connected with other devices that work in cooperation and create a smart factory scenario [22]-[23]. with reference to the wasplab®, the precocious detection of bacterial growth on incubated plates, besides and independent from image acquisition, could be an unprecedented feature that may further enhance its analytical performance. among the abovementioned techniques, impedance microbiology represents one of the most suitable ways of achieving this goal. in fact, impedance measurement allows for the acquisition of quantitative data about bacterial growth through relatively simple and cheap instrumentation [4]. in a previous work [24], we illustrated some results from a study in which we applied an impedance measuring system to analyse bacterial culture plates inside one of the wasplab® incubators. in this article, we present such system, and we describe the laboratory tests performed to evaluate the possibility of its integration in the wasplab® in more detail. in fact, the system could ensure continuous growth monitoring, bringing a great advantage to precocious detection: the opportunity to interrupt incubation could be individually determined for each single plate, avoiding pre-selection at specific time intervals. further analysis, like plate image acquisition, colony-picking, species identification, and antibiotic resistance determinations, could be performed promptly and without wasting time. in addition, this article is different with respect to the works presented in the literature review. while such works deal specifically with sensor design and related growth detection techniques, this article describes a complete system with a sensor, electronics, and a user interface. then, it focuses on system characteristics, combining impedance measurement capability with high flexibility and reconfigurability. finally, it stresses the system features that enhance collaboration and/or integration with other platforms (e.g. the wasplab®) and connectivity with external devices. therefore, it presents a case study about the application of industry 4.0 principles to microbiology. at the same time, this article proposes an improved system compared with previous versions illustrated in other works [25]-[26], regarding both electronics and user interface development. the article is structured as follows. in section 2, we provide a detailed description of the measuring system, also highlighting the progress accomplished with respect to its previous versions. then, in section 3, we illustrate how we carried out the tests on the system, whose results are reported in section 4. finally, the work’s achievements are summarised and concluding remarks are given. 2. the developed measuring system figure 1 provides a picture of the measuring system. three main parts can be identified. the first part is a petri dish instrumented with a pair of electrodes. the second part is an electronic module designed to measure petri dish impedance. finally, the third one is a laptop running a user interface program. 2.1. petri dish figure 2 shows an employed petri dish which was realised by inserting two stainless steel electrodes in a plate of the same type as those analysed with the wasplab®. the inserted electrodes laptop electronic module petri dish figure 1. picture of the developed measuring system. rm rct rct cdl cdl electrode 60 mm 60 mm screw culture medium figure 2. instrumented petri dish with its equivalent model. acta imeko | www.imeko.org june 2019| volume 8 | number 2 | 37 are 60 mm long and have a rectangular section. they are partially immersed in the agar culture medium contained in the petri dish, at a height of about 4 mm. in addition, they are kept positioned at a distance of 60 mm in order to allow the generated electric field to cover the greatest possible medium surface. finally, they are fixed to the plate cover, from which two steel screws protrude as contact terminals. all petri dishes were sterilised with γ rays after the insertion of electrodes. we fitted the petri dish frequency response with the equivalent circuit model represented in figure 2 [25]. it describes the electrode/medium interface through the parallel between the double layer capacitance cdl and charge transfer resistance rct. then, the resistance rm defines the medium’s own conductance. it is derived from the randles circuit [27], without warburg impedance, whose contribution can be neglected in the frequency range analysed in our work, going from some tens of hertz to less than 1 khz. in this model, bacterial growth is associated with a change in the characteristics of both medium and electrode/medium interface, which is reflected by rm, cdl, and rct variations. such parameters are calculated from petri dish impedance measurements carried out at two constant frequencies f1 and f2, by implementing the following mathematical formulae: 2 1 2 2 2211 ))(im(/))(im(/ ff fzffzf c dl − − = (1) ))(im(44 ))(im( 1 22 1 2 1 1 fzcfcf fz r dldl ct + −= (2) 222 2 22 41 2 ))(re( ctdl ct m rcf r fzr + −= (3) where im(z(f1)) and im(z(f2)) are the values of the impedance imaginary part measured at f1 and f2, respectively, whereas re(z(f2)) is the value of the impedance real part at f2. the need to always measure im(z) and re(z) at two frequencies allows for working with a system characterised by simpler electronics. 2.2. electronic module figure 3 reports the block scheme of system electronic module, which excites petri dish electrodes with sinusoidal waveforms and allows measuring related impedance z. such module was assembled from off-the-shelf components. the main component is the ad5933 integrated circuit (analog devices, inc.) [28]. it integrates elements that otherwise should be included as separate parts. an example is the direct digital synthesiser for excitation waveforms generation, which was a distinct element in the previous electronic unit of system version proposed in [25]. it is a part of the same component in the present version. this results in benefits concerning reduced dimensions, which enhance system portability. in fact, the electronic module is contained in the black box shown in figure 1, whose dimensions are 120 x 120 x 55 mm, and it is more compact than the electronic unit described in [25]. integrated circuit ad5933 generates excitation waveforms characterised by a dc offset equal to 1.65 v, but whose amplitude and frequency can be set according to user necessity, albeit within a specific range, comprising f1 and f2. this gives a certain degree of flexibility to the entire system that is able to fit different operating conditions. furthermore, ad5933 performs z measurements and makes the results available in terms of impedance real part and impedance imaginary part. other components guarantee electronic module operation (figure 3). firstly, the power conversion unit makes a 3.3 v proper supply available to all electronic circuits, from the 5 v supply provided by the laptop, which is connected by a usb port. then, the ael 4303 (ael crystals ltd) 16 mhz oscillator in combination with the adf4001 (analog devices, inc.) phaselocked loop (pll) divider guarantee the ad5933 clock frequency that is required according to the f1 and f2 range i.e. 200 khz [28]. anyway, other values of clock frequency can be achieved by changing the pll division ratio (currently equal to 80), enabling reconfiguration of the measuring system (if necessary). furthermore, a modifiable analog frontend was inserted between the ad5933 output and the instrumented petri dish to filter and amplify the signals. finally, the microcontroller (mcu in figure 3) acts as an interface between ad5933 and laptop for data transmission, and it manages global module operation. before starting a new measurement session, the electronic module needs to be tuned. this step is necessary for finding the value of its internal gain factor and for compensating any parasitic effect due to the presence of cables. tuning is carried out by connecting discrete elements (e.g. a resistor, a capacitor, or a combination thereof) of known value to module terminals, instead of a petri dish. since the electronic module comprises commercial components, more identical samples can be replicated and activated simultaneously if there is a need to work on an increasing number of petri dishes at the same time. this functionality favours the enhancement of measuring system modularity. 2.3. laptop the laptop runs a labview virtual instrument (vi) user interface program, which implements multiple functionalities. although we used a laptop in this work to augment system portability, any computer that is able to run the vi can be data 5 v power conversion mcu ad5933 16 mhz oscillator pll analog front end vin vout petri dish 3.3 v 3.3 v 3.3 v 3.3 v usb figure 3. block scheme of the system electronic module. acta imeko | www.imeko.org june 2019| volume 8 | number 2 | 38 potentially employed for the proposed measuring system. this vi is a more advanced version than the one previous characterising system prototype illustrated in [26]. in particular, we added some features that increase measuring system connectivity with collaborating devices, following an architecture that is compliant with industry 4.0 paradigms. in general, the vi allows for: • setting the values of the measurement parameters, such as the working frequencies f1 and f2 and the amplitude of excitation waveforms. in addition, the user can choose to put the system in continuous operation by setting the measurement cycle duration and the number of cycles that will be performed. from that moment, the system is able to work autonomously until the last cycle terminates. the opportunity to select the values for different parameters makes the system very flexible, as it can be reconfigured to fit a particular need. • calculating the value of the electronic module gain factor. • driving the electronic module operation in such a way that system can perform automatic z measurements at frequencies f1 and f2. • elaborating on the measurement data in order to obtain the values of model parameters cdl, rct, and rm by implementing equation (1), equation (2), and equation (3), respectively. • showing real-time data on the z and model parameters. data is reported by using either graphical tools or numerical indicators. therefore, the user can observe their variations at any moment during the entire measurement session. • evaluating cdl, rct, and rm trends in order to detect bacterial growth in an automatic way. in fact, at the beginning of the test, the user must set a threshold for the variation of each model parameter with respect to a reference value. when variations are relevant so as to exceed the thresholds for all parameters at the same time, an alarm is generated to inform the user that system has detected bacterial growth. furthermore, once the alarm occurs, the vi creates a notification email automatically and sends it to a previously defined address by means of an smtp server (only in the present vi version). • saving all data in *.txt files at the end of every cycle. such files are contained in a specific folder on the laptop, which is shared on the cloud (only in the present vi version). in this way, they are available for elaboration and visualisation just after they are saved, not only on the laptop, but also from any permitted device with an internet connection (including other pcs, smartphones, and tablets). in addition, in this version, the laptop is accessible for remote monitoring and control at any time in the measurement session from external connected devices that have permission to access it. therefore, even a user who is not in the laboratory can observe parameter trends in real time and be informed about bacterial growth events. 2.4. connectivity compared to the previous versions illustrated in [25]-[26] in which connectivity was not an addressed issue, the solutions implemented by the measuring system proposed in this paper promote information exchange between the actors that play a role in the analytical process, as presented in figure 4. this exchange is beneficial. on the one hand, they allow collaboration between the system and the wasplab®, or an integration of the former into the latter, as illustrated in [24]. in fact, the wasplab® can give a notification that one or more petri dishes have been positioned inside an incubator for analysis. then, the measuring system monitors the bacterial growth in those plates. finally, the system alerts the wasplab® about growth detection, and positive petri dishes are analysed through image acquisition, or they are simply removed from the incubator for further examination. such communication between them helps the development of a structure that is typical of the industry 4.0 model and its application to the microbiology field. on the other hand, data access that is granted to external connected devices permits selected personnel (e.g. engineers and microbiologists) to have information about bacterial growth at their disposal at any time and to be alerted once growth is detected, regardless of their location when the analysis is taking place. in addition, they will know if a problem is occurring, and they can exert remote control on system operation; for instance, they may start or stop data acquisition according to a particular need. 3. the performed tests we tested the impedance measurement performance of the electronic module. we found an accuracy of 0.8 % by comparing its response with the values measured by a reference instrument (i.e. the hp4194a impedance analyser) for the same device under test [26]. this is comparable with the value provided by the ad5933 datasheet [28]. in addition, the repeated measurements on fixed components permitted the estimation of a standard deviation of |z| in the order of 10 mω. in this paper, we focus on system capability in bacterial growth detection, which was evaluated through different tests. 3.1. system preparation for every test, three instrumented petri dishes were filled with a tryptone soy agar (tsa) culture medium. we inoculated two plates with a solution containing equivalent values of an initial concentration c0 of s. aureus atcc 6538. indeed, we took into account different orders of magnitude for c0 in different tests i.e. 4.5.108 cfu/ml, 4.5.107 cfu/ml, and 4.5.106 cfu/ml. inoculation took place in a biological safety cabinet at ambient temperature. we did not inoculate the third plate as a control. wasplab® system remote devices information on bacterial growth, control data data control, information data figure 4. communication between connected systems. acta imeko | www.imeko.org june 2019| volume 8 | number 2 | 39 we employed three electronic modules, one for each petri dish. in this way, we were able to evaluate the system operation when gathering data about more samples at the same time. through the vi user interface, we set the amplitude of the provided excitation waveforms to 1 vpp, while f1 and f2 were set as equal to 50 hz and 150 hz, respectively. then, we found the internal gain factor of each module by connecting a 100 ω commercial resistor to its terminals and evaluating the corresponding impedance output. finally, the vi was programmed for automatic bacterial growth detection based on the obtained values of cdl, rct, and rm related to each petri dish. in particular, the working cycle duration was set to two minutes, while the number of cycles was defined so as to drive the system to work autonomously for 24 hours. 3.2. preliminary tests firstly, preliminary tests were performed with an ad hoc laboratory setup in order to evaluate system operation (specifically in terms of autonomous growth monitoring over a day) under given conditions recreating the wasplab® environment. figure 5 shows setup block scheme. once the steps described in section 3.1 were completed, we placed all petri dishes in a fd 23 thermostatic chamber (binder gmbh). this chamber was exploited to simulate an incubator guaranteeing the environmental conditions (in terms of temperature and air ventilation) necessary for bacterial growth. it was turned on one hour before starting every test in order to achieve uniform conditions inside it. in addition, its internal temperature was set to 35 °c. afterwards, we connected each petri dish to a single electronic module. finally, we started automatic data acquisition. 3.3. tests with the wasplab® then, we conducted a series of tests in copan italia s.p.a., aiming to monitor bacterial growth while the instrumented petri dishes were contained in one of the wasplab® incubators. figure 6 shows the measuring system together with the wasplab® during one of these tests. after the system was prepared (see section 3.1), we put the petri dishes in the incubator, as highlighted in figure 7. as evidenced therein, there is a rack that allows for easy positioning of the petri dishes, and it prevents them from moving during the test. the incubator had been already working when the petri dishes entered, assuring uniform internal conditions. in particular, its control system maintained its internal temperature at 35 ± 2 °c despite the occurrence of disturbances e.g. when opening the incubator at the beginning of the test to put the plates inside. again, we connected the stacked petri dishes to the electronic modules, and data acquisition began. binder fd 23 chamber @ 35 °c laptop petri dish 1 electronic module 1 petri dish 3 petri dish 2 electronic module 2 electronic module 3 figure 5. scheme of the setup realised for preliminary growth tests. image acquisition station incubator stackers measuring system user interface figure 6. measuring system with the wasplab® platform during one of the tests conducted in copan italia s.p.a. figure 7. petri dishes stacked in one of the wasplab® incubators. acta imeko | www.imeko.org june 2019| volume 8 | number 2 | 40 4. results 4.1. preliminary tests figure 8 shows the petri dishes that were analysed during one of the preliminary tests (in this case, c0 = 4.5.108 cfu/ml). the inoculated plates are identified by the ‘+’ label. the picture was taken at the end of the test, when the inoculated plates with bacterial growth can be easily distinguished from the noninoculated one (in the centre) according to a visual inspection of the plate. in addition, the bacterial growth produced a colour (b) (c) transient detection time transient detection time (a) transient detection time figure 10. time trend of resistance rm normalised with respect to its value rm,0 at time = 0 h, resulting from a test in which (a) c0 = 4.5.108 cfu/ml; (b) c0 = 4.5.107 cfu/ml; (c) c0 = 4.5.106 cfu/ml. (b) transient detection time (a) transient detection time figure 9. time trend of electrode/medium interface parameters, resulting from one of the tests in which c0 = 4.5.108 cfu/ml. values are normalised with respect to those found at time = 0 h: (a) cdl/cdl,0; (b) rct/rct,0. inoculated not inoculated inoculated figure 8. petri dishes at the end of a preliminary test in which c0 = 4.5.108 cfu/ml. acta imeko | www.imeko.org june 2019| volume 8 | number 2 | 41 change in tsa culture medium, in correspondence of the inoculated surface, due to microbial presence. this discrimination between ‘growth’ and ‘no growth’ conditions, according to inoculated/not inoculated plate preparation, was sufficient for the purpose of this article’s specific activity. figure 9 reports the time trend that characterises ing cdl (figure 9a) and rct (figure 9b) for a petri dish inoculated with 4.5.108 cfu/ml and a non-inoculated plate. in both cases, the values are normalised with respect to those found at time = 0 h (i.e., cdl,0 and rct,0). in this way, discrepancies in the absolute values are eliminated, allowing for better graph visibility. figure 9 shows that all curves present an initial transient in which cdl increases and rct decreases rapidly. this is caused by the culture medium’s adaptation to a change in external conditions, when petri dish passes from being inoculated at ambient temperatures to being put in the thermostatic chamber at 35 °c. then, the curves referring to the non-inoculated plate are characterised only by irrelevant variations after the initial transient, which are mostly due to medium partial drying. in fact, we always noticed water droplets on plate covers at the end of each test, because the plastic bag containing the plate does not allow air exchange with the outside. on the other hand, the curves related to the inoculated petri dish show the typical phases of a bacterial growth curve [6]: • a lag phase, in which cdl and rct are almost constant as the bacteria are adapting themselves to the surrounding environmental conditions; • a log phase, in which cdl increases and rct decreases, since the bacteria multiply and reach a number that is large enough to be detected. this is in agreement with what is reported in the literature, since one of the by-products of bacterial metabolism is an increase of ion concentration in the medium, which modifies the electrochemical properties of the electrode/medium interface [6]; • a stationary phase (barely visible in figure 9), characterised by a lack of variations in cdl or rct, because of the equilibrium between live and dead bacteria present in the culture medium; and • a death phase, in which cdl decreases and rct increases, because a growing number of bacteria die, since there is no more medium available for all. the most interesting tract of these curves is the one comprising the lag and log phases, as the evident change in the curve slope observed between them identifies when the system is able to detect bacterial growth. figure 9 highlights that the detection time is equal to four hours given the considered initial concentration. figure 10 shows the rm time behaviour, referring to the petri dishes inoculated with 4.5.108 cfu/ml (figure 10a), 4.5.107 cfu/ml (figure 10b), and 4.5.106 cfu/ml (figure 10c), together with the curves related to the non-inoculated plates. as in figure 9 for cdl and rct, rm is normalised with respect to its value rm,0, assumed at time = 0 h. the reported curves have an initial transient caused by the medium settling to the (b) (c) (a) figure 11. vi user interface reporting rm real-time information during a test: (a) a global view for monitoring all parameters; (b) a case related to an inoculated petri dish; and (c) a case related to a non-inoculated petri dish. acta imeko | www.imeko.org june 2019| volume 8 | number 2 | 42 environmental conditions. afterwards, those related to the inoculated plates present a decreasing log phase until the end of the test (in fact, rm has slower dynamics than cdl and rct). furthermore, the behaviour of rm supports what is stated in the literature: ion release through bacterial metabolism contributes to an increase of medium conductance [6]. on the contrary, the curves referring to the non-inoculated petri dishes maintain a constant trend (it may even be increasing in the case that the medium is drying as the conductance falls). in addition, figure 10 evidences that the log phase begins later; therefore, the system detection time augments when c0 decreases, as expected. the results of the preliminary tests demonstrate that the measuring system is able to automatically detect bacterial growth in petri dishes and discriminate between plates inoculated with different pathogen concentrations. in addition, it can monitor growth continuously over time, during its various phases. 4.2. tests with the wasplab® this section especially focuses on system interaction with the user and other devices in order to better highlight its operation in an industry 4.0 framework. figure 11 shows the vi user interface while the system is working during a test. figure 11a provides a global view of the interface, allowing for the monitoring of all parameters. figure 11b and figure 11c focus on real-time information concerning rm related to an inoculated and a non-inoculated petri dish, respectively. in particular, besides the growth curve reported in the graph, the ‘last rm’ indicator refers to the last rm value obtained when the pictures were taken, while ‘transitorio rm’ is the reference that is considered for comparison with the last value in order to recognise whether bacterial growth has occurred. furthermore, in figure 11b, ‘crescita rm’ led (indicated by an arrow) is on, which means that the system generated an alarm due to its detection of bacterial growth in the analysed petri dish. as a proof, the displayed curve is similar to those shown in figure 10 for the inoculated plates. on the contrary, at the same time, led in figure 11c is off, and the reported curve has an increasing trend. this proves that the system successfully indicated that the examined plate had not been inoculated. table 1 reports some data extracted from a test in which c0 = 4.5.107 cfu/ml. the first column on the left shows the time in hours and minutes from the test’s commencement. the second, fourth, and sixth columns contain the cdl, rct, and rm values, respectively, related to an inoculated petri dish. finally, the third, fifth, and seventh columns include the parameter percentage variations with respect to their corresponding values at 1 h i.e. when it is reasonable to consider the initial transient as terminated. in this way, such variation is calculated starting from the lag phase (reference). for space reasons, table 1 shows only the data obtained during the sixth hour of the examined test i.e. the interval including the detection time. as explained in section 2.3, the system exerts its monitoring function by comparing the parameter variations with proper thresholds. based on statistical table 1. data on the model parameters for an inoculated petri dish obtained during the sixth hour of a test in which c0 = 4.5.107 cfu/ml. time cdl (f) % var, cdl rct () % var, rct rm () % var, rm 5 h 00 min 195.20 2.04 22.23 -1.90 74.70 -1.75 5 h 02 min 195.98 2.44 22.23 -1.91 74.72 -1.73 5 h 04 min 196.04 2.47 22.22 -1.94 74.71 -1.74 5 h 06 min 195.85 2.38 22.21 -2.00 74.70 -1.76 5 h 08 min 196.08 2.50 22.19 -2.07 74.69 -1.76 5 h 10 min 196.11 2.51 22.18 -2.11 74.68 -1.78 5 h 12 min 196.32 2.62 22.21 -1.98 74.68 -1.78 5 h 14 min 196.17 2.54 22.20 -2.06 74.66 -1.81 5 h 16 min 196.32 2.62 22.20 -2.04 74.65 -1.82 5 h 18 min 196.65 2.80 22.18 -2.14 74.65 -1.82 5 h 20 min 196.32 2.63 22.11 -2.42 74.61 -1.88 5 h 22 min 196.95 2.96 22.11 -2.45 74.61 -1.88 5 h 24 min 197.51 3.25 22.08 -2.58 74.61 -1.88 5 h 26 min 197.42 3.20 22.06 -2.67 74.59 -1.90 5 h 28 min 197.12 3.04 22.04 -2.76 74.56 -1.94 5 h 30 min 197.23 3.10 22.00 -2.91 74.55 -1.95 5 h 32 min 197.84 3.42 22.02 -2.85 74.56 -1.94 5 h 34 min 197.65 3.32 21.96 -3.11 74.53 -1.98 5 h 36 min 197.77 3.38 21.96 -3.10 74.52 -1.99 5 h 38 min 198.58 3.81 21.98 -3.01 74.54 -1.96 5 h 40 min 198.77 3.91 21.95 -3.15 74.53 -1.98 5 h 42 min 198.38 3.70 21.90 -3.38 74.49 -2.03 5 h 44 min 198.93 3.99 21.89 -3.41 74.50 -2.02 5 h 46 min 199.40 4.23 21.85 -3.58 74.49 -2.03 5 h 48 min 199.59 4.33 21.80 -3.82 74.47 -2.05 5 h 50 min 200.28 4.70 21.78 -3.91 74.47 -2.05 5 h 52 min 200.62 4.87 21.75 -4.02 74.47 -2.06 5 h 54 min 200.67 4.90 21.69 -4.28 74.46 -2.06 5 h 56 min 200.59 4.85 21.66 -4.42 74.49 -2.03 5 h 58 min 200.67 4.90 21.64 -4.50 74.53 -1.97 6 h 00 min 201.57 5.37 21.65 -4.48 74.60 -1.89 acta imeko | www.imeko.org june 2019| volume 8 | number 2 | 43 considerations on the obtained data, we set these thresholds to 2.50 %, -2.00 %, and -2.00 % for cdl, rct, and rm, respectively. such values represent a good trade-off. in fact, increasing them would postpone the detection time too much, while decreasing them would increase the probability of false positives, since the system would generate an alarm even if the bacterial growth did not occur. considering the data of table 1 in depth, it may be observed that cdl is the first parameter that exceeds its threshold, after 5 hours and 18 minutes. we activated a control, ensuring that the threshold was consecutively overcome for 10 minutes in a continuative way. this prevents the signal noise from leading to wrong conclusions. then, rct did the same after 5 hours and 24 minutes. finally, the rm threshold was exceeded after 5 hours and 52 minutes. at this point, the system generated the alarm, as it detected bacterial growth. waiting for all parameters to overcome their own thresholds increases the reliability of the results. in fact, cdl and rct provide a quicker response than rm, but the latter is more stable over time. for this reason, the system records the events regarding cdl and rct, but it alerts the user about bacterial growth detection only once the data on rm reports it. figure 12 shows the email message that is sent automatically by the system (from a gmail account created ad hoc) to a specified address (in this case, the corresponding author’s), when it detects bacterial growth in a petri dish. such message is simply a notification, which invites the recipient to check real-time data (which is shared on the cloud) and to extract the petri dish of interest from the wasplab® incubator for further analysis. in general, these achievements show how the developed measuring system can continuously monitor bacterial growth. furthermore, the connectivity between the system, the wasplab®, and other devices that have a common internet connection (e.g., pcs, tablets, smartphones) allows a user to be informed about the analyses that are in progress and to act on the system at any time and from remote locations. in fact, all data about the monitored parameters and information on the system operating conditions is shared on the cloud, and authorised devices can access the system remotely. 5. conclusion this article has presented a measuring system that allows for the automatic detection of bacterial growth in petri dishes, while they are inside the incubators of the wasplab® platform (copan italia s.p.a.). system operation relies on real-time monitoring of three parameters derived from the impedance measurement related to analysed petri dishes. furthermore, the system can be easily reconfigured according to user requirements. finally, it presents features that enable its connectivity with the wasplab® and other devices, creating a structure that is compliant with the industry 4.0 model and applicable to the microbiology field. we undertook different tests aiming to evaluate system detection abilities. in particular, we explained how the system is prepared for the tests. we provided a description of the experimental setups and test protocols used. we then presented the obtained results. this work’s achievements open the possibility for a collaboration between the proposed system and the wasplab® or an integration of the former with the latter, implementing the model of a smart factory. this could lead to many potential benefits. in fact, the generation of quantitative data about bacterial growth from continuous impedance measurements and precocious growth detection in incubated petri dishes, besides and independent from image acquisition by the wasplab®, could further enhance platform analysis performances and reduce time wastage. then, the huge amount of generated data can be exchanged between the system and other devices, permitting more effective supervision by users who are in remote locations, too. acknowledgement the authors would like to thank roberto paroni and giorgio triva from copan italia s.p.a. for their support during the different phases of the research activity. references [1] world health organization, 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[28] ad5933 datasheet [online] available: http://www.analog.com/media/en/technicaldocumentation/data-sheets/ad5933.pdf http://products.copangroup.com/index.php/products/lab-automation/wasplab http://products.copangroup.com/index.php/products/lab-automation/wasplab http://www.analog.com/media/en/technical-documentation/data-sheets/ad5933.pdf http://www.analog.com/media/en/technical-documentation/data-sheets/ad5933.pdf an iot framework for the assessment of indoor conditions and estimation of occupancy rates: results from a real case study acta imeko issn: 2221-870x june 2019, volume 8, number 2, 70 79 acta imeko | www.imeko.org june 2019 | volume 8 | number 2 | 70 an iot framework for the assessment of indoor conditions and estimation of occupancy rates: results from a real case study stefano rinaldi1, alessandra flammini1, lavinia c. tagliabue2, angelo l. c. ciribini2 1 department of information engineering university of brescia, via branze 38, 25123, brescia, italy 2 department of civil, environmental, architectural engineering and mathematics university of brescia, via branze 43, 25123, brescia, italy section: research paper keywords: smart city; cognitive building; internet of things; indoor comfort; energy saving; indoor air quality; sensor network; occupancy rate model citation: stefano rinaldi, alessandra flammini, lavinia c. tagliabue, angelo l. c. ciribini, an iot framework for the assessment of indoor conditions and estimation of occupancy rates: results from a real case study, acta imeko, vol. 8, no. 2, article 10, june 2019, identifier: imeko-acta-08 (2019)-02-10 section editor: emiliano sisinni, university of brescia, italy received july 30, 2018; in final form june 06, 2019; published june 2019 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was partially supported by university of brescia as part of the research activities of the laboratory ‘energy laboratory as university expo – elux and by research grant sin00665 ‘virtual-egateway’, funded by the italian ministry of university, research, and education (miur). corresponding author: stefano rinaldi, e-mail: stefano.rinaldi@unibs.it 1. introduction an indoor space is a place in which lives are spent and activities developed. a building can be considered as a passive container of activities where requirements of safety and security could be improved compared with external or partially covered spaces. the most important criteria related to comfort should be considered in building spaces to support health, wellbeing, and productivity. in addition, the building is actively involved in the delivery of services, such as air conditioning, ventilation, illumination, connectivity, communication, video support, and exchange of information in the connected building towards fruitful interaction of devices and functions. the optimisation of flows (i.e. people, data, energy, materials) leads to a better and easier way of developing our lives, and the evolutionary level is not the smart building in the smart city, but the cognitive building [1] in the cognitive city [2]. the cognitive building approach is an evolution of traditional home automation: a cognitive building can understand a user’s needs directly based on their habits. this new paradigm implements interactions between the building and the user, and it is based on a behavioural design [3] that can lead to the improved use of spaces and to a bi-directional relationship in an adaptive environment that is able to support users’ choices and preferences according to wider objectives of wellbeing and energy saving. the first step towards such a scenario is a building that is able to identify and estimate its state autonomously and to share such information with other buildings. abstract in recent years, energy-savings policies have affected many aspects of everyday life. considering a typical building, the heating, ventilation, and air conditioning (hvac) system is the most energy-consuming system. this consideration is especially true for large public-access buildings, such as schools, and public administrations. in these cases, the energy saving of buildings depends on the capability to optimise the behaviour of the hvac. typically, the hvac control system is based on static models of the building, which consider the average occupancy rate of each of the rooms. on the contrary, in this research work, a cognitive system based on an occupancy rate model that is able to take into consideration user habits and indoor air quality (iaq) provided by iot sensors is considered for the control of hvac systems. this approach has been applied to the elux lab building of the university of brescia, italy. data provided by iot iaq sensors (temperature, relative humidity, co2) is used to refine the results of the occupancy rates for the models of the rooms of this building. the experimental results show, as in 22.15 % of the samples, the co2 concentration exceeded the 1,000 ppm threshold of the perception of fresh air and good conditions. mailto:stefano.rinaldi@unibs.it acta imeko | www.imeko.org june 2019 | volume 8 | number 2 | 71 in the past, the problems related to indoor air quality (iaq) have been labelled as sick building syndrome, affecting 30 % of new buildings, as the world health organization alerted in 1984. the problems related to ventilation and materials were overcome in the 1990s; nevertheless, many existing buildings have outdated heating, ventilation, and air conditioning (hvac) systems, and the use and intensity of occupancy has challenged the systems and equipment. indoor environmental quality (ieq) is affected by iaq as well as other physical and psychological aspects of indoor conditions (e.g. lighting, visual quality, acoustics, and thermal comfort) [4], [5]. in addition, iaq has a strong impact on health and concentration in educational institutions. research has demonstrated that iaq significantly impacts the learning performance of the students. the parameter of carbon dioxide (co2) is used to assess the iaq [6] and is easily calculated [7] and measured [8]. specific dedicated sensors network can be used to monitor and extract information by the data collection [9]. this research aims to investigate the possibility of integrating iaq data generated by iot sensors installed into the learning spaces of an educational building adopted as a case study to estimate the number of people and occupancy rate, which is crucial for modulating and customising the operational range of the hvac systems. the implementation of iot architecture to connect the sensors to the building management system has been described in previous research work [10], while the current research aims to improve the evaluation of the occupancy rate of traditional probabilistic models adopted for energy simulations. the proposed methodology has been applied to the educational laboratories and lecture classrooms of the ‘modulo didattico’ building located at the university of brescia’s smart campus, headquarters of elux lab. in the preliminary phase of analysis of the case study building and considering the behavioural factor as a crucial element to develop comfort models and diagnostics, interviews were conducted and discomfort was often been noted during the whole year (with a changing level in the different seasons nonetheless widely recurrent). the learning spaces are frequently too warm and humid, furthermore intensive occupancy causes an increase of co2 concentration during the day and a perception of the air as stuffy and malodorous. for that reason, users in spring and summer open windows and doors to increase ventilation and promote additional air changes coupled with mechanical ventilation. in order to assess the degree of user satisfaction, considering, as reported by ashrae [11], that ‘an acceptable air quality is one for which a substantial majority of people (less than 80 %) do not express dissatisfaction’ a further aim is to promote compliance checking of the indoor conditions related to users’ request for iaq for improved learning performance. the critical indoor conditions reported for educational buildings have also increased interest in the development of the interactive bi-directional app [1] where formal and informal feedback is collected in order to understand the level of comfort (i.e. thermal, visual, acoustic, olfactory), and customised comfort conditions could be provided according to energy-saving measurements. 2. methodology the iot framework hvacs in large buildings, such as schools, universities, and public administrations, are equipped with sensors for monitoring the status of rooms and thus for controlling the quality of the comfort of the user. generally, the data generated by these sensors is not available from third-party applications. the iot paradigm allows us to overcome this situation thanks to the proper definition of a data model that can be exchanged over an ict infrastructure. the ict infrastructure for supporting iot sensors is shown in figure 1. such an infrastructure is based on five layers: the device; the device abstraction; the data; and the cognitive and the visualisation layers. the device layer includes sensors and actuators placed on the systems in the building. the device layer includes also the feedback provided by the user of the building, explicit or implicit, on the quality of comfort. each of the devices is characterised by a data model, which is used to exchange information with the upper layers using a proper communication protocol. the system is protocol agnostic i.e. it supports any communication protocol through the device abstraction layer, which provides the capability to support different devices to the upper layer. among the communication protocols supported, message queue telemetry transport (mqtt), webservice rest, advanced message queueing protocol (amqp), and modbus should be mentioned. the data layer includes the information system used to store real-time data from sensors (device data db); the model and information regarding the building and its components as well as the position of iot sensors (bim db); and the information about the installed iot devices (including metrological characteristics) (asset mgt db). the information stored in bim db and in asset mgt db are fundamental for extracting the correct information from the device data db and for sharing such information between different buildings. the data layer has been structured using different databases because of the different characteristics of data that is to be stored; for example, real-time data requires a database that is able to store a huge amount of data from hundreds of sensors in parallel, while information about the building is quite static. the data stored in the device data db can be used by the cognitive layer to infer additional information from the raw data coming from the sensors. for example, the number of people in a room and the flow of people in a building can be inferred from co2 concentration. information inferred by the raw data is uploaded in device data db as a virtual sensor. the information stored in the data layer can be processed by the cognitive layer and visualised by the visualisation layer. the visualisation layer takes the benefit of grafana, an open framework for data analytics and monitoring. the cognitive layer is able to analyse the data and to operate and control the systems of the building to close the figure 1. the reference ict architecture used for the integration of iot devices and the estimation of building parameters. acta imeko | www.imeko.org june 2019 | volume 8 | number 2 | 72 control loop (in the case, control feature is enabled). the interactions among the upper layers (device abstraction layer, data layer, visualisation layer, and cognitive layer) are provided by webservice rest. thanks to the modular and service-oriented architecture of the informative system, each of the components could be located on different physical or virtual machines, promoting the scalability of this approach. indoor air quality indices in the pilot building, the installation of the co2, temperature (ti), and relative humidity (rh) indoor sensors allowed for the possibility to check iaq conditions. the standard uni 10339 [12] establishes air flow rates; the crowding index; indoor and outdoor temperatures; indoor and outdoor rh; and air velocity in the conventional occupied volume. the iaq significantly deteriorates as the temperature and rh increase [13]. the surveys for air quality checks could be carried out by traditional methods (questionnaires) or by checking the quantitative values of the parameters, including ventilation air flow rate, intake air temperature, quantity of co2, air temperature, and rh, as well as a check on the pattern of flow distribution in the environment. in table 1, the co2 concentration has been correlated to indoor conditions and iaq. ‘perfect’ fresh air is not free of co2. the co2 concentration of fresh air is low, around 350-400 ppm; acceptable conditions have a co2 concentration of 600 ppm, while the upper limit is 1,000 ppm. a concentration of 1,500 ppm is considered as unacceptable since air becomes stuffy from this level. problems for users are reported at 2,000 ppm, and 10,000 ppm is the concentration at which bad air conditions occur and breath and headaches increase. nausea issues could be detected, with repercussions on intellectual performance and attention. the hvac parameters and values provided by international normative for university classrooms are shown in table 2 and table 3. 3. case study: modulo didattico building building configuration and use the adopted case study is an educational building located in the smart campus of the university of brescia used for lectures and educational activities, included in the didactic programme and independently by the students. the building can be classified in the national regulation framework as e.7 – an educational facility [16]. the spaces used for the indoor activities are distributed in three levels: the underground floor, where computer labs (mlab1, mlab2) are equipped with pc. the ground floor is where two lecture rooms (mta, mtb) are used every day for the didactic programme. they are connected to the exterior through a south-east oriented totally glazed atrium, which is used for the students’ shared learning activities. the first floor is where the ‘aula magna’ (m1) is located and used both for daily educational activities and for graduation day ceremonies. the atrium has also a first floor for independent study and social activities, and it is equipped with desks and electrical outlets. moreover, on the underground floor, the interdepartmental lab elux – energy laboratory as university expo – is installed and used as hot spot of research and community activities. the pilot building that is used as a case study shows the photovoltaic system installed on the rooftop during the previous experimental project smart campus as urban open labs s.c.u.o.l.a (figure 2). the total useful surface is 1,786 m2. in table 4, the spaces, that are officially designated for student’s use, are listed with the names, the actual number of users hosted in the educational spaces, the area of the spaces, the people density, and the capacity of the air handling units (ahus) to provide air to the spaces. table 1. correlation between co2 concentration and iaq conditions in the indoor space. co2 conc. in ppm iaq description and detected issue iaq conditions 350 400 fresh air perfect conditions < 600 almost fresh air acceptable conditions < 1000 upper limit of fresh air limit of co2 concentration < 1500 stuffy and not fresh air not acceptable < 2000 weak people can faint and cough bad air condition < 10000 increase of breath rates, respiratory problems, headaches, nausea very bad air condition table 2. parameters for educational building iaq conditions. parameter unit value minimum air flow rate vmin m3 / (h · p) 25.2 crowding index p / m2 0.6 correction factor for ventilation 0.51 average daily presence factor 8/24 internal gains sensitble heat w / m2 4 volumetric perc. of co2 conc. % 0.1 acceptable noise levels dba 30-40 system noise levels dba 25-30 / 30-35 table 3. parameters for thermo-hygrometric wellbeing during the winter and summer periods. parameter unit winter summer dry bulb external temp. tbse °c -7 32 external rh ure % 60 48 dry bulb indoor temperature tbsa °c 20 26 indoor relative humidity rha % 35 < rha < 45 50 < rha < 60 metabolic activity mr w / m2 ≥ 70 ≤ 116 thermal res. of clothing icl m2°c / w ≥ 0.14 ≤ 0.09 air velocity v m / s 0.05 < v < 0.15 0.05 < v < 0.15 figure 2. university of brescia smart campus. the pilot building is located on the left-hand side. acta imeko | www.imeko.org june 2019 | volume 8 | number 2 | 73 the atrium has not been designed for the allocation of desks; nevertheless, the two levels are intensively occupied by the users. the list of spaces follows the order from the underground floor to the first floor. digital twin a bim model of the building has been realised for multiple purposes. as first, the model was realised (figure 3) by means of terrestrial laser scanner technology in order to acquire the geometrical information of the spaces. the model has been compared to the data available in the technical office of the university through paper-based traditional documentation. additionally, the model has been used to calculate energy performances and compare refurbishment scenarios during the s.c.u.o.l.a. project – smart campus as urban open lab (national funding 2014-2016). the same model has been also used to locate the installed sensors into the building and propose a methodology to visualise and communicate the data promoting indoor data-mapping to track comfort conditions [17]. a dynamic structure for the communication of data to the bim model by means of the ifc standard has been proposed [18], and the test and verification phase is still in progress. the bim model can be implemented to translate the data gathered by the sensors into useful information for building operation and management. iot and bim-based building management enable the cognitive building [2], [19] vision in which the asset would learn from the analysed data and would setup the indoor conditions in the different spaces according to optimised schemes and protocols for energy saving [20] and improved user experience [1]. the case study building dates back to the 1980s, and the documentation lacks details about performance, the actual situation of the systems, and recent distribution of spaces. the envelope performance information was not present in the main documentation, and assumptions have been made to define the envelope based on visual inspections (figure 4). ahus are installed (n. 8 units) to provide fresh air to the indoor spaces, and fan coils are the emission terminals installed (in the bathrooms, and some additional spaces are equipped with radiators). systems and issues the air conditioning and ventilation system in the underground floor was refurbished in 2012, and two ahus have been positioned in additional service spaces in the northern part of the pilot building (figure 5). the other two floors have autonomous ahus, n. 4 in the ground floor, n. 1 for the firstfloor ‘aula magna’ and n. 1 for the atrium with n. 7 fan coils as terminals. every unit has a thermostat in the extract air and electric water shut-off valves. the hydraulic power supply is derived from the building's general network. a main list of interventions for the hvac system is proposed to improve the actual iaq situation and comfort conditions in the spaces: 1) verifying the operational phase of the systems related to the users’ profile; 2) verifying the control system setting aimed at optimising the comfort conditions; 3) verifying the sensing system to monitor and maintain an adequate iaq level; and 4) verifying the possible integration of the hvac system with natural ventilation. table 4. functional spaces of the ‘modulo didattico’ building. name places users dimension people density ahu n. n. in m2 in p/m2 in m3/h mlab 1 56 76 151.79 0.5 5000 mlab2 82 104 207.94 0.5 2000 mta 168 89 178.27 0.5 2000 mtb 168 89 177.48 0.5 2000 atrium 0 56 180.78 0.3 5500 0 56 144.61 0.4 m1 169 262 337.46 0.8 5500 figure 3. bim model of the ‘modulo didattico’ building at the unibs smart campus. the three floors are shown together with the rooftop level. figure 4. visual inspection and survey on the building to gather information about envelope performance. figure 5. ahus for mlab1 and mlab2 (functional scheme) as an example of the system configuration for air exchange and iaq. acta imeko | www.imeko.org june 2019 | volume 8 | number 2 | 74 sensing system the abovementioned issues could be addressed by a dynamic and responsive building that monitors user presence; collects data about temperature and humidity levels; correlates the co2 concentration with the users’ attendance at the lectures; and activates an integrated natural and mechanical ventilation to reduce the energy and environmental impacts of the building, thereby increasing the comfort level [21]. the pilot building [22] has thus been equipped with sensors to monitor the parameters that are useful for implementing the abovementioned concepts. the monitoring system installed in the building has n. 96 nodes of measurement controlling the main systems of the building, such as lighting, ventilation, heating, and cooling systems (figure 6). the systems are monitored as listed below: • lighting system control through presence: n. 24; and daylighting control through illuminance: n. 4; • ventilation system control sensing the inlet air temperature: n. 15; the air temperature n. 8; the rh n. 9 (in each zone, and the ‘aula magna’ has two sensors of humidity); and • control of heating and cooling systems with sensors of presence, co2 concentration, ventilation, and a conditioning damper n. 18 for each service. the sensors have been installed and adopted to verify previous occupancy models, to define discomfort conditions reported by the users, and to enable strategies to increase the building ieq and user satisfaction. iaq in the educational spaces the specific problem with iaq has demonstrated a strong correlation with the comfort of the users. and the parameter of the co2 concentration is widely accepted as a way of assessing user presence and occupancy in the indoor spaces [21]. in the first phase of the analysis of the model of co2 concentration in different educational spaces, the calculation model has been developed by considering a standard level of occupancy and then by comparing the results of the model with the actual one. in addition, the real schedule of the educational programme in the building for a didactic week has been applied to understand the intensity of use of the spaces. nevertheless, the real attendance at the lectures has not been measured, and thus, only a theoretical result is achievable. the use of the building and the spaces is actually intense, and the average number of occupations of the spaces during the week is 7.3 h per day, with a peak on tuesday (8.5 h) and wednesday (7.6 h). the atrium is considered constantly occupied, and the most occupied spaces are the pc laboratory and the aula magna (mlab1 and m1), which have an average daily occupancy hours of 8.1 and 7.2 respectively (figure 7). theoretical model the iaq of the building spaces has been studied assuming deterministic and probabilistic occupancy levels. the building is intensively exploited for the lectures and laboratories, which are occupied by students studying for their courses with the tutors, performing their individual learning path, or working with study groups. the occupancy scenarios have been developed starting from a survey on the maximum availability of seats in the different spaces given at the beginning of the research; however, specific people monitoring has not been conducted. the probabilistic scenarios of occupancy have been generated on the maximum number of people, assuming a triangular probability distribution function, which is: min = 0,3 no,max; mode = 0.6 no,max; max = 1.0 no,max. the calculation model assumed the air changes and the following data: • air change vmin in the deterministic scenario is equal to 21.6 m3 / (h · p) as required by the national standard, while in the probabilistic scenario, it is assumed to be equal to 25.5 m3 / (h · p). • people density in the deterministic scenario is equal to 0.5 p / m2 as required by the national standard for educational buildings, while in the probabilistic scenario, it is assumed as described above, starting with the maximum value of possible users. since the typology of users and the activity of learning in the considered spaces, the co2 per occupant qp is assumed as 0.05 m3/h, and the co2 at the starting time (t=0) is assumed to be 350 ppm, with no co2 supplied by the inlet air from the ventilation system. c1 and clim is 1,000 ppm—the threshold for the acceptance of the co2 in the learning spaces to promote a healthy and productive indoor environment. the calculation model could estimate the iaq conditions for the different spaces, showing highly critical conditions in the non-refurbished spaces, such as mta and mtb, which reach 3,000 ppm on a standard wednesday in a learning week at the campus (figure 8). the aula magna has also critical conditions reaching 2,500 ppm, while the maximum values for the computer laboratories mlab1 and mlab2 are both around 1,500 ppm, which exceeds the 1,000 pm acceptable situation for an optimal iaq in learning spaces (figure 9). figure 6. examples of the installed sensors in the building spaces: sensor of presence and sensor of temperature and humidity. figure 7. daily occupied hours per day during the learning week at the ‘modulo didattico’ building in the university of brescia’s smart campus. acta imeko | www.imeko.org june 2019 | volume 8 | number 2 | 75 monitored results the co2 concentration in the indoor spaces for learning is the main parameter that is used in this research to assess the presence of people in the spaces, and a data mining procedure was applied to the data collected, allowing us to discover how the different rooms were occupied during the monitoring campaign (from july 2017 to may 2018). the data gathered did not have the same continuity and precision for each classroom, and we had to start with a data cleaning process that excluded some spaces due to a lack of reliability in the data or a serious lack of information. the atrium and mlab1 have been excluded from the following analyses because of the lack of significance of the amount of data monitored, while mtb was subjected to strong data cleaning so as to avoid data ghosts with no actual relevance. the other learning spaces, mlab2, mta, and m1, showed reliable data. the data collection and the necessary cleaning process enabled the analysis of a large amount of data about the learning spaces, structured as follows: • analysis of the cumulative distribution function (cdf) of the data in the different spaces to understand how many times the registered values are below or above the comfort level and iaq threshold; the analysis includes co2 concentration, indoor air temperature, and indoor rh; • analysis of the minimum, maximum, and average values of co2 concentration in the different occupied hours during the day in the analysed period; and • definition of the number of people in the learning spaces based on the registered co2 concentration. the adopted methodology allows us to evaluate the iaq and is related to yet more significant parameters, such as temperature and rh. moreover, the calculation permits us to monitor the actual number of people in the spaces by providing information for setting up, checking, and adapting the ventilation system and the window openings related to the use of the spaces (figure 10). ai applications could be implemented based on the selflearning of the system by targeting the data collection at the development of predictive control settings. 4. results and discussion the data collected has been analysed in order to verify the critical discomfort conditions that could be experienced by the users and to understand which parameters are clearly beyond the comfort range conditions and how frequently these conditions are experienced by the users. cumulative distribution function the data analysis has been extended to the indoor comfort parameters, and thus, a cumulative distribution of the following parameters has been plotted: • co2 concentration in ppm; • indoor temperature ti in °c; • rh in %. the cdf demonstrates the probability that the considered variable assumes a value that is lower than a comfort threshold or between a range of values defining the comfort range during the year. for the variables listed above, we set the first threshold as 600 ppm for the co2 concentration, which is an acceptable condition with almost fresh air perception. we set the second threshold at 1,000 ppm, which is the upper limit of fresh air and the limit for co2 concentration before an unacceptable iaq and user perception of stuffiness is reached [22], [23]. in mlab2, 60 % of the data showed an indoor co2 concentration above 600 ppm, and in about 20 % of cases, it is above 1,000 ppm. the mta lecture room has values above 600 ppm in 50 % of the records and above 1000 ppm in 10 %. 80 % of the data concerning m1 is between 600 and 1,000 ppm, never exceeding the second threshold. nevertheless, it is noteworthy that the monitoring period does not include the graduation days, and for that reason, the m1 (the aula magna used on those days) does not present such critical conditions as experienced and reported by the students in the university of brescia smart campus. the figure 9. concentration of co2 in the mlab2 during the learning week. figure 10. analytical methodology and workflow of the research. figure 8. concentration of co2 in the ‘modulo didattico’ building in the different classrooms on a standard wednesday of the learning week. acta imeko | www.imeko.org june 2019 | volume 8 | number 2 | 76 most critical conditions have been registered concerning the mtb lecture classroom, where the data reached 5,299 ppm as a maximum value and an average value of 2,226 ppm. 80 % of the co2 concentration values in mtb were above 1,000 ppm (figure 1). for indoor temperature, the two standard values for internal comfort in the winter (20 °c) and summer (26 °c) periods have been considered; however, in the learning spaces, a temperature of around 20 ± 2 °c is considered as preferable based on studies on learning ability and concentration (figure 12). the cdf for the indoor temperature in the learning spaces shows that 60 % of the indoor temperature values in mlab2 and mta were above 20 °c, and 5 % above 26 °c, with a similar distribution for the mta lecture room. 50 % of the data concerning m1 was above 20 °c and 10 % above 26 °c. 40 % of the values for mtb were above 20 °c, and less than 5 % were above 26 °c. the rh parameter has a similar distribution in the different spaces in the ‘modulo didattico’ building: the 60 % of the data shows a co2 concentration lower than 35 % (an indoor environment that is too dry) and in the 20-30 % of the data, the co2 concentration was higher than 45 % (too humid). these values refer to the winter period, while in summer period, a range between 50 % and 60 % is required. in the diagram in figure 13, the comfort bands are defined by the four values assumed as lower and upper thresholds in the winter and summer periods, as clearly defined in table 2. the distribution shows that a great part of the data referring to the learning spaces is located outside of the bands; however, a detailed in-depth analysis of the two periods should be performed, with a focus on hygro-thermal comfort. moreover, the rh reaches values that are higher than 60 % in 10 % of the cases of use of the learning space, with a significant and critical level of discomfort. it is noteworthy that this first analysis on big data is able to estimate the percentage of discomfort hours during the use of the learning space of the educational building; however, detail on the moment of the day when the conditions are most critical and strategies that should be applied to improve the iaq condition are crucial for understanding the daily trends of the comfort parameters. for that reason, an analysis of the hourly distribution of the co2 concentration is performed and discussed in section 4.2. hourly distribution of the data of co2 concentration an analysis of the co2 concentration data in the different classrooms has been conducted to outline when the values exceed the stated thresholds and how the data is distributed during the day. in figure 14, the computer lab (mlab2) and the lecture rooms (mta, mtb) along with the aula magna (m1) are analysed. maximum, minimum, and average values of co2 concentration obtained from the sensors are plotted. the trend of the distribution is mostly the same in the four classrooms, and at 10:00 am, a peak co2 concentration is registered, with an average value of around 1,000 ppm and with maximum values reaching 1,500-1,600 ppm in mlab2, mta, and m1. it is also noteworthy that 1,500 ppm is reported as stuffy and not considered fresh air, since it is an unacceptable level (table 1). during lunchtime (1:00 pm), a lower value is registered in each classroom on average, and restarting with the lectures in the afternoon, we can see at 4:00 pm a second peak point. the maximum values are 1,500 ppm from 9.00 am to 6:00 pm for mlab2 and mta and to 4:00 pm in m1. the most critical situation is reported in mtb. the average values are similar to the other classrooms; however, the maximum value monitored in mtb is 5,299 ppm at 11:00 am, and in the case that there is no additional ventilation and intense occupancy, after three hours of lectures, it is possible to detect such a high level of co2 concentration that is associated with very bad air conditions and problems with headaches; nausea; and reduced attention and concentration. figure 11. cdf of co2 concentration for the different spaces and curve calculated on the whole amount of data (tot). figure 12. cdf of the indoor temperature for the different spaces, calculated on the whole amount of data (tot). figure 13. cdf of the rh for the different spaces, calculated on the whole amount of data (tot). acta imeko | www.imeko.org june 2019 | volume 8 | number 2 | 77 definition of occupancy rate based on co2 concentration the use of sensors to record the co2 concentration supports and verifies the presence of people in the indoor spaces, where other monitoring systems are unavailable or mainly used to validate data from different sensors (e.g. people-counting solutions). in many cases, these devices present some problems in the recognition of user presence due to positioning and background noise issues. the accuracy of the recognition abilities of these sensors can be limited due to illuminance (i.e. in the case of optic sensors) and the cost can be very high (i.e. especially where there are multiple entrances to/exits from the classroom). the use of the co2 concentration parameter as a reference for occupancy has been demonstrated and verified in many studies [24]-[26], and it represents an effective method of evaluating the presence of people in indoor spaces. the co2 sensors could thus be adopted as informative iot devices and correlated to actuators in order to improve ventilation (i.e. natural ventilation through windows opening or mechanical ventilation by increasing fan power). information about user presence can also be used to estimate and customise indoor comfort conditions without the installation of additional detection systems. the importance of accuracy of data in the performance of relevant analyses is crucial so as to avoid the garbage-in-garbage-out effect. in the present research, the co2 concentration monitored in the classrooms has been used to calculate the presence of people in accordance with the methodology described in prior research [27]. the comfort parameters of temperature and rh, obtained from the sensors, have been correlated to the occupancy range estimated based on co2 data. a lecture room has been adopted as an example (i.e. mta) checking the two sample days in spring and autumn (figure 15 and figure 16). it is important to note that with a 46 % occupancy rate, the threshold of 1,000 ppm is exceeded, and with a 62 % occupancy rate, the level can reach 1,500 ppm. these results refer to mta, which has not been refurbished and uses a ventilation system that is outdated and is inadequate for such intensive use. nevertheless, the spaces are not designed to provide further natural ventilation through the windows, and if in mlab2 it is possible to open the below grade basement windows, in mta, mtb, and m1, the building does not provide opening surfaces for natural ventilation. this situation means that only with mechanical ventilation is it possible to respond (or try to respond) to the required level of air changes related to the students’ presence. 5. conclusion this article demonstrates the capability of iot architecture to exploit the use of sensor data for different application domains adopting the co2 concentration parameters in combination with temperature and rh as the main indicator for iaq using sensors to monitor the variation of the parameters in the indoor spaces figure 14. minimum, maximum, and average hourly values of co2 concentration (in ppm) in the learning spaces based on the monitored data. acta imeko | www.imeko.org june 2019 | volume 8 | number 2 | 78 of an educational cognitive experimental building at the smart campus of the university of brescia. the data gathered by the sensors is used to enhance the accuracy of estimation of the occupancy range in the learning spaces, at the same time providing information about environmental quality and thus potentially preventing discomfort and conditions that are not optimal for students’ attention and learning performance. co2 concentration is adopted internationally as a factor for verifying and calculating indoor presence and for use as a basis for improving comfort conditions for users. in a user-centred design and strategies, it is relevant to have reliable information about the presence of users that could be integrated by customised requests in future. it could also be possible to deliver a tailored service to the users in the indoor space, which is the target of an increasing number of advanced studies on energy saving and operation and management optimisation. acknowledgments the authors would like to mention and acknowledge the smart campus school project team, including eng. marco pasetti, eng. paolo bellagente, and eng. federico bonafini. special thanks go to prof. fulvio re cecconi for his valuable collaboration in developing the data analysis models. references [1] f.bittenbinder, c.liu, s.rinaldi, p.bellagente, a.l.c.ciribini, l.c.tagliabue, ‘bi-directional interactions between users and cognitive buildings by means of a smartphone app’, proc. of the ieee second international smart cities conference (isc2 2016), improving the citizens quality of life, 12-15 sept., 2016, trento, italy. [2] l.c.tagliabue, a.pasquinelli, g.m.di giuda, v.villa, a.l.c.ciribini, e.de angelis, ‘cognitive adaptive urban systems for the living built environment’, proc. of the 2nd annual international conference on urban planning and property development (uppd 2016), 10-11 oct., 2016, singapore. doi: 10.5176/0000-0000_uppd.43. [3] p.j.cash, c.gram hartlev, c.boysen durazo, behavioural design: a process for integrating behaviour change and design, design studies, 48 (2017) pp. 96-128. [4] m.c.lee, k.w.mui, l.t.wong, w.y.chan, e.w.m.lee, c.t.cheung, student learning performance and indoor environmental quality (ieq) in air-conditioned university teaching rooms, building and environment 49 (2012) pp. 238-244. [5] j.d.spengler, iaq handbook, new york: mcgraw-hill co., 2001, i sbn: 9780074455494. [6] a.szczurek, et al. ‘co2 and volatile organic compounds as indicators of iaq’, proc. of the 6th aivc conference, 5th tightvent conference, 3rd venticool conference, 23-24 sept., 2015, madrid. [7] d.s.dougan, l.damiano, co2-based demand control ventilation do risks outweigh potential rewards? ashrae journal, 46 (2004), pp 47-53. [8] n.mahyuddin, et al. a review of co2 measurement procedures in ventilation research, international j. of ventilation, 10 (2012) pp. 353-370. [9] w.torresani, n.battisti, a.maglione, d.brunelli, d.macii, ‘a multi-sensor wireless solution for indoor thermal comfort monitoring’, in proc. of ieee eesms, 11-12 sept., 2013, trento, italy, pp. 25-30. [10] p. bellagente, p. ferrari, a. flammini, s. rinaldi, "adopting iot framework for energy management of smart building: a real testcase", in proc. of ieee rtsi, turin, italy, sept. 16-18, 2015, pp. 138-143. [11] ashrae 62/89, ventilation for acceptable iaq. [12] uni 10339:1995 impianti aeraulici a fini di benessere – generalità, classificazione e requisiti – regole per la richiesta dell’offerta, l’offerta, l’ordine e la fornitura. [13] f.cumo, g.caruso, l.ferroni, e.paladino, ‘l’indice di valutazione dell’indoor air quality come indicatore di sicurezza in ambienti lavorativi confinati, con particolare riferimento al terziario avanzato’, conferenza vgr 2006 valutazione e gestione del rischio negli insediamenti civili e industriali, 17-19 ott., 2006, pisa, italy. [14] uni/ts 11300-1:2014, prestazioni energetiche degli edifici – parte 1: determinazione del fabbisogno di energia termica per la climatizzazione estiva ed invernale. [15] uni 10349:1994, riscaldamento e raffrescamento degli edifici – dati climatici. [16] d.p.r. 26 agosto 1993, n. 412. regolamento recante norme per la progettazione, l'installazione, l'esercizio e la manutenzione degli impianti termici degli edifici ai fini del contenimento dei consumi di energia, in attuazione dell'art. 4, comma 4, della l. 9 gennaio 1991, n. 10, gazz. uff. 14 ottobre 1993, n. 242, s.o. [17] d.pasini, s.mastrolembo, s.rinaldi, p.bellagente, a.flammini, a.l.c.ciribini, ‘exploiting internet of things and building information modeling framework for management of cognitive buildings’, proc. of the ieee international smart cities conference (isc2 2016), 12-15 sept. 2016, trento, italy. [18] m.scheffer, m.konig, t.engelmann, l.c.tagliabue, a.l.c.ciribini, s.rinaldi, m.pasetti, ‘evaluation of open data models for the exchange of sensor data in cognitive building – dynamic linkage sensors – bim model for the elux lab’, proc. of the ieee international workshop on metrology for industry 4.0 and iot, 16-18 apr. 2018, brescia, italy. [19] s.rinaldi, a.flammini, l.c.tagliabue, a.l.c.ciribini, m.pasetti, s.zanoni ‘metrological issues in the integration of heterogeneous figure 16. number of people related to co2 in mta (12 september 2017). figure 15. number of people related to co2 in mta (20 march 2018). acta imeko | www.imeko.org june 2019 | volume 8 | number 2 | 79 iot devices for energy efficiency in cognitive buildings’, proc. of the i2mtc – 2018 ieee international instrumentation & measurement technology conference, 14-16 may, 2018, houston, texas, usa. [20] f.bittenbinder, c.liu, n.moretti, f.re cecconi, l.c.tagliabue, a.l.c.ciribini, i.kovacic, ‘a vision for a cognitive campus network of universities: the learnscapes of poveglia island’, proc. of the 3rd sdewes conference, 30 jun.-3 jul., 2018, novi sad, serbia. [21] s.rinaldi, a.flammini, l.c.tagliabue, a.l.c.ciribini ‘on the use of iot sensors for indoor conditions assessment and tuning of occupancy rates models’, proc. of the ieee international workshop on metrology for industry 4.0 and iot, 16-18 apr. 2018, brescia, italy. [22] e.de angelis, a.l.c.ciribini, l.c.tagliabue, m.paneroni, the brescia smart campus demonstrator: renovation towards a zero energy classroom building, procedia engineering, 28 (2015) pp. 735-743. [23] a.szczurek, et al. ‘co2 and volatile organic compounds as indicators of iaq’, proc. of the 6th aivc conference, 5th tightvent conference, 3rd venticool conference, 23-24 sept., 2015, madrid, spain. [24] d.calì, p.matthes, k.huchtemann, r.streblow, d.müller, co2 based occupancy detection algorithm: experimental analysis and validation for office and residential buildings, building and environment 86 (2015) pp. 39-49. [25] d.chen, s.backer. a.subbaswamy, d.irwim, p.shenoy, ‘nonintrusive occupancy monitoring using smart meters’, proc. of the 5th acm workshop on embedded systems for energy-efficient buildings, 11-15 nov., 2013, roma, italy, pp.1-8. [26] k.akkaya, i.guvenc, r.aygun, n.pala, a.kadri, ‘iot-based occupancy monitoring techniques for energy-efficient smart buildings’, proc. of the 2015 ieee wireless communications and networking conference (wcnc) workshop, energy efficiency in the internet of things, and internet of things for energy efficiency, 9-12 mar. 2015. [27] l.c.tagliabue, m.manfren, a.l.c.ciribini, e.de angelis, probabilistic behavioural modelling in building performance simulation: the brescia elux lab, energy and building, 128 (2016) pp. 119-131. acta imeko, title acta imeko issn: 2221-870x march 2018, volume 7, number 1, 86-88 acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 86 cross-floating calibration study. trim masses: on the standard balance or on the balance under calibration francisco flores, jorge torres centro nacional de metrologia km 4.5 carretera a los cues, mpio. el marques, c:p. 76246 queretaro, mexico section: technical note keywords: pressure balances; cross-floating calibration; trim masses citation: francisco flores, jorge torres, cross-floating calibration study. trim masses: on the standard balance or on the balance under calibration, acta imeko, vol. 7, no. 1, article 17, march 2018, identifier: imeko-acta-07 (2018)-01-17 section editor: paul regtien, measurement science consultancy, the netherlands received july 27, 2017; in final form november 22, 2017; published march 2018 copyright: © 2018 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: francisco flores, e-mail: fflores@cenam.mx 1. introduction the pressure balances are used as reference standards to calibrate secondary manometers and other balances. there are different types of pressure balances covering ranges from 3 kpa to 1 gpa. the pressure balances measure relative, absolute or differential pressure and some measure all types of pressure. this paper is a part of a project within mexico to aid secondary calibration laboratories to better use their high accuracy pressure balances for calibration and internal traceability. the calibration of pressure balances or dead weight balances is of high importance in secondary laboratories, as well as in industry. this type of calibrations is performed by the crossfloating method. the method is for two pressure balances, connected together using a manometric fluid (i.e. gas or liquid) which transmits the pressure between them; the equilibrium must be achieved by adding or removing trim masses (small masses), either in the standard balance or in the balance under calibration. the piston-cylinder of each balance must be located at its floating level. in mexico, more than 12 secondary laboratories have high accuracy pressure balances as calibration standards. this paper presents the results of a study of high accuracy pressure balances cross-floating calibration comparing the use of the trim masses on the standard balance or on the balance under calibration [1]-[3]. 2. study two piston-cylinder calibrations (dh instruments, 5300 model, 8666 serial number, 20 mpa maximum range, cenams property) were made (table 1). the reference standard used was a dh instruments, 5300 model, 5716 serial number pistoncylinder, 100 mpa maximum range. on the first calibration, the trim masses were added on the pressure balance under table 1. standards used. description balance under calibration standard balance manufacturer dh instruments dh instruments model 5300 5300 serial number 8666 5716 range 0.04 mpa to 20 mpa 0.2 mpa to 100 mpa abstract the pressure balances are used as reference standards to calibrate secondary manometers and other balances. there are different types and models of pressure balances that cover measuring ranges from 3 kpa to 1 gpa. the pressure balances are designed to measure relative, absolute or differential pressure and some balances can measure all types of pressure. this paper presents the results of a study of high accuracy pressure balances cross-floating calibration comparing trim masses on the standard balance or on the balance under calibration. acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 87 calibration (table 2). in the second calibration, the trim masses were added on the standard pressure balance (table 3). 3. results to evaluate the results obtained from the two calibrations the criteria used was the normalized error equation method. 𝐸n = 𝑥cal−𝑥ref �𝑈cal 2 −𝑈ref 2 (1) where: en is the normalized error; xcal is the area obtained by one method of calibration; xref is the area obtained by the other method of calibration; ucal is the expanded uncertainty estimated for one method of calibration, (k = 2); uref is the expanded uncertainty estimated for the other method of calibration, (k = 2). from the normalized error equation model, if: |en| ≤ 1 the results are compatible, |en| > 1 the results are not compatible. by means of the results included in tables 2 and 3, we can obtain the effective area and its uncertainty, for each type of calibrations as shown in tables 4 and 5. figure 1 shows the results obtained in tables 4 and 5 for the effective area aeffe and its uncertainty. normalized error equation method results can be calculated by means of the data in tables 4 and 5, as presented in table 6. figure 2 graphs the results included in table 6. additionally to the calibrations made with the mexican national standard shown before (figures 1 and 2), using the results from the original calibration made by the manufacturer for a0, we can compare the results from both calibrations for a0, as shown in table 7, where a0 is the effective area at zero pressure. 4. discussion during the calibrations, all considerations for effective area pressure balance procedure were taken into account. from the two calibrations made, there are only one point bigger than one, en, with not compatible results. the form to realize the table 4. results adding trim masses to the balance under calibration. pressure / kpa aeffe (20 °c) / m2 uncertainty / m2 relative uncertainty / aeffe pressure due to the trim masses / kpa 1 995.560 4.902 72 e-05 ± 1.8 e-09 ± 37 e-06 0.835 3 990.331 4.902 82 e-05 ± 1.7 e-09 ± 35 e-06 1.343 5 985.121 4.902 81 e-05 ± 1.7 e-09 ± 35 e-06 1.700 9 974.638 4.902 78 e-05 ± 1.7 e-09 ± 35 e-06 0.880 13 964.110 4.902 79 e-05 ± 1.7 e-09 ± 35 e-06 1.131 17 953.648 4.902 82 e-05 ± 1.7 e-09 ± 35 e-06 1.134 19 948.393 4.902 85 e-05 ± 1.7 e-09 ± 35 e-06 1.413 table 5. results adding trim masses to the standard balance. pressure / kpa aeffe (20 °c) / m2 uncertainty / m2 relative uncertainty / aeffe pressure due to the trim masses / kpa 1 994.499 4.903 35 e-05 ± 1.8 e-09 ± 37 e-06 1.473 3 989.208 4.903 08 e-05 ± 1.7 e-09 ± 35 e-06 1.658 5 984.078 4.902 86 e-05 ± 1.7 e-09 ± 35 e-06 1.733 9 973.585 4.902 80 e-05 ± 1.7 e-09 ± 35 e-06 1.718 13 962.929 4.902 83 e-05 ± 1.7 e-09 ± 35 e-06 1.575 17 952.378 4.902 82 e-05 ± 1.7 e-09 ± 35 e-06 1.548 19 947.078 4.902 82 e-05 ± 1.7 e-09 ± 35 e-06 1.523 figure 1. results of effective area and its uncertainty for both calibrations made. table 6. results adding trim masses to the standard balance. pressure / kpa trim masses on the standard balance aeffe (20 °c) / m2 trim masses on the balance under calibration aeffe (20 °c) / m 2 en 1 994.499 4.903 35 e-05 4.902 72 e-05 2.5 3 989.208 4.903 08 e-05 4.902 82 e-05 1.1 5 984.078 4.902 86 e-05 4.902 81 e-05 0.22 9 973.585 4.902 80 e-05 4.902 78 e-05 0.08 13 962.929 4.902 83 e-05 4.902 79 e-05 0.16 17 952.378 4.902 82 e-05 4.902 82 e-05 0.02 19 947.078 4.902 82 e-05 4.902 85 e-05 0.14 table 2. adding trim masses to the balance under calibration. nominal pressure / mpa masses, standard balance / kg masses, balance under calibration / kg 2.0 2.0 10.004 050 4.0 4.0 20.004 570 6.0 6.0 30.004 910 10.0 10.0 50.005 100 14.0 14.0 70.005 060 18.0 18.0 90.006 250 20.0 20.0 100.007 120 table 3. adding trim masses to the standard balance. nominal pressure / mpa masses, standard balance / kg masses, balance under calibration / kg 2.0 1.998 900 10.0 4.0 3.998 870 20.0 6.0 5.998 940 30.0 10.0 9.998 930 50.0 14.0 13.998 770 70.0 18.0 17.998 750 90.0 20.0 19.998 730 100.0 acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 88 adjustment with trim masses is compatible using one or the other method for most of the target points calibrated. as shown in figures 1 and 2, comparing both results, the most significant difference for high accuracy pressure balances is in the low range of the calibration. on the other hand, the pressure generated by the trim masses, as shown in tables 4 and 5, is in the order of 1.7 kpa, which is less than 0.009 % of the minimum range measured. 5. conclusions when comparing the a0 results between the two methods with the mexican national standard and alternatively with the results from the original calibration made by the manufacturer, both methods are compatible. although, slightly better results were obtained by the method using the trim masses on the standard balance. there is still more experiments to be carried out to discriminate other possible effects (i. e. different ranges, uncertainties, etc.). as general recommendation, it is better to use the minimum possible number of masses and trim masses. references [1] sabuga w., bergoglio m., rabault t., waller b., torres guzman j. c., olson d. a., agarwal a., kobata t., bandyopadhyay a.k. [ptb, imgc, bnm-lne, npl, cenam, nist, inms/nrc, nmij/aist, npli], final report on key comparison ccm.pk7 in the range 10 mpa to 100 mpa of hydraulic gauge pressure. metrologia 2005, 42, tech. suppl., 07005. [2] torres-guzman j. c., olvera-arana p., olson d., hydraulic gauge pressure sim comparison for a range up to 100 mpa. the 4th ccm international conference on pressure metrology from ultra-high vacuum to very high pressure. england, 2005. [3] torres guzmán j. c., guía y lineamientos generales para comparaciones de patrones de medición, memorias del segundo congreso internacional metrocal. concepción, chile. april 2001. figure 2. normalized error equation results for the two calibrations. table 7. normalized error equation results of the two calibrations using the manufacturer’s first calibration information as reference. description a0 (20 °c) / m2 uncertainty / m2 relative uncertainty / m2 en manufacturer 4.902 67 e-05 ± 3.9 e-09 ± 80 e-06 -- trim masses on the standard balance 4.903 17 e-05 ± 2.1 e-09 ± 42 e-06 1.1 trim masses on the balance under calibration 4.902 76 e-05 ± 1.8 e-09 ± 37 e-06 0.20 -2 -1 0 1 2 3 0 5 000 10 000 15 000 20 000 25 000 n or m al iz ed e rr or pressure / kpa normalized error, comparison between adding trim masses on the standard balance and on the balance under calibration cross-floating calibration study. trim masses: on the standard balance or on the balance under calibration << 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/hrv (za stvaranje adobe pdf dokumenata najpogodnijih za visokokvalitetni ispis prije tiskanja koristite ove postavke. stvoreni pdf dokumenti mogu se otvoriti acrobat i adobe reader 5.0 i kasnijim verzijama.) /hun 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can be opened with acrobat and adobe reader 5.0 and later.) >> /namespace [ (adobe) (common) (1.0) ] /othernamespaces [ << /asreaderspreads false /cropimagestoframes true /errorcontrol /warnandcontinue /flattenerignorespreadoverrides false /includeguidesgrids false /includenonprinting false /includeslug false /namespace [ (adobe) (indesign) (4.0) ] /omitplacedbitmaps false /omitplacedeps false /omitplacedpdf false /simulateoverprint /legacy >> << /addbleedmarks false /addcolorbars false /addcropmarks false /addpageinfo false /addregmarks false /convertcolors /converttocmyk /destinationprofilename () /destinationprofileselector /documentcmyk /downsample16bitimages true /flattenerpreset << /presetselector /mediumresolution >> /formelements false /generatestructure false /includebookmarks false /includehyperlinks false /includeinteractive false /includelayers false /includeprofiles false /multimediahandling /useobjectsettings /namespace [ (adobe) (creativesuite) (2.0) ] /pdfxoutputintentprofileselector /documentcmyk /preserveediting true /untaggedcmykhandling /leaveuntagged /untaggedrgbhandling /usedocumentprofile /usedocumentbleed false >> ] >> setdistillerparams << /hwresolution [2400 2400] /pagesize [612.000 792.000] >> setpagedevice acta imeko issn: 2221-870x september 2019, volume 8, number 3, 25 – 29 acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 25 calibration values uninfluenced by the kind of pressure medium and the setting posture for quartz bourdon-type pressure transducers hideaki iizumi1, hiroaki kajikawa1, tokihiko kobata1 1 national metrology institute of japan (nmij), aist, 1-1-1 umezono, tsukuba, ibaraki, 305-8563, japan section: research paper keywords: pressure transducer; pressure medium; setting posture; high gas pressure citation: hideaki iizumi, hiroaki kajikawa, tokihiko kobata, calibration values uninfluenced by the kind of pressure medium and the setting posture for quartz bourdon-type pressure transducers, acta imeko, vol. 8, no. 3, article 5, september 2019, identifier: imeko-acta-08 (2019)-03-05 editor: rugkanawan wongpithayadisai, nimt, thailand received august 31, 2018; in final form may 8, 2019; published september 2019 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: hideaki iizumi, e-mail: h.iizumi@aist.go.jp 1. introduction high gas pressure is measured in various contexts, such as hydrogen stations, natural gas pipelines, industrial plants, and scientific research [1]-[4]. for ensuring reliable measurements, the national metrology institute of japan (nmij), aist has developed a high gas pressure standard for pressures up to 100 mpa using a liquid-lubricated pressure balance [5]-[7]. a quartz bourdon-type pressure transducer [8]-[10] is one of the highest precision pressure gauges in the high gas pressure range. the accuracy of these pressure transducers is about 0.01 % of the full scale. the characteristics of the pressure transducer have been evaluated in many institutes [11]-[14]. the quartz bourdon-type pressure transducer is used as a transfer standard in some international comparisons of high gas pressure standards and hydraulic pressure standards [15]-[20]. in an international comparison of high gas pressure standards, we found that the calibration values of the quartz bourdon-type pressure transducers are affected by the kind of gas medium used [15]. in this comparison, the transducers are previously calibrated both with nitrogen and with helium for the characterisation of the transducers, and the calibration values of the transducers depend on the kind of gas medium used. a recent article described the reason for these effects [21]. the indications of the transducers are affected by the weight of the pressure medium inside a sensing element. the calibration values of the horizontally mounted transducers also depend on the rotation angle around the central axis. it is found that the effects of the kind of gas medium can be reduced by adjusting the rotation angles of the transducers. in this study, we investigate how to ensure that the calibration values are uninfluenced by the kind of gas medium and setting posture used. we evaluate the effect of the setting posture on the calibration values of transducers mounted horizontally [21]. in this paper, we evaluate the effects of both the kind of gas medium and setting posture used on the transducer mounted vertically. using these evaluation results, the methods of reducing the effects of the kind of gas medium abstract the effects of the kind of gas medium and the setting posture on the calibration values of a quartz bourdon-type pressure transducer mounted vertically are evaluated in this study. the transducer, at the upward and downward settings, was calibrated both with nitrogen and with helium. the difference between the calibration values of the transducer with nitrogen at the upward and downward settings was about 7.0 kpa at 100 mpa. at the same setting posture, the maximum difference in the calibration values between nitrogen and helium was 3.4 kpa. for precise pressure measurement, it is recommended that the transducers are used with the same pressure medium and the same setting posture with which they were calibrated. the methods of reducing the effects of both the kind of gas medium and the setting posture are discussed. the average of two calibration values at the upward setting and at the downward setting was not affected by the kind of gas medium. when the sensing elements of two pressure transducers arranged in point symmetry with each other, the average values of two transducers were independent of both the kind of gas medium and the setting posture. mailto:h.iizumi@aist.go.jp acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 26 used are discussed. we also propose an appropriate arrangement for two transducers, with which the effects of both the kind of gas medium and the setting posture are highly reduced. 2. reason for the effects of the kind of gas medium and setting posture used the sensing element of a quartz bourdon-type pressure transducer [8]-[10] mainly consists of a bourdon tube and a quartz crystal oscillator. in the sensing element, the quartz crystal oscillator is attached across the root and tip of the bourdon tube, as shown in figure 1. pressure applied to the bourdon tube generates an uncoiling force that applies tension to the quartz crystal. the change in the frequency of the quartz crystal oscillator as a function of the tension is a measure of the applied pressure. the reason for the effects of the kind of gas medium used has been described in a recent article [21]. the indications are affected by the weight of the pressure medium inside the bourdon tube. the weight of the pressure medium can cause the extensional or compressional deformation of the bourdon tube, thereby affecting the oscillating frequency and, hence, the pressure indication. in this case, the indication of the transducer can be expressed as the sum of two contributions: the force acting on the bourdon tube purely from the applied pressure and an extensional (or compressional) gravitational force arising from the weight of the pressure medium inside the bourdon tube. since the gas density largely varies depending on the kind of gas medium used [22], [23], the second term depends on both the kind of gas medium and the relationship between the directions of the gravitational force and the deformation of the bourdon tube. therefore, it is thought that the indication of the pressure transducer, ip, mounted vertically, is expressed by the following equation: ip = i’p ± ∆ip. (1) (1) here, the first term i’p represents the contribution purely from the applied pressure, and δip represents the effect of the weights of pressure medium inside the bourdon tube. δip depends on both the kind of gas medium and the setting posture used. δip depends on the relationship between the directions of the gravitational force and the deformation of the bourdon tube. when the transducer is set at the upward setting, as shown in figure 1, the sign of δip is negative because the gravitational force arising from the weight of the pressure medium causes the compressional deformation of the bourdon tube. conversely, when the transducer is set at the downward setting, δip is positive because the gravitational force causes extensional deformation. in the next section, the above assumption was experimentally confirmed by calibrating the vertically mounted pressure transducer, both with nitrogen and with helium. 3. the effects at vertical settings 3.1. calibration methods figure 2 shows a schematic drawing of a high-gas pressure calibration system in nmij [5]. a pressure supply source is gas cylinder. a gas booster with a maximum compression ratio of 25:1 pressurises the gas to about 100 mpa. pressurised gas is supplied to the pressure controller, which is used to accurately adjust the gas pressure and supply a sufficiently stable pressure to a liquid-lubricated pressure balance and the pressure transducer. a variable volume is also used to manually adjust the piston’s floating position. the liquid-lubricated gas pressure balance is used as a standard device. the calibration is conducted by comparing the applied pressure from the liquid-lubricated pressure balance with the indication of the pressure transducer. the target pressure was changed from 0 mpa to 100 mpa in steps of 10 mpa. at each target pressure, the indication of the pressure transducer was sampled 18 times at 10 s intervals, after waiting 7 mins following the establishment of a steady pressure. the difference between the pressure applied by the standard and the indication of the pressure transducer after offset correction at nominal gauge pressure p mpa, rp, is calculated as rp = (ip − sp) − (i0 − s0). (2) here, sp is the pressure applied by the standard. the mean value of rp in three ascending processes is used as the calibration value. the quartz bourdon-type pressure transducer (paroscientific inc., model 9000-15k, pressure range 100 mpa) [8] was calibrated at the upward and downward settings as shown in figure 1. the reference level for each pressure transducer was determined as the level at the end face of the inlet port. nitrogen and helium were used as the pressure media. figure 1. quartz bourdon-type pressure transducer, set at upward and downward settings. figure 2. schematic drawing of a high gas pressure calibration system. sensing element quartz crystal bourdon tube pressure upward downward pressure pressure acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 27 3.2. calibration results the calibration results of the pressure transducers at the upward and downward settings are shown in figure 3. rp depends on both the pressure medium and the setting posture. the standard deviation of rp in the three ascending processes was less than 0.6 kpa in all measurement pressure. the difference between rp at the upward and downward settings was larger than the standard deviations. the same difference with nitrogen at 100 mpa was about 7.0 kpa, relatively 7.0 × 10-5 of the full scale. when the transducer is set at the downward setting, the gravitational force arising from the weight of the pressure medium causes the extensional deformation of the bourdon tube, and when the transducer is set at the upward setting, the gravitational force causes compressional deformation. therefore, rp at the downward setting was larger than rp at the upward setting. the effects of the setting posture with helium were smaller than that with nitrogen because the density of helium is smaller than that of nitrogen. at the same setting posture, the maximum difference between rp with nitrogen and rp with helium was 3.4 kpa, relatively 3.4 × 10-5 of the full scale. 4. methods for reducing the effects 4.1. using a single pressure transducer since the quartz bourdon-type pressure transducer was affected by the kind of gas medium and the setting posture, it is recommended that the transducers are used with the same pressure medium and the same setting posture with which they were calibrated. the two methods for reducing the effect of the kind of gas medium are discussed. when the transducer is mounted vertically, the deformation of the bourdon tube by the gravitational force arising from the weight of the pressure medium at the upward setting is opposite to that at the downward setting. therefore, it is thought that the effect of the weight of the pressure medium can be eliminated by averaging the calibration values at the upward and downward settings. when the transducer is mounted horizontally, there is a particular rotation angle, θ0, where there is no effect of the gas medium. the method of identifying the rotation angle θ0 by the experiment has been reported in other studies [21]. at the rotation angle θ0, the direction of the deformation of the bourdon tube is perpendicular to the direction of gravitational force, as shown in figure 4. figure 5 shows the average value of rp at the upward and downward settings both with nitrogen and with helium. the calibration values of the horizontally mounted transducer at the rotation angle θ0, 𝑅𝑝,𝜃0, both with nitrogen and with helium are also shown. each value showed good agreement within 0.6 kpa in all measurement pressures. 0.6 kpa was comparable to the standard deviations of the calibration value. the average value of the rp at the upward and downward settings agreed well with 𝑅𝑝,𝜃0 because both values only included the contribution purely from the applied pressure. the effect of the kind of gas medium on the calibration value can be reduced by averaging the calibration values at the upward and downward settings or by adjusting the setting posture of the transducers. 4.2. using two pressure transducers in the international comparison, two pressure transducers are used as the transfer standard for securing redundancy. we discuss here the case in which two transducers can be used for measurements. it is thought that the effect of the kind of gas medium used can also be reduced by averaging the calibration values of two transducers at the upward setting and at the downward setting. in addition, when the sensing elements of the two pressure transducers are arranged in point symmetry with each other, the deformations of the two bourdon tubes by the gravitational force arising from the weight of the pressure medium are opposite to each other in any setting posture. therefore, it is thought that the effects of both the kind of gas medium and the setting posture used can also be reduced by averaging the calibration values of the two arranged transducers. figure 6 shows a point-symmetry arrangement of two transducers. the inlet port of two transducers a and b at figure 3. calibration results rp at the upward and downward settings, both with nitrogen and with helium. figure 4. setting posture of sensing element, θ0. direction of deformation of bourdon tube is perpendicular to direction of gravitational force. figure 5. average values of calibration values at the upward and downward settings. calibration values at horizontal settings with rotation angle θ0. 0 5 10 15 20 0 20 40 60 80 100 r p / k p a pressure / mpa downward: n₂ downward: he upward: n₂ upward: he bourdon tube pressure gravity quartz crystal acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 28 rotation angle θ0 were arranged face to face with each other. this posture was defined as θ = 90°, φ = 90°. the set of two transducers arranged in point symmetry were mounted vertically (φ = 0° and 180°), and both were calibrated with nitrogen and with helium. the rotation angle θ was fixed to 90°. when φ = 0°, transducer a was set at the upward setting and transducer b was set at the downward setting. when φ = 180°, transducer a was set at the downward setting, and transducer b was set at the upward setting. the rp of the two transducers were averaged. figure 7 shows the average value of the two transducers, rp,ave, both with nitrogen and with helium at φ = 0° and 180°. at the same setting posture, rp,ave with nitrogen and rp,ave with helium showed good agreement within 0.7 kpa in all measurement pressures, relatively 7.0 × 10-6 of the full scale. 0.7 kpa was comparable to the standard deviations of the calibration value. in addition, rp,ave at φ = 0° and 180° showed good agreement. two transducers a and b, arranged as shown in figure 7, were calibrated at different setting postures. the pressure medium was nitrogen. in the first experiment, the rotation angle φ was fixed to 90°. the rotation angle θ was changed in order of 90°, 45°, 0°, 90°, 135°, 180°, 90°. figure 8(a) shows the calibration values at 100 mpa, r100, of transducer a and b at each rotation angle θ. the average values of the two transducers are also shown. the three r100 at θ = 90° showed good agreement within 0.5 kpa in both transducers a and b. although the maximum difference by the rotation angle θ of transducer a was 7.2 kpa and transducer b was 7.0 kpa, the average values were 0.5 kpa. the average values were not affected by the rotation angle θ. in the second experiment, the rotation angle θ was fixed to 90°. the rotation angle φ was changed in order of 90°, 45°, 0°, 90°, 135°, 180°, 90°. figure 8(b) shows the r100 of transducers a and b at each rotation angle φ. the average values of the two transducers are also shown. the three r100 at φ = 90° showed good agreement within 0.5 kpa in both transducers a and b. although the maximum difference of rotation angle φ of transducer a was 7.8 kpa and transducer b was 9.5 kpa, the average value was 1.0 kpa. in both cases, the effect of the setting posture on the calibration value can be reduced by averaging the calibration values of the two transducers. when the sensing elements of the two pressure transducers were arranged in point symmetry, their average values were almost independent of both the kind of gas medium and the setting posture used. if two transducers that have the same effects are used, the average values would be completely independent of both the kind of gas medium and the setting posture used. in this study, the inlet ports of the two transducers were arranged face to face with each other. however, the same results can be obtained as long as the sensing elements are arranged in point symmetry with each other. 5. conclusions a quartz bourdon-type pressure transducer is one of the highest precision pressure gauges in a high gas pressure range. this study found that the calibration values of the quartz bourdon-type pressure transducers are affected by the kind of figure 6. two pressure transducers at rotation angle θ0 are positioned in point symmetry with each other. this posture is defined as θ = 90°, φ = 90°. figure 7. average value of two transducers, rp,ave, at φ = 0° and 180° both with nitrogen and with helium. figure 8. calibration values at 100 mpa, r100, of transducers a and b (a) at different rotation angles θ; (b) at different rotation angles φ. the average values of the two transducers are also shown. gravity φ θ a b 0 5 10 15 20 25 0 20 40 60 80 100 r p ,a v e / k p a pressure / mpa φ=0°: n₂ φ=0°: he φ=180°: n₂ φ=180°: he acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 29 gas medium used. this is because the indications of the transducers are affected by the weight of the pressure medium inside a sensing element. the weight of the pressure medium can cause extensional or compressional deformation of the bourdon tube in the sensing elements, affecting the oscillating frequency and, hence, the pressure indications. the indications depend on both the kind of gas medium and the setting posture used. in this study, the effects of the kind of gas medium and the setting posture on the vertically mounted transducer were evaluated. the transducer at the upward and downward settings was calibrated both with nitrogen and with helium. the difference between the calibration values of the transducer at the upward and downward settings was about 7.0 kpa, relatively 7.0 × 10-5 of full scale. at the same setting posture, the maximum difference in the calibration values between nitrogen and helium was 3.4 kpa, relatively 3.4 × 10-5 of the full scale. for precise pressure measurement, it is recommended that the transducers are used with the same pressure medium and the same setting posture with which they were calibrated. the methods of reducing the effects of the kind of gas medium used have been discussed. since the deformation of the bourdon tube due to the gravitational force arising from the weight of the pressure medium at the upward setting is opposite to that at the downward setting, it was found that the effect of the kind of gas medium used can be reduced by averaging the calibration values at the upward and downward settings. the average value of the calibration values at the upward and downward settings with nitrogen agreed well with the average value with helium. in addition, the effects of the kind of gas medium used were reduced by averaging the calibration values of two transducers at the upward setting and at the downward setting. when the sensing elements of the two pressure transducers are arranged in point symmetry with each other, the deformations of the two bourdon tubes by the gravitational force arising from the weight of the pressure medium are opposite to each other in any setting posture. therefore, it was thought that the effect of both the kind of gas medium and the setting posture used can also be reduced by averaging the calibration values of the two arranged transducers. the two arranged transducers were calibrated at different settings with nitrogen. the maximum difference in the transducer’s calibration values due to the setting posture was 9.5 kpa. however, the maximum difference of the average values of the two arranged transducers was 1.0 kpa. when the sensing elements of the two pressure transducers were arranged in point symmetry, the average values of the two transducers were independent of both the kind of gas medium and the setting posture used. references [1] un/ece, global technical regulation no. 13: global technical regulation on hydrogen and fuel cell vehicles, 2013. [2] nist handbook 44, 2013 edition: specifications, tolerances, and other technical requirements for weighing and measuring devices. section 3.39: hydrogen gas-measuring devices – tentative code. [3] emrp call 2013 – energy and environment: srt-g05 metrology for hydrogen transport, 2013. online [accessed 20190920] http://www.emrponline.eu/call2013/srts/srt-g05.pdf. [4] n. sakoda, k. onoue, t. kuroki, k. shinzato, m. kohno, m. monde, y. takata, transient temperature and pressure behavior of high-pressure 100 mpa hydrogen during discharge through orifices, international journal of hydrogen energy 41 (2016) pp. 17169-17174. [5] h. iizumi, h. kajikawa, t. kobata, a high gas pressure calibration system using a liquid-lubricated pressure balance, measurement 102 (2017) pp. 106-111. [6] t. kobata, improved methods for comparing gas and hydraulic pressure balances, metrologia 46, 5 (2009) pp. 591-598. [7] p. delajoud, m. girard, ‘a new piston gauge to improve the definition of high gas pressure and to facilitate the gas to oil transition in a pressure calibration chain’, proc. of the international symposium on pressure and vacuum (imeko tc16), beijing, china, 21 24 september 2003, pp.154-159. [8] paroscientific inc., 2013: user’s manual for digiquartz broadband intelligent instruments with dual rs-232 and rs-485 interfaces. [9] paroscientific inc. 2009: user’s manual for model 735 intelligent display and model 745 high accuracy laboratory standard. [10] fluke calibration 2012: rpm4 operation and maintenance manual. [11] t. kobata, characterization of quartz bourdon-type highpressure transducers, metrologia 42 (2005) s235. [12] h. kajikawa, t. kobata, effects of pressurization procedures on calibration results for precise pressure transducers, meas. sci. technol. 21 (2010) 065104. [13] i. kocas, m. bergoglio, an investigation of quartz type pressure transducer behavior under continuous pressure conditions and metrological characterization, measurement 45 (2012) pp. 24862489. [14] w.w. chadwick jr., s.l. nooner, m.a. zumberge, r.w. embley and c.g. fox, vertical deformation monitoring at axial seamount since its 1998 eruption using deep-sea pressure sensors, journal of volcanology and geothermal research 150 (2006) pp. 313–327. [15] h. kajikawa, d.a. olson, h. iizumi, r.g. driver, m. kojima, final report on supplementary comparison apmp.m.p-s6 in gas gauge pressure from 10 mpa to 100 mpa, metrologia 53-ts (2016) 03002. [16] t. kobata, a. k. bandyopadhyay, k. moore, a. a. e. eltawil, s. y. woo, t. k. chan, w. jian, j. man, n. n. con, c. s. fatt, w. permana, m. aldammad, w. sabuga, t. changpan, c. c. hung, z. pengcheng, final report on key comparison apmp.m.p-k7 in hydraulic gauge pressure from 10 mpa to 100 mpa, metrologia 42-ts (2005) 07006. [17] t. kobata, h. kajikawa, s. kimura, m. fitzgerald, d. jack, c. sutton, w. a. m. w. mohamed, m. m. mansor, c. s. fatt, final report on key comparison apmp.m.p-k7.1 in hydraulic gauge pressure from 10 mpa to 100 mpa, metrologia 46-ts (2009) 07008. [18] j. c. g. romero, m. c. neira, j. c. torres-guzman, final report of supplementary comparison sim.m.p-s7: hydraulic pressure comparison from 7 mpa to 70 mpa, metrologia 50-ts (2013) 07014. [19] j. c. torres-guzman, ‘pressure standards comparison between germany and mexico’, proc. of the 2002 ncsl international workshop and symposium, san diego, usa, 4-8 august 2002. [20] euramet project no 1252: comparison in the range of 10 mpa to 100 mpa of liquid pressure. online [accessed 20190920]: https://www.euramet.org/technical-committees/search-tcprojects/details/?eurametctcp_project_show%5bproject%5d= 1026&eurametctcp_project%5bback%5d=188&chash=af8693 5f61350bdaacb958e4a0a05289 [21] h. iizumi, h. kajikawa, t. kobata, effect of the kind of gas medium on calibration values of high gas pressure transducers, measurement, 131 (2019) pp. 358-361. [22] r. span, e. w. lemmon, r. t. jacobsen, w. wagner, a. yokozeki, a reference equation of state for the thermodynamic properties of nitrogen for temperatures from 63.151 to 1000 k and pressures to 2200 mpa, j. phys. chem. ref. data 29-6 (2000) pp.1361-1433. [23] r. d. mccarty, v. d. arp, a new wide range equation of state for helium, adv. cryog. eng. 35 (1990) pp. 1465-1475. http://www.emrponline.eu/call2013/srts/srt-g05.pdf http://www.emrponline.eu/call2013/srts/srt-g05.pdf https://www.euramet.org/technical-committees/search-tc-projects/details/?eurametctcp_project_show%5bproject%5d=1026&eurametctcp_project%5bback%5d=188&chash=af86935f61350bdaacb958e4a0a05289 https://www.euramet.org/technical-committees/search-tc-projects/details/?eurametctcp_project_show%5bproject%5d=1026&eurametctcp_project%5bback%5d=188&chash=af86935f61350bdaacb958e4a0a05289 https://www.euramet.org/technical-committees/search-tc-projects/details/?eurametctcp_project_show%5bproject%5d=1026&eurametctcp_project%5bback%5d=188&chash=af86935f61350bdaacb958e4a0a05289 https://www.euramet.org/technical-committees/search-tc-projects/details/?eurametctcp_project_show%5bproject%5d=1026&eurametctcp_project%5bback%5d=188&chash=af86935f61350bdaacb958e4a0a05289 https://www.euramet.org/technical-committees/search-tc-projects/details/?eurametctcp_project_show%5bproject%5d=1026&eurametctcp_project%5bback%5d=188&chash=af86935f61350bdaacb958e4a0a05289 https://www.euramet.org/technical-committees/search-tc-projects/details/?eurametctcp_project_show%5bproject%5d=1026&eurametctcp_project%5bback%5d=188&chash=af86935f61350bdaacb958e4a0a05289 https://www.euramet.org/technical-committees/search-tc-projects/details/?eurametctcp_project_show%5bproject%5d=1026&eurametctcp_project%5bback%5d=188&chash=af86935f61350bdaacb958e4a0a05289 https://www.euramet.org/technical-committees/search-tc-projects/details/?eurametctcp_project_show%5bproject%5d=1026&eurametctcp_project%5bback%5d=188&chash=af86935f61350bdaacb958e4a0a05289 microsoft word 37-daponte-ga.doc acta imeko  july 2012, volume 1, number 1, 3  www.imeko.org    acta imeko | www.imeko.org  july 2012 | volume 1 | number 1 | 3  welcome to acta imeko  pasquale daponte   department of engineering, university of sannio, palazzo bosco‐lucarelli, piazza roma, 82100 benevento, italy     keywords: journal; imeko; president elect   citation: pasquale daponte, welcome to acta imeko, acta imeko, vol. 1, no. 1, article 3, july 2012, identifier: imeko‐acta‐01(2012)‐01‐03  editor: paul regtien, measurement science consultancy, the netherlands  copyright: © 2012 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  corresponding author: pasquale daponte, e‐mail: daponte@unisannio.it    welcome to acta imeko! acta imeko is a new scientific journal exploring all the fields of measurement science; a science that drives and underpins much of what we do and experience in our everyday lives, though often unseen and beyond our awareness. acta imeko was established to provide reviews of current topics in measurement science in order to share the latest research results among scientists of all over the world, and to be a reference point of the state of art in the measurement field for all people involved directly or indirectly in measurements and to assist them in the translation of science into practice. acta imeko aims to conduct science cooperation across the world to bolster confidence in the identification of potential measurement and testing requirements at an early stage during research, innovation or development, so that measurement solutions can be developed in a timely manner in any technical field. acta imeko is a free on-line peer-reviewed journal. by making it readily available online, imeko hopes to reach the largest number of readers, including those who cannot afford printed journals and online subscriptions in countries and regions with little or no library resources, but only internet access. acta imeko is a great opportunity for all imeko technical committees to offer the contributions of the events they organize to a wider audience. revised papers, after presentation to an imeko event, are strongly solicited. thanks to acta imeko the penetration of our organization in the measurement science international community may improve considerably. this is a further step in the imeko policy toward an effective globalization of knowledge. i wish to thank all the imeko officers that made this result possible. i am sure that this young journal will attract young authors wishing to contribute to the success of this initiative, although i am aware that it requires much more active participation by the international academic community. many scientists in the measurement field have shared and supported this project by joining the journal’s scientific committee, forming a selected pool of reviewers for the incoming contributions; this is a promising start. i hope you will find the journal interesting and will actively contribute to its development. acta imeko, title acta imeko issn: 2221-870x june 2018, volume 7, number 2, 84-89 acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 84 positional uncertainty in optical-based metrology systems in large volume manufacturing ainsley miller1, anthony j. mulholland2, stephen gareth pierce1, ben hughes3 and alistair b. forbes3 1department of electronic and electrical engineering, university of strathclyde, glasgow, united kingdom, g1 1rd 2department of mathematics and statistics, university of strathclyde, glasgow, united kingdom, g1 1xh 3national physical laboratory (npl), teddington, london, united kindgom, tw11 0lw section: research paper keywords: photogrammetry; laser tracker; large volume; refraction; uncertainty citation: ainsley miller, anthony j. mulholland, stephen gareth pierce, ben hughes and alistair b. forbes, positional uncertainty in optical-based metrology systems in large volume manufacturing, acta imeko, vol. 7, no. 2, article 14, june 2018, identifier: imeko-acta-07 (2018)-02-14 section editor: franco pavese, italy received december 12, 2017; in final form april 6, 2018; published june 2018 copyright: © 2018 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: this work is funded by a strathclyde university and national physical laboratory (npl) studentship corresponding author: ainsley miller, email: ainsley.miller@strath.ac.uk 1. introduction as the world enters the fourth industrial revolution, the autonomous age, automated systems are being increasingly used for component positioning and assembly [1]. industries are interested in autonomous and robotic manufacturing as it potentially reduces costs and increases productivity [2]. the robots used in large volume manufacturing need to know their own spatial position in the working volume and the spatial position of the object that they are working on. one possible method for achieving this “local gps” system is using lightbased sensors. large volume metrology involves using light rays which travel through the air volume to gather data on the distance or angle between the measurement apparatus and the object being measured [3]. popular optical-based metrology systems such as photogrammetry and laser trackers are crucial in improving the accuracy and quality associated with robotic assembly [4]. photogrammetry systems can simply be thought of as systems which use photographs to calculate angles between the camera and the reflector [5] and a laser tracker uses laser beams to measure both the angle and the distance from the tracker to the reflector [4], [6]. in an industrial setting these positional uncertainties can in many instances be greater than the required tolerances. one source of uncertainty that arises when considering large scale industrial settings is light refraction (bending of the light ray path) due to, for example, temperature fluctuations in the air; these fluctuations create a heterogeneous refractive index map. as photogrammetry systems and laser trackers both use angular based measurements to determine coordinates, they are both affected by refraction. this source of uncertainty could contribute to difficulties in deploying robotic systems in high abstract modern manufacturing increasingly utilizes automated systems for component positioning and assembly and a vital aspect of autonomous precision manufacturing is large volume metrology. one popular approach uses light rays, which travel through the volume of air, to calculate the position of an object of interest. these optical-based metrology systems such as photogrammetry and laser tracking are crucial in improving the accuracy and quality associated with robotic assembly. however, in an industrial setting, these positional measurement systems give rise to uncertainties which can in many instances be greater than the required tolerances. one source of this uncertainty is light refraction due (in part) to temperature fluctuations in the air. this paper will describe how comsol multiphysics can be used to create simulated data with complete ground truth on the refractive index field in the volume, and will discuss the measurement uncertainties associated with these simulations. over industry relevant distances (tens of metres) the uncertainty in the position measurement due to light refraction is of the order of a millimetre. acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 85 precision industrial settings. there are of course other sources of uncertainty in optical based metrology systems, but refractive effects in the air volume will form the focus of the work in the paper. therefore, accounting for the refractive index effects of air is important to improving positional accuracy and this is a long standing and open problem in large volume metrology. the impact that the fluctuations of the refractive index in air has on industrial measurements can be significant. the recently completed international collaboration on the project “large volume unified metrology for industry and novel applications and research (luminar)” [6] focused on the refractive index effects in an industrial setting. they found that in a typical aircraft industrial space the positional measurement uncertainty can be significant [7]. in addition, [8] found that a light beam will deviate from a straight line by 0.2 mm for every 15 m it travels horizontally when there is a vertical temperature gradient of approximately 2 k/m (and a vertical height of 6m). additionally, it was found that in a working volume of 100 m3 the average positional error of the vicon t160 photogrammetry system was millimetre scale [9]. this paper will look first at characterising the errors associated with optical-based metrology systems due to refraction. the following section will present the methodology used by simulation software to examine the effect that varying parameters that characterise a temperature profile in the air volume have on the positional error. following that the results of the simulations will be presented and discussed. the paper will conclude by summarising the work done and future directions. 2. characterising the errors in opticalmetrology systems let us start by defining the positional error as the euclidean distance between the true (known in the calibration stage) position and the estimated position of the reflector. opticalbased metrology systems assume that the volume of air is homogeneous and that light rays travel in straight lines (so no refractive effects). in addition to an angular measurement, the laser tracker calculates the distance (that is, the distance as perceived by the laser tracker) of the light ray from the laser tracker ( )il to the reflector ( )jr (denote this distance by ijd ). this distance can be used to produce a straight line (see the straight dashed lines in figure 1). if we restrict attention to just distance measurements then, in a two dimensional plane, one can then deploy two laser trackers and use the intersection point of the two straight rays to estimate the position of the reflector eir . the error in the estimated position is shown in figure 1 and is denoted by 1 lε where 1 l e j ir rε = − . these errors will typically be much smaller than the errors due to angular based calculations. similarly, the photogrammetry system calculates the angle (that is, the angle as perceived by the camera denoted by ijθ ) that the line from the camera ( )ic to the reflector ( )jr makes with the normal to the camera plane. this angle can also be used to produce a straight line (see the straight dashed lines in figure 2). in a two-dimensional plane one can then deploy two cameras and use the intersection point of the two straight rays to estimate the position of the reflector eir . the error in the estimated position is as shown in figure 2 and is denoted by 1 cε where 1 c e j ir rε = − . the remainder of this paper will focus mainly on the photogrammetry case, but all the work is transferrable to the laser tracker case. 3. comsol modelling as a precursor to experiments and to allow us to explore the relationship between the measurement uncertainties and the underlying temperature field a virtual simulation of a large volume light-based metrology setting was constructed wherein spatial variations in the refractive index of the air volume can figure 1. figure showing the errors caused by refraction of the light rays when using just the distance measurement in the laser tracker. here we have two laser trackers (l1 and l2) tracking a reflector r1. the red dashed lines show the true ray path where the light rays bend towards the localised heat source and have distance d1 and d2 respectively. the black lines show the equivalent straight ray path that the laser trackers use in their calculations since this assumes the volume is homogeneous (has a constant refractive index throughout and hence no refraction takes place). the intersection of the curved solid lines therefore gives the estimated position of the reflector (r1 e). the positional error is then given by ε1 l = | r1 – r1 e|. figure 2. figure showing the errors caused by refraction of the light rays when using the photogrammetry (angular) system. here we have two cameras (c1 and c2) tracking reflector r1. the red dashed lines show the true ray path where the light rays bend towards the localised heat source and have initial angles θ1 and θ2 (this is the angle that the line from camera to the reflector makes with the normal to the camera plane). the black lines show the equivalent straight ray path that the photogrammetry systems use in their calculations since this assumes the volume is homogeneous (has a constant refractive index throughout and hence no refraction takes place). the intersection of the dashed black lines therefore gives the estimated position of the reflector (r1 e). the positional error is then given by ε1 l = | r1 – r1 e|. acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 86 be prescribed. this carefully controlled environment will supply the ground truth needed to accelerate the development of algorithms and methodologies ahead of their experimental deployment. the forward model (ray-tracing) was carried out using simulations in comsol multiphysics® 5.3 ray tracing toolbox [10] and for computational expediency the investigation was restricted to a two dimensional plane (it can very simply be extended to three dimensions in comsol). it is clear from these simulations that gradients in the refractive index that are orthogonal to the light ray’s path dominate the resulting measurement uncertainties. in other words, it is not sufficient to simply consider a light ray traversing a series of layers that have constant but differing refractive indices; the refraction in this case would simply follow snell’s law. it is important to include the transverse gradients and this is well documented [11]. to mimic the presence of a single source of heat a gaussian temperature profile was placed in the plane containing the light ray (so we are assuming that as the light ray refracts and bends that it remains within this plane) via 2 2 max min min2 ( ) ( , ) ( ) exp x y t x y t t t σ  + = − +     (1) where x and y give the co-ordinates of the point in the plane, σ is the variance of the distribution measured in metres, maxt is the maximum temperature that the domain can take and similarly mint is the minimum temperature that the domain takes ( mint was kept at 295 k throughout and both maxt and mint are measured in kelvins). the refractive index is a function of many atmospheric parameters such as temperature, humidity, pressure and carbon dioxide concentration as captured via the ciddor equation [12][14]. comsol requires the refractive index map in order to define the material properties, so the temperature values in (1) were transformed to a refractive index map using the ciddor equation [14]; all other parameters in the equation were set at the nist average values [15]. the comsol simulation will shortly be used to examine the relationship between the resulting positional uncertainty and the initial ray angle, the value of maxt and the spread of the temperature distribution (by varying σ in (1)), but first the simulation framework will be described. 3.1. setting up the simulation in comsol all the simulations described in this paper were carried out in two dimensions and the comsol model wizard was used to create each new simulation. in all of the simulations the size of the domain was kept constant. the size chosen was in keeping with a laboratory scale photogrammetry area; 3 m in the horizontal ( x ) direction and 4 m in the vertical ( y ) direction. a rectangular domain was created in comsol it should be noted that the bottom left corner of the domain has coordinates (-1.5,-2) m such that the hottest part of the domain can be positioned at the origin (see figure 3). the next step was to input the temperature profile as defined in (1). this was achieved by entering the component option in the model builder window and adding a new variable in the definitions section. once in this menu, the right-hand side of (1) was input into the expression box and this variable was named as t_domain. the next step was to convert the temperature map into a refractive index map. this was achieved by selecting an interpolation from the function menu of the global definitions functionality. once in this menu we named our function n_int and input temperatures ranging from 293.15 k to 323.15 k in the column labelled t. in order to fill in the f(t) column the refractive index values were calculated for each of the temperatures using the ciddor equation [14], [15]. to ensure that these refractive index values were used in the model the geometrical optics (gop) functionality was accessed. in this functionality the medium properties option was located and in the medium properties heading the refractive index real part was defined as user defined from the drop-down menu. finally, the refractive index was defined to take the values of n_int(t_domain). following this, the rays propagating within this geometry had to be defined. in the geometrical optics (gop) functionality of the model builder the release from grid option was used. this allows the user to prescribe the start point of a ray and its directional vector, and one is needed for each ray which propagates within the domain. in the first study 25 rays were simulated with each one having a different initial direction but each one starting from the same point (-1.5, 0) m. the vector describing the light rays’ initial figure 3. figure showing the errors caused by refraction of the light rays caused by a gaussian temperature profile. the size of the volume is 3 metres in the horizontal direction and 4 meters in the vertical direction. here we have an optical-based metrology sensor positioned at s1 (this can be either the photogrammetry system or the laser tracker) which is tracking a reflector positioned at r1. the temperature profile in this diagram causes the light ray to bend towards the heat source (the red colours) as shown by the solid black line; where the ray has initial angle θ1 (this is the angle that the line from camera to the reflector makes with the normal to the camera plane) and total distance d1. when s1 is the photogrammetry system the dashed black line shows the equivalent straight ray path that the photogrammetry system uses in its calculations since this assumes the volume is homogeneous (has a constant refractive index throughout and hence no refraction takes place). the intersection of the dashed black line and the back wall therefore gives the estimated position of the reflector (r1 e, c). the positional error is then given by ε2 c = | r1 – r1 e,c|. similarly, when s1 is the laser tracker, the dot dashed lines show the equivalent straight ray path that the laser tracker uses in its calculations since this assumes the volume is homogeneous, as in the photogrammetry system. the laser tracker gives an angular component and a distance component as output that gives an estimated position of the reflector (r1 e,l). the positional error is then given by ε2 l = | r1 – r1 e,l|. acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 87 direction is [ ]0 01,x y= where 0y takes the values 0, ± 0.05, ± 0.10, ± 0.15, ..., ± 0.6. the upper limit here is 0.6 as any higher values lead to the light ray not reaching the other side of the domain, i.e. having an end co-ordinate as (1.5, )y m with [ 2, 2].y ∈ − figure 4 shows the 25 rays propagating in the geometry. in this case, the values of maxt and σ were kept constant at 314 k and 1.3107 m respectively. the value for σ was chosen at it ensures that at the four corners of the domain the temperature is exactly mint . the second simulation examined the impact that maxt has on the positional error. in this case the number of rays propagating through the geometry was reduced to one. the chosen ray was the one whose initial ray angle led to the biggest positional error in the first set of simulations. all other parameters in this case remain as before apart from max .t in these simulations maxt takes values from 299 k to 374 k. the final simulations examined the effect that varying the spread of the temperature profile (σ ) has on the positional uncertainty. as in the second set of simulated experiments there was again only one ray propagating through the geometry (in fact, the same ray which was used in the previous case). in this case maxt is set as 314 k as in the first case and the values for σ range from 0.5 to 3.5 m. in all of the simulations the output was a set of ),( yx co-ordinates for each ray. 4. results of comsol simulations in order to analyse the results, we consider figure 3 which shows how the positional uncertainties (errors) were defined. this paper only considers the results from the error in the photogrammetry system, c2ε . firstly assume that there is a photogrammetry camera positioned at s i and it is tracking a reflector at known position jr . if we assume that the true temperature distribution of the environment is represented by the gaussian temperature profile shown, the ray will take the path shown by the solid black line. this temperature distribution causes the light rays to bend towards the heat source (the red colours in figure 3). as mentioned in section 2, the photogrammetry system calculates the angle that the line from the camera ( )ic to the reflector ( )jr makes with the normal to the camera plane (denote this angle by ijθ ). this angle produces a straight line (see the straight dashed line in figure 3) which shows the equivalent straight ray path that the photogrammetry system uses in its calculations since this assumes the volume is homogeneous. the intersection of the dashed black line and the back wall therefore gives the estimated position of the reflector ,e cir the positional error is then given by , 2 c e c j ir rε = − . the results of the three parametric studies are provided in figures 5-7. in the first case, the aim was to deduce if the initial ray angle impacted the positional error εc. the results (shown in figure 5) are very intuitive. from this figure we see that as the initial angle of the light ray increase so too does the positional error associated with this. the maximum 2 cε value seen here is approximately 60 μm when the ray has an initial angle of approximately 31°. at an initial angle of 0°the ray does not experience any transverse refractive index gradients. since it is perpendicular to each isothermal contour, and the transverse temperature gradient is zero then it does not undergo any refraction and so the positional uncertainty is zero. the second study varied maxt (keeping all other variables constant). this simulation only had one ray propagating through the geometry and the ray that was chosen was the ray whose initial angle led to the largest 2 cε value in the first case. the results in figure 6 show that as maxt increases so too does the error 2 cε . as maxt increases so too does the local gradient of the temperature (refractive index) profile. it can be seen that the positional uncertainty is now of submillimetre figure 4. comsol simulation of the 25 light rays (black lines) propagating through the gaussian temperature profile described in (1). the size of the domain is 3 metres by 4 metres in the x and y direction respectively. in this simulation the light rays have starting position (-1.5, 0) m and directional vector [ ]0 01,x y= where 0y takes the values 0, ± 0.05, ± 0.10, ± 0.15, ..., ± 0.6. figure 5. graph showing the positional errors of the light ray travelling through the gaussian temperature described in (1) when the initial ray angle is varying. the size of the domain is 3 m by 4 m in the x and y direction respectively. there are 25 rays propagating the domain each with a starting position of (-1.5, 0) m and directional vector [ ]0 01,x y= where 0y takes the values 0, ± 0.05, ± 0.10, ± 0.15, ... , ± 0.6. the parameters maxt , mint and σ are kept constant throughout at 295 k, 314 k and 1.3107 m respectively. acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 88 scale and would start to cause concern in high precision manufacturing setting (over tens of metres this would rise to be of millimetre scale). finally, the spread of the temperature profile was varied via the parameterσ . the results of this are shown in figure 7. for σ values between 0.5 m and 1.1 m the positional uncertainty 2 cε value increases, until it reaches a maximum at around 60 μm when σ is approximately equal to 0.9 m. thereafter, the 2cε value decreases as σ increases. as σ increases the temperature gradient spreads out becoming less steep in the spatial region we are considering, and so it is gradually approaching a constant temperature. so the temperature domain resembles that of a homogeneous one, resulting in reduced refractive index effects and hence a lower 2 cε value. in a similar way when σ is decreased the temperature profile narrows until it forms an isolated peak in the spatial domain we are considering. the ray now traverses a domain that is far from this peak and hence there is a very flat temperature profile; so the resulting positional error is significantly reduced. 5. conclusions and future work light based metrology systems are subject to measurement uncertainty due to refractive index fluctuations in the volume (a heterogeneous refractive index map) which are due primarily to thermal and humidity fluctuations. over typical measurement distances and temperature profiles found in practice these positional uncertainties can be millimetre scale in the case of photogrammetry. this would suggest that developing methods that can discount the effects of refraction from such measurement systems is essential for their successful deployment in high precision manufacturing environments. future work will focus on experimentally validating these simulated results. thereafter, we will deduce if it is possible to use an array of sensors to reconstruct the refractive index map and use this map to discount the refractive effects and therefore reduce the uncertainty in this positioning problem. acknowledgements this work was supported by a studentship from the national physical laboratory (npl), teddington, london and industry collaboration from vicon motion systems, oxford, england. the authors would like to thank comsol support, oxford for their continued support and guidance throughout this work. references [1] m. r. pedersen, l. nalpantidis, r. s. andersen, c. schou, s. bøgh, v. krüger and o. madsen, “robot skills for manufacturing: from concept to industrial deployment, robotics and computer-integrated manufacturing”, no. 37, (2016), pp.282-291. [2] h. park, and n. tran, “an autonomous manufacturing system for adapting to disturbances”, the international journal of advanced manufacturing technology 56, no. 9 (2011), pp. 11591165. [3] w. t. estler, k. l. edmundson, g. n. peggs, and d. h. parker, “large-scale metrology–an update”, cirp annals-manufacturing technology 51, no. 2 (2002), pp. 587-609. [4] g. n. peggs, p. g. maropoulos, e. b. hughes, a. b. forbes, s. robson, m. ziebart, and b. muralikrishnan, “recent developments in large-scale dimensional metrology”, proc. of the institution of mechanical engineers, part b: journal of engineering manufacture 223, no. 6 (2009), pp. 571-595. [5] t. schenk, "introduction to photogrammetry." the ohio state university, columbus 106 (2005). [6] a. lewis et al “luminar: large volume in industry” [online] available at http://projects.npl.co.uk/luminar/the-project/ (2016) [accessed 28th november 2017]. [7] s. robson, l. macdonald, s. kyle and m.r. shortis, “close range calibration of long focal length lenses in a changing environment.” isprsinternational archives of the photogrammetry, remote sensing & spatial information sciences, no 41 (2016) pp. 115-122. figure 6. graph showing the positional errors of the light ray travelling through the gaussian temperature described in (1) when the range of the temperature profile is varied (via the parameter maxt ). the size of the domain is 3 m by 4 m in the x and y direction respectively. the ray in this instance has a directional vector of [ ]0 1, 0.6x = as this led to the largest results in the first case. the parameters mint and σ are kept constant throughout at 295 k and 1.3107 m respectively. the values for maxt are 299 k, 304 k, 390 k, …, 374 k. figure 7. graph showing the positional errors of the light ray travelling through the gaussian temperature profile described in (1) when the spread of the profile (as determined via parameter σ) takes the values from 0.5 to 3.5 metres. the size of the domain is 3 m by 4 m in the x and y direction respectively. in each of these simulations there is one light-ray propagating through the geometry, with initial position (-1.5, 0) m and directional vector [ ]0 1, 0.6 .x = the variables mint and maxt are kept constant throughout the simulations taking the values 295 k and 314 k respectively. http://projects.npl.co.uk/luminar/the-project/ acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 89 [8] s. kyle, s. robson, l. macdonald and m.r. shortis. “compensating for the effects of refraction in photogrammetric metrology” 14th international workshop on acceleration alignment (2016) [online] available at https://indico.cern.ch/event/489498/contributions/2217509/ [last accessed on 11th december 2017]. [9] r. summan., s. g. pierce, c. n. macleod, g. dobie, t. gears, w. lester, p. pritchett, and p. smyth. “spatial calibration of large volume photogrammetry based metrology systems”, measurement 68 (2015), pp. 189-200. [10] comsol multiphysics® v. 5.3. www.comsol.com. comsol ab, stockholm, sweden. [11] american national standards institute b89.4.19-2006, “performance evaluation of laser-based spherical coordinate measurement systems”, american society of mechanical engineers, (2006). [12] k.m. nasr, b. hughes, a. forbes, and a. lewis. "determination of laser tracker angle encoder errors." in epj web of conferences, vol. 77, (2014) pp. 00002. edp sciences. [13] m. holá, j. hrabina, m. sarbort, j. oulehla, o. cíp, and j. lazar. "contribution of the refractive index fluctuations to the length noise in displacement interferometry." measurement science review 15, no. 5 (2015) pp. 263-267. [14] p.e. ciddor, "refractive index of air: new equations for the visible and near infrared." applied optics 35ti, no. 9 (1996) pp. 1566-1573. [15] j.a. stone and j.h. zimmerman, “index of refraction of air: vacuum wavelength and ambient conditions based on the ciddor equation”, national institute of standards and technology (nist) [online] available at: http://emtoolbox.nist.gov/wavelength/ciddor.asp [last accessed on 28th november 2017]. https://indico.cern.ch/event/489498/contributions/2217509/ http://emtoolbox.nist.gov/wavelength/ciddor.asp positional uncertainty in optical-based metrology systems in large volume manufacturing microsoft word 283-2051-2-le-rev acta imeko  issn: 2221‐870x  april 2016, volume 5, number 1, 36‐44    acta imeko | www.imeko.org  april 2016 | volume 5 | number 1 | 36 label free affinity sensing: application to food analysis simona scarano 1 , stefano mariani 1 , maria minunni 1,2   1 dipartimento di chimica “ugo schiff”, via della lastruccia 3‐13, 50019 sesto f.no (fi), italy   2 istituto nazionale di biostrutture e biosistemi, firenze, italy      section: review  keywords: affinity‐based biosensors; food analysis; surface plasmon resonance; piezoelectric; gmos; pathogens  citation: simona scarano, stefano mariani, maria minunni, label free affinity sensing: application to food analysis, acta imeko, vol. 5, no. 1, article 8,  april 2016, identifier: imeko‐acta‐5 (2016)‐1‐08  section editor: claudia zoani, italian national agency for new technologies, energy and sustainable economic development affiliation, rome, italy  received june 15, 2015; in final form december 22, 2015; published april 2016  copyright: © 2016 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  corresponding author: maria minunni, email: maria.minunni@unifi.it    1. introduction  food and diet are in close relation with human health, and analytical systems aiming to control food quality and safety are always welcome. food safety is a broader term, which means an assurance that food will not cause harm to the consumers when it is prepared and/or eaten according to its intended use as expressed by the codex alimentarius commission [1]. food safety is eventually a global issue that affects the health of populations in both industrialized and developing countries [2], becoming one of the highest priorities of public health at national and international level. food crises that have occurred over the last 20 years (bovine spongiform encephalopathy, dioxins, foot and mouth disease, etc), changing nutritional habits, new food production processes, increasing international trade and emerging risks, have led consumers to be more sensitive to food safety issues and risk managers to develop and strengthen a more effective food safety system. at a governmental level, food safety control for public health protection covers the range of different food chains related to a certain food product or product group, including all relevant producers, manufacturing sites and food service establishments within the country as well as those importing into the country. with the increasing international trade in food and the fact that manufacturing sites in one country may provide raw materials to other manufacturers or finished goods (products) for large numbers of consumers living in importing countries, it is critically important that there be a harmonization of food safety control procedures. the diagnostics industry has focused on the development of analytical methodologies endowed with high sensitivity, speed and portability. in this direction goes the development of fast, user friendly and eventually low cost devices with application to food industry, in field, in supermarkets or, ideally, at consumer places that could be used to obtain information about food freshness, food contamination, food safety, food origin, food content in certain compounds, i.e. allergens, gluten, etc. in this paper we will introduce biosensor devices and we will focus on label free and real-time affinity–based sensing. at this regard, some work will be reported using optical and gravimetric sensors for the detection of pesticides, genetically modified organisms (gmos), and pathogens. 2. biosensors  biosensors are devices exploiting biological reactions for detecting target analytes. such devices couple a biological recognition element (interacting with the target analyte) to a physical transducer, which then translates the biorecognition event into a useful electrical signal [3], [4] (figure 1). the biological elements immobilized on the sensor can be proteins abstract  in this paper real‐time,  label free sensing principle based on surface plasmon resonance (spr) and gravimetric,  i.e. quartz crystal  microbalance (qcm) are described and some applications to food analysis problems are reported. affinity receptors are introduced  and examples using antibodies, nucleic acid probes and biomimetic receptors i.e. molecular imprinted polymers (mip) are reported. in  particular pesticides, endotoxins, pathogens, genetically modified organism (gmos) detection and food origin analysis are reviewed.   acta imeko | www.imeko.org  april 2016 | volume 5 | number 1 | 37 (e.g., an antibody, a cellular receptor), nucleic acids or biomimetic receptors, i.e. aptamers or molecular imprinted polymers (mips). the biological element recognizes the ligand by affinity interactions such as immunoreactions, hybridization reactions, or protein/nucleic acids interactions. depending on the element of transduction, biosensors can be divided into electrochemical, optical, and piezoelectric biosensors, among others. the interaction between some of these biosensors and ligand can be monitored in real time without use of any label, i.e. through label-free detection. such interactions allow for monitoring the binding event and can be used to evaluate the relative kinetics. for this reason, label free biosensors have been employed for studies of affinity interactions. in particular, with such real-time and label free biosensors, it is possible to evaluate the rate constants for the binding (forward) reaction and dissociation (backward) reaction, represented as ka and kd, respectively; and ultimately obtain the affinity constants between the bioreceptor and the ligand expressed as ka and kd [5], i.e. association and dissociation constants, respectively. 2.1. spr‐based biosensors  in the field of affinity biosensors, one of the most popular successful transducer belongs to the biacore family, which since early 1990’s revolutionized the way to look at the affinity interactions, by means of coupling optical transduction based on the surface plasmon resonance (spr) with solid-state chemistry, for the immobilization of biomolecules on the surfaces of gold chip. over years, the technology of real-time and label-free detection has emerged as a useful and important tool for studying the affinity interactions, between nucleic acids, and nucleic acids and proteins [6]. a further advance in optical label-free sensing has been represented by spr imaging (spri) [7], also known as spr microscopy [8], first introduced by rothenhäusler and knoll in 1988 [9]. thenceforth spri has been applied to investigate interactions of biomolecules arrayed onto the chemically modified metal surfaces. likewise, spri can be made to a real-time, label free method, which additionally allows the simultaneous analysis of many interactions (up to hundreds), by using a ccd camera for signal detection. in such a way, both sensorgrams (i.e., signal vs. time) and spr images inferred from the arrayed surface can be recorded, which has important impact in high throughput analysis (figure 2) [10]. thus spri represents an interesting system for multi-analyte detection, allowing parallel analysis of many targets such as different contaminants, i.e. pathogens, pesticides, etc. in food matrices. from a physical point of view, spr biosensors detect optical signals generated as a consequence of the excitation of surface plasmons (sps) at the interface of a metal and a dielectric, either a liquid or air. sps are confined at the interface and they vanish at both sides of the metal surface. 2.2. piezoelectric‐based sensing  behind optical-based sensing [11], an interesting label free and real-time approach is provided by gravimetric sensors. in literature there is a wide number of papers based on commercially available or in house developed devices, using crystals ranging from 5, up to 10 or 30 mhz, using in qcm in the fundamental frequency or in the relative overtones [12]. piezoelectric quartz crystals are the basic elements of the quartz crystal microbalance (qcm) device. quartz resonators are the most used crystals, a crystal variant of silicon-dioxide (sio2). however, independently from the material uses, the term “piezoelectric” is derived from the greek word πιεζω (“piezo”) meaning “to press”. the first investigation on piezoelectricity was performed in 1990 by jacques and pierre curie [13], who observed that a mechanical stress applied to the surfaces of various kinds of crystal caused a corresponding electrical potential across the crystal, whose magnitude was proportional to the applied stress. the curies also verified the converse piezoelectric effect, in which application of a voltage across these crystals cause a corresponding mechanical strain. this causes a vibrational or oscillatory motion in the crystal, resulting in the generation of acoustic standing waves at a characteristic resonant frequency. the wave is called bulk acoustic wave (baw) or surface acoustic wave (saw) in the case of propagation through the substrate or on the surface, respectively. the quartz crystal microbalance is a bulk acoustic wave device based on the converse piezoelectric effect, in which a quartz crystal is sandwiched between two electrodes (figure 3). the resonant frequency of the quartz crystal depends on several parameters, such as size, density and cut. the most used devices employ at-cut quartz crystals. at-cut quartz crystals show a high frequency stability and a temperature coefficient close to zero between 0 and 50 °c [14]. figure 1. biosensor scheme: bio or biomimetic receptors are immobilized on the transduction surface and are responsible of the system selectivity. the binding with the analytes generates an analytical recordable signals whose nature depends on the transduction principle.  figure 2. spri technology. the receptor  is  immobilized on the sensor chip gold surface while the  ligand  is added  in solution. from the  interaction, a change in the signal is recorded, in label free and real time mode. parallel  analysis  on  different  spots  can  be  performed.  the  image  analysis  also visualizes changes on the different spots, providing spatial resolution to the sensorgram changes.    acta imeko | www.imeko.org  april 2016 | volume 5 | number 1 | 38 at-cut crystals oscillate in the thickness shear mode (tsm) [15]. the application of a voltage between the two electrodes causes a shear deformation of the crystal, which is maximized at the crystal faces, making the device sensitive to surface interactions. the resonant condition with the acoustic wave is satisfied by including the crystal into an oscillation circuit, where the frequency of the alternating potential difference applied to the electrodes matches the fundamental frequency of the crystal. the fundamental frequency depends upon the thickness of the wafer, its chemical structure, its shape and its mass [16]. since the oscillation frequency depends on the crystal mass, deposition of thin films on the crystal surface increases the resonator thickness and decreases the frequency in proportion to the film mass. measurements of the crystal frequency allow the detection of the film mass; therefore the device operates like a ‘microbalance’. the first quantitative investigation of the piezoelectric effect was performed by sauerbrey [17], who derived the relationship for the change in frequency ∆f (in hz) caused by the added mass ∆m (in g) in vacuum or in air, as reported in (1): ∆ 2 ∆ , 1 where f0 is the fundamental resonance frequency of unloaded quartz, µq is the shear modulus of at-cut quartz (2.947 × 1011 g cm-1s-2), ρq is the density of the quartz (2.648 g.cm-3) and a is the surface area in cm2. the sauerbrey equation assumes a uniform distribution of mass on the entire electrode portion of an at-cut quartz crystal. mass sensitivity decreases monotonically with the radius, in a gaussian manner becoming negligible at and beyond the electrode boundary [18]. another assumption of this equation is that the mass added or lost at the crystal surface does not experience any deformation during the oscillation: this is true for thin, rigid layers in vacuum or in air. for thicker, less rigid layers, as it happens for quartz crystals operating in liquid, a more complex theory is necessary. many factors such as density, viscosity, conductivity and dielectric constant of the liquid may influence the oscillating behaviour. when a quartz crystal oscillates in contact with a liquid, a shear motion on the surface generates motion in the liquid near the interface. the resonance frequency change of a quartz crystal having one face in contact with liquid is described by the kanazawa and gordon equation [19], (2): ∆ /  / / , (2) where ρl is the density of the liquid and l is the viscosity of the liquid. piezoelectric crystals have been used as microbalance and as a microviscometer owing to their small size, high sensitivity, simplicity of construction and operation, low cost, lightweight and the low power required [20]. 3. affinity sensing  in affinity biosensors, recognition of the analyte in solution by the immobilized biological element is based on an affinity reaction, that could be an antigen–antibody binding, a nucleic acid hybridization, or synthetic receptor/target recognition (molecular imprinted polymers (mips), aptamers, or artificial dna (xna)). biosensor selectivity primarily depends on the intrinsic features of the bioreceptor immobilized on the sensing surface. on the basis of these different interactions, the affinity biosensors can be divided into immunosensors, dna sensors, aptasensors, etc. an immunosensor uses the affinity interaction between an antigen and an antibody [21]. this interaction has been widely used in food analysis. a recent review reports some interesting application to a wide range of analytes using different transduction principles, both label free and not [22]. nucleic acid (na) sensors are based on oligonucleotidic probes and na hybridization reactions. in na sensors, artificial nucleosides (xna) oligomers have also been employed as bioreceptors for application to gmos detection [23]. xnas increase sensor stability by avoiding biodegradation or exploiting improved affinity of xna versus the target sequence compared with conventional dna probes. d’agata et al. reviewed works dealing with the great potential of artificial dna (pna, lna, hna, and morf) in nucleic acid spr-based sensing, exhibiting higher selectivity and sensitivity in detecting complementary or mismatching nucleic acid targets [24]. on the other hand, aptamers are single-stranded dna or rna (ssdna or ssrna) molecules that can bind to predetermined targets including small molecules, as proteins and peptides, with high affinity and specificity. such propensity helps code the name of aptamer (from the latin aptus fit, and greek µέροςpart) [25]. these nucleic acid aptamers have been engineered through repeated rounds of in vitro selection or equivalently, selex (systematic evolution of ligands by exponential enrichment). this class of binders plays a role in bioanalysis [26][28] as receptors in heterogeneous phase-based assays [29], [30], in biosensors, in affinity chromatography, in enzyme linked aptamer assay (elaa), in macroarrays [31], [32], as well as in solution as molecular beacons [33]. aptamers are obtained through a combinatorial method alternating steps of selection and amplification. initially, a combinatorial library of oligonucleotides (most typically 30 to 50 bases) is constructed and improved variants are identified through an iterative selection process performed in vitro, the selex process, which involves iterative cycles of selection and amplification (usually 12-18 cycles). during these cycles, the nucleic acid molecules with the highest affinity for the target are separated from the nonbinding species. the enriched library (1030 individual sequences) is finally cloned and sequenced, and the single sequence with the highest affinity for the target molecule is isolated. aptamers exhibit strong affinity and high specificity for a predetermined target. behind na aptamers, artificial combinatorial proteins, consisting of a variable peptide sequence inserted within a figure  3.  quartz  crystal  microbalance  (qcm):  the  crystal  is  sandwiched between two gold electrodes, on which the receptors are immobilized (left). the sensor is inserted in the measuring cell able to operate in static or flow mode (right).    acta imeko | www.imeko.org  april 2016 | volume 5 | number 1 | 39 constant scaffold protein, have been reported. in analogy to the nucleic acid counterpart, they are called peptide aptamers [34], [35]. 4. assay design  similar to other bioanaytical based assays, affinity sensing is generally a performer using two assay formats, direct and indirect, were mostly adepted on spr or qcm biosensor for the detection of analytes in food analysis (figure 4). the assay selection is based on the molecular weight (mw) of the analyte, on the bioreceptors/analyte affinity, and on the matrices under investigation. in the direct assay, bioreceptors are immobilized on the sensing surface and analytes are added in solution. this strategy is usually adopted for analyte with high mw, sufficient to develop a refractive index variation and resulting in detectable signal. indirect assays are preferred for low mw analytes where purified analytes (or conjugates) are immobilized on the sensor surface, while specific bioreceptors are incubated with the samples to be analyzed and injected to the sensing surface; when the solution reaches the surface, only the remaining free bioreceptors bind to immobilized analyte, providing a signal inversely proportional to its concentration in the sample. in both cases, i.e. direct and sandwich-based detection, secondary bioreceptors are exploited in multi-step assays to improve detection limits [21]. 4.1. pesticides  pesticides are relevant pollutants due to their large amounts released into the environment. for this reason, many countries have established maximum residue levels (mrls) in food products [36]. drinking water analysis can be achieved by spr sensing. the first example for sensitive, selective analysis of atrazine was early reported by our group where an indirect immunosensor was developed thanks to the availability of monoclonal antibodies (mabs) developed in mice; ppb levels of detection were achieved [37] with very good selectivity tested with terbutylazine and simazine. later other spr immunosensor examples were eventually reported detecting atrazine with a detection limit of 20 ng/l (ppt) for the optimized assay, and results validated by gas chromatography coupled to mass spectrometric detection (spe-gc-ms). another spr affinity sensor, still for atrazine, was realized by a direct detection system for herbicides, inhibiting photosynthetic electron transfer developed using the photosynthetic reaction centre (rc) from the purple bacterium, rhodobacter sphaeroides, immobilized on the chip and atrazine in solution [38]. atrazine was detected down to 1 μg/ml and, to evaluate the binding specificity to atrazine, chlorinated aromatic herbicides, (dcmu), n’(3,4-dichlorophenyl)-n,n-dimethylurea and 2-(2methyl-4-chlorophenoxy)propionic acid (mcpp) were investigated as well as other water pollutants, i.e. dioxins, polychlorinated biphenyls (pcb) [39][41]. similarly, competitive immunosensing for parallel analysis of endocrinedisrupting compounds (atrazine, 2,4-dichlorophenoxyaceic acid (2,4-d), 4-nonylphenol, and benzo[a]pyrene) to respective protein-analyte conjugates was realized [42], [43]. an analysis time of 45 min (including 30-min incubation of the sample with antibodies) and limits of detection as low as 0.05, 0.07, 0.16 and 0.26 ng ml−1 (ppb) were demonstrated for benzo[a]pyrene, atrazine, 2,4-d and 4-nonylphenol, respectively [43]. 2,4-d immunosenensing has been eventually set up with several mab clones set up using qcm transduction. biomimetic receptors were used as recognition elements, instead, when the dendritic nano-fibers based piezoelectric sensor was developed for the analysis of dichlorodiphenyltrichloroethane and hexachorobenzene prevalent together as organochlorinated pesticide residues in real samples (human blood serum, and river water samples) [44]. detection limits of dichlorodiphenyltrichloroethane and hexachlorobenzene were realized as low as 0.75 and 0.69 ng ml−1 (ppb) and linearity observed in the concentration ranges 5.0–150.0 and 5.0–75.0 ng ml−1, respectively. 4.2. endotoxins  endotoxins, chemically lipopolysaccharides usually found in food, environment, and clinical products of bacterial origin, are unavoidable ubiquitous microbiological contaminants [45]. an interesting review describes the application of different sensing principles including spr and qcm for the detection of endotoxins. detection of endotoxin is essential for quality control in biological products, medical devices, serological products, parenteral drugs, recombinant therapeutic products and food and water security. escherichia coli and salmonella paratyphi produce endotoxins causing sepsis, cholangitis, and neonatal meningitis. the contact with population may come from contaminated food and water. reports of reusable qcm based sensors for detection of staphylococcal enterotoxin a have been established [46]. endotoxin detection using qcm with a detection limit of 0.01 pg/ml has been accomplished [45]. most of the mentioned application deals with the use of antigen-antibody binding, i.e. immunosensing. however, as mentioned before, nucleic acid probes can also act as bioreceptors in affinity sensing targeting dna or rna sequences as analytes of interest. from late ’90 we have assisted to the increase of nucleic acid based sensors where a molecular probe is immobilized on the surface and the interaction with the complementary analyte leads to the hybridization reaction with resulting double helix formation. 4.3. genetically modified organisms (gmos)  an interesting problem studied by our group is the detection of genetically modified organisms (gmos). with the advent of biotechnology, new “products” have appeared such as gmos. this achievement has been realized by manipulating the dna molecule, by introducing new genes into a host, which would never naturally exchange the dna figure 4. schematic representation of different strategies for biomolecular recognition,  i.e. by direct and  indirect detection. both approaches can be based  on  single‐  or  multi‐step  assays,  with  or  without  the  presence  of labels.    acta imeko | www.imeko.org  april 2016 | volume 5 | number 1 | 40 with the donor. gmos are defined as living organisms whose genome has been altered in a way that does not occur naturally by mating or natural recombination. the modification involves the introduction of an exogenous sequence in the host genome either capable of expressing an additional protein that confers new characteristics, or able to hybridize specifically to target mrna, inactivating it (e.g gmos with delayed ripening). the foreign dna is “usually” inserted in a gene cassette consisting in an expression promoter (p), a structural gene (“encoding region”) and an expression terminator (t). one or more cassettes can be introduced into the host genome [47], figure 5. analytical approaches based on dna detection of gmos are based on the detection of markers of transgenosis, i.e. one of afore mentioned elements, the promoter, the terminator or the coding gene [48]. the gene cassette (one or more, i.e. cassettes) is built up by assembling some dna sequences, with a known function, coming from different species. so a cassette can be made by assembling bacterial or viral dna (generally for the promoter and terminator region) together with an eukaryotic dna codifying for the inserted gene conferring the desired characteristics. in patented gmos very established and stable promoters and terminators cassettes are generally cloned. the promoter of the subunit 35s of ribosomal rna of the cauliflower mosaic virus (p35s) and the terminator nos from agrobacterium tumefaciens are widely used for the production of many transgenic vegetables, such as soy roundup ready™, maize maisgard, and the tomato flavr savr. this has generated an interesting debate on the use, application, risk and analysis of gmos. for this, there is an increasing number of papers dealing with gmos detection, since many countries have strict legislation worldwide allowing only the presence of gmos within a certain threshold, and requiring labelling of transgenic organisms at all stages of their placing on the market. reliable and sensitive methods for gmos detection, from protein to dna based methods have been reported in the last decade. in particular, several biosensors were developed, using different transduction principles, over the last fifteen years, targeting the genetic sequences identified as markers of transgenosis, i.e. the promoter and the terminators ones. this work has been very nicely summarized in a recent review [49]. in particular spr [50], [51] and qcm-based sensing [52] have been developed by our group targeting both p35s and tnos sequences previously extracted, fragmented, and denatured [53], [54]. specific sequence detection has been achieved in pcr amplified samples and in genomic dna (10 ppm) by using as negative control the wild type genome (i.e. not genetically modified) of nicotiana glauca. the european union (eu) has elaborated a legislation for genetically modified (gm) food control, which establishes both the legal basis for the approval procedure of gmos and the post market traceability and labelling requirements for gmos and gmo-derived food and feeds [55], [56]. in this perspective, the application of biosensors to real matrices analysis for gmos detection has been successfully attempted, as well as the monitoring of the gm soya food processing (from seed to end product) in a pilot plan where the presence of target sequences in soya was followed over steps [57]. it should be noted that correct traceability of the transgenic events in food processing can be achieved only by monitoring target sequences during the whole manufacturing process, where the target analytes are followed from the initial source to the end product, in a systematic and controlled manner passing through a series of intermediate phases. in fact, false negatives can arise in processed food. several factors can negatively affect the detection of transgenes, on dependence of the considered matrices (mechanical procedures i.e. milling, thermal treatments, chemical degradation, presence of inhibitors of the pcr reaction, etc.). moreover, the use of harsh conditions in the processing steps could impact on the integrity of genomic dna, which represents the starting point of the whole analysis [58]. it is thus of primary importance to verify the possibility to amplify (amplificability) the extracted dna. the dna degradation can be caused, for example, by heating treatments [59] and exposure to ph changes [60]. furthermore different inhibitors of the taq polymerase enzyme such as proteins, fats, polysaccharides, polyphenols, and other compounds that may be present in dna may inhibit the dna template amplification [61]-[64]. to manage these problems, the traceability of several selected dna fragments differing in length, mapping on the inserted cassette, was studied along a complete industrial soybean processing chain (figure 6). in particular, samples were collected from the different levels of an industrial soybean processing plant working with 80 % gm roundupready-soybean. the genetic modification is the roundup ready-soybean event gts 40-3-2 (notification c/uk/94/m3/1 of monsanto). eventually, dna extraction procedures from the different matrices and pcr amplification of selected fragments differing in length (figure 7) were set up. as expected, the amplificability of the fragments was inversely related to their size since the degree of template dna integrity affected the success of the pcr reaction, especially in the case of long amplicons. the qcm affinity sensor, carrying a p35s immobilized probe on the sensor chip, was able to detect the 195 bp fragment (p35s) complementary sequence in the samples selected along the complete industrial soybean processing chain. this sequence was confirmed to be present amplified in the samples by end-point pcr analysed in parallel by gel electrophoresis. more recently, other authors reported about optical based sensing for ultrasensitive genomic dna detection of transgenes sequences. in particular spr imaging (spri) [65], the evolution of conventional spr instrumentation, with the advantage of parallel analysis of many targets, has been applied, or by optic fiber confirming the possibility of detecting directly, by passing   figure 5. the insertion of exogenous genes into the host genome comprises an element called promoter (p) allowing the expression of the coding region (cr)  containing  the  modification  i.e.  gene  or  sequence,  with  desired  characteristics  to  be  introduced  into  gmos.  finally  a  terminator  (t)  is added.  these  three  elements  p,  cr  ,  t    are  defined  “gene  cassette”.    in gmos one or more gene cassettes can be present.    acta imeko | www.imeko.org  april 2016 | volume 5 | number 1 | 41 the amplification step, gmos using label free, real time nucleic acid-based sensing [23]. 4.4. detection of food origin  species-specific sequences can be targeted for the identification of food origin. modern methods for meat identification are based on dna analysis. this is the case of a study conducted on highly repeated sequence called satellite 13, present in bos taurus [66]. in the bovine genome, eight highly repetitive and several minor repetitive sequences have been detected comprising a total of 27 % of the dna [67]. they allow species-specific dna sequence identification, which has some advantages over protein analysis [68]. molecular biology methods allow the determination of dna also in heat-treated nourishment and are, therefore, suitable for the identification of species-specific dna in meat and bone meal and concentrate mixture [69]. dna analysis discriminates between related species, such as sheep and goat or chicken and turkey [70]. by qcm based–sensing it is possible to detect highly repetitive and species-specific dna sequences present in bovine (b. taurus) genomic (non-amplified 10 µg/ml dna) dna, extracted from meat samples. the selectivity of the sensor was tested with porcine (sus scrofa) dna, and no interaction was observed. similar results were obtained by spr sensing using the same microsatellite sequences [54]. the system is able to discriminate between bovine and pork meats in real samples. this approach can be transferred to any other sequence since the system selectivity relies on the probe immobilized on the sensor surface. 4.5. microbial contamination  ideally, the success of food safety management should be reflected in the health status of the population concerned. epidemiological data show that major outbreaks of food safety problems persist, e.g., the contamination of ground turkey by salmonella enterica serotype heidelberg in the usa (us centers for disease control and prevention, 2011) and the contamination of sprouts by escherichia coli o104 in europe [2]. moreover, the most recent data on zoonoses, collected in the european union in 2011, showed that confirmed cases of human campylobacteriosis were reported and this number has followed a significant increasing trend in the last 4 years. moreover, verotoxigenic escherichia coli human cases has been increasing in the eu since 2008. the number of salmonellosis cases in humans decreased in 2011 by 5.4 % compared with 2010, but still 95,548 confirmed human cases were reported. interesting work has very recently been reported using molecular imprinted polymers for e.coli detection by qcm and spr sensing. amino acid based recognition element, nmethacryloyl-l-histidine methylester (a polymerizable form of histidine) was used for mip synthesis [71]. response times (time required to reach 95 % of steady state) were calculated as 113 s for spr and 56 s for qcm so both systems are very rapid compared to the classical bacterial culture methods which may take up to 7–8 days for the answer. the limit of detection (lod) and the limit of quantification (loq) of the qcm system were found as 3.72 × 105 cfu/ml and 1.24 × 106 cfu/ml, respectively. lod and loq of spr system were also calculated and were found as 1.54 × 106 cfu/ml and 5.13 × 106 cfu/ml, respectively. spiked apple juices samples were reported. lods for bacterial detection e. coli, salmonella thyphimurium, span from 102 cfu/ml to 108 cfu/ml as indicated by these authors. affinity interactions are also involved in the case of mannose-containing oligosaccharides with the fimbrial lectin of e. coli and this property can be used for designing novel biosensors. modified carbohydrate ligands figure 6. sampling plan along the complete  industrial soybean processing chain. samples were collected  at pilot industrial plan with the help of dr. r. onori, istituto superiore di sanità‐ iss, rome, italy.  figure 7. fragments selected for the traceability of the transgene along the industrial  soybean  processing  chain,  spanning  from  118  bp  to  1006  bp  in length, allowing to evaluate the optimal length to be traced in the different resulting matrices (upper). qcm‐base sensing detection of p35s sequence (195  bp)  in  the  samples  collected  along  a  complete  industrial  soybean processing (lower).    acta imeko | www.imeko.org  april 2016 | volume 5 | number 1 | 42 were synthesized and immobilized onto gold electrodes and spr surfaces. a detection limit of 1 cfu/ml was reported for this bacterium. the relative selectivity of these ligands for e. coli, citrobacter freundii, were 100 %, 2.6 %, and 8.6 % respectively. the biosensor was validated using spinach leaves at 3.0 cfu/ml [72]. immunosensing has been reported using spr for e.coli o157 h7, salmonella enteritis and lysteria monocytogens detected in culture media [73]. lods were 0.6 ×106, 1.8 × 106, and 0.7 × 107 cfu/ml, respectively, in the presence of non-target pathogens at concentrations of 105 to 108 cfu/ml. displacement assay applied to qcm for listeria was also reported with a working range between 105107 and applied in milk (2 % fat) [74]. using nucleic acid qcm sensing, bacterial detection can be also achieved as demonstrated for staphylococcus aureus (mrsa) and antibiotic resistance gene can be eventually targeted i.e. gene meca, codifying for an anomalous protein [75]. spr nucleic acid sensing using a sandwich format for inva gene of salmonella thyphimurium, using both dna and aptamer probe with streptavidin (sa) as mass enhancer, has been very recently reported [76]. this strategy was successfully applied to the determination of salmonella at levels as low as 60 cfu/ml. comparing to salmonella, no significant response was obtained when e.coli, s. aureus and s. pneumoniae were tested using the same assay. furthermore, the sensor chip could be regenerated 200 times. 5. conclusions  affinity sensing can be achieved using different bioreceptors immobilized on the chip. most frequently used are antibodies (or antigens) and thus immunosensors are developed. since early ’90 nucleic acid sensing has appeared as an interesting approach for target sequence detection, and more recently other biomimetic/synthetic receptors (aptamers, mip) are used to account for harsh conditions where bioreceptos may be not the optimal choice. here we reported about two popular detection principles in label free and real time sensing which can be indifferently used as demonstrated in provided examples. this paper is far from exhaustive since vast literature is available, but the aim was to provide some applications for different classes of analytes of 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[76] p. lei, h. tang, s. ding, x. ding, d. zhu, b. shen, q. cheng, y. yan, determination of the inva gene of salmonella using surface plasmon resonance along with streptavidin aptamer amplification, microchim. acta 182 (2015) pp. 289-296. preserving the history of science – journey of old psychological instruments acta imeko issn: 2221-870x october 2018, volume 7, number 3, 117 120 acta imeko | www.imeko.org october 2014 | volume 7 | number 3 | 117 preserving the history of science – journey of old psychological instruments oliver tošković laboratory for experimental psychology, faculty of philosophy, university of belgrade,serbia section: technical note keywords: old scientific instruments; psychological laboratory; experimental psychology; history of science citation: oliver tošković, preserving the history of science – journey of old psychological instruments, acta imeko, vol. 7, no. 3, article 18, october 2014, identifier: imeko-acta-03 (2014)-01-18 editor: sabrina grassini, politecnico di torino, italy received february 25, 2018; in final form september 20, 2018 published: october 2014 copyright: © 2014 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: this work was supported by ministry of education and science of republic of serbia, project number on179033 corresponding author: oliver tošković, otoskovi@gmail.com 1. birth of a new scientific discipline at the end of the ninetieth century, approximately at the same time, two new laboratories were formed, in leipzig by wilhelm wundt and in chicago by william james. this was the beginning of a new scientific discipline – psychology. problems that were investigated in these newly formed laboratories were sensory perception and thinking. these topics were not new in the history of science; they were borrowed from physics and philosophy [1]. problems of sensory perception often interlaced with those studied in physics. in that sense optics is related to vision, acoustics to hearing, and thermodynamics to the perception of heat… in the past researchers did not separate those two classes of problems. for instance, euclid and ptolemy dealt with space perception, kepler with function of eye lens, galileo with link between sound frequency and tone pitch, newton investigated relationship between light wavelengths and perceived color… but, since physics was formed as independent science, most physicists defined their aims in describing an objective, so called, outside world. in this sense, subjective reality, how these physical phenomena are related to our sensations, was out of their focus. most of them would be interested in describing structure and nature of light, but they would not be interested in explaining why we perceive only wavelengths approximately between 400nm and 700nm. this way some problems are left unattended and a gap for a new discipline was created. that discipline was psychology [3]. discovering relationship between the structure of physical reality and our experiences of it was the aim of first psychologists. so, why is it that we can only see certain range of light wavelengths, while some other animals perceive other wavelengths? does it have something to do with adaptation to conditions in which we live? why do we perceive a mixture of two colors as a new color, while mixture of tones as a chord? why some chords sound harmonious and others do not? how do we perceive distances of objects if we only have two dimensional images on our retina? how come that we sometimes perceive physically equal lines as different? why some chemically similar substances taste so different, while some chemically abstract creating of collection of old scientific instruments of laboratory for experimental psychology, faculty of philosophy, university of belgrade is an attempt to preserve a part of history of science in serbia. there are around 100 instruments in collection, which mostly came to belgrade within german war reparations to kingdom of yugoslavia, after the world war i. most of the instruments were made in workshop of e. zimmermann, precise mechanic of the first psychology laboratory in the world, founded in 1879 by wilhelm wundt in leipzig. they can be grouped on those aimed for examining visual and auditory perception, memory and learning, kimography and ergography and those designed for investigating emotions. together with books and journals from 19th and beginning of 20th century, instruments create an ensemble based on which it is possible to reconstruct one psychological laboratory from the very beginning of development this scientific discipline. mailto:otoskovi@gmail.com acta imeko | www.imeko.org october 2014 | volume 7 | number 3 | 118 different tastes pretty much the same? how come that we adopt easily to smell and almost never to pain, and what are the boundaries of our senses? all these questions lead us to more general one, how do we get to know external world which we live in, and what is the link between physical reality (stimulation), physiology and our consciousness? the other set of problems was inherited from philosophy. from aristotle to modern times, various philosophers were dealing with structure of concepts and judgments. they tried to develop formal axiomatic systems of mathematical logic and to establish rules of valid thinking and reasoning. one of the basic questions in this approach would be under what conditions we can infer certain conclusions from some premises [3]. on the other hand, psychologist shifted the focus from how thinking should work to how thinking actually works. what are the processes which occur between asking questions and giving answers? how do we recognize and understand letters, word and sentences? how do we learn, memorize and forget? what are the types of learning and how do they occur? how memory is structured and where is it located? how does the process of forgetting developing through time and how long does memory last? basic method for investigating above mentioned questions was taken from natural sciences. researchers used experiment as basic method, trying to evoke and examine certain phenomena in controlled conditions. only novelty in those experiments was that they required human or animal participants. this kind of approach required measurements, which further on asked for new instruments. this need for measuring and searching for answers on abandoned questions led to designing new, psychological instruments. at the beginning, researchers used existing instruments to measure and they were borrowed mostly form physics and physiology. during research of perception there was a need to generate various stimuli and record participants answers. for instance, they could generate precise optical or acoustic signals and record time taken to detect or recognize them. sometimes emotional responses were measured through various physiological instruments. as research questions were multiplied instruments were often modified to fulfill new needs. also, some new instruments were designed and used, since existing ones could not provide adequate measures. this process asked for institutionalization and first psychological laboratories were formed [1]. 2. measures of the mind – first laboratories as it was previously said, approximately at the same time two psychological laboratories were formed, in chicago by wilhelm james and in leipzig by wilhelm wundt [1]. both of them, james and wundt had philosophical background, bringing lots of above mentioned questions. beside that wundt had education from medicine. since first published paper, showing results of psychological research, came from wundt’s laboratory, we consider that first psychological laboratory was founded in 1879, in leipzig (figure 1). first printed article from psychological laboratory was published by max friedrich in the first issue of wundt’s journal “philosophical studies” (“philosophische studien”). in leipzig laboratory many significant names in the history of psychology worked as wundt’s junior associates. some of them were oswald külpe, emil kraepelin, stanley hall and james cattell [4]. few years after founding laboratory moved into newly built university premises. it contained 15 rooms and 2 lecture halls, half of which was intended for experimentation. other rooms were library, conference room, offices, workroom and wardrobe. most of the topics to be researched came from wundt, which he tried to formulate according to interests of his senior associates. in this facility various instruments were settled. some of them were used just for demonstrations while others were used in research. devices that were used in psychophysical research could generate regular and measurable stimuli for different senses since they were used to investigate intensity of sensations. these devices were gravitational phonometers, photometers, pressure scales and so on [6]. in acoustic studies sets of tuning forks were used, devices for overtones, for sounding chord with amplifiers… in visual experiments researchers used device for extracting spectral components and various so-called color-mixers (such as helmholtz color-mixer). also, various instruments for physiological measure were used, primarily to track physiological correlates of emotions. they were used for tracing the oscillations of pulse, breading and blood pressure volume – plethismographic, sphygmographic and pneumographic recording devices. chronometers were quite often used since many psychological experiments included, and they still include, reaction time. under the term reaction time we consider a period form presentation of certain stimuli until participant’s reaction to it and it is important sine enables researchers to track time course of psychological processes and even sometimes differentiate between them. some instruments were used to generate auditory signals at certain time intervals, or to present stimuli at some limited time frame, such as tachistoscopes. final group of instruments consisted of devices for study of processes of recognition and recall. they are called mnemometers. during first 30 years of its existence wundt’s laboratory published more than 100 articles. main topics in those articles were problems of classical fechner’s psychophysics, visual and acoustic studies, and researches of smell, taste and touch, perception of time, attention, memory, emotions and experimental aesthetics. most of those articles were published in already mentioned journal “philosophical studies”, which was renamed into “psychological studies” in 1905 [6]. wilhelm wundt’s laboratory had enormous influence on laboratories all over the world, and we can say that it disseminated psychological research which was born in leipzig. one of the most famous examples of this dissemination was laboratory of experimental psychology which was founded at cornell university by one of wundt’s best-known student, edward titchener [5]. figure 1. interior of a laboratory room. acta imeko | www.imeko.org october 2014 | volume 7 | number 3 | 119 3. living in oblivion – belgrade laboratory professor borislav stevanović received his phd in 1926 in experimental psychology from king’s college, london [2]. his dissertation was on cognitive processes of reasoning. one year later, in the faculty of philosophy in belgrade, department of psychology was formed and prof. stevanović becomes assistant professor. at that time, university of belgrade granted books and equipment from the war reparation funds, after the world war i. since germany was giving goods to yugoslavia through war reparation funds, prof. stevanović decided to order scientific equipment catalogues containing instruments form the first psychological laboratories in the world. based in those catalogues he made a list and wrote a letter to the dean of faculty of philosophy, prof. veselin čajkanović in which he asks a certain number of instruments, valued at 4994 reichsmarks (figure 2). this letter and wish to provide psychological instruments probably indicates intention of prof. stevanović to form laboratory at the faculty of philosophy in belgrade. his intentions were probably not to establish specialized laboratory for the study of certain area, such as perception, but rather to create laboratory which would deal with various aspects of experimental psychology. equipment that prof. stevanović ordered can be grouped into devices for investigating auditory and visual perception, memory and learning, kymographic and ergographic recording and those for studying emotions. all these instruments did arrive in belgrade, but unfortunately laboratory was not formed at that moment. many years later, in year 1974, prof. predrag ognjenović establishes laboratory for experimental psychology, at the department of psychology, faculty of philosophy at university of belgrade. instruments were at the faculty for more than 40 years, but traces of their origin were lost, and they were threatened by oblivion [7]. journey through space and time collection of old scientific instruments of laboratory for experimental psychology after creating laboratory for experimental psychology in belgrade, prof. ognjenović collects instruments and moves them into new laboratory. in 1982 prof. aleksandar kostić establishes collection of old scientific instruments of laboratory for experimental psychology, faculty of philosophy, university of belgrade and instruments finally become a part of history of psychological science in serbia and world. in 1992 collection became part of the association of museums of science and technology of serbia and instruments were granted that status of state-protected cultural asset [2]. forming the collection was only the beginning of reconstruction, which lead to new discoveries and questions. at the time when collection was formed, many instruments were in poor condition, affected by rust, and most of them had some parts missing. purpose of some instruments was hard to guess. based on catalogues ordered by prof. stevanović, which were kept in the library of the department of psychology in belgrade, and with help of prof. dejan todorović, prof. kostić managed to reconstruct some instruments and to discover their purpose. other instruments were reconstructed with the help of professors from the department of physics since they contained parts from old physical instruments. for some instruments purpose could only be guessed, since they were not listed in catalogues [7]. on the other track, in archive documentation, letter of prof. stevanović in which he orders instruments was found. this led to discovery of how instruments ended up in belgrade and prof. stevanović intention to create psychological laboratory at the beginning of 20th century. after the creation of collection of old scientific instruments of laboratory for experimental psychology, most of the instruments were reconstructed and their purpose is known. also, their unusual journey through space and time was discovered, and we found their origins. 4. thrill of discovery – instruments in the collection at present collection of old scientific instruments of laboratory for experimental psychology includes sixty instruments from various areas of experimental psychology (figure 3). figure 2. list of instruments with prices made by prof. borislav stevanović based on catalogue of zimmermann company from leipzig. figure 3. old psychological instruments: ranschburg’s mnemometer (upper left), hand ergograph after klemm (down left), chronometer after schultze (right). acta imeko | www.imeko.org october 2014 | volume 7 | number 3 | 120 most of the instruments in the collection were made in workshop of e. zimmermann, precise mechanic of the first psychology laboratory in the world, which was founded in year 1879 by wilhelm wundt in leipzig. other instruments were made by various manufacturers such as diel, boulitte, palmer and marx & berndt. all instruments within collection can be grouped on those aimed for examining visual and auditory perception, memory and learning, kimography and ergography and those designed for investigating emotions [2]. research of cognitive processes often required measuring of reaction time and controlled presentation of stimuli. therefore, instruments for investigating learning and memory and associations include chronometers, mnemometers and tachistoscopes. probably most valuable instrument in the collection is chronometer after schultze (figure 3), often used along with ranschburg’s mnemometer (figure 3). for instance, chronometer was used to measure participant’s reaction time to certain stimuli, presented briefly by mnemometer. studying perception requires controlled generation of visual, auditory, tactile and other types of stimulation. these instruments include stereoscopes for investigating depth perception, perimeters for measuring visual field, tuning forks and acumeter for auditory sensitivity, esthesiometers for measuring tactile sensitivity and colour mixers (figure 4). such mixers were, for example, used to rotate a two-coloured disc, which would be perceived as painted in neither of those, but in some third colour (a mixture of previous two). some fields of psychological research, such as experimental phonetics and the study of physiological parameters, included tracing time variations. in these fields kimography was widely applied. various kymographs and chronometers constitute this part of the collection. as experimental research in psychology advanced, some practical applications of their results became possible. therefore, instruments for study of fatigue and manual skills were invented and they are also included in the collection. those instruments are such as mcdougal-schuster apparatus, tremometer, tappingtest device, ergographs and sinusoid after bonnardel. and the last part of the collection includes general purpose devices, such as reaction keys, commutators, electric circuit switches, metronomes, instruments for some physiological parameters such as pneumographs, and anthropometric compasses. they are applied in the study of various psychological phenomena and therefore are not limited to any specific domain. together with books and journals from 19th and beginning of 20th century, which are situated in the library of the department of psychology, instruments create an ensemble based on which it is possible to reconstruct one psychological laboratory from the very beginning of development this scientific discipline. looking into the ways of function of these old instruments, their construction solutions and ways of making them, give us insight into spirit of one epoch, but they also tell us a story about beginnings of psychology as empirical science and its standards. through these instruments we can forebode ways of thinking of scientists from the end of 19th century; we can hear them asking questions and solving problems. if we listen carefully, we will feel that they always tell us the same story on adventure of seeking and thrill of discovery. references [1] e. j. haupt, laboratories for experimental psychology, in wilhelm wundt in history. r. w. rieber, d. robinson (editors). springer us, 2001, isbn 978-1-4613-5184-9, pp. 205-250. [2] a. kostić, d. todorović, sense, mind and measure, museum of science and technology, belgrade, 1997, isbn 86-82977-02-8. [3] m. kusch, psychological knowledge: a social history and philosophy, routledge, london, 2005, isbn 9781134738687. [4] d. k. robinson, reaction-time experiments in wundt’s institute, in wilhelm wundt in history. r. w. rieber, d. robinson (editors). springer us, 2001, isbn 978-1-4613-5184-9, pp. 161-204. [5] e. b. titchener, the equipment of a psychological laboratory, the american journal of psychology, 11 (1900) pp. 251-265. [6] w. wundt, the institute for experimental psychology at leipzig, psychological studies, 5 (1910) pp. 279-293. [7] o. tošković, ghost in the shell collection of old scientific instruments of laboratory for experimental psychology, proc. of 3rd imeko international conference on metrology for archaeology and cultural heritage, oct. 23-25, 2017, lecce, italy, pp. 24-28. figure 4. colour mixer – instrument for investigating perception of colours. microsoft word 319-2202-1-le-rev acta imeko  issn: 2221‐870x  april 2016, volume 5, number 1, 59‐63    acta imeko | www.imeko.org  april 2016 | volume 5 | number 1 | 59  pressure gauge calibration applying 0‐a‐0 pressurization to a  reference gauge  hiroaki kajikawa, tokihiko kobata  national metrology institute of japan (nmij), national institute of advanced industrial science and technology (aist), central 3, 1‐1‐1  umezono, tsukuba, ibaraki, japan      section: research paper   keywords: calibration; pressure gauge; hysteresis  citation: hiroaki kajikawa, tokihiko kobata, pressure gauge calibration applying 0‐a‐0 pressurization to a reference gauge, acta imeko, vol. 5, no. 1, article  11, april 2016, identifier: imeko‐acta‐05 (2016)‐05‐11  editor: paolo carbone, university of perugia, italy; section editor: marco tarabini, politecnico di milano, italy  received february 3, 2016; in final form april 8, 2106; published april 2016  copyright: © 2016 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  corresponding author: hiroaki kajikawa, e‐mail: kajikawa.hiroaki@aist.go.jp    1. introduction  electromechanical pressure gauges are essential for pressure measurement and control in various industrial applications, and also for pressure calibration. although pressure balances are the first choice as reference devices for pressure calibration because of their high resolution and longand short-term stabilities, expertise is necessary to operate them correctly and efficiently. to make pressure calibrations easier, high-accuracy electromechanical pressure gauges are becoming more commonly used as reference devices. when pressure gauges are used as the reference device, several characteristics should be evaluated in advance including repeatability, hysteresis, long-term shift, and environmental effects. we have been focusing on the time-dependent behavior of pressure gauges and the effect of pressurization on the outputs [1], [2]. the effects of the pressurization procedures on the calibration results have been quantitatively evaluated for stepwise pressurization (figure 1(a)) [1], and for 0-a-0 pressurization (figure 1(b)) [2]. pressure gauges are typically calibrated by stepwise pressurization [3], [4]. when a pressure gauge is used as the reference device, the reference gauge is calibrated in advance against a pressure balance by stepwise pressurization, and then used for calibrating other gauges (figure 2(a)). although this figure  1.  pressurization  procedure  for  pressure  gauge  calibration.  (a) stepwise pressurization, (b) 0‐a‐0 pressurization.  time p re ss ur e pmax 0 (decreasing)(increasing) time pmax 0 p re ss ur e (decreasing)(increasing) (a) stepwise pressurization (b) 0−a−0 pressurization abstract  a calibration method is proposed to improve pressure calibration that uses a pressure gauge as the reference device. the reference  gauge is pressurized by a 0‐a‐0 pressurization procedure, whereas the test gauge can be pressurized through various procedures. the  calibration results with this method were consistent with those calibrated against a pressure balance. the method is expected to help  develop low‐cost pressure calibration systems that are more precise.   acta imeko | www.imeko.org  april 2016 | volume 5 | number 1 | 60  conventional system is simple and useful, the reproducibility of the reference gauge may be insufficient in some cases because of the effects of pressurization procedures. first, even with a similar stepwise procedure, differences in the pressurization procedures, such as time intervals and the presence or absence of preliminary pressurization, can affect the reference gauge output and the calibration results of the test gauge. second, the test gauges are used under various pressurization conditions in industrial sites, not always with stepwise pressurization. some users want their gauges calibrated using a pressurization process corresponding to the actual conditions of use. one way to overcome the possible effects of the different procedures and to cope with user requirements is to prepare reference pressure gauges that have much higher accuracy and reproducibility or lower hysteresis than the test gauges. when higher-grade pressure gauges are not available or applicable, the hysteresis characteristics of the reference gauge need to be evaluated in detail to compensate for the effects of different pressurization procedures, increasing the workload of calibration laboratories. in this study, a calibration method [5] is proposed for improving reproducibility and making calibration with reference pressure gauges more precise and useful. in this method, applying 0-a-0 pressurization to the reference pressure gauge eliminates the effect of the pressurization procedure on the reference gauge, and enables us to calibrate test pressure gauges precisely with various pressurization procedures. the advantages of using 0-a-0 pressurization in pressure gauge calibrations are summarized in section 2. the crucial factors for obtaining reproducible results are also discussed, and then our calibration method using 0-a-0 pressurization is explained. demonstration experiments are presented to confirm the effectiveness of the method. the experimental scheme and results are shown in section 3, followed by a discussion of the advantages and applications of the new method in section 4. the findings are summarized in section 5. 2. new calibration method  2.1. dissemination process using 0‐a‐0 pressurization  our method using electromechanical pressure gauges was developed to disseminate pressure standards with small uncertainty for use in industry. figure 2 shows dissemination processes of the pressure standard to industrial users with the pressure gauges. figure 2(a) shows a conventional process using stepwise pressurization. figure 2(b) shows the new process using 0-a-0 pressurization. in both the processes, pressure gauge a is calibrated against a pressure balance in advance, and then used as the reference for calibration of pressure gauge b. there are two key points in the new process. first, 0-a-0 pressurization is used to calibrate the reference pressure gauges from the pressure balance. second, different pressurization procedures are used for the reference and test pressure gauges in the next calibration of the test gauges. these points are explained in detail in the following subsections. 2.2. advantages in 0‐a‐0 pressurization  an advantage in using 0-a-0 pressurization is the higher reproducibility of calibration results irrespective of pressurization history, for example, the order of calibration pressure, interval between the calibration cycles, waiting time at each calibration pressure, and the presence or absence of a preliminary pressurization [2]. figure 3 shows calibration results for clamped thin-film pressure sensors (s-10, wika). the calibration results with the stepwise (figure 1(a)) and 0-a-0 (figure 1(b)) pressurization are shown for two pressure sensors using different materials for the sensing element, and the results for sensor a1 are shown in figure 3(a) and those for a2 are shown in figure 3(b). for both procedures, the maximum pressure in a calibration cycle was set at 100 mpa, 70 mpa, or 50 mpa. the relative deviation of the sensor's output from the standard pressure applied by a pressure balance is shown. the hysteresis, which is the difference between the results for the pressure increase and decrease, was much smaller with the 0-a-0 pressurization than with the stepwise pressurization. the results with 0-a-0 pressurization almost fell on a single calibration curve, although the data scattering was slightly larger at lower pressures. even when the pressure point was changed in a random sequence instead of in sequential order, the calibration results also fell on the same calibration curve created in sequential order. similar results were obtained for quartz bourdon-type pressure transducers [2], for which the data scattering, defined as the relative standard deviation of three cycles’ data, was several parts per million. the offset correction using the measurement data at the atmospheric pressure just after the measurement at each target pressure is a key to achieving high reproducibility. in addition, the operations and time intervals between the measurements at the target pressure and at atmospheric pressure need to be fixed precisely. using a fully automated calibration system in an figure 2. dissemination process of the pressure standard using pressure gauges: (a) conventional process using stepwise pressurization, (b) new process  using 0‐a‐0 pressurization for the reference pressure gauge.  acta imeko | www.imeko.org  april 2016 | volume 5 | number 1 | 61  unattended environment reduced changes in ambient conditions and operations, and provided reproducible results for the respective pressurization procedures. 2.3. new calibration method applying 0‐a‐0 pressurization to  reference gauge   to explain the new calibration method, we discuss a test gauge calibrated with the stepwise pressurization in this section. figure 4 shows a schematic of the pressures applied to the reference and test pressure gauges. in the conventional method (figure 4(a)), the same pressure is always applied to both the reference and test gauges. in the proposed method (figure 4(b)), the two gauges are pressurized with different procedures; the test gauge is pressurized with the stepwise procedure, as in figure 4(a), whereas the reference gauge is pressurized with the 0-a-0 procedure. the pressure lines for the two gauges are isolated from each other by closing the valves between the two gauges while the pressure is changed and adjusted to the target pressures, and then the gauges are connected by opening the valves when the measurement data are obtained at the target pressure. the new calibration method can be implemented easily by using a pressure controller and by switching the opening and closing of the valves between the two gauges. moreover, this method is applicable to various kinds of pressurization procedures for test gauges other than stepwise pressurization. the appropriate procedure can be selected according to the needs of users and the actual usage of test pressure gauges in industry. 3. demonstration experiments   3.1. demonstration experiments   demonstration experiments were conducted to check that the correct calibration results were obtained with the new method. the test pressure gauge was a foil strain gauge (p3mb, hbm). the test gauge was calibrated with method i or ii, and then the calibration results were compared. in method i, the test gauge was calibrated against a pressure balance. in method ii, the same test gauge was calibrated by using the new method. two clamped thin-film sensors, the calibration results for which are shown in figure 3, were used as the reference gauges. the two gauges were calibrated in advance against a pressure balance with the 0-a-0 pressurization (figure 2(b)). figure  4.  pressurization  procedures  for  reference  and  test  gauges  during  calibration.  (a)  conventional  method,  (b)  proposed  method.  for  both  procedures, the test gauge is calibrated by stepwise pressurization. timing  of the measurements is shown by open circles for the reference gauge, and  filled circles for the test gauge.  figure 3. calibration results for two clamped thin‐film pressure sensors with maximum pressures of 50 mpa, 70 mpa, or 100 mpa. (a) pressure gauge a1, (b)  pressure gauge a2. results for stepwise and 0‐a‐0 pressurization procedures are compared.   time p re ss u re test gauge reference gauge 0 time p re ss u re test gauge reference gauge 0 (a) (b) 10 20 30 40 50 60 70 80 90 100 −2000 0 2000 pmax = 100 mpa pmax = 70 mpa pmax = 50 mpa pressure [m pa] r el at iv e d ev ia ti o n f ro m s ta n d ar d [1 0− 6 ] (increasing) (decreasing) <0−a−0 p ressurization> 10 20 30 40 50 60 70 80 90 100 −2000 0 2000 pressure [m pa] <step wise p ressurization> pmax = 100 mpa pmax = 70 mpa pmax = 50 mpa r el at iv e d ev ia ti o n f ro m s ta n d ar d (increasing) (decreasing) [1 0− 6 ] 10 20 30 40 50 60 70 80 90 100 −6000 −4000 −2000 0 2000 pmax = 100 mpa pmax = 70 mpa pmax = 50 mpa pressure [m pa] r el at iv e d ev ia ti o n f ro m s ta n d ar d [1 0− 6 ] (increasing) (decreasing) <0−a−0 p ressurization> 10 20 30 40 50 60 70 80 90 100 −6000 −4000 −2000 0 2000 pressure [m pa] <step wise p ressurization> pmax = 100 mpa pmax = 70 mpa pmax = 50 mpa r el at iv e d ev ia ti o n f ro m s ta n d ar d (increasing) (decreasing) [1 0− 6 ] (a) pressure gauge a1 (b) pressure gauge a2 acta imeko | www.imeko.org  april 2016 | volume 5 | number 1 | 62  3.2. stepwise calibration with different maximum pressures  in the first demonstration experiment, the test gauge was calibrated with the stepwise pressurization for maximum pressures pmax of 70 mpa and 100 mpa. we expected that the results for the pressure decrease would depend on the maximum pressure. the results for the test gauge with the two methods are compared in figure 5. for the both conditions at the maximum pressure, the results for the two methods agree well, showing that the new calibration method and calibration system work appropriately and that the effects of the maximum calibration pressure on the test gauge were correctly evaluated without using pressure balance. for the conventional pressure calibration using pressure gauges as the reference (figure 2(a)), the reference gauge should be calibrated in advance with the two maximum pressures to obtain the correct calibration results. otherwise, the results for the test gauge during the pressure decrease process would deviate from the correct values due to the reference gauge hysteresis. with the new method, it is not necessary to consider the effects of pressurization conditions, such as the change in the maximum pressure and direction of the pressure change, on the reference gauge. 3.3. calibration with random pressure sequence  the proposed calibration method can also be applied to any pressurization procedure, unlike the conventional stepwise calibration procedure. demonstration experiments were conducted with a random pressure sequence. figure 6(a) shows the pressures applied to the test gauge during the experiment. measurements were conducted at 10 pressures from 10 mpa to 100 mpa in steps of 10 mpa. the sequence of the measurement pressures was determined at random and 10 measurements were taken at each pressure, providing 100 measurements in total. in addition, the pressure was released to atmospheric pressure (zero gauge pressure) once every 20 measurements. the output of the test gauge at atmospheric pressure was used in the offset correction for the calibration results. the waiting time at each pressure point was 10 min. figure 6(b) shows the relative difference between the two sets of calibration results (method ii – method i) in parts per million. the x-axis is the data number, and the y-axis shows the relative difference between the results. the differences were less than 0.02 % for all the measurement points. for most of the measurement points, the differences were within 0.01 %, although the differences were larger at lower pressures and for large pressure changes from the previous measurement. the two reference pressure gauges showed hysteresis of 0.32 % and 0.58 % relative to the applied pressure at 10 mpa (figure 3). this experiment showed that the proposed method obtained correct calibration results with much higher accuracy compared with the original hysteresis of the reference gauges, even when the test gauge was pressurized randomly. 4. disscussion and applications   controlling the time intervals of the reference gauge is a key to obtaining reproducible results, because the outputs of some gauges change rapidly with time after a pressure release to atmospheric pressure. the demonstration experiments were all performed with our custom-built and programmed automatic system. it may be difficult for calibration staff to conduct this kind of calibration continuously with rigidly fixed time intervals. thus, for our method to be widely used for calibration and in industry, pressure calibration systems equipped with an automatic control system for pressure and valve operation should be developed. the type of reference pressure gauge in this study is generally used in industry, but not for calibration because its hysteresis is more than 0.3 %. however, by using 0-a-0 pressurization, these gauges can be used as if their hysteresis were 0.01 %, much less than one-tenth of the original value. the method could help reduce the cost of reference pressure gauges, leading to precise, low-cost calibration devices for pressure gauges. our method can also be used for calibration with other types of pressure gauges as the reference. when a high-end figure  5.  calibration  results  for  the  demonstration  experiment  with stepwise pressurization for pmax of (a) 70 mpa and (b) 100 mpa.  figure  6.  demonstration  experiment  with  random  pressure  sequence.  (a)  sequence  of  measurement  points,  (b)  difference  between  the  results  obtained with methods i and ii (method ii ‐ method i).  10 20 30 40 50 60 70 80 90 100 0 2000 4000 10 20 30 40 50 60 70 80 90 100 0 2000 4000 r el at iv e d ev ia ti o n f ro m s ta n d ar d method i pressure [mpa] (a) (b) method ii pressure [mpa] (decreasing) (increasing)[ 1 0− 6 ] r el at iv e d ev ia ti o n f ro m s ta n d ar d [1 0− 6 ] method i method ii(decreasing) (increasing) 0 50 100 0 50 100 0 50 100 −200 0 200 data number data number p re ss u re [ m p a] (10 −6 ) r el at iv e di ff er en ce (a) (b) acta imeko | www.imeko.org  april 2016 | volume 5 | number 1 | 63  pressure gauge is used as the reference gauge with the new method, the magnitude of the pressurization effects on the results is at most several parts per million, which is comparable to the reproducibility of pressure balances. in some cases, pressure gauges at industrial sites need to be calibrated or tested on-site. the lack of stable places for installation and factors such as wind and vibration make it difficult to use pressure balances as standards. in such cases, calibration with pressure gauges is more appropriate than with pressure balances because some pressure gauges are more robust to environmental conditions. thus, our method offers a further advantage in improving the accuracy of on-site calibration. in addition to further developments in the sensing elements of the pressure gauges by manufacturers, improving measurement procedures can enhance the performance of the pressure gauges and provide more effective and accurate calibration methods, leading to reliable pressure measurements at industrial sites. 5. conclusions   we have described a new calibration method using pressure gauges as the reference that is more precise and useful. the reference gauge was pressurized by 0-a-0 pressurization, whereas the test gauge could be pressurized through various procedures according to the user's requirements and the measurement conditions of the test gauge at industrial sites. the 0-a-0 pressurization of the reference gauge greatly reduced the effect of the pressurization history on the output of the reference gauge, and produced highly reproducible outputs. in the demonstration experiments, the calibration results using the new method were consistent with those calibrated against a pressure balance. the new method should be widely applicable to any kind of reference pressure gauges and pressurization procedures applied to the test gauge. the method is expected to help develop low-cost calibration systems that are more precise. references  [1] h. kajikawa and t. kobata, effects of pressurization procedures on calibration results for precise pressure transducers, meas. sci. technol., 21 (2010), 065104. [2] h. kajikawa and t. kobata, reproducibility of calibration results by 0-a-0 pressurization procedures for hydraulic pressure transducers, meas. sci. technol., 25 (2014), 015008. [3] “guidelines of the calibration of electromechanical manometers”, euramet/cg-17/v.02, march 2011. [4] “procedures of characterization and calibration for digital pressure gauges”, japanese industrial standard, jis b 7547, 2008 (currently only in japanese). [5] “pressure gauge-calibrating device" wo/2013/042607, 2013. template for an acta imeko event paper acta imeko issn: 2221-870x december 2016, volume 5, number 4, 73-79 acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 73 outage severity analysis and ram evaluation of italian overhead transmission lines from a regional perspective mohamed khalil, christian laurano, giacomo leone, michele zanoni deib, politecnico di milano, piazza leonardo da vinci 32, 20133 milano,italy section: research paper keywords: dependability; hazard rate; multi-regional overhead transmission lines analysis; rams analysis; severity factor citation: mohamed khalil, christian laurano, giacomo leone, michele zanoni, outage severity analysis and ram evaluation of italian overhead transmission lines from a regional perspective, acta imeko, vol. 5, no. 4, article 11, december 2016, identifier: imeko-acta-05 (2016)-04-11 section editor: lorenzo ciani, university of florence, italy received september 27, 2016; in final form december 6, 2016; published december 2016 copyright: © 2016 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: michele zanoni, e-mail: michele.zanoni@polimi.it 1. introduction transmission networks represent the core of the electrical system and their failure implies a waste of energy and significant financial losses [1], [2]. consequently, the aim of the transmission system operators (tso) is to increase the network maintainability and plan its development in order to minimize outages occurrence and make the grid robust to failures preventing serious consequences on the loads. overhead transmission lines (ohtl) are definitely the most important elements in the high voltage (hv) network, since they are responsible for the transport of the electrical energy from the generating plants to the loads. it follows that industries and facilities give a great attention to their reliability [3]-[7]. at the same time, however, from a study conducted by the same authors of this paper in [8]-[9], it results that ohtls are still the most critical components from both number of failures and the corresponding interrupted power points of view. the analysis presented in [8]-[9] were based on the data published by terna, the italian tso, about the outages of the italian ohtls in the period from 2008 to 2014. in this paper, a new analysis is performed. in particular, much emphasis is put on the geographical distribution of the ohtls and the related operating voltage level in order to investigate the effect of local factors such as environmental conditions or the regional maintenance strategy on the network reliability. further, a longer period is surveyed, from 2008 to 2015, during which 12594 outages were registered, with a total power interrupted of 81379 mw. the tools exploited for the outage analysis are the severity factor (sf), already introduced by the the same authors in [8]-[9], and the reliability, availability, maintainability (ram) analysis. the paper is structured as follows: in section 2, a general overview of the italian tso terna is provided. in section 3, a brief overview on the transmission systems reliability metrics available in literature is provided. in section 4, the severity factor is introduced and the differences with respect to the traditional metrics are highlighted. the sf is then applied in section 4 where the outage data analysis is performed. in section 5, a different analysis based on the ram technique is carried out. in particular, it is performed independently for each abstract power transmission lines represent the core of the high voltage network since they are responsible for the transport of the electrical energy from the generation power plants to the electrical substations. in this paper, an analysis of the outages occurred to the italian overhead transmission lines (ohtls) from 2008 to 2015 is carried out. a new simple and effective reliability index, namely the severity factor, is introduced with the aim to drive the prioritization of the failure causes and the enhancement of the maintenance activities. the analysis has been performed focusing on the geographical distribution of the ohtls. for each analyzed region, the voltage levels more prone to failure have been determined. the proposed methodology, thanks to the introduction of the severity factor, is a useful and effective tool for the identification of the transmission network criticalities and the enhancement of the related maintenance activities. finally, an evaluation of the reliability, availability, safety and maintainability (rams) of the italian ohtl network has been performed from a regional point of view. acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 74 ohtls voltage level and a comparison of the values of reliability, availability and maintainability obtained for the different regions is given. finally, the contribution ends in section 6 where the conclusions are provided. 2. italian network overview italian network operator, terna, is one of the most important in europe in terms of lines extent (63,900 km of high voltage lines), power generation (120 gw) and energy flowing (300 billion kwh per year). until 2009, the italian transmission network was divided in two parts: rtn, property of terna, and telat, property of enel, the italian national agency for the electrical energy. since 2009, however, the grid has been unified under the control of terna. as result of its transparency policy, terna yearly publishes an annual report about the outage events occurred in its transmission network [10]. in particular, for each failure event the following data are provided: identification number of the outage, event location, date and time of occurrence, damaged component, voltage level, outage category, kind of interruption (transient, short or long), grid configuration, amount of interrupted power and time to repair. 2.1. regional partition in order to make a uniform division of the italian territory, terna identifies in its reports eight macro regions (mrs), different from the political regional division of the country. in table 1 the eight mrs, along with the corresponding italian regions, are presented. 2.2. voltage subpopulations for an efficient statistical analysis of the ohtls outages data, it is useful to consider, besides the geographical partition of the failures, also the voltage level at which they occur. it is reasonable, indeed, to expect that the predominant outage causes for a transmission line may depend on its rated voltage. the classification of the voltage ratings adopted by terna is the following: 380 kv, 220 kv, 150 kv, 132 kv and less than 100 kv. in table 2, the ohtls territorial distribution for each voltage subpopulation is reported. such data have been obtained from the terna report of 2013. as the surveyed period, ranging from 2008 to 2015, is limited, however, the eventual grid expansions that may have occurred in such years are assumed negligible with respect to the total network extent, hence the reported values can be considered an accurate estimation of ohtls distribution. it must be specified that, as it can be observed in table 2, for what regards 150 kv and 132 kv voltage levels, the mr roma is the only one in which both 150 kv and 132 kv ohtls are present. this observation, along with the fact that 150 kv and 132 kv ohtl networks are subject to similar operating conditions, justifies the choice of the authors to consider in the followings the voltage subpopulations 150 kv and 132 kv as a unique population. 2.3. outage categories transmission lines outages can be grouped in two main categories: forced outages and scheduled outages. forced outages of ohtls are mainly due to automatic switching operations performed by the protection systems. they are defined by ieee std. 493-1990 [11] as outages that cannot be deferred and result in power interruption or loss of service. on the other side, planned and scheduled outages are interruption planned in advance for routine maintenance or equipment inspection. it follows that planned and scheduled outages should be excluded from any statistical study of ohtls as they do not represent any criticality on the network management, so that, for the sake of brevity, in the following, the term “outages” refers exclusively to “forced outages”. in its annual reports [10] terna specifies for each failure event the corresponding outage category. in particular, it distinguishes five different categories, listed in table 3. among these, only category 5dp corresponds to scheduled outages, so that the failure events belonging to this category are not taken into account in the statistical analysis proposed in the following of the paper. 3. transmission systems reliability metrics in literature, different metrics for the evaluation of transmission systems reliability and outages impact on the grid are proposed. an important source is represented by the ieee standard 1366 [12] published by the transmission and distribution committee with the scope to create indices specifically designed for transmission and distribution systems, based on [13]-[14]. the indices proposed in [12] are widely used by organizations and national operators in the assessment of table 1. terna geographical division. mr italian regions ca (cagliari) sardegna fi (firenze) part of emilia romagna toscana mi (milano) lombardia – part of emilia romagna na (napoli) campania puglia basilicata calabria pa (palermo) sicilia pd (padova) friuli venezia giulia veneto trentino alto adige rm (roma) lazio umbria abruzzo molise – marche to (torino) piemonte liguria valle d’aosta ca (cagliari) sardegna fi (firenze) part of emilia romagna toscana mi (milano) lombardia – part of emilia romagna table 2. length of ohtls (in km) per each mr and voltage level. mr 380 kv 220 kv 150 kv 132 kv < 100 kv ca 314.199 554.390 1995.282 200.066 fi 2052.499 703.060 5673.969 79.021 mi 1514.724 2176.880 5669.887 46.544 na 2675.879 1101.710 6646.029 704.217 pa 252.59 1530.090 3170.542 40.980 pd 782.031 2591.993 5564.712 459.177 rm 1953.383 1005.968 3626.473 5203.620 168.582 to 1136.753 1741.560 4343.212 119.274 table 3. terna geographical division. category description 1cd lack of resources 2fm unpredictable events 3ce external causes 4ac other causes 5dp scheduled maintenance acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 75 the quality of the provided service. in particular, in its annual quality report terna focuses its attention in the following three specific indices: 1) saifi (system average interruption frequency index) which is expressed as: saifi i t n n = ∑ , (1) where ni the number of customer interrupted for each interruption, and nt the total number of customers. it indicates how often the average customer experiences a sustained interruption (any interruption that lasts more than five minutes) over a predefined period of time. 2) saidi (system average interruption duration index) which indicates the total duration of the interruption for the average customer over a predefined period of time: saidi i i t r n n = ∑ , (2) where ri is the restoration time for each interruption event. 3) maifi (momentary average interruption frequency index), which takes into account the frequency of momentary interruptions, defined as the interruption with a restoration time ri lower than 5 minutes. given imi as the number of momentary interruptions and ni the number of interrupted customers for each momentary interruption i, maifi is mathematically defined as: maifi i i t im n n = ∑ . (3) it is immediately observable from their definition that such metrics provide information only about single features of the outages events, such as frequency of interruptions (saifi and maifi) or interruptions duration (saidi). in particular, terna computes the value of these indices for each mr, without discriminating about the operating voltage level of the ohtls. furthermore, such metrics are evaluated only for failures belonging to category 4ac. from all these considerations it comes the idea of the authors to carry out a more detailed reliability analysis based on the data of all the ohtl forced outages events (i.e. also categories 1cd, 2fm and 3ce are taken into account) and the proposal of a new reliability index, the severity factor, that synthetizes in a unique number all the information related to the criticality of outage events: frequency of occurrence, interruption duration and corresponding interrupted power. moreover, the analysis is performed focusing not only on the geographical distribution of the ohtls but also on the operating voltage level, differently than in [10]. 4. severity factor the severity factor has been introduced by the authors in [8]-[9], defining it as an index that enables a rapid and effective comparison of the impact of different outage causes for ohtls at the same voltage level. in particular, the symbol sfi,j denoted the severity factor of a generic outage category i for the ohtls of the voltage category j. it is interesting to highlight that, because of how it is defined, the concept of severity factor can be exploited also for other kinds of failure data analysis. an interesting example, indeed, is the analysis of the distribution of the failures in a given mr among the different voltage levels. in this case, the notation sfi,j would refer to the severity of the failures occurred at voltage level i for the j-th mr. in the following, for the sake of the clearness, with the notation sfi,j, the authors will always refer to the severity of the failures related to category i and affecting the ohtl belonging to the macrocategory j. in this paper, the sf is defined with a different analytical expression with respect to [8], where it was defined in an analogous way with respect to the risk priority number (rpn) adopted in the failure mode, effects and criticality analysis (fmeca) [15]. in particular, it will be adopted the definition of severity factor sf related to a category i and macrocategory j introduced in [9] and reported in (4) as: γ γ = = ∑ j i i j i j n n n j n e sf e , , , 1 , (4) where n=1, 2, …, nj is the index of each category composing the macrocategory j and ei,j is the total not-transmitted energy consequent to outage events affecting category i inside macrocategory j. in particular, ei,j can be computed as: , 1 en i j e e e e p t = = ⋅∑ , (5) where ne is the related total number of outage events, and pe and te are, respectively, the interrupted power and inactivity time caused by each single event. as for the factor γ in (4), it is a weighting factor that takes into account the different ohtls kilometric extent of each single category inside macrocategory j. in particular, given a category i and a macrocategory j, γi is expressed as: j i i l l γ = , (6) where li is the length of the ohtls belonging the category i and lj the total length of the othls related to the macrocategory j. if, for example, the objective is to compute the sf for the failures occurred at different voltage levels in the mr j, li would refer to the total length of the lines operating at voltage level i in the macro region j. the sum of the lines length at different voltage levels would provide the value of lj. putting li at the denominator allows to give more weights to the outage events related to categories less represented (in terms of length) inside the subset defined by the macrocategory j. this makes the comparison of the sfs for the different categories more fair, since it is reasonable to expect that statistically the number of outage events recorded for a given category i is proportional to the length of the related lines, so that ei,j tends to increase with the length. the new formulation of the sf given in (4) leads to different improvements with respect to the definition provided in [8]. the generic sfi,j, indeed, has now a physical meaning since it represents a weighted percentage of not-transmitted energy associated to a category i, for a macrocategory j. furthermore, the sum of the sf of the different categories i for a macrocategory j is now equal to 1 by definition. these improvements make the severity factor easier to understand and able to offer a more intuitive representation of the ohtls acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 76 outages scenario resulting in a useful tool for the maintenance personnel in the coordination of the maintenance activities. nevertheless, it is important to emphasize how the sf differs from the other indices presented in section 3 and available in literature as in its definition different features characterizing the criticality of failure events such as failure frequency, interrupted power and interruption duration are involved at the same time. another issue to take into account is that traditional indices such as saifi, saidi and maifi require the knowledge of number of customers of the network nt and the number of interruptions experienced by each customer ni. very often, these data are difficult to be obtained and may prevent the computation of the indices starting from public reports as the annual quality report published by terna [10]. conversely, the sf requires for its computation the knowledge of only the number of registered outages and the related downtime and interrupted power, which are data commonly provided by the organizations. 5. outage data analysis once having defined the new sf, it is possible to carry out a statistical analysis on the ohtl outages. in this paper, two different analyses are proposed. first, for each mr the severity factor of ohtls operating at different voltage levels is evaluated. in the second analysis, instead, the focus moves towards the comparison of the sf for the different mrs given a specific voltage level. 5.1. voltage subpopulation analysis the aim of this analysis is to study the severity of the outages occurring at the different voltage levels in a given mr. therefore, following the notation described in section 4, the mr of interest is indicated with the index j, whereas the voltage population is indicated with the index i. analogously, for what concerns the weighting factor γi, lj is the total length of the othl belonging to mr j, whereas li represents the total length of the lines of such mr operating at voltage level i. thanks to the new definition (4) of the severity factor, it is possible to carry out a meaningful sf comparison between different mrs. furthermore, it is possible to compare the sfs obtained for each mr with the results obtained at national level. the results are shown in figure 1. it is interesting to observe how the analysis carried out at national level emphasizes the criticality represented by the ohtls operating at voltages below 100 kv (row it in table 4), when compared to other voltage populations. moving the analysis to a regional level, however, allows to highlight that this is not the case for the mrs firenze and napoli, for which the most severe situations are represented, respectively, by the 132 kv grid (sf equal to 0.951) and the 380 kv and 150/132 kv grids (0.319 and 0.420). another important result derived from the outage analysis carried out at regional level is the relevant sf computed for the 380 kv population for the mrs napoli and palermo, equal to 0.319 and 0.169 respectively. this result highlights the need for more detailed analysis in order to understand the root causes of the outages and reduce the related energy losses. similar considerations can be made for firenze where the 132-150 kv grid, having an sf value equal to 0.951, represents a critical case for which a deep investigation is needed. it can be concluded that the outages analysis performed at mr level has emphasized that an analysis carried out from a national point of view leads to approximate results that, generally, can be misleading since they are not replicated in the different mrs, because of the different operating conditions and/or maintenance strategies. 5.2. mrs analysis a different point of view is given by the analysis of the sf of the ohtls among the different mrs for a given voltage level (i.e. considering the regional subdivision as category i, and the voltage population as macrocategory j). in this kind of analysis, for the sake of the computation of the weighting factor γi (the length of the lines at a given voltage level are not the same in each region), lj corresponds to the total length of the ohtls at a specific voltage level j, whereas li is the length of the lines at that voltage level operating in the i-th mr. the achieved results are shown in figure 2 and reported in table 5. the results confirm what already was described in the previous section 4.1 but allow to highlight also further considerations. it is evident, in fact, that for what concerns the 380 kv voltage grid, palermo is the most affected mr with an sf equal to 0.60. this statement was not evident by previous analysis (figure 1) as napoli presented a higher sf than table 4. severity factor per each voltage level for a given mr. mr 380 kv 220 kv 150–132 kv < 100 kv ca 0 0.293 0.126 0.581 fi 0 0.024 0.951 0.025 mi 0.001 0.001 0.002 0.996 na 0.319 0.076 0.420 0.185 pa 0.169 0.012 0.105 0.714 pd 0 0.007 0.100 0.893 rm 0.002 0.072 0.090 0.836 to 0 0.002 0.208 0.790 it 0.054 0.020 0.118 0.808 figure 1. voltage level sf for different mrs. figure 2. geographical impact for different voltage levels. ca fi mi na pa pd rm to it mrs 0 0.2 0.4 0.6 0.8 1 sf 380 kv 220 kv 150-132 kv < 100 kv 380 kv 220 kv 150-132 kv < 100 kv voltage levels 0 0.2 0.4 0.6 0.8 sf ca fi mi na pa pd rm to acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 77 palermo. this difference in the results is justified by the different focus characterizing the two analyses. in particular, the previous analysis focused on the sf of the ohtls at different voltage levels in the same mr. it can be concluded that napoli mr is the area for which the 380 kv grid represents the most critical case, but moving at national level it is the mr palermo that mostly contributes for the severity of the outages occurred at this voltage level. an analogous situation occurs for the ohtls operating at less than 100 kv. the analysis performed in section 5.1 showed similar sfs for most of the mrs (milano, padova, roma, torino had sf between 0.790 and 0.996). these results would let think that such mrs contribute almost equally for the severity of the outages at this voltage level. results depicted in figure 2, however, deny this theory as they highlight that milano mr is by far the most solicited area, presenting an sf equal to 0.80. finally, it can be concluded that both presented analyses are meaningful, providing two complementary points of view on the same problem. on one side the severity of a voltage grid over the failures of a given mr is depicted, whereas on the other the regional severity over a specified voltage subpopulation is considered. the analysis of the results has highlighted the effectiveness of the proposed sf index for the identification of the most critical voltage levels for the transmission levels at both national and regional levels, resulting as a useful tool for an optimal planning of maintenance activities. 6. ram analysis reliability, availability and maintainability (ram) are parameters that are widely used for system design specifications and as an operational performance indicator for utility assets. reliability is the probability that a system will be available at a given time. based on reliability theory [15]-[18], the following definitions are applied: • failure rate: during the useful-life phase of a product, failure rate can be defined as the number of random (unscheduled) occurrences of failure of the product to perform its intended function divided by the length of time the product was functioning. • maintainability is a property of repairable systems defined as the facility in which a system can be repaired once a malfunction (or failure) is manifested. maintenance should be performed by personnel having specified skill levels, using prescribed procedures and resources, at each prescribed level of maintenance and repair. a common way to express the maintainability is the reciprocal of the mean time to repair (mttr). it is important to note that maintainability is not the same as maintenance. • availability is the percentage of time the item is available to perform its required functions. availability deals with up-time for operations and is a measure of how often the system is alive. it is a function of both mean time between failures (mtbf) and mttr. high availability, indeed, does not always mean high reliability, since a system with frequent failures (low reliability, small mtbf) but with very small mttr (with respect to the mtbf) would be characterized anyway by a high availability. 6.1. reliability indicator: failure rate a good indicator of the ohtl reliability is the failure rate. it is calculated based on the following equation (7): λ = ⋅ n l t (7) where n is the total number of outages occurred during the surveyed period t and l is average of the lengths of ohtls during the surveyed period divided by 100 km. the failure rates of each voltage level of ohtl for all italian regions are illustrated in figure 3. it is depicted that 380 kv and 220 kv grids are quite robust to outages as the hazard rate computed for this voltage levels is very low for every italian mr. the hazard rate increases when the focus moves to the 150-132 kv voltage level, with an average value of about 3 failures per year per 100 km, except a small peak related to palermo mr, which is affected by about 6 failures per year per 100 km. the most critical situation is represented by the voltage level below 100 kv where it is recorded an average of 15 yearly failures per 100 km. the worst case is related to the mrs of milano and napoli where the hazard rate is twice as much as in the rest of the country. looking at the analysis performed in the previous subsection 5.2, this result was predictable for mr milano, but not for the mr napoli. this region, indeed, is characterized by a high average number of failures at this voltage level but a very low sf (equal to 0.05), especially if compared to the one of milano. the reason for this apparent discrepancy has to be found in the weighting factors computed for the definition of the sfs. such factor, indeed, is very low for the region of napoli, since it is characterized by a transmission grid operating below 100 kv long about 704 km (out of a total national extent of the grid at that voltage level equal to 1818 km). conversely, for the milano mr a large weighting factor is assigned since the extent of such grid is only 46 km. these considerations highlight the importance to perform, besides the computation of the sf as described in the previous section 4, also a ram analysis in order to have a complete set of table 5. severity factor per each mr for a given voltage level. mr 380 kv 220 kv 150-132 kv < 100 kv ca 0 0.090 0.009 0.002 fi 0 0.005 0.048 0.000 mi 0.033 0.048 0.042 0.798 na 0.362 0.207 0.257 0.005 pa 0.599 0.105 0.201 0.062 pd 0 0.025 0.086 0.035 rm 0.006 0.512 0.145 0.061 to 0 0.008 0.212 0.037 figure 3. hazard rate for different voltage subpopulations and macro regions. voltage level [kv] 380 220 150-132 <100 fa ilu re r at e [y ea rs -1 ] 0 10 20 30 ca fi mi na pa pd rm to acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 78 information about the national and regional scenario of the outages affecting the transmission system. 6.2. availability the availability a(t ) is calculated by equation (8) as: = + mtbf t a t mtbf t mttr t ( ) ( ) % ( ) ( ) (8) the availability for ohtls for various voltage subpopulations in the italian grid is illustrated in figure 4. the availability is concluded to be lower for the ohtls operating at less than 220 kv, even if it never goes below the value 96 %. a different scenario is depicted for the 220kv and 380 kv ohtls where the availability is very close to 1 in all the regions, except for the case of torino othls at 220 kv (availability below the 98 %). this case is very explicative about the importance of the mttr in the definition of the availability. the torino 220 kv grid, indeed, presents a very low failure rate equal to about 0.20 failure per year per 100 km, synonym of quite a large mtbf and high reliability. the key parameter in the evaluation of the availability, however, as can be seen from (8), is represented by the ratio mttr/mtbf since the lower it is the higher is the availability. this ratio has been computed for all the voltage levels and all the italian mrs and the obtained results are depicted in figure 5. it can be observed that for what concerns the 220 kv grid, torino is characterized by a ratio much higher than other mrs (almost 2 orders of magnitude), resulting in a lower availability. such a higher mttr should reflect also in a higher sf, but observing the results reported in figure 2 and table 5 this is not the case. the reason lays in the fact that in the computation of the sf a role is played also by the amount of interrupted power at each outage event occurrence, that for the specific case of torino ohtl at 220 kv is quite low. further, the weighting factor associated to such grid is also low, due to the considerable length of the considered system. 6.3. maintainability evaluation different environmental conditions and major load classes according to the main activities in each zone characterize the considered geographical mrs. some zones are mainly agricultural, while others are of composite load classes. therefore, maintainability for various voltage subpopulations differs from one zone to another. maintainability is here expressed as the reciprocal of the mean time to repair (mttr). the results obtained from the computation of the maintainability of the ohtls operating at different voltage levels and different mrs are reported in figure 6. for ohtls operating at 380 kv in the mrs cagliari, firenze, padova and torino, the maintainability is not definable as the related mttr is equal to zero, since no outages were registered during the surveyed period. in general, it seems that it is not possible to denote a characteristic trend of the maintainability as function of the voltage level, valid for all the mrs. for instance, if on one hand for torino the maintainability decreases when considering ohtls at higher voltage levels, suggesting that mttr increases with voltage level as the complexity of equipment is going up, the opposite situation occurs for mr roma for which the maintainability increases as the voltage level increases. this highlights the strong dependence of the maintainability on the geographical area of interest and the operating voltage of the ohtls as the typology of supplied loads and energy demand may change drastically. furthermore, each region has its own policy and environmental conditions that have their impact on the maintenance management. 7. conclusions in this paper, an analysis of the outages occurred to the italian overhead transmission lines from 2008 to 2015 has been carried out. in particular, two different analyses, one based on a new reliability index, namely the severity factor, and one based on a traditional ram technique, have been performed, focusing on the geographical distribution of the ohtls. the severity factor has been introduced in order to provide a useful tool for the identification of the most critical ohtls affecting and preventing the optimal operation of the national transmission system. the main differences with respect to other transmission systems reliability metrics found in the literature have been highlighted. the evaluation of the sf has shown that the impact of the outages on the ohtls reliability is generally not uniform across the country but depends on the considered figure 4. availability for different voltage subpopulations and macro regions. figure 5. mttr/mttf for different voltage subpopulations and macro regions. figure 6. maintainability for different voltage subpopulations and macro regions. voltage level [kv] 380 220 150-132 <100 a va ila bi lit y [% ] 96 97 98 99 100 ca fi mi na pa pd rm to voltage level [kv] 380 220 150-132 <100 m m t r /m t t f 0 0.01 0.02 0.03 ca fi mi na pa pd rm to voltage level [kv] 380 220 150-132 <100 m ai nt ai na bi lit y [h -1 ] 0 5 10 15 ca fi mi na pa pd rm to acta imeko | www.imeko.org december 2016 | volume 5 | number 4 | 79 region. further, for each analysed region, the voltage levels more prone to failure have been determined. the same analysis has been carried out also at a national level and the results have shown that the situation depicted at such level is not necessarily replicated at the regional level, justifying the classification of outages data according to regional criteria, as proposed in this contribution. for what concerns the ram analysis, it has been performed starting from the same outage data. the most important conclusion that can be derived is that the joint evaluation of the results obtained from these two different approaches provides a set of information that enables a complete evaluation about the impact of outages on the ohtls operation, involving at the same time key concepts of the reliability theory as mttr, mtbf, availability, failure rate and maintainability but also other physical and economical concepts as interrupted power and nottransmitted energy. it can be stated, therefore, that the proposed methodology is a useful and effective tool for the identification of the transmission network criticalities and the prioritization of the research activities aimed at a better understanding of the failure modes and failure mechanisms affecting the lines, with the final objective to increase their reliability and enhance the maintenance activities planning and performances, at both local and national levels. references [1] t. gonen, “electrical power transmission system engineering: analysis and design”, crc press, third edition, 2014. [2] j.c. pohlman, k.e. lindsey, r. f. corpuz, “controlling the economic risk from catastrophic failure of overhead transmission lines”, ieee esmo, 1998, pp. 300-310. [3] d. o. koval, a. a. chowdhury, “assessment of transmissionline common-mode, station-originated, and fault-type forced-outage rates”, ieee transactions on industry applications, vol. 46 , no.1, jan. feb. 2010, pp.313-318. [4] b. shen, d. koval, w. xu, j. salmon, s. shen, “an analysis of extreme-weather-related transmission line outages”, ieee canadian conference on electrical and computer engineering, vol.2, pp.697-700, vol.2, 24-28 may 1998. [5] d. koval, b. shen, s. shen, a. a. chowdhury, “modeling severe weather related high voltage transmission line forced outages”, in ieee transmission and distribution conference and exhibition, 2006, pp.788-793. [6] y. cheng, c. niu, c. li, “the reliability evaluation method of high voltage overhead transmission lines”, international conference on condition monitoring and diagnosis, 2008, pp. 566-569. [7] i. dobson, b. a. carreras, “number and propagation of line outages in cascading events in electric power transmission systems”, 48th annual allerton conference on communication, control, and computing, 2010, pp.1645-1650. [8] m. faifer, m. khalil, c. laurano, g. leone, s. toscani, “outage data analysis and rams evaluation of the italian overhead transmission lines”, ieee international energy conference and exhibition (energycon), 2016. [9] c. laurano, g. leone, m. zanoni, “outage severity analysis on italian overhead transmission lines from a regional perspective”, 14th imeko tc10 workshop technical diagnostics “new perspectives in measurements, tools and techniques for system’s reliability, maintainability and safety”, milan, italy, june 27-28, 2016. [10] rete elettrica nazionale, annual reports of terna [online].available: http://www.terna.it/default/home_en/electric_system/statistica l_data.aspx [11] ieee std. 493-1990, “ieee recommended practice for the design of reliabile industrial and commercial power systems”, 1991. [12] ieee std. 1366-2012, “ieee guide for electric power distribution reliability indices”, 2012. [13] r. billinton, r. n. allan, “reliability assessment of large electric power systems”, 1st edition, kluwer academic publishers, 1988. [14] h. pham, “handbook of reliability engineering”, springer 2003. [15] r. billinton, r.n allan, “reliability evaluation of engineering systems”, 2nd edition, plenum press, 1996. [16] b. s. dhillon, “maintainability, maintenance, and reliability for engineers”, crc press, 2006. [17] a. birolini, “reliability enghineering: theory and practice”, 5th edition, springer, 2007. [18] r. f. stapelberg: “handbook of reliability, availability, maintainability and safety in engineering design”, springer, 2009. http://www.terna.it/default/home_en/electric_system/statistical_data.aspx http://www.terna.it/default/home_en/electric_system/statistical_data.aspx outage severity analysis and ram evaluation of italian overhead transmission lines from a regional perspective lock-in amplifier with a high common-mode rejection ratio in the range of 0.02 to 100 khz acta imeko issn: 2221-870x march 2019, volume 8, number 1, 103 110 acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 103 lock-in amplifier with a high common-mode rejection ratio in the range of 0.02 to 100 khz pavel baranov1, valeriy borikov1, veronica ivanova1, bien bui duc1, sergey uchaikin2, cheng-yang liu3 1 division for electronic engineering, national research tomsk polytechnic university, 30 lenin avenue, 634050 tomsk, russia 2 ibs center for axion and precision physics research., kaist munji campus, 193 munji-ro, yuseong-gu, daejeon 34051, south korea 3 department of biomedical engineering, tamkang university, no. 151, yingzhuan road, tamsui district, new taipei city, taiwan section: research paper keywords: lock-in amplifier; differential input; voltage dividers; common-mode rejection ratio; calibration citation: pavel baranov, valeriy borikov, veronica ivanova, bien bui duc, sergey uchaikin, cheng-yang liu, lock-in amplifier with a high common-mode rejection ratio in the range of 0.02 to 100 khz, acta imeko, vol. 8, no. 1, article 14, march 2019, identifier: imeko-acta-08 (2019)-01-14 section editor: alexandru salceanu, technical university of iasi, iasi, romania received october 03, 2018; in final form february 19, 2019; published march 2019 copyright: © 2019 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: the research was carried out at tomsk polytechnic university within the framework of tomsk polytechnic university competitiveness enhancement program grant # viu-ink 76/2017 and funded by russian science foundation (rsf), project # 17-79-10083. corresponding author: pavel baranov, e-mail: bpf@tpu.ru 1. introduction one of the classic problems of instrumentation is the comparison of alternating voltage amplitudes [1]-[4]. inductive voltage dividers (ivds) are usually used for precision ac [5][11]ratio measurements. to calibrate ivd ratio kivd errors, the calibrated ivd referencing method is the most widely used [12], [13]. for example, this method is used in the national institute of standards and technology (nist, usa) to calibrate decade ivds (54120с – 54131с services) in the range of 50 hz to 20000 hz [14]. in the calibration process, an ivd ratio is compared with a ratio from a reference ivd using the differential method of measurement. comparisons of two ac voltages vx(t) and v0(t) with the same frequency ω lock-in amplifiers with a differential input (liadi) are usually applied. [15], [16]. liadi defines the measurement precision and permits the separation of a low differential signal from a high common-mode interfering signal. а block diagram of a measuring ivd ratio error using the liadi system is shown in figure 1. figure 2 shows a block diagram of a simple liadi. according to the scheme, the amplitudes of in-phase signals vx(t) and v0(t) are compared, the signal is then amplified and the signal difference detected. lock-in amplifiers are mainly used in physical and chemical sensing applications [17]-[26]. however, liadi inputs are also used for calibration measurement instruments [15], [16], [27]. equation (1) below defines the output voltage of simple liadi (figure 2): lock-in amplifier generator standard ivd dut ( ) sin( ) g gm v t v t=  ( ) sin( ) ref refm v t v t=  δv v0(t) vx(t) vref(t) figure 1. block diagram of a measuring ivd ratio error system. synchronous detector subtracter low-pass filter v0(t) vx(t) δv vref(t) δv(t) δv(t)·vref(t) figure 2. a simple lock-in amplifier with a differential input (a basic liadi). abstract the paper presents a new lock-in amplifier with a differential input. the lock-in amplifier has a high common-mode rejection ratio of 200 db at 1 khz and 160 db in the frequency range of 20 hz to 100 khz. a method for extending the dynamic range of comparing signals is suggested. the experimental results of metrological characteristic tests are discussed. mailto:bpf@tpu.ru acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 104 0 0 0 0 0 0 ( ) ( )1 1 ( ) ( ) ( ) ; 2 2 ( ) sin( ); ( ) sin( ); ( ) sin( ), x x ref x xm x m ref refm ref v t v t v v t v t v t dt u cmrr v t v t v t v t v t v t  +    − +       =  +  =  +  =  +   (1) where u is the multiplier denominator voltage, v; v0m and vxm are the compared signals amplitudes, v; vrefm is the reference voltage; v; φ0, φx, are the compared signal phases, rad; φref is the reference signal phase, rad; and cmrr is the common-mode rejection ratio. as shown in equation (1), the signal amplitude difference δv has an error that depends on the phase shifts of the input signals φx, and φ0 is a common-mode rejection ratio and phase shift of reference signal φref. [16] and [27] explain the possibility of reducing the error of measuring the absolute difference between the compared voltages induced by a phase error. in this current work, methods of reducing the errorconditioned measurements by using the finite value of the common-mode rejection ratio in a liadi are suggested. a new circuit is proposed for the input stage of the liadi input based on an instrumentation amplifier and a voltage follower. 2. the common-mode rejection ratio for ivds requirements for the common-mode rejection ratio can be improved, specifically when the verification and calibration of the ivd at a 1-10 nv maximum resolution of the lock-in amplifier within the medium frequency range at the upper level of 10√2 v of the compared voltage dynamic range [28]-[30]. the instrumentation amplifier application is the standard method for input differential signal extraction. the output voltage is determined by equation (2) because its response delay can be neglected in frequency range c out d d c c d d , v v a v a v a v cmrr   = + = +    (2) where ad is the differential-mode gain; ac is the commonmode gain; vd is the differential voltage, v; and vc is the common-mode voltage, v. the finite value of cmrr is determined by the value of relative error с. in this case, the ultimate requirements for cmrr are c c d . v cmrr v =  (3) according to equation (3), the common-mode rejection is (200–220) db at с = 0.1, but it is not realised in commercially available lock-in amplifiers. other important characteristics of a lock-in amplifier that must be taken into account are the bandwidth, high input impedance at measurement inputs, high stability, and low nonlinearity of the differential signal gain. for example, one of the best commercially available lock-in amplifiers, sr830 of stanford research systems, has a common-mode rejection of 100 db up to 10 khz, decreasing by 6 db per octave above 10 khz, and an input impedance 10 mohm//25 pf [31]. in [32], a special guarded vector voltmeter (a lock-in amplifier) with a bootstrapping-guarding technique [33] is proposed for the calibration of the ivd. it has a high common-mode rejection of 180 db at 1 khz. with our ivd [28], we must compare voltages of up to 10·2 v with a resolution of up to 10 nv at a frequency of 1 khz. therefore, by means of the lock-in amplifier, a common-mode rejection of about 200 db should be obtained. we suggest the use of the instrumentation amplifier in its integrated form with a programmable gain ratio for ivd comparison. in such amplifiers, cmrr does not depend on the output impedance of the compared signal sources because its dependence on the internal impedances of the cascaded instrumentation amplifier is technologically minimised. after analysing the parameters of the different instrumentation amplifiers performed for the differential signal extraction, texas instrument pga207 has been chosen [34]. however, pga207 and similar amplifiers have a low common-mode rejection of 100 db at ad = 10. experimental studies of cmrr on eight pga207 samples were carried out. equal values of voltage (5 v) were applied to both the pga207 inputs, and the value of voltage δv by the lock-in amplifier sr830 was measured on the pga207 output at ad = 10. all tests were undertaken three times for each frequency at an environment temperature of 22±2 с. the common-mode signal rejection ratio was calculated by 5 20 lg ,cmrr v =  (4) the results of the experimental studies (figure 3) of the pga207 instrumentation amplifier have confirmed the low value of the common-mode rejection ratio and have shown significant dispersion of the common-mode rejection ratio from sample to sample. in summary, it has been shown, both theoretically and empirically, that the circuit technique must be used for further increasing the common-mode rejection. 3. application of tracking power supply to differential signal extraction the common-mode rejection in lock-in amplifiers can be increased by the addition of the voltage follower to the scheme, as shown in figure 4, which provides the tracking power supply of the instrumentation amplifier. the efficient common-mode rejection ratio is ef f ( ) ( ) , 1 ( ) cmrr t cmrr t a t = − (5) where af(t) is the voltage follower voltage gain. the common-mode rejection decrease to 160-180 db can be achieved within the frequency range of practically inertialess figure 3. results of experimental studies concerning the pga207 instrumentation amplifier. 60 110 100 1.000 10.000 100.000 c o m m o n -m o d e r e je c ti o n ( d b ) frequency (hz) pga1 pga2 pga3 acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 105 circuits at easily reachable values of 0.999-0.9999 of the voltage follower voltage gain. this approach has serious disadvantages. the transmission factor module of the voltage follower and the instrumentation amplifier decreases with the increase in frequency, resulting in the cmrr of the whole circuit [35]. thus, we can see the high rejection of the common-mode signal only in the narrow frequency band. tracking a power supply circuit with the use of a voltage follower also provides an increase in the input impedance of the measuring channels in the wide frequency range, especially the vx channel of the instrumentation amplifier. figure 5 shows the novel technical implementation of the input stage of the lock-in amplifier that is used for tracking the symmetrical power supply of the instrumentation amplifier. the connection circuit of the instrumentation amplifier is presented in figure 6. the high-quality operating amplifier (da1) in the voltage follower circuit, as shown in figure 5, must be used to solve the posed problem. this amplifier is provided with an acceptable voltage profile of no less than ±18 v of the tracking symmetrical power supply and is presented by op285. a conventional representation (figure 5) of the uncomplemented input supply of this amplifier is conditioned by means of the flexible diode protection circuit of the voltage follower inputs in a number of cases so as to provide its normal quiescent mode at the disconnection of a signal source. the effect of cable capacitance is reduced due to the suspension of their braiding to the voltage follower input (figure 6). as shown in figure 6, the output signal is to be obtained relative to the original ground. it can be carried out, at least, by using optical or transformer isolation. the first method leads to an abrupt loss of sensitivity due to the properties of diode optrons. therefore, the second method is more expedient. 4. investigation of the differential signal extraction circuit the differential signal extraction circuit (see figure 5 and figure 6) was simulated in the multisim software, applying common-mode input signals with amplitudes v0m = vxm = 10·2 v. the frequency dependences of the cmrr from |af(t)|, phase af(t), and output voltage amplitude |δv(t)| were estimated. the results of the simulation are shown in table 1. the proposed circuit permits increasing the common-mode rejection up to 141 to 220 db at a frequency range of up to 100 khz (see table 1). the obtained resolution is <1 nv in the frequency range of up to 10 khz, <10 nv in the frequency range of up to 20 khz, <100 nv in the frequency range of up to 60 khz, and <1000 nv in the frequency range of up to 100 khz. the differential signal extraction circuit implementation is shown in figure 7. the experimental investigation setup for the differential signal extraction circuit is shown in figure 8. the ch0 output of the fluke 5520a calibrator supplied a voltage of 10·2 v and a frequency of 1 khz. the same signal was applied to the di-3m [28] inductive divider with a ratio of 0.100000. the δv values were measured at the lock-in amplifier sr830 resolution up to 2 nv, five times for 35 different frequencies (figure 8). +v3 -v3 + -v1 +v2 -v2 op285 3.6 k 0.15 0.15 bd139 bd140 bd140 bd139 100 100 240 240 1 k bzx55c5v6 bzx79-c6v2 300 300 470 3.6 k 22 k 22 k 240 1 k bzx79-c6v2 gnd1 gnd1 gnd1 gnd2 1 k v0(t) vout(t)da1 figure 5. voltage follower circuit for tracking power supply formation. gnd1 gnd2 gnd2 1t gnd1 +v2 -v2 + gnd2 δv(t)/2 δv(t)/2 ref instrumentation amplifier -v1 + voltage follower +v1 gnd1 v0(t) vx(t) gnd1 gnd1 figure 4. differential signal extraction scheme. 1t 200 k 820 gnd2 gnd1 +v2 -v2 + pga207gnd1 200 k v0(t) vx(t) gnd1 gnd2 δv(t)/2 δv(t)/2 ref figure 6. connection circuit of the instrumentation amplifier. table 1. results of the differential signal extraction circuit simulation. frequency (hz) |af(t)| phase of |af(t)| (deg) |δv(t)| (v) cmr (db) 100 1.000001 -0.8410-6 1.0110-10 220 1000 1.000002 -8.6710-6 2.0110-10 214 10000 1.000003 -83.110-6 1.0110-9 200 20000 1.000010 -16110-6 6.5710-9 184 30000 1.000021 -20110-6 1.9110-8 174 40000 1.000037 -13710-6 4.1110-8 168 50000 1.000048 11210-6 6.9710-8 163 60000 1.000062 78310-6 1.0510-7 160 70000 1.000065 1.8410-3 1.4410-7 157 80000 1.000058 3.7910-3 2.0710-7 157 90000 1.000040 6.4210-3 4.8510-7 146 100000 1.000016 11.510-3 8.8910-7 141 acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 106 the common-mode rejection of the differential signal extraction circuit is calculated as 10 2 20 lg ,cmrr v =  (6) where 102 is calibrator fluke 5520a nominal voltage, v. figure 9 shows the implementation of the differential signal extraction scheme that allows us to increase the common-mode rejection up to 100 to 190 db at a frequency range of up to 100 khz. the obtained maximum resolution is <10 nv in the frequency range of up to 1 khz. the sample and hold circuit can be added to the lock-in amplifier for achieving the required common-mode rejection. one of the technical realisations of the sample and hold circuit for the lock-in amplifier us described in [35]. experimental investigations for differential signal extraction circuit with sample and hold circuit [35] are also provided by the scheme shown in figure 8. the results of the differential signal extraction circuit experimental investigation with sample and hold circuit are shown in figure 10. figure 10 shows the implementation of a differential signal extraction circuit with sample and hold circuit, which allows us to increase the common-mode rejection up to 125 to 215 db in a frequency range of up to 100 khz. the obtained maximum resolution is < 1 nv in the frequency range of up to 1 khz and < 10000 nv in the frequency range of up to 100 khz. 5. new liadi 5.1. block schematic the new liadi structure, which uses a developed differential signal extraction circuit with sample and hold circuit, is introduced. a block diagram of the new liadi is shown in figure 11. the liadi consists of a voltage follower (vf); an instrumentation amplifier (ia); a programmable gain amplifier (pga), a synchronous detector (sd); a low-pass filter (lpf); a sample and hold device (sh); an analogue-to-digital converter (adc); and a microcontroller (mc). the vf is assembled on the op285 and pga207 ics; the pga207 also accommodates ia, pga, and sh. the synchronous detector is based on the ad734 ic, and its denominator voltage is set close to 1 v for improved sensitivity [36]. the lpf has a cut-off frequency of 0.2 hz and is based on the op270 ic. to achieve a uniform transient response, we use a third-order bessel filter. the liadi can automatically be calibrated to reduce the effect of a residual in-phase component on the lpf output. to calibrate, the same voltage v0(t) is applied on both the inputs of the differential signal extraction scheme (figure 4). the output voltage is stored in the sh and serves as a correction during measurements. the adc is assembled on the max110 ic. an mc (atmega128) controls the operation of the liadi’s pga, implements an additional digital filter, and indicates the measurement results. gnd1 pga207 op285 bd140 bd139 bd140bd139 vx(t) v0(t) figure 7. a photo of the differential signal extraction circuit implementation. differential signal extraction circuit fluke 5520a ch 0 0 ( )v t ( )xv t ( )refv t v di-3m out 2 sr830 ( )v t figure 8. experimental investigation setup. figure 10. results of the differential signal extraction circuit experimental investigation with sample and hold circuit. v0(t) vx(t) vref(t) vf ia pga sd lpf sh adc mc δv figure 11. new liadi block diagram. figure 9. results of the differential signal extraction circuit experimental investigation. acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 107 5.2. liadi input resistance the characteristics of the liadi are measured experimentally using the di-3m [28] reference divider. the di3m has auxiliary and main outputs connected to the output of the first decade (out.1) and the minor decade (out.2). a vx channel input resistance was determined using the fluke 5520 a calibrator with a two-step indirect measurement method (figure 12). the calibrator’s main (ch0) and auxiliary (ch1) outputs supplied the 1 khz sinusoidal voltage of 10√2 v and 5√2 v, respectively. the ratio kivd of the di-3m was set to 0.100000. first, the same signal was applied to the v0 and vx inputs to calibrate the liadi. second, the input vx was connected via a precision reference resistance r of 1 kω (usf340 1.00k 0.01 % 5 ppm/°c [37]), and the voltage difference δv was recorded according to the data provided by the liadi indicator. the liadi input resistance was calculated according to , / in ivd in v k r v r =  (7) where vin is the calibrator output voltage ch0, v. the v0 channel-based input resistance was determined in a similar way, connecting the input v0 via reference resistor r. the difference in voltages δv was recorded. the calculated input resistances are given in table 2. 5.3. the liadi resolution the liadi resolution was determined according to the scheme shown in figure 13. to measure the liadi resolution, the indirect measurement method was used. the ivd di-3m working calibration standard was used to set up nominal voltage vdi3m between outputs 1 and 2. the voltage difference δv was recorded according to the data provided by the liadi indicator. the ch0 and ch1 outputs of the calibrator supplied voltages of 10√2 v and 5√2 v, respectively, with a frequency of 1 khz. the di-3m ratio of output 1 was set to 0.100000. to determine resolutions 10 nv, 100 nv, and 1 µv, the ivd ratio of output 2 was varied in the range of 0.100001 to 0.100005. to determine resolution 10 µv, the ivd ratio of output 2 was varied in the range of 0.100010 to 0.100050. the liadi relative deviation was calculated according to ( )3 3 100, di m di m v v v −   =  (8) where vdi3m is the ivd di-3m nominal voltage between outputs 1 and 2, v. the results of the measurements are presented in table 3. 5.4. liadi dynamic range the dynamic range of the compared voltages was defined according to the scheme shown in figure 13. again, the ch0 and ch1 outputs of the calibrator supplied voltages of 10√2 v and 5√2, respectively, with a frequency of 1 khz. the di-3m ratio of output 1 was set to 0.999999. the ivd ratio of output 2 was varied in the range of 0.999998 to 0.999994. δv values were measured at the liadi resolution of 100 nv. the liadi relative deviation was calculated according to equation (8). the minimum amplitude of the compared voltages was determined with similar calibrator settings. the di-3m ratio of output 1 was set to 0.000000. the ivd ratio of output 2 was varied in the range of 0.000001 to 0.000005. δv values were measured at the liadi resolution of 100 nv. again, the liadi relative deviation was calculated according to equation (8). the results of the measurements are presented in table 4. liadi fluke 5520 a ch 0 ch 1 r di-3m v0(t) δv vx(t) vref(t) figure 12. diagram of the setup to measure the vx channel input resistance of the liadi. table 2. calculation data on input resistances. resistor r indicator reading, (µv) input resistance, (mω) in circuit v0 5 200 in circuit vx -0.5 2000 out 1 out 2 liadi fluke 5520 a ch 0 ch 1 di-3m v0(t) vx(t) δv vref(t) figure 13. diagram of the setup for resolution measurement. table 3. liadi resolution. out-of-balance voltage in µv liadi output δu in µv relative reading deviation γ in % liadi resolution of 10 µv 100 100 < 0.5 200 200 < 0.3 300 300 < 0.2 400 400 < 0.2 500 500 < 0.1 liadi resolution of 1 µv 10 10 < 5.0 20 20 < 2.5 30 30 < 1.7 40 40 < 1.3 50 50 < 1.0 liadi resolution of 100 nv 10 10.1 1.0 20 20.4 2.0 30 30.7 2.3 40 41 2.5 50 51.2 2.4 liadi resolution of 10 nv 10 10.21 2.1 20 20.34 1.7 30 overload acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 108 5.5. liadi frequency range the frequency range of the compared voltages was defined according to the scheme in figure 14. again, the ch0 output of the calibrator supplied a voltage of 10√2 v with a frequency of 1 khz. the same signal was applied to the second di-3m ivd2 with a ratio of 0.500000. the ivd1 ratio of outputs 1 and 2 was set to 0.100000 and 0.100005, respectively, to compare the voltage amplitude of about 1√2 v. the δv values were measured at the liadi resolution of 100 nv for ten different frequencies (table 5). the relative frequency deviation of the liadi readings was calculated accordingly: ( )(1) (1) 100, f f f f v v v  −  =   (9) where δvf(1) and δvf are ivd out-of-balance voltages at frequencies of 1 khz and f, v. next, we set the ivd1 ratio of outputs 1 and 2 to 0.001000 and 0.001005, respectively, to compare the voltage amplitude of about 0.01√2 v. the measurements were repeated. the results of the frequency range measurements are given in table 5. the measurements show that for all of the frequency range of 0.02 to 100 khz, the relative error of comparison is less than 5 % (table 5). 5.6. liadi common-mode rejection the common-mode rejection was defined according to the scheme in figure 15. again, the ch0 output of the calibrator supplied a voltage of 10√2 v with a frequency of 1 khz. the same signal was applied to the di-3m with a ratio of 0.500000. the δv values were measured at the liadi resolution of 10 nv for 20 different frequencies (table 6). the common-mode rejection of the liadi was calculated according to equation (6). 6. conclusion in this article, we have proposed a new scheme for a lock-in amplifier with a differential input, liadi. the amplifier is based on two instrumentation amplifiers and a voltage follower and has a common-mode rejection that is better than 200 db at table 5. frequency range. a voltage comparison at voltage amplitude of 1√2 (v) frequency in khz 1 0.2 0.08 0.04 0.02 10 20 40 80 100 ivd out-of-balance voltage in µv 51.3 50.3 49.9 48.7 48.9 53 50.3 50 48.9 48.8 error in % – 1.9 2.7 5.0 4.7 3.3 1..9 2.5 4.7 4.9 a voltage comparison at voltage amplitude of 0.01√2 (v) ivd out-of-balance voltage in µv 50.5 50.1 49.7 48.8 48.5 52.7 51 51 48 48.2 error in % – 0.8 1.6 3.4 4.0 4.3 1.0 1.0 4.9 4.5 out 1 out 2 liadi fluke 5520 a ch 0 di-3m di-3m out 2 v0(t) vx(t) δv vref(t) figure 14. diagram of the setup for measurement of the frequency range. table 4. maximum and minimum amplitude of the liadi. out-of-balance voltage in µv liadi output δv in µv relative reading deviation γ in % maximum amplitude 10 10.2 2.0 20 20.3 1.5 30 30.6 2.0 40 40.7 1.8 50 51.1 2.2 minimum amplitude 10 10.1 1.0 20 20.2 1.0 30 30.4 1.3 40 40.8 2.0 50 50.9 1.8 liadi fluke 5520 a ch 0 di-3m out 2 v0(t) vx(t) δv vref(t) figure 15. diagram of the setup for measurement of the common-mode rejection ratio. table 6. common-mode rejection ratio. frequency in khz liadi output δv in nv cmr in db 0.02 < 10 > 200 0.04 < 10 > 200 0.06 < 10 > 200 0.08 < 10 > 200 0.1 < 10 > 200 0.2 < 10 > 200 0.4 < 10 > 200 0.6 < 10 > 200 0.8 < 10 > 200 1.0 < 10 > 200 2 < 10 > 200 4 < 10 > 200 6 < 10 > 200 8 < 10 > 200 10 < 10 > 200 20 10 > 200 40 40 > 180 60 90 > 180 80 210 > 160 100 820 > 160 acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 109 1 khz and greater than 160 db in the frequency range 20 hz to 100 khz. the amplifier has a high input impedance of 2000 mohm and 200 mohm for vx and v0 inputs, respectively. the new liadi has the following characteristics: • frequency range: 0.02 to 100 khz • input voltage range: 10√2 µv to 10√2 v • voltage resolution: 10 nv at a frequency of 1 khz • resolution: 100 nv, 1 μv and 10 μv. the developed liadi is used to measure harmonics in planar fluxgate sensors. it is also used to calibrate the ivd ratio at tomsk polytechnical univesity and at the all-russian scientific research institute of physical and radio technical measurements (vniiftri, moscow). acknowledgment the research was carried out at tomsk polytechnic university within the framework of the tomsk polytechnic university competitiveness enhancement program (grant # viu-ink 76/2017) and funded by the russian science foundation (rsf) (project # 17-79-10083). references [1] s. h. tsao, comparator for calibration of inductive 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[37] the usf ultra-stable low tc film resistors 200 series and 300 series: http://caddock.com/online_catalog/mrktg_lit/typeusf.pdf, p. 1 [access date: 25.12.2017]. https://doi.org/10.1109/ifost.2012.6357635 http://ieeexplore.ieee.org/document/4768713/ http://ieeexplore.ieee.org/document/4768713/ http://ieeexplore.ieee.org/document/4768713/ https://doi.org/10.1109/tim.2008.2011084 https://doi.org/10.1109/tim.2004.843377 http://www.thinksrs.com/downloads/pdfs/manuals/sr830m.pdf http://www.thinksrs.com/downloads/pdfs/manuals/sr830m.pdf https://doi.org/10.1109/tim.2002.802258 https://doi.org/10.1109/tim.2003.810716 https://www.ti.com/lit/ds/symlink/pga207.pdf https://dx.doi.org/10.1051/matecconf/201710201006 http://www.analog.com/media/en/technical-documentation/data-sheets/ad734.pdf http://www.analog.com/media/en/technical-documentation/data-sheets/ad734.pdf http://caddock.com/online_catalog/mrktg_lit/typeusf.pdf mobile manipulator control through gesture recognition using imus and online lazy neighborhood graph search acta imeko issn: 2221-870x december 2019, volume 8, number 4, 3 8 acta imeko | www.imeko.org december 2019 | volume 8 | number 4 | 3 mobile manipulator control through gesture recognition using imus and online lazy neighbourhood graph search padmaja kulkarni1, boris illing1, bastian gaspers1, bernd brüggemann1, dirk schulz1 1 fraunhofer fkie, fraunhoferstraße 20, 53343 wachtberg section: research paper keywords: imu-based gesture recognition; online lazy neighbourhood graph; olng; robot control citation: padmaja vivek kulkarni, boris illing, bastian gaspers, bernd brüggemann, dirk schulz, mobile manipulator control through gesture recognition using imus and online lazy neighborhood graph search, acta imeko, vol. 8, no. 4, article 2, december 2019, identifier: imeko-acta-08 (2019)-04-02 editor: yvan baudoin, international cbrne institute, belgium received november 22, 2018; in final form april 2, 2019; published december 2019 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: padmaja kulkarni, e-mail: padmaja.kulkarni@fkie.fraunhofer.de 1. introduction for robots to be able to work in unstructured environments, areas dangerous to humans, or disaster sites, human intelligence is still vital. in such cases, the teleoperation of robots could be a solution. with recent advancements in robotics, the complexity of using robots has also increased. nevertheless, currently used technology limits the majority of man-machine interfaces to text or gui-based interfaces and joysticks. such types of control can become cumbersome in the case of, for example, robots with a heavy control box or high degrees of freedom. often, working in disaster areas could be stressful for an operator. hence, alternate and intuitive control paradigms need to be developed. with recent progress in the field of hardware and software, focus has been given to easy interfaces wherein non-expert users can efficiently use the system. in order for new users to be able to use otherwise complex systems, gesture-based control seems particularly useful as it can be very intuitive [1], [2]. such a control can potentially eliminate the need for physical interfaces and enables richer human-robot interaction [3]. in well-established approaches for gesture recognition, vision-based gesture control is well researched, but the setup time and dependency on controlled environmental conditions, like lighting, make teleoperation unsuitable for disaster areas. on the other hand, hoffmann et al. [4] developed an inertial measurement unit (imu)-based control for a robot manipulator, which does not need any infrastructure. they transformed human arm motions into corresponding robotic manipulator motions using five imus attached to the sleeve of a wearable jacket. they showed that teleoperation performed in this way is very efficient and intuitive [4]. however, this direct control method cannot be used to trigger some predefined manipulator motion or to trigger robot-based motions. this paper presents an extension of the work done by hoffmann et al. [4] in the area of wearable imus. we present a framework based on an online lazy neighbourhood graph (olng) search, which can identify and classify dynamic gestures in real time and can be used to trigger predefined robot motions. the main contribution of this work is the implementation and evaluation of an olng search-based algorithm for gesture recognition and robotics application. additionally, we develop a software architecture that allows the integration of robotic platforms with our gesture recognition algorithm to trigger robot motions corresponding to the gestures with robot operating system (ros) middleware. prior to this olng search, the abstract gesture-based control potentially eliminates the need for wearisome physical controls and facilitates easy interaction between a human and a robot. at the same time, it is intuitive and enables a natural means of control. in this paper, we present and evaluate a framework for gesture recognition using four wearable inertial measurement units (imus) to indirectly control a mobile robot. six gestures involving different hand and arm motions are defined. a novel algorithm based on an online lazy neighborhood graph (olng) search is used to recognise and classify the gestures online. a software framework is developed to control a robotic platform through integrating our gesture recognition algorithm with a robot operating system (ros), which is in turn used to trigger predefined robot behaviours. experiments show that the framework is able to correctly detect and classify six different gestures in real time with average success rates of 81.61 % and 81.67 %, while keeping the false-positive rate low by designing and using only 126 training samples. mailto:padmaja.kulkarni@fkie.fraunhofer.de acta imeko | www.imeko.org december 2019 | volume 8 | number 4 | 4 algorithm was primarily used in the area of computer graphics [5]. 2. related work gesture recognition has applications in a wide range of areas, such as human-computer interaction, sign language recognition, gaming, household device control, and robot control [6]. most approaches in the field of gesture recognition are based on vision, imu, and electromyography (emg) signals [7], [8], [9]. furthermore, depending on the gesture types, gesture recognition techniques can be divided into static and dynamic gesture recognition. dynamic gestures are more difficult to recognise due to their temporal variations. however, they ascertain more natural interaction and are more practical [10], [11]. for imu-based approaches, many use glove-mounted sensors. mummadi et al. [12] used an imu-based glove for realtime sign language recognition. they used various machine learning algorithms, such as support vector machines, naive bayes, multi-layer perceptron, and random forest, to classify the gestures. wu et al. [13] used a data glove with perception and hidden markov models (hmms) to classify hand gestures. however, they only tested the results on simulated data. georgi et al. [8] coupled imu-based motion with emg muscle activity to recognise hand and finger gestures. they used hmms for the gesture recognition and obtained 74.3 % in accuracy with different users. shin et al. [14] developed a system based on imu and emg sensors for controlling a mobile robot. they employed hmms as the underlying algorithm for gesture recognition. however, these methods either only classified static gestures with the hand and fingers [8], [12], [14] or needed a huge database (about 1000 samples for each kind) [12]. in the domain of commercial products, the myo armband by thalmic labs uses emg signals along with imus to detect up to five different gestures and motions of the arm. various learning approaches like support vector machines [15], k nearest neighbour (knn), and dynamic time warping (dtw) [16] are used for gesture recognition with up to 86 % accuracy. however, a set of only five pre-defined gestures is supported. for vision-based gesture recognition, microsoft kinect is widely utilised. openni or kinect sdk software are used for motion tracking [17]. for gesture identification, algorithms like dtw, hmms, or artificial neural networks (anns) are implemented. for an overview thereof, see [18]. amin et al. [19] developed a vision-based technique to identify hand gestures using principal component analysis (pca) and gabor representation. yu. et al. [20] devised a control architecture for gesture-based control of uavs using the asus xtion camera. they defined nine gestures and used ros for integrating gesture recognition with uavs. they achieve a recognition rate of over 85 % in an indoor setting and with static gestures. lai et al. [21] developed a hand gesture recognition system using the microsoft kinect camera with opencv libraries. they obtained an accuracy of about 95 %. however, vision-based approaches have limitations like occlusion and are vulnerable to bad performance due to ambient lighting and background changes. considering the limitations of vision sensors and the lack of commercially available robust sensors, imu-based gesture control suits our control requirements. 3. approach in order to control a mobile robot with gesture recognition, a two-step procedure is necessary. the first step involves recognition of the gesture and the second involves designing the control architecture to trigger pre-defined robot behaviours. accordingly, the approach description is divided into two sections. the first one explains the underlying algorithm we developed for gesture recognition and the second section describes the complete system, including the robot control framework. 3.1. gesture recognition we assume that the imu readings are available in the form of vectors at a discrete time interval (… , �⃗�𝑡−2, �⃗�𝑡−1, �⃗�𝑡 , … ), where �⃗� is a vector of euler angles and t is time. this vector is referred to as input vector in this paper. an underlying training database consists of euler angles obtained from the four imus. a 12dimensional vector forms a data point in the database. for building the database, sequences of such vectors labelled with the corresponding gesture names are saved while the gestures are being performed. every vector in the database has a unique index i, which is later used for the identification of a particular vector. for sequence matching, a window of m input vectors is defined. the distance between the current input vector and each vector in the training database is calculated, and from this set of distances, the indices of the k nearest vectors (knns) and their spatial distances from the input vector are obtained and stored using fast library for approximate nearest neighbours (flann) in real time [22]. flann allows a fast nearest neighbours search by combining two existing approaches: i) kmeans trees and ii) randomised kd-trees. additionally, flann provides automatic selection of the algorithm and configuration of parameters for a given dataset and the desired precision for the task. a matrix is created for the knns in the training database and for a window of m input vectors. considering these k × m vectors as nodes, a graph can be created, as shown in figure 1. for each input vector, its k corresponding graph nodes are then sorted in ascending order of their database index i. for the sequence matching, nodes are chosen in such a way that their database indices are in strictly ascending order. a path is a sequence connected from the first vector in the window to the last vector. figure 1. k × m nodes used for the construction of the olng. k is the number of neighbours in the window of the latest m sensor readings [5]. acta imeko | www.imeko.org december 2019 | volume 8 | number 4 | 5 figure 2 shows the possible sequences for three input vectors. if no valid neighbour index for a particular input vector exists, then the neighbours of the next input vector are considered for the path. in that case, an extra path cost is added for skipping one input vector. in this way, all possible paths are listed, and the best path among them is chosen based on its minimum cost. olng search offers extremely fast sequence matching and is suitable for real-time applications due to its linear complexity [5]. for gesture recognition, the following procedure is applied. 1) knns to the current input vector are calculated using flann for the whole database. 2) olng is then used to find a matching and valid sequence for a window of m input vectors. 3) the best path found is saved along with its cost and sequence-matching length for comparison. 4) an arm and hand movement is considered to be a gesture if the sequence matching length is more than a certain threshold value and the cost of the path is less than another threshold. gesture recognition restarts when the robot signals the completion of a motion. all needed thresholds were chosen based on initial informal experiments and were tweaked and validated during our evaluation. in the next section, we present the overall system architecture including the robot control. 3.2. robot control architecture the control architecture is such that the user controls with a serial device in their hand whether or not the gesture recognition is triggered. a user with this serial device is shown in figure 3. the serial device has two buttons, coloured red and green, to start and stop the gesture recognition respectively. a flowchart of the control scheme used for achieving gesture-based control of a mobile manipulator is shown in figure 4. a user starts the gesture recognition by pressing a green button on the serial device. this enables data publishing with imus. the gesture recognition algorithm reads the imu data and extracts movements in the specified window size, including the latest data. then the algorithm checks if the input data corresponds to a gesture. if ‘yes’, then a pre-defined robot figure 2. path finding using olng search with k nearest neighbours the nodes of valid continuous paths are connected. the first valid path shown in the figure skips a column and hence it would be penalized [5]. figure 3. a user wearing the jacket with imus attached to it and holding a serial device. the green button is used to start the gesture recognition and the red button is used to stop recognizing gestures. figure 4. complete system flowchart. after starting the gesture recognition, the stop signal for the gesture recognition and system exit is continuously checked in different threads using ros. acta imeko | www.imeko.org december 2019 | volume 8 | number 4 | 6 motion is triggered. otherwise, the input gesture window is modified to include the latest imu data frame, and this new input window is checked for gesture recognition. each recognised gesture triggers a specific motion of the robot. during the robot motion, no gestures are detected. at the same time, a check for an exit signal from the user or a red button press is performed continuously. when the red button on the serial device is pressed, the imu measurement is stopped, and the system waits for a green button signal from the user to start the gesture recognition process again. until the green button is pressed, the user’s motion has no effect on the gesture recognition system. when the system exit is signalled, the system needs to be restarted. the programming was done in c++ using a linux operating system and ros as a middleware. ros is a software architecture that supports robotic platforms and includes various libraries to plan and execute robot motions. for generating and commanding motions to the robot moveit! motion planning framework is used. different software programs called nodes communicate in ros with published messages. using these messages, we establish a link between gesture recognition node and the robot moveit! interface. moveit! is a ros-integrated software architecture that provides collision-free trajectory generation for a robotic manipulator. these trajectories in turn can be used for various tasks, including picking up and placing objects while making sure that the geometric and hardware constraints of the robot are not violated [23]. 4. experimental setup we validated our olng search-based algorithm by testing it for online gesture recognition. xsens mtw sensors were used for the motion capturing and fo building a database. the sensor is a 9-axis imu consisting of a 3-axis gyroscope, acceleration sensor, and magnetometer, and it includes an extended kalman filter. internally, the sensors operate at 75 hz, but such a high rate does not offer much additional information during arm movements. hence, we used a frequency of 15 hz to update the input. four such sensors were mounted on a jacket at the humerus, radius, hand, and finger. the training database used consisted of 21 motions of each gesture from three different users for a total of 126 samples. the test database for obtaining the threshold cost values consisted of ten different motions of each gesture plus ten random arm movements. once the threshold cost value was obtained, the algorithm was tested with a different dataset consisting of 20 motions of each gesture. for olng search we considered k = 40 nearest neighbours, a window of m = 10 vectors, and a path length of 9. these parameters were tweaked and fixed based on our initial informal experiments. given these fixed parameters, the path cost was varied from 0 to 4 in steps of 0.05 square radians. we defined six gestures to test our algorithm: hurry, drive, gather, disperse, forward, and down. these are based on internationally used hand signals. figure 5 shows the gestures used. for the gesture ‘hurry’, the user performs an up and down motion of the arm with a closed fist above the shoulder. for the gesture ‘drive’, a closed fist left-to-right motion is performed. for the gesture ‘gather’, the arm rotates in a circular way above the shoulder. for the gesture ‘disperse’, the arm is moved up and down from the elbow keeping the hand straight. for the gesture ‘forward’, the arm is stretched downwards and with the hand facing outside, the forward motion of the arm is performed. for the gesture ‘down’, the hand is moved down from the wrist, keeping the rest of the arm steady. to apply robot motions, a telemax manipulator from telerob was used. the robot with the telemax arm is shown in figure 6. the robotic arm has seven degrees of freedom, including one telescopic joint. the gestures defined are used to control the robot manipulator to perform the behaviours listed in table 1. figure 5. (a)-(f) show the six different gestures defined in our experiment. figure 6. the robot hardware used for demonstrating the gesture-based control. acta imeko | www.imeko.org december 2019 | volume 8 | number 4 | 7 5. results the receiver operating characteristic (roc) curves for all six gestures with varying path costs are shown in figure 7. the areas under these curves are defined in table 2. it can be observed that these areas are significantly above 0.5 (value for a random guess); hence, we conclude that the algorithm we used is able to distinguish the gestures well. based on the roc curves, the threshold path cost was chosen to be 1.80 square radians with 81.61% true-positive rate and a falsepositive rate of 15.12 %. by including random arm movements in the test database, all thresholds were also validated to only recognise a gesture when one was performed. the confusion matrix corresponding to the chosen cost is shown in figure 8. ‘down’ was considered the best at the chosen cost, with a 90 % prediction rate. it can be observed that the gesture ‘disperse’ is more difficult to recognise, as it shares similarity with the whole gesture ‘down’. hence, the recognition rate of the gesture ‘disperse’ was observed to be 75 %, the lowest among all other gestures. the same partial similarity is observed between the gestures ‘gather’ and ‘hurry’, which leads to the non-symmetrical confusion matrix. nevertheless, we were still able to obtain a recognition rate of 80 % for both gestures. the recognition rates for all the gestures are given in table 3. it can be observed that the gesture recognition rate is above 75 % for all the gestures. 6. conclusion and future work a novel algorithm based on olng search was implemented and evaluated for the application of gesture recognition in real time and used for mobile manipulator control. experiments were performed to validate our approach for gesture recognition. areas under the roc curve for all gestures were observed to be higher than the random-guess value of 0.5. the highest area obtained was 0.97, and the lowest was 0.75, proving the viability of the algorithm. the results showed that we could obtain a gesture recognition rate of above 75 % for all six gestures, with a maximum recognition rate of 90 % at the chosen cost. the table 1. the gestures and corresponding robot motions. gestures robot behaviour drive drive slightly forward. hurry increase the speed of the robot base. gather lookout position for the robot manipulator. disperse fold arm into transport position. forward extend arm forward for inspection. down decrease the speed of the robot base. table 2. areas under the roc curve for the gestures with variable threshold path costs. gestures robot behaviour drive drive slightly forward. hurry increase the speed of the robot base. gather lookout position for the robot manipulator. disperse fold arm into transport position. forward extend arm forward for inspection. down decrease the speed of the robot base. figure 7. roc curve for all gestures with variable threshold path costs. a path cost corresponding to high true-positives and low false-positives is chosen. table 3. gesture recognition rates with the chosen threshold path cost. gestures gesture recognition rates drive 80.0% hurry 80.0% gather 80.0% disperse 75.0% forward 85.0% down 90.0% figure 8. confusion matrix for gestures with the chosen threshold path cost. acta imeko | www.imeko.org december 2019 | volume 8 | number 4 | 8 average recognition rate obtained in the later tests was 81.67 % compared to the initial rate of 81.61 % computed from the roc curve, while keeping the false-positive rate low by design. a video showing our gesture recognition algorithm is available online at https://youtu.be/uilnmsn46di. as a part of this work, we also developed a software framework to trigger predefined robot motions based on a detected gesture. this framework was integrated with our gesture recognition in ros middleware. the developed architecture is robot independent; hence, it facilitates the integration of various robotic platforms. the developed framework also allows easy definition of new gestures and their addition to the existing database. in the future, we aim to expand our training database by adding correctly recognised gestures to it. we would also like to extend our algorithm to match parts of the start, middle, and end of a gesture to counter similarities between different gestures. this will enable us to further minimise false-positives and increase the robustness of the algorithm. for additional efficiency of finding the nearest neighbours, nanoflann [24], an optimised version of flann, will be integrated into the algorithm in the future. from a software architecture point of view, we would also like to include user feedback in the loop before sending commands to the robot based on a detected gesture. this will enable us to avoid robot motions due to wrongly detected or wrongly classified gestures, making the control more robust to both the false-negatives of the algorithm and human errors. references [1] e. coronado, j. villalobos, b. bruno, f. mastrogiovanni, gesture-based robot control: design challenges and evaluation with humans, proc. of the ieee international conference on robotics and automation (icra), 2017, singapore, pp. 27612767. 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[24] nanoflann, https://github.com/jlblancoc/nanoflann [accessed on: 20.11.2018] https://github.com/jlblancoc/nanoflann introductory notes for the acta imeko special section on the “22nd symposium on the measurement of electrical quantities” and the ‘’20th workshop on adc/dac modeling and testing’’ acta imeko issn: 2221-870x december 2018, volume 7, number 4, 1 2 acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 1 introductory notes for the acta imeko special section on the “22nd symposium on the measurement of electrical quantities” and the ‘’20th workshop on adc/dac modeling and testing’’ alexandru salceanu1, vilmos pálfi2 1 “gheorghe asachi” technical university of iasi, iasi, romania 2 budapest university of technology and economics, budapest, hungary section: editorial citation: alexandru salceanu, vilmos pálfi, introductory notes for the acta imeko special section on the “22nd symposium on measurement of electrical quantities” and the “20th workshop on adc/dac modelling and testing”, acta imeko, vol. 7, no. 4, article 1, december 2018, identifier: imeko-acta-07 (2018)-04-01 editor: dušan agrež, university of ljubljana, slovenia received december 20, 2018; in final form december 20, 2018; published december 2018 copyright: © 2018 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: alexandru salceanu, e-mail: asalcean@tuiasi.ro distinguished readers, this finalizing issue of acta imeko’s volume 7 presented here is based on the 22nd international symposium and 20th workshop on adc/dac held in iasi, romania in september 2017 by imeko technical committee number 4, (measurement of electrical quantities). this joint event was organized and hosted with the aim of supporting world development through electrical and electronic measurements and of bringing to the debate among the global community of specialists the latest results achieved by professors and researchers from 17 countries with significant experience and results achieved within the realm of the imeko confederation. out of the 109 presented papers, the section chairmen and members of the scientific committee rigorously selected the 20 most suitable papers according to the standards, requirements, and reputation of acta imeko journal. after much hard work, in the company of a very demanding panel of reviewers but with a constructive approach, the best 12 papers were finally polished and consequently accepted for this issue. a common feature of the articles presented here is their innovative approach and technically and financially affordable solutions. in this vein, the paper presented by sandu et al. describes the benefits of and principles underlying measuring defects handling metrics in automotive programs and organizations that have adopted agile software development, presenting the iteration containment effectiveness, program increment containment effectiveness, and defect debt trend metrics. the acquired advantages are demonstrated by a detailed reallife application of how to measure the classic phase containment effectiveness metric. vremera at al. propose a new improved design of a dualchannel broadband twin micro-calorimeter based on the design of hardware and new suitable algorithms, consequently allowing for the self-calibration of the system without the dismounting/mounting operations between measurement cycles. the main benefits include its shorter measurement time, maintenance of at least the usual accuracy of microcalorimeters, improvement in thermal noise rejection, and perfect correlation of the measured data series with the calculated data. möwius et al. present an introspection into the universe of permanent magnets from the perspective of the newest techniques and methods that allow their properties to be measured after final processing, often even in situ, within the sub-assemblies in which they actually operate. the synthetic analysis here-presented balances the advantages and limitations of the considered methods, related to the achievable accuracy level. the aim of the paper submitted by bongiorno et al. is to deal with the variability in railway track-to-ground conductance measurements, mainly due to the connection of the negative terminal of the power supply to a grounding electrode instead of the behind section and due to the location chosen for the voltage terminal. a simulation model including unideal soil is mailto:asalcean@tuiasi.ro acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 2 proposed and used to maintain appropriate correspondence between realistic simulation and accurate measurements. de zanet et al. focus on the fast methods of blood impedance determination used in the pre-analytical stage of blood sample analysis, where the cause of most laboratory errors is found. a fast and mobile device is a desirable task for any blood analysis, and electrical impedance measurement has considerable applicative potential. a biosensor to separate different blood components, to identify hemolysis in serum, to evaluate blood quality, and to promptly quantify hematocrit levels is proposed. “andante” is the smart walker intended to monitor and analyze human gait in real time, developed by viegas et al. the system makes use of e-textile electrodes to sense the user’s heart rate, load cells to measure the forces applied on the walker legs, and an inertial measurement unit to sense motion and orientation. all this data is sent via bluetooth to a local host and can be stored after processing to reconstitute the individual's initial walking style, in other words, the gait legacy. the proposed design might be easily integrated into any existing walker on the market. tavčar et al. detail with conviction the implementation and testing of an acoustic thermometer with a single waveguide, presenting its small size, improved sensitivity, and maintenance of an acceptable common sound path. solutions for treating some weaknesses (low return signals and possible coincidences of the return signals) are also proposed. a standard deviation of 0.050 °c with 66 independent readings per second could be described as an astonishing result. the study of riess et al. concludes that while heating an electric car during winter time, the energy savings achieved by using a bioethanol fuel-operated heater instead of an electrical heater is up to 30 %. the design, actual installation in the car, measurements, and test drives are presented in detail, mainly supporting the much-desired range extension. the experience of santobono-pausilipon hospital regarding technology-based rehabilitation pathways is shared with our readers by iuppariello et al. children with motion impairments (like gait disorders following obesity and neurological diseases) could benefit from specialized assistance. physiological parameters (such as gait or balance) might be registered and modelled, robotics and virtual reality systems providing alternatives to conventional therapy. two such complementary and innovative approaches are described in detail. castello et al., starting from the knowledge of the ongoing conditions of an electric grid, have implemented a new transmission logic for the transfer of data between phasor measurement units and phasor data concentrators. the innovative approach proposed here is to increase the measurement reporting rate only when the electric system changes from a steady-state condition to a dynamic one (which might be a defect or unwanted transition). the danger of losing the important characteristics of the surveyed transition is mitigated by sending to the concentrator, after it is revealed, supplementary data related to the pre-trigger time interval in a burst of data packets, which are relevant and providing information about its previous evolution. a characterization of the close magnetic field produced by various home appliances and a possible cause-effect relationship with the drawn current is investigated by ursache et al. an ingenious experimental setup allows for the ease of determination of the correspondence between current consumption and the generated magnetic field in the near environment. forty-seven home appliances were tested, covering almost all the entire spectrum, and they were divided into two main categories: 26 with an electric motor in normal functioning and 21 without. as the authors anticipated, no linear dependence between the current consumption and the magnetic field generated by the household appliances from both classes was found. plausible and reasoned explanations for apparent non-compliance with the biot-savart law are provided. a prolonged and rigorous activity of monitoring eight photovoltaic plants over seven consecutive years provided interesting results that are displayed and processed here by carullo et al. the obtained data is relevant mainly due to the appropriate selection of the plants that were included in the monitoring: units with different applied technologies, with fixed or mobile panels, in various environments. the multi-channel data acquisition system, which measures both the environmental-specific parameters and the supplied electricity in real time, is efficiently used to determine the rate of degradation of each plant that is being monitored. the authors drew conclusions about choosing the most effective solutions for photovoltaic plants, in terms of costs, lifetime, as well as the statistical weather characteristics of the environment in which they are located. we are deeply honored to have acted as guest editors for this innovative and challenging issue of acta imeko. the release presented here is the result of the tireless efforts of a large but homogenous team, integrating authors, reviewers, copyeditors, layout editors, and, last but not least, the new editor in chief, prof. dušan agrež, who provided outstanding support during the whole process. we felt that this quasi-large team is entirely devoted to this wonderful ascendant project known as acta imeko. our greatest joy is that together we have managed to bring to our readers worldwide a considerable number of scientific articles of undeniably new added value, which are rigorously substantiated, accessible to a wide range of specialists, and have a high practical applicability potential. alexandru salceanu vilmos pálfi guest editors acta imeko, title acta imeko issn: 2221-870x december 2017, volume 6, number 4, 121-130 acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 121 markov process reliability model for photovoltaic module failures loredana cristaldi, mohamed khalil, marco faifer dipartimento di elettronica, informazione e bioingegneria, politecnico di milano, milano 20133, italy section: research paper keywords: failure analysis; markov process; photovoltaic module; reliability model citation: loredana cristaldi, mohamed khalil, marco faifer, markov process reliability model for photovoltaic module failures, acta imeko, vol. 6, no. 4, article 19, december 2017, identifier: imeko-acta-06 (2017)-04-19 section editor: lorenzo ciani, university of florence, italy received: october 13, 2016; in final form october 6, 2017; published december, 2017 copyright: © 2017 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: mohamed khalil, e-mail: mohamedmahmoud.khalil@polimi.it 1. introduction photovoltaic (pv) systems are installed all around the world to produce electricity from solar energy. the evaluation of their long-term reliability is one of the fundamental values of their assets [1] and should be inclusive of both a complete and partial outage of pv systems in addition to their operation at a level below expectations. in the last years, pv systems have been changing from small isolated systems to large grid-connected power stations. therefore, any failure associated with pv modules can affect the overall performance of the power system and generate safety issues. consequently, comprehensive studies have been carried out to explain the failures of pv modules. for instance, quintana et al. [2] reported the different degradation mechanisms of aged pv modules in the field, chattopadhyay et al. [3] presented an analysis of degradation data of the indian survey on pv modules, cristaldi et al. [4] proposed a diagnostic monitoring architecture based upon the analysis of the pv module failure causes, and a detailed failure analysis of pv module was reported by an international energy agency in [5], etc. based on the available studies, seven main failure causes for a pv module can be identified in the field: dust, host spot, corrosion, cell cracks, semiconductor ageing, and broken interconnects and soldier busses. pv encapsulation failures contribute as well in the failure of pv modules. this includes encapsulation delamination, discoloration, moisture ingress, and module broken glass. the failures of module encapsulation are excluded in this analysis in order to have a clear view on the reliability of the pv module without the encapsulation. this will allow the manufacturers and decision-makers to have the capability to enhance the lifetime of pv modules separately. in addition, it will provide them with a clear view on the impact of each failure mode on pv reliability, and probability of occurrence for each failure mode. different techniques from the literature were used to estimate the pv module reliability. for instance, charki et al. [6] and lorande et al. [7] used a petri network to represent the failures of various components in a pv system and estimate its availability. theristis et al. [8] constructed a markov model for a standalone photovoltaic system in order to evaluate loss of load probability (lolp) based upon the failure and repair rate of each component in the system. zini et al. [9] used a fault tree technique in order to understand the impact of balance of system (bos) components on the system overall reliability. reliability block diagrams were considered in [10], [11] to measure loss of load probability, availability, and capacity, associated with the performance degradation in pv systems. abstract this paper presents a markov process reliability model for a photovoltaic module. this model includes all the possible failures associated to the operation of a photovoltaic (pv) module. failure modes are identified in detail, afterwards all the possible failure states of the pv module during the service are considered. finally, a complete markov process is attained to assess the probability of each failure mode occurrence and estimate the mean time to failure, probability density function of the time to module failure, hazard and survival functions of a pv module. acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 122 generally, two different reliability scenarios are widely encountered during the useful life time and wear out phase of systems based on the warranty limits. the first scenario is related only to the full performance so any failure within the pv modules will have a significant impact on reducing the whole system output below the warranty limits. as a result, the system states associated with these failures will be treated as a down state, and these failures are called reliability failures. the second scenario considers the degrade performance as up states, and the associated failures are called durability losses or soft failures. this is applicable as long as the performance of the pv module meets the warranty limits. both scenarios are based on nonrestorable systems and contain only transitions in the direction towards system failures. more details on reliability failures and durability losses are available in [12], [13]. in this paper, the second scenario is highlighted. for the purpose of abbreviation, the term ‘failure’ refers to ‘soft failure’. in this work, a complete framework for evaluating the reliability of pv modules using a markov process is presented based on the aforementioned failure causes. it is aimed to estimate the reliability of the system on the long-term and the probability of each failure mode occurrence, in addition to the calculation of the probability density function, hazard rate, and mean time to failure (mttf) of a pv module. it is very important to mention that the whole framework is distinct from the general reliability modelling methodologies in [14], as the operational and reliability aspects of photovoltaic modules are considered based upon the failures that affect performance requirements. moreover, the markov model processed, in this work, is totally different from the one in [8] whose authors provided a component transition diagram to measure the loss of load probability (lolp) based on the failure and repair rates of each component in a standalone pv system. in our analysis, a markov model on the different failure modes of pv module components is constructed, regardless the pv system arrangement, in order to measure the previously mentioned reliability indices, where the failure rates of these failure modes are the transition rates between the different functional states of the pv module component. monitoring activities are essential for repairable systems and components with high probability of failures. however, pv modules cannot be repaired, and they are replaced once they fail. therefore, it is quite hard to implement the necessary monitoring activity, especially if the pv plant is not easily accessed by the maintenance crew. the proposed markov model can be used as a statistical tool to measure the failure frequency once the probabilities of occurrence are estimated [14] in addition to the contribution of each failure mode in the failure frequency of the system. a markov process is a stochastic process that allows to model systems characterized with several states and the transitions between states. for a random variable x(t) that denotes the state of the system at time t and the collection of all possible states is called state space denoted by {x= 0,1,..,r}, the conditional probability of the state x(s)=i at time s to be at state j at time t+s is: 𝑃{𝑋(𝑡 + 𝑠) = 𝑗|𝑋(𝑠) = 𝑖, 𝑋(𝑢) = 𝑥(𝑢), 0 ≤ 𝑢 ≤ 𝑠} (1) equation (1) shows the necessity to know the past history of the process to know the current state. however, a markov process requires only the knowledge of the current states regardless the past history of the system as shown in (9). this feature is an advantage of using a markov process. through this work, manufacturers and designers will have a clear view on the reliability indices of pv modules in order to have more reliable pv modules at the end of the design phase. in addition, it assists decision makers during the operational phase to enhance the maintenance schedules and monitoring strategies by prioritizing the failure modules in terms of their occurrence and their impact on the functional reliability of pv modules. this paper is organized as follows: section 2 presents the different failure modes of pv module, section 3 introduces the markov process. section 4 discusses the markov analysis of module failures and section 5 presents the reliability evaluation of pv module. finally, section 6 includes conclusions. 2. pv module failures a pv module consists of solar cells interconnected together by ribbons, and an encapsulant material to protect the components of a pv module from foreign impurities and moisture. the failure causes of pv modules are briefly described in the following subsections. 2.1. dust the accumulation of dust on the pv module’s surface area can produce spots with varying concentrations. these spots vary in shape, location and concentration density. the variation in dust accumulation in any place can lead to different transmittance of light into the module. this will result in small random areas on the pv module with less exposure to solar radiation [15], [16]. dust deposition depends on its density and size distribution [17]; accordingly, the average degradation of a pv module performance due to dust fouling is not uniform and highly dependent upon the location and weather conditions. for instance, elminir et al. [18] observed a reduction in pv output power up to 17.4 % per month in a separate field experiment in egypt during the months of december 2004 to june 2005 while kymakis et al. [19] monitored the performance of a pv park in crete for one year and they found out the soiling losses were 4– 5 % during the winter and 6–7 % during the summer period. cabanillas et al. [20] observed a reduction of modules power up to 13 % within 3 months in mexico. on the other hand, experimental investigation on the reduction of the pv output efficiency presented in [21] shows that the reduction of efficiency reached up to 11.6 % when the dust deposition density was fixed at about 8 gm-2. although dust is a detrimental agent whenever solar-energy applications are concerned, in literature reviews in [22], [23], all the attention was given to estimate the degradation rate of a pv module, in short periods up to one year, and no studies are available for the failure rate of the dust when the output power is below the expectations. accordingly, the failure rate of dust, in the work is calculated based on two important performance parameters: the guaranteed output power and the annual degradation rate under the effect of dust. currently, many warranties guaranty that the output power will be at least 90 % of its initial nominal power in 10-12 years and reaches 80 % after 20-25 years. consequently, if the output acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 123 power is below 80 % of the nominal power, the pv module fails [24]. regarding the degradation rate, an interesting six-year experimental work by ryan et al. [25] concluded an annual degradation rate of 1.4 %, due to dust, that can be considered as an accepted value over long time. generally, pv module power follows a gaussian distribution [26], [27]. accordingly, the associated probability density function is 𝑃(𝑝) = 1 √2𝜋𝜎 𝑒𝑥𝑝 �– 1 2 ( 𝑝– µ 𝜎 )� (2) where p is the power of module, µ is the average power and σ is the standard deviation of the pv module’s power. as the average power of the pv module decreases linearly versus years of operation [28], [29], the average power can be calculated from, µ(𝑡) = 𝑃0 – 𝐴𝑡 (3) where po is the average power at t=0, a is a parameter that reflects the annual reduction in power such that a/po represents the annual degradation rate per year, therefore a/po =0.014. the value of σ can be related to po using the concept of manufacturing tolerance for the module output power [24], so σ =0.0167 po. taking into account the time variation, the power distribution (2) at a defined instant t will fit the following distribution: 𝑃(𝑝, 𝑡) = 1 √2𝜋𝜎 exp �− 1 2 �𝑝− (𝑝0−𝐴𝐴) 𝜎 � 2 � (4) by definition, r(t) = 1−f(t) where f(t) is the cumulative distribution function. thus, 𝑅(𝑡) = 1 − 1 2 [1 + 𝑒𝑒𝑒 (𝑝−𝑝0+𝐴𝐴 𝜎 √2 )] (5) where erf is the gauss error function. substituting p=0.8 po , a= 0.014 po, σ =0.0167 po in (5), gives 𝑅(𝑡) = 0.5 − 0.5𝑒𝑒𝑒(0.5928𝑡 − 8.4683) (6) hence the failure rate function is estimated as follows: h(t)= − d d𝐴 𝑙𝑙𝑅(𝑡) = 0.3344𝑒𝑒𝑝 [− (0.5928𝐴−8.5746)2] 0.5−0.5𝑒𝑒𝑒 (0.5928𝐴−8.4746) (7) 2.2. corrosion corrosion of the conductive parts of the cells and the interconnections is responsible for the deterioration of the pv module and it happens frequently among the pv modules. for instance, gobind h. et al. [31] observed a significant corrosion in 31 out of 640 photovoltaic modules at 15 kwp gridconnected photovoltaic system after three years in service. corrosion results in an increase of ohmic resistance and a decrease of the shunt resistance [30]. in general, it contributes in 45 % of the pv module's failures [32], [33]. 2.3. hot spots shading conditions, mismatching between cell electrical characteristics, and bypass diode failure contribute in the occurrence of hotspots [34]. in the field, solar cells arrays might be subjected to shadows from predictable sources such as weather and environmental conditions, and unpredictable sources such as bird droppings or fallen leaves. hot spots in pv modules result in contact delamination, melting of encapsulation layers and cells damage. 2.4. broken interconnect and solar buses failures solar cells consist of two basic elements: front and rear contacts, to deliver current to the external circuit. electrical current is carried by bus strips that are soldered to the front and back contacts. interconnect breaks occur as a result of thermal expansion and contraction or repeated mechanical stress. also, kinks in interconnects contribute to breaking of interconnects [35]. broken interconnects result in short circuited cells and open circuited cells. moreover, solder bond failures may cause a rise of 10 °c in the operating temperature of the cell. this increase in temperature will reduce the operational efficiency to about 97 % of the healthy conditions [36]. 2.5. cells cracks cell cracking is a common problem encountered in photovoltaic modules operation. it may develop through different stages of the module’s lifetime: during manufacturing, the soldering induces high stresses into the solar cells [37], [38], and during transport through handling and vibrations [39]. in addition, a module in the field experiences mechanical loads due to wind (pressure and vibrations) and snow (pressure). cracking of cells occurs at a rate of about 1 % per year. although 1 % failure rate is small, it leads to significant power degradation; because it causes around 1-10 % open circuit cell failures, a decrease in the filling factor and cells mismatching [40], [41]. generally, cell cracks are formed in different lengths and orientation. cracks parallel to busbars have a high probability, 16 to 25 %, of generating a potentially separated cell area [42][44]. consequently, the failure rate of cell cracks parallel to bus bars are only considered in this work. 2.6. semiconductor aging of pv material thermal ageing of semiconductors is a result of exposure to uv rays for long intervals of time. high temperature and electric fields experienced by the solar cells in a pv module result in the transport of atoms and ions that cause lattice deformation [45]. the lattice affects the structure of the solar cell and changes the electrical properties of the cell. therefore, the old pv module performance is reduced due to semiconductor material ageing. ageing results in an increased series resistance or decreased shunt resistance in addition to antireflection coating deterioration. it is well known that the lifetime of photovoltaic modules follows a weibull distribution with β= 2.6 [46]. meanwhile, the lifetimes of semiconductor devices of special applications are expected to be from 50 to 100 years [47]. taking into account the environmental extremes in which the pv modules operate, η is assumed to be 50 years for β= 2.6. 3. markov analysis the aforementioned failure causes occur during the operation of pv modules and result in poor performance and power losses. therefore, it is crucial to include these failures in the reliability model. as already stated in the introduction, several reliability techniques were considered in the literature and they highlighted the component failures. in this paper, the markov process is selected to carry out the functional reliability analysis. accordingly, the pv module is represented by the number of possible states, where each state represents the occurrence of one or more failures. markov chains are sequences of random variables in which the future variable is determined by the present variable and acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 124 independent on the way in which the present state arose from its predecessors. the analysis looks at a sequence of events and analyzes the tendency of one event to be followed by another [14]. this tendency is the probability evaluation of transition from one state to another until the system has reached the final state. thus, a markov process is defined by a process {p(t), t≥0} with state space x= {0, 1, 2, 3….r} and stationary transition probabilities, 𝑃𝑖𝑖 = 𝑃𝑒(𝑝(𝑡) = 𝑗│ 𝑝(0) = 𝑖) for all 𝑖, 𝑗 ∈ 𝑋 (8) where p(t) is a random variable that denotes the state and belongs to state space x. in this work, the state space is finite, for instance a system of n failures will have 2n states. for a process of probability state vector p(t) = [ p0(t), p1(t),….pr(t)], the distribution p(t) can be estimated from kolmogorov forward equation as follows, d𝑝(𝐴) d𝐴 = 𝑝(𝑡). 𝐴 (9) and ∑ 𝑝(𝑡)𝑛𝑖=0 = 1 (10) where dp(t)/dt is a vector that represents the state probability p(t) at time t, and a is the matrix of failure rates between states. considering failure rates from a state i to state j, the failure rates can be arranged in the matrix form           = rrr r a λλ λλ    0 000 (11) a is also known as a generator matrix. its diagonal elements are negative and they represent the rate of staying at the same state. it is worth to mention that the sum of the raw entries is equal to zero. as the number of possible states is finite, (10) is necessary since the probabilities of all states at any time t should be one and the system can be in one and only one of these states [48]. in case of zero repair rates, i.e. poisson birth-death process, (9) can be rewritten as 𝑑𝑝𝑖(𝐴) 𝑑𝐴 = – ∑ 𝜆𝑖,𝑖 𝑖≠𝑖 𝑃𝑖(𝑡) + ∑ 𝜆𝑖,𝑖 𝑖≠𝑖 𝑃𝑖(𝑡) (12) through the markov process, system characteristics can be measured like the state visit frequency, system availability, mean time to failure, probability distribution of failures and system performance. although a markov process, from the theoretical viewpoint, is flexible and versatile, special precautions are necessary to deal with the difficulties of practical applications. the main problem is that the number of system states and possible transitions increases rapidly with the number of events in the system [49] and therefore some assumptions are necessary. the usual assumptions considered by current standards and references, i.e. iec 61508 [50], iec-61165 [49], [48], and [14], can be summarized as follows: i) failure and repair rate are constant, ii) failure and repair events are independent, iii) the transition probability from one state to another state occurs within a very small time interval, iv) only one event occurs at the same time. furthermore, absorbing states can be used as an assumption that helps in numerical mathematical limitations [14], [51]. states are absorbing when they are once reached and the system will remain there forever. the model states are divided into three groups of states: up, degrade and down, as shown in figure 1. this kind of models with zero repair rate [49] focuses on reliability evaluation r(t), mttf, and hazard rate. the same assumptions and limitations considered in section 3 are followed up: i) environmental and operational conditions are constant, consequently, failure rates of different failure causes are constant, with an exception to module aging and dust whose failure rates are affected by the ageing and accumulation factors respectively. ii) the repair rates, in this model, are equal to zero; because system analysts pay a lot of attention to mttf;, also the failure of a pv module means a replacement. iii) the degrade state is still an up state. the failures occurrence leads to a drop in pv module performance but still not below the 80 % warranty. table 1 illustrates the possible states of a pv module associated with various failures. although all failure causes are independent, ageing of a pv module is assumed to be a failure which affects the operation of all module states. this is a reasonable accepted assumption if we consider the replacement of the whole pv module as a solution to eliminate that failure. therefore, each state including ageing failure will be treated as an absorbing state, as shown in figure 2. from the state transition diagram in figure 2 and (12), the state space equations can be written by inspection as shown below: 𝑃0•(𝑡) = −(𝜆1+ 𝜆2+ 𝜆3 + 𝜆4 + 𝜆5 + 𝜆6) 𝑃0(𝑡) (13a) 𝑃1•(𝑡) = 𝜆6𝑃0(𝑡) – (𝜆1+ 𝜆2+ 𝜆3 + 𝜆4 + 𝜆5) 𝑃1(𝑡) (13b) 𝑃2•(𝑡) = 𝜆5𝑃0(𝑡) – (𝜆1+ 𝜆2+ 𝜆3 + 𝜆4+ 𝜆6 ) 𝑃2(𝑡) (13c) where 𝑃•(𝑡) is the rate of change of state probability with respect to time. absorbing states are of no interest from a reliability point of view, since they are reachable from other states and the limit of their probabilities with respect to time is one: lim𝐴→∞ 𝑃absorbing (𝑡) = 1. according to [52], the 64 states of the markov process can be reduced into two states: success and failure states, as shown in figure 3, where the probability of failures is the summation of all down states. therefore, the absorbing states probability pd(t) represents the probability the system is down, where pd(t) is evaluated from: 𝑃d(𝑡) = ∑𝑃absorbing (𝑡) = 1 − ∑𝑃non-absorbing(𝑡) (14) up state degraded state down state ( absorbing state) figure 1. state transition diagram with three states for evaluation of reliability for one element. figure 3. equivalent two-state markov process from an eight-state markov process [52]. acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 125 4. markov process of pv module failures the manufacturer warranty period typically exceeds 20 years for crystalline silicon modules and 15 years for thin-film modules. unfortunately, there is little field monitored data and independent accelerated test data available to support most of these warranty claims. the failure rates of pv modules were evaluated and listed in table 2. the failure rates are estimated based on the frequency of the pv module failures, noted in literature, for certain population over a determined interval. 4.1. system state probability based on the state space equations (13) and failure rates in table 2, the probabilities of pv module states that are associated with one and two simultaneous failure occurrences are shown in figure 4 and figure 5, respectively. pv module states associated with more than two simultaneous failure occurrences are avoided because of their low probabilities of occurrence. for example, the state probability of three and four simultaneous failure events ranges from 0 to 0.19 and 0 to 0.025 simultaneously. figure 4 shows that cell cracks failure event, represented by system state p2(t) has the highest probability of occurrence and increases steeply. this is due to its high failure rate and its associated significant power degradation. on the other hand, failures due to dust events of state p16(t) occur, as expected, over a large range of operating time because dust is considered one of pv environmental extremes which has its impact on the operation of the pv system as long as the pv module is in service even if its probability of occurrence is small. accordingly, the probability of state p18(t) , in which cell cracks and dust failure events might occur simultaneously, is table 1. system states. state a geing ( λ1) d ust (λ2) h otspot (λ3) corrosion (λ4) cells cracks (λ5) broken interconnects (λ6) system state 0 n n n n n n s 1 n n n n n y s 2 n n n n y n s 3 n n n n y y s 4 n n n y n n s 5 n n n y n y s 6 n n n y y n s 7 n n n y y y s 8 n n y n n n s 9 n n y n n y s 10 n n y n y n s 11 n n y n y y s 12 n n y y n n s 13 n n y y n y s 14 n n y y y n s 15 n n y y y y s 16 n y n n n n s 17 n y n n n y s 18 n y n n y n s 19 n y n n y y s 20 n y n y n n s 21 n y n y n y s 22 n y n y y n s 23 n y n y y y s 24 n y y n n n s 25 n y y n n y s 26 n y y n y n s 27 n y y n y y s 28 n y y y n n s 29 n y y y n y s 30 n y y y y n s 31 n y y y y y f state a geing ( λ1) d ust (λ2) h otspot (λ3) corrosion (λ4) cells cracks (λ5) broken interconnects (λ6) system state 32 y n n n n n f 33 y n n n n y f 34 y n n n y n f 35 y n n n y y f 36 y n n y n n f 37 y n n y n y f 38 y n n y y n f 39 y n n y y y f 40 y n y n n n f 41 y n y n n y f 42 y n y n y n f 43 y n y n y y f 44 y n y y n n f 45 y n y y n y f 46 y n y y y n f 47 y n y y y y f 48 y y n n n n f 49 y y n n n y f 50 y y n n y n f 51 y y n n y y f 52 y y n y n n f 53 y y n y n y f 54 y y n y y n f 55 y y n y y y f 56 y y y n n n f 57 y y y n n y f 58 y y y n y n f 59 y y y n y y f 60 y y y y n n f 61 y y y y n y f 62 y y y y y n f 63 y y y y y y f table 2. pv module failure rates. failure mode parameter ageing of pv module β η β λ t=1 yrs50;6.2 == ηβ [46-47] dust )(2 th=λ hot spot 1 3 012.0 −= yrλ [42] corrosion 1 4 023.0 −= yrλ [3] cells cracks 1 5 091.0 −= yrλ [53] broken interconnect & soldier bus failures 1 6 0031.0 −= yrλ [54] acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 126 very high as shown in figure 5. through figure 4 and figure 5, it is possible to measure the probability of each failure mode occurrence after a certain period of operation. for example, figure 4 shows that the probability of occurrence of dust failure and cells cracks are 0 and 0.4945, respectively after 10 years of operation. these probabilities of failure occurrence change after 15 years of operation to be 0.1529 and 0.1358 for dust and cells cracks, respectively. generally, it can be concluded from figures 4 and 5 that the more operating years, the higher is the probability of occurrence for states that have more numbers of failure events. the peaks of system states probability curves in figures 4 and 5 decrease after a certain time because the probability of the pv module being in the absorbing state pd(t) increases by time. 4.2. reliability evaluation for a system of states x, the up states of the system can be grouped in a set b of functioning sets. the failed states, therefore are grouped into f=x-b of failed states. figure 2. state transition diagram of pv module failure. acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 127 the survivor function r(t) determines the probability that a system does not leave the set b of functioning states during the time interval. also it can be evaluated from the subtraction of the probability of being into the set f from unity. thus, the survivor function is: 𝑅(𝑡) = ∑ 𝑃𝑖𝑖∈𝐵 (𝑡) = 1 − 𝑃d (𝑡) (15) the probability density function (pdf) indicates the distribution of the failure over the entire time range. the larger is the value of pd(t), the more failures that occur in the small interval time around t. if the time to failure has a probability density function, then 𝑃𝑃𝑃 = − d𝑅 d𝐴 (16) figure 6 and figure 7 show the survivor function and the probability density function f(t) over the time of the pv module failure, respectively. this is calculated by substituting the failure rates declared in table 2 into (13). the reliability of the pv module is 0.985 and 0.767 after 10 and 30 years, respectively, as shown in figure 6. the mttf is a basic measure of reliability for non-repairable systems that describes the expected time to failure under specified experimental conditions. mttf is given by 𝑀𝑀𝑀𝑃 = ∫ 𝑅(𝑡)d𝑡 ∞ 0 (17) meanwhile the hazard rate h(t) is the instantaneous failure rate, and is defined by ℎ(𝑡) = 𝑒 (𝐴) 𝑅(𝐴) (18) the hazard function indicates the change in the failure rate over the life of a population. figure 8 shows the hazard function of a pv module. by analyzing figure 8 and (18), it can be noticed that the pv module follows up a constant failure figure 4. state probability of one failure event. figure 5. state probability of two simultaneous failure events. figure 6. pv module survivor function. figure 7. pdf of the time to pv module failure. acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 128 rate in the quarter of the first year of the pv module’s operation followed by a pronounced wear out behavior. it is worth to mentioning that the aforementioned pv module reliability results did not include the encapsulate failures in order to have a clear view on the reliability of the pv module only. the encapsulation of the pv module is subjected to failures due to discoloration and delamination (d&d), moisture ingress and module broken glass. a complete separate reliability analysis using a markov process for failures of a pv module encapsulation is conducted in [55]. the reason behind the very short constant failure rate period is the dust failure that will degrade the output power if the module is left uncleaned. by applying (17) it results that the mttf of pv modules is 44.4 years regardless module encapsulation failures. this value is relatively higher than the current warranty periods declared by manufacturers for pv modules. 5. conclusion it is well known that a high degree of reliability needs to identify the possible failure modes of a component or a system. a pv module is a critical component in a pv system. hence, the different failure modes of pv modules are discussed in detail. a markov process is used to analyze the probability of occurrence for each failure mode in addition to the evaluation of survivor function, hazard rate, meantime to failure, and probability density function based on these failure modes. the markov process shows that cell cracks have a high probability of occurrence. this requires to pay more attention to the technology of cell implementation in the module, the factory tests, and means of transportation to the site. dust is the dominant failure cause of a pv module along its operating life time and results in a very short useful life time phase, three months, if the pv module is not cleaned. in addition, both dust and cell cracks failures contribute significantly in the probability of two simultaneous failure events. for example, the highest probability for a pv module to have two simultaneous failure events is assigned to both cell crack and dust, p18. this is followed by p10, where hotspot and cell cracks occur, and p24, where dust and hotspot occur simultaneously. the reliability of a pv module is 0.985 and 0.767 after 10 and 30 years, respectively. meanwhile, the calculated mttr is 44.4 years because a pv module follows up the same aging characteristics of semiconductors, when the encapsulation failures are excluded from the pv module reliability analysis. this highlights the impact of encapsulant failures that reduces the lifetime of pv modules to the current values declared by manufacturers. references [1] mougharbel, i.; makkawi, a.; ghazal, h., 2012. optimal solution for the connectivity of pv systems on a scheduled availability of the grid: case study for lebanon. proceedings 2012 international conference on renewable energies for developing countries (redec). pp.1-5. 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can be opened with acrobat and adobe reader 5.0 and later.) >> /namespace [ (adobe) (common) (1.0) ] /othernamespaces [ << /asreaderspreads false /cropimagestoframes true /errorcontrol /warnandcontinue /flattenerignorespreadoverrides false /includeguidesgrids false /includenonprinting false /includeslug false /namespace [ (adobe) (indesign) (4.0) ] /omitplacedbitmaps false /omitplacedeps false /omitplacedpdf false /simulateoverprint /legacy >> << /addbleedmarks false /addcolorbars false /addcropmarks false /addpageinfo false /addregmarks false /convertcolors /converttocmyk /destinationprofilename () /destinationprofileselector /documentcmyk /downsample16bitimages true /flattenerpreset << /presetselector /mediumresolution >> /formelements false /generatestructure false /includebookmarks false /includehyperlinks false /includeinteractive false /includelayers false /includeprofiles false /multimediahandling /useobjectsettings /namespace [ (adobe) (creativesuite) (2.0) ] /pdfxoutputintentprofileselector /documentcmyk /preserveediting true /untaggedcmykhandling /leaveuntagged /untaggedrgbhandling /usedocumentprofile /usedocumentbleed false >> ] >> setdistillerparams << /hwresolution [2400 2400] /pagesize [612.000 792.000] >> setpagedevice microsoft word 478-3338-1-le-rev acta imeko  issn: 2221‐870x  december 2017, volume 6, number 4, 17‐24    acta imeko | www.imeko.org  december 2017 | volume 6 | number 4 | 17  bandwidth limits in hall effect‐based current sensors  marco crescentini 1,2 , marco marchesi 3 , aldo romani 1,2 , marco tartagni 1,2 , pier andrea traverso 1   1 department of electrical, electronic and information engineering “g. marconi” (dei), university of bologna, 47521 cesena, italy   2 advanced research center on electronic systems (arces), university of bologna, 47521 cesena, italy  3 analog and smart power technology, stmicroelectronics, 20010 castelletto (mi), italy      keywords: hall sensor; current sensor; silicon hall probe; wide bandwidth; hall sensor model  citation: marco crescentini, marco marchesi, aldo romani, marco tartagni, pier andrea traverso, bandwidth limits in hall effect‐based current sensors,  acta imeko, vol. 6, no. 4, article 3, december 2017, identifier: imeko‐acta‐06 (2017)‐04‐03  section editor: alexandru salceanu, technical university of iasi, romania  received may 5, 2017; in final form may 5, 2017; published december 2017  copyright: © 2017 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: this work was supported by stmicroelectronics, italy  corresponding author: marco crescentini, e‐mail: marco.crescentini3@unibo.it    1. introduction  current sensors are key elements in the design of a large family of power systems, such as motor drivers and power converters [1]. modern power applications, like power management in electric vehicles, are demanding for small, or even silicon integrated, current sensors with state-of-the-art performance in terms of bandwidth, linearity, isolation, power consumption, and many other requirements [2]–[5]. among these specifications, wide bandwidth (i.e., from dc to several mhz), together with reduced dimension, is the most challenging [1]–[3]. broadband current sensors are usually implemented by means of either a resistive shunt or a current transformer (ct) [6], [7]. the former is relatively cost-effective but is quite bulky and realises a non-isolated measurement. at the best of our knowledge, the only cmos-integrated resistive shunt is reported in [6], but heat dissipation is still an issue. on the other hand, current transformers realise isolated measurements but are bulky, quite expensive and suffer from magnetic saturation. in this framework, current sensing based on the hall effect is very promising in terms of size, cost and power consumption, since the hall probe can be easily integrated into a cmos system-on-chip (soc): in particular, power consumption can be reduced down to just a few mw. however, practical realizations still show limited bandwidth. commercially available cmos-based hall sensors are usually limited to less than 250 khz [8]–[10]. in the literature it is possible to find some particular hall-based solutions with a -3db bandwidth of 1 mhz or above, but these are either implemented by using non-standard semiconductor compounds [11] or combine the hall element with cts (or coils) to achieve the broadband capability, with the hall probe still covering only the low-frequency sub-band [12]. abstract  modern power applications are demanding for small and broadband current sensors. hall sensors are a good solution, but practical  implementations are limited to a few hundred khz. the literature offers a theoretical knowledge about the dynamic effects acting on  the  hall  probe  but  does  neither  define  nor  experimentally  assess  the  bandwidth  fundamental  upper  limit,  since  many  parasitic  dynamic effects perturb the inherent time response of the hall sensor. this paper experimentally investigates the bandwidth upper  limits in cmos hall effect‐based current sensors. based on the physics‐based description of the hall probe, the paper defines a novel,  special‐purpose,  measurement  technique,  which  is  able  to  experimentally  evaluate  the  inherent  response  time  of  the  hall  probe  without triggering the main parasitic effects. the paper also proposes an equivalent electrical model describing the dynamic response  of the hall probe so as to better explain and understand the measurement results. specifically, the paper identifies two bandwidth  upper limits: a fundamental limit set by the intrinsic capacitance, which models the transversal charge accumulation due to the hall  effect, and a more practical limit set by the capacitive input of the electronic readout interface. some main parasitic effects are then  assessed and added in the proposed model.  acta imeko | www.imeko.org  december 2017 | volume 6 | number 4 | 18  the physics-based description of the hall probe (i.e., the magnetic field sensitive element within the sensor) suggests three main bandwidth-limiting high-frequency phenomena: i) relaxation time of the carriers; ii) inductive effects; iii) capacitive effects [13]. the first limit (in the thz-ghz range) is due to scattering effects attempting to restore the energy equilibrium [13], and represents only a theoretical cut-off since it cannot be achieved by any practical implementation of the sensor. the second and third limits are associated with the inherent reactive behaviours shown by a real hall probe. to these physicsbased/technological effects, other architectureand operationrelated limits must be added. for instance, the well-known and commonly used spinning-current bias technique, which is devoted to offset rejection, introduces another bandwidth upper-bound, which is set by the time required by the technique to correctly sample and cancel out the offset voltage at the output of the hall probe [14]–[16]. this paper aims at empirically investigating the bandwidth fundamental and practical limits in hall-based current sensors, regardless of the geometry of the hall probe as well as the specific architectural readout solution. hence, limits due to the spinning-current technique, or other readout procedures, will not be considered. the paper exploits special-purpose experimental techniques (supported by numerical analysis) in order to assess these main bandwidth limits. an equivalent electrical model describing the dynamic response of the hallbased current sensor is also proposed, for a better interpretation of experimental results. with the support of both physics-based description and the proposed empirical model, the paper identifies a test, which is able to experimentally demonstrate, in the time domain, that capacitive effects actually determine the two main frequency limits. specifically, the intrinsic equivalent capacitance, which models the transversal charge accumulation occurring in the probe due to the hall effect, defines what in the following will be addressed as the bandwidth “fundamental” limit, i.e. the theoretical bandwidth of the sensor in the case the hall probe were interfaced with an ideal readout circuit. however, the capacitive input of the actual readout interface connected to the output of the probe sets a more “practical” bandwidth limit at lower frequencies, since it increases the total capacitance influencing the hall probe response. in addition, several parasitic dynamic effects superimpose and further perturb the time response of the hall sensor, degrading it with respect to the inherent rc-like time response: the modelling approach offers hints on how to take them into account. the paper is organized as described in the following. section 2 reviews the basic theory of the hall effect and describes the geometrical and physical properties of the hall probe prototype employed in this work. section 2 also presents the proposed circuit-based equivalent model. section 3 investigates the bandwidth limits in hall probes by means of numerical analyses. finally, section 4 reports all the experimental measurements carried out to assess the bandwidth fundamental and practical limits, making also use of the model and comparing results with the discussion in the previous section. conclusions are drawn in section 5. 2. theory and modelling  2.1. hall effect and hall probe  the hall probe (figure 1) is a magnetic field sensitive device relying on the well-known hall effect. basically, the hall effect is a manifestation of the lorentz force applied by an external magnetic field b to the charge carriers flowing through a material [13]. this force bends the carriers from their original path and causes a transversal accumulation of charge with an associated potential difference vh (namely, the hall voltage) that (in static operation) nominally follows the formula: h i bias z v s i b   , (1) where si is the current-related sensitivity of the hall probe, ibias is the bias current, which defines the flow of charges and their original path, and bz is the component of the magnetic field along the direction orthogonal to the plane on which the bias current is flowing. the hall probe is commonly realized by means of semiconductors, rather than metals, due to their lower charge carrier mobility [13]. in the following, we will refer to a standard square-shaped silicon hall probe realized by a thick nwell, which constitutes the sensing area, surrounded by a lowly doped p-substrate and connected to the embedding electronic circuits via four square contacts realized by means of highly doped n+ implantations and placed at the angles of the well (figure 1). the hall sensors employed in this work are provided by stmicroelectronics and are realized in bcd (bipolarcmos-dmos) 0.16-µm technology. the chosen hall probe is only a possible and quite standard realization in cmos technology, but many other implementations, with different geometries or even completely different structures, can be considered without affecting the fundamental results that are described in this paper, since the shape of the response time does not depend on the particular hall probe. the current-related sensitivity si depends on geometrical and physical properties. it is usually expressed as  h i eff r s g qnt , (2) where g is a correction factor accounting for the specific geometry of the probe, rh is the hall scattering factor expressing the ability of the material to generate the hall voltage, q is the elementary charge, n is the doping level of the n-well and teff is the effective thickness of the well, which takes into account the thickness reduction due to depletion region occurring at the pn-junction [17]. as defined in (1), the hall probe is a magnetic sensor. due to the vector formulation of the lorentz force, there is a wellknown angular relationship between the three electromagnetic quantities in (1); hence, a hall effect-based current sensor must x y z ibias -vh bz p-substrate n-well contacts ibias vh f b hall probe b) l w figure 1. a) angular relationships between electromagnetic quantities due  to the hall effect. b) simplified cross section of the hall probe provided by  stmicroelectronics  and  top  view  of  the  hall  probe  connected  to  surrounding circuits.   acta imeko | www.imeko.org  december 2017 | volume 6 | number 4 | 19  be carefully realized so as to maximize the orthogonal component of the magnetic field. to this end, a copper strip is deposited dvert above and dlat far from the centre of the probe, as shown in figure 2. the relationship between the sensed current i flowing into the strip and the magnitude b of the related magnetic field is given by the algebraic biot-savart law: 2 sb i r    , (3) where r is the radial distance between the hall probe and the copper strip, and μs is the silicon magnetic permeability (that is almost equal to the vacuum permeability). obviously, (3) is strictly valid only if both the copper strip and the hall probe widths were negligible with respect to their displacement r, thus a full electromagnetic simulation is needed in practical cases, as considered in the following. even though the realistic i-b relationship is more complicated, fem analyses showed that it can be still considered algebraic. hence, time variations in the measurand i instantaneously results in time variations of the orthogonal component of the magnetic field bz. under static operative conditions the hall voltage can be written in the practical case as:    h i bv s i f i , (4) where f(i) is an algebraic function with respect to the measurand. 2.2. electrical model  starting from the considerations made in the previous subsection, we propose the three-port, parallel rc-based circuital model of figure 3 for the description of the hall probe dynamic behaviour. the three ports are: i) the measurement port, i.e., the port through which the measurand i flows, ii) the bias port, for the injection of the bias current, and iii) the output port, at which the hall voltage can be sensed. this model is in agreement with the static theory of the hall effect described above, given  h eq h v r i (5)     eq sq h h bias bias w r r l l i g i f i k i f i w         , (6) where rsq is the square resistance of the n-well, l and w are the well sizes and μh is the hall mobility defined as   h n hr . the equivalent resistance req is the resistance seen between the two sensing contacts. the equivalent current ih can be defined as the hall current, whose ac contribution represents (under general dynamic operating conditions) the transversal local flow of carriers due to the lorentz force. this current is modelled as a current-controlled current generator, for which the controlling parameters are both the bias current ibias and the measurand i. this relationship is intuitively correct since the amount of deflected carriers depends on both the magnitude of the magnetic field (thus on the magnitude of the measurand i) and the number of charge carriers that are flowing in the probe (thus on the magnitude of the bias current ibias). following (5), the static hall voltage is given by a pseudo-ohmic relation between the equivalent resistance req and the hall transversal current ih. note this is not a real ohmic relation since there is not a real transversal dc current flowing from one sense contact to the other, but it is a mere mathematical equivalent. in dynamic operation, the lorentz force immediately reacts to the measurand i, but the charge carriers need time to (de)accumulate on one side of the probe in response to the timevariant lorentz force. more precisely, the instantaneous equivalent accumulated charge shows non-negligible memory effects with respect to the force and cannot be adequately described according to a static model. this dynamic behaviour can be reasonably modelled by inserting a capacitor cacc in parallel to the equivalent resistance. it is well known that a significant dc voltage contribution is present on the output port of the hall probe, which depends on the level of the bias current, even though no magnetic field is applied. this offset voltage vos is mainly due to technology issues that generate asymmetry in the probe and can be order of magnitude higher than the actual hall voltage, complicating the readout process. for this reason, the offset voltage in hall probes is extensively investigated in the literature [14], [18]–[21]. the common model for the offset in a hall probe is an unbalanced resistive bridge. this model is fully recognized by the community but it cannot be easily implemented in our model, since the latter decouples the bias port from the output port. hence, a voltage generator vos controlled by the bias current ibias is directly placed in series to the resistance req in figure 3. the other two ports of figure 3 can be basically modelled by two simple resistors, which denote the resistance of the copper strip (rin) and the resistance of the probe along the bias direction (rbias). finally, all the intrinsic ports are surrounded by a global black-box that takes into account all the extrinsic parasitic physical phenomena that are not described by the intrinsic core of the model, such as induced electromotive force (emf), magneto-resistivity, additional inductive effects, electrical coupling among the pins of the hall probe and so on. the capacitance cacc accounts for the only one dynamic effect dealt with by the proposed intrinsic model, thus we implicitly assume that it sets the bandwidth fundamental upper hall probe copper strip b a) i vh strip hall probe sio2 b) dlat dvert i figure 2. a) the sensed current  i flows through a copper strip placed dvert above  and  dlat  far  from  the  centre  of  the  hall  probe.  b)  cross  section highlighting  the  vertical  displacement  between  the  copper  strip  and  hall probe.     ih vos req cacc rin rbias vh cele ibias i measurement port bias port output port ih=g1(i, ibias) vos=g2(ibias) figure  3.  three‐port  equivalent  model  of  the  hall  effect‐based  current  sensor. acta imeko | www.imeko.org  december 2017 | volume 6 | number 4 | 20  limit. this hypothesis will be validated by both simulations and measurements in the next sections. it is not easy to investigate this dynamic effect by means of common techniques, since the accurate generation of fast time-varying magnetic fields is a difficult task. in addition, fast variations of b would excite parasitic phenomena described by the black-box in figure 3, perturbing the response of the equivalent circuit. nevertheless, based on the physics-based description and accordingly to the proposed model, another measurement procedure able to investigate the dynamic behaviours at the output port can be identified. since the offset voltage is due to spurious charge flow and accumulation throughout the same physical region associated with hall accumulation, and is dependent on the bias current, then time variations of ibias trigger the same rc time constant triggered by the variations of the magnetic field (i.e., of the current i), without exciting most of the other (parasitic) dynamic effects. this particular operative condition will be exploited to experimentally characterize the rc time constant of the hall element. 3. numerical simulations  the prototype of the hall probe, without i-b transduction, was designed to show an equivalent resistance req = 3 kω and an intrinsic capacitance cacc lower than 1 pf, accordingly to the probe description of section 2.1 and the equivalent electrical model of section 2.2. the probe was modelled and simulated using the physical simulator synopsys sentaurus device®. this tool takes into account all the typical phenomena occurring in a semiconductor device, from carrier transport model to electronhole recombination, carrier scattering, and mobility degradation. moreover, the tool embeds an enhanced formulation of the current density that models the lorentz force applied by a magnetic field b to the carriers flowing through the semiconductor:   0 0 0 2 1 n n h h h n nh j j j j b b b b                              , (7) where 0 j  is the current vector due to only the applied bias. a full description of the simulation environment, together with validation of the approach by means of static analysis and a full static characterization of the hall probe can be found in [17], [22]. for completeness purposes, figure 5 reports the static characteristic of the hall probe when biased with a constant dc ibias = 500 µa and varying the magnitude of the incident magnetic field along the z-axis from – 50 mt to +50 mt. the figure shows a constant sensitivity of the probe of 124 mv/t with a non-linearity error of less than 2 µv. even though the simulated probe is perfectly symmetric, the static simulation reports an offset of less than 1 mv when no magnetic field is applied, which is due to numerical errors. there are no particular problems in the simulation of fast magnetic fields, so standard investigation procedures of the dynamic behaviour can be realised. a 40-mt magnetic step was applied to the probe varying the capacitive load on the output port (cele in figure 3). the simulated time-domain responses of the hall voltage are shown in figure 5. the probe response is quasi-static when no capacitors are coupled to the sensor (we can estimate a time constant τ of less than 2 ns), while it shows a time constant τ of about 18 ns when it is loaded with 6 pf (3 pf on each sense contact). the figure reports the simulation points fitted in matlab® with spline functions. the simulation results, where main parasitic effects have been neglected, suggest that the bandwidth upper limits of the hall probe are defined by rc time constants [22]. explaining these simulation results with the equivalent electrical model of section 2.2 leads to two bandwidth limits: a fundamental limit, which is defined by the product of r = req and the intrinsic capacitance cacc, and a more practical limit that is defined by the product of r = req and the total capacitance c = cacc + cele facing to the output port. note that the equation for c is rigorously true under the assumption that cacc is directly connected to the output port. nonetheless, in a realistic architecture cele is the differential input capacitance of amplifier stage(s) plus capacitive parasitics. accordingly to the proposed electrical model, the time constants extracted from the abovedescribed simulation leads to req = 3 kω and cacc ≤ 0.7 pf which are in agreement with the design values. the simulation also defines that the bandwidth fundamental upper limit for the simulated hall probe is about 100 mhz. numerical simulations were also exploited to prove the novel measurement procedure that is proposed at the end of section 2.2. the current ibias acts on both the offset generator vos and the transversal current generator ih, as shown in figure 3. thus, it is still possible to prove the proposed measurement technique even though the offset is not modelled in the numerical simulator, since the probe is perfectly symmetric. under a constant dc magnetic field, a step change of ibias causes a step change of ih, triggering the same rc circuit triggered by a step change of the magnetic field. from a physical standpoint, a variation of the bias current changes the total sensitivity i bias s i of the hall probe and, consequentially, the amount of accumulated charge, which must re-allocate in the probe triggering the same physical phenomena. figure 4. simulated time response of the hall probe to a magnetic step of 40  mt with different loading capacitances connected to the output port. the  sensor is dc biased with 500 µa. interpreting the resulting time constants  with the proposed electrical model leads to electrical parameters that are in  good agreement with design values: req= 3 kω, cacc< 0.7 pf, cele = 6 pf. figure 5. simulated static characteristic of the probe.   ‐8 ‐6 ‐4 ‐2 0 2 4 6 8 ‐60 ‐40 ‐20 0 20 40 60 h a ll  v o lt a g e  v h (m v ) magnetic field (mt) acta imeko | www.imeko.org  december 2017 | volume 6 | number 4 | 21  figure 6 compares the time responses of the hall probe to a step change of magnetic field with the time response to a step change of the bias current, with no readout circuits connected (i.e. cele = 0 f), demonstrating that both stimuli trigger the same dynamic behaviour. dark circles reported in both figure 5 and figure 6 are the results of the same simulation, but values on the ordinate in figure 6 are normalized to the steady-state value, so as to easily compare the two time responses. 4. experimental investigation  two batches of cmos hall effect-based current sensors were experimentally analysed. the hall probe is the same in both batches, but they integrate different circuits. more precisely, batch a is an hybrid solution, which integrates only the hall probe, with bias and signal conditioning implemented by off-the-shelf components (figure 6a); while batch b monolithically integrates the hall probe together with all the auxiliary circuits i.e., bias network and voltage amplifiers, into a single cmos integrated circuit (ic) (figure 6b). given the implementation described above, batch a has obviously much bigger load capacitance cele, in the order of several tens of pf, and it is expected to experimentally exhibit a slower time constant, while batch b is a solution that minimizes the value of cele. the technological parameter cacc is the same for both sensors. the analysis was based on the measurement procedure described in section 2.2 and validated in section 3, that consists in the measurement of the output voltage response to a step of the bias current ibias. the measurement procedure was as follows: a square-wave current ibias with zero mean was applied to the sensor through contacts a and b (bias port) while no current i is injected into the sensing strip (measurement port). in this way, the output voltage vout is an amplified version of the response of the rc network of figure 3 to a step change of the offset voltage vos, allowing the estimation of the inherent time constant of the sensor. bandwidth limitations of the amplifiers were taken into consideration during result analysis. an ni-pxi 5124 data acquisition board operated at 200 msa/s with ac input coupling acquires the output voltages. 4.1. hall sensing element with external circuits  the result of the test applied to the hall probe with external auxiliary circuits (batch a) is reported at the top of figure 7. the bias current ibias was generated off-chip by applying a squarewave voltage on a resistor connected in series to the hall probe. this hybrid setup is affected by parasitic and coupling effects. in particular, the high bumps shown in figure 7(top) are due to parasitic capacitive coupling between bias port (nodes a and b) and output port (nodes c and d); this coupling is mainly due to the particular pin-out of the chip (i.e. pins connected to the bias port are close to pins connected to the output port). although the output voltage suffers from high bumps at the beginning of the step response, the expected exponential response is clearly visible. from this result it is possible to estimate a time constant τ = (200±40) ns and a 3-db bandwidth of (800±200) khz, which, accordingly to the proposed model, leads to c = (66±12) pf given req = 3 kω. this is a reasonable value for a hybrid system made by discrete components. x5 x20 ad8429 opa656 vout ibias hall ic x10 x10 cmos amplifier vouthall ic a) b) cele a a b b c c d d cmos amplifier = + ibias cele figure  6.  batch  a):  the  auxiliary  circuits  are  external  to  the  cmos  chip  integrating the hall effect‐based current sensor. batch b): both hall effect‐ based current sensor and auxiliary circuits are monolithically integrated on the same substrate.  figure 7. measured response to a step change of ibias, giving insights into the  inherent  rc  time  response  for  both  batches  of  sensors.  parameters  extracted from the measurements are: batch a) r=3 kω, c=66 pf; batch b)  r=3 kω, c=4 pf.  figure 8. simulated time response of the hall probe to a step change of b while ibias is kept constant (black circles) and simulated time response of the hall probe to a step change of ibias while b is kept constant (blue line). the hall voltage  is normalized to the stationary value so as to easily compare the  kind  of  time  response.  the  simulations  demonstrate  that  both  the magnetic field and the bias current trigger the same dynamic phenomena.  acta imeko | www.imeko.org  december 2017 | volume 6 | number 4 | 22  4.2. hall sensing element with integrated circuits  the step response of the hall probe integrated with conditioning circuits (batch b) is shown at the bottom of figure 7. the bias current ibias is now generated on-chip by means of a high-compliance current mirror. also in this case, the measured voltage is not a pure exponential function since there are other residual dynamic effects acting on the response (e.g., the residual capacitive coupling between contacts and the ringing of the amplifiers which are underdamped second order systems). however, by fitting the output voltage with an exponential function it is possible to estimate an inherent time constant τ = (12±3) ns, after de-embedding 4 ns as time response for each cmos amplifier stage (which have 40 mhz bandwidth). the estimated time constant leads to a total capacitance c = (4±1) pf, which is in good agreement with the numerically estimated values of cacc = 0.7 pf and cele = 2 pf (those parameters are extracted from actual architecture and layout of the system). since batch b minimizes the value of cele, this test identifies the bandwidth practical upper limit of the hall sensor prototype, which is between 9 and 18 mhz. in conclusion, both tests were carried out in correspondence with operative conditions that activate a few parasitic dynamic effects, while the inherent parallel rc is fully excited. the experimentally obtained values for time constant and capacitance c, both for the implementation in which the capacitive load is maximized (batch a) and for the case in which such a load is minimized (batch b), are in agreement with numerical analysis, thus validating the model shown in figure 3 and confirming that the bandwidth upper limits are set by the parallel rc circuit. parasitic dynamic effects are superimposed to such a fundamental response, and they degrade the actual bandwidth, as investigated in subsection 4.4. 4.3. analysis in the frequency domain  frequency analyses were performed on both batches to further investigate the dynamic response of the hall probe. in this test, no current i was applied to the strip, hence vout is an amplified version of the thermal noise generated by the hall probe and the output amplifiers, only. the voltages were recorded by the ni-pxi5124 board at 12-bit resolution and 100-msa/s sampling frequency. then, noise power spectrum densities (psds) were estimated in matlab® using the welch algorithm (flat top window with 4096-long segments and 90 % overlap between segments [23]). the measured dc – 50 mhz output power spectra sout are shown in figure 9. although the psds cannot be interpreted, strictly speaking, as the frequency response of the entire current-measuring system, nonetheless interesting information can be derived by observing their shapes: more precisely, it is possible to identify two important points in the noise psd of batch b (black line): a first cut-off frequency around 15 mhz and a second cut-off frequency around 40 mhz. the former relates to a time constant of nearly 10 ns, in agreement with the rc time constant of the hall probe measured in section 4.2, while the latter relates to the bandwidth limitation of the output amplifiers. this noise psd confirms the result of the timedomain test obtained in section 4.2. on the contrary, the noise psd of batch a (red line) shows a single cut-off frequency, between 1 and 2 mhz, and then a typical 20-db roll off. this cut-off frequency corresponds to an estimated time constant between 80 and 160 ns, in line with the time constant estimation from the time-domain test of section 4.1. the noise psd of batch a shows a cut-off at frequencies lower than what was reported by the noise psd of batch b, confirming all the above discussed theory. since the dynamic performance is mainly defined by the rc time constant, the higher capacitive load, which is due to the external electronic readout, slows down the time response of batch a with respect to batch b. the cut-off frequency of batch a is approximately one order of magnitude lower than the cut-off frequency of batch b, in agreement with time-domain tests reported in sections 4.1 and 4.2. 4.4. time response to actual magnetic transitions  the batch b was tested also in the presence of an actual fast (but not instantaneous) time transition of magnetic excitation. in this test the hall element is dc biased, i.e. the current ibias is kept constant so that output dynamics are related only to changes of the magnetic field. we designed a simple voltage-tocurrent (v/i) converter based on a voltage follower and a 1-ω power resistor (figure 10). the generated time-varying current i(t) is monitored by means of the voltage drop across the power resistor, and then it flows through the metal strip on the top of the hall probe, generating the desired transition in the magnetic field b. the designed v/i converter is characterized by a rise time of 210 ns, short but one order of magnitude longer than the practical response time estimated in sections 4.2 and 4.3. hence, the hall probe, as described by the model of figure 3, works in the quasi-static regime as far as the parallel rc is concerned. however, the parasitic dynamic effects described by the black-box are now fully triggered and will degrade the voltage response. this experiment provides useful information about these parasitic dynamic effects embedding the core of the sensor behaviour. figure 9. measured noise power spectrum densities of vout for both batches when no current i is applied to the copper strip. the power spectra confirm that the bandwidth practical upper limit is defined by the rc time constant where the c is mainly due to the capacitive load of the readout electronics.   figure 10. measurement setup for time response to magnetic step.   acta imeko | www.imeko.org  december 2017 | volume 6 | number 4 | 23  we acquired the voltage across the power resistor, translated it into the current i(t) and fed it into the model of figure 3 implemented in spice. the first version of the model takes into account the parallel rc core only, without any element in the black-box. the predicted output voltage out v is then compared to the measured output voltage vout. the voltage out v (figure 11, solid blue line) shows a quasi-instantaneous response to the measured current stimulus (top of figure 11), as expected, but does not fit measurement data (red circle), since the model neglects parasitic effects. specifically, out v does not foresee the high bump opposite to the exponential transition. this mainly because the model still does not describe the parasitic emfs induced by time-varying magnetic induction (which must be modelled in the black box). in fact, when the model is made more realistic by inserting proper inductive equivalent elements in order to take into account parasitic emfs, then the predicted voltage (solid black line) accurately fits the measured data. in this case, the effect of parasitic emfs is well fitted by adding a transformer to the model, as shown by the refined model of figure 12. the primary winding is placed in series to rin in the measurement port while the secondary winding is connected in series to the rc core (secondary winding being 180 degrees rotated). the measurand i(t) flows into the measurement port generating an inductive effect on the output port that opposes the transversal hall current ih. this test, when compared with that of figure 7 (bottom), shows an example of the degradation of the inherent rc time response when parasitic effects are triggered in realistic operation of the sensor. suitable design methodologies must be employed in order to minimize the deviation from the practical bandwidth limit. for example, since the induced emfs are strictly related to the layout design of the implemented prototype, they can be reduced by proper geometrical redesign (e.g., minimizing the area described by connections from the sensor to amplifiers). 5. conclusions  this paper experimentally investigated the bandwidth upper limits in hall effect-based current sensing. to this end, an adhoc measurement technique, which was derived from both a physics-based description of the hall effect and an equivalent electrical model, was proposed and exploited. the test was performed on two different batches of sensors, with the same hall probe but different capacitive loads. the proposed electrical model allowed better interpretation of the measurement results giving insights on the underlying phenomena. according to the equivalent electrical model, the experimental tests demonstrated that the intrinsic capacitance cacc, which models the charge accumulation occurring in the probe due to the hall effect, defines the fundamental upper bandwidth limit of the hall-based current sensor. this fundamental limit degrades to a more practical limit (placed at lower frequencies) due to the unavoidable capacitive load added by the readout circuit. these results fully agree with numerical simulations and frequency-domain measurements, both reported in this manuscript. a real-operation test was performed on one of the two batches, showing that the actual time response of the hall sensor is degraded by parasitic dynamic effects. hence, suitable design methodologies are needed to achieve the fundamental time response. one of the parasitic effects degrading the sensor response, i.e. the generation of electromotive forces due to time-variant magnetic fields, has been investigated and an extension to the original model, which takes such effect into account, has been proposed. acknowledgement  the authors would like to acknowledge the support from michele biondi for the numerical simulations, and paolo alberti for the practical realization of the testing setups. the authors would also like to thank domenico cristaudo and roberto canegallo for their technology support. references  [1] l. dalessandro, n. karrer, j.w. kolar, high-performance planar isolated current sensor for power electronics applications, ieee trans. power electron. 22 (2007) 1682–1692. doi:10.1109/tpel.2007.904198. [2] a. patel, m. ferdowsi, current sensing for automotive electronics—a survey, ieee trans. veh. technol. 58 (2009) 4108–4119. doi:10.1109/tvt.2009.2022081. [3] p. ripka, electric current sensors: a review, meas. sci. technol. 21 (2010) 112001. doi:10.1088/0957-0233/21/11/112001. [4] s. ziegler, r.c. woodward, h.h.-c. iu, l.j. borle, current sensing techniques: a review, ieee sens. j. 9 (2009) 354–376. doi:10.1109/jsen.2009.2013914. [5] b. whitaker, a. barkley, z. cole, b. passmore, d. martin, t.r.   figure 11. measured response (red dots) to a step change of i (batch of case b) compared with the simulated response given by the core model of figure 3 (blue line) and with the simulated response given by the enhanced model of figure 12 with  inductive transformer (black  line).   parameters used for the simulation are r=3 kω, c=4 pf, lprim= 10 nh, lsec= 30 nh.   ih vos req cacc rin rbias vh cele ibias i measurement port bias port output port lprim lsec figure  12.  modified  three‐port  equivalent  model  with  the  insertion  of  a transformer between measurement port and output port so as to take into account the parasitic emf generation.  acta imeko | www.imeko.org  december 2017 | volume 6 | number 4 | 24  mcnutt, a.b. lostetter, j.s. lee, k. shiozaki, a high-density, high-efficiency, isolated on-board vehicle battery charger utilizing silicon carbide power devices, ieee trans. power electron. 29 (2014) 2606–2617. doi:10.1109/tpel.2013.2279950. 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[13] r.s. popovic, hall effect devices, 2nd ed., crc press, boca raton (fl), 2004. [14] a. bilotti, g. monreal, r. vig, monolithic magnetic hall sensor using dynamic quadrature offset cancellation, ieee j. solid-state circuits. 32 (1997) 829–836. doi:10.1109/4.585275. [15] p.d. dimitropoulos, p.m. drljaca, r.s. popovic, a 0.35umcmos, wide-band, low-noise hall magnetometer for current sensing applications, in: 2007 ieee sensors, ieee, 2007: pp. 884–887. doi:10.1109/icsens.2007.4388543. [16] m. crescentini, m. biondi, m. bennati, p. alberti, g. luciani, c. tamburini, m. pizzotti, a. romani, m. tartagni, d. bellasi, d. rossi, l. benini, m. marchesi, d. cristaudo, r. canegallo, a 2 ms/s 10a hall current sensor soc with digital compressive sensing encoder in 0.16 µm bcd, in: eur. solid-state circuits conf., ieee, lausanne, switzerland, 2016: pp. 393–396. doi:10.1109/esscirc.2016.7598324. 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[22] m. crescentini, a. romani, e. sangiorgi, physical simulations of response time in hall sensor devices, in: 2014 15th int. conf. ultim. integr. silicon, ieee, stockhol, sweden, 2014: pp. 89–92. doi:10.1109/ulis.2014.6813923. [23] p. magnone, p.a. traverso, c. fiegna, optimal non-parametric estimation of 1/f noise spectrum in semiconductor devices, in: 20th imeko tc4 symp. meas. electr. quant. res. electr. electron. meas. econ. upturn, together with 18th tc4 int. work. adc dca model. testing, iwadc 2014, 2014. acta imeko, title acta imeko issn: 2221-870x june 2018, volume 7, number 2, 102-109 acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 102 a vision-based navigation system for landing procedure silvio del pizzo, umberto papa, salvatore gaglione, salvatore troisi, giuseppe del core department of science and technology, university of naples “parthenope”, naples, italy section: research paper keywords: uas; close range photogrammetry; camera images; flight mechanics; embedded electronic platform citation: silvio del pizzo, umberto papa, salvatore gaglione, salvatore troisi, giuseppe del core, a vision-based navigation system for landing procedure, acta imeko, vol. 7, no. 2, article 18, june 2018, identifier: imeko-acta-07 (2018)-02-18 section editor: marcantonio catelani, university of florence, italy received january 11, 2018; in final form march 26, 2018; published june 2018 copyright: © 2018 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: this study was partially financed by a grant in the framework of the project “studio e prototipazione di servizi innovativi meteo-marini in supporto alla navigazione” of university of naples, italy corresponding author: salvatore troisi, e-mail: salvatore.troisi@uniparthenope.it 1. introduction during the last years, the market of drones has been continually growing up. the faa (federation aviation administration) has recently proposed the term unmanned aircraft system (uas) for the definition of an aircraft without a human pilot on-board and controlled by an operator on the ground. the term uas includes the vehicle (uav – unmanned aircraft vehicle), a number of subsystems such as its payload, the control station, communication system, navigation system and support system. in this work the attention was particularly focused on the navigation system and the performance of the proposed approach was investigated using a vtol (vertical take-off and landing) uas. in most of the introduced applications the possibility to hovering the surveyed area is very important. vtol aircrafts provide many advantages over conventional take-off and landing (ctol), the most notable ones are the capability of hovering in place and the small area required for take-off and landing. among vtol aircraft such as conventional helicopters and crafts with rotors like the tiltrotor and fixed-wing aircraft with directed jet thrust capability, the quadrotor is very frequently chosen, especially in the academic research on mini or micro-size uas: it is an effective alternative to the high cost and complexity of the conventional rotorcrafts, due to its ability to hover and move in absence of the complex system of linkages and blade elements present in a standard single-rotor vehicle [1]. in open sky the uas’s position can be easily obtained by gnss receivers that can assure the required accuracy, but when the uas operates in harsh environments it is necessary to carefully schedule the flight with the reception of at least four satellites for all the session, but sometimes this is not enough as in the case of harsh environments [2], [3]. in such conditions the use of stand-alone positioning systems such as inertial or vision-based systems is strongly recommended. in other papers some vision-based techniques assisting the automatic landing phase have been defined. in [4] the authors use the recursive multi-frame planar parallax algorithm to obtain accurate dense elevation and appearance models of terrain, making use of a single camera placed on board an aerial platform; in theory, with perfectly registered imagery, such abstract an autonomous vision-based landing system was designed and its performance is analysed and measured by an unmanned aircraft system (uas). the system equipment is based on a single camera to determine its position and attitude with respect to a well-defined landing pattern. the developed procedure is based on photogrammetric space resection solution, which provides the position and camera attitude reckoning starting from at least three, not aligned, reference control points whose image coordinates may be measured in the image camera frame. five circular coloured targets were placed on a specific landing pattern, their 2d image frame coordinates were extracted through a particular algorithm. the aim of this work is to compute uas precise position and attitude from a single image, in order to have a good approach to landing field. this procedure can be used in addition or for replacement of gps tracking and can be applied when the landing field is movable or located on a moving platform, in which case the uas will follow the landing pattern until the landing phase will be closed. acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 103 algorithm can produce range data with error expected to increase between linearly and with the square root of the range. in [5] a vision system able to provide position and attitude of uas within respectively 5 cm in each axis and 5 degrees in each axis of rotation is presented; such system uses a planar landing target. tang et al presented an algorithm for estimating the position and attitude of a uas relative to a runway by projecting arbitrary points from the runway into the image frame [6]. in [7] the vision-based tracking and landing approach utilizes enhancement of red, green and blue (rgb) colour information of the landing target: this approach shows fast and robust performance in different lighting conditions. in [8] the authors describe a 3d vision system able to estimate the height over the ground. in [9] a forward-looking camera is used to guide the landing vehicle on a runway. in [10] an approach for guidance and safe landing of an uas based on the concept of reuse of local features extracted from the vision system is proposed. in [11] the authors present an approach based on the visual slam (simultaneous localization and mapping) algorithm that enables a mav (micro aerial vehicle) to autonomously determine its location and consequently stabilize itself. this approach does not require any a priori information on the environment or any known pattern in order to obtain a mav control. in [12] a wii remote infrared (ir) camera is used as main sensor, which allows robust tracking of a pattern of ir lights in absence of direct sunlight. the position and orientation relative to the ir pattern is estimated at a frequency of approximately 50 hz. in this paper an alternative approach is suggested to search location and orientation of onboard camera (and consequently of the uas equipped with camera), specifically for a quadcopter, through a vision-based methodology. in particular, the photogrammetric method of space resection solution (srs) is chosen. the proposed approach produces a series of continuous output that guide the uas towards the landing area on which is placed a specific pattern. the developed system provides, for the whole landing procedure, the deviation from the planned track that could be used by an autopilot to correct the flight path. lastly, it is important to specify that the whole system is onboard embedded, so that on the ground there is only a landing pattern that contains specific coded targets. 2. visual estimation procedure a vision-based landing system is based on the processing of a single image. the goal is to detect camera orientation parameters (position and attitude) in a fixed reference system. a specific landing pattern, composed by coloured circular targets (figure 5), was designed in order to implement such reference system. indeed, the determination of full orientation assumes the availability of enough information in object space. the mapping of a point in object space 𝐱 into a point in camera space 𝐱′ (expressed in homogenous coordinates) is fully described by a projection matrix 𝐏 [13]: 𝐱′ = 𝐏𝐱 (1) the projection matrix is a 3x4 matrix that describes the orientation of a pinhole camera. in photogrammetry the orientation is divided in two different types: • external orientation, which describes the position and the attitude of a camera • internal orientation, which describes the internal camera parameters such as focal length, sensor size etc. both these parameters are absorbed in the projection matrix. in photogrammetry two methods are known to compute the correct orientation of a single camera starting from object coordinate points and the homologous in camera space: direct linear transform (dlt) and space resection solution. the former computes directly the projection matrix, solving a linear system, but needs at least six corresponding image and object points. furthermore, the solution is not possible if the object points are coplanar [14]. the space resection method is not linear, but it assures a solution with only three not aligned points. furthermore, the theoretical precision of srs is better than the dlt one [13]. the adopted vision system described in this paper uses a customized space resection based vision algorithm. all procedures for a correct landing occur in the following stages: • camera calibration to compute all internal orientation parameters and lens distortion coefficients; • image coordinates point extraction (image space); • computation of external orientation parameter extrapolation (orientation phase using srs); • internal orientation parameters are extracted from calibration procedure of the camera. external orientation parameters are extracted by means of space resection method. the following section explains the overall system setup and the initial procedure that must be performed for a correct data extraction. 2.1. camera calibration in order to obtain external orientation parameters with good accuracy, it is mandatory to calibrate the camera. the calibration is the process that allows to determine the internal orientation of a camera and the distortion coefficients, which are, in particular: • principal point or image centre (x0’, y0’); • focal length (f); • radial distortion coefficient (k1, k2). one of the most used analytical camera calibration techniques was originally developed by brown [15]. this method is often used in close range photogrammetry to obtain the internal parameters with high accuracies [16]. the raspberry camera module allows to manually control the focal length, in order to set such parameters on a specific and constant value. furthermore, it can acquire a photo every second on a full resolution (about 5 megapixel); to avoid overloading the buffer and the computation and to obtain images with low brightness, the resolution was set to 1920x1080 (about 2 megapixel) in according to an hd video. the calibration was carried out using a set of coded circular targets, in order to obtain subpixel precision and to automatize the procedure of calibration. table 1 shows the results of the calibration procedure. specifically, the standard deviations provided by the selfcalibration procedure are reported in the third column. 2.2. points extraction the object recognition, consisting in the identification of a specific object on an image, is an important task in computer vision. both srs and dlt need to identify the ground control point position on image frame. circular targets were designed both to facilitate automatic detection and to assure a good marking accuracy: indeed, it is acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 104 well known that sub-pixel accuracy can be achieved [17]. all the measurements are referred to the centre of a circular target. several methods exist to automatically detect circular targets on an image and to find the centre. they can be divided in two categories: • no-initial approximation required methods: these approaches allow to obtain the coordinates of the centre of a circular target on a given image without any initial approximation; • initial approximation required methods: these approaches start from an approximated position of the target centre, being able to detect the centre with high accuracy. of course, the target coordinate centre is not sufficient to detect the correspondent gcp. the matching is performed using a unique code for every target. in this work the codification is based on the colour of the target. 2.3. orientation the camera orientation is computed by resection with respect to a landing frame. the resection is a linearization of collinearity equations (2) that describe the transformation of object coordinates (x, y, z) into corresponding image coordinates (x’, y’). 𝑥′ = 𝑥0 + 𝑐 𝑟1,1(𝑋 − 𝑋0) + 𝑟1,2(𝑌 − 𝑌0) + 𝑟1,3(𝑍 − 𝑍0) 𝑟3,1(𝑋 − 𝑋0) + 𝑟3,2(𝑌 − 𝑌0) + 𝑟3,3(𝑍 − 𝑍0) + ∆𝑥′ 𝑦′ = 𝑦0 + 𝑐 𝑟2,1(𝑋 − 𝑋0) + 𝑟2,2(𝑌 − 𝑌0) + 𝑟2,3(𝑍 − 𝑍0) 𝑟3,1(𝑋 − 𝑋0) + 𝑟3,2(𝑌 − 𝑌0) + 𝑟3,3(𝑍 − 𝑍0) + ∆𝑦′ (2) where ri,j are elements of the 3d rotation matrix r, which performs a rotation from the image camera to the object reference system, while (x0, y0, z0) are the coordinates of the camera perspective point in object reference frame (figure 1): � 𝑥′ 𝑦′ 𝑐 � = 𝑹� 𝑋 − 𝑋0 𝑌 − 𝑌0 𝑍 − 𝑍0 � (3) the collinearity equation system (2) can be rewritten in the following system of correction equations: 𝑥′ = 𝑥0′ + 𝑣𝑥′ 𝑦′ = 𝑦0′ + 𝑣𝑦′ (4) where 𝑥0′ , 𝑦0′ are the image coordinates computed using approximated orientation parameters, while 𝑣𝑥′, 𝑣𝑦′are the adjustment parameters needed to obtain the correct 𝑥′ and 𝑦′ image coordinates, equation (5). the adjustment parameters can be obtained as a linearization of the equation (2), using as initial point the approximated orientation parameters employed to compute 𝑥0′ , 𝑦0′ : 𝑣𝑥′ = � 𝜕𝑥′ 𝜕𝑋0 � 0 𝑑𝑋0 + � 𝜕𝑥′ 𝜕𝑌0 � 0 𝑑𝑌0 + � 𝜕𝑥′ 𝜕𝑍0 � 0 𝑑𝑍0 + � 𝜕𝑥′ 𝜕𝜕 � 0 𝑑𝜕 + � 𝜕𝑥′ 𝜕𝜕 � 0 𝑑𝜕 + � 𝜕𝑥′ 𝜕𝜕 � 0 𝑑𝜕 𝑣𝑦′ = � 𝜕𝑦′ 𝜕𝑋0 � 0 𝑑𝑋0 + � 𝜕𝑦′ 𝜕𝑌0 � 0 𝑑𝑌0 + � 𝜕𝑦′ 𝜕𝑍0 � 0 𝑑𝑍0 + � 𝜕𝑦′ 𝜕𝜕 � 0 𝑑𝜕 + � 𝜕𝑦′ 𝜕𝜕 � 0 𝑑𝜕 + � 𝜕𝑦′ 𝜕𝜕 � 0 𝑑𝜕 (5) where, 𝜕, 𝜕, 𝜅 are the euler angles between the camera reference system and object one. generally, the approximated position of the camera does not provide good results, therefore position and attitude are corrected using a linearization procedure. indeed, the distance between the computed coordinates, using the above-mentioned linearization procedure, and the measured coordinates approaches zero only after numerous iterations. at the end of iterative process, the method is able to estimate the camera attitude and position [18]. in order to solve the system in equation (4), at least 3 points (not aligned) are necessary. indeed, such observations provide 6 equations, which allow to estimate the six external orientation parameters. further observations could be considered to obtain an overdetermined measurement model; in this case the solution is estimated using the classical least-square adjustment method. 3. test setup 3.1. system overview the acronym uas covers all vehicles flying in the air with no person onboard controlling the aircraft. this term is commonly used in the computer science and artificial intelligence community, but terms like remotely piloted table 1. internal orientation parameters of raspberry camera module. camera name raspberry camera module standard deviation focal length [mm] 5.9409 0.003 sensor size [mm] width: 6.0035 height: 3.3750 0.006 0.005 principal point [mm] x: 2.997 y: 1.6808 5.5·10-4 lens distortion radial coefficients k1: -1.9·10 -3 k2: 1.6·10 -4 1.3·10-4 1.9·10-5 figure 1. two reference systems (rs): the object rs (xyz) and the camera one (x’y’z’). acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 105 vehicle (rpv), remotely operated aircraft (roa), remotely controlled helicopter (rc-helicopter), unmanned vehicle system (uvs) are often used. in this section the main components of the uas quadrotor, which include the basic quad-rotor and the avionic system with wireless/bluetooth shield [19], [20], are introduced. a simple electric quadrotor model (quadcopter arf 450 by conrad [21]) is as far as possible pre-assembled. in the professional field, such uas are already used for various tasks (surveillance, mapping 3d, search and rescue, etc.). this high quality "toy" is manually controlled by a remote control, while a microprocessor stabilizes it through position controls and acceleration sensors. its size is reported in table 2. the uas employed in this work (as shown in figure 2) is a mini uas, in accordance with the classification provided in [22], [23]. 3.2. landing system design the design of the proposed landing system is based on two main elements: • an embedded system that contains all hardware for image acquisition and data extrapolation; • a landing pattern, which is composed by a metallic platform where are located several circular targets. the embedded system is made up of a raspberry camera module and a raspberry pi. the sensors are placed at the bottom of the uas structure, in order to ensure that the camera optical axis is close to the nadiral direction (figure 3). the camera module is connected to the csi port, located behind the ethernet port, and enables the camera software. figure 4 shows a scheme of the onboard landing system and the applied sensors. the system works in standalone mode, with a microcontroller arm11 running 700 mhz which processes the algorithm. an optional sensor suite for the landing platform consists of low-cost pressure, temperature and humidity sensors. for the evaluation of the current vertical coordinate, a barometric height measurement msl (mean see level) was used. in alternative, it is possible to use an ultrasonic sonar sensor, as designed in previous works [24]. furthermore, it is possible to add a gps module to extract the correct altitude and position of the uas for comparing and merging data. an ad-hoc software was loaded on this system, allowing to compute the vehicle position and the attitude starting from the extraction of the circular target contour and centre present on the generic image acquired. indeed, as above mentioned, the system is based on a specific landing field as well (as shown in figure 5), where several coloured circular targets are located. the software is able to recognize a circular target, coded by colour, and to extract its coordinates (in pixels). this is figure 3. the bottom of the uas is equipped with the designed system. figure 4. on-board vision system scheme. figure 5. basic landing pattern. figure 2. the adopted small quad-rotor (conrad 450 arf 35 mhz). table 2. size of the adopted uas. weight 670 g max weight 1300 g main rotor ⦰ 260 mm height 165 mm length/width 450x450 mm2 acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 106 accomplished using a circular hough transform function developed through opencv libraries. the input parameter needed for a good finding of the circle in the image is the approximated circle radius expressed in pixels, which depends on range of altitudes of uas. figure 6 shows how the software can detect the target centre on a generic image in real time. the landing pattern dimensions were designed for a range of altitudes between 20 and 150 cm, since this range is typical for a characterization of the uas landing procedure. 4. experimental set-up in order to evaluate the accuracy and precision achievable employing the proposed methodology, a validation procedure was designed and then performed. the aim of this validation procedure is to detect the potential accuracy and precision achievable. therefore, a comparison between srs and bundle adjustment (ba) solutions was carried out. ba is a well-known numeric method used to compute a multi-images position using tie-points and ground control points; this technique assures high reliability and integrity on the solutions. the validation procedure has been conducted following these four steps: 1. design and building of landing pattern, using two types of circular targets: ring coded circular targets and coloured circular targets. 2. photogrammetric survey of landing pattern, following the classical procedure employed in close-range photogrammetry. 3. camera positions and orientations by ba, using all the circular targets, and by srs, using only the coloured circular targets. 4. comparison of position and orientation for each camera, according to the ba and srs results. 4.1. design of landing pattern the landing pattern is composed by an aluminium plane where at least five circular coloured targets were attached. in order to obtain high reliability and precision during the validation procedure, about 40 circular ring coded targets were attached both on the plane face and on the three raised points (figure 5). generally, the precision in the object space can achieve an accuracy in the 1:10 000 to 1:50 000 range, using metric cameras and employing a sufficient number of wellspaced targets. although the srs is not affected in terms of solution stability by the planar arrangement of the targets, the ba provides results more reliable and precise if the targets are located in space rather than on the plane. a cross at the centre of the coded target (figure 7) allows to measure the distance from a point to another with a simple calliper. indeed, the ba in free-network adjustment (no constraints solution) determines positions and orientations of the cameras within an arbitrary reference system and up to a scale factor. the measured distance is then applied to the solution in order to lock the photogrammetric model scale. 4.2. photogrammetric survey photogrammetry is a technique for obtaining an object 3d model through a dataset of images. the operational stages for a photogrammetry survey can be summarized in three steps: • image acquisition, performed in-field and based on several photos to be taken with a calibrated camera. • object measurements, also performed in-field and necessary to scale the photogrammetric model. • image measurement, performed in laboratory: during this step the operator has to recognize homologues points on at least two images. image acquisition can be performed with a consumer camera, but a calibration procedure should be carried out before the image acquisition, to obtain reliable results. the acquisition procedure was performed following the classical guidelines for a photogrammetric close-range survey: the 3d network of images should be optimized according to the precision, reliability and accuracy of the measurements [25]. the 3d network realized for this work is shown in figure 8. the crosses, located at the centre of coded targets, allow to easily obtain precise measurements of distances between the targets. a calliper was used to measure the distance between two targets (rather, between the associated crosses), in order to figure 7. coded circular target. figure 8. 3d network of photogrammetric survey. figure 6. image target recognition and its centre detection. acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 107 give a correct scale to the photogrammetric model. the image measurements are practically fully automated: they can be carried out employing object coded targets or the natural texture of the object. circular coded targets offer a subpixel image point measurement, which allows to obtain high accuracy during the next stages. specifically, the detection accuracy of the centre of a circular target on the image can be up to 1:50 of a pixel, yielding typical measurement accuracy on the object in the range of 1:100 000 to 1:200 000 [17]. the first one (the lower) corresponds to an accuracy of 0.1 millimeters for an object of 10 meters. in this work, the measurements of the image coordinates of the targets were carried out employing two different methods: • lsm: least square matching is a powerful technique for all kinds of data matching problems [26]. here, its application to image matching was used through the software photomodeler scanner. due to the perspective angle, the circular targets present on the scene could appear on the image as ellipses. the lsm is able to detect the correct centre because it applies an affine transformation to recognize the shape correctly. such approach achieves sub-pixel precision, and it is widely employed in industrial photogrammetry. unfortunately, the lsm is not available in the opencv library. • cht: circle hough transform is a feature extraction technique for detecting circles on images. this method is based on application of the hough transform [27], [28] to edge map. the latter is obtained applying the canny edge detector [29] with automatic thresholding on a single image. this approach is suitable for automatic recognition in real time, although the centre of the circle is not always accurate due to the perspective angle of view, as previously described. furthermore, it is already developed in the opencv library. 4.3. camera orientation the orientation is the fundamental step of the photogrammetric procedure, in this work two types of orientations were performed: bundle adjustment and space resection. the former is generally performed in laboratory and it takes as input a large image dataset and many object measurements; the output is the external orientation parameters of the entire dataset of images. in photogrammetry the terms “orientation parameters” express both the position and aspect of a camera. the bundle adjustment was performed using all circular target available on each image. this computation was carried out in post-processing with the commercial software photomodeler. on the other hand, all image orientation parameters were determined with the srs, employing only the coloured circular target and the object measurements. such methodology is completely independent from the ba and it works on a single image without any relationship with other images. the srs algorithm was initially developed in matlab environment. it is a hybrid procedure between the direct solution and the iterative one. in first instance, the algorithm searches the centres of the 3 targets visible on the generic image, so that they form a triangle with the greatest possible area. such points are used to determine the external orientation parameters solving the problem with a non-iterative solution [30], [31]. this method solves the position of the camera by ferrari’s solution of quartic equations, finding the roots of a quartic polynomial function. among the four possible solutions, the algorithm rejects the complex solutions and provides only the real ones. in order to solve the possible ambiguities, the iterative method is performed using the solutions obtained with the direct method as an estimation of the initial parameters. both the convergence factor and a statistic analysis on the reprojection residual provide a robust index to solve the ambiguity. this procedure is completely developed by the authors of the paper and it was transferred on the embedded pc of the drone. the non-iterative method allows to speed up the convergence of the iterative approach, obtaining a stable solution with high attitude angles also. 4.4. comparison the comparison is the last and fundamental step to compute the precision and accuracy achievable. the results obtained with the bundle adjustment procedure are more robust than those given by the space resection one, because the redundancy of the bundle adjustment is too high and the network is well designed. in order to evaluate the results accuracy, the ba solution is taken as reference. 5. results as reported in previous section, to estimate the achievable precision and accuracy, a comparison between the reference position and attitude, provided by ba solution, and the srs estimated values was carried out. the error analysis was performed in terms of error mean, standard deviation and rmse (root mean square errors) for each image acquired and considering two different methods of target detection: lsm and cht. of course, first of all, a deep inspection about the photogrammetric survey is indispensable. the survey was carried out using 14 images taken at 1.5 meters and using 37 coded targets. the obtained network is showed in figure 8, which assures an intersection angle of 67 degrees in average among all rays. furthermore, each image is covered by at least 28 points. targets coordinates are estimated with high precision: indeed, the overall rms vector length in 3d is 0.082 mm, while each camera precision values are reported in table 3. such values are estimated using a classical least square bundle block adjustment, using all 37 coded targets. table 3. precision values about the camera position (expressed in mm) and camera attitude (expressed in degrees), after a classical photogrammetric bundle adjustment procedure. name precision in mm precision in degrees x y z 𝝎 𝝋 𝜿 c1 0.35 0.36 0.26 0.0239 0.0226 0.0092 c2 0.34 0.33 0.20 0.0236 0.0238 0.0086 c3 0.32 0.28 0.22 0.0212 0.0197 0.0099 c4 0.32 0.35 0.21 0.0250 0.0214 0.0090 c5 0.27 0.28 0.19 0.0219 0.0197 0.0087 c6 0.26 0.32 0.24 0.0235 0.0203 0.0098 c7 0.24 0.32 0.27 0.0230 0.0186 0.0110 c8 0.22 0.25 0.26 0.0211 0.0165 0.0120 c9 0.28 0.24 0.26 0.0190 0.0170 0.0110 c10 0.34 0.38 0.26 0.0250 0.0222 0.0105 c11 0.30 0.34 0.23 0.0231 0.0223 0.0086 c12 0.32 0.32 0.20 0.0231 0.0232 0.0090 c13 1.61 1.60 0.32 0.0584 0.0584 0.0092 c14 1.21 1.19 0.21 0.0504 0.0510 0.0090 acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 108 the achieved high precision (millimetres and sub-millimetres for the position) allows to take such solution as reference. indeed, for each camera position an independent solution was computed using the developed srs algorithm. therefore, an accuracy analysis was performed as differences between the reference solution (ba) and the srs solutions. specifically, such analysis was conducted following two different approaches. in both cases only 5 circular targets are considered. a first approach was executed using the lsm to determine the image coordinates of the targets, while the second approach was performed using the cht method for target detection. the first one provides good results almost in every image, especially when the image is too tilted, because it is able to determine the correct target centre even if the circular target appears on the image as an ellipse [32]. the comparison among the ba solution and both srs solutions is described in table 4, table 5 and figure 9, where the absolute differences in 3d space of cameras position and the angle differences are reported. the error analysis reported in table 4 and table 5 provides the potential accuracy of the designed landing system. specifically, the lsm detection mostly achieves a subcentimetre accuracy, while the accuracy reached by the chc approach loses one order of magnitude. such differences are caused by the more precise detection of target centroid obtained by lsm detection; specifically, in equation 1 the image coordinates are estimated with more accuracy than through the chc approach. consequently, the camera position obtained with chc approach is less accurate than the lsm one, as shown in table 4, table 5 and figure 9. 6. conclusions unmanned aircraft systems are of very widespread use by civil and scientific community. to ensure suitable accuracy to uas navigation system, very expensive sensors could be used. the present study proposes a system based on commercial and cheap camera sensors able to guide uas during landing procedure. the vision-based navigation system performance was studied using real data. the system is part of an embedded platform for uas landing path and obstacles detection as reported in [33] and [34]. the inspection has highlighted that the proposed methodology is able to achieve high precisions in position domain and in terms of attitude angles, not comparable with performance of other commercial navigation systems (i.e. gnss positioning systems). another reported analysis was the comparison between two different photogrammetry methods, lsm and cht, implemented to process camera images. from such analysis, it can be seen that lsm approach provides more accurate results than the cht one for all considered figure of merit. the cht approach provides a mean accuracy of 2.3 cm, against the lsm one of 0.23 cm, but the former approach is completely autonomous, while the latter one needs an initialization procedure. acknowledgement this study was partially financed by a grant in the framework of the project “studio e prototipazione di servizi innovativi meteo-marini in supporto alla navigazione” of the university of naples parthenope. references [1] nonami, k., et al. autonomous flying robots: unmanned aerial vehicles and micro aerial vehicles, springer, tokyo, dordrecht, heidelberg, london, new york, 2010. [2] ackermann, s., et al. digital surface models for gnss mission planning in critical environments. journal of surveying engineering, 140.2 (2014): 04014001. figure 9. histogram report which shows the differences among the 3d positions of perspective centre computed by ba and srs. the differences using cht and lsm approaches are reported in green and blue, respectively. table 4. accuracy values about the camera position (expressed in mm) and the camera attitude (expressed in degrees) using lsm, the fifth column reports the relative error. name lsm detection 𝜕 [deg] 𝜕 [deg] 𝜅 [deg] 3d error [mm] re [1:] c1 0.002 -0.014 -0.034 0.24 7292 c2 -0.046 0.116 0.023 1.96 893 c3 0.080 0.052 0.029 1.70 1029 c4 0.020 -0.034 -0.009 0.75 2333 c5 0.009 0.110 0.014 1.57 115 c6 0.124 -0.124 0.064 2.71 646 c7 0.046 -0.037 -0.032 1.05 1667 c8 -0.059 -0.102 0.008 2.08 841 c9 -0.321 -0.083 -0.004 5.16 339 c10 -0.027 0.077 0.009 1.25 1400 c11 0.026 0.016 0.045 0.55 3182 c12 -0.157 -0.134 -0.057 10.69 164 c13 -0.008 0.046 0.082 1.31 1336 c14 -0.032 0.084 0.061 2.11 829 table 5. accuracy values about the camera position (expressed in mm) and the camera attitude (expressed in degrees) using cht; the fifth column reports the relative error. 8 cht detection name 𝝎 [deg] 𝝋 [deg] 𝜿 [deg] 3d error [mm] re [1:] c1 1.588 -2.615 -0.946 53.84 7292 c2 0.044 -1.190 -0.732 15.67 893 c3 -3.075 -0.331 0.365 43.25 1029 c4 0.404 -1.298 0.010 22.43 2333 c5 -1.287 0.536 -0.086 18.77 1115 c6 0.365 -1.063 0.110 16.95 646 c7 0.021 0.137 0.188 4.92 1667 c8 0.524 0.428 0.269 12.00 841 c9 0.591 -1.732 -0.189 32.98 339 c10 -0.885 1.275 -0.429 26.75 1400 c11 -0.171 -0.232 0.097 7.44 3182 c12 0.048 0.009 0.045 1.30 164 c13 -1.643 -0.342 0.593 38.24 1336 c14 0.269 -0.461 -0.940 35.66 829 acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 109 [3] gaglione, s., innac, a., pastore carbone, s, troisi, s. robust estimation methods applied to gps in harsh environments. in proceedings of european navigation conference, lausanne, 2017. 10.1109@euronav.2017.7954169 [4] geyer, c., templeton, t., meingast, m., sastry, s.s. the recursive multi-frame planar parallax algorithm. proceedings of third international symposium on 3d data processing, visualization, and transmission, 3dpvt (2006), pp.17-24.doi: 10.1109/3dpvt.2006.135 [5] sharp, c. s., shakernia, o., sastry, s. s. a vision system for landing an unmanned aerial vehicle. in proceedings of ieee international conference on robotics & automation, seoul, 2011. 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"dronesbench: an innovative bench to test drones", in ieee instrumentation & measurement magazine, vol. 20, no. 6, pp. 8-15, december 2017. doi: 10.1109/mim.2017.8121945 [25] del pizzo, s., and troisi. s. automatic orientation of image sequences in cultural heritage. int. archives of photogrammetry, remote sensing and spatial information sciences 38.5/w16 2011. [26] grun, a. adaptive least squares correlation: a powerful image matching technique. south african journal of photogrammetry, remote sensing and cartography, 1985, 14(3): 175–187. 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[33] papa, u., del core, g., giordano, g., & ponte, s.. obstacle detection and ranging sensor integration for a small unmanned aircraft system. metrology for aerospace (metroaerospace), 2017 ieee international workshop (2017): 571-577 [34] papa, u., del core, g., & picariello, f. (2016). atmosphere effects on sonar sensor model for uas applications. ieee aerospace and electronic systems magazine (2016): 31(6), 34-40.m. fazio, s.l. rota, metrology on stamps, phys. educ. 30 (1995) pp. 289-297. mailto:10.1109@euronav.2017.7954169 a vision-based navigation system for landing procedure acta imeko, title acta imeko issn: 2221-870x march 2018, volume 7, number 1, 42-49 acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 42 analysis of the rbf ann-based classifier for the diagnostics of electronic circuit bartosz połok, piotr bilski institute of radioelectronics and multimedia technology, warsaw university of technology, ul. nowowiejska 15/19 00-665 warsaw, poland section: research paper keywords: fault identification; rbf networks; classification citation: bartosz połok, piotr bilski, analysis of the rbf ann-based classifier for the diagnostics of electronic circuit, acta imeko, vol. 7, no. 1, article 8, march 2018, identifier: imeko-acta-07 (2018)-01-08 section editor: lorenzo ciani, university of florence, italy received october 4, 2017; in final form january 6, 2018; published march 2018 copyright: © 2018 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: this work was partially financed from the statutory grant, institute of radioelectronics and multimedia technology, warsaw university of technology corresponding author: piotr bilski, e-mail: pbilski@ire.pw.edu.pl 1. introduction artificial neural networks (ann) are currently the most popular artificial intelligence (ai) tools used in the diagnostics of analog systems [1]-[3]. their numerous advantages include the ability to extract generalized knowledge from the available measurement data, autonomous operation (making them useful in the automated online diagnostics) and the ability to accurately process data in uncertainty conditions. limited memory usage and high processing efficiency make them useful in embedded applications (implemented in the digital signal processor or the fpga array [4]). their disadvantage is the obscure form of stored knowledge (illegible for the human operator), making the explanation of generated decisions difficult. however, this is not crucial as long as the diagnostic system accurately detects and identifies faults. despite introducing hybrid approaches (such as hierarchical or convolutional ann [5]), traditional architectures are still in use, due to their simple operation principle and high efficiency proven in multiple cases. the most popular ann is the multilayered perceptron (mlp), successfully applied to solve biomedical, financial and technical problems [6], [7]. their implementations in the diagnostics cover power lines [8] or electrical machines [9]. they are currently widely exploited and work as the standard diagnostic tool, or the self-diagnostic module [10]. training the mlp classifier consists in selecting one of available algorithms (such as error backpropagation or levenberg-marquardt) and adjusting the available data to the predefined architecture. it is defined by the specific number of layers that contain the selected type of neurons (computational units). optimizing these parameters is the main design challenge, as the optimal mlp architecture has to be determined individually for each abstract the paper presents the application of the radial basis function (rbf) artificial neural network (ann) to the diagnostics of analog circuits. such networks are in most cases useful in the approximation tasks as the alternative to multilayered perceptrons (mlp) or support vector machines (svm). in this work the analysis of various rbf ann-based classifier configurations for the fault detection and identification module is conducted. the considered parameters included the optimal number of neurons in the hidden layer, coding schemes for the output layer and operation duration during the training and testing of the classifier. the efficiency of the diagnostic system was verified using the electronic circuit, i.e. the fifth order lowpass filter. it was analyzed in terms of testability, depending on the set of accessible nodes, confronted against the availability of the output node only. experiments also covered accuracy comparison between the rbf, mlp and svm classifiers. results show advantages and drawbacks of the rbf ann-based diagnostic module, compared to other available solutions. acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 43 problem. similar issues are encountered during the application of support vector machines (svm) to the classification task. they are considered the optimal ann-based classifiers in the uncertainty conditions [11]. their disadvantage is the time consuming process of selecting the optimal kernel and its parameters [12]. despite such challenges, svm are also popular in diagnostics, used to identify faults in electrical machinery [13], electronic circuits [14] or power plants [15]. on the other hand, radial basis function (rbf) networks are considered simpler in design. their training is faster and structure simpler, as only the single hidden layer is present. therefore, the following parameters are to be determined: • the number of hidden computational units, depending on the size of training data; • the number of output neurons, depending on the number of categories to distinguish and the selected coding scheme; • the activation function width (identical for all hidden neurons). in most cases the rbf network is used for the approximation task, as it has linear neurons in the output layer. its application to classification may require substituting computational units with their sigmoidal counterparts. such approaches are rare and not well described in the literature [16]. this calls for the thorough investigation of the rbf ann classification characteristics during the fault detection and identification in analog systems. the aim of this paper is to assess performance of various rbf ann configurations during the fault detection and location in analog circuits. the presented work is an extension of the research published in [17]. all introduced parameters of the classifier were tested to find the optimal configuration maximizing the diagnostic accuracy, verified on the model of the 5th order electronic lowpass filter. the testability of the circuit was checked by changing the set of accessible nodes (at which system responses can be measured). the accuracy of the ai-based diagnostic module depends on the quality of the training data extracted from the system. increasing the number of accessible nodes leads to the improvement of the diagnostic module. on the other hand, the number of pins in the circuit casing should be minimized. therefore, a compromise between these two goals must be made. finally, the rbf ann-based classifier is compared to its well-established counterparts, i.e. mlp and svm to determine in which situations it outperforms its rivals and should be selected for the task. the structure of the paper is as follows. section 2 presents the applied diagnostic architecture. details of the implemented network are described here. training and testing data sets structure is presented in section 3. the analysed circuit is introduced in section 4, while experimental results are in section 5. conclusions about the conducted analysis are in section 6. 2. ann-based diagnostic architecture the typical diagnostic scheme using any type of the annbased classifier is presented in figure 1. here the system under test (sut) is monitored by the hardware/software module, which performs data acquisition on the accessible nodes. in this way the extraction of the vector of features (symptoms) e={s1, …, sm} (also called examples) from measured signals is performed. they are subsequently processed to make the diagnostic decision (hypothesis) h. it is assumed that knowledge stored by the module allows for the automatic fault detection and identification. this requires implementation of the machine learning algorithms. during this operation the ann-based classifier produces the binary output, in which every fault category is represented by the unique sequence of values {0,1} or {-1,1}, depending on the used type of sigmoidal neurons. application of the rbf network for the fault classification task requires adjusting its structure to the specific problem. this involves selection of the number of neurons in the hidden layer and the coding scheme for binary units in the output layer. as opposed to mlp and similarly to svm, the rbf network contains only one hidden layer with the gaussian activation function φ(si) units (1). the function width (defined as the standard deviation σ, where c is the centre, usually set to 0 for all neurons) determines the network performance. these parameters are optimized during the design stage. ( )         ⋅ − −= 2 2 2 exp σ ϕ cσ σ ii (1) 2.1. minimization of the number of hidden neurons the number of neurons k in the hidden layer (usually greater for the rbf ann than for mlp) is automatically adjusted in the result of the training. although there are approaches to find it during one training sweep [18], in most cases the predetermined network structure is trained and validated, then the process is repeated for another structure. during this time consuming (though automated) process, the maximum acceptable value is determined. it is initially equal to the overall number of examples in the set l (2), which ensures the high classification accuracy, but makes the network large and slow. to avoid this, the number of hidden neurons should be minimized. the training examples are obtained after simulating the sut, which allows for connecting its behaviour with the internal configuration of parameters. in the presented work, the clustering of examples was applied to group the most similar ones. it is assumed that training data from the sut simulation is redundant, caused by the following factors: • low sensitivity of the sut for changes of the specific parameter, which results in multiple examples describing the same category, as they are close to each other (in terms of distance). such a group can easily be substituted by the single example representing all original members. it becomes their centroid. • existence of ambiguity groups (ag) [19], which contain examples belonging to different categories that are close to each other. in this case examples cannot be replaced by the single representative and must all remain in the set. figure 1. ann-based diagnostic system architecture. acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 44 data preprocessing aims at discovering all groups of similar examples by the cluster analysis, which results in the set of groups g1 easily distinguishable from each other, based on the similarity calculations. the detailed scheme is presented in figure 2. redundancy in the set l may be exploited in two ways. the first one consists in providing the original data set for the rbf network training, but limiting the maximum number of neurons in the hidden layer to the number of clusters nc. alternatively, the reduced data set l’ may be provided for training. the distance-based similarity calculation was used in the presented case. its only parameter is the threshold θ, below which two examples ei and e j are considered close to each other (2). the measure applied to create clusters exploits euclidean de (3) and cosine dc (4) distances, treating every example as the point in the m-dimensional space. this way groups located close to each other are easily identified. thresholds θ1 and θ2 for both distances should be selected adaptively to minimize the number of clusters containing examples belonging to different classes (i.e. forming ag). ( ) ( ) 21 ,,, θθ <∧<⇔∈ jiejielji eedeedgee (2) ( ) ( )∑ = −= m k jkikjie σσeed 1 2, (3) ∑∑ ∑ == = ⋅ ⋅ = m k jk m k ik m k jkik c σσ σσ d 1 2 1 2 1 (4) the illustration of the clustering method for m=2 is presented in figure 3. 2.2. output layer coding schemes the important issue during the design of the ann-based classifier is the representation of multiple categories by binaryvalued neurons in the output layer. among multiple coding schemes, three were selected for experiments [20]. it is assumed that only single faults are considered (the most probable case). schemes differ in the number of output neurons o: • one-vs-all (ova) – here each output unit is responsible for a separate category. the number of fault codes l determines the number of neurons o in the output layer (see table 1). during the fault identification, only one neuron should be active. if multiple neurons become active, they can be interpreted as fault candidates in the uncertainty conditions. this scheme makes the classifier suitable for the multiple fault diagnostics (i.e. when more than one parameter is out of tolerance margins). • one-vs-one (ovo) – each neuron is responsible for distinguishing between the pair of categories. this time o classifiers with one output neuron are created and simultaneously trained. their number is determined by the number of distinct fault code pairs: l·(l-1)/2. in this scheme both active and inactive neuron determines the specific fault. the diagnostic decision is made based on the majority voting – the category pointed by the greatest number of networks supporting it. note that each network in this scheme is trained on a different subset, extracted from l. in each, only examples belonging to two fault categories are present. for instance, three fault codes require the following units (trained on the corresponding subsets): 1: c1 vs c2, 2: c1 vs c3, 3: c2 vs c3. • minimum output coding (moc) – the minimal set of neurons, which represents each fault code by the unique combination of active units. the subsequent codes are represented by the binary representation of integer numbers (see table 2). the more sophisticated coding schemes, such as error correcting output coding (ecoc) [21] were excluded from experiments, as their implementation would complicate the output layer even more, not necessarily increasing diagnostic accuracy. ann was implemented using the matlab environment including the neural networks toolbox. 3. data sets description learning l and testing t datasets are required to train the rbf network classifier and verify its accuracy. both have the figure 2. learning data set preprocessing for the rbf classifier training. table 1. applied ova coding scheme. category coding scheme c1 1,0,0, …,0 c2 0,1,0, …,0 … … cl 0, …,0,0,1 table 2. applied moc coding scheme. category coding scheme c1 0,0,0, … ,0,1 c2 0,0,0, … 1,0 c3 0,0,0, … 1,1 … … figure 3. illustration of the clustering method. l clustering l’ nc rbf training rbf training knowledge knowledge acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 45 same structure, containing n feature vectors e with m attributes (symptoms s), supplemented by the fault code, describing the state of the sut. the generic form of the set (5) is the table, whose structure is identical for l and t.           =≡ lnmn m cσσ cσσ tl    1 1111 (5) the single example is generated after simulating the sut model with the fault introduced to its structure. this way the effect of the fault on the responses measured in accessible nodes can be observed. the key issue is the generation of the fault code for each example, based on the actual value of the faulty parameter. as before [22], it is the integer number, being the combination of the parameter identifier and its deviation from the nominal value. the code contains the identifier of the faulty parameter and the discrete degree of its change, both negative and positive (for values greater and smaller than the nominal one). the number of parameter states depends on the assumed thresholds for deviations. for instance, if the parameter value is above ten percent of nominal, the degree is labeled with “1” or “-1“, indicating it being “larger than” or “smaller than”, respectively (depending on the direction of deviation). additional intervals may be introduced to increase the resolution of the diagnostic module. for instance, if the value of the parameter is above fifty percent of the nominal one, the degree “2” may be assigned. this way the second parameter with the value larger than the nominal one is represented by the code “21”, while the first parameter with the value much smaller than the nominal one obtains the code “12”. the exception is the nominal state, encoded with the value “0”. the number of simulated parameter states depends on the expected data set size and the accuracy of the sut modeling. in the presented work, both sets are of the same size, containing mutually exclusive examples. this way the generalization of the ann rbf-based diagnostic module may be verified. diagnostic accuracy acc of the classifier is measured using the set t, for which the number of incorrectly classified examples (i.e. the ones for which the hypothesis h(e) is not equal to the fault code c(e)) determines the sample error es: ( ) ( ) t hct eacc σ eee ≠∈ −=−= : 11 (6) each sut simulation was performed after changing the single parameter value beyond the tolerance margins (here 10% of the nominal value) with all other parameters remaining nominal. a subset of fault-free examples was also prepared to check the resilience of the module to false alarms. 4. analyzed sut the fifth-order analog filter is a good example to implement the data processing methods. similar active circuits are still popular in military or acoustic applications, therefore their analysis is justified [23]. it is complex (because of the large number of nodes and sut elements) and difficult to diagnose based only on the analysis of the output node. therefore, multiple symptoms are needed to identify the state of all elements (resistances and capacitances). the circuit in figure 4 contains ten elements with the following nominal values: r1=r2=r3=r4=r5=1 kω, c1=16 nf, c2=19 nf, c3=13 nf, c4=51 nf and c5=49 nf. subsequent resistances were labelled with numbers from 1 to 5 respectively, while capacitances were referred to as parameters no. 6 to 10. the cutoff frequency of the filter for such values is 10 khz. the model of the circuit was implemented in the simulink environment with the operational amplifiers realized as equivalent circuits with controlled voltage source. simulations were performed to obtain examples of the sut behaviour for different values of elements (changed up to 90 percent of the nominal value). the excitation signal provided at node no. 1 was a sinusoid with 9 khz frequency (i.e. close to the cutoff frequency). the filter was analysed in the time domain, where at the accessible nodes sinusoidal responses were recorded. measurements were taken at nodes 2, 3, 5, 6, 8 and 9. from each response the first three maximal and minimal values of the signal with their time instants and time instants of zero crossings are extracted (figure 5). this gives the total number of 54 symptoms for each example (nine for each node), assuming all discussed nodes are accessible. to evaluate the dependency between the size of the set and the accuracy of rbf-based classifier, various sizes of the sets were prepared, starting from 70 examples (7 simulations for each parameter, including the nominal state) for the set l1 to 180 (18 simulations for each parameter) for the set l2. to consider tolerances, additional sets were created with results of simulations affected by the random value added to the actual parameter. this allows for verifying if random deviations of parameter values influence the ability of the diagnostic module to distinguish faults. the number of different fault codes to distinguish was 41, as not all 5 were used for each parameter due to their smaller sensitivity. 5. experimental results conducted experiments consisted of four stages. first, coding schemes were compared. next, relation between the size figure 5. examples of responses from the filter for various values of the resistor r1. figure 4. scheme of the 5th order lowpass filter. acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 46 of the data set and the diagnostic accuracy was determined. the testability of the circuit, depending on the number of analysed nodes, was also verified. finally, comparison between rbf, svm and mlp (also implemented in matlab) was performed to determine efficiency of all classifiers and find their advantages and drawbacks. the ann training is a random procedure, consisting in the adjustment of weights between neurons to the training data. knowledge structure depends on the initial (randomly selected) values of weights, therefore every training sweep results in different content of weights matrices. therefore, to gain the general information about the ann efficiency, training and testing experiments were repeated ten times for each architecture and data set. results presented in the following subsections refer to average accuracy values. 5.1. coding schemes verification coding schemes presented in section 2.2 were implemented to design the rbf network, train it on the set l and test using the set t. the first operation is done according to the simulation before test (sbt) [24], [25] paradigm, i.e. when knowledge is extracted prior to the diagnostic module implementation in the industrial environment. duration of the operation is of secondary importance here. the trained network is then used to process measurement data and make decisions. as the operation duration of ann in the testing phase is negligible, there are virtually no restraints, even for their on-line application (where the diagnostic decision must be made within the specific time limitations). optimal results for all configurations (regarding the width of the rbf σ, number of output neurons o, number of hidden neurons k, the maximum achieved training error e and obtained accuracy on the set t) for the set l2 (180 examples) are in table 3. in most considered cases, the ovo coding is the best scheme, producing the minimum error for all network configurations. this method is the most complex, requiring multiple ann, each responsible for only two faults. this leads to many networks, but with relatively simple structure. the average number of hidden neurons for the optimal ovo network structure was 8. the most complex is moc, as multiple output neurons have to participate in producing the fault category. for each coding the best width of the gaussian function was also determined. the full sweep optimization procedure was applied to obtain it (although more sophisticated approaches, such as simulated annealing [12] or genetic algorithm can be used). the optimal rbf width and the number of hidden neurons are in all cases in the middle of the verified range (for σ it is between 0.1 and 2.0). figure 6 shows the relation between σ and the obtained accuracy (with all other parameters set as in table 3). for all coding schemes, there is the best σ value, for which the minimum error is obtained. results show that narrow gaussian functions are preferred (like σ =0.5 for ova). the cost of the highest ovo accuracy is the greater computation effort, caused by the large number of trained networks in comparison to the single network used for remaining coding schemes. the experiments with the optimization of the number of hidden neurons show similar effects as during the selection of the gaussian function width. the discrete sweep was implemented here to train and test the network while the number of hidden neurons was incremented (i.e. increased by one). at the specific point the maximum accuracy is reached. adding more computational units does not improve the ann efficiency but increases the training duration. this concludes the structure optimization stage, with results of the ovo coding presented in figure 7. as each network distinguishes between two categories only, it is simpler than the ones implementing other coding schemes. the detailed analysis of the network performance shows that most diagnostic errors are made because of mistaking the first element (r1) with the sixth one (c1). in such a case, the direction of changes is correct (for instance, the typical error is producing the code “-62” instead of “-12”). changes in elements for which the sut has low sensitivity (especially r2, r3 and c3) are difficult to detect and the classifier regards the circuit as nominal. in such cases (when all symptoms are within tolerance margins) categories of such examples should be changed to “0”. there are also errors of minor importance, i.e. the incorrect identification of the intensity of changes, although the element is correctly located (for instance, when the code “52” instead of “-51” is generated). such mistakes should be treated as less important and regarded with smaller weights during the accuracy (6) calculation. 5.2. data set size experiments regarding the size of the training set l were divided into two steps. in the first one, various numbers of figure 6. influence of the rbf width on the diagnostic accuracy. table 3. comparison of performance of rbf network output coding schemes. coding scheme σ k o e acc [%] ova 0.5 82 41 0.0 81.11 ovo 0.6 8 820 0.0 83.33 moc 0.3 95 6 0.02 80.56 figure 7. influence of the number of hidden neurons on the diagnostic accuracy (ovo coding). acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 47 examples were inserted to the set and then used to train the rbf classifier (starting from the set l1 with the minimal number of symptom vectors, through the medium set with added some new examples, and ending with the maximal set l2). the initial diagnostic results for l1 are significantly improved by introducing additional examples, provided they carry useful information. table 4 compares the diagnostic accuracy for data sets of different sizes. as in other applications, the amount of training data influences the accuracy of the classifier, which may be a problem if they are difficult to collect (for example, when the model of the sut does not exist or requires large amount of calculations). during the second stage, the original set is processed to group the most similar examples (see section 2.1) to compress their number. additionally, dependency between the training duration and the size of processed data should be established. this is done using the clustering algorithm. in this way it is possible to eliminate redundant examples, which in turn would be represented by the single vector of symptoms. results of the diagnostic procedure for the rbf ann trained on the minimized version of the set are presented in table 5. increasing the number of examples improves the fault detection and identification performance, but to obtain the accuracy equal to the rbf trained on the l2 set, the density of clustering must not be large. comparable diagnostic results are obtained on data sets with size close to l2. again, the most effective is the ovo scheme. relation between the rbf network structure and training duration for the ova and moc coding is in figure 8. all computations were performed on the computer equipped with the amd fx-8320 processor (eight cores, 3.50 ghz) and 16 gb of ram. the processing time for both mentioned coding schemes depends on the number of hidden neurons in the network structure. the relation between the size of the data set and the number of neurons is more complex, as even larger data sets may be optimally processed by smaller networks. the training duration of the rbf ann-based classifier with ovo coding is significantly longer, requiring between 10 and 20 minutes to complete on the same hardware configuration. the time curves are close to linear, which is the positive effect. the ovo rbf classifier requires the greatest amount of time to train because of a large amount of networks to process. this calls for the parallel implementation of the training procedure [26], where each network would be processed by a separate processor, core or computer. as this procedure is implemented off-line, its duration is of secondary importance, although considering the thorough values optimization of σ and k for each network will complicate the process even more. another problem is the testing duration, as it determines the usability of the diagnostic module in the on-line mode. average times required by the rbf ann with subsequent coding schemes to process the single set of symptoms are in figure 9 (on the same computer configuration as before). although much faster for all schemes, the processing time for ovo coding is significantly longer, making it difficult to implement in the embedded system working under strict time limited conditions. 5.3. testability verification experiments presented so far were conducted on the complete data sets, i.e. containing symptoms extracted from all circuit nodes, as discussed in section 4. because of practical reasons it is rarely possible to use all of them (especially in the integrated system). therefore, the number of diagnostic pins available from the circuit casing should be minimal. the compromise between the accuracy and the set of accessible nodes must be made. figure 10 presents comparison between different coding schemes of the optimal rbf network structure for different sets of accessible nodes. besides the node 9 (always observable, therefore usable in the input-output analysis [24]), all others can be theoretically made accessible. because the filter has the sequential structure, it is reasonable to assume figure 8. training duration of the rbf network (ova and moc coding schemes). table 4. influence of the data set size on the rbf classifier performance. data set size accova [%] accmoc [%] accovo [%] 70 68.57 67.14 65.71 120 75.78 73.86 75.54 180 81.12 80.56 83.33 table 5. influence of the data clustering on the rbf classifier performance. data set size accova [%] accmoc [%] accovo [%] 70 68.57 67.14 65.71 88 70.67 69.56 68.23 103 72.23 68.34 71.12 109 70.00 70.56 71.12 136 75.89 73.45 75.78 153 76.67 72.23 77.42 162 81.12 72.78 81.89 180 81.12 80.56 83.33 figure 9. duration of processing the single vector of symptoms by the rbf ann-based diagnostic module. acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 48 that more information can be recovered from the nodes close to the output of the sut. therefore, the order of adding them to the analysis was as follows: 8, 6, 5, 3, and 2. the key step for improving the accuracy was the addition of the nodes 5 and 8. further extensions led to only small change in diagnostic outcomes. again, in all experiments the ovo coding proved to be the most effective. the procedure for other similar circuits (such as other filters) should be the same. if the system is more complex, more sophisticated node selection method should be used [27]. 5.4. comparison against other classifiers besides the rbf-based network classifier, also svm and mlp can be used to process the same data sets. in tab. 6 diagnostic outcomes of the optimal network configurations for the largest set t2 are presented. performance of classifiers is comparable, the main difference lies in the training time, which is the longest for the mlp and comparable for rbf and svm. in the latter case, additional parameters must be determined (such as the kernel function and its parameters), which significantly increases the training duration. the rbf annbased classifier is the fastest one to design and has simpler training algorithm than competitors. its optimization is also less complex than svm. this classifier can then be used during the fault analysis of suts, which are moderately difficult to diagnose. in the case of wide element tolerances (for cheap circuits) or high level of noise, more sophisticated ann (such as svm) may be preferred. 6. conclusions the paper presented analysis and optimization of various rbf network-based classifier in the diagnostics of analog circuits. multiple parameters, including the width of the activation function, number of neurons in the hidden layer and coding scheme in the output layer were verified. the testability of the circuit was also analysed. the networks with ovo scheme were the best in most cases, but required the greatest amount of time to be trained. this is because this scheme requires creating a large number of simple networks, distinguishing between only two fault categories. the moc, although requiring the smallest number of output neurons is more susceptible to random classification errors. the ova scheme is the most popular one. its accuracy is similar to ovo, but it is much faster trained. selection of the scheme is a compromise between the expected accuracy and available time for training and processing the testing vector of symptoms (when the on-line diagnostics is considered). the optimization of the rbf training process is similar to other ann-based classifiers. the supervised learning is implemented here as the standard parameterized algorithm. the designer’s task is to configure the network (by determining the number of hidden neurons or values of the activation function parameters) to ensure the highest diagnostic accuracy. this process is time consuming, but results in the improvement in the diagnostic accuracy. multiple continuous or discrete optimization methods may be used for this purpose, regarding both network structure and the sut testability. future research should cover analysis of other systems belonging to various technical domains. successful implementation of the presented diagnostic scheme to their analysis would confirm usefulness of the rbf ann in 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[27] a. bilski and j. wojciechowski, “automatic parametric fault detection in complex analog systems based on a method of minimum node selection,” int. j. appl. math. comput. sci., 2016, vol. 26, no. 3, pp. 655–668. analysis of the rbf ann-based classifier for the diagnostics of electronic circuit << /ascii85encodepages false /allowtransparency false /autopositionepsfiles true /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /error /compatibilitylevel 1.4 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.0000 /colorconversionstrategy /cmyk /dothumbnails false /embedallfonts true /embedopentype false /parseiccprofilesincomments true /embedjoboptions true /dscreportinglevel 0 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can be opened with acrobat and adobe reader 5.0 and later.) >> /namespace [ (adobe) (common) (1.0) ] /othernamespaces [ << /asreaderspreads false /cropimagestoframes true /errorcontrol /warnandcontinue /flattenerignorespreadoverrides false /includeguidesgrids false /includenonprinting false /includeslug false /namespace [ (adobe) (indesign) (4.0) ] /omitplacedbitmaps false /omitplacedeps false /omitplacedpdf false /simulateoverprint /legacy >> << /addbleedmarks false /addcolorbars false /addcropmarks false /addpageinfo false /addregmarks false /convertcolors /converttocmyk /destinationprofilename () /destinationprofileselector /documentcmyk /downsample16bitimages true /flattenerpreset << /presetselector /mediumresolution >> /formelements false /generatestructure false /includebookmarks false /includehyperlinks false /includeinteractive false /includelayers false /includeprofiles false /multimediahandling /useobjectsettings /namespace [ (adobe) (creativesuite) (2.0) ] /pdfxoutputintentprofileselector /documentcmyk /preserveediting true /untaggedcmykhandling /leaveuntagged /untaggedrgbhandling /usedocumentprofile /usedocumentbleed false >> ] >> setdistillerparams << /hwresolution [2400 2400] /pagesize [612.000 792.000] >> setpagedevice acta imeko, title acta imeko issn: 2221-870x march 2018, volume 7, number 1, 13-19 acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 13 software separation in measuring instruments through security concepts and separation kernels daniel peters, patrick scholz, florian thiel physikalisch-technische bundesanstalt, abbestrasse 2 12, 10587 berlin, germany section: research paper keywords: selinux; apparmor; mic; mac; mils; legal metrology; welmec 7.2 software guide; separation kernel; hypervisor; mid citation: daniel peters, patrick scholz, florian thiel, software separation in measuring instruments through security concepts and separation kernels, acta imeko, vol. 7, no. 1, article 4, march 2018, identifier: imeko-acta-07 (2018)-01-04 section editor: lorenzo ciani, university of florence, italy received september 25, 2017; in final form january 14, 2018; published march 2018 copyright: © 2018 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: daniel peters, e-mail: daniel.peters@ptb.de 1. introduction embedded systems play an increasingly important role in industry and economy and, in europe, currently have an annual growth rate of 12% [1]. software security is becoming a major focus of research, because today's embedded systems are often connected to insecure networks such as the internet. nevertheless, software architectures are becoming more and more complex, making it increasingly difficult to design secure systems. focusing on measuring instruments in europe, in germany alone, more than 100 million legally relevant measuring instruments are in use, mainly electricity, gas, water and heat meters which in total lead to an annual gain between €104 billion and €157 billion [2]. a malfunction or manipulation of these measuring devices could cause considerable damage. therefore, it is particularly important to focus on the security of such systems and their software. manufacturers of measuring instruments prefer to use general-purpose operating systems (gposs) such as windows or linux due to the comfortable software infrastructure and the broad availability of device drivers. these operating systems include several million source lines of code (sloc). studies have revealed that on average a software bug can be found in every 2300 lines of code [3]. the presence of only one bug can be sufficient for an attacker to gain unauthorized access, which leads to the assumption, that measuring devices running on gposs are under great risk of manipulation. the focus of the basic requirements for measuring instruments under legal control is the protection of consumers and the correctness of the measurements. in europe that trust is ensured by institutions, so-called notified bodies, which devote themselves to the validation of measuring instruments before commissioning and the assessment of conformity. the tests in laboratories primarily focus on the hardware, such as the physical sensors. testing the software proves to be more difficult, since operating systems and their additional software abstract in the age of the internet of things and industry 4.0, more and more embedded systems are connected through open networks, which also concerns measuring instruments under legal control (e.g. smart meters). therefore, cyber-security for measuring instruments is becoming increasingly important. in this paper, possibilities to design secure measuring software running on general-purpose operating systems are analyzed according to legal requirements set up by european directives, e.g., the measuring instruments directive (2014/32/eu), which define the mandatory security level. technical interpretations for the security concepts described in this paper are derived from these legal requirements with the aim to provide manufacturers the architectural guidance to construct systems which easily pass a conformity assessment at a notified body. in this paper security concepts, i.e., selinux, apparmor and mandatory integrity control (mic) are being described, which are based on mandatory access control (mac) strategies. additionally, high-security methodologies and concepts, e.g., multiple independent layers of security (mils) and security kernels, are highlighted. in the examples given, software separation, which enhances overall security, is achieved by using selinux mechanisms in the modules (virtual machines) atop a separation kernel. acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 14 have too many lines of code and are often not open source. to support this challenge, gposs enhance their systems with security concepts. in legal metrology most of the times through malfunction or manipulation financial loss or gain can be caused. still in other domains, which are also concerned about the right measuring of sensors, loss of life can be the result of a malfunction. an example is railway level crossings [18]. the analysis this paper provides focuses on how “mandatory integrity control” (windows), selinux and apparmor (linux) can be used to fulfil the requirements in legal metrology. especially, security frameworks for measuring instruments under legal control are described that can be used to fulfil the requirements of legal metrology documents. furthermore, highsecurity methodologies and concepts, e.g., multiple independent levels of security/safety (mils), are highlighted and a more sophisticated framework based on a separation kernel design is analysed. it enables the use of drivers and network stacks of gposs with the help of virtualization. in this framework, the legally relevant parts are separated to allow higher degrees of freedom for the unregulated software. afterwards, a communication framework, which provides services to these vms is created. this framework, also consisting of separated vms, monitors the information flow, correctly delegates requests from and to i/o devices and helps control agencies (such as the market surveillance) to verify system integrity. the latter security framework does not hinder the additional mac control mechanisms described in this paper, which can still be applied additionally in the vms, as shown in section 6, on an armv8 board. 2. legal metrology to help consumers to rely on the correctness of measurement results without having to check them themselves, legal regulations at national and international level are necessary. legal metrology creates the prerequisite for producing high-quality products and contributes significantly to the functioning of an economy. it is estimated that in industrialized countries, 4% to 6% of the gross national income is settled by measuring instruments and the related measurements [2]. at the international level, the "organisation internationale de métrologie légale" (oiml) is an intergovernmental organization that has set up a world-wide technical set of proposals to help with the harmonization of legal metrology. for example, software requirements for measuring instruments are formulated in the oiml d 31 [4] guide. in europe, the measuring instruments directive (mid) [5] of the european union formulates concrete legal requirements. in addition, there is welmec a committee at the european level in legal metrology, which includes the member states of the eu and efta. the guidelines for the conformity assessment of measuring instruments, the so-called welmec guides, help the manufacturers and the notified bodies to build or check measuring instruments, respectively. here, welmec 7.2 [6] is the software guide for measuring instruments. it states that the software-related requirements of the mid are fulfilled if the requirements of the guide are upheld [6]. 3. background in legal metrology, the risks of manipulating measuring instruments and the theft of sensitive data are increasing, because of the integration of general-purpose operating systems such as windows or linux, and because many devices are connected to the internet. so nowadays, security is becoming an important focus. in general, security is the ability of a component to protect resources for which it announces protection responsibility, and the component's security policies are its security-enforcing properties and requirements. generally, security policies try to enforce three points: • confidentiality ensures no unauthorized disclosure of information; • integrity prevents modifications or corruptions of a resource without authorization; • availability ensures that authorized users have access to resources whenever needed. by using access control strategies and a secure boot mechanism, it is possible to ensure adequate security (upholding the three points from above) for connected devices. in addition, separation of functionalities in it systems can provide even more security. for example, a further concept for high security it systems is the “multiple independent levels of security” (mils). mils is a high-assurance security architecture based on the concepts of separation and controlled information flow. the foundation of the system is a small kernel implementing a limited set of critical functional security policies [7], described in section 3.3. 3.1. legal requirements the welmec 7.2 software guide tries to break down the requirements for legal metrology software of the mid to special technical examples and recommendations. one important point is verifying measuring instruments in commission. validating the software identification ensures that software was not switched or manipulated. out of the mid and the assessment of hazards, the welmec 7.2 software guide defines six risk classes from a to f, evaluating the need for software protection, software examination and software conformity. the risk classes are ascending (from a to f) in their demand for security. for our purposes, the best way to start is to construct a system for risk class f measuring instruments, because it conforms to all requirements of the other classes. hence, the solution provided here can be downscaled to lower risk classes. furthermore, the welmec 7.2 clarifies what legally relevant parts are. according to welmec 7.2, all modules are legally relevant that make a contribution to or influence measurement results. these modules facilitate auxiliary functions, like displa ying da ta , protecting da ta , sa ving da ta , identifying the softwa re, executing downloa ds, tra nsferring da ta a nd checking received or stored da ta . based on these seven points, we construct our security concepts in the next section. additionally, the welmec 7.2 differentiates between measuring instruments that are built solely for the measuring purpose and the ones that run universal software. the two classes are called p and u. normally one can say, if a measuring instrument has an operating system installed, it is a type u instrument, else it belongs to the p class. for both classes, four subclasses are defined which deal with following it functions: • l: long-term storage of measurement data, • t: transmission of measurement data, • d: software download, • s: software separation. https://www.oiml.org/ https://www.oiml.org/ acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 15 to be more specific, the welmec 7.2 software guide tries to break down the requirements for legal metrology software of the mid to technical examples and recommendations, looking more closely at the four points from above. the important mid software requirements, we spotted, are: 1) the measuring instrument must guarantee reproducibility, i.e., measurement results of the same input must yield the same output, even if handled by different users. this implies that a measurement result should not depend on the user/consumer employing the instrument. from the software point of view, different processes with varying access rights performing the same measurement should yield the same result. 2) a measuring instrument shall be designed to reduce as far as possible the effect of a defect (e.g. software bug) that would lead to an inaccurate measurement result, unless the presence of such a defect is obvious. 3) a measuring instrument shall have no feature to facilitate fraudulent use, and possibilities for unintentional misuse shall be minimal. 4) software identification shall be easily provided by the measuring instrument. additionally, evidence of an intervention shall be logged to verify measuring instruments in commission. 5) if a measuring instrument has associated software which provides other functions besides the measuring function, the software that is critical for the measurement purpose shall be identifiable. 6) measurement data and software that is critical for measurement characteristics and stores or transmits metrologically important parameters, shall be adequately protected against accidental or intentional corruption. 7) for utility measuring instruments, the display of the total quantity supplied or the displays from which the total supplied quantity can be derived, which form the basis for payment, shall not be resettable during use. 8) the indication of any result shall be clear and unambiguous, i.e., easy reading of the presented result shall be permitted under normal conditions. still, additional information may be shown, if they cannot be confused with the metrologically important results. 9) a durable proof of the measurement result and the information to identify the transaction shall be available. additionally, there are specific metrological requirements for the measuring instruments analysed in the mid. to give an example, we look at taximeters, which measure the time, the distance and calculates the fare for a trip based on the applicable tariffs. hereby it is stated that in case of a reduction of the voltage supply to a value below the lower operating limit as specified by the manufacturer, the taximeter shall: • continue to work correctly or resume its correct functioning without loss of data available before the voltage drop if the voltage drop is temporary, i.e. due to restarting the engine; • abort an existing measurement and return to the position "for hire" if the voltage drop is for a longer period; • the taximeter needs to have a long-term storage, the data shall be available in the taximeter for at least one year. an acceptable solution, for example, would be that the voltage level detector fires an interrupt when the voltage level drops for a time of 15s. the assigned interrupt routine collects measurement values, state values, and other relevant data and stores them in a non-volatile storage e.g. eeprom. after the voltage level rises again the data is restored and the functioning continues or is stopped. 3.2. access control strategies in an information technology (it) system, access control strategies determine which access type (e.g., read, write, or execute) a subject (e.g., user, process, or device) may have to an object (e.g., file, table, or subject). the discretionary access control (dac) strategy, for example, is a strategy in which access rights are defined only by the identity of a subject. in legal metrology, where the focus is the integrity of measurement data and the confidence in the correctness of a measurement, this access control strategy may not be sufficient. here, one should use the mandatory access control (mac) strategy, in which an access decision is additionally determined by means of object properties and rules. one of the mac concepts is the multilevel security (mls). in this concept, all subjects and objects are classified into four categories (security level): unclassified, confidential, secret and top secret. for these categories, rules for read and write access are defined in different models. examples are given bellow. the first example is the bell-lapadula model, which deals with the confidentiality of data. it is constructed with the purpose to protect information from unauthorized access. following, security rules are being formulated: • simple-security rule (no read up policy): a subject can read the information of an object if its security level is equal to or greater than that of the object. • *-rule (no write down policy): a subject is allowed to write to an object if its security level is equal to or less than that of the object. another example is the biba model, which deals with the integrity of data. information shall be protected against manipulation of unauthorized persons. the following security rules are defined: • integrity-*rule (no read down policy): a subject can read the information of an object if its security level is equal to or less than that of the object. • simple-integrity policy (no write up policy): a subject can write to an object if its security level is equal to or greater than that of the object. in a role based access control (rbac) strategy the users have no direct access to objects. instead, permissions are directly linked to tasks by assigning roles. each user is initially assigned a role and can have access to several of these roles. in addition, at least one group is assigned to each role. by dividing everything into users, roles, and groups, an access decision is based on the roles assigned to the user and the groups that are accessible to these roles. 3.3. multiple independent levels of security/safety (mils) multiple independent levels of security/safety (mils) is a high-assurance security architecture based on the concepts of separation and controlled information flow. the foundation of the system is a small kernel implementing a limited set of critical functional security policies. this special kernel, often called a separation kernel (see section 5), implements the policies for information flow control, data isolation, damage limitation and period processing [14], which are defined as follows: • information flow control ensures that information acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 16 cannot flow between partitions unless explicitly permitted by the system security policy. • data isolation ensures that a partition is provided with mechanisms, whereby isolation within it can be enforced. • damage limitation ensures that a bug or attack damaging a partitioned application cannot spread to other applications. • period processing ensures that information from one component is not leaked into another one through resources, which may be reused across execution periods. a separation kernel should be small enough to be thoroughly evaluated. the applications managing sensitive data are then built atop the secure separation kernel. an advantage of this approach is its modularity. software of varying security demands runs on the same microprocessor, by means of software partitioning through the separation kernel. 4. security concepts 4.1. selinux in linux, the discretionary access control (dac) is used by default. nevertheless, mandatory access control (mac) and role based access control (rbac) strategies can additionally be used. one example is security-enhanced linux (selinux), a linux kernel module. selinux is based on an advanced security kernel called flask, which was developed by a research group at the university of utah named flux [8]. in the flask framework, a security server, which contains the security policies, communicates via interfaces to a so-called object manager. at each request of a subject to an object the object manger checks the policies, with the help of the security server, to grant or deny access. in selinux, every user is assigned to exactly one role at any given moment in time. if allowed, the user can him/herself switch into another role. each role has access to a set of domains and types. with the construct of user, roles and domains (or types), well-defined security policies can be constructed. the strict policy, which is available in selinux, is well suited to build a secure framework, because it upholds the least privilege principle, which states that every component should only have the access permissions it really needs to have to function properly, no more. hence, each subject and each object are controlled by their own domain and have just the access rights they need. in figure 1, our framework of user, roles and domains based on the legal requirements listed in section 3, is depicted. as can be seen, we created a user called legalsoftwareuser_u in selinux that can be in one of three roles: staff_r, system_r and legalsoftware_r. with the roles staff_r and system_r the user can have access to non-legally relevant objects, which are not needed for the measuring purpose. in our construct, only the legalsoftwareuser_u user has access to the role legalsoftware_r and only in this role the legally relevant domains can be used, which represent software processes that are needed for the measuring task. the legally relevant software parts can interact with each other in the way shown in figure 2, and explained in the text above the figure. users without the role legalsoftware_r cannot influence the legally relevant tasks, because they have no access to them. 4.2. apparmor a second way to use mandatory access control (mac) in linux is the applicationarmor (apparmor) framework. apparmor is like selinux a linux kernel extension. it does not use role based access control (rbac) strategies. in apparmor, profiles [9] and individual security rules are used to implement the mac and to determine whether the access of a subject to an object is allowed or not. each profile is created for just one process. in apparmor only the processes with profiles are supervised. because only newly started processes are monitored, apparmor must be started at the beginning. in general, apparmor divides processes into trustworthy and untrustworthy, and just guards the untrustworthy processes, because it is assumed that the others are not a danger to the it system. apparmor can operate with much less rules then selinux, making it easier to use. a disadvantage is that rules in apparmor are based on absolute file-names. by renaming a file, the protection of apparmor can be subverted. it should also be noted that apparmor and selinux cannot be used at the same time. if both modules are compiled into the linux kernel, one must be deactivated. the framework we use to fulfill the legal requirements of the welmec 7.2 software guide is shown in figure 2. here, only a user (user), which can execute the figure 2. legal software parts and their access permissions in selinux and apparmor. figure 1. framework of the user, roles and types in selinux for legal and non-legal tasks. acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 17 measurement task, has access to the legally relevant software (ls) and through this, access to the legally relevant software modules, like the download manager (dm), the data transfer module (dtm) and the data storage manager (dsm). the dtm and the dsm are responsible for the transmission of measurement data and for their long-term storage. both modules have access to the measurement data (md) and can use a key & signature manager (ksm) for deand encryption of data. in the broken measurement data protocol (bmdp), errors, like damaged measurement data can be recorded. the dm is used for the transfer of downloaded data (dd), the subsequent verification procedure and the following installation. to verify this data, the dm uses hash values to ensure authentication. also, a download protocol (dp) is being created for each download process. 4.3. mandatory integrity control since windows vista, mac is available by a framework called mandatory integrity control (mic). it can be used additionally to the dac, which windows implements by default, like linux. hereby, an access allowed from the dac can still be prevented by the mic. in mic each object is considered a securable object and is classified into one of five classifications, called the integrity level (il) that is part of the security id (sid) stored in a mandatory label of an object [10]. these five classifications are: untrusted, low, medium, high and system. with these integrity levels and mandatory labels an access decision is made. mic enforces system-wide no-up policies: no-write up, no-read up and no-execute up, meaning that a user with a lower il is not allowed to access an object with a higher level. the il system is preserved for system-related processes such as kernel processes. no user can acquire this il [10]. high is, e.g., for the administration of a system, but can be acquired from normal processes if needed. medium is the default setting of most processes. the il untrusted is for unidentified processes and low for low-trusted processes with higher attack-risk. the mandatory label of an object which includes the sids, can be found in the security descriptor. the sids of subjects are stored in an access token alongside a set of privileges and a list of group memberships. figure 3 gives an overview of the ils we use for our software framework, seen in figure 2. here, the legally nonrelevant software (nls), which is not used for the measurement, is classified as il medium. the legally relevant modules, that are used for measuring processes, are classified with il high. because of the no-up policy in mic, a manipulation of the legal relevant modules is not possible. 5. separation kernel system architecture as mentioned in section 3.3, a separation kernel is the foundation of a mils architecture, it needs to fulfil following properties: • non-bypassable: the security monitor is not bypassable by a component through a hidden communication path, including lower level mechanisms. • evaluatable: any component that is critical for the trustworthiness of the system can be evaluated to the level of high assurance. • always-invoked: each and every access/message is checked by the appropriate security monitors. • tamperproof: the system controls rights to the security monitor code, e.g., configuration and data; preventing unauthorized changes. an acronym, which is often used, for these characteristics is neat. 5.1. virtualization the mils system architecture, especially the separation kernel, together with virtual machines (vms) as its components, furnishes a great base for high-assurance software. through virtualization gpos applications can be directly executable. in general, virtualization is divided into two main approaches [14]. pure virtualization supports unmodified guest operating systems, running on top of a kernel, which is called hypervisor. hereby, closed-source operating systems are directly executable without the need for software changes. unfortunately, many processors do not have adequate support for pure virtualization. therefore, the second approach, called paravirtualization, presents the guest operating system with an interface that is nearly identical to the underlying hardware to make virtualization possible. to improve performance and allow virtualization at all, the guest operating system is often modified. in the past, embedded systems and also measuring instruments used to be relatively simple devices, which did not support virtualization. nowadays, many measuring devices have the characteristics of general purpose systems. this power together with the low development costs for a board combined with hardware virtualization technologies makes virtualization in measuring instruments, attractive. the motivation for virtualization is in our case also security. by running an operating system in its virtual machine (vm) safely encapsulated, the damage of an attack cannot propagate to other vms. 5.2. system architecture a separation kernel architecture well suited for measuring instruments is described for example in [16]. here, the legally relevant parts, which are needed for the measuring purpose, are separated from the irrelevant ones by putting them in different virtual machines. afterwards, it is made sure that their virtual machines have no direct access to i/o devices. figure 4 depicts such a stripped system architecture. the framework consists of three virtual machines that run atop the separation kernel. in the l (egally relevant) vm, all the programs are running that are important for the measuring purpose. in the n(on-legally relevant) vm everything else is figure 3. framework of the user, roles and types in selinux for legal and non-legal tasks. acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 18 running. to communicate with the separation kernel and through it with sensors and perhaps the internet, the communication supervisor is used, the third vm. it can be used to monitor the information flow, correctly delegate requests from and to i/o devices and help control agencies (such as the market surveillance) to verify system integrity. with the vm approach communication between the individual vms can take place through network protocols already implemented in the gpos, and the gpos device drivers can be used, creating virtual networks connections. the disadvantage of this concept is that attacks or bugs in the individual vms are the same as in the normal gposs, the separation kernel only makes sure that an attack/bug in one vm cannot propagate into the others. the disadvantage should be counteracted by using minimal configurations for the gposs and by configuring the individual vms with the security frameworks described in this paper. an example is given in the next section, showing how a separation kernel can execute under a linux with activated selinux. 6. practical tests to show the practicality of our approach, we have built two test systems. with the minimal implementation principle in mind, we constructed the first system with a minimal linux and selinux support, to see how much space is needed. for demonstration purposes we used qemu, which is a generic and open source machine emulator and virtualizer. we compiled the linux kernel (version 4.11.0-rc4) for x86 systems, which most desktop computers (e.g. based on intel chips) are nowadays. additionally, we used buildroot to create our root filesystem. buildroot is a tool that helps to generate embedded linux systems. with this tool, it is also simple to cross-compile everything, constructing for example also binaries for arm platforms. the compilation of busybox is also supported that is often used in embedded systems to replace more than 300 common linux commands, e.g., ls, sh, rmdir, etc. into one executable. in the root filesystem of our linux system, we added many additional packages to the core busybox-based system: the bash shell, a gui (x.org display server, matchbox window manager), mdev as device manager, and of course the necessary infrastructure to activate selinux (linux kernel modules, selinux support options in busybox). afterwards, we downloaded example policies (from [11]) and added them to our root filesystem. finally, we activated selinux by adding selinux=1 to the kernel commandline and created a test file /etc/selinux/config with following parameters: selinux=permissive and selinuxtype=targeted, to set permissive mode, which does not automatically block unauthorised access, but just logs it for our demonstration purposes. by setting selinux=enforce, access can be if the device is in real operation mode. the resulting root filesystem including the kernel had a total size of 45.8 mb. for our second test, we used a new hypervisor called phidias (“provable hypervisor with integrated deployment information and allocated structures”). this hypervisor was developed at the chair for security in telecomunications (sect) of the tu berlin and has been designed to exhibit a maximum of runtime staticity, i. e. all dynamic elements usually found in system software are either omitted altogether or carefully altered to behave statically. to obtain a bootable image, the hypervisor implementation requires an accompanying configuration framework centred around a simple, humanreadable xml configuration file, which is integrated into the build process and generates the data structures for a deployment setting. this technique pushes all dynamic decisions (e. g. the placement of memory allocations) from runtime to compile-time and thus enables phidias to be a static, yet very versatile hypervisor. the goal of this design principle is the ease of automated verification against malfunctions and bugs. it was mainly developed in c, with some assembler code for low-level operations. it is kept small (6,134 lines of c + 1,305 lines of assembler) which of course also increases the tractability of automated verification. due to this small size and the inclusion of all data structures for a given configuration, the bootable image can be verified to satisfy certain integrity and noninterference properties through symbolic execution. our test board was the hikey (lemaker version) board which is equipped with an arm cortex-a53 consisting of 8 cores, 1.2 ghz and 2gb of lpddr3 ram. the arm cortexa53 is a 64-bit-multicore-processor with an armv8-a instruction set. with armv8-a, an additional exception level was implemented, the hyp mode for hypervisor support on which the phidias hypervisor runs on. so, pure virtualization is easily possible through hardware support. we compiled phidias with multicore support and again used the open source tool buildroot to generate ram disks for two vms. first, we added a virtual network driver to the linux kernel (version 4.11.0-rc4) for inter-vm communication which uses the communication buffer system and the signalling mechanism that the phidias configuration framework offers. no further modifications to the linux source code were necessary. in the root filesystem of the first linux vm we added many additional packages to the core busybox-based system, like in our first test: the bash shell, a gui (x.org display server, matchbox window manager), mdev as device manager, and of course the necessary infrastructure to activate selinux (linux kernel modules, selinux support options in busybox). afterwards, we used the same example policies (https://rpmfind.net/linux/rpm2html/search.php?query=selin ux-policy-targeted) and added them to our root filesystem. finally, we also activated selinux by adding selinux=1 to the kernel commandline and created a test file /etc/selinux/config with following parameters: selinux=permissive and selinuxtype=targeted. the resulting root filesystem of our first vm including the kernel had a total size of 44.6 mb. the second vm was a minimal linux (without selinux), but still constructed with buildroot with a size of 11.7 mb, which led to a total size of 56.3 mb for both vms. phidias itself had a final total size (compiled hypervisor binary, data structures, pre figure 4. framework with a separation kernel running three vms. acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 19 generated pagetables) of less than 250 kb. in our opinion, the ground framework of around 57 mb is no restriction for measuring instruments as our small board (costs of around 100 $) has already 2gb of ram and an 8gb of nand flash storage. therefore, we think that our framework which combines macs with a separation kernel is not only feasible but should be the next step to enhance security in measuring instruments, which are connected to the internet. 7. conclusions in this paper, secure software constructions for general purpose operating systems, i.e., linux and windows are analysed. concretely, we look at measuring instruments under legal control, which have become powerful devices running under such operating systems. the frameworks presented here, are constructed to fulfil the requirements of legal metrology such as the measuring instruments directive mid 2014/32/eu and the welmec 7.2 software guide. the security methods shown in this paper are selinux, apparmor and mic, which are based on mandatory access control and/or role based access control strategies (mac and rbac). with these methods well-defined rules for roles, classifications for subjects and objects, and access rights are being defined for legally relevant software parts that fulfil measuring tasks. additionally, a component-based architecture is described, in which every component receives only least privilege rights. together with the before mentioned welldefined rules, all requirements of legal metrology directives for software are fulfilled. our framework ensures that non-legal processes, which run on measuring instruments, have no effect on the legally-relevant processes. this is further enhanced using a hypervisor with separation kernel properties. the tests in this paper describe how an installation of such a framework is easily possible. the goal is to make current measuring instruments more resistant against software vulnerabilities and attacks from open networks like the internet. acknowledgement we thank the anonymous reviewers for several helpful suggestions that improved the quality of this article. references [1] idc france (l'institut du comportement), “special study final study report: design of future embedded systems (smart 2009/0063)”, idc analyze the future (international data corporation), final interim study report 2, deliverable d4, april 2012. [2] n. leffler, f. thiel, “im geschäftsverkehr das richtige maß das neue messund eichgesetz”, monatsbericht, 11-2013. [3] b. chelf, “measuring software quality a study of open source software”, chief technology officer, san francisco, ca, usa, 2011. [4] oiml d 31, “general requirements for software controlled measuring instruments”, organisation internationale de métrologie légale, edition 2008 (e). [5] official journal of the european union, “directive 2014/32/eu of the eurpean parliament and of the council”, l 96/149, european commission: brussels, belgium, 2014. [6] welmec (european cooperation in legal metrology), “softwareleitfaden (europäische messgeräterichtlinie 2014/32/eu)”, welmec 7.2, 2015. [7] d. kleidermacher, m. kleidermacher, “embedded systems security practical methods for safe and secure software and systems development”, newnes imprint of elsevier, 2012, pp. 27-31. [8] r. spencer, s. smalley, p. loscocco, m. hibler, d. andersen, j. lepreau, “the flask security architecture: system support for diverse security policies”, secure computing corporation, national security agency, university of utah, usa, august 1999. [9] r. spenneberg, “selinux & apparmor mandatory access control für linux einsetzen und verwalten”, 2008, addisonwesley verlag. [10] f. kuhn, “mandatory integrity control”, ernw newaletter 17, juli 2007. [11] selinux policies, accessed: 28.04.2017, link: https://rpmfind.net/linux/rpm2html/search.php?query=selinuxpolicy-targeted. [12] j. alves-foss, w. s. harrison, p. oman, and c. taylor, "the mils architecture for high-assurance embedded systems", journal of embedded systems, 2:239-247, 2006. [13] j. barr, "the flask security architecture", in computer science 574, 2002. [14] r. w. beckwith, w. m. vaneet, and l. maclaren, "high assurance security/safety for deeply embedded, real-time systems", embedded systems conference, 2004. [15] k. doeornemann, and a. von gernler, "cybergateways for securing critical infrastructures", in proceedings of international etg-congress 2013, symposium 1: security in critical infrastructures today, berlin, germany, 5-6 november 2013. [16] d. peters and f. thiel, "software in measuring instruments: ways of constructing secure systems", sensor 2016, nuremberg; 05/2016. [17] f. thiel, u. grottker, and d. richter, "the challenge for legal metrology of operating systems embedded in measuring instruments", oiml bull. 2011 [18] t. addabbo, a. fort, c. d. giovampaola, m. mugnaini, a. toccafondi and v. vignoli, “on the safety design of radar based railway level crossing surveillance systems”, in acta imeko, vol. 6, no. 4, 2017. https://rpmfind.net/linux/rpm2html/search.php?query=selinux-policy-targeted https://rpmfind.net/linux/rpm2html/search.php?query=selinux-policy-targeted software separation in measuring instruments through security concepts and separation kernels << /ascii85encodepages false /allowtransparency false /autopositionepsfiles true /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /error /compatibilitylevel 1.4 /compressobjects /tags /compresspages true /convertimagestoindexed true 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/hrv (za stvaranje adobe pdf dokumenata najpogodnijih za visokokvalitetni ispis prije tiskanja koristite ove postavke. stvoreni pdf dokumenti mogu se otvoriti acrobat i adobe reader 5.0 i kasnijim verzijama.) /hun 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can be opened with acrobat and adobe reader 5.0 and later.) >> /namespace [ (adobe) (common) (1.0) ] /othernamespaces [ << /asreaderspreads false /cropimagestoframes true /errorcontrol /warnandcontinue /flattenerignorespreadoverrides false /includeguidesgrids false /includenonprinting false /includeslug false /namespace [ (adobe) (indesign) (4.0) ] /omitplacedbitmaps false /omitplacedeps false /omitplacedpdf false /simulateoverprint /legacy >> << /addbleedmarks false /addcolorbars false /addcropmarks false /addpageinfo false /addregmarks false /convertcolors /converttocmyk /destinationprofilename () /destinationprofileselector /documentcmyk /downsample16bitimages true /flattenerpreset << /presetselector /mediumresolution >> /formelements false /generatestructure false /includebookmarks false /includehyperlinks false /includeinteractive false /includelayers false /includeprofiles false /multimediahandling /useobjectsettings /namespace [ (adobe) (creativesuite) (2.0) ] /pdfxoutputintentprofileselector /documentcmyk /preserveediting true /untaggedcmykhandling /leaveuntagged /untaggedrgbhandling /usedocumentprofile /usedocumentbleed false >> ] >> setdistillerparams << /hwresolution [2400 2400] /pagesize [612.000 792.000] >> setpagedevice microsoft word 260-1772-1-le-rev acta imeko  issn: 2221‐870x  september 2015, volume 4, number 3, 65 ‐ 71    acta imeko | www.imeko.org  september 2015 | volume 4 | number 3 | 65  painting authentication by means of a biometric‐like  approach  giuseppe schirripa spagnolo 1 , lorenzo cozzella 1 , maurizio caciotta 2 , roberto colasanti 3 , gianluca  ferrari 3   1  università roma tre – dipartimento di matematica e fisica, via della vasca navale, 84, 00146 roma, italy  2  università roma tre – dipartimento di scienze, via della vasca navale, 84, 00146 roma, italy  3  expert in protecting cultural heritage, roma, italy       section: research paper   keywords: painting authentication; rfid application; random features; smartphone application; biometric‐like approach  citation: g. schirripa spagnolo, l. cozzella, m. caciotta, r. colasanti,  g. ferrari, painting authentication by means of a biometric‐like approach, acta imeko,  vol. 4, no. 3, article 11, september 2015, identifier: imeko‐acta‐04 (2015)‐03‐11  editor: paolo carbone, university of perugia, italy  received february 17, 2015; in final form april 23, 2015; published september 2015  copyright: © 2015 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: this work was supported by miur  corresponding author: g. schirripa spagnolo, e‐mail:  giuseppe.schirripaspagnolo@uniroma3.it    1. introduction  the artwork market is very complex and variegated, in which a single piece can have an incredible high value. in general, the value of an artwork is not always related to its intrinsic quality or characteristics, but on the possibility to demonstrate it was made by a famous artist. therefore, the amount of money that can be paid for paint or artwork is strictly related to the expertise made by a well-known and authorized art expert. the output of an expertise is always a certificate of authenticity (coa). unfortunately, often these certificates are exchanged among similar artworks: the seller, to certificate the originality of more than one single artwork, supplies the same document. in this way, the buyer could have a copy of an original certificate to attest that the “not original artwork” is an original one. unluckily, most people believe that art with a certificate is automatically genuine, but that is not even close to truth [1]. a possible fraud can be put the following way into effect. an art merchant, starting from an original artwork with original certificate of authenticity, can duplicate both and sell false artwork as genuine using false certificate of authenticity as clue of originality. abstract  artwork counterfeiting is a wide problem in the art market, both for private subjects and museums. for this reason, it is important to  introduce  innovative  authentication  solutions,  based  on  state‐of‐the‐art  technologies.  in  particular,  in  this  paper,  the  proposed  solution is based on a mobile architecture, starting from the consideration that nowadays mobile phones include quality photo and  video cameras, access to wireless networks and the internet, gps assistance and other innovative systems. the proposed solution uses  smartphones as simple, robust and efficient sensor for artworks authentication. when we buy an artwork object, the seller issues a  certificate  of  authenticity,  which  contains  specific  details  about  the  artwork  itself.  unscrupulous  sellers  can  duplicate  the  classic  certificates of authenticity, and then use them to “authenticate” non‐genuine works of art. in this way, the buyer will have a copy of  an original certificate to attest that the “not original artwork”  is an original one. a solution for this problem would be to  insert a  system that  links together the certificate and the related specific artwork. to  do this  it  is necessary, for a single artwork, to  find  unique, unrepeatable, and unchangeable characteristics.  in this article we propose an  innovative and non‐invasive method for the  authentication of artworks based on random intrinsic object characteristics. this approach is based on biometry paradigm (analogue  fingerprinting). the paper presents a stand‐alone solution, and an internet‐based one, necessary for granting security verification also  in case of problems with the used rfid tag. the proposed method uses an rfid tag and a 2d barcode, in conjunction with an internet‐ based authentication archive.  acta imeko | www.imeko.org  september 2015 | volume 4 | number 3 | 66  museum objects are generally identified using some sort of cataloguing system. the objects may be (digitally) photographed, and then marked using a sticker, perhaps with a barcode, or a marker. this information can be considered as a fingerprint of the artwork and they may be entered into a paper or modern software catalogue/database along with other descriptive and historic information, condition reports, etc. to authenticate an artwork, starting from the marker affixed on it, it is necessary to consult the museum archive. for private collections or works produced by living artist, we ust use a slightly different approach. the artist, or artwork expert, may photograph the artwork, describe it, and take photos at high resolution of the surface texture, yield a picture of him near the item and so on. in other words, an identity document is created and a unique set of fingerprints has to be identified [2], [3]. this file is archived, with the information and the author digital signature, in an opportune artwork digital archive (ada). the ada software generates a unique artwork identification number and a dedicated url (universal resource locator), where this information is deployed. this process is similar to the digital object identifier (doi) schema [4]. a doi is a character string (a "digital identifier") used for uniquely identifying an object such as an electronic document. metadata about the object is stored in association with the doi name and these metadata may include a location, such as a url, where the object can be found. in this case, the ada sends back to the author (or to the certification authority) the artwork url and the author can put it on the lithography itself (for example on its back) by means of a 2d barcode or rfid tag attached to the artworks. by using an rfid tag with a high memory capacity, all (or part of) the information contained in the identity document can be duplicated in the chip bonded on the artwork. a way to simplify the described solution would be the use of the technology offered by modern smartphones to connect to a proper website which would thus allow checking the origin of the artwork. the website, in this case, will be the ada database, designed to contain information about the artwork and a digital certificate of authenticity. this digital certificate links information on non-cloneable features of the specific artworks. in this way the inappropriate usage will not be possible and the buyer will be able to verify the originality by himself [5]. to do this it is necessary, for a single artwork, to find unique, unrepeatable, and unchangeable characteristics. if these characteristics are present, we have the possibility to identify the artwork and to distinguish it from another one [6]-[9]. by choosing the appropriate characteristic, such kind of identification can be applied to many types of artwork objects. the rest of this paper is structured as follows: section 2 outlines rfid tags and remote database authentication schema. section 3 defines the artworks fingerprints and hylemetric template. section 4 defines the safeart system and obtained result. finally, section 5 concludes the paper. 2. rfid system  radio frequency identification (rfid) is a technology that allows a small radio device attached to an item to carry an identity for that item [10]. the first known use of rfid-like technology dates back to world war ii time (1939), when british royal air force used it for friend or foe aviation identification [11]. radio frequency identification has attracted considerable press attention in recent years, and for good reasons: rfid not only replaces traditional barcode technology, it also provides additional features and removes boundaries that limited the use of previous alternatives. printed bar codes are typically read by an optical system that requires a direct line-of-sight to detect and extract information. with rfid, however, a scanner can read the encoded information even when the tag is concealed for either aesthetic or security reasons—for example, embedded in an artwork (example: sandwiched between painting layers). rfid is a wireless/contactless technology, avoiding remote manipulations, requires special protection service and risk management. a typical rfid system includes at least a tag (transponder), a reader (interrogator with antenna) and a data processing environment, which operates the obtained data. the rfid-enabled mobile phones may function as processing unit. in this case, the reader and the processing unit are integrated to one handheld device (figure 1) [12]. due to many possible uses of rfid, there are a lot of differences in its system components: different types of tags as well as a variety of readers. the function of rfid systems may be described in the following way. reader’s antenna emits radio waves and once a tag (passive) is within the working range, it receives the radio signal. then the tag responds back with its own data message. the reader decodes the received data from the tag and these data are passed to further processing. even the most important and characteristic feature of rfid systems—their unique identifier—is susceptible to attacks. although in theory you cannot ask an rfid manufacturer to create a clone of an rfid tag [13], in practice replicating rfid tags does not require a lot of money or expertise considering the wide availability of writable and reprogrammable tags. an ominous example is the demonstration, by a german researcher, of the vulnerability of german passports [14]. in this paper, the problem of possible cloning of the rfid is not important. authentication is done with the information contained into an rfid and robustness against forgery is granted to the fact that such information is related to specific characteristics of an individual work. as well as the fingerprints contained in an identity document, also hylemetry approach avoids clone problems; fingerprints on the document are compared with those of the individual. similarly, the fingerprints of the work (high resolution photo, surface texture, etc.) contained in the document of identity (rfid) are compared with those extracted from the work. in order to make an authentication system robust, a 2d barcode is linked to the rfid. in the 2d barcode is memorized the url where a copy of the authentication information is stored. to avoid copy attack, duplication/replacement of the fingerprint file, the use of a digital signature is also necessary [15]. a digital signature grants that a document is original. in figure 1. the mobile device as part of the rfid system.  acta imeko | www.imeko.org  september 2015 | volume 4 | number 3 | 67  this paper the template constructed from the artwork highresolution image is original (i.e. constructed by the artwork author or by the certifier company). it links the identity of the underwriter with the file and provides an official stamp (unalterable otherwise the digital signature verification fails), which legally determines the author of the document. these characteristics can be efficiently exploited to combat the counterfeiting. the template (i.e. the fingerprint file) is digitally signed by an asymmetric key algorithm, an encrypting “two keys system”, which exploits devices able to producing two different, but linked keys, one private (internal to the device and irretrievable) and the other public. with the digital signature, we obtain an encrypted and signed copy of the fingerprint file itself; in this way, the data present in the certification media cannot be used for copy attack. obviously, for verifying the artwork originality, it is necessary, using the associated public key kpub, to decrypt the encoded information. the whole verification procedure can be implemented in an opportune application (app) that exploits the elaboration potentiality of smartphones. figure 2 shows the schema of the artwork’s authentication step. as shown in figure 2 the verification phase is based on the extraction of the fingerprint file directly from a precise portion of the artwork image. this leads to possible geometrical distortions. for this reason, the verification app on the smartphone, will be based on the fourier miller transformation, which automatically solves rotations, translations and scales, which are the most common errors introduced during the acquisition phases. we have used this kind of registration because our subjects are flat objects and, for avoiding any other image distortion (e.g., barrel), we have taken only the central part of the image itself (i.e., 1200 × 1200 pixels, starting from an acquired image of 3264 × 2448 pixels, typical dimensions obtained using an iphone 5 or 5s and an appropriate external objective kit). obviously in case of 3d objects to be authenticated, a more sophisticated image registration will be necessary, to still allow an automatic procedure. at this point a possible stand-alone solution is based on using only a new generation smartphone and the information contained in the rfid. new generation smartphones are able to read information contained in passive rfid tags, using the nfc technology (if rfid tag data are conform to iso 15693) or external wireless (i.e. bluetooth) rfid reader. the smartphone can acquire from the rfid the information related to the verification area to be acquired, in low resolution, and the template, digitally signed, to be used in the verification phase. a dedicated application running on the smartphone can now acquire the requested area at high resolution, applying image registration and locally calculating the template on the basis of patterns obtained from the high resolution image (tt). eventually, a threshold-based correlation can be applied to verify if the two templates are as equal as necessary to be considered as related to the same artwork. the presence of a template digitally signed in the rfid (tc*), requests for the verification application to know the public key of that template, for extracting the “readable” version to be compared with the locally calculated one (tc). this step grants the protection against the copy attack, where an rfid tag is bounded onto a false artwork, containing false information: to do this the counterfeiter has also to have the private key used by the author for digitally signing all her/his work. in any case, also for stand-alone solutions, the presence of the 2d barcode containing the ada url with all the necessary verification information is necessary. this because rfid lifetime, also in case of a passive one, cannot be compared with the artwork one, but the system has to grant a correct verification also in case of rfid reading failure. in this last case the verification is made using the data retrieved by the secure url, given by the ada. the ada connection can be also available directly from the verification app. in fact, the rfid can contain also a unique identification code, such as the doi used for bibliographical purposes [16], that allows the smartphone app to access the remotely maintained information on the ada database for the investigated artwork, and obtain not only the data necessary to verify the artwork originality, but also artwork’s and author’s information as well as museum interesting data and so on. in this way the app suits both for verifying artwork authenticity and for acquiring artworks and authors information.   figure  2.  schema  showing  the  authentication  step.  in  the  figure  not  all information stored in the rfid is reported for easy understanding. tt is the  template  extracted  and  geometrically  corrected  during  authentication phase,  tc  is  the  original  template  present  in  the  coa  and  tc *   is  the  encrypted version of tc..   acta imeko | www.imeko.org  september 2015 | volume 4 | number 3 | 68  3. fingerprinting  in human authentication, the sampled characteristic should have the following properties: • universal: every person should have the characteristic; • permanent: the characteristic should not vary over time; • distinctive: samples corresponding to different persons should be as different as possible, that is, the interclass variability should be as large as possible; • robust: samples corresponding to the same person should be as close as possible, that is, the intraclass variability should be as small as possible; • accessible: the sample should be easy to be presented to the sensor; • acceptable: it should be perceived as nonintrusive by the user; • hard to circumvent: it should be hard for an impostor to fool the system. the fingerprints of an individual fully respond to these properties. as no two people in fingerprinting history have been found to have the same fingerprint, it can be said that a fingerprint may be used to uniquely identify a person. similar properties are required in the technique used for authenticating the inanimate objects. in particular, in the artworks authentication the sampled characteristic should have the following properties [17]: • uniqueness: every object should be identifiable and distinguishable from all others; • consistency: the feature vector should be verifiable by multiple parties over the lifetime of the object; • conciseness: the feature vector should be short and easily computable; • robustness: it should be possible to verify the feature vector even if the object has been subjected to harsh treatment; • resistance to forgery: it should be very difficult and costly or impossible for an adversary to forge a document by coercing a second object to express the same feature vector as the original. each texture, that is highly random and difficult/impossible to reproduce, can be potentially used as hylemetric characteristic. obviously, good characteristics for authenticate inanimate objects have to satisfy the following requirements: • it has to be simple repeatable and reliable to implement the feature vector (template); • the cost of creating and signing the feature vector has to be small, relative to a desired level of security; • the cost to create an artwork clone able to generate the same template of the original one, has to be greater than the value of the object under forgery; • the cost of verifying the authenticity of a signed feature vector has to be small, again relative to a desired level of security. the method to authenticate an inanimate object is called hylemetry, from the greek “hyle” that means “non-living matter”, and “metros”, which means measurement. in this paper, the proposed authentication system is based on the identification and correct acquisition of a hylemetric characteristic and the related creation of a hylemetric template to be used inside the “rfid-based” safeart system. if we consider the certification of lithography we have considered as hylemetric unique characteristics the colorful “stains”, acquired by means of a common high-level smartphone. therefore, with the potentiality offered by modern smartphones referring to the processor power and image elaboration, these can be easily transformed into excellent biometric (hylemetric) sensors [18]. the smartphone used in this paper is a common iphone 5 equipped with external objective olloclip® 10× macro lens. figure 3 shows the smartphone system during the acquisition, with the olloclip external lens system added on it. this external lens system is necessary due to the actual limitation of the iphone camera for macro acquisition. using different kinds of smartphones (e.g. nokia lumnia 1200 or higer) this external lens kit is no more necessary. subsequently, the colorful “stains”, acquired in rgb 24 bit format, are transformed to a uniform cielab color space [19]. after that, we use only the l channel, normalized with dynamic 0 to 1. in this way, we are sure that the obtained image is not affected by the environmental illumination. the obtained image has a typical speckle-like structure. this procedure is a one-way function. the hylemetric patterns, extracted from two job’s dog lithography (the 18th and 19th on a total number of 20 copies made by the author), used as examples are shown in figure 4. figure 3. example of smartphone used during acquisition.  figure 4. hylemetric hash pattern of the stone lithography dog 18/20 and  dog 19/20.  acta imeko | www.imeko.org  september 2015 | volume 4 | number 3 | 69  starting from the obtained pattern, the proposed authentication system wants to introduce a new digital certificate of authenticity, uniquely connected with a specific lithography using the pattern itself. the author (or the certification authority) decides which part of the artwork has to be acquired. this is acquired at high definition; in this way it is possible to extract the related hylemetric pattern. this is sent, with the artwork information and the author digital signature, to the ada server. the ada software generates a unique artwork identification number and a dedicated universal resource locator (url), where the digital certificate is deployed. this process is similar to the digital object identifier (doi) schema, as defined before in section 2. a metadata file about the object is stored in association with the doi name and this file may include a location, such as a url, where the object can be found. the doi to be used for retrieving artwork authentication and bibliographical information can be stored in a secure way inside an rfid. in the following section the complete system and the security introduced by rfid approach will be clearly explained. 4. safeart system  rfid has considerable potential in product authentication. to resist against cloning and forgery are the most important security properties of authentication tags. in rfid product authentication techniques many ways are achievable to conduct a cloning attack. these include side channel attack [20], reverse-engineering and cryptanalysis [21], brute-force attack [22], physical attacks [23] and different active attacks against the tag [24]. in addition, shared secrets based product authentication approaches are always vulnerable to data theft, where the secret pin codes or encryption schemes of valid products are stolen or sold out by insiders, which would enable criminals to create phony tags. this scenario is especially interesting for adversaries, because it would allow them to clone a large number of tags. instead of fighting against cloning, it is possible using a different approach. in this approach, the authentication is based on writing on the tag memory a digital signature that combines the identification number and product specific random non cloneable features. in particular, artworks, due to the way in which they are produced, have an intrinsic randomness, due to the hand-made process. for painting, these can be the surface texture or a high resolution photo of a small piece of the painting. if we refer to the oil painting reported in figure 5, just for example, the acquisition and hylemetric pattern (i.e. hylemetric template) creation leads to a speckle-like appearance (the same analysis can be made also for lithography in figure 4 obtaining a similar speckle pattern template). to avoid copy attack, duplication/replacement of the template file, the use of a digital signature is necessary. digital signature grants that a document (in this paper the template constructed from the interested area) is original (i.e. constructed by the artwork author or by the certifier authority) and links the identity of the underwriter with the file and provides an official stamp (unalterable otherwise the digital signature verification fails) which legally determines the author of the document. in other words, the authentication template (speckle-like structure) ct is digital signed by the asymmetric key algorithm [15], an encrypting “two keys system”, which exploits devices able to produce two different, figure 5. (a) oil painting with authentication area; (b) area acquired during  verification phase; (c) authentication template.  acta imeko | www.imeko.org  september 2015 | volume 4 | number 3 | 70  but linked to each other, keys, one private (internal to the device and irretrievable) and the other public. during the verification phase, the verifier can read the rfid, obtain the doi with the related ada url, and retrieve the related public key. at this point, the verifier, from the rfid has also obtained the encrypted version of the hylemetric template, which could be decrypted using the obtained public key so it is possible having *ct ; in this way the data present in the certification media cannot be used for copy attack. during the authentication phase the verifier captures an image similar to the one used to create the authentication template ( )tt . in order to extract a vector to compare with that stored in the rfid, it is necessary to correct any possible distortion and acquisition error before calculating the template related to the test image to be used for verifying the painting authenticity. using an automatic image registration algorithm [25], it is possible to obtain an “adjusted” image usable to extract the verification template. obviously, the captured image has residual geometrical distortion noise, which can lead to obtain a template different in comparison to that present in the authenticity media, also in case of original artwork verification. therefore, considering that the template has a casual structure to allow the comparison between the calculated template and the authentication one retrieved from the rfid, a verification approach based on digital phase correlation calculation is proposed in this paper, similar to the one used in speckle field measurement [26]. in this work, the used fourier-based phase correlation is: * 1 * ( ) ( ) ( , ) ( ) ( ) c t c t f t f t c x y f f t f t              . (1) in (1), δx and δy are the correlation peak coordinates, and are forward and backward fourier transform operators, respectively, and * means the complex conjugate. the coefficient α controls the correlation peak width. optimum values range are from α=0 for images characterized by high spatial frequency content and high noise level, to α=0.5 for low noise images with less fine structure. for values greater than 0.5 the high frequency noise is magnified. in our experiment we have always used α=0.5 values, also in case of noisy test images, obtaining in any case good results. as in fingerprint approach, also in our procedure we introduce a correlation threshold, necessary to define if the two templates are similar enough to be considered as the same. figure 4 shows the previously described process. 5. conclusions  in this paper we presented an innovative system for verifying painting and drawing authenticity (and artwork in general), based on smartphone application, smartphone internal optical sensor (corrected with external macro lens, if necessary) and rfid tags. in addition to the stand-alone solution, also a web-based one is presented, to cope with rfid lifetime problems. the proposed solution can be used by living authors who want to protect their artworks from fraudulent copies marketed by dishonest sellers. furthermore, the system can be used by foundations that deal with the protection of an artist. as well as from museum or large collection owners, for both certifying artworks authenticity and cataloguing them in an easy and secure way. in the further a deep analysis of limitations using nfc (near field communication) reader instead of rfid ones will be presented, for better understating the optimum solution for non-contact artwork verification based on analogue fingerprint. references  [1] j.h. merriman, “counterfeit art”, international journal of cultural property, 1992, 1(1), 27-28; http://dx.doi.org/10.1017/s0940739192000055. [2] g. schirripa spagnolo, l. cozzella, c. simonetti, hylemetry versus biometry: a new method to certificate the lithography authenticity, proc. spie 8084, o3a: optics for arts, architecture, and archaeology, iii, 2011, 80840s; http://dx.doi.org/10.1117/12.889387 [3] l. cozzella, g. schirripa spagnolo, f. leccese, biometric-like approach for verifying artworks authenticity, applied physics research, 2013, 5(6), 118-130; http://dx.doi.org/10.5539/apr.v5n6p118 [4] m. langston, j. tyler, linking to journal articles in an online teaching environment: the persistent link, doi, and openurl, the internet and higher education, 2004, 7(1), 51–58; http://dx.doi.org/10.1016/j.iheduc.2003.11.004 [5] g. schirripa spagnolo, l. cozzella and d. papalillo, smartphone sensors for stone lithography authentication, sensors 2014, 14, 8217-8234; http://dx.doi.org/10.3390/s140508217 [6] t. haist, h.j. tiziani, optical detection of random features for high security applications, opt. comm., 1998, 147, 173−179; http://dx.doi.org/10.1016/s0030-4018(97)00546-4 [7] r. d. melen, record document authentication by microscopic grain structure and methods, us patent us5325167 a, 1999; available online: http://www.google.com/patents/us5325167, last accessed 02/14/2015 [8] j. buchanan, r.p. cowburn, a. jausovec, d. petit, p. seem, g. xiong, d. atkinson, k. fenton, d.a. allwood, m.t. bryan, fingerprinting’ documents and packaging. nature, 2005, 436; http://dx.doi.org/10.1038/436475a [9] r.p. cowburn, laser surface authentication—reading nature’s own security code, contempor. phys. 2008, 49, 331−342; http://dx.doi.org/10.1080/00107510802583948 [10] b. glover and h. bhatt, 2006. rfid essentials. o'reilly media, inc., sebastopol (ca, usa), 2006, isbn 0-596-00944-5 [11] k. finkenzeller, “rfid handbook: fundamentals and applications in contactless smart cards and identification”, second edition, john wiley & sons ltd: hoboken, new jersey, usa, 2003, isbn: 9780470844021 [12] i.k. ibrahim, “handbook of research on mobile multimedia”, information science reference, hershey, new york – usa, 2008, isbn 978-1-60566-046-2 [13] a. laurie, practical attacks against rfid, network security, 2007, 9, 4-1; http://dx.doi.org/10.1016/s1353-4858(07)70080-6 [14] european digital rights, cloning an electronic passport, edrigram, digital civil rights in europe, 2006, no. 4–16. available online: http://history.edri.org/edrigram/number4.16/epassport, last accessed 02/14/2015 [15] fips (federal information processing standards). digital signature standard (dss) 2013, pub 186−4; http://dx.doi.org/10.6028/nist.fips.186-4 [16] m. langston, j. tyler, linking to journal articles in an online teaching environment: the persistent link, doi, and openurl. internet high. educ. 2004, 7, 51–58; http://dx.doi.org 10.1016/j.iheduc.2003.11.004 [17] w. clarkson, t. weyrich, a. finkelstein, n. heninger, j.a. halderman, e.w. felten “fingerprinting blank paper using commodity scanners”, sp 2009 proceedings of 30th ieee symposium on security and privacy, 301-314. http://dx.doi.org/10.1109/sp.2009.7 [18] c. stein, c. nickel, c. busch, fingerphoto recognition with acta imeko | www.imeko.org  september 2015 | volume 4 | number 3 | 71  smartphone cameras. in proceedings of the international conference of the biometrics special interest group (biosig), darmstadt, germany, 6-7 september 2012 , pp. 1−12. available online: http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=63 13540, last accessed 02/14/2015. [19] c. connolly, t. fliess, a study of efficiency and accuracy in the transformation from rgb to cielab color space, ieee trans. image process. 1997, 6, 1046−1048, http://dx.doi.org/10.1109/83.597279 [20] m. c. o'connor, epc tags subject to phone attacks, rfid journal february 24, 2006. available online: http://www.rfidjournal.com/articles/view?2167, last accessed 02/14/2015. [21] s. bono, m. green, a. stubblefield, a. juels, a. rubin and m. szydlo, security analysis of a cryptographically enabled rfid device, 14th usenix security symposium, baltimore, md, 2005, available online at the url: https://www.usenix.org/legacy/events/sec05/tech/bono/bono. pdf, last accessed 02/14/2015 [22] a. juels, rfid security and privacy a research survey, ieee journal on selected areas in communications, 2006, 24(2), 381 – 394; http://dx.doi.org/10.1109/jsac.2005.861395 [23] s. weingart, “physical security devices for computer subsystems: a survey of attacks and defense” , in cryptographic hardware and embedded systems — ches 2000 lecture notes in computer science volume 1965, 2000, pp 302-317 proceedings of ches'00, volume 1965 of lecture notes in computer science, pp. 302—317; http://dx.doi.org/10.1007/3-540-44499-8_24 [24] h. gilbert, m. robshaw, and h. sibert, (2005). an active attack against hb+ – a provably secure lightweight authentication protocol. electronics letters, 2005, 41( 21), 1169 – 1170; http://dx.doi.org/10.1049/el:20052622 [25] b. zitová, j. flusser, image registration methods: a survey. image and vision computing 2003, 21(11), 977–1000; http://dx.doi.org/10.1016/s0262-8856(03)00137-9 [26] m. sjödahl, “digital speckle photography trends” in optical non-destructive testing and inspection, p. k. rastogi and d. inaudi (editors). elsevier publishing, amsterdam, 2000, isbn 9780080430201, pp. 179-195. template for an acta imeko event paper acta imeko issn: 2221-870x march 2018, volume 7, number 1, 3-4 acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 3 editorial to selected papers from the 15th imeko tc10 workshop on technical diagnostics – “technical diagnostics in cyber-physical era” lorenzo ciani, marcantonio catelani department of information engineering, university of florence, italy section: editorial citation: lorenzo ciani, marcantonio catelani, editorial to selected papers from the 15th imeko tc10 workshop on technical diagnostics – “technical diagnostics in cyber-physical era”, acta imeko, vol. 7, no. 1, article 2, march 2018, identifier: imeko-acta-07 (2018)-01-2 editor: dušan agrež, university of ljubljana, slovenia received march 23, 2018; in final form march 24, 2018; published march 2018 copyright: © 2018 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: lorenzo ciani, email: lorenzo.ciani@unifi.it dear reader, a section of this issue is dedicated to eight papers that were originally presented at 15th imeko tc10 workshop on technical diagnostics in budapest (hungary) and that are here presented in their extended versions. technical diagnostics is taking in the years an increasingly important role. behind this, the fact that in high-tech industry and in different fields of application it is mandatory to fulfil the requirements related to diagnostics, reliability, maintainability and logistic support as well as risk and safety assessment. the capability to monitor and diagnose a component, a system, an equipment or an industrial plant – in general term, an item – with the aim to verify its functions represents the starting point for more complex rams (reliability, availability, maintainability and safety) evaluations and assessment. the 15th imeko tc10 workshop was held at the danubius hotel gellért in budapest, hungary, on june 6-7, 2017. this workshop aims to create a forum for advancing knowledge and exchange ideas on methods, principles, instruments and tools, standards and industrial applications on technical diagnostics as well as their diffusion across the scientific community. participants have an excellent opportunity to meet top specialists from industry and academia all over the world and to enhance their international cooperation. the program will feature industry leading keynote speakers and selected presentations. the first paper by yi huang and clemens gühmann proposes a method to estimate the temperatures of the stator winding, the rotor cage and the stator core of an asynchronous machine using kalman filter. a 4th-order kalman filter was implemented in matlab and both the simulation experiment and the test bench experiment are performed well under s1 and s6 condition. the second paper by daniel peters, patrick scholz and florian thiel analyses secure software constructions for general purpose operating systems, i.e., linux and windows. the authors look at measuring instruments under legal control, which have become powerful devices running under such operating systems. the frameworks presented here, are constructed to fulfil the requirements of legal metrology such as the measuring instruments directive mid 2014/32/eu and the welmec 7.2 software guide. the main goal is to make current measuring instruments more resistant against software vulnerabilities and attacks from open networks like the internet. the third paper by gábor kohlrusz, krisztián enisz, bence csomós and dénes fodor investigates and discusses the input and output capacitors of a digitally controlled buck converter have been tested in 4 different cases with unused and worn out capacitors. a complex simulation environment has been implemented where digital and analogue domains can be connected. the presented diagnostic approach aims to analyse signals that are used by the control loop in order to avoid expensive additions to existing circuits. the fourth paper in this issue is authored by balázs scherer introduces a novel approach, and the first part of a toolset for non-intrusive diagnostics of cyber-physical systems. the main goal is to create a toolset, which supports multiple nonintrusive diagnostic interface options for testing cyber-physical systems. the sample implementation is currently a basic version enabling only few functionalities of the xcp protocol, but it is acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 4 suitable to evaluate the concept. first measurements shown that the concept is good, and this method enables the diagnostic of low cost, resource frugal cyber-physical systems with a relatively good data bandwidth. the fifth contribution by dario pasadas, artur lopes ribeiro and helena geirinhas ramos reports a study concerning the penetration of eddy currents in metallic nonferromagnetic materials in the presence of linear sub-surface cracks. simulations were performed for a set of different lengths and depths of the crack using a sinusoidal excitation current with uniform magnetic field distribution. the sixth paper by piotr bilski and bartosz polok investigates and discusses the optimization of various rbf network-based classifier in the diagnostics of analogue circuits. multiple parameters, including the width of the activation function, number of neurons in the hidden layer and coding scheme in the output layer were verified. the testability of the circuit was also analysed. the seventh paper is the result of an international collaboration among researchers from tecnalia research and innovation, industry and transport division, miñano (araba), spain, the division of operation and maintenance engineering, luleå university of technology, sweden and the department of information engineering, university of florence, italy. this paper reviews the challenges of maintaining the linear assets, focusing on inspections. it also provides a conceptual framework for the use of autonomous inspection and maintenance practices for linear assets to reduce maintenance costs, human involvement, etc., whilst improving the availability of linear assets by effective use of autonomous robots and data from different sources. finally, the eight paper in this issue is authored by timotei erdei, zsolt molnár, nwachukwu c. obinna and géza husi. this paper proposes a design of an augmented reality based navigation system, as well as investigate its potential areas of application within the industry. with the ascent of industry 4.0 (iot), systems such as augmented reality and virtual reality benefit from the availability of bulk sensory data. it was a great honour for us to act as guest editors for this issue of acta imeko, both from the perspective of working for a high-profile scientific journal and also, of promoting tc10’s great tradition workshop, now reaching the 15th edition. let us also take this occasion to remind everyone of the next xxii world congress of the international measurement confederation (imeko) that will be held in belfast, uk, on september 3rd6th, 2018 and we are pleased to invite you to attend the conference. lorenzo ciani guest editor marcantonio catelani imeko tc10, chair editorial to selected papers from the 15th imeko tc10 workshop on technical diagnostics – “technical diagnostics in cyber-physical era” << /ascii85encodepages 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/hrv (za stvaranje adobe pdf dokumenata najpogodnijih za visokokvalitetni ispis prije tiskanja koristite ove postavke. stvoreni pdf dokumenti mogu se otvoriti acrobat i adobe reader 5.0 i kasnijim verzijama.) /hun 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can be opened with acrobat and adobe reader 5.0 and later.) >> /namespace [ (adobe) (common) (1.0) ] /othernamespaces [ << /asreaderspreads false /cropimagestoframes true /errorcontrol /warnandcontinue /flattenerignorespreadoverrides false /includeguidesgrids false /includenonprinting false /includeslug false /namespace [ (adobe) (indesign) (4.0) ] /omitplacedbitmaps false /omitplacedeps false /omitplacedpdf false /simulateoverprint /legacy >> << /addbleedmarks false /addcolorbars false /addcropmarks false /addpageinfo false /addregmarks false /convertcolors /converttocmyk /destinationprofilename () /destinationprofileselector /documentcmyk /downsample16bitimages true /flattenerpreset << /presetselector /mediumresolution >> /formelements false /generatestructure false /includebookmarks false /includehyperlinks false /includeinteractive false /includelayers false /includeprofiles false /multimediahandling /useobjectsettings /namespace [ (adobe) (creativesuite) (2.0) ] /pdfxoutputintentprofileselector /documentcmyk /preserveediting true /untaggedcmykhandling /leaveuntagged /untaggedrgbhandling /usedocumentprofile /usedocumentbleed false >> ] >> setdistillerparams << /hwresolution [2400 2400] /pagesize [612.000 792.000] >> setpagedevice a new method for accurate platelet thrombi volume measurement using a confocal microscope acta imeko issn: 2221-870x march 2019, volume 8, number 1, 48 55 acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 48 a new method for accurate platelet thrombi volume measurement using a confocal microscope denise de zanet1,2, monica battiston2, elisabetta lombardi2, ruben specogna1, francesco trevisan1, luigi de marco2, antonio affanni1, mario mazzucato2 1 polytechnic department of engineering and architecture, university of udine, italy 2 department of translational research, stem cells unit, national cancer institute cro irccs, aviano, italy section: research paper keywords: image processing; volume measurement; calibration; blood coagulation; confocal microscopy citation: denise de zanet, monica battiston, elisabetta lombardi, ruben specogna, francesco trevisan, luigi de marco, antonio affanni, mario mazzucato, a new method for accurate platelet thrombi volume measurement using a confocal microscope, acta imeko, vol. 8, no. 1, article 8, march 2019, identifier: imeko-acta-08 (2019)-01-08 section editor: rosario morello, university mediterranea of reggio calabria, italy received january 24, 2018; in final form january 19, 2019; published march 2019 copyright: © 2019 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was supported by grant from the italian ministry of health rf-2010-2316198 and rf-2010-317993 (l.d.m., m.m.). corresponding author: antonio affanni, e-mail: antonio.affanni@uniud.it, denise de zanet, e-mail: dezanet@cro.it 1. introduction live-cell microscopy has become essential in the analysis and reconstruction of the intraand inter-cellular dynamic processes. the numerical approach to these events represents the basis of the emerging studies about medical science. advances in technologies and computational methods have moved cell biology gradually toward a quantitative view [1]. numerical computation offers to medicine the possible benefit of quantifying and assessing a broad variety of diseases and medical conditions by means of innovative automated methods of image analysis [2]-[5]. in an attempt to measure cellular functions and morphological features, samples can be studied by using various microscopy techniques [6]. if the specific aim is to measure the volumetric structure of an object, a confocal microscope provides, as output, a sequence of axial fluorescent images that should facilitate three-dimensional volume reconstruction and measurement, rejecting the out-of-focus fluorescent light [7]. however, digital fluorescent images show misrepresented objects because they are modified and transformed by the acquisition system. therefore, the advanced technology of confocal microscopes hides its limitations, which are often difficult to clearly identify and adjust as well as move biologists far from accurate and realistic measurements [7], [8]. blood coagulation is a complex mechanism whose alteration can produce diseases such as thrombosis, embolisms, and haemorrhages. in particular, thrombosis is one of the major causes of death and implicates a series of morphological and functional changes in different temporal and spatial dimensional scales. platelets have a central haemostatic role due to their ability to rapidly adhere to a damaged substrate and to activate and aggregate forming thrombi. improvements in abstract the accuracy of quantitative measurements represents an essential pre-requisite to the characterization and definition of the complex dynamic phenomena occurring in the field of cell biology. in research projects that involve the induction of blood coagulation under flow in microfluidic artificial channels, thrombus volume is an important quantity for estimation as a significant index related to the individual thrombotic risk profile. concerning its importance in the early diagnosis of cardiovascular diseases, the estimated thrombus volume should reflect and represent reality. in 3d confocal microscopy, systematic errors can arise from distortions of the a xial distance, whose accurate calibration remains a challenge. as a result, the 3d reconstructions show a noticeable axial elongation, and the volume measurements are thus overestimated. in this paper, a 400-600 % volume overestimation is demonstrated, and a new easy to use and automatic calibration procedure is outlined for this specific microfluidic and optical context. the adaptive algorithm proposed leads to the automatic compensation of the elongation error and to the accurate thrombus volume measurement. the method has been calibrated using fluorescent beads of known volume, validated with groups of several distinct platelets and finally applied on platelet thrombi. mailto:antonio.affanni@uniud.it mailto:dezanet@cro.it acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 49 microscope technologies have enabled the investigation of platelet functionality and thrombus formation starting from fluorescent confocal images. however, errors are introduced by the optical system due to a lack of convergence between marginal and paraxial rays and by the mismatch of refractive indexes between the immersion medium and the biological object at the interface. consequently, spherical aberration phenomena lead to a misrepresentation of the objects, and their effects can be shown as an axial elongation in the threedimensional reconstructions [7], [9]-[12]. in specific scientific literature (e.g. in [13]-[19]) starting from sequences of confocal images acquired along the axial zdimension, thrombus volume has often been measured without due attention given to the presence of an intrinsic inaccuracy and distortions that have not been compensated for. this inaccuracy has never represented a problem because thrombus volume quantification has always aimed for a comparison between an untreated situation (control) and variations from it due to the effects of biological or pharmaceutical treatments. however, at the same time, medical research has gradually moved its attention to new non-invasive and real-time devices with the aim of extracting and monitoring the individual thrombotic risk profile and the whole dynamic of thrombus formation events [20]-[31]. some of these new technologies use confocal microscope results as a reference to calibrate their system numerical outputs; however, in this specific context, standardisation and reproducibility still remain open and unresolved issues [32], [33]. herein, a 400-600 % volume overestimation using available post-processing software is demonstrated. hence, in order to improve measurement accuracy, a new easy to use and fast calibration method is proposed. an adaptive automatic algorithm is introduced. it allows standardised and accurate volume quantification and a realistic morphological 3d reconstruction starting from the fluorescence intensity signals of the images. the algorithm is calibrated using fluorescent beads of known volume, and it is then applied to platelet aggregates. accuracy is evaluated, and the related improvement is quantified. the paper is organised as follows: in section 2, we describe how the experiments and the analysis have been performed. section 3 deals with the description of the new proposed algorithm. in section 4, the results and the major outcomes are given. finally, in section 5, conclusions are drawn and defined. 2. materials and methods 2.1. beads preparation volume calibration was performed using yellow-green carboxylate-modified microspheres (2 µm diameter; molecular probes, eugene, or). the product was supplied as an aqueous suspension containing 2 % solids and sodium azide (2 mm final concentration). beads were ultraclean polystyrene microspheres coated with a hydrophilic polymer containing multiple carboxylic acids for the covalent attachment of ligands. each sphere contained fluorescent dye molecules in its whole volume and showed little or no photobleaching, even when excited with the powerful illumination required for fluorescence microscopy. following the protocol suggested by molecular probes, the stock solution (with about 4.6 × 109 beads/ml) was diluted. considering the planar experimental spreading of the final fluid sample (1 cm2, about 3500 fields of view), in order to obtain about 20 beads in each field of view, a total amount of 7 × 103 beads/µl was achieved in the suspended solution. thereafter, a droplet of prolong diamond antifade mountant (molecular probes) was added onto a glass slide (menzel-gläser, braunschweig, d), while a coverslip (menzel-gläser) with 1 µl of diluted solution was placed down on it. the sample was kept in the dark, at room temperature, for 24 hours. 2.2. blood sample preparation and perfusion chamber for each experiment, 20 ml of venous blood from medication-free healthy volunteers were collected into dphenylalanyl-lprolyl-l-arginine chloromethyl ketone dihydrochloride (ppack, 50 µm final concentration; calbiochem, la jolla, ca). informed written consent was obtained from the healthy blood donors according to the declaration of helsinki and the dms of the italian ministry of health 2 november 2015 (concerning the quality and safety of blood and blood donors). the ethics committee cro-irccs aviano also approved the study. after obtaining the blood samples, 200 µl of acid-insoluble fibrillar type i collagen (sigma aldrich, st. louis, mo) was homogeneously distributed onto a coverslip (menzel-gläser) for 60 minutes. the coverslip was thus positioned in a flow chamber, forming the bottom wall of an artificial microchannel with a rectangular section (height 0.125 mm, width 15 mm). the fluorescent dye quinacrine dihydrochloride (mepacrine, 10 µm final concentration; sigma aldrich) was added to whole blood to label platelets. a syringe pump (harvard apparatus, boston, ma) aspirated blood from the chamber outlet for a time interval of three minutes at a flow rate q = 3.5 ml/min in order to achieve the shear rate γ of γ = 6q wh 2 = 1500 s -1 , (1) where w and h are the width and the height of the channel, respectively. 2.3. images acquisition the perfusion flow chamber was positioned on the stage of a confocal laser scanning microscope (clsm, eclipse te300; nikon, tokyo, j) based on nipkow disk technology (ultra view; perkin elmer). a 40x oil-immersion objective (na 1.30; nikon) was mounted on a piezoelectric driver controlled by the andor iq acquisition software (andortm technology, belfast, uk). images were obtained and digitalised in real time with a ccd camera (ixonem+, andortm technology) with a 512 × 512 pixel sensor. the field of view area was 28547.48 µm2. confocal optical images were detected using an excitation laser wavelength of 488 nm. the laser power was set in order to achieve an adequate emission intensity and to avoid the pixel intensity saturation of the ccd sensor. images were acquired along the z-dimension (figure 1(a)) with z-step δz = 0.2 µm, starting from the bottom, with dark slices as the baseline (z = 1) and ending with dark slices above the objects (z = n). each sequence of the confocal images acquired along the z-dimension is called stack (figure 1(a)). 2.4. volume measurement in order to evaluate the systematic errors in the state-of-theart measurements, the confocal digital images were elaborated using imagej software (http://imagej.nih.gov/ij), subjectively setting an intensity threshold to select the areas covered by the objects in all the images of a stack acquired along the zdimension. the volume was then measured as the sum of the detected areas in the images, multiplied by δz [13]-[15], [17][19]. adjusting the intensities histogram, volumes were also acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 50 quantified using volocity software (perkin elmer, waltham, ma) in a similar way. the proposed new algorithm was firstly applied to the images of the known volume beads in order to achieve an accurate evaluation of its performance and to find an optimum value for the calibration factor. consequently, it was applied to small aggregates to verify its robustness with non-spherical samples and finally to platelet large aggregates. along the text, all the results are expressed as mean ± standard deviation, each computed with n = 10 sample stacks. in order to identify significant differences among the results, the student t-test was used. 3. algorithm description 3.1. image elaboration and signal extraction for each stack (figure 1(a)), a preliminary control was performed to exclude sequences of images with saturated pixels. for this purpose, the maximum intensity projection (mip) of all the values (figure 1(b)) was extracted and visualised. thereafter, for each stack, a three-dimensional matrix was defined. the xand y-positions of each pixel were assigned to the first and the second dimensions of the matrix, respectively, while the intensity values of each related pixel assumed along the z-axis were allocated in the third dimension of the matrix, starting from the fluorescence value computed in the first slice (z = 1) and continuing until the value of the last slice (z = n). a graphical representation of the matrix is shown in figure 1(c). according to the ccd sensor physical dimensions (a square matrix having d × d = 512 × 512 pixels) and to the number n of slices acquired for a stack, the matrix had the dimensions d × d × n. since the intensities along the z-dimension are affected by noise, they have been locally smoothed with a finite impulse response (fir) low-pass filter to compensate for the high frequency noise. according to the necessity of regularising the z-intensities without losing information, the smoothing filter was designed with 13 coefficients and applied to each intensity z-signal, performing a weighted mobile averaging. centring the kernel of coefficients in every z-position, the associated smoothed value sz was obtained from the neighbouring original data vz+i as s z = ∑ v z+i c i 6 i = -6 ∑ c i 6 i = -6 , z = 1, 2, … n, (2) where ci refers to the coefficients of the filter (figure 2). hence, in order to accomplish the limits that occur in the regularisation of the first and last samples of a generic signal with a kernel of coefficients [34] and to consider the variability of the background fluorescence in all three dimensions, two values of background, b1 and b2 (shown in figure 2), were quantified as the average of the first six and the last six intensity values of each z-signal. for every stack, a binary mask was defined, whose dimensions were the xand y-positions of each pixel and were consequently connected to the first and the second dimensions of the related three-dimensional matrix defined before. a value of 1 was assigned to every pixel whose maximum intensity m in the z-dimension reached a specific threshold over its background b1 (figure 3(a)). the threshold, automatically obtained, was considered adaptive because the intrinsic figure 1. (a) example of a stack of n confocal images acquired along the zdimension. (b) bi-dimensional maximum intensity projection (mip) showing the maximum value that each pixel assumes along the z-dimension. (c) graphical representation of the three-dimensional matrix defined for a stack: the xand the y-position of each pixel are assigned to the first and second dimensions, respectively, while the n intensity values of each related pixel assumed along the z-axis are located in the third dimension. figure 2. intensity smoothing by means of the low-pass fir filter that allows us to obtain the filtered signal sz. two values of background, b1 and b2, are quantified for every z-signal as averages of the first and the last six intensity values of the signal, respectively. acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 51 variability of the behaviour of biological samples in light emission does not allow us to set a fixed intensity threshold. considering the maximum intensity peak mxm,ym among all the intensity peaks in the three-dimensional matrix (figure 3(a) and figure 3(b)) as a reference value, each pixel value in the binary mask was set to 1 if the related intensity peak m over its background b1 reached at least 1/3 of the reference value mxm,ym-b1xm,ym (figure 3(a), figure 3(b), and figure 3(c)). structuring the binary mask as described, it was possible to identify only the xand the y-positions related to a real presence of volume, excluding those affected by the presence of background, noise, and artefacts. the expression used to define the binary mask is mask j,k = { 1 𝑖𝑓 m j,k b1 j,k m xm,y b1 xm,ym > 1 3 , j, k = 1, 2, … d, 0 𝑒𝑙𝑠𝑒𝑤ℎ𝑒𝑟𝑒 (3) where j and k are the indexes identifying the generic xand ypositions. 3.2. calibration of the z-elongation after having applied the binary mask to the threedimensional matrix, a bi-dimensional matrix was initialised to attribute to every pixel the height of the volume portion located there. referring to figure 3(d), the positions z1 and z2 defined the distance d = z2 z1, which had to be calibrated according to their related signals of intensity. for each intensity signal, z1 and z2 were measured starting from the first level of background b1, advancing forward along the signal, and from the second value of background b2, moving backward. considering the intensity gaps d1 = m-b1 and d2 = m-b2, the two points along the z-axis were chosen as the z-positions whose intensity reached an elevation of d1/10 and d2/10, respectively. the height of the volume portion for each xand y-position was first quantified as the product of the z-distance d with the ratio between the intensity peak and the maximum among all the intensity peaks, related to their background values. afterwards, the quantity thus obtained was multiplied by an experimental optimal factor ĉ estimated as described in section 4. the heights hj,k associated with each xand y-position were evaluated as h j,k = ĉ d j,k m j,k b1 j,k m xm,ym b1 xm,ym , j, k = 1, 2, … d, (4) where h and d are expressed in µm and ĉ is the optimal calibration factor, obtained by means of an iterative computation applied on beads stacks in order to satisfy, at the same time, a z-dimensional condition and a volumetric condition, as specified in the following subsection. the described calibration model was applied to small platelets aggregates and finally to more complex aggregates. according to the physical dimension of pixel r = 0.33 µm, the expression for the final total platelet thrombi volume v of a stack was quantified as v = r2 ( ∑ ∑ dk = 1 d j = 1 h j,k ), (5) where the result is expressed in µm3. figure 3. steps of calibration. (a) graphical representation of the three-dimensional matrix defined for a sequence of images after applying the binary mask. (b) calculation of the adaptive threshold. the threshold is set considering 1/3 of the elevation of the maximum peak mxm,ym over its background b1xm,ym. (c) example of a selected pixel (top) whose z-signal shows a peak of intensity m that raises over the threshold. example of a pixel excluded (bottom) because its related z-signal does not go beyond the threshold. (d) definition of the axial distance d along z-dimension, for a generic z-signal related to a generic xand yposition. the two z-points (z1 and z2) are identified as the points whose intensity reaches the 10 % of the related peak m, with respect to the backgrounds b1 and b2. acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 52 4. results 4.1. optimal calibration factor estimation the optimal calibration factor ĉ was estimated from n = 10 confocal images sequences representing a single bead of known volume 4.19 µm3. in particular, ĉ was obtained with an iterative computation, which minimised the error function defined as the absolute difference between the real bead volume and the estimated one. the iterations stopped when the maximum height of each bead was at least equivalent to its real diameter (z-dimensional condition) and the correspondent volume was at least equal to its real volume (volumetric condition). the optimum factor resulted in ĉ = 0.10 ± 0.01. 4.2. calibration with fluorescent beads in order to validate the model on a different dataset from that which was outlined in the previous subsection, calibration with the optimal factor ĉ was applied on a further set of n = 10 bead stacks that had not been previously used in order to verify the robustness of the method. figure 4(a) and figure 4(b) report the numerical estimation and a graphical representation of a single bead volume performed following the procedures presented in section 2. the results were compared with those obtained using the imagej and volocity software packages. as shown in figure 4(b), the bead volumes were quantified applying the model calibration settings 5.9 ± 0.6 µm3, while the volumes calculated using imagej and volocity were 25 ± 1 µm3 and 18.3 ± 0.8 µm3, respectively. all three methods appear significantly different, as confirmed by the t-test: the p value, for all the comparisons, is less than 0.001. each single volume was evaluated assuming the real known volume (4.19 µm3) of a single bead as a reference. defining the volume overestimation as the ratio between the measured value and the real one, it has been quantified as 6.1 ± 0.3 with imagej, 4.3 ± 0.2 with volocity, and only 1.4 ± 0.1 with the new proposed algorithm (figure 4(c)). a perfect reconstruction would yield to the optimal unitary ratio of overestimation (the dashed line in figure 4(c)). the proposed method permits the significant reduction of the overestimation error and to obtain realistic volume measurements and representation. the data continues to be significantly different, because each ratio is referred to the same real absolute volume: the p value for all the population comparisons remains less than 0.001. 4.3. platelet aggregates the algorithm was then applied to n = 10 confocal stacks related to small aggregates of platelets (small groups of platelets), i.e. from 3 to 10, that adhere to the substrate as a mono-layer and whose number can be clearly observed and determined. for each small aggregate, it was thus possible to estimate a realistic absolute volume, multiplying the number of platelets observed by the representative volume of a platelet, considering its semi-spherical discoidal shape and its average diameter of 2 µm [35]. in the case of small aggregates, the volume overestimation is 5.5 ± 0.8 with imagej, 4.3 ± 0.8 with volocity, and 1.3 ± 0.2 with the proposed method, as shown in figure 5(a). this confirms the robustness of the algorithm. the new value thus obtained is significantly different to those estimated using the traditional measurement methodology, with a related p value that is less than 0.001. figure 6(a) visibly highlights the abovementioned differences measuring and reconstructing the volume of a representative small aggregate of five platelets. the last step in the proposed method was represented by its application to n = 10 stacks of images of multi-layer aggregates, composed of a considerable number of platelets. due to the clear impossibility of accurately pre-estimating a real reference volume, the measurements were expressed as the overestimation achieved using benchmark methods with reference to the measurement performed using the new algorithm. this expression of the results as an overestimation among the methods has also been done for the measurements presented in the previous subsections related to beads and small aggregates (figure 5(b) and figure 5(c)). according to this approach, the accuracy and the repeatability of the proposed calibration algorithm were confirmed by the maintenance of comparable overestimation ratios. the overestimation measured using imagej is, in fact, 4.2 ± 0.2 for beads, 4.1 ± 0.5 for small platelet aggregates, and 3.9 ± 0.6 for large platelet aggregates. this means that the figure 4. calibration and measurement results. (a) 3d rendering of a representative bead (top, on the left). 3d reconstruction of the same bead performed with benchmark methods (on the right) and obtained using the new method (bottom, on the left). (b) the beads’ absolute volume quantification performed using imagej (25 ± 1 µm3), volocity (18.3 ± 0.8 µm3), and the new proposed method (5.9 ± 0.6 µm3). measurement techniques appear significantly different among each other, as confirmed by the t-test: p value < 0.001 (***). the dashed line indicates the real volume of each single bead (4.19 µm3). (c) bead volume overestimation, with reference to the real volume, obtained using imagej (6.1 ± 0.3), volocity (4.3 ± 0.2), and the new proposed method (1.4 ± 0.1). all the results are expressed as mean ± standard deviation. the p value remains < 0.001 (***). the dashed line indicates the unitary optimum ratio, which represents perfect matching. acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 53 metrological compatibility between the measurements of overestimation is respected (figure 5(b)). similarly, considering the ratios refer to the volocity overestimation of each volume, the results obtained are 3.1 ± 0.2 for beads, 3.1 ± 0.6 for small aggregates, and 2.9 ± 0.5 for large aggregates (figure 5(c)). the overestimation of the benchmark methods is thus proven by compatible intervals. figure 6(b) compares two representative 3d reconstructions of large platelet aggregates, obtained with the commercial method and with the proposed calibration algorithm. 4.4. uncertainty evaluation the uncertainty on the volume estimation has been evaluated by means of the propagation of uncertainty outlined in equation (3), equation (4), and equation (5). in particular, the main contributions of uncertainty are related to three uncertain quantities: the identification of the intensity threshold for the binary mask, as expressed in equation (3); the identification of intensity thresholds d1 and d2 to obtain the distance d in equation (4); and the uncertainty on ĉ. regarding the threshold for the binary mask, the uncertainty in extracting the pixels belonging to the base area depends on the quantisation of the intensities mj,k and b1j,k in (3); since the intensities are quantised in 255 levels, the quantisation uncertainty assumes a uniform distribution in [-1, 1], 𝑢(𝑀𝑗,𝑘 ) = 𝑢(𝑏1𝑗,𝑘 ) = 1/√3. (6) thus, the uncertainty in the volume estimation due to the uncertainty in the binary mask can be expressed as 𝑢(𝑉)|𝑚𝑎𝑠𝑘 = √(∑ 𝜕𝑉 𝜕𝑀𝑗,𝑘 𝑢(𝑀𝑗,𝑘 )𝑗,𝑘 ) 2 + (∑ 𝜕𝑉 𝜕𝑏1𝑗,𝑘 𝑢(𝑏1𝑗,𝑘 )𝑗,𝑘 ) 2 . (7) in order to evaluate the two sensitivities in equation (7), we varied the values of mj,k and b1j,k around the operating points, and we evaluated the change in the readout of the volume estimated. the two sensitivities result in 0.02 m3/step for beads, 0.05 m3/step for small aggregates, and 0.95 m3/step for large aggregates. in the same way, for the uncertainty in the height estimation, we calculated the uncertainty of the volume related to intensities d1 and d2 in (4). since d1 and d2 are, again, intensities quantised in 255 levels, their uncertainty is the same as in equation (6), and the contribution to uncertainty on volume due to height estimation is thus 𝑢(𝑉)|ℎ𝑒𝑖𝑔ℎ𝑡 = √(∑ 𝜕𝑉 𝜕𝑑1𝑗,𝑘 𝑢(𝑑1𝑗,𝑘 )𝑗,𝑘 ) 2 + (∑ 𝜕𝑉 𝜕𝑑2𝑗,𝑘 𝑢(𝑑2𝑗,𝑘 )𝑗,𝑘 ) 2 . (8) the sensitivities result in 0.03 m3/step for beads, 0.14 m3/step for small aggregates, and 1.73 m3/step for large aggregates. referring to the uncertainty in the volume estimation due to ĉ extraction, we note that this calibration coefficient is a multiplying factor in equation (4), so the uncertainty in volume due to ĉ is 𝑢(𝑉)|�̂� = 𝑢(�̂�) �̂� 𝑉, (9) where u(ĉ) = 0.01 has been evaluated as the standard deviation of ĉ, as reported in section 4.1. the overall uncertainty in the volume finally results in 𝑢(𝑉) = √(𝑢(𝑉)|𝑚𝑎𝑠𝑘 ) 2 + (𝑢(𝑉)|ℎ𝑒𝑖𝑔ℎ𝑡 ) 2 + (𝑢(𝑉)|�̂� ) 2 (10) the last term in a previous equation prevails since the relative uncertainty on ĉ is 10 %; thus, the overall uncertainty is in the order of 10 %, as shown in table 1. table 1: estimated volumes for different objects and their uncertainties. volume (µm3) uncertainty (µm3) beads 5.9 0.6 small aggregate #1 13.3 1.3 small aggregate #2 13.8 1.4 small aggregate #3 11.7 1.1 small aggregate #4 23 2.3 small aggregate #5 28 2.8 small aggregate #6 16.7 1.7 large aggregate #1 122 12 large aggregate #2 355 35 large aggregate #3 70 7 figure 5. (a) platelet small aggregates volume overestimation (n = 10 aggregates stacks), with reference to their real volume, computed with imagej (5.5 ± 0.8), volocity (4.3 ± 0.8), and the new proposed method (1.3 ± 0.2). the last result is significantly different to those estimated using the benchmark methods, with a p value < 0.001 (***). the dashed line indicates the unitary optimum ratio, which represents perfect matching. (b) volume overestimation with imagej with respect to the volume quantified with the new method (n = 10 stacks): 4.2 ± 0.2 for beads, 4.1 ± 0.5 for platelet small aggregates, and 3.9 ± 0.6 for large aggregates. (c) volume overestimation with volocity with respect to the volume quantified with the new method (n = 10 stacks) measures 3.1 ± 0.2 for beads, 3.1 ± 0.6 for small aggregates, and 2.9 ± 0.5 for platelet thrombi. the results are expressed as mean ± standard deviation. all the intervals are metrologically compatible. acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 54 despite that the uncertainty of the proposed method seems high, it allows us to obtain considerable improvements in terms of measurement reliability and accuracy compared to the traditional methods used to quantify thrombus volume. it is important to specify that, in any case, all the measurements regarding the biomedical field require the critical interpretation and confirmation of the physician due to the high intraand inter-variability of the patients. consequently, while the inaccuracy of 400-600 % with the benchmark methods makes them inapplicable in real medical cases, we can affirm that the uncertainty of 10 % on volume measurements does not represent a problem with regard to the applicability of our method in clinics. 5. conclusion the accuracy and the standardisation of thrombus volume measurements starting from confocal microscope images have always been open and problematic issues. in the specific biological field of blood coagulation and cardiovascular diseases, the possibility of reconstructing and measuring the individual platelet thrombi formed on different adhesive surfaces is of considerable importance for studying platelet functions and the dynamic processes that involve them. the proposed method allows for the fast and accurate compensation of distortion effects and of the related volume overestimation. the algorithm described computes a realistic and accurate volume estimation offering new perspectives in terms of standardisation, inter-laboratory communication, calibration of other devices, medical diagnostics, measurements reliability, and fluid dynamic models definition. the obtained results aim at compensating for the systematic error arising when a volume is estimated from confocal stacks of images with benchmark commercial methods like imagej or volocity. the analysis indicates that the overestimation of all the benchmark methods is proven by compatible intervals for all the populations of considered objects (beads, small platelet aggregates, and large platelet aggregates). the overestimation error could be considered acceptable and treated as a common offset between measurements only in investigations about the functional effect of a substance. in medical diagnostics, instead, this overestimation is not acceptable (e.g. in tumour mass estimation). hence, it is mandatory to compute a realistic and accurate volume measurement. considering the instrumental benchmark that is described herein, the new fast calibration algorithm presented can be positively applied to different biological objects, to those characterised by an irregular shape, or to clusters of objects. references [1] r. eils, c. athale, computational imaging in cell biology, j. cell biol. 161 (2003) pp. 477-481. 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[31] w. s. nesbitt, s. giuliano, s. kulkarni, s. m. dopheide, i. s. harper, s.p. jackson, intercellular calcium communication regulates platelet aggregation and thrombus growth, j. cell biol. 160 (2003) pp. 1151-1161. [32] m. roest, a. reininger, j. j. zwaginga, m. r. king, j. w. heemskerk., flow chamber-based assays to measure thrombus formation in vitro: requirements for standardization, j. thromb. haemost. 9 (2011) pp. 2322-2324. [33] a. l. plant, l. e. locascio, w. e. may, p. d. gallagher, improved reproducibility by assuring confidence in measurements in biomedical research, nat. methods 11 (2014) pp. 895-898. [34] c. h. chen, signal processing handbook, marcel dekker, new york, 1988, isbn 0-8247-7956-8. [35] j. n. thon, l. mazutis, s. wu, j. l. sylman, a. ehrlicher, k. r. machlus, q. feng, s. lu, r. lanza, k. b. neeves, d. a. weitz, j. e. italiano, platelet bioreactor-on-a-chip, blood 124 (2014) pp.1857-1867. introduction to the acta imeko special issue on the ‘imeko tc3, tc5 and tc22 international conference 2017’ acta imeko issn: 2221-870x march 2019, volume 8, number 1, 12 acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 1 introduction to the acta imeko special issue on the ‘imeko tc3, tc5 and tc22 international conference 2017’ maija ojanen-saloranta1, petri koponen2 1 vtt mikes, tekniikantie 1, espoo, p.o.box 1000, fi-02044 vtt, finland 2 vtt mikes, tehdaskatu 15, puristamo 9p19, fi-87100 kajaani, finland section: editorial citation: maija ojanen-saloranta, petri koponen, introduction to the acta imeko special issue on the ‘imeko tc3, tc5 and tc22 international conference 2017’, acta imeko, vol. 8, no. 1, article 1, march 2019, identifier: imeko-acta-08 (2019)-01-01 editor: dušan agrež, university of ljubljana, slovenia received march 28, 2019; in final form march 28, 2019; published march 2019 copyright: © 2019 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: petri koponen, e-mail: petri.koponen@vtt.fi distinguished readers, the here finalized issue of acta imeko presents the highlights of the joint conference of imeko technical committees 3 (measurement of force, mass and torque), 5 (hardness measurement), and 22 (vibration measurement). the conference was hosted by the finnish society of automation, and took place on may 30 to june 1, 2017, in spring like but still chilly helsinki. this event invited experts and presentations around the theme “measurements facing new challenges”. out of the 97 presented papers, from 24 countries, the section chairpersons and members of the scientific committee made the selection of the most suitable 17 papers, with the standards, requirements and profile of acta imeko journal. as result of the review process with highly committed referees, the extended versions of the best 5 papers were accepted for this issue. schiavi et al. presented an experimental procedure and calculation model for the measurement of the indentation modulus by using the primary hardness standard machine at inrim in the macro-scale range at room temperature. the indentation modulus calculation is based on the doerner-nix linear model and from accurate measurements of indentation load, displacement, contact stiffness, and hardness indentation imaging. the geometrical dimensions of both the vickers and brinell indentations are measured by means of a micromechanical system and optical microscopy imaging techniques. a method for the evaluation of the response of torque transducers to dynamic load profiles is proposed by oliveira et al. this paper presents a method for providing torque traceability to rotating sensors under high torque variation rates using the principle of applying acceleration pulses to rotating shafts, with mounted reference discs with known mass moments of inertia. the research gives a very good idea of the necessary different future approaches to guarantee traceability for mechanical quantities. a set-up for the dynamic calibration of bridge amplifiers from dc up to 10 khz was presented by klaus. this paper describes a dynamic bridge amplifier calibration set-up based on the new ptb dynamic bridge standard. the calibration is carried out by synchronous sampling of the bridge amplifier output voltage and a reference signal provided by the calibrated dynamic bridge standard. the dynamic bridge standard enables calibrations in a frequency range from dc (static calibration) up to 10 khz. a new method for the calibration of strain cylinders using laser interferometry was presented by cui et al. their research investigates the possibility of calibrating strain cylinders using laser interferometry, thus providing a new type of transducer that can provide both force and deformation indications. this new method for the calibration of strain cylinders is based on the application of a dual channel laser interferometer in a force standard machine. using the proposed new method, the relationship between force, deformation, and strain can be calibrated in parallel when calibration forces are applied according to the procedure outlined in iso 376. liu et al. presented a tri-axial primary vibration calibration system that has been set up at the chinese national institute of metrology for simultaneous calibration of motion transducers. the system is driven by three electrodynamics exciters that are mounted along the three orthogonal axes. the tri-axial measuring system can simultaneously measure the three orthogonal vibration quantities based on the band-pass sampling method. the results show that a variety of spatial orbits can be generated by efficiently reducing the cross-coupling of the triaxial vibration exciter. in addition to presenting new, innovative approaches to calibration work, the selected articles address the challenges and experimentation in dynamic calibrations and measurements field whose importance is becoming increasingly important and where the si traceability must be brought. mailto:petri.koponen@vtt.fi acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 2 we are deeply honoured to be able to give our contribution to the measurement science community to act as guest editors for this innovative and challenging issue of acta imeko. the here presented release is the result of hard but rewarding scientific work, with authors, reviewers, copy-editors, layout editors and, last but not least, the editor-in-chief, prof. dušan agrež, providing his support for all this period. maija ojanen-saloranta petri koponen guest editors editorial to additional papers dear reader, eight additional papers are closing the first issue of acta imeko in 2019. the paper submitted by tarabini et al. is to deal with the possibility of monitoring the activity of and falls among people in dwellings using three or more accelerometers fixed on the ground. the main improvement of the proposed method is that it uses acceleration to estimate the impact force by using the apparent mass of the floor, experimentally identified in each room in which the tests were performed and by applying the heel drop test. the results show the possibility of identifying the presence of subjects inside the room and the falls in the majority of dwellings. de zanet et al. present measurement problems with accurate estimation of thrombus volume, which is an important quantity for estimation as a significant index related to the individual thrombotic risk profile. in 3d confocal microscopy, systematic errors can arise from distortions of the axial distance, whose accurate calibration remains a challenge. as a result, the 3d reconstructions show a noticeable axial elongation, and the volume measurements are thus overestimated. the adaptive algorithm proposed leads to the automatic compensation of the elongation error and to the accurate thrombus volume measurement. the paper submitted by affanni describes the design and characterisation of a dual-channel electrodermal activity (eda) and ecg sensor for acquiring data from the hands. this choice reduces the invasiveness of ecg measurement with respect to the usual vests or chest bands. the characterisation demonstrates high-level performance of the sensor in terms of linearity and jitter, even if the measurement on the hands provides a weaker ecg signal with respect to chest derivations. shestakov in his paper presents an overview of the directions in the development of dynamic measurement methods. he analyses the concept of dynamic measurements and outlines a short historical background. the author describes several types of dynamic characteristics of measuring equipment, which fully characterise their dynamic properties. additionally, he analyses the types of dynamic errors and their corrections, and in conclusion, an overview of the identification of dynamic characteristics is given. the aim of the paper submitted by kekalainen is to deal with a time delay estimation method based on the discrete time domain approach. in this dual-channel time delay estimation model, the criterion function compares the time differences of time sequences between channels, not the magnitude values of time functions as in the conventional cross-correlation method. an estimation task is formulated as an extreme value problem in discrete index space. thus, there are no theoretical problems in the determination of the average time delay or velocity in the non-constant or changing time delay case as long as a sufficient statistical correlation exists between channel signals. spagnolo et al. focused on the ability to process the image, what is a crucial skill in many measurement activities. in image processing or pattern recognition, fast fourier transform (fft) is widely used. in applications in which the use of memory is a critical element, fast hartley transform (fht) seems to be a good substitute. authors present a comparison of the implementations of the phase correlation function using fft and fht. shading is one of the most critical factors that produces a reduction in power in photovoltaic (pv) modules. the main aim of work presented by bucci et al. is the development of a testing procedure for the performance evaluation of commercial pv modules in the presence of partial shading on one cell. tests were carried out using thermographic and electric measurements and by varying the shading levels according to iec standards. as a case study, a complete characterisation of a 180wp polycrystalline pv module was performed according to the proposed testing procedure. the paper submitted by baranov et al. presents a new lock-in amplifier with a differential input. the lock-in amplifier has a high common-mode rejection ratio of 200 db at 1 khz and 160 db in the frequency range of 20 hz to 100 khz. authors suggest a method for extending the dynamic range of comparing signals supported by experimental results of metrological characteristic tests. hope you will have an interesting and a fruitful reading of this issue of acta imeko. dušan agrež, editor-in-chief of acta imeko microsoft word 353-2394-1-le-rev acta imeko  issn: 2221‐870x  september 2016, volume 5, number 2, 4‐13    acta imeko | www.imeko.org  september 2016 | volume 5 | number 2 | 4  from observed successions to quantified time: formalizing  the basic steps of chronological reasoning  bruno desachy  université paris‐1 and umr 7041 arscan (équipe archéologies environnementales), paris, france      section: research paper   keywords: archaeological stratigraphy; dating; chronological reasoning  citation: bruno desachy, from observed successions to quantified time: formalizing the basic steps of chronological reasoning, acta imeko, vol. 5, no. 2,  article 2, september 2016, identifier: imeko‐acta‐05 (2016)‐02‐02  section editors: sabrina grassini, politecnico di torino, italy; alfonso santoriello, università di salerno, italy   received march 20, 2016; in final form july 31, 2016; published september 2016  copyright: © 2016 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  corresponding author: bruno desachy, e‐mail: bruno.desachy@univ‐paris1.fr    1. introduction  since the concepts rethinking carried out by e.c. harris in the 1970s about archaeological stratigraphy [1], [2], the stratigraphic observations have become more rigorously recorded and processed. on the other hand, it is well known that the possibilities to get “absolute” (more exactly: quantified) indications of time about archaeological remains have been in constant progress for the past sixty years, thanks to the laboratory dating techniques. however, these advances in relative and “absolute” chronology still leave some dark zones in the basic chronological reasoning used in field archaeology. as some authors said [3]-[5], usual archaeological discourses are widely implicit and sometimes ambiguous about dating (i.e. positioning in the quantified time) the observed stratigraphic units and the historical material entities that archaeologists deduce from these units. this paper, derived from a work in progress about formalisation of stratigraphic data processing and chronological reasoning in field archaeology [6]-[9], presents some elements specifically devoted to this moving from the stratigraphic relative chronology analysed from field observations to a quantified frame of time using different dating sources. after reminding some notions of formalization of stratigraphic relative chronology – which is the basis of the chronological reasoning discussed here – in section 2, we will expose the proposed framework and process to integrate the observed relative chronology in a quantified time frame, taking in account the variety of available dating (section 3). the next two sections are about grouping, used to get a more global chronological view (section 4), and about the double nature of time in the chronological reasoning used by field archaeologists: stratigraphic time related to the formation of ground units and historical time related to the material entities use life (section 5). section 6 returns to one important issue: the uncertainties tied to the dating. in the last section, computerized tools derived from this work are briefly presented (section 7). 2. reminders about formalisation of relative  stratigraphic time  2.1. harris matrix  the stratigraphic analysis proposed by harris [1] is based on the systematic recording and processing of chronological order abstract  this paper is about the chronological reasoning used by field archaeologists. it presents a formalized and computerizable but simple  way  to  make more  rigorous  and  explicit  the  moving  from  the stratigraphic  relative  chronology  to  the  quantified  “absolute”  time,  adding to the usual terminus post quem and terminus ante quem notions an extended system of inaccuracy intervals which limits the  beginnings, ends and durations of stratigraphic units and relationships. these intervals may be processed as an inequations system,  integrating stratigraphic order relationships and available dating sources. questions about chronological units grouping, about the  differences between stratigraphic time and historical time, about the extension of the exposed chronological frame to historic entities,  and about the processing of uncertainties, are discussed. finally, the present state of computerized tools using this way  is briefly  indicated.  acta imeko | www.imeko.org  september 2016 | volume 5 | number 2 | 5  relationships deduced from observed interfaces between archaeological (and natural) layers. it involves a precise definition of the stratigraphic unit as a discrete unit of time identified by these interfaces, and also characterized by a spatial location, and a functional, social or cultural interpretation corresponding to the human or natural action behind the formation of the unit. the stratigraphic analysis process is summarized by a wellknown harris' example (figure 1): in the field, interfaces are observed and recorded, resulting in a set of units and order relationships. this set may be simplified by removing the “redundant” relationships (i.e. transitively deductible relationships) following the harris' so called “fourth law of archaeological stratigraphy”. a graph (“harris matrix”) displays this simplified (but complete) relative chronology. 2.2. mathematical and computerized approaches  the analytical concepts proposed by harris and the logical framework behind the making of a harris matrix made possible mathematical formalizations. different searchers have worked on such formalizations, from the earliest characterizations of the harris matrix as a partially ordered set [10], to recent works integrating formalized processing of stratigraphic sequences with gis and spatial analyses [11], [12]. these works resulted in some computerized tools, able to draw automatically a harris matrix, for instance stratify [13], matrixcomposer [14], or our application le stratifiant [6]. these proposals have in common to consider observed stratigraphic relationships as order relationships with their properties of reflexivity, asymmetry (or antisymmetry) and transitivity, and to use applications of graph theory to process these relationships. in particular, the harris matrix is formally close to task scheduling graphs developed in the 1950s to manage complex production processes [15], [16]. beyond this common basis, the way we chose to process stratigraphic data [6]-[9], has some peculiarities. the first is an algorithm (derived from the mpm graph method [17]) using an adjacency matrix to calculate a critical path value (i.e. irreducible relative time distance) for each couple of units and a stratigraphic rank for each unit; these values and ranks allow to provide a graph corresponding to a harris matrix. a second peculiarity is the display of the graph, chosen to avoid broken lines (which may become visually quite complex and ambiguous in big graphs) and using only vertical lines to order relationships and horizontal links to synchronisms [7] (figure 2). 2.3. the choice of admitting uncertain stratigraphic relationships  the most important peculiarity of our approach, materialized in the application le stratifiant, is the possibility of processing uncertain relationships. indeed, some doubts may appear in the field observations. it is the case with the notion of “correlation”, derived from geology, and used by harris to record a synchronic relationship between two layers, considered as the same stratigraphic step. formally, a stratigraphic synchronism is a mathematical equivalence relation with its properties of reflexivity, symmetry and transitivity. if the synchronism is certain, the order relationships of the synchronic units may be merged, and there is no need to display on the graph the order relationships deductible by this synchronism. but as s. roskams noticed [18], a stratigraphic “correlation” may be extrapolated rather than observed. for instance, in the original example retaken figure 1, harris assumed that the units 7 and 8 may originally be a same floor, so they are correlated. but they were cut by the trench 6 so that the original continuity can no more be observed; thus this synchronism is only a guess. even if this assumption is based on significant indications, it is less certain than an observed continuity. to take in account this lower level of certainty, practically used by field archaeologists when they assume (and not observe) a synchronism, we have developed a simple way, derived from modal logic: an “uncertain” (or “estimated”) logical modality can be recorded for a synchronism. in this case, a same stratigraphic rank is calculated for the “perhaps” synchronic units but their order relationships are not merged (figure 3). these two levels of certainty – certain and “estimate” (uncertain) – are logically applicable also to the order stratigraphic relationships. practically, they correspond to some figure 2. matrix processing to generate a stratigraphic graph [7]: observed  relationships  (corresponding  to  figure  1)  are  recorded  on  an  adjacency matrix; then the matrix is processed to calculate critical path values for each  couple, providing a stratigraphic rank for each unit. the processed matrix is then scanned to draw the graph, using stratigraphic ranks to place the units  and displaying only the non‐deductible relationships (value: one).  figure 1. left: simplified example of stratification (slightly modified from [2]  p.87 fig.28); 2 (horizontal cut) and 6 (foundation trench) are not deposits but erosion units. middle: observed order relationships; italics: "redundant" (deductible) relationships, not displayed on the graph. right: harris matrix.  figure 3. from the example figure 1, stratigraphic  graph  in case of certain  synchronism  between  units  7  and  8  (left),  or  in  case  of  uncertain  synchronism (right).   acta imeko | www.imeko.org  september 2016 | volume 5 | number 2 | 6  cases of difficult or limited conditions of observation, so that the archaeologist can make assumptions but he has too few indications to record a certain relationship (for instance during a standing building study, if the archaeologist is not allowed to destroy the building to confirm a relationship…). indeed, as m. carver [19] said: “stratification is not always obvious and our readings of it are often uncertain”. in every case, distinguishing between certain and uncertain is the responsibility of the field archaeologist, who must not exceed his limits of skill [9]. then, the rule of elimination of deductible relationships (on the graph) is applied only within the same level of certainty: if a certain order relationship is “redundant” compared to an uncertain one, it is not eliminated, in order to display the continuity of the certain relative chronology (figure 4). during the processing, possible contradictions between relationships provided with their certainty / uncertainty modalities result in logical cases of confirmation, invalidation (of an uncertain relation by certain ones), contradictory uncertainty (between uncertain relationships), or contradiction (between certain relationships). the two last cases need a user intervention to correct (or eliminate) the wrong data. 3. quantified stratigraphic time  3.1. dating relative time: the usual tpq‐taq way  after identifying, recording and processing the stratigraphic relative chronology, the second step of the chronological reasoning is to place this succession in a quantified time frame. as said in the introduction, the great progresses of the laboratory dating techniques are well known. however, the quantified time indications from these laboratory techniques (as well as those provided by more classical – historical, numismatic, etc. – chronometric sources), are not direct solutions to the problem of relative chronological units dating; they are only parameters of this problem. a lot of literature has been written about using contextual (thus stratigraphic) data to date archaeological objects, since the 19th century and the cross-dating method. the reverse way – dating a stratigraphic unit from different possible quantified time indications, which is our subject here – is curiously less explored, and usually limited to the tpq use. indeed, the most common way to apply dating indications to a stratigraphic unit is the terminus post quem (tpq) notion [3]. a tpq is a “not before” date: an earlier endpoint for the unit chronological position. usually, the tpq of a unit is provided by the most recent dated object found in this unit. a complementary notion –terminus ante quem (taq) or “not after” date – refers to the later endpoint for the unit chronological position. it is usually provided by an extrinsic source concerning the unit, for instance historical documentation. the couple tpq-taq forms an inaccuracy interval, bounding a precise (and generally unknown) date assignable to the stratigraphic unit. well-known rules are applicable to transfer these endpoints by the way of stratigraphic relationships: a tpq may be applied to each subsequent unit (if it has no later tpq), and a taq may be applied to each prior unit (if it has no earlier taq). these rules are the traditional way to integrate stratigraphic constraints into the dating of objects or structures. they were taken in account by some computerized dating formalizations (for instance [20], [21]), and the tpq endpoint is de facto used in the probabilistic models integrating stratigraphic constraints in laboratory dating techniques, like chronomodel [22] or oxcal. however, the use of this [tpq, taq] interval raises some issues. first, as noticed by several authors [3], [4] and as it still appears in many excavation reports, tpq is sometimes confused with the real chronological position of the deposit, in statements like “the ditch filling contains a roman sherd, so the ditch is roman”; that is at least a dangerous shortcut, because a deposit may be much more recent than the object it contains. the tpq using rules are well known, but sometimes we forget them, especially if it is convenient for our chronological hypothesis [4]. more fundamentally, there is an ambiguity about the position of the tpq related to the deposit. it is recognized that the tpq is a limit at the earliest for the position of the unit; but what limit exactly: the beginning or the end of the unit formation? actually, the most recent object found in a given unit (usual definition of the tpq, as said above) corresponds to the earliest date to its formation end, and not to its formation beginning (because an object made after the formation beginning of a unit may be deposited in this unit, if the formation duration is long enough). however, by another dangerous shortcut, the tpq is often taken as an absolute “no before” date, relevant also to the beginning of the deposit formation. another aspect of this ambiguity is the no accounting (or the rare accounting) for the duration of formation of the considered unit. the [tpq, taq] interval limits a moment (the final moment) of the unit, not its duration. so, indications of formation duration, when they are available (geoarchaeological observations for instance) are not or bad taken in account in the whole chronological reasoning of dating. finally, a reasoning only based on the [tpq, taq] interval tends to reduce intellectually the unit temporality to a simple date – just a moment in time without duration – what is all the more untoward that the duration is actually long. in other words, we think that the usual tpq-taq notions are an incomplete frame for the stratigraphic units dating. 3.2. taking durations in account: a system of inaccuracy intervals  our proposal consists in adapting the quantified frame of time used in the industrial applications of graph theory mentioned above [17]. this frame includes only four main variables. three concern each single chronological unit i: the beginning bi , the end ei and the duration di . a fourth variable is necessary: the duration dij allocated to the order relationship (if it exists) between a unit i and a unit j (which may be zero if the succession is immediate). theses variables are linked by basic equalities: . (1) for the order relationship i < j . (2) in a sequence i < j < k, the relationship i < k , transitively deductible, doesn't appear on the graph but it has a duration dik figure 4. example of an uncertain order relationship.  acta imeko | www.imeko.org  september 2016 | volume 5 | number 2 | 7  so that: . (3) this simple frame is necessary but not sufficient to deal with archaeological data. as said above, the quantified time indications, with very few exceptions, do not directly provide values to these variables. they just can be used as limits of inaccuracy intervals including each basic variable (figure 5). the limits of these inaccuracy intervals are be (beginning at the earliest), bl (beginning at the latest), ee (end at the earliest), el (end at the latest), ds (duration at the shortest), dl (duration at the longest), so that, for a unit i: , (4) , (5) . (6) and for an order relationship i < j: . (7) we find again the [tpq, taq] interval, which actually corresponds to the [ee, el] interval (cf. section 3.1 below). stratigraphic order relationships, basic equalities of quantified time (1), (2), (3) and their inaccuracy intervals (4), (5), (6), (7) may be integrated in a whole system of inequations, with its unknown values (basic variables bi ei di dij for each unit and relationship) and its valued parameters (limits of inaccuracy intervals, provided by dating indication). if there is no indication for an endpoint, a default value is used. these default values must be previously defined to include the whole period concerned, between an “absolute beginning” (for instance: the big bang date, or perhaps a little later) used for the endpoints at the earliest, and an absolute end (for instance: now) used for the endpoints at the latest. the default value for the durations at the longest is the difference between the absolute end and the absolute beginning; the default value for the durations at the shortest is zero. in the examples below, the unit of quantified time is the year. the main inequations are, for a unit i: , solution of , (8) , solution of , (9) , solution of . (10) and for an order relationship i < j , solution of , (11) , solution of ,(12) , solution of .(13) we can notice about the three last inequations that if we have no indications of duration (other than the default values), we find again the traditional rules of tpq and taq transfer – cf. section 3.1 above; but if we have some duration indications, we can improve the intervals of the related units. for an order sequence i < j < k: , solution of . (14) the simple algebraic processing of this inequations system results in the best possible reduction of the inaccuracy intervals. 3.3. impossible, possible, and certain time  after processing, each unit has a slot of “possible time”, between the beginning at the earliest be and the end at the latest el (figure 6). it is important to note that if we do not have enough accurate indications of beginning at the latest and end at the earliest (so that bl > ee), the existence of the unit will be uncertain at each moment in this “possible time”; because at any moment, the formation of the unit may be already finished or not yet started. the only certainty is negative: the formation of the unit is impossible before and after this “possible time”. it must not be confused with the unit duration: the duration at the longest is limited by the possible time, but may be shorter. in favorable cases (if bl < ee: the latest date for the beginning of the unit is before the earliest date for the end), a slot of “certain time” exists, in whom it is certain that the unit was in the making. the certain time gives of course a minimal duration at the shortest. but a wider duration at the shortest may be known for the same unit. in this case, the position of this duration is partially known because it necessarily covers this figure  5.  stratigraphic  ordered  time  (that  archaeologists  can  record  and process),  stratigraphic  quantified  time  (that  archaeologists  search)  and inaccuracy  intervals  in  quantified  time  (that  archaeologists  can  know  and process).  figure  6.  a‐c:  possible  formation  time  of  a  unit  i  (a);  with  duration  indications: possible positions of i at the earliest (b) or at the latest (c); d‐f:  certain formation time for a unit i, if bli < eei (d); with duration indications:  possible positions of i at the earliest (e) and at the latest (f).  acta imeko | www.imeko.org  september 2016 | volume 5 | number 2 | 8  certain time (figure 6). 3.4. example and graphic display  let us return to the example of figure 1 and let us admit that a document – an old photography dated 1860 – shows the place without the wall corresponding to the unit 3. it means that this wall was already destroyed in 1860. this is a taq corresponding to the end at the latest of the destruction cut (unit 2). this limit (el2 = 1860) allows to reduce the possible time for the unit 2; and thus for the stratigraphically previous units. if we admit now that a coin minted in 1600 was found in the unit 3, this coin provides a tpq for this unit i.e. a limit at the earliest only for its end (ee3 = 1860). consequently, a single eei value (with no other indications) does not allow reducing the “possible time” [bei, eli]. in other words, contrary to what is often said, a tpq does not precise the timeslot of the unit which contains it (because, as said above, it limits the end and not the beginning of this unit formation). however, it improves the “possible time” of the stratigraphically later units (here, units 2 and 1): if i < j, then of course eei ≤ bej. to precise the position of the unit 3, we need explicit duration indication. let us admit that field observations indicate a short formation duration for the whole sequence formed by the units 3, 4, 5 and 6 (for instance because the foundation trench 6 is not collapsed, so it must have been filled quickly): certainly less than a year. so we have a dl value for the unit 3. with these two known limits (ee3 = 1600, dl3= 1), we get a beginning at the earliest (be3 = 1600 – 1 = 1599) which reduces the “possible time” for the unit 3 (cf. inequation (8) above). this duration estimate – necessary to get a beginning at the earliest from a tpq provided by objects found in the units – is actually implicit and elided in many dating statements (the tpq is then, as said above, improperly used to give directly an “absolute beginning” to the unit). we get finally a [1599, 1860] “possible time” interval for the unit 3; and for the other units with a tpq = 1860 and a no more than 1 year duration of formation. we added to this example a case of “certain time” interval, admitting that the unit 1 is a contemporary floor of which the dates of beginning and end of construction are known with total certainty (1980, thus with a less than a year duration construction). in this case, the two endpoints of the inaccuracy interval have the same value (the inaccuracy range is zero). so there is no “possible time”, only a “certain time” for this unit 1. the result may be displayed by a graph on a quantitative timescale (figure 7), complementary to the stratigraphic graph. 4. changing timescale: subset relation   4.1. grouping stratigraphic units  practically, the archaeologists have to deal with stratigraphic time at different scales. after the analytic stage of identifying, recording and processing the stratigraphic units and relationships, the harris matrix has also been designed to be a basis for the synthesis of the stratigraphic and chronological information [2]. elementary units may be grouped in wider sets, providing a synthetic view of the site. this issue of grouping attracted much comment, and different ways exist. according to e. harris [2], stratigraphic units can be grouped in chronological phases and periods. in a more functional way, m. carver [19] suggests grouping the units in "features" (e.g. pit, ditch, grave, etc.) and "structures" (sets of features). a. carandini [23] combines these two approaches in a system including grouping in functional "activities" and "groups of activities", and chronological phases and periods. this is also the case with the proposals from the centre national d'archéologie urbaine [24] including a double grouping scale in chronological sequences (i.e. elementary successions), phases, periods, and functional features and structures. the grouping process itself is formally seen by i. sharon [25] as an "agglomerative model", after the "ordinal model" of stratigraphic units ordering. to a. carandini [26], it reflects the progressive rise of the archaeological discourse, from basic observations until a historic level. here, we consider this process from the point of view of its possible formalization and computerization. this point of view makes necessary an explicit distinction between two kinds of time: the stratigraphic temporality of the site formation process and the historic temporality linked to the evolution of the use of the site space (to which we will return in the section 5 below). if the archaeologist considers the stratigraphic time of the formation of the material tracks, the grouping of the stratigraphic units may be simply formalized by a subset relation between a macro unit and its included units, allowing as many grouping levels as wished. some computerized tools may display such subset relationships on the harris matrix, as frames containing units, like stratify [27]. we propose to use the adjacency matrix presented above (cf. section 2.2) to formalize this subset relation processing (figure 8). it allows to calculate the inclusion levels and to detect possible inconsistencies (unit including and included in the same other unit) (figure 8). we have chosen to make possible the transfer of the order figure 7. harris' example: “possible time” (in grey) calculated only with tpq‐ taq  intervals  (left),  and  with  duration  intervals  (right).  in  black:  “certain time” of the unit 1 (formation date accurately known).  figure 8. example of stratigraphic grouping at two levels: the stratigraphic  group 1 (sg1) includes units related to the construction of the wall 3; sg2 includes  unit  related  to  the  construction  of  the  floor  6;  sg3  includes  the  whole sequence of construction. from the straightforward inclusions coded  on the adjacency matrix, the indirect inclusions and the inclusion levels may be calculated.  acta imeko | www.imeko.org  september 2016 | volume 5 | number 2 | 9  relationships from elementary units to an upper level of grouping, so that, for the units i, j, k with i included in k: , ⊂ , ⊄ ⇒ . (15) in other terms, performing a stratigraphic grouping means that the order relationships between included and non-included units are merged and attributed to this grouping considered as a global unit. the internal order relationships (between units included in the grouping) are no more taken in account. this processing is similar to the synchronisms processing (cf. above); indeed, we can say that grouping a set of units is considering these units as synchronic, at a wider scale of time. a reverse way is sometimes practised (especially in large excavations in “flat” rural sites without deep stratification): a whole stratigraphic feature may be first identified (for instance a ditch with its filling) and relationships with other features may be recorded at this level (for instance this whole ditch cuts another ditch), before a finer stratigraphic analysis (for instance a trench practised into this ditch, in order to observe its internal stratification, providing more detailed stratigraphic units and relationships). in this case, the reverse using of the same simple subset relation allows integrating these different recording levels in the most analytical possible chronology, at the included units level; so that, for the units i, j, k with i included in k: , ⊂ ⇒ . (16) in other words, included units inherit order relationships from their including unit (figure 9). 4.2. grouping in the stratigraphic quantified time  to achieve stratigraphic groupings in the quantified time, inclusion relationships may be integrated in our inequations system. from a first simple principle – the duration of an included unit is limited by the duration of the including unit – we can deduce the relations between the basic time variables of an including unit i and its included units j(i): , (17) . (18) a second simple principle is, if order relationships exist between the included units, that the formation duration of the including unit can't be less than the critical path (minimum irreducible total duration) of the partially ordered set formed by the included units and relationships. so that, for an including unit i, its included units j(i) ⊂ i and k(i) ⊂ i and an order relationship j(i) < k(i): . (19) from those basic inequalities result some inequalities linking the intervals limits of i and j(i) ⊂ i: , (20) , (21) , (22) . (23) and for an including unit i, its included unit j(i) ⊂ i, k(i) ⊂ i and the order relationship j(i) < k(i): , (24) . (25) from these inequalities and from the inequations exposed above (8 to 14), adapted inequations may be drawn to reduce the range of the inaccuracy intervals of the included and including units, following the internal and external order relationships of the including unit. for instance, in the harris example (figure 1), we previously assumed that a set of units (3 to 6) has a whole duration at longest (1 year), which is an interval endpoint dl recorded at a grouping level; then we have transferred this duration at longest from this grouping level to each elementary units (cf. above section 3.4), which is a direct application of inequality (22); and we have grouped these units in a more synthetic unit (of construction) sg10 (figure 9). in this simple example, the order relationship sg10 < 2 inherited from the included units provides a taq: elsg10 = 1860. to find the possible time of sg10, we can find its tpq, using a simple inequation derived from the inequalities (20) to (23): ⊂ , . (26) we have 3 ⊂ sg10, dlsg10 = 1 and be3 = 1600, thus besg10 = (1600-1) = 1599. in this way, this tpq may be then directly transferred to the sg10 included units (under the inequality 20). 5. inferring historical time from stratigraphic  time  5.1. stratigraphic units and historical entities  as said above, we have to deal with a double temporality: beyond the making of the stratigraphic chronology (including dating and grouping), there is another stage of chronological synthesis, which implies a change of nature of time. the stratigraphic time, discussed until here, is strictly related to the formation of field units. for instance, in the figures 1 and 9, the duration referred to the unit 3 (wall), as stratigraphic unit later than the unit 2 (foundation trench) and prior to the unit 4 (backfill), is only the formation duration of this unit 3 (i.e. the wall construction duration). but if we want to approach the historical time of societies, we have to consider no more only the formation time of the material remains, but also their whole “cultural life”. in the same example, the historical time related to the wall corresponding to the unit 3 includes not only the construction duration, but also the period of use of this wall, as a part of an inhabited or used structure, until its destruction. this whole period of “cultural life” of a material remain, including use time, corresponds to the “systemic context” notion developed by m. schiffer [28], opposed to the “archaeological context” of the figure 9. in the harris example (cf. figure 1 above), the set of units 3 to 6 may be grouped  in a macro unit of construction (stratigraphic group  10). the graph may be displayed at the synthetic scale of the sg (which inherits relationships  from  its  included  units),  or  at  the  analytical  scale  of  the elementary  units  (which  inherit  relationships  possibly  recorded  at  the  sg level).  acta imeko | www.imeko.org  september 2016 | volume 5 | number 2 | 10  buried remains which no more interact with an alive society. we can notice that this time in systemic context longer than the duration formation is a characteristic of units functionally interpretable as building or fitting out, by opposition to the “occupation” units whose formation duration corresponds to their whole time in systemic context (like material use tracks for instance). however, the stratigraphic time, limited to the formation durations of field units, does not take in account all the time in systemic context. this limit also applies to the harris matrix, which displays only the stratigraphic time (that it is why some archaeologists like m. carver [29], p. paice [30] or j. collis [31] have criticized it). de facto, archaeologists deduce historical material entities (e.g. “structure”, “ditch”, “well” “house”, “temple”, “architectural ensemble”, etc.) from stratigraphic units, adding (explicitly or more often implicitly) use durations to formation durations, even if, as in the examples above, there are just construction units with no preserved material tracks (and so recordable as stratigraphic units) of their “occupation” (i.e. their use timespans). the functional groupings mentioned above (“feature”, “structures”…) correspond to such historical (and not only stratigraphic) entities, because they correspond to a more synchronic and historic (the life on the site at a given period) than diachronic and stratigraphic (the site formation process) view. it is also these historical entities, more than strictly the stratigraphic units (even if the shapes are the same), which are displayed on phase or period plans (or gis) of an excavated site, for the same reason: because these plans aim to represent the use of the space at a given period (and not the process of the formation of the site as the harris matrix). however, this moving is not so clear in the field archaeological reasoning. a first point usually not enough explicit is that recognizing such historical entities from a set of stratigraphic units is not a simple grouping based on elementary subset relations. there is a more complex n-n relation between the stratigraphic units and the related historical material entities, so that these two temporalities – stratigraphic and historical times – must not be confused (figure 10). indeed, a material remain may stay in “systemic context” longer than another one more recently formed. furthermore, the materiality of a stratigraphic unit may remain in use or may be reused through several successive phases of a historical entity (what is formalized for instance in the oh-fet urban evolution analysis model [32]). in other words, there is a fundamental difference between stratigraphic and historical times: the first one totally excludes cyclical phenomena (the same unit is never formed twice, and it cannot be both after and before another unit – or it is a logical fault). but the second one admits such cyclical phenomena, including not only reusing in successive phases, but also round trips between systemic and archaeological contexts: the same remain or object may stay in systemic context after its formation for a first “cultural life”, then fall into archaeological context, and then come back into systemic context (e.g. an excavated and restored archaeological site) [28]. 5.2. quantified time of historical entities  clarifying the moving from stratigraphic to historical time, and thus using explicitly stratigraphic or historical units, is all the more important when we consider quantified dating. otherwise, a new source of confusion may appear: confusing formation duration and use duration. when we write or read “this hypocaust is dated first century”, what does it mean exactly? is the first century possible time for the construction? or is it really the whole use time span? this kind of confusion, added to confusions mentioned above (between tpq and real unknown beginning, or between “possible time” and real unknown duration of the unit), is a large part of dating incorrectnesses noticed by a. ferdière [4] in the french archaeological literature. once again, this question of formation (or production) and/or use time in the chronological reasoning has been more studied by archaeologists for the objects dating than for the contexts dating. however, provided that the duration di attributed to a unit (or entity) i is clearly defined either as a formation duration or as a wider timespan in systemic context, the quantified chronological frame discussed above for the stratigraphic units may be used for the historical entities. if we go back to the harris example (figure 7), concerning the unit 1 (dated as a contemporary floor), we can consider not only its formation duration, but its known use duration (from 1980 to 2016, if we assume that excavations started this year). then we consider not a stratigraphic unit, but a historical material entity (a: figure 11) that has a totally certain time, as the related unit 1, but wider, because it includes its whole time in systemic context (26 years until now). we have added to this example a case of dating from historical sources, to illustrate the difference between stratigraphic units dating and historical material entities dating. (figure 11). as seen above, the units 3, 4, 5 and 6 may be grouped in a construction stratigraphic group (sg10), and we figure 11. stratigraphic time (left) and historical time (right); the historically  dated  house  b  has  a  “certain  use  time”  provided  by  historical documentation;  the  “possible  time”  of  the  related  stratigraphic  units  of  construction and destruction is consequently limited.  figure 10. if we add a phase of refitting (new floor) of the building showed in  the  example  figure  8,  we  see  there  is  a  complex  relation  between stratigraphic  units  (and  their  groupings)  and  material  entities  in  the historical  time  (and  their  groupings):  the  same  unit  (3)  is  related  to  two  successive historical (use time) phases.  acta imeko | www.imeko.org  september 2016 | volume 5 | number 2 | 11  can infer that this group 10 belongs to a historic material entity: the building with its use duration after its construction and before its destruction (b). let us admit that this building is mentioned in two documents dated 1710 and 1790. these documents directly give a slot of certain time for this house as historical entity (between 1710 and 1790). in other words, if we consider the inaccuracy intervals applied to this historical entity (and not to the stratigraphic units), these documents give directly the beginning at the latest and the end at the earliest of this entity (blb = 1710; eeb= 1790). the other endpoints may come from related stratigraphic units: the beginning at the earliest from the stratigraphic group (sg 10) and the end at the latest from the destruction unit (2) logically limit the b entity timespan; so beb = 1600 ; elb= 1860. we can transfer these historical dating indications to the related stratigraphic units, but not to the same endpoints. the earlier document gives logically 1710 as the end at the latest to the stratigraphic group sg 10 (and thus to the prior units) because if the building is mentioned as existing, its construction is already achieved; reciprocally, the later document gives 1790 as the beginning at the earliest to the destruction cut 2 (because if the building is mentioned as existing, its destruction has not begun yet). we can see again that stratigraphic and historic times are not the same: this interval [1710, 1790], which is a certain timespan for a material entity in the historical time, is, on the other hand, a gap in the stratigraphic quantified time because no observed occupation stratigraphic unit corresponds to the use time of this historical entity. 6. dealing with uncertain dating: the estimated  time  as said above, we have to deal with uncertainty at each step of the chronological reasoning. we saw that inaccuracies of dating result in a kind of uncertainty (“possible time”) linked to the formation of a stratigraphic unit – or to the use time of a historical or functional entity. furthermore, if we admit an uncertain modality for stratigraphic relationships (cf. section 2.3), and more generally for succession time constraints, it must be logically extended to the quantified frame of time by a distinction between “certain” and “estimated” (understood as uncertain) quantified time data. a double system of intervals (certain and estimated) is then necessary. the estimated intervals, more accurate but hypothetical, are limited by the certain ones. they result in an intermediary “estimated” time between the “possible” and the “certain” time (figure 12). the advantage is a more flexible process, able to take in account uncertain observations; but also to take in account indications, more often hypothetical than certain, about the temporality of the objects found in the units, involving their use time and notions such as “primary deposit” or “closed find”. for instance, the graph (figure 13) shows the dating of our example if we add this assuming about the coin dated 1600 found in the layer 3: it is a primary deposit, and it is not very worn, so that its circulation duration before its deposit is probably less than 50 years after its minting date. for any dating element, we have to ask clearly the question: what endpoint is informed by this indication? here, it is not an estimated tpq, but an estimated beginning at the latest. indeed, this maximum estimated duration since the tpq and before the deposit gives directly an estimated beginning at the latest for the unit 3, because the formation of the unit 3 has necessarily already begun when the coin is deposited. so we can write: ebl3 = 1649 (estimated beginning at the latest of unit 3 = minting date of the coin added to less than 50 years). then, as we have admitted a duration at the longest for the unit 3 (dl3 = 1 cf. section 3.4), we get an estimated end at later for the unit 3: eel3 = 1649 +1 = 1650 (cf. inequation (8) above). this deduced estimated taq defines an “estimated time” – more accurate, but less certain than the “possible time” – for the unit 3 and the stratigraphically related units and groups. another utility of this notion of “estimated time” is to deal with the heterogeneous quality of the documentation which sometimes characterizes the archaeological field data, and which inevitably affects the researches using various sources to identify historical and functional entities (especially at the wider scale of an archaeological map). such a chronological frame including estimated inaccuracy intervals is experimented by j. gravier in a current research about the city of noyon (france) at this wider scale of urban historical and functional entities [33]. finally, it is also a way to display a chosen chronological hypothesis, between the two extreme possible chronologies, at the earliest and at the latest. 7. practical applications: computerized tools  the approach presented here needs computing tools, because, if the underlying formalization is simple, the calculations – particularly the manipulation of the complete system of inaccuracy intervals – are quite boring manually. there again, it is a work in progress, and the complete integration of the process within a single application is not finished yet. nevertheless, two applications already exist and allow handling automatically a wide part of the process. the first one is le stratifiant: a stratigraphic data processing application developed since 2005, as an add-on to the microsoft excel software [6], [7]. its functionalities include stratigraphic graphs generation from sets of data (units and relationships) figure  12.  “estimated  time”  provided  to  a  unit  i  (between  an  estimated beginning  at  the  earliest  ebe  and  an  estimated  end  at  the  latest  eel)  if  estimated inaccuracy intervals have been valued besides the certain limits.  figure 13. possible time (grey) and “estimated time” (darker grey).  acta imeko | www.imeko.org  september 2016 | volume 5 | number 2 | 12  entered by the user or imported from a database, uncertain relationships processing, and logical faults detection. it also performs limited processing of units grouping (phasing) and inaccuracy intervals of quantified dating (figure 14). its use in operational conditions since a few years by some french archaeologists (in the institut de recherches archéologiques preventives [34], bibracte archaeological center, or territorial archaeological units) has provided some experience feedbacks. in its present state, this application does not include subset relationships processing nor extended dating intervals (other than [tpq-taq]) processing. the development of a new version containing these features has begun. from now, an experimental simple tool – chronophage – has been developed in order to explore the calculation of all the inaccuracy dating intervals of a unit (stratigraphic unit or historic entity), on the free software libreoffice /openoffice calc. it contains formula to solve inequations and to reduce the intervals from known endpoints and default values chosen by the user. it is not yet integrated in le stratifiant and it doesn’t process stratigraphic relationships (or relative time constraints); but for each unit, it detects logical faults of dating, gives the possible, certain and/or estimated time; and it provides quantified time graphs (figure 15chronophage tool (formulas and macros in libreoffice calc free software)). these tools are free and available (in french) at: https://cours.univ-paris1.fr/fixe/03-40-doctorat-archeologieatelier-sitrada (section: “outils téléchargeables”). 8. conclusion  this is only a preliminary paper: beyond the description of the approach briefly presented here, it is necessary to test it with real and enough numerous data. it will be the next step of our work, using the computerized tools presented above and their current developments. in a further step, other developments are possible: especially the application of such a systematic approach using inaccuracy intervals to the temporality of the archaeological objects (and not only to the contexts). finally, this work in progress is an attempt to explicit basic steps of the chronological reasoning, upstream to the advanced chronological modelling. indeed, these basic steps, that are fundamental, remain often implicit and ambiguous. formalizing them – as simply as possible – is a practical issue, in order to get some computer-based aids; it is also a methodological and epistemological issue, in order to make our chronological statements more rigorous and explicit. figure 14. le stratifiant: stratigraphic graph with phases and tpq thresholds (up); tpq‐taq intervals graph (down).  figure 15. chronophage tool (formulas and macros in libreoffice calc free software).  acta imeko | www.imeko.org  september 2016 | volume 5 | number 2 | 13  references  [1] e. harris, the laws of archaeological stratigraphy, world archaeology, 11-1 (1979), pp. 111-117. 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[30] p. paice, extensions to the harris matrix system to illustrate stratigraphic discussion of an archaeological site, journal of field archaeology, 18-1 (1991) pp. 17‑28. [31] j. collis, “constructing chronologies: lesson from the iron age”, in: construire le tempshistoire et méthodes des chronologies et calendriers des derniers millénaires avant notre ère en europe occidentale, a.lehoërff (editor), bibracte, glux-en-glenne, 2008, isbn 978-2-909668-60-4, pp.85-104. [32] x. rodier, l. saligny, modélisation des objets historiques selon la fonction, l’espace et le temps pour l’étude des dynamiques urbaines dans la longue durée, cybergeo: european journal of geography, 502 (2010), url: http://cybergeo.revues.org/23175 [33] j. gravier, “recognize temporalities from formalization of dating urban units: three case studies”, proc. of 42nd annual conference on computer applications and quantitative methods in archaeology, 2014, paris, france, (2015) pp.371-379. [34] a. bolo, m. muylder, c. font, t. guillemard, de la tablette pc à la cartographie de terrain: exemple de méthodologie sur le chantier d’archéologie préventive de noyon (oise), archeologia e calcolatori, supplemento 5 (2014), pp. 247-256. dual-channel electrodermal activity and an ecg wearable sensor for measuring mental stress from the hands acta imeko issn: 2221-870x march 2019, volume 8, number 1, 56 63 acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 56 dual-channel electrodermal activity and an ecg wearable sensor for measuring mental stress from the hands antonio affanni1 1 polytechnic department of engineering and architecture, university of udine, via delle scienze 206, 33100 udine, italy section: research paper keywords: electrodermal activity; stress measurement; ecg sensors citation: antonio affanni, dual-channel electrodermal activity and an ecg wearable sensor for measuring mental stress from the hands, acta imeko, vol. 8, no. 1, article 9, march 2019, identifier: imeko-acta-08 (2019)-01-09 section editor: sergio rapuano, university of sannio, italy received march 06, 2018; in final form october 23, 2018; published march 2019 copyright: © 2019 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: antonio affanni, e-mail: antonio.affanni@uniud.it 1. introduction in the recent past, studies focusing on stress or distress among healthy people in several conditions (e.g. in the workplace, while driving, playing videogames) have exponentially increased. several bio-signals are measured while the subjects of such studies are doing tasks e.g. photoplethysmogram (ppg) [1], electroencephalogram (eeg) [2], electromyogram (emg) [3], electrocardiogram (ecg) [4], [5], electrodermal activity (eda) [6]-[10], and many others. the present paper presents the design and characterisation of a sensor for stress recognition, focusing on drivers, using two eda channels and an ecg channel acquiring data from the hands. the need for two channels for eda measurement is derived from the fact that usually the motion of the hands (from where eda is mainly acquired) provides a huge motion artefact, which makes the acquired data unreliable if hand motions are not tracked [8]; for this reason, in most scientific literature, the subjects are sitting, with still hands. however, when doing several everyday tasks (e.g. driving a car), people need two hands, and this is a problem in the acquisition of eda. for this reason, in the present paper, we present our development of a dual-channel eda sensor with the aim of using both channels to discriminate the motion artefact from sympathetic activity. in order to evaluate heart rate (hr) and its variability (hrv), in scientific literature, subjects usually need to wear a chest strap or a vest, which makes the measurement setup uncomfortable for the subject, especially if the measurement is acquired in real life (e.g. driving) instead of a laboratory for scientific experiments. in this work, different to previous works [5]-[8], we have two goals: the first is to acquire the ecg trace from the hands using the same electrodes used for eda; and the second is to process two eda signals in order to increase the robustness of the data with respect to motion artefacts. in the recent past, we presented a study [5] in which eda and ecg were acquired simultaneously using traditional electrodes disposition on the chest by means of a special t-shirt; however, dressing in a special t-shirt with wet electrodes is not very comfortable for the user. on the other hand, in [8], we demonstrated that the motion artefact during driving is as high as the eda signal or higher; thus, some correction to the acquired data must be applied. adding a second channel to the eda, we are able to discriminate eda peaks in a more robust way (e.g. using independent component analysis or other algorithms) than previous works. the measurement of eda reads a signal controlled by the sympathetic nervous system. eda measurement methodologies can be exosomatic (skin conductance response [scr] or galvanic skin response [gsr]) or endosomatic (namely skin potential abstract the paper describes the design and characterisation of a dual-channel electrodermal activity (eda) and ecg sensor for acquiring data from the hands. the need for dual-channel data acquisition is due to the removal of motion artefacts that may happen when eda is measured on subjects when they are moving their hands in their everyday activities. the ecg channel is measured from the hands using the same electrodes that have already been used for eda. this choice reduces the invasiveness of ecg measurement with respect to the usual vests or chest bands. the characterisation demonstrates high-level performance of the sensor in terms of linearity and jitter, even if the measurement on the hands provides a weaker ecg signal with respect to chest derivations. even when the subject is using their hands, no artefacts were found in extracting the heart rate from ecg. mailto:antonio.affanni@uniud.it acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 57 response [spr]). the first method measures the conductance of the skin, which changes when the subject is stimulated by external events; this happens because the sympathetic nervous system activates the sweating glands in order to sweat. the second method directly measures the nervous pulse, which activates the sweat glands. this second method provides more complex signals than the first, but its readout is faster since it directly acquires the nervous pulse from the sympathetic nervous system [11]; moreover, it is less prone to electrode impedance changes due to temperature or sweating of the hands. the spr measurement measures the differential voltage on the skin between a place where there is a high density of sweat glands (mainly the palms of the hands) and a place where sweat glands are fewer (mainly the backs of the hands). in recent literature [12]-[14], several papers have measured ecg and eda simultaneously (also with commercial solutions). however, ecg is acquired on the chest, and eda is measured by means of scr, which cannot provide a fast readout of the signal. the study of ecg and the consequent hrv has been a matter of study of hundreds of scientific papers. heart activity is mainly controlled by the sympathetic nervous system and partially by the parasympathetic nervous system (vagus nerve). these two signals control hrv with different aims and speeds of intervention. the sympathetic nervous system slowly varies, while the parasympathetic nervous system varies quickly. the post-process of hrv is still an open issue in the literature (frequency domain, time domain or other methods are discussed), especially in short-term measurements, where there is a need to know beat-to-beat information [15]-[19]. eda is a good estimator of short-term sympathetic activity, and simultaneous measurement of ecg and eda can improve knowledge of hrv, especially in short-term data acquisition. in this scenario, it is important to have a wearable, wireless, and comfortable instrument that is able to measure both the ecg and eda without the need for wearing many electrodes on the body. the paper is organised as follows: section 2 provides a description of the system. section 3 provides the metrological characterisation of the sensor and shows the efficacy of the simultaneous measurement of ecg and dual-channel spr with moving hands. finally, conclusions are drawn. 2. sensor description the wearable eda + ecg system is shown in figure 1(a). three disposable ag/agcl electrodes are placed on each hand: on the palm, on the back, and on the wrist. the wrist electrodes provide a reference potential vref, and the bio-signals are acquired as differential voltages present on the skin. in particular, three analogue front-end blocks, shown in figure 1(b), acquire the difference between the palm and the back of the right hand (spr1 signal); the difference between the palm and the back of the left hand (spr2 signal); and the difference between the back of the left hand and the back of the right hand (ecg signal). each conditioning analogue front end has the architecture shown in figure 1(c). the conditioned analogue signals are acquired by means of the a/d on board a dsp, which sends data via wifi to a laptop (tablet or smartphone). the system is operated by a single 3.7 v 2000 mah lithium polymer battery and is rechargeable by means of a usb plug. since current consumption of the sensor in transmission mode is 200 ma, the system can transmit data for 10 hours of continuous acquisition, by far longer than typical eda sessions, which are in the order of one hour. 2.1. analogue front end description referring to figure 1(c), each analogue front end acquires a low-level differential voltage vin (either spr1, spr2, or ecg) and, by proper signal conditioning, provides voltage vad, which can be acquired by the a/d converter on the dsp. the vin voltage is firstly filtered with passive first-order highpass filters, whose input impedance is 100 m, by far higher than skin output impedance (in the order of 100 k), thus reducing the load uncertainty to less than 1 %. the cut-off frequency of the input filter is (2l)-1=0.05 hz. then, an instrumentation amplifier with gain g (throughout the paper, gspr and gecg are used for the spr and ecg signals, respectively) amplifies the differential signal, and at its output, there is a third order, sallenkey, anti-alias filter with time constant h (h,spr and h,ecg for the spr and ecg signals, respectively). the dc compensation block acts as a time integrator (with time constant i) of the difference between the amplifier output and vref. the feedback loop composed of the amplifier and the dc compensation block acts as a high-pass filter with cut-off frequency (2i) -1, the chosen coincident with (2l) -1=0.05 hz. in this way, all the dc non-idealities of the amplifier (offset voltage, bias current, offset current) are removed by the dc compensation block [10]. the overall transfer function of the analogue front end thus, in the laplace domain, results in 𝑉𝐴𝐷 (𝑠) = 𝑉𝑅𝐸𝐹 (𝑠) + 𝜏𝐿 2𝑠2𝐺 (1 + 𝜏𝐿 𝑠) 2(1 + 𝜏𝐻 𝑠) 3 𝑉𝐼𝑁(𝑠). (1) the analogue front end is supplied with 3.3 v, and vref = 1.65 v, at half-supply range. figure 1. (a) overall system block diagram; (b) sensor block diagram; (c) analogue front-end block diagram. acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 58 the gain of the spr1,2 channels is 𝐺𝑆𝑃𝑅 ≈ 160, since the input voltage that has a maximum amplitude of 10 mv must be converted into 3.3 vpp. the bandwidth of the spr channels results in the range [0.08, 40] hz with slopes +40 db/dec for the lower cut off (0.08 hz) and –60 db/dec for the higher cut off (40 hz). the gain of the ecg channel has been designed as 𝐺𝐸𝐶𝐺 ≈ 680, since the input voltage that has a maximum amplitude of 2.5 mv must be converted into 3.3 vpp. it is noticeable that ecg acquisition on the chest can provide signals with an amplitude of up to 5 mv, but the measurement on the hands (despite the comfort of using less electrodes) provides a weaker signal; thus the gain of the analogue front end is the quadruple with respect to traditional ecg chest bands. the bandwidth of the ecg channel results in the range [0.08, 160] hz with slopes +40 db/dec for the lower cut off (0.08 hz) and –60 db/dec for the higher cut off (160 hz). 2.2. dsp and a/d conversion ecg and spr, once filtered and amplified, are sent to the analogue input port of a digital signal processor (dsp), which acquires vad and sends the acquired data to the wifi module via a universal asynchronous receiver transmitter (uart). the dsp, a dspic 30f3013 from microchip, has an on-board 12-bit analogue-to-digital converter and operates at 8 mips. the sample rate has been set at 500 sa/s, which is high enough to capture ecg signals without overloading the cpu and the graphic card of a medium/low-performance tablet when used in real-time mode. the a/d conversion time has been set at 100 µs with the aim of reducing the sampling transient effects far lower than 1 lsb. 2.3. wifi module the wifi module is the usr-wifi232 from usr iot, which can operate both as an access point and as a station. in this application, it is configured as an access point so that several laptops or tablets can acquire data at the same time without the need for a server. the baud rate of the data transfer has been set at 115.2 kbps in order to allow for data transmission without crowding the channel. 2.4. software description the control panel used to acquire the data is shown in figure 2. it has been developed in the .net framework due to the ease of programming the graphical user interfaces (guis). the software is designed to communicate with the spr+ecg sensor and to plot, in real time, the signal showing the graphical markers with their comments when the subject receives some stimuli. at the top left of the gui, it is possible to connect to the sensor by inserting the ip address of the access point and network ssid, and to start and stop the acquisition. in the middle left of the screen, it is possible to manually insert markers with their description and save the recorded data or load saved data. the top graph shows the left spr, the middle graph shows ecg, and the bottom graph shows the right spr. 3. sensor characterisation and experimental results characterisation was done using three different test setups. the first test setup aimed to characterise the analogue and digital circuitry in terms of linearity and bandwidth, sending sinusoidal signals at inputs and measuring the sensor response with the oscilloscope. the second test setup sends, at the input of the sensor, a synthesised ecg generated by a waveform generator. it evaluates the r-r interval of the output with respect to the input using commercial software for ecg and hrv analysis. the aim of this second test setup was to quantify the accuracy of the hr readout by means of the jitter introduced by the sensor. the third test setup showed the necessity of measuring spr with two channels when hands are moving and shows, for example, the extraction of the hr from ecg acquired from the hands. 3.1. metrological characterisation in this subsection, we present the experimental characterisation of the sensor in terms of linearity and bandwidth. for the characterisation, an automatic test benchmark has been developed, connecting an oscilloscope through the lan and the sensor by means of wifi. an ad hoc program developed in the matlab environment controls the built-in waveform generator of the oscilloscope, measures the output of the measurements from the oscilloscope, and a robot code controls saving of the data transmitted from the sensor (by wifi) to the panel described in section 2.4. the aim is to automatise all the measurements in order to acquire the largest possible amount of data. after the data collection, the performance was evaluated in accordance with the guide to the expression of uncertainty in measurement (gum). 3.1.1. linearity the waveform generator built-in digital oscilloscope (rigol ds2302a) has been set to provide a sinusoidal voltage vg with amplitude variable from 0 to 3.3 vpp in 100 linearly spaced steps. the frequency f0 of the sinusoid is fixed at the centre of the bandwidth, i.e. 𝑓0 = √𝑓𝐿 𝑓𝐻, with value 2 hz for spr and 4 hz for ecg. at the output of the generator, a resistive (0.1 % tolerance of the resistors) attenuator a with output impedance 1 m (to simulate skin behaviour) is connected to each channel of the sensor. in the case of spr, a = 5.56810-3 (810-6), and in the case of ecg, a = 1.49710-3 (310-6). the uncertainty on the vector vin (throughout the paper, vectors will be expressed in bold type) thus results in 𝑢(𝑽𝑰𝑵) = √[𝐴 ∙ 𝑢(𝑽𝑮)] 2 + [𝑽𝑮 ∙ 𝑢(𝐴)] 2. (2) the rms value of the output of the analogue front-end vad is automatically measured with the same oscilloscope over 10 periods of the sinusoid per each input amplitude. the gain is estimated by the least-squares method, supposing that the best estimation of vad is figure 2. control panel for the data acquisition. acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 59 𝑽𝑨�̂� = 𝐺𝑽𝑰𝑵 + 𝑏 . (3) using 〈∙〉 to identify the mean value, the gain g is estimated accordingly as 𝐺 = 〈𝑽𝑰𝑵𝑽𝑨𝑫〉 − 〈𝑽𝑰𝑵〉〈𝑽𝑨𝑫〉 〈𝑽𝑰𝑵 2〉 − 〈𝑽𝑰𝑵〉 2 . (4) since both vin and vad are affected by uncertainty (from the manual of the oscilloscope u(vad) = 0.02  vad [20]), the uncertainty on the gain is obtained by propagating (4) as 𝑢(𝐺) = √∑ ( 𝜕𝐺 𝜕𝑉𝐼𝑁,𝑖 𝑢(𝑉𝐼𝑁,𝑖 )) 2 + ∑ ( 𝜕𝐺 𝜕𝑉𝐴𝐷,𝑖 𝑢(𝑉𝐴𝐷,𝑖 )) 2 , (5) where subscript i means the i-th value of the vector corresponding to the i-th input amplitude. after having characterised the gain, the linearity error is estimated as (referring to equation (3)) 𝑬 = 𝑽𝑨𝑫 − 𝑽𝑨�̂� = 𝑽𝑨𝑫 − 𝐺𝑽𝑰𝑵 − 𝑏 . (6) the uncertainty on the linearity error e can be obtained by propagating in equation (6) the uncertainties in equation (2) and equation (5). since the quantities vin and vad are the input and output of an amplifier, respectively, their correlation is expected to be very close to one (in the measured data, it results in 0.999986). therefore, according to the propagation of uncertainty, the uncertainty estimation of the linearity error e results 𝑢(𝑬) = √[𝑢(𝑽𝑨𝑫) − 𝐺 ∙ 𝑢(𝑽𝑰𝑵)] 2 + [𝑽𝑰𝑵 ∙ 𝑢(𝐺)] 2 + [𝑢(𝑏)]2 . (7) regarding the spr channels, using equation (2) to equation (7) results in gspr = 145.4  0.5, and the linearity error is in the order of 0.5 % fs, corresponding to 50 v on vin. figure 3(a) shows the linearity behaviour of the analogue front end. the error bars represent the uncertainty in equation (7), where the most significant contribution is due to the uncertainty on the oscilloscope u(vad). regarding the ecg channel, using equation (2) to equation (7) results in gecg = 679  3 and the linearity error is in the order of 0.2 % fs, corresponding to 5 v on vin. figure 4(a) shows the linearity behaviour of the analogue front end; the error bars represent the uncertainty in equation (7). again, the most significant contribution is due to the uncertainty on the oscilloscope u(vad). referring to figure 3(a) and figure 4(a), there is a common trend in linearity between the channels spr and ecg, which is related to the linearity behaviour of the oscilloscope used for the characterisation. using a similar procedure, we characterised the linearity of the overall system, considering the uncertainty introduced by the a/d converter. we acquired the sent data with the control panel and evaluated the rms value of the data sent over a time interval of 10 periods. naming nq the quantized values of vad acquired during the characterisation and vfs the full scale of the a/d (i.e. 3.3 v), the estimation of nq, according to the least-squares method (3), is 𝑁�̂� = 212(𝐺𝑽𝑰𝑵 + 𝑏) 𝑉𝐹𝑆 ≡ 𝛼 ∙ (𝐺𝑽𝑰𝑵 + 𝑏) . (8) considering from the dsp datasheet that the uncertainty introduced by the a/d is uq = 2 lsb (which takes into account the quantisation and inl), we have 𝑢(𝑁𝑄 ) = √( 𝜕𝑵𝑸 𝜕𝐺 𝑢(𝐺)) 2 + ( 𝜕𝑵𝑸 𝜕𝑽𝑰𝑵 𝑢(𝑽𝑰𝑵)) 2 + 𝑢𝑄 2 . (9) similar to equation (6), we calculate the linearity error of the quantized values eq as (a) (b) figure 3. linearity error of the spr channels (a) for the analogue front end and (b) for the overall acquisition system. error bars represent uncertainty in the data. (a) (b) figure 4. linearity error of the ecg channel (a) for the analogue front end and (b) for the overall acquisition system. error bars represent the uncertainty on the data. acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 60 𝑬𝑸 = 𝑵𝑸 − 𝑵�̂� = 𝑵𝑸 − 𝛼𝐺𝑽𝑰𝑵 − 𝛼𝑏 (10) and the uncertainty on eq as 𝑢(𝑬𝑸) = √[𝑢(𝑵𝑸) − 𝛼𝐺 ∙ 𝑢(𝑽𝑰𝑵)] 2 + [𝛼𝑽𝑰𝑵 ∙ 𝑢(𝐺)] 2 + [𝛼𝑢(𝑏)]2 . (11) regarding the spr channels, using equation (8) to equation (11), the linearity error is in the order of 0.7 % fs, corresponding to 70 v on vin. figure 3(b) shows the linearity behaviour of the quantised values. the error bars represent the uncertainty in equation (11). linearity and its uncertainty are in the same order as those estimated for the analogue front end. regarding the ecg channel, using equation (8) to equation (11), the linearity error is in the order of 0.7 % fs, corresponding to 18 v on vin. figure 4(b) shows the linearity behaviour of the quantised values; and the error bars represent the uncertainty in equation (11). 3.1.2. bandwidth for bandwidth characterisation, the waveform generator has been set to generate a sinusoid with an amplitude of 3.3 vpp and a frequency varying from 0.1 hz up to 1 khz, with 10 points per decade, logarithmically spaced. the attenuator described in the previous subsection is placed at the output of the generator. the frequency response of the gain is obtained by evaluating the rms value of the output over 10 periods with respect to the rms value of the input. again, the oscilloscope acquires the rms value of vad, and the developed control panel acquires the rms value of the transmitted data. figure 6 shows the bandwidth behaviour of the spr channels. the solid line represents the analogue front end, and the square markers represent the data sent to the laptop. the horizontal line represents the -3 db amplitude, which defines the bandwidth. the upper cut-off frequency is 40 hz, and the lower cut-off frequency is lower than 0.1 hz, which is the limit of the waveform generator (according to the design, it is 0.08 hz). figure 5 shows the bandwidth of the ecg channel, the solid line represents the analogue front end, and the markers represent the entire system. the bandwidth results in the range [0.08, 160] hz. 3.2. characterisation with synthesised ecg for the hrv analysis, accurate recognition of the r-peaks is crucial. for this reason, one of the main problems related to hrv analysis is the jitter introduced by the sensor analogue bandwidth and the sample rate. in this subsection, we show the results of an rr-tachogram acquired by the proposed sensor when the input is a synthesised ecg provided by an arbitrary waveform generator. for the purposes of comparison, the data is simultaneously acquired and saved by an oscilloscope from the sensor and from the generator, used as references. the synthesised ecg has a frequency of 1 hz and an amplitude of 2 vpp. the input voltage (after the attenuator) thus results in 3 mvpp, and the analogue front-end output is 2 vpp. figure 7 shows, as an example, a comparison between the pqrst wave acquired by the oscilloscope from the generator and from vad. the r-peak is shifted because of the analogue bandwidth of the sensor. in order to quantify the jitter introduced by the sensor, the signal was acquired for 2 mins, and the data of the generator and sensor were processed using kubios software [21]. figure 8 shows a comparison between the rr-tachogram acquired by the oscilloscope from the sensor and from the generator. with the same input, the jitter on the r-peak recognition from the proposed sensor is slightly higher than the reference generator. the standard deviation of the tachogram in figure 8 is 900 µs for the generator and 1.9 ms for the sensor. the correlation between the two signals is lower than 4 %; therefore, we can conclude that the jitter introduced by the sensor is in the order of √1.92 + 0.92 ms ≈ 2 ms. the quantised data acquired by the overall system exhibits similar behaviour. it is slightly worse, presenting a jitter in the order of 2.3 ms; in any case, the jitter introduced by the sensor is close to the sample rate i.e. the lower bound for jitter performance. figure 5. bode plot of the gain relative to the ecg channel. continuous line: analogue front end. markers: overall acquisition system. the horizontal line represents the -3 db cut off. figure 6. bode plot of the gain relative to the spr channels. continuous line: analogue front end. markers: overall acquisition system. the horizontal line represents the -3 db cut off. figure 7. pqrst wave acquired at the generator (dashed line) and at the output of the analogue front end (solid line). it is possible to see a small jitter introduced by the analogue bandwidth. acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 61 3.3. signal acquisition from hands in motion figure 9(a) shows a comparison between spr1 and spr2 when the hands are moving in the time interval of 2 mins. in particular, the rectangles evidence a small movement of the right hand and a strong movement of the left hand, with high disturbance of electrode contact. in the case of the small movement, the hand was just grasping an object; in the case of the strong movement, the person was tightening their fist against the electrode. in figure 9(a), it is shown that without the movement, both channels are identical (trace in the interval 0 – 60 s), and when tightening the fist (trace in the interval 80 – 95 s), the artefact can be up to five times the actual spr signal. with a single-channel eda sensor, these peaks could be identified as stress peaks instead of motion artefacts. with a dual spr channel, instead, it is quite simple, with several algorithms (e.g. independent component analysis, principal component analysis, adaptive filtering) used to discriminate artefacts from sympathetic pulses. using adaptive filtration techniques, for example, it is possible to improve the accuracy to 10 %. figure 9(b) shows the ecg, in the same session over the same time interval, acquired from the hands. even if the trace is a little bit noisier with respect to traditional derivations on the chest, the pqrst wave is easily visible and processable. figure 9(c) in fact, shows the extracted hr without any artefact due to noise or movements. 3.4. comparison with chest acquisition in previous subsections, we have characterised the performance of the sensor itself, neglecting the uncertainty due to the ecg-on-hands methodology. in this subsection, we compare the ecg readout from the proposed sensor on the hands, with a reference instrument acquiring ecg on the chest. in order to verify the performance of the proposed sensor, the ecg has been acquired simultaneously from the hands and from the chest, using the shimmer system as a reference [23]. a shimmer 3 exg unit has been configured, with sample rate 1024 sa/s posing three electrodes on the chest, at standard ecg positions v1 and v2, and the reference electrode was posed on the lower right side of the chest. the proposed sensor was connected as shown in figure 1, and the acquisition ran simultaneously with the shimmer system. the acquisition duration has been set to five minutes, which is typical for hrv analysis. during acquisition, no artefacts were recorded on both systems. figure 10(a) shows the ecg traces from the chest (red line) and from the hands (blue line). the ecg acquired from the hands is a little bit noisier (due to the acquisition being far from the heart), but the r-peaks are well identifiable on the trace. for this reason, it is inconvenient to implement other hardware/firmware filters on the sensor that could increase the jitter. filtering instead in post processing, the noise can be easily removed without introducing jitter by filtering in forward and reverse directions. figure 10(b) shows the extracted tachograms from the chest (red line) and from the hands (blue line). from a qualitative point of view, it is plain to see that all the r-peaks have been identified without artefacts. from a quantitative point of view, we compared the hrv analysis computed by kubios from both systems. in table 1, we show the obtained results. table 1 compares several standard parameters for hrv analysis, like the mean rr interval, the standard deviation normal-to-normal (sdnn), the mean hr, and the root mean square of successive differences (rmssd). as evidenced, even if the signal on the hands is noisier, the results differ less than 4 %. in addition, we evaluated the difference between the traces shown in figure 10(b) and obtained a standard deviation on the difference of 4 ms. this result has the same order of magnitude of the sample rate; therefore, the ecg figure 8. rr-intervals acquired from the sensor (dashed line) and from the generator (solid line). (a) (b) (c) figure 9. (a) motion artefacts on the spr channels; combining the two signals, it is possible to discriminate the sympathetic pulses; (b) ecg acquired from the hands; the box is a zoom of the ecg signal; (c) extracted heart rate. acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 62 on-hands methodology can provide results with an accuracy comparable to the usual measurements on the chest. 4. conclusion the paper describes the design and characterisation of a wearable sensor for the simultaneous measurement of dualchannel eda and ecg from the hands. thanks to the dualchannel measurements, it is possible to improve the removal of motion artefacts that may affect eda measurements. the choice of ecg measurement from the hands improves the invasiveness of usual ecg sensors (which require the user to wear a vest or a chest band), and it is obtained using the same electrodes used for eda. the characterisation of the sensor showed high-level performance in terms of linearity, bandwidth, and jitter, even if the ecg is acquired where the signal-to-noise ratio is much lower than usual chest derivations. references [1] d. s. lee, t. w. chung, b. g. lee, stress events detection of driver by wearable glove system, ieee sensors journal 17, 1 (2017) pp. 194-204. 398 doi:10.1109/jsen.2016.2625323 [2] n. yoshimura, o. koga, y. katsui, y. ogata, h. kambara, y. koike. decoding of emotional responses to user-unfriendly computer interfaces via electroencephalography signals, acta imeko 6 (2017) 2, pp. 93-98, doi: https://dx.doi.org/10.21014/acta_imeko.v6i2.383. 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[15] a. porta, e. tobaldini, s. guzzetti, r. furlan, n. montano, t. gnecchi-ruscone, assessment of cardiac autonomic modulation during graded head-up tilt by symbolic analysis of heart rate variability, am j physiol heart circ physiol 293, 1 (2007) pp. 702-708. [16] t. ziemssen, m. reimann, j. gasch, h. rudiger, trigonometric regressive spectral analysis: an innovative tool for evaluating the table 1. comparison of hrv analysis performed from the proposed sensor and the shimmer system. proposed sensor shimmer % mean rr (ms) 739.85 736.58 0.44 sdnn (ms) 49.545 48.904 1.2 mean hr (bpm) 81.453 81.808 -0.43 rmssd (ms) 23.825 22.976 3.6 (a) (b) figure 10. (a) comparison between ecg acquired on the chest (red line) and on the hands (blue line); (b) rr tachogram extracted from the chest (red line) and from the hands (blue line). https://dx.doi.org/10.21014/acta_imeko.v6i2.383 https://doi.org/10.1109/mhs.2015.7438258 https://doi.org/10.1109/ivs.2016.7535570 https://doi.org/10.1016/j.measurement.2013.06.024 https://doi.org/10.1016/j.measurement.2016.06.007 https://doi.org/465%2010.1111/srt.12397 acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 63 autonomic nervous system, j neural transm 120, 1 (2013) pp. 2733. [17] h. rudiger, l. klinghammer, k. scheuch, the trigonometric regressive spectral analysis-a method for mapping of beat-to-beat recorded cardiovascular parameters on to frequency domain in comparison with fourier transformation, computer methods and programs in biomedicine 58 (1999) pp. 1-15. [18] a. l. smith, h. owen, k. j. reynolds, heart rate variability indices for very short-term (30 beat) analysis. part 1: survey and toolbox, j clin monit comput 27 (2013) pp. 569-576. [19] a. l. smith, h. owen, k. j. reynolds, heart rate variability indices for very short-term (30 beat) analysis. part 2: validation, j clin monit comput 27 (2013) pp. 577-585. [20] rigol technologies inc., mso2000a/ds2000a series digital oscilloscope user's guide, august 2015, http://int.rigol.com/file/techdoc/20160830/mso2000a&ds 2000a_userguide_en.pdf [21] kubios, kubios hrv software http://kubios-hrv.software.informer.com/ [22] w. boucsein, electrodermal activity, springer, new york dordrecht heidelberg london, 2012, isbn 978-1-4614-1126-0 https://www.springer.com/us/book/9781461411253 [23] shimmer realtime technologies ltd, shimmer user manual, 2017, https://www.shimmersensing.com/ http://int.rigol.com/file/techdoc/20160830/mso2000a&ds2000a_userguide_en.pdf http://int.rigol.com/file/techdoc/20160830/mso2000a&ds2000a_userguide_en.pdf http://kubios-hrv.software.informer.com/ https://www.springer.com/us/book/9781461411253 https://www.shimmersensing.com/ acta imeko, title acta imeko issn: 2221-870x december 2017, volume 6, number 4, 54-60 acta imeko | www.imeko.org december 2017 | volume 6 | number 4 |54 estimation of the signal component from random equivalent and non-coherent sampling measurements dušan agrež university of ljubljana, faculty of electrical engineering, trzaska 25, 1000 ljubljana, slovenia section: research paper keywords: random equivalent time sampling; non-coherent condition; signal component; interpolated dft; parametric estimation citation: dušan agrež, estimation of signal component from random equivalent and non-coherent sampling measurements, acta imeko, vol. 6, no. 4, article 8, december 2017, identifier: imeko-acta-06 (2017)-04-08 section editor: alexandru salceanu, technical university of iasi, romania received may 5, 2017; in final form june 27, 2017; published december, 2017 copyright: © 2017 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: dušan agrež, e-mail: dusan.agrez@fe.uni-lj.si 1. introduction high-speed digitizing of waveforms plays an important part in broadband measurements and one of the possibilities to increase the sampling frequency is the equivalent time sampling with reconstruction of the signal in the prolongation of the acquisition time [1]. in many applications of practical interest, we often wish to estimate signal parameters and reconstruct a signal from incomplete time samples, which can be attained non-uniformly and randomly as in the case of random equivalent sampling [2], [3]. unlike the uniform sampling case, where the aliases are simply periodic replicas of the signal’s original spectrum when the sampling theorem is fulfilled, random sampling theory shows that sparsing and randomization of the sampling intervals shape the aliases into a noise floor in the sampled spectrum [4]. by sampling signals non-uniformly, the average sample rate can be more than a magnitude lower than the nyquist rate, provided that these signals have a relatively low information content as measured by the sparsity of their spectrum. random equivalent time sampling is a technique which is used for achieving high sample rates with reduction of the equivalent sampling interval eqs,t . it is applicable only for periodic signals. it is usually used in digital oscilloscopes and some other high-speed sampling applications [5], [6]. the general idea behind equivalent time sampling is shown in figures 1 to 6. figure 1 shows one sinusoidal signal component g, which can be generally expressed as ( ) ( )∑ −= +⋅ 1 0 eqs,eqs, 2sin m m mmm tnfatnw ϕπ ( mf , ma , and mϕ are frequency, amplitude, and phase of the particular component). the signal is presented by randomly sampled points with low ‘duty ratio’ nnd rsrs = , where rsn is the number of random samples from a total of n samples in the non-coherent measurement interval .eqs,m ntt = the basic raster of n samples is equidistant with the sampling interval eqs,t [7]. the whole acquired signal contains 3.8 cycles of the sine 8.3eqs, =⋅=∆= tnfff mmmq on 512=n points and samples are captured randomly to the triggering point (signal abstract estimations of the signal component parameters in the case of random equivalent time sampling and under non-coherent sampling condition comprise two main error contributions: spectral leakage effect due to non-coherency and additional noise due to the randomization of sampling intervals. in the estimation procedure the non-parametric interpolated dft approach has to be used first to estimate the component frequency and, after that, an iterative 4-parametric sine-fit algorithm should be used for other component parameters (amplitude and phase). their estimations are possible when the duty ratio of random samples from the total samples in the non-coherent measurement interval is above 0.1. with these duty ratios of random samples it is possible to achieve error levels of 0.001 bins of the frequency estimations in relation to the estimation on full ensemble of points. acta imeko | www.imeko.org december 2017 | volume 6 | number 4 |55 zero crossing with phase deg0 ) in two sweeps with fix sampling raster eqs,s 26 tt = . in figure 1 the samples of the first sweep are represented by solid lines and the samples of the second sweep by dotted lines). in this figure the sampling theorem is fulfilled with a relative sampling frequency of mseqs,ss 27.1926512 qq ⋅>===∆= ttnff . although the samples are randomly captured (essentially the first sample in one sweep) the systematic relation between the signal frequency and sampling frequency is clearly visible in the spectrum of the equivalent sampled signal (figure 2 line b) where we have well-known replicas of signal components with frequency mq around integer values of the sampling frequency mk qqq ±=′ s ( ...,2,1=k ). with increasing the number of sweeps the duty ratio approaches 1rs →d and the component rises from the noise floor and replicas diminish (figure 2 line a). if the analog-to-digital conversion is relatively slow compared to the signal period, the sampling raster relatively increases and the sampling theorem could not be fulfilled (see figure 3: →= eqs,s 85 tt mseqs,s 20.685512 qq ⋅<=== ttn ). to fill the empty points by signal values more sweeps are needed for the same duty ratio (in figure 3: six sweeps are needed for 07.0rs ≈d ) but the systematic relation between sampling frequency and component frequency is still recognized (figure 4 line b). if one reduces the number of samples of one sweep to 1, what is the limit case, we get ‘pure’ random sampling (figure 5: →= eqs,s 512 tt ms 21512512 qq ⋅<<== ). due to relative sampling frequency of 1s =q all amplitude dft coefficients are contaminated by a systematic sampling effect mk qq ±=′ ( ...,2,1=k ) and beside noise of the acquisition signal we have also the ‘noise’ of random sampling (figure 6 line b). this fact has to be considered in estimations of the signal components. estimations of the component signal parameters are distorted due to the missing samples in the time domain [8] or due to large random sampling noise in the frequency domain (figure 6 line b) where snr for the investigated component is very low when rsd is low. considering also the non-coherent sampling condition with leakage effect, where the measurement interval mt is not an integer multiple of the signal period mmmm itt δq +== m , parameter estimations are not an easy task [9], [10]. the leakage signal energy additionally spreads figure 1. signals of random sampling: a–original signal, b–random sampled signal; 512=n , 7.19s =q , 8.3=mq , 1=a , deg0=ϕ , 07.0rs ≈d . figure 3. signals of random sampling: a – original signal, b – random sampled signal; 512=n , 0.6s =q , 8.3=mq , 1=a , deg0=ϕ , 07.0rs ≈d . figure 4. spectra of signals from figure 3: a – original signal, b – random sampled signal, c – reconstructed signal; 0.6s =q , 8.3=mq , 07.0rs ≈d . figure 5. signals of random sampling: a – original signal, b – random sampled signal; 512=n , 1s =q , 8.3=mq , 1=a , deg0=ϕ , 07.0rs ≈d . figure 2. spectra of signals from figure 1: a – original signal, b – random sampled signal, c – reconstructed signal; 7.19s =q , 8.3=mq , 07.0rs ≈d . acta imeko | www.imeko.org december 2017 | volume 6 | number 4 |56 along the frequency axis and disturbs the dft coefficients (figure 6) [11], which cannot be removed as in the case of compressive sensing with the coherent sampling model [12], [13]. 2. analysis for parameters’ estimations accurate measurement of a multi-sine waveform is a classic problem in spectral analysis. it is well known that nonparametric algorithms based on the discrete fourier transform (dft) have to contend with spectral leakage, which affects both amplitude estimation accuracy and frequency resolution [14]. these methods usually estimate the signal component frequency as key parameter by searching for the maximum in the spectrum by using dft [15]. advanced windowed functions and interpolation schemes have been applied to compensate for the effects of leakage caused by incoherent sampling and finite frequency resolution problems [16], respectively. it is also common knowledge that approaches based on a parametric signal model can achieve much better frequency resolution, but this is obtained at the price of greater complexity [17]. for estimations of the signal parameters from sparse sampled periodic signals two methods are investigated and compared: the non-parametric interpolated dft and the sinefit parametric approach [18]. in the first approach, one can choose the number of dft coefficients in interpolation and the window function w(*) [16]. in the second approach, a number of parameters has to be defined: non-iterative 3parametric sine-fit estimation when the component frequency is known or 4-parametric estimation when also the frequency has to be estimated in the iterative procedure [17]. in the first approach typically two or three largest dft coefficients )1( −mig , )( mig , )1( +mig and rife-vincent windows class i with suitable order p are used [16]: two-point frequency estimation where s is a sign of displacement mδ : ( ) ( ) ( ) ( ) ( )sigig igpsigp sii mm mm mmmm ++ ⋅−+⋅+ +=+= 1 2 δq (1) three-point frequency estimation: ( ) ( ) ( ) ( ) ( ) ( )121 11 13 +++− −−+ ⋅++=+= mmm mm mmmm igigig igig pii δq (2) one-point amplitude estimation: ( ) ( ) ( ) ( )mpl m m m p m iglp a ∏ = −⋅⋅= 1 22 2 1 πsin π !2 2 2 δ δ δ  (3) three-point amplitude estimation: ( ) ( ) ( ) ( ) ( ) ( )( )121 πsin π !22 2 2 1 1 22 2 3 +++−⋅ −⋅⋅ + = ∏ + = mmm p l m m m p m igigig l p a δ δ δ  (4) three-point phase estimation with the arguments of the three largest local dft coefficients ( )[ ]mi igm arg=ϕ : ( ) ( ) 2 π 3 2 6 141 11 3 +− +++− = +− mimiimm a mmm δϕδϕϕδ ϕ (5) since the sparse sampling considerably increases the noise in the frequency domain (figure 6) it is not suitable to increase the number of interpolation dft coefficients in interpolations due to their distortions. figure 7 shows the mean values of the absolute relative errors of the amplitude dft coefficients ( ( )mige rs , ( )1rs +mige , and ( )1rs −mige ) around the local peak in relation to the original spectrum on 512=n samples normalized by the duty ratio ( ) ( ) rs * digig ⋅= in (6) (see figures 2,4,6 line c). ( )( ) ( ) ( ) ( )* * rs rs ig igig ige − = (6) analyses of figure 7 show that it is better to use two-point estimation for frequency to determine the position of the measurement component mmm i−=qδ between dft figure 6. spectra of signals from figure 5: a – original signal, b – random sampled signal, c – reconstructed signal; 1s =q , 8.3=mq , 07.0rs ≈d . i) 8.3=q , 2.0−=mδ . ii) 4.4=q , 4.0=mδ . figure 7. mean values of the absolute relative errors of the amplitude dft coefficients; a – ( )ige rs , b – ( )1rs +ige , c – ( )1rs −ige , 1=a , deg0=ϕ , 100repeat =n trials. acta imeko | www.imeko.org december 2017 | volume 6 | number 4 |57 coefficients )( mig and )1( +mig surrounding the component m as the key parameter, due specific distortion in sparse sampling. this conclusion can be supported also by analyzing the error characteristics using two and threepoint estimations (figure 8) [16]. figure 8 shows collected minimal curves of the maximal bias errors of the frequency estimations due to phase changing with the two-point (idft-2p) and three-point (idft-3p) interpolations of the dft using rv-i windows and the corresponding orders 6...1=p with which these minimal values have been obtained. it can be seen that bias errors are on the same level with two (1) or three-point (2) estimations but window order p has to be increased with two-point estimation. whenever there is no information of frequency content of the signal, the component frequency estimation is the first step and the key to estimate other parameters (amplitude, phase, and sometimes also damping). this is valid for both the parametric and the non-parametric approach. results in figures 9 and 10 confirm above presumptions, where the frequency estimation error is defined as ( ) trueest. qqq −=e . it can be noticed that the relative number of random samples nnd rsrs = has to increase at least to 0.1 when the frequency estimation errors step-like drop below 0.1 bin but this is still far from the target values when we have the whole ensemble of samples (figures 9 line a and 10 line a). with increasing rsd errors slowly drop but the number of iterations in the process of randomly adding additional samples relatively substantially increase (for 512=n we typically need 2000 iterations to achieve 98.0rs ≈d ). due to the randomness, some positions may be repeatedly sampled, whereas other positions are left blank. the parametric estimation approach (figure 9 line c: 4parametric estimation with 9 iterations) gives better results than the non-parametric idft approach (figures 9 line b and 10 line d) but the frequency has to be estimated first (typically by symmetric 3-point estimation using the hann window 1=p and (2)). if we don’t have a good initial value of the component frequency the parametric approach gives useless results (figure 10 line c). after the point where the frequency component exceeds the random sampling noise (figure 6 line b: 074.0rs =d ) twopoint idft estimation (figures 9 line d and 10 line d) gives lower errors than the three-point estimation and especially in the case of large non-coherency (figure 10 line d: 4.0=mδ ). if the sampling frequency relatively increases to the signal frequency as in the case in figures 1 and 2 ( 28.37.19ms >=qq ) the ‘random noise’ effect around the component diminishes and the non-parametric idft approach gives better results at lower values of rsd (figure 11: lines b, d). the same behavior of the estimation errors can be noticed also for the amplitude estimations (figures 12 and 14: ( ) 1trueest. −= aaae ) and phase estimations (figure 13: ( ) trueest. ϕϕϕ −=e ) where the same test conditions were used as in figures 9 and 10. errors drop very fast where rsd starts increasing and after the point around 1.0rs ≈d the decrease of the errors slowed down. figure 9. mean values of the absolute errors of the frequency estimation; a – original signal – idft(f-3p), b – random sampled signal – idft(f-3p), c – random sampled signal – 4-par. sine-fit (start with estimated f-3p), d – random sampled signal – idft(f-2p); 1s =q , 8.3=q , 1=a , deg0=ϕ , 100repeat =n trials. figure 10. mean values of the absolute errors of the frequency estimation; a – original signal – idft(f-3p), b – random sampled signal – idft(f-3p), c – random sampled signal – 4-par. sine-fit (start with 2=q ), d – random sampled signal – idft(f-2p) ; 512=n , 1s =q , 4.4=q , 1=a , deg0=ϕ , 100repeat =n trials. p 5 8 4 5 6 7 1 2 3 q )(max qe 2p -idft 3p -idft 2p -idft 3p -idft 1010− 010 210− 410− 610− 810− 1 3 figure 8. minimal curves of the maximal errors of the frequency estimations with the two(idft-2p) and three(idft-3p) interpolations of the dft using rv-i windows and the corresponding orders 6...1=p . acta imeko | www.imeko.org december 2017 | volume 6 | number 4 |58 the iterative 4-parametric estimation (figures 12 line c and 13 line c) gives a better result than one-step 3-parametric estimation (figure 13 line d) when the sampling frequency is relatively low but the initial frequency has to be very well estimated. if we fix the value of the estimated frequency by idft the one step 3-parametric sine-fit estimation gives the error values as with the non-parametric approach (figure 13 line d). 3. experimental results to demonstrate the behavior of estimations of the periodic signal parameters in the case of random sampling and under non-coherent sampling condition in a real measurement environment a digitizing oscilloscope (do), agilent 54501a [19] was used to randomly acquire signals. the signal was generated by a stable voltage generator, keysight 33500b [20], with a nominal sine voltage of amplitude v900.0v =a . first, the signal frequency was fixed to mhz9.1v =f and with time constant of divns200time =c and 10 divisions in the x-axis, the do acquires and shows 3.8 periods of the signal in the duration of μs2m =t ( 3.8μs2mhz9.1mv =⋅=⋅= tfq ). since the do display uses 500=n points to represent the signal in the x-axis the equivalent sampling frequency was mhz250meqs, == tnf what is 25-times larger than the do real sampling frequency mhz10s =f with 2025500eqs,ss ==⋅= ffnq . sampling points were acquired with the multiple ‘randomly triggered’ sweeps to fill the points for display presentation [19]. to fulfil all 500=n points typically 150 sweeps were needed and this acquisition procedure was controlled remotely by scpi commands: :tim:mode sing and :run [19]. figures 15 (frequency estimation) and 16 (amplitude figure 11. mean values of the absolute errors of the frequency estimation; a – original signal – idft(f-3p), b – random sampled signal – idft(f-3p), c – random sampled signal – 4-par. sine-fit (start with estimated f-3p), d – random sampled signal – idft(f-2p) ; 512=n , 7.19s =q , 8.3=q , 1=a , deg0=ϕ , 100repeat =n trials. figure 12. mean values of the relative errors of the amplitude estimation; a – original signal – idft(f-3p, a-3p), b – random sampled signal – idft(f-3p, a-3p), c – random sampled signal – 4-par. sine-fit (start with estimated f3p), d – random sampled signal – 3-par. sine-fit (f-3p), e – random sampled signal – idft(f-2p, a-1p) ; 512=n , 1s =q , 8.3=q , 1=a , deg0=ϕ , 100repeat =n trials. figure 13. mean values of the absolute errors of the phase estimation; a – original signal – idft(f-3p, ph-3p), b – random sampled signal – idft(f-3p, ph-3p), c – random sampled signal – 4-par. sine-fit (start with estimated f3p), d – random sampled signal – 3-par. sine-fit (f-3p) ; 512=n , 1s =q , 8.3=q , 1=a , 100repeat =n trials. figure 14. mean values of the relative errors of the amplitude estimation; a – original signal – idft(f-3p, a-3p), b – random sampled signal – idft(f-3p, a-3p), c – random sampled signal – 4-par. sine-fit (start with 2=q ), d – random sampled signal – 3-par. sine-fit (f-3p), e – random sampled signal – idft(f-2p, a-1p); 512=n , 1s =q , 4.4=q , 1=a , deg0=ϕ , 100repeat =n trials. figure 15. mean values of the maximal absolute errors of the frequency estimation; a – target estimation error, b – random sampled signal – idft(f3p), c – random sampled signal – 4-par. sine-fit (start with estimated f-3p), d – random sampled signal – idft(f-2p); 500=n , 20s =q , 8.3=q , deg0=ϕ , 150sweeps =n . acta imeko | www.imeko.org december 2017 | volume 6 | number 4 |59 estimation) show estimation errors. the reference value for the specific parameter is the one attained with estimation on the whole number of the filled points 1rs =d after 150 sweeps with 3-point estimations ((2) and (4)). estimation errors are defined as ( ) 1,3p,est rsrsrs =−= ddde qqq for the relative frequency (figure 15) and as ( ) 11,3p,est rsrsrs −= =ddd aaae for the amplitude (figure 16). estimation error behaviors in figure 15 confirm simulation results from figures 9 and 11 with a very fast decrease at low values of 1.0rs <d to the target level (figure 15 – line a: ( ) 310 rs −=qde ) and after increasing 1.0rs >d errors drop slowly to the final estimation result with the whole ensemble of points at 1rs =d . the same estimation error behaviors can be noticed also in the case of the amplitude estimation (figure 16). the best results are obtained by parametric sine-fit estimation procedures where the initial frequency was estimated first by the 3-point interpolated dft approach (2). to analyze the estimation error behavior when the relative frequency is changing the testing conditions from figure 15 were enhanced from one frequency of 8.3=q to a frequency range 64.1 ÷=q with frequency increments 05.0=∆q (figure 17). for this testing the signal frequency was changed from mhz7.0minv, =f to mhz0.3maxv, =f with steps of khz25v =∆f and at each frequency the estimation error was defined as difference between estimation value around 2.0rs =d and estimation value when 1rs =d (figure 17: ( ) 1,est2.0,est1/2.0 rsrsrs === −= ddde qqq ). the frequency estimation errors in figure 17 show that by changing the relative frequency the relation of errors between estimations with low sampling duty cycle and estimations with the whole ensemble of points remain on the same level. 4. conclusions estimations of the signal component parameters from random equivalent and non-coherent sampling periodic signals comprise additional problems of random sampling noise. the non-coherency is more problematic due to frequency leakage than in the case of the coherent sampling where the problem of frequency estimation is much reduced like in approaches of compressive sensing. to achieve the lowest estimation errors the non-parametric idft estimation of frequency has to be used first for detection of the component frequency in both the parametric and non-parametric approach and after that the iterative parametric method should be used for other parameters when the sampling frequency is relatively lower than the nyquist frequency and the sampling procedure becomes pure random. simulations and experimental results show that the estimations of the component frequency, amplitude and phase are possible when the duty ratio of the random samples from the total number of samples in the non-coherent measurement interval is very low around 0.1. references [1] y.-c. jenq, perfect reconstruction of digital spectrum from nonuniformly sampled signals, ieee trans. instrum. meas. 46(3) (1997) pp. 649-652. 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[10] d. bao, p. daponte, l. de vito, s. rapuano, frequencydomain characterization of random demodulation analog-to figure 16. mean values of the maximal relative errors of the amplitude estimation; a – target estimation error, b – random sampled signal – idft(f3p, a-3p), c – random sampled signal – 4-par. sine-fit (start with estimated f-3p), d – random sampled signal – 3-par. sine-fit (f-3p), e – random sampled signal – idft(f-2p, a-1p); 500=n , 20s =q , 8.3=q , deg0≈ϕ , 150sweeps =n . q ( )q1/2.0rs =de 1 2 3 4 10-4 10-2 1 b 102 6 c a d 5 figure 17. mean values of the maximal absolute errors of the frequency estimation; a – target estimation error, b – random sampled signal – idft(f3p), c – random sampled signal – 4-par. sine-fit (start with estimated f-3p), d – random sampled signal – idft(f-2p) ; 500=n , deg0≡ϕ , 150sweeps =n . http://ieeexplore.ieee.org/search/searchresult.jsp?searchwithin= http://ieeexplore.ieee.org/search/searchresult.jsp?searchwithin= http://ieeexplore.ieee.org/xpl/articledetails.jsp?arnumber=6638701&newsearch=true&querytext=random%20sampling acta imeko | www.imeko.org december 2017 | volume 6 | number 4 |60 information converters, acta imeko. 4(1) (2015) pp. 111120. 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[17] p. stoica, r. moses, spectral analysis of signals, prentice-hall, nj, usa (2005). [18] standard ieee-1241-2010, ieee standard for terminology and test methods for analog-to-digital converters (2011). [19] agilent technologies, 54501a digitizing osciloscope programming reference (1989). 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can be opened with acrobat and adobe reader 5.0 and later.) >> /namespace [ (adobe) (common) (1.0) ] /othernamespaces [ << /asreaderspreads false /cropimagestoframes true /errorcontrol /warnandcontinue /flattenerignorespreadoverrides false /includeguidesgrids false /includenonprinting false /includeslug false /namespace [ (adobe) (indesign) (4.0) ] /omitplacedbitmaps false /omitplacedeps false /omitplacedpdf false /simulateoverprint /legacy >> << /addbleedmarks false /addcolorbars false /addcropmarks false /addpageinfo false /addregmarks false /convertcolors /converttocmyk /destinationprofilename () /destinationprofileselector /documentcmyk /downsample16bitimages true /flattenerpreset << /presetselector /mediumresolution >> /formelements false /generatestructure false /includebookmarks false /includehyperlinks false /includeinteractive false /includelayers false /includeprofiles false /multimediahandling /useobjectsettings /namespace [ (adobe) (creativesuite) (2.0) ] /pdfxoutputintentprofileselector /documentcmyk /preserveediting true /untaggedcmykhandling /leaveuntagged /untaggedrgbhandling /usedocumentprofile /usedocumentbleed false >> ] >> setdistillerparams << /hwresolution [2400 2400] /pagesize [612.000 792.000] >> setpagedevice template for an acta imeko event paper acta imeko issn: 2221-870x july 2017, volume 6, number 2, 80 acta imeko | www.imeko.org july 2017 | volume 6 | number 2 | 80 editorial to additional papers paolo carbone university of perugia, italy section: editorial citation: paolo carbone, editorial to additional papers, acta imeko, vol. 6, no. 2, article 14, july 2017, identifier: imeko-acta-06 (2017)-02-14 section editor: paul regtien, measurement science consultancy, the netherlands received july 14, 2017; in final form july 14, 2017; published july 2017 copyright: © 2017 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: paolo carbone, email: paolo.carbone@unipg.it dear reader, three additional papers are closing this issue. the first one covers the interesting area of human computer interaction. it is entitled ‘a preliminary approach to study the behaviour of human fingertip at contact via experimental test and numerical model’ and it is authored by maria laura d’angelo et al. the paper describes the measurement results associated with the deformation of the fingertips, as a function of the exercised force on a flat surface. it includes information on the contact area and of the fingertip pressure contour. both theoretical and experimental results are described. the next paper, authored by valeriy didenko, is entitled ‘improved evaluation of uncertainty for indirect measurement’. it covers the subject of uncertainty evaluation in indirect measurements and presents the analysis of several practical cases, occurring when using multi-channel adc-based acquisition and measurement systems. the last paper in this issue is authored by natsue yoshimura et al. it is entitled ‘decoding of emotional responses to userunfriendly computer interfaces via electroencephalography signals’. it contains a statistical analysis of emotional responses of users of unfriendly computer interfaces. the analysis is supported by electroencephalography derived data, performed by stressing users in target-reaching tasks. have a fruitful reading of this second issue of acta imeko in 2017! paolo carbone, editor-in-chief of acta imeko. editorial to additional papers performance of force standard machines with compensation of lever arm distortion acta imeko june 2014, volume 3, number 2, 19 – 22 www.imeko.org performance of force standard machines with compensation of lever arm distortion ulrich kolwinski, daniel schwind gtm gassmann testing and metrology, philipp reis strasse 6, 64404 bickenbach, germany section: research paper keywords: force; lever arm; calibration citation: ulrich kolwinski, daniel schwind, performance of force standard machines with compensation of lever arm distortion, acta imeko, vol. 3, no. 2, article 6, june 2014, identifier: imeko-acta-03 (2014)-02-06 editor: paolo carbone, university of perugia received february 13th, 2013; in final form december 16th, 2013; published june 2014 copyright: © 2014 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: this work was supported by measurement science consultancy, the netherlands corresponding author: daniel schwind, email: daniel.schwind@gtm-gmbh.com 1. introduction gtm's force standard machines (fsm) for higher loads use a lever system to amplify forces with high accuracy, by having strain controlled support of the lever. on each force introduction point of the lever, the bending moments are measured by means of strain gauges which are bonded to elastic hinges. force introduction points are the connection point of the masses (mass hinge), the support of the lever at its fulcrum (support hinge) and the connection point on the loaded side of the lever (load hinge). the sum of the bending moments of the hinges is the feedback value for a closed loop control system of the drive of the lever-side crosshead, which is always in active mode while amplifying the force, with a setpoint of zero. 2. description a fsm with lever amplification has been designed and developed by gtm gassmann testing and metrology, germany, and has been installed at the national physical laboratory, india in 2010. the machine consists of a deadweight part with 100 kn capacity, a lever part with 1 mn capacity, and a lever with a ratio of 1:10 supported by strain controlled hinges, see figure 1. the main construction principles of the 100 kn / 1 mn fsm of lever type with strain figure 1. 100 kn / 1 mn fsm of npl india. abstract the demand for lower uncertainties in force measurement is increasing. deadweight force standard machines are known to yield the lowest measurement uncertainty in force realization. however, their capacities cannot be extended indefinitely as they become too large in size and prohibitively expensive. lever amplification of a deadweight force is an option for higher capacities in force realization. this paper describes the performance of a force standard machine with a new method (patent pending) for automatic compensation of lever arm distortion in order to keep the amplification ratio constant. acta imeko | www.imeko.org june 2014 | volume 3 | number 2 | 19 controlled hinges were previously described in [1]. in order to reduce the expanded uncertainty of the lever amplification forces to the range of 10-5, the machine design was completely re-developed. many optimized or newly designed features have been incorporated in the machine. these include: 1) a machine frame with different supporting beams for the deadweight side and the lever side respectively, for higher stiffness. 2) further developed mass disks to avoid asymmetric distortions, to reduce the risk of mass contacts, to improve the smooth asymptotic load change and to reduce side-ways motion. 3) a lever designed for higher stiffness and higher stability of the lever ratio, which is covered by an enclosure to reduce ambient influences (figure 2). 4) the mass stack comprising 26 mass disks of various denominations, adjusted to the local ‘g’ value and air density with a relative uncertainty of 5·10-6 (k=2), so that all the forces in the ranges given in table 1 can be applied sequentially. 5) a four column hanger made of an aluminium and steel combination to realize a low lying balance point and to allow a temperature chamber installation. 6) a load-depending compensation of the lever arm distortion was realized with special strain controlled hinges. 3. lever arm distortion it is known that a lever arm will deform when loads are applied to its ends. this distortion reduces the effective lever length especially on the long side of the lever, see figure 3. the distortion of the lever arm directly influences the lever amplification ratio. as the generated force at the lever side of the machine is a result of the deadweight force multiplied by the amplification ratio, a systematic error in measurement results from any distortion of the lever arm which is not taken into consideration. in order to reduce this systematic error it is known from practise that some degree of mechanical compensation can be applied. however, great effort, usually by trial and error, is necessary for this. to improve this situation gtm has designed an automatic system to compensate the lever arm distortion, using strain controlled hinges (figure 4). the lever bearings with elastic hinges are completely free of friction and wear and define the pivot and load introduction points of the lever with the best possible precision, including long term. the ratio can be set exactly through the adjustment of the long lever arm length. the strain controlled hinges are equipped with measuring circuits for bending moments and additional measuring circuits for axial loads. the measurement of axial loads can be achieved relatively easily since it is not necessary to measure the axial force with extreme precision as only a fraction of the axial measurement signal is necessary for the compensation. the different strain controlled hinges must however have their sensitivity exactly adjusted. with the help of these strain controlled hinges in this type of machine the systematic effect of the lever ratio reduction caused by distortion can be compensated. the load-dependent distortion of the lever arm is compensated by the evaluation and sensitivity adjustment of the measured bending moments and axial forces. a fraction of the table 1. measuring ranges of the fsm. machine part deadweight lever measuring ranges 1-10 kn 2-20 kn 5-50 kn 10-100 kn 10-100 kn 20-200 kn 50-500 kn 100-1000 kn figure 3. lever arm reduction by distortion. figure 2. schematic view of the 1 mn fsm at npl(i). figure 4. gtm lever amplification system. lever mass stack support hinge movable crosshead control cabinet tare weight crosshead drive load suspe nsion gear base frame enclosure f m support hinge mass hinge load hinge acta imeko | www.imeko.org june 2014 | volume 3 | number 2 | 20 measured axial force signal is added to the sum signal of the bending moments. this is the corrected feedback value for the closed loop control system of the drive of the lever-side crosshead. as the corrected feedback value is generated in real time and automatically during operation of the machine no additional time and effort for calibration tasks is necessary. 4. description of compensation equations (1) and (2) describe the balanced system. in the balanced system the sum of all moments is zero 0 1 a b m m m g l f a m f a ρ ρ   = = ⋅ ⋅ − ⋅ + ⋅ + − ∆ ⋅∑     . (1) the force on the lever side is: 1 a b m ml f m g f a a ρ ρ   − = ⋅ ⋅ − ⋅ + − ∆    (2) with f: force on the lever side [n] m: mass on the deadweight side [kg] g: local acceleration of gravity [m/s²] ρa: density of air [kg/m³] ρm: density of mass [kg/m³] l : long lever arm [m] a : short lever arm [m] mb: sum of bending moments of lever support [n·m] ∆f: systematic force error of f due to distortion [n] a previously developed method was that the closed loop control system of the drive of the lever-side crosshead moves the crosshead in such a way that the sum signal of all bending signals of the elastic hinges is zero. the systematic force error out of the lever arm distortion had to be minimized by other mechanical means. for the new electronic method for the compensation of lever arm distortion a correction force fcorr is determined from the axial load signal and added to the sum of the bending signals. this is done in such a way that a modified sum of the bending moments m´b includes the correction moment mcorr which is generated by fcorr. corr corrm f a= ⋅ . (3) the modified sum of the bending moments m´b is ´b b corr b corrm m m m f a= + = + ⋅ . (4) the correction force fcorr has to be determined in such a way that it is nearly equal to the systematic force error δf. in that case the modified sum of the bending moments m´b compensates the systematic force error δf. ´ 1 a b m ml f m g f a a ρ ρ   − = ⋅ ⋅ − ⋅ + − ∆    . (5) from a practical point of view a small deliberate deflection of the hinges is used to achieve the compensation. the force which is needed for the deflection of the hinges is equal to the systematic force error. this compensation method can only be used with strain controlled hinges and not with knife edge bearing designs. the necessary correction force fcorr can be described by a polynomial function and can be calculated from the measured axial force of the elastic hinge. the measurement error of the axial force has a negligible effect on the measurement uncertainty of the fsm. for example: if a correction of 3.5·10-4 is necessary and the measurement error of the axial force is 0.5% the effect on the measurement uncertainty of the fsm is 1.75·10-6 . 5. self-calibration of lever forces in order to determine the dependency between load and correction force fcorr a self-calibration procedure of the fsm can be carried out. this procedure may be important for an independent traceability of the forces in national metrology institutes. for this purpose several force transfer standards of similar type can be calibrated with smallest measurement uncertainty in the deadweight part of the fsm. afterwards these calibrated force transfer standards are combined to buildup systems and measurements can be done in the lever amplification part of the fsm. the measurements can be performed with a single transducer and by use of build-upsystems with up to nine transducers (figure 5). examples of the measuring ranges of build-up-systems according to this procedure are as follows: 10 100kn (with a single transfer standard) 30 300kn (with a system of three transfer standards) 100 900kn (with a system of nine transfer standards) 6. linearity of lever forces an example for the linearity error of lever forces of a 1000 kn fsm with and without the described lever arm compensation is shown in figure 6. in this case the linearity errors are not identified with the self-calibration method, but by comparison with the calibration results of several force transfer standards calibrated at ptb. figure 5. build-up-system 900kn. acta imeko | www.imeko.org june 2014 | volume 3 | number 2 | 21 without any lever arm compensation the relative linearity error (means the relative deviation to ptb) increases up to more than 3·10-4 at 1000 kn. in case the electronic lever arm distortion compensation is used the linearity error is nearly constant and less than 5·10-5 across the whole measuring range. the electronic compensation of lever arm distortion reduces the linearity error significantly. 7. uncertainty of lever forces the achieved uncertainties for the lever amplification forces of the npl(i) fsm were calculated according the euramet/cg-04 calibration guide [3]. chapter 4.3 in the guide describes the uncertainties to be considered. the detailed uncertainty contributions for the fsm of npl(i) have been previously discussed in detail in [2]. the following table 2 presents the achieved uncertainty contributions for the smallest force on lever side and the calculated expanded relative uncertainty, where m: mass on deadweight side g: local acceleration of gravity ρa: density of air ρm: density of mass ∆fdw: uncertainty contribution due to measurements on deadweight side ∆fl: uncertainty contribution due to measurements on lever side ∆fdis: uncertainty contribution due to the distortion of the lever system ∆finst: uncertainty contribution due to the instability of the control system ∆fecc: uncertainty contribution due to an eccentric load introduction ∆fsens: uncertainty contribution due to the response sensitivity of the lever system 8. conclusions load-dependent compensation of a lever arm distortion can be achieved with special strain controlled hinges. this compensation reduces the measurement uncertainty of lever force standard machines. machines with this type of compensation of lever arm distortion can achieve relative expanded uncertainties of less than 9·10-5 on lever side. references [1] g. navrozidis, f. strehle, d. schwind, h. gassmann, “operation of a new force standard machine at hellenic institute of metrology”, imeko 2001, istanbul. [2] s.k. jain et al., “metrological characterization of the new 1mn force standard machine of npl india”, measurement 45 (2012) pp.590-596 [3] uncertainty of force measurements, calibration guide, euramet/cg-04/v.01, march 2010. figure 6. linearity error of lever force. table 2. uncertainty of 10kn on lever side. -4,00e-04 -3,00e-04 -2,00e-04 -1,00e-04 0,00e+00 1,00e-04 0 100 200 300 400 500 600 700 800 900 1000 force / kn li ne ar ity e rr or without compensation with compensation quantity estimate relative half width distribution divisor relative standard uncertainty sensitivity coefficient relative uncertainty contribution m 102,14678 kg gaussian 2,51e-06 1 2,51e-06 g 9,791245 m/s² 5,00e-07 rectangular √3 2,89e-07 1 2,89e-07 ρa 1,15 kg/m³ 1,50e-03 rectangular √3 8,66e-04 1,84e-04 1,59e-07 ρm 6255 kg/m³ gaussian 1,12e-02 1,84e-04 2,06e-06 ∆fdw 100 kn gaussian 1,10e-05 1 1,10e-05 ∆fl 100 kn gaussian 1,20e-05 1 1,20e-05 ∆fdis 0 kn 5,00e-05 rectangular √3 2,89e-05 1 2,89e-05 ∆finst 0 kn 1,00e-05 triangle √6 4,08e-06 1 4,08e-06 ∆fecc 0 kn 1,00e-05 triangle √6 4,08e-06 1 4,08e-06 ∆fsens 0 kn 1,50e-05 rectangular √3 8,66e-06 1 8,66e-06 3,5e-05 0,0070% rel. uncertainty: expanded relative uncertainty (k =2): acta imeko | www.imeko.org june 2014 | volume 3 | number 2 | 22 performance of force standard machines with compensation of lever arm distortion an active beacon-based leader vehicle tracking system acta imeko issn: 2221-870x december 2019, volume 8, number 4, 33 40 acta imeko | www.imeko.org december 2019 | volume 8 | number 4 | 33 an active beacon-based leader vehicle tracking system stanislaw goll1,2, elena zakharova 1,2 1 department of information-measuring and biomedical engineering, ryazan state radio engineering university (rsreu), gagarin street, 59/1, ryazan, russia 2 llc kb avrora, skomoroshinskaja street 9, of 3, ryazan, russia section: research paper keywords: ultrasound; mobile robot convoying; ultrasonic beacon; kalman filter; rauch-tung-striebel smoother citation: stanislaw goll, elena zakharova, an active beacon-based leader vehicle tracking system, acta imeko, vol. 8, no. 4, article 7, december 2019, identifier: imeko-acta-08 (2019)-04-07 editor: yvan baudoin, international cbrne institute, belgium received november 23, 2018; in final form june 26, 2019; published december 2019 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: elena zakharova, e-mail: zaharova@kb-avrora.ru 1. introduction the convoying scenario is one of the most important tasks in modern robotics. a mobile robot autonomously following a leader can be widely used in such areas as agriculture, transportation, and the military. many well-known international trials, such as elrob, include this scenario either as an independent one or as a part of a more complex scenario. the scenario is focused on the mobile robot autonomously following some leader, which can be either a human operator or another vehicle – either autonomous or remote-controlled. to detect the leader, different systems can be used, including gnss systems [1], [2], video and infrared cameras [3]-[7], lidars [8]-[10], directional antennas [11], radars, ultrasonic rangefinders [12]-[14], and their combinations [15], [16]. the nature of these systems, however, can impose various restrictions on the conditions of their use. gnss systems perform poorly in urban areas and inside buildings. lidars and radars can be used to measure the distance between the vehicle and the surrounding objects, but it is not an easy task to detect the leader based on range data. cameras, both video and infrared, can help with leader detection but depend greatly on environmental conditions. moreover, if the obstacle appears between the leader and the robot, the accuracy of their relative positions’ estimate can be severely decreased. the mathematical methods used for localisation, obstacle detection, and occupancy grid mapping are not reliable enough to build a robust convoying system. such methods usually require a redundant sensor data of a different nature. the leader detection suffers from the same constraints and, to make things even more difficult, the rough weather conditions, dense vegetation, smoke, and other factors should also be considered. in section 2, we discuss the implementation of an ultrasonicbased leader detection system that consists of an active beacon carried by the leader and a set of several ultrasonic receivers mounted on the convoyed robot. in section 3, different methods of computing the estimate of the beacon’s position are described. conclusions of this study, experimental results, and dynamic errors for all methods are illustrated in section 4. finally, recommendations for future work are given in the last section. 2. the approach in this article, we propose an ultrasonic-based leader detection system that includes an active beacon carried by the abstract this article focuses on mobile robot convoying along a path travelled by a certain leader carrying the active ultrasonic beacon. the robot is equipped with the three-dimensional receiver array in order to receive both the ultrasonic wave and the rf wave marking the beginning of the measurement cycle. to increase measurement reliability, each receiver contains two independent measurement channels with automatic gain control. the distance measurements are pre-processed in order to identify the artefacts and then either remove them or replace them with the interpolated value. to estimate the position of the beacon in the robot’s local coordinate system, several methods are used, including the least squares method with subsequent exponential smoothing, the linear kalman filter, the rauch-tung-striebel smoother, the extended kalman filter, the unscented kalman filter, and the particle filter. the experiments were undertaken in order to estimate the estimation method preferable for following the leader’s path. mailto:zaharova@kb-avrora.ru acta imeko | www.imeko.org december 2019 | volume 8 | number 4 | 34 leader and a set of n ultrasonic receivers mounted on the convoyed robot (figure 1). the active beacon transmits the ultrasonic waves with constant time intervals between the waves. at the same time that the ultrasonic wave is sent, the radio frequency wave marking the beginning of the measurement cycle is also transmitted. for the each of the n receivers, the time interval between the moment of the radio frequency wave and ultrasonic wave arrivals is measured according to the time-offlight principle. these intervals are proportional to the distances between the beacon and the respective receivers and can be calculated as n nr c= , n 1, , n= , where c is the speed of the ultrasound in the air, and n is the time interval measured for the th n receiver. to estimate the beacon’s coordinates in the robot’s local coordinate system, the system of n nonlinear equations is solved. 2 m 2 m 2 m 2 n n n nr ( x x ) ( y y ) ( z z )= − + − + − , (1) where n 1, , n= , t m m m u x y z =   is a vector containing the coordinates of the beacon, and   t n n nx y z are the coordinates of the nth receiver in the convoyed robot’s local coordinate system. during the system operation, an obstacle can cause line-ofsight loss between one or several receivers and the beacon. moreover, the ultrasonic wave can be reflected by the surrounding objects, causing the problem of multipath propagation. for these reasons, acquired measurements can contain artefact distances nr (figure 2(a)), which are identified using the threshold constant p. the distance measurement n ,kr is considered an artefact if the following condition is met: n ,k n ,k 1r r −−  , (2) where p is an adjustable threshold constant, and k is the consequent measurement number. if the artefact is detected, the linear least squares extrapolation is used to calculate the substitute estimate based on the last v estimates  n ,k 1 n ,k vr , ,r− − . if the series of m artefacts is detected in the receiver’s measurements, or the substitute estimate cannot be calculated due to the unreliable measurements present in the last v estimates, the corresponding equations are excluded from the system (1). when the reliable measurements (i.e. condition (2) is not met for the two consecutive measurements) arrive, the receiver is included back in the system (1), and the artefact removal procedure becomes applicable again. the system must contain at least three reliable measurements from different receivers to calculate the beacon’s position estimate u . if there are not enough reliable measurements available, the robot stops until the beacon’s position can be calculated again. to make more robust measurements, each receiver contains two independent measurement channels with partly overlapping beam patterns and automatic gain control. if both channels succeeded at the distance measurement at some moment of time k , then the average value is used as a resulting measurement. if one of the channels failed to provide a measurement, then the over channel’s measurement is used as a resulting measurement. 3. position estimation many of the effective estimation methods are based on the linear models and use normally distributed values. figure 2b and 2c show the histograms of centred values for the measured distances nr between the beacon and the th n receiver with the b a ... beacon 3 n 2 z 1 x y rn r1 figure 1. (a) the active beacon and the receiver array mounted on the mobile robot (b) convoying scenario with the two robots. b artifacts a c figure 2. histogram of centered values for the single receiver’s measurements: a – measurement series containing artifacts, b – 11 meter distance between the beacon and the receiver, c – 3 meter distance between the beacon and the receiver. acta imeko | www.imeko.org december 2019 | volume 8 | number 4 | 35 beacon positioned along the axis of the receiver’s beam pattern at a distance of 11 and 3 metres respectively. clearly, the histograms are multimodal, with the constant distances c  between the modes, where  is the period of ultrasound. since the envelope’s shape can be approximated with the gaussian function, we can assume that the distances nr are distributed normally, with the variance depending on the beacon’s position. we can use a linear combination of the equations to transform the nonlinear equations of (1) into the linear ones [17]. for each pair of receivers, the difference of the squared distances to the beacon is calculated as follows: 2 2 m 2 m 2 m 2 i j i i i m 2 m 2 m 2 j j j r r ( x x ) ( y y ) ( z z ) ( x x ) ( y y ) ( z z ) , − = − + − + − − − − − − − where i , j 1, , n= , i j , 2 nc – binomial factor, 2 nl c= – the number of receiver combinations. as a result, system (1) can be written in the form of the linear system bu g= , (3) where b is the system matrix, with l rows containing j i j i j i2( x x ) 2( y y ) 2( z z ) − − −  , and g is the column vector of l constant terms ( )2 2 2 2 2 2 2 2i j j i j i j ir r x x y y z z− + − + − + − , i , j 1, , n= , i j . generally, the system (3) is inconsistent due to the measurement noise. hence, to estimate the position of the beacon, the least squares method can be used: ( ) 1 t t u b b b g . − = to compute the estimate u of the beacon’s position, different methods can be used; therefore, several estimates were computed in the course of this research. during the research, the receiver array of n 4= receivers was used; therefore, the system (3) contains 2 4l c 6= = equations. 3.1. least squares method with exponential smoothing ( )s sk k k 1u u 1 u  −= + − , ( )0; 1  , (4) where  is the adjustable smoothing factor. 3.2. kalman filter with the non-stationary measurement noise matrix the overall filter design is undertaken according to [18]. the stochastic system model can be described as follows: k 1 k k k k k x ax , g cx ,   +  = +  = + , (5) where t y zm m m x k k k k k k kx x y z v v v =   is the state vector containing the beacon’s coordinates and velocity components; 1 0 0 t 0 0 0 1 0 0 t 0 0 0 1 0 0 t a 0 0 0 1 0 0 0 0 0 0 1 0 0 0 0 0 0 1          =           is the stationary transition matrix of the dynamic model, where t is the time interval between the consecutive measurements; t k 1,k l ,kg g g=    is the measurement vector; c is the stationary measurement model, with l rows j i j i j i2( x x ) 2( y y ) 2( z z ) 0 0 0 − − −  , i , j 1, , n= , i j ; y zx k k k k0 0 0 a a a  =   is process noise, ~ℕ ( | 0⃗ ); ( )2 2 2a a aq diag 0, 0, 0, , ,  = is the stationary process noise covariance matrix; kp is the covariance matrix of the state; k is the observation noise vector, ~ℕ( | 0⃗ ); kr is the non-stationary measurement noise covariance matrix. to estimate the measurement noise covariance [19], we assume that t t t k k k k ke s s e cp c    = −    , where k k ks g cx= − is residual, when 0 k k t t d d k d k d r , k d, r 1 s s cp c , k d, d = −   =  +    d is the estimation window size. the initial state vector is assumed to be t m m m 0 0 0 0x x y z 0 0 0 =   , input: output: begin for prediction update if then end else end end end 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 0 0 0x , p , r , a, q ,c , d k kx , p k 1k k 1 x ax −− = t k 1k k 1 p ap a q−− = + kk k 1 k k 1 s g cx − − = − ( ) 1 t t k kk k 1 k k 1 g p c r cp c − − − = + k kk k 1 k k 1 x x g s − − = + ( )k k k k 1p i g c p −= − k k ks g cx= − k t t k 1 d d k d k d 1 r s s cp c d + = − = + k d k 1 0r r+ = k 1 k= → acta imeko | www.imeko.org december 2019 | volume 8 | number 4 | 36 where ( ) t 1 m m m t t 0 0 0 0x y z b b b g −   =   . the posteriori error covariance matrix and the measurement noise covariance matrix are written as ( )2 2 2 2 2 20 u u u v v vp diag , , , , ,     = and 2 2 0 g g l r diag , ,  =     respectively. 3.3. rauch-tung-striebel smoother the sequence of the beacon’s state estimates  k k 1 k tx , x , , x− − can be used to set the desired path for the mobile robot. if the corresponding covariance matrices  k k 1 k tp , p , , p− − are also known, then this path-to-be can be smoothed using the rauch-tung-striebel smoother. the estimate and covariance sequences are rewritten as  t t 1 0x , x , , x− and  t t 1 0p , p , , p− accordingly. the initial smoothed estimate is s t tx x= with the covariance s t tp p= . then the smoother is applied from the last time step to the first (i.e., t t 1, ,0= − ) according to the description given in [18]. input: output: begin for end end 1 2 3 4 5 6 7 8 9 10 11    t t 1 0 t t 1 0x , x , , x , p , p , , p , a, q,t− − t t 0= →    s s s s s st t 1 0 t t 1 0x , x , , x , p , p , , p− − tt 1 t x ax + = t tt 1 t p ap a q + = + t 1 t t t 1 t s p a p − + = ( )s st t t t 1 t 1 tx x s x x+ += + − ( )s s tt t t t 1 tt 1 tp p s p p s+ += + − 3.4. extended kalman filter according to [18], the stochastic system model can be described as follows: ( ) k 1 k k k k k x ax , r h x ,   +  = +  = + (6) ( )kh x is the vector-valued function containing n elements m 2 m 2 m 2 k n k n k n( x x ) ( y y ) ( z z )− + − + − , n 1, , n= ; t k 1,k n ,kr r r=    is the measurement vector; kp is the covariance matrix of the state; kr is the non-stationary measurement noise covariance matrix. to estimate the measurement noise covariance [20], we assume that t t t k k k k k k ke s s e h p h    = −    , where ( )k k ks r h x= − is residual, when 0 k k t t d d k k k d k d r , k d, r 1 s s h p h , k d, d = −   =  +    (7) d is the estimation window size, k k k 1 h dh dx − = is the jacobian matrix with n rows t m nk k 1 m 2 m 2 m 2 n n nk k 1 k k 1 k k 1 m nk k 1 m 2 m 2 m 2 n n nk k 1 k k 1 k k 1 m nk k 1 m 2 m 2 m 2 n n nk k 1 k k 1 k k 1 ( x x ) ( x x ) ( y y ) ( z z ) ( y y ) ( x x ) ( y y ) ( z z ) ( z z ) ( x x ) ( y y ) ( z z ) 0 0 0 − − − − − − − − − − − −  −    − + − + −    −    − + − + −     −     − + − + −             , n 1, , n= . on the initial step t m m m 0 0 0 0x x y z 0 0 0 =   , where ( ) t 1 m m m t t 0 0 0 0x y z b b b g −   =   ; ( )2 2 2 2 2 20 u u u v v vp diag , , , , ,     = ; 2 2 0 r r n r diag , ,  =     (8) input: output: begin for prediction update if then end else end end end 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 0 0 0x , p , r , a, q , h , d k kx , p k 1k k 1 x ax −− = t k 1k k 1 p ap a q−− = + k kk k 1 k k 1 x x g s − − = + ( )k k k k k 1p i g h p −= − k d k 1 0r r+ = k 1 k= → ( )kk k 1 k k 1s r h x− −= − k k k 1 h dh dx − = ( ) 1 t t k k k k kk k 1 k k 1 g p h r h p h − − − = + ( )k k ks r h x= − k t t k 1 d d k k k d k d 1 r s s h p h d + = − = + acta imeko | www.imeko.org december 2019 | volume 8 | number 4 | 37 3.5. unscented kalman filter in the initial step, the set of sigma points is computed as 0 0 0 0 0 u 1 u u u u 2u 1 x x u 0 p p     +  = + + −       , where t m m m 0 0 0 0x x y z 0 0 0 =   , (9) ( ) t 1 m m m t t 0 0 0 0x y z b b b g −   =   . each sigma point has its own weight, calculated by the formulas ( )m m c c0 2u 2u 1 t m m 2u 1 w w w diag w , , w w w ,   + +   = −           −       (10) input: output: begin for prediction update if then end else end end end 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 0 0 0x , p , r , a, q ,w , ,u , d, h k kx , p k d k 1 0r r+ = k k 1a  −= m kk k 1 x w − =  k kk k 1 p w q    − = + m kk k 1 k k 1 s r w − − = −  ( )k k 1 k k 1h − −= ( ) 1 t t k kk k 1 k k 1 k k 1 k k 1 g w w r    − − − − − = + k kk k 1 k k 1 x x g s − − = + ( )t tk k k kk k 1 k k 1 k k 1p p g w r g − − −= − + m k k ks r w= −  k t t k 1 d d k k d k d 1 r s s w d  + = − = + k 1 k= →  k k 1 k k 1 k k 1x x − − −= +  k k kx x = + k ku 0 p p  + −  k k 1 k k 1 u 0 p p − −  + −   where t m m m 0 2uw w w =   , m 0w u   = + , c 2 0w 1 u     = + − + + , ( ) m c i i 1 w w 2 u  = = + , i 1, , 2u= ; u 6= refers to the number of components in the state vector kx ; ( )2 u u  = + − is a scaling parameter; , ,   are the parameters of the method; kr is the non-stationary measurement noise covariance matrix. to estimate the measurement noise covariance [21], we assume that t t t k k k k k ke s s e w      = −    , where m k k ks r w= −  is residual, 0 k k t t d d k k d k d r , k d, r 1 s s w , k d, d   = −   =  +    (11) d is the estimation window size, ( )k kh = , kx is the array of sigma points. the initial value of ( )2 2 2 2 2 20 u u u v v vp diag , , , , , ,     = 2 2 0 r r n r diag , ,  =     . the filter implementation complies with [18]. 3.6. particle filter to describe the beacon’s position, the following state vector t y zm m m x x x y z v v v =   is used, where ( )0x ~ x x , . the initial estimate is t m m m 0 0 0 0x x y z 0 0 0 =   , where ( ) t 1 m m m t t 0 0 0 0 0u x y z b b b g −  = =   , ( )2 2 2 2 2 2u u u v v vdiag , , , , ,      = . based on the distribution ( )0x x , 1 , the set of particles 0x is generated. for each particle j ,kx , the weight j ,0w 1 j= is set, with the sum of the weights being j j ,0 j 1 w w 1 = = = , j 1, , j= . acta imeko | www.imeko.org december 2019 | volume 8 | number 4 | 38 input: output: begin for for for end end for end the particles with the weighs less than are removed where – is the size of the new particle set new particles are added to the set according to the normal distribution end end 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20      0 1 n 1 n 1 nx , a, , n , j , , x x , y y , z z  k 1 k= → kx k k 1x ax −= ( ) ( ) ( ) 2 2 2 m m m n , j ,k j ,k n j ,k n j ,k nr̂ x x y y z z= − + − + − ( ) ( )j ,k 1, j ,k 1, j ,k n , j ,k n , j ,kˆ ˆw r r , r r , =   k k j ,kj 1 w w w  = =   ( )k xx x , k j ,k j ,kj 1 x w x ,  = =  n 1 n= → j 1 j= → j 1 j= →  old j ,k j ,kx x w , j 1, , j=  = ( ) new j ,k k xx x x x , , j 1, , j = = − k old newx x x= +  4. results during the experiments, an array of n 4= receivers was statically placed, while a mobile robot carrying the active beacon was moved in front of it following a rectangular path (figure 3) with a width of 4 m and a length of 7 m (1st experiment) or 10.5 m (2nd experiment). the robot moved clockwise, starting and finishing at the coordinates ( )2; 1− in the array’s local coordinate system. for each beacon’s position, we computed a total of six estimates using the methods described above. the number of particles in the particle filter is j 5000= . to compare the performance of these methods, we calculated the average of the root mean square errors (rmse). dynamic errors in the form of distances between the estimate of the active beacon trajectory and the exemplary trajectory in the two-dimensional 2 and three-dimensional coordinate systems 3 in time are presented in figure 3. the grey area on the dynamic error graphs marks the parts of the active beacon path corresponding to the smallest sides of the rectangle. on the left-hand side, the path estimates and their dynamic errors are shown, which are calculated based on the shifted differences of the square distances between the beacon and each of the receivers ng . on the right-hand side, the path estimates and their dynamic errors are shown, which are calculated based on the distances between the beacon and each of the receivers nr . table 1 shows the rmses of the beacon’s position estimates during the first experiment. errors rmse(|δ2|) and rmse(|δ3|) correspond to the estimates made in the twodimensional and three-dimensional coordinate systems respectively. the rauch-tung-striebel smoother outperforms other considered methods in terms of rsmes and dynamic errors. however, the path estimates shown in figure 3 appear to be more parallelogrammatic than rectangular, altering the shape of the original path. these alterations were caused by the side wind during the experiment. during the second experiment, no wind was present. furthermore, the length of the ground truth rectangle was increased to 10.5 m. the path estimates and the corresponding dynamic errors are shown in figure 4. the rsmes are shown in table 2. clearly, the rauch-tung-striebel smoother demonstrates the best estimate again, despite changes in the experimental conditions. – 3 – 2 – 1 0 1 2 3 0 1 2 3 4 5 6 7 8 – 3 – 2 – 1 0 1 2 3 0 1 2 3 4 5 6 7 8 1 2 3 4 5 6 0.5 1.5 y,m y,m x,m x,m 5 15 25 35 0 1 2 0.5 1.5 5 15 25 35 0 1 2 0.5 1.5 5 15 25 35 0 1 2 5 15 25 35 0 0.5 1 1.5 2 t,s t,s t,s t,s |δ2| |δ3| |δ2| |δ3| figure 3. path estimates and the corresponding dynamic errors in the 1st experiment: 1. least squares method with exponential smoothing. 2. kalman filter. 3. kalman filter with rauch-tung-striebel smoother. 4. unscented kalman filter. 5. particle filter. 6. extended kalman filter. table 1. the root mean square errors of the beacon’s position estimates during the first experiment rmse(|δ2|), m rmse(|δ3|), m es 0.7365 0.8948 kf 0.6161 1.0029 rts 0.4776 0.7410 ekf 0.5569 0.9255 ukf 0.4922 0.8875 pf 0.4830 1.0422 acta imeko | www.imeko.org december 2019 | volume 8 | number 4 | 39 5. future research the main goal of our future research in this area is to eliminate biases caused by the wind and measurement errors during the estimation of the system’s structural parameters. to estimate these parameters, we plan to perform a calibration procedure involving the 3d lidar mounted on the robot. given the 3 cm accuracy of the lidar measurements and usage of the machine learning algorithms, such calibration should outperform the current manual measurements made with the slide rule. moreover, it should also allow for estimating the size and the shape of the receivers. currently, the effective measurement range is up to 20 m; however, it can be extended to 100 m. this can be achieved by combining the ultrasonic measurements with the gnss-based measurements. in this case, it is possible to mitigate the relative displacement between the robot’s and beacon’s gnss receivers by mixing the ultrasonic and the gnss data, while the distance between the robot and the beacon is relatively small. references [1] m. spencer, d. jones, m. kraehling, k. stol, trajectory-based autonomous vehicle following using a robotic driver, proc. of the australasian conference on robotics and automation, 2009, brisbane, australia, australian robotics and automation association, pp. 1-10. [2] n. wong, k. a. stol, c. chambers, r. halkyard, autonomous vehicle following using a robotic driver, proc. of the ieee 15th international conference on mechatronics and machine vision in practice, 2-4 december 2008, auckland, new zealand, pp.115-120. [3] z. changchen, c. weihai, z. zhiwen, l. jingmeng, an rgbd data based vehicle detection algorithm for vehicle following systems, proc. of the 2013 8th ieee conference on industrial electronics and applications (iciea), 19-21 june 2013, melbourne, australia, pp. 1506-1511. [4] r. vidal, o. shakernia, s. sastry, following the flock: distributed formation control with omnidirectional vision-based motion segmentation and visual servoing. ieee robotics & automation magazine 11(4) (2004) pp. 14-20. [5] j. krejsa, s. vechet, infrared beacons based localization of mobile robot. electronics and electrical engineering 1(117) (2012) pp. 17-22. [6] c. fries, h.-j. wuensche, monocular template-based vehicle tracking for autonomous convoy driving, proc. of the ieee/rsj international conference on intelligent robots and systems (iros), 2014, pp. 2727-2732 [7] l. zhang, t. ahamed, y. zhang, p. gao, t. takigawa, visionbased leader vehicle trajectory tracking for multiple agricultural vehicles, sensors 16(4) (2016) p. 578. [8] t. c. ng, j. ibanez-guzman, j. shen, z. gong, h. wang, c. cheng, vehicle following with obstacle avoidance capabilities in natural environments, proc. of the ieee int. conf. robotics and automation icra '04, april 2004, new orleans, la, usa (5) pp. 4283-4288. [9] c. fries, h.-j. wuensche, autonomous convoy driving by night: the vehicle tracking system, proc. of the ieee conf. technol. pract. robot appl. tepra, 2015, pp. 1-6. [10] m. himmelsbach, a. muller, t. luttel, h.-j. wunsche, lidarbased 3d object perception, proc. of the 1st international workshop on cognition for technical systems, october 2008, munchen, pp. 1–7. [11] b.-c. min, e. t. matson, robotic follower system using bearingonly tracking with directional antennas, proc. of the 2nd international conference on robot intelligence technology and applications (rita 2013) 18-20 december 2013, denver, colorado, usa, pp. 37-58. [12] s. shoval, j. borenstein, measuring the relative position and orientation between two mobile robots with binaural sonar, proc. of the 9th ans international topical meeting on robotics and remote systems, 4-8 march 2001, seattle, washington, usa. [13] f. hoflinger, l. m. reindl, tdoa based localization using interacting multiple model estimator and ultrasonic transmitter/receiver, proc. of the 9th ieee international multiconference on systems, signals and devices, 20-23 march 2012, chemnitz, germany (1) pp.1063-1069. [14] e. s. ningrum, r. hakkun, a. alasiry, 2016, tracking and formation control of leader-follower cooperative mobile robot based on trilateration data. international journal of engineering technology emitter 3(2) (2016) pp. 88-98. [15] t. c. ng, j. i. guzman, m. d. adams, 2005, autonomous vehicle following systems: a virtual trailer link model, proc. of the ieee/rsj international conference on intelligent robots and systems, 2-6 august 2005, edmonton, alta., canada, pp. 30573062. 0.5 1.5 20 40 60 80 0 1 2 t,s 20 40 60 800 0.5 1 1.5 2 t,s20 40 60 800 0.5 1 1.5 2 t,s 20 40 60 800 0.5 1 1.5 2 t,s -3 -2 -1 0 1 2 30 2 4 6 8 10 12 y,m x,m 4 5 6 -3 -2 -1 0 1 2 30 2 4 6 8 10 12 1 2 3 y,m x,m |δ2| |δ3| |δ2| |δ3| figure 4. path estimates and the corresponding dynamic errors in the 2nd experiment: 1 – least squares method with the exponential smoothing, 2 – kalman filter, 3 – kalman filter with rauch-tung-striebel smoother, 4 – unscented kalman filter, 5 – particle filter, 6 – extended kalman filter. table 2. the root mean square errors of the beacon’s position estimates during the second experiment. rmse(|δ2|), m rmse(|δ3|), m es 0.7502 1.0236 kf 0.5318 0.7447 rts 0.3682 0.5431 ekf 0.4386 0.6350 uk f 0.4471 0.6552 pf 0.5005 0.6052 acta imeko | www.imeko.org december 2019 | volume 8 | number 4 | 40 [16] d. simon, b. theisen, convoy active safety technology: environmental understanding and navigation with use of low cost sensors, proc. of the ndia ground vehicle systems engineering and technology symposium, 2012. [17] a. montanha, a. m. polidorio, f. j. dominguez-mayo, m. j. escalona, 2d triangulation of signals source by pole-polar geometric models, sensors 19(5) (2019) p. 1020. [18] j. hartikainen, s. sumo, optimal filtering with kalman filters and smoothers, a manual for the matlab toolbox ekf/ukf, department of biomedical engineering and computational science, aalto university school of science, espoo, finland, august 2011, p.129. [19] j. wang, w. ding, j. wang, improving adaptive kalman filter in gps/sdins integration with neural network, proc. of the ion gnss 2007, 25-28 september 2007, fort worth, tx, usa (1) pp. 571-579. [20] s. akhlaghi, n. zhou, z. huang, adaptive adjustment of noise covariance in kalman filter for dynamic state estimation, arxiv preprint arxiv:1702.00884, 2017, pp.1-5. [21] b. zheng, p. fu, b. li, x. yuan, a robust adaptive unscented kalman filter for nonlinear estimation with uncertain noise covariance, sensors 18 (2018) p. 808. microsoft word article 10 tc4 paper 6b.docx acta imeko  august 2013, volume 2, number 1, 32 – 39  www.imeko.org    acta imeko | www.imeko.org  august 2013 | volume 2 | number 1 | 32  variability and uncertainty of track circuit band‐pass  modeling for interference evaluation  andrea mariscotti  università di genova, via opera pia 11a, 16145 genova, italy    keywords: conducted interference; guideway transportation systems; power quality; spectral analysis; supply transients; time domain analysis  citation: andrea mariscotti, “variability and uncertainty of track circuit band‐pass modeling for interference evaluation”, acta imeko, vol. 2, no. 1,  article 10, august 2013, identifier: imeko‐acta‐02(2013)‐01‐10  editors: paolo carbone, university of perugia, italy; ján šaliga, technical university of košice, slovakia; dušan agrež, university of ljubljana, slovenia  received january 13 th , 2013; in final form june 10 th , 2013; published august 2013  copyright: © 2013 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: none reported  corresponding author: andrea mariscotti, e‐mail: andrea.mariscotti@unige.it      1. introduction  when a new locomotive is introduced on existing lines it is essential to ensure that the locomotive and the signalling systems are compatible under normal operation and exceptional conditions. a key point is the evaluation of the interference mechanism, where the locomotive is the source of the disturbance (and new locomotives equipped with electronic static converters have pushed the spectrum of disturbances at higher frequency), the signalling circuit is the victim (new signalling systems operate in a wide frequency range at power frequency and audio-frequency up to some tens of khz) and the line and track are the means of propagation. several are the relevant parameters and the degrees of freedom, related to:  the infrastructure parameters that modify in terms of transfer function and impedance the return path of the current leaving the train axles (called “cold path”) and the supply circuit feeding current to the pantograph (called “hot path”) [1]-[3];  the complex architecture of the on-board converters and drives, for traction and auxiliaries, the related static and dynamic operations, considering in particular on transient conditions [4][5];  the susceptibility of the victim track circuits (tcs), not only for steady conditions (that can be simulated at the manufacturer’s workshop), but also including various types of transients and the variability of the characterizing parameters [2][3][6]. different vehicles or trains absorbing current in the same supply section represent a possible and relevant situation: the return current components (flowing through the analyzed track circuit section) may be led back to the respective sources with different summation rules to account for the phase relationship (depending on the degree of synchronization of the sources, the relative distance and the frequency response of the traction line) [7]. while the absolute limit on the disturbing current in the tc operating bandwidth is set by the characteristics of the tc itself with the due safety margins, the limits for each source are normally apportioned by the standardization bodies and infrastructure owners, following justified summation rules [3] and adding further safety margins. the entire process of combined testing is particularly critical if the exigency of a high degree of reproducibility and repeatability is considered, in particular to ensure the interoperability of rolling stock and the equivalence of the test abstract  when evaluating track circuits (tcs) interference, the latest normative approach indicates the use of band‐pass filters that emulate the  tc receiver response. the rms value of the output, when compared to the interference limit, indicates whether a specific signal might  create interference or not. the implementation of the filter has some degrees of freedom and needs thus to be characterized in terms  of added uncertainty to the interference evaluation process. the latter is relevant to safety, strictly related to the incorrect assignment  of the status of the track section monitored and protected by the tc itself. when giving a definitive answer about the immunity of a  specific tc and the compatibility with the tested rolling stock, it is thus necessary to completely evaluate the uncertainty associated to  the chosen model and to the related signal processing operations, even if – it is recognized – their influence may be considered of  second order with respect to infrastructure and rolling stock non‐idealities. acta imeko | www.imeko.org  august 2013 | volume 2 | number 1 | 33  results by different operators and manufacturers, to address the cross-acceptance of rolling stock on existing traction lines in europe. while the variability of infrastructure parameters and related uncertainty was already considered in the past [1], the focus of this paper is on the processing circuits and algorithms indicated by the european standards [2][3] to model the susceptibility of the victim tc and applied to the recorded pantograph current and its spectrum. a tc has an intrinsic band-pass behavior due to the internal filters and protection circuits. the standardized method for the evaluation of interference is to model the tcs by means of an equivalent band-pass filter built around the nominal tc parameters [2][3]. the processing method used for testing can also be applied in real time, while the locomotive is subject to tests or to monitor normal service. 2. problem description  the problem considered in this paper is reduced to two elements, the victim (represented by the tc and its susceptibility) and the source (represented by the rolling stock, by its conducted emissions and the variability of their spectral properties). the interest is in quantifying the uncertainty of the test outcome with respect to the degrees of freedom offered by the implementation of the band-pass filter that models the specific tc. 2.1. signalling circuit fundamentals  the considered signalling circuits are track circuits (tcs), all based on a common operating mechanism, that of transmitting a modulated signal from a transmitter (tx) to a receiver (rx) over the track, and sensing track occupation by measuring the amount of signal that reaches the rx. the track is detected as occupied, when the rails are shunted by the low resistance axles of the entering rolling stock, if the rx signal drops below a given threshold. the tcs frequency interval ranges from some tens of hz for power frequency tcs up to about 20 khz for audio-frequency tcs, with only a few exceptions. the longitudinal voltage drop along the track is mainly inductive and is thus roughly proportional to the frequency; for this reason, to maximize the amount of signal reaching the rx in free conditions, some techniques are normally adopted (capacitive compensation of the track impedance, matching of tx impedance to track impedance, directionality of tx signal towards the respective rx by use of track bonds) [8]. the signal is nowadays always modulated to increase the robustness against the superimposed conducted disturbance of circulating rolling stock, that may occur in the operating band of the tc itself. moreover, the rx has an intrinsic band-pass behavior determined, first, by the electrical coupling method itself often obtained by using resonant circuits tuned including the track itself, and, second, by additional analog or digital filters to increase noise rejection. the so-called relays connected to the rx output cable in the interlocking cabin are responsible for additional processing, since they often include a delay, to filter out transient components (such as in motor relays or slow pickup relays) or a phase/frequency matching circuit between the track and a “clean” parallel cable between tx and rx (such as for vane relays). the modern relays are digital implementations on an interlocking computer and their functions are even more extended; however, the continuity with respect to “traditional functions”, often required by railways, has led to standardization on old functionalities. an entire tc is a complex, potentially non-linear, system that can be characterized with a black box approach by a series of curves of susceptibility, only if a certain degree of approximation is acceptable. susceptibility tests with steady signals are easier, in that swept sinusoidal signals are applied first, and then followed by multi-sine signals, all of variable amplitude, until the effects of interference are detected. if transient signals are considered too, the characterization is more complex, since the time-frequency properties of the signal and the tc time domain response are strongly related, and the identification of the suitable subspaces, indexes and mathematical representations is still under investigation [3]. pragmatically, in this paper, the band-pass approach followed by the cited standards is considered, thus fixing the method of evaluation, while the uncertainty related to the variability of the applied steady and transient signals is analyzed. 2.2. rolling stock  the rolling stock (i.e. a locomotive or electrical multiple units) represents the source of disturbance as the return current leaving its axles and coupling along the track with the victim tc. coupling is conductive and occurs mainly in differential mode, while the return current leaves in principle symmetrically the axles, so representing a common mode variable. a certain amount of asymmetry exists in the rail-wheel contact resistances, in the rail self and mutual impedance and in the grounding electrical terms, and represents a first important assumption for an administration or standardization committee, while fixing the safety margins for the evaluation methods and limits. moreover, the pattern of conducted emissions is basically determined by the characteristics of the on-board converters and of the adopted modulation schemes. even in perfectly steady conditions, modulations can change in an attempt of optimization, depending on the motor speed and torque, or even pantograph voltage level. normal rolling stock operations are divided into acceleration, coasting and braking; acceleration and braking may occur at different rates, and for braking a regenerative action is possible up to a maximum allowed line voltage, then dissipative electric braking is applied; below a certain speed, on the contrary, braking is normally pneumatic. coasting at a fairly constant speed may occur at different line slopes, positive and negative, so with different amounts of however small absorbed power. transient conditions may be various, such as pantograph bounce, wheel slip and slide and input transformer and filter inrush current. the former causes a series of electric transients in the input circuitry, in particular transformer and filters, influencing also traction and auxiliary converters. the second may be quite complex, in particular if an anti-skidding mechanism is implemented, quickly changing the traction converter modulation. the latter occurs when the rolling stock is stationary and raises its pantograph (or more properly closes its main circuit breaker), or while passing neutral sections (similar to pantograph bounce). in the presence of transients (thus non-stationary signals) a fourier based analysis may be inadequate and unsatisfactory, taking into account in particular the trade-off between the time duration of the transient, the desired frequency resolution and the time constants of the rx and relays. acta imeko | www.imeko.org  august 2013 | volume 2 | number 1 | 34  2.3. traction supply systems and track circuits  rolling stock may circulate on different supply systems: dc systems at 1500 and 3000 v rated voltage and ac systems at 16.7 hz / 15 kv (used in german speaking countries and also in sweden and norway) or at 50 hz / 25 kv (for many railway systems, of the high speed type or conventional). while in the past a given rolling stock was bounded to one or few countries with the same supply system, several modern locomotives and emus can be supplied by two (or even three, recently) different supply systems. this has raised the problem of interoperability even more and of the compatibility of variable combinations of supply system frequency, type of locomotive and type of track circuit. while there are evident impediments for the use of a 50 hz tc under a 50 hz supply system, track circuits and supply systems combinations are all possible, even if for historical reasons and opportunities only a subset of them is really implemented [6]. however, some tcs equip for example only high speed lines or some others are not put by the administration in the list of the preferred ones [3][6]. in any case the exhaustive verification of compatibility of a new locomotive or emu against the whole set of the installed tcs is an expensive and time consuming mission; moreover, any rolling stock unit requires an initial period of preliminary verifications and fine tuning that can be performed in-house. this justifies the approach followed by cenelec of a shared framework for tc modeling, where the tc band-pass behavior is simulated by adopting the correct band-pass filter implementation. 3. setup for track circuit interference testing  test methods generally assess receiver immunity through signal processing of the recorded variables. a distinction may be made between type tests performed on reference railway tracks with the mentioned test methods and on-line real time monitoring of traction return current to prevent signalling interference. on-line monitors implement the same limit curves and processing methods, and even the same algorithm, used for type testing and they are aimed at detecting possibly critical interference situations in real time. when a locomotive is undergoing the certification process to run inside some country, circulation is restricted and authorized during the many tests if such a monitor is installed on-board, to prevent hazardous situations due to excessive disturbance onto the infrastructure. in general, an administration might require that any locomotive or unit running on the network is permanently equipped with monitoring equipment for an increased level of safety. the considered track circuit receivers have all a band-pass behavior. the band-pass function may be obtained with different solutions: an analogue filter, a digital filter or a discrete fourier transform (dft). in general the analogue solution is not preferable for reasons of flexibility and implementation costs. the clc/ts 50238-2 [3] proposes the digital band-pass filter solution and this will be considered here, against a set of synthetic and real test signals. in the 50238 standards also the use of dft is considered, as dictated by some national standards, but not recalled explicitly in the last issue [3]. the standardized filter for the evaluation of interference has some degrees of freedom in its specification, also depending on the specific target tc:  attenuation or frequency response and filter order are the most relevant and obvious; attenuation is normally given in one or two frequency points on the sides of the filter band, thus defining its bandwidth (at –3 db points) and its roll-off (e.g. adding another point at –6 or –10 db);  the filter type is sometimes specified, thus fully identifying the filter behavior, and including the phase response, that is particularly relevant for transients and for the correct combination of the sinusoidal components located within the filter band. the block diagram of the interference evaluation system (ies) includes an input band-pass filter (bpf) followed by a routine for the computation of the total rms in the filter band (rms detector), and is shown in figure 1. the signals are indicated here as continuous time signals, even if they are in reality discrete time signals. rms computation of the bpf output y may be in general performed in two ways: in the frequency domain the root of the sum of the squared dft components is used; in the time domain the exact definition of rms is used, taking the average of y2. the latter is preferred here because it is closer to the rms definition and the integration time ti specified in [3] can be readily implemented (using modified simpson rule over a moving integration window of amplitude ti). the test signal that will be used in the following is the absorbed pantograph current i(t). real signals are the result of recordings on various locomotives and trains and cover real situations, including any type of transients as they occur in normal circulation conditions or purposely generated. synthetic signals are conceived to be similar to known already observed spectral signatures, but with controllable parameters, to test the sensitivity of the filter response to their variations. the bpf, when a precise specification is lacking, may be implemented in any of infinite impulse response (iir) architectures, provided that the requisites on attenuation, rolloff and number of poles are met [6][9]. in some cases the tc band-pass behaviour is specified by the administration or the manufacturer that clearly indicates the required filter architecture, that is the butterworth one; in other cases the indication is not explicit or unique, or even the specified tc frequency response cannot be led back to a “classical” iir architecture. 4. characterization of the selected track circuits  the set of selected victim tcs is defined in table 1; the criteria followed for the selection consist of identifying various frequency intervals where train emissions behave differently and where different specification schemes apply [3][6]. the bpf transfer functions are analyzed for three different iir bpf rms calc. i(t) y(t) z(t)    itt ti dy t tz )( 1 )( 2 figure  1.  block  diagram  of  the  ies  with  the  bpf  and  the  rms  calculation  block  (the  possible  implementations  in  the  time  or  frequency domain are outlined in the upper right position).  acta imeko | www.imeko.org  august 2013 | volume 2 | number 1 | 35  architectures, bessel (bs), butterworth (bw) and chebyshev (cb): the filter order is the same and the bandwidth is adjusted for i) equivalence (see below the two criteria used, the first with respect to the –3 db points only, the second including the rest of the bandwidth by means of the equivalent noise bandwidth, enbw, estimation) and ii) to comply with the attenuation specifications of table 1. the two tcs selected for the analysis feature either a direct bw implementation (ugsk 3) or an unsatisfactory implementation within the iir framework (ti 21). a synthetic noise signal with a flat power spectral density in the filter band is first used to derive the basic bpf properties and to preliminarily rate the performance of the different bpf implementations. 4.1. tc with implicit butterworth implementation  in the first example (focusing on ugsk 3), the three bpf architectures have all the same number of poles (n = 3) and the same central frequency (fc = 208.75 hz). the frequency response is shown in figure 2. concerning the definition of filter bandwidth and the equivalence between the three architectures, two criteria were followed (the equivalence is established either for equal –3 db attenuation or for equal equivalent noise bandwidth) and two series of tests were performed. the enbw difference for the –3 db criterion is 2.83% between bs and bw and 1.14% between cb and bw, the bw always taken as reference. the condition of –3 db bandwidth is imposed when the band-pass filter is created from the low-pass prototype: it is evident that the bw architecture meets also the specification at –20 db, thus indicating the bw as the underlying filter architecture for the ugsk 3 specification. the difference in z(t) for the –3 db and enbw bandwidth criteria was evaluated as the rms deviation divided by the average value of z(t) for a set of random test sequences (see figure 3). while there is a general agreement between the curves (accounting for not so much different rms values over the 5 s records), there are some small parts of the traces showing an amplitude discrepancy (like the second one between 2 and 2.5 s, with a spread of up to 20% for the three architectures and a negligible difference between the two bandwidth criteria). in table 2 the results for twenty test sequences are shown in aggregate form: maximum deviation zmax1 and zmax2 and rms deviation zrms1 and zrms2 between bs/bw and cb/bw. being the difference among the three bpf in terms of enbw less than 3% (2.83% and 1.14%, as said above), the larger rms variability is explained with differences in the phase response. 4.2. tc without a direct iir implementation  the test is repeated for the ti 21 track circuit, whose frequency response is peculiar and cannot be led back to one of table 1. reference tcs at power and audio frequency used in the foregoing  analysis.  tc f0/hz δf/hz ti/s order n limit/a u g sk 3 208.75 6.5 @ -3 db 14 @ -20 db 0.5 2×3 4 222.45 6.5 @ -3 db 14 @ -20 db 0.5 2×3 4 242.15 6.5 @ -3 db 14 @ -20 db 0.5 2×3 4 t i 2 1 1549 12 @ -3 d b 60 @ -2 0 db 2.0 un sp ec ifi ed 0.09 1699 2.0 0.09 1848 2.0 0.09 1996 2.0 0.09 2146 2.0 0.09 2296 2.0 0.09 2445 2.0 0.09 2593 2.0 0.09 198 200 202 204 206 208 210 212 214 216 218 220 10 -2 10 -1 10 0 frequency [hz] figure  2.  frequency  response  of  the  three  bpf  implementations  for  ugsk 3: bs (black), bw (blue), cb (red).  0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 0 1 2 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 0 1 2 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 0 1 2 3 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 0 1 2 time [s] figure  3.  calculated  rms  signal  z(t)  for  ugsk 3  and  four  random  sequences: “‐3 db criterion” (blue) and “enbw criterion” (red); bs (dark),  bw (medium), cb (light).  table  2.  statistical  analysis  of  deviations  of  signal  z(t)  for  the  bpf  architectures: rms value and maximum value over 5 s records for bessel and  chebyshev, divided by the average z(t) of the butterworth implementation.  parameter bessel chebyshev zrms/zav(butt.) 0.1075 0.0538 zmax/zav(butt.) 0.5144 0.2810 frequency/hz time/s acta imeko | www.imeko.org  august 2013 | volume 2 | number 1 | 36  the three considered iir architectures (as shown in figure 4). the specified tc bandwidth is remarkably larger and adopting one of the three iir architectures would imply an underestimation of the real interference coupled onto the tc. in this case there is no implicit or explicit indication for the bpf architecture and after having tried the three bs, bw and cb architectures, the fulfilment of the frequency response specifications is reached with a high order fir filter, synthesized with the inverse dft technique [9] (see figure 5). additional points were added to the specification of the nominal frequency response only to obtain a better shape of the roll-off slope. different filter lengths (i.e. number of filter taps) were considered and the approximation is satisfactory for our tests beginning with n = 4096, so that any deviation from the nominal frequency response is smaller than 1% and can be considered negligible. the added frequency response points (shown as hollow circles in figure 5) impose a stop-band amplitude of the filter frequency response below 1% of the center band amplitude (normalized to 1). the relative error between the filter and the nominal frequency responses is kept smaller than 1%. 5. test with real locomotive currents  some pantograph current recordings are considered: they were recorded by different types of locomotives in real operating conditions and under different supply networks. the variability due to the filter architecture and the performance of the various implementations are thus tested and evaluated in real conditions, by using both steady and transient signals. the instrumental uncertainty behind the measurement setups is irrelevant to the analysis presented in this work, related only to the uncertainty given by the chosen filter architecture and parameters. a complete analysis of the uncertainty of the measurement equipment and setup behind the used recordings can be found in [10]. 5.1. steady pantograph current  some test signals are used, selected from the recordings performed on-board a german and a french train running on the respective supply networks with fundamental frequency of 16.7 and 50 hz [10][11]. the signal waveforms do not show any remarkable local non-stationarity or transient behaviour, while some step changes are visible for some spectral components (see figure 6). spectra are calculated versus time with short time fourier transform: the frequency resolution is one third of the fundamental and a hamming window was used to control the frequency leakage. the spectra are characterized in general by odd harmonics of the fundamental supply frequency [10]. the low frequency portion of the spectrum of figure 6(a) shows clearly the 11th and the 15th harmonics with amplitude of about 0.3 a; their amplitude is quite constant over time, while the 13th harmonic is somewhat lower and has an intermittent behaviour. similar considerations can be made for the high frequency portion located around two of the ti 21 channels. 1480 1500 1520 1540 1560 1580 1600 1620 10 -3 10 -2 10 -1 10 0 frequency [hz] figure 4. frequency response of the three bpf implementations for ti 21:  bs  (black),  bw  (blue),  cb  (red);  frequency  response  specifications  (solid  circles) and additional points (hollow circles).  1450 1500 1550 1600 1650 10 -3 10 -2 10 -1 10 0 frequency [hz] n = 512 n = 1024 n = 2048 n = 4096 figure 5. frequency response of the fir implementation for ti 21.  (a) (b) figure 6. short time fourier transform spectra of the (a) german and (b)  french pantograph current  in the frequency  intervals used to test ugsk 3  and ti 21 (the intensity is expressed in dba peak).  frequency/hz frequency/hz time/s time/s time/s time/s f re q u e n c y /h z f re q u e n c y /h z time/s time/s f re q u e n c y /h z acta imeko | www.imeko.org  august 2013 | volume 2 | number 1 | 37  considering figure 6(b), all the odd harmonics of the 50 hz fundamental are visible and are of approximately equal amplitude. the amplitude is particularly high (nearly 1 a) and the traces are shortly interrupted by rapid transients, typical of the french recordings, as described in [11]. the influence of the same interruptions may be recognized in the time domain filter response shown in figure 7, even if attenuated by the filter transient response. in figure 7 the time domain response of the different implementations is shown for the two test signals: the response of the ugsk 3 filters working at the lower frequency is always characterized by the larger uncertainty for the chosen filter implementation. the first three traces of figure 7(a) show variability between the red, the blue and the black waveforms of up to 100%. this represents a large variability that is to be accounted for in this kind of analysis. the different behaviour of the time response for the three ugsk 3 channels is explained by observing the characteristic harmonics and their sidelobes of the upper spectrum in figure 6(a):  the large clear horizontal band at 183 hz is the 11th harmonic of the 16.7 hz supply frequency, and some leakage to the 208 hz channel is observed only at the end of the trace;  the 222 hz channel on the contrary is nearly centered on the 13th harmonic, from which a larger amplitude in the filter output, characterized also by wider changes;  the 242 hz channel is in similar conditions, but at a more intense characteristic harmonic, the 15th, with a more regular amplitude profile versus time, and this explains the larger and more regular values in the output. the spread between the three filter implementations that is observed in particular in the third channel may be explained by observing that not only, as a general fact, the phase relationship between the spectral components is important, but also that any change of the slope of the amplitude and phase response (rolloff) around the corner frequency (242.15+3.25 hz at –3 db), changes the amount of the captured 15th located at 250 hz. the same may be observed in the first ti 21 channel at 1549 hz, so centered exactly on a particularly intense characteristic harmonic of the 50 hz french system, from which the largest amplitude of the filter output and the spread due to different roll-off at the corner frequency. the same channel under a 16.7 hz supply (last trace of figure 7(a)) features a ten times smaller interference and less variability for the chosen filter implementation, due to the smaller content of high frequency harmonics of the 16.7 hz supply systems. 5.2. transient pantograph current  the analysis of sec. 5.1 is repeated considering transients observed in the pantograph current during tests performed in sweden on a 16.7 hz supply system in 2008. scheduled ramps up and down were applied, so non-stationarity is expected, at least localized on some spectrum components. during the tests some short-time transients occurred as well, such as pantograph bounces, due to the mechanical interaction of the catenary and pantograph systems. the pantograph current waveforms used are shown in figure 8 and their spectra in figure 9; rapid changes of the amplitude of some harmonic components are visible in particular in figure 9(a). tests are performed on the (a) (b) figure 7. time response of the three bpf implementations for (a) germany  (ugsk 3 with the 208.75 hz, 222.45 hz and 242.15 hz channels, ti 21 with  the 1549 hz channel) and (b) france (ti 21 with the 1549 hz, 1699 hz and  1848 hz channels).  1 2 3 4 5 6 7 8 -30 0 -20 0 -10 0 0 1 00 2 00 3 00 c u rr e n t [a ] 1 2 3 4 5 6 7 8 -4 0 -2 0 0 20 40 c u rr e n t [a ] tim e [s ] figure  8.  pantograph  current  waveforms  used  for  testing  the  ugsk 3  filter implementations; time intervals where transients can be observed  are marked by a horizontal grey line; the two graphs show different kind  of transients while accelerating from standstill (top) and during sudden  changes of the driving/catenary conditions.  12 14 16 18 20 22 24 26 28 30 -0.02 -0.01 0 0.01 0.02 12 14 16 18 20 22 24 26 28 30 -0.02 -0.01 0 0.01 0.02 12 14 16 18 20 22 24 26 28 30 -0.02 -0.01 0 0.01 0.02 12 14 16 18 20 22 24 26 28 30 -0.02 -0.01 0 0.01 0.02 time [s] 12 14 16 18 20 22 24 26 28 30 -0.5 -0.25 0 0.25 0.5 a m p lit u de [v ] 12 14 16 18 20 22 24 26 28 30 -0.2 -0.1 0 0.1 0.2 a m p lit u d e [v ] 12 14 16 18 20 22 24 26 28 30 -0.02 -0.01 0 0.01 0.02 time [s] a m p lit u de [v ] time/s time/s time/s c u rr e n t/ a c u rr e n t/ a time/s a m p lit u d e /v a m p lit u d e /v a m p lit u d e /v acta imeko | www.imeko.org  august 2013 | volume 2 | number 1 | 38  ugsk 3 channels at 208.75 hz, 222.45 hz and 242.15 hz. in figure 10 the rms output is shown, obtained by time window integration of the squared output, as indicated in figure 1. the curves of figure 10(a) show the step changes due to the heavy transient behaviour of some spectral components, in particular around 3, 4 and 6.5 seconds. the integration time ti is 2 s for the ugsk 3 track circuit receiver and this introduces a time delay in the rms output response, that may be slightly variable depending on the time behaviour of the specific spectral components on the specific channel: for this reason the peaks of the rms output z of the three channels might be not exactly aligned in time. a large spread is observed, up to 100% for bs, 30% for cb, both during short and long time intervals, as it can be seen in figure 10(a) around 6.5 s and in figure 10(b) over the entire time interval for the first two channels. during transients, extraneous components may appear and the existing ones are subject to frequency leakage, from which the larger spread of the bessel architecture that features the highest sidelobes. on the contrary, what is observed in the output cannot be put in direct relationship with the enbw that is not so different for the three iir filter architectures. the supply harmonics occupy a large percentage of the frequency interval and there is no much space left between them, so that steep sidelobes more than any other feature are relevant for a tc manufacturer. 6. conclusions  the focus of this work is the evaluation of the uncertainty related to the choice of the implementation of the band-pass filter that simulates track circuit behaviour. the rms value of the band-pass filter output is used for the evaluation of conducted disturbance produced by rolling stock in the track circuit operating bands, as dictated by standards [2][3]. the tests are carried out using synthetic noise sequences and real pantograph current waveforms. this uncertainty, related to the post-processing of the recorded signal in order to assess possible interference to the safety relevant track circuit systems, is one of the terms of the uncertainty budget and in some cases is not negligible if compared to other elements of the measurement chain. the variability of the rms value z(t) of the filter output evaluated with respect to the butterworth architecture using synthetic test signals gave 10.8% and 5.3% for bessel and chebyshev implementations, respectively. the application of real waveforms of recorded pantograph currents have shown a variability in the output response of the three bpf implementations of up to 100%. the bessel implementation shows the largest response due to its highest   (a)    (b)   figure  9.  short  time  fourier  transform  spectrum  of  the  swedish  pantograph  current  in  the  frequency  intervals  used  to  test  ugsk 3  (the  intensity is expressed in dba peak).  1 2 3 4 5 6 7 8 0 0.02 0.04 0.06 a m p lit u d e [ v ] 1 2 3 4 5 6 7 8 0 0.02 0.04 0.06 0.08 a m p lit u d e [ v ] 1 2 3 4 5 6 7 8 0 0.01 0.02 0.03 0.04 time [s] a m p lit u d e [ v ] (a) 1 2 3 4 5 6 7 8 0 0.02 0.04 0.06 a m p lit u d e [ v ] 1 2 3 4 5 6 7 8 0 0.05 0.1 a m p lit u d e [ v ] 1 2 3 4 5 6 7 8 0 0.01 0.02 0.03 0.04 time [s] a m p lit u d e [ v ] (b) figure 10. time response of the three bpf implementations for the swedish  transients  for  ugsk 3  with  the  208.75  hz,  222.45  hz  and  242.15  hz  channels.  time/s time/s time/s time/s a m p lit u d e /v a m p lit u d e /v a m p lit u d e /v a m p lit u d e /v a m p lit u d e /v a m p lit u d e /v f re q u e n c y /h z f re q u e n c y /h z acta imeko | www.imeko.org  august 2013 | volume 2 | number 1 | 39  sidelobes, which do not allow filtering out the leakage of supply harmonics and other non-harmonically related components during transients. it is thus sensible to assume that a track circuit manufacturer implements a more selective frequency response for its product, that is best simulated by a butterworth or chebyshev architecture. the ti 21 track circuit is an exception. the frequency response curves of butterworth and chebyshev may still exhibit in some cases a deviation of up to 30%, but only a few % on average. it is thus evident that specifying the attenuation at a few frequency points is an insufficient characterization of the track circuit, for which the required band-pass filter architecture should always be specified by the standards. references  [1] a. mariscotti, “distribution of the traction return current in ac and dc electric railway systems”, ieee transactions on power delivery, vol. 18, oct. 2003, pp. 1422-1432. [2] “cenelec std. fpren 50238-2, railway applications – compatibility between rolling stock and train detection systems – part 2: compatibility with track circuits”, 2008-06. [3] “cenelec std. clc/ts 50238-2, railway applications – compatibility between rolling stock and train detection systems – part 2: compatibility with track circuits”, 2010-07. [4] a. steimel, “electric traction motive power and energy supply: basic and practical experience”, oldenbourg industrieverlag, munich, germany, 2008. [5] ccitt (international telegraph and telephone consultative committee), 1989, “directives concerning the protection of telecommunication lines against harmful effects from electric power and electrified railway lines”, volume iv (isbn 92-6104051-9), geneva, switzerland. [6] “cenelec std. clc/tr 50507, railway applications – interference limits of existing track circuits used on european railways”, 2007-05. [7] b. hemmer, a. mariscotti, d. wuergler, “recommendations for the calculation of the total disturbing return current from electric traction vehicles”, ieee transactions on power delivery, vol. 19, april 2004, pp. 1190-1197. [8] a. mariscotti, m. ruscelli and m. vanti, “modeling of audiofrequency track circuits for validation, tuning and conducted interference prediction”, ieee transactions on intelligent transportation systems, vol. 11, march 2010, pp. 52-60. [9] s.k. mitra, “digital signal processing – a computer based approach”, 1st ed., mcgraw-hill, new york, 1998. [10] a. mariscotti, “direct measurement of power quality over railway networks with results of a 16.7 hz network”, ieee transactions on instrumentation and measurement, vol. 60 n. 5, may 2011, pp. 1604-1612. [11] a. mariscotti, “results on the power quality of french and italian 2x25 kv 50 hz railways”, i2mtc 2012, graz, austria, may 13-16, 2012. acta imeko, title acta imeko issn: 2221-870x march 2018, volume 7, number 1, 1 acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 1 section: a message to our readers dear reader, the first issue of volume 7 of acta imeko is in your hands. it is the seventh year of publishing this open access scientific journal exploring all the fields of measurement science, methods, functionality, and applications; a science that drives and underpins much of what we do and experience in our everyday lives. this is a small but a rather important achievement for all the people involved in this venue. let me take this occasion to thank all of them and especially prof. paul p.l. regtien, founding editor-inchief, and prof. paolo carbone, former editor-in-chief of this journal who have ended their terms. they both worked together for making the journal grow with the help of an excellent editorial board, which is now upgraded with dr. dirk röske in the role of associated editor, prof. francesco lamonaca and dr. nicola pompeo in the role of copyeditors and prof. rosario morello, former and current layout editor, and prof. sabrina grassini in the role of article editor. acta imeko has now a steady flux of submitted papers from imeko events and scientific articles on measurement and instrumentation not related to an imeko event. it is listed under elsevier scopus and papers are cited and consistently read by the community of scientists working in measurement around the world. acta imeko was established to be a reference point of the state of art in the measurement field for all people involved directly or indirectly in measurements and to assist them in the translation of science into practice. many scientists in the measurement field have shared and supported this project by joining the journal’s scientific committee, forming a selected pool of reviewers, who are the fundamental and most important factor in the process of reviewing and editing to improve the quality of papers. papers are sought that address innovative solutions to the development of measurement science, includes all aspects both theoretical and practical with reference to measurements, whatever their uncertainty are, and in whatever fields of science or technology they occur. published papers have to show advancing knowledge in the field of measurements in comparison themselves with other available measurement methods and techniques, discusses measurement uncertainty, and shows that it provides a new insight into the field of measurements. it is worth to spend time and energy to make sure that the overall quality of the acta imeko will improve in the only acceptable way: through a timely and strict review process, driven by the most competent and meticulous editors, and assigned to competent reviewers, whose comments will invaluably help authors to improve their papers. thank you all for your contributions to acta imeko, as authors and readers. let me now conclude with a warm thank to paul and paolo for the wonderful job that they have done and for the honour of this nomination and challenging position for me as the new editor-in-chief of acta imeko. let me also thank the wonderful team of editors and reviewers for their hard and, mostly, obscure work. they have most of the merit for the present quality of this journal, and i hope that they will continue cooperating with me in the same great way that they did with paul and paolo. let me thank some of the people that worked hard over the last years to produce this journal: dirk roeske and rosario morello. last but not the least, let me thank also to german ptb for the continuous support provided to acta imeko and to judit farago at imeko. sincerely, dušan agrež editor-in-chief of acta imeko acta imeko issn: 2221-870x march 2018, volume 7, number 1, 2 acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 2 editorial to the issue vol.7.1 dušan agrež university of ljubljana, slovenia section: editorial citation: dušan agrež, editorial to the issue vol 7, no 1 (2018), acta imeko, vol. 7, no. 1, article 1, march 2018, identifier: imeko-acta-07 (2018)-01-1 section editor: paul regtien, the netherlands received march 27, 2018; in final form march 28, 2018; published march 2018 copyright: © 2018 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: dušan agrež, email: dusan.agrez@fe.uni-lj.si dear reader, this first issue in 2018 of acta imeko comprises the extended and updated versions of 8 selected papers from 15th imeko tc10 workshop on technical diagnostics: ‘technical diagnostics in cyber-physical era’, which took place in budapest, hungary, on june 6-7, 2017, and 5 selected papers and one technical note from the 5th international conference imeko tc16, measurement of pressure and vacuum, which took place in pereira, colombia, on may 8-10, 2017. we would like to thank all those who have contributed to this issue: authors, reviewers, copyeditors, layout editors and journal managers. special thanks go to the section editors lorenzo ciani and marcantonio catelani (tc10) and jorge c. torresguzman (tc16). on behalf of the editorial board i sincerely hope that you will enjoy the scientific content of this acta imeko issue. dušan agrež editor-in-chief of acta imeko editorial to the issue vol.7.1 << /ascii85encodepages false /allowtransparency false /autopositionepsfiles true /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /error /compatibilitylevel 1.4 /compressobjects /tags 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/pdfxoutputintentprofileselector /documentcmyk /preserveediting true /untaggedcmykhandling /leaveuntagged /untaggedrgbhandling /usedocumentprofile /usedocumentbleed false >> ] >> setdistillerparams << /hwresolution [2400 2400] /pagesize [612.000 792.000] >> setpagedevice template for an acta imeko event paper acta imeko issn: 2221-870x june 2018, volume 7, number 2, 60 acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 60 editorial to selected papers from the imeko tc21 conference amctm 2017 franco pavese tc21 chair, imeko, torino, italy section: editorial citation: franco pavese, editorial to selected papers from the imeko tc21 conference amctm 2017, acta imeko, vol. 7, no. 2, article 10, june 2018, identifier: imeko-acta-07 (2018)-02-10 editor: dušan agrež, university of ljubljana, slovenia received june 22, 2018; in final form june 22, 2018; published june 2018 copyright: © 2018 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: franco pavese, email: frpavese@gmail.com dear reader, this special issue includes five papers, originating from subject matters presented at the imeko tc21 international conference amctm 2017 held in glasgow in 2017 and selected for publication in acta imeko. the initial paper is a review about how knowledge is gained progressively and about where, in this process, the intercomparisons are an essential step. the paper of shestakov is an ample review of the viewpoint of a russian major university about dynamic measurement. the paper of miller et al. concerns a problem in largevolume metrology (lvm) manufacturing, where the uncertainty must be evaluated in the frame of optical-based metrology. the paper of franceschini and maisano illustrates, also in the frame of lvm systems, a new mathematical/statistical model for the real-time localization of the probe. the paper of shestakov et al. proposes basic ideas for the mathematical modelling of optimal dynamic measurements, the key issue being the construction of a mathematical model of the measuring transducer. franco pavese imeko tc21, chair editorial to selected papers from the imeko tc21 conference amctm 2017 acta imeko, title acta imeko issn: 2221-870x september 2017, volume 6, number 3, 35-44 acta imeko | www.imeko.org september 2017 | volume 6 | number 3 | 35 vision metrology and structure from motion for archaeological heritage 3d reconstruction: a case study of various roman mosaics mauro lo brutto, gino dardanelli dept. of civil, environmental, aerospace, materials engineering (dicam), university of palermo, viale delle scienze, 90128 palermo, italy section: research paper keywords: mosaic; photogrammetry; computer vision; camera calibration; 3d model; accuracy citation: mauro lo brutto, gino dardanelli, vision metrology and structure from motion for archaeological heritage 3d reconstruction: a case study of various roman mosaics, acta imeko, vol. 6, no. 3, article 6, september 2017, identifier: imeko-acta-06 (2017)-03-06 section editor: sabrina grassini, politecnico di torino, italy received march 3, 2017; in final form june 15, 2017; published september 2017 copyright: © 2017 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: mauro lo brutto, e-mail: mauro.lobrutto@unipa.it 1. introduction a 3d survey of archaeological heritage is a fundamental step in increasing our knowledge of archaeological finds and sites. much information can be obtained from a correct 3d documentation, including: measurements regarding the shape and size of objects, information relating to the state of preservation, data related to material deterioration, etc. the techniques available to us as part of a 3d survey of cultural archaeological heritage permit us to obtain very accurate and detailed metric data for large and small dimension objects [1]. in archaeology a 3d survey of objects requiring a very high level of accuracy (<1 mm) can be performed typically with active sensors (such as triangulation-based range sensors and pattern projection sensors) or passive sensors (such as image-based techniques) [2]. in recent years, image-based techniques have become increasingly used in 3d surveys, thanks to the integration of a photogrammetric and computer vision approach. this integration has enabled the development of a fully automated pipeline. some examples are reported in [3] where the advantage of photogrammetric/computer vision approach is demonstrated for stratigraphic archaeological excavations. this work outlines how such approach can be applied to map archaeological excavations in a very fast and uncomplicated way, using imagery shot with a standard compact digital camera (even if the images were not taken for this purpose). photogrammetric and computer approach in archaeology has become increasingly popular due mainly to the availability of abundant low-cost and open-source software and the ease of processing stages. in [4] some well-known opensource software packages, a web service and a low-cost software for image-based generation of point clouds or 3d mesh were compared in order to analyse the accuracy and the abstract vision metrology and computer vision can be successfully used for archaeological heritage 3d reconstruction in very high uncertainty 3d measurement projects. of those archaeological objects requiring very accurate measurements (<1 mm), ancient mosaics comprise some of the most important. the aim of this paper is to assess the photogrammetric/computer vision approach in a vision metrology context as part of a 3d mosaics survey. in order to evaluate the optimal photogrammetric/computer vision workflow in this work, three different surveys were performed on three mosaics of different sizes and locations. two of these are stored at the antonino salinas regional archaeological museum in palermo (italy) and the other is located at the baglio anselmi regional archaeological museum in marsala (italy). the mosaics survey was undertaken in order to obtain a very detailed 3d model and a full-scale orthoimage (scale 1:1), which would be useful for documentation and restoration processes. the research involved an evaluation of the potential and the related issues of the photogrammetric/computer vision approach for 3d mosaic documentation, particularly regarding the issue of camera calibration. acta imeko | www.imeko.org september 2017 | volume 6 | number 3 | 36 reliability of 3d model of archaeological finds and objects. further tests have been done using web service in [5] for 3d modelling of architectural heritage. some researchers have aimed at investigating the potentials of the computer visionbased software package agisoft photoscan, for the 3d documentation of archaeological surfaces and contexts during excavations, for the visualisation of excavation data during the post-excavation process and for the visualisation of the unmovable archaeological heritage for a professional and a wider audience [6]. furthermore, several recent studies have demonstrated that photogrammetric and computer vision approach has improved its performance to the point of being comparable with that of active range sensors [7], [8]. as is widely known, one of the main objectives of photogrammetry is measurement accuracy and, for this reason, the use of close-range photogrammetry in metrology applications is not new. the term vision metrology is also often used to describe this technology, when applied to tasks involving higher accuracy 3d measurement [9]. for example, vision metrology is typically applied to engineering and manufacturing contexts where it is necessary to obtain a measuring accuracy in the range of a few tens of micrometres to tenths of a millimeter, and where object size is in the range 1÷10 m [10]. vision metrology applications could be for quality control [11], for reverse engineering [12] or for soil microtopography evaluation [13], [14]. the measurement results must be accurate, repeatable and traceable to national or international standards (such as the german standard vdi/vde guidelines) [15]. to achieve such a degree of performance, vision metrology systems adopt carefully designed image networks, calibrated cameras, coded targets for point measurements and very accurate scale bars to scale the photogrammetric model and to check for accuracy [16]. computer vision, however, aims at the automatic image orientation of large unordered and uncalibrated image sequences, using a structure for motion (sfm) algorithm [17] and it does not place any emphasis on measurement accuracy. photogrammetry and computer vision enable us to produce final products with comparable features; originally, computer vision, and particularly the sfm approach, were not considered suitable for application to vision metrology due to a lack of results in terms of accuracy and the reliability of the process. recently, advances have been made in several software packages that permit the integration of the sfm and photogrammetric approach into the same pipeline. examples of these include: commercial software packages like agisoft photoscan, pix4d, contexcapture, 3df zephyr or open source software packages like micmac. thus, the photogrammetric/computer vision approach could also be deployed in a metrological context, even if many aspects (like the camera network or the camera calibration) must be taken into account. a number of tests have been reported in a metrological context for both outdoor and environmentally controlled conditions [18] and for geoscience applications [19]. in a 3d survey of archaeological heritage, there are various circumstances in which the metrological aspect is very important: for example, the survey and documentation of ancient mosaics, ancient paintings and frescoes requires submillimetre uncertainty with which to plan a restoration and conservation activities [20], [21]. these objects will have to be measured and monitored to a high level of accuracy because small geometric variations could cause very serious damage. of those objects requiring very accurate measurements (<1 mm), mosaics are a special case due to their variability in size (mosaics can range from a few decimetres to several metres) and the diverse conditions of conservation (the mosaics can be in an original site or preserved in a museum). moreover, the presence of the tesserae, the reference unit of these objects, renders a mosaics survey challenging. although the dimensions of the mosaics may be measured in metres, the single tessera is always very small (generally a few millimetres). the tesserae of ancient mosaics are generally unique elements, which are characterized by a high geometric and material complexity. the traditional survey approach involves single tessera, using a transparent polyester film that rests in contact with the mosaic. all the tesserae are drawn on the film in order to obtain a fullscale (scale 1:1) drawing. whilst extremely detailed and accurate, this type of documentation has several drawbacks: the lengthy time required for drawing, the difficulty of operating in situ and operator subjectivity. in addition, any drawing produced is unwieldy to use and reproduce, and not easily portable due to its size. the use of image-based techniques is a great support in surveying mosaics, specifically regarding the documentation of single tesserae and a full-scale (scale 1:1) drawing. various applications have been performed in recent years as part of a close-range photogrammetry survey of the mosaics of saint mark's basilica in venice [22]. in this work, the researchers obtained a 3d model and an ortho-image on a scale of 1:1 of the entire mosaic of the basilica (2,100 m²), using a traditional close-range photogrammetric approach. more recently, additional tests have been undertaken in order to compare different mosaic point clouds, from laser scanners and from sfm. this has enabled the documentation of three ancient roman mosaics and the evaluation of the best point cloud resolution for a detailed shape analysis of each tessera [23]. the aim of this paper is to define the workflow of a mosaic survey, using the photogrammetric/computer vision approach in a vision metrology context. an accuracy assessment relating to the various camera calibration processes has also been performed. the work involved a 3d survey of three ancient roman mosaics, which were used as test data, and it permitted us to obtain 3d models and ortho-images of the mosaics to a high level of accuracy. such products could assist in the restoration and preservation process. the paper is organized as follows: section 2 provides the main information relating to the three ancient mosaics herein discussed. section 3 describes data acquisition. section 4 analyses data processing and accuracy results. section 5 describes 3d mosaic reconstruction phases. the concluding section contains comments. 2. the ancient mosaics the research involved the use of three ancient roman mosaics as test data. two mosaics are preserved at the antonino salinas regional archaeological museum in palermo (italy) and the third is located at the baglio anselmi regional archaeological museum in marsala (italy). the former two mosaics belong to the piazza della vittoria archaeological site, which is located within the city centre of palermo (italy). the discovery of this archaeological site in one of the oldest and most central parts of palermo has revealed two buildings: building a and building b. the construction of the former dates back to the early 3rd century ad, that is, the roman imperial age; the latter can be dated to the late 2nd acta imeko | www.imeko.org september 2017 | volume 6 | number 3 | 37 century bc, that is, the hellenistic period. the visible remains today of building a are composed of parts of walls and decorations, which were unearthed during archaeological excavations. building a is characterized as having rich mosaic pavements, which were removed and preserved in part in the antonino salinas regional archaeological museum and in part left in situ. the two mosaics discussed in this paper belong to building a. the first mosaic is a bichrome opus tessellatum mosaic with geometric decorations (figure 1). the mosaic is made of square tesserae of black and white marble. the mosaic decoration is a geometric drawing, obtained by the arrangement of black tesserae on a white background. the black tesserae are arranged to form rows of recumbent, spindle-shapes that constitute a grid with squares whose sides’ measure approximately 40 centimetres; each square has a crosslet with chevrons in the centre. the mosaic is about 5.50 m by 4.20 m. the second mosaic is a geometric polychrome mosaic, composed of different colour tesserae (figure 2). the basic element of the mosaic decoration is the lozenge shape with different colour tesserae arranged to form this repeating, geometric element; thus a slanting direction is formed which creates a regular pattern. the materials used for the mosaic are white, black, green, yellow, orange, pink and red limestone. this mosaic is approximately 2.30 m by 1.75 m. the third mosaic (figure 3), preserved at the baglio anselmi regional archaeological museum in marsala (italy), belongs to the capo boeo archaeological site. this archaeological area extends for 28 hectares within the new city of marsala, much of it overlapping the settlement of the ancient city of lilybaeum. the carthaginians founded the latter in the 4th century bc and it later became a roman colony. the archaeological surveys have uncovered: ancient fortifications, the thermae (baths), the decumanus maximus (the main east-west-oriented road in a roman city), and various examples of domus (the private homes in a roman city). the mosaic had originally been placed in compartment n. 36 of a domus in the i insula; it is considered to be the most important due to its considerable architectural complexity and elaborate decoration. the fine details of this mosaic render it a most refined mosaic, characterized by different colours, which paint a geometric polychrome decorative motif. its dimensions are 4.50 m by 5.50 m. 3. data acquisition the data acquisition has been carried out with the same parameters (same camera, same focal lens, same camera network, same camera-to-object distance, etc.) in order to obtain similar datasets for the three mosaics. in all three locations, the logistic conditions have not presented particular problems for the photogrammetric survey; to get the best lighting conditions the photogrammetric survey was performed without the use of artificial lights or spotlights but only using natural light. the images acquisition has been carried out using a nikon d5200 digital camera equipped with a 28 mm nikkor af-s f/2.8g fixed focus lens; the camera has a ccd sensor with size of 23.5 mm × 15.7 mm, a pixel size of 3.9 µm and an effective resolution of 6000 pixels × 4000 pixels. the camera-to-object distance was chosen equal to 1.5 m; the image scale was 1/54 and the coverage of each image was about 1.2 m × 0.8 m. because the camera focal length was 28 mm, each pixel was about to 0.2 mm in the object space. during the images acquisition the lens was set to manual figure 1. the bichrome opus tessellatum mosaic at the antonino salinas regional archaeological museum in palermo (italy). figure 2. the geometric polychrome mosaic at the antonino salinas regional archaeological museum in palermo (italy). figure 3. the geometric polychrome mosaic at the baglio anselmi regional archaeological museum in marsala (italy). acta imeko | www.imeko.org september 2017 | volume 6 | number 3 | 38 focus after adjusting it to the average camera-to-object distance of the project and then adequately fixed throughout the shooting process. a nadiral stereoscopic coverage has been planned for the three mosaics with strips parallel to the longer side of the mosaic. the photogrammetric strips have been selected providing a forward overlap and a lateral overlap of 70 %. some additional convergent strips have been also planned along the edge of the mosaic to increase the redundancy of the measures at the edges of the photogrammetric block and to limit bowl-effect in the 3d model [24]. in this way three photogrammetric datasets have been obtained, called “mosaic1” for the first mosaic, “mosaic-2” for the second and “mosaic3” for the third. figure 4 shows the camera network for the three datasets. the images have been taken with the camera mounted on a tripod (figure 5); this condition has allowed to acquire nadiral images maintained the sensor position parallel to mosaic’s plane. during data acquisition, some extra convergent images were also taken; overall mosaic-1 and mosaic-3 have almost the same number of images, mosaic-2 has about a quarter of images than the others (table 1). some photogrammetric coded targets have been positioned around and along the edge of the mosaics to improve the photo orientation step for the images taken along the edge of the mosaics. the use of coded targets, encoded with a unique identifier, is very common in vision metrology to signalize feature points [25]. concentric rings coded targets with a 12 bit pattern have been used (figure 6); these targets are relatively easy to recognize and decode. it was not possible to measure the position of the targets with accuracy suitable to the purpose of the work but three coded targets were put to define a local reference system (figure 7); the coordinates of these three targets were arbitrarily (a) (b) (c) figure 4. planimetric view and cross-section view of camera network geometry for the three datasets: (a) mosaic-1, (b) mosaic-2 and (c) mosaic-3. figure 6. concentric rings coded targets with a 12 bit pattern. figure 7. placement of coded target for mosaic-2; in the lower left corner the three targets for defining the reference system. (a) (b) (c) figure 5. data acquisition: (a) mosaic-1, (b) mosaic-2 and (c) mosaic-3. table 1. photogrammetric blocks. datasets images nadiral strips convergent strips mosaic-1 401 17 4 mosaic-2 100 8 4 mosaic-3 433 18 4 x y acta imeko | www.imeko.org september 2017 | volume 6 | number 3 | 39 assigned. moreover, eleven calibrated bars have been placed along the edge of the mosaics, two near each corner and one along the edge in the center position (only for three edges) (figure 8). calibrated bars are of aluminium and are long 50 cm; every bar has four circular holes. the distance between the center of the holes was measured by a computer numerical control machine mazak vertical center nexus 410a-ii with an accuracy of ±50 µm. for each bar two calibrated distances, one of 48 cm and another of 46 cm were measured (figure 9); one distance has been used to scale the photogrammetric model, the other to check the accuracy of the photogrammetric survey. to evaluate the theoretical uncertainty of the survey the traditional photogrammetric formulas for so-called “normal case of stereo-photogrammetry” have been used [26]. the theoretical uncertainty σx and σy, respectively in x and y coordinates, has been calculated from: 𝜎𝑋 = 𝜎𝑌 = 𝐷 𝑐 𝜎𝑥′ , (1) where d is the camera-to-object distance, c is the focal length and σx’ is the image measurement uncertainty. the theoretical uncertainty σz along the z direction depends also on the ratio d/b, where b (baseline) is the distance between the two camera stations, and has been calculated from: 𝜎𝑍 = 𝐷2 𝑐∙𝐵 𝜎𝑝𝑥′ , (2) where σpx’ is the image measurement uncertainty of the xparallax. these formulas give reasonable approximation of achievable uncertainty and depend primarily on image measurement uncertainty. for a close range photogrammetric/ sfm approach the image measurement uncertainty depends on the accuracy of feature extraction and feature matching, that generally can achieve a sub-pixel accuracy [27]. for this work it was supposed σpx’=σx’ and an image measurement uncertainty of 0.5 pixel, corresponding to 1.95 µm; the theoretical uncertainty has been estimated at 0.10 mm for x and y and 0.42 mm for the z direction. the theoretical uncertainty of 2d point measurement σxy and 3d point measurement σxyz can be calculated as 𝜎𝑋𝑌 = �𝜎𝑋 2 + 𝜎𝑌 2 (3) and 𝜎𝑋𝑌𝑍 = �𝜎𝑋 2 + 𝜎𝑌 2 + 𝜎𝑍 2 . (4) these values are respectively equal to 0.14 mm and to 0.44 mm and, in particular for the xy accuracy, are compatible with the accuracy of photogrammetric measurements required for the production of an ortho-image at full-scale (scale 1:1). it should be noted that these values are only theoretical values; in fact, the theoretical uncertainty of the so-called “normal case of stereo-photogrammetry” is computed assuming that camera interior orientation parameters and image external orientation parameters are free of error. the actual accuracy depends also on other parameters as the number of images, the metric constraint, etc. and theoretical values must be always validated in the real case. 4. data processing the images processing has been done, through the typical sfm workflow, using the well-known commercial software package photoscan professional edition by agisoft llc [28] (figure 10). this software package provides a sequence of automatic steps for image orientation and image matching; moreover, photoscan allows extracting 3d models and orthoimages with a very high level of detail. during image orientation (called “photo alignment” in photoscan) the software can estimate both internal camera parameters and external camera orientation for each image. photoscan is the typical sfm software that has also integrated photogrammetric procedures; in fact, it is possible to recalculate the orientation parameters through a bundle figure 8. calibrated bars arrangement along the sides of the mosaic. figure 9. detail of the calibrated bar. figure 10. data processing workflow. image acquisition camera calibration image orientation camera calibration and image orientation image matching and point cloud generation mesh generation and texture mapping orthophoto production acta imeko | www.imeko.org september 2017 | volume 6 | number 3 | 40 adjustment with self-calibration (called “optimization” in photoscan), to use coded targets for points detection and known distances as metric constraints, to obtain some parameters about the uncertainty of the process and to assign an accuracy value to the metric constraints (ground control points or distances). for camera calibration the software package photomodeler scanner by eos systems inc. has been used too [29]. photomodeler scanner is a typical and well-known close-range photogrammetry software package. the data processing was used to test some different camera calibration approaches and to produce very detailed and accurate 3d models and ortho-images. 4.1. camera calibration the calibration process is one of the most important aspects of the photogrammetric workflow. a correct estimation of camera interior orientation parameters and lens distortion parameters is necessary in order to obtain accurate photogrammetric measurements. this aspect becomes even more important when the accuracy of the photogrammetric survey is less than one millimetre [30]. in close-range photogrammetry the camera is generally calibrated in advance, using a test field, and generally performed with coded targets. the camera interior orientation parameters and the lens distortion parameters were calculated independently from the photogrammetric survey. a self-calibrating bundle adjustment, in which interior orientation parameters and distortion parameters are unknown, was used to solve the camera calibration [31]. according to the sfm approach, the camera calibration process is not considered a particularly important task, and the camera parameters are almost always calculated as part of the orientation process [32]. this approach was developed mainly with the aim of achieving maximum process automation and not the best accuracy, as is the case with photogrammetry. therefore, camera calibration issues have invariably been not particularly considered. as part of the sfm approach, the use of a huge number of feature points (rather than a small number of targets) can yield enhanced internal uncertainty of the calibration parameters but reduced reliability of the camera parameters. this is mainly due to the poor reliability of the camera network, which may be involved. on the other hand, the high accuracy required for mosaic surveys places great emphasis on camera calibration. for this reason different camera calibration methods were tested to determine the most suitable workflow for this phase. specifically, two approaches were evaluated: the first used a set of coded targets as part of a typical photogrammetric approach (pre-calibration); and the second made use of a standard sfm approach (self-calibration) (figure 11). the typical photogrammetric approach was performed using either photoscan (ps) and photomodeler scanner (pm). two different test fields were prepared: one with pm-coded targets and another with ps-coded targets (figure 12). the images were taken in accordance with the classic rules for camera calibration in photogrammetry (multi-station, convergent imaging network, images rotated by ±90°, etc.) [21], thereby obtaining two different calibration datasets for each mosaic. accordingly, many feature points were measured in the precalibration process with ps, in addition to the coded targets. moreover, an additional camera calibration was performed, using the typical sfm approach. the camera parameters were calculated during the self-calibration by the ps optimization process. three different sets of calibration parameters were, therefore, calculated for each mosaic (pm pre-calibration, ps pre-calibration and ps self-calibration). the standard photogrammetric camera model for camera calibration, which consists of three elements of interior orientation (principal point coordinates and principal distance) and three types of lens distortion (radial, tangential and affine), was taken into consideration in the analysis. after this process, all parameters were analysed according to the camera model for ps and the parameters which had been calculated with pm were converted into the ps standard, using the agisoft lens software package. this software is necessary because various software packages use slightly different camera models and parameter sets for their calibrations; the parameters are not, however, directly comparable. the camera model used in ps assume a central projection camera where non-linear distortions are modelled using the self-calibration model proposed by brown [33]. a camera model specifies the transformation from point coordinates in the local camera coordinate system to the pixel coordinates in the image frame. the model used to project points in the local camera coordinate system is 𝑥 = 𝑋 𝑍 (5) 𝑦 = 𝑌 𝑍 (6) 𝑟 = �𝑥2 + 𝑦2 (7) 𝑥′ = 𝑥(1 + 𝐾1𝑟2 + 𝐾2𝑟4 + 𝐾3𝑟6 + 𝐾4𝑟8) + (𝑃1(𝑟2 + 2𝑥2) + 2𝑃2𝑥𝑦)(1 + 𝑃3𝑟2 + 𝑃4𝑟4) (8) 𝑦′ = 𝑦(1 + 𝐾1𝑟2 + 𝐾2𝑟4 + 𝐾3𝑟6 + 𝐾4𝑟8) + (𝑃1(𝑟2 + 2𝑦2) + 2𝑃2𝑥𝑦)(1 + 𝑃3𝑟2 + 𝑃4𝑟4) (9) 𝑢 = 𝑤 2 + 𝑥𝑝 + 𝑥′𝑐 + 𝑥′𝐵1 + 𝑦′𝐵2 (10) 𝑣 = ℎ 2 + 𝑦𝑝 + 𝑦′𝑐 , (11) figure 11. camera calibration process. (a) (b) figure 12. camera calibration test field for pm (a) and ps (b). camera calibration pre-calibration self-calibration pm ps ps acta imeko | www.imeko.org september 2017 | volume 6 | number 3 | 41 where (x, y, z) are the point coordinates in the local camera coordinate system, (u, v) are the projected point coordinates in the image coordinate system (in pixels), c is the principal distance, (xp, yp) are the principal point coordinates, kn are the radial distortion parameters, pn are the tangential distortion parameters, bn are the affinity and shear distortion parameters, and (w, h) are the image width and height in pixels. two procedures can be followed in determining the most reliable values of camera calibration: the first concerns the recovery accuracy of the individual parameters of the calibration model, while the second relates to the accuracy impact on any subsequent photogrammetric measurements, i.e. the effect of calibration errors on object point determination [34]. the first approach could be applied to close-range photogrammetric software packages (like pm), which reports the standard error of the unknown parameters computed during the camera calibration. these errors can provide us with information regarding the inner reliability of the process but it does not give any information about the accuracy of the object point coordinates.. moreover, the software package from a computer vision, like ps, lacks information regarding this step. for these reasons, it is not possible to evaluate the camera calibration according to the first procedure. the second procedure requires various independent checking measures to estimate the accuracy in object space and to evaluate camera calibration reliability. this second procedure will be described in detail in subsection 4.2. the values obtained in the calibration with the test field (pre-calibration) are always very similar to each other. on the other hand, the values obtained by self-calibration are different from each other, particularly regarding the principal distance and principal point coordinates. moreover there exist differences in the lens distortion coefficients (on occasions of one or two orders of magnitude), specifically, the radial distortion parameters k3 and k4 and the affine distortion coefficients b1 and b2. 4.2. image orientation images were oriented with the aim of assessing the most suitable camera calibration procedure with which to obtain the maximum accuracy of the final products. three different projects were created for each dataset: one with the parameters calculated by pm (pm pre-calibration project), another with the parameters calculated by ps (ps pre-calibration project) and the third with the parameters calculated by ps during image orientation (ps self-calibration project). images were oriented using only ps and this procedure was divided into several steps: i. image orientation with an sfm approach, which was performed considering the first 1/16 of the original image resolution (low photo alignment), without any image pair preselection for feature points detection. thereafter, considering 1/2 of the original image resolution (high photo alignment) and the generic preselection mode, which only used the overlapping pairs of photos to detect feature points (no camera calibration parameters were estimated at this stage) ii. an automatic measurement of the coded targets and manual measurement of the calibrated bars iii. importing a different set of camera calibration parameters, which had been held constant throughout the next step (no parameters were imported in the ps self-calibration project) iv. a re-computation of exterior orientation parameters by means of a bundle adjustment, taking the calibrated bars into consideration as constraints. in this step the camera calibration parameters were also calculated in the ps self-calibration project v. an analysis of the results. all the projects were calculated according to an arbitrary reference system by applying a free-network solution bundle block adjustment and by using only the 48 cm calibrated distances of the scale bars with which to scale the photogrammetric model. however, the 46 cm calibrated distances of the scale bars were used as independent check, control distances with which to evaluate the accuracy of the projects. this procedure was chosen in agreement with german standards (vdi/vde 2634 part 1) [15] for evaluating the object space accuracy of 3d point measurement systems, which are based on a length measurement error (lme). this value is the difference between the distance calculated by the photogrammetric process and the calibrated distance of the scale bar; the root mean square error (rmse) of all the differences provides us with an assessment of the accuracy of the photogrammetric measurement. table 2 reports the rmse of the scale bars for 48 cm calibrated distances, which were used to scale the photogrammetric model, and the 46 cm calibrated distances, which were used as an independent check. the mean of the residuals was also checked to verify the presence of systematic effects. the mean of the residuals for all projects was very close to zero for the distances used to scale the photogrammetric model and for distances used as an independent check. the rmse of the calibrated check distances for almost all projects was lower than 0.2 mm. the diverse tests indicated that the table 2. rmse for scale bars. pm pre-calibration [mm] ps pre-calibration [mm] ps self-calibration [mm] mosaic-1 rmse scale distances 0.453 0.050 0.030 rmse check distances 0.440 0.054 0.033 mosaic-2 rmse scale distances 0.137 0.083 0.230 rmse check distances 0.118 0.068 0.180 mosaic-3 rmse scale distances 0.158 0.181 0.183 rmse check distances 0.162 0.180 0.191 acta imeko | www.imeko.org september 2017 | volume 6 | number 3 | 42 projects with camera calibration parameters obtained as part of an independent process could provide greater accuracy. there was a notable improvement in mosaic 2 (of approximately one order of magnitude for the ps pre-calibration project) while the differences with mosaic 3 were less evident. only mosaic 1 revealed different results: very high residuals for the pm precalibration project (the calibrated check distances reached about 0.4 mm) but very low residuals (in the order of few hundredths of a millimetre) for the other two projects. the accuracy evaluation has, however, some critical issues. the arrangement of the calibrated bars is not the most favourable because the bars are only along the edges of the mosaics. theoretically, they should be placed also inside the mosaic; this condition was not possible because in this way they would cover the tesserae of the mosaic. the accuracy verification should be performed along three dimensions, as required in the vdi/vde guideline; this was challenging for a survey of mosaics and the scale bar distribution was indeed coplanar with the mosaics. finally, the dimension of the mosaic would have required some more long reference distances, similar to the dimension of the mosaics (i.e. 4 m or 5 m). in our study, the short calibrated distance was due to the limit of the computer numerical control machine used for the distance measurement. this issue is generally very common in metrology applications with close-range photogrammetry; in fact, it is very difficult to find scale bars suitable for large measuring objects (dimension > 3 m) [26]. for large measuring objects, accuracy evaluation could be checked using very accurate checkpoints measured with total station that allowed recording data with high angular and distance accuracy, and with very redundant topographic schemes. this approach is not always possible because of the logistical conditions of the site, such as in our situation where the photogrammetric surveys were conducted in museums. as can be noted from table 2, the accuracy obtained in almost all tests was better than the theoretical 3d uncertainty σxyz, which had been calculated in (4); however, as previously underlined, the scale bars arrangement is not the most suitable for assessing 3d accuracy. the results are comparable with the theoretical 2d uncertainty σxy, which was calculated in (3). the accuracy results could however be considered as satisfactory in obtaining a full-scale (scale 1:1) ortho-image. 5. 3d models and ortho-images of ancient mosaics the 3d models and the ortho-images of the ancient mosaics were obtained using the projects which obtained the greatest accuracy (that is, the ps pre-calibration projects for mosaic 1 and mosaic 2, pm pre-calibration for mosaic 3). the 3d models were achieved according to the following steps: calculating of a dense point cloud by dense image matching, building of a 3d mesh and texturing of the 3d mesh. the point clouds were calculated by taking into account: the theoretical gsd, the residuals of the orientation phase and the level of detail suitable for the 3d model. thus, a point cloud with a ¼ of an image resolution was calculated as corresponding to about 0.8 millimeters (figure 13). three point clouds with 74 million points (mosaic 1), 16 million points (mosaic 2) and 63 million points (mosaic 3) were obtained, and three meshes with 15 million polygons (mosaic 1), 3 million polygons (mosaic 2) and 12 million polygons (mosaic 3) were calculated respectively from the point clouds. the rgb value was also associated with the 3d position for each point during the 3d reconstruction process. this value in the mesh generation process permitted the assignment of a mean value of rgb to each polygon. the more defined the mesh, in terms of numbers of polygons and geometry, the better the appearance of the 3d model. generally, a mesh with a coloured vertex will be sufficient in providing an approximate idea of the condition of the mosaic and the type of material from which it was made. in order to increase this level of detail, it is necessary to provide texture to the 3d model with the images of the photogrammetric survey. five textures of 4,096 pixels × 4,096 pixels each were generated for each 3d model with the texture number ensuring the resolution of a sufficiently detailed 3d model to analyze even the smallest details (figure 14). (a) (b) (c) figure 13. detail of mosaic-1 (a), mosaic-2 (b) and mosaic-3 (c) point clouds. (a) (b) (c) figure 14. detail of mosaic-1 (a), mosaic-2 (b) and mosaic-3 (c) 3d models. acta imeko | www.imeko.org september 2017 | volume 6 | number 3 | 43 finally, the ortho-images of the three mosaics were calculated with a geometric resolution of 0.5 mm (figure 15). good chromatic continuity and enhanced definition of the details of the mosaics were obtained in all the cases under investigation. despite being 2d data, ortho-images still constitute the main type of documentation of single tessera: they allow us to examine numerous irregularities, the size and position of an individual tessera and the presence of erosion and patina deterioration with non-invasive and more efficient procedures. 6. conclusions the research outline in this paper has evaluated the surveying of mosaics using a photogrammetric/computer vision approach in the context of vision metrology. different camera calibration processes have been tested in order to identify the most appropriate workflow. the research was performed on three ancient roman mosaics as test data. aspects relating to the camera calibration process were evaluated by virtue of different sets of internal orientation parameters. moreover, the camera calibration parameters were calculated in accordance with two photogrammetric test fields (performed with coded targets) and a sfm approach. the tests revealed slight changes between the camera parameters, particularly for the principal distance and for principal point coordinates. an accuracy evaluation was conducted by the residuals obtained from various scale bars, which were used as checks. these results also demonstrate that the projects with camera calibration parameters, which had been obtained via an independent process, could offer enhanced accuracy. differences among the different datasets were also observed. however, an rmse of the calibrated check distances for almost all projects was observed to be lower than 0.2 mm. the survey facilitated the obtaining of a 3d reconstruction and an ortho-image for each of the three mosaics, which was compatible with one at full scale (scale 1:1). this type of documentation could be repeated over the time to monitor and analyse mosaics even after restoration processes. the high accuracy obtained with a photogrammetric/sfm approach demonstrates that these techniques could also be applied to a context of vision metrology relating to archaeological 3d documentation. an evaluation of the accuracy of this approach also confirms that camera calibration is still an open issue and additional tests are required to improve any evaluation of the accuracy of the photogrammetric/sfm approach. acknowledgement thanks to directors of antonino salinas regional archaeological museum in palermo and baglio anselmi regional archaeological museum in marsala for allowing the execution of the surveys. thanks to maria grazia spera for the help in data processing and in the survey of the mosaic 2, to lorella pellegrino, alessandra garraffa, barbara di natale for the collaboration in the study and in the surveys of mosaic 1 and mosaic 3, and also to enza mancuso for the help in the survey of mosaic 2. references [1] g. guidi, f. remondino, “3d modelling from real data”. in modelling and simulation in engineering, edited by catalin alexandru, intech, 2012, pp. 69–102. 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[34] s.k. nouwakpo, m. r. james, m. a. weltz, c.-h. huang, i. chagas, l. lima, “evaluation of structure from motion for soil microtopography measurement”. photogrammetric record, vol. 29, 2014, pp. 297–316. acta imeko contacts acta imeko issn: 2221-870x october 2018, volume 7, number 3, 0 acta imeko | www.imeko.org october 2018 | volume 7 | number 3| 0 journal contacts about the journal acta imeko is an e-journal reporting on the contributions on the state and progress of the science and technology of measurement. the articles are mainly based on presentations presented at imeko workshops, symposia and congresses. the journal is published by imeko, the international measurement confederation. the issn, the international identifier for serials, is 2221-870x. editor‐in‐chief dušan agrež, slovenia founding editor‐in‐chief paul p. l. regtien, netherlands associate editor dirk röske, germany editorial board section editors (vol. 6 and 7) leopoldo angrisani, italy filippo attivissimo, italy eulalia balestieri, italy eric benoit, france paolo carbone, italy lorenzo ciani, italy catalin damian, romania pasquale daponte, italy luca de vito, italy luigi ferrigno, italy edoardo fiorucci, italy alistair forbes, united kingdom helena geirinhas ramos, portugal sabrina grassini, italy fernando janeiro, portugal konrad jedrzejewski, poland andy knott, united kingdom francesco lamonaca, italy massimo lazzaroni, italy fabio leccese, italy rosario morello, italy michele norgia, italy pedro miguel pinto ramos, portugal nicola pompeo, italy sergio rapuano, italy dirk röske, germany alexandru salceanu, romania constantin sarmasanu, romania lorenzo scalise, italy emiliano schena, italy enrico silva, italy krzysztof stepien, poland marco tarabini, italy marcantonio catelani, italy paolo carbone, italy lorenzo ciani, italy egidio de benedeto, italy alessandro germak, italy sabrina grassini, italy konrad jędrzejewski, poland min-seok kim, korea fabio leccese, italy rosario morello, italy vilmos palfi, hungary nicola pasquino, italy franco pavese, italy paul regtien, the netherlands alexandru salceanu, romania jan saliga, slovakia jorge c. torres-guzman, mexico ian veldman, south africa claudia zoani, italy final editing dirk röske, ptb, braunschweig, germany about imeko the international measurement confederation, imeko, is an international federation of actually 41 national member organisations individually concerned with the advancement of measurement technology. its fundamental objectives are the promotion of international interchange of scientific and technical information in the field of measurement, and the enhancement of international co-operation among scientists and engineers from research and industry. addresses principal contact prof. dušan agrež university of ljubljana faculty of electrical engineering tržaška 25, 1000 ljubljana, slovenia e-mail: dusan.agrez@fe.uni-lj.si acta imeko dr. sabrina grassini dept. of applied science and technology, politecnico di torino corso duca degli abruzzi 24, 10129 torino, italy e-mail: sabrina.grassini@polito.it support contact dr. dirk röske physikalisch-technische bundesanstalt (ptb) bundesallee 100, 38116 braunschweig, germany e-mail: dirk.roeske@ptb.de mailto:dusan.agrez@fe.uni-lj.si mailto:sabrina.grassini@polito.it mailto:dirk.roeske@ptb.de the determination of the volume of weights in the range of 1 g – 5 kg: a comparison of hydrostatic weighing and double weighing in air using the monte carlo simulation acta imeko issn: 2221-870x march 2020, volume 9, number 1, 61 68 acta imeko | www.imeko.org march 2020 | volume 9 | number 1 | 61 the determination of the volume of weights in the range of 1 g – 5 kg: a comparison of hydrostatic weighing and double weighing in air using the monte carlo simulation yi su1, kilian marti2, christian wüthrich2 1 simt shanghai institute of measurement and testing technology, shanghai, china 2 federal institute of metrology metas, lindenweg 50, 3003 bern, switzerland section: research paper keywords: volume determination; double weighing; hydrostatic weighing; monte carlo citation: yi su, kilian marti, christian wüthrich, the determination of the volume of weights in the range of 1 g – 5 kg: a comparison of hydrostatic weighing and double weighing in air using the monte carlo simulation , acta imeko, vol. 9, no. 1, article 10, march 2020, identifier: imeko-acta-09 (2020)-01-10 section editor: yon-kyu park, kriss, republic of korea received august 15, 2019; in final form february 25, 2020; published march 2020 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was supported by the national key r&d program of china for national quality infrastructure under grant 2017yff0205006. corresponding author: kilian marti, e-mail: kilian.marti@metas.ch 1. introduction there are six accepted methods for the determination of the density of weights, which are described in the recommendation of the organisation internationale de métrologie légale (oiml) r111 and are denoted in methods a to f [1]. these methods can broadly be classified into three categories: the hydrostatic method, the geometric measurement, and density estimation. here, the hydrostatic method, which traces the volume/density to the reference volume weight or the water density, is considered the most accurate measurement method. it is called the reference method of volume/density determination and has been implemented by many national metrology institutes (nmis) worldwide [2]. even though hydrostatic weighing is the most accurate method among those described in oiml r111 for determining the volume of the weights, it does have some disadvantages. the method is time consuming, it is expensive, and it changes the surface of the weight. after immersion in water, it can lead to instabilities in the weight. furthermore, the immersion of stainless steel weights in water can remove the adsorbed contaminants or the surface oxide layer, thus changing the surface properties [3]. however, immersion in water is unlikely to cause significant corrosion of stainless steel weights [3], [4]. besides the hydrostatic method, there are some alternative methods: optical interferometry [5], which is probably the most accurate method available; weighing with a balance immersed in fluorocarbon fluid [5]; using an acoustic volumeter [7]–[9]; or double weighing in air [10]–[12]. like hydrostatic weighing, double weighing in air is based on archimedes’ principle. however, the main difference between the two methods is the medium to which the weight is exposed. although the double weighing in air has the advantage of being clean and efficient, its consistency with the hydrostatic method requires verification by means of experiments. previous publications by clarkson [10], malengo [11], and ueki [12] have described the principle and feasibility of determining the volume of weights by double weighing in air. clarkson described a method of volume determination of weights of 1 kg nominal mass. malengo adopted a modified version of clarkson's method by considering mass and volume as two measurands that are determined simultaneously in a multivariate context. he validates the calculations experimentally by using stainless steel and platinumabstract we have investigated two methods for the determination of the volume of weights in the range of 1 g – 5 kg: double weighing in air and hydrostatic weighing. we present the mathematical equations of both methods, showing that the monte carlo simulation is a suitable way of determining the measurement uncertainties and of overcoming the difficulties in dealing with correlated variables. we found that the measurement uncertainties of the two methods are comparable and that double weighing in air is an efficient method of determining the volume of weights below 1 kg. mailto:kilian.marti@metas.ch acta imeko | www.imeko.org march 2020 | volume 9 | number 1 | 62 iridium weights of a 1 kg nominal mass. ueki investigated the simultaneous calibration of mass and volume of weights in the range of 1 kg to 20 kg. we extend the range from 5 kg down to 1 g and compare the results from double weighing to those from hydrostatic weighing. in addition, we explain how to use support weights both in double weighing and hydrostatic weighing when measuring test weights that are too small to be loaded on the weighing pan. we compare the measurement results and uncertainties of these two methods and show that the monte carlo simulation is a suitable and simple way to overcome the difficulties of laboriously calculating correlations between input quantities. 2. setup we now describe the measurement sites that we used to perform the double weighing and hydrostatic weighing. all measurement sites are located in the mass laboratory of metas. 2.1. hydrostatic method the volume apparatus vk1 at metas consists of an at1005 comparator from mettler-toledo with a maximum capacity of 1011 g and a readability of 1 µg. the suspension system is immersed in a water basin (figure 1). the water comes from an ultrapure water system (elga labwater, purelab). the suspension system carries both the reference weight and the test weight and loads them individually on the weighing pan in water. the weighing pan is connected with the comparator in air through a thin metal duct. the comparator measures the resulting force – the difference between the gravitational force and the buoyancy due to the water. the buoyancy is determined by the water density, which is measured experimentally by means of the reference weight. we use a silicon sphere as the reference volume. we do three weighings and measure (i) the reference volume, (ii) the test weight, and (iii) the suspension only. to compensate for the balance's linearity and for the different buoyancy forces and masses during the three weighings, we use auxiliary weights that are placed on the four-position weighing carousel of the comparator. the temperature, pressure, and relative humidity of the air are recorded during the measurement. additionally, four thermistors from ysi inc. measure the water temperature on the front and rear sides of the reference and test weights. the volume apparatus vk10 consists of a pr10003 comparator from mettler-toledo with a maximum capacity of 10100 g and a repeatability of 2 mg (figure 2). the operating principle is similar to that of vk1, except that the reference volume (the middle image of figure 1) is not present. the traceability of the density is obtained by calculating the formula of tanaka for the density of water [13]. 2.2. double weighing in air for the double weighing measurements, we use an at10005 comparator (mettler-toledo) with a repeatability of 0.02 mg for the 2 kg and 5 kg weights; an m_one (mettler-toledo) with a repeatability of 0.5 µg for weights from 200 g to 1 kg; and an at106 (mettler-toledo) with a repeatability of 1.5 µg for weights from 1 g to 100 g. the comparators are in airtight enclosures, in which the air density can be varied by changing the air pressure (figure 3). to change the air pressure, we use a membrane pump (813.3, knf neuberger). 3. measurement procedures in our experiment, we used oiml stainless steel weights with nominal mass settings of 1 g, 10 g, 20 g, 50 g, 100 g, 200 g, 500 g, 1 kg, 2 kg, and 5 kg (figure 4). for each weight, we first carried out a double weighing measurement at 950 mbar and 750 mbar. then, we performed a hydrostatic weighing. thereafter, we carried out a second double weighing measurement at 950 mbar and 750 mbar to check the stability of the weight and to observe possible adverse effects after immersing the weight in water. for both methods, we calculated the air density according to the cipm formula [14], which we verified by using air buoyancy artefacts before comparing the double weighing and hydrostatic weighing methods (cipm = comité international des poids et mesures, the international committee for weights and measures). figure 1. principle of the volume apparatus vk1 at the federal institute of metrology metas for the determination of volume of test weights up to 1 kg by using hydrostatic weighing. left: the weighing of the suspension only. middle: the weighing of the reference weight (silicon sphere). right: the weighing of the test weight. figure 2. vk10 apparatus at metas for volume determination of weights up to 10 kg. only the test weight is immersed in water. acta imeko | www.imeko.org march 2020 | volume 9 | number 1 | 63 3.1. hydrostatic method in the hydrostatic method, we used the silicon sphere in gravimetric weighing to determine the water density. the water density was then used to determine the volume of the test weight. small weights from 1 g to 5 g could not be loaded directly on the weighing pan because of their geometrical dimensions. instead, we used a 10 g stainless steel disc as a support weight (see section 4.1.1). 3.2. double weighing method in the double-weighing method, we compared every test weight to at least one reference weight of the same nominal mass and density. weights smaller than 10 g could not be loaded directly on the weighing pan because of their dimensions. for this reason, we used a 100 g oiml weight as a support (see section 4.2.1). 4. calculations we now explain the basic mathematical formulae we used in the hydrostatic and double weighing methods. the uncertainty calculations are presented in section 5. 4.1. hydrostatic method the basic force equations for the three weighings of the suspension only (f0), the suspension plus the test weight (f1), and the suspension plus the reference weight (f2) are | 𝐹0 = 𝐺o − 𝐴o + 𝐺z − 𝐴z 𝐹1 = 𝐺t − 𝐴t + 𝐺z − 𝐴z + 𝐺b − 𝐴b 𝐹2 = 𝐺r − 𝐴r + 𝐺z − 𝐴z + 𝐺a − 𝐴a |, (1) where gi and ai denote the gravitational force and the buoyancy, respectively. the indices represent the mass of the suspension (z) and its auxiliary weights (o); the mass of the test weight (t) and its auxiliary weights (b); and the mass of the reference weight (r) and its auxiliary weights (a). all the auxiliary weights are in air. as the weights are placed at different heights above the surface, we correct the force equations for the gravitational acceleration of each weight with 𝛼𝑖 = 𝑔e 𝑔𝑖 | | 𝑚o 𝑔e 𝛼o − 𝜌lo 𝑉o 𝑔e 𝛼o + 𝜆𝐾 = 𝑀0 𝑔e 𝛼h γ 𝑚t 𝑔e 𝛼t − 𝜌wt𝑉t 𝑔e 𝛼t + 𝑚b 𝑔e 𝛼b − 𝜌lb 𝑉b 𝑔e 𝛼b + 𝜆𝐾 = 𝑀1 𝑔e 𝛼h γ 𝑚r 𝑔e 𝛼r − 𝜌wr𝑉r 𝑔e 𝛼𝑅 + 𝑚a 𝑔e 𝛼a − 𝜌la 𝑉a 𝑔e 𝛼a + 𝜆𝐾 = 𝑀2 𝑔e 𝛼h γ | |, (2) where ge represents the gravitational acceleration at the load cell and λk equals 𝐺z − 𝐴z. the uncorrected mass difference, i.e. the indication of the balance, is denoted as mi and is corrected by the standard densities for air and material γ = (1 1.2/8000) [14]. as the suspension is present in every weighing, its influence is denoted by 𝜆. | | 𝑀0 = [ 𝑚o 𝛼o − 𝜌lo𝑉o 𝛼o + 𝜆] 𝛼hγ −1 𝑀1 = [ 𝑚t 𝛼t − 𝜌wt𝑉t 𝛼t + 𝑚b 𝛼b − 𝜌lb𝑉b 𝛼b + 𝜆] 𝛼hγ −1 𝑀2 = [ 𝑚r 𝛼r − 𝜌wr𝑉r 𝛼r + 𝑚a 𝛼a − 𝜌la𝑉a 𝛼a + 𝜆] 𝛼hγ −1 | | (3) we can now solve the volume of the test weight, vt, at 20 °c by using the thermal expansion coefficients ci of the weights. 𝑉t,water ∶= 𝑉t = [ (𝑀0−𝑀1) 𝛼h γ + 𝑚t 𝛼t + 𝑚b 𝛼b − 𝑚o 𝛼o − 𝜌lb𝑉b20𝐶b 𝛼b + 𝜌lo𝑉o20𝐶o 𝛼o ] 𝛼t 𝜌wt𝐶t (4) the coefficient ci is given by the linear thermal expansion coefficient β and the measured temperature in air or in water 𝐶𝑖 ∶= 1 + 3 𝛽 (𝑇meas − 20 °c). (5) we must know the density of water, 𝜌wt, at the test weight's position inside the water basin. to calculate 𝜌wt, we determine the density of water, 𝜌wr, at the position of the silicon sphere in two ways: (i) we use the well-known silicon sphere as the volume reference in gravimetric weighing 𝜌wr = [ (𝑀0−𝑀2) 𝛼h γ + 𝑚r 𝛼r + 𝑚a 𝛼a − 𝑚o 𝛼o − 𝜌la𝑉a20𝐶a 𝛼a + 𝜌lo𝑉o20𝐶o 𝛼o ] 𝛼r 𝑉r20eff𝐶r (6) figure 3. at106 mass comparator with an airtight enclosure in the mass laboratory at metas. figure 4. oiml stainless steel weights used for the comparison. acta imeko | www.imeko.org march 2020 | volume 9 | number 1 | 64 and (ii) we calculate the density �̃�wr by using tanaka's formula [13]. we also calculate the water density at the test weight's position, �̃�𝑊𝑇, by using tanaka's formula. now, we can establish the following relation 𝜌wt = �̃�wt �̃�wr 𝜌wr . (7) the difference between the calculated (�̃�wr) and measured (𝜌wr) water densities is about 2.5 ppm, which is in line with the uncertainty contributions related to the temperature and the isotopic distribution. the volume, vr20eff, of the silicon sphere was determined by optical interferometry and ellipsometry at the national metrology institute of japan in 2006. 4.1.1. hydrostatic method with support weights smaller than 10 g cannot be measured directly on our volume comparator because their geometrical dimension is too small. instead, a support weight must be used. by doing this, we measure the total volume of the support weight and the small test weight. the mass mt in equation (4) becomes 𝑚t = 𝑚small + 𝑚support . (8) the volume of the small test weight is simply the difference between the total volume and the volume of the support weight. 𝑉small = 𝑉total − 𝑉support . (9) the latter was a 10 g stainless steel disc and was determined separately through hydrostatic weighing. 4.2. double weighing in air the weighing equation for mass determination in air on a comparator is given by 𝑚t = 𝑚r + 𝜌𝑎 (𝑉t − 𝑉r) + δ𝑚𝑤 (1 − 𝜌0 𝜌𝑐 ) (10) where mr and vr are the true mass and volume of the reference weight, and vt is the volume of the test weight. δmw is the uncorrected (conventional) weighing difference indicated by the balance, which needs to be corrected for the reference air density ρ0 = 1.2 kg/m³ and the reference material density ρc = 8000 kg/m³ to calculate the true mass mt of the test weight [15], [16]. after a double weighing measurement at two different air densities, ρa1 and ρa2, we can write the following system of equations | 𝑚t = 𝑚r + 𝜌a1(𝑉t − 𝑉r) + δ𝑚w1 (1 − 𝜌0 𝜌c ) 𝑚t = 𝑚r + 𝜌a2(𝑉t − 𝑉r) + δ𝑚w2 (1 − 𝜌0 𝜌c ) | (11) from which we can derive the volume vt of the test weight 𝑉t,air ∶= 𝑉t = 𝑉r + δmw2−δmw1 𝜌a1−𝜌a2 (1 − 𝜌0 𝜌c ). (12) 4.2.1. small weights with support if the test weight is too small to be loaded on the weighing pan, a support weight is necessary. in our experiment, we used a 100 g oiml weight as support and put the small test weight (1 g or 10 g) on top of the oiml weight. this combination can be compared to another 100 g reference weight, providing that the weighing difference is still within the weighing range of the balance. firstly, we performed a double weighing with the support weight only and calculated its volume according to equation (12). then, we added the small test weight, repeated the double weighing, and calculated the volume of the weight combination according to equation (12), which is the sum of the two volumes. the volume of the small test weight is given by the total volume of the weight combination (small weight plus support weight) minus the volume of the support weight 𝑉small = 𝑉total − 𝑉support . (13) the support weight can either be determined separately by double weighing or by any other method, such as hydrostatic or interferometric measurements. 5. uncertainty calculations we used the monte carlo simulations to estimate the measurement uncertainties in the hydrostatic weighing and double weighing methods. we assume that the input quantities xi in equations (4) and (12) are normally distributed 𝑋~𝒩(𝜇, 𝜎 2). (14) for the mean value, μ, and the standard deviation, σ, we use the average value, �̅�, and the standard deviation, s, from the measurement or certificate. for the simulation, we used n = 10000 iterations. 5.1. hydrostatic weighing equation (4) can be written as 𝑉t,water (𝑗) ∶= 𝑉t(𝑗) = [ (𝑀0(𝑗)−𝑀1(𝑗)) 𝛼h(𝑗) γ + 𝑚t(𝑗) 𝛼t(𝑗) + 𝑚b(𝑗) 𝛼b(𝑗) − 𝑚o(𝑗) 𝛼o(𝑗) − 𝜌lb(𝑗)𝑉b20(𝑗)𝐶b(𝑗) 𝛼b(𝑗) + 𝜌lo(𝑗)𝑉o20(𝑗)𝐶o (𝑗) 𝛼o(𝑗) ] 𝛼t(𝑗) 𝜌wt(𝑗)𝐶t(𝑗) . (15) where j represents the iteration. if a support weight is used, equations (8)and (9) become table 2. typical values of the uncertainty components in the double weighing of a 1 kg oiml weight (k = 1). uncertainty component value unit 𝑢(𝑉r) 0.20 mm³ 𝑢(δ𝑚w1 ) 12.90 ng 𝑢(δ𝑚w2 ) 28.25 ng 𝑢(ρa1) 0.0001 kg/m³ 𝑢(ρa2) 0.0001 kg/m³ table 1. typical values of uncertainty components in the hydrostatic weighing of a 1 kg oiml weight (k = 1). uncertainty component value unit 𝑢(𝑀0) 0.66239 mg 𝑢(𝑀1) 0.82361 mg 𝑢(𝑚t) 0.00658 mg 𝑢(𝑚b) 0 mg 𝑢(𝑚o) 0.16279 mg 𝑢(𝛼h) 0.032965 × 10 -6 (m/s²) / (m/s²) 𝑢(𝛼t) 0.017215 × 10 -6 (m/s²) / (m/s²) 𝑢(𝛼b) 0 (m/s²) / (m/s²) 𝑢(𝛼o) 0.032622 × 10 -6 (m/s²) / (m/s²) 𝑢(𝜌lb) 0.002 kg/m³ 𝑢(𝜌lo ) 0.002 kg/m³ 𝑢(𝜌wt ) 0.00108 kg/m³ 𝑢(𝑉b20) 0 mm³ 𝑢(𝑉o20) 1.1344 mm³ 𝑢(𝐶t) 0.0223 × 10 -6 mm³/ mm³ 𝑢(𝐶b) 0.1187 × 10 -6 mm³/ mm³ 𝑢(𝐶o) 0.1187 × 10 -6 mm³/ mm³ acta imeko | www.imeko.org march 2020 | volume 9 | number 1 | 65 𝑚t(𝑗) = 𝑚small(𝑗) + 𝑚support (𝑗) 𝑉small (𝑗) = 𝑉total(𝑗) − 𝑉support (𝑗). (16) the volume of the test weight we are looking for and the estimated standard uncertainty are given by the mean and standard deviation of the simulated values vt(j) and vsmall(j). the typical values of the uncertainty components in hydrostatic weighing of a 1 kg oiml weight are listed in table 1. 5.2. double weighing to estimate the measurement uncertainty in the double weighing method, we proceed in the same way as for the hydrostatic weighing. equations (12) and (13) can be written as 𝑉t,air(𝑗) ∶= 𝑉t(𝑗) = 𝑉r(𝑗) + δmw2(𝑗)−δmw1(𝑗) 𝜌a1 (𝑗)−𝜌a2 (𝑗) (1 − 𝜌0 𝜌c ) (17) 𝑉small (𝑗) = 𝑉total(𝑗) − 𝑉support (𝑗). (18) the typical values of the uncertainty components in the double weighing of a 1 kg oiml weight are listed in table 2. 6. results 6.1. verification of the air density we verified our air density calculation according to the cipm formula [14] by using air buoyancy artefacts. the pair of artefacts consisted of a tube and a hollow body with volumes of (124.80558 ± 0.00023) cm³ and (412.63374 ± 0.02563) cm³, respectively. we used the artefacts to determine the air density experimentally in the range of 650 mbar to 1060 mbar and compared the results to the air density obtained using the cipm formula (figure 5). the results are in good agreement. for the figure 5. air density determined experimentally by using buoyancy artefacts and calculated according to cipm formula. error bars represent expanded uncertainties with a coverage factor k = 2. figure 6. stabilization time of a 1 kg oiml weight after hydrostatic weighing and the change in mass measured gravimetrically in the air before (black triangle) and after (red circle) hydrostatic weighing. the error bars represent expanded uncertainties with a coverage factor k = 2. the dashed line is an exponential fit. figure 7. the simulated values of the volume are normally distributed in all four scenarios. (a) 1 kg in double weighing without support, (b) 1 kg in hydrostatic weighing without support, (c) 1 g in double weighing with support, and (d) 1 g in hydrostatic weighing with support. -0.0006 -0.0004 -0.0002 0.0000 0.0002 0.0004 0.0006 d e g re e o f e q u iv a le n c e ( k g /m ³) 11001000900800700600 air pressure (mbar) cipm ba acta imeko | www.imeko.org march 2020 | volume 9 | number 1 | 66 subsequent calculations in the comparison of the double weighing and hydrostatic weighing, we use the cipm formula. 6.2. stabilization time after we complete the hydrostatic weighing, the test weight was taken out of the water. at that moment, the surface of the weight is chemically unstable. gravimetric measurements in the air immediately after the hydrostatic weighing show the change in mass and thus the instability of the weight very clearly (figure 6). the weight must achieve thermal equilibrium and stable surface conditions. therefore, a stabilization time of several hours after hydrostatic weighing is necessary (also observed by malengo [11]). 6.3. a comparison of double weighing and hydrostatic weighing we measured our test weights in air and in water by using double weighing and hydrostatic weighing, respectively. then, we used monte carlo simulations to determine the volume and uncertainty of each test weight by varying the input quantities according to equation (14). the output value, i.e. the volume of the test weight we are looking for, is normally distributed as well. this applies to all four scenarios: double-weighing with/without a support weight and hydrostatic weighing with/without a support weight (figure 7). to compare the results between the two methods, we calculated the degree of equivalence and its standard uncertainty according to cox [17]. 𝑑𝑖 = 𝑉𝑖 − 𝑉ref (19) 𝑢2(𝑑𝑖 ) = 𝑢 2(𝑉𝑖 ) − 𝑢 2(𝑉ref) (20) where i represents the measurement in air or in water. the comparison reference value, vref, and its standard uncertainty, u(vref), are given by 𝑉ref = 𝑉air 𝑢2(𝑉air) ⁄ + 𝑉water 𝑢2(𝑉water) ⁄ 1 𝑢2(𝑉air) ⁄ + 1 𝑢2(𝑉water) ⁄ (21) 1 𝑢2(𝑉ref) = 1 𝑢2(𝑉air) + 1 𝑢2(𝑉water) , (22) where we use the average value of the volumes and their uncertainties from the double weighing before and after the hydrostatic measurement: 𝑉air = 𝑉air,before+𝑉air,after 2 (23) 𝑢(𝑉air) = 𝑢(𝑉air,before)+𝑢(𝑉air,after) 2 . (24) the analysis shows that the volume of the test weights determined by double weighing and hydrostatic weighing are in good agreement for all the weights in the range of 1 g to 5 kg (figure 8). the standard uncertainties are comparable between the two methods. the relative uncertainties are the largest for the 1 g weight and decrease with an increasing nominal mass up to 1 kg. above 1 kg, the relative uncertainties increase again (figure 9). 6.4. the influence of the different material densities so far, we compared test weights and reference weights of the same nominal mass and density. now, we compare the 1 kg oiml test weight with a double weighing measurement to reference weights of different material densities: an air buoyancy artefact with (2424.69 ± 1.03) kg/m³ from mettler-toledo, an oiml 1 kg stainless steel weight with (7914.37 ± 0.01) kg/m³ from häfner and a platinum-iridium stack of discs with (21535.19 ± 0.07) kg/m³ from the bureau international des poids et mesures. we get three values and their corresponding measurement uncertainties for the volume of the test weight. the three values are consistent. however, the measurement uncertainties are different. to compare the results, we calculate the degree of equivalence. the smallest measurement uncertainty in the volume of the test weight is achieved when the material density of the test and reference weights are the same (table 3). figure 8. a direct comparison of the volumes of the test weights determined by double weighing in air and hydrostatic weighing. error bars represent expanded standard uncertainties (k = 2). figure 9. the relative uncertainties (k = 2) of the volume of the test weights determined in double weighing and hydrostatic weighing. 1 10 100 1000 10000 1g 10g 20g 50g 100g 200g 500g 1kg 2kg 5kg u (v )/ v ( p p m ) air water acta imeko | www.imeko.org march 2020 | volume 9 | number 1 | 67 7. discussion we have demonstrated that the double weighing measurement is a suitable alternative to the hydrostatic measurement for the determination of the volume and density of weights ranging from 5 kg down to 1 g. the achieved results of the two methods are in good agreement, a result that was also found by malengo [11]. the measurement uncertainties given by the two methods are comparable, and they are similar to those found by clarkson [10], who performed the double weighing measurements between 950 mbar and 1050 mbar. double weighing offers several advantages: it is less time consuming than hydrostatic measurement to prepare and perform the measurement; it is easier to do the calculations; and it keeps the weight stable, both thermally and chemically. furthermore, corrections for the volumetric expansion coefficient are not necessary. firstly, the air-tight enclosure of the comparator is an isolated system that can be considered as a canonical ensemble representing a system in thermal equilibrium with the heat bath. secondly, both the test weight and reference weight (usually) have the same nominal density and are exposed to the same environmental conditions. an expansion coefficient of γ = 50 × 10-6 c-1 – as suggested for stainless steel weights in oiml r-111 [1] – for the test and reference weight results in a difference of less than 0.1 ppm between the corrected and uncorrected volume. the major contribution to the measurement uncertainty in the volume of the test weight comes from the uncertainty of the air density followed by the measured weighing difference and the volume of the reference weight. for instance, reducing the air density uncertainty by a factor of 100 from 10-4 to 10-6 kg/m³ leads to a decrease in the volume uncertainty of about 64 %. a decrease of the uncertainties of the measured weighing difference or the volume of the reference weight by a factor of 100 results in a decrease of the volume uncertainty of only 3.5 % or 1 %, respectively (figure 10). thus, the only way to significantly reduce the volume uncertainty of the test weight is to reduce the uncertainty of the air density. the cipm formula provides an uncertainty of 22 ppm [14], whereas we used a more conservative value of 100 ppm. in future, we will try to reduce the air density uncertainty by using additional air buoyancy artefacts. however, the air density uncertainty is dominated by the standard deviation of the weighing difference. so, the challenge is to improve the repeatability of the comparator. we have also demonstrated that the monte carlo simulation is useful for estimating the measurement uncertainty of the measurand by varying the input quantities. in this way, we can overcome the difficulties in dealing with correlations among the variables. acknowledgement this work was supported by the national key r&d program of china for national quality infrastructure under grant 2017yff0205006. references [1] tc 9/sc 3 weights, oiml r 111-1. 2004. [2] bipm, kcdb database, 2020 [online] https://www.bipm.org/kcdb/cmc/quick-search [3] m. p. seah, j. h. qiu, p. j. cumpson, j. e. castle, stability of reference masses ii: the effect of environment and cleaning methods on the surfaces of stainless steel and allied materials, table 3. influence of different material densities on the determination of the volume and standard uncertainty of a 1 kg oiml test weight in double weighing measurements. di represents the degree of equivalence. density (kg/m³) di (cm³) u(di) (cm³) k = 2 2424 0.07333 0.35417 21536 0.02362 0.09480 7914 -0.00001 -0.00004 figure 10. major contribution to the measurement uncertainty of the test weight: (top) influence of the measurement uncertainty of the air density; (middle) influence of the measurement uncertainty of the weighing difference; (bottom) influence of the measurement uncertainty of the reference volume. 1.000 0.998 0.996 0.994 0.992 0.990 r e la ti v e c h a n g e o f u (v t ) 2 3 4 5 6 7 8 9 0.01 2 3 4 5 6 7 8 9 0.1 2 u(vr) (mm³) 1.00 0.99 0.98 0.97 r e la ti v e c h a n g e o f u (v t ) 3 4 5 6 7 8 9 0.001 2 3 4 5 6 7 8 9 0.01 2 3 u(dmw) (g) 1.0 0.8 0.6 0.4 r e la ti v e c h a n g e o f u (v t ) 10 -6 2 3 4 5 6 7 8 9 10 -5 2 3 4 5 6 7 8 9 10 -4 u(air) (kg/m³) https://www.bipm.org/kcdb/cmc/quick-search acta imeko | www.imeko.org march 2020 | volume 9 | number 1 | 68 metrologia, 31(2) (1994) pp. 93 https://doi.org/10.1088/0026-1394/31/2/002 [4] angelini, e, p bianco, und m plassa. instability of stainless steel reference weights due to corrosion phenomena. corrosion science 40, no. 7, july 1998: 1139–48. https://doi.org/10.1016/s0010-938x(98)00017-1 [5] n. kuramoto, y. azuma, h. inaba, k. fujii. volume measurements of 28si-enriched spheres using an improved optical interferometer for the determination of the avogadro constant, metrologia 54(2) (2017) pp. 193-203. https://doi.org/10.1088/1681-7575/aa5379 [6] r. m. schoonover, r. s. davis, quick and accurate density determination of laboratory weights, proc. of the 8th conference of imeko tech. comm. tc3 – measurement of force and mass. weighing technology, 1980, krakow, poland, pp. 1123-1127. [7] t. kobata, m. ueki, a. ooiwa, y. ishii, measurement of the volume of weights using an acoustic volumeter and the reliability of such measurement, metrologia, 41(2) (2004) pp. s75–s83, https://doi.org/10.1088/0026-1394/41/2/s08 [8] t. kobata, m. ueki, y. nezu, a. ooiwa, y. ishii, characterization of an acoustic volumeter for measuring the volume of weights, presented at the proc. xv imeko world congress, osaka, japan, 1999, vol. 3, pp. 205–212. [9] m. ueki, t. kobata, s. mizushima, y. nezu, a. ooiwa, y. ishii, application of an acoustic volumeter to standard weights, proc. of the xv imeko world congress, 1999, osaka, japan. [10] m. t. clarkson, r. s. davis, c. m. sutton, j. coarasa, determination of volumes of mass standards by weighings in air, metrologia, 38(1) (2001) p. 17, https://doi.org/10.1088/0026-1394/38/1/3 [11] a.malengo, w.bich, simultaneous determination of mass and volume of a standard by weighings in air, metrologia, 49(3) (2012) p. 289, https://doi.org/10.1088/0026-1394/49/3/289 [12] m. ueki, s. mizushima, j.-x. sun, k. ueda, simultaneous calibration of the mass and the volume of weights in the range from 1 kg to 20 kg, proc. of the 41st sice annual conference, 2002, vol. 1, pp. 480-482, https://doi.org/10.1109/sice.2002.1195448 [13] m. tanaka, g. girard, r. davis, a. peuto, n. bignell, recommended table for the density of water between 0 °c and 40 °c based on recent experimental reports, metrologia, 38(4) (2001) p. 301, https://doi.org/10.1088/0026-1394/38/4/3 [14] a. picard, r. s. davis, m. gläser, k. fujii, revised formula for the density of moist air (cipm-2007), metrologia, 45(2) (2008) p. 149 https://doi.org/10.1088/0026-1394/45/2/004 [15] m. kochsiek, m. gläser, massebestimmung. vch verlagsgesellschaft mbh, 1997, isbn 3-527-29352-3. [16] borys, michael, roman schwartz, arthur reichmuth, und roland nater. fundamentals of mass determination. springer heidelberg, 2012, https://doi.org/10.1007/978-3-642-11937-8 [17] m. g. cox, the evaluation of key comparison data, metrologia, 39(6) (2002) p. 589 https://doi.org/10.1088/0026-1394/39/6/10 https://doi.org/10.1088/0026-1394/31/2/002 https://doi.org/10.1016/s0010-938x(98)00017-1 https://doi.org/10.1088/1681-7575/aa5379 https://doi.org/10.1088/0026-1394/41/2/s08 https://doi.org/10.1088/0026-1394/38/1/3 https://doi.org/10.1088/0026-1394/49/3/289 https://doi.org/10.1109/sice.2002.1195448 https://doi.org/10.1088/0026-1394/38/4/3 https://doi.org/10.1088/0026-1394/45/2/004 https://doi.org/10.1007/978-3-642-11937-8 https://doi.org/10.1088/0026-1394/39/6/10 application of non-destructive techniques. the madonna del latte case study acta imeko issn: 2221-870x october 2018, volume 7, number 3, 52 56 acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 52 application of non-destructive techniques. the madonna del latte case study maria celeste leuzzi1, mila crippa1, giorgio andrea costa1,2 1 department of chemistry and industrial chemistry (dcci), university of genoa, genoa, italy 2 national research council-spin, genoa, italy section: research paper keywords: madonna del latte; baccio della porta and the school of san marco; artist’s palette; imaging techniques; x-ray fluorescence citation: maria celeste leuzzi, mila crippa, giorgio andrea costa, application of non-destructive techniques. the madonna del latte case study, acta imeko, vol. 7, no. 3, article 9, month year, identifier: imeko-acta-07 (year)-03-09 section editor: egidio de benedetto, university of salento, italy received april 4, 2018; in final form september 5, 2018; published october 2018 copyright: © 2018 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: maria celeste leuzzi, celesteleuzzi@gmail.com 1. introduction the madonna del latte is a tempera on wood representing the virgo lactans, namely the virgin nursing the child, a traditional iconography of christian art, widely used until the mid-16th century when progressively disappeared because the virgin’s bare breast was considered indecorous by the council of trent [1] (figure 1). according to historical documents like the painting bill of sale dated 1971, the madonna del latte would belong to the school of the florentine mannerism and especially to the san marco school, an artistic workshop within the dominican convent of san marco, florence. in the early 16th century, this workshop was a prestigious artistic centre headed by the artist and friar baccio della porta (1472-1517), also known as fra’ bartolomeo, one of the leading florentine painters in late fifteenth century [2]. fra’ bartolomeo, as well as the school of san marco were under the influence of girolamo savonarola, a former prior at the convent of san marco. savonarola preached against the hypocrisy and luxury of nobles and popes, encouraging the production of simple artistic forms in order to spread renovated spiritual, political and aesthetic values [3]. in this cultural context, young and minor artists worked under the guidance and supervision of fra’ bartolomeo and his co-worker mariotto albertinelli, creating masterpieces for public and private clients [4]. extensive researches concerning the production of fra’ bartolomeo and the school of san marco were carried out in the last decades on the occasion of restoration campaigns and exhibitions, allowing more technical assessment of the artists’ paintings [5, 6, 7]. more than thirty works of art were studied combining connoisseurship, technical examination and non-invasive analysis, thus providing new attributions, as well as giving an insight into the workshop’s painting technique. abstract in the frame of a research project involving a private collection of artworks a panel painting representing the nursing madonna (madonna del latte) was studied by means of scientific examination. the department of chemistry and industrial chemistry (dcci) of the university of genoa carried out non-invasive exams, combining imaging techniques with analytical analyses. uv fluorescence (uvf), infrared reflectography (irr), infrared false colour (irfc), x-ray fluorescence (xrf) and reflectance spectroscopy in the visible range (vis-rs) were used to analyse the painting palette thus obtaining more information on a possible creation date of the painting. some of the most interesting results are the identification of a freehand underdrawing and the usage of lead white, cinnabar, ironbased pigments for yellows and browns and copper-based pigments for blue and green colours. the scientific results, as well as historical information allowed the hypothesis that the madonna del latte was authored in the early 16th century by a florentine artist, likely a scholar of baccio della porta. mailto:celesteleuzzi@gmail.com acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 53 starting from art historical and technical information, provided by the painting bill of sale and the literature cited above, a scientific investigation was performed on the madonna del latte, aiming at studying the painting’s materials and technique applied by the author [8]. for this purpose, the painting was examined by using ultraviolet fluorescence (uvf), infrared reflectography (irr), infrared false colour (irfc), a digital microscope (dinolite), x-ray fluorescence (xrf) and reflectance spectroscopy in the visible range (vis-rs). this paper presents the analyses results on the madonna del latte, which confirm materials and technique consistent with early-sixteenth century paintings. this finding would give a significant contribution to further art historical studies on who within the san marco workshop might have contributed to the execution of this painting. 2. materials and methods the research methodology was based on non-invasive scientific examination. a preliminary investigation with ultraviolet light was followed by infrared reflectography (irr) and infrared false colour (irfc) in order to provide information about the subsurface layers, such as areas of retouching, the underdrawing and the nature of certain pigments with similar optical behaviour in visible light but different reflection in infrared light. furthermore, an examination through a digital microscope (dinolite, usually around 50 x magnification) showed the sequence of paint and ground layers, as well as coarsely ground pigments and pigments mixtures. uvf and ir images were respectively acquired using a mercury vapour lamp with block wood’s glass (maximum emission at 365 nm) and near-infrared leds (maximum emission at 980 nm). infrared images were recorded with a nikon coolpix camera professionally modified to remove the infrared filter and then combined with those in the visible light in order to obtain false colour images. representative points recorded by the imaging techniques were chosen to carry out xrf and vis-rs analysis, aiming at the investigation of the artist’s palette. elemental analysis (xrf) was performed with a lithos 3000 portable xrf system by assing and the lithos software to process the experimental data. the apparatus consists of a molybdenum tube, a zirconium filter and semiconductor silicon (li) detector, cooled by peltier effect. the operating parameters were: 25kv, 0.1 ma and 120 seconds of acquisition time. reflectance measurements (vis-rs) were acquired in the range 400-700 nm with the minolta cm-2600d spectrophotometer endowed with the software spectra magic nx. illuminant d65, d/8 geometry, observer angle 10° and the specular component included (sci) were the experimental conditions adopted. 3. results and discussion the painting was firstly investigated under ultraviolet light, revealing several discontinuities throughout the painting surface which show up as dark patches, as visible in figure 2a representing the virgin’s lips. these black areas correspond to retouching and reintegration of paint losses carried out during previous restoration works. the infrared reflectography revealed an extensive underdrawing (i.e. the artist’s preliminary sketch made before the paint layers are applied), done freehand and visible to the naked eye in localised areas where the paint has become more transparent over time. the underdrawing consists of soft and fluent strokes that define the figures, the drapery folds, the shadows and presumably the virgin’s cloak and the architecture, where thick and dark paint layers make the underdrawing not visible in these areas. some reworkings have been observed in the sketch of the baby’s hands revealing the author’s attempts at depicting this detail. the infrared reflectograms exposed other pentimenti (i.e. changes between the initial design and the final painting) in the virgin’s lips which under ir-light appear bigger and fleshier than the depicted one (figure 2b). this change in the composition can also be observed in the infrared false colour image (figure 2c), where the virgin’s lower lip is thinner in the final painting than in the underdrawing, that consequently, it remains visible as a curved line on the ground. a close figure 1. madonna del latte. figure 2. set of multispectral images of the virgin’s lips: a) uvf, b) irr, c) irfc, d) photomicrograph of the lower lip. acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 54 observation of this area with a digital microscope allowed to identify a finely ground red pigment, likely mixed with a white one to obtain pinkish shades, applied over a yellowish ground layer (figure 2d). microscopic examination of the painting surface provided preliminary information about the nature of the pigments used by the artist. among the pigments examined in depth were those of the virgin's cloak, which in visible light are dark blue in the outer part and light green in the inner part. under the digital microscope, the dark blue area exhibited a blue pigment coarsely ground and mixed with a greenish one (figure 3a), whereas the light green area showed a finely ground green pigment (figure 3b). starting from this evidence, xrf and vis-rs analysis were performed to gain insight into the painting’s palette. figure 4 illustrates the position of the most representative points selected for xrf analysis and table 1 reports the detected elements, where those with higher concentration are highlighted in bold. in a small lacuna calcium (ca) and strontium (sr) were detected. this result allowed identifying the presence of a cabased ground, more likely natural gypsum [9], which is typically found in the italian paintings’ ground layer [10]. the presence of lead (pb) in all the measurements indicates that lead white (2pbco3·pb(oh2) was probably used in a lower lying layer such as the imprimitura, and in paint layers to change the tonality of other colours and also to improve the drying properties of the paint. in light green areas (points no. 2, 3 and 6) mainly copper (cu) was found. a straightforward identification of the pigment used was not possible, as xrf analysis is an elemental technique that does not permit to make the distinction between copper-based compounds. thus, additional investigation was required. vis-rs absorption band around 700 nm suggests the presence of copper acetate (cu(ch3coo)2·2cu(oh)2), verdigris) usually mixed with yellows or lead white, and painted in many overlying layers due to its little covering power (figure 5, curves 2 and 6) [11, 12]. this pigment is one of the most widely used greens by the majority of the 16th century italian painters, including fra’ bartolomeo [7, 13]. points no. 4 and 5 (light green areas) revealed the presence of a chromium (cr) based pigment, such as chrome green figure 3. photomicrographs of the virgin’s cloak: a) blue pigment from the outer cloak, b) light green pigment from the inner cloak. table 1. pigments identified on the madonna del latte at each analysis site. point colour description xrf detected elements xrf pigments identification 1 white lacuna ca, sr, pb ca-based pigment (chalk or gypsum), lead white 2 light green virgin’s inner cloak cu, pb cu-based pigment, lead white 3 light green virgin’s inner cloak cu, pb cu-based pigment, lead white 4 light green retouching cr, fe, zn, ca, pb, cu, ti, sr cr-based pigment, zinc white, ca-based pigment 5 light green retouching cr, fe, zn, ca, pb, cu, ti, sr cr-based pigment, zinc white, ca-based pigment 6 light green virgin’s shoulder cu, pb cu-based pigment, lead white 7 dark blue virgin’s cloak cu, pb, fe cu-based pigment, lead white 8 dark blue virgin’s cloak cu, pb cu-based pigment, lead white 9 red virgin’s sleeve hg, pb, cu cinnabar, lead white, cu-based pigment 10 dark red virgin’s dress hg, pb, cu cinnabar, lead white, cu-based pigment 11 red collar hg, pb cinnabar, lead white 12 red virgin’s dress hg, pb cinnabar, lead white 13 pink cheek pb, fe lead white, red ochre 14 yellow hair pb, fe lead white, yellow ochre 15 brown architecture pb, hg, fe lead white, brown ochre, cinnabar 16 brown architecture pb, hg, fe lead white, brown ochre, cinnabar figure 4. madonna del latte with indication of the xrf measurement points. acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 55 (fe4[fe(cn)6]3+pbcro4) or chromium oxide (cr2o3) mixed with zinc white (zno). all these pigments have been produced in the 19th century and are usually found in ancient paintings as restoration materials [14, 15]. this is confirmed by the uvf images, in which the presence of dark areas that do not fluoresce are characteristic of retouching. the reflectance spectra recorded in these two areas (reflectance peak around 540 nm and absorption band around 630 nm) (figure 5, curves 4 and 5), as well as the irfc image (light green in visible light turns red in false colour), suggested the use of chrome green [16]. in dark blue areas (points no. 7 and 8) a copper-based pigment was used. vis-rs absorption band around 640 nm and infrared false colour (blue in visible light turns red-violet in false colour), allowed to identify the presence of azurite (figure 5, curve 8) [11, 16]. azurite is a basic carbonate of copper, 2cuco3·cu(oh)2, widely used throughout the renaissance as one of the most important blue pigments [17]. as reported in literature, azurite is widely used along with ultramarine by fra’ bartolomeo depending on the degree of fineness to which it is ground, azurite gives a wide range of blues: coarsely ground azurite produces dark blue, whereas fine grinding produces a lighter tone [18]. the greenish-blue and dark blue shade observed in the virgin's cloak under magnification, may be explained with the presence of green malachite, a basic carbonate of copper, that is usually associated in nature with azurite or even developed due to azurite degradation in humid conditions. in in red areas (points no. 9, 11 and 12) a high amount of mercury (hg) suggested the presence of cinnabar (hgs). this result was confirmed by vis-rs analysis which shows a shoulder around 600-610 nm, typical of cinnabar (figure 6, curves 9, 11, 12) [19]. an additional proof of the use of this pigment in red areas was provided by infrared false colour, which exhibits a characteristic yellowish shade, as visible in the virgin’s lips (figure 2d). in point no. 10 (dark red) along with mercury, a high amount of copper was detected probably due to the use of azurite to underline the drapery folds and to create shadows. the use of combine reds with azurite, iron-based pigments or red lakes is reported in literature as a feature of several florentine paintings, like those of raffaello, perugino and fra’ bartolomeo [12, 13]. although the presence of manganese (mn), calcium and iron detected in point no. 10 suggested the use of these compounds, it was not possible to identify univocally their presence. the last analyses, points no. 13, 14, 15 and 16 indicated the presence of fe-based pigments. in these areas, both red, yellow and brown ochre, were likely mixed with lead white and cinnabar to depict the flesh tone, the virgin’s hair and the surrounding architecture. 4. conclusions the multi-analytical approach while studying the madonna del latte allowed to assess its condition and gain insight into the painting materials and technique. even though several of retouches and repairs were observed throughout the surface by uvf and irr, the application of non-destructive techniques, such as xfr and vis-rs, gave a crucial contribution to the identification of the painting’s palette. it comprises lead white, azurite, verdigris, cinnabar, red and brown ochre. these traditional pigments have been historically used by italian painters for centuries and were found in several masterpieces of fra’ bartolomeo and paintings of his scholars at the san marco school. the results concerning the features of the underdrawing, especially its freedom and the evidence of changes made between the first sketch and the finished painting, suggests that the madonna del latte is an original work and not a copy of an existing model. this finding may confirm the belonging of the madonna del latte to a florentine workshop of the early 16th century and possibly as work of a minor artist, as reported by historical documents. the evidence provided by scientific examination together with a more comprehensive historical and artistic research about the florentine painters of the first mannerism would allow the more specific definition of the author of the madonna del latte. references [1] p. berruti, madonna del latte. la sacralità umanizzata, polistampa, firenze, 2006, isbn 88-596-0043-x. [2] g. c. argan, storia dell’arte italiana, sansoni, firenze, 1997, isbn 88-383-1619-8. [3] mrs. jameson, lives of the early painters: fra bartolomeo, called also baccio della porta and il frate, the american art journal, vol. 6, n. 11, (1867) pp. 169-170. [4] g. vasari, le vite, ed. 1568, iv parte, pp. 89-113. [5] e. lucchesi ragni, p. bolpani, fra’ bartolomeo. sacra famiglia a modello, sagep, genova, 2014, isbn 978-88-6373-311-2. [6] s. padovani, l’età di savonarola. fra’ bartolomeo e la scuola di san marco, marsilio ,venezia, 1996, isbn 88-317-6413-6. figure 5. vis-rs spectra of dark blue and light green pigments. identified pigments: 8azurite; 2, 6 – verdigris; 4, 5 – chrome green. figure 6. vis-rs spectra of red colours. identified pigment: 9, 11, and 12 – cinnabar. acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 56 [7] p. moioli, c. seccaroni, f. persia, applicazione di tecniche non distuttive: l’adorazione del bambino di fra’ bartolomeo, energia ambiente e innovazione, vol. 3 (2007), pp. 38-49. [8] m. c. leuzzi, m. crippa, g. a. costa, “application of nondestructive techniques. case study: the madonna del latte”, proc. of 3rd imeko international conference, oct. 23-25, 2017, lecce, italy, pp. 308-311. [9] e. franceschi, f. locardi, strontium, a new marker of the origin of gypsum in cultural heritage?, jch, no. 15, 2014, pp. 522-527. [10] d. bomford, art in the making. underdrawings in renaissance paintings, the national gallery, london, 2002, isbn 1-85709987-7. [11] m. l. amadori, p. baraldi, s. barcella, g. poldi, new studies on lorenzo lotto’s pigments: non-invasive and micro-invasive analyses, proc. of 7th national archeometry congress, feb. 2224, 2012, modena, italy, pp. 1-23. [12] p. moioli, c. seccaroni, pigmenti a base di rame: fonti storiche e analisi scientifiche, opd restauro, vol. 7 (1995) pp. 216-252. [13] p. moioli, c. seccaroni, analisi di fluorescenza x su sei dipinti di fra’ bartolomeo, in: l’età di savonarola. fra’ bartolomeo e la scuola di san marco. s. padovani (editor). marsilio,venezia, 1996, isbn 88-317-6413-6, pp. 314-316. [14] h. kuhn, zinc white, in: artists’ pigments. a handbook of their history and characteristics. r.l. robert (editor). national gallery of art, washington, 1986, isbn 978-1-904982-74-6 pp. 169-186. [15] r. newman, chromium oxide greens, in: artists’ pigments. a handbook of their history and characteristics. e. west fitzhugh (editor). national gallery of art, washington, 1997, isbn 978-1-904982-74-0 pp. 273-295. [16] a. cosentino, identification of pigments by multispectral imaging; a flowchart method, heritage science, vol. 2, no. 8, 2014, pp. 1-12. [17] n. eastaugh, v. walsh, t. chaplin, r. siddal, pigment compendium. a dictionary and optical microscopy of historical pigments, butterworth-heinemann, oxford, 2008, isbn 978-0-7506-8980-9. [18] c. cennini, il libro dell'arte, 5th edition, neri pozza, vicenza, 2009, isbn 978-88-7305-910-3. [19] g. poldi, la scienza del colore del giovane lotto: pigmenti e analisi non invasive https://www.academia.edu/5402806/g._poldi_la_scienza_del_ colore_del_giovane_lotto_pigmenti_e_analisi_non_invasive access date: 15/09/2017. template for an acta imeko event paper acta imeko issn: 2221-870x december 2017, volume 6, number 4, 68 acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 68 editorial to the last papers from the imeko world congress 2015, prague paolo carbone university of perugia, italy section: editorial citation: paolo carbone, editorial to the last papers from the imeko world congress 2015, prague, acta imeko, vol. 6, no. 4, article 10, december 2017, identifier: imeko-acta-06 (2017)-04-10 section editor: paul regtien, measurement science consultancy, the netherlands received december 12, 2017; in final form december 12, 2017; published december 2017 copyright: © 2017 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: paolo carbone, email: paolo.carbone@unipg.it dear reader, nine additional papers close this issue of acta imeko. the first paper by miha hiti addresses the problem of calibrating the train gauge bridge amplifier when measuring force, torque and pressure. a detailed procedure is described that provides a reduced calibration uncertainty with respect to the conventional single-point calibration. experimental results are presented to support the idea described in the paper. the paper by christiaan s. veldman describes a calibration problem to be applied to an accelerometer used in laser interferometry. the technique analyzed by the author allows extension of the calibration frequency range from 10 to 20 khz. performances are evaluated using experimental data. tomasz tarasiuk describes a procedure to perform voltage dip tracking in the third paper of this series. the method provides its usefulness in the context of power quality. the paper also considers the impact of practical requirements, such as simplicity and ease in the implementation of the already available and newly proposed techniques. the paper authored by sebastian baumgarten, dirk röske and rolf kumme describes how to compile the measurement uncertainty budget of a multi-component measuring facility, when it is applied to the measurements of more than one force or torque component in vectorial physical quantities. a thorough analysis is presented, based on the research experience of the authors working at ptb, in germany. the paper by christian müller-schöll covers the calibration of weight pieces. it provides a detailed procedure to evaluate the mass comparator sensitivity and to assess the related component uncertainty. numerical examples and experimental results are described that highlight how to interpret the information provided by the makers of the used equipment. the paper by m. becchetti, r. marsili, f. cannella and a. garinei deals with measurements of the gripping force associated with human fingers. an optical measurement system is described that is capable to relate contact pressure with changes in the grey-level of taken images. the following paper is authored by kamol wasapinyokul, santhad chuwongin, ajchara charoensook. when making flux measurements, baffle reflection and shadows in an integrating sphere may result in inaccuracies in the uniformity of the generated illuminance of the light. a thorough analysis is made and experimental results described so that these non ideal phenomena results in predictable behaviours. the paper by loredana cristaldi, mohamed khalil and payam soulatiantork deals with the reliability and root cause analysis of photovoltaic systems and balance of system subsystems. several techniques are applied (rpn, fmeca, markov) to predict and analyse the reliability properties of these entities. the paper closing this issue is written by loredana cristaldi, mohamed khalil and marco faifer. it uses markov processes to model the reliability of photovoltaic modules, and reports expressions of corresponding mean-time-to-failure, probability density functions, hazard and survival functions. have a fruitful reading of the last and fourth issue of acta imeko in 2017! paolo carbone, editor-in-chief of acta imeko. editorial to the last papers from the imeko world congress 2015, prague << /ascii85encodepages false /allowtransparency false /autopositionepsfiles true /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /error /compatibilitylevel 1.4 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends 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can be opened with acrobat and adobe reader 5.0 and later.) >> /namespace [ (adobe) (common) (1.0) ] /othernamespaces [ << /asreaderspreads false /cropimagestoframes true /errorcontrol /warnandcontinue /flattenerignorespreadoverrides false /includeguidesgrids false /includenonprinting false /includeslug false /namespace [ (adobe) (indesign) (4.0) ] /omitplacedbitmaps false /omitplacedeps false /omitplacedpdf false /simulateoverprint /legacy >> << /addbleedmarks false /addcolorbars false /addcropmarks false /addpageinfo false /addregmarks false /convertcolors /converttocmyk /destinationprofilename () /destinationprofileselector /documentcmyk /downsample16bitimages true /flattenerpreset << /presetselector /mediumresolution >> /formelements false /generatestructure false /includebookmarks false /includehyperlinks false /includeinteractive false /includelayers false /includeprofiles false /multimediahandling /useobjectsettings /namespace [ (adobe) (creativesuite) (2.0) ] /pdfxoutputintentprofileselector /documentcmyk /preserveediting true /untaggedcmykhandling /leaveuntagged /untaggedrgbhandling /usedocumentprofile /usedocumentbleed false >> ] >> setdistillerparams << /hwresolution [2400 2400] /pagesize [612.000 792.000] >> setpagedevice template for an acta imeko event paper acta imeko issn: 2221-870x july 2017, volume 6, number 2, 50-53 acta imeko | www.imeko.org july 2017 | volume 6 | number 2 | 50 the establishment of a 1 n·m torque standard machine at nim zhang zhimin1, zhang yue1, meng feng1, zhang wei1,hu gang1, li tao2, ji honglei2 1national institute of metrology, beijing 100029, china 2shanghai marine equipment research institute, shanghai 200031, china section: research paper keywords: torque standard machine; air bearing; weight loading system; repeatability; uncertainty citation: zhang zhimin, zhang yue, meng feng, zhang wei,hu gang, li tao, ji honglei, the establishment of a 1 n·m torque standard machine at nim, acta imeko, vol. 6, no. 2, article 8, july 2017, identifier: imeko-acta-06 (2017)-02-08 section editor: min-seok kim, research institute of standards and science, korea received june 29, 2016; in final form may 24, 2017; published july 2017 copyright: © 2017 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: this work was supported by the general administration of quality supervision, inspection and quarantine of p.r.china, the project number: 201110002 corresponding author: zhang zhimin, e-mail: zhiminzhang@nim.ac.cn 1. introduction torque, as a basic parameter, is widely used in automobile, communications, aerospace, shipping and other fields. in recent years with the development of the electronic, automobile, aerospace and medicine industry, there is a growing demand for small torque measurements. micro-motors have wide applications in the aerospace and automobile sectors, where accurate torque measurements of micro-motors is essential. slight torque tightening tools are widely used for mobile phones, hard drives, cameras, laptop computers, automobile electronics and medicine devices. by using the tightening tools equipped with torque sensors, precise torque control is achieved and product quality is guaranteed. the establishment of a small torque standard machine may provide reliable technical support for various industries. the national metrology institute is responsible for establishing and maintaining national torque standards, ensuring accuracy and consistency of torque dissemination and providing calibration services for customers in various sectors of industry. in order to meet the requirements for small torque measurements, a set of 1 n·m torque standard machines has been established at nim in 2013. the range of the small torque machine is from 1 mn·m to 1 n·m. the expanded uncertainty (k = 2) is smaller than 5 × 10-5 in the range of 100 mn·m 1 n·m, and smaller than 1 × 10-4 in the range of 10 mn·m 100 mn·m. 2. the construction of a 1 n·m torque standard machine the machine consists mainly of an air bearing, a momentarm part, 2 weight suspension parts, 2 weight loading systems, a counter bearing part, transducer couplings, a mounting platform and a pedestal part. the mechanical construction of the 1 n·m torque standard machine is shown in figure 1. an x-type air bearing is adopted to support the momentarm in order to minimize the friction at the fulcrum. the air bearing is in axial positioning, the gap of the stator and the rotor is 5 μm, the friction torque is smaller than 0.3 μn·m abstract 1 n·m torque standard machine was established at national institute of metrology (nim) in 2013. the torque standard machine adopts deadweight and moment-arm type. the static air bearing with low friction is used to support the moment-arm, the invar alloy with the low expansion coefficient as the material of the moment-arm, the specially designed weight suspension part and weight loading system may ensure the applied force by small weights accurately and reliably. the mechanical structure of the machine is introduced, the results of performance test and uncertainty assessment are described. the expanded uncertainty (k = 2) is smaller than 5 × 10-5 in the range of 100 mn·m 1 n·m, smaller than 1× 10-4 in the range of 10 mn·m 100 mn·m. acta imeko | www.imeko.org july 2017 | volume 6 | number 2 | 51 according to the test report of the manufacturer (ceh xl60_45s, germany). the work pressure of compressed air supplied to the air bearing is 4.2 bar. the moment-arm adopts the single beam structure; its nominal length is 250 mm. the balance beam is equipped in a way that the mass center of the moment-arm may be adjusted. the overload protection part is used to control the swing amplitude of the moment-arm to ensure the moment-arm in the normal work status. invar alloy with low thermal expansion coefficient is adopted for the moment-arm materials in order to reduce the uncertainty caused by the length change of the moment-arm due to the temperature variations. the designed weight suspension assembly may keep the force, applied by small weights, vertically downward and free from parasitic forces. the machine includes two sets of weight loading systems which are at two sides of the moment-arm and can generate clockwise and anticlockwise torque separately. the torque range of the machine runs from 1 mn·m to 1 n·m, divided into five torque range segments: 1 n·m, 500 mn·m, 200 mn·m, 100 mn·m and 10 mn·m. each torque range segment includes 10 torque steps. each weight loading system consists of 5 groups of weights, a load frame and a weight support frame. each group of weights includes 10 pieces of 0.4 n weight, 10 pieces of 0.2 n weight, 10 pieces of 0.08 n weight, 10 pieces of 0.04 n weight and 10 pieces of 0.004 n weight. the nominal mass values and tolerances of the weights are shown in table 1. the load frame has 10 layers of weight trays on which the working weights are placed. the weight support frame includes a 10-story pentagon weight stand. five groups of weights are placed on the paws of the pentagon weight stand. the different groups of weights may be selected by rotating the weight support frame. before loading, the support frame with groups of weights is moved horizontally to the set position. the selected weights are transported from the weight support frame to the load frame and torque is applied by lowering or lifting the weight support frame. the structure of the weight loading system is shown in figure 2. the transducer that is calibrated on the 1 n·m torque machine is very small. to prevent damage of the calibrated transducer, caused by parasitic forces acting on the transducer, a three-dimensional precision mounting platform is designed which may precisely adjust the position of the transducer in x, y, z directions and keep the transducer axis aligned with that of the torque machine so that the transducer is protected from the impact of parasitic forces while mounting. 3. performance experiments 3.1. repeatability test repeatability experiments were carried out by using a high precision torque transducer and measuring amplifier. at present the minimum nominal torque of the high precision torque transducer which may be used as torque reference is 1 n·m. generally, the transducer meets its technical performance requirements within 10 % to 100 % of its nominal load, but cannot guarantee an accuracy less than 10 % of its nominal load. for this machine, repeatability experiments were carried out in the range of 1 mn·m to 1 n·m. a tt1 1 n·m torque transducer and a dmp41 measuring amplifier were used in the tests. since the lowest ranges of two torque segments are far beyond the lower limit of the 1 n·m torque transducer, only the test results in the torque range segment of 1 n·m, 500 mn·m and 200 mn·m are given. the measurements were done in clockwise and anticlockwise direction. the measurement sequence includes three preloadings and three measurements at the initial mounting position of the torque transducer (0°), one preloading and one measurement at each of another two rotational positions of the torque transducers (120° and 240°). figure 1. mechanical structure of the 1 n·m torque standard machine. figure 2. weight loading system. table 1. the nominal mass values and tolerances of the weights. the weights at the right side the weights at the left side 5 groups of weights nominal mass values (g) tolerances (mg) 5 groups of weights nominal mass values (g) tolerances (mg) 0.4 n × 10 40.822090 ±0.82 0.4 n × 10 40.821719 ±0.82 0.2 n × 10 20.411045 ±0.41 0.2 n × 10 20.410860 ±0.41 0.08 n × 10 8.164418 ±0.16 0.08 n × 10 8.164344 ±0.16 0.04 n × 10 4.082172 ±0.08 0.04 n × 10 4.082172 ±0.08 0.004 n × 10 0.408221 ±0.008 0.004 n × 10 0.408217 ±0.008 acta imeko | www.imeko.org july 2017 | volume 6 | number 2 | 52 the repeatability is calculated by %1001 )( 1 2 ×− − = ∑ = x n xx r n j j (1) where n is the number of the increasing series at 0° position, xj and x are the deflection and average value of deflections with increasing torque at 0° position, respectively. the results of the repeatability test are shown in figures 3 to 5. the results indicate that the repeatability of the machine is better than 2.32 × 10-5 in the 1 n·m torque range segment, 2.77 × 10-5 in the 500 mn·m torque range segment, and 3.80 × 10-5 in the 200 mn·m torque range segment. 3.2. sensitivity test the sensitivity tests were carried out by means of the milligram weights as well as torque transducers and the measuring amplifier. the torque transducer is mounted on the torque machine and torques as shown in table 2 were applied. after the output signal of the torque transducer was stabilized, additional small weights (as small as possible) were added on the top weight until the output signal showed a visible change. table 2 shows the results of the sensitivity test of the machine. 4. evaluation of uncertainty the uncertainty evaluation of the deadweight torque standard machine has been discussed in former papers [3]-[6]. the source of uncertainty, the probability distribution, distribution factor and relative standard uncertainty as well as the relative combined standard uncertainty and the relative combined expanded uncertainty are listed in table 3. the uncertainties are calculated according to equations (2) to (4). the uncertainty caused by the arm lever’s length is obtained by 2 , 2 , 2 , 2 ,, 4321 lrlrlrlrlr wwwww +++= (2) the relative standard uncertainty is calculated as follows: 2 , 2 , 2 , 2 , 22,2,2 , 2 ,, )( ()()( f wa mrtrbrlr aw a w r a r grmrcr wwww ww www ++++ −     +++= rr r rr rr (3) and the relative expanded uncertainty (k=2) is calculated by crcr ww ,, 2= (4) 5. conclusion the 1 n·m torque standard machine adopts an air bearing with low friction to support the moment-arm to minimize the friction at the fulcrum. invar alloy with low expansion coefficient is used as the material of the moment-arm to reduce the uncertainty caused by the moment-arm length change due to temperature variations. the designed weight suspension system keeps the force generated by small weights vertically downward and free from parasitic forces. the distinctive weight loading system ensures the applied force by small weights accurately and reliably and avoids a reverse process during loading. the torque machine is capable of generating clockwise and anticlockwise torques in a range from 1 mn·m to 1 n·m. the expanded uncertainty (k = 2) is smaller than 5 × 10-5 in the range from 100 mn·m to 1 n·m, and smaller than 1 × 10-4 in the range from 10 mn·m to 100 mn·m. references [1] z.m. zhang, t.li, y.zhang, e.a., “a full-automatic high accuracy 100 nm torque standard machine”, proc. of 19th imeko world congress, sept. 6-11, 2009, lisbon, portugal. figure 3. the results of the repeatability test in the 1 n·m torque range segment. figure 4. the results of the repeatability test in the 500 mn·m torque range segment. figure 5. the results of the repeatability test in the 200 mn·m torque range segment. table 2. the results of the sensitivity test of the machine. torque transducer applied torque corresponding weights mass added small weights mass relative sensitivity (mn·m) (g) (mg) tt1/1 nm 50 20.041105 1 5.0e-05 100 40.822100 2 5.0e-05 500 200.411045 5 2.5e-05 1000 400.822090 10 2.5e-05 0.00e+00 5.00e-06 1.00e-05 1.50e-05 2.00e-05 2.50e-05 -1000-900 -800 -700 -600 -500 -400 -300 -200 -100 0 100 200 300 400 500 600 700 800 900 1000 torque (mn•m) r e p e a t a b i l i t y 1000 mn•m 0.00e+00 5.00e-06 1.00e-05 1.50e-05 2.00e-05 2.50e-05 3.00e-05 -500 -450 -400 -350 -300 -250 -200 -150 -100 -50 0 50 100 150 200 250 300 350 400 450 500 torque (mn•m) r e p e a t a b i l i t y 500 mn•m 0.00e+00 5.00e-06 1.00e-05 1.50e-05 2.00e-05 2.50e-05 3.00e-05 3.50e-05 4.00e-05 -200 -180 -160 -140 -120 -100 -80 -60 -40 -20 0 20 40 60 80 100 120 140 160 180 200 torque (mn•m) r e p e a t a b i l i t y 200 mn•m acta imeko | www.imeko.org july 2017 | volume 6 | number 2 | 53 [2] z.m. zhang, y.zhang, t.li, h.l.ji, “the design of 1 n·m torque standard machine at nim”, proc. of asia-pacific symposium on mass, force and torque, nov. 20-22, 2013, chinese taibei. [3] d. röske, uncertainty considerations for the physical quantity torque, china industry system—technical seminar of torque metrology technology, 2006, shanghai, china, pp.193 – 201. [4] d. roske, “metrological characterization of a 1 n·m torque standard machine at ptb”, germany [j]. metrologia 51, no.1 (2014) 87–96. [5] atsuhiro nishino, koji ohgushi and kazunaga ueda, “design and component evaluation of the 10 n·m dead-weight torque standard machine”, proc. of asia-pacific symposium on mass, force and torque, oct 24 25 2007, sydney, australia. [6] yon-kyu park, min-seok kim, dae-im kang, “development of a small capacity deadweight torque standard machine”, measurement science and technology, vol. 18, no. 11 (2007) 3273-3278. table 3. uncertainty budget. source of uncertainty irw , probability distribution coverage factor relative standard uncertainty the mass measurement of weights mrw , normal 3 1.2 × 10 -5 the gravitational acceleration measurement grw , normal 3 6.6 × 10 -8 the variety of air density ar w r, rectangular 3 1.9 × 10 -2 the density measurement of the weights material wr w r, normal 3 8.7 × 10 -3 arm lever’s length lrw , length measurement 1,lr w / 2 2.0 × 10-6 the influence by temperature change 2,lr w rectangular 3 6.9 × 10 -7 the influence by deformation 3,lr w rectangular 3 1.9 × 10 -9 the influence by lever inclination 4,lr w rectangular 3 4.5 × 10 -9 the influence by weight swing brw , triangle 6 1.0 × 10 -6 the influence by non-coaxality of rotation axis and counter axis trw , rectangular 3 5.0 × 10 -10 the influence by sensitivity of the machine smr w , rectangular 3 1.44 × 10-5 （500 mn·m） 2.89 × 10-5 （50 mn·m） the relative combined standard uncertainty crw , 1.95 × 10-5（500 mn·m） 3.15 × 10-5（50 mn·m） the relative combined expanded uncertainty (k = 2) crw , 3.9 × 10-5（500 mn·m） 6.3 × 10-5（50 mn·m） analysis of the influence of the current drawn by the appliance on the close magnetic field acta imeko issn: 2221-870x december 2018, volume 7, number 4, 70 74 acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 70 analysis of the influence of the current drawn by the appliance on the close magnetic field silviu ursache1, eduard lunca1, alexandru salceanu1, ionel pavel1 1 ”gheorghe asachi” technical university of iasi, prof. dr. doc. d. mangeron street, 700050 iaşi, romania section: research paper keywords: citation: silviu ursache, eduard lunca, alexandru salceanu, ionel pavel, analysis of the influence of the current drawn by the appliance on the close magnetic field, acta imeko, vol. 7, no. 4, article 12, december 2018, identifier: imeko-acta-07 (2018)-04-12 editor: vilmos pálfi, budapest university of technology and economics, hungary received april 24, 2018; in final form october 2, 2018; published december 2018 copyright: © 2018 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: silviu ursache, e-mail: silviu_ursache@tuiasi.ro 1. introduction the rapid development of technology has been materialized over time by, for example, the creation and widespread use of household appliances. at home or in the office, we are surrounded by the most sophisticated equipment and devices, without which it would be impossible for us to imagine our everyday living. once connected to a power supply network, these devices generate low-frequency electric and magnetic fields, complementary to (operational) radio frequency fields. there is a qualitative perception that as power consumption increases, the generated (electro)-magnetic fields become more profound. concerning control of a person’s exposure to electromagnetic fields during working hours, directive 2013/35/eu [1] stipulates precise responsibilities for employers and presents an overview of action levels at 50 hz (1,000 µt rms) as recommended by the icnirp 2010 guidelines. at home, where a person is supposed to be in a “corner” for rest and relaxation, it is a priori supposed that exposure to various harmful factors, including electromagnetic fields, is lower, without additional checks. is this true, given that the number and diversity of equipment found in a modern home can in some cases be much higher than at work? sometimes, they are much closer to two of the most sensitive parts of the human body: the eyes and the brain. most of the research undertaken on this issue has been focused on the work environment, with much less work dedicated to the private home environment. the variety of these low-frequency magnetic field sources involves a wide range of measurement methods, with the aim of realistically characterizing the electromagnetic home environment. even if numerous studies have been published over the last 25 years related to human exposure to low-frequency electric and magnetic fields [2-6], the possible connection between the magnetic field generated by appliances and the current consumption has not been investigated in detail. more recent studies have started to approach this issue, making the necessary difference between occupational and residential exposure [7, 8]. several scientific papers published in the last five years have taken into account the classification of household appliances operation cycles, which is beneficial to the smart grid concept abstract the development of technology and the corresponding increase in living standards imply the generalized use of the most diversified and sophisticated equipment, a distinct, significant category being household appliances. the frequent presence of these devices close to a person’s heart, eyes, or brain naturally gives rise to concerns about the level of the associated (low-frequency) magnetic fields. the main objective of our paper is to study the possible correspondence between the low-frequency magnetic field generated by some household appliances and their current consumption. the proposed measurement system consists of the hand-held triple axis 480826 emf tester manufactured by extech instruments and the 80i-400 fluke clamp meter. the performed measurement methodology takes into account the operating principles of the appliance. we perform measurements for a large selection of devices (47). the measuring point for the magnetic field is set at 30 cm from the source. finally, by applying the biot-savart law to the previously determined drawn current, we calculate the magnetic field. we also compare the corresponding, directly measured values and analyze them. mailto:silviu_ursache@tuiasi.ro acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 71 [9], or classification using probabilistic neural networks [10]. systems monitoring electrical energy consumption have also been developed, allowing for automatic home appliance detection (and even identification) by means of the analysis of domestic power consumption [11-13]. recent papers have presented techniques and various algorithms for monitoring electric power consumption in individual households [14, 15]. an important concern of european organizations and authorities is the characterization and limitation of household electricity consumption, promoting energy-saving techniques by means of adequate policies and recommendations [16]. in the relevant literature, the two most important factors that contribute to the characterization of a household electrical appliance, the absorbed current and the magnetic field emitted during standard operation, have been treated as separate. what we attempt in this paper is to study the possible relationship between the two previously mentioned characteristics. in particular, we select 47 representative household appliances, mainly because they work in very close vicinity to the human body. we establish their normal, stable operating regime and measure both the current absorbed from the outlet and the resulting magnetic field, at a point located 30 cm from the apparatus. we chose this distance as a reasonable compromise for the cases in which the user stands close to the equipment for comparative consistency. consequently, the test setup consists of an adequate current sensor and one magnetic flux density measuring equipment. several factors are taken into account: the specifications of the measurement devices, the measurement methods, and the operating mode of the appliances. 2. methodology and instrumentation all magnetic field measurements were performed using a hand-held triple axis 480826 emf tester manufactured by extech instruments (figure 1), which operates in the frequency range from 30 hz to 300 hz, the rms values of the magnetic flux density being calculated by equation (1): 222 zyx bbbb  , (1) where bx, by, and bz are the rms values of the magnetic flux density measured in three orthogonal directions. the intensity of the drawn current was measured using a 80i400 ac true rms current clamp meter produced by fluke, with the output level set at 1 ma per 1a of input current (figure 2). the measuring setup for studying the possible relationship between the magnetic field generated by the home appliance (equipment under test [eut]) and current consumption is shown in figure 3. both in terms of the average and rms values of the absorbed current, from the point of view of the density of the produced magnetic flux and of the desired modeling simulation, the household appliances can be divided into two main categories (that may also have common elements): (quasi-) linear devices based on the thermal effect of electric energy and appliances with a motor that converts electrical energy into mechanical energy. there are, of course, appliances that meet both criteria of having both heating resistance and an electric motor (washing machines, air-conditioning devices, microwave ovens, etc.). consequently, the 47 studied appliances have been categorized as either class 1 (c1) appliances that mainly operate based on the thermal effect of the electric current and class 2 (c2) appliances that essentially operate based on an electric motor(s), with more details in [17]. for both categories, the real, direct measurements have been performed with the handheld triple-axis 480826 emf tester. for the purpose of comparison, the magnetic field generated by a linear conductor or an operating electric motor could be calculated using various commercial software packahes, mostly based on the finite element method. we have also performed the theoretical calculation of the magnetic flux density generated by c1-type appliances based on their drawn current by using the biot-savart law. this calculation provides that the derivative of the magnetic flux density bd is generated at a point m, at distance r (direction is given by the unit vector r̂ ) from the current i (which crosses the infinitesimal figure 1. a triple-axis 480826 emf tester manufactured by extech instruments. figure 2. 80i-400 ac true rms current clamp meter manufactured by fluke. figure 3. experimental setup. acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 72 length), with the current carrying wire ld  , as shown in equation (2): 2 ˆ 4 0 r rldi bd     (2) the relationship between the drawn current and the magnetic field is studied considering a long straight wire, which is carried out by an electric current i in the positive direction along the yaxis. we essentially want to calculate the magnetic field b  , generated at perpendicular distance r from the wire due to the current running in the positive direction along the y-axis. we have chosen the origin o as being the separation between the negative and positive parts of the wire related to the x-axis. the wire is divided into an infinite number of small segments dl, each of which is driven by an infinitesimal current, known as the “current element”. this current element creates an infinitely small magnetic field, given by db at point m. if we want to find the total magnetic flux density at this point, we add all the quantities of bd  as a result of all the small current elements given by dl, as shown in equation (3):  bdb  (3) because we have considered an infinitely long wire, equation (3) can be written in integral form, as shown in equation (4):         2 0 2 0 sin 4 ˆ 4 r ldi r rldi b       (4) where φ is the angle between ld  and r  . we simply have to use the right triangle from figure 4, where r is the distance from point m to the origin point o, and l is the distance from origin o to the dl quantity. by using the pythagorean theorem, we have: 222 lrr  (5) where r is considered constant, while l and φ are variables. both l and φ are variables appearing in equation (4), and it is necessary to express one in terms of the other. we want to express l in terms of φ using the tangent function, as shown in equation (6): l r tan . (6) we observe that: tan r l  (7) we also want to express dy in terms of angle φ, deriving both members of equation (7) as presented in equation (8): r dr r r rdrd dl    2 22 sin         . (8) using (8) in (4), we get the equation (9):         0 0 0 2 2 0 sin 4 sin 4 d r i rr dri b (9) in equation (9), we observe that the integral is defined from 0 to π. in figure 4, we observe that when the negative part of the wire goes to -∞, the angle φ becomes very small, in fact, very close to zero. while the wire tends to +∞, the angle goes to π radians. after some related transformations, we get equation (10): r i r i d r i b         24 cos sin 4 0 0 0 0 0   . (10) in essence, the biot-savart law establishes the direct proportionality between the electric current and the generated magnetic field. 3. measurements and results 3.1. household appliances based on the electro-thermal effect using the setup in figure 3, we have determined the absorbed current and the magnetic field emitted by 21 devices. the registered data is summarized in table 1. for comparison, the last column on the right, bc, is introduced. this column contains the magnetic field values, calculated on the basis of the biotsavart law, for the measured currents, at the points with the previously stated spatial coordinates. the household appliances have been modeled as a long straight wire through which a known electric current passes. the values recorded for the drawn current range from 1.5 a (a washing machine in extortion mode) to 22.3 a (an electric hob). for comparable (even quasi-similar) appliances, it may be that both the drawn current and the magnetic field differ significantly. furthermore, the direct proportional dependence that theoretically might be established between the current and the field, according to the biot-savart law, is by no means respected. for instance, the two irons, labelled “iron3” and “iron 4” have quasi-similar values for the drawn current (about 8.6 a), while associated magnetic field values differ with an order of magnitude (628.89 and 52.81, respectively). the fact that there are significant differences between the magnetic fields emitted by devices with perfectly comparable features is evidence that this issue has been considered by many manufacturers and designers, who have tried to find the right technological solutions (shielding, wiring, grounding, positioning, etc.) with the aim of reducing the (electro)magnetic emissions. 3.2. household appliances based on an electric motor figure 4. applying biot-savart law for calculating the magnetic flux density. m o r̂ dl l dl i r r bd  acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 73 we performed measurements on the drawn current and the emitted magnetic field for 26 household appliances, class c2, summarized in table 2. most of the recorded values for the drawn current are smaller than 1 a, excluding microwave ovens, electric vegetable cutters, and vacuum cleaners, for which the current consumption ranges from 2.9 a to 8.34 a. perhaps the results regarding the two vacuum cleaners belonging to the same class should be considered. although the absorbed currents are in the 1:2 ratio, the magnetic fields emitted are virtually equal. we also remarked that in the case of a hand-mixer, the drawn current has a very low value, while the magnetic flux density is greater than 9 µt. in the case of several household appliances, such as refrigerators, fruit juicers, or vertical freezers, very low values for both the drawn current and magnetic flux density have been recorded. as we anticipated, there is no linear dependence between the current consumption and the magnetic field generated by the household appliances from class c2. figure 6 graphically expresses this result. it is appropriate to suggest the absence of any law of dependence between the current absorbed by the appliance and its emitted magnetic field by combining the graphical representations of figure 5 and figure 6, thus resulting in figure 7. table 1. class c1 devices: typical values of magnetic flux density and drawn current at 30 cm distance. household appliance i in a b in nt bc in nt kettle mug 1 8 4696.8 5333.33 kettle mug 2 8.6 5517.25 5733.33 sandwich-maker 2.85 144.57 1900 toaster 1 5.2 361.53 3466.67 toaster 2 3.37 259.62 2246.67 coffee-maker 4.1 67.82 2733.33 electric boiler 5.1 708.59 3400 hairdryer 1 3.26 42.43 2173.33 hairdryer 2 2.16 274.40 1440 hairdryer 3 3.94 746.79 2626.67 hairdryer 4 6.3 1723.36 4200 electric hob 22.3 87.74 14866.67 electric heating 6.25 64.03 4166.67 iron 1 5.02 120.83 3346.67 iron 2 6.3 2800 4200 iron 3 8.65 628.89 5766.67 iron 4 8.62 52.81 5746.67 electric tap 24.1 946.04 16066.67 electric oven 8.22 522.78 5480 washing machine extortion 1.5 960.10 1000 washing machine lavation 9 204.21 6000 figure 5. the dependence between the magnetic field (measured vs. calculated) and drawn current for c1 class appliances table 2. typical values of magnetic flux density and drawn current at 30 cm for c2 class appliances. household appliance i in a b in nt vegetable chopper 1 0.1 207.12 mixer 0.29 142.48 hand blender 0.26 580.68 electric oven 8.34 383.27 freezer 1 6.4 158.42 freezer 2 1 136.38 freezer 3 1.8 400.50 refrigerated display case 1 0.21 238.74 refrigerated display case 2 0.26 81.24 vertical freezer 1.02 74.83 microwave oven 1 5.75 5101.96 microwave oven 2 4.9 9053.86 microwave oven 3 2.9 10080 microwave oven 4 5.88 3379.18 refrigerator 1 0.3 17.32 refrigerator 2 0.34 99.50 refrigerator 3 0.73 64.03 refrigerator 4 0.4 75.49 hand-mixer 0.45 9129.75 vacuum cleaner 1 6.15 756.96 vacuum cleaner 2 3.05 780.12 coffee grinder 0.4 433.7 fruit juicer 0.13 78.74 massage machine 0.05 264.76 meat machine 0.22 677.86 cooker hood 0.75 354.96 figure 6. correspondence between the magnetic field and drawn current for class c2 appliances. b i n n t a t 3 0 c m i in a b measured b calculated b i n n t a t 3 0 c m i in a acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 74 4. conclusions by means of a simplistic application of the well-known biotsavart law, at first glance, a user with good knowledge of electrical engineering would be tempted to consider that a highpower appliance (which consumes a higher current from the network) would produce a high magnetic field in its vicinity. rigorous measurements made for 47 household appliances have shown that the situation is different, primarily due to designers who have adopted technological solutions (shielding, wiring, cabling, grounding, positioning, etc.) to reduce or simply compensate for low-frequency magnetic field pollution. it should also be borne in mind that the distribution of the magnetic field around an electric motor rarely has spatial uniformity. thus, the (horizontal) position of the measuring point, located on a 30 cm radius circumference, might influence the measured value. in future research, we aim to 3d model the spatial distribution of magnetic fields produced by electric motors that typically equip household appliances. furthermore, a comparative analysis of the efficiency of the methods adopted by various designers regarding the diminution of low-frequency magnetic fields will be performed. references [1] a. salceanu, e. lunca, m. paulet, affordable evaluation of lowfrequency electric fields from the standpoint of directive 2013/35/eu, acta imeko 6(4) (2017) pp. 37-45. [2] e. lunca, s. ursache, a. salceanu, study of the power-frequency magnetic fields in residences and schools, bulletin agir 3 (2012) pp. 689-694. [3] e. ali, a. r. memari, “effects of magnetic field of power lines and household appliances on human and animals and its mitigation”, proc. of the ieee middle east conference on antennas and propagation, 20-22 oct., 2010, cairo, egypt, doi: 10.1109/mecap.2010.5724167 [4] w. t. kaune, assessing human exposure to power-frequency electric and magnetic fields, environmental health perspectives 101 (1993) pp. 121-133. [5] a. cieśla, w. kraszewski, m. skowron, p. syrek, determination of safety zones in the context of the magnetic field impact on the surrounding during magnetic therapy, przegląd elektrotechniczny 87(7) (2011) pp. 79-82. [6] p. syrek, m. skowron, “the impact of overhead lines for employees with stents”, proc. of the iop conference series: materials science and engineering, 2017, 200(012013), pp. 1-6. [7] s. ursache, a. salceanu, o. neacsu, “indoor and outdoor measurements of the low frequency magnetic fields in an urban area”, proc. of the international conference and exposition on electrical and power engineering (epe), 2016, pp. 1-4. [8] m. chen, y. du, “magnetic field environments at power frequency inside modern buildings”, proc. of the asia-pacific symposium on electromagnetic compatibility (apemc), 2013, doi: 10.1109/apemc.2013.7360626. [9] z. wang, r. srinivasan, classification of household appliance operation cycles: a case-study approach, energies 8 (2015), pp. 10522-10536. [10] n. a. rosdi, f. h. nordin, a. k. ramasamy, n. b. mustafa, “classification of electrical appliances using magnetic field and probabilistic neural network”, proc. of the 5th ieee control and system graduate research colloquium, 2014, doi: 10.1109/icsgrc.2014.6908735. [11] r. lukaszewski, k. liszewski, w. winiecki, “methods of electrical appliances identification in systems monitoring electrical energy consumption”, proc. of the 7th ieee international conference on intelligent data acquisition and advanced computing systems (idaacs), doi: 10.1109/idaacs.2013.6662630. [12] r. streubel, b. yang, “identification of electrical appliances via analysis of power consumption”, proc. of the third international conference on computer, communication, control, and information technology (c3it), 2015, doi: 10.1109/c3it.2015.7060219. [13] h. serra, j. correia, antónio j. gano, antónio m. de campos, isabel teixeira, “domestic power consumption measurement and automatic home appliance detection”, proc. of the ieee international workshop on intelligent signal processing, 2005, doi: 10.1109/wisp.2005.1531645. [14] m. fitta, s. bizaet, m. lehtonen, g. jacucci, exploring techniques for monitoring electric power consumption in households”, proc. of ubicomm 2010: the 4th international conference on mobile ubiquitous computing, systems, services, and technologies, 2010, pp. 471-477. [15] w. j. jin, y. feng, n. zhou, x. zhong, “a household electricity consumption algorithm with upper limit”, proc. of the international conference on wireless communication and sensor network, 2014, doi: 10.1109/wcsn.2014.93. [16] a. almeida, p. fonseca, barbara schlomann, nicolai feilberg, characterization of the household electricity consumption in the eu, potential energy savings, and specific policy recommendations, energy and buildings 43(8) (2011) pp. 18841894. [17] s. ursache, e. lunca, a. salceanu, i. pavel, “study on the relationship between magnetic fields generated by home appliances and associated drawn currents”, proc. of the 22nd imeko tc4 international symposium and 20th international workshop on adc modelling and testing, sept. 14-16, 2017, iasi, romania, pp. 305-308. figure 7. the dependence between the magnetic field and drawn current both for c1 and c2 class appliances. b i n n t a t 3 0 c m i in a c1 c2 https://doi.org/10.1109/mecap.2010.5724167 https://doi.org/10.1109/apemc.2013.7360626 https://doi.org/10.1109/icsgrc.2014.6908735 https://doi.org/10.1109/idaacs.2013.6662630 https://doi.org/10.1109/c3it.2015.7060219 https://doi.org/10.1109/wisp.2005.1531645 https://doi.org/10.1109/wcsn.2014.93 https://www.sciencedirect.com/science/article/pii/s0378778811001058#! https://www.sciencedirect.com/science/article/pii/s0378778811001058#! microsoft word 429-2552-1-le acta imeko  issn: 2221‐870x  november 2016, volume 5, number 3, 2    acta imeko | www.imeko.org  november 2016 | volume 5 | number 3| 2  introduction to the special section tc21 of the xxi imeko  world congress 2015  franco pavese  torino, italy    section: editorial   citation: franco pavese, introduction to the special section tc21 of the xxi imeko world congress 2015, acta imeko, vol. 5, no. 3, article 1,  november 2016, identifier: imeko‐acta‐05 (2016)‐03‐01  section editor: franco pavese, italy  received october 20, 2016; in final form october 20, 2016; published november 2016  copyright: © 2016 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  corresponding author: franco pavese, email: frpavese@gmail.com      dear reader, this special issue includes five papers, originating from subject matters presented at the xxi imeko world congress 2015 held in prague, during sessions organized by technical committee 21 on mathematical tools for measurements, and selected for publication in acta imeko. the first is a short review of concepts of probability related to measurement. the paper of cundeva-blajer is devoted to optimization methods in electrical metrology. the paper of shirono concerns the maximum likelihood concept applied to evaluate the uncertainty in proficiency tests. the paper of volosnikov illustrates the use of neural networks for error correction in dynamic measurements. the paper of witkovsky proposes an alternative tool to form the probability distribution of output quantities by numerical inversion in linear models. franco pavese guest editor imeko tc21, chair microsoft word 447-3162-1-le-rev acta imeko  issn: 2221‐870x  september 2017, volume 6, number 3, 12‐17    acta imeko | www.imeko.org  september 2017 | volume 6 | number 3 | 12  drawing materials studied by thz spectroscopy  andrea taschin 1,2 , paolo bartolini 1 , jordanka tasseva 3 , jana striova 4 , raffaella fontana 4 , cristiano  riminesi 5 , renato torre 1,2   1 european lab. for non‐linear spectroscopy (lens), univ. di firenze, via n. carrara 1,i‐50019 sesto fiorentino, firenze, italy.  2 dip. di fisica e astronomia, univ. di firenze, via sansone 1, i‐50019 sesto fiorentino, firenze, italy.  3 infn, istituto nazionale di fisica nucleare, sez. di napoli, complesso univ. di m. s. angelo, ed. 6‐ via cintia, 80126 napoli.  4 istituto nazionale di ottica, ino‐cnr, largo fermi 6, i‐50125 firenze, italy.  5 istituto per la conservazione e la valorizzazione dei beni culturali, icvbc‐cnr, via madonna del piano 10, i‐50019 s sesto fiorentino, italy.      section: research paper   keywords: ink; drawing material; thz spectroscopy  citation: andrea taschin, paolo bartolini, jordanka tasseva, jana striova, raffaella fontana, cristiano riminesi, renato torre, drawing materials studied by  thz spectroscopy, acta imeko, vol. 6, no. 3, article 3, september 2017, identifier: imeko‐acta‐06 (2017)‐03‐03  editor: sabrina grassini, politecnico di torino, italy  received february 13, 2017; in final form july 27, 2017; published september 2017  copyright: © 2017 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: regione toscana, por‐crofse‐unifi‐26 ;  ente cassa di risparmio firenze, n. 2015‐0857 and iperion ch ‐ ga 654028  corresponding author: r. torre, e‐mail: torre@lens.unifi.it    1. introduction  during the last years, thz spectroscopy has been intensely developed thanks to the realization of new laser sources capable of generating coherent radiation. because the thz radiation is of low energy, thz spectroscopic techniques are nondestructive tools able to provide valuable information in the cultural heritage field. recently, some studies [1]-[3] have demonstrated the possibility to use this radiation in the investigation of painting and drawing media. in particular, the thz-time domain spectroscopy (thz-tds) proved to be a powerful thz spectroscopic technique able to measure different optical parameters of the artistic works. these pulsed thz techniques can be implemented by using both the transmission or reflection configurations [4]-[8]. when a thz pulse passes through the sample, the material induces modifications in its temporal waveform; from the analysis of these variations the optical parameters of the material can be recovered. nevertheless, the extraction of the transmission parameters (i.e. absorption coefficients and indexes of refraction) from the thz pulse waveform is not a trivial task [9]. it depends on the experimental conditions and sample characteristics; in particular, the procedure becomes very complex for thin film samples [10]. recently, we have presented a thz spectroscopic study of layered ink samples [7], [8]. we implemented an innovative experimental and analytical method enabling a reliable extraction of the thz parameters from a tds experiment. we report here on the investigation of a series of black and colored inks by means of this innovative thz-tds approach. we measured both bulk inks in pellet samples and inks layered as thin film samples. moreover, we compared the ink thz spectra with those of the separate specific constituents of the ink pigments. this enables one to identify the origin of the spectroscopic peaks present in the thz absorption spectra of the inks. abstract  thz time‐domain spectroscopy in transmission mode was applied to study dry and wet drawing inks. in specific, cochineal‐, indigo‐ and  iron‐gall based inks have been investigated; some prepared following ancient recipes and others by using synthetic materials. the thz  investigations have been realized on both pellet samples, made by dried inks blended with polyethylene powder, and layered inks,  made by  liquid deposition on polyethylene pellicles. we  implemented an  improved thz spectroscopic technique that enabled the  measurement of the material optical parameters and thicknesses of the  layered  ink samples on absolute scale. this experimental  investigation shows that the thz techniques have the potentiality to recognize drawing inks by their spectroscopic features.  acta imeko | www.imeko.org  september 2017 | volume 6 | number 3 | 13  2. materials and methods  2.1. samples preparation  we investigated samples prepared in the laboratory using home-made and commercial inks. the former inks have been prepared using both ancient recipes (giovanni alcherio 1411) and synthetic materials. four groups of inks, according to the colour they render, might be distinguished: red, blue, black and white. in table 1, we report the compositions of the studied inks and the recipes followed for their preparations. the black inks, all iron gall based, were prepared in the laboratory following two main recipes containing either oak galls as a source of the gallo-tannic acid (recipe a in table 1) or preliminary synthesized gallic acid (recipe b in table 1). details about the ink preparation are reported in [7]. dried inks were blended with polyethylene (pe) powder (merck), ground, and pressed in pellets of 13.2 mm diameter and thickness around 1 mm. pe is the ideal support for absorption spectroscopy in the thz region thanks to its negligible absorption coefficient (below 1 cm-1 see [11] and references therein). the analyte concentration in the pe pellet was chosen to be approximately 33 wt. % that, for most of the studied inks, is an optimal concentration, rendering detectable any eventual features in a relatively wide spectral range. as an attempt of getting closer to a real-practice experiment on ancient manuscripts and drawings, some of the above inks have been studied also in the form of thin films, of the order of tens of μm, layered on 10 μm pe pellicles (polyethylene far-ir sample cards by sigma-aldrich). 2.2. terahertz time domain spectroscopy  a standard thz-tds set-up in transmission configuration has been used to extract the values of the refractive index, n, and the absorption coefficient, , of inks in the frequency range 0.1 – 4 thz. thz pulses are produced by exciting a biased photoconductive antenna with femtosecond optical laser pulses (= 780 nm, t = 120 fs at 100 mhz). the outgoing thz radiation is guided, as usual, by two couples of off-axis parabolic mirrors. the thz field is collimated, focused on the sample, collimated again, and finally focused on a second photoconductive antenna for its detection. a second optical laser pulse, sent to the detection antenna, acts as the current gate for the thz detection. the temporal evolution of the photocurrent amplitude in the detection antenna, acquired by changing time delay between exciting and gate pulses, is directly related to the electric field amplitude of the thz radiation. this current is amplified by a lock-in amplifier and digitized by an acquisition board. the processed signal and delay line encoding are acquired by homemade software for retracing the final time dependent thz field. the whole thz set-up is enclosed in a nitrogen purged chamber for removing the water vapour contribution present at the thz frequencies spanned by the experiment. more details about the used thz-tds set-up can be found in [7], [8] and the supplementary information therewith. 2.3. materials optical parameters extraction  the transfer function of the material, h(ω) [7], [8] is the ratio between the thz field transmitted after the sample, et(), and the incident field, ei(). it expresses how the phase and amplitude of an electromagnetic plane wave of frequency  change due to the absorption and refraction of the crossed medium. for a homogeneous dielectric slab of thickness d and complex refractive index , surrounded by nitrogen, and for normal incidence, the theoretical expression of the transfer function can be written as ∙ with ∑ ′ 2 (1) being: 2/ and ′ 2 / the nitrogensample and sample-nitrogen complex transmission coefficients, respectively; ′ / the reflection coefficients, with , where is the refractive index, the extinction coefficient, and the refractive index of nitrogen, is the vacuum speed of light. the multiple reflections inside the sample are taken into account by the fabry-pérot effect defined by the function. the optical properties of the material, and and its thickness, , in principle, can be completely characterized by measuring the experimental transfer function, , obtained by the ratio of the complex fourier transform of the sample and the reference signal. unfortunately, the expression for is not expressed in a closed-form and the real thickness of the sample is not generally known with enough accuracy. some numerical iterative process has to be used to extract , and [12]-[16] and this process depends on the nature of the thz signal and the thickness of samples. figure 1 reports typical signals measured by our thz-tds set-up on black b pellet sample. both the temporal evolution of the thz signal (main panel of figure 1, oscillations in the 5-10 ps range of the red curve) and its spectral content (inset of figure 1, holes in the spectrum) show modifications due to the table 1. compositions and recipes of the studied inks.  color ink  ink type  ink recipes  red  1  commercial cochineal carmine: possibly based on carminic acid (c22h20o13).   2  red ochre: containing iron(iii)oxide‐hydroxide. blue    commercial blue ink: based on 2,2‐bis(2,3‐dihydro‐3‐oxoindolyli‐den),  indigotin.  black  a  recipe a ‐ iron gall ink with arabic gum: 70 ml water, 10 ml white wine, 10 ml red vinegar, 5 g powdered  oak galls (bizzarri, firenze), 1.25 g arabic gum, 1.25 g  feso4∙7h2o  (bizzarri, firenze) melanterite.    b  recipe b ‐ iron gall ink with arabic gum: 14 ml distilled water, 1.14 g gallic acid (bizzarri, firenze), 0.29 g  arabic gum, 0.29 g  feso4∙7h2o  (bizzarri, firenze).    c  commercial iron gall ink (zecchi). white    white lead: 2pbco3∙pb(oh)2. acta imeko | www.imeko.org  september 2017 | volume 6 | number 3 | 14  absorption of the ink. in time evolution of the sample thz signal (red curve), the additional little pulse at around 15 after the main pulse is due to the internal reflections of the thz pulse between the sample surfaces. these appear in the spectrum as fast oscillations in the low frequency window. if the reflections are clearly distinguishable in the time domain and they are not superimposed on the oscillating modifications due to the sample absorption (contrary to what is observed in figure 1), the analysis for extraction of the optical parameters is relatively simple [7], [8]. usually, this occurs with thick samples and weak absorption. contrarily, in a thin-layered sample, the reflection signals are close in time and partially superimposed, so a correct extraction of the material parameters requires a complex data analysis where the multi-reflection processes are properly taken into account [7]-[9]. the transfer function becomes more complex and the material parameters, and , can be extracted only using an articulated data analysis based on iterative fitting procedures [7]-[10]. otherwise, the refractive index and absorption coefficient spectra are distorted by fake oscillations. recently, we have implemented an innovative experimental procedure and numerical method to analyse the transmission thz-tds signal of samples composed of multiple thin layers [7], [8]. the numerical method is based on a polynomial fit of the optical parameters, by which we can catch the real physical frequency behaviour and remove the fp oscillations. we perform a preliminary evaluation of  ,  and : neglecting the fp term and the imaginary part of the refractive index in the fresnel coefficients of (1), an analytical expressions for  and  can be obtained as a function of  with measured by a micrometer screw. then we perform a polynomial fit of  and  in order to remove the fp oscillations by which they are affected. then we minimize the difference between the calculated and experimental h functions integrated over all the frequencies, ∆ , to infer new best values for  ,  and . the final step concerns the refinement of the frequency dependence of parameters  and  obtained performing a minimization of the transfer function difference, ∆  , at variable frequencies and keeping fixed the value. this data analysis enables a reliable and robust extraction of the frequency dependence of the optical parameters, opening the possibility to measure even weak and broad peaks present in the  and  functions. moreover, it allows measuring the sample layer dimensions, even when it is not sufficiently thick to generate visible reflections in the time-domain data. in this experimental investigation, we applied this method on the thz-tds data from a double layered sample: an ink film deposited on polyethylene (pe) pellicles. all details about the data analysis on single and dual thin layers are reported in [7] and the supplementary information enclosed therewith and in [8]. 3. results and discussion  due to the complexity in calculating the molar concentration of inks in pellets, only the relative molar absorption coefficient and the refractive index may be extracted and these are reported in relative units (figures 2-6). for the pellet samples, scattering processes due to inhomogeneities in the sample could also affect the optical properties. in particular, the mismatch between the ink and pe indices gives rise to a spectral distortion called christiansen effect [17]. the effect can be greatly reduced by a fine grinding of the powders. usually, the scattering introduces an additional absorption contribution, which grows with frequency, and distorts the absorption peaks and their dispersion curves. yet, new fake absorption peaks could arise, as a result of interference of mie scattering that, however, do not exhibit the characteristic dispersion in the refractive index. as long as the peaks in the measured figure  2.  thz  absorbance  spectra  for  red  1,  red  2,  blue,  and  white  pigments  measured  on  pellets.  some  weak  spectral  contributions  are evident in all the inks except in the cochineal carmine ink.    figure 1. main panel: typical time evolution of the thz electric field of the reference (black line) and signal (red line) in the black b pellet sample. inset: amplitude spectra of the signals obtained by their fourier transforms. in the signal  data,  the  oscillations  after  the  main  pulse  are  due  to  the  ink absorption  effects  whilst  the  little  pulse  at  14.5  ps  to  the  fp  effect,  see enlargement.  0.3 0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7 3.0 10 20 30 40 50 60 70 80 90 100 frequency (thz) white 2.0 1.95 1.5 blue a b so rb an ce ( ar b . un .) red 2 2.2 red 1 wavenumber (cm-1) 0 5 10 15 20 25 0.5 1.0 1.5 2.0 2.5 3.0 3.5 11 12 13 14 15 16 17 s ig n al i n te n si ty ( ar b .u n. ) time (ps) frequency (thz) acta imeko | www.imeko.org  september 2017 | volume 6 | number 3 | 15  absorbance spectra are of a symmetric lorentzian line shape and are also present in the relative dispersions of the refractive index, the scattering processes can be considered negligible. for the films, instead, the employed analysis allows the measurement of the optical properties of the same ink layer, and the absolute absorption coefficients and refractive indices are reported (figure 7). in figure 2 we report the absorbance spectra of red 1, red 2, blue, and white inks in pellets. only red 1 shows featureless spectra in the measured thz range whilst the others present a few spectral peaks. in particular, red 2 shows a peak around 2.2 thz, blue two very weak peaks at 1.5 and 1.95 thz and the white ink a broad band centred at 2 thz. these features are confirmed by the dispersion sample visible in figure 3. figure 4. thz absorbance spectra for the iron gall inks measured on pellets.  the pigments prepared following the ancient recipe (recipe a) containing oak galls (black a) do not show any particular spectral feature contrary to what is observed in the inks containing synthesized gallic acid (black b and  black c) which instead shows well pronounced peaks.  figure  6.  thz  absorbance  spectra  for  the  black  b  and  black  c  pigments  together with those of the individual constituents gallic acid, iron(ii) sulfate commercial, and iron(ii) sulfate synthesized in the laboratory.    figure 3. frequency dependence of the refractive indices for red 1, red 2, blue, and white pigments measured on pellets. the dispersion behaviour confirms  the  absorptive  origin  of  the  features  found  in  the  absorbance spectra of the red 2, blue, and white samples in the 1.5‐2.5 thz range.    figure 5. frequency behaviour of the refractive indices for the iron gall inks  measured on pellets. the dispersive features of the refractive index for the  black  b  and  black  c  inks  confirm  what  measured  in  the  respective  absorbance spectra.  0.3 0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7 3.0 10 20 30 40 50 60 70 80 90 100 black c frequency (thz) wavenumber (cm-1) 1.92 1.52 2.57 2.061.50 black b a b so rb an ce ( ar b . u n .) 1.04 black a 0.3 0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7 3.0 3.3 3.6 10 20 30 40 50 60 70 80 90 100 110 120 iron (ii) sulfate frequency (thz) 3.01 3.09 2.572.06 1.50 wavenumber (cm-1) 1.04 1.12 1.52 1.92 iron (ii) sulfate (comm.) black c a b so rb an ce ( ar b . u n .) gallic acid black b 0.3 0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7 3.0 10 20 30 40 50 60 70 80 90 100 frequency (thz) white blue r ef ra ct iv e in de x (a rb . u n. ) red 2 red 1 wavenumber (cm-1) 0.3 0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7 3.0 10 20 30 40 50 60 70 80 90 100 black c frequency (thz) wavenumber (cm-1) black b r ef ra ct iv e in d ex ( ar b. u n .) black a acta imeko | www.imeko.org  september 2017 | volume 6 | number 3 | 16  figures 4 and 5 show the absorbance spectra and frequency dependence of the refractive indexes for all three black inks measured on pellets. the iron gall inks prepared following the old recipe of mixing mainly iron(ii) sulfate and oak galls powder, black a, shows a featureless response in the thz range. on the other hand, the ink containing synthetic gallic acid, black b, and the commercial iron gall ink, black c, show structured spectra with several peaks. our results are in perfect agreement with those reported by bardon et al. [6]. in order to ascribe each spectral component of black b and black c, we compared them with those of the individual constituents, gallic acid, iron(ii) sulfate commercial, and iron(ii) sulfate synthesized in the laboratory (figure 6). gallic acid shows well-defined bands around 1.04, 1.50, 2.06, 2.57 and 3.09 thz. in the spectra of commercial ferrous sulfate, features at 1.12, 1.52, 1.92 thz are present. for the in-laboratory synthesized ferrous sulfate, instead, the peak at 1.12 thz disappears and an additional band around 3.01 thz is detected. the iron gall inks prepared following the recipe a do not show any features related either to gallic acid or to iron(ii) sulfate. it is known that under hydrolysis gallic acid is extracted from the oak galls. most likely all ferrous sulfate has oxidized to ferric sulfate, resulting in the formation of iron-tannic complexes. that hypothesis is well supported by bardon et al. [6], reporting no contribution of tannic acid in the 0.15 – 3 thz range and a single, questionable feature in the spectrum of iron(iii) sulfate. on the other hand, if gallic acid was extracted from the oak galls, and the iron sulfate to gallic acid molar ratio was close to 1, the two components might have been consumed in the formation of iron gallate complexes, possibly featureless in the studied range. in the spectra of the laboratory-made iron gall ink prepared from ferrous sulfate and synthesized gallic acid (recipe b, table 1), we can see the response of the gallic acid, while no ferrous sulfate contribution is present (figure 6). following what was reported by other researchers [6], we could attribute this to the low iron sulfate to gallic acid molar ratio that in our case is 0.1. the commercially available iron gall ink, instead, shows features of the iron sulfate and lacks those of gallic acid as reported in figure 6. as a next step we decreased the analyte quantity, preparing thin films on transparent pe pellicles. the analysed inks were: the three black inks, black a, black b, and black c and their optical parameters are shown in figure 7. we were able to obtain also in this case a very good signal-to-noise ratio and to get agreement with the results from bulk samples. the bilayer analysis procedure [7], [8] enables us to measure thicknesses down to 10 m. iron-gall ink, prepared with oak galls powder, once again does not manifest features of either gallic acid or ferrous sulfate. similarly to the pellet sample, the layered irongall ink prepared following recipe b reveals the presence of gallic acid by peaks in absorbance at 1.04, 1.50, 2.06 and 2.57 thz. the film from commercial iron-gall ink shows the 1.92 thz feature of ferrous sulfate, while the one at 1.52 thz seems to be strongly damped. it is worth pointing out that, thanks to the study on the thin films on pe pellicles, the refractive index and absorption coefficient values that we measured are absolute values different to what was measured in the pellet samples. 4. conclusions  thz time-domain spectroscopy in transmission configuration was applied to investigate inks commonly used in artworks, i.e. red, indigotin-based, iron-gall and lead white inks. pellet samples were prepared, blending the bulk dried pigments with pe powder. thin film samples were made by liquid inks layered on pe pellicles and subsequently dried. the thz-tds technique achieved high signal-to-noise ratio for both pellet and layered inks, enabling a particularly meaningful analysis of the data. the thz spectra measured show similar features in both sample types, pellet and film. in particular, the red cochineal carmine does not show any evident spectral signature in the 0.1-3.2 thz range, whereas the red ochre shows a weak but distinct absorption peak at about 2.2 thz. the blue ink presents two weak absorption peaks at 1.5 and 1.95 thz. the absorbance spectrum of white lead shows a shallow feature at around 2 thz. figure 7. absorption coefficient (left panel) and refractive index (right panel) as a function of frequency for the black inks on pe pellicles.  0.3 0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7 3.0 0 50 100 150 0 50 100 150 0 100 200 1.2 1.4 1.6 0.3 0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7 3.0 1.2 1.4 1.6 1.2 1.4 1.6 black c frequency (thz) black b 2.57 a b so rb an ce c o ef f. ( cm -1 ) black a 2.061.50 1.04 d = 17 m d = 16 m 1.52 1.92 d < 10 m r ef ra ct iv e in d ex frequency (thz) acta imeko | www.imeko.org  september 2017 | volume 6 | number 3 | 17  the spectroscopic features of the iron gall inks are more articulated and pronounced; we want to stress how the inks prepared by the ancient recipe (recipe a), can be immediately distinguished from those prepared with synthetic materials (recipe b) or commercial ones. in fact, the black inks prepared by recipe a show a featureless response. on the contrary, black inks prepared by recipe b or commercially available inks reveal the spectral signatures of their elementary constituents. moreover, the possibility to measure in absolute scale the thz spectra of layered inks brings these spectroscopic investigations closer to real-practice problems. in these respects, the present research is a rare attempt to improve the thz-tds method in order to overcome the gap between the purely scientific uses and the operative applications in the field of cultural heritage. acknowledgement  this work was funded by regione toscana, prog. porcrofse-unifi-26 and by ente cassa di risparmio firenze, prog. 2015-0857. the authors of ino-cnr acknowledge iperion ch project ga 654028, funded by the eu community’s h2020-research infrastructure program. references  [1] k. fukunaga, y. ogawa, s.i. hayashi, i. hosako, application of terahertz spectroscopy for character recognition in a medieval manuscript, ieice electronics express, 5, (2008), pp. 223-228. [2] c. seco-martorell, v. lopez-dominguez, g. arauz-garofalo, a. redo-sanchez, j. palacios, j. tejada, goya's artwork imaging with terahertz waves, opt express, 21, (2013), pp. 17800-17805. [3] g.c. walker, j.w. bowen, w. matthews, s. roychowdhury, j. labaune, g. mourou, m. menu, i. hodder, j.b. jackson, subsurface terahertz imaging through uneven surfaces: visualizing neolithic wall paintings in catalhoyuk, opt express, 21, (2013), pp. 8126-8134. [4] k. fukunaga, m. picollo, terahertz spectroscopy applied to the analysis of artists' materials, appl phys a-mater, 100, (2010), pp. 591-597. [5] i. cacciari, j. agresti, s. siano, combined thz and lips analysis of corroded archaeological bronzes, microchemical journal, 126 (2016), pp. 76-82. [6] t. bardon, r.k. may, p.f. taday, m. strlic, systematic study of terahertz time-domain spectra of historically informed black inks, analyst, 138, (2013), pp. 4859-4869. [7] j. tasseva, a. taschin, p. bartolini, j. striova, r. fontana, r. torre, thin layered drawing media probed by thz time-domain spectroscopy, analyst, 142, (2017), pp. 42-47. [8] a. taschin, j. tasseva, p. bartolini, r. torre, thz time-domain spectroscopic investigations of thin films, measurement (2017), http://dx.doi.org/ 10.1016/j.measurement.2017.05.074. [9] w. withayachumnankul, m. naftaly, fundamentals of measurement in terahertz time-domain spectroscopy, journal of infrared, millimeter, and terahertz waves, 35, (2013), pp. 610-637. [10] j.f. o’hara, w. withayachumnankul, i. al-naib, a review on thin-film sensing with terahertz waves, journal of infrared, millimeter, and terahertz waves, 33, (2012) pp. 245-291. [11] y.s. lee, principles of terahertz science and technology, springer us, new york, (2009), isbn 978-0-387-09540-0. [12] w. withayachumnankul, b .ferguson, t. rainsford, s. p. mickan, d. abbott, material parameter extraction for terahertz time-domain spectroscopy using fixed-point iteration proc. spie, 5840, (2005), pp. 221-231. [13] i. pupeza, r. wilk, m. koch, highly accurate optical material parameter determination with thz time-domain spectroscopy, opt. express, 15, (2007), pp. 4335-4350. [14] m. scheller, real-time terahertz material characterization by numerical three-dimensional optimization, opt. express, 19, (2011), pp. 10647-10655. [15] m. scheller, c. jansen, m. koch, analyzing sub-100-�m samples with transmission terahertz time domain spectroscopy, opt. commun., 282, (2009), pp. 1304 – 1306. [16] m. scheller, m. koch, fast and accurate thickness determination of unknown materials using terahertz time domain spectroscopy, j. infrared millim. terahertz waves, 30, (2009), pp. 762-769. [17] m. franz, b.m. fischer, m. walther, the christiansen effect in terahertz time-domain spectra of coarse-grained powders, app. phys. lett., 92, (2008) pp. 021107. evaluation of pressure effects on an acoustic thermometer with a single waveguide acta imeko issn: 2221-870x december 2018, volume 7, number 4, 42 47 acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 42 evaluation of pressure effects on an acoustic thermometer with a single waveguide rok tavčar1, dušan agrež1, samo beguš1 1 university of ljubljana, faculty of electrical engineering, laboratory of metrology and quality, ljubljana, slovenia section: research paper keywords: sound; temperature; acoustic; pat citation: rok tavčar, dušan agrež, samo beguš, evaluation of pressure effects on an acoustic thermometer with a single waveguide, acta imeko, vol. 7, no. 4, article 8, december 2018, identifier: imeko-acta-07 (2018)-04-08 editor: alexandru salceanu, "gheorghe asachi" technical university of iasi, romania received march 29, 2018; in final form may 31, 2018; published december 2018 copyright: © 2018 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: rok tavčar, e-mail: rok.tavcar@fe.uni-lj.si 1. introduction acoustic thermometers are better known as primary thermometers [1] and have a very high level of accuracy. however, they are not particularly useful for practical measurements because they are too cumbersome. several proposals have been put forward with the aim of constructing similar thermometers that would be similarly accurate but more practical for everyday use. examples of designs of practical acoustic thermometers include the twin-tube design [2 4] or the use of a measured medium as a waveguide [5 6]. acoustic thermometers with waveguides are useful in harsh environments, such as nuclear reactors [7] or other environments that could influence the measurements, such as a high magnetic field. they are also useful when it is necessary to measure the average temperature of a large volume, such as a chemical reactor. they also benefit from low drift and inexpensive maintenance, as only the gas inside must be replaced to restore operation. besides the twin-tube design, which is the most intuitive, there are other designs, such as the joined twin-tube design and single-tube waveguide. the motivation behind exploration of the the single-tube waveguide design is that other designs of acoustic thermometer have inherent flaws that are hard to overcome. for example, the early twin-tube design designed by the authors had fluctuations in readings of approximately 0.5 °c at room temperature, depending on environmental influences, such as room temperature. this occurred despite the good thermal connection between both tubes, where copper wire was wound around both tubes at the common sound path. in the case of the joined twin-tube design, it was difficult to construct it in such a way that sound split in both tubes evenly. another motivation is that this design is compact and has a small measurement end, as it is coiled into a helix as opposed to being bent in the form of the letter u. our proposed design has several advantages compared to other designs, such as its smaller size, improves sensitivity, and the same common sound path. unfortunately, this design also has some disadvantages, such as the low amplitude of the return signal due to reflection and possible coincidences of the return signals. section 3 explains how to mitigate these limitations. the heart of the problem with acoustical thermometers is determining the speed of sound. there are two main approaches to this process: measuring acoustic resonance [8 9] or measuring time delay [10 11]. this design measures a time delay, more precisely, the time delay between the first and second reflections of soundwaves in the tube. abstract this paper presents the implementation and testing of an acoustic thermometer with a single waveguide. the proposed design has several specific advantages compared to other acoustic designs with waveguides, such as its smaller size, improved sensitivity, and the same common sound path. this paper describes each advantage and the tests used to determine their usefulness. it also shows how to deal with the disadvantages of this design, such as its weak return signals and possible coincidences concerning the return signals. the instrument was calibrated at ice point and then tested in an oil bath from -30 °c to 120 °c. in steady state, it had a standard deviation of 0.050 °c, with 66 independent readings per second. the stability of the bath was 10 mk and the uncertainty of the reference thermometer was 0.003 °c. mailto:rok.tavcar@fe.uni-lj.si acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 43 2. acoustic thermometer the scheme of the proposed design is shown in figure 1. the blue colour indicates the common sound path, which consists of a tube with a larger inner diameter. the green colour indicates the measurement end, a tube with a smaller inner diameter. this tube is coiled into a helix with the smallest bending radius possible for the given tube material. a microphone is placed at the left end of the blue tube. at the junction at which tubes with a different inner radius meet, the first sound reflection is generated, and the second reflection is generated from the end point of the measuring end, where the tube is closed. 2.1. speed of sound the speed of sound in a medium is mostly influenced by the medium itself and its temperature [12-15]. the relationship is expressed as: 𝑐𝑠 = √ 𝑇 𝛾 𝑅 �̅� , (1) where t is the temperature in kelvin, r is the gas constant, γ is the ratio of specific heats cp/cv, and �̅� is the molar mass. equation 1 is valid in the free field, while in the narrow waveguide, there is an additional dependency on thermal and viscous boundary layers [13], which is a factor at low frequencies (below 5 khz). 2.2. calculating temperature in the proposed design, the speed of sound is calculated using the time of flight method: 𝑐𝑠 = 2𝑙 𝜏 , (2) where l is the length of the tube with the smaller inner radius and τ is measured transit time. the factor 2 is necessary, as reflected soundwaves travel the same path twice. when inserted into equation 1, it is transformed to: 𝑇 = 𝑚 (2𝑙)2 𝛾 𝑅 ∙ 1 𝜏2 . (3) equation 3 assumes that the first part is constant during temperature measurements, which is valid because the instrument has a slight overpressure in the waveguide compared to the outside atmospheric pressure. the only parameter that changes is the length of the tube. changes in length are compensated for by the available thermal linear expansion data for stainless steel (aisi 304). the constant in equation 3 is determined by calibration at an ice point. this is necessary because measurement of the length between the microphone and the sound reflective surfaces is too difficult due to the curvature of the tube. 2.3. reflections in tubes the positions of the microphone and the reflections are indicated in figure 2. only reflections occurring due to the change in the inner radius and those at the end of tube are considered. echoes through metal are not considered, as the speed of sound in steel is more than 10 times faster than in gas. because of this high difference in speed, most of the soundwaves reflect at the boundary between a gas and a metal. 2.4. measuring the transit time in this design, the microphone has the dual role of both microphone and speaker. the microphone used in the experiment is a brüel & kjær type 4189 [16] with a custom preamplifier that allows for its use as a speaker. a soundcard emu-0404 was used to generate and capture electric signals. the microphone was positioned on the tube end (the left side of the blue tube in figure 1 and figure 2) with the diaphragm parallel to the soundwaves. the microphone was secured inside the tube with teflon sealing tape. the transit time measurement was commenced by generating a signal on a computer. the soundcard generated an electric signal that was transmitted by a microphone operating in a speaker mode. the microphone then switched to microphone mode to receive the reflected sound signals. the received signal then travelled to a microphone preamplifier and the soundcard. thereafter, the microphone switched back to speaker mode. the computer started another measurement after it calculated the transit time. the excitation signal was a linear sweep, with a frequency range from 5 khz to 15 khz. both boundaries were determined in order to ensure the best possible results, since low frequency soundwaves have a different speed and high-frequency soundwaves have a higher attenuation when traveling through a tube. the signal duration was limited by the length of the smaller tube, around 2.5 ms in this design. the pulse compression technique and matched filter were used to improve the signal-to-noise ratio (snr) and to determine the transition time. because the soundcard sampling frequency was 192 khz, the received signals were up-sampled by a ratio of 10, and quadratic fit was used in searching for the peaks in the results of the matched filter. thus, approximately 100 ns of time delay resolution could be secured. 3. relevant studies other attempts [1 3], [17] at the construction of acoustic thermometers with waveguides have given consistent results over a wide range of temperatures and designs. the results show that the uncertainty and deviation arising in the model described in equation 3 increased along with a temperature increase. results in the range of 30 °c to 50 °c were a 0.08 °c deviation from the model and uncertainty amounted to 0.005 °c. in the range up to 600 ° c, the deviation from the model increased to 6 °c and the uncertainty to 0.5 °c. in the range from room temperature to 1000 °c, the deviation from the model increased figure 1. schematic diagram of the acoustic thermometer with a single waveguide. figure 2. sound path inside the tubes. m marks the position of a microphone, a marks the position of the first reflection, and b marks the position of the second reflection. acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 44 to 13 °c and the uncertainty to 1 °c. results exceeding 1000 °c were somewhat different across the designs, but all indicated an increasing deviation from the model along with an increase in temperature. 4. instrument design the instrument was constructed from two main mechanical parts. the first is the measuring part, shown in figure 3 on the left-hand side in both images. the basic design consisted of a tube bent into a helix, with one end closed and the other end straightened out and left open. the open end was connected to the second part with another tube (shown in blue in figure 1 and figure 2), which had a larger inner and outer diameter than the first part. the second part was cylindrical, housing the microphone and its preamplifier. this part also held the electric and the pneumatic connection to the outside. inside the helix, the tube and microphone case were filled with air with slight overpressure to minimize the intrusion of the surrounding air. the dc power supply, soundcard, amplifier, and control electronics were separated from the microphone box. the calculations and data storage were maintained on a pc. 4.1. processing the received signal the received signal was filtered and synchronized with the transmitted signal. thereafter, the matched filter model was applied. the matched filter model used was the same as the transmitted signal, which consisted of a linear sine sweep from 5 khz to 15 khz, and was additionally filtered by the inverse frequency filter before matching. the parameters of the inverse frequency filter were handpicked for ensuring the best performance. 4.2. the lengths of the tubes and their ratios as mentioned above, one major problem with this design is coincidence of the return signals. the proposed solution is to carefully choose the dimensions of the tubes and the ratios between them. in table 1 and table 2, the calculations for two optimal options are displayed. length 1 is the length of the straight tube in figure 1, and length 2 is the length of the helix. table 1 and table 2 show the sound path lengths for two different tube length ratios. figure 2 shows the sound paths in the tubes, with the length ratios calculated as in table 2. the sound started at m and travelled to point a. here, the path split, and half of the sound was reflected back to m (blue arrow), and half travelled further to point b. at point b, all sound was reflected back to m. in this case, the first reflected signal was from the first reflection, and the second from the second reflection. in the second case, the first reflected signal was from the first reflection, the second was the second return from the first reflection, and the third was from the second reflection. both cases were chosen so that the signal return from the second reflection was between two returns from the first reflection to minimize the influence between both reflections. for this instrument, it was better to choose the second ratio (with the longer measuring end) to ensure improved sensitivity. improved sensitivity at the longer measurement end was a consequence of the measurement change in the time of flight for the transmitted signals. the longer measurement end delays the returned signal more so than the short one. the total length was limited by the attenuation of the returned signals, and this depended on the radii of the tubes, the microphone, and the frequency of the used sound waves. the preliminary tests showed that the maximum length of the tubes and microphone was 1.5 m, i.e., 0.6 m for the common path and 0.9 m for the measuring end. 5. experimental setup before the main test could be performed, a few other preliminary tests that were not temperature-related were required to be performed to determine the thermometer parameters, such as microphone snr and the effect of switching the microphone preamplifier between the microphone and the speaker mode. the main test consists of two parts. the first is the thermometer calibration at an ice point to determine equation 3’s parameters. the second part is the actual temperature test in the oil bath: from -30 °c to 120 °c. the bath used had a temperature stability of 10 mk, and the reference thermometer was 0.003 °c. one test cycle included 16 uniformly spaced steady states, each two hours long. in total, there were four cycles, which resulted in one week of testing. this test showed the standard deviation and matching with the model. it also showed the presence of any hysteresis or drift. after the main test was complete, three additional tests for measuring acoustic snr versus pressure were undertaken, increasing the temperature range and gas mixing time. the test setup for measuring the acoustic snr and gas mixing time was similar to the ice point calibration setup. the test included maintaining the measuring end at a constant temperature (0 °c) and changing the pressure in the tube. to measure usability at a higher temperature, the thermometer was put in an oven with a controlled temperature. 6. experimental results in figure 4, the readings from the reference thermometer versus the acoustic thermometer are plotted. the line is almost linear, with a few bumps. these bumps were consequences of the larger time constant of the reference thermometer compared to the acoustic thermometer, and they were only present in the transient states. figure 3. an image of the thermometer with its dimensions, created using cad (top). a photograph of the finished thermometer (bottom). table 1. calculations for sound paths with length ratio 1:0.5. length 1=1 m length 2=0.5 m first return 2 m 3 m second return 4 m 6 m third return 6 m 9 m table 2. calculations for sound paths with length ratio 1:1.5. length 1=1 m length 2=1.5 m first return 2 m 5 m second return 4 m 10 m third return 6 m 15 m acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 45 considering figure 4 and figure 5, the model used (equation 3) was good in the temperature range of -30 °c to 20 °c, but at higher temperatures, there was a deviation that increased with the temperature. further analysis showed that above 60 °c, the deviations from the model linearly increased along with the temperature increase, with a slope of 0.05 °c /°c. figure 5 shows the temperature error of the acoustic thermometer versus the measured temperature compared to the reference thermometer. the deviations from the model above 60 °c are clearly shown. figure 6 shows a small part of the measurements around 0 °c. the readings from the reference thermometer are marked in red, and the readings from the acoustic thermometer are marked in black. readings from the acoustic thermometer were averaged to better show the drift in the figures. the time at 0 s was the start of the experiments. the steady state shown in figure 6 was 3.1 days after calibration. it shows that the thermometer did not have a longterm drift, but it did have some short-term drift. figure 7 shows a small part of the readings around 100 °c. the difference compared to the previous figure is that the scale for the acoustic thermometer is on the right-hand side of the graph. the difference between the scales is 1.8 °c. it is presented in this manner for ease of comparison. figure 7 shows that at high temperatures, a similar short-term drift occurred as was seen for lower temperatures. furthermore, at these higher temperatures, the model deviated from the measured results. the snr tests showed that the signal duration of 2.5 ms was enough in order to ensure sufficient snr after the pulse compression for temperature measurements. switching between microphone and speaker mode did produce spikes in the returned signal, but they were harmless due to a long pause between one mode and another, and they did not influence the peak position after cross-correlation. heating the housing (but not the measuring end) with a heat gun did not cause any temperature error, but it did produce noise in the measurements while a heat gun was turned on. 6.1. length of tubes the lengths and ratios of the tubes were selected before the production of the thermometer and were based on a prototype without overpressure. because of the effects of overpressure on acoustic snr (details described in section 8), the selected lengths were not ideal. the results of the selected lengths included an overlap in the return signal starting from 200-300 °c (depending on the temperature in the common sound path). because of the increased acoustic snr, the length of the transmitted signal can be shorter, which directly influences the lengths of the tubes. as signal overlap happens only when the temperature in the measuring end is too high compared to the common sound path, it is possible to raise the temperature limit higher by heating the common sound path. this did not influence how the temperature was calculated in this experiment because the temperature in the common sound path does not have a direct influence on the measured temperature. this method can practically raise the temperature range by 100 °c, as it is limited by a maximum permissible working temperature of the microphone and electronic components used. figure 8 shows the problem that limits high-temperature operations. at 4 ms, the first return from the change in radius started, as shown in blue in figure 2. at 8 ms, the second return from the change in radius started, and at 9 ms the reflection from figure 4. a plot of the reference thermometer versus the acoustic thermometer temperature readings. figure 5. temperature error versus acoustic thermometer temperature readings. figure 6. small part of the steady state around 0 °c. figure 7. small part of the steady state around 100 °c. acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 46 the end of the measurement end started, as shown in green in figure 2. the reflection from the end of the measurement end was faster than the second reflection from the change in the inner radius return. this created a coincidence in the acoustic signal and distorted the temperature measurements. 6.2. revised calculation of tube lengths for high temperatures while the ratio calculations described in section 4.2 are important for a design, they do not consider changes in temperature between a common sound path and measurement end. the temperature in a common sound path has a greater influence on the delay and amplitude of reflected signals than is estimated using our thermometer [18]. this simplifies tube length calculations, because the main focus is reducing the length of the common sound path as much as possible. the lowest length is limited by the signal duration and expected temperatures in a common sound path. 7. gas mixing in tubes we conclude that the drift occurred due to the mixing of air with pure gas (argon), as shown in figure 6 and figure 7. the duration of this process was long because the gas exchange in the thermometer was only accomplished by changing pressure in the tube without creating a vacuum in it. gas mixing in a tube is a slow process [19], having a significant effect on the speed of sound [20]. this process was over two weeks for this thermometer and uncertainty level. in this case, the calibration and tests were undertaken one week after the start of the gas exchange, so gas mixing had not completed finished by this point, resulting in drift. figure 6 and figure 7 also show that the temperature readings from the thermometer fell during the steady state, which supports our hypothesis. the speed of sound in argon is lower than in air, so when the gas mix included more argon, the sound needed more time to travel the same distance. this resulted in a lower temperature being calculated using equation 3. figure 9 shows the start of the effect caused by mixing two gases. at this point, the thermometer was filled with air at room pressure, and then it was filled with argon until the pressure inside reached four bars. thereafter, it was left at a constant temperature and pressure. 8. acoustic snr dependance on pressure the snr of the acoustic signal in the tubes depend on the gas pressure. figure 10 is a graph of the measured snr of the acoustic signal from the thermometer versus the pressure of the gas inside of the thermometer. the acoustic snr increased along with an increase in pressure, or attenuation in the tube was falling. this occurred because of the reduced absorption of the medium, which is inversely proportional to the square root of pressure. this has both good and bad implications for temperature measurements. temperature measurements with a higher acoustic snr have a smaller standard deviation, but the measurement frequency has to be reduced because acoustic signals need more time to settle down after each measurement with lower attenuation. this is because additional reflections need to be attenuated to minimize the impact on the next measurement. figure 11 shows the temperature standard deviation versus pressure. despite the reduced measurement frequency, it was better to increase the pressure in the thermometer than to undertake faster measurements and then average them. the measurement frequency was 66 readings per second at room pressure and 15 readings per second at 4 bars. the standard deviation was much smaller than in the bath test because this test was done in a less noisy environment than the bath, which had some noisy moving parts. the other reason is that the bath had a stability of 10 mk. figure 8. received acoustic signal at 400 °c. figure 9. measuring delay at a constant temperature while the gases inside the thermometer mix. figure 10. signal-to-noise ratio of the acoustic signal versus the pressure inside the thermometer. figure 11. temperature standard deviation at ice point versus pressure inside the thermometer. acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 47 9. conclusion the design proposed in this work offers new advantages compared with similar acoustic thermometers, while also retaining their benefits. the key change and advantage of this design is a compact coiled measuring end, since it enables this design to have very high sensitivity with a small size. other important changes include the use of sound frequencies at which sound speed is less dependent on frequency. furthermore, the dual use of a microphone as a speaker improves the mechanical stability of the system. the experimental results showed two problems with the current implementation. the first is the gas mixing in the tubes, particularly its timescale. the second is the deviation from the model used. gas mixing can be accelerated by using a vacuum pump, and once the process is complete, it does not have an influence until the gas composition changes again. after mitigating the deviation from the model, the calculated standard deviation on 66 samples (1 second of measurement) at 100 °c was 70 mk with the oil bath running and 30 mk with the oil bath powered down. it was also shown that with a higher gas pressure in the waveguide, it is possible to reduce the temperature standard deviation if the measurements are slowed down to compensate for the reduced attenuation of sound in the tubes. the range of the thermometer can be increased to 300 °c without changes in the design or gas used. by heating the common sound path, the range can be increased to 400 °c. using the described algorithms and tube designs, it is possible to achieve a temperature error of less than 2 °c temperature error in the -30 °c to 120 °c range with a calibration at one temperature. with a calibration at more reference temperatures or by applying the correction to the measured temperature, it is possible to achieve a temperature error below 0.4 °c at 120 °c. the tests showed three areas in which improvements are necessary: using a speaker with a higher spl, improved acoustic insulation between the outside and inside of the housing, and filling the acoustic tube with a pure gas, e.g., argon or nitrogen, to eliminate drift due to gas mixing. another possibility is to use xenon gas, which has a much lower speed of sound: 178 m/s compared to 319 m/s for argon and 349 m/s for nitrogen, at a gas temperature of 20 °c. increasing the sound pressure level and improving the acoustic insulation would reduce the standard deviation of the measurements and susceptibility to outside noise. a greater snr can also be achieved by a higher gas pressure and improved acoustic insulation. changing the gas inside of the housing and tubes would also reduce the short-term drift and deviation from the model. using xenon would also increase the sensitivity by almost factor of two compared to nitrogen or air. the problem of acoustic signal coincidence can be solved by heating a common sound path or using gas with lower speed of sound. the first method is limited by a microphone’s tolerance to temperature, as it is at one end of a common sound path. the second method requires finding a usable gas with a lower speed of sound. another solution to this problem is to use acoustic noise cancellation techniques to cancel or reduce the amplitude of the second return from the first reflection, thus eliminating or reducing acoustic signal coincidence. references [1] b. fellmuth, c. gaiser, j. fischer, determination of the boltzmann constant status and prospects, meas. sci. technol. 17 (2006), pp. 145-160. [2] h. j. aulfes, akustisches gasthermometer mit dunnen sensorrohren, phd dissertation, university of paderborn, 1993 [3] m. de podesta, g. sutton, r. underwood, s. legg, a. steinitz, practical acoustic thermometry with acoustic waveguides, int. j. thermophys 31 (2010), pp. 1554-1566. [4] n. amir et al., “inspecting u-tube bundles using acoustic pulse reflectometry”, proc. of the asme power conf., jul. 21-23, 2009, albuquerque, new mexico, usa, pp. 141-145. [5] r. underwood, t. gardiner, a. finlayson, j. few, j. wilkinson, s. bell, j. merrison, j. j. iverson, m. de podesta, a combined noncontact acoustic thermometer and infrared hygrometer for atmospheric measurements, meteorol. appl. 22 (2015), pp. 830835. [6] r. underwood, t. gardiner, a. finlayson, s. bell, m. de podesta, an improved non-contact thermometer and hygrometer with rapid response, metrologia 54 (2017), pp 9-15. [7] j. b. roes, d. l. peat, the development of an acoustical thermometer for a graphite matrix nuclear fuel element, ieee transactions on nuclear science 14 (1967), pp. 348-359. [8] a. elliott, passive acoustic thermometry, proc. of the internoise 2016, aug. 21-24 2016, hamburg, germany, pp. 622-629. [9] j. h. apfel, acoustic thermometry, rev. sci. instrum. 33 (2004), pp. 428-430. [10] l. c. lynnworth and e. h. carnevale, “ultrasonic thermometry using pulse technique”, in: temperature: its measurement and control in science and industry, volume 4. instrument society of america (editor), 1972, pp. 715-732. [11] x. b. mi, s. y. zhang, j.j. zhang, and y. t. yang, “automatic ultrasonic thermometry”, journal of nanjing university 39 (2003), pp. 517–523. [12] j. w. s. rayleigh, theory of sound, vol ii, sects. 255-266, macmillan, london, 1894. [13] l. e. kinsler, a. r. frey, a. b. coppens, and j. v. sanders (editors), fundamentals of acoustics, vol. xii wiley, new york, u.s.a., 2000, p. 548. [14] h. tijdeman, on the propagation of sound waves in cylindrical tubes, j. sound vib. 39 (1975), pp. 1-33. [15] t. yazaki, y. tashiro, t. biwa, “measurements of sound propagation in narrow tubes”, proc. r. soc. a 463 (2007), pp. 2855-1862. [16] https://www.bksv.com/en/products/transducers/acoustic/micr ophones/microphone-cartridges/4189. [17] g. sutton, m. de podesta, r. i. veltcheva, p. gélat, h. d. minh, g. edwards, twin-tube practical acoustic thermometry: theory and measurements up to 1000 °c, meas. sci. technol. 26 (2015), pp 1-15. [18] r. tavcar, d. agrez, s. begus, “acoustic thermometer with single waveguide”, proc. of the 22nd imeko tc4 international symposium & 20th international workshop on adc modelling and testing: supporting world development through electrical and electronic measurements, sept. 14-15, 2017, iasi, romania, pp. 244-248. [19] photoacoustic, photothermal and photochemical processes in gases. ed. p. hess, springer-verlag, berlin a.o.1989. pp. 166 [20] d. a. bohn, environmental effects on the speed of sound,j. audio eng. soc. 36 (1988), pp. 223-231. . https://www.bksv.com/en/products/transducers/acoustic/microphones/microphone-cartridges/4189 https://www.bksv.com/en/products/transducers/acoustic/microphones/microphone-cartridges/4189 a field-measurements-based lora network planning tool acta imeko issn: 2221-870x december 2020, volume 9, number 4, 21 29 acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 21 a field-measurements-based lora network planning tool susanna spinsante1, luca gioacchini2, lorenzo scalise3 1 dipartimento di ingegneria dell’informazione, università politecnica delle marche, via brecce bianche 12, 60131 ancona, italy 2 dipartimento di elettronica e telecomunicazioni, politecnico di torino, 10129 torino, italy 3 dipartimento di ingegneria industriale e scienze matematiche, università politecnica delle marche, via brecce bianche 12, 60131 ancona, italy section: research paper keywords: lora; network planning; path loss; lorasim tool citation: susanna spinsante, luca gioacchini, lorenzo scalise, a field-measurements-based lora network planning tool, acta imeko, vol. 9, no. 4, article 4, december 2020, identifier: imeko-acta-09 (2020)-04-04 section editor: leopoldo angrisani, university of naples federico ii, italy received: october 15, 2019; in final form: february 3, 2020; published: december 2020 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: s. spinsante acknowledges support by the italian national technology cluster project shell (shared interoperable home ecosystems for a green, comfortable and safe living), project code: ctn01 00128 111357. corresponding author: susanna spinsante, e-mail: s.spinsante@staff.univpm.it 1. introduction industry 4.0 encompasses several key technological components and substantiates in the so-called smart factory, where computer-driven systems monitor physical processes, create a virtual copy of the physical world and make decentralised decisions based on self-organisation mechanisms. the internet of things (iot) is considered an essential element of industry 4.0, providing the connection among the different sub-systems and enabling both human-to-machine and machine-to-machine communications. as such, large-scale iot deployments are expected to become widespread, not only within smart industrial plants, but also in similar scenarios, such as multi-site production districts [1], smart construction sites and even mines [2]. among the communication technologies enabling iot, low power wide area networks (lpwans) have been established as a good trade-off between performance and deployment costs [3]. in fact, widely distributed sensor networks relying on 3g/4g cellular infrastructures benefit from reserved frequencies (thus being free from interferences), network-based synchronisation and a pervasive and reliable connectivity; however, they are associated with prohibitively high costs in the case of dense networks. meanwhile, lpwans, such as the long range network (lora) [4], provide license-free long-range communication (possibly prone to interferences) with low power demands (essential for iot sensors), but may require a dense deployment of gateways (gws) to ensure adequate data transfer performance and to minimise the data losses. numerous examples of iot applications relying on lora transmission technology can be found in the recent literature. addabbo et al. [5], [6] present a low-power iot architecture for the monitoring of chemical emissions, employed to set up monitoring infrastructures in industrial plants or public buildings. within the context of smart abstract long range (lora) transmission technology enables energy-constrained devices such as the tiny sensor systems used in internet-ofthings applications that are distributed over wide areas while still being able to establish appropriate connectivity. this has resulted in the development of an exponentially increasing number of different solutions and services based on lora, be they dedicated to the long-term monitoring of distributed plants and infrastructures or to human-centred applications such as safety-oriented sensor systems for use in the workplace. in dense lora networks, predicting the number of supported nodes in relation to their position and the propagation environment is essential for ensuring reliable and stable communication and minimising costs. in this paper, after comparing different path loss models based on a field measurement campaign for lora received signal strength indicator values within a university campus, two main modifications of the lora simulator tool were implemented. these were aimed at improving the accuracy of the prediction of the number of sustainable nodes in relation to the target data extraction rate set. the simulations based on field measurements demonstrated that through an improved path loss evaluation and the use of three gateways, the number of nodes could be increased theoretically from around 100 to around 6,000. mailto:s.spinsante@staff.univpm.it acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 22 cities, the use of lora provides the advantage of allowing for the deployment of a large quantity of sensor nodes while keeping the structure of the acquisition network relatively simple and flexible. a lora-based smart bin architecture for waste management within the smart city context is presented in [7], where an ultrasonic sensor is used to check the trash level inside the bin. using a single-channel lora network, the authors demonstrated how a system composed of five sensor nodes was deployed in the historical centre of the city of florence, italy. another example, where the problem of covering long distances among lora nodes and gws in sensor networks deployed to monitor gas pipelines was addressed, was recently presented in [8]. here, in order to overcome the limitations of transmission coverage, the authors suggest a multi-hop linear topology supported by an appropriately adapted synchronisation protocol. other examples of distributed plants and systems monitored by means of a lorabased solution are provided in [9] and [10]. at the same time, lora has been adopted to support sensor network solutions aimed at human monitoring, as presented in [11], where a wearable system for noise assessment in the workplace is proposed, which makes it possible to notify a subject of a potentially dangerous exposure to high noise levels for a prolonged period of time. in [12], the capability of low-cost lora transceivers to schedule the transmission of frames with a standard uncertainty of less than 3 μs and an acceptable longterm clock stability for applications such as industry processing was demonstrated. lora is also being used and evaluated in terms of mobility conditions, as presented in [13]. lora is based on the chirp spread spectrum modulation technique, with sinusoidal signals with a frequency that changes over the transmission time (chirp signals). this provides inherent robustness to interference, allowing the coexistence of several active nodes within the same frequency channel, with the communication robust even in the presence of high noise levels [14]. when the sensor network to be deployed needs to include a huge number of tiny devices, as could be the case in industrial or construction process monitoring, the possibility of accurately predicting the attainable data extraction rate (der) prior to setting up the real network can be critical. the der is the key criterion used to evaluate the scalability of a network and is defined as the ratio of received messages to transmitted messages over a given period of time. the der depends on the position, number and behaviour of the nodes and sinks, and takes values of between 0 and 1, where, in a perfect deployment, der = 1. once estimated, the predicted der value can be checked against the specific requirements, and, if the check fails, the network design can be modified (e.g. by adopting a specific topology, as in [15]) before carrying out the final deployment, thus avoiding additional costs and delays associated with repeated test installations and failures. in this paper, with reference to [16] and the lorasim simulation tool presented therein, we implement various modifications based on real measurements, with the aim of improving the capability of the tool to predict the scalability performance of lora networks. such modifications will enable a more realistic modelling of the propagation losses and the positioning of the nodes, as well as a more accurate determination of the number of nodes that allow acceptably reliable and stable communications and the network configuration that best adapts to different propagation scenarios. the remainder of the paper is organised as follows. in section 2, the original lorasim tool and the applied changes are briefly introduced before section 3 describes the experimental tests performed and the results obtained, both by comparing the theoretical models to the field measurements and implementing different path loss models within the lorasim tool. finally, section 4 concludes the paper. 2. the lora simulator and applied modifications lorasim, a discrete-event simulator based on simpy, was implemented according to the technical specification of the sx1272 lora module provided by semtech [18]. the software operations also apply to later modules, such as the recent sx1276 that features additional frequency bands of operation (169 mhz and 433 mhz, in addition to the 850 mhz–1 ghz already supported by the sx1272), more options for the programmable bandwidth, and a better receiver sensitivity (down to −148 dbm vs. −137 dbm of the sx1272 module). the software tool can be used to model the communications between randomly placed nodes and gws, the number of which can be selected by the user. one of the tool's main features is the possibility of choosing different configurations for the lora module through a combination of settings, such as transmission power (tp), carrier frequency (cf), spreading factor (sf), bandwidth (bw) and coding rate (cr). for each data transmission occurring over a node-to-gw link, the lorasim estimates the path loss using an embedded model and simulates collisions among the data packets in terms of signal power (through the so-called ‘capture effect’), carrier frequency, spreading factor and received power. in order to obtain a more realistic evaluation of network scalability, first, we implemented additional path loss models to allow lorasim to adapt to different environmental scenarios. then, a function enabling the insertion of nodes and gw positions from a file was added to analyse the der performance changes in relation to the default random node positioning assumption. 2.1. path loss models a path loss model is an equation describing the decrease in signal power density due to its propagation in different environments. we focused on three main models, which reasonably apply to the scenarios noted in section 1 and are valid in the frequency range used by lora. 1) log-distance model. according to [16], the log-distance path loss model refers to built-up and densely populated areas, expressing the signal attenuation as a function of the distance 𝑑 between node and gw (given in km): 𝑃𝑙(𝑑) = 𝑃�̅�(𝑑0) + 10 𝛾 log ( 𝑑 𝑑0 ) + 𝑋𝜎 , (1) where 𝑃𝑙(𝑑) is the path loss in db, 𝑃�̅�(𝑑0) is the mean path loss at the reference distance 𝑑0, 𝛾 is the path loss exponent and 𝑋𝜎 ≅ 𝒩(0, 𝜎 2) is the normal distribution with zero mean and 𝜎 2 variance, to account for any shadowing. 2) okumura–hata model. the main enhancement introduced by the okumura–hata model [11] is the dependence of the path loss on the carrier frequency that characterises the transmission and on the terminal (node and gw) height. as such, we can model a wider range of node distribution in more detail while accounting for the height of the antennas in four different scenarios: acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 23 𝑃𝑙(𝑑) = 𝐴 + 𝐵 log(𝑑) + 𝐶 , (2) where a, b and c are frequencyand antenna-height-dependent terms. factor a increases according to carrier frequency and decreases with an increase in gw and node height. in addition, the path loss exponent (proportional to b) decreases with an increase in gw height: 𝐴 = 69.55 + 26.16 log(𝑓𝑐 ) − 13.82 log(ℎ𝑏 ) − 𝑎(ℎ𝑚 ) (3) and 𝐵 = 44.9 − 13.82 log (ℎ𝑏 ) (4) where 𝑓𝑐 is the carrier frequency in mhz, 𝑑 is the distance between node and gw (in km), and ℎ𝑏 and ℎ𝑚 are the gw and node height (in m), respectively. the model is only intended for large areas, with the gw placed higher than the surrounding rooftops. the values assigned to the different terms were obtained by interpolating the results of extensive measurement campaigns carried out in propagation scenarios corresponding to the model requirements (see [19], appendix 7.a). different expressions of the term 𝐶 and the 𝑎(ℎ𝑚 ) function refer to four different scenarios or propagation environments: • small and medium size cities: 𝑎(ℎ𝑚 ) = (1.1 log(𝑓𝑐 ) − 0.7)ℎ𝑚 − 1.56 log(𝑓𝑐 ) − 0.8) 𝐶 = 0. (5) • metropolitan areas: 𝑎(ℎ𝑚 ) = { 8.29(log(1.54 ℎ𝑚 )) 2 − 1.1 𝑓𝑜𝑟 𝑓𝑐 ≤ 200 mhz 3.2(log(11.75 ℎ𝑚 )) 2 − 4.97 𝑓𝑜𝑟 𝑓𝑐 ≥ 400 mhz 𝐶 = 0. (6) • suburban environments: 𝐶 = −2 [log ( 𝑓𝑐 28 )] 2 − 5.4. (7) • rural areas: 𝐶 = −4.78[log(𝑓𝑐 )] 2 + 18.33log (𝑓𝑐 ) − 40.98. (8) the function 𝑎(ℎ𝑚 ) in suburban and rural areas is the same as with urban (small and medium-sized cities) areas. 3) 3gpp. both 3gpp and 3gpp2 [20] developed a path loss model to evaluate the performance of cellular systems. here, again, the equation considers the carrier frequency and the antenna height: 𝑃𝑙 = (44.9 − 6.55 log(ℎ𝑏 )) log(𝑑) + 45.5 + (35.46 − 1.1 ℎ𝑚 ) log(𝑓𝑐 ) − 13.82 log(ℎ𝑚 ) + 0.7 ℎ𝑚 + 𝐶 , (9) where ℎ𝑏 and ℎ𝑚 are the base station (gw) and mobile station (node) antennas’ height, respectively. unlike in the two previous models, the distance 𝑑 is expressed in metres. according to the value of the constant term 𝐶, a suburban macrocell system (𝐶 = 0 db) or an urban macrocell system (𝐶 = 3 db) is assumed. these models were implemented in lorasim, starting with the log-distance model, as the following coding shows: #log-distance model pl = pld0+10*gamma*math.log10(distance/d0) the variables used in the code have the same meaning as those in equation (1). the distance variable represents the distance between the gw and each node; considering that models 2) and 3) include the antenna height, transforming the 2d spatial representation of the log-distance model into a 3d representation, we changed the variable definition by applying the pythagorean theorem. the following coding was used to implement the okumura– hata and 3gpp models, respectively. the user can select them through a simple if…elif control, which was omitted here for convenience. #okumura–hata model #small and medium-size cities ahm=(1.1*(math.log10(self.freq)-math.log10(10**6))+ -0.7)*hm-(1.56*(math.log10(self.freq)+ -math.log10(10**6))-0.8) c = 0 #metropolitan areas if (self.freq <= 2*10**8): ahm = 8.29*((math.log10(1.54*hm))**2)-1.1 elif (self.freq >= 4*10**8): ahm = 3.2*((math.log10(11.75*hm))**2)-4.97 c = 0 #suburban environments ahm=(1.1*(math.log10(self.freq)-math.log10(10**6))+ -0.7)*hm-(1.56*(math.log10(self.freq)+ -math.log10(10**6))-0.8) c = -2*((math.log10(self.freq)+ -math.log10(2.8*10**7))**2) 5.4 #rural area figure 1. the lora node used in the experiments, which was based on an arduino board equipped with a gps receiver and a lora feather. figure 2. the 37 planned lora rssi measurement positions along eight different directions within the university campus. acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 24 ahm=(1.1*(math.log10(self.freq)-math.log10(10**6))+ -0.7)*hm-(1.56*(math.log10(self.freq)+ -math.log10(10**6))-0.8) c=-4.78*((math.log10(self.freq)+ -math.log10(10*6))**2)+ +18.33*(math.log10(self.freq)+ -math.log10(10*6))-40.98 a = 69.55+26.16*(math.log10(self.freq)+ -math.log10(10**6))-13.82*math.log(hb)-ahm b = 44.9-6.55*math.log10(hb) #3gpp model #suburban macro c = 0 #urban macro c = 3 where self.freq is the carrier frequency, and hb and hm are the base station (gw) and node height, respectively. all the equations are the same as in section 2.1. the result of the path loss computation was passed to the pl variable by the following line of code for the okumura–hata model, pl=a+b*(math.log10(distance)-math.log10(1000))+c, and by the following for the 3gpp model: pl = (44.96.55*math.log10(hb))*math.log10(distance/1000)+45.5 +(35.46-1.1*hm)*(math.log10(self.freq)+ -math.log10(10**6))-13.82*math.log10(hm)+0.7*hm+c 2.2. node placement the second relevant change applied to the lorasim tool allows for specifying the placement of the nodes. in fact, the original tool only allows for a random distribution of the nodes, according to which the gw is positioned at the centre of a circular area, with the nodes randomly placed inside it based on a simplified poisson point process (ppp) [17]. we modified the simulator to feed it with the positions of the ‘hot spots’ recorded as gps coordinates in a file. then, in order to account for any possible misplacement of the lora nodes around these hot spots (e.g. due to physical obstacles or constraints encountered during the installation), each hot spot was assumed to be the centre of a circular area in which an evenly distributed number of nodes was randomly placed, with a minimum distance between the nodes of 10 m, while their height was between 1 m and 2 m. 3. experimental results 3.1. measurement campaign before adding the different path loss models into the lorasim software tool, an extensive measurement campaign aimed at evaluating the validity of the models was carried out within our university campus. lora received signal strength indicator (rssi) measurement values (in dbm) were collected over one-hour intervals at each position and were subsequently gps-referenced using a transmission module (lora node) based on the adafruit feather m0 with a rfm95 lora radio (900 mhz) equipped with a gps receiver (itead royaltek reb4216/reb-5216 gps shield breakout board for arduino mega) and connected to a laptop, as shown in figure 1. the rssi parameter (i.e. the measured received signal strength indicator) represents a measure of the signal power on a radio link. the link can be affected by several channel conditions causing variation at the rssi level, such as the distance between the nodes, the radio transmission medium (e.g. air, water), physical obstacles, the geometrical orientation of the nodes and interference from other radio transmission equipment and reflected radio waves. the rssi is related to the path loss pl in terms of the following equation: 𝑃𝑙(𝑑) = |𝑅𝑆𝑆𝐼| + 𝑆𝑁𝑅 + 𝑃tx + 𝐺rx [db], (10) where snr is signal-to-noise ratio, 𝑃tx is the effective isotropic radiated power, and 𝐺rx is the receiver’s antenna gain. both the rssi and the snr are measured by the receiving module, while the 𝑃tx is known, and the 𝐺rx is 5 dbi for the specific antenna installed in the gw. table 1. average rssi value and variance measured at each position within the university campus, over a one-hour data collection session. position avg rssi (dbm) σ2 (dbm2) 1 −99.7 4.0 2 −94.7 3.6 3 −92.0 14.5 4 −95.6 6.0 5 −105.9 23.6 6 −107.3 19.2 7 −98.4 8.5 8 −109.1 12.6 9 −94.7 8.5 10 −88.5 2.9 11 −90.0 5.0 12 −100.9 18.4 13 −87.0 4.5 14 −110.6 12.3 15 −93.5 9.8 16 −86.8 4.4 17 −88.9 70.3 18 −101.7 13.4 19 −89.3 12.0 20 −99.5 14.1 21 −105.5 4.4 22 −106.4 7.7 23 −83.1 6.9 24 −84.0 2.4 25 −97.2 5.3 26 −97.7 7.1 27 −92.7 6.6 28 −84.8 3.5 29 −86.5 3.4 30 −101.5 62.5 31 −99.5 30.0 32 −103.0 13.9 33 −81.2 3.6 34 −105.5 5.1 35 −108.0 18.9 36 −90.4 10.8 37 −91.4 4.6 acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 25 the multi-channel lora gw located on top of the campus tower was implemented using a raspberry pi and an ic880a board, which integrates two semtech sx1257 transceivers and an sx1301 baseband processor, thus allowing for simultaneously receiving up to eight lora packets transmitted with different sf values and on different channels. the single gw was positioned on top of the highest tower within the campus (200 m a.s.l.), with the 37 different outdoor measurement positions planned, as shown in figure 2, moving along eight different directions at a distance of 25 m, 50 m, 75 m, 100 m, and 125 m from the tower where the gw was positioned, along each direction. this system collected more than 7,000 measures in total. for practical reasons, the measurements were collected during several days of good weather conditions (i.e. almost clear sky, no rain or fog). the measurement positions identified in this preliminary step, as based on the observation of the university campus plan, were adhered to as far as possible during the measurement campaign. when the identified measurement position was indoors, the nearest possible outdoor position was chosen to collect the rssi measurement values since we were only interested in checking the signal power distribution in outdoor conditions and referencing each measurement via gps coordinates. table 1 lists the average rssi values measured at each of the 37 real positions selected within the campus, together with their corresponding variance. figure 34 shows the distribution of the rssi values measured at each real position in graphical terms, encoded by coloured dot clouds of different gradations and associated with their gps coordinates. it should be noted that while the lora node remained fixed at each position during the one-hour measurement collection period, the resulting gps coordinates provided by the on-board receiver fluctuated slightly due to the gps module’s horizontal position accuracy of 2.5 m [21], [22]. the positions located in the line-of-sight (los) to the gw returned the highest rssi values (violet to red dots, from −97.4 dbm to −74 dbm) even when at a longer distance. conversely, the positions that were located near the gw but were obstructed by buildings (yellow to light green dots, from −121 dbm to −97.5 dbm) exhibited the lowest rssi values. this was further confirmed by the box plots presented in figure 4, where the measured rssi values were grouped based on the distance from the gw, irrespective of the direction along which the measurement position was located. it is clear that the median rssi (horizontal red line inside each box) did not always decrease with an increase in distance, which was due to the propagation effects resulting from the buildings and other surrounding obstacles. as noted above, in this study, the rssi measurement values were collected during several days of good weather conditions and it is important to note that bad weather conditions, specifically rain, could increase the lora packet loss rate due to the stronger attenuation of the transmitted signal, as presented in [23], where the authors reported that light rain led to an additional 20 % reduction in the packet delivery ratio. the impact of the weather factor should be taken into account in applications that require high reliability at any given time, and higher-layer protocols should be used to overcome the effects by automatically adjusting the lora configuration parameters. data related to the number of packets lost during transmission was also collected through setting the number of transmitted packets at each measurement position and then checking the number of packets received at the backend server via the gw. in fact, each lora packet carries several data fields, among which, the frame counter field allows for assigning a counter to each packet, which is incremented sequentially by the transmitter [4]. once the transmission from a specific measurement site was finished, the sequence of received packets at the server was checked to obtain the number of lost packets. figure 5 shows the resulting packet loss percentage at each measurement direction. here, direction no. 1 is the south-west direction in relation to the gw in figure 2, while direction no. 8 is the north-west direction shown in the same image, as identified in a clockwise fashion. figure 3. heat map showing the gps-referenced lora rssi measurements in the selected real field positions. figure 4. distribution of lora rssi measurements at different distances from the gw. figure 5. percentage of packet loss values along the eight different directions (see figure 2) considered during the lora field measurements at the university campus. 0% 2% 4% 6% 8% 10% 12% 14% 1 2 3 4 5 6 7 8 % p a ck e t lo ss measurement direction acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 26 as the graph shows, the percentage of lost packets was, on average, under a 7 % threshold, with the exception of direction no. 6, where the highest packet loss occurred (around 12 %). given that the measurements at the 37 different positions were carried out in a randomised manner, we can assume that, during a single week, the highest loss value along direction no. 6 was motivated by the propagation environment and not by occasional or sporadic conditions. 3.2. comparison of the path loss models in relation to the field measurements before implementing a modified version of the lorasim tool to improve the network planning performance, the results of the rssi field measurements were processed in order to check which theoretical model among those presented in section 2.1 can be considered as more reliably describing the lora signal propagation behaviour. to this end, the average rssi values at each position reported in table 1 were considered in relation to the corresponding distance 𝑑 between the measurement position and the gw while assuming a fixed lora node height of 1 m. two different polynomials were used to interpolate the available values, a thirddegree polynomial and a sixth-degree polynomial, as described by the following equations: 𝑅𝑆𝑆𝐼3(𝑑) = −5.49 ∙ 10 −63𝑑3 + 2.94 ∙ 10−32𝑑2 − 0.50𝑑 + −70.75 [dbm], (11) and 𝑅𝑆𝑆𝐼6(𝑑) = 3.69 ∙ 10 −12𝑑6 + 5.99 ∙ 10−11𝑑5 − 1.38 ∙ 10−6𝑑4 + 5.13 ∙ 10−4𝑑3 − 7.32 ∙ 10−2𝑑2 + 4.25𝑑 − 171 [dbm]. (12) both the models are shown in figure 6 together with the measured average rssi values. equation (11) allows for correctly describing 14.6 % of the measured data, while the sixth-degree model given by equation (12) allows for correctly describing 23 % of the measured data. following the generation of the interpolation models based on the measured rssi values, they were compared to the theoretical models previously introduced, as shown in figure 7. as figure 7 shows, the log-distance model generally overestimated the path loss, providing the lowest estimated rssi values in relation to the measured values. the sixth-order polynomial model was not applicable for distances shorter than 35 m, while the third-degree polynomial model appeared to better represent the measured rssi trend. both of the 3gpp path loss models provided acceptable results, with a reasonable underestimation of the rssi. finally, all the okumura–hata models overestimated the rssi, with an average excess of 60 db in relation to the measured values. based on the experimental rssi measurements, the 3gpp path loss models (urban and suburban) provided the best approximation for predicting the propagation behaviour of the lora signals within the campus. however, to ensure completeness, all the theoretical models were implemented within the lorasim tool and were used to predict the performance of the network. 3.3. simulations with the modified lorasim tool the lorasim tool considers n lora end-nodes and m gws, each of which feature specific configurations of tp, cf, sf, bw and cr parameters. together with the average rate of transmitted packets (λ) and the packet load (b), these parameters identify a network setting of sn = {tp, cf, sf, bw, cr, λ, b}. following the modifications to the lorasim tool, an initial simulation campaign was conducted aimed at evaluating how the network der was affected by the selection of the path loss model. for a better comparison with the previous studies, we used the same settings as chosen in [16]. here, assuming that n = 1,000, each lora node is able to send a packet of 20 bytes every 16.7 minutes to a single gw (m = 1), with the simulated transmission time = one hour. the signal carrier frequency was 860 mhz. a total of 100 simulation runs were executed for each path loss model. path losses and collisions determine the communication behaviour of lora nodes. in lorasim, the so-called simple model (s.m.) variant assumes an infinite communication range and the collisions occurring whenever any two transmissions overlap in time at the receiver with the same cf, sf and bw, which means both transmissions are lost. figure 6. the thirdand sixth-order polynomials interpolation models based on the measured rssi values. figure 7. comparison between the rssi values estimated through different theoretical path loss models and the interpolation models based on the field measurements. the okumura–hata model for small cities has been excluded as it reports the same values as the urban areas model. acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 27 when, during a lora transmission, a signal arrives at the receiver before the previous packet has been processed, the receiver is unable to decode one or both of them. denoting this overlap between packets x and y as o(x,y), the condition according to which the lorasim tool decides on a successful packet decoding can be expressed as: c = o(x,y) ∧ c freq ∧ c sf ∧ c pwr ∧ c time , where the different symbols represent an overlap (leading to a collision) in frequency, spreading factor, power and timing, respectively. the use of the and logic operators implies that a packet is suppressed at the receiver (i.e. lost) if, and only if, all the overlapping conditions occur. each overlap is represented by an independent random variable, which means, according to the central limit theorem, that their sum tends toward a normal distribution. the s.m. variant allows for establishing a baseline that can be analytically described. in an initial experiment, the configuration of the sn1 parameters detailed in table 2 was considered, albeit in two different simulation settings. here, the first setting, labelled sn1pl, assumes the combination of selectable parameters that enables the strongest transmission (i.e. the most robust to channel quality degradation), with the longest possible airtime of 1712.13 ms (sf = 12), the selection of different path loss models, and the collisions as defined above. meanwhile, the second setting, labelled sn1 s.m., assumes the same node configuration as the s.m. variant, but with a fixed log-distance path loss model (default model). the mean der value obtained for n = 1,000 nodes randomly placed around a single gw was then evaluated. as figure 8 shows, the s.m. variant consistently provided der values of <4 % due to the underestimation of the communication channel, which means a good packet reception is highly unlikely. meanwhile, the results for the sn1pl configuration indicated a dependence on the path loss model used. the okumura–hata model, which considers different terrains, provided the highest der in the rural scenario (as was expected), albeit that it was still very low (around 12 %). figure 9 shows the expected mean der trend with an increase in the number of nodes for different path loss models. in line with the previous experiment, the generic sn1 s.m. configuration returned the lowest probability of receiving packets, while the configuration incorporating the 3gpp model was the most favourable. when assuming a der of ≥0.8 to be an acceptable value for a realistic deployment, the log-distance model predicted that there were 64 supported nodes, while the other two models predicted around 100. in accordance with section 2.2, we then modelled a more realistic lora deployment by placing nodes in the same gpsreferenced positions used for the rssi field measurements (hot spots). given the poor der results obtained from the simulations discussed above, and in order to increase the number of nodes supported by the network, we assumed a scenario that included three gws, one located on top of the highest tower (as with the measurement campaign) and two additional gws, both positioned at a height 150 m a.s.l. the new scenario is shown in figure 10. we first tested the sn1 configuration by comparing the der results provided by the random positioning of the nodes (default option in lorasim) and the gps-based positioning in both the single-gw network and the three-gw network. from figure 11, it is clear that with the gps-based node placing method and the single-gw network, a 34.6 % increase in mean der (0.11– 0.148) was obtainable. the network performances were further improved using the three-gw network. as such, we can obtain a 91.94 % increase in the der using the gps-3gw network compared to when using a single gateway (gps-1gw), and a table 2. configuration settings. parameter sn1 sn5 transmission power (dbm) 14 14 carrier frequency (mhz) 860 860 spreading factor 12 best of 7-12 bandwidth (khz) 125 best of 125/250/500 coding rate 4/8 4/5 figure 8. comparison of the der results obtained by setting different path loss models in the lorasim tool for the two configurations, sn1 s.m. and snpl1, for n = 1,000. figure 9. comparison of mean der results obtained by setting different path loss models, for an increasing number of nodes. figure 10. the simulated scenario with three gws located in the campus area: hot spots (yellow markers) and gws (white circled red markers) placement. acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 28 global 158.4 % increase compared to when using the random placement method (random-1gw), thus increasing the mean der value from 0.11 to 0.285. the use of the gps-based location of the nodes improved the simulated performance as it allowed for fully exploiting the results of the rssi field measurement campaign, with a realistic distribution of the received signal power described by the propagation model used. the last simulated configuration, labelled sn5 in table 2, relies on dynamic parameters and allows for minimising both the airtime and the tp. as shown in table 3, this is the best option for ensuring a high reception rate, since the mean der ranges from 0.97 to 0.99. according to the simulations, a der of ≥0.8 is obtainable with up to 6,000 nodes deployed. 4. conclusion in this paper, we presented two effective modifications to the lorasim network simulator aimed at improving the accuracy of the software tool in predicting the number of nodes that can be sustained by a dense lora network, given a target mean der. the applied modifications consisted of introducing the possibility of testing different path loss models, previously verified through an extensive field measurement campaign for lora rssi values and through providing a gps-based positioning of the nodes rather than the default random positioning available in the simulator. the simple default model configuration tended to underestimate the link quality, while the 3gpp path loss models were found to be the most favourable, allowing for placing around 100 nodes in a metropolitan area as opposed to the 64 nodes supported by the default log-distance model. meanwhile, the okumura–hata model introduced an 83 % increase in der compared to the log-distance model in rural scenarios. the gpsbased location of the nodes allowed for simulating a more realistic network deployment that optimised the estimation of the mean der. as such, irrespective of which path loss model was applied, a 34.6 % increase in the mean der was obtained. the study relied on measurement results obtained outdoors during several days of good weather conditions; however, it is known that rain, fog and other phenomena may impact the received signal levels and the packer delivery rate of the lora link. even if the huge link budget margin ensured by the lora technology makes it less sensitive to weather conditions and signal attenuation than other wireless communication systems, it is important to not underestimate the weather-related effects, especially if the target application requires high reliability in packet transmission at any given time. to address this issue, it is crucial to remain aware that, in addition to the propagation environment, the physical-level configurable settings of lora also determine different trade-offs among range, consumption and data rate. future activities could involve a more thorough analysis of the weather-related effects on the capacity of a lora network, the deployment of an adequate number of nodes to measure the simulator performance, and the introduction of additional capabilities, such as accounting for the imperfect orthogonality among the node transmissions and any additional attenuation due to poor weather conditions. acknowledgement s. spinsante acknowledges the support from the italian national technology cluster project shell (shared interoperable home ecosystems for green, comfortable and safe living), project code: ctn01 00128 111357. references [1] c. liu, s. yan, t. lai, the establishment of an integrated automation production line with multiple monitoring and control loops, ieee international conference on applied system invention (icasi), chiba, japan, 13 – 17 april 2018, pp. 968-971. doi: https://doi.org/10.1109/icasi.2018.8394432 [2] r. laskier, modernizing the mining industry with the internet of things, in: internet of things and data analytics handbook. h. geng (editor). wiley, 2017. isbn: 978-1-119-17364-9 doi: https://doi.org/10.1002/9781119173601.ch32 [3] g. margelis, r. piechocki, d. kaleshi, p. thomas, low throughput networks for the iot: lessons learned from industrial implementations, ieee 2nd wf-iot, milaan, italy, 14-16 december 2015, pp. 181-186. doi: https://doi.org/10.1109/wf-iot.2015.7389049 [4] lora alliance, lora alliance website, 2019. online [accessed 28 october 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okum. small city, metropolitan 0.03 0.31 0.99 okum. suburban 0.04 0.28 0.99 okum rural 0.03 0.29 0.98 3gpp urban, suburban 0.03 0.29 0.99 https://doi.org/10.1109/icasi.2018.8394432 https://doi.org/10.1002/9781119173601.ch32 https://doi.org/10.1109/wf-iot.2015.7389049 https://lora-alliance.org/about-lorawan https://doi.org/10.21014/acta_imeko.v8i2.642 https://doi.org/10.1109/metroi4.2018.8428325 acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 29 ieee sensors applications symposium (sas), sophia antipolis, france, 11 – 13 march 2019, pp. 1-6. doi: https://doi.org/10.1109/sas.2019.8705980 [8] a. abrardo, a. fort, e. landi, m. mugnaini, e. panzardi, a. pozzebon, black powder flow monitoring in pipelines by means of multi-hop lora networks, ii workshop on metrology for industry 4.0 and iot (metroind4.0&iot), naples, italy, 4 – 6 june 2019, pp. 312-316. doi: https://doi.org/10.1109/metroi4.2019.8792890 [9] e. l. d. medeiros, c. a. d. s. filho, f. b. s. d. carvalho, r. m. s. 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https://doi.org/10.1109/metroi4.2019.8792890 https://doi.org/10.1109/i2mtc.2019.8827080 https://doi.org/10.1109/i2mtc.2019.8826885 https://doi.org/10.1109/i2mtc.2019.8826907 https://doi.org/10.1109/tim.2017.2746378 https://doi.org/10.1109/iwmn.2019.8805037 https://doi.org/10.1109/rtsi.2017.8065952 https://doi.org/10.1109/tim.2018.2814082 https://www.lancaster.ac.uk/scc/sites/lora/lorasim.html https://www.etsi.org/committee/1418-3gpp https://doi.org/10.1109/lwc.2016.2647247 https://www.u-blox.com/sites/default/files/products/documents/neo-6_datasheet_%28gps.g6-hw-09005%29.pdf https://www.u-blox.com/sites/default/files/products/documents/neo-6_datasheet_%28gps.g6-hw-09005%29.pdf https://www.u-blox.com/sites/default/files/products/documents/neo-6_datasheet_%28gps.g6-hw-09005%29.pdf https://doi.org/10.1109/vtcfall.2017.8288154 acta imeko, title acta imeko issn: 2221-870x september 2017, volume 6, number 3, 76-81 acta imeko | www.imeko.org september 2017 | volume 6 | number 3 | 76 ion microbeam analysis in cultural heritage: application to lapis lazuli and ancient coins alessandro lo giudice1,2, alessandro re1,2, debora angelici2,3, jacopo corsi1,2, gianluca gariani1, marco zangirolami1, emma ziraldo1 1dipartimento di fisica, università di torino, via pietro giuria 1, 10125, torino, italy 2infn sezione di torino, via pietro giuria 1, 10125, torino, italy 3tecnart s.r.l., via pietro giuria 1, 10125, torino, italy section: research paper keywords: ion beam analysis; microbeam; trace elements; ancient coins; lapis lazuli citation: alessandro lo giudice, alessandro re, debora angelici, jacopo corsi, gianluca gariani, marco zangirolami, emma ziraldo, ion microbeam analysis in cultural heritage: application to lapis lazuli and ancient coins, acta imeko, vol. 6, no. 3, article 12, september 2017, identifier: imeko-acta-06 (2017)-03-12 section editor: sabrina grassini, politecnico di torino, italy received march 15, 2017; in final form august 3, 2017; published september 2017 copyright: © 2017 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: this work was supported by university of torino and infn, italy corresponding author: alessandro lo giudice, e-mail: alessandro.logiudice@unito.it 1. introduction ion beam analyses (iba) are performed by using a proton beam provided by small accelerators as probe and collecting the products of the interaction between the particles and the atoms of the sample [1]. depending on the applications and techniques, a broad beam (a few mm2) or a micro-beam (2-20 µm in diameter) could be used as probe; a scanning system permits to map the properties of the material under investigation in an area of about 2×2 mm2 when micro-beam is employed. the energy of protons useful for iba techniques is around 2-3 mev whereas the current is of the order of few hundreds of pa, low enough to avoid any damage in the majority of the materials. iba techniques can be carried out both in vacuum, preparing the sample as in electron microscopy, and in atmosphere, allowing the non-invasive analysis of objects such as artworks of practically any shape and dimension without sample preparation. among the most used iba techniques rbs (rutherford back-scattering), pixe (proton induced x-ray emission), pige (proton induced gamma-ray emission), erda (elastic recoil detection analysis) and nra (nuclear reaction analysis) [1], [2] could be used in many scientific fields for elemental analysis, il/ibil (ionoluminescence or ion beam induced luminescence) [3], [4] for characterization of optical properties of materials and ibic (ion beam induced charge) [5] for the characterization of electronic properties of devices. in the cultural heritage field many of these techniques are used, though pixe has a particular importance for the elemental composition analysis (especially for trace elements). in this paper an application of micro-pixe and il (both micro-il and broad beam) to solve outstanding archaeological issues is described. in particular two case studies are reported. the first is related to ancient silver-copper alloy coins from roman and pre-roman italian cultures. the coins were studied abstract ion beam analyses (iba) techniques, for example pixe (particle induced x-ray emission) and il (ionoluminescence), are a powerful analytical tool used to investigate the composition and structure of materials in cultural heritage. these techniques could be applied both in vacuum, preparing the sample as in electron microscopy, and in air in a non-invasive way allowing to analyse artworks of practically any shape and dimension without sample preparation. moreover the use of a focused beam (microbeam) permits to reach an analysis resolution of few micrometers in vacuum and ten micrometers in air. in this work, instruments and methodologies are described and two examples of case study are reported: i) the mapping of elemental distribution in ancient roman coins; ii) the trace elements measurement in lapis lazuli for provenance determination. acta imeko | www.imeko.org september 2017 | volume 6 | number 3 | 77 by means of micro-pixe to investigate their composition in function of the depth to obtain information on manufacturing and degradation processes. the second case study is related to provenance study of lapis lazuli, that is a blue semi-precious stone that has been used since neolithic era (vii millennium bc) for the manufacturing of precious carved artefacts. the aim was to identify the extraction sites of the rock obtaining information on ancient trade routes. 2. instruments and methods ion beam analyses were carried out mainly by means of two scanning proton micro-beam lines at infn (national institute for nuclear physics) facilities having different acceleration voltages: the 2.5 mv an2000 accelerator at the legnaro national laboratory (infn-lnl) in padua (figure 1) widely used for material characterization/modification and ion implantation [6]-[8] and the 3 mv accelerator of the infnlabec laboratory in florence (figure 2) widely used for multi-disciplinary studies [9]-[11]. in few cases also the in air aglae microbeam facility of the c2rmf in paris was used [12]. analyses at the micro-beam line in legnaro are performed in a vacuum chamber. therefore, the samples need to be prepared in a way similar to scanning electron microscopy (sem), i.e. a conductive carbon coating is applied (figure 1). pixe measurements were carried out both on coins and on lapis lazuli rocks using 2 mev protons and a current below 500 pa. the beam was below 5 µm in diameter with a 3×3 mm2 maximum scanning area. in florence, the measurements were carried out in air (heflux) using a 3 mev proton beam and a current below 200 pa. the beam was of about 10 µm in diameter with a 2×2 mm2 maximum scanning area. only lapis lazuli were analysed both by means of pixe and il. in the case of il also the broad beam ion microscopy [13] was employed. working in air the samples don’t need any preparation and rocks or artworks of almost any shape and dimension can be analysed in a noninvasive way. both pixe data acquired in legnaro and in florence were quantitatively analysed by means of gupixwin (version 2.1.3) and by the use of a set of reference standards for the determination of the experimental parameters. 3. case studies 3.1. ancient coins a study to analytically characterise ancient coins has been started in 2010. this topic is particularly interesting for numismatic and archaeometry, because it allows to evaluate commercial exchanges among tribes, to hypothesize relative chronologies, to verify the presence of a debasement along the years and to investigate the exchange ratios among different emissions of coins [14]. until now the study has been focused on coins made of a silver-copper alloy in the roman (figure 3) and pre-roman period. the main activity was carried out using neutron diffraction, neutron activation analysis and p-xrf, that are non-invasive techniques useful to measure the elements content [15]-[17]. the coins made in this material are often characterised by a surface layer with an ag content higher than the bulk. to better study this surface silver-enriched layer, corrosion layers or inhomogeneity between surface and bulk, some silver coins have been chosen and prepared using a standard metallographic procedure, to analyse directly their section. at first, they have been cut with a rotating diamond wheel and their sections have figure 2. in air micro-beam line at infn-labec laboratory during the analysis of a lapis lazuli artefact. figure 1. in vacuum analysis chamber at infn-lnl laboratory and a coin sample cut, embedded in resin and covered with a carbon coating. figure 3. obverse and reverse images of one of the investigated victoriatus jc1 (rrc: 166) acta imeko | www.imeko.org september 2017 | volume 6 | number 3 | 78 been mounted in resin. subsequently, surfaces were polished with papers having different grit and also with a diamond crystal paste (with grains 6-3-1 µm). a carbon coating has then been applied to make the surfaces conductive. µ-pixe measurements were carried out in vacuum on these crosssections at the an2000 microbeam facility of infn-lnl. µ-pixe elemental maps allow to analyse the distribution of major elements in few minutes, as shown for example in figure 4. thanks to this feature, the presence of a very thick surface silver-enriched layer (up to 250 μm) has been confirmed, for example, on a specific roman coin emission: the victoriati (4 samples analysed). all these coins are characterised by a silver content clearly higher in the surface layer and by peculiarities, such as for example the presence of voids, that suggest a probable intentional depletion occurred with acid chemicals before minting operations, as reported in other works [18]-[20]. this is a different process in respect of the production of suberatus coins where a copper core is wrapped with a silver foil. anyway, the added value of µ-pixe is the possibility to detect minor and trace elements inside the sample with a resolution of few micrometers. similar techniques that make use of electron and x-ray beams show for some aspects lower performances than μ-pixe. using sem-eds only cu and ag elements are detected due to the low sensitivity to trace elements, whereas µ-xrf has a beam diameter greater than 10 μm and then a lower spatial resolution than μ-pixe. moreover, although not used in this work, using proton beam, it is possible to carry out rbs analysis at the same time, which in some cases is capable of resolve the distribution of chemical elements in depth [21]. in figure 5 typical pixe spectra of the external and internal regions of the coin are shown. the distribution of the elements along the section was measured, analysing small areas of about 40×40 µm2, recreating a profile of all the detected elements along the coin section, as shown e.g. in figure 6b for silver and copper in the same victoriatus figure 6. µ-pixe results of a profile of the same victoriatus shown in figure 3 and figure 5; a) optical microscopy section showing the analysed areas (each yellow square is about 40×40 µm2); b) concentration profiles of the main elements (ag and cu); c) ratio of peak areas au/ag and au/cu; d) ratio of peak areas cl/ag and cl/cu. figure 4. optical microscopy section of a victoriatus compared to µ-pixe maps (area ~ 1.5 × 1.5 mm2). figure 5. example of pixe spectra (in linear and in logarithmic scale) of internal and external regions of the victoriatus shown in figure 3 and figure 4. acta imeko | www.imeko.org september 2017 | volume 6 | number 3 | 79 figure 3. the ratio minor element / main element (using the area of one of the main characteristic peaks) has been used to evaluate the correlation of each minor and trace element with the main ones (silver and copper). some examples of the results are shown in figure 6c and figure 6d. in this way it is possible to understand where each element comes from: if the ratio on the main element is constant along the profile, one can suppose that the trace/minor element was present in the related raw material (for example gold in silver, figure 6c). if both the ratios (on cu and on ag) vary along the profile, the element is not correlated to the raw material (for example chlorine in figure 6d) but it could be entered in the coin during its burial time or during chemical treatments. in this way important information was obtained on the analysed coins about the raw material used to mint them. in particular bromine, silicon, chlorine and iron were mainly detected close to the surface but not necessarily in strict correlation with the silver-enriched layer. this means that these elements, commonly present in soil and water, probably entered in the coin during its burial time and are not related to copper or silver ores. for example, in figure 6d, it can be observed that in one side of the coin (right) chlorine is present only in two of the six areas related to the silver-enriched layer, whereas it is present in all the other side (left). on the other hand, as expected, gold and lead were clearly linked to silver in all the coins we analysed but not to copper for which a correlation with nickel and zinc was observed. in table 1 the values measured for two of the four victoriati studied are shown; in this case zn was under lod (limit of detection). increasing the number of analysed samples, ratios can be used as a semi-quantitative way for provenance study of the raw material or as distinctive elements to identify production places and mints. in our case, for example, pb/ag was observed to be up to 0.3 (one order of magnitude higher than observed in the two victoriati of table 1) in coins from other regions. 3.2. lapis lazuli lapis lazuli is a blue semi-precious stone that has been used since neolithic era (vii millennium bc) for the manufacturing of precious carved artefacts [22], [23]. the possibility to associate the raw material to man-made objects could help historians and archaeologists to reconstruct trade routes, especially for the ancient time when written testimonies are scanty or absent at all. to solve the issue we started in 2008 a long-term research, involving an interdisciplinary team. due to the low probability of geological conditions in which it can be formed, only few sources of lapis lazuli exist in the world [24]. until now, 45 lapis lazuli rocks from 4 quarry districts (afghanistan, tajikistan, siberia and chile) have been analysed, creating a database on this rock. the research has been focused in searching markers on single mineral phases using microscopic techniques, which allow to observe and to analyse single crystals in a non-invasive way. on the basis of the markers found, a protocol was established [25] in which the luminescence properties and the quantification of trace elements in pyrite and diopside crystals play a crucial role in the provenance determination of lapis lazuli. the protocol was successfully applied on historical and archaeological objects from medici’s collection (16th-18th century) and from ancient egypt (first millennium bc) [25], [26]. it is not the aim of this paper to report all the markers of the protocol, but as an example diopside crystals of the afghanistan rocks show higher quantity of titanium (> 710 ppm) or vanadium (>210 ppm) or chromium (>220 ppm) than tajik samples or a strong ionoluminescence band at about 770 nm. otherwise only siberian samples have a strontium content in diopside crystals higher than about 180 ppm. in particular it was verified that all the markers found (presence of a mineralogical phase, peculiar trace elements inside pyrite or diopside crystals, luminescence bands) are simultaneously detectable by means of ion beam analysis techniques, in particular µ-pixe [18] and il. moreover, iba techniques are very suitable techniques because if applied in air are non-invasive methods usable on precious archaeological objects, contrarily to other techniques used in provenance studies that need a sampling and are destructive [28], [29]. also µ-xrf have produced good results [30], but limited to elemental analysis because x-ray luminescence (xrl) is normally not present in benchtop instruments. moreover xrf allows to analyse only relatively large crystals (more than about 100-150 µm in diameter, not always available in archaeological objects) due to the beam dimension and to the higher depth of analysis with respect to iba techniques that are able to analyse crystals down to 20-50 µm in diameter. for example, in figure 7 the pixe map of a portion (about 125×125 µm2) of lapis lazuli is shown in which a pyrite crystal (fes2) is visible, that is characterized by the presence of selenium and the absence of copper (under the limit of detection). a content of selenium in pyrite crystals higher than 40 ppm is a marker for a chilean provenance of lapis lazuli [27]. such result cannot be obtained neither by means of µ-xrf due to the small dimension of the crystal, nor by means of sem-eds because the nickel and selenium contents are too low to be detected. table 2 shows another typical result in element quantification, obtained by means of sem-eds and µ-pixe on the diopside crystal shown in figure 8. all the trace elements, that are the only usable for provenance table 1. ratio of minor/trace elements with main elements for two victoriati coins. victoriatus 1 is the same of figures 3-6. for these coins zn was under lod (limit of detection). elements ratio victoriatus 1 (jc1) victoriatus 2 (jc13) au/ag (37 ± 9) × 10-3 (25 ± 3) × 10-3 pb/ag (23 ± 3) × 10-3 (29 ± 5) × 10-3 ni/cu (2.6 ± 0.3) × 10-3 (1.8 ± 0.3) × 10-3 zn/cu under lod under lod figure 7. µ-pixe map of some elements in a lapis lazuli from chile (sample chile 16980), µ-pixe spectrum of a pyrite crystal and elements content. acta imeko | www.imeko.org september 2017 | volume 6 | number 3 | 80 determination, are measurable only by means of µ-pixe analysis. in this sample, the features of an afghan provenance of the rock are evident, i.e. the high content in titanium and vanadium and the low quantity of strontium [26]. recently, to increase the markers, the trace element analysis by means of µ-pixe was extended to other mineralogical phases in addition to diopside and pyrite, in particular to calcite and feldspar that are easily recognizable in a lapis lazuli by means of their ionoluminescence properties. in fact, as shown in figure 8, calcite shows a strong red luminescence at about 625 nm whereas feldspar has a characteristic blue luminescence centered at about 450 nm 4. conclusions thanks to the use of iba techniques it is possible to measure the elemental composition and luminescence properties of materials in cultural heritage with a micrometric resolution and, if necessary, in a non-invasive way using in-air facilities. this is very useful to answer many of the questions posed by archaeologists, art historians and restorers. similar techniques that make use of electron and x-ray beams show for some aspects lower performances than iba and in many cases are not able to solve adequately the issues. in this paper it was shown how the use of iba techniques has been suitable to contribute in solving two archaeological issues. μ-pixe analysis carried out on ancient roman and preroman coins was useful to know their composition in function of the depth. in particular the profile of the silver-enriched layer has been reconstructed inside the coin obtaining information on manufacturing and degradation processes. moreover trace elements have been associated to the contaminants of the soil (br, si, cl, fe) or to the raw minerals (pb, au in silver; ni, zn in copper). finally the ratios pb/ag, au/ag, ni/cu and zn/cu have been measured and they will be used in future works to identify production places and mints or provenance of raw materials. about lapis lazuli a protocol to identify the provenance of the stone used for carved artefacts has been carried out. it is based on the measurement of trace elements and luminescence properties of minerals inside lapis lazuli at a micrometric scale. currently the protocol is completely applicable only by means of iba techniques, in particular μ-pixe an il analysis, also on precious objects. it has been successfully applied on artworks and in particular on precious carved objects from ancient egypt (first millennium bc). at now all the analysed objects are ascribable to afghanistan (badakhshan province) that is a further confirmation of trading of this stone up to 4000 km from its extraction site in the first millennium bc. acknowledgement the authors wish to acknowledge silvia calusi, nicla gelli, lorenzo giuntini, mirko massi, francesco taccetti, as the experts of the external scanning microbeam line at infnlabec laboratory; leonardo la torre, valentino rigato as the experts of the in vacuum microbeam line at infn-lnl laboratory; thomas calligaro, claire pacheco, quentin lemasson, laurent pichon, brice moignard as the experts of the external scanning microbeam line at cnrs-aglae laboratory; giovanni pratesi, maria cristina guidotti for having made available the lapis lazuli samples and objects. the authors wish to warmly thank chnet, the infn network of laboratories working in the cultural heritage field, for supporting this research, both in terms of instrumentations, competencies and grants. moreover, the infn experiment chnet-imaging is acknowledged with gratitude. references [1] m.b.h. breese, d.j. jamieson, p.j.c. king, “materials analysis using a nuclear microprobe”, john wiley & sons, new york, (1996) [2] s.a.e. johansson, j.l. campbell, k.g. malmqvist, particle induced 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heritage: application to lapis lazuli and ancient coins establishment of torque realisation up to 5 kn·m with a new design of the torque standard machine acta imeko issn: 2221-870x september 2019, volume 8, number 3, 30 – 35 acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 30 establishment of torque realisation up to 5 kn·m with a new design of the torque standard machine nittaya arksonnarong1, nattapon saenkhum1, pramann chantaraksa1, tassanai sanponpute1 1 national institute of metrology (thailand), pathumthani, thailand section: research paper keywords: torque standard machine; sensitivity of fulcrum; elastic hinge citation: nittaya arksonnarong, nattapon saenkhum, pramann chantaraksa, tassanai sanponpute, establishment of torque realisation up to 5 kn·m with a new design of the torque standard machine, acta imeko, vol. 8, no. 3, article 6, september 2019, identifier: imeko-acta-08 (2019)-03-06 editor: jeerasak pitakarnnop, nimt, thailand received: august 31, 2018; in final form june 10, 2019; published september 2019 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: nittaya arksonnarong, e-mail: nittaya@nimt.or.th 1. introduction in general, the fulcrum of the torque standard machine (tsm) is one of the most important parts of the machine. national metrology institutes (nmis) usually provide torque standard machines by using air bearing [1]-[5] or using flexure bearing as a fulcrum [6]-[7]. an air bearing is best suited for the fulcrum because it is a good method of minimising friction [8][10]. however, it is expensive, needs continuous maintenance, and requires the operator to have strong experience in its operation. even though the flexure bearing has not been confirmed as being as accurate as air bearing [11], there are obvious advantages, such as its lower cost, easier operation, and lower maintenance requirements. it has been developed continuously so as to enhance the bearing sensitivity. recently, a tsm with a suspended fulcrum was developed in the range of 0.1 n∙m to 10 n∙m [12], as shown in figure 1. a bilateral comparison between the 10-n·m-dwtsm of the national metrology institute of japan (nmij) and the 10-n·m-dwtsm of the thai national institute of metrology (nimt) in the calibration range of 0.1 n·m to 1.0 n·m were conducted to confirm nimt’s capability [13]. however, the length of long thin metal sheet that used as the suspended fulcrum mainly affected the position of the fulcrum and the lever arm stability. thus, the authors of this article are interested in designing and developing a 5 kn∙m tsm with the flexure bearing as a main part thereof. it was designed on the principle of an eightfold elastic hinge, which was found to be more rigid and stable than the suspended one. the machine capability is confirmed by two informal comparisons. abstract a torque standard machine (tsm) with a rated capacity of 5 kn∙m was designed and constructed by the torque laboratory, national institute of metrology (thailand), nimt. the machine had initially used a flexure bearing as a fulcrum. it had been developed based on the research of a 10 n·m suspended fulcrum tsm. however, the bearing structure was changed to a combination of eight elastic hinges in order to withstand larger cross-forces for providing greater strength and providing a shorter stabilising time, consuming the lever arm’s swing. with a three-column weightlifting system, the machine provides five measuring ranges ranging from 100 n·m to 5,000 n·m in the same set of stacked weights. the measurement results showed the sensitivity of the fulcrum within ± 0.005 n·m from 10 % to 100 % of the measurement range. the sensitivity of the fulcrum is one of the main sources of the uncertainty evaluation of the torque measurement. the calibration and measurement capabilities (cmcs) of the torque measurement were 0.01 % (k=2) in the measurement range from 500 n·m to 5,000 n·m. to confirm the capability of the measurement, an informal comparison with physikalisch-technische bundesanstalt (ptb) was conducted. the results were satisfactory, with the |en| less than 1. mailto:nittaya@nimt.or.th acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 31 2. conceptual design the 5 knm tsm (model: pramann 02 [named to honour the machine’s constructor] s/n: 02), is shown in figure 2. the main parts are the flexure bearing, lever arm, two sets of stacked weights, two motor gears, and magnetic damping. the machine can have the torque transducer installed on both sides of the lever arm because it is equipped with two sets of motor gears at either side. the advantage of this flexure bearing is that it can be used as a part of a torque calibration machine. there are five measurement ranges, ranging from 100 nm to 5 knm, in a selectable mode. the swing of the stacked weights was restrained by magnetic damping. 2.1. flexure bearing the fulcrum is one of the main components of the tsm. this machine was designed by the flexure bearing used for the lever fulcrum. its structure is based on the principle of an eightfold elastic hinge, as shown in figure 3. it consists of eight pieces of elastic hinge, which are independently and simultaneously moveable when applying the force on the lever arm. the design was developed based on previous research on the existing 10 nm suspended-fulcrum tsm in order to reduce the swing, endure the cross-force effect, and give the lever arm greater strength. 2.2. lever arm the design of the lever arm of the tsm is shown in figure 4. the nominal length of the full lever arm was 2 m. the hollow shape was designed to reduce the weight of the lever arm. the strength of the lever arm was confirmed by using finite element analysis before construction. the lever arm and stacked weight were bound to each other by a thin metal plate, figure 1. suspended-fulcrum torque standard machine of nimt (10-n·mdwtsm). (a) (b) figure 2. the schematic diagram (a) and photograph (b) of 5 knm tsm. figure 3. schematic diagram of flexure bearing with eight elastic hinges (four pieces, highlighted in red, and four pieces highlighted in pink). figure 4. a schematic diagram of the lever arm. 1st red 2nd 3 rd 4th 5th 8th pink 6th 7th acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 32 which had the following dimensions: 300 mm × 400 mm × 0.05 mm. 2.3. stacked weights the machine consists of two sets of stacked weight (for clockwise and counter-clockwise directions) with a threecolumn weightlifting system. each set of stacked weight comprises 50 pieces of 100 n weight disks, as shown in figure 5. it can be operated in five measurement ranges from 100 nm to 5 knm in the same stacked weight. the weights were automatically loaded according to the calibration sequence. firstly, the lifting system was inserted under the selected stacked weight and then the motor drove the column upward, lifting the stacked weight. the lever arm was free, without torque applied. thereafter, the column was driven downward, and the lifting system was removed. in this step, the stacked weight was loaded onto the lever arm. 2.4. operational system a torque measuring device was connected to the machine, using flexible coupling to avoid misalignment. the machine applied force to the torque measuring device using the motor gears, which was controlled by the labview program. the calibration was performed according to din 51309: 2005-12. magnetic damping was used to stabilise the lever arm when applying the force. the automatic weight loading was controlled by computer using the labview program, as shown in figure 6. 3. measurement results 3.1. the uncertainty of the torque standard machine the 5 knm tsm is directly traceable to the si units of mass, length, acceleration, and density by weighing the calibrated mass disks onto a frictionless supported lever. the lever support and the force coupling were based on the principle of flexure bearing. the machine was designed for the calibration of a torque measuring device. the mathematical model for the generated torque is as follows: 𝑀𝑠𝑡𝑑 = 𝑚𝑐 ⋅ 𝑔𝑙𝑜𝑐𝑎𝑙 ⋅ (1 − 𝜌𝑎 𝜌𝑚 ) ⋅ 𝑙 + 𝑀𝑟𝑒𝑠 (1) where: stdm is the torque standard in newton metres, nm cm is the conventional mass in kilograms, kg localg is the local gravitational acceleration in metres per second squared, m/s2 a is the density of moist air in kilograms per cubic metre, kg/m3 m is the density of mass in kilograms per cubic metre, kg/m3 l is the lever arm length in metres, m resm is the residual torque in newton metres, nm the uncertainty of the torque standard machine was evaluated with a combination of force, length, air density, mass density, local gravitational acceleration, and residual torque, as shown in equation (1). the relative expanded uncertainty of torque standard machine was expressed at a coverage factor k = 2 for measurement range 200 n·m to 2 knm as shown in equation (2). figure 6. the labview program used for controlling the machine. (a) (b) (c) figure 5. (a) a schematic overview of the stacked weight with the lifting system. (b) one piece of the weight disk. (c) three-column (highlighted in red, yellow, and green columns) weightlifting system. stacked weight three-column weight lifting system. lifting system in-out area for stacked weight acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 33 𝑊(𝑀𝑠𝑡𝑑 ) = {0.0042 + 26 ∙ [𝑀 (n∙m)⁄ ]} −1.935 % (2) where: 𝑊(𝑀𝑠𝑡𝑑 ) is the confirmed uncertainty of torque standard m is the nominal torque 0.0042 is the expanded uncertainty of torque standard equation (2) was determined based on interpolation of the relative expanded uncertainty of the torque standard at any nominal torque value. 3.2. comparison results of a measurement range 200 n·m to 2,000 n·m an informal comparison between nimt and physikalischtechnische bundesanstalt (ptb) was conducted in 2016 to confirm the capability of nimt’s 5 kn·m tsm. the comparison method was made in accordance with din 51309: 2005-12 [14]. the artefact was the torque transducer, capacity: 2 kn·m, model: tb2/2000 n·m and s/n: 122530099. the tsm of ptb is a 20 kn·m deadweight torque standard machine, with its claimed uncertainty ranging from 0.002 % to 0.003 %. the environmental conditions and the measurement results of the comparison are shown in table 1 and table 2, respectively. the comparison results were reported at the temperature of 21.1 °c with the artefact’s temperature coefficient of 0.00004/k [15]-[16]. the |en| of the comparison results between 0.18 and 0.91 are shown in figure 7. 3.3. comparison results of the measurement range 500 n·m to 5,000 n·m the informal comparison was conducted in 2016 in the measurement range of 200 n·m to 2,000 n·m. the comparison results were satisfactory [17], as described in section 3.2. thereafter, the authors extended the scope of the work up to 5,000 n·m, which is the maximum capacity of the standard machine with its claimed cmcs of 0.01 %, as confirmed by the comparison results. since early 2018, the laboratory has sent the torque transducer (model: tb2/5000 n·m, s/n: 123330449) for calibration at ptb. the calibration ranges are 500 n·m to 5,000 n·m according to din 51309: 2005-12. to compare the results, nimt carried out a calibration of the torque transducer again according to the same procedure. the environmental conditions and the measurement results of comparison are shown in table 3 and table 4, respectively. the comparison results of the measurement range from 500 n·m to 5,000 n·m in 2018 were reported at a temperature of 21.1 c with the artefact’s temperature coefficient as described in section 3.2. the difference in arm length between both sides of the machine (l) evaluated by using the weight balancing technique [18]-[19] are shown in figure 8. the results showed positive l, which shows that the lever arm length in the clockwise table 2. the measurement results of nimt and ptb, ranging from 200 n·m to 2,000 n·m. torque (nm) nimt ptb output signal (mv/v) w (k = 2) output signal (mv/v) w (k = 2) 0 -200 -0.100026 0.010 -0.100009 0.005 -400 -0.200049 0.010 -0.200022 0.003 -600 -0.300071 0.010 -0.300042 0.003 -800 -0.400095 0.010 -0.400067 0.003 -1000 -0.500121 0.010 -0.500086 0.003 -1200 -0.600151 0.010 -0.600115 0.003 -1600 -0.800210 0.010 -0.800174 0.003 -2000 -1.000288 0.010 -1.000240 0.003 0 200 0.100025 0.010 0.100010 0.004 400 0.200045 0.010 0.200029 0.003 600 0.300065 0.010 0.300046 0.003 800 0.400087 0.010 0.400068 0.003 1000 0.500114 0.010 0.500094 0.003 1200 0.600141 0.010 0.600124 0.003 1600 0.800197 0.010 0.800178 0.003 2000 1.000268 0.010 1.000247 0.003 table 1. the environmental conditions during the comparison carried out in 2016. environmental condition nimt ptb temperature (c) 22.5 (cw) and 22.6 (ccw) 21.1 (cw) and 21.1 (ccw) relative humidity (%) 52.4 (cw) and 51.8 (ccw) 40.5 (cw) and 40.5 (ccw) atmospheric pressure (hpa) 1006.5 ± 18 996 cw means a clockwise direction, and ccw means a counter-clockwise direction figure 7. the |en| evaluated based on the comparison, ranging from 200 n·m to 2,000 n·m. table 3. the environmental conditions during the comparison carried out in 2018. environmental condition nimt ptb temperature (c) 21.6 (cw) and 22.4 (ccw) 21.1 (cw) and 21.1 (ccw) relative humidity (%) 51.6 (cw) and 53.9 (ccw) 38.9 (cw) and 38.9 (ccw) atmospheric pressure (hpa) 1006.5 ± 18 983 cw means a clockwise direction and ccw means a counter-clockwise direction acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 34 direction was longer than it was in the anti-clockwise direction. consequently, the comparison results were corrected due to the difference in the arm length of the machine. after the correction of the results due to the temperature sensitivity and the difference in arm length, the reference values were calculated according to procedure a suggested in the guidelines for the evaluation of key comparison data [20]. the |en| from 0.07 to 0.66 was observed from the informal bilateral comparison between nimt and ptb on the torque measurement ranging from 500 n·m to 5000 n·m as shown in figure 9. since the range of this comparison was close to the machine’s capability, the results showed better agreement than one compared in the range of 200 n·m to 2000 n·m. 4. conclusions nimt developed the calibration capability of a torque measuring device up to 5 knm by using a 5 knm tsm with its cmcs at 0.01 % (k = 2) in the measurement range of 500 nm to 5,000 nm. the flexure bearing was designed as a fulcrum in order to withstand larger cross-forces and to provide a shorter time for damping the lever arm’s oscillating movement. the capability of the tsm was confirmed (see figure 10), with satisfactory results of |en| obtained from the informal comparison with the well-recognised nmi. acknowledgement the authors would like to express their sincere thanks to dr. sompop saengphueng for his assistance on a preliminary spell check. the utmost gratitude goes to ms. rugkanawan wongpithayadisai for her contribution and support for proofreading the article and consequently improving the quality of language and presentation. references [1] k. ohgushi, t. tojo, a. furuta, development of the 1 kn·m torque standard machine, proc. of the xvi imeko world congress, vienna, austria, 25-28 september 2000. figure 9. the |en| evaluated from the comparison, ranging from 500 n·m to 5,000 n·m. figure 8. the difference in arm length (l) of both sides of the machine. 1st represents the first series of the measurement. 2nd represents the second series of the measurement. table 4. the measurement results reported by nimt and ptb ranging from 500 n·m to 5,000 n·m. torque (nm) nimt ptb output signal (mv/v) w (k = 2) output signal (mv/v) w (k = 2) 0 -500 -0.100059 0.011 -0.100050 0.005 -1000 -0.200129 0.010 -0.200117 0.003 -1500 -0.300198 0.010 -0.300185 0.003 -2000 -0.400267 0.010 -0.400256 0.003 -2500 -0.500339 0.010 -0.500324 0.003 -3000 -0.600416 0.010 -0.600400 0.003 -4000 -0.800558 0.010 -0.800549 0.003 -5000 -1.000728 0.010 -1.000709 0.003 0 500 0.100061 0.011 0.100056 0.004 1000 0.200133 0.010 0.200121 0.003 1500 0.300207 0.010 0.300185 0.003 2000 0.400286 0.010 0.400257 0.003 2500 0.500363 0.010 0.500332 0.003 3000 0.600445 0.010 0.600409 0.003 4000 0.800606 0.010 0.800562 0.003 5000 1.000801 0.010 1.000726 0.003 figure 10. the confirmed uncertainty (solid pink line) and the expanded uncertainty (dash blue line) of the 5 knm torque standard. acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 35 [2] d. röske, metrological characterization of a 1 n m torque standard machine at ptb, germany, metrologia 51, 2014, pp. 8796. [3] k. ohgushi, t. ota, k. ueda, e. furuta, design and development of 20 kn·m deadweight torque standard machine, proc. of the imeko world congress, celle, germany, 24-26 september 2002. [4] zh. zhang, y. zhang, b. guo, f. meng, t. li, h. ji, m. dai, the development of 100 n m torque standard machine at nim, proc. of the xix imeko world congress, lisbon, portugal, 6-11 september 2009. [5] k. ogushi, a. nishino, k. maeda, k. ueda, calibration chain for hand torque screwdrivers, proc. of the imeko 22nd tc3, 12th tc5, and 3rd tc22 international conferences, cape town, south africa, 3-5 february 2014. [6] n. saenkhum, t. sanponpute, the improvement of nimt’s 1 kn·m tsm, proc. of the asia-pacific symposium on measurement of mass, force & torque (apmf 2017), krabi, thailand, 19-23 november 2017. [7] a. c. p. bitencourt, r. theska, a. wagner, h. a. lepikson, l. a. g. junior, w. l. weingaertner, a novel approach in the application of flexure bearings in primary torque standard machines, proc. of the 11th euspen international conference, como, italy, 23-27 may 2011. [8] a. nishino, k. ogushi, k. ueda, d. röske, d. mauersberger, bilateral comparisons of measurement capabilities for the calibration of low nominal capacity torque measuring devices between nmij and ptb in the range from 0.1 n·m to 10 n·m using different procedures, measurement 68, 2015, pp. 32-41. [9] a. nishino, k. ogushi, k. ueda, uncertainty evaluation of a 10 n·m deadweight torque standard machine and comparison with a 1 kn·m deadweight torque standard machine, measurement 49, 2014, pp. 77-90. [10] d. peschel, determination of the friction of aerostatic radial bearings for the lever-mass system of torque standard machines, proc. of the xiii imeko world congress, torino, italy, 5-9 september 1994. [11] a.c.p. bitencourt, r. theska, a. wagner, h.a. lepikson, w.l. weingaertner, alternative concepts for high precision bearings in torque standard machines, 10th euspen international conference, delft, netherlands, 31 may – 4 june 2010. [12] t. sanponpute, p. chantaraksa, n. saenkhum, n. arksonnarong, suspended-fulcrum torque standard machine, proc. of the xix imeko world congress, lisbon, portugal, 6-11 september 2009, pp. 343-346. [13] a. nishino, k. ogushi, t. sanponpute, n. arksonnarong, interlaboratories comparison between nmij and nimt for calibration capability of a low nominal capacity torque measuring device in the range from 0.1 n·m to 1 n·m, proc. of the sice annual conference 2016, tsukuba, japan, 20-23 september 2016. [14] din 51309: 2005-12, werkstoffprüfmaschinen – kalibrierung von drehmomentmessgeräten für statische drehmomente, december 2005, [in german]. [15] n. arksonnarong, t. sanponpute, a study of torque transfer wrench stability, proc. of the 3rd tsme international conference on mechanical engineering, chiang rai, thailand, october 2012. [16] t. sanponpute, n. arksonnarong, temperature and humidity dependence of stability of torque measuring devices, imeko conference tc3, tc5 & tc22, cape town, south africa, 3-5 february 2014. [17] n. arksonnarong, n. saenkhum, p. chantaraksa, c. schlegel, t. sanponpute, 5 kn·m torque standard machine of nimt, proc. of the asia-pacific symposium on measurement of mass, force & torque (apmf 2017), krabi, thailand, 19-23 november 2017. [18] t. sanponpute, n. arksonnarong, a. brüge, n. saenkhum, suspended-fulcrum torque standard machine, 2nd report, proc. of the xxi imeko world congress, prague, czech republic, 30 august-4 september 2015. [19] a. nishino, k. ogushi, k. ueda, evaluation of actual sensitivity limit in a 10 n·m dead weight torque standard machine and stability of a new 1 n·m torque transducer, proc. of the imeko 2010 tc3, tc5 and tc22 conferences “metrology in modern context”, pattaya, chonburi, thailand, 22-25 november 2010. [20] m.g. cox, the evaluation of key comparison data, metrologia 39, 2002, pp. 589-595. acoustically-shaped laser: a machining tool for industry 4.0 acta imeko issn: 2221-870x december 2020, volume 9, number 4, 60 66 acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 60 acoustically shaped laser: a machining tool for industry 4.0 salvatore surdo1, alessandro zunino1,2, alberto diaspro1,2, martí duocastella1,3 1 nanoscopy, cht erzelli, istituto italiano di tecnologia, via enrico melen 83, building b, 16152 genova, italy 2 department of physics, university of genoa, via dodecaneso 33, 16146 genova, italy 3 departament de física aplicada, universitat de barcelona, c/martí i franquès 1, 08028 barcelona, spain section: research paper keywords: laser–matter interaction; resonant cavity; digital industry; cyber-physical systems; acousto-optic citation: salvatore surdo, alessandro zunino, alberto diaspro, martí duocastella, acoustically-shaped laser: a machining tool for industry 4.0, acta imeko, vol. 9, no. 4, article 8, december 2020, identifier: imeko-acta-09 (2020)-04-08 section editor: leopoldo angrisani, university of naples federico ii, italy received october 31, 2019; in final form june 5, 2020; published december 2020 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding authors: salvatore surdo, e-mail: salvatore.surdo@iit.it, martí duocastella, e-mail: marti.duocastella@iit.it 1. introduction the fourth industrial revolution or industry 4.0 (i4) exploits machines that are augmented by means of internet connections and sensor networks and are controlled by intelligent systems capable of monitoring the entire production chain and making autonomous decisions [1], [2]. this enables workpiece production with high levels of adaptability in terms of manufacturing conditions and design adjustments, which ultimately results in high-quality manufacturing at high yields [3], [4]. in detail, the i4 paradigm encompasses technologies that belong to two broad categories, often known as digital and physical worlds. the former includes the internet of things (iot), big data, and cloud computing and allows wireless collection of data from physical objects (sensors, machines, and products) as well as their real-time processing, and is aimed at improving production and logistics efficiency. the second group comprises robots and manufacturing tools that are digitally controlled and perform machining operations that are capable of shaping materials according to a specific design. over the past few decades, significant efforts have been directed toward the growth of the digital world, which currently offers multiple iot elements (sensors, actuators, and nodes) [5], [6] and wireless communication protocols such as zigbee, 5g, and lte [7]-[9]. the physical world can now include autonomous robots, although it is still looking for a suitable manufacturing method, often referred to as a ‘machine tool 4.0’ (mt4.0) [10]. the ideal candidate should be capable of performing multiple operations, such as the cutting, punching, forming, and welding of a large range of functional materials, ideally at high speed and with design flexibility. however, traditional tools only fulfil a few of these requests. for instance, computer numerical control (cnc) milling machines can cut several materials, such as wood, metals, plastics, glass, and foams, but operate only in subtractive mode, typically with one material at a time, and suffer from mechanical wear issues. their additive counterparts, such as fused deposition modelling and ink-jet printing, are significantly more flexible in pattern selection but typically operate with a reduced number of materials; their compatibility with metals, arguably the most widely used abstract the high versatility of laser direct-write (ldw) systems offers remarkable opportunities for industry 4.0. however, the inherent serial nature of ldw systems can seriously constrain manufacturing throughput and, consequently, the industrial scalability of this technology. here we present a method to parallelise ldws by using acoustically shaped laser light. we use an acousto-optofluidic (aof) cavity to generate acoustic waves in a liquid, causing periodic modulations of its refractive index. such an acoustically controlled optical medium diffracts the incident laser beam into multiple beamlets that, operating in parallel, result in enhanced processing throughput. in addition, the beamlets can interfere mutually, generating an intensity pattern suitable for processing an entire area with a single irradiation. by controlling the amplitude, frequency, and phase of the acoustic waves, customised patterns can be directly engraved into different materials (silicon, chromium, and epoxy) of industrial interest. the integration of the aof technology into an ldw system, connected to a wired-network, results into a cyber-physical system (cps) for advanced and high-throughput laser manufacturing. a proof of concept for the computational ability of the cps is given by monitoring the fidelity between a physical laser-ablated pattern and its digital avatar. as our results demonstrate, the aof technology can broaden the usage of lasers as machine tools for industry 4.0. mailto:salvatore.surdo@iit.it mailto:marti.duocastella@iit.it acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 61 resource in industry, is limited [11], [12]. recent attempts to address these issues with hybrid systems that combine additive and subtractive methods have led to complex machine tools with limited speed and high costs [13]. a family of technologies capable of solving the problems encountered with traditional methods is laser direct writing (ldw) [14]. ldw can use a laser beam to either alter a controlled volume of material causing localised damage to a surface (subtractive operation) or can transfer a specified amount of material onto a workpiece (additive operation). in both cases, the desired pattern is first drawn with computer-aided design (cad) and then sequentially etched or built by scanning the sample with the laser beam. these peculiarities make ldw a good choice as a machine tool for i4 [15]. ldw is compatible with metals [16], ceramics [17], polymers [18], and more exotic materials such as wood [19] and leather [20]. furthermore, being capable of both subtractive and additive operations, ldw can implement multiple machining processes, such as printing [21], depositing [22], the joining [23] of various materials, and the polishing [24], coating [25], and cleaning [26] of a surface. finally, lasers are controlled numerically and can be connected to other physical or digital objects, enabling the realisation of laserbased cyber-physical systems (cpss) for advanced manufacturing. so far, the application of ldw in i4 is limited to a few operations, namely laser ablation and sintering, and the reasons can be ascribed to its serial nature and limited flexibility and speed in terms of beam shape selection, which limit the manufacturing throughput. these constraints have spurred the development of methods for shaping a laser beam in a tuneable and controlled way. examples include laser beam parallelisation with passive optical elements (gratings [27] and beams splitters [28]) or beam shaping by means of mask projection systems [29]. however, these methods lack tunability in terms of the number, position, and shape of the laser beams, thus impeding the realtime correction and adjustment of the manufacturing process. this problem can be solved with active optical elements such as digital micromirror devices (dmd) [30] and spatial light modulators (slm) [31] that allow for the generation of tuneable light patterns; however, they suffer from pixellation issues and long response times and can be easily damaged. we recently proved that rapid (in the sub-microsecond timescale) both-beam parallelisation (up to 35 beams) and the generation of complex and tuneable light patterns can be achieved through the interaction of laser and acoustic waves in a liquid [32], [33]. our technology, acousto-optofluidics (aof), enables high-throughput material processing without suffering from the problems (damage, pixellation, and speed) of conventional methods. here, we present a detailed characterisation of the acousto-optofluidic cavity and the first results of its integration into an ldw system, aimed at proving its feasibility as a machine tool for i4. specifically, we illustrate a simple (two-level) laser-based cps with aof functionalities for the high throughput additive and subtractive machining of various materials such as metals, polymers, and semiconductors. 2. acoustically shaped laser beam 2.1. acoustic cavity: design, implementation, and characterisation the key element of our technology is an acoustic resonant cavity capable of inducing stationary pressure waves in a liquid. figure 1 a) shows a schematic of the cavity that encompasses two figure 1. implementation and characterisation of the acoustic cavity. a) schematic description of the main components of the aof cavity, which consists of four rectangular (2 cm × 1 cm) piezoelectric plates, a metallic cylindrical container (ø = 2.54 cm), and two circular glass windows. b) optical images of a disassembled aof cavity. the inset shows a magnified view of the piezoelectric plates, assembled according to rectangular symmetry. c) transfer functions (amplitude and phase) of the cavity. d) crosstalk (amplitude) between the xand y-axis of the cavity. the blue symbols denote the experimental data, while the yellow lines are the best-fit resonance functions. acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 62 couples of rectangular piezoelectric plates, with each pair being perpendicular to the other. the cavity is placed in a metallic cylinder and two glass circular windows seal the device with the help of o-rings. the two holes in the cylinder are for electrical connections between the piezoelectric elements and external instrumentation (waveform generator and amplifier). figure 1 b) shows an optical image of the main components of the cavity, which has a final size of 2 cm3. the application of sinusoidal signals to the piezoelectric pair generates vibrations in the liquid. propagation of these acoustic waves along the xand y-axes can be described by solving the damped acoustic wave equation with appropriate boundary conditions [33], [34]. on resonance, standing acoustic and, hence, density waves are present in the cavity at resonant frequencies given by the following equation: 𝑓r = 𝑚 𝑐s 2 𝑤 , (1) where 𝑐s is the speed of sound in the liquid (𝑐s = 1490 m/s in water at room temperature), 𝑤 = 1 cm is the distance between the piezoelectric plates of each couple, and 𝑚 is an odd integer that identifies the resonance cavity order. figure 1 c) shows the measured transfer function for both amplitude and phase between the piezoelectric plates of the same couple. a resonance is clearly visible at 𝑓r ≈ 1 mhz that corresponds to the 13 th harmonic. notably, the resonance has a good quality factor (q ≈ 290), which helps to filter out undesired contributes such as noise and environmental signals. interestingly, the crosstalk, namely the transfer function between the piezoelectric plates of different pairs, has also a peak at such a resonant frequency but with a significantly smaller amplitude – roughly one order of magnitude (see figure 1. d). 2.2. laser diffraction through the acoustic cavity the previous section enables the prediction of the resonant conditions for the cavity and the driving frequencies of the piezoelectric plates that result in standing acoustic waves in the liquid. what remains is understanding how the laser beam that travels through the cavity (along the z-axis) is diffracted by sound. to shed light on this point, we must first determine how the stationary acoustic waves alter the refractive index of the liquid. on resonance, the induced vibrations cause a periodic modulation of the density of the liquid, which, according to the lorentz–lorenz model, translates into a periodic refractive index that can be calculated with [33]: 𝑛(𝑥, 𝑡) = 𝑛0 + δ𝑛 cos(2 π 𝑓r 𝑡) cos ( 2 π 𝑓r 𝑐s 𝑥), (2) where 𝑛0 is the static refractive index of the liquid and δn the peak of the induced variations. the latter depends on both the amplitude and frequency of the driving signals [32], [33]. based on equation 2, we can anticipate that the cavity behaves as an oscillating periodic grating capable of diffracting the incident light. specifically, figure 2 shows the effects of the aof cavity on a gaussian laser beam when combined with a telescope. in the focal plane of the first lens, the oscillating grating diffracts the laser beam into multiple beamlets [32]. the number, spacing, and intensity of each diffracted spot can be controlled by properly adjusting the driving parameters, such as amplitude, frequency, and the phase of the signals applied to the piezoelectric plates. as such, the aof cavity enables laser parallelisation, provides an enhancement in the throughput roughly equal to the number of spots, and is only limited by the energy of the laser. the aof system also enables more complex light shaping. specifically, after passing through the second converging lens, the diffracted beamlets interfere, generating an intensity pattern. notably, the generated pattern can be controlled with the same parameters that control the beamlets. the phase, in particular, allows ultra-fast pattern selection when a pulsed laser with a duration much smaller than the temporal period of the driving signals is used [32], [33]. in this case, the laser pulse interacts with an instantaneous refractive index profile (see equation 2) that can be selected by delaying the arrival of the laser with respect to the temporal period of the driving signal and, thus, with nanosecond resolution. in this configuration, the aof system can boost the throughput of an ldw machine by scanning the material surface region-by-region rather than point-by-point. 3. cyber-physical system for laser manufacturing figure 3 a) schematically describes the architecture of a laserbased cyber-physical workstation with aof functionalities. the station is organised in two levels: physical and digital. the former handles the operations that are necessary for the manufacturing of the workpiece and is connected to the digital one through a network (a private network in the current setup), which provides the physical world with access to various users, computers, and data-storage systems. the mutual interaction between these levels results into a cyber-physical system in which information and actions can be monitored and synchronised between the “factory floor” and cyberspace. for instance, the digital level can collect data from the physical objects that, once processed, can be used to monitor the manufacturing process, compute the usage or wear of a physical object, or tag a workpiece. figure 2. the optical setup used to generate multiple beamlets and interference patterns consists of an aof cavity and two converging lenses. in the focal plane of the first lens, the aof cavity diffracts the incident gaussian beam (top) into multiple beamlets (middle) that, after passing through the second lens, interfere, forming a light pattern (bottom). the beamlets and interference in the figure were generated at the acoustic frequencies of 1.37 mhz and 1.8 mhz, respectively. scale bars are 200 µm. acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 63 3.1. implementation of the physical space of a laser-based cps the physical part of a laser-based cyber-physical system comprises several elements. the key component is an ldw station that consists of an ultrafast pulsed laser emitting 70-fs pulses at an operational wavelength of 800 nm, an upright microscope equipped with a long working-distance microscope objective (numerical aperture 0.55), a complementary metal– oxide–semiconductor (cmos) camera for real-time inspection of the workpiece, and an aof cavity. a waveform generator provides the driving signals for proper aof operation. for laser manufacturing using the interference patterns, the aof cavity is conjugated to the workpiece plane by means of relay lenses. simply removing one of these lenses enables the generation of multiple diffracted spots at the focal plane of the microscope objective. in both cases, high-throughput material processing can be achieved by using a motorised xyz stage to displace the sample relative to the acoustically shaped light (see figure 3 b). complex patterns can be prepared by varying the resonances, i.e. by controlling the driving parameters while scanning the sample surface. 3.2. digital assessment of the aof-generated pattern fidelity the proof of concept for the computational operations of the digital level of the laser-based cps is provided by successfully monitoring the fidelity between a physical pattern and its digital avatar. in this experiment, we ablated a large-area pattern onto a chromium substrate by scanning its surface with up to 19 aofgenerated beamlets and using a cmos camera to record the machining process. figure 3 c) shows an optical image of a fraction of the physical pattern that was obtained by stitching together 11 frames of the recorded video. the corresponding digital counterpart (a binary pattern in this case) is shown in figure 3 d). at each time step, we used cross-correlation to evaluate the fidelity or similarity between the instantaneous physical pattern and the final digital copy. as shown in figure 3 e), initially (t = 0), the fidelity is 0 because the physical pattern does not yet exist and it increases at each time step as soon as the ablation proceeds, indicating that the laser-machining process is performing well. notably, deviation from the expected trend can be used to predict errors or the failure of one or more physical objects in the ldw station. our results clearly prove that the functionalities of the aofldw station can be augmented by collecting data from the physical space and using it to monitor the similarity between the machined workpiece and its digital copy. in the future, the digital pattern could be decomposed into fundamental patterns, for instance, by means of fourier transformation, in order to directly provide the aof system with suitable driving parameters. in conjunction with appropriate xyz scanning, this strategy will enable the realisation of totally automatised and digitally controlled laser-based machining tools. as we implement more computational operations and improve data collection capabilities at the physical level, with sensors or advanced imaging methods, interesting possibilities will continue to emerge, providing novel opportunities for laser machining tool development and optimisation. figure 3. cyber-physical system for aof-enabled laser manufacturing. a) the architecture of the laser-based cps comprises a physical (top) and digital (bottom) level. the former consists of an ldw station integrating an aof cavity that is conjugated with the plane of the workpiece by means of a series of converging lenses. the physical and digital levels are connected through a network that offers access to the aof-ldw station to users, workstations, or data-storage devices. b–e) digital assessment of the workpiece fidelity. b) schematic description of the strategy used for large-area patterning with an aof-shaped laser beam. c) large field-of-view image of the ablated pattern. d) digital copy of the target pattern. e) fidelity between the physical and digital patterns at different time steps. acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 64 4. material processing with the aof-ldw station the previous sections demonstrate that aof technology can be effectively integrated into an ldw station and then into a cyber-physical system. what remains is proving if the aof technology enables high-throughput laser manufacturing. to answer this question, we used the aof-ldw to machine several materials (metals, semiconductors, and polymers) in both additive and subtractive mode. figure 4 shows two examples of subtractive laser manufacturing of semiconductors and metals, e.g. patterns ablated in silicon [figure 4. a)] and palladium [figure 4. b)]. for the ablation of si, we used both piezoelectric couples of the aof cavity to generate a 2d distribution of the diffracted beamlets. in detail, driving the piezoelectric plates at the resonant frequency of 𝑓𝑟 = 4.5 mhz led to a 5-fold enhancement in the throughput of the manufacturing process. alternatively, for the ablation of palladium, we used only one axis of the aof cavity, which we modulated between the on and off states while snakescanning the sample surface. this strategy resulted in a more complex pattern and a 9-fold enhancement (in the on state) in the throughput of the laser ablation. as shown in figure 5, the aof-ldw station also allows for the machining of an entire area with single-laser irradiation when interference patterns are used. we tested this functionality in both additive [figure 5 a) and b)] and subtractive [figure 5 c) and d)] modes. figure 5 a) in particular shows photo-polymerised structures formed by scanning an epoxy resin along the x-axis with an intensity pattern generated at an acoustic frequency of 1.2 mhz. to increase the complexity of the polymeric structures, we exploited the binary modulation (on–off and vice versa) of the aof cavity while snake-scanning the resin. figure 5 b) shows a 3d map of a representative portion of the photopolymerised structures that highlights the high quality of the prepared surfaces without evident defects or irregularities. figure 5 c) shows an example of a metallic pattern that we obtained with the laser-ablation of a palladium substrate. in this experiment, the substrate surface was snake-scanned with two alternating interference patterns generated by driving a single piezoelectric pair at the acoustic frequencies of 1.2 mhz and 1.8 mhz, respectively. this strategy resulted in two closely periodic microstructures (aligned with the interference pattern) with periods of 2.1 µm and 2.9 µm, respectively. interestingly, the atomic force microscopy (afm) measurements in figure 5 d), on a fraction of these structures, reveal the existence of periodic nanoripples (width ~ 250 nm and period ~ 500 nm) orthogonal to the interference pattern. also known as laserinduced periodic surface structures or lipss, they are of great interest for industrial applications such as structural coloration, control of surface wetting, engineering bacterial and cellular proliferation on substrates, and optimisation of the tribology of a surface [35]-[39]. 5. conclusion an aof-ldw system is a feasible and functional machine tool for i4. in particular, the high speed and versatility, in terms of beam shaping, offered by the aof cavity enables highthroughput laser manufacturing of various materials of industrial interest with both additive and subtractive functionality. the shape of the laser beam depends on the amplitude, frequency, and phase of the oscillating refractive index of the liquid in the cavity. as such, rapid pattern selection is possible by simply adjusting the driving signals. as our results demonstrate, integrating the aof cavity into an ldw station is simple, and, with few optical elements (lenses), either beam parallelisation or the generation of an interference pattern can be selected. the aof-ldw station is digitally controlled and can be connected to cyberspace, enabling the realisation of a laser-based cyberphysical system with advanced manufacturing functionalities. we can anticipate that in conjunction with the use of intelligent systems [40] and improved wireless connections, the aof-enabled cyber-physical system will enable the realisation of autonomous manufacturing tools that adapt their operations in response to feedback, collected data, or their own learning abilities. 6. appendix – materials and methods 6.1. optical characterisation setup light intensity distributions produced with the aof cavity were recorded with a cmos camera (thorlabs, dcc1545m) placed after the lens system used to generate either beamlets or interference patterns. a 445 nm laser was used in this experiment (coherent cube 445‐40c). 6.2. laser polymerisation pentaerythritol triacrylate and isopropylthioxanthone were purchased from sigma‐aldrich and used as a negative resin and photo initiator, respectively. photopolymerisation was induced by scanning the resin with an aof-generated interference patter with a dose of 0.3 µj per pulse. to remove the uncured resin, the sample was immersed in methanol for five minutes and then rinsed with isopropanol. figure 4. laser material processing with multiple beamlets. a) optical image of a pattern ablated into silicon with a beamlet distribution obtained by driving the xand y-axis of the aof cavity at the same resonant frequency. b) laser ablation of a chromium substrate generated by snake scanning the sample surface while the aof cavity was alternating between the on and off states. the insets show a magnified view of the patterns, ablated with a single laser irradiation and corresponding laser intensity distributions. acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 65 6.3. sample characterisation laser-modified materials were inspected with a scanning electron microscope (jsm‐6390, jeol) at an acceleration voltage of 10 kv. polymeric samples were sputter‐coated with 10 nm of gold to inhibit charging effects. the height map of the polymeric structures was measured with an optical profilometer (zeta‐20, zeta instruments). the morphology of the metal surfaces was imaged with an atomic force microscope (mfp‐3d, asylum research) operating in tapping mode in air. the probes (ppp‐nchr, nanosensors) were aluminium-coated si cantilevers with a resonance frequency of 330 khz, spring constant of 40 n m−1, and tip‐radius of 5 nm. amplitude images were collected by scanning 10 × 10 µm2 areas with a resolution of 256 × 256 pixels. 6.4. measurement of the transfer function to acquire the amplitude and phase of the transfer function of the resonant cavity, a sinusoidal signal was applied to one of the piezoelectric plates (transmitters) while acquiring the signal of the opposed plate (receiver) with an oscilloscope. a sinusoidal function was fitted to both recorded signals to measure both their amplitude ratio, 𝐴𝑅 /𝐴𝑇 , and phase shift, 𝜙𝑅 − 𝜙𝑇 . the transfer function was obtained by sweeping the frequency of the driving signal in the interval of 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https://doi.org/10.1016/j.pmatsci.2019.100573 https://doi.org/10.1016/j.optlaseng.2019.105897 https://doi.org/10.1364/oe.15.014488 https://doi.org/10.1038/s41598-019-41902-x https://doi.org/10.1364/ome.4.002032 https://doi.org/10.1364/ol.40.004875 https://doi.org/10.1016/j.optlaseng.2018.09.002 https://doi.org/10.1002/admt.201900623 https://doi.org/10.1002/advs.201900304 https://doi.org/10.1063/1.2763947 https://doi.org/10.1038/s41598-017-08788-z https://doi.org/10.3390/ma9060476 https://doi.org/10.3390/lubricants7080070 https://doi.org/10.1016/j.nano.2019.102036 https://doi.org/10.1371/journal.pone.0048556 https://doi.org/10.1016/j.rcim.2019.101842 novel concepts and strategies in skull base reconstruction after endoscopic endonasal surgery acta imeko issn: 2221-870x december 2020, volume 9, number 4, 67 73 acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 67 novel concepts and strategies in skull base reconstruction after endoscopic endonasal surgery domenico solari 1, ida papallo2, lorenzo ugga2, luigi m. cavallo1, ilaria onofrio3, renato cuocolo2, giovanni improta4, arturo brunetti2, massimo martorelli5, antonio gloria6, paolo cappabianca1, teresa russo6 1 division of neurosurgery, department of neurosciences, reproductive and odontostomatological sciences, university of naples federico ii, naples, italy 2 department of advanced biomedical sciences, university of naples federico ii, naples, italy 3 department of neurosciences, reproductive and odontostomatological sciences, university of naples federico ii, naples, italy 4 department of public health, university of naples federico ii, naples, italy 5 department of industrial engineering, fraunhofer jl ideas, university of naples federico ii, naples, italy 6 institute of polymers, composites and biomaterials – national research council of italy, naples, italy section: research paper keywords: endoscopic endonasal surgery; skull base reconstruction; csf leakage; reverse engineering; additive manufacturing; design of injectable systems citation: domenico solari, ida papallo, lorenzo ugga, luigi m. cavallo, ilaria onofrio, renato cuocolo, giovanni improta, arturo brunetti, massimo martorelli, antonio gloria, paolo cappabianca, teresa russo, novel concepts and strategies in skull base reconstruction after endoscopic endonasal surgery, acta imeko, vol. 9, no. 4, article 9, december 2020, identifier: imeko-acta-09 (2020)-04-09 section editor: leopoldo angrisani, university of naples federico ii, italy received november 4, 2019; in final form august 5, 2020; published december 2020 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding authors: domenico solari, e-mail: domenico.solari@unina.it, ida papallo, e-mail: i.papallo89@gmail.com 1. introduction the skull base is located between the brain and the extracranial compartment and consists of several anatomical structures. from an anatomical and surgical point of view, the skull base represents one of the most complex areas of the human body. it is reported that a broad variety of lesions, either neoplastic or not, may arise primarily from this area or, subsequently, involve it. the surgical management of these lesions can be extremely difficult, especially of deep-seated lesions, despite a variety of innovative craniofacial approaches that have been adopted to access the entire skull base in recent decades 0-[9]. additionally, it should be underlined that tissue disruption and neurovascular manipulation often characterise the access routes to the skull base, increasing perioperative morbidity and/or mortality rates. the surgical advances and technological innovations, together with the progress in diagnostic imaging techniques and the intraoperative neuronavigational systems, have progressively reduced the surgical invasiveness of the skull base approaches. specifically, a trans-sphenoidal technique was developed for the treatment of lesions previously amenable only to a transcranial route. in fact, over the past few years, the endonasal endoscopic route has been considered as a viable corridor to access first the sellar region, and then the surrounding areas. presently, this route represents a suitable approach for several skull base lesions [10]-[12]. abstract recently, a variety of craniofacial approaches has been adopted to enter the skull base, including the endonasal endoscopic technique. an effective watertight technique, the reconstruction can be performed using different materials, both autologous and non-autologous, individually or combined in a multilayer fashion. the current study focuses on the development of new advanced devices and techniques that help to reduce the postoperative cerebrospinal fluid leak rate. additive manufacturing allows the design of devices with tailored structural and functional features, as well as injectable semi-interpenetrating polymer networks and composites; therefore, specific mechanical/rheological and injectability studies are valuable. accordingly, we propose new additive manufactured and injectable devices. mailto:domenico.solari@unina.it mailto:i.papallo89@gmail.com acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 68 these surgical refinements and technological advances, together with an increasing comprehension of the region’s anatomy, have led to a revolution in skull base surgery [13]. in further detail, the main advantage of skull base surgery performed through the nose, using an endoscope, is the direct visualisation of the neurovascular structures of several areas of the skull base while minimising brain displacement and manipulation. the use of an endoscope allows for a wider and multiangled close-up view of the surgical field. as a result, patients treated with this approach may benefit in terms of early discharge, reduced postoperative morbidity, and a faster return to work [14]. however, the main issue of this approach lies in the reconstruction, as a consequence of a higher risk of postoperative cerebrospinal fluid (csf) leakage in comparison with the conventional transcranial method [15]. the removal of a skull base lesion via the endonasal route requires a wider osteo-dural opening and, especially, extensive opening of the arachnoid cisterns and/or, sometimes, the third ventricle. consequently, a large communication between the septic cavity (i.e. the sinonasal tract) and the sterile intradural compartment is created. an effective watertight closure is fundamental to isolating the intracranial cavity for restoration of the natural intra and extradural compartment division, to prevent postoperative csf leakage and further complications (i.e. brain herniation, meningitis, tension pneumocephalus) [16]. skull base reconstruction after extended endoscopic endonasal surgery should be optimised taking into account some crucial points (i.e. isolation of the intradural compartment from the sinonasal tract, water and airtight closure, obliteration of dead spaces, promotion of the healing process, preservation of function and cosmesis, and management of the risk factors of increased intracranial pressure) [15],[17],[18]. accordingly, the reconstruction can be performed using different devices that consist of autologous grafts and nonautologous materials, individually or combined in a multilayer strategy. the purported multilayer techniques have to deal mostly with the major issues of the irregular shape of both bony and dural defect boundaries, and the presence of optic nerves, internal carotid arteries, and the neurovascular structures of the cavernous sinus, surrounding them. furthermore, the removal of the tuberculum sellae and of the posterior part of the planum sphenoidale, along with the floor of the sella, creates a defect that does not lie on a flat plane. for these reasons, especially in the cases with a very large skull base opening, a semisolid buttress – adopted to sustain the dural foil – does not adhere properly. thus far, the recent trend is moving toward the use of autologous materials [19]-[21]. autologous grafts can interact with the surrounding structures of the osteo-dural defect, favouring the migration of fibroblasts and, consequently, a complete recovery. clearly, each component plays its specific role. cartilage and bone offer support, whereas the mucosa provides the matrix for the tissue vascularisation. the harvesting of a nasoseptal flap is considered to be one of the most effective techniques: it is able to bolster the skull base closure, ensuring the isolation of the surgical field, and plays an important role in reducing the csf leak rate after endoscopic skull base surgery [22]. nevertheless, it should be performed only in cases of wide skull base defects, as it may lead to nasal morbidity [19],[23]. the development of novel closure techniques and the availability of reliable reconstructive materials may contribute to reducing the postoperative csf leak, which represents one of the most feared complications of this type of surgical procedure [22],[24],[25]. in the last few months, we have modified our reconstruction method according to the concept of ‘3f’, each addressing a different compartment of the defect to be sealed. the first ‘f’ stands for autologous fat, to be used as a cork stopper across the intra–extradural space; the second ‘f’ refers to the nasoseptal flap, used to cover the skull base defect – we harvest it at the end of the surgical procedure; and, finally, the third ‘f’ represents our idea of ‘flash’ patient mobilisation out of the bed [26]. the major advantages arise from the possibility of moulding the fat inside the surgical cavity, having it pierce the entire diameter of the breach and, above all, fit and fill the outer shady areas, namely the irregular contours at the level of the tuberculum sellae and the optic canals [26]. in this context, further efforts are needed to develop novel strategies and devices for reducing the csf leak rates after endoscopic skull base surgery, as well as the post-operative patient discomfort and morbidity. moreover, benefiting from the development of advanced technologies [27] and methodologies of analysis [28]-[31] in different fields, innovative systems may also be designed for biomedical applications. the research has been driven toward the engineering of materials [32],[33] and biomedical devices with improved and tailored functional properties [34],[35]. novel and multifunctional devices may be developed in the form of injectable or solid systems, according to the specific application and the surgical approach. concerning the skull base reconstruction after endoscopic endonasal surgery, intriguing strategies would involve advances in the development of injectable semi-interpenetrating polymer networks (semi-ipns) and composites, computer-aided design (cad), reverse engineering, and 3d additive manufactured devices with controlled architectures and mechanical properties [34],[35]. the current research is focused on the development of novel approaches and devices, aiming at preventing the postoperative csf leakage in skull base reconstruction after endoscopic endonasal surgery, which represents one of the most feared complications related to this procedure. a design strategy was proposed involving the use of an injectable tool as a filling system and a ‘solid’ customised device as a closure system for skull base defects. 2. materials and methods the semi-ipns were prepared by promoting the polymer network formation. natural and synthetic polymers were employed and also properly modified. micro/nanoparticles were also used as reinforcement. the selection of the system components was made with the aim of promoting a clinical translation of the injectable devices as dura mater substitutes and sealant systems able to reduce the risk of csf leakage. acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 69 a rheometer (bohlin gemini; malvern instruments, malvern, uk) equipped with parallel-plate geometry was used to evaluate the viscoelastic properties of the materials at 37 °c. to determine the linear viscoelastic region, strain sweep tests were carried out at a fixed oscillation frequency. small amplitude oscillatory shear tests were performed, and the frequency varied from 0.01 to 2 hz. the storage modulus (g’) and the loss modulus (g”) were evaluated as follows: 𝐺′ = 𝜏0 𝛾0 cos 𝛿 (1) 𝐺′′ = 𝜏0 𝛾0 sin 𝛿 (2) with δ representing the phase shift between the input and output signals, whereas γ0 and τ0 represent the strain and stress amplitudes, respectively. different clinical needles were considered, and the effect of the injection on the viscoelastic properties was analysed, as it plays a crucial role in the design of injectable systems. steady shear measurements were carried out, and viscosity as a function of shear rate was evaluated at 37 °c (figure 1). the shear rate ranged from 0.01 to 10 s-1. to simulate clinical practice, injectability tests were also performed using an instron 5566 testing machine. by contrast, image capture and analysis techniques were used to generate 3d virtual models of the skull base defects. in particular, 3d reconstruction of the skull base region and defects was performed using computed tomography (ct) and a dedicated software. the model of a region of the skull base and customised devices for the defects were manufactured by fused deposition modelling using a 3d printer. in particular, z-glass filament was used to manufacture models of a skull base region, whereas a very durable and flexible material certified for medical use (bioflex, filoalfa) was employed for the fabrication of customised closure devices for the skull base defects. 3. results starting from some basic concepts and principles, novel strategies may be proposed towards the design of injectable systems and additively manufactured devices for skull base reconstruction after endoscopic endonasal surgery. recent literature stresses the important role of reverse engineering [36]-[39], computer-aided design (cad), and finite element analysis [40]-[43], as well as the potential of pushing the research toward the development of design strategies and methodologies of analysis in different fields of application [44][47]. the strategy related to the design of injectable semi-ipns and composites with tailored properties has benefited from specific rheological/mechanical and injectability studies [48]. injectable devices can also be developed through the combination of conventional methods and additive manufacturing techniques. some examples of rheological results obtained from analyses performed on semi-ipns consisting of different materials are reported below. the materials are not specified, the aim being only to stress the importance and the role of the viscoelastic properties and the flow behaviour in the design of the proposed systems, as it is widely reported [48] that the potential to tailor their rheological characteristics clearly depends on the employed material combinations. an example of one of the mechanical spectra achieved for the developed semi-ipns is reported in figure 2. the g’ values were always higher than the g” values in the analysed frequency range (figure 2). furthermore, the presence of micro/nanoparticles generally improved both viscoelastic moduli, until a threshold limit value was reached for the particle concentration. figure 1 reports typical values of the viscosity as a function of the shear rate obtained for some engineered semi-ipns. figure 1. example of viscosity as a function of shear rate for the developed semi-ipns. the data are reported as the mean value; the error bar represents the standard deviation. figure 2. example of the storage modulus (g’) and the loss modulus (g’’) as a function of the frequency for the developed semi-ipns. the data are reported as the mean value; the error bar represents the standard deviation. acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 70 the results showed that the viscosity decreased with the increasing shear rate (shear thinning behaviour), thus suggesting the possibility of injecting the developed materials. load–displacement curves were obtained from injectability tests. at low displacements, the curves obtained for the semiipns showed a linear region until the load reached a maximum value. successively, the load sharply dropped to a plateau value as the displacement further increased. at the end of the plateaulike region, the material was completely injected. the obtained values for the maximum and plateau loads were in the range of 6.6 – 3.7 n and 2.0 – 0.9 n, respectively, depending on the clinical needle employed and the material composition. by contrast, additive manufacturing techniques offer the opportunity of developing customised devices with complex geometry. additive manufactured devices with several architectural features can be fabricated by the 3d fibre deposition technique/fused deposition modelling (fdm) [34],[35]. starting from ct analysis, the surgical approach and the creation of a bone defect (i.e., cavity) were planned (figure 3 and figure 4). virtual models of a region of the skull base without and with the defect (figure 5 and figure 6), as well as of the customised closure device (figure 7) were created. initially starting from the shape and size of the defect, the geometry of the closure device was appropriately designed to be fitted in the cavity to prevent csf leakage. the feasibility of the proposed technical solutions was first assessed through virtual models (figure 8). additive manufactured models of a skull base region with the defect were developed using z-glass filament (figure 9). 3d customised devices with appropriate flexibility were also manufactured by fdm, using bioflex filament, as a closure system for the created skull base defect (figure 10). in comparison with conventional fabrication methods, additive manufacturing techniques allow strict control of the structural features and, consequently, of the properties of the devices, satisfying all the requirements [34],[35]. the customised closure devices were suitably developed to provide high flexibility and relatively high strength, according to the specific application. figure 3. results from image capture and analysis – skull base region. figure 4. results from the image capture and analysis: planning of the surgical approach and preparation of a cavity as a skull base defect. figure 5. 3d reconstruction of a skull base region. figure 6. 3d reconstruction of a skull base region with the created defect. figure 7. 3d model of the customised device as a closure system for the skull base defect. figure 8. 3d model of the device closing the skull base defect – different views. feasibility assessment of the proposed technical solutions. acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 71 the physical models were used to simulate clinical practice, and the feasibility of the proposed approach was consequently demonstrated (figure 11). 4. conclusions an insight into the development of novel strategies and devices for skull base defects was provided by integrating rheological/mechanical concepts, image capture and analysis techniques, the cad approach, and additive manufacturing. specifically, a systematic study of the design of multifunctional systems in the form of injectable tools and ‘solid’ customised devices was reported in the current research. the focus was set on the importance of viscoelastic properties and the flow behaviour of materials in the case of injectable systems, as well as on the potential to start from the geometry of skull base defects to design additive manufactured closure devices 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https://doi.org/10.1016/j.measurement.2016.02.018 https://doi.org/10.1016/j.dental.2012.06.005 https://doi.org/10.3390/en11113079 https://doi.org/10.3390/en11092303 https://doi.org/10.1163/1568562041753025 on the performance of opc ua and mqtt for data exchange between industrial plants and cloud servers acta imeko issn: 2221-870x june 2019, volume 8, number 2, 80 87 acta imeko | www.imeko.org june 2019 | volume 8 | number 2 | 80 on the performance of opc ua and mqtt for data exchange between industrial plants and cloud servers murilo silveira rocha1, guilherme serpa sestito1, andre luis dias1, afonso celso turcato1, dennis brandão1, paolo ferrari2 1 department of electrical engineering, university of são paulo, são carlos, brazil 2 department of information engineering, university of brescia, brescia, italy section: research paper keywords: open platform communication-unified architecture; message queuing telemetry transport; machine-to-machine (m2m); iot; industry 4.0; protocol comparison; data exchange citation: murilo silveira rocha, guilherme serpa sestito, andre luis dias, afonso celso turcato, dennis brandão, paolo ferrari, on the performance of opc ua and mqtt for data exchange between industrial plants and cloud servers, acta imeko, vol. 8, no. 2, article 11, june 2019, identifier: imeko-acta-08 (2019)-02-11 section editor: emiliano sisinni, university of brescia, italy received july 30, 2018; in final form june 17, 2019; published june 2019 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was supported by the laboratório de automação industrial, lai, from the school of engineering of são carlos. corresponding author: murilo s. rocha; murilo.silveira.rocha@usp.br, paolo ferrari; paolo.ferrari@unibs.it 1. introduction the process of digital transformation known as industry 4.0 is ongoing. its main objective is the implementation of a more advanced, flexible, and efficient manufacture [1]-[3]. in recent years, the use of sensors, actuators, controllers and supervisory systems has become common in industrial automation systems, with a typical communication infrastructure organised like a pyramid, where these layers can exchange data vertically from layer to layer. nowadays, on the contrary, the information flows as in a mesh communication structure, where the industrial automation devices are able to communicate with each other without a strict hierarchy. this architecture is the basis for a wider collection of information, with feature extraction, and enhanced knowledge about the process by means of intelligent systems and big-data techniques [4], [5]. the huge quantity of data to be exchanged between the industrial devices deployed in the field and the cloud servers imposes the use of new machine-to-machine (m2m) protocols, oriented to be more efficient and reliable. moreover, determinism and low complexity are two important characteristics required by embedded systems for industrial applications [6]. it is common, for the most diffused industrial automation protocols and networks to have some performance indicators that show their suitability for the demanding communication task in the industrial environment. robustness and determinism have been studied in general terms by several researchers. a specific focus on the estimation of communication delays can be commonly found [7]-[9]. the new m2m protocols that are now abstract the internet of things (iot) is a key technology in the development of industry 4.0. an increasing number of new industrial devices are expected to communicate with each other by means of local (edge) and cloud computing servers. in this article, two well-known protocols used for iot and industrial iot (iiot) are compared in terms of their performance when they are used to send/receive data to/from cloud servers. due to their wide diffusion and suitability, the considered protocols are open platform communication-unified architecture publisher-subscriber (opc ua pubsub) (purposely developed and maintained by industrial consortia) and message queuing telemetry transport (mqtt), the most well-known message protocol originally developed by ibm. the performance comparison is carried out considering the overall quantity of the data transferred (user payload plus overhead) and the roundtrip time required to send in data and receive a feedback message in return. the experimental results include the evaluation of several cloud computing server and application scenarios, highlighting how each protocol is particularly suitable for certain situations. finally, conclusions about the best choice for data exchange between devices are given. mailto:murilo.silveira.rocha@usp.br mailto:paolo.ferrari@unibs.it acta imeko | www.imeko.org june 2019 | volume 8 | number 2 | 81 starting to be used in industrial systems for internet connection with cloud servers should be evaluated with similar metrics, related to time, in order to estimate communication performance. a prime example of this approach is given in [10], [11]. on this subject, the current article expands the work in [12] about open platform communication-unified architecture (opc ua) and message queuing telemetry transport (mqtt) (two widely used protocols in industry 4.0 and iot applications [13]) with the aim of comparing them in the publish/subscribe mode of operation. the time characteristics of each protocol are discussed, and the distribution of the round-trip delay for sending and receiving messages is analysed in different scenarios. note that for opc ua, only the transport part of the specification is taken into account in this article. the article is organised in five sections. section 2 briefly introduces the two protocols, showing basic operation concepts for the sake of understanding the proposed tests. section 3 describes the proposed tests and their purpose. section 4 reports the results. finally, section 5 shows the overall results of the comparison between opc ua and mqtt. 2. brief introduction to mqtt and opc ua this section describes the two protocols used in the proposed tests (mqtt and opc ua), together with a brief introduction to the publish/subscribe model, which is used for data exchange in both protocols. 2.1. the publish/subscribe general model the publish/subscribe model has been introduced to facilitate communication between two or more devices. data exchange is organised according to ‘topics’. the publish/subscribe model is one of the most popular paradigms in the industry 4.0 environment [14]-[17], which is the reference scenario in this article. in the publisher-subscriber model, there are two kinds of stations: 1) the subscribers, which are interested in receiving the information, may subscribe to the topic of interest and then wait for new messages to come; and 2) the publishers, which wish to send information, may publish a message identified by a topic [18]. the publishing and the message distribution to subscribers may be done through a specific server, also called a broker [19], or by using some services offered by the transport layer (e.g. multicast communication). 2.2. overview of mqtt ibm designed a message protocol called mqtt for applications in the consumer market. currently, it is widely applied, for instance, to office and home automation and healthcare applications. mqtt is also attractive for low-power and low-latency applications, especially those based on wireless devices (e.g. smartphones) [20]. the protocol is designed for ‘transporting’ data; hence, no differentiation/organisation of data type is provided in the protocol specification. the coding/meaning/modelling of data is demanded by the participants in the mqtt data exchange. the mqtt architecture is completely built on the publish/subscribe model. for instance, a client cannot freely read variables, but it must wait until the information (topic) is published by the system [7]. in the mqtt specification, the device’s participation in the data exchange can either be as a server or a client. the server is the message broker who manages and delivers the messages. it is the centre of the architecture, and all clients are related to it. the mqtt server is also called the broker. on the other hand, a client can act as a publisher (sender) and/or subscriber (destination) of messages. mqtt defines the quality of service (qos) modes. the sender and the receiver of the messages can reach an agreement about the certainty of the delivery. three levels of qos are defined. qos 0 does not guarantee that the message reaches the destination, as shown in figure 1(a). in this case, the reliability of the data exchange depends only on the underlying transport protocol (which is usually tcp). qos 1 guarantees that the sent message arrives at the destination at least once. since the transaction ends only when receiver responds with the confirmation message back to the sender to acknowledge receipt of the message (as shown in figure 1(b)), it may happen that the message is sent more times to the receiver. furthermore, qos 2 guarantees that the message is delivered only once to the receiver. this is also the most complex transaction, since four messages are exchanged, as shown in figure 1(c). the message contains the data for the receiver; the acknowledgement message that informs the sender about receipt; the request for the sender to release the message; and finally, the confirmation from the receiver that the sender can release the message and close the transaction [8]. the mqtt overhead is expected to be minimal because since its beginning, ibm’s goal was to minimise it. the mqtt published message has a maximum of 9 bytes of overhead plus the ‘topic’ string. the acknowledgement/confirmation messages used for the qos implementation have a maximum size of 2 bytes. the mqtt is transported over tcp/ip, so another (20+20) bytes for the headers is required. the layer 2 overhead is 18 bytes for ethernet connections. in sum, a minimum overhead of about 60 to 80 bytes per message is highly probable. 2.3. opc ua in brief the opc ua is widely used in industry applications to interconnect equipment present in different networks or at different levels of the automation pyramid. this protocol provides object-oriented data modelling as well as organisation of the address space of these objects inside the server [21]. it allows the creation of variables of certain data types within each object on the server, allowing clients to read the variable values or to subscribe to the variables for which it would like to receive all updated values [22]. the transport layer of opc ua is based on tcp, and it allows for the binary encoding of data or the use of http with soap (simple object access protocol) (binary and xml). figure 1. mqtt publish flow for different qos (a) qos 0; (b) qos 1; (c) qos 2. acta imeko | www.imeko.org june 2019 | volume 8 | number 2 | 82 the opc ua was designed to use a client-server architecture, but recently (2017), the opc foundation released the opc ua pubsub specifications that enable full support for the publisher/subscriber model. the specification defines two modes of operations depending on the way in which the publisher/subscriber mechanism is implemented: the ‘brokerbased model’ or ‘broker-less model’. the former is based on message protocols (like the advanced message queuing protocol [amqp] or even mqtt) while the latter uses udp (user datagram protocol) multicast. binary or json (javascript object notation) encoding of data can be used. in this paper, only the opc ua pubsub functionalities implemented using the ‘broker-based model’ have been considered in order to be directly comparable with the mqtt. however, despite that opc ua offers many other functionalities, the main objective of this article is data exchange in the publish/subscribe model. therefore, it will not consider all the other features provided by the protocol. this choice, in turn, also limits the scope of the considered overhead to what is visible in the messages collected on the network at layer 2 (i.e. a black-box approach). in literature, [12] investigated the topic of an opc ua client server overhead, concluding that the encapsulation of data due to the structured information model and the security coding can cause an additional overhead (up to 15 times greater under some conditions) with respect to straight tcp binary encoding. for this reason, the overhead of opc ua pubsub is expected to be higher than mqtt, since the same opc ua data structure is maintained over the message protocol used by opc ua pubsub. 3. the proposed measurement methodology in order to achieve this study’s goal, the most recent investigation about delay estimation for data exchange across the cloud has been taken into account. since iot and industry 4.0 are hot research topics, there are several important concepts that must be considered: the estimation of internet delay is investigated in-depth in [23]-[26], and in particular, mqtt seems to be one of the most studied protocols, as shown in [27]-[30]. this section describes the measurement methodology used to compare the protocols by taking into account the previous achievements described in [31]-[34]. all the designed tests have been created to use only the publish/subscribe model to exchange data using the same procedure. regarding mqtt, all three available qos levels are used. 3.1. experimental setup architecture for the tests the experimental setup general architecture is shown in figure 2. it is composed by a sender-server machine connected to the internet. in the cloud, the virtual server implements the mqtt broker and the opc ua message distribution server. the network connection is monitored (at layer 2) by means of a network tap. all the traffic to and from the machine is recorded by a monitoring machine. the round-trip delays are measured using the local clock of the sender machine. since the duration of the measured delay is very small (a few hundreds of milliseconds) the local oscillators can be considered stable for the duration of the measurements (i.e. it is supposed that the shortterm stability of crystal oscillator is better than 10-5). 3.2. test 1: protocol overhead this test analyses all the data transmitted by the source machine in order to estimate the number of bytes to be transferred for transmitting a given useful payload. the monitoring station captures all the network frames and calculates the total number of bytes exchanged to carry out the message delivery for the transaction under test. then, the metric related to the protocol overhead can be computed as 𝑀1 = 𝑇𝑜𝑡𝑎𝑙 𝐵𝑦𝑡𝑒𝑠 𝑇𝑟𝑎𝑛𝑠𝑚𝑖𝑡𝑡𝑒𝑑 𝑆𝑖𝑧𝑒 𝑜𝑓 𝑃𝑎𝑦𝑙𝑜𝑎𝑑 . (1) the test is carried out several times in order to obtain meaningful statistics, and the payload size can be varied in order to achieve a deeper understanding of the protocols. 3.3. test 2: round-trip delay per telegram length this test aims to determine the time required by a message to reach a server/broker and then to be sent to a subscriber. hence, the overall delay is composed by two contributions that may make it difficult to measure. for this reason, the proposed metric is the round-trip time calculated by a client that publishes a topic/variable to whom the client itself is also a subscriber. in this way, all the necessary time references are taken by the same client. the procedure requires that once the publication is done, the t1 timestamp is saved in the source machine. then, when the message that comes back from the server/broker activates the callback function of the subscribed topic, the t2 timestamp is saved in the destination machine, which is also the source machine. finally, the round-trip time metrics are calculated by means of equation (2): 𝑀2 = 𝑇2 − 𝑇1. (2) 3.4. test 3: round-trip delay for different regions of the world the relationship between the location of the server and the round-trip delay is studied in test 3. the delay is expected to be dependent on the packet routing over the internet. this test involves five servers created in different locations (i.e. in different parts of the world) with the same architecture based on google cloud platform. in test 3, the round-trip time has been calculated with a message with a payload of 16 bytes. the metric for this test is the same for test 2, explained in section 3.3. the servers have been created in the following five locations: são paulo (brazil), oregon (usa), frankfurt (germany), tokyo (japan), and finally, a local server in the same laboratory network, são carlos (brazil) for comparison. 3.5. test 4: round-trip delay for multiple clients participating in the publish/subscribe protocol test 4 is designed to highlight the effect of an increasing number of subscribers. in order to verify if the server processing time for the distribution of the message to all clients (subscribed to the same topic/variable) varies when the number of clients increases, a test with n clients has been designed. given n clients, the first client (c0) is a subscriber of topic a, and all other clients (c1 to cn-1) are subscribers to topic b. figure 2. a proposed experimental setup for interaction with cloud servers. acta imeko | www.imeko.org june 2019 | volume 8 | number 2 | 83 in order to initiate one run of the experiment, client c0 publishes a 16-byte message on topic b and at the same time saves the t3 timestamp. when clients c1 to cn-1 receive the message from topic b, they instantly publish a message in topic a. client c0 waits for receiving all n-1 messages on topic a; then, it saves the timestamp t4. the total round-trip delay metrics can be calculated as: 𝑀4 = 𝑇4 − 𝑇3. (3) 4. experimental results the experimental setup described in section 3.1 and shown in figure 2 is customised differently depending on the protocol that is being tested. the different scenarios use opc ua and mqtt clients executed on a linux computer with a high-speed internet connection located at the university of são paulo in the city of são carlos (approximately 200 km from são paulo, brazil). on the contrary, the servers of both protocols are implemented by means of a virtual linux machine inside the google cloud platform. most of the experiments are on the google cloud server located in são paulo, brazil. however, for some tests, other locations of the google cloud server have been used (see the section on test 3 for further details). for the software implementation, python 3 has been used as a programming language. this article has used open-source libraries. in particular, the opc ua server and client (required to create the publisher-subscriber architecture) are based on the python-opc ua library with pubsub extensions. the mqtt server (broker) uses the hbmqtt library, while the mqtt client is built with the paho-mqtt library. the monitoring machine is a linux machine running the network capture software wireshark. the network uses ethernet layer 2. 4.1. test 1: experimental results test 1 requires assessing the overhead of the protocols varying the payload length. for the implementation, four different payloads (10 bytes, 100 bytes, 1 kb and 1 mb) have been considered. a graphical comparison of the results is shown in figure 3. the main point is that in the case of messages with small payloads, the protocol overhead is dominant. most of the transmitted bytes on the network (ethernet in the experiment cases) are used by other supporting protocols (e.g. ethernet, ip, and tcp headers) or by headers/trailers and acknowledgement/confirmation messages of mqtt and opc ua. mqtt qos 0 and 1 have clear advantages over the quantity of the transmitted bytes compared to mqtt qos 2 and opc ua. this is highly relevant in scenarios in which the main limit of the connection is the amount of data transmitted; for instance, when stations are using mobile network operators that charge depending on the total exchanged bytes. however, there are situations in which the behaviour of opc ua is very similar to that of the mqtt. it is in the case of mqtt with qos 2 in which the mqtt guarantees the message delivery only once to the subscriber. since this is the normal behaviour of opc ua, it is reasonable to expect the same value for the considered metric. as matter of fact, the ratio of the payload and the total size of the ethernet frame is practically the same in the two protocols. consequently, if the application scenario has poor quality connection between stations, and messages can be lost, the use of mqtt qos 2 or opc ua is strongly recommended, and both methods are equivalent from the point of view of their figure 3. overhead of the considered protocols as a function of the useful payload size. table 1. results of test 1 with the cloud servers located in são paulo. all values are in ms. payload size protocol mean median st. dev max min 100 bytes mqtt qos 0 6.08 6.08 0.57 28.63 5.42 mqtt qos 1 54.50 54.07 9.02 636.33 51.97 mqtt qos 2 65.91 65.50 5.76 512.41 62.78 opc ua 67.22 67.07 1.06 553.90 60.31 10 kb mqtt qos 0 8.62 8.34 1.46 47.10 7.70 mqtt qos 1 14.14 14.09 0.89 52.11 12.87 mqtt qos 2 25.63 25.47 3.19 331.98 23.63 opc ua 42.23 40.11 2.72 60.65 17.27 100 kb mqtt qos 0 35.60 35.21 5.00 385.32 33.56 mqtt qos 1 34.79 34.30 2.55 89.07 31.10 mqtt qos 2 46.57 46.15 2.44 88.47 43.72 opc ua 58.01 55.40 5.86 85.04 37.97 acta imeko | www.imeko.org june 2019 | volume 8 | number 2 | 84 measured performance. a typical situation in which these conditions are met are the wireless connections (or even mobile networks) with low received signal strength indicator (rssi) and unpredictable interference. 4.2. test 2: experimental results test 2 aims to calculate the round-trip time required by the protocols. in this test, the round-trip time for a self-published topic is calculated by changing the payload size between 100 bytes, 10 kb, and 100 kb. the round-trip times for the calculation of the statistics are shown in table 1, while the probability density function estimates are reported in figure 4, figure 5, and figure 6. each test considers 10000 samples, which means 10000 published messages with a 5 s wait interval between transmissions. the results show that the mqtt with qos 0 has advantages when transmitting smaller and medium packets of up to 10 kb. with larger packets, mqtt qos 0 and qos 1 perform the same, while mqtt qos 2 are beneficial for transmitting larger packets (more than 10 kb). the opc ua protocol is slower than the mqtt in all situations in terms of the round-trip time, and its distribution has a long tail and several peaks, demonstrating the presence of several sampling intervals and timeouts [35] inside the opc ua stacks. it should also be noted that the results for opc ua pubsub are in the same order of magnitude of the results obtained in [36] and [37] for the opc ua client-server architecture, demonstrating the inherent complexity of opc ua protocol stacks. in conclusion, for situations in which the speed in updating variables is a decisive factor in the process, and the size of this variable is small, the use of the mqtt protocol with qos 0 is suggested. it has round-trip time statistics with a lower mean, a smaller standard deviation, and (most importantly) a distribution with a single and narrow peak. 4.3. test 3: experimental results test 3 evaluates the influence of the cloud server location on communication performance. as in test 2, the same code was used to create the opc ua architecture and the mqtt broker on different locations by means of virtual machine instances hosted by google cloud platform. the test measured the roundtrip time for each one of these cloud servers, as shown in table 2, while the probability density function estimates are reported in figure 7, figure 8, figure 9, figure 10, and figure 11. comparing the different servers, it is clear that for this test, the delay is caused by the routing of the messages across the world. the round-trip time increases with the distance and the number of routers between the server and client. multiple peaks appear in the distribution of all the protocols because the cyclical behaviour of internet routers along the path is now relevant. the presence of multimodal distributions is an impairment in the use of such architectures in high-speed industrial control loops. it is clear that an mqtt with qos 0 again has the best performance for all the locations. the reason is that it does not need any kind of confirmation of receipt. the mqtt with qos 2 presented a similar result to opc ua, but the latter has the longest tails when the distributions are compared. figure 4. distribution estimate for the round-trip time of a single variable with a size of 100 bytes. figure 5. distribution estimate for the round-trip time of a single variable with a size of 10 kb. figure 6. distribution estimate for the round-trip time of a single variable with a size of 100 kb. acta imeko | www.imeko.org june 2019 | volume 8 | number 2 | 85 table 2. results of test 3 with cloud servers around the world. all the results are in ms. location of the cloud server protocol mean median st. dev max min local area network mqtt qos 0 5.33 5.31 0.46 8.19 4.87 mqtt qos 1 15.82 15.77 0.50 22.05 13.47 mqtt qos 2 25.44 25.51 0.53 27.51 22.91 opc ua 29.12 29.27 0.61 26.83 26.83 são paulo mqtt qos 0 33.80 31.69 1.82 204.50 25.89 mqtt qos 1 58.93 54.12 2.57 382.85 50.52 mqtt qos 2 62.80 57.78 2.82 357.82 59.50 opc ua 62.11 60.01 2.64 515.79 59.21 oregon mqtt qos 0 175.39 173.79 3.44 610.21 152.44 mqtt qos 1 210.02 211.52 3.17 759.37 189.17 mqtt qos 2 238.90 232.16 4.85 749.84 215.75 opc ua 251.61 241.11 4.11 812.40 222.65 frankfurt mqtt qos 0 243.27 243.12 2.88 437.77 218.94 mqtt qos 1 311.94 307.06 3.30 811.85 288.09 mqtt qos 2 372.61 365.61 5.53 756.41 352.76 opc ua 389.17 388.49 5.26 899.63 333.71 tokyo mqtt qos 0 327.79 317.27 7.79 760.25 295.48 mqtt qos 1 408.04 395.88 8.29 1025.10 385.04 mqtt qos 2 416.40 401.98 9.76 1135.43 398.56 opc ua 433.13 419.78 12.16 1235.61 401.58 figure 7. distribution estimate for the round-trip time of a server located in the same local area network. figure 8. distribution estimate for the round-trip time of a server located in são paulo (bra). figure 9. distribution estimate for the round-trip time of a server located in oregon (usa). figure 10. distribution estimate for the round-trip time of a server located in frankfurt (ger). acta imeko | www.imeko.org june 2019 | volume 8 | number 2 | 86 4.4. test 4: experimental results test 4 has been designed to verify the impact of the number of clients on the publisher/subscriber architecture of the two protocols. in order to perform the test, the message payload has been set to 16 bytes, and the number of clients connected to the topics/variables a and b has been varied. the results are shown in figure 12 in the form of a box-plot graph. there are seven groups depending on the number of clients used in the tests. each test uses 1000 samples, which means 1000 round-trip transactions as explained in section 3.5. it is interesting to note that for ten clients, the time between publishing on a topic and receiving the response of all clients on a second topic is practically the same for the mqtt (any qos) and opc ua. however, the behaviour changes as soon as the number of clients (subscribers) increases. starting from 100 subscribed clients, the considered metric begins to increase faster with the opc ua. the maximum value for opc ua is a response time of up to 1.2 seconds for 1000 subscribed clients, while for the mqtt, the worst case was less than 500 ms. in can be concluded that the current implementations of mqtt the protocol are more efficient for the distribution of messages in the publish/subscribe model when there is a large number of clients subscribed to the same topic. moreover, there are no considerable differences between the available qoss. on the other hand, the available opc ua implementations of the pubsub architecture suffer when the number of subscribers increases, showing once again the higher complexity of opc ua protocol stacks. 5. final remarks and recommendations the results of the proposed tests highlight that the mqtt has the advantage of using less data to transmit the same payload and slightly lower transmission times compared to opc ua. the most interesting result is the large difference between the two protocols when the transmission of same message to multiple clients is considered: the mqtt protocol has a great advantage (at least three times faster) in sending the messages to a large number of clients. when the topic/variable is small, the mqtt is better. when a small delay is important for an application, the mqtt has more advantages than opc ua. however, this paper has evaluated only the publish/subscribe model. the view of the opc ua protocol is therefore limited only to the data exchange part. the opc ua protocol stack is complex, and it has several other services besides data exchange (such as data modelling; address space; alarm and event management; variable history; and access control), which will be evaluated in future works. on the contrary, the mqtt is basically unstructured, and it implies the use of additional tools for the development of methods, for the definition of data types sent between devices, for the sequencing of messages, and for the creation of historical data services. even if it is clear that the comparison of complete solutions based on the mqtt or opc ua must take into account all the components and tools used, the results related to the data exchange presented in this article are fundamental, since they may constitute a reference for the best performance that is obtainable. 6. conclusions today, iot is the most important technology for the implementation of industry 4.0. new industrial devices communicate by means of local (edge) and cloud computing servers. in this article, opc ua and the mqtt (two well-known protocols used for iot and industrial iot) are compared in terms of performance when they are used to send/receive data to/from cloud servers. the performance comparison is carried out considering the overall quantity of data transferred (user payload plus overhead) and the round-trip time required to send in data and receive a feedback message in return. the measurement methodology was fully described, and the experimental results, including the evaluation of several cloud computing server and application scenarios, were reported. considering specific use case studies, the mqtt protocol was found to be faster than opc ua for pure data exchange. opc ua pays the complexity of its stack, which is also designed for ancillary services. generally speaking, the data exchange between industrial plants and cloud servers may take from less than 100 ms to more than 1 s depending on the internet path length, with lower values obtained by the mqtt qos 0 and the small size of the exchanged variables. figure 12. comparison of performance with the multi-client test. opc ua performance quickly worsens when the number of clients increases. figure 11. distribution estimate for the round-trip time of a server located in tokyo (jap). acta imeko | www.imeko.org june 2019 | volume 8 | number 2 | 87 7. references [1] l.d.xu, w.he, s.li, internet of things in industries: a survey, ieee transactions on industrial informatics, 10(4) (2014) pp. 2233-2243. 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[34] s.savage, a.collins, e.hoffman, j.snell, t.anderson, the end-toend effects of internet path selection, sigcomm comput. commun. rev., 29(4) (1999) pp. 289-299. [35] p.ferrari, a.flammini, d.marioli, a.taroni, f.venturini, ‘experimental analysis to estimate jitter in profinet io class 1 networks’, proc. of the ieee conference on emerging technologies and factory automation etfa, 2006, prague, czech republic, pp. 429-432. [36] s.cavalieri, f.chiacchio, analysis of opc ua performances, computer standards & interfaces, 36(1) (2013) pp. 165-177. [37] s.cavalieri, evaluating overheads introduced by opc ua specifications, in advances in intelligent and soft computing, 98 (2012) pp. 201-221 applying heuristics to the selection and prioritisation of security assessment items in software assessment: the case of iso/iec 27001 acta imeko issn: 2221-870x june 2019, volume 8, number 2, 12 20 acta imeko | www.imeko.org june 2019 | volume 8 | number 2 | 12 applying heuristics to the selection and prioritisation of security assessment items in software assessment: the case of iso/iec 27001 ferrucio de franco rosa1,2, mario jino2, paulo marcos siqueira bueno1,2, rodrigo bonacin1,3 1 cti renato archer, campinas sp, brazil 2 feec-unicamp, campinas sp, brazil 3 unifaccamp, campo limpo sp, brazil section: research paper keywords: assessment; security; standard; software; assessment dimension; security property; heuristics; coverage; ontology citation: ferrucio de franco rosa, mario jino, paulo marcos siqueira bueno, rodrigo bonacin, applying heuristics to the selection and prioritisation of security assessment items in software assessment: the case of iso/iec 27001, acta imeko, vol. 8, no. 2, article 3, june 2019, identifier: imeko-acta-08 (2019)-02-03 section editor: emiliano sisinni, university of brescia, italy received july 13, 2018; in final form june 11, 2019; published june 2019 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this study was financed in part by the coordenação de aperfeiçoamento de pessoal de nível superior – brasil (capes) – finance code 001 corresponding author: ferrucio de franco rosa, e-mail: ferrucio.rosa@cti.gov.br 1. introduction nowadays, cars are also computers on wheels. smartphones have become bank agencies, replacing even wallets and credit cards. interconnected devices have massively increased in prevalence. the new industrial revolution is characterised by strong automation based on artificial intelligence. in this environment of complexity and ubiquity, we are faced with serious attacks on important infrastructures that make use of smart devices [1]-[3]. fraud related to information security is commonplace. nevertheless, security receives increased attention only after software has been developed or even deployed [4], [5]. in this context, information security is a critical issue. security standards are essential instruments in the security assessment process [6], [7]. key questions to overcome security issues remain open, such as: which test cases are the most effective for security assessment? how can the effectiveness of a security assessment be verified? how can security assessments be made to be less dependent on human skills and intuition? systematic approaches based on quantitative analysis to support security assessment activities are needed to deal with current it complexity. coverage measures are important instruments in quantifying key security aspects in software testing. we propose the identification of security properties (e.g. confidentiality and availability) and assessment dimensions (e.g. business logic and system in runtime) covered by assessment items (e.g. test cases) from sources such as security standards, by using a security assessment ontology [8], [9]. a set of heuristics can be applied to the analysis, selection, and prioritisation of items from security assessment designs abstract security standards are essential instruments for security assessment. to create security assessment designs with suitable assessment items, we need to know the security aspects that are covered by a particular standard. we propose an approach to the selection and prioritisation of security assessment items. assessment heuristics are proposed, aiming to increase the coverage of assessment dimensions and security characteristics in assessment designs. the main contribution of this paper to the field is the set of security assessment heuristics. our approach can be applied to security standards in order to select or to prioritise assessment items with respect to 11 security properties and 6 assessment dimensions. the approach is flexible and allows the inclusion of security dimensions and properties. our proposal is applied herein to a well-known security standard, iso/iec 27001, and its assessment items were analysed. the proposal is intended to support (i) the generation of high-coverage assessment designs, by including security assessment items with assured coverage of the main security characteristics and (ii) the evaluation of security standards with respect to the coverage of security aspects. mailto:ferrucio.rosa@cti.gov.br acta imeko | www.imeko.org june 2019 | volume 8 | number 2 | 13 (ads). this systematic and measurable approach has the purpose of developing security ads, which aim to be comprehensive and low time-consuming. in this sense, this article proposes a set of security assessment heuristics to support the generation of high-coverage ads and the evaluation of the coverage of security standards. our proposal is intended to be applied in the context of measurable security assessments (encompasses test and verification) in measurable applications. we have applied it here to the well-known security standard (ss) iso/iec 27001 [10]. 2. literature review and related work a literature review has been carried out. articles were selected from the main research databases following known systematic review methods [11], [12]. related works with characteristics or objectives similar to the proposed approach were selected. a more extensive view of the related works and conceptual foundations can be found in [5], [9]. an integrative security knowledge model and a heuristic approach to security requirements is presented in [13]. a method based on natural language analysis is proposed to refine and adapt security knowledge. the approach aims to identify vulnerabilities in natural language requirements based on reported security incidents. the approach has two parts: (1) security assessment and (2) extraction of security knowledge. this work focuses on the identification of known vulnerabilities and their variations in natural language requirements. although the focus is technical aspects of security, the approach is also capable of incorporating aspects related to humans. important studies and their main security concepts are presented. however, the success of the approach depends on the quality of the security knowledge. a quantitative method of assessing and prioritising security is presented in [14], aiming to evaluate a specific security aspect (authenticity) for access control in web applications. the analytic hierarchy process is used as a mathematical tool to transform intangible measures into tangible measures. the main objective is to assess the security aspect authenticity for access control in web applications i.e. to evaluate the authentication process of a system in test runtime. the method does not consider other assessment dimensions or security issues, such as the operating environment, network, business rules, source-code analysis, availability, and integrity. security requirements for access control are presented by means of a checklist, and they can be used to support the security assessments of banking systems. a methodology for the automatic generation of it security metrics based on iso 27001 is presented in [15], aiming to enable organisations to assess compliance with information sss and the effectiveness of control implementations. it is based on the security ontology presented in [16]. a method of mapping the information security knowledge of the french standard ebios [17] and the german manual grundschutz [18] for a security ontology is presented in [19]. knowledge provided by these standards is transformed into web ontology language (owl) code. the proposed method allows for the reusing of information security knowledge bases (kbs) and for the mapping thereof to standardised data structures. according to the authors, although there are information security ontologies, no method has been proposed to map best practice guidelines or information sss to an ontology. the proposed approach is based on the nist handbook [20]. to simplify the mapping process, top-level threats (e.g. data disclosure, data tampering, and data loss) that affect security attributes (e.g. confidentiality, integrity, and availability) have been defined. the open source security testing methodology (osstmm) is proposed in [21] and aims to characterise operational security by examining and correlating test results consistently. an osstmm audit is an operational level security measure. a secondary objective is to provide guidelines to ensure that (i) the test has been conducted completely; (ii) the test includes all the necessary channels; (iii) the test complies with the law; (iv) the results are measurable; (v) the results are consistent and repeatable; and (vi) the results contain only facts derived from the tests. the risk assessment value (rav) is a scale measure of an attack surface. it is calculated by the quantitative equilibrium between limitations and controls. on this scale, 100 rav is a perfect balance; less than 100 represents a lack of control (or countermeasures) and therefore a large attack surface. more than 100 rav shows more controls than are needed, which can be a problem because controls often add interactions within a scope. there are also complexity and maintenance issues. our proposal, which is described in the next section, provides a feasible and systematic approach based on measurable coverage of security properties. 3. hcapp-sec – heuristics and a criteria-based approach to the selection and analysis of security assessment items when there is a large quantity of tests to do, assessment may become inaccurate, laborious, and expensive. clear criteria are required for the selection and prioritisation of assessment items according to the relevant security requirements. hcapp-sec is an approach to the selection and prioritisation of security assessment items, devised to support the development of security ads in a systematic, comprehensive, quantifiable way. it is rooted in assessment heuristics. measured diversity (from coverage measures) is ensured by considering security properties and possible perspectives (dimensions) in a security assessment. 3.1. security assessment ontology the approach makes use of conceptual formalisation by means of the security assessment ontology (secaonto) [8], [9]. the main objective thereof is to represent and formally structure the knowledge on security assessments. further information about secaonto can be found in [9], [22]. the essential concepts used in the context of this paper are described. we consider knowledge sources (ks) as either sss or other documents that can be used in security assessments. a ks is composed of a set of assessment items (ais) that can be treated as unities of knowledge. the 11 security properties (pps) initially considered are: availability; integrity; confidentiality; authenticity; nonrepudiation; traceability; privacy; auditability; legality; resilience; non-retroactivity. the six assessment dimensions (dms) considered are: business logic; system architecture; process; system in runtime; source-code structure; and operating environment. an ai is said to cover a pp or a dm if it explicitly evaluates that pp or dm. secaonto is applied to determine which pps and dms a given ai covers. acta imeko | www.imeko.org june 2019 | volume 8 | number 2 | 14 3.2. coverage measures the assessment results indicate that in most situations highdiversity sets achieve more efficiency and a larger coverage than those obtained by randomly generated sets of the same size [23]. we propose an approach for quantifying the coverage of security properties and assessment dimensions of each ai of security kss. it is used as the basic input to define the security assessment heuristics. by measuring the coverage of each ai, our approach enables the application of systematic and quantifiable heuristics, aiming at the development of comprehensive and low time-consuming security ads. for instance, an ai that covers availability, integrity, and confidentiality is more comprehensive than one that only covers integrity. the process of defining heuristics begins with coverage calculus. coverage calculus considers the diversity of the assessment scope (the dms) and the diversity of security aspects (pps), as well as their conceptual distances, proposed in adjacency matrices (i.e. distances between dms and distances between pps). these distances (the degree of diversity) were initially proposed by a security expert based on a previous literature review and their professional experience, and they were refined by four system analysts and security experts. their values range from 0.0 to 1.0. the aim of the adjacency matrices is to model the conceptual distances between pairs of the security concepts: pairs of pps and pairs of dms. for example, the property confidentiality has a distance value from authenticity and authenticity has a distance value from the integrity property. the rationale is that a greater diversity (and therefore the assessment comprehensiveness) is achieved when concepts are semantically dissimilar. for example, confidentiality and availability are distinct and distant concepts, whereas privacy and confidentiality are closer concepts, since privacy is confidentiality of personal information. the proposed assessment coverage measures, based on the distances between security properties and assessment dimensions, are: (i) coverage of dms (covdm) is a measure concerning the dms addressed by the ai. in equation (1), the numerator is the sum of the conceptual distances (or diversity) of all pairs of dms, and c is the total number of dms covered by the ai. the alpha constant () represents the total number of dms (equal to 6). hence, covdm is a value between 0 and 1. it is greater if more dms are covered, weighted by the diversity of covered dms. (ii) coverage of pps (covpp) is a measure concerning the pps addressed by the ai. in equation (2), the numerator is the sum of the conceptual distances (or diversity) of all pairs of pps, and c is the total number of pps covered by the ai. the beta constant () represents the total number of pps (equal to 11). hence, covpp is a value between 0 and 1. it is greater if more pps are covered, weighted by the diversity of the covered pps. (iii) local coverage of ais (covloc) is a measure of the coverage of the ai, taking the average between covdm and covpp. in equation (3), the numerator is the sum of the previous coverage values for the ai. therefore, covloc combines covdm and covpp. (iv) global coverage of ks (covglo) is a measure of the coverage of the ks, taking the average of covloc for all ais of the ks. in equation (4), the numerator is the sum of the coverage of all ais of a given ks; n is the total number of ais in ks. therefore, covglo is the average coverage of the ais in the ks. (v) total coverage of ads (covtot) is a measure of the coverage of the ad, taking the average of the covloc of all selected ais (sais) of kss. in equation (5), the numerator is the sum of the coverage of all ais in a specific ad; n is the total number of ais in the ad. therefore, covtot is the average coverage of the ais in the ad. we propose three classes of coverage measures, each class with a different application. measures (1), (2), and (3) refer to ais and can be used for the prioritisation and selection of ais that will be used. measure (4) characterises a ks and can be used for choosing kss according to the organisation security goals. measure (5) characterises an ad and can be used for choosing ads. covdm(dm)= ∑ ∑ d(dmi, dmj) c j=(i+1) c-1 i=1 α (1) covpp(pp)= ∑ ∑ p(pp i , pp j )cj=(i+1) c-1 i=1 β (2) covloc(ai)= (covdm +covpp) 2 (3) covglo(ks)= ∑ covloc(ai)ni=1 n (4) covtot(ad)= ∑ covloc(sai)ni=1 n (5) 3.3. heuristics for selecting the ais of sss heuristics are methods of solving problems. heuristic reasoning is good, but we cannot say it is suitable for rigorous proofs [24]. heuristics, in general, do not guarantee that the resulting set is the best set or even the smallest set; rather, this set has a high probability of being efficient concerning the requirements. the prioritisation techniques of test cases (ais) are typically heuristics [25]. the role of heuristics in the context of exploratory software assessment is discussed in [26]. according to the authors, every decision of an evaluator is made under conditions of uncertainty and insufficient knowledge. therefore, all decisions have some probability of being incorrect, and this probability means that we cannot mechanically choose the next thing to do. instead, since each decision brings an element of risk, we can use heuristics. even the process of evaluating a heuristic for its applicability can be an extremely useful process to help an evaluator think about what they are trying to achieve from different viewpoints and help them find a good solution to the problem [26]. in this work, we detail and apply the heuristics proposed in [27]. our heuristics have low computing costs for effective solutions, given that they consider the diversity of security aspects. in the context of this work, the term ‘assessment heuristics’ encompasses both test heuristics and verification heuristics. each heuristics process is an approximation to an objective. the proposed heuristics aim to select or prioritise (i) n better ais by considering covdm; (ii) n better ais by considering covpp; (iii) n better ais, by considering covloc; (iv) n better kss by considering covglo; (v) n better ads (selected ais) by considering covtot; (vi) n better ais by considering coverage (covdm, covpp, or covloc) above average; (vii) n better ais acta imeko | www.imeko.org june 2019 | volume 8 | number 2 | 15 by considering higher coverage with a lower amount of ais; and (viii) n better ais by considering covdm and covpp concurrently. with these objectives in mind, the following security assessment heuristics are proposed. coverage of ad heuristics (h-covdm) aims to select ais with more diversity in covdm. this heuristics method determines ais with a high range of dms by addressing at least one pair of dms among those with highest distance of a ks. hcovdm relies on covdm. this coverage is based on the diversity of the ai. if the ai addresses only one dm, the coverage value will be 0, but if it is more than one dm, its coverage value will depend on the distances between dms. the larger the distance, the greater the diversity of dms, and the higher the coverage value is. an example is ‘select the ten best ais by considering covdm’. coverage of pp heuristics (h-covpp) aims to select the ais with greater diversity in covpp. this heuristics method determines the ais with high range of pps by addressing at least one pair of pps among those with highest distance of a ks. hcovpp relies on covpp; this coverage is based on the diversity of the ai. if the ai addresses only one pp, the coverage value will be 0, but if it is more than one pp, its coverage value will depend on the distances between pps. the larger the distance, the greater the diversity of pps, and the higher the coverage value is. an example is ‘select the ten best ais by considering covpp’. in this case, there would be a cut line, and the ten ais with the higher covpp values would be selected. in this context, it is not guaranteed that the resulting set is the best set; rather, this is a set that is efficient and considers the diversity of pps. local coverage heuristics (h-covloc) aims to select the ais with the higher average of the sum of covdm and covpp (i.e. covloc). this heuristics method determines the ais among those with the highest arithmetic mean of covdm and covpp. consider three ais (a, b and c) with the following values: (a) covdm=0.450, covpp=0.450, then covloc=0.450. (b) covdm=0.900, covpp=0.0, then covloc=0.450. (c) covdm=0.450, covpp=0.800, then covloc=0.625. in this case, ai (c) will be selected. an example is ‘select the first ten ais by considering covloc’. in this case, there would be a cut line, and the ten ais with the highest covloc values would be selected. in this context, it is not guaranteed that the resulting set is the best set; rather, it is a set that is efficient and considers the diversity of ads and pps. global coverage heuristics (h-covglo) aims to select the kss with the highest average of the sum of the covlocs of all ais. this heuristics method determines the kss with the higher values of covglo. an example is ‘select the first two kss by considering covglo’. in this context, it is not guaranteed that the resulting set is the best set; rather, it is a set that is efficient and considers the average of the covlocs of each ks (covglo) of the database. total coverage heuristics (h-covtot) aims to select the ads with the higher values of covtot. as an ad is composed of sais of kss, this heuristics method selects the ads that have, in their set of ais, the highest average of the sum of covlocs (covtots). an example is ‘select the first two ads by considering covtot’. in this case, the two ads with the highest covtot values will be selected. in this context, it is not guaranteed that the resulting set is the best set; rather, this is a set that is efficient and considers the average of the covlocs of each ad (covtot) of the database. this heuristics method can be interesting, for example, when we want to reuse designs that have proven efficient in a certain context. in addition, experienced and robust ads may be candidates for becoming kss. above average heuristics (h-aboveavg) aims to select or prioritise ais that are above average by considering covdm, covpp, or covloc of all the ais from a specific ks. this heuristic is an approximation to the objective of identifying n ais that have higher coverage values by considering the average of one of the coverages (covdm, covpp, or covloc) as the cut line. some examples of using the heuristics method are ‘select all ais with above-average covdm values’; ‘select all ais with above-average covpp values’; ‘select the top five ais with above-average covdm, covpp, and covloc values’. in these cases, ais with higher coverage values are selected. in this context, it is not guaranteed that the resulting sets are the best possible sets; rather, they are efficient (based on the coverage, either covdm, covpp, or covloc) and consider the average as a cut line. this heuristics process can be useful, for example, when we need to create efficient designs in a certain context by considering the average. pareto percentage heuristics (h-paretopercentage) aims to select the ais among those with the best covloc of a ks. this heuristics process selects a set of ais among those with highest covloc and a lowest quantity of ais by disregarding the average (covglo), and it considers the total sum of the covlocs of all ais of a ks. examples are ‘select the ais that represent 40 % of the sum of the covlocs of a ks’ and ‘select all ais up to the optimum point of the sum of the covlocs of a ks’. in these cases, the ais with the higher values of covloc would be selected. in this context, it is not guaranteed that the resulting set is the best set; rather, it is efficient and considers the sum of all the covlocs of a ks of the database. pareto frontier heuristics (h-paretofrontier) aims to select the ais with the higher values by considering covpp and covdm. this heuristics process selects a set of ais among those with the best values of covdm and covpp concurrently (biobjective). consider two ais (a and b) with the following values: (a) covdm=0.450, covpp=0.450, then covloc=0.450. (b) covdm=0.900, covpp=0.0, then covloc=0.450. in this case, ai (a) will be selected or prioritised. some examples of scenarios are ‘select the ais with covdm and covpp > 0’, ‘select the ais with covdm and covpp > 0.400’, and ‘select the ais with covdm > 0.200 and covpp > 0.350’. in these scenarios, the ais with higher values of covdm and covpp at the same time would be selected. in this context, it is not guaranteed that the resulting set is the best set; rather, it is an efficient set and considers the values of covdm and covpp pairs of a certain ks of the database. in table 1, we present a synthesis of the proposed security assessment heuristics. acta imeko | www.imeko.org june 2019 | volume 8 | number 2 | 16 4. applying heuristics to iso/iec 27001 in this section, we present a case study of an application of hcapp-sec to the selection and prioritisation of ais for ads. among the well-known and accepted information security kss, iso/iec 27001 [10] was selected (identified as ks1). the proposed heuristics were applied to ks1. the complete dataset can be found in [28]. concerning usage examples of the heuristics, we expect (i) to verify the possibility of selecting better ais and (ii) analyse how heuristics can be used in selection or prioritisation based on the measures of coverage of dms and pps. table 2 presents an example of the application of the hcovdm heuristics, which resulted in the five best ais and the five worse (informative) ais of the ks1 dataset, according to the proposed heuristics. this heuristics attains higher diversity with respect to dms, based on covdm. table 3 presents an example of application of the h-covpp heuristics, which resulted in five best ais and the five worse (informative) ais of the ks1 dataset, according to the proposed heuristics. this heuristic attains high diversity with respect to pps, based on covpp. table 4 presents an example of an application of h-covloc heuristics and values for covdm and covpp, which resulted in the five best ais and the five worse (informative) ais of the ks1 dataset, by considering covloc. this heuristics method attains high diversity with respect to dms and pps, based on the average of covdm and covpp. h-covglo (table 5) aims to select or prioritise kss that attain the highest value of covglo i.e. it aims to select the kss that are the most comprehensive by considering the average covloc values for all the ais. covglo for ks1, used in this study case, is 0.252. we intend to compare this value with other kss in future works. this comparison can be useful for choosing, for example, which kss should be used in certain compliance assessments. one example of the heuristics process is ‘select the three of the best kss by considering h-covglo’. in table 5, a simulation of a selection of all the sources of the database is presented. in this hypothetical situation, we present the selected kss, highlighted based on h-covglo. the covglo values of the kss are simulated to demonstrate the use of the heuristics, with the exception of ks1, which presents the real value. h-covtot aims to select or prioritise the ads that attain high covtot (the average of the values of covloc for the selected ais). an example of an application of the above is ‘select the three best ads for considering covtot’. table 6 shows all three ads prioritised for covtot by using the covloc valueshaboveavg aims to select or prioritise among the most comprehensive ais of a ks by considering the ais with values above the average of covloc, covdm and covpp. an example of the heuristics process is ‘select the ais with values above the average, by considering covpp’. table 7 presents all the ais selected according to the h-aboveavg heuristics by using covpp; additionally, h-aboveavg can be used to select ais by using covdm and covloc. for ks1, the averages are: covdm = 0.220; covpp = 0.284; and covloc = 0.252. h-paretopercentage aims to select the smallest set of ais of a ks, which achieves high comprehensiveness percentage. the table 1. synthesis of the proposed security assessment heuristics. heuristics objective h-covdm n better ais, by considering covdm. h-covpp n better ais, by considering covpp. h-covloc n better ais, by considering covloc. h-covglo n better kss, by considering covglo. h-covtot n better ads (selected ais), by considering covtot. h-aboveavg n better ais, by considering coverage (covdm, covpp, or covloc) above average. hparetopercentage n better ais, by considering higher coverage with lower amount of ais. h-paretofrontier n better ais, by considering covdm and covpp at the same time. table 2. application of h-covdm to ks1. ks ai covdm obs. 1 11.6.2 0.967 better 1 12.3.2 0.850 1 11.7.1 0.650 1 11.5.6 0.650 1 10.3.1 0.650 1 6.1.8 0.000 worse 1 8.2.1 0.000 1 8.2.2 0.000 1 8.3.1 0.000 1 6.1.7 0.000 table 4. application of h-covloc to ks1. ks ai covdm covpp covloc obs. 1 12.3.2 0.850 0.673 0.761 better 1 10.10.1 0.483 1.000 0.742 1 15.1.3 0.483 1.000 0.742 1 14.1.3 0.483 0.900 0.692 1 11.6.2 0.967 0.336 0.652 1 6.1.8 0.000 0.045 0.023 worse 1 8.2.1 0.000 0.045 0.023 1 8.2.2 0.000 0.045 0.023 1 8.3.1 0.000 0.045 0.023 1 6.1.7 0.000 0.000 0.000 table 3. application of h-covpp to ks1. ks ai covpp obs. 1 10.10.1 1.000 better 1 15.1.3 1.000 1 13.2.3 1.000 1 15.1.5 1.000 1 9.2.4 0.982 1 10.10.2 0.018 worse 1 13.1.2 0.018 1 10.8.4 0.018 1 10.8.5 0.018 1 6.1.7 0.000 acta imeko | www.imeko.org june 2019 | volume 8 | number 2 | 17 goal is to achieve improved completeness, but with less effort (the number of selected ais). table 8 presents an example of an application of this process, which selects ais such that the sum of covloc values reaches 40 % of the total for ks1”. in figure 1, an application of h-paretopercentage to the ks1 dataset (covloc) is presented. as shown therein, after sorting the ais by considering the individual covloc values of all ais, a cut point can be identified i.e. a point at which a smaller number of ais presents a certain percentage of covloc, among all the ais of ks1. h-paretofrontier aims to select or prioritise ais considering both covdm and covpp at the same time (bi-objectively). after obtaining the sum of distances between dms and pps, values of covdm and covpp are obtained. table 9 presents an example of an application of the above, which selects ais that have covdm and covpp values that are greater than 0.430. in figure 2, we present a visualisation of the ais of ks1, with the values of covdm and covpp. in figure 2, we can see all of the ks1 ais plotted in the graph. the x-axis of the graph (values from 0 to 1) represents covpp. the y-axis of the graph (values from 0 to 1) represents covdm. in this view, we identify the points that can be selected or prioritised because they simultaneously achieve improved results by considering two coverage objectives (covdm and covpp). for example, in figure 2, the ais in the upper-right quadrant are more adequate when we consider the values of covdm and covpp at the same time. in [29] we detail the implementation of coverage calculus algorithms that calculate the coverage of security characteristics. this is an important step in the generation of security assessment criteria. we shared it in the github repository [30]. this table 5. application of h-covglo: selection of 3 kss of the database. id ks covglo <8> <fips (nist) (140-2)> <0.397> <5> <sans critical security controls> <0.373> <4> <owasp testing guide> <0.295> 2 iso/iec 15408 0.277 6 sbis/cfm moea 0.276 1 iso/iec 27001 0.252 12 slti/mpog eping-security 0.245 11 bacen/stn manual de segurança da rsfn 0.234 7 pci/dss 0.211 13 csa/caiq 0.198 9 sox audit checklist 0.194 10 cybersecurity capability maturity model (c2m2) 0.144 14 med-sec-awa checklist 0.128 3 mitre ten strategies of a csoc 0.110 table 6. ads selected or prioritised by considering h-covtot. ad ai ks covdm covpp covloc covtot 3 12.3.2 1 0.850 0.673 0.761 0.718 3 10.10.1 1 0.483 1.000 0.742 3 15.1.3 1 0.483 1.000 0.742 3 14.1.3 1 0.483 0.900 0.692 3 11.6.2 1 0.967 0.336 0.652 2 10.10.1 1 0.483 1.000 0.742 0.680 2 15.1.3 1 0.483 1.000 0.742 2 13.2.3 1 0.283 1.000 0.642 2 15.1.5 1 0.283 1.000 0.642 2 9.2.4 1 0.283 0.982 0.633 1 12.3.2 1 0.850 0.673 0.761 0.528 1 11.6.2 1 0.967 0.336 0.652 1 11.7.1 1 0.650 0.318 0.484 1 11.5.6 1 0.650 0.173 0.411 1 10.3.1 1 0.650 0.018 0.334 table 7. ais selected by considering h-aboveavg (covpp). id covpp id covpp 10.10.1 1.000 12.2.1 0.518 15.1.3 1.000 14.1.5 0.491 13.2.3 1.000 9.1.4 0.473 15.1.5 1.000 9.2.1 0.473 9.2.4 0.982 6.2.3 0.464 14.1.3 0.900 7.1.2 0.445 15.3.2 0.900 12.4.3 0.409 8.1.3 0.855 11.4.4 0.409 12.2.2 0.845 10.1.3 0.409 11.4.5 0.818 12.4.2 0.364 10.4.2 0.745 15.1.1 0.364 12.3.2 0.673 8.1.2 0.364 10.9.2 0.673 9.2.3 0.355 10.7.3 0.645 9.2.6 0.355 15.2.1 0.636 11.6.2 0.336 10.8.3 0.591 11.5.4 0.336 10.9.1 0.591 11.7.1 0.318 12.5.5 0.573 11.4.1 0.318 15.1.6 0.573 7.2.1 0.309 10.2.2 0.573 8.1.1 0.309 10.4.1 0.518 15.1.4 0.309 10.6.1 0.518 12.5.4 0.300 10.9.3 0.518 10.2.1 0.300 acta imeko | www.imeko.org june 2019 | volume 8 | number 2 | 18 implementation needs low computational resources, once the algorithms have quadratic time complexity (polynomial) considering a relatively small input (from the computational point of view) of words representing dm and pp. 5. discussion of the results by using the well-known ss (iso/iec 27001), we observed that the proposed approach is pragmatic and realistic, and it can be useful in real-world situations of security assessments. the h-covdm heuristics process attains high diversity with respect to dms. as shown in table 2, the best set of five ais by following h-covdm is: 11.6.2, 12.3.2, 11.7.1, 11.5.6, and 10.3.1. the h-covpp heuristics process attains high diversity regarding pps. as shown in table 3, the best set of five ais by following h-covpp is: 10.10.1, 15.1.3, 13.2.3, 15.1.5, and 9.2.4. as shown in table 4, by using h-covloc, it is possible to select the five best ais from the ks1 dataset based on the average of covdm and covpp. the best set of five ais by following h-covloc is: 12.3.2, 10.10.1, 15.1.3, 14.1.3, and 11.6.2. h-covglo attains a high covglo by considering the average of covloc values for all the ais. covglo for ks1 is 0.252. in table 5, the covglo values of the kss are simulated to demonstrate the use of the heuristics, with the exception of ks1. h-covtot aims to select the ais with the highest covtot of an ad, i.e. the average of the values of covloc for the selected ais. as shown in table 6, three ads were prioritised for covtot by using the covloc values. h-aboveavg aims to select or prioritise ais by considering the ais with values above the average of covloc, covdm, or covpp. in table 7, all ais selected are presented according to the h-aboveavg heuristics by using covpp. for ks1, the table 8. ais selected considering h-paretopercentage. id covloc % 12.3.2 0.761 2.27 % 10.10.1 0.742 4.49 % 15.1.3 0.742 6.71 % 14.1.3 0.692 8.77 % 11.6.2 0.652 10.72 % 13.2.3 0.642 12.64 % 15.1.5 0.642 14.55 % 9.2.4 0.633 16.44 % 15.3.2 0.592 18.21 % 10.4.2 0.589 19.97 % 11.4.5 0.551 21.62 % 10.4.1 0.542 23.24 % 12.2.2 0.514 24.77 % 12.5.5 0.511 26.30 % 15.1.6 0.503 27.80 % 12.4.3 0.488 29.26 % 14.1.5 0.487 30.71 % 11.7.1 0.484 32.16 % 10.9.2 0.478 33.59 % 10.7.3 0.464 34.97 % 15.2.1 0.460 36.34 % 10.6.1 0.451 37.69 % 8.1.3 0.444 39.02 % 10.8.3 0.437 40.32 % figure 1. application of h-paretopercentage to the ks1 dataset. table 9. ais selected by considering h-paretofrontier. id covdm covpp 10.10.1 0.483 1.000 15.1.3 0.483 1.000 14.1.3 0.483 0.900 10.4.2 0.433 0.745 12.3.2 0.850 0.673 12.5.5 0.450 0.573 15.1.6 0.433 0.573 10.4.1 0.567 0.518 14.1.5 0.483 0.491 acta imeko | www.imeko.org june 2019 | volume 8 | number 2 | 19 average covpp is 0.284. therefore, 46 ais were selected because the covpp values thereof are higher than the average. h-paretopercentage aims to achieve improved completeness, but with less effort. in table 8, we present the ais such that the sum of covloc values reaches 40 % of the total for ks1. as shown in table 8, by including the ai 10.8.3, we achieved the objective of 40 % (40.32 %). h-paretofrontier aims to select or prioritise ais considering both covdm and covpp at the same time (bi-objectively). table 9 presents the ais that have covdm and covpp values that are greater than 0.430. as shown in table 9, the best set of ais by following h-paretofrontier is: 10.10.1, 15.1.3, 14.1.3, 10.4.2, 12.3.2, 12.5.5, 15.1.6, 10.4.1, 14.1.5. 6. conclusion in this article, we presented the hcapp-sec coverage heuristics for selecting and prioritising ais. our proposal focuses on applying heuristics to make the use of sss systematic, ensuring that security characteristics are covered by the security ads; in this way, a small number of ais can be selected for testing a comprehensive set of security characteristics. our proposal can be applied to the selection and prioritisation of the ais of any ss. we considered 11 pps and 6 dms. other pps or dms can be included by making the additional conceptual formalisations and by defining the semantic distances from the new concepts to those already defined. hcapp-sec was applied to a well-known security ks (iso/iec 27001 – ks1). we characterised ks1 by identifying which dms and pps that their 133 ais addressed. we also presented the quantities of dimensions and properties addressed by all ais. all the proposed heuristics were exercised in the selection or prioritisation of the ais of security ads, thus demonstrating the feasibility of the application of our proposal. the approach proposed herein can assist security researchers in the generation of ads with assured coverage of key security characteristics and a priori assessment of security kss with respect to scope and security. specifically, the proposal can support (i) the selection or prioritisation of items with respect to the coverage and comprehensiveness of pps and dms; (ii) the selection of items that combine specific pps or dms; (iii) coverage analysis of items in an ad for more than one objective (scope or security); (iv) analysis of a source of security knowledge with respect to the coverage of important scope and security characteristics. in future work, we hope to include pps related to e-voting domains, such as anonymity, uniqueness, transparency, and noncoercibility [31]. since heuristics are approximations of objectives, one can think of replacing them with exact solutions that, for example, select the best set of 10 ais or one of the smaller sets of ais covering 80 % of the pps. our proposal is to develop algorithms to improve the selection of ais. for example, meta-heuristic methods can be applied to indicate sets of viable solutions. references [1] s. mertl, 'how cars have become rolling computers', the globe and mail, 05.03.2016. 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[accessed: 12.01.2017] [31] p. salini, s. kanmani, a knowledge-oriented approach to security requirements engineering for e-voting system, international journal of computer applications, 49(11) (2012) pp. 21–25. influences on torque measurement in nacelle test benches and their effect on the measurement uncertainty and consequences of a torque calibration acta imeko issn: 2221-870x september 2019, volume 8, number 3, 59 – 68 acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 59 influences on torque measurement in nacelle test benches and their effect on the measurement uncertainty and consequences of a torque calibration gisa foyer1, stefan kock2, paula weidinger1 1 physikalisch-technische bundesanstalt, bundesallee 100, 38116 braunschweig, germany 2 chair for wind power drives, campus-boulevard 61, 52062 aachen, germany section: research paper keywords: torque measurement; measurement uncertainty; nacelle test bench calibration; wind energy citation: gisa foyer, stefan kock, paula weidinger, influences on torque measurement in nacelle test benches and their effect on the measurement uncertainty and consequences of a torque calibration, acta imeko, vol. 8, no. 3, article 10, september 2019, identifier: imeko-acta-03 (2019)-01-10 editor: maija ojanen-saloranta, vtt technical research centre of finland, national metrology institute mikes, finland received august 23, 2018; in final form july 1, 2019; published september 2019 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work has been funded by the empir initiative which is co-funded by the european union’s horizon 2020 research and innovation programme and the empir participating states. corresponding author: gisa foyer, e-mail: gisa.foyer@ptb.de 1. introduction wind energy is a developing market initiated by a worldwide move towards renewable energies. this is leading to more as well as larger wind turbines, which must be tested prior to their market launch. several nacelle test benches (ntbs) have been constructed worldwide in the last few years. these ntbs aim to accelerate the testing phase of innovative concepts and enable an overall system test compared to component testing. one example of an ntb is shown in figure 1. it consists of a prime mover, a load application system (las), and a device under test (dut). the prime mover in figure 1 is an electrical direct drive; other ntbs have a geared drive that reduces the speed and increases the torque between the electrical drive and the dut. the las shown in figure 1 is not found in all ntbs. its purpose is the generation of additional load components (axial and lateral forces as well as bending moments) to the main shaft in order to simulate the mechanical loads acting on the rotor hub of a nacelle during field operation. a dut, which is normally a nacelle but can also be a gearbox or a main bearing, is connected to the main shaft of the ntb for testing. figure 1. example of a nacelle test bench: cwd aachen, 4 mw system test bench (source: cwd aachen) prime mover load application system main gearbox generator (load machine) device under test abstract torque measurement in the mn·m range is important in nacelle test benches, as it constitutes the primary value for the efficiency determination of wind turbine drive trains. however, no traceable measurement is possible above 1.1 mn·m. at the moment, alternatives to direct torque measurement in the mn·m range are, therefore, used. several of these torque measurement methods are presented and discussed in this paper with respect to traceability and measurement uncertainty. it was found that these methods cannot fully substitute direct measurements. therefore, detailed recommendations for a torque calibration procedure based on the specific conditions in nacelle test benches (such as rotation and additional mechanical loads) are given so as to enable the traceability of torque measurement using a torque transfer standard. mailto:gisa.foyer@ptb.de acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 60 the tests on ntbs generally include torque measurements to determine the input power to the dut. depending on the purpose of the measurements, a measurement uncertainty of 0.1 % for the torque measurement is envisioned [1]. however, the nominal torque load on most ntbs is in the multi-mn∙m range, e.g. 13 mn∙m at the fraunhofer institute for wind energy and energy system technology (iwes), 6 mn∙m at the centro national de energías renovables (cener), and 3.4 mn∙m at the center for wind power drives of rwth aachen university, whereas the largest torque standard is only able to exert torque loads up to 1.1 mn∙m [2]. moreover, the input torque to the dut should ideally be measured directly at the dut. this location is subjected to additional mechanical loads if an las is present on the ntb. due to these constraints, ntb operators use different methods to determine the torque that is applied onto the dut. these measurements are often not traced to national standards, and, sometimes, this is not even possible. even if traceability is ensured, the measurement uncertainty for the torque measurement is most likely to be > 0.1 %. however, for most ntbs, no uncertainty budget for the torque measurement exists at all. in september 2015, the ‘torque measurement in the mn∙m range’ project started within the framework of the european metrology programme for innovation and research (empir). it aims to improve the abovementioned situation by evaluating it and by enabling a torque traceability of up to 5 mn∙m [3]. this article is based on the preliminary outcomes of the project, and it describes the different methods of torque measurements on ntbs. the traceability and measurement uncertainty of these methods are discussed in order to enable the evaluation of the present measurement uncertainty of ntbs, which is otherwise not possible. based on this discussion, it is shown that a torque calibration is needed, and recommendations for a calibration procedure on ntbs using a torque transfer standard (tts) are given to form the basis for traceable torque measurements in the mn∙m range. 2. overview of current torque measurement methods on ntbs the main objective of torque measurements on ntbs is to determine the torque load to which the dut is submitted. therefore, a measurement directly between the las and the dut would be preferable. however, this is not possible for all the different torque measurements, as they do not tolerate additional mechanical loads or need be performed at another location. to account for the special requirements for the different torque measurement methods, alternative locations have been chosen. in figure 2, an overview of the locations for torque measurements on ntbs is given together with the three most common measurement methods based on [4]. in the following part, the three methods are described, and their location of application is discussed. direct torque measurement: a torque transducer consisting of a deformation body and strain gauges is used. this measurement method is feasible at any position of the main shaft of an ntb (see figure 2). torque loads occurring directly at the dut are in the multimn∙m range and, therefore, cannot be traced to any torque standard. moreover, the influence of the multi-component loading by the las cannot be taken into account. a torque measurement between a prime mover and an las (without a gearbox) still lacks traceability due to the large torque loads. the only currently available option for traceable measurements using direct torque measurement is to measure the torque load between the prime mover and the gearbox, as this is much smaller than at the dut (see figure 3). if this last option is used, the mechanical efficiency of the gearbox ηgear must be considered as an additional influence. force lever: a rotating force lever system consists of several force transducers that are connected to the main shaft of the ntb at a certain distance to the centre of rotation. technically, the same locations as for direct torque measurements are possible for force lever measurements. however, due to the increased measurement uncertainty compared to direct torque measurements, using force lever systems only makes sense at locations where direct torque measurement is not possible, e.g. due to traceability issues for multi-mn∙m measurements. these locations are either between the prime mover and the las figure 2. locations and methods of torque measurements in nacelle test benches figure 3. example of a gearbox in an ntb drive (source: cener) prime mover device under test n a c e ll e t e st b e n ch torque measurement electrical power direct torque* direct torque/force lever direct torque/force lever both with additional loads** *,** measurements depend on existence of some components mechanical components rotational speed for electrical power gearbox * load application system ** *,** optional components gearbox with gear ratio i torque m1 rotational speed n1 torque m2=m1∙i ∙ηgear rotational speed n2=n1/i prime mover device under test acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 61 without a gearbox or between the las and the dut. at the second location, the loads from the las must be transmitted by the force lever system as well. theoretically, a non-rotating force lever measurement at the prime mover in the form of a reaction force measurement would be possible. however, in practice, this is not sensible, as it entails a myriad of other issues from the location of the measurement to the quantification of effects, such as torque bypasses and additional mechanical loads. electrical power: instead of measuring the input torque to the dut, it is possible to calculate the input power based on the measurement of electrical power. for the calculation of the input torque, the measurement of rotational speed is also needed. the electrical power must be measured as the input to the prime mover. the rotational speed should be measured at the location for which the torque load is calculated: between the las and the dut. this difference between the locations of the two components involved in the torque calculation is the main drawback of this method. it entails influences such as friction and other losses between the place of measurement of the electrical power and the input to the dut. traceability of the abovementioned methods: not all of the described methods provide complete traceability for the torque measurement. direct torque measurement is only traceable up to 1.1 mn∙m, which is only sufficient for measurements between the prime mover and the gearbox. all measurements between the las and the dut (direct and force lever measurement) are influenced by the additional mechanical loads, which cannot yet be considered sufficiently. hence, these methods are not considered in the following section. 3. torque measurement uncertainty in ntbs without a torque calibration of the setup this section focuses on the estimation of the uncertainty of torque measurements in an ntb without prior calibration of the entire setup. as the torque measurement methods in ntbs differ greatly, several different influences and their effect on the measurement uncertainty must be accounted for. in addition, some specific influences are found in all ntbs. an overview of the influences on the torque measurement in ntbs is given in figure 4. in the following sections, the ntb-specific influences are discussed as well as the method-dependent influences. only traceable torque measurement methods that are not inflicted with a large temporal shift due to their location in respect to the dut are considered. this leaves two methods: the direct torque measurements between the prime mover and the gearbox with torque loads in the kn∙m range as well as the force lever measurements between the prime mover and the las without large additional loads, as measurements under these conditions would not be traceable. 3.1. general influences essentially, the measurements in ntbs are under the same influences as those in the reference torque standards using one or several calibrated transducers. however, the effects on the torque measurement are sometimes even larger, and additional contributions to the measurement uncertainty of torque measurement are found in ntbs. in [5], an overview of the measurement uncertainty in reference torque standards is given. this includes the uncertainty based on transducer calibration as well as the ambient conditions, such as temperature and humidity, which are often unstable in ntbs. a correction of the torque measurement based on a temperature and humidity measurement should therefore be made, which requires investigations of the measuring system under different conditions. another ntb-specific influence is rotation. a detailed analysis of the effect of rotation on the measurement results can be found in [6]. according to this paper, it can be assumed that the rotation influence for torque transducers in an ntb is mostly negligible, as the rotational speed is fairly small (< 25 turns per minute). only for torque transducers between the prime mover and the gearbox can an effect of up to 0.1 % occur, according to [6], due to the much higher rotational speed (400 – 700 turns per minute). an estimation of the effect of rotation on the force lever torque measurement was performed in [7] and was found to be < 0.1 %. figure 4. ishikawa diagram showing the influences of current torque measurement methods on the measurement results in nacelle test benches calculated torque displayed signal temporal shift meas.  calc. load cycle static  dynamic measurement installation conditions parasitical loads rotation/rotational speed gearbox orientation (compared to calibration) temperature environmental conditions humidity surrounding pressure measurement conditions amplifier connection/interconnection transducer uncertainty load application system measurement on high speed shaft force lever measurement load application system influences depending on the existence of certain components negligible influences acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 62 the influence of dynamic mechanical loading compared to a static calibration is extremely difficult to estimate and take into consideration. scenarios such as an emergency brake lead to impact torque loads that cannot be measured accurately with strain gauge-based transducers. in the case of small acceleration of the drive train or slight changes in the torque load, the effect on the measurement should be relatively small. however, as a dynamic calibration is not possible at the moment, dynamic loading has to be minimised. a measurement uncertainty for the torque measurement can therefore only be given for stationary states in the ntb e.g. only when a constant rotational speed is possible, not for braking scenarios. especially for the determination of the efficiency of the dut, only stationary loads are used. a further influence that can be observed in all ntbs is the load cycle, which differs greatly from the standard calibration cycle and which can have an effect on the torque measurement, e.g. by affecting the hysteresis. in general, this effect is relatively small, but if it is combined with alternating torques (which occur, for example, in brake tests), a larger effect on the torque signal and thus on the measurement uncertainty is expected [8]. the calibration of the transducers should therefore account for this influence. the influence of parasitical loads and a proposal to consider them are described in [8]. however, this approach is purely empirical, and more research is needed to include this influence into a torque uncertainty budget. the last general influence is caused by the amplifier. this should be chosen according to the targeted torque measurement uncertainty, meaning a sufficiently precise amplifier has to be employed. however, due to the large number of the electrical components of the ntbs, electromagnetic compatibility (emc) is an issue for the measurements. consequently, the emc effect on the amplifier signal should be checked. 3.2. influence of the las the las is responsible for the application of additional static and dynamic wind loads (e.g. thrust, radial forces, and bending moments). there are different construction principles of the las regarding the load application and the suspension of the ntb main shaft, e.g. load application by several preloaded hydraulic cylinders (figure 5) or by a hydraulic hexapod in the form of a stewart platform (e.g. 10 mw iwes dynalab). the suspension of the ntb main shaft can be designed with hydrostatic plain bearings or with roller bearings (e.g. arranged tapered roller bearings). hydrostatic plain bearings are more expensive, but their use is preferred because of the lower friction torque, lack of stick slip, lower deformation, longer life durability, and high reliability. even when no additional loads are applied, the las is used for stabilisation, leading to power losses and friction torque during rotation along the main shaft. the calculation of the power loss is crucial for torque measurement between las and drive (e.g. electrical input measurement, see figure 2). this loss is mainly caused by the ntb main shaft suspension and can be accounted for with an efficiency parameter ηlas, which depends on the design of the las, the rotational speed, and the torque on the drive train as well as the loads applied by the las. because of the abovementioned influences, the traceable measurement of the las friction torque under different operation conditions is not possible. however, the efficiency parameter ηlas can be estimated by analytical or numerical methods that are based on fluid dynamics. in the case of suspension with roller bearings, the bearing manufacturer provides analytical calculation methods for the estimation of friction torque. in the case of hydrostatic plain bearings, the calculation methods are based on the descriptions of the hagen-poiseuille flow and the corresponding drop of pressure in the plain bearing. for both the suspension methods of the ntb main shaft, the load-dependent parameters – such as the temperature and viscosity of the oil as well as lubricant thickness, local pressure, and the entire system friction coefficient – are estimated. consequently, the estimation of ηlas requires close consultation between the ntb operator and the las manufacturer. an exemplary estimation of the las effect on the torque measurement based on numerical simulation was made in [10]. there it was found that the analysed las leads to a friction torque of up to around 1 kn∙m for a 1.5 mn∙m torque load. assuming an uncertainty of 10 – 20 % for such a numerical estimation, it is possible to evaluate the overall effect of the las on the torque measurement uncertainty. 3.3. influence of the gearbox to determine the torque load at another part of the drive train, e.g. right at the dut, the influence of the gearbox has to be considered. for a measurement between the prime mover and the gearbox, this can be expressed in the following equation: m = mt⋅i⋅ηgear (1) with: m = input torque for dut mt = torque measured at transducer between the prime mover and the gearbox i = gear ratio ηgear = mechanical efficiency of the gearbox gear ratio i is a parameter that is determined by the layout of the gearbox. as this is a fixed constant, apart from small variations due to imprecisions in the manufacturing process, the uncertainty of the gear ratio i is extremely small. this can be neglected if the measurement values are averaged over an integer number of rotations of the shaft where the measurement is taken and, if possible, even over an integer number of rotations of the figure 5. load application system at cwd aachen (source: mts systems corporation) preloaded hydraulic cylinders ntb main shaft axis acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 63 shaft for which the torque load is calculated. ideally, several full rotations of the gearbox are used for the measurement. the mechanical efficiency of the gearbox ηgear is dependent on the load (torque and rotational speed) as well as on the condition of the gearbox (e.g. oil condition and temperature). therefore, precise information on ηgear for the entire load and condition range of the ntb is necessary. ideally, the measurement uncertainty is provided, too. this is, however, mostly not the case, and an uncertainty for ηgear must be assumed based on experience. if a transfer torque transducer for the mn∙m range is available, this assumption can be verified. an example of the measurement uncertainty of a torque measurement caused only by the presence of a gearbox is given in the following section. the assumed input is based on the general information known about ntbs and gearboxes. i is considered to be a constant factor without a measurement uncertainty: m = 4750 kn∙m mt = 100 kn∙m i = 50 ηgear = 0.95 measurement uncertainty of torque transducer: umt = 0.1 kn∙m (normal distribution) measurement uncertainty of gearbox efficiency: uη = 0.02/√3 (rectangular distribution) the standard measurement uncertainty of the calculated torque is: um =√umt 2 ⋅cmt 2 +uη 2⋅cη 2 (2) the sensitivity coefficients for the torque transducer and the gearbox efficiency are: cmt = i⋅ηgear cη = mt⋅i um =√ (4.75 kn⋅m)2 +(57.73 kn⋅m)2 = 58 kn⋅m (3) the expanded relative measurement uncertainty of the calculated torque is: wm(k = 2) = 2⋅ 58 kn⋅m 4750 kn⋅m = 2.4% (4) the standard uncertainty of the mechanical efficiency ηgear cannot be determined correctly, so the result has to be used with care. however, the effect of the uncertainty of the mechanical efficiency ηgear on the overall result is clarified, as it constitutes the main part of the overall torque measurement uncertainty. 3.4. influence of force lever measurement one way of avoiding the problem of the traceability of direct torque measurement in the mn∙m range is to employ a torque transducer consisting of force transducers at a fixed distance to the main centre of rotation. the number of force transducers is variable. a definition sketch for the setup of a force lever system and its parameters is shown in figure 6. sketch and parameter definition for a force lever system.. equation (5) presents the calculation of the resulting torque assuming equally distanced transducers. m = ∑ (ft,i⋅ri⋅ηcon) n i=1 (5) with: m = input torque for dut ft,i = force measured at transducers ri = distance of force measurement from the main axis n = number of force transducers used for the measurement ηcon = mechanical efficiency of the connection between the force transducers and the main shaft the measurement uncertainty of the force measurement can be based on a calibration. however, the influence of parasitic loads on the force transducer due to alignment errors and deformation under load cannot be sufficiently accounted for. some recommendations and a mathematical model for determining the additional measurement uncertainty for force measurements in the mn range were given in the emrp project called ‘force traceability in the meganewton range’ [11]. the influence of the slightly dynamic loading of the force transducers cannot be taken into account at all. the effect of the uncertainty of the distance r on the overall torque measurement uncertainty is very difficult to estimate, as r changes under load. this change ∆r is load-dependent, making it a systematic effect that cannot be measured. moreover, the value of r only decreases under pure torque load if the force transducers are under a tension load. as a correction of this effect is not possible without detailed measurements of r under load, ∆r must be estimated and taken as a direct contribution to the overall measurement uncertainty of m according to [12], [13]. ∆r is much bigger than the uncertainty of r itself, which can be obtained by a calibration, and it is also bigger than its own uncertainty. the influence of the connection between the force transducers and the main shaft can be accounted for using a factor ηcon. this factor can technically be treated similarly to ηgear. however, the effect on the force lever system torque measurement is expected to be much smaller as the effect is only indirect. moreover, ∆r has a much larger influence. therefore, ηcon can be considered to be negligible as long as ∆r cannot be quantified satisfactorily. in the following, an example of a calculation of the measurement uncertainty contribution caused by a torque measurement using a force lever system is presented. similar to the example of the influence of a gearbox given above, the values given are based on estimations: m = 4800 kn∙m n = 3 ft1 = ft2 = ft3 = 1600 kn r1 = r2 = r3 = 1 m figure 6. sketch and parameter definition for a force lever system. ft1 ft2 ft3 r1 r2 r3 m force trans ducer effecti ve l ever l ength acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 64 ηcon = 1 (neglected in the example) measurement uncertainty of distance r: ur = 0 (neglected in the example) measurement uncertainty of force transducers: uft = 1 kn (normal distribution) ∆r = 0.005 m (systematic error) the standard measurement uncertainty of the calculated torque is: um =√3⋅uft 2 ⋅cft 2 +3⋅δr2⋅cr 2 (6) the sensitivity coefficients for force transducers and the distance from the centre of rotation are: cft = r cr = ft um =√ 3⋅(1 kn⋅m)2 +3⋅(8 kn⋅m)2 = 14 kn⋅m (7) the expanded relative measurement uncertainty of the calculated torque is: wm(k = 2) = 2⋅ 14 kn⋅m 4800 kn⋅m = 0.6% (8) the influence of the change of distance r under load might have been overestimated in the example; however, the significance of the influence is clearly shown. for the overall torque measurement uncertainty, the uncertainties of r itself and of ∆r are not considered, as they should be comparatively small. even without these components, the expanded relative uncertainty of m results in wm(k = 2) = 0.6 %. in further studies within the same project framework, several force lever systems have been designed and analysed (see [7] and [14]). additional influences on the measurement that have been found in these studies are the additional mechanical loads and the temperature. these influences should be considered with care when using a force lever system in an ntb. especially for the additional mechanical loads, a specific design of the force lever system to transmit the load through the force lever system has to be chosen (e.g. roller bearing, as in [7]). 3.5. summary of influences on torque measurement and their effect on measurement uncertainty most of the influences shown in figure 4. ishikawa diagram showing the influences of current torque measurement methods on the measurement results in nacelle test benches and their effect on the uncertainty for torque measurements on ntbs can be estimated. the uncertainty is larger than the demanded 0.1 %, due to a variety of influences on the measurement. moreover, influences derived from an las, such as friction and additional loads, are still not accounted for, and a verification of the estimation is not possible. in summary, it can be stated that a calibration of the torque measurement on ntbs is necessary to obtain better precision of the torque measurements through improving the estimation of the measurement uncertainty, thus also improving the efficiency determination of nacelles. for the torque calibration of an ntb, a tts in the mn∙m range is required. moreover, a calibration procedure considering all occurring influences on ntbs is necessary. in the subsequent section, recommendations for such a procedure are summarised. 4. development of a torque calibration procedure for nacelle test benches based on the descriptions and discussions in previous sections, recommendations for a torque calibration procedure of ntbs are given in the following subsections. the calibration recommendations are based on euramet cg-14 [15] and iso 7500-1 [16]. the recommended procedure focuses on torque calibration without any additional mechanical loads but also includes a section on further tests to analyse the multicomponent effects on the torque measurement. 4.1. requirements for a torque transfer standard as was shown in the previous sections, it is possible to measure mn∙m torque loads in ntbs; however, the measurement uncertainty is not small enough for this purpose. therefore, it is inevitable that a tts should be used to calibrate the ntb. this transducer must be characterised over the torque range of the ntb. this was not possible before the start of the project ‘torque measurement in the mn∙m range’, but it was found to be a requirement for the improvement of the torque measurement in ntbs. within the project, two options of traceability with tts have been developed: extrapolation and force lever systems. these can be used for the purpose of calibrating the torque signal of an ntb. it is also advantageous to have knowledge of the behaviour under temperature (and humidity) of the tts, as the ntb calibration cannot be performed under laboratory conditions. furthermore, the transducer should withstand the occasionally occurring additional mechanical loads in the ntb, as it has to be installed directly in front of the dut. the general requirements for the tts are its weight, size, and adaptability to the ntb, which has to be checked for each tts and ntb combination individually. 4.2. general measurement conditions the tts has to be placed directly between the ntb and the dut (see figure 2. locations and methods of torque measurements in nacelle test benches) for the torque calibration measurements because the influences of an existing gear box or las (see sections 3.2 and 3.3) would otherwise have to be estimated. the alignment of the tts on the ntb has to be very precise, as misalignments can lead to additional permanent loads, such as axial force, lateral forces, or bending moments. furthermore, a same-time measurement of the ntb torque and tts signal is necessary either over one shared data acquisition system or two synchronised ones. moreover, the rotational speed has to be recorded in order to determine any influences due to rotation and to perform a calibration with calibration maps (see section 4.6). the torque signals (ntb and tts) and the rotational speed have to be recorded in an adequate frequency, taking into account the rotational speed of the test bench and the envisioned resolution of the encoder or rotation speed signal. all results should be averaged for each load step for at least one turn of the main shaft. an averaging over a larger number of turns results in the filtration of mechanical noise but also makes time-dependent effects like creeping or an instable test bench control invisible. good knowledge of the ntb behaviour is, therefore, essential for determining the precise number of turns. as the recorded (non-averaged) data can be used to determine any temporal phase shifts between the measurements, it should be stored in order to also analyse the directly measured signals for the entire testing time. acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 65 it should be ensured that the measurement and recording of humidity and temperature values are as close to the tts and the ntb-torque measurement as possible during the measurement. ideally, the temperature gradient on the tts should be measured. all transducer signals should be corrected for changes in the environmental conditions (see [5]). to ensure relatively stable environmental conditions, the ntb should be in operation with the entire setup prior to the calibration. as the calibration of an ntb is not possible without a dut, a suitable test object should be available for the calibration. ideally, a dut with full control access is used for the calibration, as several scenarios depend on the controlled change of electrical braking power asserted by the generator. an alternative to using an entire nacelle for the calibration is using only a generator. this often provides the necessary access to the control but comes with the disadvantage of not being able to perform full multicomponent measurements, as the generator is not built for this purpose. 4.3. preparatory tests as there are always high-powered electrical machines in multimw ntbs, the emc of the tts should be tested by purposely causing possible interference e.g. a test with a tts in an uninstalled situation prior to installation. furthermore, the emc for the ntb-torque measurement should be ensured. it is also advised that some general test with a range of torque loads and rotational speed in advance of the torque calibration should be undertaken to optimise the control system, as the additional tts leads to a change in the mechanical behaviour of the entire setup. furthermore, the influence of the warming up of the ntb can be studied during this time if no reliable knowledge of the behaviour is available. 4.4. determination of the zero signal a crucial step in the calibration of the ntb torque is the determination of the zero signal. the quality and correctness of the signal affect all other results during the calibration. there are two main issues in determining the zero signal. firstly, the tts cannot be dismounted after each measurement cycle; therefore, the zero signal has to be established with the entire setup. secondly, the transducer is loaded by a bending moment and a lateral force due to its own weight, which is different to the situation under rotation, where the averaging of the signal of an integer number of turns leads to the deletion of this effect. two options are suggested to account for these issues: static and rotational zero point determination. ideally, both are performed for comparison purposes. 4.4.1. static zero signal all zero signals of the ntb torque measuring device and the tts have to be determined as an average over differently rotated positions with respect to the measurement axis. the rotation should be performed for the entire main shaft of the ntb without the mounting and dismounting of the tts. depending on the measurement method, 6∙60 ° rotations (force lever system) or 3∙120 ° rotations (direct torque measurement) are suitable. this option is visualised in figure 7(a). the static zero signal can be calculated using the following equations: szero.stat = 1 p⁄ ⋅ ∑ s̅i p i=1 with s̅i = 1 n⁄ ⋅ ∑ sj n j=1 and n = tmeas⋅fsample (9) where p is the number of measurement points, tmeas is the duration of each measurement, and fsample is the sampling frequency of the measurement. 4.4.2. rotational zero signal a rotational zero-point determination can be made either before connecting the tts to the dut or with the entire setup. in both cases, the signal is recorded over an integer number of turns and averaged in the postprocessing (see figure 7(b)). the disconnected option has the advantage that no friction torques occur, but it has the disadvantages of the transducer needing to be disconnected, which cannot be ensured between calibration sequences, and of the installation condition not being exactly the same as the condition during the calibration. the connected option leads to an additional friction torque but can easily be performed between tests. in [17], it was found that the friction torque in the connected zero-signal procedure can be minimised by employing the dut generator as a second engine to compensate for the losses in the drive train. for this control mode, full access to the ntb control is required. the rotational zero signal can be calculated using the following equations: szero,rot = 1 m⁄ ⋅ ∑ si m i=1 with m = nmin⋅k⋅fsample (10) where nmin is the minimum speed of rotation, k is the number of turns of the drive train, and fsample is the sampling frequency of the measurement 4.5. quasi-static torque calibration to achieve traceability to the tts calibration and for a comparison with the static calibration of the tts, it is advised that a quasi-static calibration of the ntb should be performed first. this is a normal torque calibration according to [15], under a constant minimum rotational speed. instead of using three different installation positions as requested by [15], three repetitions without dismounting the tts are sufficient. the repeated installation of the tts would be helpful but is not necessary, as the tts is rotated during the calibration, and the control system applies additional lateral forces and bending moments. figure 7. explanatory sketch of the procedure for zero signal determination: a) static zero and b) rotational zero. a) stati c zero s i gnal b) rotati onal zero si gnal x y z x y z acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 66 4.6. calibration maps identifying the influence of the rotational speed on the torque measurement is an important part of the calibration of an ntb, as most ntbs cannot apply torque loads without rotating. a simple solution to account for this influence in the ntb calibration is the use of calibration maps. this is a combination of all possible torque and rotational speed loads. a similar procedure has been suggested for rotary power measurements [18]. in the first step, the typical testing range (rotational speed and torque values) of the ntb has to be determined. this is represented by the blue rectangle in figure 8. the calibration should then cover the entire range by repeating the standard calibration with different combinations of torque load and rotational speed, as shown by the crosses in figure 8. if this procedure is not possible due to insufficient access to the control values, the calibration must be performed in the median of the typical testing range requesting a standard dut. load steps for the torque calibration should be based on the typical maximum torque loads of the ntb for different duts. several standard calibrations, one for each typical load, should be performed. the increments of the load steps should also represent the typical ntb situation; e.g. if a torque load of 4 – 5 mn∙m is typical, more load steps in this range and fewer below 4 mn∙m are required. it is reasonable to omit rotational speed steps that are near to an eigenfrequency of the system and would lead to dynamic effects on the torque signal. during the performance of one calibration map test, each measurement point should preferably be reached twice to evaluate the hysteresis. the order of the measurement points within the calibration map must be defined with care, as control influences might lead to different results depending on the order. in figure 9, two examples of the resulting load-step sequences are given, where the torque load is the fixed parameter, and the rotational speed is altered on each torque step. it is also possible to fix the rotational speed and alter the torque load on each speed step. thus, several different measurement sequences based on the same basic calibration map are possible. generally, the typical load sequences of the ntb under normal conditions should be used to decide for a load sequence. each load step has to be held for the time that the ntb needs to stabilise its control plus a minimum of six rotations. a longer measurement time would also give an indication of the stability of the ntb control. a repetition of the calibration map sequence of at least three times is recommended to determine the repeatability of the ntb. 4.7. multi-component effects on the torque measurement it is expected that additional mechanical loads of the ntb cannot be ignored. a deviation into purposely applied loads and loads due to control issues has to be made. it is assumed that the occurrence of las-induced loads due to control issues is random if the setup is operated in pure torque mode for the calibration. their effect is therefore accounted for in the torque measurement uncertainty within the part of repeatability. if the aim of the calibration is an additional calibration of the ntb torque under larger multi-component loads, which are applied on purpose, and a similar procedure for this calibration as for the effect of rotational speed is proposed. examples for torque and axial force as well as for torque and axial and lateral force are given in figure 10. 4.8. additional tests if the ntb is used for a wide range of duts, it would be sensible to perform calibrations in the full torque range of the ntb and in partial ranges. ideally, different duts are used for the different ranges. most ntbs have one main direction of rotation. therefore, only one direction of torque loading is needed. however, the figure 8. schematic drawing of a calibration map based on the typical testing range of an ntb and with a uniform distribution of the measurement points. figure 9. example of two different load-step sequences for a calibration with torque being increased (and decreased) in one (two) staircase(s) and increasing and decreasing rotational speeds on each torque step. figure 10. example of a load sequence for the combination of lateral force, axial force, and torque load. t o rq u e rotati onal speed typi cal testi ng range meas urement poi nt acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 67 influence of possible zero-crossings on the torque measurement system of the ntb should be tested, e.g., by torque loads in the opposite direction. the zero-return and the hysteresis can thus be determined. furthermore, the influence of braking scenarios on the torque measurement should be tested and analysed. these tests should not be included in the calculation of a torque measurement uncertainty as long as they are not required in a typical loading scenario that is in need of high-precision torque measurement. continuous measurement to determine creeping is suggested. this can be performed by recording the torque signals after the end of a calibration map for several minutes. 4.9. evaluation as already mentioned on several occasions in this section, it is important to use averaged torque values over an integer number of rotations for the evaluation. a constant number should be chosen for all evaluations. it is almost impossible to predict all resonance frequencies of such a complex experimental setup in advance; therefore, the resulting data has to be checked for possible dynamic influences. this data should best be deleted from the evaluation. no class definition should be made. an evaluation of all the determined components, indication deviation, and measurement uncertainty is necessary. 5. conclusions this paper gives an overview of torque measurement methods in ntbs and specific influences on the torque measurement uncertainty. it was found that the current measurement methods do not fulfil the envisioned measurement uncertainty of ntb operators. therefore, a torque calibration with a tts is inevitable. in the final section, a detailed description of recommendations for performing and evaluating such a calibration are given based on existing calibration guidelines. the project ‘torque measurement in the mn∙m range’ addressed several of the issues that were discussed in this article. the main outcome is the test of a torque calibration of an ntb. a 5 mn∙m tts with multi-component capabilities was calibrated up to 1.1 mn∙m [19], and an extrapolation to 5 mn∙m was applied [20]. this tts was then installed in the 4 mw ntb at the cwd aachen, germany. results and a detailed description of the measurement campaign can be found in [17] and [21]. the torque measurement uncertainty of force lever systems has been further investigated within the project, and recommendations for the design and determination of a measurement uncertainty are given. several publications on this topic have been made ([7], [14], [22]) and are being made ([23][25]) to develop a design that fulfils the requirements of ntb operators. acknowlegements the empir initiative is co-funded by the european union’s horizon 2020 research and innovation programme and the empir participating states. the discussions and input of all the partners in the project as well as of their colleagues are greatly appreciated. this paper is an extended version of a conference contribution for the 2017 tc3 imeko conference in helsinki, finland [1] references [1] s. kock, g. jacobs, d. bosse, p. weidinger, torque measurement uncertainty in multi-mw nacelle test benches, proc. of the conference for wind power drives, aachen, germany, 7-8 june 2017. [2] d. peschel, d. mauersberger, d. schwind, u. kolwinski, the new 1.1 mn·m torque standard machine of the ptb braunschweig / germany, proc. of the 19th conference on "force, mass and torque measurement", cairo, egypt, 19 23 february 2005. online [accessed 16 september 2019]: https://www.imeko.org/publications/tc3-2005/imeko-tc32005-kl-001u.pdf [3] empir project "torque measurement in the mn·m range". online [accessed 16 september 2019]: https://www.ptb.de/emrp/torquemetrology.html [4] s. kock, d. bosse, n. eich, g. foyer, n. medina, j. m. quintanilla crespo, l. vavrecka, j. ala-hiiro, report on existing nacelle test benches and their boundary conditions, deliverable in empir 14ind14 project [unpublished] 2016. [5] d. röske, messunsicherheiten bei der drehmomentmessung mit referenzmesseinrichtungen, tech. mess. 78, 2 (2011) pp. 77-87 [in german]. [6] a. brüge, influence of rotation on rotary torque transducers calibrated without rotation, proc. of the xiv imeko world congress, tampere, finland, 2-6 june 1997. [7] s. kock, g. jacobs, d. bosse, j. gnauert, conception of 5 mnm torque transducer for wind turbine test benches, conference "5. tagung innovation messtechnik", vienna, austria, 11 may 2017. [8] d. röske, d. peschel, investigations into the alternating torque calibration of torque transducers, proc. of the xiv imeko world congress, tampere, finland, 2-6 june 1997. [9] g. wegener, j. andrae, measurement uncertainty of torque measurements with rotating torque transducers in power test stands, proc. of the 19th conference on "force, mass and torque measurement", cairo, egypt, 19 23 february 2005. online [accessed 16 september 2019]: https://www.imeko.org/publications/tc3-2005/imeko-tc32005-049u.pdf [10] s. kock, g. jacobs, d. bosse, g. foyer, influences on mn·m torque measurement in multi-mw nacelle test benches, proc. of the dewek 2017 13th german wind energy conference, bremen, germany, 17-18 october 2017, poster. [11] emrp project: "force traceability within the meganewton range wp4 improved traceability". online [accessed 13 october 2016]: http://www.ptb.de/emrp/2387.html [12] i. h. lira, w. wöger, the evaluation of standard uncertainty in the presence of limited resolution of indicating devices, 1997 meas. sci. technol. 8 441. [13] jcgm 100:2008: joint committee for guides in metrology, evaluation of measurement data — guide to the expression of uncertainty in measurement, 2008. [14] r. m. lorente, n. medina, m. a. sáenz, m. á. sebastián, torque traceability for nacelle’s test benches: a design proposal, proc. of the 23rd imeko tc3, helsinki, finland, 30 may 01 june 2017. online [accessed 16 september 2019]: https://www.imeko.org/publications/tc3-2017/imeko-tc32017-004.pdf [15] euramet e.v., guidelines on the calibration of static torque measuring devices, 2011. [16] beuth, din en iso 7500-1 metallic materials calibration and verification of static uniaxial testing machines part 1: tension/compression testing machines calibration and verification of the force-measuring system. [17] p. weidinger, g. foyer, s. kock, j. gnauert, r. kumme, development of a torque calibration procedure under rotation for nacelle test benches, conference "the science of making torque from wind", milano italy, 20-22 june 2018. [18] a. brüge, h. pfeiffer, a standard for rotatory power measurement, acta imeko 8 (2019) 3, pp. 48-58. https://www.imeko.org/publications/tc3-2005/imeko-tc3-2005-kl-001u.pdf https://www.imeko.org/publications/tc3-2005/imeko-tc3-2005-kl-001u.pdf https://www.ptb.de/emrp/torquemetrology.html https://www.imeko.org/publications/tc3-2005/imeko-tc3-2005-049u.pdf https://www.imeko.org/publications/tc3-2005/imeko-tc3-2005-049u.pdf http://www.ptb.de/emrp/2387.html https://www.imeko.org/publications/tc3-2017/imeko-tc3-2017-004.pdf https://www.imeko.org/publications/tc3-2017/imeko-tc3-2017-004.pdf acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 68 [19] c. schlegel, h. kahmann, r. kumme, mn · m torque calibration for nacelle test benches using transfer standards, acta imeko 5 (2016) 4, pp. 12-18. online [accessed 16 september 2019]: http://dx.doi.org/10.21014/acta_imeko.v5i4.414 [20] p. weidinger, g. foyer, j. ala-hiiro, c. schlegel, r. kumme, investigations towards extrapolation approaches for torque transducer characteristics, proc. of the xxii imeko world congress, belfast, united kingdom, 3-6 september 2018. online [accessed 16 september 2019]: http://dx.doi.org/10.1088/1742-6596/1065/4/042057 [21] p. weidinger, g. foyer, s. kock, j. gnauert, r. kumme, procedure for torque calibration under constant rotation investigated on a nacelle test bench, conference "sensoren und messsysteme", nuremberg, germany, 26-27 june 2018. [22] g. foyer, h. kahmann, design of a force lever system to allow traceable calibration of mn ∙ m torque in nacelle test benches, conference "sensoren und messsysteme", nuremberg, germany, 26-27 june 2018. [23] r. m. lorente, n. medina, m. a. sáenz-nuño, m. a. sebastiánpérez, study of influences in cem’s new transfer standard for torque measurements in the mnm range, proc. of the xxii imeko world congress, belfast, united kingdom, 3-6 september 2018. online [accessed 16 september 2019]: http://dx.doi.org/doi:10.1088/1742-6596/1065/4/042039 [24] j. gnauert, g. jacobs, s. kock, d. bosse, measurement uncertainty estimation of novel torque transducer for wind turbine test benches, proc. of the xxii imeko world congress, belfast, united kingdom, 3-6 september 2018. online [accessed 16 september 2019]: http://dx.doi.org/10.1088/1742-6596/1065/4/042050 [25] g. foyer, h. kahmann, a finite element analysis of effects on force lever systems under nacelle test bench conditions, proc. of the xxii imeko world congress, belfast, united kingdom, 3-6 september 2018. online [accessed 16 september 2019]: http://dx.doi.org/10.1088/1742-6596/1065/4/042006 [26] g. foyer, s. kock, measurement uncertainty evaluation of torque measurements in nacelle test benches, proc. of the 23rd imeko tc3 conference, helsinki, finland, 30 may 1 june 2017. online [accessed 16 september 2019]: https://www.imeko.org/publications/tc3-2017/imeko-tc32017-010.pdf http://dx.doi.org/10.21014/acta_imeko.v5i4.414 http://dx.doi.org/10.1088/1742-6596/1065/4/042057 http://dx.doi.org/doi:10.1088/1742-6596/1065/4/042039 http://dx.doi.org/10.1088/1742-6596/1065/4/042050 http://dx.doi.org/10.1088/1742-6596/1065/4/042006 https://www.imeko.org/publications/tc3-2017/imeko-tc3-2017-010.pdf https://www.imeko.org/publications/tc3-2017/imeko-tc3-2017-010.pdf microsoft word article 8 tc4 paper 4.docx acta imeko  august 2013, volume 2, number 1, 21 – 26  www.imeko.org    acta imeko | www.imeko.org  august 2013 | volume 2 | number 1 | 21  development of a fast and reliable system for the automatic  characterization of giant magnetoimpedance samples  eduardo c. silva 1 , joão h.c.c. carneiro 2 , luiz a.p. gusmão 1 , carlos r.h. barbosa 3 , elisabeth c.  monteiro 3   1  department of electrical engineering, pontifícia universidade católica do rio de janeiro, rua marquês de são vicente 225, 22451‐900 rio  de janeiro, brazil  2  pee ‐ coppe/ufrj, universidade federal do rio de janeiro, centro de tecnologia ‐ bloco h, sala 321, 21941‐972 rio de janeiro, brazil  3  postgraduate program in metrology, pontifícia universidade católica do rio de janeiro, rua marquês de são vicente 225, 22451‐900 rio  de janeiro, brazil      keywords: giant magnetoimpedance; automatic characterization system; impedance measurements; magnetic sensors  citation: eduardo c. silva, joão h.c.c. carneiro, luiz a.p. gusmão, carlos r.h. barbosa, elisabeth c. monteiro, “development of a fast and reliable system for  the automatic characterization of giant magnetoimpedance samples”, acta imeko, vol. 2, no. 1, article 8, august 2013, identifier: imeko‐acta‐02(2013)‐01‐ 08  editors: paolo carbone, university of perugia, italy; ján šaliga, technical university of košice, slovakia; dušan agrež, university of ljubljana, slovenia  received january 10 th , 2013; in final form march 16 th , 2013; published august 2013  copyright: © 2013 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: this work was supported by the brazilian funding agencies cnpq and faperj  corresponding author: eduardo c. silva, e‐mail: edusilva@ele.puc‐rio.br    1. introduction  the successful use of giant magneto-resistive (gmr) materials in the construction of precision magnetometers in the area of spintronics, in conjunction with the recent award of the 2007 nobel prize in physics to gmr's co-inventors, albert fert and peter grünberg, has spurred much activity by research groups in the gmi effect. even though similarly named, the gmr and gmi effects differ in their nature. while gmr has explanations in quantum mechanics, gmi is derived from the skin effect. the gmi effect is a physical phenomenon characterized by large variations in the electrical impedance when soft amorphous ferromagnetic samples of specific composition are subjected to an external magnetic field [1-2]. in spite of the gmi effect being a recent discovery, gmi magnetometers have already been developed for various applications, among which stand out: presence detectors [3], industrial process control [4], space research and aerospace applications [1, 5], navigation systems [6], high-density memories and hds [7], traffic control [8], cracks detection in materials [9], and biological and biomedical applications [10-16]. researches in the gmi effect has shown that, with further sophistications, magnetometers based on gmi technology may have enough resolution to replace even squids (superconducting quantum interference devices), the currently most sensitive magnetometers [1, 5, 17-18], in some applications. in the area of biomagnetism, an eventual gmi magnetometer has promising potential uses, because of the resolution in the order of some picoteslas [15, 17-19]. abstract  giant magnetoimpedance (gmi) magnetometers are one of the most recent families of magnetic transducers, being characterized by  their potential to achieve high sensitivities. the sensitivity of magnetic transducers is directly related to the sensitivity of its sensor  elements. thus, optimizing the sensitivity of these sensor elements is a critical part of the magnetometers development chain. this  paper describes an automatic characterization system designed for the measurement of the electrical impedance of giant magneto‐ impedance  samples.  the measurement  uncertainties  of  the  system  were verified  and  discussed.  the  high  speed  of measurement  attained with the use of this system, implemented in labview, allows for the rapid determination of the optimal operational point of  gmi magnetometers.  acta imeko | www.imeko.org  august 2013 | volume 2 | number 1 | 22  the sensitivity of a magnetic transducer is directly related to the sensitivity of its sensor elements. thus, the optimization of the sensitivity of the sensing elements is essential. however, to date there is no accurate mathematical model for the impact of the various physical parameters that influence the gmi effect. thus, the optimization process is usually empirical [20]. among that set of parameters, it can be mentioned: chemical composition and geometry of the sample (length, width and thickness); dc level, amplitude and frequency of the current used to excite the samples; components of the external magnetic field (longitudinal, orthogonal and perpendicular) and temperature [2]. in the research carried out by labiomet at puc-rio, aiming at defining the set of parameters that optimizes the sensitivity of the gmi sensor elements, the impact of various parameters is estimated via multiple experimental measurements. the large number of variables that should be analyzed in the measuring procedure motivated the development of an automatic characterization system, which accepts as input a list of measurement configurations, and outputs the resulting measurement data – readings of magnitude and phase of the impedance of gmi samples [21]. 2. giant magnetoimpedance  in general, the impedance of a conductor depends on the distribution of the current inside the material. when the frequency increases, it is common for the current to concentrate in the surface of the conductor. in magnetic materials, the value of the current skin depth depends not only on the applied current amplitude and frequency value, but on the conductor geometry and on its magnetic permeability, which can vary with the applied magnetic field. this implies that, in samples of high permeability materials, even at moderate frequencies, a variation of the conductors’ impedance with the value of the applied magnetic field can be expected [22]. in the gmi effect, the alternating current applied along the sample length creates a transverse magnetic field (hac). this field magnetizes the material, increasing its permeability. the permeability grows until the external magnetic field (h) becomes sufficiently high to rotate the magnetic domains of the sample along its direction. the permeability dependence with the magnetic field and with the current modifies the skin depth of the current inside the material and, consequently, the impedance of the sample [23-26]. ribbons and wires of soft ferromagnetic alloys of the series co75-xfexsi15b10, which present low magnetostriction, exhibit the giant magnetoimpedance (gmi) phenomenon. the gmi samples can be electrically modelled by a simple rl model of a resistor rsens(h) in series with an inductor lsens(h) as defined by equation (1). ( ) ( ) ( )sens sens sensz h r h j l h  , (1) where ( ) ( ) cos ( )sens sens sensr h z h h , (2) ( ) sin ( ) ( ) sens sens sens z h h l h    (3) and θsens is the impedance phase of the gmi samples. experimentally the gmi phenomenon is induced by the application of an alternating current (i) along the length of ferromagnetic samples, which are then submitted to an external magnetic field (h), as shown in figure 1. the impedance of gmi samples changes as a function of h, and h can be inferred by measuring the difference of potential (v) between the extremities of the gmi sample. thus, the magnitude and phase values can be acquired via conventional impedance measurements using an adequate device [19, 21]. as mentioned in the introduction, there is a wide variety of external factors that can impact the gmi effect in a sample. for example, the frequency of the current that flows through the sample, iac, directly modifies the skin effect and therefore the gmi behavior. also, the dc current, idc, is responsible for agmi (asymmetric giant magnetoimpedance) effects [27], while small changes in the chemical composition of the sample material can often have dramatic effects on gmi percentages [26]. as in most materials that are influenced by magnetic phenomena, gmi samples suffer from magnetic hysteresis [2], which certainly accounts as an obstacle for the construction of a magnetometer, and consequently should be carefully analyzed. 3. description of the system  aiming to make the characterization process of the gmi samples more agile, it was developed an automatic characterization system [21]. as shown in figure 2, the system is composed by six parts essential to its functionality: a helmholtz coil for the generation of the external magnetic field; an lcr meter that electrically excites the sample and, simultaneously, measures its impedance; a current source (ih) to excite the helmholtz coil; a polarity inverter to change the direction of the current source; digital outputs of a daq (data acquisition device) module to control the polarity inverter; and, finally, the software developed in labview to serve as a virtual interface between the operator and the other modules. the selection of the experimental configurations in the labview software is made by the user. specifically, the user defines, via a simple graphical interface, a list of experiments to carry out. the uniaxial helmholtz coil has 48 turns and a radius of 15 cm and it is responsible by generating the dc magnetic field used to excite the gmi samples, according to equation (4). this coil is powered by a dc current source (e3648, agilent), controlled by a gpib interface, which has a maximum power capacity of 100 w and a dual output, each one capable of providing 8 v / 5 a or 20 v / 2.5 a. as it allows a broader range of magnetic fields during the characterization process of the gmi samples, the current source was used in the mode where imax = 5 a (corresponding to hmax ≈ 14.4 oe inside the coil). to analyze the behaviour of gmi samples excited by higher magnetic fields, the system must be redesigned by figure  1.  typical  measurement  of  the  gmi  effect:  the  difference  of  potential  (v)  between  the  extremities  of  a  gmi  sample,  submitted  to  an  excitation current (i), as a function of the magnetic field (h).  acta imeko | www.imeko.org  august 2013 | volume 2 | number 1 | 23  increasing the source power and/or the number of turns of the helmholtz coil. 2.88 oe a   h ih . (4) the developed system also contains a polarity inverter to reverse the direction of the current that flows through the helmholtz coils and, consequently, enabling to generate magnetic fields in both directions. as shown in figure 2, this polarity inverter is powered by a 9 v voltage source and the polarity reversal is controlled by a ttl signal (vin) generated by an acquisition board (ni usb-6221, national instruments). the electronic circuit designed for this polarity inverter, shown in figure 3, consists of a double relay (et2-b3m1s, nec), able to support currents up to 25 a and with switching times of less than 5 ms, junction diodes d1 and d2 (1n4001) and npn junction bipolar transistors q1 and q2 (bc547). the control voltage vin is not applied directly to the terminals of the relay coils because the acquisition board is not capable of providing more than 24 ma per digital output and the switching coils of the relays require currents of 50-80 ma to be magnetized. therefore, the transistors q1 and q2 were employed to allow the proper excitation of the switching coils. in figure 3, the terminals indicated by f+ and fare, respectively, the connection points of the positive and negative terminals of the controlled current source. in turn, the outputs h+ and hare, respectively, the connection points of the positive and negative terminals of the helmholtz coil. the core of the system is a precision lcr meter (4285a, agilent), responsible for measuring the impedance magnitude and phase of gmi sensor elements, for each magnetic field generated by the helmholtz pair. the lcr meter is also responsible by the excitation of the gmi samples, being capable of generating currents with adjustable amplitudes (100 μarms to 20 marms), frequencies (75 khz to 30 mhz) and dc levels (0 to 100 ma). finally, in figure 2 it is also depicted a gpib-usb converter (82357b, agilent) used to allow the communication between the gpib interfaces of the controlled current source and of the lcr meter and the usb bus of the computer which is running the developed labview software, responsible by controlling the entire measuring process and properly processing the acquired information. at the end of each characterization test an excel output file is automatically generated, containing a header with the parameters defined for the test and the measurement results of the impedance magnitude |zsens(h)| and phase θsens(h) of the analyzed gmi sensors, as a function of the magnetic field h. in addition to the experimental measurements of magnitude and phase, these files also contain the mathematically calculated resistance rsens(h), equation (2), and inductance lsens(h), equation (3), of the gmi sensors, assuming that they can be electrically modelled as a resistance rsens(h) in series with an inductance lsens(h) as according to the electrical model defined by equation (1), which is useful to simulate the behaviour of the sensor elements in a spice program. it is noteworthy that, as magnetic sensors usually have hysteresis, the characterizations were performed in such a way to obtain the hysteresis curves of the samples. then, the output files include graphs with hysteresis characteristics and respective averaged values. the system was tested with ribbon-shaped samples having a chemical composition of co70fe5si15b10. the studied samples are of the lmi (longitudinal magnetoimpedance) type, which are much more sensitive (about 100 times) to the component of the magnetic field parallel to their lengths. thus, it is recommended that, during the measurement process, the gmi ribbon be positioned in such a way that the earth's magnetic field is perpendicular to its length. in this way, noise caused by the earth’s magnetic field is minimized. as an example, consider the following experiment: a gmi sample of co70fe5si15b10 alloy with 5 cm length, 2 mm width and 60 μm thickness is subject to a measurement temperature of 298k and excited by a current with a dc level of 80 ma, 15 ma amplitude and 1.5 mhz frequency. figure 4a show the details of a typical output excel file, while figures 4b and 4c highlight, respectively, the hysteresis curves and the mean curves. to recognize how each parameter affects the gmi behaviour it is required the analysis of a significant amount of experimental data, which demands long periods of time. thus, aiming at improving the analysis of the output data, it was also developed a program named “organize”, written in perl, which can be directly called by the labview interface of the main program. it processes the experimental data from multiple measurements and presents the results in a friendlier way, helping their interpretation by the user. the developed program allows the individual assessment of the influence of each of the parameters that affect the behaviour of the gmi samples. the user informs which parameter he wants to analyze and, based on this information, the program accesses the full database of figure  2.  basic  setup  of  the  automatic  system  for  gmi  sample characterization.  figure  3.  schematic  diagram  of  the  electronic  circuit  developed  for  the  polarity inverter.   acta imeko | www.imeko.org  august 2013 | volume 2 | number 1 | 24  previously acquired measurements searching for identical configurations, where the only parameter differing among them is the one informed by the user. as a result it generates concise and simplified graphs, which highlights how the parameter of interest affects the behaviour of the gmi samples. it should be noted that, after several measurements, it has been shown that, if certain techniques and circuitry are used, then the phase characteristic of gmi can be much more sensitive to magnetic fields than its counterpart magnitude characteristic [15-16, 19]. 3.1. uncertainty analysis  considering that, in the characterization studies, the impedance measurements were performed by the lcr meter 4285a (agilent), the uncertainties of the impedance magnitude (uz) and phase (uθ) measurements of the gmi samples are directly attributed to the uncertainties of the lcr meter, which are, respectively, defined in its operational manual as (%) ( )z n c tu a a k    and (5) 180 ( ) (degrees) 100 n c ta a ku        , (6) where an is the component of the uncertainty due to the equipment intrinsic characteristics, ac is the cable length factor and kt is the temperature factor. the temperature factor kt is equal to one in the range of 18°c to 28°c. the measurements were always performed within this temperature range, then it can be admitted that kt = 1. for impedance magnitudes below 5 kω, ac is given by (%) 15 m c f a  , (7) where fm is the frequency, in mhz, used to excite the sample. on the other hand, knowing that all of the experimental measurements of the impedance of the gmi sensors returned magnitude values between 10 mω and 100 ω, the parameter an is defined by the instrument maker as 2 1(%) % 3% 30 50 0.02% 0.1% 30 m n m i osc m f a n f k k z                       , (8) where |zm| is the absolute value of the measured impedance in ohms and n1 is a frequency-dependent factor which can be equal to 0.15 – for 75 khz < fm < 200 khz or 3 mhz < fm < 5 mhz; to 0.08 – for 200 khz < fm < 3 mhz; or to 0.30 – fm > 5 mhz. the constant ki is related to the integration time used by the lcr meter, which can be set to short (30 ms), medium (65 ms) or long (200 ms). if the integration time is defined as long then ki = 1, else ki = 2. on the other hand, for impedance magnitudes between 10 mω e 100 ω, kosc is given by 1 , if (20 ) 1 20 , if (20 ) 1 osc osc osc osc v k v v       , (9) where vosc is the rms value of the ac component of the voltage used to excite the samples, expressed in mv. assuming a gaussian distribution, the standard uncertainty of the impedance magnitude measurements uz is given by equation (10) and the one of the impedance phase uθ is given by equation (11). (%) ( ) 2 m z z z u u    (10) (degrees) 2 u u   . (11) figure 5a shows the expanded uncertainty of the impedance magnitude measurements uz (%) and figure 5b shows the expanded uncertainty of impedance phase measurements uθ (degrees). both of them are functions of the impedance magnitude of the samples and of the frequency of the current used to excite the samples. those graphs were generated assuming that the integration time was defined as long, ki = 1, figure  4.  (a)  typical  output  file  (b)  hysteresis  curves  of  the  impedance components  of  the  sensor  (c)  averaged  curves  of  the  impedance components of the sensor.  acta imeko | www.imeko.org  august 2013 | volume 2 | number 1 | 25  and that the amplitude of the excitation current was kept in 10.6 marms, thus allowing to define vosc = |zm| × 10.6. by observing figure 5, it can be noticed that the measurement uncertainties tend to decrease for low frequencies and high impedance values. on the contrary, high frequencies and small impedances increase the measurement uncertainties. for the experimental measurements performed, the impedance measurement uncertainty uz of the results, obtained by applying equation (5) to the experimental data set, is always, at least, ten times smaller than its respective impedance value. also, all of the measured impedance phase values presented uncertainties uθ,, obtained by applying equation (6), equal or smaller than ± 1°. the standard uncertainty of the magnetic field (uh) generated by the helmholtz pair is dependent of the standard uncertainty of the dc current source (agilent, e3648a), which is equal to ±2.0 ma. then, supposing, by simplicity, that the geometric configuration of the helmholtz coils is satisfactorily close to the one considered on the theoretical model and knowing that the relation between the current and the magnetic field generated by the helmholtz pair is given by equation (4), uh is expressed as 2.88 ( ) 5.76 h iu u a moe     . (12) thus, the expanded uncertainty uh, for a confidence level of 95.45%, is 2 11.52 h hu u moe    . (13) the smallest magnetic field step used for the gmi samples characterization was 0.1 oe, which is about 10 times larger than uh. it can be noticed that, in order to reduce the magnetic field step, it will be essential to improve the uncertainty of the current source. 4. results  the automated system has shown to be capable of acquiring a much larger amount of data than the manual process conventionally used, in the same timeframe. for instance, to generate the data shown in figure 6 by means of a manual process, it would take approximately 19 hours of sequential experiments, while the automated system managed to acquire the same data in approximately 95 minutes. in other words, an experiment that would consume 1 hour using manual procedures takes 5 minutes using the current version of the automated characterization system. in turn, figure 6 illustrates an example of output file of the “organize” program, where the parameter of interest, chosen by the user, is the frequency. the presented results are for a gmi sample of co70fe5si15b10 alloy with 5 cm length, 2 mm width and 60 μm thickness. the measurements were performed at a room temperature of 298k and, in all of them; the dc level was kept at 80 ma and the amplitude of the ac current at 15 ma. this figure shows four comparative graphs generated by the program, referring to variations, as a function of applied magnetic field, of the resistance rsens(h), inductance lsens(h), magnitude |zsens(h)| and phase θsens(h) of the impedance gmi samples in relation to their respective values at h = 0. the clear tendencies of the impedance components as a function of the frequency, shown in figure 6, highlight the impact of this parameter on the gmi effect. in this stage of research, the vast amount of data and variables that should be analyzed for optimizing the sensitivity of gmi sensor elements indicates the relevance of an automated system for the characterization of gmi effect. 5. conclusions  the developed system for automated characterization of gmi samples, designed in the labview environment, allows a reliable and high speed identification of the impedance behaviour as a function of specific measurement parameters. the attained performance is essential for the use of this tool to identify the optimal operational point of gmi magnetometers. the number of external parameters controlled by the current system can be expanded, incorporating other aspects discussed in the literature [2], and this expansion is an objective of the labiomet team. also, while the current version has shown a figure  5.  impedance  magnitude  uncertainty  uz(%)  and  impedance  phase uncertainty  uθ(degrees)  as  a  function  of  the  frequency  fm  and  of  the  impedance magnitude |zm|. (a) uz(%) for 75 khz < fm < 30 mhz and 2 ω <  |zm| < 50 ω; (b) uz(%) for 75 khz < fm < 30 mhz and 0.5 ω < |zm| < 2 ω; (c)  uθ(degrees) for 75 khz < fm < 30 mhz and 2 ω < |zm| < 50 ω; (d) uθ(degrees) for 75 khz < fm < 30 mhz and 0.5 ω < |zm| < 2 ω.  figure 6. example of an output file of the program “organize”, where the  frequency is the parameter that the user wants to analyse. variation of the (a) resistance, (b) inductance, (c) magnitude and (d) phase as a function of the magnetic field, for several frequencies.  acta imeko | www.imeko.org  august 2013 | volume 2 | number 1 | 26  great improvement in speed for the characterization measurements, optimization in the labview code itself is being discussed in order to increase its performance even more. finally, the large amount of raw information requires some type of data filtering and analysis, since the tendencies identified by human eye recognition need to be captured by optimization algorithms. in this way, a future version of the automated characterization system would incorporate a feedback analysis to generate a new – and hopefully even more insightful – list of experiment configurations, aiming at reaching an optimal point, which is necessary for the development of a high sensitivity gmi magnetometer. acknowledgement  we thank the brazilian funding agencies cnpq, faperj and finep, for their financial support, and prof. fernando machado (dept. of physics/ufpe) for the gmi samples provided. references  [1] a. e. mahdi, l. panina, d. mapps, “some new horizons in magnetic sensing: high-tc squids, gmr and gmi materials”, sensors and actuators a: physical 105 (2003) pp. 271-85. 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[21] j.h.c.c. carneiro, e. costa silva, l.a.p. gusmão, c.r.h. barbosa, e. costa monteiro, “system for automatic characterization of giant magneto-impedance samples”, proc. of xx imeko world congress, sept. 9-14, 2012, busan, republic of korea, pp. 1-5. [22] l.a.p gonçalves, j.m. soares, f.l.a machado, w.m. de azevedo, “gmi effect in the low magnetostrictive cofesib alloys”, physica b 384 (2006) pp. 152-54. [23] h. hauser, l. kraus, p. ripka, “giant magnetoimpedance sensors”, ieee instrumentation & measurement magazine, 4(2) (2001) pp. 28-32. [24] v. knobel, k.r. pirota, “giant magnetoimpedance concepts and recent progress”, journal of magnetism and magnetic materials 242 (2002) pp. 33-40. [25] f.l.a. machado, s.m. rezende, “a theoretical model for the giant magnetoimpedance in ribbons of amorphous softferromagnetic alloys”, journal of applied physics 79 (1996) pp. 6958-60. [26] k.c. mendes, f.l.a. machado, “enhanced gmi in ribbons of co70.4fe4.6si15b10 alloy”, journal of magnetism and magnetic materials 177 (1998) pp. 111-12. [27] f.l.a. machado, a.r. rodrigues, a.a. puça, a.e.p. de araújo, “highly asymmetric giant magnetoimpedance”, materials science forum 302-303 (1999) pp. 202-8. acta imeko july 2017, volume 6, number 2, 21-31 www.imeko.org acta imeko | www.imeko.org july 2017 | volume 6 | number 2 | 21 inter-laboratory comparison between nmij and kriss of calibration capabilities for torque measuring devices in the range from 50 n·m to 2 kn·m koji ogushi1, atsuhiro nishino1, kazunaga ueda1, min-seok kim2 and yon-kyu park2 1national metrology institute of japan, aist, tsukuba central 3, 1-1-1 umezono, tsukuba, ibaraki, 305-8563, japan 2korea research institute of standards and science, 267 gajeong-ro, yuseong-gu, daejeon, 305-340, rep. of korea section: research paper keywords: torque standard machine; torque; comparison; transfer standard; bridge calibration unit citation: koji ogushi, atsuhiro nishino, kazunaga ueda, min-seok kim and yon-kyu park, inter-laboratory comparison between nmij and kriss of calibration capabilities for torque measuring devices in the range from 50 n∙m to 2 kn∙m, acta imeko, vol. 6, no. 2, article 5, july 2017, identifier: imeko-acta-06 (2017)-02-05 editor: paul regtien, the netherlands received june 13, 2016; in final form february 1, 2017; published july, 2017 copyright: © 2017 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: koji ogushi, e-mail: kji.ogushi@aist.go.jp 1. introduction torque standard machines (tsms) in national metrology institutes (nmis) have been established in many countries over the last two decades. the korea research institute of standards and science (kriss) has developed two deadweight-type tsms (dwtsms), which have rated capacities of 100 n∙m and 2 kn∙m (100-n∙m-dwtsm(k) and 2-kn∙m-dwtsm(k), respectively) [1]. a new deadweight-type tsm with a rated capacity of 20 kn∙m (20-kn∙m-dwtsm(k)) is also being developed [2]. the national metrology institute of japan (nmij) has also developed three dwtsms, which have rated capacities of 10 n∙m (10-n∙m-dwtsm(j)) [3], 1 kn∙m (1kn∙m-dwtsm(j)) [4], and 20 kn∙m (20-kn∙m-dwtsm(j)) [5]. the first bilateral comparison between nmij and kriss was performed in 2004 for the range from 200 n∙m to 2 kn∙m by using 1-kn∙m-dwtsm(j) and 20-kn∙m-dwtsm(j) at nmij and 2-kn∙m-dwtsm(k) at kriss [6]. after that, 1-kn∙mdwtsm(j) was improved to reduce the relative expanded uncertainty (reu) of the realized torque [7]. in order to confirm this improvement and to confirm the stability of other tsms after the first comparison, a second bilateral comparison between nmij and kriss was conducted in 2010 for the range from 50 n∙m to 2 kn∙m. although the evaluation was put on hold after the great east japan earthquake on march 11, 2011, the evaluation was resumed in 2014. this paper describes the second comparison results. we also investigated the influence abstract an inter-laboratory comparison of the calibration capability for torque measuring devices (tmds) was conducted between the national metrology institute of japan (nmij) in the national institute of advanced industrial science and technology (aist) and the korea research institute of standards and science (kriss). three high-performance torque transducers having rated capacities of 100 n∙m, 1 kn∙m, and 2 kn∙m, and a bridge calibration unit (bn100a) were used as the transfer devices. the 1 kn∙m and 2 kn∙m transducers had one bridge, but the 100 n∙m transducer had two bridges that acquired data more reliably. an identical indicator/amplifier type (dmp40) owned by each laboratory was used. all of the transducers and bn100a were transferred from nmij to kriss. for the comparison, nmij used 1 kn∙m and 20 kn∙m deadweight torque standard machines (tsms), and kriss used a 2 kn∙m deadweight tsm. in particular, the capability of the 1 kn∙m tsm at nmij was examined after some improvements. in the calibration range from 50 n∙m to 2 kn∙m, relative deviations were less than 3.0 × 10-5 for increasing torques. sufficiently small deviations were obtained between the calibration results in the two laboratories, as compared with their calibration and measurement capabilities (cmcs), which were 3.5 × 10-5 for the 1 kn∙m tsm at nmij, 7.0 × 10-5 for the 20 kn∙m tsm at nmij, and 5.0 × 10-5 for the 2 kn∙m tsm at kriss (as relative expanded uncertainties). acta imeko | www.imeko.org july 2017 | volume 6 | number 2 | 22 of the voltage span of the indicator/amplifiers by using a bridge calibration unit as well as the temperature and humidity coefficients of the transducers. 2. torque standard machines 2.1. nmij/aist two deadweight machines, 1-kn∙m-dwtsm(j) and 20kn∙m-dwtsm(j), were used for the comparison at nmij. figure 1 shows 1-kn∙m-dwtsm, which has a calibration range from 5 n∙m to 1 kn∙m. the lower limit of the range was extended to 0.5 n∙m by developing a new small linkage series of weights; 1-kn∙m-dwtsm(j) was also improved to reduce the uncertainty of the moment arm length by changing the metal band thickness at the end of the arm from 100 µm to 50 µm and re-evaluating the sensitivity limit of the fulcrum (aerostatic bearing). as a result of these evaluations, reus (k = 2) of 7.3 × 10-5 and 2.9 × 10-5 could be obtained for the ranges from 0.5 n∙m to 20 n∙m and from 5 n∙m to 1 kn∙m, respectively [7]. after some other minor changes, such as the environmental conditions, an reu of 3.4 × 10-5 for the realized torque was obtained in the range from 5 n∙m to 1 kn∙m. figure 2 shows 20-kn∙m-dwtsm(j), which has a calibration range from 200 n∙m to 20 kn∙m. the reu of the torque realized with the tsm was 6.7 × 10-5 [5]. the calibration and measurement capabilities (cmcs), which equal the reus (k = 2) of the calibration with almost ideal torque measuring devices (tmds), were 3.5 × 10-5 for 1-kn∙m-dwtsm(j) from 5 n∙m to 1 kn∙m and 7.0 × 10-5 for 20-kn∙m-dwtsm(j). 2.2. kriss the 2-kn∙m-dwtsm(k) deadweight machine was used for the comparison at kriss. the cmc of 2-kn∙m-dwtsm was evaluated as 5.0 × 10-5 [1]. figure 3 shows 2-kn∙mdwtsm(k). the characteristics of 2-kn∙m-dwtsm can be found in references [1] and [6]. 3. transfer devices 3.1. torque transducers three torque transducers of different capacities were used as the transfer devices for this comparison. the three transducers, which have rated capacities of 2 kn∙m (tn/2knm), 1 kn∙m (tb2/1knm), and 100 n∙m (tn/100nm), were transferred from nmij to kriss by air transport. figures 4 and 5 show figure 1. components of 1-kn∙m-dwtsm at nmij. figure 2. components of 20-kn∙m-dwtsm at nmij. figure 3. components of 2-kn∙m-dwtsm at kriss. figure 4. 2 kn∙m transducer and its container. figure 5. 1 kn∙m and 100 n∙m transducers and their container. torque transducer installation comp. of the torque transducer arm comp. weight loading comp. bearing comp. for fulcrum (aerostatic bearing ) bearing comp. for counter torque pedestal comp. torque transducer arm comp. w eight loading comp. bearing comp. for fulcrum (aerostatic bearings) bearing comp. for counter torque pedestal comp. arm comp. w eight loading comp. pedestal comp. bearing comp. for fulcrum (aerostatic bearings) tn/2knm accumulators desiccant tb2/1knm tn/100nm accumulators desiccants acta imeko | www.imeko.org july 2017 | volume 6 | number 2 | 23 the transducers and their special containers. tn/2knm and tn/100nm are shaft-type transducers, whereas tb2/1knm is a disk-type transducer. the adapter flanges were kept fastened to both sides of tb2/1knm for more than one year before the comparison. the 100 n∙m transducer of tn/100nm has double bridges so that two series of outputs were available. the output from bridge 1 is expressed as tn/100nm(md1), whereas that of bridge 2 is expressed as tn/100nm(md2). the containers sealed the transducers from the outside environment. accumulators (20 °c to 25 °c) and desiccants (40 % relative humidity) were put into the containers to maintain the lab environment as much as possible. thermohygrometers and a 3-d accelerometer were placed in the container to monitor the environmental conditions. the sampling period of the thermo-hygrometers was 30 minutes. the variations in temperature and relative humidity during transportation were from 10 °c to 24 °c and from 28 % to 63 %, respectively. the sampling period of the accelerometer was 0.5 s, and the maximum acceleration was recorded for every hour. the containers inadvertently received a maximum shock of 76 m/s2 (one time) during transportation. however, no serious damage was found in the transducer after unpacking. figures 6(a)–6(d) indicate the long-term stabilities of tn/2knm, tb2/1knm, and tn/100nm(md1 and md2) before the comparison. the long-term stability was expressed by relative deviations of the calibration results at the rated capacities from the mean values of all calibration results during a certain period. tn/2knm and both bridges of tn/100nm were stable, and the maximum variation over approximately three months was less than 1.0 × 10-5. the variation of tb2/1knm over approximately one and half years was 7.0 × 10-5. although the variation was relatively large, the tendency of the variation was approximately linear, and the change in calibration results within three months was less than 2.0 × 10-5, so the authors decided that the transducer could be used as a transfer standard. the temperature and humidity coefficients of all transducers were investigated after the comparison in order to correct the results depending on the environmental conditions during each measurement. the detailed procedure and results are explained and discussed in sections 4.2 and 5.3. 3.2. amplifier/indicators and bridge calibration unit the bridge calibration unit bn100a(j), which calibrates the ac bridge voltage with an excitation voltage of 5 v and a carrier frequency of 225 hz, was also transferred from nmij to kriss by air transport. the amplifier/indicator of each nmi (dmp40s2(ja) and dmp40s2(jb) at nmij and dmp40(k) at kriss) was connected to the transfer transducers during each torque calibration. at nmij, dmp40s2(ja) was used for the calibrations of tb2/1knm and tn/100nm, whereas dmp40s2(jb) was used for the calibrations of tn/2knm. at kriss, dmp40(k) was used for the calibrations of all transducers. the amplifier/indicators were calibrated just before and immediately after each torque calibration by bn100a(j). figure 7 shows bn100a(j) and dmp40(k). the reference voltage ratios of the bridge calibration unit were measured by the amplifier/indicators at steps of +0, +0.1, +0.5, +0.7, +0.8, +1.0, +1.4, and +1.6 mv/v and then -0, -0.1, -0.5, -0.7, -0.8, -1.0, -1.4, and -1.6 mv/v. bridge voltage calibration was carried out 10 times during this comparison calibration, as (a) tn/2knm (b) tb2/1knm (c) tn/100nm(md1) (d) tn/100nm(md2) figure 6. long-term stabilities of the torque transducers. r el at iv e de vi at io n, x 10 -6 -100 -75 -50 -25 0 25 50 75 100 20 05 /9 /5 20 06 /3 /2 4 20 06 /1 0/ 10 20 07 /4 /2 8 20 07 /1 1/ 14 20 08 /6 /1 20 08 /1 2/ 18 20 09 /7 /6 date cw ccw -100 -75 -50 -25 0 25 50 75 100 20 08 /1 2/ 18 20 09 /3 /2 8 20 09 /7 /6 20 09 /1 0/ 14 20 10 /1 /2 2 20 10 /5 /2 20 10 /8 /1 0 20 10 /1 1/ 18 date r el at iv e de vi at io n, x 10 -6 cw ccw -100 -75 -50 -25 0 25 50 75 100 20 09 /1 0/ 14 20 09 /1 2/ 3 20 10 /1 /2 2 20 10 /3 /1 3 20 10 /5 /2 20 10 /6 /2 1 date r el at iv e de vi at io n, x 10 -6 cw ccw -100 -75 -50 -25 0 25 50 75 100 20 09 /1 1/ 23 20 10 /1 /2 20 10 /2 /1 1 20 10 /3 /2 3 20 10 /5 /2 20 10 /6 /1 1 date r el at iv e de vi at io n, x 10 -6 cw ccw acta imeko | www.imeko.org july 2017 | volume 6 | number 2 | 24 follows: (v1) dmp40s2(ja) calibrated by bn100a(j) before pre-cal. at nmij (v2) dmp40s2(ja) calibrated by bn100a(j) after pre-cal. at nmij (v3) dmp40s2(jb) calibrated by bn100a(j) before pre-cal. at nmij (v4) dmp40s2(jb) calibrated by bn100a(j) after pre-cal. at nmij (v5) dmp40(k) calibrated by bn100a(j) before calibration at kriss (v6) dmp40(k) calibrated by bn100a(j) after calibration at kriss (v7) dmp40s2(ja) calibrated by bn100a(j) before post-cal. at nmij (v8) dmp40s2(ja) calibrated by bn100a(j) after post-cal. at nmij (v9) dmp40s2(jb) calibrated by bn100a(j) before post-cal. at nmij (v10) dmp40s2(jb) calibrated by bn100a(j) after post-cal. at nmij the above procedure was able to evaluate the stability and differences of amplifier/indicators during the entire comparison calibration. results are discussed in section 5.1. 4. experimental procedure 4.1. comparison calibration procedure preand post-calibrations were conducted at nmij (we call the pre-calibration “j1” and the post-calibration “j2”) by three transducers (four bridges) before and after the calibration at kriss (we call the calibration “k”). loading timetables for the individual calibrations are shown in figure 8. the timetable used in the cipm key comparison of ccm.t-k2 [8] was adopted. the torque calibration was conducted separately in both the clockwise (cw) and counterclockwise (ccw) directions. table 1 shows the overall schedule of the comparison. the rotational mounting position of each transducer was changed to three pitch directions (every 120°) and rotated twice. first, after three pre-loading cycles up to the maximum torque (the rated capacity of the torque transducer) in two steps of 50 % and 100 % of the maximum torque, the combination of one pre-loading and three measurement loading cycles was conducted at the 0° direction. in all cycles, the torque steps were increasing only. the combination of one pre-loading and one measurement loading cycle was performed in the directions of 120°, 240°, 360°, 480°, 600°, and 720°. the time intervals were strictly maintained to exclude the influence of the creep characteristics of transducers. the interval from the start of loading to data acquisition was six minutes. the interval from the last maximum torque reading at the present mounting position to the first zero reading at the next mounting position was ten minutes. 4.2. determination procedure of temperature and humidity coefficients the principle of measurement for the torque transducers used in this comparison was the strain gauge. the output of this type of transducer generally depends on the environmental temperature and humidity [9]. after the comparison calibration, therefore, temperature and humidity coefficients were experimentally determined. we prepared the following five conditions in the torque calibration rooms at nmij, and torque was loaded on the transducers in steps of 50 % and 100 % of the maximum torque, as was done in the comparison calibration procedure (but with only three pre-loading cycles and one measurement loading cycle at only the 0° position, and the time interval was approximately 40 seconds, but not six minutes). data 1: 23 °c, 40 % data 2: 25 °c, 40 % data 3: 27 °c, 40 % data 4: 27 °c, 50 % data 5: 27 °c, 60 % from the measurement values of the five conditions, temperature coefficient αt and humidity coefficient βh were estimated for both the 50 % and 100 % steps and for both the cw and ccw directions of all bridges. 5. results and discussion 5.1. stability and differences of amplifier/indicators all measurement results are summarized in figures 9(a) and 9(b), where the relative deviation of each step from the start of measurement is expressed. figures 9(a) and 9(b) are the cases referring to dmp40s2(ja) and dmp40s2(jb), respectively. in both cases, very small voltage span variations occurred except at the steps of ± 0.1 mv/v. from the differences of voltage figure 7. bridge calibration unit and amplifier/indicator. table 1. schedule of measurements and transportation. pre-calibration at nmij (j1) september 13 – september 24, 2010 transportation september 24 – october 5, 2010 calibration at kriss (k) october 5 – october 21, 2010 transportation october 26 – october 29, 2010 post-calibration at nmij (j2) november 1 – december 2, 2010 figure 8. loading timetable of calibration for each torque transducer. bridge calibrator: bn100a(j) amplifier/indicator: dmp40(k) torque, % of rated capacity *1) interval to the first zero reading at next mounting positions 6 6 6 6 6 6 10 0o 6 6 6 6 6 6 6 6 6 6 6 6 66 100 50 pre-loadings one preloading three meas.loadings time, min. time, min. 120o one preloading 100 50 6 6 6 6 6 10 6 6 6 6 6 10 6 6 6 6 6 10 *1) *1) *1) one meas. loading one preloading one meas. loading one preloading one meas. loading 240o 360o time, min. 480o one preloading 100 50 6 6 6 6 6 10 6 6 6 6 6 10 6 6 6 6 6 10 *1) *1) *1) one meas. loading one preloading one meas. loading one preloading one meas. loading 600o 720o 120o 240o 0o 0 0 0 acta imeko | www.imeko.org july 2017 | volume 6 | number 2 | 25 ratios between dmp40s in nmij and kriss, we calculated correction factors and uncertainties for the torque calibration values obtained in kriss according to the procedure in reference [6], as shown in table 2. the correction factors are so small that they would not affect the final results of the comparison. nevertheless, these factors were used for the correction of the results in sections 5.3 and 5.4. 5.2. calibration results of torque without correction the calibration results were calculated by using the following equation for the mean of the measured values which are defined as each indicated value minus the zero value at the prior loading cycle, at the measurement loading cycles for all mounting positions except the 0° direction: ∑ = = rot 1e i1e rot i ' 1 ' n s n s (1) where s’ije is the measured value in the measurement loading cycle for step i, cycle j (=1), and series e (where “series” means the successive calibration sequence within the same mounting position), and nrot is the number of rotational mounting positions (nrot = 6). the relative deviations of the calibration results obtained at kriss from the mean results of the preand post-calibrations at nmij are shown in figures 10(a)–10(d) (with white squares) for tn/2knm, tb2/1knm, tn/100nm(md1), and tn/100nm(md2), respectively. the short-term drift, which is expressed by (11) in section 5.4.5, is defined as the difference in the results between the preand post-calibrations at nmij. the relative values of the short-term drift are also shown in figure 10 (with white circles). at the calibration points for 2 kn∙m, 1 kn∙m, 500 n∙m, 100 n∙m, and 50 n∙m, the relative deviations were from 0.2 × 10-5 to 5.0 × 10-5. at some points, very small deviations were obtained, whereas relatively large deviations occurred in the 50 n∙m and 100 n∙m steps. the deviation tendency in tn/100nm(md1) was similar to that in tn/100nm(md2). the deviation of tn/2knm in the cw direction became larger than the others. the authors presume that the differences in such deviation levels are due to the differences in environmental conditions, humidity in particular. the environmental conditions during each torque calibration are summarized in table 3. some of the short-term drifts of the transducers for approximately two months were also larger than the others (0.3 × 10-5 to 3.8 × 10-5). 5.3. determination of temperature and humidity coefficients figure 11 shows the actual temperature and relative humidity measured at each condition and for each transducer (as described in section 4.2). because the torque calibration rooms are very huge, the temperature and humidity are very stable, but it was difficult to precisely set the temperature and humidity to objective values. the dispersions and deviations occurred even table 2. correction factors due to differences of voltage ratios. transducer torque steps in n∙m equivalent voltage ratio steps in mv/v correction factor for kriss values relative standard uncertainty tn/2knm 1000 +0.7 to +0.8 1.000003 1.2.e-05 2000 +1.4 to +1.6 1.000003 1.2.e-05 -1000 -0.7 to -0.8 1.000000 1.1.e-05 -2000 -1.4 to -1.6 1.000005 1.1.e-05 tb2/1knm 500 +0.5 0.999993 1.1.e-05 1000 +1.0 0.999992 1.1.e-05 -500 -0.5 0.999992 1.2.e-05 -1000 -1.0 1.000006 1.1.e-05 tn/100nm(md1) 50 +0.7 to +0.8 0.999997 1.1.e-05 100 +1.4 to +1.6 0.999996 1.1.e-05 -50 -0.7 to -0.8 0.999998 1.0.e-05 -100 -1.4 to -1.6 1.000005 1.0.e-05 tn/100nm(md2) 50 +0.7 to +0.8 0.999997 1.1.e-05 100 +1.4 to +1.6 0.999996 1.1.e-05 -50 -0.7 to -0.8 0.999998 1.0.e-05 -100 -1.4 to -1.6 1.000005 1.0.e-05 (a) referring to dmp40s2(ja) (b) referring to dmp40s2(jb) figure 9. stability and differences of voltage span in amplifier/indicators. -50 -40 -30 -20 -10 0 10 20 30 40 50 13-sep 24-sep 6-oct 21-oct 8-nov 2-dec measurement date +0.1 mv/v +0.5 mv/v +0.7 mv/v +0.8 mv/v +1.0 mv/v +1.4 mv/v +1.6 mv/v -0.1 mv/v -0.5 mv/v -0.7 mv/v -0.8 mv/v -1.0 mv/v -1.4 mv/v -1.6 mv/v r el at iv e de vi at io n in 1 06 j1 (v1) (v2) (v5) (v6) (v7) (v8) k j2 -50 -40 -30 -20 -10 0 10 20 30 40 50 13-sep 24-sep 6-oct 21-oct 2-nov 24-nov r el at iv e de vi at io n in 1 06 measurement date +0.1 mv/v +0.5 mv/v +0.7 mv/v +0.8 mv/v +1.0 mv/v +1.4 mv/v +1.6 mv/v -0.1 mv/v -0.5 mv/v -0.7 mv/v -0.8 mv/v -1.0 mv/v -1.4 mv/v -1.6 mv/v (v3) (v4) (v5) (v6) (v9) (v10) j1 k j2 acta imeko | www.imeko.org july 2017 | volume 6 | number 2 | 26 under the same conditions. nevertheless, data 3, 4, and 5 have comparably similar temperature values (around 27.3 °c). so, we decided to carry out the following procedures: (1) humidity coefficient βh for each transducer, each direction and each step was estimated by fitting the linear curve for three measurement results of data 3, 4, and 5. (2) measurement values of data 1 and 2 were corrected to values at the humidity of data 3 by using the humidity coefficient obtained by procedure (1). (3) corrected measurement values of data 1, 2, and 3 have the same humidity conditions and different temperature (a) tn/2knm (b) tb2/1knm (c) tn/100nm(md1) (d) tn/100nm(md2) figure 10. relative deviation and short-term drift of calibration results. table 3. temperature and humidity conditions at each measurement. j1 j1 k k j2 j2 temp. in oc r. h. in % temp. in oc r. h. in % temp. in oc r. h. in % tn/2knm cw min. 22.9 36 23.1 49 22.8 37 max. 23.1 37 23.5 53 22.9 39 ccw min. 22.7 37 23.3 36 22.9 37 max. 22.9 45 23.5 42 23.0 39 tb2/1knm cw min. 23.1 41 23.3 47 22.7 39 max. 23.3 42 23.6 50 23.0 40 ccw min. 23.2 40 23.4 48 22.8 40 max. 23.3 40 23.6 51 23.0 44 tn/100nm(md1) cw min. 23.1 40 22.6 50 22.8 39 max. 23.2 47 22.9 53 22.9 40 ccw min. 23.0 42 23.0 52 22.8 40 max. 22.9 39 22.9 51 22.7 40 tn/100nm(md2) cw min. 23.1 40 22.6 50 22.8 39 max. 23.2 47 22.9 53 22.9 40 ccw min. 23.0 42 23.0 52 22.8 40 max. 23.0 42 23.0 52 22.8 40 -0.010 -0.008 -0.006 -0.004 -0.002 0.000 0.002 0.004 0.006 0.008 0.010 -2000 -1000 0 1000 2000 r el at iv e de vi at io n in % drift dev. drift, cor. dev., cor. torque in n m. -0.010 -0.008 -0.006 -0.004 -0.002 0.000 0.002 0.004 0.006 0.008 0.010 -1000 -500 0 500 1000 drift, % dev., % drift, cor. dev., cor. r el at iv e de vi at io n in % torque in n m. -0.010 -0.008 -0.006 -0.004 -0.002 0.000 0.002 0.004 0.006 0.008 0.010 -100 -50 0 50 100 drift, % dev., % drift, cor. dev., cor. r el at iv e de vi at io n in % torque in n m. -0.010 -0.008 -0.006 -0.004 -0.002 0.000 0.002 0.004 0.006 0.008 0.010 -100 -50 0 50 100 drift, % dev., % drift, cor. dev., cor. r el at iv e de vi at io n in % torque in n m. acta imeko | www.imeko.org july 2017 | volume 6 | number 2 | 27 conditions. then, temperature coefficient αt for each transducer, each direction and each step was estimated by fitting the linear curve for three corrected measurement results of data 1, 2, and 3. for example, the estimated αt and βh were obtained from the gradient of slopes shown in figure 12, for all steps and all directions in the case of tn/2knm. here, the blue values (circles) correspond to the left ordinate axis and the red ones (diamonds) to the right one. the coefficients for other transducers were also successfully obtained. all values of αt and βh, and their standard uncertainties (standard deviations of the fitting curves) are summarized in table 4. all of the comparison calibration results in figure 10 were corrected by the following equations: ( )%40'' hi 04h i −−= hss β and (2) ( )°c23'' ti 23t i −−= tss α , (3) where h and t are the mean values of the relative humidity and the temperature, respectively, which were measured during torque calibration for obtaining the result i's , and 04hi 's and 23t i's are corrected values at relative humidity h = 40 % and temperature t = 23 °c (reference humidity and reference temperature, respectively). in addition, calibration results were corrected by referring the influence of the voltage ratio differences by multiplying correction factors in table 2. the calculation results of the short-term drift (with black circles) and the relative deviation (with black squares) between nmij and kriss obtained by using the above corrected values are shown in figure 10. almost all relative deviations became smaller than the values without correction. in particular, correction of the humidity achieved good results. on the other hand, short-term drifts did not change much because temperature and humidity conditions were not very different between preand post-calibrations at nmij. 5.4. evaluation of en numbers the uncertainties of calibrations including corrections were calculated according to the following equations and procedure. then, we could finally compare the results of the two nmis by using the en numbers. 5.4.1. reproducibility with changing mounting position the relative reproducibility with changing mounting position b was estimated by defining an experimental standard deviation in the measured values for the first measurement loading cycles in all directions, 120°, 240°, 360°, 480°, 600°, and 720°, as follows: ( )∑ = − − = rot 1e 2 ii1e roti i ''1 1 ' 1 n ss ns b . (4) the relative standard uncertainty wrot is calculated as “an experimental standard deviation of the mean” by the following equation: 2 i rot 2 irot, 1 b n w = . (5) 5.4.2. repeatability with unchanged mounting position the relative repeatability with the unchanged mounting position b’ was estimated by defining the experimental standard deviation of the measured values for three measurement loading cycles in the 0° direction as follows: ( )∑ = − − = rep 1j 2 repi,ij0 reprepi, i ''1 1 ' 1 ' n ss ns b , (6) where repi,'s is the mean value of the three measured values in the 0° direction (e = 0, nrep = 3). the relative standard uncertainty wrep is also calculated as “an experimental standard deviation of the mean” by the following equation: 2 i rep 2 irep, ' 1 b n w = . (7) 5.4.3. zero point shift the relative zero point shift f0 was estimated by defining the deviation between the zero signals prior to the increasing torque and after the decreasing torque in the first and second cycles of the 0° direction, as follows: nj0 0j00j0 j00, ' '" s ss f − = , (8) figure 11. actual temperature and relative humidity at each experimental condition to determine the temperature and humidity coefficients of the torque transducers. (a) βh for cw direction (b) αt for cw direction (c) βh for ccw direction (d) αt for ccw direction figure 12. estimated temperature and humidity coefficients (tn/2knm)). 35 40 45 50 55 60 65 22 23 24 25 26 27 28 r el at iv e hu m id ity / % temperature / oc tn/100nm(md1)r tn/100nm(md1)l tn/100nm(md2)r tn/100nm(md2)l tb2/1knm r tb2/1knm l tn/2knm r tn/2knm l data1 data2 data3 data4 data5 1.50260 1.50265 1.50270 1.50275 0.75120 0.75125 0.75130 0.75135 35 45 55 65 1.50260 1.50265 1.50270 1.50275 0.75120 0.75125 0.75130 0.75135 22 24 26 28m ea s. r es ul t / ( m v /v ) relative humidity / % temperature / oc 50 % step 100 % step -1.50260 -1.50255 -1.50250 -1.50245 -0.75135 -0.75130 -0.75125 -0.75120 35 45 55 65 -1.50260 -1.50255 -1.50250 -1.50245 -0.75135 -0.75130 -0.75125 -0.75120 22 24 26 28 relative humidity / % temperature / oc m ea s. r es ul t / ( m v /v ) acta imeko | www.imeko.org july 2017 | volume 6 | number 2 | 28 where i = n is the maximum torque step. the zero point shift cannot be calculated in the last measurement loading cycles because the zero signals after the decreasing torque were read after changing the mounting positions (see figure 8). the relative standard uncertainty wzer was calculated according to the following equation, considering the mean deviation f0,mean in two f0,j0 as the half-width of the rectangular distribution: 2 mean0, 2 zer 3 1 fw = . (9) 5.4.4. resolution in the case of a digital scale, resolution r is defined as one increment in the last active number of the amplifier/indicator when the indication does not fluctuate under the no-load condition. if the indication fluctuates, then r is determined to be half the range of the fluctuation. here, r is stated in units of torque (n∙m). the uncertainty due to the resolution should be taken into account twice, because each measured value is obtained as the difference between the step-indicated value and the zeroindicated value. the relative standard uncertainty wres was calculated by the following equations for the applied torque ti (n∙m) at step i: 1) considering the resolution r as the whole width of the rectangular distribution when the indication does not fluctuate under the no-load condition: 2 i 2 ires, 23 2       = t r w . (10a) 2) considering the resolution r as the half-width of the rectangular distribution when the indication fluctuates: 2 i 2 ires, 3 2       = t r w . (10b) the low-pass frequency of the amplifier/indicators was always set to “0.1 hz bessel” during this bilateral comparison. in fact, the peak-to-peak fluctuation was only three digits (0.000003 mv/v) in all calibrations. 5.4.5. short-term drift the uncertainty due to the short-term drift of the torque transducer during preand post-calibrations (j1 and j2) was calculated by the following equation for the increasing torque: 2 ij, ipre,ipost,2 idft, '2 '' 3 1         − = s ss w , (11) where ij,'s is the mean of ipre,'s and ipost,'s . 5.4.6. stability of the amplifier/indicators the relative standard uncertainty due to the stability of the amplifier/indicators was included as wamp by using the relative standard deviation in table 2. 5.4.7. torque by using the tsms from the description in section 2, the relative standard uncertainties of realized torque by using the tsm wtsm are 1.7 × 10-5 for 1-kn∙m-dwtsm and 3.4 × 10-5 for 20-kn∙m-dwtsm at nmij and 2.5 × 10-5 for 2-kn∙m-dwtsm at kriss with a coverage factor of k = 2 (equivalent to a confidence level of approximately 95 %). 5.4.8. influence of temperature and humidity the standard uncertainties u(αt) or u(βh) in table 3 show just the uncertainty of the fitting curves (slopes). the units are (mv/v)/°c or (mv/v)/%. the relative standard uncertainties wtmp and whmd were calculated by using the following equations: )()()'( ' 1 ' 22 hh 22 04h i 2 ii ihmd, ihmd, huuhsu ss u w ∆⋅+⋅∆+== ββ and (12) )()()'( ' 1 ' 22 tt 22 32t i 2 ii itmp, itmp, tuutsu ss u w ∆⋅+⋅∆+== αα , (13) table 4. temperature and humidity coefficients for torque transducers and their uncertainties. transducer torque steps temperature coefficient standard uncertainty humidity coefficient standard uncertainty n∙m (mv/v)/k (mv/v)/k (mv/v)/% (mv/v)/% tn/2knm 1000 4.0.e-06 2.5.e-06 -1.8.e-06 2.7.e-07 2000 1.0.e-05 2.6.e-06 -4.7.e-06 2.2.e-07 -1000 -4.6.e-06 1.8.e-06 2.2.e-06 1.3.e-07 -2000 -8.7.e-06 4.4.e-06 4.4.e-06 6.7.e-07 tb2/1knm 500 4.8.e-06 1.8.e-06 -1.2.e-07 5.2.e-07 1000 1.0.e-05 4.1.e-06 -4.3.e-07 1.1.e-06 -500 -3.2.e-06 5.3.e-07 -5.1.e-07 1.3.e-07 -1000 -6.0.e-06 1.8.e-06 -1.1.e-06 1.3.e-07 tn/100nm(md1) 50 3.3.e-06 3.1.e-06 -2.0.e-06 1.3.e-07 100 8.0.e-06 2.5.e-06 -5.2.e-06 3.4.e-07 -50 -4.1.e-06 2.9.e-08 2.2.e-06 1.3.e-07 -100 -7.3.e-06 2.0.e-06 4.5.e-06 3.7.e-07 tn/100nm(md2) 50 -9.0.e-07 3.8.e-07 -3.1.e-06 3.2.e-07 100 -2.9.e-06 2.0.e-06 -6.8.e-06 5.2.e-07 -50 3.0.e-06 3.1.e-07 2.8.e-06 9.2.e-07 -100 4.8.e-06 3.6.e-07 6.3.e-06 1.3.e-06 acta imeko | www.imeko.org july 2017 | volume 6 | number 2 | 29 where ∆h and ∆t are the relative humidity and temperature deviations from the reference values of 40 % and 23 °c, respectively, )'( 40 i h su and )'( 32t isu are just the measurement uncertainties obtained by the procedure in section 4.2, and u(∆h) is 1.0 % and u(∆t) is 0.05 k. 5.4.9. evaluation of en number the relative expanded uncertainty of the calibrations at nmij (denoted as the j1 and j2 calibrations) was calculated by the following equation: 2 ires, 2 izer, 2 irep, 2 irot,ic_j,ij, wwwwkwkw +++⋅=⋅= 2 tmp 2 hmd 2 tsm 2 idft, wwww ++++ . (14) the relative expanded uncertainty of the calibration at kriss (denoted as the k calibration) was calculated by the following equation: 2 ires, 2 izer, 2 irep, 2 irot,ic_k,ik, wwwwkwkw +++⋅=⋅= 2 amp 2 tmp 2 hmd 2 tsm wwww ++++ . (15) the en number was also evaluated according to the following equation: ( ) 2 ij, 2 ik, ij,ij,ik, in, ''' ww sss e + − = , (16) where ik,'s denotes the result of the calibration at kriss. table 5 summarizes the results of the en number evaluation. the relative deviations and the total uncertainties are compared in the results obtained in the two nmis in figures 13(a)-13(h). these figures show the relative deviations of kriss values from nmij values. the error bars express the relative expanded uncertainty of measurements obtained from (14) and (15). the en numbers were all less than unity in the calibration range from 50 n∙m to 2 kn∙m. therefore, the equivalence of the torque standards between nmij and kriss were confirmed. however, the total measurement uncertainties became much larger than the cmcs of the two nmis because the uncertainties of the corrections were added; in particular, a somewhat large influence occurred in the results having large differences of the relative humidity. it might arouse much controversy that the more corrections could be considered, the better the result of en evaluation becomes, because the total uncertainty necessarily becomes larger than original value (see (16)). as an example of one of solutions for this problem, see the reference [9]. the steps of 500 n∙m and 1000 n∙m in this comparison coincide with those of ccm.t-k1 (one of the cipm key comparisons) [10]. a subject of future study will be to link the comparison results to ccm.t-k1. 6. conclusions an inter-laboratory comparison of the calibrations for torque measuring devices was conducted between nmij/aist and kriss in the range from 50 n∙m to 2 kn∙m. both nmis have well-established deadweight-type tsms. in the calibration range from 50 n∙m to 2 kn∙m, the relative deviations were from 0.2 × 10-5 to 2.4 × 10-5. sufficiently small deviations could be obtained between the calibration results of the two laboratories as contrasted with their cmcs, so the equivalence of the torque standards between nmij and kriss was confirmed again. the influences of the stability and difference of voltage ratio spans in the amplifier/indicators as well as the temperature and humidity dependency of the transducer outputs could also be successfully evaluated. references [1] y. k. park, m. s. kim, j. h. kim, j. h. choi and d. i. kim, “establishment of torque standards in kriss of korea”, proc. 20th int. conf. imeko tc-3, merida, 2007, cd-rom. [2] m. s. kim and y. k. park, “design of the 20 kn·m deadweight torque standard machine”, proc. 22nd int. conf. imeko tc3, cape town, 2014, usb flash drive. table 5. en evaluation results. torque cal. j cal. k rel. dev. s. t. drift u j u k e n in n･m in mv/v in mv/v in % in % in % in % tn/2knm 1000 0.751265 0.751260 -0.0006 -0.0017 0.0076 0.0064 -0.06 2000 1.502633 1.502645 0.0008 -0.0013 0.0071 0.0058 0.08 -1000 -0.751244 -0.751233 -0.0014 -0.0013 0.0075 0.0062 -0.14 -2000 -1.502522 -1.502508 -0.0010 -0.0036 0.0072 0.0057 -0.10 tb2/1knm 500 0.500185 0.500191 0.0012 -0.0009 0.0046 0.0064 0.15 1000 1.000439 1.000448 0.0008 -0.0008 0.0038 0.0060 0.12 -500 -0.500193 -0.500182 -0.0021 -0.0011 0.0046 0.0063 -0.27 -1000 -1.000467 -1.000465 -0.0002 -0.0010 0.0038 0.0057 -0.02 tn/100nm(md1) 50 0.757805 0.757809 0.0006 0.0020 0.0048 0.0064 0.07 100 1.515693 1.515684 -0.0006 0.0011 0.0040 0.0057 -0.08 -50 -0.757807 -0.757824 0.0024 -0.0037 0.0049 0.0063 0.29 -100 -1.515706 -1.515743 0.0024 -0.0042 0.0044 0.0058 0.34 tn/100nm(md2) 50 0.755867 0.755877 0.0013 0.0000 0.0046 0.0067 0.16 100 1.511800 1.511815 0.0010 -0.0003 0.0039 0.0058 0.15 -50 -0.755883 -0.755894 0.0015 -0.0008 0.0046 0.0073 0.18 -100 -1.511855 -1.511872 0.0011 -0.0011 0.0039 0.0062 0.15 ccw direction ccw cw ccw cw cw ccw cw acta imeko | www.imeko.org july 2017 | volume 6 | number 2 | 30 (a) tn/2knm, cw (b) tn/2knm, ccw (c) tb2/1knm, cw (d) tb2/1knm, ccw (e) tn/100nm(md1), cw (f) tn/100nm(md1), ccw (g) tn/100nm(md2), cw (h) tn/100nm(md2), ccw figure 13. relative deviations and uncertainty of comparison calibration. -0.015 -0.010 -0.005 0.000 0.005 0.010 0.015 nmi and torque step +1000 n∙m nmij +1000 n∙m kriss +2000 n∙m nmij +2000 n∙m kriss r el at iv e de vi at io n an d r el at iv e ex pa nd ed u nc er ta in ty in % -0.015 -0.010 -0.005 0.000 0.005 0.010 0.015 nmi and torque step -1000 n∙m nmij -1000 n∙m kriss -2000 n∙m nmij -2000 n∙m kriss r el at iv e de vi at io n an d r el at iv e ex pa nd ed u nc er ta in ty in % -0.015 -0.010 -0.005 0.000 0.005 0.010 0.015 nmi and torque step +500 n∙m nmij +500 n∙m kriss +1000 n∙m nmij +1000 n∙m kriss r el at iv e de vi at io n an d r el at iv e ex pa nd ed u nc er ta in ty in % -0.015 -0.010 -0.005 0.000 0.005 0.010 0.015 nmi and torque step -500 n∙m nmij -500 n∙m kriss -1000 n∙m nmij -1000 n∙m kriss r el at iv e de vi at io n an d r el at iv e ex pa nd ed u nc er ta in ty in % -0.015 -0.010 -0.005 0.000 0.005 0.010 0.015 nmi and torque step +50 n∙m nmij +100 n∙m kriss +50 n∙m nmij +100 n∙m kriss r el at iv e de vi at io n an d r el at iv e ex pa nd ed u nc er ta in ty in % -0.015 -0.010 -0.005 0.000 0.005 0.010 0.015 nmi and torque step -50 n∙m nmij -100 n∙m kriss -50 n∙m nmij -100 n∙m kriss r el at iv e de vi at io n an d r el at iv e ex pa nd ed u nc er ta in ty in % -0.015 -0.010 -0.005 0.000 0.005 0.010 0.015 nmi and torque step +50 n∙m nmij +100 n∙m kriss +50 n∙m nmij +100 n∙m kriss r el at iv e de vi at io n an d r el at iv e ex pa nd ed u nc er ta in ty in % -0.015 -0.010 -0.005 0.000 0.005 0.010 0.015 nmi and torque step -50 n∙m nmij -100 n∙m kriss -50 n∙m nmij -100 n∙m kriss r el at iv e de vi at io n an d r el at iv e ex pa nd ed u nc er ta in ty in % acta imeko | www.imeko.org july 2017 | volume 6 | number 2 | 31 [3] a. nishino, k. ogushi and k. ueda, “uncertainty evaluation of a 10n·m dead weight torque standard machine and comparison with a 1 kn·m dead weight torque standard machine”, measurement, 49, pp.77-90, 2014. [4] k. ohgushi, t. tojo and e. furuta, “development of the 1 kn∙m torque standard machine”, proc. xvi imeko world congress, vol. 3, vienna, 2000, pp.217-223. [5] k. ohgushi, t. ota and k. ueda, “uncertainty evaluation of the 20 kn∙m deadweight torque standard machine”, proc. 19th int. conf. imeko tc-3, cairo, 2005, cd-rom. [6] k. ohgushi, t. ota, k. ueda, y. k. park and m. s. kim, “international comparison of torque standards between the national metrology institute of japan (nmij) and korea research institute of standards and science (kriss)”, proc. asia-pacific symposium on mass, force and torque (apmf), jeju, 2005, cd-rom. [7] k. ohgushi, a. nishino, t. ota, k. ueda, “expansion of the calibration range and improvement of the uncertainty in the 1 kn·m deadweight torque standard machine”, proc. 20th int. conf. imeko tc-3, merida, 2007, cd-rom. [8] d. röske and k. ogushi, “final report on the torque key comparison ccm.t-k2 measurand torque: 0 kn·m, 10 kn·m, 20 kn·m”, metrologia, 53, technical supplement 07008, 2016. [9] d. röske, “a german torque comparison from 20 n·m to 200 n·m and its results”, proc. 22th int. conf. imeko tc-3, cape town, 2014, http://www.imeko.org/publications/tc32014/imeko-tc3-2014-015.pdf. [10] d. röske, “final report on the torque key comparison ccm.t-k1 measurand torque: 0 n·m, 500 n·m, 1000 n·m”, internet: http://www.bipm.org/utils/common/pdf/final_reports/m/tk1/ccm.t-k1.pdf, 2009. http://www.imeko.org/publications/tc3-2014/imeko-tc3-2014-015.pdf http://www.imeko.org/publications/tc3-2014/imeko-tc3-2014-015.pdf http://www.bipm.org/utils/common/pdf/final_reports/m/t-k1/ccm.t-k1.pdf http://www.bipm.org/utils/common/pdf/final_reports/m/t-k1/ccm.t-k1.pdf coverage path planning for a flock of aerial vehicles to support autonomous rovers through traversability analysis acta imeko issn: 2221-870x december 2019, volume 8, number 4, 9 12 acta imeko | www.imeko.org december 2019 | volume 8 | number 4 | 9 coverage path planning for a flock of aerial vehicles to support autonomous rovers through traversability analysis dario calogero guastella1, luciano cantelli1, domenico longo1, carmelo donato melita1, giovanni muscato1 1università degli studi di catania, dipartimento di ingegneria elettrica, elettronica e informatica, viale andrea doria 6, 95125 catania, italy section: research paper keywords: unstructured environments; photogrammetry; terrain traversability analysis; uav/ugv cooperation citation: dario calogero guastella, luciano cantelli, domenico longo, carmelo donato melita, giovanni muscato, coverage path planning for a flock of aerial vehicles to support autonomous rovers through traversability analysis, acta imeko, vol. 8, no. 4, article 3, december 2019, identifier: imeko-acta-08 (2019)-04-03 editor: yvan baudoin, international cbrne institute, belgium received november 22, 2018; in final form april 2, 2019; published december 2019 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: the presented work is in the framework of the clara pon project. the project clara (cloud platform and smart underground imaging for natural risk assessment) is supported by miur under the program pon r&c scn_00451 corresponding author: d. c. guastella, e-mail: dario.guastella@dieei.unict.it 1. introduction the problem with the autonomous navigation of an unmanned ground vehicle (ugv) in outdoor environments, which are most of the times unstructured environments, cannot be considered as having been fully solved in the current robotics literature. in particular, path planning, one of the key elements in the general problem of robot navigation, is still difficult in such scenarios due to the difficulty of taking into account many issues simultaneously e.g. robot kinematics and stability and terrain geometry [1]. furthermore, most robotics research has been carried out on structured environments, such as roads, indoor spaces, and factories, where the vehicle is expected to move within clearly defined regions. a rather common approach to coping with the problem of autonomous outdoor navigation has been to employ unmanned aerial vehicles (uavs) in order to provide an aerial overview of the considered environment or for drone-based mapping [2], [3], [4]. at the same time, photogrammetric 3d reconstruction from aerial surveys has gained more and more relevance over the years, thanks to the enhanced quality of the results and the increased computation power available [5], [6]. such 3d modelling, belonging to the wider class of structurefrom-motion approaches, can provide an accurate representation of the environment across which the ground vehicle has to move, both from the geometrical perspective and from the appearance perspective, thanks to texture mapping. these features allow the performance of a reliable terrain traversability assessment [1] on the recreated environment model. although such processing of the environment on 3d models has been widely adopted in the literature [4], [7], only recent examples exploit photogrammetry from airborne imagery for this purpose [8], [9]. abstract in rough terrains, such as landslides or volcanic eruptions, it is extremely complex to plan safe trajectories for an unmanned ground vehicle (ugv), since both robot stability and path execution feasibility must be guaranteed. in this paper, we present a complete solution for the autonomous navigation of ground vehicles in the mentioned scenarios. the proposed solution integrates three different aspects. the first is the coverage path planning for the definition of uav trajectories for aerial imagery acquisition. the collected images are used for the photogrammetric reconstruction of the considered area. the second aspect is the adoption of a flock of uavs to implement the coverage in a parallel way. in fact, when non-coverable zones are present, decomposition of the whole area to survey is performed. a solution to assign the different regions among the flying vehicles composing the team is presented. the last aspect is the path planning of the ground vehicle by means of a traversability analysis performed on the terrain 3d model. the computed paths are optimal in terms of the difficulty of moving across the rough terrain. the results of each step within the overall approach are shown. mailto:dario.guastella@dieei.unict.it acta imeko | www.imeko.org december 2019 | volume 8 | number 4 | 10 in this article, an integrated strategy for solving the problem of rover path planning in unstructured environments is presented based on the previous experience of the authors in the field. the three main aspects considered herein are: 1) the coverage path planning for aerial vehicles, discussed in section 2; 2) the assignment of the coverage regions to each member of a flock of uavs, described in section 3; 3) the costmap derived from the traversability assessment on the environment model, reported in section 4. finally, in section 5, a brief description of the experimental hardware setup is given, while some considerations about the proposed approach are reported in the concluding section. 2. coverage path planning coverage path planning refers to a special kind of planning algorithm used for large regional surveys. although such planning has been used for a large variety of applications on several kinds of vehicle (ranging from ground [10], [11], to underwater vehicles [12], [13]), here, we focus on aerial vehicles only. in particular, an offline coverage path planning method is proposed, designed for trajectory planning with static 3d models of the environment. such approaches are essentially the opposite of the so-called online methods, where the environment model is dynamically built and can change over time [14]. in our solution, the first step is to define the region of interest (roi) to survey as a 2d top-view area over a georeferenced map. in particular, we consider a georeferenced digital elevation model (dem) of the environment. a dem is a gridded representation of the environment, and its cells contain the height values of the 3d structure. although this type of model does not allow us to include multi-layered or overhanging structures, it offers a helpful 3d representation of the environment, being a fair compromise between the complexity and detail of the model. after that, non-coverable zones within the roi are defined, namely those zones above which we do not want the uav to fly. these zones can be defined as a sequence of vertices, thus obtaining polygonal zones. at this point, the whole area is decomposed into free-to-fly subregions via an exact cell decomposition algorithm i.e. the morse-based decomposition [15]. it is an exact decomposition because all the available free area is included in the subregions, without overlaps. as suggested by the name, this decomposition algorithm is based on an analysis of the singular points of a suitably defined morse function. by considering different morse functions, a wide variety of decomposition patterns can be obtained. in this work, two simple functions have been considered, thus obtaining two linear decompositions: one parallel to the vertical axis and the other parallel to the horizontal axis of the map frame. an example of georeferenced dem, the roi defined over it (in green), and the two types of decompositions are shown in figure 1. easting and northing coordinates (in metres) are respectively reported along the x and y axes, according to the utm (universal transverse of mercator) convention. eventually, coverage trajectories within each region are defined. these trajectories are used for the photogrammetric analysis and reconstruction. in fact, by taking regularly spaced aerial pictures, if a suitable overlap is guaranteed, it is possible to derive the 3d structure of the environment with its realworld size. nowadays, several mapping programs are available that can deliver different kinds of 3d models by processing the acquired pictures, once the details of the acquisition camera are given. in this work, the pix4d mapper program has been used [16]. it can derive textured meshes as well as digital elevation models. as a coverage pattern, the so-called back-and-forth motion has been considered (figure 2), and the trajectories along each side of each region are computed. among these trajectories, the one with the minimum number of turns is chosen. in fact, according to [5], [6], and [16], the manoeuvres involved in a turn (decelerate, move to the following line, and accelerate again) are the main losses of energy and time during the backand-forth motion execution. the resulting trajectories for the vertical decomposition example, chosen according to the figure 1. examples of vertical and horizontal decompositions. figure 2. the back-and-forth pattern chosen as coverage trajectory within each subregion. figure 3. example of the environment decomposition and related coverage trajectories. acta imeko | www.imeko.org december 2019 | volume 8 | number 4 | 11 principle described, are shown in figure 3. 3. coverage subregions assignment once the coverage paths for all the regions have been defined, our approach proposes the adoption of a flock of uavs in order to parallelise and then speed up the mission for the imagery acquisition. a negotiation rule is then used to assign each region to one of the members of the team. the strategy described in [18] has been adopted. it assumes a number of vehicles equal to the number of regions to be surveyed. this strategy involves computing the lengths of the paths to reach each region for all the uavs of the flock from their starting positions. a 3d implementation of the a* algorithm has been used for this purpose [18]. after that, all the possible combinations of uavs/region are derived and, eventually, the combination with the minimum total path length for the whole swarm is chosen. this is obtained by simply summing up the computed trajectory lengths of all the uavs for a certain combination. the enhancement in this work with respect to [18] consists of considering both ends of the coverage trajectories as potential target positions while computing uavs/region combinations. in fact, as underlined by [5], coverage paths can be travelled indistinctly along the two possible directions, thus providing a further variable element in the negotiation task. in figure 4, an example of the target points, the first points of the coverage trajectories assigned to each uav, and the paths to reach them are shown. it is important to note that the information about the terrain geometry given by the dem is considered in trajectory computations. the trajectories in red are those obtained after a line-of-sight length reduction approach, as described in [18]. 4. traversability costmap generation after the coverage mission is carried out by the flock and the environment model is reconstructed by the mapping software [16], a terrain traversability assessment is performed on such model, as reported in [8]. the dem of the terrain is also considered; however, this is the model obtained by the aerial photogrammetric reconstruction, which is much finer than the dem used for the flock mission planning in terms of resolution. this level of detail allows us to perform an analysis of the geometric properties of the environment: the slope of the terrain and the presence of height discontinuities [8]. the outcome of this processing is a map that includes the traversing costs, which is used for the path planning of the rover within the unstructured environment in order to avoid steep paths as well as obstacles. the latter may be either physical obstacles, such as walls; rocks or trees; or non-traversable zones, such as cliffs or areas that are too steep. combining the costmap with classical grid-based path planning algorithms, the trajectories for the ugv can be computed. in this work, the d* algorithm has been used, thus obtaining paths that are optimal in terms of the difficulty of travelling through the environment, according to the traversing costs derived. clearly, the costmap derived by the georeferenced dem includes geographical data, which is fundamental during the autonomous gnss (global navigation satellite system) navigation of the rover. the resulting costmap from the terrain traversability analysis of a subregion within the considered roi is shown in figure 5. darker areas are easily traversable zones, grey areas are those zones that are difficult to figure 4. three-dimensional model of the terrain and uav-to-region paths for a flock with six vehicles. figure 5. map of traversing costs of a subregion within the roi. acta imeko | www.imeko.org december 2019 | volume 8 | number 4 | 12 cross, and white areas are absolutely unpassable zones. 5. experimental setup and testing for the experimental testing, the u-go rover, a rough terrain tracked vehicle, was adopted [19] (figure 6). it embeds a gps-based navigation system, an imu (inertial measurement unit) for the attitude data acquisition, and a frontal 2d laser range finder for the detection and avoidance of local or dynamic obstacles. an on-board companion computer is used to run the robot operating system infrastructure [20] that allowed us to verify and monitor the proper behaviour of the autonomous navigation algorithm during the on-field tests. from the obtained costmaps, the vehicle was capable to autonomously reach a goal in the environment map while avoiding the static obstacles and the steepest paths, which may expose the rover to instability. 6. conclusions in this paper, an integrated strategy for the autonomous navigation of a rover in an outdoor unstructured environment has been discussed. it exploits the 3d photogrammetric reconstruction of the considered area with the help of a flock of uavs for fast coverage missions and, therefore, the subsequent reconstruction. once the 3d model of the environment was recreated, a geometry-based traversability assessment was carried out, and the obtained 2d costmap was combined with a d* algorithm for the path planning of the rover. although geometric terrain traversability analysis represents a well-known approach in robotics research, the novelty of the present work lies in the combination of such approach with drone-based photogrammetry. another contribution is related to the suggestion of a flock of uavs for the coverage mission. most of the single research has been focusing on single-vehicle solutions rather than multi-vehicle approaches. as future developments, we aim to improve the region/uav assignment strategy by taking into account the coverage path lengths within each subregion, besides the distance to reach the region itself. a further enhancement will be to consider the more general case of a number of uavs different from the number of regions to be surveyed. this involves the definition of flock management routines to be applied before the coverage path planning step. references [1] p. papadakis, terrain traversability analysis methods for unmanned ground vehicles: a survey, engineering applications of artificial intelligence 26(4) (2013) pp. 1373-1385. [2] l. cantelli, p. laudani, c. d. melita., g. muscato, uav/ugv cooperation to improve navigation capabilities of a mobile robot in unstructured environments, advances in cooperative robotics (2017) pp. 217-224. [3] b. grocholsky, j. keller, v. kumar, g. pappas, cooperative air and ground surveillance, ieee robotics & automation magazine 13(3) 2006, pp. 16-25. [4] h. balta, g. de cubber, d. doroftei, y. baudoin, h. sahli, terrain traversability analysis for off-road robots using time-offlight 3s sensing, proc. of the 7th iarp international workshop on robotics for risky environment-extreme robotics, 2013. [5] m. torres, d. a. pelta, j. l. verdegay, j. c. torres, coverage path planning with unmanned aerial vehicles for 3d terrain reconstruction, expert systems with applications 55 (2016) pp. 441-451. [6] c. di franco, g. buttazzo, coverage path planning for uavs photogrammetry with energy and resolution constraints, journal of intelligent & robotic systems 83(3-4) 2016, pp. 445-462. [7] t. braun, h. bitsch, k. berns, visual terrain traversability estimation using a combined slope/elevation model, proc. of the annual conference on artificial intelligence, 2008, pp. 177-184. [8] d. c. guastella, l. cantelli, c. d. melita, g. muscato, a global path planning strategy for a ugv from aerial elevation maps for disaster response, icaart 1 (2017) pp. 335-342. [9] r. fedorenko, a. gabdullin, a. fedorenko, global ugv path planning on point cloud maps created by uav, proc. of the 3rd ieee international conference on intelligent transportation engineering (icite), 2018, pp. 253-258. [10] j. prado, l. marques, energy efficient area coverage for an autonomous demining robot, proc. of the robot2013: 1st iberian robotics conference, 2014, pp. 459-471. [11] j. jin, l. tang, optimal coverage path planning for arable farming on 2d surfaces, transactions of the asabe 53(1) 2010, pp. 283-295. [12] l. paull, s. saeedi, m. seto, h. li, sensor-driven online coverage planning for autonomous underwater vehicles, ieee/asme transactions on mechatronics 18(6) 2013, pp. 1827-1838. [13] b. englot, f. s. hover, three-dimensional coverage planning for an underwater inspection robot, the international journal of robotics research 32(9-10) 2013, pp. 1048-1073. [14] a. khan, i. noreen, z. habib, on complete coverage path planning algorithms for non-holonomic mobile robots: survey and challenges, journal of information science and engineering, 2017, 33 1, pp. 101-121. [15] e. u. acar, h. choset, a. a. rizzi, p. n. atkar, d. hull, morse decompositions for coverage tasks, the international journal of robotics research 21(4) 2002, pp. 331-344. [16] pix4d mapper, https://pix4d.com. [17] e. galceran, m. carreras, a survey on coverage path planning for robotics, robotics and autonomous systems 61(12) 2013, pp. 1258-1276. [18] d. c. guastella, n. d. cavallaro, c. d. melita, m. savasta, g. muscato, 3d path planning for uav swarm missions, proc. of the 2nd international conference on mechatronics systems and control engineering, 2018, pp. 33-37. [19] f. bonaccorso, d. longo, g. muscato, the u-go robot, proc. of the world automation congress, 2012, pp. 1-6. [20] m. quigley, k. conley, b. gerkey, j. faust, t. foote, j. leibs, e. berger, r. wheeler, a. y. ng, ros: an open-source robot operating system, proc. of the icra workshop on open-source software 3(3.2), 2009, pp. 5-10. figure 6. the u-go rover adopted for the experimental tests. a layered iot-based architecture for a distributed structural health monitoring system acta imeko issn: 2221-870x june 2019, volume 8, number 2, 45 52 acta imeko | www.imeko.org june 2019 | volume 8 | number 2 | 45 a layered iot-based architecture for a distributed structural health monitoring system francesco lamonaca1, carmelo scuro2, paolo francesco sciammarella3, renato sante olivito2, domenico grimaldi3, domenico luca carnì3 1 ding department of engineering, university of sannio, 82100 benevento, italy 2 dinci – department of civil engeneering, university of calabria, 87036 rende (cs) italy 3 dimes department of informatics modelling electronics and systems science, university of calabria, 87036 rende (cs) italy section: research paper keywords: structural health monitoring, internet-of-things, synchronization, multi-agent system, signal processing, acoustic emission. citation: francesco lamonaca, carmelo scuro, paolo francesco sciammarella, renato sante olivito, domenico grimaldi, domenico luca carnì, a layered iotbased architecture for a distributed structural health monitoring system, acta imeko, vol. 8, no. 2, article 7, june 2019, identifier: imeko-acta-08 (2019)02-07 editor: alessandro depari, university of brescia, italy received july 26, 2018; in final form june 24, 2019; published june 2019 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: domenico luca carnì, e-mail: dlcarni@dimes.unical.it 1. introduction structural collapse of buildings can be caused by the sum of minor damage caused by aging phenomena, prolonged pressure to the structural load curve limit, and many other stress causes which, if promptly identified, could prevent loss of human life [1]. with this aim, several inspections and building maintenance programmes are carried out and regulated by law, according to importance, ownership, use, risk, and hazard [2]. these solutions are insufficient for guaranteeing high safety levels because the periodicity of the human inspections carried out are generally unrelated to the unpredictable time in which structure damage events can occur. the need to carry out continuous and more accurate monitoring has led to increased research and development in the field of structural health monitoring (shm) [3]-[4]. the efforts have been devoted to developing methods, techniques, and systems designed to perform real-time and automated monitoring of buildings [5], detecting the occurrence of structural damaging events (such as a crack in a concrete pillar) that could provoke structural failure or those in any other concrete structures. the sensing part of an shm system generally constitutes a set of wired or wireless sensors, typically based on fibre bragg gratings [6] or micro-electro-mechanical-systems (memss) [2]. both estimate the impact of static and dynamic loads on a pillar, measuring the structural vibration response. other innovative techniques are based on the analysis of the natural frequency of the structure [7]-[10] and investigate the information transmission problem [11]. abstract structural health monitoring (shm) is responsible for identifying techniques and for prototyping systems performing a state diagnosis of structures. its aim is to prevent sudden civil infrastructure failure as a result of several invisible sources of damage. since structural damage is often caused by ground phenomena involving circumscribed geographical areas, it is useful to extend shm systems to allow for the exchange of information among nearby buildings and then to increase the timeliness of the alerts. to this end, in this article, an shm based on the iot paradigm is proposed (shm-iot). shm-iot carries out both localised monitoring on a single building, and it uses information collected by several sensors correlated in time, aiming to identify potentially dangerous damage. it also performs a wider monitoring on a group of buildings in order to alert a larger number of people. shm-iot also sends a remote notification, which is finalised in order to alert the authorities and rescuers about the status of each monitored building. in this context, synchronisation problems arise because the information collected by each sensor of the shm-iot needs to be correlated in time in order to be used for damage evaluation and alert generation. in this paper, the hardware and software architectures of the proposed shm-iot are presented together with the synchronisation requirements and the methods of satisfying them. experiments are undertaken to validate the shmiot in real scenarios. mailto:dlcarni@dimes.unical.it acta imeko | www.imeko.org june 2019 | volume 8 | number 2 | 46 nowadays, techniques based on the use of acoustic emission (ae) associated with a crack event show interesting results in pillar damage detection. aes are elastic radiations generated by the release of energy within the material [12], converted to voltage signals by using piezoelectric sensors applied to the different faces of the pillar. the acquired signal contains information about fracture and plastic deformations, impacts, friction, corrosive film rupture, and other aging processes [13],[14]. the pillars and beams are considered as critical points of the structure that must be monitored in order to prevent critical damage. the number of pillars is determined by several factors, such as the architecture of the structure, in order to transfer the load to the foundation. the use of multiple sensors for each monitoring point determined by the designer of the infrastructure permits detection and localisation of the damage. in the implementation of the monitoring system, the use of wireless networks allows for easy allocation of wireless sensors in the space, increasing the volume of the monitored structure and reducing hardware costs (compared to the adoption of a wired ad hoc network technology). in this article, an improvement on the author’s previous aebased shm system [15],[16] is proposed. the overall system can be divided into two subsections: the first deals with the monitoring of the state of a single structure, and the second, instead, is responsible for sending a notification alarm not only within the building but also to the competent authorities and to the other building alarm systems within the neighbourhood. in fact, if a subsoil event affects a certain area, it is likely that more than a single building may have been damaged. communication among structures is a useful feature for improving the reliability of the shm system by covering possible failure in the detection of damaging events occurring in the building. in fact, if a structure receives a critical event notification from a neighbour, the preventive alert is performed, even if its sensing part has not detected any event. according to the iot paradigm [17],[18], the proposed shm system is designed as a distributed measurement system which follows a layered implementation. each layer has a specific role and requires the fulfilment of specific temporal constraints to work properly, according to the goals that it has to achieve. this is a preliminary study to determine the applicability of the technologies considered therein. future research will be focused on the dislocation of the sensor by analysing a scaling model of the structures and then the number of sensors required to monitor a structure. the paper is structured as follows. section ii covers the state of the art in the use of iot system for shm is provided. section iii presents an overview on the methodology used to develop each layer of an shm. in section iv the results of the experimental validation of the proposed shm are carried out. finally, in section v the conclusions are drawn. 2. state of the art in the iot system for shm in the field of shm, different technologies and techniques are presented in the literature. the information used to infer the health state of a structure are related to global or local structural properties. the methods used to determine these properties are typically based on: ultrasonic or ae; vibration; strain; comparative vacuum monitoring; lamb wave; and e/m impedance. among the architectures proposed in the literature to apply the iot paradigm to the shm, in [19], a real-time platform for shm combining the information provided by two techniques implemented on wireless sensors is proposed: the pitch-catch and pulse-echo sensors. the former is used to detect damage, the second to localise the damage. one limitation of this proposal is that each node needs a cable to connect the ultrasonic transmitter and receiver to the opposite side of the critical point under monitoring and it is tested only on homogeneous material. in [20], an open platform to implement a bridge structural health monitoring system based on iot is proposed. similarly, in [21], a smart wireless structural monitoring system for a self-anchored suspension bridge is proposed. the system in [21] uses bluetooth technology and a tcp/ip network protocol. among the methods proposed in the literature for shm, which can be used in an iot-based system, in [22] an ultrasonic inspection system for the shm is proposed. additionally, in [23], the huanghilbert transform is used to analyse the sensor data to infer the health of a structure. however, the analysis was only made on simulated data. in [1], the hilbert transform was used on ae signals, and it has enabled the experimental highlighting of two different phases in the damage to the concrete. further possibilities of iot in civil engineering for shm include the recording and storage of a large quantity of data related to the engineering constants of the materials of a structure (elastic modulus; poisson coefficient; and compressive and tensile strength). these constants can thus be used to improve the analysis of numerical models based on a finite element [24]. 3. shm based on multi-agent iot the proposed system considers the shm system as a network of interconnected smart objects (sos) that can be installed in a non-invasive way on the pillars of a structure in order to carry out real-time structural monitoring. each so is modelled by following a hierarchical layered architecture (see figure 1), comprising: • a physical part that includes all the sensors and actuators; and • a cyber part that includes all the algorithm and software protocols. challenging in this design is the matter of ensuring that all the time constraints on the synchronisation accuracy required by each layer to work properly are satisfied. moreover, although it is going up in the hierarchy, the growing level of abstraction is coupled with the relaxation of the time constraints, and problems could arise from the synchronisation functioning among different layers (for example, in operations such as data figure 1. hierarchical layered implementation of a shm system. application layer event detection layer signal processing layer sensing layer cyber part physical part acta imeko | www.imeko.org june 2019 | volume 8 | number 2 | 47 exchange), which limits the reactivity of the overall system, highlighting a separate block: non-holistic functioning. by collecting, processing, and combining the information coming from the individual sos, it is possible to achieve complex monitoring applications. in particular, the operations computed by each so are gathered into two groups. the first group includes all the tasks finalised to the local detection of damage, such as: • acquisition of ae signals; • processing of acquired signals to classify whether they are associated with a critical event or not; and • correlation of multiple critical events to infer whether a dangerous damage event has occurred. after the identification of a critical event or the reception of a dangerous event notification, the second group of operations performs tasks aimed at guaranteeing people’s safety. exploiting the iot paradigm, sos can communicate with each other through the network [25],[26], and the performed actions include: • the propagation of an alarm throughout the building in order to evacuate it; • the sending of a remote message to advise the competent authorities; and • the sending of a remote notification to the other buildings included in a fixed neighbourhood to suggest their evacuation. the latter feature is introduced to increase the reliability of a group of sos installed in the same building and to extend the implementation of an shm system on a set of structures. in fact, since events generated in the subsoil generally affect a wide geographic area, it is probable that not only has one single structure been damaged, but also its neighbours have suffered damage. therefore, if a building detects structural damage in itself, it spreads the news towards its neighbours to advise their preventive evacuation. this is useful for avoiding risks due to a possible failure in the detection of a dangerous event. all local and remote communication operations performed by the sos are carried out exploiting the agent programming paradigm. as defined in [27], an agent is a software entity that is capable of performing an autonomous operation in order to reach an established behavioural goal. each so is equipped with an agent that, through its properties (such as reactivity and proactiveness [27]), supervises and manages the monitoring operations, choosing autonomously when it is necessary to interact with other sos. the main properties that agents offer that are useful for modelling the functioning and the dynamics of a distributed system [27] are: • sociality and mobility, which simplify the implementation of data exchange and offer a distributed view of the application; • concurrent data processing, which allows the acquisition of a high computational throughput; • extensibility, which allows the addition of new features to the application or updating of the used technologies, avoiding any redesign cost; • easy detection of malfunctions by simple problem isolation; • role decomposition, which permits the system to be scaled. the whole shm system can be seen as a federation of interacting agents, also called multi-agent system (mass). in such a system, critical aspects relate to the implementation and synchronisation issues connected with the development of a single so. these aspects can be addressed by analysing each layer of figure 1 in detail. 3.1. sensing layer to detect the structural damage, avoiding the use of invasive techniques, the proposed architecture uses the detection of the ae generated in a pillar and in the evaluation of its characteristics. as shown in figure 2, if a pillar is over-stressed, a set of microcracks will be generated within it. if the pressure is exceeded for a long time or if it increases in its intensity, these micro-cracks tend to be generated at multiple points inside the pillar. consequently, if the micro-cracks couple with each other, highly critical damage can be caused inside the pillar, which can be recognised by detecting the occurrence of the cracks in a time window. as shown in figure 1, the lowest level of the proposed hierarchy is the sensing layer. that layer is responsible for carrying out continuous real-time monitoring of the structure, exploiting the sos installed on the pillars, and raising the alarm if a critical event occurs. low-level ae acquisition is achieved using a set of piezoelectric sensors (pss) equipped on each so. the main problems resolved in this layer by using the proposed architecture are concerned with: • the identification of the signals of interest, distinguishing it among environmental noise and sounds produced by a mini-crack i.e. a crack that has low amplitude with respect to a preestablished threshold and cannot be considered significant in the evaluation of the pillar health; • the storage and acquisition only of signals associated with potentially critical events in order to save computational resources; and • the synchronisation of measurements coming from different sos in order to ensure that they are related to the same phenomenon. figure 2. example of crack generation. figure 3. distributed structural health monitoring architecture. acta imeko | www.imeko.org june 2019 | volume 8 | number 2 | 48 figure 3 presents that all these issues have been solved by using the logic flat amplifier and trigger (l-fat) component, described in [15]. l-fat works together with the data acquisition system (daq), extending its capabilities and guaranteeing no loss of signal and no wastage of storage memory. the wastage of memory is related to the requirements of continuous storage of the input signals to guarantee no loss of events. l-fat allows for the implementation of a suitable logic for the memorisation. the l-fat component is responsible for conditioning the input signals coming from pss, amplifying their values, and managing the beginning of the acquisition operations by sending a trigger to the daq. that trigger is generated only when one of the signals perceived on the input channels exceeds an experimentally fixed threshold. by appropriately calibrating that threshold, it is possible to store and process only signals associated with a potential event of interest. to avoid the loss of signal of interest samples due to the trigger propagation delay (estimated in the order of ns), it is possible to set daq functioning in a pre-trigger mode. in this way, the board is enabled to acquire a fixed number of samples before the trigger occurrence, estimated for the aes using the hsu-nielsen test [28]. all the operations involved in the acquisition phase are timecritical, and a synchronisation accuracy in the order of the s is needed to establish that the different measurements are related to a same event/phenomenon. the parallel-channel architecture of the l-fat component is able to satisfy that constraint, introducing a propagation delay among the acquired signals estimated in the order of 20 ns, with an uncertainty of a few ns [15], which can be considered negligible. 3.2. signal processing layer the processing layer includes the algorithms needed to perform low-level processing of the acquired signal. in particular, the sensing layer using the l-fat acquires the signals related to a crack. this level acts as a filter, discarding among the signals received all those with an intensity that is insufficient for consideration as being associated with the occurrence of a dangerous event. to achieve this goal, the signal processing layer implements all the mathematical operations and the procedures finalised to: • achieve the processing of the acquired signals; and • make the information about the number of the crack identified as dangerous available to the higher software levels. exploiting the similarity among the crack ae signals and those associated with earthquakes [29], it is possible to use the gutenberg richter (gbr) law in order to estimate the crack intensity and to determine the damage level. using a variant of the gbr law [30], reported in equation (1), critical cracks are characterised by a value of b in the neighbour of 1 [13]. 𝐿𝑜𝑔(𝑁) = 𝑎 − 𝑏 ∙ 𝐴𝑑𝑚 (1) where n is the number of hits higher than the threshold noise, experimentally fixed at 40 db. the adm variable represents the maximum amplitude peak of the ae signal. the a and b parameters are two constants that are experimentally fixed by using the techniques reported in [13]. considering that for an event characterised by a higher magnitude and the b-value tends to 1, the a value is experimentally evaluated by using equation (1). with this value, b is also estimated by using equation (1) for the non-critical events. with this technique, the a value obtained for a sample of rck25 is 14.45. as shown in figure 4, the signal processing layer keeps track of the number of potentially dangerous cracks within a counter variable (cv). the value of the cv is needed by the higher software level in the hierarchy for it to carry out its own operations. to make the system as reactive as possible, the decoupling among the updating time of the signal processing layer (determined by the event occurrence) from the periodicity associated with the event detection layer is required. to perform the data exchange, non-blocking mechanisms are used, specifically to increase parallelism and to improve performance. the cv variable is stored in a shared memory location, accessible in windows by using other software, using the dynamic link library. consequently, the main problem in this layer lies in synchronising the read-and-write access of the shared variable among the different software components. to overcome this issue, the dekker mutual exclusion algorithm was used [31]. 3.3. event detection layer continuing to rise in the hierarchy of figure 1, the next level is the event detection layer. it represents a front end between the functioning of the single smart object and the remaining part of the system. the event detection layer: • allows for the identification of the occurrence of effective dangerous damage in the pillar, which is monitored locally by the specific so; and • sends a notification to the node hosting the application layer through the lan. the operation of this tier and the algorithms needed to achieve its goals are schematized in figure 5 and implemented within the behaviour of a software entity called the event detection agent (eda). each so is equipped with one eda, which exploits the agent message-passing protocol in order to figure 4. layer interactions to perform the low-level crack identification. figure 5. event detection layer functioning. acta imeko | www.imeko.org june 2019 | volume 8 | number 2 | 49 exchange data with the next layer. to detect dangerous damage within a pillar, the first operation that the eda algorithm performs is the continuous monitoring of the cv value according to an observation period to=1 s. this periodicity is acceptable, and a greater resolution is not required because to is compatible with the time constraints needed by the higher layer to perform its operations. since the recent literature [32] assesses that the structure can be considered dangerously damaged if at least three events of interest are detected in a time interval of 60 s, if the eda detects a variation in the cv of 3, it automatically detects the hazard situation and sends the appropriate notifications. 3.4. application layer when the eda in the event detection layer identifies dangerous damage, it sends a message to the monitoring agent (ma), which implements the high-level operations of the application layer (see figure 6). that ma is responsible for: • enabling the actuators (such as alarms) to command the evacuation of the building. this is performed by sending an alert message to all the sos spread throughout the structure, using the lan; • sending two types of remote notifications using the wan: o the first to the authorities (such as firefighters), to require their intervention; and o the second to the monitoring agents installed on the structures included in the neighbourhood. no strict time constraints are required to implement the notification mechanism via lan and globally via wan, because any propagation delay, estimated in hundreds of ms [32], is negligible. the reaction-actions that will be performed by humans do not in fact require the fulfilment of hard real-time constraints. in fact, the addition of a few ms has no impact on the time needed to carry out an evacuation, the implementation, or the arrival of the authorities for the inspection. the only critical constraint that must be satisfied is that both remote and local notification messages must arrive at their destination. in order to also implement the propagation of the warning to nearby structures, it is necessary that the ma knows and can contact the mas installed thereon. to solve this problem, the proposed shm system offers two solutions. according to the first solution, each ma statically memorises information about the neighbours’ ips in a map, which are appropriately initialised during configuration. the advantage of this approach is that no further network communication is required to discover the neighbourhood, and the disadvantage is that if a new structure is built or if the neighbourhood radius is extended, manual reconfiguration in needed. the second proposed solution is a compromise between the advantage and disadvantage of the previous solution. following the agent philosophy, a single ma can periodically update its list of neighbouring nodes, requesting it for a yellow pages service, which acts as a directory facilitator. issues concerning fault tolerance and possible reduction of incoming traffic on the node that offers this service are resolved with node replication mechanisms. 3.5. remote transmission protocol if all the agents are on the same so, they can exchange information locally, exploiting their sociality propriety by means of local message passing. instead, if they are deployed on different sos, the data exchange requires the use of the network and mechanisms that support the correct message delivery in a distributed environment. the mas architecture proposed in [33] includes the gateway component, which has the task of managing the distributed data exchange, exploiting different protocols. in particular, it exposes to the agents the basic read-and-write operations needed to interact with physical devices or other remote cyber components, hiding all the details about the communication protocols used. to develop the proposed shm, the gateway has been improved with the introduction of a new module, which enables data exchange on the internet. according to the iot paradigm, in order to avoid the active waiting due to the polling cycle between sender and receiver agents (i.e. different mas or among edas and mas) and to guarantee no packet loss in the communication, the message queue telemetry transport (mqtt) protocol is used [23]. mqtt is based on the publish-subscribe communication paradigm. the architecture does not allow direct data exchange between publisher and subscriber but provides an entity called a broker, which acts as a mediator. subscribers register themselves to the broker, specifying the topic of the data that they want to receive. when the publisher makes a piece of data available, it dispatches that information to the broker, adding a topic string label that summarises its content. when the broker receives the data, it forwards and delivers the data to the appropriate receivers only if the associated topic is the same as the one that was requested. it is worth noting that by using the broker, publishers ignore the details related to the subscribers’ locations and vice-versa, guaranteeing the operations asynchronicity. mqtt was also chosen because it is designed for networks with low bandwidths and high latency. it uses reduced header and payload for the packet transmission, estimating the transmission’s upper bounds delays in the order of 56 ms [35]. furthermore, mqtt also offers three quality-of-service (qos) levels for the reliability of message delivery [36], summarised as follows: level 0 guarantees a best-effort performance because a message is delivered once at the most, and no acknowledgement of receipt is required. this level ensures reduced transmission times, but there is no reliability concerning delivery. in level 1, every message is delivered at least once to the receiver, and confirmation of message receipt is required. this level ensures that the message arrives with the receiver, but duplicates can occur. level 2, by means of a four-way handshake mechanism, guarantees that each message is received only once by the receiver. it is the safest and also the slowest qos level, and it ensures that delivery occurs and that network congestion and packet duplication are avoided. figure 6. application layer communication. acta imeko | www.imeko.org june 2019 | volume 8 | number 2 | 50 to ensure that the notification alarm produced by an so of the application layer reaches the other destinations, it is appropriate to use qos level 2. this level ensures delivery and reduces network traffic at the cost of a slightly longer transmission delay. the increase in the delay is acceptable compared with the timing of human reactions. 4. experimental validation experiments were carried out to validate the proposed shm system. figure 7(a) shows the experimental test bed. it comprises: • compression system: o matest high stiffness compression machines with load control (mod. yimc109ns); and o pc. • monitoring system: o sensing layer: ▪ four ae transducers r15, operating in the frequency range [50, 200] khz, with a peak sensitivity of 69v/(m/s), a resonant frequency of 150 khz, and a directionality of ±1.5 db; ▪ the l-fat component with four input channels; and ▪ the data acquisition board daq (ni 6110 pci), allowing a sampling frequency of 5ms/s for each input channel and a resolution of 12-bit. o signal processing layer: ▪ hp pc-desktop, 2gb of ram, windows xp. o application layer: ▪ macbook, 8gb of ram. figure 7(b) shows the detail of the sensors’ positioning over the concrete specimen. in particular, the four sensors were placed on the free faces of the specimen. according to [37],[38], the b-value acceptability parameter for the detection algorithm, is selected in the range [0.9-1.2]. the threshold of the l-fat for sending the acquisition trigger is established with the hsu-nielsen test [39], which is 0.7 v. the number of pre-trigger samples settled in the daq is 1000. tests were conducted on six specimens, and four are characterised by the typical resistance used in concrete structures. two of them are characterised by a very high resistance with respect to the typical values. the results show that in the case of typical resistance, the three dangerous cracks in the time interval of 60 s detected around 80 % of the maximum load curve (figure 8). in the other two cases, however, the ishm system identified only two cracks instead of three. this may be due to the a and b values used in the identification of the events that are established on the basis of typical resistance values. the validation of the sensing layer with respect to the time constraint is performed by comparing, for each event, the time difference among the p-wave of the signals acquired by the four sensors and by verifying that this time difference is compatible with a position of the crack inside the specimen according to the speed of the wave in the concrete. in order to evaluate the one-way delay from publisher to subscriber, it is considered that the iot devices operate in the same lan. in this scenario, the one-way delay is evaluated by executing multiple instances of mqtt publishers and subscribers. the subscribers provide multiple messages that flow through the broker to the subscribers. figure 9 depicts the architecture of the test bed. several instances of mqtt publishers run equally distributed on two raspberrypi minipcs called raspberry#0 and raspberry#1. one instance of the selected mqtt broker (mosquito) is executed on pc#1. several instances of mqtt subscribers run equally distributed on two raspberrypi systems called raspberry#2 and raspberry#3. the delay measurement system is installed on pc#2. the delay measurement system deploys the open-source network analyser tool wireshark [39] in order to (i) capture network packets in real time, (ii) select only the packet exchanged by the agents running on the hp and mac computers, (iii) save figure 8. load vs. time. the round marker highlights 80% of the load and the triangular marker indicates the maximum load. figure 9. application layer communication. (a) (b) figure 7. (a) experimental test bed overview; (b) details of the sensors’ positioning over the concrete specimen. acta imeko | www.imeko.org june 2019 | volume 8 | number 2 | 51 the acquired information in human-readable format, together with the acquisition timestamp, and (iv) evaluate the one-way delay from the transmission of a packets up to its receipt. it is worth noting that the packets are timestamped by wireshark by using the clock that equips pc#1. this solution does not require the use of protocols to synchronise the clocks that equip the raspberrypi board in order to evaluate the packet delay [38]. consequently, we avoid that the synchronisation uncertainty that characterises the actual realisation of such protocols would degrade the accuracy of the packet delay measurement [40],[41]. all components in the testbed are connected to a network hub. this connection allows each packet to be captured as soon as it is sent by the raspberrypi board and then the one-way delay values are considered as a function only of the message flows. the proposed application scenario is the worst case because in a typical scenario, all the subscribers and publishers have a dedicated device. the implementation on the same device of difference publisher or subscriber processes has the following effects: (i) all the publishers/subscribers share the reduced computational resources and (ii) several messages sent by the publishers are queued to the same network interface. table 1 shows the results obtained by the experimental testbed considering different numbers of message flows produced by the publishers and received by the subscribers. as expected, the mean  and standard deviation  values increase along with an increase in the number of message flows. the maximum delay, in the order of tens of ms, is acceptable to correlate in time dangerous events registered on different components of the same structure. the evaluation of the one-way delay from publisher to subscriber on the internet is not useful, since it is related to human reaction in the case of an event. therefore, a delay in the order of some seconds, typically in the case that the internet is used, is fully acceptable on this level. conversely, guarantee of the receipt of the alarm by the authorities is a fundamental requirement. such a need is satisfied by using the mqtt with acknowledgment of receipt. 5. conclusions in this article, a shm system based on an iot paradigm was proposed. the features provided by the proposed system include the localisation of the damage; the classification of the damage to recognise a possible danger for a single building and for neighbouring buildings; and the sharing of the information with the authorities to prevent the critical degradation of the building status. particular attention was given to the synchronisation problems arising from the distributed sensor network composing the proposed system. in this regard, a hardware and software architecture is proposed that fulfils all the synchronisation requirements. experimental tests validated the effectiveness and suitability of the proposed system in a real context. references [1] g.siracusano, et al., a framework for the damage evaluation of acoustic emission signals through, mechanical systems and signal processing, elsevier, 75 (2016) pp.109-122. [2] j.m.brownjohn, structural health monitoring of civil infrastructure, philosophical transactions of the royal society of london a: mathematical, physical and engineering sciences, 365 (2007), pp. 589-622. [3] c.u.grosse et al, “wireless acoustic emission sensor networks for structural health monitoring in civil engineering”, proc. of european conf. on non-destructive testing (ecndt), 25-29 september, 2006, berlin, pp. 1-8. [4] r.morello, et al., ‘remote monitoring of building structural integrity by a smart wireless sensor network’, international instrumentation and measurement technology conference (i2mtc 2010), 3-6 may, austin, texas, 2010. [5] g.t.webb, et al., categories of shm deployments: technologies and capabilities, journal of bridge engineering, 20 (2014), pp. 115. [6] r.c.tennyson, et al., structural health monitoring of innovative bridges in canada with fiber optic sensors, smart materials and structures, 10 (2001), pp. 560-573. 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[17] r.mahmoud et al., “internet of things (iot) security: current status, challenges and prospective measures”, proc. of 10th international conference for internet technology and secured transactions (icitst), 14-16 dec., london, uk, 2015, pp. 336341. [18] l.atzori, et al., the internet of things: a survey, computer networks, 54(15) (2010) pp. 2787-2805. [19] w.j.staszewski, b.c.lee, l.mallet, f.scarpa, structural health monitoring using scanning laser vibrometry: i. lamb wave sensing, smart materials and structures, 13(2) (2004) pp. 251-260. [20] c.lazo, p.gallardo, s.céspedes, ‘a bridge structural health monitoring system supported by the internet of things’, proc. of the ieee colombian conference on communications and computing (colcom '15), may, 2015, popayán, colombia, pp. 1-6. table 1. packet delays with respect to multiple publishers and subscribers. #publisher #subscriber max (ms) min (ms)  (ms)  (ms) 1 1 14.72 0.01 0.09 0.79 1 3 13.35 0.01 0.6 0.42 3 3 13.14 0.01 0.39 2.11 5 10 23.98 0.01 3.62 4.03 10 5 22.01 0.01 5.12 4.27 10 10 21.02 0.01 7.14 6.55 acta imeko | www.imeko.org june 2019 | volume 8 | number 2 | 52 [21] g.heo, j.jeon, a smart monitoring system based on ubiquitous computing technique for infra-structural system: centering on identification of dynamic characteristics of self-anchored suspension bridge, ksce j. civil eng., 13(5) (2009) pp. 333-337. [22] e.la malfa ribolla, m.rezaee hajidehi, p.rizzo, g.fileccia scimemi, a.spada, g.giambanco, ultrasonic inspection for the detection of debonding in cfrp-reinforced concrete, structure and infrastructure eng. j., 14(6) (2018) pp. 807-816. [23] h.zhang, j.guo, x.xie, r.bie, y.sun, ‘environmental effect removal based structural health monitoring in the internet of things’, proc. of the 7th international conference on innovative mobile and internet services in ubiquitous computing (imis ’13), july, 2013, taichung, taiwan, pp. 512-517. [24] r.s.olivito, r.codispoti, c.scuro, a seismic analysis for masonry constructions: the different schematization methods of masonry walls, aip conference proceedings, 1906(1) (2017) p. 090007. [25] j.p.lynch, et al., a summary review of wireless sensors and sensor networks for structural health monitoring, shock and vibration digest, 38 (2006) pp. 91-130. 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[34] k.tang, et al., ‘design and implementation of push notification system based on the mqtt protocol’, proc. of the international conference on information science and computer applications, 2013, pp. 116-119. [35] l.de vito, et al., ‘an iot-enabled multi-sensor multi-user system for human motion measurements, in proc. of medical measurements and applications (memea), 2017, pp. 210-215. [36] d.thangavel, et al., ‘performance evaluation of mqtt and coap via a common middleware,’ proc. of intelligent sensors, sensor networks and information processing, 2014, pp. 1-6. [37] f.leccese, et al., a smart city application: a fully controlled street lighting isle based on raspberry-pi card, a zigbee sensor network and wimax, sensors (switzerland), 14 (2014) pp. 24408-24424. [38] d.l.carnì, d.grimaldi, g.guglielmelli, f.lamonaca, ‘synchronization of measurement instruments co-operating into the w-dms’, proc. of imtc 2007 – ieee instrumentation and measurement technology conference, 1-3 may, 2007, warsaw, poland, pp.1-6. [39] wireshark [online] available: https://www.wireshark.org [40] f.lamonaca, d.grimaldi, r.morello, a.nastro, ‘sub-µs synchronization accuracy in distributed measurement system by pda and pc triggers realignement’, proc. of i2mtc 2013, ieee international instrumentation and measurement technology conference, 6-9 may, 2013, minnesota, usa, pp. 801-806. [41] f.lamonaca, d.l.carnì, m.riccio, d.grimaldi, g.andria, preserving synchronization accuracy from the plug-in of nonsynchronized nodes in a wireless sensor network, ieee transactions on instrumentation and measurement, 66(5) 2017, pp.1058-1066. https://www.wireshark.org/ acta imeko, title acta imeko issn: 2221-870x december 2017, volume 6, number 4, 37-45 acta imeko | www.imeko.org december 2017 | volume 6 | number 4|37 affordable evaluation of low frequency electric fields from the standpoint of directive 2013/35/eu alexandru salceanu, eduard lunca, marius paulet ”gheorghe asachi” technical university of iasi, mangeron str. 67, 700050, iasi, romania section: research paper keywords: directive 2013/35/eu; low frequency electric fields; health and safety citation: alexandru salceanu, eduard lunca, marius paulet, affordable evaluation of low frequency electric fields from the standpoint of directive 2013/35/eu, acta imeko, vol. 6, no. 4, article 6, december 2017, identifier: imeko-acta-06 (2017)-04-06 editor: paul regtien, measurement science consultancy, the netherlands received june 23, 2017; in final form november3, 2017; published december 2017 copyright: © 2017 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: alexandru salceanu, e-mail: asalcean@tuiasi.ro 1. challenges for engineers emerging from directive 2013/35/eu on the 26th of june 2013, in the official journal of the european union, the directive 2013/35/eu of the european parliament and of the council [1] was published, focused on the minimum health and safety requirements regarding the exposure of workers to electromagnetic fields. this directive already entered into force on the 1st of july 2016, being mandatory for any enterprise in the european union to apply and comply with its provisions. consequently, there is a large concern between employers, stakeholders and authorities regarding the effective application of the associated settlements. how could be decided that we have to deal or not with a dangerous situation? for the very beginning, it is the responsibility of the employer, but in the second phase, a state protection authority should check the orderliness and the conformation to the imposed limits. basically it is about health and safety at every workplace, a fundamental conditioning all over europe. even since june 1989, the european union (eec at that time) has unequivocally assumed the priorities in the field of employees’ health and safety, through the adoption of the framework directive 391 on improving the conditions for the general protection of employees at work, the so called “occupational safety and health” (osh). within this general directive, the directive 2004/40/ec, the 20-th specific one, has been also adopted, strictly focused on the risks arising from human exposure to electric and magnetic fields. due to the impressive growth rate of electromagnetic traffic and the quasi-generalized presence of equipment that uses either electricity or transmits information or energy through abstract since the 1st of july 2016, the directive 2013/35/ eu on the employees’ health protection in terms of non-ionizing electromagnetic radiations acquired the force of law. an accessible methodology for the characterization of a workspace is proposed here, in terms of exposure to low-frequency electric fields. firstly, the means whereby an external electric field can induce electrical processes in the human body are presented, followed by a comparative summary of differently expressed exposure levels (icnirp, european directive, ieee-ices). further on two electric field sensors are presented that can easily be hand-crafted in any laboratory, useful for extending the capabilities of a budget, low-frequency handheld spectrum analyzer. a realistic exposure metric is developed that cumulates the influence of all e-fields in the environment. a case study is presented on the cumulative assessment of exposure to low frequency electric fields produced in a laboratory-class where a network of 16 computers was working. a simple numerical approach based on femm 4.2 has also been developed, for evaluating the e-field produced by overhead high voltage transmission lines. this paper is an extended version of the original contribution to the imeko tc 4 2016 symposium in budapest, hungary. acta imeko | www.imeko.org december 2017 | volume 6 | number 4|38 radio waves, the interest of both the general public and the specialists or authorities has increased in view of the potential negative effects on human health. this required the replacement of the above-mentioned directive only after 9 years of applicability, which is a record in the “world” of european legislation. this unusual dynamic is a convincing and plain argument for the actuality but also for the subtleties of the addressed issue. more specifically, the new directive assumes more rigorous defined values for the so-called exposure limit values (elvs) and action levels (als) as regarding the induced electric fields in the body and respectively, the measured non-ionizing field strengths in the controlled environment. in essence, this document does not deal with the possible long-term effects of variable electric and magnetic fields, a domain in which fundamentally different views and approaches exist in the international community of the strongest interdisciplinary collectives of researchers. the directive mainly establishes in an engineering manner the limitation of exposure to electrical and magnetic fields at values below those at which biophysical (direct or indirect) effects were scientifically proven. one basic principle stipulates: better to prevent than to attempt restoring health! the original guidelines established by international commission on non-ionizing radiation protection (icnirp) are based on the recording of short-term sensory effects related to action of internal electric fields on the central nervous system, the most well-known phenomenon being the “retinal phosphenes”. there are many (quasi-different) approaches regarding the “correlation” between the electric and magnetic field strengths in the human’s environment and the induced electric fields and current densities in the human body. obviously, they are considered as unacceptable, current densities that become comparable to currents produced by heart beats or producing excitations of the optic or acoustic nerve. any employer should adopt actions aiming to protect his employees, by not overcoming these (measurable) values for the als of the electric and magnetic fields in the workplace neighborhood. the elvs and als, stipulated in this directive, are founded on the suggestions released by icnirp, in 1998 and later in 2010, even if the terms and the limit themselves are quite different. more specifically, the elvs refer to the maximum levels accepted for the electric fields induced in the human body due to the presence in the workers’ environment of any alternative magnetic or electric fields, corresponding to the icnirp basic restrictions, while als represent the corresponding “icnirp reference values” for the external electric and magnetic fields that could be directly measured. the effects might be: direct (thermal, mainly heating but also non-thermal, like electric fields induced in the body determining stimulation of muscles, nerves or sensory organs or like currents established in the limbs) or indirect, caused by the action of a specific object placed in the field (pacemakers, other implants, detonators or fires produced by sparks). stake-holders’ expectations in relation to the part of the verification and control that must be done by the engineers concern first of all the correct measurement of the incident fields and, in the next stage, the calculation, analytical or by modeling, of the current densities or electric fields induced within the human body. 2. support for micro, small and medium-sized enterprises the directive in its second chapter, entitled “obligations of employers” is executive, with three consecutive types of action: the assessment of risks, (identification of places with possible high exposure levels followed by their determination, by measurement or calculation), actions that should be done in order to avoid or at least to reduce the assessed risks and the effective participation of the workers at miscellaneous programs of training, also including continuous information and consultation. for the effective implementation of directive 2013/35/eu, the commission has elaborated various practical guides, accepted by the regulation authorities in the domain that should provide a reliable scientific approach of the effective exposure to non-ionizing radiations, coupled with the evaluation of the efficiency of the adopted reducing methods [2]. there are two fundamental aspects that this directive wants to regulate. of course, mainly it is the health and safety of all eu workers. but equally important is the creation of a nondiscriminatory business environment which, by imposing the same restrictions on all (industrial) operators, supports the framework of a fair economic competition. is there really a need for technical and logistic support for micro, small and medium-sized enterprises (msmes)? undoubtedly yes, the answer being argued by the following statistics. in the eu, over 92 % of enterprises are micro or small (having less than 10, respectively 50 employees), providing about 67 % of jobs, thus being vital for the eu economy. in order to survive and withstand the market, they have to face many challenges, one of them being the compliance of the same quasi-tough restrictions from the point of view of osh as the large enterprises, which have completely different human and financial resources. evidences of european agency for safety and health at work (eu-osha) show that msmes have greater difficulties in controlling risks than larger ones. various studies, including the eu-osha’s own european survey of enterprises on new and emerging risks (esener) [3], show that the challenges in handling osh are particularly significant as the enterprises get smaller. in other words, it is expectable as a medium or small employer cannot afford to permanently engage high qualified workforce, needing expensive instrumentation and sophisticated software for a reliable certification of the adherence to the stipulated exposure limits. pre-compliance tests and methodologies should be easier to be performed, offering a good preparation for the strictly specialized measurements. the main objective of the here presented paper is to propose a practicable sequence and an affordable device for assessing the low frequency (up to 100 khz) electric fields (lfef) in the residential or occupational environment. additionally, a modeling done by using open source software, which allows comparison with effectively performed measurements, leading to approximately equivalent or just only comparable results should be beneficial for validation. acta imeko | www.imeko.org december 2017 | volume 6 | number 4|39 3. e-field exposure, from elv to al the guidelines evolved by international authorities, mainly focused on health effects of the electric and magnetic fields, generally consider three “frequency milestones”, 1 hz, 100 khz and 10 mhz that divide the spectrum in four ranges: static, low frequency, medium frequency and high frequency. this partition is justified by the intrinsic nature of the dominant effects: non-thermal (nerve or muscle stimulation), dominant for frequencies lower than 100 khz or thermal (heating of the tissue as a result of energy absorption, firstly in the skin region), for higher frequencies, with different measurement techniques and associated instrumentation used for an accurate determination. for frequencies up to 100 khz, the near field -far field border is at least at 500 m distance from the source, involving the compulsoriness of different measurements for the separated electric and respective magnetic fields [4]. in principle, high voltages (meaning high linear density of charge, measured in c/m) are generating dominant electric fields, while large currents are mainly sources of dominant magnetic fields. obviously, the electric or magnetic fields have different units of measurement, so they cannot be compared as such. they could, however, be compared by means of the energy values (volumetric density) associated with them, εe2/2 and respective µh2/2. if the densities of these stored energies are equal (in free space) the value 120π (377 ω) is obtained for the ratio between electric and magnetic field strengths, the wellknown characteristic impedance of free space. a higher value for this ratio means the preponderance of the electric field. by establishing the need for separate approaches, in this paper we only deal with human exposure to electric fields having frequencies lower than 100 khz. the electric fields are generated by the electric charges that produce a potential difference between the “plates” of a permanent or just occasionally appeared capacitor. the biunivocal link between voltage and electric field could be very easy disturbed by any object with some electric properties placed between the source and the measuring point. more specifically, the human body, a good conductor at low frequencies, significantly disturbs the distribution of the electric field lines in its vicinity. there are many international agencies and organizations involved in establishing exposure restrictions on scientific basis: icnirp [5], [6], institute of electrical and electronics engineers-international committee on electromagnetic safety (ieee-ices) [7], [8], world health organization (who) or international agency for research on cancer (iarc). an external (alternative) electric field induces a superficial electric charge on the surface of the exposed body. this phenomenon implies the emergence of internal currents required to “move” the induced electric charges on the body surface. physicians and biologists have ascertained internal electric fields with the smallest sensory effects and respective higher values at which adverse health effects may occur [1], table 1. it should be here recalled that in the previous documents elaborated by icnirp in 1998 [5] and assumed by eu immediately after 2000, are mentioned as base restrictions not the induced internal electric fields but the induced current densities that could produce stimuli of the central nervous system. these values were around 10 ma/m2. anyway, the established current density j [a/m2] and induced internal electric field eint [v/m] are vector related by the frequency dependent electric conductivity σ [s/m] of the medium, equation (1), a customized expression of ohm's law: 𝐽 = 𝜎𝐸int (1) initiating from these basic restrictions, reference levels (maximum accepted values for the incident, exterior electric field, also called als) have been set, which should not be exceeded. the basic idea is that an accurate and non-invasive measurement of about 1 v/m electric fields or 10 ma/m2 current densities (all set inside the human body) represents a virtually impossible task. measuring four orders of magnitude higher electric fields in the free space is a much more accessible assignment. these reference levels are calculated on the basis of simplified models, taking into account the maximum possible coupling of the field to the exposed person, representing a well desired safety margin. experts dealing with the dosimetry for exposure to lfef, in principle agree with some general statements [9], [10]: • considering a certain external field, the highest values of the internal electric field are registered when the body is firmly electrically grounded, while the lowest values are associated with the “free space” situation (body insulated from the ground); • the current established in the grounded body is significantly influenced by physical dimensions (height and weight) and also by the posture (seated, standing or even lying on the floor) [11], [12]; • the distribution of the induced currents inside the body is not uniform, being decisively determined by the conductivity of various organs and tissues; • the maximum coupling is met when the external electric field is parallel to the body vertical axis. it is useful to remember that a contact with a good conductor placed in an electric field could be a source for indirect currents set up in the body. as order of magnitude, the internal induced field is much smaller than the exterior one (for instance, about six times smaller at the power frequency). the internal electric field strength is very complicated to be assessed but could be computed by using electrically sophisticated heterogeneous models. it is advisable to apply a supplementary reduction factor of 3 to the so calculated values, in order to cover the great uncertainties in the dosimetric domain [13], [14]. at first glance, the field internally induced in the human body seems far too small compared to the outer electric field. this can be easily understood by imposing the continuity condition on the electric flux density, which is the product of the dielectric permittivity and the electric field strength. at low frequencies, the relative permittivity of human tissues and table1. health and sensory effects elvs for internal electric field strengths (base restrictions); f is the frequency expressed in hertz (hz). frequency range (hz) sensory effects elvs, peak values (v/m) health effects elvs, peak values (v/m) 1-10 0.7/f 1.1 10-25 0.07 1.1 25-400 0.0028 x f 1.1 400-1000 0.0028 x f 1.1 103-3x103 1.1 3x103-107 3.8 x 10-4 x f acta imeko | www.imeko.org december 2017 | volume 6 | number 4|40 organs has values in the order of 105-107 [15]. these action levels could be measured or calculated (analytical or by modeling), representing the highest values encountered at the employees’ body position. we have to note that in specific non-sinusoidal conditions, the exposure evaluation must be based on the weighted peak method, involving filtering in the time domain, [16]. two levels of exposure limits are to be considered: • for general population, also called ”public” or “residential” (people of various ages and state of health, including children and grey-headed persons, untrained and uninformed, being possible to stay 24 hours a day in that environment); • for certain groups of workers (the so-called occupational exposure, in a controlled environment), for trained, informed, well-conditioned adults, working only a few hours per day in that restricted environment while respecting some protection rules. in table 2 there are summarized the reference values for the external e-field strength, accepted by icnirp [6]. the frequency f included in these formulae is expressed in hz. the maximum permissible exposure (mpe) levels, the ieee designation, corresponding to a certain degree to icnirp reference levels, for the human whole body exposure to sinusoidal electric fields in the background, stipulated in [8], are reported in table 3. for occupational situations, ieee utilizes the around synonymous ”controlled environment”. the ratio between occupational and public acceptable level is 4(frequencies up to 272 hz) and varies from 2 (icnirp recommendations) to 3 (ieee restrictions) for the higher spectrum. the european directive 2013/35/eu is mainly concentrated on occupational exposure, with two levels of ”alarm”, low and high, presented in table 4. while reaching them, some actions are strictly recommended, aiming to reduce the exposure, e.g. to increase the distance or to screen. anatomically realistic or simpler phantom models could be used for the calculation of induced current densities or induced fields, an example being the maxwell surface-based phantom used in the practical guide associated with the directive 2013/35/eu. 4. main sources of lfef the most frequently encountered sources of lfef (being significant even in terms of the power involved or due to their omnipresence in our daily environment) are: • 50 hz, mains power transport and distribution system (north america and partly, japan use the frequency of 60 hz); • the power supplied for traction systems (there is a certain variety, there are trams supplied at 600 volts dc, electric locomotives in romania are supplied with 50 hz power while in germany, deutsche bahn network uses power having the frequency of 16.7 hz); • electrical welding installations or other equipment requiring higher voltages, use working frequencies between 50 hz and 200 hz; • most power converters have operating frequency between 200 hz and 1 khz; • older generation cathode ray tube monitors and tvs have 31 khz as operating frequency; • the working frequency of economic lamps is varying in the range 40-45 khz, depending on power and manufacturer; • the working frequency of lcd monitors made in tft (thin film transistor) technology is in the range 42 khz 57 khz (depending on the manufacturer, but also on the diagonalsize); • switched mode power supplies, equipping pcs, laptops or printers have their working frequencies in the range 30 70 khz; • ignition systems of injection engines; • a specific type of radio-frequency service system (rfid), which uses electromagnetic fields to identify and automatically track a specific labeled object (which received a "tag" that contains electronically stored information), usually uses the 125-135 khz frequency range. anyway, the distance d between the emitting source and the measuring point is essential. the electric field is inversely proportional to d3 for high impedance sources, or only to d2 for sources having low impedance (rough approximation). 5. determination of the exposure metric as we have seen, aiming to predict and to avoid health risks in epidemiology, some recommended exposure limits were established, associated with a frequency domain. the issue is very complicated as the electric field strength is a multidimensional quantity that varies over space and time, while its interaction with the human body is considerably influenced by the frequency. the approach is even more challenging if the field polarization is non-linear, the sources have multiplefrequencies, the fields are non-uniform (spatially or temporally), non-sinusoidal or with high harmonic content. it is strongly desirable to have a single number summing up a certain table 2. icnirp reference levels (both occupational and public, rms values) for exposure to time varying electric fields. frequency range (hz) e-field strength (v/m), occupational exposure e-field strength (v/m), general public exposure 1-50 20.000 5.000 50-3x103 5 x 105/f 2.5 x 105/f 3x103-107 170 83 table 3. maximum permissible exposure (mpe) to lfef, f is the frequency expressed in hertz (hz). frequency range (hz) e (v/m)-rms, controlled environment e (v/m)rms, general public, (residential) 1-272 20.000 5.000 272-368 5.44 x 106/f 5.000 368-3x103 5.44 x 106/f 1.84 x 106/f 3x103-107 1810 610 table 4. action levels (rms values) for occupational exposure, according to directive 2013/35/eu/2013 (f expressed in hz). frequency range (hz) e-field strength (v/m), low als e-field strength (v/m), high als 1-25 20.000 20.000 25-50 5 x 105/f 20.000 50-1640 5 x 105/f 106/f 1640-3000 5 x 105/f 610 3000-107 170 610 acta imeko | www.imeko.org december 2017 | volume 6 | number 4|41 exposure that should be compared with the action levels. many measurements and an algorithm for data processing and analysis are necessary in the low frequency domain (up to 100 khz), the metrics with high significance are the peak vector magnitude or, mainly, the rms vector magnitude, averaged over the specified period. in situations of simultaneous exposure to e-fields having different frequencies or non-sinusoidal fields, having significant harmonics, compliance with the imposed limits should be additively checked as stipulated in (2): ∑ 𝐸imeas 𝐸𝑖ref ≤ 1100 khz𝑖=1 hz (2) where eimeas represents the rms values of electric field measured for every frequency of the spectrum and eiref is the corresponding reference value for that frequency, as it is specified in the appropriate table 2, table 3 or table 4. aiming to decompose the fields with complex, pulsed patterns, the fast fourier transformation techniques are the best option, usually included in the firm software of now-a-days (even hand-held) spectrum analyzers. 6. assessing the exposure to lfef in a laboratory class with 16 networked pcs. as basic device we have used a hand-held, low frequency spectrum analyzer, spectran nf 5305, by usb connected to a laptop running the mcs dedicated software produced by aaronia ag electronics manufacturer, germany. even if this spectrum analyzer already has an internally integrated sensor for electric fields, we have complementary used an external capacitive sensor, battery powered, connected at the provided external input (sma socket) of the device, figure 1 [17]. unlike the case of close magnetic field sensors, the physical detection principles for low frequency electric fields are less numerous. very important is the influence that the sensor itself exerts on the detected field strength, the frequency related sensitivity, detection range and linearity. a modern approach for electric field measuring should be the electro-optical effect or the variability of gate to source voltage for some special field effect transistors. quasi affordable for manufacturing in an ordinary laboratory is the measurement of induced electrical charges on a sensing capacitor (with magnitude order of 100-200 pf), followed by a differential amplifier. the sensor’s geometry could be plate (rectangular) or even spherical. the induced charge on one half of this field meter is directly proportional to the electric field that is parallel to its axis, the sensitivity depending on the effective area where the electrical charge could be induced: 𝑄 = 𝑆𝜀0𝐸 (3) where, as we have previously stated, s is proportional to the sensor’s active surface area, in our case 50 cm2 (the surface of a circle with 4 cm radius). the current established between the electrodes of the capacitive sensor is the time derivative of the induced charge (the field being time-harmonic) while the meter is considered as “free body” (isolated from the ground): 𝐼 = 𝑗ωs𝜀0𝐸 (4) another type of sensor, this time non-directional that can also be easily hand-crafted, is presented in figure 2. due to the relatively small length (and surface), this sample has little sensitivity compensated by the advantage of accurately locating the emitting source. if a more sensitive sample is desired (which can "pick" lower value fields) at the cost of reducing the resolution, the key might be the "sphere" sample. the handle is made of a semirigid coaxial cable of 50 ω connected to the bnc connector by means of a resistor of 50 ω as impedance adaptation. at the other end of the coaxial cable, the central conductor is attached to a conductive sphere having a diameter of 3-5 cm. the assembly becomes rigid due to the plastic insulator material, also preventing short circuits that might occur during measurements. due to the presented spatial symmetry, both stub and sphere probes are omnidirectional. with the hand-held spectrum analyzer, using its own e-field sensor or coupled with our hand-crafted ones, we have traced the profiles of the electric field strength in the laboratory class, the contours and a 3d map. we proposed to measure the exposure around the regions where the heads of the 15 students are expected to be placed. the electric field could be easily perturbed by any (conductive) object; consequently, the classroom was without students, the sensor was mechanically introduced in the area of interest by using a 1.5 meter fiberglass extension pole and the pcs were running a repetitive routine. at the first survey, e-fields having three frequencies were encountered: 50 hz (the mains power), 56 khz (working frequencies of the 16 tft-lcd displays) and 62 khz (much figure 2. stub type, low sensitivity but omnidirectional, near electric field probe. figure 1. capacitive e-field sensor made from pcb double layer, 2mm thickness. conductive shield bnc 50 ω connector rg coaxial cable electric insulator from pvc polyethylene cooper, inner conductor (6-10 mm length) – active element acta imeko | www.imeko.org december 2017 | volume 6 | number 4|42 lower values than the previous ones, being associated with the switched mode power supply of the pcs, easy to be shielded). the rms readings were averaged over space and time (5 minutes). these values, expressed in v/m, for the 16 places of interest and 2 significant frequencies are shown in figure 3. their spatial distribution in the laboratory room is presented in figure 4a (for 50 hz) and figure 4b (for 56 khz). we have applied (2) as the sum of ratios between the highest e-field values measured in the laboratory class, at 50 hz, 56 khz and 62 khz and the corresponding reference levels accepted for public exposure: ∑ 𝐸imeas 𝐸𝑖ref = 230 5000 + 36 83 + 9 83 = 0.588 ≤ 162khz𝑖=50hz (5) this value is significantly below the accepted unit value, even for public, non-occupational exposure. 7. “in silico” measurement of the electric field produced by overhead power lines the overhead power lines (opls) are major sources of lfef which might reach, in specific conditions, the action levels stated by directive 2013/35/eu. we have performed direct measurements with the device and the external sensor previously described for various high voltage opls. the registered values were used for checking and validation of the proposed numerical method. in the scientific literature, there are many papers dealing with the computation of lfef produced by overhead power lines, e.g. [18]-[23], employing both analytical and numerical methods. we developed here a simple numerical approach based on finite element method magnetics 4.2 (femm 4.2) [24], which is open source finite element software for solving low frequency electromagnetic problems on two-dimensional planar and axisymmetric domains. as can be seen from the very recent data literature, concerns about the calculation of electric fields produced by overhead power lines are still topical. there are either variants of using very powerful general software [25], alongside specific methods (such as charge simulation method, csm) implemented in general mathematical computation software [26] or using simpler, 2d or more elaborate 3d models [27]. this domain is also very topical from the reverse perspective, by developing reliable and cost-effective noncontact monitoring techniques for high voltage transport and distribution networks [28], [29]. in a previous study [30], we have also applied femm 4.2 to compute low frequency magnetic fields associated with common configurations of opls used in romanian power transport and distribution systems. 7.1. double-circuit power line and associated femm model opl selected for this case study is a typical 110 kv doublecircuit line (dcl) with the xy coordinates of the 7 conductors presented in table 5.the architecture is determined by the steel pylons sn 110.252 type, [31]. the active 2 × 3 aluminum conductor steel reinforced (acsr) conductors have a crosssection 240/40 mm2 (core diameter 8.1 mm and exterior diameter 21.70 mm). the acsr ground wire has across-section 160/95 mm2(core diameter 12.75 mm and exterior diameter 20.75 mm). the sag effect is taken into consideration by specifying three ground clearances, namely h = 8 m, h = 12 m figure 3. the measured values of the e-field strength, at 50 hz (light yellow) and 56 khz (dark brown). figure 4a. the 3d map of the 50 hz e-fields. table 5. input data for the considered 110 kv double-circuit line. conductor i xi (m) yi (m) phase i, 110 / √3 ej0° 1 -3.05 h 2 3.05 phase ii, 110 / √3 ej240° 3 -5.00 h + 4.60 4 5.00 phase iii, 110 / √3 ej120° 5 -3.05 h + 10.30 6 3.05 ground wire, 0 7 0.00 h + 16.25 figure 4b. the 3d map of the 56 khz e-fields. acta imeko | www.imeko.org december 2017 | volume 6 | number 4|43 and h = 15 m. the power line femm model is shown in figure 5a, meshed by a net of 252900 nodes and 505375 triangular elements. the boundary conditions here applied assume that the scalar electric potential at the ground level and along the semi-circular boundary is zero (u = 0). the air region surrounding the conductors is taken large enough to neglect the side effects, having a radius of at least 10 times the arrangement dimensions. all conductors are modeled as presented in figure 5b, using the following electric parameters: for aluminum (al),σ = 3.77×107 s/m and εr = 1; for steel (ol, internal symbol), σ = 0.56×107 s/m and εr = 1. the analysis type is set to “current flow” (f = 50 hz), which generally allows to determine electric field distributions, voltage distributions, current densities, etc. 7.2. electric field simulation results figure 6 compares lateral profiles of the rms electric field strength calculated at the standard height of 1 m above the ground for the aforementioned ground clearances. for h = 8 m, a relatively low clearance, the maximum electric field strength under the power line is 2.18 kv/m, which is 2.3 times below de eu reference (public) level, 5 kv/m. at 18.5 m distance from the centerline, which represents the semi-width of the safety / protection zone for 110 kv overhead high-voltage lines (according to the national regulations) [31], the electric field strength is much lower, about 90 v/m. for h = 12 m, an intermediate clearance, the maximum electric field strength under the power line is 1.25 kv/m, while at 18.5 m distance from the centerline the field strength falls below 50 v/m. the distribution of the electric field around the power line is presented in figure 7 (h = 8 m), where the horizontal contour indicates the width of the safety zone. as it can be concluded from both figure 6 and figure 7, the electric field exposure levels outside this area are very low, fast decreasing with distance. note that the e-field density plot in figure 7 is given in terms of peak value, as directly generated by femm 4.2. results obtained with this femm approach have been compared with similar calculations carried out with software tools based on quasi-static analytical methods [20], as well as with measurements conducted in vicinity of several power lines. all the approaches lead to approximately comparable results. 8. conclusions and further attempts eu member states have currently transposed directive 35/2013/eu into their legislation. it is necessary to provide, both for the employers and for the appropriate authorities, an applicable procedure, based on a simple metric and budget figure 6. lateral profiles of the rms electric field strength from the considered 110 kv dcl, calculated for a height of 1 m above the ground. figure 5b. femm 4.2 model for a 110 kv double-circuit power line (conductor’s cross-section details). figure 5a. femm 4.2 model for a 110 kv double-circuit power line: meshed global model. figure 7. electric field distribution around the 110 kv dcl (h = 8 m). ground (u = 0) air u = 0 power line acta imeko | www.imeko.org december 2017 | volume 6 | number 4|44 equipment, aiming to decide by measurements, if a specific location is risky or not from the point of view of the exposure to electric (and also magnetic) fields. we have presented the approach intended to deal this question only for lfef, based on an affordable handheld spectrum analyzer, easy to be made additional sensors and a cumulative metric derived from the rms vector magnitude. the paper also stimulates the use of free-open source software, which can model complex configurations from the point of view of the generated electric (or magnetic) fields. to avoid systematic errors, it is advisable to verify the comparability between directly measured values and those obtained by modeling and simulation. these domestic tests have the role of inexpensive internal control being namely oriented to small businesses. if there are found values close to the maximum allowed ones, it could be decided on the opportunity of calling for specialized firms. as a further concern, we want to present a modeling proposal for the current densities simultaneously induced in the human body by the electric and magnetic fields. concretely, through the hv-opls presented in the paper, usually pass currents of hundreds of amperes, so the generated magnetic field is also considerable. if the currents induced in the human body by the electric field produced by these hv-opls are vertical and linear, the ones produced by the magnetic field are shaped like circular loops. there might be an exposure risk in certain organs (e.g. one lung or one kidney). it is possible as the overlapping of these currents to be cumulative-additive, a matter that should need to be taken into account both in modeling and in legal regulations. references [1] european commission, "directive 2013/35/eu of the european parliament and of the council of 26 june 2013 on the minimum health and safety requirements regarding the exposure of workers to the risks arising from physical agents (electromagnetic fields)”, official j. eur. union, 2013, 1791, pp. 1-21. [2] european commission, "non-binding guide to good practice in implementing directive 2013/35/eu electromagnetic fields, volume 1: practical guide", 2015 [3] european agency for safety and health at work, "european survey of enterprises on new and emerging risks", https://osha.europa.eu/en/node/6745/file_view. [4] m. morega, i. m. baran, a. m. morega, "evaluation of environmental low frequency magnetic fields in occupational exposure", proc. of 8th int. conf. and exposition on electrical and power engineering epe-2014, oct. 16-18, 2014, iasi, romania, pp. 522-527. [5] icnirp, "guidelines for limiting exposure to time-varying electric, magnetic and electromagnetic fields (up to 300ghz)", health physics 74(4), 1998, pp.494-522. [6] icnirp, "guidelines for limiting exposure to time-varying electric and magnetic fields (1 hz 100 khz)", health physics 99(6), 2010, pp.818-836. [7] ieee-ices standard c95.6-2002, "standard for safety levels with respect to human exposure to electromagnetic fields, 0-3 khz", 2002. [8] ieee-ices standard c95.3.1-2010, "ieee recommended practice for measurements and computations of electric, magnetic, and electromagnetic fields with respect to human exposure to such fields, 0 hz to 100 khz", 2010. [9] j.d. bowman, m.a. kelsh, w.t. kaune, manual for measuring occupational electric and magnetic field exposures, dhhs (niosh) publication no. 98-154, 1998. [10] v. de santis, x.l. chen, "on the issues related to compliance assessment of icnirp 2010 basic restrictions" journal of radiological protection, 2014, 34 (2), pp. n31-n39. [11] p. dimbylow, r. findlay, "the effects of body posture, anatomy, age and pregnancy on the calculation of induced current densities at 50 hz.", journal of radiation protection dosimetry, 2010, 139 (4), pp. 532-538. [12] r.p. findlay, "the effects of human height and mass on the calculated induced electric fields at 50 hz for comparison with the emf directive 2013/35/eu", journal of radiological protection 2017, volume 37, issue 1, pp. 201-213. [13] iso/iec guide 98-3:2008, "uncertainty of measurement-part 3: guide to the expression of uncertainty in measurement", 2008. [14] s.w.park, k. wake, s. watanabe, "calculation errors of the electric field induced in a human body under quasi-static approximation conditions", ieee transactions on microwave theory and techniques, 2013, volume 61, issue 5, pp. 2153-2160. [15] c.gabriel, s. gabriel, e.courthout, "the dielectric properties of biological tissues, i. literature survey", physics in medicine&biology, 1996, 41, pp.2231-2249. [16] icnirp, "icnirp statement-guidance on determining compliance of exposure to pulsed and complex non-sinusoidal waveforms below 100 khz with icnirp guidelines", health physics, 2003, vol. 84, no. 3, pp. 383-387. [17] a. salceanu, m.m. poenaru, m.a. anghel, m. paulet, "approach on the evaluation of exposure to low frequency electric fields", proc. of 21st imeko tc4 international symposium and 19th international workshop on adc modelling and testing, sept. 79, 2016, budapest, hungary, pp. 32-36. [18] b. ali rachedi, a. babouri, f. berrouk, "a study of electromagnetic field generated by high voltage lines using comsol multiphysics", proc. of 2014 international conference on electrical sciences and technologies in maghreb (cistem), nov. 3-6, 2014, tunis, tunisia, pp. 1-5. [19] l. xiao, k.e. holbert, "development of software for calculating electromagnetic fields near power lines", north american power symp. (naps), sep. 7-9, 2014, pullman, u.s., pp. 1-6. [20] s.s. razavipour, m. jahangiri, h. sadeghipoor, "electrical field around the overhead transmission lines", world academy of science, engineering and technology 2012, 6, pp. 168-171. [21] h. ahmadi, s. mohseni, a.a. shayeganiakmal, "electromagnetic fields near transmission lines problems and solutions", iran. j. environ. health. sci. eng. 2010, 7, pp. 181-188. [22] m. milutinov, a. juhas, m. prša, "electromagnetic field underneath overhead high voltage power line", proc. of 4th international conference on engineering technologies, april 2830, 2009, novi sad, serbia, pp. 1-5. [23] s. vujević, d. lovrić, t. modrić, "2d computation and measurement of electric and magnetic fields of overhead electric power lines", proc. of the joint inds'11 & istet'11, july 2527, 2011, klagenfurt, austria, pp. 1-6. [24] d. meeker, "finite element method magnetics, version 4.2, user's manual", october 25, 2015, http://www.femm.info/archives/doc/manual42.pdf. [25] i.n.ztoupis, i.f.gonos, i.a.stathopulos, "measurement and calculation of power frequency electric fields generated by high voltage overhead power lines", proc. of international conference on high voltage engineering and application (ichve), september 08-11, 2014, poznan, poland. [26] a. rankovic, "novel multi-objective optimization method of electric and magnetic field emissions from double-circuit overhead power line", international transactions on electrical energy systems, 2017, volume 27, issue 2. [27] t. modric, s. vujevic, "computation on the electric field in the vicinity of overhead power line towers", electric power systems research, 2016, volume 135, pp. 68-76. [28] k. zhu, w. k. lee, w.t. philip pong, "non-contact capacitivecoupling-based and magnetic-field-sensing-assisted technique for monitoring voltage of overhead power transmission lines", ieee sensors journal, 2017, volume 17, issue 4, pp. 1069-1083. acta imeko | www.imeko.org december 2017 | volume 6 | number 4|45 [29] m.a. noras, "electric and magnetic field monitoring for power line diagnostics", proc. of 51-st annual meeting of the ieeeindustry applications society, october 11-22, 2015, addison. [30] e. lunca, m. istrate, a. salceanu, "comparative analysis of the extremely low-frequency magnetic field exposure from overhead power lines", environmental engineering and management journal, 2013, 12, pp. 1145-1152. [31] anre, nte 003/04/00 "normative document regarding the construction of overhead electric lines exceeding ac 1kv" (in romanian), 2014, url: http://www.anre.ro/ro/legislatie/documente-de-discutieee1/regl-tehnice-regulamente/normativ-pentru-constructialiniilor-aeriene-de-energie-electrica-cu-tensiuni-peste1000v&page=2. affordable evaluation of low frequency electric fields from the standpoint of directive 2013/35/eu << /ascii85encodepages false /allowtransparency false /autopositionepsfiles true /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /error /compatibilitylevel 1.4 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.0000 /colorconversionstrategy /cmyk /dothumbnails false 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can be opened with acrobat and adobe reader 5.0 and later.) >> /namespace [ (adobe) (common) (1.0) ] /othernamespaces [ << /asreaderspreads false /cropimagestoframes true /errorcontrol /warnandcontinue /flattenerignorespreadoverrides false /includeguidesgrids false /includenonprinting false /includeslug false /namespace [ (adobe) (indesign) (4.0) ] /omitplacedbitmaps false /omitplacedeps false /omitplacedpdf false /simulateoverprint /legacy >> << /addbleedmarks false /addcolorbars false /addcropmarks false /addpageinfo false /addregmarks false /convertcolors /converttocmyk /destinationprofilename () /destinationprofileselector /documentcmyk /downsample16bitimages true /flattenerpreset << /presetselector /mediumresolution >> /formelements false /generatestructure false /includebookmarks false /includehyperlinks false /includeinteractive false /includelayers false /includeprofiles false /multimediahandling /useobjectsettings /namespace [ (adobe) (creativesuite) (2.0) ] /pdfxoutputintentprofileselector /documentcmyk /preserveediting true /untaggedcmykhandling /leaveuntagged /untaggedrgbhandling /usedocumentprofile /usedocumentbleed false >> ] >> setdistillerparams << /hwresolution [2400 2400] /pagesize [612.000 792.000] >> setpagedevice integrated approach to structural diagnosis by non-destructive techniques: the case of the temple of minerva medica acta imeko issn: 2221-870x october 2018, volume 7, number 3, 13 19 acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 13 integrated approach to structural diagnosis by nondestructive techniques: the case of the temple of minerva medica ivan roselli1, angelo tatì1, vincenzo fioriti1, irene bellagamba1, marialuisa mongelli1, roberto romano1, gerardo de canio1, mariarosaria barbera2, marina magnani cianetti2 1 enea casaccia research center, rome, italy 2 mibact soprintendenza speciale per i beni archeologici di roma, rome, italy section: research paper keywords: non-destructive tests; integrated approach; archaeological building citation: ivan roselli, angelo tatì, vincenzo fioriti, irene bellagamba, marialuisa mongelli, roberto romano, gerardo de canio, mariarosaria barbera, marina magnani cianetti, integrated approach to structural diagnosis by non-destructive techniques: the case of the temple of minerva medica, acta imeko, vol. 7, no. 3, article 4, october 2018, identifier: imeko-acta-07 (2018)-03-04 section editor: sabrina grassini, politecnico di torino, italy received february 25, 2018; in final form september 13, 2018; published october 2018 copyright: © 2018 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: ivan roselli, ivan.roselli@enea.it 1. introduction in the context of restoration and conservation of archaeological heritage it is crucial to extract experimental data to characterise the physical and chemical properties of the materials that constitute historical objects. this allows more effective and optimized design of eventual interventions needed to repair or preserve these objects for future generations avoiding any risk of damage. providing as much as possible complete and accurate characterisation of historical structures is particularly important in urban areas, which pose relevant problems in executing the restoration interventions. for example, this kind of situations are faced regularly in big cities that are rich in archaeological assets, subjected to potential degradation phenomena caused by urban pollution, traffic vibrations and excavation works that might threaten their structural stability. such cases are very common in italy, which presents the greatest concentration worldwide of historical constructions in urban areas, as documented by unesco world heritage list, and in particular in the city centre of rome, where the needs of a modern european capital and its archaeological treasuries must coexist side by side. this is the case of the monument studied in the present paper, the socalled temple of minerva medica, which is subjected to remarkable urban traffic vibration and pollution [1]. in order to give a contribution to the structural characterisation of the studied building without any risk of damaging the monument, a variety of non-destructive tests (ndts) was carried out. the applied ndts are the most advanced in their respective fields and are lately increasingly abstract in the present paper an integrated approach of a wide range of non-destructive tests (ndts) was applied to study an archeological ruined building located in rome, the so-called temple of minerva medica. applied ndts focused on the monument properties and environmental conditions affecting its structural behavior (geometry, thermal and mechanical properties, microclimatic monitoring, ambient vibration response). two surveys were performed in august and december 2016 combining 3d laser scanning, thermal infrared, air temperature and relative humidity acquisitions. in addition, high resolution digital images were acquired and processed by stereo-photogrammetry structure from motion (sfm) methodology, carried out for detailed reconstruction of the crack pattern of the monument. in order to obtain information on the integrity and consistency of the masonry, a sonic testing of each pillar was also performed. the integration of the above ndts provided quite complete and comprehensive understanding of the structural behavior and state of the building, reducing the amount of invasive measurements further needed. mailto:ivan.roselli@enea.it acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 14 utilized. nonetheless, they are mainly applied separately with no substantial integration. on the contrary, here ndts were designed together and collected in an organic data repository.also, taking into account a proper timing of acquisition of different kinds of data allowed to establish mutual relations and interdependences, which makes the whole experimental program systemic and integrated. the executed measurements focused on surveying the geometry, the thermal properties and the integrity of the material constituting the monument. a particular focus was put on the physical properties that affect the dynamic characterization of the main hall, which is subjected to relevant vibration intensity induced by the surrounding tramways and railways traffic (figure 1). a detailed 3d geometric reconstruction of the monument was achieved by integrating information from laser scanner surveys and from high resolution stereo-photogrammetry. contemporarily, also thermographic imagery by infrared camera was collected, as well as data of microclimatic parameters, such as air temperature and relative humidity. such measurements were important to evaluate the thermal behaviour of the masonry, which have a relevant influence also on the structural behaviour of the building, as well known [2]. in order to characterize the influence of microclimatic conditions on the dynamic response of the building, also ambient vibration measurements were carried out for modal identification of the structure. sonic testing was performed to investigate the interior consistency of the pillars using the sonic transmission procedure [3]. 2. the studied monument since the 16th century, this ruined building was misinterpreted as a temple dedicated to minerva medica (italian for “minerva the doctor”), as in this site an ancient statue with the likeness of this roman goddess was found. nevertheless, according to more recent studies, archaeologists hypothesize it might have been part of the horti liciniani [4] or of another complex of imperial rome. it is located in the city-centre of rome, on the esquiline hill, between via labicana and aurelian walls. the structure was initiated in early 4th century a.d. as a majestic building. the main hall of the complex was probably a nymphaeum. it had a decagonal polylobate plan with a diameter of 25 m and an overall height of 32 m. after several partial collapses during the past centuries, the size of the main hall is today almost unchanged, except for the height, which is currently reduced to only 24 m after a major dome collapse occurred in mid-19th century. the main hall presents two orders of arcades: on each of the ten sides (named as in figure 2) a large arched niche is at ground level (first order arcade) over which there is a smaller arched window (second order arcade). the structure was initially built in opus latericium, which was a common construction technique of that time, based on the use of roman bricks and mortar. as the building started presenting structural problems, in the following centuries the construction was restored and reinforced. in this context, interventions in opus mixtum of tuff bricks and roman bricks were carried out. in particular, most niches were closed and some walls with the function of buttresses were added on the southeast side of the monument, which testify that a certain structural weakness of this part already arose in ancient times. a further confirmation of such weakness in the south side is that the major historical damages, some of which were documented also by paintings in the 16th through the 18th century, were concentrated here, where recent investigations indicated that the foundations are less consistent [4]. also, some major restoration interventions were carried out in the past. in 1846 first floor arcade of the southeast side was reconstructed, while the upper floor arcade of the southeast side was reconstructed in the years 2012-2013. the relevance of such a complex and long history of collapses and reconstructions lies in the fact that the monument now appears quite heterogeneous both in terms of construction materials and techniques, as well as irregular in shape. 3. experimental data the survey strategy comprised the design of data acquisition using a variety of in-situ measurement techniques capable of providing indications on the mechanical behaviour and the conservation state of the monument. given the historical importance of the studied monument, only ndts were considered. data were recorded in a time span of one year, from july 2016 until july 2017, making possible to investigate the seasonal behaviour of the structure. in addition, also the eventuality of structural changes of the building due to the possible effects of the seismic sequence that hit central italy from august 2016 to february 2017 could be considered. in particular, in the present paper the results from the following measurements will be illustrated and integrated: 1) geometric survey by 3d laser scanner and stereophotogrammetry; 2) thermal imaging; figure 1. aerial photo of the so-called temple of minerva medica, rome. figure 2. decagonal plan of the main hall with sides (s1-s10) and pillars (p1p10) nomenclature. acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 15 3) microclimatic characterization (air temperature and relative humidity); 4) ambient vibration measurements; 5) sonic testing of the pillars. microclimatic and ambient vibration data were acquired at the same time in order to characterize the environmental conditions under which the dynamic behaviour was identified. 3.1. geometric surveys 3.1.1. 3d laser scanner surveys the laser scanner measurements were acquired in summer of 2016 (august 5th) and in winter of 2016 (december 20th), when air temperatures were supposed to be the highest and the lowest of the year, respectively. both acquisitions were carried out at 12 a.m., so that solar beams direction was essentially affected only by the season and not by the daytime, in order to analyse the seasonal effects neglecting the day/night cycle effects. the instrumentation used comprised a riegl z360 equipped with a nikon d100 digital camera. the nominal angular resolution is 0.0025° horizontal and 0.002° vertical, while range accuracy was +/6 mm. through the use of retroreflecting targets, which were located at each pillar at the height 2.5 m from ground level, the two surveys could be compared by matching the targets positions as reference points. this 3d geometrical reconstruction by laser scanner was the base both for measurements of distances between walls at unreachable points and for the mathematical modelling for successive structural analyses. the plan of the main hall with sides (s1s10) and pillars (p1-p10) nomenclature is depicted in figure 2. in particular, the ten diameters obtained as the distances between the internal walls of the opposite sides were calculated from both summer and winter surveys in order to assess the masonry expansion in each horizontal direction. the overall expansion is the combination of the effects of masonry temperature and moisture deformations. in fact, construction materials are known to expand when heated and contract when cooled. in particular, ancient roman masonry, such as the one constituting the walls of the temple, have typical values of the thermal expansion coefficient that lie in the range of 3-8 x 10-6 °c [5]. on the other hand, this historic masonry is also capable of absorbing water and, consequently, expand and contract again on drying. the thermal expansion is generally more intense than moisture deformations in case of high temperature excursion and dry climate. nonetheless, as the december survey was carried out after a two-day rain of 12 mm, the masonry was substantially wet to near saturation. on the contrary, the august survey was performed in a very hot and dry period. the found masonry expansion, expressed in terms of the percent variation (%) between 3d laser scanning surveys of august and december 2016, is illustrated in figure 3. 3.1.2. stereo-photogrammetric survey in addition to 3d laser scanning surveys, also a stereophotogrammetric survey was executed. about 500 digital images (10 mpx each) were acquired by means of a nikon d60 camera. subsequently, they were processed by structure from motion (sfm) methodology [6] using the agisoft photoscan software via eneagrid, available on cresco (computational research center for complex systems) hpc infrastructure [7]. a dense cloud of 30 million points was obtained and mesh returned more than 200,000 tria element. stereo-photogrammetry was used as a complementary tool to laser scanning, since it is able to provide more detailed documentation of the crack pattern [8], even if it provides a less accurate 3d geometry reconstruction. figure 4 shows a comparison between the raw digital image of side s4 and its reconstructed 3d rendering with texture after sfm processing. background was eliminated so as to facilitate crack pattern recognition and analysis. 3.2. temperature and humidity measurements 3.2.1. thermal imaging thermographic data of the inner walls of the monument were acquired using a flir t440 thermal infrared camera. the building images were recorded from the centre of the main hall and were subdivided in 10 ground floor and 10 upper floor views, corresponding to the sides of the decagonal plan (figure 5). the thermal images were captured at about 12 a.m. on the same two dates as the laser scanner surveys. a temperature diagram was obtained as a function of the walls solar exposure (ground and upper floor sides) for all acquired seasons, putting into evidence the thermal differences between walls. minimum, maximum and average values of temperature excursion (°c) between thermal imaging of august and december 2016 are depicted in figure 6. figure 4. digital photo (left) and related 3d texture (right) of northwest side s4 of the monument. figure 3. variation (%) of opposite sides distances between 3d laser scanning surveys of august and december 2016. acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 16 temperature excursions were more than 30 °c between the maximum temperature values of the masonry recorded in august and in december surveys. the temperature of the walls was higher in the upper arcade and particularly in sides s2, s3 and s4 (north-western sides of the monument), where temperatures reached 40 °c in august. this was a result of the direct solar beam exposure of their upper inner wall-faces due to the big hole left by the collapsed dome. conversely, the walls on the eastern and southern sides (s6, s7, s8, s9 and s10) revealed maximum values of only 3435 °c. in the december survey the walls temperature resulted more uniform, increasing only slightly, from 8 °c at the bottom to 12 °c at the upper arcade. solar beam direction was approximately the same, since the daytime of measurements was the same, but the effect of direct solar irradiation was very limited because it was a very cloudy day. 3.2.2. air temperature and relative humidity two microclimatic monitoring stations were positioned on the west side (pillar p4 on side s2) and on the east side (pillar p8 on side s7). the microclimatic stations recorded air temperature and relative humidity by means of msr145 mini data loggers during august and december surveys. consequently, the measured air temperatures could be correlated with the corresponding temperatures in the thermal images at the same positions. in figure 7 the obtained correlation is illustrated. it shows that masonry temperatures resulted slightly lower than air temperatures in the december survey and, vice versa, slightly higher in august. the excursion of the air temperature recorded by the microclimatic stations is indicated in figure 6 where it can be compared with the temperature excursion by thermal imagery. air temperature varies with less ample excursion than the walls temperature, as expected in consequence of typical masonry thermal behaviour. as for the relative humidity, it resulted in the range 51-55 % in the august survey, while 71-72 % in december. to be noticed that higher values of humidity (55 % in august and 72 % in december) were always measured at the station located in side s7, which indicates that on this side of the monument microclimatic conditions tend to induce more humid air. 3.3. ambient vibration measurements to assess the dynamic behaviour of the building, several measurement stations were located on the façade of the monument to acquire ambient vibration. they were made up of a digital recorder equipped with a triaxial velocimeter provided with a gps antenna for time synchronization. the instruments were located as in figure 8, simply laid on horizontal surfaces reachable by scaffoldings, at ground level, at windows and on the roof. each measured position was acquired for at least 20 minutes at a sampling frequency of 200 hz, as in previous similar studies of historic masonry buildings [9]. the recorded vibration data were processed and analysed by several timeand frequency-domain modal analysis techniques [10-12] for mutual validation of results [13]. in fact, all modal analysis methods provided very similar values of the modal parameters, confirming the reliability of the results. consequently, the average values calculated by the applied methods were considered in the following as the most reliable estimate of the natural frequencies of the monument. in the present study only the first four modes were considered, because higher modes are less relevant for the overall state of figure 5. thermal image of upper floor arcade on the northwest side (s3) of the monument (left) and corresponding photo (right). figure 6. minimum, maximum and average values of temperature excursion (°c) between thermal imaging of august and december 2016 at 12 a.m. air temperature excursion is also shown (blue dots). figure 7. correlation between air temperature and thermal imaging temperature at the same points. figure 8. ambient vibration measurement points on the main façade of the monument (view from north). acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 17 health of the structure, as they are representative of more local modes. the initial outcomes showed that the considered modal frequencies resulted quite stable, though a slight variation was actually revealed. in fact, modal frequencies were a bit higher in spring and summer acquisitions. consequently, the influence of the masonry temperature on the obtained frequencies was investigated. in particular, a remarkable correlation was found between the first four modal frequencies and the temperature, as showed in figure 9. as quite common in historic buildings, the modal frequencies increased slightly with the temperature. however, recorded variations were limited in the range of 2-6 %, which is substantially coherent with a temperature excursion between about 10 °c and 30 °c. 3.4. sonic testing of pillars one of the more critical uncertainties in the analysis of a historic masonry construction to assess its structural behaviour lies in the inner consistency or structure of walls, which may present voids or unknown materials. in these circumstances it is crucial to apply a ndt measurement technique able to provide indications on the inner structure of walls. the ndt methods based on the measurement of the velocity of sonic waves propagating through compact materials have such potentiality. in february 2017 a series of sonic measurements was executed on all the pillars of the studied monument using the direct transmission procedure [14]. this technique implies the use of an instrumented hammer and a probe, both equipped with an accelerometer. once the wanted path through the thickness of the wall is decided, the probe is laid on one end and the other end is hit by the hammer. the sonic wave propagates from one end to the other and the acquisition system records the acceleration time-histories of both sensors. after processing such time-histories the time of flight can be calculated and used as dividend of the path length to extract the sonic velocity. the results of the direct sonic tests provide indications on the walls consistency, both in terms of masonry local mechanical properties (in particular, the young´s modulus can be estimated) and in terms of walls inner structure (voids, cavities, layers etc.). in general, values lower than 800 m/s indicate quite poor mechanical properties, while values in the range 1500-2500 m/s are typical of good quality historic masonry. inner voids and cracks induce sonic velocity decrease in comparison to surrounding masonry. since this technique is especially effective for investigating heterogeneities in the construction materials, such as historic masonry, it allows the identification of layering of the sections, the presence of voids, cavities and anomalies in material density. in particular, it was applied to the base of each pillar in order to verify the state of compacity of the masonry and to estimate its compression strength. the used equipment comprised an instrumented hammer with a pcb icp accelerometer, as well as the probe, both cabled to the acquisition unit. data were acquired with a sampling frequency of 500 khz. the applied measurement procedure was strictly in accordance to the uni 10627-1997 standard [15]. the acquisition configurations were as depicted in figure 10 for pillar p1. it shows the sonic path length and the meaning of d, which is the distance from internal pillar face to the path line. because of the particular geometry of pillars, the acquisition with the utilized instrumentation was possible only on the two radial faces. in particular, as the points on the outer sides of pillars could not be acquired, a rectangular grid could not be performed and a representative 2d tomography could not be obtained. consequently, only 1d sonic wave velocity distribution could be detected (in radial direction), which however provided interesting indications. in particular, in figure 11 a graph of the sonic test velocity recorded at each pillar versus d is depicted. initial values (d = 0) tend to decrease at higher values of d, indicating poorer material properties of the core (opus caementicium) with respect to the wall-face brickwork layers (opus latericium or mixtum). the low initial value of pillar p4 is due to a crack in the wall-face layer. the lowest values were measured in pillars p9 and p10 at d in the range 80-120 cm. for d > 120 cm the lowest values were obtained for pillars p1 and p2. the diagram in figure 12 illustrates the average values of sonic velocity recorded at each pillar. it suggests that pillars from p3 to p8 (clockwise from southwest to northeast) are characterized by higher average consistency than the remaining figure 9. influence of temperature on first four modal frequencies of the monument. figure 10. pillar p1 plan view with configurations of sonic wave velocity measurement (unit is m). figure 11. sonic test velocity recorded at each pillar (p1 to p10) in function of the distance from internal face (d) defined as in figure 10. acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 18 pillars in east and south sides. it is worth noting that pillar p1 was entirely reconstructed in 1846, when a major restoration intervention was carried out to rebuild the portion of the ground floor arcade that goes from p10 to p2 clockwise. on the other hand, p9 and p10 values are very low, but they do not apparently show any external defect, as confirmed by their good values in the first 40 cm of d, which may indicate that these pillars have a different internal structure. it is also worth noting that the sonic test results can be considered as local measurements of the material stiffness between the measurement points (i.e. they are essentially related to the average young´s modulus of the material within the sonic wave path). even if it is a measurement of substantially local nature, it gives the structural engineers indications on the building process of the entire structure. especially, considering that the material stiffness practically is the main factor that determines the dynamic behaviour of the structure, once that mass and geometry remain unchanged. consequently, sonic velocity measurements are very relevant to understand the structural behaviour in relation with the modal frequencies identified through the ambient vibration acquisitions. all above information is essential to create a representative numerical model of the building that could be used for further structural analyses and simulations. 4. conclusions the present paper illustrates how the integration of different kinds of measurements extracted from the application of a variety of ndt techniques provided a significant contribution to the study of the so-called temple of minerva medica. the several investigated physical properties about the monument and its local environmental conditions were integrated in an effective database for a comprehensive understanding of the overall structural behaviour. in particular, the influence of the geometry and of the environmental conditions (especially, in terms of solar exposure) on the thermal behaviour of the structure was highlighted. the thermal conditions are among the main factors affecting the dynamic behaviour of historic masonry structures, which was evidenced by the high correlations found between the monument’s modal frequencies with the temperature. the integration of thermal imagery with microclimatic stations providing the air temperature and the relative humidity, taking into account also the weather records (e.g. rainfall records) for the acquisition dates, allowed a more comprehensive understanding of the actual environmental conditions and their effect on the structural properties of the studied structure. moreover, the mechanical properties of the masonry, investigated by sonic tests and the surveyed crack pattern by stereo-photogrammetric tools, will be fundamental for the production of representative numerical models, e.g. finite element models (fems), to be used for more reliable subsequent structural analyses. the presented study confirms that an integrated multisensor and multi-disciplinary approach can be successfully applied to the study of a heterogeneous and complex object, such a historic structure that was subjected to a variety of major changes and interventions during its age-old life. acknowledgement the present work was conducted within the framework of the co.b.ra. project, which focused on the development of advanced technologies and methods for the conservation of cultural heritage assets, funded by lazio region, italy. references [1] i. roselli, v. fioriti, i. bellagamba, m. mongelli, a. tatì, m. barbera, m. magnani cianetti, g. de canio, “urban transport vibrations and cultural heritage sites in rome: the cases of the temple of minerva medica and of the catacomb of priscilla”, wit transactions on ecology and the environment 223(2017) pp. 335-343, doi: 10.2495/sc170291. [2] h. nandan, m.p. singh, “effects of thermal environment on structural frequencies: part i – a simulation study”, eng. struct. 81(2014) pp. 480-490, doi: 10.1016/j.engstruct.2014.06.046. [3] m. r. polimeno, i. roselli, v. a. m. luprano, m. mongelli, a. tatì, g. de canio, “a non-destructive testing methodology for damage assessment of reinforced concrete buildings after seismic events”, eng. struct. 163(2018) pp. 122–136, doi: 10.1016/j.engstruct.2018.02.053. [4] m. barbera, m. magnani cianetti, s. barrano, “da massenzio a costantino: le indagini in corso nel c. d. tempio di minerva medica”, proc. of the international conference of cisem la villa restaurata e i nuovi studi sull'edilizia residenziale tardoantica (in italian), november 7-10, 2012, piazza armerina, italy, pp. 1613-1624. [5] k. van balen, i. papayianni, r. van hees, l. binda, a. waldum, “introduction to requirements for and functions and properties of repair mortars”, materials and structures 38 (2005) pp. 781785, doi: 10.1007/bf02479291. [6] g. verhoeven, “taking computer vision aloft—archaeological threedimensional reconstructions from aerial photographs with photoscan”, archaeol. prospection 18(2011) pp. 67–73, doi: 10.1002/arp.399. [7] g. ponti et al., “the role of medium size facilities in the hpc ecosystem: the case of the new cresco4 cluster integrated in the eneagrid infrastructure”, proc. of the international conference on high performance computing and simulation (hpcs), july 21–25, 2014, bologna, italy, doi: 10.1109/hpcsim.2014.6903807. [8] m. mongelli, g. de canio, i. roselli, m. malena, a. nacuzi, g. de felice, “3d photogrammetric reconstruction by drone scanning for fe analysis and crack pattern mapping of the bridge of the towers, spoleto”, key engineering materials 747(2017) pp. 423-430, doi: 10.4028/www.scientific.net/kem.747.423. [9] i. roselli, m. malena, m. mongelli, n. cavalagli, m. gioffrè, g. de canio, g. de felice, “health assessment and ambient vibration testing of the ponte delle torri of spoleto during the 2016-17 central italy seismic sequence”, int. j. civil structural health monitoring 8-2(2018), doi: 10.1007/s13349-018-0268-5. [10] c. w. de silva, vibration and shock handbook, crc press, 2005, ebook isbn: 9781420039894. figure 12. average values of sonic test velocity recorded at each pillar acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 19 [11] g. de canio, m. mongelli, i. roselli, a. tatì, d. addessi, m. nocera, d. liberatore, “numerical and operational modal analyses of the ponte delle torri, spoleto, italy”, proc. of the 10th international conference on structural analysis of historical constructions (sahc), 2016, pp. 752-758, ebook isbn: 978-1-315-61699-5. [12] m. r. gallipoli, m. mucciarelli, m. vona, “empirical estimate of fundamental frequencies and damping for italian buildings”, earthquake engineering & structural dynamics 38-8(2009) pp. 973–988, doi: 10.1002/eqe.878. [13] i. roselli, v. fioriti, m. mongelli, i. bellagamba, g. de canio, “mutual validation between different modal analysis techniques for dynamic identification of the so-called temple of minerva medica, rome”, iop conf. series: materials science and engineering 364-1(2018), doi: 10.1088/1757899x/364/1/012004. [14] s. hum-hartley, nondestructive testing for heritage structures, bulletin of the association for preservation technology 103(1978) pp. 4-20, doi: 10.2307/1493664. [15] uni 10627:1997. sistemi di tomografia computerizzata per indagini strutturali (italian standard), http://hdl.handle.net/11573/65727. http://hdl.handle.net/11573/65727 additive manufacturing and technical strategies for improving outcomes in breast reconstructive surgery acta imeko issn: 2221-870x december 2020, volume 9, number 4, 74 79 acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 74 additive manufacturing and technical strategies for improving outcomes in breast reconstructive surgery nicola rocco1, ida papallo2, maurizio bruno nava1, giuseppe catanuto1, antonello accurso3, ilaria onofrio4, olimpia oliviero4, giovanni improta5, domenico speranza6, marco domingos7, teresa russo8, roberto de santis8, massimo martorelli9, antonio gloria8 1 g.re.t.a. group for reconstructive and therapeutic advancements, milan, naples, catania, italy 2 department of advanced biomedical sciences, university of naples federico ii, 80131 naples, italy 3 breast unit, university hospital federico ii, 80131 naples, italy 4 department of neurosciences, reproductive and odontostomatological sciences, university of naples federico ii, 80131 naples, italy 5 department of public health, university of naples federico ii, 80131 naples, italy 6 department of civil and mechanical engineering, university of cassino and southern lazio, cassino (fr), italy 7 department of mechanical, aerospace and civil engineering & henry royce institute, school of engineering, faculty of science and engineering, university of manchester, uk 8 institute of polymers, composites and biomaterials – national research council of italy, 80125 naples, italy 9 department of industrial engineering, fraunhofer jl ideas, university of naples federico ii, 80125 naples, italy section: research paper keywords: additive manufacturing; breast reconstructive surgery; fat grafting; reverse engineering; scaffold design; pore geometry and lay-down pattern; mechanical and functional properties citation: nicola rocco, ida papallo, maurizio bruno nava, giuseppe catanuto, antonello accurso, ilaria onofrio, olimpia oliviero, giovanni improta, domenico speranza, marco domingos, teresa russo, roberto de santis, massimo martorelli, antonio gloria, additive manufacturing and technical strategies for improving outcomes in breast reconstructive surgery, acta imeko, vol. 9, no. 4, article 10, december 2020, identifier: imeko-acta-09 (2020)04-10 section editor: leopoldo angrisani, university of naples federico ii, italy received november 17, 2019; in final form july 30, 2020; published december 2020 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: nicola rocco, e-mail: nicolarocco2003@gmail.com 1. introduction breast reconstruction provides a significant improvement in the quality of life of women undergoing mastectomy for breast cancer [1]. immediate reconstruction does not negatively impact on oncological outcomes, does not delay the commencement of adjuvant therapies, and does not impair postoperative oncological surveillance [2], [3]. conservative mastectomies (nipple areola complex (nac)sparing, skin-sparing and skin-reducing) allow the complete preservation of the breast envelope in patients, without involvement of the skin or the nac [4]-[6]. abstract it has been widely reported that breast reconstruction improves the quality of life of women who undergo mastectomy for breast cancer. this approach provides many psychological advantages. today, different techniques are available for the breast oncoplastic surgeon that involve the use of breast implants and autologous tissues, also offering interesting results in terms of aesthetic and patient-reported outcomes. on the other hand, advanced technologies and design strategies (i.e. design for additive manufacturing, reverse engineering) may allow the development of customised porous structures with tailored morphological, mechanical, biological, and mass transport properties. for this reason, the current study deals with the challenges, principles, and methods of developing 3d additive manufactured structures in breast reconstructive surgery. specifically, the aim was to design 3d additive manufactured poly(ε-caprolactone) scaffolds with different architectures (i.e. lay-down patterns). preliminary mechanical and biological analyses have shown the effect of the laydown pattern on the performances of the manufactured structures. mailto:nicolarocco2003@gmail.com acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 75 currently, breast surgeons can choose among many surgical techniques for breast reconstruction, with the opportunity of offering women the best results in terms of aesthetic results and postoperative quality of life, using both silicone breast implants and autologous tissues [7]. the patient’s wishes must drive every choice in the decisionmaking for breast reconstruction, aiming at tailored surgical treatments [8], [9]. implant-based reconstructive options are widely used, even though they could be associated with postoperative complications and with a worsening of the aesthetic results over time [7]-[10]. capsular contracture represents the most common complication. other complications have also been described in association with breast implants, such as ruptures, infections, late seromas, and the development of an extremely rare form of lymphoma, the breast implant-associated anaplastic large cell lymphoma (bia-alcl), [11]. in particular, the possible development of bia-alcl has recently raised some doubts about the safety of silicone breast implants. researchers are searching for alternatives to implants for breast reconstruction. a possible alternative to breast implants is represented by myocutaneous pedicled and muscle-sparing free flaps, already used by several reconstructive surgeons with good outcomes. however, breast reconstruction with autologous tissue flaps is not exempt from complications and reinterventions. moreover, the autologous tissue-based surgical procedures are time consuming and expensive when compared to implant-based reconstruction. the mastectomy reconstruction outcomes consortium (mroc) study, a prospective multi-centre trial, recruited patients undergoing breast reconstruction (implant-based and autologous-tissue-based) following mastectomy from 11 centres across north america from february 2012 to july 2015 [12], [13]. the mroc study reported two-year complications rate following autologous tissue-based reconstruction of 47 % versus 26.6 % with implant-based techniques and reoperation rates of 27.4 % with autologous tissues versus 15.5 % with implants. these data must be taken into account when considering autologous flaps as the first possible alternative to silicone implants for breast reconstruction. we developed a new reconstructive technique with the aim of reducing surgical aggressiveness, improving cosmetic outcomes, and achieving long-lasting results, involving tissue regeneration supported by three-dimensional (3d) additively manufactured scaffolds with controlled morphology. this method could represent a further evolution of the ‘hybrid reconstructive’ option that we have already presented, with the combined use of silicone implants and autologous fat tissue transplantation [14], [15]. 3d-printed bioresorbable scaffolds will be positioned subcutaneously following nac-sparing or skin-sparing mastectomies and filled with autologous fat tissue over 2–3 sessions to achieve a natural-shaped breast mound with a soft consistency and long-lasting aesthetic results. this reconstructive option could also be offered to women undergoing postmastectomy radiotherapy (pmrt), representing a significant advantage when compared with silicone implants in the radiotherapy setting [16]. future clinical applications of the 3d-engineered breast reconstruction will validate this innovative technique that will probably become a standard for the next generation breast surgeon. in this scenario, the development of innovative breast devices would involve the reverse engineering approach and additively manufactured scaffolds combined with autologous fat grafting. thus, 3d scaffolds with autologous adipose-derived stem cells have been proposed for breast tissue engineering, as the strategy should be to appropriately place the device and to fill it with autologous fat tissue over a few sessions. the customised porous structure filled with autologous fat tissue should be capable of maintaining the breast shape, as well as its natural consistency. further technical improvements (i.e. stromal vascular fraction derived growth factors) will be also taken into consideration. for this reason, an integration of the basic concepts of regenerative medicine with advanced technologies (i.e. additive manufacturing), image capture, and analysis techniques should potentially lead to the design of 3d scaffolds with tailored architectural features and properties for breast tissue regeneration. in brief, 3d virtual models of the breast may be generated starting from medical scans (i.e. computed tomography (ct), magnetic resonance imaging (mri)) and 3d porous structures can be fabricated by fused deposition modelling (fdm/3d) fibre deposition, layer-by-layer, according to specific lay-down patterns. in this context, to design innovative systems, the research may clearly benefit from the development of advanced technologies [17] and methodologies of analysis [18]-[21] in different fields, as well as from the engineering of biomaterials [22], [23] and 3d porous structures with enhanced and tailored properties for tissue regeneration [24], [25]. accordingly, the aim of the current study was to design 3d additively manufactured poly(ε-caprolactone) scaffolds with different architectures (i.e. lay-down patterns). preliminary mechanical and biological analyses were carried out to analyse the effect of the lay-down pattern on the performances of the manufactured structures. 2. materials and methods poly(ε-caprolactone) (pcl, capa 6500, mw = 50,000) pellets were employed and 3d scaffolds (length l of 7.0 mm, width w of 7.0 mm, height h0 of 8.0 mm) were fabricated by fdm/3d fibre deposition technique, using an extrusion-based system. the pellets were heated to 100°c and the 3d scaffolds were built layer by layer, as the molten pcl was extruded/injected through a needle with an inner diameter of 300 µm. the pcl filaments were deposited according to three sequences of stacking (i.e. lay-down patterns) (0°/90°, 0°/60°/120° and 0°/45°/90°/135°). a filament distance and a slice thickness of 650 and 280 µm were employed, respectively. further process parameters were: extrusion pressure of 5 bar, deposition speed of 10 mm/s, screw velocity of 30 rpm. three different lay-down patterns were adopted that maintained a constant filament distance (i.e. centre-to-centre distance) of 650 µm. thus, different pore geometries were obtained. mechanical compression tests were carried out on the manufactured scaffolds at a cross-head speed of 1 mm/min, up to a strain of 0.4 mm/mm, using an instron 5566 testing machine. acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 76 the apparent stress (σ) and strain (ε) were calculated as reported below [25], [26]: 𝜎 = 𝐹 𝐿 ∙ 𝑊 (1) 𝜀 = ∆𝐻 𝐻0 (2) with f being the measured force, whereas ∆h represents the height variation of the pcl scaffold. the slope of the initial linear portion of the stress–strain curve was considered to determine the compressive modulus. the biological performances of the manufactured scaffolds were evaluated to analyse the effect of the lay-down patterns. in brief, the pcl scaffolds were prepared for cell seeding following a protocol that has already been reported in the literature [25], [26]. the pcl scaffolds were seeded with adiposederived mesenchymal stem cells (amscs) using 1 × 104 cells/sample. the cell viability was evaluated at different time points using the alamar blue assay (abd serotec ltd,uk). confocal laser scanning microscopy (clsm) and rhodamine phalloidin staining were employed to analyse the cell adhesion and spreading at different time points. the image j software and a shape factor were used to analyse the clsm images of the cell-scaffold constructs [25], [26]. the shape factor was calculated as follows: 𝜙 = 4 π 𝐴 𝑃2 (3) with p and a representing the perimeter and the area of a cell, respectively. as frequently reported, circular objects have the greatest areato-perimeter ratio, and a shape factor of 1 represents a perfect circle. thin thread-like objects are characterised by the lowest shape factor, which approaches zero [25], [26]. all the data were analysed by anova, followed by the bonferroni post hoc test. statistically significant values were defined as p < 0.05. 3. results the gel-filled breast implants generally consisted of a shell made of an elastomeric material (i.e., polydimethylsiloxane, pdms) and a gel-like core (i.e. pdms with a lower cross-linking degree). with regard to the development of novel breast devices, the use of advanced technologies and the increasing knowledge of the structure–property relationship of the materials have led to the possibility of reproducing the complex viscoelastic properties, as well as the nonlinear and large-deformation behaviour. several strategies have been proposed for breast tissue repair/reconstruction and regeneration, also involving the combination of additive manufacturing techniques with an appropriate selection of materials that are already used for tissue engineering and prosthetic applications. specifically, over the past few years, many efforts have been made to develop devices in the form of gels/hydrogels and 3d structures with controlled morphology [27]-[29]. a first approach should involve nondegradable polymers and the design of customised prostheses using the reverse engineering approach. the material–geometry design plays a crucial role in reproducing the mechanical behaviour of the native tissue, as well as the exact shape and size of the defect. shell-core or multilayer devices can be designed, employing rubber-like and gel-like materials or the combination of conventional fabrication methods with additive manufacturing techniques. in the current research, a second design strategy was reported, involving the development of customised porous structures and lattices with tailored morphological, biological, mechanical, and mass transport properties. pcl, which is an aliphatic polyester, was considered as among synthetic polymers it has been widely studied for tissue engineering applications. in the field of additive manufacturing, 3d fibre deposition, which represents a modified technique of 3d plotting to extrude highly viscous polymers, is a fused deposition technique in which the material is extruded/injected through a nozzle and appropriately deposited according to the selected lay-down patterns. scaffolds with different internal pore geometries (i.e. quadrangular, triangular, and complex polygonal) were designed adopting three lay-down patterns (0°/90°, 0°/60°/120° and 0°/45°/90°/135°, respectively), while the filament diameter, filament distance, and slice thickness were maintained as constant. the stress–strain curves (figure 1) obtained from the compression tests were similar to those reported for 3d fibredeposited scaffolds [25], [26]. the compressive modulus is reported in table 1 as the mean value ± standard deviation. at fixed filament diameter, filament distance and slice thickness, the structures with a 0°/90° pattern provided a compressive modulus that was significantly higher than those found for the other types of scaffolds (i.e. 0°/60°/120° and 0°/45°/90°/135°). furthermore, with regard to the scaffolds with the 0°/60°/120° and 0°/45°/90°/135° patterns, statistically significant differences were observed in terms of the compressive modulus. figure 1. the effect of the lay-down pattern. stress–strain curves for additive manufactured pcl scaffolds tested up to a strain of 0.4 mm/mm. acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 77 it is well known that 3d fibre-deposited scaffolds are characterised by a column-like behaviour of the filament junctions under compression [25], [26]. spanning from 0°/90° to 0°/45°/90°/135°, a larger contact area (i.e. fused area) was a clear consequence of a decrease in the amplitude of the deposition angle between the struts of adjacent layers. this led to a reduction of the local stress experienced by the structure, and in the scaffolds with a smaller amplitude of deposition angle, the filament sliding was easier, increasing the deformability of the structure. in vitro biological analyses were carried out to investigate the influence of the adopted lay-down pattern on the behaviour of the amscs. figure 2 reports typical results obtained from the alamar blue assay. a redox reaction occurred in the cell mitochondria and the percentage of alamar blue reduction could be related to the number of viable cells. amscs may have survived and proliferated throughout the pcl structures as a significant increase of the alamar blue reduction was achieved over time. even though at day one no significant differences were found among the pcl structures, at three and seven days the scaffolds with a lay-down pattern of 0°/90° showed values with a percentage of reduction of the alamar blue that were significantly greater than those found for the other structures (p < 0.05). accordingly, a lay-down pattern of 0°/90° significantly improved the cell viability/proliferation (figure 2). thus, pore geometry influenced the cell viability and an increase in cell viability was generally evident when the lay-down pattern varied from 0°/45°/90°/135° to 0°/90°. to provide further insight into the effect of the lay-down pattern on cell morphology, clsm was carried out. as an example, figure 3 reports typical results obtained at the end of one, three, and seven days of incubation. using the clsm images, cell adhesion and spreading were further analysed to evaluate the shape factor. values of the shape factor are reported (mean value ± standard deviation) at one, three, and seven days after the cell seeding (figure 4). a significant decrease of the shape factor was evident over time for all kinds of cell-scaffold constructs. a decrease of the shape factor should suggest better cell adhesion and spreading since the lower the shape factor, the more elongated the cell [25]. however, in terms of the cell shape factor, no significant differences were observed among the scaffolds with different patterns. nevertheless, the obtained findings confirmed a strong influence of the lay-down pattern and, hence, of the pore geometry on the viability/proliferation of the amscs seeded and cultured on the pcl scaffolds. figure 2. percentage of alamar blue reduction for the pcl scaffolds at one, three, and seven days: the effect of the lay-down pattern. the data are reported as the mean value; the error bar represents the standard deviation. table 1. effect lay-down pattern on compressive modulus of 3d additively manufactured pcl scaffolds. results are reported as mean value ± standard deviation. lay-down pattern e (mpa) 0°/90° 39.1 ± 3.4 0°/60°/120° 29.1 ± 2.8 0°/45°/90°/135° 20.1 ± 2.2 figure 3. typical results from the clsm analysis on the pcl scaffolds at one (a), three (b), and seven (c) days. images of rhodamine phalloidin-stained cells/actin cytoskeleton (red). scale bar of 100 µm. figure 4. values of the shape factor obtained from clsm images of the amscs on the pcl scaffolds with different lay-down patterns. data are reported as mean value, and the error bar represents the standard deviation. acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 78 in the literature, many studies frequently report the important roles of reverse engineering [30]-[33], computer-aided design (cad), and finite element analysis [34]-[37], as well as the advances in methodologies of analysis and design strategies to develop smart devices for different applications [38]-[41]. for this reason, although the results suggest that a pattern of 0°/90° would improve in vitro amsc proliferation, the combination of lay-down patterns and, hence, of pores with different geometries within a single scaffold may represent a potential solution for the design of functionally graded structures that are capable of promoting optimal tissue regeneration while providing the required flexibility and strength. 4. conclusions taking into account the current scenario and a critical analysis on the breast reconstruction techniques, principles, and design methods for the development of 3d additively manufactured structures were proposed for breast reconstructive surgery. in particular, an insight into the design and analysis of 3d scaffolds with different lay-down patterns was provided to improve surgery outcomes. the possibility of fabricating 3d scaffolds with tailored architectures, functional and structural features by varying the lay-down pattern and, hence, the pore geometry, was stressed. biological analyses also demonstrated the effect of the laydown pattern on the viability/proliferation of the amscs. the reported experimental results, together with the image capture and analysis techniques (i.e. reverse engineering) would suggest the feasibility of developing customised structures and technical solutions in breast reconstructive surgery. acknowledgments the authors gratefully acknowledge mrs mariarosaria bonetti (institute of polymers, composites and biomaterials – national research council of italy) for her contribution related to cad/cam systems. references [1] a. 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editor‐in‐chief dušan agrež, slovenia founding editor‐in‐chief paul p. l. regtien, netherlands associate editor dirk röske, germany editorial board section editors (vol. 7 and 8) leopoldo angrisani, italy filippo attivissimo, italy eulalia balestieri, italy eric benoit, france paolo carbone, italy lorenzo ciani, italy catalin damian, romania pasquale daponte, italy luca de vito, italy luigi ferrigno, italy edoardo fiorucci, italy alistair forbes, united kingdom helena geirinhas ramos, portugal sabrina grassini, italy fernando janeiro, portugal konrad jedrzejewski, poland andy knott, united kingdom francesco lamonaca, italy massimo lazzaroni, italy fabio leccese, italy rosario morello, italy michele norgia, italy pedro miguel pinto ramos, portugal nicola pompeo, italy sergio rapuano, italy dirk röske, germany alexandru salceanu, romania constantin sarmasanu, romania lorenzo scalise, italy emiliano schena, italy enrico silva, italy krzysztof stepien, poland marco tarabini, italy marcantonio catelani, italy paolo carbone, italy lorenzo ciani, italy egidio de benedeto, italy alessandro germak, italy sabrina grassini, italy konrad jędrzejewski, poland min-seok kim, korea fabio leccese, italy rosario morello, italy vilmos palfi, hungary nicola pasquino, italy franco pavese, italy paul regtien, the netherlands alexandru salceanu, romania jan saliga, slovakia jorge c. torres-guzman, mexico ian veldman, south africa claudia zoani, italy about imeko the international measurement confederation, imeko, is an international federation of actually 41 national member organisations individually concerned with the advancement of measurement technology. its fundamental objectives are the promotion of international interchange of scientific and technical information in the field of measurement, and the enhancement of international co-operation among scientists and engineers from research and industry. addresses principal contact prof. dušan agrež university of ljubljana faculty of electrical engineering tržaška 25, 1000 ljubljana, slovenia e-mail: dusan.agrez@fe.uni-lj.si acta imeko petri koponen vtt mikes, tehdaskatu 15, puristamo 9p19 fi-87100 kajaani, finland e-mail: petri.koponen@vtt.fi support contact dr. dirk röske physikalisch-technische bundesanstalt (ptb) bundesallee 100, 38116 braunschweig, germany e-mail: dirk.roeske@ptb.de mailto:dusan.agrez@fe.uni-lj.si mailto:petri.koponen@vtt.fi mailto:dirk.roeske@ptb.de electrical method for intelligent cooling liquid control system acta imeko issn: 2221-870x march 2020, volume 9, number 1, 56 60 acta imeko | www.imeko.org march 2020 | volume 9 | number 1 | 56 an electrical method for intelligent cooling liquid control systems maryna mikhalieva1, liubomyra odosii1, bohdan seredyuk1, vasyl zalypka1, lidiya parashchuk1, hanna lunkova1 1 national academy of land forces, department of rocket troops and artillery, lviv, ukraine section: research paper keywords: electrophysical characteristics; immittance; conductometric sensor; cooling liquids citation: maryna mikhalieva, liubomyra odosii, bohdan seredyuk, vasyl zalypka, lidiya parashchuk, hanna lunkova, electrical method for intelligent cooling liquid control system, acta imeko, vol. 9, no. 1, article 9, march 2020, identifier: imeko-acta-09 (2020)-01-09 editor: giovanni bucci, university of l'aquila, italy received december 28, 2019; in final form february 18, 2020; published march 2020 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: marina mikhalieva, e-mail: galmih@ukr.net 1. introduction water mixtures are the most commonly used objects for measurement and control in industry, medicine, agriculture, and environmental monitoring. the standard control characteristics in these industries are concentrations – the mass of substances, per unit volume of the object of control. to objectively determine the concentration of the dissolved substance, the object should be uniform throughout the volume. therefore, analytical control methods are based on studies of homogeneous liquids. in cases where the object of research is in a solid or loose state, its composition is extracted in a liquid for further long-term analytical manipulations in a laboratory. another important requirement for a liquid under standard controls is a twocomponent system – with the content of the control substance and the unchanged composition of the solvent. the implementation of standard analytical definitions (electrical, optical, and chemical methods) for the rapid and automated control of the composition of multicomponent liquids, such as real objects, is complicated by lengthy analysis or inaudibility. the scientific research conducted to improve the rapid control methods of the composition of dissolved substances in liquids during the course of technological processes and environmental monitoring is outlined in this article. the standard conductometric method is designed for expressive studies for binary solutions of concentrations of substances that affect the conductivity of a liquid. this method allows to determine the concentration of a single controlled substance in a multicomponent fluid, using a single measured value of the specific conductivity at a predetermined in the laboratory frequency of the electromagnetic field. investigation of the electrical properties of aqueous solutions as liquid dielectrics and an analysis of the components of complex electrical parameters at different frequencies of the electromagnetic signal deepens the theory of the study of the dependencies of the electrical properties of liquids on their composition. attention to such research has intensified due to the improvement of experimental technology and the need to improve the methods of control of technological processes [1]. hence, there is a danger of overheating the engine, which leads to its failure. the antifreeze resource directly depends on its composition (the brand of the manufacturer) and the mileage of automotive equipment. when exhaust gases, air or water abstract the ability to determine and control normalized concentrations is established in this study using experimentally obtained dependencies of the active and reactive constituents of conductivity in the wide frequency range of the electromagnetic field on the composition of the multicomponent fluid and the impurity concentrations. a method for controlling the qualitative and quantitative composition is developed for highly aqueous substances dissolved in water (cooling liquids). it is based on a comparison of the measured and experimentally established active conductivity component and a comparison of the corresponding dependencies in the frequency field of the reactive component of conductivity. the developed electric method allows the quantitative and qualitative estimation of the composition of the coolant on the contents of controlled components in a short time (up to two seconds) in a non-laboratory environment and to ensure the uninterrupted operation of the equipment. mailto:galmih@ukr.net acta imeko | www.imeko.org march 2020 | volume 9 | number 1 | 57 evaporates into the cooling system, its "aging" is particularly intense. such a coolant should be replaced urgently. the technical certification of cooling liquids in our country is optional, so the validity of antifreeze (a cooling liquid) is not regulated by state standards. standard techniques for controlling their composition require routine laboratory work. 2. review of the concept known non-destructive methods for the study of liquids are based on the use of conductometry and aquametry. these two methods are applied to non-electrolytes (some substances of inorganic composition and mainly all organic composition) for non-selective generalized analysis. only the immittance method, which actually contains both conductometric and dielectric methods, is theoretically capable of studying liquids of different chemical natures. the conductometric method is based on the measurement of the electrical conductivity of the entire measuring system, which depends on the concentration of only those substances that affect the electric current (electrolytes). this method is applicable for dual component liquids only. in the dielectric method (aquametry), according to which the informative parameter is electrical permeability, the generalization of this indicator makes it difficult to control the composition of multicomponent fluids. the complex electrical parameters of the immittance method would give more information about the composition of such liquids. immittance spectroscopy is used to analyse semiconductive, dielectric, and ion-electronic materials as well as electrochemical systems. the specific physical mechanisms of the response of systems to the applied sine wave signal in different external physical fields are studied. the mechanisms of electrical and kinetic phenomena occurring under the given conditions are investigated as a result of the analysis of the immittance data [4]–[7]. the authors, having completed the theoretical analysis, have proposed the study of the composition of objects of a nonelectric nature (liquids) employing the method of immittance spectroscopy. in other words, for the experimental studies, a physical-chemical study with elements of conductometry has been employed. aquametry and immittance spectroscopy have been carried out using a capacitive primary transducer. the novelty of the research conditions is the use of a new approach of dielectric properties of objects of a non-electric nature. the authors do not describe in this work all the scientific facts that were obtained during the research and used in practice. the purpose of this article is to describe only some of the scientific results that illustrate the possibility of developing a new approach to immittance studies of objects of a non-electric nature – multicomponent liquids containing non-electrolytes. 3. results an investigation of the electrical parameters of liquids in a frequency-tuned electromagnetic field allowed us to obtain information about their composition in non-laboratory conditions for a short period of time. therefore, the development of advanced methods for the operational control of cooling liquids, which can be carried out in field conditions is of great interest. the method involves measuring the value of the component of the electrical conductivity of the controlled fluid and comparing it with the corresponding values established in the laboratory by means of a simulation study. the experimental measuring system consisted of a standard measuring device rlc-meter, a primary converter with capacity for the fluids studied, and a computer with software for processing the results of measurements (figure 1). the cell converter (sensor) consists of a specially graduated glass with a cover and permanently fixed immersed electrodes. the cell is placed on a grounded screen. for a sensor with carbon electrodes, the material obtained by high-temperature sintering of graphite powder designed for industrial galvanic baths is used. this technical solution provides the possibility of changing the volume of the test fluid (the amount of substance) located between the electrodes [2], [3]. as a result of the study of the immittance of model and real liquids with a composition of substances of different electrical natures (high and low), it is possible to identify the qualitative and quantitative composition by comparison with the experimentally determined normalized values of the active component with the established value of frequency. the methodology is based on the reaction of the features of the alcohol molecule of the homologous series, which is dissolved in water to the electrical properties of the mixture with its composition. as a result of the experiment, an algorithm for the method of operational control of the composition was established. to determine the normalized parameters, a model sample study was performed. the under laboratory conditions installed value of the active component gnorm is determined at an arbitrarily selected frequency ν of the electromagnetic field, and the reactive component bnorm is measured at all possible frequencies. this value of the active component gnorm and the form of the plotted graphic dependence bnorm = f(ν) are the normalized control parameters. the algorithm for controlling the presence of dangerous high-resistance impurities in the liquid mixture contains measurements of the value of the active component gm at the normalized signal frequency fnorm. the result of the comparison gm > gnorm and the absence of differences between the bnorm = f(ν) and bm = f(ν) dependencies figure 1. primary transducer and experimental setup. acta imeko | www.imeko.org march 2020 | volume 9 | number 1 | 58 indicate the presence of a toxic additive – methyl alcohol – in the solution. on the other hand, the result gm < gnorm and the absence of differences among the bnorm = f(ν) and bm = f(ν) dependencies indicate the presence of butyl or propyl alcohol in the solution. the presence of differences in the comparison of bnorm = f(ν) and bm = f(ν) imply the presence of inorganic impurities in the test mixture [8]. 4. methods the purpose of these studies is to develop an electrical method for the quantitative and qualitative analysis of the composition of cooling liquids. dependencies of the reactive and active components of conductivity on the frequency of the electromagnetic field for the model liquids of a given composition have been studied. using the experimentally obtained data, a method for the operational control of the composition and cooling liquids has been proposed. the main results of the study are shown in figure 1 and figure 2. changes in the value of the reactive component are only due to the presence of the excessive water, causing higher values of the reactive conductivity than those established by industry standards (figure 2). on the other hand, inhibitors lower the reactive component down to negative values. the values of the active component of the object, as the function of the field frequency (figure 3), indicate the presence of the poisonous ethylene glycol (if the value of the active component is in the range of min m.s. to max m.s.) or water (if the value of the active component is within the range of min m.s. to min e.s.). the measured value of the reactive component indicates the presence of water (if the value of the reactive component is above row 1) or pollutants, for example, unauthorized inhibitors (if the value of the reactive component is in the range of row 1 to row 2). the control methods vary depending on the chemical properties of the controlled substance (electrolytes or nonelectrolytes) [9]–[15]. the developed methods for the operational control of the concentration of controlled substances in technical liquids and purified sewage water consist of two main stages: the study of control model – liquids with a boundary permissible concentration (bpc) of controlled substances carried out in laboratory conditions – and the most operational control process for a real object (figure 2). in the first stage, a method for investigating liquids is used, where the dependencies of the active (reactive) conductivity component on the volume of the multicomponent fluid in the converter; the concentration of the controlled substance; the frequency of the electromagnetic field; and the concentrations of the matrix and temperature are applied. the second stage is the process of the operational control of normalized concentrations of controlled substances in a real object, determined in laboratory conditions (in stage 1) by the parameters, as described here: stage 1. determination of control parameters: gcontr(ω-1); bcontr(ω-1); f(hz) by investigating g = f(f); b = f(f) ml (multicomponent liquids) with the control substance cbpc(g/l). stage 2. the process of the control of the bpc of nonelectrolytes in a real object: 1. measurement of gcontr(ω-1); bcontr(ω-1) by f(hz); 2. conclusions drawn from the measurement data: comparison of g ≤ gcontr, or g ≥ gcontr, by bcontr: c ≥ bpc or c ≤ bpc (depending on the chemical nature of the non-electrolyte). let us consider the method of calibration of the control channels of the electrical concentrators for the software and hardware monitoring of the equipment. the method of determining the k (concentration) of a controlled substance in ml requires clarification of the value of the volume of the investigated ml. to estimate the uncertainty in determining the volume and the k of components, we constructed a mathematical model of the measuring cell. in the electrolytic cell, l is the distance between the electrodes; a and b are its height and width respectively and thus the geometrical dimensions of the electrodes; and a1 is the height of the liquid part of the capacitor. these parameters form the volume of the investigated ml. the capacity of such an electrolytic cell, which consists of the air cx1 and liquid cx2 parts is defined as: 𝐶𝑥 = 𝐶𝑥1 + 𝐶𝑥2 = 𝜀п 𝜀0 (𝑎 − 𝑎1)𝑏 𝑙 + 𝜀𝑝 𝜀0 𝑎1 𝑏 𝑙 , (1) where n, p is the dielectric constant of air and liquid solution and 0 is the dielectric constant of the vacuum. in the mode of a given sinusoidal current for the converter, the expression for the output conductivity can be represented by the relation: figure 3. dependence of the reactive conductivity components on the frequency of the electromagnetic field. figure 2. dependence of the active conductivity components on the frequency of the electromagnetic field. acta imeko | www.imeko.org march 2020 | volume 9 | number 1 | 59 𝐺out = 1 𝑍out = 𝑗𝜔(𝐶𝑥1 + 𝐶𝑥2 ) + 1 𝑅𝑥 + 1 𝑅ic , (2) where rx and ric are the active component of the resistance and the insulation resistance of the cell, respectively. the mathematical dependence of the function of the transformation of the device (reactive and active constituents of conductivity) on the concentrations and parameters of the metering circuit is as follows: 𝑁𝐺 = 𝑘a 𝜔 (𝐶𝑥1 + 𝐶𝑥2 ) = 𝑘a𝑏 𝜔 𝜀0 𝑎 𝑏 𝑙 [𝜀a(1 − 𝑎1 𝑎⁄ ) + 𝜀l а1 а ] , 𝑁𝐺𝑎 = 𝑘a𝑎 ( 1 𝑅𝑥 + 1 𝑅ic ) , (3) where 𝑐𝑥1 and 𝑐𝑥2 are the concentrations of the first and second substances of the liquid system, respectively; a, a1, l, b are the known design parameters; ka is the conversion factor of adc; kaa and kab are the conversion factors of the geometric parameters of the electrodes; εa and εl are the dielectric constants of the air and the liquid, respectively; rx is the active component of the resistance; ric is the resistance of the cell’s isolation; and ε 0 is the dielectric constant of the vacuum ε 0 = 8,8510-12 f/m. in general, the mathematical dependence of the transformation function of the reactive component of the digital immittance meter can be given in the form of 𝑁𝐺 𝑏 = 𝑘a 𝜔𝑖 𝜀0 𝑎1𝑏 𝑙 [𝜀a ( 𝑎1 𝑎 − 1) + (𝑚1𝑐𝑥1 + 𝑚2𝑐𝑥2)] + 𝛥a , (4) where ngb is the device display on the i-th frequency ωi = 2 fi; m1 and m2 are the coefficients of transformation of the concentration of the test substance (hereinafter k) into the capacitive components for 𝑐𝑥1 and 𝑐𝑥2 , respectively; 𝛥a is the additive component of the admittance meter, reduced to its output. in order to correct the additive component of the error of the measuring component of the reactive component, one measurement should be made in the absence of the electrodes of the test substance: 𝑁𝐺0 = 𝑘a 𝜔𝑖 𝜀 𝑎1 𝑏 𝑙 𝜀a ( 𝑎1 𝑎 − 1) + 𝛥a , (5) where ε is the dielectric constant of the investigated liquid. from this point, the display of the device ngi should be adjusted by the value of the additive component ng0, and the value of ng is thereby obtained: 𝑁𝐺 = 𝑁𝐺𝑖 − 𝑁𝐺0 = 𝑘a 𝜔𝑖 𝜀 𝑎1 𝑏 𝑙 (𝑚1𝑐𝑥1 + 𝑚2𝑐𝑥2) . (6) taking into account the nonlinear relationship between the display of the device and the concentrations of the investigated substances, it is possible to calibrate any gauge of the admittance in the vicinity of the frequency of the polarity swap of the reactive component. for this purpose, in the vicinity of this frequency ω0 of the polarity change of the reactive component, an admittance measurement is carried out for the case of n concentrations of control models of liquids, and the value of the frequency ω0n is registered, according to which the value of the reactive component is measured: 𝑁𝑖𝑧𝑛 = 𝑘𝐴 𝜔0п 𝜀0 𝑎1𝑏 𝑙 (𝑚1z𝑛𝑐1𝑛 + 𝑚2z𝑛𝑐2𝑛) = 0 , (7) where ω0n is the value of the frequencies at which the meter of the reactive conductivity component has zero indications; c1n and c2n are the n-th concentration of the first and second controlled substances in the model sample, respectively; m1zn, and m2zn are the coefficients of the concentration of the test substance (hereinafter k) for the first and second substances into the reactive constituent of conductivity for k c1n and c2n, respectively. from equation (6), we calculate 𝑚1z𝑛 𝑚2z𝑛 = − 𝑐2𝑛 𝑐1𝑛 . (8) then, in the vicinity of the values of frequencies ω0n, the function of transforming the reactive component of the device can be written as follows: 𝑁𝑖z𝑛 = 𝑘a 𝜔𝑖 𝜀0 𝑎1 𝑏 𝑙 𝑚1 (𝑐𝑥1 + 𝑚2 𝑚1 𝑐𝑥2) = 𝑘a 𝜔𝑖 𝜀0 𝑎1 𝑏 𝑙 𝑚1 (𝑐𝑥1 − 𝑐1𝑛 𝑐2𝑛 𝑐𝑥2) . (9) an analysis of equation (9) enables a conclusion that the accuracy of the determination of k in the multicomponent substance (ms) is determined by the error of the preparation of the control models with the required k, the error in determining the geometric dimensions of the capacitive sensor, and the errors in the frequency measurement as well as the digital gauge of the reactive component of the impedance. for modern measuring instruments, the error may not exceed values from several tenths to 1 %, depending on the value of k and the frequency at which the research is carried out. in order to improve the accuracy of the results of the analysis of the composition of liquids, both a method for planning a measuring experiment and an evaluation of the characteristics of the errors (uncertainty) of constructing linear grading characteristics are proposed. an electronic measuring unit can be made ‘intelligent’ by introducing a microprocessor into its structure. by the instrumentality of the corresponding microprogram support, the function of the electrochemical sensor can be expanded considerably in this case. this may enhance and enable the following features: the processing of data during the calibration (graduation) of ion-selective electrodes; the construction of calibration charts; and the memorization of the acquired calibration information. on the other hand, other features may also improve: automatic calculation according to the measurement of the difference in the potentials of ion concentrations taking into account both the actual temperature of the investigated solution as well as its dilution; the corrective supplements used and other side effects; fixing the date and time of measurement; the accumulation and formatting of data; the issuance of the acquired data in any of the user-specified units, not only on their digital display, but also to an external computer or to the communication network [16]–[18]. 5. conclusions the results of studies using rlc-meters allow us to determine the dependence of electrical parameters of liquids in wide ranges of frequencies of the electromagnetic field on the chemical nature and concentrations of their components. the use of the established electrical parameters corresponding to the normalized concentrations of the components improves the informativity of conductometric studies of multicomponent liquids. this allows for the expansion of the list of controlled acta imeko | www.imeko.org march 2020 | volume 9 | number 1 | 60 substances of different electrical properties, increasing the selectivity, precision, and efficiency of analysis. the introduction of an electrical method for the rapid determination of the characteristics of liquids will enhance the following issues: determining the required level of cleaning of technical liquids; predicting the quality of the return water of the enterprise in the future; and monitoring the normalized parameters of treated waste water for compliance with their normative documents related to production and specific categories of water use. descriptions of the experimental studies of the electrical acoustic properties of model liquids in an electromagnetic field of variable frequencies provide improvements such as: 1. determining the concentration of a substance in a multicomponent fluid according to the measured values of admittance; 2. controlling the maximum permissible concentration of the controlled substance in the multicomponent mixture according to the value determined by the measurement of the reactive component of the conductivity as well as to the value of the experimentally established frequency of the electromagnetic field. 3. expanding the list of substances, the identification or concentration of which is controlled by the electric method. 4. implementation of a new approach for the method of dielectric and conductometric measurement for the operational control of the composition of technical liquids, food products, medical products, and sewage characteristics. on the basis of theoretical and practical research, the method of operational control of the composition of cooling liquids has been proposed based on the dependence of the value of the active and reactive components of the conductivity on the signal frequency. this method allows both the quantitative and qualitative evaluations of the composition of the liquid on the contents of the controlled components as well as the maintenance of the uninterrupted operation of the equipment in a small period of time (up to two seconds). references [1] d. de zanet, m. battiston, e. lombardi, a. da ponte, r. specogna, f. trevisan, a. affanni, m. mazzucato, fast blood impedance measurements as quality indicators in the pre-analytical phase to prevent laboratory errors, acta imeko, 7(4) (2018) pp. 26-32. doi: http://dx.doi.org/10.21014/acta_imeko.v7i4.574 [2] l. gonella, problems in theory of measurement today, acta imeko, 3(1) (2014) pp. 19-22. doi: http://dx.doi.org/10.21014/acta_imeko.v3i1.192 [3] l. dinia, f. mangini, m. marco, f. frezza, fbg multifunctional ph sensor – monitoring the ph rain in cultural heritage, acta imeko, 7(3) (2018) pp. 24-30. doi: http://dx.doi.org/10.21014/acta_imeko.v7i3.560 [4] i. i. grigorchak, g. v. ponedylok, impedance spectroscopy, lviv polytechnic, lviv, 2011, p. 352. [5] e. s. hixson, mechanical impedance, in: shock and vibration. c.m.harris (editor). mcgraw hill, new york, 1988, chapter 10. [6] e. barsoukov, j. r. macdonald, impedance spectroscopy – theory, experiment and application, wiley interscience, canada, 2005, p. 585. [7] applications of kramers—kronig transforms in the analysis of electrochemical impedance data—iii. stability and linearity, mirna urquidi-macdonald, silvia real, digby d. macdonald., electrochimica acta.1990, vol. 35, p. 1559. [8] p. stolyarchuk, m. mikhalieva, t. bubela, o. basalkevych, measurement of spirit solution immitance, in proc. of the 6th ieee international conference on intelligent data acquisition and advanced computing systems: technology and applications, idaacs 2011. [9] impedance spectroscopy. theory, experiment and application/ed. e. barsoukov and j. r. macdonald, wiley interscience (canada), 2005, p. 585. [10] i. m. kobasa, l. i. odosiy, i. v. kurdyukova, a. a. ishchenko, s. s. kurek, electrochemical and energetic characteristics of new dye-sensitizers for photovoltaic cells, j. funct. mater. lett. vol. 8, no. 6. (2015)1550067 (5 pages). [11] m. mikhalieva, m. mykyichuk, t. bubela, t. boyko, o. malyk, qualitative and quantitative research on pesticide chemical admixture in liquids, proc. of the 7th international ieee conference on intelligent data acquisition and advanced computing systems, 2013, idaacs 2013. [12] p. stolyarchuk, v. yatsuk, y. pokhodylo, m. mikhalieva, t. boyko, o. basalkevych, electric express-method for liquid quality level monitoring, proc. of the 5th international ieee workshop on intelligent data acquisition and advanced computing systems: technology and applications, 2009, idaacs 2009. [13] b. o. seredyuk, r. w. mccullough, h. b. gilbody, collision mechanisms in one-electron capture by he2+ ions in collisions with hydrocarbons, phys. rev. a. 71 (2005) pp. 022713-1–0227135. [14] b. seredyuk, r. w. mccullough, h. b. gilbody, mechanisms of one-electron capture by slow he2+, c4+ and o6+ ions in collisions with ch4. brazilian journal of physics 36(2b) (2006) pp. 509-510. [15] f. o. ivashchyshyn, i. i. grygorchak, o. v. balaban, b. o. seredyuk, the impact of phase state of guest histidine on properties and practical applications of nanohybrids on inse and gase basis, materials science-poland, 35(1) (2017) pp. 239-245. [16] p. stoliarchuk, v. yatsuk, m. mikhalieva, g. barylo, intelligent data acquisition system error correction in working external conditions, proc. of the third workshop – 2005 ieee intelligent data acquisition and advanced computing systems: technology and applications, 2005, idaacs 2005. [17] p. stolyarchuk, v. yatsuk, m. mikhalieva, v. druziuk, intelligent system of temperature field ecological monitoring, proc. of the 4th ieee workshop on intelligent data acquisition and advanced computing systems: technology and applications, 2007, idaacs 2007. [18] mikhalieva m., shabatura y., lunkova h., hots n., przystupa k., atamaniuk v., electrical method for the cyberphysical control system of non-electrical objects przeglad electrotechniczny. 2019, no 12, pp. 200 – 203. http://dx.doi.org/10.21014/acta_imeko.v7i4.574 http://dx.doi.org/10.21014/acta_imeko.v3i1.192 http://dx.doi.org/10.21014/acta_imeko.v7i3.560 acta imeko, title acta imeko issn: 2221-870x december 2018, volume 7, number 4, 15-20 acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 15 measurement technologies for permanent magnets stefan möwius1, nicolas kropff1, mircea velicescu1 1 bec gesellschaft für produktmanagement mbh, zechenstraße 55, 47443 moers, germany section: research paper keywords: permanent magnets, hysteresis graph, helmholtz coil citation: stefan möwius, nicolas kropff, mircea velicescu, measurement technologies for permanent magnets, acta imeko, vol. 7, no. 4, article 4, december 2018, identifier: imeko-acta-07 (2018)-04-04 editor: alexandru salceanu, "gheorghe asachi" technical university of iasi, romania received march 26, 2018; in final form september 13, 2018; published december 2018 copyright: © 2018 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: nicolas kropff, e-mail: nkropff@bec-gmbh.de 1. introduction permanent magnets are electrotechnical devices that can induce a magnetic flux in an air gap without the need for any other energy except that which is used for their magnetization. for the manufacture of permanent magnets, a magnetic material must combine optimal intrinsic hard magnetic properties with a suitable metallurgical microstructure. useful magnetic materials are expected to have: • a curie temperature, tc, that is high enough to enable the operation of permanent magnets at temperatures of up to 200 °c, without appreciable irreversible polarization losses, • a high saturation polarization, js, in the temperature range from -40 °c to 200 °c to obtain high remanent polarization, br, and to store the maximum magnetic energy density, (bh)max, and • a strong uniaxial magneto-crystalline anisotropy, ha, to achieve high coercive field strength hcj. due to the remanent polarization, the magnet induces a magnetic flux φ, which is the sum of the magnetic force lines through a defined cross-section in a heterogeneous magnetic field [2], 𝛷 = ∫ 𝐵 𝐴 0 , (1) where 𝐵 = magnetic flux density or the magnetic induction and 𝐴 = cross-section. in the case of a homogenous magnetic field 𝛷 = 𝐵 × 𝐴, (2) the characteristic lines of a permanent magnetic material are the hysteresis loops of the polarization j(h), the intrinsic magnetic property of the material, and the induced flux density b(h), plotted versus the magnetic field strength h. the b(h) and j(h) loops are correlated by the relationship 𝐵 = 𝐽 + µ0𝐻, (3) where µ0 = 1.256 ∙ 10-6 vs/am., the universal electromagnetic constant. users of permanent magnets are mainly interested in the second quadrant of the hysteresis curve, i.e., the demagnetization curves of the used permanent magnetic material as well as the load line of the permanent magnet, which depends on the specific magnet geometry. the demagnetization curve j(h) shows how the external magnetic field changes the remanent polarization of the magnet, while the b(h) demagnetization curve shows the magnet induction in a real air gap and is used especially for designing the magnet systems. in terms of application, the users have a plethora of permanent magnetic materials at their disposal. the most commonly used hard magnetic materials that are available in mass production are alnico; hard ferrite; and the permanent magnet materials based on rare earth alloys, namely smco5, sm2co17, and ndfeb. the characteristic magnetic properties of the permanent magnetic materials are provided by manufacturers permanent magnets have a broad application in many important fields in modern technology. they have become indispensable in the automotive, aerospace, acoustic, telecommunications, energy generation, and many more industries. physically, a permanent magnet is a metastable system. fluctuations in composition and processing parameters can cause fluctuations in magnetic properties. to obtain the optimal performance in their application, users require careful control of non-machined parts and of the finished machined magnets. in most cases, the measurement of the final control of the assembled magnetic systems must be performed. the aim of this paper is to review the most common measurement methods used for the magnetic properties of permanent magnets, to comment on their advantages and limitations, and to discuss the level of accuracy that they can achieve. mailto:nkropff@bec-gmbh.de acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 16 in their corresponding data sheets. the principal magnetic properties; the remanence br; the coercive forces of the polarization hcj and of the induction hcb; and the maximum energy density (bh)max are specified according to the typical experimental demagnetization curve j(h) and to the resulting curve b(h) according to equation (2). the properties of permanent magnets are generally very sensitive to their exact composition and the manufacturing parameters. each type of permanent magnet that is mass manufactured fluctuates, leading in turn to fluctuations in their microstructure, and, further, to fluctuations in the magnet properties. the increasing use of the permanent magnets in essential applications involves implementing safety actions to assure their optimal performance. users of permanent magnet technology are demanding quality control of the magnet properties from product development to serial production. in this paper, the most typical magnetic measurement methods and adequate equipment for quality control and monitoring of permanent magnets from raw sintered parts to finished magnets are reviewed. their advantages, disadvantages, and achieved accuracies are also discussed. 2. measurement methods for permanent magnets as mentioned above, the role of a permanent magnet is to induce a magnetic flux. permanent magnet behavior, under the influence of temperature, opposite magnetic fields, and environmental conditions, is decisive in its performance in all applications. users need a high magnetic flux that is created by a high remanence and high stability in opposite magnetic fields (especially at high) temperatures. the latter depends on the coercive force of the material and on its temperature coefficients and corrosion stability against the environment operating conditions, assured by the permanent magnet compositions and by surface protections. for the characterization of the permanent magnets roughly two ways can be used: in a closed or in an open magnetic circuit. in a closed magnetic circuit, the magnet is in closed contact with magnetic conducting parts, for example between the poles of an electromagnet, and so the magnet flux is closed, without any airgaps between magnet and the ferrous parts. in an open circuit, the magnet is not in contact with ferrous parts, the measurement signals depend on the induced magnetic flux, i.e. is dependent on the magnet shape and thus on its load line. these signals must be converted by simulation models in signals corresponding to the closed circuit [1]. different international and national standards are recommending measurement methods of the magnetic properties of permanent magnetic materials, for example [3-5]. they recommend the measurement in a closed circuit, commonly performed in a hysteresis graph. the highest accuracy is given only for magnets having a straight magnet axis and being produced with a constant cross section along the axis of magnetization. furthermore, the magnet volume of the used samples must be not less than 1 cm³ and the smallest magnet dimension shall be 5 mm [3,4]. the performance of a magnetic application depends on the used permanent magnet material, as well as on their dimensions, air-gaps and on the soft magnetic components of the circuit. the number of new applications is continuously rising, very often leading to special and critical geometries and to requirements of very narrow magnetic tolerances. the aim of a measurement method is to determine the magnetic properties of magnets or of a magnetic system. depending on the application, the magnet characterization means not imperatively to determine its principal magnetic parameters resulting from the characteristic lines j(h) and / or b(h). to ensure the high quality demanded by users, additional methods to control special magnetic properties are used. in table 1, an overview of the most typical measurement methods used for the characterization of the permanent magnets inspired from [6] and adjusted for this paper is presented. in this table the methods used for free magnets and those for magnets which have been assembled are separately mentioned. 3. choice of measuring method in choosing a measurement method, several factors must be considered. in terms of quality control in mass production, economic factors (e.g., the throughput of the pieces that must be characterized, the measurement time required per magnet, the relevant costs) and the required level of accuracy must be considered. for serial characterization, non-destructive measurement methods are more economical and are thus preferred. depending on these requirements, the most suitable commercially available equipment must be used. for development and testing in laboratory conditions, equipment with improved accuracy increases the magnitude order of accuracy. 4. a short description of measurement methods besides the classical hysteresis graph, demagnetization curves can be traced by newly developed pulse-filed magnetometers for series measurement, while the foner magnetometer is usually the preferred equipment for material research and development activities. for measurement of magnetic field strength and of magnetic induction, a gaussmeter + hall probe has been the universally preferred equipment for decades. the use of helmholtz coils for measurement of remanence in open circuits and of the magnetic moment makes this equipment quite indispensable in laboratories and for the incoming controls of magnets. furthermore, the homogeneity and 3d magnetic profiles can be controlled by 3d magnetic field mapping systems [7]. 4.1. hysteresis graph the working principles of a hysteresis graph according to the iec’s recommendation is shown in figure 1 [3]. the measurement of the samples occurs in a closed circuit between two polar pieces with an adjustable air gap. the intensity of the magnet field is measured by a hall sensor or by a coil, while a surrounding coil detects the flux density b(h). modern equipment uses only one compensated coil for the measurements of both parameters j(h) and b(h). it is additionally equipped with heating plates for tracing the demagnetization curves at high temperatures up to 200 °c. 4.2. pulse field magnetometer the first experiments on pulse fields for the measurement of high coercive forces were performed in 1988 by grössinger [8]. the intensive research and development efforts of a european group [9] have shown that this method can be applied to the industrial characterization of permanent magnets. a diagram of the working principles of the pulse field measurement method following [10] is presented in figure 2. a sinusoidal magnetic acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 17 damped field is generated by discharging a bank of capacitors trough the solenoid. the magnetic polarization of the sample is measured by pickup coils, while a hall probe or another field sensor measures the field strength, allowing for the tracing of the j(h) curve. this method has several important advantages, namely the high magnetizing field strength, which ranges between 7 and 15 t, and the facility to measure, in a non-destructive way, samples with arbitrary geometric forms, e.g., arc segments, a bread shape, and diametrical-oriented cylinders. even though this method has not been recognized by an international norm until now, this equipment is produced serially today and is already used on a large scale for research and development as well as serial quality control. the measurement can be performed at temperatures of up to 200 °c. specials facilities have been developed to perform measurements in the low temperature range of 20-40 °c [11]. table 1: measurement methods used for the characterization of permanent magnets measurement method parameters units equipment accuracy [%] measurement of free magnets demagnetization curves j(h), b(h) t, ka/m hysteresis graph (+/-) 1 pulse field magnetometer (+/-) 1 foner magnetometer (vsm) (+/-) 0.5 remanence in open circuit joc t helmholtz coil (+/-) 1 magnetic moment mm vs cm helmholtz coil (+/-) 1 angle deviations j degree helmholtz coil (+/-) 0.5° 3d mapper (+/-) 0.5° h(b) in a test point b t gauss meter / hall probe (+/-) 1 3d mapper (+/-) 1 surrounding coil φ vs cm coil and fluxmeter (+/-) 0.5 holding force f n dynamometer 2 – 5 measurement of assembled systems, e.g. rotors h(b) in air gap b t gaussmeter / hall probe 1 – 2 emf ueff v motor coil / voltmeter 1 figure 1: diagram of the working principles of a hysteresis graph. figure 2. diagram of the working principles of a pulse field magnetometer. figure 3. diagram of the working principles of a foner magnetometer. acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 18 in terms of electrical conducting metallic magnets like smco or ndfeb, this method induces eddy currents. this phenomenon influences the value of the remanent polarization. to correct the measured data, modeling calculations have been developed. 4.3. foner or vibrating sample magnetometer (vsm) the vibrating sample magnetometer (vsm) was developed in the 1950s by foner [12]; therefore, it is also known as the foner magnetometer. figure 3 shows the working principles of this method. initially, a loudspeaker membrane [13] was used for the vibration of a sample holder and thus of the sample. an electromagnet creates the variable external magnetic field. its strength is measured by a hall sensor. other sources of a variable magnetic field have been developed, e.g., superconducting solenoids [14], bitter coils [15], and even permanent magnet assembled systems [16,17]. with a vertical vibration of 80 hz [12], the sample induces a signal that is proportionate to its magnetic polarization in the pickup coil. the samples are measured in open circuit and the signal requires corrections caused by the shearing of the demagnetization factor of the polarization. for a long time, the foner magnetometer was the most common measurement method for soft and hard magnetic materials in the temperature range of 2-1000 k. the sample size is limited to any mm3. the overall measurement accuracy depends on the measurement accuracy of the sample volume. nowadays, the vsm method is mostly used in research centers. 4.4. helmholtz coil (hhc) the helmholtz coil (hhc) was invented at the end of the 19th century to generate small, highly uniform magnetic fields. it consists of two identical coils connected in a series, placed symmetrically along an axis, and separated by a distance that is equal to their radii. its initial idea has been reversed. for magnetic measurements, the coils are used as flux-sensing coils to accurately measure magnetization in open circuit [17]. figure 4 shows a typical configuration of the hhc. by connecting it with an electronic fluxmeter, the hhc allows for the measurement of the remanence in open circuit, of the magnetic moment, and of the deviation angle between the magnetic and geometric axes with a high level of accuracy. such measurement can be performed for serial production. to collect data, different calculations and integrations must be conducted. the most important parameters are the coil constant, the demagnetization factor, and the volume of the analyzed magnet. based on the easily comprehensible measurement process, the magnet is inserted between both coils and the measured field is reset to zero. thereafter, the magnet is rotated 180 °. by turning the magnet inside the coils, the magnetic field is turned 180 °, too, and as a result, the hhc can measure the signal entirely [18]. the hhc has become indispensable equipment for incoming control of permanent magnets and for laboratory tests. 4.5. hall probe the hall probe, figure 5, is used to measure magnetic field strength or flux density. it is also deployed as a sensor device in various applications. the inherent hall generator consists of a semiconducting material and is passed by a control current. if a magnetic field is applied perpendicularly to the measurement surface of the hall probe, the lorentz force acts on the moving charge carriers passing the generator. the lorentz force is expressed as: 𝐹𝐿 = 𝑞 (𝑣 × 𝐵), (4) where q is the particle charge, v is the particle velocity, and b is the magnetic flux density. the resulting force pushes the charge carriers sideways, thus enabling the measurement of the corresponding hall voltage uh, expressed as: 𝑈𝐻 = 𝑅𝐻 × 𝐼×𝐵 𝑑 , (5) where rh is the hall constant, i is the passing control current, b is the magnetic flux density, and d is the thickness of the hall generator [19, 20]. while carrying out a measurement using a hall probes, the user must consider the sensitivity of the measuring device. the volume of the hall generator itself is crucial, as indicated in equation (5). flux density b is a temperature-dependent parameter [21]. 4.6. gaussmeter a gaussmeter combined with a hall probe is a universally preferred equipment type that is used for measuring magnetic flux density b or magnetic field strength h statically, in a desired location or space. both parameters can be determined in air gaps, too, e.g., in engines or loudspeakers. a gaussmeter is a suitable device for the quality control of serial produced magnets as it carries out measurements of magnetic field strength at a definite point and at a definite distance from the magnet surface. the “quality” of the tested magnet is evaluated comparatively with the flux density of a calibrated reference magnet. the resolution and the accuracy of a hall gaussmeter depend on the size and the properties of the hall sensor [22]. newly developed hall gaussmeters that use miniaturized horizontal figure 4. a typical configuration of the helmholtz coil according to [18]. figure 5. schematic configuration of a hall probe according to [19]. acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 19 and vertical hall devices, the so-called integrated 3-axis hall probes, achieve extremely high resolution and accuracy [23]. 4.7. 3d mapper in many permanent magnet applications, the magnetic field distribution over an axis, area, or volume is essential in the magnetic system. this parameter is concerned with rotation and linear motors, especially magnets assembled in positioning sensor systems. control of the magnetic field near and around a magnet can be performed using a magnetic field mapper. a block diagram of a commercially available 3d mapper is presented in [24]. the main parts of such a mapper are the moving platform; the hall probe; the electronic box (with digital processing); the motor drives and power supply; and a pc and software for collection of the measured data and the visualization and analysis thereof, see figure 6. one of the most important benefits of such 3d mappers is the measurement of complete systems and the visualization of all magnetic components. thereby, it is possible to evaluate, e.g., the field distribution in a magnetic system or to analyze the magnetic pole symmetry in a multiple magnet. all these measurements are performed with a 3-axis hall probe. besides the magnetic measurements, such measurement equipment can be used with other sensors, e.g., for detecting cracks within the magnet body or measuring the dimensions of the analyzed samples. the sensors can be changed by using a plug-and-play system, and the measured data can be combined in one system for detailed analysis. the so-called defectoscope can identify cracks and inhomogeneities within magnetized and unmagnetized magnets by using an eddy current measurement probe. the probe measures the eddy current distribution and creates a visual scan of the surface to show the defects [25,26]. 5. measurement results all of the measurement technologies explained above have one thing in common: they are used to examine the magnetic properties of a magnet or a magnetic system in an open or closed circuit. the compliance of the special magnetic properties with quality control requirements is important for magnet suppliers and essential for customer. the choice of measurement technology made is according to these parameters. generally, the measured results can be divided into two groups. the first results group is for mass production, where the parts are only differentiated between “good” and “not good” items and are sorted accordingly. therefore, there is no further evaluation of the collected data. such measurement methods are highly cost-sensitive and require a high output of analyzed permanent magnets to be economically viable. the second results group is for research and development. the collected data is analyzed in a highly detailed way, as it is required for further improvement of and modifications to the materials or application. in addition to the influence of different measurement technologies, the results obtained by magnetic measurements can be influenced by the interpretation and consideration of the sample’s intrinsic parameters as well. figure 7 shows the measured magnetic remanence br over an increasing temperature t, obtained by applying the metis hympulse measurement. two measurements of the same magnet sample indicate significantly varying results, such variance depending on the underlying sample volume. the curve for the sample measurement with the calculated sample volume (based on the technical drawing) shows a reduced magnetic remanence compared to the measurement curve with the precisely measured sample volume (volume measured by using the archimedes method). as the investigated sample was produced at the upper limit of the drawing-specified dimensions, the measured sample volume turned out larger than the calculated sample volume. based on the dependency of the magnetic remanence br on volume, different measurement results were obtained. 6. conclusion we reviewed the most commonly used methods for magnetically characterizing permanent magnets in laboratories, e.g., in their development state, and for quality control in the serial production thereof. different measurement methods and suitable measurement equipment are commercially available for most required characterizations related to the magnetic properties and magnetic behavior of permanent magnets. by choosing the most adequate measuring method, unnecessary controlling costs can be avoided. the combined use of multiple measurement methods can contribute to the improvement of knowledge concerning the behavior of permanent magnets and thus to the achievement of good quality assurance. the bec laboratory is equipped according to this regard. figure 6. block diagram of a commercially available magnetic mapper, reprinted with permission of senis ag. figure 7. metis hympulse measurement of br over increasing temperature t with the measured and calculated sample volume, performed in 2017 at the bec laboratory. 1,15 1,20 1,25 1,30 1,35 20 40 60 80 100 120 r e m an e n c e b r [t ] temperature [°c] br with measured sample volume br with calculated sample volume acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 20 7. acknowledgment the authors wish to thank dr. dragana popovic renella for a helpful discussion during preparation of this paper. 8. references [1] b. grieb, hartmagnetische werkstoffe. magnet werkstoffe für technische anwendungen. essen, haus der technik, 2016. [2] e. steingroever, magnetic measuring techniques, [3] iec international standard 60404 part 5: permanent magnet (magnetically hard) materials – methods of measurement of magnetic properties, 2015. [4] din en 60404 – 5vde 0354-5: 2016-02 “magneti sche werkstoffe”. [5] mmpa standard 0100-00: “standard specifications for permanent magnet materials”. [6] k. kuntze, chapter 6, in tae band 672, “dauer magnete”, expert verlag, 2015 [7] s. r. trout, ieee trans. mag. 24(4) (1988), p. 2108. [8] r. grössinger, c. gigler, a. keresztes, ieee trans. mag. 24(2) (1988), p. 970 [9] r. cornelius, j. dudding, r. grössinger, b. enzberg mahlke, w. fernengel, m. p. knell, m. küpferling, m. taraba, j. c. toussaint, a. wimmer, and d. edwards, ieee tans. mahn. 38(5) (2002), p. 2462. [10] f. fiorillo, c. beatrice, o. bottauscio, e. patroi, ieee trans. mag. 43(7) (2007), p. 3159. [11] l. van bockstal, technical presentation hypulse technology, 2017. [12] s. foner, rev. sci. instrum. 30 (1959) p. 548. [13] d. feldmann, r. p. hunt, z.instr. 72(11) (1964), p. 313. [14] s. foner, j. appl. phys. 79 (1996) p. 4740. [15] d. k. das, w. j. harrold, ieee trans. mag. 17 (1981) p. 281. [16] o. cugat, p. hansen, j. m. d. coey, ieee trans. mag. 30 (1994) p. 4602. [17] j. m. d. coey, j. magn. magn. mat. 248 (2002) p. 441. [18] brockhaus messtechnik data sheet “measurements using the helmholtz coil” [19] s. johnstone, the analyst 133 (2008) pp. 293-296. [20] w. zhenxing, s. mehrdad, s., m. otto, d. schall, d. neumaier, nanoscale 8 (2016) pp. 7683-7687. [21] k. kuntze, chapter 3, in tae band 672, “dauer magnete”, expert verlag, 2015. [22] r. s. popovic (editor), hall effect devices, 200, crc press, taylor & francis group. [23] d. popovic renella, s. dimitrijevic, s. spasic, r. s. popovic, 20th imeko int. symp. 2014, italy. [24] s. spasic, magnetics, business & technology, spring (2015) p. 16. [25] y. g. guo;, j. g. zhu, z. w. lin, j. j. zhong, measurement and modeling of core losses of soft magnetic composites under 3-d magnetic excitations in rotating motors, ieee trans. mag. 41(1) (2005). [26] w. xin, k. ding. magnetic measurement method on structure fatigue damage based on the material magnetic characteristics, chinese journal of scientific instrument 38(6) (2017) pp. 1474-1481. http://ieeexplore.ieee.org/search/searchresult.jsp?searchwithin=%22authors%22:.qt.%20youguang%20guo.qt.&newsearch=true http://ieeexplore.ieee.org/search/searchresult.jsp?searchwithin=%22authors%22:.qt.%20jian%20guo%20zhu.qt.&newsearch=true http://ieeexplore.ieee.org/search/searchresult.jsp?searchwithin=%22authors%22:.qt.%20zhi%20wei%20lin.qt.&newsearch=true http://ieeexplore.ieee.org/xpl/recentissue.jsp?punumber=20 https://www.researchgate.net/scientific-contributions/2133130714_w_xin?_sg=wttkxutt7p0iq77qt3sheyiz2nbol_5p1zvjr76uygzgarroqcbihwhzhdxdvqzhpy2t0-4.cb5ljsz2q4oasjvrihkyfwgey_bcbd1b07sk155qnzmxjwhtzsmtslxr7rappqlfokokme7jtg77tdd2f_twvw an ecg signal model based on a parametric description of the characteristic waves acta imeko issn: 2221-870x june 2020, volume 9, number 2, 3 9 acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 3 an ecg signal model based on a parametric description of the characteristic waves pavol dolinský1, imrich andráš1, linus michaeli1, ján šaliga1 1 department of electronics and multimedia telecommunications, faculty of electrical engineering and informatics, technical university of košice, letná 9, 042 00 košice, slovakia section: research paper keywords: synthetic ecg; ecg signal processing; ecg model; pqrst complex citation: pavol dolinský, imrich andráš, linus michaeli, ján šaliga, an ecg signal model based on a parametric description of the characteristic waves, acta imeko, vol. 9, no. 2, article 2, june 2020, identifier: imeko-acta-09 (2020)-02-02 editor: alexandru salceanu, technical university of iasi, romania received december 9, 2019; in final form april 9, 2020; published june 2020 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: ján šaliga, e-mail: jan.saliga@tuke.sk 1. introduction mathematical modelling allows for a description – by mathematical tools – of the properties of complex mechanical, electric, and any systems in which the final behaviours result from the physical relationships of various origins acting in a complex manner. these include behaviour analysis and the prediction of electronic [1], mechanical [2]-[4], biological [5] system components, thermal systems [6][7], data transmission channels [8], and more. a mathematical model gives concise information about basic processes, without the effects of low significance. such modelling is beneficial in that its accuracy can be controlled according to user needs. electrical modelling is especially suitable for biomedical systems, where the noise and several inherent artefacts influence the measured signal from the output of the observed biological object. the best examples of applying mathematical modelling of biological signals is the evaluation and extraction of electrocardiogram (ecg) signals [9]-[11] and compressed sensing [12]-[14][16]. the human heart generates spatio-temporal electric potential during contractions in the cardiac pump cycle. the measured potential propagates through human tissues on the skin surface. the difference of electric potential between electrodes placed on the selected positions of the body is acquired and converted to a standardized ecg. because ecg is a result of the propagation through the human body, it contains artefacts caused by muscle and respiration movement; powerline and other external noise sources; and spurious distortion from non-perfect electrode contacts [15]. databases of real recordings, such as phisionet [17], are often used for model evaluation, but it may be difficult to assess how a particular method reacts in the case of the same basic signal, with different levels of noise and artefacts. an alternative approach is to generate synthetic biomedical signals by using the suitable signal model and comparing its output signal with the measured one. in the past, a wide variety of different ecg signal models have been developed and tested. one of the most basic ecg models was published in [18]. in [19], the rational bézier curves abstract this article introduces a new electrocardiogram (ecg) signal model based on geometric signal properties. instead of the artificial functions used in common ecg models, the proposed model is based on the modelling of real ecg signals divided into time segments. each segment has been modelled using simple geometrical forms. the final ecg signal model is represented by the sequence of parameters of the base functions. parameter variations allow for the generation of different waveforms for each subsequent heartbeat without mixing up the pqrst waves order. two basic models utilize slightly modified elementary functions, which are computationally simple. a combination of both models allows for the modelling of irregularities in the consecutive heartbeats of the specific ecg waveforms. respiratory, noise, and powerline interference can be added in order to make the generated ecg signal more realistic. the model parameters are estimated by differential evolution optimization and a comparison between the modelled ecg and the acquired signal. the proposed models are tested by the database included in the labview biomedical toolkit and ecg records in the mit-bih arrhythmia database. mailto:jan.saliga@tuke.sk acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 4 and bernstein polynomials were used to model heartbeat curves. the gaussian curve is also a very common method of modelling ecg signal. an evaluation and comparison concerning mathematical models for the extraction of qrs features was done in [21]. gaussian, mexican hat, and rayleigh functions were used for the whole heartbeat period of the modelling. adaptive weighted hermit functions were used in [22]. an original, alternative method was introduced in [23], where a qrs wave group was modelled using the ant colony algorithm. the researchers in [24] simulated real-time ecg waveforms by means of the superposition of bounded functions. this property is a defining feature of most periodic functions. an alternative method of the simulation of ecg signals the spectrum-based one [25]. among the well-known, most widely used ecg signal models, the dynamic model [26] is based on three coupled differential equations. the approach presented in this paper is significantly different to all of these. it is based on the elementary trigonometric and linear functions or the derivation of the gaussian pulse to model each individual wave. computational simplicity is the main advantage of these models, which allows for the implementation on embedded devices thereof for denoising, automated patient diagnostics, etc. 2. ecg signal the ecg signal is described by five consecutive waves, labelled with the letters p, q, r, s, and t, as is standard (figure 1). these waves correspond to the depolarization and repolarization of heart muscles [26] during one heartbeat. the initial p wave represents the activation of atria and is followed by a relatively short isoelectric segment, while the cardiac impulse passes through the a-v node and the hispurkinje system. the excitation and muscle contraction represent the main force behind blood circulation to the organs and creates the tallest qrs complex. it has three components. the first downward deflection is called the q wave, the consecutive upward deflection is called the r wave, and the final downward deflection is called the s wave. the polarity of each wave in the qrs complex may vary according to the position of sensing leads as well as body abnormalities. a normal ecg beat ends with a low-frequency t wave, during which the ventricles return to their electrical resting state. 3. the proposed ecg signal model the authors initially proposed two signal models, in which each wave of a pqrst complex is modelled by elementary concatenated mathematical functions. the models differ in the modelling of the q and s waves. the first one uses the fixed shape of q and s waves with the adjustable time of origin, width, and amplitude. the second ecg signal model additionally allows for the adaptation even of their shapes. finally, the third custom ecg signal model is based on linking the cardiac cycles of the first two models, where the vectors of their parameters may differ in each pqrst complex. it allows for the generation of a model of a different waveform for each subsequent heartbeat or the superposition of different heart rate disturbances at predefined positions. these models can include the interference of gaussian and powerline noise as well as interference caused by respiration. 3.1. fixed-shape q and s waves the heartbeat starts with a short isoelectric segment. it is modelled by the simple constant: 𝐵(𝑘) = 0 for 0 ≤ 𝑘 ≤ 𝐾𝐵 () where kb is the width and k is the sample number within one wave segment. the consecutive p wave is modelled by the cosine function: 𝑃(𝑘) = −𝐴𝑃 cos ( 2𝜋𝑘+15 𝐾𝑃 ) for 0 ≤ 𝑘 ≤ 𝐾𝑃  () where ap is the amplitude and kp is the width of the p wave. a short isoelectric segment follows: 𝑃𝑄 (𝑘) = 0 for 0 < 𝑘 ≤ 𝐾𝑃𝑄  () where kpq is the width. the q wave is modelled using a segment created by a section of a gaussian monopulse (a differentiated gaussian pulse): 𝑄(𝑘) = 𝐴𝑄(k−0.1kq+0.1)19.78𝜋 𝐾𝑄 𝑒 −2( 6𝜋 𝐾𝑄 (k−0.1𝐾𝑄+0.1)) 2 () for 0 ≤ 𝑘 ≤ 𝐾𝑄 , where aq is the amplitude and kq is the width of q wave. the r wave is modelled using a sinewave segment: 𝑅(𝑘) = 𝐴𝑅 sin ( 𝜋𝑘 𝐾𝑅 ) for 0 ≤ 𝑘 ≤ 𝐾𝑅  () where ar is the amplitude and kr is the width of r wave. the s wave is modelled by a segment of a gaussian monopulse: 𝑆(𝑘) = −𝐴𝑆0.1𝑘 19.78𝜋 𝐾𝑆 𝑒 −2( 6𝜋 𝐾𝑆 0.1𝑘) 2 for 0 ≤ 𝑘 ≤ 𝐾𝑆 − 𝐾𝐶𝑆 () where parameter as is the amplitude, and ks is the basic width of the s wave. kcs is a parameter that allows for the slight adjustment of the s wave shape by cutting away a portion at the end. the transition between the s and t wave is described as a linear function: 𝑆𝑇 (𝑘) = −𝑆(𝐾𝑆 − 𝐾𝐶𝑆) 𝑘 𝑠𝑚 + 𝑆(𝐾𝑆 − 𝐾𝐶𝑆) () for st kk 0 , where sm is the slope parameter, and kst is the width of the transition segment. the t wave is modelled using a segment of the cosine function: 𝑇(𝑘) = −𝐴𝑇 cos ( 1.48𝜋k+15 𝐾𝑇 ) + 𝐴𝑇 + 𝑆𝑇 (𝐾𝑆𝑇 ) () figure 1. pqrst complex of a normal lead ii ecg recording. acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 5 for t kk 0 , where at is the amplitude and kt is the width of the t wave. the final transition from the t wave back to the isoelectric line is modelled using the function: 𝐼(𝑘) = 𝑇(𝐾𝑇 ) 𝑠𝐼 𝑘+10  for 0 ≤ 𝑘 ≤ 𝐾𝐼  () where si is the parameter for setting the transition slope between the t wave and isoelectric line, and ki is the width of the ending section. the concatenation of all modelled h(1b,n) (equations 1-8) describes the complete heartbeat. the beginning of each segment is appended to the end of a previous one, while their order is always the same starting with equation (1) and ending with equation (8). here, parameter 1b denotes the set of heartbeat parameters for the first variant: 𝑏 1 = {𝐾𝐵 , 𝐴𝑃 , 𝐾𝑃, 𝐾𝑃𝑄 , 𝐴𝑄 , 𝐾𝑄 , 𝐴𝑅 , 𝐾𝑅 , () 𝐴𝑆, 𝐾𝑆 , 𝐾𝐶𝑆 , 𝑠𝑚, 𝐾𝑆𝑇 , 𝐴𝑇 , 𝐾𝑇 , 𝑠𝐼 , 𝐾𝐼 } and n denotes the sample number within one generated heartbeat (n = 0, 1, 2, …, n1-1). each heartbeat segment has a maximum length defined by its width parameters (kx). thus, the sum of all segment lengths is n1=kb+kp+kpq+kq+kr+ks–kcs+kst+kt+ki. 3.2. the pqrst complex with adjustable q and s waves the modelling of the q and s waves by their linear functions is the main enhancement on the previous model. using linear functions instead of a gaussian monopulse allows for the adjustment of the shape more precisely. all other modelling functions in this model are the same as those in the previous model, except for equations (4) and (6). equation (4) is replaced by concatenation of two functions: 𝑄1(𝑘) = −𝐴𝑄 𝑘 𝐾𝑄1  for 0 ≤ 𝑘 ≤ 𝐾𝑄1 () 𝑄2(𝑘) = 𝐴𝑄 𝑘 𝐾𝑄2 − 𝐴𝑄  for 0 ≤ 𝑘 ≤ 𝐾𝑄2 () where kq1 is the width of the downward deflection, kq2 is the width of upward deflection, and aq is the amplitude of the q wave. equation (6) is replaced by a concatenation of two functions: 𝑆1(𝑘) = −𝐴𝑆 𝑘 𝐾𝑆1  for 0 ≤ 𝑘 ≤ 𝐾𝑆1 () 𝑆2(𝑘) = 𝐴𝑆 𝑘 𝑠𝑠 − 𝐴𝑆 for 0 ≤ 𝑘 ≤ 𝐾𝑆2 () where ks1 is the width of the downward deflection, and ks2 is the width of the upward deflection. slope parameter ss represents the upward deflection. parameter as is the amplitude of the s wave. the argument of s(ks-kcs) in equation (7) is replaced here by ks2. a complete heartbeat h(2b,n) is then adjusted using the modified set of parameters 2b for the second variant: 𝑏 2 = {𝐾𝐵 , 𝐴𝑃 , 𝐾𝑃, 𝐾𝑃𝑄 , 𝐴𝑄 , 𝐾𝑄1, 𝐾𝑄2, 𝐴𝑅 , 𝐾𝑅 , () 𝐴𝑆, 𝐾𝑆1, 𝑠𝑠, 𝐾𝑆2, 𝑠𝑚, 𝐾𝑆𝑇 , 𝐴𝑇 , 𝐾𝑇 , 𝑠𝐼 , 𝐾𝐼 } here, n denotes the sample number n = 0, 1, 2, …, n2-1, and n2=kb+kp+kpq+kq1+kq2+kr+ks1+ks2+kst+kt +ki is the sum of all segment width parameters. 3.3. creating a custom ecg the custom ecg signal model is created by the following method. the function of one heartbeat is generated using any of the described variants as h(b,n), where b is the set of parameters 1b or 2b and n = 0, 1, 2, …, n=n1-1 in the case of 1b or n=n2-1 in the case of 2b. a complete ecg signal can now be built by concatenating multiple heartbeats hf(bi,n). the set of parameters bi can vary in each consecutive heartbeat or even continuously throughout a heartbeat. the beginning and end segment of each heartbeat aligns smoothly with the isoelectric line, so there are no artefacts present at the position of their joint. the concatenation of the functions forming the sequence of the heartbeats may lead to unnatural sharp edges present in the resulting signal, so the generated heartbeat is filtered using a savitzky-golay smoothing filter with a window size of 7: 𝐻𝑓 (𝑏, 𝑛) = 1 21 {−2𝐻(𝑏, 𝑛 − 3) + 3𝐻(𝑏, 𝑛 − 2) + +6𝐻(𝑏, 𝑛 − 1) + 7𝐻(𝑏, 𝑛) + 6𝐻(𝑏, 𝑛 + 1) + () +3𝐻(𝑏, 𝑛 + 2) − 2𝐻(𝑏, 𝑛 + 3)} for n =0, 1, 2, …, n, where h(b,n±j)=0 if the sum n ± j is out of the specified interval for n. the filter plays a significant role while using the second model variant, because it forms the smooth shape of the q and s waves. this ecg model allows for the creation of a completely custom-defined ecg signal, which can include various irregular heartbeats present at custom positions. even mixing the heartbeats generated from both the proposed model variants is possible. for example, a short signal containing a premature ventricular contraction (pvc) is modelled in figure 2 using the first model variant. figure 2. a modelled ecg containing a pvc heartbeat. figure 3. a modelled example of hypokalaemia ecg with noise and respiration added. acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 6 3.4. modelling additive signals caused by respiration and external noise sources to make the modelled signal more realistic, different artefacts can be easily added to it. the respiration causes a baseline wander in the ecg signal. this means that the isoelectric line periodically changes its position, relatively slowly. a simple way of modelling this effect is to add a sinewave of the respiration frequency into the ecg signal, which may vary from about 0.2 hz to 0.5 hz (approximately 12 to 30 breaths per minute). the interfering parasitic signal from external sources is modelled by the white noise and the inducted periodic signal from the powerline. if necessary, powerline noise can also be added in the form of a small sinewave with a powerline frequency of 50 hz or 60 hz. an example of a synthetized ecg signal in relation to hypokalaemia, with the addition of noise and the effect of respiration, can be seen in figure 3. 4. an evaluation of the proposed ecg signal models at the beginning of this study, the proposed models were evaluated using a database in the labview biomedical toolkit. the database represented the reference signals created from large-scale experimental data by medical experts. to test the possibilities of qrs pattern generation using the proposed model, its parameters were adjusted to fit typical irregular heartbeats. as a reference, irregular heartbeats associated with various diagnoses were generated using the ecg signal generator of the labview biomedical toolkit [27]. each reference pattern hr(n) was extracted from a 60 bpm signal, generated with a sampling frequency of 512hz and containing one qrs complex with a length of 1 s. the set of model parameters b was found using the differential evolution (de) optimization algorithm [28] using the principle in [29]. the optimal model parameters for the best fit are found by means of the iterative minimization of each parameter b: 𝑏 = 𝑎𝑟𝑔 𝑚𝑖𝑛 𝑏 𝑃𝑅𝐷 () where prd is the normalized mean square difference between the reference heartbeat ecg signal and the modelled heartbeat ecg signal: 𝑃𝑅𝐷 = ‖𝐻𝑟(𝑛)−𝐻𝑓(𝑏,𝑛)‖2 ‖𝐻𝑟(𝑛)‖2 100% () in each iteration, the de algorithm takes the reference ecg signal, compares it to the ecg generated based on the current parameter set b, and updates the parameter set, as summarized in figure 4. this is repeated until a sufficiently low prd or maximum iteration count is reached. the initial bounds of the b parameters were set in the following way: -0.2 to 2 for the amplitude parameters, 0-150 for the segment length parameters, and 0-200 for the slope parameters. after the first few iterations of de, the b parameter bounds were adjusted according to the data shown in table 1. the length parameter of the last segment ki is calculated so that the total sum of length parameters n=n1-1 or n=n2-1 is always equal to 512; thus, it is excluded from the set of parameters that are being optimized. the signal output of a model with such a configuration does not need resampling. artificial qrs patterns were used as a test reference, corresponding to (a) atrial tachycardia, (b) ventricular tachycardia, (c) junctional tachycardia, (d) atrioventricular block, (e) hyperkalaemia, (f) hypokalaemia, (g) hypercalcemia, and (h) hypocalcaemia. the resulting ecg waveforms are shown in figure 5. the corresponding parameters found by the de optimization and the resultant prd for the first model are listed in table 2. for the second model, the identified parameters and prd are listed in table 3. the results were obtained using a simulation in the labview programming environment. the de optimization used the uniform crossover method, a population size of 500, and a maximum number of iterations of 200 for all the testing results provided. the prd values listed in all the tables are given as an average of ten runs of the de algorithm. figure 4. ecg patterns generated by the first model variant: a) atrial tachycardia; b) ventricular tachycardia; c) junctional tachycardia; d) atrioventricular block; e) hyperkalaemia; f) hypokalaemia; g) hypercalcaemia; h) hypocalcaemia. table 1. de parameter bounds for the first and second model. 1b min max 2b min max kb 0 130 kb 0 130 ap -0.2 0.1 ap -0.2 0.1 kp 10 100 kp 10 100 kpq 0 60 kpq 0 60 aq 0 0.5 aq 0 0.5 kq 10 150 kq1 0 70 ar 1 2 kq2 0 50 kr 10 150 ar 1 2 as 0 1 kr 10 150 ks 10 200 as 0 1 kcs -5 150 ks1 0 50 sm 1 150 ss 1 110 kst 0 110 ks2 0 50 at -0.5 1 sm 1 150 kt 50 200 kst 0 100 si 0 50 at -0.5 1 kt 50 200 si 0 150 acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 7 the second evaluation method is based on a comparison between the modelled signal and the real signal taken from a database of real patients’ signals. the evaluation of the real ecg patterns is similar to that which was described in the previous subsection. here, the reference pattern is made by means of extraction of a single heartbeat from the beginning of the first channel of each massachusetts institute of technology – beth israel hospital (mit-bih) database record [17]. the database records use a sampling frequency of 360 hz. the reference pattern hr(n) is obtained by the resampling for a duration of 1 s at a sampling frequency of 512hz. the de algorithm remains the same as listed in table 1 for the previous experiments. the modelling accuracy, evaluated by prd, was assessed for the real ecg patterns taken from the mit-bih database and is presented in table 4. as evidenced in table 4, each model is suitable for different types of patterns. the average prd is better for the second model variant, although it should be noted that the extracted heartbeats possibly contain noise and artefacts that were not included in the signal model during this test. noise causes a higher prd even if there is a good fit between the original and modelled signal [30]. table 3. the second variant model parameter sets for each pattern. 2b diagnosis a b c d e f g h kb 10 0 117 61 121 121 117 124 ap 0.03 0 0.0045 0.053 0.035 0.035 0.05 -0.02 kp 93 23 79 91 73 69 79 75 kpq 1 1 14 50 13 43 14 17 aq 0.13 0.33 0.065 0.065 0.047 0 0.06 0 kq1 55 55 12 12 12 0 11 0 kq2 31 37 7 7 7 0 7 0 ar 1.15 1.09 1.52 1.55 1.17 1 1.55 1.37 kr 77 137 22 22 22 12 23 32 as 0.38 0.27 0.18 0.16 0.12 0.48 0.1 0.19 ks1 32 44 9 7 7 12 5 15 ss 62 108 4 4 6 6 2 8 ks2 33 31 4 4 6 7 4 4 sm 53 97 138 64 14 1 29 55 kst 52 52 100 56 53 74 1 62 at 0.12 0 0.2 0.13 0.665 -0.1 0.12 0.23 kt 119 87 137 126 126 126 133 183 si 17 0 0 128 32 0 23 0 ki 9 45 11 76 72 48 118 0 prd in % 9,5 10,6 9,18 9,59 6,69 13,9 10,6 11,4 figure 5. an ecg fitting algorithm for finding the set model parameters. table 2. the first variant model parameter sets for each pattern. 1b diagnosis a b c d e f g h kb 10 0 117 61 121 121 117 124 ap 0.03 0 0.0045 0.053 0.035 0.035 0.05 -0.02 kp 93 23 79 91 73 69 79 75 kpq 0 0 0 48 6 13 5 0 aq 0.135 0.325 0.065 0.04 0.04 0.02 0.03 0 kq 85 140 25 21 21 22 20 15 ar 1.15 1.09 1.52 1.55 1.17 1 1.55 1.37 kr 84 133 23 23 23 15 22 36 as 0.35 0.28 0.16 0.13 0.11 0.75 0.6 0.16 ks 114 182 15 15 15 26 14 54 kcs 61 100 5 2 4 -3 5 27 sm 61 119 96 17 26 35 1 87 kst 52 57 101 52 56 64 6 42 at 0.13 0 0.19 0.132 0.685 -0.1 0.115 0.225 kt 127 77 126 116 112 112 116 184 si 0 0 2 9 9 7 10 19 ki 8 0 31 87 89 67 138 9 prd in % 7,8 7,61 11,12 11,3 14,0 9,89 13,4 7,65 acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 8 5. conclusions two basic ecg signal models have been presented in this paper based on a set of basic function parameters. the models allow for the description of irregularieties in the shapes of the characteristic waves in one heartbeat. irregularities in the consecutive heartbeats can be modelled by combining both models. in addition, the respiratory and external electromagnetic interferences can be included in the models. the model parameters were identified from real ecg records using the differential evolution optimization strategy. the normalized mean square difference prd was the parameter describing the modelling accuracy. besides the physically acquired ecg signals, the modelling accuracy was evaluated by the ecg signals generated by the labview biomedical toolkit and real ecg records from mit-bih. the ecg model evaluation using signals from databases showed that modelling the low-frequency q and s waves is better in the first model. the second ecg model, with linearly modelled segments q and s, is more suitable in the case of steeper waveforms. a lower number of model parameters is the advantage of the first model, also an advantage in the de optimization. the relatively complicated adjustment of the heart rate of long-term signals is another drawback of the second variant presented. resampling the acquired ecg signal to a suitable rate is the easiest way of suppressing this disadvantage for both model variants. acknowledgement the work is a part of the project supported by the science grant agency of the slovak republic (project no. 1/0722/18). references [1] m. crescentini, m. marchesi, a. romani, m. tartagni, 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parameters estimators returned by the interpolated discrete fourier transform algorithm daniel belega1, dario petri2, dominique dallet3 1 department of measurements and optical electronics, university politehnica of timișoara, bv. v.pârvan, nr. 2, 300223, timișoara, romania 2 department of industrial engineering, university of trento, via sommarive, 9, 38123, trento, italy 3 ims laboratory, bordeaux inp, university of bordeaux, cnrs umr5218, 351 cours de la libération, bâtiment a31, 33405, talence cedex, france section: research paper keywords: error analysis; interharmonic component; interpolated dft algorithm; parameter estimation; windowing citation: daniel belega, dario petri, dominique dallet, contribution of an interharmonic component to the sine-wave parameters estimators returned by the interpolated dft algorithm, acta imeko, vol. 9, no. 3, article 4, september 2020, identifier: imeko-acta-09 (2020)-03-04 editor: jan saliga, technical university of košice, slovakia received january 15, 2020; in final form april 1, 2020; published september 2020 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: daniel belega, e-mail: daniel.belega@upt.ro 1. introduction frequency-domain algorithms based on the discrete fourier transform (dft) return highly accurate sine-wave parameters estimates under coherent sampling conditions [1]. however, in practice, coherent sampling is often difficult to achieve, since the sine-wave frequency can be time-variant, and the waveform can be affected by spurious tones. time-domain approaches, such as those implemented by the prony, pisarenko, and music algorithms [2], [3], also ensure accurate sine-wave parameter estimates when non-coherent sampling occurs. unfortunately, the required processing effort is quite high, and the estimation accuracy depends on the order of the adopted waveform model, which is normally unknown a priori. therefore, these approaches do not provide optimal performance in real-time applications, for which frequency domain algorithms are often preferred due to their lower processing effort. unfortunately, when noncoherent sampling occurs, the sine-wave parameter estimates provided by dft-based algorithms are affected by both spectral leakage [4] and picket-fence effects [5]. the spectral leakage effect can be significantly reduced by windowing [4], and cosineclass windows are frequently used, since they can be easily implemented and ensure a good spectral leakage suppression. conversely, the picket-fence effect can be reduced by applying one of the so-called interpolated dft algorithms [5]-[21]. the classical ipdft algorithm [8] firstly estimates the sine-wave frequency by interpolating the two highest samples of the signal spectrum. then, the sine-wave amplitude and phase are determined considering the signal dft and the estimated frequency. when a small number of sine-wave cycles is acquired under non-coherent sampling conditions, the obtained parameter estimates are significantly affected by the spectral interference raising from the fundamental image component. the related contribution can be reduced by using more interpolation points [15]-[21], but at the expenses of an estimator variance increase [17]. ipdft estimators are expressed by very simple expressions when maximum sidelobe decay (msd) windows are employed [4], [22]. these are cosine windows that abstract this article investigates the contribution of a small-amplitude interharmonic component to the sine-wave parameter estimators returned by the classical interpolated discrete fourier transform (ipdft) algorithm. the analytical expressions for the frequency, amplitude, and phase estimation errors are derived herein by considering the ipdft algorithm based on the maximum sidelobe decay (msd) windows and by assuming the interharmonic frequency located at least one bin apart the unknown sine-wave frequency. the derived expressions allow us to analyse the impact of an interharmonic on the accuracies of the ipdft frequency, amplitude, and phase estimators. the accuracies of the derived expressions are verified by means of both computer simulations and experimental results. mailto:daniel.belega@upt.ro acta imeko | www.imeko.org september 2020 | volume 9 | number 3 | 18 exhibit the maximum spectrum sidelobe decay rate for a given number of terms, thereby effectively reducing the detrimental effect on the estimated parameters of long-range spectral leakage due to harmonics or spurious tones. however, the effectiveness of windowing on short-range leakage is very low, such that the influence of nearby interfering tones on the estimated parameters can be significant. when the classical ipdft algorithm based on an msd window is applied, expressions for the frequency error and the amplitude and phase errors due to short-range interference from both the image component and harmonics have been derived in [23] and [24], respectively. however, reallife sine-waves are also affected by interharmonics. an expression for the frequency error affecting the ipdft algorithm when the analysed sine-wave is corrupted by a smallamplitude interharmonic located at least one bit apart the unknown frequency has been derived in [25]. our aim in this article is to extend that result by deriving the expressions for the amplitude and phase estimation errors that affect the classical ipdft algorithm based on an msd window. the derived expressions generalise the results related to the sine-wave affected by one harmonic. they allow us to easily analyse the contribution of an interharmonic on the amplitude and phase estimates returned by the ipdft algorithm. to the best of the authors’ knowledge, such an analysis has not yet been published. the aforementioned expressions are outlined in section 2, where some remarks about the contribution due to a nearby interharmonic are also reported. the accuracies of the derived expressions and the correctness of the remarks are then verified by means of computer simulations in section 3. finally, conclusions are proposed in section 4. 2. sine-wave affected by an interharmonic: ipdft frequency, amplitude, and phase estimation errors the analysed discrete-time waveform is modelled as: 𝑥(𝑚) = 𝐴 cos (2π 𝑓0 𝑓s (𝑚 + 1 2 ) + 𝜙) + 𝐴ih cos (2π 𝑓0ih 𝑓s (𝑚 + 1 2 ) + 𝜙) 𝑚 = − 𝑀 2 , − 𝑀 2 + 1, … , 𝑀 2 − 1 (1) where a, f0, , aih, f0ih, and ih are the amplitude, frequency, and initial phase of the fundamental and the interharmonic components, respectively, and fs is the sampling frequency, while m is the acquisition length. the ratios f0 / fs and f0ih / fs can be expressed as: 𝑓0 𝑓s = 𝜈 𝑀 = 𝑙 + 𝛿 𝑀 , (2) 𝑓0ih 𝑓s = 𝜈ih 𝑀 = 𝑙ih + 𝛿ih 𝑀 , (3) where  and ih are the number of acquired cycles; l and lih are the integer parts of  and ih, respectively. finally,  (-0.5   < 0.5) and ih (-0.5  ih < 0.5) are the corresponding fractional parts, i.e. the inter-bin frequency locations. it is noteworthy that the reference time (i.e. the instant m = 0) is chosen in the centre of the observation interval so that the ipdft phase estimator is independent of the frequency estimator, and more accurate estimate can be obtained [11], as it will be shown in the next section. in practice, both the fundamental and the interharmonic components are normally non-coherently sampled (i.e.   0 and ih  0), so the discrete spectrum of (1) is affected by spectral leakage. to mitigate the issue, the windowed signal xw(m) = x(m)  w(m) is analysed, where w() is a suitable window function. an h-term msd window (h  2) is considered due to its good spectral characteristics [4], [22]. it is defined as: 𝑤(𝑚) ≝ ∑ 𝑎ℎ cos ( 𝐻−1 ℎ=0 2π ℎ 𝑀 (𝑚 + 1 2 )), 𝑚 = − 𝑀 2 , − 𝑀 2 + 1, … , 𝑀 2 − 1 (4) where the window coefficients ah are given by [12]: 𝑎0 = 𝐶2𝐻−2 𝐻−1 22𝐻−2 , 𝑎ℎ = 𝐶2𝐻−2 𝐻−ℎ−1 22𝐻−3 , ℎ = 1, 2, … , 𝐻 − 1, in which 𝐶𝑎 𝑏 = 𝑎! (𝑎−𝑏)! 𝑏! . the dft of the windowed signal xw() is given by: 𝑋𝑤 (𝑘) ≝ ∑ 𝑥𝑤 (𝑚)𝑒 −𝑗 2π 𝑀 𝑘(𝑚+ 1 2 ) 𝑀 2 −1 𝑚=− 𝑀 2 = 𝐴 2 𝑊(𝑘 − 𝜈)𝑒𝑗𝜙 + 𝐴 2 𝑊(𝑘 + 𝜈)𝑒 −𝑗𝜙 + 𝐴ih 2 𝑊(𝑘 − 𝜈ih )𝑒 𝑗𝜙ih + 𝐴ih 2 𝑊(𝑘 + 𝜈ih)𝑒 −𝑗𝜙ih , (5) where w() is the discrete time fourier transform (dtft) of the adopted window w(). for || ≪ m and m ≫ 1, w() is given by [12]: 𝑊(𝜆) = 𝑀𝑠𝑖𝑛(π𝜆) 22𝐻−2π𝜆 (2𝐻 − 2)! ∏ (ℎ2 − 𝜆2)𝐻−1ℎ=1 . (6) which shows that w() exhibits even symmetry. it is important to note that the second and the fourth terms in (5) represent the contributions of the fundamental spectral image and interharmonic components, respectively. the inter-bin frequency location estimator returned by the considered ipdft algorithm is given by [12]: �̂� = (𝐻 − 1 + 𝑖)𝛼 − 𝐻 + 𝑖 𝛼 + 1 , (7) where i = 0 if |xw(l – 1)|  |xw(l + 1)| and i = 1 if |xw(l – 1)| < |xw(l + 1)| and 𝛼 ≝ |𝑋w(𝑙+𝑖)| |𝑋w(𝑙−1+𝑖)| . the related ipdft amplitude and phase estimators are given by [11], [12]: �̂� = 2|𝑋w(𝑙)| |𝑊(−�̂�)| = 2|𝑋w(𝑙)| |𝑊(�̂�)| , (8) and �̂� = ∡ {𝑋w(𝑙)}, (9) where �̂� is the inter-bin frequency location estimate returned by (7). when lih  l + 1 and aih ≪ a, due to the interharmonic component, that estimate is affected by the following error [25]: acta imeko | www.imeko.org september 2020 | volume 9 | number 3 | 19 δ𝛿 ≅ (𝐻 + (−1)𝑖 𝛿) × [ (−1)𝑖2ν 2𝜈−𝛿+(−1)𝑖+1𝐻 𝑊(2𝜈−𝛿) 𝑊(𝛿) cos(2𝜙) + (−1)𝑖(𝜈ih−𝜈) 𝜈ih−𝜈+𝛿+(−1) 𝑖𝐻 𝐴ih 𝐴 𝑊(𝜈ih−𝜈+𝛿) 𝑊(𝛿) cos(𝜙ih − 𝜙)] . (10) the corresponding relative amplitude and phase estimation errors are given by (see appendix a): 𝐴 ≝ �̂� − 𝐴 𝐴 = 𝑊(2𝜈 − 𝛿) 𝑊(𝛿) cos (2𝜙) + 𝐴ih 𝐴 𝑊(𝜈ih − 𝜈 + 𝛿) 𝑊(𝛿) cos(𝜙ih − 𝜙) − 𝑊′(𝛿) 𝑊(𝛿) δ𝛿, (11) and (see appendix b): δ𝜙 ≝ �̂� − 𝜙 = − 𝑊(2𝜈 − 𝛿) 𝑊(𝛿) sin(2𝜙) + 𝐴ih 𝐴 𝑊(𝜈ih − 𝜈 + 𝛿) 𝑊(𝛿) sin(𝜙ih − 𝜙), (12) where 𝑊 ′(𝛿) is the derivative of w() with respect to  and  is given by (10). (11) and (12) show that the amplitude error a depends on the frequency error , while the phase error  is independent of it. by using expression (8), after simple calculations, the error a becomes: 𝐴 = 𝑊(2𝜈 − 𝛿) 𝑊(𝛿) cos (2𝜙) × [(𝐻 + (−1)𝑖𝛿) (−1)𝑖+12ν 2𝜈 − 𝛿 + (−1)𝑖+1𝐻 𝑊′(𝛿) 𝑊(𝛿) + 1] + 𝐴ih 𝐴 𝑊(𝜈ih − 𝜈 + 𝛿) 𝑊(𝛿) cos (𝜙ih − 𝜙) × [(𝐻 + (−1)𝑖𝛿) (−1)𝑖+1(𝜈ih − 𝜈) 𝜈ih − 𝜈 + 𝛿 + (−1) 𝑖 𝐻 𝑊′(𝛿) 𝑊(𝛿) + 1]. (13) observe that both equations (12) and (13) contain two terms that depend on the phases of the fundamental and interharmonic components through a sine function. the first term is the contribution of the spectral interference from the fundamental image component. it decreases as the number of acquired sinewave cycles  increases. the second term is the contribution of the spectral interference from the interharmonic component. it decreases as the difference between the observed number of interharmonic and fundamental cycles ih  increases. moreover, both errors a and  linearly increase with the relative interharmonic amplitude aih / a. if the distance ih  is small, and the relative amplitude aih / a is quite high, then the contribution of the interharmonic component dominates that of the fundamental image component. it is also important to note that when acquiring a single signal record, the errors , a and  exhibit a systematic behaviour since they depend on the constant parameters of the fundamental and the interharmonic components, the window dtft and its derivative. unfortunately, the proposed analysis only allows for an evaluation of that contribution (and so providing a type b method of uncertainty evaluation) when the difference between the recorded number of cycles of the fundamental and the interharmonic components is greater than about three. indeed, below that threshold, the interference from the interharmonic component is significant, and the ipdft algorithm does not return accurate parameter estimates, so the derived relationship cannot be effectively applied to evaluate – and possibly mitigate – the related systematic contribution. 3. computer simulations and experimental results in this section, both meaningful computer simulations and experimental results are reported in order to check the accuracies of the derived expressions for relative amplitude error (13) and phase error (12) and to verify the correctness of the related conclusions. furthermore, equation (10) for the inter-bin frequency estimation error is analysed since it affects the amplitude estimation error. only simulations and experiments related to specific values of the parameters are reported in the following for the sake of conciseness. however, behaviours very similar to those shown here were obtained in all the considered situations. 3.1. simulation results the amplitude and the phase of the simulated sine-waves are a = 1 p.u. and  = /3 rad, respectively. the acquisition length is m = 512 samples. each simulation considers 1000 runs in which the initial phase of the interharmonic component ih changes at random in the range [0, 2) rad. the two-term msd (a) (b) figure 1. the simulation and theoretical results related to the maximum values of the error magnitudes ||, |a|, and || versus the inter-bin frequency  when l = 2 cycles, ih  = 1.4 cycles (a) and ih  = 2.8 cycles (b). aih / a = 0.1 and 1000 runs of m = 512 samples each are considered. the hann window is adopted. acta imeko | www.imeko.org september 2020 | volume 9 | number 3 | 20 window, known also as hann window [4], [22], is used. figure 1 shows the maximum values of the error magnitudes ||, |a|, and || obtained by simulations and theoretical results as a function of the inter-bin frequency location  when l = 2 cycles, aih = 0.1 p.u., ih  = 1.4 cycles (figure 1(a)) and ih  = 2.8 cycles (figure 1(b)). figure 1 shows a very good agreement between the simulation and the theoretical results except when  is close to zero and ih  = 1.4 cycles. that behaviour is probably due to an incorrect selection of the second interpolation point between the two samples |xw(l – 1)| and |xw(l + 1)|, which exhibit very close values. it is noteworthy that the same behaviour was observed for other values of ih  < 2.4 cycles. moreover, this occurred in a range of  values that became shorter when ih  increases. notice also that the simulation and theoretical results related to the phase estimation error are always close each other. indeed, they do not depend on the estimated inter-bin frequency, see (12). figure 2 shows the maximum values of the error magnitudes ||, |a|, and || obtained by the simulation and theoretical (a) (b) (c) (d) (e) (f) figure 2. the simulation and theoretical results related to the maximum values of the error magnitudes ||, |a|, and || versus the distance ih  between the observed cycles of the interharmonic and the fundamental components when  = 1.7, 5.7, 10.7, 15.7 cycles (a) and  = 2.3, 6.3, 11.3, 16.3 cycles (b). aih / a = 0.1 and 1000 runs of m = 512 samples each are considered. the hann window is adopted. acta imeko | www.imeko.org september 2020 | volume 9 | number 3 | 21 results versus the difference ih  of the observed cycles of the two components when aih = 0.1 p.u.,  = 1.7, 5.7, 10.7, 15.7 cycles (figure 2(a), (c), (e)) and  = 2.3, 6.3, 11.3, 16.3 cycles (figure 2(b), (d), (f)). notice that figure 2(a) and (b), (c) and (d), and (e) and (f) are related to the same values of l and  = −0.3 and  = +0.3, respectively. from figure 2, it follows that there is a very good agreement between the simulation and the theoretical results. furthermore, it can be observed that all estimation errors are quite significant when the two tones are close to each other (specifically, when ih  is less than about 1.5 cycles) and they mainly depend on the interference due to the interharmonic. conversely, when the differenceih  increases, the contribution of the fundamental spectral image becomes significant. it is worth noticing that this behaviour holds regardless the value of the number of sine-wave cycles  as soon as it is greater than one cycle, except when  is close to zero and the difference ih  is enough high (see figure 1). figure 3 shows the simulation and theoretical results related to the maximum values of the estimation error magnitudes ||, (a) (b) (c) (d) (e) (f) figure 3. the simulation and theoretical results related to the maximum values of the error magnitudes ||, |a|, and || versus the ratio aih / a when  = 1.7 and 5.7 cycles (a), and  = 2.3 and 6.3 cycles (b). ih  = 1 cycle and 1000 runs of m = 512 samples each are considered. the hann window is adopted. acta imeko | www.imeko.org september 2020 | volume 9 | number 3 | 22 |a|, and || as a function of the ratio aih / a between interharmonic and fundamental amplitudes, when  = 1.7 and 5.7 cycles (figure 3(a), (c), (e)),  = 2.3 and 6.3 cycles (figure 3(b), (d), (f)) and ih  = one cycle – the smallest value considered for the difference between the observed cycles of the interharmonic and the fundamental components. in figure 3, it can be observed that the simulation and the theoretical results are very close to each other when aih / a  0.1, except for the phase estimation errors for which there is a very good agreement for all the considered values of  when aih / a < 0.65. this is due to the fact that the expression for the phase error is accurate also when the interharmonic amplitude is much smaller than the fundamental component. furthermore, all estimation errors linearly increase as the ratio aih / a increases, as expected from the theory. finally, figure 4 compares the maximum error magnitudes ||, |a|, and || due to both an interharmonic and a second harmonic as a function of ; ih  = 1.7 cycles and both the interharmonic and harmonic have the same amplitude – equal to 0.1 p.u. the theoretical expressions for the contribution of the second harmonic are obtained from (10), (13), and (12), assumingih = 2. as we have shown, the sine-wave parameter estimation errors due to the interharmonic are almost independent of the integer part of the number of observed sine-wave cycles, as they mainly end on the inter-bin frequency location . indeed, the second terms in (10), (12), and (13) dominate, and for fixed values of ih , they depend on the inter-bin frequency . therefore, the error magnitude ||, |a|, and || have an almost periodic behaviour with respect to . conversely, the errors ||, |a|, and || due to the effect of the second harmonic decrease as the number of observed sine-wave cycles  increases. moreover, the contribution of the interharmonic component dominates, except when  < 2 cycles, where the contribution due to the fundamental spectral image becomes important. 3.2. experimental results a block scheme of the experimental setup is shown in figure 5. the fundamental component was set to an amplitude of a = 2 v and a frequency of f0 = 2 khz, and it was provided at the signal generator out1 output, while an interharmonic component of amplitude aih = 0.175 v and frequency f0ih = 2.293 khz were provided at the out2 output. the analysed signal, given by the sum of the fundamental and the interharmonic, was obtained at the output of the passive adder. this signal was sampled at a rate of 100 khz by an ni6023e acquisition board with a full-scale range equal to 10 v. fifty records of m = 512 samples each were acquired. figure 6 shows the experimental and theoretical estimation errors , a, and  affecting the ipdft algorithm. the theoretical results were obtained by applying (10), (12), and (13), respectively, in which the involved signal parameters were returned by a linear least squares two-tone fit algorithm. according to that algorithm, the number of the acquired cycles of the fundamental and the interharmonic components,  and ih respectively, are firstly determined by (2) and (3), in which f0 and f0ih are the frequencies of the generated signals. then, the signal offset and the (b) (c) figure 4. the simulation and theoretical results related to the maximum values of the error magnitudes ||, |a|, and || versus  when the sine-wave is affected by both an interharmonic and a second harmonic with the same amplitudes, equal to 0.1 p.u. the interharmonic is 1.7 cycles from the fundamental component. a = 1 p.u. and 1000 runs of m = 512 samples each are considered. the hann window is adopted. figure 5. a block scheme of the experimental setup. acta imeko | www.imeko.org september 2020 | volume 9 | number 3 | 23 amplitudes and phases of the two components are estimated by applying a linear least squares approach. it is important to note that in the considered situation, the ipdft algorithm does not return accurate parameter estimates due to the strong interference from the interharmonic component, which abruptly increases as soon as ih   is smaller than about three cycles. it follows that the derived equations (10), (12), and (13) do not allow for an effective compensation for the systematic contribution due to the spectral interference that arises when the ipdft algorithm is used. figure 6 shows that the experimental and theoretical results for the errors  and a are close to each other, while those related to the error  almost coincide, thereby validating the performed analysis. 4. conclusions the analytical expressions for the errors that affect the sinewave amplitude and phase estimates returned by the classical ipdft algorithm based on a generic msd window have been derived in the case when the sine-wave is corrupted by a smallamplitude interharmonic located at least one bin apart the unknown frequency. the derived expressions show that the amplitude error depends on the inter-bin frequency location error, while the estimation of the phase referred at the centre of the observation interval is independent of the sine-wave frequency error. both expressions for the amplitude and phase errors contain two terms that exhibit sine-like behaviour. one term represents the contribution of the spectral interference from the fundamental image component, while the other one is the contribution of the spectral interference from the interharmonic component. in particular, the obtained expressions show when the contribution due to an interharmonic with a frequency located closely to the sine-wave one can dominate the detrimental effect of the second harmonic. both computer simulations and experimental results confirm the accuracies of the derived expressions. appendix a derivation of the expression for the relative amplitude error a when lih  l + 1 and aih ≪ a, we have [25]: |𝑋w(𝑙)| ≅ 𝐴 2 𝑊(𝛿) + 𝐴 2 𝑊(2𝜈 − 𝛿)cos(2𝜙) + 𝐴ih 2 𝑊(𝜈ih − 𝜈 + 𝛿)cos(𝜙ih − 𝜙). (14) since �̂� = 𝛿 + δ𝛿, with || ≪ ||, 𝑊(�̂�) can be accurately expressed by a first-order taylor’s series about : 𝑊(�̂�) ≅ 𝑊(𝛿) + 𝑊 ′(𝛿)δ𝛿, (15) where 𝑊 ′(𝛿) is the derivative of w() with respect to . by using expressions (14) and (15) in (8), the estimator �̂� becomes: �̂� ≅ [𝐴 + 𝐴 𝑊(2𝜈 − 𝛿) 𝑊(𝛿) cos(2𝜙) +𝐴ih 𝑊(𝜈ih − 𝜈 + 𝛿) 𝑊(𝛿) cos(𝜙ih − 𝜙)] × [1 + 𝑊 ′(𝛿) 𝑊(𝛿) δ𝛿] −1 . (16) since | 𝑊′(𝛿) 𝑊(𝛿) δ𝛿| ≪ 1, using the approximation (1 + x)-1  1 x, when |x| ≪ 1, (a.3) becomes: �̂� ≅ [𝐴 + 𝐴 𝑊(2𝜈 − 𝛿) 𝑊(𝛿) cos(2𝜙) +𝐴𝑖ℎ 𝑊(𝜈ih − 𝜈 + 𝛿) 𝑊(𝛿) cos(𝜙ih − 𝜙)] (17) figure 6. experimental and theoretical errors , a, and  affecting the sine-wave parameters ipdft estimator obtained when considering 50 records of m = 512 samples each. real sine-waves affected by an interharmonic are separated by about 1.5 cycles. theoretical results are achieved by using the linear least squares two-tone fit algorithm. acta imeko | www.imeko.org september 2020 | volume 9 | number 3 | 24 × [1 − 𝑊 ′(𝛿) 𝑊(𝛿) δ𝛿]. the second and third terms in the first square brackets of (17) become negligible when multiplied by 𝑊′(𝛿) 𝑊(𝛿) δ𝛿. thus, (17) becomes: �̂� ≅ 𝐴 + 𝐴 𝑊(2𝜈 − 𝛿) 𝑊(𝛿) cos(2𝜙) + +𝐴𝑖ℎ 𝑊(𝜈 − 𝜈ih − 𝛿) 𝑊(𝛿) cos(𝜙ih − 𝜙) − 𝐴 𝑊 ′(𝛿) 𝑊(𝛿) δ𝛿. (18) from (18), the amplitude estimation error results in: δ𝐴 ≝ �̂� − a ≅ 𝐴 𝑊(2𝜈 − 𝛿) 𝑊(𝛿) cos(2𝜙) + +𝐴ih 𝑊(𝜈ih − 𝜈 + 𝛿) 𝑊(𝛿) cos(𝜙ih − 𝜙) − 𝐴 𝑊 ′(𝛿) 𝑊(𝛿) δ𝛿. (19) finally, by using (19), equation (11) can be easily obtained. appendix b derivation of the expression for the phase error  from (7), the expression of xw(l) is given by: 𝑋w(𝑙) = 𝐴 2 [𝑊(𝛿) + 𝑊(2𝜈 − 𝛿)] cos(𝜙) + 𝐴ih 2 [𝑊(𝜈ih − 𝜈 + 𝛿) + 𝑊(𝜈ih + 𝜈 − 𝛿)] cos(𝜙ih) +𝑗 𝐴 2 [𝑊(𝛿) − 𝑊(2𝜈 − 𝛿)]sin (𝜙) +𝑗 𝐴ih 2 [𝑊(𝜈ih − 𝜈 + 𝛿) − 𝑊(𝜈ih + 𝜈 − 𝛿)] sin(𝜙ih) (20) by using (20), observing that the contribution of w(ih +  ) can be neglected compared with the other terms, after some simple calculations, equation (9) becomes: �̂� ≅ tan−1 {tan(𝜙) [1 − 𝑊(2𝜈 − 𝛿) 𝑊(𝛿) + 𝐴ih 𝐴 𝑊(𝜈ih − 𝜈 + 𝛿) 𝑊(𝛿) sin(𝜙ih) sin(𝜙) ] × [1 + 𝑊(2𝜈 − 𝛿) 𝑊(𝛿) + 𝐴ih 𝐴 𝑊(𝜈ih − 𝜈 + 𝛿) 𝑊(𝛿) cos(𝜙ih) cos(𝜙) ] −1 }. (21) observing that | 𝑊(2𝜈−𝛿) 𝑊(𝛿) + 𝐴ih 𝐴 𝑊(𝜈ih−𝜈+𝛿) 𝑊(𝛿) cos(𝜙ih) cos(𝜙) | ≪ 1 and using the approximation (1 + x)-1  1 – x, when |x| ≪ 1, (21) becomes: �̂� ≅ tan−1 {tan(𝜙) [1 − 𝑊(2𝜈 − 𝛿) 𝑊(𝛿) + 𝐴ih 𝐴 𝑊(𝜈ih − 𝜈 + 𝛿) 𝑊(𝛿) sin(𝜙ih) sin(𝜙) ] × [1 − 𝑊(2𝜈 − 𝛿) 𝑊(𝛿) − 𝐴ih 𝐴 𝑊(𝜈ih − 𝜈 + 𝛿) 𝑊(𝛿) cos(𝜙ih) cos(𝜙) ]}. (22) the second and third terms in the first square brackets of (22) multiplied by [ 𝑊(2𝜈−𝛿) 𝑊(𝛿) + 𝐴ih 𝐴 𝑊(𝜈ih−𝜈+𝛿) 𝑊(𝛿) cos(𝜙ih) cos(𝜙) ] are very small compared with the other terms so that they can be neglected. thus, after some calculations, the estimator �̂� becomes: �̂� ≅ tan−1 {tan(𝜙) − 2𝑊(2𝜈 − 𝛿) 𝑊(𝛿) tan(𝜙) + 𝐴ih 𝐴 𝑊(𝜈ih − 𝜈 + 𝛿) 𝑊(𝛿) sin(𝜙ih − 𝜙) cos2(𝜙) }. (23) by expressing the tan-1() function using its first-order taylor’s series about tan(), the following expression for �̂� is achieved: �̂� ≅ 𝜙 − 𝑊(2𝜈 − 𝛿) 𝑊(𝛿) sin(𝜙) + 𝐴ih 𝐴 𝑊(𝜈ih − 𝜈 + 𝛿) 𝑊(𝛿) sin(𝜙𝑖ℎ − 𝜙). 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international symposium, xi’an, china, 17-20 september 2019. https://www.imeko.org/publications/tc4-2019/imeko-tc42019-034.pdf https://doi.org/10.1016/j.measurement.2011.09.010 https://doi.org/10.1109/tim.2010.2046594 https://doi.org/10.1109/imtc.2004.1351218 https://doi.org/10.1109/19.997826 https://doi.org/10.1109/tim.2010.2060870 https://doi.org/10.1016/j.cpc.2008.12.019 https://doi.org/10.1016/j.measurement.2008.04.007 https://doi.org/10.1109/tie.2014.2316225 https://doi.org/10.1016/j.sigpro.2016.08.013 https://doi.org/10.1109/tassp.1981.1163506 https://doi.org/10.1016/j.ymssp.2015.05.025 https://doi.org/10.1109/amps.2018.8494892 https://www.imeko.org/publications/tc4-2019/imeko-tc4-2019-034.pdf https://www.imeko.org/publications/tc4-2019/imeko-tc4-2019-034.pdf process modeling for historical buildings restoration: an innovation in the management of cultural heritage acta imeko issn: 2221-870x october 2018, volume 7, number 3, 95 103 acta imeko | www.imeko.org october 2014 | volume 7 | number 3 | 95 process modeling for historical buildings restoration: an innovation in the management of cultural heritage ada malagnino1, giovanna mangialardi1, giorgio zavarise2, angelo corallo1 1 department of engineering for innovation, university of salento, campus university ecotekne, lecce, italy 2 department of structural engineering, construction and geotechnics, polytechnic of turin, torino, italy section: research paper keywords: restoration; historical building information modeling; business process modeling citation: ada malagnino, giovanna mangialardi, giorgio zavarise, angelo corallo, process modeling for historical buildings restoration: an innovation in the management of cultural heritage, acta imeko, vol. 7, no. 3, article 15, october 2018, identifier: imeko-acta-07 (2018)-03-15 section editor: egidio de benedetto, university of salento, italy received april 30, 2018; in final form september 21, 2018; published october 2018 copyright: © 2018 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: this work was supported by italian ministry of university and research, through the prin 2015 funding scheme (project 2015jw9njt – advanced mechanical modeling of new materials and structures for the solution of 2020 horizon challenges corresponding author: ada malagnino, ada.malagnino@unisalento.it 1. introduction cultural heritage is a complex ecosystem requiring innovative methods and techniques to facilitate its management and valorisation. in particular, restoration processes are very complex, considering that the original characteristics of historical buildings or sites must be preserved and that different professional figures, having different roles, responsibilities and skills, are involved [1]. the correct management of the informative flows and knowledge, potentially accumulated for decades, becomes fundamental for the cultural heritage enhancement, since it is an expression of the ways of living, developed by a community and passed on from generation to generation, including customs, practices, places, objects, artistic expressions and values [2]. the restoration of historic buildings is particularly relevant in italy, because it is a country characterized by a big cultural patrimony unique in the world. a long and laborious legislative path has resulted in the laws issuance aimed at defining the concept of cultural heritage and its preservation. in italy, for the cultural heritage, it is currently in force the legislative decree no. 42/2004 named “codice dei beni culturali e del paesaggio”. the decree regulates cultural and landscape heritage and also defines the “minor architectures” as monuments and environmental assets in close proximity of other monumental architectures, by referring to the building’s importance, considered historical even if humble. in addition, the legislative decree no. 310/1964, named “costituzione di una commissione d'indagine per la tutela e la valorizzazione del patrimonio storico, archeologico, artistico e abstract cultural heritage is a complex ecosystem requiring innovative methods and techniques to facilitate its restoration, management and valorization. in detail, restoration processes have to preserve the original characteristics of historical buildings or sites, by involving different professional figures, each one with different roles, responsibilities and skills. in this light, a system that facilitates the information management, supported by new technologies, could allow more precise, efficient and traceable information exchange and could optimize the construction and management operative phase. starting from these assumptions, the research proposes the application of a new integrated approach to a restoration process of a historical building. it consists of applying business process management (bpm) approach and building information modeling (bim) methodology to provide valuable insights to optimize the data flow, to gather information and to share knowledge during the restoration process, by optimizing the whole lifecycle management process, by facilitating the ordinary and extraordinary maintenance. all these aspects are essential and strategic for the italian cultural heritage. mailto:ada.malagnino@unisalento.it acta imeko | www.imeko.org october 2014 | volume 7 | number 3 | 96 del paesaggio”, identifies the historical town centres as a new class, defining them as “urban settlements that constitute cultural unity or the original and authentic settlements part”. the building cultural interest is not equivalent to the historical-artistic high value. for this reason, not all the historical buildings are bound by law [3]. the cultural heritage context is quite varied and complex. it involves, in fact, different actors and different procedural processes in relation to the kind of intervention and to the kind of historical building. another structural characteristic is that cultural and historical heritage is exposed to a continuing degradation risk. degradation process may be aggravated and accelerated by environmental conditions or catastrophic events and by the lack of adequate forms of continuous monitoring and maintenance. in italy, these continuous care practices of cultural assets and the good practices diffusion among institutions have been neglected by owners and property managers, as the following pictures show (figure 1,2,3). the lack of maintenance operations not only implies the aesthetic degradation of materials and buildings, but also the structural and functional characteristics loss. therefore, when a catastrophic event occurs, such as an earthquake, the probability of the structural and non-structural elements collapse is much higher. figure 1. historical buildings in piazza garaffello, palermo (pa), italy. (source: https://commons.wikimedia.org/wiki/file:piazza_garraffello__panoramio.jpg) figure 2. pompei archeological site (na), italy. (source: https://culture.globalist.it/arti/2017/10/12/come-cambiano-isaperi-e-la-cultura-grazie-alla-rete-2013000.html) figure 3. guglielmo marconi's telegraphic radio station, coltano (pi), italy. (source: http://www.5avi.net/2018/01/18/la-palazzina-marconi-coltanoriflessione-sulla-tutela/) the adoption of maintenance and conservation dynamic plans (like a building dossier) can give a response to the sector development need. these plans would provide cognitive, conservative and economic benefits for a better management of cultural heritage, including preventive and predictive maintenance. the new non-invasive technologies for surveys or the new methodologies of remote representation, more developed in other sectors, represent a potential to support such management. examples are low-cost sensors from the internet of things, widespread in the automotive and aerospace sectors, and digital twins [4]. the need for new scenarios in the cultural heritage field is suggested by the awareness that maintenance should not be considered as a set of activities linked to a catastrophic event, but it should be considered as a set of complex and continuous activities that includes observation, evaluation, registration, programmed conservation and requires specialist skills during the building lifecycle. as regards built heritage, historic buildings and monuments, the maintenance activities have always been considered as the prevailing mode of intervention on the existing city, starting from the restoration debate at the end of the nineteenth century, as well as various international restoration charters (charter of athens 1931, italian restoration charter 1932, charter of venice 1964, etc.) [5]. the legislative decree no. 42/2004, mentioned above, is the main legislative reference of the sector and includes the following maintenance definition (art. 29, paragraph 3): "complex of activities and interventions for the control of the cultural heritage and the maintenance of integrity, functional efficiency and the identity of the good and its parts". this definition underlines the idea that also maintenance is based on collection and management of information produced during the activities and the interventions. technological innovation, and especially process innovation (new management and prevention approaches), could allow to reduce, delay and, in particular, prevent degradation and, at the same time, reduce the maintenance and restoration costs. process management could represent the first step to innovation in this field, generally slow to change, to redefine the govern modality for the cultural heritage. in this light, a methodology that facilitates the information management, supported by new technologies in the internet of acta imeko | www.imeko.org october 2014 | volume 7 | number 3 | 97 things and industry 4.0 era, could allow more precise and traceable information exchange, by optimizing the building management. this paradigm shift could represent the key for processes innovation of the cultural heritage, such as restoration, management and enhancement, by adopting theories and practices usually applied to other fields, such as business process management (bpm) discipline, that manages processes to improve business performance outcomes and operation agility. the data digitalization, whose proper utilization strongly depends on their analysis and interpretation, could optimize knowledge management and sharing among the multidisciplinary team, according to the building information modeling (bim) approach, which aims to manage the whole building lifecycle in a common data environment, based on a three-dimensional informative and parametric model. a common data environment is a kind of online platform for collecting, managing and sharing (historical) building information through different people and process phases. for example, it facilitates the information exchange from the restoration phase to the management phase, where even the stakeholders involved are different. in detail, bpm technique enables the processes modelling and the identification of roles and relationships among different stakeholders and among different types of documentation, while bim methodology enables a collaborative common environment, where a 3d virtual model is the building data container. starting from these assumptions, this research proposes the application of a new integrated approach to a restoration process of a historical building. it consists in applying bpm approach and bim methodology to provide valuable insights to optimize the data flow, gather information and share knowledge during the restoration process through lifecycle management. the paper is divided into six sections, including the introduction. section 2 presents a brief review of the bim, bpm, bpmn theoretical background and a review of the application of these approaches on cultural heritage process management. section 3 introduces the research settings and the methodology. section 4 presents the application of bpm approach and bim methodologies for the optimization of the restoration process. section 5 discusses findings, and finally, section 6 concludes the paper and identifies challenges and future developments. 2. bim and bpm approach: a brief state of the art the new methodologies and technologies, together with theories and practices usually applied to a limited phase of the process or in other fields, can be the key for cultural heritage process innovation. 2.1. building information modeling (bim) the building information modeling, defined as a process related to the creation and use of digital models for design, construction and operations of projects [6] has changed the paradigm of construction industry as regards not only technological aspects (software and design tools), but also the approach by users and stakeholders that are becoming more closer to the concept of standardization [7]. as the bim model is a “shared information repository for collaboration throughout the facility’s lifecycle” [8], it allows all design team members (owners, architects, engineers, contractors, suppliers, etc.) to collaborate more efficiently than using traditional processes [9]. recently, the building lifecycle management (blm) [10] [11] is emerging as a new methodology, aiming at improving information sharing among all the phases of the building process, by providing an integrated it environment to manage the whole building lifecycle [12]. even if bim/blm has been usually adopted by the scientific community for the design and lifecycle management of buildings, some researchers have focused their studies on the value of bim in the management and documentation of cultural heritage monuments [13]. according to maddigan [14], the following benefits could be obtained by a 3d bim environment: remote reviewing of the building; possibility to study the new structures in the environmental context; having surveys from different periods of time; better structures analysis thanks to images that give information about texture, massing and form; estimation of restorations and adaptations costs and effects before construction operations start; better comprehension of the building. so, bim methodology gives a further contribution in order to improve the sustainable valorization of the cultural heritage. historic building information modeling (hbim) is a solution that allows to create parametric objects from the analysis of historic data to represent architectural elements. the design of these parametric objects is based on architectural manuscripts ranging from vitruvius to palladio to the architectural pattern books of the 18th century. the virtual architectural elements contain information about their methods of construction and the materials used. through a semi-automatic method, using a platform for the management of different software applications, the 3d historic building information model is realized by mapping the parametric objects onto the point cloud and image survey data [15]. “very little work has been done in relation to modeling historic buildings and also generating bim models from laser scan survey data” [16]. however, thanks to the diffusion of laser scanning and photogrammetry for recording cultural heritage sites and to the evolution of digital information systems, the hbim approach is becoming a topic of great interest. 2.2. business process management (bpm) and bpmn the main challenge of process management in a new field, like the management of cultural heritage, is the knowledge creation. “this is widely viewed as a creative, ‘black box’ activity that is difficult to manage as a process but not impossible. perhaps there are circumstances in which knowledge creation is totally unstructured, unmeasured, and unrepeatable – but in most situations, progress can still be made in this direction. one common approach to knowledge creation processes is simply to decompose them into several pieces or stages” [17]. therefore, the business process management (bpm) discipline is useful to reach the research aim, because it manages processes to improve business performance outcomes and operation agility in the manufacturing sector. the european association of bpm (eabpm) gives the following definition in its reference work, bpm common acta imeko | www.imeko.org october 2014 | volume 7 | number 3 | 98 body of knowledge: “business process management is a systematic approach for capturing, designing, executing, documenting, measuring, monitoring, and controlling both automated and non-automated processes to meet the objectives and business strategies of a company” [18]. in addition to that, the approach creates and delivers value by linking together people, information flows, documents, changes, systems and other assets. “because bpm is dependent on the internet and various internet protocols, there have been a variety of efforts to generate software standards that would support bpm development. as part of process modelling techniques, first attempts towards standardization have emerged, and the most prominent candidate for such a process modelling standard is the business process modeling notation (bpmn)” [17]. the business process model and notation is an international and standard common language, bridging the communication gap that occurs between business process design and implementation, which has been maintained by the object management group (omg) 1. the notation allows hierarchical structuring of the process, distributing information for describing a process among several levels and several actors, from the general process flow of the highest level of the hierarchy to the process details on the lowest one [17]. the bpmn provides a detailed top-down description of a business process model, along with information flows between activities. the model is described by graphical notations in a business process diagram (bpd) [19]. bpmn was developed to help businesses to understand their external and internal procedures, so that decision makers see their processes globally, and to communicate these procedures according to the specific standard [20] [21]. the application of the bpm approach for modelling in bpmn language the restoration process is an innovative way to map the current methodology and consequently optimizing them with bim innovation. 2.3. systematic literature review about bim and bpm application to the cultural heritage processes management to pursue the main goals of the research, namely the modeling of a cultural heritage restoration process using bpmn language and bim methodology, a research among literature works has been carried out through a systematic literature review. a set of keywords, related to the simultaneous application of bim and bpm methods in the construction sector, were chosen and then used to search papers in web of science and scopus bibliographic databases. very few papers that correlate bim and bpm or bpmn were found and no one about the application of bpm/bpmn to hbim processes. therefore, found papers refer to the application of bpmn and bim to different topics related to the construction industry. for instance, they deal with the development of a process mapping procedure for planning bim implementation on a 1 https://www.omg.org/ project [21], the development and validation of a holistic energy performance evaluation framework [22], the investigation of the requirements for bim governance and the development of specifications for a cloud-based governance platform [23], the analysis of work process and information exchange in construction projects by focusing on the project owner [24], the definition of a building materials reuse workflow [25], the encoding of regulatory knowledge related to fire safety into a computable representation [26]. whilst analyzed papers integrated bim and bpm concepts for different purposes, they all agreed that mapping processes by bpmn improve the information exchange among stakeholders [24], describes business processes in detail [25] and supports transition to bim implementation [22]. the literature review about the simultaneous application of bim and bpm approaches to the construction sector has revealed that cultural heritage processes are not managed by bpm techniques. furthermore, there is no integration between bim and bpm approaches in this field. considering the literature review results, it would be useful to apply bpmn to hbim process in order to point out critical points of traditional processes and to underline strengths and weaknesses of the same processes in a bim environment. mapping a hbim process using bpm method results to be strategic and acquires relevance considering that very little has been done in this field. 3. research method the research activities goals have been focused on the understanding of the processes through observation in the natural settings of the analyzed subject, according to qualitative research methods [27]. data collected have been then studied in depth, in order to propose a methodology for construction sector processes analysis, which integrates knowledge from different research fields. the target of the research work has been, in particular, the restoration process of a historical building. the present work research method includes the main qualitative characteristics reported by creswell. more specifically, the authors have chosen a case study qualitative approach [28]. among the multiple forms of data that a qualitative researcher can gather [29], authors have selected interviews and document analysis as sources of information. in addition to that, to ensure natural settings, data are collected at the site where participants face the issues that researchers are studying. in particular, the italian context has been considered and engineers and architects that work in the italian regulatory framework have been interviewed. the researcher has a key role in data collection, management and analysis. he/she gives his/her own interpretation to data, and this often means a lack of transferability, reliability, and objectivity. to explore different perspective of the topic, authors have used investigator triangulation and data triangulation [30]. inductive and deductive approaches have both played an important role in the data analysis [28]. firstly, data have been organized into categories and unit of information and, secondly, hbim principles have been included in the specific case study to be processed. acta imeko | www.imeko.org october 2014 | volume 7 | number 3 | 99 3.1. research methodology following the research method design, the first stage of this work has been the study of the actual historical building restoration process. guidelines for the modeling of the analyzed process are those given by the italian legislative framework. in detail, the legislative decree no. 42/2004, above mentioned, the legislative decree no. 380/2001 (d.p.r. testo unico delle disposizioni legislative e regolamentari in materia edilizia) and the legislative decree no. 50/2016 (codice dei contratti pubblici di lavori, forniture e servizi) were complied with. to correctly map the restoration process, the regulation code has been therefore studied and information has been integrated with data collected through qualitative interviews with ten subject matter experts (architects and engineers) working as freelancers on the italian territory. these interviews have involved unstructured and open-ended questions in order to acquire knowledge from current practice. the process has been first divided into macro-phases and for each macro-phase actors involved, their roles and responsibilities, activities and their interconnection, information exchange, resources have been identified. particular attention has been given to documentation produced during activities, to the format used and to the sharing mode. it is in fact in this area that benefits could be significant as a bim based system optimizes time and costs related to paper consumption, redigitation of data, redundant information and archived data for future works. in the second stage bpmn notation has been used to represent graphically and in an international standard language the traditional cultural heritage restoration process, in its current state (as is) bpd. the authors have translated and integrated italian regulation codes and stakeholders’ activities into bpmn, classifying process information in levels of detail, in order to give a graphic representation of the process at different informative layers. after a critical analysis of the current process (as is), weaknesses, bottlenecks or possible legislative code integration have been identified and studied in depth. to improve restoration process management in the specific field of cultural heritage, hbim requirements have been investigated and, by applying a deductive approach, changes in the traditional process have been proposed. accordingly, the final stage of the research work has consisted in a to be cultural heritage restoration process proposal, which integrates hbim methodology and business process management and which is illustrated in bpmn language. 4. case study the described methodology can be applied to different processes in the construction field and specifically to cultural heritage processes. to give an example of how process modeling could be a useful communication and analysis tool, authors have chosen a specific case study, hypothesizing that: the process analyzed is that one referred to a private property building, identified by law as a building having historical, cultural or artistic value; architectural, structural, plant engineering projects are managed by a design team; construction works are given to a building contractor, which is external to the design team. the specific process has been then studied and mapped in bpmn language as it is in current practice, and weak points have been identified and analyzed in a critical light. the made observations have been then used to design the new restoration process (figure 4), integrated with hbim methodology. both the as is and the to be processes have been mapped using signavio software and hypothesizing a starting simplification, namely that all professional figures have accepted their designation by the owner. the bpd illustrated in figure 4 shows the process complexity with actors involved and their interactions in the regulatory italian framework. the bpd is a high-level representation, which gives an idea of how many professionals and stakeholders have a role in the process, when and how they have to interact, according to the italian law, and indicates the documentation required in a certain point of the process to a specific actor by another actor or stakeholder. however, the process is much more complex, and some activities represented in figure 4 have been analyzed in greater depth and mapped as sub-processes, indicated in the diagram with a “+”, under the graphical rules of bpmn language. in the as is process six actors have been identified: the owner, assumed to be a private in this case study; the superintendence, which could authorize or not the restoration of the building; the local authority, which gives the authorization for the restoration works; the design team, considered as a single actor to simplify the representation of the process, but often it has to be replaced with different professional figures (architect, structural engineer, plant engineer, etc.); the contractor, in charge of the construction works; the works supervisor, who has to verify the correct execution of works. to optimize the as is, bim, and more specifically hbim principles, are evaluated and introduced in the to be process, resulting in the introduction of new professional figures, as regards actors, namely the bim manager and the facility manager, in order to have a bim process supervisor and a manager for use and maintenance phase management. a “pool” has been assigned to each process actor. a pool is a rectangular box that can contain flow objects such as task and activity. therefore, a pool is a graphical representation of the activities that participants who take part in the process have to do. dotted lines represent communications between pools and the email message has been assumed as the main communication tool. another change to traditional process is the introduction of a common database, the hbim db (figure 5), that contains the hbim model, a 3d parametric and informative model and the relative documentation, which are dynamically uploaded by actors involved in specific phases. in fact, actors are allowed to manage the model in defined phases and only specific operations are available for each actor in the hbim system. when an actor adds information to the hbim db, this activity is associated to a specific task and illustrated in the bpd. thanks to the graphical representation, it becomes clear who has to enrich the hbim db, which piece of information he/she has to add and when, during the whole building lifecycle. in this way information sharing is in real time and data are traceable, avoiding waste of time and optimizing costs. acta imeko | www.imeko.org october 2014 | volume 7 | number 3 | 100 b u il d in g r e s to ra ti o n p ro c e s s owner o w n e r o w n e r o w n e r r e s to ra ti o n n e e d d e s ig n t e a m d e s ig n a ti o n b im m a n a g e r d e s ig n a ti o n f a c il it y m a n a g e r d e s ig n a ti o n w o rk s s u p e rv is o r d e s ig n a ti o n a c q u is it io n o f d o c u m e n ta ti o n fo r th e a u th o ri z a ti o n re q u e s t a u th o ri z a ti o n r e q u e s t a u th o ri z a ti o n r e q u e s t d e n ie d a u th o ri z a ti o n c o n fi rm e d a u th o ri za ti o n c o n fi rm e d a u th o ri z a ti o n d o c u m e n ta ti o n r e q u e s t d o c u m e n ta ti o n re q u e s t to d e s ig n t e a m s c ia a u th o ri z a ti o n r e q u e s t a u th o ri z a ti o n n o ti c e a u th o ri z a ti o n n o ti c e a u th o ri z a ti o n r e q u e s t d e n ie d a u th o ri z a ti o n c o n fi rm e d a u th o ri z a ti o n c o n fi rm e d a u th o ri z a ti o n d o c u m e n ta ti o n r e q u e s t d o c u m e n ta ti o n re q u e s t to d e s ig n t e a m c o n tr a c to r d e s ig n a ti o n a u th o ri z a ti o n n o ti c e a u th o ri z a ti o n n o ti c e s u p e ri n te n d e n c e c o m m u n ic a ti o n a b o u t c o n s tr u c ti o n w o rk s w o rk s e n d c o m m u n ic a ti o n h b im m o d e l d e li v e ry design team d e si g n t e a m d e si g n t e a m d e si g n t e a m d e s ig n a ti o n n o ti c e d e s ig n p ro je c t c o n c e p t p ro je c t h b im d b d o c u m e n ta ti o n r e q u e s t e la b o ra ti o n o f d o c u m e n ta ti o n r e q u e s te d h b im m o d e l a u th o ri z a ti o n n o ti c e a u th o ri z a ti o n n o ti c e e la b o ra ti o n o f s c ia d o c u m e n ta ti o n d o c u m e n ta ti o n r e q u e s t e la b o ra ti o n o f d o c u m e n ta ti o n r e q u e s te d h b im d b a u th o ri z a ti o n n o ti c e a u th o ri z a ti o n n o ti c e superintendence s u p e ri n te n d e n c e s u p e ri n te n d e n ce s u p e ri n te n d e n c e a u th o ri z a ti o n r e q u e s t c o n fi rm a u th o ri z a ti o n r e q u e s t fu rt h e r d o c u m e n ta ti o n d e n y a u th o ri z a ti o n v e ri fy d o c u m e n ta ti o n c o n fi rm a u th o ri z a ti o n d e n y a u th o ri z a ti o n h b im d b h b im d b h b im d b h b im d b h b im d b w o rk s d e ta il s n o ti c e s u p e rv is io n a n d c h e c k s w o rk s e n d n o ti c e bim manager b im m a n a g e r b im m a n a g e r b im m a n a g e r d e s ig n a ti o n n o ti c e r e q u ir e c o n s u le n c e d e fi n e p ro je c t re q u ir e m e n ts s u p e rv is io n d e s ig n p h a s e a c ti v it ie s h b im d b h b im d b h b im d b s u p e rv is io n c o n s tr u c ti o n p h a s e a c ti v it ie s d e fi n it iv e r e p o rt c h e c k h b im d b c o n tr a c to r d e s ig n a ti o n n o ti c e facility manager f a ci li ty m a n a g e r f a c il it y m a n a g e r f a ci li ty m a n a g e r d e s ig n a ti o n n o ti c e c o n s u le n c e a c ti v it y c o n s u le n c e r e q u e s t c .e .l . f a c il it y m a n a g e m e n t h b im d b works supervisor w o rk s s u p e rv is o r w o rk s s u p e rv is o r w o rk s s u p e rv is o r d e s ig n a ti o n n o ti c e e la b o ra ti o n o f d e fi n it iv e r e p o rt h b im d b s u p e rv is io n c o n s tr u c ti o n w o rk s c o n tr a c to r d e s ig n a ti o n n o ti c e contractor c o n tr a c to r c o n tr a ct o r c o n tr a c to r c o n s tr u c ti o n w o rk s h b im m o d e l d e s ig n a ti o n n o ti c e local authority (municipality) l o c a l a u th o ri ty ( m u n ic ip a li ty ) l o ca l a u th o ri ty ( m u n ic ip a li ty ) l o c a l a u th o ri ty ( m u n ic ip a li ty ) c o n fi rm a u th o ri z a ti o n r e q u e s t fu rt h e r d o c u m e n ta ti o nd e n y a u th o ri z a ti o n v e ri fy d o c u m e n ta ti o n c o n fi rm a u th o ri z a ti o n d e n y a u th o ri z a ti o n h b im d b h b im d b h b im d b h b im d b h b im d b w o rk s e n d n o ti c e figure 4. historical building restoration process bpd. acta imeko | www.imeko.org october 2014 | volume 7 | number 3 | 101 figure 5. process detail: the hbim database for data sharing. according to bim logic, the bim manager elaborates a bim execution plan (bep) to describe and establish the project requirements, that must be communicated to the design team and to managers involved at each level within the organization. to give a building lifecycle vision to the project, the facility manager has a role in the design phase and gives a contribution in order to define the bep (figure 6). the bim manager requests advice from the facility manager at the beginning of the design phase and considers the point of view of a specialist that deals with the building use and maintenance phase, optimizing the project as regards for example energy efficiency or plants requirements. figure 6. definition of project requirements (bep). the facility manager is a relevant figure within the new process because a great importance has been given to building preservation, conservation and maintenance. the facility management has been, in fact, enabled through the hbim model, delivered by the owner to the facility manager (figure 7). the hbim model is the container of all the information associated to the building. these data are useful to better plan the building management and to design all the possible changes during its lifecycle. for a historical building, it is essential to preserve its original characteristic in order to keep its artistic and historical value. storing data about materials, structure, but also previous restoration activities in a common database is a key factor for hbim methodology application. figure 7. facility management phase. 5. discussions the application of the methodology to the cultural heritage restoration process has led the authors to underline the benefits of an integrated management approach. firstly, the digital model allows knowledge sharing among all the stakeholders involved in the whole building lifecycle and facilitates document exchange during the lifecycle of the building. secondly, this methodology allows to integrate into one single process different laws that are not connected to each other, although all these laws are referred to restoration processes. in fact, a critical point of this study has been to find correspondence among laws, as regards above all the interactions between actors and documentation. in addition to that, the use of an open standard graphical approach, such as bpmn, facilitates communication and makes easier the full comprehension of processes by stakeholders. nevertheless, bpmn language is not considered suitable for the detailed description of documental exchange in this case. the methodology proposed is also replicable, since it could be applied to other similar processes in the construction industry, as new construction, urban renovation, etc. furthermore, the link with hbim environment simplifies the documentation, archived in the hbim database, and operations management related to the historical building, by enabling ordinary and extraordinary dynamic maintenance. this aspect is an important objective for the management of the building historical heritage. thanks to the hbim db, in fact, the facility manager has a picture of the state of the building and can plan maintenance or restoration works giving priority to the interventions which are more necessary, if economic resources are limited. a programmed maintenance is also a way restoration process 2 o w n e r o w n e r o w n e r o w n e r r e s t o r a t io n n e e d d e s i g n t e a m d e s i g n a t i o n b i m m a n a g e r d e s i g n a t i o n f a c i l i t y m a n a g e r d e s i g n a t i o n w o r k s s u p e r v i s o r d e s i g n a t i o n a c q u i s i t i o n o f d o c u m e n t a t i o n f o r t h e a u t h o r i z a t i o n r e q u e s t a u t h o r i z a t i o n r e q u e s t a u t h o r i z a t i o n r e q u e s t d e n ie d a u t h o r i z a t i o n c o n f i r m e d a u t h o r i z a t io n c o n f i r m e d a u t h o r iz a t i o n d o c u m e n t a t i o n r e q u e s t d o c u m e n t a t i o n r e q u e s t t o d e s i g n t e a m s c ia a u t h o r i z a t i o n r e q u e s t a u t h o r iz a t i o n n o t i c e a u t h o r i z a t io n n o t ic e a u t h o r i z a t i o n r e q u e s t d e n ie d a u t h o r i z a t i o n c o n f ir m e d a u t h o r i z a t i o n c o n f i r m e d a u t h o r i z a ti o n d o c u m e n t a t i o n r e q u e s t d o c u m e n t a t i o n r e q u e s t t o d e s i g n t e a m c o n t r a c t o r d e s i g n a t i o n a u t h o r i z a t io n n o t i c e a u t h o r i z a t io n n o t i c e s u p e r i n t e n d e n c e c o m m u n i c a t i o n a b o u t c o n s t r u c t i o n w o r k s w o r k s e n d c o m m u n i c a t i o n h b im m o d e l d e l i v e r y d e s ig n t e a m d e s ig n t e a m d e s ig n t e a m d e s ig n t e a m d e s ig n a t io n n o t i c e d e s i g n p r o j e c t c o n c e p t p r o j e c t h b im d b d o c u m e n t a t io n r e q u e s t e l a b o r a t i o n o f d o c u m e n t a t i o n r e q u e s t e d h b i m m o d e l a u t h o r iz a t i o n n o t i c e a u t h o r i z a t io n n o t ic e e l a b o r a t i o n o f s c ia d o c u m e n t a t i o n d o c u m e n t a t i o n r e q u e s t e l a b o r a t i o n o f d o c u m e n t a t i o n r e q u e s t e d h b i m d b a u t h o r i z a t io n n o t i c e a u t h o r i z a t io n n o t i c e s u p e ri n te n d e n c e s u p e r in t e n d e n c e s u p e r i n t e n d e n c e s u p e r in t e n d e n c e a u t h o r iz a t io n r e q u e s t c o n f i r m a u t h o r i z a t i o n r e q u e s t f u r t h e r d o c u m e n t a t i o n d e n y a u t h o r i z a t i o n v e r i f y d o c u m e n t a t i o n c o n f i r m a u t h o r i z a t i o n d e n y a u t h o r i z a t i o n h b im d b h b im d b h b i m d b h b i m d b h b i m d b w o r k s d e t a i ls n o t i c e s u p e r v i s i o n a n d c h e c k s w o r k s e n d n o t ic e b im m a n a g e r b i m m a n a g e r b im m a n a g e r b i m m a n a g e r d e s ig n a t io n n o t ic e r e q u i r e c o n s u l e n c e d e f i n e p r o j e c t r e q u i r e m e n t s s u p e r v i s i o n d e s i g n p h a s e a c t i v i t i e s h b im d b h b im d b h b i m d b s u p e r v i s i o n c o n s t r u c t i o n p h a s e a c t i v i t i e s d e f i n i t i v e r e p o r t c h e c k h b i m d b c o n t r a c t o r d e s ig n a t io n n o t i c e f a c il it y m a n a g e r f a c ilit y m a n a g e r f a c i lit y m a n a g e r f a c il i t y m a n a g e r d e s i g n a t i o n n o t ic e c o n s u l e n c e a c t i v i t y c o n s u l e n c e r e q u e s t c .e .l . f a c i l i t y m a n a g e m e n t h b i m d b w o rk s s u p e rv is o r w o r k s s u p e r v is o r w o r k s s u p e r v is o r w o r k s s u p e r v is o r d e s i g n a t i o n n o t ic e e l a b o r a t i o n o f d e f i n i t i v e r e p o r t h b im d b s u p e r v i s i o n c o n s t r u c t i o n w o r k s c o n t r a c t o r d e s i g n a tio n n o t i c e c o n tr a c to r c o n t r a c t o r c o n t r a c t o r c o n t r a c t o r c o n s t r u c t i o n w o r k s h b i m m o d e l d e s i g n a t i o n n o t ic e l o c a l a u th o ri ty ( m u n ic ip a li ty ) l o c a l a u t h o r it y ( m u n ic ip a l it y ) l o c a l a u t h o r i t y ( m u n ic ip a lit y ) l o c a l a u t h o r it y ( m u n i c ip a l it y ) c o n f i r m a u t h o r i z a t i o n r e q u e s t f u r t h e r d o c u m e n t a t i o n d e n y a u t h o r i z a t i o n v e r i f y d o c u m e n t a t i o n c o n f i r m a u t h o r i z a t i o n d e n y a u t h o r i z a t i o n h b i m d b h b i m d b h b im d b h b i m d b h b i m d b w o r k s e n d n o t i c e acta imeko | www.imeko.org october 2014 | volume 7 | number 3 | 102 to prevent serious damage that requires more expensive operations. particular attention has been given to the management of use and maintenance phases in order to cover all the dimensions related to the building, giving to the construction industry sector methodologies and tools to innovate processes and increase construction processes sustainability. however, there are some critical points in hbim models, that have to be overcome, in order to reach better results in the application of the proposed methodology. for instance, the lack of standardization and the difficulty in the management of a huge quantity of surveys, geospatial and administrative data have been considered crucial aspects. solutions to these issues could be: a) enrich hbim models with the web ontology language (owl) standards; b) use open libraries, that could be customized according to local materials and constructive techniques [31]. 6. conclusion and future research after an analysis of current cultural heritage restoration process, this research describes a methodology proposal for the optimization of the historical buildings restoration as is process, consisting in the mapping of the to be process using bpm technique and hbim approach. this methodology has been proposed with the aim of innovating cultural heritage management. the main goal has been the reengineering of current procedures and activities, characterized by several actors, many procedures, long work periods and fragile assets to be maintained. bpm approach has allowed to study in depth the process related to the restoration of a historical building, by fostering activities standardization and facilitating the identification of stakeholders, documents, procedures and bottlenecks that would be otherwise hardly identified. this analysis has included the reengineering of the process thanks to the integration of the bim methodology, focused on the creation and management processes of a digital model. the challenge of a bim-based approach is the centralized database. the hbim db is proposed as a strategy to manage the as-built heritage and to store the main information and documents regarding the historical buildings. the proposed methodology bim and bpm based could be useful for the public authorities, responsible for the cultural heritage management, and for the owners for planning the maintenance of the historical buildings in a structured and dynamic common data environment. the next steps will focus on the process improvement, the analysis of other processes related to cultural heritage and on the automatization of some hbim functionalities for document exchange among different stakeholders. acknowledgement this paper is based on a greater research started inside the core lab, a research laboratory of the department of innovation engineering, university of salento. to this aim, authors would thank colleagues antonio margarito, computer engineer, and marco esposito, management engineer, who provided insight and expertise on bpmn that greatly assisted the presented work. authors gratefully acknowledge the support received from the italian ministry of university and research, through the prin 2015 funding scheme (project 2015jw9njt – advanced mechanical modeling of new materials and structures for the solution of 2020 horizon challenges). references [1] a. malagnino, g. mangialardi, a. corallo, g. zavarise, business process management and building information modeling for the innovation of cultural heritage renovation process, proc.of the 3rd imeko international conference on metrology for archaeology and cultural heritage, pp. 184-189, 23-25 october 2017. 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[31] c. di biccari, a. malagnino, a. corallo, g. zavarise, building information modeling or cultural heritage: beyond asset modeling. a pragmatic comparison of literature case studies, in proc. of the 3rd imeko international conference on metrology for archaeology and cultural heritage. metroarchaeo 2017, lecce, 2017. gold and silver joining technologies in the moche tombs “señor de sipán” and “señora de cao” jewellery acta imeko issn: 2221-870x october 2018, volume 7, number 3, 3 7 acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 3 gold and silver joining technologies in the moche tombs “señor de sipán” and “señora de cao” jewellery roberto cesareo1, angel bustamante2, régulo franco jordán3, arabel fernandez3, soraia azeredo4, ricardo t. lopes4, walter alva5, luis chero6, antonio brunetti7, giovanni e. gigante8, stefano ridolfi9 1 università di sassari, sassari, alghero, italy 2 universidad nacional mayor de san marcos, lima, perù 3 paceb museo cao e fundacion wiese, trujillo, perù 4 coppe, universidade federal do rio de janeiro, rio de janeiro, brazil 5 museo “tumbas reales de sipán”, lambayeque, perù 6 museo de sitio huaca rajada, sipán, lambayeque, perù 7 dipartimento di chimica e farmacia, università di sassari, sassari, italy 8 dipartimento di scienze di base ed applicate per l’ingegneria, università di roma “la sapienza”, rome, italy 9 arsmensurae, rome, italy section: research paper keywords: edxrf; radiography; soldering; welding; pre-moche citation: roberto cesareo, angel bustamante, régulo franco jordán, arabel fernandez, soraia azeredo, ricardo t. lopes, walter alva, luis chero, antonio brunetti, giovanni e. gigante, stefano ridolfi, gold and silver joining technologies in the moche tombs “señor de sipán” and “señora de cao” jewellery, acta imeko, vol. 7, no. 3, article 2, october 2018, identifier: imeko-acta-07 (2018)-03-02 section editor: egidio de benedetto, university of salento, italy received march 11, 2018; in final form august 16, 2018; published october 2018 copyright: © 2018 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: giovanni e. gigante, e-mail: giovanni.gigante@uniroma1.it 1. introduction on the north coast of present-day peru developed between 1200 bc and 1375 ad approximately, various relevant civilizations. among them the most important, from the point of view of metallurgical ability, was the moche civilization (also called mochica). the moche civilization flourished mainly in the moche and chicama valley (figure 1), where its great ceremonial centres have been discovered, from around 100 bc to 600 a.d., producing painted pottery, gold-silver ornaments and beautiful tissues. the moche were known as sophisticated metal smiths, both in terms of their technology, and of the beauty of their jewels. the moche metalworking ability was impressively demonstrated when walter alva and co workers discovered in 1987 the “tumbas reales de sipán” [ 1 , 2 ] and, more recently, when regulo franco jordan discovered in 2005 the tomb of the “lady of cao” ( figure1) [ 3-6]. abstract about 200 gold and silver funerary ornaments from the moche tombs “señor de sipán” and “señora de cao” were analyzed to determine their metallurgic characteristics. of particular interest was the question about the gold-silver joining process. to this aim, following methods were employed, all based on the use of x-rays: energy dispersive x-ray fluorescence; transmission of monoenergetic fluorescent x-rays; radiography. at least three joining methods were possibly identified: of gluing gold and silver sheets; of brazing using a proper solder; of using a mercury amalgam. mailto:giovanni.gigante@uniroma1.it acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 4 in the late 1960, tombs rich of metals, attributed to the moche, were discovered and located in a site that came to be known as loma negra, district of la arena, piura province. hundreds of objects attributed to loma negra were sold to uscollectors and finally partially transferred to the metropolitan museum of new york [ 7 , 8 ]. the site of loma negra was approximately dated about 2nd-3th century a.d., possibly before the tombs of the lady of cao (~300 a.d.) and of the lord of sipán (~350 a.d.). as explained in previous papers [9,10], the manufacturing technology of the moche was very various and partially unknown, and, also after six campaigns of analysis, many questions remain open: why the moche used to put relatively high quantities of gold in the silver sheets (tomb of cao)? why the gold artifacts from the tomb of cao have approximately the same composition, differently from the gold from the tomb of sipán? how are joint together gold and silver sheets? we can only make hypothesis on the first two questions; to answer to the last question a sub-millimetric edxrf portable equipment was developed to carry out detailed millimetric measurements on the au/ag interface; further radiographs were carried out with special attention to the interfaces. 2. experimental set-up 2.1. energy dispersive x-ray fluorescence: characteristics and experimental set-up energy dispersive x-ray fluorescence analysis is able to quantify the composition of a gold, silver, or copper alloy by using, for example, standard samples of the same alloys (section quantitative analysis of gold and silver alloys). edxrf-analysis is also able to detect, and qualitatively determine trace elements, when approximately present a level of 0.1 % or more. edxrf is a surface analysis, in the sense that the thickness of the alloy involved in the analysis is of the order of microns to a maximum of tens of microns; the results are therefore related to this depth and are generally valid in absence of surface enrichment phenomena (patina and ions migration processes). for the measurements carried out in the museum of cao, the portable equipment was composed of a mini x-ray tube by amptek (bedford, ma, usa), which is characterized by a ag anode and works at 40 kv and 200 μa maximum voltage and current, and a 123-si-drift detector [11]. bias supply and electronics of both x-ray tube and detector are located in the case of tube and detector, respectively; in such a manner, the equipment is extremely compact (figure 2). the si-drift is a thermoelectrically cooled detector, with 450 μm thickness and 7 mm2 area of the si-crystal respectively, and a thin be-window, of typically 12.5 μm thickness (0.5 mil) with about 125 ev energy resolution at 6.4 kev [11]. this detector has an efficiency of 97 %, 39 %, and 14 % at 10, 20, and 30 kev, respectively. the irradiated and analyzed area is of about 20 mm², when the object is at a distance of a few centimeters. the x-ray beam intensity with a not collimated and not filtered x-ray tube is largely in excess for analysis of alloys. for this reason, the x-ray beam is collimated and filtered, also to better its ‘form’, and to partially monochromatize the x-ray beam. the object to be analyzed is positioned at 1.5–3.0 cm distance from both x-ray tube and detector. the measuring time ranges from about 50 s to about 200 s, mainly according to the sample composition and size. standard gold and silver alloys were employed for calibration and for quantitative determination of alloy composition. figure 1. map of the moche territory, covering an area of about 50000 km2, showing the moche sites loma negra, sipán and el brujo. the tomb of the lady of cao is located in the “complejo el brujo”. (courtesy of fundacion wiese). figure 2. energy-dispersive x-ray fluorescence portable equipment, composed of a si-drift detector (on the right; 123 sdd: 450 μm thickness, 7 mm2 area, and 125 ev energy-resolution at 5.9 kev) and an ag-anode x-ray tube (40 kv, 100 μa maximum voltage and current) [10]. electronics including bias supply and multi-channel analyzer (mca) are in the case of the detector; x-ray tube bias supply is in the case of the x-ray tube. a typical measurement takes 50–100 s. acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 5 to measure gilding thickness of gilded copper or silver, the cu (kα/kβ)-ratio or ag(kα/kβ)-ratio and the (au-l/cu-k) or (au-l/ag-k)-ratios were employed. further, commercial gold leaves were employed (each au-foil 0.125 μm thick) to simulate gilded copper or tumbaga, and silver foils (each ag-foil 0.25 μm thick) to simulate the ‘silvered copper’ or ‘ag-tumbaga’. also, calibrated au-leaves were employed, and several gilded-copper samples with calibrated gilding thickness. thick sheets of pure copper and silver were also employed. 2.1.1. x-ray spectrum of gold alloys the final result of edxrf – analysis of a sample is an x-ray spectrum containing a set of x-lines for each detectable element present in the analyzed object [12,13]. when k-shells are excited, four characteristic lines are emitted by each element: kα1, kα2, kβ1, and kβ2. from a practical point of view, and because of the finite energy resolution of semiconductor detectors, the line combinations k1 – k2, coincide in a unique peak, and the same happens for the lines pairs or triplets kβ1 – kβ2, lα1 – lα2, and lβ1 – lβ2 – lβ3. k-lines are therefore identified by two x-rays kα and kβ, and l-lines by six x-rays lη, lα, ll, lβ, lγ1 and lγ3 of which only lα and lβ are of high intensity. by analyzing a gold alloy, containing au-ag-cu, and using a proper filter at the x-ray tube output to attenuate the low-energy part of the spectrum, a typical x-ray spectrum is obtained, as shown in figure 3. 2.1.2. quantitative analysis of gold and silver alloys in the case of gold or silver alloys, composed by au-ag-cu and ag-cu-au respectively, and assuming that au(%)+cu(%) +ag(%)=100 % (both for au and ag-alloys), by plotting the ratios (cu-kα/au-lα)-counts versus (cu-kα/au-lα)concentration, (ag-kα/au-lα)-counts versus (ag-kα/au-lα)concentration and the same for ag-alloys, it turns out that there is approximately a linear relationship, at least for agconcentrations in au up to about 30%, cu-concentrations in au up to 20%, for auconcentrations in ag up to 30%, and cuconcentrations in ag up to 20%. when other elements are present as trace elements, their concentrations can be determined by using the method of fundamental parameters. 2.2. radiography x-ray radiography is an imaging method that uses x-rays to reveal the structure of an object based on the different densities of its constituent materials. in the case of the objects from the tomb of the lady of cao, we should consider the fact that the large majority of the objects are sheets, basically on gold or silver alloys, and the approximate thickness was measured to be about 100 μm for au-layers and 200–300 μm for ag-layers. radiographs were carried out with the following portable equipment: • a portable mini x-ray tube by amptek, working at 50 kv and • 100 μa maximum voltage and current; • a flat panel detector by schick technologies; and • image analysis software isee. to image the moche jewels under study the equipment worked at 25, 35, 45 and 50 kv. the first value appeared to be ideal to visualize the ag-areas, while the last values (45 and 50 kv) were able to image the gold areas. 3. results by analyzing the gold-silver interfaces of many of the forty nose ornaments with the internal x-ray method [14], at least four typical situations were identified, and an additional fifth hypothesis was considered: figure 3. x-ray spectrum of an au-ag-cu alloy, obtained with the equipment shown in figure 2; the x-ray tube was working at 35 kv and 30 μa. the x-ray peaks are, from left: cu-kα line (8 kev), au-ll (8.5 kev), cu-kβ (8.9 kev), au-lα (9.7 kev), au-lη (10.3 kev), au-lβ (11. kev), au-lγ (13. kev), ag-kα (22 kev), and ag-kβ (25 kev). au x rays. figure 4. front side of the nose decoration paceb-f4-0002 from the tomb of the lady of cao. it is composed of a body on a gold-alloy sheet with, superimposed, two shields on silver and, partially superimposed, two huts on silver. silver huts and shields, these last only present in the front side, were probably glued to the gold body with an organic resin. figure 5. radiography of nose-ornament shown in figure 4, and behaviour of grey numbers in the interface au/ag. the grey number shows a sharp discontinuity between the ag hut and the area where ag is glued to au. then there is a discontinuity between this area and the au area. 0 500 1000 1500 2000 2500 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 c ts / ch an n e l energy (kev) cu k cu kb a u l  a u l b ag k ag kb a u l h au lg acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 6 1. silver sheets glued to gold sheets; this is the case of nose decoration paceb-f4-00002 (see figures 4 and 5), where the silver huts and disk are glued to the gold body; this is not a common case and was also observed only in nose decoration paceb-f4-00006. 2. silver and gold sheets brazed together; this is the case of nose decoration paceb-f4-00106 (figures 6 and 7) and peanut mb-8404 from the tombs of sipán (see figure 8), where the presence of additional silver in the au/ag interface was identified in the x-ray spectrum, and also can be hypothesized from several radiographic measurements; this is a common case, and was observed in many other nose ornaments. 3. silver and gold sheets joint together by using mercury amalgam; this is the case of nose decoration paceb-f400011 (figure 9), where the systematic presence of mercury, identified in the x-ray spectra of the au/ag interface, and only there, indicates the use of mercury for soldering together gold and silver. a test carried out by joining a gold to a silver foil by using mercury confirms this hypothesis. 4. it cannot be completely excluded that in a few cases a unique sheet on silver or on gold could have been employed, and a process of depletion gilding or silvering applied to the desired area. this possibility was considered in the case of nose decoration paceb-f4-00018, where attenuation of x-rays by silver and gold areas, determined by radiographic measurements, are similar. 4. conclusions from all measurements, following may be deduced on the forty nose ornaments: the moche artisans employed different techniques to join gold and silver: by gluing, by using an ag brazing alloy and by using a mercury amalgam; a simultaneous process of heating can never be excluded. figure 8. two peanuts on gold, from the royal tombs of sipán, code mb-9404 and mb-9405. each peanut is composed of two parts soldered with a ag-alloy containing cu and br. the same method was employed by the moche for soldering front and rear side of the famous mask mb-09398. figure 9. front side of nose ornament paceb-f4-00011, composed of two sheets on ag and au, joined together by using a mercury amalgam; along the red line in the upper figure, it was observed the systematic presence of mercury. figure 7. radiography of nose decoration paceb-f4-100106 (second ag-head from the left and interface gold-silver, see figure 4). the region between a and b corresponds to the soldering region, possibly carried out with an agalloy. figure 6. front and rear side of the nose decoration paceb-f4-00106 from the tomb of the lady of cao. it is composed of a body on a gold-alloy sheet, soldered on five heads on silver, which are further soldered to a half-moon sheet on gold. the soldering between the head in the middle and the upper gold shows a gap, which was covered in the rear side by a tape. acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 7 the gold sheets of the forty nose ornaments from the tomb of the lady of cao have a quite similar composition; the silver sheets of these forty nose ornaments have a completely erratic composition, with large differences also inside the same object. also thickness differences were observed; obvious deductions from above points are: the forty-two nose decorations were produced in different periods and by different artisans; this is not surprising, because it is likely that the lady of cao may have inherited several of the nose decorations, and that the others may have been made in different times; the moche possibly knew how to reproduce gold-alloys with the same composition. this consideration also derives from the previous point; the moche were possibly trying to yield silver alloys with special characteristics of visual aspect and/or resistance to oxidation, by putting relatively high concentrations of gold; the presence of gold in silver is almost constant, while its concentration is highly variable. this could indicate a continuous investigation to produce an ideal silver-alloy; the extraordinary metallurgic ability of the moche, at least in the period characterized by the tombs of span and cao must be finally pointed out. references [1] w. alva, sipan, descrubrimiento y investigation, quebecor world perù s.a: lima 2004. [2] w. alva, c.b. donnan, the royal tomb of sipán, los angeles fowler museum of cultural history; university of california: los angeles 1993 isbn-13: 978-0930741303. [3] r. franco jordan, in “señores de los reinos de la luna”, ed. krzysztof makowski, banco de crédito del perù: lima 2008, pp. 280-287. [4] r. franco jordán, los mochica: los secretos de la huaca cao viejo. lima, fundación wiese y petrolera transoceánica. s.a.: lima 2009. [5] r. franco jordan, pour la science 2010, 390, 8 paris, france. [6] r. franco jordan, la dama de cao. investigacion y ciencia: 6874, bercelona, spain 2011. [7] h. lechtman, new perspectives on moche metallurgy: techniques of gilding copper at loma negra, northern peru, american antiquity 1982 47(1) pp. 3-30, doi: 10.2307/280051 [8] d. schorsch, silver-and-gold moche artifacts from loma negra, peru, metropolitan museum journal, 1998, 33 pp. 109136. [9] r. cesareo, r. f. jordan, a. fernandez, a. bustamante, j. fabian, s. zambrano a., s. azeredo, r.t. lopes, g. ingo, c. riccucci, g. di carlo, g.e. gigante, analysis of the spectacular gold and silver from the moche tomb “señora de cao”, x-ray spectrometry 2016 45(3) pp.138-154, doi 10.1002/xrs.2680. [10] r. cesareo, a. bustamante, r. f. jordan, a. fernandez, s. azeredo, r. t. lopes, w. alva, . chero, a. brunetti, g. e. gigante, s. ridolfi welding brazing and soldering of gold and silver by the moche tombs “señor de sipán” and “señora de cao”, imeko international conference on metrology for archaeology and cultural heritage lecce, italy, october 23-25, 2017 [11] amptek inc., 6 de angelo drive, bedford, ma 01730-2204, usa. [12] r. cesareo, x-ray physics in la rivista del nuovo cimento, ed. compositori, bologna, italy, 2000. [13] a. markowicz, x-ray physics in handbook on x-ray spectrometry: methods and techniques, (eds: r. van grieken, a. markowicz), m. dekker inc, new york, basel, hong kong, 1992, chapter 1, isbn 0-8247-8483-9. [14] r. cesareo, g. buccolieri, a. castellano, r. t. lopes, j. t. de assis, s. ridolfi., a. brunetti, a. bustamante: the structure of two-layered objects reconstructed using edxrf-analysis and internal x-ray ratios, x-ray spectrometry (2015) 44 (4), pp. 233238 doi: 10.1002/xrs.2611. cyber-physical manufacturing systems: an architecture for sensors integration, production line simulation and cloud services acta imeko issn: 2221-870x december 2020, volume 9, number 4, 39 52 acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 39 cyber-physical manufacturing systems: an architecture for sensor integration, production line simulation and cloud services mariorosario prist1, andrea monteriú1, emanuele pallotta1, paolo cicconi2, alessandro freddi1, federico giuggioloni3, eduard caizer3, carlo verdini3, sauro longhi1 1 department of information engineering, università politecnica delle marche, via brecce bianche, 60131 ancona, italy 2 department of industrial engineering and mathematical science, università politecnica delle marche, via brecce bianche, 60131 ancona, italy 3 syncode s.c.ar.l., spin-off università politecnica delle marche, via brecce bianche, 60131 ancona, italy section: research paper keywords: industry 4.0; smart factory; osgi framework; cyber physical system; wireless sensor networks citation: mariorosario prist, andrea monteriù, emanuele pallotta, paolo cicconi, alessandro freddi, federico giuggioloni, eduard caizer, carlo verdini, sauro longhi, cyber-physical manufacturing systems: an architecture for sensors integration, production line simulation and cloud services, acta imeko, vol. 9, no. 4, article 6, december 2020, identifier: imeko-acta-09 (2020)-04-06 section editor: leopoldo angrisani, university of naples federico ii, italy received: october 30, 2019; in final form: january 17, 2020; published: december 2020 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: andrea monteriù, e-mail: a.monteriu@staff.univpm.it 1. introduction in the industry 4.0 era, smart factories are powered by cyberphysical systems (cpss) and cloud computing (cc) technologies. cpss using cc technologies enable smart objects (wireless or cabled) and software modules to both communicate and interact with each other. the european h2020 research agenda considers cpss to be ‘the next generation of embedded information and communication technology (ict) systems that are interconnected and collaborating, providing citizens and businesses with a wide range of innovative applications and services’ [1]. cpss applied to the fields of manufacturing or production are often referred to as cyber-physical manufacturing systems (cpmss) or cyber-physical production systems (cppss) [2] and are expected to increase efficiency, precision, and performance as well as bring unprecedented flexibility to the industrial manufacturing process. today, the automated manufacturing facilities are commonly based on hardware solutions from different vendors. this often leads to a heterogeneous and inconsistent mix of automation technologies, each one addressing one or more specific automation problems within the facility. while each of these single systems can acquire and transmit data, they are generally not designed to easily convey the available information to other production lines or manufacturing systems located within the abstract the pillars of industry 4.0 require the integration of a modern smart factory, data storage in the cloud, access to the cloud for data analytics, and information sharing at the software level for simulation and hardware-in-the-loop (hil) capabilities. the resulting cyberphysical system (cps) is often termed the cyber-physical manufacturing system, and it has become crucial to cope with this increased system complexity and to attain the desired performances. however, since a great number of old production systems are based on monolithic architectures with limited external communication ports and reduced local computational capabilities, it is difficult to ensure such production lines are compliant with the industry 4.0 pillars. a wireless sensor network is one solution for the smart connection of a production line to a cps elaborating data through cloud computing. the scope of this research work lies in developing a modular software architecture based on the open service gateway initiative framework, which is able to seamlessly integrate both hardware and software wireless sensors, send data into the cloud for further data analysis and enable both hil and cloud computing capabilities. the cps architecture was initially tested using hil tools before it was deployed within a real manufacturing line for data collection and analysis over a period of two months. mailto:a.monteriu@staff.univpm.it acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 40 same facilities or in another place, state or country. indeed, most of the actual automation systems are based on monolithic architectures with very limited communication ports and reduced local computational capabilities, which make it difficult to acquire data for further data processing. in recent years, the use of a wireless sensor network (wsn) as a bridge between the automation technologies at the production level and the cc infrastructure for data analysis [3] (i.e. control, coordination, supervision and management) has become common practice and has even been extended to different environments (e.g. smart grids, smart homes, etc.) [4]-[11]. consequently, the market is seeking optimised solutions based on industrial wsns to create parallel backbones to the programmable logic controller (plc) infrastructure in order to collect valuable data and send them to the cloud for further processing [12]. for this reason, cpmss have to be modular, dynamic, flexible, networked, and large-scaled. they are also increasingly based on software, which has become the largest and most complex aspect [13]. meanwhile, the software development environment needs to be somewhat different from that of standard embedded software given that cpss integrate computational capabilities and networks in addition to physical processes controlled by embedded computers. cpss provide abstractions and modelling, design, and analysis techniques for the integrated whole [14]. the interaction and data exchange between computers and physical systems requires new architectures and innovative design approaches [15]. in addition, due to the increased system complexity, it has become crucial to model and simulate the physical system, or part of it, to verify the new algorithms and to reduce the development and production times. finally, there is the cloud infrastructure. cloud solutions provide a powerful platform for harmonising incompatible connected devices. at the factory level, gateways are used to provide an intermediate layer between the proprietary protocols integrated in many automation systems and the standard internet protocols that are required to access the cloud. at the cloud level, different solutions are available for realising large-scale widespread software systems that elaborate data from different sources to control and manage real systems. thus, the cps-related challenges faced within the industry 4.0 paradigm can be summarised as follows [16]-[18]: a connected supply chain (csc) for real-time data analysis, new modular and dynamic software architectures, the capacity for testing software and hardware before actual deployment via simulation-in-the-loop (sil) and hardware-in-the-loop (hil) systems, wsns and data integration at the cloud level. in this paper, we focus on a cpps-related case study based on a real production line in order to address the challenging cps complexity. specifically, we present a new software architecture together with a cc layer. the cooja wsn simulator is used to simulate the data flow from a real production line, with the simulator integrated into a more complex system composed of a modular and extensible and interoperable software for managing heterogeneous devices based on the open service gateway initiative (osgi) framework. the cloud level is used to manage and analyse the data as well as to expose business data and services via an external interface. this proposed solution has various advantages. on one side, it allows for verifying the correctness of the wsn design using different simulated scenarios, while on the other, it allows for executing a cloud architecture stress test prior to commercial deployment. the remainder of the paper is organised as follows. section 2 reviews the relevant works in the existing literature before section 3 outlines the proposed architecture and how each part is integrated within the system as a whole. the osgi framework and the developed software modules for managing heterogeneous types of sensors, both real and simulated, are then described in section 4 before the cloud architecture is outlined in section 5, with a focus on data management and performance. the wsn for industrial appliances simulated in the cooja simulator and the gateway model for exchanging data with the cloud are then explained in section 6. the preliminary experimental tests and results are reported in section 7 before the paper is concluded with remarks and possible future works in section 8. 2. related works there exist numerous research papers on cps modelling techniques. here, a great deal of effort has been put into constructing compact and accurate mathematical models for complex processes, such as in [19]-[22]. in [23], the authors define a cyber-physical production system architecture, and a virtual factory is modelled and simulated using etri cps model language. the parallel development of computer science (cs) and information and communication technology on one side and of manufacturing science and technology on the other are described in [24], where the authors note the convergence of two worlds, that is, the virtual and the physical realms, in the field of manufacturing. a unified five-level architecture, namely, 5c architecture, is proposed by [25] as a guideline for the implementation of cpps for manufacturing applications. meanwhile, the current status, the expectations, the advancement of cpss in manufacturing and the related r&d challenges are described in [26]. however, only a small number of works demonstrate a simulation or a hybrid architecture of a cps oriented to industry 4.0, namely, cpms. for example, in [27], the authors focus on a collaborative function dimension and describe a list of cps challenges (e.g. the cps architecture of the towers of hanoi). finally, in [28], the authors describe an algorithm for managing the energy consumption in autonomous electric vehicles. generally, the state-of-the-art research related to cps applications deals with two different approaches, that is, topdown or bottom-up approaches to aspects related to the flow of control and monitoring [29]. top-down applications are concerned with the processes that are executable at a high level [30]. however, while they are widely adopted in the world of industry, the real industrial processes are bottom-up since they are concerned with hard-programmed and low-level devices [31]. an example of a bottom-up model was proposed in [32], with both event-driven and service-based models considered in the authors’ architectures. the early developments in industry 4.0 were based on top-down architectures, where the devices notified the events but the process control was defined at a high level [33]. moreover, in the initial stages, csp applications were also considered as part of a computational [34] integration for closing the feedback loop of physical process. nowadays, the implementation of an agent-oriented framework has led to a shift in the views on control loops [35]. an interesting bottom-up architecture was proposed in [36], where the authors also considered the hardware components such as the wsn, the storage units, and the computing system. however, the interoperability and the integration level must be improved to ensure a better and reconfigurable connection between each hardware component. a bottom-up service-based cps acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 41 architecture was proposed in [37], where the traditional architecture, composed of sensors, a physical platform, and a control system was combined with an internet service framework to manage the control tasks. while the two systems are fully integrated, the architecture does not provide an efficient and flexible solution that meets industry 4.0 standards. another cps architecture that follows a bottom-up approach is the eurocps that was the result of a european project [38]. here, the architecture is based on a networked collection of hardware platforms used to acquire information in every context (factory, agriculture, smart home, etc.). while the project is industry-4.0ready, it lacks the interoperability to integrate the existing industrial commercial solutions and the flexibility to use custom machine learning algorithms for each monitored production line. meanwhile, the top-down approach has been followed within the development of iso-iec architecture, which was proposed by the iso organisation and is based on a previously realised internet of things architecture [39]. the innovation here lies in the integration of the internet-service-oriented architecture with the virtualisation; however, the proposed solution is not lowlevel independent and the prevalent focus is on the business level. even if cps applications would appear to be replacing older supervisory control systems using bottom-up models, the existing applications reveal certain limitations related to the interoperability and modularity. while a top-down model has several limitations regarding the communication with single devices, bottom-up models can prove difficult to implement due to the different developmental interface, language, and protocols related to the sensors and each bottom-unit. within this context, a tentative solution is provided by the reference architecture model for industry 4.0 (rami 4.0) [40]. however, even if the rami 4.0 is used in different industrial contexts, it adopts the upc ua standard to communicate with the production lines, while most production lines do not currently use this standard. in the present paper, we present a new software architecture composed of a modular and extensible and interoperable software for managing heterogeneous devices based on the osgi framework along with a cc layer. the proposed architecture is not aimed at defining an industry 4.0 standard but at demonstrating how the integration of the osgi framework within industry 4.0 enables the system to be interoperable, extensible and flexible. as a solution, we analyse the employment of a gateway module for enhancing the interoperability and modularity following the osgi approach. 3. architectural approach this section provides a preliminary description of the modules comprising the proposed architecture for cps, which are as follows: • physical sensor network at the production line level • gateway • cc the proposed architecture can monitor a production line using a physical sensor network. the gateway sends data to the cc module, which includes features and functions for data storage, data management, analysis, simulation, and visualisation (figure 1). the proposed architecture does not perform direct control actions on the production lines; rather, it allows for performing monitoring tasks that can be used at the supervisory level or for the decision support systems. it should be noted that the information provided by this architecture may be also used at the control level but not in the low-level control field, where stability, reliability and synchronism are of utmost importance; these aspects are not the focus of our paper, and are well described in the literature related to sensor networks, as in [41]. the last module, labelled api tools, pertains to the collection of programming procedures and algorithms for providing the access and connection to cc, enterprise resource planning, material requirements planning, and other enterprise repositories, including the gateway, wireless sensors, and front-end applications such as apps and web-apps used for data visualisation [42], [43]. in the remainder of this section, we describe the modules of the architecture while highlighting the main characteristics they should possess but without a detailed description of their implementation (which are provided in sections 4 and 5). here, whenever the term ‘cps’ is used, it refers to ‘a cps compliant with the proposed architecture’. meanwhile, each of the following subsections first provide a description of the modules at the architecture level before providing a more technical overview of the actual implementation. 3.1. physical sensor network at the production line level the production line is the physical system that is monitored by a physical sensor network. the proposed cps system aims to integrate the physical system into a virtual environment. here, data are gathered by the physical sensor network connected to the physical system. data acquisition can be performed in two different ways, the first of which involves the use of a plc interface with industrial protocols, while the second involves the implementation of a wsn, which can be directly connected to the cc via the gateway. the approach proposed in this work provides both modalities, and the format of the file used for data exchange is the same in both modalities. this flexibility is required in view of enhancing the installation of a sensor network in different industrial scenarios [44]. in fact, in some cases, if the employed plc does not have external communication ports for data exchange, plug-in wireless sensors are suitable for the implementation of a cps system. a wsn is a simple solution in many cases where it is difficult to add new sensors to the electrical layout of a physical system, such as a production line. in fact, while the use of a wsn is suitable for the integration of physical sensors in the cc module, the remote control of the production line, after closing the feedback provided by data analysis in the cloud, requires connected machineries or plc interfaces, an aspect that is not considered in this paper. the implementation of the architecture presented in this paper considers a production line monitored by a wsn, and a gateway performing the bidirectional connection from/to the cc module. a wsn simulator was implemented to test the proposed architecture at the cloud and gateway levels prior to actual deployment in a production line. since the virtual nodes figure 1. block diagram of the proposed architecture. acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 42 are simulated with the same firmware and properties as the physical nodes, the wsn simulator can also be used to test the sensor network itself before physical deployment. a physical gateway was employed in the architecture for sharing data between the simulated network and the cc. however, this aspect requires the gateway to be developed in a flexible and modular way to perform a re-configurable cloud connection, involving both physical and virtual nodes in the loop, as detailed in the following section. 3.2. gateway the gateway (gw) is the embedded device that can connect the plc interface with the cc. the plc–gw communication is provided by drivers based on the involved specific protocols, such as the process field bus (profibus), process field network (profinet), fieldbus foundation, modbus, highway addressable remote transducer, controller area network, ethernet for control automation, and the industrial ethernet [45], [46]. meanwhile, the cc is based on internet connection through a wireless network, ethernet, 3g, or lte, etc. in this case, the protocols are based on the involved architecture, which includes representational state transfer or message queue telemetry transport [47]. however, the next generation plc can also send log files via file transfer protocol, email, or text message, etc. a modular programming approach of the gw system based on the osgi framework is proposed in our architecture (section 4), which allows for the integration of different types of sensors to be rapidly implemented. in fact, the connection with a new sensor can be performed through the addition of programming modules for data reading and for sending instructions. the modular programming of the gateway based on the osgi framework also allows for simulated models to be integrated into a gw system. the integration of simulations and simulated data is suitable in the early phase of the system learning, allowing for performing the implemented algorithms prior to installation in the physical production line. 3.3. cloud computing cc is a necessary information technology (it) instrument for the digitalisation of industry 4.0 factories [48]. the concept of cc includes the storage of data in an ‘external’ repository and services related to data analysis, simulation, and visualisation. therefore, within the context of smart factories, cc allows for digitising the data to be sent from the manufacturing processes, such that they can be stored and accessed by different smart devices [49]. in the proposed architecture, we present an http server that provides all the functionality required by the cloud service (section 5). the server contains the database with all the information on the connected facilities and their own sensor network. in addition, it provides an interface for any client-side application that may be required to manage values that have been stored over time. due to the complex queries on large amounts of data, a high level of performance is needed and a database that allows for efficient handling of time series data has thus been used [50]. 4. gateway the gw connects the production line to the cloud services by considering each device, be it physical or virtual, as a software module via the osgi framework. 4.1. the osgi framework the non-profit osgi was initiated in 1999 through the union of ibm, oracle, philips, sun, etc., with the aim of standardising the approach to modular application development in the field of home and industrial automation. today, the osgi technology has been used in vehicles, the final generation of mobiles, and in software, both in terms of desktops and servers [51]. from the programming point of view, the osgi present a modular system for java language, which defines the modality for creating modules and how they mutually interact during runtime [52]. the osgi architecture is composed of the osgi framework and a set of bundles. the osgi implements a centralised serviceoriented architecture with loosely coupled services. the advantages of using the osgi framework include platform and application independence, multiple service support, dynamic updates, service collaboration support, and security, with more details on this provided in [53]. 4.2. implementation details the application developed with the osgi framework is based on an extensible core to manage the dataflow related to multiple bundles that cooperate via the use of various services. the proposed approach provides a supervisor bundle and a collection of bundles that are related to sensors, devices, database management, data analysis, cloud communication and notification mechanisms. the supervisor bundle represents the active extensible core to integrate the sensors and notification mechanisms into the software platform. this core is also used for the management of security events related to the sensors, while it can also run notification events such as messages to the administrator, where threats are logged into the software platform. specifically, notification events are related to emails, push and text messages, alarms, etc. the management of the supervisor bundle can be also performed using a web-access that provides tools for local and remote bundle administration. it is important to note that while a message notification associated with specific events is logically sent to the final users, it will be physically sent via a cloud infrastructure. the gw creates the message with a specific payload and an alert tag that is interpreted by a dedicated cloud function. figure 2 shows the proposed bundle architecture, where sensors and devices are involved in the monitoring of any security-related events. in the proposed scenario, several combinations of sensors and devices are possible. for example, a sensor can be connected to a single device or to many, while it can be active, directly sending data to the supervisor, or passive, requiring the polling of the supervisor module for initiating the acquisition. the local database, which is used to save data from the production line sensors before figure 2: bundles architecture. acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 43 transmission to the cloud, consists of both static and dynamic tables. static tables include the collection of every device involved in the system, while dynamic tables contain data and statistics. the information stored in static tables includes the device’s id, credential accesses, and statistical indicators. this information is updated when two types of events are notified: a new cloud setting or a new device added to the osgi framework (in this case, a new bundle is installed in the software). meanwhile, dynamic tables are updated during runtime. the approach proposed in this paper provides the direct transmission of the production data to the cloud computing module. production data contain every variable related to the production process monitored by the sensor network. therefore, while every raw data is sent and stored in the cloud repository, the statistical information is first elaborated by a local machine before being sent to the cc for further elaboration and analysis. static tables also include information on the processing time for statistical analysis. here, an asynchronous scheduler integrated within a dedicated bundle was used to implement this functionality. this scheduler can also both schedule commands to run after a given delay and periodically execute the production data processing, which increases the computational efficiency. the number of tasks is directly related to the number of different devices installed within a given production line and to the number of indicators to be monitored. when multiple tasks are needed, or when additional flexibility or capabilities are required, one thread is adopted to manage the execution timer and check the tasks to be run at every second. a thread starts only if there is an indicator to be calculated and, as such, the complexity and the number of threads in the runtime are reduced. in addition, the data processing has to be performed with some consideration of a consistent time interval for the extracted information. for example, while it is important to monitor the power consumption during the day, it is also important to observe the status of the production line in terms of the true performance of the equipment’s productivity. in these cases, the task can be executed daily, weekly and monthly in order to verify the correct trend. the proposed bundle architecture for the asynchronous scheduler consists of three levels: scheduler, dispatcher, and threshold. here, the scheduler module runs a thread when each event is verified, while the dispatcher module decides whether the current event has to be associated with a specific indicator and notifies all subscribed services to run the related data processing. meanwhile, the threshold module executes the elaboration process. when the gw starts for the first time, it must be identified within the cloud infrastructure using its authentication key. the cloud login allows for all setting parameters to be downloaded and saved to the local machine. here, the settings contain information related to any alerts and the devices to be elaborated. the supervisor module manages the login of the cps. the sequence of all handshake phases such as the gw setting requests are related to the first installation of the software platform within an enterprise’s system. a front-end web-based interface was implemented to perform the registration of the gw related to a production line in the cloud. the bundles, which manage the external connection with the production line, were tested via a bench test using a real plc and within a simulated scenario using a wsn as part of an hil architecture, as detailed in section 3. the development of each bundle was implemented using the eclipse rich client platform, which is a multi-language software development environment for implementing general purpose applications. finally, the hardware employed in the research described in this paper is an advantech embedded pc. it provides two usb ports (one for the integration of the bench test and one for connecting the external hardware with the cooja simulator for hil) and two ethernet ports (one for the internet connection and one to integrate industrial communication protocols such as profibus or profinet). the osgi framework, the attendant bundles, and the wsn simulator were implemented in this system. 5. cloud computing architecture an http server provides all the functionalities required by the cloud service. here, the communication is handled through http requests that conform to a pre-specified rest application programming interface (api). the cloud service is comprised of various software components that receive, store and analyse the data streams created by the sensors. it hosts the databases that contain the information on the users, associating them with their own sensor network. in addition, it provides an interface for client-side applications that wish to access the data that have been stored over time. 5.1. origin of the information the cloud service receives data through a pre-determined rest api. any external component that complies to the api is able to use the entire storage and analysis pipeline. this means that the data acquired by the underlying system will be treated the same, regardless of whether the system is real or simulated, enabling usage of the system by both real and simulated components at the same time. the behaviour of the cloud service is consistent for both types of component, provided the api is respected. however the data is acquired, it is ignored by the service, as it is not important for the following steps. 5.2. forwarding once the data has been acquired, a complete http post request can be built and executed with the data as the payload. the server is always ‘listening’ for these requests, waiting to execute its pipeline on the incoming data. the server also stores all the information regarding the users of the system and their devices inside a mysql database. the information will be used to handle each request according to the user and the device that produced the event, to decide, for example, where to store the newly received value. due to the geographical replication of the database, the system becomes scalable, as the requests can be offloaded to the server that is nearest to the sender. 5.3. persistent storage all the incoming values will have an associated timestamp. in fact, each sensor will generate a constant stream of values and timestamps, which can be referred to as time series data. for this reason, a database that allows efficient handling of time series data is required, one that also maintains a high level performance while executing complex queries on large amounts of data, which arises especially when the sensors operate at a high sampling frequency. moreover, the database must be intelligent enough to cope with large amounts of constant data, which are typically generated by binary or low-resolution sensors that monitor processes operating at a steady state. mongodb and cassandra are the most likely candidates here, largely due to their key-value acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 44 nature, which ensures empty values are not stored on the disk. repeating values can then be mapped onto the empty values to save huge amounts of memory. as shown in table 1, while both are schema-free nosql databases, they differ in terms of the method they use to physically store the data. mongodb is a document store database, storing all data in json documents on the disk, while cassandra is a wide column store, which means that any values with the same partition key are stored on the same row. here, cassandra was chosen over mongodb after comparing the scalability of the two systems. in fact, mongodb is more efficient with small amounts of data, while cassandra outperforms it when large datasets are involved [55]. since handling large amounts of data is vital to the system, cassandra was clearly the best option. in addition, cassandra does not follow a master-slave type architecture, making the addition of a new node as simple as creating and connecting it to the rest of the nodes. as the data is replicated across the nodes based on cassandra’s configuration, it is also less likely that any catastrophic failure could result in the total loss of data. 5.4. primary keys in cassandra unlike many other database management systems, cassandra uses the primary key, which is composed of partition and cluster keys, to decide how to physically store the data. in fact, in selecting the timestamp as a clustering key, the data will be physically ordered on the storage based on the date the value was received. this ensures any query requesting the latest available is deal with rapidly, even when huge amounts of data are involved. to further improve the speed of more complex queries, a partition key that separates each day’s worth of data was used, which ensures even time range queries are scalable, regardless of the size of the database. to verify the effectiveness of the adopted solution, an experimental test was carried out to compare the execution time performance and the memory usage of mongodb and cassandra when saving raw data from production lines. we used a droplet model (ssd-based virtual machines) from digital ocean with an installed ubuntu server (14.04 64 bit), 2 gb ram, 40 gb of hard disk, and two virtual cpus (vcpu) (a virtual processor, which is a physical central processing unit [cpu] that is assigned to a virtual machine). we assumed the monitoring of 10 production lines with an acquisition time of 1 s and a window acquisition time of 1 h. in this scenario, 3.600 (one production line), 10.800 (three production lines) and 108.000 (thirty production lines) records were collected and sent to the cloud to update the cassandra and mongodb databases. mongodb version 3.0 and cassandra version 2.2 were used. the results are shown in table 2, where it is highlighted that, with an increased number of records, cassandra has a more stable performance than mongodb, with a two-fold improvement, while for small amounts of data mongodb, has better time performance but consumes a great deal of memory, as shown in table 3. 5.5. data visualisation the rest api provided by the server not only regulates data acquisitions but also enables the development of client-side applications to visualise the values permanently stored on the table 1: comparison between cassandra and mongodb [54]. name cassandra mongodb description wide-column store based on ideas of bigtable and dynamodb one of the most popular document stores available as a fully managed cloud service or for deployment on selfmanaged infrastructure primary database model wide column store document store secondary database models search engine license open source open source implementation language java c++ data scheme schema-free schema-free secondary indexes restricted yes sql sql-like select, dml and ddl statements (cql) read-only sql queries via the mongodb connector for bi apis and other access methods proprietary protocol thrift proprietary protocol using json server-side script no javascript triggers yes yes partitioning methods sharding sharding replication methods selectable replication factor master-slave replication mapreduce yes yes consistency concepts eventual consistency immediate consistency eventual consistency immediate consistency foreign keys no no transaction concepts no multi-document acid transactions with snapshot isolation concurrency yes yes durability yes yes in-memory capabilities no yes table 2: the calculated time to write key-value pairs. 3.600 10.800 108.000 cassandra 1.73 ms 3.51 ms 28.22 ms mongodb 1.06 ms 2.89 ms 15.16 ms acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 45 server. once the client conforms to the rest apis provided by the cloud service, it can execute a certain number of queries on the stored values, the results of which will then be visualised through a template that can be chosen by the user themselves. a limited number of templates is provided for various purposes, such as variable visualisation, comparison with another unrelated variable, or simply the display of the current state of the variable. every instance of a template connected to a variable is referred to as a ‘widget’ (see figure 3). in addition, the client application allows for the definition of the notification rules for each variable, with these rules checked by the server whenever a new value is received. if the notification rule matches the newly received value, the user who created it is alerted via the specified message. in short, the conditions are all at the user’s discretion. 5.6. data analysis two types of analysis are possible in this system: online and offline analysis. to execute online analysis, the algorithms should be constantly running on a dedicated server for each data stream currently in use. for each new value (or window of values) the algorithms will update their state and will generate notifications if needed. for example, one of these algorithms could be tasked with sending an alert whenever a device deviates from its normal behaviour due to a fault, failure or malfunction. these algorithms provide insight into the current state of each device in real time, making it possible to detect problems before they negatively affect the system performance, according to the so-called ‘preventive maintenance’ paradigm. meanwhile, in terms of offline analysis, after the system has been running for a certain period of time, there will be large amounts of data that describe the behaviour of the physical or simulated system. the analysis of this data could provide information on the efficiency of the process as a whole, which can then be used to make better decisions. due to the nature of the problem, the amount of data to be analysed will be large, which could involve prohibitive amounts of execution time. to resolve this issue, so-called map-reduce methods are required, which improve the performance even when the algorithm needs to go over all the stored data to reach a useful conclusion [56]. the algorithm is run in parallel on each node of the cluster on a different part of the data, and the results are then unified to provide the results of the algorithm as a whole. this is only possible because the chosen database management system provides apis for user defined map-reduce methods. 6. data management at production level: testing and debugging a production line can be modelled using a set of inputs and outputs, be they digital or analogue. an embedded pc or plc is largely involved in a production line in terms of controlling all operations. generally, the controller of a production line system, which is based on a plc, reads input data at every fixed timestep and saves them to its memory. the data temporally stored in the memory are elaborated, and the results of the logical or mathematical operations are exported to other memory locations. following this, another thread reads the values at each memory location and sets the physical output. the same approach also works if variables are acquired via fieldbus. however, some plc devices do not have a communication interface for integration with external applications, while some have proprietary protocols that hinder their integration into software platforms for industry 4.0. in contrast, many recent plcs can integrate external applications using standard and known protocols. this scenario continues to characterise the automatic machines market, with old-style machines without any communication interface on one side, and new machines that provide advanced interaction features on the other. as stated in section 3, the presence of a wsn that runs parallel to the cabled sensor network in the production line and is connected to the gw (and thus to the cloud), opens up new hil scenarios. the proposed approach allows data to be acquired from the production line without limitations related to the plc device employed in the system. once the specifications are available for the specific industrial process, then the wsn must be designed in view of acquiring the variables needed to comply with the specifications (e.g. providing certain performance indexes). then, according to the proposed architecture, the compliant osgi bundle can be developed, such that the data from the sensor network can be parsed in the gw, while the same applies to software wireless sensor nodes. as such, the information from the production line is stored in a cloud database, where analytics are developed and performed. finally, according to our proposed architecture, the information that is compliant with the specifications is sent back to the production line, where it can be used for decision support and/or automated supervision. as is clear, our architecture is both system independent and flexible, thus allowing for dealing with different industrial processes with the same functional blocks. moreover, testing and debugging at both the cloud communication and the gw system levels can be enabled prior to deployment. at the cloud communication level, a software module has been developed to generate random data packets that are sent/received for the testing of the cloud communication. this test is necessary for evaluating the communication performance, including in terms of response time. at the gw level, a wsn simulator has been implemented for testing and debugging the system behaviour in cases such as delay and loss of information. the proposed test case uses the cooja simulator to reproduce the behaviour of a wsn with different nodes. in this test, the production line and its outputs are simulated by the wsn (software part), and the gw and the cloud communication (hardware part) are tested prior to connection to the production line within a hil scenario. it should be noted that in the simulated wsn, virtual nodes are simulated with the same firmware and properties of the physical nodes, which means it can be used to test the sensor network prior to actual deployment on a real production line. the schemes for the two tests are presented in figure 4. figure 3: example of a single value widget. table 3: memory usage for write operation (mb). 3.600 10.800 108.000 cassandra 6.04 8.1 212.16 mongodb 63.56 281.02 373.36 acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 46 6.1. testing and debugging of the cloud communication this section describes the first level of simulation, which relates to the generation of data to be sent to the cc for the application testing and debugging. specifically, the osgi bundle is proposed and analysed in view of describing the communication formalism. the proposed module creates a background service application that initially connects to the cloud using the login access to download the configuration filesetting and the variable representation. this service application generates random data for saving on a memory buffer. a synchronised service-thread can read data from the same buffer and format them into a json data packet that will be sent to the cc. as is described in the next sections, this json packet contains information related to the production line, the reference to the plc memory location, the acquisition timestamp, and the set of each reading. each data can be represented by the same variable type used in plc programming, such as word (2 byte), double word (4 byte), byte, and int e char, while for the float type, a custom interpretation is used where the data is separated into two parts (e.g. the type f5.3 is formed by five digits for the integer part and three for the decimal part). using the proposed data simulator, it is possible to simulate the interconnection of many production lines to the cc system. this approach also allows for testing and debugging the cloud features prior to the development of a physical production line. the performance of the cc in terms of response time, elaboration time, and data saving can be evaluated and compared with different database solutions such as mongodb and cassandra. 6.2. testing and debugging of the gateway system in this section, we describe the testing and debugging of the gateway system using a working example involving a small-scale cps prototype. a cooja wireless sensor network simulator was used in the cps prototype, while the system as a whole is composed of the following: 1) osgi software bundles to manage the connection with the simulated wsn, as described in the previous section. 2) a gw (pc embedded) used to interconnect the production line with the cloud and other machines (see section 4). 3) a wsn simulator, namely, cooja, and 10 simulated nodes that exchange data using the collect tree protocol (ctp): a. five simulated nodes used as a repeater or wireless extender, b. four simulated nodes with hil (interface with real i/o), c. one simulated node used as a base-station, d. an hil bench test. this scenario is represented in figure 5, which shows the relationship between the bundles running in the osgi framework and the simulated infrastructure based on cooja and the hil model. the wsn simulation was implemented using a cooja cross simulator, which is an emulator of the programming code of the real nodes of a physical sensor network [57]. the cooja simulator allows certain features, such as the transmission and connections, to be evaluated using virtual experiments. this activity is suitable for the debugging of the entire firmware since it allows for any possible problems and bottle necks to be identified prior to the release of the firmware and the hardware deployment. sky motes were used as nodes to reproduce a sensor network in the proposed simulation. the advanced sky gui plugin, an extension of the contiki cooja for sky motes [58], was modified and used. this new plugin includes additional features such as the option to save i/o messages into a log file in addition to the standard functionalities (user button, leds pin, serial communication, eight adc ports and a virtual joystick). this feature is necessary to reproduce the behaviour of the basestation node when it saves a radio-message from the nodes to a log file. the cooja simulation workflow also includes the osgi bundle, which reads this log file and sends data to the cloud computing system. figure 6 shows the wsn designed using the cooja-based simulator. this network includes 10 sky motes as wireless sensor nodes. the communication protocol is based on a stable version of ctp, which provides a datagram routing layer used to gather data and the tree routing for the higher level routing [59] and the transport protocols [60], [61]. each node can be a candidate for the base station that initiates the data collection. every analysed configuration considers 1-node as the reference node of the figure 4: the two simulated scenarios. (a) left: testing and debugging of cloud communication and database performances using random data. (b) right: testing and debugging of cloud communication and database performances using wsn. figure 5: simulation test architecture. acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 47 wsn network, where the proposed plugin is run to save the data collected from other nodes on a file. the log file content is already formatted following the json format; thus, the osgi bundle can directly send data to the cc module for further analysis. the data described above represents the states of a production line. 7. pilot case and preliminary results as described in section 6, preliminary testing and debugging at both the cloud communication and gw system levels can be performed using simulated data. these data can be generated randomly (cloud communication test) or via a wsn simulator (gw test). in the latter case, the wsn can also be used to test the network capabilities prior to implementation in a real production line. in this section, the focus is on the pilot case of a real industrial automation system. various tests were conducted to demonstrate the feasibility of the architecture in terms of the functionality of the osgi framework, correct storage of information into the cloud, and the definition of services through data analysis techniques. 7.1. pilot case ctf automazioni of matelica (italy) – an italian company leader in the design and construction of special machines, robotised production lines and automatic assembly islands – was monitored for a period of two months via the architecture proposed in this paper. the main focus was on the automatic line for the production of metal filters (aluminium, stainless steel, etc.) designed for sifim srl, an italian company leader specialised in the production of metallic filters and in the realisation of quality aesthetic metallic components for household and industry and community kitchen appliances (restaurants, hotels, canteens, etc.). in terms of the production line, the machine used as a pilot case produces metal filters for the domestic appliance sector (see figure 7). the sifim production line can be divided into five functional blocks associated with the five production phases, as shown in figure 8: a. the decoilers on which the metal coils are positioned will be used to assemble three cutters, which will process the sheets with a maximum dimension of 1100 × 1100 mm2. b. the robot will pick up the cut sheets and place them on a table, creating a stack of 13 sheets. c. a manipulator will take over the stack and will secure the fourth angular cutter that will scrape the 1100 × 1100 m2 sheets in the required format. figure 6: the cooja cross simulator. the simulated wsn and the new version of the advanced sky gui plugin. table 4: communication settings: standard format. start length type measure key description 0 2 w cust00 client code 2 2 w lin000 code line 24 20 c codprod product code 44 4 d 1/10 s tmpcod cycle time 48 1 b start run 49 1 b stop stop 50 1 b mnl manual 51 1 b em000 emergency 52 2 w al000 alarm code 54 2 w npzok good items 56 2 w npzko discarded items 60 4 d cycles ba_001 motor siemens 1fk7105-2af71-1qa0 76 4 d cycles ba_002 cylinder smc cdq2b40-100dz … … … … …. figure 7. sifim’s production line. acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 48 d. the sheets processed by the cutter will be positioned on a linear axis made up of several trolleys that will be used to assemble the series of presses that will make the bevel on all sides of the filter mat, including on the part reserved for the handle. e. finally, a 3-axis manipulator will stack the finished product on an empty pallet which, once filled, is unloaded onto the discharge strip for the finished product. the environment and the production line structure presented the following integration problems: • presence in the production area of an ethernet connection. • the production line has to mount a pure plc and not a panel pc. generally, a panel pc is an embedded pc with a touch panel that integrates plc runtime and visual software. in this case, to avoid interference between plc runtime and gw client, a separated solution was adopted. • integration with a siemens plc s7 series. this type of plc was selected since it is one of the most commonly used plcs in production lines, while the plc used for the demo (panasonic) is essentially installed for small applications; serial rs232 communication is not the most stable bus solution for the transmission of data in an automation context. • availability to integrate the gateway on the electrical panel near to the plc. • different type of sensors. • process identified by the state machine, meaning it is possible to match the working state and the process variables. • process stability. the production line has to work in production mode and not in test mode. • fixed work shifts. the evaluation of performance indexes starts from the definition of the range times in which the machine should work. the main objectives to be achieved during the test stage were: • monitoring the production line state. • integration of new bundles in the osgi framework and a new communication protocol in a plug and play mode. • data transmission policy (acquisition from plc and sending to the cloud rate). the main variables for estimation during the monitoring stage for performance index calculation and maintenance support were as follows: cycle time, average cycle time expected, product code, start, stop, alarm, emergency, good object, discard object, principal actuators installed (motors, pumps, etc.). in order to standardise the settings used by the bundle to access the production lines memory block for data reading, a standard format was used as shown in table 4. in terms of software integration, this stage required developing a new osgi bundle that can, on the one hand, communicate with the coordinator bundle and, on the other, manage the communication between the siemens plc and the gateway. the objective was to validate the concept of extensibility, flexibility and interoperability of the developed system and to define a standard for the line configuration process in order to replicate the same solution on other new lines but with different plcs and communication protocols. the flexibility of the architecture due to the osgi framework offers the possibility of developing one or more bundles associated with the new production line for integration. the developed osgi bundle differed from that managing the panasonic plc in terms of the communication protocol, which, in this case, was the s7comm. the bundle periodically retrieves plc data, saves them to a buffer and then sends a complete packet containing all the buffer content to the cloud. the scanning time is defined as 1/3 of the machine’s ideal cycle time related to the product code that is in production, while the data transmission time to the cloud is three times the cycle time. all parameters can be configured from the cloud and downloaded to the gw during the device authentication phase or when a changing configuration occurs. 7.2. analytical aspects the possibility of evaluating the consumption of the production line’s critical components, monitoring the performance indicators, diversifying the causes of the downtime via the functional area and systematically identifying where more stop problems are created, allows for a reduction of the losses. in order to analyse the data and the results of the computational analysis, an ad hoc web user interface was developed. specifically, two main sections were defined, the first of which relates to the efficiency indicators, while the second is used for the management of the component consumption rules. the part associated with the stop causes is directly shown on the overall equipment effectiveness (oee) web page since a direct visual inspection allows for better identifying the amount of job loss occurring as a result of the stops and for grasping the direct link between the loss trend and the causes. in this first phase, two data analysis algorithms were implemented: one for the analysis of the efficiency indexes and one for the cycle time analysis for the of micro stop calculation. in terms of efficiency indexes, the analysis consists of quality rate, utilisation level of the equipment, efficiency, availability, and oee, which are measures of the manufacturing operations performance and productivity, expressed in percentage. the oee indicates the degree to which a manufacturing plant is truly productive and serves as a general and inclusive measurement of how well a company’s manufacturing operations are performing. cycle time analysis relates to how, for any manufacturer, the idle time that occurs during production results in a loss of money. work stoppages reduce the productivity rates and increase the cycle times. identifying micro stops or reductions in the job speed is one of the most difficult parts of the monitoring process. in order to handle this type of loss, cycle time analysis can be used. in the developed osgi bundle, an automatic data gathering was implemented to enable the cycle time measurement. by comparing all the real cycles against an ideal cycle time and figure 8. computer-aided design schema of sifim’s production line. acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 49 through matching the data of the micro stops, a total loss due to speed reduction could be calculated. 7.3. results the pilot case was monitored over a period of two months, from april 2018 to may 2018. during this period, all the macro activities – such as stoppage, efficiency reduction, set-up/adjustments, and quality impairments – and the micro activities – such as the plc variable state (start, stop, cycle time, etc.) with a resolution of 10 seconds – were stored in the cloud for analysis. the production analysis consisted of computing and monitoring the following indexes: quality rate (qr), efficiency (e), availability (a) and oee, which are measures of the performance and productivity of the manufacturing operations, expressed in percentage. in brief, the above indexes can be defined as follows [12]: 𝐴 = 𝑁𝑒𝑡𝑂𝑝𝑒𝑟𝑎𝑡𝑖𝑛𝑔 𝑇𝑖𝑚𝑒 𝑂𝑝𝑒𝑟𝑎𝑡𝑖𝑛𝑔 𝑇𝑖𝑚𝑒 ∙ 𝑃𝑙𝑎𝑛𝑛𝑒𝑑𝐷𝑜𝑤𝑛 𝑇𝑖𝑚𝑒 ∙ 100 % (1) 𝐸 = 𝐼𝑑𝑒𝑎𝑙𝐶𝑦𝑐𝑙𝑒 𝑇𝑖𝑚𝑒 ∙ 𝑇𝑜𝑡𝑎𝑙𝐶𝑜𝑢𝑛𝑡 𝑁𝑒𝑡𝑂𝑝𝑒𝑟𝑎𝑡𝑖𝑛𝑔 𝑇𝑖𝑚𝑒 ∙ 100 % (2) 𝑄𝑅 = 𝐺𝑜𝑜𝑑𝐶𝑜𝑢𝑛𝑡 𝑇𝑜𝑡𝑎𝑙𝐶𝑜𝑢𝑛𝑡 ∙ 100 % (3) 𝑂𝐸𝐸 = 𝐴 ∙ 𝐸 ∙ 𝑄𝑅 (4) where netoperatingtime is the time for which the equipment was available for operation, operatingtime is the total calendar period for the oee calculation, planneddowntime is the scheduled maintenance time, idealcycletime is the theoretical minimum time to produce one part, goodcount is the produced parts that meet the quality standard (without reworking), and totalcount is the total of all produced parts (including defects). the total production output, the losses, and the operating time are listed in table 5, while the effectiveness indexes and the oee values are shown in table 6. finally, the total number of stoppages and the attendant causes within the total time the production line was stopped were analysed in the cloud using the cycle time analysis, with the results recorded in table 7. the results of the preliminary tests indicated that through using the proposed infrastructure for the management of the data, the cloud and the physical devices, it is possible, via a simple method, to acquire all the information associated with the production line, including production quality (goods vs. discarded objects), cycle time, start, stop, emergency, alarm and device status (e.g. inverters, motors, etc.). all the data in the cloud was used to define tailored services for the firm. as an example, the data and signals from the plc and the operator shifts (see figure 9) could be merged to provide a consistent job activity, which will be useful for the performance index estimation, while, at the same time, support for the production line maintenance was proposed using the abc classification to identify the stop causes. 8. conclusion and future work the fourth industrial era is taking the automated factory to a whole new level through the introduction of modular, extendible and customised mass production technologies. this means that production lines or machines will be integrated in all management processes to ensure independent operation or cooperation with other machines or humans, with the main objective being to realise and optimise a fully customer-oriented production while constantly maintaining its condition. following the paradigm of the new smart factories, the main challenges related to cps complexity were addressed in this paper through developing both a new modular, extensible and interoperable software architecture based on the osgi framework for managing heterogeneous devices, simulation and hil prototype and a cc system for managing and analysing data as well as for exposing business data and services via external apis interfaces. specifically, the testing of a smart production line prior to deployment was developed based on two levels of analysis. the first allowed for the development of an osgi software module for generating random data packets that will be table 5: april-may 2018. manufacturing line data. data type line a operating time 232 h 0 m 0 s set up and down time 28 h 41 m 40 s planned down time 0 h 0 m 0 s net operating time 203 h 18 m 14 s average cycle time 0.76 m number of good units 16148 rejects 0 table 6: efficiency indexes for april–may 2018. efficiency index line a actual availability 87.6 % performance efficiency 82.1 % rate of quality 100.0 % oee 72.0 % figure 9. comparison between operator shift (left) and real production line activity (right). table 7: april-may 2018. micro stoppage analysis. data type line a robot 989 m generic 546 m decoiler 4 95 m manual 53 m decoiler 3 21 m decoiler 2 18 m decoiler 1 0 m total 1680 m acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 50 sent for the testing of the cloud communication system and the database performances. meanwhile, the second was realised on a simulated wsn using a cooja cross simulator integrated with an hil infrastructure. the proposed cps architecture was tested in terms of a pilot case, the sifim srl production line developed by ctf automazioni srl. here, the performance was evaluated, while high-level cloud analytics were developed to exploit the versatility of the cps to produce a proactive maintenance scenario. eight weeks of data, collected during april and may of 2018, were used to undertake the analysis of the production line performances in terms of oee and machine failure. these analyses will prove useful to improving customer care and maintenance management and for evaluating the structural improvements to new production lines. further tests and functional integrations will be carried out in the future to enhance the performance of the proposed system. acknowledgements special thanks go 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high-voltage power lines; induced currents; cst simulation citation: marius valerian paulet, alexandru salceanu, constantin dan neagu, dragos florin bordeianu, on the coupling influence of the relative position of human trunk with respect to the overhead high-voltage power line, acta imeko, vol. 9, no. 3, article 9, september 2020, identifier: imeko-acta-09 (2020)03-09 editor: jan saliga, technical university of košice, slovakia received february 20, 2020; in final form march 23, 2020; published september 2020 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: alexandru salceanu, e-mail: asalcean@tuiasi.ro 1. magnetic and electric coupling between overhead high-voltage power lines and the human body high-voltage (and also high current) power transmission networks are sources of considerable magnetic and electric fields. specifically, these are voltages of hundreds of kv, generating significant electric fields and currents above 1000 a, which produce magnetic fields that should be taken into account. most of the relevant concerns relate to the influence that these fields could have on living matter. shielding (and, implicitly, protection) from electric fields is easier to achieve than from magnetic ones. any „shell” (be it very thin or with numerous apertures) with conductive properties can play the protective role of a faraday cage. matters are more complicated in the case of magnetic fields, which are associated with moving electrical charges. the techniques for reducing the possible inductive coupling are few and expensive. those that do not impose unacceptable costs or dimensions are primarily based on increasing the distance from the source. in romania, the national power transmission network, operated by the national company ‘transelectrica’, measures approximately 9000 km of overhead power lines (the vast majority being 220 kv and 400 kv lines) and 81 power stations (approximately the same parity, 42 of 220 kv, 38 of 400 kv, and only one of 750 kv). in addition, thousands of kilometres of 110 kv overhead power lines (managed by various distribution companies) should be also considered. of course, 110 kv represents the theoretical, conventional ‘boundary’ between medium and high voltage, but due to the lower height of the supporting pylons and the greater proximity to the inhabited areas, the 110 kv lines must be also included due to their possible harmful effects on health in the same category as 220 kv, 400 kv, or even 750 kv lines. abstract in this article, we present our study of the relevance of the relative position of the human torso with respect to the plane of the supporting pylons of various overhead high-voltage power lines from the perspective of currents induced by the generated magnetic fields. a homogenous model of the human body that considers the trunk as an elliptic cylinder has been developed in cst studio software. due to the necessary theoretical brevity, the results obtained by the simulation for the loop currents along the perimeters associated with the large (28 cm) and, respectively, the small (16 cm) axis of the elliptical cross-section of the trunk are presented and discussed. simulations have been performed for five relative positions (0°, 30°, 45°, 60°, 90°) of the human trunk with respect to the plane of the transmission towers and for two types of symmetric, double three-phase networks (110 kv and 640 a, 220 kv and 960 a, respectively), with the normal supporting poles sn 110252. we present the solutions and the selected boundary conditions with the aim of using cst software in the domain of industrial frequency. the obtained results make it possible to formulate recommendations on the reduction of human exposure to magnetic fields. mailto:asalcean@tuiasi.ro acta imeko | www.imeko.org september 2020 | volume 9 | number 3 | 54 values that are not considered dangerous and therefore acceptable, for current densities induced in the human body by electric fields with frequencies lower than 100 khz (capacitive coupling) or by magnetic fields with the same frequency spectrum (inductive coupling) are known in the literature under the generic name of ‘basic restrictions’. these values have mainly been established by considering the instant, short-term health effects: the excitation of nerves and muscles. the human body is electrically non-homogeneous; therefore, the values of the basic restrictions are expressed in the form of the current density obtained by averaging the current over a 1 cm2 cross-section, perpendicular to the current flow. a complicated and debatable issue is setting the correspondence between the electric and magnetic fields that exist in the surrounding environment (the so-called reference levels) and the current density actually induced in the human body. numerous models based on the human anatomy (often based on magnetic resonance imaging, mri) have been developed with millimetre resolutions. using these models as well as numerical methods of solving (partial) differential equations (the finite element method or finite difference method), the currents induced in the human body model by the electric and magnetic fields in the immediate vicinity of the human subject can be calculated. we must then compare the value of the current density induced in various organs with the typical limits accepted by the standards [1]: 10 ma/m2 (professional, occupational exposure) and only 2 ma/m2 (residential, public exposure). since coupling mechanisms are fundamentally different, the vast majority of simulation/modelling programs have different modules for low and, respectively, high frequencies. moreover, at a low frequency, there are two different solvers: one is (quasi)electrostatic and is recommended when the dominant field is the electric one, and the other is (quasi)magnetostatic and suitable for the dominant magnetic field occurrence. here, we must specify the criterion that allows us to determine the nature of the dominant field because due to the different units of measure, v/m and a/m, it is not possible to compare them directly with each other. what could be compared are the associated volumetric energy densities, expressed in j/m3: 𝑊𝐵 = 𝐵 ∙ 𝐻 2 = 1 2 𝜇0𝐻 2 and 𝑊𝐸 = 𝐷 ∙ 𝐸 2 = 1 2 𝜀0𝐸 2 . (1) in the case of high-voltage power lines, the problem of the nature of the dominant field is of particular importance. we have both very high voltages and very high currents. in the case of the equivalence of the spatial energy densities of the two fields, we obtain the well-known value of the wave impedance [2] as the ratio between the electric field strength and the magnetic field strength (in free space): 1 2 𝜇0𝐻 2 = 1 2 𝜀0𝐸 2 ⇒ 𝐸 𝐻 = √ 𝜇0 𝜀0 = √ 4π ∙ 10−7 1000 36π 10−12 ω = 120 π ω ≃ 377 ω . (2) if at the point of interest, the e / h ratio is greater than 377 ω, the dominant field is the electric one, but if this ratio is less than 377 ω, the dominant field becomes the magnetic one. the electric field strength is proportional to the scalar electrical potential and is higher if electric charges are nearer to the measuring point. some approximations, which are very close to reality, should be made, especially for 50/60 hz frequencies (the so-called mains frequency), which fall under the extremely low frequency category. most important for the (quasi)electrostatic approach is neglecting the time respect variations of the electric flux density in the ampere law (written in harmonic mode). in this way, the influence of the electric field on the current density in conductors in general and in the human body in particular can be properly modelled. ∇⃗⃗ 𝑋 �⃗⃗� = 𝐽 + 𝜕�⃗⃗� 𝜕𝑡 = 𝜎�⃗� + 𝜀 𝜕�⃗� 𝜕𝑡 = 𝜎�⃗� + 𝑗𝜔𝜀�⃗� = (𝜎 + 𝑗𝜔𝜀)�⃗� (3) around the industrial frequency spectrum, the displacement current in the human body has a much smaller influence than the conduction current. for the human body at the 50 hz frequency, the conductivity σ is about 0.2 s/m, and the relative permittivity r is approximately 107. an approximate calculation of the product  gives the value: 𝜔𝜀 = 2π𝑓𝜀0𝜀𝑟 = 2π ∙ 50 ∙ 1000 36π ∙ 10−12 ∙ 107 = 1 36 ≃ 0.027 (4) this value represents about 14 % of the conventionally accepted value for electrical conductivity, which justifies the lower weight of displacement currents. the main weight in the complex conductivity formula belongs to the conduction component rather than to the displacement one. this finding is also valid at frequencies above 50 hz; regarding the  value, the increase in frequency is offset by the decrease in permittivity. essentially, the external electric field produces two effects in the human body: • it establishes a conduction current of the existing charges whose amplitude is determined by the electrical conductivity σ, and • it establishes a displacement current, both by forming new charges (bound charges) and by orientating existing dipoles. permittivity is an expression of the magnitude of polarisation processes in dielectric bodies. it should be noted that in the case of the human body, both electric conductivity and permittivity vary in a very wide range, depending both on the nature of the tissue and the frequency of the external applied field. additionally, an external field due to coulomb forces will induce a movement of the charges at the surface of the body. this alternate polarisation of superficial charges will cause induced currents, the distribution of which will be determined by both the geometric dimensions of the body and its position relative to the field. if the magnetic field is dominant (being directly proportional to the current), a magnetostatic solver is used. the most important approximation for the (quasi)magnetostatic approach is to ignore the magnetic flux density variation in the faraday-lenz law, the induced voltage being practically zero: ∇⃗⃗ 𝑋�⃗� = − 𝜕�⃗� 𝜕𝑡 ≃ 0 (5) acta imeko | www.imeko.org september 2020 | volume 9 | number 3 | 55 the interaction between the external variable magnetic field and the human body means the induction of voltages that cause current loops. the larger is the surface intersected by the variable magnetic flux, the higher are the values of the induced voltage. consequently, the circulating currents have higher values. an indicative formula (deduced from faraday-lenz's law) shows that the circular current density (2πr being the length of the quasi-circular path) is directly proportional not only to the radius of the intersected surface but also to the conductivity of the tissue and to the amplitude and frequency of the magnetic flux density. 𝐽 = π𝑅𝑓𝐵𝜎 (6) due to the extensive resources of modern computing systems, most actual simulation/modelling programs of the electromagnetic fields also have a module that takes into account, even at 50 hz frequencies, both variations of magnetic flux density and electrical flux density. it should be noted that this option is (computer) resources and time consuming. in the specific case of the cst em studio ® software [3], this solver is called ‘fullwave’, with the name suggesting that both components of the electromagnetic wave are taken into account. 2. elliptic cylinder model of the human trunk suited for the study of inductive coupling the issue of the electric and magnetic fields generated by overhead high-voltage power lines concerns the most diverse teams of researchers, spread across all over the world, using different approaches, but finally reaching convergent conclusions. in the frequency range 50-60 hz, the biological effects of the electric fields [4] must be treated differently to those of the magnetic fields [5]. a challenging matter is to study the cumulative effects of currents induced by both fields [6]-[8]. fortunately, the currents induced by the electric field are vertical, and those induced by the magnetic field are in the loop. in addition, there is a phase shift between them, so their maximum values should not be arithmetically summed up. the studies undertaken must be differentiated in terms of the two fundamentally different categories: (1) occupational – for a limited time, a healthy, trained, and protected worker can be found near the high-voltage lines due to some service tasks [9], [10] and (2) residential – when any person, regardless of their age or health status, is located in the vicinity of these lines, which might be placed within newly developed residential districts [11]. even if 110 kv can be considered the upper limit of medium voltage [12], the generated fields at soil level are comparable with those emitted by higher-voltage networks (as the supporting pylons are lower). one of the problems that called for the attention of researchers is the influence of soil, floor, or footwear on the subject (due to different conductivity and permittivity values) in determining the amplitude of the induced currents in the human body [13]. in [14], we developed an axis-symmetric rotation 3d model consisting of a hemisphere, four cylinders, and four trunks of cone. from the perspective of the current density induced by quasi-homogeneous electric fields along the vertical axis of the body, this model provided values not only in accordance with the analytical calculations but also in accordance with the values in the extant literature. in [15], we developed a more refined model – closer to the real geometric shapes of the human body. in order to accomplish this target, the trunk has been designed in the shape of an elliptic cylinder (the cross-section is an ellipse with a large radius of 28 cm and a small one of 16 cm). the hands and legs have also been distinctly shaped. this model has proven useful in determining the current density at the body surface and the loop current induced by the horizontal component of the magnetic field generated by overhead high-voltage power lines. the fact that the origin of the three coordinate axes should be considered outside the human body (modelled with two distinct feet) represents a difficulty in determining the vertical induced current. in the present article, we have aimed to study the influence of the relative position of the human trunk versus the vertical plane determined by the supporting pillars of the network on the magnitude of the induced currents in the body. emphasis is given to the loop currents induced by the horizontal component of the magnetic field generated by overhead high-voltage power lines. we have developed an intermediate model between the two previously presented ones, also using the cst suite studio cad module in the ‘modelling’ section. in particular, we kept the trunk as an elliptic cylinder; however, since the vertical axis of human symmetry should coincide with the vertical axis of the coordinate system, the two legs were modelled together in the form of a truncated cone – which was also elliptic. 3. the cst-developed model and the acquired results and their relevance we have considered five representative positions of the human trunk – the angles between the major axis of the elliptic cross-section and the plane determined by the network pillars: 0°, 30°, 45°, 60°, and 90° respectively. we have taken into account a double 110 kv three-phase network, with a single grounding conductor, supported by sn 110252 pylons. we have assumed (even for the sake of comparison), two values for the maximum root mean square (rms) phase current: 640 a (active conductors 2 x 3 acsr 240/40) and 960 a (active conductors 2 x 3 acsr 380/50). for the model development, we have started from the geometric dimensions of a power transmission network supported by the normal poles sn 110252 (we have chosen the standard case – the normal level to the ground). in table 1, there are presented the coordinates in the lateral cross-sectional plane; the spatial cartesian axes have been previously, conveniently fixed. in this study, we are concerned about the magnetic field generated by overhead high-voltage power lines. for this reason, we did not consider the ground (protective) wire, its contribution being negligible from this perspective. in [16] and [17], the importance of a correct ground consideration due to the effective contributions of charge and current images was underlined. we have modelled the ground (in a covering way) in the form of a right quadratic prism, with the side of the base being 100 m and the height (or, more correctly, the depth) being 50 m. we have chosen the soil relative table 1. coordinates (m) of the 2 x 3 active wires, the transverse lateral plane, the -x0x axis being parallel to the wires and normal to this plane. r1 s1 t1 r2 s2 t2 y z y z y z y z y z y z -3.05 27.3 -4.55 21.6 -3.05 17 3.05 27.3 4.55 21.6 3.05 17 acta imeko | www.imeko.org september 2020 | volume 9 | number 3 | 56 permittivity r = 40, while electric conductivity has been fixed at the medium value 10 s/m. the variation margin for the permittivity of the soil is reduced in comparison with that of the conductivity, which, depending on the nature of the soil and the degree of humidity, can vary from 0.1 to 1000 s/m. with the aim of verifying the correctness of the model construction, we have simulated (for two usually encountered rms values of the current intensity) the lateral profile of the magnetic field generated by the three-phase symmetrical double untransposed network, at the standard height of 1 m from the soil level. at 1 m above the ground, most of the vital internal organs (heart, liver, kidneys, stomach) of a man of average height are located. the result is shown in figure 1 and agrees well with other simulations presented in the literature [10], [11] and with the individually performed analytical calculations. the next step was to place the human model in the symmetry centre of the grounding plane (the intersection of the three xyz coordinate axes) in order to set its electrical properties (the relative permittivity r was 107 while the electric conductivity was fixed at the medium value 0.2 s/m) and primarily to establish the right boundary conditions (a good balance between accuracy and duration). more exactly, in the magnetostatic solver, on the –x0x axis boundaries of the electric type were imposed on both ends, while on the –y0y and –z0z axes, the ‘open-boundary’ option was also chosen for both ends. as for the soil-air working space, we have taken into account two restrictions. along the –x0x axis, due to the conductive electric type boundaries, it is compulsory for these extremities to be connected to soil, allowing the current loop to close through the soil. this condition is not for axes –y0y and –z0z, where a gap between the ground and the open boundary is accepted. in figure 2(a), the general defining framework and the previously described human model being placed on the earth in the centre of the selected square surface are presented. the six considered three-phase currents are parallel to the soil. in [18], we presented compelling simulations and results that demonstrate the irrelevant influence of the trunk orientation upon the surface current density distribution. this conclusion was, in principle, expected in this current study, with the incidental electric field variations being virtually negligible. in figure 2(b) and (c), there is a progressive colour-coded presentation of these ‘scatterings’ for the extreme positions of the trunk (parallel and normal to the network plane). in the same article, we also investigated the vertical currents induced by these fields – they were four orders of magnitude smaller than the accepted basic restrictions, which makes them practically irrelevant. we have focused on the loop currents induced by the generated magnetic field (see figure 1) in the trunk of the elliptic cylinder torso model accepted here. as shown in equation (6), the induced current density is directly proportional to the radius of the surface perpendicular to the horizontal lines of magnetic field. in figure 3, we present the induced current density in the horizontal elliptical cross-section made 1 m high from the ground. the great axis of the ellipse is parallel to the network plane. following this assumption, the magnetic inductive coupling reaches its highest value (according to network orientation). it should be noted that for both current intensity rms values, the induced current density considerably exceeds figure 1. lateral profile of the magnetic field strength produced at a height of 1 m from the soil, by two double three-phase 110 kv transmission networks, untransposed. the balanced currents have the rms values 640 a and 960 a. figure 2. maximal coupling along the perimeter associated with the large radius, the trunk being parallel to the plane of the pillars. a b c figure 3. working space of the simulation (a). the current density distribution on the model surface is very slightly influenced by the trunk relative orientation (b) parallel to the network plane and (c) normal to network. 0 1 2 3 4 5 6 7 -100 -50 0 50 100 h [ a /m ] -yy 640 a 960 a 0 0,0001 0,0002 0,0003 0,0004 0,0005 0,0006 -0,4 -0,2 0 0,2 0,4 c u rr e n t d e n si ty [ a /m 2 ] -xx 640 a 960 a acta imeko | www.imeko.org september 2020 | volume 9 | number 3 | 57 the value of 2 ma / m2 – the accepted admissibility threshold for residential exposure. the corresponding area to be intersected by the magnetic field lines is the largest possible, as shown in figure 4. in figure 5, we present the induced current density in the same horizontal elliptical cross-section performed 1 m high from the ground, this time with the great axis of the ellipse being normal to the network plane and, consequently, the coupling being minimal. this situation is suggestively represented in figure 6, the human trunk being rotated 90° from the condition presented in the previous figure. up and down these two extremes are the intermediate values associated with the angles of 30°, 45°, 60° between the large axis of the cross-section ellipse and the plane determined by the power network pillars. as expected, the value of the current density induced in the trunk decreases in proportion to the surface exposed to the field lines. 4. conclusions the modelling and simulations presented in this article are consistent with those obtained through the use of other software or communicated in significant journals, demonstrating the important resources of cst studio suite and the importance of the study of extremely low frequency (elf) electrical and magnetic fields, such as 50 hz. because the human trunk has been modelled as an elliptic cylinder, it has been possible to study the distribution of the loop currents induced by the horizontal component of the magnetic field generated by overhead power transmission networks, both on the peripheral perimeter associated with the large axis of the cross-section and on that associated with the small one. the graphic representations in figure 3 and figure 5 are in the form of a v – with almost straight arms. interpreting these results, a good correspondence with the theory can be established, the induced current density being proportional to the radius of the exposed surface, under the theoretical condition of uniform electrical conductivity. in addition, an increase in the rms value of the current by 50 % (from 640 a to 960 a) resulted in an increase of approximately the same percentage of the current density induced (obviously, at the same point of the selected model). the results herein obtained provide useful recommendations, especially for maintenance employees, who work close to highvoltage lines and should therefore avoid (as much as possible) the position with the large axis of the trunk being parallel to the plane of the supporting pillars. specifically, in the cases presented herein, there is a 57 % reduction in exposure when the large axis of the elliptical cylinder that shapes the human trunk is oriented perpendicular to the active wires compared to the situation when it is oriented parallel to them – in other words, a reduction of the current density from 3.75 to 2.15 ma/m2 for the case of the 640 a network, and a reduction from 5.5 to 3.2 ma/m2 for the 960 a network. according to the theoretical predictions, this reduction is approximately equal to the ratio between the small and the large radius of the ellipse representing the cross-section of the modelled trunk. this reduction is all the more important as the involved values are within the sensitive range between occupational and residential basic restrictions. the recommendation becomes even more stringent for workers who are closer to the wires during the execution of urgent interventions. in future work, we aim to perform a more refined study, which, based on successive 10° rotations, will allow us to identify the position for which the total cumulative inductive coupling is minimal, simultaneously taking into account the induced currents on both the maximum and minimum perimeter. figure 4. the trunk of the human body, parallel to the plane of the power network, in the most exposed position to inductive coupling. figure 6. the trunk of the human body, normal to the plane of the power network, in the least exposed position to inductive coupling. figure 5. the trunk of the human body, parallel to the plane of the power network, in the most exposed position to inductive coupling 0 0,00005 0,0001 0,00015 0,0002 0,00025 0,0003 0,00035 -0,2 -0,1 0 0,1 0,2 c u rr e n t d e n si ty [ a /m 2 ] -xx 960 a 640 a acta imeko | www.imeko.org september 2020 | volume 9 | number 3 | 58 we will also consider that at a height of about 1 m from the ground, the human body has sensitive organs, such as the liver, spleen, and lungs, which have different conductivity and permittivity (and different basic restrictions). taking these nonuniformities into account should lead to significant and even asymmetric nonlinearities in the two arms of the v-shaped graphs presented in figure 3 and figure 5 herein. it is worth studying the possible overcoming of basic restrictions in certain organs considered more sensitive. references [1] european commission, directive 2013/35/eu of the european parliament and of the council of 26 june 2013 on the minimum health and safety requirements regarding the exposure of workers to the risks arising from physical agents (electromagnetic fields), official j. eur. union, 2013, 1791, pp. 1-21j. 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https://www.imeko.org/publications/tc4-2019/imeko-tc4-2019-039.pdf https://www.imeko.org/publications/tc4-2019/imeko-tc4-2019-039.pdf https://doi.org/10.1109/sielmen.2019.8905880 https://doi.org/10.1109/sielmen.2019.8905895 external magnetic field measurements in ume kibble balance acta imeko issn: 2221-870x september 2020, volume 9, number 3, 33 39 acta imeko | www.imeko.org september 2020 | volume 9 | number 3 | 33 external magnetic field measurements in the ume kibble balance hacı ahmedov1, beste korutlu1, lev dorosinskiy1, recep orhan1, özlen tuncel1 1 tübi̇tak national metrology institute (ume), kocaeli, turkey section: research paper keywords: kibble balance; magnetic flux density; measurement techniques; planck’s constant citation: haci ahmedov, beste korutlu, lev dorosinskiy, recep orhan, özlen tuncel, external magnetic field measurements in ume kibble balance, acta imeko, vol. 9, no. 3, article 6, september 2020, identifier: imeko-acta-09 (2020)-03-06 editor: jan saliga, technical university of košice, slovakia received january 17, 2020; in final form march 24, 2020; published september 2020 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was supported by tübi̇tak national metrology institute, turkey. corresponding author: haci ahmedov, e-mail: haji.ahmadov@tubitak.gov.tr 1. introduction the redefinition of the kilogram unit, approved at the 26th meeting of the general conference on weights and measures (cgpm), held in november 2018, has replaced its previous artefact definition via the mass of the international prototype of the kilogram (ipk) with one based on the fixed numerical value of the planck constant [1], [2]. extensive studies have been performed at several national metrology institutes (nmis) across the world to ensure a smooth transition to the revised definition of the kilogram. currently, there are two independent primary methods for realising the kilogram with an uncertainty of a few parts in 108: kibble balance [3]-[23] and x-ray-crystal density (xrcd) [24]-[29]. the content of this paper draws upon the kibble balance principle. originally devised at the national physical laboratory (npl) by brian kibble in 1975 [30], [31], the kibble balance relates mechanical and electrical powers where the electrical power is measured in terms of planck’s constant by using two macroscopic quantum phenomena known as the quantum hall effect [32] and the josephson effect [33] to link the macroscopic mass to the planck constant. a number of kibble balances with different geometries and experimental protocols have been constructed in various nmis [3]-[23]. the national metrology institute of turkey (ume) has constructed a kibble balance with a stationary coil suspended from the load cell of a mass comparator and a surrounding oscillating magnetic circuit. the prominent features of the ume kibble balance can be seen in the adopted novel measurement procedure whereby the oscillating parameters are continuously averaged over the magnet oscillation half-cycles (see [34]-[37] for details of the adopted measurement procedure). as a result, the variations in magnetic field and temperature are suppressed, which in turn enables the construction of both the magnetic circuit and the apparatus in smaller dimensions. the oscillatory motion of the magnetic circuit also allows the simultaneous operation of weighing and moving phases (i.e. concurrent testing of ampere’s force law and faraday’s law of induction), which eliminates the need to abstract the new definition of the kilogram in terms of the fixed value of planck’s constant ensures the long-term stability of the si mass unit and enables traceability from more than one source. kibble balance experiments offer an effective primary realisation method for the new definition of a kilogram. the kibble balance apparatus at the national metrology institute of turkey is designed with a stationary coil and an oscillating magnet. in contradistinction to traditional moving coil kibble balance experiments, the external magnetic field introduces an asymmetry between the ampere’s law of force and the faraday’s law of induction in moving magnet experiments. in this paper, we develop a method based on external magnetic flux density difference measurements in a vertical direction by using a magnetic gradiometer to take into account the effect of the external magnetic field on the realisation of kilogram. we have examined the time variations of the external magnetic flux differences and non-liner effects to guarantee the validity of the model. the proposed model fits well with the data, and the kilogram realisation requirement is fulfilled within the accuracy level of the electrical measurements taken. we conclude that, when placed in close proximity to the kibble balance, the magnetic gradiometer may be used for monitoring purposes. mailto:haji.ahmadov@tubitak.gov.tr acta imeko | www.imeko.org september 2020 | volume 9 | number 3 | 34 precisely quantify the variations in the environmental and experimental conditions between the two phases. in addition, the system does not require a fine adjustment between the magnetic centre of the magnetic circuit and electric centre of the coil. instead, the misalignment is treated as a parameter contributing to the faraday’s voltage across the ends of the coil generated due to the relative motion between the coil and magnetic circuit [36], [37]. aside from its distinctive properties, there is an important difference between the ume kibble balance and traditional twophase, moving coil kibble balances in that the external magnetic field introduces an asymmetry between the ampere’s law of force and faraday’s law of induction. in the present work, we develop a convenient approach for taking into account the effect of earth’s magnetic field on the oscillating magnet kibble balance. the approach guarantees the desired uncertainties in the realisation experiment within the accuracy level of electrical measurements. in section 2, the asymmetry between faraday’s law of induction and ampere’s force law in relation to the moving magnet is described. in section 3, the ume kibble balance external magnetic force measurements are presented, in which the importance of the stability of external magnetic flux density difference and non-linear effects is emphasised. in section 4, the monitoring of the magnetic flux density is explained. finally, we offer some conclusions in section 5. 2. asymmetry in moving magnet kibble balances currently, in all kibble balance experiments, a permanent magnetic circuit is adopted as the most practical solution when balancing the mechanical power with the electrical one. originally designed by the international bureau of weights and measures (bipm) [38], a typical symmetrical permanent magnetic circuit is widely used by worldwide kibble balance experiments. generally, smco is used as the permanent magnetic material since it has high performance and a low temperature coefficient. thanks to the magnetic circuit, the magnetic flux density generated by the permanent magnetic material is directed towards the narrow air gap around the coil parallel to the inner and outer soft-iron yokes. thus, a strong, vertically uniform, radially magnetic field is generated in the vicinity of the coil [39]. the cross-sectional view of the ume permanent magnetic circuit is given in figure 1. the coil located in the air gap of the magnetic circuit is stationary as it is suspended from the load cell of the mass comparator. the magnetic circuit, on the other hand, oscillates in the direction of the gravitational acceleration. the soft-iron yoke not only guides the generated magnetic flux density towards the coil volume but also effectively shields the coil from the effect of external magnetic flux density. the exposure of the yoke itself, a magnetically non-linear material, to the external magnetic flux density gives rise to an additional magnetic flux density in the air gap, which is called the magnetisation flux density. the magnetisation flux density compensates for the external magnetic flux density in the air gap, which is the manifestation of the magnetic shielding principle for cavities in magnetic materials. in moving magnet kibble balance experiments, there occurs an asymmetry between ampere’s law of force and faraday’s law of induction due to the external magnetic field. we describe this asymmetry in the following sections. 2.1. ampere’s law of force there are three sources of magnetic flux density created in the air gap: a) magnetic flux density generated by the magnetic circuit (�⃑� mc), b) magnetisation flux density (�⃑� m), c) external magnetic flux density (�⃑� e), which contribute to the ampere’s force law given by �⃗� = 𝐽 ∮ 𝑑𝑙 × �⃑⃗�, where �⃑⃗� = �⃑� mc + �⃑� m + �⃑� e is the total magnetic flux density in the air gap, 𝐽 is the dc current on the coil and 𝑙 is the vector having a magnitude equal to the total length of the coil and being in the direction of electric current 𝐽. the force on the coil in the direction of gravitational acceleration is obtained by the dot product 𝐹 = �̂� ∙ �⃗�, where �̂� is the unit vector representing the direction of gravitational acceleration yielding. 𝐹 = 𝐹mc + 𝐹m + 𝐹e. (1) here, 𝐹𝑚𝑐 is the force generated by the magnetic flux density of the magnetic circuit, and 𝐹m and 𝐹e are the forces induced on the coil by the magnetisation and external magnetic flux densities, respectively. 2.2. faraday’s law of induction as the coil is stationary with respect to the external magnetic flux density, only �⃑� mc and �⃑� m contribute to the faraday’s law of induction. as a result, for an oscillating-magnet kibble balance, the fundamental equation takes the following form: ℎ ℎc 𝑉 = 𝐹 − 𝐹e 𝐽 𝑢, (2) where 𝑉 is a faraday’s voltage across the ends of the coil and 𝑢 is the velocity of the coil with respect to the magnet. here ℎ is the planck constant and ℎc is the consensus value of the planck constant [2]. the ratio ℎ/ℎc appears since the faraday’s voltage is measured in terms of the consensus value of the planck constant by using the josephson effect. in the kibble balance experiment, the quantities measured directly are the total vertical force 𝐹, the electrical current 𝐽, the vertical velocity 𝑢 and the faraday voltage 𝑉. the force produced by external magnetic field 𝐹e on the other hand is not a directly measured quantity in the figure 1. cross-sectional view of the ume magnetic circuit. the yoke is made from iron, and the permanent magnets are made from smco. the colour scale in the colour bar goes from high magnetic flux density given in red to low given in dark blue. acta imeko | www.imeko.org september 2020 | volume 9 | number 3 | 35 kibble balance experiment, and an indirect measurement procedure should be developed to take this factor into account. one way to handle this effect is put forward by the chinese joule balance team, in which they place two compensation coils connected in series with the suspended coil in the air gap of the permanent magnet system. one of the compensation coils is fixed to the inner yoke while the other one is fixed to the outer yoke. the number of turns in each compensation coils is equal to half of the number of turns of the suspended coil. when the magnet moves, the compensation coils move together with the magnet. thus, the relative motion between the external magnetic field and the compensation coils induces faraday’s voltage across the ends of the compensation coils. as they are connected in series with the suspended coil, the asymmetry in (2) is eliminated. this is ensured by placing the compensation coils sufficiently close to the suspended coil. although the idea of compensation coils is neat, mechanical difficulties may arise while placing three coils in the air gap [40], [41]. in this paper, we propose another method for taking into account the effect of an external magnetic field on a stationary coil and oscillating magnet system without compensation coils. one may rewrite (2) as ℎ ℎc = 𝑄 〈𝐹〉 〈𝐽〉 {𝑢|𝑉}, (3) by following the dynamical measurement procedure of the ume kibble balance [34], [35], where the integration time is chosen to be a multiple of oscillation fundamental period 𝑇 such that 𝜏 = 𝑁𝑇. here 〈𝐹〉 and 〈𝐽〉 are the average of the total ampere’s force on the coil and current over the integration time, respectively, {𝑢|𝑉} represents the average of the velocity 𝑢 of the coil with respect to the magnet over the half-cycles of faraday’s voltage 𝑉, and 𝑄 is the geometric factor, which includes inhomogeneities and the effect of external magnetic flux density on the planck constant. using (3), the effect of external magnetic field on the planck constant is found to be 𝑞 = − 〈𝐹e〉 〈𝐹〉 , (4) where 〈𝐹e〉 is the average of the force induced on the coil by the external magnetic flux density over the integration time. practically speaking, the force created by the external magnetic flux density is measured when there is no magnetic circuit, while kibble balance measurements are performed when the coil is surrounded by the magnetic circuit. the two conditions for this method to be valid are: a) the variations of 𝐹e in time should be small enough to be negligible. b) the nonlinear effects caused by the interaction of the magnetic circuit with external bodies should be small enough to be negligible. if these conditions are met, there will be no need for compensation coils. we clarify this statement in the next section. 3. external magnetic force measurements the schematic circuit diagram of the measurement set-up for external magnetic force measurements is given in figure 2. below we give a step-by-step description of the research conducted. 1. the 𝑞 factor in (4) is determined by simultaneous measurement of 𝐹e and 𝐽 when there is no magnetic circuit around the coil. 𝐹e is determined by mass measurements with a mass comparator, and the current is determined by measuring the voltage across the ends of a standard resistor connected to the coil in series. 2. the time variations of 𝐹e are determined by measuring external magnetic flux density differences via magnetic gradiometer. over the short term, the variations in 𝐹e are found to be negligible. 3. the possible non-linear effects when the magnetic circuit is placed around the coil are simulated by ems magnetic software. they are found to be small enough to disregard. 4. the correlation between 𝐹e and the magnetic flux density difference measurements by the gradiometer sensor is determined when an additional axial magnetic field source is used to simulate abrupt changes in external magnetic field. once the correlation coefficient is determined, the magnetic gradiometer can be used for monitoring the variations in 𝐹e. it is important to emphasise that there is no magnetic circuit surrounding the suspended coil. therefore, the ampere’s force in (1) reduces to 𝐹 = 𝐹e. the force measurements are carried out with the load cell of an ax5006 mass comparator with 1 μg resolution that is integrated into the ume kibble balance. dc current is measured by keysight 3458 a digital multimeter across the two tinsley 5658a 100 ω standard resistors connected in parallel and placed in an oil bath. the external magnetic flux density difference measurements are performed with a bartington grad601 magnetic gradiometer, which will be explained in the following subsections. dc calibration of the keysight 3458 a digital multimeter with programmable josephson voltage standard (pjvs) yields a total relative measurement uncertainty of 0.1 ppm. the uncertainty components in the force, the standard resistance and the change in magnetic flux density difference measurements are small compared to the uncertainty from to the digital multimeter. therefore, the total uncertainty is restricted by the uncertainty in the dc voltage measurements. figure 3 illustrates the ratio of the force by the external magnetic flux density 𝐹e and the dc current 𝐽 averaged every 10 minutes. there is a 10 min. delay between each consecutive piece of data. the current direction has been reversed every 100 s to handle the drifts in the balance. we obtain the mean value of the ratio as 〈𝐹e 𝐽⁄ 〉 = −0.295 mn/a and the standard deviation as 𝜎(𝐹e 𝐽⁄ ) = 0.005 mn/a. since the geometrical factor 〈𝐹 𝐽⁄ 〉 in our experiment is about 150 n/a, using (4), we obtain 𝑞m = 1.97 × 10−6 with a standard deviation of 3 × 10−8. figure 2. the schematic circuit diagram of the external magnetic flux density measurement procedure. acta imeko | www.imeko.org september 2020 | volume 9 | number 3 | 36 3.1. stability of external magnetic flux difference as emphasised in section 2, the variation of 𝐹𝑒 in time should be small enough to allow the proposed procedure to be followed by taking into account the effect of the external magnetic flux density. one may easily find that the vertical component of the ampere’s force is proportional to the magnetic flux through the lateral surface of the coil by using simple vector algebra. based on the fact that the divergence of magnetic flux density over a closed surface is equal to zero, the ampere’s force is expressed in terms of the magnetic flux difference 𝛿𝛷 between the top and the bottom surfaces of the coil as 𝐹e(𝑡) = 𝐽 𝑀 𝛿𝛷e(𝑡) 𝑑 , (5) where 𝑑 is the height of the coil and 𝑀 is the number of turns in the coil. substituting (5) in (4) yields 𝑞 = − 〈𝛿𝛷e〉 〈𝛿𝛷〉 , (6) where the effect of the external magnetic field on the planck constant is reflected in the ratio of the flux differences instead of the forces. we estimate 〈𝛿𝛷〉 as the product of the radial magnetic flux density 〈𝛿𝐵〉 in the centre of the air gap and the lateral area of the coil, while 〈𝛿𝛷e〉 is estimated by the product of external magnetic flux density difference 〈𝛿𝐵e〉 and the horizontal area of the coil. thus, one may write 𝑞 ∝ (𝑟/2𝑑)〈𝛿𝐵e〉/〈𝛿𝐵〉, where 𝑟 is the radius of the coil. the height of the coil is around 2 cm and radius is around 7 cm. the radial magnetic flux density in the centre of the air gap is around 0.5 t. thus, if the variations of 〈𝛿𝐵e〉 are smaller than 0.5 nt, we may neglect the variations in q-factor. we use a commercial bartington single-axis vertical component fluxgate gradiometer for flux density difference measurements whereby two cylindrical gradiometer sensors are mounted on a rigid beam fixed at a distance of 90 cm. we modified the rigid beam so that two sensors could be fixed at different positions. the minimum possible distance between the two sensors is 7.5 cm after this modification. figure 4 demonstrates the temporal changes in the difference of the external magnetic flux density between the two sensors of the fluxgate gradiometer averaged every 100 s, where the sensors are separated at a distance of 7.5 cm. the mean value of the temporal changes in the difference of external magnetic flux density between the two sensors is 〈𝛿𝐵e〉 = 141.388 nt, and the standard deviation is 𝜎〈𝛿𝐵e〉 = 0.160 nt. the linearity tests performed at three different distances between the sensors of the magnetic gradiometer are given in figure 5. now that the linearity is observed, it is possible to interpolate the standard deviation obtained by the fluxgate gradiometer at the minimum distance between the sensors of the fluxgate gradiometer to the one over the top and bottom surfaces of the coil by linear approximation, which yields 𝜎〈𝛿𝐵e〉 ~ 0.05 nt. 3.2. non-linear effects since the effect of the external magnetic flux on the planck constant is determined in the absence of the magnetic circuit, it is important to make the possible non-linear effects generated in the presence of the magnetic circuit negligible. the non-linear effects include the influence of the magnetic circuit on the nearby soft magnetic materials, which in turn is reflected in the external magnetic flux. this can be achieved by keeping the magnetic materials away from the permanent magnet. the simulations are carried out in the ems magnetic software to support this idea (see figure 6). we simulated the non-linear effect of the external magnetic flux by using ferromagnetic material of a cylindrical form with a diameter of 20 cm and height of 10 cm. the distance between the center of the ferromagnetic material and the magnetic circuit was taken to be 62.5 cm. the centre of the ferromagnetic material is placed on the same symmetry axis as the magnetic circuit. we use pure iron as the ferromagnetic material. the figure 3. the ratio of the force by the external magnetic flux density and the dc current averaged every 10 min. there is a 10 min. delay between each consecutive piece of data. the current direction has been reversed every 100 s to account for drifts in the balance. the mean value of the ratio 〈𝐹e 𝐽⁄ 〉 = −0.295 mn/a is given in the blue straight line. the standard deviation is σ(𝐹e 𝐽⁄ ) = 0.005 mn/a. figure 5. the average of external magnetic flux density difference with respect to the distance between the sensors. figure 4. the temporal changes of the difference of external magnetic flux density between the two sensors of the fluxgate gradiometer averaged in every 100 s. the mean value of external magnetic flux density difference 〈𝛿𝐵e〉 = 141.388 nt is given in blue. the standard deviation is 𝜎〈𝛿𝐵e〉 = 0.160 nt. 0 100 200 300 400 500 -0.31 -0.305 -0.3 -0.295 -0.29 -0.285 -0.28 time (min) f o rc e b y e x te rn a l m a g n e ti c f lu x d e n s it y o v e r c u rr e n t ( m n /a ) data mean 0 5 10 15 20 0 50 100 150 200 250 300 350 400 distance (cm) a v e ra g e o f e x te rn a l m a g n e ti c f lu x d e n s it y d if fe re n c e ( n t ) data linear fit 0 10 20 30 40 50 60 141.1 141.2 141.3 141.4 141.5 141.6 141.7 141.8 time (min) e x te rn a l m a g n e ti c f lu x d e n s it y d if fe re n c e ( n t ) data mean acta imeko | www.imeko.org september 2020 | volume 9 | number 3 | 37 magnetic circuit oscillates vertically around the centre of the coil with an amplitude of 1 mm. the vertical distance given in the xaxis of figure 6 represents the distance between the centre of the magnetic circuit and the centre of the coil. as the height of the coil is around 𝑑 = 2 cm, we simulated the region within a displacement of ±1 cm from the origin where the two centres coincide. the flux through the horizontal disk with a radius of 7 cm is simulated. the y-axis on the right represents the flux in the air gap for vertical displacements of ±1 cm from the origin, while the one on the left represents the differences between the flux in the presence and in the absence of ferromagnetic material. the effect of the non-linear flux is estimated to be 3 × 10−9. it is important to note that in the actual experiment we expect the non-linear effects to be much smaller as there would be no heavy ferromagnetic materials near the magnetic circuit. 4. monitoring the change in magnetic flux density the external magnetic flux density differences are measured on different days. in normal conditions, we observe that the time variations in external magnetic flux density differences are negligible. however, in case of geophysical natural phenomena, the variations may be high. in addition, moving heavy magnetic materials near the kibble balance system may cause additional variations in the external magnetic field since the magnetic circuit may magnetise these materials, and in turn, they may contribute to the external magnetic field. this is why monitoring of the external magnetic flux density differences may be necessary. the temporal changes in the flux through the suspended coil may be characterised by using the magnetic gradiometer locally. this is possible since the characteristic scales of the factors mentioned above that produce variations in external magnetic flux density are very large compared to the distance between the coil and the magnetic gradiometer sensors. therefore, we expect the ampere’s force on the suspended coil to be strongly correlated with the magnetic flux density difference measured by the magnetic gradiometer sensors. using (5), we may write this correlation in the following form 〈𝐹e〉 = 𝜁〈𝐽〉〈 𝛿𝐵e〉, (7) where 𝜁 is a model parameter that depends on the geometry of suspended and sensor coils and also on the local inhomogeneities in the external magnetic flux density differences. we determine the value of the model parameter 𝜁 by minimising the quadratic form 𝔉(𝜁)2 = 𝐷(〈𝐹e〉 − 𝜁〈𝐽〉〈𝛿𝐵e〉), (8) where 𝐷 is the standard deviation. since the target uncertainty in the realisation of a kilogram is twenty parts per billion, the modelling given in (7) will meet the requirements provided that 𝔉/〈𝐹〉 is much less than 10−8. it is important to note that the model parameter 𝜁 is determined in the absence of a magnetic circuit. the presence of a magnetic circuit may directly affect the gradiometer sensors such that an offset may occur. however, it is possible to determine this offset independently by measuring the difference in the presence and absence of the magnetic circuit. then, 𝛿𝐵e will be modified accordingly in the optimisation procedure. in this way, using (7), we will be able to trace the variations of the 𝑞-factor. the asymmetry between faraday’s law and amperes law can be treated with compensating coils, as proposed by the nim joule balance group [40], [41]. we have introduced an alternative approach where, under certain circumstances, the asymmetry could be accounted for without compensation coils by means of a magnetic gradiometer. however, we are planning to use both the compensating coils and the proposed magnetic gradiometer method to increase the reliability of the results. the comparative analysis between these two methods will be conducted after the ongoing modifications on the handler of the mass comparator are complete. we demonstrate the correlation in (7) with the help of an additional axial magnetic field source, which is introduced to simulate the effect of possible extreme variations in the external magnetic field due to the reasons mentioned above. the source is composed of a pair of identical square coils with a side length of 1.5 m separated at a distance equal to the side length of the square that are used to create an additional axial magnetic flux density, 𝐵axial. in this way, we produce short term variations of the external magnetic flux density of a sufficient magnitude to be measured by magnetic gradiometer. a magnetic flux density of 1 gauss is produced at the center of the coil pair provided that they are connected in series. the coil is placed next to the ume kibble balance apparatus. for our purposes, 𝐼 = 4 a is switched on and off every 10 min to create this additional flux density where 𝐵axial = 0 is equivalent to the case of the coil under the sole influence of the external magnetic flux density. in figure 7 we show the temporal changes of the difference in magnetic flux density averaged every 10 min in the presence and absence of the axial flux density. it is clear from figure 7 that the variations in external magnetic field density difference can be traced by the magnetic gradiometer. the apparent difference of approximately 400 nt in the external magnetic flux density in the presence figure 6. the simulation carried out in the ems magnetic software. given in blue dashed line with circle markers are the non-linear effects of external magnetic field and in green solid line with star markers indicate the magnetic flux in the air gap. figure 7. the temporal changes of the external magnetic flux density difference averaged in every 10 min when the axial field is on and off. given in red star is given the actual data with 𝐵axial = 0 and in green star is with 𝐵axial ≠ 0. the separation between the sensors of gradiometer is 90 cm. -1 -0.5 0 0.5 1 6.9 6.92 6.94 6.96 6.98 n o n -l in e a r m a g n e ti c f lu x ( 1 0 -1 1 w b ) vertical distance (cm) -1 -0.5 0 0.5 1 -2 -1 0 1 2 m a g n e ti c f lu x i n a ir g a p ( 1 0 -4 w b ) 0 100 200 300 400 500 -900 -800 -700 -600 -500 time (min) e x te rn a l m a g n e ti c f lu x d e n s it y d if fe re n c e ( n t ) acta imeko | www.imeko.org september 2020 | volume 9 | number 3 | 38 (lower data) and in the absence of the axial field (upper data) allows the ampere’s force measurements, which otherwise cannot be detected within its resolution, to be monitored with the mass comparator. with this data at hand, we solve for the optimisation parameter 𝜁 by minimising the quadratic form in (6), which yields 𝜁 = 583.7 m. the actual data and model fitted data of the ratio 〈𝐹e〉/〈𝐽〉 are given in figure 8. additionally, the actual data for 𝐵axial = 0 and 𝐵axial ≠ 0 are shown as red and green stars, respectively, and the model fitting data using 𝜁 = 583.7 m is shown as blue circles. the actual data and the fitted data coincide with each other. this is an indication of the strong correlation (0.9986) between the force from the external magnetic flux density over the current and the magnetic flux density differences. the target accuracy in (7) is achieved since we obtain 𝔉 /〈𝐹〉 = 4 × 10−9. 5. conclusion to deal with the impact of the external magnetic field on the determination of the planck constant within a kibble balance experiment, we have proposed an approach based on vertical external magnetic flux density difference measurements. the proposed method offers a convenient and practical solution for taking into account the effect of earth’s magnetic field on the moving magnet in kibble balance experiments. a magnetic gradiometer placed in close proximity to a kibble balance with a moving magnet may be used for monitoring the variations in external magnetic flux density and reflecting such variations in planck constant measurements. although, the proposed method is very effective in dealing with the asymmetry between the faraday’s and ampere’s laws, the results would be more rigorous if a comparative study with a compensation coil were conducted. the results of this study indicate that the model fitting the data meets the uncertainty requirement expected in the realisation of a kilogram. however, the accuracy of the measurements is restricted by the pjvs used in the calibration of the 3458a digital multimeter, and 0.1 ppm of total relative uncertainty is obtained. the current measurement accuracy will be improved by using differential measurements after integrating pjvs into the measurement set-up. currently, there are ongoing mechanical modifications being made in our setup to the handler of the mass comparator from which the coil is suspended and the sensors of the magnetic gradiometer. the comparative analysis will be performed after these modifications are completed. acknowledgement this research is funded by the tübi̇tak national metrology institute of turkey. references [1] m. j. milton, r. davis, n. fletcher, towards a new si: 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https://doi.org/10.1016/0031-9163(62)91369-0 https://doi.org/10.1109/cpem.2016.7540780 https://doi.org/10.1088/1681-7575/aab23d https://doi.org/10.1109/cpem.2018.8500794 https://doi.org/10.1109/tim.2019.2899185 https://doi.org/10.1109/cpem.2018.8500899 https://doi.org/10.1109/tim.2012.2230771 https://doi.org/10.1109/tim.2015.2406055 https://doi.org/10.1109/tim.2019.2893717 acta imeko, title acta imeko issn: 2221-870x march 2018, volume 7, number 1, 20-26 acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 20 electric energy converter development and diagnostics in mixed-signal simulation environment gábor kohlrusz, krisztián enisz, bence csomós, dénes fodor dept. of automotive mechatronics, institute of mechanical engineering, university of pannonia, egyetem 10, 8200 veszprém,hungary section: research paper keywords: discrete fourier transform; digitally controlled converter; capacitor degradation diagnostics; mixed-signal simulation citation: gábor kohlrusz, krisztián enisz, bence csomós, dénes fodor, electric energy converter development and diagnostics in mixed-signal simulation environment, acta imeko, vol. 7, no. 1, article 5, march 2018, identifier: imeko-acta-07 (2018)-01-05 section editor: lorenzo ciani, university of florence, italy received september 28, 2017; in final form february 3, 2018; published march 2018 copyright: © 2018 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: this work was supported by the hungarian government and co-financed by the european social fund and european union corresponding author: gabor kohlrusz, e-mail: gkohlrusz@almos.uni-pannon.hu 1. introduction a mixed-signal simulation environment is capable to prognosis failure or wear out phenomena during the design phase. the simulation results affect the controller design process and circuit components sizing. the purpose of this paper is to present the possibilities of a mixed-signal simulation in power converter or electric drive diagnostics. the analyzed converters or circuits are modelled in spice (simulation program with integrated circuit emphasis) design environment introducing realistic nature of circuit. the connection of the spice model to an evaluation platform creates a mixed-signal simulation which is suitable for signal analysis based diagnostics in power converters. signal-based diagnosis looks for the known fault signatures in quantities sampled from the actual system instead of analyzing capacitor material [1]. in order to identify fault or degradation in the analyzed system the sampled signals are evaluated by signal processing techniques. typically, frequency analysis is used, but time-frequency analysis and other advanced methods can also be advantageous according to the literature [2]. in this work the input and output capacitors of a digitally controlled step-down dc-dc converter (buck converter) have been tested in 4 different cases with unused and worn out capacitors. the applied diagnostic technique focuses on both the detection of dominant frequency components occurred by degradation and change in operation-related components due to capacitor aging. the tests were performed by exploiting the advantages and possibilities of a mixed-signal simulation environment which is well-suited for energy conversion system modelling and analysis. many contributions can be found in literature dealing with capacitor aging diagnosis in pwm (pulse-width modulation) converters, but they have a lack of considering the effect of signal conversion and signal processing or realistic noisy operational circumstances [3], [4], [5], [6]. in [3] the authors also analysed capacitor aging in switch mode dc-dc converter. however, the observation of frequency spectrum of capacitor current and voltage has been performed in a converter without digital control. in [4] the authors analyze the effect of capacitor degradation in digitally controlled pwm converter, but without abstract in this work the input and output capacitors of a digitally controlled buck converter have been tested in 4 different cases with unused and worn out capacitors. the information extracted from interactions between the subsystems of an electric energy conversion system requires the implementation of the whole system from network to load. a complex simulation environment has been implemented where digital and analogue domains can be connected. the input and output capacitor aging have a significant influence on time domain characteristics, but the evaluation of frequency-domain data is supposed to be required to extract information from the noisy and distorted time-domain signal. the presented diagnostic approach aims to analyze signals that are used by the control loop in order to avoid expensive additions to existing circuits. acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 21 considering the discrete-time operation of the controller. it also assumes ideal dc voltage at the converter input. in the related area, most of the diagnostic methods utilize dynamic model of the analyzed system [7], [8] while the presented diagnostic approach requires only signals that are necessary for normal operation of the controller. in [9] the author made experiments with frequency analysis of dc-link capacitor signal, but only the capacitor current has been analysed. 2. modelling in mixed-signal simulation environment the digitally controlled power converters and electric drives are mixed-signal systems containing both analogue and digital signals. in order to analyze these systems close to realistic behaviour it is worthy to design and to model the analogue and digital subsystems separately. the electric part of the system can be modelled properly in a circuit design environment, while the digital environment and the control algorithm should be implemented in an environment that is capable to run numerical computations. in such a complex development and test environment it is possible to simulate signal conversion and signal processing processes which have a significant effect on the system behaviour and generally are neglected. 2.1. electric energy converter development and diagnostics the mixed-signal environment fosters the hardware and software development and testing (figure 1). the interconnected environment plays an important role in hardware and software development process of electric drives and converters that has been highlighted by red transparent boxes in the figure. the control algorithm and the circuit have been modelled individually at the modelling level. mil (model-in-the-loop) testing ensures the execution of functional tests of the developed control algorithm. the model of the control algorithm is tested at mil level utilizing the interconnected orcad schematic which represents the controlled energy conversion system. the separate design of analogue and digital subsystems are connected via a software interface which handles data flow between the two platforms. as a result of mil tests, a software model has been created that meets functional and fundamental safety requirements. therefore, the software model can be implemented on hardware in order to test it in real environment. a real-time executable prototype of the software has been created by rcp (rapid control prototyping) process. since the control algorithm is developed in simulink environment the suitable target hardware components are dspace or ni veristand realtime environments. the rcp is not vital to create or generate code from software model. the schematic of the energy conversion system can be finalized with or without using rcp. during the sil (software-in-the-loop) test, the interaction of deployable software and schematic has been verified. the features of the interconnected simulation environment facilitate and support the target code generation and pcb design for hardware implementation and hil (hardware-in-the-loop) tests. the mixed-signal simulation environment also assists in product development processes. it makes possible to design, size or diagnose a circuit component if its model is integrated into the circuit schematic in orcad and is simulated together with the digital subsystem in order to form realistic operational environment. in this work a digitally controlled buck converter has been analyzed using orcad circuit design environment and matlab/simulink environment interconnected (figure 2). this case study is intended to present the features and possibilities of the simulation environment for diagnostic purposes. 2.2. modelling of step-down dc-dc converter the analogue part of the analyzed electric energy conversion system is a buck dc-dc converter fed by a single phase diode rectifier. the converter is designed for providing 16 vdc output voltage when 24 vdc input voltage is supplied by the rectifier. the design specifications can be found in table 1. the circuit of the converter has been implemented in figure 1. electric energy converter development process. table 1. buck converter specifications. input voltage (uin) 24 vdc output voltage (uout) 16 vdc operating frequency (f) 20 khz output current (iout) 2.25 a inductor current ripple (δil) 0.5 a output voltage ripple (δuc) 0.5 v acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 22 orcad environment. it was extended with additional components in order to reflect the realistic behaviour: 1) wire resistances and parasitic inductances; 2) inductor model extended by coil resistance; 3) standard model for input and output capacitor. aluminium electrolytic capacitors are modelled by an electrical equivalent circuit (figure 3) which is the so-called “standard model” and is used in the industry by several manufacturers such as epcos, jianghai, nichicon or vishay. the substitution of the input and output capacitor by the standard model makes component aging simulation possible. 2.3. dc-dc converter digital controller design and implementation the digital part of the system has been implemented in matlab/simulink. it includes the controller algorithm, the a/d converters etc. the controller is responsible for holding the 16 vdc at the converter output. the closed-loop control system is implemented in the structure as it can be seen in fig. 2. based on the difference between reference voltage and measured output voltage signal, the error signal can be calculated that serves as the input of the pi regulator. the regulator has been tuned by direct synthesis method that required the transfer function between output voltage and inductor current. the output of the pi regulator is the reference current of the inductor. comparing the controller output and the measured inductor current, an error signal can be generated. the duty cycle is calculated by considering inductor current error signal (1), [10]: [ ] [ ] [ ]( ) [ ]cref l,avg in s in u k 1l d k i k 1 i k 1 u t u − = − − − + (1) where d is the duty cycle, l is the inductance of the inductor, uin is the input dc voltage of the converter, ts is the sampling time of the digital platform, il,avg is the measured current, iref is the current reference provided by the pi regulator, uc is the output voltage of the converter and k is the discrete time. as it can be found in table 1, the operating frequency of the converter is 20 khz, hence the control algorithm runs in every 50 μs. therefore, 50 μs timeframe is available for realistic signal conversion, data processing and duty cycle calculation. it introduces 50 μs time delay that is treated by smith-predictor. 3. modelling of capacitor degradation due to the complexity of electric drives, faults can be occurred related to several components. focusing only to the power converter, faults are categorized as sensor faults, conducting part faults, passive component faults and switching element faults. the effect of circuit component degradation can be analyzed easily in the presented mixed-signal environment. changed behaviour of a component can be considered in wide-range that can be reflected in different component models. for instance, capacitor anode foil degradation can be observed by a variation in corresponding circuit parameters if the equivalent capacitor model properly describes its physical structure. in this paper, a passive component aging is introduced to the power converter, more precisely, capacitor degradation has been modelled by the assistance of the standard model (figure 3). it is a commonly used and simple capacitor model that conforms to industrial experiments and contains the main parameters of the analysed electrolyte capacitor. these are the capacitance (c), equivalent serial resistance (rs), self-inductance (l) and the oxide layer resistance (rp). in the model the capacitor specifies the rated capacitance of the analyzed component. the leakage current inside the component is modelled by the parallel resistance. the series resistance is a construction dependent parameter which contains the inner resistance of the conductive components (dielectric losses and ohmic resistance of raw materials, electrolyte, terminals and connections). figure 2. mixed-signal simulation environment for digitally controlled electric drive and power converter analysis. figure 3. standard model of electrolytic capacitor. acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 23 even the capacitor has significant degradation, the catastrophic failure should be avoided. it is sufficient to notice the wear out before it causes short-circuit through the capacitor and affects other components. in order to avoid damage, it is sufficient to notice faults as early as possible. the aim of the presented analysis is to find quantities which may vary with the aging or carries information about degradation. the mixed-signal model environment is a tool to analyze signals that may not be accessible on a real device. four different case studies were performed during converter operation: 1) new/unused input and output capacitors; 2) only output capacitor is degraded; 3) degraded input and output capacitors; 4) only input capacitor is degraded. in the four different simulations the buck converter was operated under conditions detailed above, the new and aged capacitor values were determined based on table 2. due to the pwm operation of converter and the harmonics occurred, the aging of the capacitor can be observed. the main wear out failure mechanism is the evaporation of the electrolyte solution which causes a decrease in capacitance (c) and an increase of the equivalent series resistance (esr). in the case of aged capacitor, the series resistance was increased by the factor 5 while the capacitance was decreased by 10 % compared to the unused capacitor values. the parallel resistance models the leakage current of the capacitor. the leakage current should be around 1-2 μa in the case of a new capacitor, however, this value is 8-10 μa for an aged capacitor [11]. the inductance of the capacitor foil is irrelevant since its value is less than 1 nh. the values shown in table 2 belong to a capacitor used in practical application and do not concern with theoretical domain. 4. signal analysis based degradation detection the mixed-signal environment allows the execution of frequency spectrum analysis of measured signals due to realistic system modelling. the presented approach utilizes measurement of voltage and current that are also required by the control system and it does not mean extra effort or cost. consequently, the diagnostic approach aims to analyze the output voltage waveform, inductor current and input capacitor voltage. these signals serve as a basis of a diagnostic technique which focuses on the detection of dominant frequency components or distorted frequency spectrum of the measured signals caused by the aging. figure 3 shows the output voltages of the buck converter when a 16 v output reference has been applied. simulations were run to all the four cases. the input and output capacitor aging have a significant influence on time domain characteristics (figure 4). the spikes on the characteristic are caused by the minimum and maximum values of the rectified voltage ripple that change corresponding to aging. the evaluation of frequency-domain data is supposed to be required to extract information from the noisy and distorted time-domain signal. the signal analysis has been performed by dft (discrete fourier transform) when the recorded signals are in steady state and under load change. the input data of dft was sampled by 10 mhz sampling frequency and sample count (n) was equal to data length. the frequency analysis was run on only the steady-state region in order to supress the effect of the transient part of the response. the dc component was removed by subtracting the average voltage from the data. the amplitude-frequency diagram of the frequency-domain data has been augmented to include a second chart which emphasizes the dominant frequency values. the dft of the steady-state signals resulted in frequency components with amplitude less than 0.1 v. in order to distinguish the dominant frequencies easier and help to recognize and extract the relevant information from the inspected region (2) has been introduced. 10(i 1) aux i n n 10i 1 a (f ) a(f ) + = + = ∑ (2) where aaux(fi) is the manipulated amplitude of the i-th downsampled component with unit v , a(fn) is the voltage amplitude of n-th frequency component according to the results of dft, i = 0,1,… it means ten times down-sampled frequency-domain data whereas the points are square-rooted. the square-root reduced the difference between the components with high and small amplitudes. since the frequency intervals are δf = 20 hz, the summary in (2) results in amplitudes valid for 200 hz intervals. 5. simulation results the output voltage of the capacitor has been analyzed in the first case. besides evaluating the steady-state waveforms, it seemed to be beneficial to gather information about capacitor aging upon load-change. 5.1. analysis of steady-state output voltage waveform the upper part of figure 5 shows the frequency-amplitude characteristics of voltage output of the converter in steady-state for 4 different capacitor conditions. in all cases, the switching frequency fundamental at 20 khz and its higher order harmonics can be observed. the fundamental component is operational-related property, so it was reasonable to track its figure 4. time-domain output voltages in four test cases. table 2. parameters of new and aged capacitor. input capacitor output capacitor new aged new aged rs 0.02 ω 0.1 ω 0.03 ω 0.15 ω rp 40 mω 2.4 mω 16 mω 1.6 mω c 10 000 μf 9 000 μf 12.5 μf 11.25 μf lc 0.7 nh 0.7 nh 0.3 nh 0.3 nh acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 24 amplitude in all test cases. when the output capacitor was selected for degraded component, the amplitude of fundamental component started to increase. nevertheless, when the input capacitor was degraded, the amplitude of 20 khz fundamental frequency component decreased. however, the capacitor parameter variation does not have significant influence on 20 khz component within the range set in table 2, therefore, the variation of fundamental component amplitudes is not practically measurable. there are considerable harmonics in the 0-4000 hz interval due to aperiodic spikes in the voltage time-domain waveform (figure 4). the worn out input capacitor amplifies the amplitudes of spikes because the filtering function of the capacitor is getting insufficient. it leads to increased harmonic amplitudes in the 0-4000 hz region. the effect of aged output capacitor on the inspected spectrum is negligible compared to the new one. the corresponding phase chart has also been generated. tracking the phase of aperiodic signals is unreliable and thus, unreasonable, consequently, the phase of fundamental and its higher order harmonics were investigated. these phase values have completely overlapped in all test cases. 5.2. analysis of transient state output voltage waveform the change in the capacitance due to aging is supposed to have an influence on the time constant of system output that in this case is an lc filter. in order to retrieve information about degraded capacitors in the circuit, analysis of voltage signal response during transient seemed to be reasonable. the transient has been introduced by load change that was twice as many as in the beginning. the load change was applied at the time t = 0.075 ms (figure 6). as the figure shows the figure 5. frequency spectrum of output capacitor steady-state voltage. figure 6. effect of load-change in output capacitor voltage waveform. acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 25 capacitor aging has no effect on the transient of the timedomain signal. the transient spike becomes dominant in frequency-domain and therefore, the differences between harmonics in 0-4000 hz interval have vanished. 5.3. analysis of inductor current waveform besides the output voltage, the inductor current is also used by the control loop so it was analysed in frequency-domain, as well. the frequency spectrum of inductor current is very similar to the spectrum of output voltage but the amplitudes in the region of lower harmonics are less significant. the output capacitor does not play an important role in the harmonic content because the capacitance of the output capacitor is much smaller than the input one. consequently, simulations that subjected the aged output capacitor and unused input and output capacitors together show the same results, however, the degraded input capacitor has noticeable effect on harmonic amplitudes. the fundamental component of the switching frequency at 20 khz varies in amplitude due to the dominance of input capacitor. in the case of unused input capacitor the amplitude of 20 khz component is 0.1 a, while this value is 0.118 a when input capacitor is degraded. 5.4. aging effects of input capacitor according to (1), the calculation of duty cycle requires the input voltage that can be considered constant in certain circumstances or can be measured for more reliable operation. although the converter can operate without knowing the accurate value of input voltage, it is practical and easy to measure this and applicable for diagnostics. the function of the input capacitor is smoothing the voltage ripple at the converter input. the filtering effectivity and characteristic of the capacitor is strongly related to its parameters. if a change occurs in its capacitance, the input voltage characteristic of the converter is distorted. figure 7 shows different results of aging of the input and output capacitors. the voltage signal contains the ripple caused by the 50 hz ac voltage as well as the 20 khz pwm switching harmonics. both of the mentioned voltage ripple have been amplified by aging but only the effect of the input capacitor degradation is outstanding. this phenomenon can be explained by the reduction of capacitance. the amplitude difference between the minimum and the maximum value of the ripple is 1.5 v when input capacitor is new and 2.5 v when it is aged. the peak-to-peak voltage ripple is 75 mv and 340 mv, respectively. in frequency domain, the fundamental component and its higher harmonics are remarkably increased (figure 8). in case of new capacitors and degraded output capacitor, the component’s amplitude is 26 mv at 20 khz and 18 mv at 40 khz and in case of degraded input and degraded input and output capacitor it reaches 112 mv at 20 khz and 72 mv at 40 khz. the voltage values presented previously have been figure 8. frequency-domain data of input capacitor voltage signal. figure 7. input capacitor voltage waveform. acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 26 determined from the measurement data which also serve as a basis for the calculation of root-squared voltages. note that the amplitudes on figure 8. are manipulated according to (2) and the role of the figure is to reflect the main changes in frequency spectrum. the amplitude of the smaller additional harmonic components has been also slightly increased when degraded input capacitor has been chosen. 6. conclusions the present paper concerns a buck converter that has been undergone a simulation analysis where the aging phenomenon was inspected in the filtering input and output capacitors for diagnostic purposes. the information extracted from interactions between the subsystems requires the implementation of the whole electric energy conversion system from network to load. it led to a universal and modular structure of simulation environment. a complex simulation environment has been implemented where digital and analogue domains can be connected and run simultaneously. it supports the analysis with practical aspects introducing cycle time and delays to the model and also, establishes connection between continuous and discrete domains. the continuous part of the simulation environment is based on orcad while the discrete domain is implemented in simulink. consequently, the circuit has been implemented in orcad and the controller algorithm has been realised in simulink. the diagnostic capabilities and features of the simulation environment have been validated on a buck converter because its structure is quite general to model most of the switch-mode power converters. for diagnostic purposes, the simulation has also included the rectifying stage to make the converter output effect at the input realistic. to avoid expensive additions to existing circuits or to wellintegrated designs, it is advantageous to use signals for diagnostics that are already present in the controller. after the analysis of input and output voltages and inductor current of the converter, it can be concluded that the capacitor aging can be observed in the amplitude variation of voltage spikes. based on observations of time-domain signal distortion it was reasonable to analyze the frequency-domain of the measured signals. in frequency-domain the signatures of capacitor aging appeared in the 0-4000 hz region of output voltage signal, however, during load-change this phenomenon become invisible since the harmonics due to load-change become dominant. this can be handled by the application of frequency analysis using sliding window where it can be seen how frequencies change through time. the amplitude of the operation-related 20 khz harmonic component changes when the output or input capacitor degrades. the 20 khz component amplitude of the output voltage waveform changes with a small amount with the aging of the capacitors. the ripple in the input voltage waveform has been amplified by aging of input capacitor. the variation of input voltage can be well-observed, but it can be difficult the measurement of this change in time-domain. in frequency domain, the fundamental component and its higher harmonics are remarkably increased so the detection of aging-related signal distortion is effortless. acknowledgement this research is supported by efop-3.6.1-16-2016-00015 project. the project is supported by the hungarian government and co-financed by the european social fund. the project has been supported by the european union, cofinanced by the european social fund. (efop3.6.2-16-201700002). references [1] m. marracci, b. tellini, m. catelani, l. ciani, “ultracapacitor degradation state diagnosis via electrochemical impedance spectroscopy”, ieee trans. on instrumentation and measurement, vol. 64 (7), pp. 1916-1921, july 2015. [2] y. gritli, “diagnosis and fault detection in electrical machines and drives based on advanced signal processing techniques”, ph.d. dissertation, department of electrical, electronic, and information engineering, university of bologna, 2014. [3] a. m. imam, t. g. habetler, r. g. harley, d. divan, “failure prediction of electrolytic capacitor using dsp methods”, applied power electronics conference and exposition (apec 2005), austin, texas, usa, 2005. [4] r. marschalko, d. fodor, p.teodosescu, m. bojan, “influence of dc-link capacitor aging on the pwm converters operation”, acta electrotehnica, vol. 52 (4), 2011. [5] a. braham, a. lahyani, p. venet, n. rejeb, “recent developments in fault detection and power loss estimation of electrolytic capacitors,” ieee trans. power electronics, vol. 25 (1), pp. 33-43, jan. 2010. [6] m. l. gasperi, “a method for predicting the expected life of bus capacitors,” presented at the ieee industry applications society annual meeting, new orleans, louisiana, usa, oct. 5-9, 1997. [7] k. abdennadher, p. venet, g. rojat, j. retif, c. rosset, “a realtime predictive-maintenance system of aluminum electrolytic capacitors used in uninterrupted power supplies”, ieee transactions on industry applications, institute of electrical and electronics engineers, 2010, vol. 46 (4), pp. 1644 1652. [8] j. poon, i. c. konstantakopoulos, c. spanos, s. r. sanders, “real-time model-based fault diagnosis for switching power converters”, ieee applied power electronics conference and exposition (apec), 2015. [9] m. l. gasperi, “life prediction modeling of bus capacitors in ac variable-frequency drives”, ieee transactions on industry applications, vol. 41 (6), pp. 1430-1435, nov./dec. 2005. [10] k. wan, “advanced current-mode control techniques for dcdc power electronic converters” ph.d. dissertation, missouri university of science & technology, 2009. [11] l. kovács, d. fodor, estimation of the maximum applicable voltage level of aluminium electrolytic capacitors by automated spark-detection measurement, hungarian journal of industrial chemistry, 2011, vol. 39 (1), pp. 73-78. electric energy converter development and diagnostics in mixed-signal simulation environment << /ascii85encodepages false /allowtransparency false /autopositionepsfiles true /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /error /compatibilitylevel 1.4 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.0000 /colorconversionstrategy /cmyk /dothumbnails false /embedallfonts true /embedopentype false /parseiccprofilesincomments true /embedjoboptions true 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can be opened with acrobat and adobe reader 5.0 and later.) >> /namespace [ (adobe) (common) (1.0) ] /othernamespaces [ << /asreaderspreads false /cropimagestoframes true /errorcontrol /warnandcontinue /flattenerignorespreadoverrides false /includeguidesgrids false /includenonprinting false /includeslug false /namespace [ (adobe) (indesign) (4.0) ] /omitplacedbitmaps false /omitplacedeps false /omitplacedpdf false /simulateoverprint /legacy >> << /addbleedmarks false /addcolorbars false /addcropmarks false /addpageinfo false /addregmarks false /convertcolors /converttocmyk /destinationprofilename () /destinationprofileselector /documentcmyk /downsample16bitimages true /flattenerpreset << /presetselector /mediumresolution >> /formelements false /generatestructure false /includebookmarks false /includehyperlinks false /includeinteractive false /includelayers false /includeprofiles false /multimediahandling /useobjectsettings /namespace [ (adobe) (creativesuite) (2.0) ] /pdfxoutputintentprofileselector /documentcmyk /preserveediting true /untaggedcmykhandling /leaveuntagged /untaggedrgbhandling /usedocumentprofile /usedocumentbleed false >> ] >> setdistillerparams << /hwresolution [2400 2400] /pagesize [612.000 792.000] >> setpagedevice acta imeko, title acta imeko issn: 2221-870x march 2018, volume 7, number 1, 72-75 acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 72 measurement of the sensitivity for diving-bell manometer jose eduin culma caviedes instituto nacional de metrología, avenida carrera 50 # 26-55 interior 2, bogotá d.c., colombia section: research paper keywords: diving-bell manometer; sensitivity; low pressure; low-gauge measurement citation: jose eduin culma caviedes, measurement of the sensitivity for diving-bell manometer, acta imeko, vol. 7, no. 1, article 14, march 2018, identifier: imeko-acta-07 (2018)-01-14 section editor: jorge torres-guzman, cenam – centro nacional de metrologia, santiago de querétaro, mexico received july 14, 2017; in final form november 8, 2017; published march 2018 copyright: © 2018 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: this work was supported by instituto nacional de metrología, colombia. grant: c-3502-0200-1 fortalecimiento investigación y desarrollo metrología nacional, sist-inm corresponding author: jose eduin culma caviedes, e-mail: jeculma@inm.gov.co 1. introduction in general terms, according to lipták [1] and dryjańska [2], in all inverted bell pressure measuring devices, the bell is immersed into a sealed liquid. the immersion produces a vertical movement that is related to a difference of pressures between the bell’s inner and outer surfaces. the bell’s movement can be compared to a calibration spring or to its own weight. depending on the reference side where pressure is measured, this device can detect absolute, positive, negative or differential pressures. different kinds of this type of device include single or double bells, with mercury or oil as the immersion liquid, and those designs where the bell’s movement is connected to a differential pressure in a linear or square root way (ledoux) [1]. moreover, inverted bell pressure measuring devices are used extensively to evaluate very low pressure [2]. the diving bell manometer found at the instituto nacional de metrología – inm (colombian national institute of metrology) is a veb junkalor dessau tmg 25/1104 model, made in 1982 [3]. this device has a range of measurement between – 1 000 and 1 500 pa [4]. this range can be extended to values from – 1 000 to 3 000 pa when flourinert fc 40 is used as manometric liquid. currently, kerosene is used as the operational liquid and this influences the produced pressure values obtained with this device [4]. this diving bell manometer is a single bell device, where the bell is suspended on the right the arm of an equal-arm balance (see figure 1), weights can be placed on the both arms. according to constantino et al. [5], small mass variations between one arm and the other are related to positive and negative pressure differentials. diving bell manometers, like the one found at inm, have been on the industry for a considerable amount of time. they have a variety of uses, including, but not limited to, air conditioning cost reduction, industrial leakage control and, as a result of recent quality control system implementations, serving as the standard in the calibration of other devices. they are also useful in medical equipment, pressurized rooms, and cleanroom certifications because these places need a controlled atmosphere in order to offer proper working conditions and abstract diving-bell manometers are devices mainly used for low-gauge measurements, with a variety of industrial and medical applications. at the colombian instituto nacional de metrología, a junkalor dessau tmg 25/1104 diving-bell manometer was subjected to sensitivity verification, with 200 mg as the discrimination threshold. considering that this manometer works analogously to an equal arm balance, sensitivity is defined as the weight change on the balance that is needed to produce a movement larger that the smallest estimable fraction of a scale interval determined by visual interpolation component (resolution on the indicator element). three set data points were evaluated: the first, with 20 g placed on each plate; the second, with 19.7 g on the left plate and 20 g placed on the right plate; and the third, with 20.3 g on the left plate and 20 g on the right plate. recommendations for use of the diving bell are made and the resolution for the device was determined. acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 73 low infection risk [5]. however, inverted bell pressure measuring devices authentic importance is that, even considering that these devices are not traded like in the past, they can be simplified and updated to meet metrology’s current needs [5]. several articles show that they have a consistent use in different laboratories, as seen in works by rantanen et al. [9], ahrendt et al. [10], perkin et al. [11] and zvizdic et al. [12]. most of these researchers worked on comparisons between inverted bell pressure measuring devices and other equipment used for analysing low pressure. nowadays there are others equipment available in the market that fulfill the diving-bell characteristics for lowpressure measurement. some of the tmg 25/1104 model’s characteristics, compared with similar devices can be seen in table 1. inverted bell manometers are highly valued instruments and this is a reason to preserve and maintain them. sensitivity verification is one of a series of efforts by inm toward this end. sensitivity is defined as the smallest weight change on the balance (it is considered that a diving-bell manometer works analogously to an equal arm balance—see figure 1). this is necessary to produce a movement of a unit of change on the indicator element (in this particular case, the indicator needle) [13]. when the weight is on the left plate, the needle moves to the right. when the weight is on the right plate, the needle moves to the left. in section 2, we will discuss the methodology followed to evaluate the sensitivity in the diving-bell manometer, considering its similarity to an equal-arm balance. in the next two sections, results and conclusions will be shown. 2. methodology 2.1. sensitivity analysis for each plate, 30 observations were taken: 10 for 20.0 g, 10 for 19.7 g and 10 for 20.3 g. certified weights were used, considering established reference parameters [14]. in total, 60 observations were taken. when weights varied in one of the plates, the other maintained a 20.0 g weight constant, following the instructions on oiml r 76-1 [15]. three set data points were evaluated: a) no difference: 20.0 g in both plates b) negative difference: 19.7 g and 20.0 g c) positive difference: 20.3 g and 20.0 g these values were taken according to oiml r 76-1 [15]: “sensitivity evaluation must be done adding additional weights with a small impact in order to avoid any effects by discrimination threshold.” keeping in mind that the threshold is 200 mg, then 300 mg is a slightly higher value above this threshold. observations were taken following the methodology by zvizdic et al. [12]. moreover, these observations were randomized to ensure their independence. according to its calibration certification, the diving-bell manometer is sensitive as long as a weight above 200 mg [16] produces a movement of a segment of scaling. the aim is to prove that a slightly higher weight (300 mg) can produce a movement higher than a segment of scaling. the best estimate of an analogue indicating device is determined by visual interpolation we call “ap” the number of times that the needle falls within two adjacent graduation lines [13] [17] [18]. in a practical sense, each segment of scaling was subdivided into 5 ap (see figure 2). accordingly, one hypothesis can be considered: the diving-bell manometer is sensitive if there is a significant difference among ap values for no difference versus ap values for positive and negative differences. in other words, if any ap value is not higher than 5 with additional weights above 200 mg, then the diving-bell manometer is not sensitive and it is not under its calibration conditional. 2.2. statistical analysis statistically, data was analyzed with a kruskal-wallis test, considering that their distribution was nor normal and that there are repeated values [19]. keeping this in mind, with the kruskal-wallis test, it was possible to establish whether there were or were not significant differences on ap values obtained with no differences in weight and weight differences above the minimum weight defined for this diving-bell manometer sensitivity, according to its calibration certification [16]. 3. results anova statistical methodology might produce unreliable outcome due to the fact that the data is about counting. this fact can affect some anova characteristics such as normality and homoscedasticity. therefore, the kruskal-wallis test is applied to avoid problems with these aforementioned figure 1. similarity between a diving bell manometer and an equal-arm balance. table 1. comparison of measuring equipment for low pressure. diving-bell manometer tgm 25/1104 piston gauge manometer fpg8601 [6] low-pressure micromanometer fco510 model 2 [7] portable commercial manometer manoair 1001 [8] meaurement pressure range -1 000 to 3 000 pa [16] -1 000 to 1 500 pa [4] 0 to 15 000 pa 200 to 2 000 pa -2 000 to 2 000 pa current status discontinued discontinued on the market on the market uncertainty not less than 0.3 pa [16] ± 0.25 pa + 0.003 reading percentage between ±0.05 y ±0.2 pa special feature positive and negative pressures measurement reduced noise portability (weight is just 6 kg) positive and negative pressures measurement and portability manufacturer veb junkalor dh instruments furness controls pce instruments figure 2. ap is the number of times that the needle falls within the lines. acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 74 characteristics. the null hypothesis is: ho = the ap averages for different levels of weight variation on the left and right plate should not exceed a minimum ap value of 5. for the left plate, obtained data (table 4), was analyzed with r software (table 6), concluding the information given in (table 2). moreover, this test showed that the highest ap value was for the 19.7 g level with a 6.5 average, and the lowest ap value was for the 20.3 g level with a 5.7 average. for the right plate, obtained data (table 5), was analyzed with r software (table 7), concluding the information provided in (table 3). moreover, this test showed that the highest ap value was table 2. kruskal-wallis test results for the left plate. statistic test 𝜒2 = 23.822 p value 𝑝 = 6.7 × 10−6 conclusion with a 95% confidence level, it is possible to reject the null hypothesis. this means that there is a highly significant difference between average ap values for the different levels of weight variation for the left plate. table 3. kruskal-wallis test results for the right plate. statistic test 𝜒2 = 25.215 p value 𝑝 = 3.3 × 10−6 conclusion with a 95% confidence level, it is possible to reject the null hypothesis. this means that there is a highly significant difference between average ap values for the different levels of weight variation for the right plate. table 5. right plate data and kruskal-wallis test. weight (g) ap value range rm 20.0 0 1 5.5 20.0 0 2 5.5 20.0 0 3 5.5 20.0 0 4 5.5 20.0 0 5 5.5 20.0 0 6 5.5 20.0 0 7 5.5 20.0 0 8 5.5 20.0 0 9 5.5 20.0 0 10 5.5 19.7 5 11 16.0 19.7 5 12 16.0 19.7 5 13 16.0 19.7 5 14 16.0 19.7 5 15 16.0 19.7 5 16 16.0 19.7 5 17 16.0 19.7 5 18 16.0 19.7 5 19 16.0 20.3 5 20 16.0 20.3 5 21 16.0 19.7 6 22 24.0 20.3 6 23 24.0 20.3 6 24 24.0 20.3 6 25 24.0 20.3 6 26 24.0 20.3 7 27 28.5 20.3 7 28 28.5 20.3 7 29 28.5 20.3 7 30 28.5 table 6. kruskal-wallis test data from left plate. weight (g) 𝒓� 𝒓�𝒊 𝒓�𝒊 − 𝒓� 𝒏𝒊 𝒏𝒊(𝒓�𝒊. −𝒓�)𝟐 19.7 15.5 23.6 8.1 10.0 656.1 20.0 15.5 5.5 -10.0 10.0 1000.0 20.3 15.5 17.4 1.9 10.0 36.1 h 21.834 correction 0.916 h' 23.822 p value 6.715e-06 table 4. left plate data and kruskal-wallis test. weight (g) ap value range rm 20.0 0 1 5.5 20.0 0 2 5.5 20.0 0 3 5.5 20.0 0 4 5.5 20.0 0 5 5.5 20.0 0 6 5.5 20.0 0 7 5.5 20.0 0 8 5.5 20.0 0 9 5.5 20.0 0 10 5.5 19.7 5 11 12.5 20.3 5 12 12.5 20.3 5 13 12.5 20.3 5 14 12.5 19.7 6 15 19.5 19.7 6 16 19.5 19.7 6 17 19.5 20.3 6 18 19.5 20.3 6 19 19.5 20.3 6 20 19.5 20.3 6 21 19.5 20.3 6 22 19.5 20.3 6 23 19.5 20.3 6 24 19.5 19.7 7 25 27.5 19.7 7 26 27.5 19.7 7 27 27.5 19.7 7 28 27.5 19.7 7 29 27.5 19.7 7 30 27.5 table 7. kruskal-wallis test data from right plate. weight (g) 𝒓� 𝒓�𝒊 𝒓�𝒊 − 𝒓� 𝒏𝒊 𝒏𝒊(𝒓�𝒊. −𝒓�)𝟐 19.7 16.0 17.8 1.8 10.0 32.4 20.0 16.0 6.0 -10.0 10.0 1100.0 20.3 16 25.2 9.2 10.0 846.4 h 22.887 correction 0.907 h' 25.215 p value 3.345e-06 acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 75 for the 20.3 g level with a 6.2 average, and the lowest ap value was for the 19.7 g level with a 5.1 average. 4. conclusion considering that both plates have significant differences and it is possible to conclude that the diving-bell manometer is sensitive due to the fact that the ap values on the plates are always higher than 5.0 ap when weights slightly heavier than 200 mg were placed on the plates. on average, for the left plate, the highest ap values were 6.5 ap while for the right plate the highest values were 6.2 ap. the sensitivity of 300 mg is equivalent to a pressure of 0.15 pa, however as the lowest measuring uncertainty for the divingbell manometer is 0.3 pa, then the contribution of the sensitivity must be included in the uncertainty budget of the diving-bell manometer. references [1] b. g. lipták, “5.9 manometers”, in: process measurement and analysis. b. g. lipták. crc press, boca raton, 2003, isbn 08493-1083-0 (v. 1), pp. 766-773. [2] a. dryjańska, “pressure measurement”, metrology laboratory (m-4), (2015), p. 6. [3] veb junkalor dessau, “bedienungsanleitung für feindruck-prüfwaage”, (1982), pp. 1-10. [4] laboratorio de presión-inm, “instructivo operación de equipos m1-01/l04/i-01”, (2015), pp. 1–72. [5] p. constantino, j. camacho, a. maciel, a. acquarone “diseño de un instrumento de bajo costo para la calibración de micromanómetros diferenciales”, innotec 2014, 9 (14 18), vol. 9, (2014), pp. 1–5. [6] d. h. instruments and fluke company, “fpg8601tm/vlpctm operation and maintenance manual”, (2007), pp. 1-166. [7] furness controls, “microprocessor micromanometer fc0510”, (2017), pp.1-2. [8] p. c. e. instruments, “manómetro de presión manoair100-1,” (2017), pp.1-2. [9] m. rantanen, s. semenoja, g. peterson, j. busk, “low gauge pressure comparisons between mikes, metrosert and force technology range -2000 pa to +2000”, mikes publication, 2007, pp. 1-22. [10] h. ahrendt, j. könemann, w. sabuga, et al., “final report on coomet key comparison of national pressure standards in the range 100 pa to 5 kpa of gauge pressure (coomet. m.pk14)”, versión 1, 2011, pp. 1–27. [11] m. perkin, c. rendle, j. jäger, et al., “comparison of low differential pressure standards in the range 3 pa to 1000 pa,” metrologia, 36, 1999, pp. 1-34. [12] d. zvizdic, l. g. bermanec, w. schultz, t. veliki, “characterization of lpm diving-bell manometer,” imeko 20th tc3, 3rd tc16 1st tc22 int. conf., p. 6, 2007. [13] s. v. gupta, “chapter 2 two-pan equal-arm balances”, in: mass metrology. springer series in materials science. springer-verlag, berlin heidelberg, 2012, isbn 978-3-642-23412-5, pp. 19-30. [14] sci, "proceso de calibración m-004 para balanza de dos platos con brazos iguales", madrid, 1991, pp. 1-22. [15] international organization of legal metrology, “oiml r 761:2006, non-automatic weighing instruments. part 1: metrological and technical requirements”, oiml, paris-francia, 2006, pp. 25-27. [16] physikalisch-technische bundesanstalt, “calibration certificate ptb 30050”,(2014), pp. 1–4. [17] dkd-r 6-1, “calibration of pressure of gauge”,(2014), p. 28. [18] r. idrovo, “informe de prueba y puesta en funcionamiento del manómetro de campana de inmersión nim 45008.,” bogotá d.c.-colombia, 2011, pp. 1-40. [19] w. kruskal, w. wallis, “use of ranks in one-criterion variance analysis”, journal of the american statistical association, vol 47, (1952), pp. 583-621. measurement of the sensitivity for diving-bell manometer << /ascii85encodepages false /allowtransparency false /autopositionepsfiles true /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /error /compatibilitylevel 1.4 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.0000 /colorconversionstrategy /cmyk /dothumbnails false /embedallfonts true /embedopentype false /parseiccprofilesincomments true /embedjoboptions true /dscreportinglevel 0 /emitdscwarnings false /endpage -1 /imagememory 1048576 /lockdistillerparams false /maxsubsetpct 100 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can be opened with acrobat and adobe reader 5.0 and later.) >> /namespace [ (adobe) (common) (1.0) ] /othernamespaces [ << /asreaderspreads false /cropimagestoframes true /errorcontrol /warnandcontinue /flattenerignorespreadoverrides false /includeguidesgrids false /includenonprinting false /includeslug false /namespace [ (adobe) (indesign) (4.0) ] /omitplacedbitmaps false /omitplacedeps false /omitplacedpdf false /simulateoverprint /legacy >> << /addbleedmarks false /addcolorbars false /addcropmarks false /addpageinfo false /addregmarks false /convertcolors /converttocmyk /destinationprofilename () /destinationprofileselector /documentcmyk /downsample16bitimages true /flattenerpreset << /presetselector /mediumresolution >> /formelements false /generatestructure false /includebookmarks false /includehyperlinks false /includeinteractive false /includelayers false /includeprofiles false /multimediahandling /useobjectsettings /namespace [ (adobe) (creativesuite) (2.0) ] /pdfxoutputintentprofileselector /documentcmyk /preserveediting true /untaggedcmykhandling /leaveuntagged /untaggedrgbhandling /usedocumentprofile /usedocumentbleed false >> ] >> setdistillerparams << /hwresolution [2400 2400] /pagesize [612.000 792.000] >> setpagedevice new approach to agile cycles containment effectiveness metrics in automotive software development acta imeko issn: 2221-870x december 2018, volume 7, number 4, 3-8 acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 3 new approach to agile cycles containment effectiveness metrics in automotive software development ionut-andrei sandu1, alexandru salceanu1 1 “gheorghe asachi” technical university of iasi, mangeron str. 67, 700050 iasi, romania section: research paper keywords: iteration containment effectiveness; program increment containment effectiveness; defect debt trend; phase containment effectiveness; software quality metric; agile; defects handling citation: ionut-andrei sandu, alexandru salceanu, new approach to agile cycles containment effectiveness metrics in automotive software development, acta imeko, vol. 7, no. 4, article 2, december 2018, identifier: imeko-acta-07 (2018)-04-02 editor: vilmos pálfi, budapest university of technology and economics, hungary received march 13, 2018; in final form october 2, 2018; published december 2018 copyright: © 2018 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: this work was supported by “gheorghe asachi” technical university of iasi, faculty of electrical engineering corresponding author: ionut-andrei sandu, e-mail: siandrei@gmail.com the importance of measuring the phase containment effectiveness metric in agile software development in the automotive industry, agile software development began to be used among developers as far back as 12 years ago, according to kugler maag cie”s study in 2015 [1]. since then, more and more automotive companies (oems and suppliers) that develop software-based electronic components are implementing the agile methodology. this is mainly because companies must keep pace and be flexible with constantly changing requirements, especially in current times, when the time to market is decreasing. organizations adopting the agile methodology also implicitly implement continuous process improvement, as teams and organizations must be effective and efficient. agile process transformation also implicitly triggers improvement actions and measures for software development processes. in this way, projects and organizations support and successfully fulfill the requirements of the process assessment models (e.g., automotive software process improvement and capability determination, a-spice® [2]). one of the principles of the agile manifesto is “working software is the primary measure of progress [3].” the ideal situation is that deliveries have no faults that affect the end user or faults that are introduced due to the incorrect implementation of the requirements. for the minimization of the defects rate, the classic software development approach is to use the phase containment effectiveness (pce) metric to calculate the effectiveness of the verification in each of the development phases. the pce metric answers the following questions: how efficient is the verification process? which phases escaped defects? which phases found/did not find those defects? how can the above questions be answered concerning programs and organizations that have adopted agile software development? we propose here a method of how to apply the pce metric to organizations and teams that have developed software for the automotive industry using the agile software development approach. abstract in an ideal agile development team, defects should not exist. however, in reality, especially in automotive agile software development, there must be a mechanism for handling defects and tracking them to closure. in this article, we describe the benefits of and principles underlying the measurement of defects handling metrics in automotive programs and in organizations that have adopted agile software development. we present the iteration containment effectiveness, program increment containment effectiveness, and defect debt trend metrics. the advantages acquired thereby are demonstrated by a detailed example of a real application concerning how to measure the classic phase containment effectiveness metric on the iteration (sprint) and program increment (scum of scrums / scaled agile) level. mailto:siandrei@gmail.com acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 4 previous achievements in this field pce was introduced in 1997 as a software quality improvement metric by a. r. hevner [4]. in 2003, it was also adopted in the six sigma (a disciplined, data-driven methodology for eliminating defects) [5]. this metric provides the ability to measure verification (a review, inspection, or unit-testing software method ensuring that each software unit satisfies its design) effectiveness and allows the software development team to improve their software development process. the pce metric can also be used for measurement in automotive software development by applying it in the specific software development and test phases [6]. faults can be classified as either errors or defects depending on the phase into which they were injected and the phase in which they were found. errors are faults that are discovered in the proper phase into which they were injected (e.g., design faults caught by design reviews). defects are faults that were not identified in the development phase (e.g., design faults caught in code reviews or software tests). ideally, all faults should be discovered in the phase in which they were introduced, leading to an idealistic pce of 100%. considering that in the automotive industry the rate of softwarerelated recalls increased from 5 % in 2011 to 15 % in 2015 (stout risius ross”s study, based on data from the united states national highway traffic safety administration [7]), improved phase containment is needed in automotive software development. increasing the faults detection rate within the development phases will reduce the problem resolution effort and the test effort. more precisely, detection of 10 % more defects in software design or coding phases can lead to a potential saving of 3 % in the total product development cost [8]. the error correction cost can even increase up to 90 times in the post-production phase compared to the concept phase [9]. the price of recalls comprises, besides fault-fixing costs, legal costs and image costs. at present, researchers have not decided how to apply the pce metric in agile software development. our specific approach agile software development is executed in iterations (according to scaled agile framework safe® model [10] or sprints in scrum [11]). working software (ideally fault-free) should be delivered at the end of every iteration. because incremental build and continuous integration is undertaken in agile software development [12], current delivery is used in the next iteration to add features on top. however, if a delivery containing undiscovered faults from iteration n is used to add new functionalities for the upcoming deliveries >n, the undiscovered faults are also implicitly translated to these deliveries. because defects can escape from one iteration to another in agile software development, the iteration itself can be considered as a phase in the classic pce metric. faults that have escaped identification from one iteration to another (i.e., those that are inherited by the next iteration) can be monitored and reduced by analyzing and taking appropriate actions when measuring the pce for iterations, which we call iteration containment effectiveness (ice): • iteration errors: faults caused during iteration n and discovered/solved during iteration n (e.g., during architecture review, code review, software testing). • iteration defects: faults caused during iteration n and detected/solved during iteration >n (the upcoming iterations) or by the customer the total number of faults is obtained by equation (1): 𝐼𝑡𝑒𝑟𝑎𝑡𝑖𝑜𝑛 𝐹𝑎𝑢𝑙𝑡𝑠 = 𝐼𝑡𝑒𝑟𝑎𝑡𝑖𝑜𝑛 𝐸𝑟𝑟𝑜𝑟𝑠 + 𝐼𝑡𝑒𝑟𝑎𝑡𝑖𝑜𝑛 𝐷𝑒𝑓𝑒𝑐𝑡𝑠 (1) ice can be calculated for each iteration by calculating equation (2): 𝐼𝐶𝐸 𝑓𝑜𝑟 𝐼𝑡𝑒𝑟𝑎𝑡𝑖𝑜𝑛 𝑁 = 𝐼𝑡𝑒𝑟𝑎𝑡𝑖𝑜𝑛 𝐸𝑟𝑟𝑜𝑟𝑠 𝐼𝑡𝑒𝑟𝑎𝑡𝑖𝑜𝑛 𝐹𝑎𝑢𝑙𝑡𝑠 ∙ 100% (2) how can defects that will be fixed in the next iterations be handled? a problem report (pr) should be opened, the defect should be added to the program backlog, and it should be prioritized accordingly. if the defect debt (the number of open prs) is increasing, a special iteration might be required for fixing bugs. how can defects (issues caused from previous iterations) that have been discovered and will be fixed in the current iteration be handled? if a defect that is not related to the current iteration but is severe enough that an immediate fix is required, a pr should be created. this will ensure transparency and alignment with all the stakeholders, as the scope of the current iteration may be affected. it will also support the project team concerning future situations in which similar problems may be detected. if the user story is closed, it is advised that a fix task or a pr is created anyway and linked with the initial story. [13] how can errors that are discovered and fixed in the same iteration be handled? valuable data would be lost if issues found during testing activities are not captured. however, entering and administrating formal pr entries is probably an overhead. this issue can be handled in either of the following ways: 1) such errors should be tracked to closure in a separate list 2) the current workflow used by the team should be followed. for example, when the development of a current user story is complete, the user story should be transitioned to a verifying state. if it did not pass the verification, the ticket should be returned to the implementing state. only after the developer fixes the discovered error is the user story transitioned again to the verifying state. synthesis of obtained results in the following example, we outline a program increment with five execution iterations. iteration 15 is the last one, executed on the creation date of the report. equations (1) and (2) allow us to calculate the ice for each iteration. this is how the values in table 2 were obtained for a specific program. acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 5 furthermore, in agile software development, the classic pce metric can be applied at the iteration level. to understand an iteration in whose development and testing phases defects were not identified, the classic pce for development metric [5] (based on the development and test phases used in the iteration) and the classic pce for testing metric [5] (considering only the development and test phases used in the iteration) should be used at the iteration level. as a program increment (pi) consists of several iterations and as the unit of a program execution is represented by the pi [10], ice can also be applied to the pi level by applying the same mechanism. we therefore propose program increment containment effectiveness (pice) in equation (3), which involves • pi errors: faults discovered during pi n • pi defects: faults that escaped from pi n and were detected during pi > n (the upcoming pis) or by the customer 𝑃𝑟𝑜𝑔𝑟𝑎𝑚 𝐼𝑛𝑐𝑟𝑒𝑚𝑒𝑛𝑡 𝑁 𝐶𝑜𝑛𝑡𝑎𝑖𝑛𝑚𝑒𝑛𝑡 𝐸𝑓𝑓𝑒𝑐𝑡𝑖𝑣𝑒𝑛𝑒𝑠𝑠 = program increment errors program increment errors+program increment defects ∙ 100% (3) the ice and pice metrics can also be analyzed together with the trend of open defects for each iteration/pi. in short, figure 1. ice values for a specific program using agile. table 1. ice values for a specific program using the agile methodology. it.6 it.7 it.8 it.9 it. 10 it. 11 it. 12 it. 13 it. 14 it. 15 customer total errors total defects total faults % ice program increment 2 iteration 6 20 3 0 0 1 10 2 0 0 0 7 20 23 43 47 iteration 7 100 10 0 5 22 3 1 0 0 8 100 49 149 67 iteration 8 21 3 0 0 0 0 1 4 1 21 9 30 70 iteration 9 36 3 1 0 1 0 0 3 36 8 44 82 iteration 10 78 2 0 1 0 0 2 78 5 83 94 program increment 3 iteration 11 60 0 0 0 2 6 60 8 68 88 iteration 12 57 1 0 0 3 57 4 61 93 iteration 13 54 0 0 5 54 5 59 92 iteration 14 63 10 4 63 14 77 82 iteration 15 52 1 52 1 53 98 figure 2. program increment containment effectiveness values for a specific program using agile. 0 10 20 30 40 50 60 70 80 90 100 % i te ra ti o n c o n ta in m e n t e ff e ct iv e n e ss iterations iteration containment effectiveness for agile sw development % ice goal 0 10 20 30 40 50 60 70 80 90 100 pi2 pi3% p i c o n ta in m e n t e ff e ct iv e n e ss program increment program increment containment effectiveness for agile sw development % pi containment effectiveness goal acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 6 the number of open inherited defects that is increasing over time from one iteration to another can be used as a starting point for analyzing the result of the ice metric with the ultimate goal of detecting in which iteration defects escape. as best practice, if the software development team does not have the capacity to solve the issues detected during the current iteration, open defects should be planned for implementation in the next iteration. furthermore, if the number of defects is high, the team can decide to designate one iteration for bug-fixing activities in order to reduce the defect debt. in the long term, improvement actions and measures for continuous process improvement at the iteration level should be defined. only by improving the pce for development for the phases used in the iteration can the fault debt and, implicitly, the development costs be reduced. this is how both pce and ice can be applied complementarily if the defect debt trend (ddt) indicates that a root cause analysis is necessary. in the following section, we present the ddt using data from the example mentioned above. this metric shows the cumulative number of open defects in each iteration. it also considers the number of resolved defects from past iterations. similar to ice, the ddt metric can also applied be applied at the program level. ddt iteration n = (previous iteration defect debt) + (total number of defects introduced in the previous iterations and discovered by iteration n) – (number of defects caused by the previous iterations and solved in iteration n) (4) by analyzing the ddt using equation (4), we can easily identify that in sprint (iteration) 8, the agile team took corrective actions to reduce the number of defects inherited from the previous iterations. we can also forecast that the number of defects should be reduced starting with iteration 11 or in the upcoming iterations. in , in the rows, we list the defects entered and solved in the same iteration and the number of defects entered in iteration n and discovered in the following iterations >n. we apply equation (4) in order to calculate the ddt for each iteration. we consider that the first iteration has a 0 defect debt. we present here the data starting with iteration 6. when the report was generated, the last iteration was iteration 15. root cause analysis with ice and pce metrics when measuring number of customer defects per lines of code metric in order to monitor the quality of the delivered product, the customer defects per lines of code (cdlc) metric can also be used in automotive software development projects by applying equation (5) [14]. cdlc = number of customer defects/(kilo lines of code) (5) the number of lines of code can be calculated by using equation (6): source lines of code (sloc) = total project lines of code – number of empty lines and commented lines (6) the sloc is used to determine the size and cost of software development projects. considering this metric, we can compare the size and complexity of different projects. figure 3. defect debt trend. table 2. defect debt values for each iteration. it. 6 it. 7 it. 8 it. 9 it. 10 it. 11 it. 12 it. 13 it. 14 it. 15 past iterations defects solved defects from past iterations defect debt program increment 2 iteration 6 20 3 0 0 1 10 2 0 0 0 4 0 39 iteration 7 100 10 0 5 22 3 1 0 0 4 0 43 iteration 8 21 3 0 0 0 0 1 4 16 50 9 iteration 9 36 3 1 0 1 0 0 8 0 17 iteration 10 78 2 0 1 0 0 21 0 38 program increment 3 iteration 11 60 0 0 0 2 44 40 42 iteration 12 57 1 0 0 6 0 48 iteration 13 54 0 0 10 0 58 iteration 14 63 10 6 50 14 iteration 15 52 28 15 27 0 10 20 30 40 50 60 70 it e r. 6 it e r. 7 it e r. 8 it e r. 9 it e r. 1 0 it e r. 1 1 it e r. 1 2 it e r. 1 3 it e r. 1 4 it e r. 1 5 n u m b e r o f o p e n d e fe ct s iterations defects debt trend ddt acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 7 by applying sloc, the cdlc metric is normalized and can be used for all types of projects, regardless of the complexity and during the entire project lifecycle. as cdlc values can be compared, they can be used for identifying the lessons learned from completed projects for future projects [15]. when cldc is measured in agile software development, for every iteration, it can be used for continuous improvement. it provides early feedback of the built-in quality achieved by the delivered working system. it is a highly powerful metric, which allows for the comparison of results between different types of project complexities. fault severity classification (major or minor [16]) needs to be defined either at the project or organization level. in order to determine what must be filtered out from the calculation, the business goal(s) and the questions that it is hoped that this metric should answer should be used as the starting point. it should be documented in the metric definition which type of defects will be counted (e.g., all, only major ones, only customer-visible defects). a company that is using this metric should establish its targets and monitor cdlc in projects accordingly. companies should take corrective actions [17] and improvement measures to fulfil the defined targets. if deviations appear, a root cause analysis can be undertaken by applying the ice and pce metrics. the efficiency of the agile cycles and development phases can be measured using these metrics. in this way, the number of defects discovered by the customer is reduced, and the consequent value and customer satisfaction increase significantly. in lean agile development [18], the results of the ice, ddt [19], and cdlc metrics can be used at the retrospective ceremony as process efficiency metrics. by analyzing the output of these metrics, the agile software development team can continuously improve the development and testing processes. even more so, these metrics can be used to provide objective evidence of improvement. in order to support organizations and agile software development teams to improve continuously, we detailed in this article the concepts and principles of ice and ddt metrics [19]. we presented the latest experimental data for ice, ddt, and how cdlc can be used together with these metrics. conclusions and recommendations for future research in this article, we showed how classic pce can be applied to agile software development by considering iterations instead of phases. the ice metric was defined, and its application was explained in the form of an example. by increasing ice through continuous process improvement, an agile software development team will not be overwhelmed by the increasing number of defects in the backlog, delivery commitments will be fulfilled, and the quality of the developed product will improve. we also presented how the ice metric can be used to analyze the results of the ddt metric, which shows the trend of open defects over time. even more so, as a result of the improvement measures, the increased rate of ice should lead to reduced values and lower trends in the ddt. we scaled the ice metric usage to the pi level by describing the relevant equation and manner of usage. fault debt (remaining open problem reports) from one iteration to another can be monitored and reduced by monitoring, analyzing, and taking appropriate action when measuring the ice metric. the lowest rates should indicate that a root cause analysis is necessary. we presented an approach of how to handle defects from previous iterations that are discovered and must be fixed within the current iteration. we also indicated how to track to closure the errors discovered and fixed within the same iteration. if these problem reports are not fixed, the defect debt increases, and the development process becomes unsustainable. variability increases, while for the entire product development system, predictability decreases. for root cause analyses and to understand an iteration in which defects were not identified in the development or testing phases, the classic pce for development (based on the development and test phases used in the iteration) and the pce for testing (considering only the development and test phases used in the iteration) can be used. if organizations want to improve their processes, their products, and customer trust, they should first focus on including in the ice and ddt metrics only faults that are visible to the end customer, regardless of whether they are critical or not. this suggestion implies that fault severity classification also needs to include customer visibility. concerning future research and development, we have clearly proposed how to measure ice and ddt metrics at the iteration and pi levels. in the future, we intend to investigate how data should be aggregated in order to measure these metrics also at the upper levels of the scaled agile methodologies in the organizations (e.g., the scaled agile framework safe® model. references [1] kugler maag cie, “agile in automotive – state of practice 2015”, may 2015, [online]. available: http://www.kuglermaag.com. [2] vda qmc working group 13/automotive sig, “automotive spice process assessment/reference model, version 3.0”, 2015, pp. 74-75 [online]. available: http://www.automotivespice.com/ fileadmin/softwaredownload/automotive_spice_pam_30.pdf. [3] k. beck, m. beedle, a. van bennekum, a. cockburn, w. cunningham, m. fowler, j. grenning, j. highsmith, a. hunt, r. jeffries, j. kern, b. marick, r. martin, s. mellor, k. schwaber, j. sutherland, d. thomas, ”manifesto for agile software development”, 2001 [online]. available: http://www.agilemanifesto.org. [4] a. r. hevner, phase containment metrics for software quality improvement, information and software technology, 39(13) (1997), pp. 867-877 [online]. available: http://www.sciencedirect.com. [5] d. l. hallowell, “six sigma software metrics maturity, part 1”, six sigma advantage inc, 2003, [online]. available: https://6sigma.com. [6] i. a. sandu, a., salceanu, “metrics improvement for phase containment effectiveness in automotive software development process”, proc. of the 10th international symposium on advanced topics in electrical engineering (atee 2017), mar. 23-25 2017, bucharest, romania, pp. 661-666. [7] stout risius ross, 2016, apr. 25, automotive warranty and recall report 2016 [online], available: http://www.srr.com. [8] c. ebert and r. dumke: software measurement, springer, heidelberg, new york, 2007, pp. 245-300. [9] d. seidler, t. southworth, ibm rational automotive engineering symposium 2013, source herstellerinitiative software (audi, bmw, daimler, porsche and volkswagen) 2016, nov. 21 [online]. available: https://www.ibm.com. [10] scaled agile inc, “safe® 4.0” [online]. available: http://www.scaledagileframework.com. http://www.kuglermaag.com/ http://www.agilemanifesto.org/ http://www.sciencedirect.com/ https://6sigma.com/ http://www.srr.com/ https://www.ibm.com/ http://www.scaledagileframework.com/ acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 8 [11] scrum.org, “the home of scrum” [online]. available: https://www.scrum.org. [12] j. shore, s. warden, the art of agile development, o”reilly media inc, 2008 [13] scrumcrazy.com, “one way to handle bugs and production support in scrum” [online]. available: http://www.scrumcrazy.com/one+way+to+handle+bugs+and +production+support+in+scrum. [14] i. a. sandu, 2018, “new approach of the customer defects per lines of code metric in automotive sw development applications”, xxii world congress of the international measurement confederation (imeko) sept. 3-6, 2018, belfast, ireland. [15] l. w. walker, “learning lessons on lessons learned”, paper presented at the pmi® global congress 2008 north america, 2008, denver, colorado, newtown square, pennsylvania. [16] k. el emam, the roi from software quality, auerbach publication taylor & francis group, 2005, pp. 14-16. [17] d. e. robitaille, j. rothman, corrective action for the software industry: a pragmatic approach to effective problem solving, 2004, paton professional. [18] d. p. oosterwal, “the lean machine: how harley-davidson drove top-line growth and profitability with revolutionary lean product development”, 2012, amacom. [19] i. a. sandu, a. salceanu, 2017, “applications of the phase containment effectiveness metric in automotive industry agile software development”, proceedings of the 22nd imeko tc4 international symposium and the 20th international workshop on adc modelling and testing supporting world development through electrical and electronic measurements, sept. 14-15, 2017, iasi, romania. https://www.scrum.org/ http://www.scrumcrazy.com/one+way+to+handle+bugs+and+production+support+in+scrum http://www.scrumcrazy.com/one+way+to+handle+bugs+and+production+support+in+scrum https://www.google.ro/search?tbo=p&tbm=bks&q=inauthor:%22denise+e.+robitaille%22&source=gbs_metadata_r&cad=7 https://www.google.ro/search?tbo=p&tbm=bks&q=inauthor:%22johanna+rothman%22&source=gbs_metadata_r&cad=7 acta imeko, title acta imeko issn: 2221-870x december 2017, volume 6, number 4, 25-30 acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 25 hardware and software implementation aspects for low-cost multi-channel interface for passive resistive sensors zivko kokolanski, vladimir dimcev, dimitar taskovski ss. cyril and methodius university in skopje, faculty of electrical engineering and information technologies, rugjer boskovic 18, 1000 skopje, republic of macedonia section: research paper keywords: multivibrator; sensor interface; passive sensors; multi-electrode sensors; labview citation: zivko kokolanski, vladimir dimcev, dimitar taskovski, hardware and software implementation aspects for low-cost multi-channel interface for passive resistive sensors, acta imeko, vol. 6, no. 4, article 4, december 2017, identifier: imeko-acta-06 (2017)-04-04 section editor: alexandru salceanu, technical university of iasi, romania received may 12, 2017; in final form august 19, 2017; published december, 2017 copyright: © 2017 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: z. kokolanski, e-mail: kokolanski@feit.ukim.edu.mk 1. introduction very often there is a need for measurements of a lot of physical quantities by a single data acquisition system, such as when performing sensor fusion. these systems must have multiple input channels in order to interface all of the sensors. regarding the application, the implementation of the systems ranges from individual sensor signal conditioning and separate analog-to-digital (ad) or time-to-digital (td) conversion for each channel, to sensor multiplexing and interfacing with only one input channel. however, quite often the overall performance of the system, its size, and cost, relies on the implementation of the sensor signal conditioning module. multi-channel sensor interfaces are used in many applications: in multi-electrode sensor systems [1], sensor data fusion [2], [3], multi-sensor systems regulated in standards [4], etc. generally, the sensor interfaces can be divided in two groups: ones based on ad conversion that utilize analog-todigital converters (adc) [5], and other based on relaxation oscillators (multivibrators) [6] and time-to-digital (td) [7] or frequency-to-digital (fd) [8] conversion. the second approach can be sometimes advantageous, especially when the sensors are directly interfaced to a digital programmable device (such as microcontrollers) [9]-[11]. microcontrollers usually include peripherals for accurate measurement of time events, which can be used for td or fd sensor interfacing without the need of adc. 2. multivibrator-based sensor signal conditioning generally, a multivibrator is an oscillating electronic circuit with discrete signal output, which is rather suitable for direct digital interfacing. it can be configured in monostable or astable operation. when triggered by an external trigger pulse, the monostable multivibrator (figure 1a) changes state and remains in this second state for an amount of time determined by the external rc components used. once this time interval has passed, the monostable multivibrator returns back to its original state awaiting a new trigger pulse. the result is an output pulse, which width is proportional to the rc time constant, arising each time the multivibrator is triggered. in astable operation (figure 1b), the multivibrator constantly switches between two states. the time duration of each stage is determined by the external rc network. now, the multivibrator delivers a pulse train which frequency, time interval, or duty cycle is determined by the rc network. abstract the paper elaborates the hardware and software design and implementation aspects of a low-cost multichannel interface for passive resistive sensors. the interface is based on a multivibrator in a monostabile configuration and is suitable for direct interface to a programmable devices (e.g. microcontrollers, field programmable gate arrays, etc.). in the paper, details concerning the sensor interface are presented, and different calibration techniques are described. the theoretical analysis is supported by a practical realization of a prototype multichannel resistive sensor interface and software implementation in a labview environment. acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 26 when used as a passive sensor interface circuit, the sensor (resistive or capacitive) is placed in the multivibrator rc network, whereas the other remaining component (r or c) is fixed. despite the fact that both circuits (monostable or astable) can be used as sensor signal conditioners, they have different metrological performances. namely, the measurement uncertainty of such interface circuit is analyzed in [6], where it is proven that the standard deviation of the output pulse width (type a measurement uncertainty) of the monostable multivibrator is lower than that of the astabile configuration. therefore, the proposed signal conditioning circuit in this paper is based on the monostabile multivibrator topology. in monostable operation, the multivibrator generates a single output pulse whenever the trigger input has been triggered by an external pulse. in such case, the voltage across the external capacitor increases exponentially with time constant τ=rc, where r is the value of the external resistor and c the value of the external capacitor. the waveforms of the characteristic signals during monostabile operation are given in figure 2. the output pulse width of the monostabile multivibrator t is given with: ,krct ≡ (1) where the constant k depends on the power supply voltage vcc and the threshold voltage vth of the “threshold” port in figure 1a. hence, assuming that r in (1) is a resistive sensor, the output pulse width t is directly proportional to the sensor resistance. from a metrological point of view, the instability of k (and c when analysing a resistive sensor interface) in (1) is problematic. this is usually solved by calibration [12]. another challenge is the ability to interface a larger number of sensors with less interface connections. 3. multichannel sensor interface description one possible solution towards increasing the number of sensors in the multivibrator-based sensor interfaces, while keeping a low number of interface connections to the programmable logic, is by time multiplexing. such solution is given in figure 3, which allows interfacing n×n passive resistive sensors, where n is the length of the ring counters 1 and 2. the ring counters change the state of their outputs d1 to dn on every clock (clk) period. when the counter reaches the final state (dn is “high”), it returns back to the initial position and generates a carry-out bit (cout). the carry-out bit signal acts as a clock of the ring counter 2. therefore, the clock frequency of the ring counter 2 is n times lower than the frequency of ring counter 1. even though there are commercially available integrated solutions of ring counters, a discrete implementation with d flip-flops is given in figure 4. the sensors are wired to the ring counters through mosfet transistors forming a sensor matrix, where ring counter 1 activates the rows and ring counter 2 activates the columns of the matrix. at a given moment, only a single sensor in the intersection row-column point is brought at the input of the monostabile multivibrator. an example of a simple monostabile multivibrator is given in figure 5. after “selection” of the sensor, the time constant of the multivibrator is determined by the sensor resistance rx and the fixed capacitor c. in this way, a consecutive sensor read-out is performed in a form of a pulse train at the output of the multivibrator. it is important to note that, in this solution, the speed of the measurements decreases proportionally with the number of sensors in the matrix. however, having in mind the speed of the actual digital programmable devices, time multiplexing is often acceptable, especially when measuring a slow-varying physical quantity (such as temperature or humidity). when a particular resistive sensor is selected with the switching matrix, and the monostabile multivibrator is triggered, the output pulse width is given with: ( ) ,1ln2       −+−= cc th dsonxx v v crrt (2) where rdson is the drain-to-source on resistance of the mosfet transistors, considering that all transistors in the matrix are identical. in order to implement the simplest “single point” calibration, a second measurement with a fixed calibration resistor is performed. this time the output pulse width is given with: ( ) ,1ln2       −+−= cc th dsoncc v v crrt . (3) the sensor resistance is calculated by dividing (2) and (3), that is: ( ) dsondsonc c x x rrrt t r 22 −+= , (4) where rc is the value of the calibration resistor. by considering the following approximations: rx>>2rdson and rc>>2rdson, (4) becomes: c c x x rt t r = . (5) it is clear that (4) suggests errors in a form of offset and gain in the transfer characteristics. however, these errors can be reduced by implementing a two point calibration [12] or the three signals method [13]. the two point calibration is performed in three phases: measurement of the sensor resistance rx, measurement of the first calibration resistor rc1, and measurement of the second calibration resistor rc2. figure 1. a) monostabile multivibrator, b) astabile multivibrator. figure 2. waveforms of the characteristic signals of a multivibrator in monostabile configuration. acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 27 figure 3. multivibrator-based multi-channel resistive sensor interface circuit. figure 4. state diagram and implementation of a six bit ring counter with d flip-flops. acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 28 the measured time intervals in the three measurement phases will be tx, tc1 and tc2 respectively (which can be expressed similarly as (2) and (3)). according the two point calibration, the sensor resistance can be estimated by using a two point linear approximation given with: ( ) 221 21 2 ccc cc cx x rrrtt tt r +− − − = . (6) this time the offset component will not be in the order of the drain-to-source on resistance (as with the single point calibration), rather of its mismatch between different mosfet transistors. the three signals method can be considered as a special case of the two point calibration where the second calibration resistor is zero. so, (6) becomes: 1 21 2 c cc cx x rtt tt r − − = . (7) however, in order to implement the three signals method it is necessary to use a current protection resistor in all measurement phases (resistor in series to c in figure 3). 4. software implementation the software programming architecture and implementation is a very important part for a successful multichannel sensor interface implementation. in fact, any digital programmable device containing a general purpose digital input/output ports and a timer module can be used for the software control algorithm implementation. in this paper, the software implementation in a labview environment is presented. however, such software implementation concepts can also be used for implementation on any other digital programmable device. the labview software implementation for the multichannel resistive sensor interface containing nine sensors is given in figures 7 and 8. having in mind that the sensor signal conditioning circuit can be divided into several phases, one of the most suited programming architecture is based on the state machine principles. the state machine state transition diagram is given in figure 6. the virtual instrument was realized for controlling the multichannel sensor interface containing nine resistive sensors and a single calibration resistor for the single point calibration implementation. the state machine contains four programming states which are represented with a simple state transition diagram (figure 6). the program starts with the “initialize and reset” state. in this phase, the virtual instrument resets the ring counters in order to start from the first sensor measurement and sets the default behaviour of the controls and indicators of the virtual instrument. then, the virtual instrument enters the second state named “trigger and measure”. since the first sensor is “selected” with the mosfet matrix, triggering the monostabile multivibrator results in delivering a output pulse which width is given by (2). afterwards, the output pulse is measured with a timer in the same program state. once the pulse width is measured, the virtual instrument enters the “increment clock” state where a clock pulse shifts the matrix column by one position, thus, selecting the second resistive sensor. whenever the clock port makes ten pulses (the ring counter overflows) then the virtual instrument enters the “calculate sensor resistance” state. otherwise it returns back into the “trigger and measure” state. once the sensor resistances are calculated, the virtual instrument resets the ring counters and starts the programming sequence from the beginning. in the current implementation the sensor resistance is calculated by using the single point calibration given with (5). however, the two point calibration or the three signals method can also be easily implemented by using one more external element and one additional programming state in the state transition diagram. on the other hand, when “low” drain-tosource on-resistance mosfets are used for the matrix realization, it is expected that the improvement of accuracy of the two point calibration and the three signals method will not be very significant. this is especially emphasized when measuring resistive sensors of relatively “high” resistance (say higher than 10 kω). the block diagram of the realized state machine programming architecture is given in figure 8. in the particular example, the control ports of the multichannel interface are controlled by using a commercial data acquisition (daq) card from national instruments. 5. experimental results and discussion to support the theoretical analysis, a prototype multichannel resistive sensor interface (figure 9) was designed. the prototype system was realized by using the timer ne555p from texas instruments wired in a monostabile multivibrator configuration. low drain-source on-resistance mosfet transistors bs250d for all transistors in the switching matrix were used. figure 5. example of a simple monostabile multivibrator based on the 555 circuit. figure 6. programming architecture state transition diagram. acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 29 the boards were supplied from a “clean” 9 v battery with lm78l05 positive voltage stabilizer to obtain 5 v. as recommended from the manufacturer, a 1 µf electrolytic and 100 nf ceramic capacitor was placed between the 555 power supply pins. the ring counters were realized with integrated hcf4017 10-bit decade counters. in the current implementation, the resolution of the ring counters allows theoretically the interfacing of 100 resistive sensors, which are modularly connected in groups of ten (figure 9). the output pulse period was measured with a data acquisition card usb6218 from national instruments connected to a pc in a labview environment. the daq card contains two 32 bit timers/counters with a selectable time base and two 8-bit digital input/output ports. the time base of the timers was 20 mhz with 50 ppm accuracy. the digital output pins p1.0, p1.1, and p1.2 were used as drivers (“clock”, “reset”, and “trigger” pins). the experiments were performed by interfacing six ntc temperature sensors with nominal resistance of 2000 ω, one ntc temperature sensor with nominal resistance of 10 kω, and figure 7. realization of the virtual instrument in labview for read-out of nine resistive sensors. figure 8. block-diagram of the labview virtual instrument implementation with a focus on the measure programming state. acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 30 two resistors with nominal values of 1 kω and 10 kω (variable). the 1 kω resistor was used for calibration. the transfer characteristic of the interface circuit was measured in the range from 1000 ω to 15 kω in ten discrete steps (by using the variable resistor). the maximal relative error recorded was 0.43 %, which is satisfactory considering the simplicity of the signal conditioning circuit. however, the metrological performances can be improved even more, by using lower drain-to-source resistance mosfet transistors, or by implementing a two point calibration or a three signals method. 6. conclusions in the paper, a hardware and software implementation principle for a multi-channel interface for passive resistive sensors was elaborated. the sensor interface incorporates a multivibrator in monostable configuration, and a sensor multiplexer implemented with ring counters and a mosfet switching matrix. three calibration techniques were described: single point calibration, two point calibration, and the three signals method. the software programming procedure was implemented in a labview environment by using a state machine programming architecture. key concepts for implementation of this programming architecture were identified and a prototype realization was proposed. the theoretical analysis was supported by the practical realization of a multichannel measurement system. the experimental results showed that, when implementing a sensor interface for nine resistive temperature sensors in the range from 1 kω to 15 kω, and by the implementation of the simplest “single point” calibration, a maximal relative error of 0.43 % was achieved. the performances of the signal conditioning circuit can be improved by using a two point calibration or the three signals method. authors believe that such sensor interface is advantageous for low-cost, small size implementations with medium accuracy. references [1] e. danicki, y. tasinkevych, “electrostatics of planar multielectrode sensors with application to surface elastometry”, sensors, vol.12, pp. 11946-11956, 2012, doi:10.3390/s120911946. [2] m. e. stieber, canadian space agency, canada, e. petriu, g. vukovich, “instrumentation architecture and sensor fusion for systems control”, ieee trans. instr. and meas., vol.47, iss.1, pp.108-113, 2002. doi:10.1109/19.728801. [3] marin b. marinov, todor s. djamiykov, volker zerbe: „sensor and actuator nodes for multi-zone demand-controlled ventilation” electrotecnica & electronica, vol. 49. no 78/2014, p. 26 – 32. [4] iec 60335-1, “safety of household and similar electrical appliences”, iec:1999(e). [5] r. p. areny, j. webster, “sensors and signal conditioning”, john wiley & sons inc., ney work, usa, 2001. [6] z. kokolanski, c. gavrovski, v. dimcev, m. makraduli, “simple interface for resistive sensors based on pulse width modulation”, ieee trans. instrum. meas., vol. 62 pp. 2983– 2993, 2013. doi:10.1109/tim.2013.2266025. [7] f. reverter, r. p. areny, “ direct sensor to microcontroller interface circuits”, marcombo s.a, barselona, spain, 2005. [8] h. danneels, f. pitte, v. de smedt, w.dehaene, g.gielen “a novel, pll-based frequency-to-digital conversion mechanism for sensor interfaces”, sensors and actuators a: physical, vol. 172, iss. 1, dec. 2011, pp. 220-227. [9] f. reverter, o. casas, “interfacing differential capacitive sensors to microcontrollers: a direct approach”, ieee trans. instrum. meas., october 2010. [10] f. reverter, o. casas, “interfacing differential resistive sensors to microcontrollers: a direct approach”, ieee trans. instrum. meas., 2009. [11] a. custodio, r. bragos, r. pallas-areny, “a novel sensorbridge-to-microcontroller interface”, ieee instrumentation and measurement technology conference, budapest, may 2001 [12] a. custodio, r. p. areny, r. bragos, “error analysis and reduction for a simple sensor-microcontroller interface”, ieee trans. instrum. meas., vol.50, no.6, 2001. doi: 10.1109/19.982960. [13] f.m.l. van der goes: “low-cost smart sensor interfaces”, ph.d thesis, delft university of technology, 1996. figure 9. prototype thirty channel resistive sensor interface. hardware and software implementation aspects for low-cost multi-channel interface for passive resistive sensors << /ascii85encodepages false /allowtransparency false /autopositionepsfiles true /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /error /compatibilitylevel 1.4 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.0000 /colorconversionstrategy /cmyk /dothumbnails false /embedallfonts true /embedopentype false /parseiccprofilesincomments true /embedjoboptions true /dscreportinglevel 0 /emitdscwarnings false 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/hrv (za stvaranje adobe pdf dokumenata najpogodnijih za visokokvalitetni ispis prije tiskanja koristite ove postavke. stvoreni pdf dokumenti mogu se otvoriti acrobat i adobe reader 5.0 i kasnijim verzijama.) /hun 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can be opened with acrobat and adobe reader 5.0 and later.) >> /namespace [ (adobe) (common) (1.0) ] /othernamespaces [ << /asreaderspreads false /cropimagestoframes true /errorcontrol /warnandcontinue /flattenerignorespreadoverrides false /includeguidesgrids false /includenonprinting false /includeslug false /namespace [ (adobe) (indesign) (4.0) ] /omitplacedbitmaps false /omitplacedeps false /omitplacedpdf false /simulateoverprint /legacy >> << /addbleedmarks false /addcolorbars false /addcropmarks false /addpageinfo false /addregmarks false /convertcolors /converttocmyk /destinationprofilename () /destinationprofileselector /documentcmyk /downsample16bitimages true /flattenerpreset << /presetselector /mediumresolution >> /formelements false /generatestructure false /includebookmarks false /includehyperlinks false /includeinteractive false /includelayers false /includeprofiles false /multimediahandling /useobjectsettings /namespace [ (adobe) (creativesuite) (2.0) ] /pdfxoutputintentprofileselector /documentcmyk /preserveediting true /untaggedcmykhandling /leaveuntagged /untaggedrgbhandling /usedocumentprofile /usedocumentbleed false >> ] >> setdistillerparams << /hwresolution [2400 2400] /pagesize [612.000 792.000] >> setpagedevice acta imeko, title acta imeko issn: 2221-870x june 2018, volume 7, number 2, 3-9 acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 3 high accuracy attitude and navigation system for an autonomous underwater vehicle (auv) enrico petritoli, fabio leccese science department università degli studi “roma tre” via della vasca navale n°84, 00146, rome, italy section: research paper keywords: accuracy; attitude; navigation; autonomous; underwater; vehicle; glider; auv citation: enrico petritoli, fabio leccese, high accuracy attitude and navigation system for an autonomous underwater vehicle (auv), acta imeko, vol. 7, no. 2, article 2, june 2018, identifier: imeko-acta-07 (2018)-02-02 editor: dušan agrež, university of ljubljana, slovenia received january 12, 2018; in final form march 21, 2018; published june 2018 copyright: © 2018 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: this work was supported by the science department of the università degli studi roma tre, rome, italy corresponding author: fabio leccese, e-mail: fabio.leccese@uniroma3.it 1. introduction the exploration of the sea and the discovery of the sea floor requires a great deal of data and the carrying out of very long research campaigns that can go from a few weeks to several months. for this reason, self-propelled autonomous underwater vehicles (auvs) are unsuitable because of their too short endurance; the rovs (remotely operated underwater vehicle) do not allow a high operative range and therefore require the constant presence of a supporting ship [1]. alace was one of the first solutions adopted to solve these problems: a drifting buoy that could suitably regulate its operational depth; once it reached the surface, it transmitted the data collected to the argos satellite communication system [2]. the need to be free of the randomness of the currents, and the increase of the miniaturization of electronic components, have led to the natural development: the underwater gliders [3]. an underwater glider is an auv that, by changing its buoyancy, moves up and down in the ocean like a float. unlike a float, an underwater glider uses hydrodynamic wings to convert the vertical motion to horizontal, moving forward with very low power consumption. figure 1 shows the example of our auv called squid [4]. 2. the sub glider while not so fast as conventional auvs, sub gliders using buoyancy-based propulsion have a high range and endurance compared to motor-driven vehicles, and may extend the mission to months, and to several thousands of kilometres of figure 1. squid tug (tailless underwater glider) view. abstract this paper examines the development of an attitude and control system for a tailless autonomous underwater vehicle (auv) without movable control surfaces. as the auv does not have movable surfaces, the buoyancy system and the center of gravity displacement manage the entire maneuvering system. acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 4 range [5]. the squid is a tailless sub vehicle: the cylindrical hull has a constant external diameter of 220 mm (approx.), with a radome on the nose and a shaped hydrodynamic fairing in the tail [6]. the hull (fuselage) is made of aluminium 6061-t6: it has excellent joining characteristics, good acceptance of applied coatings and combines relatively high strength, good workability and high resistance to corrosion [7]. the wings are in ultem 1000 (polyetherimide high-density polymer): it has a good dielectric strength, high inherent flame resistance and extremely low smoke generation; furthermore, it has high mechanical properties and performs in continuous use to 170 °c [8]. the vehicle has no movable control surfaces [9]. 2.1. fuselage the fuselage is composed by five coaxial cylindrical compartments (or bays): • payload bay: the nose cone has a prolate hemispheroid shape and contains the (customizable) payload and the ancillary systems, based on the new raspberry system [10] [14]. the section up to the first bulkhead is an available space that can be filled with all the needed instrumentation up to a diameter of 200 mm. if an active sonar is required, an alternate version of the radome is available [15]. the second part of the bay accommodates all the ancillary services such as payload power packs, thermal control, and other instrumentation recording and storage devices. • navigation and communications bay: it contains the glider integrated control system (gics), the ins (inertial navigation system) platform and the radio communication systems (global positioning system-gps, iridium rtx and hf emergency beacon) [16]. the gics oversees all the functions of navigation, guidance and vehicle control. • battery bay: it contains the battery pack and the servomotors to trim and regulate pitch and heading (yaw) of the auv. the batteries are mounted on a special support (cradle) and actuated by servomotors (controlled by the obc – on board computer) that allow the forward/backward scrolling (for pitch control) but also the right/left tilt for intrinsic direction control (figure 2). • buoyancy control bay: it contains the buoyancy motor and the oil tank. it balances the system longitudinally by adjusting the level in the reservoir. the bay also supports bulkhead load. the buoyancy motor has the task of pumping oil into the bladder and, in the event of a serious emergency, if ordered by the obc, it can swell the bladder to the maximum in order to reach rapidly the surface. • hydrodynamic fairing: it contains the oil bladder, is open to the water and provides a slender shape to the auv. the fairing has the task of not disturbing the hydrodynamic flow of the fuselage and closing the fuselage in closure. in any case, it must withstand considerable loads: for this reason, there are several reinforcements. it also protects the bladder from the flow and its dynamic loads, which could deform it. 2.2. wings the wing has a high aspect ratio with no dihedral and a swept of λ = 30°: the thickness is constant [17]. the wing profile is symmetric, type eppler e838 hydrofoil (see figure 3): it was intended for use at high reynolds’ numbers; the foil is optimized for use as a hydrofoil wing and at low speeds of the auv it expresses its best lift/drag rate. the choice of such a thick profile is due to two factors: first, the wing is subjected to considerable loads due to the aspect ratio even though the speed remains modest. the second is that in such a thick section it is possible to accommodate a hollow tubular aluminium spar, which increases flexural rigidity. each wing tip is provided with a küchemann carrot and a symmetric winglet in order to increase lateral stability and to reduce drag by partial recovery of the tip vortex energy. a karman aerodynamic fitting connects the wing to the fuselage. 3. buoyancy system the drone control system is essential: to vary the buoyancy of the vehicle, it is sufficient that the buoyancy motor modifies the quantity (volume) of oil in the bladder: (e.g.) if the overall volume increases, the drone begins to float. in order to reduce the force required to actuate the oil piston, which pushes the oil in the bladder at high depth, is necessary to reduce the piston surface (diameter) and increase the stroke. so, the buoyancy engine resembles to a “shotgun” [18]. the evaluation of the buoyancy b of the drone is made considering the buoyancy of the naked glider (as a rigid body) and the variable component due to the bladder and the buoyancy motor. therefore, the vertical balance of total forces on the glider is: 𝑊𝑇𝑇𝑇 = 𝑊𝐷𝐷 − 𝐵𝐺𝐺 − 𝐵𝐺𝐺 (1) where: 𝑊𝑇𝑇𝑇 = net total “weight” in the water. 𝑊𝐷𝐷 = dry weight of the glider. 𝐵𝐺𝐺 = buoyancy of the oil bladder. 𝐵𝐺𝐺 = buoyancy of the naked glider. 4. pitch control system for the pitch setting, is necessary to move the battery pack (and therefore the centre of gravity cg) forward or backward. however, in order to control the heading, it is necessary to tilt figure 2. fuselage profile view (cutaway). figure 3. the eppler e838 hydrofoil profile. acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 5 the pack at a certain angle [19]. we have to consider the attitude of the drone with a pitch angle ϑ (see section). for the balance of the forces on the zaxis, we have (figure 4): ∑ 𝐹𝑧 = 𝑊𝐺𝐵 + 𝑊𝑇𝑇 + 𝑊𝐺𝐺 + 𝐵𝐺𝐺 + 𝐵𝐺𝐺 = 0 (2) where: 𝑊𝐺𝐵= weight of the battery pack. 𝑊𝑇𝑇 = weight of the oil tank. 𝑊𝐺𝐺 = weight of the naked glider (without oil tank and batteries). at the equilibrium, we have: � 𝑀𝑌 = (𝑙𝐺𝐵 + 𝑧𝐺𝐵 tan 𝜃)𝑊𝐺𝐵 + 𝑙𝑇𝑇 𝑊𝑇𝑇 + 𝑙𝐺𝐺 𝑊𝐺𝐺 − 𝑙𝐺𝐺 𝐵𝐺𝐺 − 𝑙𝐺𝐺 𝐵𝐺𝐺 = 0 (3) for the position of the centre of buoyancy cb and the centre of gravity cg we have: the centre of gravity on the x-axis is defined as: 𝐶𝑋 𝐺 = 𝑙𝐵𝐵𝐷𝐵𝐵+𝑙𝑂𝑂𝐷𝑂𝑂+𝑙𝐺𝐵𝐷𝐺𝐵 𝐷𝐵𝐵+𝐷𝑂𝑂+𝐷𝐺𝐵 (4) the centre of gravity on the z-axis is defined as: 𝐶𝑍 𝐺 = 𝑧𝐵𝐵𝐷𝐵𝐵 𝐷𝐵𝐵+𝐷𝑂𝑂+𝐷𝐺𝐵 (5) the centre of buoyancy on the x-axis is defined as: 𝐶𝑋𝐺 = 𝑙𝐺𝐵𝐺𝐺𝐵+𝑙𝐵𝐵𝐺𝐵𝐵 𝐺𝐺𝐵+𝐺𝐵𝐵 (6) the centre of buoyancy on the z-axis, (by definition): 𝐶𝑍𝐺 = 0 (7) 5. heading control system to obtain a variation of the heading of the vehicle, due to the absence of the tail, rudder or ailerons, is necessary to move the centre of gravity around of the x-axis [20]. at the equilibrium, for the roll φ angle we have for the roll forces balance (figure 5): 𝜑 = 𝑧𝐵𝐵 𝑐𝑐𝑐 𝛼 𝐷𝐵𝐵 𝑍𝐺𝐵(𝐷𝐺𝐵+𝐷𝑂𝑂)+𝑍𝐵𝐵𝐷𝐵𝐵 (8) in almost-static conditions, if we move the wba slowly (or not fast), the only effect is simply the increase of the attitude φ angle. when the auv is “gliding” (i.e.: diving at constant speed), the effect is completely different. in fact, when the attitude φ angle increases, the up-going wing increases the local angle of attack αatt so that the lift and the drag increase while the down-going wing reduces the angle of attack, the lift and the drag, producing a rolling (also called 'banking') moment about the aircraft longitudinal axis [20]. in our case, the nose of the drone rotates towards the upgoing wing. in particular, the roll motion is characterized by an absence of natural stability; in fact, there are no stability derivatives that generate moments in response to the inertial roll angle (figure 6). a roll disturbance induces a roll rate and water viscosity cancels it: this takes place with insignificant changes in some sideslip and a yaw rate. 6. dynamic vertical forces balance here, we consider the drone diving (or emerging) at constant speed (figure 7) [21]. figure 6. banking effect scheme. figure 7. dynamic vertical forces balance (immersion (top) and emersion (bottom) phases). figure 4. pitch balance diagram. figure 5. roll forces balance. z acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 6 at the equilibrium, the dynamics on the vertical plane is, at constant speed: 𝑊𝑇𝑇𝑇����������⃗ + 𝐿�⃗ + 𝐷��⃗ = 0 (9) the expression for the lift is: 𝐿 = 1 2 𝜌𝑣2 𝑆𝐶𝐿 (10) according to the e838 characteristic “cl vs. alpha” (figure 8) when the angle of attack αatt = 0° (is null) the cl is zero so that the lift force l is null. this shows that the drone cannot progress horizontally at constant speed (straight and level): the only mission profile allowed is a sawtooth curve. 6.1. glider typical trajectory because its motion is due to the difference between the forces of weight and buoyancy, the glider is unable to proceed straight and level, thus being forced to follow a dive/climb trajectory made smooth by the wings [45]. moreover, unlike gliders in air, auvs can have ascending glide slopes if the net buoyancy is positive, producing a negative sink rate. the buoyancy engine of the glider allows changing its net buoyancy into alternating positive and negative states, thereby imparting it with the ability to string together a succession of descending and ascending glide slopes referred to as a sawtooth glide path (figure 9). 7. auv functional modes the particular mission of the auv allows us to consider it as a simple "finite-state machine" (fsm). the fundamental normal states of the machine (figure 10) are essentially two: the "diving mode" and the "rising mode". during these functional modes, the glider describes the sawtooth glide path [23]. the "floating mode" is used only to connect the glider with the support ship and/or used to download payload data through the satellite communications system. the "emergency mode" manages all possible problems that here we could reassume in dynamic instability, catastrophic failure, loss of batteries, loss of the payload, etc. in this case, the only contemplated action is to bring up the glider for preparing it to be rescued by a support boat [24]. a. floating mode the drone is afloat: at the beginning, from the gps system, it acquires position coordinates. then it sends them through another satellite communication system (e.g. iridium). secondly, it waits for a support boat and turn on wi-fi for any download of new commands or software update. b. diving mode the drone is diving: the programmed maximum depth is reached by running the drone at constant speed. c. rising mode the drone is rising: it proceeds at constant speed until it reaches the programmed minimum depth. d. emergency mode this is an emergency mode: if an unrecoverable attitude is detected, the system automatically goes to this emergency mode setting the buoyancy to the maximum and placing the trim in the maximum lift. this allows reaching the surface as soon as possible. e. transition conditions a the drone receives the command to submerge and to reach the programmed maximum depth. b the drone has reached the programmed maximum depth. c the drone has reached the programmed minimum depth. d an unrecoverable attitude is detected by attitude system control. e an unrecoverable attitude is detected by attitude system control. figure 10. functional modes scheme. figure 8. eppler e838 hydrofoil characteristics (re = 106). figure 9. sawtooth typical glide path (qualitative). acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 7 f the drone has ended the mission and receives the command to emerge to wait for new communications. g the attitude is trimmed on the maximum ascend ratio and the buoyancy is set to the maximum. the glider integrated control system (gics) manages the functional modes status and the transition modes [25]. 8. glider integrated control system (gics) the aim of the glider integrated control system is to perform the attitude and navigation control and to manage the data handling & control, including payload and communications management functions [26]. the architecture of the gics is shown in figure 11. it is built around the glider central unit (gcu), which is a central processing unit and includes several remote terminal units that interface the payload and glider equipment [27]. the functions performed are: • attitude determination and control. • buoyancy and other propulsion components control. • telemetry data acquisition, formatting and encoding. • command detection, decoding, distribution and actuation. • battery management. • payload management. except for the communications, all other functions are performed when the drone is in autonomous operation [28]. 9. navigation system the navigation system is composed by a strapdown ins (inertial navigation system) system corrected by the gnss (global navigation satellite system receiver using the gps, glonass, galileo or beidou systems) data every time the vehicle enters in the “floating mode” [29]. all inertial navigation systems suffer from integration drift: small errors in the measurement of acceleration and angular velocity are integrated into progressively larger errors in velocity, which are compounded into still greater errors in position [30]. since the new position is calculated from the previous calculated position and the measured acceleration and angular velocity, these errors accumulate roughly proportionally to the time since the initial position was input. therefore, the position must be periodically corrected by input from gnss (gps and other) satellite navigation systems when the auv is in the floating mode [31]. the benefits of this technology are lower cost, reduced size and greater reliability compared with equivalent platform systems. as a result, small, lightweight and accurate inertial navigation systems may now be fitted to small auv [32]. the general arranging of an ins is quite simple (figure 12): the output of a 3-axis accelerometer is rotated (axis transform) with the attitude angles supplied by three gyrometers: after a double integration we have the position offset with respect to the initial point [33]. the incurred major penalties are a substantial increase in computing complexity and the need to use high dynamic range sensors capable of measuring much higher rates of turn. 9.1. ins errors assessment the essential general error equations block diagram representation of the error model is given in figure 13: the diagram shows the schuler’s loop and other cross-coupling terms, which give rise to long-term oscillations [34]. an inertial navigation system over long periods has three type of errors, which propagate in time and are characterized by three distinct frequencies: • schuler’s oscillation the schuler’s oscillation has a period of: figure 12. strapdown ins unit block diagram. figure 13. ins long period errors correction block scheme. figure 11. gics general arrangement. axyz ωxyz 3 gyros 3 accelerom eters axis transform com pute attitude vector standard equations position velocity anev acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 8 𝜔𝑐 = � 𝑔 𝑅0 𝑇 ≈ 84.4 min (11) and it is present in both horizontal channels. • foucault’s oscillation the foucault’s oscillation has a period of: 𝜔𝑓 = ωsin𝐿 𝑇 = 2𝜋 ω𝑠𝑠𝑠𝐿 ≈ 30 h (12) and can be considered a modulation of the schuler’s oscillation. • 24 h oscillation the 24 h oscillation has a period of: 𝜔𝑒 = 15°/h 𝑇 = 24 h (13) and it is obviously equal to the period of rotation of the earth. generally, the full error model in the previous section is required to assess the performance of inertial navigation systems operating for long periods: for our glider applications, flight times are typically of the order of 2-3 days rather than weeks [35]. unlike an unmanned aerial vehicle (uav) system, in our case the inertial navigation error terms cannot be neglected: an aerial drone has a rather limited mission time and therefore all long-term oscillation periods can be absolutely deleted in the calculation of the position [36]-[38]. in our case, these errors also have a considerable weight, as they would critically contribute to the overall error [39]-[42]. all the following fixes help to minimize the ins platform error: however, at every denomination, the error is cancelled due to the connection to the gnss system (figure 14) [43][45]. the gnss measurements are used to update the kalman’s filter estimates the ins position, velocity and attitude errors [46][47]. these errors are then subtracted from the indicated position, velocity and attitude provided from the ins forming an optimal estimate of the true position [48]. 10. conclusions an underwater glider has been presented. for it, we studied both the buoyancy control and the attitude demonstrating that, since the motion equation directly affects the lift force and hence the pitch, it is not possible to separate the buoyancy control system and the attitude control system. in addition, we demonstrated that, during the banking, the speed is directly proportional to the lateral displacement angle of the centre of gravity and to the rate of descent. given the above, w.r.t. previous similar systems, ours shows the following novelties: • the presented attitude system control is optimized for a tailless underwater glider with a non-negligible wingspan: in fact, the wide wing surface allows a smooth glide slope and an excellent operational range; • the auv has no movable control surfaces in the hydrodynamic flow: therefore, the actuation of the attitude is based only on the variation of buoyancy and on the displacement of the centre of gravity. furthermore, a centralized control system called "glider integrated control system (gics)" was developed for the glider management. the gics monitors the buoyancy and attitude control (subject of another paper), handles the payload by taking care of the entire data package that it provides and of 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[48] s. wood, “state of technology in autonomous underwater gliders”, marine technology society journal, volume 47, number 5, september/october 2013. high accuracy attitude and navigation system for an autonomous underwater vehicle (auv) acta imeko, title acta imeko issn: 2221-870x march 2018, volume 7, number 1, 50-56 acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 50 smart maintenance and inspection of linear assets: an industry 4.0 approach dammika seneviratne1, lorenzo ciani3, marcantonio catelani3, diego galar1,2 1tecnalia research and innovation, industry and transport division, miñano (araba) 01510, spain 2division of operation and maintenance engineering, luleå university of technology, sweden 3department of information engineering, university of florence, via di s. marta 3, 50139, florence, italy section: research paper keywords: linear assets; autonomous robots; inspection; maintenance citation: dammika seneviratne, lorenzo ciani, marcantonio catelani, diego galar, smart maintenance and inspection of linear assets: an industry 4.0 approach, acta imeko, vol. 7, no. 1, article 9, march 2018, identifier: imeko-acta-07 (2018)-01-09 section editors: paul regtien, measurement science consultancy, the netherlands lorenzo ciani, university of florence, italy received october 30, 2017; in final form january 5, 2018; published march 2018 copyright: © 2018 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: dammika seneviratne, diego galar, e-mail: dammika.ndb@gmail.com, diego.galar@ltu.se 1. introduction a linear (or continuous) asset is an engineering infrastructure that usually spans long distances and can be divided into segments, all of which perform the same function but may be subject to different loads and environmental conditions. typical linear assets include railway lines, roads, pipelines and cables. modern society relies heavily on linear assets so it is imperative to manage them effectively. research on linear asset management has attracted considerable attention from asset management practitioners and academic researchers, although previous research has focused on specific types of linear assets. for example, ahern and anandarajah [1] developed a model for prioritising railway transportation infrastructure using weighted integer goalprogramming. li et al. [2] developed a management information system for mining railway transportation equipment. brito and almeida [3] attempted to rank risks for natural gas pipelines based on multi-attribute utility theory. their model can incorporate the decision-maker’s preferences and behaviours quantitatively. to model the variable condition of pipeline segments along a route, such as the pipeline failure rate, they divided the pipelines into a number of smaller sections. to address failure issues, cagno et al. [4] conducted a case study to estimate the prior distributions of gas pipeline failures using the analytical hierarchy process (ahp) and a bayesian approach. coffen-smout and herbert [5] presented an overview of submarine cable management challenges, while jones and mcmanus [6] conducted a life-cycle analysis of five different 11 kv electrical power cables. link [7] studied the renewal costs of motorways in germany using an econometric analysis but did not explicitly consider the road condition. finally, francesco and leccese [8] analysed electrical powerlines due to the seismic disaster using fmeca approach. conventional asset management systems or decision support tools are not suitable for linear assets; their hierarchical asset structure uses parent–child relationships to link system, abstract linear assets have linear properties, for instance, similar underlying geometry and characteristics, over a distance. they show specific patterns of continuous inherent deteriorations and failures. therefore, remedial inspection and maintenance actions will be similar along the length of a linear asset, but because as the asset is distributed over a large area, the execution costs are greater. autonomous robots, for instance, unmanned aerial vehicles, pipe inspection gauges, and remotely operated vehicles, are used in different industrial settings in an ad-hoc manner for inspection and maintenance. autonomous robots can be programmed for repetitive and specific tasks; this is useful for the inspection and maintenance of linear assets. this paper reviews the challenges of maintaining the linear assets, focusing on inspections. it also provides a conceptual framework for the use of autonomous inspection and maintenance practices for linear assets to reduce maintenance costs, human involvement, etc., whilst improving the availability of linear assets by effective use of autonomous robots and data from different sources. mailto:dammika.ndb@gmail.com mailto:diego.galar@ltu.se acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 51 assembly, component, and part type hierarchies and is not suitable for managing linear assets [9]. to address linear asset issues, ibm began to develop a generic approach to dynamic segmentation using existing standards such as iso 19133 (information technology geographic information framework data content standard) [10]. they discussed this idea among several development partners, including infrastructure managers, academics and government authorities, and decided to segregate the assets into linear and non-linear ones, making them more easily controlled by users. the maximo linear asset manager 7.1 was released after 3 years; it is based on the concept of ‘treating linear assets differently than pumps’ through the dynamic segmentation of the former [11], [12]. however, this tool does not integrate inspection and maintenance data and decision analysis functions. various types of engineering systems have been developed to acquire data on inspections or to perform maintenance tasks. these are mostly semi-automated systems, i.e., robotic applications. the fields of application range from welding rails to inspecting steel bridges, railways or nuclear power plants. in general, robots need to be developed for specific tasks and fields of application, as these define which locomotion principles or adhesion systems are suitable and which dimensions the robot must have. while the research results up to date are useful for maintenance management decision support, an effective framework for linear asset management inspection and maintenance needs to be developed to integrate field operations into decision support. as this paper argues, this could be generated using the emerging ict technologies, for instance, big data technologies. the paper discusses a framework to incorporate inspection data with maintenance data using big data technologies to manage linear assets by means of autonomous robots. the paper is set up as follows. section 2 discusses current industrial practices; section 3 explains the industrial challenges; section 4 discusses autonomous inspections and maintenance and explains the framework for the autonomous inspection and maintenance of linear assets; section 5 provides conclusions. 2. current industrial practice in today’s industrial setting, engineering assets are divided into three categories: 1) non-linear assets: non-linear assets can be further classified as component based assets, which include production assets such as machines, mobile assets such as vehicles, and fixed physical assets such as manufacturing facilities [8]; 2) linear assets: linear assets can be defined as engineering structures or infrastructures that often cross a long distance and can be divided into different segments that perform the same function but are subjected to different loads and conditions; 3) hybrid systems: these comprise a combination of linear and non-linear assets; the differences between the asset categories are summarised in table 1. linear assets have the following characteristics: • linear assets often form networks which consist of a number of ‘lines’. these lines are functionally similar, but can have different characteristics due to various construction materials, operational environments or geometric sizes; • linear assets may not have a clear physical boundary. for example, a pipeline is often partitioned using technical, jurisdictional and organisational criteria, and different criteria can result in different, perhaps overlapping, ‘pipelines’ being identified for the same pipeline network; • linear assets usually span long distances and are divided into segments. these segments are often virtually defined based on maintenance activities and may vary across maintenance actions. an example is given in figure 1. even though linear assets are constructed of basic elements such as individual pipes in a pipeline, the segments are not necessarily the same as the construction elements. failures, maintenance events and associated costs are usually recorded for segments, not for the whole linear asset; • linear assets are usually long-lived. as a result, when conducting reliability analysis, we may have sparse failure records for a few segments only, as the majority of segments have not failed; • if any single section of a linear asset malfunctions, the entire asset will not function properly. table 1. three asset categories. category example system configuration characteristics linear assets roads, railway tracks, pipelines, power cables, canals and waterways tree structure or networks -maintained and renewed in place and in segments -no clear physical boundary for segments -all segments normally perform the same function -maintenance costs, failures and maintenance and operational events are associated with segments -segments can be dynamic and are usually long lived non-linear assets pumps, cars, machine tools complex physical structure -installed and replaced as a whole, but maintained at base maintainable unit (bmu) level, often in workshops -bmu has clear physical boundaries -bmus often play different functions -maintenance cost, failures and maintenance and operational events are associated with bmus -bmus are usually static -lifetime varies hybrid assets power plant boilers, refrigerators, refineries complex physical structure, but linear and nonlinear parts have a clear boundary -installed, maintained and replaced in parts for large assets and as a whole for small assets -linear subsystems and nonlinear subsystems can often be separated acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 52 these characteristics of linear assets mean that their registry (categorisation), reliability and cost analysis, and maintenance decision modelling methods are different from those of nonlinear assets. 2.1. asset registry models in asset intensive industries where there are strict regulations, the use of a top/down or hierarchical approach to the management of linear assets is both challenging and multifaceted. these types of assets have exclusive requirements that demand a unique asset management approach, often called continuous or linear asset management [13]. in current industrial practice, engineering assets are registered using a hierarchical approach or a ‘parent–child’ model. using this model, maintenance attributes, such as maintenance history, repair costs, replacement costs and operational history, are directly linked to individual systems, subsystems, equipment or components. the traditional ‘parent– child’ hierarchical model works well for most non-linear assets, as there is normally a clearly fixed physical boundary between components. however, a parent–child hierarchy is not suitable for linear assets because of their virtually defined and dynamically changing segments. a network model approach may be useful in addressing these issues [8]. figure 2 applies the parent-child model to the linear asset hierarchy. the linear reference point (lrp) shown in the figure 2 is used as a marker schema for the asset. functional location is the identifier for the functionality of the asset segment. hca stands for the high consequence areas of the sub system. 2.2. data processing and segmentation of linear assets data grouping plays an important role in the management of linear asset inspection and maintenance. data grouping demands can be divided into: • dividing linear assets into proper segments; • correctly grouping segments. important data on inspection and maintenance can be gathered by investigating the main factors influencing asset degradation and failure. for water pipelines, for example, these could include pipe materials, soil types, water pressure, age, and pipe diameters. segment grouping is critical to scheduling the maintenance of linear assets. a major challenge is how to group similar but geographically-separated segments, especially with respect to how these groupings influence maintenance decisions and executed maintenance, e.g., sending a repair team out to replace all segments made of the same material and with the same amount of wear in a particular local. making maintenance decisions for a linear asset network involves the following: • determining which parts of the asset should be renewed together (i.e., a contiguous line or a group of separated but similar segments); • determining the best time to renew each part. the current industrial segmentation of linear assets is generally not suitable for maintenance management statistical analysis, as the assets can have various lengths, and the subdivisions relating to maintenance requirements may overlap. see figure 3. 3. industrial challenge 3.1. challenges of linear assets the main areas of linear asset use include railways, roads, pipelines, electricity distribution networks, canals and waterways. the major challenges of linear asset management are the following [14]: 1. structure. long and straight linear assets are different from up and down ones. 2. use of gis. linear asset management requires data from different sources. geographic information systems (gis) is one method to acquire data and information. but the reporting of maintenance activities should combine linear asset management data and gis data. 3. dynamic segmentation. this is the ability to define changing characteristic features along the length of an asset. for example a segment of pipeline will have changing features along its length. pipeline is the asset and the features can dynamically change across with piping segment which is maintained. 4. linear reference methods. the location on a linear asset can be signified in a several ways. the location can be absolute point (start of the segment) or relative to some defined marker such as a kilometre post or distance from a known marker. this referencing is useful to identify faults in the system (figure 4). 5. mass changes. tools are needed to effect mass changes to figure 1. dynamic maintenance segment boundaries of a linear asset. figure 3. segmentation of linear assets for maintenance management. figure 2. hierarchical parent-child approach for linear assets. figure 4. different methods used to record distance on a linear asset. acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 53 the linear assets under management – this is particularly relevant where costing boundaries change and both the linear assets and point assets need to be updated with new cost collectors. 6. location information. this can vary depending on the asset type, e.g.: roads: markers, e.g. kilometre posts, lanes, direction, offsets, bridges, tunnels, signs etc. rail: kilometre or chain markers, track number, offsets, switches, crossings, signal etc. pipelines: kilometre points, compressor stations, valves, pig stations etc. transmission lines: substations, traces etc. waterways: distribution points, gates, wires, dams, barrages, bridges, culverts, banks etc. 7. reflection of linear asset: the linear aspect has to be reflected in: • inspections • condition monitoring • maintenance and repair the maintenance of any linear infrastructure brings many challenges [15]: 1. modelling linear assets: modelling is the major challenge as assets are not limited to a single location and may span geographical areas. 2. assigning non-linear assets to linear assets: inclusion of non-linear assets within linear assets is difficult as it affects the behaviour of the latter. 3. parallel networks: with parallel networks, a failure of any one has consequences, such as work stoppage. 4. intersecting networks: networks can cross one another, so maintenance carried out on one asset will have an impact on another network. this happens, for example, when road and railways intersect pipelines, waterways and transmission lines. 5. planned maintenance time: stoppage for maintenance will result in an increased load on other networks as traffic is diverted; more usage may lead to reductions in performance. 6. skilled personnel to perform maintenance: maintaining linear assets requires groups from different departments to work on the same assets and coordination may be difficult. 7. special equipment for maintenance: maintaining these assets may require special equipment. 8. work order planning and execution: the planning and execution of maintenance tasks with multidisciplinary teams is a challenge. 9. varying load: the modelling of varying loads on linear assets to differentiate the changes in their maintenance needs. 10. maintenance analytics: in addition to planning and executing the work, other analytics may be required, for example, determining the downtime trend, cost of maintaining the network, allocation of costs to different networks etc. 3.2. typical attributes of a linear asset • a linear asset has an associated length dimension represented by means of start points and end points or asset length. components (or features) can be ‘installed’ along its length. • attributes can change frequently along its length. • it is typically connected with other linear assets to represent an infrastructure network or route. • a linear asset is subject to dynamic segmentation; multiple sets of attributes can be associated with any portion of an existing linear feature independently of where it begins or ends. • a linear referencing method is required to describe and locate a position along its length. • permanent referencing locations (markers or referencing locations) plus offsets are used to describe an exact position along the linear asset. 3.3. challenges in inspection and maintenance industrial maintenance service providers find that creating a technical structure to accurately represent the linear assets in their infrastructure network is a highly complex process, making maintenance and inspection strategies difficult to plan and report. yet maintaining the integrity of these assets ensures performance optimisation and compliance with health safety and environmental (hse) regulations. managing linear assets, such as waterways or rail networks, comes with a unique set of requirements. asset management functionality is associated with conducting detailed inspections, planning maintenance schedules, devising overhaul schedules and prioritising work to meet safety, budget and customer expectations. to maintain linear assets, asset managers must do the following: • split assets into sections based on a definable length breakdown. • add records for associated assets along lengths, such as locks, gates, footpaths etc. • record multiple ownership records for single assets. • import historical maintenance records, images, and any additional data in a dynamic manner. • perform on-site, roaming inspections using autonomous devices to record defects, identify issues, add images, log gps data, and add any other information required. • perform maintenance activities through a remotely operated interconnected environment. 4. autonomous inspection and maintenance remotely controlled and autonomous inspection and maintenance devices are used in different sectors for different purposes. for instance, the military uses unmanned aerial vehicles for inspection, and offshore oil and gas industries use underwater robots for maintenance. the following list lists some autonomous or remotely controlled devices used in the inspection and maintenance of linear assets and explains their purpose: railways: • identification of obstacles and track irregularities using drones. • inspection of rail profile, cracks, irregularities, missing components using an autonomous robot vehicle. • replacement of missing components, crack welding, etc. using an autonomous maintenance robot vehicle. roads: • identification of obstacles and damage using drones. • inspection of roadway, road alignment, road profile etc. using an autonomous robot vehicle. • repair of roadway (placement of asphalt/concrete), repair of pavement, maintenance of embankments, maintenance and cleaning of ditches, etc. using autonomous maintenance robot vehicle. canals and waterways: acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 54 • identification of debris, obstacles and damages for the infrastructure through drones. • inspection of waterway, sidewalls, berm, gates, etc. using an autonomous robot vehicle, both land and water. • removal of debris and obstacles, repair of sidewalls, berm, etc. using an autonomous maintenance robot vehicle (both land and water). power lines: • identification/inspection of power line damage, insulator defects, tower damage using drones. • cleaning of insulators and repair of line damage using an autonomous drone robot vehicle. 4.1. smarter drones unmanned aerial vehicles (uavs) have attractive features, such as flexibility, adaptability, and a range of payloads. sensors include high-resolution digital and infrared cameras, light detection and ranging (lidar), geographic information systems (gis), sonar sensors, and ultrasonic sensors; most can be adapted to a uav platform. a close-up photograph of a structure on an offshore platform, difficult for inspectors to reach, will show maintenance personnel how much corrosion/erosion has built up and suggest the situation of welds and other structural elements. drones equipped with forward-looking infrared (flir) or ultraviolet sensors can detect hot spots or corona discharge on conductors and insulators, signalling a potential defect or weakness in the component. lidar can be integrated with drones to survey a proposed right-of-way, show the infrastructure situation when seismic conditions are changing, or monitor the encroachment of vegetation. there are many more potential uses and the examples are only a small fraction of the possible applications. at present, uavs are remotely operated; the next phase of uav technology will be to deploy ‘smarter’ machines that can fly autonomously. this technology will allow uavs to sense and avoid other objects in their path, recognise features or components through various sensors (including cameras) using complex software algorithms such as image processing algorithms, and achieve situational awareness. this advanced technology will foster calculated decision making, such as initiating focused inspections, issuing work orders for repairs, and starting maintenance work with the same robot or another autonomous robot integrated in the system. in any industry, safety and cost are two of the most significant drivers of operation and maintenance and, thus, are always of high importance. many industrial work areas are hazardous, so measures must be taken to secure the safety of users. hse indicators can mitigate the risks, but the situation remains challenging when new technologies are introduced. for instance, working on energised high-voltage transmission lines, sometimes hundreds of feet up in the air, can make the consequences of a mistake deadly. according to the us bureau of labor statistics, 15 linemen were fatally injured in 2013 as a result of ‘exposure to harmful substances or environments’ [16]. unmanned systems have the potential to greatly reduce the amount of risk exposure of the operational workforce. the safety of personnel involved in risky operational tasks can be ensured with this new technology. 4.2. autonomous robots many different robots have been developed to handle various situations on linear assets, buildings, ship hulls, or other human-made structures. however, most are limited to special situations or applications. to execute the desired tasks, autonomous robots, as well as all other technical systems, have to fulfil certain requirements. the requirements and their importance and focus depend on the individual application or tasks. however, we can formulate a general set of requirements as follows: 1. velocity and mobility: vehicle speed and dynamics (ability to move) are two main aspects of robot design. depending on the dimension of the linear asset, it may have to reach a relatively high velocity for sufficiently fast navigation between inspection areas or similar points of action. another requirement is related to the desired manipulation and positioning capabilities of the system. this includes the precision of locomotion and its trajectory, since some inspection sensors need to be moved in a smooth and continuous way over the surface. the robot may also need to move sideways or to turn 360° to position sensors or tools. the system dynamics should be able to handle the various terrains and reach all positions of the asset. 2. payload: depending on the application, the system must be able to carry payloads of different weights. for example, in the case of steel piping, a payload of 5 kg or more is mandatory to carry ultrasonic inspection sensors. this requires a much bigger robot than a system which just needs a simple camera with a weight of several hundred grams. in other words, the dimension, adhesion, and motion components of the robot need to be adapted for the application. 3. reliability and safety: a further important non-functional aspect is the robustness of the system. if the autonomous robot fails frequently during one inspection task, it is not usable in practice. the requirements of reliability and safety include robust hardware, optimal controllers, and methods to detect and handle hazardous situations and to recover from them. 4. usability: velocity, manoeuvrability, and the capability of carrying a certain payload are important, but they are only the basis of the general operability of the system. to bring a robotic system into application, it has to be more powerful, more efficient, and less dangerous than common approaches, e.g., in terms of inspection devices. this includes aspects of maintainability and a broad range of other tasks. therefore, it must be able to carry different payloads (e.g. inspection sensors or tools) depending on the desired task, parts need to be easily replaceable, and the operation must be faster and less complicated than existing approaches. aspects like energy consumption, weight, or dimension of the system can be important as well. based on the individual task, a robot developer has to decide which requirements have to be fulfilled and select a suitable locomotion and attraction principle. 4.3. autonomous inspections traditionally, electric power suppliers have inspected power lines for encroaching trees, damage to structures, and deterioration of insulators by having employees traverse the lines on foot and climb the poles. this is time-consuming and arduous, with a considerable element of risk. now the task is often carried out by crews in manned helicopters using binoculars and thermal imagers to detect the breakdown of insulators. this too is not without hazard. recently, trials have tested the use of uavs to inspect power lines, with considerable success. uavs offer lower costs, acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 55 do not create a hazard for aircrews, can operate in more adverse weather conditions, and are less obtrusive to neighbouring communities or animals. hover flight is essential for the inspection task. the uav carries an electro-optic and thermal imaging payload, the data from which are available in real-time to the operator and recorded. the uav is automatically guided along the power line within a limited volume of airspace close to the lines using a distance measuring device. an important requirement of uavs deployed in this role is that they must be flown close to high voltage power lines, that is, within their electromagnetic fields, without adverse effects upon their control system or payload performance. oil and gas supplying companies are interested in uavs for inspection and exploration purposes. uavs offer a less expensive means of surveying the land where pipelines are installed. they also offer a means of patrolling the pipes to look for disruptions or leaks caused by accidents such as landslides or lightning strikes or for damage caused by vehicles or falling trees. in certain areas of the world, sabotage is not uncommon, so they look for this as well. uavs could be used in road and railway inspections and for certain maintenance purposes by traffic infrastructure agencies. in addition to being less expensive to operate than manned aircraft, they are more covert and will avoid distracting drivers. irrigation projects, river authorities, and water boards could use uavs to monitor canals, waterways, pipelines, and rivers. uavs could be used to monitor reservoirs for pollution or damage or to monitor pipelines for security purposes. however, the use of uavs in many of these cases will depend on the approval of the relevant regulatory authorities. 4.4. autonomous maintenance autonomous maintenance activities are mainly associated with robotic applications. various industries, especially those dealing with high risk activities, are already using remotely operated robots for maintenance activities, for instance, marine repairs (repairs of ships offshore, offshore oil and gas platform maintenance, deep sea pipeline and cable maintenance), oil refinery repairs, nuclear power plant repairs etc. at the moment, because of the limited development of robots for maintenance purposes, complete maintenance cannot be performed in the above mentioned industries. 4.5. conceptual framework with autonomous inspection devices and autonomous maintenance robots, a dynamic asset maintenance and management plan can be deployed with the help of big data technologies and available analytics. right now, industries are using the devices separately for inspection and maintenance; the two have not yet been integrated. by integrating the two operations with the available ict technologies, the asset maintenance and management process can be automated. the possible architecture for the ict framework is shown in figure 5. moreover, the incorporation of artificial intelligence tools can make the whole process dynamic and autonomous. since linear assets have a common behaviour and architecture across their length, the implementation of the concept may reduce costs, ensuring more effective operation and maintenance. the proposed framework is shown in figure 6. 5. conclusions the importance of autonomous inspections and maintenance in linear assets is increasing because many of these assets are aging and, at the same time, asset managers are struggling to operate effectively and maintain costs. reliable inspection and maintenance methodologies incorporating new technologies, specifically, the emerging ict technologies, would facilitate cost effective and efficient asset management. the industry movement towards new and more sophisticated inspection, condition monitoring, analysis and maintenance technologies, together with the development of autonomous robotics, will provide a platform to maintain large assets (i.e. liner assets) more efficiently. the technology is available but integration remains a key concern. references [1] a.ahern, g. anandarajah, ‘an optimisation model for prioritising transport projects,’ in proc. of the institution of civil engineers-transport, thomas telford ltd, 2008, vol. 161, no. 4, pp. 221-230. [2] m.y.li, k.q.han, x.y.zhang, x.l.li, y.j.zhai, w.h.yu, ‘a management information system for mine railway transportation equipment,’ journal of china university of mining and technology, 2008, vol. 18, no. 3, pp. 373-376. linear assets inspection robot (uav) inspection/maintenance/surveillance robot data storage/ processing/ analysis/ transmit facility data storage/ processing/ analysis/ transmit facility analytics and tools (big data analytics) cloud data storage figure 6. proposed conceptual framework for autonomous inspection and maintenance of linear assets. se ns or n et w or ks cloud data analysis agents analytics and tools (big data analytics) cloud information storage secure data access cloud web portal ( decision support portal) knowledge base maintenance requirment identification modules maintenance resource allocator maintenance task scheduler figure 5. proposed ict infrastructure for the autonomous inspection and maintenance of linear assets. acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 56 [3] a.j.brito, a.t.de almeida, c.m.mota, ‘a multicriteria model for risk sorting of natural gas pipelines based on electre tri integrating utility theory,’ european journal of operational research, 2010, vol. 200, no. 3, pp. 812-821. [4] e.cagno, f.caron, m.mancini, f.ruggeri, ‘sensitivity of replacement priorities for gas pipeline maintenance,’ in robust bayesian analysis, springer new york, 2000, pp. 335-350. [5] s.coffen-smout, g.j.herbert, ‘submarine cables: a challenge for ocean management,’ marine policy, 2000, vol. 24, no. 6, pp. 441448. [6] c.i.jones, m.c.mcmanus, ‘life-cycle assessment of 11kv electrical overhead lines and underground cables,’ journal of cleaner production, 2010, vol. 18 no. 14, pp. 1464-1477. [7] h.link, ‘an econometric analysis of motorway renewal costs in germany,’ transportation research part a: policy and practice, 2006, vol. 40, no. 1, pp.19-34. [8] e.de francesco, f.leccese, ‘risks analysis for already existent electric lifelines in case of seismic disaster’, in proc. of the 11th international conference on environment and electrical engineering, eeeic, 2012, art. no. 6221490, pp. 830-834. doi: 10.1109/eeeic.2012.6221490. [9] y.sun, l.ma, w.robinson, m.purser, a.mathew, c.fidge, ‘renewal decision support for linear assets,’ in engineering asset management and infrastructure sustainability, springer, london, 2012, pp. 885-899. [10] iso 19133, ‘geographic information -location-based services tracking and navigation,’ international organization for standardisation, switzerland, 2005. [11] e. jones, ‘linear asset manager a quick history,’ [online]. available: https://www.ibm.com/developerworks/community/blogs/a9ba 1efe-b731-4317-9724 a181d6155e3a/entry/linear_asset_manager_a_quick_history18?la ng=en [12] y.sun, l.ma, w.robinson, m.purser, a.mathew, c.fidge, ‘renewal decision support for linear assets,’ in engineering asset management and infrastructure sustainability, springer, london, 2012, pp. 885-899. [13] ibm, ‘better manage your linear assets with ibm maximo linear asset manager,’ [online]. available: https://www31.ibm.com/solutions/cn/traveltransportation/pdf/3better_ma nagemaxio.pdf [14] duncan, ‘five reasons why linear assets are different,’ [online]. available: https://millsone.wordpress.com/2010/03/01/fivereasons-why-linear-assets-are-different/ [15] a. kaliappan, ‘managing linear assets with oracle eam,’ [online]. available: http://www.aioug.org/sangam14/images/sangam14/presentati ons/201593_kaliappan.pdf [16] d.schmidt, k.berns, ‘climbing robots for maintenance and inspections of vertical structures—a survey of design aspects and technologies,’ in robotics and autonomous systems, 2013, vol. 61, no. 12, pp. 1288-1305. https://www-31.ibm.com/solutions/cn/traveltransportation/pdf/3better_managemaxio.pdf https://www-31.ibm.com/solutions/cn/traveltransportation/pdf/3better_managemaxio.pdf https://www-31.ibm.com/solutions/cn/traveltransportation/pdf/3better_managemaxio.pdf 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can be opened with acrobat and adobe reader 5.0 and later.) >> /namespace [ (adobe) (common) (1.0) ] /othernamespaces [ << /asreaderspreads false /cropimagestoframes true /errorcontrol /warnandcontinue /flattenerignorespreadoverrides false /includeguidesgrids false /includenonprinting false /includeslug false /namespace [ (adobe) (indesign) (4.0) ] /omitplacedbitmaps false /omitplacedeps false /omitplacedpdf false /simulateoverprint /legacy >> << /addbleedmarks false /addcolorbars false /addcropmarks false /addpageinfo false /addregmarks false /convertcolors /converttocmyk /destinationprofilename () /destinationprofileselector /documentcmyk /downsample16bitimages true /flattenerpreset << /presetselector /mediumresolution >> /formelements false /generatestructure false /includebookmarks false /includehyperlinks false /includeinteractive false /includelayers false /includeprofiles false /multimediahandling /useobjectsettings /namespace [ (adobe) (creativesuite) (2.0) ] /pdfxoutputintentprofileselector /documentcmyk /preserveediting true /untaggedcmykhandling /leaveuntagged /untaggedrgbhandling /usedocumentprofile /usedocumentbleed false >> ] >> setdistillerparams << /hwresolution [2400 2400] /pagesize [612.000 792.000] >> setpagedevice measuring the farm's profitability after the adoption of precision agriculture technologies: a case study research from italy acta imeko issn: 2221-870x september 2020, volume 9, number 3, 65 74 acta imeko | www.imeko.org september 2020 | volume 9 | number 3 | 65 measuring a farm's profitability after adopting precision agriculture technologies: a case study from italy giorgia bucci1, deborah bentivoglio1, matteo belletti1, adele finco1 1 department of agricultural, food and environmental (d3a), marche polytechnic university, ancona, italy section: research paper keywords: precision agriculture, measurement; cost-benefit analysis; profitability, case study research citation: giorgia bucci, deborah bentivoglio, matteo belletti, adele finco, measuring the farm's profitability after the adoption of precision agriculture technologies: a case study research from italy, acta imeko, vol. 9, no. 3, article 11, september 2020, identifier: imeko-acta-09 (2020)-03-11 section editor: mauro d'arco, university of naples federico ii, italy received march 9, 2020; in final form august 31, 2020; published september 2020 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was supported by the rural development programme (rdp) of marche region 2014-2020, which financed the operational group (op) s.a.t. (smart agriculture team) precision agriculture: reduction of the environmental impact of production systems (id n°29000). corresponding author: deborah bentivoglio, e-mail: d.bentivoglio@univpm.it 1. introduction as a means of producing on-site data to guide decisionmaking, precision agriculture (pa) is a whole-farm management approach that allows for managing crops growth for better yield and quality through measuring physical parameters and collecting data [1]. starting in the 1990s, several definitions of pa have been given in the literature, [2]-[6], but all the authors agree that this practice allows the site-specific management of the agronomic inputs and practices within a field through accurate measurements [7]. in detail, we can consider pa as integrated information and as a production-based farming system, designed to deliver high-end technology solutions to increase farm production efficiency and profitability while minimising environmental impacts. pa technologies (pats) are all those innovations that incorporate recent advances in modern agriculture, providing evidence for lower production costs, increased farming efficiency and reduced impacts. accuracy and precision are two relevant factors to consider when taking data measurements. they both reflect how close a measurement is to an actual value, but accuracy refers to how close a measurement is to a known or accepted value, while precision reflects how reproducible measurements are. for a long time in the field of pats, digital devices, which can take more accurate and precise measurements, generally corresponded to higher investment costs. this economic constraint initially caused a limited diffusion of pa. today, however, there are a wide range of different low-cost devices is available on the market that allow for meeting measurement accuracy requirements. further, policymakers at the national and european level have established a set of measures and initiatives to encourage and facilitate the purchase and use of digital technologies, including pats, throughout the agri-food supply chain (figure 1). abstract precision agriculture (pa) offers the opportunity for farmers to improve both efficiency in managing resources and optimisation of process inputs, thus increasing their whole farm’s profitability. despite these well-known benefits, the adoption of pa technologies (pats) is still challenging due to socio-economic barriers and unique characteristics of the farms: cropping systems, technical developments, field sizes and farm scale. the economic aspect is undoubtedly one of the most important aspects to consid er before adopting pats. in most of the cases, farmers are reluctant to introduce precision farming systems since the costs and uncertainty about the profitability and advantages need to be addressed. this study aims to explore how pats could affect the profitability of a representative italian farm specialising in the production of cereals, making this a case study. in detail, an economic analysis was applied to determine the profitability of the farm, which showed that the adoption of pat’s increased the yield of durum and soft wheat and significantly reduced the cost of mechanical operations and technical means. therefore, the potential gains from the adoption of pats challenges policymakers to design targeted interventions which could encourage their uptake. this paper is an extended version of the original contribution presented to the 2019 ieee international workshop on metrology for agriculture and forestry (metroagrifor) in portici, italy. mailto:d.bentivoglio@univpm.it acta imeko | www.imeko.org september 2020 | volume 9 | number 3 | 66 from european level, on 19 april 2016, the european commission launched the first industry-related initiative known as the digital single market strategy, part of the digital agenda, aiming to make the agriculture sector and rural areas of europe digitised and data-empowered. another fundamental contribution to the diffusion of these technologies is mainly provided under horizon 2020 through the societal challenges and industrial leadership pillars. however, when we refer to implementation and adoption of pats by end-users, and then by farmers, this is mostly channelled through the eu’s common agricultural policy (cap). for instance, different rural development measures under pillar ii of cap can foster the development of these technologies. pa can contribute to meeting the requirements put forward within the greening measures (pillar i of cap) in which farmers receive payment to undertake practices that benefit the environment and the climate. at the national level, each member state of the eu has developed industry 4.0 policies to strengthen industrial competitiveness and modernise the manufacturing and agriculture sectors. this policy especially supports the digitalisation of agriculture based on the development and introduction of new tools and machines in the production process. however, even if there are affordable pats available on the market and policy support for the acquisition of the technologies, the application remains circumscribed at few farms. in fact, in addition to the cost of investment, the adoption of the pats has encountered other difficulties, such as additional application or management costs and investment on new equipment, employee training for using the technologies and uncertainties found within the farming community [8]-[14]. given these premises, this paper discusses the economic benefits of pa, as they concern the accuracy of the measurements taken by different technologies, while trying to answer the following question: “what is the economic effectiveness deriving from the adoption of high-accuracy pats?”. to reach this goal, we attempt to quantify the economic benefits of pa based on a case study – a representative cereal farm in central italy that manages the whole-farm system with a mixed approach of conservation agriculture and precision farming. the case study method allows researchers to explore and investigate a contemporary real-life phenomenon through a detailed contextual analysis of a limited number of events or conditions and their relationships. the methodology adopted for evaluating the profitability, the cost– benefit analysis, derives from the introduction of pats. this paper extends a previous study presented during the 2019 ieee international workshop on metrology for agriculture and forestry (metroagrifor) in portici, italy. here, a more extensive and detailed economic analysis is provided by the authors. the remainder of the paper is organised as follows: section 2 shows the primary precision farming tools and their adoption. section 3 presents a brief literature review on the profitability of pa. section 4 provides the methodology and data, and section 5 discusses the results. finally, the conclusions and some policy implications are detailed in section 6. 2. precision agriculture techniques and technologies rather than referring to the development of new digital technologies, pa refers to the need to collect geo-referenced information necessary to monitor or manage spatially variable agricultural fields. in fact, pa is a management concept based on observing, measuring and responding to intra-field variability in crops through the use of technology. pats allow farmers to recognise variations in the fields and to apply variable-rate treatments with a greater degree of precision than before [15][16]. the management of pa can be divided into four phases (figure 2): 1. understanding and identifying variability 2. determination of homogenous zones 3. decisional phase 4. agricultural operation management each of these phases requires specific technology. to achieve a better understanding of within-field variability, there is a set of different instruments and tools that allow farmers to generate and manage big data from the field. the development and implementation of pa has been made initially possible by combining the global positioning system (gps) and geographic information systems. these technologies allow the combination of real-time data collection with positional information. remote and proximal sensing are the two most common techniques used for the acquisition of information related to variability within crop fields. satellite, airborne or uav platforms, using different type of cameras, are the most popular technologies used in agriculture. in terms of proximal sensing, in which the sensor is close to the object to be monitored, it is possible to directly analyse soil and crop data in real-time. typical examples of proximal sensing are as follows: • watermark and sentek soil moisture sensors – used measuring soil humidity. figure 1. policy instruments for pats adoption acta imeko | www.imeko.org september 2020 | volume 9 | number 3 | 67 • the green seeker system – used to measure the normalised difference vegetation index and to quantify crop variability via optical sensors; • on-harvester grain quality sensors – sued to estimate protein, oil and moisture within the grain by using infrared spectroscopy. by collecting this data from different sensors, farmer can be aware of the spatial and temporal variability of the fields, as represented through maps, and it is possible to recognise homogeneous areas within a field that could be treated in a diversified way. all the data collected from the sensors and maps can be stored in a decision support system (dss), a softwarebased system that allows farmers to analyse all the agricultural data and consider them as inputs for the decision-making process [17]. after the decision phase, the farmer is therefore ready to intervene in the field through the use of advanced agricultural machinery with serial control and communications data network (commonly referred to as "iso bus" or "isobus"), the standard protocol that makes it possible to manage the communication between tractors, software and equipment of major manufacturers, allowing the exchange of data and information with a universal language through a single control console in the tractor’s cab [18]. another suitable application for agricultural operations management is variable-rate technology, which provides the capability to vary the rate of soiland crop-applied inputs for site-specific applications. these technologies consent the recording of spatial differences in relevant factors to crop growth, such as the quality of soil, availability of water and fertilisers, and crop yield. this greatly improves the efficiency of resources and adjustability of biological-technical systems as well as leads to reduced waste of inputs. other studies highlight how positioning accuracy represents a key factor for the precise management of agricultural operations [19], [20]. in the engineering field, accuracy refers to how close a measurement is to the true value, but a more rigid definition is applied by the international organization for standardization (iso), which defines accuracy as a measurement with both true and consistent results. the iso definition means that an accurate measurement has no systematic error and no random error. a key component of the precision farming management approach is the use of a wide array of digital devices that allow taking accurate agricultural measurements, including gps guidance, sensors, control systems, robotics, drones, autonomous vehicles, variable rate technology, gps-based soil sampling, automated hardware, telematics and software. pa applications can be classified into three categories, taking into account the different degree of accuracy in the positioning systems [21]: • low accuracy (meter level) – used for asset management, tracking and tracing; • medium accuracy (sub-meter level) – used for tractor guidance, via manual control, for lower accuracy operations such as spraying, spreading, harvesting bulk crops and area measurement/field mapping; • high accuracy (cm level) – used for auto-steering systems on tractors and self-propelled machines, like harvesters and sprayers. these technologies also differ in cost and the knowledge or skills required to use the tool. proximity sensors, depending on the type of sensor, have a commercial price between 50–60 € (watermark sensor) up to 1,000 € (sentek sensor). a drone for professional agricultural use has an average cost of 5,000 €, while a tractor isobus application via the automatic steering systems can cost up to 20,000 €. considering these relatively high costs and the skills needed to manage technologies, which not all farms still have, technology providers are increasingly making these technologies available as services. this is the case for yield maps or dsss, which are generally made available in the form of annual fees, depending on the services requested. the use of technologies as a service is a way to reduce the costs of technology and to spread their use among farmers who possess knowledge gaps regarding management of the equipment. focusing on the adoption of pats at worldwide level, the us is the top player in this sector, followed by australia and canada. nevertheless, the percentage of pa adoption has increased in europe, with a rate of 15–20 %. based on region, the eu pa figure 2. technologies for pa acta imeko | www.imeko.org september 2020 | volume 9 | number 3 | 68 market is segmented into the uk, germany, spain and france. in italy, only 1 % of the agricultural surface is managed through precision farming techniques. the work of [22], focused on the level of adoption of pa among italian farmers, showed that pats adopters were characterised by an average farm size of 143 ha, showing that farmers are more likely to manage big farms with ap. in line with these results, it is possible to highlight that pa follows the model of a capital-intensive technology, characterised by both high entry and large fixed transaction costs, and by an overly long payback period. however, although the adoption rate of technology among farmers is still low due to these socioeconomic barriers [23]-[26], the market for smart agriculture technologies is growing since technology providers are increasingly developing solutions that can cover the entire field of the agri-food supply chain (afsc). in particular, most of the solutions cover the first step of afsc, the production phase, from cultivation to storage of the product to processors. according to a recent survey, currently, the available technologies on the market are those that support the growing phase of the crop (79 %) followed by seeding/plantation phase (37 %) and harvesting (33 %) [27]. the most widespread technologies on the market are related to the soil mapping (29 %), machine control (27 %) and precision interventions (21 %), such as planting, fertilising and distributing pesticides. the remaining part of these technologies are reserved for farm and crop management and monitoring (18 % and 5 %, respectively). the main crops treated with pa are fruit and vegetables (38 %), cereals (35 %), grapes (23 %) and olives (4 %) [28]-[30]. for fruit and vegetable crops, machine vision methods allow growers to grade products as well as monitor food quality and safety with automation systems recording parameters related to product quality (colour, size, shape, external defects, sugar content, acidity, etc.). additionally, the tracking of field operations, such as the chemicals sprayed and use of fertilisers, can provide for a complete fruit and vegetable processing method. the use of pats on arable land is one of the most successful applications and is the most advanced amongst farmers. the technology allows farmers to control the number of inputs in arable lands, such as the optimised amounts of fertilisers like nitrogen, phosphorus and potassium. the development and adoption of pats and methodologies in grape and olive orchards are more recent than in arable lands. for these high-value crops, precise irrigation methods are developing rapidly to save water while improving yields and fruit quality; for example, grape quality and yield maps are of great importance during harvest to avoid mixing grapes of different potential wine qualities [31]. the "guidelines for the development of precision agriculture in italy" [32] calls for expanding management through precision agriculture to up to 10 % of the agricultural area cultivated nationally by 2021. therefore, it becomes essential to identify the factors limiting their diffusion and to analyse profitability from using these technologies. 3. the profitability of precision agriculture: a brief review three different pa research focus areas are represented in the literature [33]: studies aiming to prove the profitability and the positive environmental impacts of pa [34]-[38], studies investigating the technical aspects of product development and process improvement, and studies focusing on the implementation of pa at the single-farm level. in the first research focus area, pa has the potential to help farmers improve input allocation decisions, thereby lowering production costs or increasing outputs and, potentially, increasing profits. however, there is still scant knowledge about the relative magnitude of the overall costs and benefits of pats on individual farms. previous studies [39]-[44] tried to evaluate the savings and revenues caused by pa, but only by considering either the average savings from the application of a single technology or a specific growth phase of the crop (table 1) according to [45], the impact of pats on agricultural production is expected in two areas: • profitability for farmers; • ecological and environmental benefits to the public. however, both the profitability and the environmental benefits of pa continues to be difficult to predict, evaluate and measure [46], [47]. according to the literature, the profitability of pa depends on different aspects, including farm size, the type of crop, the technology adopted, the degree of spatial variability of soil attributes (e.g. soil types, fertility and organic matter) and yield response [48]-[53]. studies on pat adoption emphasise that adopters tend to operate a larger agricultural area and subsequently generate a higher income. this indicates the ability to accommodate some risk in investment of newer and larger technologies. some studies have highlighted that farms specialising in high-income crops, such as vineyards and olive groves, are more likely to adopt pats. the major benefits of pa management derive from the increase of crop yields and reduced inputs as well as more efficient farm management with improved communication possibilities and higher quality of work with machine-guided systems. the implementation of precision farming concepts may mitigate production risks because inputs are applied only where they are needed. while risk mitigation with precision farming is intuitive, the implementation of precision farming typically requires substantial investments that may increase financial risk [54]. investments in precision farming are further associated with the irreversibility of the capital cost, which should be taken into account where appropriate; farmers might prefer to wait for better information on the costs and benefits of the new technology before investing in precision farming technologies [55]. while the costs of precision farming technologies can, in table 1. economic benefits of pa year author pats case study crop average savings 2000 bongiovanni & lowenbergdeboer rtv for fertilisation usa soya and corn 17.60 € / ha 2003 godwin et al. assisted/sem i-automatic driving uk arable crops 25 € / ha 2003 godwin et al. ctf uk soft wheat from 18 to 45.5 € / ha 2009 biermacher et al. rtv for fertilisation usa soft wheat 13.2 € / ha 2010 wagner et al. rtv for fertilisation de soft wheat 16 € / ha 2011 robertson et al. rtv for fertilisation aus arable crops 9.4 € / ha 2012 shockley et al. rtv for seeding usa soya and corn 31.67 € / ha acta imeko | www.imeko.org september 2020 | volume 9 | number 3 | 69 many cases, be estimated precisely, it is more challenging to evaluate the benefit of the system in management. the willingness of farmers to trust the technology is a fundamental behavioural factor in achieving positive results. several studies found that a low level of trust in the technology could be a key limitation for pat adoption when compared to other factors. thus, farmers are waiting for research results on the profitability of various pats before deciding to invest significantly to adopt new technologies. on the one hand, pa is aimed at large holdings with a farm and capital structure that enables them to invest in expensive systems. on the other hand, it is a means to move farm management back to small-scale farming processes with detailed knowledge about small units and management zones. it enables farmers to treat each unit, whether it is a piece of land or an animal, with the same care as farmers did in previous times. this development is facilitated by the help of smart technologies that allow the farmer to gain detailed knowledge about the field and subsequently treat the field accordingly. despite these advantages, pa is adopted only by innovative farmers and the intelligent usage of precision farming data is still rather limited. the introduction and uptake of technologies require new skills and knowledge for farmers and advisers. raising awareness and organising training on a regional/local level is essential, especially to reach small and medium-sized farms where the use of digital technologies is not always thought of as profitable however, taking advantage of pats will depend not only on the willingness of farmers to adopt new technologies but also on each farm’s potential, in terms of scale economies, since profit margin increases with farm size. this concept is widely explained in the work of [56], which analysed the socio-cultural and complexity factors that affect the probability of an italian farmer adopting new pats. the authors found that the farmers most prone to technological innovation all had similar characteristics: big size farms (average dimension equal to 143 hectares) and young managers with the highest level of education. 4. data and methods to determine the profitability of applying pa, a case study was conducted. according to [57], the case study research method is ‘an empirical inquiry that investigates a contemporary phenomenon within its real-life context; when the boundaries between phenomenon and context are not clearly evident; and in which multiple sources of evidence are used.’ due to the limited availability of other cases for replication, in this study, we adopt a single-case design. while not reflecting a statistical representation, a single-case study can contribute to scientific development through a deep understanding of a still-rare context of inquiry, such as that of adopting pats in the italian agriculture sector. cost–benefit analysis was selected evaluate the economic implications of adopting pats and was carried out on an innovative farm specialising cereals production located in the centre of italy. this study, conducted in 2019, takes into consideration durum wheat and soft wheat production on an agricultural area of 537 flatland hectares applying a conservative production system (i.e., sod seeding). the farmer was interviewed and asked to characterise the farming practices before and after the adoption of pats. also, specific questions were asked to find out the technological investments. from 2010 to 2016, the farm has invested in highly accurate pats, costing approximately 184.000 €, to be used to make decisions with greater precision and to optimise crop yields. the main investments include assisted steering (isobus); services for georeferencing, production and soil mapping system; a variable rate fertiliser spreader; machinery for weeding; and treatment with variable dosage distribution. the description of the phases of the cost-benefits analysis are follows: 1) definition of the time horizon under study: • 2005–2009 – pre-adoption period; • 2010–2016 – progressive investment period in the pa technological ‘package’; • 2013–2017 – post-adoption (progressive) period of the pa package. 2) definition of average land productivity (both for durum wheat and soft wheat): • for pre-adoption period (2005–2009); • for post-adoption period (2013–2017). 3) definition of a 10-year amortisation schedule (and related constant annual payment) of the pa technology ‘package’. this phase is aimed at defining the annual capital cost of the investment in pa. 4) definition of the pre-adoption average total cost: • per hectare (ha); • per product unit, in tonne (t). 5) estimation of the post-adoption monetary savings at a level of average total cost induced by pa adoption: • per hectare (ha); • per product unit (t). 6) estimation of the post-adoption average total cost (atc): • per hectare (ha); • per product unit (t). 7) definition of the market price time series (2012–2019) for durum and soft wheat. 8) estimation of the operating margin generated by the adoption of the pa package: • per hectare (ha); • per product unit (t). 9) sensitivity analysis on the cost–benefit analysis results so as to evaluate the economic and financial effectiveness of the investment, according to the changes in • production scale; • unit product cost; • land productivity. 5. results comparing the pre-adoption and post-adoption period of pats, the main empirical evidence is relating to two main issues: • the variation in land productivity; • the change in cost. relating to the first aspect, as shown in table 2, an increase in the average land productivity in the post-adoption period is observed. in particular, the post-adoption land productivity enhancement is considerably greater in the case of soft wheat (+ 23.3 %) compared to the durum wheat (+ 14.2 %). consequently, we decided to assess separately the supposed effect of pa in terms of economic effectiveness for both durum and for soft wheat. in analysing the crop yield trends, it is not possible to establish with certainty whether this productivity enhancement is due to acta imeko | www.imeko.org september 2020 | volume 9 | number 3 | 70 the technological change. we are aware that crop productivity is influenced by a complex set of factors, such as climatic conditions or the type of grain variety, certainly not only by the possible introduction of a specific technology. however, it must be noted that the increase in land productivity is measured over a five-year post-adoption period on average, which is a fairly reliable period to assume the presence of some level of impact from the introduced technology. indeed, the improvement of crop yields could be associated with both the direct and indirect effects of pats. the direct effects derive from the optimisation of production processes. the indirect effects derive from greater knowledge about the state of soils and crops. in this way, the farmer can make more timely decisions. in fact, the farmer in this case study stated that the georeferenced mapping of both the farmlands and working time allowed them to quantify how much of the farm area was actually worked upon due to overlapping errors from different cultivation operations. further, both the mapping of production and the soil analysis allowed the farmer to optimise seeds, fertilisers and herbicides according to the real need of the plants and the productivity of the soils. to evaluate the effect on production costs due to the introduction of pats, the pre-adoption and post-adoption atc have been estimated and then compared. in summary, table 3 shows a comprehensive picture of the pa cost-benefit analysis results. the pre-adoption atc equalled 794.56 €/ha for durum wheat and equivalent to 768.98 €/ha for soft wheat. further, the average saving (as) on atc in the post-adoption period, hypothetically attributable to the cost efficiency of the pats, was found to be 77.55 €/ha, a cost reduction of 10.08 % on average compared to atc in the pre-adoption period. in particular, through the use of fertiliser spreaders, machines for weeding and treatments, and seeders with variable dosage distribution, it was possible to reduce the cost of mechanical operations (labour, diesel, lubricants, etc.) and technical means (spreading seeds, fertilisers and pesticides). however, in order to evaluate the net savings due to pa adoption, the capital cost (cc) of the technology introduced has to be estimated and then discounted from the average savings on the operating production costs. thus, a 10-year amortisation schedule at a 5 % annual interest rate (plus related constant annual payments) of the introduced pats package has been calculated. we calculated a cc of the total investment in pats to be 44.08 €/ha, calculated based on the agricultural area invested in the cereal production within the case study, i.e. 537 ha. we then calculated the net savings (ns) per hectare on the production cost, hypothetically due to the technology package introduced, as follows: as – cc = ns = 33.47 €/ha (1) thus, when cc is deducted from the total cost reduction (or as) between preand post-adoption atcs, a reduction of 10.08 %, the total ns is 4.3 %/ha. the first interesting notion is that this net effect of pa in terms of cost efficiency is relatively modest and in line with the previous studies examined in the literature review. a possible explanation is the fact that the case study farm is an entrepreneurial farm, already fully functional before pa adoption with a high level of efficiency with respect to the cost of production per unit of land. that said, the most significant effect attributed to pa seems to concern productivity. finally, to measure the net gains per unit of production in the post-adoption period, the operating margin (om) per tonne of production has been calculated as the percentage difference between the average revenue (ar) – corresponding to the average market price for the period considered – and the atc as follows: om (%) = (ar atc)/ar (2) table 4 shows the main indicators from the pa cost–benefit analysis. the om increases from 40.5 % in the pre-adoption period to 50.1 % in the post-adoption period for soft wheat while it increases from 42.9 % in the pre-adoption period to 55.7 % in the post-adoption period for durum wheat. this performance is due almost entirely to the increase in land productivity registered for the post-adoption period. table 3: cost-benefits analysis results cost soft w. durum w. atc pre-adoption (per hectare) 768.98 € / ha 794.56 € / ha average saving on atc (per hectare) 77.55 € / ha 77.55 € / ha incidence capital cost (per hectare) 44.08 € / ha 44.08 € / ha incidence capital cost (per tonne) 6.0 € / t 7.7 € / t average saving on atc due to pa cost efficiency (per hectare) 33.47 € / ha 33.47 € / ha average saving on atc due to pa cost efficiency (per tonne) 4.6 € / t 5.9 € / t total average saving on atc (per tonne): cost efficiency plus productivity effect 29.0 € / t 25.5 € / t atc pre-adoption (per tonne) 129.6 € / t 158.6 € / t atc post-adoption (per tonne) 100.7 € / t 133.1 € / t table 2: variation of crop yields over the entire study period crop pre-adoption 2005/2009 average yield (t / ha) post-adoption 2013/2017 average yield (t / ha) increase (t / ha) increase (%) durum w. 5 5.71 0.71 14.2 soft w. 5.93 7.31 1.38 23.3 table 4: cost-benefits analysis indicators indicators soft w. durum w. land productivity increase postadoption 23.18 % 14.13 % average saving on atc (%) 10.08 % 10.08 % average saving on atc (%) net of the cc 4.3 % 4.3 % total average saving on atc (per tonne): cost efficiency plus land productivity effect (%) 22 % 16 % om per tonne (%) pre-adoption 40.5 % 42.9 % om per tonne (%) post-adoption 50.1 % 55.7 % net gains in monetary terms preadoption 114.8 € / t 98.7 € / t net gains in monetary terms postadoption 140.3 € / t 127.7 € / t net gains in monetary terms due to pa 25.49 € / t 28.97 € / t acta imeko | www.imeko.org september 2020 | volume 9 | number 3 | 71 the om is the summary result of this cost–benefit analysis; therefore, it seems important to offer some assessments on the basic meaning of this measurement. looking at figure 3, it is possible to visualise in a comparative way the net gains derived from the production of wheat in the pre-adoption period and the post-adoption period. the first interesting note is that we analyzed a farm able to generate income both for the preadoption (thus regardless of the pa adoption) and the postadoption period respect to italian conventional cereal farms that are rarely effective. the second interesting note is that in the post-adoption period, the ns increases, in monetary terms, by 9.6 % for soft wheat and 12.8 % for durum wheat. this means, in absolute terms, the net gains in monetary terms is higher in the case of soft wheat (in both the pre-adoption and post-adoption periods). the increase of the operating margin due to pa (in %) for durum wheat could be due to a more favourable average level of the durum wheat market price in the post-adoption period (as compared to the soft wheat market price). a further consideration which may deserve more attention is the following: the present case study is a cooperative farm that dedicates more than 500 ha to the production of wheat. this means that the farm is a ‘large farm’ and that it can be roughly considered a case of minimum efficient scale, with respect to a fixed investment like the pats package. for this specific case study, we observed that the impact of cc on the unit of production turns out to be minimised thanks to the optimal farm dimensions. however, if the farm size decreases, the impact of cc on the unit of product will increase accordingly. this occurs because, as we have verified, the cost of the pats package can be considered, with good approximation, a fixed cost, not reducible depending on the reduction of the firm size. based on this hypothesis, we performed a sensitivity analysis to explore the variation of the cc impact on atc per unit of production with respect to changes in farm size. the goal of this sensitivity analysis was to identify the minimum farm size needed to balance the farm budget, with respect to the post-adoption market price levels. table 5 shows the results of the sensitivity analysis performed on the balanced minimum farm size, assuming as constant all the variables considered in the present case study – i.e., the cc invested in the pats package, the atc of production per hectare, the average saving on atc per hectare and the productivity levels; only production scale changed. the minimum production scale necessary to balance the farm budget appears to be strongly influenced by land productivity. accordingly, based on a post-adoption soft wheat productivity level that is 22 % greater than durum wheat (7.3 t vs. 5.7 t), the ‘virtual’ minimum production scale necessary to balance the firm budget results in 60 ha for durum wheat and 30 ha for soft wheat. it is interesting to note that the minimum farm size required to balance the budget – in this case, regardless of the distinction between soft and durum wheat – is significantly smaller compared to the size of the real case study farm. this means that pats could be financially sustainable even for ‘medium’ production scales when keeping the cost efficiency and productivity levels, as expressed by the study case farm, fixed. finally, as a further point of reference related to the minimum production scale necessary to obtain a positive result by adopting pats, table 6 illustrates a simulation consisting of the results of a sensitivity analysis to identify the minimum farm size needed to balance the farm budget with respect to pat adoption by a cereal farm producing durum wheat in a hilly area. thus, the fundamental analytical elements that distinguish this ‘virtual’ farm from our real case study are as follows: • hilly area (vs. flat land for the case study); • minimum tillage (vs. no tillage for the case study). • unit cost of production equal to 170 € / t (vs 133.1 € / t for the case study); • land productivity equal to 5 t / ha (vs. 5.7 t / ha for the case study) the results show that the minimum farm size necessary to balance the farm budget is considerably greater than the minimum farm size for the case study farm (200 ha in a hilly area versus 60 ha in a flat area). this result is indicative of how the economic effect of pats changes as the environmental conditions, in which the production takes place, change. particularly, in this hypothetical scenario characterised by minimum tillage of a hilly area, the unit production cost is assumed to be 35 % greater than the unit production cost of the case study farm, and the land productivity level is assumed to be 15 % lower. based on these results, one can conclude that pa adoption in a hilly area using minimum tillage could be worth the investment only for large farms (> 200 ha) or for cooperative systems capable of bringing together many producers in a common management organisation. figure 3. prices and atcs for soft and durum wheat table 5: minimum farm size in balance indicators soft scale 30 ha durum scale 60 ha incidence capital cost / ha 790.0 € / ha 395.0 € / ha incidence capital cost / t 108.1 € / t 69.1 € / t atc post-adoption (ha) 1,481.4 € / ha 1,112.0 € / ha atc post-adoption (t) 202.7 € / t 194.5 € / t table 6: minimum farm size balance in hilly area indicators based on atc condition of minimum tillage average cost (ha) and hilly area durum 200 ha incidence capital cost / ha 118.5 € / ha incidence capital cost / t 23.7 € / t atc cost post-adoption (ha) 968.5 € / ha atc post-adoption (t) 193.7 € / t acta imeko | www.imeko.org september 2020 | volume 9 | number 3 | 72 6. conclusion pa may offer important opportunities toward more sustainable agriculture. however, the diffusion of pats in the agricultural sector is still insufficient due to the scarce knowledge regarding economic and environmental benefits that pats may have. in this regard, the case study in this work contributes to the body of research aimed at identifying important points of reference for cost-effectiveness and efficiency in pa-derived production inputs. the case study shows how a large farm can effectively exploit the returns to scale associated with adopting pats packages, generating income as a consequence. indeed, pa requires a large investment of capital, time and learning. thus, costs associated with pats may prevent smaller farms from being able to invest in these technologies. in this context, the farm-scale is a crucial variable in the analysis tools to evaluate the adoption and profitability of pats. however, insofar as how pats were able to reconcile production requirements and environmental protection, questions arise on how best to support pa adoption. it is clear that there are still no specific measures for the diffusion of pats in the agriculture sector, but there are generic measures of sector innovation and digitisation of the agri-food chain. while several studies have begun to demonstrate the economic effectiveness of pats, the assessment and quantification of the environmental benefits are almost totally lacking in the literature. some farmers do consider these benefits as part of their overall viability decision, but this is based upon their personal values. apart from general qualitative statements, there is no quantified environmental benefit assessment that can underpin an investment decision; this appears to be a significant omission that could be addressed by developing a methodology and/or tool for the management decision-making process. finally, in pa, 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https://doi.org/10.1017/s1074070800027887 https://doi.org/10.1016/s0168-1699(02)00143-6 https://doi.org/10.1017/s1074070800008555 https://doi.org/10.1016/j.agsy.2009.02.001 https://doi.org/10.1016/j.landusepol.2020.104481 template for an acta imeko event paper acta imeko issn: 2221-870x june 2018, volume 7, number 2, 90 acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 90 editorial to selected papers from the international conference imekofoods 2016 claudia zoani italian national agency for new technologies, energy and sustainable economic development, rome, italy section: editorial citation: claudia zoani, editorial to selected papers from the international conference imekofoods 2016, acta imeko, vol. 7, no. 2, article 15, june 2018, identifier: imeko-acta-07 (2018)-02-15 editor: dušan agrež, university of ljubljana, slovenia received june 27, 2018; in final form june 28, 2018; published june 2018 copyright: © 2018 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: claudia zoani, email: claudia.zoani@enea.it dear reader, this special section of acta imeko is dedicated to a selection of papers related to the international conference imekofoods “metrology promoting objective and measurable food quality and safety”, which took place in benevento (italy) on 2015, organised by the university of sannio, enea (the italian national agency for new technologies, energy and sustainable economic development) and the imeko tc 23 “metrology in food and nutrition”. the general objective is to promote the discussion, the scientific debate and the encounter between the different realities that revolve around the "world of measures", promoting the harmonization and integration and addressing the "world of research" to the emerging needs of the civil society and the productive sectors. in particular, this issue contains 2 papers representing the extended versions and advancements of selected papers presented at the 2nd imekofoods conference. the first paper describes the application of a catalytic fuel by researchers from the department of chemistry of the university of rome “la sapienza” (italy) to determine the ethanol content in several samples of commercial wines, beers and hard liquor drinks and its comparison with two other conventional biosensors, based on catalase or alcohol oxidase enzyme respectively. the second contribution describes a cooperative study performed by different research institutes from portugal (insa, cerena, inst. superior te ́cnico, escola superior de hotelaria e turismo do estoril), focused on the determination of acrylamide in portuguese bread by uplcms/ms and the association of the acrylamide content with the place of production, formulation, flour quality and the varieties of processing techniques. i would like to thank all those who contributed to this issue and made possible its realisation: the authors, the reviewers, the layout editors, and the journal managers. hoping you will have a fruitful and interesting reading! claudia zoani editorial to selected papers from the international conference imekofoods 2016 acta imeko contacts acta imeko issn: 2221-870x september 2019, volume 8, number 3, 0 acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 0 journal contacts about the journal acta imeko is an e-journal reporting on the contributions on the state and progress of the science and technology of measurement. the articles are mainly based on presentations presented at imeko workshops, symposia and congresses. the journal is published by imeko, the international measurement confederation. the issn, the international identifier for serials, is 2221-870x. editor‐in‐chief dušan agrež, slovenia founding editor‐in‐chief paul p. l. regtien, netherlands associate editor dirk röske, germany editorial board section editors (vol. 7 and 8) leopoldo angrisani, italy filippo attivissimo, italy eulalia balestieri, italy eric benoit, france paolo carbone, italy lorenzo ciani, italy catalin damian, romania pasquale daponte, italy luca de vito, italy luigi ferrigno, italy edoardo fiorucci, italy alistair forbes, united kingdom helena geirinhas ramos, portugal sabrina grassini, italy fernando janeiro, portugal konrad jedrzejewski, poland andy knott, united kingdom francesco lamonaca, italy massimo lazzaroni, italy fabio leccese, italy rosario morello, italy michele norgia, italy pedro miguel pinto ramos, portugal nicola pompeo, italy sergio rapuano, italy dirk röske, germany alexandru salceanu, romania constantin sarmasanu, romania lorenzo scalise, italy emiliano schena, italy enrico silva, italy krzysztof stepien, poland marco tarabini, italy marcantonio catelani, italy lorenzo ciani, italy egidio de benedetto, italy alessandro depari, italy alessandro germak, italy sabrina grassini, italy konrad jędrzejewski, poland min-seok kim, korea fabio leccese, italy rosario morello, italy vilmos palfi, hungary nicola pasquino, italy franco pavese, italy jeerasak pitakarnnop, thailand alexandru salceanu, romania emiliano sisinni, italy jan saliga, slovakia jorge c. torres-guzman, mexico ian veldman, south africa rugkanawan wongpithayadisai, thailand claudia zoani, italy about imeko the international measurement confederation, imeko, is an international federation of actually 42 national member organisations individually concerned with the advancement of measurement technology. its fundamental objectives are the promotion of international interchange of scientific and technical information in the field of measurement, and the enhancement of international co-operation among scientists and engineers from research and industry. addresses principal contact prof. dušan agrež university of ljubljana faculty of electrical engineering tržaška 25, 1000 ljubljana, slovenia e-mail: dusan.agrez@fe.uni-lj.si acta imeko dr. jeerasak pitakarnnop, rugkanawan wongpithayadisai department of mechanical metrology national institute of metrology, thailand (nimt) 3/4-5 moo 3 klong 5, klong luang pathumthani 12120, thailand e-mail: jeerasak@nimt.or.th, rugkanawan@nimt.or.th support contact dr. dirk röske physikalisch-technische bundesanstalt (ptb) bundesallee 100, 38116 braunschweig, germany e-mail: dirk.roeske@ptb.de mailto:dusan.agrez@fe.uni-lj.si mailto:dusan.agrez@fe.uni-lj.si mailto:jeerasak@nimt.or.th mailto:jeerasak@nimt.or.th mailto:rugkanawan@nimt.or.th mailto:rugkanawan@nimt.or.th mailto:dirk.roeske@ptb.de mailto:dirk.roeske@ptb.de ultrasonic rangefinder with resolution in hundredths of the probing signal's wavelength for the mobile rescue robot acta imeko issn: 2221-870x december 2019, volume 8, number 4, 41 46 acta imeko | www.imeko.org december 2019 | volume 8 | number 4 | 41 ultrasonic rangefinder with resolution in hundredths of the probing signal's wavelength for the mobile rescue robot stanislaw goll1,2, julia maximova1,2 1 department of information-measuring and biomedical engineering, ryazan state radio engineering university (rsreu), gagarin street 59/1, ryazan, russia 2 llc kb avrora, skomoroshinskaja street 9, of.3, ryazan, russia section: research paper keywords: ultrasound; rangefinder; resolution; envelope; phase spectrum; rescue robot citation: stanislaw goll, julia maximova, ultrasonic rangefinder with resolution in hundredths of the probing signal's wavelength for the mobile rescue robot, acta imeko, vol. 8, no. 4, article 8, december 2019, identifier: imeko-acta-08 (2019)-04-08 editor: yvan baudoin, international cbrne institute, belgium received november 23, 2018; in final form june 26, 2019; published december 2019 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: julia maximova, e-mail: maximova@kb-avrora.ru 1. introduction extreme mobile robotic systems, safety-critical systems (particularly those designed for drivers, pilots, and motormen), and medical equipment (e.g. diagnostic and therapy instruments) need low-cost precision rangefinders with an effective range of up to 1 m and an output data rate of at least 20 hz to measure both absolute and relative distances. such rangefinder sensors can be used to provide the noncontact measurements of a person’s pulse and respiratory rates (figure 1) – information that is critical for planning rescue operations, the prevention of human-factor accidents, and the synchronisation of medical equipment with a patient’s biorhythms [1]. search and rescue (sar) operations caused by industrial disasters and military conflicts can be dangerous for sar crew members. the main need here is to minimise the human presence in the danger zone by relying on telemetry data (i.e. victims’ vital signs and locations) to optimally plan and execute the rescue operation, including the sar crew members’ efforts and the rescue robots, both autonomous and remote controlled [2], [3]. detection of the injured person’s vital signs, mainly pulse and respiratory rates, poses a nontrivial task even for a remote controlled rescue robot [4], [6]. for that task, video and thermal cameras are considered to be the most informative sensors. due to the amount of data broadcasted by these sensors, they require a wired connection (e.g. a lan) between the robot and the human operator or the broadband wireless connection in the case that the former option is not possible. however, the radio wave-based data transmission can lead to detection mistakes due figure 1. positioning of the ultrasonic beam pattern by the mobile robot in noncontact registration of the person’s pulse and respiratory rates. abstract the main goal of this research is to increase the measurement resolution of ultrasonic rangefinders to meet the needs of vital signs noncontact registration based on chest movements. the two-phase method is proposed to make distance estimates by sending probe pulse trains, calculating the phase spectrum of the echo signal’s envelope, and tracking its relevant components. during the first phase, rough time-of-flight (tof)-based estimates are made. during the second phase, this estimate is corrected based on the phase spectrum of the echo signal’s envelope, the phase ambiguity is removed, and the relevant components are determined. the final estimate of the human chest displacement is calculated based on these relevant components. the output data rate is the same as for the tof-based measurements, but the measurement resolution is increased to one hundredth of the ultrasonic wavelength. the experiment results are provided for the both model and the real human chest displacements caused by the respiration and heartbeat processes. mailto:maximova@kb-avrora.ru acta imeko | www.imeko.org december 2019 | volume 8 | number 4 | 42 to interferences and non-line-of-sight and near-line-of-sight transmissions. moreover, the usage of thermal cameras to detect body temperature can sometimes lead to a dead body being mistaken for a living person. another way of measuring human vital signs is to make contact measurements [7]. in that case, the robot must place electrodes on particular parts of the human body, which leads to many problems, like the process of electrode placement or the detection and recognition of the body parts where the electrodes should be placed. clearly, these can be quite dangerous operations to be performed on an injured person. in section 2, we will discuss the relevant prior research in this area, including the advantages and disadvantages thereof. in the next section, the essence of the proposed method is described. the following section presents some experiments and their results. in the final section, conclusions are made and recommendations for further research are outlined. 2. related work the noncontact measurements of the pulse and respiratory rates can be taken by means of short-range radars [8], [10], since it is possible for a single integrated circuit to contain two 77 ghz uhf transmission lines (one transmitting line and one receiving line) as well as the low-frequency processing channel and the adc [11]. the data gathered that way can be affected by electromagnetic interference and other factors; hence, we propose to increase the robustness of the radar measurements by means of the secondary noncontact measurement channel based on ultrasonic sensors. the commonly used time-of-flight (tof) principle [12], [13], is based on measurement of the time interval between the start of the ultrasonic wave emission and the moment when the reflected wave causes the difference between the current air pressure and the air pressure at rest to exceed the given threshold for the first time. the measurement resolution of the tof-based sensor depends both on the properties of the timer/counter used and the ultrasonic signal wavelength. the tof measurement resolution for the 40 khz ultrasound is, at a rough estimate, 8 mm. at the same time, many of the tof-based sensors are featured, with the resolution being up to one fourth of the ultrasonic wavelength. to achieve such resolution, various methods can be used: the adaptive threshold, comparisons for the positive and negative half-waves, etc. however, all these modifications are not enough to achieve the submillimetre resolution. further attempts to improve the measurement resolution of the ultrasonic rangefinders are based on linear frequency modulation or composite modulation [14]; binary frequency shift-keyed signal and phase detection [15]; vernier caliper phase metre [16]; and so on. however, these methods require broadband ultrasonic receivers and transmitters, sufficiently increasing the hardware cost. 3. the approach in this article, we propose an algorithm to improve the measurement resolution of the low-cost single-tone 40 khz ultrasonic rangefinders. these on the inverse piezoelectric effect based sensors are commonly used in the automotive industry to build advanced driver assistance systems (adass) [17]. the cyclic movements of the human chest are caused by both the respiration process and cardiac activity, with movement amplitudes of 4 to 12 mm and 0.5 mm accordingly [18]. the cardiac cycle is more frequent than the respiration cycle and can occur up to 200 times per minute. according to the nyquistshannon-kotelnikov sampling theorem, the measurement rate of 20 hz is sufficient to perform vital signs detection. due to the comparatively high sound speed in the air medium, it is possible to obtain up to 100 measurements per second at a distance of up to 1 m using the tof method. nevertheless, the measurement resolution of one fourth of the 40 khz ultrasonic wavelength is deficient in pulse and respiratory rates registration. we suggest an approach to improve the measurement resolution without changing the output data rate. first, some method (for example, the tof method) is used to obtain a rough figure 2. (a) probe pulse trains (b) received reflected signal (c) a single frame as it appears in the receiver. acta imeko | www.imeko.org december 2019 | volume 8 | number 4 | 43 distance estimate refd . second, the proposed method is used to estimate the displacement d relative to the rough estimate refd . contrary to the tof method, which uses the point in time at which the threshold is exceeded as the input data, the proposed method uses all the readings of the echo signal gathered with the sampling rate df 200= khz. figure 2(a) shows pulse trains containing n = 8 probe pulses each with the probe pulse frequency set to usf 40= khz. the pulse trains are transferred to ultrasonic transmitter with the frame period being t = 10 ms. the black-coloured regions in figure 2(b) represent the digital signal u , which is formed from the received frame sequence and can be described as the amplitude overmodulated signal. the white line in figure 2(b) represents the demodulated envelope of the receiver’s signal. to compute the envelope, the modulated signal is multiplied by the sinusoidal carrier signal ( )sin us vv v 2πf t φ= + sampled with rate df ; the low pass recursive filter with the transfer function 2 0 1 2 2 1 2 b z b z b z a z a + + + + is then applied to the multiplication result. figure 2(c) shows an enlarged version of the frame bordered by the dotted rectangle in figure 2(b). each frame contains the deterministic transmitted pulse received when the transmitter emits the ultrasonic wave. this signal starts from the beginning of the frame and ends at time 1t . in the case that some object or group of objects is present in the area covered by the rangefinder’s beam pattern and these objects are large enough to reflect the ultrasonic wave with the frequency usf , the frames also contain the reflected echo signal at the time interval  2 3δt t t,= . moment 1t is when the transmitted pulse ends is constant for every frame and is computed as 1 1 us n n t f + = . we define the frame containing the rough tof-based measurement of the distance refd to the closest object in the sensor’s beam pattern as a reference frame. the rough estimate refd is proportional to the time interval reft , which is determined when the echo signal exceeds the comparison threshold refu after time 1t . for every frame received after the reference frame time 2 2 ref us n t t f = − remains constant until the new reference frames arrive, changing the value of reft . the coefficients 1n and 2n represent the adjustable offsets with the integer values. the reference frame is used as a starting point for the proposed method to compute its distance estimate and can be set (and changed later) either by the human operator’s request or automatically if the absolute distance measurements acquired by means of the tof method lie in some predetermined intervals. the moment 3 2t t δt= + sets the right boundary of the echo signal’s time interval of the constant duration δt const= . on the  2 3δt t t,= interval of each frame, fast fourier transform is applied to the echo signal’s envelope, and then, for each of its frames, the phase spectrum is computed. the next step is the selection of the m = 10 sequential spectrum components (starting with the second component), which are expressed as , where k is the number of frames relative to the reference frame. this selection takes place at the end of each frame period t . each component  mφ k is used to estimate the displacement  md k relative to the rough estimate refd . this estimate is calculated as    m m d k β φ k=  , where β is the conversion coefficient (m/rad);      m m mφ k φ k λ k 2π= +  is the absolute value of the phase spectrum thm component after unwrapping; and  mλ k is the number of thm component phase unwrappings and is calculated as follows:      ( )m m mλ k λ k 1 g δφ k= − + , (1) where      m m mδφ k φ k φ k 1= − − , ( ) , , , , , , ref ref ref ref 1 ξ φ g ξ = 1 ξ φ 0 φ ξ φ −  −    −   refφ is the adjustable threshold of the mδφ k[ ] . figure 3 illustrates the computation of  mφ k . when the new reference frame is set, the number of phase unwrappings  mλ 0 is zeroed for each phase component, which provides the recurrent equation (1) with the initial condition. therefore, for each received frame, m = 10 estimates are calculated for the absolute distance mh between the sensor and the object of interest. each of these estimates is calculated as    m ref mh k d d k= + , where and apparently figure 3. phase unwrapping. acta imeko | www.imeko.org december 2019 | volume 8 | number 4 | 44 represents the sum of the reference distance refd and the displacement  md k computed using the proposed method. let us note that n , 1n , 2n , refu , δt , refφ , , , m are the structural parameters of the algorithm and are estimated empirically before the algorithm is applied. 4. results the proposed method was used to process data gathered from the ultrasonic rangefinder in the three experiments. for the first experiment, a rectangular aluminium sheet was used as the object of interest, shown in figure 4. a step motor with a step of 1.56 µm was used to cause the periodic sawtooth-like motion of the sheet with an amplitude of 50 µm along the axis of the rangefinder’s beam pattern. figure 4. the experimental setup and the sawtooth-like displacements of the sheet. figure 5. changes in absolute values of the phase spectrum components  mφ k caused by aluminium sheet displacement (proposed method): a – amplitude of 50 µm, b – amplitude of 4 mm; c – tof-based displacement estimates for the movements with the amplitude of 4 mm. algorithm: displacement estimation acta imeko | www.imeko.org december 2019 | volume 8 | number 4 | 45 figure 5(a) contains ten graphs showing the absolute values of the phase spectrum components  mφ k changing due to the aluminium sheet displacement with the amplitude of 50 µm. figure 5(b) shows the absolute values of the phase spectrum component  mφ k for the sheet’s displacement with a 4 mm amplitude. figure 5(c) shows the tof-based displacement estimates for the movements with an amplitude of 4 mm. for the second and third experiments, the human chest was used as the object of interest. figure 6 shows ten graphs demonstrating the absolute values of the phase spectrum components  mφ k , changing due to human chest displacement during the respiration process, such values having been computed using the proposed method. figure 7 contains the ten graphs of absolute values of the phase spectrum components  mφ k , changing due to cardiac activity while a person was holding their breath. 5. conclusions it is possible to empirically select the relevant (i.e. those that are the most correlated with the object’s movements) components of  mφ k from the series computed by the proposed method. for the first experiment, these estimates are figure 6. absolute values of the phase spectrum components changing due to the human chest displacement during the respiration process. figure 7. absolute values of the phase spectrum components  mφ k changing due to the human chest displacement caused by cardiac activity. acta imeko | www.imeko.org december 2019 | volume 8 | number 4 | 46 under numbers 5, 6, 10; for the second experiment, these estimates are under numbers 7, 8, 9; and for the third experiment, these estimates are under numbers 5, 6, 8. since the aluminium sheet used in the first experiment was moving with an amplitude of 50 µm, we can make a rough estimate of the proposed method’s resolution, which is less than 50 µm and, consequently, less than one hundredth of the ultrasonic wavelength with a frequency of usf 40= khz, making it sufficient for the purposes of respiration and heartbeat detection. however, the aforementioned estimate is highly conservative and will be a subject of future research. according to the experimental results, the tof method is inadequate for plotting the respiration and heartbeat graphs, due to its measurement resolution. it should be also noted that in all of the experiments, a considerable part of the target object was covered by the sensor’s beam pattern, and the waveform of the echo signal’s envelope depends highly upon object’s shape. so, in the first experiment, all the surface points move equally linearly, but in the second and third experiments, the shape of the surface is subject to the nonlinear three-dimensional changes, causing sufficient changes in the waveform of the corresponding echo signal’s envelope. for that reason, the tof-based results contain more noise for the second and third experiments than for the first. consequently, for most of the applications, the measurement resolution of this method is in close proximity to the wavelength. since the displacements caused by the respiration and heartbeat can be considered to be concentrated in the small neighbourhood of refd , it is possible to provide accurate estimates based on the phase components. the proposed method is robust to such displacements, since the only thing that changes during the process is the set of the relevant phase spectrum components. the main source of such robustness is the fact that both the time at which the envelope exceeds the comparison threshold refu and all the information it contains is used to perform calculations. however, to survive the significant nonlinear changes in the object’s surface over time, some combinations of the specific phase components should be used. therefore, the proposed method is suitable for small movement detection happening in any region of the sensor’s beam pattern and can be applied by mobile robots to detect human’s vital signs during a sar operation. an algorithm for the automatic selection of the most relevant phase spectrum components or combinations thereof based on a priori information about the target object and the measuring conditions poses another subject for a future research. topics of interest also include the impact of the rough estimate refd on the relevant phase components selection and the compensation of the distortions caused by the air streams on the territory covered by the sensor’s beam pattern. references [1] e. m. grigoryev, s. g. gurzhin, v. i. zhulev, m. b. kaplan, v. g. kryakov, e. m. proshin, technology and methods for formation of the complex magnetotherapy impact by the inductor array, proc. of the 5th mediterranean conference on embedded computing, meco 2016, june 2016, bar, montenegro, pp. 419422. [2] t. bagosi, k. v. hindriks, m. a. neerincx, ontological reasoning for human-robot teaming in search and rescue missions, proc. of the 11th acm/ieee international conference on human-robot interaction (hri), 7-10 march 2016, christchurch, new zealand, pp. 595-596. [3] f. çakmak, e. uslu, m. f. amasyali, s. yavuz, hazmat charts detection on search and rescue league of robocup competitions, proc. of the 25th signal processing and communications applications conference (siu), 15-18 may 2017, antalya, turkey, pp. 1-4. [4] d. j. nallathambi, comprehensive evaluation of the performance of rescue robots using victim robots, proc. of the 4th international conference on control, automation and robotics (iccar), 2023 april 2018, auckland, new zealand, pp. 60-64. [5] r. ventura, p. u. lima, search and rescue robots: the civil protection teams of the future, proc. of the 3rd international conference on emerging security technologies, 5-7 september 2012, lisbon, portugal, pp. 12-19. [6] m. m. saad, c. j. bleakley, s. dobson, robust high-accuracy ultrasonic range measurement system, ieee trans. instrum. meas. 60(10) (2011) pp. 3334-3341. [7] k. yasuda, k. takamura, t. masuda, k. magatani, a remote measurement method of the human biosignals, proc. of the ieee embs asian-pacific conference on biomedical engineering, 2022 october 2003, kyoto, japan, pp. 288-289. [8] c. li, w. chen, g. liu, r. yan, h. xu, y. qi, a noncontact fmcw radar sensor for displacement measurement in structural health monitoring. sensors 15(4) (2015) pp. 7412-7433. [9] j. h. lee, j. m. hwang, d. h. choi, s. o. park, noninvasive biosignal detection radar system using circular polarization. ieee transactions on information technology in biomedicine 13(3) (2009) pp. 400-404. [10] pr. k mane, k. n. rao, computational modelling of biosignalbased occupancy sensing system using doppler radar, proc. of the international conference on trends in electronics and informatics (icei), 11-12 may 2017, tirunelveli, india, pp. 25-31. [11] texas instruments datasheet iwr1443 single-chip 76to 81ghz mm wave sensor, swrs211b – may 2017 – revised february 2018. [12] j. c. jackson, r. summan, g. i. dobie, s. m. whiteley, s. g. pierce, g. hayward, time-of-flight measurement techniques for airborne ultrasonic ranging. ieee transactions on ultrasonics, ferroelectrics, and frequency control 60(2) (2013) pp. 343-355. [13] a. saad, a. radwan a, s. sadek, s. obeid, g. zaharia, g. e. zein, remote monitoring of heart and respiration rate using a wireless microwave sensor, proc. of the 12th international healthcare, hospital supplies and medical equipment shows (saudi medicare 2009), may 2009, riad, saudi arabia. [14] s. hirata, m. k. kurosawa, t. katagiri, accuracy and resolution of ultrasonic distance measurement with high-time-resolution cross-correlation function obtained by single-bit signal processing. acoust. sci. & tech. 30(6) (2009) pp. 429-438. [15] s. s. huang, c. f. huang, k. n. huang, m. s. young, a high accuracy ultrasonic distance measurement system using binary frequency shift-keyed signal and phase detection. the review of scientific instruments 73(10) (2002) pp. 3671-3677. [16] k. y. lee, c. f. huang, s. s. huang, k. n. huang, m. s. young, a high-resolution ultrasonic distance measurement system using vernier caliper phase meter. ieee transactions on instrumentation and measurement 61(11) (2012) pp. 2924-2931. [17] v. agarwal, n. v. murali, c. chandramouli, a cost-effective ultrasonic sensor-based driver-assistance system for congested traffic conditions, ieee transactions on intelligent transportation systems 10(3) (2009) pp. 486-498. [18] y. p. huang, j. s. wang, k. n. huang, c. t. ho, j. d. huang, m. s. young, envelope pulsed ultrasonic distance measurement system based upon amplitude modulation and phase modulation. the review of scientific instruments 78(6) (2007) p. 06510. acta imeko, title acta imeko issn: 2221-870x april 2017, volume 6, number 1, 64 acta imeko | www.imeko.org april 2017 | volume 6 | number 1 | 64 section: advertisement nmisa celebrates 70 years of accurate scientific measurement (metrology) and 10 years of institution existence! in 2017, the national metrology institute of south africa (nmisa) is celebrating 10 years as an independent entity and 70 years as the official metrology institute in south africa. the ceo of nmisa and the technical infrastructure department of the department of trade and industry (the dti), invites you to share in the commemoration events planned for the week of 31 july 2017. in 1947, the national physical laboratory was formed as a division of the council of scientific and industrial research (csir) under the tutelage of dr meiring naude. by 1950, a brochure indicated that the laboratory was responsible for “the ultimate standards of length, mass and pressure, as well as fundamental electrical and temperature scale standards”. by the end of the 1970’s, the laboratory was well established as the csir national metrology laboratory, providing traceability to the international system of units (si) for most of the physical parameters. in 1995 a chemistry programme was added to provide traceability for chemical measurements. in 2007, after the promulgation of the new measurement units and measurement standards act (act no. 18 of 2006), nmisa was established as a technical (quality) infrastructure public entity reporting to the department of trade and industry (the dti). consequently, 1947 is the year metrology was formally established in south africa and 2007 is the year nmisa was established as a metrology institute. the commemoration celebrations will take place from 31 july to 4 august 2017 at conference venues near the nmisa premises, in pretoria. the local test & measurement conference is also planned to link to the celebrations and the intra-africa metrology system (afrimets) annual general assembly and related technical committee meetings will be hosted at the same time, thus ensuring that the “think-tanks” of metrology in africa will be in attendance. this also provides an opportunity to commemorate the africa year of quality. a scientific conference will be held from 02 to 04 august and a gala dinner the friday evening, celebrating nmisa achievers. representatives of the national, regional and international institutions in the science and metrology fraternity, high-ranking government representatives and prominent industry experts will be attending. nobel laureates and prominent metrologists will add a high-level scientific flavour to the occasion. information about the venue, accommodation and the social programmes are available on the conference website http://seventyten.nmisa.org/ contact information: dr wynand louw: wlouw@nmisa.org sophia mabela (communications) comms@nmisa.org register online at http://seventyten.nmisa.org/ editorial to selected papers from the 3rd imeko international conference on metrology for archaeology and cultural heritage 2017 acta imeko issn: 2221-870x october 2018, volume 7, number 3, 1 2 acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 1 editorial to selected papers from the 3rd imeko international conference on metrology for archaeology and cultural heritage 2017 egidio de benedetto1, sabrina grassini2 1 university of salento, department of engineering for innovation, lecce, italy 2 politecnico di torino, department of applied science and technology, torino, italy section: editorial citation: egidio de benedetto, sabrina grassini, editorial to selected papers from the imeko tc4 international conference on metrology for archaeology and cultural heritage 2017, acta imeko, vol. 7, no. 3, article 1, october 2018, identifier: imeko-acta-07 (2018)-03-01 editor: dušan agrež, university of ljubljana, slovenia received october 22, 2018; in final form october 22, 2018; published october 2018 copyright: © 2018 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: sabrina grassini, e-mail: sabrina.grassini@polito.it dear reader, this special issue of acta imeko includes selected papers presented at the 3rd imeko international conference on metrology for archaeology and cultural heritage – metroarchaeo2017, which took place on october 23-25, 2017 in lecce (italy). the conference was held at the castello carlo v, a majestic fortification in the heart of lecce. all over the world, and in particular in the mediterranean countries, there is a growing attention on the need to promote and strengthen the importance of developing multidisciplinary measurement approaches, which are the key for an effective preservation, management and understanding of cultural heritage. today more than ever, it is generally agreed upon that heritage scientists, engineers, conservators and archaeologists have to share their common interests and understanding in the cultural heritage areas, while maintaining their different perspectives. for instance, heritage scientists and engineers’ focus is often oriented towards the development of measurement system and to the interpretation and validation of the measurement data. on the other hand, conservators and archaeologists concentrate more on a pragmatic understanding of heritage assets and of the social/historical contexts in which they originated. metroarchaeo gives all these actors of the scientific and humanistic world the opportunity to meet, share knowledge and discuss new ideas to provide effective and comprehensive solutions to age-old problems in research, protection, monitoring and exploitation of cultural heritage artefacts and sites. in this perspective, metroarchaeo2017 confirmed to be an important annual meeting collecting experts and researchers with different expertise, who are interested in open-minded discussion on the recent advances in research on instrumentation and measurements in the cultural heritage field. more than 200 papers were presented in lecce and after a careful peer review 16 papers and one technical note were eventually accepted for the acta imeko special issue. these papers give an interesting overview of the integrated analytical and measurement approaches in cultural heritage and archaeology. most of the papers published in this special issued focus on innovative methods and on enhancements of state-of-the-art systems for the characterization of cultural heritage, as well as on methods for the preservation of archaeological artefacts and historical sites. in this regard, the paper presented by gigante et al. addressed the use of x rays for the characterization of the metallurgic characteristics of moche tombs “señor de sipán” and “señora de cao”. cesareo et al. address the use of a portable edxrf-device for a 3d map of chemical elements distribution. the chemical and physical characterisation of cultural heritage assets is as important as the development of tailored methodologies for their long-lasting preservation. in this contest, a new fibre optic sensor for monitoring the ph of water is proposed by dinia et al. to assess the effect of corrosion on different materials. di francia et al.’s work focused on a low-invasive and selective laser cleaning procedure for the removal of reactive corrosion products on cu-based artefacts without damage the substrate. eventually, degradation of cultural heritage materials can be studied both on real fragments as well as on reference samples as in the case of the infrared thermography investigation performed mailto:sabrina.grassini@polito.it acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 2 by forestieri et al. to assess the water absorption/desorption mechanisms in building stones. several papers employed current investigation methods (or their combination) to bring to light new discoveries and features of invaluable art and monumental pieces. for instance, leuzzi et al. addressed non-invasive exams, combining imaging techniques (including uv fluorescence (uvf), infrared reflectography (irr), infrared false colour (irfc), x-ray fluorescence (xrf) with analytical analyses, for the scientific examination of the nursing madonna (madonna del latte). di giacomo et al. employed integrated geophysical surveys for the study and the reconstruction of the sixteenthcentury fortifications of the city of lecce. de giorgi et al. addressed non-destructive testing to assess the conservation state of the elements constituting the roofing of the cathedral of foggia (italy) by means of ground penetrating radar. diaferio et al. discussed the results of an in-situ dynamic measurement campaign on a stocky masonry clock tower situated in the swabian castle of trani (italy) for assessing the effects of the shaking device on the vibrations of the tower. eventually, an integrated measuring approach was used by rosselli et al. for the structural diagnosis of the temple of minerva medica, in roma (italy). finally, other papers focused on software and management tools for optimal investigation and/or fruition of cultural heritage. for example, de nunzio’s paper addresses a software tool for interactive and semi-automatic segmentation, characterization, and annotation of 3d models produced by photogrammetric surveys, while buzi et al. present a new digital approach also based on photogrammetry to study a paleoanthropological specimen: the neanderthal skull saccopastore 1. malagino et al. addressed the application of a new integrated approach (resorting to business process management (bpm) and building information modelling (bim)) to optimize the data flow, to gather information and to share knowledge during the restoration process, thus facilitating the ordinary and extraordinary maintenance. the work by tanasi et al. addressed the strategies for the optimization of the dissemination of 3d models on cultural heritage: they presented the case of the archaeological museum ‘paolo orsi’ in syracuse (italy), as best practice on digital dissemination through platforms. eventually, angelini et al. presented some effective strategies to reduce the errors inherent to range-data and image-based systems applied to the archaeological heritage, with particular focus on photogrammetry from drones compared to traditional acquisitions. 3d survey and metric analyses also allows more reliable interpretation of historical data, as in the case study of late roman fort of umm al-dabadib (egypt), discussed by fiorillo et al. last but not least, the technical note on ancient psychological instruments by tošković highlighted the importance of these artefacts, which allow the reconstruction of a psychological laboratory from the very beginning of this scientific discipline, as well as of the partnership between science, measurements and psychological research. we hope that this special issue will give the reader new points of view in metrology and measurement applied to cultural heritage, taking also into account their fundamental role in our society. the special issue wants to highlight the importance of the synergy among all heritage scientists and actors and among the different disciplines, which provide important tools for the monitoring, preservation, management and safeguard of cultural heritage assets. hope you will have an interesting reading! sabrina grassini egidio de benedetto microsoft word 430-2580-1-le acta imeko  issn: 2221‐870x  november 2016, volume 5, number 3, 45‐46    acta imeko | www.imeko.org  november 2016 | volume 5 | number 3| 45  editorial to selected papers from the xxi imeko world  congress 2015   paolo carbone  university of perugia, italy    section: editorial   citation: paolo carbone, editorial to selected papers from the xxi imeko world congress 2015, acta imeko, vol. 5, no. 3, article 7, november 2016,  identifier: imeko‐acta‐05 (2016)‐03‐07  editor: paolo carbone, university of perugia, italy  received october 27, 2016; in final form october 27, 2016; published november 2016  copyright: © 2016 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  corresponding author: paolo carbone, email: paolo.carbone@unipg.it      dear reader, in the second part of this issue, acta imeko publishes eight papers that were originally presented at the 2015 world congress in prague and that are here presented in their extended versions. the first paper by dušan agrež, damir ilić and janko drnovšek considers the problem of estimating efficiently the parameters of a sinusoidal signal in the frequency domain. this is a very common issue in many practical measurement problems solved by means of numerical procedures. the paper provides both simulation and experimental results allowing the user to grasp the practical implications of estimating the amplitude, phase and frequency of a sampled and quantized sinewave. the second paper by leonard klaus addresses the problem of estimating the parameters of a model used in dynamic torque measurements. several estimators are considered including the least squares and maximum likelihood estimators. a procedure is then described and applied to a set of data obtained experimentally at the ptb, where the author works. the third paper is the result of an international collaboration among researchers in the egyptian, and german national metrological institutes and in the faculty of engineering of the cairo university. it presents a new approach in force measurement standards that includes a force transducer having three adjustable capacities. the design and realisation of the prototype is described, also including results from a finite element analysis. the fourth paper by qiao sun and hong-bo hu addresses the problem of low intensity shock acceleration calibration. the authors performed a pilot comparison study collaborated by four calibration laboratories, showing good compatibility between the obtained measurement results. the fifth contribution by xiao-ping res et al. considers the issue of mass standard design and the difficulties associated to the inclusion of adsorption in the design models. the effects of sorption over time and its implications when measuring the standard mass are taken into account in this paper. experimental results are shown that also include the effect of humidity on the sorption velocity. the sixth paper by jiří přibil, anna přibilová and ivan frollo considers magnetic resonance imagers and the analysis of the human vocal tract structure together with its dynamic shaping. the noise generated by the physical mechanisms excited by the interaction of the generated magnetic fields with the machine gradient coils is analysed experimentally in this paper. the seventh paper in this issue is authored by marija cundevablajer. it describes a framework for the design of a three-cycle university program focused on metrology and the results of an international collaboration addressing this aspect. it can be considered as a useful reference for all those involved also in educational activities. the eighth paper by nataliya a. gavrilenko et al. describes the usage of an optical analytical method to detect chromium (vi) using polymeric sensors. since chromium compounds tends to acta imeko | www.imeko.org  november 2016 | volume 5 | number 3| 46  be irritating and corrosive, finding a new method to detect their presence that is alternative to the usage of spectrophotometry widens the amount of available measurement techniques. let me take this occasion to remind everyone that acta imeko accepts freely submitted papers in all areas of metrology and not only extended versions of papers previously presented at imeko conferences and congresses, as in the recent past. i encourage all authors in these research areas to consider our journal as the possible publication medium for their researches. have a fruitful reading of the third issue of acta imeko in 2016! template for an acta imeko event paper acta imeko issn: 2221-870x september 2017, volume 6, number 3, 2-3 acta imeko | www.imeko.org september 2017 | volume 6 | number 3 | 2 introduction to the special issue of the metroarchaeo 2016 conference sabrina grassini1, alfonso santoriello2 1department of applied science and technology, politecnico di torino, torino, italy 2dipartimento di scienze del patrimonio culturale, università di salerno, italy section: editorial citation: sabrina grassini, alfonso santoriello, introduction to the special issue of the metroarchaeo 2016 conference, acta imeko, vol. 6, no. 3, article 1, september 2017, identifier: imeko-acta-06 (2017)-03-01 section editor: paul regtien, measurement science consultancy, the netherlands received september 19, 2017; in final form september 19, 2017; published september 2017 copyright: © 2017 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: sabrina grassini, e-mail: sabrina.grassini@polito.it dear readers, this special issue of acta imeko contains 14 papers presented at the imeko tc4 international conference on metrology for archaeology and cultural heritage – metroarchaeo2016, which took place on october, 19-21 2016 in torino (italy). the conference was held at the valentino castle, an historical building inscribed in the unesco world heritage list in 1997, which actually hosts the faculty of architecture of politecnico di torino university. started in 2015 in benevento (italy), metroarchaeo is becoming a successful international annual meeting, which puts together heritage scientists, engineers, conservators and archaeologists to foster exchanges of ideas and information on metrology and measurements for archaeology and cultural heritage. heritage scientists, engineers, conservators and archaeologists have common interests and understanding in many areas, but at differing levels. conservators and archaeologists have less understanding of experimental science and measurement, but more experience and pragmatic understanding of heritage assets and their context, practicality and ethics. heritage scientists and engineers with high expertise in metrology, have greater understanding of interpretation and validation of data, taking into account their reliability and uncertainty. today it appears more and more clear that multidisciplinary approaches coupled with appropriate measured survey and imaging data are fundamental requirements for the effective preservation, management and understanding of our cultural heritage, offering also new scenarios for territorial growth from the social-economic point of view. metroarchaeo gives all these actors of the scientific and humanistic world the opportunity to discuss and exchange knowledge and to develop integrated systems able to provide answers to historiographical questions and face real problems in research, protection, monitoring and exploitation of cultural heritage and archaeological sites. more than a hundred papers were presented in torino and after a careful peer review fourteen papers were eventually accepted for this acta imeko special issue. these papers give an interesting overview of the integrated analytical and measurement approaches in cultural heritage and archaeology and well represents the worldwide attendance to the conference. the paper presented by calicchia et. al., illustrates a new model of a territorial system, the cultural heritage open laboratory system (chelabs), based on open access policy to the cultural assets grounded on the territory and on the technological and social economic context, with the final aim of attracting competencies and generating excellence in heritage science. two papers emphasize the contributions of 3d approaches and digital techniques for the restoration and measurements of cultural heritage artefacts. stamatopoulos et. al. illustrate a new digital approach for reassembling ancient ceramic pottery based acta imeko | www.imeko.org september 2017 | volume 6 | number 3 | 3 on 3d models of their fragments and the exploitation of their thickness profile, with particular attention to the accuracy of the thickness measurements. dondi et. al. present a new protocol and a 3d model suitable for acquiring precise measurements on historical violins applied to the stradivari “cremonese” (1715) kept in “museo del violino”, cremona (italy). the papers of lo brutto et. al. and a. kubicka highlight the potentialities of digital photogrammetry and laser scanning techniques for the full-scale 3d modelling and representation of archaeological artefacts and sites. a. angelini et. al. discuss a new algorithm for the 2d representation and surface reconstruction of heritage buildings. 2d and 3d reconstructions are important tools for the documentation and the restoration of cultural heritage, as well as the development of tailored wireless sensors networks, such as the one proposed by l. d’alvia et. al., are fundamental instruments for the long time preservation of multimateric artefacts displayed in museums. five papers are focused on the application of different advanced analytical techniques for the characterisation of inks, ceramics, glasses, stones and metallic artefacts. thz spectroscopy is proposed by taschin et. al. to study the absorption coefficient and the reflective index of different inks in form of thin films. r. pitonzo and co-authors use gas chromatography coupled with mass spectroscopy for the analysis of archaeological ceramic amphorae belonging to the carthaginian period. the mcx beamline at elettra is employed by plaisier et. al. for non destructive x-ray diffraction measurements on stained glasses. the application of ion microbeam analysis in the cultural heritage field is discussed by lo giudice et. al., while r. chapoulie work is focused on photons and electrons for the study of drawings on prehistoric wall caves. eventually, two papers by e. shaykhutdinova discuss interdisciplinary studies on silver coins and iron casting technologies. we hope that this special issue gives the reader new points of view in metrology and measurement applied to cultural heritage, taking also into account their fundamental role in our society. the special issue wants to highlight the importance of the synergy among all heritage scientists and actors and among the different disciplines, which provides important tools for the monitoring, preservation, management and safeguard of cultural heritage assets. hope you will have an interesting reading! sabrina and alfonso introduction to the special issue of the metroarchaeo 2016 conference acta imeko, automated lung segmentation on digital tomo-synthesis images with complex method acta imeko issn: 2221-870x december 2017, volume 6, number 4, 46-53 acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 46 improved approaches of automated lung segmentation on digital tomosynthesis images bence tilk budapest university of technology and economics 1111 budapest, műegyetem rkp. 3., hungary section: research paper keywords: medical imaging; digital tomosynthesis; lung segmentation; computer-aided diagnosis citation: bence tilk, improved approaches of automated lung segmentation on digital tomosynthesis images, acta imeko, vol. 6, no. 4, article 7, december 2017, identifier: imeko-acta-06 (2017)-04-07 section editor: alexandru salceanu, technical university of iasi, romania received may 5, 2017; in final form october 9, 2017; published december 2017 copyright: © 2017 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: bence tilk, e-mail: bence.tilk@gmail.com 1. introduction lung cancer is one of the leading causes of death. early detection of the lung nodules can increase the survival chance of the patient. the most common screening method used to inspect the lungs is chest radiography, which produces single summation images. these x-ray images do not give in-depth information about the lung, so abnormalities like lung nodules cannot easily be separated from other anatomical shadows like vessels, bones, etc. although computed tomography (ct) is a much more sensitive imaging modality, its cost and the radiation dose a patient obtains during a ct scan are obstacles for largescale screening use. these drawbacks are true even for its low dose version (ldct). chest digital tomosynthesis (dts), as a relatively new imaging modality, can be positioned between chest radiography and ct [1]. while it produces (coronal) slice images of the chest similarly to ct, its slice thickness is larger, it creates a bit more blurred slices, and the radiation dose produced during a dts scan is much lower than that of a ct scan. a further consequence of using dts is that, instead of a single x-ray image, a dts scan produces about 100-400 coronal slice images. so, reading the images of a complete scan needs more time and thoughtfulness from the radiologists/pulmonologists. computer-aided diagnosis (cad) can help physicians, can improve radiologists' efficiency as it detects nodule-like areas, and helping radiologists to decide if this area can be regarded as a potential abnormality or not. a modern cad system has 3 main parts. to create regions of interest (rois), robust methods are needed to segment the lungs from other parts of the image, and to determine areas of the images where any abnormality is looked for. the second part is to find suspicious regions inside the previously segmented area. the third part is to classify the candidate nodules with machine learning tools and to provide a list of potential lesions which can be overlooked by doctors. this paper focuses on the first step of the procedure. it proposes a complex lung segmentation method of dts slice images. dts is considered as a 2.5d imaging technique. it creates projections similar to standard x-ray radiography, however, instead of getting a single projection of a patient, 40−60 projections are taken from different angles. from these abstract for lung screening the most common method is chest radiography, which produces summation images without giving any depth information about the lung. computed tomography (ct) creates excellent slice images, which give volume data that makes ct a more sensitive nodule detection system. however, ct has disadvantages, it is too expensive and its x-ray emission is too high to be used as an everyday screening method. digital tomosynthesis (dts), as a relatively new chest imaging modality, can be positioned between chest radiography and ct. while it produces slice images of the chest similarly to ct, its slice thickness is larger, it creates a bit more blurred slices, it has much lower radiation than ct. this blurring makes it hard to segment the lung areas automatically, which is essential for an efficient computer-aided diagnosis system. the paper proposes a combined method, which starts from a previously published approach, extends it using snake methods and adjacent images’ segmentation information to improve lung segmentation. experiments show that the combination of methods reduces the incorrectly segmented lung region. acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 47 projections reconstruction algorithms are used to compute the coronal slice images of the chest. because the projections are taken from a limited angle range, the coronal slices are not ideally thin, and there is some blurring between the neighbouring slices too. although the spatial resolution of the images is rather good (the coronal slices are at least 1000×1000 pixel images) their in-depth resolution is poor making difficult to obtain 3d information about the lung. central slices have relatively high quality, but further from the central slice, image quality is getting poorer, what can mislead the segmentation methods on non-central images. this paper proposes a complex segmentation procedure, which starts from a method published in [2], but as the result of this previous method does not seem satisfactory, further steps are introduced. the complex algorithm applies three different approaches: an intensity-based, a gradient-based and an energy-based segmentation in order to make the algorithm more robust and accurate. the efficiency of the method is increased by combining information from the adjacent images. this paper is organized as follows: section 2 is a short summary of the previous results; it describes the most promising solutions and shows their deficiency. in section 3. the improvements of the previous method are described. an evaluation is presented in section 4, where simulated dts images are used, which are generated from ct images, to get at least acceptable standard reference lung contours. 2. related results in the literature dts is a relatively new medical imaging modality. its main application is related to breast screening [3], while only a few products can be found where dts is used for chest screening [4]. previous results that are dealing with computer-aided detection or diagnostic (cad) systems based on dts are even fewer. according to the knowledge of the author, there are only very few papers dealing with the problem of lung segmentation in chest dts slice images. seung-hoon et al. [5] proposes segmentation using only intensity information with a region growing method on central images. this paper recommends using a previously selected fixed pixel as an inner point, which is the initial point of the growing method. this approach is not considered as a robust method because it is sensitive to the position of the patient. it also suggests rolling-ball, where a ballshaped element is rolled along the edge of the segmented area to get smooth lung borders in the end, as a useful postprocessing step. automated lung segmentation in chest dts [2] uses the gradient information of the image and searches the lung masks along the edges by dynamic programming. the gradient image is transformed into polar coordinates where an inner point determined previously is used as the origin of the polar coordinate system. this approach works quite well in the central slice images, as in these images the lung area has rather definite and sharp borders. this paper also proposes a method on how to handle non-central images. the basic idea is that the neighbouring slices are rather similar, so the gradient images in these slices must be similar too. the results using these approaches are quite remarkable, however, the robustness of these methods should be improved. as the variability of dts images obtained from different patients is rather large, the performance of these methods varies greatly. a typical gradient image of a dts slice is shown in figure 1 and its version in the polar coordinate system is shown in figure 2. as it can be seen, there are brighter paths and many pale paths in the image, and these paths are going through the gradient image near horizontally, as the lung contour, where there may be larger gradients, surrounds the inner point. in this unfolded image, dynamic programming is used to find a route connecting the brightest points. the drawback of this algorithm is that it assumes that one column of this image belongs to one angle, that is, only convex shapes can be identified using dynamic programming. a cost function is assigned to each pixel, which has two components. one of them is the intensity of the pixel; it is used as a reward. the second component is the distance difference from the previous pixel; this is used as a penalty. more details about the cost function are available in [2]. the result of the method for a slice is shown in figure 3. concave segments may show up around the heart, so when using this approach the area under the heart can’t be segmented correctly and additional methods are required to find this figure 2. gradient image of one dts slice in the polar coordinate system; white pixels mark the calculated path. only the right lung is shown in this image. figure 3. after reconstructing the found path, the center of the lung and some points are highlighted with red dots. figure 1. gradient image of one dts slice in the cartesian coordinate system; it is squeezed along the vertical dimension to set each lung’s width and height to have similar sizes. this step is done in order to eliminate as many jumps on the path as possible. acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 48 region. there is another disadvantage of dynamic programming. two paths can be very different while they may have similar costs. the consequence is that there may be very similar slice images, but the segmented masks can be significantly different, what makes this method less robust. this study [2] also suggests a method to reduce wrong segmentation on non-central slice images, by fading unlikely position pixels on the polar coordinate images, as the assigned positions on the adjacent slices must be close to each other. on the current gradient image, the pixel values are multiplied by a gaussian function, where pixel distances from the adjacent selected points in that column are assigned as gaussian function’s variable, see (1). 𝐼′(𝑥, 𝑦) = 𝐼(𝑥, 𝑦)𝐺(𝑂(𝑥) − 𝑦) (1) where i(x,y) is the intensity value of one pixel of the polar coordinate image, at the x-th column and y-th row. g is a gaussian weighting function and o(x) is the neighbouring row index in the gradient path, which image is towards the center slice. so o(x)-y is a distance between the two selected pixels in the x-th column. images are processed from the center slice towards the non-central slices. this model eliminates problems in many cases, but sometimes a wrong central image could mislead the whole process. it is more desirable to have more images around the central one instead of exactly one because a poorly chosen slice as a center image can reduce the accuracy. this is based on the assumption that, choosing such images where at least half of them are considered as blurred ones, is less likely. an important and hard problem of lung border detection is to validate the results. the study [2] uses reference lung contours, which are created by radiologists. however the real lung contours in dts slice images are very difficult to determine even for skilled radiologists. on the central slice images it is quite easy to determine the lung contour but on non-central images the expected lung area based only on dts can hardly be determined. this is why more information is needed to get at least approximately correct lung borders on non-central images. 3. model-free method 3.1. pixel intensity based segmentation using only pixel intensity values to determine the lung area is impossible as in contrary to the ct images on the dts images the intensity values are not standardized. this means that on dts images a universal intensity limit, correct for differentiating the lung area from non-lung areas on every image cannot be identified. instead, picture-dependent limits should be defined. pixels below this limit are considered as part of the lung. the correct value of the intensity limit can be determined by using the histogram of the image, which is shown in figure 4 for a typical slice. first, the histogram is smoothed using a low-pass filter to reduce noise and the effect of missing intensity values. a simple averaging filter with a 5pixel kernel is used. the histogram has two valleys after filtering. a proper limit value in the smoothed histogram will be the local minimum of the first valley. after that, morphological methods are used to get continuous lungs and to remove noise. experiments show that eroding with a smaller structure-element and dilating with a bigger structure-element gives the best results. next, some inner point of each lung, called center point, must be determined. the image is split vertically at the center into two parts (the two (half) lungs), so one lung is processed by the method, then the formulas below are used; p(x,y) is the value of the binary mask at pixel (x,y): 𝐶𝐶𝐶𝐶𝐶𝐶𝑥 = �∑ 𝑥2 (𝑥,𝑦) 𝑃(𝑥, 𝑦) (2) 𝐶𝐶𝐶𝐶𝐶𝐶𝑦 = �∑ 𝑦2 (𝑥,𝑦) 𝑃(𝑥, 𝑦) (3) results show that this point isn’t optimal because it is near to the arteries and the heart, which can be misleading for the further steps of the method. the centers are translated to the outer side of the lung by 8 % of the image width and translated upwards by 10 % of the image height. (an example of the selected center points is shown in figure 5). 3.2. gradient-based segmentation the next step is using a path finder algorithm. due to the previously mentioned problem of ambiguity of getting a path, additional smoothing is needed. get every point of the lung contour on each slice and make a function from it; the values of the function will be the distances from the central point. generate these functions fx(y) belongs to the x-th point of the boundary and y is the index of the slice, as can be seen in figure 6 -, then convolve the function as described in (4) with a smoothing filter (h kernel, which can be any vector; the chosen one is described in (5)), so it decreases the big steps on the masks between the adjacent images. after the convolution, figure 5. segmented area based on pixel intensity, after morphological steps; red dots are the calculated centers. figure 4. original histogram of a chest tomosynthesis; some intensity values are missing. figure 6. construct fx(y) function based on images in polar coordinates. acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 49 convert the values back to the cartesian coordinate system to get the suggested path. 𝐹𝑥’(𝑦) = 𝐹𝑥(y) ∗ 𝐻 (4) 𝐻 = 1 5 [1; 1; 1; 1; 1] (5) in the end, the presented method works almost perfectly on the outer side of the lung, but it gives rather bad results near the mediastinum. 3.3. energy based segmentation fortunately, there is a method that works better on the inner side of the organ: we can use an active contour model (sometimes referred to as snake) to find the region under the heart on the left lung and it also works more or less on the mediastinum. the active contour model [6] is based on energy minimization; it creates smooth contours and, at the same time, it takes the edges into consideration as can be seen in figure 7. the energy formula is the sum of the smoothness of the contour, the continuity of the contour (first two components known as internal energy) and image energy. furthermore, with correct parametrization on images that differ slightly, it creates almost identical masks. it is an iterative method and needs an initial mask. an intensity-based segmentation is considered as a satisfying initial mask. it contains the region under the heart, but it isn’t accurate enough. to combine the two methods, use the active contour model to define the inner side of the lung and use the gradient-based method to determine the others. the regions are determined by the range of angles and the chosen inner point is used as a center point. 3.4. information from adjacent slices the information of adjacent images can be used to fit the segmentation to obtain more reliable masks. the basic method [2] is modified to get a better model. the image in polar coordinates is modified, fading the unlikely pixels. unlike the method in [2], two gaussian functions are used. the first one gives a weight to the adjacent images, which depends on how far it is from the calculated image; only the images to the central direction are relevant, weights in other directions are zero. the second gaussian function fits vertically to the image like in the referred article. the center of the function is the path of the adjacent image in the selected pixel of the current column. here the main modification of the function is: using an asymmetric gaussian function (shown in figure 8), with a lower width parameter on the farther side from the origin and a higher σ on the nearer side. this approach is used because the lung area is bigger on central slices and it shrinks when going further from the central slice. i(x,y) is the current pixel, n is a constant for normalization, g is a standard gaussian function, g2 is the asymmetric gaussian function and o(x,i) is the x-th point of the found path on the i-th image: 𝐼(𝑥, 𝑦) = 𝑁 ∑ 𝐼(𝑥, 𝑦)𝐺(𝑖)𝐺2(𝑂(𝑥, 𝑖) − 𝑦)𝑖 (6) in the end, the masks may contain acute angles. it is desirable to minimise them using morphological closing to accomplish this. usage of different structure sizes on different regions of the lung gives the best result the biggest structure is used at the inner side of the lungs and a smaller one used on the top of the lungs; the lower area is skipped completely. this approach produces similar results as those of the rolling ball, but with less computational complexity [5]. 4. model-based improvements correct segmentation of the lung can be interpreted as a measurement with error. the measurement error can be reduced as the expected distribution of the measurement is known. in order to make a correct segmentation, prior knowledge is needed about shape, size, the location of the lung and characteristics of tomosynthesis. to build a model the reference shapes of the lungs on each slice are needed. section 5 of this paper shows the method of generating these references from an image set [7]. because the generated references are on simulated images, features of the images cannot be used because it may add a bias to the model. considering these facts, the final method is an extension of the snake algorithm [6]. it adds size and shape energy to the model as can be seen the in (7), where x iterates through the boundary. 𝐸𝑠𝑠𝑠𝑠𝑠 = ∫ (𝐸𝑖𝑠𝑖𝑠𝑖𝑠𝑠𝑖(𝑥) + 𝐸𝑖𝑖𝑠𝑖𝑠(𝑥) + 𝐸𝑠ℎ𝑠𝑎𝑠(𝑥) + 1 0 𝐸𝑠𝑖𝑠𝑠(𝑥))d𝑥 (7) the snake algorithm minimizes the energy; the internal energy describes the smoothness of the shape and the image energy describes how well the shape fits to the image edges. to optimize the energy of the boundary, the gradient or its approximation must be provided as well. modelling the size of the lung on each slice can be done by modelling the relative size of the lung on each slice. the relative size of an average left lung on each slice can be seen in figure 9. the model stores the relative size of the segmentation, figure 8. asymmetric gauss function, with different standard deviation on each side; both sides are normalized to give the same results in the origin. figure 7. active contour result after 30 iterations. acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 50 compared to the chosen interval of the slices. the central interval is used as the reference size and other slices are compared to it because its size is the easiest to determine with model-free methods. the size energy is calculated on each slice as the absolute difference of the relative reference lung size and the current relative size (compared to the central images) of the segmentation. of course the size energy on the central interval is zero. equation (8) declares 𝛂, which is a normalization coefficient. it shows the size of the patient’s lung compared to the reference lung. cstart and cend are the endpoints of the interval of central slices, mstart and mend are the same for the modelled reference lung: 𝛼 = 𝑀𝑒𝑒𝑒−𝑀𝑠𝑠𝑠𝑠𝑠 𝐶𝑒𝑒𝑒−𝐶𝑠𝑠𝑠𝑠𝑠 ∗ ∑ 𝑆𝑖𝑠𝑠(𝐼𝑖𝑠𝑖𝑠(𝑖)) 𝐶𝑒𝑒𝑒 𝑖=𝐶𝑠𝑠𝑠𝑠𝑠 ∑ 𝑀𝑀𝑀𝑠𝑖𝑆𝑖𝑠𝑠(𝑖) 𝑀𝑒𝑒𝑒 𝑖=𝑀𝑠𝑠𝑠𝑠𝑠 (8) considering the model lung and the current image set have the same number of slices, (9) shows the size energy on the current slice: 𝐸𝑠𝑖𝑠𝑠(𝑥) = |𝛼 𝑀𝑀𝑀𝐶𝑀𝑀𝑖𝑀𝐶(𝑖) − 𝑀𝑖𝑀𝐶�𝐼𝐼𝐼𝐼𝐶(𝑖)�| (9) the gradient of the size energy directs towards the normal vector of segmentation; its magnitude equals the calculated size energy. the shape energy is calculated as dtw (dynamic time warping) distance of two time series [8]. time series are generated for each lung on each slice; an example for this time series is shown in figure 10. dtw is a dynamic programming technique, calculated as below. dtw(i,j) means the cost of time series one(t1) till the i-th point and time series two(t2) till the j-th point. the cost of two values is calculated as absolute distance. the value of the dtw function is 0 if the value of i or j equals zero. 𝐷𝐷𝐷(𝑖, 𝑗) = 𝑐𝑀𝑐𝐶�𝐶1(𝑖), 𝐶2(𝑗)� + 𝐼𝑖𝐶{𝐷𝐷𝐷(𝑖 − 1, 𝑗), 𝐷𝐷𝐷(𝑖, 𝑗 − 1), 𝐷𝐷𝐷(𝑖 − 1, 𝑗 − 1)} (10) one of these time(t1) series is the reference series on the given slice, and the other(t2) is the calculated one from the current boundary and normalized to have the same average for both time series. the boundary is transformed to time series similarly as the image in polar coordinates is calculated. the same center point is used but this time the distance for every point of the boundary is calculated by iterating the points from a starting point to a given direction. this method creates a time series shown in figure 10. the shape energy can be calculated according to the above information and an approximate gradient is needed for the iterative method. the equations below show how to calculate the gradient. equation (11) describes the magnitude of the gradient for the i-th point. equation (12) shows the gradient calculation: the first part shows the gradient direction, which directs inwards and its magnitude is 1; the third part decides whether it should direct inwards or outwards. |∇p(i)| ≈ cost�𝐶1(𝑖), 𝐶2(𝑗)� (11) ∇p(i) = � 𝑂𝑥 𝑂𝑦 � − � 𝑃𝑥(𝑖) 𝑃𝑦(𝑖) � �� 𝑂𝑥 𝑂𝑦 � − � 𝑃𝑥(𝑖) 𝑃𝑦(𝑖) �� ∗ |∇p(i)| ∗ ∗ �if 𝐶1(𝑖) > 𝐶2(𝑗): 1 else: −1 � (12) 5. results validation of the results for non-central slices is challenging, where blurring could mislead radiologists, so without any further information lung contours created by them cannot be considered as reliable segmentation. this problem can be partially solved with simulated tomosynthesis images, which are generated from axial ct slices. the simulated image characteristics are close to those of the original dts images. on axial ct images, as can be seen in figure 11, lung masks are generated based on pixel intensity and morphological closing. as a next step, these images were projected to coronal slices with similar geometry as the geometry of the dts. masks are projected as well; they are used as an at least acceptable standard reference, which is useful to calculate metrics too. 5.1. center slices two metrics are calculated: overlapping and mean average distance (mad). mad is calculated as an average of each distance of discovered points to the nearest reference point. figure 10. this time series is generated from the left lung on the 130th slice of 250 slices. more than 2500 boundary points are used and the distance is measured in pixels: the x-axis shows the ordinal number of the boundary point and the pixel distance of the boundary point from the center point is shown on the y-axis. figure 9. the relative size of the lung on each slice: the x-axis shows the ordinal number of the slice, and the relative (compared to the biggest seen lung area) size of the lung area is shown on the y-axis. this graph is calculated from an average of 8 lungs. 250 slices are used, many of them don't contain lung volume. the peak on the graph is caused by the area of the apex. acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 51 these standard deviations are calculated as well. overlapping is calculated by: ∑ 𝑹(𝒊,𝒋)∩𝑴(𝒊,𝒋)𝒊,𝒋 ∑ 𝑹(𝒊,𝒋)∪𝑴(𝒊,𝒋)𝒊,𝒋 (13) and the mean average distance (mad) is calculated by: 1 𝑁 ∑ �(𝑅𝑥(𝑖) − 𝑀𝑥(𝑖))2+(𝑅𝑦(𝑖) − 𝑀𝑦(𝑖))2𝑁𝑖=1 (14) in (14) r(i,j) denotes the pixel of the reference mask at the i-th row and j-th column and m(i,j) is the segmented mask with the same parameters. the evaluation is done by five central images of the image sets. the results are weighted by the number of central images. an image is considered as a central image if the cross-sections of the ribs are sharply visible. the number of central images on the used image set with 223 slices varies from 41 to 57. the aggregated results are shown in table 1. each image set of a patient has been investigated; results depend on images quality. table 2 shows the results for the patient whose image set shows the least blurry. images are compared inside one image set. figure 12 shows a segmentation of a central image, figure 13 shows a segmentation of a non-central image from the same image set. the next section provides more information about the accuracy trend inside one image set. 5.2. result on noncentral images table 3 shows the results for non-central slices; central borders have been identified manually. figure 11. axial ct slice segmented by the intensity of the pixel; the green line is separating lungs from other parts. table 1. averaged results using 5 central images of the image set. image resolution is 512x512. result overlap mad (pixel) standard deviation both lungs 0.7976 5.452 6.630 left lung 0.8089 4.507 4.770 right lung 0.7882 6.109 7.019 table 2. results of one image set out of 57 central images from 223 images, 130 of them containing lung area according to ct images. resolution of the images is 512x512 pixels. result overlap mad (pixel) standard deviation both lungs 0.8508 5.175 5.943 left lung 0.8462 5.125 6.117 right lung 0.8557 5.169 5.510 table 3 segmentation with a model on non-central images, which contain lung area. result overlap mad (pixel) average 0.4922 20.529 standard deviation 0.2027 17.258 max 0.8314 109.485 min 0 3.744 figure 12. on a central image successful segmentation, the green line is the method’s boundary and the red one is the reference lung, which comes from a segmented ct-scan. figure 13. on a simulated non-central slice, the red line is the reference lung based on ct; it is hard to be separated by radiologists without any help. acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 52 figure 14 shows the difference between the model-based and the model-free approach. it suggests that modelling lung features improve the results on non-central images and create more robust segmentation on these slices. that is why model results tested only on non-central slices. results are calculated from 8 image sets each of them containing 225 slices only those slices are used that contain lung area. figure 15 shows sample lung segmentations on various slices. figure 15. segmentation on digital tomosynthesis images; red is the model-free version, the green line is the segmentation with a model. image’s order is anterior—posterior. the method using the model seems more natural for humans. figure 14. model-based and model-free approaches comparison based on mad. the method with a model is more robust, but on central slices the image itself is more reliable than the model suggested segmentation. acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 53 6. conclusion a combined method can improve the results and may result in more robust solutions. three different model-free approaches are used to take advantage of each method on different areas of the lungs: intensity-based segmentation for center point identification, gradient-based segmentation for segment outer side of the lung and snake for segment outer side of the lung. using more complex models is to gain information from adjacent images to improve the incorrectly segmented region on the non-central images. the inspection mostly focused on the central images, because without them, non-central slices cannot be accurately segmented, and most of the information is concentrated on central images; blurring through adjacent images here can be useful. it is proved that further improvement can be reached on non-central images by creating a model of the lung using statistical methods. this paper showed two statistical methods to model the lung’s size and shape. these methods can increase accuracy on non-central slices and make the numbers slightly worse on central images. combining these two approaches can produce the highest accuracy, but models have many difficulties. one of the biggest problems is the large number of images that are required to get a correct model. 7. acknowledgement the author thanks the help of gábor horváth, who introduced the problem, and dániel hadházi for generating simulated tomosynthesis images from lidc ct images [7]. references [1] j.t. dobbins, “tomosynthesis imaging: at a translational crossroads”, medical physics, 36, (2009), pp.1956-1967. [2] wang, jiahui, j.t. dobbins, q. li, “automated lung segmentation in digital chest tomosynthesis”, medical physics, 39, (2012), pp.732-741. [3] park, m. jeong, e. a. franken, m. gargand, l.l. fajardo, and l.t. niklason, ”breast tomosynthesis: present considerations and future applications”, radiographics, 27, (2007), pp.231-240. [4] j.t. dobbins, h.p. mcadams, j.w. song, c.m. li, d.j. godfrey, d.m. delong, s.h. paik, and s. martinez-jimenes, “digital tomosynthesis of the chest for lung nodule detection: interim sensitivity results from an ongoing nih-sponsored trial”, medical physics, 35, (2008), pp.2554-2557. [5] c. seung-hoon, l. jeongwon, w. chulho, p.s. bum, “lung segmentation using prediction-based segmentation improvement for chest tomosynthesis”, international journal of bio-science and bio-technology, 6, (2014), pp.81-90. [6] m. kass, a. witkin and d. terzopoulos, “snakes: active contour models”, international journal of computer vision, 1, (1988), pp. 321-331. [7] a. g. samuel, g. mclennan and l. bidaut, m.f. mcnitt-gray, c.r. meyer, a.p. reeves, b. zhao, d.r. aberle, c.i. henschke, e.a. hoffman, “the lung image database consortium (lidc) and image database resource initiative (idri): a completed reference database of lung nodules on ct scans”, medical physics, 38, (2011), pp.915-931. [8] h. sakoe, s. chiba: “dynamic programming algorithm optimization for spoken word recognition”, ieee transactions on acoustics, speech, and signal processing, 26, (1978), pp.43-49. improved approaches of automated lung segmentation on digital tomosynthesis images << /ascii85encodepages false /allowtransparency false /autopositionepsfiles true /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /error /compatibilitylevel 1.4 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.0000 /colorconversionstrategy /cmyk /dothumbnails false /embedallfonts true /embedopentype false /parseiccprofilesincomments true /embedjoboptions true /dscreportinglevel 0 /emitdscwarnings false /endpage -1 /imagememory 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/pdfxoutputintentprofileselector /documentcmyk /preserveediting true /untaggedcmykhandling /leaveuntagged /untaggedrgbhandling /usedocumentprofile /usedocumentbleed false >> ] >> setdistillerparams << /hwresolution [2400 2400] /pagesize [612.000 792.000] >> setpagedevice microsoft word article 9 tc4 paper 5.docx acta imeko  august 2013, volume 2, number 1, 27 – 31  www.imeko.org    acta imeko | www.imeko.org  august 2013 | volume 2 | number 1 | 27  concept of mobile lab for electric vehicle systems research  and testing  mario vražić, marinko kovačić, hrvoje džapo, damir ilić  university of zagreb faculty of electrical engineering and computing (fer), unska 3, hr‐10000 zagreb      keywords: electric vehicle, measurement system, can bus communication  citation: mario vražić, marinko kovačić, hrvoje džapo, damir ilić, “concept of mobile lab for electric vehicle systems research and testing”, acta imeko,  vol. 2, no. 1, article 9, august 2013, identifier: imeko‐acta‐02(2013)‐01‐09  editors: paolo carbone, university of perugia, italy; ján šaliga, technical university of košice, slovakia; dušan agrež, university of ljubljana, slovenia  received january 10 th , 2013; in final form july 11 th , 2013; published august 2013  copyright: © 2013 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: none reported  corresponding author: mario vražić, e‐mail: mario.vrazic@fer.hr    1. introduction  the main scope of this research is the development of a system that is capable of various measurements on the electric vehicle. the requirement for research on the electric vehicle is to test different types of the motor and battery configurations, thus the measurement system has to be universal and adaptable to fulfil this requirement. different control systems as well as power conversion systems have to be considered. there are several papers that are dealing with different topologies of the vehicle measurement system or with the individual components such as current or voltage measurements [1-3]. in their paper ying et al. [4] report an implementation of the monitoring system based on the pc daq unit. zhilong et al. describe a monitoring system built around a can-bus as the communication interface [5]. weiss [6] describes an internal combustion vehicle conversion procedure and the precautions that must be taken into account. it was decided that an old (cheap) but suitable car should be bought. the main reason for buying such a car was not its low price, but lack of modern technology and electric devices and aids. this particularly includes servo aids for brakes and for turning of the wheel. without these aids there are no additional unnecessary loads for electric power supply and researchers can focus on the drive train instead providing power to the auxiliary systems of modern cars. also it is desirable that the car is equipped with rear wheel drive transmission and a longitudinally placed internal combustion engine. this enables easy exchange of different electric motors instead of the internal combustion engine. the gearbox is then placed outside the engine space (front vehicle figure 1. opel kadett c – test vehicle.   abstract  when research of different systems (measurement, control, etc.) on an electric vehicle is required, it is very useful to have some sort  of a test bed for the system under investigation. in order to enable research of this kind, at the closest way possible to a real situation,  the design and a large part of the actual realization was made on a test electric vehicle that should pass homologation and be able to  drive on public roads. the vehicle with the systems required for the realization of the concept, as well as the possibilities of the future  updates, will be presented in this paper.  acta imeko | www.imeko.org  august 2013 | volume 2 | number 1 | 28  volume) so this space is left available for additional equipment such as torque sensors, power converters, batteries, etc. after a careful study of those requirements an oldtimer was chosen and purchased. as shown in fig. 1 it is an opel kadett c from 1978. when buying a car of that age there are lots of precautions which have to be taken into account, the most important of them is the quality of the bodywork and the suspension. the main concern was its bodywork so it was carefully checked and was proven to be excellent. the selected car has most of the requested features. its weight is only 750 kg, it has no servo for brakes nor for wheels, and it has rear transmission that includes a gearbox with 4 gears, a cardan shaft and a rear differential transmission. so its internal combustion engine could be replaced with an electric motor (fig 2). the electric motor is an induction motor with peak power of 37 kw. a frequency converter has the ability of regenerative braking with the motor thus returning energy into the battery pack. the induction motor is installed onto gearbox (fig. 3). a battery pack with a nominal dc voltage of 102.4 v, a capacity of 10.24 kwh (or 100ah), lifemnpo4 chemistry, and a weight of 100 kg is installed in the vehicle. the battery pack is divided into two equal parts. one is installed in the front part (in front of the motor – fig. 4), and the other in the back. the total weight of the equipment installed in the car is practically the same as the parts which have been removed from the vehicle (internal combustion engine, fuel tank, battery, cooler, all liquids, exhaust system, etc.). with the additional modification it is possible to add front wheel drives, preferably independent electric hub motors for each front wheel. this will enable research on the electric differential transmission as well as on the other electronically controlled features on vehicles such as regenerative braking. lots of other possibilities are open with this project but the primary goal was to design a measurement system capable to fulfil all requirements for this research. there are many research studies reported [7-10]. before modification some measurements were done on the vehicle. the drive wheel torque and the rotational speed for different driving regimes have been measured. the noise level within the vehicle was measured at the same time as well. 2. measurement system possibilities  during the design and development of this vehicle lots of different ideas and concepts were considered. one of them was to use a programmable logic controller (plc) as a core of the measurement system as well as a control system (fig. 5). such a system has one major shortcoming due to the significant lack of memory suitable for long-term measurement data logging. moreover, it is rather an expensive solution. another approach for the data acquisition system based on standard daq components is depicted in fig. 6. this is a much better solution but it is also, due to current transducers and isolation amplifiers, a rather expensive system. finally, an idea that incorporates can communication as a backbone for the measurement system is proposed (fig. 7). namely a frequency converter for motor control has the possibility of can communication, and therefore the measurement of vital physical quantities such as input and output voltage, current and power, as well as converter and motor temperatures, motor rotational speed, etc. additionally, a battery charger as well as a battery management system (bms) has also the possibility of can communication. it measures temperature and voltage for each battery cell, as well as the voltage and current of the whole battery pack. a plan is to obtain a power converter for a solar panel that would also use can to communicate with the vehicle systems. a torque sensor, together with the rotational speed measurement, gives mechanical power and is also essential for research. in the first step those physical quantities will be measured via a small data figure 2. induction motor with frequency converter installed in the vehicle. figure 3. induction motor with gearbox.  figure 4. battery pack in front of the induction motor.  acta imeko | www.imeko.org  august 2013 | volume 2 | number 1 | 29  acquisition system connected to a pc. a low cost national instruments ni usb-6009 data acquisition device will be used for the ad conversion. this daq will measure 12 vdc consumption (voltage and current) as well as the ambient temperature. such concept proves to be the cheapest and yet the most open system so it is chosen for installation. 3. development of mechanical power  measurement system  monitoring of motor torque and rotational speed in realtime provides a valuable insight of the vehicle behaviour and characteristics. the torque and rotational speed measurement subsystem was designed to accommodate the requirements for easy installation, low price, modularity and flexibility for various figure 5. plc based measurement system.  figure 6. daq system based measurement system.  to daq ch1 96v / 12v power converter for auxiliary loads to daq ch2 current transducer current transducer power converter for photovoltaic panels to daq ch3 current transducer photovoltaic panel power converter for electric motor to daq ch4 current transducer electric motor torque sensor gearbox to daq ch8 to daq ch6 torque rotational speed to daq ch7 isolation amplifier 1 to differential transmission and drive wheels to daq ch5 isolation amplifier 6 motor temperature temperature probe 4 usb usb analog inputs digital i/o intel atom touch panel acta imeko | www.imeko.org  august 2013 | volume 2 | number 1 | 30  test purposes. the block diagram of the measurement system is depicted in fig. 8. the system consists of two main parts: the wireless torque measurement node, mounted directly on the cardan shaft, and the receiver unit which processes the measurement signals and transmits the results to the main processing unit of the vehicle. the receiver unit also has an input for interfacing with the rotational speed sensor. torque is measured directly by means of strain gauges mounted on the shaft in a full bridge configuration, which inherently provides high sensitivity and good temperature compensation [11]. the voltage u measured on the bridge diagonal can be converted into torque m as follows: 0 0 44 )( 32 mc mu ct t uu uu d dd rk gr m          (1) where rt is the strain gauge resistance, g the shear modulus (n/m2), kt the gauge factor, rc a calibration resistance, d the outer shaft diameter, du the inner shaft diameter, uc the gain calibration voltage and um0 the zero bridge voltage. equation (1) enables torque calculations without the application of the reference momentum, under the assumption that all factors in (1) are known. this enables a simple two-step calibration procedure which is easy to automate: the zero calibration is performed in a first step (measurement of voltage um0 with unloaded shaft when the calibration resistance rc is disconnected), while the gain is calibrated in a second step (measurement of voltage uc with unloaded shaft when calibration resistance rc is connected to simulate bridge unbalance under known load). zero voltage um0 represents the initial bridge unbalance due to the differences in the nominal resistance values of the four strain gages, as well as any residual strain. alternatively, the torque measuring channel calibration can be performed by a two-point measurement and application of the reference momentum. the power can be calculated from the measured torque and the rotational speed by means of the known relation: nmp  2 . (2) the block diagram of the shaft-mounted wireless torque measurement unit is depicted in fig. 9. the module consists of a precision instrumentation amplifier, a high-resolution  a/d converter (adc), a reference voltage source, a microcontroller, a radio-frequency (rf) transceiver, a highefficiency switching-mode regulator, and the battery. due to the oversampling operation principle of the adc, a simple passive first-order low-pass antialiasing filter is sufficient. the sampling rate of the torque measurement channel is programmable up to 2 ks/s. the microcontroller itself controls the sampling process, the data stream packaging and the two way rf communication through the 2.4 ghz ism rf link. the radio communication was implemented by using the xbee module that provides a simple transparent uart operation mode, without the need for developing complex dedicated packet oriented rf protocols. the data rate of 115.2 kbps is sufficient for transmission of digitized torque samples to the stationary receiver unit. the total current consumption of the torque measurement module is less than 70 ma (with 350  strain gage), thus providing autonomous operation of about 30 hours under full load, assuming that the system is powered from four 2000 mah nimh batteries. this figure can be further improved for practical purposes by driving the torque measurement module into the idle state when it is not in use. the vehicle control system can bring the torque measurement module into sleep mode with negligible power consumption by means of a two way rf communication link. the module can be woken up on demand by sending the appropriate control packet through the rf link. the ultra-low power operation in sleep mode with remote wake-up capability is achieved by means of a low duty cycle listening algorithm. the receiver module contains a microcontroller, an xbee rf module, a rotational speed sensor input, analogue current loop outputs (4 to 20 ma), a can interface, and the power supply. the microcontroller receives torque samples through the uart interface connected to the xbee module. the data stream is decoded in real time and the torque measurement signals are transmitted to the main vehicle control unit by means of the analogue current loop. an inductive proximity figure 7. can based system.  figure  8.  the  block  diagram  of  the  torque  and  rotational  speed  measurement subsystem.  figure  9.  the  block  diagram  of  the  wireless  torque  measurement  unit  mounted on shaft.  strain gage sensor wireless torque measuring unit stationary receiver module shaft torque (4-20 ma) rot. speed (4-20 ma) proximity switch sensor input can interface vref �� adc cpu strain gage rf transciever antenna battery switching regulator enclosure rc acta imeko | www.imeko.org  august 2013 | volume 2 | number 1 | 31  switch is used to measure the rotational speed. the output of the rotational speed measurement sensor is connected directly to the microcontroller which performs high precision measurements by simultaneous measurement of time and frequency from the incoming pulses [12]. both quantities are available as analogue current outputs 4-20 ma. alternatively, these quantities can be transmitted through the can interface but the basic prototype implementation favoured analogue outputs due to software and hardware simplicity of the implementation. the can interface is also useful for sending control messages from the vehicle control unit for e.g. powering down or waking up the torque measurement module. the power management of the stationary receiving module is not an issue, because the power is supplied by the vehicle main power supply. 4. mobile laboratory possibilities    the main idea is to establish a platform for research on different systems contained in a modern electric vehicle. therefore an ordinary car with an internal combustion engine is transformed into an electric car with lots of equipment that enables the intended research. at first, the car will be equipped with systems necessary for passing homologation and registration. therefore it will be fully driveable in everyday traffic. afterwards, additional equipment will be added in order to enhance research possibilities. several research goals are now planned. for example, the drive range detection will be investigated. in order to do that properly, one should know the battery model, the energy stored in the battery pack, the current energy consumption, etc. since this system can be linked with maps (for example with google maps via internet connection enabled by gprs, edge or similar), the user could enter the desired destination and check if it is possible to reach this destination and under what circumstances (estimated arrival time, average velocity, etc). also the user could see, using the same application, the reachable destinations such as hotels, towns, gas stations, etc. of course, it would be possible to limit the vehicle speed in order to expand the drive range at users will. another research that will be achievable with this vehicle is the study of the usage of solar panels as alternative source for battery charging. for example, most of the cars are used for driving to work and back home. since they stand still at a parking lot, solar panels installed on them can be used for battery charging thus reducing battery charging from the power grid. if there is a solar panel with an area of 2 m2 it can, in ideal conditions, produce a power of 300 w. when this value is reduced to a more realistic level, 150 w average power can be taken into consideration. therefore approximately 1200 wh of electric energy can be produced, or 11.7 ah for 102.4 vdc. that is almost 12% of the vehicle battery pack capacity, and that is not something that can be ignored. the efficiency of the different system parts can be measured and determined for different operating points thus enabling their optimal usage. in this research, the general idea is to increase the total efficiency in order to enhance the drive range (or to reduce the weight of the vehicle). 5. conclusion  the presented system mounted on the described vehicle is capable of carrying out research on different problems such as optimal energy distribution, efficiency of separate parts as well as the whole vehicle, vehicle range prediction, etc. since the vehicle will be used in everyday traffic, the obtained data will be closer to reality than in laboratory tests. use of the can enables easy integration of the new equipment and the signals to the measurement and control systems of the vehicle. this system will also enable research on vehicle control strategies, e.g. optimization of the motor torque (vehicle acceleration) in order to safely reach the destination. this system will leave a lot of interesting research possibilities open as such vehicle actually represents a mobile laboratory for vehicle systems research. references  [1] x. wang, t. stuart, “charge measurement circuit for electric vehicle batteries”, ieee transactions on aerospace and electronic systems, volume: 38, issue: 4, pp. 1201 1209, 2002, doi 10.1109/taes.2002.1145743 [2] a. hande, s. kamalasadan, a. srivastava, “selective voltage measurement system for series connected battery packs“, proceedings of the ieee southeastcon, pp. 22 – 27, 2006, doi 10.1109/second.2006.1629317 [3] t.p. bohn, r.d. lorenz, e.r. olson, “measurement of in-situ currents in a hybrid electric vehicle integrated power module using giant magnetoresistive sensors“, power electronics in transportation, pp. 55 – 59, 2004, doi 10.1109/ pet.2004.1393795 [4] yin ying, xin tang, xiaonan li, xiaodong li, h. weiss, “implementation of rugged measurement system on electric vehicle“, international conference on actual problems of electronic instrument engineering, pp. 166 – 171, 2006, 10.1109/apeie.2006.4292413 [5] yu zhilong, li dongsheng, he long, wang haiying, “hardware design on electric vehicle lcd display system based on can bus“, 6th international forum on strategic technology (ifost), vol. 1, pp. 283 – 286, 2011, doi 10.1109/ ifost.2011.6021023 [6] h. weiss, “revitalization, performance measurement, and improvement of electric vehicles“, international conference on actual problems of electronic instrument engineering, pp. 108 – 113, 2008, doi 10.1109/apeie.2008.4897068 [7] k. muehlbauer, f. bachl, d. gerling, “comparison of measurement and calculation of power losses in ac/dcconverter for electric vehicle drive“, international conference on electrical machines and systems (icems), pp. 1 – 4, 2011, doi 10.1109/icems.2011.6073652 [8] m. gougani, m. chapariha, j. jatskevich, “locking electric differential for brushless dc machine-based electric vehicle with independent wheel drives“, ieee vehicle power and propulsion conference (vppc), pp. 1 – 6, 2011, doi 10.1109/vppc.2011.6043122 [9] l.w. juang, p.j. kollmeyer, t.m. jahns, r.d. lorenz, “system identification-based lead-acid battery online monitoring system for electric vehicles“, ieee energy conversion congress and exposition (ecce), pp. 3903 – 3910, 2010, doi 10.1109/ecce.2010.5617776 [10] j.t.b.a. kessels, b. rosca, h.j. bergveld, p.p.j. van den bosch, “on-line battery identification for electric driving range prediction“, ieee vehicle power and propulsion conference (vppc), pp. 1 – 6, 2011, doi 10.1109/vppc.2011.6043022 [11] i. j. garshelis, “torque and power measurement”, in measurement, instrumentation, and sensors handbook crcnetbase 1999, crc press llc, 1999. [12] h. dzapo, z. stare and n. bobanac, “portable wireless measuring system for monitoring motor shaft parameters”, in proc. 21th ieee instrumentation and measurement technology conference, como, italy, 2004, pp. 901-906. template for an acta imeko event paper acta imeko issn: 2221-870x march 2018, volume 7, number 1, 63-64 acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 63 editorial to selected papers from the 5th international conference imeko tc16 jorge c. torres-guzman1, tokihiko kobata2 1centro nacional de metrología, queretaro, mexico. 2national metrology institute of japan (nmij) tsukuba, japan section: editorial citation: jorge c. torres-guzman, tokihiko kobata, editorial to selected papers from the 5th international conference imeko tc16, acta imeko, vol. 7, no. 1, article 11, march 2018, identifier: imeko-acta-07 (2018)-01-11 editor: dušan agrež, university of ljubljana, slovenia received march 29, 2018; in final form march 29, 2018; published march 2018 copyright: © 2018 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: jorge c. torres-guzman, e-mail: jorge.torres@cenam.mx 1. introduction this section is centred on the 5th international conference imeko tc16 which was held in the city of pereira, colombia on may 2017. the conference was performed in conjunction with the 6th international conference of the ccm wg pressure and vacuum. the national institute of metrology (inm) colombia organized these conferences. the main objectives of these scientific events and contributions were to present the latest research, to stimulate information exchanges on current activities dedicated to the measurement, calibration and development of pressure and vacuum. the members of the imeko tc16 board identified 8 valuable contributions presented in the pereira events, for eventual publication in acta imeko. their authors were invited to submit extended and updated versions of their selected papers. all of the authors that gave positive responses to this challenge started sending their extended papers to the acta imeko online submission system at middle of 2017. a great team of reviewers proceeded in successive stages, their task of assessing the papers and submitting their recommendations. the final list of 6 published papers in this issue is a high-quality exhibit of the papers submitted to our conferences. all the reviewers involved in the process contributed to improve the shape as much as the intrinsic content of the papers. we invite you to continue and increase your partnership with acta imeko in all levels. 2. about tc16 the realization of the pressure scale comprises 18 decades, from 10-9 pa to 109 pa, this is one of the reasons for which the applications of pressure measurement technology reach into many fields. one example is at the process industry, where all types of manometers and transmitters are found, to measure absolute, gauge and differential pressure. these devices are not only used for the measurement of pressure, but also to control the process. practice shows that there is an increasing need to carry out international work to homogenize criteria, procedures and uncertainty evaluation to facilitate technical information exchange; to develop projects among groups from different nations; and to support measurements and calibrations for mutual recognition between country's national metrology institutes or accreditation bodies. this technical committee pursues this goal by the organization of different activities. the current members of tc16 are: dr. tokihiko kobata, chairperson japan dr. jay h. hendricks, vice chairperson usa dr. maría nieves medina martin, scientific secretary spain dr. dominik prazak, scientific secretary czech republic dr. jorge c. torres-guzman, advisor (past chairman) mexico acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 64 dr. sam-yong woo republic of korea dr. a. k. bandyopadhyay india prof. giorgio buonanno italy dr. anita calcatelli italy dr. marcello caravaggio italy mr. tawat changpan thailand mr. michael d. collins united kingdom mr. alexandre filipe da silva lourenco portugal dr. alaa el-din a. e. el-tawil egypt dr. peter farar slovakia dr. john greenwood united kingdom dr. seung-soo hong republic of korea dr. karl jousten germany dr. rifat kangi turkey dr. alper elkatmis turkey dr. yasin durgut turkey dr. jean-claude legras france dr. gianfranco molinar italy dr. akira ooiwa japan mr. luiz hemrique paraguassu brazil dr. wladimir sabuga germany dr. marina sardi italy dr. janez setina slovenia mr. schalk smit south africa brian yalisi south africa dr. jiri tesar czech republic mr. jos c. g. a. verbeek the netherlands dr. christian wuthrich switzerland dr. martin wuest liechtenstein mr. alvaro bermudez coronel colombia mr. mayckol j. morales castro colombia mr. pengcheng zhang china prof. davor zvizdic croatia dr. kenta arai japan dr. chris sutton new zealand ms. yanhua li china dr. in-mook choi republic of korea mr. pierre otal france dr. fredrik arrhen sweden mr. hideaki iizumi japan dr. hiroaki kajikawa japan mr. martin girard usa mr. michael bair usa mr. jacob ricker usa dr. james a. fedchak usa mrs. julia scherschlight usa mr. aykurt altintas denmark mr. ambrose okoth kenya mrs. sari saxholm finland 3. the papers the papers included in this special section range from vacuum to high pressure, including barometric pressure. in addition, the papers present work performed experimentally as well as analytically. it is also important to note that their applications are at industrial level and at national metrology institute level. we sincerely hope that all the contributions you will find in the next pages provide you with up-to-date information of your interest. 4. conclusions it was an honour for us to act as guest editors for a section of acta imeko, from the perspective of working for a highprofile scientific journal and to promote tc16’s activities. we have to highlight the quality and great flexibility of the electronic submission platform, the most valuable improvement recommendations and comments of the reviewers, the elevated standards for copyediting, proofreading and layout editing services of acta imeko. we want to express our deepest gratitude and appreciation to the two editors in chief that work very hard to make this section possible, prof. paul p. l. regtien and prof. dušan agrež. editorial to selected papers from the 5th international conference imeko tc16 << /ascii85encodepages false /allowtransparency false /autopositionepsfiles true /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /error 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/hrv (za stvaranje adobe pdf dokumenata najpogodnijih za visokokvalitetni ispis prije tiskanja koristite ove postavke. stvoreni pdf dokumenti mogu se otvoriti acrobat i adobe reader 5.0 i kasnijim verzijama.) /hun 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can be opened with acrobat and adobe reader 5.0 and later.) >> /namespace [ (adobe) (common) (1.0) ] /othernamespaces [ << /asreaderspreads false /cropimagestoframes true /errorcontrol /warnandcontinue /flattenerignorespreadoverrides false /includeguidesgrids false /includenonprinting false /includeslug false /namespace [ (adobe) (indesign) (4.0) ] /omitplacedbitmaps false /omitplacedeps false /omitplacedpdf false /simulateoverprint /legacy >> << /addbleedmarks false /addcolorbars false /addcropmarks false /addpageinfo false /addregmarks false /convertcolors /converttocmyk /destinationprofilename () /destinationprofileselector /documentcmyk /downsample16bitimages true /flattenerpreset << /presetselector /mediumresolution >> /formelements false /generatestructure false /includebookmarks false /includehyperlinks false /includeinteractive false /includelayers false /includeprofiles false /multimediahandling /useobjectsettings /namespace [ (adobe) (creativesuite) (2.0) ] /pdfxoutputintentprofileselector /documentcmyk /preserveediting true /untaggedcmykhandling /leaveuntagged /untaggedrgbhandling /usedocumentprofile /usedocumentbleed false >> ] >> setdistillerparams << /hwresolution [2400 2400] /pagesize [612.000 792.000] >> setpagedevice uncertainty assessment for measurement and simulation in selective laser melting: a case study of an aerospace part acta imeko issn: 2221-870x december 2020, volume 9, number 4, 96 105 acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 96 uncertainty assessment for measurement and simulation in selective laser melting: a case study of an aerospace part giulio d’emilia1, antoniomaria di ilio1, antonella gaspari1, emanuela natale1, antonios g. stamopoulos1 1 department of industrial and information engineering and economics, university of l’aquila, 67100, l’aquila, italy section: research paper keywords: additive manufacturing; selective laser melting; simulation; experimental validation; measurement techniques; uncertainty citation: giulio d'emilia, antoniomaria di ilio, antonella gaspari, emanuela natale, antonios g. stamopoulos, uncertainty assessment for measurement and simulation in selective laser melting: a case study of an aerospace part, acta imeko, vol. 9, no. 4, article 13, december 2020, identifier: imeko-acta09 (2020)-04-13 section editor: francesco bonavolonta, university of naples federico ii, italy received october 11, 2019; in final form december 10, 2019; published december 2020 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: giulio d’emilia, e-mail: giulio.demilia@univaq.it 1. introduction additive manufacturing (am) technologies enable the fabrication of parts with complex geometries without the need for tooling and assembly, thus allowing for a reduction of costs and time compared with conventional manufacturing processes [1], [2]. with am methods, it is possible to realise components using a range of materials, such as steels, aluminium or titanium alloys, and metal-based or ceramic-matrix composites [2]. due to its versatility and potentiality, am plays a key role as one of the central paradigms of industry 4.0, but it still requires further research and development [3]. among the most frequently addressed problems related to am are residual stresses. these are mainly caused by the thermal cycle of the process itself, which entails rapid heating, several cooling rates and a relative re-melting of the secondary layer [4]. this whole procedure generates thermal stresses, which, in turn, produce residual stresses after the production of a certain part of the component, leading also to notable distortions, cracks and variations on the relative dimensions of the produced part [5], [6]. numerical tools aimed at predicting residual stresses and geometrical irregularities exist, but these are mainly based on multi-physics and are either complicated and time consuming or simplified and case-specific [7], [8]. furthermore, the phenomena am is based on are typically non-stationary, increasing the difficulty of simulation. these models could support process optimisation, provided they are able to discriminate the effects of redesign or treatments on the piece being produced. for this purpose, a metrological approach that includes an uncertainty analysis of simulation data abstract in this work, the additive manufacturing process selective laser melting is analysed with the aim of realising a complex piece for aerospace applications. in particular, the effect of the manufacturing process and of the following thermal treatments on the dimensions of the workpiece is evaluated. the study is based on a hybrid approach including a simulation of the whole manufacturing process by advanced software packages and the dimensional measurements of the realised pieces taken by a coordinate measuring machine (cmm). the integrated use of simulation and measurements is carried out with the aim of validating the simulation results and of identifying the operational limits of both approaches; this analysis is based on metrological evaluation of the results of both the simulation and the tests, taking into account the uncertainty of the data. in addition, the main causes of uncertainty for the simulation activity and the experimental data have been identified, and the effects of some of them have also been experimentally evaluated. based on the experimental validation, the simulation seems to predict the absolute displacement of the supports of the piece in a satisfactory way, while it is unable, in the actual configuration, to assess the conformity of the surface to its very tight shape tolerances. conformity assessment of the surface should be carried out by cmm measurement. integrated use of simulation and experimental results is expected to strongly improve the accuracy of simulation results for the effective and accurate design and control of the additive manufacturing process, including dimensional control and thermal treatments to mitigate induced thermal stresses. mailto:giulio.demilia@univaq.it acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 97 is necessary to evaluate the ability to resolve even small variations in the properties of the object being studied [9]. the way the mesh is realised, the accuracy of data on physical characteristics of the materials and the process parameters can all produce variability in the results, which in turn can affect the dimensional knowledge of the piece. furthermore, simulation data should be validated after part manufacturing, especially after additional thermal treatments, by comparison with measurements of different possible parameters such as microstructural variations, induced stresses and deviations on the part dimensions [10], [11]. this comparison requires the theoretical/experimental analysis of all the uncertainty contributions involved in the measurement process in order to assess the compatibility between model results and measurement data in probabilistic terms [12]. many measurement techniques for validation exist that examine the piece characteristics at a microor macro-scale level [8], [10], [13]. the former group includes x-ray diffraction analysis, which is mainly based on computed tomography analysis of both the microstructure and the relative part dimensions [8], [10]. techniques measuring the effects of residual stresses on a macroscopic scale, including distortions, cracks and variations on the relative dimensions of the produced part, belong to the latter group. in particular, the three-prong method, digital image correlation and the use of optical measuring machines are very popular in the industry [13], [14]. coordinate measuring machines (cmms) are also of general use for measuring the nominal geometry of the pieces. furthermore, in some cases, it might be necessary to resort to a merging of many measurement techniques in order to take into account all the possible aspects of interest for process control and optimisation [10]; thus, it is important to be able to fuse measurement data from different sensors. additionally, the characterisation of the part shape is particularly critical for quality control of am products and for successful optimisation of am processes as new materials, treatments and geometries are developed [14]. shape deformation is one of the most noticeable effects following most metal am processes due to the material thermal stresses. hence, post-process characterisation methods would be highly beneficial in understanding and contributing to the reduction of these effects, especially for industries with stringent requirements, such as the aerospace and automotive industries. contact systems, such as mechanical probe-based cmms, can measure forms with guaranteed high accuracy and have been widely used in such industries for many years, but they have limitations. for example, they can only measure a limited number of points on an object’s surface [14], [15]. the measurement uncertainty for cmms strongly depends on the measurement procedure [16]. for this reason, a design of experiments (doe) that controls for all of the parameters that influence uncertainty along with an evaluation of the resulting measurement uncertainty is necessary for assessing the conformity of the piece to the requirements. knowledge of the measurement uncertainty is also needed to validate the theoretical model when the simulation is used as an instrument to optimise the am process. validation of models by experimental techniques can also be supported by advanced data processing techniques, including advanced machine learning, deep learning and neural network algorithms, which are commonly used in many experimental fields [17], [18]. based on the above considerations, establishing a hybrid methodology that is able to integrate contributions from simulations and experiments is a non-trivial task. it should be taken step by step, with iterative merging of simulation and experimental data. in summary, the main issues with regards to simulation are: • simplifying the approach in order to guarantee that the models are reliable and efficient from the processing load point of view; • identifying the most meaningful quantities and aspects to be considered; • evaluating the uncertainty of the simulation data in order to define the ability of the model to discriminate the effects of changes in the piece’s design or manufacture process; • assessing the interaction with experimental activity in order to allow for mutual validation. as for measurements, the following are topics of interest: • sensor fusion and integration of different data bases, depending on specific measuring chains; • data fusion between measurement and simulation data [19]; • optimisation of experimental activity (e.g. doe, control of the environmental and procedural effects, in-situ calibration, virtual instruments and data validation) [20]; • the synthesis of information to determine meaningful features for the application of advanced machine and deep learning algorithms; • uncertainty evaluation of experimental data. the present work will study the optimisation and realisation of an aerospace part using am technologies. simulation and measurement aspects will be considered in order to provide the most informative basis for validation of the model results. this study will discuss a hybrid approach including a simulation of the whole manufacturing process by advanced software packages and dimensional measurements of the realised pieces taken by cmms. the integrated use of simulation and measurements will be carried out with the aim of validating the simulation results and identifying the operational limits of both approaches. this analysis is based on metrological evaluation of results of both simulation and tests, taking into account the uncertainty of the data. indeed, the uncertainty of the data derived from both sources will be the driving criterion for success, and the causes of uncertainty for data provided by both simulation and measurement actions will be investigated in order to realise the best scenario for mutual validation. as for measurements, cmm dimensional data will be analysed, taking into account all the steps of the measurement procedure. it is expected that the uncertainty analysis will increase the reliability of the simulation approach as it will determine the accuracy of the simulation predictions, which can then be used to determine the feasibility of specific actions for am process improvement. the requirements of the measurement techniques in order to obtain reliable measurements for the validation of the simulation model and for understanding the physical phenomena involved in the process are also discussed. the interactions between simulation and experimental data will be critically discussed in the results section, including recommendations for the identification of the operational limits of both approaches and for mutual improvement. acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 98 2. materials and methods the piece under investigation is depicted in figure 1. the bracket forms a part of the closing device for the antenna of a satellite, developed in close collaboration with thales alenia space. the measurement campaign aimed at guaranteed shape tolerances in the order of ± 0.05 mm for the belt surface. the dimensional characterisation of the piece was carried out by means of the specification of 66 points along the belt support surface, as seen in figure 2, which were placed on a grid, as depicted in figure 3. in order to understand the effect of a thermal treatment applied to the component for the purpose of reducing residual stresses, four measuring lines were identified, namely ‘a’, ‘b’, ‘c’ and ‘d’ in figure 3. analysis along each measuring line allowed us to investigate the effect of material stress on different sections of the bracket in the (x-z) plane. am using metallic materials causes some residual stress, which is usually removed by means of a stress-relieving thermal treatment on the component. the thermal cycle is composed of three main phases: 1. heating ramp: from 20 °c, up to 870 °c, 0.5 l/min argon, partial pressure 0.1 mbar (duration 1 h); 2. maintenance: 870 °c, partial pressure 0.1 mbar (duration 1 h); 3. cooling: natural cooling, firstly down to 150 °c, partial pressure 0.1 mbar (duration 3 h) and then down to room temperature, partial pressure 1 bar (duration 40 min). in order to highlight the effect of the thermal treatment, the component was characterised before and after the treatment by means of a cmm as follows: • d0 measurements refer to the status before the thermal treatment; • d1 measurements refer to the status just after the thermal treatment, as soon as the bracket has been posed in the measurement area of the production plant; • d2 measurements refer to the status after d1, around 2 weeks after the thermal treatment (thermal stabilisation at the measurement area); • d3 measurements refer to additional measurements of the status after d1 in order to assess repeatability. it has to be pointed out that every measurement after the thermal treatment was carried out without moving the bracket from the cmm measuring support. the main measurement steps can be summarised as follows: a) specification zones were identified, including the surface, the guiding spheres and the fixing bases. it is worthwhile to note that there were also some support zones in the workpiece that were linked to the requirements of the am process but did not serve the functional characteristics of the bracket itself. b) the origin of the ideal x-y-z reference coordinate system was chosen with reference to the probe access possibilities. c) the cmm geometry was integrated with the nominal geometry, which included the nominal coordinates of some reference points of the bracket (e.g. the origin of the reference coordinate system, the centres of holes, etc.). this required aligning the part coordinate system and the machine coordinate system. d) the positioning errors of the component reference system were minimised to find the best fit for the alignment of the nominal reference coordinate system and the artificial coordinate system created on the cmm software in point c). e) the measurement points were defined as a grid on the surface and other zones of the bracket. f) the measurement campaigns were executed. g) best fit was determined a second time for the reconstruction of the nominal belt surface from the cad model, starting from the cloud of measured points. h) the distances were evaluated between pairs of corresponding points that had been measured and located on the nominal surface. for the belt surface, the correspondence was obtained by a geometrical projection. figure 1. the component under analysis: a belt support for a satellite antenna. figure 2. the point grid on the belt surface of the designed part. figure 3. schematic representation of the distribution of the grid points on the belt surface. acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 99 according to the previous steps, figure 3 shows the cmm measurement points fitted to the catia v5 model, which was used as a basis for comparison with the model results. deviations between the measurement points and the nominal geometry were used to check dimensional and shape tolerances. it has to be pointed out that all of these actions allowed us to fit, rotate and shift surfaces represented by the measurement coordinates to the reference system of the model. 3. simulating the additive manufacturing process in this section, the dependency of displacements while simulating the additive fabrication of the previously presented component seen in figure 1 is described. after defining the structural loads this component was subject to, a topological optimisation was carried out, leading to a geometry characterised by high complexity, which did, therefore, make am indispensable. the simulation of the fabrication process was made mostly to foresee the deformations and distortions of the component caused by the process itself. considering the drawbacks of selective laser melting, the accurate prediction of residual stresses and distortions caused by the heating and cooling of the several layers into which the component is divided may be crucial. to this end, the amphyon 2018 fe code was utilised [21]. this particular software, developed by additive works, is said to be accurate for predicting the distortions not only from the fabrication process but also from any potential thermal treatment afterwards. consequently, two simulations were performed using the mechanical process simulation tool. in one, the mesh density was adjusted, and in the second, the adaptive meshing method was retained. the external surfaces of the component were refined, and the model supports were considered to ensure closest fit to those used in the real case. to this end, the first model created had 244,262 elements of 1.294 mm each, while the second was meshed with 116,694 elements, sized 1.542 mm each. both models had a similar number of layers, and a potential thermal treatment after the fabrication was considered. the purpose of this parametric analysis was the investigation of the sensitivity of the results with regard to mesh size and density as well as the necessary processing time in each case. the results in terms of distortions are presented in figure 4(a) and figure 4(b), respectively, alongside the corresponding displacement scale. in both cases, the magnitude of the observed distortions is the same. moreover, a significant displacement is reported at the supports of the component. in figure 4(a), the red area of the support on the left has a maximum displacement of 0.37 mm, while in figure 4(b), the displacement reaches the value of 0.47 mm. nevertheless, by comparing the distribution of the displacements, a significant difference is observed in the lower region of the component near the through hole; in figure 4(a), a displacement up to 0.45 mm is registered, compared to almost 0.3 mm in the output shown in figure 4(b). thus, the two simulations describe a different situation in terms of the overall deformation of the component and the upper surface. considering the above, each simulation fe model created is quite sensitive to the model parameters themselves, such as the mesh size and density. if the effect of the mesh size is considered, the uncertainty contribution for the displacement provided by the simulation can therefore be evaluated as: (0.45 – 0.30) mm / √3 = 0.087 mm. other contributions should be considered, due, for example, to the choice of material characteristics and process parameters. a complete evaluation of the uncertainty of the simulation results should be carried out by running the program using different values for the parameters of influence. in any case, a first validation of the model was made by a measurement campaign assessing the component’s dimensions and distortions, as presented in the following sections. 4. results the results of the measuring campaign are represented in figures 5–8. in particular: • figures 5 a), b), c) and d) show the distances between cmm measured points and nominal ones before thermal treatment (d0), immediately after treatment (d1), approximately two weeks after treatment (d2) and in a repeated measure approximately two weeks after treatment (d3), respectively, for all four of the established measuring lines. • figures 6 a), b), c) and d) show the distances between cmm measured points and nominal ones along each of the four established lines. it can be seen that significant differences arise between data before treatment and data after treatment, in the order of 0.3 mm as the maximum difference. • figures 7 a), b), c) and d) show the distances between best fitted points and nominal ones in cases d0, d1, d2 and d3, respectively, for all of the four measuring lines. • figures 8 a), b), c) and d) show the distances between best fitted points and nominal ones along each of the four lines. it can be seen that the best fitting procedure removes the differences between before and after treatment. a) b) figure 4. deformation obtained by simulations with different mesh size and discretisation. acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 100 on the basis of the measuring data, some parameters of interest were calculated. first of all, the repeatability of the distances between cmm measured points and nominal ones, calculated on the basis of d2 and d3 data, was evaluated and found to equal 0.0094 mm in terms of max difference/√3; the value of 0.012 mm is found in terms of the root mean square (rms) of the differences between the data. since the shape tolerance for the surface of the piece is equal to ± 0.05 mm, the expanded uncertainty of measurements found in the contribution of repeatability, which are in the order of 0.02 mm at a confidence level of 95 %, appears to be a critical value. that suggests the need for careful control of the whole measurement process to optimise it from a metrological point of view. in particular, attention should be paid to the following aspects: • workpiece alignment, which is the process of relating the part coordinate system and the machine coordinate system; • the workpiece fixing procedure; • the determination of the number of measurement points; • the determination of the reference points on the nominal surface for the calculation of the distances of the experimental points from the surface itself. in table 1, the rms of differences between regression lines and experimental points for the cmm data are shown for each line and in correspondence to each condition (d0, d1, d2, d3). in case d0, the fitting line has a slope other than zero; in cases d1-d3 (see figure 5 b), 5 c), 5 d)) experimental distances are well fitted by horizontal lines, a sign that the experimental point clouds are parallel to the nominal surface. therefore, measurements suggest that the main effect of the thermal treatment was a mitigation of the roto-translation of the surface of interest due to a reduction in thermal stresses in the supports. with reference to figure 5 and figure 7, the max difference between measuring lines for each condition (d0, d1, d2, d3) for both cmm and best fitted data is shown in table 2. it can be seen that the variability between measuring lines is the same both for measured points and best fitted ones and both before and after the thermal treatment. this indicates that the surface irregularities were practically unaffected by the thermal treatment. finally, the average distance of the experimental points from the nominal surface in the area of the hole (figure 4. deformation obtained by simulations with different mesh size and discretisation.) was calculated and found to be equal to 0.39 mm in case d1 and 0.34 mm in cases d2 and d3. these values are comparable with the values obtained by means of the simulation, as said in section 3 (0.45 mm and 0.30 mm for the highest displacement, corresponding to different mesh sizes and densities); therefore, the simulation results seem to be satisfactory. 5. conclusions this study aimed to analyse some practical issues of both technological and metrological type in order to optimise the am process of a component for aerospace applications. surface tolerances in the order of ± 0.05 mm have to be satisfied on specific parts of the specimen and should be checked by means of a hybrid approach based on both simulation of the process and dimensional measurement by cmm. in particular, the effect of thermal treatment after the realisation of the piece was of interest with regard to the possibility of reducing surface irregularities and absolute displacement of the parts under tolerance. two kinds of analysis were carried out: 1. the evaluation of the causes of variability connected to the simulation approach and to the experimental measurements and their effect, in order to estimate the uncertainty of data for conformity assessment purposes; 2. the evaluation of the capability of simulation and cmm measurements to contribute to the monitoring of the process, in order to use their indications for process optimisation. the main results related to point 1) are the following: • with regard to the simulation, the greatest effect of changing the mesh is in the order of 0.1 mm, without considering the uncertainty contributions due to material characteristics and process variability. • with regard to the cmm measurements, repeatability is in the order of 0.01 mm. this does not take into account the effects of fixing or positioning or of the best fitting procedure that was done to evaluate the reference surface and its modification due to the process; these effects could increase the variability of the measurements. the main results related to point 2) are the following: • in the area of interest, the differences between the predictions of absolute displacement, in the order of 0.45 mm, and the measured absolute displacements are negligible; therefore, the behaviour of the simulation seems to be satisfactory. of course, variability of prediction should be taken into account. measurements indicate that the main effect of the thermal treatment is to substantially mitigate the roto-translation of the surface of interest, which is caused by thermal stresses in the supports during manufacturing. • the surface irregularities seem practically unaffected by the thermal treatment, being the same before and after it. due to these considerations, simulation was useful for predicting the behaviour of the supports, as we were unable to achieve the needed accuracy in the order of ± 0.05 mm for the conformity assessment of surface tolerances. cmm measurements could be used for this assessment, provided that the procedure is optimised from a metrological point of view. table 1. rms (mm) of differences between regression lines and experimental points for each line and in correspondence to each condition (d0, d1, d2, d3) for cmm data (ref. figure 5). condition line ‘a’ line ‘b’ line ‘c’ line ‘d’ d0 0.089 0.11 0.11 0.088 d1 0.050 0.063 0.037 0.033 d2 0.052 0.064 0.036 0.033 d3 0.053 0.064 0.036 0.033 table 2. max difference between lines (mm) for each condition (d0, d1, d2, d3). d0 d1 d2 d3 cmm data 0.17 0.17 0.16 0.16 best fitted data 0.16 0.17 0.17 0.17 acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 101 a) b) c) d) figure 5. distances between cmm points and nominal ones: a) before thermal treatment; b) immediately after treatment; c) two weeks after treatment; d) two weeks after treatment (repeatability trial). acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 102 a) b) c) d) figure 6. distances between cmm points and nominal ones along measuring lines a), b), c) and d), respectively. acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 103 a) b) c) d) figure 7. distances between best fitted points and nominal ones: a) before thermal treatment; b) immediately after treatment; c) two weeks after treatment; d) two weeks after treatment (repeatability trial). acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 104 a) b) c) d) figure 8. distances between best fitted points and nominal ones along measuring lines a), b), c) and d), respectively. acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 105 future work will be devoted to improving the whole hybrid procedure in order to reduce the uncertainty effects both on the simulation, taking into account all causes, and on the cmm measurements, including measurement data processing techniques. this way, the uncertainty of the simulation process will be improved by validation from cmm measurements for better process control and definition. the cmm measurements will also enable assessment of the component’s conformity to the strict tolerances required for aerospace applications. acknowledgement the authors are grateful to thales alenia space (italy), renishaw s.p.a. 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g. d’emilia, a. gaspari, measurement-uncertainty as an innovative management tool in modern engineering systems, 14th imeko tc10 workshop on technical diagnostics 2016: new perspectives in measurements, tools and techniques for systems reliability, maintainability and safety, milan, italy, 27-28 june 2016, pp. 197-202. online [accessed 02 december 2020] https://www.imeko.org/publications/tc10-2016/imekotc10-2016-040.pdf [21] m. c. kushan, o. poyraz, y. uzunonat, s. orak, systematical review on the numerical simulations of laser power bed additive manufacturing, sigma journal of engineering and natural sciences 36 (2018), pp. 1197-1214. online [accessed 02 december 2020] https://eds.yildiz.edu.tr/ajaxtool/getarticlebypublishedarticle id?publishedarticleid=2734 https://doi.org/10.1016/j.precisioneng.2016.06.001 https://doi.org/10.1016/j.jmst.2018.09.002 https://doi.org/10.1016/j.procir.2018.05.039 https://doi.org/10.1016/s0007-8506(07)60677-5 https://doi.org/10.1108/13552540610707013 https://doi.org/10.1111/ffe.12560 https://doi.org/10.1016/j.addma.2019.06.021 https://doi.org/10.17265/2159-5275/2018.02.001 https://doi.org/10.1109/metroi4.2019.8792876 https://doi.org/10.1016/j.measurement.2017.12.034 https://doi.org/10.1080/17452759.2017.1392682 https://doi.org/10.1177/0020294016644479 https://doi.org/10.1016/j.ijmachtools.2004.06.017 https://doi.org/10.1088/1742-6596/944/1/012061 https://doi.org/10.1016/j.mfglet.2019.02.001 https://doi.org/10.1016/j.procir.2017.12.221 https://doi.org/10.1109/metroi4.2018.8428341 https://www.imeko.org/publications/tc10-2016/imeko-tc10-2016-040.pdf https://www.imeko.org/publications/tc10-2016/imeko-tc10-2016-040.pdf https://eds.yildiz.edu.tr/ajaxtool/getarticlebypublishedarticleid?publishedarticleid=2734 https://eds.yildiz.edu.tr/ajaxtool/getarticlebypublishedarticleid?publishedarticleid=2734 the improvement of an elastic hinge-type torque standard machine in nimt acta imeko issn: 2221-870x september 2019, volume 8, number 3, 36 – 41 acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 36 the improvement of an elastic hinge-type torque standard machine in nimt nattapon saenkhum1, tassanai sanponpute1 1 national institute of metrology (thailand), 3/4-5 moo 3, klong 5, klong luang, pathumthani 12120, thailand section: research paper keywords: torque standard machine; elastic hinge; torque transducer; bending moment; fulcrum sensitivity citation: nattapon saenkhum, tassanai sanponpute, the improvement of an elastic hinge-type torque standard machine in nimt, acta imeko, vol. 8, no. 3, article 7, september 2019, identifier: imeko-acta-08 (2019)-03-07 editor: rugkanawan wongpithayadisai, nimt, thailand received august 31, 2018; in final form june 6, 2019; published september 2019 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: nattapon saenkhum, e-mail: nattapon@nimt.or.th 1. introduction nimt’s 1 kn·m elastic hinge-type torque standard machine (tsm) [1,2,3,4] was established in 2004. the machine was designed and assembled by gtm gmbh in germany. unfortunately, the left elastic hinge was damaged during the commissioning process at nimt’s bangkok site. in 2005, the nimt moved it to a new site in pathumthani. the machine was reconstructed, including the replacement of the damaged parts and recalibration. the calibration and measurement capabilities (cmcs) were 0.010 % (k = 2) in the measurement range of 10 n∙m to 1 kn∙m and 0.030 % (k = 2) in the measurement range of 1 n∙m to 10 n∙m. the cmcs were proven by the ccm.tk1.2 comparison in 2008 [5]. the final report was published in 2015. in 2013, it was found that the machine had problems caused by the residual torque signal from the fulcrum cross-elastic hinge. the problem was resolved by using the displacement sensor to detect the balance position of the lever instead of the damaged elastic hinge. in 2014, the mechanical fulcrum crosselastic hinge completely malfunctioned due to the deformation of its plastic, and this must be replaced. the new fulcrum crosselastic hinge was procured from the manufacturer and was replaced by nimt staff in the same year. the cmcs were confirmed again by some bilateral comparisons (compared with a ptb certificate and the 10 n·m tsm of nimt [6,7,8]). this was our chance to study the strain-controlled elastic hinge characteristic based on each repair and maintenance exercise. the most important point of the machine’s realisation was a residual torque that was a dominant uncertainty contribution. nimt was interested in determining the method of reducing the uncertainty of measurement caused by the residual torque signal. abstract the national institute of metrology of thailand’s (nimt) strain-controlled elastic hinge-type torque standard machine was designed to cover a measuring range of 1 n·m to 1 kn·m. the elastic hinge was used both at the fulcrum and the hanger of the lever arms. the designed elastic hinge’s thickness, 0.50 mm, caused a higher stiffness than a sheet metal plate of other types of torque machines. the bending moment of all elastic hinges affected the sum of the torque signal on the lever arm that was used to observe the balancing of the lever. the residual torque sensitivity, which was no better than 0.20 mn·m, significantly affected the uncertainty of the low-range torque realisation. the calibration and measurement capabilities of the machine were 0.010 % (k = 2) in the measurement range of 10 n·m to 1 kn·m and 0.030 % (k = 2) in the measurement range of 1 n·m to 10 n·m. in the transducer calibration, the influence of the random bending moment of the elastic hinge affected the repeatability, reproducibility, and linearity of the low torque measurements. the cause of the bending moment of the elastic hinges was a result of the deviation of the centre of gravity (cg) of the weight on the pan from the reference line. to improve cmcs, separate signal calibrations were selected for this experiment i.e. the left hinge, the right hinge, and the fulcrum. the torque in each signal calibration was combined by software and was used to correct the calibration value of the torque. mailto:nattapon@nimt.or.th acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 37 2. residual torque configuration the original residual torque signal was configured by the manufacturer. the output residual torque signal combined four signals (left hinge, channel 1; right hinge, channel 2 and fulcrum front and rear, channels 3 and 4) by using the electric circuit. the sensitivity of the signals of the left and right hinges was reduced tenfold by using normal resistors with the same resistance on both sides. the reduced signals and the fulcrum signals were combined. the manufacturer aimed to reduce the influence of the left and right elastic hinges signals that would affect the combined signals. the combined signals were converted to a torque unit by an analogic an3060 digital weight indicator [9], as shown in figure 1. the original residual torque configuration should be realised by the separate signal calibrations as performed in this experiment: left hinge, right hinge, and fulcrum. the residual torque in each signal calibration was combined by software and was used to correct the calibration torque value. 3. bending moment of elastic hinge 3.1. asymmetric load the imperfection of the manufacturing process of the weight set caused an asymmetric load distribution. additionally, small steel balls were used to adjust the mass of the weight set. the weight stack on the pan touched the column, and many steel balls inside changed the position independently, as shown in figure 2. both of these cases might cause the bending moment at the elastic hinge of lever arm. the hypothesis of the asymmetric load was proven by the finite element analysis (fea) at nimt. static analysis was selected for studying the stress, the displacement, and the strain of the elastic hinges. the fea simulation was designed by applying the 0.5 n forces on the centre position, at 60 mm from the centre and 120 mm from the centre. the elastic hinge geometry was divided into a finite element by a curvature-based mesh algorithm. these small meshes were controlled in the areas in which there is a rapid change in stress, while the large meshes were controlled in the areas in which there was little change in stress [10]. obviously, the bending value of the elastic hinge depends on the distance of the force on the pan, as shown in figure 3. 3.2. the moment of force as evident in figure 4, the centre of the gravitational force acting on the hinge shifted from the original position due to the stacked weights. in this case, the bending moment, mb, was not equal to zero. that variable was effected by the calibration torque, mt, as in equation (2). the measured bending moment of the elastic hinge should be considered: 0=m (1) 0b11t =−− mlfm (2) figure 2. asymmetric load on the pan due to the imperfection of the manufacturing and the steel balls inside. figure 1. residual torque configuration. figure 3. finite element analysis. figure 4. moment of a force. touching disc weight column steel balls ch3, ch4 ch1 ch2 circuit box indicator f1 l1 mt mb strain gauge strain gauge reference line acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 38 11tb lfmm −= (3) where: f1 is the force, and l1 is the lever length (approx. 500 mm). 4. experiment the measured moment of the strain-controlled elastic hinges is realised by considering the signal separation. the indicating device was changed to a four-channel hbm mgcplus synchronised amplifier (1×ml38 and 3×ml30b) [11]. the signal was separated from the combined circuit box to: ch1; right hinge, ch2; left hinge and ch3+ch4; fulcrum front and rear. the ch1, ch2, ch3+ch4, and the transducer were connected to the amplifier, as shown in figure 5. the transducer did not connect to the tsm. next, we performed the activities as described in sections 4.1 to 4.4. 4.1. fulcrum cross-elastic hinge realisation the fulcrum was preloaded three times at approximately 5 mn·m. then, the weight was applied onto the reference position (as shown in figure 6), as the calibration steps are shown in figure 7, three times. the weight steps were 50 mg, 100 mg, 200 mg, 200 mg, and 500 mg, respectively. the measurement process was done using the left and right pans. in the meantime, the data channel was recorded. together, the strain signals of clockwise and anticlockwise torque were expressed as a function of the torque by the first-order linear interpolation equation, without the absolute term. 4.2. left and right elastic hinges realisation a recognised torque transducer (rauter; tt1/10 n·m) [12] was used to transfer the torque value to both sides of the elastic hinges. the tt1/10 n·m was calibrated by the 10 n·m tsm of nimt [6,7] in the measurement range of 0.1 n·m to 1 n·m. the repeatability and linearity of the measurement results of tt1/10 n·m at 0.2 n·m were within 0.002 % and 0.004 %, respectively. the measurements should be carried out separately for the right and left torques. the next steps were to connect the transducer to the machine, as shown in figure 8, preload three times at approximately 0.25 n·m (50 g weight), and wait for three minutes. then, we put the same weight on the reference position of the pan and applied the load by using a motor gear until (0.00000 ± 0.00010) n·m of ch3+ch4 was indicated. in the meantime, all the channel data was recorded. we performed the same procedure but changed the weight position to p1, p2, p3, p4, p-1, p-2, p-3, and p-4, as shown in figure 6. the interval of the positions was set to 30 mm. the whole process was repeated three times. the first order of the linear interpolation equation was selected to transform the strain signal to express it as a function of the torque. 4.3. the difference in arm length evaluation the difference in arm length between the left and right arms was evaluated by a moment balancing method [6,7,13]. this method was implemented by loading the weight on both the left and right pans simultaneously in three ranges: 2 n to 20 n (step 2 n, 4 n, 8 n, 12 n, 16 n, 20 n), 20 n to 100 n (step 20 n, 40 n, 60 n, 80, n 100 n), and 100 n to 1,000 n (step 100 n, 200 n, 400 n, 600 n, 800 n, 1,000 n), as shown in ch1ch2 ch3+ch4 mgcplus ch2 ch1 ch3+ch4 0.00000 mv/v transducer figure 5. experiment setup. reference position p-1 p-2 p-3 p-4p1p2p4 p3 reference position p1 p2 p3 p4p-1p-2p-4 p-3 pan left, ch2 pan right, ch1 figure 6. position of the weight on the pan. figure 8. transducer setup. step: (50, 100, 200, 200 and 500) mg figure 7. calibration step. acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 39 figure 9. the measurement in each range must be done for three cycles. if the system is in equilibrium, the clockwise torques, m1 and anti-clockwise torque, m2 shall be equal. the difference in arm length, ld, was calculated by equations (4) to (9). m1 = m2 (4) f1 · (l ld) = f2 · (l + ld) (5) f1 · l f1 · ld = f2 · l + f2 · ld (6) f1 · l f2 · l = f1 · ld + f2 · ld (7) f1 · l f2 · l = ld · (f1 + f2) (8) 𝑙𝐷 = ( 𝐹1·𝐿− 𝐹2·𝐿 𝐹1+𝐹2 ) (9) 4.4. torque transducer calibration the realised residual torque from ch1, ch2, and ch3+ch4 was combined by a software program (shown in figure 10) used to monitor and correct the calibration torque value in the calibration process. the proposed method was proven reliable by comparing the transducer calibration results characteristics: the repeatability, reproducibility, and linearity of the measurement according to the german industrial standard documentation din 51309 [14]. 5. experimental results 5.1. fulcrum cross-elastic hinge realisation results two cross elastic hinges’ signals at the fulcrum (ch3+ch4) were expressed as a function of torque by using the first order of the interpolation equation without the absolute term, as shown in figure 11. the interpolation deviation results were within ± 0.0002 n·m, as shown in figure 12. that was a limitation of the fulcrum sensitivity of the machine, which was due to the designed elastic hinge’s thickness, 0.50 mm, causing a higher stiffness compared to the sheet metal plate of the other types of tsms. 5.2. left and right elastic hinges realisation results the calibration results for the bending moment of the elastic hinge are shown in table 1 and table 2. it is clear that the torque calibration in which the weight was not put on the pan did not change. that meant the moment of the hinge was zero. the weight placed at each position on the pan from the reference line in any direction caused a varied torque vector, as shown in figure 13. it was therefore possible to determine the sensitivity, as shown in figure 14. it was found that the sensitivities of the left and right elastic hinges were different. figure 11. linear regression of the ch3+ch4 signal. f2 f1 ll l-ld ld l+ld figure 9. difference in arm length between the left and right arms. figure 10. software program. figure 12. interpolation deviation. table 1. ch1; clockwise torque measurement results. mass position transducer ch3+ch4 output signal in mv/v in m in n·m in n·m ch1 ch2 p-4 -0.12 0.24037 -0.00003 0.11646 0.00010 p-3 -0.09 0.24269 -0.00004 0.08790 0.00008 p-2 -0.06 0.24500 -0.00006 0.05934 0.00005 p-1 -0.03 0.24748 -0.00005 0.02929 0.00009 ref. 0 0.24996 -0.00005 -0.00077 0.00013 p1 0.03 0.25220 -0.00004 -0.02797 0.00014 p2 0.06 0.25444 -0.00004 -0.05517 0.00014 p3 0.09 0.25680 -0.00001 -0.08353 0.00011 p4 0.12 0.25917 0.00002 -0.11189 0.00008 y = 0.4431x r² = 1.0000 -0.006 -0.004 -0.002 0.000 0.002 0.004 0.006 -0.015 -0.010 -0.005 0.000 0.005 0.010 0.015 c a li b ra ti o n t o rq u e i n n ·m output signal in mv/v -0.0003 -0.0002 -0.0001 0.0000 0.0001 0.0002 0.0003 -0.006 -0.003 0.000 0.003 0.006 in te rp o la ti o n d e v . in n ·m calibration torque in n·m acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 40 the residual torque of the machine, mr, was calculated by a summation of the torque vector signals at the fulcrum (ch3+ch4), a bending moment of the right elastic hinge (ch1), and a bending moment of the left (ch2). the calibration torque, mt, was calculated by a summation of the torque vectors, including the generated torque (f1·l1) and the residual torque value mr. 5.3. difference in arm length evaluation results the difference in the arm length between the left and right arms varied between 23 µm and 33 µm, as shown in figure 15. the difference in the lever arm length in each couple force step was expressed as a function of length by a third order interpolation equation, including the absolute term. the difference in arm length must be used to correct the effective lever arm length, l1, for each calibration torque value. the shift in the difference of arm length had a direct effect on the linearity of the calibration. the difference in the arm length, ldi (in micrometres), of each calibration torque step, mti, was calculated by equation (10). 𝑙𝐷𝑖 = −0.0000007 𝑀𝑖 3 + 0.0002 𝑀𝑖 2 − 0.0773 𝑀𝑖 + 32.593 (10) the influence of the variation in the difference in arm length (10 µm) affected the linearity of the measurement by about 2 ppm relative to the lever arm of 500 mm and affected the certified output signals of the transducer under calibration by about 7 ppm. however, the difference in arm length of each calibration torque step was used to calculate the effective lever arm length, l1i, by summing it with half of the entire measured lever length. the uncertainty in the different length evaluation and entire lever length measurement must be taken into account. 5.4. the comparison results of the torque transducer calibration characteristics the calibration torque, mt, was corrected by the residual torque value and the difference in arm length of both configurations (original and this research’s concepts). however, the original configuration of the residual torque did not concern the bending moment from the asymmetrical load. the measurement results for the torque transducer calibration were interpolated to the nominal torque value. the comparison results for the repeatability, reproducibility, and linearity characteristics are shown in figure 16, figure 17 and figure 18, respectively. those results were a calibration of the same transducer, but different configurations for the residual torque signal. the random bending moment affected the repeatability, reproducibility, and linearity of measurement. it was found from the experiment that the residual torque combined with the bending moment could be used to correct the calibration torque value, because the actual residual torque sensitivity was calibrated. considering the above three parameters, this research proposed a method of significantly improving the torque calibration’s characteristics. table 2. ch2; anti-clockwise torque measurement results. mass position transducer ch3+ch4 output signal in mv/v in m in n·m in n·m ch1 ch2 p-4 -0.12 -0.24009 -0.00001 0.00019 -0.11620 p-3 -0.09 -0.24257 0.00002 0.00018 -0.08699 p-2 -0.06 -0.24504 0.00006 0.00006 -0.05779 p-1 -0.03 -0.24750 0.00004 0.00015 -0.02902 ref. 0 -0.24995 0.00003 0.00018 -0.00025 p1 0.03 -0.25233 0.00000 0.00019 0.02776 p2 0.06 -0.25470 -0.00002 0.00016 0.05576 p3 0.09 -0.25721 0.00001 -0.00004 0.08557 p4 0.12 -0.25973 0.00003 -0.00018 0.11537 figure 15. change in the difference in arm length. figure 14. linear regression of the ch1 and ch2 signals. figure 13. varied torque by mass position. figure 16. the repeatability characteristic. y = -2e-07x3 + 0,0002x2 0,0773x + 32,593 r² = 0,9999 0 10 20 30 40 0 100 200 300 400 500 d if fe re n c e o f ar m l en g th i n µ m applied calibration torque in n·m raw data poly. (raw data) y = -0.0823x r² = 1.0000 y = -0.0849x r² = 1.0000 -0.015 -0.010 -0.005 0.000 0.005 0.010 0.015 -0.15 -0.10 -0.05 0.00 0.05 0.10 0.15 d e v . to rq u e f ro m r e fe re n c e p o si ti o n i n n ·m output signal of elastic hinge in mv/v ch1 ch2 linear (ch1) linear (ch2) -0.265 -0.260 -0.255 -0.250 -0.245 -0.240 -0.235 0.235 0.240 0.245 0.250 0.255 0.260 0.265 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 t o rq u e i n n ·m mass position on pan in m ch1:cw ch2:ccw 0.000 0.030 0.060 0.090 0.120 0.150 re l. r e p e a ta b il it y i n % torque in n·m original config. this research config. acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 41 6. conclusions the bending moments of the right and left elastic hinges were the results of the force-applied position that deviated from the reference line. the cause of the misposition of the applied force was a result of an imperfection in the manufacturer’s process and a small steel ball inside the weight. the proposed method in this research can resolve the undesirable influence of the bending moment of elastic hinges by significantly improving the repeatability, reproducibility, and linearity of the measurement. in the future, the problem with the asymmetric load should be corrected by the fabrication of new standard weights. the holder of the strain controlled elastic hinge type torque standard machine should be considered the influence of the bending moment, which affected the uncertainty of the measurement in a low torque calibration, especially when it was lower than 1.0 % of the maximum capacity of the machine. the calibration and measurement capabilities (cmcs) was still maintained at 0.030 % (k = 2) at the measurement range 1 n∙m to 10 n∙m. however, the cmcs at the measurement range 2 n∙m to 10 n∙m was improved to 0.020 % (k = 2). references [1] h. gassmann, t. allgeier, u. kolwinski, a new design of primary torque standard machines, proc. of the xvi imeko world congress, vienna, austria, 25-28 september 2000. [2] r. oliveira, l. cabral, u. kolwinski, d. schwind, performance of the new primary torque standard machine of inmetro, brazil, proc. of imeko tc3 “force, mass and torque measurements”, cairo, egypt, 19-23 february 2005. [3] h. w. werner, d. schwind, 25 kn·m torque calibration machine reaches ubmc = 0.008 % using new design features, proc. of the xviii imeko world congress, rio de janeiro, brazil, 17-22 september 2006. [4] t. allgeier, u. kolwinski, d. schwind, jockey-weight lever machines for force and torque, proc. of the joint international conference on “force, mass, torque, hardness, and civil engineering metrology in the age of globalization”, celle, germany, 24-26 september 2002. [5] d. röske, final report on the torque key comparison ccm.tk1.2 measurand torque: 0 n·m, 500 n·m, 1000 n·m, metrologia 52 (2015) tech. suppl. online [accessed 31 august 2018] https://kcdb.bipm.org/appendixb/appbresults/ccm.t-k1/ccm.tk1.2.pdf [6] t. sanponpute, p. chantaraksa, n. saenkhum, n. arksonnarong, suspended-fulcrum torque standard machine, proc. of the xix imeko world congress “fundamental and applied metrology”, lisbon, portugal, 6-11 september 2009. [7] t. sanponpute, n. arksonnarong, a. brüge, n. saenkhum, suspended-fulcrum torque standard machine, 2nd report, proc. of the xxi imeko world congress, prague, czech republic, 30 august 4 september 2015. [8] a. nishino, k. ogushi, t. sanponpute, n. arksonnarong, interlaboratories comparison between nmij and nimt for calibration capability of a low nominal capacity torque measuring device in the range from 0.1 n·m to 1 n·m, sice 2016, tsukuba, japan, 20-23 september 2016. [9] analogic: the world resource for precision signal technology, an3060 digital weight indicator user manual, analogic corporation, ma 01960-7987, usa, p/n 82-5111 rev.3. [10] solidworks corporation, solidworks simulation, dassault systemes solidworks corporation, ma 02451, usa, pmt1540eng. [11] hbm, operating manual amplifier system mgcplus with display and control panel ab22a/ab32, hottinger baldwin messtechnik gmbh, darmstadt, germany, b0534-14.1 en. [12] n. saenkhum, t. sanponpute, the optimization of continuous torque calibration procedure, measurement 107 (2017) pp. 172178. [13] n. arksonnarong, n. saenkhum, p. chantaraksa, c. schlegel, t. sanponpute, 5 kn·m torque standard machine of nimt, proc. of the asia-pacific symposium on measurement of mass, force & torque (apmf 2017), krabi, thailand, 19-23 november 2017. [14] din 51309, materials testing machines – calibration of static torque measuring devices, deutsches institut für normung e. v., rev. 05, 2005-12-01. figure 17. the reproducibility characteristic. figure 18. the relative interpolation deviation of results. 0.000 0.030 0.060 0.090 0.120 0.150 re l. r e p ro d u c ib il it y i n % torque in n·m original config. this research config. https://kcdb.bipm.org/appendixb/appbresults/ccm.t-k1/ccm.t-k1.2.pdf https://kcdb.bipm.org/appendixb/appbresults/ccm.t-k1/ccm.t-k1.2.pdf https://kcdb.bipm.org/appendixb/appbresults/ccm.t-k1/ccm.t-k1.2.pdf https://kcdb.bipm.org/appendixb/appbresults/ccm.t-k1/ccm.t-k1.2.pdf https://kcdb.bipm.org/appendixb/appbresults/ccm.t-k1/ccm.t-k1.2.pdf https://kcdb.bipm.org/appendixb/appbresults/ccm.t-k1/ccm.t-k1.2.pdf acta imeko contacts acta imeko issn: 2221-870x december 2018, volume 7, number 4, 0 acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 0 journal contacts about the journal acta imeko is an e-journal reporting on the contributions on the state and progress of the science and technology of measurement. the articles are mainly based on presentations presented at imeko workshops, symposia and congresses. the journal is published by imeko, the international measurement confederation. the issn, the international identifier for serials, is 2221-870x. editor‐in‐chief dušan agrež, slovenia founding editor‐in‐chief paul p. l. regtien, netherlands associate editor dirk röske, germany editorial board section editors (vol. 6 and 7) leopoldo angrisani, italy filippo attivissimo, italy eulalia balestieri, italy eric benoit, france paolo carbone, italy lorenzo ciani, italy catalin damian, romania pasquale daponte, italy luca de vito, italy luigi ferrigno, italy edoardo fiorucci, italy alistair forbes, united kingdom helena geirinhas ramos, portugal sabrina grassini, italy fernando janeiro, portugal konrad jedrzejewski, poland andy knott, united kingdom francesco lamonaca, italy massimo lazzaroni, italy fabio leccese, italy rosario morello, italy michele norgia, italy pedro miguel pinto ramos, portugal nicola pompeo, italy sergio rapuano, italy dirk röske, germany alexandru salceanu, romania constantin sarmasanu, romania lorenzo scalise, italy emiliano schena, italy enrico silva, italy krzysztof stepien, poland marco tarabini, italy marcantonio catelani, italy paolo carbone, italy lorenzo ciani, italy egidio de benedeto, italy alessandro germak, italy sabrina grassini, italy konrad jędrzejewski, poland min-seok kim, korea fabio leccese, italy rosario morello, italy vilmos palfi, hungary nicola pasquino, italy franco pavese, italy paul regtien, the netherlands alexandru salceanu, romania jan saliga, slovakia jorge c. torres-guzman, mexico ian veldman, south africa claudia zoani, italy about imeko the international measurement confederation, imeko, is an international federation of actually 41 national member organisations individually concerned with the advancement of measurement technology. its fundamental objectives are the promotion of international interchange of scientific and technical information in the field of measurement, and the enhancement of international co-operation among scientists and engineers from research and industry. addresses principal contact prof. dušan agrež university of ljubljana faculty of electrical engineering tržaška 25, 1000 ljubljana, slovenia e-mail: dusan.agrez@fe.uni-lj.si acta imeko prof. alexandru salceanu “gheorghe asachi” technical university of iasi mangeron str. 67, 700050 iasi, romania e-mail: asalcean@tuiasi.ro support contact dr. dirk röske physikalisch-technische bundesanstalt (ptb) bundesallee 100, 38116 braunschweig, germany email: dirk.roeske@ptb.de mailto:dusan.agrez@fe.uni-lj.si mailto:asalcean@tuiasi.ro mailto:dirk.roeske@ptb.de design of technology-based rehabilitation pathways: the experience of santobono-pausilipon hospital acta imeko issn: 2221-870x december 2018, volume 7, number 4, 55 61 acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 55 design of technology-based rehabilitation pathways: the experience of santobono-pausilipon hospital l. iuppariello1, m. cesarelli2, g. faiella3, s. esposito1, m. nespoli1, l. foggia1, f. clemente4,3 1 department of rehabiliation, aorn santobono-pausilipon, naples, italy 2 department of electric engineering and information technologies (dieti), school of engineering, university of naples federico ii, naples, italy 3 fondazione santobono pausilipon, naples, italy 4 institute of cristallography, italian national research council (ic-cnr), rome, italy section: research paper keywords: physiological measurement; rehabilitation; clinical pathways; virtual reality; gait analysis; health management citation: l. iuppariello, m. cesarelli, g. faiella, s. esposito, m. nespoli, l. foggia, f. clemente, design of technology-based rehabilitation pathways: the experience of santobono-pausilipon hospital, acta imeko, vol. 7, no. 4, article 10, december 2018, identifier: imeko-acta-07 (2018)-04-10 editor: alexandru salceanu, "gheorghe asachi" technical university of iasi, romania received march 30, 2018; in final form september 21, 2018; published december 2018 copyright: © 2018 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: luigi iuppariello, e-mail: l.iuppariello@santobonopausilipon.it 1. introduction the concept of patient-based rehabilitation is a crucial issue for every healthcare provider. its aim is to overcome a diseasebased approach to healthcare that often does not consider patients’ preferences and their emotional needs and life issues. assistive technology-based rehabilitation is an area of research relating to the use of technology within the realm of physiotherapy [1-3]. whatever the cause of impairment, the main aim of most neurological rehabilitation is to facilitate the learning or relearning of motor skills. the process of gaining motor skills involves repeating the same movement hundreds or even thousands of times. traditional rehabilitation is time consuming and requires lengthy sessions with therapists that are demanding on personnel and healthcare budgets. patients often tire of rehabilitation, finding it frustrating or dull. integrating virtual, augmented, and mixed reality environments into rehabilitation protocols has the potential to change the nature of rehabilitation, particularly in the neurorehabilitation field [4-5]. in recent years, beyond conventional physical treatment, various new rehabilitation strategies, such as robotics and virtual reality (vr), have been introduced and proven effective in improving walking performance. disabilities, residual motor function, and efficacy of treatment are often quantified using semi-quantitative evaluation scales, without reliable quantitative measures. especially in the field of rehabilitation medicine, robotics and vr systems have the potential to objectively measure abilities and to serve as therapeutic tools. they provide users with repetitive, contextualized, task-specific training in an exciting and entertaining manner. such exercises are fundamental in neurorehabilitation as they aid in promoting neuroplasticity and recovery. robotic systems can facilitate or assist the movement of paretic limbs in performing functional movements and actions, enabling patients to increase their social engagement and therefore their autonomy and independence. abstract the fields of rehabilitation robotics and virtual reality are becoming a growing area of interest in the clinical rehabilitation of people with motion impairments. these systems have the potential to assess abilities through physiological measurements and modelling activities, such as posture, gait, and balance. they can also be used as rehabilitative tools by providing patients with task-specific training in a motivating and engaging way. despite the potential advantages of such systems, until now, there has been a general limitation of their use in rehabilitative practice. robotics and virtual reality systems can be challenging, engaging, and fun all at the same time, particularly for children with disabilities (since they are often not very motivated to comply with conventional therapies). the aim of this work is to accurately describe the clinical use of innovative rehabilitative technologies and their use in the development of two technology-based rehabilitation pathways for the treatment of gait disorders following obesity and neurological diseases in the treatment of pediatric patients. mailto:l.iuppariello@santobonopausilipon.it acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 56 vr systems are based on three-dimensional computer-generated simulations of the real world. they offer a variety of mechanisms for therapeutic gain, including repetitive practice of movements; engagement in problem-solving, memory, and attention tasks; and exposure to anxiety-provoking stimuli or events. moreover, innovative computerized systems can define physiological models of the human body and provide a quantitative measure of biomechanical features of human movements. this is useful for quantitatively assessing baseline motor conditions; individually adapting the rehabilitation program with residual motor function and treatment objectives; and measuring improvements. vr systems, for instance, offer real-time feedback regarding performance that can be provided to patients and clinicians (which is useful for addressing the rehabilitation targets) by monitoring changes during the rehabilitation pathways (reports can be produced to provide objective data regarding patient progress in comparison to themselves or other users). the appropriate use of robotics and vr systems [6] can be challenging, engaging, and fun all at the same time, particularly for children with disabilities, who are often not very motivated to comply with conventional therapies that they do not think are meaningful or fun. moreover, the use of vr systems in pediatric patients is suitable thanks to the high plasticity of neuronal circuits in children. despite these advantages, the practical introduction of new technologies requires introducing an ad hoc quality control management plan for multidimensional and multidisciplinary procedures (i.e., new clinical pathways) in the clinical system. clinical pathways are predefined, articulated, and coordinated sequences of services provided at the outpatient, hospitalization, and/or geographical level. they involve the integrated participation of various specialists and professionals (in addition to the patient themselves) at the hospital and/or geographical level in order to achieve the most appropriate diagnosis and therapy for a specific pathological situation. they are specifically recommended by the italian national health service with the aim of guaranteeing the essential levels of healthcare [7]. most traditional rehabilitative pathways are performed in a passive way, without a patient-centered approach; consequently, improvements of the patients’ health cannot be maintained [811]. to overcome this limitation, this paper introduces specific patient-centered pathways, designed using an innovative standard of patient flow through the healthcare system [12]. the aim of this work is to describe the clinical implementation of two technology-based rehabilitation pathways in the rehabilitation unit of the pediatric hospital santobonopausilipon in naples, italy. this paper includes (i) a description of the technologies; (ii) an explanation of the implementation of the new standardized procedures based on innovative physiological measurements and activities; and (iii) the main results from the clinical implementation of the procedures. 2. materials the foundation of a successful rehabilitation program and its monitoring is a well-developed plan of action based on the specific limitations of each patient. even if two persons have the same physical limitation or the same disease, different circumstances may impair one more than the other. therefore, it is necessary to construct a personalized and optimized rehabilitation program for each patient. with this consideration in mind, the rehabilitation unit of the pediatric hospital santobono-pausilipon in naples, italy has developed several individual rehabilitation pathways combining conventional physiotherapy with robotics and vr systems for the treatment of children with gait disorders. the following robotic and virtual reality systems have been used: • gait real-time analysis interactive lab (grail), for both gait analysis (baseline and follow-up evaluations) and rehabilitation. • lokomat, erigo, and alterg systems for motor rehabilitation. these technologies have been used to flank and support clinicians in the implementation of two technology-based rehabilitation pathways in the treatment of gait disorders following neurological diseases and obesity in childhood. in the following sections, the technologies are described in detail. 2.1 grail 2.1.1. architecture grail [5], produced by motek medical, has shown encouraging prospects in terms of its evaluative and rehabilitative benefits. grail offers clinical gait analysis and gait training using the latest technologies, which are solidly integrated into one functional system, providing the full functionality of a traditional gait lab and more. it can be used as a standard for a variety of patient groups, including those with neurological, orthopedic, and muscular-skeletal complaints and elderly persons with an increased risk of falling. the architecture consists of a self-paced instrumented dual-belt treadmill, an integrated motion-capture system, and synchronized vr environments next to three video cameras. the entire grail setup can be classified into three categories: i) “system operating area,” ii) “operator desk,” and iii) “background processing” (figure 1). the “system operating area” includes all the hardware with which the subject has direct interaction. almost all components in this part are present in order to create an immersive experience for the subject. the “operator desk” includes all the hardware with which the operator has direct interaction. it includes two computers (d-flow and mocap) to manage the entire system. the d-flow computer uses two dedicated monitors, one mouse and one keyboard. the operation of the d-flow computer is like that of any other pc. the dflow software controls the whole system, managing the relationship between the subject, the scenario, and the interactive feedback and simulations [13]. as well as the d-flow computer, grail includes many other computers. these computers are all connected to one monitor, one mouse, and one keyboard. to control all computers with one set of monitors, a mouse, and a figure 1. the grail system. acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 57 keyboard, a kvm switch is used. from here, the operator controls what happens in the “system operating area.” finally, “background processing” includes all the hardware that is needed to control the system. this system permits the calculation of all gait parameters in real time (spatio-temporal parameters, kinematics, kinetics, and muscle-activation). the self-paced mode permits the subject to walk at a self-selected speed, while the treadmill and the vr environment are perfectly synchronized. accurate gait parameters are ensured due to a natural walking pattern, facilitated by an immersive vr environment projected onto a 180° screen. this gives the subject the impression of walking in a natural environment with a peripheral view, while measuring gait behavior, thereby providing a “functional gait analysis” (figure 2). the accuracy of the data can be verified in real time. an offline analytical tool offers dedicated data analysis. 2.1.2. calibration a grail gait analysis session requires appropriate preparation with additional steps in the motion capture software and d-flow. first, the motion capture system and the force plates need to be calibrated. two steps are commonly involved in the motion capture system calibration: static calibration and camera calibration. static calibration is performed by using a wand embedded with five markers emitting red led lights. this step allows us to set the camera position and the global origin of the acquisition volume where the subjects will be analyzed. camera calibration involves moving the wand in the acquisition volume so that each camera collects the required number of refinement frames in the acquisition volume. finally, the force plates are calibrated with a specific module on the d-flow computer, when no weight is exerted on them. 2.1.3. gait analysis and gait training using grail once the calibrations of both the motion capture system and the force plates are complete, the system is ready to perform a gait analysis session. reflective markers are placed on the subject’s skin (figure 3) and the cartesian position of each marker relative to the global coordinate system is tracked using high-speed infrared cameras. before starting the physical task, the subject performs a range of motion (rom) trial on the treadmill to minimize real-time labelling errors and to create the virtual skeleton or full human body model (hbm), as shown in figure 4. the hbm uses inverse dynamics to estimate muscle forces and visualize them in a virtual environment in real-time. it allows for both the visualization and calculation of muscle forces, joint angles, moments, and powers in real time during the gait sessions. the subject measurements as well as the marker and force plate data are input into the model. inverse dynamics are used to calculate the biomechanical outputs. the hbm can be displayed on the screen to represent the patient in the vr environment, and as muscles are activated, the muscles light up and change color depending on the amount of force applied. the patient walks on the treadmill at either a fixed, comfortable speed that can be adjusted by the operator or in a self-paced mode (i.e., the treadmill speed is controlled by the patient’s position on the treadmill). the operator sets a vr scene that is a futuristic, colorful, non-distracting environment. the ability of the grail system to simulate real-life learning while providing augmented feedback and a high intensity of massed practiced tasks can be used for a variety of patient groups with neurological or orthopedic disorders. it can help in the development of technology-based rehabilitation pathways in which a multidisciplinary team composed of neurologists, rehabilitation specialists, orthopedists, engineers, and physiotherapists can collaborate with one another to both evaluate patients’ functional behaviors and help restore or improve that function. 2.2. lokomat the lokomat (figure 5) is a robotic treadmill training system that uses augmented biofeedback and a body weight support system to suspend individuals while their legs are attached to robotic legs that assist them with basic walking functions. it allows the therapist to control how fast the patient walks, how much body weight the patient can support, and how much assistance the robotic legs give the patient through a range of motions. the customized support and limb guidance provided by the lokomat allows patients who have a wide variety of figure 2. functional gait analysis using the grail system. figure 3. the position of the markers on the body subject for gait analysis and gait training sessions. figure 4. the virtual skeleton (on the left) and the full human body model. acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 58 walking impairments to use this device both in inpatient and outpatient settings. it is used to improve mobility in individuals following a stroke, a spinal cord injury, a traumatic brain injury, multiple sclerosis, cerebral palsy, and other neurological diseases and injuries. 2.3 erigo the erigo (figure 6) is a robotic system used in the rehabilitation of neurological patients. it accelerates the recovery process with intensive sensorimotor stimulation and prevents potential damage due to immobilization. 2.4 alterg the alterg is a treadmill (figure 7) formed of two compressors, a protective transparent chamber, safety locks, and different sizes of pants to hold the patient firmly in the chamber. the patient can walk without bearing their entire weight, thereby reducing the impact on the body, in turn optimizing rehabilitation and physical therapy outcomes. its differential air pressure (dap) technology applies a lifting force to the body that reduces the weight on the lower extremities and allows for precise unweighting of up to 80 % of a person’s body weight, finding exactly where the pain stops and making natural movement feel good again. 3. methods the use of technology-based rehabilitation pathways has clear positive effects on the safety and quality of care, particularly for children, thereby improving their motor function outcomes and disability. these considerations led the pediatric hospital santobonopausilipon to define and approve two new rehabilitation models of care combining different robotics and vr systems for the treatment of patients suffering from neurological diseases and obesity. the idea underpinning the models is to construct two organized, goal-defined, and time-specified plans of management of the gait disorders in which a multidisciplinary team of clinicians and engineers works together to achieve better quality outcomes by applying new technologies. in this way, children progress along a technology-based pathway designed according to the healthcare standards of the italian ministry of health and the regional health system. 3.1. neurological diseases at the santobono-pausilipon hospital, the only pediatric rehabilitation center for post-acute treatments (cod.56 of the regional health system) and brain injuries (cod.75 of the regional health system) has recently been opened, according to the regional plan of programming of the hospital network. the mission of this center is to guarantee global care for children with severe diseases. participants in this study were recruited from both the inpatient and outpatient settings of the rehabilitation figure 5. the driven gait orthosis lokomat guides the legs of the subject walking on the treadmill. a body weight support system further facilitates the walking movements. figure 6. the erigo device combines progressive verticalization and cyclic leg movements in combination with step-synchronized muscle functional electric stimulation at the lower limb level to ensure safe stabilization when the patient is upright. figure 7. the alterg anti-gravity treadmill. it uses differential air pressure technology to provide lower body positive pressure unweighting. acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 59 unit of the pediatric hospital santobono-pausilipon. for inclusion in the study, the participants were required to have neurological diseases (ataxia, post-stroke spasticity, cerebral palsy, etc.) associated with gait disorders. the study excludes those who are unable to walk without direct assistance or those who have major cognitive deterioration that would impact participation. all the patients were enrolled in the study by clinicians. further information was given to the parents of those satisfying the entry criteria, and they were asked to declare their willingness for their children to comply with whichever treatment option to which they are assigned and to attend all the follow-up visits. the baseline and follow-up evaluations consist of traditional clinical scales (i.e., the gross motor function measure, the berg scale), and gait analysis was assessed using grail (figure 8). inpatient and outpatient children, depending on their residual motor capabilities, completed gait training sessions using the erigo, lokomat, alterg, and grail. 3.2. obesity the rehabilitation pathways for children suffering from obesity have been developed with the aim of achieving improvements in postural balance, gait, and the reduction of excess weight or fat mass. these children were recruited from an outpatient setting (day hospital rehabilitation) of the children obesity and endocrine disorders unit of the pediatric hospital santobono-pausilipon. for inclusion in the study, the participants were simply required to be suffering from obesity. the study excludes those who are unable to walk without direct assistance or those who have major cognitive deterioration that would impact participation. 3.3. rehabilitation pathways a flowchart of the implemented technology-based rehabilitation pathways is represented in figure 8. the clinical assessments described above were performed at the beginning (baseline) and at the end of an eight-week rehabilitation treatment program (follow-up). the baseline evaluation of all eligible patients consisted of a clinical evaluation performed by the clinicians and physiological measurements of gait patterns recorded by grail. all the spatiotemporal parameters of gait as well as the articular ranges of motion, force, and muscle activation were used as outcome measures together with the clinical scales to assess the baseline state and the effect of the rehabilitation program. figure 9 shows a representative grail gait analysis report. the grail rehabilitation sessions, common to both neurological and obesity pathways, consist of a set of different, serious games with the aim of training balance control and dual tasking for patients facing problems in coordination, posture, gait, balance, and/or stability (figure 10). for example, in the active balance application, the patient is represented as a red ball, and they are required to steer the red ball through the maze by shifting their weight or by flexion, extension, and lateral flexion of the trunk. the pathway includes up to four hours of individual rehabilitative training for five days a week, which included traditional neuro-motor techniques, articular mobilization, stretching exercises, “sit-to-stand” exercises, and step figure 8. a flowchart showing the patient-based model of technology-based care. the baseline and follow-up evaluations consist of traditional clinical scales (bmi, the berg scale, the barthel index, etc.) and a grail gait analysis. depending on their residual motor capability, the participants completed gait training sessions using grail. figure 9. gait offline tools grail analysis (at the top); an example of such kinematics angles during grail gait analysis sessions. the shaded area denotes normal range (at the bottom). acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 60 decomposition exercises. the outpatients receive the same rehabilitation program for about one or two days a week (day hospital rehabilitation). the neurological rehabilitation pathways include balance and gait training using erigo, lokomat, alterg, and grail. the erigo device is used for early rehabilitation and mobilizations of the patients, while lokomat is used for intensive locomotion therapy. the alterg allows the patients to undertake gait training on a treadmill with different percentages of body weight support (body weight can be reduced by up to 80 % in 1 % increments), thereby reducing ground reaction forces in the lower limbs and improving balance and walking capacity. in this way, the system encounters patients both physically and cognitively in realistic, interactive, and controlled environments. children interact with the system due to the integrated force plates measuring weight shifting and body motion. engagement is further enhanced using realistic sounds and scents. additionally, grail allows the child to identify problematic balance situations that occasionally occur in everyday life but cannot be replicated in traditional physical therapy. measuring the changes in gait pattern during all these conditions allows for the identification and quantification of pathology specific compensation strategies or determining dynamic stability. then, clinicians can manipulate the environment interaction conditions, customizing the challenges applying varying levels of difficulty that change automatically according to patient performance, thereby facilitating the learning process in both virtual and real environments. results since 2017, a biomedical engineer has participated in the regional network for paediatric rehabilitation (netcrip) to study a rehabilitation assistance network for pediatric patients. his role has been to setup the entire rehabilitation laboratory system and to introduce new technology-based pathways in clinical activities. in this framework, the enrolment for patients started according to the abovementioned neurological pathways. to date, over fifty children with gait disorders arising due to neurological diseases have been enrolled in the technology-based rehabilitation program. following the encouraging results from this implementation, the obesity rehabilitation pathways have been designed and introduced in clinical activities. to date, the pathway has been tested on one patient. discussion the primary objective of the current study was to outline a methodology for introducing new robotics and vr systems, such as erigo, lokomat, and grail, in patient pathways at the pediatric hospital santobono-pausilipon. these technologies are the basis of designing and implementing two technology-based rehabilitation pathways for children with gait disorders arising due to obesity and neurological diseases. in recent years, several robotics and vr systems for rehabilitation treatments have been developed successfully to help patients with functional disability recover or compensate for loss of limb motor function [14]. these systems allow the patient to perform safe and intensive training that can be done in combination with other treatments. this is reflected in the opportunities for play that can be enjoyable, stimulating, and non-threatening. such rehabilitation ensures that patients have access to the professional support, advice, and intervention they need to accomplish their personal rehabilitation targets, maximizing their independence relating to activities of daily living. the benefit of using vr for rehabilitation is that the type of action, exercise, and feedback can be tailored to the needs and capabilities of the child for more effective motor learning and other rehabilitation outcomes. however, even if these technologies are available in medical departments, they are usually used only sporadically in gait disorder rehabilitation. this is due to a lack of multidisciplinary teams in clinical practice and to the fact that customized devicebased therapy processes still contrast strongly with typical ways of working in rehabilitation, still oriented towards manual activities (the traditional approach for rehabilitation). moreover, given the labor intensiveness of creating technology-based rehabilitation pathways, the use of these tools is still not common in rehabilitation. the use of technologybased custom care pathways can often reduce in-hospital complications and can lead to clear positive effects on safety and quality of care, particularly for children. the rapid involvement of healthcare professionals in such projects has shown that the design of a new clinical pathway is useful in promoting a patient-centered approach. this is especially true if pathways are technologically driven by new physiological measurements and modeling methods. now the operative procedure has been defined, it is now necessary to define robust indices [15] to evaluate the pathway itself and its introduction in clinical activities. in order to do so, the next step is to analyze the clinical results using a robust statistical analysis in order to investigate and gain feedback on patient acceptance of this new integrated and technology-based clinical pathway acknowledgements this research was supported by regione campania regional network for paediatric rehabilitation (netcrip). we give special thanks to the management of aorn santobonopausilipon for the rapid reception of the proposed rehabilitation clinical pathway. figure 10. rehabilitation grail sessions for patients facing problems in coordination, posture, gait, balance, and/or stability. acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 61 references [1] d. f. higginbotham, h. shane, s. russell, k. caves, access to aac: present, past, and future, augment. altern. commun. 23 (2007) pp. 243-257. [2] l. ball, a. nordness, s. fager, et al., eye gaze access of aac technology for persons with amyotrophic lateral sclerosis, j. med. speech lang. pathol. 18 (2010) pp. 11-23. [3] s. fager, l. bardach, s. russell, j. higginbotham, access to augmentative and alternative communication: new technologies and clinical decision-making, j. pediatr. rehabil. med. 5 (2012) pp. 53-61. [4] p. langhorne, f. coupar, a. pollock, motor recovery after stroke: a systematic review, lancet neurol. 8(8) (2009) pp. 741-754. [5] l. iuppariello, p. bifulco, m. cesarelli, s. esposito, m. nespoli, l. foggia, f. clemente, “new measurement techniques for gait analysis: the grail experience”, proc. of the 22nd imeko tc4 international symposium, sept. 14-15, 2017, iasi, romania, pp. 249-252. [6] r. l. o'neil, r. l. skeel, k. i. ustinova, cognitive ability predicts motor learning on a virtual reality game in patients with tbi, neurorehabilitation 33(4) (2013) pp. 667-680. [7] v. d. tozzi, f. longo, g. pacileo, d. salvatore, n. pinelli, v. morando. patient workflow for chronic care model (in italian), fiaso italian federation of health services, rome, 2014. [8] w. ming et al. “locomotor training through a 3d cable-driven robotic system for walking function in children with cerebral palsy: a pilot study”, proc. of the 36th annual international conference of the ieee, aug. 26-30, 2014, chicago, u.s.a., pp. 3529-3532. [9] j. a. day, et al. locomotor training with partial body weight support on a treadmill in a non-ambulatory child with spastic tetraplegic cerebral palsy: a case report, pediatric physical therapy 16(2) (2004) pp. 106-113. [10] i. borggraefe, et al. robotic-assisted treadmill therapy improves walking and standing performance n children and adolescents with cerebral palsy. european journal of paediatric neurology 14(6) (2010), pp. 496-502. [11] i. borggraefe, et al. improved gait parameters after roboticassisted locomotor treadmill therapy in a 6-year-old child with cerebral palsy, movement disorders 23(2) (2008) pp. 280-283. [12] g. k. r. berntsen, d. b. gammon, a. steinsbekk, n. foss, c. ruland, v. fønnebø, what are the goals of care in individual patient pathways? an qualitative analysis of how different health concepts may affect goals for care”, international journal of integrated care 15(5) (2015) pp. 1-2. [13] t. geijtenbeek, f. steenbrink, b. otten, o.e.zohar, “d-flow: immersive virtual reality and real-time feedback for rehabilitation”, proc. of the 10th international conference on virtual reality continuum and its applications in industry, dec. 11-12, 2011, hong kong, china. [14] m. cesarelli, g. d’angelo, m. romano, p. bifulco, f. clemente, “a new system to deliver vibration, measure motion and emg reaction for forearm muscles”, 20th imeko tc4 symposium on measurements of electrical quantities and the 18th tc4 international workshop on adc and dca modeling and testing, iwadc, sept. 15-17, 2014, benevento, italy, pp. 606609. [15] f. clemente, m. d’arco, e. d’avino. the use of a conceptual model and related indicators to evaluate quality of healthcare, in intensive care units”, qual. eng. 26, (2014) pp. 196-205. acta imeko, title acta imeko issn: 2221-870x march 2018, volume 7, number 1, 65-67 acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 65 system for measuring vacuum-pump performance using the standard throughput method sheng-jui chen, sheau-shi pan, ta-chang yu center for measurement standards, industrial technology research institute, hsinchu, 30011, taiwan, r.o.c. section: research paper keywords: vacuum-pump performance; pumping speed; throughput; vacuum calibration; volume flow rate citation: sheng-jui chen, sheau-shi pan, ta-chang yu, system for measuring vacuum-pump performance using the standard throughput method, acta imeko, vol. 7, no. 1, article 12, march 2018, identifier: imeko-acta-07 (2018)-01-12 section editor: jorge torres-guzman, cenam – centro nacional de metrologia, santiago de querétaro, mexico received july 12, 2017; in final form november 15, 2017; published march 2018 copyright: © 2018 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: this work was supported by bureau of standards, metrology & inspection, m.o.e.a., r.o.c. corresponding author: sheng-jui chen, e-mail: sj.chen@itri.org.tw 1. introduction the pumping speed is one of performance parameters of a vacuum pump. it specifies the volume flow rate at which gas is evacuated through the pump inlet port. the pumping speed is pressure and gas dependent. it has a dimension of volume per unit time such as m3/s, l/s or m3/h (cubic meter per hour, cmh). another important parameter is the ultimate pressure which is the lowest pressure a pump can achieve. in most applications, the pumping speed given in the pump specification is usually sufficient for designing a vacuum system. however, in some applications, there are still needs to measure the pumping speed in order to verify the design or to check whether the pump is properly installed and conditioned. in this work, we measured the pumping speed of a screw pump according to the standard throughput method described in iso 21360-1 [1]. the screw pump is installed in a semiconductor fabrication building and designed to evacuate gas at a very high speed. to verify the pump performance on site, we designed and fabricated a pumping speed measurement system to fit this purpose. in the following sections, we introduce the principle of measurement, experimental setup and the results of this work. 2. measurement principle and experimental setup since mass is conserved in a flow system, the rate of change of gas mass is the same at any cross section in the flow system. using ideal gas equation, the rate of change of gas mass can be written as:       = tk pv dt d m dt dn m b )( , (1) where m is the molar mass of the gas, n is the number of gas molecules in a given volume v, p is the absolute pressure, t is the absolute temperature and kb is the boltzmann constant. assuming the number n of gas molecules is also conserved, i.e. no chemical reactions between gas molecules, and the abstract ultimate pressure of a vacuum system is determined by two parameters, namely the total gas load of vacuum system and the pumping speed (volume flow rate) of vacuum pump. after the total gas load of a system is estimated, the required pumping speed can be set. in this study, we constructed a system for measuring the pumping speed of vacuum pump according to iso 21360-1:2012, in which three methods are described, i.e. the throughput method, the orifice method and the pump-down method. the vacuum pump under test is designed to be used in low vacuum range for evacuating a chamber at high pumping speed. for this reason, the throughput method was selected as the main method. the system consists of pressure gauges, thermometers, a flow meter and a test chamber. the system was used to measure the pumping speed at the inlet of the vacuum pump at several pressure points. we present the system setup, uncertainty evaluation and vacuum-pump performance results of this work. acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 66 temperature t is the same in the given volume, the rate of change of gas molecules is written as: ( ) tk q dt pvd tkdt dn bb == 1 , (2) where q = d(pv)/dt is the throughput of the gas. when pressure is held at a constant value, the throughput can be expressed as vpqdt dv pq == , (3) where qv is the volume flow rate. according to the throughput method in iso 21360-1, we set up a vacuum-pump pumping speed measurement system to testing the screw pump (sullair vs20-75). the measurement system consists of a vacuum chamber, a connection vacuum pipe, a capacitive absolute pressure gauge (mks 690a13tra), a portable pressure gauge (ruska 6220-19) and a rotary position gas flowmeter (elster instromet irm-a g160). each instrument used in the system is traceable to the corresponding standard in national measurement laboratory (nml). the schematic is shown in figure 1. considering the height of the screw pump and the easiness of installation, the connection vacuum pipe was used in the practical setup, making the setup slightly deviate from the standard configuration. however, the pressure and temperature were measured at positions very close to the air inlet of the screw vacuum, and therefore the conductance of the vacuum pipe will not affect the measured pumping speed at the air inlet. figure 2 shows the practical configuration of the measurement system, along with parameters to be measured at corresponding positions. the parameters include pressures p0, p1 and temperatures t0 ,td at the input port of the flowmeter and at the air inlet of the pump under test, respectively. the gas flowmeter is a high accuracy positive displacement meter which is insensitive to changes in the relative density of the gas to be measured. a fixed volume vvf = 1/806.313 m3 of the gas is displaced by each revolution of rotors inside the flowmeter. a frequency counter (hp 53132a) was used to measure the frequency of the rotor revolution signal output from the flowmeter. from the measured frequency fvf, the volume flow rate qvf (m3/h) at the flowmeter can be obtained by qvf = 3600 ⋅ vvf ⋅ fvf . to allow the correct use of the flowmeter, the measurements were made with increasing pressure. to begin the measurement, firstly the base pressure pb of the system was determined by evacuating the system with all gas inlet valves closed and observing the pressure p1. when no pressure drop was observed, the final pressure p1 was the base pressure pb. the pressure range for pumping speed measurement was from 10 torr to 110 torr with an increment of 10 torr. the adjustable gas inlet valve was opened to make pressure p1 equal to the set pressure point. at each pressure point, all the measurement parameters were repeatedly acquired and statistically processed by a labview program. 3. data analysis and results from equation (2) and (3), the rate of change of gas molecules at the input port of the flowmeter is the same as that at the air inlet of the screw pump: ( ) v db b vf b q tk pp q tk p − = 1 0 0 , (4) where the base pressure pb is introduced in the equation to figure 1. schematic of the pumping speed measurement system. figure 2. practical configuration of the measurement system. 1 2 3 4 5 6 8 5 7 key: 1 vacuum chamber 2 flow meter 3 pressure gauge 4 thermometer 5 gas inlet valve 6 pump under test 7 capacitive pressure gauge 8 thermometer p0 t0 tdp1 qv pump under test air inlet working bench vacuum chamber vacuum pipe gas inlet valves flowmeter q qvf acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 67 reflect the fact that there will be no gas flow when the ultimate pressure pb of the system is reached. the pumping speed qv at the air inlet is calculated as )( 10 0 b dvf v ppt tpq q − = . (5) the uncertainty of pumping speed is estimated by [2] 2/1 2 1 222 0 22 0 2 )( 10 0               − + +      +       +      +      = b ppt d tp vf q vq pp uu t u t u p u q u qu b dvf v , (6) where ui is the uncertainty of parameter i. one of the measurement results are listed in table 1. the highest pumping speed was measured to be (1661 ± 45) m3/h, and this result agrees with the specification of the screw pump. two pumping performance curves are measured and plotted in figure 3 along with the data given in the specification of the screw pump. 4. discussion in this work, we demonstrated the on-site measurement of the pumping performance of the screw pump used in semiconductor industry. the standard throughput method was used in this measurement. the measured performance curve is compatible with the specification of the pump under test. the relative expanded uncertainties of the results are all within 5 % meeting the requirement stated in iso 21360-1:2012. references [1] iso 21360-1:2012, vacuum technology – standard methods for measuring vacuum-pump performance – part 1: general description. [2] iso/iec guide 98-3:2008, uncertainty of measurement — part 3: guide to the expression of uncertainty in measurement. table 1. pumping speed measurement results. the base pressure pb was measured to be 2.75 torr. p1 (torr) p1 std (torr) fvf (hz) fvf std (hz) qvf (m3/h) t0 (deg. c) t0 std (deg. c) td (deg. c) td std (deg. c) qv (m 3/h) expanded uncertainty of qv (m 3/h) 9.95 3.53e-2 2.19 1.27e-02 9.76 22.75 9.24e-03 21.68 8.04e-03 1007 42 19.99 2.10e-2 6.62 1.04e-02 29.56 22.76 6.46e-03 21.69 2.42e-03 1274 39 29.96 2.49e-02 11.42 1.46e-02 51.01 22.76 1.44e-03 21.64 2.05e-03 1392 40 39.89 3.80e-02 16.36 1.79e-02 73.06 22.74 8.82e-04 21.56 6.20e-03 1459 41 49.85 3.46e-02 21.65 1.36e-02 96.67 22.7 1.79e-03 21.37 2.92e-03 1521 42 59.90 3.23e-02 27.06 4.00e-02 120.80 22.68 6.48e-03 21.26 4.18e-04 1565 43 69.70 4.20e-02 32.34 8.25e-02 144.37 22.51 5.12e-04 21.22 4.27e-03 1595 44 79.80 4.90e-02 37.61 7.71e-02 167.91 22.43 2.16e-03 21.25 4.92e-03 1611 44 90.12 4.07e-02 43.22 2.79e-02 192.98 22.35 1.59e-03 21.16 1.55e-03 1631 44 99.75 5.07e-02 48.62 5.02e-02 217.10 22.26 4.36e-03 21.13 5.16e-04 1651 44 106.65 6.29e-02 52.43 9.08e-02 234.08 22.18 1.39e-03 21.12 4.18e-03 1661 45 figure 3. two pumping performance curves measured in this study. 0 200 400 600 800 1000 1200 1400 1600 1800 0 20 40 60 80 100 120 140 pu m pi n g s pe ed (m 3 / h) inlet absolute pressure (torr) p u m p i n g s p e e d m ea s u r e m e n t r e s u lt s run-1 run-2 spec. system for measuring vacuum-pump performance using the standard throughput method << /ascii85encodepages false /allowtransparency false /autopositionepsfiles true /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /error /compatibilitylevel 1.4 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false 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/hrv (za stvaranje adobe pdf dokumenata najpogodnijih za visokokvalitetni ispis prije tiskanja koristite ove postavke. stvoreni pdf dokumenti mogu se otvoriti acrobat i adobe reader 5.0 i kasnijim verzijama.) /hun 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can be opened with acrobat and adobe reader 5.0 and later.) >> /namespace [ (adobe) (common) (1.0) ] /othernamespaces [ << /asreaderspreads false /cropimagestoframes true /errorcontrol /warnandcontinue /flattenerignorespreadoverrides false /includeguidesgrids false /includenonprinting false /includeslug false /namespace [ (adobe) (indesign) (4.0) ] /omitplacedbitmaps false /omitplacedeps false /omitplacedpdf false /simulateoverprint /legacy >> << /addbleedmarks false /addcolorbars false /addcropmarks false /addpageinfo false /addregmarks false /convertcolors /converttocmyk /destinationprofilename () /destinationprofileselector /documentcmyk /downsample16bitimages true /flattenerpreset << /presetselector /mediumresolution >> /formelements false /generatestructure false /includebookmarks false /includehyperlinks false /includeinteractive false /includelayers false /includeprofiles false /multimediahandling /useobjectsettings /namespace [ (adobe) (creativesuite) (2.0) ] /pdfxoutputintentprofileselector /documentcmyk /preserveediting true /untaggedcmykhandling /leaveuntagged /untaggedrgbhandling /usedocumentprofile /usedocumentbleed false >> ] >> setdistillerparams << /hwresolution [2400 2400] /pagesize [612.000 792.000] >> setpagedevice an integrated circuit to null standby by using energy provided by mems sensors acta imeko issn: 2221-870x december 2020, volume 9, number 4, 144 150 acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 144 an integrated circuit to null standby using energy provided by mems sensors roberto la rosa1, a. y. s. pandiyan2, carlo trigona3, bruno andò3, salvatore baglio3 1 stmicroelectronics, italy 2 department of electrical and electronics engineering, imperial college london, uk 3 department of electrical, electronics and computer engineering, university of catania, italy section: research paper keywords: mems; zero-energy standby; power management; wireless sensor networks; lithium ion battery; wireless sensor networks; internet of things citation: roberto la rosa, a. y. s. pandiyan, carlo trigona, bruno andò, salvatore baglio, an integrated circuit to null standby using energy provided by mems sensors, acta imeko, vol. 9, no. 4, article 19, december 2020, identifier: imeko-acta-09 (2020)-04-19 section editor: francesco bonavolonta, university of naples federico ii, italy received october 31, 2019; in final form june 23, 2020; published december 2020 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: roberto la rosa: roberto.larosa@st.com; carlo trigona: carlo.trigona@dieei.unict.it 1. introduction in response to the advent of emerging technology such as iot, industry 4.0 and smart solutions in electronics and measurement, an interesting development has recently appeared regarding ultra-low-power solutions, self-powered methods, energy harvesting and novel conditioning circuits for sensors and transducers [1]-[4]. the common goal is to realise a sensing node having the prerogative to absorb small amounts of electrical power. these kinds of circuits are prominent in several applications, such as biomedical [5], industrial [6], smart solutions such as laptops, smartphones and gaming devices [7] and intelligent systems and novel measurement architectures such as smart homes, smart cities, etc. [8]. along with reducing the size of electronic components and sensors (mems), manufacturers also focus on producing devices that consume less power. it is worth noting that battery life must be maximised, since replacement poses a real problem in node maintenance. in fact, this task is often very costly and difficult to perform; at times it is even impossible because sensor nodes are often positioned in inaccessible areas. various strategies have been addressed in order to reduce maintenance issues in measurement systems that are often disseminated. some solutions, such as those implementing real time sensors, may require batteries; other systems, such as low duty cycle sensors and measurement architectures, may be able to function appropriately without batteries in specific working conditions [9]-[14]. other solutions regard methods able to reduce or eliminate the power consumption during standby mode [15], [16]. in fact, several electronic parts need to be turned on only when an event occurs, being in standby the rest of the time with an associated power consumption. it must be observed that during standby, most of the electronic parts are turned off except for the power management abstract the advent of smart measurement systems and innovative wireless sensor networks evinces the necessity to develop novel solutions for conditioning circuits to be used in autonomous or quasi-autonomous measurement systems and sensing nodes. the main problem these systems still face is the question of how to supply the nodes in a cost-effective way, considering that, very often, a battery is required, and consequently, the maintenance labor and cost to replace or recharge it may be high. the main target is to increase battery lifetime by decreasing unnecessary energy consumption as much as possible. in this context, several solutions, including energy harvesters, have already been proposed. one of the main solutions is the reduction of power consumption by the measurement device while in standby, which, in most cases, represents a significant amount of the total power dissipation. to this end, authors have already addressed a zero-energy standby solution able to supply the power requested by the measurement equipment only when the appliance is turned on. in this paper, we present an integrated circuit solution suitable to be used with mems scale transducers. the validation and the characterisation of the system will be shown to demonstrate the suitability of the proposed method. mailto:roberto.larosa@st.com acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 145 components that supply the microcontroller unit (mcu) and the front-end transmission, such as bluetooth and rf [17]. for this reason, power management circuits are in fact permanently on in standby, and, depending on the duty cycle, can consume unnecessarily high percentages of battery charge [18], [19]. standby leads to a significant savings, which can be translated into prolonged battery life or a reduction of battery size. this is beneficial in terms of cost and system miniaturisation [20]. with the aim of overcoming this problem, several solutions have been proposed in the literature regarding suitable design and circuit configuration [21]. a zero-standby solution, with discrete approach, has been presented by authors in order to remove the power dissipated during the standby of a measurement system, such as [15], [16] and [22]. authors have also addressed the idea of using the zerostandby solution in the presence of mems; however, this approach has only been demonstrated through simulation with the aim of implementing an integrated chip [23]. in contrast to that study, the solution presented here involves more exhaustive simulations, the design of an integrated circuit and the experimental validation of the principle. it is worth noting that the proposed solution is suitable for mems and microscale transducers able to generate very weak voltage signals. this study used a device based on piezomumps technology [24], which represents an interesting solution in order to realise self-generating integrated sensors. this family of device can produce output voltages from tens to a few hundreds of mv. it is worth noting that a more generalised study and characterisation (based on various voltage levels) of the designed and realised integrated conditioning circuit will be addressed in the experimental section, showing very intriguing features. the paper is organised as follows: section 2 describes the working principle. section 3 reports the system description and simulations. section 4 shows the setup and the results, section 5 presents an analysis of the system, and the conclusions are given in section 6. 2. working principle and mems technology this section addresses the idea of turning on a device that is completely off using only the energy provided by a mems device used as an energy transducer. the concept presented in this section goes beyond the standby functionality of a device, since it is intended to toggle the device from its off to its on state without an intermediary standby state. a mems device is advantageous compared to other existing technologies, like pzt, due to the maturity and the wide diffusion of this technology. this inherently leads to low cost solutions and higher reliability. furthermore, mems can also be adapted for sensing applications, creating an integrated system for cost optimisation. thus, the aim is to use the mems device both as an energy transducer to turn on the application and as a sensor while the system is active. however, there are major challenges to achieving this. in fact, a mems device is normally used as a sensor rather than as an energy transducer due to its small size and energy density (in the order of few nw). it also generates very low open circuit voltage, as low as 100 mv. these issues pose problems in the very early stage of the design. to tackle this, the mems must be designed in order to be energy efficient while also having an open circuit voltage high enough to trigger the system to turn on. in this context, piezomumps technology [24] was used for the simulation and as a reference voltage level for the integrated circuit in the zero standby method. figure 1 shows some details of the mems process. as can be noted, the mems device was based on a silicon on insulator wafer, which was used as a substrate. the 10 μm of silicon was a doped layer used to contact the aln layer, having a thickness of 0.5 μm. an insulation layer of thermal oxide with a thickness of 200 nm separated the silicon layer and the aln. the last layer was a metal stack composed of 200 nm chrome and 1,000 nm of aluminium. it was used as a second electrical contact for the aln material. a deep reactive ion etching procedure was used to create a suspended structure. figure 2 shows the layout of the mems, and figure 3 shows the realised prototype that figure 1. mems process used to realise integrated-scale sensors with output voltage thanks to the aln layer. figure 2. layout of the mems, with several transducers in order to generate several output voltages. figure 3. picture of the realised bonded device. acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 146 was used for simulations (see section 3) and as reference voltage source for the experiments. it is worth mentioning that the realised mems can generate a voltage output between 10 mv and hundreds of mv. because of this, a suitable design for the conditioning circuit for the zero standby device must be addressed. this discussion will be presented with the design, realisation and characterisation in section 4. 3. system description and simulations figure 4 represents a battery driven system such as a wsn that is composed of three main sections: 1) the vibration-based transducer, 2) a power management and 3) the final load, which in a typical application consists of sensors, micro controller units and generic transceivers (i.e. bluetooth, rf, wi-fi, etc.). the proposed vibration-based transducer for zero-energy standby applications is composed of two sections: (a) a mems device that works as a mechanical to electrical energy transducer able to convert external vibrations such as shocks and induced movements into electrical energy; and (b) a logic architecture able to efficiently manage power and convert the low voltage output of the mems device into level switching voltage. the idea is to supply the power requested by the electronic equipment only when the appliance is turned on by inducing kinetic energy through an impressed movement mems energy transducer that converts kinetic movement to an electric signal, vmems. when enough energy is provided to the mems harvester, it is able to connect the power path between the battery source, vcc, and the output of the power management block, vout. this, in turn, supplies a regulated voltage to the loading section. the logic section of the power management block does not have any static power consumption as long as the enable signal is low, and the leakage current flowing through the battery (few pa) is, in practice, negligible. this block also ensures that the enabling signal of the voltage regulator can stay on even when the kinetic energy feeding the mems device has faded out. since the output voltage of a mems is very low (~ 100 mv), it would be insufficient to enable the voltage regulator, which normally requires a voltage as high as vcc to be turned on. for this reason, a special level shifter is needed to interface the mems-based transducer with the enable digital input of the voltage regulator. the main purpose of the level shifter is to convert the small voltage provided by the mems device when moved by a mechanical force into a suitable logic level to enable the power management to bias the sensor node. in order to null standby-power consumption, it is fundamental that the circuit does not consume any power when the system is off. with reference to figure 5, a visual inspection of the circuit shows that if the voltage vmems is zero, then transistor m9 is off. also, the voltage vmid will be high and equal to vcc, accounting for the leakage in the transistor. therefore, the inverter with transistors m2 and m3 is in a digital stable state with vmid1 set to zero and no current flowing through the transistors. the rest of the circuitry is digital, and thus does not consume any power, as shown in figure 6a. as shown in figure 6b, the final flip-flop stays in its reset status so that the enable signal ven is low and the power management is off; therefore, the rest of the system does not consume any power. the working principle of the level shifter can be explained with reference to the simulated output as shown in figure 7. once displacement has been induced in the system, the mems device provides the signal vmems to the input at the gate of transistor m9. this, working sub-threshold, is able to pull down the voltage vmid when a small current flows through resistor r1. the following transistors will add gain to the signal vmid at each stage, so that the signal vmid3 will have sharp edges to latch the signal ven through the d type flip-flop (f1). consider a stationary system where there is no displacement. therefore, vmems = 0, and the current through resistor r1 is zero. after giving an external displacement to the system att = 550 µs, the vmems has a peak voltage that allows a small current to flow through the large resistor r1. this, in turn, provides a voltage figure 4. system block diagram. figure 5. schematic diagram of the proposed level shifter. a) b) figure 6. simulation results of the circuit with respect to the system standby and active state. a) current consumption during start up, b) shift of enable signal from mems device. mems en cc out acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 147 drop in the resistor of 2 v and brings vmid to 1 v, as the bias across the branch is maintained by the supply at 3 v. since the voltage threshold of transistor inverters m2 and m3 is higher than 2 v, the signal vmid gets inverted to vmid1. the following transistors will buffer the signal vmid1, providing the signal vmid3 that is used to latch the d type flip-flop for a high voltage output q, so that the signal ven goes high. it can be observed that once turned on, even if the system becomes stationery, the enable signal is maintained at vcc irrespective of vmems. when needed, the flip-flop can be turned off by providing the high signal to vclr in order to reset it. this resets the signal ven to zero, which will turn off the system with no current consumption until the next impulse displacement is probed. 4. integrated circuit and experiments in order to support the simulation results, the proposed schematic was fabricated in the layout shown in figure 8a into a die of dimensions 4 mm × 4 mm. a picture of the die is shown in figure 8b. the die was then mounted on a pcb for external circuitry, as shown in figure 8c. to characterise the circuit alone, an initial experimental setup was made to evaluate the triggering voltage of the level shifter with controlled dc voltages. this test setup consisted of a 3 v battery supply for vcc, powering the level shifter and a voltage divider circuit. the input of the circuit was fed by the voltage across the network of resistors varying from 52 mv to 800 mv. the fabricated die was also covered by an opaque sleeve to reduce the leakage current due to light, and the circuit appeared to toggle at voltages as low as 52 mv. however, the required duration of the input was observed to be more than the simulation results, at an input voltage of 100 mv. it was also noticed that at slightly higher voltages (~ 270 mv) the toggle of the circuit was almost instantaneous, in the range of ms. the results of the toggling speed at 92 mv and 270 mv are compared in figure 9. following the above observations, the integrated conditioning circuit was characterised based on the behaviour of the vin, and vout in terms of response time δt using the same setup. the results of this experiment are as summarised in figure 10. it can be understood that the behaviour of the chip was as anticipated, but the resulting capacitance of the die was more than expected. in the next section, the experimental study of the integrated circuit will be conducted, considering several voltage levels in accordance with the amplitude generated by the mems used in the simulation (see figure 3) and the fabricated chip (see figure 8). figure 7. analysis of the different node voltages in the circuit during turn-on sequence. a) b) c) figure 8. integrated conditioning circuit for zero standby: a) layout view, b) picture of the fabricated die, c) fabricated die mounted on a printed circuit board. acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 148 5. system analysis in order to study the chip for the zero standby device, an inverter (st m74hc132b1) and a d type flip-flop (st m74hc74b1) were connected to the setup as shown in figure 11. the board included the voltage divider branches, the die with the pcb, the inverter, the d type flip-flop and the battery. figure 12 shows the experimental setup used to characterise the system. in accordance with the simulation, the output q of the flip-flop was set to high as long as the flip-flop was externally reset through vclr. figure 13 represents the different results obtained from the entire circuit shown in the set up. clearly, there is a delay time and a need of constant input at very low voltages (100 mv). but interestingly, the ven does not toggle to digital high unless the vclr signal is given. this gives the user control over the active time of the sensor node/iot device, allowing it to stay on until a vclr is programmed from the mcu. to accurately test and characterise the chip, the voltage divider bridge was replaced by an amplified piezoelectric actuator (apa). apa 50xs from cedrat technologies was used in this experiment. the apa consisted of a rigid external shell used to produce stimulus at various voltage levels, casing a multi-layered piezoelectric stack. therefore, when subjected to vibration, the kinetic force was translated into electrical voltage due to the piezoelectric effect, and the voltage was fed into the input terminal of the chip. the apa was given a short mechanical impulse, and the response of the whole system at vin, vclr, vmid3, and ven was measured. the results are as shown in figure 14, and they show very good agreement with the simulation results from the previous section. 6. conclusions in this paper, we addressed a zero-energy standby solution that is able to supply the power required by the measurement equipment to turn on an appliance only when needed. in particular, an integrated circuit (ic) solution suitable to be used with mems scale transducers was pursued. both the mems transducer and the integrated conditioning circuit were a) b) figure 9. results from the characterisation of the level shifter at various input voltages (vin): a) at vin = 92 , δt = 9 s, b) at vin = 270 , δt = 221 ms. figure 10. response time analysis for the input and output voltages of the mems level shifter. figure 11. experimental board combining the chip with st m74hc132b1 inverter and st m74hc74b1 d flip-flop. figure 12. experimental setup. acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 149 simulated, designed, realised and characterised, so that the zero-energy standby solution has been experimentally studied by using the realised devices. the ic is capable of working as an integrated level shifter with a very low threshold voltage (< 100 mv). this is advantageous and easily compatible with the majority of mems devices, as this value of voltage can be easily produced by several mems technologies. in this specific case, we chose the most commonly used mumps process, called piezomumps. moreover, the tests showed that the performance obtained positively confirmed the purpose of the feasibility study. in addition to validating the methodology and the architecture of the system, the experiment provided relevant ideas for future improvements and optimisation for the performance of both the mems and the integrated conditioning circuit. in particular, it should be noted that the level shifter requires minor improvements in terms of switching times, whereas for mems it is necessary to increase the open circuit voltage for low mechanical stresses. continued study is in progress with a new design of both silicon-based devices in flip chip configuration in order to implement small scale measurement systems based on a zero-energy standby solution. references [1] a. burdett, ultra-low-power wireless systems: energy-efficient radios for the internet of things, ieee solid-state circuits magazine 7 no. 2 (2015), pp. 18-28. doi: https://doi.org/10.1109/mssc.2015.2417095 [2] s. park, s. w. heo, w. lee, d. inoue, z. jiang, k. yu, k. fukuda, self-powered ultra-flexible electronics via nano-grating-patterned organic photovoltaics. nature 561 no.7724 (2018), pp. 516–521. doi: https://doi.org/10.1038/s41586-018-0536-x [3] c. trigona, n. dumas, l. latorre, b. andò, s. baglio, p. nouet, exploiting benefits of a periodically-forced nonlinear oscillator for energy harvesting from ambient vibrations, procedia 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https://doi.org/10.1109/access.2017.2677521 acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 150 [13] t. soyata, l. copeland, w. heinzelman, rf energy harvesting for embedded systems: a survey of tradeoffs and methodology, ieee circuits and systems magazine 16 no. 1 (2016), pp. 22-57. doi: https://doi.org/10.1109/mcas.2015.2510198 [14] r. la rosa, c. trigona, g. zoppi, c. di carlo, l. di donato, g. sorbello, rf energy scavenger for battery-free wireless sensor nodes, proc. of ieee international instrumentation and measurement technology conference (i2mtc), houston, usa, 14-17 may 2018, pp. 1-5. doi: https://doi.org/10.1109/i2mtc.2018.8409738 [15] c. trigona, b. andò, s. baglio, r. la rosa, g. zoppi, vibrationbased transducer for zero-energy standby applications, proc. of ieee sensors applications symposium (sas), catania, italy, 2022 april 2016, pp. 1-4. doi: https://doi.org/10.1109/sas.2016.7479817 [16] c. trigona, b. andò, s. baglio, r. la rosa, g. zoppi, sensors for 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24-26 august 2016, pp. 166-172. doi: https://doi.org/10.1109/cse-euc-dcabes.2016.179 [20] r. la rosa, n. aiello, g. zoppi, rf remotely-powered integrated system to nullify standby power consumption in electrical appliances, proc. of iecon 2016, 42nd annual conference of the ieee industrial electronics society, florence, italy, 24-27 october 2016, pp. 1162-1164. doi: https://doi.org/10.1109/iecon.2016.7793079 [21] k. h. jones, j. gross, reducing size, weight, and power (swap) of perception systems in small autonomous aerial systems, proc. of 14th aiaa aviation technology, integration and operations conference, atlanta, usa, 16-20 june 2014, aiaa 2014-2705, pp. 1404-1412. doi: https://doi.org/https://doi.org/10.2514/6.2014-2705 [22] r. la rosa, p. livreri, c. trigona, l. di donato, g. sorbello, strategies and techniques for powering wireless sensor nodes through energy harvesting and wireless power transfer. sensors 19 no. 12 (2019) 2660. doi: https://doi.org/10.3390/s19122660 [23] r. la rosa, c. trigona, b. andò, s. baglio, mems based transducer for zero-energy standby application, proc. of ii workshop on metrology for industry 4.0 and iot (metroind4. 0&iot), naples, italy, 4-6 june 2019, pp. 12-15. doi: https://doi.org/10.1109/metroi4.2019.8792888 [24] memscap, piezomumps. online [accessed 30 october 2020] http://www.memscap.com/products/mumps/piezomumps https://doi.org/10.1109/mcas.2015.2510198 https://doi.org/10.1109/i2mtc.2018.8409738 https://doi.org/10.1109/sas.2016.7479817 https://doi.org/10.1109/tim.2016.2644838 https://doi.org/10.1109/tmc.2014.2315788 https://doi.org/10.1109/tpel.2011.2162530 https://doi.org/10.1109/cse-euc-dcabes.2016.179 https://doi.org/10.1109/iecon.2016.7793079 https://doi.org/https:/doi.org/10.2514/6.2014-2705 https://doi.org/10.3390/s19122660 https://doi.org/10.1109/metroi4.2019.8792888 http://www.memscap.com/products/mumps/piezomumps acta imeko, title acta imeko issn: 2221-870x june 2018, volume 7, number 2, 65-72 acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 65 numerical investigation of optimal dynamic measurements aleksandr l. shestakov, georgy a. sviridyuk, alevtina v. keller, alyona a. zamyshlyaeva, yurii v. khudyakov south ural state university,lenina, 76, chelyabinsk, russia section: research paper keywords: mathematical model of measuring transducer; optimal dynamic measurement; set of admissible measurements citation: aleksandr l. shestakov, georgy a. sviridyuk, alevtina v. keller, alyona a. zamyshlyaeva, yurii v. khudyakov, numerical investigation of optimal dynamic measurements, acta imeko, vol. 7, no. 2, article 12, june 2018, identifier: imeko-acta-07 (2018)-02-12 section editor: franco pavese, italy received december 16, 2017; in final form may 11, 2018; published june 2018 copyright: © 2018 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: the work was supported by act 211 government of the russian federation, contract № 02.a03.21.0011 corresponding author: alevtina keller, e-mail: kellerav@susu.ru 1. introduction currently, the theory and practice of dynamic measurements are developing in various directions. for a long time the theory of inverse problems [1], methods of direct and inverse fourier transforms were the mathematical basis of dynamic measurements. in addition, for more than 20 years methods of the remote control theory were successfully applied for solving various problems of dynamic measurements. in the recent years, a new approach was developed [2] to restore the input signal distorted by the measuring transducer and external interference, called by the authors “the theory of optimal dynamic measurements”. we emphasize that the theory of optimal dynamic measurements is based on three fundamental components: 1) the solution of dynamic measurements problems using the ideas and methods of the remote control theory; 2) the development of the descriptor systems theory and its applications [3]; 3) the development of methods for the solution of sobolev type equations and corresponding optimal control problems, including numerical methods for solving optimal control problems for leontief type systems. leontief type systems are finite-dimensional case of sobolev-type equations and a special case of descriptor systems (with constant matrices). this report presents recent results in the theory of optimal dynamic measurements. 2. mathematical modeling of optimal dynamic measurements we first introduce mathematical model of optimal dynamic measurements (figure 1). mathematical model of measuring transducer (mt) is represented by the leontief type system of equations , , lx ax bu g y cx d ς η = + +  = +  (1) where l and a are matrices that characterize the structure of the mt. in some cases it is possible that det 0l = (this will be discussed in the next section); ( )x t and ( )x t are vector functions of the state of the mt and the velocity of the state change, respectively; y(t) is a vector-function of observation; c and d are rectangular matrices characterizing the interrelation between the system state and observation; ( )u t is a vectorfunction of measurements; b is the matrix characterizing the abstract the basic ideas of mathematical modeling of optimal dynamic measurements are presented. the key thing is to construct a mathematical model of the measuring transducer, which allows simulating also a complex measuring transducer. in this manuscript we propose such a mathematical model and we discuss its results obtained through numerical studies in a variety of cases: from the simplest, without considering disturbances, to the case when a deterministic disturbance, for example resonances, occur at the input and at the output of the measuring transducer. lastly, the importance of the description of the set of admissible measurements in the modeling of dynamic measurements is discussed. acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 66 interrelation between the system state and measurement; ( )tς is a vector-function of resonances in the circuits of the mt (see figure 1); g is the matrix characterizing influence of resonances in the circuits of the mt; ( )tη is a vector-function of disturbance at the output of the the mt (see figure 1). if l is not degenerate then system (1) can be reduced to , , x mx fu h y cx d ς η = + +  = +  (2) where 1 ,m l a−= 1 ,f l b−= 1 .h l g−= this system appeared in the dynamic measurements theory from the remote control theory, where (2) was obtained in the study of the transfer function of the mt. finally, in accordance to new approaches in the field of measurements, it is possible to replace the terms "input signal" with "measurement" and "output signal" with "observation" [4], respectively. we emphasize that all observations and measurements in systems (1) and (2) are simulated or "virtual". the initial showalter – sidorov condition ( ) ( ) 11 0(0) 0 p l a l x xα +− − − =  (3) for some 0 , nx r∈ ( )l mα ρ∈ , reflects the initial state of the mt. the initial showalter – sidorov condition is equivalent to the initial cauchy condition ( ) 00x x= when 0detl ≠ . for the investigation of problem (1), (3) introduce the space of states 2 2{ ((0, ), ) : ((0, ), )} n nx x l r x l rτ τ= ∈ ∈ and the space of observations [ ]y c x= . the key concept is the optimal measurement simulated by solving the optimal control problem for (1), (3). to find it, we introduce the space of measurements ( 1) 2 2{ ((0, ), ) : ((0, ), )} n p nu u l r u l rτ τ+= ∈ ∈ and allocate it in a closed convex set of admissible measurements u u∂ ⊂ 2( ) 0 0 ( )q q u u u u t dt d τq ∂ =   = ∈ ≤    ∑∫: . (4) the latter contains a priori information about measurements (this will be discussed in the sixth section of the paper). in the optimal measurements theory, as in the control theory, several types of cost functionals can be used depending on the goals of restoration of a dynamically distorted signal. consider a general cost functional ( ) ( )1 0 2( ) ( , , , ) ( ) , ,j u j y u t y t j uς η ς= − + (5) where 0 ( )y t , [0, ]t τ∈ is a continuously differentiable function constructed on the basis of the observed values 0iy at the output of the real mt, which is called “real observation” in the mathematcal model. the first term of the cost functional reflects the closeness of the real observation 0 ( )y t and the virtual observation ( )y t , obtained on the basis of mathematical model of the mt. the second term acts as a filter in the presence of resonances ( )tς in the chains of the mt. in case of lack of resonances ( )tς in the chains of the mt, functional (5) consists only of the first term. the minimum point ( )v t of the functional (5) on the set u∂ , being a solution of optimal control problem ( ) min ( ) u u j v j u ∂∈ = (6) is called an optimal dynamic measurement. in practice, there is only indirect information about ( )v t . note that quadratic functionals containing the observational error and its derivative have also been considered in the remote control theory, moreover issues of stability arising in connection with this were actively investigated. one of the first works that used the criterion of quadratic error is the article by a. a. kharkevich [5]. it describes methods for estimating distortions introduced by linear systems. in the 50s the integral criteria of quadratic error began to be used: 1) to search the optimal parameters of the system modeling the dynamics [6], [7] and 2) to solve the problem of synthesis of the regulator [8]. it should be noted that numerical methods for the calculation of the real automatic systems were developed by k. merriam [9]. solutions were also made by working in the frequency domain and by using the spectra of disturbances and formulas for squared deviations, which allowed to reduce the problem of finding the best operator to the search of a function, delivering an extremum to the mean-square functional [10]. 3. mathematical model of a complex measuring transducer as a leontief type system the mathematical model of a complex measuring transducer involves several subsystems, each of which represents a model of one measuring transducer (or simple mt). differential equations in the system reflect a complex of dynamic system elements (subsystems), and algebraic equations represent the connection between dynamic elements. theoretical aspects of the construction of mathematical models of the complex mt are based on the ideas of the descriptor systems theory [11]. as an example, we present the mathematical model of the complex mt, using the iterative principle for the restoration of a dynamically distorted signal, which allows to achieve a high dynamic accuracy during dynamic measurements [12]. a structural scheme of the iterative mt is shown in figure 2 for the case n=3. the mathematical model of each simple transducer is represented by system: figure 1. structure elements of mathematical model of optimal dynamic measurements. acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 67 , . i mi i mi mi mi mi i z a z b u y c z = +  =  the error identified by the previous iteration 1 , 1mi m iu u y −= − becomes the input of the i-th simple transducer miu , = 2, 3....i n . matrices , , , 1, 2,...mi mi mia b c i n= have the same meaning as the matrices , ,a b c in (1). therefore a mathematical model of the iterative mt can be represented by system 1 1 1 1 1 1 1 1 1 1 1 1 2 1 1 2 2 2 2 2 2 2 2 2 2 3 1 2 1 1 2 1 1 2 , , , , , , , ... , , ... ... . m m m m m m m m m m m m m m m m m m m n mn n mn mn mn mn n mn mn m m mn m m mn u u z a z b u y c z d u u y z a z b u y c z d u u y z a z b u y c z d y u u u u y y y η η η =  = +  = +  = −  = +  = +  = −    = +  = +  = + + + + − − − −    (7) which reflects the idea of correction of dynamic error in the iterative principle, which consists in the sequential use of a number of simple mts. the output of the system is presented by the sum of the observed signal and errors simulated by iterative chains. we rewrite the mathematical model of the mt, which is represented by the scheme of the iterative mt, in the form of matrix equations for the case = 2n . by introducing the notation ( )1( ) ( ), ( ), ( ), t m mx t z t u t y t y= , 1 1 2( ) ( , 0, , ) t m mu t u η η= , 1( ) ( )y t y t= . (7) can be represented in the form of a leontief type system , , lx ax bu y cx = +  =  where 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 i i l           =             , 1 2 1 2 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 m m d b d i i i d d           =             , (0, 0, 0, 0, 0, 0, )c i= . the use of leontief type systems in modeling of the mt allows: 1) to model complex measuring devices with different types of connections between the transducers, considering the various types of feedback; 2) to solve not only the problem of restoration of dynamically distorted signals, but also problems connected with restoration of the system states. 4. mathematical model of optimal dynamic measurements with inertia and methods for its numerical study subsections considering the mathematical model of optimal dynamic measurements taking into account only inertia [14] of the mt, i.e. under the assumption of absence of disturbance or their insignificance, system (1), which models the mt, can be written in more simple way , . lx ax bu y cx = +  =  (8) the initial condition (3) remains unchanged. by putting ( ) ( , )y t y u t= in the cost functional (5), we obtain 1 2( ) ( ) 1 0 1 0 ( ) ( ) ( , ) ( )q q q j u j u sy u t sy t dt τ = = = −∑∫ (9) where ( )0 01 02( ) ( ), ( ),..., ( ) t ory t y t y t y t= are real observations, 0 ( )sy t are significant real observations, i.e. the ones that are used for searching the optimal measurement (though one can watch a greater number of characteristics of the process); ( )y t are virtual observations, obtained during mathematical modeling of the restoration of dynamic measurement; ( )sy t are significant virtual observations, i.e. the ones that are simulated by "the virtual mt", which are used for the comparison with significant real observations. we emphasize that the cost functional (9) minimizes the difference between the real and virtual observations, and the difference between the derivatives of these processes ( ) min ( ) u u j v j u ∂∈ = . an approximate solution to the optimal measurement problem is searched in the form of vector-polynomials 1 2 0 0 0 ( ) , ,..., . t j j j j j nj j j j u t a t a t a t = = =   =     ∑ ∑ ∑     (10) figure 2. structural scheme of the iterative measuring transducer. acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 68 the polynomials power is determined based on the requirements of numerical methods and calculation accuracy. in the numerical procedure [13] we search the coefficients of the vector-polynomials for which the cost functional is minimal. note that the actual observation 0 ( )y t was constructed by an interpolation procedure based on observed values 0iy at the output of the real mt. the initial values of the coefficients vector of the polynomial (10) were taken equal to zero. because of this, the search for optimal dynamic measurement with a given accuracy used to spend considerable time. we suggested the modification of the numerical method by introducing an additional procedure into the algorithm that uses linear splines [14]. as a result, the algorithm contains the following steps. step 1. procedures of input and calculation of auxiliary parameters. step 2. splitting [0, ]τ into n intervals. consistent solving of the optimal measurement problem at each i -th interval 1[ , ]i iτ τ− (where 1,...,i n= ) using linear functions for real observation 0 ( )iy t , measurement ( )iu t and optimal measurement ( )iv t . as a result, on [0, ]τ we get a piecewise linear function with the values 0 0 ( )i i iv v τ= (figure 3). step 3. determination of the vector-function of real observations 0 ( )y t using the values 0iy and the interpolation procedure (figure 4). step 4. determination of the vector-function (10), whose coefficients will be the starting values for the numerical algorithm for the solution of the optimal measurement problem in the whole interval [0, ]τ (figure 4), using the values 0iv and the interpolation procedure. step 5. search for virtual optimal measurement on the segment [0, ]τ . it uses the idea of the multistage alternatingvariable descent method with memory (figure 5). 5. mathematical model of optimal dynamic measurements with inertia and resonances on the output of the measuring transducer with different a priori information one of the most important problems in the theory of dynamic measurements is the restoration of the input signal distorted by deterministic disturbance, i.e. interference that does not change from experiment to experiment. by deterministic we refer to disturbance caused by • resonances in the mt, which lead, for example, to the blur of the lance-like input signals. note that in this case it is assumed that frequencies of resonance disturbance are known; • impacts of different nature, with a known form of useful signal, for example, a variable of voltage power sources. we consider the modeling of optimal dynamic measurement under the assumption that the impact of deterministic disturbance is only on the output of the mt. in this case, the system modeling the mt has the form , . lx ax bu y cx dη = +  = +  (11) the initial condition (3) remains unchanged. by putting ( ) ( , , )y t y u tη= in the cost functional (5), we obtain 1 2( ) ( ) 1 0 1 0 ( ) ( ) ( , , ) ( )q q q j u j u sy u t sy t dt τ η = = = −∑∫ (12) note that the functionals (9) and (12) differ only by the functions of virtual observations, which are defined in systems (8) and (11) respectively. the virtual optimal measurement is determined by finding the minimum of the cost functional (12) ( ) min ( ) u u j v j u ∂∈ = . if the frequency of resonances in the output of the mt is known as a priori information then the function of real figure 3. step 2 of the proposed algorithm. figure 4. steps 3 and 4 of the proposed algorithm. figure 5. step 5 of the proposed algorithm. acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 69 observations 0 ( )y t is constructed by an interpolation of the observed values 0iy at the output of the real mt. if the form of the measurement function is known as a priori information, then the function of real observations 0 ( )y t is constructed by an approximation procedure of the observed values 0iy at the output of the real mt. the method of numerical solution of the optimal measurement problem taking into account inertia and disturbance at the output of the mt in detail was described earlier. an approximate solution is sought in the form ( )1 2( ) , ,..., t nu t u u u= =     1 2 0 0 0 sin , sin ,..., sin t j j j j nj j j j j a t a t a tϖ ϖ ϖ = = =   =     ∑ ∑ ∑    . (13) the deterministic disturbance is considered as a vector function, each component of which is the sum of the resonances at known frequencies 1 2 1 21 2 0 0 0 ( ) sin , sin ,..., sin m m tmm m j j j j j mj j j j t a t a t a tω ω ωη ω ω ω = = =   =     ∑ ∑ ∑ . (14) for the conduction of computational experiments, we construct a mathematical model of the mt, specified by a transfer function ( )gw p = ( )( )( )( )2 2 2 21 1 1 2 2 2 3 4 1 2 1 2 1 1 1t p t p t p t p t p t pξ ξ = + + + + + + with known parameters 1 0, 01t s= , 1 0, 6ξ = , 2 0, 002st = , 2 0, 2ξ = , 3 0, 0005st = , 4 0, 0001st = . the mathematical model of the mt taking into account resonances has the form: 1 1 2 2 1 2 3 2 3 4 4 3 4 5 4 5 6 5 6 6 0, 9881 , 10000 119 , 0, 999204 , 250000 119 , 2000 2000 , 10000 10000 , . x x x u x x x x x x x x x x x x x x x x y x η = − − +  = −  = − −  = −  = −  = −  = +       (15) for simplicity, b, c and d will be identity matrices. to evaluate the accuracy of the method, the found approximate optimal dynamic measurement is compared to the known input signal (precise dynamic measurement). note that in real problems the precise dynamic measurement is not known. example 1. we consider the case when the resonance appears only at the output of the mt. its frequency 5000ω = is known, so sin 5000a tωη = . figure 7 presents the real observation 0 ( )y t found as a result of computational experiment, the optimal virtual measurement ( )kv t  and a precise dynamic measurement ( )v t . in (13) frequencies 6, 25j jϖ = , 0,1, 2,... ,j =  32,= were taken, due to the amplitude-frequency properties of the transfer function of the mt (figure 6). as a result of computational experiment (figure 7) the amplitude of the resonance disturbance 0, 30151aω = was found, so 0, 30151sin 5000tη = . a virtual optimal measurement as an approximate solution of the optimal dynamic measurement problem was obtained 0,15638671sin(2 6,25 ) 0,24740745 sin(2 12,5 )v t tk π π= ⋅ + ⋅ +  0, 25821506 sin(2 18, 75 ) 0, 21717253sin(2 25 )t tπ π+ ⋅ + ⋅ + 0,15941696 sin(2 31, 25 ) 0,10642029 sin(2 37, 5 )t tπ π+ ⋅ + ⋅ + 0, 06576689 sin(2 43, 75 ) 0, 03746629 sin(2 50 )t tπ π+ ⋅ + ⋅ + 0, 018902669 sin(2 56, 25 ) 0, 00722448 sin(2 62, 5 )t tπ π+ ⋅ + ⋅ 0, 00014199 sin(2 68, 75 ) 0, 00397622 sin(2 75 )t tπ π+ ⋅ − ⋅ − 0, 00622392 sin(2 81, 25 ) 0, 007312764 sin(2 87, 5 )t tπ π− ⋅ − ⋅ − 0, 00769625 sin(2 93, 75 ) 0, 00766148 sin(2 100 )t tπ π− ⋅ − ⋅ − 0, 00738759 sin(2 106, 25 ) 0, 0069865873 in(2 112, 5 )t s tπ π− ⋅ − ⋅ − 0, 00652671sin(2 118, 75 ) 0, 00604986 sin(2 125 )t tπ π− ⋅ − ⋅ − 0, 00558016 sin(2 131, 25 ) 0, 00513176 sin(2 137, 5 )t tπ π− ⋅ − ⋅ − 0, 00471147 sin(2 143, 75 ) 0, 00432264 sin(2 150 )t tπ π− ⋅ − ⋅ − 0, 00396555 sin(2 156, 25 ) 0, 00363959 sin(2 162, 5 )t tπ π− ⋅ − ⋅ − 0, 00334281sin(2 168, 75 ) 0, 00307338 sin(2 175 )t tπ π− ⋅ − ⋅ − 0, 00282881sin(2 181, 25 ) 0, 002607121 in(2 187, 5 )t s tπ π− ⋅ − ⋅ − 0, 00698658726010777sin(2π 112,5t)⋅ − 0, 0073875884086711sin(2π 106,25t)− ⋅ − 0, 00240593sin(2 193, 75 ) 0, 00222346 sin(2 200 ).t tπ π− ⋅ − ⋅ figure 6. the amplitude-frequency properties of the transfer function of the mt. figure 7. results of computational experiment for the case of resonances at the output of the mt with known resonance frequency. acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 70 example 2. we consider the case when the resonances appear at the output of the mt, their frequencies are not known, and the form of the signal 22 1 3( ) a tu t a t e a−= + is known. figure 8 presents the observed values 0iy at the output of the the real mt, real observation 0 ( )y t approximating these values, virtual optimal measurement ( )kv t  found as a result of the computational experiment and precise dynamic measurement ( )v t . as a result of the computational experiment an approximate solution of the optimal dynamic measurement problem 2 250,24( ) 115002, 332 tkv t t e −= was obtained. 6. mathematical model of optimal dynamic measurements with inertia and resonances at the output and input of the measuring transducer we consider the model of an optimal dynamic measurement under the assumption on the impact of deterministic disturbances (resonances) both in the circuits of the mt and at the output of the mt. note that it is assumed that the frequency of resonance disturbance is known. in this case, the system that models the mt has the form (1) , . lx ax bu g y cx d ς η = + +  = +  the initial condition (3) remains unchanged. the cost functional (5) has the form ( )j u = ( ) 1 2 ( ) ( ) ( ) 0 0 1 0 ( , , , ) ( , , , ) ( , , )q q q q sy u t sy u t sy t dt τ ς η ς η ς η = = − − +∑∫ 1 ( ) ( ) 1 0 ( ) ( ), ( ) ( )q qq q n u t u t dt τ ς ς = + + +∑∫ (16) where 0 ( , , )ty ς η are real observations with zero input signal, 0 ( , , )y ts ς η are those real observations with zero input signal that are used for restoration, 0 ( , , , )u ty ς η are real observations with measurement, 0 ( , , , )y u ts ς η are those real observations that are used for restoration of a dynamically distorted signal with nonzero input signal; ( , , , )y u tς η are virtual observatins obtained in mathematical modeling of dynamical measurement, ( , , , )sy u tς η are those virtual observations that are used for restoration of a dynamically distorted signal in mathematical model. virtual optimal measurement is determined by finding the minimum of the cost functional (16): ( ) min ( ). u u j v j u ∂∈ = example 3. we consider the case when the frequencies of resonance at the output of the mt 5000ω = and resonance in the circuits of the mt 500ϕ = are known, so sin 5000a tωη = and sin 500a tϕς = . figure 9 represents the real observations 0 0( ) ( , , , )t uy y tς η= with zero input signal (this function is constructed by an interpolation procedure on the basis of observed values 0iy at the output of the real mt with zero values of measured signal). the real observations are 0 0( ) ( , , , )t uy y tς η= with nonzero input signal (this function is constructed by an interpolation procedure on the basis of observed values 0iy at the output of the real mt with nonzero values of measured signal); the virtual optimal measurement ( )kv t  is found in the computational experiment; the precise dynamical measurement is ( )v t . as a result of computational experiment the approximate amplitudes of the resonances were found: 0, 0300098aω = and 0, 2999991aϕ = , so 0, 0300098 sin 5000tη = and 0, 2999991sin 500tς = . 7. the set of admissible measurements in mathematical modeling of dynamical measurements an essential part of the optimal measurement model is a set of admissible measurements u ∂ , which in fact is a compact convex subset in the space of observations u and plays the same role as the set of admissible controls in optimal control theory: 2( ) 0 0 ( )q q u u u u t dt d τq ∂ =   = ∈ ≤    ∑∫: , (17) where q can take integer values from 0 to 1p + . the integral form of the set of admissible measurements allows to use a priori information about the nature of the measured process in time. note that the time integral of the square of the measured figure 8. results of computational experiment for the case of disturbances at the output of the mt with the known form of measured signal. figure 9. results of computational experiment for the case of disturbances at the input and output of the mt with the known resonance frequences. acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 71 process is the energy released during this process in the time interval [ ]0,τ . this information can be obtained on the basis of laboratory tests, physical model, values of its parameters, etc. moreover, this information may allow to allocate time intervals 1[0, ]τ , 2[0, ]τ [0, τ2], ..., [0, ]τ (or 1[0, ]τ , 1 2[ , ]τ τ , ..., 1[ , ]i iτ τ− ,…), reflecting several stages of the process, and to assess the set of admissible measurements at each of these intervals with constants 1d , 2d ,..., d , respectively. note that the more accurate estimates are the more significant is the use of the set of admissible measurements at various stages of the measuring process. as a measure of accuracy of the mathematical model one can use the difference 2( ) 0 0 ( )qk q d d d v t dt τq = − = −∑∫  where ( )kv t  is an approximate virtual optimal measurement. this approach was proposed to be used in a numerical algorithm to determine the stopping criteria of the algorithm, and selection of the sets of admissible measurements at different stages for more effective finding of an approximate solution [15]. we provide here an example about a combustion chamber: when comparing the speed and volume of delivery of the fuel mixture into the chamber, the calculated expected energy and burning rate, we can allocate time periods defining the stages of the combustion process, which will allow to estimate the value of energy released during these intervals. data from laboratory tests, physical models of process, etc, can be used. in the combustion process two phases can be selected – transition or start of combustion and the operating mode of the mixture combustion in the combustion chamber. the transition process begins at the moment of ignition, when the contained working mixture is ignited. the operating mode of combustion starts with the moment when the volume of flammable mixture becomes constant in time (i.e. the pressure in the chamber becomes constant in time). information about these phases allows to specify the set of admissible measurements, which will further improve the accuracy of virtual dynamic measurements. we consider two cases of definition of the set of admissible measurements. s1. the set of admissible measurements u∂ is given in the form (4) with 211, 6097d = , 0, 08τ = and 1q = , i.e. 0,081 2( ) 0 0 ( ) 211, 6097q q u u u u t dt∂ =    = ∈ ≤     ∑ ∫: . s2. the set of admissible measurements *u∂ is given as follows: 3 * 1 ,i i u u u u u∂ ∂ = = ∈ ∈       1: 0,0041 2( ) 1 0 0 ( ) 123, 006q q u u u u t dt∂ =    = ∈ ≤     ∑ ∫: , 0,0141 2( ) 2 0 0,004 ( ) 124, 6636q q u u u u t dt∂ = = ∈ ≤       ∑ ∫: , 0,081 2( ) 3 0 0,014 ( ) 38, 9579q q u u u u t dt∂ = = ∈ ≤       ∑ ∫: . figure 10 represents the solution of the optimal measurement problem in both cases s1 and s2: ( )kv t  and * ( )kv t  are the virtual optimal measurement for the case s1 and s2 respectively; 0 ( )y t is the real observation. 8. conclusion this paper presents a number of mathematical models of optimal dynamic measurements with the inclusion of deterministic disturbances in various cases. the results of computational experiments show the adequacy of the mathematical models and the efficiency of computational methods. in addition, we note that the theory of optimal dynamic measurements is developing in other directions, the results of which were not included in the paper: development of numerical methods using parallel computing; study of mathematical models of optimal dynamic measurements with a deterministic multiplicative effect on the state of the mt; study of mathematical models of optimal dynamic measurements with "noise" [16]. references [1] v.a. granovsky, dynamical measurements: fundamentals of metrology software (energoizdat, leningrad, 1984). (in russian) [2] a.l. shestakov and g.a. sviridyuk, a new approach to measurement of dynamically perturbed signals, bulletin of the south ural state university. series: mathematical modelling, programming and computer software 16 (192), 116 (2010). (in russian) [3] a.a. belov and a.p. kurdyukov, descriptor systems and control problems (fizmatlit, moscow, 2015). (in russian) [4] k.h. ruhm, measurement plus observation – a modern structure of metrology, in measurement in research and industry world congress international measurement confederation (xxi imeko), (prague, czechia, 2015). [5] a. a. kharkevich, application of quadratic error criterion to estimate a linear distortion, technical physics 7 (5), 515 (1937). (in russian) [6] v.v. solodovnikov, introduction to the statistical dynamics of figure 10. results of computational experiment for different sets of admissible measurements. acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 72 remote control systems (gostekhteorizdat, moscow, leningrad, 1952). (in russian) [7] sh. chang, synthesis of optimal systems of remote control (mashinostroenie, moscow, 1964). [8] d. newton, l. gould and l.d. kaiser, theory of linear tracking systems (fizmatgiz, moscow, 1961). [9] k. merriam, optimization. theory and calculation of systems of feedback control (mir, moscow, 1967). [10] a. bryson and ho yu-chi, applied optimal control theory (translated from the edition of 1969, mir, moscow, 1972). [11] guang-ren duan. analysis and design of descriptor linear systems, advances in mechanics and mathematics 23 (springer science+business media, llc, 2010) [12] a. l. shestakov, measuring transducer of the dynamic parameters with the iterative principle for signal restoration, devices and systems. management, control, diagnostics 10, 23 (1992). [13] a.l. shestakov, a.v. keller and e.i. nazarova, numerical solution of the optimal measurement problem, automation and remote control 1, 107 (2012). [14] a.a. ebel, on algorithm for numerical solution of optimal measurement problem using linear splines, journal of computational and engineering mathematics 3 (1), 37 (2016). [15] yu.v. khudyakov, оn adequacy of the mathematical model of the optimal dynamic measurement, journal of computational and engineering mathematics. 4 (2), 14 (2017). [16] a.l. shestakov, g.a. sviridyuk, a.v. keller and yu.v. khudyakov, the numerical algorithms for the measurement of the deterministic and stochastic signals, in springer proceedings in mathematics & statistics, vol. 113 (2015), pp. 183-195. https://elibrary.ru/item.asp?id=29454742 https://elibrary.ru/item.asp?id=29454742 https://elibrary.ru/contents.asp?issueid=1841980 https://elibrary.ru/contents.asp?issueid=1841980 https://elibrary.ru/contents.asp?issueid=1841980&selid=29454742 numerical investigation of optimal dynamic measurements infrared thermography technique (irt) for the evaluation of the hydric behavior of building stones acta imeko issn: 2221-870x october 2018, volume 7, number 3, 20 23 acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 20 infrared thermography technique (irt) for the evaluation of the hydric behavior of building stones giulia forestieri1, mónica álvarez de buergo2 1 facultad de ingeniería, universidad de la sabana, campus universitario del puente del común, chía, cundinamarca, colombia 2 instituto de geociencias igeo (csic-ucm), spanish research council – complutense university of madrid, madrid, spain section: research paper keywords: building stones; non-destructive tests; infrared thermography; hydric behaviour citation: giulia forestieri, mónica álvarez de buergo, infrared thermography technique (irt) for the evaluation of the hydric behavior of building stones, acta imeko, vol. 7, no. 3, article 5, october 2018, identifier: imeko-acta-07 (2018)-03-05 editor: sabrina grassini, politecnico di torino, italy received february 28, 2018; in final form september 14, 2018; published october 2018 copyright: © 2018 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: this work was supported by “programa de geomateriales 2 (s2013/mit-2914)” and by “fondi 5 per mille d.p.c.m. 23/04/2010” corresponding author: giulia forestieri, giulia.forestieri@unisabana.edu.co 1. introduction hydric tests are very important in the characterization of the porosity of stones, especially in terms of the movement of water through the pore system, which is the main factor controlling water uptake and its transport inside the stone itself. in particular, the open porosity of stones and their effective pore size determine the movement of water, as obtained by authors who based their research on the comparison of pore size distribution values of various building materials [1]. in this study, the infrared thermography technique (irt) is used to better individuate the water absorption/desorption of different stones simulating real conditions. a not so frequent approach is pursued focusing on water exchange properties of capillary rise uptake and evaporation of two different stones, with the aim of simulating masonry prototypes in laboratory as well as integrating traditional standardized techniques with nondestructive irt [2], [3], [4]. 2. materials and methods two types of building stones are investigated and tested by different techniques. 2.1. stone materials for the stone selection, different criteria are considered: the historical importance and the role of building stone materials in the calabrian (southern italy) context; the main schools of stonemasons and the related architectural models; the role of the quarry activity in the past and how it still influences the calabrian quarry market through the presence of active quarries [5]. two lythotypes of sedimentary stones are selected: 1) san lucido calcarenite (cs), a medium porous miocenic calcarenite quarried in “motta lupo” quarry in san lucido (39°18′ n and 16°03′ e), with micritic matrix and visible fossils. cs is classified as “biopelmicrite” and “with pores” [6] and its carbonate content is quite high, abstract the water distribution into stone specimens in laboratory conditions is evaluated through the infrared thermography method (irt). porous building stones samples (calcarenite and sandstone) are examined under stable laboratory conditions (controlled temperature and relative humidity) in order to simulate the same hydric behavior in real scale of material systems in situ. hydric tests monitored through irt are performed in order to analyze the capillary water absorption and evaporation transport phenomena into stone samples. irt technique allows to record thermal images at different intervals of time highlighting the internal capillary and evaporation rise heights, responsible for the majority of decay processes occurring in masonries. the geometric shape of the damped area and the time of spreading are directly related to the open porosity of the investigated stone materials. hydric tests are repeated for each splitting plane of the specimens (faces), in order to obtain useful results that could be applied for real masonries. results demonstrate the usefulness of irt as a non-destructive and portable technique in the field of new construction and for restoration purposes, as well as its importance in characterizing the physical stone features and the effectiveness of applied conservation treatments. mailto:giulia.forestieri@unisabana.edu.co acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 21 reaching 94 % at the maximum. it is also known as “mendicino stone” and due to its architectural importance has been investigated by many authors [7], [8]. 2) fuscaldo sandstone (af), a porous miocenic sandstone quarried in outcrops located in fuscaldo (39°24′ n and 16°01′ e). af contains silico-clastic matrix (50 %) and clasts (50 %), it is classified as “graywacke” and “with many pores” [6]. it is also commercially known as “sweet stone” due to its easy workability and principally employed as building material for structural elements such as buildings, arches and portals of many calabrian historical centres [5]. 32 cubic specimens (50 ± 5 mm side), are prepared for each lithotype, and their spatial coordinates (x, y and z) marked according to the quarrymen’s convention that is related to the anisotropy of the stones [9]. 2.2. experimental procedure specimen dimension is measured with a mitutoyo digital caliper with a precision of ±0.01 mm. measurements are recorded along each of the three orthogonal directions and averaged. the stone porosity structure is evaluated by means of mercury intrusion porosimetry (mip) with an autopore iv micromeritics mercury porosimeter, being the measured pore diameter range between 0.001 and 400 micrometers. the hydric tests performed are: i) capillary water absorption; and ii) desorption; iii) porosity accessible to water, real and bulk densities. the first one is carried out longitudinal and perpendicular to the anisotropic planes of stone specimens (x and z). specimens are placed in a water sheet of 3 (±1) mm height and the weight increase is measured at the time intervals indicated by current standards [10] until reaching constant mass. the capillary coefficients c1 (along x), c2 (along z) and the mean value cc are calculated. the second hydric test consists in determining the water desorption content (wi(ds)) [11]. after reaching the saturated conditions through the capillary test, the same specimens are let to drying under constant laboratory conditions, and their weight water content is registered and plotted as a function of time. open porosity is obtained employing an evacuation vessel for the water absorption by total immersion test under vacuum [12]. during the hydric tests, in order to provide thermal maps of the specimen surface areas in a black and white scale, in relation to a temperature scale and to visualize the moisture distribution by water movement (absorption, evaporation) into samples, an irt camera (thermacam b4 flir systems) is used. water areas present lower temperatures that can be detected by irt. the detection of water is registered as a thermal difference and irt images provide the measurement of the change of temperature due to the presence of water [13]. 3. results and discussion open porosity, pore size distribution and hydric parameters are reported in table 1 while the capillary absorption curve and the kinetic of desorption are plotted in figure 1 and figure 2, respectively. the first part of each curve of figure 1 defines the capillary water absorption, while the second part defines the water saturation. according to the values reported in table 1 and as shown in figure 1, cs shows the highest values of capillary absorption. while af demonstrates a different hydric behaviour according to the sample orientation, in contrast to cs that shows similar absorption curves along the two investigated directions. according to figure 1, af absorbs water more quickly along the x direction than when oriented with the z axis parallel to the capillary rise direction. by analysing figure 1 it is evident that cs absorbs water more quickly than af (if comparing the first part of the curve of the two stones), while af absorbs the major water quantity due to its higher open porosity (the second part curve), coherently to the porosity values reported in table 1. the difference of the capillary suction rate between cs and af is related to the pore diameter and open porosity [14]. cs, due to the presence of macroporosity, absorbs water faster than af, characterized by microporosity (table 1) [15]. the difference in the absorbed water quantity is to be related to the open porosity: as af exhibits higher open porosity than cs, it absorbs more water [1]. thus, capillary test curves perfectly plot what expected by the open porosity values obtained by the mip test. as well as for the capillary absorption, cs displays higher evaporation rate than af (figure 2) during the desorption process. within the first 3 hours the saturation is still about 90 %, for both materials. after the evaporation of the water from their surfaces, a pretty fast drying rate still keeps due to the internal figure 1. capillary water absorption (g/m2) vs. square root of the time (s0,5) along the two investigated directions (x and z). figure 2. water desorption curves. saturation (%) vs. square root of the time (h0,5) along the two investigated directions (x and z). 0 2000 4000 6000 8000 0 200 400 600 800 c a p il la ry w a te r a b so rp ti o n ( g /m 2 ) √ time (s0,5) cs_x af_x cs_z af_z 0 20 40 60 80 100 0 2 4 6 8 10 12 14 sa tu ra ti o n ( % ) √ time (h0,5) af_ds_x cs_ds_x cs_ds_z af_ds_z acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 22 absorbed water: within 21 hours (interval of time from 3 h until 24 h) a 60 % of their water content is lost. probably, this high percentage is due to a good connectivity of the porous system [1]. after this point, there is an abrupt change in the drying rate with a constant saturation of 10-15 %. thus, it could be summarized that each stone presents the same hydric behaviour during the absorption and desorption process: the higher the water absorption velocity, the higher the water desorption velocity and vice versa [16]. irt thermal images, obtained during hydric tests are shown in figure 3 and figure 4. irt thermal maps show a clear correlation among porosity, water absorption and evaporation. the cooling down of the damp areas on the specimen surface is the effect of the water (both in liquid and vapour phase) transport phenomena. irt images provide the measurement of the change of temperature due to these effects. on cs surface, the wet area is larger than on af specimens; the regular shape is due to the liquid, which regularly fills up all the open pores at disposal. the largest area is due to the rapid absorption of the drop by the surface rich in macropores, where water capillary spreading is faster. on af specimens, the wet area has undergone the largest increase during the observation time, nevertheless the shape becomes irregular and the contour line is unreadable and vague, so that it is difficult to measure. this behaviour can be attributed to the pore size distribution. the extension and shape of the wet moisture over the intervals of time changed for any tested materials, depending on their surface characteristics. moreover, comparing hydric absorption process figure 3. irt thermal images taken during the capillary water absorption test along the x direction of san lucido calcarenite (cs) and fuscaldo sandstone (af). figure 4. irt thermal images taken during the capillary water absorption test along the z direction of san lucido calcarenite (cs) and fuscaldo sandstone (af). acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 23 along the two investigate directions trough irt images, it can be observed how along z the water uptake (figure 4) is more irregular and slower than along x (figure 3), especially for af. this fact indicates that water rises with higher difficulty inside samples along z due the foliation planes orientated perpendicularly to the flux direction [17]. in the case of cs, no significant differences are detected for the two directions, coherently to the similar values obtained for the capillary coefficients c1 and c2 (table 1) and to the quite parallel slope of the curves reported in figure 1. 4. conclusions irt images result suitable and effective tests to evaluate the absorption capability and the evaporation of liquid water into building stones together with the performed hydric tests. the presented techniques allow to measure different characteristics of the exterior layer of stone building materials. irt shows good results indicating the variation into stones by measuring the changes of the surface temperatures due to absorption, diffusion and evaporation of water. the most influencing factor during the capillary test is related to pore features, especially in the very exterior layer of the materials, where the water exchanges occurs much more frequently than in the inner part of the stone. af shows a more anisotropic hydric behaviour than cs and absorbs more water than cs, coherently to the porosity values and the pore size distribution. each stone presents the same hydric behaviour during the absorption and desorption process. results obtained are useful when considering decay processes due to the water uptake, like salts or ice crystallization. for what concerning the hydric behaviour variability along the investigated directions, the way in which stones are positioned in construction could influence the capillary rise and, consequently, their durability. to minimize decay induced by water ingress and at the same time to improve stone durability, it is suggested to place (or lay) stones with their higher anisotropic hydric behaviour vertically along the z axis, where the water uptake registered is lower. in the case of af, as resulted more anisotropic than cs from a hydric point of view, this stone should be positioned along the z axis in order to minimize the water content by capillary absorption. moreover, irt might be used as an important non-destructive technique to evaluate the performance of conservation interventions and materials, in compatibility to the original materials on the level of the structures. indeed, irt recording thermal maps of real surfaces could provide useful information on the differential behaviour of the various materials on the masonry scale regarding the water impregnation and evaporation phenomena, which control decay effects in porous media. acknowledgements this study is supported by “geomateriales 2 programme (s2013/mit-2914)” and by “fondi 5 per mille d.p.c.m. 23/04/2010”. references [1] e. molina, g. cultrone, e. sebastián, f.j. alonso, l. carrizo, j. gisbert, o. buj, the pore system of sedimentary rocks as a key factor in the durability of building materials, eng. geol. 118 (2011) pp. 110-121. [2] n. p. avdelidis, a. moropoulou, p. theoulakis, detection of water deposits and movement in porous materials by infrared imaging, infrared phys. tech. j. 44, 3 (2003), pp. 183-190. [3] a. tavukçuoğlu, a. düzgüneş, e.n. caner-saltık, ş. demirci, use of ir thermography for the assessment of surface-water drainage problems in a historical building, ağzıkarahan (aksaray), turkey, ndt&e int., 38, 5 (2005), pp. 402-410. [4] n. ludwig, e. rosina, a. sansonetti, evaluation and monitoring of water diffusion into stone porous materials by means of innovative ir thermography techniques, measurement (2017), dx.doi.org/10.1016/j.measurement.2017.09.002. [5] g. forestieri, a. campolongo, m. ponte, analisi storica e materica del materiale lapideo nel territorio di cosenza, proc. of 2nd history of engineering international conference, vol.1, apr. 22-23, 2016, naples, italy, pp. 213-222. [6] g. forestieri, m. álvarez de buergo, m. ponte, idoneidad de rocas como material constructivo y restauración mediante la evaluación de su comportamiento hídrico y fíisco-mecánico, proc. of ecpc national congress, vol.1, nov. 16-19, 2015, málaga, spain, pp. 23-27 [7] f. rodolico, le pietre delle città d’italia, second edition, le monnier, florence, 1995, pp. 427-429. [8] g. m. crisci, a. m. de francesco, c. gattuso, d. miriello, un metodo geochimico per la determinazione della provenienza di lapidei macroscopicamente omogenei. un esempio di applicazione sui monumenti del centro storico di cosenza, arkos 2 (2003), pp. 52-59. [9] g. forestieri, d. m. freire-lista, a. m. de francesco, m. ponte, r. fort, strength anisotropy in building granites, int. j. archit. herit. 8 (2017), pp. 1-13, doi.org/10.1080/15583058.2017.1354096. [10] en 1925, natural stone test methods determination of water absorption coefficient by capillarity, italian standards, milan, 2000. [11] normal 29, measurement of drying index, icr cnr, rome, 1988. [12] en 1936, natural stone test methods determination of real density and apparent density, and of total and open porosity”, italian standards, milan, 2001. [13] n.p.avdelidis, a.moropoulou, “applications of infrared thermography for the investigation of historic structures”, j. cult. herit. 5 (2004), pp. 119-127. [14] s. siegesmund, h. dürrast, physical and mechanical properties of rocks, in: stone in architecture. properties, durability, 5th edition. s.siegesmund, r.snethlage (editors.). springer, berlin, 2014, pp. 107-224. [15] f. de quervain, “technische gesteinskunde. lehrbücher und monographien aus dem gebiete der exakten wissenschaften”, mineralogisch-geotechnische reihe, bd 1, birkhäuser, basel, 1967. [16] r. fort, m. álvarez de buergo, m.j. varas-muriel, m. gomezheras, “anisotropy indices and the effects on the hydric behaviour of natural stone”, g.r.a. 12 (2010), egu2010-6830-1. [17] g. forestieri, m. ponte, “evaluation of hydric behavior into porous building stones by means of infrared thermography (irt)”, proc. of imeko international conference on metrology for archaeology and cultural heritage, october 23-25, 2017, lecce, italy, pp. 269-273. table 1. test results of san lucido calcarenite (cs) and fuscaldo sandstone (af). porosity values: macro (> 5 µm) and micro (< 5 µm) porosity [15], open porosity to mercury (po) values in percentage obtained by the mip test. hydric parameters: open porosity to water (pow), capillary coefficients along x (c1), along z (c2) and mean value (cc) expressed in g/m2s0.5, water content evaporated (wids) expressed in %, obtained by the capillary absorption and evaporation tests. samples > 5 µm (%) < 5 µm (%) po (%) pow (%) c1 (g/m2s0.5) c2 (g/m2s0.5) cc (g/m2s0.5) wids (%) cs 31.4 68.6 21.0 20.1 49.4 48.3 48.9 2.5 st. dev. 1.0 0.2 0.7 2.3 10.1 3.4 0.8 0.3 af 3.0 97.1 13.0 16.1 33.5 24.9 29.1 1.9 st. dev. 0.9 0.8 2.5 0.8 9.4 5.8 6.1 0.1 http://dx.doi.org/10.1016/j.measurement.2017.09.002 https://doi.org/10.1080/15583058.2017.1354096 3d survey and metric analysis of the late roman fort of umm al-dabadib (egypt) acta imeko issn: 2221-870x october 2018, volume 7, number 3, 57 63 acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 57 3d survey and metric analysis of the late roman fort of umm al-dabadib (egypt) fausta fiorillo1, corinna rossi1 1 politecnico di milano, department abc (architecture, built environment and construction engineering), milan, italy. section: research paper keywords: late roman forts; metrology; units of measurement; egypt; photogrammetric survey citation: fausta fiorillo, corinna rossi, 3d survey and metric analysis of the late roman fort of umm al-dabadib (egypt), acta imeko, vol. 7, no. 3, article 10, month year, identifier: imeko-acta-07 (2018)-03-10 section editor: sabrina grassini, politecnico di torino, italy received april 8, 2018; in final form october 1, 2018; published october 2018 copyright: © 2018 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: this work was supported by the european research council consolidator grant no. 681673. corresponding author: fausta fiorillo, fausta.fiorillo@polimi.it 1. introduction this article presents the latest results of the ongoing investigation and study of the late roman site of umm aldabadib, located in the kharga oasis, in egypt’s western desert. this major archaeological site lay undisturbed until recently [1, 2], mainly because of its remote position and challenging environmental conditions [3]. it is characterized by the well-preserved remains of both the ancient settlement and its corresponding agricultural system, thus making it an ideal candidate for a comprehensive study encompassing both aspects [4]. the site is now the focus of a 5-year project funded by and erc grant, and is being studied thanks to a combination of traditional and innovative archaeological survey and 3d modelling techniques [5-7]. in particular, the adoption of 3d survey techniques allowed the creation of an accurate, metric 3d model of the entire site, that is now being used to carry out further research on these remains, including a first metrological study, that is yielding extremely interesting results, able to cast a new light on this archaeological site and on the antiquities of the kharga oasis. 2. history of the survey of the site umm al-dabadib, located at the outskirts of the kharga oasis (figure 1), contains the impressive archaeological remains of a vast and well-preserved roman installation, active in the iii and iv centuries ad and abandoned at the beginning of the v century. the site was probably inhabited already during the ptolemaic period, as attested by ceramics and grave goods. the vast majority of the visible architectural remains date to the roman period, in particular to the iv century ad, when this site was selected to become an important element of a chain of aggressive-looking settlements, probably meant to keep under control the caravan routes that crossed the kharga oasis [8]. the site covers a total area of 10 square km and consists of three settlements (the fortified settlement, the northern settlement and the eastern settlement) served by an extensive agricultural system. the northern settlement, the oldest core of the site, consists of a group of large houses, all different from one another; the fortified settlement dates instead to the iv century, and consists of an extremely compact group of houses, surrounded by a continuous wall, in the middle of which rises the fort (figure 2). abstract this paper presents a metric analysis and interpretation of the 3d survey of the late roman fort of umm al-dabadib (kharga oasis, egypt). the aim is to verify if a modular measure was used in the construction of the fort and whether this was congruent with roman or egyptian units of measurement. horizontal and vertical sections were extracted from the 3d model of the fort derived from a closerange photogrammetry survey method. the resulting technical drawings were used for the study and interpretation of the dimensional patterns of the fort that revealed the correspondence of the units of measurement of the building to egyptian reformed cubit. this research is part of the project life (living in a fringe environment), funded by the erc cogrant 681673. mailto:fausta.fiorillo@polimi.it acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 58 a first survey by theodolite in 2003 [9] of the entire archaeological site was followed by a complete photogrammetric terrestrial survey of the fortified settlement in 2014 [3] and 2015, with the aim to obtain a complete and accurate 3d model of this portion of the archaeological remains. this survey was prepared with extreme care as the logistic and environmental difficulties relating to the site dictated rather precise rules. the extraction and interpretation of the ensuing, accurate technical drawings offered the chance to perform a study on the unit of measurement employed in the construction of the fort (presented here), which will be extended in the near future to the rest of the fortified settlement. 3. the 3d survey of the fort the survey was planned to ensure the minimum requirement of a 1:50 scale. it means that according to a plotting error of 0.2 mm, the minimum ground sample distance (gsd) and measurement accuracy should not exceed 10 mm. the fort, located in the middle of the fortified settlement (figure 2), consists of a (nearly) square building endowed with two rectangular towers on either side along the southern façade, and consists of 5 floor levels (including the ground floor, labeled 0). during the 2014 season, more than 500 photos were acquired using a canon eos 5d mark iii with a fix 35 mm lens, in order to survey the exterior of the fort [3]. the camera network geometry corresponded to a closed circular acquisition of images taken around the external wall of the fort. the average capture distance from the object was of about 15-20 m and the baseline between adjacent images was about 1,5 m (baseline/distance ratio of about 0,1). related to the camera sensor resolution, this distance ensured a gsd of 1 mm for the fort’s façade and a maximum of 5 mm for the portion in background. the gsd is a very important parameter to establish the details that we can later measure on the output restitutions (final point cloud, orthoimages, etc.). in order to georeference the models in the same coordinate system, to scale them and to check the reconstruction accuracy, a net of gcps (ground control points) was established and measured [10]. the global accuracy of the topographic network was about 4 mm. the 2014 exterior survey of the fort was integrated by the 2015 survey of the accessible internal spaces, located at level 0 and 1. it was possible to perform a complete photogrammetric survey of: (a) four rooms on the east side and (b) four rooms in the north-east corner of level 0; (c) two rooms on the east side of level 1; (d) the entire staircase in the south-east tower. due to the distribution of the accessible inner rooms and the conformation of the space, the photogrammetric acquisitions were divided into the four blocks described above. the minimum internal dimension of the rooms was about 4×5 m2 with a maximum height of about 2.4 m and a minimum height of about 1.1 m (impost line of the vault). the staircase, the corridors and the passages connecting the rooms with one another are even narrower, even down to 40 cm only. in these critical conditions, achieving an accurate and complete dense 3d model which could match the exterior represented a very complex task. the lack of operating space made the use of rectilinear lenses extremely complicated. fisheye lenses, instead, ensure a wider field of view in comparison with a rectilinear lens. this allows to simplify the survey phase and to reduce the number of photos needed for a complete 3d restitution. for these reasons, two fisheye lenses were used: i) a canon eos 350d coupled with a fix 8 mm focal length for the level 0 and the staircase; ii) a nikon d810 coupled with a fix 12 mm focal length for the level 1. during the acquisition with the fisheye optics, the team paid special attention to the base/distance ratio and to the degradation of the gsd in the boundary part of the frame [11]. generally, a capture distance from the object of less than 1 m was used. the indoor survey was designed in order to have a sufficient overlap with the external wall surface. the model of the exterior of the fort, covered by the topographic network, was used as reference block to align the four inner block-models in the same coordinate system. the 2014 and 2015 photogrammetric surveys were planned paying special attention to the camera network geometry [12] and to some important photogrammetric rules [13]. the photos were acquired with an average overlap between two consecutive normal views of about 80 %, in order to ensure the automated detection of homologous points. convergent images were also acquired. the acquisitions were performed using encoded markers. the markers are very important as external constrains during the image orientation and to optimize the calculation of lens distortion parameters during the self-calibration process (for details see the next section). the markers must be uniformly distributed in the area (especially where the geometry is complex) and visible in a sufficient number of photos. figure 1. the kharga oasis, indicated by the arrow, lies 650 km south of cairo and 250 km west of luxor. figure 2. the fortified settlement with the fort in the centre (photograph by paolo viviani, 2013). acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 59 4. the photogrammetric process for the photogrammetric processing, we used agisoft photoscan, a powerful software able to successfully manage a large number of photos organized in groups of images. the first step of the photogrammetric pipeline is the automatic orientation of images, performed using the full resolution of the frame. the second step is the automatic detection of markers on the photos and the consequent optimization of the camera calibration parameters. before the optimization, a human intervention is necessary to verify and eventually adjust the correct position of the markers on the image and/or manually add undetected markers. in the third phase, in order to scale and georeference the external model of the fort, the coordinates acquired by total station are associated to the markers. the adoption of this work sequence avoids accidental topographical errors affecting the orientation. instead, in order to scale the four indoor models, many reference distances among markers had to be measured, since it was not possible to perform topographic survey. up to this third step, each “block” of images was processed separately. in the fourth step, instead, the indoor blocks were aligned to the fort’s referenced model, using a “marker-based” method. several couples of homologous architectural points taken on the external surface of the fort were picked manually on the photos. we used a minimum of six markers shared by both the external and the internal survey. these markers were used as reference points to roto-translate the four indoor blocks into the reference system of the external model of the fort. the accuracy of the alignment of the blocks of images of the exterior of the fort is about 3 mm, calculated from the deviation of topographic gcps. the accuracy of the indoor blocks (level 0 east side, level 0 n-e corner, level 1 east side, staircase) varies but keeps below 3 mm. the general accuracy of the final model made of the aligned blocks (exterior and interior) has been established by checking the deviation of the manual marker used for the roto-translation: therefore, the final georeferencing average error has been increased to about 1 cm. finally, for each oriented block of images the dense matching process was elaborated to produce a dense point cloud model and a mesh model. from the textured polygon model, it was possible to extract the orthoimages of the main façade of the fort (figure 3) with a maximum detail of about 2 mm that ensured a graphic representation to a scale 1:20 (see also figure 4). this precision is due to the fact that the orthoimages were extracted from the 2014 survey only, which had an average gsd of 1.27 mm (table 1). the possibility to visualize the four façades offered the chance to better understand the overall design of the building, to analyze its internal distribution around the central courtyard, and to reconstruct the organization into superimposing levels. figure 3. the orthoimages of the fort façades showing the elevation of the horizontally-cut sections. table 1. photogrammetric images blocks. images block camera focal (mm) photos average gsd (mm) point cloud fort exterior canon 5d markiii 35 509 1.7 32,685,339 level 0 east side canon 350d 8 210 0.7 16,417,253 level 0 n-e corner canon 350d 8 208 0.8 11,862,279 level 1 east side nikon d810 12 508 0.3 77,273,568 staircase canon 350d 8 243 0.6 12,623,173 acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 60 5. the (re)construction of the model the full photogrammetric process of the captured data allowed to obtain a series of dense point clouds and textured mesh models of the interior and exterior of the fort that were referenced in the same coordinate system. the unified 3d model of the building consists of more than 150 million points, with many redundant points and a variable resolution, due to the overlapping areas among the different model-blocks, necessary to align them with one another. in order to homogenize the resolution of the point model and in order to make it more manageable for the successive elaborations, it was subsampled with a minimum point-to-point step of 5 mm. therefore, it was possible to reduce the final point cloud to 37.041.824 points. this operation was useful for the next step: the extraction of the horizontal sections. in fact, using a more uniform and homogenous point cloud, the algorithm of interpolation of the polyline from the point section works better. the final point cloud, obtained in this way, was imported in autocad to extract horizontal sections for each floor level. in figure 4 it is possible to see the five planes on the point cloud model and the extracted horizontal sections. the height of the section plan has always been set below the vault’s impost line in order to have a correct representation and measurement of the internal spaces (clearly visible in the upper part of the transparent frontal view in figure 5). in order to ensure the adequate density of points for the extraction of the polyline, the vertical thickness of the slices of point cloud was set equal to 1cm: double of the point-to-point spatial resolution. figure 4. horizontal sections for each floor levels, extracted from the final 3d point cloud. figure 5. from above: transparent south façade with the cut section levels and top view of the different levels. acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 61 in this way we obtained a comprehensive 3d model of the entire building, from which we could extract plans at all levels, as well as meaningful sections cut across specific points or areas (figures 4, 5 and 6). the three-dimensional study of the building allowed the detection of irregularities and misalignments that had not been previously noted: not all rooms belonging to the same level, for instance, share the same floor level, and the same happens to the vaults, that sometimes reach different heights. this means that the internal design of the building was influenced by several factors, including probably the original ground level, that was only partly flattened before the construction began. the ground floor is the best-documented level, as it can be entered (and thus surveyed) from various sides, and yet many important details on the internal arrangement of the building are uncertain, including the precise dimensions of the internal courtyard. the first floor can be partly reconstructed from the accessible remains, and partly by projecting the alignment of the upper and lower rooms. the remains of the third and fourth levels stick out of the collapsed mass of debris, so that they can be reconstructed up to a certain point: the precise design and extent of the central courtyard remains obscure. all floors were served by the elaborated staircase that occupied the south-eastern tower; four flights of stairs, covered by a sequence of small vaults, revolved around a central pillar and continued to the roof level (see figure 6). even if the staircase is half-filled with sand, the 3d model of its internal space is extremely useful to reconstruct the original floor levels. the only portion that is currently inaccessible is the ground floor, totally engulfed by sand. the vertical sections of the building show very well the marked slant of the external surface of the outer wall, in stark contrast with the vertical alignment of its internal face and of all the internal walls (see figure 6). figure 6. north-south and east-west sections of the fort across the staircase. acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 62 6. the metric analysis of the fort the search for a possible unit of measurement started from the analysis of the mudbrick building technique employed for the entire settlement. the most common brickwork type (at umm al-dabadib, as well as at all the contemporary sites in northern kharga) is the two headers + one stretcher wall with a thickness of about 52-53 cm. in fact, the bricks have average dimensions of 34-36 cm × 16-18 cm × 8-10 cm [2]. this metric information was tested on different portions of the 3d model of the building. figure 7, for instance, shows a detail of the orthoimage of the south façade (picked up on the internal face of the northern wall), where it is possible to see the measures of the brick and the two headers + one stretcher brickwork type. this metric module of 52-53 cm does not seem to match the basic unit of roman linear measurement which was the pes (the foot, about 29 cm): instead, it corresponds exactly to the egyptian royal cubit (here abbreviated as c) [14, 15]. in order to verify whether this module can be also detected in the dimensions of rooms and walls, a reference grid of 52 cm was mapped on the floor sections (figure 8). a grid based on the unit of measurement does not necessarily reflect a grid actually used by the builders; it rather represents a graphic tool to easily visualise the presence of that unit of measurement. in this case, for instance, it immediately showed that the width of the rooms at level 0 corresponds to 5.5 or 6 c; that of the central court probably to 13.5 c; that the east tower measured 3.5 c × 4.5 c, and the west tower 3 c × 4 c (figure 9). a detailed metrological analysis, including a discussion on the units of measurement employed, will appear elsewhere [16, 17]. the level of accuracy in the dimensions of the masonry is relatively high (figure 7), considering that looking for extreme precision makes little sense in a building made of plastered mudbrick walls. figure 8. the royal cubit grid overlapped on the top view. figure 7. detail of the orthoimage on figure 3: it is possible to visualize the measures of the bricks (in cm) and the two-header brickwork type. figure 9. a detail of figure 5 relating to level 0: in red the dimension of the rooms in royal cubit unit and in black the corresponding in meters. acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 63 7. conclusions the results of the metric analysis offered an important insight into the interpretation of the historical data. first of all, it is important to note that this is, so far, the latest attestation of the use of the ancient egyptian unit of measurement in architecture. the use of the royal cubit in this late roman period fortified site suggests several directions for future research: first of all, umm al-dabadib is one element of a chain of late roman fortified sites that punctuate the kharga oasis, and the same conclusions might therefore apply to the other sites as well. moreover, the use of the royal cubit in a late roman fortified installation (or chain of installations) in the middle of the western desert suggests that this building programme was the result of a strategic project evidently funded by the roman authorities, but deeply rooted in the local building techniques [18]. the umm al-dabadib settlement is an interesting case study to analyse the late roman strategy to control and manage the southern border of the empire. the interpretation of the archaeological remains based on an egyptian or roman unit of measurement aims at understanding the history of this wellpreserved site and offers important clues for the interpretation of other similar but less-preserved sites. in addition, the presence of modular measurements may facilitate the graphic restitution of the current state of the remains, and the representation of constructive hypotheses, as basic and repetitive elements (vaults, arcs, etc.) might have been designed according to the module. our future aim is to extend the metric analysis presented here to the rest of the fortified settlement, in order to study how the same unit of measurement was employed in the construction of the other buildings. in conclusion, although traditional, two-dimensional surveys of buildings can be extremely precise, the advantage of carrying out a metrological analysis on a metric 3d model is quite evident. it is hoped that the adoption of this survey technique will increase in the near future, in order to open the way to further analyses of this type. acknowledgement we wish to thank thank francesco fassi and cristiana achille for many fruitful discussions; the egyptian ministry of the antiquities for their support; in particular, the former general manager of the antiquities of the kharga oasis, mr ahmed ibrahim bahgat, and the current general manager mohamed ibrahim ahmed, for their constant encouragement and support. this paper presents the results of the research currently carried out by the project life (living in a fringe environment), funded by the erc cogrant 681673. references [1] c. rossi, umm el-dabadib, roman settlement in the kharga oasis: description of the visible remains. with a note on ‘ayn amur, mitteilungen des deutschen archäologischen instituts kairo 56 (2000), pp. 235-52. [2] c. rossi, s. ikram, north kharga oasis survey. explorations in egypt’s western desert, british museum publications on egypt and sudan 5, peeters, leuven, 2018, isbn 978-90-429-3621-8. [3] f. fassi, c. rossi, a. mandelli, emergency survey of endangered or logistically complex archaeological sites, int. arch. photogramm. remote sens. spatial inf. sci., xl-5/w4 (2015), pp. 85-91. [4] c. rossi, a. migliozzi, f. fassi, a. mandelli, g. b. chirico, c. achille, s. mazzoleni, archeologia, scienza e tecnologia: lo studio del sito tardo-romano di umm al-dabadib (oasi di kharga, egitto), proceedings of the seminar l.o.s.a.i. (laboratori open su scienza, arte e innovazione) 2015, isbn 9788899130206, pp. 221-8. [5] f. remondino, heritage recording and 3d modeling with photogrammetry and 3d scanning, remote sensing, 3.6 (2011), pp. 1104-1138. [6] s. gonizzi barsanti, f. remondino, d. visintini, 3d surveying and modeling of archaeological sites – some critical issues, ann. photogramm. remote sens. spatial inf. sci. ii-5/w1 (2013), pp. 145-150. [7] c. rossi, italian mission to umm al-dabadib, season 2014 – preliminary report, mitteilungen-deutschen archaologischen instituts abteilung kairo, 72 (2016), pp. 153-72. [8] c. rossi, controlling the borders of the empire: the distribution of late-roman ‘forts’ in the kharga oasis, in: r. bagnall, p. davoli, c. hope (eds.), the oasis papers 6, proceedings of the sixth conference of the dakhla oasis project, dakhla oasis project monograph 15, oxford: oxbow, 2013, pp. 331-6. [9] c. rossi, s. ikram, north kharga oasis survey 2003. preliminary report: umm el-dabadib, mitteilungen des deutschen archaologischen instituts abteilung kairo, 62 (2006), pp. 279. [10] f. fassi, 3d modeling of complex architecture integrating different techniques a critical overview, int. arch. photogramm. remote sens. spatial inf. sci., xxxvi-5/w47 (2007), pp. 1-11. [11] l. perfetti, c. polari, f. fassi, fisheye photogrammetry: tests and methodologies for the survey of narrow spaces, int. arch. photogramm. remote sens. spatial inf. sci., xlii-2/w3 (2017), pp. 573-580. [12] e. nocerino, f. menna, f. remondino, accuracy of typical photogrammetric networks in cultural heritage 3d modeling projects, int. arch. photogramm. remote sens. spatial inf. sci., xl-5 (2014), pp. 465-472. [13] f. remondino, e. nocerino, i. toschi, f. menna, a critical review of automated photogrammetric processing of large datasets, int. arch. photogramm. remote sens. spatial inf. sci. xlii-2/w5 (2017), pp. 591-599. [14] t. pommerening, weights and measures, pharaonic egypt, in: r. s. bagnall, k. brodersen, c. b. champion, a. erskine and s. r. huebner (eds.), the encyclopedia of ancient history. blackwell, oxford, 2013, pp. 7087–7092. [15] a. p. hirsch, ancient egyptian cubits origin and evolution. phd dissertation, university of toronto, 2013, unpublished. [16] f. fiorillo, c. rossi, metric analysis and interpretation of the unit of measurement in the late roman fort of umm al-dabadib (egypt), in: metroarchaeo 2017, pp. 139-144, isbn 9789299008409. 3rd imeko international conference on metrology for archaeology and cultural heritage, lecce 23-25 ottobre 2017. [17] c. rossi, f. fiorillo, “a metrological study of the late roman fort of umm al-dabadib, kharga oasis (egypt)”, nexus network journal “architecture and mathematics”, 20, 2, (2018), pp. 373391. [18] c. rossi, “searching for the right words: what happened in kharga in the iv century ad?”, proceedings of the colloquium marges et frontières occidentales de l’égypte (cairo, 2-3 december 2017), bibliothèque d’étude series, in-press. smart wearable devices for human exposure vibration measurements on two-wheel vehicles acta imeko issn: 2221-870x december 2020, volume 9, number 4, 121 127 acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 121 smart wearable devices for human exposure vibration measurements on two-wheel vehicles marco carratù1, antonio pietrosanto1, paolo sommella1, vincenzo paciello2 1 university of salerno, via giovanni paolo ii 132, 84084 fisciano (sa), italy 2 university of cassino and southern lazio, via di biasio 43, 03043 cassino (fr), italy section: research paper keywords: vibrations; wearable device; accelerometer; iso5349; suspension system citation: marco carratù, vincenzo paciello, antonio pietrosanto, paolo sommella, smart wearable devices for human exposure vibration measurements on two-wheel vehicles, acta imeko, vol. 9, no. 4, article 16, december 2020, identifier: imeko-acta-09 (2020)-04-16 section editor: francesco bonavolonta, university of naples federico ii, italy received october 16, 2019; in final form december 2, 2019; published december 2020 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: marco carratù, e-mail: mcarratu@unisa.it 1. introduction body vibration is a term used when vibrations (mechanical oscillations) of any frequency are transferred to the human body. people are exposed to many different types of vibrations in their daily lives; for example, consider the time spent in a vehicle (car, train, plane, motorcycle, etc.) or the use of power tools. vibration experienced as part of a particular type of work can become a potential form of professional risk, particularly after years of exposure [1]. humans are exposed to vibration through a contact surface that is in a mechanical vibrating state. in recent years, human exposure to vibrations has increased progressively with the development of agricultural, industrial mechanisation and the use of vehicles. due to this increased use of vehicles in recent decades, many people spend at least an hour a day in a vehicle, so it has become crucial to analyse the phenomenon of vibration and the types of stress it might place on the human body. mechanical vibrations are produced by the oscillatory movement of a body around a position of equilibrium; they are characterised by frequency (hz), amplitude (m/s2) and the length of time that the body is exposed to the vibration. in most cases, vibrations are unwanted effects that can dissipate energy and create noise. the study of the impact of vibrations is very complex [2]-[4]; there are many researchers focused on the effects of vibrations on certain parts of the human body [5], [6]. these studies consist of the analysis and control of all possible ways the vibrations might interfere with the human body to cause discomfort or negatively affect the activities and well-being of a person, including consideration of the effects due to a prolonged exposure over years. there are different effects if the human body is exposed to short-term or long-term vibrations. in the first case, the vibrations cause small physiological effects, such as an increase in heart rate or an increase in muscle tension. in the second case, they produce effects such as alteration of the vertebral column, degenerative processes of the lumbar segments, arthrosis, problems of the digestive system and problems with the genital and urinary system [7], [8]. some international regulations supervise the exposure of human beings to different sources of vibration. iso 2631-1 [9] and iso 5349 [10] are the reference standards for the analysis of abstract the comfort experienced while driving a motorcycle is becoming a subject of great importance; indeed, the driver is exposed to vibrations that are typically caused by irregular profiles or wear of the road surface as well as by the aerodynamic influence and highfrequency rotation of the motorcycle engine. this paper discloses an original solution that allows the driver to monitor their exposure to vibration during a ride using a low-cost wearable device (smartwatch). a suitable measurement system has been designed and tested using a real motorcycle. the system captures acceleration signals in real time through bluetooth communication and interfaces with a wearable device with a microcontroller unit that calculates vibrations transmitted through the driver’s hands. different indexes proposed in the literature are adopted for the comfort analysis in both time and frequency domains. the hand transmitted vibrations are also experimentally compared with those measured through a fixed accelerometer according to the prescription included in the standard iso 5349 to show the feasibility of the proposed approach in typical application conditions. mailto:mcarratu@unisa.it acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 122 human exposure to vibrations. the standards define methods and quantify the vibrations transmitted to the body with regard to human health and well-being. the standards do not indicate limits of vibration exposure, but they do describe vibration assessment methods for exposure risk that can be used as a basis for determining these limits. iso 2631-1 and iso 5349 each address different types of vibration and different affected body parts: 1) iso 2631-1 assesses exposure to whole-body vibration (wbv) [11], [12]. 2) iso 5349 evaluates exposure to vibration transmitted to the upper limbs, known as hand–arm vibration or handtransmitted vibration (htv). vibrations transmitted to the whole body can create feelings of discomfort and unease, influencing the standard capacity for human performance or exposing the human body to health risks, such as pathological damage or psychological changes. stress can occur in different directions, contain many frequencies and vary over time. the standards establish the criteria for measuring the vibrations transmitted to the body, considering the following types: random, periodic and transient vibrations. when the vibrations have frequencies close to the resonance frequencies of the human body, then the harmful effects increase. consider that the resonance frequency of the spine, which is about 5 hz, is similar to that produced by many earth-moving machines [13]. within the subset of vehicles, motorcycles represent a vehicle of great interest for the analysis of vibrations due to their increased sensitivity to the roughness of the road when compared to other vehicles [14], [15]. the vibration analysis band for a motorcycle varies from 0.25 hz to 20 hz [16], [17]. excitations at very low frequencies (below 0.25 hz) are caused by natural variations in the slope of the road and are not transmitted to the human body. frequencies above 20 hz can be considered noise, which does create different sensations in the body while driving, but these are not caused by the movement of the motorcycle. the remaining frequency range, between 0.25 hz and 20 hz, could also be divided into two other bands: frequencies below 1.5 hz, which can be set aside due to their low contribution in terms of vibrations, and frequencies from 1.5 hz to 20 hz, which include all the primary vibrations foreseen by the dynamics of a motorcycle [18], [19]. the most significant component affecting the human body’s exposure to vibrations during a motorcycle ride is vertical acceleration. indeed, the inertia forces to which the internal organs of the passenger’s body are subjected are proportional to the vertical acceleration, while their relative displacements are influenced by the frequency of excitation [20]. this paper presents an experiment with the use of a wearable device, an object that today is commonly used in everyday life, to evaluate the vibrations transmitted to a human body during a motorcycle ride. furthermore, the results will be compared those derived from a more typically used fixed accelerometer through the calculation from both of several comfort indexes in the time and frequency domains. the paper is organised as follows: section 2 describes the measurement system used for the experiment, section 3 reports the experimental tests, showing the calculation of several comfort indexes for different types of road, and, finally, sections 4 and 5 present a discussion of the results followed by the conclusion. 2. the measurement setup a typical urban motorcycle was used for the evaluation of human body exposure to vibrations during the ride. a suitable data acquisition system was set up, including a wearable device worn by the driver and a data acquisition system able to sample the acceleration data measured on the arm of the driver. a fixed accelerometer mounted across the contact point between the driver’s hand and the motorcycle steering was used as a reference for the vibration measurement according to [21]. for more details, see figure 1. the measurement system is composed of: 1) wearable device st steval-wesu1 (equipped with a 3d accelerometer and a 3d gyroscope lsm6ds3) mounted on the wrist of the driver. 2) stm32f401re nucleo board equipped with a bluetooth module. 3) fixed accelerometer (lsm6ds3) mounted on the motorcycle’s steering. 4) data logger mdlog (spring off). the wearable device steval-wesu1 [22] includes a mems accelerometric sensor for the measurement of accelerations with different ranges: ± 2 g / ± 4 g / ± 8 g / ± 16 g. the sensor is characterised by an accuracy equal to ± 90 mg at full scale. the acceleration measurements used to evaluate the vibrations transmitted by the motorcycle to the driver would normally be carried out directly on the points of contact between the motorcycle and the driver, but in this experiment, we decided to put the sensible device directly on the driver’s arm, as is typical of a wearable device. the axis reference system of the accelerometer sensor according to the wearable device arrangement (located on the left wrist) is shown in figure 2. the fixed accelerometer has been mounted at the end of the handlebar. the stm32f401re nucleo board [23] was responsible for the data recording, carried out at the sampling frequency of 400 hz, which was accomplished through the adoption of several methods of data compression in order to reduce the a) b) c) figure 1. the measurement setup: a) the wearable device, b) the nucleo board equipped with the bluetooth module c) the data logger. a) b) c) acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 123 power consumption and the computational burden [24]-[26]. the communication channels used for data transfer were a bluetooth bridge with the wearable device and the spi with the fixed accelerometer. as for data logging, a suitable data acquisition system was designed to store the data collected by the stm32f401re nucleo board. the data acquired at a frequency of 400 hz are: 1) acceleration value on the x-axis ax 2) acceleration value on the y-axis ay 3) acceleration value on the z-axis az. several tests were carried out on a mixed urban-suburban road, specifically chosen to obtain data on different road surfaces. data was collected for about 30 km, classifiable by road typology as follows (see figure 3): 1) 10 km of urban road with an irregular road surface 2) 10 km of cobblestone road 3) 10 km of highway. 3. experimental test as previously explained, considering the massive growth of wearable devices in recent years, the experiment aimed to analyse the use of such a device in the field of human exposure to vibration and compare the results with those obtained from a fixed accelerometer such as would typically be used in the field. specifically, attention was paid to the vertical accelerations transmitted to the motorcycle driver's body. with this aim, several indexes were evaluated for both accelerometers (i.e. the reference accelerometer and the one included in the wearable device) in order to highlight the annoying frequency components and the daily vibration dose to which the human body is exposed on different road surfaces. in particular, the following indexes were calculated: • calculation of htv in the time domain, which is the rms acceleration weighted in frequency for the evaluation of comfort concerning an observation window of 8 hours as reported in [10]. • evaluation of vibration dose value (vdv), useful for the evaluation of vibration transmitted as a result of shock events caused by concentrated obstacles. • analysis in the frequency domain of the mean combined auto-spectrum on a range of frequencies of interest. 3.1. htv index for the measurement of the vibrations generated by the motorcycle and transmitted through the handlebar, we adopted the techniques described in iso 5349, which estimate the vibrations conducted to the hand-arm system (i.e. the htv). the vibration assessment described by the standards is calculated according to the following equation: 𝑎𝑤 = √𝑘x 2𝑎w,x 2 + 𝑘y 2𝑎w,y 2 + 𝑘z 2𝑎w,z 2 (1) where aw,x , aw,y , aw,z are the rms acceleration components on the three axes weighted in frequency, and kx, ky, kz represent a multiplicative factor defined by iso 5349 that depends upon the position of the subject and the contact point between the vibrating surface and the body. in our case, those multiplicative factors were considered equal to 1, as we disregarded the influence of the subject on the contact point [27] and instead considered that all the vibrations generated to the handlebar were transmitted to the driver. the values of the rms accelerations aw,x , aw,y , aw,z are calculated for each axis (x, y and z) applying a weighting factor according to (2)-(4): 𝑎w,x = √∑ (𝑊h𝑖 𝑎h𝑖,x) 2 𝑖 (2) 𝑎w,z = √∑ (𝑊h𝑖 𝑎h𝑖,z) 2 𝑖 (3) 𝑎w,x = √∑ (𝑊h𝑖 𝑎h𝑖,x) 2 𝑖 (4) where ahi,x , ahi,y , ahi,z are the accelerations in m/s2 measured along the x, y and z axes at the i th frequency; whi represents the weighting factors. table 1 presents the frequency weighting in terms of the evaluation of htv laid out in iso 5349. figure 2. the reference system for the wearable device. a) b) c) figure 3. the road experimented: a) urban road with irregular profile, b) cobblestones, c) highway. x y z a) b) c) acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 124 the weighting aims to highlight specific, particularly troublesome frequencies for the human body. in the present case, the weighting was carried out as shown in table 1, which was extracted from the weighting curve reported in iso 5349. the application of the table consisted of filtering the acceleration signal with an appropriate set of filters centred in the next i th frequency defined in table 1. the acceleration signal was filtered for each i th band to calculate the value ahix,hiy,hiz. finally, the acceleration was multiplied for the i th frequency with the appropriate weighting value whi according to table 1. from this operation, it was possible to calculate awx,wy,wz, which represents the frequency-weighted rms acceleration along the three x, y, z axes. the index, calculated for the three orthogonal directions x, y, z, was combined into a single index that defined the effective value of the frequency-weighted acceleration expressed as a combination of the accelerations measured in each direction using (1). the htv index was calculated for each road profile by considering consecutive time windows of 30 s in order to achieve a set of 30 values, the mean value and corresponding standard deviation of which are summarised in table 2. moreover, the percentage coefficient of variation (cov, i.e. the standard deviation normalised to the mean value) for each result is reported. 3.2. vdv index the vdv index was evaluated when the motorcycle crossed a concentrated obstacle, which was a generic bump such as is often encountered in an urban scenario (see figure 4). to analyse the data of interest, a 3-second time window centred from the peak of the oscillation was taken, and 20 tests were considered. the vdv is a parameter used for the assessment of comfort in case of vibrations due to shock events caused by a concentrated obstacle such as a bump. the mathematical equation used to calculate the vdv index is the following: 𝑉𝐷𝑉 = √ 1 𝑇 ∫ 𝑎w 4 (𝑡)𝑑𝑡 𝑇𝑠 0 4 (5) where ts is the duration of the observation window expressed in seconds and aw(t) is the measured acceleration. specifically, the calculation uses the fourth power of acceleration, which, compared to the rms value, is more sensitive to the peaks generated by these shock events. passing over the concentrated obstacle will generate a spike on the acquired dataset of accelerations, which can be seen from the graph reported in figure 5. for example, it is possible to recognise the peak of accelerations at around 35.3 s on the acquisition record. the mean vdv index calculated with both of the accelerometers is reported in table 3 along with the corresponding standard deviation and percentage cov. 3.3. frequency domain analysis the index analysed is the combined medium spectrum, calculated from the signals measured by the accelerometers during 15 minutes of data acquisition. in detail, the autospectrum was calculated by applying the discrete fourier transform (dft) to the signal samples and multiplying the value obtained by its conjugated complex, as reported in (6): table 1. weighting factors for the htv index. frequencies in hz weighting factor whi 1 0.0235 2 0.1000 4 0.3981 5 0.5450 6.3 0.7270 8 0.8730 10 0.9510 12.5 0.9580 16 0.8760 20 0.7820 25 0.6470 31.5 0.5190 40 0.4110 50 0.324 figure 4. the bump used for the calculation of the vdv index. table 2. htv indexes calculated on different types of roads with the proposed measurement setup. typology fixed accelerometer wearable device mean std cov mean std cov in m/s2 in m/s2 in % in m/s2 in m/s2 in % cobblestone 2.58 0.15 6 3.07 0.11 4 highway 2.64 0.10 4 1.69 0.05 3 urban 4.48 0.18 4 1.55 0.07 4 figure 5. spike in the acceleration signal due to a bump. acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 125 𝑆𝑥𝑥 = 𝐴(𝑓) ∙ 𝐴 ∗(𝑓) (6) where a(f ) is the fourier transform of the signal, and a*(f ) is the complex conjugate of a(f ). the auto-spectrums were calculated for each axis of the accelerometers x, y, z (the data pre-processing was performed through the application of a suitable high pass filter aiming to suppress the continuous component). subsequently, the individual auto-spectrums were combined using the following formula: 𝑆comb = √𝑆xx 2 + 𝑆yy 2 + 𝑆𝑧𝑧 2 (7) where sxx is the auto-spectrum in the direction x, syy is the autospectrum in the direction y and, finally, szz is the auto-spectrum in the direction z. finally, the quantitative evaluation was performed, considering as an objective index the integral of the autospectrum calculated in the frequency range 0 hz to 50 hz: 𝐼 = ∫ 𝑆combd𝑓 50 0 (8) the frequency range was selected to highlight through the combined spectrum the expected vibration peaks associated with: • frequency of suspended masses • frequency of unsuspended masses • frame frequencies as an example, the combined spectrum of the signals measured by the fixed accelerometer is reported in figure 6. the frequency analysis was performed in such a way as to calculate the integral i (8) of the combined average spectra for each type of road. the results are reported in table 4 in terms of the mean value, standard deviation and percentage cov derived from 30 samples of the integral i (8). 4. discussion of results the experimental results summarised in table 2–table 4 show the satisfying data quality of the measurement campaign, as highlighted by the limited coefficient of variation exhibited by the proposed indexes, particularly when computed on the signal outputs from the wearable device (cov < 15 %). moreover, as determined by a mean test with a confidence level not lower than 95% for the varying road typology, the corresponding values for each index type are statistically significant since they result from different distribution (i.e. the null hypothesis h0, according to which two indexes may result from the same population, is refused regardless of which combination of device and/or road typology is considered). as already highlighted in [21], the htv index was shown to be the most reliable and significant, since: similar results in terms of small cov (< 10 %) are exhibited with both the devices under test; the greatest span is exhibited for each device when different road typologies are considered, making it easier to identify the corresponding discomfort levels. moreover, all three indexes confirm that the comfort level as interpreted by the device on the driver's wrist is strongly influenced by the roughness of the road, which may be assumed to be the primary source of vibration at the lowest frequencies and motorcycle speeds. indeed, with regard to the wearable device, all three indexes assume the most significant value in evaluation of the ride on cobblestone (here we assume as similar the behaviour exhibited in correspondence with the bump). furthermore, for the first road type, both the htv and the (combined spectrum integral) i index are characterised by greater value when both devices under test are compared, whereas the opposite behaviour is exhibited when riding on urban and highway roads is considered. in other words, both the time domain and frequency domain analysis highlight that the vibrations transmitted to the hand (as measured by the fixed accelerometer) while riding a motorcycle at high speed increase with the influence of the aerodynamics and the contribution of the frame vibration; these components are more significant than the vertical dynamics introduced by the road roughness. at the same time, the former vibration contribution is muffled by the driver mass, which reduces the corresponding index values computed on the signals outputted from the wearable device. 5. conclusions the evaluation of the vibration transfer to the human body during a motorcycle ride is of considerable interest in order to give relevant information to the rider about the stress accumulated during a ride. considering the technological progress regarding the development of new tiny and wearable technologies, the paper focuses on the feasibility of the use of a table 3. vdv indexes calculated by the devices under test. fixed accelerometer wearable device mean std cov mean std cov in m/s1.75 in m/s1.75 in % in m/s1.75 in m/s1.75 in % 2.25 0.08 4 4.39 0.64 14 table 4. auto-spectrum indexes calculated on different types of roads with the proposed measurement setup. typology fixed accelerometer wearable device mean std cov mean std cov % in m2/s5 in m2/s5 in % in m2/s5 in m2/s5 in % cobblestone 0.97 0.18 19 1.09 0.04 4 highway 1.03 0.33 32 0.36 0.05 14 urban 1.55 0.54 35 0.38 0.03 9 figure 6. the spectrum of the acceleration signal. acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 126 typical wearable device (smartwatch) for the calculation of htv indexes for different road profiles. from the time and frequency domain analysis represented by the most commonly adopted indexes, it is clear that the highest value of vibration is relative to the cobblestone; thus, a long ride on a road surface of this type will create more significant discomfort. conversely, driving the motorcycle on a suburban road will involve less discomfort. considering all the results described, the method is also useful for the evaluation of the quality of the road surface that a driver is obliged to use for daily commuting in order to give feedback and choose the best road for reduced exposure to vibrations (for example a suburban road instead of a highway). the measurement system described could also be used for the evaluation of the vertical vibration reduction introduced by a semi-active suspension system [28] instead of the classical suspension system in a motorcycle or for the detection of faults in a suspension system [29]-[32]. indeed, the use of wearable devices (as opposed to the installation of expensive fixed sensors) offers the possibility of calculating the htv index in real time (the most reliable method) for different kinds of vehicles, just by wearing a smartwatch. on the other hand, the main hypothesis of the proposal is the fixed position of the wearable device on the left wrist. different installation positions should be investigated in order to give the end-user a multiplicative coefficient for the correction of the index. future research will also concern the analysis wbv according to iso 2631-1: it 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[21] m. carratu, a. pietrosanto, p. sommella, v. paciello, a wearable low-cost device for measurement of human exposure to transmitted vibration on motorcycle, proceedings of the ieee international workshop on metrology for industry 4.0 and iot, metroind 4.0 and iot, naples, italy, 4 – 6 june 2019, pp. 329-333. doi: https://doi.org/10.1109/metroi4.2019.8792855 [22] stmicroelectronics, wearable sensor unit reference design for fast time to market, online [accessed 29 october 2020] https://www.st.com/en/evaluation-tools/steval-wesu1.html [23] stmicroelectronics, stm32 dynamic efficiency mcu, online [accessed 29 october 2020] https://www.st.com/en/microcontrollersmicroprocessors/stm32f401re.html [24] l. angrisani, f. bonavolonta, m. d'apuzzo, r. schiano lo moriello, m. vadursi, a compressive sampling based method for power measurement of band-pass signals, ieee international instrumentation and measurement technology conference (i2mtc), minneapolis, usa, 6 – 9 may 2013, pp. 102-107. doi: https://doi.org/10.1109/i2mtc.2013.6555390 [25] l. angrisani, f. bonavolonta, r. schiano lo moriello, a. andreone, r. casini, g. papari, d. accardo, first steps towards an innovative compressive sampling based-thz imaging https://doi.org/10.1016/0169-8141(94)90017-5 https://doi.org/10.1080/10937404.2018.1557576 https://doi.org/10.21014/acta_imeko.v3i4.152 https://www.imeko.org/publications/wc-2009/imeko-wc-2009-tc22-431.pdf https://www.imeko.org/publications/wc-2009/imeko-wc-2009-tc22-431.pdf https://doi.org/10.1109/caidcd.2008.4730545 https://doi.org/10.1109/mace.2010.5535864 https://doi.org/10.1006/jsvi.2001.4256 https://doi.org/10.1109/i2mtc.2013.6555457 https://doi.org/10.1109/i2mtc.2017.7969943 https://doi.org/10.1109/indin.2018.8472039 https://doi.org/10.1109/metroi4.2019.8792855 https://www.st.com/en/evaluation-tools/steval-wesu1.html https://www.st.com/en/microcontrollers-microprocessors/stm32f401re.html https://www.st.com/en/microcontrollers-microprocessors/stm32f401re.html https://doi.org/10.1109/i2mtc.2013.6555390 acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 127 system for early crack detection on aereospace plates, 2014 ieee metrology for aerospace (metroaerospace), benevento, italy, 2014, pp. 488-493. doi: https://doi.org/10.1109/metroaerospace.2014.6865974 [26] s. krug, m. o’nils, modeling and comparison of delay and energy cost of iot data transfers, in ieee access 7 (2019) pp. 58654-58675. doi: https://doi.org/10.1109/access.2019.2913703 [27] r. taiar, c. b. machado, x. chiementin, m. bernardo-filho, whole body vibrations: physical and biological effects on the human body, crc press, boca raton, 2018, isbn 9781138500013. 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https://doi.org/10.1109/i2mtc.2018.8409829 https://doi.org/10.1109/i2mtc.2018.8409833 https://doi.org/10.1109/imtc.2010.5488034 https://doi.org/10.1109/tim.2019.2905945 in-field monitoring of eight photovoltaic plants: degradation rate over seven years of continuous operation acta imeko issn: 2221-870x december 2018, volume 7, number 4, 75 81 acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 75 in-field monitoring of eight photovoltaic plants: degradation rate over seven years of continuous operation alessio carullo1, antonella castellana1, alberto vallan1, alessandro ciocia2, filippo spertino2 1 politecnico di torino, dipartimento di elettronica e telecomunicazioni, corso duca degli abruzzi n. 24, 10129 torino, italy 2 politecnico di torino, dipartimento energia, corso duca degli abruzzi n. 24, 10129 torino, italy section: research paper keywords: photovoltaic power systems; degradation; electric variables measurement; data acquisition; uncertainty citation: alessio carullo, antonella castellana, alberto vallan, alessandro ciocia, filippo spertino, in-field monitoring of eight photovoltaic plants: degradation rate over seven years of continuous operation, acta imeko, vol. 7, no. 4, article 13, december 2018, identifier: imeko-acta-07 (2018)-04-13 editor: alexandru salceanu, "gheorghe asachi" technical university of iasi, romania received april 24, 2018; in final form september 21, 2018; published december 2018 copyright: © 2018 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: alessio carullo, e-mail: alessio.carullo@polito.it 1. introduction the continuous worldwide increase in installations of photovoltaic (pv) plants [1] demands a reliable estimation of their long-term performance. the main parameters of interest are the payback time (pbt) and the energy payback time (epbt) [2-6]. such parameters are largely affected by the actual degradation to which the pv modules are subject, which reduces efficiency during their lifetime. as an example, for a small-size 3 kw plant whose expected energy production is 4500 kwh/year, in the conditions defined in the web tool [7], a payback time of nine years and eight months is estimated using a pv module decay of 0.70 %/year, as suggested in the iea document [8] for mature module technologies. however, if the pv modules used exhibit a degradation rate of 2.40 % per year, the estimated payback time becomes 10 years and 4 months. with the aim of estimating the degradation rate of the commercially available pv technologies, the authors monitored eight different outdoor pv plants between 2010 and 2017. electrical and environmental quantities were continuously monitored by means of a specifically conceived data acquisition system, which is subject to a metrological confirmation program [9]. this allows for measurement traceability to be ensured and uncertainty to be stated for each of the measured parameters. preliminary results are reported in [10-12]. the results reported here refer specifically to the degradation rate of the monitored pv plants, estimated from october 2010 to december 2017. furthermore, three of the plants based on thin-film technologies, which exhibited the worst degradation rate, have been characterized at module level in order to verify the average degradation of all the involved pv modules and to highlight the presence of possible “outliers.” this characterization was undertaken in march 2018. in section 2, the test facility is described, providing information about the monitored pv plants, the experimental setup, and the estimated parameters. in section 3, the results obtained in terms of the degradation rate at plant level are provided for each of the monitored pv plants, while section 4 reports the results in terms of the degradation rate at module abstract the results of more than seven years (october 2010-december 2017) of continuous monitoring are presented in this paper. eight outdoor photovoltaic (pv) plants were monitored. the monitored plants use different technologies: mono-crystalline silicon (m-si), policrystalline silicon (p-si), string ribbon silicon, copper indium gallium selenide (cigs), thin film, and cadmium telluride (cdte) thin film. the thin-film and m-si modules are used both in fixed installations and on x-y tracking systems. the results are expressed in terms of the degradation rate of the efficiency of each pv plant, which is estimated using the measurements provided by a multi-channel data acquisition system that senses both electrical and environmental quantities. a comparison with the electrical characterization of each plant obtained by means of the transient charge of a capacitive load is also made. in addition, three of the monitored plants are characterized at module level, and the estimated degradation rates are compared to the values obtained with the monitoring system. the main outcome of this work can be summarized as the higher degradation rate of thin-film based pv modules with respect to siliconbased pv modules. mailto:alessio.carullo@polito.it acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 76 level for three of the monitored plants. finally, in section 5, the main outcomes related to the obtained results are summarized. 2. test facility 2.1. monitored pv plants the eight pv plants that were monitored are located in piemonte (italy) at a latitude of about 45 °n, in the cwc köppengeiger climate zone [13]. table 1 summarizes the main characteristics of the monitored pv plants, which include siliconbased modules (plants a, b, c, and ats) and thin-film based modules (d, e, dts, and ets). the subscript “ts” denotes the plants whose modules are installed on a x-y tracking system, while the others are oriented towards south (azimuth angle of about 0°) and mounted in a fixed position with a tilt angle of 35°. in table 1, apv, pnom, and nom refer to the nameplate area (m2), power (kw), and efficiency (%), respectively. immediately after the installation of the pv plants, their i-v electrical characteristics were measured in natural sunlight by means of the acquisition of the transient charge of a capacitive load [14], and the actual power pact and efficiency act in the standard test conditions (stcs) were then estimated and are also reported in table 1. the relative expanded uncertainty (coverage factor k = 2) for both parameters is about 3.5 %. 2.2. monitoring system the monitoring system is able to measure direct voltage vdc and direct current idc (to upstream the inverter) and temperature tm of a pv module for each of the eight pv plants. solar irradiance gm was also measured on the plane of the pv modules by means of two secondary-standard pyranometers: the first one was mounted with the same orientation as the fixed plants, while the second one was mounted on the x-y tracking system of the plant ats. a measurement of each quantity was carried out every 10 s and stored in a daily file. the architecture of the monitoring system is described in [10], and its metrological confirmation process is described in [9]. the calibration is performed with a periodicity of one year and includes the initial verification of each measuring chain, the adjustment that allows offset and gain drifts to be compensated, and the final verification. the maximum admitted errors used during the verifications are: • (0.5 % reading + 0.2 v) for voltage vdc in the range of 100 v to 450 v; • (0.4 % reading + 5 ma) for current idc in the range of 0.5 a to 7 a; • (2.0 % reading + 5 w/m2) for irradiance gm in the range of 500 w/m2 to 1200 w/m2; and • (0.6 % reading + 0.55 °c) for temperature tm in the range of 10 °c to 80 °c. 2.3. data processing the efficiency  of each plant at stc is obtained by: stcpv stcmax, ga p   (1) where gstc = 1000 w/m2 is the reference irradiance value, apv is the area of the pv modules as reported in table 1, and pmax,stc is the maximum measured power reported at stc through the simplified model:      tgactsc,acts,tdc tgactsc,dcstcmax, ccircv cciip     (2) where isc,act and rs,act are the short-circuit current and the series resistance of each plant obtained during the preliminary i-v characterization;  (a/°c) and  (v/°c) are the absolute current and voltage temperature coefficients, respectively; and the correction coefficients cg and ct are obtained by: mstct stc m g ;1 ttc g g c  (3) starting from the available data, a clear day was selected in each month in the period from january 2014 to december 2017, and equations (1)-(3) were calculated for each plant using the measured parameters that correspond to the values of irradiance gm that are greater than 800 w/m2. during the initial monitoring period (from september 2010 to december 2013), the same procedure was implemented, but with different sampling, since a clear day was selected for each couple of months. the standard uncertainty u(pmax,stc) of each pv plant was estimated according to the procedure described in [11], which takes into account both the repeatability contributions and the uncertainty of each measuring chain. the last contribution corresponds to the maximum admitted errors that are periodically verified. one should note that the uncertainty of the efficiency  obtained by means of equation (1) mainly depends on the uncertainty of the parameter pmax,stc, since gstc is a conventional reference value, while apv is known with a negligible uncertainty. 3. plant degradation rate an example of the obtained results that refer to the selected day, 17 january, 2016, is reported in table 2, where ua(p) represents the standard deviation of the set of values pmax,stc,i, corresponding to the gmi values that are greater than 800 w/m2. meanwhile, ubx(p) is the uncertainty contribution related to the measured quantity x. among the different contributions, the one that mainly accounts for the uncertainty of the parameter pmax,stc and, in turn, of the estimated efficiency  is that related to the correction coefficient cg, which depends on the measured quantity gm. since this quantity is sensed through a secondary standard pyranometer, this contribution cannot be significantly reduced for monitored outdoor pv plants. in the same table, the estimated efficiency  at stc is reported for each plant with the corresponding standard uncertainty u(). similar uncertainty values for the parameters pmax,stc and were obtained during the whole monitored period. 3.1. m-si based plants the estimated efficiency  of plant a (fixed installation) based on m-si pv modules during the monitored period is reported in figure 1, while figure 2 refers to the plant ats (x-y tracking system). the red circles represent the estimated  values, the black squares are the values obtained after the adjustment of the table 1. main characteristics of the monitored pv plants. plant pv technology apv (m2) pnom (kw) pact (kw) nom (%) act (%) a m-si 11.2 2.03 1.93 18.1 17.2 b p-si 13.8 1.85 1.80 13.4 13.1 c string ribbon si 17.9 2.28 2.16 12.7 12.1 d cigs 17.5 1.70 1.68 10.3 9.6 e cdte 17.3 1.74 1.61 10.1 9.3 ats m-si 11.2 2.03 1.93 18.1 17.2 dts cigs 17.5 1.70 1.68 10.3 9.6 ets cdte 17.3 1.74 1.61 10.1 9.3 acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 77 monitoring system, and the black stars represent the value obtained after the pv modules were washed. the former intervention does not show significant effects on the estimated efficiency, thus highlighting that the drift of the measuring chain characteristics during the calibration period is acceptable. in figure 1 and figure 2, the efficiency values obtained with the capacitive load technique in months 1 and 78 are also reported (black diamonds), and they demonstrate behavior that is in agreement with the observed season variability. the black triangles identify invalid results, due to a fault in the monitoring system in the month-interval (80-85) and a fault in the plant ats during months 46, 47, and 48. the continuous straight line is obtained by fitting only the valid  values through a least square algorithm. one should note that for plant ats, the straight line was fitted from month 40, since previous efficiency estimations were affected by a fault in the board that connects the positive pole of the pv modules to ground. due to this improper grounding of the pv modules, the two poles of the plant were floating, thus exposing the modules to potential induced degradation (pid) [15], one of the causes of efficiency loss. after the grounding board was replaced (month 40), plant ats exhibited efficiency values very similar to plant a. the fitted straight line can be represented as:     tstt   0 (4) where t is the time (months), while s is the slope of the fitted line (%/months). the parameter s and its standard uncertainty u(s) were obtained according to the procedure described in [11], taking into account the standard uncertainty u(i) corresponding to each monthly estimation. starting from the obtained values, the yearly percentage degradation rate dr (%/year) was estimated following:  s dr   12 100 (5) for the m-si based plant a, a degradation rate dra equal to -0.03 %/year, expanded uncertainty (95 % confidence level) u(dra) = 0.35 %/year, was obtained. these values are summarized in table 3, where the degradation rates and the corresponding uncertainties of the pv plants under investigation are reported for different monitoring periods [11-12]. taking the uncertainty into account, the estimated degradation rates of plant a are compatible during the different monitored periods, thus showing excellent behavior of the plant based on m-si pv modules in fixed installations. for plant ats, the obtained value is drats = -0.78 %/year, and the expanded uncertainty is u(drats) = 0.40 %/year. this updated value is significantly different from the previous one (+0.20 %/year at the month 72, as indicated in table 3), thus highlighting an important degradation of the m-si modules installed on the x-y tracking system. the obtained results are in good agreement with the efficiency values estimated for month 78 with an independent measuring system (black diamonds in figure 1 and figure 2). 3.2. cigs-based plants table 2. an example of the uncertainty budget, selected day: january 17, 2016. plant ua(p) (w) ubidc(p) (w) ubvdc(p) (w) ubcg(p) (w) ubtm(p) (w) u(pmax,stc) (w) pmax,stc (w)  (%) u() (%) a 8.2 5.0 5.7 26 3.2 28 1940 17.33 0.25 b 8.4 4.4 5.0 40 4.1 42 1725 12.50 0.21 c 11.7 5.4 6.2 27 4.9 31 2110 11.77 0.16 d 10.9 3.4 4.1 14 3.2 19 1375 7.95 0.16 e 4.9 3.0 4.3 8.5 3.1 12 1415 8.07 0.16 ats 8.0 5.9 5.7 24 3.2 26 1990 17.78 0.25 dts 3.7 4.0 3.8 10.8 3.5 13 1350 7.70 0.16 ets 3.9 3.9 3.9 11.0 2.6 13 1360 7.84 0.16 figure 1. estimated efficiency of plant a based on m-si pv modules in a fixed installation (red circles); black squares: values after adjustment of the monitoring system; black stars: values after module washing; black diamonds: values obtained by means of the capacitive load technique; black triangles: invalid data. table 3. summary of the degradation rates of the plants under investigation estimated during different monitored periods acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 78 the results in terms of efficiency  of plant d (fixed installation) based on cigs thin-film pv modules are reported in figure 3. the meaning of the symbols is the same as in figure 1, and the invalid results are the same as those of plant a. furthermore, there is good agreement with respect to the values obtained using the capacitive load technique (black diamonds for months 1 and 78), and the results obtained after the adjustment of the monitoring system (black squares) do not significantly differ from the others. the estimated degradation rate drd is equal to -1.83 %/year (expanded uncertainty u(drd) = 0.20 %/year), which is compatible with the values obtained in the previous monitored interval (see table 3). unfortunately, the results obtained after month 65 for plant dts that was installed on a x-y tracking system are unreliable. this is due to a misalignment between the tracking systems of plants ats and dts in the month-interval (65-72) [12] and to a fault in the electrical measuring chain of this plant in the month-interval (73 87) that the plant owner has not fixed. for this reason, the estimated degradation rate for plant dts is the same as the value stated in [12], i.e., drdts = -3.34 %/year, u(drdts) = 0.26 %/year. the obtained uncertainty is low enough for us to confidently state that cigs modules are subject to an important degradation and that the modules installed on the x-y tracking system exhibit a larger degradation than the same modules in a fixed position. 3.3. cdte-based plants figure 4 shows the efficiency  of the plants based on cdte thin-film pv modules. the left chart refers to plant e (fixed position) and the right chart to plant ets (x-y tracking system). the same symbols used in the previous figures are used here, and the reasons for the invalid results (black triangles) are the same as for the other plants. the efficiency values obtained by means of the capacitive load technique for months 1 and 78 (black diamonds) are in good agreement with the observed season variability. the estimated degradation rates are dre = -2.36 %/year, u(dre) = 0.30 %/year, and drets = -2.41 %/year, u(drets) = 0.20 %/year. as summarized in table 3, these values are in agreement with the degradation rates estimated during the first 72 months of operation, while they seem better than the values estimated after 34 months of operation. however, the large uncertainty of the first estimated values does not allow us to distinguish the degradation rates estimated during the different monitored periods, since they are compatible. furthermore, in this case, we can conclude that the degradation of the cdte thin-film modules is higher than the degradation exhibited by the m-si modules. however, due to the uncertainty, it is not possible to clearly distinguish between the behavior of the modules in a fixed position with respect to the modules installed on the x-y tracking system. 3.4. p-si and string ribbon si based plants the results of the last two monitored plants are shown in figure 5: the left chart refers to the plant based on p-si pv modules, while the right chart refers to the plant that uses stringribbon si modules. the same symbols used in the previous figure 2. estimated efficiency of plant ats based on m-si pv modules on an x-y tracking system (red circles); black squares: values after adjustment of the monitoring system; black stars: values after module washing; black diamonds: values obtained by means of the capacitive load technique; black triangles: invalid data. figure 3. estimated efficiency of plant d based on cigs thin-film pv modules in a fixed installation (red circles); black squares: values after adjustment of the monitoring system; black stars: values after module washing; black diamonds: values obtained by means of the capacitive load technique; black triangles: invalid data. acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 79 figures are used here, and the reasons for the invalid results (black triangles) are the same as for the other plants in fixed positions. one should note that the adjustment of the monitoring system does not significantly change the efficiency estimations. the fitted straight lines of the two plants are very similar, which is confirmed by the numerical results: for the p-si based plant, the estimated degradation rate drb is -0.87 %/year, u(drb) = 0.35 %/year, while for the string-ribbon si-based plant, the obtained value is drc = -0.68 %/year, u(drc) = 0.25 %/year. both degradation rates are very similar to the values previously estimated (see table 3), thus confirming a medium degradation for these plants. 4. module degradation rate with the aim of confirming the estimated degradation rates for the plants that exhibited the worst behavior, an electrical characterization at module level was performed for plants d and dts, which are based on cigs thin-film modules, and for plant ets, which is based on cdte thin-film modules. the i-v characteristic of each of the 24 modules that made up each plant was obtained by means of an experimental setup that is based on a programmable electronic load connected to the pv modules under test. each i-v characteristic is obtained at the measurement conditions (air temperature in the range of 15.3 °c to 18.0 °c and irradiance in the range of 800 w/m2 to 1030 w/m2) and then reported to stc in order to obtain the parameter pmax,stc,i of each pv module. the experimental setup and the correction algorithm are the same as those described in [16]. figure 4. estimated efficiency of the plants based on cdte pv modules: e in fixed installation, ets on an x-y tracking system (red circles); black squares: values after the adjustment of the monitoring system; black stars: values after module washing; black diamonds: values obtained by means of the capacitive load technique; black triangles: invalid data. figure 5. estimated efficiency of the plants based on p-si modules (left chart) and string ribbon si modules (right chart) (red circles); black squares: values after adjustment of the monitoring system; black stars: values after module washing; black diamonds: values obtained by means of the capacitive load technique; black triangles: not valid data. acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 80 this operation was performed on march 23, 2018, approximatively 90 months after the installation of the pv modules. once the parameter pmax,stc,i was obtained, the actual degradation rate of each pv module was obtained as: )%/year( 90 12 100 inom, inom,istc,max, i    p pp dr , (6) where pnom,i is the nameplate power of the investigated pv modules, which is 70 w for cigs thin-film modules and 72.5 w for cdte thin-film modules. the obtained results are summarized in figure 6 in the form of a histogram of occurrences of the parameters dri, which were obtained according to (6). the top histogram, which refers to the pv modules of the plant d (cigs thin-film in fixed position), shows the lowest standard deviation (about 0.7 %/year) and an average value of -2.43 %/year. this value is very similar to the degradation rate estimated at plant level, which is -2.35 %/year, thus confirming the obtained outcomes. the middle histogram refers to the pv modules of the plant dts (cigs thin-film on x-y tracking system) and exhibits a standard deviation of about 0.8 %/year and an average value of -2.90 %/year. in this case, the average value is lower than the degradation rate estimated at plant level (3.34 %/year), but the two values are not comparable, since the monitoring of the plant dts only refers to the first 65 months of operation, for the reasons described in subsection 3.2. the bottom histogram, which refers to the pv modules of the plant ets (cdte thin-film on x-y tracking system), shows the highest standard deviation (about 0.9 %/year) and an average value of -2.72 %/year, which is comparable with the degradation rate at plant level (-2.41 %/year). the high standard deviation is mainly due to a pv module that shows a degradation rate of about -5.0 %/year. however, this is a biased value, since this pv module, which is positioned at the lower-left corner of the plant, presents a visible break, as shown in figure 7. the average value of the degradation rate recalculated after the broken module has been excluded does not change significantly. 5. conclusion the results of the monitoring of eight outdoor pv plants based on different technologies along a period longer than seven years have been presented in this paper. the parameters that have been taken into account in order to assess the performance of the pv plants under investigation are the maximum power and the pv efficiency at stc. the latter parameter allowed the degradation rate of each plant to be obtained, which provided interesting information related to the behavior of the different types of plants. the plants that use silicon-based pv modules are subject to a degradation rate that is significantly lower than the plants based on thin-film pv modules. taking the estimated uncertainty into account, the pv plants that use m-si, p-si and string-ribbon si pv modules exhibited a degradation rate that is in agreement with the indication given in [8] for mature module technologies. on the contrary, the plants based on thin-film pv modules exhibited degradation rates that exceed the typical value for mature technologies. the estimated uncertainty does not allow significant conclusions to be made about the different behavior of the same pv modules installed in fixed positions and on tracking systems. an estimation of the degradation rate at module level was also performed for three of the eight monitored plants, obtaining results that are in good agreement with the degradation rate at plant level. references [1] international energy agency, photovoltaic power systems programme, annual report, 2016. [2] l. francke, m. s. armand, et al., “ghg emissions and energy payback time of ac electricity generated by the sunpower® oasis® photovoltaic power plant”, proc. of the 42nd ieee figure 6. degradation rate (%/year) of the 24 pv modules of plants d, dts, and ets expressed in the form of a histogram of occurrences. figure 7. detail of the broken module in plant ets (cdte thin-film on an x-y tracking system). acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 81 photovoltaic specialist conference (pvsc), 2015, new orleans, l.a., u.s.a. [3] m. held, r. ilg, update of environmental indicators and energy payback time of cdte pv systems in europe, progress in photovoltaics 19(5) (2011) pp. 614-626. [4] a. f. sherwania, j. a. usmanib, et al., life cycle assessment of solar pv based electricity generation systems: a review, renewable and sustainable energy reviews 14(1) (2010) pp. 540544. [5] l. lu, h. x. yang, environmental payback time analysis of a roofmounted building-integrated photovoltaic (bipv) system in hong kong, applied energy 87(12) (2010) pp. 3625-3631. [6] a. nishimuraa, y. hayashia, et al., life cycle assessment and evaluation of energy payback time on high-concentration photovoltaic power generation system, applied energy 87(9) (2010) pp. 2797-2807. [7] sunearthtoll.com, photovoltaic payback [online] available: https://www.sunearthtools.com/solar/paybackphotovoltaic.php#top [8] international energy agency, methodology guidelines on life cycle assessment of photovoltaic electricity, report iea-pvps t12-03:2011. [9] a. carullo, s. corbellini, et al., in situ calibration of heterogeneous acquisition systems: the monitoring system of a photovoltaic plant, ieee trans. on instrum. and meas. 59(5) (2010) pp. 10981103. [10] a. carullo, a. vallan, outdoor experimental laboratory for longterm estimation of photovoltaic-plant performance, ieee trans. on instrum. and meas. 61(5) (2012) pp. 1307-1314. [11] a. carullo, a. vallan, et al., uncertainty analysis of degradation parameters estimated in long-term monitoring of photovoltaic plants, measurement 55 (2014) pp. 641-649. [12] a. carullo, a. castellana, et al., “degradation rate of eight photovoltaic plants: results during six years of continuous monitoring”, proc. of the 22nd imeko tc4 international symposium, sept. 14-15, 2017, iasi, romania. [13] f. rubel, m. kottek, observed and projected climate shifts 19012100 depicted by world maps of the kãüppen-geiger climate classification, meteorologische zeitschrift 19(2) (2010) pp. 135141. [14] f. spertino, j. ahmad, et al., capacitor charging method for i–v curve tracer and mppt in photovoltaic systems, solar energy 119 (2015) pp. 461-473. [15] m. martin, r. krause, et al., “investigation of potential induced degradation for various module manufacturers and technologies”, proc. of the 27th european photovoltaic solar energy conference and exhibition, 2012, frankfurt, germany, pp. 3394-3398. [16] a. carullo, a. castellana, et al., “uncertainty issues in the experimental assessment of degradation rate of power ratings in photovoltaic modules”, measurement 111 (2017), pp. 432-440. https://www.sunearthtools.com/solar/payback-photovoltaic.php#top https://www.sunearthtools.com/solar/payback-photovoltaic.php#top a new method for the calibration of strain cylinders using laser interferometry acta imeko issn: 2221-870x march 2019, volume 8, number 1, 25 – 32 acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 25 a new method for the calibration of strain cylinders using laser interferometry junning cui1, rolf kumme2, holger kahmann2 1 harbin institute of technology, harbin, pr china 2 physikalisch-technische bundesanstalt, braunschweig, germany section: research paper keywords: strain cylinder; laser interferometry; calibration; deformation; compression testing machine. citation: junning cui, rolf kumme, holger kahmann, a new method for the calibration of strain cylinders using laser interferometry, acta imeko, vol. 8, no. 1, article 5, march 2019, identifier: imeko-acta-08 (2019)-01-05 editor: petri koponen, mikes metrology, finland received august 23, 2018; in final form january 31, 2019; published march 2019 copyright: © 2019 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: rolf kumme, e-mail: rolf.kumme@ptb.de 1. introduction a strain cylinder, also referred to as a strain gauged column, is a type of force transducer with four channel outputs. it is often used for the alignment verification of compression testing machines. specifications of strain cylinders and the proof procedure for the verification of compression testing machines are specified in international and national standards [1]-[2]. the calibration of strain cylinders can be done in a force standard machine according to iso standard 376 (iso 376)[3]. strain cylinders are used in two main ways. first, since the difference between the signals of the four bridges represent the distribution of the force applied on the strain cylinders, they are used as standards for the alignment verification of compression testing machines, including the self-alignment of upper machine platens, the alignment of component parts of machines, and so on. second, they can also be used as normal force transducers for force measurement, taking the average signals of the four bridges. therefore, during the indication calibration of the compression testing machines, strain cylinders can also be used as force transfer standards. normally, only mechanical qualities like force and strain are of interest during the calibration and application of strain cylinders, while deformation is not given special attention. however, since the platens of the testing machines are sufficiently large and strong, we can ignore integral deformation, and the compression deformation of strain cylinders and local deformation distribution in the surface of platens is mostly decided according to material characteristics. therefore, in many applications, the deformation is highly significant, and it is often hoped that deformation indication is provided. methods and tools in precision engineering have frequently been introduced into the field of force metrology over recent years. for example, laser interferometry has been used in the measurement of the deformation or displacement of key parts [4]-[6], dynamic calibration [7]-[9], and so on. the coordinate measuring technique has been used in on-machine or offmachine measurements of key dimensional parameters [10], [11]. these methods and tools show great capability and advantages in solving certain problems. abstract this research investigates the possibility of calibrating strain cylinders using laser interferometry, thus providing a new type of transducer that can provide both force and deformation indications. this new method for the calibration of strain cylinders is based on the application of a dual channel laser interferometer in a force standard machine. using the proposed new method, the relationship between force, deformation, and strain can be calibrated in parallel when calibration forces are applied according to the procedure outlined in iso 376. experimental results show that the deformation of a strain cylinder has a definite and stable relationship with the force applied and can be calibrated and directly traced to the wavelength of the laser. therefore, a new standard that can be used for both alignment verification and indication verification of compression testing machines and other uniaxial testing machines is proven. the research also illustrates the possibility of providing a new deformation-type force transducer using a non-gauged steel cylinder together with a multi-channel laser interferometer. mailto:rolf.kumme@ptb.de acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 26 figure 1. structure of a strain cylinder and its strain bridges [1]. in this paper, a new method for the calibration of strain cylinders using laser interferometry is proposed and investigated. the new method is based on the application of a dual channel laser interferometer in a force standard machine and shows key benefits and provides important information. 2. basic concept the structures of a strain cylinder and its strain bridges are shown in figure 1. a strain cylinder consists of a steel cylinder, 16 strain gauges, and other accessories. it has profound demands in terms of its materials, manufacture, assembly, and calibration. the steel cylinder should be precisely manufactured and assembled with 16 matched and temperature-compensated electrical resistance strain gauges. each four strain gauges are wired in the form of a full bridge, electrically and thermally balanced, and assembled at one of the ends of a pair of orthogonal diameters halfway up the cylinder. furthermore, the strain cylinder needs to be precisely calibrated on a force standard machine together with the dedicated strain measuring equipment. this calibration qualifies the strain cylinder to be used as a standard for both the alignment verification and indication verification of testing machines. the conversions of physical quantities during the calibration and application of the strain cylinders are shown in figure 2. it can be seen that several conversions of physical quantities are involved, during which process the deformation and strain serve as the most important intermediate quantities. at present, strain cylinders are calibrated and used focusing only on the strain rather than the deformation. however, in many applications, the deformation is important and represents key information about alignment, force distribution, and so on. this is the reason why it is often hoped that deformation indication is also provided by strain cylinders. normally, the platens of compressing testing machines are designed in such a way that they are sufficiently large and strong so as to avoid integral deformation. in turn, the local deformation in the surface of the platens and strain cylinders is only related to their material characteristics. therefore, important information can be derived from data on deformation. the aim of the new method for the calibration of strain cylinders is to introduce laser interferometer to a force standard machine and obtain deformation information when calibration forces are applied. it is also hoped that the method will allow for the calibration of the relationship between force, deformation, and strain in parallel. in this way, a new standard that can provide both force and deformation indication is provided, and its use for both the alignment verification and indication verification of compression testing machines is proven. furthermore, this research illustrates the possibility of providing a new deformation-type force transducer using a non-gauged steel cylinder together with a multi-channel laser interferometer. this new force transducer is much simpler in structure and usage, and it does not require the same complicated manufacturing and assembly processes as strain gauges. this idea is applicable on the condition that the deformation of a strain cylinder has a definite and stable relationship with the force applied and can be calibrated using a laser interferometer in a force standard machine with good repeatability and reproducibility. this will be proven through experiments and the assessment of this new type of transducer in the following sections. 3. perimental setup and calibration procedure 3.1. experimental setup the experimental setup is shown in figure 3. the experiments were carried out in a 5 mn force standard machine. the maximum force applied on the cylinder is 2 mn. the strain cylinder, laser heads, and reflectors of the laser interferometer are assembled between the upper and lower platens of the machine, and the strain cylinder is centrally assembled on the lower platen. figure 2. conversions of the physical quantities of strain cylinders. a) b) figure 3. experimental setup in the 5 mn force standard machine. a) experimental setup. b) top view of the strain cylinder with four bridges and the four corresponding measurement points. acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 27 the strain cylinder is 100 mm in diameter and 200 mm in height. it conforms to the requirements for material and dimensional tolerances in [1]. four full bridges of strain gauges, labelled channels 1 to 4, are assembled at fours ends of a pair of orthogonal diameters halfway up the cylinder. each bridge consists of two elements measuring the axial strain and two elements measuring the circumferential strain. the outputs of the four bridges are recorded by the dedicated strain measuring equipment dmp41. a mi-5000 dual channel laser interferometer from sios meßtechnik gmbh is used. its relative length measurement accuracy is up to 110-6; therefore, the length measurement error introduced during the experiments is negligible, showing the great advantage of this precision engineering tool. for each channel, interferometric optics, excluding the measuring reflector, are assembled together as a laser head. the laser is transmitted to each of the two laser heads through a length of optical fibre. corresponding to the orientations of the four bridges on the strain cylinder, there are four measurement points for the laser heads on the lower platen, which are at four ends of a pair of corresponding orthogonal diameters of a circle and designated as a, b, c, and d in figure 3 b). in such a way, the strain cylinder together with the laser heads work as a new type of transducer with four ‘deformation sensing elements’. the four ‘deformation sensing elements’ are uniformly distributed over 360 and labelled as channels 1 to 4 respectively. the distance between the measurement points and the sidewall of the strain cylinder should be kept as small as possible. in this study, it is 60 mm. the influence of this distance and the resultant deformation distribution in the surface of the platens can be taken into consideration in two ways: 1) when the strain cylinder is used in a compression testing machine, the same parameter should be maintained. usually, the platens of the compression testing machine and the force standard machine are made of material similar to young’s elastic modulus. the error introduced should be corrected if there is an obvious difference; 2) the deformation to the central position of the platens for both the force standard machine and the compression testing machine should be corrected. 3.2. calibration procedure the procedure for calibrating a strain cylinder using a dual channel laser interferometer is proposed with reference to iso 376. the cylinder is rotated around its axis to four uniformly distributed positions over 360, as shown in figure 4. deformation and strain data at the four rotation positions are averaged to eliminate the influence of the non-uniform force distribution characteristics of the force standard machine. a series of increasing calibration forces and then a series of decreasing calibration forces are applied at each rotation position. because strain cylinders are only used for compression testing, the data corresponding to the decreasing calibration forces are not used for interpolation, but are only used for the assessment of the relative reversibility error in section 4.3. the two laser heads are first assembled at point a and point b, and the strain cylinder is calibrated at the four rotation positions. then, the two laser heads are reassembled to point c and point d, and the strain cylinder is calibrated at another four rotation positions. in such a way, the outputs of the four channel ‘deformation sensing elements’ of the new type of transducer can be obtained at each rotation position. the series of increasing calibration forces are set from 0 to 2000 kn, with a 200 kn increment, and the series of decreasing calibration forces are set from 2000 kn down to 0 kn, with a 200 kn decrement. in total, 11 calibration forces (including zero force), uniformly distributed over the calibration range, are applied. there is a wait of ~15 s after a calibration force is applied, waiting for the force to become stable. then, the calibration force is maintained, unchanged, for ~45 s before it is changed to the next successive value. the data recorded by dmp41 is designated as xchi, j and xchi, j in the following sections, where i is the channel number of strain bridges (i = 1, 2, 3, or 4), j is the rotation position of the strain cylinder (j = 0, 90, 180, or 240), xchi, j corresponds to the increasing calibration forces, and xchi, j corresponds to the decreasing calibration forces. similarly, the data recorded by the laser interferometer is designated as ychi, j and ychi, j, where i is the channel number of ‘deformation sensing elements’. 3.3. data sampling setting it is a good idea to use a high data sampling rate based on the strategy of ‘more data, more information’. however, a high sampling rate results in a high demand for hardware and software, making the data processing difficult. on the other hand, according to general knowledge concerning the force standard machine, the natural frequency of the micro-vibration of the lower platen is ~0.27 hz, so the period of vibration is ~3.7 s. therefore, the data sampling rates of the laser interferometer and dmp41 are both set at 10 hz. therefore, the sampling can be regarded as ‘dynamic’ compared to the period of platen vibration, thereby including any important information. 4. data processing and experimental results 4.1. data pre-processing the abundance of data continuously sampled, at a sampling rate of 10hz, are pre-processed first so as to obtain the values of deformation and strain corresponding to the 11 calibration forces during each incremental force loading and decremental force loading. for the data on the laser interferometer during each calibration, first, the corner points corresponding to the moments when a calibration force begins to increase or decrease to the next successive value are determined with a slope-judging algorithm, marked with a green * in figure 5. then, the average values of the 25 s data before each corner point are calculated, marked with green lines in figure 5. this averaging operation works as a low-pass filter (lpf) with a cut-off frequency of ~ 0.04 hz, thus eliminating the influence of the resultant noise from the mechanical vibration and other factors. the deformation values corresponding to 10 calibration forces during each incremental force loading and decremental force loading are obtained. figure 4. the procedure for calibrating a strain cylinder using a dual channel laser interferometer. acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 28 theoretically, the indication of the laser interferometer should be reset to zero at the moment at which the upper platen comes into contact with the strain cylinder i.e. the moment at which the calibration force begins to increase from zero. it is a good solution to send a trigger signal to the laser interferometer either automatically or manually, or to use one of the following two methods: (1) during data pre-processing, the data corresponding to zero force should be recognised and all successive data should be subtracted by the corresponding zero data, which is the case in this research; (2) the values of deformation corresponding to 10 calibration forces from 200 kn to 2000 kn should be used for interpolation, and the deformation value from 0 to 200 kn through extrapolation should be calculated. all deformation values from 0 to 2000 kn should be subtracted by the value corresponding to zero force. the data on dmp41 is similarly pre-processed, as shown in figure 6. the only exception is that the strain value corresponding to zero force can be directly calculated by averaging the 25 s data before the corresponding corner point. in total, 11 strain values corresponding to the 11 calibration forces (including zero force) are used for interpolation. 4.2. data processing results by means of data pre-processing, the four channels’ values of deformation and strain corresponding to the 11 calibration forces at each rotation position, either increasing or decreasing, are obtained. the interpolation curves are determined using the average values of deformation and strain at the four rotation positions. as mentioned above, data corresponding to the decreasing calibration forces is not used for interpolation. threedegree polynomial fitting is carried out to obtain third-degree equations about the relationship between force, deformation, and strain. 4.2.1. the polynomial fitting results of force vs. deformation the equations for calculating the deformation as a function of strain are as follows: (1) (2) where f is the calibration force in kn; ychi is the deformation of channel i in µm; i = 1, 2, 3, or 4; and y is the average deformation of the four channels. the curves showing the four third-degree equations are shown in figure 7. it can be seen that the four channels of the strain cylinder have very good consistency. it can be concluded that the deformation of a strain cylinder has a definite and stable relationship with the force applied by the force standard machine. equation (1) and equation (2) also illustrate the possibility of providing a new deformation-type force transducer using a non-gauged steel cylinder together with a multi-channel laser interferometer, using either average or nonaveraged values of the deformation of the four channels. the average value represents the amplitude of the force applied on the strain cylinder, while the non-averaged values of the four channels illustrate the force distribution. 4.2.2. the polynomial fitting results of deformation vs. strain and force vs. strain the equations for calculating the deformation as a function of strain are as follows: (3) (4) where xchi is the strain indication of channel i in mv/v; i = 1, 2, 3, or 4; and x is the average strain of the four channels. figure 5. pre-processing of the data on the laser interferometer (0 rotation position). figure 6. pre-processing of the data on the four channels of dmp41 (0 rotation position). figure 7. curves showing the relationship between force and deformation. acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 29 the curves showing the four third-degree equations are shown in figure 8. since the four strain bridges are well matched as well as thermally and electrically balanced, so too do these four curves have good consistency. equations giving the calibration force as a function of strain can be similarly calculated, as follows: (5) (6) during real applications, either the average signal or the nonaveraged signals of the four channel strain bridges can be used. the average signal represents the amplitude of the force applied on the strain cylinder, while the non-averaged values of the four channels illustrates the force distribution applied on the strain cylinder. using equation (3), equation (4), equation (5), and equation (6), a new type of transducer that provides both force indication and deformation indications is provided. this new type of transducer can be used as a standard for both the alignment verification and indication verification of compression testing machines. 4.3. assessment of the deformation calibration results the calibration results of deformation y are assessed with reference to iso 376. the relative length measurement accuracy of the mi-5000 laser interferometer is up to 1  10-6, and the uncertainty of the calibration forces applied by the 5 mn force standard machine is up to 1  10-4 (k = 2); therefore, this assessment can be used to verify the proposed new method. data corresponding to decremental force loading is used during the assessment of relative reversibility error. 4.3.1. relative reproducibility error the relative reproducibility error of deformation value y assesses the variation of the deformation indication at different rotation positions. the average deformation of the four channels is used for assessment. the error is calculated using the following equations in accordance with iso 376: (7) the relative reproducibility errors are shown in figure 9. the maximum relative reproducibility error bmax results when the calibration force is 200 kn and bmax = 6.4 %. we found that the reproducibility error mostly results due to the non-uniform force distribution characteristics of the force standard machine. the indication variation of deformation at different rotation positions and the relative reproducibility error can be significantly reduced if the force distribution is more uniform. 4.3.2. relative interpolation error the relative interpolation error is calculated for each channel using the following equation: (8) where yachi is the value of deformation of channel i computed by means of inverse functions of equation (1). these functions give the deformation as a function of the calibration force. the relative interpolation errors of channels 1 to 4 are shown in figure 10. the maximum relative interpolation error |fc|max results in channel 4 when the calibration force is 200 kn, |fc|max = 0.84 %. the relative interpolation error can be averaged if the average deformation of the four channels is used. 4.3.3. relative zero error the relative zero error of channel i at j degree rotation position is calculated using the following equation: (9) where yochi is the deformation taken before the calibration force is applied, and yfchi,j is the deformation taken after the calibration figure 8. curves showing the relationship between deformation and strain. figure 9. relative reproducibility errors in %. figure 10. relative interpolation errors of the four channels. acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 30 force is completely removed. ynchi,j is the deformation corresponding to the 2000 kn maximum calibration force. the relative zero errors of channels 1 to 4 are shown in figure 11. the maximum relative zero error fomax results in channel 3 when the cylinder is set at a 270 rotation position and fomax = -3.5 %. 4.3.4. relative reversibility error the relative reversibility error of channel i at j degree rotation position vchi,j and the relative reversibility error of channel i vchi are calculated using the following equations respectively: (10) the relative reversibility errors of channels 1 to 4 are shown in figure 12. the maximum relative reversibility error vmax results in channel 4 when the calibration force is 200 kn and vmax = 13 %. the reversibility error mostly results from the hysteresis between the incremental loading and the decremental force loading. since strain cylinders are only used for compression testing, this is unproblematic. 4.3.5. relative creep error the relative creep error of channel i is calculated using the following equation: (11) where y20chi and y200chi are deformation indications obtained 20 s and 200 s after the maximum calibration force is applied; and ynchi is the maximum deformation corresponding to the 2000 kn maximum calibration force. remark: according to iso 376, the creep should be measured within 300 s, but in this experiment, the creep was measured within 200 s in order to reduce the measurement time. the calculation of the average of the 25 s data that works as a 0.04 hz lpf is used. the experimental results of the relative creep error test are shown in figure 13. the maximum relative creep error cmax = 0.091 %. 4.4. analysis of influential factors several main influential factors concerning the experimental results can be analysed. this analysis should reveal the value of the influence of the force cylinder tolerance and how the influence of the non-ideal characteristics of the force standard machine [12]-[15] is greatly reduced by means of the calibration procedure design proposed in iso 376. 4.4.1. influence of the rotation of the strain cylinder according to the experimental procedure design, the influence of the rotation of the strain cylinder can be evaluated by means of the reproducibility of a single laser head assembled at a fixed position, while the strain cylinder is calibrated at four rotation positions. as shown in figure 14, the measurement results of the laser head fixed at point c have very good reproducibility when incremental forces are applied on the strain cylinder at the four rotation positions. this proves that the rotation of a force cylinder, which conforms to the requirements of material and dimensional tolerance, has nearly no influence on the measurement results. we can ignore the non-parallelism of its upper and lower surfaces and other dimensional tolerance, unlike the dimensional tolerance of the force standard machines. 4.4.2. averaging the different rotation positions the method of averaging measurement results at multiple rotation positions is proposed in iso 376. theoretically, the influence of non-uniform force distribution, contact effect, and so on can be significantly reduced using this method. in this study, the difference between the characteristics of the different channel strain bridges in the strain cylinder is less than 0.1 %. therefore, the influence of the averaging of the different rotation positions can be evaluated by means of a comparison between the different channel calibration results. figure 11. relative zero errors of the four channels. figure 12. relative reversibility errors of the four channels. figure 13. experimental results of the relative creep error test. acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 31 the calibration results of the four-channel relationship between strain and force are shown in figure 15. the maximum deviation of strain when 200 kn force is applied is 0.007 mv/v. the maximum strain when the 2000 kn maximum force is applied is 1.6445 mv/v; therefore, the relative difference is ((0.007 mv/v) / (1.6445 mv/v))  100 % = 0.4 %. it is proven that the error introduced by non-uniform force distribution characteristics, contact effect, and so on are effectively reduced by averaging the different rotation positions. 4.5. proposals in this research, the calibration procedure, data processing method, and assessment of the transducer calibration are designed with reference to iso 376. however, some suggestions and proposals should be noted: (1) the cylinder should be rotated symmetrically around its axis to at least three uniformly distributed positions over 360, thus eliminating the influence of the non-uniform force distribution characteristics of the force standard machine; (2) in this research, the incremental and decremental force loading are both applied on the strain cylinder. since strain cylinders are only used for compression testing, the data corresponding to decremental force loading is only used for assessment rather than interpolation. in order to get as much information as possible, ‘dynamic’ data sampling is advisable, but not mandatory; (3) during data pre-processing, the calculation of deformation and strain corresponding to zero force should be given special attention. readings of strain bridges can be directly taken before the force is applied or after the force has been completely removed. however, readings of the laser interferometer corresponding to zero force should be taken at the moment at which the upper platen comes into contact or loose contact with the cylinder. the indication of the laser interferometer can be reset to zero at this moment, using the methods recommended in section 4.1; and (4) if a non-gauged steel cylinder is calibrated using the proposed method and used together with a multi-channel laser interferometer, a ‘deformation-type’ force transducer is provided. the distance between measurement laser beams and the sidewall of the cylinder should be kept as small as possible, and the error introduced can be considered using the methods recommended in section 3.1. if the deformation of platens is negligible because the calibration force is too small or the platens are sufficiently strong, this does not prove problematic. 5. conclusion a new method for the calibration of strain cylinders using laser interferometry was proposed and investigated in this study. this new method was illustrated by means of calibrating a strain cylinder with a sios dual channel laser interferometer in a 5 mn force standard machine at ptb. the calibration procedure, data processing method, and assessment of calibration results were designed with reference to iso 376. experimental results show that the deformation of a strain cylinder has a definite and stable relationship with the force applied and can be calibrated and directly traced to the wavelength of laser. therefore, a new type of transducer that can provide both force and deformation indications has been proven herein, and it can be used for both the alignment verification and indication verification of compression testing machines and other uniaxial testing machines. this method can also be applied to non-gauged standard cylinders, thus providing a new deformation-type force transducer using a non-gauged steel cylinder together with a multi-channel laser interferometer. acknowledgement the authors gratefully acknowledge the valuable help of heiko wunderlich of ptb on the experimental operation, and a constructive technical discussion with michael wagner of ptb. references [1] en 12390-4: testing hardened concrete – part 4: compressive strength – specification for testing machines, 2000. [2] din 51302-2: strain cylinder test for verifying compression testing machines for testing concrete, 2000. [3] iso 376: metallic materials – calibration of force-proving instruments used for the verification of uniaxial testing machines, 2011. [4] c. junning, r. kumme, h. kahmann, proposal and investigation of a new method for calibration of strain cylinders using laser interferometry, proc. of the 23rd imeko tc3 conference on the measurement of force, mass and torque, helsinki, finland, 30 may – 01 june 2017. [5] gordon a. shaw, koo-hyun chung, douglas t. smith et al., methods for transferring the si unit of force from millinewtons to piconewtons. proceedings of the sem annual conference, albuquerque new mexico usa. 1 – 4 june 2009. [6] c. bartoli, m. f. beug, t. bruns et al., traceable dynamic measurement of mechanical quantities: objectives and first results of this european project, int. j. metrol. qual. eng. 3 (2012), pp. 127-135. figure 14. deformation at point c when incremental force is applied at the four rotation positions. figure 15. the calibration results of the four-channel relationship between strain and force. acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 32 [7] l. zhang, r. kumme, investigation of interferometric methods for dynamic force measurement, proc. of the xvii imeko world congress, dubrovnik, croatia, 22 28 june 2003, pp. 315318 https://www.imeko.org/publications/wc-2003/pwc-2003tc3-015.pdf. [8] y. fujii, j. d. r. valera, impact force measurement using an inertial mass and a digitizer, meas. sci. technol. 17 (2006), pp. 863868. [9] y. fujii, optical method for accurate force measurement: dynamic response evaluation of an impact hammer. optical engineering 45, 2 (2006), 023002-1-7. [10] s. baumgarten, d. röske, h. kahmann et al., concept and setup of a multi-component facility for force and torque in the range of 1 mn and 2 kn·m, acta imeko 3 (2014) 2, pp. 39-43, doi: https://doi.org/10.21014/acta_imeko.v3i2.66. [11] d. röske, k. adolf, d. peschel, lever optimization for torque standard machines, proc. of the xvi imeko world congress, vienna, austria, 25 – 28 september 2000, https://www.imeko.org/publications/wc-2000/imeko-wc2000-tc3-p100.pdf. [12] y.-k. park, d.-i. kang, oscillating signal components of a deadweight force-standard machine and reduction techniques, meas. sci. technol. 10 (1999), pp. 748-754. [13] y.-k. park, d.-i. kang, pendulum motion of a deadweight forcestandard machine, meas. sci. technol. 11 (2000), pp. 1766-1771. [14] s. k. jain, h. kumar, s. s. k. titus, et al., metrological characterization of the new 1 mn force standard machine of npl india, measurement 45, 3 (2012), pp. 590-592. [15] t. hayashi, y. katase, h. maejima, recent renovations of deadweight type force standard machines at nmij, measurement 46, 10 (2013), pp. 2127-2134. https://www.imeko.org/publications/wc-2003/pwc-2003-tc3-015.pdf https://www.imeko.org/publications/wc-2003/pwc-2003-tc3-015.pdf https://doi.org/10.21014/acta_imeko.v3i2.66 https://www.imeko.org/publications/wc-2000/imeko-wc-2000-tc3-p100.pdf https://www.imeko.org/publications/wc-2000/imeko-wc-2000-tc3-p100.pdf clinical computed tomography and surface-enhanced raman scattering characterisation of ancient pigments acta imeko issn: 2221-870x march 2021, volume 10, number 1, 15 22 acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 15 clinical computed tomography and surface-enhanced raman scattering characterisation of ancient pigments sveva longo1,2, silvia capuani2, francesca granata3, fortunato neri1, enza fazio1 1 department of mathematical and computational sciences, physics science and earth sciences (mift), university of messina, viale f. stagno d’alcontres, 98166 messina, italy. 2 nuclear magnetic resonance and medical physics laboratory cnr-isc, department of physics, sapienza university of rome, piazzale aldo moro 5, 00185 roma, italy. 3 neuroradiology unit-department of biomedical sciences and morphological and functional images (biomorf), university of messina, via consolare valeria 1, 98125 messina, italy. section: research paper keywords: x-ray computed tomography; hounsfield units; sers; ftir; optical absorbance; ancient pigments; conservation science citation: sveva longo, silvia capuani, francesca granata, fortunato neri, enza fazio, clinical computed tomography and surface-enhanced raman scattering characterisation of ancient pigments, acta imeko, vol. 10, no. 1, article 4, march 2021, identifier: imeko-acta-10 (2021)-01-04 editor: eulalia balestrieri, university of sannio, italy received march 24, 2020; in final form august 25, 2020; published march 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was supported by po fse regione siciliana 2014-2020. corresponding author: sveva longo, e-mail: sveva.longo@unime.it 1. introduction the identification and characterisation of pigments provide essential information for the dating, authentication and conservation of wood-based objects and for the study of art history in general [1], [2]. spectroscopic techniques are typically used to characterise pigments. however, different methods are used to describe inorganic and organic pigments due to the intrinsic limitations of the techniques: x-ray fluorescence cannot provide a definitive elemental signature; fourier transform infrared spectroscopy is limited by the interference of binders and extenders [3] and high-performance liquid chromatography [4] requires relatively large samples (0.5–5 mm in diameter). in the last few years, portable raman setups have regularly been used to study a wide variety of materials, including paper, in order to follow degradation processes or create spectral reference libraries of organic and inorganic pigments. however, some laser source wavelength is absorbed by specific pigments, leading to extensive fluorescence backgrounds, which obscure the weak raman signals, or in worse cases, cause photo-degradation of the sample, preventing pigment identification [5]. surface-enhanced raman scattering (sers) addresses these difficulties since the presence of noble metal nanoparticles induces a giant amplification of the raman signal [6]. clinical computed tomography based on multislice technology (msct), which has been used for decades in the medical field, can be successfully applied for the localisation of abstract a systematic and complete chemical and physical characterisation of painted pigments on wood samples was carried out using multislice x-ray computed tomography (msct) and surface-enhanced raman scattering (sers) techniques. inorganic and organic pigments present on the wooden tablets were differentiated by msct determinations of hounsfield units, a semi-quantitative method for measuring xray attenuation that, in turn, offers an indirect estimation of a material’s density. however, the msct technique is not as reliable as traditional spectroscopic techniques for recognising and classifying organic pigments. nevertheless, a strength of the msct approach was its ability to simultaneously provide a volumetric view of the wood and segment the layers of the specimen using suitable reconstruction methods such as is generally done for biomedical applications. furthermore, the sers technique made it possible to identify the type of material present in the pigments (for both inorganic or organic materials) with a high spatial resolution, even pigments in mixtures or those applied directly on the investigated wooden support. the combined msct and sers data obtained through this systematic investigation constitutes the basis for the assembly of larger reference databases that will ultimately support the development of long-term conservation protocols. mailto:sveva.longo@unime.it acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 16 structures on or under the painting's surface [7] without removing the paint or scratching the surface of the wood. msct can also be used for densitometric estimation, which is expressed in hounsfield units (hu). in the biomedical field, the density values of different structures and tissues in the human body and their variations have been estimated primarily through the use of msct techniques. specifically, using msct scans, it is possible to reconstruct images through two main techniques: 2d images can be generated using multiplanar reconstruction (mpr), and 3d models can be generated using volume rendering technique (vrt). the vrt images are generated using 3d filters calibrated with reference to known densities (measured in hu) (e.g. of various human body tissues). the technique is used by medical doctors to navigate inside a human body virtually. using vrt reconstruction, it is possible to select a specific density and virtually erase the others. in this way, an object can be virtually dissected and studied from every angle [8]. nevertheless, there are some limitations, which are primarily related to the minimum slice thickness (0.6 mm in our case), although the newest generation of siemens computer tomography (ct) scanners have a spatial resolution of 0.2 mm, and we think that this capability will continue to improve. the gantry opening can also be a limiting factor; the maximum size available on the market is 78 cm in diameter. with an opening of this size, the method can be applied to small and medium-size objects, which includes the majority of artefacts. moreover, the widespread international adoption of this instrumentation makes it possible to carry out this type of analysis in any city, and scans can be completed very quickly (only 12 s per scan). for these reasons, improvements to this method should be explored to make this innovative, valuable and useful tool more available for cultural heritage investigations. in this paper, clinical ct and its dedicated software (designed for medical use) were used for the first time to systematically investigate the microstructure of two wooden tablets painted with ancient pigments in accordance with cennino cennini's instructions [9]. ct analyses were used to investigate the status of the wood and, in particular, to provisionally classify the detected pigments. however, we found that some of the identifications made using the hu values (indirect indices of the pigment densities determined according to conventional approaches) were not realistic. therefore, the combination of msct and spectroscopic analyses was used to ensure accurate identification. in particular, raman/sers investigations were used to confirm the dubious results of the msct analysis with regard to organic pigments that were present on the tablets, especially those at low concentrations. for correct identification, sers spectra of several varieties of pigments were acquired as references. in this way, we discriminated the various pigments and identified their regular contribution on wooden tablets. 2. materials and methods 2.1 sample preparation two chestnut oak (castanea sativa) wooden tablets measuring 33 × 14 cm2 and painted with inorganic and organic pigments from zecchi florence were investigated [10]. the pigments we used were red ochre, natural ultramarine, white lead, cinnabar, madder lake, malachite, red lead and azurite. rabbit skin tempera glue was chosen as a binder, and two types of wooden support were utilised, one prepared with bologna plaster and one prepared with incamottatura, which was widely used on wooden paintings in the 11th to the 14th centuries and consists of a layer of linen canvas applied on the wooden support to simplify the colour expanse. 2.2 multislice computed tomography in our experiments, a somatom sensation 16 msct (siemens healthcare) with a 70 cm gantry opening located in the neuroradiology unit-department of policlinic g. martino of messina was used (figure 1). a ct scanner is composed of three units (figure 2): 1 the scanning unit (gantry) in which the high voltage generator, x-ray tube, collimation system, detectors and data acquisition system are collocated, 2 the computers used to scan and reconstruct the data and 3 the control console, comprising systems for the display, storage and playback of images. after the acquisition parameters were optimised to obtain well-resolved images, all of the msct images were acquired using the following parameters: 62 ma, 120 kv and b60f smooth kernel. a 0.6 mm slice thickness was used in order to obtain the highest resolution. the kernel number determines the sharpness of the edges and the smoothness of the images. in our case, we used soft tissue algorithms performed with a kernel setting of 20–40, and bone algorithms, which use a 60 or 70 kernel setting. these settings were chosen because bones and soft tissues have characteristics similar to wood. thus, we expected that a low kernel setting (smoother image) would generate good contrast resolution, a low noise level and poor edge definition, as happens in biomedical diagnosis. moreover, a higher number of kernels (sharper image) should provide better spatial resolution, a higher noise level and better edge definition [11]. the resulting images were evaluated using mpr, vrt and siemens syngo software. several regions of interest were selected on the mpr images, and the hu mean values of the pigments were estimated. during vrt imaging post-processing, different ct medical filters with densitometric characteristics close to those of our materials, such as those for tissue and skin, were explored. it was expected (as already proved with preliminary tests) that conventional densiometric similarities between wood and tissues would correspond to analogous figure 1. wooden tablets painted with the most commonly used ancient pigments placed on the ct scanner bed for msct scanning. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 17 behaviour in terms of attenuation of the x-ray beam during ct characterisations. a further advantage of msct is the short time needed to perform image scans. as it was developed for human analysis, it was designed to generate quality images quickly. for example, images of organs are acquired in 1 second, those of the heart in 5 seconds and those of the whole body in less than 10 seconds [12]. longer scans do not improve measurements because the technique is not time dependant. in this study, we scanned the two wood specimens in 12 seconds. the most important innovation that follows from this method is the possibility of estimating the different densities of materials through attenuation measurements [13]. attenuation, which measures the decrease in x-ray beam intensity as it passes through matter, is the most crucial parameter of msct. this reduction in x-ray intensity is a consequence of absorption, deflection or scattering processes, and, thus, it is affected by several different factors including the beam energy, the atomic number of the absorber and the thickness of the scanned object. the measurement of the electron density is made using the hounsfield scale, expressed in hu, which includes 2001 different shades of grey, from black to white. the hu scale is a linear transformation of the original linear attenuation coefficient measurement into one in which the radiodensity of distilled water at standard pressure and temperature is defined as 0 hu, while the radiodensity of air at standard pressure and temperature is defined as −1000 hu and the radiodensity of bone is set to +1000 hu. therefore, for any material with linear attenuation coefficient μ, the corresponding hu value is given by 𝐻𝑈 = 1000 𝑥 𝜇 − 𝜇w 𝜇w − 𝜇air , (1) where μw and μair are the linear attenuation coefficients of water and air, respectively; consequently, the hounsfield unit is a relative parameter in which water is taken as a reference substance (figure 3). 2.3 spectroscopic techniques: uv-vis optical absorption, fourier transform infrared spectroscopy and surfaceenhanced raman scattering uv-vis optical absorption spectra were obtained using the perkinelmer lambda 750 uv/vis/nir spectrometer in the 190–1100 nm range. spectra were acquired at room temperature in a few minutes, and each measurement was conducted twice to test its repeatability. uv-vis optical absorption spectra of au colloids were acquired three times: immediately after the ablation process using quartz cuvettes, one day after this to monitor their stability over time and, finally, after the au substrates were soaked in the pigments. ftir spectra were collected at room temperature using a perkinelmer spectrum 100 spectrometer in the 450–4000 cm−1 range. the spectrometer was used in an attenuated total reflectance (atr) configuration. powdered pigments were used as a reference, and pigments painted on the wooden support were analysed without any treatment of the samples. they were placed directly on the atr support. ftir data were collected in a few minutes, and the resulting spectra were the average of ten acquisitions. to carry out the sers analysis, gold (au) nanocolloids were prepared using a green laser ablation synthesis procedure [14] and deposited on glass and silicon substrates. sers spectra were acquired after the au substrates were soaked in pigments such as pure alizarin, carmine lake and purpurin solutions at different concentration levels (between 10−3 and 10−5 m) for about 1 h and then rinsed with deionised water and left to dry in air. each sample shows a specific molecular fingerprint and a high homogeneity, which was confirmed by performing measurements on a 60 × 60 µm2 surface area. after the acquisition of the raman spectra of the powdered pigments used as references, sers measurements were carried out on the painted wooden tablets. raman spectra were collected at room temperature using a horiba xplora spectrometer. the diode lasers were focused on a 50 μm diameter spot with a power of about 1.4 w and 0.5 w for 532 nm and 785 nm excitations, respectively. the integration time for each measurement was 80 and 150 s, with an accumulation time of 3 s. 3. results and discussion 3.1 multislice computed tomography data analysis in table 1, the hu values estimated by msct and the density in g/cm3 reported in [15] of the investigated pigments are presented. most of the investigated pigments are inorganic and made with metallic oxide. for example, white lead is made from basic lead carbonate [2pbco3·pb (oh)2] and has a radiodensity of about 2852 hu, the highest value of the studied samples. red lead (2pbo·pbo2) is also made from lead, but, due to its different chemical coordination, it has a radiodensity of 1456 hu. the radiodensities of the other samples can be listed in descending order as follows: 1534 hu for cinnabar; 775 hu for red ochre; 674 hu for malachite; 356 hu for azurite; 297 hu for natural ultramarine and, finally, 218 hu for madder lake. as shown in figure 4, all of these values closely follow the density trend figure 2. multislice computed tomography schema. figure 3. hounsfield unit scale. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 18 estimated by conventional methods (jolly balance, beam balance, pycnometer and heavy liquids). as expected, higher density values correspond to higher hu values. moreover, metals are in the positive range of hu values starting from 1500 hu. however, some materials of equal density have different hu values. this experimental evidence demonstrates an added value provided by msct technique. for example, cinnabar and red lead have the same density, but their radiodensities are 8.2 hu and about 1500 hu, respectively. similarly, azurite and malachite both have a density of 3.8 g/cm3, but they have different radiodensities (356 and 674 hu, respectively). in this case, however, the observed differences were probably due to inhomogeneities in the pigment layer measuring less than 0.6 mm (less than the ct slice thickness). azurite is less opaque than malachite, and it is probable that the concentration of azurite in its measured area was less than the corresponding concentration of malachite. finally, a drawback of msct must be mentioned. the estimated hu values of madder lake are not realistic, and this can be explained by considering the underlying mechanisms of the msct technique. attenuation coefficient values are affected by the absorption and scattering of the incident radiation. at the xray energy level used in our ct investigation, 120 kev, a competition between compton scattering (which results in incoherent scattering for elements of z < 20) and photoelectric absorption (which increases for z > 20) occurs. thus, the influence of these mechanisms seems relevant when considering organic pigments such as madder lake. the madder lake value was also invalidated by the presence of calcium sulphate hydrate (bologna plaster), which was used during the sample preparation and has a density of around 2.9 g/cm3. in this work, we chose medical vrt filters calibrated on the human body that include density values similar to those of the investigated materials. the upper panel of figure 5 shows a virtual map of all of the materials present on the samples. this map was obtained using skin and bone vrt filters, which divide the sample into white zones (pigments based on lead and mercury), blue zones (wood) and peach zones (linen canvas and organic substances). the second filter that was used, a bone filter, highlights the pigments only (bottom panel of figure 5), especially those with densitometric hu values closest to the hu value of bone (+1000 hu) (see also table 1). the third filter that was used is typically applied for evaluating the pulmonary system. this filter was selected because wood is a very porous material that contains a lot of air (see figure 6). in this case, the densitometric values are on the negative range of the hu scale, since the hu value of air is −1000. for example, the radiodensity of chestnut oak wood was measured as −300 hu. densitometric measurements were also performed on treerings, and investigations of latewood and earlywood found radiodensities of −157 hu and −459 hu, respectively. latewood has a greater radiodensity than earlywood, lung tissue or air, and of these materials its radiodensity is the closest to that of water (0 hu). this finding is in agreement with the microstructural features of growth rings, as latewood is stronger and more compact than earlywood [15]. the average radiodensity of the latewood and earlywood regions is about −300 hu, which corresponds to the total hu value of chestnut oak. to summarise, msct was able to provide qualitative information about materials on wood specimens. in detail, the application of msct to a cultural heritage object made it possible to 1) view structures, 2) understand the manufacturing method, 3) recognise subsequent restoration, 4) assess the conservation status of the object and its materials (for example, tunnels caused by xylophagous insects were perfectly visible) and 5) in the case of wooden artefacts, identify the types of wood used and perform dendrochronological analysis [16]. mpr reconstruction allowed us to identify the different pigments by mapping hu values corresponding to x-ray coefficient attenuation onto the known density of each material [17]. in addition, vrt filters can be used to virtually dissect objects without compromising their integrity. this approach made it possible to obtain details that, in some cases, were not visible and to generate a complete map of all of the materials on the artefacts. it is important to emphasise table 1. nomenclature, chemical formula, density and hu values of the pigments painted on the wooden support. pigments chemical formula density (g/cm3) [15] hu values madder lake c14h8o4 or c14h8o5 1.5 218 ultramarine na7al6si6o24s3. 2.4 297 azurite cu3(co3)2(oh)2. 3.8 356 malachite cu2co3(oh)2 3.8 674 red ochre ∝-fe2o3 5.3 775 red lead 2pbo·pbo2 8.2 1456 cinnabar hgs 8.2 1534 white lead 2 pbco3·pb(oh)2 11.37 2852 figure 4. hu values and density expressed in g/cm3 for all the detected pigments on the wooden tablets. figure 5. msct results after processing with vrt filters. a) skin and bone filter, b) bone filter. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 19 that this type of discrimination could be extended to any other kind of material found on artistic objects. 3.2 analysis of the uv-vis optical absorption and fourier transform infrared spectroscopy data the uv-vis optical absorption spectra of some of the powder samples are shown in figure 7a–d. the analysed samples show similar uv-visible absorption spectra characterised by a structured band located between 400 and 700 nm. the different chromophores cannot be discriminated and, thus, cannot be ascribed to a specific pigment. only carmine lake powder shows two defined absorption features at about 300 nm and 370 nm as well as a structured group in the 400–600 nm range. these bands are probably due to the different acid–base forms of carminic acid when it is dispersed in water (as was true in our case): ca, ca−, ca2−, ca3−and ca4− [18]. figure 7e–g shows the ftir spectra of dyes on the painted tablets. figure 7 displays a prominent feature at about 1400 cm−1 and two well-defined contributions at around 1600 cm−1, which is similar to the spectra of pure white lead inorganic pigment [19], while the bands in figure 7f and g are similar to those ascribed to copper carbonates and previously observed for azurite and malachite [20]. finally, the ftir spectra of cinnabar (not shown) is analogous to spectra reported in [21]. the infrared analysis detected complex fingerprints in all of the samples. some ftir contributions are due to features of wood, such as lignin (around 1275 cm−1) and cellulose-hemicellulose (around 1650 cm−1). hence, it is not possible to completely differentiate the pigments using only uv-vis optical absorption and ftir spectra. microraman analysis provided information to complement the complex results of ftir. raman spectroscopy has an advantage over ftir when identifying pigments in painted wooden tablets. both organic and inorganic paints can be infrared active, but their absorptions are so low that it is difficult to distinguish between two samples using only the ir spectra. however, pigments show a strong response in the raman spectra, especially below 600 cm−1, where ir does not strongly absorb. the ability to detect these low-frequency vibrations makes raman spectroscopy a useful tool for differentiating pigments in painted tablets. furthermore, raman spectroscopy has another advantage over ftir. since the raman peaks are sharp and do not overlap, it is much easier to resolve mixtures of pigments using raman spectroscopy rather than ftir. also, the technique is sensitive to even small differences in the intensities of various peaks. since paints are a complex mixture of many components, raman spectroscopy can be used to distinguish between two similar paint samples that have slightly different amounts of each component. 3.3 analysis of raman spectroscopy and surface-enhanced raman spectroscopy data figure 8 shows the raman spectra of nominal malachite, cinnabar and red lead powders, used as references for a subsequent comparison with raman spectra collected on the painted tablets. the spectra plotted in red in figure 7a shows the characteristic bands of malachite [22], [23] (see also table 2) together with the typical features, centred at 1300 and 1600 cm−1, of carbon black. in the 300–550 cm−1 range shown in figure 8b, the raman spectrum is characterised by features typical of red lead [mixed pb (ii) and pb (iv) oxide with the general formula figure 6. a) results of vrt pulmonary filter application (top), b) mpr image showing the points at which densitometric wood measurements were carried out (bottom). figure 7. a–d) uv-vis absorption spectra of nominal cinnabar, carmine lake, red lead and malachite powders, e–g) ftir spectra of pigments on tablets. some ftir contributions could also be ascribed to features of wood, such as lignin (1275 cm−1) and cellulose-hemicellulose (1650 cm−1). figure 8. raman spectra of nominal malachite, cinnabar and red lead powders. the spectra plotted in red (panel a) shows the characteristic bands of malachite, whose assignments are reported in table 2. in some points of the same powder, the typical raman bands of a carbon black material have been collected. the raman spectra shown in panel b are characterised by a mixed pb (ii) and pb (iv) oxide phase and by the typical hgs stretching modes, which are constituents of red lead and cinnabar, respectively [24], [25]. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 20 pb3o4] [24]. finally, the raman spectra plotted in green in figure 8b displays prominent bands at about 278, 290 and 353 cm−1, which are assigned to hgs bending and stretching modes [25] and are characteristic of cinnabar pigments. at this point, raman measurements were also carried out on the pigments painted on the tablets to identify their composition in terms of chemical configurations. however, no clear raman signals were collected. this result is in good agreement with previously reported analyses of natural and synthetic dyes found in textiles, inks and paints. the low raman scattering crosssections and the significant fluorescent background put severe limits on the quality of raman spectra [26], [27]. in order to overcome these limitations, we used surfaceenhanced raman scattering (sers). it is well known that raman cross-sections of molecules adsorbed on artificially roughened noble metal surfaces show dramatic enhancements thanks to the amplification of the incident field produced by the excitation of the localised plasmon resonance modes corresponding to the metallic nanostructure. thus, we expected that this approach could be effective for characterising the pigments present on the tablets. raman spectra acquired from the powder of commercial dyes were used as a reference for sers measurements carried out on the painted tablets. in particular, the sers behaviour of two dyes of interest in the cultural heritage field, alizarin and purpurin, adsorbed on gold nanostructured substrates were analysed. their molecular structures are similar except for the presence of an additional hydroxy group in purpurin. figure 9 shows the chemical structure of alizarin and purpurin (panels a and d), a scanning electron microscope image of the surface morphology of the gold nanostructured film used for sers, characterised by islands (agglomerates) with smooth edges (panel b) and the raman spectra acquired from the surface of substrates soaked in the alizarin and purpurin aqueous solutions (panels c and e). the spectra acquired from the bare glass substrate show no evidence of raman peaks related to the presence of dyes. in contrast, sers spectra obtained from the surface of the gold-covered substrates show clear raman contributions typical of the investigated dyes [28], [29]. the precise sers spectra acquired from the powders gave us hope for similar results from the tablets. therefore, we proceeded to collect the raman spectra from the wooden support, which had previously been prepared using firenze zecchi pigments, covered with gold colloid and then air-dried before raman analyses were carried out. using the sers approach, it was possible to identify different red pigments (alizarin, purpurin, madder lake, lac and ochre) and white lead, whose raman features are shown in figure 10 and figure 11, respectively. the raman spectra in figure 10a show the characteristic vibrational bands of both alizarin and purpurin. alizarin peaks are located at about 580 (skeletal vibrations), 1015 (c-c stretching, c-c-c bending), 1185 (c-c stretching, c-h and c-cc bending), 1285 (c-o and c-c stretching) and 1558 (c-c stretching) cm−1, while those centred at about 740, 980, 1027, 1218 and 1398 cm−1 nearly coincide with purpurin features [30], table 2. positions (cm−1) and assignments of raman bands typical of malachite [24], [25]. malachite frost et al. (2002) mattei et al. (2008) assignments 130, 142, 151, 166, 176, 205, 217, 249, 267, 294, 320, 389, 429, 514, 531, 563, 596 157, 171, 182, 204, 224, 272, 352, 435, 513, 537, 601 t cu, co3 718 750 723 753 ν4-asymmetric co3 bending mode 807 817 ν 2-symmetric co3 bending mode 1098 1058 1101 ν1-symmetric co3 stretching mode 1365 1423 1493 1514 1370 1463 1497 ν3asymmetric coν stretching mode o-h bending mode 3349 3380 3380 o-h stretching mode figure 9. a, d) chemical structure of alizarin and purpurin, b) sem image of the synthesised gold film, c, e) raman spectra of alizarin and purpurin powders adsorbed on the surface of au nanostructured substrate. figure 10. raman spectra of (a) madder lake (which contains both alizarin and purpurin) and (b) ochre/lac on painted tablets covered with gold nano colloids and then dried in air. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 21 [31]. thus, the madder lake dye was correctly identified by sers, as was the mixture of red ochre and lac dyes (see figure 10b) that had been brushed on the wooden tablets. finally, the sers spectra shown in figure 11 nearly coincide with those of gypsum (hydrated calcium sulphate), whose raman bands are generally located at about 181, 415, 493, 617, 670, 1008 and 1134 cm−1, and those of a carbonate material (i.e. 2pbco3) [32]. ultimately, all of the pigments present on the wooden tables were correctly identified, confirming the reliability of the msct data. 4. conclusions for the first time, clinical msct was applied to the investigation of pigments painted on a wooden support. the presented results indicate that msct is a valuable and promising diagnostic method for the analysis of painted objects [33]. mpr and vrt post-processing techniques make it possible to ‘dissect’ the wood without contact, allowing for a complete analysis. moreover, msct can be adapted for the non-destructive characterisation of coatings and coating pigments, which can be differentiated based on hounsfield units obtained from a linear transformation of the measured attenuation coefficients. the only drawback – the inability to recognise organic pigments – was overcome in this work by associating the results of the raman/sers scans with the msct ones. then, by combining x-ray densitometric and sers measurements, it was possible to identify and characterise both inorganic and organic materials present on the wooden tablets. these techniques can be extended to the investigation of other objects of interest in cultural heritage, and prospective studies will continue to explore and refine the method. acknowledgement the authors would like to thank full prof. marcello longo of the policlinico g. martino of messina, italy. references [1] l. bonizzoni, s. bruni, m. gargano, v. guglielmini, c. zaffino, a. pezzotta, a. pilato, t. auricchio, l. delvaux, n. ludwig, use of integrated non-invasive analyses for pigment characterisation and indirect dating of old restorations on one egyptian coffin of the xxi dynasty, microchemical journal 138 (2018), pp. 122-131. doi: 10.1016/j.microc.2018.01.002 [2] c. denoël, p. roger puyo, a. m. brunet, n. p. siloe, illuminating the carolingian era: discoveries as a result of scientific analyses, heritage science 6(1) (2018), art. 28. doi: 10.1186/s40494-018-0194-1 [3] m. mantler, m. schreiner, x-ray fluorescence spectrometry in art and archaeology, x-ray spectrometry 29(1) (2000), pp. 3-17. doi: 10.1002/(sici)1097-4539(200001/02)29:1<3::aidxrs398>3.0.co;2-o [4] j. wouters, a. verhecken, the coccid insect dyes: hplc and computerised diode-array analysis of dyed yarns, stud. conserv. 34 (1989), p. 189. doi: 10.2307/1506286 [5] l. h. oakley, s. a. dinehart, s. a. svoboda, k. l. wustholz, identification of organic materials in historic oil paintings using correlated extraction less surface-enhanced raman scattering and fluorescence microscopy, anal. chem. 83 (2011), p. 3986. doi: 10.1021/ac200698q [6] e. fazio, f. neri, a. valenti, p. m. ossi, s. trusso, r. c. ponterio, raman spectroscopy of organic dyes adsorbed on pulsed laser deposited silver thin films, appl. surf. sci. 278 (2013), pp. 259264. doi: 10.1016/j.apsusc.2012.10.133 [7] c. parisi, c. pelosi, u. santamaria, p. pogliani, g. agresti, s. longo, the conservation project of a liturgical object: the case of infant jesus of prague in the church of saint mary of providence at macchia giarre (italy), european journal of science and theology 12(2) (2016), pp. 235-244. 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panel support of a seventeenthcentury dutch painting by clinical multislice computed tomography, studies in conservation 66(3) (2021), pp. 174-181. doi: 10.1080/00393630.2020.1757881 https://doi.org/10.1016/j.vibspec.2005.08.002 http://lisa.chem.ut.ee/ir_spectra/paint/pigments/lead-white/ http://lisa.chem.ut.ee/ir_spectra/paint/pigments/verdigris/ http://lisa.chem.ut.ee/ir_spectra/paint/pigments/cinnabar/ https://doi.org/10.1016/s1386-1425(02)00034-3 javascript:void(0) https://doi.org/10.1002/jrs.1845 https://doi.org/10.1039/b008302j https://doi.org/10.1002/jrs.848 https://doi.org/10.1021/ar100019q https://doi.org/10.1007/11378235_18 https://doi.org/10.1002/jrs.1576 https://doi.org/10.1098/rsta.2016.0051 https://doi.org/10.18434/t4d01f https://doi.org/10.1080/00393630.2020.1757881 acta imeko, title acta imeko issn: 2221-870x june 2018, volume 7, number 2, 73-83 acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 73 data inter-comparisons in the context of the knowledgegaining process: an overview franco pavese1, abderafi charki2 1torino, 10139, italy 2university of angers, laris 62 avenue notre dame du lac 49000 angers, france section: research paper keywords: inter-comparisons; infra-comparisons, proficiency testing; accuracy; reliability; uncertainty; validation; competency; knowledge citation: franco pavese, abderafi charki, data inter-comparisons in the context of the knowledge-gaining process: an overview, acta imeko, vol. 7, no. 2, article 13, june 2018, identifier: imeko-acta-07 (2018)-02-13 editor: dušan agrež, university of ljubljana, slovenia received january 16, 2018; in final form may 29, 2018; published june 2018 copyright: © 2018 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: franco pavese, e-mail: frpavese@gmail.com 1. introduction the measurand is a concept that is shared in the scientific and technical communities [1]–[4]. any given measurand is supposed to be the object of replicated measurements that must be comparable with each other. in other words, it is, or should be, recognized as a quantity having a current recognizable meaning for the specific community. in the jargon of science philosophers, this means that it should be projected onto a “social framework” [5]. in the scientific experimental context, this means that the measurand model must be of the “prescriptive” type, meaning “giving directions or injunctions” [6], which does not always denote the “physical” model. the confidence that can be ascribed to a measurement process is normally based on an estimation of uncertainty [7], [8]. however, uncertainty can often be difficult to assess properly in certain complex situations for which unique physical models do not a priori exist. and yet laboratories, customers and/or instructing parties need to have accurate information about the confidence level associated with measured data in order to make important decisions that are justified, technically and scientifically, about the validity of a method or the conformity of a procedure, to authorize the sale of a product, and endorse and maintain an individual’s competence and skills. the core concept of measurement needs to be inseparably complemented by the need to ensure reliability of results (qualitative and quantitative) and, whenever possible, measurement traceability. that need obviously also implies a sound understanding of the concept of calibration and of comparison. calibration [9] is an operation that, under specified abstract this paper deals with the principle of data inter-comparisons, the object of which is to increase knowledge continuously with respect to time. although the principle is as such nothing new to metrology and testing laboratories, which carry out experimental measurements, a degree of clarification is nonetheless called for in view of the numerous questions that arise concerning ways of implementing and utilizing it to improve knowledge capitalization. the acquisition of knowledge relative to any measurand involves a series of steps: studying the state of knowledge of the measurand, choosing a working method (typically, by establishing a design of experiment), obtaining the measurements, and analyzing them. following this, an action plan is established in order to reduce (or if possible avoid) weaknesses or over sensitivity. comparisons are already conducted using various approaches within a laboratory. it is therefore important to understand that to assess the accuracy of a method and validate it, it is necessary to compare the results obtained by several laboratories for a given method and measurand with the correct type of inter-comparison. it is this comparison between several laboratories that, when using different methods, produces the most up to date knowledge with the highest confidence level. this paper goes over the steps that allow developing knowledge, presenting the aims and characteristics of the various inter-laboratory comparison methods, notably referring to the tools established by documents such as the bipm mra (the mutual recognition arrangement), the iso 5725 and the iso 13528. acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 74 conditions and as a first step, establishes a relationship between the quantity values, associated with measurement uncertainties provided by the measurement standards, and the corresponding indications with their associated measurement uncertainties. as a second step, this information is used to establish a relationship for obtaining a measurement result from an indication. the accreditation standard for testing and calibration laboratories is iso/cei 17025 [10], which contains the requirements relating to management aspects and the technical requirements necessary to ensure the accuracy and reliability of results obtained by a laboratory. many factors can influence the accuracy and reliability of testing and/or calibration carried out by a laboratory [10]. some of these influence factors may include the following: • level of competence of personnel • equipment and ambient conditions • handling and storage of objects for test and calibration • measurement traceability • sampling • collection • testing and calibration methods (whether developed or adopted, standardized or non-standardized) • means to be used to ensure the quality of testing and calibration results:  regular use of certified reference materials and/or internal quality control with use of secondary reference materials  participation in comparison programs between laboratories, or proficiency tests  tests or calibration repeated using identical or different methods  renewed tests or renewed calibration of stored objects  correlation of results for different characteristics of an object. it is important to remember that a quantity value [9] is the basis for the subsequent comparison of values of quantities of the same kind. in many situations, a known (conventional) quantity value can be used for a true quantity value of the measurand, in lieu of the true value that is always unknown. a conventional (reference) quantity value with associated uncertainty is usually assigned, for example, to a certified reference material, or for the characteristics of a device, e.g., a stabilized laser, or in the case of a reference measurement procedure or, often, for a comparison of measurement standards. the concept of comparison has become widely used in many sectors, and many new proficiency test (pt) schemes are currently launched every year worldwide [11], [12]. a detailed study of comparison testing has been conducted [12]. the international organization for standardization (iso) has published a reference document on general requirements for proficiency testing [13] and a standard on statistical methods for use in proficiency testing [3]. the international laboratory accreditation corporation (ilac) has also published a document on requirements for proficiency testing [14]. in the testing field, the questions most frequently asked about the subject of comparisons are the following. does the possibility of a comparison in my case exist? with whom can i or should i compare? what should i do if no comparison solution has been organized? who might be able to set up the comparison? is an interor intra-laboratory comparison sufficient in my case? is there a recognized reference? is there a reference laboratory? what should be done if no reference laboratory exists? who can prepare the samples to be tested? how can the stability and homogeneity of the reference material and/or samples sent to each laboratory be ensured? can i compare with a single other organization? how many laboratories need to be involved for a comparison to be reliable? do all the laboratories taking part in a comparison need to have iso/iec 17025 standard [10] accreditation? does the organizer of the inter-laboratory comparisons have the necessary competences? should the organizer be independent? the principle of comparison has long since been used and considered effective by national metrology institutes (nmi), notably, since 1999, in the form of key comparisons [1] organized under the auspices of the international bureau of weights and measures [1]. key comparisons of national standards are carried out to establish the degrees of equivalence of measurement results obtained by the nmis [4], [15], [16]. in certain situations or fields (especially in testing), comparisons may be made complicated either by the fact that no reference exists or because no person or organization is taking the initiative of setting up a comparison campaign. in that case, e.g., shirono et al. [17], thompon et al. [11] and ponomareva et al. [19] provide performance evaluation methods for pts with uncertainty information when there is no reference laboratory available. in the absence of knowledge of a “true value”, the fundamental aim of comparison measurements is to increase as much as possible the confidence level associated with the evaluation of the measurement result. this may be expressed using different terms depending on the branch of statistics involved, but there are basically two common ultimate aims: – to obtain a measure of the degree of confidence (or degree of believe) for the differences found in the measured numerical values – to obtain a measure of the degree of reliability of the uncertainty evaluations associated to those values. note that the attribute “best” often associated to the evaluation of a measurement can only have the meaning associated with (or derived from) the hypotheses under which the analysis is performed, which vary according to subjective preferences. without entering into the vast subject of subjectivity vs. objectivity (see, e.g., [19]), it is enough to note here that various degrees of subjectivity are possible, depending on the overall ‘level of knowledge’—here a term not necessarily used in the bayesian sense—associated with a specific measurand. strangely, however, this fact is barely referred to, if not totally ignored, in the main approaches to statistics of interest to metrology and testing, i.e., the “error approach” and the more recent “uncertainty approach”. below a roadmap follows to illustrate the role of inter-comparisons in the overall process of gaining progressive knowledge, and the main features of these exercises. the article maps (in section 2) the different steps that contribute to the development of knowledge. looking at a single laboratory, this section explores the existing situation with the choice of method, implementation of a degree of equivalence (doe), and the necessity to obtain several series of measurements to allow an assessment of repeatability and reproducibility. the interpretation of these concepts, as well as that of accuracy, is the role of the field of metrology and testing. subsequently, by looking at several laboratories together, we demonstrate that comparison of knowledge is a https://jcgm.bipm.org/vim/fr/1.19.html https://jcgm.bipm.org/vim/fr/1.1.html https://jcgm.bipm.org/vim/fr/1.1.html https://jcgm.bipm.org/vim/fr/2.11.html https://jcgm.bipm.org/vim/fr/2.3.html https://jcgm.bipm.org/vim/fr/2.26.html https://jcgm.bipm.org/vim/fr/5.14.html https://jcgm.bipm.org/vim/fr/2.7.html https://jcgm.bipm.org/vim/fr/5.1.html acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 75 way of increasing the total level of available knowledge. section 2 also briefly goes through what a comparison of methods consists of and the advantages this offers. section 3 presents the principal methods of key inter-comparisons (mra kc) [1], iso 5725 [2], and iso 13528 [3]), and their specificities. 2. how knowledge is gained in steps a feature of the measurement process, namely in metrology where it is implicit without always being explicit, is the fact that the acquisition of knowledge always develops with time: knowledge is gained in steps. 2.1. within-(infra-)laboratory knowledge (i) in a single laboratory, the serious job of considering a new measurand is undertaken. first the published/’gray’ literature is examined in a search of whatever information may be available on that measurand. (i-1) if nothing useful is available, proceed to step (ii). (i-2) if something useful is available, proceed to step (iv): this will be affected by type b (“other than statistics” method) uncertainty—see also section 3.2; (ii) a draft of the "experiment design" is prepared. almost invariably, the experimental setup (mathematical model ⇒ experimental model, instrumentation) includes in part information already available; (iii) case (i-1) does not occur. otherwise, proceed to step (v); (iv) prior to the start of measurement, the measurand may be unknown, and the uncertainty level is affected by partial ignorance regarding the response of the equipment; (v) if (i-2) applies, the difference from (iii) is that partial ignorance also applies to the measurand-value and uncertainty is too high; (vi) 1st step of the measurement procedure – a first run generates a series of replicated data. within this knowledge level (kl) the data can be considered repeated only in the absence of adverse evidence from inference within the knowledge level. (v-1) if nothing changes in the procedure go to step (vi). (v-2) model adjustment can only be possible relative to discrepant behaviour of the equipment: proceed to step (ix); (vii) 2nd step of the measurement procedure – more runs are replicated in the same laboratory by the same staff—obtaining new series of replicated data—they may be repeated or not, see step (viii) and (ix). if (v-1) applies, the set of two series of data should be considered as repeated, but only after the same scrutiny is performed in (v). if (v-2) applies proceed to (ix); however, the possibility of occurrence of betweenseries significant differences in the representative parameters of the sample distributions (or of the inferred probability distribution) is more likely than for within-series non-random differences between data. more tools are available for this scrutiny, concerning not only the position parameter but also the dispersion parameter: the scrutiny should be considered as the first level of data inter-comparison. this between-series within-laboratory analysis can be considered as the first level of data comparison, characterized as follows [20]. (viii) repeatability. the set can be formed by several series of data taken over a period of time that is much longer than the “short period” indicated for "repeatability" of data to apply. [9] in metrology, this is the typical case of a standard constructed with the aim of preserving a stable value with time. in testing, this is the typical aim of a laboratory issuing test results over time under assumed repeatability conditions. the testing case is different, because the test material changes each time, but applies to checks made using a "reference material": however the latter additional information is external to the within-laboratory knowledge, therefore see step (xi). the repeatability condition is assumed obtained by correctly performing the test according to an approved procedure, and this is basically why the result of a test can be obtained as a single value associated with an acceptance limit (tolerance interval), in contrast with the situation in metrology; (ix) reproducibility. a reproducibility study consists in preparing a “design of experiment” that can obtain sensitivity coefficients for the different influence quantities. it consists in varying by known amounts each influence factor separately, and checking the overall effect. these coefficients can also be computed without experimentation by differentiating the model expressed in closed form (analytically): this method may suffer from model imperfections. the results do not directly inform about the actual variability of an experimental setup in each specific real condition. the set of results of a run form a single series of non-repeated data [21]. a variability level of the setup should be obtained by performing a specific experimental condition, called “reproducibility condition” (or “intermediate condition” when focused on only specific effects). it is assumed that an evaluation of reproducibility is achieved, but the truth of this assumption is not particularly easy to check; (x) accuracy. measurements are normally assumed to provide a measure of the true value of the measurand, but this assumption cannot be verified. consistency of results is indifferent to truth; one can be entirely consistent and still be entirely wrong. consistent within-laboratory data can only provide a value that may be used as a "laboratory reference value" for internal use, with an associated dispersion that is lower than that of inconsistent data. only when a “reference value” is provided as the target value, one can talk about “precision” or “trueness” [2]. however, it can only be a “conventional” or “accepted” reference value, which shows how unavoidable inter-subjectivity is. in this respect a within-laboratory reference is, in itself, less reliable than a between-laboratory reference. acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 76 for a specific measurand using a specific method, that is the maximum knowledge level (kl) that can be gained withinlaboratory (kl_w). at that level, systematic errors/effects can only be inferred from the dispersion level of uncertainty obtained compared with the target uncertainty, but not proved—except when the target value is an accepted (conventional) reference value. except also, to a limited extent, when the experience accumulated in the laboratory shows the lack of repeatability between the position parameters of subsequent series of data (called “bias between series”), which is typically an instability of the value of the measurand—called “drift” when the change is essentially monotonic with time. the main factor limiting the value of these within-laboratory comparisons is the fact that the different series are strongly correlated with each other. the next step toward increasing the kl is to compare similar results on the same (or similar) measurand obtained by other laboratories [22], [23]. 2.2. between-(inter-)laboratory knowledge (xi) this is knowledge that is not available to any of the participants to the comparison prior to the (first) exercise being finalized. therefore, it is additional to the kl_w. the comparison is supposed to be performed between non-repeated measurements. the exception to this is in the testing situation, where all participants apply the same standard procedure: this is assumed to generate repeated measurements within a stated uncertainty, an assumption that is valid until there is evidence to the contrary. an intercomparison can provide such evidence (of a nonstandard condition); (xii) the maximum increase in knowledge from the results of inter-comparison is achieved when all results are fully uncorrelated. the reason for this is that correlated results tend to be less dispersed simply because at least some of the same systematic errors/effects are paired together. therefore, the degree of correlation must be carefully scrutinized in order to attribute the correct significance to the results; (xiii) a comparison concerns one of basically two classes of measurands: (a) artefacts, or (b) physical/chemical/ biological/ etc. states. the comparison design should be specific to the class of measurand; (xiv) the aim of a comparison must be clearly identified, since this affects several aspects of the exercise. (xv) a chain of comparisons can occur, involving the same participants, either fully or in part; (xvi) the level of additional knowledge brought in by the participants depends not only on the number of participants but also on the target uncertainty of the exercise or the dispersion of uncertainty levels among the participants; (xvii) the knowledge level gained in between-laboratory comparisons (kl_b) is additional to the kl_w, and is the maximum up-to-date level that can be obtained experimentally. replication of comparisons may compound knowledge, enriching the kl_b. however, the wider the range of uncertainties among the participants in the comparison, the less information the comparison outcome can provide about the participants with the lowest uncertainty; (xviii) from the kl_b the participants can gain information about hidden errors in their own realizations: when discrepant results occur, this can help the relevant laboratory to identify systematic errors/effects and correct its model or detect equipment behaviour anomalies. the kl_b is the maximum kl that can be gained for a specific measurand from between-laboratory (kl_w) exercises. these usually aim to detect differences when using a specific method. however, there exists the so-called “method-bias”, which is generally not detected by inter-comparisons – except, for example, those designed for the specific task of providing a value for a reference material by using different methods. in these cases, using different approaches/methods can still enhance the level of knowledge. this is particularly important for measurands of fundamental importance. this applies, for instance, to the constants of nature, and allows a critical analysis of the experimental numerical values such as that provided by the codata task group [24]. 2.3. contribution of diversity: comparison of methods (i.e., of models) (xix) in order to detect “method bias”, [2] the same measurand needs to be subjected to measurements based on different methods. the comparison between the results is not generally the task of a specific exercise, but happens when a critical review is made of the literature on the specific subject. a review may typically concern the diversity of experimental approaches or the diversity of approaches to data-analysis of the same experimental method—or both; (xx) each experimental approach is based on a different model and involves, either partially or totally, different influence quantities—and consequently different measurement units. another reason for diversity as regards experimental methodology is the possible division of the measurands into two broad categories that have been demonstrated to require different tools for their analysis: artefacts, and “natural states” (physical, chemical, biological, etc.). such diversities are a source of possible discrepant results, and can help in identifying missed influence quantities in other methods; (xxi) the results of any specific experimental method require an analysis, at least in part of a statistical nature; (xxii) however, the existence of type b (in the sense of the gum [25]) components of uncertainty implies that the analysis extends also over the statistical frame. when this type of exercise concerns a specific method, it is the frame of the inter-comparisons (see xi-xviii above). however, very seldom does such an exercise include a substantial diversity of data analysis; most commonly, especially in the testing frame, an agreed single method for the data analysis is used. the commonest statistical diversity factor is between the "frequentist" and the "bayesian" approaches, which typically give non-identical results because of the acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 77 different inference methods on which they are based [26]. another common approach is to consider all inter-comparison data as pertaining to a single population or, less commonly, to a mixture of populations. in actual fact, the diversity can be wider, including non-probabilistic methods; (xxiii) the above contributions to total kl cannot be included either in the kl_w or in the kl_b, but bring about a level kl_d, with higher understanding. kl_d is the summary of up-to-date knowledge, time remaining the only factor allowing a possible increase of kl-d level thanks to replication of some of the above steps—which may invalidate some or all the previous findings. 3. different types of inter-comparisons for different aims 3.1. direct inter-comparisons there are basically three different types of between-(inter)laboratory direct comparison that can be found in general prescriptive documents, each with different aims: (incidentally, gum [25] is not considered here, not because it is only a guideline, but because it deals only with withinlaboratory single-series data treatment and not with comparisons) 1) mra key comparison (kc): inter-comparison in metrology in the frame of the mutual recognition agreement (mra), to establish the “degree of equivalence” between national metrology institutes [1]; 2) iso 5725: inter-comparison to establish the “accuracy (trueness and precision)” of a method in the field of testing [2]; 3) iso 13528: proficiency test between laboratories in the field of testing, “to determine the performance of individual laboratories for specific tests or measurements, and to monitor the continuing performance of laboratories” [3]. in table 1 the main features of the methods used are summarized for each type [27]. 3.1.1. mra key comparisons (metrology field) the key comparisons were introduced in 1999 as a basic requirement in the text of the mutual recognition arrangement (mra) by the bipm, as the end of the very complex process leading to the calibration and measurement capabilities (cmc) of the nmis. the whole process is pictured in figure 1: the very beginning of the process relates to the contents of section 2 of this paper— a full illustration can be found in [21]. kc meaning. the full meaning of a kc remains to some extent controversial. on the one hand, in fact, it may be perceived as a scientific exercise or as a proficiency test. the text of the mra glossary reads: “key comparison: one of the set of comparisons selected by a consultative committee to test the principal techniques and methods in the field (note that key comparisons may include comparisons of representations of multiples and sub-multiples of si base and derived units and comparisons of artefacts)”. in the preamble: “… key comparisons carried out using specified procedures which lead to a quantitative measure of the degree of equivalence of national measurement standards”. it would seem that the above indications are not sufficient to avoid current interpretations of the two meanings of a kc. one interpretation is that a kc is a scientific exercise establishing a quantitative measure of the international degree of equivalence (doe) of a given standard, as currently maintained in each nmi under its normal – though generally “best” – local working operations and methods. the other interpretation considers a kc equivalent to a “proficiency test” (pt), a tool of normal use in testing. the definition of a proficiency test, as given in iso 13528 [3], is as table 1. different types of direct inter-comparisons for different aims: main features of the different methods. mra kc [1] iso 5725 [2] iso 13528 [3] the aim is to estimate the differences between nmi standard realizations. mra key comparison (kc): inter-comparison in metrology in the context of the mutual recognition agreement (mra), to establish the “degree of equivalence” between national metrology institutes the aim is to estimate the accuracy (trueness and precision) of a specific method in the field of testing the aim of the proficiency test is to estimate the accuracy and precision of a specific method in the field of testing the number of participants is generally small and a random choice of participants cannot be assumed. non-hierarchical the number of participants, of top level, is generally sufficiently high to allow the assumption of a random choice of participants. non-hierarchical the number of participants is generally high. when there are a few participants (e.g.,< 30), the iupac/ citac recommendation should be followed. hierarchical measurements can be performed using different methods all participants strictly use the method under test the same method is used by all participants, as the test concerns the ability to use it correctly nmis with different levels of uncertainty (up to > 10:1) can take part in each exercise. the term "bias" is not used estimation of the laboratory bias is one of the aims. this bias is assumed to be a random effect the laboratory bias is estimated each local sample is implicitly assumed to represent the local population estimation of the method bias is not one of the aims homogeneity and stability of the samples are checked a kcrv is normally – but not always – defined for each exercise, based on the results of (all) participants, usually with an associated uncertainty estimation. the results of the exercise are the general mean of the test and an estimation of the between-laboratory variance (in addition to the within-laboratory variance) an assigned value is used, which can be achieved in a variety of ways (reference or consensus) the difference of each nmi result with respect to the kcrv is called “degree of equivalence” (doe). the pair differences, called “bilateral degrees of equivalence”, do not require a kcrv the precision of the model is established one of several scoring methods can be used to evaluate the results no result can be considered an outlier, nor discarded, but is noted. outlying results are detected and can be rejected outlying results are detected and can be rejected acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 78 follows: “proficiency testing by inter-laboratory comparisons is used to determine the performance of individual laboratories for specific tests or measurements, and to monitor the continuing performance of laboratories. the introduction to iso 17043 [13] should be consulted for a full exposition of the purposes of proficiency testing. in statistical language, the performance of laboratories may be described by three properties: laboratory bias, stability and repeatability.” however, proficiency testing should only serve to answer the basic question: can an individual laboratory continuously demonstrate its ability to correctly conduct a specific procedure? a proficiency test should not be designed to test the limits of laboratory ability to minimize its uncertainty, nor to establish how the laboratory capability ranks within any given community of laboratories. a proficiency test simply establishes that the laboratory in question can do its job at its assessed level of competency. in addition, the term “specified procedures” in the mra extract quoted above does not refer to a requirement to use a “standard method” in the sense of iso 5725 [2], the latter associating to it a default uncertainty (and often to a reference value used as the “true value”). in fact, each nmi participating in a kc is allowed to use different methods and the group of participants can have very different known levels of uncertainty associated to their standards and use different experimental techniques. the kc protocol is only intended to ensure verification of the transfer standard/s being stable with time (or brought to a specific “ground state” before being passed to the next participant), and verification of the transfer standard/s, or the local standard/s being used in a correct way (consensus best-practice rules). in addition, the nmis participating into a kc do not necessarily represent the total population of nmis, but are only a de facto group. therefore, the key comparison reference value (kcrv) prescribed by default by the mra is not an unbiased estimate of the “best approximation” value of the si quantity. consequently, the does relative to the kcrv (unilateral does) are strictly relative to the computed kcrv, regardless of how it is computed. only the does between pairs of nmis (bilateral doe) generally do not suffer from kcrv bias. on the other hand, the kc represents the only technical basis (and experimental evidence) to allow an nmi adhering to the international arrangement to accept the degree of equivalence of any other nmi with respect to its own standards. this implies that the measurements leading to that recognition are not occasional (i.e., possibly fortuitous), but represent the limits of the typical capability of a specific nmi. therefore, unless a local standard is occasionally in non“normal” conditions, undetected within the nmi, the measurement data specifically provided by each nmi to a kc (its input data, or in other words, each local sample), is assumed to measure the exact state-of-the-art local standard, i.e. these are its typical values. the fact that often a limited number of measurements are specifically provided for the kc does not affect the statistical significance of the nmi data provided to the kc. in fact, the limited number of measurements is performed only to check that the standard is at its “normal” operational capability. the supplied local value is a representative value of the local population; it is not a summary statistic of the (few) specific measurements performed for the kc, any more than is the expected value of the occasional probability density function (pdf) inferred from these specific measurements. it is assumed, instead, to be consistent with the expected value of the local standard as currently maintained, i.e., of the local population of samples normally drawn from that standard. in a word, it is not a “special” value nor is specific to the kc. similarly for the associated uncertainty, which is the local capability of realizing the standard. it is not the uncertainty associated to the (few) specific measurements performed for the kc, nor is the second moment of the occasional pdf figure 1. the process leading to the mra kc. for acronyms see text, and the following: sud = systematic unresolved deviations; kcdb = key comparison database; type a, type b: uncertainty components according to gum (from [21]). acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 79 inferred from these specific measurements. it is evaluated from the local standard as currently maintained, i.e., from the local population of samples normally drawn from that standard. additionally, only an uncertainty component arising from artefacts arising from the comparison itself (e.g., transfer standard uncertainty, comparison apparatus uncertainty) should be added. in all instances the kc is a non-hierarchical exercise. nature of the standards. standards can have very different characteristics, depending on the physical or chemical quantity, and they can behave and be used in different ways. these differences reflect on the type of kc [28]. however, as to their intrinsic nature, they can be grouped essentially into two types: (1) artefacts and (2) realization of a physical state or law. this distinction, which should normally generate a substantial difference in the statistical treatment of the data, was introduced back in 2002 and again more recently. every other characteristic pertains to the specific measurand used in a kc (e.g., single travelling standard) or to an empirical behaviour of a standard realization (e.g., stability with time of the value). to put it briefly (for more, see [29] and references therein): (1) artefact standards (e.g., a piece of metal as a mass standard or as a length standard): a measurement device a “natural” value of which does not exist (device value). each specific standard carries its own value, though standards can be made with very close values to each other. in the kc statistical treatment, each artefact is regarded as a distinct random variable. (2) standards realizing a physical state or law (e.g., phase transition of an ideal substance, vapour pressure law, state related to a fundamental constant, chemical composition of a mixture): a measurement device aimed at accurately realizing the physical state or law. the value of the state or law is unknown but unique for all samples and realizations; it is a physically-based value, and therefore, all realizations aim at approximating the same value. in a kc statistical treatment, each standard is regarded as sampling from the same random variable. the above has a basic influence on the definition of the measurand of the kc. data required as the input to the key comparison. input data can be required in essentially two different ways. figure 2 summarizes the case of kcs based on artefact(s): (i) using local synthetic input data: this the most usual way, required by most protocols (upper input type in figure 2a). within each nmi, data, xij, are made available (account being taken of the above discussion about sampling the local standard(s)). the following summary statistics are supplied: • the expected value yi of yi from the xij pertaining to xi, the local random variable, not based only on the xi, kc data acquired for the kc; yi is a new random variable of higher hierarchical rank than xi • an uncertainty ui(yi), the variance of the summary variable y. (ii) using the local pdf ℱi: it is seldom used (lower input type in figure 2(a). the statistics of the nmi data are fully conveyed through the pdf, which provides: • an expectation value e(ℱ(xi)), the random variable is xi; • an uncertainty ui, the second moment of the local pdf (accurate only if that can be described by just two moments). in the case of a kc involving instead the comparison of realizations of a physical state or law, there is a single stochastic variable q, with an associated single pdf ℱ. all xi,n ∈ q , where n = 1 … n refer to the nmis. therefore, the model of figure 2 does not apply. however, each nmi contributes to ℱ with a local ℱi, not necessarily equal one to another. the output data of each participant nmi forms the set of the kc input data. it is analysed by the kc pilot following an agreed statistical method according to a specific model – most often not already specified in the kc protocol. it enables the obtaining of the kcrv (when this is agreed, as in most cases it is) and the does. figures 3-4 show typical outcomes of a kc. the computed kcrv and the doe (unilateral and bilateral) of each kc are available at the bipm kcdb, with their links to previous kcs of the same type: the possible sud are not discussed there, because, the kc being a non-hierarchical exercise, no outliers can be defined nor can the doe be modified (outliers may only be noted in the final report). this peculiarity does not reflect on the computation of the kcrv, which should be computed with a free-chosen statistical method, but on the representative input values of all participants. there is a line of thinking that instead suggests possibly computing the kcrv on the “maximum consistent set” of the participant results [4] but the effect of such a selection would bias the kcrv, and thus the unilateral does, while the bilateral does are not affected by such a selection. 3.1.2. iso 5725 inter-comparisons (testing field) the aim of inter-comparisons is indicated in iso 5725 [2]: “iso 5725 presents a method of analysis of inter-laboratory comparisons that can be adapted to intra-laboratory comparisons. users’ criteria, concerning operator, equipment, etc. for the tests can be different. in particular, intra-laboratory comparison is realized under specific laboratory conditions. the objective of the iso 5725 series is: a) to provide useful definitions, b) to provide procedures to assess accuracy (trueness and precision) of measurement methods and results; c) to give guidance and examples to use in practice of trueness and precision data” (emphasis added). the inter-laboratory comparison aims at validating a single specific method, by means of a non-hierarchical structure and requires top-level laboratories to perform it in order to assess accuracy (trueness and precision) of the method, which can then be used in proficiency tests. in itself “this standard is not (a) (b) figure 2. (a) input-output in a key comparison of artefacts. (b) the output in (a) can be considered as a “virtual artefact”, corresponding to an associated new random variable, z. [27]. acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 80 applicable to proficiency testing or reference material certification”. its use is also restricted, as “the principles of iso 5725 should also apply to discrete quantities where the quantity is measured on a scale and be considered as continuous variables” (emphasis added). accordingly, the number of participants is not so high as in the case of proficiency testing, but it is assumed (without statistical check) that the choice of the participants among the population of the laboratories makes it possible to consider their results sparse at random, so that the normal distribution is used. the basic feature of iso 5725 is that the normal distribution object of the treatment is that of the expectation provided by the participants, as illustrated in figure 5. as is clarified in the sentence quoted from iso 5725, an inter figure 4. results of a ccqm kc showing on the right side the individual results and on the left the marginal distributions (light lines) and the overall mixture distribution (thick line) [32]. figure 3. compound pdf for 8 temperature fixed points of the its-90 in a cct.kcs, obtained as mixtures of several pooled local pdfs [31]. acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 81 laboratory comparison does not much differ in principle from an intra-laboratory exercise when organized for directly comparing in one place one (or more) standards provided by the different laboratories. in this case, one of the purposes is to establish the differences of the standards of different laboratories. the iso 5725 data model is written, where the subscript i refer to the i-th standard: xij = a + mi + єij (1) with i = 1,…, i; j = 1,…, j. see below for the meaning of a and mi,. the term mi not depending on j (i.e., having the same value µi for all j = 1,…, j) expresses the variability of the i-th standard and indicates that part of the model (4) does not apply to all the measurements, but only to the subset concerning the specific ith standard. the index i in equation (1) refers to the i-th laboratory, participating with one standard in the exercise (i = 1,…, i). in the calibration frame these exercises admit the use of different methods in different laboratories, while in the testing frame it is generally not admitted: in fact the “reproducibility” definition mainly differs in this respect between vim [9] and iso 5725 [3]. considering equation (1), the term a in the model has the same meaning that it would have in an intra-laboratory comparison of standards. in the testing frame, a is called the general mean (expectation value) in iso 5725 standard. when a true value µ can be defined (e.g., an accepted reference value in the sense of iso 3534 [30], i.e., a known value), in general, a = µ + δ where δ is the bias of the measurement method used with respect to the accepted measurement value (iso 5725). the term µi might have the meaning, ∑ µin, i.e., of the overall effect of the influence parameters in the i-th laboratory. in the testing frame, µi is called the “laboratory component bias under repeatability conditions”. when a δ exists in this frame, representing the between-laboratory variation, the laboratory bias becomes δi = δ + µi. the term єij in equation (1) is the random error occurring in every j measurement of the i-th participant. єij is called the “random error occurring in every j measurement of the i-th participant under repeatability conditions”. 3.1.3. iso 13528 proficiency tests (testing field) figure 6 shows a typical outcome of a proficiency test. iso 13528 [3] provides detailed descriptions of statistical methods for proficiency testing providers to use to design proficiency testing schemes and to analyse the data obtained from those schemes. it provides recommendations on the interpretation of proficiency testing data by participants in such schemes and by accreditation bodies. the procedures in iso 13528 [3] can be applied to demonstrate that the measurement results obtained by laboratories, inspection bodies, and individuals meet specified criteria for acceptable performance [32]. iso 13528 [3] is applicable to proficiency testing where the results reported are either quantitative measurements or qualitative observations on test items.” the different methods for determining this "standard deviation" for the evaluation of proficiency are contained in the iso 13528 standard. depending on which model is chosen, the standard deviation for the evaluation of proficiency is not an experimental standard deviation in the mathematical sense of the term. the iso 13528 standard suggests five different methods for determining the standard deviation for the evaluation of proficiency as well as eight statistical performance calculations. these choices should be made known to the participants for the interpretation of their performance. the iso 13528 standard can serve as the basis for the implementation of the homogeneity and stability evaluation figure 5. in the lower part the results of each participant are shown: their outcome is the expectation, e, of the respective pdf, mi, and its variance. the upper part represents the distribution of the expectations, mi, distribution assumed to be normal: its expectation is m and the variance is σl2. figure 6. proficiency test on lead concentration: many more participants laboratories, here shown in the order arising from the obtained numerical result for the same reference material, lead concentration in the matrix [3]. acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 82 procedures. the data model proposed in [3] is written as follows: xi = µ + εi (2) where: xi = participant result, laboratory i, µ = true value for measurand, εi = random error, laboratory i. iso 5725 [2] and iso 13528 [3] are complementary and are recommended by iso 17043 [13]. the proficiency tests are a hierarchical exercise, meaning that the pilot of the pt provides samples from the reference material that is the object of the pt with an assigned target value. the aim of the exercise, differently from that of the one illustrated in the iso 5725, is to test the current ability of the participants to obtain the reference value. 3.2. indirect comparisons here “indirect” means comparisons lacking a single physical location, for example the result of a search for results in the literature (as in step (i) of the process illustrated in section 2), where one compares published results without the possibility of checking the comparison outcome by means of a direct comparison. this case is very common. it is used, for example, by codata [24] for the input data of their least squares analysis (lsa) on the numerical values of the physical constants. only inference about their mutual consistency is possible by means of a critical review and of an expert judgment (type b component of uncertainty according to the gum [25]). in some cases, the measurand is such that, when in doubt, a further direct comparison can be planned and performed (by one of the preceding three methods). in other cases, this is not possible, for instance for the numerical values of the physical constants. the latter issue is very limiting when it affects the confidence level of the judgment, since it is impossible to adequately check the possible existence of systematic effects that may affect some of the results (e.g., like the current problem with the planck constant). this issue is postponed to further studies. 4. concluding remarks the importance and need of inter-comparisons has been illustrated in the frame of the process of progressively increasing the knowledge level, and its main features. we have also indicated how they can be of different types fulfilling different purposes, by limiting the examples to comparisons being the object of three international codes. references [1] cipm mra-d-05, measurement comparisons in the context of the cipm mra, bipm document 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[21] f. pavese, a metrologist viewpoint on some statistical issues concerning the comparison of non-repeated measurement data, namely mra key comparisons, measurement 39 (2006) pp. 821–828. [22] a.g. chunovkina, n.d. zviagin, n.a. burmistrova, interlaboratory comparisons, practical approach for data evaluation, acta imeko 2 (2013) pp. 28–33. [23] a.g. chunovkina, a.v. stepanov, n.a. burmistrova, evaluation of inconsistent data: comparison of two adjustment algorithms, measurement 91 (2016) pp. 707–712. [24] http://www.codata.org/about-codata [25] jcgm, evaluation of measurement data – guide to the expression of uncertainty in measurement, bipm, 2008. https://www.bipm.org/utils/common/documents/jcgm/jcgm _100_2008_e.pdf [26] r. willink, forming a comparison reference value from different distributions of belief, metrologia 43 (2005) pp. 12–20. [27] f. pavese, on hierarchical vs. non-hierarchical comparisons in metrology and testing, int. j. metrol. qual. eng. 1 (2010) pp. 7– 10. doi: https://doi.org/10.1051/ijmqe/2010004 [28] p. ciarlini, m.g. cox, f. pavese, g. regoliosi, the use of a mixture of probability distributions in temperature interlaboratory comparisons, metrologia 41 (2004) pp. 116–121. https://www.bipm.org/ https://doi.org/10.1016/j.shpsa.2017.05.003 https://doi.org/10.1051/ijmqe/2012017 https://doi.org/10.1051/ijmqe/2012029 http://www.codata.org/about-codata https://www.bipm.org/utils/common/documents/jcgm/jcgm_100_2008_e.pdf https://www.bipm.org/utils/common/documents/jcgm/jcgm_100_2008_e.pdf acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 83 [29] f. pavese, dependence of the treatment of systematic error in inter-laboratory comparisons on different classes of standards, acqual 1 (2010) pp. 305–315. [30] iso 3534, statistics – vocabulary and symbols – part 1 and part 2, 2006. [31] a.g. steele, k.d. hill, r.j. douglas, data pooling and key comparison reference values, metrologia 39 (2002) pp. 269–277. [32] d.l. duewer, a robust approach for the determination of ccqm key comparison reference values and uncertainties, ccqm-doc/2004-15, bipm, sèvres, 2004. the core concept of measurement needs to be inseparably complemented by the need to ensure reliability of results (qualitative and quantitative) and, whenever possible, measurement traceability. that need obviously also implies a sound understanding o... many factors can influence the accuracy and reliability of testing and/or calibration carried out by a laboratory [10]. some of these influence factors may include the following:  level of competence of personnel  equipment and ambient conditions  handling and storage of objects for test and calibration  measurement traceability  sampling  collection  testing and calibration methods (whether developed or adopted, standardized or non-standardized)  means to be used to ensure the quality of testing and calibration results:  regular use of certified reference materials and/or internal quality control with use of secondary reference materials  participation in comparison programs between laboratories, or proficiency tests  tests or calibration repeated using identical or different methods  renewed tests or renewed calibration of stored objects  correlation of results for different characteristics of an object. it is important to remember that a quantity value [9] is the basis for the subsequent comparison of values of quantities of the same kind. in many situations, a known (conventional) quantity value can be used for a true quantity value of the measuran... the concept of comparison has become widely used in many sectors, and many new proficiency test (pt) schemes are currently launched every year worldwide [11], [12]. a detailed study of comparison testing has been conducted [12]. the international orga... in the testing field, the questions most frequently asked about the subject of comparisons are the following. does the possibility of a comparison in my case exist? with whom can i or should i compare? what should i do if no comparison solution has be... the principle of comparison has long since been used and considered effective by national metrology institutes (nmi), notably, since 1999, in the form of key comparisons [1] organized under the auspices of the international bureau of weights and measu... in certain situations or fields (especially in testing), comparisons may be made complicated either by the fact that no reference exists or because no person or organization is taking the initiative of setting up a comparison campaign. in that case, e... in the absence of knowledge of a “true value”, the fundamental aim of comparison measurements is to increase as much as possible the confidence level associated with the evaluation of the measurement result. this may be expressed using different terms... – to obtain a measure of the degree of confidence (or degree of believe) for the differences found in the measured numerical values – to obtain a measure of the degree of reliability of the uncertainty evaluations associated to those values. note that the attribute “best” often associated to the evaluation of a measurement can only have the meaning associated with (or derived from) the hypotheses under which the analysis is performed, which vary according to subjective preferences. without entering into the vast subject of subjectivity vs. objectivity (see, e.g., [19]), it is enough to note here that various degrees of subjectivity are possible, depending on the overall ‘level of knowledge’—here a term not necessarily used in th... strangely, however, this fact is barely referred to, if not totally ignored, in the main approaches to statistics of interest to metrology and testing, i.e., the “error approach” and the more recent “uncertainty approach”. below a roadmap follows to i... added value of harmonization and consistency of laboratories acta imeko issn: 2221-870x march 2020, volume 9, number 1, 49 55 acta imeko | www.imeko.org march 2020 | volume 9 | number 1 | 49 the added value of the harmonisation and consistency of laboratories’ accreditation to the development of the global market álvaro ribeiro1, jeff gust2, antónio vilhena1, john wilson3 1 national laboratory for civil engineering, av. do brasil 101, 1700-066 lisbon, portugal 2 fluke, everett, wa, united satetes of america 3 national laboratory association, pretoria, south africa section: research paper keywords: accreditation; laboratory; conformity assessment; testing citation: alvaro ribeiro, jeff gust, antonio vilhena, john wilson, added value of harmonisation and consistency of laboratories, acta imeko, vol. 9, no. 1, article 8, march 2020, identifier: imeko-acta-09 (2020)-01-08 editor: lorenzo ciani, university of florence, italy received november 25, 2019; in final form february 6, 2020; published march 2020 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: álvaro ribeiro, e-mail: asribeiro@lnec.pt 1. introduction there are many reasons why it is important to understand the relevance of accreditation for the economy. one of the most important reasons is that global markets require a growing level of confidence in goods and services. trade operations are supported by trust in the quality of goods and services provided by suppliers and agents of the transactions (namely, buyers and sellers) [1]. to provide the assurance of goods and services, a quality control infrastructure is needed, bringing added value to citizens and consumers through the development and use of tools able to fulfil the expectations of safety and quality of life. quality infrastructure is ‘the system comprising the organisations (public and private) together with the policies, relevant legal and regulatory framework, and practices needed to support and enhance the quality, safety and environmental soundness of goods, services and processes’ [2]. according to the “bureau international des pois et mesures" (bipm) [2], ‘the quality infrastructure is required for the effective operation of domestic markets, and its international recognition is important to enable access to foreign markets. it is a critical element in promoting and sustaining economic development, as well as environmental and social wellbeing’, considering that it relies on five domains: 1) metrology; 2) standardisation; 3) accreditation; 4) conformity assessment; and 5) market surveillance in regulated areas. the relationship between conformity assessment and accreditation significantly impacts the global economy in that conformity assessment ensures that suppliers consistently meet industry standards and therefore can be relied upon, and accreditation serves as a reinforcement of conformity assessment using external entities (national accreditation bodies [nabs]). abstract laboratories are the largest group of stakeholders of the international organization for laboratory accreditation (ilac). the impact of laboratory activities as independent entities that ensure the safety and quality of products and services is growing and becoming a key issue in many fields. new challenges of globalisation, international trade, technological developments, and increasing expectations of citizens and consumers require a robust implementation of accreditation to promote the fair competition and sustainability of laboratories. for this purpose, the harmonization and consistency of accreditation is a major task that can be achieved through the cooperation of all stakeholders. the ilac laboratory committee has conducted a survey to evaluate the present state of harmonization of accreditation activities. the main results are presented in this paper, and some recommendations for future activity are given at the end. mailto:asribeiro@lnec.pt acta imeko | www.imeko.org march 2020 | volume 9 | number 1 | 50 the nabs are able to provide an independent way to validate the competence of conformity assessment bodies (cabs), with services such as calibration, testing, inspection, and certification. the accreditation of cabs is implemented by using system standards that allow, at the international level, the recognition of conformity assessment providers’ competence. for laboratories, iso/iec 17025 [3] is the reference, and in the case of inspection bodies, iso/iec 17020 [4] supports the accreditation. the framework of these standards is called the iso casco toolbox (figure 1). the recent revision of iso/iec 17025 introduced relevant changes [5], [6], namely an update to the vocabulary, a new structure harmonised with related system standards (e.g. iso 9000 and others), the concept of impartiality, risk-based thinking, a process-based approach, a higher focus on clients, an interpretation of and requirements for sampling activities, the implementation of decision rules associated with statements of conformance, and a flexible structure for organisations considering the digital transition. the new structure of the standard modified the previous version published in 2005, which separated the requirement for the laboratory management and technical domains, introducing an approach that considers a composition of five main sections, related to general requirements (focused on impartiality and confidentiality); structural requirements related to the model type of organisations and the way that develop the support for the management system; the resources and processes provided in order to fulfil the requirements for the implementation of the management system; and the requirements oriented to promote the harmonisation with related management systems based on other schemes (e.g. iso 9001). this structure is briefly presented in figure 2. 2. new trends today, laboratories face the same challenges as many organisations, overwhelmed by the growing development of technologies, products, materials, and services, which creates new expectations that require innovation as a new tool applied to conformity assessment. the dynamic and sometimes unpredictable world of today, described by the acronym vuca (volatility, uncertainty, complexity, and ambiguity) [7], requires leadership, vision, and knowledge to face daily shifts in economy, trade, and international relations. the increasing velocity of change of technologies through innovation is becoming overwhelming, requiring action upon a future that remains unclear. to prepare strategies, organisations need reliable information about the emerging technology trends in order to provide solutions for a new world of people’s expectations [8]. this development is strongly related to the so-called digital transformation, which is in the spotlight of government strategies worldwide. at the european level, the digital single market (dsm) designates the strategy of the european commission for the best possible access to the online world for individuals and businesses [9] (see figure 3), expected to have a huge impact on economic growth, boosting jobs, competition, investment, and innovation [10]. to achieve goals related to the digital economy, the smart cities concept [10]; a data-driven society; a reshaping of communication and businesses through 5g; artificial intelligence; and many other challenges, laboratories need to be involved in the digital transition and need to be able to adapt to many daily changes that happen. urban development and smart cities are key concepts for the understanding of the relations to be established between digital technology, disruptive innovation, and society [11], [12]. to understand and to foresee the impact and development of expected world transformations, specifically in relation to economic models, a global perspective of the trends and the catalysts becomes a key issue [13] for developing strategies for science and technology, which are the support for conformity assessments performed by laboratories. in this new framework, iso/iec 17025 brought to the scene the tools to allow laoratories to create the right conditions to develop management systems and to promote conformity assessments fulfilling the growing expectations of future markets and society. however, the nature of laboratories will have to figure 2. iso/iec 17025:2017 requirements. figure 1. iso casco toolbox (source: iso). figure 3. the eu digital single market strategy [6]. acta imeko | www.imeko.org march 2020 | volume 9 | number 1 | 51 adapt to the dynamic nature of the changes, which will require flexibility, responsibility, knowledge (scientific and technical) with higher levels of specialisation and competence in new fields (e.g. data science; data management; artificial intelligence; nano and bio materials; large sensor networks using big data approaches; and quantum computing). 3. ilac forum and the role of the ilac laboratory committee the international laboratory accreditation cooperation (ilac) was created in 1977 in copenhagen ‘with the aim of developing international cooperation for facilitating trade by promotion of the acceptance of accredited test and calibration results’ [14] as the international organisation for accreditation. it includes the accreditation bodies operating in accordance with iso/iec 17011, able to support the accreditation of entities such as cabs (using iso/iec 17025 [3]), medical laboratories (using iso 15189 [15]), and inspection bodies (using iso/iec 17020 [4]). the main added value is given by the mutual recognition arrangement (ilac mra) [16]; the evolution of accreditation of cabs in recent years (figure 4) shows an increasing interest in accreditation and in the ilac mra as a way that these entities found to stand out from competitors in the global market. this mutual recognition gives a unique approach that allows the recognition of the competence of laboratory activity providers from the signatory countries, based on mutual evaluation and acceptance of the national laboratory accreditation systems. the global network of services relies on this system, which expects that they can assure safety and quality of life for citizens and consumers by testing, inspection, and certification (tic). the tic sector market is a large contributor to the economy (see figure 5), as mentioned in [17]: ‘it provides greater consumer protection, safer products and industrial installations, reduces compliance costs for [smalland medium-sized enterprises] smes and increases brand reputations and consumers’ trust and confidence in a product by ensuring that products, infrastructures and processes meet the required standards and regulations in terms of quality, health and safety, environmental protection and social responsibility and can, therefore, also be a facilitator to international trade’. according to ilac, ‘in 2019, almost 80,000 laboratories, 11,000 inspection bodies and over 450 proficiency testing providers were accredited by ilac mra signatories’. as these numbers show, laboratories have a special role as stakeholders in the ilac organisation; this role is also recognised with the laboratory committee (ilac lc) in ilac’s structure, which provides a platform for interaction with the laboratory community. 4. concerns, expectations, and challenges of laboratories: the 2017 international survey 4.1. framework as described above, accreditation is essential for modern trade, which requires harmonisation and consistency in the application of its rules worldwide to have a fair and balanced competition. however, in some circumstances, different practices are found at national and regional levels. to support the development of an ilac strategy, ilac lc decided in 2017 to develop an international survey on the harmonisation and consistency of accreditation in order to provide a global view of the concerns, expectations, and challenges of laboratories and cabs, between may and june 2017. the main conditions to measure the impact of the survey were [18]: − the number of replies should be statistical relevant; − ideally, geographical representation should be by countries and by regions; − it should cover a wide range of activities; − it should represent a range of small, medium, and large companies and institutions; − it should cover a range of small to large numbers of accreditation parameters (both testing methods and calibration parameters); and − it should capture respondents’ experience regarding the accreditation process. the target was to obtain responses from at least 25 countries and 4 continents. 421 responses from 35 countries and the economies of 5 continents were received [18] (figure 6) from an estimated sample of 1500 laboratories questioned. the answers received allowed for an analysis divided into three parts: 1) the characterisation and validation of the sample of participants; 2) an accreditation performance analysis; and 3) an analysis of satisfaction regarding the services provided by nabs and an improvement analysis. figure 5. 2015 tic market (source: sgs report). figure 4. evolution of accredited cabs worldwide (2010–2019) (source: https://ilac.org/about-ilac/facts-and-figures). acta imeko | www.imeko.org march 2020 | volume 9 | number 1 | 52 results are presented in an ilac lc report [18] and include an analysis of the following issues identified by stakeholders of ilac lc regarding the lack of consistency observed: scope definition; cycle and frequency of assessment; quality of assessment; translation barriers; non-uniform interpretation of iso 17025 at the national level by accreditation bodies; accreditation body policies; use of pt/ilc and similar quality control tools in assessment; and lack of recognition of ilac mra. in the next sections, highlights of those results are presented. 4.2. characterisation of laboratories a critical issue regarding the development of the survey was how the sampling of the participating laboratories would represent the whole community and their activities. the distribution of laboratories by activities showed that a large spectrum of domains was included, although there was higher participation in some fields, namely agro-food, environment and metrology (figure 7). a target for this survey was also to have entities with different perspectives and experience levels regarding the length of accreditation. the answers (figure 8) show a higher number of responses from laboratories with more years of accreditation. the relatively higher rate of responses from laboratories with long-established accreditation indicates a keen interest in the issues explored in the survey. another main topic that was intended to be evaluated was the application of flexible accreditation [19] in testing and calibration contexts, because it is one of the differentiation elements pointed out by the community of laboratories as having different approaches in different countries and economies. the laboratories’ answers allowed for verification that at the end of 2017, only 32 % of test laboratories and 14 % of calibration laboratories have flexible accreditation scopes. some entities replied that such a scope would not be applicable in their case (18 % of the test laboratories and 37 % of the calibration laboratories). a more detailed analysis of the ‘not applicable’ answer shows, in some cases, a lack of awareness of this concept. the comments received show that in some countries and economies, a flexible scope has not yet been implemented, and in the case of calibration, some countries and economies apply it (as semi-flexible) and others do not allow it. this promotes a distortion on the market affecting competitiveness, and in some cases, accreditation bodies do not provide information or promote this possibility. 4.3. evaluation of the performance of accreditation bodies another relevant feature of the survey was to develop a performance analysis of accreditation bodies considering the main conditions for accreditation, namely the time it takes to get accreditation, use of extra requirements for accreditation, cooperation with stakeholders, impact of quality control tools, surveillance time interval, and cycle of accreditation. 4.3.1. cycle and frequency of assessment a set of survey questions addressing the cycle and frequency of assessment was prepared. it was intended to evaluate how these two aspects, which are very relevant to the competitiveness of laboratories, are applied in distinct countries and economies. the average duration of accreditation cycles varies from 12 months to more than 60 months (figure 9). in 54 % of the countries and economies, the average duration of the accreditation cycles is equal to or greater than 48 months. of the remaining countries and economies, 40 % have an average accreditation cycle of 24 months or less. the average surveillance time interval between audits for main laboratories is mostly 12 months. the main criticism noted from the comments is that many laboratories consider an interval of 12 months between audits to be ‘very small’ and that the significant repetitive effort securing ongoing accreditation does not contribute real value to the laboratory’s activity. one of the concerns that laboratories have in common is that quality control tools, such as proficiency testing [20], often have figure 7. replies by types of activities [18]. figure 8. distribution of responses by years of accreditation [18]. figure 6. laboratories by location of main office [18]. acta imeko | www.imeko.org march 2020 | volume 9 | number 1 | 53 no impact on the external evaluation programs. in fact, only 20 % of the laboratories experienced a reduction of the assessment effort when previous audits demonstrated that quality control tools were applied with success. 4.3.2. the performance of accreditation bodies another major concern about the competitiveness of laboratories in the market is the time it takes to get accreditation. 80 % of the laboratories surveyed obtained their accreditation in 12 months or less. on the other hand, 6 % of the laboratories took more than two years to become accredited. an obstacle to competitiveness and equality in international markets is the existence of accreditation body guides that contain extra national requirements beyond the requirements of the accreditation standards, which may also lead to difficulties in the ilac mra process. often, the laboratories are not consulted in the development of these accreditation body guides. over half the laboratories (61 %) responded that their accreditation body has such guides. again, the results show a wide variety of situations. national (standards) interpretation guides are often not supported by dialogue with stakeholders; 21 % of the participating laboratories answered that the guides are approved without stakeholder participation. only 29 % answered that their accreditation body develops guides with the agreement/participation of the laboratory stakeholders. the results also show concerns regarding the introduction of extra requirements by accreditation bodies outside the requirements of the accreditation standards. over half the laboratories that reported that their accreditation body has national interpretation guides for the reference accreditation standard said that those guides establish extra requirements that are not supported by that standard. less than half of the laboratories (46 %) reported that they are invited to participate in the definition of national strategies for accreditation. for those entities that do participate in the definition of the national accreditation strategy, they are generally involved at lower levels (participation in committees and meetings), with less than 5 % participating at management or director level, as is the case with accreditation bodies (figure 10). 4.3.3. satisfaction of the laboratories and improvement areas the final section of the survey aimed to establish a procedure for evaluating the degree of satisfaction of the laboratories regarding the accreditation process in different spheres, such as: − quality and competence of the auditors [21]; − quality of the interpretation guides; − accreditation process; and − complaints procedure [22]. the first question addressed how laboratories perceive the lack of ilac mra as a trade barrier in its activity. the results show that about two thirds of the laboratories considered that this issue was not directly applicable to them, meaning that those laboratories do not carry out activities abroad. considering only the remaining laboratories with activities abroad, 18 % thereof felt that the lack of recognition of the ilac mra has impact on their activity. considering the improvement of the accreditation body’s provided service, when questioned, the participants responded that the service was mostly unchanged from one year to the next or improved. only 10 % of the respondents indicated that the service provided by the accreditation body worsened. still, there are big differences when the performance of accreditation bodies is compared worldwide. a more detailed analysis carried out for countries and economies with more than five responses shows, particularly in europe, that the accreditation process has not improved from one year to the next, and there are even many responses stating that the process has worsened (figure 11). however, in south and north america, africa, and asia, the process either remained the same or led to greater satisfaction. for the different specific accreditation body performance aspects questioned, all received a positive evaluation. the best qualified are the ‘quality of the auditors’ and ‘the reporting of the assessment’ (figure 12). at the other end are the aspects related to the laboratories’ interaction with the accreditation bodies, namely the way in which the accreditation body deals with the complaints, how it adapts procedures to the laboratories’ different business needs, and how the accreditation body promotes the accreditation. the final set of questions look at the aspects considered most positive and identifies the aspects of the accreditation process that need improvement; the classification obtained is presented figure 9. distribution by cycle of accreditation [18]. figure 10. distribution by type of participation in the definition of national accreditation strategy [18]. figure 11. comparison of accreditation services with the previous year, by country (countries with five or more answers) [18]. acta imeko | www.imeko.org march 2020 | volume 9 | number 1 | 54 in figure 13 and figure 14. in line with the results presented previously, figure 13 shows that the major aspects that need improvement by accreditation bodies are the public promotion of the added value of accreditation that leads to a better cost benefit for the organisations. furthermore, the interpretation of the iso / iec 17025 requirements that are transposed to national interpretation guides is a parameter that should be the subject of additional research. the quality of the assessment, in line with the answers about the quality of the auditors, is the top-rated aspect of the accreditation procedure (figure 14). also noteworthy is the cycle and frequency of assessment, but a more careful analysis is necessary in this parameter in order to identify the countries and economies to which this response applies. a final important aspect considered in the survey was whether the laboratories were comfortable filling a complaint or appeal of the accreditation process with your accreditation body and not be concerned about reprisal. about one third of the laboratories (34%) answered that they are not comfortable of filing a complaint, mainly because they believe the complaint will not alter any ab operating procedure 5. conclusions and outlook accreditation plays a fundamental role in the economy and is expected to be an engine of economic development on a fair trade basis [23], [24]. in order to achieve this objective, it is necessary to ensure that the application and assessment of the standard requirements of iso/iec 17025 (the reference standard for laboratory accreditation) is applied worldwide in a balanced, harmonised, and consistent manner. nowadays, on the edge of a new industrial revolution, industrialisation 4.0, ‘the response to it must be integrated and comprehensive, involving all stakeholders of the global polity, from the public and private sectors to academia and civil society’ [25]. the collaboration of the laboratory community for this purpose is necessary, and the role of entities such as ilac lc is particularly relevant, as demonstrated by the action of promoting a survey to this community in order to understand the current state of implementation of accreditation as well as its positive aspects and opportunities for improvement. it is also known that ‘the conformity assessment community including accreditation bodies is a conservative community’ [25], which reinforces the importance of and need for the diagnosis provided by this survey, pointing out the main factors that the community feels are the most urgent ones to change. this approach has also enabled an independent evaluation of nabs’ performance as a way of supporting the evolution of accreditation and its recognition in the international context. 6. acknowledgements to the members of ilac laboratory committee, the coauthors of the report ‘2017 international survey on harmonisation and consistency of accreditation: a global view of laboratories and conformity assessment bodies – concerns, expectations, and challenges’. to the laboratories and other stakeholders; national and international laboratory and conformity assessment bodies; and associations and international organisations that supported and contributed 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[18] ilac laboratory committee, 2017 international survey on harmonization and consistency of accreditation: a global view of laboratories and conformity assessment bodies – concerns, expectations and challenges, april 2018. [19] ilac g18:04/2010, guideline for the formulation of scopes of accreditation for laboratories, ilac, silverwater, australia, 2010. [20] ilac p9:06/2014, policy for participation in proficiency testing activities. ilac, silverwater, australia, 2010. [21] ilac g3:08/2012, guidelines for training courses for assessors used by accreditation bodies, ilac, silverwater, australia, 2012. [22] ilac r5:04/2016, ilac procedure for handling complaints, ilac, silverwater, australia, 2016. [23] ilac-iaf, accreditation: adding value to supply chains, ilaciaf joint publication, world accreditation day 2019, ilac, silverwater, australia and iaf, chelsea, canada, 2018. 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[25] m. m. nilsson, background paper for break-out 1: future trends in conformity assessment and casco standards in industrialisation 4.0, iso, annex 2 to devco 10/2019, july 2019. template for an acta imeko event paper acta imeko issn: 2221-870x june 2018, volume 7, number 2, 96-101 acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 96 determination of acrylamide in portuguese bread by uplcms/ms: metrological and chemometric tools susana jesus1,2, inês delgado1,3, andreia rego1,2, carlos brandão4, rui galhano dos santos2, joão bordado2, isabel castanheira1 1departamento de alimentação e nutrição, instituto nacional de saúde doutor ricardo jorge, i.p., lisboa, portugal 2cerena centre for natural resources and the environment , universidade técnica de lisboa, instituito superior técnico, lisboa, portugal 3chemical engineering department, instituto superior técnico, lisboa, portugal 4escola superior de hotelaria e turismo do estoril, estoril, portugal section: research paper keywords: acrylamide; occurrence; uplc-ms; efsa citation: susana jesus, inês delgado, andreia rego, carlos brandão, rui galhano dos santos, joão bordado, isabel castanheira, determination of acrylamide in portuguese bread by uplc-ms/ms: metrological and chemometric tools, acta imeko, vol. 7, no. 2, article 17, june 2018, identifier: imeko-acta-07 (2018)-02-17 section editor: claudia zoani, italian national agency for new technologies, energy and sustainable economic development affiliation, rome, italy received february 16, 2017; in final form april 6, 2018; published june 2018 copyright: © 2018 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: this work was supported by elementaria-2013dan850 project corresponding author: isabel.castanheira@insa.min-saude.pt 1. introduction acrylamide (aa) is mainly used as a monomer in the polymerization reaction of polyacrylamide. it is present in cooked foods and cigarette smoke; nevertheless, the occurrence of acrylamide in foods was only firstly reported in 2002 [1], [2]. in 1994, the international agency for research on cancer (iarc) classified acrylamide as a carcinogen in animals and probably to humans (group 2a)[1], [3]–[7]. also, the european commission in 2002 classified this compound as category 2 for carcinogenicity and category 2 for mutagen [8]. the harmful effect of such compound on humans was later confirmed, and nowadays the acrylamide is, in fact, considered a neurotoxic substance. in people overexposed to such compound, the peripheral nervous system can be impaired as well as the central nervous one [2], [9]. besides that, the acrylamide metabolism is considered genotoxic [10]–[17]. after being absorbed and distributed by cells, a process of oxidation by cytochrome p450 2e1 is initiated, leading to the transformation of acrylamide in glycidamide, a genotoxic agent, which reacts with the dna. the reactional products can originate dna adducts that are associated with cell damage. nevertheless, the untransformed acrylamide and its metabolites are eliminated by urine [10], [11], [15], [16], [18]. abstract acrylamide (aa) is known for its potential health hazards and it is present in processed potatoes, coffee, and bread. wheat is still the most important cereal for bread making, however, the interest in its substitution by other grains is growing due to the consumer demand for novels and healthy foods, like oat and rye flour. the aim of this study was to analyze the aa content of confectioned bread in fifty-four different pastries and make an association of acrylamide contents and the place of production to identify the quality of the flour used. there was a wide variability in aa levels among different breads and within different bread varieties. the median of aa values was 787 µg/kg for whole grain bread, 783 µg/kg for oat bread and 253 µg/kg in rye bread. with the cluster analysis, it was possible to conclude that besides the factors like baking temperature-time and fermentation time which affects aa formation in bread products, several other parameters such as the formulation, flour quality and the varieties of processing techniques, among others, play a crucial role in the aa formation. acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 97 there are three pathways for the formation of acrylamide in processed foods [19]-[20]. one is the thermal decomposition of lipids which results in a significant amount of acrolein. afterward, the oxidation of acrolein generates acrylic acid that in the presence of ammonia conducts to the formation of acrylamide. another pathway is carnosine pyrolysis, which liberates acrylic acid. as in the case of the lipid route, the acrylic acid originates acrylamide in the presence of ammonia. finally, the main route for acrylamide formation in foods is a consequence of the maillard reaction which involves a free amino acid (asparagine) and reducing sugars. the reaction of these precursors results in a schiff base which undergoes through decarboxylation to afford 5-1-oxazolidine. subsequently, the decarboxylated product could decompose either to acrylamide, 3-oxopropanamide or 3aminopropionamide (3-apa). the excessive sugar amount could also be responsible for the formation of acrylamide (figure 1) [4]. m. cengiz, c. gündüz reported acrylamide levels of 225 µg/kg and 495 µg/kg in bread and cookies, respectively [21]. in finland, s. eerda, k. hollebekkers, hallikainen a. and k. peltonen obtained acrylamide levels in bread and sweet biscuits of 645 µg/kg and 310 µg/kg [22], respectively, but a study in belgium showed lower results in bread with 34 µg/kg and biscuits with 154 µg/kg [23]. dietary acrylamide exposure of population is mainly dependent on the contaminant amounts in foodstuffs. for the risk supervision of acrylamide, it is necessary to identify and characterize it. for non-neoplastic, benchmark dose (bmdl10) is 0.43 mg/kg b.w. per day, but for neoplastic effects, it is 0.17 g/kg b.w. per day [25]. in this year, the european commission published a regulation about the acrylamide mitigation measures and benchmark levels in foodstuffs [26]. this regulation establishes mitigation approaches for foodstuff, considering 3 phases, agronomical, product design and processing. however, until this date, efsa did not set maximum limits but stated indicative values for various food groups such as cereals (table 1) [24]. occurrence data is required to support efsa decisions since the data correlation between acrylamide intake and biological biomarkers of exposure become crucial to evaluate the dietary exposure: it is therefore important to accurately assess the level of acrylamide in various processed food. to this date, in portugal as in other european countries, occurrence studies in cereal group are scarce. therefore, the aim of this study is to assess the levels of acrylamide in bread confectioned in several pastries and make an association of acrylamide contents with the place of production to further identify the quality of flour. these values are discussed and compared with those available in others european countries. 2. materials and methods 2.1 sampling and sample preparation bread varieties were collected randomly from 54 industrial producers located in the district of lisbon. producers were asked about dough and bread preparation. the sampling plan is present in table 2. samples were analyzed as pool composed of 10 breads from the same producer. 2.2 dough and bread preparation the bread products collected were further categorized according to the flour: rye bread, oat bread and whole grain bread (bread with wheat flour and others in minor quantity). the time and temperature of the baking process did not vary between producers, ranging from 190 ⁰c to 200 ⁰c. however, no information is available about either all ingredients used in recipe or the fermentation step. 2.3 reagents, chemical standards and materials aa analysis for this study, the following reagents were used: acetonitrile (merck gradient grid is liquid chromatography), formic acid (group carlo erba reagents, 99% for analysis) methanol (merck, hypergrade for lc-ms) and ultrapure type 1 water (captured from a milli-q water purification system). the standard used was the standard of acrylamide (99%) dr. ehrenstorfer gmbh). for the homogenization of samples, a knife mill grindomix gm 200 was used. in the extraction of the compound to be analyzed in this study, an eppendorf table 1. indicative values set by european commission for acrylamide content in cereal-based foods [24]. foodstuff indicative values (µg/kg) soft bread a) wheat based bread b) soft bread other than wheat based bread 80 150 breakfast cereals bran products and whole grain cereals wheat and rye-based products maize, oat spelt, barley and rice based products 400 300 200 biscuits and wafers crackers crispbread gingerbread products similar 500 500 450 1000 500 biscuits and rusks for infants and young children 200 processed cereal-based foods for infants and young children 50 figure 1. acrylamide formation of the maillard reaction. table 2. sampling plan applied in this study. species nº producer number of collected samples number of pools oat 23 230 23 rye 24 240 24 mixture 7 70 7 total 54 acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 98 centrifuge 5804 r, a vortex shaker type stirrer, a kline stirrer and spe columns (bondelut plexa, 6 ml, 200 mg) were used. in the identification and quantification of acrylamide the ultraefficient liquid chromatograph (acquity uplc) was used and the analytical column acquity uplc beh c (18, 1.7 μm, 2.1 x 150 m) was obtained from waters. 2.4 acrylamide extraction bread was immediately prepared after the reception in the laboratory. each sample was milled separately using a highspeed grinder (knife mill grindomix gm), homogenized and stored in vacuum bags. 2 g of homogenized sample were weighed into a centrifuge tube, and 20 ml water/ formic acid (0.1%) was added. the sample was stirred in a vortex for 2 minutes and then in an oscillating shaker for 30 min at 70 oscillations per minute. then, it was centrifuged at 10,000 rpm for 15 minutes. oasis hlb spe cartridges (waters) were conditioned with 3.5 ml of methanol and equilibrated with 3.5 ml of acidified water. 1.5 ml of the sample were loaded and eluted with 3 ml of acidified water to a flask of 10 ml, and the volume was completed with acidified water. all samples were prepared and analyzed as three replicates. the results are expressed as mean and standard deviation. 2.5 lc-ms/ms analysis analysis was carried out in an ultra performance liquid chromatography coupled to a mass spectrometry (uplcms/ms) with electrospray ionization source (esi). the method was operated using an isocratic elution with 90% water and 10% acetonitrile at a flow rate of 0.2 ml/min. retention and separation of acrylamide were achieved in a uplc beh c18 column (2.1 × 50 mm) with 3 minutes of analysis and a retention time of 0.84 minutes. the uplc-ms/ms was operated in the positive ion electrospray at the following conditions: capillary voltage of 3 kv, cone voltage 29 v, source temperature 120 °c, desolvation gas temperature 350 °c, desolvation gas flow of 5 l/hr, cone gas flow at 30 l/hr and the pressure of the collision gas was 3 × 10-3 mbar. 2.6 quality assurance the laboratory technical competence needed to carry out the acrylamide assay was provided by iso/iec np 17025:2005. equipments used during experiments were calibrated according to approved calibration procedures and external standards traceable to national measurements standard, when available. all volumetric glassware belongs to class a. certified reference material erm –bd273 (toasted bread powder) was purchased from irmm and used as a quality control of the method. the laboratory has participated in several pt schemes launched by accredited provider fapas to guarantee the laboratory performance. the performance of laboratory was expressed by the z-score, expressing the difference between the laboratory value and target value. 2.7 statistical analysis statistical analysis was performed using statistica 13 software (statsoft ibérica, lisboa, portugal). data were expressed as mean and standard deviation. one-way analysis of variance (one-way anova) was carried out in order to determine significant differences (p<0.05) between data. to make an association between acrylamide content in bread and the producer’s recipe the obtained data were analyzed by multivariate hierarchical cluster analysis by linkage ward’s method. the distance between samples was calculated using euclidean distances. 3. results and discussion in this study, accuracy was assessed through certified reference material (erm-bd273 toasted bread, certified value: 425 ± 29 ng/g) where the obtained analytical values are within 95% of the confidence level of certified value. laboratory performance was demonstrated by participation in adequate pt schemes, t3067 (biscuit (cookie)) and t3059 (biscuit (cookie)). in these assays, results were rated as satisfactory in all schemes. these results demonstrated the adequacy of the analytical procedure for acrylamide quantification. the present study focused on portuguese bread in common commercial market. a total of 54 bread samples divided into 3 groups (oat bread, rye bread, and whole grain bread) were analyzed for acrylamide concentration. the occurrence data is shown in tables 3, 4, and 5. in general, the acrylamide level in bread ranged between 60 and 1273 µg/kg depending on the bread types. the acrylamide concentrations in all bread types were order from high to low as whole grain bread (786,86 µg/kg)> oat bread (783,00 µg/kg)> rye bread (252,90 µg/kg). these results revealed high variation in acrylamide amount in different bread types, suggesting that differences in the so-called bread road-map, including types and quality of raw materials, formulations, baking methods can affect the acrylamide formation in bread. in the whole grain bread was observed a median of 786,86 µg/kg (table 3). however, there were two samples (producer 1 and 3) in the range of 200 and 600 µg/kg and three samples higher than 900 µg/kg. once this type of bread had a high percentage of wheat flour, the differences in acrylamide levels can be attributed to the fact that this bread may have various levels of some fractions of wheat, such as germ and bran, which contain high levels of the main precursor (asparagine) for the acrylamide formation in bread [27]. relatively to the oat bread (table 4), it was found a median of 783 µg/kg and a maximum level of 1258 µg/kg, results similar to the whole grain bread. nevertheless, the producers 13, 19, 22, 23 and 51 confectioned bread with acrylamide range between 200 and 600 µg/kg. also, breads with acrylamide amount in the range of 600 µg/kg and 900 µg/kg were observed. on the other hand, the bread from producers 16 and 53 were below the 200 µg/kg level of this contaminant. the variation among results may be justified through the differences in oat flour brands because the composition of acrylamide table 3. acrylamide levels in whole grain bread collected in several bakeries. producer acrylamide (µg/kg) 1 328.78 ± 3.14 2 786.86 ± 2.24 3 247.50 ± 18.02 4 1159.66 ± 5.38 5 1235.80 ± 52.30 6 741.18 ± 35.26 7 1149.82 ± 3.32 acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 99 precursors in flours are influenced by cultivars, year to year variations, harvest period, climate and soil composition [28]. in table 5, the rye bread had a median of 252,90 µg/kg and a maximum of 751 µg/kg. most samples were below 600 µg/kg, even so, acrylamide levels lower than 200 µg/kg were achieved by producers 36, 44, 48, 49, 50. these differences between acrylamide levels in rye bread can indicate that there is a difference in the asparagine content among the varieties of rye flours used by the producers. as mentioned earlier, the acrylamide indicative value set by the european commission for soft bread other than wheatbased bread was 150 µg/kg [24]. some of the oat and rye bread results were around this value, namely from producers 53, 16, 50, 49, 48, 44 and 36. in 2009, efsa reported a maximum level of acrylamide in bread group around 2430 µg/kg and in 2011 a maximum value of 2565 µg/kg and 910 µg/kg for bread non-specified and soft bred, respectively [29], [30]. the results obtained were lower than the occurrence levels of acrylamide reported by efsa. more recently, based on analytical results from a total of 24 european countries and six associations, efsa reported maximum levels of acrylamide in unspecified soft bread around 141 µg/kg, which is similar to bread samples analyzed from the producers 16, 53, 50, 48, 49, 36 and 44 [25]. nevertheless, efsa reported occurrence values for pumpernickel (rye bread) with a maximum of 245 µg/kg [25]. the majority of bread analyzed, 13 out of 24, contained lower levels of acrylamide which may be explained by the difference between rye bread recipe in portugal (flour mixture) and the pumpernickel (rye flour). in the literature, meghavarnam et al evaluated bread samples from the local market in india, obtaining a range between 1210 – 1365 µg/kg [31]. similar results were achieved by european commission in 2007 in bread and toast samples, 1987 µg/kg [32]. all the bread samples collected and analyzed in this work were below the maximum value of meghavarnam et al and wenzl studies. furthermore, the study published by krishnakumar and visvanathan reported a high variability of acrylamide concentration in bread group, <10 – 3200 µg/kg [2]. also, in this case, the results obtained in the three types of bread were lower than 3200 µg/kg. table 4. acrylamide levels in oat bread collected in several bakeries. producer acrylamide (µg/kg) 13 269.40 ± 5.22 14 1182.42 ± 14.06 15 834.50 ± 18.48 16 59.94 ± 4.02 17 807.88 ± 2.06 18 715.66 ± 2.30 19 344.62 ± 12.44 20 937.32 ± 15.12 21 1105.38 ± 23.56 22 546.18 ± 2.98 23 579.96 ± 7.60 24 1273.10 ± 28.92 25 783.38 ± 17.30 26 1208.28 ± 48.20 27 851.10 ± 5.92 28 1168.96 ± 69.48 29 843.38 ± 10.26 30 723.56 ± 9.26 31 1259.00 ± 11.66 51 387.82 ± 26.88 52 195.42 ± 4.76 53 119.70 ± 4.14 54 749.58 ± 47.10 table 5. acrylamide levels in rye bread collected in several bakeries. producer acrylamide (µg/kg) 8 536.08 ± 44.48 9 638.34 ± 23.54 10 505.98 ± 2.28 11 712.96 ± 65.94 12 645.94 ± 25.72 32 749.58 ± 47.10 33 325.96 ± 6.22 34 209.54 ± 14.18 35 751.28 ± 35.36 36 168.32 ± 12.92 37 319.38 ± 10.16 38 368.44 ± 9.20 39 205.44 ± 18.36 40 252.82 ± 13.58 41 278.30 ± 20.28 42 210.72 ± 13.16 43 216.22 ± 17.24 44 161.98 ± 6.78 45 252.96 ± 21.30 46 204.84 ± 16.54 47 201.46 ± 13.14 48 160.38 ± 12.70 49 176.86 ± 16.56 50 159.72 ± 5.80 acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 100 on the other hand, other authors had observed minor levels of acrylamide in bread. normandi et al analyzed the acrylamide concentration in canadian bread and found a maximum value of 107 ng/g. also, in bread from syria and iran, similar results were achieved, 119 – 263 µg/kg and 166 – 290 µg/kg, respectively [33] [34]. these levels are identical to the results observed in the bakeries 3, 13, 34, 36, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 52 and 53. in a higher range, pacetti et al obtained an acrylamide content between 102 – 594 µg/kg, which is higher than more than half of the bread samples (producers 1, 3, 8, 10, 13, 19, 22, 23, 33, 34, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 52 and 53) [35]. moreover, gündüz et al analyzed bread types from turkey and obtained a maximum of 695 µg/kg for all bread types. in this study, most of the bread samples were below this value. gündüz et al also reported values for rye bread with a range of 209 – 624 µg/kg, which is higher than the rye bread levels shown in the present work [36]. to study the relationship between the acrylamide levels and the recipe of producers, a cluster analysis was performed. the results of the cluster analysis (figure 2) in the present study are derived from the tree clustering analysis. the hierarchical tree observed in figure 2 provides a key to understand the relation between the bread samples and the producers. it was possible to identify the presence of 5 major clusters. the first cluster grouped the bread samples (4, 5, 7, 14, 21, 24, 26, 28 and 31) with the highest values, 1105.38 to 1273.10 µg/kg, where two types of bread, oat, and whole grain bread were included. these results can indicate that the oat bread producers of 14, 21, 24, 24, 26, 28 and 31 used flours with a similar amount of asparagine. relatively to the whole grain bread, producers can be grouped with respect to the similarity of cooking phase. nevertheless, the relation between these two types of bread can be explained by various reasons, such as the addition of other ingredients in the whole grain bread that influences the formation of acrylamide. most samples identified in the second cluster were from rye bread producers (8, 9, 10, 12, 22 and 23). in this cluster, the acrylamide content varies between 505.98 and 645.94 µg/kg. the similarity of the results observed among the rye bread producers suggests that the flour and the cooking method were identical. in the case of oat bread producers (bakeries 22 and 23) included in this cluster, the similar results can be related to the asparagine content in oat flours. cluster 3 included the three varieties of bread (2, 6, 11, 15, 17, 18, 20, 25, 27, 29, 30, 32, 35, 54), but mostly oat bread, where the values are among 712.96 and 937.32 µg/kg. in this cluster, the oat bread values are lower than in cluster 1, which can be related to the lower asparagine content of the varieties of oat flour. the two bakeries that produce whole grain bread should have a large amount of oat flour than wheat flour because the values are close to the oat bread. cluster 4 was represented mostly by rye bread (3, 13, 16, 34, 36, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 52 and 53). in this group, the values of acrylamide are between 59.94 and 269.40 µg/kg, which corresponds to the lowest values of acrylamide in general. to obtain the lowest value the producer may have used flour with low quantity of asparagine, but these results could also be caused by the cook confection, like the use of a high fermentation time. the cluster 5 grouped the three varieties of bread analyzed in this study (1,19, 33, 37, 38, 51), where the rye bread producers stand out. the acrylamide levels were between 319.38 and 387.82 µg/kg, which represented the average values obtained from bread samples. the cluster analysis grouped the samples with similar values of acrylamide, which allowed a preliminary conclusion about the association of acrylamide contents with the bread producers. however, there are many factors, such as crop management, fermentation process, that need more attention to comprehend the high variability of the results. 4. conclusions food is an important source of exposure to acrylamide of the population. thus, in this study the acrylamide content in confectioned bread in several bakeries was determined and an association of acrylamide contents with the producers was made. the applied quality control procedures, framed by iso 17025 requirements, demonstrated adequacy to guarantee the reliability of the obtained data. hierarchical cluster analysis, as applied to the data acquired from the content of acrylamide in three variants of bread, was useful. chemometric tools allowed identifying factors that could influence the acrylamide formation. this work is aligned with those that advocate the need for national data to estimate the real dietary exposure to acrylamide. publication part of this work was presented at 2nd imekofoods, benevento 2016, as poster. acknowledgement misage project (lisboa-01-0145-feder-024172). this project has received financial support from the fundação para a ciência e a tecnologia (fct), portugal. references [1] r. c. alves, c. soares, s. fernandes, and m. oliveira, acrylamide in espresso coffee: influence of species, roast degree and brew length, food chem., vol. 119, (2010), pp. 929–934. 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[36] c. gündüz, m. cengiz, acrylamide contentes of commonly consumed bread types in turkey, vol.18, (2015), pp.833-841. determination of acrylamide in portuguese bread by uplc-ms/ms: metrological and chemometric tools variability of measured railway track conductance acta imeko issn: 2221-870x december 2018, volume 7, number 4, 21-25 acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 21 variability of measured railway track conductance due to test setup jacopo bongiorno1, andrea mariscotti2 1 università di genova, via opera pia 11a, 16145 genova, italy 2 astm sagl, via comacini 7, 6830 chiasso, switzerland keywords: conductivity measurement; dc power systems; electric variables measurement; grounding; guideway transportation power systems; guideway transportation testing; stray current citation: jacopo bongiorno, andrea mariscotti, variability of measured railway track conductance due to test setup, acta imeko, vol. 7, no. 4, article 5, december 2018, identifier: imeko-acta-07 (2018)-04-05 editor: alexandru salceanu, "gheorghe asachi" technical university of iasi, romania received march 28, 2018; in final form september 26, 2018; published december 2018 copyright: © 2018 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: jacopo bongiorno, jacopo.bongiorno@edu.unige.it 1. introduction the stray current phenomenon, particularly significant for dc electrified transportation systems, is directly related to the insulation level of the traction current return path from surrounding structures and soil [1][2]. a known effect of stray current is the initiated and/or accelerated corrosion on metallic structures and on the rails themselves. this process is more evident and serious for larger leaking traction return currents [3][5]. a correct measurement of rail-to-earth conductance is therefore crucial during the entire life of an electrified transportation system. the iec 62128-2 international standard [6] identifies some measurement methods for track-to-ground conductance, which, however, are affected by profound variability due not only to the characteristics of the measured system, but also to the applied method itself. in a previous work, measurement variability introduced by various parameters was numerically evaluated [7] and estimated as lower than 1 %. rail-to-earth conductance values, however, depend on several factors, which can hardly be thoroughly modeled or accurately predicted [8]. therefore, confirmation of the results using on-site measurements is necessary. variability due to the location of the voltage terminal by performing track-to-earth measurement following a.3. of iec 62128-2 is thus experimentally assessed herein. the simulation model used in [7] and [9] is improved by including a simple soil model. as is commonplace, when performing measurements during the installation phases of a system [10], the installed tracks behind the measured section, which iec 62128-2 identifies as the grounding path for the negative terminal of the power supply, may be unavailable. this negative terminal must be connected to grounded structures or to an electrode driven into the soil. consequently, the resistance to ground of the power supply’s negative circuit may take highly different values, which has an impact on the measured track insulation [7]. iec 62128-2 [6] does not impose a unique location that is related to section length where the voltage reading is needed for the measurement. the choice of location introduces variability in track-to-ground conductance measurement results. this variability has been estimated in a simulation as being lower than 1 % of the measured value. in this work, the abovementioned variabilities are demonstrated and quantified using on-site measurements and are compared with the estimate obtained by the simulation. 2. the track-to-ground conductance measurement method reported in iec 62128-2 iec 62128-2 [6] proposes and describes in its appendix a, section a.3, a track conductance measurement method, shown in the schematic in figure 1. its use is suggested in the case of a abstract one of our previous studies, published in the acta imeko journal, pointed out the probable variability in track-to-ground conductance measurements due to the use of the method indicated in a.3 of the international standard iec 62128-2. in this work, the presence of measurement variabilities due to the connection of the negative terminal of the power supply to an earthing electrode instead of the behind section and due to the location chosen for the voltage terminal was proven by on-site measurements. a simulation model that includes undesirable soil conditions is proposed and used herein in the estimation of the correct value of track-to-ground conductance using the measured values. mailto:jacopo.bongiorno@edu.unige.it acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 22 track section that is delimited by an electrical separation of running rails, e.g., by rail cuts or insulated rail joints (irjs). the section length l must be less than 2 km. a dc voltage power supply (ps) is connected across the first rail cut and is used to obtain a nearly constant current, which flows from the measured track to the earth. the higher the trackto-earth conductance within the measured section, the larger the current that leaks from track to earth. in this paper, we identify with the term “measured section” or “track section” either the track or single rail, if there is no need to specify the real configuration. the ps current i and voltage to ground vp at a distance d from the current injection point are the measured quantities. the vp voltage is measured against a reference electrode (re) or a grounding circuit that is used as reference concerning which track conductance is quantified (e.g., concrete reinforcement, stray current collector, stray current mesh). the standard considers the ground reference as ideal and gives precise indications about its distance from the track, which, however, cannot be always fulfilled in real situations and are applicable to point electrodes, not distributed circuits. iec 62128-2 does not require a specific distance d to use for the vp measurement or the characteristics of the track behind section to which the negative terminal of the power supply should be connected. the conductance to earth is estimated as: lv i g p  . (1) 3. simulation model the model used for theoretical calculations that was originally proposed and used in [7] and [9] is improved herein in terms of its representation of the return path resistance (the parallel connection of various conductive parts, unideal soil, and concrete). the improved model is implemented using matlab simulink. the equivalent circuit is shown in figure 2. the simulink model is solved by using the ode15 method for ordinary differential equation systems. 4. variability of results in this section, variabilities due to the earthing resistance of the ps negative terminal (r0), the change in location of the voltmetric measurement point p, and the effect of the different resistance levels of the return path (e.g. for different soil types) are analyzed and experimentally verified. as pointed out in section 1 and in [7], there are many sources of variability in railto-earth conductance measurements using the method of iec 62128-2. some variabilities can be avoided by taking precautions when performing measurements, but others depend on contingency, site characteristics, the availability of connections, etc. 4.1. description of the measured sections measurements from four different sites are considered herein, named a through d. two sites were used to verify the influence of the resistance to earth of the ps negative terminal (sections a and b). the results for two other sites are reported (sections c and d) to support the assessment of variability of track-to-earth conductance for different positions of the volt-metric probe p. section lengths for short sections are rounded to the first decimal place, with  5 cm accuracy that is better than 1 %. when the length is longer than 100 m, the rounding is to 1 m, again with accuracy that is better than 1 %. the measured resistance values are expressed with 0.5 % precision compared to a combined uncertainty of the voltage and current measurements with accuracy that is better than 1 % (with coverage factor k=2). estimated values, characterized by a larger and inaccurately quantified uncertainty due to the many quantities and conditions involved, are always rounded to the nearest integer. the calculated conductance-to-earth values are expressed in s/km with a precision of three decimal places (about 0.1 %). the estimated variability of conductance-to-earth values has similar precision. the length of section a is 74.9 m, and the measured rail p.u.l. resistance is 43.15 mω/km. section a is characterized by a very short section of behind tracks connected to the negative terminal of the ps, and the resistance to ground of this section is estimated in 93 ω. as the resistance to ground of the ps negative terminal connected to the behind tracks is high, one copper electrode driven into the soil was used as alternative grounding, with a resistance to ground of about 60 ω (see section 4.2.1). the length of section b is 60.8 m, and the measured rail p.u.l. resistance is 42.27 mω/km. this section, unlike the other section a, has a very long track section behind it, with an estimated resistance to ground of lower than 5 ω (see section 4.2.1). for both sections a and b, a copper electrode was driven into the soil to be used as alternative grounding means for the ps and its resistance to earth was measured (see section 4.2.1). section c is a tunnel section of 1917 m. it is very well insulated and was measured using the stray current collector as a reference. the measurement current flows from the rails into the slab beneath them and then returns to the negative terminal of the ps using the preferential low resistance method represented by the stray current mesh and the stray current collector. in this case, the measurement current return path resistance has the order of magnitude specified by 0.07 ω/km, estimated based on the premise that the longitudinal stray current mesh and collector are electrically in parallel. section d is a cut-and-cover section of 454 m, measured using copper sulfate electrodes positioned on an almost dry concrete wall (surfaces at the electrode-concrete interface were figure 1. test setup for track conductance (iec 62128-2 annex a.3). figure 2. equivalent circuit of the measured track section. voltage source e is applied with a positive pole on the track and a negative pole on the “grounding electrode (ge)”/“behind section.” the potential v is measured at the volt-metric point p. acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 23 kept humid by pouring water on them). in this case, the return path resistance is higher, estimated in the range of 10-1000 /km, using table 7 of [11]. it worth stressing that this section was purposely chosen to emphasize the effect of different p locations and to demonstrate the effect of poor conductive soil or material used as return conductor. 4.2. measurements the site measurements of the rail-to-earth conductance were performed with the aims of demonstrating the variability due to the ps earthing resistance and of validating the numerical models used. the measurements of rail-to-earth conductance for sections a to d are considered. the preliminary steps were the measurement of the resistance to ground of the ps grounding electrode (ge) and the estimation of the resistance to earth of the track section behind. 4.2.1. influence of grounding electrode (ge) resistance the used ge is a copper cylindrical rod with a length of 1.50 m and a diameter of 2.54 cm. the ge resistance to ground was measured using an aemc/chauvin arnoux mod. 6471 “digital ground resistance and soil resistivity tester” [12]. the method used is the three-pole earth/ground measurement with the 62 % rule [13]. two auxiliary electrodes (eh and es) were placed in a straight line from the electrode being tested, complying with the 62 % rule. the electrode s, the closest to the measured electrode ge, was placed at a distance d=10.50 m, more than eight times the driven depth of the ge (1.10 m). the electrode eh was placed in a straight line with electrodes ge and es at a distance d-62%d. the measurement was repeated, rotating the electrodes eh and es in three different positions around the electrode ge for verifying the homogeneity of the surrounding soil and the absence of interference from external sources. the track-to-earth resistance of the track section behind the injection point was estimated using, as input values, the track length and an estimated conductance value of 0.092 s/km, in agreement with the values obtained from conductance measurements performed on other similar tracks and assuming the homogeneous behavior of the insulation. site a. the measured value of the ge earthing resistance is 59.0 ω, with an instrument error declared by aemc of ±2 %. the spread of the three measured soil resistivity values around the ge is ±1.5 % of the mean. the low variability indicates a good homogeneity of the soil as well as low instrument uncertainty. the track section behind the injection points in this site was very short, 117 m, leading to an estimated resistance to earth of 92.9 ω. site b. the measured value of the ge earthing resistance is 61.4 ω, with an intrinsic error declared by the manufacturer (aemc) of ±2 %. the spread among the three measured soil resistivity values is ±1.7 % of the mean. the low variability again indicates good soil homogeneity and low instrument uncertainty. the track section behind the injection point was very long in this site, more than 2 km. as the precise length could not be measured, a value of 5 ω is assumed for its resistance to ground. the results of the rail-to-ground conductance measurements for both sites a and b are shown in table 1. the volt-metric measurement point p is fixed at 60 m from the injection point, and the reference electrode (re) is placed 30 m away from the track axis. the column “δgmeas” represents the spread in % around the calculated mean. the values of the resistance to earth of the ps negative pole are shown in column “r0.” the measured conductance variability, depending on r0, is clearly visible for all measurements. the measured rail-to-ground conductance increases for larger r0 values. the observed spread around the mean value is about ±1-2 % with a peak value of ±8.0 % for the measurement a2, where, however, the re for the volt-metric measurement at point p was placed at the opposite side of the measured rail. in a different way, for all other measurements, the reference electrodes were placed always at the same side of the track of the measured rail. the presence of the other rail in the path between the measured rail and the reference electrode can disturb the electric field created by the measured rail, and the measurements are affected by this distortion. 4.2.2. track-to-ground conductance value vs. potential measurement point p measurements at sections c and d were performed, moving p along the track, with a ps earthing resistance of  3 ω. site c. the first p point was located at 63.0 m from the injection point and the other three locations were chosen at 177 m, 487 m, 1126 m from the injection point. the ps current return path resistance is calculated as 0.07 ω/km, as anticipated in section 4.1. the measured section track-to-earth conductance values were very low, giving 0.00132 1.5 % s/km. the variability is thus comparable with the instrumental uncertainty, and it is difficult to link it back to p moving between locations. the measurement is, however, used as a check for consistency in the simulation results because, for these values of conductance and return path resistance, the simulated variability should not be larger than the measured one. site d. site d is more interesting. the measurement was performed with a specific return path through concrete. p was moved in four 35 m steps from 135 to 310 m from the ps injection point. the measurement results, together with the error bars that represent the 5 % measurement variability, are shown in figure 3 for the five different p locations. the results take into account the single measurement point and the least-square fit of the data in order to compensate for the random errors in the measurements. the measurement was repeated later in the same section d using method a.2 of iec 62128-2 in order to confirm the order of magnitude of the results obtained using the first measurement. the track-to-earth conductance using method a.2 and similar environmental conditions was 0.206 s/km 5 %. 4.3. simulation as mentioned in the previous section, the numerical calculation performed in [7] shows that the variability of the measured conductance due to the test setup highly depends on table 1. rail-to-ground conductance measurement and variations against different resistance to ground of the power supply. site r0 in ω gmeas in s/km δgmeas gsim in s/km δgsim a1 92.9 0.380 ±1.6 % 0.374 < ±0.1 % 59.0 0.368 0.374 a2 92.9 1.299 ±8.0 % 1.204 ±0.1 % 59.0 1.107 1.204 b1 5 2.639 ±0.6 % 2.657 < ±0.1 % 61.4 2.672 2.628 b2 5 2.516 ±1.0 % 2.543 < ±0.1 % 61.4 2.569 2.545 acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 24 the physical characteristics of the measured system (e.g., real conductance, soil characteristics, and track length). new numerical calculations using an improved simulink model (which includes a soil model) were performed in order to obtain the results of theoretical variability to compare with the measurements for sites a through d. the input parameters of the model are therefore set according to the physical parameters of the system discussed so far. the results of the numerical calculations for the r0 effect are shown in table 1, where the column “δgsim” represents the variability of the simulated values in percentage, considering the two different values of r0 when using the track section behind and a vertical ge. the simulated variability of the resulting conductance is always lower than ±0.03 % around the mean of the two calculated results. the simulation results for site c, varying the location of the volt-metric point p, show a variability lower than ±0.0001 %, with a simulated conductance of 0.00132 s/km). the whole dataset is not reported here in the form of a figure or table since, due to the very low variability, it is considered insignificant. the simulation results for site d considering variability due to the volt-metric point p are shown in figure 4. as the value assigned to the concrete resistivity is an estimation based on typical values identified in table 7 of [11], but no measurements were performed to assess this point, a sensitivity analysis is reported in figure 4, considering 220 /km, 110 /km, 55 /km, 22.75 /km, and 13.75 /km, using as input track-toearth conductance value of 0.167 s/km. 4.4. comparison the measurement results reported in table 1show that for both sites a and b, the simulated variability of the track-to-earth conductance due to r0 highly underestimates the variability that occurs in real measurements. the underestimation is about two orders of magnitude, and for measurement a2, it increases to three orders of magnitude. when the first simulations with the lumped circuit simplified were done, it was thought that the extreme simplicity of the model, neglecting soil resistivity, discontinuities, field distortion due to conductors in the nearby area, etc., would have justified such difference. however, using the improved model that includes the resistivity of the current return path through structure and soil, the simulation results have not changed. the observed variability thus reflects that different positions of the negative terminal of the ps imply different current distributions in the soil, intercepting different layers at different depths with variable resistivity. the simulation results for site c, varying the location of the volt-metric point p, are comparable with the measured results. setting in the model a track-to-earth conductance value that is equal to the one measured, 0.00132 s/km, the simulated variability is lower than ±0.0001 % and is thus insignificant because the measurement variability of such measurement has the order of magnitude specified by ±1 % of the measured value (see section 4.2.2). the whole dataset is not reported here in the form of a figure or table, as it is considered insignificant. figure 4 shows that the simulation results, varying the voltmetric point p, are in accordance with the measurement results for site d, using an estimated resistivity of the measurement current return path of 55 /km. after having determined the simulated curve, which shows very good similarity between the simulation results and the measurement least-square interpolation, estimation of the real track-to-ground conductance is possible, considering the input value of the simulation, 0.167 s/km, as real. this conductance is comparable with that measured using the a.2 method, as detailed in section 4.2.2. 5. conclusion the variability in rail-to-earth conductance measurements due to different resistance to ground levels in the ps and the different locations of the volt-metric terminal used to apply the methods described in a.3 of iec 62218-2 [6] has been experimentally verified. a comparison with numerical simulations concerning the variability due to the resistance of the negative terminal of the ps has shown that, in real cases, the variability is larger than numerical calculations predict. it was anticipated that an improved model that includes the soil resistivity of the measurement current return path would have improved the figure 3. rail-to-earth conductance for site d by moving v terminal position: measured values (error bars 5 % of measured value) and least-square interpolation (line). figure 4. comparison of rail-to-earth conductance values of site d: measured (error bars 5 % of measured value), measured with least-square interpolation (black line), and predicted (curves for concrete resistivity values of 220 /km [blue], 110 /km [light blue], 55 /km [red], 27.5 /km [green], 13.75 /km [magenta]). acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 25 similarity between the measured and simulated data, but the expected improvement was not obtained. the observed variability thus reflects that the different positions of the negative terminal of the ps imply different current distributions in the soil, intercepting different layers at different depths, with variable resistivity. our advice is that, in case the earthing electrode for the negative terminal of the ps is used instead of the behind section of the track, two checks should be performed: the resistance to earth of the electrode should be much lower than the measured track-to-ground resistance, and the absence of highly resistive soil zones near the track should be ascertained. the variability in rail-to-earth conductance measurements due to the grounding resistance of the ps in real cases is about a few percent but increases up to about 10 % when the reference electrode for the volt-metric measurement is placed opposite the measured rail. this behavior indicates that the volt-metric reference electrode should be placed on the same side of the measured rail. in other cases, a variability of some percent of measured conductance results can, however, be considered acceptable for the type of measurement. variability that occurs due to different volt-metric point (p) locations along the measured track section is generally irrelevant, since it is lower than the variability that is due to random measurement errors. however, it was seen to have increased in the case of high resistance of the measurement current return path (through concrete instead of soil in the measured scenario). a correction factor based on the proposed model might be proposed to find the correct track-to-earth conductance value in the unideal case of low current return path resistance. future researchers could analyze the behavior of the simulated results (that are now validated) by changing the relevant parameters (the resistivity of the measured current return path, rail resistivity, or the track-to-earth conductance itself) in order to extrapolate some general rules about conditions in which variability due to test setup becomes relevant. 6. references [1] p. aylott, the application of modelling systems at the design stage to the mitigation of stray current interference, corrosion 2003, nace international. [2] a. ogunsola, a. mariscotti, l. sandrolini, estimation of stray current from a dc-electrified railway and impressed potential on a buried pipe, ieee transaction on power delivery 27(4) (2012) pp. 2238-2246. [3] t. barlo, a. zudnek, stray current corrosion in electrified rail systems, final report, northwest engineering publications, may 1995. [4] l. bertolini, m. carsana, p. pedeferri, corrosion behaviour of steel in concrete in the presence of stray current, corrosion science 49(3) (2007) pp. 1056-1068. [5] a. ogunsola, l. sandrolini, a. mariscotti, evaluation of stray current from a dc-electrified railway with integrated electricelectromechanical modeling and traffic simulation, ieee transactions on industry applications 51(6) (2015) pp. 54315441. [6] iec 62128-2, railway applications fixed installations electrical safety, earthing and the return circuit part 2: provisions against the effects of stray currents caused by d.c. traction systems, 2012. [7] j. bongiorno, a. mariscotti, accuracy of railway track conductance and joint efficiency measurement methods, acta imeko, 4(4) (2015) pp. 82-87. [8] c. charalambous, i. cotton, p. aylott, a simulation tool to predict the impact of soil topologies on coupling between a light rail system and buried third-party infrastructure, ieee transactions on vehicular technology 57(3) (2008) pp. 14041416. [9] j. bongiorno, a. mariscotti, variability of track-to-ground conductance measurement, proc. of 22nd imeko tc4 symposium, pp. 106-110, sept. 14-15, 2017, iasi, romania. [10] p. aylott, i. cotton, c. charalambous, impact and management of stray current on dc rail systems, iet seminar digest, 2011. [11] ieee std 80, ieee guide for safety in ac substation grounding, feb. 2013. [12] aemc/chauvin arnoux, digital ground resistance and soil resistivity tester mod. 6471 (user manual). [13] f. dawalibi, c. mukhledkar, ground electrode resistance measurements in non-uniform soils, power apparatus and systems (1973) pp. 109-115. experimental and numerical investigation of single point incremental forming of aluminium alloy foils acta imeko issn: 2221-870x march 2020, volume 9, number 1, 25 31 acta imeko | www.imeko.org march 2020 | volume 9 | number 1 | 25 experimental and numerical investigation of the single-point incremental forming of aluminium alloy foils imre paniti1, zsolt jános viharos1,2, dóra harangozó3, sherwan mohammed najm4 1 institute for computer science and control, h-1111 budapest, kende u. 13-17., hungary 2 john von neumann university, h-6000, izsáki str. 10, kecskemét, hungary 3 széchenyi istván university, h-9026 győr, egyetem tér 1., hungary 4 budapest university of technology and economics, h-1111 budapest, műegyetem rkp. 3., hungary section: research paper keywords: incremental sheet forming; crack monitoring; optical strain measurement; design of experiments citation: imre paniti, zsolt j. viharos, dóra harangozó, sherwan m. najm, experimental and numerical investigation of the single-point incremental forming of aluminium alloy foils, acta imeko, vol. 9, no. 1, article 5, march 2020, identifier: imeko-acta-9 (2020)-1-5 editor: lorenzo ciani, university of florence, italy received november 5, 2019; in final form february 5, 2020; published march 2020 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was supported by h2020 project epic under grant no. 739592, ed_18-2-2018-0006 and efop-3.6.2-16-2017-00016 project grants. corresponding author: imre paniti, e-mail: imre.paniti@sztaki.hu 1. introduction incremental sheet metal forming, with its main variants single-point incremental forming (spif) and two-point incremental forming (tpif), is an interesting research topic in material science due to the extreme and complex mode for deformation, the flexibility of the process, and the high forming limits compared to traditional forming processes. several articles have dealt with experimental study on force measurements for spif, like [1] or [2], but only a couple of them focus on sheets with an initial thickness of less than 0.5mm ([3]-[6]). furthermore, as gatea et al. [7] highlighted regarding the fracture forming limit curve (fflc) in a review, further investigation should be carried out concerning the effect of initial sheet thickness on the tool radius ratio (t0/r) on the fflc and whether it is enough to describe fflc in spif. the abovementioned factors and statements inspired this study to conduct experiments on almn1mg1 sheets with an initial thickness of 0.22 mm and to run some preliminary numerical simulations with the given process parameters. former results of this research with the same material explained what kind of control system has been used to execute the tool path on this part [11], and how flat-end tools can improve e.g. the accuracy of the part [12]. therefore, the aim of this research work is twofold. the primary goal is to apply a non-traditional force monitoring to almn1mg1 foils. the secondary goal is to experimentally validate the main process parameters on sheets thinner than 0.5 mm. the first part of this paper focuses on the material characterisation, introducing a forming limit curve (flc) measured by the nakazima test and some numerical simulation results. the second part of this article presents preliminary investigations on the formability of truncated conical shapes with a continuously increasing wall angle as a function of major operating parameters. in addition, forming forces have abstract single-point incremental forming (spif) is a flexible process for manufacturing sheet metal parts that is well adapted and profitable for prototypes or small batch production. compared to traditional sheet forming technologies, this relatively slow process can be used in different applications in the automotive and aircraft industries; architecture engineering; and medical aid manufacture. in this article, the indirectly obtained axial forming force on the spif of variable wall angle geometry is studied using different process parameters. the estimation of the forces on almn1mg1 sheets with an initial thickness of 0.22 mm is performed by continuous monitoring of servomotor currents. the deformation states of the formed parts were analysed using the argus optical strain measurement system of gom, while the roughness measurements were carried out by a mitutoyo system. some initial finite element analysis simulations and a crack monitoring method together with an interaction plot of forming speed, incremental depth, tool diameter, and lubrication were also reported. mailto:imre.paniti@sztaki.hu acta imeko | www.imeko.org march 2020 | volume 9 | number 1 | 26 been investigated experimentally with servomotor acquisitions, and a simplified crack monitoring system is also introduced. 2. description of the method spif experiments were carried out on a rieckhoff computer numerical control (cnc) milling machine. the forming tool and a fast-clamping system are shown in figure 1. the number of experiments required to determine the forming limit of a sheet can be reduced by using a part geometry with a variable wall angle as explained in [8]. for this reason, a conical frustum with circular generatrix (model generating curve) design was used, as shown in figure 2. the cnc machine control was realised with an open-source real-time control software called linuxcnc. this control allowed us to send the tool coordinates to a data acquisition program, which also collected the servomotor current data of the z axis. the chemical composition of almn1mg1 used for this study is given in table 1. the tensile tests were carried out according to en iso 68921:2010 standard at room temperature using an instron 5582 universal testing machine. specimens were cut from sheet in 0 °, 45 °, and 90 ° to a rolling direction. the planar anisotropy values (r) were evaluated from longitudinal and transversal strains measured by the advanced video extensometer (ave). the mechanical properties are listed in table 2. the result of the erichsen cupping test is ie = 6.79, and the limiting draw ratio obtained from the cup drawing test according to swift is ldr = 1.7. figure 3 illustrates the flc and fflc of the tested sheet, which were constructed from the results of the nakazima test using a hemispherical tool with a diameter of 50 mm and a gom aramis digital optical measuring system. the flc was evaluated according to the en_iso_12004-2-2009 standard. it is well known from the literature that with spif much larger strain values can be achieved compared with a conventional forming process like deep drawing. therefore, the forming limit in spif should be represented by fflc. the fracture limit strains of the tested sheet were also determined from the nakazima test using ε2-ε1 plots of the gom evaluation system. it should be noted that this fflc seems unrealistic as a limit of incremental sheet forming for many reasons. the main argument is that during the nakazima test, local stretching deformation causes a positive stress triaxiality factor, while with spif, the tool generates compressive stress in the sheet metal, which might influence the stress triaxiality in a negative direction and therefore higher strain limits can be achieved. this effect is stronger with double point incremental forming (dpif) where a compressive load caused by tools is more significant. to realise this enhanced process, two parallel kinematic machines (pkms) [13] or two industrial robots should be used [14]. in both cases, the synchronisation of the two tools have to be solved. however, the same results can be achieved with one industrial robot, a cframe, a linear actuator, and a mechanical copying device [15]. more realistic values for limit strains can be found in the existent literature. at plane strain (ε2 = 0), the limit strain (ε1 = fld0) reaches 2.3 for aa1050-o (filice et al., [16]), 0.84 for aa6114-t4, while 3.0 for aa3003-o (micari [17]). these values show that significant scatter is among the empirical values. applying the classic equation tf = t0∙sin(90 °ϕ) (where tf and t0 are final and initial thicknesses, and ϕ is the wall angle), it can figure 1. the setup of the experiments. figure 2. section view of the test geometry. table 1. chemical composition of the sheet material. al si fe cu mn 96.90 0.201 0.448 0.212 0.807 mg zn cr ni others 1.260 0.071 0.022 0.006 0.073 table 2. results of the tensile tests. direction rp0.2 in mpa rm in mpa ag, % 0o 88.3 183.0 16.44 45o 90.0 155.5 9.27 90o 86.3 170.3 12.48 a50, % n5 r10 16.88 0.297 0.554 10.45 0.266 0.580 12.98 0.268 0.594 figure 3. forming and fflc. acta imeko | www.imeko.org march 2020 | volume 9 | number 1 | 27 be calculated that if ϕ = 60 °, then the logarithmic thickness reduction strain (fld0) is 0.9, and if ϕ= 70 °, then fld0 = 1.08. figure 4 shows that the lower alloy content of aa3003 enables higher wall angle limits than aa5754, with a 3.5 % mg content. the initial wall thickness influences the limit strain; for example, kim et al. [18] showed that if thickness decreases from 0.5 mm to 0.3 mm, the limit strain also decreases by 23 % to fld0 = 0.92. jeswiet et al. [19] elaborated empirical formulae for the calculation of a maximum wall angle for truncated conical specimens. equations 1(a) and (b) show the influence of the sheet thickness on the wall angle as a function of material. ϕmax = 8.5 t0 + 60.7 for aa3003-o, (1)(a) ϕmax = 3.3 t0+ 58.3 for aa5754-o (1)(b) at the same time, it is also visible that as the sheet thickness decreases, the formability also decreases. the empirical evaluation of local deformations was measured by the gom argus system [20]. this technique uses a regular mesh on the surface of blank material. after forming process, the local deformations are calculated using argus software. the results from the argus system provide full-field information about the major-minor strain, the thickness reduction, and the geometric parameters of the sheet metal part. 3. numerical simulations the effects of process factors considered as input parameters, such as the tool diameter, incremental depth, and forming speed, were investigated using finite element method (fem) approaches. in [31], gál and lukács gave an overview of the parameters of the high-strength aluminium cold-forming simulations. great advances in fem have been related in the literature in terms of predicting the influence of parameters and variables involved in spif for the evolution of the process. examples of explicit [21]-[23] and implicit [24] solutions can also be found in publications. in [23], we also find a comparison of three yield functions. for analysing the process by fem, it is necessary to know the plastic behaviour of the material when it is subjected to severe deformations. the determination of the true stress – the true strain curve, which describes the strain hardening of materials (even in the post-necking period) – can be realised experimentally e.g. by the watts-fords test or by the extrapolation of the true stress and true strain data of the linear period recorded from the tensile tests. usually, typical functions like swift, voce, hollomon, ludwik, hockett-sherby, or the proper combination thereof are suitable for the approximation of the post-necking period of the yield curve. other, similar functions are also used, such as johnson-cook’s, but some terms that consider the test velocity and temperature are also introduced. specific methods for determining the material behaviour model can also be taken into consideration, such as photographic techniques (digital image correlation [dic]) or reverse engineering methodologies [25]. in [26], a comprehensive methodology with two developed analytical models for a multi-stage deformation pass design was proposed. the models were compared with finite element analysis (fea) and experimental tests concerning the evaluation of the process formability of the final part and thickness strain distributions. publication [28] presented the process of the fea of producing the truncated pyramid parts of aa1100 sheets with a thickness of 1 mm by spif. the tools of the experiments are similar to those used by us. the article gives an explicit solution to the simulation of spif by ansys (a finite element analysis software). the authors prepared a geometric model of four parts: the forming tool, the blank, the blank holder, and fixture by using different solid and elastic elements offered by the software. a permanent coulomb friction of 0.04 was used in the simulations, and the researchers analysed the effects of altering the tool diameter and the incremental depth on the final thickness of the part, von mises stresses and strains. it was revealed that the maximum stresses occur at the contact region between the forming tool and sheet during deformation, and the increase in the forming tool diameter results in a decrease in the von mises stresses and an increase in the formed thickness of the workpiece. they found that small stepover values are preferred for increasing the formability and for reducing the forminginduced stresses. the prediction of the occurrence of fracture is a significant impediment in the adoption of incremental sheet forming (isf) technologies in industry. well-developed approaches that study the mechanics of isf technologies can be found, such as those in [29] and [30]. emmens and van den boogaard [32] studied the aspects of incremental forming (the effects of speed and bending angle) in continuous bending under tension (cbt) tests (see figure 5) and claimed that material thickness had only a minor effect. however, foils (like in our specific case) have not been examined. figure 4. wall angle vs. sheet thickness. figure 5. cbt setup with three rolls. acta imeko | www.imeko.org march 2020 | volume 9 | number 1 | 28 this is one of the reasons why we considered to model the experimental conditions (up-down speed: 66.7 mm/s, up-down stroke: max. 140 mm, pulling speed 0.15 mm/s, pulling distance max. 160 mm, roll diameter: d = 2.381 mm, roll distance: l = 5.56 mm, depth setting: p = 0.094 mm) in deform. the same specimen geometry was used by modelling the quarter of the effective part and applying a simple clamping of the sheet. after five hours of calculation, a similar strain distribution appeared (see figure 6) as in [32]. this confirms the assumption that the same phenomena can be observed with thinner sheets too. the forming of an aa6061 sheet with a thickness of 1 mm to a conical shape using spif is presented in [27]. this type of aluminium alloy is primarily an al-mg-si-based precipitationhardened alloy commonly used in aerospace and automotive industry due to its high strength. the maximum formability achieved in aa6061 aluminium alloy was found in the range of 12 % to 34 % with traditional forming processes. to further enhance the formability, the authors performed spif experiments; used the digital image correlation (dic) technique to measure the major and minor strains; and compared the results with fem results. the contact area (between blank and tool) in this process is very small and localised, which requires a very fine mesh size. by lowering the mesh size, the variation in strain decreased, and smooth surfaces were observed; however, the simulation could not be completed successfully due to limitations in computational capabilities. therefore, a relatively coarse mesh size of 1 mm was used to complete the spif simulation successfully. using traditional solid elements in the pre-calculations (see figure 7), a simulation time of several weeks has been predicted for us to form a single part, so we decided to stick to experiments with real-time monitoring enhancements. 4. results of the experiments and discussion table 3. shows the process parameters (f: forming speed, z: incremental depth, d: tool diameter, and lubricant number) applied in the design of experiments (doe, using an l9 orthogonal array of taguchi) and the examined output parameter. the z coordinate is where fraction occurred on the formed part (-zfrac.). lubricant number 1 corresponds to a lubricant with sommerfeld number s = 17.6, number 2 corresponds to s = 33.4, and number 3 corresponds to s = 58.4. z axis loads were obtained to monitor the necking and fracture, as in [9]. figure 8 shows the thickness reduction of the first formed part (from the gom argus system), while figure 9 shows the result of the forming, also indicating the fracture caused by necking. by using the data of the motor and the drive train, the force applied by the axle as a function of the motor current is the following equation: 𝐹𝑧−current = 2𝜋 ℎ ⋅ 𝑖𝑠 ⋅ (𝑘𝑀 ⋅ 𝐼 − 𝑀𝑅 ) (2) where: • fz-current – axial force applied by the ball screw nut in n, • h – ball screw pitch in mm, • is – transmission ratio of the belt drive • km – motor constant in mn·m/a • i – motor current a • mr – torque loss due to friction in the motor in mn·m. table 3. process parameters and results. exec. order f in mm/min z in mm d in mm lubricant -zfrac. in mm 1. 500 0.1 2.381 1 22.32 5. 500 0.3 4 2 20.16 9. 500 0.5 6 3 19.60 4. 1750 0.1 4 3 19.81 8. 1750 0.3 6 1 19.93 3. 1750 0.5 2.381 2 20.01 7. 3000 0.1 6 2 18.95 2. 3000 0.3 2.381 3 20.01 6. 3000 0.5 4 1 20.10 figure 6. strain distribution on the effective part of the specimen model. figure 7. simulation of the first forming contour. figure 8. thickness reduction of the first part. acta imeko | www.imeko.org march 2020 | volume 9 | number 1 | 29 similar methodology was used by rauch et al. [10] to evaluate tool loads in a parallel kinematic machine. figure 10 shows the validation of the measurement concept, by comparing measurement results from a jr3 force cell (fz) with the calculated forces (fz-current) from the motor current measurements. the reaction force (and the current) increases in the first phase of the forming as the sheet becomes harder to form. figure 11 shows the results of the first forming, indicating the values of the fracture (the oval mark). a simplified monitoring solution for the crack detection could be the implementation of some light sensors under the sheet (see figure 12). current measurements were realised with a 0.33 ohm electrical measurement resistance. from ohm’s law, the voltages on the cnc’s z axis can be obtained. the peaks in the fz-current indicate the z-level changes, where the tool pushes the sheet to reach the next depth level up to the fracture. the fz values are pre-filtered in quasi-real time to get a smoother value change. in case local necking or fracture occurs, the voltage increases around the starting value. in a simplified case, the supporting rig of the setup would be a dark chamber where the cover is the clamped sheet. of course, a proper illumination from the forming tool side and an emergency/cycle stop circuit with a trigger signal from the light sensors would be necessary to stop the machine and avoid wasting machining time. direct and indirect force monitoring systems are already known, but this new method would be a cost-effective solution with an easier system integration. figure 9. picture of the first formed part, indicating the fracture. figure 10. validation of the measurement concept. figure 11. measured voltage on the z axis with fracture indication [11]. figure 12. flowchart of a light sensor-based monitoring system with a cycle stop signal. figure 13. major strain distribution of the first part in a section. acta imeko | www.imeko.org march 2020 | volume 9 | number 1 | 30 the major strain distribution in a dedicated section of the same part can be seen in figure 13. the major strain increased up to 126 % (1.26) in the area of the fracture. similar phenomena occur with thicker sheets. this value is similar to that cited from the literature. as it shown in table 3, the z coordinate, where the fracture occurred on the formed part (-zfrac.), is about 20 mm. using the part dimensions from figure 2, the final wall angle can be calculated, giving a value of 78.46 °. this is higher than the published wall angle values of regular truncated conic shape, which is used as reference geometry for evaluating the limit strains. roughness measurements (see table 4) were also carried out on the inner wall of the formed parts to compare the results of the different forming tests. it shows that the best surface roughness can be achieved with the fastest tool movement by using the smallest incremental depth and the biggest forming tool. however, according to the previous table, this combination gives the worst result regarding the forming depth (formability). to summarise the results of the experiments, an interaction plot of the factors for -zfrac. is given in figure 14. experimental results showed that the tool diameter has the primary influence on the forming depth in the case of spif on almn1mg1 sheets with an initial thickness of 0.22 mm, which reflects the importance of the t0/r (initial sheet thickness-to-tool radius) ratio. the second factor in the line of the influencing parameters is the lubrication (with respect to the sommerfeld number), which is followed by the feed rate and the incremental depth. further analysis can be carried out by correlation matrix of parameters, which is displayed on table 5. therefore, it follows that the highest correlation index is between the tool diameter and forming depth, and the negative sign indicates that the lower the diameter, the higher the depth. this finding supports the hypothesis that compressing stresses play a key role in the increased formability of spif, as a smaller diameter induces compressive stresses more effectively on the surface of the sheet. second highest indices can be regarded to feed rate and lubrication, but the effect of z incremental depth is less significant. this ranking is in good agreement with the conclusions derived from figure 14. in case we analyse the tool diameter and feed rate by multiple regression, then equation (3) gives a simple form to calculate the depth where fracture occurs: −𝑍frac. = 22.8 − 0.42 ⋅ 𝑑 − 0.0005 ⋅ 𝐹 . (3) the coefficient of regression is 0.826, which is acceptable for the further estimation of forming depth if similar geometry and the same sheet metal are used. 5. conclusions in this paper, the characterisation of almn1mg1 and spif of the same material with an initial thickness of 0.22 mm have been conducted, applied by a doe, using a l9 orthogonal array of taguchi. initial numerical simulations gave an unacceptable computing time in the case of spif, but the cbt simulations (as an approximate model for incremental sheet forming) gave similar results for the examined foil, as in other documented cases, using thicker sheets. the monitoring of servo-motor currents allowed the estimation of the forming forces, and a new crack monitoring method based on light sensors was also given. all results regarding the estimation of fractures caused by necking are consonant with the results obtained in the spif of thicker sheets. acknowledgement this research has been supported by the ed_18-2-2018-0006 grant on ‘research on prime exploitation of the potential provided by the industrial digitalisation’. the research in this paper was (partially) supported by the european commission through the h2020 project epic (https://www.centre-epic.eu/) under grant no. 739592. the research presented in this paper was also carried out as part of the efop-3.6.2-16-2017-00016 project in the framework of the new széchenyi plan. the completion of this project is table 4. results of the roughness measurement. 1 st measurement in µm 2nd measurement in µm average in µm test no. ra rz ra rz ra rz 1 0.95 4.10 0.73 3.80 0.84 3.95 2 1.58 9.40 1.47 7.00 1.53 8.20 3 2.55 9.70 2.58 9.50 2.57 9.60 4 0.46 2.70 0.47 2.70 0.46 2.70 5 1.10 4.60 0.98 4.60 1.04 4.60 6 1.65 6.70 1.53 6.00 1.59 6.35 7 0.28 1.80 0.31 1.80 0.30 1.80 8 0.45 2.80 0.50 2.90 0.48 2.85 9 1.50 8.10 1.22 5.00 1.36 6.55 table 5. correlation matrix of the parameters. f δz d lubr. -zfrac. f 1 δz 0.130 1 d 0.126 0.126 1 lubr. -0.130 -0.130 -0.126 1 -zfrac. -0.574 -0.385 -0.661 0.564 1 figure 14. interaction of the factors for –zfraction https://www.centre-epic.eu/ acta imeko | www.imeko.org march 2020 | volume 9 | number 1 | 31 funded by the european union and co-financed by the european social fund. the support provided by viktor gál, t. győgér, and sz. szalai in the simulations/experiments is also greatly acknowledged. references [1] j. duflou, y. tunçkol, a. szekeres, p. vanherck, experimental study on force measurements for single point incremental forming, journal of materials processing technology 189(1-3) (2007) pp. 65-72. 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[32] w. c. emmens, a. h. van den boogaard, incremental forming by continuous bending under tension – an experimental investigation, journal of materials processing technology 209(14) (2009) pp. 5456-5463. https://www.gom.com/metrology-systems/argus.html low-cost implementation and characterisation of an active phasor data concentrator acta imeko issn: 2221-870x june 2019, volume 8, number 2, 21 27 acta imeko | www.imeko.org june 2019 | volume 8 | number 2 | 21 low-cost implementation and characterisation of an active phasor data concentrator paolo castello1, carlo muscas1, paolo attilio pegoraro1, sara sulis1 1 department of electrical and electronic engineering, university of cagliari, cagliari, italy section: research paper keywords: phasor data concentrator (pdc); ieee c37.118.2; synchrophasor technology; single-board computer; raspberry pi, smart grid citation: paolo castello, carlo muscas, paolo attilio pegoraro, sara sulis, low-cost implementation and characterisation of an active phasor data concentrator, acta imeko, vol. 8, no. 2, article 4, june 2019, identifier: imeko-acta-08 (2019)-02-04 section editor: alessandro depari, university of brescia, italy received july 14, 2018; in final form june 17, 2019; published june 2019 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: paolo castello, e-mail: paolo.castello@diee.unica.it 1. introduction the main components of the synchronised distributed measurement systems used in power network monitoring systems that are based on synchrophasor technology are phasor measurement units (pmus) and phasor data concentrators (pdcs) [1]-[4]. the pmu is the ‘sensor’ of this measurement system and gives several measurements: synchronised phasor magnitude and phase angle; frequency; and rate of change of frequency (rocof), evaluated based on voltage and current signals [1]-[3]. all the measurements are time tagged using an absolute time reference in coordinated universal time (utc) format and usually provided by the global positioning system (gps). measurements are encapsulated in data packets that are compliant with standard [2] and forwarded to the pdc. the data coming from pmus in the field is thus collected by the pdc, aligned with the same timestamp and typically forwarded in suitable streams to the upper levels of the architecture (either a higher level pdc or the control centre). different target applications, running either online or offline, can be implemented based on such data. synchrophasor technology was originally designed for widearea monitoring systems (wamss) used in transmission systems [5]. currently, commercial pdcs suitable for wamss allow for the management of hundreds of incoming streams, and their cost is in the order of thousands of dollars. these devices can implement mathematical functions (power calculations and evaluations of sequence components) and other utility functions, such as alarms, that are specific for electric substations [6]. however, such equipment is not usually programmable by users for specific functions. nowadays, research is pushing to extend the benefits of synchronised measurements to distribution networks. due to the different scales and economic constraints of the two kinds of systems, ongoing industrial and academic research projects have recently been working on the exploitation of low-cost architectures for the evaluation of electrical quantities in pmulike devices [7]-[13] and in power quality metres [14], [15]. a pmu project based on low-cost hardware can be developed using the most common single-board computer (sbc) platforms. thanks to the low cost, good performance, and availability of the various hardware equipment that can be integrated into the boards (as wireless and wired communication), these devices are abstract the main components of an advanced measurement system based on synchrophasor technology for the monitoring of power systems are the phasor measurement unit (pmu), which represents the ‘sensor’, and the phasor data concentrator (pdc), which collects the data forwarded by pmus installed on the field. for the purpose of extending the benefit of synchrophasor technology from transmission grids to distribution networks, different projects are seeking to use low-cost platforms to design devices with pmu functionalities. in this perspective, in order to achieve a complete synchrophasor-based measurement architecture based on low-cost technologies, this work presents a pdc design based on a low-cost platform. despite the simplicity of the considered hardware, advanced pdc functionalities and innovative control logics are implemented in the prototype. the proposed device is characterised by several experimental tests aimed at assessing its performance in terms of both time synchronisation and capability of managing several pmu data streams. the feasibility of some additional functionalities and control logics is evaluated in the context of different possible scenarios. mailto:paolo.castello@diee.unica.it acta imeko | www.imeko.org june 2019 | volume 8 | number 2 | 22 pushing the development of prototypes that are specifically designed for different applications in iot architectures [16]. considering that the pdc is the first element of the synchrophasor system with a global view of an entire portion of the electric system and thanks to measurement data received from different locations in the field [17], the pdc could be engineered as a new measurement device that is able to make decisions that rely on the information received from different sources. in [18], an active low-cost pdc that is suitable for distribution networks and is based on the most common sbc, the raspberry pi [19], is presented and characterised in terms of synchronisation accuracy (provided by the global navigation satellite system [gnss] receiver) and computational burden (in terms of the number of manageable data streams). the pdc allows for the implementation of advanced functionalities and, in particular, time-related measurements, which enable advanced stream management as in [17]. in this paper, the design of the sbc-based pdc is expanded upon in order to implement a control logic relying on measurement data processing and digital output. a synchronised calibrator is used to test a complete measurement and control chain in a real scenario, in which the pdc relies on a simple frequency change detection to trigger its digital output. an overall delay characterisation is performed to show the relationship between measurement device configuration, input data latency, and processing time. 2. a pdc with control capability in a typical synchrophasor monitoring system, a commercial pdc is generally implemented as a physical device that is specifically designed to operate in an electrical substation. however, pdc functionality can also be implemented as a software tool [20] that is generally installed at the top of the synchrophasor hierarchy in the control centre of the grid operators. as well as the functionalities of aligning and forwarding the data streams with the same timestamp, the pdc can implement additional functionalities. in the guide for pdc requirements [4], the ability to implement control logic is not specified as a functional requirement; rather, it is recommended as an additional optional function. in this context, the pdc uses the data provided by the pmu in real time and can become a crucial element not only for wams applications that are based on the synchrophasor measurements, but also for the wide-area monitoring protection and control system (wampac). in particular, the pdc can monitor the behaviour of the underlying measurement system, evaluating and tracking performance indices as, for example, the mean latency of each input stream. it can include specific outputs, similar to digital interfaces operating in a supervisory control and data acquisition system or an energy management system (ems) [4]. the pdc can thus support advanced monitoring applications, and it might implement the logic for event detection as part of protection and control. in [21], the architecture of a pdc that plays an important role in a wampac vision is described. in [22], a synchrophasor system with a single level of pdcs, which can manage variable input reporting rates, is introduced. in [23], a pdc specific for the real-time state estimation application is presented. 3. the proposed architecture the proposed architecture is based on two low-cost hardware devices and on a software application designed in the labview environment. the cost of the overall hardware is less than € 80. figure 1 shows the main blocks of the proposed active pdc prototype based on a sbc raspberry pi 3b and uputronics synchronisation board. 3.1. hardware the active pdc prototype is developed using the low-cost sbc raspberry pi (figure 2), produced by the raspberry pi foundation [19]. the model 3b has been used here, equipped with a system on chip (soc) bcm2837 with a cpu quad-core arm up to 1.2 ghz and 1 gb of ram. the sbc is also equipped with 10/100 ethernet and 2.4 ghz wi-fi 802.11n modules, which make the device able to receive the streams provided by the pmus. a 40 pins general purpose input/output (gpio) is used to communicate with the peripherals and to generate the digital signal shown in the subsequent section. the device does not comprise a real-time clock, but, for the implementation of the active functionalities defined in [17], an accurate time reference is necessary to correctly evaluate the latency of the data packets received from each connected pmu. to overcome this lack, the prototype is equipped with a uputronics gps expansion board that includes a gnss receiver. the gps receiver has been chosen for timekeeping due to its availability and low price [24]. the u-blox max-m8q module can provide the absolute time reference pulse per second signal (pps) with an accuracy of ± 60 ns. generally, when the pdc cannot rely on the timestamps provided by a pmu because of the lack of synchronisation (the sync bit set in the data frame header), the pdc discards all the data frames for that pmu [25]. instead, during a loss of the gps signal feeding the active pdc, the prototype can still operate as a data concentrator using the timestamps inside the received data packets to the alignment and forwarding process. during this interval, the latency evaluation of the input data stream is suspended until the restoration of a good gps signal. figure 1. proposed active pdc prototype based on sbc raspberry pi 3b and uputronics synchronisation board. figure 2. hardware details of the proposed active pdc prototype (sbc raspberry pi 3b and uputronics synchronisation board). acta imeko | www.imeko.org june 2019 | volume 8 | number 2 | 23 3.2. software the operating system used in the sbc is the linux raspbian, based on debian. the software of the proposed active pdc is developed in the labview environment in an external pc before being installed in the low-cost board. communication between the development pc and the labview project running in the prototype is managed by the hardware abstraction layer provided by the linx firmware installed in the sbc. the virtual instrument (vi) running in the sbc exploits the linx firmware that provides the necessary logic to access the sbc and gpio interfaces [26]. 4. characterisation of the pdc prototype to validate the feasibility of the proposed approach, the performance of the prototype is firstly characterised by means of a suitable controlled test setup and two different test sections. the first one concerns the capability to evaluate the latency of the incoming data streams and the associated accuracy level. the second testing stage is focused on the performance of the prototype while dealing with multiple incoming data streams. in this case, the cpu utilisation level of the sbc is used as a performance index 4.1. tests on time synchronisation a good level of time synchronisation is a prerequisite to evaluate the latency of every incoming stream. the synchronisation is provided by the gps receiver to give the network time protocol (ntp)-pps discipline to the entire system. the basic version of ntp daemon supplied on the raspberry pi does not support the pps signals in the standard version, and it is thus necessary to recompile the module. the operating system is synchronised with the utc by means of the ntp daemon (running locally on a loopback port) synchronised with the pps signals from the gps receiver. in this way, accurate time information can be obtained during the time characterisation test. the proposed pdc, provided with latency evaluation functionality, reads the time-tag included in every data packet (in the soc and fracsec fields of the ieee c37.118.2 data frame header) of the incoming pmu streams and evaluates the delay between it and the arrival time of the data packet. this approach requires that the arrival time is measured with a sufficient accuracy, and a suitable source of synchronisation is therefore needed for the pdc time base [17]. figure 3 shows the test setup for evaluating the accuracy of the time synchronisation of the prototype. the test setup is composed of a gps receiver symmetricom xl-750, able to generate a synchronisation time signal of up to 50 pps with an accuracy of ± 100 ns. the pdc prototype is programmed to generate a digital signal every second using the gpio of the sbc. this digital signal is generated to verify the pace keeping of the occurrence of the utc second. the gps receiver is a reliable solution when the time coordination is difficult and when the network topology does not provide a continuous, reliable, and stable link that is either wired or wireless [27]. the two synchronisation signals (from the gps receiver and from the prototype) are acquired, and the time offset is evaluated using the counter functionality of the national instruments daq board usb-6211, which is connected to a pc desktop via usb. figure 4 shows the time offset measured over five hours, taking into account the 18,000 measurements corresponding to second occurrences. the distribution of the time offset is shown in figure 5: the mean value is 164.4 µs, and the standard deviation is 14.1 µs. the maximum offset obtained in the test is 297.4 µs, and the jitter, defined as the difference between maximum and minimum time offset, is 170.7 µs. in this test, the time offset is directly obtained by comparing the pps signals, and no correction has been added. nevertheless, it is clear from the results that a compensation of the average time offset would be effective in reducing the synchronisation error. this can be done by adjusting the generation trigger in the labview application. in any case, the synchronisation level obtained by the prototype is fully adequate for evaluating the latency of a pmu data stream, as reported in [3], where a 2 ms time accuracy is required. 4.2. tests on cpu utilisation figure 6 represents the test setup for evaluating the performance of the pdc prototype in terms of cpu utilisation with ten incoming data streams. the data of the different streams is individually collected and evaluated in terms of latency. during this test, no other operations run in the pdc prototype. the pmus simulator, which relies on a pmu data stream simulator software developed in labview, can generate data streams that figure 3. experimental setup designed to evaluate the synchronisation performance of the active pdc prototype. figure 4. time offset between the active pdc prototype and gps receiver. figure 5. histogram of the time offset between the pps generated by the active pdc prototype and that of the gps receiver (5 h measurements). acta imeko | www.imeko.org june 2019 | volume 8 | number 2 | 24 are compliant with [2], with different reporting rates. the tcp communication is used between the pmus simulator and the pdc prototype. the pmus simulator runs in a pc desktop that is synchronised by the local-area ntp every 15 minutes, and its clock is also used to countercheck for possible anomalous behaviour during tests. table 1 reports the results in terms of the average cpu utilisation (at five-minute observation intervals) and the data rate. ten streams and different reporting rates up to 50 fps are considered. it is clear that the prototype can easily manage ten streams up to the maximum reporting rate prescribed by [1], which is 50 fps for a system operating at the nominal frequency of 50 hz. 5. prototype behaviour in a real scenario in the second step of the validation procedure, the pdc prototype is included in real synchrophasor architectures based on two commercial pmus that are compliant with the last version of synchrophasor standard [3]. another pdc is also included with the aim of evaluating the overall latency of the synchrophasor monitoring system while varying the architecture and considering the latency contributions for each component. furthermore, a protection logic based on the monitored quantities provided by a pmu without a digital interface is developed and implemented in the prototype of the active pdc. the last part of this section shows the results relevant to this logic. 5.1. synchrophasor test scenarios in the test scenarios, two network topologies are considered. in the first one, the commercial pmus are connected to the pdc prototype by means of a commercial ethernet switch, using the ieee c37.118.2-2011 communication protocol encapsulated in a tpc transport layer (figure 7). in the second test scenario, a computer with pdc functionalities is connected between the pmus and the pdc prototype using two different ethernet 10/100 interfaces in order to emulate a more complex synchrophasor architecture (figure 8). in the first test scenario, the pdc prototype is configured to collect and evaluate the pmu latency for two incoming streams of measurement data provided by two commercial pmus (figure 7). each element of this architecture is synchronised with the utc so that the pdc prototype can evaluate the synchrophasor data latency. including both pmu and network latencies. the limits of pmu latency (the so-called pmu reporting latency) given in [3] are reported in 0the latency value is associated with the measurement process of the devices and strictly depends on two main pmu characteristics: the performance class and the reporting rate. the pmus are directly connected to the pdc by means of a 10/100 commercial ethernet switch in a laboratory test setup without other types of network traffic. the protocol used for the characterisation is the ieee c37.118.2-2011, which is specifically used for synchrophasor communication in power systems. the synchrophasor data latency reported in the following section is evaluated as the time difference between the arrival time of each data packet measured by the pdc (thanks to the time synchronisation functionality developed in the prototype) and the time tag included in the corresponding frame sent by the pmu. the results of the synchrophasor data latency evaluation over 1,000 tcp messages are shown in table 3 and table 4 for reporting rates (𝐹𝑠) of 10 fps and 50 fps (i.e. the minimum and maximum mandatory values prescribed in [1]) respectively. the figure 6. experimental setup designed to evaluate the cpu utilisation of the active pdc prototype, with ten streams from the simulated pmus. figure 7. experimental setup designed to evaluate synchrophasor data latency with the active pdc prototype. table 1. values of average cpu utilisation and data rates with ten incoming streams of pmu data and different reporting rates. reporting rate (fs) in fps data rate in kb/s cpu utilisation in % 1 7 6 10 70 9 25 175 15 50 351 23 table 2. maximum pmus reporting latency in [3]: fs is the reporting rate expressed in frames per second (fps) performance class maximum pmu reporting latency in s p 2/fs m 7/fs table 3. evaluation of the synchrophasor data latency (fs = 10 fps). pmu max in ms mean in ms st. dev. in ms limit of prl in ms pmu a p 66.7 66.2 0.2 200 m 583.4 582.7 0.2 700 pmu b p 73.8 73.2 0.1 200 m 718.2 713.4 0.2 700 table 4. evaluation of the synchrophasor data latency (fs = 50 fps). pmu max in ms mean in ms st. dev. in ms limit of prl in ms pmu a p 71.0 70.5 0.1 40 m 134.9 134.8 0.1 140 pmu b p 73.4 73.0 0.1 40 m 168.0 167.0 0.1 140 acta imeko | www.imeko.org june 2019 | volume 8 | number 2 | 25 tables report the results for both performance classes in terms of the maximum and mean values and standard deviations. the tables also report the pmu reporting latency (prl) limits for the different considered cases. it is important to highlight that the prl does not consider the network latency introduced by the network devices between the pmus and the pdc prototype, and it gives only an indication of the latency associated with the first step of measurement architecture. it is worth recalling that the p-class is specific to applications that require low latency, and its accuracy requirements do not significantly change for the used reporting rates. for this reason, in the implementation, the same algorithm is adopted by manufacturers, and this situation leads to two very similar latency results, as indicated by the experiment results. on the contrary, the requirements for the m-class are different for each reporting rate, and the latency values largely increase with lower reporting rates, as shown in table 3 and table 4. the second test scenario is based on a synchrophasor architecture comprising two pmus and an intermediate pdc layer (figure 8). the commercial pmus send their outputs to a workstation, which is equipped with an i7 quad-core intel processor, 8 gb ram, windows 7 operating system, and openpdc software [20]. openpdc implements the pdc functionality of collecting, aligning, and forwarding, in a single output stream, the received input data to the low-cost pdc, following the hierarchical architecture suggested in [2]. the aim of the test is to evaluate the capability of the active pdc prototype to receive data not only from commercial pmus but also from other devices of the synchrophasor architecture, in order to verify the feasibility of the proposal. the workstation is equipped with two ethernet interfaces: the first one is connected to a switch in order to receive the incoming pmu streams, while the second one is directly connected by means of an ethernet cable to the pdc prototype. since the pmus are connected to the unsynchronised openpdc system, the pdc prototype is responsible for evaluating the overall latency of the input stream, which practically corresponds to the output latency of the intermediate pdc. comparing these new results with those of the previous test case, it is possible to evaluate the latency generated by the intermediate pdc. table 5 reports the latency measurements over 1,000 tcp messages when either the por m-class is considered for both pmus and with reporting rates of 10 fps and 50 fps. the maximum latency is similar for the two performance classes. this may be attributed to the openpdc software, whose default configuration includes a one-second waiting time (lag time) before transmitting the aligned data. any data arriving after that interval has elapsed is considered as late and is discarded. it is clear from the results that pdc configurations can play a crucial role in overall system performance. the above openpdc configuration is useful for sending data within a fixed time frame, but it strongly affects the propagation through the system layers. if measurements are used for fast response applications, then this configuration may render the benefits of the p-class pmus useless in terms of speed. the proposed synchronised pdc proves a valuable tool for evaluating the operating conditions of complex architectures in real time. 5.2. latency evaluation in a protection application scenario this test scenario (figure 9) aims to verify the performance of an application implemented in the pdc prototype and thus to test the feasibility of the pdc as an intelligent device that can send commands or protection signals. as mentioned above, the pdc prototype can evaluate the measurements contained in the data packets provided by the pmus to implement different logics based on the input data and thus on the actual state of the electric grid. the gpio interface is used in this test to send a digital control signal in reaction to an input variation. the omicron cmc 256plus synchronised calibrator [28], used specifically for test protection and measurement devices of class 0.2, is utilised to estimate the time difference between the occurrence of an electrical event and the activation of a digital output signal provided by the pdc. in this context, the calibrator generates a synchronised three-phase voltage signal used as input table 5. evaluation of the latency with an intermediate pdc. fs in fps input streams type max in ms mean in ms st. dev. in ms 10 p 1015.4 1012.6 0.4 m 1001.3 998.8 0.4 50 p 1013.8 1009.5 0.4 m 1024.2 999.7 0.4 figure 8. experimental setup designed to evaluate the pdc reporting latency of the wams. figure 9. experimental setup designed for the protection application scenarios. figure 10. frequency signal test and digital signal output. acta imeko | www.imeko.org june 2019 | volume 8 | number 2 | 26 to a commercial pmu. it is also able to receive binary input signals, with a time resolution of 100 µs and a time stamp accuracy of ±0.00015 % of reading ± 70 µs [28]. the test voltage magnitudes (controlled by using the omicron power quality signal generator) are 50 vrms, and the frequency evolution is represented in figure 10. from t = 0 s to t = 10 s, the test signal is characterised by a nominal 50 hz system frequency, while a -2 hz frequency step change occurs at t = 10 s. the commercial pmu follows the variation of the frequency and sends the measured frequency to the pdc in data packets at regular intervals. following this configuration, the pdc prototype is programmed to check all the packets provided by the pmu and to set the digital output to a high level when the frequency variation exceeds a given threshold, which is equal here to ±0.5 hz. the gpio interface of the pdc is connected to the binary input of the synchronised calibrator that, thanks to the time synchronisation provided by the gps receiver, can evaluate the time difference between the generated event (frequency step change) and the status change of the pdc digital output (low-tohigh transition). the pmu used in the test is fully compliant with [3], without a digital interface. the device is connected to the pdc prototype with a high-performance ethernet switch, used specifically for industrial application (the switch is not reported in the figure). 0reports the maximum, mean, and standard deviation values of the data latency evaluated by the pdc prototype over 1,000 data packets for four different configurations. the results are compliant with the specification of standard [3] (summarised in table 2). to highlight how different configurations of the pmu can influence the detection of an event by the pdc, the tests have been performed by generating the same electrical event and using the pmu with the two different compliance classes (pand mclasses) and two different reporting rates. it should be recalled that high reporting rates allow evaluating dynamic events in an electrical system, but they also require a pdc capable of handling the streams without increasing overall latency. table 7 shows the results of the overall delay of the protection application in ten different tests, each obtained for all the configurations. it should be noted that the overall delay results are close to the latency results in table 6. with the m-class and the lowest reporting rate, the delay between the detection of the event and the binary events reaches the highest value. increasing the reporting rate while keeping the class, the delay is lower. this change is due to the reduced value of the synchrophasor data latency in the input of the pdc as shown in table 6. this behaviour is due to the fact that the pmu needs to change the algorithm to adapt its performance to the different accuracy requirements of the m-class for different reporting rates. the last two columns are related to the p configuration and are characterised by the lowest delay values. the results are similar for the two different reporting rates. this is because the electrical event is synchronised, and it is always located at the occurrence of second 10. the pmu sends its measurements at multiples of the nominal cycles second with respect to the utc second and thus, in the considered test, the event instant and the timestamp of the corresponding measurement are equal. indeed, it is possible to make the different contributions of delay explicit, as follows: tdelay = tl + tp ∆t (1) the overall delay tdelay of the event detection depends on the pmu reporting latency tl and the processing time tp of the pdc that is necessary for evaluating the data and for setting the digital output. the last term of the expression (∆t) is the difference between the timestamp related to the first measurement, where the change of frequency is detected, and the time location of the electrical event. if the processing time tp is low (as it is in the case at hand) and the event is located at the occurrence of the second, the overall delay is close to the pmu reporting latency and does not depend on the reporting rate. in this scenario, the pdc prototype, thanks to the continuous evaluation of the latency of the input streams, also provides a good evaluation of the delay. a second test is then performed to emphasise the role of the event occurrence instant in the overall delay focusing on the pclass, since for this class, tl is independent of fs. in this scenario, the time position of the frequency step change is moved forward of 50 ms from t = 10 s to 10 = 10.05 s. the detection of the event is located at different measurement instants, depending on the configured reporting rate. table 8 reports the delay between the frequency step change and the digital output signals provided by the pdc prototype when the reporting rates are 10 or 50 fps. the p-class algorithm of the commercial pmu does not change its latency as shown in table 6, but the overall delay depends on the chosen reporting intervals. comparing the first and second columns, the impact of the reporting instants to the promptness of the system emerges. furthermore, in this case, equation (1) holds true, and the low delay contribution introduced by the table 6. evaluation of the latency of a pmu with different configuration. fs in fps input streams type max in ms mean in ms st. dev. in ms 10 p 39.3 38.9 0.1 m 679.7 679.4 0.1 50 p 39.4 39.2 0.1 m 135.82 133.3 0.1 table 7. evaluation of the latency in the protection test scenario for a pmu with different configuration. test m class 10 fps in ms m class 50 fps in ms p-class 10 fps in ms p-class 50 fps in ms 1 679.5 134.0 39.4 39.2 2 678.8 134.3 39.2 39.2 3 679.3 134.1 39.3 39.1 4 679.3 134.1 39.3 39.4 5 680.1 133.3 39.1 39.3 6 679.3 133.9 39.0 39.2 7 679.5 133.6 39.2 39.3 8 679.4 133.4 39.1 39.5 9 679.4 133.6 39.3 39.5 10 679.7 133.3 39.4 39.0 table 8. evaluation of the overall delay in the protection test scenario for a p-class pmu with different reporting rates. test p10 fps in ms p50 fp in ms 1 89.4 49.2 2 89.3 50.0 3 89.4 49.2 4 89.4 49.2 5 89.3 49.1 6 89.7 49.0 7 89.3 48.9 8 89.2 49.0 9 89.6 49.6 10 89.3 49.4 acta imeko | www.imeko.org june 2019 | volume 8 | number 2 | 27 pdc is confirmed. as a practical rule, a fast response algorithm and high reporting rates are important prerequisites for minimising the overall delay of the protection system. 6. conclusion synchrophasor technology is becoming an important tool for operators to monitor electric systems over a wide area in real time. currently, this technology is viewed with interest not only for transmission grids, but also for distribution networks. in this context, with the aim of exploring the possibilities for synchrophasor-based monitoring architectures, this paper has presented a prototype of an active pdc developed with low-cost hardware. the feasibility of the proposal was evaluated in terms of the capability of the system to deal with time synchronisation at a good level of accuracy, thus also enabling nontrivial data management. the prototype is also able to safely manage several pmu data streams with different reporting rates and is thus promising as a building block for complex smart grid monitoring architectures. the pdc allows data latency estimation and is thus particularly useful for both real-time monitoring and control. the active pdc is equipped with a digital i/o interface and supports digital protection outputs with low additional delay. it can thus implement decision strategies based on input measurement data and latency evaluation. acknowledgement the research activities described in this paper have been conducted within the r&d project ‘cagliari2020’, partially funded by the italian university and research ministry (grant# miur_pon04a2_00381). references [1] ieee std c37.118.1-2011, ieee standard for synchrophasor measurements for power systems, 2011. 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[28] cmc 256plus technical data [online]. available: https://www.omicronenergy.com/en/download/document/69 c95e19-2233-4610-b86e-ca2c87dbbba8/ indentation modulus at macro-scale level measured from brinell and vickers indenters by using the primary hardness standard machine at inrim acta imeko issn: 2221-870x march 2019, volume 8, number 1, 3-12 acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 3 indentation modulus at the macro-scale level measured by brinell and vickers indenters by using the primary hardness standard machine at inrim alessandro schiavi1, claudio origlia1, alessandro germak1, giulio barbato2, giovanni maizza3, gianfranco genta2, roberto cagliero3, gianluca coppola1,2 1 inrim – national institute of metrological research, str.delle cacce 91, 10135 torino, italy 2 politecnico di torino digep, c.so duca degli abruzzi 24, 10129 torino, italy 3 politecnico di torino disat, c.so duca degli abruzzi 24, 10129 torino, italy section: research paper keywords: hardness; indentation modulus; macro-scale citation: alessandro schiavi, claudio origlia, alessandro germak, giulio barbato, giovanni maizza, gianfranco genta, roberto cagliero, gianluca coppola, indentation modulus at the macro-scale level measured by brinell and vickers indenters by using the primary hardness standard machine at inrim, acta imeko, vol. 8, no. 1, article 2, march 2019, identifier: imeko-acta-08(2019)-01-02 editor: petri koponen, mikes metrology, finland received july 31, 2018; in final form january 31, 2019; published march 2019 copyright: © 2019 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was supported by euramet empir project 14ind03 “strength-able”. corresponding author: alessandro schiavi, e-mail: a.schiavi@inrim.it 1. introduction the elastic properties of materials in mechanical engineering and material science can be evaluated by means of several different experimental techniques that are based on static, quasi-static, and dynamic methods. these techniques involve, for example, measurements of tensile or compressive stress strain, resonance methods, and ndt-based methods, such as measurements of acoustic wave propagation speed in solids or phonon detection. moreover, techniques involving instrumented indentation (from the nano to the macro scale) that are used to evaluate the elastic properties of materials and procedures are embodied in iso 14577-1 [1] (for metallic materials). as it is known, the elastic response of materials may vary as a function of the different experimental techniques, measurement procedures, and other boundary conditions used. consequently, the relevant differences in young’s modulus values can be easily achieved. in the case of instrumented indentation, iso selected the specific name of “indentation modulus” and the symbol eit for underlining possible differences. from a metrological point of view, “hardness” is actually the only quantity (although conventionally defined) collected in the calibration and measurement capabilities (cmc) of the bipm and supported by international key comparisons. consequently, the possibility of evaluating the elastic properties from consolidated and accurate experimental procedures is a promising attempt to reduce some sources of uncertainty and dispersion in experimental results. observation of elastic recovery effects in indentation tests dates back to 1961, specifically the study of stilwell and tabor abstract in this paper, the experimental procedure and calculation model for the measurement of the indentation modulus by using the primary hardness standard machine at inrim in the macro-scale range at room temperature is described. the indentation modulus is calculated based on the doerner-nix linear model and from accurate measurements of indentation load, displacement, contact stiffness, and hardness indentation imaging. measurements are performed with both pyramidal (vickers test) and spherical indenters (brinell test). test force is provided by a dead-weight machine, and the occurring displacement is measured by a laser-interferometric system. the geometrical dimensions of both the vickers and brinell indentations are measured by means of a micro-mechanical system and optical microscopy imaging techniques. applied force and indentation depth are measured simultaneously, at a 16 hz sampling rate, and the resultant loading-unloading indentation curve is obtained. preliminary tests are performed on metal and alloy samples. considerations and comments on the accuracy of the proposed method and analysis are discussed. mailto:a.schiavi@inrim.it acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 4 [2]. the first attempts to measure hardness and elastic modulus by instrumented indentation were made in 1983 by pethicai, hutchings, and oliver, investigating a method of evaluating hardness at a nano-scale level. their findings showed that depth-sensing indentation allows the construction of a loaddisplacement curve, which is strongly related to the typical stress-strain diagrams of materials [3]. in 1986, doerner and nix improved the existing methods by using a high-resolution depth-sensing instrument [4]. in 1992, oliver and pharr introduced a practical model for measuring hardness and elastic modulus by instrumented indentation [5]. at present, these models are included in iso 14577; nevertheless, several important changes have been proposed during past decades, improving both the accuracy and application field of the models [6]-[11]. in this paper, the elastic properties of copper alloy, aluminum alloy, brass and stainless steel samples are investigated in terms of the indentation modulus using the macro-indentation test. measurements are performed using the primary hardness dead weight machine, designed and realized at inrim. 2. indentation modulus the method for measuring the indentation modulus by means of the indentation technique was introduced by oliver and pharr in 1992 [5]. the indentation modulus, eit, is properly the elastic response of a material when subjected to the action of a concentrated load in a single point. occurring deformations are not linear; consequently, indentation modulus represents a reasonably close estimation of young’s modulus, in particular, at the macro-scale level. indentation modulus eit depends on several parameters and boundary conditions, and it is expressed as: 𝐸it = 1−𝜈𝑠 2 2√𝐴𝑝 𝑆√𝜋 − 1−𝜈 𝑖 2 𝐸𝑖 (1) where s is the poisson ratio of tested material; i and ei are the poisson ratio and the young’s modulus of the indenter material respectively; s is contact stiffness, i.e., the incremental ratio between unloading force and related displacement at the maximum depth of indentation; and ap is the projected contact area, i.e., the value of the indenter area function at the contact depth. the contact stiffness s is experimentally determined from the best fit of the unloading indentation curve, as the incremental ratio s = f / h|hmax. the projected contact area ap depends on the depth hc of the contact of the indenter with the sample at fmax and on the type of indenter. for the vickers diamond pyramidal indenter, with a vertex angle , the projected contact area is determined by: 𝐴𝑝 = (2ℎ𝑐 ∙ tan 𝛼 2 ) 2 (2) and the brinell tungsten carbide spherical indenter, with a radius r, can be determined by: 𝐴𝑝 = 𝜋ℎ𝑐 ∙ (2𝑅 − ℎ𝑐) (3) in both cases, the depth hc of the contact of the indenter with the sample at fmax is determined as a function of frame compliance cf as follows: ℎ𝑐 = ℎmax − 𝜀 ∙ 𝐹max 𝑆 − 𝐶𝑓 ∙ 𝐹max (4) in which hmax is the maximum indentation depth; fmax is the maximum applied force;  is a value depending on the indenter geometry and the extent of plastic yield in the contact (for both vickers and brinell  = 0.75); s is the contact stiffness; and cf is the frame compliance. once the maximum applied force fmax and the maximum depth of indentation hmax are known, indentation modulus eit can be calculated by using the indentation geometrical dimensions (in particular, the length of the two diagonals and radius) from vickers and brinell’s experimental data and from the slope of the indentation curve (during the unloading path). in particular, an accurate evaluation of contact stiffness s depends on the best fit of the unloading path. in figure 1, an experimental loading-unloading indentation curve, as a function of true applied force and displacement, is shown. iso 14577-1 recommends two methods of fitting the curve: by means of a linear model (doerner-nix method), taking into account the initial 20 % of the unloading curve, and by a power-law model (oliver-pharr method), taking into account a range between 50 % and 80 % of the unloading curve. in the linear model, it is assumed that the first portion of the unloading curve is linear and only the linear portion to intercept the displacement axis is extrapolated. the power-law model relies on the depth of the unloading point, which has a high uncertainty both in terms of its measurement and the last part of elastic recovery; therefore, in this paper, only the linear model is used in order to fit the unloading curve. 3. evaluation of frame compliance frame compliance cf is an experimental quantity that takes into account the whole deformation occurring in the testing machine during the indentation test. frame compliance affects the accuracy of the indentation modulus measurement [10] as well as the elasto-plastic deformations occurring in the sample under investigation. the elasto-plastic behavior of the materials is investigated from the evaluation of piling-up and sinking-in effects on the resulting hardness indentations. in order to estimate the actual frame compliance, in this paper, two methods are compared: the first according to iso 14577-1, based on a series of loading and unloading cycles, the second according to the existent literature [11], [12], based on the actual indentation depth of the vickers and/or brinell indentation. in general terms, frame compliance cf can be considered as the difference between the total compliance ctot and the sample compliance cs [13] as follows: 𝐶𝑓 = 𝐶𝑡𝑜𝑡 − 𝐶𝑠 = 𝜕ℎ 𝜕𝐹 | 𝐹=𝐹𝑀𝐴𝑋 − √𝐴𝑝 𝑆 √𝐻𝐼𝑇 𝐹𝑀𝐴𝑋 (5) figure 1. loading-unloading indentation curve, as a function of applied force and displacement, with an indication of the quantities used in the model. acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 5 in which total compliance ctot is determined as the reciprocal of contact stiffness, measured after a series of loading and unloading cycles on a single point, and sample compliance cs depends on indentation hardness hit, on contact stiffness s, and on projected contact area ap of the vickers or brinell indenters. the frame compliance is determined after several cycles of loading and unloading until the slope of the unloading curve is constant, according to the standard [1], on a stainless steel sample, as shown in figure 2. the reciprocal of contact stiffness, as shown in equation (5), allows for calculating the value of the frame compliance. in this study, cf = 22.1x109 m/n. frame compliance cf is also calculated based on indentation depth (determined from the actual indentation geometry) and the maximum indentation depth (measured during the test). assuming that, for many metallic materials, the elastic recovery upon unloading induces a very small elastic deformation at the corners of a vickers indentation, between the loaded and unloaded condition, a negligible change in the diagonal dimensions is therefore expected [13]. an estimation of the frame compliance is as follows: 𝐶𝑓 = ∆ℎ 𝐹𝑀𝐴𝑋 = ℎ𝑀𝐴𝑋−ℎ𝑣 𝐹𝑀𝐴𝑋 (6) in which h is determined from the difference between the indentation depth reached during the indentation test hmax and the indentation depth hv derived from the resulting indentation geometry. a similar approach is also proposed for the brinell indentation; nevertheless, in this case, a relevant change in the radius dimension is expected. in fact, pile-up and sink-in effects influence the radius calculation as well, and they must be taken into account. in figure 3, typical vickers and brinell hardness indentations are shown. diagonal lengths of a vickers pyramidal indentation and the diameter of a brinell spherical indentation are measured with an accuracy of 0.1 m. a micro-mechanical system and an optical microscopy imaging technique (inrim patent) are used for measurement of the indentation geometry. 3.1. frame compliance from the vickers indentation in order to evaluate the frame compliance from the vickers indentation by means of equation (6), it is necessary to accurately evaluate the resultant indentation depth hv from the geometrical dimensions of the indentation. in particular, by measuring the length of indentation sides l, indentation depth hv is given by: ℎ𝑣 = 𝑙 2 ∙ cot 𝛼 2 (7) where the square-based diamond pyramid indenter that is actually used has a measured vertex angle of  = 135.9° between opposite faces, and the side indentation length l is calculated from the two diagonal ds, since a greater accuracy can be achieved, 𝑙 = 𝑑1+𝑑2 2 ∙ sin 𝜋 4 , (8) where d1 and d2 are the two measured diagonals of the vickers indentation. 3.2. frame compliance from the brinell indentation it is also possible to determine the frame compliance from the brinell indentation. in this case, the indentation depth hv depends on the radius r of the spherical indenter used and on the radius r of the resulting indentation, as follows: ℎ𝑣 = 𝑅 − √𝑅 2 − 𝑟2 (9) for spherical indentation, the material around the contact area can be deformed upwards (piling up) or downwards (sinking in) along the axis to which load is applied [14]. indeed, such surface deformation modes influence the measurements of hardness and the indentation modulus, as the true contact area between the indenter and the specimen increases in the case that piling up predominates and decreases in the event that sinking in predominates [15]. the following relationship links the contact area and the indenter displacement measured during the continuous indentation test: 𝑐2 = 𝑟2 2ℎ𝑣𝑅 (10) in this equation, c2 quantifies the degree of piling up and sinking in during the indentation test, c2 > 1 indicates piling up, and c2 < 1 accounts for sinking in. in general terms, it is expected that if c2  1, the surface deformations can be considered negligible and the frame compliance can consequently be determined on the basis of the proposed equation (6). in the graph shown in figure 4, the distribution of figure 2. series of loading-unloading cycles on a single point and linear regression of the last reversed unloading curve. figure 3. microscopic image of a typical pyramid indentation of a vickers hardness test and a spherical indentation of a brinell hardness test, on a metallic surface. figure 4: experimental data on the c2 parameter as a function of indentation depth, evaluated by the brinell test with indenter diameters of 1.0 mm and 2.5 mm. acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 6 parameter c2 as a function of indentation depth hv, for all tested materials, is shown. the experimental data values are shown in table 6, table 7, table 8, and table 9. for each single measurement, it is clear that it is possible to evaluate the degree of the piling up and sinking in that is occurring. in particular, sinking-in effects increase as a function of increasing indentation depth. 3.3. experimental evaluation of frame compliance the data on frame compliance cf calculated by means of equation (6) from the shapes of both the vickers pyramidal indentations and the brinell spherical indentations and from the maximum applied force, ranging from ~10-10 m/n to ~107 m/n, as shown in tables 2-9. data in brackets (referred to as hv3) are considered outliers. it is clear that both the vickers and brinell methods return a coherent dispersion of the frame compliance values as a function of the applied force; in particular, the effects of frame compliance are relevant to the low values of applied force. the proposed method allows us to accurately quantify the actual frame compliance for each single measurement, taking into account the whole deformation occurring during the indentation test as a function of maximum indentation depth. average values, calculated by equation (5), cf  0.022 m/n and by equation (6) cf  0.031 m/n, can be considered as in agreement. in the graph shown in figure 5, a comparison between the standard method and the proposed method is shown. 4. primary hardness standard machine measurements of indentation modulus are performed by using the inrim primary hardness standard dead weight machine, shown in figure 6. the standard machine at inrim was realized in the early 1970s and improvements have continued to be made to it until the present day. its technical features and metrological characterization are summarized in detail in [16]-[18]. the system generates force by moving a series of dead weights, and a laser interferometric system is used for indentation depth measurements. test forces are generated by dead weights and are measured along the scale by a force transducer; experimental values are determined with an accuracy range of 0.01 %. the laser beam is aligned on the measurement axis, and experimental values are determined with a resolution of 0.02 m. force and indentation depth are monitored in real time, and dates are recorded with a sampling rate of 16 hz. the micro-mechanical system, the optical microscopy imaging technique (inrim patent), and the image processing technique used for the measurement of the indentation geometry are shown in figure 7. 5. materials the materials tested in this work are copper alloy, aluminum alloy, stainless steel, and brass. young’s modulus e and poisson ratio s of the tested materials were previously determined on the basis of accurate measurements of longitudinal cl and transversal ct sound speed waves in solids [19], at room temperature. although some systematic differences between dynamic and static moduli can be achieved, the reference data can be considered sufficiently accurate and useful for the proposed comparison method, since overall uncertainties are lower than 1 %. in table 1, the reference data of tested alloys are shown. the dynamic poisson ratio s is calculated by means of the following equation: 𝜈𝑠 = 1−2( 𝑐𝑙 𝑐𝑡 ) 2 2−2( 𝑐𝑙 𝑐𝑡 ) 2 (11) the values of poisson ratio s, listed in table 1 have been used in equation (1) for the indentation modulus calculation. dynamic young’s modulus e is calculated by means of the following equation: 𝐸 = 2𝜌𝑐𝑙 2(1 − 𝜈𝑠) (12) figure 5: comparison between frame compliances calculated by equation (5), continuous line and single values from vickers and brinell measurements, determined by equation (6). the dotted line is the average value of frame compliance measured by equation (6). figure 7: optical microscopy system and imaging techniques for measuring geometrical dimensions of the indentations. figure 6. inrim primary hardness standard dead weight machine (details of the interferometric system and the anvil). acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 7 6. experimental results in order to define the indentation modulus eit, vickers hardness measurements are performed with maximum forces fmax of 980.6 n, 294.2 n, and 29.4 n. the brinell hardness measurements, with maximum forces of 1838.6 n, 612.9 n, and 305.9 n, are performed by using the spherical indenter of radius r = 1.25 mm and 294.2 n, 98.1 n, and 49.0 n, by using the spherical indenter of radius r = 0.5 mm. the young’s modulus and poisson ratio of the vickers diamond pyramidal indenter are ei = 1140 gpa and vi = 0.07 [1] respectively and the same values for the brinell tungsten carbide spherical indenter are ei = 634 gpa and vi = 0.22 respectively [20]. the values of the young’s modulus and poisson ratio of the spherical indenter refer to the specific tungsten carbide-cobalt alloy (composition wc 94%/co 6%), density 14800 kg/m3 (iso code k10, ansi code c3). the maximum indentation depth hmax is measured by means of the laser interferometric system and hv from the hardness indentation, based on equation (7) and equation (9). values of frame compliance cf, are calculated by equation (6), and contact area ap and sample compliance cs are collected for each single measurement. the value of contact stiffness s = f / h, is calculated based on the doerner-nix linear model. indentation hardness hit is determined, as stated in iso 14577-1 [1], from the following equation: hit = fmax / ap, where the contact area is expressed as a function of contact depth hc, equation (4). in the graphs shown in figure 8, figure 9, and figure 10, the values of the maximum indentation depth hmax measured as a function of the maximum applied load fmax are depicted. in figure 8, the values are determined based on the vickers method, and in figure 9 and figure 10 the values are measured by means of the brinell method, with both spherical indenter radii being 0.5 mm and 1.25 mm. in the following tables, all the experimental average values and empirical data of four metallic samples, used in equation (1) for model implementation, are collected. figure 8: experimental data of maximum indentation depth, measured from vickers method, as a function of applied load. table 1. reference data on the tested metal alloys. parameter stainless steel aluminum alloy copper alloy brass density /kgm-3 7914.1 2806.4 8932.5 8296.1 longitudinal speed wave cl/ms-1 5759.1 6294.7 4779.0 4520.3 transversal speed wave ct/ms-1 3146.2 3082.4 2247.8 1842.1 young’s modulus e/gpa 201.7 71.6 122.6 78.9 poisson’s ratio s− 0.287 0.342 0.358 0.400 figure 9: experimental data of maximum indentation depth, measured from brinell method (r = 0.5 mm), as a function of applied load. figure 10: experimental data of maximum indentation depth, measured from brinell method (r = 1.25 mm), as a function of applied load. acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 8 table 3. vickers hardness test: data on stainless steel. experimental data hv3 hv30 hv100 maximum applied load fmax/n 29.4 294.2 980.6 maximum indentation depth hmax/m 43.5 112.1 181.0 maximum indentation depth hv/m 23.2 80.1 147.8 square impression side l/m 114.8 395.7 729.9 contact area ap/m 4.38∙10 -8 2.83∙10-7 7.19∙10-7 sample compliance cs/mn-1 5.74∙10 -12 5.08∙10-13 1.51∙10-13 hardness hit /gpa 67.1 104.0 136.4 frame compliance cf / mn-1 (6.89∙10 -7) 1.09∙10-7 3.38∙10-8 contact stiffness s/ nm-1 1.80∙107 4.75∙107 7.56∙107 indentation modulus eit/ gpa 154.5 114.8 98.6 table 5. vickers hardness test: data on copper alloy. experimental data hv3 hv30 hv100 maximum applied load fmax/n 29.4 294.2 980.6 maximum indentation depth hmax/m 43.6 155.3 320.4 maximum indentation depth hv/m 41.7 151.9 293.3 square impression side l/m 206.0 749.8 1448.1 contact area ap/m 4.44∙10 -8 5.64∙10-7 2.39∙10-6 sample compliance cs/mn-1 2.69∙10 -12 2.92∙10-13 1.15∙10-13 hardness hit /gpa 66.3 52.2 41.0 frame compliance cf / mn-1 (6.31∙10 -8) 1.16∙10-8 2.77∙10-8 contact stiffness s/ nm-1 2.15∙107 6.15∙107 9.14∙107 indentation modulus eit/ gpa 90.2 69.4 52.8 table 4. vickers hardness test: data on aluminum alloy. experimental data hv3 hv30 hv100 maximum applied load fmax/n 29.4 294.2 980.6 maximum indentation depth hmax/m 32.0 89.6 187.7 maximum indentation depth hv/m 25.4 80.9 149.1 square impression side l/m 125.4 399.6 736.4 contact area ap/m 2.16∙10 -8 1.64∙10-7 7.27∙10-7 sample compliance cs/mn-1 7.12∙10 -12 6.36∙10-13 2.39∙10-13 hardness hit /gpa 136.4 179.1 134.9 frame compliance cf / mn-1 (2.26∙10 -7) 2.95∙10-8 3.94∙10-8 contact stiffness s/ nm-1 9.32∙106 2.86∙107 4.75∙107 indentation modulus eit/ gpa 68.5 66.0 59.7 table 2. vickers hardness test: data on brass. experimental data hv3 hv30 hv100 maximum applied load fmax/n 29.4 294.2 980.6 maximum indentation depth hmax/m 32.5 103.0 187.8 maximum indentation depth hv/m 227.1 719.2 1311.6 square impression side l/m 160.6 508.5 927.4 contact area ap/m 1.47∙10 -8 2.39∙10-7 7.79∙10-7 sample compliance cs/mn-1 2.63∙10 -12 3.79∙10-13 1.30∙10-13 hardness hit /gpa 199.8 122.9 125.9 frame compliance cf / mn-1 (-2.26e -07) 2.49∙10-9 6.92∙10-10 contact stiffness s/ nm-1 1.62∙107 4.51∙107 7.21∙107 indentation modulus eit/ gpa 85.3 74.7 65.4 acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 9 table 8. brinell hardness test: data on aluminum alloy. experimental data hbw (5) r=0.5 hbw (10) r=0.5 hbw (30) r=0.5 hbw (31.2) r=1.25 hbw (62.5) r=1.25 hbw (187.5) r=1.25 maximum applied load fmax/n 49.0 98.1 294.2 305.9 612.8 1838.6 maximum indentation depth hmax/m 14.9 27.8 71.4 41.8 66.6 174.1 maximum indentation depth hv/m 11.3 19.0 53.5 25.7 47.6 138.6 radius of the impression r/m 105.5 136.5 225 252.5 341.7 572.2 contact area ap/m2 3.29∙10-8 6.55∙10-8 1.72∙10-7 2.41∙10-7 3.94∙10-7 1.07∙10-6 sample compliance cs/mn-1 5.22∙10-12 2.96∙10-12 1.13∙10-12 9.38∙10-13 4.45∙10-13 1.56∙10-13 hardness hit /gpa 149.2 149.6 170.6 127.2 155.6 171.2 frame compliance cf / mn-1 7.5∙10-8 8.93∙10-8 6.1∙10-8 5.24∙10-8 3.1∙10-8 1.93∙10-8 contact stiffness s/ nm-1 0.84∙107 1.14∙107 1.67∙107 2.52∙107 2.97∙107 4.74∙107 indentation modulus eit/ gpa 39.7 38.0 34.2 43.0 40.5 39.0 table 7. brinell hardness test: data on stainless steel. experimental data hbw (5) r=0.5 hbw (10) r=0.5 hbw (30) r=0.5 hbw (31.2) r=1.25 hbw (62.5) r=1.25 hbw (187.5) r=1.25 maximum applied load fmax/n 49.0 98.1 294.2 305.9 612.8 1838.6 maximum indentation depth hmax/m 16.5 34.5 73.9 39.3 71.0 177.6 maximum indentation depth hv/m 10.0 19.6 56.7 28.0 51.9 136.2 radius of the impression r/m 99.5 138.5 231.2 263.5 356.7 567.5 contact area ap/m2 4.34∙10-8 9.36∙10-8 1.94∙10-7 2.62∙10-7 4.75∙10-7 1.15∙10-6 sample compliance cs/mn-1 3.64∙10-12 2.02∙10-12 6.98∙10-13 4.79∙10-13 2.73∙10-13 1.18∙10-13 hardness hit /gpa 112.8 104.7 152.0 116.6 128.9 159.5 frame compliance cf / mn-1 1.33∙10-7 1.52∙10-7 5.85∙10-8 3.69∙10-8 3.11∙10-8 2.25∙10-8 contact stiffness s/ nm-1 1.47∙107 1.96∙107 2.78∙107 4.15∙107 5.09∙107 6.53∙107 indentation modulus eit/ gpa 62.5 56.3 55.3 72.9 65.7 53.0 table 6. brinell hardness test: data on brass. experimental data hbw (5) r=0.5 hbw (10) r=0.5 hbw (30) r=0.5 hbw (31.2) r=1.25 hbw (62.5) r=1.25 hbw (187.5) r=1.25 maximum applied load fmax/n 49.0 98.1 294.2 305.9 612.8 1838.6 maximum indentation depth hmax/m 28.3 49.4 134.5 69.0 125.7 323.7 maximum indentation depth hv/m 24.7 45.7 123.3 63.8 115.4 306.1 radius of the impression r/m 155.3 208.8 328.8 394.2 524.7 819.5 contact area ap/m2 8.02∙10-8 1.37∙10-7 3.48∙10-7 4.88∙10-7 8.75∙10-7 2.09∙10-6 sample compliance cs/mn-1 2.64∙10-12 1.61∙10-12 6.11∙10--13 4.15∙10-13 2.45∙10--13 1.02∙10-13 hardness hit /gpa 61.1 71.6 84.5 62.6 70.0 87.7 frame compliance cf / mn-1 7.36∙10-8 3.81∙10-8 3.82∙10-8 1.71∙10-8 1.67∙10-8 9.59∙10-9 contact stiffness s/ nm-1 1.75∙107 1.97∙107 2.88∙107 4.33∙107 5.17∙107 6.84∙107 indentation modulus eit/ gpa 54.1 46.0 42.0 54.2 47.9 40.4 table 9. brinell hardness test: data on copper alloy. experimental data hbw (5) r=0.5 hbw (10) r=0.5 hbw (30) r=0.5 hbw (31.2) r=1.25 hbw (62.5) r=1.25 hbw (187.5) r=1.25 maximum applied load fmax/n 49.0 98.1 294.2 305.9 612.8 1838.6 maximum indentation depth hmax/m 30.7 57.9 193.3 93.2 196.6 561.5 maximum indentation depth hv/m 25.0 50.3 168.2 77.2 158.1 468.3 radius of the impression r/m 156 218.5 374 432.7 608.5 975.5 contact area ap/m2 8.74∙10-8 1.62∙10-7 4.78∙10-7 6.7∙10-7 1.37∙10-6 3.34∙10-6 sample compliance cs/mn-1 2.67∙10-12 1.45∙10-12 5.12∙10-13 4.01∙10-13 2.27∙10-13 8.9∙10-14 hardness hit /gpa 56.1 60.3 61.6 45.6 44.6 54.9 frame compliance cf / mn-1 1.16∙10-7 7.83∙10-8 8.56∙10-8 5.2∙10-8 6.28∙10-8 5.06∙10-8 contact stiffness s/ nm-1 1.80∙107 2.25∙107 3.45∙107 4.78∙107 5.98∙107 7.98∙107 indentation modulus eit/ gpa 53.2 48.5 43.0 50.8 43.9 37.1 experimental data hv3 hv30 hv100 acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 10 7. evaluation of indentation modulus eit as is evident from the experimental data summarized in tables 2-9, all the data shows a systematic dependence as a function of applied load. in general terms, the data suggests that evaluations of an indentation modulus at lower values of applied load (fmax  100 n) are less accurate than other values, for both the vickers and brinell methods, at the macro-scale level. the uncertainty is due to the determination of contact stiffness s. in fact, estimation of the unloading indentation slope is less accurate for low values of applied force (since the indentation depth is small) than indentation curves of greater indentation depth. consequently, the number of points of the curve on which the regression is applied is exiguous for low indentation depth. for example, in figure 11, a comparison between the doener-nix model and the actual best fit of the unloading curve returns two different slopes of about 2.5%, from which contact stiffness s is evaluated. as is evident from the diagram, by applying the doener-nix method to the raw data (taking into account the initial 20% of the unloading curve), the resultant contact stiffness is s = 18.05∙106 n m-1 (as shown in table 9), while if the actual best fit is applied, the resultant contact stiffness is s = 18.51∙106 n m-1. this difference (of only 2.5%) results in a difference in the indentation modulus of about 5% or more. moreover, the effects of frame compliance are relevant to the low values of indentation depth [21]. nevertheless, it is known that the poorly defined tip shape of the vickers indenters at low indentation depths is a cause of hardness measurement errors [22]. on the other hand, the piling-up effects in the brinell tests, at low values of indentation depth (hmax  50 m), allow for evaluations of the geometrical dimensions of the spherical impression to be less accurate than the impressions affected by the sinking-in effects as well as the indentation size effects [23]. more in-depth analyses are currently underway, considering the validity of the assumption of a load-independent value for cf and further checking the homogeneity of the tested materials. in the graphs shown in figure 12, figure 13, and figure 14, the indentation modulus eit (with the related expanded uncertainties) of the tested materials is demonstrated. in the graph in figure 12, the indentation modulus is determined on the basis of the vickers method, and in figure 13 and figure 14, it is determined on the basis of the brinell method. the indentation modulus is expressed as a function of applied load. in the following graphs shown in figure 15, figure 16, figure 17, and figure 18, the average indentation modulus eit of each sample is compared with young’s modulus e. values for the indentation modulus are determined by the data shown above on the basis of equation (1). data on the young’s modulus is evaluated on the basis of equation (11) and equation (12), based on measurements of the speed of sound in solids. the observed load dependence of all experimental data allows for the achievement of a relevant load-dependent indentation modulus. figure 11: determination of contact stiffness on the basis of the doener-nix method and from the actual best fit of the unloading curve (for copper, hbw (5), r=0.5mm). figure 12: indentation modulus calculated using the vickers method. figure 13: indentation modulus from brinell method (r=0.5 mm). figure 14: indentation modulus calculated using the brinell method (r=1.25 mm). acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 11 8. conclusion this paper has described a method for the evaluation of the indentation modulus by using the primary hardness standard machine at inrim in the macro-scale range, with both vickers and brinell indenters. the indentation modulus is calculated based on the doerner-nix linear model and from accurate measurements of indentation load, indentation depth, and projected contact area. vickers and brinell hardness impression imaging methods were measured by means of optical microscopy imaging techniques. in particular, a detailed analysis of frame compliance was performed and discussed. preliminary tests were performed on brass, stainless steel, aluminum alloy, and copper alloy samples and experimental data and results were presented. the primary finding of this study is that the indentation modulus at a macro-scale level is strongly load/indentation depth-dependent. large plastic effects occurring at high values of applied load affect the evaluation of the indentation modulus, since the method is based on the determination of the elastic response of the material under investigation. consequently, the indentation modulus is greatly underestimated with respect to young’s modulus. on the contrary, at lower values of applied load, values of indentation modulus tend to increase, since elastic behavior tends to be predominant with respect to the plastic behavior. acknowledgement the research leading to these results was conducted in the frame of empir 14ind03 “strength-able”. the empir is jointly funded by the empir participating countries within euramet and the european union. references [1] iso 14577-11:2002. metallic materials: instrumented indentation test for hardness and materials parameters part 1: test method. [2] n. a. stilwell, d. tabor, elastic recovery of conical indentations. proc. of the physical society 78, 2 (1961) pp. 169-179. [3] j. b. pethicai, r. hutchings, w. c. oliver, hardness measurement at penetration depths as small as 20 nm. philosophical magazine 48, 4 (1983) pp. 593-606. [4] m. f. doerner, w. d. nix, a method for interpreting the data depth-sensing indentation instruments. j. mater. res 1, 4 (1986) pp. 601-609. [5] w. c. oliver, g. m. pharr, an improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments. journal of materials research 7, 6 (1992) pp. 1564-1583. [6] y.w. bao, w. wang, y.c. zhou, investigation of the relationship between elastic modulus and hardness based on depth-sensing indentation measurements. acta materialia 52 (2004) pp. 5397– 5404. 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[9] t. chudoba, n. m. jennett, higher accuracy analysis of instrumented indentation data obtained with pointed indenters. journal of physics d: applied physics 41, 21 (2008) p. 215407. figure 17: indentation modulus and young’s modulus for aluminum. figure 16: indentation modulus and young’s modulus for stainless steel. figure 15: indentation modulus and young’s modulus for brass. figure 18: indentation modulus and young’s modulus for brass. acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 12 [10] c. ullner, e. reimann, h. kohlhoff, a. subaric-leitis, effect and measurement of the machine compliance in the macro range of the instrumented indentation test. measurement 43, 2 (2010) pp. 216-222. [11] r. cagliero, g. barbato, g. maizza, g. genta, measurement of elastic modulus by instrumented indentation in the macro-range: uncertainty evaluation. international journal of mechanical sciences 101 (2015) pp. 161-169. [12] a. schiavi, c. origlia, a. germak, g. barbato, g. maizza, g. genta, r. cagliero, g. coppola, “measurement of macro-scale indentation modulus using the primary hardness standard machines at inrim,” proc. of imeko tc3, tc5, and tc22 international conference, 2017, helsinki, finland. [13] k. herrmann, n. m. jennett, w. wegener, j. meneve, k. hasche, r. seemann, progress in determination of the area function of indenters used for nanoindentation. thin solid films 377-378 (2000) pp. 394-400. [14] x. hernot, o. bartier, y. bekouche, r. el abdi, g. mauvoisin, influence of penetration depth and mechanical properties on contact radius determination for spherical indentation. international journal of solids and structures 43, 14-15 (2006), pp. 4136-4153. [15] a. p. g. m bolshakov, g. m. pharr, influences of pile-up on the measurement of mechanical properties by load and depth sensing indentation techniques. journal of materials research 13, 4 (1998) pp. 1049-1058. [16] g. barbato, s. desogus, r. levi, design and performance of a dead-weight standard rockwell hardness testing machine. journal of testing and evaluation 6 (1978) pp. 276-279. [17] a. germak, c. origlia, metrological characterization of the vickers hardness calibration machine and gal-vision measuring system realised by ltf for nim (china). inrim tech. report 16, 2006. [18] a. germak, a. liguori, c. origlia, “experience in the metrological characterization of primary hardness standard machines,” in proc. of hardmeko, 2007. [19] s. lago, s. brignolo, r. cuccaro, c. musacchio, p.g. albo, p. tarizzo, “application of acoustic methods for a non-destructive evaluation of the elastic properties of several typologies of materials,” applied acoustics 75 (2014), pp. 10-16. [20] http://www.wesltd.com/divisions/hardmetal/html/tungstencarbide.html (last accessed 21/12/2018) [21] a. schiavi, c. origlia, a. cackett, c. hardie, d. signore, o. petrella, a. germak, comparison between tensile properties and indentation properties measured with various shapes indenters of copper-chromium-zirconium alloy at macroscale level. journal of physics: conference series 1065, 6 (2018) p. 062010. [22] i. spary, a. bushby, n.m. jennett., on the indentation size effect in spherical indentation. philosophical magazine 86, 33-35 (2006) pp. 5581-5593. [23] a. j. cackett, j. j. h. lim, p. klupś, a. j. bushby, c. d. hardie, using spherical indentation to measure the strength of copperchromium-zirconium. journal of nuclear materials (2018) pp. 17. http://www.wesltd.com/divisions/hardmetal/html/tungsten-carbide.html http://www.wesltd.com/divisions/hardmetal/html/tungsten-carbide.html phase correlation functions: fft vs. fht acta imeko issn: 2221-870x march 2019, volume 8, number 1, 87 92 acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 87 phase correlation functions: fft vs. fht giuseppe schirripa spagnolo1, lorenzo cozzella1, fabio leccese2 1 università roma tre – dipartimento di matematica e fisica, via della vasca navale, 84, 00146 roma, italy 2 università roma tre – dipartimento di scienze, via della vasca navale, 84, 00146 roma, italy section: research paper keywords: fft; phase correlation; fast hartley transform; digital image elaboration citation: g. schirripa spagnolo, l. cozzella, f. leccese, phase correlation functions: fft vs. fht, acta imeko, vol. 8, no. 1, article 12, march 2019, identifier: imeko-acta-08 (2019)-01-12 editor: alistair forbes, national physical laboratory, uk received may 05, 2018; in final form march 20, 2019; published march 2019 copyright: © 2019 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was supported by miur. corresponding author: f. leccese, e-mail: fabio.leccese@uniroma3.it 1. introduction the ability to accurately process images and video signals is an important skill in many research fields. one of the more widespread techniques used is the fourier transform [1], [2]. in fact, fourier’s theorem is not only one of the most impressive results of analysis and one of the most important mathematical tools available to physicists and engineers, but it may be considered an indispensable instrument in digital image processing. in this field, the fourier’s transform and correlationbased analysis are crucial elements of the pattern-matching process [3], [4]. in many image-processing applications, it is necessary to determine the correspondence between two or more images. the different images can either be images of the same object taken from different sensors or images of the same object taken at different times. this matching analysis is typically performed using either cross-correlation in the spatial domain or phase correlation in the frequency domain. in general, spatial cross-correlation [5], [6] yields several broad peaks and a main peak whose maximum is not always clearly defined; therefore, it is difficult to evaluate and locate it. on the other hand, correlation in the frequency domain is preferable in the case that the images are corrupted by noise. figure 1 shows the correlation function between two images, one of which is corrupted by noise. in this figure, it is possible to note the difference between the cross-correlation in the spatial domain and the phase correlation performed in the frequency domain. figure 1. example of correlation between two images (a) image ‘1’; (b) image ‘2’; (c) correlation in the spatial domain; (d) correlation in frequency domain. abstract the ability to process an image is a crucial skill in many measurement activities. in image processing or pattern recognition, fast fourier transform (fft) is widely used. in particular, the phase only correlation (poc) method demonstrates high robustness and subpixel accuracy in pattern matching. however, there is a disadvantage in the required memory machine because of the calculation of 2d-fft. in applications in which the use of memory is a critical element, fast hartley transform (fht) seems to be a good substitute. in this context, the use of hartley’s transform can be of interest for apps implemented on portable systems e.g. smartphones. in this article, we present a comparison of the implementations of the phase correlation function using fft and fht. particular attention is given to the analytical steps necessary to implement the poc by means of the hartley transform. mailto:fabio.leccese@uniroma3.it acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 88 the phase correlation in the frequency domain is efficiently performed by phase-only correlation (poc) [7], [8]. this correlation method demonstrates high robustness and accuracy in pattern matching and in image registration. however, there is a disadvantage in terms of the required machine memory because of the calculation of 2d-fft. the fast hartley transform (fht) can be a valid alternative to the fast fourier transform (fft) [9]. the hartley transform resembles a fourier transform, but it is free from the need to process complex numbers. the hartley transform also has better properties and faster algorithms than the fourier one; therefore, it can represent a valid alternative, particularly useful when a greater efficiency in memory requirements is needed. in this paper, we analyse the equivalence of the poc computation by means of fft and fht. the hartley transform, in some cases, is a valid alternative to the fourier transform, in particular in all those applications implemented on embedded systems or smartphones. the remaining of this paper is organised as follows. the definition of the poc function and its basic properties are described in section 2. the properties of the harley transform and the definition of poc in hartley space are given in section 3. in section 4, we present a set of experiments for evaluating the performance of the proposed methods. finally, conclusions are drawn in section 5. 2. phase-only correlation (poc) poc is a digital image processing technique that is normally used to estimate the ‘similarity’ between two images. it is extensively used in many practical applications. in particular, poc is employed in image registration [10]-[12], fingerprint matching [13], [14], iris recognition [15], palmar recognition [16], piv analysis [17], digital image stabilisation [18], and security applications [19]-[24]. phase correlation is based on the well-known fourier shift property [25]. if we consider two x yn n digital images ( )1 ,i x y and ( )2 ,i x y . we assume that the index ranges are , ,x xx m m= − , and , ,y yy m m= − for mathematical simplicity. hence, 2 1x xn m= + and 2 1y yn m= + . let ( )1 ,f u v and ( )2 ,f u v denote the 2d discrete fourier transforms of the two images ( )1 ,i x y and ( )2 ,i x y . ( )1 ,f u v and ( )2 ,f u v are given by: ( ) ( )1 1 1 , , exp 2 yx x y mm x m y mx y x y u x v y f u v i x y j n n n n  =− =−    = − +           (1) ( ) ( )2 2 1 , , exp 2 yx x y mm x m y mx y x y u x v y f u v i x y j n n n n  =− =−    = − +           (2) where , ,x xu m m= − , , ,y yv m m= − the cross-phase spectrum (or normalised cross spectrum) is defined as [26] ( ) ( ) ( ) ( ) ( ) 1 2 1 2 * * , , exp , , f u v f u v j u x v y f u v f u v  = − +    (3) with ( )2 * ,f u v being the conjugate function of ( )2 ,f u v . the poc function is the 2d inverse discrete fourier transform of equation (3). in other words, the poc surface is defined as ( ) ( ) ( ) ( ) ( ) 1 21 1 2 * * , , , , , f u v f u v r x y f f u v f u v −    =     (4) if both images are similar, the poc function ( ),r x y gives a distinct sharp peak. conversely, if the two images are not similar, the peak significantly drops. the height of the peak gives a similarity measurement of the matching, and the position of the peak in the x e y axis shows the translation displacement of two images. the most remarkable property of poc compared to the ordinary correlation is that it is not influenced by brightness change, and it is highly robust against noise. poc, in some cases, is defined as [27], [28] ( ) ( ) ( ) ( ) ( ) 1 21 1 2 * * , , , , , f u v f u v r x y f f u v f u v  −     =      (5) in equation (5), the coefficient α controls the correlation peak width. optimum values range from α = 0 (for images characterised by high spatial frequency content and a high noise level) to α = 0.5 (for low-noise images with a less fine structure). for values greater than 0.5, the high frequency noise is magnified [29]. figure 2 shows two examples of poc outputs. in some applications, instead of fft, it is possible to use fht. using fht, equation (4) and equation (5) can be computed efficiently and without loss of information [30]. 3. the hartley transform the hartley transform, introduced in 1942 [31], became widely known after the publication of bracewell’s study in 1983 [32]. it is an integral transform closely related to the fourier transform; keeps all the useful properties thereof; and can be used to obtain the power spectrum and perform convolution directly through the output real-valued data without firstly calculating the real and imaginary parts. the use of the hartley transform also reduces the processing time and needs less memory. therefore, it is particularly well suited for application on mobile devices and embedded systems. the hartley transform is one type of integral transformation that is closely related to the fourier transform. it has the same integral core of the transform and the inverse transform, and it also has most of the characteristics of the fourier transform. the hartley transform can be obtained from the fourier integral by replacing the exponential function figure 2. examples of the poc function. acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 89 ( ) ( ) ( )exp 2 cos 2 sin 2j ux vy ux vy j ux vy  − + = + − +           with ( ) ( ) ( )cas 2 cos 2 sin 2ux vy ux ux  + = +   . the hartley transform ( ),hs u v of a real signal ( ),s x y and its inverse transform are defined as ( ) ( ), ( , ) cas 2hs u v s x y ux vy dx dy + + − − = +    (6) ( ) ( ), ( , ) cas 2hs x y s x y ux vy du dv + + − − = +    . (7) the fourier and hartley transforms are very similar and share many properties. consequently, many applications using the fourier transform can be performed by the hartley one [33]. to derive the relationship between the fourier and hartley transforms, their symmetry must be considered. let us split the hartley transform ( ),hs u v into its even and odd parts ( ),he u v and ( ),ho u v , so that ( ) ( ) ( ), , ,h h hs u v e u v o u v= + . (8) the even part of the function is what we get by reversing the function (changing u to u− and v to v− ), adding the reversed function to the original and dividing by two, that is ( ) ( ) ( ), , , 2 h h h s u v s u v e u v + − = . (9) the odd part is formed by subtracting the reversed function and dividing by two: ( ) ( ) ( ), , , 2 h h h s u v s u v o u v − − = . (10) any function may be split uniquely into even and odd parts and from the even and odd parts, if given, the original function may be uniquely reconstituted. obviously, the fourier transform can also be divided into even and odd parts: ( ),fe u v and ( , )fo u v . furthermore, ( ) ( ), re ,f fe u v s u v=    , (11) ( ) ( ), im ,f fo u v s u v=    . (12) by considering the definitions of the hartley and fourier transforms, we have ( ) ( ), ,h fe u v e u v= and ( ) ( ), ,h fo u v o u v= − . therefore, ( ) ( ) ( ), re , im ,h f fs u v s u v s u v= −       , (13) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( )  , , , , , 1 , , 2 , , . f f f h h h h h h s u v e u v j o u v e u v j o u v s u v s u v j s u v s u v = + = − = + − −   − − − −   (14) the generalisation of splitting single-variable functions into even and odd parts is the decomposition into symmetric and antisymmetric parts ( ) ( ), ( , ) ,symm antisymmg x y g x y g x y= + (15) with ( ) ( ) 1 ( , ) , , 2 symm g x y g x y g x y= + − −   , (16) ( ) ( ) 1 ( , ) , , 2 antisymm g x y g x y g x y= − − −   . (17) the real part of a 2d fourier transform is symmetrical, and the imaginary part is antisymmetric. therefore, ( ) ( ) ( ), , , 2 h h h g u v g u v e u v + − − = , (18) ( ) ( ) ( ), , , 2 h h h g u v g u v o u v − − − = (19) with ( ) ( ) ( ), , ,f h hg u v e u v j o u v= − , which is the 2d extension of equation (14) the cross-correlation ( ),r x y of the two functions ( ),p x y and ( ),q x y is ( ) ( ) ( ), , ', 'r x y p x y q x x y y dx dy + + − − = + +  , (20) where x and y are variable shift components along the xdirection and the y-direction respectively. evaluating the cross-correlation by means of the integral in equation (20) is complicated. this task can be significantly simplified in the fourier space, where ( ) ( )*( , ) , ,f f fr u v p u v q u v=  ; with ( , )fr u v , ( ),fp u v and ( ),fq u v being the fourier transforms of ( ),r x y , ( ),p x y , and ( ),q x y , respectively. therefore, ( )   ( ) ( ) -1 -1 *, fft fft , ,f f fr x y r p u v q u v= =  . (21) in general, the presence of noise makes the ‘exact’ localisation of the cross-correlation peak difficult. in other words, using the poc function, ( ) ( ) ( ) -1 , , fft , f f r u v r x y r u v    =      (22) and we obtain a good compromise between peak sharpness and noise tolerance. using the hartley transform, fr can be written as [34] ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) , , , , , 2 2 , , , , 2 2 , , , , 2 2 , , , , 2 , , , , 2 h h h h f h h h h h h h h h h h h h h h h r u v r u v r u v r u v r u v i p u v p u v p u v p u v i q u v q u v q u v q u v i p u v q u v p u v q u v p u v q u v p u v q u v j + − − − − − = − + − − − − −  = +    + − − − − −   −     + − −  − − =  − − − − −  + (23) where ( ),hr u v is the hartley transform of ( ),r x y . from equation (23), we obtain ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) , , , , , , , , , , , , h h h h h h h h h h h h r u v r u v p u v q u v p u v q u v r u v r u v p u v q u v p u v q u v + − − =  −  − − − − − = − −  −  − − (24) and eventually acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 90 ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) , , , , , 2 , , , , 2 h h h h h h h h h p u v q u v p u v q u v r u v p u v q u v p u v q u v  −  − − = − −  + − −  − − + (25) therefore, the cross-correlation can be computed by means of the hartley transform. indicating with fht, we have ( )   ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) -1 -1 , fht , , , , fht 2 , , , , 2 h h h h h h h h h r x y r p u v q u v p u v q u v p u v q u v p u v q u v =  −  − − =   − −  + − −  − −  +   (26) introducing the poc function, we have ( ) ( ) ( ) -1 , , fht , h h r u v r x y r u v    =      . (27) 4. results and discussion a number of fht algorithms have been developed [9], [35][39]. to test the proposed technique, an fht was implemented by using the algorithms reported in [40]. furthermore, the application of the hartley poc function, in comparison with fourier poc, was tested. figure 3 shows the poc obtained by two fingerprint patterns. in this example, the two images are each 512 × 512 pixels. the poc was computed in the fourier space by means of equation (22) and in the hartley space by means of equation (27). the fft poc and the fht poc are very similar, virtually identical. in banknotes and other documents, metal fibres are present, as security features, inside the paper pulp. these fibres can only be observed under ultraviolet light. after image enhancement and some segmentation processes, it is possible to obtain a binary controlling grid that indicates the position of the majority security fibres. the distribution of these metallic fibres is random and not reproducible; each banknote can be told apart from the others using this random distribution. obviously, the captured image has residual geometrical distortion and noise. therefore, a verification approach based on digital phase correlation calculation can be a good solution [20], [21]. figure 4 shows the poc used in the banknote identification. in this example, the fluorescent fibres (embedded in the paper) of two different 20-euro banknotes are compared. the poc functions obtained by means of fft and fht is practically identical. the examples illustrated do not prove that the fht is superior to the fft, but they do demonstrate that the fht is fully compatible with the fft. figure 5 shows an example of a drug package watermark realised with florescent points printed with ultraviolent lightsensitive ink. this watermark has a structure similar to the one used in speckle measurement [41]. in this example, the robustness of the poc approach against geometrical distortions and watermark partial disruption (also in the case of the hartley transform poc) is highlighted. in particular, the correct watermark identification is highly relevant even in case c), where more than half of the watermark was unavailable. figure 3. examples of the poc function obtained from two fingerprint images. figure 4. examples of the poc function obtained from two different banknotes. in this example, the two images are each 512 × 1024 pixels. figure 5. examples of the poc functions obtained from two different acquisitions of drug package watermarks. in this example, it is possible to note the robustness of the poc approach against geometrical distortions. acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 91 in general, to perform fft and fht, the same number of operations are required. in other words, fht requires a comparable number of steps to execute and is of comparable complexity to the fft. in any case, the fht algorithm requires only real arithmetic computations compared to complex arithmetic operations in any standard fft. therefore, the speed of performing an fht should be about twice as fast as the fft. however, it is hard to compare the run times of these two programs for the transform process itself because the programs used in this effort are implemented differently. on the other hand, the use of only real-value functions implies fewer computational resources and less computer memory; usually, the memory requirement by typical fft algorithm is twice as high as for fht. 5. conclusion in this paper, we have shown the correspondence between the poc function obtained by means of fft and fht. the hartley transform is similar to the fourier transform, but it is free from the need to process complex numbers. furthermore, the fht performs the transformation itself, and the convolution and the cross-correlation are undertaken with fewer additions and multiplications than fft. using fht to compute cross-correlation, there is no loss of information, and the results are identical to those obtained by fft. unfortunately, the numbers of all other operations are the same. differences in time consuming exist between fht and complex-valued fft, and in any case, even compared with newly real-valued fft algorithms, the possibility to use an identical programming solution for forward and inverse transformation is still an important point for using fht instead of fft in image correlation. in addition, the fact that the hartley transform does not use complex numbers leads to the need for less memory to store numbers than the fourier one. the hartley transform can therefore be considered a valid alternative to fft, particularly useful when a greater efficiency in memory requirements is needed, as, for instance, in embedded systems and smartphone apps. references [1] s. bochner, k. chandrasekharan, fourier transforms, annals of mathematics studies (am-19), princeton university press, 41 william street, princeton, new jersey, 08540, usa, 1950, isbn: 9780691095783. 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in final form august 16, 2018; published october 2018 copyright: © 2018 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: giovanni e. gigante, e-mail: giovanni.gigante@uniroma1.it 1. introduction many artefacts have a multilayered structure: not only all types of paintings, which are composed by at least two-three layers, but also gilded or silvered metals, tumbagas (poor gold-alloy subject to depletion gilding), bronzes with patina, corroded silver and so on. the most useful analytical information about these multilayered structures can only be three-dimensional, although the third dimension is of the order of mm for paintings and of a few micrometers for metals. 2d scanning of paintings using energy-dispersive x-ray fluorescence (edxrf) analysis, are currently carried out, by using collimated x-ray beams (µedxrf). in such a manner, maps of chemical elements in a painting are obtained, strongly increasing the level of information. 2d scanning are carried out in the best manner by using synchrotron radiation, but also xray tubes may be employed, due to the general high concentration of chemical elements in pigments [1, 2, 3,4]. edxrf-analysis is also able to reconstruct a 3d-map of the elemental distribution by a confocal set-up realized by adding two optics in the excitation (synchrotron light) and in the detection channel [5, 6, 7, 8]. however, this approach requires a high intensity x-ray source and the related device is, of course, not portable. therefore, an alternative approach has been developed, based on the use of a portable edxrf-equipment [9] and accurate analysis of the ratios of x-rays emitted by the chemical elements, which depends on the position and, in many cases, can help to reconstruct a 3d-image of the artefact. figure 1 shows how edxrf-analysis versus depth works in confocal configuration and using the x-ray ratios. the edxrf-spectrum of an irradiated and analyzed volume of a painting contains the x-ray peaks of all elements present in that volume, with a dependence on the depth from which x-rays of the element are coming. in several cases, and when the number of pigment layers is not too great, this dependence of xray peaks from the depth can be determined, by carefully processing the x-ray spectrum, in terms of elemental kα/kβ and/or lα/lβ-ratios [10]. in fact, these ratios depend on the location in the corresponding elemental layer, according to all the attenuation processes to which the x-rays emitted by this abstract a 3d map of chemical elements distribution from energy-dispersive x-ray fluorescence (edxrf) analysis would be a perfect conclusion in a detailed study of any type of artifact. this map can be easily achieved by using synchrotron light as source of radiation, and microoptics both at the source and at the detector. in such a manner a micro-voxel is irradiated and detected, which can be at any depth with respect to the surface of the artifact. this method is effective but needs a high-intensity x-ray source; therefore, its use in archaeometry is limited. an alternative method is proposed in the present paper, which uses a portable edxrf-device to measure the altered kα/kβ or lα/lβ-ratios, which allow to locate the chemical elements. several examples are described. acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 9 element are subject, i.e. self-attenuation in its own layer, and attenuation by the other superimposed layers. the real novelty of this work is in the proposal of a procedure for reconstruction of 3d images from xrf spectra processing, with the extraction of characteristic peaks for each element and the calculation of the thickness of each layer by systematically using the peak ratios. the image is not reconstructed automatically but iteratively on the base of structural model of artefact that could be modified taking into account the obtained results. in this paper, some examples are given of 3d reconstruction of paintings and of alloys subject to some surface alteration or discontinuity. 2. theoretical background 2.1. kα/kβ and lα/lβ –ratios kα are defined as the x-rays produced by transitions from the n = 2 to n = 1 atomic levels. kβ are the x-rays produced by transitions from the n = 3 to n = 1 atomic levels [10, 11]. lα may be defined as the x-rays produced by transitions from the n = 3 to n = 2 level, and lβ are the x-rays produced by transitions from the n = 4 to the n = 2 level [10, 11]. the kα/kβ and lα/lβ–ratios for all elements have been calculated and in several cases were measured [13]. theoretical values are calculated for infinitely thin samples, i.e. when secondary interactions in the sample are negligible. this corresponds to very low thicknesses, generally lower than 1 µm. the ratios kα/kβ and lα/lβ can vary: when the considered elemental layer is not “infinitely thin” (self-attenuation effect); this effect depends on the selfattenuating material and thickness [11, 13]; when a layer of different material is covering the considered element; this effect depends also on the attenuating material and thickness [10]. in a multi-layered sample, kα/kβ and lα/lβ ratios allow, therefore, to measure the position of any element and the position and thickness of the corresponding layer. the thickness can be also determined from the attenuation of individual kα or kβ (or lα and lβ). this is strictly true for metal layers [10]. for pigment layers, where the pigment contains one or more chemical elements, an “equivalent thickness” may be determined, i.e. the elemental (metal) thickness which would give the same attenuation as the pigment [10]. in figure 2 the average kα/kβ ratio versus atomic number z, for infinitely thin (blue rhombi) and thick (red squares) single element layer. the points for thin layers were deduced from various calculations. the points for thick layers were calculated according to the following equation: (kα/kβ)thick = (kα/kβ)thin · µ2 / µ1, where µ1 and µ2 are the attenuation coefficients at energy of kα and kβ radiation respectively. the possibility to measure an elemental thickness by self-attenuation is related to the difference between thin and thick samples. the area between thin and thick curves defines the self-attenuation region, while the region below the thick sample defines the attenuation by a second layer, when the first one is thick. when the first layer is not thick, then the attenuation region starts at the corresponding value. the green points in the figure show the kα/kβ ratio of zn (white zinc pigment “wzp” of intermediate thickness), of wzp covered by a red pigment “rp” containing se and cd (wzp+rp), and finally of wzp + rp covered by a green pigment containing cu and br. figure 3 shows the average lα/lβ ratio versus atomic number z, for infinitely thin (blue) and thick (red) single element layers. the points for thin layers were deduced from various calculations. the points for thick layers were calculated according to the following equation: (lα/lβ)thick = (lα/lβ)thin · µ2 / µ1, where µ1 and µ2 are the attenuation coefficients at energy of lα and lβ radiation respectively. the possibility to measure an elemental layer thickness from self-attenuation is connected to the different response between thin and thick samples. the area between thin figure 1. left image: micro-edxrf-analysis in confocal configuration: a micro-volume is identified by two optics at the x-ray source and detector, and a 3d-map is obtained when moving the measuring table orthogonally with respect to the artefact to be scanned. voxels at any depth are then analyzed. right image: all chemical elements excited in the truncated cone can reach the detector and no information is obtained about the originating depth, unless the internal x-ratios are processed; in fact, x-rays emitted by an internal point (p) are characterized by their altered kα/kβ or lα/lβ-ratios, which allows to correctly localize it and, finally, to obtain a 3d-image. figure 2. kα/kβ peaks ratios vs atomic number for thin and thick samples [9]. green point are results of measurements of zn k peaks ratios (see figure 7). figure 3. lα /lβ ratios vs atomic number for thin and thick samples [10]. acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 10 and thick curves defines the self-attenuation region, while the region below the thick sample defines the attenuation by a second layer, when the first one is thick. when the first layer is not thick, then the attenuation region starts at the corresponding value. 3. experimental set-up the experimental set-up for scanning a painting to obtain a 3d-image is mainly composed of (figure 4): a well collimated x-ray beam, mono energetic or bremsstrahlung, generated by a synchrotron or by an x-ray tube [9]; this incident beam irradiates the painting surface orthogonally; a well collimated x-ray detector, with excellent energy resolution and efficiency [14]; the detector collects x-rays at an angle ϑ as close as possible to the normal of the painting surface; the angle ϑ between surface normal and detector; a x-y movable table to translate the painting in the x-z plane; a sophisticated software to process with high accuracy and precision the x-ray spectra [15]; more into details, a complex set of x-rays is generally collected in the detector and processed in the pulse height analyzer. this complex spectrum requires to be processed by the software, in order to determine the chemical elements present in the picture and to quantitatively evaluate the kα/kβ and/or lα/lβ ratios of these elements (figure 5). a test sample was realized using the following pigments spread on a wood substrate (figure 6): a first layer of white zinc of average thickness d1; part of the first layer covered with a protective plastic layer, of mean thickness d2; a second layer of a green pigment composed of copper and bromine, of mean thickness d3; a third layer of red pigment containing selenium and cadmium, of mean thickness d4. the thickness inhomogeneity of the pigments was considered; for example, a layer of white zinc (area 4 in figure 7) was measured in twenty different areas, giving the following result: zn(kα/kβ) = 6.0 ± 0.2, value internal to the selfattenuation area of figure 2. a linear scan of the painting surface is shown in figure 7, where the values of zn(kα/kβ) versus position are shown and where scanning was carried out in areas 1 to 4. changes between points of the same pigment reflects the variable thickness of the pigments layer. the following average values were measured: zn(kα/kβ) = 6.0 ± 0.1 (zinc-white, area 4 in figure 7), zn(kα/kβ) = 5.65 ± 0.2 (zinc-white covered by a protective resina, area 1 in figure 4), zn(kα/kβ) = 4.6 ± 0.25 (zn crossing the br-pigment, area 3 in figure 5), zn(kα/kβ) = 2.9 ± 0.3 (zn crossing the cd-se pigment plus the br-pigment, area 2 in figure 7). then, from the relative intensity of zinc-lines, attenuated and not, the approximate thickness of br and cd-pigments can be calcolate; it ranges between 80 µm and 110 µm for the cd-se pigment and between 60 µm and 100 µm for the br-pigment. similar results were obtained using a test sample involving l transitions, having the following sequence of pigment layers (starting from the wood substrate): white lead, bromine pigment, cadmium – selenium pigment. scanning of this sequence gave the results for the pb(lα/lβ) ratio shown in figure 8. differences of pb(lα/lβ) ratio inside the same pigment reflects the thickness variation of the pigment layer. figure 4. experimental set-up to scan a painting composed of various pigment layers. it is composed of a collimated x-ray beam emitted by an xray tube and a collimated si-drift detector that collect the secondary fluorescent x-rays. the intersection of the two beams identifies the irradiated and analyzed voxel of the painting. by moving the painting in the xz plane, the whole painting is edxrf-analyzed. figure 5. layered structure of a painting composed of various pigments, and detection procedure. figure 6. pigment layers; from the surface: zinc-white, br-cu pigment, cd-se red pigment. on the left side, zn-white is covered by a protective resin. analysed voxel painting x-z translation collimated x-ray beam collimated fluorescent x-ray beam x-ray tube x-ray detector table with x-ray tube and detector orthogonal to the painting  acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 11 the following average values were measured: pb(lα/lβ) = 0.8 ± 0.02 (lead-white), pb(lα/lβ) = 0.45 ± 0.03 (pb crossing the br-pigment), pb(lα/lβ) = 0.36 ± 0.03 (pb crossing the cd-se pigment plus the br-pigment). in many cases a 3d-map is not required, and the described methods for the evaluaton of the self-attenuation and attenuation effects, were largely employed in the case of gilding, silvering, tumbaga and so, by using single measurements only [10, 16, 17, 18]. the same method was applied in the case of the halos of giotto’s fresco in the chapel of the scrovegni in padua, where the multilayered structure was reconstructed, on the basis of k and l-internal ratios [19]. in conclusion, accurate evaluation of kand l-internal ratios when carrying out edxrf scanning of a multilayered artifact is able to generate 3d imaging. 4. proposed procedure to obtain a 3d image the following step-procedure is proposed to obtain a 3dimaging of a painting: 1) scanning of the painting or of an area of a painting with an adequate space resolution 2) processing the edxrf-spectra with a proper software 2.1) determining the elements of interest present in the spectra 2.2) determining the area of the k(kα and kβ) or l(lα and lβ) peaks of these elements 2.3) analysing the kα/kβ and lα/lβ-ratios with a dedicated software 3) according to point 2.3, classification of the elements versus depth 4) according to point 2.2, calculation of the layer thickness 5) according to point 2.2, a 3d-map of chemical elements can be constructed. 5. conclusions the simplicity with which a scanning system can currently be realized and the information that can be obtained by using it in the case of a stratified object (such as a painting), implies that its use will quickly become a rule rather than an exception. in this context it is important, as previously done for many microanalytical techniques, to verify which additional information can be extracted from the acquired data, in addition to elemental maps of various elements at the surface. this work shows how it is possible to obtain, in addition to the traditional elemental maps, also maps of the ratios between peaks of the same spectrum that contain various additional information. references [1] m. mantler, m. schreiner, f. weber, r. ebner, f. mairinger, advances in x-ray anal., 1992, 35, pp. 987–993. [2] k. h. a. janssens, f. c. v. adams and a. rindby, microscopic x-ray fluorescence analysis, wiley, chichester, 2000, isbn 0471-97426-9. [3] m. alfeld, j. v. pedroso, m. van eikema hommes, g. van der snickt, g. tauber, j. blaas, m. haschke, k. erler, j. dik, k. janssens, a mobile instrument for in situ scanning macro-xrf investigation of historical paintings, j. anal. at. spectrom., 2013, 28, pp. 760-767, doi:10.1039/c3ja30341a. [4] m. alfeld, j. a. c. broekaert, mobile depth profiling and subsurface imaging techniques for historical paintings – a review spectrochimica acta part b (2013) 88 pp. 211–230, doi:10.1016/j.sab.2013.07.009. [5] d. l. howard, m. d. de jonge, d. lau, d. hay, m. varcoecocks, c. g. ryan, r. kirkham, g. moorhead, d. paterson, d. thurrowgood, anal. chem., 2012, 84, 3278–3286. [6] j. dik, k. janssens, g. van der snickt, l. van der loeff, k. rickers, m. cotte, anal. chem., 2008, 80, 6436–6442. 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[10] r. cesareo, 2000.x-ray physics: interaction with matter, production, detection, in: la rivista del nuovo cimento, ed. compositori, bologna. figure 7. linear scanning of zinc (kα/kβ) ratio, for the set of pigments shown in figure 5. figure 8. linear scanning of pb(lα/lβ) ratio. from left: pb(lα/lβ) ~ 0.8, for pb alone; pb covered by a pigment containing cd, plus a pigment containing br, pb(lα/lβ) = 0.36; pb covered by a pigment containing br, pb(lα/lβ) = 0.45; pb alone, pb(lα/lβ) =0.8. 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 8 9 z n ( k α / k β ) position (mm) 1 3 4 2 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 0 1 2 3 4 5 6 7 8 9 10 11 12 13 p b (l α / l β ) position (mm) cd + br br pb l acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 12 [11] b. ertugral et al., kβ/kα x-ray intensity ratios for elements in the range 16-92 excited by 5.9, 59.5 and 123.6 kev photons, radiation phys. and chem. 2001 76 pp. 15-22 doi:10.1016/j.radphyschem.2006.04.003. 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[18] cesareo, r. non-destructive edxrf-analysis of the golden haloes of giotto’s frescos in the chapel of the scrovegni in padua nucl. instrum. and methods in phys. res. b (2003) 211 pp.133-137, doi:10.1016/s0168-583x(03)01165-0. automatic measurement of the hand dimensions using consumer 3d cameras acta imeko issn: 2221-870x june 2020, volume 9, number 2, 75 82 acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 75 automatic measurement of hand dimensions using consumer 3d cameras daniele marchisotti1, pietro marzaroli1, remo sala1, michele sculati2, hermes giberti3, marco tarabini1 1 department of mechanical engineering, politecnico di milano, via la masa 1, milan, italy 2 private medical doctor, bergamo, via ferruccio galmozzi 12, italy 3 dipartimento di ingegneria industriale e dell'informazione, university of pavia, via ferrata 1, pavia, italy section: research paper keywords: hand; kinect v2; intel realsense; ni labview; diet; calibration; measure. citation: daniele marchisotti, pietro marzaroli, remo sala, michele sculati, hermes giberti, marco tarabini, automatic measurement of the hand dimensions using consumer 3d cameras, acta imeko, vol. 9, no. 2, article 12, june 2020, identifier: imeko-acta-09 (2020)-02-12 editor: dušan agrež, university of ljubljana, slovenia received may 2, 2020; in final form june 12, 2020; published june 2020 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this paper was supported by politecnico di milano and by private doctor studio of michele sculati. corresponding author: daniele marchisotti, e-mail: daniele.marchisotti@polimi.it 1. introduction there are several applications in medicine that require knowledge of the dimension of the human hand. oedema of the hand, resulting in an increase of the hand volume, often accompanies upper-extremity lymphedema, which develops after breast cancer-related surgical interventions [1][2]. in this case, measurement of the hand volume and its changes over time can provide objective measures about the effectiveness of the therapy; however, to date, this measurement is not a part of the routine assessments due to the absence of a readily available method for the estimation of the hand volume. given the rapid increase in prevalence of obesity [3] and the costs associated therewith [4], the benefits of a low-cost measurement system would also be consistent in the assessment of correct food portions. in fact, food portions have increased their energetic content from two to eight times in the past 40 years [5], and it has been found that body weight is related to food portion size [6]. great benefits may be derived from assessing correct food portions using a simple intuitive comparison [7][8], and the size and geometric parameters of the hand can be used for this purpose [9][10]. it could be possible to determine food portions also with a portable reference object (a ball or a cube), but the hand offers difference reference volumes and areas (fist, palm, fingers) and cannot be left at home or forgotten somewhere. since the anthropometric measurements of the hand vary greatly from person to person (up to 400 % according to [11]), it is necessary to verify the food volume to obtain precise diet indications. different measurement methods were proposed for the automatic identification of one or more geometrical parameters of the hand. the different technologies that can be used for the abstract this article describes the metrological characterisation of two prototypes that use the point clouds acquired by consumer 3d cameras for the measurement of the human hand geometrical parameters. the initial part of the work is focused on the general description of algorithms that allow for the derivation of dimensional parameters of the hand. algorithms were tested on data acquired using microsoft kinect v2 and intel realsense d400 series sensors. the accuracy of the proposed measurement methods has been evaluated in different tests aiming to identify bias errors deriving from point-cloud inaccuracy and at the identification of the effect of the hand pressure and the wrist flexion/extension. results evidenced an accuracy better than 1 mm in the identification of the hand’s linear dimension and better than 20 cm3 for hand volume measurements. the relative uncertainty of linear dimensions, areas, and volumes was in the range of 1-10 %. measurements performed with the intel realsense d400 were, on average, more repeatable than those performed with microsoft kinect. the uncertainty values limit the use of these devices to applications where the requested accuracy is larger than 5 % (volume measurements), 3 % (area measurements), and 1 mm (hands’ linear dimensions and thickness). mailto:daniele.marchisotti@polimi.it acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 76 measurement of the hand shape for biometric identification have been reviewed by duta [12]. existing works typically focus on the identification of the hand silhouette (hand shape systems) or on the identification of specific geometric features (hand geometry systems). the measurement chain of both these systems is based on cameras or scanners for capturing a subject’s hand image and compares this information with the database records to verify the subject’s identity. after the review of duta was published, several works focused on similar topics. sanchezreillo et al., in their work [13], described a system capable of extracting hand parameters from a 2d colour image. kumar et al. [14] proposed the extraction, from a single image, the palmprint and the hand geometry. the different hand features are then used to verify the capability of classification on a dataset of 100 users. the length of fingers is measured in several studies; the most widely used finger parameters are the distal extent of the fingertips and the length of the fingers. in a pilot study, peters et al. [15] used a device first built by george [16] to identify the distance from the middle fingertip to the index and ring fingertips. the device is based on a sliding calliper with a resolution of 0.5 mm. manning et al. [17] took photocopies of hands and made measurements based on the photocopies. the uncertainty of the method based on photocopies was evaluated [15] by assessing the measurement reproducibility. four observers took ten measurements of the hands of four subjects. the observed parameters were the finger lengths of the index, middle, and ring fingers on both hands and from these parameters it was possible to compute the difference in length among fingers. the standard deviation of the length measurement across the fingers and the different subjects was 0.64 mm. the standard deviation of the tip measure was 0.52 mm, very close to the resolution of the instrument based on the calliper. the hand volume has been measured by hughes using archimedes principle [18]: the technique involves the immersion of a hand in a water container placed on an electronic balance, with errors lower than 0.3 %. mayrovits et al. [19] proposed the estimation of the hand volume using geometric algorithms. the hand’s linear dimensions were measured using a calliper. the hand volume was calculated using an elliptical frustum model. the results of the model were compared to volumes determined by water displacement, with errors in the estimation typically lower than ±30 ml for hands with volumes between 220 and 600 ml. the performance of a biometric system is typically evaluated by verifying the correct classification rates of images; however, as evidenced by duta in [12], a comparison between the performances of different systems is almost impossible for different reasons. from a metrological point of view, the most important issue is that the different experimental setups used for the acquisition strongly affect the classification capabilities. however, the different works do not focus on the accuracy of the systems used to acquire the images. furthermore, all the biometric systems are based on 2d hand images, thus preventing their usage in all the applications that require knowledge of the hand thickness or volume. a typical example is dietary applications, which are those for which the systems described in this work were developed. this article proposes the extraction of the following features (hereinafter parameters) necessary for the identification of food portions in a volume diet: 1) volumes a) hand volume b) palm volume 2) areas a) area of the hand b) area of the palm c) middle finger area 3) dimensions a) hand width b) hand length c) index, middle and ring fingers width d) thumb length e) fingers’ length f) height (thickness) of the fingers, computed at the nails or at the first phalanx many of the above parameters have no clear definition, and the presence of soft tissues implies a potentially relevant loading effect in the presence of contact measurement devices, such as the callipers. the main idea of our work is to derive the above listed parameters from point clouds describing the hand dorsum. the accuracy of different parameters is assessed by the metrological characterisation of two consumer 3d cameras. although in the literature there are several works focused on the identification of metrological performances of 3d cameras [20][23] and the classification capabilities of point cloud-based methods for hand silhouette identification, in this work, we focus on the accuracy in the identification of hand geometrical parameters using point clouds acquired by commercial 3d cameras. the article is structured as follows: the measurement method is described in section 2; experimental results are presented in section 3 and discussed in section 4. the study’s conclusions are drawn in section 5. 2. measurement method the measurement system described in this work is based on acquiring the point cloud of the hand dorsum with the two 3d vision systems, i.e. kinect v2 and the intel realsense d415 and d435. 2.1. sensors’ characteristics kinect v2 is a time-of-flight (tof) camera that uses infrared (ir) emitters and sensors to reconstruct a 3d scene from the elapsed time between the emission of a light ray and its collection after reflection from a target. the metrological qualification of the camera [20] evidenced that the measurement uncertainty increases linearly with the distance between kinect v2 and the measured point. the repeatability (expressed as standard deviation on the distance of planar objects perpendicular to the optical axis) is 1.2 mm at 1.5 m and 3.3 mm at the maximum distance (4.2 m). moreover, there is a relationship between the measurement uncertainty and the radial coordinate of the sensor, as the ir light cone of emission is not homogeneous [20][21], and this might affect the volumes, areas, and dimensions of the hand parts. the minimum working distance of kinect is 0.70 m. in these conditions, the field of view implies an observed area that is much larger than the hand size (a suitable measurement area can be defined by the size of an a4 piece of paper). in order to limit the effects of the different reflectivity of the reference surface and of the human skin, the subject is required to wear black silk gloves and to keep the instrument far from obstacles that may reflect or scatter ir radiation, as described in [23]. two 3d cameras belonging to the intel realsense d400 series were initially tested (d415 and d435). the cameras are acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 77 based on stereo vision depth technology and consist of left and right imagers assisted by an ir projector. the latter emits an invisible static ir pattern to improve depth accuracy in scenes with low texture. the 3d scene is reconstructed by correlating points on the left and right images using built-in calibration equations [22]. the intel realsense data acquisition program allows for the selection between different settings for the 3d scene reconstruction: following the user manual, all the acquisitions were performed with the ‘short-range’ depth preset (given the fixed camera focal length and the necessity of filling scene with the subject hand) and the automatic exposure computation. the optimal distances for obtaining hand 3d images using realsense d435 and d415 cameras are 0.4 m and 0.5 m respectively. distances are slightly larger than the minimum distances: 0.35 m and 0.45 m. the sensors’ characteristics are summarised in table 1. kinect v2 has a smaller depth measurement range with respect to realsense cameras. the kinect v2 resolution and sampling rate are lower than those of the real sense cameras, and its minimum focusing distance is larger. the kinect v2 field of view (fov) is similar to that of the d415 and smaller than that of the d435. the smaller fov of the d415 camera makes it more suitable for measuring small objects and was consequently chosen for the identification of the hand size. preliminary tests evidenced that realsense cameras are less sensitive to lighting conditions and are not affected by obstacles close to the target, although the light reflection on the plane that supports the hand often leads to disturbances on the point cloud. 2.2. measurement procedure the geometry of the hand is obtained by subtracting the 3d image of the plane that is supporting the hand from the 3d image of the hand laying on the plane. with reference to the coordinated axes of figure 1, the measurement process consists, therefore, of the identification of two point clouds: 1. the point cloud of the reference plane, with coordinates [xp], [yp], [zp]; 2. the point cloud of the hand lying on the reference plane, with coordinates [xh], [yh],[zh]; the point cloud to be analysed, whose coordinates are [x], [y], [z], is defined by: (1) 2.3. algorithms fit-for-purpose algorithms were implemented in labview 2017 in order to recognise the main parts of the hand and to compute the parameters listed in section 1. the automation of the measurement process allows for the reduction of the variability due to the observer; although, the lack of a standardised model for the identification of specific hand parameters might introduce a discrepancy between different measurement algorithms. the dimensions, areas, and volumes of the hand were computed with the algorithms that are extensively described in [23]: the different hand features are identified using simple image processing algorithms that are based on the extraction of the hand silhouette identified from the binarized 2d image of the hand by detecting the borders of the hand. the flowchart of the method is shown in figure 2. multiple point clouds of the hand are acquired by the sensor. a temporal average is used to derive the point cloud for analysis. the reference plane is subtracted from the observed scene, as described in section 2.2. the image is binarized to obtain the hand area; a particle filter was used to delete measurement                         h p h p h p x = x x y = y y z = z z table 1. kinect v2 and realsense d400 characteristics. characteristic kinect v2 realsense d415 realsense d435 working principle time-of-flight active ir stereo active ir stereo depth range 0.7-4.2 m 0.4-10 m 0.2-10 m max depth resolution 512x424 1280x720 1280x720 max colour resolution 1920x1080 1920x1080 1920x1080 fov h: 70°, v: 60° h: 69°, v: 42° h: 91°, v: 65° max acquisition frequency 30 hz 90 hz 90 hz working conditions indoor indoor/outdoor indoor/outdoor figure 1. scheme of the plane and image reference systems. figure 2. flowchart of the analytical method for 3d point clouds. acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 78 artefacts. conventional image processing algorithms (image derivatives or contours extraction starting from the binary image) were used to identify the hand features parameters described in section 2.1 [23]. 2.4. calibration tests have been performed by comparing the lengths, distances, and volumes measured with the 3d cameras with the standard measurement systems; more than 50 subjects participated in the experiments, which were performed in accordance with the politecnico ethical guidelines. 2.4.1. thickness the hand thickness measured by the 3d cameras was compared to that which was measured by a triangulation laser sensor micro-epsilon optoncdt 1402, with a measurement range of 50 mm and a resolution of 5 µm [23]. the results reported in this work refer to the thickness measured at the centre of the middle fingernail; the finger represents a challenging position for measuring the thickness, given that small pressure variations at the fingertip result in a large variation of the measured height. however, similar experiments were performed also in different hand positions and provided similar results. the thickness of different subjects’ right hands (18 for the realsense, 33 for the kinect) was measured; given that tests were performed in different time periods, the subjects were not the same, thus preventing a direct comparison between the two measurement systems (described for a single subject in section 2.4.4). 2.4.2. volume the reference volume of the hand was computed by immersing the hands of different subjects (33 for the realsense, 30 for the kinect) in water using a graduated beaker according to the procedure described by [18]. the resolution of the graduated scale was 10 ml. furthermore, in this case, since the tests were performed in different experimental sessions, the subjects were not the same. 2.4.3. disturbances a third series of tests were performed to investigate the effect of disturbances. the quantities that may worsen the measurements’ reproducibility are the position of the forearm (i.e. the wrist flexion/extension) and the pressure between the hand and the supporting surface. both these factors, because of the compliance of the hand soft tissues, modify the local height of the hand and consequently might vary the hand volume. tests were performed with a single subject under reproducibility conditions: the hand volume has been measured with the kinect v2 and realsense d415 cameras upon varying: a) the elbow extension (i.e. the forearm angle with respect to the horizontal plane); the wrist extension angles were -10 °, 0 °, 10 °, 20 °, 30 °; a total of 40 measurements were performed; b) the hand pressure against the reference plane (0 kpa, 1.30 kpa, 1.94 kpa, 2.65 kpa, 3.31 kpa, 3.97 kpa). the setups for the identification of the effects of the forearm orientation and of the hand pressure are shown in figure 3. tests were performed both with the kinect v2 and realsense d415. wrist extension was controlled by positioning supports of different heights below the elbow. the extension α has been computed as the arctangent of the support height h divided by the forearm length w: (2) the hand contact pressure was obtained by dividing the net force measured by the scale (difference between the force measured by the balance f and the device weight w) by the hand area measured by the proposed 3d measurement system a. (3) given that the contact area is smaller than the total hand area (because of the palm concave shape), the reported values are an overestimation of the actual hand pressure; however, reporting pressure data allows for a comparison of the measurement results with possible forthcoming studies. 2.4.4. system comparison the final analyses were performed to compare the results of measurements performed with the kinect and realsense cameras. different parameters of the hands of a single subject ( )arctan /h w = f w p a − = figure 3. scheme of the setup for the identification of the disturbances generated by the forearm angle (a) and hand pressure against the surface (b). figure 4. correlation between the height of the hand measured by the laser sensor and the height of the hand measured by realsense d415. (a) (b) acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 79 were automatically extracted from 30 repeated measurements by means of the computation algorithms described in [23]. the compatibility of measurements was verified through the hypothesis testing about the equality of the means using minitab. 3. results 3.1. hand thickness calibration the fingers’ thickness ranged between 9 and 12 mm. the root mean square (rms) of the difference between the height measured with the kinect and the height measured by the laser is 1.1 mm. this value is compatible with the kinect resolution (1 mm) and with the instrument uncertainty identified in [20], which was 1 mm when the measurand was placed at 0.75 m from the sensor. as shown in figure 4, hand thickness h can be determined starting from the thickness measured by the kinect hk using the following linear regression model: (4) the standard error of the estimate is 0.85 mm. the r² value is 36.8 %; such a low value reasonably derives from the large variability of the measurand (uncontrolled pressure of the fingers) and to the limited variation of the fingers’ thickness in comparison with the kinect resolution. the first order term of the regression model is between 0.9 and 0.95 with a 95 % confidence level. the distribution of residuals and the probability plot of standardised residuals evidenced the validity of the proposed regression model [25][28]. the same calibration was performed for the realsense camera. the rms of the difference between the height measured with the realsense and the height measured by the laser is 1.1 mm. this value is compatible with the expected resolution for realsense d415 at a 0.5 m distance and is equal to the value obtained with the kinect. as shown in figure 5, similar to what was performed with the kinect, the finger thickness h was fitted by a linear regression model starting from the thickness measured by the realsense hrs: (5) the standard error of the estimate is 0.88 mm. r2 is also low in this case (47.7 %). the distribution of residuals and their probability distribution evidenced the validity of the regression model. 3.2. volume calibration the results of the calibration of kinect are shown in figure 6. hand volumes ranged between 180 and 500 cm3. the rms of the difference between the volume measured with the kinect v2 and the volume measured by the beaker is 35 cm3. the hand volume v (in cm³) can be estimated starting from the volume vk measured by the kinect using the following quadratic regression model: (6) the standard error of estimate is 17 cm3. the r² value is 96.7 %, evidencing a better correlation with respect to the thickness data presented in the previous paragraphs. the distribution of residuals and the probability plot of standardised residuals evidenced the validity of the quadratic regression model; conversely, the adoption of a linear model led to a parabolic distribution of residual versus fits. a similar regression operation was performed with the realsense camera, and the results are shown in figure 7. the rms of the difference between the volume measured with the realsense d415 and the reference volume was 64 cm3. to increase the accuracy of the volume results, data were modelled with a linear regression model: (7) the standard error of estimate was 16 cm3, i.e. comparable to the value of 17 cm³ obtained with kinect. r² was 95.2 %, and the analysis of the residuals evidenced the validity of the linear model. 3.3. influencing quantities the results (figure 8 and figure 9) evidenced the necessity of controlling both the forearm position and the contact pressure. 0.93* k h h= 3.53 0.62 rs hh = +  2 48.44 0.63 0.00095 k k v v v= +  +  33.71 0.75 rs v v= +  figure 5. correlation between the height of the hand measured by the laser sensor and the height of the hand measured by kinect v2. figure 6. correlation between the hand volume measured by the kinect camera and the one measured with the reference system. figure 7. correlation between the hand volume measured by the realsense camera and the one measured with the reference system. vk [cm³] v [c m ³] vrs [cm³] v [c m ³] acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 80 the wrist extension (figure 8) affects the hand volume in a complex manner. the volume is maximum when the wrist is in neutral position, i.e. when the forearm is parallel to the plane supporting the hand palm. in the presence of elbow flexion or extension, the measured volume decreases, reasonably because of the change in pressure distribution on the hand palm and fingers. results also evidence that kinect v2 seems more sensitive to the elbow angle with than realsense d415; however, given that the tests were performed by different subjects, it is not possible to conclude that the bias is generated by an instrumental effect. the effect of pressure is summarised in figure 9; the increase in contact pressure results in a decrease of measured volume because of an average reduction of the hand thickness deriving from the compression of the hand soft tissues. the trends measured by the two measurement systems are comparable, although the volume reductions measured by the kinect (figure 9 a) are, on average, larger than those measured by realsense. the reduction of volume in correspondence to a pressure of 1.48 kpa is 13.8 % for kinect and 9.5 % for realsense; similar differences occur at higher pressures. 3.4. systems comparison the comparison between the hand’s parameters extracted in 30 repeated measurements from the two 3d cameras are shown in table 2. data show that the differences between the average measurements obtained with the kinect v2 and the realsense range between 1 % (hand area, flat fist volume) and 16 % (index finger thickness). in most cases (6 out of 9), the hypothesis testing about the equality of the means of the measurements performed by the two measurement systems lead to a rejection of the null hypothesis k=rs. the most critical measurement was the thickness of the index: the difference between the two measurement systems was large in percentage (16 %) but was comparable to the resolution of the two measurement systems (2 mm). measurements of the kinect were performed wearing a silk glove because of the necessity of keeping the sensor as close as possible to the hand (to grant a decent resolution) and to avoid saturation problems in specific hand areas. this did not lead to a systematic overestimation of hand dimensions/areas/volumes. 4. discussion the results presented in this work evidence the accuracy limitations of the use of commercial 3d cameras for the identification of hands’ geometrical parameters. both the calibrations versus the golden standard and the comparison between two different measurement systems evidenced accuracy limitations that are in the order of millimetres for the hand thickness, of a few cm2 in area measurements, and 20 cm3 for volume measurements. the volume measurements were affected by the wrist extension and by the hand pressure against the surface; consequently, in order to increase the measurement reproducibility, it is necessary to perform all the measurements with a neutral wrist extension (and lateral deviation) and by controlling the hand pressure. the possibility of compensating the disturbances given by these factors seems limited and deserves future investigations. the adoption of the fit-to-purpose calibration procedure allowed reducing the measurement uncertainty of both the systems. the kinect v2 thickness calibration improved the accuracy of the measurement system, reducing the rms of the errors from 1.1 mm to 0.8 mm; for the same system, the volume calibration reduced the standard uncertainty from 35 cm3 below 20 cm3. the realsense d415 volume calibration lowered the rms of errors from 64 cm3 to 16 cm3 for hand volume and from 1.1mm to 0.9 mm for thickness. these values are tolerable for figure 8. boxplot of the hand volume measured by kinect v2 (a) and realsense d415 (b) upon varying the forearm angle. data refer to two different groups of people. figure 9. boxplot of the hand volume measured by kinect v2 (a) and the realsense d415 (b) upon varying the pressure between the hand and the supporting surface. data refer to two different groups of people. forearm angle [°] v [c m ³] forearm angle [°] v [c m ³] (a) (b) pressure [kpa] v [ cm ³] v [ cm ³] pressure [kpa] (a) (b) acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 81 the identification of the food portions and are lower than the values reported in the literature for measurements based on the archimedes principle (around 30 ml). the possibility of using more accurate sensors or scanning both sides of the hand is currently under evaluation and will be the topic of forthcoming studies. the comparison between the two measurement systems evidenced that the intel realsense d 415 camera allows for the acquisition of more repeatable results (average cov 3 %) with respect to the kinect v2 (average cov 5 %), thanks to the higher resolution and to the lower measurement distance. the main limitation in the use of the kinect v2 was related to the necessity of wearing black gloves and keeping the system far away from obstacles that could cause ir reflection. these limitations were not present when using realsense d415 sensor, which reconstruct the 3d scene with an active ir stereo vision camera instead of the tof principle of the kinect. 5. conclusions this work described the characterisation of an automated system that can be used to determine the volume, silhouette, and other geometrical parameters of the hand, starting from the point cloud acquired with commercial 3d cameras. this automated system is composed of kinect v2 or intel realsense d415 sensors, which acquire a point cloud that is analysed using purposely developed algorithms tests performed with the microsoft kinect v2 and intel realsense d415. we evidenced metrological performances comparable to those currently used in the literature (for instance, measuring the hand volume by using a graduated beaker or using a calliper for dimensions). the main limitation of the systems based on the acquisition of the point cloud derives from the presence of disturbances that can have a strong influence on the final results. measurements performed with the kinect v2 were unreliable when not wearing gloves with optical proprieties similar to those of the reference plane; stereoscopic cameras did not evidence similar limitations. the accuracy of the measurement systems was increased by direct calibration, adopting a regression model for the compensation of the hand thickness and volume bias error, obtaining an accuracy lower than 1 mm for height and lower than 20 cm3 for volume. area measurements did not require any compensation, being the in-plane measurement free from bias component. further efforts will be focused on the accuracy in the identification of the silhouette of the hand starting from 3d images. references [1] h. n. mayrovitz, assessing local tissue edema in postmastectomy lymphedema, lymphology 40(2) (2007) pp. 87-94. 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[12] n. duta, a survey of biometric technology based on hand shape, pattern recognition 42(11) (2009) pp. 2797-2806. table 2. kinect v2 and realsense d400 comparison concerning the measurements performed with the two measurement systems on 30 repeated measures of the same subject. microsoft kinect intel realsense 95% ci difference mean st. dev cov mean st. dev cov difference (%) min max p value hand volume in cm3 311 15 5% 317 10 3% -2% -13.5 -0.3 5% fist volume in cm3 322 16 5% 327 10 3% -1% -10.8 2.8 24% flat fist volume in cm3 344 16 5% 346 10.1 3% -1% -9.0 5.0 56% hand area in cm2 137 4 3% 146 4 3% -7% -10.9 -7.0 0% hand without thumb area in cm2 118 3.8 3% 123 2.9 2% -5% -6.8 -3.3 0% index thickness in cm 1.2 0.1 8% 1.0 0.1 9% 16% 0.1 0.2 0% middle finger width in cm 2.0 0.2 9% 1.8 0.1 3% 9% 0.1 0.3 0% hand width in cm 8.4 0.1 2% 8.0 0.2 3% 6% 0.4 0.6 0% hand length in cm 17.4 0.4 2% 18.2 0.2 1% -5% -0.9 -0.6 0% https://www.who.int/en/news-room/fact-sheets/detail/obesity-and-overweight https://www.who.int/en/news-room/fact-sheets/detail/obesity-and-overweight https://doi.org/10.3205/psm000087 acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 82 [13] r. sanchez-reillo, c. sanchez-avila, a. gonzalez-marcos, biometric identification through hand geometry measurements, ieee transactions on pattern analysis & machine intelligence, 10 (2000) pp. 1168-1171. 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[23] m. tarabini, d. marchisotti, r. sala, p. marzaroli, h. giberti, m. sculati, a prototype for the automatic measurement of the hand dimensions using the microsoft kinect v2, proc. of the 2018 ieee international symposium on medical measurements and applications (memea), 2018, pp. 1-6. [24] laser instruction manual and datasheet. [online]. available: https://www.micro-epsilon.fr/download/manuals/man– optoncdt-1402–en.pdf [25] d. c. montgomery, g. c. runger, applied statistics and probability for engineers, john wiley & sons, 2010. [26] g. moschioni, b. saggin, m. tarabini, 3-d sound intensity measurements: accuracy enhancements with virtual-instrumentbased technology, ieee trans. instrumentation and measurement, 57(9) (2008) 1820-1829. [27] b. saggin, d. scaccabarozzi, m. tarabini, instrumental phasebased method for fourier transform spectrometer measurements processing, applied optics 50(12) (2011) pp. 1717-1725. 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[29] g. moschioni, b. saggin, m. tarabini, j. hald, j. morkholt, use of design of experiments and monte carlo method for instruments optimal design, measurement 46(2) (2013) pp. 976-984. https://www.micro-epsilon.fr/download/manuals/man–optoncdt-1402–en.pdf https://www.micro-epsilon.fr/download/manuals/man–optoncdt-1402–en.pdf microsoft word 681-5061-1-le.docx acta imeko issn: 2221-870x december 2019, volume 8, number 4, 13 19 acta imeko | www.imeko.org december 2019 | volume 8 | number 4 | 13 a proof of the concept of the in-flight launch of unmanned aerial vehicles in a search and rescue scenario niels nauwynck1, haris balta2, geert de cubber2, hichem sahli1,3 1 department of electronics and informatics, vrije universiteit brussel, brussels, belgium 2 department of mechanics of the royal military academy, brussels, belgium 3 interuniversity microelectronics centre (imec), heverlee, belgium section: research paper keywords: unmanned aerial vehicles; control; autonomous stabilisation; search and rescue drones; heterogeneous systems citation: niels nauwynck, haris balta, geert de cubber, hichem sahli, a proof of concept of the in-flight launch of unmanned aerial vehicles in a search and rescue scenario, acta imeko, vol. 8, no. 4, article 4, december 2019, identifier: imeko-acta-08 (2018)-04-04 editor: yvan baudoin, international cbrne institute, belgium received november 23, 2018; in final form february 12, 2019; published december 2019 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was supported by the royal military academy of belgium and the vrije universiteit brussel, belgium. corresponding author: geert de cubber, e-mail: geert.de.cubber@rma.ac.be 1. introduction 1.1. problem statement as more and more unmanned aerial vehicles (uavs) are being used in our everyday lives, we are also seeing increasing variety in the systems that are being developed in different applications. this variety should come as no surprise, as it is impossible to create one system that would fit all user needs. heterogeneous systems (which are all used concurrently) are therefore the way forward. however, this recommendation comes with the problems of interoperability and the search for optimal collaboration strategies between all these different systems. in this article, we focus on the collaborative action between two uavs. one acts as a mothership/carrier/launch platform, capable of launching a smaller child system in-flight, which can then be used for close-to-ground missions. the in-flight launch of one aerial system by another is no easy task and requires the careful consideration of the aerodynamics and control of the two systems. indeed, in terms of aerodynamics and flight performance, the mothership and child uavs impose important forces and constraints on one another. these constraints are very different when the aircraft are mechanically interlinked and when they are separated. the autonomous control concept that is implemented for this research experiment on the child uav needs to be able to cope with these sudden changes in real time at the moment of release in order to prevent a crash. one example application domain where the use of an in-flight launched rotorcraft could be of great use is the field of search and rescue, where multiple heterogeneous uavs are being used already [3], [5]. we therefore decided to validate the applicability of the in-flight launch concept in a search and rescue scenario, where a larger carrier uav is requested to carry a close-in inspection uav to a certain location (the location at which the incident is supposed to have taken place). the close-in inspection uav is then released and starts the task of autonomously inspecting a disaster-struck area, searching for human victims by segmenting the visual images it acquires in real time. based on this analysis, alarms can then be sent to the search and rescue workers when victims are found. in this paper, we do not focus on the autonomous navigation capabilities of the uav; rather, abstract this article considers the development of a system to enable the in-flight-launch of one aerial system by another. the article discusses how an optimal release mechanism was developed taking into account the aerodynamics of one specific mothership and child unmanned aerial vehicle (uav). furthermore, it discusses the proportional–integral–derivative (pid)-based control concept that was introduced in order to autonomously stabilise the child uav after being released from the mothership uav. finally, the article demonstrates how the concept of a mothership and child uav combination could be taken advantage of in the context of a search and rescue operation. acta imeko | www.imeko.org december 2019 | volume 8 | number 4 | 14 we concentrate on the real-time semantic segmentation methodology. 1.2. previous work in the field of collaborative uavs, lacroix et al. studied in 2007 the multi-agent decision-making process between the different systems in [1]. however, applying these concepts in practical applications and the reality in the field has proven difficult due to the complex nature of operating multiple heterogeneous platforms simultaneously. serrano et al. proposed in [2] an interoperability concept that enables the messagepassing and collaborative control for multiple heterogeneous uavs and applied that concept to heterogeneous systems developed within the context of the icarus project [3]. they put the interoperability and collaboration concept in [4] into practice in a search and rescue case for the eurathlon challenge [5], where multiple heterogeneous systems (though not all airborne) were validated in a fukushima-like response simulation scenario. while these operations entailed the use of heterogeneous uav operations, none of the systems featured an in-flight launch capability. the in-flight launch of one uav by a mothership is has been considered most commonly for military operations. even in 1975, lempert reported in [13] on a series of flight tests where a target drone was released from a military aircraft. roberts et al. described in [6] similar flight tests to determine the flight envelope and launch system configuration for which a small (maximum gross weight of 80 lbs), unpowered uav glider could be safely launched from the cargo ramp of a c-130 transport aircraft. safe separation from a c-130 aircraft was demonstrated as well as uav stability for successful deployment of wings and fly-out. doi described in [14] the development of an air-launched multi-role uav system for the japanese ministry of defense. a successful separation between the mothership and child aircrafts was demonstrated here. however, all of the three tests campaigns discussed above considered a manned aircraft as a mothership and only a fixed wing aircraft as the child system. limited research exists on the particular use of rotorcrafts due to the difficult aerodynamic constraints imposed by the downwash underneath the mothership platform. in may 2018, the us army was the first military agency to present plans to launch uavs from manned helicopters [15]. in the field of semantic segmentation, important advances have been made in recent years, mainly due to the advent of more powerful machine learning-based approaches. semantic segmentation assigns a label to each pixel in the output image that represents a specific object from an available class of objects depending on the dataset. with the rise of easy-to-use and lowcost uavs, a platform can be created for the remote sensing of forests, with an accuracy of 89 % for seven tree types with basic rgb images. this is a real improvement in comparison with previous studies that use expensive hardware [16]. not only is it possible to calculate the tree type, the tree height and canopy width could be crucial information for search and rescue missions with uavs [17]. there is a method known as regions with convolutional neural networks (cnns), which utilises region-based methods. with results obtained from object detection, it can perform semantic segmentation. regions with cnns is worthwhile because it first extracts a large quantity of object classifications with a selective search and only then computes the cnns feature. afterwards, it can classify each region [18]. the principle behind fully convolutional networks (fcns)based semantic segmentation is pixel-to-pixel map learning without the extraction of region proposals. contrary to cnns, an fcn only has pooling and convolution layers. due to these layers, a prediction can be made on inputs of arbitrary sizes. therefore, the size of the output is dependent on the size of the input instead of a fixed-sized output, which makes it perfect for semantic segmentation or object detection [19]. one semantic segmentation methodology that was specifically adapted to treat visual data coming from uavs is the segnet algorithm [20]. the first phase of this algorithm consists of up-sampling the input using the transferred pool to produce the sparse feature map(s). the second phase involves performing convolution to densify the feature map. the last phase is processing the feature maps to pixel-wise classifications with a soft-maximum classifier [21]. the remainder of this paper is organised as follows. section two discusses the overall conceptual design of the proposed system. section 3 then focuses on the design of the mechanism that was developed in order to drop the child uav from the mothership uav. section 4 then discusses the control concept that was used to stabilise the child uav after being dropped. section 5 then shows how the child uav can execute a useful mission after being dropped by performing an environmental segmentation in a search and rescue context. finally, in section 6, some conclusions and indications for future work are given. 2. conceptual design 2.1. hardware and software used the main aim of this research is to show the concept of the autonomous in-flight launch stabilisation system on commodity hardware multi-copters as opposed to the heavy military systems where in-flight launch systems have already been shown (see the discussion in section 1.2). therefore, we chose to work with modest, low-cost equipment, as presented here. the platform used for the parent uav is a dji phantom 2. this ready-to-fly, multi-functional quadcopter is easy to fly, offers precision flight, and has stable hovering – without too much interaction. throughout this research, this system remained a closed system, where the only communication was done through the included controller. the dji phantom 2 is a consumer product not specifically equipped to carry any load but did offer the requirements for the proof of the concept. by removing the pre-installed camera, the total mass of the mothership uav is 1093 g. the platform used for the child uav is the parrot ar drone 2.0. this uav is mostly intended for use as a toy, which makes it quite popular and affordable. this uav has a starting mass of 501 g. by sacrificing security and durability, we were able to reduce the weight by 58 g. however, this weight reduction meant that no protection hull was present during crashes, bringing the lowest mass to 443 g. the parrot ar drone figure 1. dji phantom 2 used as mothership uav (left); parrot ar drone 2.0 used as child uav (right). acta imeko | www.imeko.org december 2019 | volume 8 | number 4 | 15 2.0 is used frequently in research, because it is programmable in a ros [7] interface, making use of wifi communication for input and output. a ros-driver is provided to create a communication channel with the uav. this communication driver offers a great deal of functionalities that were used for the in-flight launch software, such as: a) three-dimensional rotation values from the x, y & z axis; b) magnetometer readings in three-dimensional space; c) pressure from the barometer; d) linear velocity in three-dimensional space; e) linear acceleration in three-dimensional space; f) estimated altitude; g) motor pulse width modulation values; h) forward and downward facing camera stream; i) yaw, pitch, and roll control. 2.2. aerodynamic constraints the main problem with the in-flight launch of one rotorcraft from another rotorcraft is the intense turbulent airflow caused by the rotors of the mothership aircraft. one approach to this problem is to experimentally measure the so-called downwash area, as performed in [8]. another approach is a simulation. high-fidelity computational simulation tools are required to investigate the complex physics of multirotor interactions and interactions with the airframe(s). these simulations require highperformance computing infrastructure and can provide an insight in the turbulence layer underneath the aircraft. figure 2 gives an example of such an analysis, made by nasa on the pleiades supercomputer, simulating the airflow underneath the dji phantom drone that was used in this study as well [23]. while such simulations are extremely useful, they do not tell the whole story, as they do not incorporate the real-time influence of environmental factors such as wind. indeed, in order to do so, one would need to measure the wind, calculate the airflow model in real time, and then act upon this updated model. this real time computation capability is currently beyond the capabilities of even modern supercomputers. however, as shown by these research studies [23], these external factors have an important effect on the turbulence layer underneath the aircraft in real-life conditions. therefore, a modular and flexible design is needed in order for the devices to be able to cope with various environmental conditions. therefore, we decided to develop a winch-based release mechanism that would allow us to change the release distance between the mothership and child uavs in terms of environmental parameters. 3. design of the release mechanism as the child uav still has a task to complete after being launched, as much weight as possible should be left on the mothership uav. this means that a design was necessary for the actual launch mechanism to hang on the mothership uav. a major issue in the process of designing and developing a release mechanism on the child uav was to prevent any unwanted rotations due to wind, for example, which would cause system instability. therefore, a child uav release mechanism was designed, consisting of a base plate and a locking mechanism, terminating in an o-ring to which a hook can be attached. once the design was fully made, it was 3d printed. the design turned out to be 44 g. adding the 44 g to the 443 g of the child uav allowed us to ensure that the child uav now had a total mass of 487 g. note that it is technically not possible for a dji phantom 2 to support such a payload; therefore, it is necessary for the child uav to help with lifting its own mass pre-release by spinning its rotors. figure 3 shows the results of this design: a lightweight, stern, and rotation resistant component capable of carrying the child uav. as discussed above, the child uav can be carried through an o-ring. this was specifically done to create an easy-to-use launch mechanism on the mothership uav. the major difficulty on the mothership side was to include a mechanism that can increase or decrease the distance between the mothership and child uav. as discussed before, due to the effects of turbulence under the mothership aircraft, it is necessary to release the child uav at a reasonable distance from the mothership uav, sufficiently far from the turbulence zone. a winch system consisting of a pcb-controlled servo-motor was developed. once 3d-printed, the base plate extension creates a functional winch system, as seen in figure 2. the parent uav now has the possibility to lower the uav to any desired launch height from a remote site. the final design of the parent uav release mechanism has a mass of 245 g, bringing the total mass of the mothership uav to 1338 g. 4. autonomous stabilisation 4.1. controller design in order to be platform-independent, a new proportionalintegral-derivative (pid) controller was created [22] to take over figure 2. airflow for around a dji phantom quadcopter in hover mode (nasa ames graphic/patricia ventura diaz) [23] figure 3. release mechanism on the child uav (left), release mechanism on the mothership uav (right). acta imeko | www.imeko.org december 2019 | volume 8 | number 4 | 16 the default hovering function embedded in the used devices, taking into account the constant turbulence by the mothership uav. since we wanted a platform-independent solution, we did not rely on this on the built-in stabilisation method, which also makes use of the downward facing camera. for the creation of the pid controller, a custom package that subscribed to the navigation data and odometry was created. in return, it could publish the necessary yaw, pitch, and roll values, calculated as control commands to stabilise the uav. in the implementation, the maximum reference speed of the uav was limited to 0.6, which prevented it from performing jerky movements. the velocity error was calculated by the difference between the navigation commands of yaw, pitch, and roll and the incoming odometry values. this value was assigned to the proportional gain. the integral gain was calculated based on the previous integral gain and the proportional gain. by using the proportional gain, we were able to determine the integral gain as shown in figure 4, based on a set limit, the current situation of the error (new err), and the previous integral gain (i term). lastly, the derivative gain was calculated by filtering the incoming odometry data. 4.2. validation protocol in the first phase of the experiment, we wanted to validate the robustness of the implemented control mechanism by manually destabilising the child uav in-flight. figure 5 shows the results of such an experiment, where the uav was pushed backwards twice. the graph shows the value of the pitch angle and clearly shows how the pitch angle is stabilised after each of the disturbances. assured by the robustness of the system, actual in-flight launch experiments were performed. in order to study the effect of the turbulence layer, we experimented with difference release altitudes, measured between the mothership and child uavs. the different separation distances between the both uavs that were considered were: 140 cm, 100 cm, and 60 cm. 4.3. behaviour with 140 cm separation using a 140 cm launch distance, the pid controller does not need to change the yaw, pitch, or roll values (https://youtu.be/hvxir1gvgtc). its only task is increasing the power on all four motors in order to counteract the descent. this is a fairly easy task, and the release therefore goes quite smoothly. the pulse-width modulation (pwm) motor data displayed in figure 7 shows the values from the launch till the landing. as seen in the top-left graph, during the lift, the child uav helps with the overload by throttling the motors to their maximum speed. after the recover loop is initialised, the uav stabilises. figure 4. calculation of the pid controller values. figure 5. pitch angle over time, with a person manually pushing the uav backwards twice. figure 6. in-flight launch of the child uav by the mothership uav: uavs before launch (left); uavs after launch (right). figure 7. in-flight launch with a 140 cm distance, with pwm values in the time domain. acta imeko | www.imeko.org december 2019 | volume 8 | number 4 | 17 when releasing the uav, we can see a clear increase of power on all four motors for counteracting the descent. 4.4. behaviour with 100 cm separation using a 100 cm launch distance, the behaviour is, in most cases, similar to the previous case, a distance of 140 cm (https://youtu.be/-hsyfgzbpow). however, we observed that the child uav sometimes needs to already compensate the prerelease for the extra downward forces induced by the downwash of the mothership uav. this is the case in the experiment shown in figure 8. the result is that the pid controller acquires the correct height by lifting its own weight, not relying on the strength of the parent uav. 4.5. behaviour with 60 cm separation using a 60 cm launch distance, the distance between the mothership and child uav becomes very small (https://youtu.be/3xvp1fmt6tg). as a result, the child uav is directly within the turbulent area underneath the rotors of the mothership uav. consequently, the pid controller is no longer capable of recovering the turbulence induced by the rotors of the mothership uav, and the child uav always crashes upon release. figure 9 displays the pwm values of the motors while helping lift the weight, which was a requirement for in-flight launch. as seen in figure 9, the motor pwm values are highly variable. in all of the four runs, the release was never possible, because the mothership uav caused turbulence on the child uav. this turbulence interfered with the spinning propellers of the child uav which made it move all over the place. because of the movement of the child uav, the mothership uav also started to wiggle, which only increased the movement on the child uav, repeating this pattern until a crash occurred. obviously, this means that we have reached the limits of what was possible with the given platforms and the proposed control and stabilisation paradigm. 5. validation in a search and rescue scenario in order to present a meaningful case for the validation of the proposed system, the field of search and rescue was chosen. this specific domain was not chosen by accident. the specific requirements of search and rescue workers often demand the deployment of multiple heterogeneous robotic tools. indeed, large fixed wing systems are required to have a permanent eye in the sky and to create a map of the area, while rotorcrafts are generally more suited for outdoor victim searches or dropping rescue kits. small rotorcrafts are excellent for indoor victim searches. in this context, we envision a search and rescue operation whereby a large uav launches a smaller one at a specific site such that this small uav can go and search for victims. a necessary requirement for using a uav for victim searches is the capability to detect human survivors in a totally unstructured environment. for scene analysis, using the onboard camera, the uav has to detect and classify the objects seen by the camera. for this purpose, a deep neural network (dnn) is used to achieve semantic segmentation, assigning a class label to every pixel. a dnn is another form of an artificial neural network that has shown spectacular accuracy on datasets with large feature and solution spaces. since dnns often have more vanishing gradient problems and exploding gradient problems, they are harder to train than other networks. for this application, we use the enet semantic segmentation algorithm [10], which uses a dnn architecture to provide realtime semantic segmentation for self-driving vehicles. this algorithm runs 18 times faster than existing models by early downsampling, nonlinear operations, changing the decoder size, regularisation, etc. to train from a dataset, a modified version of caffe was used, which supported all the necessary layers for enet. this requires a training and testing set whereby the encoder is first trained with pre-labelled objects from the dataset. after about 75,000 iterations, we noticed a convergence with a minimum training accuracy of 80 %. after the decoder had been trained, the encoder was further trained specifically to obtain a training accuracy of 80 %. after launching the child uav from the mothership uav, the enet semantic figure 8. in-flight launch experiment with a 100 cm distance. the four colours show the pwm values of the four motors. figure 9. in-flight launch experiment with a 60 cm distance experiment. the four colours show the pwm values of the four motors. figure 10. enet’s semantic segmentation input image of a lost person on small road in open countryside. the visual image frame from the child uav (left), enet segmentation of the image frame (red=victim) (right). acta imeko | www.imeko.org december 2019 | volume 8 | number 4 | 18 segmentation algorithm was activated on the images of the parrot ar drone 2.0 front-facing camera, which has a resolution of 1280 x 720 at 30 fps. the first test, shown in figure 10 shows an example of how the output on a small access road to a building mimics the idea of a small road in open countryside. the second experiment set can be seen in figure 11 and 12 and displays the detection possibilities in front of tunnels and shows that while inside a dark tunnel, person detection becomes less obvious. 6. conclusions in this article, an in-flight launch concept was proposed for a child rotorcraft uav by a mothership rotorcraft uav. the solution was developed not only in theory, but also in practice, by the design of a release mechanism and a control concept in order to stabilise the child uav after the launch procedure. the system was extensively validated by multiple launch experiments, evaluating the limits of the control concept. furthermore, a practical use case was elaborated whereby this concept could be put into practice: search and rescue. therefore, a dnn was implemented in order to perform a semantic segmentation of the video data of the child uav (after being released in a disaster area by the mothership uav), enabling autonomous victim search operations. it must be stressed that the objective of this research work was to provide a proof of concept by using cheap hardware. future work will thus mainly focus on applying this concept to higher-performing hardware platforms, such that real use cases can be performed. acknowledgement this research was institutionally supported by the royal military academy of belgium and the vrije universiteit brussel, belgium. references [1] s. lacroix, r. alami, t. lemaire, g. hattenberger, j. gancet, decision making in multi-uavs systems: architecture and algorithms, in: multiple unmanned aerial vehicles, ser. tracts on advanced robotics. a. ollero, i. maza (editors). springer, 2007. [2] d. serrano lópez, g. moreno, j. cordero, j. sanchez, sh. govindaraj, m. monteiro marques, v. lobo, s. fioravanti, a. grati, k. rudin, m. tosa, a. matos, a. dias, a. martins, j. bedkowski, h. balta, g. de cubber, interoperability in a heterogeneous team of search and rescue robots, in: search and rescue robotics-from theory to practice. in-tech, 2017. [3] g. de cubber, d. serrano, k. berns, k. chintamani, r. sabino, r. ourevitch, d. doroftei, c. armbrust, t. flamma, y. baudoin, search and rescue robots developed by the european icarus project, proc. of the 7th int. workshop on robotics for risky environments, 2013. [4] m. m. marques, r. parreira, v. lobo, a. martins, a. matos, cruz, j. m. almeida, j. c. alves, e. silva, j. bedkowski, k. majek, m. peka, p. musialik, h. ferreira, a. dias, b. ferreira, g. amaral, a. figueiredo, r. almeida, f. silva, d. serrano, g. moreno, g. d. cubber, h. balta, h. beglerovi, use of multi-domain robots in search and rescue operations contributions of the icarus team to the eurathlon 2015 challenge, proc. of oceans 2016, april 2016, shanghai, pp. 1-7. [5] a. f. t. winfield, m. p. franco, b. bruggemann, a. castro, m. c. limon, g. ferri, f. ferreira, x. liu, y. r. petillot, j. roning, f. e. schneider, e. stengler, d. sosa, a. viguria, eurathlon 2015: a multi-domain multi-robot grand challenge for search and rescue robots, in: taros, ser. lecture notes in computer science, vol. 9716. springer, 2016, pp. 351-363. [6] d. w. roberts, a. d. judy, separation flight tests of a small unmanned air vehicle from a c-130 transport aircraft, proc. of the nato sto meeting, vol. mp-sci-162-19, 2012. [online] https://www.sto.nato.int/publications/sto%20meeting %20proceedings/rto-mp-sci-162/mp-sci-162-19.pdf [7] m. quigley, k. conley, b. p. gerkey, j. faust, t. foote, j. leibs, r. wheeler, a. y. ng, ros: an open-source robot operating system, proc. of the icra workshop on open source software, 2009. [8] m. sjholm, n. angelou, p. hansen, k. h. hansen, t. mikkelsen, s. haga, j. a. silgjerd, n. starsmore, twodimensional rotorcraft downwash flow field measurements by lidar-based wind scanners with agile beam steering, journal of atmospheric and oceanic technology 31(4) (2014) pp. 930-937. [9] d. doroftei, a. matos, g. de cubber, designing search and rescue robots towards realistic user requirements, in: advanced concepts on mechanical engineering (acme), 2014. [10] a. paszke, a. chaurasia, s. kim, e. culurciello, enet: a deep neural network architecture for real-time semantic segmentation, corr abs/1606.02147 (2016). [11] y. jia, e. shelhamer, j. donahue, s. karayev, j. long, r. girshick, s. guadarrama, t. darrell, caffe: convolutional architecture for fast feature embedding, arxiv e-prints, jun. 2014. figure 11. enet’s semantic segmentation output image of a lost person in front of a tunnel. visual image frame from the child uav (left), enet segmentation of the image frame (red=victim). figure 12. enet’s semantic segmentation output image of a lost person inside a tunnel. visual image frame from the child uav (left), enet segmentation of the image frame (red=victim) (right) acta imeko | www.imeko.org december 2019 | volume 8 | number 4 | 19 [12] m. cordts, m. omran, s. ramos, t. rehfeld, m. enzweiler, r. benenson, u. franke, s. roth, b. schiele, the cityscapes dataset for semantic urban scene understanding, proc. of the ieee conference on computer vision and pattern recognition (cvpr), 2016. [13] m. lempert, air launch characteristics of the mqm-74c target drone from dc-130 airplane, ntis report ad/a004, january 1975. [14] h. doi, tacom – air-launched multi-role uav, proc. of the 24th congress of international council of the aeronautical sciences, 29 aug.-3 sept. 2004, yokohama, japan. [15] j. judson, us army to launch drone from helicopter for first time this year, defensenews, 1 may 2018. [16] m. onishi, t. ise. automatic classification of trees using a uav onboard camera and deep learning. arxiv e-prints, 1804.10390 (2018). [17] y. seul lim, h. la, j. soo park, m. hee lee, m. wook pyeon, j.-i. kim, calculation of tree height and canopy crown from drone images using segmentation. journal of the korean society of surveying, geodesy, photogrammetry and cartography 33 (2015) pp. 605-613. [18] r. girshick, j. donahue, t. darrell, j. malik, rich feature hierarchies for accurate object detection and semantic segmentation, proc. of the ieee computer society conference on computer vision and pattern recognition, 2013. [19] j. long, e. shelhamer, t. darrell, fully convolutional networks for semantic segmentation, cvpr (2015). [20] b. luque, j. r. morros, j. ruiz-hidalgo, spatio-temporal road detection from aerial imagery using cnns, proc. of the international conference on computer vision theory and applications, 2017, porto, portugal. [21] v. badrinarayanan, a. kendall, r. cipolla, segnet: a deep convolutional encoder-decoder architecture for image segmentation. ieee transactions on pattern analysis and machine intelligence 39(12) (2017) pp. 2481-2495. [22] n. nauwynck, h. balta, g. de cubber, h. sahli, in-flight launch of unmanned aerial vehicles, proc. of the international symposium on measurement and control in robotics ismcr2018, sept. 2018, mons, belgium. [23] p. ventura diaz, s. yoon, high-fidelity computational aerodynamics of multi-rotor unmanned aerial vehicles, proc. of the 2018 aiaa aerospace sciences meeting, aiaa scitech forum (aiaa 2018-1266), 2018. a low-power iot architecture for the monitoring of chemical emissions acta imeko issn: 2221-870x june 2019, volume 8, number 2, 53 61 acta imeko | www.imeko.org june 2019 | volume 8 | number 2 | 53 a low-power iot architecture for the monitoring of chemical emissions tommaso addabbo1, ada fort1, marco mugnaini1, lorenzo parri1, stefano parrino1, alessandro pozzebon1, valerio vignoli1 1 department of information engineering and mathematical sciences, university of siena, via roma 56, 53100 siena, italy section: research paper keywords: chemical emissions; lora; iot citation: tommaso addabbo, ada fort, marco mugnaini, lorenzo parri, stefano parrino, alessandro pozzebon, valerio vignoli, a low-power iot architecture for the monitoring of chemical emissions, acta imeko, vol. 8, no. 2, article 8, june 2019, identifier: imeko-acta-08 (2019)-02-08 editor: alessandro depari, university of brescia, italy received july 26, 2018; in final form june 04, 2019; published june 2019 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: alessandro pozzebon, e-mail: alessandro.pozzebon@unisi.it 1. introduction it is now apparent that the man-made pollution of the atmosphere can have serious consequences. therefore, monitoring air quality is of the utmost importance. among other pollutants, co and nox, which are by-products of combustion, are extremely harmful and dangerous, even if they are very diluted in the atmosphere and appear in very low concentrations (including below the ppm range) [1]. for this reason, toxic gas monitoring in ambient air is a difficult task. the required measurement accuracy and resolution are challenging [2]-[4]. it is obviously convenient to also monitor the sources of toxic gas emissions from the point of view of a policy of emission abatement. in this context, there is great interest in developing systems that are able to continuously monitor the combustion processes that are a certain source of air pollutants. moreover, the monitoring of combustion products also allows us to optimise combustion operations and to alleviate the instabilities and severe consequences thereof [5]. there are many different technologies and instrumentations that can be used to measure gaseous emissions of combustion processes, but they are usually expensive and complex, and they require frequent, cumbersome, and time-consuming calibration procedures. in practice, gaseous emissions are rarely monitored on a continuous basis, even if the advantages of such a measurement are apparent. in this work, an iot framework for the continuous monitoring of chemical emissions is presented. such an infrastructure may be extremely important in several different contexts. the most important scenario is clearly the industrial one. industrial plants contribute significantly to the introduction of pollutants into the atmosphere, and their vast size makes the realisation of pervasive monitoring infrastructures complex and expensive, while the maintenance of these systems may be just as onerous as the installation phase. nevertheless, the industrial scenario is not the only one that could benefit from the realisation of a system as that which is proposed in this paper. such an architecture could also be employed to monitor the chemical emissions and environmental conditions in a smart city abstract in this work, a low-power iot architecture for the monitoring of chemical emissions is presented. this system is expected to be employed to set up monitoring infrastructures in industrial plants or public buildings. the proposed system has been designed to embed different sensors. in particular, each sensor node manages a humidity sensor and an array of temperature and electrochemical gas sensors for the detection of carbon monoxide (co), nitrogen oxides (nox), and oxygen (o2). moreover, it exploits some dedicated processing algorithms to mitigate the dependence of the sensor response on temperature. the sensor node has been designed to minimise power consumption as much as possible, and it is provided with lora lpwan connectivity, which allows for wide-area data transmission. tests carried out in urban areas proved that a 3 km communication range is achievable in noisy environments. a network architecture and a data acquisition and management structure are then described. a multilayer modular topology that combines the features of lora technology with shorter and larger range telecommunication channels in order to develop an iot framework that can be customised according to the physical and technical features of the deployment scenario. mailto:alessandro.pozzebon@unisi.it acta imeko | www.imeko.org june 2019 | volume 8 | number 2 | 54 scenario. in this case, monitoring the emissions of public buildings may be useful in terms of improving the quality of life of citizens and optimising the energy consumption of buildings. finally, such a system may be used to monitor the impact of every single citizen on the environment by measuring the emissions of private buildings pervasively and in real time. this practice may lead, for example, to the implementation of taxation policies. wireless sensor networks (wsns) for gas monitoring have been the subject of an increasingly large number of research studies [6]. the majority of such studies that aim to implement gas emission monitoring in industrial environments are based on local networks rather than on internet-connected architectures. many works have proposed local area wsns based on resistive prototypal or commercial metal oxide (mox) chemical sensors, mostly for combustible and toxic gas detection. however, since mox sensors are required to be properly heated, they are usually power-consuming devices. the wsns proposed in the literature attempt to solve this problem by exploiting energy harvesting and/or optimised heating strategies [7]-[10]. this last approach can result in many problems, since the operation of sensors with a low average temperature enhances the cross-sensitivity to environmental conditions, such as humidity, and discontinuous heating can start very long transients [10]. on the other hand, electrochemical gas sensors are low-power devices and are surely more suitable for the development of wsns. nevertheless, few works have presented solutions based on this technology. an example of a wsn for the detection of co and ch4 based on radio frequency (rf) energy harvesting is presented in [11]. in [12], a portable bluetooth device was proposed for the crowdbased detection of co, s2o, and no2. therefore, it is not specifically tailored for industrial applications. regarding the internet of things (iot) domain and the development of monitoring structures to make data remotely available by means of the internet, some interesting examples of infrastructures can be found concerning environmental monitoring. a large number of studies deal with zigbee-based networking architectures, according to which local wsns are integrated over the internet through either a wireless [13] or wired connection [14]. some solutions based on other microwave (2.45 ghz) [15] or sub-ghz [16] radio technologies can also be found. wifi and bluetooth have also been exploited to set up environmental monitoring architectures based on the iot paradigm [17]. with the emergence of so-called low-power wide area network (lpwan) technologies, several solutions have been presented using lora technology in particular for environmental monitoring. most of these systems focus on smart cities [18][20], with applications measuring parameters like temperature, humidity, or pollutant concentrations. nevertheless, all these solutions use low-quality, off-the-shelf devices, mainly focusing on the single data acquisition platform rather than on the overall network architecture. very few solutions deal with iot applied to the monitoring of gas emissions. moreover, all of the relevant studies approach the problem by focusing on the design of the single sensor node rather than on the whole monitoring infrastructure [21], [22]. in this article, we propose a monitoring infrastructure that allows for wide-area internet-based data transmission, focusing on the monitoring of combustion by-products mainly in industrial plants, which is based on low-power electrochemical sensors and does not need any energy harvesting system. in fact, by accurately designing the whole network node electronics comprising the front end, a node powered with off-the-shelf 1.5 v aa batteries was developed, which can work for the whole sensor life. the proposed system has been designed with a modular structure that enables it to embed different low-power sensor systems based on a simplified version of a measurement device that is developed and tested as a standalone instrument [23]. the sensor node is low power and manages an array of electrochemical gas sensors of carbon monoxide (co), nitrogen oxide (nox), and oxygen (o2), and it exploits some processing algorithms designed ad hoc to mitigate the sensor drift and dependence of the sensor response on temperature and humidity. the sensor node has been studied not only to measure the required parameters but also to remotely transfer, in real time, the collected data, based on previously developed pervasive environmental monitoring architectures [24], [27]. in particular, different architectures have been studied for the communication module of the node, ranging from local area (zigbee) to wide area (lora) and global connectivity (the internet). while all these solutions have proved to be employable, suggesting their use for a modular, multilayer network topology, the final sensor node prototype has been provided with lora connectivity. indeed, the lora technology has proven to be the most flexible one, allowing for the deployment of both localand city-scale networks [28], [29]. in the context of industrial monitoring, this approach allows for the deployment of wireless monitoring infrastructures for both smalland large-scale industrial plants as well as multi-plant networks. similarly, in the context of smart cities, such connectivity allows for the deployment of a large quantity of sensor nodes while keeping the structure of the acquisition network relatively simple and flexible. by deploying a very small number of gateway nodes (according to the dimensions of the city), it is possible to provide connectivity to a relatively large urban area, managing data collection from a large number of public and private end-users. 2. iot framework architecture as explained in the previous section, an ad hoc multilayer iot network architecture has been studied and designed to fit different scenarios according to different levels of capillarity. the overall network architecture is based on a cluster-tree topology, where each level of the tree corresponds to a different communication technology. the three communication layers of the proposed architecture are shown in table 1, while the network topology can be seen in figure 1. the three communication technologies present different technical features that make them suitable for employment for different functions within the network infrastructure. zigbee is a network protocol based on the ieee 802.15.4 protocol [30] and designed for the realisation of local-area mesh wsns, adopting multi-hop as a strategy to improve network efficiency and lifetime. zigbee nodes are generally provided with short-range data transmission. in general, they can transmit data table 1. communication technology layers. layer communication technology transmission range bottom layer zigbee < 100 m middle layer lora 100 m … 3 km upper layer ethernet-gprs/umts > 3 km acta imeko | www.imeko.org june 2019 | volume 8 | number 2 | 55 at a distance of 100 m in line of sight, even if this value can drop down to a few metres in indoor environments, especially in the presence of large walls [31], [32]. this means that in this context, zigbee networks can be employed for the monitoring of a single room rather than a whole building. lora, an acronym of long range, is a physical-level proprietary protocol patented by semtech [33], which belongs to lpwan technologies. such systems have been designed to provide long-range data transmission (in the order of some kms) and very low-power data transmission according to the two most significant system requirements for iot infrastructures. lora systems operate in the unlicenced industrial, scientific and medical (ism) bands of 868 mhz in europe, 433 mhz in asia, and 915 mhz in the us. they are based on star network topologies, according to which a large number of lora nodes are able to transmit data to one or more lora gateways that are responsible for forwarding data to any kind of data management system. the performance levels of lora devices can vary, both in terms of bitrate and data transmission range according to the values of a set of parameters. in any case, lora systems generally feature low bitrates, and they can achieve 20 km distance in line of sight and a 3-4 km distance in urban areas. moreover, they can also be employed to monitor large buildings due to their efficiency, as will be shown in section 4. it must be underlined that lora technology has a legal requirement for a mandatory 1 % duty-cycle (in europe for eu 868 end devices [34]). this means that the number of packets transmitted by the networks is quite low in any case. finally, the last layer comprises internet-connected technologies, which are generally characterised by high power consumption. their purpose is mainly the transmission of data collected by the whole network to a remote data collection centre or cloud infrastructure. according to the three layers, three sensor nodes and three cross-layer gateway typologies have been designed. in order to test the functionalities of the monitoring infrastructure, one sensor node and one cross-layer gateway have been fully developed. the lower level of the network structure comprises the clusters of zigbee-connected nodes arranged in mesh subnetworks. due to the limited communication range, these networks are expected to be employed for the monitoring of single rooms or for the deployment of sensor arrays positioned at limited distances. each node integrates a zigbee radio module that can collect data from one or more sensors. the zigbee nodes represent the lower level of the network architecture, acting either as zigbee end devices or zigbee routers according to their function in the mesh network. they are designed to be as simple as possible, optimised in terms of power consumption by enabling duty-cycling and characterised by a low data rate, since only the numerical values acquired by the sensors need to be transmitted. the zigbee nodes interface with the upper levels through two different cross-layer gateways: the zigbee-internet gateway and the zigbee-lora gateway. the zigbee-internet gateways can be based on both wired and wireless connections. the first ones are designed for integration on a single microcontroller-driven board of a zigbee radio module, acting as a zigbee coordinator, and an ethernet module. meanwhile, the second ones can be based on either a wifi module or a simbased gprs/umts connection. the zigbee-lora gateways are based on a multi-protocol board that re-routes the zigbee data packets to the middle layer. since these nodes present an ad hoc microcontroller for the packet management, they can also be responsible for data filtering. indeed, a first check, based on threshold values (for example), can be performed directly on the gateways through ad hoc data check protocols. this practice leads to the discarding of the unnecessary packets and the forwarding to the lora network only a subset thereof, with a notable reduction in the overall network traffic. the middle layer of the iot framework is represented by the lora infrastructure. this is basically a tree network that can be employed to cover larger areas than the zigbee one. in particular, with adequate positioning of the lora gateway nodes, transmission ranges of up to 20 km can be achieved. since the main task of the lora nodes is to collect the sensor data and transmit it, their structure (and that of the zigbee nodes) is very simple, integrating the microcontroller unit responsible for data acquisition and transmission; the sensors; and the lora radio module. since lora modules are developed to minimise power consumption and the lora protocol is more energy efficient than the zigbee one due to the lack of routing algorithms required to implement multi-hop, these nodes are expected to be the most energy efficient, even if they are less fault tolerant due to their intrinsic network architecture. the middle layer is provided with two cross-layer gateways: the abovementioned zigbee-lora gateway and the lora-internet gateway. this second gateway is similar to the zigbee-internet one even if due to the larger transmission ranges, the ideal configuration is the wired one. in many cases, ethernet connectivity will surely be available. the wired connection is intrinsically more fault tolerant than the wireless one, and the gateway could be powered by power-over-ethernet technology. the upper level is represented by the internet backbone. in this context, internet-connected nodes have been designed, allowing for the collection of sensor data and forwarding thereof directly to the internet through a wired (ethernet) or wireless (gprs/umts or wifi) connection. these nodes are obviously more complex and power hungry than the previous ones due to the need to implement the tcp-ip protocol stack, and the advantages that are achievable in terms of data rate and efficiency are not fully exploited due to the limited amount of data for transference. this suggests their use only in the presence of a power source, otherwise other solutions should be preferred. the cross-layer gateways interfacing with this layer are those described earlier i.e. the zigbee-internet and the lora-internet ones. figure 1. multi-layer network architecture. acta imeko | www.imeko.org june 2019 | volume 8 | number 2 | 56 this architecture fits well within the proposed monitoring context (monitoring chemical emissions in industrial complexes or cities) thanks to the unique features of each technology. in particular, the lower layer can be used to monitor small closed environments (single rooms or very small buildings). this layer is especially important since it can operate data filtering. in particular, the zigbee gateway can act as a filter, choosing the packets for forwarding to the upper layers according to a set of rules that can lead to discard the majority of data packets, transmitting only the relevant ones. in large monitoring infrastructures, this can be crucial so as to avoid a network overload. the lora layer is expected to be used within two different contexts: when the area to be monitored is so large that it cannot be covered by a zigbee network and when data cannot be forwarded directly by the zigbee clusters to the internet. in particular, in the first case, lora networks can be used to monitor wide outdoor environments (up to some kms) as well as large buildings [35]. in this last context, while zigbee has proven unable to interconnect two devices placed even in two adjoining rooms [31], [32], in section 4, we show that lora is able to cross a large number of walls and rooms, providing connection to a whole large building with a single gateway node. the internet layer is in charge only of forwarding a sub-set of data packets to a remote data collection centre or to a cloud infrastructure. in the proposed scenario, the internet gateways simply transfer the packets to a remote server through an http post. a java web application deployed on a glassfish server provides a web service in charge of receiving the packet, extracting the data and storing them into a mysql database. data are then made available through an ad-hoc web page that allows to retrieve and analyse them. while the whole infrastructure could be set up in case of large and articulate monitoring architectures, the minimal sufficient subset is represented by a sensor node and an internet-enabled gateway. the modules developed as a proof-of-concept in this research satisfy this requirement being them a lora node and a lora-internet gateway. even if in this paper we used lora technology for data transmission, it is well known that lora alliance has also created an ad-hoc protocol at medium access control (mac) and application layer, called lorawan [36]. this protocol is expected to be adopted for the proposed scenario when designing the overall monitoring infrastructure, since its benefits are clearly visible mainly when large dimension networks have to be deployed. indeed, the adoption or lorawan protocol can add a wide range of features to the overall system that can make it more reliable and efficient. in particular, lorawan protocol adds anti-collision features that notably reduce the risk for packet losses in presence of large quantities of sensor nodes. it foresees the use of two layers of security, one for the network and one for the application, based on advanced encryption standard (aes) encryption. another key feature of lorawan when designing large network architectures is the independence of the sensor nodes from the single gateway: in lorawan networks, any packet transmitted by the nodes is received by multiple gateways which forward it to a standardised backend in charge of managing them. the standardised backend is composed of two different servers: the network server, which manages the reception and transmission of packets from the nodes, and the application server, which is the final destination for the data contained in the payload of the packets. in order to adapt the proposed architecture to the lorawan protocol, few changes have to be done. in particular, lora nodes are expected to be turned in class a lorawan devices, while lora-internet gateways are simply replaced by lorawan gateways that are provided themselves with internet connectivity. for what concerns the zigbee-lora gateways, their structure can be based too on class a lorawan devices since they are expected to receive very few downlink messages, with a large number of uplink transmissions. this means that the overall structure of the zigbee-lorawan gateways is expected to be very similar to the zigbee-lora ones. 3. sensor node architecture each node within the proposed iot architecture consists of a communication module and a micro-module based on a lowpower stm32l432 microcontroller, connected to a series of different sensor units. a mosfet-based switch is used to set up a duty-cycling for the radio module, thus turning it off when no data transmission is required. a sensor unit contains one of the electrochemical sensors listed in table 2 or a humidity sensor coupled with a sensor for the measurement of the local temperature [37]. 3.1. sensor node each sensor unit contains an electrochemical (or humidity) sensor and a temperature sensor, plus the dedicated front-end electronics (whose architecture for the case of the electrochemical sensors is shown in figure 2): its outputs are then the analogue measurement signals of the local temperature figure 2. sensor node architecture. table 2. sensors used in the proposed system. electrochemical sensors humidity sensor temperature sensor alphasense co−a4 alphasense no2 −a43f alphasense no−a4 alphasesne o2 −a1 honeywell hih4000 national semiconductors lm32 acta imeko | www.imeko.org june 2019 | volume 8 | number 2 | 57 and of the target gas concentration (or humidity value). the selected humidity and temperature sensors require a very simple and conventional low-power front-end electronics that will not be discussed in this paper. on the other hand, the electrochemical sensors need ad-hoc electronics, specifically designed to grant high accuracy and very low-power consumption. the front-end electronics has the structure shown in figure 3 where, as an example, the co sensor is considered. as shown in the figure, the front end consists of an i-v converter and of a biasing circuit. the i-v converter is used to convert the current flowing through the sensor across the working electrode (we) and the counter electrode (ce) into an analogue voltage, which is then fed to the a/d converter in the micro-module. this current is caused by the electrochemical reaction and is theoretically proportional to the target gas concentration. the biasing circuit assures a constant voltage across the weelectrolyte interface, irrespective from the current flowing through the sensor, by a feedback loop that sets the ce voltage on the basis of the comparison of the reference electrode (re) voltage with the voltage vbias. being the reaction at the we highly influenced by the voltage at the we-electrolyte interface, this control circuit highly improves the sensor output stability. the design of the sensor node circuit is similar to the one presented in [38] but with significant improvements in terms of power consumption reduction. to this purpose, nano-power st tsu10x operational amplifiers have been used. these devices are particularly suitable for the low-power i-v converters, since they are characterized by a sub-micro ampere current consumption (580 na per channel at 25 °c at vcc=1.8 v), by a very low supply voltage in the range (1.5 v 5.5 v), by rail-to-rail input and output operations, and by a very low input bias current (5 pa max at 25 °c). a special attention in terms of power consumption is also required for the voltage reference circuits. a band gap reference voltage (lt1379), combined with the operational amplifier previously described (figure 3) is used to generate an adjustable reference for both the biasing voltage (vbias) and the reference voltage (vref) of the circuit. the front-end power consumption was determined by spice simulations (figure 4b), considering the components previously described and the standard electrical model used for the electrochemical sensors (figure 4a). from simulations, the overall current consumption of the analogue front end is approximately 7 µa with a 3 v supply voltage. this very lowpower consumption allows for maintaining the front end active also when the microprocessor is in sleep mode and the communication module power supply is turned off. this solution is advantageous in terms of accuracy and measurement speed: indeed, when turning off and on the biasing of the electrochemical sensors (especially the no sensor which requires a vbias = 0.3 v), long chemical transients are started and long settling time (hours) is required before reaching the final accuracy. 3.2. a/d conversion by micro-module and measurement management the measurement signals from the front-end circuits are a/d converted by the stm32l432 low-power microcontroller. this microcontroller sinks only few μa in standby mode whereas in run mode the current consumption depends on the clock frequency and is approximately 84 μa/mhz of cpu clock. the microcontroller is equipped with a 12 bits a/d converter with maximum sampling frequency of 5 msps. exploiting oversampling the resolution can be enhanced and an equivalent 16-bits resolution can be obtained with a current consumption of 200 µa/msps. for the current application, the sampling frequency is not a critical issue, since electrochemical sensors are slow devices, with a response time constant τ in the order of few seconds. on the contrary, accuracy is required, hence a high ad conversion resolution is important. in particular, since the sensor noise floor is 20 equivalent ppb, with a full scale up to 500 ppm figure 3. sensor node structure in case of electrochemical gas sensors. (a) (b) figure 4. front end: a) electronic front-end structure. b) spice model used for the simulations and the evaluation of the power consumption. analog front end we ce re i-v converter e0 co co2 2 h + h2o 1/2 o2 2 e 2 e i0 i0 i 0 v bias microcontroller stm32l4 v sens spi a d c v ref v bias biasing circuit ae voltage references switch 3v battery electronic front end 1electrochemical sensor 1 radio analog front end we ce re i-v converter e0 co co2 2 h + h2o 1/2 o2 2 e 2 e i0 i0 i 0 v bias v sens v ref v bias biasing circuit ae voltage references electronic front end 2electrochemical sensor 2 3v 3v we re ce ae 3v 3v 3v vbias vref vref vwe vaux 3v vref electrochemical sensor rgain rgain road road lt13891.25 vbias 3v lt13891.25 i_v converters biasing circuit voltage references acta imeko | www.imeko.org june 2019 | volume 8 | number 2 | 58 for the co sensor, to preserve the measurement resolution an ad converter with an effective number of bits larger than 14.6 is required. a solution can easily be found using averaging. in fact, it is well known that considering a white quantisation noise when averaging n n-bit samples of a constant analogue voltage, the mean value can be represented with a neq-bit sample, such that 𝑛𝑒𝑞 = 𝑛 + 10 log10 𝑁 6.02 𝑑𝐵 = 𝑛 + 𝑛 (1) where ∆n represents the resolution increment. using the 12 bits analog to digital converter (adc) (n = 12) of the microprocessor, to obtain a 16-bit resolution (neq = 16), a resolution increment ∆n, equal to 4, is needed. hence, n = 256 is the required number of samples for the average or, alternatively, the oversampling ratio. considering the output of the electrochemical sensors constant in time intervals much shorter than their time constants, we can sample the sensor output during a period tm = 0.5 s (≈ τ/5), that means that the needed sampling frequency of the adc is 512 hz. in these conditions, the expected adc current consumption during the sampling period is around 100 na that is a negligible value with respect to cpu current consumption. the cpu is turned on only for the time required to sample the signal and transmit it over the radio link. the radio module is a libelium 868 mhz, sx1272 lora module, whose current consumption is approximately 𝐼𝐿𝑜𝑅𝑎 =28 ma in transmission. overall current consumption of the node during the wake-up period will be 𝐼𝑂𝑁 = 𝐼𝐿𝑜𝑅𝑎 + 𝐼𝜇𝑈 + 𝐼𝑆𝑈 , (2) where 𝐼𝜇𝑈 is the current absorption of the micro-module and 𝐼𝑆𝑈 is the current absorption of the sensor unit. assuming the previously discussed values for these parameters, the overall current absorption of the sensor node when on is calculated as follows: 𝐼𝑂𝑁 ≅ 28.00 𝑚𝐴 + 0.84 𝑚𝐴 + 0.01𝑚𝐴 ≅ 28.85 𝑚𝐴 . (3) since in its simplest configuration the sensor node is expected to be battery-powered, in order to estimate its life time, it is possible to analyse the power consumption in terms of battery capacity dissipation, measured in mah. in order to wake up and transmit three packets, the module requires around 5 seconds. if assuming an hourly sampling rate (and then an activity period of 5 seconds each hour), the average hourly current absorption of the node can be calculated as follows: 𝐼ℎ𝑡𝑜𝑡 = 𝐼𝑂𝑁 ∙ 𝑡𝑂𝑁 + 𝐼𝑂𝐹𝐹 ∙ 𝑡𝑂𝐹𝐹 1 ℎ𝑜𝑢𝑟 , (4) where 𝐼𝑂𝐹𝐹 is the current absorbed when the sensor node is sleeping and 𝑡𝑂𝐹𝐹 is the sleeping period. this value is calculated as follows: 𝐼ℎ𝑡𝑜𝑡 = 28.85 𝑚𝐴 ∙ 5 𝑠 + 0.007 𝑚𝐴 ∙ 3595 𝑠 3600 𝑠 = 0.047 𝑚𝐴 . using two 1.5 v aa lithium batteries, whose nominal capacity is 𝐶𝑏 = 3000 mah, the average life time 𝑡𝑙𝑖𝑓𝑒 of the node is: 𝑡𝑙𝑖𝑓𝑒 = 𝐶𝑏 𝐼ℎ𝑡𝑜𝑡 = 3000 𝑚𝐴ℎ 0.047 𝑚𝐴 ≅ 63830 ℎ ≅ 2660 𝑑𝑎𝑦𝑠 . this means that the sensor node can theoretically operate for more than 7 years. if taking into account the impact of environmental conditions, the non-ideal behaviour of the batteries and the discharge rate of the batteries that does not allow to fully exploit their capacity, and halving this value, the life time of the node is still larger than the one of the sensors. the node is then able to satisfy the initial requirements. the lora-internet gateway receives all the packets transmitted by the node and through an ad-hoc api transfers them to the cloud infrastructure in charge of storing them into a mysql database. the prototype of the gateway has been realised exploiting an arduino mkr1000 wifi board connected to a libelium 868 mhz, sx1272 lora module. while this prototype is based on wifi connection, alternative solutions could have been set up by using either wired ethernet connection or wireless gprs/umts connection, by simply using dedicate transmission modules. in case of the gateway, no energy consumption analysis has been performed since this kind of device is expected to be always on to receive the packets from the single sensor nodes. in this case, it must be mandatorily connected to a continuous source of energy such as the electricity grid or some kind of reliable energy harvester as for example solar cells. 4. tests and validation the tests on the operation of the sensor node have focused on both data acquisition by the sensor unit and data transmission reliability by the communication module. the sensor unit was characterised in laboratory, in terms of response to the target gases: co, no, no2 and o2. cross-sensitivity was tested and found to be negligible as declared by the producer. moreover, the influence of environment temperature and humidity was assessed. these measurements are obtained by analysing test gases and by means of a characterisation system, which allows for generating reference mixtures of air and target gases, with (a) (b) figure 5. tests of the sensor nodes. a) laboratory measurements on reference gas mixtures. no and co mixtures in air (rh = 50 %, flow rate 200 ml/min). the ‘true’ concentrations of the two gases are indicated with green and black lines. b) test of sensor unit in field (industrial exhaust gases). acta imeko | www.imeko.org june 2019 | volume 8 | number 2 | 59 known compositions, starting from reference gas cylinders and using mass-flow controllers bronkhorst (accuracy 1 % of the full scale). the total flow during a measurement is constant (in the results reported in this paper equal to 200 ml/min), whereas its composition can change dynamically during a measurement. the gas humidity is set by means of a bubbler containing ultrapure water, kept at known temperature in which the dry air is saturated; the desired relative humidity (rh) value is obtained by mixing the flow from the bubbler with a dry gas. the data shown in figure 5a, concern an example of calibration measurements obtained using a mixture of oxygen, nitrogen, co and no, simulating possible operating conditions. the oxygen in the test had a fixed concentration equal to 16 % (volume/volume) whereas the concentrations of both co and no were varied during the measurement, following the theoretical profiles represented in the figure by the black and the green line respectively. the magenta and the blue lines represent the measured co and no concentrations: this test allows for assessing the very low cross-sensitivity of the co and no sensor. the electrochemical sensors exhibit in general a linear dependence of the output current on the gas concentration, and the measurements can be effectively corrected for the effect of temperature. in figure 5b an example of data coming from an in-field test of the monitoring system concerning the emission of a combustion process is shown. note that in the test phase the sample rate used for measuring gas concentrations is higher than the one that will be used in the final deployment. after the tests on the sensor unit, the overall sensor node architecture has been tested, focusing on the reliability of the lora minimal communication infrastructure described in section 2. the data transmission was validated with indoor tests, performed in the main building of the department of information engineering and mathematical sciences of the university of siena (see figure 6). this structure has been identified as suitable for the tests due to its size and features that make it similar to a real deployment scenario. indeed, the building has 80 m × 100 m rectangular shape, with a total surface of around 7.000 m2. the long sides are roughly oriented northsouth, while the short ones are oriented east-west. it is structured on five floors divided into a large number of rooms separated by large concrete walls that in some cases reach a 1 m width. the rooms host scientific laboratories where electromagnetic instrumentation is used: moreover, the whole structure is covered with wifi connection. the level of electromagnetic noise is therefore very high. in order to test the reliability of the communication channel, the sensor sampling rate was set at six samples per minute, i.e.1 sample each 10 s. table 3 shows the radio settings for the lora modules, chosen to maximise the reading range according to the networking guide [39]. the packet loss rate was calculated by counting the number of packet losses across four min time span. the gateway node was placed inside a room positioned in the south-western corner of the third floor of the building, in order to test the functioning in a worst-case scenario: indeed, in a real deployment the ideal positioning would have been in the centre of the building. the data loss rate was checked across all the five floors, by checking the received packet rate in ten different spots: four at the corners of the building, four in the intermediate points of the long sides and two in the intermediate points of the short sides. figure 7 shows the spots where packet reception was tested on the map of the building, marked with yellow dots, while the star marks the position of the gateway. figure 7. building map with the positions of the reception spots (marked by the yellow dots) and the gateway (marked by the star). table 3. current absorption of the sensor node sub-systems. current absorption for one measurement cycle (1 hour) phase device absorption (a) total (a) period (sec) measurement (ion) microcontroller unit (u) 840 28847 5 sensor unit (su) 7 radio module (lora) 28000 standby (ioff) microcontroller unit (u) 0 7 3595 sensor unit (su) 7 radio module (lora) 0 average cycle current absorption 47.06 a figure 6. aerial view of the main building of the department of information engineering and mathematical sciences of the university of siena, acting as a test site for the lora network. acta imeko | www.imeko.org june 2019 | volume 8 | number 2 | 60 a 0 % data loss rate was achieved on all the floors for the tests achieved in eight out of ten spots: the only two points where data losses occurred were the north-eastern corner (the farthest from the gateway) and the adjacent point on the northern side. the data loss rates for the two spots are shown in table 4. 5. conclusions the aim of this paper was to propose and test an iot infrastructure to be employed for the monitoring of chemical emissions in different contexts, from industrial plants to smart cities scenarios. the two main requirements to be accomplished were the low-power consumption of the sensor nodes and their modularity. the first requirement has been satisfied by using low-power components and adopting duty-cycling policies to switch off the more energy-hungry parts of the node (i.e., the radio module). calculations show that the architecture described in this paper allows to achieve a life time for the node as long as the life time of the sensors, that need to be changed every 24 months. the modularity requirement has been satisfied adopting an architecture for data transfer from the sensors to the micromodule that allows the transparent addition of a new sensing device. the architecture described in this paper is then expected to be seen as a general-purpose framework to be shaped on different scenarios, ranging from small dimension industrial plants to large urban areas. tests were performed to demonstrate the effectiveness of the proposed solution: while they were focused on a sub-set of the proposed infrastructure, it is expected that the results can easily be extended to a larger scale scenario. indeed, these tests have demonstrated the effectiveness of the proposed approach to setup pervasive infrastructures for the monitoring of chemical emissions. references [1] i. linnerud, p. kaspersen, t. jaeger, “gas monitoring in the process industry using diode laser spectroscopy”. appl. phy.s b: lasers o., 1998, 67(3), pp. 297-305. 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[20] r. almeida, r. oliveira, d. sousa, m. luis, c. senna, s.a. sargento, “multi-technology opportunistic platform for environmental data gathering on smart cities”. in 2017 ieee globecom workshops (gc wkshps), 2017, pp. 1-7. [21] s. dong, s. duan, q. yang, j. zhang, g. li r. “mems-based smart gas metering for internet of things”, in ieee internet things, 2017, 4(5), pp. 1296-1303. [22] n.p.g. bhavani, k. sujatha, r.s. ponmagal, t.k. reddy, “monitoring of so2 emissions in power plants using internet of things”, 2017 international conference on energy, communication, data analytics and soft computing (icecds), 2017, pp. 1064-1067. [23] t. addabbo, b. bardi, s. cioncolini, a. fort, m. mugnaini, l., parri, v. vignoli “multi-sensors exhaust gas emission monitoring system for industrial applications”, to be published in lecture notes in computer science, springer. [24] a. pozzebon, aicappelli, a. mecocci, d. bertoni, g. sarti, f. alquini, “a wireless sensor network for the real-time remote measurement of aeolian sand transport on sandy beaches and dunes”, sensors, 2018, 18(3), 820. [25] a. pozzebon, a. andreadis, d. bertoni, c. “a wireless sensor network framework for real-time monitoring of height and volume variations on sandy beaches and dunes” isprs int. j. geo-inf., 2018, 7(4), 141. table 4. data loss rates for the test points. test point 1-8 9 (north side) 10 (north-east corner) first floor 0.0 % 25.0 % not achievable second floor 0.0 % 12.5 % 87.5 % third floor 0.0 % 12.5 % 41.7 % fourth floor 0.0% 29.8 % 50.0 % fifth floor 0.0 % 50.0 % 100.0 % acta imeko | www.imeko.org june 2019 | volume 8 | number 2 | 61 [26] a. pozzebon, “integrating rfid transponders as data loggers in wireless sensor nodes for outdoor remote monitoring operations”, international journal of wireless information networks, 2015, 22(4), pp. 399-406. [27] a. mecocci, g. peruzzi, a. pozzebon, p. vaccarella, “architecture of a hydroelectrically powered wireless sensor node for underground environmental monitoring”, iet wireless sensor systems, 2017, 7(5), pp. 123-129. [28] a. augustin j. yi t. clausen, w.m. townsley “a study of lora: long range & low power networks for the internet of things”, sensors. 2016, 16:1466. [29] m. cerchecci, f. luti, a. mecocci, s. parrino, g. peruzzi, a. pozzebon, “a low power iot sensor node architecture for waste management within smart cities context”, sensors, 2018, 18(4). [30] zigbee alliance, zigbee specification, 2012. [31] m. tabassum, k. zen, “performance evaluation of zigbee in indoor and outdoor environment”, in 2015 9th international conference on it in asia (cita), 2015, pp. 1-7. [32] i. kuzminykh, a. snihurov, a. carlsson, “testing of communication range in zigbee technology”, in experience of designing and application of cad systems in microelectronics (cadsm), 2017 14th international conference, 2017, pp. 133-136. [33] semtech, sx1272/73 860 mhz to 1020 mhz low power long range transceiver datasheet, 2017. [34] f. adelantado, x. vilajosana, p. tuset-peiro, b. martinez, j. meliasegui, t. watteyne, “understanding the limits of lorawan”, ieee commun. mag., 2017, 55(9), pp. 34-40. [35] j. haxhibeqiri, a. karaagac, f. van den abeele, w. joseph, i. moerman, j. hoebeke, “lora indoor coverage and performance in an industrial environment: case study”, in 2017 22nd ieee international conference on emerging technologies and factory automation (etfa), 2017, pp. 1-8. [36] lora alliance, a technical overview of lora and lorawan, 2015. [37] t. addabbo, a. fort, m. mugnaini, e. panzardi, a. pozzebon, m. tani, v. vignoli, “a low cost distributed measurement system based on hall effect sensors for structural crack monitoring in monumental architecture”, measurement, 2018, 116, pp. 652-657. [38] t. addabbo, a. fort, m. mugnaini, l. parri, s. parrino, a. pozzebon, v. vignoli, “an iot framework for the pervasive monitoring ofchemical emissions in industrial plant”, proc. of ieee international workshop on metrology for industry 4.0 and iot, 2018, pp. 269-273. [39] libelium, waspmote lora 868 mhz-915 mhz sx1272 networking. age determination and authentication of ceramics: advancements in the thermoluminescence dating laboratory in torino (italy) acta imeko issn: 2221-870x march 2021, volume 10, number 1, 32 39 acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 32 age determination and authentication of ceramics: advancements in the thermoluminescence dating laboratory in torino (italy) laura guidorzi1,2, fulvio fantino3, elisabetta durisi1,2, marco ferrero2,4, alessandro re1,2, luisa vigorelli1, lorenzo visca1,2, monica gulmini5, giovanni dughera2, giuseppe giraudo2, debora angelici3, elisa panero6, alessandro lo giudice1,2 1 dipartimento di fisica, università di torino, via pietro giuria 1, 10125 torino, italy 2 infn sezione di torino, via pietro giuria 1, 10125 torino, italy 3 tecnart s.r.l., via modena 58, 10153 torino, italy 4 università del piemonte orientale, largo guido donegani 2, 28100 novara, italy 5 dipartimento di chimica, università di torino, via pietro giuria 5, 10125 torino, italy 6 musei reali di torino, ministero per i beni e le attività culturali, piazzetta reale 1, 10122 torino, italy section: research paper keywords: thermoluminescence; dating; customised instrumentation; authentication citation: laura guidorzi, fulvio fantino, elisabetta durisi, marco ferrero, alessandro re, luisa vigorelli, lorenzo visca, monica gulmini, giovanni dughera, giuseppe giraudo, debora angelici, elisa panero, alessandro lo giudice, age determination and authentication of ceramics: advancements in the thermoluminescence dating laboratory in torino (italy), acta imeko, vol. 10, no. 1, article 6, march 2021, identifier: imeko-acta-10 (2021)-01-06 editor: eulalia balestrieri, university of sannio, italy received april 9, 2020; in final form september 9, 2020; published march 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: alessandro re, e-mail: alessandro.re@unito.it 1. introduction the dating process of archaeological records can be of the utmost importance for an in-depth understanding of ancient civilisations and their development. given the need for accurate temporal collocation, absolute dating techniques have, over time, completely complemented the archaeological relative approach [1]-[3]. luminescence is a suitable parameter for characterising cultural heritage objects, being directly proportional to some of the intrinsic characteristics of the material. therefore, out-ofcontext objects can also be investigated [4], [5]. the team at the university of torino extensively applies various luminescencebased techniques, such as ionoluminescence [6]-[9] and x-ray luminescence [10], to this field, always pursuing the development of customised instrumentation tailored to archaeological and artistic samples. in 2007, a thermoluminescence (tl) dating laboratory was opened in the physics department, in collaboration with the national institute of nuclear physics (infn) and tecnart srl, a small enterprise dedicated to the scientific analysis of cultural heritage founded as a spin-off of the university of torino. in its first decade of activity, the tl dating laboratory has provided excellent support to archaeological surveys in the nearby piedmont area [11], [12]. the laboratory, the only one in piedmont and one of the few providing tl dating and authentication in italy, has been progressively expanded with new instruments and methods, which are presented in this work. abstract classified as an absolute dating method, thermoluminescence (tl) is a well-established radiation-based technique for the age determination and authentication of ceramic materials. specifically, this method allows the determination of the time elapsed since kiln firing (or later fire events) by evaluating the luminescent emission of ceramics under heating at high temperatures. this paper provides a comprehensive presentation of the tl laboratory developed over the last decade at the physics department of the university of torino. the laboratory was set up in collaboration with tecnart s.r.l. and is also currently operating within the cultural heritage network of the national institute of nuclear physics (infn-chnet). more than 10 years of experience in the field has resulted in improvements in procedures, with the development of customised αand β-irradiation systems and the optimisation of sampling approaches and chemical pre-treatment. thanks to tecnart s.r.l., the laboratory has been employed for dating and authenticating hundreds of archaeological sites and artworks, some of which are discussed in this work and compared, when possible, with radiocarbon dating. mailto:alessandro.re@unito.it acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 33 the use of the tl process for dating purposes was introduced by aitken et al. in the 1960s [13], [14] and is widely employed nowadays [15]-[18]. the technique allows the determination of the age of a fired clay object by measuring the natural radiation absorbed in some of its compositional minerals (e.g. quartz and feldspars) since the last firing. the luminous signal emitted by the material, when heated at a few hundred degrees celsius in the laboratory, is proportional to the absorbed dose [19]. this means that not only the absorbed dose (palaeodose) is discovered, but also the dose rate to which the artefact has been subjected over time (annual dose), which depends on environmental conditions and varies according to the geographical area [20]-[22]. consequently, the age can be calculated by the simple ratio between the two quantities. several measurements are therefore required to provide a complete dating. first, the palaeodose should be evaluated by recording the tl signal from the subsamples exposed to increasing doses of artificial irradiation from β sources. this is known as the additive method [13]. the effective dose in grays (the only component of the palaeodose in which there are no supralinearity effects) can then be extrapolated via a linear regression of the integrated tl signal versus the corresponding value of the added artificial dose. the temperature interval to be used for integration is defined by the ‘plateau test’, which identifies the most thermally stable defect energy levels to be taken into account for a correct evaluation of the age [13], [14], [23]. the data needed to calculate the annual dose are then collected by measuring the artefact α activity (caused by the decay chain events of uranium and thorium atoms contained in the clay matrix), the environmental dose rate and the 40k-isotope concentration in the material. in addition, further corrections for water absorption, the supralinearity of the tl signal and anomalous fading [13], [14], [24] are conducted. the high accuracy of all these measurements yields a final-date error that is generally lower than 15%, or even 5% in more favourable cases [25]. thermoluminescence is an invasive technique and, for a complete dating, needs the sampling of about 1–2 grams of powdered material for the preparation of subsamples via the fine grain method [26]. clay material from archaeological objects is often contaminated by organic matter and carbonates (the latter enhanced by long exposure to highly humid burial grounds). this requires a chemical pre-treatment of the powder before the fine grain selection to eliminate these sources of spurious luminescence [13], [27], [28]. in the following section, the equipment and procedures adopted are presented; the results of two recent case studies, one for a complete dating and one for an authentication, will then be illustrated in sections 3 and 4. 2. the tl laboratory at the university of torino in establishing the tl laboratory in the physics department of the university of torino, it was decided not to use an all-inone tl analyser so that specific components (tl reader, irradiation systems, etc.) could be adapted to the needs of analyses and samples as they arose. with the exception of the irradiation systems, which were specifically developed in our laboratory, the employed instrumentation follows the schematics and specifics illustrated in the comprehensive textbook by aitken [13]. the main instruments currently used in the laboratory are described in the next sections. all the equipment is operated under red light, as are the sampling procedures. if necessary, sampling is followed by a chemical pre-treatment of the powder, which is outlined in section 2.7. 2.1. tl measurement apparatus the instrument setup for thermoluminescence measurement consists of a sample-holding chamber, a 9235qa electron tubes photomultiplier powered at -1100 v by an hivo voltage supply and the ipses-tl2000 control unit, regulated by dedicated software. two filters are inserted between the sample and the phototube: a kg5 filter that cuts the ir radiation and a bg25 filter that selects only photons in the blue range. the sample is placed in the chamber on a heating metal strip; a thermocouple allows the monitoring of the temperature and the control of the heating rate (generally kept constant at 5–10 °c/s). during measurements, a vacuum is generated in the chamber by a rotative pump, and nitrogen is inserted up to a pressure of 0.4– 0.6 bar to avoid spurious luminescence and to achieve homogeneous heating for the sample. glow curves, i.e. the intensity of the tl emission vs time, are obtained to record the tl signal every second as the temperature increases from 50 °c to 450 °c at a fixed heating rate. 2.2. alpha-irradiation system a customised α-irradiation system (figure 1) was designed, realised and optimised to evaluate the α-particle efficiency (k) [29] for each analysed object. this parameter is calculated by comparing the effective doses obtained by α and β irradiation on the same sample [13]. the upper part of the steel chamber contains a 241am source with nominal 4 mbq activity and an active surface of 16 cm2. the α source is shielded at the top and sides; the active surface, facing down towards the sample, is protected by a 2 µm au-pd layer. in addition, a steel and lead shield is situated between the source and the sample-holding wheel with an automatic shutter that opens only during measurements. since the penetration range of an α particle in air is just a few centimetres, a vacuum is needed for the particle to reach the samples. with an optimal pressure of 170 mbar, the particles reach the sample with a mean energy of about 2.9 mev, providing a dose of 2.0 ± 0.1 mgy/s. the emission rate and spectrum are measured by means of an ortec particle silicon detector installed in one of the sampleholder positions. 2.3. beta-irradiation system for the determination of the palaeodose, β irradiation is conducted using a 90sr/90y source with nominal activity of 1.48 gbq and a dose rate of 13.2 ± 0.1 mgy/s. the irradiation apparatus was designed and developed by the infn (figure 1). the new system replaced an older one, which was also custom made but bearing a source with 0.79 gbq activity and a 7.0 ± 0.1 mgy/s dose rate. as a consequence, irradiation times are now almost halved. the source is installed in a brass box, irradiating downwards and facing the sample holder. the sample holder is a wheel with four slots that is accessible through the external brass case by a hole with a screw top. additional shielding is provided by lead plates contained in the black plexiglass case, as shown in figure 1. the whole system is fully automated, controlled by dedicated labview-based software, and four irradiations in a row can be set up without reopening the case. furthermore, a quartz window with a thickness of 8 mm works as a shutter between the source and the sample wheel when the system is not in use. quartz was chosen as a compromise between the β stopping power of the material used as a shutter acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 34 and the need to minimise braking radiation production. before the realisation, the whole apparatus was simulated using mcnp4c software [30] to calculate the irradiation dose rate and to verify the absence of radiation leakages. in addition, the high repeatability of irradiations was tested and confirmed using (lif:mg,ti) tld-100 dosimeters. 2.4. alpha-activity measurements the uranium (u) and thorium (th) atoms contained in the clay matrix of the object decay by α-particle emission and contribute to the annual dose. the total α activity can be easily determined by covering a zns scintillator with a homogeneous layer of ceramic powder (at least 600 mg) [13]; the scintillator emits photons each time it is struck by the α particle, and these photons are collected by a photomultiplier powered at -1100 v. the generated electric signal is transferred to an amptek mca 8000a multichannel analyser, and the real-time spectrum of the α counts is created. the system can also evaluate the u/th activity ratio using the ‘pairs’ technique [13]. this measurement usually lasts at least 7–10 days for suitable statistics, as the typical count rate �̇� for ceramics generally varies between 10 and 20 counts/ks. 2.5. environmental dose measurements the dose provided to the artefact by the radioisotopes present in the surrounding environment can easily be evaluated using thermoluminescent dosimeters (tlds). these are doped crystals, typically lif or caso4/caf2, which indiscriminately accumulate energy from α, β and γ particles, and their cumulative dose can be measured by means of thermoluminescence [31]. tlds need to be placed in the exact spot the sample was collected from, recreating the environment in which the sample was conserved before discovery, and kept in place for a long time (approximately one month). if it is not possible to place dosimeters in situ, a good alternative is to conduct γspectroscopy measurements on the excavation soil [32], [33]. tl laboratory users have access to the biomedical physics instrumentation at the university of torino, which is equipped with a cooled cylindric high purity germanium (hpge) detector with a diameter of 48.2 mm; special beakers with ‘marinelli’ geometry [34] are used because of the cylindrical symmetry. the analysis supplies the activities of each radioisotope and allows the measurement of the annual dose provided by the u and th chains as well as by 40k. finally, if it is not possible to obtain a sufficient amount of soil for the analysis, the activity can be set as equal to that of the artefact as a first approximation. 2.6. other measurements if the sample is conserved in a highly humid environment, the open porosity of the ceramic material can be filled with water that partially absorbs the ionising radiation. a correction to the annual dose is then needed, using the ratio 𝑊 = 𝑤wet − 𝑤dry 𝑤dry , (1) where w is the weight of the sample. for an evaluation of w, a small fragment of the archaeological sample is saturated with distilled water, then a sartorius ma35 moisture analyser is used to heat it for 15 minutes up to 150 °c. the weight is measured every second with an accuracy of 1 mg; when no further change in weight is detected, the w ratio is displayed. the moisture fraction content f additionally takes into account the environmental history of the artefact and specifies the average fraction of the open porosity that has actually been filled with water. a typical value for medium–high humidity environments is f = 0.8 ± 0.2 [13]. the quantification of potassium (k) content for the sample is obtained via icp-oes analysis performed in the chemistry department of the university of torino with a perkin-elmer optima 2000 spectrometer. the resulting percentage by mass for k is then used to directly calculate the contribution to the annual dose of the radioisotope 40k, up to some multiplicative constants [13], [35]. 2.7. sample pre-treatment often, the sampled powder, collected with a low-speed small drill, needs a chemical treatment for the removal of carbonates and organic matter to avoid spurious luminescence [13], [27], [28]. at first, the procedure developed by the iramat-crp2a laboratory of bordeaux was employed, using three different reagents: hydrochloric acid (hcl), hydrogen peroxide (h2o2) and hydrofluoric acid (hf) [15]. in recent years, our tl laboratory has customised this procedure on the basis of application experience. it was verified that the use of the same reagents in higher concentrations but for a shorter time provides the same results, simultaneously speeding up the method. moreover, it is not always necessary to proceed with the three reagents in a sequence; depending on the expected composition of the sample and of the burial environment, one single treatment, either with hcl or with h2o2, can be chosen. acetic acid (ch3cooh) in concentrations between 30 % and 10% w/w can also replace hydrogen peroxide in organic matter removal. considering the shorter reaction time required for this preparation, it can be more convenient when the presence of organic matter is prevalent. it must also be stressed that each step of the pre-treatment procedure causes the loss of part of the material. this is a critical issue, as only small samples are usually available from artefacts in the cultural heritage domain, particularly if authentication of a precious object is the goal of the whole procedure. to limit these losses, a vacuum system coupled with a 1 µm pore-sized paper filter is used to completely retrieve and rinse the powder in one single step. the treated powder is then detached from the filter by a quick transfer in an ultrasonic bath, directly immersing the filter in the next reagent or in acetone if the treatment procedure is accomplished. figure 1. on the left: α-irradiation system with a view of the inner sampleholder wheel. on the right: β-irradiation system and a scheme of its internal structure. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 35 after the chemical treatment, the powder is deposited on the analytical aluminium disks using the fine grain technique described in [26]. 2.8. final age evaluation the age of an object subjected to the complete dating procedure is obtained by calculating the ratio between the palaeodose and the annual dose. expanding these quantities into their composite terms, the age can be written as 𝐴𝑔𝑒 = 𝐸𝐷 + 𝑆 𝑘 ∙ 𝐷α + 𝐷β + 𝐷env , (2) where ed is the effective dose measured using β irradiation and s is the supralinearity [13], [14]. all contributions to the total annual dose, both internal (α and β) and external (environmental) to the sample, are set out in the denominator, taking also into account the corrective coefficient k for α dose. to be able to evaluate the annual dose from all the measurements listed above, equation 2 should be made explicit, considering the numerical coefficients extensively described in [13] and [35], which provide a dose rate in µgy/year and a final age in years, when w is expressed as a fractional number: 𝐴𝑔𝑒 = (𝐸𝐷 + 𝑆)⸱106 𝑘 ∙ 1594 ∙ �̇� 1 + 1.5 ∙ 𝑊 ∙ 𝐹 + (72.2 ∙ �̇�) + (782 ∙ 𝐾) 1 + 1.25 ∙ 𝑊 ∙ 𝐹 + 𝐷env . (3) capital k is the measured potassium content, expressed as percentage by mass. the calculation of age using equation 3 is supported by an ad hoc notebook written in mathematica software [36]; simply by inserting the measured values and running the program, all the resulting quantities and associated uncertainties are provided in a short time. 3. case study i: dating of bricks from an excavation site in trino (vc), italy in 2016, the excavation for the construction of a new gas pipeline in vercelli province, piedmont (italy), revealed the presence of some ancient brick structures. the institution in charge of the rescue of the archaeological site (soprintendenza archeologia belle arti e paesaggio delle province di biella, novara, verbano-cusio-ossola e vercelli) was interested in obtaining tl-dating analyses for bricks from a tomb and from a building found outside the city of trino, a small town about 60 km from torino, at geographical coordinates 45°11’30.5”n– 8°16’51.1”e. tl-dating results were especially important, as the site was not particularly rich in moveable findings and was therefore lacking in age reference materials. 3.1. structures and in situ sampling the first of the two structures chosen for tl analysis was a burial chamber with w–e orientation, built with an outer stone wall and an inner brick one (figure 2). inside the tomb, a brick curb was also present. according to the archaeologists, this had been added later to the main construction in order to create a new space for another, smaller corpse. four bricks were sampled, one from each wall and all from the deepest level of the tomb. in particular, sample t1_c and t1_d were extracted from the main structure, whereas t1_a and t1_b were collected from the inner curb. tlds were placed in the sampling spots, covered with excavation soil and left in situ for 28 days. a short distance away from the burial, well-preserved brick flooring with two wooden beams was found (figure 2). the shape of the bricks and the type of construction suggested to the archaeologists that this building could be much more recent than the tomb. due to the short time available before the closing of the rescue excavation, only one sample (e1) was retrieved. as it was impossible to leave a dosimeter on site, the environmental dose was obtained, in this case, by means of γ-spectrometry measurements on both the powdered brick and the surrounding soil. figure 2. samples from the trino excavation site and mean tl glow curves. on the left: tomb t1, its four samples and their original position. on the right: the studied brick from the building (e1) and optical microscope images of its ‘hard’ face (upper row) and ‘soft’ face. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 36 3.2. tl-dating procedures for each brick from the grave, at least three values of effective dose (ed) were calculated using the additive dose method [13] with β irradiations (figure 3); the effective dose is exactly equal to the palaeodose if supralinearity is absent. the collected powder (about 1 g per sampling by drilling) was treated only with hcl 15 % w/w for 45 minutes, but no evidence of the formation of gaseous carbon dioxide was observed. the brick from the building presented a peculiar hardness on one of the larger faces, whereas the specular face had a soft ceramic body and was easy to sample with the drill. the comparison of microscopical observations did not highlight differences beyond a more intense reddish hue of the soft part; however, the tl natural signals were quite different, as the one from the hard side seemed to be lower than the one from the soft side at temperatures below 300 °c (see figure 2). powder from the bulk was also analysed, and the tl response was compatible with the soft side. it could be assumed that this brick was subjected to an inhomogeneous kiln firing or that a high temperature blaze had reached it. based on this hypothesis, only results from the soft face and the bulk were considered for statistical purposes in the effective dose determination, although the chemical treatments were different for the two parts of the brick. powder from the surface was treated for 45 minutes with hcl 15 % w/w; however, in the bulk, the organic component was high enough to alter the tl natural signal, so a 60-minute treatment with ch3cooh 10 % w/w was preferred. in addition, for all the samples, about 1 g per brick was subjected to α spectroscopy. the absence of supralinearity was verified using the regenerative method [13], anomalous fading was checked and k quantifications with icp-oes were performed. finally, the effective dose via α irradiation was also obtained in order to calculate the k coefficient. all the values resulting from the tl-dating procedure can be found in table 1. these are directly measured (ed β, supralinearity, anomalous fading, α counts, w %, k % w/w, ed α, environmental dose) or derived via calculation using the same measured values (palaeodose, k coefficient, annual dose and, finally, the age). 3.3. tl-dating results the age determination for samples coming from tomb t1 led to a very interesting result: bricks t1_a and t1_b from the inner curb were dated to the end of the eighth century ad (790 ± 110 ad and 750 ± 80 ad, respectively) but the other two samples to the fourteenth century ad (1300 ± 50 ad for t1_c and 1340 ± 70 ad for t1_d). in such a case, tl dating assigns the burial as contemporary or subsequent to the more recent component; hence, reused material had been employed for building the curb. it should be noted that the medieval bricks could have been reused as well; a comparison with radiocarbon dating on the skeletons could then be crucial but, unfortunately, the bones and teeth had deteriorated too much as a result of the highly acidic soil. the case of this tomb highlights how important the in-situ sampling phase can be in results interpretation. if only bricks near t1_a and t1_b had been collected, they would not have been representative of the real age of the tomb and would have led to an inaccurate attribution. due to the impossibility of using an environmental dosimeter, the age determination for the brick from the floor led to two values, depending on whether the annual dose was determined by means of γ-spectroscopy measurements on the brick itself or on the surrounding soil: 1670 ± 50 ad using the brick and 1620 ± 60 ad using the soil. the two calculated ages are compatible and confirm in both cases the archaeological hypothesis of a more recent construction. this is additionally supported by the results of radiocarbon-dating analysis performed at the infnlabec laboratory (florence) on one of the wooden beams. the measured radiocarbon age (125 ± 25 years bp) classifies the sample as modern, meaning that its calibrated calendar age falls between approximately 1650 ad and 1955 ad. it is worth noting that, in this case, tl led to a more precise date in respect table 1. results of tl-dating measurements for samples from trino. for sample e1, a distinction is reported for results obtained using γ spectroscopy on soil (s) and on brick (b). sample t1_a t1_b t1_c t1_d e1 (soft surface and bulk) chemical pre-treatment hcl 15 % w/w hcl 15 % w/w hcl 15 % w/w hcl 15 % w/w hcl 15 % w/w (surface) ch3cooh 10 % w/w (bulk) # ed β evaluations 3 3 4 3 3 largest t-integration interval from plateau test (c°) 320–380 350–390 330–370 350–400 350–380 ed β (gy) 4.27 ± 0.28 4.73 ± 0.11 2.75 ± 0.08 2.57 ± 0.19 1.10 ± 0.18 supralinearity none none none none none anomalous fading none none none none none palaeodose (gy) 4.27 ± 0.28 4.73 ± 0.11 2.75 ± 0.08 2.57 ± 0.19 1.10 ± 0.18 α counts (ks-1) 16.1 ± 0.1 13.3 ± 0.1 13.0 ± 0.1 12.3 ± 0.1 11.0 ± 0.1 w (%) 16.65 ± 0.01 13.65 ± 0.01 11.48 ± 0.01 15.29 ± 0.01 13.52 ± 0.01 k (% w/w) 1.26 ± 0.08 1.16 ± 0.08 1.30 ± 0.09 1.27 ± 0.09 1.19 ± 0.08 ed α (gy) 233 ± 35 115 ± 23 66 ± 3 37 ± 5 23 ± 5 k coefficient 0.015 ± 0.003 0.034 ± 0.007 0.035 ± 0.002 0.058 ± 0.009 0.040 ± 0.010 environmental dose (μgy/year) 1290 ± 40 1460 ± 60 1430 ± 190 1210 ± 250 s 670 ± 90 b 1040 ± 100 annual dose (μgy/year) 3470 ± 230 3740 ± 240 3840 ± 270 3820 ± 350 s 2800 ± 240 b 3160 ± 250 age (ad) 790 ± 110 750 ± 80 1300 ± 50 1340 ± 70 s 1620 ± 60 b 1670 ± 50 acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 37 of radiocarbon. however, although the flooring appeared very homogeneous in material constitution, the analysis of only one brick from the building cannot represent the whole structure. 4. case study ii: authentication of pre-columbian pottery in 2019, a ceramic artefact from the dominican republic was studied to be authenticated. the analysed object (figure 4) is a symmetrical shaped bottle about 60 cm high, and it was attributed to the pre-columbian population inhabiting the island before the european conquest in the late fifteenth century. clay pottery was introduced to the caribbean islands, from the lesser antilles to puerto rico, by people from the ‘early ceramic age’ around 500 bc. in hispaniola, for reasons still unknown, ceramics production was introduced about one millennium later, although older ceramics have been found resulting from interisland transport, as evidenced by the exchange of heirlooms [37], [38]. the assignation of this artefact to the pre-columbian period and related populations could enrich the knowledge and studies on stylistics and trade routes. 4.1. sampling and analysis the authentication procedure can be described as a test to verify if measurable tl parameters, in particular ed, are compatible with values expected for an artefact belonging to the period suggested by archaeologists on the basis of stylistic features [4], [39]-[41]. usually, due to the value of the objects to be authenticated, only a small quantity of powder can be sampled, weighing some few hundreds of milligrams. the scarce availability of material to be analysed prevents the measuring of all the parameters described above for tl dating. moreover, authentication is usually requested only for decontextualised objects, i.e. when the exact burial spot or the conservation place is unknown. in these cases, it is not possible to retrieve the specific environmental dose, and the whole set of values for one or more possible provenance areas must be considered. if necessary, data can be partially obtained from the scientific literature, although, in many cases, these data are scarce or absent for a specific typology of object or geographical area. in the case under investigation, it was possible to collect 480 mg of powder from the base of the artefact. the whole amount was first used to measure the α activity, which is sufficient in many cases, together with the ed measurement, to address the authentication issues, and is, nonetheless, useful additional information to complement the literature. the same powder, unmodified in any way, was then used to determine the effective dose via the additive method using 9 gy and 18 gy β doses. usually, pre-treatments in authentication are reduced to a minimum in order to preserve the small quantity of powder available. no chemical pre-treatment was performed in this case due to the suitable tl response of the material. 4.2. authentication results the glow curves obtained with the additive method are reported in figure 4, and the corresponding effective dose resulted in 8.5 ± 1.0 gy. anomalous fading was negligible. the annual dose for similar objects from hispaniola was not found in the scarce literature, but by using the maximum spread of the values indicated for neighbouring areas [37], [38], [42], [43], a reasonable interval of 2.2–6.2 mgy/year could be considered. the value obtained for the effective dose leads to a minimum age of about 1,000 years, when the minimum ed at 2 σ and the maximum annual dose values are considered. it is worth noting that the measured �̇� rate from the α activity can also be used to roughly estimate the minimum age of the artefact. in this case, the �̇� rate was 9.6 ± 0.1 counts/ks, and the subsequently calculated α contribution to the annual dose was 1.4 ± 0.7 mgy/year, considering a wide range for k (k = 0.10 ± 0.05) and moisture (f = 0.8 ± 0.2 and w = 0.1 ± 0.1) values reported in the general literature [13]. moreover, by setting the potassium content arbitrarily, albeit reasonably, to k = 2 ± 2% and the cosmic ray contribution to 160 ± 160 µgy/year [13], [38], [42]-[44], a total annual dose interval of 0.4–9.2 mgy/year was obtained for the studied sample, taking the ± 2 σ range into account. this result is in accordance with the values found in the literature for the neighbouring area. the annual dose range can be reduced by also measuring the k value and potassium content (if the amount of powder is sufficient, as around 40 mg is needed for icp-oes analysis). in this case, it was not necessary since the results obtained by means of α counting were adequate to address the authentication issue. in fact, considering a value of ed-2 σ and the maximum annual dose in the new range, the minimum age found was about 700 years. in both cases, by means of α counting or referring to the literature, the results led to a compatibility with the tl measurements of an artefact that was last fired in a period prior to the arrival of europeans in the americas. it is not possible to identify a more precise timeframe attribution because, in terms figure 3. resulting fit for the additive method applied to one measured set of sample t1_a. corresponding mean glow curves are reported in figure 2; the temperature interval for signal integration was 340–380 °c. the intercept on the x-axis, i.e. the effective dose (ed), is 4.75 ± 0.66 gy. figure 4. on the left: the object intended for authentication with thermoluminescence. on the right: mean glow curves from the additive method. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 38 of the minimum annual dose in the range, all the sub-periods in the early caribbean ceramic age are possible. it must be stressed that authentication does not provide an age (expressed as date ± error) but only allows the confirmation or denial of the hypothesis of the experts attributing the artefact to a specific historical period. 5. conclusions improvements made in the thermoluminescence dating laboratory at the university of torino have led to faster and partially automated methods, but they still remain tailored to the specific artefact under investigation. the approach to dating starts with a representative sampling in situ and a flexible chemical pre-treatment, if needed. the new irradiation systems allow more data to be collected in the same amount of operating time, enlarging the dataset employed for statistical treatment. analyses of brick samples from the rescue excavation site in trino led to a dating result with less than 10 % of sigma for the tomb and 15 % of sigma for the brick flooring. the tomb appears to be contemporary or subsequent to the fourteenth century, and the reuse of building material is evident for part of the construction. for the adjacent flooring, the archaeological hypothesis of a more recent construction was proved, it being dated at least to the seventeenth century ad. this result is in accordance with the radiocarbon dating of a structural wooden beam. moreover, this case study verified that, when on-site dosimetry is not available, it is possible, in some cases, to achieve good results considering as a first approximation the annual dose obtained by γ-spectrometry measurements on both the powdered sample and the surrounding soil. dating results aided the archaeologists to define the habitation period of trino’s surroundings. the instrumentation is also extremely useful for authentication purposes where the effective dose is generally compared to bibliographic values in order to verify artefact compatibility with the attributed period. in addition, the possibility of measuring α activity allows a rough calculation of the annual dose. this can be very important when relevant literature is scarce, as in the case of the caribbean artefact presented here, which was confirmed to be compatible with precolumbian production. tl measurements in the cultural heritage field can produce a considerable amount of data that can be combined with literature sources and subjected to an extensive statistical analysis, providing additional information for archaeological issues. acknowledgements the authors wish to warmly thank chnet, the infn network of laboratories working in the cultural heritage field, for supporting this research in terms of instrumentation, competencies and grants and, in particular, the infn-labec laboratory in florence for performing radiocarbon dating analysis. thanks are due to prof. ettore vittone for his support for the activities. the authors are grateful to compagnia di san paolo for their financial support, prof. roberto 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https://doi.org/10.1016/j.radmeas.2013.02.011 https://doi.org/10.1111/j.1475-4754.1979.tb00243.x https://doi.org/10.1093/oxfordjournals.rpd.a006683 https://doi.org/10.1007/bf02041927 https://www.wolfram.com/mathematica/ https://doi.org/10.1023/a:1009403127753 https://doi.org/10.1016/j.jas.2008.08.007 https://doi.org/10.1111/j.1475-4754.1972.tb00059.x https://doi.org/10.1111/j.1475-4754.1973.tb00094.x https://doi.org/10.1016/j.radmeas.2004.05.012 https://doi.org/10.1093/oxfordjournals.rpd.a032782 https://doi.org/10.1016/j.jas.2009.12.019 microsoft word article 5 tc4 paper 1.docx acta imeko  august 2013, volume 2, number 1, 7 – 11  www.imeko.org    acta imeko | www.imeko.org  august 2013 | volume 2 | number 1 | 7  testing gps generated 1pps against a rubidium standard  vukan ogrizović 1 , jadranka marendić 2 , snežana renovica 2 , siniša delčev 1 , jelena gučević 1  1  university of belgrade, faculty of civil engineering, bulevar kralja aleksandra 73, 11000 belgrade, serbia  2  directorate of measures and precious metals, mike alasa 14, 11000 belgrade, serbia      keywords: gps, 1pps, time‐keeping  citation: vukan ogrizović, jadranka marendić, snežana renovica, siniša delčev, jelena gučević, “testing gps generated pps against a rubidium standard”,  acta imeko, vol. 2, no. 1, article 5, august 2013, identifier: imeko‐acta‐02(2013)‐01‐05  editors: paolo carbone, university of perugia, italy; ján šaliga, technical university of košice, slovakia; dušan agrež, university of ljubljana, slovenia  received january 10 th , 2013; in final form july 11 th , 2013; published august 2013  copyright: © 2013 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: this work was supported by the ministry of education and science of the republic of serbia (project of the technological development tr‐36048)  corresponding author: vukan ogrizović, e‐mail: vukan@grf.bg.ac.rs    1 introduction  distributed industrial and technological systems request precise time synchronization between their sectors. devices connected to each other by a communication link have to share the same time system in order to complete their tasks [1]. the tightest synchronization requirements lead to the need of highly accurate clock settings, that can be accomplished by means of gps [2]. the components of the distributed system can easily use the readily available gps time standard, wherever they are located. computers in a local or a wide-area network synchronize their times using network time protocol (ntp). in such clientserver configuration time matching between the workstations should be much below a second. on a wide-area network (wan), ntp provides time synchronization from 10–50 ms, nominally, while 1 ms absolute accuracy can be achieved if a gps is used as the time source within a local area network (lan) [3]. in order to test the accuracy of time synchronization using gps, an experiment with five gps receivers, connected to the same antenna, was performed [4]. all five clocks were synchronized with an accuracy better than 50 ns [4]. here, direct measurement of 1 pulse per second (pps) was used, instead of the transmission of the clock corrections via ntp. an example of a high-precision time-keeping is monitoring of bridge deviations due to a heavy load. the surveyors mount gps receivers to the characteristic points on the bridge. receivers collect data for a long period under different loads. the relative accuracy of baseline vectors between the receivers depends on a lot of factors that decrease the accuracy of the vectors. since the field crew do not have any impact on ambient conditions, the accuracy can be increased only by adapting the measuring method. time synchronization is a tool for determining of pseudo-ranges and phase differences. accurate time is necessary for the astrometry applications, like the determination of the earth's orientation [5]. also, due to the earth’s rotation, relative positions of the observed radiosources or stars, depending on the measuring method, change constantly. an error of 100 ms in determining the epoch of the observed object corresponds to an error of tens of meters in determining the position at the earth’s surface. in astrometry applications, an accuracy of the order of microseconds (or better) is requested. the gps receivers manufactured for timing applications or geodetic class gps receivers provide a 1pps signal with an accuracy of up to 10 ns for 5 day average values, according to the manufacturers' specification. this accuracy is often misunderstood as the accuracy in real-time, which is much worse due to the large noise in the gps signal. here we performed a 24-hours test of such gps receiver, comparing its 1pps against 1pps of a abstract  gps  receivers  are  often  used  in  time‐keeping  applications,  due  to  their  availability,  low  price  and  high  accuracy.  we  tested  the  capability  of  the  gps  receiver  to  deliver  a  time‐keeping  accuracy  needed  for  the  time‐critical  applications,  such  as  astrometry  measurements, when a microsecond or better level of accuracy is needed in real‐time. we tested a geodetic class gps receiver against  a  rubidium  standard,  over  a  24  hours  period.  in  the  overall  view,  the  accuracy  corresponds  to  the  nominal  values.  however,  we  experienced outliers with a certain regularity that we could not explain with cycle‐slips or the experimental set‐up.   acta imeko | www.imeko.org  august 2013 | volume 2 | number 1 | 8  rubidium standard. the test should show whether the gps receiver can provide reliable, consistent and accurate time signals over a 24-hour period. many other applications need precise time-keeping: event reconstruction, synchrophasors, system time and frequency deviation, multi-rate billing, power quality incentives, time tagging recorded data, wide area measurement systems, travelling-wave fault detection, end-to-end relay testing [6], and even electronic betting systems [7]. in section 2 we will present the background and theoretical concepts of our research, with the focus on time-keeping methods and the resolving of cycle slips that occur during gps measurements. also, we will give the necessary equations needed for calculating the parameters of the device under test. section 3 gives the results of our experiment. we presented the experiment set-up and the measurements we performed. section 4 comprises the relevant conclusions we have drawn from the results, with some remarks and suggestions for further research. 2 background and methods  2.1 time‐keeping and transmission of time  time marks are transferred to users by many means, including terrestrial or satellite systems [8]. the gps receivers usually transfer their time via an ntp network, serving as a stratum 1. depending on the configuration, the gps receiver can work as a node in the network, delivering the time signals using the tcp/ip protocol, or directly via a serial or a usb port. both methods assume the 24/7 configuration, with regular corrections of the clock in computers being synchronized. several manufacturers of gps equipment offer a special class of gps receivers capable of delivering 1pps via a bnc output, disciplining a computer clock applied in the specific timing application. those 1pps outputs are often steered using the receiver's gps data, and do not represent the 1pps output of the internal clock [9]. the application is not ready-made, but rather a user writes a custom low-level routine, written in lowlevel c or, even, assembler [10]. the features of the operating system influence the latencies appearing during the transfer of time signals from the source of 1pps to the computer. for that reason, such time-keeping routine often runs under a real-time operating system (rtos). the rtos treats software and hardware interrupts in a special way, minimizing the latencies. synchronization by catching the pps ticks with interrupts implies permanent synchronization with minimal latencies [11]. all methods used for time-keeping by the gps receivers share the same issue. the configuration requires a minimum of 15 minutes after the start of the synchronization to adjust the computer clocks to required accuracy [9], [10], [12]. 2.2 resolving cycle slips  cycle slips represent sudden jumps in gps integrated phase measurements. when the cycle slip occurs, a new unknown appears in the mathematical model of an autonomous or relative solution. increasing the number of unknowns degrades the internal reliability of the adjustment solution. because of that, an extreme attention is paid to the cycle-slips resolving. here we applied the bias optimizing method [13]. if 1b and 2b are frequency biases for gps frequencies l1 and l2, respectively, after the cycle slips in the amounts 1n and 2n , new biases become 1'b and 1'b , which can be written as:    1 2 1 1 2 2, ' , 'n n b b b b     , (1) for dual-frequency gps receivers, a wide-lane linear phase combination  of carrier-phase data on frequencies 1 and 2 is created:  1 2    , (2) which, after the development given in [14], yields the bias of the wide-lane combination:  1b l p      , (3) where  is the wavelength, l the carrier-phase data expressed in cycles, and p the code pseudo-range. subscript b stands for the wide-lane combination. the time-average value (3) is then calculated before and after the cycle slip. the difference between these two values should be close to an integer value:   1 2'n b b n n       . (4) 2.3 determining the clock's stability  we determined the short-term stability of the 1pps output of the gps receiver's clock. commonly used measures of stability are different kinds of variances or their square roots – deviations. we used the allan variance [12]:      2 22 22 1 1 22 y x y       , (5) or its square root y allan deviation (adev); and modified allan variance [12],     2 2 2 2 1 mod. 2 y x     , (6) or its square root modified allan deviation (mdev); time variance [12]       2 2 2 2 21= mod. 3 6 x y x        , (7) or its square root time deviation (tdev); and overlapping hadamard variance, following the notation given in [15]:       3 22 3 2 12 1 1 3 3 6 3 n m y i m i m i m i i h x x x x n m               . (8) the overlapping hadamard variance is used for characterization of rubidium oscillators, because the linear frequency drift, characteristic for rubidium oscillators, does not affect it [15]. also, the power spectral density is calculated, with a population consisting of 86400 samples. 3 results and discussion  3.1 device under calibration and measurement  we tested a geodetic class, dual frequency gps receiver trimble netrs (depicted as dcm in fig. 1). the receiver collects the code and phase gps data on 24/7 basis within the serbian continuously operating reference stations (cors) network. since a computer running under linux is incorporated in the netrs, it is attached directly to the serbian academic network and, thus, available throughout the internet. the gps antenna is mounted to the highest point of the roof of the building. no radio-sources or voltage cables exist in the vicinity of the antenna. we use a high-performance trimble zephyr geodetic gps antenna, connected to the receiver with acta imeko | www.imeko.org  august 2013 | volume 2 | number 1 | 9  a low-noise cable. zephyr geodetic antenna owns a groundplane, specially constructed for minimizing possible multipath effects. further improvement could be made by constructing a counterpoise under the antenna. everyday use of netrs is in surveying, serving as one of the nodes in the cors network. however, it is equipped with a 1pps output, which makes it convenient for time-keeping purposes. the receiver delivers 8 µs wide pulses, with rise and fall times of 100 ns. the resolution is 40 ns, but factors like position uncertainty, antenna and cable delay uncertainty, may limit the accuracy of the 1 pps signal up to ± 1 µs [16]. 3.2 experimental set‐up  the experimental set-up is depicted in figure 1, and the list of instruments used is given in table 1. one day prior to the start of the measurements, the rubidium frequency standard was calibrated against the serbian time and frequency standard utc(dmdm). the 10 mhz signal from the rubidium standard is divided down to 1pps in the digital clock. the 1pps from the digital clock is applied to the start input, the 1pps from the dcm (netrs gps receiver) is applied to the stop input of the time interval counter (tic). input signal conditioning is as follows: dc coupling, 50 , rising edge; trigger level set at 50% of the signal amplitude. 3.3 time interval measurement  the frequency of the dcm is compared to the reference frequency by measuring the phase difference between the two 1 pps signals and its change in time. the phase difference is estimated by measuring the time interval between the zero crossings of the 1 pps signals using the tic. the relative frequency offset is calculated by fitting the least-square line to the data and taking the slope of the line. the measurement time is 86400 s. the raw data diagram is given in the figure 2. the data acquisition and processing procedure was performed fully automatic. time view software was used for data logging and processing. the indoors ambient temperature was constant throughout the experiment, 20°c. during the 24 hours session, 16 outliers occurred depicted as sudden jumps in the phase-time diagram (fig. 2). the outliers showed a certain regularity in the amount, which was not expected. their values were grouped around three points: approximately -0.4 ms, -0.6 m, and -1 ms, in respect to the mean value. after the removal of the outliers, and normalization (because the two 1 pps signals were not synchronized) we obtain the plot in fig. 3. fig. 4 presents the power spectral density. since the tic cnt-90 has virtually zero dead-time between two successive sample measurements, stability features may be calculated from its measurement data. for this data processing we used stable 32 software. in fig. 5 various types of stability measures in the time domain (adev, mdev, tdev, and hdev) are presented. if the receiver is moving, the accuracy of its 1pps is decreasing [17]. since the receiver used in the experiment is stationary, that source of error is neglected. after the notification of the outliers, we processed the rinex file with raw gps data for cycle slips detection, in order to find the correlation with the outliers in the time-phase diagram. we followed the method described in [13], using the geometry-free and wide-lane linear combinations of raw dual frequency pseudorange and carrier phase data. we used the modules of gps toolkit [15], [18]. there were 56 cycle slips in total, during the 24 hours of data logging. we estimated the clock’s stability independently by calculating allan variances from rinex data. the results are given in fig. 6. no correlation between the epochs of the cycle slips and outliers in 1pps were found, which means that the satellite constellation did not affect the 1pps performance. figure 1. experimental set‐up.  figure 2: time view graph. the two 1 pps signals are not synchronized – their difference is about 32 ms. apparent outliers.   table 1: equipment used in the experiment.  name model manufacturer rubidium frequency std. 6689/021 pendulum time interval counter cnt-90 pendulum netrs 6609/021 trimble digital clock cadm rohde-schwartz oscilloscope tds 3032 tektronix acta imeko | www.imeko.org  august 2013 | volume 2 | number 1 | 10  considering the experiment set-up, ambient conditions (indoors and outdoors), examining the change of the satellite constellation, we did not find what caused the outliers in the 1pps coming from the netrs. they do not affect the everyday use of netrs (surveying, positioning, and time-keeping in lan), but could seriously degrade the accuracy of certain timecritical applications. therefore, we are preparing the extension of the experiment, in order to investigate the problem. 4 conclusions and remarks  according to overall results of our 24 hours measuring session, it can be concluded that a geodetic class gps receiver is capable of permanent delivering time marks within ms accuracy. examples of such application are permanent monitoring of an industrial process, or time synchronization between distant computer nodes in a wide-area network. still, when higher accuracy is needed, in the order of microseconds or better, a special attention should be paid to the set-up of the equipment, keeping in mind what disturbances of the gps signal could occur during the exploitation. the outliers that occurred in our experiment could seriously degrade the accuracy of real-time applications. we did not find any relation between the obtained results and the experiment (set-up, ambient conditions, or satellite constellation). the grouping of the outliers in the mean of values and their irregularity considering the time epochs when they occurred, should be investigated further. we suggest another, longer experiment, which would last for seven days, in order to find the factor that causes outliers. the mentioned outliers could degrade also the positional accuracy. the solution for high-precision surveying purposes is to discipline an external high stability oscillator, such as rubidium standard, with the gps signal. acknowledgement  the authors of the paper express their gratitude to the ministry of education and science of the republic of serbia, who supported the work on this paper by participating in the project of technological development tr-36048. references  [1] j. elson, d. estrin, “time synchronization for wireless sensor networks”, international parallel and distributed processing symposium (ipdps), san francisco (ca), usa, 2001, pp. 19651970. [2] a. carta, n. locci, c. muscas, f. pinna, s. sulis, “gps and ieee 1588 synchronization for the measurement of synchrophasors in electric power systems”, computer standards & interfaces vol. 33, no. 2, 2011, pp. 176-18. [3] h. wohlwend, g. crispieri, y.s. li, “factory and equipment clock synchronization and time-stamping guidelines: version 2.0”, international sematech manufacturing initiative, inc, 2008. [4] k. behrendt, k. fodero, “the perfect time: an examination of time synchronization techniques”, http://www.pespsrc.org/i/tp6226_perfecttime_kb_20050922.pdf. [5] d. mccarthy, “using utc to determine the earth's rotation angle, in decoupling civil timekeeping from earth rotation”, science and technology series, vol. 113, american astronautical society, isbn 9780877035756, pp. 105-117. [6] b. dickerson, “time in the power industry: how and why we use it”, white paper, arbiter systems, http://www.arbiter.com/files/product-attachments/ timeinthepowerindustry.pdf. [7] p. hämälainen, m. hämälainen, t.d. hämälainen, r. soininen, r. rautee, “design and implementation of real-time betting figure 3. phase‐difference data after removal of outliers and normalization.  figure 4. power spectral density values.  figure  5.  stability  measures  of  the  1  pps  output  of  the  netrs:  allan  deviation, modified allan deviation, time deviation, overlapping hadamard  deviation.  figure 6. allan variances from raw gps data.  1 10 100 1000 10000 1e-12 1e-11 1e-10 1e-09 1e-08 adev mdev tdev hdev   acta imeko | www.imeko.org  august 2013 | volume 2 | number 1 | 11  system with offline terminals”, electronic commerce research and applications vol. 5, no. 2, 2006, pp. 170-188. [8] m. lombardi, “nist time and frequency services”, nist special publication 432, nist, 2002. [9] t. schildknecht, g. dudle, “time and frequency transfer: high precision using gps phase measurements”, gps world, feb. 2000, pp.48-52. [10] v. ogrizović, “a construction of an advanced measuring system for astro-geodetic determinations”, publications of the astronomical observatory of belgrade, vol. 86, 2009, pp. 145150. [11] a. heursch, “using standard operating systems for time critical applications with special emphasis on linux”, phd dissertation, universität der bundeswehr münchen, fakultät für informatik, 2006. [12] d.w. allan, n. ashby, c.c. hodge, “the science of timekeeping”, hewlett packard application note 1289, 1997. [13] g. blewitt, “carrier phase ambiguity resolution for the global positioning system applied to geodetic baselines up to 2000 km”, journal of geophysical research, vol. 94, no. 88, 1989, pp. 10,187-10,203. [14] g. blewitt, “an automated editing algorithm for gps data, geophysical research letters”, vol. 17, no. 3, 1990, pp. 199-202. [15] t. craddock, r.j. broderick, c.p. petersen, a. hu, “the gps toolkit: open source clock tools”, 40th annual precise timing and time interval (ptti) meeting, reston (va), usa, 2008, pp. 275274. [16] “trimble: netrs gps receiver user guide”, trimble navigation limited, 2004. [17] p. hightower, “motion effects on gps receiver time accuracy”, instrumentation technology systems, northridge (ca), usa, 2008. [18] t. gaussiran, d. munton, b. harris, b. tolman, “an open source toolkit for gps processing, total electron content effects”, measurements and modelling, international beacon symposium, trieste, italy, 2004. experimental characterization of pantograph arcs and transient conducted phenomena in dc railways acta imeko issn: 2221-870x june 2020, volume 9, number 2, 10 17 acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 10 experimental characterization of pantograph arcs and transient conducted phenomena in dc railways andrea mariscotti1, domenico giordano2 1 astm sagl, via comacini 7, 6830 chiasso, switzerland 2 istituto nazionale di ricerca metrologica (inrim), strada delle cacce 91, 10135 torino, italy section: research paper keywords: arc discharge; electric arc; power quality; rail transportation. citation: andrea mariscotti, domenico giordano, experimental characterization of pantograph arcs and transient conducted phenomena in dc railways, acta imeko, vol. 9, no. 2, article 3, june 2020, identifier: imeko-acta-09 (2020)-02-03 editor: alexandru salceanu, technical university of iasi, romania received december 19, 2019; in final form march 13, 2020; published june 2020 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was developed within the 16eng04 myrails project and supported by the empir programme (european union’s horizon 2020 research and innovation programme) and by the swiss state secretariat for education, research and innovation (seri) under contract number 17.00127. corresponding author: andrea mariscotti, e-mail: andrea.mariscotti@astm-e.ch 1. introduction power is transferred from supply substations to the rolling stock (locomotive or electro-train) through a sliding contact between the supply line (either a catenary or a third rail) and the current collection system (correspondingly, a pantograph or a shoegear). overhead conductors (the so-called catenary or overhead contact line [ocl]) are used in many railway systems, including tramways, metro applications, and some light railways, in which a third rail distribution is preferred. there are cases in which the distribution in tunnels occurs through a rigid catenary, which is an overhead busbar. both the pantograph and the sliding shoe are controlled to ensure good mechanical contact, exerting an adequate force and thus contact pressure. mechanical oscillations and bounces, including tilting in curves and irregularity of the track, cause detachments that in turn can cause arcing; other reasons for electric arcing include defects or oxidation on the surfaces of the catenary or of the sliding contact and the presence of ice. the large current transferred through the sliding contact makes the current collection and the control of arcing some of the most critical elements of electrified railways: the contact cross-section may account for up to a tenth of square millimetres, whereas the current intensity of a single collection point may reach values of several thousand amps. predictive maintenance and monitoring can help the prevention of the degradation of the sliding contact and reduce the failure rate of the elements involved [1][2]. failures are often caused by excessive heating and erosion, of which electric arcing is a contributory cause. arcing is also a consequence of excessive heating: the higher the arcing, the higher the heating and the wearing of surfaces, with increased oxidation and erosion, from which there is a local increase in the arcing rate. the number of electric arcs is indicated as a direct index for the assessment of the quality of current collection in en 50367 [3], sec. 7.3 ‘percentage of arcing’; thus, their identification is a relevant problem: arcs can be detected by measuring the light emissions (mostly blue and ultraviolet) [4]-[6], the electromagnetic emissions [7][8], or by processing the pantograph electrical quantities [9]-[11], looking for typical distortions, such as voltage jumps and oscillations. the presence of many similar transients can confuse detection algorithms, which can be beneficial in an analysis of the related phenomena and system response, as carried out in this work. abstract an electric arc is an example of a transient event that is quite common in electrified transportation systems as by-product of the current collection mechanism. as a broadband transient, an electric arc excites a wide range of (often oscillatory) responses related to the substation and onboard filters, as well as the line resonances and anti-resonances. similarly do the charging of onboard filter and other related inrush events. this work considers the electrical characteristics of these transients and of the excited responses in order to define their typical spectral signatures in dc railways and take them into account concerning their impact on power quality measurements and the measurements of instruments deployed onboard. mailto:andrea.mariscotti@astm-e.ch acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 11 an electric arc, as anticipated, is a source of electromagnetic emissions and disturbance [7][8], both conducted and radiated, impacting the power quality of the line [12] as well as the measurement of electrical quantities [12]-[15] and radio systems for signalling and communications [16]-[19]. an electric arc, as with any other steep transient, is not a standalone event; rather, it is a broadband excitation for the entire electric traction system and triggers other conducted phenomena, such as resonances, oscillations, and overvoltages. the majority of the works in the relevant literature have covered ac applications, driven by the interest in and the rapid development of high-speed lines as well as by critical scenarios of winter frozen lines and long-distance lines, which require expensive maintenance and surveillance [20]-[22]. however, dc railways (the term ‘railways’ includes all the rapid transit, light railways, tramways, etc. with a dc supply) feature more intense arcing due to two factors: power is supplied through a dc voltage of a significant amplitude (between 600 v and 3000 vdc, with maximum permanent voltages up to about 4000 v), and the current intensity can be an order of magnitude larger than ac railways. within the empir 16eng04 myrails research project [23], which aims to develop the metrological framework to foster and demonstrate the energy efficiency of railway systems [24], the focus has been on dc electric arcs and their influence on the behaviour of the system, measurements, and measurement accuracy. to this end, an essential activity is the definition of a reliable and comprehensive electrical model of the system, in which the arc events originate and propagate. this system includes all the elements of a dc railway [25]-[28]: • electrical substations (esss) equipped with diode rectifier groups, representing a non-linear element; • the catenary and the return circuit, extensively modelled by means of lumped and distributed parameter models for the propagation of conducted emissions; and • the locomotive or electro-train, subdivided between the pantograph interface and the onboard circuits, including the onboard filters and the power converters. the objective is the characterization of the interaction of the electric arc at the pantograph-catenary interface of a dc vehicle with the rest of the network. the desired result is a better understanding of the circuit dynamic response, of the variability caused by varying parameters or parameters that are not known to be accurate, and of the assessment of the impact on connected circuits, particularly measurement systems such as those uses for energy consumption [24]. this work goes beyond the evaluation in [12] of electric arcs impacting pantograph voltage waveform and its power quality (pq) features. the approach of this work is practical, using circuit modelling for a better understanding of measured waveforms, with the aim of classifying the typical transients, including the identification of worst-case scenarios, as shown in the imeko tc4 symposium preliminary work [29]. 2. electric arc and filter transients as explained in the introduction, although the focus of this paper is the electric arc as a source of radiated and conducted disturbance and as a cause of the wearing of the sliding contact elements, there are other transients in dc railways that have a similar excitation in the network response and can be assimilated to electric arcs from a phenomenological viewpoint. the general equivalent circuit of a dc vehicle fed by a dc line is shown in figure 1, where the elements contributing to the overall transient response are highlighted, particularly those that determine the oscillatory response, as described in the following section. 2.1. electric arc a typical v-i characteristic of a dc arc can be summarized as follows: at a low current (tens of amperes), a constant arc power law can be assigned; for larger currents above a specific threshold, the voltage level reaches a plateau and no longer depends on the current value. an interesting discussion of more complete dynamic models (starting from mayr’s model) can be found e.g. in [30]. various v-i arc characteristic curves for lowvoltage dc arcs with static electrodes were provided by paukert [31]: he collected data provided by seven research institutes with currents ranging from 100 a to 100 ka. the electric arc can thus be represented with a combination of equivalent circuits: a resistor variable with the intensity of the flowing current (getting smaller along with the increasing current) and a voltage source that stabilizes to the plateau for a current exceeding a threshold value. as detailed in section 3.4, the arc resistance is extremely small and does not contribute significantly to circuit damping. the arc conduction voltage instead appears suddenly at arc ignition and represents the main electric phenomena related to the arc event. 2.2. pantograph operation and onboard filter charge whenever the pantograph is operated by raising or lowering it, a transient occurs: the former is more interesting than the latter, because the onboard measuring system can thoroughly measure the development of the transient responses through the established electrical connection at the sliding contact. pantograph raising is carried out with the onboard circuit breaker (high-speed circuit breaker [hscb]) open in order to avoid the sudden intense charging current of the onboard filter; rather, a charging resistor rch is used to control it, as shown in figure 2. figure 1. a simplified network equivalent circuit (traction line, substation, and vehicle) for supporting the interpretation of the observed phenomena. figure 2. electrical scheme of the e464 locomotive, showing the onboard lc filter, the main circuit breaker, and the pre-charging circuit with the limiting resistor rch [32]. m l x ess ess l-x acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 12 during the pre-charge, the capacitors are charged with the pantograph current flowing through the limiting resistor rch and the filter inductor lfilt of 8.5 mh. after 0.8 s, the voltage across the capacitor bank (made of all parallel-connected capacitors) reaches a threshold that is sufficient to accelerate charging through the main hscb, closing it in order to bypass the charging resistor. a smaller current impulse is thus absorbed by the capacitor bank, where the limiting elements remain the filter inductor lfilt and the line impedance zp. the equivalent rlc series circuit, which during pre-charge was over-damped thanks to the rch resistor, becomes thus under-damped, with visible oscillations. 3. catenary – vehicle electric interaction a simplified equivalent circuit has been introduced in section 0 in order to discuss and interpret the observed phenomena related to the transient response of the network. network dynamics require that sophisticated models are used, including, at least, the main characterizing elements: the non-linearity of the substation diodes, the extended frequency response of the traction line accounting for the first two or three resonance modes, and a correct estimate and inclusion of damping factors at resonances. studies on the characterization of dc arcs have always used simpler lumped-parameter models with a single pi cell for the line and with the inductance and capacitance values fixed to a predetermined line length [33]-[35]. it is known that lumped-parameter models are less accurate than distributedparameter models, particularly with circuit resonances, and that they are subject to a maximum frequency constraint: the higher the desired upper frequency limit, the higher the number of required cells [36]. having observed repeated resonance oscillations triggered by transient events, a circuit model is used for the analysis of system response; the model was introduced in the imeko tc4 symposium paper [29], where some more details on expressions can be found. a traction line, electrically continuous and fed at esss, has a frequency behaviour characterized by a series of resonances and anti-resonances, starting at a low frequency with a resistiveinductive behaviour. the frequency response of the traction line is influenced by the said esss with lc resonant filters, representing a shunting point thanks to the low impedance [37]: starting from around several hundred hz (some octaves above the resonance frequency of the substation lc filter, which in italy is located slightly above 100 hz), the portion of the line between two esss is electrically decoupled from the adjacent sections. concerning our problem, the line section is seen as tapped by the pantograph, with left and right parts in electric parallel: the equivalent impedance made of the two left and right sections in electric parallel is known as the ‘pantograph impedance’ zp. due to the different lengths of the two sections, the frequency responses of the two sections mix, resulting in an overall response that has resonance frequencies approximately fixed and independent of the train position, whereas antiresonances move with the train. at resonances, what changes with the train position is the damping factor, which is at its maximum when the train is in the middle – farthest from the two esss. approaching one of the esss, the peak flattens and broadens. the train position is thus quite important in the interpretation of the experimental results and the verification of the agreement with the simulation. the theoretical expression of zp is: (1) where zc is the characteristic impedance of the line, x is the train position, and l is the length of the line section between two esss. for the quantification of zp and the variability of its main characteristics, the set of parametric curves is as follows. the model is implemented in a compact matlab code used for line response study [37][38], based on transmission line equations for the line (a distributed parameter model, including line losses due to e.g. skin effect in the running rails) and lumped circuits described by kirchhoff equations for the terminal conditions (substations and train). with an assumed return path through the running rails, rail-to-rail capacitance and its significant variability [39] is not a relevant parameter. the program was used for the evaluation of the spread of line responses depending on substation electric parameters and the interaction with the vehicle filter [37][38]. the train is modelled focusing on its input filter, as explained later in section 3.3. the electric arc is modelled as a generator and a series resistor that simulates the arc resistance. the geometrical and electric line parameters, including the onboard vehicle filter, considered for the parametric analysis of this section 3, are shown in table 1. 3.1. influence of the line’s geometrical and electrical parameters the analysis of the sensitivity of the electrical behaviour and in particular of the pantograph impedance have been carried out a few times in the last 20 years [40]-[42]. in general, changes to the geometry of about 10 % (height of contact wire, height of running rails from the reference plane, and horizontal displacement) influence the location of the resonance and antiresonance marginally (about 1-1.5 % sensitivity, so one-tenth) and more remarkably affect the impedance value at resonance (10 %, so a 1:1 proportion). similarly, the variability of the electrical parameters (such as soil resistivity and the magnetic permeability of rails) mainly affects the impedance at resonance (20 % of a variation against more than a tenfold variation of the said electrical parameters), whereas the resonance and anti])1([tanh)(tanh ])1([tanh)(tanh lxxl lxxl zz cp −+ − = table 1. electrical and geometrical parameters of the traction line and onboard filter. parameters values contact wire (copper): height (m), cross section (mm2), resistivity (/m) 5.2, 240, 0.018 suspension wire (bronze): height (m), cross section (mm2), resistivity (/m) 5.6, 200, 0.05 running rail: gauge (m), height above ref (m), cross section (mm2), relative permeability, resistivity (/m) 1.5, 0.1, 7600, 50, 0.22 rectifier: rated power (mw) 5.4, transformer: rated power (mva), rated sec. voltage (kv), short-circuit voltage (%) 5.75, 2.71, 10% substation filter: induct. (mh), capac. (f) 6, 360 rolling stock filter: induct. (mh), capac. (mf) e402b 16, 3.2 e412 21, 9.47 taf 18, 2.5 etr460 21, 4 acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 13 resonance frequencies are almost unaffected, changing less than 1 %. the variability of the rail internal inductance, amounting to the geometrical external inductance term, is probably the most relevant influencing factor and was implicitly taken into account in [40] by varying the magnetic permeability of the rail steel. 3.2. influence of the train position the train position influences not only the anti-resonance frequency but also the height of the resonance peak i.e. the factor of merit or damping (not the resonance frequency, which depends instead on the length of the line section). accurate expressions can be found in [37]. the factor of merit at resonance may be large, especially when the train is far away from the substations, with a significant effect in terms of voltage amplification when excited by the pantograph transient. three sets of curves are shown in figure 3 for three different ess separation distances l, with curves for four train positions x, measured relative to l: x = 0.1, 0.2, 0.33, 0.5 l. positions beyond the mid-point in the 0.5-1.0 l interval give identical results. the pair of resonance/anti-resonance at a low frequency is due to the ess filter and may be slightly variable, depending on the values of inductance and capacitance implemented at the ess. as a consequence, a significant 100 hz component is visible, generated by the ess rectifier and amplified by the filter resonance. at a high frequency, the anti-resonance frequency is variable and evenly distributed approximately between 6.5 khz and 20 khz, depending on length l and train position x. as for the voltage components susceptible to the line resonances, there should be current components enhanced by anti-resonances: the pantograph current will show a persistent response at these frequencies, continuously excited by various types of transients and high-frequency components, including the steep front of the electric arc event. the increasing line resistance is remarked: at 120-200 hz, anti-resonances caused by the interaction of the substation and vehicle filter, the values are 0.5-2 , whereas at the first highfrequency anti-resonance, the minima are about 20 , so they are larger by an order of magnitude due to increased losses and skin effect. 3.3. onboard equivalent circuit and input filter effect the onboard input filter is a low-pass lc filter with a typical resonance between 15 hz and 25 hz. regardless of the loading of traction and auxiliaries (seen as an equivalent resistor parallel to the capacitor bank), the shunting effect across the capacitor is negligible for frequencies above some tens of hz; for this reason, only the lc on-board filter is considered. the onboard filter is parallel to zp when seen from the pantograph port. ideally, its loading effect is negligible for frequencies above a hundred hz, since the filter inductance is larger than the overall equivalent line inductance seen at the pantograph for lengths up to about 25-30 km. as shown in figure 4, the curves are quite variable – up to 100 hz; from here onward, only a negligible shift between curves can be seen. (a) (b) (c) figure 3. zp for 3 esss separations l: (a) 15 km, (b) 20 km, (c) 25 km; no loco filter; 4 train positions (black to light grey) x=0.1, 0.2, 0.33, 0.5 l [29]. figure 4. sensitivity analysis of zp for the varying vehicle input filter: e402b, e412, taf, etr460 (see table 1) [29]. acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 14 3.4. influence of the electric arc resistance the electric arc is a dynamic phenomenon with a resistance that varies during the occurrence of the transient. arc resistance values have been estimated for dc arcs at various inter-electrode gaps. the arc resistance ra is, in general, negligible, as shown in [2], figure 14, where values of 10 mω to 100 mω are reported. in the case of various types of wearing after long use, the electric contact at the sliding surfaces worsens, and the resistance increases up to ten times [43]. the observed values during preliminary measurements [13] are of the same order (about 0.2 ω to 0.5 ω). in the equivalent circuit for the pantograph voltage and current at the sliding contact interface, the arc resistance is always in series with zp (the overall network impedance); voltage and current are measured immediately after it, either on the roof or inside the rolling stock. it is observed that arc resistance values larger than observed will not cause any appreciable change of the overall series impedance and of current flow, being small compared to the overall line resistance and reactance (see zp values in figure 3). 3.5. overview of the electric phenomena justified by the model based on the examined characteristics of the frequency response of the traction line feeding the vehicle through the pantograph, the following considerations are anticipated regarding the conducted phenomena following a transient. 1) the low-frequency resonance related to the substation filter is responsible not only for the low-frequency voltage oscillation but also for the persistent 100 hz ripple, where the excitation signal comes from the rectification of the negative sequence component of the 50 hz fundamental of the high-voltage side. 2) another low-frequency oscillatory response is observed due to the transient response of the onboard filter; considering the filter parameter values and based on experience, the resonance frequency is located between about 15 hz and 25 hz for a broad range of vehicles. the large inductance value accounts for several km of traction line and can thus drive its oscillatory current into the system. 3) the onboard filter oscillation has slightly non-linear behavior in that its positive and negative half-cycles are unequal, with the positive ones slightly longer. the reason is the nonlinearity of the diodes in the substation rectifiers, allowing for the conduction of the current pulled from the line but necessitating the bias of other trains loading the same line to let a ‘negative’ current flow in the opposite direction. unless the bias current is large (a nearby train pulling traction current), bringing the diodes to a full conduction state, the non-linearity is visible and can cause significant distortion. 4) the initial steep leading edge (at the contact detachment for an electric arc or at the insertion of the charging resistor) has a fairly broad range of components able to excite the highfrequency resonances of the line; the first resonance is located between 5 khz and 10 khz depending on the separation of substations (up to 30 km), the successive ones roughly at its multiples, with a variable pattern of peaks, depending on train position. the reaction is a burst of faster and slower oscillatory modes with a rapid decay (overall resistance is quite large thanks to several causes of loss), causing rapid voltage changes (positive or negative depending on the direction of the flowing current, leaving or entering the vehicle pantograph, respectively). if it is not an issue for dielectric breakdown, it is a source of significant electromagnetic interference and possibly a cause of increased stress to components. reference will be made to this classification when showing and commenting on the experimental results in the next section. 4. experimental results the measurements of the pantograph voltage and current were performed along various italian 3 kv dc lines: a sampling rate of 50 ks/s and probe bandwidth of about 20 khz. data were digitized at 16 bits. 4.1. electric arc event as shown in figure 5, the arc event triggers voltage oscillations at a frequency slightly larger than 100 hz (around 0.14-0.15 s), corresponding to the predicted anti-resonance of figure 3; such faster oscillations (at about 120 hz) are damped and disappear in about five cycles, and the regular 100 hz resumes (e.g. beyond 0.19 s). the first peak of the current is caused by the excitation of the on-board filter and ess antiresonances, visible in figure 3 and figure 4 at about 10-30 hz and 120-150 hz. the broader oscillation that follows the current peak is due to the transient response of the on-board filter tuned to about 15 hz, which is not visible in the voltage waveform (besides a slight fluctuation) due to the train position being close to the ess, the very low line impedance, and, consequently, the voltage drop. the persistent low-frequency oscillations on the pantograph voltage (caused by the substation ripple mainly at 100 hz and 300 hz) hide the high-frequency components that should be located at line resonances that are visible only with a more detailed spectral analysis. to narrow the analysis around the arc event, considering the short duration of the arc, the larger damping at high frequency, and the underlying low-frequency components that cause the local non-stationarity of the extracted time windows, tools with a short duration of the kernel function and yet sufficient frequency resolution are needed: suitable tools may be wavelets and joint time-frequency transforms. another example with a much longer distance between the loco and the ess is shown in figure 6, where the peak voltage reached by the triggered oscillations is much higher, and the oscillations are more persistent: the 15 hz oscillation of the figure 5. electric arc event during braking (positive voltage jump). the numbers refer to the estimated half-cycle duration in ms during the oscillatory transient, passing from the transient response of the ess filter back to the underlying 100 hz ripple. 3 .3 2 3 .6 6 4 .0 2 3 .8 8 4 .0 8 4 .3 2 4 .2 6 acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 15 onboard filter is surrounded by repetitive oscillations with the ess filter of a similar amplitude. although it is not easy to see, the change of period occurs as well, passing from about 3.2 ms at the very beginning (about 150 hz of resonance frequency) to the steady 5 ms half-cycle of the 100 hz component. 4.2. onboard filter charging process a similar behaviour is expected for the initial transient when the pantograph line is connected for the charging of the onboard filter. the recorded waveform is shown in figure 7, focusing on the initial voltage spike (negative, scale divided by 10 in the figure): the dynamics are very fast, and there are only a few samples along the waveform. the interpretation is that the steplike current absorbed through the charging resistor excites the line resonances, namely the fastest ones at about 20 khz and above (responsible for the first peak) and then the slower first and second resonance. 5. disturbance to measured electrical quantities as discussed in the previous section, the examined transients following the arc or filter pre-charge events may be characterized from three viewpoints: • they cause significant spikes in the pantograph voltage, both positive and negative, the former being more significant for the stress on components in terms of the maximum temporary voltage; a 5 kv peak was reported in figure 6; a very fast negative peak is visible in figure 7. • fast peaks indicate a broadband excitation of the line and bring with them fast line oscillations, damped because of line losses at higher frequencies. these oscillations appear as narrowband components but are located generally above the usual harmonics, approximately above 5 khz, as shown in figure 3. • low-frequency oscillations are related to the transient response of the resonant filters, both at substations and onboard. in this case, the damping is much less, and the oscillations may persist for longer time intervals, such as the 100 ms shown in figure 6. in general, the transient response of the loco filter is always an issue and may cause frequent out-of-scale readings when carrying out pantograph current measurements for pq purposes and for demonstrating compliance to limits of emissions based on signalling susceptibility, as explained below. the voltage spikes may thus represent a significant stress for the components, particularly the sensors connected to the pantograph line, but all these elements must be tested per en 50124-1, and, in terms of rigorousness, they are exposed to transients related to the line. reading the standard more accurately, a sensor connected to the pantograph line is ‘not powered directly by the contact line’, as table a.1 of en 501241 says, and this may lead to select a comfortably lower – but more critical – test voltage. the observed overvoltages, although included and covered by the test voltages for dielectric strength, should be considered as a source of dielectric stress. the observed oscillations have two consequences for the measurement process: • they add spectral components with a narrowband behaviour, which may be interpreted as emissions from some kind of apparatus, conveying false information in pq indexes and assessment procedures that are not prepared to exclude such distortions (preliminarily identified in [12]). • in the case of an expanded scale for better ac reading (after removing the dc component e.g. by measuring using a capacitive divider for the voltage and a rogowski coil for the current), such transients will cause out-of-scale readings, corrupting the acquired signal for the whole duration of the transient itself. in addition, this underlying signal variability causes issues in spectral leakage by modifying and making the two ends of the time record fed to the fourier or equivalent analysis quite different. figure 6. another electric arc event during traction (negative voltage jump): triggered oscillations with the ess filter are more intense and persistent and are modulated by the onboard filter transient response, lasting about two cycles (140 ms). figure 7. measured waveforms of pantograph voltage vp, pantograph current ip and capacitor voltage downstream of filter vf; zoom of the first insertion with resistor rch. acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 16 6. conclusions this paper has presented typical transients of pantograph quantities (voltage vp and current ip) in dc railways. such transients (electric arcs and onboard filter charging) have been considered for the excitation of line, substation, and vehicle transient responses, discussing and characterizing the components observable in the measured signals. these components were considered in relation to lowand highfrequency responses, specifically the response of the substation and onboard resonant filters as well as the excitation of the line resonances and anti-resonances, respectively. the observed phenomena consist of rapid overvoltages/ undervoltages and repeated oscillations, in some cases, with very low damping, thus lasting for 100 ms and longer. the rapid voltage changes are steep and intense, possibly causing the accelerated aging of connected measuring equipment if they are too frequent, but they are still below the insulation voltage level required for equipment connected to the pantograph line. the observed oscillations correspond to low-frequency components, approximately in the range of 10 to 150 hz, due to the excited transient response (poles and zeros) of the combined substationnetwork circuit and of the onboard filter. the spectral behaviour is thus the one typical of harmonic emissions that might affect the evaluation of pq and the calculation of indexes if not properly separated and considered. this work also addresses indirectly the problem of arc detection, identifying a peculiar spectral behaviour that may be exploited to define a spectral signature of such events, but the onboard filter oscillations alone can be related to other types of transients, such as mechanical and loading transients. references [1] g. wu, j. wu, w. wei, y. zhou, z. yang, g. gao, characteristics of the sliding electric contact of pantograph/contact wire systems in electric railways, energies 11(1) (2018) pp. 17. 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[20] g. wu, w. wei, g. gao, j. wu, y. zhou, evolution of the electrical contact of dynamic pantograph-catenary system, journal of modern transportation 24(2) (2016) pp. 132-138. [21] l. buhrkall, dc components due to ice on the overhead contact wire of ac electrified railways, electrische bahnen 103(8) (2005) pp. 380-389. [22] s. midya, d. bormann, t. schutte, r. thottappillil, pantograph arcing in electrified railways – mechanism and influence of various parameters – part i: with dc traction power supply, ieee trans. on power delivery 24(4) (2009) pp. 1931-1939. [23] myrails, metrology for smart energy management in electric railway systems, [24] cenelec en 50463-2, railway applications – energy measurement on board trains, 2017. [25] r. j. hill, m. fracchia, p. pozzobon, g. sciutto, a frequency domain model for 3 kv dc traction dc-side resonance identification, ieee trans. on pow. sys. 10(3) (1995) pp. 13691375. [26] a. m. walczyna, p. janiszzwski, time domain analysis of harmonic interactions between traction vehicle and dc supply, intern. conf. on electric railways in a united europe, 27-30 march, 1995, amsterdam, netherlands. [27] a. szelag, m. steczek, analysis of input impedance frequency characteristic of electric vehicles with a.c. motors supplied by 3 kv dc system for reducing disturbances in signalling track circuits caused by the harmonics in the vehicle’s current, przegląd elektrotechniczny 89(3) (2013) pp. 29-33. [28] v. šmídl, š. janouš, z. peroutka, improved stability of dc catenary fed traction drives using two-stage predictive control, ieee trans. on ind. electronics 62(5) (2015) pp. 3192-3201. [29] a. mariscotti, d. giordano, electrical characteristics of pantograph arcs in dc railways: infrastructure influence, proc. of acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 17 the 23rd imeko tc4 intern. symp., 17-20 september, 2017, xi’an, china. 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[40] g. d'addio, m. fracchia, a. mariscotti, p. pozzobon, sensitivity analysis of railway line impedance to variations of electrical and geometrical parameters, proc. of world congress on railway research wcrr, 19-23 october, 1999, tokyo, japan. [41] m. brenna, f. foiadelli, sensitivity analysis of the constructive parameters for the 2 25-kv high-speed railway lines planning, ieee trans. on power delivery 25(3) (2010) pp. 1923-1931. [42] r. garg, p. mahajan, p. kumar, sensitivity analysis of characteristic parameters of railway electric traction system, intern. j. of electronics and electrical eng. 2(1) (2014) pp. 8-14. [43] v. v. terzija, h. j. koglin, on the modeling of long arc in still air and arc resistance calculation, ieee trans. on power delivery 19(3) (2004) pp. 1012-1017. [44] cenelec en 50124-1, railway applications – insulation coordination – part 1: basic requirements – clearances and creepage distances for all electrical and electronic equipment, 2017. acta imeko  september 2014, volume 3, number 3, 50 – 56  www.imeko.org    acta imeko | www.imeko.org  september 2014 | volume 3 | number 3 | 50  contactless high frequency inductive position sensor with  dsp read out electronics utilizing band‐pass sampling  bernhard aschenbrenner, bernhard g. zagar  institute for measurement technology, johannes kepler university, 4040 linz, austria        section: research paper   keywords: inductive sensor; contactless; band‐pass‐sampling; synchronous demodulation, resonant circuit  citation: bernhard aschenbrenner, bernhard g. zagar, contactless high frequency inductive position sensor with dsp read out electronics utilizing band‐pass  sampling, acta imeko, vol. 3, no. 3, article 11, september 2014, identifier: imeko‐acta‐03 (2014)‐03‐11  editor: paolo carbone, university of perugia   received february 15 th , 2013; in final form april 10 th , 2014; published september 2014  copyright: © 2014 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: this work was supported by the austrian center of competence in mechatronics (accm)  corresponding author: bernhard g. zagar, bernhard.zagar@jku.at    1. introduction  many industrial applications demand for precise absolute position measurement of objects placed in a harsh environment. sensors for industrial applications have to meet criteria such as reliability, small size, wide temperature ranges, and insensitivity to moisture, vibrations, dust, chemicals and mechanical offset even in rough conditions. ideally, the measurement system should fulfill these criteria without any mechanical contact to maintain long service life and should be applicable for highly dynamic drives. prime candidates to meet these requirements are contactless inductive sensors. one of these inductive sensors is based on inductive resonance [1–4]. the developed sensor [5] can be used for absolute position measurement and measures the position accurately right from power up. this sensor works on a similar principle to contactless resolvers and utilizes the mutual inductance between an elongated antenna on a printed-circuit-board (pcb) and a target which acts as a passive lc resonant circuit. figure 1 shows a sketch of the principle of operation. the target is a passive high-q lc resonant circuit with a planar rectangular inductance (lt) formed by tracks on a pcb and a discrete capacitance (ct). furthermore, the antenna pcb contains five different planar coils which operate as transmission and receive circuits (see figure 2 and figure 3). the sine and cosine shaped coil pair with the single period over the full antenna pcb length la is used for absolute position measurement. already developed and used contactless high frequency inductive position sensors use a single pair of receive coils only and are thus not able to attain such a good resolving power [1,4,6]. through adding two additional sinusoidal shaped receive coils with a shorter cycle length, the measurement precision and measurement range can be increased. however, these multicycle coils are only able to generate an incremental position signal and thus a more elaborate signal processing is necessary which is performed by a digital signal processor (dsp). the envelopes of the amplitude modulated double sideband suppressed carrier (dsbsc) receive coils output signals contain the position information of the target. these voltages are abstract  this paper presents a precise, reliable, low cost and contactless inductive absolute position measurement system for rough industrial  environments. it offers a high inherent resolution (0.04 % of antenna length), and measures absolute position over a relative wide  measurement range. the main property for this kind of sensor is its good immunity to external noise and target misalignment off the  measurement axis. the measurement range and the precision are extended by adding additional and finer pitched receive coils. this  sensor works on similar principles as resolvers but consists of a rectangular antenna pcb, a small moveable passive lc resonant circuit  and a signal processing unit. furthermore, the used read out electronics utilizes under‐sampling to demodulate the sensor output  signals and the corresponding position is estimated from a lookup table (lut) implemented on a digital signal processor (dsp) to avoid  singularities in the inverse tangent and cotangent calculation. moreover, the mechanical transducer arrangement, the signal condition  electronics design and measurement results of the transmitter to receiver signal coupling and relative position error will be presented.  acta imeko | www.imeko.org  september 2014 | volume 3 | number 3 | 51  amplified in an analog front-end, analog to digital converted, and digitally demodulated to calculate the position using the dsp. this way the measurement system attains an update rate of approximately 60 khz. conventional resolver-to-digital converters seem to be ideal for high–frequency inductive position sensors but the carrier frequency of these converters, which is usually set between 2 and 30 khz, is too low for the intended application of the sensor. in [1-3], an asic front end was developed for this kind of sensor which demodulates, amplifies and filters the dsbsc signals from the receive coils, offers a differential mixer topology for suppressing common mode voltages and provides an excitation voltage that supplies the transmitter coil. however, this asic cannot simultaneously sample the four receive coil signals, which is necessary for fast moving objects requiring high update rates. after this introductory part, section 2 describes the basic theory of operation for this kind of sensor. in section 3 the problem of the transmitter-to-receiver coupling is described by a simple model and measurement results are shown. section 4 describes the demodulation method of the received sensor signals utilizing band-pass-sampling. section 5 gives an overview of the already developed signal conditioning electronics, the measurement setup and measurement results. finally, conclusions are suitably drawn in section 6. 2. operating principles  the inductive position sensor operating principle is similar to the resolver operating principle. however, instead of a rotor winding and two stator windings there are an excitation coil and further four receive coils on the same antenna pcb. furthermore, the coils are planar on the antenna pcb instead of a spatial arrangement like the resolver windings in a motor stator. a typically planar arrangement of tracks which forms transmission and receive coils is illustrated in figure 2. the absolute receive coil cycle length is la = 390 mm, and consequently the theoretical measurement range is lth = 390 mm. furthermore, the vernier multi-cycle receive coils contain 10 cycles with cycle length lp = 40 mm, and its overall length is 400 mm. thus, the total length of the vernier circuits is 10 mm longer than the absolute receive coils, due to via placing reasons at the right most end of the antenna pcb. hence, the precise receive circuits have a cycle period of lp = la / 9.75 = 40 mm. furthermore, the single cycle receive coils have a width of 17 mm, the multi-cycle receive coils have a width of 18 mm, and the antenna pcb width is 26 mm. the transmission coil is formed by tracks which extend around the periphery of the antenna pcb forming three loops. the sine shaped tracks are the receive coils. all traces have a width of 0.254 mm on the antenna and target pcb, and the copper coating is 35 µm thick on both pcbs. the number of cycles is chosen such that the sensor resolution improves significantly and that the absolute coil signals detect the correct incremental index reliably. as already mentioned the target lc resonant circuit is a simple passive and serial lc resonant circuit where the inductance lt is a multilayer planar coil which is formed by a set of straight conductive tracks on a pcb, which can be seen in figure 3. targets are relatively short as compared to the antenna pcb and are typically attached to a moving host component. the two single cycle coils of the antenna pcb are used to determine the absolute position and the multi-cycle windings improve the sensor resolution but are incremental. thus, the single cycle or absolute coils must resolve smaller displacements than half the period of the multi-cycle coils to obtain the correct incremental index. consequently, the determined multiple of the multi-cycle period length plus the relative target position delivers the precise and absolute position. hence, the combination of two different pitches improves the sensor for high resolution position measurement. usually, a sine or square wave signal supplies the rectangular transmission coil which has a frequency range between 100 khz and 4 mhz where the fundamental oscillation can be represented as utx = atx sin(2πf0t), where atx and f0 are the amplitude and the fundamental frequency of the excitation signal [1,2]. in the region of the transmission coil, a uniform, elongated and alternating electromagnetic field is formed due to the excitation signal. when the target enters this alternating field, currents are induced to flow in the resonant circuit. these currents generate their own magnetic field, which induces a voltage in the receive coils. the magnetic coupling between the resonant circuit and each of the sensor receive windings varies with position, so that by applying an oscillating signal to the excitation winding at the target resonant frequency, a signal is induced in the four receive windings which oscillates at resonant frequency but whose amplitude varies as a function of the target position relative to figure 1. the picture shows a contactless inductive position sensor in linear arrangement,  consisting  of  an  antenna  pcb,  a  target  pcb  which  is  a movable,  passive,  high‐q  lc  resonant  circuit  and  a  signal  conditioning electronic.  figure 2. in the antenna pcb picture can be seen the transmission / excitation coil and the different sine and cosine structures which are manufactured on a  four layer pcb and offers a theoretical absolute measurement length of 390 mm. the two single cycle sine and cosine structures are for absolute position  measurement and the multi‐cycle sine and cosine structures are for increased precise position measurement.   acta imeko | www.imeko.org  september 2014 | volume 3 | number 3 | 52  the antenna pcb. because of the sine shaped receive coil design, the carrier signals are in phase with each other but the complex envelopes are in-phase and quadrature as a function of the target-position along the antenna pcb. theoretically, the two amplitude modulated receiver signals from the absolute structure can be represented in the time domain as follows: )( 2 sin )(2 cos 2 )( acos a a acos tct l txa tu                 , (1) )( 2 sin )(2 sin 2 )( asin a a asin tct l txa tu                 , (2) where x(t) is the absolute position of the target along the measurement axis, la is the cycle length of the absolute position coil, ω = 2·π·f is the angular frequency of the target resonance and excitation signal frequency and cacos(t) and casin(t) are the capacitive coupling terms directly from the transmission to the absolute position windings [1,2]. the two amplitude modulated receive coil signals from the multi-cycle structure can be represented in the time domain as follows: )( 2 sin )(2 cos 2 )( pcos p p pcos tct l txa tu                 , (3) )( 2 sin )(2 sin 2 )( pcos p p psin tct l txa tu                 , (4) where lp is the pitch of the multi-cycle windings and cpcos(t) and cpsin(t) are the capacitive coupling terms directly from the transmission to the multi-cycle receive windings. the target position is determined from the relative amplitudes of the components of the signal induced in the sensor windings corresponding to the in-phase and the quadrature signal. if the signal pairs are doubled or halved, the calculated position remains unchanged. this ratiometric format yields immunity to variations in the excitation, multiplicative disturbances, temperature, and the sensitivity of the processing electronics. the high operation frequency allows to avoid ferromagnetic coupling elements that in alternative designs can cause hysteresis effects thus decreasing both resolution and precision especially if an extended temperature range is demanded. furthermore, without the target circuit there are no induced currents in the receive windings, because the conducting tracks of the sine and cosine receive coils are twisted properly. therefore, the electrical current induced by the excitation field to flow in one loop of a receive coil is canceled out by an equal and opposite in phase induced current of the other loop. using such twisted coils has the advantage that the influence of inductive crosstalk from the excitation coil current and by external electromagnetic noise is significantly reduced, if the number of loops is even [7,8]. a further main property of the ratiometric transducer design offers a greatly reduced influence of mechanical tolerances such as gap variations between target and antenna pcb or target displacements lateral to the measurement axis. therefore, lateral misalignment variations to the measurement axis within ±10% of the sensor affect the measurement result only marginally because this invariance results from equal signal reduction in channel signals generated by the oscillating target [9]. the vertical distance between stator and target pcb depends on the application and is usually between 0.5 mm and 3 mm [1, 2]. the two single period receive windings offer absolute position measurement, but the effective measurement length should be limited to 80% of the structure length la to avoid edge effects. the system allows resolutions of 0.1% to 0.01% of the antenna length and the absolute linearity of the transducer is between 0.1% and 1% [4]. 3. capacitive crosstalk  if there are two or more electrical circuits, there will be a capacitive coupling between them and if there are two or more current loops, there is an inductive coupling between each of them [8,11]. the geometry of the printed circuit board traces has to be such as to avoid to the greatest extends any of these unwanted coupling mechanisms. since the sensor principle relies on the wanted inductive coupling effect due to the target pcb influence any unwanted cross-talk, be it inductive or capacitive is deteriorating sensor performance. especially this sensor design suffers from the direct capacitive transmission to receive coil crosstalk, and this influences the linearity of the arc-tan ratio significantly [1, 2], whereas the inductive transmission to receive coil crosstalk is strongly suppressed by the receive circuit design as already explained in the previous section. figure 4 illustrates the amplitudes due to direct inductive transmission to receive coil crosstalk in dependence of the frequency and absence of the target pcb. the signals were measured for each receive winding by an impedance / gain phase analyzer (hp 4194a), which also supplied the excitation signal (a 2.4 v peak–to–peak sine wave). the receive windings were shorted out one after another and the induced currents were measured by a tektronix tcp312 current clamp and a tektronix tcpa300 amplifier. figure 3. resonant target pcb with the rectangular planar coil windings to form the inductance for the serial lc resonant circuit.   figure 4. the plot shows the im/itx, m = {asin, acos, psin, pcos} behavior of  the  four  receive  windings  due  to  direct  inductive  transmitter  to  receive coupling. iasin and iacos are the single cycle coil currents, ipsin and ipcos are the  multi‐cycle coil currents and itx is the excitation current.  acta imeko | www.imeko.org  september 2014 | volume 3 | number 3 | 53  it is easy to see, that the coupling is rather weak and the sine windings suppress the inductive coupling current even better by 15 to 20 db than the cosine windings. furthermore, the multicycle cosine receive winding offers a better suppression characteristic than the cosine single cycle winding, as well as the sine multi-cycle winding, due to the smaller loop areas which improves the crosstalk current cancellation. moreover, the better suppressed amplitudes of the sine circuits results from the better receive circuit symmetry (see figure 2). this happens because the multi-cycle and single cycle cosine windings contain loops with larger surfaces at the antenna leftmost position than the loops at the rightmost position. these differences occur due to the connection traces from the cosine windings to the connector p2. capacitive coupling can be separated as a first approximation from inductive terms by using open-loop measurements thus limiting current loops. the voltage due to capacitive coupling (see coupling terms in equations (1), (2), (3) and (4)) appears in phase with the induced voltages uasin, uacos, upsin and upcos due to the target oscillating field. because of this capacitive crosstalk, an offset voltage appears after demodulation, and thus, the linearity of the arc–tan ratio becomes distorted. these capacitive transmitter-to-receive crosstalk voltages are shown in figure 5 and figure 6 for a square and a sine wave excitation signal in absence of the target pcb. the square wave excitation signal causes significantly higher disturbances than the sine wave excitation due to the higher harmonics of the square wave signal fundamental frequency (3 f, 5 f, 7 f,...). therefore, a sinoidal excitation is recommend although a square wave signal is more simply to generate. to describe this phenomenon theoretically, figure 7 shows two pcb traces in close proximity. one trace is supplied by a sinusoidal voltage source, ∙ sin and the other trace is connected to ground via a resistor r2 and a parallel capacity c2. furthermore, a transfer function which describes the coherent capacitive interference between these two parallel conductors illustrated in figure 7 is given in [10] and can be used as a simple mathematical model to describe the capacitive coupling (c12) between transmitter and receive coils. the transfer function for the coherent interference to any receive coil is given by:     rx 12 2 tx 12 2 2 1 u s c r s u s c c r    (5) where c12 is the capacitance between the transmission and receive coils, which picks up the interference from the transmission coil. resistance r2 and capacitance c2 are parallelconnected, and c2 is the capacitance with respect to ground and r2 the ohmic receive coil resistance. the variable urx represents the crosstalk voltages from each receiver channel (uasin, uacos, upsin and upcos) and is measured across r2. c12 is typically on the order of 2.3 pf to 3.1 pf for the singleand multi-cycle coils, c2 is on the order of 17 pf, and r2 is approximately 1.8 ω for the single and approximately 2.8 ω for the multi-cycle receive coil. from inspection of equation (5), it is easy to see that in order to minimize the voltage urx, due to capacitive crosstalk for all ω > 0, c12, r2 and utx have to be small. however, the coupling capacity c12 is difficult to reduce due to the closely spaced transmission and receive coil arrangement, and increasing the distance between the excitation and receive coils will increase the dimensions of the antenna pcb in width and length. the ohmic resistance can be reduced by reducing the number of vias (two layers a receive coil) and choosing thicker copper layers. in figure 8 the bode diagram shows the capacitive transmitter-to-receive coil crosstalk behavior of the four receive windings in absence of the target pcb. the capacitive crosstalk behavior was measured for each receive coil one after another by an impedance analyzer (hp 4194a), which supplied the excitation signal of a 2v peak to peak sine wave. the frequency was swept from 100 khz to 100 mhz. the receive coils are assumed unloaded. in figure 8 is easy to see that the voltage at the receive coils due to capacitive crosstalk increases with increasing frequency with approximately 20 db per decade from 100 khz to 10 mhz. between 20 and 30 mhz is a resonance peak and for frequencies beyond 30 mhz the crosstalk decreases again. the figure 5. the measured coupling voltages on the absolute sine receive coil  show  voltage  transients  of  150  mv  amplitude.  the  excitation  voltage  is approximately a 4 v peak–to–peak 1 mhz square wave signal.  figure 6. the measured coupling voltages on the absolute sine receive coil  show  voltage  transients  of  15  mv  amplitude.  the  excitation  voltage  is approximately a 4 v peak–to–peak 1 mhz sine wave signal.  figure  7.  simple  model  to  describe  the  capacitive  coupling  between  two  parallel conductors.   acta imeko | www.imeko.org  september 2014 | volume 3 | number 3 | 54  cause for this phenomenon is under research and cannot be explained yet. therefore, equation (5) describes well the crosstalk behavior of this kind of sensor in the usual operating range between 100 khz and 4 mhz. the cosine coils exhibit stronger crosstalk than the sine coils due to edge effects (straight traces at the antenna pcb ends) 4. amplitude demodulation by undersampling  for obtaining the position of the target, the two absolute receiver signals uasin and uacos (equation (1) and equation (2)) and precise receive signals upsin and upcos (equation (3) and equation (4)) have to be demodulated to extract the envelope from the dsbsc receiver signals. the amplitude modulated receiver signals can be demodulated by using any synchronous demodulating technique. one basic method utilizing digital demodulation is introduced in this section. the uasin, uacos , upsin and upcos signals are sampled at the same frequency as the carrier frequency f. this, so called under-sampling, synchronously demodulates the four amplitude modulated receiver signals prior the quantization. the demodulated and quantized envelopes of the absolute amplitude modulated signals can be represented as:    aqa 0 a 2π ( ) sin δ 2 i a x t u i t it l           , (6)    aia 0 a 2π ( ) cos δ 2 i a x t u i t it l           , (7) where δ is the kronecker delta, uia and uqa the in-phase and quadrature envelopes of the absolute receiver signals. furthermore, the demodulated and quantized envelopes of the multi-cycle amplitude modulated signals can be represented as:    p pqp 0 p 2π ( ) sin δ 2 i a x t u i t it l           , (8)    p pip 0 p 2π ( ) cos δ 2 i a x t u i t it l           , (9) where uip and uqa are the in-phase and quadrature envelopes of the multi-cycle receiver signals. due to the synchronized excitation, sampling, and processing of all the signals it is guaranteed that the receive coil signals uasin, uacos, upsin, and upcos are sampled very close to their respective maximum thus yielding an optimal signal-to noise ratio [11]. a second order rc–band–pass filter rejects signal components outside the band of interest fin = f ± fb = 1 mhz ± 50 khz before the modulated signals are sampled. the argument arg{ua} of the complex signal, ua=uia+juqa is computed digitally on a dsp to obtain the absolute position. this has to be also done for the multi-cycle coil. for arguments arg{·}=mÿπ/2; m={1, 2, 3,…} the tangent used to calculate the phase exhibits singularities that need to be circumvented. such singularities are avoided by use of a lookup table which uses the symmetry properties of the tangent and co-tangent functions by correctly selecting the quadrant, octant, and the tangent or cotangent part of  = 2πx(t)/la (see figure 8). therefore, the tanand cotangentfunctions are always bounded by ±1. the conditions for selecting the quadrant are given by: i q i q i q i q 1, if 0 0 2, if 0 0 3, if 0 0 4, if 0 0 i i u u i u u q i u u i u u                     (10) figure 8. the bode diagram shows  the capacitive crosstalk behavior from the excitation winding to each of the four receive channels.  figure 9. arc–tan computation for measurement range [0, 2πx/la ] [8].  figure 10. functional block diagram of the signal condition electronic for the  absolute or precise position receive coils.  acta imeko | www.imeko.org  september 2014 | volume 3 | number 3 | 55  which give a full position calculation of the target without any discontinuities at singular points [12]. 5. measurement system and preliminary  measurement results  figure 10 illustrates the main components of the signal conditioning electronics for two receive coils. each receive channel consists of a second order band–pass filter, an amplifier and a 12–bit adc. the two coarse and the two fine pitched receive channels are digitalized by a dual channel simultaneous sampling adc. a dev–bf537eda–lite eval board and the extender board ext–bf5xx–ad/da from bluetechnix gmbh [13] provide the digital evaluation electronics, which sample at the same frequency as the carrier frequency f. one end of the receive coils is connected to gnd and the other is connected to a second order band–pass filter. the functional block diagram also shows a pwm generated transmit signal, but for the current measurement results and analysis, the excitation was done using an agilent 33250a function waveform generator. the pwm excitation has not yet been implemented, but will be done in further development steps. figure 13 shows the single target test arrangement for precise absolute position measurement where the isel linear translation stage with 17 µm minimum incremental motion, 20 µm repeatability accuracy and 90 cm travel can be seen. furthermore, the test arrangement is able to move the target pcb off the measurement axis into the y-direction by a second linear translation stage with shorter travel to test the influence of lateral target to antenna misalignment. the antenna pcb is mounted on a perspex–glass bar and the target is mounted on a perspex–glass holder to get sufficient stand–off to electrically conductive and ferromagnetic materials. this offers to neglect eddy currents and makes possible to assume that the sensor coils are air–cored coils. figure 11 and figure 12 show the demodulated receive coil signals over the full measurement range from 0 mm to 390 mm. these signals where obtained when the transmitter coil was excited by a 1 mhz sine wave signal with 4 v peak to peak. the demodulated and amplified by a factor 10 measured output voltages of the two absolute and precise position receive channels compared to ideal sine and cosine signals show the behavior of the target resonance circuit along the antenna pcb axis. the target pcb was shifted relatively to the antenna pcb by a linear stage, in 1 mm increments, from the current position to the new position along the antenna measurement axis. furthermore, the voltage was measured at the cosine and sine coils for every position. figure 11 shows distortions at the beginning and at the end of the measurement range. especially, the measured cosine– signal deviates from the ideal at both measurement range ends. these distortions occur due to the non–continued cosine structure by the straight track at the 0 mm and 390 mm measurement positions (see figure 1). the measured sine-signal fits much better to the ideal signal than the cosine-signal because its structure is not as prone to the edge effect as the cosine structure. due to these distortions the practical measurement range is limited to approximately 80 % of the theoretical measurement range. figure 11. demodulated output voltages uia (red) and uqa.(blue) of the two  absolute  receive  channels  compared  to  ideal  sine  and  cosine  (both  black  dashed) signals.  figure 12. demodulated output voltages uip (red) and uqp (blue) of the two  precise    receive  channels  compared  to  ideal  sine  and  cosine  (both  black  dashed) signals.  figure  13.  the  picture  shows  a  measurement  setup  of  a  high–frequency  inductive  position  sensor  for  absolute  linear  position  measurement.  the used antenna design corresponds to the illustrated antenna pcb in figure 1. acta imeko | www.imeko.org  september 2014 | volume 3 | number 3 | 56  the measured signals of the multi-cycle scale coils which are illustrated in figure 12 behave almost like the ideal signals and distortions are barely to be seen. however, the amplitude increases slightly over the measurement range. this is due to an unintended decreasing target antenna gap over the measurement range in our experiment but due to the ratiometric principle the measurement uncertainty is affected only marginally. figure 14 and figure 15 show the position error along the measurement axis over a range of 350 mm and approximately 90% of the theoretical absolute measurement length la is used by applying precise and absolute structures. moreover, the preliminary measurement results show that the errors are smaller than 0.05 %. this corresponds to an absolute measurement error of approximately 190 µm. 6. conclusion  an inductive position sensor based on pcb techniques which can operate in harsh environments has been presented. this sensor technology provides many advantages, for instance relatively wide measurement range, high resolution, reasonable production costs, and the ability to use pcb techniques, that offers supplier independency. the measurement results clearly demonstrate that the sensor, offers a measurement error smaller than 0.05 % over 90 % of the theoretical measurement length without further digital signal processing. however, further analog and digital signal processing techniques reduce the measurement error by applying narrower band pass filters, type ii servo loop or tracking observers. active devices are not utilized, the resonant loop is passive and just a capacitance is used to tune the desired resonant frequency. the desirable properties contactless, absolute and non–optical make this sensor a good alternative to other linear sensors. acknowledgement  the authors gratefully acknowledge the partial financial support for the work presented in this paper by the austrian research promotion agency and the austrian comet program supporting the austrian center of competence in mechatronics (accm). references  [1] m. rahal, a. demosthenous, an asic front end for planar high-frequency contact-less inductive position sensors, instrumentation and measurement, ieee transactions on, 58 (2009) pp.3021 –3030. [2] m. rahal, a. demosthenous, an integrated signal conditioner for high-frequency inductive position sensors, measurement science and technology, 21 (2010) 015203 (12pp) [3] m. rahal, a. demosthenous, a synchronous chopping demodulator and implementation for high-frequency inductive position sensors, instrumentation and measurement, ieee transactions on, 58 (2009) pp.3693 – 3701. [4] j. golby. induktive weg–sensoren –– neues messprinzip ermöglicht maßgeschneiderte entwicklungen, elektronik, 6 (2009),pp.22-26, www.elektroniknet.de/automation/sonstiges/artikel/28107/2. [5] b. aschenbrenner, b. g. zagar, dsp based read out system for contactless high-frequency inductive position sensors, proc. xx imeko world congress, busan, republic of korea, sep. 2012 [6] r.-p. jones, r. a. doyle, m. a. howard, d. a. james, d. kreit, and c. s. sills, sensing apparatus and method, us patent no. 7208945 b2, apr. 24 2007. [7] p. clayton, j. mc knight, prediction of crosstalk involving twisted pairs of wires part ii: a simplified low-frequency prediction model, electromagnetic compatibility, ieee transactions on, 21 (179), pp.105 – 114. [8] p. clayton,introduction to electromagnetic compatibility,wileyinterscience, hoboken n.j, 2006, isbn 0-471-75500-1 [9] b. aschenbrenner, b. g. zagar, planar high–frequency contactless inductive position sensor, ieee international instrumentation conf., minneapolis, usa,mai 2013 [10] r. b. northrop, introduction to instrumentation and measurements, 2013 taylor & francis, boca raton, 2005, isbn 978-0-8493-3773-4 [11] j. w. howard, m. graham, high speed digital design. a handbook of black magic, prentice hall ptr , englewood cliffs nj, 1993, isbn 0-13-395724-1. [12] s. sarma, v.k. agrawal, s. udupa, k. parameswaran, instantaneous angular position and speed measurement using a dsp based resolver-to-digital converter, measurement, 41 (2008), pp.788 – 796. [13] bluetechnix gmbh, www.bluetechnix.com/rainbow2006/site/blackfin_family/__de v__eval_boards/__dev-bf5xxda-lite/380/dev-bf5xxda-lite.aspx. figure 14. position error of the absolute coils along the measurement axis.  figure  15.  position  error  of  the  multi‐cycle  coils  along  the  measurement  axis.  a hybrid tree sensor network for condition monitoring system to optimize maintenance policy acta imeko issn: 2221-870x march 2020, volume 9, number 1, 3 9 acta imeko | www.imeko.org march 2020 | volume 9 | number 1 | 3 a hybrid tree sensor network for a condition monitoring system to optimise maintenance policy lorenzo ciani1, alessandro bartolini1, giulia guidi1, gabriele patrizi1 1 university of florence, department of information engineering (dinfo), via di s.marta 3, 50139, florence, italy section: research paper keywords: wireless sensor network; condition monitoring; maintenance; wind turbine, redundancy citation: lorenzo ciani, alessandro bartolini, giulia guidi, gabriele patrizi, a hybrid tree sensor network for a condition monitoring system to optimise maintenance policy, acta imeko, vol. 9, no. 1, article 2, march 2020, identifier: imeko-acta-09 (2020)-01-02 editor: lorenzo ciani, university of florence, italy received october 30, 2019; in final form january 14, 2020; published march 2020 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: gabriele patrizi, e-mail: gabriele.patrizi@unifi.it 1. introduction in recent years, the diagnostic process became fundamental in several different fields of applications. diagnostics are now an essential part of the performance requirements in industry 4.0, especially for industrial, energy, and transportation systems [1][7]. diagnostics are used to evaluate the current health status of the system under test, predicting its remaining useful life based on features that capture the gradual degradation of its operational capabilities. the diagnostic procedure allows the improvement of the reliability, availability, maintainability, and safety (rams) parameters of complex systems, ensuring that the equipment works successfully and reducing downtime due to unexpected failures [8]-[12]. condition monitoring (cm) is one of the most effective and powerful diagnostic procedures. cm is the process of monitoring one or more condition parameters in machinery to identify some changes that are indicative of an incipient fault or equipment health degradation [13]. in the past, cm was applied simply through routine manual diagnostic actions; however, with the introduction of low-cost sensors and automated monitoring systems, online data-driven cm was adopted. cm is a type of condition-based maintenance (cbm) used to select and survey parameters from the sensors placed in the system in order to detect a change in the health machine condition [14][15]. cm, as well as reliability and availability, is mandatory in several manufacturing fields, such as energy and industry applications, where products are forced to endure extreme process and environmental conditions [7][16]-[18]. during the design phase of a product or plant, assessment of the optimal diagnostic and failure management policy is a critical phase that could be implemented using the reliability-centred maintenance (rcm) procedure. rcm allows the efficient and effective achievement of the required safety, availability, and economy of operation. rcm provides a decision process to identify applicable and effective diagnostic and preventive maintenance requirements for equipment in accordance with the safety-related, operational, and economic consequences of identifiable failures as well as the degradation mechanisms responsible for those failures. rcm uses a failure modes and effects analysis (fmea) to lead the diagnostic and maintenance assessment towards the most cost-effective solution. according to the international standard iec 60300-3-11, the rcm procedure follows five steps, providing a comprehensive programme that addresses not only the analysis process but also the preliminary and follow-on activities necessary to ensure that the rcm effort achieves the desired results [19]-[22]: 1. initial definition (availability requirements, maintenance specification, environment, and so on); 2. failure modes and effects analysis; abstract wind energy is the leading candidate among renewable energy sources as an alternative to burning fossil fuels. a proper and accurate condition monitoring plan is necessary to ensure the high reliability and availability required by energy production systems. this article proposes a wireless mesh network to implement a widely distributed condition monitoring system for a wind farm. using different types of sensor, the condition monitoring system evaluates the health state of each turbine. the aim of the work is to propose an architecture that can identify possible incipient failures in the most critical turbine’s components. using this system, it is possible to guarantee continuity of service minimising the unplanned maintenance operation due to hidden failure. mailto:gabriele.patrizi@unifi.it acta imeko | www.imeko.org march 2020 | volume 9 | number 1 | 4 3. identification of the optimal maintenance task using the decision-making diagram; 4. implementation of corrective actions; and 5. continuous improvements that give information to improve steps 2 and 3. steps 2 and 3 are the core of rcm. indeed, fmea is a systematic procedure for the analysis of a system to identify the potential failure modes, their causes and effects on system performance. it allows the identification of the criticality of each failure mode, and it provides an input to mitigate the risk associated with the modes [23]. the ‘identification of the optimal maintenance task’ (step 3) is guided by the fmea report, which provides important information for selecting the most appropriate and effective failure management policy. in compliance with international standard iec 60300-3-11, the main maintenance tasks are [22]: • cm: the continuous or periodic task of evaluating the condition of an item in operation against preset parameters in order to monitor its deterioration. it may consist of inspection tasks, which are an examination of an item against a specific standard. • scheduled restoration: the work necessary to return the item to a specific standard. since restoration may vary from cleaning to the replacement of multiple parts, the scope of each assigned restoration task must be specified. • scheduled replacement: the removal from service of an item at a specified life limit and the replacement thereof by an item meeting all the required performance standards. • failure finding: the task of determining whether or not an item is able to fulfil its intended function. it is solely intended to reveal hidden failures. a failure-finding task may vary from a visual check to a quantitative evaluation against a specific performance standard. some applications restrict the ability to conduct a complete functional test. in such cases, a partial functional test may be applicable. • no preventive maintenance: in some situations, no task is required, depending on the effect of failure. the result of this failure management policy is corrective maintenance or no maintenance at all. sometimes, this approach is called ‘run-to-failure’. cm and failure finding belong to the cbm procedure. cbm is the most effective policy because its goal is to monitor the health state of the system under test and to start a maintenance intervention only in the case that it is necessary [24]-[27]. in fact, international standard iec 60300-3-11 proposes a decision diagram for the task assessment that assigns the cm as the optimal maintenance solution for improving system availability and minimising unexpected failures. for this reason, this article proposes a cm network for a wind farm based on a customised low-cost wireless sensor network. 2. system under test a wind farm, also known as a wind park, is a group of wind turbines (wts) in the same location used to produce electricity. a large wind farm may consist of several hundred individual wts and may cover an extended area of hundreds of square kilometres, but the land between the turbines may be used for agricultural or other purposes [28]-[30]. this results in a very critical issue during the design of cm and diagnostic systems. a wt is a complex equipment made up of both mechanical and electrical/electronic components. the efficiency and practicality of horizontal-axis three-blade turbines have resulted in domination of the market, and in almost all the new installations of wind farms, this is the only type that is employed for energy generation. such a wind turbine is comprised of four major components (the rotor, tower, blades, and nacelle) in an enclosure that contains the electrical, electronical, mechanical, and hydraulic components needed to produce electricity (e.g. gearbox, shafts, rotor brake, and nacelle turning system, usually known as yaw, generator, power lines, control system, bearings, pitch controller, and so on). figure 1 shows the main components of a horizontal-axis, three-blade wt. this article focuses on a spanish wind farm located near zaragoza in the region of aragon (spain). the first phase of the project has the aim of analysing all the components included in the turbine and to carry out a fmea, which highlights several different types of failure modes and causes. many causes have a great influence on the electronic and mechanical components; for instance, failures of the mechanical items may be affected by high vibration levels on the shafts, the very extreme temperature generated during the rotation speed conversion by the gearbox, corrosion, and fatigue. the damage to the electrical and electronic components may be affected by the environmental conditions, such as temperature, humidity, and vibration. 3. proposed wireless sensor network to monitor the wind farm under test monitoring systems play a fundamental role in any wind farm, because they are crucial to guaranteeing the turbine functionality and to providing quick advice in the case of failure. furthermore, the recent development of offshore wind farms makes the diagnostic implementation to control and manage the status of the turbines even more difficult. for example, the use of a wired monitoring system would entail higher additional costs compared to the wireless solution. moreover, in a fixed installation, technological and structural change is more difficult. therefore, it is essential to use a low-cost plug-and-play system, such as a wireless sensor network (wsn). wsns are a network characterised by a distributed architecture of small nodes. each node can host multiple sensors and is equipped with figure 1. internal view of a horizontal-axis wind turbine highlighting the most critical components. acta imeko | www.imeko.org march 2020 | volume 9 | number 1 | 5 a computational and wireless communication unit [31][32]. the monitoring system acquires several condition parameters to evaluate the health state of the turbine. it must be able to detect and notify potential breakdowns in time, thereby preventing critical damage and increasing the wt’s operational lifetime. however, the design of a monitoring system for a wind farm has four main issues: • the large dimensions of the covered area; • the great number of parameters that must be monitored; • the condition parameters rapidly change, and they therefore require a high sampling frequency; • the possibility of increasing the number of sensor nodes in the case of new turbine installations inside the wind farm. the problems explained previously require a proper trade-off in terms of the cover area and transmission protocol. in fact, the great number of parameters and the high sampling frequency require a high-frequency protocol with a high bitrate that usually is characterised by a limited cover area. wireless mesh networks (wmns) are an optimal solution for providing broadband internet access to large geographical areas using a great number of near nodes and dynamic routing tables to guarantee highfrequency transmissions and a high bitrate, despite the large geographical area [33]. moreover, wmns allow the increase of the number of nodes inside the network in the case of a new turbine installation without requiring any change in the already existing architecture, with a consequent decrease of management cost. the differences between a traditional wsn and a wmn are as follows [34]-[37]: • a traditional wsn (figure 2) is a point-tomultipoint (star) network where a single central node, known as the access point (ap), is directly connected to all other nodes known as stations or, generally, nodes. traditional infrastructure wi-fi networks have the disadvantage of a limited coverage area because every station must be in a range directly connected with the ap. furthermore, overloading is another relevant drawback of traditional wireless networks, because the maximum number of stations permitted in the network is limited by the capacity of the ap. • a wmn (figure 3) is a self-organised and selfconfigured sensors network. it is composed of several sensor nodes and a single root node that manages the entire network and allows for data uploading to the cloud. each node can communicate with all the neighbouring nodes. consequently, the network can cover very large areas. another advantage of this network is that it is naturally a fault-tolerant system by definition; in fact, when a node stops working, the whole network does not fail, but the ap can be reached by different paths. this article proposes the design of a wmn composed of lowcost and low-power sensor nodes based on a commercial system on a chip that integrates a high computational capability microcontroller; multiple analogue and digital inputs; and a multi-protocol radio unit. a block diagram of the proposed architecture is shown in figure 4. figure 4 highlights the main components that make up this system, as follows: • power unit: it provides power to all units of the sensor node. in particular, it is composed of a charge regulator, two batteries, and a dc/dc converter. the batteries are necessary for guaranteeing a continuous power supply to the monitoring system. using this architecture, the monitoring system can provide a continuous flow of information regarding the state of the turbine, even in the case of major damage to the turbine’s uninterruptible power supply (ups). such a redundancy is mandatory for guaranteeing diagnostic self-sufficiency and consequently allowing the proper monitoring of the system under test. • processing unit: the esp32 powered by espressif is the system on a chip used to process the acquired data. this unit is composed of a microprocessor that elaborates on the data from the sensors and transmits it using the radio unit. there is also a nonvolatile memory for storing the firmware and a faster volatile memory (ram) for the data elaboration. figure 2. traditional wireless sensor network. figure 3. wireless mesh network topology. figure 4. block diagram of the sensor node. acta imeko | www.imeko.org march 2020 | volume 9 | number 1 | 6 • radio unit: once the elaboration on the sensor information is complete, the data is transmitted to the central base station using the radio unit. it consists of a rf transceiver and an antenna for transmitting and receiving packets. in order to optimise energy efficiency, the communication unit is set at ‘idle and sleep mode’. the esp32 includes (in the same chip) the processing unit and the radio unit. the protocol used is the ieee 802.11 (wi-fi). • sensor unit: it could include several different kinds of transducer, such as thermal, vibration, magnetic sensors, etc., based on the physical quantities under analysis. if one or more sensors provide analogue signals, then they will be converted to digital form using an analogue-to-digital converter (adc). the wmns provide continuous and near real-time data acquisition and autonomous data acquisition. it can be used to [38]: • maintain process tolerances; • verify and protect machines, systems, and process stability; • detect maintenance requirements; • minimise downtime; • prevent failures and consequently save costs and time; and • plan a maintenance policy based on the prediction of failure. each turbine requires hundreds of different sensors to monitor the condition parameters that influence the wear-out process; therefore, a hybrid tree-mesh network is proposed. figure 5 shows the network architecture divided into three layers: • the first level is composed only of the root node; • the second layer is the wi-fi mesh network where each node identifies a specific turbine; and • the third layer is a wired network directly connected only to the second layer node of the specific turbine. the root node is the data collection point of the network and also includes the router and server, as shown in figure 6. each of the nodes in the second layer manages the data collection of a single wind turbine. it is also able to work as bridge node for the other second-layer devices receiving data from a different node and retransmitting that to the root node in order to cover the entire wind farm area. inside each turbine, there are a few thirdlayer nodes with the aim of managing various sensors, as shown in figure 6 and figure 7. this level uses a shielded wired connection to carry information to the second-layer node and to avoid interference caused by electronic and mechanical components of the turbine. figure 7 also slows an example of diagnostic implementation using the proposed network in the nacelle of the wt under test. the red circle shows some possible locations for the processing unit of the nodes to manage the sensors of a specific turbine section. over the years, the power generation capability of the wt has increased, as well as the dimension of the blades and the overall wt extension. therefore, the distance between two neighbouring wts could be significant (over hundreds of metres) [39]. the esp32 board is equipped with an integrated patch antenna, which is definitely not enough to cover such distances, so it is necessary to introduce another antenna located above the nacelle used for the transmission of the acquired data (figure 7). it is not possible to use a directional antenna heading towards a close node, because the wmn is a self-organised and self-configured network in which every device must be able to communicate with many nodes located in different positions. therefore, a power amplifier and a high-gain omnidirectional antenna are used as a radio unit. consequently, the second-layer node is located outside the wt, close to the antenna. it is very important to protect the electronic device by the weathering effects using a protective case to minimise the acceleration of the wear-out process due to temperature, humidity, rain, wind, and solar radiation. 4. optimization of maintenance policy an rcm procedure was carried out on the wind farm under test to identify the failure mode that could be monitored using condition parameters [40]. table 1 presents an extract of the rcm assessment, containing: figure 5. hybrid tree-mesh network architecture. figure 6. proposed architecture with first-, second-, and third-layer node location. figure 7. possible third-level sensor (red circle) deployment inside the nacelle of a wind turbine. acta imeko | www.imeko.org march 2020 | volume 9 | number 1 | 7 • a description of the functionality of the items useful for a functional failure analysis; • a list of the main failure modes (how we can observe the inability of the item to perform a required function) and mechanisms (the physical, chemical, or other processes that have led to a failure); • one or more failure effects related to each failure mode; • the output of the procedure (the assessed maintenance task) evaluated following the guidelines of international standard iec 60300-3-11 [22]. the table highlights that cm is a fundamental management policy for the wt under test, and almost all the subsystems of the turbine can be monitored using the appropriate sensors. table 2 deals only with the components for which rcm suggests cm as the optimal maintenance task in terms of availability, management cost, safety, and downtime. the table shows the types of measurements used to monitor each item and the proposed sensor technology for the relative measurement. the wmn proposed in the previous paragraph is used to implement the cm procedure, and each sensor node is provided by several different types of sensing elements. an ieee 802.11n lan protocol was implemented in the sensor network because it represents the optimal trade-off between the geographical coverage, the transmission of great amount of data, and the power dissipation [41]. 5. reliability improvements using redundancy the wmn proposed in this paper is a diagnostic tool fundamental for the wind farm under test. the entire network is composed of several components forced to endure extreme conditions. usually, during the reliability analysis of a system or plant, diagnostics involve high-quality components and are considered failure-free. this article also considers the reliability of the diagnostic network to highlight the possible criticality of the proposed configuration. the mesh network is a fault tolerant system by definition. in fact, when a node fails, the other nodes reconfigure the network to find another path to reach the root node and therefore the server. in this kind of architecture, it is clear that the most critical component becomes the root node because a failure in the root node causes the failure of the whole network. to maximise network reliability and reduce the downtime of the cm system of the mesh network, a warm standby redundancy was considered. this configuration is composed of two root nodes and a switch that detects the state of the main root node and reconfigures the architecture, activating the standby module to maintain the network’s operation in case of failure [42][43] (figure 8). both root nodes are connected to power sources, but only the main power source is used. the standby is half-operative and ready to work in the case that the main power source fails. for table 1. extract of rcm assessment for the most critical component of the wt component description/function failure modes and mechanisms failure effects maintenance task blades power source of the wind turbine that catch the wind energy and convert it in mechanical motion. • fatigue failure • microfracture • corrosion • delamination loss of aerodynamics and effiency property condition monitoring gearbox increases the rotor speed up to the appropriate generator speed using gears, pinions and shafts. • binding/sticking • excessive wear • fails to move • mechanical damage inoperative wt condition monitoring combined with failure finding and predetermined maintenance restoration every 12 months brake stop the turbine movement on demand • excessive wear • leaking • cracked housing impossibility to brake the nacelle movement scheduled maintenance every 6 months hydraulics system moves the nacelle towards the yaw axis (wind direction) • leakage • contamination • improper flow • stuck valve loss of pressure in the hydraulic system impossible to move the nacelle toward the wind direction condition monitoring combined with predetermined maintenance restoration every 6 months tower supports the other parts and holds them off the ground. • fatigue failure • corrosion • fracture serious damage of the structure. possible turbine collapse condition monitoring bearings achieve a smooth, low-friction rotary motion or sliding action (linear motion) between two surface • binding/sticking • excessive play • contaminated possible mechanichal damage overheating of the nacelle limited turbine functionality scheduled maintenance every 6 months ups provides emergency power when the input power source or mains power fails. • dead batteries cells • no output • cooling fan can not move no impact in the short term if standard power supply is working, otherwise inoperative wt overheating of the nacelle condition monitoring combined with failure finding (only for cooling fan) power converter convert electrical energy from the grid to supply the control system • no output • short circuit • parameters drift no impact in the short term if ups is working, otherwise inoperative wt loss of power converter capability condition monitoring combined with predetermined maintenance restoration every 12 months control system contains all the electrical components in the turbine, such as power converter, pfc system, transformers, contactors etc. • input open • no output • short/open circuit • error in data storage wrong output data elaborated by the control system inoperative wt condition monitoring acta imeko | www.imeko.org march 2020 | volume 9 | number 1 | 8 this reason, it is not necessary to wait for the standby unit startup because the equipment is ready to be used [43]. the failure rate of the two different configurations was evaluated using a reliability prediction procedure, and the reliability results are shown in figure 9. the reliability of the root node remains very high, even after several years of the network life cycle because of the limited number of devices that make up the board. the difference between the single item and the warm standby redundancy reliability trends is quite remarkable. in particular, the warm standby allows the enhancement of the network’s life cycle, maintaining the reliability as very close to one. 6. conclusions wireless sensor network and condition monitoring represent the optimal maintenance solution for an extended wind farm. this is because of the limited installation and management costs and the possibility of easy installation and/or future upgrade, even if the park is already in use. the article focused on the reliability-centred maintenance procedure, which leads to the optimal maintenance task choice. the first part of the article described the advantages of the proposed hybrid tree wireless mesh network and illustrated its possible deployment for the sensor nodes inside a wind farm. an extract of the reliabilitycentred maintenance procedure for the wind turbine is proposed, and table 1 shows that for most of the failure modes, the optimal maintenance task is condition monitoring. indeed, the use of condition monitoring allows the minimisation of scheduled maintenance and consequently saves time and money. using this approach, maintenance and tasks policies are managed by the diagnostic mesh network. corrective maintenance is performed only in the case that the system identifies an intervention request. the article suggested a group of measurement quantities and relative appropriate sensors to monitor the failure modes identified through the reliability-centred maintenance procedure. due to the limited costs of the proposed elaboration unit, the introduction of a fault-tolerant design guarantees a constant service over time without affecting the overall cost. so, finally, a warm standby architecture was designed for the root node, and reliability with and without redundancy is compared in order to show the high increase of the reliability with the warm standby architecture. the root node has to manage all the network functionalities; therefore, the proposed warm standby configuration activates the redundancy in the case of failure, guaranteeing a probability of failure of the root node lower than 0.01 %. references [1] d. 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[42] m. catelani, l. ciani, a. bartolini, g. guidi, g. patrizi, standby redundancy for reliability improvement of wireless sensor network, proc. of the 5th international forum on research and technologies for society and industry, 2019. [43] o. tannous, l. xing, p. rui, m. xie, s. h. ng, redundancy allocation for series-parallel warm-standby systems, proc. of the 2011 ieee international conference on industrial engineering and engineering management, 2011, pp. 1261-1265. acta imeko, title acta imeko issn: 2221-870x march 2018, volume 7, number 1, 76-79 acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 76 evaluation of supplementary comparison euramet.m.p-s14 in the range 50 mpa to 1 gpa of hydraulic gauge pressure j. könemann1, d. pražák2, r. fira3, c. wüthrich4, t. rabault5, w. sabuga1 1physikalisch-technische bundesanstalt (ptb), bundesallee 100, 38116 braunschweig, germany 2český metrologický institut (cmi), okružní 31, 638 00 brno, czech republic 3slovenský metrologický ústav (smu), karloveská 63, 842 55 bratislava 4, slovakia 4eidgenössisches institut für metrologie (metas), lindenweg 50, 3003 bern, switzerland 5laboratoire national de métrologie et d'essais (lne), 1 rue gaston boissier, 75015 paris, france section: research paper keywords: high pressure; comparison; pressure transducers citation: j. könemann, d. pražák, r. fira, c. wüthrich, t. rabault, w. sabuga, evaluation of supplementary comparison euramet.m.p-s14 in the range 50 mpa to 1 gpa of hydraulic gauge pressure, acta imeko, vol. 7, no. 1, article 15, march 2018, identifier: imeko-acta-7 (2018)-1-15 section editor: jorge torres-guzman, cenam – centro nacional de metrologia, santiago de querétaro, mexico received july 18, 2017; in final form december 17, 2017; published march 2018 copyright: © 2018 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: jens könemann, e-mail: jens.koenemann@ptb.de 1. introduction ptb organized a supplementary comparison between five national metrology institutes (nmis) from euramet – ptb (germany), smu (slovakia), cmi (czech republic), lne (france) and metas (switzerland) – in the pressure range up to 1.0 gpa. this comparison was embedded into the joint research project ind03 “high pressure metrology for industrial applications” which was carried out within the european metrology research programme. the objective of this project was to create a primary pressure standard for the range up to 1.6 gpa using pressure multipliers [1], and besides this, to develop a transfer standard (ts) for pressures up to 1.5 gpa on the basis of modern high-pressure transducers. it was aimed to determine the applicability of high-pressure transducers as a ts for pressure comparisons, i.e. testing of typical parameters like drift, hysteresis, sensitivity, repeatability, long-term stability and load cycling effects, see [2]. due to the structure of a ts consisting of a set of eight different and parallel pressure transducers with individual behaviour, the evaluation of the degree of equivalence of the participants required a special procedure for the analysis of the rich data. it was agreed that ptb would be the pilot laboratory. the analysis of data on the basis of the results from the participants was performed by the pilot institute, hence providing uniform treatment for all participants. 2. organization of the comparison information about laboratory standards (lss) including their piston-cylinder units (pcus) is given in table 1. abstract in this work, we present a mathematical procedure to evaluate a hydraulic gauge pressure comparison in the range to 1 gpa which was piloted by the physikalisch-technische bundesanstalt. the comparison used a transfer standard consisting of two series of modern high-pressure transducers, i.e. eight pressure transducers in total. this set of parallel arranged transducers should ensure the reliability of the transfer standard at high pressures and provides rich data for testing the performance of modern high-pressure transducers. the analysis of the comparison results was based on the evaluation of the individual measurement deviations of these transducers with respect to the laboratory standards, whereas the corresponding comparison reference values and their uncertainty were determined separately at each pressure point and pressure transducer. all these results were summarized to derive the degree of equivalence for each laboratory at each pressure. the degree of equivalence was found to be consistent for all laboratories at almost all pressures. acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 77 table 1. laboratory standards of the nmis. nmi type of reference standard relative standard uncertainty in ppm at 0.5 gpa and pmax #1 pressure balance: harwood, modified, pcu: desgranges et huot, controlled clearance 29 at 0.5 gpa 52 at 1.0 gpa #2 pressure balance, pcu: desgranges et huot, controlled clearance 26 at 0.5 gpa 51 at 1.0 gpa #3 pressure balance, pcu: dhi, free deformation 100 at 0.5 gpa pressure multiplier: desgranges et huot, controlled clearance 145 at 0.8 gpa 1 #4 pressure balance, pcu: dhi/fluke, free deformation 32 at 0.5 gpa #5 pressure balance, pcu: own production, free deformation 200 at 0.5 gpa 1 nmi #3 could reach only 0.8 gpa due to leakage problems. its measurements in the 1 gpa range were therefore withdrawn. the transfer standard was calibrated at ptb at the beginning and end of the comparison. the participants had minor technical problems during the comparison, but the ts did not significantly change its metrological properties, and the comparison was completed successfully. 3. transfer standard and calibration ptb developed and provided the transfer standard for the comparison. it consisted of eight high-pressure transducers of two manufacturers: type a (foil strain gauge, transducers s1 to s4) and type b (thin layer strain gauge, transducers s5 to s8), see figure 1. thus, from each manufacturer, there were four pressure transducers with different pressure ranges: one with (0 – 0.5) gpa, one with (0 – 1.0) gpa and two with (0 – 1.5) gpa pressure ranges. the whole device was enclosed in an aluminium box with a plexiglas window. additionally, for the read-out of the type a transducers, a measuring amplifier (range 2.5 mv/v, resolution 10-6 mv/v 1 ppmfs) with several measuring channels was circulated as well, whereas the type b transducers could be read out directly via a usb connection. measurements were carried out in two different pressure ranges, up to 0.5 gpa and up to 1 gpa. laboratories capable of measuring up to 0.5 gpa, i.e. #4 and #5, performed measurements only in this measurement range. laboratories with a measurement capability up to 1 gpa, i.e. #1 to #3, performed the measurements two times, in the range of 0.5 gpa and, additionally, in the 1 gpa range. due to their leakage problems and the associated piston fall rates which were too high at pressures above 500 mpa, nmi #3 decided to withdraw its measurements in the 1 gpa range. each laboratory performed four measurement cycles in each pressure range. after preloading, one complete measurement cycle in each pressure range consisted of 22 measurements – 11 for ascending and 11 for descending pressures. the pressures generated by the lss had to deviate from the nominal pressures by less than 0.1%. at each pressure point, the ts had to be in pressure equilibrium with the lss. the difference between the ts output (converted to pressure) and the ls pressure was used as a measure for the comparison. software was provided for readings of all eight transducers. after achieving equilibrium, seven readings from each of the eight transducers were taken. from the seven readings, the average and its standard deviation (σ) were calculated for each transducer at each pressure point by each participant and were reported together with the respective pressure value of each participant’s laboratory standard and its standard uncertainty at this pressure point. 4. comparison results 4.1. data processing for each participant each pressure transducer in each measurement cycle, a zero-pressure correction was applied. the result of each participant obtained in cycle c (xc) was expressed as the difference between the pressures of the ts and the laboratory standard: lsts ppx c −= (1) the average (x) and its standard deviation (σ(x)) from the results in four cycles were calculated: ( ) ( ) 5.04 1 2 4 1 3 ,4         −== ∑∑ == c c c c xxxxx σ . (2) the analysis of the ptb results obtained at the beginning and the end of the comparison revealed that the changes of the eight pressure transducers were rather irregular, so that an assumption of a linear change in properties of the transducers can hardly be justified. therefore, the difference in the ptb results from 2013 and 2014 was handled as an additional uncertainty contribution due to the ts drift, udrift : ( ) ( )32ptb,2013ptb,2014drift xxu −= . (3) the uncertainty of the transfer standard, uts, was determined for each participant, each pressure transducer in each measurement cycle and for each pressure, taking four components into account: • the standard deviation of the four xc values, σ(x), calculated according to (2), • uncertainty due to the ts drift, udrift , as defined above by (3), figure 1. image of the transfer standard. for more details see [2]. acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 78 • the zero deviation of the ts, uzerodev : ( ) ( )[ ]{ ( ) ( )[ ]} ( )320abs ,,...0absmax up44 up11zerodev =− =−= pxpx pxpxu (4) • and the instability of the ts, uinstab , taken as the resolution or the fluctuation of readings, ( )fluctresolinstab ,max uuu = (5) with the resulting uncertainty contribution due to ts ( )[ ] 5.02drift2instab2zerodev2ts uuuxu +++= σ . (6) the uncertainty of the ts pressure transducers measured by the laboratories especially in the 1 gpa range was observed to be very different. this was caused, to high extent, by very different fluctuations of readings. 4.2. deriving the degrees of equivalence the following considerations were made to account for the special setup of the ts: • the ts comprises eight pressure transducers. each of the eight transducers has different properties at increasing and decreasing pressures. the same pressure transducer has different properties when used in 0.5 gpa and 1 gpa measurements. it is thus only possible to directly compare results of laboratories which were obtained for the same pressure transducer, in the same pressure change direction and in the same maximum pressure comparison to determine one degree of equivalence at each pressure from the numerous “degrees of equivalence” available at the same pressure. • at each nominal pressure value, 16 comparison reference values (crvs), (i.e. 8 pressure transducers, 2 pressure change directions) can be generated for the 0.5 gpa, and 12 crvs (i.e. 6 pressure transducers, 2 pressure change directions) for the 1 gpa measurements. as some nominal pressures of the 0.5 gpa and 1 gpa measurements coincide, the number of crvs can be reduced at certain pressures. • for each of these crvs, a participant deviation from it and the uncertainty of this deviation can be calculated, which represents the degree of equivalence for this particular crv. • results of the nmis were considered to be independent. the method by cox [3] was used (uncertainty weighted mean) to calculate each particular crv rj. • results of an nmi obtained in n measurements at the same pressure were considered as fully correlated. this is justified by the observation that the performance of all eight transducers in 0.5 gpa and six pressure transducers in 1 gpa measurements, e.g. fluctuations of readings, is strongly correlated. with the following designations and definitions, • xij – result of nmi i for measurement j according to (2), • i = 1 … n with n being equal to 5 at pressures (50500), and 2 at pressures (600-1000) mpa, • j = 1 … n, n = 16, 28, 16, …, 12 at pressures of p = (50, 100, 150, …, 1000) mpa, as described above, • u(xij) – uncertainty of xij with ( ) [ ] 5.02ls2ts uuxu ij += , uts according to (6) with uls as the uncertainty of ls, • uij = u2(xij), • rj – crv of measurement j, and • u(rj) – uncertainty of rj, one obtains the expressions below: ( ) ( )∑∑ == = n i ij n i ijijj uuxr 11 1 , (7) ( ) ( )∑ = == n i ijjj uruv 1 2 11 . (8) accordingly, the deviation of the result j of laboratory i from the crv and its corresponding uncertainty are: jijij rxd −= , (9) ( ) ( ) 5.0jijij vudu −= . (10) equation 9 (and equation 10 resp.) represents the deviation of each nmi at each pressure from each of the n reference values rj with its associated uncertainty. a calculation of a mean crv of n rj values is not reasonable because each of them is individual in respect to the individual pressure transducers and measurement conditions. however, deviations dij and their uncertainties u(dij) can be processed to determine a mean deviation id with its uncertainty )( idu . an analysis of the uncertainty contributions revealed a very high variability in the ts uncertainties not only between the nmis but also for the same nmi in different measurements. for this reason, to minimize the uncertainty of the mean deviation, it was calculated as a weighted mean: ( )[ ] ( )[ ]∑∑ == −−= n j jij n j jijiji vuvudd 11 1 , (11) ( ) ( )[ ] ( )[ ]∑∑ == −−= n j jij n j jiji vuvudu 11 5.0 11 . (12) from equation 7, one can straightforwardly derive the following condition: ( )[ ] ( )[ ] 01 1 2 1 2 =∑∑ == n i i n i ii dudud . (13) however, the situation is different for the actual case of a series of several transducers. it can be shown that the id values may be shifted by a value ∆. this depends on the results obtained by an nmi in different measurements at the same nominal pressure, in particular when some results lie below and some above rj and the uncertainties of the single measurements σc of the same nmi and/or of different nmis strongly vary, as was the case in this comparison. the deviations corrected for the shift ∆ are then given as ( )[ ] ( )[ ]∑∑ == −=∆ n i i n i ii dudud 1 2 1 2 1 and (14) ∆+= ii dd (15) with )()( ii dudu = and the expanded uncertainties u(di) taken as u(di) = 2×u(di). acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 79 4.3. results from the raw data of the comparison presented in [2], one can derive the degrees of equivalence according to equation 15. as a result, it can be found that most results are in agreement with each other. there is one exception: participant #5 deviates from the crv at 150 mpa. the degrees of equivalence with respect to the crv are displayed in figure 2 for some nominal pressures. in some cases, one has to note a strong imbalance between the uncertainties of the ls according to the cmcs within the cipm mra and the uncertainties estimated in the comparison. that means, in the case of nmi #2, the latter uncertainties were too optimistic, but at the same time they also appeared quite small compared with the corresponding deviations from the crv. this was presumably related to the rather poor performance of the transfer standard in its measurements mostly due to fluctuations of readings at higher pressures. this might indicate that the stability of the generated pressure at the nmis was different, which usually cannot be seen when performing cross-float measurements. 5. conclusions the measurements of this supplementary comparison euramet.m.p-s14 for high pressures in the range up to 1 gpa were conducted during 2013 and 2014 with five participating institutes: ptb, lne, metas, cmi and smu. the comparison was piloted by ptb. the transfer standard comprised eight pressure transducers. some of these, in some of the measurements, had performance as low as 14⋅10-6, comparable with the uncertainty of the laboratory standards of the participants. but in general, the uncertainty of the transfer standard appeared too high to completely verify the uncertainty of the national standards claimed by the laboratories. these elevated uncertainties of the transfer standard might be influenced by pressure instabilities in the pressure generation systems of the laboratories, as concluded from high fluctuations of the transfer standard readings. a special procedure was derived to calculate the degree of equivalence on the basis of numerous results obtained with different pressure standards, different pressure ranges and pressure change directions. to summarize, most of the results of the participants were equivalent. the results show that a comparison of very high pressures by means of high-pressure transducers is possible, but special measures are required have stable generated pressures. however, the comparison also reveals the limits which laboratories have when calibrating high-pressure transducers. acknowledgement the authors thank a. gluschko and t. konczak at ptb for the development and characterization of the transfer standard. references [1] a. d. s. ahmed, “development of pressure multipliers for pressure measurement up to 1.6 gpa”, ptb-bericht th-9, fachverlag nw, bremen, isbn 978-3-95606-307-7, 2017. [2] j. könemann et al., “investigations of high-pressure transducers as transfer standards evaluated by a 1 gpa pressure comparison”, in ptb-bericht th-6, fachverlag nw, bremen, isbn 978-3-395606-232-2, pp. 57-79, 2015. [3] m. g. cox, “the evaluation of key comparison data”, metrologia,. 39, pp. 589-595, 2002. figure 2. degrees of equivalence with respect to the crv of each laboratory averaged over all pressure transducers of appropriate pressure ranges. the error bars show the expanded uncertainty of the degree of equivalence for each calibrated value. evaluation of supplementary comparison euramet.m.p-s14 in the range 50 mpa to 1 gpa of hydraulic gauge pressure table 1. laboratory standards of the nmis. << /ascii85encodepages false /allowtransparency false /autopositionepsfiles true /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /error /compatibilitylevel 1.4 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true 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can be opened with acrobat and adobe reader 5.0 and later.) >> /namespace [ (adobe) (common) (1.0) ] /othernamespaces [ << /asreaderspreads false /cropimagestoframes true /errorcontrol /warnandcontinue /flattenerignorespreadoverrides false /includeguidesgrids false /includenonprinting false /includeslug false /namespace [ (adobe) (indesign) (4.0) ] /omitplacedbitmaps false /omitplacedeps false /omitplacedpdf false /simulateoverprint /legacy >> << /addbleedmarks false /addcolorbars false /addcropmarks false /addpageinfo false /addregmarks false /convertcolors /converttocmyk /destinationprofilename () /destinationprofileselector /documentcmyk /downsample16bitimages true /flattenerpreset << /presetselector /mediumresolution >> /formelements false /generatestructure false /includebookmarks false /includehyperlinks false /includeinteractive false /includelayers false /includeprofiles false /multimediahandling /useobjectsettings /namespace [ (adobe) (creativesuite) (2.0) ] /pdfxoutputintentprofileselector /documentcmyk /preserveediting true /untaggedcmykhandling /leaveuntagged /untaggedrgbhandling /usedocumentprofile /usedocumentbleed false >> ] >> setdistillerparams << /hwresolution [2400 2400] /pagesize [612.000 792.000] >> setpagedevice modelling of a dynamic torque calibration device and determination of model parameters acta imeko june 2014, volume 3, number 2, 14 – 18 www.imeko.org acta imeko | www.imeko.org june 2014 | volume 3 | number 2 | 14 modelling of a dynamic torque calibration device and determination of model parameters leonard klaus, thomas bruns, michael kobusch physikalisch-technische bundesanstalt (ptb), bundesallee 100, 38116 braunschweig, germany section: research paper keywords: dynamic torque calibration, mass moment of inertia, torsional stiffness, sinusoidal excitation, modelling of mechanical systems. citation: leonard klaus, thomas bruns, michael kobusch, modelling of a dynamic torque calibration device and determination of model parameters, acta imeko, vol. 03, no. 2, article 5, june 2014, identifier: imeko-acta-03 (2014)-02-05 editor: paolo carbone, university of perugia received february 15th, 2013; in final form august 1st , 2013; published june 2014 copyright: © 2014 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: the research leading to these results has received funding from the european union on the basis of decision no 912/2009/ec. corresponding author: leonard klaus, leonard.klaus@ptb.de 1. introduction dynamic torque applications exist in many fields of industry; two examples are engine test stands and power torque tools. efficiency measurements of combustion engines and electric drives are carried out by means of rotational speed and dynamic torque measurements in engine test stands. the torque output of these drives may be significantly dynamic with a frequency content up to the kilohertz range. from experience with force transducers – which have a related mechanical design, measurement principle and installation situation – it is known that transducers may have a dynamic behaviour which can influence the measuring results. at present, static torque calibrations can be carried out in a wide range and with high precision, but there are no facilities and standards to determine the dynamic behaviour of torque transducers. for this reason, a method for primary dynamic torque calibration was developed and a proof-of-principle measuring device was designed and manufactured at ptb. 2. measuring device the dynamic torque measuring device enables dynamic torque excitations in a frequency range of up to 1 khz with up to 20 n·m sinusoidal torque. the measurement principle is in analogy to the periodic force excitation based on newton's second law [1]. with the mass moment of inertia j and the time-varying angular acceleration ( )t , the torque ( )m t is given by      .m t t j  (1) figure 1 shows the components of the calibration device and the corresponding mechanical spring-mass-damper model. the transducer under test (dut) is mounted via two coupling elements to a rotational exciter. the coupling elements are composed of an upper and a lower half connected by a steel diaphragm to reduce parasitic bending moments acting on the transducer. at the same time, this mechanical design offers a high torsional stiffness. interchangeable collets enable the mounting of transducers with shaft ends of various diameters in the measuring device. at the top of the set-up, the transducer under test carries a known mass moment of inertia (mmoi) j, which is composed of components for an angular acceleration measurement set-up and a radial air bearing. the air bearing reduces the axial load as well as bending moments acting on the transducer under test in conjunction with low friction operation. abstract for the dynamic calibration of torque transducers, a dedicated calibration device has been developed. this paper is focused on the model of the calibration device and the methods for the determination of its model parameters. the modelling is required for the identification of the corresponding model parameters of the torque transducer under test. these parameters describe the transducer's dynamic behaviour. measurement methods and auxiliary devices for the determination of mass moment of inertia and torsional stiffness are explained. this research is part of the emrp jrp ind09 – “traceable dynamic measurement of mechanical quantities”. acta imeko | www.imeko.org june 2014 | volume 3 | number 2 | 15 the primary measurement of the angular acceleration is carried out by means of a radial grating disk and a laserdoppler interferometer [1]. 3. model the measurement principle of the vast majority of torque transducers is based on strain gauges. this principle is used for force transducers as well, so the implementation of a model for torque transducers was realised based on the experience with force transducers [2]. strain gauge transducers have a structural design, in which the sensing elements are applied on a structural part of high compliance. this part can be described as a torsional spring ( tc ) for torque transducers. the remaining structural parts of the transducer can be assumed to be rigid and will be allocated to the mass moment of inertia element above ( hj ) or below ( bj ) the torsional spring, respectively. in parallel to the torsional spring, the model assumes a damping element. the behaviour of this system is supposed to be linear and time invariant (lti). the distribution of the head and base mmoi of the transducer under test ( hj and bj ) depends on the mechanical design of the transducer. because of the fact that torque transducers are always coupled to the mechanical environment at both ends, different coupled elements of different mmoi, stiffness and damping may have influence on the frequency response of the system, which affects the frequency-dependent output signal of the transducer. to be able to identify the model parameters of the transducer, it is necessary to include the entire measurement set-up in the model. again, a linear and time invariant approach is applied. as the coupling elements cannot be assumed to be totally rigid to torsional loads, they are represented in the model of the measuring device by two torsional spring and damper elements ( m m e e,   ;  ,   )c d c d coupled to mass moments of inertia at each end. the mmoi values of the rigid connections of the couplings and the transducer are split and accordingly allocated to the transducer and to the measuring device (e.g. je2 and jb in figure 1) in order to derive the model parameters of the transducer. the mass moment of inertia jm2 includes all components of the measuring device on top of the upper half of the coupling. jm1 includes the lower part of the coupling, as well as adapters for the mounting of the transducer; the same applies to je2 but in the opposite direction. only if the model parameters of the measuring device are known, the determination of the transducer's model parameters from measurement data will be possible. thus, measurement techniques for the determination of these parameters have to be developed. 4. mass moment of inertia the mass moment of inertia that generates the dynamic torque acting on the transducer, when applied to an angular acceleration, needs to be determined with high precision. for this purpose, a dedicated measuring device for the mass moment of inertia has been developed (see figures 2, 3). its measurement method is based on the principle of a physical pendulum. for small angles of pendulum excitation, the nonlinearity of the pendulum can be neglected – though this effect has to be taken into account for the measurement uncertainty evaluation. the frequency  of the pendulum's swing depends on its mass moment of inertia j and the restoring torque given by the mass of the pendulum m, the distance of the centre of mass from the axis of rotation l and the gravitational acceleration g. 2 m g l j     . (2) to minimize frictional losses, the pendulum set-up uses an air bearing [3]. the dut is mounted on the axis of rotation. the mass moment of inertia of the dut is derived from the change of the pendulum's swing frequency. for the measurement of the angular excitation, an optical angle encoder is used. its grating disk has an angular pitch of 0.04°. this figure 2. pendulum for the measurement of the mass moment of inertia. figure 1. measuring device and its model. pendulum, jpend additional mass bodies, ji air bearing scanning head radial grating disk device under test (dut), jdut g cm jm2 jm1 dt jh ct jb je2 ce je1 dm de air bearing radial grating disk coupling torque transducer (dut) coupling exciter top mmoi spring/damper of coupling mmoi of coupling head mmoi of transducer spring/damper of transducer base mmoi of transducer mmoi of coupling spring/damper of coupling buttom mmoi acta imeko | www.imeko.org june 2014 | volume 3 | number 2 | 16 angular resolution can be additionally increased by interpolation methods. the measured total mass moment of inertia total  j is composed of the mmoi of the pendulum pendj and the mmoi of all mounted components, e.g. of the device under test dut  j and of the added mass bodies ij of each test configuration i. total dut pend 0     .i ij j j j j j      (3) as the pendulum's mass moment of inertia pendj is unknown, it has to be determined first. this is to be carried out in the same procedure as used subsequently for the mounted device under test. the unknown property can be identified by adding known mass moments of inertia to the pendulum. this is measured by mounting auxiliary mass bodies at different distances from the axis of rotation into the pendulum lever. the mass bodies of the pendulum have well-known dimensions and weights. with this information, the mass moment of inertia for rotation around the centre of gravity of each mass body can be calculated. the distance of the mounting position of the masses to the rotational axis of the pendulum was measured in advance. the acting mass moment of inertia ij of each mass body at a certain mounting position is given by the huygens-steiner theorem. mounted mass bodies do not only influence the total mass moment of inertia of the pendulum, but also its total mass and its centre of gravity, i.e. the restoring torque ( )m g l  on the right-hand side of equation (2) is influenced. each test configuration is described by  0 0 0 2   ( )      . i i i i m l g m l g j j          (4) to determine the swing frequency of the pendulum in a given test configuration, the angle encoder output is sampled with a sufficiently high sampling rate for a similar range of pendulum excitation angles. after the predetermination of frequency , magnitude c, phase  and decay-rate , a four parameter levenberg-marquardt fit is calculated on a damped sine function    sin     ,ty t c e t      (5) where the predetermined values are used as initial parameters. experience gathered after the first measurements shows a stable behaviour of the fit and a very good agreement of measured data and fit. the unknown mass moment of inertia j0 can be derived by varying the configuration of the pendulum by means of mounting different auxiliary mass bodies. several measurements are necessary to vary the two parameters mass moment of inertia and restoring torque sufficiently for the identification process. this leads to a solvable bivariate regression problem in the form of  , .y f x p (6) the vector of observed values y consists of all of the measured angular frequency values i of the corresponding configuration i; the added auxiliary mass bodies contribute to the matrix of independent values x, which consist of ij and ( )i im l g  . the vector of approximated parameters p consists of the unknown mass moment of inertia of the pendulum j0 and the unknown restoring torque 0 0( )m l g  .  t 1 2,  ,  ,  n   y , (7a) 1 1 1 2 1 2 ( ) ( ) ( )n n n j m l g j m l g j m l g                           x , (7b) t 0 0 0[ , ( )]j m l g  p . (7c) for the successful identification of the unknown parameters it is beneficial to maximize the variation of the independent values. this can be achieved by choosing mass bodies and mounting positions in an optimized way. figure 4 shows the influence of different mass body configurations on the squared pendulum swing time 2 2 24π /  t  at the z-axis, and on the restoring torque  i im l g  and the added mass moments of inertia ij at the xand y-axis, respectively. figure 3. measurement set-up for the determination of the mass moment of inertia, shown with three mounted mass bodies and mounted dut. acta imeko | www.imeko.org june 2014 | volume 3 | number 2 | 17 according to equation 2, the pendulum is modelled assuming undamped oscillations. therefore, this simplification can be applied only for systems of low damping. to estimate the effect of the damping, the decay of the pendulum's amplitude was measured (see figure 5). differing from the previous description, the oscillations of the pendulum were measured by means of a laser-doppler interferometer, as the angle measurement set-up had not yet been installed. the interferometer traced the pendulum's swing at the bottom end of the pendulum. the damping of the pendulum was determined by calculating a non-linear regression approximating the measurement values with a damped sine fit function (see equation (5)). the decay of the pendulum swing amplitude for the measurement data shown in figure 5 was calculated and equals 3 12.23 10  s    . the damping slightly influences the resonant frequency of the pendulum [4]; the relationship between the damped natural frequency d and the undamped frequency ud is given by  2 2ud d     .    (8) for the measurement shown in figure 5, the relative deviation due to this effect is in the range of 8  8 10 . 5. torsional stiffness the stiffness of the components of the measuring device has great influence on the torque measurement as it affects the dynamic behaviour of the whole system. the torsional stiffness c is defined as the torque-to-torsion ratio (with torque m and the torsional angle 2 1δ      )  . δ m c   (9) the determination of the torsional stiffness is realised by a measurement set-up (see figure 6) in which a well-known torque is applied to the dut while the angle of torsion is measured with high precision by means of two autocollimators. two mirrors attached with support clamps to both sides of the dut provide the sensing surface of the autocollimators. the torque-dependent difference of the two angle readings of both autocollimators yields the torsional angle. mirrors with a planarity of 1/10  enable the determination of the angle with uncertainties of less than one arcsecond; the measurement range amounts to rotational angles of up to ± 600 arcseconds. in this measurement set-up, the statically applied torque is measured by means of a reference torque transducer. to measure the torsional stiffness, ptb's 20 n·m torque calibration machine was equipped with two electronic figure 6. torsional stiffness measurement set-up. figure 5. decay of the pendulum's swing amplitude. figure 4. measurement values for different auxiliary mass bodies mounted at different positions in the pendulum (red, green and magenta dots) and result of the bivariate regression (blue, dotted surface). figure 7. measurement set-up for torsional stiffness with indicated sensing beams of autocollimators (red). m -m autocollimator / φ2 mirror dut reference torque transducer φ1 sensing beam acta imeko | www.imeko.org june 2014 | volume 3 | number 2 | 18 autocollimators in order to perform measurements of the torsional angle. figure 7 shows a photograph of the set-up. the 20 n·m torque calibration machine is seen in the centre, on the right is the autocollimator tracing the bottom mirror, on the left, the autocollimator sensing at the top mirror. a measurement routine was developed for the evaluation of the torsional stiffness. the applied load steps are based on the din 51309 standard for torque transducer calibrations. after preloading to avoid hysteresis behaviour in the torque calibration system, the applied torque (load) increases in steps of 10% to the full load. the number of load steps shown in figure 8 was increased in comparison to the standard. for the measurement, both a clockwise and a counter-clockwise torsional loading cycle were applied. the voltage output of the bridge amplifier connected to the reference torque transducer and the angle values of the two autocollimators were acquired simultaneously. several hundred measurement values were recorded for each torque step. the first measurement results are very encouraging, as seen in figure 9. the torsional angle values for the different torque steps show a linear dependency. the first order regression line excellently fits the mean values of the different torque levels. the value for torsional stiffness is given by the gradient of the regression line. the angle offset is given by the initial absolute angle values of the autocollimators. 6. conclusions this paper describes the modelling and the methods for the determination of the model parameters of the dynamic torque calibration device at ptb. this modelling of the measuring device is a prerequisite to be able to determine the transducer's dynamic properties. the described methods and auxiliary measuring set-ups enable the measurement of the torsional stiffness and the mass moment of inertia. the torsional stiffness is determined by applying a well-known torque and by measuring the torsional angle on both sides of the dut. the mass moment of inertia is determined by means of the swing frequency of a compound pendulum. as the commissioning of the described measuring set-ups is already completed, it is now intended to subsequently determine the model parameters of the components of the dynamic torque calibration device with these measurement methods. for the future model-based calibration of torque transducers, the parameters of the measuring device need to be known in order to identify the model parameters of the torque transducer under test from measurement data. acknowledgement the authors would like to thank their colleagues mr brüge from the working group “realization of torque” for the opportunity to utilise the 20 n·m torque calibration machine for the measurements of torsional stiffness, and mr just from the working group “angle metrology” for his helpful suggestions in angle measurement and for providing the autocollimators. references [1] t. bruns, “sinusoidal torque calibration: a design for traceability in dynamic torque calibration” in proc. of xvii imeko world congress; 2003, dubrovnik, croatia, cd publication, online at: http://www.imeko.org/publications/wc-2003/pwc-2003-tc3008.pdf. [2] m. kobusch, a. link, a. buss, t. bruns, “comparison of shock and sine force calibration methods” in proc. of imeko tc3 & tc16 & tc22 international conference; 2007, merida, mexico, cd publication, online at: http://www.imeko.org/publications/tc3-2007/imeko-tc32007-007u.pdf. [3] d. peschel, d. mauersberger, “determination of the friction of aerostatic bearings for the lever-mass system of torque standard machines” in proc. of xiii imeko world congress, 1994, torino, italy, pp. 216-220. [4] g. baker, j. blackburn, the pendulum: a case study in physics, oxford university press, chapter 3, pp. 30-31, 2005. figure 9. measurement result with standard deviation (blue) and regression line (red). figure 8. load steps of the torsional stiffness measurement routine. acta imeko, title acta imeko issn: 2221-870x april 2017, volume 6, number 1, 2 acta imeko | www.imeko.org april 2017 | volume 6 | number 1| 2 editorial to selected papers from the xxi imeko world congress 2015 and freely submitted articles paolo carbone university of perugia, italy section: editorial citation: paolo carbone, editorial to selected papers from the xxi imeko world congress 2015 and freely submitted articles, acta imeko, vol. 6, no. 1, article 1, april 2017, identifier: imeko-acta-06 (2017)-01-1 section editor: paul regtien, the netherlands received april 19, 2017; in final form april 19, 2017; published april 2017 copyright: © 2017 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: paolo carbone, email: paolo.carbone@unipg.it dear reader, this year acta imeko will publish its 6th volume. this is a small but a rather important achievement for all the people involved in this venue. let me take this occasion to thank all of them. the first 6 papers in issue 6.1 originate from the 2015 world congress. the first paper is the paper by michael kobusch and sascha eichstädt who work at ptb. their contribution is aimed at illustrating a problem in system identification in the area of mechanical measurements, so to achieve a model-based calibration of a strain gauge force transducer. the following paper by yukio hiranaka, shinichi miura and toshihiro taketa takes into account the goal of determining the system input of a heater from measured data of room temperature. this leads to a backward simulator that tries to minimise processing time by taking into account the particular nature of the problem and also the usage of a forward model. the paper by sergey v. muravyov, irina a. marinushkina, and diana d. garif covers the topic of inter laboratory comparison. it proposes a software to process experimental or simulated data by means of an aggregate preference method that, according to the authors, outperforms some other known rules that allow establishment of the reference value of a measurand. the next paper by michal junek, jiří janovec and petr ducháček describes the method to measure the hardness of creep-resistant steel using ultrasound techniques to prove that the independence of the results obtained by such method from the young’s modulus of the specimen is not entirely correct. the paper by giovanni betta et al. describes the issues associated with the metrological characterisation of 3d biometric face recognition systems. the problem of the performance evaluation of these systems is not yet fully addressed and this contribution shows how to compare experimental results obtained using various architectures under actual operating conditions. the paper by dragana popovic renella et al. contains an overview of commercially available teslameters used for measuring dc and ac magnetic flux densities. a thorough discussion is presented including the performance of such measurement instruments and more details are provided on gaussmeters based on 3-axis integrated hall probes. the next two papers in this issue were freely submitted by the authors. the paper by catarina andré et al. is in the area of measurements for the food industry. it discusses the design of statistical experiments to quantify the amount of freemonomeric isocyanates in aggregated cork stoppers. experimental results are presented to validate their procedure. the last paper in this issue by dirk röske et al. is the result of a collaboration among several national metrological institutes and describes the realization of a software tool for the calculation of uncertainty in force measurements. this research was developed through a research project in the european metrology research programme. have a fruitful reading of the first issue of acta imeko in 2017! paolo carbone, editor-in-chief of acta imeko editorial to selected papers from the xxi imeko world congress 2015 and freely submitted articles acta imeko, title acta imeko issn: 2221-870x september 2017, volume 6, number 3, 1 acta imeko | www.imeko.org september 2017 | volume 6 | number 3 | 1 journal contacts about the journal acta imeko is an e-journal reporting on the contributions on the state and progress of the science and technology of measurement. the articles are mainly based on presentations presented at imeko workshops, symposia and congresses. the journal is published by imeko, the international measurement confederation. the issn, the international identifier for serials, is 2221-870x. editor-in-chief paolo carbone, italy honorary editor-in-chief paul p.l. regtien, netherlands editorial board section editors (vol. 4, 5 and 6) leopoldo angrisani, italy filippo attivissimo, italy eulalia balestieri, italy eric benoit, france catalin damian, romania lorenzo ciani, italy pasquale daponte, italy luca de vito, italy luigi ferrigno, italy edoardo fiorucci, italy alistair forbes, united kingdom helena geirinhas ramos, portugal sabrina grassini, italy fernando janeiro, portugal konrad jedrzejewski, poland andy knott, united kingdom massimo lazzaroni, italy fabio leccese, italy rosario morello, italy michele norgia, italy pedro miguel pinto ramos, portugal sergio rapuano, italy dirk röske, germany alexandru salceanu, romania constantin sarmasanu, romania lorenzo scalise, italy emiliano schena, italy enrico silva, italy marco tarabini, italy eric benoit, france marcantonio catelani, italy paolo carbone, italy lorenzo ciani, italy spartacus gomariz, spain sabrina grassini, italy konrad jędrzejewski, poland min-seok kim, korea franco pavese, italy serge rapuano, italy paul regtien, the netherlands janaina m rodrigues, brazil alexandru salceanu, romania jan saliga, slovakia krzysztof stepien, poland marco tarabini, italy ian veldman, south africa claudia zoani, italy final editing rosario morello, university mediterranea of reggio calabria, italy about imeko the international measurement confederation, imeko, is an international federation of actually 42 national member organisations individually concerned with the advancement of measurement technology. its fundamental objectives are the promotion of international interchange of scientific and technical information in the field of measurement, and the enhancement of international co-operation among scientists and engineers from research and industry. addresses principal contact prof. paolo carbone university of perugia dept. electr. inform. engineering (diei) via g.duranti, 93, 06125 perugia, italy email: paolo.carbone@unipg.it acta imeko prof. paul p. l. regtien measurement science consultancy (msc) julia culpstraat 66, 7558 jb hengelo (ov), the netherlands email: paul@regtien.net support contact dr. dirk röske physikalisch-technische bundesanstalt (ptb) bundesallee 100, 38116 braunschweig, germany email: dirk.roeske@ptb.de journal contacts introductory notes for the acta imeko special section on the asia-pacific symposium on measurement of mass, force and torque (apmf 2017) acta imeko issn: 2221-870x september 2019, volume 8, number 3, 1 2 acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 1 introductory notes for the acta imeko special section on the asia-pacific symposium on measurement of mass, force and torque (apmf 2017) rugkanawan wongpithayadisai1, jeerasak pitakarnnop1,2 1 national institute of metrology, pathumthani, thailand 2 aerospace engineering, chulalongkorn university, bangkok, thailand section: editorial citation: rugkanawan wongpithayadisai, jeerasak pitakarnnop, introductory notes for the acta imeko special section on the asia-pacific symposium on measurement of mass, force and torque (apmf 2017), acta imeko, vol. 8, no. 3, article 1, september 2019, identifier: imeko-acta-08 (2019)-03-01 editor: dušan agrež, university of ljubljana, slovenia received: june 11, 2019; in final form september 30, 2019; published september 2019 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: rugkanawan wongpithayadisai, e-mail: rugkanawan@nimt.or.th distinguished readers, this special issue of acta imeko is the second issue, which presents the research highlights of the biannual asia-pacific symposium on measurement of mass, force and torque (apmf). over the past two decades since its first organizing in 1992, the apmf symposium cosponsored by imeko tc3 has developed into one of the most active symposia in the asiapacific region. the recent apmf was organized by national institute of metrology (thailand) in november 2017 in krabi, thailand, aka the land of smiles, around the theme “metrology moving towards foundation”. the fields of the discussions and presentations were extended to cover the area of mass, force, torque, density, hardness, pressure, vacuum and gravimetry, and consequently the future formal name “asia-pacific measurement forum on mechanical quantities (apmf)”. with the contributions from 62 participants from 11 countries, 47 scientific papers were presented with a fruitful outcome. the international program committee of apmf 2017 considered and selected 10 potential papers to be published as the enhanced versions in acta imeko journal. with the journal’s standards of the review process by the expert referees, 6 articles were accepted for this special issue. in the field of mass metrology, the state-of-the-art strategy for calibrating non-automatic weighing instruments based on euramet cg-18 was precisely and clearly summarized with the explanation of its scientific principle by klaus fritsch. differing from the existing national guidelines, euramet cg-18 suggested the uncertainty of a weighing result that described the device performance during day-to-day usage not only at the time of calibration. to achieve the required accuracy complying with the quality of weighing process, the prerequisite concept of minimum weight under consideration of safety factor was introduced and described. there was the continuous development of small force metrology by nim, china. a 10 n small force standard machine with the air bearing support was proposed and investigated its performance by gang hu. the machine’s minimum force was extended to 1 mn to serve the demands of force measurements in the range of millinewton to newton, e.g., biology applications, pharmaceutical industries, nano-hardness metrology and precision manufacturing. the mechanical system’s structure and main components of the machine were newly designed to reduce the standard uncertainties, and consequently achieved the higher accuracy of force measurement in this small range. two interesting papers from nmij/aist were presented in the field of pressure metrology. hiroaki kajikawa studied the long-term drift characteristics of three kinds of hydraulic pressure gauges (different sensing structures). the results from the experimental investigation led to the practical way to reduce and compensate the long-term drift, i.e., the application of preliminary pressure that was very close to the measurement pressure. furthermore, the drift could also be corrected by tracing a zero reading during the pressure application. hideaki iizumi proposed the solution to achieve the calibration values of high pressure unaffected by the kind of gas medium and the setting posture of quartz bourdon-type pressure transducers. extending from his recent study in which the analysis was carried mailto:rugkanawan@nimt.or.th mailto:rugkanawan@nimt.or.th acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 2 out with the horizontally mounted transducers, the vertically mouthed transducers were investigated. this concept is very useful for the transfer standards used for the international comparisons. nimt’s big project of torque realization up to 5 knm of torque standard machine was conducted. nittaya arksonnarong presented the principle of the designs and the performance of the developed machine. the flexure bearing was selected as the fulcrum to improve the machine’s strength and to shorten the stabilizing time of the lever arm’s swing. less maintenance and lower construction costs than the machine using air bearing are the dominant advantages of this design. the machine’s capability and its claimed cmcs at 0.01% (k=2) had been confirmed by the satisfactory results from the informal comparison with the recognized nmi, ptb. last but not least, another project in the torque metrology was proposed by nattapon saenkum. nimt continuously attempted to improve the performance of its elastic hinge type torque standard machine. the concept of the separate calibrations of the signals observed at the left hinge, the right hinge and the fulcrum, was introduced to deal with the undesirable influences from the bending moment by obtaining the residual torque. with the corrections of the residual torque value and the difference arm length, the torque characteristics, i.e., the repeatability, the reproducibility and the linearity were significantly improved, and consequently the better cmc at 0.020 % (k = 2) at the measurement range 2 nm to 10 nm. it is our extreme honour to serve as the section editors for this knowledgeable issue of this open-access journal. we extend heartfelt thanks to the authors, reviewers, associated editor, copy editor, layout editor and article editor for their supports and dedications to this issue. the publication of this special issue would not be possible without the great supports from dr. rolf kumme, the imeko tc3 chairperson and dr. yon kyu park, the imeko tc3 vice chairperson. our huge gratitude goes to dr. dirk röske, who always provided his prompt assistances and advices on the journal’s online system. we would like to express our appreciations to dr. min-seok kim, the former section editor of the special issue of the apmf 2015 for sharing his experience of being journal’s section editor. we fully appreciate prof. dr. dušan agrež, the editor in chief, for his kind contribution to the issue. lastly, we hope you enjoy reading our work. rugkanawan wongpithayadisai jeerasak pitakarnnop section editors editorial to additional papers dear reader, five additional papers are closing the third issue of acta imeko in 2019. in the paper submitted by saher r. hassan et al. is presented investigation of the influence of the shaker armature’s mechanical structure on periodic force measurement, with emphasis on measurement uncertainty. in order to perform modal and harmonic analyses, a finite element model was iteratively developed. the results of validation show that the dynamic behaviour of the shaker armature should be taken into consideration before carrying out a periodic force measurement. andreas brüge et al. present the realisation and dissemination of the unit of rotatory power at the physikalisch-technische bundesanstalt (ptb) aimed at ensuring the safe operation of ergometers for patients. this task is carried out by means of a standard that can be operated both as a regulated brake and as a regulated drive, so that the effective torque and the revolution speed can be measured traceably. at the end, technical details and uncertainty calculations about this standard are given. gisa foyer et al. analyse the torque measurement in the mn∙m range. it is important in nacelle test benches, as it constitutes the primary value for the efficiency determination of wind turbine drive trains. however, no traceable measurement is possible above 1.1 mn∙m. several torque measurement methods are presented and discussed with respect to traceability and measurement uncertainty. it was found that these methods cannot fully substitute direct measurements. therefore, detailed recommendations for a torque calibration procedure based on the specific conditions in nacelle test benches are given so as to enable the traceability of torque measurement using a transfer standard. alessandro gallo et al. are focused on models of submerged structures and underwater archaeological finds, which environments are characterised by poor visibility conditions. performance analysis has been conducted on a multi-view technique in comparison with different image enhancement algorithms that are commonly used in air in order to highlight their limits in the underwater environment. the results have been adopted to reconstruct an area of 40 m² at a depth of about 5 m at the underwater archaeological site of baiae (italy). the aim of the paper submitted by sebastian baumgarten et al. is to deal with torque measurements. in torque standard machines, different kinds of bearings are used to reduce the influence of parasitic loads under the assumption that such loads are low. in this article, the parasitic loads of the torque standard machines at the ptb, teknologian tutkimuskeskus vtt oy (vtt), and cesky metrologicky institut (cmi) were characterised and compared. the results are presented together with a proposal for an evaluation. have a fruitful reading of this interesting issue of acta imeko. dušan agrež, editor-in-chief of acta imeko influences on amplitude estimation using three-parameter sine fitting algorithm in the velocity mode of the planck-balance acta imeko issn: 2221-870x september 2020, volume 9, number 3, 40 46 acta imeko | www.imeko.org september 2020 | volume 9 | number 3 | 40 influences on amplitude estimation using the threeparameter sine fitting algorithm in the velocity mode of the planck-balance shan lin1, christian rothleitner1, norbert rogge2, thomas fröhlich2 1 physikalisch-technische bundesanstalt, bundesallee 100, 38116 braunschweig, germany 2 institute of process measurement and sensor technology, technische universität ilmenau, 98684 ilmenau, germany section: research paper keywords: planck-balance; sine fitting algorithm; monte carlo simulation; kibble balance citation: shan lin, christian rothleitner, norbert rogge, thomas fröhlich, influences on amplitude estimation using the three-parameter sine fitting algorithm in the velocity mode of the planck-balance, acta imeko, vol. 9, no. 3, article 7, september 2020, identifier: imeko-acta-09 (2020)-03-07 editor: jan saliga, technical university of košice, slovakia received january 17, 2020; in final form march 24, 2020; published september 2020 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was funded by ‘validierung des technologischen und gesellschaftlichen innovationspotenzials – vip+’, a program of the german federal ministry of education and research (bmbf), which is managed by vdi/vde innovation + technik gmbh. corresponding author: shan lin, e-mail: shan.lin@ptb.de 1. introduction after the redefinition of the kilogram, the kibble balance is one possible approach to the calibration of mass standards in terms of the fixed value of the planck constant with zero uncertainty [1]. the planck-balance (pb) is a table-top kibble balance and is currently under development in a collaboration between physikalisch-technische bundesanstalt (ptb) and technische universität ilmenau (tuil) [2]. the kibble balance has two measuring modes: force mode and velocity mode. in the velocity mode of other kibble balance experiments, the coil is usually moved at a constant velocity [3][5]. in contrast to these kibble balances, the coil of the pb is sinusoidally moved through the magnetic (b-)field in an oscillatory manner, thus inducing an ac voltage across the coil ends. this voltage is digitised by means of a high-precision digital multimeter (keysight 3458a). if it is assumed that the coil motion and the induced voltage are perfectly sinusoidal, the force factor bl (l denotes the coil wire length, b the magnetic flux density) can be determined by the oscillation frequency, the amplitudes of the coil motion, and the induced voltage. in the pb setup, only the amplitudes need to be estimated because the oscillation frequency can be accurately measured with a frequency counter. the motion of the coil with respect to the magnet is measured with a high-precision (but commercial) laser interferometer, which provides the position data as a function of time. the measurements of voltage and position are synchronised by means of an external trigger source with a trigger frequency of 1 khz. the three-parameter sine fitting algorithm is applied to determine the amplitudes of the induced voltage and the coil motion. however, in practice, the sampled signal is not an ideal single-component sine wave due to some perturbations like, for example, additive gaussian white noise, quantisation error, harmonic distortion, frequency error, and time jitter. the fitting abstract the planck-balance is a table-top version of a kibble balance. in contrast to many other kibble balances, the coil in the planck-balance is moved sinusoidally and an ac rather than a dc signal is generated in velocity mode. the three-parameter sine fitting algorithm is applied to estimate the amplitudes of the induced voltage and the coil motion, which are used to determine the force factor bl of the voice coil of the electromagnetic force compensation balance. however, the three-parameter sine fitting algorithm is not robust against some perturbations, e.g. additive gaussian white noise, quantisation error, harmonic distortion, frequency error, and time jitter. these perturbations affect the accuracy of the amplitude estimation. based on numerical simulations and correlation analyses, the effects of these perturbations are determined. by optimising the measurement and data processing approach, the bias and standard deviation of the estimated amplitude can be effectively reduced; thus, the accuracy of the force factor bl in velocity mode can be improved. mailto:shan.lin@ptb.de acta imeko | www.imeko.org september 2020 | volume 9 | number 3 | 41 algorithm is not robust against these perturbations and may result in a bias of the amplitude, which will directly affect the accuracy of bl. in this article, the effects of aforementioned perturbations are analysed by numerical simulations. the bias and standard deviation of the estimated amplitude provided by the three-parameter sine fit are investigated by the theoretical equations and a monte carlo simulation. 2. velocity mode of the planck-balance a kibble balance establishes a link between the mass and the planck constant. this is done by compensating the mechanical force 𝐹𝑚𝑒𝑐ℎ = 𝑚 ∙ 𝑔 with an electromagnetic force 𝐹𝑒𝑚 = 𝐵 ∙ 𝑙 ∙ 𝐼. here, m denotes the mass, g the local gravity, b the magnetic flux density, l the length of the coil in the magnetic field, and i the electrical current that flows through the coil. the electromagnetic force is generated by a voice coil actuator. the electrical current can be measured to high precision via ohm’s law with an electrical resistor and a voltmeter, which can be traced back to quantum standards (the quantum hall resistor and josephson voltage standard). b and l, however, cannot be measured separately with high accuracy. this is why the so-called ‘velocity mode’ was introduced by bryan kibble (hence the name ‘kibble balance’) [1]. here, the coil is moved through the b-field that – according to faraday’s law – induces a voltage between the coil ends as uind = b  l  v, where v denotes the coil velocity. in state-of-the-art kibble balances, the motion has constant velocity. hence, the induced voltage is dc. in the pb, the motion is harmonic. the induced voltage, therefore, is ac. in the velocity mode of the pb, the coil oscillates through the b-field generated by the magnet. the coil position is measured by using an interferometer, while the induced voltage in the coil is measured by using a digital voltmeter synchronously. the measured coil motion is assumed to be an ideal singlecomponent sine wave with an amplitude s and an initial phase s: 𝑠(𝑡) = 𝑆0 + 𝑆 sin(𝜔𝑡 + 𝜑s). (1) here, s0 is the dc offset and  is the angular frequency, 𝜔 = 2𝜋𝑓sig, where fsig denotes the oscillation frequency. the coil velocity can be obtained as the derivative of (1) with respect to time t as 𝑣(𝑡) = 𝜔𝑆 cos(𝜔𝑡 + 𝜑s). (2) the induced voltage is also assumed to be a perfect sine wave with an amplitude u and an initial phase u: 𝑢(𝑡) = 𝑈 cos(𝜔𝑡 + 𝜑u). (3) assuming that the force factor bl is a constant during the whole range of the coil movement, bl can be calculated to divide the amplitude of the induced voltage by the amplitude of the coil velocity as 𝐵𝑙 = 𝑈 𝜔𝑆 = 𝑈 2𝜋𝑓sig𝑆 . (4) in the pb system, oscillation frequency fsig can be accurately measured by a frequency counter. therefore, the accuracy of bl depends mainly on the estimation of amplitudes u and s. 3. measurement setup in the current setup of the pb (see figure 1 [6]), which consists of several custom-made circuitries and a customised setup of commercially available devices, the controller is implemented on a digital signal processor (dsp) system provided by dspace gmbh. the control algorithm runs on a processor board of type ds1006, which is enhanced by an analogue-to-digital converter (adc) board of type ds2004 and a digital-to-analogue converter (dac) board of type ds2102. the available dacs and adcs all have a 16-bit resolution. the current pidt1-controller is implemented with a sampling frequency fs of 10 khz and provides an output voltage udrv in order to control the measured position sensor voltage up to a given set point value. the output voltage of the dsp is converted by an amplifier circuitry into a controlled current that is proportional to udrv. when applied to the internal balance actuator, the output current can be utilised to excite the load carrier with a sinusoidal motion that is necessary in the velocity mode for the calibration of the force factor bl of the external coil. after this calibration mode, the output current is connected via latching relays to the external coil and is then used to balance the system in force mode. in addition to the control algorithm, several different signal generators are implemented on the dsp, which provide chirp signals for testing purposes and the sinusoidal modulation of the set point position in velocity mode. since the internal clock of the dsp represents the time base for the excitation frequency, one of the dacs outputs a square wave that changes its voltage level at each sampling period of the dsp and therefore yields a frequency of fs/2. the actual frequency of this signal is measured by a frequency counter of type keysight 53220a, which receives a reference frequency fref = 10 mhz from a stable reference clock. the reference clock is an oven-stabilised quartz oscillator of type rsggo10o provided by rf suisse, which is disciplined by gps signals. the same reference frequency is supplied to a waveform generator of type keysight 33521b that provides a square wave signal with the frequency ftrig. the square wave signal is used to trigger the measurement of the laser interferometer and the digital multimeters (keysight 3458a) with a sampling frequency of 1 khz. one multimeter measures the induced voltage in velocity mode, while the other one measures the necessary compensation current during force mode as a voltage drop ur figure 1. the signal processing setup of the pb [6]. acta imeko | www.imeko.org september 2020 | volume 9 | number 3 | 42 across a pre-calibrated reference shunt resistor that is connected in a series with the coil and the current amplifier output. 4. three-parameter sine fit a sinusoidal signal can be described as 𝑦(𝑡) = 𝑌0 + 𝑌 sin(𝜔𝑡 + 𝜑), (5) where y is the amplitude,  the initial phase, and y0 a dc offset. equivalently, the signal in (5) can be written as a linear combination of two shifted sine waves as 𝑦(𝑡) = 𝑌0 + 𝐴 sin(𝜔𝑡) + 𝐵 cos(𝜔𝑡) (6) where a and b are the amplitudes of the in-phase and in-quadrature components, respectively. in (6), the signal frequency is known; thus, only three parameters y0, a, and b need to be estimated. when a set of m (with m > 3) samples y1, y2,..., ym from a sine wave is sampled at the time instants t1, t2,..., tm, a linear least squares method can be used to determine the best sine wave parameters by minimising the sum of the squares of the following errors: min 𝐴,𝐵,𝑌0 ∑(𝑦𝑖 − 𝑌0 − 𝐴 sin(𝜔𝑡𝑖 ) − 𝐵 cos(𝜔𝑡𝑖 )) 2 𝑀 𝑖=1 . (7) the estimated parameters of the sine wave can be calculated in a matrix form as: [ 𝐴 𝐵 𝑌0 ] = (𝐃t𝐃)−1𝐃t [ 𝑦1 𝑦2 ⋮ 𝑦𝑀 ], (8) with 𝐃 = [ sin(𝜔𝑡1) cos(𝜔𝑡1) 1 sin(𝜔𝑡2) cos(𝜔𝑡2) 1 ⋮ ⋮ ⋮ sin(𝜔𝑡𝑀 ) cos(𝜔𝑡𝑀 ) 1 ]. amplitude y can be determined from 𝑌 = √𝐴2 + 𝐵2. 5. influences on the amplitude estimation the amplitudes of the induced voltage and displacement are provided by the sine fitting algorithm. an overview of the possible influences of the induced voltage on the amplitude estimation is given in figure 2. due to the possible error sources, the effects of these error sources on the amplitude estimation should be investigated, and the additional contributions to the uncertainty budget of the bl in velocity mode must be considered. in this section, several error sources shown in figure 2 are investigated by using the theoretical equation and/or numerical simulations. the values of most of the parameters used for the numerical experiments have been chosen according to real measurement data [7]. 5.1. additive gaussian white noise the amplitude estimation is affected by additive gaussian white noise (agwn), with zero mean and standard deviation n. the theoretical expressions have been derived in [8] and [9] as: u,n ≈ 𝜎n 2 𝑀𝑈ref 2 and (9) 𝜎u,n = √ 2 𝑀 𝜎n, (10) where u,n and u,n are the systemic error and standard deviation of the estimated amplitude, respectively. uref is the nominal i.e. unbiased voltage, and m is the number of samples calculated from the product of sampling frequency fs and sampling period t. in order to validate the theoretical (9) and (10), the monte carlo method (mcm) is used to evaluate the relative bias and standard deviation of the amplitude. firstly, a single-component sine wave for the induced voltage is generated with the amplitude uref = 0.3051 v, the initial phase  = -1.5876 rad, and the oscillation frequency fsig = 4 hz. for the error comparison of amplitude, the value of uref is taken as the reference value. a dataset {𝑢𝑖 }𝑖=1 104 is generated from the pure sine wave with the sampling frequency fs = 1 khz and the sampling period t = 10 s. figure 2. an ishikawa diagram showing the influences of the possible error sources on the amplitude estimation of the induced voltage. acta imeko | www.imeko.org september 2020 | volume 9 | number 3 | 43 the generated dataset {𝑢𝑖 }𝑖=1 104 is taken as nominal points. the agwn with different values of n is superimposed on the nominal points. the three-parameter sine-fitting algorithm is used to estimate the amplitude �̂�. the process is repeated 104 times. finally, the relative bias of the amplitude u,n is calculated by comparison of the mean value of �̂� with uref as u,n = |�̂� − 𝑈ref| 𝑈ref⁄ . the standard deviations associated with the estimated amplitude as a function of the noise level are presented in figure 3. according to (10), the theoretical values are also calculated with m = 104 samples and different values of n. the solid line in figure 3 represents the theoretical results. from (9) and (10), it can be deduced that the bias is negligible with respect to its standard deviation. figure 3 shows good agreement between the theoretical and simulated results. it is shown that u,n increases with increasing n. in the real measurement of pb, the standard deviation of the residuals is in the order of 10-4 v after the extraction of the harmonics up to the fifth order. in such a case, the corresponding u,n is in the order of 10-6 v. when the type a evaluation is used to estimate the standard uncertainty, repeated measurements for the velocity mode can reduce the standard uncertainty of amplitudes. for the type b evaluation, u,n can be reduced by either reducing n or by increasing the number of samples m for each single measurement i.e. the sampling frequency or the sampling period. 5.2. sampling strategy when measuring the induced voltage and the coil motion, the sampling frequency fs and the sampling period t need to be determined. the number of samples m is equal to the product of fs and t. as mentioned in section 5.1, the bias of the estimated amplitude and associated standard deviation can be reduced by increasing m. the longer sampling period of measurement t can increase the number of samples with a fixed sampling frequency fs. in addition, more samples can also be obtained by increasing fs within a fixed sampling period t. however, a higher sampling frequency fs may result in a lower amplitude resolution of the digitiser with q bits of resolution, which results in an increased quantisation error. the quantisation error is assumed to be uniformly distributed with the standard deviation q [10]: 𝜎q = 1 √12 ∙ 𝑈fsr 2𝑞 , (11) where ufsr is the full-scale range. if only the quantisation noise is considered, the standard deviation of the estimated amplitude is [11]: 𝜎u,q = √ 2 𝑀 𝜎q . (12) figure 4 gives an example of the influence of the quantisation noise on the amplitude estimation. the theoretical values with respect to sampling frequencies of 1 khz and 10 khz are represented by the blue circles, which are calculated by using (12). the digital multimeter keysight 3458a is used to measure the induced voltage in a measurement range of 1 v i.e. ufsr = 1 v. when the sampling frequency fs = 1 khz, the corresponding resolution is 21 bits [12]. according to (11), q is equal to 1.38  10-7 v. when the sampling period is t = 10 s, the standard deviation of the voltage is u,q = 1.95  10-9 v. for a sampling frequency of 10 khz, the resolution is q = 18 bits, and q = 1.10  10-6 v. consequently, u,q is equal to 4.92  10-9 v. next, the mcm is implemented to evaluate the standard deviation of the amplitude. two datasets of nominal points are generated i.e. {𝑢𝑖}𝑖=1 104 for fs = 1 khz and {𝑢𝑖 }𝑖=1 105 for fs = 10 khz. the quantisation noise is generated and superimposed on the nominal points. the process is repeated 104 times. the standard deviations of the estimated amplitude are represented by red triangles in figure 4. the results show that those theoretically determined values have been validated by the mcm. when the sampling frequency fs increases by a factor of ten from 1 khz to 10 khz, the corresponding standard deviation of amplitude increases by a factor of about 2.5 (see figure 4). however, u,q is in the order of 10-9, which is much lower than the noise level. therefore, the quantisation error is negligible in this case. if fs = 10 khz is used instead of 1 khz and the noise standard deviation of n = 10-4 v, the corresponding u,n is reduced by √10 times according to (10). therefore, fs = 10 khz can be adopted to reduce the uncertainty of amplitude estimation. figure 3. the standard deviation of the amplitude as a function of the noise standard deviation. the circles represent the simulated values obtained by the mcm. the solid line represents the theoretical values given by (10). figure 4. standard deviation of the amplitude as a function of the sampling frequency. the circles represent the simulated values by the mcm. the triangles represent the theoretical values given by (12). acta imeko | www.imeko.org september 2020 | volume 9 | number 3 | 44 5.3. harmonic distortion the oscillation frequency fsig, the sampling frequency fs, and the number of samples m satisfy the relation [13] 𝑓sig 𝑓s = 𝐽 + 𝛿 𝑀 , (13) where j and  are, respectively, the integer and the fractional parts of the number of sine wave cycles. when  = 0, it is called coherent sampling, and (9) and (10) are only valid in this case. however, when non-coherent sampling (  0) occurs, the three-parameter sine fit provides a biased amplitude due to harmonic distortion [14]. multiharmonic sine fitting or the extraction of integer periods of the sine wave can be used to reduce the bias of the amplitude provided by the three-parameter sine fit. in practice, there are higher-order harmonics in the measurements of the induced voltage and the displacement. the magnitudes of higher-order amplitudes are shown in figure 5 by using a fast fourier transform (fft) – obtained from a real measurement of the induced voltage. the oscillation frequency of the measurement data in figure 5 is about 4 hz. the amplitudes of the secondup to the fifth-order harmonics are estimated by a factor of about 103, 104, 105, and 105 smaller than that of the first harmonic, respectively. the total harmonic distortion is 0.15 %. a numerical simulation is implemented to investigate the influence of the harmonic distortion on the amplitude estimation. figure 6 shows the simulation results [15]. the signal of the induced voltage is simulated by using a fundamental sine wave and simply superimposed by higher order harmonics up to the fifth order. the adopted amplitude for each harmonic is acquired from the real measurement data, which is the same amplitude as that which is shown in figure 5. the oscillation frequency fsig is changed from 3.82 hz to 4.02 hz with the same sampling frequency of fs = 1 khz and the sampling period of t = 10 s. when the oscillation frequency is fsig = 3.9 hz or 4 hz, the number of sine wave cycles is integer i.e. we have coherent sampling. figure 6 shows that sine-fitting algorithms are robust against harmonics distortion when coherent sampling is applied. in such cases, the bias of the amplitude can be negligible. however, for the non-coherent sampling, the relative bias of the amplitude provided by the three-parameter sine fit (n = 1) is in the order of 10-6. compared to the target lowest relative standard uncertainty for the pb (8.4  10-8 for 1 kg), the bias of the amplitude is relatively high. when the multiharmonic sine fit is applied, the higher the number of the included harmonics, the lower the bias of the estimated amplitude. the relative bias of the amplitude can greatly be reduced (to below 10-10) for the noncoherent sampling if the number of samples is reduced to obtain an integer number of periods (the green line in figure 6). in such cases, the bias of the estimated amplitude becomes negligible. higher-order harmonics are present in the real measurement of the induced voltage as well as in the coil motion, which may be caused by the current source, nonlinear b-field, and so on. moreover, in practice, coherent sampling cannot be perfectly achieved due to the accuracy of the generated frequency from the waveform generator – the time jitter of the sampling. therefore, the extraction of the integer periods of the sine wave or the multiharmonic sine fitting included higher-order harmonics that could be adopted to estimate the amplitudes of induced voltage and coil motion. 5.4. frequency error in the linear sine-fitting algorithm, the oscillation frequency fsig is assumed to be known, and it is taken as an input parameter to the fitting model. if there is an error fsig in the input frequency, an additional contribution will be included in the fitting model, which is described by a linearly damped oscillation [16]. a numerical simulation has been implemented in order to investigate the effect produced by a frequency error fsig. the dataset {𝑢𝑖 }𝑖=1 104 is generated with fsig = 10 hz, fs = 1 khz, and t = 10 s. a frequency error fsig is added to fsig, and the obtained frequency (fsig + fsig) is taken as the known frequency of the three-parameter sine fit to evaluate the amplitude. finally, the relative bias of the amplitude u,f is calculated and shown in figure 7. it can be seen from figure 6 that in order to keep this bias below 10-8, the frequency has to be known with the same relative uncertainty. in the measurement setup, the frequency counter of type keysight 53220a can measure the actual frequency accurately (< 10-8), which can be used as the input frequency of the model and can achieve the desired accuracies for the pb (about 10-8). experiments using other algorithms (e.g. the four figure 5. a periodogram using fft for the residuals obtained by using the three-parameter sine fit. figure 6. the relative bias of the amplitude as a function of the number of periods [15]. the amplitudes are estimated by using multiharmonic sine fitting included from firstup to fourth-order harmonics and the extraction of the integer periods of the sine wave. acta imeko | www.imeko.org september 2020 | volume 9 | number 3 | 45 parameter sine fit) showed that those algorithms can also achieve frequency estimations with errors in the order of 10-8 [13]. 5.5. time jitter when implementing the sine fitting algorithm in (7), it is assumed that the sampling instants t1, t2,..., tm have no influence due to the time jitter. however, in practice, the time jitter is an unpredictable timing noise, and this effect leads to the bias of estimated sine wave amplitude. the relative bias of the amplitude is [17] u,j = ( 1 2 − 1 𝑀 ) (𝑒 −𝜎𝜃 2 − 1) (14) where 𝜎𝜃 = 2𝜋𝑓sig𝜎t , and t is the standard deviation of the time jitter. moreover, the time jitter also leads to amplitude noise of the estimated signal. the corresponding noise standard deviation of the amplitude is calculated as [18]: 𝜎j = 2𝜋𝐴rms𝑓sig𝜎t, (15) where arms is the maximum root mean square (rms) amplitude of the effective value of an ac signal. as the induced voltage is an ac measurement, the maximum rms amplitude arms is equal to 𝑈ref √2⁄ . if only the time jitter is considered, the standard deviation of the estimated amplitude is 𝜎u,j = √ 2 𝑀 𝜋𝑈ref𝑓sig𝜎t. (16) the time jitter of a keysight 3458a dmm is 5 ns and has a rectangular distribution [19]. thus, the standard deviation of the time jitter becomes t = 2.89 ns. when fsig = 4 hz and m = 104, the relative bias and relative standard deviation of the amplitude is in the order of 10-15 and 10-10, respectively. in such a case, the influences of the time jitter are negligible. according to (14) and (16), the influences of the time jitter will be stronger for a sampled signal with a higher oscillation frequency and a lower number of samples. in the current measurement of the pb, the number of the sample points is not below 5000, and the oscillation frequency is lower than 15 hz. if fsig = 15 hz and m = 5000 are selected to estimate the relative bias and relative standard deviation, in the worst-case scenario, the values are equal to 3.7  10-14 and 2.7  10-9, respectively. therefore, the time jitter is negligible in the current measurement. 6. conclusions perturbations in the signal can result in a bias of the amplitude estimation when using the three-parameter sine-fitting algorithm. the effects of agwn, quantisation error, harmonic distortion, frequency error, and time jitter have been investigated by numerical simulations in this article. the bias and the associated standard deviation due to agwn depend on the noise level and the number of samples. in order to improve the accuracy of the amplitude estimation, a sampling frequency fs = 10 khz can be used instead of 1 khz without notable influence of the increased quantisation noise. the three-parameter sine fit is not robust against harmonic distortion with non-coherent sampling. in order to reduce the bias of the estimated amplitude, the extraction of the integer periods of the sine wave or the multiharmonic sine fitting included higher-order harmonics could be adopted. moreover, the simulation results indicate that the frequency error of the fitting mode can also cause a biased amplitude. however, the use of a frequency counter or some effective algorithms can keep this influence negligible. the effects of time jitter can also be neglected in this case. in total, it can be concluded that the three-parameter sine-fitting algorithm satisfies the requirement of the pb. as a next step, work will be carried out in order to include other important effects, e.g. a nonlinear b-field. all the investigated error sources will be taken into account in order to evaluate the uncertainty of the force factor bl in the velocity mode. the aim is to create a good model for the implementation in a digital twin of the planck-balance i.e. a monte carlo-based uncertainty estimation to the pb [2]. acknowledgement the research of this project is funded via the program ‘validierung des technologischen und gesellschaftlichen innovationspotenzials – vip+’, a program of the german federal ministry of education and research (bmbf), which is managed by vdi/vde innovation + technik gmbh. the authors are grateful to the 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[20] s. lin, c. rothleitner, n. rogge, amplitude estimation using three-parameter sine fitting algorithm in the planck-balance, proc. of 23rd imeko tc4 international symposium, 17-20 september 2019, xi’an, china. url: https://www.imeko.org/publications/tc4-2019/imekotc4-2019-019.pdf https://doi.org/10.1063/1.4953825 https://www.imeko.org/publications/tc4-2019/imeko-tc4-2019-044.pdf https://www.imeko.org/publications/tc4-2019/imeko-tc4-2019-044.pdf https://doi.org/10.1109/cpem.2018.8500904 https://doi.org/10.1016/j.measurement.2008.12.006 https://doi.org/10.1016/j.isatra.2011.10.003 https://doi.org/10.1109/imtc.2006.328187 https://doi.org/10.1109/19.387298 https://doi.org/10.5162/sensoren2019/6.4.4 https://doi.org/10.21014/acta_imeko.v4i2.173 https://doi.org/10.1109/tim.2009.2034576 https://www.imeko.org/publications/tc4-2019/imeko-tc4-2019-019.pdf https://www.imeko.org/publications/tc4-2019/imeko-tc4-2019-019.pdf are learning preferences really a myth? exploring the mapping between study approaches and mode of learning preferences acta imeko issn: 2221-870x june 2021, volume 10, number 2, 185 190 acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 185 are learning preferences really a myth? exploring the mapping between study approaches and mode of learning preferences elena fitkov-norris1, ara yeghiazarian1 1 kingston business school, kingston university, kingston hill, kingston upon thames kt2 7lb, uk section: research paper keywords: learning preferences; vark; study preferences; study habits; learning citation: elena fitkov-norris, ara yeghiazarian, are learning preferences really a myth? exploring the mapping between study approaches and mode of learning preferences, acta imeko, vol. 10, no. 2, article 25, june 2021, identifier: imeko-acta-10 (2021)-02-25 section editor: yasuharu koike, tokyo institute of technology, japan received: march 13, 2018; in final form: june 21, 2021; published: june 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: elena fitkov-norris, e-mail: e.fitkov-norris@kingston.ac.uk 1. introduction learning is an essential part of life, and since the acquisition and development of new skills throughout one’s career is crucial, scholars are continuing their attempts to identify the factors that influence the effectiveness of learning and the subsequent academic performance. many instruments have been developed to assess learner engagement and their attitudes and aptitudes at different stages of the learning process or to assess the way in which individuals learn, with the objective of identifying the optimal study strategies that can be adopted by learners at the point of interaction with their learning environment [1]-[3]. while learning can take place in various different contexts, the complex interactions that arise in formal learning sites such as classrooms are of particular interest since the classroom environment provides a unique setting in which exogenous variables can be controlled, thus potentially increasing the effectiveness of the learning process. a significant number of theories have emerged aimed at explaining the various learning approaches taken by students in the classroom and have been largely embraced by educators. however, various recent meta-analytical studies have failed to support the existence of disparate learning styles [4], [5]. here, the notion that approaches that are intuitively attractive and highly popular among educators, such as matching the mode of delivery (visual, auditory, read/write or kinaesthetic) to the student’s learning preference, can enhance performance has been largely discredited [6]. however, neuroscience-grounded evidence has emerged that indicates that students may, in fact, have an inherent preference for the mode through which they receive information [7]. specifically, functional magnetic resonance imaging (fmri) scans have demonstrated that individuals with a strong visual preference have to convert words into pictures when presented with text-based input in order to understand it, while individuals with a strong read/write text preference have to convert pictures into text to help with the comprehension [8]. further support for the existence of a preference for visual or text-based (read/write) information abstract this article tested for the presence of the conversion effect in the mapping related to the strength of students’ preferences for receiving information in a visual, auditory, read/write or kinaesthetic modality and the study approaches they adopt when taking notes in class, learning new concepts and revising for exams. the results indicated that the conversion effect is not ubiquitous but is context specific and only present when students seek to learn a new concept and revise for exams. it was present for students with strong visual and read/write preferences but only when attempting to learn a new concept. it was also present for students with a strong auditory preference when revising for exams, while these students preferred to learn a new concept by reading about it. however, the conversion effect did not emerge with kinaesthetic-leaning students in any of the contexts studied, while these students were significantly more likely to utilise auditory input when learning a new concept. overall, the findings suggest that traditional educational approaches such as lectures and tutorials can be effective in supporting the learning for diverse student groups. mailto:e.fitkov-norris@kingston.ac.uk acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 186 intake has been provided through psychology-based eye-tracking studies, which demonstrated that visualisers (students with a preference for pictures and diagrams) and verbalisers (students with a preference for text) generally examine the areas on the screen where the information is presented in terms of their specific preference, with visualizers found to be able to focus for longer on information-rich areas of diagrams than verbalisers [9]. these findings suggest that the mode of a student’s information input preference could play a role in the speed of the information processing and the way in which they interact with the learning materials. however, empirical research related to the link between students’ information-input preferences and their cognitive style and ability within the context of probabilistic reasoning suggests that the interactions between these elements of the learning process are complex and plagued by contradictions, which means that making generalisations and devising practical recommendations regarding the most efficient learning strategy is a highly difficult task [10]-[12]. furthermore, past research has indicated that students’ study strategies and habits, as well as affective factors such as attitudes and self-regulation, can play a significant role in predicting academic performance [13]. evidence from the fields of educational psychology and educational neuroscience supports these findings and indicates that effective learning takes place when the information taken in by the students is moved to the long-term memory and the behaviour associated with it is automated [14]. in light of these findings, a number of new conceptual learning frameworks have been proposed, explicitly identifying the relationship between the different elements affecting learning proposed by educational neuroscientists and psychologists, such as students’ preferred mode of information input (visual, textual) as well as their attitudes, study strategies and the habits they adopt to manage their learning [15], [16]. despite being grounded in neuroscientific research, the validity of the frameworks is yet to be tested within the context of education-based empirical evidence. this article sets out to address this gap by examining the information input preferences of students and studying their habit behaviour (classified as visual, auditory, read/write and kinaesthetic) in terms of a conversion effect within the context of taking notes in class, learning a new concept and revising for exams. it is expected that students with a strong visual input preference will be more likely to adopt a visual behavioural approach to taking notes in class and learning (e.g. summarising notes as diagrams, or learning a new concept from a diagram), while students with a strong text-based preference (read/write or verbalisers) will be more likely to adopt a corresponding approach (e.g. writing down every word the lecturer says, or learning a new concept by reading about it). the remainder of the article is organised as follows. section 2 provides an overview of the proposed conceptual framework linking the preferred learning modality with the study behaviour approaches before section 3 discusses the methodology used to classify the student’s information input preferences and study behaviour as visual, auditory, read/write and kinaesthetic as well as the results from the analysis. section 4 then discusses the implications from the findings within the context of education before the final section summarises the research findings and provides a number of suggestions for further research. 2. framework linking information input preferences and study approaches many instruments and models have been designed to assess and describe the approach taken by students when studying [3], while the terms used by academics to reflect the different elements involved in the learning process are not always well defined [17]. before introducing the learning framework demonstrating the interaction between different aspects of learning, we must first outline the definitions of the different terms used in this paper. learning preferences relate to the learner’s preference for one method of teaching over another [18] and reflect the way in which a learner prefers to receive information. conventionally, there are four ways of conveying and absorbing information: visual (via pictures and diagrams), auditory (hearing), read/write (via text) and kinaesthetic (through doing), which together are termed as vark. the preferences of students for using combinations of information input pathways are generally assessed using vark-type questionnaires [19]. meanwhile, the terms ‘study habits’ and ‘study strategies’ are used interchangeably in the academic literature and their definitions have evolved over time to encompass a number of different subconstructs and aspects of study habits or behaviour, covering various different cognitive, affective and behavioural activities [2]. here, the cognitive factors reflect the approach and techniques used by students to draw inferences from the information, which may include deep, strategic and surface approaches to learning [2], while the affective components are linked to the students’ emotions and define their motivation and anxiety as well as their propensity to avoid procrastination [13]. lastly, behavioural factors include study-related behaviours such as note taking, highlighting and reviewing [20]. the majority of study habits and strategy instruments involve the use of a combination of some, if not all of these sub-scales [2], [3]. however, evidence from empirical neuroscience research suggests that treating study habits and strategies as identical may be an oversimplification. brain scanning provides further insight into how individuals make decisions, including the way in which students react when finding themselves in a context that may or may not be familiar to them. here, the attendant research demonstrates that their behaviour is governed by the interactions of two systems in the brain: the reflective c-system and the reflexive x-system. [21], [22]. the reflective c-system is used in situations that are largely unfamiliar and provides a sequential and exacting assessment for the appropriate course of action. meanwhile, the reflexive x-system is used in familiar situations, where actions are automatic and relatively effortless, and involves the use of parallel processing. within the context of learning, this can be explained in terms of a student who is attending a lecture for the first time and is not sure how to handle the situation versus a student who has attended lectures in the past and automatically ‘knows’ what to do. students who consistently use the same study strategies could benefit in the long term by gradually strengthening their decision pathways and shifting from using their reflective system to using the more efficient and less arduous reflexive system. in addition, the repeated actions lead to situational familiarity, which, in turn, is more likely to trigger the same habitual behavioural response [22], further reinforcing the habituation of that specific behaviour. this reinforcing feedback loop could explain how students can become trapped in using inefficient study habits and strategies, acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 187 even when they make a conscious effort to study hard or work efficiently [17]. therefore, the optimal study strategy would be contextdependent in terms of both the individual’s specific information processing preference mode and the mode in which the information is presented to them, which is reflected in the conceptual framework shown in figure 1. here, the structure highlights the subconscious information processing carried out to aid comprehension that may lead to an additional informationprocessing step carried out by learners when presented with information in a mode other than their primary vark preference. it also illustrates the interaction between the reflective and reflexive decision-making systems by explicitly demonstrating the impact of study habits on the conscious information-processing behaviour learners carry out as part of their learning activities. as can be seen in figure 1, poor study habits are the result of a discrepancy between the inherent information preference output generated subconsciously to aid understanding and the study related output generated using 'conscious' information processing. the proposed framework can be used as a tool for identifying the potential for improvement in a student’s study behaviour. specifically, the student’s inherent mode of learning preference and the output produced as a result of their study efforts can be compared, with any discrepancy found indicative of potentially poor study habits. the complex interrelationship between learning approaches and information processing may account for some part of the variation in the success of students’ learning strategies. adopting a study strategy that is not optimal (e.g. a visual learner reading their lecture notes over and over again without summarising them into diagrams) could result in a highly inefficient and arduous learning experience and could ultimately lead to discouragement and increased anxiety. while the higher educational system has been deigned to instil good study habits and to encourage students to become independent learners, to the best of our knowledge, no study strategy instrument is aimed at assessing the fit between the student’s preference for seeing a specific type of information and their approach to handling it. of course, if a learner is multi-modal, the impact of the potential discrepancy between actual and optimal behaviour may be small. however, learners with a strong single preference will potentially be most at risk of adopting a suboptimal study strategy, which is supported by empirical findings indicating that learners with a single strong modal learning preference are at a much higher risk of academic failure, compared to their peers with more balanced or multimodal preferences [23]. it is worth noting that the framework focuses on examining the disparities that may exist between a learner’s inherent information processing preference and their study habit behaviour in different learning contexts. study habit behaviour is complex and may be influenced and mediated by other intrinsic factors such as a learner’s academic self-efficacy, motivation and optimism as well as external interactions such as formative and/or summative feedback and recommendations by the lecturer [20]. while the impact of these factors on study behaviour may be significant, evaluation of their impact on the interactions between a learner’s inherent information processing learning preference and their study habit behaviour in different learning contexts is beyond the scope of this study. 3. mapping of study approaches to vark modality preferences 3.1. methodology this research utilises fleming’s updated vark questionnaire [19] to measure the blend and the strength of students’ preferred learning mode. the questionnaire consists of 16 scenarios, each of which includes four options corresponding to a different information input preference, namely, vark. users are asked to tick all options that apply to them in a specific situation. the vark questionnaire’s validity has been examined using exploratory factor and rasch analyses, with the questionnaire confirmed to be suitable as a low-stake diagnostic tool, one that can be used to make study approach recommendations, while figure 1. conceptual framework linking information processing and study approaches. table 1. classification of study habits/approaches. study habit/approaches type of preference when taking notes in class i: summarise concepts by drawing pictures and diagrams. v when taking notes in class i: write down every word the lecturer says. r/w when doing homework, i tend to learn a new concept by: looking at a diagram. v when doing homework, i tend to learn a new concept by: having it explained to me. a when doing homework, i tend to learn a new concept by: reading about it. r/w when doing homework, i tend to learn a new concept by: doing exercises, examples and trying it out. k when doing homework exercises, i tend to: read about things first before i try to solve them. r/w when doing homework exercises, i tend to: get stuck straight in and try solving them. k when revising for exams i: talk to others and discuss the material in my notes. a when revising for exams i: read the notes aloud to myself or others. a when revising for exams i: read through the notes taken in class. r/w when revising for exams i: re-write/re-draw my notes. k acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 188 more research is required to confirm its suitability as a robust research instrument [24], [25]. to the best of our knowledge, no instrument has been designed to assess the approach taken by students when studying in terms of their vark modalities. however, a number of studystrategy questionnaires have been proposed with the aim of capturing students’ study approaches in different study contexts, such as note taking and revising [20], [26]. the study-strategy questionnaire designed and validated by nonis and hudson [20] was used as the basis for developing a questionnaire aimed at eliciting the information-processing approach students adopt (visual, auditory, read/write or kinaesthetic) within three different study contexts, namely, when taking notes in class, doing homework or revising (see table 1). each question was allocated to the visual, auditory, read/write or kinaesthetic categories and students were then asked to identify all the study behaviours they may adopt in any specific situation. the vark/study approach questionnaire was administered to a convenience sample of 100 postgraduate and undergraduate students, with 92 complete questionnaires gathered electronically. the majority of the respondents were undergraduate students (84 %) and female (57 %), with a mean age of 23 years. the students were from a range of faculties, with the majority studying in the faculty of business and law (57 %), followed by arts and social sciences (20 %) and science, engineering and computing (14 %). the majority of students were classified in terms of ‘using all four modes’ (57.6 %), followed by students with a single preference (40 %). no students reported using three modes, while only a small number reported utilising two preference modes (2.2 %) (see table 2). the students in this sample reported slightly weaker preference for visual study approach overall (see table 3) and very similar preference strength for aural, read/write and kinaesthetic approaches. the overall sample mean learning preference strength shown in table 3 can act as a comparative benchmark when examining the data for statistically significant variations. the data were analysed using spss version 21, with an independent sample t-test used to compare the mean strength of each vark preference across the different types of study behaviour adopted by the students. results with statistical significance of less than 5 % were also evaluated in terms of practical significance using cohen’s d effect size. 3.2. results the behaviour of the students and the strength of their vark preferences were examined within three different contexts: when taking notes in class, when learning a new concept and when revising for exams. the conversion effect was not present within the context of taking notes in class and no significant difference was observed between the behaviour of students when taking notes in class, when accounting for the strength of their vark preferences. the students’ study behaviour did not change as their respective preference for receiving information in a specific format (visual, auditory, read/write and kinaesthetic) increased. it was hypothesised that perhaps the conversion effect would be present when considering only the students who expressed a strong preference for visual or read/write preferences. however, the sample did not contain any students with a strong read/write preference, thus, the t-test was run for the subgroups of strong visual vs. mild read/write. while, there was no significant difference between the strength of the students’ preference scores and their behaviour when summarising notes as diagrams, perhaps somewhat surprisingly, the students with a strong visual preference were significantly more likely to write down every word said by the lecturer when taking notes in class (m = 12.1, sd = 3.03), compared to students with a lower visual preference, (m = 3.00, sd = 1.41, t[7] = 4.001, p = 0.005). furthermore, cohen’s effect size (d = 3.18) suggested the strong practical significance of the above result. within the context of learning a new concept, the conversion effect was present for students with strong read/write and visual preferences. students with a higher read/write preference were significantly more likely to learn a new concept by reading about it (m = 8.56, sd = 2.36) than those who reported a lower preference (m = 7.21, sd = 2.72, t[90] = 2.426, p = 0.017). here, cohen’s effect size (d = 0.53) suggested a moderate practical significance. similarly, students with a stronger visual preference were more likely to learn a new concept by looking at a diagram (m = 8.58, sd = 2.86) than those who reported a lower visual preference (m = 7.08, sd = 3.01, t[90] = 2.44, p = 0.017), with cohen’s effect size (d = 0.51) again suggesting a moderate practical significance. again somewhat surprisingly, the conversion effect was not present for students with a stronger auditory preference (m = 8.84, sd = 2.69), who were far more likely to read about a new concept rather than have it explained to them, compared to their counterparts with a lower auditory preference (m = 7.52, sd = 2.38, t[90] = 2.27, p = 0.026). here, cohen’s effect size (d = 0.52) suggested a moderate practical significance. in fact, table 2. learning preference sample distribution. preference type frequency percent cumulative percent mild aural 8 8.7 8.7 mild aural, mild kinaesthetic 1 1.1 9.8 mild aural, mild read/write 1 1.1 10.9 mild kinaesthetic 13 14.1 25.0 mild read/write 2 2.2 27.2 mild visual 7 7.6 34.8 strong visual 7 7.6 42.4 you have no particular preference. you use all 4. 53 57.6 100.0 total 92 100.0 table 3. learning preferences strength. preference type mean standard deviation visual 7.92 3.0 aural 8.42 2.7 read/write 8.13 2.5 kinaesthetic 8.73 2.6 acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 189 learning a new concept through having it explained was a behaviour that was also more likely to be adopted by students with higher read/write (m = 8.73, sd = 2.69) and kinaesthetic (m = 9.41, sd = 2.59) preferences, with the opposite the case with students with lower read/write (m = 7.65, sd = 2.33, t[90] = 2.04, p = 0.045) and kinaesthetic (m = 8.18, sd = 2.54, t[90] = 2.31, p = 0.023) preferences. meanwhile, the effect size for the behaviour of students with a high read/write preference within the context of learning through having concepts explained was found to be moderate to low (d = 0.43), which was also the case for students with a high kinaesthetic preference (d = 0.48). within the context of revision, a conversion effect was present for auditory students, with those with a higher auditory preference (m = 9.23, sd = 2.37) being more likely to read the notes aloud, compared to students who expressed a lower auditory preference (m = 7.58, sd = 2.70, t[90] = 3.13, p = 0.002). here, cohen’s effect size for the different behaviours of students with high and low auditory preferences was moderate to high (d = 0.65). in addition, students with a stronger read/write preference (m = 8.68, sd = 2.21) were also more likely to read the notes aloud, compared to their counterparts with a lower read/write preference (m = 7.56, sd = 2.76, t[90] = 2.16, p = 0.03. here, cohen’s effect size was moderate to low (d = 0.45). 4. discussion the results suggest that for this set of students, the convergence between study behaviour and learning preferences was not ubiquitous but context specific. there was no evidence of such convergence within the context of taking notes in class, with students being equally likely to write down every word said by the lecturer or summarise the ideas using a diagram, irrespective of the strength of their preference for visual, auditory, read/write or kinaesthetic information input. this was somewhat surprising since the evidence from neuroscience suggests that students with a strong visual preference tend to find a summary diagram approach more conducive to their learning, compared to their counterparts with a strong read/write preference [8]. in fact, the converse seems to be true, with students with a strong visual preference being more likely to write down every word the lecturer says. this could possibly be explained by the fact that summarising concepts requires additional processing, which the students may find difficult to achieve within the classroom context while listening at the same time. however, this behaviour could, in fact, be indicative of poor study habits, formed when students do not adopt the approach likely to suit them best. it is worth noting that this finding is based on a very small sample size as the subset of students with a strong visual preference was extremely small and it is thus recommended that further investigations using a larger sample size are undertaken before generalising the conclusions. meanwhile, within the context of learning a new concept, the conversion effect emerged with students who expressed stronger read/write and visual preferences, with the attendant behaviours prioritising reading and looking at diagrams, respectively. this suggests that students intuitively choose the approach that will help them learn more efficiently, which is in line with the findings from neuroscience where the conversion effect was observed [8]. the notion of the conversion effect was also supported by various empirical eye tracking studies related to verbalisers and visualisers, which demonstrated that students with strong visual preferences spend more time looking at diagrams, while those with stronger read/write preferences spend more time looking at text [9]. this finding suggests that in order to effectively support the learning for students with strong read/write and visual preferences, lecturers must ensure that they present balanced materials that contain both visual and read/write content. again, somewhat surprisingly, the conversion effect was not present for students with a stronger auditory preference, who were more likely to attempt learning a new concept by reading about it rather than rely on an auditory input, such as an explanation. this convergence was present when students with a stronger auditory preference attempted to revise a concept they had already learned, but not when first attempting to learn it. this implies that auditory students need to reflect on their learning in order to consolidate their understanding and that this happens more effectively when they are revising. in fact, it is the students with strong read/write and kinaesthetic preferences that would appear to rely on auditory input when learning a new concept, generally in terms of having things explained to them. while the practical effect size for this behaviour was moderate to low, the implication is that traditional lectures or materials with an auditory component will be appropriate for supporting these groups of students. it would appear that students with stronger read/write, auditory and kinaesthetic preferences may adopt learning strategies that do not always align with their expressed learning preferences. this could perhaps be partially explained by the large proportion of multimodal students in the sample, who, naturally, tend to adopt a range of learning strategies. as noted above, the conversion effect was found to be present for auditory-leaning students when revising concepts, with students with a stronger auditory preference being more likely to experience it. somewhat surprisingly, students with a stronger read/write preference were also likely to adopt this behaviour when revising, possibly due to the biased nature of the question, which described behaviour that includes both read/write and auditory aspects. it is worth noting that students with a strong kinaesthetic preference were the only group that did not support the convergence theory within any of the contexts tested, while they were significantly more likely to adopt an auditory approach when learning a new concept. further research is required to identify further ways in which the learning of kinaesthetic-leaning students can be supported. 5. conclusions this article examined the presence of a conversion effect between students’ preferences for visual, auditory, read/write and kinaesthetic information processing and their study behaviour within the context of taking notes in class, learning a new concept and revising for exams. the results indicated that the conversion effect is not ubiquitous but context specific and is only present when students seek to learn a new concept and revise for exams. the conversion effect was present for students with strong visual and read/write preferences only when they attempted to learn a new concept, which implies that there is a need for including a range of study support materials in lectures and seminars to enhance the students’ experience. meanwhile, the conversion effect was present for students with a strong auditory preference when they revised for exams, while they expressed a preference for learning a new concept by reading about it, suggesting the need for support materials with both acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 190 auditory and textual components. the conversion effect was not clear with kinaesthetic-leaning students within any of the contexts studied, while the students with a stronger kinaesthetic preference were significantly more likely to utilise auditory input when learning a new concept. overall, the findings are in line with the research from neuroscience and suggest that traditional educational approaches, such as lectures and tutorials, can be effective in supporting the learning for diverse groups of students. the findings also raise interesting questions regarding the best approaches for supporting peer-to-peer learning within the educational context by creating groups consisting of team members with balanced learning preferences (e.g. students with strong visual or read/write preferences) in order to enhance the learning experience for all students. references [1] r. dunn, learning style: state of the science, theor. pract. 23(1) (1984), pp. 10-19. doi: 10.1080/00405848409543084 [2] 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18-27. doi: 10.1002/ase.1522 https://doi.org/10.1080/00405848409543084 https://doi.org/10.1007/s10648-004-0003-0 https://www.leerbeleving.nl/wp-content/uploads/2011/09/learning-styles.pdf https://www.leerbeleving.nl/wp-content/uploads/2011/09/learning-styles.pdf https://doi.org/10.1111%2fj.1539-6053.2009.01038.x https://doi.org/10.1177%2f0098628315589505 https://doi.org/10.3389/fpsyg.2015.01908 https://doi.org/10.3389/fpsyg.2012.00429 https://dx.doi.org/10.3389%2ffnhum.2014.00015 https://doi.org/10.1016/j.chb.2016.11.028 http://dx.doi.org/10.1016/j.compedu.2011.09.016 https://doi.org/10.1088/1742-6596/588/1/012040 https://doi.org/10.12973/eurasia.2016.1265a https://doi.org/10.3102%2f0034654310362998 https://doi.org/10.1080/10528008.2016.1166441 http://dx.doi.org/10.1109/fie.2014.7044203 https://doi.org/10.1088/1742-6596/459/1/012022 https://doi.org/10.14742/ajet.1859 http://www.vark-learn.com/english/page.asp?p=questionnaire%20 https://doi.org/10.1080/08832320903449550 http://dx.doi.org/10.1016/s0065-2601(02)80006-5 http://dx.doi.org/10.1016/j.jenvp.2004.08.004 https://doi.org/10.4195/jnrlse.2010.0006u https://doi.org/10.1177%2f0013164409344507 https://doi.org/10.1088/1742-6596/588/1/012048 https://doi.org/10.1002/ase.1522 integrated geophysical and mechanical study on the wooden structures of the ceiling of the church of beata vergine maria assunta in cielo acta imeko issn: 2221-870x october 2018, volume 7, number 3, 111 116 acta imeko | www.imeko.org january 2014 | volume 3 | number 1 | 111 integrated geophysical and mechanical study on the wooden structures of the ceiling of the church of beata vergine maria assunta in cielo lara de giorgi1, giovanni leucci1, davide melica1 1 institute for archaeological and monumental heritage – national research council (ibam-cnr) c/o campus universitario, lecce, italy section: research paper keywords: wood elements; geophysics; measurements citation: lara de giorgi, giovanni leucci, davide melica, integrated geophysical and mechanical study on the wooden structures of the ceiling of the church of beata vergine maria assunta in cielo, acta imeko, vol. 7, no. 3, article 17, october 2018, identifier: imeko-acta-07 (2018)-03-17 editor: egidio de benedetto, university of salento, italy received; june 27, 2018; in final form october 3, 2018; published october 2018 copyright: © 2018 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: giovanni leucci, g.leucci@ibam.cnr.it 1. introduction a discrete number of historic building structures in italy are made of timber and the need of preservation is increasingly significant. currently, several non-destructive testing (ndt) and minor-destructive techniques (mdt) are used in situ for assessing the physical and mechanical properties of wood structures [1, 2, 3, 4, 5, 6, 7, 8]. these include ground penetrating radar (gpr) that is also used to inspect other materials such as masonry and concrete [9, 10, 11]. in addition, there are specific ndt and mdt methods for wood, such as the micro-drilling resistance tool and xylotomic analysis [12]. some of these techniques can be used for in situ evaluation of timber structures, including the gpr [13, 14] which yields results such as those presented at the third international conference on metrology for archaeology [33]. the cathedral of santa maria de fovea, which is directly linked with the patron saint "madonna dei sette veli" (madonna of the seven veils) in foggia (figure 1), was built in 1170 in romanesque-apulian style. due to the earthquake that almost destroyed the foggia town, it was strongly damaged. later, in 1731 the cathedral was re-built in baroque style. the cathedral of foggia is strictly connected to the discovery of the icona-vetere religious object, a table that depicts a rare portrait of the virgin kiriotissa with seven veils, (from which the name of the madonna with seven veils). in 2011, an intervention related to the restoration work of the roof required an in-situ investigation campaign aimed at the assessment of the conservation status of the wooden elements constituting the roofing of the cathedral. 2. ndt and mdts used in this work for this work the following methods were used: gpr (used device: ids-hi mod with 2ghz antenna, figure 2a), measurement of the drilling resistance (used device: resistograph iml f400s, figure 2b). gpr is very important and powerful method for detecting nodes, internal cavities and microfractures. gpr method uses electromagnetic waves to investigate the internal structures of natural or human-made objects. its wave frequency ranges from 10 to 2000 mhz. the gpr system consists of a data acquisition unit and of two (transmitter and receiver) antennas. the transmitter sends an electromagnetic wave pulse which is reflected to the receiver by an anomaly, if there is a difference in the complex permittivity between the anomaly and the surrounding environment. the maximum depth of wave propagation depends on the wave frequency and on the complex abstract non-destructive testing by mean of ground penetrating radar were performed on the wooden elements constituting the roofing of the cathedral of foggia. these measurements were related to the restoration work of the roof. measurements campaign aimed at the assessment of the conservation status of the wooden elements of the all major and secondary elements of the truss of the nave, as well as on those main of the dome and the apse. results show various abnormalities which correspond to nodes and/or slots. the presence of the nodes allow a rapid and accurate analysis of the electromagnetic waves velocity and the volumetric water content. mailto:g.leucci@ibam.cnr.it acta imeko | www.imeko.org january 2014 | volume 3 | number 1 | 112 permittivity of the investigated medium. the higher the frequency, the smaller the depth and the better the spatial resolution of the signal. the lower the wave frequency, the higher the depth and the smaller the spatial resolution. for example, a 2000 mhz antenna (used in this study) can reach about 1 m with a resolution of about 1 cm. the output of the gpr survey is a radar section of the investigated medium, where a point-shaped anomaly is outlined by a hyperbolic trace. the x axis corresponds to the direction of scanning, the y axis is the depth. the data coming from the reflected echoes, saved in the data acquisition unit, are processed by filtering algorithms. the interpretation of the radar section permits the recognition of radar anomalies and their spatial location. these anomalies may reflect actual discontinuities, objects or voids. resistograph is an attachment to a bosch drill that aids in the detection of decayed timber structures. it is based on a drilling resistance measuring method. a drilling needle with a diameter of 3 mm penetrates into a wooden element at a constant speed, and the drilling resistance is measured. resistograph gives drilling resistance along the drilling depth in the form of a dendrogram from which the resistographic measure (rm) parameter can be calculated. rm can be used for the estimation of modulus of elasticity (moe), density, compressive strength or bending strength but the correlations are prevailingly moderate. the local character of resistograph leads to preferable use of this technique for assessment of timber element interior state, internal voids, decay or loss in crosssection dimensions, which often cannot be seen from the outside. resistograph is influenced by moisture content and possible deviation in drilling path. data are recorded on a wax paper strip at a 1:1scale and converted by an electronic unit. graph-profiles can then be downloaded and analyzed. because the drilling needle diameter is so small, the damage to the timber is quite insignificant. 3. gpr data acquisition and analysis all gpr profiles were performed on wooden structures (figure 3) and acquired at 1024 samples / track, the other acquisition parameters were optimized on site and held constant for each profile. to eliminate the noise component present in the data and facilitate the interpretation, processing was realized through the following phases: i) background removal; ii) migration. the presence of various anomalies due to small inhomogeneities, such as nodes and fractures, can be detected through a rapid and accurate analysis of the velocity of propagation of electromagnetic waves which allows also to obtain also the depth of the anomalies. the analysis of the gpr data (figure 4) shows that: • the radar signal has an excellent penetration and crosses the element investigated for all its thickness; • the surface portion of the wooden structures, for a thickness varying from 2 to 5-6 cm, is generally characterized by reflections of weak amplitude, and this testifies to a net decrease in the density of the wood, probably related to both the activity of wood-eating insects and chemical attack; this anomaly appears to be more extensive on the extrados of the chains where the infiltration of rainwater, in the presence of large accumulations of guano, may have generated a corrosive action resulting in breakdown of the cellular elements of the wood; • the numerous reflections in the shape of hyperbole, placed at various depths, indicate hardening of woody tissue that, in size and shape, can be related to nodes, single or in groups; their number, sometimes high, determines an average density including between 1.4 and 5.9 nodes per meter, but in some elements of the apse area will reach higher values; • reflections with continuous development are generated by sub-horizontal fissures almost always oriented in the figure 2 a) the gpr instrument (left); b) the resistograph (right). figure 1. the cathedral of foggia. figure 3. the roof planimetry: the red letters refer to the position of the wooden structures. acta imeko | www.imeko.org january 2014 | volume 3 | number 1 | 113 direction of the grain; their width is generally close to 1 cm; • in some of the trusses wood elements b, c, f, g, l of the nave, the presence of cavities with dimensions of the order of 5 cm was recorded, probably due to biological attack. similar anomalies, presumably correlated to fracture larger than those usually identified, were detected in the trusses l1 (chain, strut right monaco), l3 (chain), l5 (chain) and l6 (chain), the longitudinal development of these inhomogeneities varies from 15 to 35 cm. 4. volumetric water content analysis numerous studies have made use of gpr techniques to determine material moisture [10, 15, 16, 17, 18, 19]. however, since each technique is generally considered individually and is efficient only in a specific context, it is difficult to compare the results because of different field conditions. for the gpr frequency band, the velocity 𝑣 of em-waves propagating in the ground depends on the relative dielectric permittivity (i.e. the real part of the dielectric constant, k) of the material; it is given by the simplified equation [20], 𝑣 = 𝑐/𝐾1/2 (1) where c is the em-wave velocity in a vacuum (0.30 m/ns). hence, k can be determined directly from the em-wave velocity similarly to what is done for time-domain reflectometry measurements [27, 28, 29]. for pure water, k is about 80, while for most dry geological material it varies between 4 and 10. if only a small amount of water is contained in the material, the value of k will increase considerably and, conversely, the emwave velocity will decrease significantly. thus, k is a good measure of the water-content in the various materials, such as ground. several formulae have been developed, both theoretically and empirically, to give the dielectric response of heterogeneous mixtures such as water-saturated soils. one such formula is the complex refractive index method (crim) equation, which is often used in the interpretation of em logging data [21]. the major problem with the crim formula is that it does not take into account the geometrical information about the internal structure of rocks and about the microscopic fluid distribution. this has a significant effect on the dielectric properties of partially saturated rocks [22]. the above restriction may be overcome by using the hanai–bruggeman formula [22]. the main problem with the two previous approaches is that it is not possible to derive both the porosity and the water-content from the dielectric constant. therefore, it is not possible to obtain information about the water-content without strong a priori assumptions. for this reason, it is preferable to use the well-known empirical equation, derived by topp et al. [23], relating the dielectric response k of various soil samples (with figure 4. the processed radar sections. acta imeko | www.imeko.org january 2014 | volume 3 | number 1 | 114 different degrees of saturation) to their net water-content w. this formula is given by 𝑤 =– 5.3 ∙ 10−2 + 2.92 ∙ 10−2𝐾 – 5.5 ∙ 10−4𝐾2 + 4.3 ∙ 10−6𝐾3 (2) the em-wave velocity can be estimated from gpr data in several ways [24, 25]; the conventional methods involve common depth-point (cdp) and wide-angle reflection and refraction (warr) data sets. both methods require two antennae in separate units and relatively long acquisition times. in the first case, both antennae are simultaneously moved apart on either side of the midpoint of the profile. in the second case, the position of one antenna is fixed while the other is moved along the profile direction. the em-wave velocity can be more quickly and easily determined from the reflection profiles acquired in continuous mode, using the characteristic hyperbolic shape of reflection from a point source [26]. this is a very common method of velocity estimation and it is based on the phenomenon that a small object reflects em-waves in almost every direction. in the data set, several hyperbolic reflections caused by objects of small dimensions such as node (figure 4) are present, enabling em wave velocity analysis to be performed [30, 31, 32]. figure 5 shows an example of velocity analysis performed on the data set. this represent the interactive velocity adaptation of a diffraction or reflection hyperbola using a hyperbola of defined velocity and width. the velocities are combined into a 2d model using a special interpolation method. the interpolation is performed as follows: all actual velocities are summed for every point in the x–t range, proportional to the square of their distance from the (x, t) point. this method provides only the average em-wave velocity down to the depth of the source-point reflector. the em-wave velocity was determined from the pointsource reflections. this method gives rms em-wave velocities to the depth of the point-source reflector. since in all radar sections acquired on the wooden structures, any interfaces with changes in em-wave velocities between the surface and the target depth level are recorded, and the errors involved, compared to those obtained using interval velocities, are about 0.35%. this early application, which uses a point-source reflection from a buried object to determine the average velocity, gives an accurate (qualitative) estimate of the subsoil volumetric water-content. successively, the volumetric water-content was determined from the dielectric properties of subsurface material, using the above empirical relationship (2). by applying the empirical topp figure 6. volumetric water content analysis. figure 5. em wave velocity estimation. acta imeko | www.imeko.org january 2014 | volume 3 | number 1 | 115 formula (2), we then obtain an estimate of the average volumetric water-content of the wood; this varies from about 3% to 6% (figure 6). water content was measured also through a direct mode using a wooden core and results agree with those obtained using the topp formula. 5. resistographic tests resistographic tests carried out on wooden structures of the ceiling showed, for most of the investigated elements, a mediumlow drilling resistance, attributable both to the intrinsic characteristics of the silver fir wood (softwood) and to external factors responsible for an incipient or advanced decay. an example of the results obtained by these tests is given by the curve in figure 7, in which each of the suspect points, with low resistance, are easily distinguishable. 6. conclusions the integration of the gpr and resistographic tests data allowed to detect the presence of various abnormalities which correspond to nodes and/or slots, making possible a rapid and accurate analysis of the electromagnetic waves velocity of propagation and thus to obtain the depth of these anomalies. among the main obtained data, it is worth stressing the following: • the surface portion of the beams, for a thickness varying from 2 to 5-6 cm, is characterized by abnormalities related to the activity of wood-eating insects and to chemical attack due to pigeon droppings; • the numerous reflections in form of hyperbole, located at various depths, indicate that hardening of woody tissue can be mapped, both in size and shape, as nodes; • the continuous reflections with a sub-horizontal development are generated by fissures almost always oriented in the direction of wood fibers; • in some of the trusses wooden elements of the nave was recorded the presence of cavities with dimensions of about 5 cm. volumetric water content analysis shows an approximately homogeneous distribution of the moisture in the analyzed woods elements. the resistographic tests showed a medium-low resistance to penetration for most of the wooden elements investigated, that can be attributed both to the intrinsic characteristics of the wood employed and to external factors, such as the insects attack and guano deposition, responsible for their advanced deterioration. references [1] c. m. browne, w. e. kuchar, 1985, determination of material properties for structural evaluation of trestle, in proc. 5th international nondestructive testing of wood symposium; 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[30] a. cataldo, e. de benedetto, g. cannazza, g. leucci, l. de giorgi, c. demitri, (2017), enhancement of leak detection in pipelines through time-domain reflectometry/ground penetrating radar measurements. iet science, measurement and technology, 11 (6), 696-702. [31] r. persico, g. leucci, l. matera, l. de giorgi, f. soldovieri, a. cataldo, g. cannazza, e. de benedetto, (2015), effect of the height of the observation line on the the diffraction curve in gpr prospecting. near surface geophysics, 13(3), 243-252. [32] a. cataldo, r. persico, g. leucci, e. de benedetto, g. cannazza, l. matera, l. de giorgi, (2014), time domain reflectometry, ground penetrating radar and electrical resistivity tomography: a comparative analysis of alternative approaches for leak detection in underground pipes. ndt and e international, 62, 14-28. [33] g. quarta, g. leucci, r. persico, p. durante, s. giammaruco, 2017, geophysical investigations on hypogeic monument. the case study of the crypt of st. sebastian in sternatia (lecce – southern italy). proc. of 3rd imeko international conference on metrology for archaeology and cultural heritage, 350-352, isbn: 978-92-990084-0-9. historical masonry churches diagnosis supported by an analytic-hierarchy-process-based decision support system acta imeko issn: 2221-870x march 2021, volume 10, number 1, 6 14 acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 6 historical masonry churches diagnosis supported by an analytic-hierarchy-process-based decision support system valentino sangiorgio1, silvia martiradonna1,2, fabio fatiguso1, giuseppina uva1 1 politecnico di bari, dept. dicatech, via orabona 4 bari 70128, italy 2 univerdisad de cantabria, dept. ingeniería estructural y mecánica, av. de los castros 44 santander 39005, spain section: research paper keywords: historical masonry churches; structural performance; analytic hierarchy processes; decision support system. citation: valentino sangiorgio, silvia martiradonna, fabio fatiguso, giuseppina uva, historical masonry churches diagnosis supported by an analytichierarchy-process-based decision support system, acta imeko, vol. 10, no. 1, article 3, march 2021, identifier: imeko-acta-10 (2021)-01-03 editor: eulalia balestrieri, university of sannio, italy received march 19, 2020; in final form october 26, 2020; published march 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was supported by the project manufacturing education and training governance model for industry 4.0 in the adriatic-ionian area – future 4.0 adriatic-ionian programme interreg v-b transnational 2014-2020. corresponding author: valentino sangiorgio, e-mail: valentino.sangiorgio@poliba.it 1. introduction the structural diagnostics of a masonry building should involve numerous criteria, phenomena, and criticalities. the process of knowledge starts from an in-depth investigation of various different aspects of the construction, including the building history (expansion interventions, transformations, traumatic events), the geometry, the technology used, and the characteristics of the materials as well as any degradation, cracking, or deformation. the synthesis of this information allows us to better understand the structural behaviour and to formulate a valid diagnosis, highlighting the potential weaknesses, collapse mechanisms, instability, and degradation of the structure. masonry churches are the category of buildings most vulnerable to catastrophic events, often experiencing serious damage and collapses as a result of the earthquakes that have occurred throughout history and have compromised the structural performance of the buildings [1]. the difficulty in analysing masonry building degradation is related to the heterogeneity of the factors that pertain to the building’s residual performance evaluation. as a multicriteria decision-making tool, the analytic hierarchy process (ahp) allows for performing exhaustive structural analyses due to the possibility of considering numerous criteria and subsequently affording them a calibrated weight. this method has become popular in various areas of construction. starting from the project stage, both fong and choi [2] and hsieh et al. [3] used the ahp approach to identify the key selection criteria for determining the contractor selection, plebankiewiczm and kubek [4] and kahraman et al. [5] for selecting the best material supplier, and wong and li [6] for analysing the selection among the various intelligent building abstract the structural and anti-seismic performances of historical masonry churches are related not only to structural damage and masonry quality but also to other criticalities that can jeopardise the connections, wooden elements, and non-structural elements. as such, the technical and scientific communities are interested in the development of fast visual-survey approaches for assessing these issues and for performing exhaustive diagnostics by considering all the parameters affecting the building performance. in this paper, a new procedure for performing rapid visual surveys and the diagnostics of masonry buildings using a set of condition ratings is proposed. the proposed novel approach includes three synergistically related techniques: i) the analytic hierarchy process method for measuring and analysing both qualitative and quantitative data during a visual survey, ii) a suitable survey form to perform a rapid visual inspection implementable in a decision support system (dss), and iii) a dss procedure to provide a powerful computerised tool that would be useful in large-scale data acquisition. finally, to demonstrate the potential of the proposed approach, the romanesque masonry church ss. salvatore in capurso, italy, is analysed. a comparison with a standard diagnostic workflow is also presented to validate the procedure and to emphasise the advantages of the novel diagnostics. mailto:valentino.sangiorgio@poliba.it acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 7 systems. the ahp method has also been used for the systematic evaluation of various factors, including building efficiency, safety, user comfort, reliability, functionality, and maintainability. in addition, it has been employed to characterise the design work, to evaluate the soft benefits in relation to the costs [7], [8], and to estimate the environmental work impact [9], [10]. meanwhile, teo and ling [11] and li et al. [12] used the ahp approach to evaluate the safety and administration of work sites, while das et al. [13], wu et al. [14], and kettner and diaz [15] used it in relation to construction management, and ali and nsairat [16] and chang et al. [10] used it to estimate green building ratings. in terms of existing buildings, the approach presents an effective assessment tool for supporting building energy rehabilitation [17], [18], performing large-scale structural vulnerability analysis [19], and identifying the criticalities in seismic and volcanic vulnerability assessments [20], [21]. furthermore, the ahp approach has proven useful in scheduling the building maintenance and selecting the most suitable intervention alternatives [22], since it allows for comparing multiple measures such as attribute data and both qualitative and quantitative data [13]. meanwhile, sangiorgio et al. [23] employed the approach as part of a decision support system (dss) during the performance monitoring phase, which involved a combination of the ahp method with various information technologies. here, the authors obtained data weighted via ahp in order to evaluate the building conditions. cultural heritage monitoring and preservation is a developing discipline that consists of the application of intelligent technologies in cultural sites. at present, such technologies are widely used to monitor different parameters, such as the thermohygrometric conditions [23], [24], the light, and the air quality of churches, to help preserve the wall paintings and architectural monuments [25], [26], [27], [28], while they are also employed to estimate accelerations, tensile stresses, compressive stresses, and the degradation of building materials [29], [30]. numerous parameters must be considered to obtain a complete overview of the performance of masonry churches, with the masonry quality, cracking patterns, connections, or wood elements just some of the parameters that must be analysed to obtain an exhaustive preliminary structural diagnostic of such churches. in this paper, the ahp approach [31] is used to comprehensively study the characteristics involved in the performance of masonry structures and to establish a novel visual-survey-based methodology to measure both quantitative and qualitative data and to perform the relevant diagnostics. a key point of the proposed method is the effective evaluation of suitable condition ratings, which depend on an extremely large amount of possible damage, degradations, or criticalities affecting the church. in this field, the use of the ahp approach can be particularly useful since it allows for considering both qualitative and quantitative data and for examining complex issues via multicriteria analysis. the resulting condition ratings can then be easily implemented in a dss that supports the decision-making activities related to maintenance and monitoring [32]. 2. the analytic-hierarchy-process-based methodology the ahp methodology, used to quantify the weights of the macro-element collapse modes, is based on saaty's well-known three-step method [31], which involves i) hierarchically structuring the problem, ii) a weight evaluation, and iii) a summary of the priority. starting with a decision issue, the first step consists of structuring the problem according to a hierarchical scheme to identify the macroand sub-criteria. meanwhile, the second step, the weight evaluation, presents the core of the method and provides the weights necessary to generate the ranking. in this step, it is possible to analyse each aspect of the decision issue individually by considering ordered criteria and by performing pair comparisons using saaty's fundamental scale (table 1). the result is a nxn judgments matrix a (figure 1). in terms of solving an eigenvector issue, the ahp approach allows for determining the relevant weights. in addition, saaty's method involves defining the consistency ratio (cr) to verify the coherence of the assigned judgement and the reliability of the results [31]. according to the various empirical studies conducted by saaty, a cr of < 0.10 can be regarded as acceptable [31]-[33]. finally, the third step, summarising the priority, is performed to determine the global ranking and weight. these are obtained by multiplying each criterion weight by the alternative weight and by summing the results for each alternative. 3. application of the analytic hierarchy process approach the proposed approach is particularly useful in terms of providing an exhaustive overview of the issues potentially affecting a masonry church. in addition, numerous aspects and criteria, both qualitative and quantitative, can be considered within the diagnostics structure. the main advantages of the proposed approach are the simplicity of the measurements, the affordability of the survey, and the possibility of obtaining a complete overview of the criticalities and an adequate ‘history’ of the previous surveys by implementing the approach in a dss. meanwhile, while specific investigations performed using thermal imaging cameras or ground-penetrating radar can provide a higher level of information (e.g. cracks not visible to the human eye), this type of survey is more time consuming and is rarely applicable to the entire structure. as such, the two approaches can be regarded as complementary. first, the proposed dss approach can be applied at a large scale to identify the components of masonry churches that require further table 1. saaty’s fundamental scale [31]. aij verbal scale aij = 1 equal importance aij = 3 moderate importance of one over another aij = 5 strong importance aij = 7 very strong importance aij = 9 extreme importance 1.5 4 6 8 intermediate value 1/9,1/8,….,1/2 the reciprocal expresses an opposite judgement a 1 2 … n 1 1 1/a1,2 … 1/a1,n 2 1/a1,2 1 … 1/a2,n … … … 1 … n 1/a1,n 1/a2,n … 1 figure 1. the generic matrix of judgments a. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 8 investigation. second, more in-depth surveys using thermal imaging cameras or ground-penetrating radar can be carried out. 3.1. step 1: the issue of masonry damage and quality the first step of the ahp approach was applied to investigate the deterioration in the structural performance of masonry churches that is due to the various phenomena and typologies pertaining to construction criticalities and damage. specifically, three macro-criteria were considered: a) the masonry performance, b) the effectiveness of the connections, and c) the conservation state of the wooden flooring. a) the first macro-criterion, used to determine the structural performances, relates to the quality of the masonry. here, the following five sub-criteria were considered, as shown in figure 2. i) the first sub-criterion classifies the macro elements of the churches by considering the recurrent failure mechanisms. the macro elements are as follows: chamber, façade, chapel, roof, transept, apse, bell tower, dome, and triumphal arches (indicated in blue in figure 2). ii) the second sub-criterion relates to the damaged elements. it is worth noting that recent post-earthquake observations in italy (i.e. umbria and marche, 1997; l'aquila, 2009; emilia, 2012; central italy, 2016) revealed different levels of vulnerability depending on the damaged building components [34]. the relevant elements here include walls, pillars, columns, arches, vaults, architraves, floor strips, and ribs. iii) the third fundamental criterion exploits an existing methodology known as the masonry quality index (mqi) method. this method is both effective and perfectly compatible with the ahp approach and was thus incorporated into our ahp methodology. the mqi method involves evaluating the presence, the partial presence, or the absence of certain parameters that define the ‘rule of the art’, that is, a set of construction devices that, if executed during the wall construction, provide good behaviour and ensure both compactness and a monolithic quality. in such an approach, a visual procedure is used to classify the masonry on the basis of a direct measurement of a specific group of parameters (i.e. stone properties [sm], stone dimensions [sd], stone shape [ss], wall leaf connections [wc], horizontal bed joints [hj], vertical joints [vj], and mechanical properties [mm]) before, finally, the mqi is evaluated through the various tabulated values. iv) the fourth criterion relates to the masonry performance in terms of cracking or any criticality manifestations affecting the building. here, it is necessary to take into account the type of damage affecting each church element in order to perform exhaustive diagnostics. in addition, the classification of the damage must be established according to the model devised by sangiorgio et al. [35]. in this approach, the types of building damage are decomposed and coded on the basis of crack type (vertical, oblique, etc.), with each crack-type then further classified in terms of the severity of the damage (magnitude a, b or c, where c is the most severe damage). v) finally, the sub-criterion, criticality manifestation, is considered in terms of identifying any masonry deformation, scroll back, or disconnection. b) the second macro-criterion relates to the efficiency of the connections and is also classified in terms of various sub-criteria as follows: vi) tie rods anchor systems, (vii) relative corrosion, viii) wall-to-floor connections, and ix) wall-to-wall connections. these aspects ensure the continuity of the energy path and prevent the occurrence of the majority of local collapse mechanisms. in fact, the out-of-plane behaviour of masonry walls observed in recent seismic events has demonstrated the critical importance of having appropriate connections in historical buildings. c) the third macro-criterion relates to the conservation state of the wood flooring. the evaluation of the quality and degradation of the wooden elements begins with a macroscopic examination that allows for the identification of the characteristics of the wood, such as colour, grain, and texture, as well as any anatomical characteristics. specifically, the following wood flooring qualities are considered in the evaluation of the overall performance of masonry buildings: x) the typology of the damaged element (e.g. timber truss or slab), xi) the wood criticality related to moisture, xii) surface alteration, xiii) extension of the problem, and xiv) any mechanical problem including cracks or xv) deformations. finally, a suitable classification of the wood quality is used to take into account the natural vulnerability of the material in terms of xvi) knotting and xvii) shrinkage. 3.2. step 2: weight evaluation the second step in the ahp involves analysing each aspect of the structure of the problem individually in order to weight the criteria and parameters involved. this section refers to previous studies related to every aspect of masonry structures to obtain the judgment matrices. here, seven matrices were used to analyse the sub-criteria and parameters of the masonry macro-criteria, with five matrices evaluated to obtain the criteria and parameters weights of the connections macro-criterion, and eight matrices estimated to provide the weights related to the wood flooring macro-criterion. to provide an example, the weight calculations of the tie rod anchor system related to the connections macro-criterion is figure 2. structure of the problem: historical masonry church diagnostics. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 9 provided. in fact, the effectiveness of the tie rods is related to the typology (classified by year of manufacture) and the level of steel corrosion, as discussed in the structuring of the problem. figure 3 shows the preliminary study and classification of the main tie rod typologies. by referring to the preliminary study, pairwise comparisons of the alternatives were carried out as well as qualitative analysis to obtain the judgment matrix. the weights were obtained by solving the eigenvector problem defined by saaty [31]. the resulting matrix satisfies the cr requirement (cr < 0.1), meaning it was possible to derive consistent weights, all normalised between 0 and 1 (see table 2). it is worth noting that the typology of the tie rods was combined with their state of conservation to ensure a precise evaluation of the performance. in fact, this second parameter, steel corrosion, classifies the condition in terms of three alternatives: low, medium, and high. meanwhile, a qualitative evaluation was performed to quantify the influence of corrosion on the tie rod performance based on the detailed studies contained in the existing literature. the condition ratings could be defined following the weighting phase. this operation relates to the third step of the summary of priority. the condition rating formulas were obtained by multiplying each criterion weight by the sub-criteria weight and subsequently summing the results, as in the standard ahp procedure. for this purpose, three condition ratings were defined: masonry index (im), connection index (ic), and wood elements index (iw). to provide an example, the im could be obtained by using the weights obtained in step 2 of the ahp. table 3 shows the association of the criterion, sub-criterion, or alternative with the relative weights obtained via the comparison matrices, while a complete overview of the tabulated weights is available in the supplementary material (figure s1). let us assume we have surveyed a certain component of the church, one pertaining to a specific macro-element and one that exhibits a specific combination of mqi, cracking, and manifestation. here, the condition rating for quantifying the performance of the masonry criterion (im) could be evaluated using the following equation in accordance with sangiorgio et al. (2019): 𝐼m = 𝑣1 ∙ 𝑤1,𝑗 + 𝑣2 ∙ 𝑤2,𝑗 + 𝑣3 ∙ [𝑤3,1 ∙ (∑ 𝑤3,𝑗 7 𝑗=2 ∙ 𝑤3,𝑗,𝑘 )] + 𝑣4 ∙ 𝑤4,𝑗 ∙ 𝑤4,𝑗,𝑘 + 𝑣5 ∙ 𝑤5,𝑗 (1) where vi is the weights associated with the sub-criteria and wij is the weights correlated with the parameters of the analysis. meanwhile, it was possible to evaluate the performances of the connections. here, the connection index (ic) could be evaluated using the following formula: 𝐼c = 𝑣6 ∙ 𝑤6,𝑗 + . . . +𝑣9 ∙ 𝑤9𝑗 ∙ 𝑤9,𝑗,𝑘 . (2) the wood flooring index (iw) could then be evaluated using the following formula. 𝐼w = 𝑣10 ∙ 𝑤10,𝑗 + 𝑣11 ∙ 𝑤11,𝑗 + . . . +𝑣17 ∙ 𝑤17,𝑗 . (3) finally, the overall condition rating (global index: istructure) could be evaluated, with the average of the im, ic, and iw values calculated for all the components of the structure [36]. 4. integration of analytic-hierarchy-process-based approach in a decision support system the proposed approach can be easily exploited to create a spreadsheet that would prove useful to conducting a rapid survey of a historical church. here, an appropriate survey form was developed, and a suitable flowchart was produced to explain the dss process better. figure 3. study of tie rod evolution over time. table 2. judgment matrix for tie rod typology parameters, weights, and crs. typology a b c d cr w6,1 before 1500 (a) 1.0 1.8 2.5 3.0 0.01 1.00 between 1500-1800 (b) 0.6 1.0 2.0 2.8 0.69 between 1800-1900 (c) 0.4 0.5 1.0 1.9 0.41 after 1900 (d) 0.3 0.4 0.5 1.0 0.26 table 3. connections, macro-criteria, and weights. criterion connections sub-criterion tie-rods typology v6 tie-rods steel corrosion v7 floor/wall v8 wall/wall v9 parameters locking bolt and wood rod w6,1 high w7,1 open joint w8,1 façade w9,1 locking bolt and steel squareheaded rod w6,2 medium w7,2 interlocking leaning joint w8,2 transept w9,2 square-headed or roundheaded bolt and circular rod w6,3 low w7,3 cemented joint w8,3 chapel w9,3 other bolts and threaded rods w6,4 bell tower w9,4 disconnection w9,j,1 vertical crack c w9,j,2 manufacture) and the level of steel corrosion as discussed in the structuring of the problem. fig. 3 shows the preliminary study and classification of the main tie-roads typologies. tie rods evolution over time before 1500 a.d. −wood tie-rods; −strong aesthetic impact; −high weight; −problems related to the degradation both of wood and steel; −locking bolt. between 1500 and 1700 a.d. −steel tie-rods; −locking bolt; −clamping techniques not very effective. between 1800 and 1900 a.d. −advanced clamping systems; −static or aesthetic needs; −stiffening ribs; −multiple threaded and tensioned rods; −square-headed or round-headed bolt. −bilateral connection on the wooden beams; −bolted systems. after 1900 a.d. −advanced clamping systems; −static or aesthetic needs; −corrosion protection by austenitic or duplex steels; −polymeric sheath inside between steel and masonry to avoid direct contact; −remaining voids filled by injection of resins; −chain joining via sleeve for threaded junction, clamped, clamped, bolted or grooved; −bars equipped with threaded ends for clamping; fig.3. study of the tie rods evolution over time by exploiting the preliminary study, pairwise comparisons of the alternatives are carried out performing a qualitative analysis to achieve the judgment matrix. the weights were obtained by solving the eigenvector problem in eq. (1). table 3. judgment m atrix for the typology parameter, weights, and cr typology a b c d cr w 6,1 before 1500 (a) 1.0 1.8 2.5 3.0 0.01 1.00 betw een 1500-1800 (b) 0.6 1.0 2.0 2.8 0.69 betw een 1800-1900 (c) 0.4 0.5 1.0 1.9 0.41 after 1900 (d) 0.3 0.4 0.5 1.0 0.26 acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 10 4.1. survey form a crucial point of the proposed approach is the definition of the survey form that can be implemented in an app for smart devices. such a spreadsheet-style form allows the user (technicians) to measure and evaluate both qualitative and quantitative data during a fast on-site inspection. in fact, the condition ratings can be obtained using eqs. 1–3. here, the proposed app can be used for detecting any damaged or degraded element of the structure to obtain a numerical evaluation of its condition by taking into account all of the macro-criteria, sub-criteria, and parameters of the considered ahp issue. figure 4 shows a general survey form (empty form). 4.2. decision support system architecture the architecture of the proposed dss consists of five components (figure 5): i) the survey form consisting of a suitable spreadsheet that can be implemented in an app for smart devices. iii) a data management system (dms), which collects the information provided by the users through the survey form. iii) a web-based platform that represents the intelligence of the system. this platform processes the survey form data, classifies the church's components, and calculates the condition ratings. iv) a web application used by technicians to display the reported survey form history and the register information related to the church. v) an application programming interface (api) connecting the survey form (app) and the web application with the webbased platform and the dms. 4.3. decision support system process in order to better specify the dss procedure and the use of the survey form, a suitable flowchart [37] was produced in accordance with the uml framework (figure 6). here, the rules and tasks of the actors involved in the process are highlighted. the flowchart procedure explains all the interactions between the two main components of the dss (i.e. the survey form implemented in smart devices and the web application), and the actors involved in the process (i.e. the users of the survey form and the bridge technicians and engineers). the flow is described by defining three phases of the process: the data acquisition phase, the data processing phase, and the diagnostic phase. i) during the data acquisition phase, the user first accesses the list of churches on the web-based platform to select those to be surveyed (if the tool is used on a territorial scale, a number of churches can be selected by using a drop-down list). second, the user can ask to use the survey form for the inspection. third, the user can complete the inspection by taking photographs of any damage and criticalities before sending the inspection information to the platform. figure 4. general survey form. figure 5. components of the dss architecture. figure 6. flowchart explaining the procedure of the system. location data identification data macroelement element mqi nc pc c (masonry quality index) yes no yes no yes no yes no yes no element biological criticality yes no moisture surface alteration extension mechanic criticality yes no cracks deformations classification i ii iii (uni 11119:2004) shrinkage cracks s d w d p d iw size of knots ic steel corrosion joint wall/wall disconnection/vertical crack c floor/wall photo/ plan/ section tie-rods anchor system typology typology im mortar quality cracks typology and magnitude manifestation resistance of the elements horizontal rows presence of diatones shape of the resistant elements off vertical joints dimension of the resistant elements parameters 20% 30% 50% acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 11 ii) in the data processing phase, the web-based platform uses the received surveys to store the new data and to update the survey's history. iii) during the diagnostic phase, the web-based platform sends notification of any new damage to the building staff who can read, change, or validate the inspection. at this point, the system is able to automatically evaluate the condition ratings using eqs. 5–7 and the tabulated weights obtained via the ahp method. finally, if the condition rating exceeds certain defined threshold values, a suitable alarm protocol is triggered. an alert notification is then sent to all users of the dss to set up an emergency intervention. 5. case study a representative example of a church with a typical apulian architectural style, namely, ss. salvatore, was selected in order to apply the proposed methodology. the ss. salvatore church was built in 1541 at the will of queen sforza in the town centre of capurso, located a few kilometres from bari. since it is typical romanesque church, it exhibits a basilica-type plan, which is divided into three naves, a transept, an apse, a dome, and a bell tower (figure 7a, b). the church has experienced numerous geometrical and technical alterations over time, which have resulted in the present configuration. the most crucial variation was the replacement of the wood truss roof over the principal nave with a heavy reinforced concrete ceiling. in fact, the church shows various signs of degradation due to atmospheric agents and the lack of maintenance. therefore, the novel ahp-based procedure was employed to comprehensively investigate the church’s pathologies and to quantify its criticalities. the first step involved filling in the survey form in terms of any elements that appeared to be damaged. figure 8 shows two examples of the survey form, one for the chamber pillar diagnostics (a) and one for the connection system of the bearing structure of the lateral transept vault (b). finally, an evaluation of the church’s overall condition was carried out by calculating the global index (istructure) in terms of all the components of the structure. the result was 1.52 on a scale of 0–10. this low result indicated that the general structural condition was good and that only minor damage had occurred. however, the survey form indices highlighted the dangerousness of the damages. indeed, the im (figure 8a) and ic (figure 8b) values, which were over 7 on a scale of 0–10, suggested the need for deeper investigations. 6. validation a comparison of the results obtained via the proposed ahpbased method and those obtained via the traditional survey and diagnostics approach was conducted to validate the proposed approach for the romanesque masonry church [1]-[38]. here, various specialists, who did not know the results of the innovative approach, were involved in reporting on the condition of the ss. salvatore church, performing a direct on-site inspection, creating technical plans of the church with the indications of damage marked, and compiling a report [39]. following the technical description, it emerged that the pillars near to the dome presented the severest cracks and deformations, likely due to the heavy weight of the reinforced concrete roof covering the central nave. thus, the conclusion was that these structures had been affected by compression damage. the drawing shown in figure 9, which was included in the specialists’ technical sheets, indicates the cracking in red and clearly reveals the serious damage suffered by the pillars. a) b) figure 7. the ss. salvatore church: a) principal façade, b) church plan. a) b) figure 8. survey forms for a) damaged pillar and b) damaged connection system. macroelement 10,00 element 8,65 nc pc c 1 resistance of the elements 1 horizontal rows 2 presence of diatones 1 shape of the resistant elements 3 off vertical joints 0 dimension of the resistant elements 1 mortar quality 2 yes no typology and magnitude yes no 8,75i m cracks 10,00 vertical c chamber pillar mqi (masonry quality index) location data | italy | puglia | capurso (ba) | via carone n.2 identification data | ss. salvatore's church | 1541 | late romanesque | basilical plan | three naves manifestation typology 10,00 deformation parameters yes no yes no yes no 7,64 element 10,00 biological criticality yes no moisture surface alteration extension mechanic criticality yes no cracks deformations classification i ii iii (uni 11119:2004) shrinkage cracks s d w d p d 8,00 10,00size of knots iw 6,67 interlocking leaning joint wall/wall connections anchor system typology 9,59 locking bolt and wood rod steel corrosion medium i c disconnection/vertical crack c 0,00 location data | italy | puglia | capurso (ba) | via carone n.2 identification data | ss. salvatore's church | 1541 | late romanesque | basilical plan | three naves 50% 0,00 timber truss no 4,31holes floor/wall joint 20% 30% 50% acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 12 the comparison between the results of the two approaches revealed highly similar diagnoses. in short, all of the pathologies surveyed through the survey forms and quantified via the ahp approach agreed with the damage indicated by the traditional survey. furthermore, the minor phenomena in the other structures were also comparable. despite the similarity between the results, the ahp approach had the advantage of producing a numerical quantification of the criticalities, which would prove useful for comparing the damaged elements of the same structure or identifying buildings that are most at risk on a large scale. in addition, it allows for comparing different forms of damage on the same structure and provides support for planning more in-depth investigations such as destructive investigations. finally, this approach can be implemented in smart devices and a large-scale application information platform. in this field, the ratings of the specified conditions play a key role in the comparison between different structures and building types. 7. conclusions this article presented a new procedure based on the ahp approach, which was developed to perform rapid on-site measurements and diagnostics of masonry churches through a series of condition assessment indexes. this approach could be implemented in conjunction with specific information technology to provide the advantages of simplicity, survey affordability, and knowledge of the survey’s history through implementing the approach in a dss. as such, the architecture of the dss supported by an ahp-based diagnostics approach was specified in this work. this methodology provides four innovative aspects of the visual diagnostics of masonry structures. first, the ahp approach allows for both qualitative and quantitative data to be included in the analysis, including in terms of connections, wood elements, and moisture. second, the proposed survey and diagnostics carried out with appropriate assessments of the conditions provides an extensive application for identifying buildings with the greatest amount of damage that require more detailed structural surveys. third, the ahp-based diagnostics combined with a dss procedure provides a useful tool that allows for performing analyses on a regional scale. in fact, the dss could detect the damaged elements of masonry churches under investigation using specific survey devices, such as thermal imaging cameras or ground-penetrating radar. finally, a comparison with a standard survey was carried out to validate the method and to underline the advantages of the attendant innovative diagnostics. after discussing the method, a representative case study was selected to assess the proposed methodology. here, the ss. salvatore church in capurso (near bari) was chosen since it presented the typical religious architectural characteristics of the apulian romanesque style. here, a number of specialists performed a traditional survey, from which it was deduced that the pillars near the dome presented damage that was due to compression. a comparison of this result and the defined condition ratings was then carried out to validate the proposed procedure. since the two approaches provided a comparable diagnosis, the innovative method was regarded as valid. since the proposed ahp-based approach has the advantage of obtaining numerical values for characterising the performance of the structure, it represents a promising tool for structural monitoring and diagnosis. future research will focus on a large-scale application of the proposed methodology to demonstrate the potential of the proposed dss for monitoring and diagnosing masonry structures. figure 9. technical drawing by specialists (with damage representation). acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 13 references [1] g. uva, v. sangiorgio, s. ruggieri, f. fatiguso, structural vulnerability assessment of masonry churches supported by userreported data and modern internet of things (iot), measurement 131 (2019) pp.183-192. doi: 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[27] j. shah, b. mishra, customized iot enabled wireless sensing and monitoring platform for preservation of artwork in heritage buildings, 2016 ieee international conference on wireless communications, signal processing and networking, wispnet 2016, chennai, india, 2016, pp. 361-366. doi: 10.1016/j.protcy.2016.03.025 [28] f.j. mesas-carrascosa, d. verdú santano, j.e.m. de larriva, r. ortíz cordero, r.e. hidalgo fernández, a. garcía-ferrer, monitoring heritage buildings with open source hardware sensors: a case study of the mosque-cathedral of córdoba, sensors 16(10) (2016) p. 1620. doi: 10.3390/s16101620 [29] c. scuro, p.f. sciammarella, f. lamonaca, r.s. olivito, d.l. carni, iot for structural health monitoring, ieee instrumentation and measurement magazine 21(6) (2018) pp. 414. doi: 10.1109/mim.2018.8573586 [30] s. martiradonna, i. lombillo, f. fatiguso, performance monitoring of refurbished building through innovative precast concrete modules, ieee international conference on systems, https://doi.org/10.1016/j.measurement.2018.08.014 https://doi.org/10.1080/014461900407356 https://doi.org/10.1016/j.ijproman.2004.01.002 https://doi.org/10.1061/(asce)co.1943-7862.0001033 https://doi.org/10.1108/09576050310503367 https://doi.org/10.1016/j.buildenv.2006.11.019 https://doi.org/10.6106/jcepm.2014.4.1.045 https://doi.org/10.1061/(asce)0733-9364(2005)131:12(1263) https://doi.org/10.1016/j.conbuildmat.2010.11.041 https://doi.org/10.1016/j.buildenv.2005.08.015 https://doi.org/10.1016/j.buildenv.2005.06.005 https://doi.org/10.1061/(asce)co.1943-7862.0000605 https://doi.org/10.1080/01446193.2010.501806 https://doi.org/10.1080/01446193.2010.501806 https://doi.org/10.1061/40513(279)143 https://doi.org/10.1016/j.buildenv.2008.07.015 https://doi.org/10.13033/isahp.y2011.135 https://doi.org/10.1007/s11367-012-0417-8 https://doi.org/10.4028/www.scientific.net/amm.82.639 https://doi.org/10.1016/j.compstruc.2015.06.016 https://doi.org/10.1016/j.jobe.2019.100971 https://doi.org/10.1109/soli.2017.8120999 https://doi.org/10.5194/hess-24-473-2020 https://doi.org/10.1007/978-3-030-24305-0_19 https://doi.org/10.1016/j.protcy.2016.03.025 https://doi.org/10.3390/s16101620 https://doi.org/10.1109/mim.2018.8573586 acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 14 man and cybernetics, smc 2019, bari, italy, 2019, pp. 953-957 doi: 10.1109/smc.2019.8914472 [31] t. l. saaty, decision making with the analytic hierarchy process, int. j. services sci. 1(1) (2008) pp. 83-98. doi: 10.1504/ijssci.2008.01759 [32] v. sangiorgio, g. uva, j. m. adam, integrated seismic vulnerability assessment of historical masonry churches including architectural and artistic assets based on macro-element approach. international journal of architectural heritage (2020) pp. 1-14. doi: 10.1080/15583058.2019.1709916 [33] e. e. noble, p. p. sanchez, a note on the information content of a consistent pairwise comparison judgment matrix of an ahp decision maker, theory decision 34(2) (1993) pp. 99-108. doi: 10.1007/bf01074896 [34] a. borri, r. sisti, g. castori, m. corradi, a. de maria, analysis of the collapse mechanisms of three churches in valnerina struck by the 2016 italian earthquake, international journal of architectural heritage 13 (2019) pp. 17-21. doi: 10.1080/15583058.2018.1431731 [35] v. sangiorgio, g. uva, f. fatiguso, user-reporting based semeiotic assessment of existing building stock at the regional scale, journal of performance of constructed facilities 32(6) (2018) p. 04018079. doi: 10.1061/(asce)cf.1943-5509.0001227 [36] v. sangiorgio, g. uva, f. fatiguso, a procedure to assess the criticalities of structures built in absence of earthquake resistant criteria, rehabend 2016, burgos, spain, 2016, pp. 631-639. 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https://doi.org/10.1109/soli.2017.8120999 http://hdl.handle.net/11311/501569 https://doi.org/10.1002/pse.160 microsoft word article 11 tc4 paper 7.docx acta imeko  august 2013, volume 2, number 1, 40 – 48  www.imeko.org    acta imeko | www.imeko.org  august 2013 | volume 2 | number 1 | 40  real‐time smart meters network for energy management  giuseppe del prete, daniele gallo, carmine landi, mario luiso   department of industrial and information engineering, second university of naples,via roma 29, 81031 aversa, italy      keywords: smart metering; smart grid; power and energy meter; distributed measurement system; real‐time measurement  citation: giuseppe del prete, daniele gallo, carmine landi, mario luiso, “real‐time smart meters network for energy management”, acta imeko, vol. 2,  no. 1, article 11, august 2013, identifier: imeko‐acta‐02(2013)‐01‐11  editors: paolo carbone, university of perugia, italy; ján šaliga, technical university of košice, slovakia; dušan agrež, university of ljubljana, slovenia   received january 11 th , 2013; in final form april 27 th , 2013; published august 2013  copyright: © 2013 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: none reported  corresponding author: daniele gallo, e‐mail: daniele.gallo@unina2.it    1. introduction  the traditional electricity distribution network has been designed essentially as a passive network that carries the energy one way: from a few big power stations to the consumption points of end users. the increased cost of energy production and the growing up of its demand require different management system, based on real-time measurements, which make more efficient its use. we are moving to an upgraded electricity network to which two-way digital communication between supplier and consumer, intelligent metering and monitoring systems have been added. these are known as smart grid paradigm [1][2]. an essential requirement for the application of smart grid approach is the development of an advanced metering network. basically, it consists in a set of electricity meters that records consumptions of electric energy and make this information available to the grid operator and energy supplier for monitoring and billing purposes. typically, these meters are implemented with embedded microcontrollers designed for a specific application and capable to run stand alone. in addition these meters should include a communication front-end that allows them to exchange information within a network and to communicate with external devices. then a new generation of meters is required: smart meters [2]. in this way, the implementation of smart metering network is the first step towards the creation of a "smart grid" electricity network that can intelligently integrate the actions of all logged users: generators, consumers and prosumers (those who simultaneously perform the dual role), in order to offer efficiency with a sustainable supply of electricity, cheapness and safety. an important feature of smart metering is the possibility to make available some basic information, such as current energy absorption, with stringent time constraints that reach the real time requirements. for a full functionality, another important feature that a smart meter should have is the ability of exchanging information or services, or any part of them, with other devices, also not homogeneous. this can be implemented abstract  in this paper, an architecture of a low‐cost arm‐based smart metering network is presented. the system is designed to be suitable for  smart grids applications aimed to a more efficient energy use according to the article 13 of directive 2006/32/ec. the network  is  composed by several slave smart meters that continuously monitor loads and energy generators to make available information in real‐ time such as power and energy consumption/generation and several power quality parameters communicating them via can bus to  specific master device called data aggregator. this device, integrating the information coming from field devices (energy demands of  loads, the current energy production and co‐generator status), with information obtained through the web access (a prevision on the  expected availability of energy produced by renewable sources, current and future energy price, customer remote setting), can take  decisions to  implement a suitable energy management aimed to cost saving or whatever else strategy chosen by customer. data  aggregator also allows checking current consumption  locally, thanks to a display, and remotely, using the web browser access. to  prevent external attacks a low computational burden protection software based on message authentication code (mac) has been  implemented.  finally,  characterization  test  of  realized  apparatus  have  shown  good  performance  both  in  terms  of  communication  delays and measurement uncertainty.  acta imeko | www.imeko.org  august 2013 | volume 2 | number 1 | 41  only on the basis of a set of open standards. the most important initiative is this field is the open meter project [3]. already in 2008, the european smart metering community agreed that there were several barriers that prevented the large adoption of advanced metering infrastructures at european or global level. one of the most important barriers was considered the lack of a set of widely accepted open standards for smart metering, capable to guarantee the interoperability of both systems and devices produced by different manufacturers in the metering and control industry. this barrier was also acknowledged by the european commission and as a response to overcome this barrier the m/441 mandate was issued by the ec to cen, cenelec and etsi. the objective of the mandate is to make the development of an open architecture for utility meters involving communication protocols enabling interoperability [4]. the main aim of the open meter project is to specify a set of open standards for advanced metering infrastructure (ami) to support smart meters for electricity, gas, water and heat. the project's aim is to bridge the knowledge gap for the adoption of open standards for smart multi-metering equipments, smart metering functions and all prescriptive aspects; the communication protocols and data formats are also considered part of the project. the open meter project was taken as a guideline for application layer of the implemented meters [3]. in this paper a multi-meter device for smart grid applications, capable to manage the energy consumption and to communicate in real-time with decentralized control unit by means of a modified can protocol, is proposed. to prevent external attacks, a protection software with low computational burden based, on message authentication code (mac), has been implemented. furthermore, the user is able to check several information as the current power/energy consumption or information on current intensity of load use, remotely, through the implemented web server in the data aggregator, and locally, through the display interface. the smart energy meter network is designed with the aim to obtain a low-cost equipment for a widespread usage. the paper is organized as follows. in section 2 the smart metering network architecture is presented. then, in sections 3 and 4 the hardware implementation and the measurement software of the realized smart meters are shown. in section 5 the communication protocol among smart meter is described. section 6 presents how the network is managed and, finally, section 7 show the characterization of the smart metering network, in terms of measurement accuracy and communication capability. 2. smart metering network architecture  in figure 1, the simplified scheme of the proposed smart meter network is shown [5], [6]. it includes a master data aggregator and several slaves. the data aggregator, thanks to the implemented web server contains, in a table, information regarding:  renewable source availability  co-generator status,  actual energy prices. the first information is important because the level of energy generated by renewable sources often suffers of high time variability due to climatic changes. therefore due to this time variability it is not possible to guarantee a specific level of power availability in a specific time. for this reason often the renewable source are used jointly with storage systems. monitoring of the charge level of storage systems could be an additional feature of the smart meter network that is not treated in this paper but it is one of the enhancements that the authors intend to implement in the next future. for an integrated management some information regarding the co-generator are needed such as the status, thermal energy requirement, etc. finally the data aggregator needs to know the energy price that can be updated daily or even hourly. the slave smart meters are connected to the single node of power network. each device acquires continuously voltages and currents and it calculates power and energy consumption/generation and several power quality parameters. the master microcontroller, in addition, acquires information about energy pricing and therefore it is able to make decisions for an efficient use of the energy such as to disconnect a single load or to decide the time of use of a load. 3. hardware implementation of the smart meter  from a functional point of view, it consists of the following blocks: i) a metering unit that tracks the energy usage of the customer and processes the billing, ii) a communication unit that enables two way digital communication with the energy company iii) a switching unit that starts and shuts down the energy supply. from a physical point of view, each equipment consists of: i) a transduction section composed by voltage and current sensors and level adapters, ii) an elaboration section that acquires the output of the sensors and processes the acquired samples, iii) a display monitoring several information, iv) a memory section which stores the billing value in an eeprom [7]. the transduction stage and the elaboration section are described in the following subsections. 3.1. voltage and current sensing section  voltage and current sensing section is composed a prototype of a combined voltage and current transducer (cvct) [8], [9]. the block scheme is shown in figure 2 and a photo is in figure 3. as it can be seen it is made of simple electrical and electronic components and thus its cost is very figure 1.  architecture of the proposed smart metering network.   acta imeko | www.imeko.org  august 2013 | volume 2 | number 1 | 42  low. the two sections of the cvct take supply from voltage input: a simple half-wave rectifier and a linear regulator obtain single supply 5 vdc from 230 vac. voltage transducer is a high impedance resistive divider with a differential operational amplifier, current transducer is a low resistance resistive shunt with a differential operational amplifier. input voltage is in the range 460√2,460√2 v, which corresponds to a root mean square (rms) value equal to 460 vrms, in order to make the meter able to measure power quality parameters like voltage swells. current input is in the range 30√2,30√2 a, which corresponds to a rms value equal to 30 arms. both the outputs are in the range 0,3 v, in order to be suitable for microcontroller analog inputs; with zero inputs the outputs are 1.5 vdc. the realized cvct has been simulated in multisim environment. figure 4 and figure 5 show, respectively, inputs (230 vrms and 15 arms) and outputs of the cvct. from figure 5 it can be seen that: 1) outputs are inverted with respect to inputs, 2) the signal becomes stationary after a small transient due to stabilization of supply voltage by linear regulator, 3) mean values are 1.5 v for both outputs. in figure 6 magnitude and phases of the outputs, from the ac analysis simulation, are shown: it can be seen that in the range of interest for power quality analysis, i.e. until 10 khz, frequency bandwidth of the cvct is suitable for the application for which it has been realized. 3.2. elaboration section  the hardware implementation is built around a stm32f107vct6 microcontroller, which incorporates the high-performance arm®cortex™-m3 32-bit risc core operating at a 72 mhz frequency. its main features are: 1) 256 kbytes of flash memory and 64 kbytes of general-purpose sram, 2) low power: sleep, stop and standby modes, 3) 2 × 12-bit, 1 µs a/d converters (16 channels) with conversion range from 0 to 3.6 v and up to 2 mhz in interleaved mode, 4) 2 × 12-bit d/a converters, 5) 12-channel dma controller, 6) up to 80 fast i/o ports, 7) crc calculation unit, 96-bit unique id, 8) up to four 16-bit timers, 1 × 16-bit motor control pwm timer, 2 × watchdog timers, 9) up to 2 × i2c, 5 usarts, 2 × can interfaces (2.0b active) with 512 bytes of dedicated sram, usb 2.0 full-speed device/host/otg controller, 10/100 ethernet mac with dedicated dma and sram (4 kbytes) and with ieee1588 hardware support. figure 3.  a photo of the realized cvct.  figure 4.  voltage and current at the input of cvct.  figure 5.  outputs of cvct with voltage and current input of figure 4.  figure 6.  ac analysis of the realized cvct: magnitudes and phases of the  outputs.  figure 2.  circuit diagram of the cvct.  r1 480kω r2 1.2kω r3 22kω r4 22kω v1 230 vrms 50 hz 0° c1 10µf r20 10kω d2 diode_virtual u3 lm7805ct line vreg common voltage c2 10µf v1 v1 v1 rload 15.35ω rshunt 2.5mω r8 14.2kω r7 1kω v1 u2a ad8032an 3 2 4 8 1 u2b ad8032an 5 6 4 8 7 r10 49.2kω r9 1kω v1 r5 6.6kω r6 1.2kω c3 100pf c4 150pf acta imeko | www.imeko.org  august 2013 | volume 2 | number 1 | 43  4. measurement software implementation  in this smart meter, the elaboration section is able to calculate the following parameters: i) voltage and current rms values, ii) active power (p) and power factor (pf), iii) energy consumption, iv) power consumption profile, v) frequency, vi) voltage and current total harmonic distortion (thdv and thdi) and vii) voltage dip events. as for the implemented software, it adopts on line data processing to obtain the desired quantities. for this aim, an accurate synchronization is essential because most parameters depend on the actual fundamental frequency, so frequency deviation from nominal value should be continuously monitored. the synchronization of the input sequence is implemented through: i) the hysteresis block that selects a number of samples between the first and second zero crossing and ii) least square linear regression block that rebuilds the real index position of the input samples zero crossing. defining and , respectively, the fundamental and the sampling frequencies, 1 and 0 , respectively, the index values for the first and second zero crossing, ∆ and ∆ their residuals, adopting the mathematical relation (1) it is possible to obtain the fundamental frequency value: 1 1 ∆ 1 0 ∆ 0 (1) a section monitors dip events through rms continuous processing. considering n the ratio between the sampling rate and the input signal frequency, the relations (2), based on the eulero’s equation, calculate the rms values: 1 1 (2) the algorithm adopts a sliding window technique and this leads to (3): 1 1 (3) the active power is calculated as in (4): 1 (4) the power factor is calculated with (5): (5) where p is the active power and s the apparent power. subsequently a digital re-sampling is made to obtain in exactly ten cycles of the fundamental a number of samples that is a power of two. the results of all the measurement sections are validated using flag control: flagged results are not accounted for subsequent analysis, not flagged data are grouped with reference to absolute time in order to obtain measurement with 10 min clock boundary. thdv and thdi are evaluated through fast fourier transform (fft) of voltage and current signals, after a resampling process to obtain a number of samples equal to 256 for each signal, i.e. a power of four [10]. the new samples are taken at non integer index corresponding to (6): 2 ∀ 1,…,2 (6) the integer and decimal part of the index corresponding to the k-th new sample are respectively and . the value of new samples can be calculated as in (7): ∆ 1 10 (7) where and 1 are two consecutive samples adopted to calculate , and finally the distance between and is . 5. communication protocol  5.1. can protocol  the implemented smart meter network requires a reliable low level communication interface. its main required features should be: i) low cost implementation, ii) noise immunity, iii) easy configuration, iv) multicast network. for these reasons, the authors chose the can protocol [11]. it was specifically designed to operate seamlessly even in presence of high electromagnetic disturbances thanks to the adoption of transmission signals with a balanced difference of potential. the immunity to electromagnetic interference can be further increased by using twisted pair cable type. the bit rate can be up to 1 mbit/s to less than 40-meter nets. slower speeds let you reach greater distances (125 kbit/s to 500 m) as in the considered case. the can communication protocol is standardized in iso 11898-1 [12]. if the bus is idle, any node may begin to transmit. if two or more nodes begin to send messages at the same time, the message with the higher id (which has more dominant bits, i.e., zeroes) will overwrite other nodes lower id's, so that eventually (after this arbitration on the id) only the dominant message remains and it is received by all nodes. this mechanism is referred to as priority based bus arbitration. messages with numerically smaller values of id have higher priority and they are transmitted first. the can communication was implemented by the stm32f107vct6, in order to efficiently manage a large number of incoming message [13]. the simplified architecture of the stm32 can interface is shown in the figure 7. 5.2. practical utilization of can architecture  the master data aggregator (see figure 1) sends a message, with a “remote frame”. it is a message without information content, aimed to request a data frame from the slaves. three transmit mailboxes are provided to the software for setting up messages. the transmission scheduler decides which acta imeko | www.imeko.org  august 2013 | volume 2 | number 1 | 44  mailbox has to be transmitted first, for example the energy consumption of a single load. the message is converted by a parallel-serial converter and it is sent to the can tx pin. the master receives the remote frame through the can rx pin. then, the message is converted in parallel through a serialparallel converter. the frame is sent to an acceptance filter that is composed of 14 configurable identifier filter banks for selecting the incoming messages that the software needs and discarding the others. two reception fifo (first in first out) buffers are used by hardware to store the incoming messages. three complete messages can be stored in each fifo. the fifos are completely managed by hardware. when a remote frame is received, the device sets up a response message, a data frame corresponding to the remote frame, and sends it to the other device that previously sent the remote frame. the structure of the data and remote frame is the following. the start of frame denotes the start of the frame transmission. the id is the identifier for the data and also represents the message priority. the remote transmission request is set to dominant (zero). the identifier extension bit and reserved bit must be dominant (zero). the data length code consists of four bits and indicates the number of data bytes (0-8 bytes). the data field denotes the data to be transmitted (0-8 bytes) and it only is in the data frame. the cyclic redundancy check, composed by 15 bits, is an error-detecting code used to detect accidental changes to raw data. the ack slot is sent recessive (1) from the transmitter and any receiver can assert a dominant (0); the end of frame must be recessive (1). 6. smart meter network management  6.1. operating system  a real time operating system has been implemented, to support the different operations of each smart meter. there is a main task that enables and disables all the four tasks that we have implemented [14], [15]. they are:  can manage to manage locally slave microcontrollers  uip server to manage the communication over tcp/ip protocol  measure to measure the parameters reported in tab. i.  user interface to show the power and energy profile consumption and manage the touch screen. in figure 8 and figure 9, respectively, the display of a smart meter with all tasks and the power consumption, through the task user interface, are shown. 6.2. communication over tcp/ip  in the data aggregator a web server collects the statistics of each household and extracts other information [15]. through the web server, the user can remotely monitor the power profile of a single load. for these reasons, an http web server has been implemented. it can serve dynamic web pages and files from a read-only rom file system, and provides several scripting languages. we report an example for the power consumption request. the steps of the cgi (common gateway interface) are the following: i) user clicks power graph, ii) client browser is shown, iii) request is sent to the web server. the web server: i) loads the uip packet management routine, ii) check uip packet, iii) starts the routine httpd_add_call, iv) checks the request (power graph). the service procedure of the script is the following: i) sends the file header.html, ii) writes the text “load power profile” iii) calls the function create graph, iv) terminates the script. in the development of web server application, html protocol is used to provide static web pages to the client . concrete steps are: i) users, in the client browser, make a request to the web server, ii) web server will make a judgment on the request, iii) web server will transfer file directly to the client browser, iv) the header part contains the title of the website and several links to view other pages, v) the body contains the linked pages. in figure 10 the web page is shown. it is possible to remotely monitor the instantaneous power consumption of a specific load, as shown in figure 11. figure 7.  can communication device architecture.   figure 8.  user interface for accessing to the implemented tasks.  figure 9.  graphical representation of the results from the executed task.   acta imeko | www.imeko.org  august 2013 | volume 2 | number 1 | 45  6.3. cyber security  security becomes an important issue in the new scenario of the smart grid. to maintain the steady operation for smart power grid, massive measurement devices must be allocated widely among the power grid [16], [17]. this expansion exposes more to the external attacks that can alter the load through the web server. it can create several problems, for example: disconnect a load, increasing the amount of the billing connecting the washing machine in the morning instead of the night, as the user decided to save the amount of the billing. for these reasons, it becomes fundamental to develop system control algorithms to improve cyber security of the communication network. with this aim the authors present an algorithm that does not use any encryption algorithm. the idea is to use a mac (message authentication code) used to check the message and the authenticity of the sender. the steps, as shown in fig. 6, for the message authentication are the following:  the sender adds a sync code, a non-decreasing number, and a secret key, shared with the receiver, to the original message  with a hash function is calculated the mac that the sender replaces with the secret key  therefore the receiver replaces the mac with the same secret key used to calculate mac by the sender and obtains new mac using the same hash function.  if the receiver notes that the two mac are the same, states that the message is the original one. the sync code is used to verify the authenticity of the message; in fact being a non-decreasing number, an old message has a value that should be smaller than that of a new message. if the values are different, there surely were not any external attacks. 7. smart meter network characterization  the realized smart meter network has been characterized, in terms of measurement accuracy and communication performance. they are shown in the following subsections. 7.1. characterization of measurement accuracy  in order to prove reliability of the implemented instrument, a characterization, in static and dynamic conditions, has been performed [14], [18]. the following instruments, as shown in figure 12, were used:  function generator yokogawa fg320 (features: dual channel output, frequencies range 1 µhz to 15 mhz, amplitude range ± 10v, ac amplitude accuracy ±(0.8% of setting + 14 mv), dc output accuracy ±(0.3% of setting + 20 mv)  pxi 1042 chassis with a pxi-daq 6123 (features: 16-bit, 500 khz/ch, simultaneous sampling)  pacific power source 3120 amx (features: maximum power: 12 kva; ii) frequency range: 20 hz to 50 khz; iii) line regulation: 0.027 mv; iv) load regulation: 0.00135 mv; v) thd: 0.1%; vi) voltage ripple and noise: -70 db). 7.2. static characterization  for static characterization only behavior of a/d conversion systems of microcontroller are taken into account and it is directly tested with the experimental set-up reported in figure 12. in fact the adopted function generator has two independent output channels that can be separately configured with proper values of amplitude and phase. these signals were acquired at the same time by the two a/d channels of the microcontroller and by the a/d channels a pxi-daq 6123 that was adopted as reference. through a software developed in labview environment, it was possible to monitor and store continuously the values supplied by the generator and to obtain expected measurement results. these values were then compared with those shown on the display of the power meter. in the first set of tests a dc voltage was has been generated at different level varied with a step of 10 % of full scale range. each test was repeated ten times. a systematic deviation was found for each input level with a figure 10.  the web page.  figure 11.  power graph of a specific load.  figure 12.  experimental set‐up for static characterization.  acta imeko | www.imeko.org  august 2013 | volume 2 | number 1 | 46  value that is within the range 0.1 0.8 %. starting from the obtained results it is possible evaluate, with the least squares method, gain and offset correction parameters for each input channel. after the compensation the mean error is lower than 0.05 % and the standard deviation is lower than 0.03 %. 7.3. dynamic characterization  for dynamic characterization, the output signals of function generator were amplified by the power amplifier, the pacific source 3120 amx and the pxi measurement system was again adopted as reference value as depicted in the simplified scheme reported in figure 13. the test sets were chosen accounting considerations reported in [14], [18]. the tests were performed in sinusoidal and non-sinusoidal conditions. in sinusoidal conditions the effects of frequency deviation and phase angle variation on active and reactive power were evaluated. the sinusoidal tests have been performed varying input parameters around rated values: frequency (50 hz ±15%), input voltage amplitude (50% to 100 %), input current amplitude (0% to 100 %), finally the phase displacement between –π/4 to π/2. in figure 14 and figure 15 the percentage relative uncertainties for active and reactive powers in sinusoidal conditions are reported. in non-sinusoidal conditions, the effects of the fundamental phase angle and the harmonic order variation on active and non-active power, were evaluated. for the non-sinusoidal tests, according to [19] a fixed thd to 8% has been adopted. for each tests five harmonic components spanning between 3rd and 39th harmonic order are superimposed to fundamental tone, with fixed thd. the testing procedures is well described in [14], [18], [20], [21]. in the figure 16 and figure 17 the percentage relative uncertainties for active and non-active powers in non-sinusoidal conditions are reported. a summary of the obtained results are shown in the table 1 reporting the uncertainty for each measured quantity. figure 13.  experimental set‐up for dynamic characterization.  figure 14.  active  power  uncertainty  estimation  vs  phase  angle  and  fundamental frequency.  figure 15.  reactive  power  uncertainty  estimation  vs  phase  angle  and  fundamental frequency.    figure 16.  active  power  uncertainty  estimation  vs  harmonic  order  and  phase angle.  figure 17.  non active power uncertainty estimation vs harmonic order and  fundamental frequency.  table 1. value of the mean squared deviations in the sinusoidal and non‐ sinusoidal tests.  quantity  uncertainty  (sin. test)  uncertainty   (non‐ sin. test)  units voltage (r.m.s.)  0.03  0.04  % current (r.m.s.)  0.03  0.04  % frequency  0.67  0.67  mhz active power  0.043  0.061  % apparent power  0.13  0.15  % pf (conventional)  0.002  0.002  p.u. non active power  0.60  0.62  % voltage thd  ‐‐‐  0.072  % current thd  ‐‐‐‐  0.070  % acta imeko | www.imeko.org  august 2013 | volume 2 | number 1 | 47  7.4. network communication characterization  distributed energy meters transfer measurement results to the data aggregator through can protocol. in the figure 18 an example with two slave microcontrollers that communicates with a data aggregator is reported. they continuously acquire voltage and current and calculate several parameters shows on the display. several tests have been performed to evaluate the transmission time. the measurement time in these tests were performed through a tektronix tds3012b oscilloscope. in order to check the time latency, an output bit of the dac was used to generate a square wave as signal to acknowledge the microcontroller operation. a first test was made to calculate the delay between the instant when the master microcontroller sends the request and the instant when a slave detects an interrupt for the reception of the request. the estimated time is approximately 3.6 ms (figure 19a). in the second test, the data aggregator requires data relating the active power to both slaves with two different priority levels. it receives the data from the slave with the highest priority approximately after 25 ms (figure 19b) and the data from the lower priority slave after approximately 60 ms (figure 19c). a complete analysis of the utilized protocol, in terms of communication capabilities in dependence on number of units in the network and its throughput, has been performed in [22]. regarding the throughput, it is shown that it slightly increases when the number of devices increases from 5 to 20: when the number of devices is higher than 20 the throughput remains constant. in [22] it also reported a correlation between number of devices, latency time, throughput and payload, i.e. the length of the message. in particular, it is observed that observe that the latency time remains practically constant with payload equal 8 bytes, that is the length used in the smart meter network, and number of devices. 8. conclusions  in this paper a smart energy meter network is presented with the aim to obtain a low-cost device for a sustainable use of electrical energy. the architecture provides a several slave smart meters connected to the single node of power network. each microcontroller acquires continuously voltages and currents and it calculates: level of power , energy consumption and several power quality parameters. the master microcontroller, called data aggregator, acquires information about the single load via can protocol. furthermore a web server was implemented: in addition to acquisition of the several parameters, it daily updates the energy price. with that information it calculates the amount of the billing and it is able to make decisions for an efficient use of the energy such as to disconnect a single load or to decide its time of use. however the users can locally control their consumption through the display of the data aggregator and remotely using a web browser. metrological characterization has shown good performance. furthermore the performed tests on communication network have shown negligible character error rate. considering the performance and the low cost of the realized smart meter network, its application and commercialization potential may be quite high: it can be employed in domestic and industrial application in order to have real-time control on the network and to rapidly intervene in case of failures. references  [1] t. flick, j. morehouse, “securing the smart grid: next generation power grid security”, 2011 elsevier inc., burlington, ma, usa, isbn 978-1-59749-570-7. [2] k. moslehi, r. kumar, “a reliability perspective of the smart grid”, ieee trans. on smart grid, vol. 1, 2010, pp. 57-64. [3] openmeter, “open public extended network meter”, http://www.openmeter.com/ [4] n. arcauz, a. goñi, m. adriansen, b. roelofsen, t. schaub, f. tarruel, b.schumacher, “open meter overview of the market for smart metering”. open metering deliverables public report 31/1/2012, http://www.openmeter.com. [5] g. del prete, c. landi, “energy saving smart meter using lowcost arm processor”, 18th symp. imeko tc4 – meas. of elec. quantities natal, brazil, 2011. (a)  (b)  (c)  figure 19.time latency results in the first (a) and second (b and c) test.  figure 18.  example of device connection for data transmission.  acta imeko | www.imeko.org  august 2013 | volume 2 | number 1 | 48  [6] c. landi, g. del prete, d. gallo, “real-time smart meters network for energy management”, xx imeko world congress, busan, republic of korea, 2012. [7] h. serra, j. correira, a.j. gano, a.m. de campos, i. teixera, “domestic power consumption measurement and automatic home appliance detection”, ieee international workshop on intelligent signal processing, university of algarve, portugal, 2005, pp.128-132. [8] a. delle femine, d. gallo, c. landi, m. luiso, “broadband voltage transducer with optically insulated output for power quality analyses”, ieee instr. and meas. tech. conf. imtc 2007, warsaw, poland, 2007. [9] d. gallo, c. landi, m. luiso, e. fiorucci, g. bucci, f. ciancetta, “realization and characterization of an electronic instrument transducer for mv networks with fiber optic insulation”, wseas transactions on power systems, vol. 8, no. 1, 2013, pp. 45-56. [10] d. gallo, c. landi, n. rignano, “multifunction dsp based realtime power quality analyzer”, xviii imeko world congress, rio de janeiro, brazil, 2006, pp. 38-43. [11] f. moraes, a. amory, n. calazans, e. bezerra, j. petrini, “using the can protocol and reconfigurable computing technology for web-based smart house automation”, 14th symp. on integrated circuits and systems design, pirenopolis, brazil, 2001. [12] iso 11898-1:2003, “road vehicles -controller area network (can) -part 1: data link layer and physical signalling”. [13] stm32f107vct6 datasheet, http://www.st.com/st-webui/static/active/en/resource/technical/document/datasheet/cd 00220364.pdf [14] d. gallo, g. ianniello, c. landi, m. luiso, “an advanced energy/power meter based on arm microcontroller for smart grid applications”, 17th symp. imeko tc4, 3rd symp. imeko tc19 and 15th iwadc, kosice, slovakia, 2010. [15] f. ciancetta, e. fiorucci, b. d'apice, c. landi, “a peer-to-peer distributed system for multipoint measurement techniques”, ieee inst. and meas. tec. conf., warsaw, poland, 2007. [16] “ieee recommended practice for monitoring electric power quality”, ieee std 1159-2009 (revision of ieee std. 1159-1995), 2009, pp. c1–81. [17] a. delle femine, d. gallo, c. landi, m. luiso, “power quality monitoring instrument with fpga transducer compensation”, ieee transactions on instrumentation and measurement, vol. 58, no. 9, 2009, pp. 3149–3158. [18] a. delle femine, d. gallo, c. landi, m. luiso, “advanced instrument for field calibration of electrical energy meters”, ieee transactions on instrumentation and measurement, vol. 58, no. 3, 2009, pp. 618–625. [19] iec en 50160, “voltage characteristics of electricity supplied by public distribution systems”, cenelec ,1999. [20] g. del prete, d. gallo, c. landi, m. luiso, “the use of real-time instruments for smart power systems”, ieee international energy conference and exhibition, florence, italy, 2012. [21] iec standard en 61000-4-7, “testing and measurement techniques general guide on harmonics and interharmonics measurements and instrumentation, for power supply systems and equipment connected thereto”, 2003. [22] s. dridi, b. gouissem, s. hasnaoui, h. rezig, “coupling latency time to the throughput performance analysis on wireless can networks”, international multi-conference on computing in the global information technology, 2006, p. 39. interactive model of magnetic field reconstruction stand for mobile robot navigation algorithms debugging which use magnetometer data acta imeko issn: 2221-870x december 2019, volume 8, number 4, 47 53 acta imeko | www.imeko.org december 2019 | volume 8 | number 4 | 47 interactive model of a magnetic field reconstruction stand for the debugging of mobile robot navigation algorithms using magnetometer data stanislaw goll1,2, alexander borisov1,2 1 department of information-measuring and biomedical engineering, ryazan state radio engineering university (rsreu), gagarin street, 59/1, ryazan, russia 2 llc kb avrora, skomoroshinskaja street 9, of.3, ryazan, russia section: research paper keywords: magnetic field reconstruction; helmholtz coil system; magnetometer calibration; closed loop control; interactive simulink model; adaptive regulator, frequency response estimation citation: stanislaw goll, alexander borisov, interactive model of magnetic field reconstruction stand for mobile robot navigation algorithms debugging which use magnetometer data, acta imeko, vol. 8, no. 4, article 9, december 2019, identifier: imeko-acta-08 (2019)-04-09 editor: yvan baudoin, international cbrne institute, belgium received november 23, 2018; in final form april 18, 2019; published december 2019 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: stanislaw goll, e-mail: likvon@list.ru 1. introduction simultaneous localization and mapping (slam), based on magnetometer data, is a rapidly growing field in robotics [1]-[8]. the unperturbed natural earth’s magnetic field (emf) is used to determine the orientation of the mobile robot, and local anomalies i.e. disturbances of the emf are used as features for positioning and navigation algorithms [4], [6]. magnetometer data-based algorithms and methods for navigation and mapping are relevant both for indoor usage and usage in urban environments. such areas have many objects that perturb the emf, which is expected to be homogeneous and stable in the mobile robots’ operation areas. modern research has created systems for building maps of the magnetic environment along with maps based on data from lidars and cameras. to build a map of the magnetic environment, singleor multi-axis magnetometers are used. they can be combined into magnetometer systems and integrated into the exterior receptive sensor system of the mobile robot. even a carefully calibrated magnetometer system combined with accelerometers is subject to influences of static, folding, retractable, and other elements of the robot, as well as its flowing currents. distortion of magnetometer estimations caused by such influence may be mitigated by algorithms based on machine learning and the use of redundant data recorded by proprioceptive mobile robot sensor systems [9]. the quality of the compensation depends on the model of formation and the propagation of disturbances. the perturbation model is created abstract an important addition to inertial navigation systems is the magnetometer. areas with magnetic field anomalies serve to determine the reference points. however, magnetometers can be influenced by both the robot’s configuration and its electrical equipment. compensation for the robot’s self-influence on the readings of the magnetometers is carried out by computer tools. in order to obtain the initial data, live experiments are required in a natural environment. to simplify data acquisition concerning the behaviour of the magnetometric systems of a mobile robot, a special facility that allows for the local compensation of the earth’s magnetic field is used, and an artificial magnetic field that varies according to a predetermined algorithm is created. using this facility, we can also simulate the magnetic field that will be present in the intended environment of the application of the robot. the facility features are a working space that is sufficient to place the mobile robot; a coil temperature drift correction; uniformity of the frequency response in operating frequency range; compensation for the power supply interference and similar disturbances; sensitivity equalisation of control channels; compensation for the misalignment of the sensor’s and coil system’s coordinate systems. an interactive simulink model is designed and evaluated. the automated stand is created as an experimental facility, its parameters proving the proposed model’s adequacy. mailto:likvon@list.ru acta imeko | www.imeko.org december 2019 | volume 8 | number 4 | 48 in a special environment known as ‘magnetic silence’, which significantly excludes sources of influences that are external to the robot, such as indoor engineering networks, specifically the power supply. even influences from reinforced concrete beams and a metal-coated floor may be significant. for the model evaluation in [9], the authors were obliged to build an experimental site in the utah desert. 2. magnetometers calibration stand it is common practice to calibrate magnetometers using facilities based on the helmholtz coil system (or some similar coil system) and functional generators. these facilities can form an artificial magnetic field (mf) that is comparable with emf. this homogeneous mf can be constant or variable according to a given function and may simultaneously mitigate the emf as well as its perturbations of both an anthropogenic nature and magnetic storms. the control channel is implemented, both open [10]-[12] and closed [13]. the authors used this approach to design an automated stand for magnetic field reconstruction, aiming to solve the problem of determining perturbations caused by mobile robot elements. a mobile robot is placed in the test zone of a threecomponent contour system, where a series of experiments is performed. during these experiments, the robot’s executive subsystems operate in various modes. the reproducible mf is either constant or variable according to the specified function e.g. simulating the rotations of a mobile robot. the prerecorded set of magnetic induction vectors serves as a training set for designing and evaluating various models that are invariant to the self-influence of the robot. the stand can also be used to debug and benchmark magnetic-based feature extraction algorithms. first, the changes in the magnetic field vector are recorded by the precision three-axis magnetometer during the robot’s normal operation. second, the robot is placed inside the contour system, and the recorded data is played back to debug and verify the algorithms. most emergency mobile robots can fit into a space of about 1 m3. however, we encountered several problems while building a coil system with such test zones. the current transmission coefficient decreases with the increase in the test site space. to mitigate this issue, we increased the number of coil turns. that caused an increase in the inductance of the coils and the narrowing of the operating frequency range. increasing the control currents, in turn, requires non-trivial solutions for their formation, preserving low levels of interference. furthermore, changes in currents caused by temperature drift of the coil resistance cannot be ignored in this case. this leads to deviations of the mf, with the magnitude comparable to that of the emf. the mf stabilisation in the test site space is impossible without a closed loop system. feedback can be realised either with a separate mf sensor or with the robot’s onboard sensor. the coaxial arrangement of the sensor and the helmholtz coil circuit system also ensures the correct operation of the closed loop control system. the mechanical arrangement of the coordinate systems cannot be sufficiently accurate because mobile robots are usually not equipped with tools for precise orientation estimation. in addition, in a sequence of experiments, a robot is not always installed in a similar way; rather, it takes many different positions. the residual misalignment should be mitigated by the program setting of the test site. 3. automated stand model let us dwell on the problem of stabilising the mf in the test site space. to solve it, one must compensate for the misalignment of the coordinate systems of the sensor and the contour system as well as eliminate the power supply interferences and the influence of the temperature drift of the ring resistance. the proposed solution is explained using the simulink model, which is a convenient interactive verification tool. the model of the automated stand for the reconstruction of the mf is shown in figure 1(a). the rings block defines the model of the contour system as a transmission function ( ) 1.4406 w s 0.0008s 0.8 = + for each reproducing channel. this transmission function is obtained as a result of identification of a real helmholtz coil with a diameter of 2 m, with seven turns of copper wire and a crosssectional area of 2 mm2. the mul1 block determines the differences in the current transmission coefficients of the coils by multiplying the input signals by the matrix 0.9 0 0 m 0 1.0 0 0 0 1.1     =      . the translation factors here are chosen for demonstration purposes. the sensor module simulates the transmission function of a three-component magnetic induction transducer ( ) 2 2 1 h s 0.000514 s 0.0019s 1 = + + for each of the channels ( )h s . function ( )h s is an estimate of the transmission function of the serial sensor hb0302.61a, which was used during the experiments. the time step of the measurement data is 1 ms and is defined by the zero-order hold block. it should be noted that the sampling interval of the entire simulink model is 50 µs; therefore, the channel for reproducing the mf is 20 times faster than the measuring channel. the signals of the magnetic induction vector t x y zb b b   assigned for reproduction are combined by the mux block into a vector signal; therefore, all other signals of the simulink model have three components. the rm1 block specifies the misalignment of the coordinate systems of the contour system and the magnetic induction sensor. this is done by multiplying the vector of coil currents t x y zi i i   by the rotation matrix 0.9998 0.0118 0.0146 r 0.01 0.9933 0.1152 0.0158 0.115 0.9932 −    =    − −  . the settings of the rotation matrix are based on the rotation of the mf sensor’s reference system in relation to ψ= π 23− , 27θ=π , 25φ=π in the z x z− − order. identification of this matrix in practice is performed using the formula ( ) 1 t t r ds ss − = , where d is the matrix of n test current vectors sequentially fed to the contour system, s is the corresponding matrix of the components of the magnetic induction vectors measured by the sensor t x y zb b b   . for an accurate estimation of r , it is necessary to reproduce acta imeko | www.imeko.org december 2019 | volume 8 | number 4 | 49 currents close to the borders of the ranges of the test mf, without causing saturation of the coils with respect to the current. it is necessary to take into account the displacement of the dynamic ranges of the channels by the magnetic induction, which is connected with the presence of a constant emf in the region of system installation. the main feedback loop for the mf sensor closes on sum1. before the input signal reaches that block, it is multiplied by the matrix t r , called r in simulink notation. this multiplication is done by the blocks rm2. these blocks are designed to compensate the misalignment of the axes of the sensor and the contour system. the difference signal from sum1 is fed to the regulator reg1 with a transmission function ( ) 0.2043z 0.1883 v z z 1 − = − . its amplification factor is interactively tuned with the k_gain1 block. in addition to the k_gain1 block, there are also amplitude, k_int, ph, amp, k_gain2, temp and b0 interactive tuning blocks. all are built using the standard slider gain simulink block. the background mf level and its perturbations are interactively set by the b0 block and then added by sum4 block to the field created by the coil system. the sum5 block introduces changes in the mf inside of the coil system, which are caused by the power supply interference. it is a 50 hz sine wave, set by the sin block with interactively arranged amplitude (amp block) and phase (ph block). the main feedback loop compensates for the constant or slowly changing external mf, but it does not satisfactorily mitigate the power supply interference. for that reason, another feedback loop was introduced with the regulator reg2, built as a modification of the adaptive rejection filter for suppressing this interference [14]. the regulator’s structure is represented in figure 1(b). the additive mixture of the useful signal and the power supply interference from the main feedback loop output is fed to the input of the adaptive regulator reg2. the reference 50 hz sine wave is generated by the sin generator with a 50 µs sampling interval of the simulink model. the reference signal with a delay of 100 sampling intervals, created by the delay a b figure 1. (a) simulink model of the automated stand for the reconstruction of the magnetic field. (b) the structure of the adaptive regulator reg2. acta imeko | www.imeko.org december 2019 | volume 8 | number 4 | 50 block, is converted into a 90 phase shifted signal. the reference signal and its shifted copy are multiplied with the input signal of the regulator reg2, and the results are fed to the digital integrators integrator1 and integrator2. the corresponding integration results are weight coefficients of the adaptive regulator. the weighted reference signal and its shifted copy are summed, resulting in the regulator output, which is subtracted from the output signal of the main regulator reg1. adjustable weighting values allow changing the reference signal in amplitude and phase, thus obtaining the necessary signal characteristics to suppress the power supply interference. to change the speed of adaptation, amplitude and k_int blocks can be used. the former is used to interactively adjust the amplitude of the reference sine wave. the latter adjusts the integration factor, which is the same for both integrator1 and integrator2. the temperature stabilisation loop includes system blocks associated with helmholtz coil’s currents generation. the output, which contains coil currents for each of the spatial axes, is converted with the scale factor defined by the mul2 block and then fed to the summation point sum3. the signal compensating for the power supply interference also enters at this point. the feedback transfer ratio depends on the parameters of the coils and the electronics of the regulated current source, so it must be selected during the initial system calibration. the regulator reg3 in the direct transfer circuit eliminates the residual steady-state error of the control channels, and the block k_gain2 allows for an interactive change of the parameters of the current generating unit. the temperature error is modelled by the interactive temp block and is calculated using the current signals of the loop system. figure 2 shows the results obtained at points 1, 2, 3, and 4 of the simulink model. for effective demonstration of the proposed solution, let us designate three sawtooth signals offset from each other as changes in the component of the magnetic induction vector (point 1). the signals at point 2 describe the field formed by the helmholtz coils. the graphs at point 3 show the result of the superposition of the field being formed and the local magnetic field and its perturbations. at time t 0.25= s, the amplitude and phase of the sine wave of the power supply interference are abruptly changed, and at time t 8.75= s, the level of the background mf is abruptly changed. transient processes caused by these disturbances, are shown in the graph of field reproduced by the stand (figure 2, point 3) and the graph of superposition of this field and perturbations at the sensor output (figure 2, point 4). the system needed 10 ms to adapt to a new external field and 3.5 s to eliminate the power supply interference. transient processes caused by an abrupt increase in the coil’s temperature are shown in figure 2, point 3 at 9.25 s. system stabilisation took around 5 ms. at point 4, the temperature disturbance is imperceptible due to the sensor sampling rate. such experimental conditions are stressful and not used in practice; for that reason, the values obtained can be considered sufficient for solving the designated tasks. the restored parallelism of the sawtooth signals at point 3 indicates that the compensation for the misalignment of the sensor, and the contour system was successfully performed. the resulting mf magnitude, which is shown at point 3, is equal to the programmed one (point 1). 4. automated stand implementation the automated stand for the mf reconstruction, which was made as an experimental facility, is presented in figure 3. its performance proves the proposed simulink model’s adequacy. 1 2 3 4 1 4 2 0 6 0 2 3 4 5 6 7 98 10 1 4 2 0 6 0 2 3 4 5 6 7 98 10 1 4 2 0 6 0 2 3 4 5 6 7 98 10 1 4 2 0 6 0 2 3 4 5 6 7 98 10 4 10 4 10 4 10 4 10 figure 2. signals at points 1, 2, 3 and 4 of the simulink model. x-axes in seconds, y-axes in nanotesla. acta imeko | www.imeko.org december 2019 | volume 8 | number 4 | 51 due to the small number of independent variables in the model, it is possible to interactively tune the regulators’ parameters instead of using predefined constant close-tooptimal values, which should be estimated analytically beforehand. operation of the model is preferred due to the wide variety of mobile robots and magnetometers used. the automated stand is used to experimentally evaluate the robot’s self-influence on its magnetometer system. for that, we chose a method similar to that shown in [9], where the process of compensation for the robot’s self-influence is described. according to [9], the system-immanent self-influence can be either static or dynamic. the main sources of such distortions are changes in motor currents and changes in the configuration of the robot and its parts, since they also modify the robot’s magnetic field. this, in turn, causes deviations in readings of onboard magnetometers. these readings, however, are used to describe the uniform magnetic field. moreover, the position of the magnetometer relative to the robot can also cause deviations in sensor readings. to compensate for these deviations, in [9], the authors used a neural network based on the dependency between the deviations and the corresponding self-influences. however, to create a training set for such network, one needs a magnetic field that is as close to uniform as possible. in [9], the utah desert was chosen as one. in this article, we have aimed to create a uniform magnetic field with the automated stand and to use changes in coil currents as the desired magnetometer deviations for the training set, since these changes are proportional to the deviations in magnetometer readings. the experimental procedure, consisting of several stages, is shown in figure 4. during the first stage, a reference magnetic field (figure 4, black arrows) is created inside of the contour system. the onboard magnetometer, at this point, is considered a part of the stand and is placed into the feedback loop. the contour system currents are considered to be reference ones. during the second stage, a mobile robot is placed inside the contour system. the same magnetometer, which is now mounted on the robot, is again used in the feedback loop. the distortions caused by static self-influence of the robot lead to changes in the contour system’s coil currents. the difference between the reference and the present currents is used to form the training set for the static self-influence compensation algorithm. during the third stage, all the robot’s subsystems are turned on. that causes additional distortions, which are compensated for using the same onboard magnetometer placed in the feedback loop. the difference between the reference and the present currents is used again to form the training set. based on this set, both static and dynamic system-immanent distortions can be mitigated. another use case of the stand is the determination of the frequency characteristics of the magnetometers. after transmitting a test sinusoidal signal with a linear frequency modulation to the coil system and simultaneously maintaining the specified amplitude of the coil currents, the unit will perceive all the frequency properties of the sensor block. the process of generating the frequency response of the system is shown in figure 5. in figure 5, we demonstrate the response of the system to a linear fm impact at points 2 and 4. the frequency characteristic of the magnetometer is determined by the correlation of the signal envelopes at points 2 and 4. the graphs in figure 5(a) illustrate the influence of an adaptive regulator, which eliminates power circuit noise on the response of the system. the work of the regulator distorts the recorded (a) (b) figure 3. automated stand for magnetic field reproduction. (a) a three-component system of helmholtz coils. (b) a control block. i ii iii – magnetometer – mobile robot with moving parts – reference state – static disturbances – both static and dynamic disturbances figure 4. the experimental procedure for the training set creation. acta imeko | www.imeko.org december 2019 | volume 8 | number 4 | 52 response, which is clearly evident at 50 s of the simulation process. it is possible to eliminate the influence of adaptive regulator by holding the characteristics of compensating signal (fed on the input of a summation point sum3) fixed for the whole period of estimating the frequency response of the system. the graphs in figure 5(b) illustrate the response of the system to a linear fm impact with a ‘frozen’ compensating signal. 5. future works in future developments, all components of the system (i.e. the interactive model, the supporting software, and the hardware of the stand) will be improved. the work will be focused on the three main topics enumerated below. first, it is necessary to establish a procedure for both the automated calibration of the stand and the formation matrices r and m . second, the interactive model will be used to create procedures for estimating the residual nonorthogonality of a helmholtz coil circuit system, which could be used later to precisely form the artificial magnetic field. third, it is important to analytically estimate the adaptive regulator’s influence on the estimation of the magnetometer’s frequency characteristics. with this information, we can estimate the frequency characteristics of magnetometers without changing the working mode of the automated stand. references [1] a. solin, m. kok, n. wahlström, t. b. schön, modeling and interpolation of the ambient magnetic field by gaussian processes. corr abs/1509.04634 (2015). [2] j. haverinen, a. kemppainen, global indoor self-localization based on the ambient magnetic field. robotics and autonomous systems 57(10) (2009) pp. 1028-1035. [3] i. vallivaara, j. haverinen, a. kemppainen, j. röning, ‘magnetic field-based slam method for solving the localization problem in mobile robot floor-cleaning task’, proc. of the 15th ieee international conference on advanced robotics: new boundaries for robotics, icar 2011, 20-23 june 2011, tallinn, estonia, pp. 198-203. [4] m. angermann, m. frassl, m. doniec, b. j. julian, p. robertson, ‘characterization of the indoor magnetic field for applications in localization and mapping’, proc. of the ieee international conference on indoor positioning and indoor navigation, ipin 2012, 13-15 november 2012, sydney, australia, pp. 1-9. [5] e. l. grand, s. thrun, ‘3-axis magnetic field mapping and fusion for indoor localization’, proc. of the ieee international conference on multisensor fusion and integration for intelligent systems, mfi 2012, 13-15 september 2012, hamburg, germany, pp. 358-364. [6] m. frassl, m. angermann, m. lichtenstern, p. robertson, b. j. julian, m. doniec, ‘magnetic maps of indoor environments for precise localization of legged and non-legged locomotion’, 2 4 2 4 a b 4 10 4 10 4 10 4 10 0 -2 -4 -6 2 4 6 0 -2 -4 -6 2 4 6 0 -2 -4 -6 2 4 6 0 -2 -4 -6 2 4 6 0 10 20 30 40 60 7050 80 90 100 0 10 20 30 40 60 7050 80 90 100 0 10 20 30 40 60 7050 80 90 100 0 10 20 30 40 60 7050 80 90 100 figure 5. responses to the linear frequency modulated signal. (a) response with the activated adaptive regulator: left – coil system response, right – sensor data. (b) the adaptive regulator’s output signal is ‘frozen’ before the linear fm generation. x-axis in seconds, y-axis in nanotesla. frequency sweep – 1 hz/s. acta imeko | www.imeko.org december 2019 | volume 8 | number 4 | 53 proc. of the ieee/rsj international conference on intelligent robots and systems, 3-7 november 2013, tokyo, japan, pp. 913-920. [7] n. lee, s. ahn, d. han, amid: accurate magnetic indoor localization using deep learning. sensors 18(5) (2018) p. 1598. [8] a. kemppainen, i. vallivaara, j. röning, ‘magnetic field slam exploration: frequency domain gaussian processes and informative route planning’, proc. of the ieee european conference on mobile robots, ecmr 2015, 2-4 september 2015, lincoln, united kingdom, pp. 1-7. [9] l. christensen, m. m. krell, f. kirchner, ‘learning magnetic field distortion compensation for robotic systems’, proc. of the ieee/rsj international conference on intelligent robots and systems, iros 2017, 24-28 september 2017, vancouver, bc, canada, pp. 3516-3521. [10] s. roman, s. jacek, b. adam, k. rafal, s. marcin, testing of the three axis magnetometers for measurements of the earth magnetic field. journal of automation, mobile robotics and intelligent systems 3(4) (2009) pp. 96-98. [11] p. mahavarkar, j. john, v. dhapre, v. dongre, s. labde, triaxial square helmholtz coil system at the alibag magnetic observatory: upgraded to a magnetic sensor calibration facility. geoscientific instrumentation, methods and data systems 7 (2018) pp. 143-149. [12] c. f. de melo, r. l. araújo, l. m. ardjomand, n. s. r. quoirin, m. ikeda, a. a. costa, calibration of low frequency magnetic field meters using a helmholtz coil measurement 42 (2009) pp. 1330-1334. [13] a. r. alvarez, e. f. mejía, h. c. ramírez, c. p. jaramillo, a simple geomagnetic field compensation system for uniform magnetic field applications. revista facultad de ingeniería 0(83) (2017) pp. 65-71. [14] b. widrow, s. stearns, adaptive signal processing. prentice hall, englewood cliffs, nj, 1985. acta imeko, title acta imeko issn: 2221-870x september 2017, volume 6, number 3, 52-56 acta imeko | www.imeko.org september 2017 | volume 6 | number 3 | 52 the metrological research of the machu picchu site. application of a cosine quantogram method for 3d laser data anna kubicka wroclaw university of science and technology, ul. bolesława prusa 53/55 50-317 wroclaw, poland section: research paper keywords: point cloud; length measurements, inca architecture; statistic citation: anna kubicka, the metrological research of machu picchu site. application of a cosine quantogram method for 3d laser data, acta imeko, vol. 6, no. 3, article 8, september 2017, identifier: imeko-acta-06 (2017)-03-08 section editor: sabrina grassini, politecnico di torino, italy received march 14, 2017; in final form july 31, 2017; published september 2017 copyright: © 2017 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: this work was supported by national science centre of poland corresponding author: anna kubicka, e-mail: kubicka.ania@gmail.com 1. introduction during the expansion of the inca empire from about 1430 to the spanish conquest in 1532, (chronology based on john rowe [1]) the inca created an infrastructure as well as small and large settlements like a read network, vast irrigation system, agricultural terraces, suspension bridges and many more to launch a massive construction program which marked their presence across the andean region. one of these settlements was machu picchu – an architectural investment generally associated with the inca pachacuti. beginning with the bingham publication [2] machu picchu has been identified as a fortified city, outpost into tropical forest, a sanctuary dedicated to the moon, a center for “chosen woman”, the last refuge of incas, a ceremonial canter. in spite of theoretical discussions about the function, the masonry of buildings in machu picchu is without doubts very fine and varied, without adobe bricks. these features and the presence of temples and sacred places on machu picchu indicate that it was a ceremonial center of great importance. the state wanted to show imperial investment and firm an intention of their presence in the area to the neighboring people. nonetheless, the settlement does not have distinctive characteristics for a city with a probable population of one thousand inhabitants [3]. the settlement is divided into agricultural and urban zones with an entrance to the residential area from the south (figure 1). the general layout of the settlement seems not to be achieved coincidentally but rather deliberately planned. it is visible in the spatial organization of the complex where each sector has a specific border carefully arranged on andenes (agricultural terraces). a water system must have been planned earlier in order to create a water channel system to all sacred structures on machu picchu. the purpose of this research is to look for a basic unit or units of measure (quantum), multiplication of which would help delineate the outline of the machu picchu site. search for a unit of design could determine the fact if the inca used one imperial abstract the aim of a metrological analysis of the machu picchu site is to verify the hypothesis on the functioning of the imperial system of length measurement which was used by the incas during measurement and construction processes. data for metrological analyses were obtained from 3d laser scanning as 3d point cloud from where desired length measurements were collected. as far as the research method is concerned, a statistical model of a cosine quantogram was used to find a unit of design from a data set. the method has successfully been introduced during the analysis of architectural sites of the mediterranean culture but never has been applied in regard to pre-columbian archaeology. statistical approach in this study will reveal new information about inca urban planning based on the elements of architecture design. mailto:kubicka.ania@gmail.com acta imeko | www.imeko.org september 2017 | volume 6 | number 3 | 53 measurement systems and construction rules in their urban planning or, quite the contrary. the ethnic diversity of people working on the construction of the machu picchu settlement brought with them a local tradition of measurement which caused more than one quanta in building architecture. a statistical model of a cosine quantogram has successfully been implemented during the analysis of architectural sites of mediterranean culture as well as european medieval urbanism [4]. written sources from greek, roman (e.g. isidore of charax, sextus julius frontinus, lucius junius moderatus columella) or egyptian times give an opportunity to prove the usage of the metrological system in urbanism or architectural design. 2. inca mathematic precision as john rowe said some years ago, “information on inca units of measurement is relatively abundant, but so scattered and unsystematised as to give the impression that the inca had no very precise standards. actually, inca skill in engineering works almost required a system of measurement at least as exact as that in use in 16th-century europe” [5]. the art of cutting stones is, without a doubt, a very astonishing achievement of precision. it gives an impression that all inca buildings were constructed of cut and fitted stonemasonry, but most of them were built of semi-cut or unworked fieldstones laid up in an argillaceous mortar. undoubtedly, finely worked stones show an important function of a building but it does not determine an interpretation. in the machu picchu case, most buildings were constructed with mortared fieldstone masonry. in this construction, masonry is quite varied. in the walls, corners or frames of doors or niches stones were partially worked to fit into their position as well in corners of buildings where headers and stretchers are alternated providing excellent stability. fieldstone masonry may have been coursed or uncoursed which depends on the shape and size of stones used. in coursed masonry stones are roughly the same height. they are usually 65 to 90 cm thick, but most of them measure close to 80 cm [6]. the precision in inca masonry and civil engineering was not only present in this field of inca material culture. knotted string devices: quipu and an abacus calculating devices express inca mathematical knowledge. construction and use of quipu were mentioned in many spanish sources as an extremely accurate accounting device. data was encoded there using a decimal system indicated by the size and position of the knots. marcia and robert ascher [7] provided some ideas concerning inca mathematical abilities based on their studies of quipus: arithmetic ideas used by incas must have included at a minimum: addition, division into equal parts, division into simple unequal fractional parts, and multiplication of integers by integers and fractions. 3. architecture and methods of building layout the area of the empire was covered by many different ecological zones so to conquer this varied landscape the inca introduced architectural practice which could have adapted to these diverse regions and climates. alexander von humboldt, the german naturalist from early nineteenth century described inka architecture as: “uniformity of construction… one should think that a single architect has built this large number of monuments” [8]. in inca architecture, a basic form of settlement exists which is a single-room structure. based on the older central andean building tradition this form was used to create a diversity of settlement types across the inka empire. they created a variation of form by making small but important changes, which allowed to enrich functionality and made architecture highly standardized. structural elements of inca architecture are constantly repeated. the interior of buildings is occupied with niches or windows arranged in a row in specific intervals, trapezoidal shaped with the shortest side on top. they placed them in all kinds of buildings (not only in temples like other andean cultures.) niches are usually situated on 120 to 150 centimetres above the floor level and their functions are different, depending on their location and size. the doorways are of similar shape, and on both these elements, incas located a monolithic stone or wooden lintel. last architectural detail visible inside and outside of a building structure are pegs. outside they were used to tie a roof to a gable wall and inside probably as hangers located symmetrically between the niches on the same height. a distinctive feature of inca walls, which obviously has an influence on architectural planning, is the construction of battered walls. these have a sloping or trapezoidal cross section (from 3 to 5-degree), which is also often visible in a plan of a building. the function and purpose of this feature is based on statics of a building, where the thrust of the roof beams with a thick thatch roof could be more easily absorbed. based on wedge-stones, found in situ on ollantaytambo, we can assume that some of the walls were constructed simultaneously from both ends and the working groups reached a point where was a gap to put only one more stone from the front. this type of wedge stones were set in a regular way on sites like qorikancha, saqsaywaman [9]. that is not a pattern of wall construction. yet careful observation of construction features is needed for each type of wall. based on the knowledge of inca architecture features building layout existed at least of three levels: a foundation, a level of niches or windows and occasionally a floor level where there is a doorstep in situ. these studies are focused on noninvasive techniques of documentation so it is not possible to reach a foundation level or even establish an assumption level because of the irregular bedrock of the mountain. in buildings where the floor level is preserved by a doorstep, a layout of the building is reconstructed but in most cases, it is not visible. however, a known level of construction remains on the elevation of niches and windows. they are wrapped around all four walls with a symmetrical distance between them. measurements of niches and windows can be different from figure 1. plan of machu picchu with a marked area of investigation, source: g. gasparini, l. margolies, inca architecture, indiana university press, 1980, p. 89. acta imeko | www.imeko.org september 2017 | volume 6 | number 3 | 54 building to building but their arrangement on the wall is standardized. they are always symmetrically oriented on the wall with a door in the middle or two doors on equal distance, so it means that a length of the wall has to be known previously. niles [10] presented a theory that the walls were built up to the desired height of the sills of the niches. then the frames of niches were constructed at the desired intervals. when all the frames had already been placed, gaps between them were filled and additional courses of masonry laid on the top (figure 2). for that reason types of length measurements included in the dataset were on the level of niches which most likely have standardized measurements due to their uniformity in buildings and in some cases layout of the floor level. 4. methodology 4.1. quantum model due to the survival of measuring devices like rods or written sources which confirmed a design unit of measure it is possible to determine the fundamental quantum underlying the design and construction. however, for the cultures without this kind of established knowledge, a certain solution is to derive a basic unit of length from architectural dimensions. the length of architecture elements like blocks of stones or decorative elements could be described as muliplication of a basic unit plus small error caused by ancient building execution, where a type of hand tool did not provide a great precision for workers. because of the same manual reason an error could be created by modern measurement techniques. using statistical methods it is possible to derive a basic unit of design from a set of lenght measurements. the cosine quantogram method used in this study was developed by d. g. kendall [11] for detecting a quantum (the basic unit of design) of an unknown size from a set of data. in the case of inca architecture each building dimensions of machu picchu y can be described as an integer m multiples of the basic unit: q, plus an error ɛ as an uncertainty or margin of error of a measurement. 𝑦 = 𝑞𝑞 + ɛ . (1) in (2) ɛ which is significantly smaller than q is analyzed and then the formula calculates an amount which clusters around q. results of the right candidate for a quantum (q) are presented on a line graph as the highest peak (figure 3). n in the formula is a number of dimensions: 𝑓(𝑞) = �2/𝑁 ∑ cos ( 2𝜋𝜀1 𝑞 )𝑛𝑖=1 . (2) for a purpose of the research on machu picchu site, cosine quantogram formula was implemented into a javascript to simplify the calculation. all collected data in .txt format is imported with information about searching scope and value of division. works of other scholars proof that the cosine quantogram method encountered a problem if more than one quantum exists in a set of data [12]. for this reason, the machu picchu site was divided into architectural groups with the same function or construction and further calculated separately. 4.2. the samplelength measurements from 3d point cloud. all the data has been collected from the 3d point cloud generated by 3d laser scanners and multistation in recent years and reached a final stage this year. however, for a purpose of this paper around 50 % of the scanned site was investigated and another half is currently in processing (figure 1). in spite of this a set of data should be considered as one integral part because the area of the 3d point cloud constitutes an upper part of an architectural complex of machu picchu, where the lower part is on the opposite site of the main square (plaza principal). this division in inca urbanism was presented in many settlements (hanan-upper part and hurin-lower part) [13] so these studies give an opportunity to check if a division in a metrological system of this two sectors exists for this particular settlement. the 3d point cloud obtained from 3d laser scanning or lidar as a type of documentation gives, at that moment in archaeological studies, the most precise measurement results especially if an object is not regular and simple in its form. furthermore, this allowed to create, directly from the obtained data, section and geometry reconstruction for some nonaccessible areas. based on these features for the purpose of 3d documentation and these studies mostly the 3d scanner leica scanstation p40 (accuracy 3 mm at 50 m; 6 mm at 100 m) was used by a team from 3d scanning and modelling laboratory (wroclaw university of technology) and the national archaeological park of machu picchu. the dataset collected from the 3d point cloud was first transformed into vectorised plans and sections. each plan contains desired measurements that were created as a line from the pick points (figure 4). data from the upper part of the urbanized site of machu picchu was divided into separate architectural groups, based on subdivision of modern site organization (figure 5). 5. conclusions the residential area was divided into three groups: 2a, 2b, 3 with the following numbers of measurements: 50, 84, 34. a figure 3. graph presenting a quantum for sector 2b with a score 6.7 (significance level of a peak). figure 2. view inside the inca building on machu picchu. construction level of niches marks with a line of dots; photo: author. acta imeko | www.imeko.org september 2017 | volume 6 | number 3 | 55 sector with a temple of the sun was created separately and included 128 measurements. the building of the so called royal estate has 60 numbers of length measurements. the distribution of all measurements suggests that two quanta may exist in the investigated data set: 0.27 m and 0.41 to -0.44 m. the estimation of the quantum q for the residential sectors 2a, 2b and 3 is 0.41-0.43 m (figure 6). the estimate of q for the temple of the sun and for the building of sacred square is 0.27 m (figure 7). accompanying buildings of the temple has quantum 0.42 m. for a royal estate area estimate q is 0.44 m. there is no significant difference of quantum between the residential buildings and storage buildings: 0.41 to 0.44 m. however, for sacred buildings, the quantum is completely different: 0.27 m. in order to exclude senseless quanta, lower and upper bounds were placed into a program to the limited value of quantum: 0.02 to 1.60 meters. measurement systems based on body proportions are still common in latin america. the smallest unit of measure is the finger: between 0.05 and 0.06 m and upper bounds capture the estimated value of fathom: 1.62 m [14]. the result of a quantum could be compared with the body proportion of cubit (quechua: khococ) [15] as a distance around 0.45 m. for 0.27 m an equivalent in the human proportion in latin america does not exist, because a length of the foot is usually 0.25 m. it is possible that information about this unit of measurement does not exist in hispanic sources but it fits as a part of fathom measure: 6×0.27=1.62 m. throughout the history of civilization, the system of urban planning defined the level of development. inca architects/engineers and stone masons were professionals excluded from labour tax. they were supported by the state and worked on the building project but a scale of the participation in the construction process of the settlements plans is unknown [16]. two significant length measurements were used in the residential sector of machu picchu, which suggests the stronger influence of imperial regulations. this phenomenon shall complement our knowledge of machu picchu, as well as other sites on the inca territory, in terms of function and the process of creation. references [1] j. rowe, “inca culture at the time of the spanish conquest”, in: julian h. steward (ed.) handbook of south american indians, vol. 2.,june 2012, pp.323-325. [2] h. bingham, h. thomson, lost city of the incas, phoenix press 2003. [3] g. gasparini, l. margolies, inca architecture, indiana university press, 1980, pp. 79-91. [4] j. pakkanen, “deriving ancient foot unit from building dimensions: a statistical approach employing cosine quantogram analysis”, in: burenhult, g. and j. arvidsson (eds.) archaeological informatics: pushing the envelope. caa2001; cox s. m., determining greek architectural design units in the sanctuary of the great gods, samothrace: application of and extensions to the cosine quantogram method, web source: https://etd.library.emory.edu/view/record/pid/emory:19n1r (09.12.2014); crummy p., “the system of measurement used in town planning from the north to the 13-th cent.”, in: anglo saxon studies in archaeology and history, (ed.) hawkes, d .brawn., campbel, oxford 1979. [5] s. niles, callachaca: style and status in a inca community. iowa city 1987, pp. 217-225. [6] j. rowe, “inca culture at the time of the spanish conquest”, figure 4. example of the 3d point cloud during a preparation of data in leica cyclon. figure 5. architectural groups of building measurements. figure 6. results of quantum for residential sector. figure 7. results of quantum for the sacred square and the building of the sun temple. https://etd.library.emory.edu/view/record/pid/emory:19n1r acta imeko | www.imeko.org september 2017 | volume 6 | number 3 | 56 in: julian h. steward (ed.) handbook of south american indians, vol. 2.,june 2012, p. 323. [7] j.p. protzen, inca architecture and construction of ollantaytambo, oxford 1993, pp. 157-164. [8] m. ascher, r.ascher, mathematics of the incas: code of the quipu, new york 1981, pp. 133-143. [9] a. humboldt, sites des cordillères et monuments des peoples indigènes de l’amérique, paris 1896,p. 449. [10] j.p. protzen, inca architecture and construction of ollantaytambo, oxford 1993 pp. 195-223. [11] d.g. kendall, haunting quanta, philosophical transactions of the royal society of london, a276, 1974, pp. 231-66. [12] s. mustonen, hunting multiple quanta by selective least squares, websource: http://www.survo.fi/papers/huntingquanta2012.pdf (10.10.2015). [13] g. gasparini, l. margolies, inca architecture, indiana university press, 1980, pp. 79-82. [14] agurto calvo, s., estudios acerca de la construccion, arquitectura y planeamiento incas, camera peruana de la construccion, lima 1987, pp. 255-274. [15] m. rostworowski de diez canseco, mediciones y computos en el antiguo perú, cuadernos prehispanicos 6, seminario amerycanista 1988. [16] j. hyslop, inka settlement planning, austin 1990, pp. 27-28. http://www.survo.fi/papers/huntingquanta2012.pdf the metrological research of the machu picchu site. application of a cosine quantogram method for 3d laser data acta imeko, title acta imeko issn: 2221-870x june 2018, volume 7, number 2, 10-15 acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 10 grain size of marine sediments in the environmental studies, from sampling to measuring and classifying. a critical review of the most used procedures elena romano, maria celia magno, luisa bergamin ispra, italian institute for environmental protection and research. via v. brancati 60, 00144 rome (italy) section: research paper keywords: marine sediments; sampling sediments; grain size analyses; classification of sediments citation: elena romano, maria celia magno, luisa bergamin, grain size of marine sediments in the environmental studies, from sampling to measuring and classifying. a critical review of the most used procedures, acta imeko, vol. 7, no. 2, article 3, june 2018, identifier: imeko-acta-07 (2018)-02-03 section editor: fabio leccese, università degli studi di roma tre, italy received january 15, 2018; in final form march 23, 2018; published june 2018 copyright: © 2018 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: elena romano, e-mail: elena.romano@isprambiente.it 1. introduction different marine environments are usually characterized by different sediment types because, in general, coarser sediments are present close to the coast, at shallow water depth, while finer sediments raise with increasing bathymetry. however, there are exceptions to this general rule, because fine sediments may be found not only in deep water environments but also in enclosed marine and transitional settings, due to the low environmental energy, while sandy sediments may be present offshore, as a result of quaternary glacial low-stand or gravitational processes. hydrodynamics, meteorology, climate and bottom morphology are the main factors influencing the distribution of sediment particles once they are supplied to the marine environment, where they may undergo repeated resuspension, transport and deposition over the continental shelf and slope, before being definitively deposited. since sediment texture influences a lot of important biotic and abiotic aspects in the marine environment, this is considered a key factor in many fields. for example, diversity and composition of macro and meio benthic communities [1], [2], as well as organic carbon [3] and contaminant distribution [4], are strongly conditioned by sediment grain size. consequently, the sediment texture has to be determined not only for sedimentological studies, but also for the ecological characterization of benthic habitats and/or for the environmental assessment of marine coastal areas affected by anthropogenic impact. in this context, grain size is also required for normalization procedures of metal and trace element concentrations [5]. another important application of grain size data is for the correlation of seismic units, recognized in the profiles, with the corresponding stratigraphic ones [6] or for the calibration of remote sensing data [7]. another application of sediment grain size is in the field of coastal management; in particular, the beach nourishment requires sediment characterization because the features of the borrow area must be as similar as possible to those of the native beach [8]. abstract the knowledge of sediment texture is a main issue in the marine environmental research. studies on sediment contamination, ecology of benthic communities, seismic studies, remote sensing surveys and beach nourishment are among the research areas which benefit of information on sediment grain size. in this review, the main methods used for sampling, measuring and classifying marine sediments are critically illustrated in order to highlight their strengths and weaknesses. from this revision it was deduced that it does not exist a single procedure, considered as the best one, to be applied in all the cases for obtaining reliable grain size data. this result can be achieved using as much as possible flexible strategies, and adopting the suitable sampling devices and analytical instruments based on the overall characteristics of the study area and the specific aim of the survey. acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 11 the activities finalized to the acquisition of grain size data should be planned taking into account the topic of the research, and the methodologies of sampling, analysis and classification of sediments may follow different procedures, according to specific needs. nevertheless, due the importance of the environmental studies for their repercussions on management policies, there is the need of carrying out accurate and reliable measurements [9]-[11]. aim of this review was to describe the most used procedures as regards sampling, analysis and classification of marine sediments from a critical viewpoint, in order to offer the conceptual tools to adjust the methodologies for obtaining as most as possible reliable data, according to the specific requirements of the study. 2. sampling in most cases, surveys finalized to environmental studies comprise the collection of samples for multidisciplinary analyzes, including grain size. devices and sampling methods are finalized to the collection of undisturbed marine sediments, because the preservation of physical structure and chemical composition of the sample is necessary to obtain reliable data on physical, chemical and biological characteristics. the need to maintain the original sedimentary sequence is related to its chronological significance, which allows attributing to a unique historical moment the analytical data obtained from a specific sediment layer. in addition, the chemical contamination of sediment samples, due to the use of unsuitable devices, sampling and subsampling methods, determines the attribution of unreal chemical properties to the sediment layer deposited during a definite time range. minimizing effects on the chemical and physical integrity of the sample is of great importance for studies in the field of the environmental research, because the use of unsuitable devices or sampling procedures may result in the modification of typology and bioavailability of contaminants. two main groups of sampling devices, employed for different purposes, are grabs and corers. grabs are used for collecting superficial sediment, normally in a large number of stations, to characterize spatially the study area. differently, corers, which collect a whole thickness of sediment, supply information along the vertical direction that, according to sedimentation principle, provides the chronological record of past conditions. 2.1. devices for collecting superficial sediments an appropriate sampling method for characterizing current environmental conditions of sea bottom as regards chemical, biological and sedimentological parameters implies the use of grab, which is relatively easy and fast to handle and operate, allowing the collection of a lot of samples in a short time. the most commonly used are: van veen grab, ekman-birge grab and shipek grab (figure 1). the van veen grab is constituted by two jaws. as soon as the grab comes down to the bottom, the jaws close by holding the sediment inside. some models have upper doors that allow collecting the nearly undisturbed upper centimeters of bottom sediment, directly inside the device. this kind of grab is used to collect sediment ranging from muddy to sandy. the ekman-birge grab has a box shape with upper windows and lower jaws, and the smaller type can be manually operated by means of a rod that allows the insertion into the sediment. it is mainly used in shallow water environments with muddy sediments like as lagoons or marshes [12]-[14]. the shipek grab has a bucket that rotates into the sediment when it reaches the sea bottom. it is suitable to collect fine sediments and it can be used even on sloping seabed. 2.2. devices for collecting sediment cores because sediment cores provide chronological information useful to reconstruct the environmental history of the study area, they should be collected when temporal changes of environmental conditions are investigated. the anthropogenic impact on marine coastal systems may be recognized in sediment cores where by changes of the sedimentary and chemical input, associated to the micropaleontological content. deeper core levels, deposited during pre-impacted periods, reflect the reference conditions and may be compared to the recent ones for the assessment of environmental status [15]. for this purpose, the collected core must preserve the in situ conditions of sediment and any kind of cross contamination, especially in the lower levels, reworking of sediment and alteration of the stratigraphic sequence should be carefully avoided during coring and sub sampling phases. the corers used for the collection of marine sediment are mainly of two types: gravity and vibro corers (figure 2). all devices consist of a core barrel, varying in length and diameter, with internal liner used to facilitate the extraction of the core and avoid contamination of the sediment due to the transfer of heavy metals from the core barrel; it also prevents the downward drag of any contaminants during the extrusion of the figure 1. superficial sediment samplers; types of grabs: from the left to the right, van veen, eckman-birge, shipek. figure 2. from left to right: sw-104, rossfelder® and kullemberg corers (photo by ispra). acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 12 core. the gravity corer is equipped at the head with dead weight, variable depending on the number of lead ring masses, which provides the kinetic energy needed to penetrate the sediments. a flap valve, at the barrel top, allows water to escape during coring, but it is closed during pullout, ascent and recovery, to prevent sediment being washed out. a sharp cutting edge, at the end of the barrel, bears the core catcher, necessary to hold the core during the ascent. the length of the core barrel can generally vary between 2 and 6 meters. a wide variety of gravity corers is available; among these, some models, such as the kullemberg one, may be equipped with dead weight up to 1000 kg, which allows deep sediment penetration, but determine some disturbance, mainly in upper layer and mostly at the water sediment interface. these heavy devices have the disadvantage to need a substantial secondary apparatus due to the difficulty of handling [16]. on the other hand, the sw-104 bears lower weight at the head (up to 90 kg) and penetrates sediments with scarce disturbance, preserving the water sediment interface, also thanks to the wider core diameter. nevertheless, it allows the recovery of cores shorter than 135 cm [17]. all gravity corers are mainly used to sample fine sediment bottoms in deep water to allow effective penetration and adequate recovery of sediment. in this environment, the coring method with controlled penetration speed (angel descent) allows to minimize the sediment shortening and deformation [18]. the vibro corer has a structure similar to the gravity one, but it has a different principle for sediment penetration, because it is equipped with an electric-powered mechanical vibrator at the head, which applies thousand of vibrations per minute, to help the penetration of the sharp cutting edge. this corer is able to recover cores from a wide range of sandy sediments [12]. another category of samplers, the box corer (figure 3), is generally used for collecting sub-superficial samples, no more than 30 cm thick, preserving the water-sediment interface. it consists of a stainless steel square box of variable size and a support frame that stabilizes the sampler on the seabed and ensures the vertical penetration. when the box has penetrated into the sediments by means of weights, a scoop cutting the sediment layer and closing the box is released. the large area of sampled sediment allows the recovery of nearly undisturbed samples [17]. a more recent type of sampling device is represented by multicorer (figure 3). it’s a single device constituted by multiple core barrels for collecting cores of undisturbed sediment, including the sediment-water interface. a range of models is available to sample from 4 up to 12 cores in a single deployment. it is widely used for soft sediment sampling in a wide range of environmental applications. the cores are held together by a stainless steel frame. the core tubing (from 300 to 600 mm length) can be manufactured from acrylic, polycarbonate or stainless steel, depending on requirements. the frame is held by a winch that allows a descent and the penetration is due to a hydrostatic damping system that eliminates the typical bow wave. corer weight ranges from 88 to 735 kg. during the ascent, samples are sealed, being capped both top and bottom. 3. grain size analyses the accuracy of grain size determination in the environmental studies is of basic importance to obtain reliable data. the main phases of the analytical procedure are described below. 3.1. pre-treatment a pre-treatment of the sediment sample is necessary in order to discrete the colloidal aggregates, to eliminate saline content and to remove any organic material. the saline content and the organic material, adhered to the sediment granules can be easily removed by immersing the sample in a solution of distilled or natural water and hydrogen peroxide (30 % h2o2) and leaving it for at least 24 hours; if necessary, the operation can be repeated [19]-[21]. removal of organic material may be necessary to achieve full dispersion of clay and, in sediments with a high organic content (> 3 %), to avoid organic matter being counted in the total weight of the sample, thereby affecting the grain size distribution [22]. the separation of coarse (> 63 µm) and fine (< 63 µm) fraction is carried out by wet sieving. 3.2. analysis of coarse fraction the coarse fraction (> 63 µm) can be efficiently determined by dry sieving through set of sieves, whose number and range of coverage depend on the required detail of the grain size classes, placed on a vibro-tilting mechanical stirrer. after sieving, the individual dimensional classes are weighed and recorded, and then their relative abundances are calculated. data obtained from sieving are normally integrated with those of the fine fraction and, if present, of the gravelly ones. 3.3. analysis of fine fraction as regards the analysis of the fine fraction, up to a few decades ago, sedimentation methods such as the pipette or the hydrometer based on the stokes law were used. the analysis, assumes the preparation of a homogeneous sample suspension in a dispersant solution, and subsequent particle sedimentation within a graduated cylinder. the procedure is completely manual and requires a high level of precision and competence of the operator [23]. in the last decades, several new instruments have been developed; they can characterize variable dimensional ranges based on particle electroresistence (e.g. coulter counter), photometric techniques (e.g. microtrac, malvern laser sizer, coulter ls) or on automated image analysis (qicpic). instruments based on laser diffraction have become one of the most widely used for determining particle size. the speed of figure 3. from left to right: box corer (photo by elena romano) and multiple corer (http://osil.com/home.aspx). http://osil.com/home.aspx acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 13 analysis and the possibility of using extremely small sample quantities are the strengths of such instruments. samples can be analyzed either in a liquid suspension or in a dry dispersion. a small, but representative, quantity of sample is crossed by a laser beam, which is deviated of an angle, inversely proportional to the size of the single particle. although laser diffraction can theoretically be easily applied in a dimensional range from sand to clay, some studies have shown that it is preferable to limit the analysis to lower grain size classes [24]. sedigraph, an instrument that utilizes the x-ray attenuation principle, is another widely used for grain size analysis, in particular for the fine fraction. the analysis is performed through an x-ray beam, suitably collimated in a thin horizontal band that allows calculating the concentration of particles in the liquid medium, following the principle of the stokes law. it detects the size of the particles according to their sedimentation rate [25]. the problem of comparing grain-size analyses based on different techniques and different physical principles has been discussed by several authors. for example, goossens carried out a detailed comparative study analyzing four sediment samples with ten techniques indicated that, although the trends were generally similar, minor differences between the results of grain size analyses, attributable to different instruments, were observed [26]. advantages and limitations of the two main instruments are reported in table 1. 4. classification sediment classification is necessary to include the analyzed sediments in distinct classes, according to their dimensional ranges, and to compare different types of sediments. because several classifications are known in scientific literature, the choice of the most suitable one depends on the purpose of the study, type of the studied sediment and accuracy of the analysis. the wentworth scale defines limits and names of grain size classes [27]. range limits are expressed in phi (φ), which is calculated as follows: φ = -log 2 (d/d0) (1) where d is the diameter of the particle in millimeters and d0 is a reference diameter, equal to 1 mm [28]. the binary classification of nota may be used also in case there is no possibility of analyzing the fine fraction [29]. this classification is given by the ratio between sand and mud (silt + clay): − sand: sand > 95 % − muddy sand: sand 95-70 %; mud 30-5 % − very sandy mud: sand 70-30 %; mud 30-70 % − sandy mud: sand 30-5 %, mud 70-95 % − mud: mud > 95 % one of the most commonly used classifications of marine sediments is the shepard ternary classification [30]. it considers the relative abundance of sand, silt and clay fractions with the identification of 10 sediment classes (figure 4). it is possible to use this diagram even if small percentages of gravel fraction are present. when a large amount of gravel is recorded, the modified shepard diagram, is preferable. in this ternary diagram the vertices consist of gravel, sand and mud (silt + clay). the use of this classification is recommendable for studies on contamination assessment, because of the great importance of clay minerals for the accumulation of heavy metals and organic compounds. also the folk classification is suitable for gravelly sediments [31] (figure 5). the basis of this classification is a triangular diagram on which the proportions of gravel, sand and mud are plotted. depending on the relative proportions of these three constituents, 15 major textural groups are identified. 5. grain size data data resulted from the analysis of the coarse and fine fraction should be integrated and processed for the determination not only of percentages of the main grain size fractions (sand, silt, clay), but also to reconstruct the frequency and the cumulative curves and to obtain the main statistical parameters, as defined by folk and ward [31]. figure 4. shepard ternary diagram. table 1. advantages and limitations of laser granulometer (lg) and x-ray sedigraph (xs). instruments laser granulometer x-ray sedigraph advantages wide range of measuring (variable depending on the instrument from gravel to clay) wide range of measuring (0,1 µm 300 mm) short analysis times (only few minutes) excellent spectrum analysis resolution above 1 µm small amounts of sample possibility of serial analysis (using an accessory) limitations need to make accurate splitting long measurement times (25 minutes or more) high cost relatively high cost greater range of measurements, less accuracy in finer fractions https://en.wikipedia.org/wiki/diameter acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 14 for each diameter class, the frequency curve expresses its percentage of the total sample. it could have only one (unimodal curve) or more peaks (bimodal or polimodal curves), indicative of the presence of a mixture of different sediments. the cumulative curve represents the percentage, referring to the total, of the sediment relative to each diameter class. in order to compare different sedimentary environments by quantitative viewpoint, it is necessary to calculate measures of average size, sorting, and other frequency distribution properties. these properties may be determined either mathematically by the method of moments or graphically by reading selected percentiles off the cumulative curves [31]. the main statistical parameters are mean size, standard deviation, skewness and kurtosis. important parameters are also the median (d50) and other significant percentiles (d5, d16, d25, d75, d84), recognizable in the cumulative curve, and the mode, derived from the frequency curve. different environmental contexts are characterized by distinct statistical parameters while the grain size distribution depends on the source rock. mean size, mode, median and first percentile diameters give information on transport capacity. in general, coarser sediments indicate greater energy (higher transport capacity) than the finer sediments. the standard deviation parameter indicates the selective capacity of the medium of transport. a well-sorted sediment is subjected to a constant selection. 6. conclusions this methodological review was carried out considering the importance of grain size in a wide spectrum of marine environmental studies. the acquisition of reliable data is of basic importance in these studies especially because of their consequence on the management policies. this work, highlighted advantages and disadvantages of the most commonly used sampling, analysis and classification methods for marine sediments. considering this information, it was clear that it does not exist a single procedure to be considered as the best one for the acquisition of reliable grain size data. as regards sampling and sediment analysis, the choice of appropriate instrumentation should be carefully planned. as regards the choice of sampling strategy and device, type of sediment, sea bottom bathymetry and morphology, sample volume, necessity of collecting more or less deep sediment layers and the specific aim of the study should be considered. concerning grain size analysis, although a standard method of dry sieving is generally adopted for the analysis of coarse sediment fraction (> 63 µm), for the analysis of the finer one (< 63 µm) the instrument should be selected considering several factors such as type and quantity of available sediment, speed of measurement and reproducibility of the results. finally, for the choice of sediment classification, type of analyzed sediment and availability of silt and clay data should be considered. in conclusion, this review offers the conceptual tools to plan a flexible strategy to obtain the highest reliability of grain size data of marine sediments, considering the most suitable method in sampling, measuring and classifying sediments, in relation to the specific aim of the research. references [1] m.j.baptist, 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[10] f.rolle, e.pessana, m.sega, “metrological traceability for the analysis of environmental pollutants in the atmosphere”, acta imeko, vol. 4, issue 4, 2015, pp. 62‐ 65. figure 5. folk ternary diagram and nomenclature. acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 15 [11] d.m.toma, a.garcia‐ benadí, b.j.mànuel‐ gonzález, j.del‐ río‐ fernández, “systematic quality control for long term ocean observations and applications”, acta imeko, vol. 5, issue 1, 2016, pp. 64‐ 68. [12] usepa, “methods for collection, storage and manipulation of sediments for chemical and toxicological analyses: technical manual”. us epa-823-b-01-002, 2001. [13] usepa,“operating procedure sediment sampling”. sesdproc-200-r3, 2014. [14] a.mudroch, s.d.macknight, “handbook of techniques for aquatic sediments sampling”, 2nd edition, lewis publishers, boca raton, fl., 1991. 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[25] g.f.coates, c.a.hulse “a comparison of four methods of size analysis of fine-grained sediments”, new zeal. j. geol. geoph., vol. 28, 1985, 369-380. [26] d.goossens, “techniques to measure grain size distributions of loamy sediments: a comparative study of ten instruments for wet analysis”, sedimentology, vol. 55, 2008, pp. 65-96. [27] c.k.,wentworth, “a scale of grade and class terms for clastic sediments”, j. geol., vol. 30, 1922, pp. 377-392. [28] w.c.krumbein, l.l.sloss, “stratigraphy and sedimentation”, 2nd edition, freeman, san francisco, 1963. [29] d.j.g.nota, “ sediments of the western guyana shelf”, report of orinoco shelf expedition, 2. mendedel, landbomvhogedrool, wegeningen, vol. 58, 1958, pp. 1-98. [30] f.p.shepard, “nomenclature based on sand-silt-clay ratios”, j. sed. petr., vol. 24, 1954, pp. 151-158. [31] r.l.folk, “the distinction between grain size and mineral composition in sedimentary-rock nomenclature”, j. geol., vol. 62, 1954, pp. 344-359. [32] r.l.folk, w.c.ward, “brazos river bar: a study in the significance of grain size parameters”. j. sed. petr., vol. 27, 1957, pp. 3-26. grain size of marine sediments in the environmental studies, from sampling to measuring and classifying. a critical review of the most used procedures acta imeko, title acta imeko issn: 2221-870x march 2018, volume 7, number 1, 27-35 acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 27 hardware-in-the-loop test based non-intrusive diagnostics of cyber-physical systems using microcontroller debug ports balázs scherer department of measurement and information systems, budapest university of technology and economics, h-1117 magyar tudósok krt. 2 ie444, budapest hungary section: research paper keywords: hil testing; non-intrusive measurement; arm cortex; cmsis-dap; labview citation: balázs scherer, hardware-in-the-loop test based non-intrusive diagnostics of cyber-physical systems using microcontroller debug ports, acta imeko, vol. 7, no. 1, article 6, march 2018, identifier: imeko-acta-07 (2018)-01-06 section editor: lorenzo ciani, university of florence, italy received september 29, 2017; in final form november 29, 2017; published march 2018 copyright: © 2018 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: balázs scherer, e-mail: scherer@mit.bme.hu 1. introduction most of the cyber-physical systems use one or more microcontrollers to perform the computation and control tasks. developing software for these microcontrollers requires much effort and extensive testing. one of the typical tests of the cyber-physical systems is the hil (hardware in the loop) test, where the behaviour of the integrated software and hardware of the dut (device under test) can be investigated in a simulated and stable environment. these test are originated and widespread in the automotive and transportation industry, where the duts are mostly safety critical, and performing the first tests in the real environment would be too costly and dangerous. hil tests are used for the verification and validation of systems like transmission control unit, electronic power steering systems and break systems. safety standards like iso26262 explicitly names hil as a suitable test environment for software unit tests and integration tests for such devices. although hil tests have been designed primarily for safetycritical devices, but using them for general-purpose cyberphysical systems have many advantages and getting more and more widespread [1]. hil tests can reduce the time spent to real environment testing, because most of the failures can be detected earlier in the simulated environment. this can reduce the overall testing cost significantly, because real environment setups like prepared track with staff for autonomous robots, or real plant setups for industrial controllers are very costly. hil tests also have the advantage of repeatability, controllability and stability, which is usually not given in the real environment. a typical hil test setup (shown on figure 1) consists of an environment simulator, which is usually executed in a real-time hardware, and a test control station, which is usually a general purpose pc. the environment simulator runs the model of the dut’s physical surroundings, and interacts with the dut’s hardware i/o-s. while the test control station enables the tester to configure and execute the hil test and evaluate its results. many hil test setups gather additional information from the dut beside measuring and stimulating its direct interface to the physical environment. this additional information is useful to monitor the internal behavior of the dut’s software. for example, monitoring the internal states, main input and abstract cyber-physical systems have extensive contact with the physical world. usually during the development of these systems, the testing phase cannot be done efficiently or safely in the complete real environment, and therefore hil (hardware in the loop) simulators are used. during hil testing, diagnostic protocols are used very often to gather detailed information about the dut’s (device under test) internal state. diagnostic protocols are very useful during testing, but they cause a significant load to the dut. this paper introduces a novel approach to replace traditional diagnostic protocols with a non-intrusive solution. the presented method is based on the debug capabilities of modern arm cortex m core microcontroller, and uses a cmsis-dap (cortex microcontroller software interface standard debug access port) based interface. this paper also introduces a solution to integrate this non-intrusive measurement method to ni labview based test environments and ni veristand based hil simulations. acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 28 output parameters of software modules are very useful during the testing to identify the source of problems. it is also very useful, for example, to calculate and modify calibration parameters for analog measurements and controls during testing, which also needs an interface to the core software parameters. the parameters above can be monitored or modified by using diagnostic protocols. among others, the automotive industry has many standardized protocols with extensive support tools. the most widespread automotive diagnostic protocols are the ccp (can calibration protocol), its extended version the xcp (universal measurement and calibration protocol family), kwp2000 (keyword protocol 2000) and uds (unified diagnostic services). these protocols have very similar main features for performing the functionality described above: • reading and writing ram memory locations by address. • programming the dut’s flash memory. some protocols complement these features with user authentication, periodic stimulus and measurement possibilities, and special features, like calibration page writing or diagnostic trouble code reading and clearing. the protocols above are not restricted to the automotive industry. they can be used in any industrial segment. xcp [2] for example is well suitable for testing cyber-physical system. it supports many types of communication interfaces: serial, usb, ethernet, can etc. xcp is an open, free standard and there are downloadable sample implementations of it. beside the useful functionality and features provided by xcp or another diagnostic protocols they also have disadvantages, which limit the scope of their use. this paper introduces these disadvantages and proposes a novel approach to replace traditional diagnostic servers with a non-intrusive solution based on the debug capabilities of modern arm cortex m core microcontrollers. this novel approach removes the overhead and load of the diagnostic communication from the dut, while providing similar functionality. therefore, it can be used as a solution for most of the applications including resource limited or very real-time systems. beside the description of the proposed novel approach, the paper also introduces a new labview based free to download basic toolset to enable the application of it. 2. limitations of traditional diagnostic protocols xcp and similar diagnostic protocols are very widespread and suitable for testing cyber-physical systems, but they have many drawbacks: • diagnostic protocol servers cause significant load to the dut’s processor: for example, handling tcp/ip or usb communication requires significant processor time in simple embedded systems, which can highly influence the behaviour of the investigated system. • diagnostic protocol servers require program and data memory of the dut. the simplest diagnostic server configurations start at the 10 kb range of program memory, but ones, that are more complex can easily reach the 100 kb domain, which make this technique unusable for systems with very limited flash and ram. • diagnostic protocols often use the same communication channel as the normal function, which gives a hard limitation to the diagnostic data rate. • load (processor time, communication bandwidth) caused by the diagnostic can modify the behaviour of the dut. • diagnostic communication is hard or impossible to use for monitoring low power modes of cyberphysical systems. • monitoring after reset: many times an errors cause a reset or reboot to the system. for these situations it is very useful to get information about the memory content during this reset procedure. the drawbacks above show that diagnostic protocols are not usable for some type of cyber-physical systems, especially for high data rate, or low resource, power critical systems. therefore, a solution is needed to provide the functionality of diagnostic protocols, which is essential part of modern testing, but without their limitations. the solution is to use nonintrusive diagnostic by using the features included in modern microcontrollers. 3. non-intrusive diagnostic in embedded system testing modern 32-bit microcontrollers include very capable enhanced debug features. for example, the arm cortex core devices [3] include the arm coresight on-chip trace and debug solution, where many others implement the ieee-isto 50012003 (nexus) standard [4]. these debug features among others provide interface to the internal memory of the microcontrollers without stopping, or modifying the behaviour of the core. reading from and writing to the dut’s memory and reprograming it are the most important features of diagnostic protocols. therefore, modern debug ports can be used to replace many of the functions of the diagnostic protocols, and provide a non-intrusive solution to the limitations described in chapter 2. to achieve this goal hil figure 1. typical hil test environment. real time environment simulator test control logs test configuration and control test profiles hardware i/o environment model test execution tester device under test global variables diagnostic communication acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 29 tests needs to be extended to use modern debug ports for dut diagnostic. 4. interfacing debug port from hil test environment at the embedded software development process, the ides (integrated development environment) are provided by the microcontroller manufacturers traditionally. these ides include the debug features to help software development and debugging. however, these ides can be used at the very early phase of the testing process, where the developers debug their code, but they are inappropriate to be used in later verification activities like hil tests, due to the following reasons: • testing is often done by a separate group or company to provide independence of verification required by many guidelines. the source code for them might be unavailable. • software development ides usually has a significant amount of license fee. • ides do not have real-time capabilities required by many integrated hardware-software tests • it is very hard, or even impossible to synchronize and share data between the test environment like hil environment and a software development ide. automotive tool manufacturer companies are aware to the above problems and try to provide solutions. vector’s vx1000 family is one of the very few tools capable to solve this issue [5]. the vx1000 family functions as an ethernet based xcp diagnostic server, and provides jtag (joint test action group) debug port or special, target dependent debug interface for typical automotive target processors. the vx1000 series has excellent functions, but it is a very expensive tool. depending on the configuration, its price range in the $ 2000 $ 10000, without the software tools like vector canoe, which is also very costly. other drawback of the vx1000 series that it is designed for special line of automotive processors. therefore, it cannot be used with general-purpose microcontrollers typically applied in most of the cyber-physical systems (many automotive microcontroller has special features not required by normal applications, and available only in large quantities like 10 000 pieces or more). 5. suggested solution the proposition of this paper is to follow these trends, but in an environment that is not restricted to the automotive industry. therefore, a complete novel toolset is under design and development by us, to support non-intrusive diagnostic of cyber-physical systems. this toolset is primary designed for arm cortex m core microcontrollers, which are the most well known platforms of cyber-physical systems [6], but it will be extensible to other microcontroller families too. 5.1. comparing interfacing options during the design of the toolset four main possible hil – debug port interface implementation options are identified (shown on figure 2). to understand these possibilities an overview of microcontroller debugging is needed (gnu tools based development environment is used as example): in a normal debugging there are four components: debugger, debug server, debugger hardware and target. the debugger (for example gdb the gnu debugger) is connected to the graphical ide (eclipse cdt, ddd etc.), it communicates through a special protocol called rsp (remote serial protocol) to the debug server, and debug server executes the debug command through the debugger hardware. the identified four integration ways based on figure 2 are the following: 1. it is possible to directly perform the swd (serial wire debug) or jtag debug communication using a hil test integrated hardware. the straightforward solution to this implementation way is to use an fpga based hardware integrated to the hil environment. such fpga based hardware is very costly. for example, the price of an fpga based national instruments pxi card starts at the $ 2000 range. this solution is costly but has the advantages that it provides a direct fast interface with low response time. the fpga hardware used for this purpose could be used to other interfacing too in the test environment. 2. another promising way is to use an existing usb based debugging hardware and integrate it into a hil environment. traditionally the main problem with this option is that each debugger hardware manufacturer uses its own closed communication protocol through the usb connection, like segger’s j-links or stmicroelectronics’s st-link/v2 etc. in case of arm cortex core, arm gives a solution to this problem and make the debug interface tool vendor independent. the name of this solution is the cmsis-dap (cortex microcontroller software interface standard debug access port) that will be introduced later. 3. it is also possible to emulate an external debugger, and send rsp commands to a debugger server from the hil environments. the drawback of this solution, that a debug server implementation like open-ocd is needed. therefore, this solution is not suitable for real-time hil systems (making a non-real-time program like open-ocd real-time is nearly impossible). other drawback of this solution is the more and more interfaces and layers used during the debug process makes the response time higher. figure 2. possible interfacing points of the debug port communication. debugger (gdb) in gnu environment gdb (gnu debugger) debug server most widespread open source solution is the openocd for example: rsp (remote serial protocol) the debug protocol of gdb . through tcp or rs232 interface hw interface driver debug server (openocd) debugger hw usb jtag / swd target (cortex m0-m7 core) cmsis-dap, j-link, st-link or other debug hardware 4 3 2 1 acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 30 4. it is also possible to interface to the debugger application like gdb from the hil system by the same way as the development environments does it. this solution also non fit for real-time systems, and needs huge third party software support. the advantages and disadvantages of the above options are summarized in table 1. table 1 shows that there is no a clearly best way. option 1 provides the best functionality, but it is expensive and requires significantly more development time than the others. option 3 and 4 are the easiest to implement, but have many drawbacks and these options are not usable for testing many types of duts. option 2 provides the best compromise in term of cost and functionality. based on the above, our toolset is intended to support both option 1 and 2. this paper describes the implementation of option 2 the cmsis-dap debugger hardware based solution in detail. this option was chosen for implementation, because it is relatively easy, and cheap, but it can provide a medium-high data rate and has real-time capabilities. therefore, it could be a very good solution to testing general purpose cyber-physical systems. this cmsis-dap debugger hardware based diagnostic in hil test is also a new idea first described in this paper. 5.2. suggested architecture of the cmsis-dap hil test interface implementation the vx1000 tool of vector informatik (see chapter 4) simulates an xcp server to cover the debug communication. this approach is good, because xcp is a widespread protocol, and there are many tool supports for it including hil test integrations. this paper suggests using this approach, but the xcp server functionality will not be integrated into an embedded device, like the vx1000. the reason of this decision was to make our toolset as flexible as possible. integrating the xcp server functionality to an embedded device would mean that anyone, who would like to use the toolset should have to buy or manufacture this special hardware. we decided to make a pure software implementation of the xcp server (box xcp server emulator on figure 3). this makes the tool flexible, because only one hardware component a commercial off-the-shelf cmsis-dap debugger is needed. the xcp server emulator will be written in national instruments labview [7]. labview was selected because it is one of the most widespread programing environment used for testing, and ni also has extensive hardware and software support for hil testing. the oop (object oriented programing) feature of labview is used during the implementation. the labview oop programing makes the implementation of xcp server emulator extensible. it separates the debug port interfaces, and the xcp transport layers from the xcp server functionality, therefore make it possible to use the implementation with different xcp transport layers or debug port interfaces. the simplified class diagram of the xcp server emulator is shown on figure 4. the xcp server class contains one xcp transport layer, one ecu interface and a xcp daq processor class. 6. implementation of the ecu interface the goal of the ecu interface is to provide the nonintrusive connection to the target. the various ecu interface implementations are based on the ecu interface abstract parent class. currently only two ecu interfaces are implemented, an ecu simulator interface for toolset testing purposes, and a cmsis-dap interface based one to demonstrate the concept. 6.1. the cmsis-dap interface to understand the cmsis-dap based ecu interface a short overview of the internal debug architecture of an arm cortex core microcontroller (figure 5) is needed. external debug hardware can be connected to an arm cortex microcontroller either using a 5-wire jtag or a 2-wire swd (serial wire debug) debug port (dp) [8]. the enabled functionality is independent from the debug port. through the debug port, the debugger is able to access the ahb-ap (advanced high-performance bus access port). the ahb-ap enables the debugger to transfer data through the ahb bus interconnect of the microcontroller, which means that the debugger is able to read or modify any memory location, including peripheral registers in the target systems, without causing load to the microcontroller (that was the main table 1. comparing interfacing options. figure 4. simplified class diagram of xcp server emulator. figure 3. architecture of the suggested solution. 1, direct jtag/swd interface 2, debugger hw driver if 3, rsp protocol interface 4, debugger software interface complexity high m edium low low speed high m edium high low low real-time hil simulation possible possible, real-tim e usb support needed no rt support for debug server no rt version of gdb universality m edium : µc arcitecture depended m edium : µc arcitecture depended m edium : any target with debug s. support m edium : any target with gdbsupport hw cost m edium : fpga based hardware low: com m ercial debugger low: com m ercial debugger low: com m ercial debugger + init() + process_command() + close() xcp transport layer ecu interface xcp daq processor + init() + read_byte() + read_multiple_bytes() + write_byte() + write_multiple_byte s() + close() xcp server + session_id + xcp protocol version + state + init() + send_message() + receive_message() + close() + init() + star() + stop() + write_daq() swdhil xcp interface cmsis-dap cots debug unit dut usbxcp server emulator local host ethernet hil environment simulator acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 31 condition for non-intrusive diagnostic). traditionally the external debug hardware is manufacturer depended as described in chapter 5. fortunately, arm has introduced the cmsis (cortex microcontroller software interface standard) with the cortex core [9]. the goal of this standard is to give a common support to all arm cortex based microcontroller regardless its manufacturer. as a part of this standard, cmsis-dap is intended to give an open specification for the debugger hardware with sample implementations (figure 6). price of such commercial off-the-shelf hardware is about $20. cmsis-dap also specifies an open usb-hid (usb human interface device) based communication protocol to interact with the debugger hardware. the main benefit of using the usb-hid class is that there is no need for a custom driver. the operating system will automatically identify the device like it identifies a keyboard or a mouse. the cmsis standard also provides a rddi-dap access dll, with a sample implementation to enable windows based debugger hosts an easy interfacing. 6.2. implementation of the cmsis-dap based ecu interface the cmsis dap ecu interface implements the init(), read_byte(), read_multiple_bytes(), write_byte(), write_multiple_bytes(), close() methods of the ecu interface abstract class. to perform this implementation, the connection to the debug hardware is needed. this requires an usb-hid class communication under labview. labview do not support custom usb-hid devices, therefore a raw visa usb implementation is needed [10]. this means, as a first step a special driver needs to be created by using ni driver wizard, then a raw interrupt endpoint communication needs to be performed. the commands available for interacting to the debug hardware is described in [9]. to initialize the debug hardware, the debug connection mode (jtag or swd) should be selected and the debug clock rate need to be determined. writing and reading memory values are done by sending transfer commands through the ahb-ap (debug port connection of an arm cortex m core figure 5). the ahb bus is a 32-bit aligned bus therefore this read and write operations can be done in 32-bit aligned quantities. aligned means in this case that, read and write addresses divisible with 4 (aligned to 32-bit) can only be used. this means an extra care is required for variables not keeping these boundaries. the debug port allows the transfer of multiple words, that can increase the effective data rate of the communication. 7. implementation of xcp transport layer, xcp server and xcp daq processor 7.1. xcp transport layer the goal of the xcp transport layer is to transport the xcp messages through various network communication interfaces. xcp supports: serial, usb, ethernet-tcp, ethernet-udp, can and flexray interfaces. currently one udp (user datagram protocol) implementation is created of the xcp transport layer abstract class. this implementation uses the built in udp communication vi-s of labview. the transport layer functionality is simple: receives frames from the udp port, handle the transport layer specific headers and provides data for decoding and processing in the process_command() method of xcp server. the response messages of xcp server and daq messages of xcp daq processor is also transmitted through, the xcp transport layer class. in this case the udp specific headers are added to the data and after that the message is sent to the xcp master application by using ni udp vi-s. 7.2. xcp server the xcp server class is the main component of the toolset. it processes the incoming messages of xcp master and interacts with the dut by using the ecu interface class. current implementation of the xcp server is a basic implementation, which only implements the most vital mandatory commands of xcp. these commands include the connection and disconnections to an xcp master, directly reading data from the dut’s memory, directly writing data to the memory. it also implements very basic commands for handling the xcp daq processor, and commands giving some state and configuration information about the xcp server itself. however, this is a basic implementation, but these mandatory functions are enough to demonstrate the capabilities of the concept. the xcp server class is easily extendable with new commands in the future. 7.3. xcp daq processor the purpose of the daq processor it to provide an automated data acquisition. to perform this the daq processor maintains odt-s (object descriptor table). these odt-s are used to make connection between the memory of the dut and the xcp daq messages, each odt describes a set of variables to be transmitted as part of a daq message. current implementation contains a very basic version of the daq processor, which is extensible in the future. figure 5. debug port connection of an arm cortex m core. figure 6. the cmsis-dap interface [9]. arm cortex m core based microcontroller swj-dp or sw-dp memory and peripherals jtag or swdexternal debugger hardware usb or ethernet arm cortex m processor core arm cortex m core ahb interconnenct ahb-ap acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 32 8. integrating the xcp server emulator into a hil test environment the xcp server emulator is written in labview; therefore, it is a straightforward decision to use ni veristand as hil environment form national instruments. 8.1. ni veristand the ni veristand [11] is one of the most widespread hil development environments used by many automotive and nonautomotive companies for hil testing. a typical ni veristand system consists of a host computer, where the hil test can be configured: selecting the model which describes the environment, connecting the input/output channels of the model to hardware i/o channels or communication signals, specifying the user interface and creating automated stimulus profiles. after the configuration of the hil test, typically the model, the stimulus profile execution, and hardware handling functions are deployed to a rt target (pxi real-time controller, realtime pc, or compact rio), while the user interface remains in the host pc (shown on figure 7). a special tcp/ip communication maintains the connection between the host pc (user interface), and the rt target (veristand engine) through the veristand gateway. 8.2. integration into ni veristand it is relatively easy to integrate xcp based diagnostic into ni-veristand, because there is an add-on the ni ecu measurement and calibration toolkit for this [12]. using this add-on, only the xcp server emulator from figure 3. needs to be integrated manually. this block is a gateway between xcp and the cmsis-dap debugger hardware. to integrate it into ni-veristand a so called custom device driver is needed. a ni veristand custom device functions as an interface to a special hardware. a custom device can have any number of input and output channels, and its functionality can be executed in every veristand engine cycle or can be implemented as a parallel executed task [13]. in this integration the parallel executed task version is need, because the xcp server functionality is too complicated to be implemented in in-cycle mode. for example, the daq processor functions could not be executed without running it in a parallel task. the xcp server emulator is integrated into ni veristand. the architecture of this integration is shown on figure 8. this implementation currently a minimal version, where the configuration of the xcp server is limited, but it is suitable to demonstrate the concept. 9. experimental measurements this chapter presents an experimental setup and measurements used for testing our toolset. 9.1. experimental setup the experimental setup is shown on figure 9. it consists of a test station running labview 2016, ni ecu measurement and calibration toolkit, and our new toolkit. this test station is connected to a device under test embedded system, the mbed lpc1768 [14] (it costs about $50), which contains an on-board cmsis-dap debugger and the lpc1768 cortex m3 core based microcontroller. 9.2. code running on the lpc1768 microcontroller a very basic code was used to demonstrate the concept. this code is presented on figure 10. the figure 8. integrating the xcp server functionality into ni veristand. figure 7. the architecture of ni veristand. figure 9. experimental setup for testing the toolset. veristand engine (rt target) server communication hardware i/o drivers model execution stimulus generation channel forcing, processing, calculation custom device xcp server, cmsis-dap interface workspace (host pc) veristand gateway user interface ni ecu mc toolbox localhost tcp/ip swdcmsis-dap cots debug unit dut arm cortex m core micro usb workspace (host pc) veristand gateway user interface veristand engine (rt target) server communication hardware i/o drivers fpga i/o model execution stimulus generation channel forcing processing calculation custom devices alarms test station device under test mbed lpc1768 usb xcp xcp transport layer xcp server ecu interface cmsis-dap commands embedded software to test lpc1768 dut board controller cmsis-dap debugger ni ecu mc toolbox diagnostic tester diagnostic protocol server (toolset presented in the paper) swd acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 33 my_function_to_test() function is called periodically and the diagnostic will be able to read or modify the variables without modifying the execution of the program. 9.3. configuring the diagnostic protocol server the use of our toolset can be very easy. in most of the cases only the xcp transport layer and the ecu interface type should be specified. the demo code for the presented example is shown on figure 11. the xcp server initialisation requires the type of the xcp transport layer (in this case udp), and the type of the ecu interface (in this case cmsis-dap), then the xcp command processor should be executed in a while loop. 9.4. testing and measuring the test software is written using the vi-s of the ni ecu measurement and calibration toolkit. the front panel of the example measurement window is shown on figure 12. for the measurement, the xcp transport protocol need to be determined here again, and there also a need for a database that contains the variables intended to measure or modify. this database is called the asam mcd2 database or a2l file, that contains the memory addresses and scaling of the parameters downloaded to the device under test embedded system. the response times were measured during this experimental test. the response time measurements include the writing of every parameter: a, b, x and reading back the measurement variables: counter and y. the results are shown on figure 13. this example is using a complex read, write scenario, similar to the one used in normal test cases. a complex analysis of performance comparing to traditional approach is presented in chapter 10. 10. comparing the performance of the cmsis-dap based sollution to the diagnostic protocol based sollutions 10.1. performance of a diagnostic protocol based implementation the performance of an xcp based diagnostic protocol based solution is highly depending on the used communication layer. a 1 mb/s can bus can be considered as a very typical communication layer for a calculation example. this 1 mb/s can bus data rate would mean about a maximum of 450 kb/s diagnostic data rate in case of daq message based acquisition. however, in reality this data rate in unachievable, the can bus cannot be loaded to 100 %, and in many situations the diagnostic protocol shares the communication bus with the application. therefore, usually a busload higher than 10 % 20 % is not allowed for the diagnostic communication that results a 40-100 kb/s diagnostic data rate. this data rate is enough to read the values of about 30 – 100 variables in every 10 ms. if direct memory reads or writes, are used instead of daq based synchronised measurements, then the above data rate can drop to even 1/10 due to the command – response type of operation. native usb or ethernet based xcp servers could provide a significantly higher data bandwidth, but they cause huge overhead to the device under test, and therefore cannot be used in low resourced cyber-physical systems. 10.2. performance of the cmsis-dap interface based implementation the performance of this presented implementation mainly depends on the capabilities of the cmsis-dap debug hardware. current cots debug hardware solutions and open source software and hardware versions support a relatively low bandwidth. this means a maximum of 1 mhz swd clock frequency, and a full speed 12 mbps usb communication. determining the number of clock phases required reading, or writing one 32-bit variable of the dut through the swd debug port is not easy: the number of clock phases is depending on whether the debugger wants to read or write a continuous ram area, or just a single 32-bit block. to make the calculation even harder the ahb-ap access port use an aligned data transfer, therefore variable addresses crossing a 32bit word boundary cannot be handled with one read or write figure 12. measurement and test panel using ni ecu mc toolset. figure 10. part of the code running on the lpc1768 microcontroller. figure 11. xcp server with cmsis-dap communication implemented with the new toolset. figure 13. measurement results of the experimental setup. volatile uint16_t a,b,x,y = 0; volatile uint32_t counter = 0; void my_function_to_test(void) { y = a*x+b; counter++; } acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 34 cycle. to enable simple calculations, 138 clock cycles can be considered as 32-bit variable accesses. hil tests usually use a 1 ms, or 10 ms cycle times. this means 7 pieces of 32-bit variable accessing for 1 ms cycle time, and 72 pieces for 10 ms cycle time. measurements based on an lpc1768 mbed hardware [14] show slower real data rates, as shown in figure 14. these measurements show that the data rate is mainly depend on the sdw clock rate. well organized memory architecture allowing continuous data block reads can also increase the performance significantly. however, these data rates are smaller than the calculated maximum value, but are still in the range that a 1 mb/s can diagnostic can provide. fortunately, the data rate of the cmsis-dap port based solution can be increased relatively easily. the swd clock frequency is not limited in 1 mhz. the dut’s hardware gives limitation to the maximum swd clock rate, but typically this limitation is in the n*10 mhz range. if the swd clock rate is increased, probably the full speed 12 mbps usb port became a limitation. therefore, the nxp lpc11u35 low end 32-bit microcontroller used in most of the cots debug hardware should be replaced by a more capable microcontroller, for example one from the lpc18xx series. these microcontrollers are not much costlier than the lpc11u35 (the cost difference is not higher than $ 10). this replacement means hardware redesign and software porting, but both design processes are relatively easy. probably cots debug hardware with these features will appear on the market soon. 11. conclusions this paper has introduced a novel approach, and the first part of a toolset for non-intrusive diagnostics of cyber-physical systems. our goal is to create a toolset, which supports multiple non-intrusive diagnostic interface options for testing cyberphysical systems. this toolset is under development, and in this paper a novel way, the cmsis-dap based solution is presented in detail. the cmsis-dap debug port support is integrated into the ni veristand environment as a custom device, and this custom device provides an xcp server interface to enable the use of cots diagnostic software modules. the sample implementation is currently a basic version enabling only few functionalities of the xcp protocol, but it is suitable to evaluate the concept. first measurements have shown that the concept is good, and this method enables the diagnostic of low cost, resource frugal cyber-physical systems with a relatively good data bandwidth. future work includes the improvement of the toolset and functionality provided by the cmsis-dap interface implementation. for example, the xcp server can be extended. beside the implementation of more data acquisition and stimulation xcp commands, the program download to the dut could also be implemented. the program download is one of the hardest features to implement, because it highly depends on the target microcontroller’s flash memory handling. usually xcp sample implementations need user contribution to perform this operation. the cmsis-dap based method could simplify this process. the cmsis-dap interface is used by the mbed community, and they complemented the cmsis-dap functionality, with a mass storage device based programming feature and a virtual serial port support. this new platform is called the mbed hardware development kit (mbed hdk [15]), and there are open source implementations for that. using this support the program downloading could be done easily for devices supported by the mbed community. the virtual serial port feature of mbed hdk complementation also can be used to provide more feature, for example dut – hil system synchronization, but this is out of the scope of this paper. the presented toolset is downloadable at [16]. it is tested with the mbed lpc1768 hardware with on board debug support, but any low cost (around $ 20) cmsis-dap debugger can also be used with the tool. references [1] a. biagini, r. conti, e. galardi, l. pugi, e. quartieri, a. rindi, s. rossin “development of rt models for model based controldiagnostic and virtual hazop analysis”. 12th imeko tc10 workshop on technical diagnostics. june 6-7, 2013, florence, italy. [2] asam, "asam mcd-1 xcp (universal measurement and calibration protocol) v.1.3.0", 2015. [3] j. yiu, “the definitive guide to the arm cortex-m3, second edition”, elsevier inc. isbn 978-1-85617-963-8, 2010. [4] ieee-isto 5001™ “the nexus 5001 forum™ standard for a global embedded processor debug interface version 2.0” 23 december 2003. [5] vector informatik gmbh, „vx1000 measurement & calibration hardware,” [online]. available: https://vector.com/vi_vx1000_en.html. [access date 20.11.2017]. [6] ubm “electronics embedded markets study 2015” eetimes 2015. [7] national instruments labview https://www.ni.com/gettingstarted/labview-basics/ [access date 20.11.2017]. [8] arm technical reference manual “arm® debug interface v5 architecture specification” arm ihi 0031a, arm limited 2006. [9] arm technical documentation: “cmsis version 4.5.0 cortex microcontroller software interface standard” arm limited 2017. [10] national instruments, “usb instrument control tutorial”, 2014 http://www.ni.com/tutorial/4478/en/ [access date 20.11.2017] [11] national instruments, “ni veristand fundamentals course manual“. part number 325785a-01, 2011. [12] national instruments, “ni ecu measurement and calibration toolkit”. [online] available: http://sine.ni.com/nips/cds/ view/p/lang/hu/nid/210569 [access date: 20.11.2017] [13] national instruments, “ni veristand custom device developer’s guide (beta)”, ni white paper. [14] the mbed lpc1768 development board:. https://os.mbed.com/platforms/mbed-lpc1768/ [access date: 20.11.2017]. figure 14. diagnostic data rate in function of sdw clock rate and packet size. 100kbit/sec 200kbit/sec 500kbit/sec 1mbit/sec 4 bytes 8 bytes 16 bytes 24 bytes 32 bytes 40 bytes 48 bytes 56 bytes 64 bytes sd w c lo ck r at e packet size diagnostic data rate in bits/sec 0-20000 20000-40000 40000-60000 60000-80000 https://os.mbed.com/platforms/mbed-lpc1768/ acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 35 [15] armmbed, “the mbed hdk (hardware development kit)” [online] available: https://developer.mbed.org/handbook/mbed-hdk#accessthe-mbed-hdk-repository [access date: 20.11.2017]. [16] balázs scherer, “homepage of the cmsis-dap based xcp server project”: http://mit.bme.hu/~scherer/xcp_server_cmsis_dap/introd uction.htm [access date: 20.11.2017]. hardware-in-the-loop test based non-intrusive diagnostics of cyber-physical systems using microcontroller debug ports << /ascii85encodepages false /allowtransparency false /autopositionepsfiles true /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /error /compatibilitylevel 1.4 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.0000 /colorconversionstrategy /cmyk /dothumbnails false /embedallfonts true /embedopentype false /parseiccprofilesincomments true /embedjoboptions true /dscreportinglevel 0 /emitdscwarnings false 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can be opened with acrobat and adobe reader 5.0 and later.) >> /namespace [ (adobe) (common) (1.0) ] /othernamespaces [ << /asreaderspreads false /cropimagestoframes true /errorcontrol /warnandcontinue /flattenerignorespreadoverrides false /includeguidesgrids false /includenonprinting false /includeslug false /namespace [ (adobe) (indesign) (4.0) ] /omitplacedbitmaps false /omitplacedeps false /omitplacedpdf false /simulateoverprint /legacy >> << /addbleedmarks false /addcolorbars false /addcropmarks false /addpageinfo false /addregmarks false /convertcolors /converttocmyk /destinationprofilename () /destinationprofileselector /documentcmyk /downsample16bitimages true /flattenerpreset << /presetselector /mediumresolution >> /formelements false /generatestructure false /includebookmarks false /includehyperlinks false /includeinteractive false /includelayers false /includeprofiles false /multimediahandling /useobjectsettings /namespace [ (adobe) (creativesuite) (2.0) ] /pdfxoutputintentprofileselector /documentcmyk /preserveediting true /untaggedcmykhandling /leaveuntagged /untaggedrgbhandling /usedocumentprofile /usedocumentbleed false >> ] >> setdistillerparams << /hwresolution [2400 2400] /pagesize [612.000 792.000] >> setpagedevice acta imeko, title acta imeko issn: 2221-870x december 2017, volume 6, number 4, 61-67 acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 61 frequency domain identification of data loss models lászló sujbert, györgy orosz department of measurement and information systems, budapest university of technology and economics, budapest, hungary section: research paper keywords: data loss; measurement; fft; psd; system identification citation: lászló sujbert, györgy orosz, frequency domain identification of data loss models, acta imeko, vol. 6, no. 4, article 9, december 2017, identifier: imeko-acta-06 (2017)-04-09 editor: alexandru salceanu, technical university of iasi, romania received may 5, 2017; in final form july 11, 2017; published december, 2017 copyright: © 2017 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: l. sujbert, e-mail: sujbert@mit.bme.hu 1. introduction nowadays measurement data transfer is frequently carried out in sensor networks or on the internet. in this case data can be corrupted or the transmission medium can be partially damaged, etc. [1], [2]. in the era of internet of things (iot) the significance of the problem culminates. data loss modeling has a great interest for a long while in the communication field. detailed models have been developed for video transmission [3], [4], or voice over internet (voip) applications [5]. data loss is usually modeled by a random process, where a single sample or a block of samples (a packet) is randomly available or lost. data availability or data loss is described by the so-called data availability indicator function 𝐾𝑛 [3], [4], [6], [7]. this function is a series of zeros and ones, and the data loss pattern depends on the data transmission system. in the simplest model the samples of 𝐾𝑛 are independent from each other [10], but it is often supposed that the successive samples depend on each other. comprehensive studies utilize different classes of markov models, see, e.g., [3], [4], [11]. the identification of the data loss model is based on statistical methods. first the structure and the order of the model are chosen, then the parameters of the model are determined by measurement data [3], [4]. model verification is an important task, as the process is not necessarily stationary and even the model order is to be adaptively changed. to this end, a correlation-based method [3] or off-line processing [4] can be used. the presence of data loss in measurement systems motivated the investigation of the phenomena from a signal processing point of view. our recently published paper [8] discussed one of such problems: the handling of data loss in the case of spectrum estimation. in spite of the existing methods concentrating to the spectrum estimation, this paper dealt with the characterization of distortion caused by missing data. spectrum estimation based on time records with irregular sampling has been used for a long time. records with missing data can be treated as a special irregular sampling, substituting the signal samples with zeros where the data are lost. theoretically, such records can be synthesized by the multiplication of the original signal (without data loss) and the data availability indicator function 𝐾𝑛. the spectral estimator of the damaged record is the discrete convolution of the original spectrum and the spectrum of 𝐾𝑛. thus the introduction of the distorted spectra involved the calculation of the spectra of the data availability indicator functions. three data loss models have been investigated: random independent, random independent block-based, and two-state markov model-based data loss. all their spectra have abstract recently measurement data loss has been of greater interest, due to the spread of sensor networks and the idea of internet of things. a procedure is proposed that is able to identify the most frequently employed data loss models. it is assumed that the communication protocol provides information about data loss, i.e. the so-called data availability indicator function is known. the power spectral density (psd) of the indicator function is representative for the model, and can be used for identification. spectral estimation is carried out by fast fourier transform (fft) based techniques. the paper introduces the identification procedure for random independent, random block-based and general markov model-based data loss patterns. the efficiency of the proposed method is demonstrated by simulation and measurement results. acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 62 been determined, and quantitative connection between the data loss model parameters and the spectral parameters have been calculated. in this paper we propose the inverse procedure: the data loss model can be identified by the fourier transform of the data availability indicator function. first the psd of the indicator function is calculated, then a parametric system identification method is used to get the spectral parameters. as the spectral shape is simple in most cases, this step is not critical. the last step is the calculation of the data loss parameters by the already known relations. model order selection and verification can also easily be done by comparing the measurement spectra to the model spectra. the spectra are calculated by the fast fourier transform (fft), which makes the method computationally effective. summing up, our method offers not only the spectral estimation of the lossy signal [8], but also provides an estimate for the data loss model along with its parameters. in section 2 the data loss models and their spectra are introduced in detail. the analysis of the markov model-based data loss has been expanded to the general case. section 3 deals with the identification procedure itself, while section 4 presents simulation and measurement results that confirm the procedure in practice, as well. 2. spectrum estimation in the case of data loss 2.1. power spectrum estimation the fourier transform of a sampled signal 𝑥(𝑡𝑛) can be estimated by a finite set of samples [9]. the signal 𝑥(𝑡) is usually equidistantly sampled, and the spectrum is calculated by the discrete fourier transform (dft): 𝑋(𝑓𝑘) = � 𝑥𝑛𝑒 −𝑗 2𝜋 𝑁 𝑛𝑘, 𝑛, 𝑘 = 0 … 𝑁 − 1 𝑁−1 𝑛=0 , (1) where 𝑓𝑘 = 𝑘/𝑁 and 𝑥𝑛 = 𝑥(𝑡𝑛). the dft of a signal is usually calculated by the fft. the transformed vector 𝑋(𝑓𝑘) is generally complex valued, and the spectral content of the signal is expressed by the real valued power spectral density (psd) function: 𝑆(𝑓𝑘) = 1 𝑁 |𝑋(𝑓𝑘)|2. (2) in order to reduce the variance of the psd, a long series of samples is recorded, and many consecutive blocks of 𝑁 samples are transformed, and the estimator is obtained by averaging the individual psds. the mean of the individual estimates can be calculated either by linear or exponential averaging. 2.2. formulation of data loss in order to model the data loss, a so-called data availability indicator function, 𝐾𝑛, is introduced [3], [4], [6], [7]: 𝐾𝑛 = � 1, if the sample is processed at 𝑛 0, if the sample is lost at 𝑛 . (3) samples which are not lost will be termed as processed or available samples. the data loss rate can be defined with 𝐾𝑛 as: 𝛾 = ℙ{𝐾𝑛 = 0}, (4) where ℙ{. } stands for the probability operator. the probability that a sample is available is 𝜇 = 1 − 𝛾. 2.3. spectrum estimation with missing data using the indicator function, 𝐾𝑛, (1) can be rewritten for the case of data loss: 𝑋�(𝑓𝑘) = dft(𝑥𝑛𝐾𝑛) = � 𝑥𝑛𝐾𝑛𝑒 −𝑗 2𝜋 𝑁 𝑛𝑘 𝑁−1 𝑛=0 𝑛, 𝑘 = 0 … 𝑁 − 1 . (5) this formula means that by incorporating 𝐾𝑛 into the usual form of dft, missing samples are practically substituted with zeros. equation (5) can also be evaluated via fft. the spectrum of the signal containing missing samples is obtained as the convolution of the spectrum of the lossless signal and the spectrum of the data loss indicator function. now only the latter is interesting. let 𝑋𝐾(𝑓𝑘) denote the fourier transform of the data loss indicator function: 𝑋𝐾(𝑓𝑘) = dft(𝐾𝑛). (6) thus the psd of the data loss indicator function is: 𝑆𝐾(𝑓𝑘) = 1 𝑁 |𝑋𝐾(𝑓𝑘)|2. (7) the variance of 𝑆𝑘(𝑓𝑘) can also be reduced by averaging. 2.4. data loss models and their spectra in [8], three data loss models have been investigated: 1. random independent data loss, 2. random block-based data loss, 3. two-state markov model-based data loss. the random data loss is one of the most essential data loss models; it is often used because of its simplicity [10]. blockbased data loss models are often used, e.g., when several measurement results are transmitted over packet-based communication systems. the markov model has been proven to be useful, e.g., in the description of data loss pattern in realtime data transmission over internet [11]. in [8] the two-state model has exhaustively been investigated, but real situations may require more states. in the following the main features of the data loss models are recalled. 2.4.1. random independent data loss random independent data loss can be defined as follows: 𝐾𝑛 = 1, with probability 𝜇 = 1 − 𝛾 𝐾𝑛 = 0, with probability 𝛾 for ∀ 𝑛. (8) the definition means that each sample is lost with probability 𝛾, and data losses at different time instants are independent of each other. the psd of the data loss pattern is [8]: 𝑆𝐾(𝑓𝑘) = 𝜇(1 − 𝜇) 𝑁 + 𝜇2𝛿(𝑓𝑘), (9) where 𝛿(𝑓𝑘) stands for the dirac-delta function. the psd is white, which is represented by the first term, while the second term represents the power of the mean value 𝜇 of the data loss pattern. 2.4.2. random block-based data loss to define the random block-based data loss, the indicator function is given as: {𝐾𝑘𝑘 … 𝐾(𝑘+1)𝑘−1} = 1, with probability 𝜇 {𝐾𝑘𝑘 … 𝐾(𝑘+1)𝑘−1} = 0, with probability 𝛾 for ∀ 𝑘. (10) acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 63 the definition means that each block of length 𝑀 is lost with probability 𝛾, and the data loss in different blocks are independent of each other. the power spectral density of the data loss pattern is [8]: 𝑆𝐾(𝑓𝑘) = 𝜇(1 − 𝜇) 𝑀𝑁 � sin (𝑓𝑘𝜋𝑀) sin(𝑓𝑘𝜋) � 2 + 𝜇2𝛿(𝑓𝑘). (11) 2.4.3. two state markov model-based data loss the first markov model-based data loss is described by the two-state markov chain shown in figure 1. this is the simplest stochastic model that describes the dependency of the consecutive samples of 𝐾𝑛. the states of the markov chain represent the value of the indicator function 𝐾𝑛. if a sample is available at time instant 𝑛, the next sample will be available with probability 𝑝, and will be lost with probability 1 − 𝑝. if a sample is missing at time instant 𝑛, the next sample will be available with probability 1 − 𝑞, and will be lost with probability 𝑞. the data availability rate 𝜇 is the following [12]: 𝜇 = 𝑞 − 1 𝑝 + 𝑞 − 2 . (12) note that the parameters 𝑝, 𝑞, and 𝜇 cannot be prescribed simultaneously. the spectral property of a data loss sequence generated by the markov chain shown in figure 2 can be determined according to [12]. the psd of 𝐾𝑛 is a first-order, low-pass type spectrum defined as [8]: 𝑆𝐾(𝑓𝑘) = 1 − 𝑎2 𝑁(1 − 𝑎2𝑁) ∙ 1 |1 − 𝑎𝑧−1|2 + 𝜇2𝛿(𝑓𝑘), (13) where 𝑧−1 = 𝑒−𝑗2𝜋𝑓𝑘 and 𝑎 = 𝑝 + 𝑞 − 1. (14) 2.4.4. general markov model-based data loss the two-sate markov model describes the relation of the actual and only the previous sample of 𝐾𝑛. generally, the actual state could be determined by the last 𝑟 samples. as each sample could have two distinct values (zero or one), a general model could have 2𝑟 states. the parameter 𝑟 is the order of the model. the states and the transitions can be seen in figure 2 for the case 𝑟 = 2. note that there are impossible state transitions that have formally zero probability in the model. the relation between the transition probabilities and the psd of the model can also be determined [12]. let the psd be as follows: 𝑆𝐾(𝑓𝑘) = 𝐴 |1 − ∑ 𝑎𝑚𝑧−𝑚𝑟𝑚=1 |2 ∙ +𝜇2𝛿(𝑓𝑘), (15) where 𝑧−1 = 𝑒−𝑗2𝜋𝑓𝑘. the constant 𝐴 is a function of the vector 𝑎𝑚, 𝑚 = 1, … , 𝑟 and the number of the fft points 𝑁. the state transition probabilities are: 𝐏 = �𝑃𝑖𝑗� = = ⎩ ⎨ ⎧𝜇 + � 𝑎𝑚(𝐾𝑛−𝑚 − 𝜇 ), if 𝐾𝑛 = 1 𝑟 𝑚=1 1 − �𝜇 + � 𝑎𝑚(𝐾𝑛−𝑚 − 𝜇 ) 𝑟 𝑚=1 � , if 𝐾𝑛 = 0 , (16) where �𝑃𝑖𝑗� = ℙ�𝑠𝑖�𝑠𝑗�, (17) i.e., the probability of the state 𝑠𝑖 with a condition of the previous state 𝑠𝑗. the 𝑠𝑖 and 𝑠𝑗 states are the following: 𝑠𝑖 = [𝐾𝑛, 𝐾𝑛−1, … , 𝐾𝑛−𝑟+1], 𝑖 = 1 … 2𝑟 𝑠𝑗 = [𝐾𝑛−1, 𝐾𝑛−2, … , 𝐾𝑛−𝑟], 𝑗 = 1 … 2𝑟 . (18) table 1 summarizes the psds of the data loss indicator functions for the most important data loss models. the small figures in the table illustrate the typical shapes of psd functions. general markov models can have diverse spectra, thus they are not represented in the table. 3. identification of data loss models data loss model identification consists of the model selection and the determination of the model parameters [3], [4]. the procedure utilizes some direct parameters of 𝐾𝑛 and is completed by the evaluation of 𝑆𝐾(𝑓𝑘). the theoretical basis for this method is the exact relation between spectral leakage and the parameters of different data loss models presented in the previous section. the identification process is summarized in figure 3. an essential requirement is that the communication protocol provides information about each sample whether its transfer was successful or not. without such information only qualitative assessment of the data loss can be done. in such a case the shapes depicted in table 1 can be used for identification. if 𝐾𝑛 is available, it is also known whether the protocol is block-based. in the latter case, one sample of 𝐾𝑛 is enough for each block representing the data loss. it is a kind of decimation. the block length 𝑀 is obviously available. the data availability rate 𝜇 can easily be estimated as the mean value or dc level of 𝐾𝑛. this dc level should be subtracted from 𝐾𝑛, in order to remove 𝛿(𝑓) from the psd, as its presence can impair the transfer function fitting. the next step is the calculation of 𝑆𝐾(𝑓𝑘). it can be done by the averaging of the ffts of consecutive (possibly overlapping) blocks of 𝑁 samples of 𝐾𝑛. as the dc component is removed from the psd, windowing is not necessary. figure 2. a four-state markov model of data loss. figure 1. a two-state markov model of data loss. state “1”: actual sample is available (kn = 1). state “0”: actual sample is lost (kn = 0). acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 64 the inverse fourier transform (ifft) of 𝑆𝐾(𝑓𝑘) provides the autocorrelation function of 𝐾𝑛. the fft block size should be greater than the length of the autocorrelation function. it is not a hard requirement, as the usual fft block size is much greater than required by 𝐾𝑛. the main part of the identification is the approximation of 𝑆𝐾(𝑓𝑘). as the previous investigations have shown, the transfer function can be well approached by an all pole or autoregressive (ar) system. theoretically it has no pole if 𝐾𝑛 is random independent, and has only one pole if 𝐾𝑛 is markov model-based with two states. generally a markov model with 2𝑟 states results in an ar system with 𝑟 poles. the psd 𝑆𝐾(𝑓𝑘) of a block based data loss has zeros, but after decimation 𝐾𝑛 is either random independent or markov model-based. as the system is quite simple, there are no special requirements for the identification. we have used a linear prediction filter (lpc) which determines the coefficients of a forward linear predictor by minimizing the prediction error in the least squares sense [14]. to this end, the lpc function of matlab [15] has been applied. there are 𝑟 + 1 lpc coefficients, where 𝑎0 = 1. if the lpc coefficients 𝑎1, 𝑎2, … ≈ 0, the data loss can be handled as random independent. its only parameter 𝜇 has already been calculated. however, if 𝑎1, 𝑎2, … ≠ 0, markov model-based data loss has been happened. in the general case of a 𝑟th order markov model, the transition probabilities are to be determined by the formula (16). in the special case of the two-state markov model-based data loss the parameters 𝑝 and 𝑞 can be estimated using the relations (12) and (14): �̂� = �̂�(1 − 𝑎�) + 𝑎�, 𝑞� = �̂�(𝑎� − 1) + 1, (19) where 𝑎� = −𝑎1. (20) in (19) and (20) the hat operator indicates the estimation. at the end, the information whether the data loss is block based has to be incorporated. if so, the estimators �̂�, �̂�, and 𝑞� do not change, but the parameter set has to be completed by the block size 𝑀. 4. results the procedure presented above has been intensively tested by simulations and measurements. in this section results of both tests are presented. the data processing has followed the procedure given in figure 3. figure 3. data loss model identification. table 1. summary of psds belonging to different data loss models. model 𝑆𝐾(𝑓𝑘) shape of 𝑆𝐾(𝑓𝑘) random independent data loss 𝜇(1 − 𝜇) 𝑁 + 𝜇2𝛿(𝑓𝑘) block-based data loss 𝜇(1 − 𝜇) 𝑀𝑁 � sin (𝑓𝑘𝜋𝑀) sin(𝑓𝑘𝜋) � 2 + 𝜇2𝛿(𝑓𝑘) two-state markov modelbased data loss 1 − 𝑎2 𝑁(1 − 𝑎2𝑁) ∙ 1 |1 − 𝑎𝑧−1|2 + 𝜇2𝛿(𝑓𝑘). acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 65 4.1. simulation results first a random independent data loss pattern has been generated, then it has been identified by the proposed method. the parameters of the simulation are summarized in table 2, where 𝐿 is the total length of the record in samples, 𝑁 is the fft size. the spectra have been exponentially averaged, with a smoothing factor 𝛼. the constant 𝜇 is the parameter of the data loss model. at the end a 10th order lpc model has been fitted, in order to check the dynamic behavior of the data loss. the identified model parameters are the following: �̂� = 0.9900, 𝑎� = 0.0013. (21) note that 𝑎� = −𝑎1 is the spectral parameter of the twostate markov model, according to (20). as already 𝑎1 ≈ 0, the random independent data loss has been verified. the psd of the model has also been calculated. the upper plot of figure 4 shows the psd of the data loss pattern (green line), and the psd of the model (blue line). the coefficients of the 10th order ar system are depicted in the lower plot. it can be seen that the fitted psd is in good accordance with the generated one. all the ar coefficients equal approximately zero except the first one. the second simulation example deals with two-state markov-based data loss. the parameters of the simulation are summarized in table 3, where 𝐿 is the total length of the record in samples, 𝑁 is the fft size, and 𝛼 is the smoothing factor, again. the constants 𝑝 and 𝑞 are the parameters of the markov model. a 10th order lpc model has been fitted as before. the identified model parameters are the following: �̂� = 0.9118, 𝑎� = 0.8872, �̂� = 0.9900, 𝑞� = 0.8971 . (22) the original parameters of the model are 𝑝 and 𝑞, therefore the data availability rate 𝜇 is a resulted constant. the second lpc coefficient 𝑎1 is non-zero, but the rest of the coefficients are close to zero. the main result of the identification �̂� and 𝑞� are really close to the initial parameters given in table 3. the psd of the model has also been calculated. the upper plot of figure 5 shows the psd of the data loss pattern (green line), and the psd of the model (blue line). the coefficients of the 10th order ar system are depicted in the lower plot. both the spectra and the lpc coefficients verify that the calculated first order model is appropriate for the two-state markov-based data loss. 4.2. measurement results measurements were carried out by a test system introduced in [13]. in this testbed, wireless sensors perform real-time data collection, and they transmit the data to a pc through a gateway node. in this measurement we used only one sensor. the data sent by the sensor are recorded and processed on the pc. since data transmission and collection is performed in a hard realtime manner, there is no possibility to apply any acknowledge mechanism for the indication and retransmission of lost packets, hence data loss is inevitable. the data loss is recognized by a time-out mechanism. the sampling frequency is 𝑓𝑠 = 1800 hz, and the sensor transmits data packets of 𝑀 = 25 samples. if data loss occurs, it can be described by the block-based model. in the first measurement setup, the sensor and the gateway were placed 4 m away from each other in a room, and the sensor was placed near to a big wardrobe made of metal, in order to degrade the radio transmission properties. the extensive metal surfaces of the wardrobe without any special arrangement (e.g., connection to the earth) produced data loss in the communication. depending on the physical circumstances, data loss rates in the range of [0.1 … 30]% could be detected. now the analysis results for the 3.75% case are introduced. the parameters of the measurement are summarized in table 4, where 𝐿𝐵 is the length of the record in figure 4. identification of a random independent data loss: psd of the data loss pattern, and the psd of the model (upper plot). coefficients of the identified ar system (lower plot). figure 5. identification of a markov-model based data loss: psd of the data loss pattern, and the psd of the model (upper plot). coefficients of the identified ar system (lower plot). table 2. main data of the first simulation. record length 𝐿 fft length 𝑁 smoothing factor 𝛼 𝜇 106 1024 0.01 0.9900 table 3. main data of the second simulation. record length 𝐿 fft length 𝑁 smoothing factor 𝛼 𝑝 𝑞 106 1024 0.01 0.9900 0.9000 table 4. main data of the first experiment. record length in blocks, 𝐿𝐵 duration of the record, 𝑡𝑚 fft length, 𝑁 smoothing factor, 𝛼 4638 64.4 sec 1024 0.01 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 -40 -30 -20 -10 0 10 relative frequency, f p s d , db s(f) spectrum of k_n spectrum of the identified model 0 1 2 3 4 5 6 7 8 9 10 -0.5 0 0.5 1 order, i a(i) 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 -40 -30 -20 -10 0 10 relative frequency, f p s d , db s(f) spectrum of k_n spectrum of the identified model 0 1 2 3 4 5 6 7 8 9 10 -1 -0.5 0 0.5 1 order, i a(i) acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 66 blocks, while 𝑡𝑚 is its duration. the parameter 𝑁 denotes the fft size, and 𝛼 is the smoothing factor. the result of the identification can be seen in figure 6. the estimated data loss parameters are the following: �̂� = 0.9625, 𝑎� = 0.0201, �̂� = 0.9632, 𝑞� = 0.0569 . (23) in order to check our assumptions, the estimators �̂� and 𝑞� of a two-state markov-model have also been calculated. the system is usually in the '1' state, and if it moves to '0', it has a small probability that stays also in '0' for the next time instant. the upper plot of figure 6 shows the psd of the measured data loss pattern (green line), and the psd of the model (blue line). the coefficients of the 10th order ar system are depicted in the lower plot. both the graphical result and the estimated parameters imply that this radio communication suffers from random independent block-based data loss. the second measurement aimed the investigation of a different data loss mode. a mobile phone has been placed next to the wireless sensor, and the wifi function of the phone has been activated by playing an on-line media stream. as both devices use the same 2.4 ghz radio band, the communication of the phone causes a disturbance for the wireless sensor. the parameters of the measurement are summarized in table 5, where 𝐿𝐵 is the length of the record in blocks, while 𝑡𝑚 is its duration. the parameter 𝑁 denotes the fft size, and 𝛼 is the smoothing factor. the result of the identification can be seen in figure 7. the estimated data loss parameters are the following: �̂� = 0.9533, 𝑎� = 0.2529, �̂� = 0.9651, 𝑞� = 0.2878 . (24) as 𝑎1 ≠ 0, the data loss introduced by the wifi function of the mobile phone cannot be random independent. nevertheless, a two-state markov-based model can be well fitted to this data loss pattern, as the rest of the coefficients are sufficiently small (𝑎2, … ≈ 0). the psd of the model has also been calculated. the upper plot of figure 7 shows the psd of the measured data loss pattern (green line), and the psd of the model (blue line). the coefficients of the 10th order ar system are depicted in the lower plot. the last example demonstrates yet another lossy measurement. the wireless sensor has been carried by a walking person in the building, resulting in an occasionally very large distance from the base station. the parameters of the measurement are summarized in table 6, where the parameters 𝐿𝐵, 𝑡𝑚, 𝑁, and 𝛼 have the same meaning as before. the result of the identification can be seen in figure 8. based on the psd of the data loss pattern, we have tried to fit a two-state markov model. the estimated data loss parameters are the following: �̂� = 0.8829, 𝑎� = 0.5915, �̂� = 0.9522, 𝑞� = 0.6393 . (25) figure 7. identification results of the second experiment: psd of the measured indicator function, and the psd of the model (upper plot). coefficients of the identified ar system (lower plot). table 5. main data of the second experiment. record length in blocks, 𝐿𝐵 duration of the record, 𝑡𝑚 fft length, 𝑁 smoothing factor, 𝛼 25988 361 sec 1024 0.01 figure 6. identification results of the first experiment: psd of the measured indicator function, and the psd of the model (upper plot). coefficients of the identified ar system (lower plot). table 6. main data of the third experiment. record length in blocks, 𝐿𝐵 duration of the record, 𝑡𝑚 fft length, 𝑁 smoothing factor, 𝛼 10200 142 sec 1024 0.01 figure 8. identification results of the third experiment: psd of the measured indicator function, and the psd of the model (upper plot). coefficients of the identified ar system (lower plot). 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 -40 -30 -20 -10 0 relative frequency, f p s d , db s(f) measured spectrum spectrum of the identified model 0 1 2 3 4 5 6 7 8 9 10 -0.5 0 0.5 1 order, i a(i) 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 -40 -30 -20 -10 0 relative frequency, f p s d , db s(f) measured spectrum spectrum of the identified model 0 1 2 3 4 5 6 7 8 9 10 -0.5 0 0.5 1 order, i a(i) 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 -40 -30 -20 -10 0 relative frequency, f p s d , db s(f) measured spectrum spectrum of the identified model 0 1 2 3 4 5 6 7 8 9 10 -1 -0.5 0 0.5 1 order, i a(i) acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 67 the psd of the model has also been calculated. the upper plot of figure 8 shows the psd of the measured data loss pattern (green line), and the psd of the model (blue line). the coefficients of the 10th order ar system are depicted in the lower plot. in this case the psd of the model does not fit well the measured psd. it can also be observed that the higherorder lpc parameters are non-zero, as well. finally we have found that a third-order markov model describes the data loss well. the estimated lpc parameters are the following: 𝐚 = [1, −0.6031, −0.2235, −0.1483]. (26) the psd of the model has also been calculated. figure 9 shows the psd of the measured data loss pattern (green line), and the psd of the model (blue line). here the graphical representation of the spectral coefficients has already been omitted. the model already fits to the measured spectrum, the identification is successful. now the state transition probabilities can be calculated using the formula (16): 𝐏 = ⎣ ⎢ ⎢ ⎢ ⎢ ⎢ ⎢ ⎡ 0.978 0 0.83 0 0 0 0 0 0.022 0 0.17 0 0 0 0 0 0 0 0 0 0.75 0 0.61 0 0 0 0 0 0.25 0 0.39 0 0 0.37 0 0.23 0 0 0 0 0 0.63 0 0.23 0 0 0 0 0 0 0 0 0.15 0 0 0.003 0 0 0 0 0.85 0 0 0.997⎦ ⎥ ⎥ ⎥ ⎥ ⎥ ⎥ ⎤ . (27) unfortunately, the probability matrix 𝐏 has a size of 8 by 8 as the model has 23 = 8 states. note that the zero probabilities belong to impossible state transitions. here the probabilities 𝑃11 = 0.978 and 𝑃88 = 0.997 are heightened. they can be interpreted as the probability that the system remains in the 'data loss' and the 'data available' state, respectively. during the walk the distance between the sensor node and the pc has been continuously increased and decreased, resulting in continuously increasing and decreasing data loss. it means that the data loss is not stationary which explains the necessity of the higher order markov-model. 5. conclusions recently the analysis of measurement data loss by the spread of sensor networks and internet-based technology has gained importance. the investigation of the fft based psd estimation in the case of data loss has discovered the exact relation between spectral leakage and some data loss models. this paper introduced the inverse procedure: the data loss model can be identified by the fourier transform of the socalled data availability indicator function. the identification procedure has been elaborated for random independent, random block-based, and markov model-based data loss. frequency domain identification also supports the verification process, as model fitting and checking can be accomplished simultaneously. the use of the fourier transform is the main novelty of our approach. by its use not only the parameters of the data loss model can be calculated, but the model order can be determined, as well. the method has been intensively tested by simulations and measurements. based on the experiences, the proposed procedure is a promising solution for data loss model identification. further research is required if the data availability function is not stationary or it is not directly available. references [1] l. kong et al., “data loss and reconstruction in wireless sensor networks”, proc. ieee infocom 2013, turin, apr. 14-19, 2013, pp. 1654-1662. [2] m. mathiesen, g. thonet, n. aakwaag, “wireless ad-hoc networks for industrial automation: current trends and future prospects”, proc. of the ifac world congr., prague, czech republic, july 4-8, 2005, pp. 89-100. [3] m. yajnik, et al., “measurement and modelling of the temporal dependence in packet loss”, in: proc. ieee infocom 1999, pp. 345-352. [4] m. ellis et al., a two-level markov model for packet loss in udp/ip-based real-time video applications targeting residential users, computer networks 70 (2014), pp. 384-399. [5] j. rachwalski, “analysis of packet loss pattern for concatenated transmission channels using burst ratio parameter”, phd dissertation, agh university of science and technology, 2016. [6] h. sanneck, g. carle, and r. koodli. “framework model for packet loss metrics based on loss runlengths”, proc spie int soc opt eng. 3969 (2000). [7] g. sinopoli et. al, kalman filtering with intermittent observations, ieee trans. autom. control 49 (2004) pp. 14531464. [8] l. sujbert, gy. orosz, fft-based spectrum analysis in the case of data loss, ieee trans. instrum. meas. 65 (2016) pp. 968976. [9] j. s. bendat, a. g. piersol, random data: analysis and measurement procedures, john wiley and sons, inc., new york, london, sidney, toronto, 1971. [10] t. nagayama, b. f. spencer, g. agha, and k. mechitov, “modelbased data aggregation for structural monitoring employing smart sensors”, 3rd int. conf. on networked sensing systems (inss). [11] o. hohlfeld, r. geib, g. hasslinger, “packet loss in real-time services: markovian models generating qoe impairments”, 16th int. workshop on quality of service, enschede, june 2008, pp. 239-248. [12] p. boufounos, “generating binary processes with all-pole spectra”, ieee int. conf. acoustics, speech and signal processing, honolulu, hi, vol. 3, apr. 15-20, 2007, pp. 981-984. [13] gy. orosz, l. sujbert, g. péceli, “testbed for wireless adaptive signal processing systems”, proc. ieee instrum. and meas. conf., warsaw, poland, may 1-3, 2007, pp. 123-128. [14] jackson, l.b., digital filters and signal processing, 2nd ed., kluwer academic publishers, 1989. [15] matlab (2010), www.mathworks.com figure 9. identification of a third order markov-model in the third experiment: psd of the measured indicator function, and the psd of the model. 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 -40 -35 -30 -25 -20 -15 -10 -5 0 relative frequency, f p s d , db s(f) measured spectrum spectrum of the identified model frequency domain identification of data loss models << /ascii85encodepages false /allowtransparency false /autopositionepsfiles true /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /error /compatibilitylevel 1.4 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.0000 /colorconversionstrategy /cmyk /dothumbnails false /embedallfonts true 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can be opened with acrobat and adobe reader 5.0 and later.) >> /namespace [ (adobe) (common) (1.0) ] /othernamespaces [ << /asreaderspreads false /cropimagestoframes true /errorcontrol /warnandcontinue /flattenerignorespreadoverrides false /includeguidesgrids false /includenonprinting false /includeslug false /namespace [ (adobe) (indesign) (4.0) ] /omitplacedbitmaps false /omitplacedeps false /omitplacedpdf false /simulateoverprint /legacy >> << /addbleedmarks false /addcolorbars false /addcropmarks false /addpageinfo false /addregmarks false /convertcolors /converttocmyk /destinationprofilename () /destinationprofileselector /documentcmyk /downsample16bitimages true /flattenerpreset << /presetselector /mediumresolution >> /formelements false /generatestructure false /includebookmarks false /includehyperlinks false /includeinteractive false /includelayers false /includeprofiles false /multimediahandling /useobjectsettings /namespace [ (adobe) (creativesuite) (2.0) ] /pdfxoutputintentprofileselector /documentcmyk /preserveediting true /untaggedcmykhandling /leaveuntagged /untaggedrgbhandling /usedocumentprofile /usedocumentbleed false >> ] >> setdistillerparams << /hwresolution [2400 2400] /pagesize [612.000 792.000] >> setpagedevice concept and setup of a multi-component facility for force and torque in the range of 1 mn and 2 kn·m acta imeko june 2014, volume 3, number 2, 39 – 43 www.imeko.org concept and setup of a multi-component facility for force and torque in the range of 1 mn and 2 kn·m sebastian baumgarten, dirk röske, holger kahmann, dietmar mauersberger, rolf kumme physikalisch-technische bundesanstalt (ptb), bundesallee 100, 38116 braunschweig, germany section: research paper keywords: multi-component measurement, friction coefficient sensor, screws, clamp force, torque citation: sebastian baumgarten, dirk röske, holger kahmann, dietmar mauersberger, rolf kumme, concept and setup of a multi-component facility for force and torque in the range of 1 mn and 2 kn•m, acta imeko, vol. 3, no. 2, article 10, june 2014, identifier: imeko-acta-03 (2014)-02-10 editor: paolo carbone, university of perugia received february 7th, 2013; in final form june 28th, 2014; published june 2014 copyright: © 2014 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: (none reported) corresponding author: sebastian baumgarten, e-mail: sebastian.baumgarten@ptb.de 1. introduction screw joints are an essential constructional element in nearly all fields of economy and everyday life. the screw industry aims to improve screws and to optimize the interaction between the screw and the material to be fastened, in order to make this fastening element safer and better value. in this context, friction coefficient sensors have been used for a few years. these are multi-component transducers which can measure both an axial force and at least one torque. in the case of screws, the forces involved are: the prestressing force and the tightening torque or the friction torque between the screw head and the support as well as inside the thread. industry entrusted the ptb with the qualification of the various commercially available friction coefficient sensors using a suitable calibration procedure to enable traceable measurements. in order to meet these needs and to fulfil its mandate (dissemination of the units), the ptb set up a novel measuring facility which enables the simultaneous generation of both a precise axial force and of a very accurately known torque into a friction coefficient measuring system. this paper presents the principle and the design of this multi-component measuring facility. 2. design and conception of the auxiliary device 2.1. schematic setup to attain the forces required for the selected measuring range, it was decided to place a two-armed lever with a horizontal lever side plate into the force flow of ptb's 1-mn force standard machine (1-mn-k-nme). a force couple generating the required torque is to act at the ends of this lever. the force is transmitted to the lever arm by means of thin metallic bands which act tangentially and are placed parallel to each other. two mass stacks with different load masses are located on either side of the device to generate force. the forces, which are realized as gravitational forces by means of load masses (principle of the direct deadweight effect), ensure that a sufficiently small measurement uncertainty can be achieved. the vertical force is transformed into a horizontal force by means of a converting device. to this end, an air bearing is used to minimize force losses due to friction. the limited space offers only little clearance for the positioning of the load masses. lift tables located beneath the operation platform are the optimal solution. on each lift table a linear table and a rotation disc is mounted. the load masses rest on the rotations disc. the individual weights are applied abstract this article describes the design of a measuring facility which can be used to investigate and calibrate so-called "friction coefficient sensors". these measuring facilities are used to measure the prestressing force and the tightening torque, respectively the friction torque of screws. these measurements are aimed at optimizing screw joints. the measuring facility described here is part of a force standard machine (fsm). in addition to the force which this system can realize with a very small measurement uncertainty of 0.002 % (k = 2) (in the measuring range from 20 kn to 1 mn), it can also generate an extremely precise torque (objective: better than 0.005 % at k = 2) in the range from 20 n·m to 2 kn·m. acta imeko | www.imeko.org june 2014 | volume 3 | number 2 | 39 consecutively by lowering the lift table. converting the force effect from a vertical gravitational force into tensile force acting horizontally at the end of the lever posed a problem. figure 1 shows a simplified schematic drawing of the planned set-up. the drawing shows one mass stack only – a second one is located symmetrically on the opposite side. figure 2 shows the planned set-up of the auxiliary device in the form of a cad model. the device in question is a modular principle which can also be applied to other force standard machines. there are some similarities to an older torque calibration machine at the npl [1]. with the additional facility that is being built, axial forces in the range from 20 kn to 1 mn and torques in the range from 20 n·m to 2 kn·m can be realized. 2.2. torque transmitting system to generate a pure torque mz, one needs a force couple with the same force value but of opposite direction. small deviations from the ideal orientation of the force vectors lead to additional bending moments and to a reduced value of the torque mz. the force couple needed is realized by means of two identical load masses located at the end of levers facing each other. the exact coordination of the orientation of the mass stacks and of the force bands is particularly challenging. in order to ensure an optimal positioning of the bands during the loading phase, various sensors are integrated into the system, and the direction can, if necessary, be corrected by means of stepper motors. in the first orientation phase, the spatial position of the mass stacks and of the force bands is determined as a function of the lever position. the spatial coordinates are measured using the portable coordinate measuring machine in the form of an index arm, figure 3. the index arm can be mounted at each pillar of the force standard machine frame. single points are measured with a relative uncertainty (k = 2) of δx = 10 µm. with a range of 1200 mm it is not possible to reach all characteristics of the entire system from one position so the measurement arms have to be mounted at every pillar. the software guarantees a coordinate transformation for the new point of origin. from the data of the different positions and orientation of the modules the necessary displacements and rotation angles can be calculated. first, one of the mass stacks is rotated until the band makes contact with the outer surface of the lever. this position is used as a reference; the coordinate measuring index arm is then mounted onto the opposite side. the coordinate measuring system is maintained, and the relation between the second mass stack and the lever is now determined. the force application point of both bands on the lever must be identical. the second mass stack is shifted on a linear table and rotated by means of a rotary table until this condition is fulfilled. figure 4 is a schematic representation of the completed orientation shown from above. 2.3. vectorial representation the vectors and positions in space are illustrated in figure 5. the point p0 is the centre of the lever. pb is the point in the middle of the rotor in which the band starts to roll off the rotor. the force application point is referred as pa. the points pm1, pm2, pb and p0 are measurement points which can be determined by means of the coordinate measuring arm. the points pm1 and pm2 span the vector 21 mm pp which describes the position in space of the axis of the rotor equipped with an air bearing. point pb is – geometrically – equidistant from the points pm1 and pm2. the point where the band runs off the air bearing shaft is located at the vertical distance rrotor (rrotor = radius of the air bearing shaft) from the axis. to figure 1. schematic representation of the set-up for a loading device, side view with mass stack, air bearing, metal band and lever arm in the set-up of the 1-mn-k-nme. figure 2. cad model of the set-up of the multi-component measuring facility and additional mass stacks. figure 3. the figure shows the portable coordinate measuring arm, which is mounted at a pillar of the 1-mn-k-nme frame. acta imeko | www.imeko.org june 2014 | volume 3 | number 2 | 40 simplify the description, all points are considered as lying in the xy plane. in reality, this assumption can, however, be guaranteed to a certain extent only. when orientating the system, the objective is to come as close as possible to the ideal case ∆z = 0 between all measurement points. in order to determine the rotation angle of a mass stack in such a way that the band can be applied tangentially to the lever front surface, the following steps are necessary: determining the vector 21 mm pp and the vector bpp0 . from this, it is possible to calculate the angle γ1 under which the two vectors cross each other. the triangle papbp0 must be right-angled. the length of the vector app0 is identical with the half of the lever length llever. with the lever radius r and the value of the vector bpp0 being known, the angle γ2 can be calculated. the rotation angle by which the mass stack must be rotated around pb is yielded from the difference between the two angles. the point pa is the only point that cannot be measured. it represents the force application point and must be determined from the other geometrical quantities. 2.4. control sensors during the measurement, displacements of the band, of the air bearing shaft or of the load stack may occur. the individual sensors are there to detect distance changes. with the signals they provide, various stepped motors are controlled which can then compensate for these displacements. the sensors a-d are installed in such a way that it is possible to check the orientation of the metallic bands also during a loading operation of up to 1 mn-k-nme. in more detail, sensor a is an optical distance sensor with a wavelength of 670 nm. the height change between the end of the lever and a fixed reference point at the frame of the force machine is determined with a laser. the resolution is in the range of 7 µm with a linearity error lesser then 1%. during the loading and under its own weight, the lever end may underlie a deformation, the force vector thus changing by an additional z component (see figure 6). sensor a controls a stepped motor which can vary the height of the air bearing head by the same difference in a range of 16 mm with a relative uncertainty of 6.25 · 10-4 mm. the operating principle and wavelength of sensor b are identical to that of sensor a. the resolution is in the range of 20 µm with a linearity error less than 1%. it is used to determine the distance between the topmost weight and a fixed reference point at the frame of the load stack. due to a change in the length of the band caused by thermal or mechanical influences, a change in the length of the band could cause individual weights not to be applied correctly or even not to be applied at all. the data transmitted by sensor b are used to control the lowering effected by the motor. if during the loading operation the force vector ba pp is no longer perpendicular to the air bearing axis 21 mm pp , this leads to an additional force effecting on the air bearing shaft. the shaft will then slowly drift laterally inside the air bearing stator. to detect a displacement of the air bearing shaft, sensor c is used to measure the distance capacitively and sensor d to determine the inclination (see figure 5). the inclination sensor is custom-made. it has a measuring range of ± 0.2° and a resolution of ~ 1 · 10-6 °. the measuring range of the capacitive sensor is 2 mm with a relative uncertainty (k = 2) of 4.5 · 10-2 mm. the two sensors control stepper motors which adjust the position of the roll by varying the inclination and by displacing the complete air bearing head. the optimal position is attained when no more drift displacements can be observed. figure 4. schematic top view of the lever system and bands once the orientation of the mass stacks has been completed. figure 5. representation of the measurement points in the xy plane and of the vectors and angles that can be calculated on this basis and that are necessary to orientate the system. figure 6. sensors a, c and d, used to determine the position and the inclination of the air bearing roll. displacements are measured by optical or capacitive means. acta imeko | www.imeko.org june 2014 | volume 3 | number 2 | 41 3. modules of the multi-component measuring facility 3.1. axial force of the 1-mn force standard machine the axial force is realized by the 1-mn force standard machine – a dead-weight machine. the measurement uncertainty of the 1-mn force standard machine is known sufficiently well and has been investigated on different occasions [2,3,4]. the relative measurement uncertainty (k = 2) of the 1-mn force standard machine is 2 · 10−5. the deviation from the ideal force vector development was investigated in [5]. this investigation showed that, at an axial force fz = 80 kn, shear forces fx = fy ~ 10 n can be expected. furthermore, a 6component measuring platform was implemented in the upper part of the force standard machine in order to, in future, be able to measure also parasitical influences. 3.2. lever arm the lever used to generate the torque has a length of 2 m and an uncertainty ∆llever = 1 µm. figure 7 shows a version made of aluminium. the shape has been chosen in such a way that a torque can be generated in both directions (left and right), hereby exploiting the space of installation optimally. in order to reduce changes in torque due to the dilatation of the lever arm, invar steel was chosen for the material, since it has a very low linear thermal expansion coefficient of < 2 · 10−6 k−1 at room temperature. if the temperature changes by 1 k, the length of the lever changes by ∆llever = 3.7 · 10-3 mm. the torque mz will be increased by a value of 3.7 · 10-6 n·m. 3.3. masses the additional mass stacks located on either side are composed of 10 individual weights. one stack has load masses for the following weight forces: 1 x 10 n, 2 x 20 n, 1 x 50 n, 3 x 100 n and 3 x 200 n. the maximum load mass is approx. 100 kg. each mass stack thus generates a couple of 1000 n. the material chosen for the weights is x2crnimon 18-14. this is a corrosion-resistant steel which is not easily magnetisable and is mainly used in maritime constructions. for the masses, a relative uncertainty of 5 · 10-6 can be stated. for a realistic estimation of the resulting force, also ambient influences, such as temperature, pressure, and humidity, have to be taken into account. the air buoyancy contributes to reducing the effective mass. the value of the force can be calculated by means of the following formula:       −⋅⋅= m air1 ρ ρ gmf (1) t ehp t air + ⋅⋅−⋅ = ⋅ 15,273 009024,034848,0 0612,0rρ (2) where m is the mass, g is the acceleration due to gravity (9.81253 m · s-2), p the air pressure, hr the air humidity, t the temperature and ρm the density of the weights. in order to keep ambient influences, such as changes in temperature, pressure and humidity, as low as possible, the complete measurement set-up is located in a fully airconditioned hall. the temperature varies over a range of ∆t = 0.2 k, and ∆hr = 5 % for the humidity. the value of the gravitational acceleration inside the measurement hall was determined with g = 9.81253 m·s-2 with a relative measurement uncertainty of 0.0001 %. 3.4. force-torque-sensor for the calibration of the two-component measuring equipment, a transfer standard with a sufficiently small relative measurement uncertainty of < 1 · 10-4 is required. the transfer standard is a two-component transducer covering a measuring range with axial forces fz ~ 500 kn and torques mz ~ 500 n·m. the selected force/torque transducer is realized in the form of a build-up system (figure 8). the sensor is calibrated separately with regard to the axial force and with regard to torque. the adaption parts allow it to be mounted in the 1-mn force standard machine for an axial load with a relative measurement uncertainty of 2 · 10−5 as well as in the 1kn·m torque standard machine (1-kn·m-dmnme) [6] with a relative measurement uncertainty (k = 2) of 2 · 10−5. afterwards, the cross-talk of the transducer when loaded simultaneously with fz and mz can be investigated in the twocomponent measuring equipment. 4. vectorial error analysis generating a torque by means of a couple requires a vectorial approach (see figure 5). the direction of both the force vector and the distance vector must be determined. deviations from the orthogonal orientation of the two vectors in the xy plane make it necessary to consider vectorial error figure 7. the figure shows the first version of the aluminium lever mounted on the 1-mn force standard machine. the shown shape has been retained for the invar lever. one can also see the measurement points used to determine the position towards which the coordinate measuring machine later moves. figure 8. force-torque-transducer as a build-up-system of a 500 n·m torque-transducer on the top and a 500 kn force-transducer between these and the pressure plate under it. acta imeko | www.imeko.org june 2014 | volume 3 | number 2 | 42 analysis. if only one side of the system is taken into account, the torque obtained then results from the vector product of the distance and the force vectors: f pp pp ppppppfrm ba ba abaa ⋅×=×=×= 00  . (3) in order to obtain a statement with regard to the individual moments, one needs the complete data concerning the position of the vectors app0 and ba pp . the vector product is solved in equation (4):           =           − − − = z y x baabaa baabaa baabaa ba m m m pppppppp pppppppp pppppppp pp f m xyyx zxxz yzzy 00 00 00 ·  . (4) the problem is that the exact position of the force application point pa is not known. it is, therefore, not possible to measure directly with the index arm. the point pa can, however, be calculated from points p0 and pb, from the knowledge of the value of the vector app0 , but also from the fact that the triangle papbp0 must be right-angled. the values for xa pp0 , yapp0 and zapp0 can be determined with the aid of the software of the coordinate measuring arm – an uncertainty statement is, however, not given. to obtain an uncertainty for the point pa, the internal calculation must be reconstructed. a detailed calculation of app0 , of the individual components mx, my and mz, as well as of their uncertainty will be the subject of another publication. for the torque, a relative measurement uncertainty in the range of 5 · 10-5 is aimed at. an accurate analysis of the measurement uncertainty budget from sources of errors such as, e.g., the mass, lever length and bending, orientation and length of the metallic band, and ambient influences will also be the subject of a subsequent, detailed analysis. 5. outlook the relative measurement uncertainty (k = 2) of the 1-mn force standard machine is 2 · 10−5. according to first estimations, we are expecting a relative measurement uncertainty in a range smaller than 5 · 10−5 for torque generation. the measurement uncertainty budget will be considered to a greater extent after the facility has been commissioned. the aim is to reduce the measurement uncertainty by optimizing the measurement cycle with regard to the acquisition of the spatial coordinates. it is expected that this novel measurement facility will lay the basis for the calibration of friction coefficient sensors. a first step towards this is calibration by means of a transfer standard. in addition, a comparison of diverse commercial friction coefficient sensors is aimed at. furthermore, the development of the facility is to be pursued with regard to a possible introduction of bending moments into multi-component transducers. references [1] a. robinson, “the commissioning of the first uk national standard static torque calibration machine“, 19th conference on force, mass and torque measurement, cairo, egypt, 2005, pp.1-6. [2] w. weiler, m. peters, h. gassmann, h. fricke, w.ackerschott, “die 1-mn-normalmeßeinrichtung der ptb braunschweig.”, vdi-z 120, 1978, pp.1-6. [3] a. sawla, w. weiler, m. peters, a. bray, r. levi, m. vattasso “a comparsion of force standards between the imgc and the ptb”, 6th conf imeko tc3, odessa, pp.1-13. [4] w. weiler, a. sawla “force standard machines of the national institutes for metrology”, ptb bericht me-22, 1978. [5] c. ferrero, “the measurement of parasitic components in national force standard machines”, measurement, vol. 8, no. 2, apr-jun 1990, pp.66-76. [6] k. adolf, d. mauersberger, d. peschel “specifications and uncertainty of measurement of the ptb’s 1knm torque standard machine”, proc. of the 14th imeko tc3 conference, september 5-8, 1995, warsaw, poland, pp.178-182. acta imeko | www.imeko.org june 2014 | volume 3 | number 2 | 43 concept and setup of a multi-component facility for force and torque in the range of 1 mn and 2 kn m inventory and monitoring of historical cultural heritage buildings on a territorial scale: a preliminary study of structural health monitoring based on the cartis approach acta imeko issn: 2221-870x march 2021, volume 10, number 1, 57 69 acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 57 inventory and monitoring of historical cultural heritage buildings on a territorial scale: a preliminary study of structural health monitoring based on the cartis approach renato s. olivito1, saverio porzio1, carmelo scuro2, domenico l. carnì3, francesco lamonaca4 1 dinci – department of civil engineering, university of calabria, 87036 rende (cs), italy 2 department of physics, university of calabria, 87036 rende (cs), italy 3 dimes department of informatics modelling electronics and systems science, university of calabria, 87036 rende (cs), italy 4 ding department of engineering, university of sannio, 82100 benevento, italy section: research paper keywords: seismic vulnerability; monitoring system; structural health monitoring (shm); iot citation: renato sante olivito, saverio porzio, carmelo scuro, domenico luca carnì, francesco lamonaca, inventory and monitoring of historical cultural heritage buildings at the territorial scale. a preliminary study of shm based on cartis approach, acta imeko, vol. 10, no. 1, article 9, march 2021, identifier: imeko-acta-10 (2021)-01-09 editor: eulalia balestrieri, university of sannio, italy received april 22, 2020; in final form june 18, 2020; published march 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: carmelo scuro, e-mail: carmelo.scuro@unical.it 1. introduction seismic-damage prevention is one of the main goals of researchers in the management of historical heritage buildings. however, an appropriate appraisal of the building’s heritage requires the definition of the building’s characteristics for examination, especially in terms of (i) the construction type (i.e. reinforced concrete [rc] or masonry), (ii) the intended use (i.e. residential or specialised), and (iii) the state of conservation (i.e. good or neglected) [1]-[3]. the first step in obtaining a building inventory is to ensure the survey measure is focused on non-specialist buildings. the narrowing of the investigation framework is important for optimising the analytical effectiveness 0, [5]. this paper presents a preliminary study aimed at defining a methodology for coupling the recent trends in building inventory procedures with those of structural monitoring. in the last few decades, various attempts have been made to formulate refined collecting models. at the regional scale, the census data provides the main source for a classification of building vulnerability. meanwhile, the data sources provided by interview-based or building-by-building surveys allows for an in-depth examination, despite the significant difficulty in accessing the relevant information [6]. from the collaboration between the seismic engineering university laboratories network (reluis) and the civil abstract earthquakes induce dynamic stresses in structures, and past seismic events have demonstrated that existing heritage buildings are highly vulnerable. this vulnerability applies to both reinforced-concrete and masonry buildings, which are concentrated in historic centres throughout italy. significant variations in construction account for the inadequacy of existing structures to withstand seismic actions, such as the materials used and the construction details, which can be neglected in building practices. this work focuses on the analysis of heritage buildings through an inventory using the caratterizzazione tipologica strutturale (cartis) form developed by the seismic engineering university laboratories network in conjunction with the civil protection department. on knowing a building framework, structural health monitoring (shm) systems can be applied on the town compartments (tcs) that are prone to the highest vulnerabilities. a priority criticalities scale can be devised starting from the building inventory by identifying the tcs through the cartis-based data. this approach can be used to determine a safety threshold obtained via structural parametrical analysis using commercial software (vemnl) with different building typologies. the next stage consists of the implementation of appropriate shm to provide important information regarding the structural integrity of the buildings. the proposed methodology is outlined in this paper with reference to the suggested shm system. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 58 protection department (dpc) emerged the caratterizzazione tipologica strutturale (cartis) tool. this is a territorial knowledge tool based on the concept of buildings with specific structural feature typologies and presents an approach that is remarkably adaptable to the diversity of buildings throughout italy, where each city is the result of an historic evolution leading to the current situation. the use of speedy and efficient approaches allows for the individuation of the critical town compartments (tcs) that indicate the major structural safety issues. real-time damage identification systems can help to mitigate seismic risk. here, the aim is to act exclusively on the most vulnerable buildings with minimal intervention in order to optimise both the technical and the economic resources, which can be achieved by using structural health monitoring (shm) systems combined with the internet of things (iot) paradigm [7]-[14]. this work presents a preliminary study aimed at defining an integrated methodology for inventory, monitoring, transmission, and data management. the remainder of the paper is organised as follows. sections 2–4 outline the cartis approach and the theoretical formulation of seismic vulnerability as well as the equivalent frame model. section 5 then introduces and explains the monitoring systems related to historical building construction before section 6 presents the preliminary study based on the application of cartis, with the parametric numerical modelling and the shm system presented. the conclusions are then drawn in section 7. 2. building inventory using the cartis approach defining homogeneous territorial zones, namely, the tcs, is the first step of the cartis methodology [15]. these zones group buildings of the same age and construction technique. the data collection is carried out by filling out forms subdivided into different levels: a first-level single form for the city (figure 1); second-level forms, one for each tc individuated in the local territory (figure 2); and third-level forms, one for each building type (bt) within each tc (figure 3). figures 1, 2 and 3 show the application of the various levels of cartis to a small town located in the central part of the calabria region, fuscaldo, identifying two different tcs in its historical compartment and the most ubiquitous masonry building types. qualified technicians (professionals and researchers) conduct the survey campaign both in terms of building-by-building surveys and through interviewing local experts operating in the sector with an in-depth knowledge of the territory. in third-level surveying, the classification of the building typologies of each compartment is carried out by identifying the macro-classes of the buildings, that is, rc or masonry (labelled as car and mur, respectively, on the forms). other determining factors that contribute to defining the typologies include slab-type, the number of floors, and the number (percentage) of openings. however, it is worth noting that there can be multiple masonry typologies. in rc buildings, the appropriate building details correspond to the date of construction (e.g. class of concrete strength, diameter/spacing of strips, percentage of rebars in the beams and pillars). meanwhile, for masonry buildings, a more marked variability tends to exist with the same type of material due to the texture and the cross sections, the presence of courses and diatons, and the type of corner connection and thin-bed joint, etc. [16], [17]. furthermore, these buildings are generally geometrically irregular in terms of both plan and elevation. clearly, the role of the data collector is fundamental and is highly related to their ability to access information. figure 1. first-level cartis approach. figure 2. second-level cartis approach. identification of tcs. figure 3. third-level cartis approach. identification of the characteristics of the existing buildings in the various tcs. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 59 while the collected data may appear to be accumulated, they can still be used to define vulnerability models on different scales. another aspect of the potential of cartis is the web application (available at https://cartis.plinivs.it), where all the data manually gathered through the forms are introduced and are accessible to the scientific community. the web application guides the data entry procedure, reducing the chances of human error that can affect the collection. moreover, specific ‘query’ commands allow the data to be queried. as such, the same information can be subjectively assessed to define and calibrate new refined models of seismic vulnerability [17], [18]. 3. seismic vulnerability nowadays, various approaches are employed to estimate the seismic vulnerability of buildings [19]. however, the strategy employed must establish a priori (i) the differentials that contribute to defining the seismic reactions of a typological class, (ii) the damage scale, and (iii) the extent to which these classified buildings are vulnerable to a determined level of seismic intensity [20]. therefore, researchers have attempted to standardise the vulnerability concept to ensure analogous investigations [21]. in this framework, the damage probability matrix (dpm) is one of the main tools employed [22]. this relates to the probability that a determined damage level k occurs for a seismic event of a certain intensity,      = =: | | i p p d k i k i (1) taking into account a specified building class. for each i value, which represents the vulnerability index of each building, the distribution of damage d presents a mean value of ( ) ( ) 5 | 0 1 5 k i k d i p k = =  (2) where the term 1/5 is related to the five damage degrees (dk) foreseen by this type of approach. according to the adopted model, specific curves known as vulnerability curves (figure 4), which plot the damage distribution d vs. intensity i, can be drawn. when several damage grades of dk are taken into account, the curves become fragility curves (figure 5), which are regularly developed on the basis of a lognormal distribution of d values [23]. seismic fragility is identified in terms of the chance that the seismic capacity related to a ground-motion parameter (peak ground acceleration [pga]) a of a structure is below a threshold a of that pga [24], [25].  =  =(a ) pf a a pga ap (3) the pga seismic capacity a of a structure is often expressed as a function of three variables:  = m u ra a (4) where am is the median pga seismic capacity, εu is the epistemic uncertainty of a due to a lack of knowledge and is a random variable representing uncertainty, and εr is linked to the concept of aleatory uncertainty and is a random variable representing the inherent randomness of a [26]. the two random variables are taken to be lognormally distributed with unit average values and with logarithmic standard deviations of βu and βr, respectively [27]. in accordance with eq. 4 and the assumption that εr and εu are lognormally distributed, the seismic fragility curve, which identifies the probability of failure given a pga threshold a at a confidence level of q = q, is expressed as follows in accordance with [28]:     − =  = = =    +      =         1 (a,q ) p , ln ( ) f u m r a a pga a q q a q a p (5) where φ represents the standard normal distribution function. the confidence levels q are often taken as various discrete values, such as 5 %, 50 %, or 95 %. generally, this equation identifies a family of seismic fragility curves for various levels of confidence, as shown in figure 6 [29]. it is possible to obtain a high confidence of low probability of failure (hclpf) seismic capacity in terms of a selected pga by solving eq. 5 with a 5 % probability of failure and a 95 % confidence level. the subsequent analysis will lead to the formulation of eq. 6: ( ) − + = 1.65 e r uhclpf mc a (6) here, the median capacity am, as well as the associated randomness βr and uncertainty βu, become the principal values to calculate to obtain the seismic fragility curves and the hclpf seismic capacity of a given structure [30]. the safety factor, f, which is an intermediate random variable, is used to calculate the am, βr, and βu. here, f describes the extent to which the pga seismic capacity a is above the reference earthquake level in terms of the same pga quantity aref and is defined as follows: figure 4. example of a vulnerability curve. figure 5. example of a fragility curve. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 60 =  re fa f a (6) meanwhile, the physical meaning of f is the ratio of actual seismic capacity of the structure analysed to the actual response (demand) of the same in terms of the reference earthquake chosen for the analysis. it is clear that the vulnerability studies carried out using these approaches lead to a comprehensive overview of the most critical building classes. generally, higher mean damage is experienced by the more vulnerable building classes than by the less vulnerable classes. attention should be paid to these buildings when setting up systems for monitoring the structural safety in real time. however, past earthquake data have revealed that a given building class may perform better in the case of weaker seismic events than in the case of stronger events. thus, it could prove difficult to compare different building classes in terms of vulnerability. 4. theoretical formulation of equivalent frame model the seismic analysis of masonry structures is conducted using software based on equivalent frame or macro-element models [31], [32]. in these approaches, the walls are subdivided into the vertical elements – the piers – and the horizontal elements – the spandrels interconnected by rigid nodes [33], [34]. as such, the structural response is brought back to the constitutive laws of these elements. generally speaking, in modelling and analysis techniques, the contribution of the spandrels is neglected and, even at the regulatory level, their effectiveness is often subordinated to the presence of tie-rods or rc bond-beams. thus, in assessing the safety of existing buildings, this approach leads to a large number of them being labelled as ‘unsafe’. it is clear that this outcome is not plausible, and that the hypothesis adopted is too burdensome since the possible contribution of the spandrels is not considered [35], [36]. certain modelling strategies for masonry structures are based on the identification of the macroscopic structural elements. these are defined in terms of finite elements (fes) – membranes [37]-[39] or frame solids [40], [41] – from a geometric and kinematic point of view, or in terms of generalised internal forces from a static point of view [34]. the latter models are aimed at a substantial reduction of the computational burden, as they are based on a discrete modelling in which the basic element is designed to model a given masonry panel. according to a more simplified approach, the basic macro-elements can represent complete piers and/or spandrels. this reduces the computational burden and decreases the degrees of freedom (dof) of the structural system. the so-called frame models are the most commonly used macro-models and are based on a discretisation of the walls using the macro-elements. here, the walls are regarded in terms of an idealised frame, in which rigid nodes connect the deformable elements (figure 7) [35]. meanwhile, the piers are the elements that discharge the loads to the ground, and their failure causes the collapse of the wall, while, in contrast, the spandrels – which are vertically aligned with the openings – influence the behaviour of the wall in relation to the degree of coupling with the piers without directly causing the collapse. this idealisation derives from the observation of the damage caused by an earthquake in relation to the concentration of diagonal cracks in these elements and the absence of cracking or damage mechanisms in the rigid nodes (figure 8). in fact, it is assumed that in the rigid nodes, the strains remain in the elastic field, which can result in their contribution being neglected in the evaluation of the seismic behaviour of the building [42], [43]. in the new italian technical standard (ntc 2018), the spandrels are considered as 90 ° rotated piers. however, due to the anisotropy of the material, the behaviour between the piers and the spandrels will be different, depending on whether the direction of the applied forces is orthogonal (piers) or parallel (spandrels) to the layout of the mortar bedding joints. the italian ntc 2018 assigns a resistance to damage value for the spandrels only in the presence of tensile-strength elements capable of generating a compressive stress. the bending strength of these structural elements tends to be related to the formation of the diagonal strut mechanism that guarantees the coupling with the piers. however, even in the absence of additional elements, the masonry will tend to have a considerable resistance to bending stress due to the friction force between the blocks, especially with spandrels characterised by small dimensions. to obtain moderate values of the axial load acting on the spandrels and the related buckling strength, eq. 8 was employed following modification of the formulation used for the piers (eq. 7): figure 6. fragility curve based on the formulation proposed using eq. 5 [29]. figure 7. equivalent frame idealisation [35]. figure 8. damage that occurs in piers during a seismic event. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 61    = −      2 1 2 u n t b n m a h t k fd (7)       = −    =  2 0 0 0 1 2 u t b m k fhd p t b (8) where n is the compression load applied to the piers, t is the thickness of the masonry element, b and h are the base and the height, respectively, of the pier or spandrel, a is the cross section of the element and is equal to h∙t, fd is the design compressive strength of the masonry, fhd is the design horizontal compressive strength of the masonry, p is the compressive horizontal load applied to the spandrel, and k is an imposed coefficient less than or equal to 0.85. in the case of the presence of tie-rods or rc bond-beams in the structure, the value of the buckling is calculated using eq. 9:    = −      1 2 0.85 p p u h h h m fhd h t (9) where hp is the minimum value of the tensile stress between reinforcement element (tie-rod or rc bond-beam) and 0.4 h∙t fhd. the strength and stiffness of the piers and spandrels are determined by the relative failure criteria, while the node element is considered infinitely rigid and resistant. the great advantage of this method lies in the possibility of reproducing the non-linear behaviour of the wall panels. the piers and the spandrels are modelled through two-dimensional finite macro-elements that are representative of wall panels, with two nodes and three dof per node (ux, uz, roty). the remaining wall portions are therefore considered as rigid nodes, as identified in the wall plane by two points (x, z), to which the macro-elements are connected, with the latter transmitting the forces to each node along the three dof. this approach requires a limited number of dof with a reasonable computational burden, allowing for the analysis of complex models of unrestored masonry structures obtained by assembling the walls and horizontal elements. in this approach, the loads are only applied to the nodes, meaning the element is not loaded along its own axis. in the constitutive laws, the initial elastic branch is directly determined by the shear and the bending stiffness, calculated on the basis of the geometric and mechanical properties of the masonry wall, as summarised in the stiffness matrix ke [29], [38]:                               − − − + + + + − + − − + + + + = − + + + + − + − + 3 2 3 2 2 2 3 2 3 2 2 12 6 12 6 0 0 (1 ) (1 ) (1 ) (1 ) 0 0 0 0 6 (4 ) 6 ( 2 ) 0 0 (1 ) (1 ) (1 ) (1 ) 12 6 12 6 0 0 (1 ) (1 ) (1 ) (1 ) 0 0 0 0 6 (4 ) 0 (1 ) e ej ej ej ej h h h h ea ea h h ej ej ej ej h h h h k ej ej ej ej h h h h ea ea h h ej ej h h                                     +    + + +  2 6 (4 ) 0 (1 ) (1 ) (1 ) ej ej h h (10) where ψ is the coefficient calculated as 1.2·el2/(gh2), e and g are the elastic and shear modulus, respectively, a and j are the cross-section area and the moment of inertia of the panel, respectively, l and h are the length and height of the panel, respectively, and η is the stiffness reduction coefficient [44], [45]. the employment of the equivalent frame model for the analysis of masonry structures allows for the identification of the possibility of schematising as well as buildings with onedimensional elements. to calculate the stresses, it is simply necessary to solve the structural scheme of the frame (as is performed, for example, for rc structures). since masonry walls are configured as two-dimensional elements, in which two dimensions (width and length) are preponderant compared to the third (thickness), the simplifications of the bernoulli model cannot be applied. in fact, this model is applied to the beam elements, i.e. a one-dimensional element for which two dimensions are negligible with respect to the longitudinal development. the stiffness of the elements is schematised according to timoshenko’s theory, which takes into account the bending and shear component of the strain [45]: =     +       3 1 1.2 12 k h h ei ga (11) the behaviour of the spandrels strongly influences the overall response. in fact, in the case of tensile-strength elements, the coupling between the piers is ensured while the rotational behaviour of the piers occurs in the absence of tensile-strength elements [46], [47]. 5. monitoring system for historical buildings through the application of the cartis approach, an historic city centre can be classified, ensuring the individuation of the building compartments that could be most damaged when subjected to seismic action. given this, monitoring these compartments through the application of preventive shm systems becomes an interesting pursuit. here, the modification of structural behaviour associated with the local dissimilarities in structural rigidity, which are generally the result of seismic events or dynamic loads, is achieved through monitoring the dynamic actions and assessing the dynamic parameters [48]. vibration-based shm systems are highly attractive since they can provide reliable dynamic parameter estimates from in-service response data. generally, automated operational modal analysis (oma) techniques incorporating a small number of sensors provide these results [49]-[51]. forecasting the degradation of the physical and chemical properties of the masonry – the main raw material used in historic buildings – is affected by the sensitivity of the shm sensor [52], [53]. in the near future, the new technologies will attempt to resolve the aforementioned problem by adopting the simulation of intelligent data processing by associating shms with the iot paradigm [54], [55]. at present, the events that cause great damage to the engineering structures are studied through the transformation of models that simulate their behaviour. this is because the attendant processes are often dynamic and tend to change over time. if the dynamics of the source events are greater than those in the real world, their description will prove to be difficult [56]. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 62 6. preliminary study the cartis methodology was set up in relation to a case study to define a preliminary research. here, the main objective was to implement an shm system to be used on the most vulnerable building classes, individuated using the cartis approach. the methodology is viable only for residential buildings, those that are the most ubiquitous throughout the region and that have possessed the same properties over time. specialised buildings, such as aristocratic palaces, religious buildings, and other historic buildings, require specific acumen developed using alternative methodologies [59]-[62]. this work aims to assess the subjection to seismic risk of an urban centre in the calabria region, southern italy, namely, mendicino. specifically, the study area is the ‘old town’, which is widely held to be the most vulnerable tc of the town. after analysing the building inventory and thereby identifying the tc through the cartis-based data, a study based on the pushover approach [63] applied to the most common bts of the tc was carried out. the seismic safety indexes of the patterns identified were elaborated by varying the geometric and structural parameters [36] using vemnl calculation software. as such, a qualitative interpretation of the urban fabric was carried out, supplying a classification of the most suitable strengthening for each bt configuration. with regard to the monitoring system, a possible preliminary shm system for observing the building sectors in real time is proposed in this paper, which consists of a cyber part and a physical part. within this context, the quality of analytical or numerical models, the adoption of a specific algorithm and the attendant parameters, and the accuracy of the sensor measurements are all potential sources of error and uncertainty [64]. 6.1. cartis-based methodology for a case study the case study in question relates to the municipality of mendicino, which is in close proximity to the provincial capital of cosenza. while the territory under investigation is extensive (35.7 km2), the built-up area includes the ‘old town’ and ‘expansion area’ tcs (figure 9), while further subdivisions into smaller tcs are possible. however, this work is aimed at individuating a prevalent bt within the old town (figure 10) so as to carry out relevant studies on the seismic susceptibility of these buildings and the possible reinforcement measures. thus, the focus is on the second and third levels of the cartis forms. the old town tc was the only compartment in the area until 1954. masonry bts are the principal type in this tc and are classified in terms of three types according to the formal and construction properties. nonetheless, there are examples of rc buildings that were not taken into account during the survey campaign performed with the aid of the second-level forms. in typological terms, the terraced house is the most ubiquitous bt in the area, formed by the primary unit known as a ‘tower-house’, as illustrated in figure 11. for the definition of the structural behaviour of the bts, three actual buildings (square plan, rectangular plan, and irregular plan) located at three different points of the old town tc were considered. in brief, the most common properties of the towerhouse covering all typologies are as follows: • an average storey height of 2.50 m; • twoto three-storey structures with elevations mainly built in the mid-twentieth century; • a ground floor, largely semi-underground, with a height lower than that of the intermediate levels. the intended use has changed from a workshop or a shelter for animals to a residential area; • a top floor with a further reduction in height (in m); • wide openings compared with the size of the masonry panels. figure 9. the tcs of the municipality of mendicino. figure 10. old town tc. figure 11. typological example of the tower-houses in mendicino. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 63 other applicable parameters, in addition to the shape of the plan, are as follows: • the thickness of the masonry (= 0.60 m) throughout the entire height; • the masonry type, from the weaker rubble masonry to the stronger masonry blocks; • the different ground floor heights, ranging from 2.30 m to 2.80 m; • the type of slab: wooden slab (lightweight) or steel i-beam with hollow flat blocks (stiffer). according to the pushover approach established by the italian structural codes [65], [66], the seismic safety of a building is defined by the following: = max max s u i d (11) this index represents the ratio of capacity displacements identified in the control point (umax, generally situated on the centroid of the attic floor) to seismic demand (dmax). in this paper, the aim is to identify the safety indexes by varying the percentage of openings (from 9 % to 20 %) and the mechanical parameters of the structure. here, < 1 is the non-reliability condition. 6.2. parametric analysis parametric seismic analysis was carried out using the vemnl software based on the schematisation of buildings using the equivalent frame method. the structures considered are all the tower-house type and differ in terms of plan (square, rectangular, and irregular). in addition, typological evolutions were identified for each typology based on the number of storeys, which ranged from two to three. table 1 and figure 12 show the evolution of the basic elements and the eight variants analysed, which were distinguished according to ground floor height hpt, the raw material (rubble stone or ashlars), and the type of slab (wood or rigid). the thickness of the masonry was set at a constant 0.60 m. analyses were conducted for each type of plan, for each of the eight variants, for all two-storey buildings and, subsequently, for all three-storey buildings by varying the percentage of openings in the structure. the ultimate aim was to identify the safety index (is) for each individual variant of the model. for masonry buildings, the analysis using ntc 2018 must be conducted for 16 seismic load configurations, eight along the x axis and eight along the y axis, proportional to the masses and heights, and another eight with consideration of a possible rotation of the action along the two main axes. for simplicity, only the values obtained for the configuration of the seismic analysis proportional to the masses along the positive x direction are reported. the numerical analysis led to the identification of 72 safety indices, for each typology of seismic load combination, calculated for the eight configurations of the two-storey square plan by varying the percentage of openings in the wall in a range of approximately 10 %–20 %. this was subsequently performed for each type and for each load combination. the results of the first load combination are presented in table 2 and figure 13. similar results are obtained for: three storey square plane, two storey rectangular plane, three storey rectangular plane, two storey irregular plane and three storey irregular plane. the results are presented in the table 3 table 7 and figure 14-figure 18, respectively figure 12. the parametric analysis was carried out starting from elementary cells a0 and b0, which together identified the different bts present in the mendicino area. the figure also shows a scheme of the square-shaped bt elevations and the attendant model in the vemnl software. table 1. characteristics of the eight models. masonry slab t in m hpt in m rubble stone ashlars wood rigid a 0.60 2.50 x x b 0.60 2.50 x x c 0.60 2.50 x x d 0.60 2.50 x x e 0.60 2.30 x x f 0.60 2.30 x x g 0.60 2.80 x x h 0.60 2.80 x x acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 64 table 2. safety index values for two-storey square plan. % a b c d e f g h 10.0 3.38 3.32 7.55 5.00 4.79 3.56 2.47 2.01 11.3 3.23 3.17 7.54 4.85 4.68 3.41 2.34 1.98 12.6 3.33 3.28 8.07 5.02 4.46 3.28 2.25 1.93 13.8 3.31 3.24 5.14 3.58 4.44 2.03 2.22 1.84 15.1 3.34 3.27 5.17 3.57 4.30 3.01 2.20 1.86 16.2 1.84 1.81 3.02 2.25 2.93 2.24 1.53 1.28 18.4 1.74 1.72 2.12 1.91 1.99 1.76 1.50 1.35 19.2 1.72 1.70 2.10 1.89 1.92 1.70 1.44 1.32 20.2 1.65 1.64 2.04 1.84 1.92 1.71 1.45 1.32 figure 13. square plan – two-storey: is vs. % openings. table 3. safety index values for three-storey square plan. % a b c d e f g h 8.3 1.59 1.47 2.66 2.12 1.98 1.63 1.42 1.98 9.8 1.29 1.18 1.76 1.55 1.56 1.35 1.28 1.56 11.5 1.22 1.10 1.66 1.46 1.40 1.24 1.04 1.40 12.5 1.24 1.13 1.72 1.50 1.40 1.24 1.04 1.40 14.0 1.68 1.51 2.21 1.95 1.84 1.65 1.40 1.84 15.3 1.69 1.51 2.23 1.96 1.87 1.67 1.40 1.87 17.0 1.45 1.27 1.78 1.65 1.76 1.57 0.88 1.76 18.2 1.00 0.91 1.69 1.53 1.57 1.12 0.98 1.57 19.5 1.05 0.97 1.31 1.22 1.92 1.71 1.45 1.32 figure 14. square plan – three-storey: is vs. % openings. figure 15. rectangular plan – two-storey: is vs. % openings. table 4. safety index values for two-storey rectangular plan. % a b c d e f g h 9.0 2.69 2.19 5.18 3.89 3.50 2.71 2.01 1.78 10.1 2.52 2.02 4.95 3.65 3.30 2.53 1.92 1.72 11.8 2.55 2.05 5.34 3.86 3.21 2.46 1.87 1.69 13.2 2.53 1.95 4.39 3.19 3.20 2.30 1.91 1.67 14.3 2.54 1.97 4.33 3.15 3.17 2.29 1.92 1.68 15.5 2.36 1.84 3.97 2.92 2.39 1.64 1.19 1.09 17.1 1.42 1.25 2.25 1.97 1.84 1.41 1.43 1.31 18.7 1.69 1.53 2.15 1.97 1.87 1.65 1.41 1.29 20.0 1.65 1.49 2.08 1.88 1.85 1.65 1.42 1.30 table 5. safety index values for three-storey rectangular plan. % a b c d e f g h 8.3 1.38 1.28 1.68 1.49 1.56 1.39 1.22 1.12 10.1 1.27 1.16 1.75 1.58 1.44 1.30 1.12 1.03 11.4 1.08 0.98 1.40 1.27 1.22 1.08 0.95 0.87 12.3 1.07 0.98 1.41 1.26 1.20 1.08 0.95 0.87 14.0 1.46 1.33 1.86 1.68 1.66 1.49 1.27 1.16 15.5 1.43 1.30 1.81 1.64 1.60 1.44 1.24 1.14 16.8 0.93 0.82 1.50 1.39 1.43 0.99 0.75 0.70 18.1 1.03 0.95 1.28 1.18 1.19 1.09 0.91 0.84 19.3 1.01 0.93 1.26 1.16 1.15 1.04 0.89 0.83 figure 16. rectangular plan – three-storey: is vs. % openings. 1 3 5 7 9 9 11 13 15 17 19 21 is % openings a b c d e f g h 0 1 2 3 8 10 12 14 16 18 20 is % openings a b c d e f g h 1 2 3 4 5 6 7 8 10 12 14 16 18 20 is % openings a b c d e f g h 0,6 0,8 1,0 1,2 1,4 1,6 1,8 2,0 2,2 8 10 12 14 16 18 20 is % openings a b c d e f g h acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 65 6.3. physical part a possible solution to the shm physical part [67] involves a wired sensor network characterised by two types of piezoelectric accelerometer sensor (integrated electronic piezoelectric— iepe): a ks48c-mmf with a voltage sensitivity of 1 v/g and a measurement range of ± 6 g, and a kb12vd-mmf with a voltage sensitivity of 10 v/g and a measurement range of ± 0.6 g. the data recorded with the sensors are acquired via a gateway to be sent to a remote control and service room [68], [69], where they will be analysed and compared to the results obtained via the finite elements model (fem) in order to obtain the value of the control point displacement (umax). 6.4. cyber part nowadays, natural computing is employed to automate the monitoring processes so as to reduce human error. here, the possible sources of error are identified via a sensitivity analysis of the parameters used in the dynamic models. meanwhile, natureinspired algorithms are adopted to ensure convergent solutions to the problems in a local context, with specific attention paid to the suitability of the solutions to constraints problems, such as optimal sensor placement (osp) and model-based damage identification (di). specifically, when the di strategies are used, it is possible to assess specific damage parameters from alterations in the elastic-mechanical properties of the structural system by solving the inverse problem of the system analysed [70]-[73]. vibration-based methods allow for the initial individuation of damage by means of dynamic property measurements. these methods relate to the changes in the mass, damping, and rigidity of the system, determined through the analysis of natural frequencies using accelerometers [74]-[75]. 7. conclusions the pushover analyses conducted in relation to various configurations of twoand three-storey bts returned some important results. first, the safety index always demonstrated values more significant than 1 in the case of two-storey buildings, while the reliability condition was rarely not satisfied with the three-storey buildings. an index reduction was also noted with the buildings with a regular plan compared to those with an irregular plan. as could be expected, the performance decay was due to the weaker mechanical properties of the calculation model, such as the type of masonry and the flexural strength of the floors. the increase in the storey height, as well as the percentage of openings in the main façade, led to adverse effects on the seismic safety. however, the index tended toward analogous values regardless of the structural properties of the model. specifically, the vulnerability elements were ordered as follows: the number of floors → the storey height → the stiffness of the slabs → the percentage of openings (with a focus on their vertical misalignment) → the plan shape. this is shown in figure 19–figure 22, where all configurations characterised by a non-verified safety index is are presented, the analysis of which led to defining the scale of possible vulnerability. therefore, the proposed shm system could be applied to buildings that present the above-defined elements according to the vulnerability ranking (figure 23). the identification of the buildings most exposed to seismic risk using the cartis approach will lead to table 6. safety index values for two-storey irregular plan. % a b c d e f g h 7.2 2.39 2.02 3.92 3.21 3.67 3.00 1.67 1.57 9.1 2.20 1.86 3.49 2.86 3.36 2.75 1.58 1.47 10.1 2.18 1.83 3.45 2.83 3.23 2.69 1.59 1.47 11.5 2.23 1.83 3.65 2.90 3.45 2.75 1.53 1.41 13.0 2.34 1.87 3.86 2.69 2.78 2.30 1.57 1.45 14.2 2.00 1.72 3.14 2.55 2.70 2.23 1.50 1.40 15.4 1.52 1.39 2.00 1.70 1.77 1.57 1.27 1.19 17.3 1.47 1.34 1.87 1.64 1.74 1.53 1.26 1.17 19.0 1.46 1.35 1.86 1.63 1.75 1.54 1.24 1.17 figure 17. irregular plan – two-storey: is vs. % openings. figure 18. irregular plan – two-storey: is vs. % openings. table 7. safety index values for two-storey irregular plan. % a b c d e f g h 8.1 1.25 1.15 1.59 1.47 1.40 1.28 1.15 1.11 9.3 1.45 1.09 1.49 1.37 1.37 1.25 1.04 0.99 10.1 1.18 1.07 1.47 1.37 1.30 1.19 1.08 0.99 11.4 1.14 1.03 1.37 1.30 1.24 1.14 0.93 1.00 13.1 1.11 1.00 1.33 1.25 1.18 1.09 1.07 0.96 14.0 1.12 1.01 1.33 1.26 1.20 1.10 1.02 0.95 15.4 1.09 0.98 1.32 1.22 1.21 1.11 0.95 0.88 17.0 1.02 0.95 1.23 1.17 1.15 1.09 1.21 1.13 18.4 1.00 0.94 1.19 1.14 1.11 1.05 0.59 0.56 1 2 3 4 5 7 9 11 13 15 17 19 is % openings a b c d e f g h 0,5 0,7 0,9 1,1 1,3 1,5 1,7 8 10 12 14 16 18 is % openings a b c d e f g h acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 66 a reduction in both costs and computational burden. the buildings identified would be monitored via shm after calculating the maximum displacement demand (dmax) for each. the dynamic solicitations recorded by the system would subsequently be sent in real time to a control room in order to identify whether the is is less than or greater than the unit, carrying out a highly accurate analysis via fem. references [1] a. ferrante, f. clementi, g. milani, advanced numerical analyses by the non‐smooth contact dynamics method of an ancient masonry bell tower, mathematical methods in the applied sciences, 43 (13), 2020, pp.7706-7725. doi: 10.1002/mma.6113 [2] f. clementi, g. milani, a. ferrante, m. valente, s. lenci, crumbling of amatrice clock tower during 2016 central italy seismic sequence: advanced numerical insights, frattura ed integrità strutturale, 14(51) (2020), pp. 313-335. doi: 10.3221/igf-esis.51.24 [3] v. nastro, d. l. carnì, a. vitale, f. lamonaca, m. vasile, passive and active methods for radon pollution measurements, historical heritage buildings, measurement 114 (2018), pp. 526-533. doi: 10.1016/j.measurement.2016.09.002 [4] g. standoli, e. giordano, g. milani, f. 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model for each seismic load distribution. figure 21. distribution of non-verified is for two-storey irregular plan model for each seismic load distribution. figure 22. distribution of non-verified is for three-storey irregular plan model for each seismic load distribution. number of floors average storey height slab stiffness % openings plan shape masonry properties figure 23. vulnerability ranking of various structural elements. 0,80 0,85 0,90 0,95 1,00 9 11 13 15 17 19 21 is % openings a b c d e f g h 0,60 0,65 0,70 0,75 0,80 0,85 0,90 0,95 1,00 9 11 13 15 17 19 21 is % openings a b c d e f g h 0,80 0,85 0,90 0,95 1,00 7 9 11 13 15 17 19 21 is % openings a b c d e f g h 0,40 0,50 0,60 0,70 0,80 0,90 1,00 7 9 11 13 15 17 19 21 is % openings a b c d e f g h https://doi.org/10.1002/mma.6113 https://doi.org/10.3221/igf-esis.51.24 https://doi.org/10.1016/j.measurement.2016.09.002 https://doi.org/10.1080/15583058.2020.1723735 https://doi.org/10.1016/j.ymssp.2016.05.025 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measurements acta imeko issn: 2221-870x december 2020, volume 9, number 4, 151 156 acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 151 geometrical and thermal influences on a bacterial cellulose-based sensing element for acceleration measurements giovanna di pasquale1, salvatore graziani2, antonio licciulli3, rossella nisi4, antonino pollicino5, carlo trigona2 1 d.s.c., dipartimento di scienze chimiche, university of catania, viale andrea doria 6,95125, catania, italy 2 d.i.e.e.i., dipartimento di ingegneria elettrica elettronica e informatica, university of catania, viale andrea doria 6, 95125, catania, italy 3 dipartimento di ingegneria dell’innovazione, university of lecce, via per arnesano, 73100, lecce, italy 4 biofaber, via luigi di savoia 19, mesagne, brindisi, italy 5 d.i.c.a.r., dipartimento di ingegneria civile e architettura, university of catania, viale andrea doria 6, 95125, catania, italy section: research paper keywords: bacterial cellulose; ionic liquids; greener transducers; biodegradable sensor; geometry analysis citation: carlo trigona, giovanna di pasquale, salvatore graziani, antonino licciulli, rossella nisi, antonino pollicino, carlo trigona, geometrical and thermal influences on a bacterial cellulose based sensing element for acceleration measurements for acceleration measurements, acta imeko, vol. 9, no. 4, article 20, december 2020, identifier: imeko-acta-09 (2020)-04-20 section editor: francesco bonavolonta, university of naples federico ii, italy received october 31, 2019; in final form march 4, 2020; published december 2020 copyright: ©2020 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: carlo trigona, e-mail: carlo.trigona@dieei.unict.it, salvatore graziani, e-mail: salvatore.graziani@dieei.unict.it 1. introduction the introduction of digitalisation in virtually all aspects of everyday life is revolutionising people's lifestyles. not less important, new ecologies, such as the internet of things (iot) and industry 4.0, require the mass-production of low-cost electronic devices to implement new functionalities in unexplored environments and working conditions. as a result, vigorous growth in the economy can be envisaged both in developed and developing countries, with a corresponding beneficial effect on people’s wealth. unfortunately, side effects need to be considered in such a promising scenario. the trend for the lifetime of electronics is significantly decreasing; a cell phone is used for less than 18 months before being replaced, a computer for less than 3 years. in 2007, 3.2 million tons of e-waste were generated in the us, with a corresponding increase of landfilling and environmental concerns [1]. new production strategies are required that are capable both of saving nonrenewable resources and of producing electronics that do not create environmental concerns after their end of life (eol). as it stands, existing technologies for the manufacture of integrated circuits (ic) use hazardous substances that, if not suitably processed, can pose significant risks to human health. this problem is leading to significant challenges, especially in developing countries where rudimentary treating procedures are implemented [2]. abstract the need for a sustainable economy necessitates new environmentally friendly production technologies as well as devices that can be easily recycled, disposed of, and, finally, degraded, without any release of pollutants to the environment. in this context, bacterial cellulose (bc) has recently been investigated as an intriguing solution for the creation of green motion sensors. bc has excellent mechanical properties, and it is fully biodegradable and greener than the more common plant-derived cellulose. in this paper, we investigate the influence of geometry and environmental temperature on bc-based sensing elements. more specifically, the influence of these quantities on a previously investigated bc-based accelerometer are reported. an experimental campaign and the characterization of the proposed green device for several geometries (from 7 to 22 mm of length) and various temperatures (from 5 °c to 55 °c) is addressed, obtaining very intriguing results. mailto:carlo.trigona@dieei.unict.it mailto:salvatore.graziani@dieei.unict.it acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 152 low-power, renewable production processes and easily biodegradable materials that do not pollute the environment after disposal are required to create a more sustainable economy. though polymers have attracted the interest of the scientific community because they can introduce new functionalities, such as affordability, flexibility, and biocompatibility, they are not biodegradable, and concerns exist about the treatment of polymer-based electronics after their eol. cellulose, the most widely available biopolymer on the earth, looks to be a suitable candidate for producing next-generation electronics. it is cheap, biocompatible, and biodegradable. additionally, cellulose and its derivatives have interesting mechanical and electrical properties that have been exploited in many applications [3]-[6]. notwithstanding such promising properties, plant-derived cellulose extraction by the pulp industry involves significant resource consumption in terms of energy and fresh water. according to the eu, the average energy costs for the pulp and paper industry equal around 16 % of production costs. 38 % of the energy used comes from natural gas [7]. not less important, pollutants are produced by the pulp and paper industry, raising concerns about their environmental effects. bacterial cellulose (bc) is a kind of cellulose produced by some bacteria genera when maintained in suitable culturing environments. in this case, the cellulose is directly produced by the bacteria at the interface between the growing media and the open air. the production process can be implemented in typical laboratory conditions and enables the production of very pure cellulose. bc has been proposed for manufacturing electronics and sensors [8], [9]. recently, bc has also been proposed for the manufacture of both actuators and hybrid and conventional solutions for mechanical sensors [10], [11]. the possibility of using bc-based composites for the manufacture of green sensors has been investigated [12]-[13]. composites consisting of a bulk bc membrane impregnated with ionic liquids (il) and covered on both sides by poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (pedot:pss) have been fabricated. in particular, the inertialand mass-sensing properties of such composites when used in a cantilever configuration have been investigated [12]-[14]. moreover, the influence of geometry on the sensing properties of an accelerometer has been analysed [12], [15]. polymer composites are known to be sensitive to environmental parameters, temperature and relative humidity being the most influential [16], [17]. based on such considerations, the present paper carries the investigation of the accelerometer further, by investigating the influence of environmental temperature on the transducing properties of the bc-based composites. the influence of temperature on the investigated bc-based composites is even more relevant because of the presence of ils, which can have freezing points that are similar to the environmental temperature. results reported here complete the investigation reported in our previous study, and finally provide evidence of the role ils play in the transduction properties of the bc composites we investigated [15]. the results we obtained contribute to a better understanding of the performance of such a new composite, a necessary task ahead of its use in real sensing problems. the paper is organised as follows: section 2 describes the materials and the preparation procedure. section 3 reports the characterisation setup. section 4 shows the results, and the conclusions are given in section 5. 2. materials and preparation procedure the following commercial products were explored to make the bc sensing element: • black tea bags • sucrose • vinegar • kombucha tea furthermore, sodium hydroxide (naoh) and absolute ethanol (99.8 %) were purchased from sigma-aldrich and used without further modification. pedot:pss and clevios phcv4 were purchased from h. c. starck for aqueous dispersion. emim-bf4, an il, was purchased from alfa aeser. concerning the preparation procedure for the compound, bc pellicles were obtained from the fermentation process of kombucha tea, a sweetened black tea with acetobacter strains [18]. the bacterial culture was composed of upper cellulosic pellicle and a lower liquid broth [19]. the culture medium was prepared by adding 70 g of sucrose and 4 g of black tea to 1 l of boiling water. then the mixture was left to steep for 15 min and, after removing the tea, the ph value of the broth was adjusted to 2.7–3.0 by adding 10 ml of acetic acid for each liter of broth. the addition of acetic acid at the beginning of the fermentation process prevents the formation of molds and protects against undesirable microorganisms. finally, the cellulosic pellicle pieces (3 % w/v) and liquid broth (10 % v/v) with the acetobacter strains (1 ml/l) were added to the cooled tea broth. a teflon stopper was attached to the lower part of the beaker to block the pellicle. in the initial stage, the bacteria increase their population by consuming dissolved oxygen and producing a certain amount of liquid cellulose, as observed by the appearance of turbidity. after that, a pellicle of cellulose is grown on the surface of the broth. figure 1. picture of the prepared bc, without contacts and before the drying procedure. acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 153 when the dissolved oxygen is used up, only bacteria existing near the surface can maintain their activity and produce cellulose. this medium was used to inoculate new fermentations. the pellicles of nanofibrillated bc were thoroughly washed with distilled water and boiled in 0.5 m naoh solution for 2 h in order to remove all impurities, such as the medium or endotoxins from the bc samples, contained in the cell wall of the bacteria. the pellicles of bc were then washed in water 4 times. figure 1 shows the bc after production. the bc was dried in an oven at 80 °c overnight to eliminate the water content. upon retrieving the samples, they were further dehydrated in an oven to eliminate any water absorbed during the storage phase. the samples were then cut, infused with emim-bf4 as the il, and covered with pedot:pss as flexible electrodes. 3. characterisation setup in order to analyse the sensor as a function of its geometry and in terms of the external temperature, a suitable experimental setup was conceived. in particular, figure 2 shows the entire block diagram used for the study and the characterisation. figure 3 shows the experimental setup used to characterise the system. a temperature-controlled chamber was used for the thermal characterisation of the device. the experimental setup consisted of: • a shaker to excite the structure • an hp33120a signal generator to produce a sinusoidal waveform • a digital teledyne lecroy waverunner 6050 to measure the signals • an inertial sensor to monitor the level of acceleration imposed through the shaker • a temperature-controlled chamber to investigate the effect of temperature • a b2985a electrometer/high resistance meter to measure the resistance and the output current of the sensing element figure 4 shows a sample of the bc-based compound. the black surface is the pedot:pss layer. 4. experimental results the measurement campaign was composed of two parts. the first investigation (subsection 4.1) compared the characterisation of the sensor for several geometries (from 7 mm to 22 mm of length). more specifically, the generated output voltage as a function of the imposed acceleration was examined along with the influence of the transducer length on these relationships. the second investigation, (subsection 4.2) explored the thermal characterisation of the device, using a temperature range from 5 °c to 55 °c. the influence of temperature on both static and dynamic working conditions was investigated. 4.1. geometrical influences the investigation was conducted using samples with a width of 11 mm and a thickness of about 0.25 mm. devices with lengths of 22 mm, 17 mm, 12 mm, and 7 mm were tested. an analysis of the output voltage as a function of the imposed acceleration was performed in the frequency domain for each transducer geometry. the measurements were obtained by forcing a sinusoidal signal through the signal generator (see section 3) at the mechanical resonant frequency of each device. it is worth noting that for the four case studies, the following figure 2. block diagram of the equipment used to characterise the bc-based sensing element. figure 3. experimental setup. the inset shows a sample of the bc-based compound mounted between two rigid electrodes, which were used to detect the electrical signal that was generated. figure 4. the realised device (length 35 mm, width 11 mm, and thickness about 0.25 mm) with the two electrical contacts on the top and bottom faces. the black surface is the pedot:pss layer. the bc–il composite is sandwiched between the two pedot:pss layers. acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 154 mechanical resonant frequency values were experimentally estimated: 71 hz (device #1, length of 22 mm), 110 hz (device #2, length of 17 mm), 133 hz (device #3, length of 11 mm) and 135 hz (device #4, length of 7 mm). more details concerning the spectrum of the output voltage are shown in figure 5a and figure 5b, where the fft of the output for devices #1 and #4 are presented, respectively. figure 6 shows the normalised output voltage as a function of the normalised imposed acceleration. for sake of comparison, the voltage of each sample has been normalised to the output voltage of the first sample, i.e., the device with a length of 22 mm. furthermore, the acceleration values have been normalised to the acceleration a1, which refers to the first sample. the value a2 is the acceleration level at a frequency of 110 hz normalised to a1, and so on for a3 (at 133 hz) and a4 (at 135 hz), respectively. figure 7 shows the normalised output voltage as a function of the normalised length of the beam. the normalisation has been performed using the length of device #1. 4.2. thermal influences the investigation described in this section was devoted to studying the behaviour of the bc sensing element in the presence of temperature changes. a device having a geometrical characteristic similar to prototype #1 (see section 4.1) was considered to have a significant output voltage. in particular, two aspects were investigated: 1) the static characterisation was based on an analysis of electrical properties as a function of environmental temperature variation. during this set of experiments, no motion was applied to the device, and the electrical properties of the composite were investigated. 2) the dynamic characterisation was based on the study of the generation capability of the bc device at various temperatures. in this latter case, the shaker was actuated inside the thermal chamber to mechanically excite the base of the sensing element, which deformed according to its mechanical inertia. the considered range of temperature was from 5 °c to 55 °c. the minimum value was selected due to the fact that the il used for the prototype (see section 2) has a melting point of about 15 °c [20]. for this reason, it was interesting to investigate temperatures lower than this value. the upper temperature of 55 °c was chosen due to the fact that the maximum operative temperature of the device is about 60 °c. this limit is given by the melting temperature of pedot:pss, which was used to make the contacts of the bc-based sensing element. in order to analyse the influence of temperature on the resistance of the device, an electrometer (see section 3) was used, and the device was inserted inside the thermal chamber. figure 8 shows the resistance of the bc as a function of the imposed temperatures in the absence of mechanical vibrations (static analysis). six values were considered. each temperature value was measured through an internal thermal sensor. each point in the graph represents the mean value and the measurement uncertainty (3 σ). as it can be noted, a decrement of the resistance across the bc can be observed for increasing temperature values. the observed effect is, of course, a cumulative effect, reflecting possible changes either in the bulk or at the electrodes. with regard to the electrodes’ contribution, the observed phenomenon can be explained by considering the low level of conductivity associated with the pedot:pss electrodes [21]. nevertheless, further investigation is needed to determine any contribution of the bulk to the decrement of the resulting resistance. figure 9 shows the voltage across the bc as a function of time for each of the six temperature values imposed inside the chamber. as it can be observed, the voltage decreases when the temperature value increases. it is worth noting that this behaviour is in accordance with the previous result for resistance. namely, the electrometer imposes the current, so that the voltage a) b) figure 5. fft of the output voltage (expressed in v) of a) bc with a length of 22 mm (frequency of 71 hz), b) bc with a length of 7 mm (frequency of 135 hz). two other spikes can be seen in the figure, indicating the powering line contribution (50 hz and 100 hz contributions, respectively). figure 6. normalised output voltage as a function of normalised acceleration. figure 7. normalised output voltage as a function of normalised length. acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 155 across the device changes because of the change in the resistance value of the bc. in order to study the device from a dynamic point of view, it was mechanically actuated by using the shaker and imposing a sinusoidal waveform with a fixed amplitude and frequency (tuned to the mechanical resonance) through the signal generator (see section 3). the shaker was inserted inside the chamber. the temperature values were imposed in order to study the changes in the current generated by the bc as a function of the temperature. figure 10 shows the generated current as a consequence of the mechanical vibration for different values of the environmental temperature. a low pass filter was applied in order to decrease the noise level. as it can be observed, the analysis shows that the generated current is temperature dependent. the output current starts to increase at temperature values larger than 15 °c, which coincides with the melting point of the il used in the compound. furthermore, while higher temperature values increase the current, they could have adverse effects on the electrical contacts. for these reasons, the region between 15 °c and 45 °c represents an interesting operative range for the bc-based sensing element. the effect of the freezing temperature of the ionic liquids has been also investigated, as shown in figure 11. the graph evinces that when the compound is used at a temperature lower than the meting point of the il, the output current is close to zero. it has been widely reported in the literature that in the case of bc impregnated with ils and covered with pedot:pss, the electromechanical transduction of the actuators is due both to the il’s migration inside the bc bulk and electrochemical doping processes at the polymeric electrodes [4], [22]. the results reported here clearly show that the ils also play a relevant role in the mechanoelectrical transduction properties of bc-based composites. in fact, at temperatures lower than the il’s freezing point, the charges inside the bc compound cannot move. as a consequence, no current can be generated in the presence of mechanical vibrations. to the best of our knowledge, no such results have been reported before in the literature. figure 12 shows the current waveform of the bc for two temperature values. a decrement in terms of amplitude can be observed when a decrement of temperature occurs. 5. conclusions in this paper, the mechanoelectrical transduction properties of a bc-based sensing element were investigated. more specifically, the investigation analysed the dependence of the transduction properties on the geometry of the system and the environmental temperature. figure 8. resistance of the bc as a function of temperature. the connecting segments are introduced to help the eye. figure 9. measurement of the change in voltage across the bc for six temperature values. figure 10. generated current as a consequence of the mechanical vibration, function of temperature. figure 11. generated current value as a function of number of samples. the analysis includes two values of temperature, 5 °c (blue diamonds) and 35 °c (red triangles), respectively. normalisation has been conducted with respect to the maximum value of the signal. figure 12. generated current waveform as a function of the acquired samples. the analysis includes two temperature values, 45 °c (squares) and 35 °c ( diamonds), respectively. acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 156 reported results evince for concluding that the geometry of the system can be used to modulate the expected output signal. temperature was revealed to have a strong influence on the system’s transducing properties. more specifically, reported results showed that the dependence on the temperature can be explained by the active role of mobile charges inside the composite: as a matter of fact, a reduction of the working temperature below the melting point of the il virtually annulled the sensing signal. acknowledgement this research has been partially funded by the university of catania within the project’s piani della ricerca dipartimentale. references [1] y. h. jung, t. h. chang, h. zhang, c. yao, q. zheng, v. w. yang, h. jiang, high-performance green flexible electronics based on biodegradable cellulose nanofibril paper, nature communications 6 (2015), p. 7170. doi: https://doi.org/10.1038/ncomms8170 [2] m. c. vats, s. k. singh, e-waste characteristic and its disposal. am assoc sci and techno 1(2) (2014), pp. 49-61. online [accessed 04 december 2020] http://article.aascit.org/file/pdf/9040718.pdf [3] j. kim, y. b. seo, electro-active paper actuators, smart materials and structures 11(3) (2002), p. 355. doi: 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https://doi.org/10.1109/i2mtc.2019.8826962 https://doi.org/10.1109/iwmn.2019.8805008 https://doi.org/10.1109/metroi4.2019.8792875 https://doi.org/10.1109/tim.2009.2026613 https://doi.org/10.1109/tim.2011.2118870 https://doi.org/10.1046/j.1365-2672.2000.01188.x https://doi.org/10.1021/acsomega.7b00442 https://www.sigmaaldrich.com/catalog/product/aldrich/900772?lang=it&region=it https://www.sigmaaldrich.com/catalog/product/aldrich/900772?lang=it&region=it https://doi.org/10.1109/iceee.2015.7357906 https://doi.org/10.1088/0964-1726/22/8/085026 microsoft word article 12 tc4 paper 8.docx acta imeko  august 2013, volume 2, number 1, 49 – 55  www.imeko.org    acta imeko | www.imeko.org  august 2013 | volume 2 | number 1 | 49  the implementation of fpga for data transmission between  adc and dsp peripherals in the measurement channels for  power quality assessment  janusz mindykowski, romuald masnicki, damian hallmann  gdynia maritime university, morska 81‐87, 81‐225 gdynia, poland      keywords: adc; dsp; fpga; interface; electrical power quality  citation: janusz mindykowski, romuald masnicki, damian hallmann, “the implementation of fpga for data transmission between adc and dsp peripherals  in the measurement channels for power quality assessment”, acta imeko, vol. 2, no. 1, article 12, august 2013, identifier: imeko‐acta‐02(2013)‐01‐12  editors: paolo carbone, university of perugia, italy; ján šaliga, technical university of košice, slovakia; dušan agrež, university of ljubljana, slovenia  received january 10 th , 2013; in final form july 1 st , 2013; published august 2013  copyright: © 2013 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: none reported  corresponding author: romuald masnicki, e‐mail: romas@am.gdynia.pl    1. introduction  on-line analysis of power quality in an electrical power system is associated with the need for fast acquisition and processing of data collected from the tested system. the power quality analyzers offered for this purpose enable the determination of multiple parameters. unfortunately, usually it is not specified according to which algorithms they are operating and what standards are the basis for their calculation. for example, we have indicated a few different definitions of the thd (total harmonic distortion) coefficient, concerning estimation of waveform distortions [1], a few definitions of voltage imbalance [2, 3] and some approaches related to power theories [4]. in addition, the measurement results obtained with various kinds of analyzers in some cases seem to differ substantially. the department of marine electrical power engineering of gdynia maritime university undertook a research project to develop an estimator/analyzer of power quality in which the relevant parameters are determined in accordance with the standards [2, 4, 5, 6 and 7]. one of the key points of this development is focused on implementation of the fpga (field programmable gate array) for data transmission between adc (analog-to-digital converter) and dsp (digital signal processing) peripherals in the measuring channels of the considered device. the paper is organized as follows. firstly, the main ideas concerning on-line analysis of power quality in an electrical power system and its association with fast acquisition and processing of data collected from the tested system are briefly presented. in section 2, the measurement channels are shown and discussed based on a case study – the estimator/analyzer of power quality developed, constructed and tested at gdynia maritime university. the lack of compatibility between adc output interfaces and used dsp communication ports was a reason to introduce an appropriate fpga module as an intermediate device. abstract  the paper presents the fpga xilinx spartan 3 series implementation in an analog data acquisition system, in particular consisting of  nine adcs and the tigersharc dsp processor. because of different standards of adc and dsp data interfaces, the exchange of data  between them requires the use of an intermediary device for combining their communication ports. the procedures for sending data  from the adc to the fpga buffers, their conversion and subsequent transmission to the tigersharc dsp are described. the digital  representations of adc analog input signals are sent to the fpga through three lines of serial interface at the physical layer compatible  with the spi standard. after processing in the fpga they are sent to the dsp, with the four‐line interface in the lvds standard. the  paper discusses the properties of related interfaces, standards, procedures and control of adc issues as well as the algorithms of the  fpga configuration program and functions implemented in this system. selected results of functional testing are shown.  acta imeko | www.imeko.org  august 2013 | volume 2 | number 1 | 50  in section 3 the adc and dsp communication features are analyzed, in the context of the ad7656 converter and the tigersharc processor as applied devices. the conversion process and data acquisition procedures, signals and parameters are presented. in section 4, firstly, the scope of selected fpga implementations is discussed, and secondly, the features of fpga applied in the chain adc-fpga-dsp and considered configuration program algorithms are described in detail. next, section 5 is devoted to the fundamental tests of measurement channels carried out in the estimator/analyzer for power quality. firstly, three steps of the examination of the concept of the fpga device, concerning simulation in the webpack design software environment, the functional tests of communication correctness and the estimator/analyzer functional tests are presented. secondly, three kinds of the last functional tests related to voltage quality parameters monitoring, current and power parameters analysis and maximal spi sclk frequency setting are shown. finally, the conclusions and indications for future activity are collected in section 6. 2. measurement channels configuration  measurement tracks (figure 1) of signals from the power system contain typical input circuits including a set of voltage dividers, galvanic separation systems and anti-aliasing filters. the voltage signals are fed to the inputs of adcs of type ad7656 (analog devices). a single ad7656 chip contains six adcs. determination of the individual parameter values for assessment of the electrical power quality takes place in the dsp processor adsp ts-201 tigersharc (analog devices). the dsp processor communicates with external systems via a custom protocol link port, using 128-bit data frames and the lvds interface. dsp has no native support for the spi protocol. on the other hand, the ad7656 chip has high speed parallel and serial interfaces, allowing the data to be exchanged with external devices. due to the lack of compatibility between adc output interfaces and the dsp communication port, i.e. link port, the fpga spartan 3 (xilinx) was used as an intermediate device. besides providing communication between adc and the dsp system the fpga device also mediates the exchange of information with the gpp (general purpose processor) lpc3250 microcontroller (arm926ej-s core) (figure 1), which implements the user interface features, such as support for the keyboard, graphic display, transmission via usb and ethernet ports. 3. adc and dsp communication features  the ad7656 chip contains six 16-bit, low power, fast, successive approximation adcs [8]. the ad7656 has throughput rates up to 250 ks/s. it can accommodate true bipolar input signals in the ±4×vref range and ±2×vref range. the ad7656 also contains an on-chip 2.5 v reference (figure 2). the conversion process and data acquisition are controlled from the external device using convst signals (figure 3) and an internal oscillator. three convst lines (convst a, convst b, and convst c) allow independent sampling of the three adc pairs (channels 1-2, 3-4 and 5-6). the rising edge of convst initiates simultaneous conversions on the selected adcs. the conversion time is 3μs. when the conversion is complete, the signal on the busy output goes low to indicate that the conversion results, stored in the output data registers, are ready for readout. a simultaneous sample of all six adcs can be performed by connecting all three convst a, b, c pins together. the ad7656 is equipped with two types of data output interfaces: parallel and serial. the selection of the type of interface was preceded by an analysis of the features of both interfaces. in the designed application (figure 1) nine adc channels, i.e. two ad7656, were applied, so in parallel output connection of both circuits, beside busy and convst lines, 33 lines (2x 16 data lines + 1 control line (_rd)) have to be used. in serial connection, the data output needs only 7 lines (2x3 data lines + 1 clock line (sclk)). the parallel interface tacq acquisition time (figure 3), i.e. time required to output all six 16-bit words (from v1 to v6) from adc registers, is about 700 ns. on the other hand, the serial interface tacq acquisition time is about 943 ns, as will be explained in the next section, which is sufficient according to the project assumption. figure  1.  the  measurement  channels  configuration  of  the  developed estimator/analyzer.   figure 2. the configuration of ad7656 [8].    signal conditioning unit signal conditioning unit l 1 l 2 l 3 n bus bars of main switchboard i g11 i g13 i g21 i g23 i g31 i g33 a/d converter ad7656 estimator analyzer of power quality a/d converter ad7656 dsp tigersharc gpp ram flash memory keyboard lcd display ethernet usb fpga spartan 3 xilinx dsm stsl module acta imeko | www.imeko.org  august 2013 | volume 2 | number 1 | 51  taking into account the complicity of adc output connection lines as well as the timing of data outputting, the serial interfaces of ad7656 were selected. then only 7 interface lines instead of 33 are necessary for adc data access. the three-line serial interface applied in the ad7656 chip is compatible with the standard one-line spi serial interface [8, 9]. data transfer is synchronized with a rising or falling edge clock (depending on the configuration), and their set-up on the slope opposite to the sending data. the sclk frequency used for control of data transfer can reach a few mhz, depending on a given application. figure 4 shows the signals in the spi lines. in the serial interface mode of ad7656 three dout pins (figure 5), dout a, dout b, and dout c, are available. data can be read out from the adc using one, two, or all three dout lines. figure 5 shows six simultaneous conversions and the read sequence using three dout lines. also in figure 5, 32 sclk transfers are used to access the data from all six registers of the ad7656; however, two sets of 16 sclk individually framed transfers can also be used to access the data. the tigersharc [10, 11] is a 128-bit processor, designed to perform operations on floating-point and fixed-point numbers, bearing the double sets of alu units for both types of operation, the internal memory of 24mb dram, integrated circuits i/o, 14 channels of dma, and 4 internal buses for fast exchange of data between memory and the peripheral circuits and other cpus. because of its features, it is a very suitable dsp for designation of power quality parameters. the tigersharc processor has four full-duplex type link port circuits that can operate in parallel. these ports use in the physical layer the interface lvds (low voltage differential signal) [12], in which the information (in 128-bit frames) is sent using a low voltage differential signal in the symmetrical copper cables in the system loop. the lvds protocol enables data transmission speeds of up to 4 gb/s with very low power consumption. each link port can be configured to exchange data on one or four interface lines. figure 6 shows the interface line configuration and signals in the lvds interface. as mentioned above, the two interface standards differ significantly and the only way to provide the data exchange between adc and dsp is to apply an intermediate device. 4. fpga features and configuration program  algorithms  the need for high speed data processing in many applications is becoming more and more common. the multi-channel measurements as well as the high sampling rate of the adcs demand the use of advanced acquisition techniques. the features of the current fpgas make them especially suitable to act as an interface between the set of high speed adcs and dsp. they can be adapted to different adcs or different dsp interfaces. the next advantage is the possibility to shift some tasks, usually performed in the dsp, to the fpga implementation. it lowers the load on the dsp and often speeds up the execution of certain operations. some examples related to selected fpga implementations can be found in [13, 14, 15 and 16]. in the designed system the fpga xcs1000 xilinx spartan 3 [17, 18] was used, to provide the acquisition in the tigersharc of data obtained from ad7656. in figure 7, the interface connections of the fpga circuit to the adc and dsp devices are shown. attached digits indicate the number of data lines in the respective bus. figure  3.  the  adc  registers  data  output  procedures  through  the  parallel interface [8].  figure 6. the configuration and signals in the lvds interface lines [11]. figure 4. the line driver and waveforms in the spi interface lines.  figure 5. the signals in the ad7656 serial interface lines [8].  acta imeko | www.imeko.org  august 2013 | volume 2 | number 1 | 52  the fpga is a large-scale integration system, used to create high-speed digital circuits for specialized logic functions. the power consumption of these systems is much smaller than for standard digital systems and processor devices. for these systems, as for processors, the change of the implemented function requires software changes to the fpga. the i/o lines can be configured by software to work with different voltage standards. the analog voltage comparators can act as differential input/output [17]. in general, the configuration program organizes the connection network among selected circuits of the fpga and then the device works autonomously as every hardware device. in the considered project the basic task fulfilled by the fpga is the conversion between different physical layer standards of the adc and tigersharc interfaces as well as different formats of frames transmitted by both links. for handling of the adc spi interface, the respective fpga lines were configured as lvcmos25 and for tigersharc as lvds_25 (figure 7). the output data from the adc are available at one time on three lines dout a, b, c (figure 5, figure 8). on the 1st line the adc conversion results are transmitted in sequence from input channels 1 and 2, on the 2nd line from channels 3 and 4, and on the 3rd line data from channels 5 and 6. the transfer of all data requires 32 spi sclk cycles, but in case only three channels are needed and when the input signals are connected to the channel inputs 1, 3 and 5, to transfer such data only 16 spi sclk transfers are necessary. when the spi sclk frequency is equal to 16 mhz the transfer of three 16-bit words from the adc takes approximately 943 ns. the a/d conversion takes about 3 s, so the full adc conversion cycle and output of digital data requires about 3.95s, which means the maximal frequency of a/d operations is above 253 ks/s (figure 8). such a result is possible because other operations performed in the fpga, connected with data transmission to the tigersharc, are implemented parallel in time with the next a/d conversion cycle. the data from three fpga buffers buf have to be overwritten to eight ram blocks (this takes about 480 ns), because of the configuration of the four-line lvds interface connecting the fpga with the tigersharc link port. the transmission through the link port requires approximately 160 ns. the time dependencies shown in figure 8 illustrate the location of individual operations during the a/d conversion cycle. more explanations of the procedures implemented in the fpga are presented in the algorithm in figure 9. the conversion cycle begins every 4 s with convst going high, generated in the fpga. after the digital data in cycle n of the conversion are completed and the adc line busy goes low, the fpga clock spi sclk starts pulling data v1, v3 and v5 (16-bit words) from dout registers (in the adc) to the buf buffers (in fpga). after 16 cycles the spi sclk clock stops and data are shifted from three buf to eight ram. then the state of the output line ack from the tigersharc is examined. if it is high, the fpga issues 100 mhz clock managing data transfer from the fpga ram to the tigersharc on four differential lines of the lvds interface. the operations performed in the fpga during the a/d conversion (busy at high state) take less than 1 s. they are performed in parallel with other a/d operations in the next a/d cycle; otherwise it would not be possible to obtain the assumed frequency of the a/d conversion. in figure 10, the configuration of functional blocks developed in the fpga to establish communication between the ad7656 and the tigersharc dsp is shown. the individual functions are performed sequentially, under control of the "stanu" signal (figure 10). the sequential algorithm provides correct flow of operations and elimination of the phenomenon of hazard between digital signals, unwanted pulses as well as the possibility of suspension of the device. as was previously described, the sequence of operations starts with the conv signal, generated in the fpga every 4 s, then the fpga is waiting for the busy signal. the next operations in figure 8. adc data handling during a/d conversion.  figure 7. adc and dsp interface buses in the measurement track.  figure 9. algorithms performed in the fpga.  acta imeko | www.imeko.org  august 2013 | volume 2 | number 1 | 53  sequence are performed autonomously, under control of the fpga internal clocks. finally, the data transfer to the dsp begins when the ack signal from the link port enables start of transmission, also under control of the fpga clocks. 5. functional tests   the examination of correctness of the accepted concept of the fpga device was performed in 3 stages: the simulations in the webpack design software (xilinx) environment while designing the fpga configuration program (figure 11), the functional tests of communication correctness; the adc input was fed with the signal generated in the agilent 33220a function generator and digital samples in tigersharc memory were observed (figure 12) also as a waveform, the estimator/analyzer functional tests (figure 13). the xilinx webpack fpga programming environment is a tool for configuring internal structures of the fpga as well as enabling one to perform simulations for verifying correctness of process signals on selected functional blocks in the fpga (figure 10). part of the simulation window is shown in figure 11. the satisfactory results of simulations gave the final configuration of functional blocks implemented in the fpga and proper sequence of operations connected with data transfer from adc to dsp. the examination of functionality of the structures configured in the fpga was carried out for real input signals from the agilent 33220a waveform generator. the input signals were at the same time registered with a tektronix mso 2024 oscilloscope (figure 12a). figure 12a presents an exemplary square wave with a frequency of 1 khz and vpp=6v. the samples of the input signal were converted in the adc and their digital representations were transmitted via the fpga to dsp memory. in figure 12b, the window of the test program is shown. the application in dsp in the visualdsp++ (analog devices) environment contains the exemplary data collected in the dsp in hexadecimal code as well as the respective waveform for the input signal as in figure 12a. the cut-off frequency of anti-aliasing filters in the measurement channel during this test was set at 10 khz. the examination was performed for various signals, with different frequencies and amplitudes. the observed results ensured the correctness of data transmission via the fpga and confirmed that no transmitted sample was lost. the developed estimator/analyzer of electric power quality gives some options of the device for choice by the user. these options [19] concern a kind of analysis (voltage quality monitoring or monitoring of power/current distribution), mode of operation (estimator/analyzer), and also a kind of power system and number of generators working in parallel. as was mentioned, the instrument can operate in two basic modes: estimator and analyzer. in the first mode, only the parameters whose current values exceed user-defined limits are determined, whereas in the latter mode all defined parameters are calculated [19]. starting with the option "voltage quality monitoring", we figure 10. the structure of the functional blocks configured in the fpga.  a) b) figure 12. the exemplary results of communication correctness tests: a) the  input  signal,  b)  the  samples  acquired  in  tigersharc  memory  and  the  respective waveform plotted in the visualdsp++ environment.  figure 11. the simulations in the webpack environment.  acta imeko | www.imeko.org  august 2013 | volume 2 | number 1 | 54  can choose "estimator mode" as well as "analyzer mode". choosing the option "power and current load distribution" is possible only in "analyzer" mode. for the functional tests of the measurement channel, the three-phase power signal generator chroma programmable ac source model 6590 was used (figure 13). the performed functional tests among others consisted of generating signals with known harmonic content in three phase voltage. this allows the evaluation of correctness of calculations of some parameters processed in the tigersharc dsp. some selected results of voltage quality parameter measurements are shown in table 1. these are exemplary results for the l1 phase. the values of urms, u1, thd and u5 averaged for 150 s, i.e. 750 measurement windows, are determined for a known test signal. the  factor represents the relative deviation between the quantity measured by means of the tested instrument and the reference value obtained from the pxi national instruments device. it should be strongly underlined that presented measurement results are only an illustration of the functional tests of the channel under consideration for the verification of operations performed in the fpga during a/d conversion, together with the fpga-dsp link. the designed implementation of the measurement channel in "estimator mode" allows the data to be acquired from three adcs at 250 ks/s from each, as was assumed at the start of the project. the algorithms described above concern the "estimator mode" for voltage quality monitoring. the second mode of instrument operation is "analyzer", dedicated to assessment of the proportional distribution of currents and powers. then the a/d conversions are performed in different conditions, and the required a/d frequency is 25 ks/s only, so all processes connected with adc data handling, performed in the fpga, are not critical as in "estimator mode". the tests carried out confirmed the correctness of the used algorithms and procedures applied in the fpga device. the accuracy of determining the related coefficients is satisfactory from a practical point of view. a more detailed description and analysis of the measurement tests of the designed power quality estimator/analyzer are beyond the concept and scope of the presented paper. other tests were connected with maximal spi sclk frequency. this signal is generated in the fpga. it causes output of ready data from adc registers to fpga buffers. in the technical data of the ad7656 the maximal transmission frequency is given as 18 mhz [8]. after numerous experiments the spi sclk frequency was set equal to 16 mhz. frequencies higher than 17 mhz caused distortion, especially at the first bits of data words. 6. final remarks  in this paper, the analysis of the given adc and dsp interfaces was carried out. an analysis of adc set readout timing for various interfaces and the selection of the interface type for implementation in the instrument were performed. the verification of correctness of the implemented solutions for a lossless data flow between adcs and the dsp was carried out in a few steps, at various levels of instrument functionality. in the designed instrument the configured fpga fulfills many functions, sometimes critical for all instrument operation. the adaptation of data in the fpga to enable communication between adc and dsp includes:  different speed of transmission; the adc output bit transmission speed is 16 mb/s (common three lines – 48 mb/s), while the dsp link port speed is 200 mb/s (common four-line speed is 800 mb/s),  different data frame formats; the single adc output frame consists of 8 bits, while the dsp link port communication frame consists of 128 bits,  different data transmitting signal standards; adc data carrying signals are lvcmos25 standard, while dsp link port signals are lvds_25 standard. the implementation of functions connected with data conversion using conventional discrete digital circuits (ttl or even cmos) entails a much higher demand for power supply. the operations performed in the fpga are crucial to the operation of the entire measurement channel. the operations performed in the fpga enable the efficient delivery of measurement data to the dsp. the results of tests carried out for measurement channels in the developed estimator/analyzer of power quality confirm the correctness of the applied solutions. references  [1] t. tarasiuk, j. mindykowski, “an extended interpretation of thd concept in the wake of ship electric power systems research”, measurement, vol. 45, 2012, pp. 207-212. [2] iec standard 61 000-4-30, “testing and measurement techniques – power quality measurement methods”, 2008. [3] ieee standard 1159-2009, “ieee recommended practice for monitoring electric power quality”. [4] ieee standard 1459-2010, “standard definitions for the measurement of electric power quantities under sinusoidal, nonsinusoidal, balanced or unbalanced conditions”, 2010. figure 13. laboratory stand for experimental verification of the elaborated estimator/analyzer  of  power  quality  (version  2.0)  based  on  the appropriately defined testing signals – voltage quality parameters.   table 1. measurement of voltage quality parameters. exemplary test results  for phase l1.  f = var   pxi ni  e/a  %  urms    /v  220.6  220.3  0.15  u1       /v  219.7  219.4  0.15  thd %  8.84  8.81  0.34  u5     /v  5.01  5.00  0.20  acta imeko | www.imeko.org  august 2013 | volume 2 | number 1 | 55  [5] iec standard 61 000-4-7, “general guide on harmonics and interharmonics measurement for power supply systems and equipment connected thereto”, 2007. [6] iec standard 60 092-101, “electrical installations in ships, definitions and general requirements”, 2002. [7] ieee standard 45-2002, “ieee recommended practice for electrical installations on shipboard”, 2002. [8] ad7656/ad7657/ad7658, data sheet, rev. d, analog devices, 2010. [9] analog devices, “interfacing to high speed adcs via spi”, application note an-877, rev. a 12/05. [10] analog devices, “adsp-ts201 tigersharc® processor”, hardware reference, revision 1.1, 2004. [11] analog devices. “adsp ts-201 ez-kit lite evaluation system manual”, revision 3.1, 2007. [12] national semiconductor, “lvds owner’s manual”, 4th edition; 2008. [13] t.d. shingare, r.t. patil, “spi implementation on fpga, international journal of innovative technology and exploring engineering”, vol. 2, no. 2, 2013, pp. 7-9. [14] j.k. singh, m. tiwari, v. sharma, “implementation of spi–slave on fpga, international journal of advanced engineering technology”, vol. 3, no. 4, 2012, pp. 24-26. [15] t. rahman, r. zaman, s. nowreen, i. rahman, “hardware implementation of strongarm processor interface using verilog and fpga”, 2nd international conference on electronics, biomedical engineering and its applications (ebea'2012), bali, indonesia, 2012, pp. 136-140. [16] k.harikrishna, t. rama rao, vladimir a. labay, “fpga implementation of fft algorithm for ieee 802.16e”, international journal of computer theory and engineering, vol. 3, no. 2, 2011, pp. 197-203. [17] xilinx. “spartan-3 fpga family data sheet”, product specification, application note ds099; 2005. [18] xilinx. “analog devices tigersharc link port, application note: spartan-ii and spartan-3 families, virtex and virtex-ii series”, xapp634 (v1.2), 2004. [19] j. mindykowski, t. tarasiuk, “development of dsp-based instrumentation for power quality monitoring on ships”, measurement, vol. 43, 2010, pp. 1012-1020. heating an electric car with a biofuel operated heater during cold seasons – design, application and test acta imeko issn: 2221-870x december 2018, volume 7, number 4, 48 54 acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 48 heating an electric car with a biofuel-operated heater during cold seasons: design, application, and testing christian riess1, michael simon josef walter1, stefan weiherer1, tiffany haas1, sebastian haas1, alexandru salceanu2 1 university of applied sciences ansbach, residenzstraße 8, 91522 ansbach, germany 2 gheorghe asachi technical university, bd. dimitrie mangeron, nr.67,700050 iaşi, românia section: research paper keywords: electric vehicle; range extension; air-conditioning; fuel-operated heater; biofuel; prototype citation: christian riess, michael simon josef walter, stefan weiherer, tiffany haas, sebastian haas, alexandru salceanu, heating an electric car with a biofuel operated heater during cold seasons – design, application and test, acta imeko, vol. 7, no. 4, article 9, december 2018, identifier: imeko-acta-07 (2018)-04-09 editor: alexandru salceanu, "gheorghe asachi" technical university of iasi, romania received march 29, 2018; in final form september 21, 2018; published december 2018 copyright: © 2018 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: the authors thank the biomass institute bavaria for its support in this project. corresponding author: christian riess, e-mail: christian.riess@hs-ansbach.de 1. introduction battery-powered electric vehicles (bev) offer great advantages over conventional combustion engine-driven cars. however, due to the limited charging possibilities and insufficient range during winter months, there are still some serious limits to widespread electro-mobility. various studies indicate that the power of such a battery and thus the range of the vehicle are affected by the heating of the passenger compartment during winter [1-4]. figure 1 details the driving ranges of the research vehicle (renault zoe r240, see figure 5) experienced in different driving scenarios (i.e., under varying ambient temperatures). according to the new european driving cycle (nedc), the range of a renault zoe r240 equipped with a 22 kwh battery should be about 240 km. the nedc is intended to map typical driving cycles for vehicles and consists of four urban trips and an extra suburban driving cycle [5]. figure 1 clearly shows that the realistic ranges are obviously different from the nedc. in optimal conditions, at an ambient temperature of 20 °c in eco mode (maximum velocity 94 km/h, no heating and cooling of the passenger compartment), a maximum range of 170 km can be achieved. our experience is that the vehicle has an average range of 120 km in summer (hot days, with air conditioning operational) and 80 km in winter (cold days, with the on-board heating system operational). the high-range losses can be explained by the operation of the energy-intensive on-board heating and cooling systems. furthermore, the impact on the driving range is even higher at cold temperatures, since the battery capacity decreases rapidly at lower temperatures below 0 °c. according to [7], heating significantly reduces the range of a mitsubishi i-miev. this abstract the automotive industry is currently undergoing far-reaching structural changes. automobile manufacturers are pursuing intensive scientific research and technological development in the field of alternative drive systems, such as electric powertrains. if electric car batteries are charged with regenerative generated electricity, their emission output is zero (from a well-to-wheel view). furthermore, electric drives have very high efficiency. at cold temperatures, however, the battery power drops due to energy-intensive loads, such as the heating of the passenger compartment, and this consequently reduces the range dramatically. therefore, the focus of this research work is external energy supply for the required heat capacity. the auxiliary energy may be generated by renewable energy technologies in order to further improve the co2 balance of electric vehicles. the paper deals with the design, application, and testing of a biofueloperated heater to heat the passenger compartment of a battery-powered electric car (a renault zoe r240). the practical use of the heating system is analyzed in several test drives, performed during winter 2018. the results as well as the range extension of the electric car that can be achieved by substituting the on-board heating system by the fuel-operated heater are quantified herein. mailto:christian.riess@hs-ansbach.de acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 49 clearly indicates a high demand for further research aiming to advance electro-mobility. the aim of our research is therefore to test the design and application of a fuel-operated heater in bevs and to quantify the range increase that can be achieved by means of battery relief. 2. state of the art 2.1. current state of the art on air-conditioning electric vehicles the heating of the passenger compartment of electric vehicles that are currently available is mainly carried out by commercial air-conditioning systems as in combustion vehicles. due to insufficient heat losses of the battery compared to internal combustion engines, these air-conditioning systems require a great deal of energy in winter, which is taken from the battery. since the generation of heat is easier to handle than that of cold and the maximum energy savings possible can be achieved in winter, most of the research work focuses on providing heat for the vehicle’s passenger compartment. currently, three heating concepts are in use for bevs: 1. electrical high-voltage air heaters 2. electrical high-voltage water heaters 3. heat pump heaters electrical high-voltage heaters are a specific type of electric heaters that are powered by battery energy. they can be applied in both airand water-cooled vehicle systems. high-voltage water heaters allow for energy efficient usage but reduce the driving range by up to 40 % [8]. heat pump heaters have remarkably high energy efficiency, but still need electric power for operation. this advanced technology can provide heating as well as cooling for the electric vehicle [9]. the renault zoe r240 is equipped with a heat pump system for heating and cooling. the disadvantage of the abovementioned heating systems is that the required electrical energy for their operation is drawn from the vehicle battery and thus considerably reduces the driving range of the batterypowered electric vehicle. therefore, providing a secondary source of energy is a promising area of research. heat energy from regenerative sources can take on the task of heating. furthermore, the electrical energy provided may also power alternative air-conditioning systems. various studies indicate that the use of regenerative energy for air conditioning in closed rooms is most promising [10-11]. 2.2. alternative technologies for air-conditioning electric vehicles this section gives a comprehensive overview of the most promising regenerative technologies for heat and electricity generation. these four technologies are shown in figure 2. figure 2. an alternative energy supply for auxiliary systems in bevs. several studies (such as [11]) show that there is a huge range of applications for biogas engines in electric and hybrid vehicles. furthermore, jaguar and bladon jets developed a rangeextended battery-powered electric vehicle that is supplied with electricity through two small biogas engines [12]. this application seems useful if a small size, a light weight, and high output delivery is significant. another research area is the use of fuel cells to generate energy and heat. fuel cells generate about 40 % waste heat, which can be used for cabin heating purposes [13]. the production of hydrogen (which is necessary for the fuel cell operation) is still very expensive and energy intensive. hydrogen is mainly produced in pilot plants and is thus unavailable in a sufficient amount. webasto manufactures solar sunroofs for cars that provide additional electric energy for air conditioning the passenger compartment. latest studies focus on the usage of so-called organic solar foils since no rare earths are needed. organic solar foils can be easily implemented on car roofs and, if translucent, also on windows. the generated electric power can operate several fans, which lead to a continuously fresh air supply and help to cool down the passenger compartment. furthermore, these solar foils help to reduce the direct radiation on the car. the implementation on the car is relatively easy: the foils are glued on the rooftop or windows, and the necessary cable connections can be executed on the inside of the vehicle [14]. auxiliary heaters using biofuels are another promising approach. this technology can support or substitute the air conditioning of bevs. because the heat production is mostly independent, the use of fuel-operated heaters (fohs) does not have a strong impact on the battery. heat is directly generated by burning fuel. for the most part, fohs can be implemented directly into the air-conditioning system of the vehicle. there are two possibilities for fohs: water and air heaters. the installation of fohs into the engine compartment, the passenger compartment, or at the exterior of the vehicle is quite easy. figure 3 and figure 4 show the main structure of airconditioning circuits, including fohs. the study of mimuro et al. [4] shows that fohs are a workable solution to the range difficulty. several manufacturers are working on fohs for electrified cars. eberspächer has already implemented an foh prototype in the mitsubishi imiev. they even manufacture an foh (monofuel hydronic-emobility e4s, based on ethanol e100) that leads to emission-free operation [15]. it is obvious that biofuels would be a sensible source of energy for running fohs. kohle et al. [16] point out that the energy savings that can be achieved by using an ethanol foh instead of an electrical heater is up to 30 %. therefore, a bev was tested according to the tüv süd electric car cycle (tsecc), which defines the velocity profile as a combination of three street types [17], at an ambient temperature of -7 °c. the applied fuel heater was a figure 1. the range of renault zoe r240 according to [6]. acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 50 figure 3. a water circuit incorporating a fuel-operated heater. figure 4. an air circuit incorporating a fuel-operated heater. hydronic-e-mobility e4s heater, which uses 100 % ethanol (e100) to operate. the range benefit was calculated according to the energy consumption. due to the uncomplicated production and widespread availability of bioethanol as well the easy implementation of the heater and the auxiliary systems in the vehicle, bioethanol fohs seem to offer a good contribution to the heating systems of the future [18]. apart from research on alternative energy concepts, several researchers deal with new concepts concerning air conditioning in general. decentralized air conditioning allows each passenger to individually control their surrounding temperature. this could lead to greater energy savings and thus relieve the strain on the vehicle battery. the most promising approaches are explained below. area heaters, such as resistance heating foils, can heat up cold areas around the passenger, which will lead to a higher level of comfort for the passengers due to the surface radiation instead of direct hot air streams. the required electricity can be generated by solar cells, mounted on the vehicle’s exterior. ackermann et al. [19] evidence that area heaters should be mounted close to the body to achieve perfect heat distribution to the body. their study shows how area heaters contribute to the comfort of the passenger and an energy efficient heating, ventilation, and air conditioning (hvac) system. layer heaters, a further development of common highvoltage heaters, are already in use in different industrial applications. when supplied with electrical power, they emit heat. the work of cap et al. [20] points out that the efficiency of these layer heaters can be up to 99 %. generated sun power can support continuous ventilation in the compartment, which leads to a continuously fresh air supply and lowered heating in the car cabin. the required energy can be produced by solar foils that are mounted on the car roof. adsorption cooling shows potential for efficient use in electric vehicles. it is not yet commonly used due to a few of its limitations, such as the low capacity of the most currently used adsorbents. abdullah et al. [21] point out that adsorption cooling will be common if further improvements are achieved. however, the problem concerning automotive applications is the missing systems in the required performance class. 3. research questions as mentioned above, the implementation of a bioethanoloperated heater seems to be a good contribution to the heating system of a bev. thus, two questions arise: 1. can a bioethanol-operated heater contribute to sufficient heating of the vehicle’s passenger compartment from a technical and legal point of view? 2. how large is the range increase achievable by means of battery relief? to answer these questions, an foh is implemented in the renault zoe. due to legal constraints, diesel is currently used instead of bioethanol in order to receive permission to use the vehicle on the roads in germany. the heater is hoped to contribute to sufficient heating of the vehicle’s passenger compartment. some modifications to the vehicle are necessary. from a legal point of view, the implementation is subject to conditions, but possible. however, only individual approval is possible in the current legal situation. a series approval is not achievable, especially when using bioethanol as the vehicle’s fuel. the necessary steps for the technical realization and first results on the range increase are presented and discussed in the following sections. 4. research vehilce: renault zoe r240 for practical testing and research purposes, the university of applied sciences ansbach has a battery-powered renault zoe r240 [6] at its disposal (figure 5). table 1 details the relevant technical data of the renault zoe r240 research vehicle. figure 5. research vehicle renault zoe r240. table 1. technical data of the renault zoe r240 (assembled in 2015) [22]. technical data renault zoe r240 electric motor three-phase synchronous generator cooling system air-cooled battery 22 kw nedc range 240 km power 65 kw (88 hp) peak max. torque 220 nm max. speed 135 km/h consumption 13.3 kwh/100 km acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 51 5. installation and application of the fueloperated heating system the main component of the foh implementation is a fueloperated auxiliary air heater from eberspächer (airtronic d2) with a maximum power output of 2.2 kw. the conversion measures performed at the vehicle are described herein. the auxiliary heater and corresponding peripherals (fuel tank, etc.) are mounted in the vehicle’s luggage compartment (figure 6 and figure 10) or below the floor panel of the vehicle (figure 7). the heated air is guided to the front of the passenger compartment using insulated tubes with an inner diameter of 60 mm. this diameter keeps the back pressure and thus the noise level of the built-in fan low. the tubes are routed underneath the backseat and the corresponding interior trim of the vehicle (2, figure 6) and end in the front-facing diffusors behind each front seat (figure 9). to avoid unpleasant dry air in the passenger compartment, the air intake (4, figure 6) provides fresh air from outside the vehicle into the passenger compartment. the air intake as well as the exhaust for the fuel-burning process are located underneath the vehicle to completely separate these streams from the inside of the vehicle. to keep the impact on the environment as low as possible, the combustion air and the exhaust gases are guided through appropriate silencers. the exhaust gases are additionally cleaned in an oxidizing catalytic converter, which is mounted (similarly to the fuel pump) close to the heater’s fuel supply underneath the floor panel of the luggage compartment (figure 7). figure 7. partly mounted silencer (1) and oxidizing catalytic converter (2) for exhaust gases (view from below the floor panel). the auxiliary heater and the fuel system are controlled with the “easystart timer” control unit of eberspächer. the control unit is mounted next to the steering wheel to allow complete control of the auxiliary heater while driving (figure 8). to regulate the power of the auxiliary heater automatically, a temperature sensor is mounted at the dashboard next to the vehicle’s air vents. this sensor-controlled heating protects the passengers from excessive heat and reduces the fuel consumption of the heater. to satisfy legal regulations and to keep the vehicle road legal in germany, an additional ventilated enclosure is fitted to the fuel tank in the luggage compartment (3, figure 6). a bioethanol-powered heater requires more action to keep the vehicle permanently road legal. figure 10. fuel tank (1, without enclosure) as well as wiring and piping (2, not covered by interior trim) during installation and assembly. figure 6. the assembled fuel-operated heater (1), air tubes (2), enclosed fuel tank (3), and fresh air intake (4) in the luggage compartment. figure 8. “easystart timer” (1) mounted next to the steering column. figure 9. front-facing hot air diffusor (1) on the right side of the backseat (2). 1 2  front 1 1 2 3 4 1 2 1 2 acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 52 figure 11. elevation profile of the test route. 6. experimental procedure and test results the results from the first test drives with different driving modes and heater settings in the vehicle are presented and discussed in this section. in advance of the test drives, a test route of 49.3 km was defined in order to keep the energy consumption comparable. the test route is a combination of city and country roads as well as a short highway section. this combination ensures that every traffic and load scenario is included in the test drives. the elevation profile (figure 11) of the test route is diversified but with more flat and uphill sections than downhill sections for recuperation. the overall altitude difference is 92.6 m (maximum 491.4 m, minimum 398.8 m). the selected roads are main roads, but with a lower traffic volume than other roads in the area around ansbach. the section through the city is routed in such a way that good progress is guaranteed, even during rush hour. the driving data was recorded using the can-bus by means of the vehicle’s onboard diagnosis interface (obdii) with the open-source application canze [23]. figure 12. procedure for acquiring structured driving data [24]. the procedure for acquiring the data of the relevant driving parameters has three main steps: pre-processing, processing, and post-processing (figure 12). during the pre-processing, it was necessary to establish an adequate bluetooth connection between the car’s obdii and a device that provides access to the canze application (in this case, an android tablet). the processing step includes the test drive as well as the entire monitoring activities of the driving data. the application canze receives information about driving parameters. however, the application stores those in an unstructured matter in raw files (such as *.field). by closing the app, the monitoring process (as well as the storage of the gained data in the raw files) was completed. to provide an adequate database for the further analysis and visualization of driving parameters, a filter application is applied during the post-processing phase. this application extracts the corresponding information about a driving parameter in a structured manner by sorting the raw data in accordance with the given parameter ids and time stamps. these parameter ids were previously defined by the car manufacturer’s on-board protocol and are not subject to change [24]. two test drives, one at -11 °c and one at -13 °c, are taken as examples in figure 13. both test drives were conducted between 8:00 a.m. and 9:00 a.m. on two consecutive days. the test drive at -11 °c was conducted under the defined standard test conditions (stcs). the stcs include the driving mode of the vehicle set to economy (eco) with a limited top speed of about 94 km/h, the vehicle’s heater set to 22 °c with fresh air intake from outside the vehicle, and the blower set at level 2. the vehicle’s battery was fully charged before the test drive was conducted. the settings for the compared test drive at -13 °c are the same, apart from the use of the auxiliary heater instead of the vehicle’s heater. the uneven records of canze caused the different time intervals displayed in figure 13 and figure 14. the record intervals range from four to seven seconds and are not evened out for the graphs. a comparison of the graphs in figure 13 shows that the battery’s state of charge (soc) without the auxiliary heater drops with a larger gradient and earlier than the soc with the auxiliary heater switched on. the substitution of the vehicle’s heater results in a soc difference of 0.08 after the test drive. this is equivalent to a range increase of about 21.6 %. the difference in soc at the beginning is caused by the batteries’ behaviour at 5 km 49,3 km university ansbach gps 49.305516, 10.567302 university ansbach gps 49.305516, 10.567302 500 m 480 m urban area (strong traffic, traffic lights, two-lane-roads, etc.) rural area (low traffic, no traffic lights, one-lane-roads, large variety of road users such as tractors etc.) 460 m 440 m 420 m 400 m 10 km 15 km 20 km 25 km 30 km 35 km 40 km 45 km 50 km0 km urban arearural areahighway a6 (strong traffic, two-lane-highway) establish a bluetooth connection via obd 1 p re -p ro c e ssin g start the app canze to monitor the parameters2 perform test drive3 p ro c e ssin gfinish the monitoring via app canze4 import raw-data in the filter application5 p o st-p ro c e ssin g apply parameter filter function6 acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 53 figure 13. state of charge of two test drives at -11 °c (stc) and -13 °c (test drive); (soc = 1: fully charged battery; soc = 0: entirely uncharged battery). figure 14. comparison of the state of charge in two test drives at 1 °c; (soc = 1: fully charged battery; soc = 0: entirely uncharged battery). cold temperatures, which in some instances prevented the soc from reaching 100 %. a comparison of the graphs in figure 14 delivers a very similar result as the graphs in figure 13. the gradient of both curves is lower than the gradient for colder conditions.this gradient difference is caused by the lower battery capacity in cold conditions as described in section i. the difference in soc caused by the auxiliary heater is 0.03. this means a calculated range increase of about 5.5 %. a comparison of the curves in figure 13 and figure 14 with the elevation profile in figure 11 shows that the gradient difference between the curves in both figures is solely due to whether the vehicle’s heater switched on or off. the different heater settings can be clearly identified in both figures. 7. conclusion the gained results clearly show that the impact of the vehicle’s heating unit on the achievable range during winter or cold conditions is significant and measurable. although the potential range increase observed during the first test drive is already at an acceptable level, the expected energy consumption in even colder climates (countries with ambient temperatures below -20 °c during cold seasons) is significantly higher than in the mild central european climate. the potential range increase between 5.5 % and 21.6 % (without any further optimization of the vehicle or the auxiliary heater) shows that the concept of substituting the vehicle’s heater is a promising possibility for increasing the range of bevs in todays’ state of technology. we intend to carry out further tests with the research vehicle on an air-conditioned chassis dynamometer to verify the obtained results from the test drives on the road. the chassis dynamometer allows a more exact driving performance and thus a more precise view of the heater’s impact on the vehicle’s range. furthermore, the air conditioning of the chassis dynamometer is capable of providing test scenarios with ambient temperatures as low as -35 °c. this allows an appraisal of the impact on bevs in colder countries such as norway, finland, or sweden. however, the proposed method does have several limitations. the data acquisition procedure is not fully automated and thus not yet able to provide real-time data. furthermore, the statistical reliability of the data (and the on-board computer system) is currently insufficient. a promising approach may be the implementation of artificial intelligence (such as artificial neural 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1 0 4 8 13 17 21 25 29 33 38 42 s ta te o f c h a r g e time in minutes stc, bioethanol heater: off test drive, bioethanol heater: on 0,5 0,6 0,7 0,8 0,9 1 0 4 8 13 17 21 25 29 33 38 42 s ta te o f c h a r g e time in minutes stc, bioethanol heater: off test drive, bioethanol heater: on acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 54 networks [25]) for a more realistic estimation of the remaining driving range. finally, to increase comparability between several test scenarios (according to an adequate experiment design), the development and application of a numerical test drive simulation [26] is recommended. acknowledgements the authors thank the biomass institute bavaria and eberspächer climate control systems gmbh & co kg. references [1] k. kim, w.-s. lee, y.-y. kim, investigation of electric vehicle performance affected by cabin heating. journal of the korea academia-industrial cooperation society 14(10) (2013), pp. 4679-4684. [2] t. kiss, j. lustbader, d. leighton, d. “modelling of an electric vehicle thermal management system in matlab/simulink”, proc. of the sae 2015 world congress & exhibition, apr. 2123, 2015, detroit, u.s.a., sae technical paper 2015-01-1708. [3] n. meyer, i. whittal, m. christenson, a. loiselle-lapointe, “the impact of driving cycle and climate on electrical consumption & range of fully electric passenger vehicles”, proc. of the evs26 international battery, hybrid, and fuel cell electric vehicle symposium, los angeles, u.s.a., jun. 6-9 2012, pp. 1-11. [4] t. mimuro, h. takanashi, fuel operated heaters applied to electric vehicles. international journal of automation technology 8(5) (2014), pp. 723-732. [5] european automobile manufacturing association, how do the lab tests work? [online] available: http://www.caremissionstestingfacts.eu/ nedc-how-do-lab-tests-work/?gclid=eaiaiqobchmiipn_qly n1qivqlvtch2xda74eaayasaaegl_p_d_bwe# (access date 07.31.2017). [6] t. haas, m. s. j. walter, s. weiherer, a. salceanu, “increasing the driving range of electric vehicles using secondary energies – a review”, proc. of the 22nd imeko tc4 international symposium & 20th international workshop on adc modelling and testing, sept. 14-15, 2017, iasi, romania, pp. 325-330. [7] k. umezu, h. noyama, air-conditioning system for electric vehicles (i-miev). proc. of the sae automotive refrigerant and system efficiency symposium, july 13 – july 15 2010, scottsdale, arizona, u.s.a. [8] k. beetz, u. kohle, g. eberspach, beheizungskonzepte für fahrzeuge mit alternativen antrieben. automobiltechnische zeitschrift 112 (2010), pp. 246-249. [9] q. peng, q. du, progress in heat pump air conditioning systems for electric vehicles – a review. energies 9(4) (2016), pp. 1-17. [10] m. shimada, a vehicle driven by electricity, designed for chill and snowy areas. sensors and actuators a: physical 200(1) (2013), pp. 168-171. [11] a. s. ramana, l. chidambaram, g. kamaraj, r. velray, evaluation of renewable energy options for cooling applications. international journal of energy sector management 5(1) (2012), pp. 65-74. [12] bladon jets: jaguar c-x75 concept. [online] available: http://www.bladonjets.com/applications/automotive/jaguarc-x75-concept-case-study (access date 05.20.2018). [13] h. eichlseder, m. klell (editors), wasserstoff in der fahrzeugtechnik, vieweg+teubner, wiesbaden, 2010. [14] t. haas, m. walter, s. weiherer, “potentiale zur reichweitenerhöhung eines serien-elektrofahrzeuges durch die nutzung regenerativer energien”, proc. of the biomass to power and heat conference, may 31-jun. 1, 2017, zittau, pp. 185-197. [15] eberspächer gmbh & co. kg, assembly recommendation hydronic ii – d5s commercial/e 4 s (ethanol) in mistubishis i-miev (ha0), internal document provided by eberspächer, 2017. [16] u. kohle, w. pfister, r. apfelbeck, bioethanol heater for the passenger compartments of electric cars. springer, automobiltechnische zeitschrift, 2012, pp. 58-63. [17] tüv süd tsecc, reichweitenermittlung von elektrofahrzeugen. [online] available: http://www.tuev-sued.de/uploads/images/ 1296656610031102360044/ts-am_reichweitenermittlung_4-sei ter_a4_d.pdf (access date 07.31.2017). [18] t. wolf, auslegung, konzeptentwicklung und bewertung einer anlage zur klimatisierung eines elektrofahrzeuges mittels biogener energieträger. master’s thesis, university of applied sciences ansbach, 2018. [19] j. ackermann, c. brinkkötter, m. priesel, neue ansätze zur energieeffizienten klimatisierung von elektrofahrzeugen, automobiltechnische zeitschrift 115 (2013), pp. 480-485. [20] c. cap, c. hainzlmaier, schichtheizer für elektrofahrzeuge. automobiltechnische zeitschrift 115 (2013), pp. 486-489. [21] n. o. abdullah, i. a. w. tan, l. s. lim, automobile adsorption air-conditioning system using oil palm biomass-based activated carbon: a review. renewable and sustainable energy reviews 15(4) (2011), pp. 2061-2072. [22] renault, renault zoe – preise und ausstattungen, 2017. [23] b. fisch, j. meijer, canze. [online] available: canze.fisch.lu (access date 01.09.2018). [24] m. s. j. walter, s. weiherer, t. haas, d. l. dao, a. sover, “acquision and filtering of relevant driving parameters of electric cars”, proc. of the 22nd imeko tc4 international symposium & 20th international workshop on adc modelling and testing”, sept. 14-15, 2017, iasi, romania, pp. 66-70. [25] m. walter, t. sprügel, s. wartzack, tolerance analysis of systems in motion taking into account interactions between deviations. journal of engineering manufacture 227(5) (2013), pp. 709-719. [26] m. walter, m. storch, s. wartzack, on uncertainties in simulations in engineering design: a statistical tolerance analysis application. simulation 90(5) (2014), pp. 547-559. acta imeko, title acta imeko issn: 2221-870x december 2017, volume 6, number 4, 0 acta imeko | www.imeko.org december 2017 | volume 6 | number 4| 0 journal contacts about the journal acta imeko is an e-journal reporting on the contributions on the state and progress of the science and technology of measurement. the articles are mainly based on presentations presented at imeko workshops, symposia and congresses. the journal is published by imeko, the international measurement confederation. the issn, the international identifier for serials, is 2221-870x. editor-in-chief paolo carbone, italy honorary editor-in-chief paul p.l. regtien, netherlands editorial board section editors (vol. 4, 5 and 6) leopoldo angrisani, italy filippo attivissimo, italy eulalia balestieri, italy eric benoit, france catalin damian, romania lorenzo ciani, italy pasquale daponte, italy luca de vito, italy luigi ferrigno, italy edoardo fiorucci, italy alistair forbes, united kingdom helena geirinhas ramos, portugal sabrina grassini, italy fernando janeiro, portugal konrad jedrzejewski, poland andy knott, united kingdom massimo lazzaroni, italy fabio leccese, italy rosario morello, italy michele norgia, italy pedro miguel pinto ramos, portugal sergio rapuano, italy dirk röske, germany alexandru salceanu, romania constantin sarmasanu, romania lorenzo scalise, italy emiliano schena, italy enrico silva, italy marco tarabini, italy eric benoit, france marcantonio catelani, italy paolo carbone, italy lorenzo ciani, italy spartacus gomariz, spain sabrina grassini, italy konrad jędrzejewski, poland min-seok kim, korea franco pavese, italy serge rapuano, italy paul regtien, the netherlands janaina m rodrigues, brazil alexandru salceanu, romania jan saliga, slovakia krzysztof stepien, poland marco tarabini, italy ian veldman, south africa claudia zoani, italy final editing rosario morello, university mediterranea of reggio calabria, italy about imeko the international measurement confederation, imeko, is an international federation of actually 42 national member organisations individually concerned with the advancement of measurement technology. its fundamental objectives are the promotion of international interchange of scientific and technical information in the field of measurement, and the enhancement of international co-operation among scientists and engineers from research and industry. addresses principal contact prof. paolo carbone university of perugia dept. electr. inform. engineering (diei) via g.duranti, 93, 06125 perugia, italy email: paolo.carbone@unipg.it acta imeko prof. paul p. l. regtien measurement science consultancy (msc) julia culpstraat 66, 7558 jb hengelo (ov), the netherlands email: paul@regtien.net support contact dr. dirk röske physikalisch-technische bundesanstalt (ptb) bundesallee 100, 38116 braunschweig, germany email: dirk.roeske@ptb.de acta imeko issn: 2221-870x december 2017, volume 6, number 4, 1 acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 1 section: a message to our readers dear reader, after several years, both paul regtien, current honorary editor, and paolo carbone, current editor-in-chief of this journal are ending their terms. this issue will be the last one for both of us. paul must be credited for starting this journal almost 7 years ago. paolo was added to the editorial board one year after. we both worked together for making the journal grow. with the help of an excellent editorial board, acta imeko has now a steady flux of submitted papers, it is listed under elsevier scopus and papers are cited and consistently read by the community of scientists working in measurement around the world. acta imeko is and will remain an open-access journal for the benefit of this community. a new editor-in-chief will manage this journal, starting from the first issue of 2018: dusan agrez, a professor with the university of ljubljana, was appointed to this role. we wish him success in bringing acta imeko to new achievements. this is the occasion to thank, especially, some of the people that worked hard over the last years to produce this journal: dirk roeske and rosario morello. a big thank also to german ptb for the continuous support provided to acta imeko and to judit farago at imeko. finally, thank you all for your contributions to acta imeko, as authors and readers. sincerely, paul regtien and paolo carbone journal contacts << /ascii85encodepages false /allowtransparency false /autopositionepsfiles true /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /error /compatibilitylevel 1.4 /compressobjects /tags /compresspages true /convertimagestoindexed 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/hrv (za stvaranje adobe pdf dokumenata najpogodnijih za visokokvalitetni ispis prije tiskanja koristite ove postavke. stvoreni pdf dokumenti mogu se otvoriti acrobat i adobe reader 5.0 i kasnijim verzijama.) /hun 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can be opened with acrobat and adobe reader 5.0 and later.) >> /namespace [ (adobe) (common) (1.0) ] /othernamespaces [ << /asreaderspreads false /cropimagestoframes true /errorcontrol /warnandcontinue /flattenerignorespreadoverrides false /includeguidesgrids false /includenonprinting false /includeslug false /namespace [ (adobe) (indesign) (4.0) ] /omitplacedbitmaps false /omitplacedeps false /omitplacedpdf false /simulateoverprint /legacy >> << /addbleedmarks false /addcolorbars false /addcropmarks false /addpageinfo false /addregmarks false /convertcolors /converttocmyk /destinationprofilename () /destinationprofileselector /documentcmyk /downsample16bitimages true /flattenerpreset << /presetselector /mediumresolution >> /formelements false /generatestructure false /includebookmarks false /includehyperlinks false /includeinteractive false /includelayers false /includeprofiles false /multimediahandling /useobjectsettings /namespace [ (adobe) (creativesuite) (2.0) ] /pdfxoutputintentprofileselector /documentcmyk /preserveediting true /untaggedcmykhandling /leaveuntagged /untaggedrgbhandling /usedocumentprofile /usedocumentbleed false >> ] >> setdistillerparams << /hwresolution [2400 2400] /pagesize [612.000 792.000] >> setpagedevice pre-filter design for exact linearization-based tracking controllers for variable-load systems acta imeko issn: 2221-870x december 2019, volume 8, number 4, 62 69 acta imeko | www.imeko.org december 2019 | volume 8 | number 4 | 62 pre-filter design for exact linearisation-based tracking controllers for variable-load systems na wang1, bálint kiss1 1 budapest university of technology and economics, műegyetem rakpart 3, 1111 budapest, hungary section: research paper keywords: exact linearisation; robust design; parameter uncertainty, scara robot; overhead crane citation: na wang, balint kiss, pre-filter design for exact linearization-based tracking controllers for variable-load systems, acta imeko, vol. 8, no. 4, article 11, december 2019, identifier: imeko-acta-08 (2019)-04-11 editor: yvan baudoin, international cbrne institute, belgium received january 16, 2019; in final form september 12, 2019; published december 2019 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was supported by the bme-artificial intelligence fikp grant of emmi (bme fikp-mi/sc). corresponding author: na wang, e-mail: wang@iit.bme.hu 1. introduction robotic arms and cranes serve mostly as weight-handling devices, and they move a load to a desired target location along a specified path within the workspace of the equipment. a shared property of such weight-handling tasks is the uncertainty of the load mass or inertia, as load variation from one operation cycle to another may be high. therefore, both for robots and for cranes, one of the objectives of the controller design is the limitation or full cancellation of the effects of this uncertainty on the accuracy of reference trajectory tracking. in contrast with fully actuated robotic arms, cranes are underactuated mechanisms, such that the number of configuration variables is larger than the number of actuators. nevertheless, both robots and cranes share the formalism (euler-lagrange equations) available to obtain their dynamical models. from a control point of view, it can be observed that tracking precision and speed are generally conflicting requirements, especially if uncertainty is present for inertia values, since these uncertainties are amplified due to the high-motion speeds of the mechanism. moreover, in addition to the load inertia, friction coefficients and torque constants of the (usually electric) actuators are also known with only a limited accuracy. closed-loop control is expected to ensure robust stability and tracking performance in the presence of such uncertainties. various methods have been already explored for robot controller design and implementation, including sliding mode control [1]-[3], neural networks [4], and fuzzy systems [5], [6], to name a few. a comprehensive overview of the robust control techniques of robot manipulators is presented in [7]. similarly, several control techniques are proposed for cranes to ensure the tracking of a reference load trajectory. a review is presented in [8]. the flatness-based modelling and control of crane systems similar to that considered in this paper are addressed in [9] and [10]. the authors of [11] suggested robust controllers for the approximated linear crane dynamics obtained by (idle) setpoint linearisation. setpoint linearisation remains a valid approximation for idle positions. this limits the applicability of the feedback to slow (or quasi-static) load motions. in the case of quick load trajectories, abstract several classes of weight-handling equipment with nonlinear dynamics are exactly linearisable by feedback. this implies that a controller ensuring the exponential decay of the tracking error along a reference trajectory can be also designed. the control of both fully actuated robotic arms and underactuated crane-like systems can be addressed using this method. however, the value of model parameters such as the mass of the load or the friction coefficients must be known in order to calculate the actuated inputs in closed loop. the uncertainty of these parameters may result in unstable closed-loop dynamics or in the deterioration of the tracking performance. this paper presents a design procedure to take into consideration such uncertainties using a serial pre-filter, which is determined using h∞ techniques. the procedure is applied to a scara-type robotic arm and to a two-dimensional overhead crane system. the simulation results show the applicability of the suggested method. mailto:wang@iit.bme.hu acta imeko | www.imeko.org december 2019 | volume 8 | number 4 | 63 where the nonlinear coupling effects and the rope angles become large, the linear approximation is no longer valid. dynamical models of robots and cranes are differentially flat [12]; hence, they are feedback linearisable. the linearising feedback is static for robots and dynamic for cranes (i.e. the feedback has a non-empty state vector) [9], [13]. controllers exploiting the flatness property are capable of quick and precise reference tracking if the parameters are known with accuracy. uncertainty in parameters, such as inertia, load mass, and friction coefficients, may destabilise the closed-loop system or considerably deteriorate its performance in terms of both steadystate and transient behaviour. this paper addresses the problem of the uncertainty of parameters by the design of a serial and linear pre-filter using standard h∞ synthesis techniques. the need for the robust approach is motivated by the fact that the parameters of the real system and the parameters used in the linearising feedback may be different, especially as far as the load mass is concerned. to define the underlying synthesis problem, the set of systems at which the exact linearisation is applied with a parameter misfit to the robot or crane model must be covered by a standard linear uncertainty structure. the weighting matrices of this structure are calculated by solving a set of inequalities such that the linearising feedback is applied to a large set of robot or crane models with the parameter values spanning a grid of their uncertainty range. the idea of applying a robustifying controller together with exact feedback linearisation has already been presented by the authors in [14]. the necessary design steps to obtain the pre-filter are presented in section 2. the suggested method is applied to a scara-type robot in section 3. in section 4, we discuss how the method can be used in crane control. the concluding section enumerates some topics subject to further investigation. 2. the robustifying pre-filter and its design consider a nonlinear system with a finite dimensional statespace so that its description depends on the n elements of a parameter vector 1 2[ , , , ] t np p p p= . the uncertainty of each parameter is described by a bounded real interval p i ∈ q i ⊂ ℝ such that 1 2 nq q q q=    . the vector of the nominal parameter values is denoted by 0 p q . the nonlinear dynamics are described by ( , , ) ( , )x f x u p y h x p= = (1) where x ∈ ℝ𝑛 and the dimension m of the input vector u equal the dimension of the output vector y . we suppose that a linearising feedback can be designed in equation (1) for all possible parameter values in q , such that the linearising (or flat) output is y . since the real parameter values are not known, the nominal values 0 p are used in the expressions of the feedback: 0 ( , , , )x v p  = , (2) 0 ( , , , )u x v p = . (3) thanks to the exact linearisability (or differential flatness property [12]) of the system, it is possible to express the derivatives of the output trajectory as the function of the full state vector of the closed-loop system, [ , ] t x  :  = =  − = ( ) ( )0 , ( , , ) 1, , 1 j i j j j i i j i iy j d y h x p dt y . (4) let us now denote by ry a sufficiently smooth reference trajectory for the output y. the tracking feedback law can be than set as ( )      − − = = + − =  ( ) ( 1) 0 , , , , 1 ( , , ) 1, , i i i ii i r i j i r i j j v y y h x p i m (5) which results in an exponential decay of the tracking error, since, by the construction of equation (5), ,i i r ie y y= − satisfies       − − = + + ++ ( ) ( 1) ( 2 ) ,1 ,2 ,0 i i i ii i i i i i i e e e e (6) where the coefficients ,i j are design parameters. they are set so that the characteristic polynomial of equation (6) is hurwitz stable i.e. all its roots have negative real parts. the block diagram of the closed-loop system is depicted in figure 1. it follows from the above discussions that the closedloop transfer between y r̅ and y, assuming p= p0, reads , 01 1, , ,1 , ( ) diag ( ) ( ) i i i i i i mr i i y s g s y s s s        − =     = =  + ++   . (7) due to the uncertainty of the parameters, the nominal values p0 used in the linearising feedback (equations (2)-(3)) may be different to their real values. this parameter misfit implies that the resulting closed-loop dynamics may still include nonlinearities. for the robotic arms and crane systems considered in this article, the uncertain parameters are the inertia of the moving parts of the mechanism, the mass of the load, and the friction coefficients. the serial pre-filter (see figure 1) is designed using h∞ synthesis techniques [15]. to apply such a synthesis technique, the feedback structure following the controller ( )k s in figure 1 figure 1. closed-loop dynamics with a serial pre-filter, tracking and linearising feedback. figure 2. the controller k(s) with the augmented plant. acta imeko | www.imeko.org december 2019 | volume 8 | number 4 | 64 must be replaced by an uncertainty structure as in figure 2. a sufficiently smooth grid is chosen to cover the uncertainty range q of the parameters. for each vertex pi of this grid, a linear transfer function gi(s) is obtained between the signals ry and y by a setpoint linearisation. the weighting matrices 1( )w s and 2( )w s of the output multiplicative structure in figure 2 are chosen so that = + 1 2 0( ) (1 ( ) ( ) ( )) ( ) i g s w s s w s g s holds true for some   ( ) 1s . the command ucover in matlab calculates the weighting matrices accordingly if the nominal model and the frequency response data for each ( ) i g s is given. the h∞ design problem to obtain ( )k s is also solved with matlab, such that a standard augmented plant, denoted by ( ),ag s is constructed (figure 2) with performance weighting transfer functions ( )m s , ( )ew s and ( )uw s :   11 12 21 22 ( ) ( ) t m u a r a r u p y e e e g s y g s p p p y        = =        (8) with   2 0 21 0 11 1 12 0 22 1 0 0 0 0 e e e u w g p g p w w w m p w g p w i w     =    = − =     = −        (9) where the dependence on s is omitted. applying the controller ( )k s , the closed-loop transfer matrix can be calculated using a lower fractional transformation 1 11 12 21 22( , ) ( )l af g k p p k i p k p − = + + . (10) by introducing 1 0 − = +( )os i g k , we get: 2 0 1 2 2 1 0 1 −    = −    −  ( , ) ( ) o o l a e o e o u o u o w g ks w w g ks f g k w s w w g ks m w s w w ks . (11) finally, the h optimisation consists of finding the controller ( )k s , which minimises the h norm of (11). this optimisation process is implemented in matlab through the hinfsyn command [16]. robust stability is also guaranteed if 2 0 1 1ow g ks w   (12) the next two sections present the application of the above design procedure for a four degrees-of-freedom (dof) robot arm and for a planar crane system. 3. control of a scara-type robotic arm 3.1. robot modelling let us consider a four-dofs scara-type robotic arm with three rotary joints whose axes are parallel to each other and which have a prismatic joint. to demonstrate the feasibility of the proposed method, the design procedure described in the previous section is executed on the model of a bosch turbo scara sr 60-type robot [17]. the robot scheme is shown in figure 3. the dynamics of a robot manipulator read + + + =( ) ( , ) ( ) ( )h q q c q q q b q g q (13) where ,q q and q are the vectors of joint angles, joint velocities, and joint accelerations, respectively; h(q) ∈ ℝ4×4 is a symmetric, positive definite inertia matrix; c(q, q̇ )q̇ ∈ ℝ4 is the vector of the coriolis and centrifugal torques; b(q̇ ) ∈ ℝ4 is the vector of the friction torques; g(q) ∈ ℝ4 is the vector of gravitational torques, and the control input torque is τ ∈ ℝ4. the length, the position of the centre of gravity (cog), and the mass of the segments are denoted by il , ia , and im , respectively; and r is radius of the last segment. using standard notations in robotics, ijd denotes the elements of ( )h q ( =ij jid d ) and reminds us that 4 4 1 1= = =( , )i ijk j k j k c q q q d q q . (14) considering the scara robot, such that we set 2/i ia l= , the elements of the inertia matrix read 22 2 2 41 1 11 2 2 1 1 2 2 2 2 3 4 1 2 1 2 2 2 12 2 2 1 2 2 2 2 3 4 2 1 2 2 4 2 2 2 2 2 14 4 22 2 2 3 2 4 2 33 3 4 31 32 34 1 3 3 2 ( ) )( ) 2 0 ( 2 ) 1 1 3 2 1 ( 2 1 1 1 2 3 m rm l d m l l l l c m m l l l l c d m l l l c m m l l l c m r d m r d m l m l m l r d d dm dm = + + + + + + + + + = + + + + = − = + + + = +             +       = = = (15) and the non-zero ijkd elements are  = = −  = − + +    112 1 1112 2 3 4 221 2 2 21 1 2 2 2d m m m l l ds d d . (16) figure 3. a scara robot with rrtr (r-rotational, t-translational) joint configuration. table 1. uncertain parameters of the four-dofs scara-type robotic arm. variable normal value minimum value maximum value m4 3 kg 2 kg 4 kg fv1 0.5 kg m rad-1 s-1 0.2 kg m rad-1 s-1 0.8 kg m rad-1 s-1 fv2 0.5 kg m rad-1 s-1 0.2 kg m rad-1 s-1 0.8 kg m rad-1 s-1 fv3 0.05 kg m/s 0.02 kg m/s 0.08 kg m/s fv4 0.05 kg m rad-1 s-1 0.02 kg m rad-1 s-1 0.08 kg m rad-1 s-1 acta imeko | www.imeko.org december 2019 | volume 8 | number 4 | 65 shortcut notations s2 and c2 represent 2sin q and 2cos q , respectively. only viscous friction is considered as in [18]: =( )i vi ib q f q (17) and only the motion of the third joint is influenced by gravity 3 3 4 1 2 4 0 9 81 2 ( ) . m/sg m m g g g g g= − + = = = = . (18) 3.2. pre-filter design and simulation results in most applications, the joint mass can only be estimated with limited accuracy. we assume that the parameters 1m , 2m , and 3m are known with sufficient precision, and their values are 15 kg, 12 kg, and 3 kg, respectively. the parameter m4 comprises the variable payload mass; hence, its value is uncertain. the lengths of the two links are 1 0 5. ml = , 2 0 4. ml = , and 0 2. mr = . the friction coefficients vif are also variable. the uncertain parameter ranges are given in table 1. a grid of 50 vertices is used to get the output multiplicative uncertainty structure presented in figure 2 with a second order weighting matrix 1w , with the following elements: 2 2 2 2 2 2 2 2 2 2 0.27 0.06 4.61 4 0.35 0.04 1.27 4 1,11 1,22 0.93 0.04 0.56 0.01 0.45 0.05 6.09 4 1.18 0.16 5.04 4 1,33 1,44 2.26 0.14 1.45 0.03 0.71 0.12 8.13 4 2,11 ( ) ( ) ( ) ( ) ( ) s s e s s e s s s s s s e s s e s s s s s s e s w s w s w s w s w s + + − + + − + + + + + + − + + − + + + + + + − + = = = = = 2 2 2 2 2 2 0.71 0.10 2.98 4 2,22 1.94 0.07 1.06 0.02 0.45 0.05 6.09 4 0.72 0.11 3.49 4 2,33 2,44 2.26 0.14 1.07 0.02 . ( ) ( ) ( ) s s e s s s s s e s s e s s s s w s w s w s + + − + + + + + − + + − + + + + = = = (19) a reference model is used to define the desired closed-loop tracking response for each joint angle:        =   + +   2 2 2 ( ) diag 2 m m m m m s s s (20) with 2 rad/secm = and 0 8. ,m = respectively. the remaining performance weighting transfer matrices are also diagonal (fourby-four). recall that uw is linked to the actuator bandwidth. the weighting functions used in our design read   + + = 2 2 40( 10 ) ( 200 ) ( ) diag m m s u s w s (21) and 2000 1 1 1 1 1 2 9 ( ) diag ,10 , ,10 20 m m e e e e e e s w s w w w w w  +   = =    . (22) the robot dynamics and the controller are simulated for different uncertain parameter values using matlab and simulink. the reference trajectory ry is a step function at 1sect = for each joint. the initial conditions are set to zero. figure 4 shows the tracking performance without the use of the pre-filter, whereas figure 5 shows the joint trajectories with the use of the pre-filter. for each joint, a family of curves is presented, each for a different vertex of the grid spanned over the uncertain parameter space. it can be observed that the use of the pre-filter reduces the effect of uncertainty and provides better performance. since 2 0 1 0 5025. ,ow g ks w  = the robust stability condition (12) is satisfied. 4. crane control 4.1. crane modelling we consider a 2d overhead crane [9], depicted in figure 6. the load is suspended by a massless rope, which is pulled by a figure 4. joint trajectories without the h∞ pre-filter. figure 5. joint trajectories with the application of the h∞ pre-filter. acta imeko | www.imeko.org december 2019 | volume 8 | number 4 | 66 winch. the suspension point is moved horizontally by a cart. the configuration variables are the horizontal position of the cart ,r the length of the rope l , and its angle with respect to (w.r.t.) the vertical position:  . the position of the load in the vertical x z− plane is denoted by [ , ] t m mx z and , , , , , . t x r l r l  =   the parameters of the system are [ , , , ] , t p m m j = including the mass of the load, the mass of the cart, the inertia and the radius of the winch. let /t  = denote the equivalent pulling force on the rope such that the system inputs are [ , ] t u f t= . let us denote the rope tension by lt . introducing cosc = and sins = , the dynamics read ( ) 2 2 2 2 0 0 2 . 02 m m ms mlc r j ms m l mlc ml ml c ml s f ml gmc t ml l gs r s                  +       + +            −       + − − = −       + −     (23) 4.2. pre-filter design and simulation results it is known [9], [13] that equation (23) is differentially flat and hence feedback linearisable. the coordinates of the load form the flat output: 1 2m my x r ls y z lc = = + + = = . (24) the control objective is to follow a reference trajectory specified for the flat output. the crane system can be transformed by feedback into two separated chains of integrators ( 4 ) ( 4 ) 1 1 2 2y v y v= = . (25) the linearising feedback has two states, 1 and 2 . these are the rope tension 1 lt = and its first time derivative 2 lt = . let us now calculate the linearising feedback resulting (25). referring to [19], the motion equations of the load read  = = − = = − +1 2sin cos l l m m t t y x y z g m m . (26) let us differentiate both sides of (26) twice w.r.t. the time so that we use the two states of the feedback ( 1 and 2 ) as well ( ) ( ) 2 2 2 1 1 1 2 2 2 1 1 2 1 sin 2 cos cos sin 1 cos 2 sin sin cos v m v m                       − − − + = − + + + = (27) and isolate 2 and  using the nominal 0 m . we get 1 0 2 2 1 1 1 2 0 2 1 2 1 2 2 2 s c m v s c c s m v c s                    −       − − −   = −        − +             (28) equation (24) can be differentiated twice more to get 2 1 2 1 2 2 r ls l c l c l s s m lc l s l s l c c g m                   + + + + = − − − − = + (29) which allows us to isolate the vector of accelerations as 1 1 0 2 2 1 1 0 2 2 r s s lc c c lsl m l s l c g l c l s                 −   − −      = +                  − +   + +   (30) the calculations are also illustrated in figure 7. blocks a, b, and c represent calculations equation (28), equation (30), and the first two lines of equation (23), respectively. recall that the states of the linearising feedback, 1 and 2 , must also be initialised. for the idle initial load position 0 1 0( ) m g = and 2 0 0( ) = . in addition to the linearising feedback, a tracking controller is also designed. suppose that a sufficiently smooth trajectory is given for the motion of the load as , ( )m rx t , , ( )m rz t and their derivatives, such that , ,[ , ] t m r m ry x z= . the tracking errors are defined as ,x m r me x x= − and ,z m r me z z= − . the expression of the tracking controller can be obtained by specifying the desired exponential decay of the tracking errors as 3 3 ( 4 ) ( ) ( 4 ) ( ) , , 0 0 0 0 i i x x i x z z i z i i e e e e  = = + = + =  (31) since 4 4 4 4 1 ( ) ( ) ( ) ( ) , ,x m r m m re x x x v= − = − and 4 4 2 ( ) ( ) ,z m re z v= − , equation (31) can be rewritten as 3 3 ( 4 ) ( ) ( 4 ) ( ) 1 , , 2 , , 0 0 i i m r x i x m r z i z i i v x e v z e  = = = + = +  (32) the design parameters ,x i and ,z i 0 3( , , )i = define the roots of the characteristic equations of equation (31), hence figure 6. overhead crane in 2d and forces acting on the load. figure 7. calculation blocks of the flatness-based exact linearisation of the crane system. acta imeko | www.imeko.org december 2019 | volume 8 | number 4 | 67 the decay rate of the tracking errors. to realise equation (32), one must specify how to calculate 3 3( ) ( ) , , , , ,m m m m m mx x x z z z from x and  : 1 1 0 0 ( 3) ( 3)2 1 1 2 0 0 . m m m m m m x r ls l c z lc l s x s z c g m m s c s c x z m m                     = + + = − − = − = + + − = − = (33) let us emphasise that the exponential decay of the tracking errors satisfies equation (30) only if the parameter values used in the expressions of the feedback fit the parameters of the controlled system. the method suggested in section 2 is applied to the crane model. the parameters and their uncertainty range are given in table 2. a grid of 50 vertices has been spanned over the uncertain parameter space q . it is possible to choose 2( )w s i= , and 1( )w s reads   2 2 2 2 0.5 0.005 1.9 5 0 0.32 0.01 2 6 1 14.57 0.21 0.72.7913 di( g ,a) s s e s s e s s s s w s + + − + + − + + + + = . (34) the closed-loop reference model ( )m s is set similarly to equation (20), with 0 5. rad/secm = and 0 8. .m = the 2×2 performance weighting transfer matrices ( )ew s and ( )uw s are set as 2 2 8000 500( 10 ) ( ) diag ( ) diag 2 ( 200 ) m m e u m m s w s w s s s        +  = =    + +     . (35) let 0 0 0 2[ , ] [ , ] t t m mx z = be the initial position of the reference load trajectory, which is defined as a periodic sinusoidal motion in both the x and the z directions 1 , 1 ,( ) ( ) sin( ) ( ) ( ) 3 cos( )r m r x r m r zy t x t t y t z t t = = = = − (36) with 2 rad/secx z = = . this reference trajectory results in sufficiently high rope angles staying away from the region of quasi-static motions. the derivatives of the reference trajectory that are required in equation (31) are easily calculated. the closed-loop responses are shown in figure 8 – figure 12 for the five representative parameter values in q . all figures show the transients without (top) and with (bottom) the robustifying controller ( )k s . we get 2 0 1 0 2021. ,ow g ks w  = hence, the robust stability condition equation (12) is satisfied. figure 8 and figure 9 show the trajectory of the load coordinates mz and mx . figure 10 depicts the rope angles. the input signals, namely the force applied to the cart f and to the winch t , are presented in figure 11 and figure 12, respectively. the robustifying controller ( )k s provides better performance and robustness. figure 9. horizontal displacement of the load. figure 10. rope angle. table 2. uncertain and fixed parameters of the 2d crane system. variable normal value minimum value maximum value m 1 kg 1 kg 1.6 kg m 0.3 kg 0.2 kg 0.6 kg j 0.001 kg m2 0.001 kg m2 0.001 kg m2 𝜌 0.03 m 0.03 m 0.03 m figure 8. vertical displacement of the load. acta imeko | www.imeko.org december 2019 | volume 8 | number 4 | 68 5. conclusions a novel method of increasing the robustness of exact linearisation against parameter uncertainty is presented for two variable-load mechanisms, namely for a fully actuated scaratype robot manipulator and for an underactuated 2d overhead crane. the method intends to bridge the gap between nonlinear control methods capable of stabilising fast load motions and robust synthesis techniques, ensuring robust stability and performance despite model uncertainties. as suggested by the simulation results, the proposed serial pre-filter may improve stability and performance robustness for quick reference trajectories. some issues may be further investigated. a restriction of the presented methods is the necessity to know all elements of the state vector x . if these measurements are only partly available, some observers need to be applied [20], [21]. our studies show that the choice of the nominal values of the parameters 0 p influences the uncertainty weighting functions 1( )w s and 2( )w s ; hence, it affects the overall performance of the closed-loop system as well. the selection of the best 0 p can be investigated as an optimisation problem. a single ( )k s controller may not be suitable if the uncertainty range q is too large. a possible solution for the large parameter uncertainty is the application of a gain scheduling or some adaptation techniques following the ideas presented in [22]-[24]. references [1] m. w. spong, s. hutchinson, m. vidyasagar, robot modeling and control, wiley, 2005, isbn: 978-0-471-64990-8. [2] n. adhikary, c. mahanta, sliding mode control of position commanded robot manipulators, control eng. pract. 81 (2018), pp. 183-219. [3] c. j. fallaha, m. saad, h. y. kanaan, k. al-haddad, sliding-mode robot control with exponential reaching law, ieee trans. ind. electron. 58 (2011) pp. 600-610. [4] l. jin, s. li, j. yu, j. he, robot manipulator control using neural networks: a survey, neurocomputing 285 (2018) pp. 23-34. [5] k. lochan, b. k. roy, control of two-link 2-dof robot manipulator using fuzzy logic techniques: a review, advances in intelligent systems and computing 335 (2015) pp. 499-511. [6] j. l. meza, v. santibáñez, r. soto, m. a. llama, fuzzy self-tuning pid semiglobal regulator for robot manipulators, ieee trans. ind. electron. 59 (2012) pp. 2709-2717. [7] h. g. sage, m. de mathelin, e. ostertag, robust control of robot manipulators: a survey, int. j. control 72 (2010) pp. 1498-1522. [8] l. ramli, z. mohamed, a. m. abdullahi, h. i. jaafar, i. m. lazim, control strategies for crane systems: a comprehensive review, mechanical systems and signal processing 95 (2017) pp. 1-23. [9] b. kiss, j. lévine, p. müllhaupt, modelling, flatness and simulation of a class of cranes, period. polytech. electr. eng. 43 (1999) pp. 215-225. [10] b. kolar, h. rams, k. schlacher, time-optimal flatness based control of a gantry crane, control engineering practice 60 (2017) pp. 18-27. [11] m. hičár, j. ritók, robust crane control, acta polytech. hungarica 3 (2006) pp. 91-101. [12] j. lévine, analysis and control of nonlinear systems. springer, 2009. isbn 978-3-642-00839-9. [13] j. lévine, p. rouchon, g. yuan, c. grebogi, b. r. hunt, e. kostelich, e. ott, j. a. yorke, on the control of us navy cranes, proc. of the 1997 european control conference, 1-7 july 1997, brussels, belgium, pp. 2829-2833. [14] n. wang, b. kiss, a robustness study of process control loops designed using exact linearization, proc. of the 21st international conference on process control, 6-9 june 2017, strbske pleso, slovakia, pp. 7-11. [15] k.-z. liu, y. yao, robust control: theory and applications. wiley, 2016, isbn: 978-1-118-75437-5. [16] d. w. gu, p. petkov, m. m. konstantinov, robust control design with matlab. springer, 2013, isbn 978-1-4471-4681-0. [17] g. incerti, trajectory tracking for scara robots with compliant transmissions: a technique to improve the positioning precision, proc of the 12th world congress in mechanism and machine science, 17-21 june 2007, besancon, france, pp. 1-9. [18] a. mohammadi, m. tavakoli, h. j. marquez, f. hashemzadeh, nonlinear disturbance observer design for robotic manipulators, control eng. pract. 21 (2013) pp. 253-267. [19] m. fliess, j. lévine, p. martin, p. rouchon, flatness and defect of nonlinear systems: introductory theory and examples, international journal of control 61 (1995) pp. 1327-1361. [20] t. rózsa, b. kiss, tracking control for two-dimensional overhead crane: feedback linearization with linear observer, proc. of the 8th figure 12. input f (force acting on the cart) during the motion. figure 11. input t (force acting on the which) during the motion. acta imeko | www.imeko.org december 2019 | volume 8 | number 4 | 69 international conference on informatics in control, automation, and robotics, 28-31 june 2011, noordwijkerhout, the netherlands, pp. 427-432. [21] b. patartics, b. kiss, application of laser slot sensors for the state estimation of overhead cranes, proc. of the 20th international conference on process control, 9-12 june 2015, strbske pleso, slovakia, pp. 387-392. [22] f. gao, d. dang, s. e. li, m. zhou, control of large model mismatch systems using multiple models, int. j. control. autom. syst. 15 (2017) pp. 1494-1506. [23] p. zhao, r. nagamune, switching lpv controller design under uncertain scheduling parameters, automatica 76 (2019) pp. 243250. [24] l. chen, y. jia, output feedback tracking control of flat systems via exact feedforward linearization and lpv techniques, int. j. of control, automation, and systems 17 (2019) pp. 606-616. industry 4.0 technologies and manufacturing back-shoring: a european perspective acta imeko issn: 2221-870x december 2020, volume 9, number 4, 13 20 acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 13 industry 4.0 technologies and manufacturing back-shoring: a european perspective luciano fratocchi1, cristina di stefano1 1 university of l’aquila, department of industrial and information engineering and economics, l’aquila, italy section: research paper keywords: reshoring; industry 4.0; manufacturing, citation: luciano fratocchi, cristina di stefano, industry 4.0 technologies and manufacturing back-shoring: a european perspective, acta imeko, vol. 9, no. 4, article 3, december 2020, identifier: imeko-acta-09 (2020)-04-03 section editor: leopoldo angrisani, university of naples federico ii, italy received october 11, 2019; in final form august 7, 2020; published december 2020 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: luciano fratocchi, e-mail: luciano.fratocchi@univaq.it 1. introduction manufacturing activities have been off-shored (and, often, also out-sourced) for a long time. such location decisions have been motivated primarily by efficiency seeking, and they have therefore generally involved low-cost countries (e.g. eastern europe and asia). however, in the last two decades, companies have been increasingly re-evaluating their previous off-shoring decisions having encountered several off-shoring pitfalls [1]. as a consequence, they have implemented decisions on relocations of second degree (rsd) [2], also referred to as reshoring [3], that consist of the relocation of manufacturing activities either to the home country (relocation to the home country [rhc] or backshoring) or to another host country (relocation to a third country [rtc]). in the case of rtc, the relocation may be either to a new host country in the home region (near-shoring) or to a far-away host country (further off-shoring) [3], [4]. we recognise the term ‘reshoring’ has often been adopted with very different meanings, ranging from the ‘generic change of location with respect to a previous off-shore country’ [3] to the relocation to the home country [5], [6]. however, in this paper, we use the term reshoring to refer to a generic rsd decision that is, independently of the governance mode (invs out-sourcing), adopted to manage relocated manufacturing activities. we also recognise that the term ‘near-shoring’ has been adopted to refer to the initial off-shoring in countries located in a company’s home region [5]. however, in this paper, we use the term nearshoring to refer to an rsd in a second host country located in a company’s home region. among the three rsd alternatives (back-shoring, nearshoring and further off-shoring), rhc has attracted the attention of several scholars over the last ten years [7]-[9]. more specifically, the phenomenon has been investigated in-depth from the perspective of motivations, i.e. drivers that induce companies to revise their initial off-shoring decisions. among such motivations, a growing relevance has been assumed of the adoption of automated production systems (see, for instance, [10], [11]) and additive manufacturing [12]-[14]. both these production technologies are included in the set of industry 4.0 technologies, i.e. those based on cyber physical systems, which abstract over the last two decades, companies have increasingly been re-evaluating their off-shoring strategies and implementing decisions on relocations of second degree (rsd). in some cases, the rsd decision has been implemented through back-shoring. the topic has attracted the attention of several scholars, some of whom have specifically investigated the role of industry 4.0 technologies in the backshoring decision-making process. this paper aims to shed new light on the role that industry 4.0 technologies play as a driver or an enabling factor for companies that are evaluating rsd alternatives. a two-step explorative approach has been used for this research. in the first step, a theoretical approach has been followed by developing a structured literature review based on 115 scopus-indexed journal articles. the second step of the methodology is based on the empirical evidence of european countries using the univaq manufacturing reshoring dataset. the collected data provides evidence of the growing interest of scholars in this issue; however, attention has mainly been focused on two single technologies, production automation and additive manufacturing. the empirical data offers interesting findings in terms of industry and geographic characterisation. mailto:luciano.fratocchi@univaq.it acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 14 include ‘smart machines, warehousing systems and production facilities that have been developed digitally and feature end-toend ict-based integration, from inbound logistics to production, marketing, outbound logistics and service’ [15]. in the international business literature, it is widely recognised that information and communication technologies (icts) may have a relevant impact on the company’s internationalisation since they allow for remote coordination, extending the span of control and reducing its cost [16]-[18]. moreover, such technologies permit companies to ‘fine slice’ their value-adding activities and to locate their production in different places, as in the ‘global factory’ scenario [19], [20]. however, technologies embedded in industry 4.0 may also promote a reconfiguration of the company production footprint, since they increase a company’s productivity [21], [22], which, in turn, weakens (if not eliminates) the location advantages of low cost countries [10], [11], [23]. at the same time, industry-4.0-enabling technologies support a higher level of flexibility in manufacturing processes, allowing companies to be more responsive to clients’ needs and to offer them customised products [10]-[14], [23], [24]. in this respect, it is worth noting that the influence (if any) of industry 4.0 technologies on back-shoring decisions has been increasingly proposed as a future research avenue (e.g. [7], [9], [25], [26]). therefore, it is time to define the state of the current academic debate on industry 4.0 technologies and second-degree relocations to the home country. consequently, this paper is based on the following research questions: a) does industry 4.0 represent a driver/motivation [27]-[28] for companies that are evaluating rsd alternatives? b) does industry 4.0 represent an enabling factor [26], [28] for companies that are evaluating rsd alternatives? the paper aims to shed new light on this issue, adopting an explorative approach based on a variety of data sources. more specifically, a two-step explorative research method will be used. in the first step, a theoretical approach is followed by developing a structured literature review based on 115 elsevier scopusindexed journal articles published before august 2019. the second step of the paper methodology is based on empirical evidence from the univaq manufacturing reshoring dataset (umrd), which has already been adopted in previous backshoring research [10]-[13], [29]-[31], since it is recognised as the most comprehensive set of data at a global level. since industry 4.0 phenomenon may have dissimilarities among different geographic areas, attention has been focused on reshoring evidence from european countries. findings obtained from the two-step methodology described earlier provide evidence of the growing interest of scholars in the impact of industry 4.0 enabling technologies on manufacturing reshoring decisions. however, attention has primarily focused on two single technologies, production automation (42 out of 115 sampled elsevier scopus-indexed journal articles, published between 2014 and 2019) and additive manufacturing (10 documents published in the last two years). moreover, only four journal articles (of which three were published in 2019) specifically investigated the causality (if any) of industry 4.0 technologies on back-shoring. however, the research findings emerging from these four articles are quite differentiated and not definitive. finally, it is worth noting that, while the majority of sampled articles conceptualise industry 4.0 technologies as a driver [7], these technologies have also been viewed as an enabling factor [26], [28]. the paper is structured in three sections, the first of which describes the two-step methodology adopted. in the following section, findings obtained through the chosen methodology are presented and discussed. finally, the third section contains conclusions, implications and limitations. 2. methodology as outlined above, in order to examine the investigated research questions, a two-step explorative methodology has been applied. the first one is based on a structured literature review, e.g. ‘a systematic, explicit, and reproducible design for identifying, evaluating, and interpreting the existing body of recorded documents’ [32] regarding back-shoring decisions. it has been performed according to the process model for content analysis described by [33], which has already been adopted to conduct literature reviews on back-shoring [7], [9]. attention has been focused on elsevier scopus-indexed journal articles published before august 2019. this source has been chosen since it is widely recognised as one of the top business and management databases [34]. the following research criteria have been adopted: a) english language b) publication date before august 2019 (although no limits have been applied for the starting date given the novelty of the investigated phenomenon) c) containing at least one of the following sets of terms in the title, abstract and/or keywords: “reshor*”, “re-shor*”, “backshore*”, “back-shor*”, “back-reshor*” and “back-sourc*” we found a total number of 177 journal articles. after all the co-authors had carefully read the full text of these articles, the following excluding criteria were adopted: journal articles focusing on the back-shoring of companies operating in industries not related to manufacturing (e.g. ict companies) documents published in sources without peer review systems journal articles in which the searched terms are used to indicate a different concept from the one of interest (for instance, some of the keywords belonging to the reshoring concept are used with different meanings in the maritime and building engineering research fields) documents referring to functions other than operations (e.g. human resources and research and development [r&d]). based on these criteria, 62 documents were eliminated; therefore, the total number of sampled documents was 115. the second step of the methodology is based on empirical evidence from the umrd dataset that contains data from the uni-club more reshoring dataset, already adopted for use in the research on manufacturing back-shoring (see, among others, [4], [10]-[13], [27], [29]-[31]), and the european reshoring monitor dataset by the eu foundation, eurofound, used in several recent studies [11], [30], [31]. up to the end of december 2018, the umrd dataset contained a total of 1,279 instances of evidence regarding backshoring decisions implemented at the global level. among them, 814 are related to rsd decisions implemented by european companies. 3. findings 3.1. findings from the extant literature an analysis of the 115 sampled journal articles clearly shows that the relationship (if any) between industry 4.0 technologies as a whole and back-shoring has been addressed by only four acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 15 journal articles (namely, [10], [11], [23], [35]). however, wider attention has been given to some specific industry 4.0 technologies, including automation and three dimension (3d) printing/additive manufacturing (table 1). more specifically, reshoring scholars have been increasingly conceptualising automation as a back-shoring driver and/or an enabling factor since 2014. in the sampled literature, we found a total of 42 documents referring to this issue. in contrast, attention on the role of additive manufacturing/3d printing technologies has only arisen in the last two years (2018 and 2019). this finding may be at least partially explained by the infancy of the additive manufacturing technologies [12], [13]. finally, one contribution [10] specifically refers to another two industry 4.0 technologies, sensors and simulation. at the same time, [11] investigated the opportunity for adopting cyber physical systems to connect production and development and/or buyers and suppliers. as far as production automation technology is concerned, the first evidence in the sampled journal articles is proposed by [36], who found that 47.5 % of danish companies that off-shored production activities between 2009 and 2014 found the same activities could be back-shored as a result of the advances in automation. similarly, [37] and [38] found, in a sample of danish, finnish and swedish companies, that access to technology (including production automation) is one of the ‘significantly more important drivers for back-shoring than for off-shoring (p ≤ 0.001)’ [38]. moreover, [39] and [40] found, on the basis of a swedish sample, that companies that had both offand backshored during the investigated period considered access to technology to be of a slightly lower level of importance than companies implementing only back-shoring strategies. finally, in their qualitative study, [28] found that several companies decided to back-shore in sweden due to the benefits offered by production automation. however, the huge contribution of such an enabling technology to the relocation of manufacturing activities in the home nordic countries seems to be questioned by scholars who investigated other geographic areas. for instance, [10] point out that only 13.6 % of the 840 back-shoring decisions made by the eu and us companies they analysed specifically declared at least one of the industry 4.0 technologies as a relocation driver. at the same time, [41] found that robotics has a negative impact on off-shoring decisions (at least for companies located in developed countries), but it does not yet trigger back-shoring decisions. production automation has usually been seen as a driver for relocation to the home country since it reduces labour costs and increases productivity [42]. this, in turn, ‘makes it more viable to produce in high cost environments’ [26]. another benefit offered by production automation is regarding the reduction of costs in order to implement a flexible production system [43], allowing companies to customise products for different customers’ targets and, in turn, improve companies’ responsiveness [14]. based on this, [10] state that production automation may support two types of back-reshoring strategy: ‘cost-oriented’ (i.e. relocation aimed at reducing production and logistics costs) and ‘flexibilityoriented’ (aimed at improving a company’s responsiveness to customer needs). this evidence is quite relevant since, according to these two authors, the two typologies of reshoring decisions are the most diffused among the 840 global-level back-reshoring initiatives they analysed. in contrast, ‘quality-oriented’ back-shoring strategies, i.e. when the relocation to the home country is aimed at implementing product upgrade strategies [44], are less relevant. this finding is quite at odds with the previous evidence collected by [10] and [24] with respect to uk back-shoring companies. a final question on the role of automation in back-shoring decisions is that posed by the research of [26]: ‘if the level of automation should be seen as a factor acting as a barrier or driver’, i.e. if it either boosts the back-shoring decision or its lack hinders a relocation to the home country. as far as the 3d/additive manufacturing technologies are concerned, it is expected they will have a disruptive impact on global value chains (gvc), thereby also supporting back-shoring decisions [45], [46]. in this respect, it is worth noting that [14] found that 72 % of the 50 companies investigated that were adopting additive manufacturing technologies positively evaluated the contribution it makes to back-shoring decisions. in addition, [47] states that 3d printing technologies will induce firms to locate manufacturing activities closer to customers; hence, its adoption would boost reshoring decisions. however, [10] adopt a more restrictive position, stating that this technology may support the implementation of only quality-oriented backshoring decisions. additive manufacturing better supports product development processes and the integration of r&d, design, production and marketing functions [22]. moreover, additive manufacturing allows companies to reduce prototyping costs and times [11]. in addition, [14] state that relocation to the home country is boosted by the following six benefits that additive manufacturing technologies offer in terms of supply chain management: ‘shorter lead time, responsiveness to the product and market changes, lower transportation costs, fewer miscommunications with suppliers, more customization options, fewer products stored in inventory’. at the same time, [12] and [13] present evidence that the technical and economic benefits provided by 3d printing represent an adequate response to several of the back-shoring drivers discussed in the extant literature [7]. furthermore, [12] and [13] showed that industries in which additive manufacturing technologies are implemented are generally the same ones in which earlier empirical research found evidence of back-shoring strategies. this finding completes the previous evidence presented by [48], who classified manufacturing industries according to their propensity to adopt additive manufacturing technologies. as already noted, the attention paid by scholars to the relationship (if any), and even the causality, between manufacturing reshoring and the whole set of industry 4.0 table 1. breakdown of sample articles by year and article content. a published articles; b production automation; c additive manufacturing; d sensor; e simulation; f cyber physical systems; g industry 4.0 impact on back-shoring year a b c d e f g 2007 1 2009 1 2011 1 2012 1 2013 6 2014 11 3 2015 8 1 2016 23 12 2017 16 6 2018 24 13 8 1 1 2019 23 7 2 1 1 3 total 115 42 10 1 1 1 4 acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 16 technologies is still in its infancy. among the few authors who have investigated such a linkage, [10] point out that ‘robotics is not a necessary ingredient of [back-]reshoring’, but ‘industry 4.0 supports manufacturing [back-]reshoring when design and product innovation are involved’. at the same time, [11] provide evidence that, at least until now, back-shoring decisions have been implemented without investing in new technologies, especially if the relocation was aimed at leveraging the ‘made in’ effect and/or shortening the lead time and improving the company’s responsiveness. however, authors expect industry 4.0 may, in the near future, play a specific role in supporting manufacturing relocation decisions, especially in the case of skill shortages, due to the previous de-industrialisation emerging after decades of manufacturing off-shoring and/or when companies aim to improve design and strengthen product-development linkages. these findings are also confirmed by [35], who analysed a sample of danish manufacturing companies. they found that the investigated technologies had no impact on the decision to relocate manufacturing activities to the home country. in contrast, [23] found a positive and significant association between investments in industry 4.0 technologies and backshoring decisions. moreover, their study, which focused on manufacturing companies in germany, austria and switzerland, shows that there is no causality between the two variables since both are driven by the research on higher levels of flexibility. it is worth noting that a previous investigation into a german sample conducted by [49] found that industry 4.0 technologies played a role in supporting back-shoring decisions in only 13 of the 50 sampled back-shoring decisions they analysed. furthermore, the relationship between the two issues was not adequately corroborated by a quantitative analysis, since the mean value was below 3 on a likert scale from 1 to 5 (specifically, 2.3 for companies implementing relocation to the home country coupled with in-sourcing, and 2.2 for those that out-sourced reshored production activities). findings by [49] also show that the adoption of the investigated technologies is mainly related to companies declaring the following back-shoring drivers: innovation, testing of technologies and time-to-market reduction. of specific note is the [23] study in which the authors point out that the higher level of responsiveness allowed by industry 4.0 technologies may be carefully evaluated in terms of the geographical distribution of a company’s customers. more specifically, if company customers are located in countries/regions other than the home country, the adoption of industry 4.0 technologies would induce companies to implement rtc strategies, either in the form of near-shoring or of further off-shoring. to sum up, the structured literature review conducted offers a varied set of results that are not conclusive. while several authors recognise that individual industry 4.0 technologies (mainly 3d/additive manufacturing and automation) may have an impact on manufacturing relocation decisions, their impact is highly dependent on the strategic aims pursued by the company. moreover, analyses have been focused, until now, on a restricted number of countries, mainly in europe. therefore, further investigations are required; in this respect, evidence belonging to the umrd, which will be discussed in the next section, may contribute to the academic debate. 3.2. empirical findings given the lack of homogeneity in the results emerging from the extant literature and in order to investigate more deeply the proposed research question, attention is now focused on the empirical evidence collected from the umrd. it contains secondary data belonging to back-shoring decisions implemented by companies at a global level. however, for the aims of this paper, attention has been focused on companies headquartered in europe. up to december 2018, the umrd contains a total of 814 examples of back-shoring decisions implemented by companies headquartered in 24 european countries. companies have been taking and implementing such decisions since the 1990s, but most of the examples are from this century. before analysing the impact (if any) of industry 4.0 technologies on back-shoring decisions, it seems useful to point out the main characteristics of the sampled back-shoring decisions. as far as the geographical dimension (host country/region) is concerned, the evidence is homogeneously divided between asia and europe (table 2). this finding is quite interesting and may be, at least partially, explained by the ‘region-centric’ approach often adopted by european companies when off-shoring their production activities. these location strategies were given a strong boost by the european union enlargements in 2004 and 2007, which generated an opportunity to produce in low-cost countries close to western europe. finally, it is worth noting that the majority of intra-europe relocations have been implemented among western countries, i.e. between high-cost nations. this evidence seems to confirm the idea that production offshoring has not been aimed only at reducing production costs. the breakdown by company size shows that half of the backshoring decisions have been implemented by large companies. the other half is divided equally between medium and small and micro companies (table 4). the equal distribution in terms of company size is consistent with some previous findings [9], [27], [50]. however, it must be acknowledged that the data related to germany [23], [51], [52] and the scandinavian countries [38]-[40] shows a higher backshoring propensity among large companies. table 2. breakdown of back-shoring decisions by host region. source: umrd. host region/country decisions china 33.8 % asia (other than china) 9.2 % asia (not specified) 2.2 % asia 45.2 % eastern europe & former ussr 17.6 % western europe 26.0 % europe (not specified) 0.5 % europe & former ussr 44.1 % north africa & middle east 3.7 % south africa 0.1 % africa (not specified) 0.2 % africa 4.1 % north america 2.4 % central & south america 1.5 % americas 3.8 % oceania 0.1 % not available 2.7 % total 100.0 % acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 17 the breakdown by industry (table 3) shows that at least one back-shoring decision has been implemented in 24 different sectors. the industries with the highest representation (more than 10 %) are the ‘manufacture of machinery and equipment not elsewhere classified’, the ‘manufacture of leather and related products’ and the ‘manufacture of computer, electronic and optical products’. therefore, it seems the intensity of an industry’s technology is not relevant. when considering the back-shoring drivers (table 5), three out of the four most cited belong to the ‘customer perceived values’ quadrants of the [27] framework: ‘customer responsiveness improvement’ (17.1 %), ‘made in effect’ (19.5 % of total sample) and ‘delivery time’ (17.3 %). ‘logistics costs’ (16.3 %) belong to the cost quadrant. therefore, back-shoring decisions seem to be aimed more at effectiveness than at efficiency. this finding is relevant to the theoretical positions found in the extant literature (e.g. [10]). it is worth noting that researchers have not yet conducted an analysis aimed at investigating the most relevant contingency factors (e.g. company size, geographic location, motivations) that characterise the back-shoring companies. future research should investigate this issue, which could offer useful insights. table 3. breakdown of back-shoring decisions by industry. source: umrd. nace code description decisions 28 manufacture of machinery and equipment not elsewhere classified 10.6 % 15 manufacture of leather and related products 10.6 % 26 manufacture of computer, electronic and optical products 10.1 % 14 manufacture of apparel 9.8 % 27 manufacture of electrical equipment 9.6 % 32 other manufacturing 6.6 % 10 manufacture of food products 6.0 % 30 manufacture of other transport equipment 5.9 % 25 manufacture of fabricated metal products, except machinery and equipment 5.2 % 29 manufacture of motor vehicles, trailers and semitrailers 4.8 % 22 manufacture of rubber and plastic products 4.5 % 31 manufacture of furniture 3.3 % 20 manufacture of chemicals and chemical products 2.3 % 21 manufacture of basic pharmaceutical products and pharmaceutical preparations 2.3 % 24 manufacture of basic metals 2.0 % 13 manufacture of textiles 1.8 % 23 manufacture of other non-metallic mineral products 1.2 % 17 manufacture of paper and paper products 1.0 % 16 manufacture of wood and of products of wood and cork, except furniture 0.6 % 11 manufacture of beverages 0.6 % 18 printing and reproduction of recorded media 0.6 % 12 manufacture of tobacco products 0.1 % 33 repair and installation of machinery and equipment 0.1 % 62 computer programming, consultancy and related activities 0.1 % not available 0.1 % total 100.0 % table 4. breakdown of back-shoring decisions by company size. source: umrd. company size decisions large 52.2 % medium 23.6 % small & micro 23.2 % not available 1.0 % total 100.0 % table 5. breakdown of back-shoring decisions by declared motivation. motivations declared by at least 10 companies. motivations belonging to industry 4.0 in bold. source: umrd. motivation decisions made in effect (home country) 19.5 % company’s organisational restructuring 17.6 % delivery time (including delays) 17.3 % customer responsiveness/vicinity higher service quality 17.1 % logistics costs (including freight costs) 16.3 % off-shored poor product quality 16.1 % adoption of automation and/or other innovative product/process technologies (excluding 3d printing/additive manufacturing) 13.1 % total cost of ownership 11.8 % vicinity to engineering & production + company strategies focused on product and process innovations 7.9 % increasing labour cost in the host country (including higher productivity in the home country) 7.6 % organisational flexibility 7.4 % cost and difficulties in controlling the host country activities 6.8 % government incentives (in the home country) 5.8 % poor company performance and global financial and economic crisis 5.8 % availability of research centres and universities in the home country 4.8 % availability of production capacity in the home country 4.5 % host country employees’ low skills and ‘work attitude’ 4.4 % minimum order quantity (including difficulties in forecasting sales volumes) 4.3 % intellectual property issues (including counterfeiting) 3.4 % entrepreneurs’ emotional attachment to the home country 2.5 % supply chain risks (different from exchange rate fluctuations) 2.2 % poor attractiveness of host country 2.2 % social pressure in the home country (e.g. trade unions) 2.2 % duties and tariffs 1.8 % physical distance 1.6 % availability of suppliers in the home country 1.2 % carbon footprint and other environmental issues 1.0 % exchange rate risk 1.0 % corporate social responsibility (e.g. with respect to work conditions in the host country) (this does not include environmental issues) 0.7 % adoption of 3d printing/additive manufacturing 0.5 % energy costs 0.1 % acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 18 focusing attention on individual industry 4.0 technologies, it is clear that production automation and additive manufacturing are the only two factors specifically cited by back-shoring companies. this finding is consistent with the previous statement by [11] and the results obtained by analysing the extant literature. however, while automation has been declared as a back-shoring driver in 13.1 % of the sampled decisions, the adoption of additive manufacturing technologies has been considered as a reshoring motivation in only 0.5 % of the sampled relocation decisions. it is important to note that, while [10] and [11] suggested that the adoption of production automation is more consistent with ‘cost-oriented’ and ‘flexibility-oriented’ back-shoring decisions, our data shows that such technology has been cited jointly with drivers belonging to quality oriented relocation decisions. more specifically, the adoption of the investigated technology was coupled with the ‘cost and difficulties in controlling the host country activities’, the ‘made in effect’ and the ‘delivery time’ (table 6). further insights regarding the role of production automaton emerge when considering contingency factors. among them, the breakdown by company size unexpectedly shows this technology, which generally requires high levels of investment, is adopted primarily by medium-sized companies (18.2 % of total companies in the size range) and small/micro companies (12.2 %) rather than large ones (11.5 %) (table 7). when considering the geographic contingencies (table 8), data clearly shows that the adoption of automated production technologies is not influenced by the host region in which companies have previously off-shored their production activities. this finding is partially unexpected, since one would have expected that back-shoring decisions regarding production activities located in low-cost countries (e.g. asia) would be largely supported by automation when compared with mediumand high-cost countries (e.g. europe). moreover, it is in contrast with the findings of [23] in terms of the higher ‘industry 4.0 readiness’ of large companies with respect to small and medium ones. a possible explanation for this unexpected result may be represented by the latter-day implementation of automated production systems by medium-sized companies. given that there is little evidence of back-shoring companies implementing 3d/additive manufacturing technologies, further characterisation of this technology is not relevant. 4. concluding remarks this paper aimed to investigate the relationship (if any) between industry 4.0 technologies and decisions to relocate previously off-shored manufacturing activities to the home country. to shed new light on this research question, an exploratory approach has been implemented adopting a two-step methodology. first, a structured literature review was conducted on a sample of 115 scopus-indexed journal articles published between 2007 and august 2019. this research clearly shows the topic is attracting a growing interest among scholars (at least since 2014). however, they have focused primarily on specific technologies, namely production automation and 3d printing/additive manufacturing, and the findings that have emerged are still not sufficiently conclusive. only four journal articles specifically address the relationship between industry 4.0 technologies and back-shoring decisions; moreover, their findings are somewhat different. for instance, [23] found a significant and positive relationship (but not also the causality) between the two, while [11] and [35] did not discover any connection. this finding might suggest that country-specific issues may have influenced the results, since [23] have focused on german, austrian and swiss companies, while [35] on danish ones. as clearly demonstrated by the data from the umrd, the european companies that back-shored their production based on industry 4.0 technologies are characterised by a lack of similarity, especially in terms of industry and adopted technology (production automation vs additive manufacturing). another issue emerging as relevant is the one concerning size. while it is generally expected that industry 4.0 technologies would be more readily adopted by large companies, an analysis of the umrd data provides evidence that, at least for production automation, it is primarily medium-sized companies that have adopted them (table 7). future research should further address this aspect, given the implications for policymakers. a third question is still open as regards the relationships (if any) between the adoption of a specific industry 4.0 technology and the strategic aims pursued by the back-shoring decision. while [11] and [10] suggest that production automation is more consistent with ‘cost-oriented’ and ‘flexibility-oriented’ backtable 6. back-shoring motivations cited jointly with production automation. only 10 most cited motivations. source: umrd. motivations decisions cost and difficulties in controlling the host country activities 21.8 % made in effect (home country) 18.9 % delivery time (including delays) 18.4 % vicinity of engineering & production + company strategies focused on product and process innovations 17.2 % off-shored poor product quality 13.7 % customer responsiveness/vicinity higher service quality 12.9 % increasing labour cost in the host country (including higher productivity in the home country) 12.9 % total cost of ownership 12.5 % company’s organisational restructuring 9.1 % logistic costs (including freight costs) 8.3 % table 7. back-shoring evidence citing production automation: breakdown by company size. source: umrd company size decisions large 11.5 % medium 18.2 % small & micro 12.2 % not available total 13.1 % table 8. back-shoring evidence citing production automation: breakdown by host region. source: umrd host region decisions asia 13.6 % europe & former ussr 12.8 % africa 21.2 % americas 3.2 % oceania not available 13.6 % total 13.1 % acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 19 shoring decisions, data from the umrd provides evidence that companies adopting this technology have been driven by motivations linked to the ‘quality-oriented’ back-shoring decisions. support for the adoption of industry 4.0 technologies can be given to companies by policymakers that should not only offer companies the possibility of reducing the fixed costs related to the adoption of industry 4.0 technologies but also to develop ‘the necessary digital competencies for the successful exploitation of these technologies’ [11]. in this respect, [10] point out that such digital competences may also substitute for the manual skills no longer available in western countries, given the deindustrialisation resulting from decades of manufacturing offshoring. the previous discussion leads us to conclude that further studies are required to investigate the proposed research question. our study has an explorative aim and is mainly based on secondary data; therefore, our conclusions are not generalisable. however, it may represent a useful summary of the academic debate and of back-shoring evidence that has been available up until now. in this respect, we suggest future research should couple a longitudinal case study approach with quantitative surveys. moreover, this study offers some useful insights for policymakers since it clearly shows that industry 4.0 technologies may support the back-shoring phenomenon. therefore, industrial policy aiming to support the diffusion of such 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of rome, rome, italy section: research paper keywords: fiber bragg grating; ph sensor; hydrogel; cultural heritage monitoring; fem citation: lorenzo dinia, fabio mangini, marco muzi, fabrizio frezza, fbg multifunctional ph sensor monitoring the ph rain in cultural heritage, acta imeko, vol. 7, no. 3, article 6, october 2018, identifier: imeko-acta-07 (2018)-03-6 section editor: sabrina grassini, politecnico di torino, italy received february 28, 2018; in final form september 25, 2018; published october 2018 copyright: © 2018 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: lorenzo dinia, lorenzo.dinia@uniroma1.it 1. introduction during the last decades, there has been a rising interest in the development of sensors based on optical fibers for several applications, including chemical sensing [1]. over the years, the acid rains damage monuments and other ancient artworks, destroying the historical and cultural heritage of many countries much faster. acid rains promote the metal and marble corrosion, determining a loss of mechanical strength and compactness, causing a partial dissolution of the materials themselves and ultimately resulting in the destruction of monuments [2] (figure 1). the sulfates, present in the acid rain, transform the calcium carbonate into gypsum, known as soft sulfate mineral composed of calcium sulfate dehydrate [2]. this phenomenon is very critical in countries like china with the tremendous pollution that started with the fast industrial growth of the past years, which triggered an acceleration in the destruction of buildings and artworks [2]. this has created in the last years an important interest on how to identify the starting point of the damages to prevent them [3]. in the past, optical analysis and analytical techniques have been used to identify the compound responsible for the corrosion and deterioration of the original artworks’ material, discovering that the gypsum was always present in samples of deteriorated marble [2]. moreover, traces of nitrates and sulfates were investigated on marble surfaces’ samples. it was discovered that although both sulfates and nitrates are compounds present in the acid rain in significant quantities, the concentration of sulfates on the samples was 20 times bigger than the quantity of nitrates [2]. this result provided a significant indication about the leading responsibility of the sulfates in deteriorating marbles [2]. the problem of the acidification of the air and rains was discussed and addressed many years ago during the medieval era, even if it was limited to few compounds mainly released around the cities and to the volcanic eruptions [4]. acid rain has been detected, during the second half of the nineteenth century, primarily in europe, east asia and north america [3]. abstract a new era of pollution requires an important focus on the conservation of archaeological sites and monuments. in the last years great efforts were required to develop various sensors for different tasks. the fiber bragg grating (fbg) was one of the most studied thanks to the multitude of applications and the surprising performances. an original fiber optic sensor that combines the fiber bragg gratings with a ph-responsive polymer coating for monitoring the ph of the rains on critical and prestigious monuments is proposed. the core consists of four different materials, which makes the sensor very innovative and allowing it to reach a high sensitivity. in this study, the setup arrangement of the optical sensor is modeled with comsol multiphysics (wave optics module), based on the fem (finite element method) solver. monitoring the ph of water can be used by experts to predict and control the corrosion phenomenon of specific materials, especially limestone and marble, thus scheduling the timely restoration. mailto:lorenzo.dinia@uniroma1.it acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 25 the industrial age improved the average life quality of human beings but on the other hand provoked intense atmospheric acid deposition through rain, fog and snow, threatening the environmental stability and the conservation of monuments and artworks [3]. the corrosion of different materials cannot be prevented without continuously monitoring the ph of the rain. presently, the catastrophic effects of the polluted rains are addressed merely with restoration after visible damages, such as micro cracks, discoloration and corrosion as seen in figure 2. with an innovative monitoring procedure of the water ph, maintenance and restoration programs can easily be set depending on the chemical features of the rains over the years. monitoring the ph can also help to forecast the possible damages for the different materials, in order to preserve the original status of the artworks by also using new laser cleaning method to remove the dirt resulting from acid rains [5]. the main enemy of the artworks is the acidity of the rain, [3], [6] that attacks the statues and ancient buildings, causing enormous damages without control. the calcareous building stones are one of the main materials most sensitive to pollutants [7], [8]. normal rain is not neutral like pure water but is slightly acidic at around 5.6 ph or lower [3], [6]. industrial areas have registered acidity of the rain below a ph value of 2.4 [3], [6]. rainwater becomes weakly acidic because carbon dioxide gas in the atmosphere reacts with water to form carbonic acid. moreover, the rainwater increases the acidity even more, mixing with sulphur oxide and nitrogen oxide molecules, coming from industrial pollution and automobile exhaust [2], [3], [6]-[8]. the corrosion rate on artworks is much faster in industrialized areas and areas where human activities are more intense [3], [4]. sandstone, limestone and marble of monuments and artworks are affected by acid rain because calcium carbonate, one of the main constituents of these materials, reacts with sulphur, present in dry deposition, and form calcium sulphate [3], [7], [8]. this product is very soluble, and all the acids formed thereby and deposited are washed off the stone’s surface with the following rains, damaging the world’s cultural heritage, ancient monuments, historic buildings, sculptures, and paintings [3]. therefore, acid rains are responsible for the loss of original features and the impairment of monuments and artworks [3], [7]. the different sources of so2 emissions are the burning of coal and petroleum, several industrial processes, the smelting of iron and some other metals, the manufacturing of sulphuric acid, and all the activities related to the acid concentrators in the petroleum industries [3], [7], [8]. surely the emission of nox compared to so2 are much less but still harmful in making the rain even more acidic [3], [7], [8]. there are natural sources of nitric oxide, but men contribute heavily with power plants, vehicle exhausts and industrial processes too [3], [7], [8]. in this scenario, in which the sustainability is one of the main objectives of the industrialized countries, monitoring the damages caused by the pollution is necessary for improving the quality of human life, reducing diseases as well as for economic reasons. fiber bragg grating (fbg) sensors coated with ph-sensitive hydrogel have been studied through a simulation, which showed that they can provide a linear response in the ph range of 5 to 7 with a sensitivity of 0.164 nm/ph unit [9], [10]. optical ph sensors have advantages compared to the conventional ones, being free from electromagnetic interference, having wide bandwidth, being open to miniaturization, real-time measurement and remote sensing [11]-[13]. the outstanding innovation of this paper is to monitor point by point the ph of the water in artworks in real-time with a high sensitivity. the actual ph of the water, resulting from the mixing of calcium carbonate and calcium-based compounds, coming from limestone and marble, with acid rain can be discovered. this technique allows to monitor small and critical areas like an elbow or an inner thigh of a statue. the ph sensor suffers from some limitations due to temperature and bending cross-sensitivities [14]-[17] that will be assumed to be constant and negligible, respectively. this assumption does not seem to be too problematic, as there is a wide variety of compensation techniques for such variables. the accuracy of the sensor is also affected by the multiple resonance peaks and the vast transmission resonance [15]. one of the main advantages of the fiber bragg grating coated with hydrogel is that the sensed information is encoded directly into wavelength, providing an outcome independent of the intensity of the input light and the loss along the optical system [15]. the device can be split into two sensors, one using a ph-sensitive polymer with an acidic group and the other with a polymer with a basic group. in contact with certain ph values, the polymers acquire a charge, become ionized and cause a modification of the length of the fiber bragg grating. the configuration of the device could be composed of two fiber bragg grating sensors to detect both acidic and basic variations. a further strength of the device is its size. the developed sensor has a length of 10 mm. the small size allows the sensor to be located on almost unreachable artwork elements for ph monitoring. figure 1. neptune’s fountain of giambologna – bologna, italy; a portion of stone material stained in a light blue color. the stains are caused by the percolation of the corrosion products of the overlying bronze elements. figure 2. fountain of the obelisk of republic square fondi (lt), italy. acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 26 2. materials and methods the optical fiber sensor is composed by layers of alternating materials with different refractive indices as shown in the schematic in figure 3. each layer boundary causes a partial reflection of the incident optical wave. the ph-responsive polymers respond to the changes in the ph of the surrounding water by having a volumetric variation. the expansion of the polymer coating causes a strain on the fiber, modifying the bragg grating period that results in a shift in resonant bragg wavelength, and thus in the sensor response. peak reflectance is achieved with a design wavelength 𝜆0 when the configuration of bragg grating provides constructive interference between reflections. this is reached when the bragg condition is satisfied: 𝜆0 = 2 𝑛𝑒𝑓𝑓 𝛬, (1) where 𝑛𝑒𝑓𝑓 is the effective refractive index of the optical fiber and λ is the grating period. the bragg wavelength is set to 550 nm, because it is the average wavelength of the visible spectrum and because it minimizes the loss of the light in the fiber. achieving the bragg condition requires layer thicknesses of: 𝑡𝑖 = λ0 4𝑛𝑖⁄ , (2) where 𝑛𝑖 is the refractive index of the i-th layer. the model is studied with 5, 11 and 21 layers composing the distributed bragg grating, showing the different response of the sensor. the fiber bragg grating is constituted by two different dielectric materials chosen in this study with nh=2.32 and nl=1.38 as refractive index, while air and substrate form the core of the sensor outside the fbg with a refractive index of 1 and 1.5 (figure 3), respectively. the cladding has a refractive index equal to 1.444. this model is very innovative because the core consists of four different materials. the fbg is produced inducing a periodic or aperiodic variation of refractive index in the core, using an ultraviolet source, such as a uv laser [18]. the amount of change depends on the time and intensity of the exposure to uv light [18]. normally a germanium-doped silica fiber is used in the manufacture of fiber bragg gratings [18]. the polymer is assumed to coat the sensor for the length of 10 mm. the change in ph determines a mechanical expansion of the hydrogel that stretches the dimension of the alternating material layers inside the fiber [19], [20]. the change of the layers’ thicknesses creates a shift in the bragg wavelength. the ph range between 4 and 7.1 is considered to demonstrate the performance of the sensor. the percentage reflectance and the wavelength shift are analysed to provide an estimation of efficiency and sensitivity in the measuring range. in the study both a 2-d and a 3-d geometry were analysed. the planar technology has been chosen to adapt the fbg to the examined surfaces in the synthesis of the device. in the future physical realization, the cladding material and the bragg grating will be deposited by using the standard microelectronic processes on two different polymer substrates, making two identical samples. finally, the two surfaces will be bonded achieving the final structure. external hydrogel is made up of a combination of a solid crosslinked polymeric chain and an adjacent aqueous solution [15]. the ph-sensitive hydrogel will coat the fbg to obtain the final sensor. data acquisition will be performed with an optical source, an optical coupler to prevent the reflected light to be propagated back, and an optical spectrum analyser for detecting the sensor response. 2.1. materials – sensor variant the fbg ph sensor has also been studied with other electromagnetic parameters to allow an easier manufacturability. the fiber bragg grating is comprised of different dielectric materials compared to the materials for the first sensor of the study; the grating is studied in three configurations with nh=1.4 and nl=1.38, nh=2.2 and nl=1.38, and finally nh=3 and nl=1.38. the core of the sensor outside the fbg is assumed to have a refractive index of 1.451 [21], which is characteristic of glassy material for the optic frequencies, while the cladding that surrounds the core of the sensor is set to 1.444 [21]. in this scenario the core is made out of three different materials that are realized with a constant uv light through different exposure times. the choice to set the materials of the sensor to be around 1.5 as refractive index is due to manufacturing considerations. knowing that this last value is typical of the glass, the standard microelectronic processes on two different polymer substrates will be more straightforward to be used. the sensor has a length of 100 µm. the core of the sensor was set to 6.08 µm as diameter, while the cladding was set to 125 µm [21]. the bragg wavelength is considered to be 1550 nm for this second phase, because at this wavelength the loss of any optical fiber is minimum. 3. results in order to validate the measurement system, we have conducted simulations with comsol multiphysics (wave optics module), based on the fem solver. a homemade code in matlab environment was used for the post processing of the data. the grating period λ is around 158.9 nm and the bragg wavelength is around 550 nm. in figure 4, the simulation results show an evident shift of the wavelength as a function of ph variation in a time period of 0.05 ns [22]. the chart is based on the assumption of having the nominal length of the sensor for a ph value equal to 4. it is observed that the shift in the wavelength presents an increasing behaviour, approximately linear, when the ph increases from 4 up to 7.1. as expected, the wavelength shift shown in figure 4 is identical for the three scenarios with a number of layers of 5, 11 and 21 for fbg. the shift in the peak of the wavelength is independent from the number of layers in the model. figure 3. schematic drawing of polymer-coated fiber bragg grating ph sensor. drawing is not to scale. acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 27 figure 5 depicts the peak of the percentage reflection in the ph range 4–7.1. increasing the number of layers from 5 to 21 results in an increase of the reflectance of the initial light intensity, while maintaining the same pattern. as shown in figure 5 the reflectance peak of the sensor, when 21 layers are considered, is higher than 99.99 %, reaching almost the total reflection. as depicted in figure 5, the higher the number of layers in the sensor, the narrower is the range of the reflectance where the peak values fall. figure 6 illustrates that the response of the distributed bragg grating in the ph range 4-7.1 shows a narrower reflectance range with a higher number of layers in the fbg for vacuum wavelength. the blue lines in the three charts represent the reflectance for a ph of 4, while the red lines represent the reflectance for a ph of 7.1. all the curves for the ph within 4 and 7.1 are not represented for clarity of the chart, but they are in between the blue and red lines. figure 7 shows the percentage reflectance of the sensor for the three different layers of grating. for 21 layers, the reflectance is approximately 100 as illustrated in figure 5. as depicted in figure 7, the higher the number of layers in the sensor, the closer to 100 % is the reflectance. concluding, the fbg ph sensor coated with hydrogel has been devised exploiting the hydrogel swelling as a result of the change of surrounding ph in a time interval and its mechanical effect on the fbg. 3.1. results – sensor variant in this section the behaviour of the sensor variant, which is made of different materials for the core and the cladding compared to the first one, has been studied. figure 8 shows the reflectance percentage as a function of the vacuum wavelength in the ph range 4-7.1 for 5, 11 and 21 layers of the distributed bragg grating each. in the figure 8 the performances of the sensors with different grating materials are compared to one another, for the number of layers of the grating equal to 5, 11 and 21. the ph values vary from 4 to 7.1, however only the curves for the limiting ph values are represented for clarity of the charts. figure 4. response of a fiber bragg grating coated with hydrogel as a function of ph. figure 5. trend of the peak of percentage reflectance of the distributed bragg grating in the range of ph 4–7.1 for number of layers 5, 11 and 21. figure 6. response of the distributed bragg grating for all the strains of the hydrogel in the ph range 4–7.1 for the number of layers 5, 11 and 21. for clarity, the curves for ph 4 and 7.1, blue and red respectively, are represented; all other curves are in between the two lines. figure 7. percentage reflectance of the distributed bragg grating in the ph range of 4–7.1 for the number of layers 5, 11 and 21. acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 28 in figure 8 a), b) and c) the grating of the sensor is set to nh=1.4 and nl=1.38, nh=2.2 and nl=1.38, and nh=3 and nl=1.38, respectively. in the scenario a) the reflectance peak is narrower for 21 layers compared to scenarios b) and c), with the highest peak in the three figures always being for the 21 layers. in figure 8 a) the percentage reflectance is very low for the three different configurations of grating with 5, 10 and 21 layers, even if the one with 21 shows a slightly higher peak. it is also evident in the three figures that the reflectance for the 21-layer setup is always higher compared to the 5 and 10 ones. from this analysis it can be stated that the higher the number of layers in the grating, the higher the percentage reflectance or peak reflection, as confirmed also with the first sensor studied in the prior section. moreover, the higher the difference between the refractive index of the two materials that constitute the grating, the higher the peak reflection, even if with a wider bandwidth. figure 9 shows the reflectance for the fiber with a number of layers in the grating equal to 21 only, because it is the model that shows the better performances compared to the others. the reflectance peak for the configuration with the grating equal to nh=3 and nl=1.38 shows a reflectance power approximately equal to 100%, reaching almost the total reflection in the bragg bandwidth. the fiber with nh=1.4 and nl=1.38 shows a very low reflectance, around 1.15965 %. in figure 10 the simulation results show an evident shift of the wavelength as a function of ph variation in a time period of 0.05 ns. the chart is based on the assumption of having the nominal length of the sensor for a ph value equal to 4. it is observed that the shift in the wavelength presents an increasing behaviour, approximately linear, when the ph increases from 4 up to 7.1. a) b) c) figure 8. percentage reflectance of the distributed bragg grating, with number of layers 5, 11 and 21, as a function of vacuum wavelength between 1300-1900 nm in the ph range of 4–7.1. the legend is shown only in the top figure for clarity, but it is also applied to the other two figures. grating equal to nh=1.4 and nl=1.38 (a), to nh=2.2 and nl=1.38 (b), to nh=3.0 and nl=1.38 (c). figure 9. trend of the peak of percentage reflectance for the distributed bragg grating in the range of ph 4–7.1 for a grating with 21 layers and refractive index equal to nh=1.4 and nl=1.38, nh=2.2 and nl=1.38, and nh=3 and nl=1.38, respectively. figure 10. response of a fiber bragg grating coated with hydrogel as a function of ph. acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 29 the sensitivity of the sensors for all different configurations with different materials for the grating provided the same response, because the bragg grating period is a function of the wavelength and refractive index. this fiber, compared to the one with air and substrate in the core outside the grating, shows a higher sensitivity in the spectrum range under analysis, being around 8.58 nm/ph. 4. discussion the response of the sensor in figure 4 shows approximately linear behaviour in the ph range 4-7.1. changing the number of layers in the grating does not affect the response of the fbg sensor in terms of wavelength shift, but it has an effect on the reflectance intensity as showed in figure 5. subsequently, the sensor was studied with different grating materials for maximizing its performances. in figure 8 the behaviours of the sensor with different grating materials are compared to one another, nh was set equal to 1.4, 2.2 and 3 in three sensor configurations while nl was kept to 1.38. from this analysis it was noticed that the higher the number of layers in the grating, the higher the reflectance percentage or peak reflection, and moreover, the higher the difference between the refractive indices of the two materials that constitute the grating, the higher the peak reflection, even if with a wider stop-band. in conclusion, it can be stated that the optimum sensor has the grating constituted by two materials with nl =1.38 and nh equal to a value higher than 1.4 and lower than 3. the purpose is to have the highest sensitivity, thus a high reflectance percentage but also a limited stop-band. the wavelength drift of the fbg sensor is also induced by bending cross-sensitivity and temperature changes. in this paper these two effects have not been considered. the temperature and strain parameters will be taken into account for the next generation of fbg ph sensors. these will be compensated with either two further fbg sensors or a single fbg sensor able to measure both temperature and strain simultaneously. the dedicated system will be placed in series in the measurement chain. as temperature and strain sensors, fbgs have a fast response time due to their small size and volume. knowing the real-time outputs of the sensors for the temperature and strain changes, it will be possible to accurately measure the variation of ph of the material under analysis, subtracting the effect of these two quantities. moreover, in future developments this sensor will be integrated with the optical ring resonator in order to improve its performances. even though the sensor has very small dimensions with a length of 10 mm for the original configuration and 100 µm for its variant, the goal will be to reduce the size even more for medical applications. 5. conclusion in this work, an innovative system has been presented in order to determine the ph value of the rain. the advantage of forecasting the corrosion of monuments allows optimal restoration programs. these are also depending on the materials under analysis and can have an important impact in terms of cost reductions and higher maintenance efficiency. an additional advantage, due to the small size of the sensor, is to locate it in modest areas of the artworks without a visual impact. moreover, the chain of measurement can also be located underground for protecting and conserving the original appearance. in conclusion, the swelling response of hydrogels to the change of surrounding ph allowed the development of a model of hydrogel coated fbg ph sensor. modelling the fbg ph sensor for monitoring the rain in archaeology and in cultural heritage provided innovative results in terms of high sensitivity and small dimensions of the device, allowing for better intervention planning, especially when used together with other stone conservations [23]. the sensitivity of the first sensor is around 2.91 nm/ph, as confirmed in figure 4. a second generation of sensors was developed to facilitate the manufacturability. the analysis of the performances of the sensors showed an even higher sensitivity around 8.58 nm/ph. the high variation of the refractive index in the model allowed to discover a sensor with a greatly higher sensitivity and better performances. these innovative results can be applied to all other sensors such as strain gauges and thermometers that use the same physical principle. acknowledgement the authors wish to thank samanta f. strigaro for editing the manuscript. support was provided solely from 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[8] s. j. haneef, j. b. johnson, c. dickinson, g. e. thompson, g. c. wood, effect of dry deposition of nox and so2 gaseous pollutants on the degradation of calcareous building stones, atmospheric environment. part a. general topics, 26(16), 1992, pp. 2963-2974. [9] i. yulianti, a. s. m. supa'at, s. m. idrus, n. m. kassim, a. m. alhetar, fiber bragg grating based ph sensor, enabling science and nanotechnology (escinano), international conference on, kuala lumpur, malaysia, 2010, pp. 424-428. [10] i. yulianti, a.s.m. supa'at, s.m. idrus, “characterization of fiber bragg grating sensor for ph measurement,” advanced intelligent mechatronics (aim), ieee/asme international conference on, besançon, france, (2014), pp. 1163-1166. [11] v. n. k. pabbisetti, s. s. madhuvarasu, hydrogel-coated fiber bragg grating sensor for ph monitoring, optical engineering, 55(6), 2015, pp. 066112-066112. 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[23] a. macchia, f. mangini, s. a. ruffolo, m. muzi, l. rivaroli, m. ricca, f. frezza, a novel model to detect the content of inorganic nanoparticles in coatings used for stone protection, progress in organic coatings, vol. 106, 2017, pp. 177-185. research challenges in measurements for internet of things systems acta imeko issn: 2221-870x december 2018, volume 7, number 4, 82 94 acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 82 research challenges in measurements for internet of things systems eulalia balestrieri1, luca de vito1, francesco lamonaca1, francesco picariello1, sergio rapuano1, ioan tudosa1 1 department of engineering, university of sannio, 82100 benevento, italy section: research paper keywords: internet of things; distributed measurement; compressed sensing; power measurement; mobile devices; localization; synchronization citation: eulalia balestrieri, luca de vito, francesco lamonaca, francesco picariello, sergio rapuano, ioan tudosa, research challenges in measurements for internet of things systems, acta imeko, vol. 7, no. 4, article 14, december 2018, identifier: imeko-acta-07 (2018)-04-14 editor: dusan agrez, university of ljubljana, slovenia received october 26, 2018; in final form november 28, 2018; published december 2018 copyright: © 2018 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: francesco picariello, e-mail: fpicariello@unisannio.it 1. introduction “the internet of things (iot) extends the idea of interconnecting computers to a plethora of different devices, collectively referred to as smart devices [1].” these devices are connected to physical items, usually called “things,” such as home appliances, vehicles, buildings, and wearable objects. the iot paradigm is applied in several applications, including transportation and industrial sectors [2-8], allowing users to monitor and manage complex systems in a simple way. however, iot represents a technological challenge in several respects, specifically hardware [9] and telecommunication aspects [2, 10]. the most common iot systems aim to monitor and manage physical quantities related to the “things” to which they are applied. thus, the measurement of these physical quantities plays a crucial role from the design to the test phases of an entire iot system. first, for designing an iot system, the physical quantities that require monitoring must be defined. then, the data acquisition system of each iot node is designed according to the defined quantities. the acquired data must be shared over the internet with the following constraints: (i) minimization of the amount of the exchanged data, while maintaining the entirety of the information, (ii) reduction of the time delay required for sharing the data over the network, and (iii) an increase in the lifetime of the nodes. to comply with these constraints, it is crucial to measure (i) the power consumption of each node, which depends on the hardware and software implementation of the whole iot system, and (ii) the time delay that occurs due to the data processing and the network. furthermore, in the case of mobile applications, it is crucial to measure the position of the devices such that it is possible to associate the position where the measurements are taken with the measurement information. lastly, the synchronization of the iot nodes must be guaranteed in order to allow the implementation of time division multiple access (tdma) communication protocols and to complement the measurement information with the time instant at which it has been acquired. the aim of this paper is to present and discuss the key measurement aspects for the design of an iot system. in particular, the generic design guidelines for an iot system are summarized according to two important requirements: the power supply capabilities of the infrastructure and the time delay constraints of the application. several research contributions in the measurement field are reviewed, proposing innovative solutions that attempt to satisfy the abovementioned requirements. from an analysis of the scientific literature in the measurement field, it is clear that the main research trends are abstract in this paper, an overview of the research challenges in measurements for the design of internet of things (iot) systems is proposed. to this end, a general architecture of an iot system is presented, which is specialized according to two key requirements: the power supply capabilities of the infrastructure and the time delay constraints of the application. guidelines for the design of an iot system are summarized, and the measurement needs are highlighted. a review of the research contributions is given concerning three main measurement topics: (i) energy-aware data acquisition systems, (ii) localization of mobile iot nodes, and (iii) precise synchronization protocols. mailto:fpicariello@unisannio.it acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 83 focused on (i) energy-aware data acquisition systems, (ii) localization of mobile iot nodes, and (iii) precise synchronization protocols. in this paper, an overview of these research contributions is given. the paper is organized as follows. in section 2, the general architecture of an iot system is presented and some examples of iot systems are reported. section 3 presents four iot system typologies that are defined on the basis of the power supply capabilities of the system components and the time delay constraints imposed by the application. furthermore, in section 3, design guidelines for an iot system are summarized. a brief review on the research contributions in the measurement field concerning the implementation of energy-aware data acquisition systems is presented in section 4. section 5 presents an overview of the research contributions concerning the implementation measurement systems for accurate localization of mobile iot nodes. section 6 reports the classification of the synchronization protocols available in the literature on iot systems. the final section concludes the paper. 2. general architecture of an iot system an iot system allows for digitizing the physical quantities related to a huge number of physical objects. thus, it converts the real world into a virtual world, allowing for the sharing of the object capabilities over the internet. to this end, an iot system must comply with several key requirements [11], which can be classified as (i) resource control, (ii) energy awareness, (iii) quality of service (qos), (iv) interoperability, (v) interference management, and (vi) security. resource control is related to the fact that all iot nodes must be remotely accessible and configurable. furthermore, as an iot system is composed of a huge number of nodes, it is necessary to manage the availability of redundant resources. usually, iot nodes are powered by batteries. thus, the design of an iot system from the network level to the hardware level (i.e., sensors, actuators, microcontrollers) must minimize the power consumption of the single device and of the overall system. qos in iot is ensured by assigning priorities to services and data [11]. for example, services that require real-time processing or early warning must have a higher priority than other services. moreover, a service must provide only relevant information to the users by processing and fusing the large overall amount of data that is provided by all iot nodes. an important feature of an iot system is the interoperability of devices made by several different manufacturers. this feature requires adaptation between the networking protocols of the different devices. another important characteristic of an iot system is related to interference management. the huge number of devices connected to the internet by means of multi-radio transceivers requires the design of communication protocols for minimizing the effect of electromagnetic interference among neighboring nodes. moreover, the level of security related to the exchanged data and to the access to each iot node is an essential requirement. finally, to guarantee the system’s reliability, it is important to implement procedures that automatically recognize the faults and failures of the entire iot system. the design guidelines of an iot system from the perspective of hardware, software, and communication must consider the abovementioned requirements. furthermore, once the system has been designed and implemented, it is necessary to verify compliance with the design requirements. to this end, measurement methods must be provided for measuring the parameters that quantify the performance of an iot system following such requirements. in the literature on this topic, several works have focused on the implementation of iot systems, and several architectures have been proposed [12]. all these systems have been designed in order to comply with at least two or three of the abovementioned requirements. furthermore, they have several blocks in common. such similarities allow us to draw a general scheme of an iot system. according to [2], the general architecture of an iot system consists of the following layers (figure 1): (i) the physical layer, (ii) the data exchange layer, (iii) the information integration layer, and (iv) the application service layer. the physical layer is composed of sensor and actuator nodes that allow the monitoring and the control of physical quantities related to the physical objects. furthermore, the sensor and actuator nodes communicate with each other and share data over the internet. in general, a sensor and actuator node consists of the following components: (i) a microcontroller; (ii) smart sensors that communicate the measurement data to the microcontroller via digital interfaces (such as the inter-integrated circuit [i2c], the serial peripheral interface [spi], and the universal asynchronous receiver-transmitter [uart]); (iii) actuators that consist of transducers (such as motors, relays, thermal resistors, and speakers) and electronic circuitry used to drive them by means of commands received from the microcontroller; (iv) a power supply system that, according to the application requirements, is connected directly to the electrical network or consists of batteries with a harvesting system; and (v) wireless and/or wired transceivers, which enable communication between the nodes. the data exchange layer aims to provide internet connection capabilities to the sensor and actuator nodes. in particular, data figure 1. the general architecture of an iot system [2]. acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 84 is collected from the sensor nodes and is shared over the internet. commands can be sent to the actuators. according to the network topology, the data exchange layer is implemented in two ways: (i) concentrator nodes communicate with the sensor and actuator nodes as well as with the internet gateways, which provide internet capabilities to all the devices in the physical layer; or (ii) the sensor and actuator nodes have an internet connection and can interact with the cloud server. in particular, the first network topology is used when it is needed to minimize the power consumption of the sensor and actuator nodes, especially when they are powered by batteries. the information integration layer is also known as the middleware layer. it stores, analyzes, and processes the huge amount of data that comes from the data exchange layer. it employs several technologies, including databases, cloud computing, and big data processing modules. this layer creates an abstraction of the physical layer by processing the large amount of data given by the data exchange layer and by providing the compressed and more intuitive information that is usually required by the end user. according to [13], the features of an information integration layer are: (i) interoperability and application programming interface; (ii) scalability; and (iii) the security and privacy protection. the first feature facilitates collaboration and information exchange between heterogeneous nodes. in particular, two types of interoperability should be assured: semantic and syntactic. semantic interoperability deals with abstraction of the meaning of the data within a particular domain according to the services that must be provided to the end user. syntactic interoperability ensures that the devices can communicate the data to each other even if the shared portions of data have different formats, structures, and encodings. the information integration layer must manage the system scalability by automatically making the required changes when the infrastructure scales (e.g., when a new iot node is added to the system). furthermore, the integration layer must protect the shared data by means of encryption, and it must limit access only to specific users by means of reliable user authentication methods. the application service layer consists of digital services that allow the management of the entire iot system or parts of it by using the abstraction provided by the integration layer. in particular, end users can interact with the iot nodes by visualizing the integrated information and by sending operative commands that are implemented in the information integration layer. the proposed general architecture, depicted in figure 1figure 1. the general architecture of an iot system [2]., can be used to model an iot system, and in the following section, two examples of an iot system implementing such architecture are presented and analyzed. in [14], the authors implemented an iot-based system for the real-time monitoring of a decentralized photovoltaic plant. as depicted in figure 2, the physical layer is composed by an analog/digital converter embedded system (adces) that consists of a pic18f microcontroller that includes a native usb interface as well as analog voltage, current, temperature, and solar irradiance sensors. the adces acquires the data provided by the sensors and sends it via usb to the embedded linux system (els). furthermore, the els acquires the measurements provided by the ambient temperature and relative humidity digital sensors and combines them with the measurements of the adces. all the data is shared over the internet thanks to the wi-fi interface of the els, which is connected to an internet gateway. the information integration layer is implemented on a cloud server, where the data is stored directly on a database. the application service layer, where the user can interact with the cloud server, is implemented by means of a web application. in the beginning, the web application requires user authentication. thereafter, it shows the instantaneous measured values provided by the sensor nodes and the stored historical data with a time scale of one minute. figure 2. the general architecture of an iot-based system for real-time monitoring of a decentralized photovoltaic plant. figure 3. the general architecture of an iot system applied to guardrails installed along the road infrastructure. acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 85 another example of the iot system is reported in [15]. the study in [15] proposes an iot system applied to the guardrails installed along the road infrastructure (figure 3). the physical layer consists of two types of sensor and actuator nodes: (i) the traffic safety nodes and (ii) the environment monitoring nodes. the first one contains (i) a speed measurement system, which measures the vehicle speed; (ii) a proximity measurement system, which detects when a vehicle is going to be dangerously close to the guardrail respect to a defined safety distance; (iii) an impact detection system, revealing a crash of a vehicle with the guardrail; and (iv) an alert system, consisting of a light and acoustic system, which are activated in the case of an accident, the dangerous approach of a vehicle to the guardrail, and when a remote command is received. the environment monitoring nodes provide (i) pm2.5, pm10, co2, no2, and so2 concentration measurements; (ii) ambient temperature and relative humidity measurements; and (iii) information related to the conditions of the road surface. in addition to the sensors, both types of the nodes embed (i) the iris mote platform, which includes an ieee 802.15.4 radio transceiver and the atmega1281 microcontroller; and (ii) a power management system that is connected to an external battery, charged by a photovoltaic panel. the data exchange layer consists of a concentrator that collects the data provided by the sensor and actuator nodes and sends them to a server through an internet key. furthermore, each concentrator manages a mesh network composed by the sensor and actuator nodes. for the communication, the ipv6 over a low-power wireless personal area network (6lowpan) protocol is adopted. the integration information layer is implemented on the server, which provides (i) information related to the traffic conditions by estimating the average speed on the road section according to the number of vehicles crossing the road and each vehicle speed; (ii) information related to the surface road conditions; (iii) the concentration measurements of the considered pollutants and, if they are higher than the maximum allowed value, an alert semantic command is stored on the database; (vi) the entities of the vehicle-guardrail impacts that eventually occurred; and (v) a command that can be send to each node to remotely activate the alert system. the application service layer is implemented using a web application that shows to the user the information provided by the previous layer on a map and allows them to interact with each sensor and actuator node for activating the alert system. 3. design guidelines for iot systems of course, the specific design of an iot-based system depends on the type of application; however, according to the general architecture presented in the previous section, it is possible to define some common design guidelines. in this section, the key steps required for the design of each iot layer are described. furthermore, simple guidelines for the design of an iot system are summarized. in order to define the guidelines for designing an iot system, it is necessary to look at some parameters related to the target application. in particular, as reported in figure 4, iot systems can be classified according to (i) the maximum time delay required for receiving and transmitting the information from and to the sensor/actuator nodes; and (ii) the power supply capabilities of the infrastructure or, in general, of the objects where the iot nodes will be installed. by combining these parameters, four typologies of iot systems can be identified: (i) battery-powered and real-time iot systems; (ii) battery-powered iot systems; (iii) network-powered and real-time iot systems, and (iv) network-powered iot systems. in particular, the first couple of typologies are applied in the case of applications in which the power network is unavailable or in the case that the iot system is composed of mobile devices and the time delay required for transmitting and receiving data from the sensor and actuator nodes by the end user is constrained. an example of this typology is reported in [16]. such a system consists of sensor and actuator nodes placed along the road infrastructure on the guardrails. due to power network limitations, the power system consists of a photovoltaic panel with a battery [17]. furthermore, due to the fact that the system aims to send warnings to the user and to the other sensor/actuator nodes related to the impacts between the vehicles and the guardrails, the required time delay of the data packets must be less than 110 ms [18]. a battery-powered iot system is usually applied for monitoring physical quantities (temperature, humidity, pollutant concentration, etc.) that have a low variation in time and in environments in which the power network is not available. for example, the researchers in [19] propose an iot system to monitor temperature, humidity, and co2 for implementing an air quality and weather monitoring system in a smart city. in that case, the time required for sending the temperature, humidity, and co2 concentration measurements is around 40 s, and the power supply system consists only of a battery. the adopted communication interface is the wi-fi connection. the researchers in that study implemented an algorithm for each sensor node such that it goes into several sleep states for reducing its power consumption. in this way, they increase the lifetime of the battery. applications that require the management of early warnings and the time at which the infrastructure that is intended to be monitored has a power network connection available fall within the third typology. as reported in [2], this is a smart energy application. in particular, in the case of the monitoring and management of the power grid, the iot system consists of several phasor measurement units (pmus) placed along the network itself, which measures voltages and currents simultaneously. if the smart energy system is aiming to achieve protection, it requires the implementation of early warning mechanisms from the data exchange layer up to the application service layer. in this case, the p-class pmus are adopted figure 4. the four proposed iot system typologies. acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 86 according to the ieee std c37.118.1-2011, and a networkpowered and real-time iot system is needed. for applications in which the power network is available and no time delay constraints are imposed, a simple networkpowered iot system can be used. this is the case in industrial applications, where it is necessary to extend the capabilities of an already available network (e.g., based on rs-485) of sensors and actuators with iot. for example, in [20], the researchers propose a multi-channel rs-485 iot gateway, consisting of an embedded device with an ethernet interface and 32 rs-485 ports. another example of a network-powered iot system is reported in [21]. the researcher proposes therein an iot system for sharing the data provided by biomedical devices, such as nuclear medicine images, mri images, and digitized x-rays. for example, the access point communicates with medical devices, and it is connected via ethernet to the internet. all the collected data is shared on a medical data cloud service. considering the power supply capabilities of the infrastructure where the iot nodes will be installed and the time delay constraints, in the following section, some guidelines for the design of an iot system are summarized (see figure 5): 1. definition of the system requirements in terms of power supply capabilities and time-delay constraints. 2. choice of the iot system typology, according to the abovementioned definitions. 3. definition of the requirements related to the physical layer in terms of (i) number and types of sensor and actuator nodes; (ii) maximum power consumption for each node; (iii) target uncertainty related to the physical quantities measured by each sensor; (iv) target accuracy and precision of the actuators; (v) typology of the digital interfaces for acquiring the measurements provided by the sensors and for driving the actuators; (vi) computational effort of the data processing algorithms that will be implemented on the node; and (vii) time delay required for data processing. 4. definition of the requirements related to the exchange layer, in terms of (i) maximum time delay allowed for sending or receiving a packet to or from the nodes; (ii) typology of communication, wireless or wired; (iii) network topology; (iv) maximum distances for the communications among the sensor/actuator nodes, the sensor/actuator nodes and the concentrator, the sensor/actuator nodes and the gateway, and the concentrator and the gateway; (v) maximum power consumption allowed for the communication; and (vi) type of data cryptography. 5. requirements related to the information integration layer: (i) definition of the end users; (ii) definition of the number and kinds of services that must be provided to each end user; (iii) definition of the integrated information needed for implementing each service; (iv) definition of the information integration layer architecture in terms of data processing algorithms that will be implemented on the nodes or in the cloud; (v) evaluation of the computational complexity of the algorithms for processing the measured data to provide the integrated information; and (vi) definition of the processing time required for providing each integrated information. 6. requirements related to the application service layer: (i) definition of the user interface for each provided service; (ii) definition of the computational complexity related to the processing algorithms that will be implemented in the end user device; and (iii) definition of the typologies of end user platforms that will be used for implementing the user interface. 7. choice of the data exchange layer and information integration layer architectures according to the abovementioned requirements and analysis of the time delay required for sending and receiving data from and to the nodes. 8. selection of the sensors and the actuators to be embedded on the physical layer nodes according to their metrological and electrical characteristics. 9. selection of the microcontroller and radio transceivers to be embedded on the nodes (sensor/actuator nodes, gateway and concentrator), according to their power consumption, computational capabilities, and the available digital and analog peripherals. 10. definition of the data processing algorithms to be implemented on the nodes and of the procedures performed in the cloud. 11. analysis of the computational effort of each algorithm and evaluation of the time required for performing them. 12. definition of the graphical user interface on the end-user devices according to the typology of service to be provided. these design steps require knowledge of a huge range of technologies, from telecommunication to firmware development. furthermore, they turn out to be challenging when the application requires low power consumption and real-time operations. in particular, these requirements affect the design of (i) each sensor/actuator node, (ii) the network architecture, and (iii) the data processing at each layer. several research contributions in the measurement field have proposed solutions figure 5. the proposed steps for the design of an iot system. acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 87 to satisfy these requirements. the main research trends are focused on the following topics: (i) energy-aware data acquisition systems, (ii) localization of mobile iot nodes, and (iii) precise synchronization protocols. for each of these topics, a carefully selected collection of research contributions is reviewed in the following sections, allowing us to draw on the current research trends. 4. energy-aware data acquisition systems the sensor/actuator node in an iot system has the following roles: (i) providing measurements related to the physical quantities that are intended to be monitored, (ii) driving the actuators embedded on the node, (iii) communicating with the internet gateway or the coordinator node according to the typology of the data exchange layer, and (iv) communicating with the other sensor/actuator nodes. furthermore, especially for battery-powered iot systems and in the case of early warning applications, the node must exhibit both low-power consumption and low processing time. in such cases, several solutions have been proposed in the scientific literature, with the aim of minimizing such parameters. those solutions refer to energy-aware sampling methods and power management. 4.1. energy-aware sampling methods the data acquisition system of a sensor node aims to determine the value of one or more physical quantities for monitoring. the general architecture of a data acquisition system consists of the following elements [22]: (i) sensors or transducers; (ii) signal conditioning circuitry; (iii) anti-aliasing filters; (iv) a sample and hold circuit; (v) a multiplexer; (vi) the analog-to-digital converter (adc); (vii) the clock that defines the timing unit used for the analog-to-digital conversion and the data processing; (viii) the memory; and (ix) a digital interface. the most important parameters of a data acquisition system are (i) the bandwidth, (ii) the maximum sampling rate, and (iii) the memory length. these parameters directly affect (i) the power consumption of the node; (ii) the amount of data that is processed and transmitted; and (iii) its cost. to this end, several researchers have focused on new data acquisition techniques, aiming either to enlarge the bandwidth or reduce the sampling frequency and thus the data transfer rate. one emerging technique is based on the compressed sensing (cs) theory [23]. the cs theory relies on the fact that many types of information have a property called sparseness in a proper domain [24]. thus, the required information for the specific application could be obtained from a compressed version of the sampled signals as well as the whole signals sampled according to the nyquist theory [24]. in [24], the researchers analyzed the use of cs theory for the acquisition of data in iot systems, and they propose a signal reconstruction algorithm that offers higher accuracy and lower computational effort compared with group/cluster-sparse reconstruction algorithms that are available in some studies. in the following section, the cs theory as applied to a generic iot system is briefly described. an iot system consists of n nodes, where each node j collects the yj sample, with j = 1, ..., n. the collected measurement vector is 𝒚 = [𝑦1, … , 𝑦𝑛 ] 𝑇 . the data acquisition process can be modeled as: 𝒙 = 𝜱 𝒚, (1) where 𝜱 denotes an 𝑚 × 𝑛 matrix with 𝑚 far less than 𝑛, and the measurement data x is a 𝑚 × 1 vector. according to the cs theory, by knowing the matrix 𝜱, it is possible to recover y from x. as the dimension of y is much smaller than the size of x, cs theory provides the following advantages in the data acquisition: (i) the number of acquired samples is reduced and the length of the occupied memory is consequently reduced by maintaining the same time window of a nyquist-based acquisition system; and (ii) the sampling rate of the adc can be reduced by maintaining the same bandwidth of a nyquist-based acquisition system. in this case, it is possible to minimize the power consumption, processing, and transmission time of each sensor node. on the other hand, in order to reconstruct the entire vector y from the acquired measurements stored in the vector 𝒙, the y vector must be sparse in a specific domain, (i.e. it can be defined by few coefficients in that domain): 𝚯 = 𝜳t𝒚, (2) where 𝜣 is a k-sparse 𝑛 × 1 vector, with 𝑘 ≪ 𝑛, which denotes the weights vector, 𝜃𝑖 = 〈𝒚, 𝝍𝑖 〉. 𝜳 = [𝝍1, … , 𝝍𝑛] is the basis matrix. a vector can be said to be k-sparse, if there are k nonzero coefficients in 𝜣 with 𝑘 ≪ 𝑛. furthermore, the matrices 𝜱 and 𝜳 must be incoherent, and the 𝑚 × 𝑛 measurement matrix 𝜱 must satisfy the restricted isometry property (rip). a cs theory-based sampling process requires the acquisition of a small number of samples of a sparse signal, thus allowing the reconstruction of the observed signal by using linear/convex optimization methods. in iot systems, cs theory can be applied to the following data: (i) node-dependent data and (ii) node-cooperative data. figure 6. the general architecture of a data acquisition system (a) [22] vs. the general architecture of a data acquisition system with the cs applied to node-dependent data (b). acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 88 in the case that cs theory is applied to node-dependent data, each node acquires the samples related to a signal y(t), with a sampling process working at sub-nyquist rate (see figure 6b). in this case, cs theory can be used at the physical layer to effectively reduce the sampling rate of the adc with respect to a classic nyquist-based sampling process (see figure 6a). thus, the power consumption of the iot node can be reduced according to the clock rate reduction. furthermore, the amount of the exchanged data is reduced. the reconstruction of the data from the compressed data is usually performed in the information integration layer after the data has been sent through the digital interface. for example, in [25], the researchers propose an iot system using cs theory applied to electrocardiographic (ecg) and photoplethysmography (ppg) signals acquired by an iot node. in particular, the researchers propose an adaptive cs algorithm, whereby according to the morphology and the noise and signal quality, the compression ratio related to the non-uniform sampling operation is tuned. the researchers compared the energy consumption of a system based on the commonly used discrete wavelet transform (dwt) and the proposed cs method on a fourier basis. in particular, the energy consumption in the case of dwt is from 215 mj to 830 mj, and in the case of cs theory, it is around 6 mj, with a signal-to-noise ratio (snr) on the reconstructed signal of 16 db. furthermore, the number of samples acquired by using cs theory is at least four times less than the number of samples acquired using dwt. in this way, the number of exchanged and stored data is drastically reduced. in the case of node-cooperative data, the cs can be applied to reconstruct the measurements related to several spatial distributed monitored points by taking a few measurements provided by a set of randomly activated iot sensor nodes (see figure 7). the iot sensor nodes that do not send measurements at a given time instant can be kept in sleep mode. in this way, it is possible to reduce the power consumption of the entire iot system and the amount of the exchanged data. as in the case of node-dependent data, the reconstruction of the data from the compressed data is usually performed in the information integration layer. in [26], a cs theory-based scheduling scheme is proposed for a battery-powered iot system applied to a water distribution network system. each iot node is connected by wireless with the other and communicates with a concentrator that sends the monitored data to the server, via the internet. the researchers propose an algorithm for reducing the amount of data transmitted by the iot nodes. furthermore, when an iot node is not transmitting data to the concentrator, it is kept in sleep mode. for each timeslot, the concentrator randomly calculates a sequence of m numbers to represent the number of iot nodes that will be activated in the network (n is the total number of iot nodes that can be potentially activated). the concentrator broadcasts a message to all the iot nodes, which contains the identification number of the m nodes that will be activated and that will provide the measurements. the m iot nodes transmit data to the concentrator by hopping, while the other n-m data is in sleep mode. for each timeslot, the n-m measurements of the inactive iot nodes must be recovered at the information integration layer by applying cs theory, which considers the type-iv dct basis. in this case, at each time slot, the compression ratio 𝑚 𝑛⁄ corresponds to the number of activated iot nodes with respect to the total number of nodes. the researchers showed that the proposed iot system is able to minimize the number of activated iot nodes, assuring the correct reconstruction of all the data. compared with the other methods available in the relevant literature, they increase the lifetime of each iot node by about 10%. 4.2. energy-aware power management it is especially crucial to consider the power consumption of an iot node in the case of battery-powered iot systems [27]. in order to reduce the power consumption of the sensor/actuator nodes, two strategies are applied: (i) operating on the technology adopted for implementing the integrated circuits (ics) and (ii) operating on the power management system of the node. the complementary metal-oxide semiconductor (cmos) technology is the basic part of all the ics embedded on an iot node. as reported in [28], the operating voltage is the dominant factor that affects the power consumption of an ic. on the other hand, the operating speed of cmos ics is continuously increasing due to miniaturization, in accordance with moore's figure 7. the general architecture of a data acquisition system, with cs theory applied to the node-cooperative data. figure 8. the general energy model for an iot node [32]. acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 89 law. however, the continuous reduction of the working voltage of ics slowed down after 100 nm technology, due to the increase of the threshold voltage vth variability, which has a huge effect on it [28]. for this reason, several new cmos technologies have been proposed in the relevant literature, such as the fully depleted silicon-on-insulator (fdsoi), fin field effect transistor (finfet) [29], tunneling field-effect transistor (tfet) [30], and silicon on thin buried oxide (sotb) transistors [28]. in addition to these technological advances, the power consumption of an iot node can be reduced by operating on the management of the peripherals; the microcontroller; and the sensors and/or actuators integrated on the node. of course, in battery-powered iot systems, in which small batteries are used, the lifetime must be increased as much as possible, and the power consumption of the entire iot node must therefore be minimized [31]. in [32], the generic energy model for an iot node is reported. it is shown here in figure 8 and consists of (i) energy harvesting (e.g., photovoltaic panels, wind turbines), which collects energy from the environment and converts it into usable electrical power by the subsequent blocks; (ii) the energy buffer, consisting of batteries or super-capacitors, where the energy is stored and then released in a different time; and (iii) the energy consumer, which dissipates the energy provided by the energy buffer according to the performed tasks (e.g., acquiring data from the sensors, driving the actuators, processing the data provided by sensors, transmitting and receiving data from/to the wired or wireless communication interface). usually, the two main power consumption contributors of the energy consumer are (i) the microcontroller that communicates with or drives (through its peripherals) the sensors and the actuators, respectively; (ii) the radio transceiver that communicates with the microcontroller and sends/receives data to/from antennae; and (iii) the actuators. in order to reduce the power consumption, each iot node can work in different power modes [33]. in general, these power modes are (i) active mode, when all the peripherals are active, and the microcontroller is performing tasks; (ii) idle mode, when all the peripherals are active, and the microcontroller is not performing any tasks; and (iii) sleep mode, when some or all of the peripherals are not powered. for reducing the total amount of energy dissipated by the iot node, it is usually placed in sleep mode for long periods of time. thus, the power consumption of the device in sleep mode for saving energy is crucial. for this reason, in modern microcontrollers, several sleep modes according to the type of disabled peripherals are available. for example, in [34], a power management system for ultra-low-power microcontrollers that is used for implementing smart sensors and that supports several graded sleep modes is proposed. in this case, there are three sleep modes considered: (i) low-power state, when the central processing unit (cpu) of the microcontroller is stopped by gating the system clock, (ii) power gating mode, when the microcontroller is switched off but the low-dropout regulator (ldo) is activated, and (iii) disabling ldo mode. for the radio transceiver, the power modes agree with the following radio states: (i) transmitting, (ii) receiving, (iii) standby, and (iv) off mode. for each power mode, the relative power consumption and the time required for transitioning into and out of each of them are related. switching between states can be due to synchronous or asynchronous events. an example of a synchronous event is the periodic sending of measurements performed by the iot sensor node over the network. asynchronous events are usually due either to the detection of variations of one or more monitored physical quantities or the receipt of a command from the network. according to the defined sleep modes and their power consumption and transitioning times, it is possible to determine the number of iot nodes and the time intervals for which the devices are in each mode. the aim of this exercise is to minimize the dissipated energy by keeping the time delay constraints for performing each task. in [33], the researchers propose a power-aware methodology to reduce iot node energy consumption by exploiting the sleep mode according to the prediction of asynchronous events. in particular, in the beginning of the process, the iot node is in active mode until the asynchronous events are caught. according to this principle, a probability function of missing events is constructed. when the probability is lower than a threshold value, the iot node is placed in sleep mode. by considering the proposed power-aware methodology, the researchers obtained a reduction of 50 % in the energy consumption of the iot node, embedding the strongarm processor. another important issue for implementing a battery-powered iot system is to appropriately size its harvesting system. according to that, an estimation of the power consumption of each iot node has to be evaluated. the power consumption of a complex iot system should consider that (i) several iot nodes that have different hardware implementations can be part of the iot system, (ii) each iot node can work in several states, and (iii) the events for transitioning between the states can only be defined in terms of statistics. for example, in [17], the researchers propose the power consumption analysis of an iot system for road safety with the aim of sizing a harvesting system that consists of photovoltaic panels. in the beginning, for each iot node, the researchers identified several states. for each state, they defined the events for transitioning between them. thereafter, they measured the power consumption of the devices in each state by means of a calibrated power measurement system based on an ammeter and a voltmeter. a power consumption simulator of the entire network was then developed according to the number of the iot nodes considered and the number and type of road events affecting the power consumption of each node (e.g., vehicle-guardrail impact, crossing of the vehicle in the monitored road segment). according to the specific road statistics (e.g., the number of impacts in a year, the number of vehicles crossing the road segment in a day), the power consumption of each device and of the overall network is estimated. thus, according to the peak power consumption and the average power consumption values, the battery and the photovoltaic system are sized. 5. localization of mobile iot nodes iot systems are increasingly being used to implement location-based services (lbs) in several applications, including military, environment monitoring, home automation, etc. [35]. in these applications, the location information plays an important role when it is combined with the measurements of physical quantities related to space-distributed objects [36]. in this way, it is possible to monitor the variation of physical quantities in time and space. due to the fact that there are billions of objects that must be monitored and managed in iot systems, the location information is usually adopted for filtering their provided large amount of measurements and for organizing them in a database. acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 90 in the case of non-mobile iot systems, the position of each iot node can be measured during the system installation and then provided together with the device measurements when they are sent via the internet [15]. on the other hand, for mobile iot systems, the position should be measured whenever measurements are sent by each iot node. global positioning system (gps) could be the solution to all of the iot applications for providing position measurements, but it does have the following disadvantages: (i) high hardware cost, (ii) high-power consumption, and (iii) lack of availability in indoor environments. thus, a large proportion of the relevant research is focused on the development of positioning measurement systems that overcome the drawbacks of the gps-based ones [37-43]. all the position measurement systems are based on the use of signal transmitters (e.g., radio frequency rf signals as well as optical and ultrasonic signals) and receivers. for example, in the case of gps, a gps receiver acquires at least four rf signals transmitted by four corresponding satellites. an rf signal is used for synchronizing the clock of the receiver. from the other three signals, the time-of-arrival (toa) measurements are obtained, and by means of trilateration, the position of the receiver with respect to the satellites is estimated. as reported in [44], localization techniques can be classified as (i) connectivity-based methods, which use only connectivity information to locate the entire iot system or (ii) distance or angle-based methods, which use distance or angle measurements to determine the location of the iot node. distance and angle-based methods can achieve a higher resolution than connectivity-based ones [44]. when at least three measurements of distance or angle between a receiver and the transmitters are provided, the adopted distance and angle-based methods for localizing the receiver with respect to the transmitters are [44] (see figure 9) (i) triangulation, (ii) trilateration, and (iii) multilateration. triangulation is a method by which it is possible to find the position of a receiver when the angles are measured by means of the angle of arrival (aoa) technique. the receiver measures its angles with respect to three transmitters. the measured angles form three lines along the corresponding directions of the three transmitters to the receiver. the intersection between the three lines defines the location of the receiver with respect to the transmitter reference location. the position measurement accuracy is strictly correlated to the angle accuracy. trilateration allows for the measurement of the position of a receiver when three distance measurements between the receiver and three transmitters are provided. the position of the receiver is estimated as the intersection among three circles. the diameter of each circle is twice the distance measurement between the receiver and a transmitter and the circle center is in the transmitter position. this method can be used when the distance measurements exhibit a high accuracy [44]. multilateration is based on the estimation of the receiver position by minimizing the error between more than three measured distances. in this case, the position measurement accuracy depends on the distance accuracy as well as on the number of distance measurements of the receiver with respect to several reference transmitters. this technique is the most commonly used in the case of iot node localization because it can be implemented by using a low-accuracy distance measurement system embedded on the iot node, but with a huge number of transmitters [44]. the distance measurements provided by the receiver are performed by considering the following signal characteristics [44]: (i) received signal strength indicator (rssi), (ii) toa, and (iii) time difference of arrival (tdoa). the first is based on power measurements (see subsection 5.1), while the remaining two are based on time measurements (see subsection 5.2). 5.1. power-based position measurement systems power-based position measurement systems consist of measuring the received signal strengths (rsss) for obtaining the distance between the iot node and the reference transmitters with known positions. rss is defined as the voltage measured by a receiver’s rssi circuit. often, the term “rss” is used interchangeably with “measured power.” wireless sensors communicate with neighboring sensors so that the rss of the rf signals can be measured by each receiver during normal data communication, without presenting additional bandwidth or energy requirements. if the transmitted power is known, the effective propagation loss can be calculated. theoretical and empirical models are used to translate this loss into a distance estimate. in free space, signal power decays are proportional to d-2, where d is the distance between the transmitter and the receiver. in real cases, multipath signals and shadowing are the two main sources of environmental dependence in the measured rss value. m power measurements pr,i with i = 1,...,m provided by a receiver related to m transmitters can be modeled as follows [45]: 𝑃𝑟,𝑖 = 𝑃𝑡,𝑖 − 10𝛽 ∙ log10 ( 𝑑𝑟,𝑖 𝑑𝑡,𝑖 ), (3) where pt,i is the transmitted power of the i-th transmitter at a reference distance dt,i; β is the path loss exponent (in free space it is 2); 𝑑𝑟,𝑖 = ‖𝒙𝑟 − 𝒙𝑡,𝑖 ‖2 is the distance between the receiver and the i-th transmitter; and 𝒙𝑟 = [𝑥𝑟 , 𝑦𝑟 ] 𝑇 and 𝒙𝑡,𝑖 = [𝑥𝑡,𝑖 , 𝑦𝑡,𝑖] 𝑇 are the receiver and transmitter positions, respectively. the multilateration method involves estimating the 𝒙𝑟 value according to the following minimization criterion: 𝒙𝑟 = argmin 𝒙𝑟 ∑ [𝑃𝑡,𝑖 − 10𝛽 ∙ log10 ( 𝑑𝑟,𝑖 𝑑𝑡,𝑖 )]𝑀𝑖=1 . (4) if the power of the signal transmitted by each transmitter is equal to p0 and is unknown, the previous minimization criterion can be applied, undertaking minimization according to 𝒙𝑟 and p0. figure 9. distance and angle-based methods for iot node localization [44]. acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 91 in [45], the researchers propose a novel semidefinite programming (sdp) approach for solving the abovementioned minimization criterion. they compare the results in terms of the root mean square error (rmse) with maximum likelihood (ml) estimator, least-squares (ls), and weighted total ls (wtls) approaches. the proposed sdp approach exhibits the lower rmse (around 1 m) with respect to the ls and the wtls. furthermore, its performance is comparable with the ml estimator, but with less computational effort [45]. an example of the rss-based localization method is proposed in [46]. in this paper, the researchers propose a trilateration-based localization method with a dynamic-circleexpanding mechanism (lodce) for indoor localization of an iot node. the lodce method was implemented for the octopus ii mote equipped with a ti msp430 processor and a cc2420 transceiver. furthermore, each mote is equipped with a 2.4 ghz omnidirectional antenna. during the experiment, each iot node estimates its location coordinates by measuring the rssi values with respect to the reference nodes and sends them to a gateway. the researchers performed experimental tests according to three scenarios. in one of them, an iot mobile node is moved in 10 known locations. by considering 10 reference nodes at each location, 50 position measurements were provided by each iot mobile node. at each location, the standard deviation and the average of the 50 measurements were calculated. the obtained maximum discrepancy between the measured positions and the known ones is 1.49 m for a considered area of 36 m2. the maximum standard deviation is 0.28 m. 5.2. time delay-based position measurement systems the time delay-based position measurement systems are classified as [47] (i) toa-based methods or (ii) tdoa-based methods. in the case of toa-based methods, the distance between the receiver and the transmitter is estimated from the time delay required by a signal to spread from the transmitter to the receiver and by knowing the propagation speed of the signal. by measuring three distances between the receiver and three transmitters, a trilateration localization method can be applied for obtaining the coordinates of the receiver [47]. in the case of tdoa-based methods, localization is based on the time difference between two different propagation signals, which spread from two different transmitters to the receiver [47]. when three propagation time delays are measured, the following system of equations can be written: { √(𝑥𝑟 − 𝑥1) 2 + (𝑦𝑟 − 𝑦1) 2 − √(𝑥𝑟 − 𝑥2) 2 + (𝑦𝑟 − 𝑦2) 2 = 𝑐(𝑡1 − 𝑡2) √(𝑥𝑟 − 𝑥1) 2 + (𝑦𝑟 − 𝑦1) 2 − √(𝑥𝑟 − 𝑥3) 2 + (𝑦𝑟 − 𝑦3) 2 = 𝑐(𝑡1 − 𝑡3) √(𝑥𝑟 − 𝑥2) 2 + (𝑦𝑟 − 𝑦2) 2 − √(𝑥𝑟 − 𝑥3) 2 + (𝑦𝑟 − 𝑦3) 2 = 𝑐(𝑡2 − 𝑡3) (5) where c represents the propagation speed of the signal; t1, t2, and t3 are the time of arrivals measured by the receiver respect to the three transmitted signals; [𝑥1, 𝑦1], [𝑥2, 𝑦2 ], and [𝑥3, 𝑦3] are the known transmitter positions; and [𝑥𝑟 , 𝑦𝑟 ] is the unknown receiver position. the solution to this system of equations [𝑥𝑟 , 𝑦𝑟 ] can be represented as the unique intersection point of three hyperbolic curves defined according to the toa measurements and the known transmitter positions. in both cases (i.e., toa and tdoa), each iot node requires additional hardware to guarantee synchronization between the transmitter and the receiver; otherwise, a small timing error may result in a significant distance estimation error. for this reason, such methods exhibit a higher power consumption than the rss-based ones. on the other hand, they can be adopted for measuring the position of an iot node for a higher distance range than the rss-based one. thus, they are used in the case of iot systems that adopt long-range communication protocols, such as long-range wide area network (lorawan), the longterm evolution (lte) standard, narrow-band iot (nb-iot), and so on [48]. tdoa-based methods have the following advantages over toa-based methods [49]: (i) they do not require the measurement of the global time when the signals are transmitted, (ii) the clock bias error of the receiver is mitigated, and (iii) the time delays due to the atmospheric effects are reduced. on the other hand, the measurements provided by tdoa-based methods are subject to higher uncertainty than the toa-based ones as a result of a time-subtraction operation [49]. an example of the implementation of a localization method based on tdoa for iot applications is reported in [50]. the tdoa-based method was applied to a low-power lorawan, where the current consumption of the receiver placed on the iot node (i.e., 14.2 ma) was lower than the current absorption of a common gps receiver (i.e., 50 ma). the iot node consists of a lorawan module embedded on a waspmote board based on the atmel atmega1281 microcontroller. four lorawan gateways with known locations were placed in a surveyed area of around 10 km2. the iot node measures the toa of the signals transmitted by the gateways and sends them to the gateway via lorawan. the gateway communicates to the server over the internet the received toas, and then the iot node localization algorithm is performed on the server side. the server performs non-iterative and iterative multilateration algorithms for estimating the position of the iot node. in the paper, the researcher showed the results obtained by both algorithms by comparing them with the measurements obtained using a gps receiver. in the case of the non-iterative algorithm, the maximum obtained discrepancy between the measured position and the gps measurement was 206 m. in the case of the iterative algorithm, this maximum discrepancy was 127 m. 6. precise synchronization protocols the data exchange layer of an iot system is based on the intercommunication of a huge number of devices. this is challenging in terms of scalability, sustainability, and improving efficiency [51]. figure 10. classification of the iot synchronization protocols [51]. acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 92 a critical aspect that must be considered in the design of the data exchange layer, especially in the case of battery-powered and real-time iot systems, is to assure low-latency [52] and highenergy efficiency. to this end, one major challenge is to ensure frame synchronization for avoiding collisions and for assuring deterministic latency. as reported in [51], a synchronization solution for an iot system must comply with the following features: (i) network interoperability, i.e., the synchronization massages should be accessible to all the iot nodes and for a wide range of radio transceiver technologies; (ii) efficiency, as the synchronization overhead has to be minimized as much as possible; and (iii) sustainability, as the synchronization has to guarantee low power consumption and low latency. as depicted in figure 10, the synchronization protocols available in the relevant literature can be categorized as [51] (i) reference-based synchronization protocols, where one device provides a reference signal and all the other devices synchronize their own clocks based on the received event and (ii) distributed synchronization protocols, where all the devices cooperate with each othe by sharing timing information to reach a common time reference. an example of a reference-based synchronization protocol is the flooding time synchronization protocol (ftsp) [53]. in the ftsp, a master node periodically broadcasts messages for the time synchronization of the iot nodes [53]. each broadcasted message contains the master node’s timestamp, which is the estimated global time at the transmission of a given byte. this timestamp does not include the time that is required for communication channel access. the slave iot node, receiving the message, obtains the corresponding local time from its local clock upon receipt of the message reception. the difference between the global and local time gives an estimation of the clock offset of the receiver with respect to the sender. by receiving several timestamps and by averaging the estimated differences, it is possible to achieve a one-hop synchronization accuracy of about 1.5 µs [53]. the virtual high-resolution time (vht) protocol improves the accuracy and energy performance of the ftsp [54] by considering a combination of two clocks: (i) a low-frequency, low-power clock for keeping the devices synchronized and (ii) a high-frequency, high-power clock used for high-resolution timestamping. the low-frequency clock is kept synchronized according to the timestamps exchanged between the iot nodes. on the other hand, the high-frequency clock is activated only when high-resolution timestamps are needed. in this way, it is possible to reduce the power consumption due to the exchange of the timestamps for the synchronization. as reported in [54], the mean synchronization accuracy is 0.125 µs when the highfrequency clock is used. an example of the distributed synchronization protocol is the reference broadcast synchronization (rbs) [55]. in the rbs, a broadcast reference signal is sent by a device. the reference signal does not contain timestamp information. when the iot nodes receive the broadcast signal, they calculate their local timestamps and by exchanging these values, they evaluate the relative clock offsets and compensate for them. in [55], the researchers evaluate the performance of the rbs applied to the ieee 802.11 and they obtained a synchronization accuracy in the order of 10 µs. the ieee 1588 precision time protocol (ptp) is a protocol standard for precise clock synchronization in local area networks (lans) [57]. the ptp standard defines, for each communication, a master clock device, while the others are slave clocks synchronized to it. the master clock sends a synchronization message containing information about the clock and the transmission timestamp. the clock information contains the identification and accuracy of the master clock. if the master clock exhibits an accuracy that is acceptable for the slave, the slave sends a delay request message and stores its transmission time with a timestamp. when the master clock receives the message, it sends a delay response message containing the reception timestamp. the slave calculates the master-to-slave delay according to the timestamps. this delay is used for synchronizing the slave clock according to the master one. an implementation of this standard on an excalibur chip with arm9, a programmable logic device (pld), and a medium access control (mac)-less radio transceiver compatible with the ieee 802.11b standard is presented in [57]. the researchers obtained a synchronization accuracy of 1.1 ns. a comparison of the performance of the abovementioned protocols is reported in table 1. in all of these protocols, the deterministic latency of the network is estimated, and then the clocks of the devices are synchronized according to it. thus, the protocol is designed to minimize the effect of non-deterministic latency in the network, which directly affects synchronization accuracy. according to [55] and [56], the sources of deterministic and non-deterministic latency in a network are: (i) the send time, the time spent at the sender constructing the message, (ii) the access time, the delay incurred in accessing the transmission channel, (iii) the propagation time, the time delay needed to transfer the message from the transmitter to the receiver, and (iv) the receiving time, the processing time required by the receiver to read the message from the channel and notify the host about it. table 1. performance comparison of the analyzed synchronization protocols. synchronization protocol accuracy advantages disadvantages flooding time synchronization protocol (ftsp) 1.5 µs it requires only a single master synchronization clock. low-energy performance. virtual high-resolution time (vht) 0.125 µ high-energy performance. it requires two master synchronization clocks, one at a high frequency and one at a low frequency. reference broadcast synchronization (rbs) 10 µs the synchronization procedure does not depend solely on a single master node. the nodes exchange their timestamps with each other for estimating clock errors. ieee 1588 precision time protocol (ptp) 1.1 ns high accuracy; the master node changes in each communication step. in each communication step, the master node must send the accuracy of its clock at all the slave nodes. acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 93 usually, the propagation time is neglected in the clock offset estimation. on the other hand, the send and access times are dependent on the instantaneous load on the sender processing unit and on the state of the network, respectively. this makes them the most non-deterministic and difficult-to-estimate parameters. thus, all the synchronization protocols act to minimize the effects of these two time delays. for this reason, as reported in [54], the protocols act at the mac layer. an important figure of merit to evaluate the performance of a synchronization protocol is the time interval required to synchronize all the iot nodes after a topology change. in [58] and [59], the researchers propose a method for minimizing the time interval required for a synchronization; thereafter, the non-synchronized devices perturb an already synchronized iot network. the method was applied to zigbee-based iot nodes. by considering the repetition period for sending the synchronization messages among the iot nodes of 10 s and a percentage of non-synchronized devices of 5 %, the 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[59] f. lamonaca, d. grimaldi, “synchronization for hot-plugging node in wireless sensor network”, proc. of the 2013 ieee international workshop on measurements and networking, oct. 7-8, 2013, naples, italy, pp. 13-18. acta imeko  september 2014, volume 3, number 3, 9 – 14  www.imeko.org    acta imeko | www.imeko.org  september 2014 | volume 3 | number 3 | 9  an approach to determining the brinell hardness indentation  diameter based on contact position   li ma 1, 2 , samuel low 1 , john song 2   1  national institute of standards and technology, 100 bureau drive, gaithersburg, md 20899‐8553,  usa  2  kent state university, kent, oh 44242, usa      section: research paper  keywords: brinell hardness test; indentation contact radius; finite element analysis (fea); pile‐up/sink‐in  citation: li ma, samuel low, john song, an approach to determining the brinell hardness indentation diameter based on contact position, acta imeko,  vol. 3, no. 3, article 4, september 2014, identifier: imeko‐acta‐03 (2014)‐03‐04  editor: paolo carbone, university of perugia   received february 5 th , 2013; in final form april 22 nd , 2014; published september 2014  copyright: © 2014 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: (none reported)  corresponding author: li ma, e‐mail: li.ma@nist.gov    1. introduction  although the brinell hardness test has been widely used by industry for quality control and acceptance testing of metallic materials and products since it was proposed in 1900 by a swedish researcher j. a. brinell [1], significant measurement differences have been continually observed worldwide, even among the national metrology institutes that calibrate test block reference standards. these differences increase as they propagate down to the industrial level. the main cause of this problem is that the edge of the indentation is not a distinct boundary, but is instead a curved surface, from either material piling up (pile-up) or sinking in (sink-in), caused by plastic flow of the material surrounding the ball indenter. this makes it difficult to clearly resolve the edge of the indentation and thus to determine the indentation diameter from optical microscope measurement. the phenomenon of pile-up or sink-in in the indentation process is known to significantly influence the contact area and indentation contact diameter. early experimental studies of surface deformation around a spherical indenter found that the amount of pile-up and sink-in was related to the strainhardening exponent of the materials [2-3] and these were expressed as functional relationships [4]. from finite element analysis (fea) simulations, the pile-up/sink-in is found to be related, not only to strain hardening n [5-6], but also to yield strain (the ratio of yield strength y to elastic modulus e), indentation ratio (indentation depth h over indenter radius r) [7-8] and contact friction [9-10]. we investigated the brinell indentation contact diameter using confocal microscope measurements and fea modelling in our former research [13]. this paper continues our formal research to determine the most appropriate definition of the diameter of a brinell indentation that also allows its unambiguous measurement in the unloaded condition. fea models were developed to study the location of contact boundaries at the edges of brinell hardness indention crosssectional profiles. from the fea models, the location of the contact boundary under load is determined and is then tracked after removing the load. the indention profile shapes from fea models were confirmed by examining actual brinell indentations. from the fea model parameter study results, we developed a new method to effectively determine the abstract  significant differences in brinell hardness results have been observed worldwide, largely due to the curved surface at the indentation  edge making it difficult to measure its diameter. the indenter/material contact boundary under the test force should be the basis for  the brinell indentation diameter; however, the contact boundary cannot be observed using an optical microscope after the indenter is  removed  as  is  required  by  the test  methods.  finite  element  analysis  (fea)  models  were  used  to  develop  a method  to  effectively  determine  the  location  of  the  indentation  contact  boundary  after  unloading,  allowing  the  indentation  diameter  to  be  physically  defined and measured.  acta imeko | www.imeko.org  september 2014 | volume 3 | number 3 | 10  indentation contact boundary after removing the test force and that can be applied to experimental measurements. 2. brinell hardness tests  brinell hardness tests were made by a secondary calibration laboratory in accordance with astm e10 [14] using a calibration brinell hardness machine. the nominal brinell hardness value for the test material was 503 hbw 10/3000. the test material was indented using a standard 10 mm diameter tungsten carbide ball indenter and an applied force of 29.42 kn (3000 kgf ). the indentation diameters were measured automatically using an image analysing system by the laboratory with a reported expanded uncertainty of better than 3 m with a confidence level of 95%. the brinell hardness number, hbw, can be calculated [14] as: )( 2 102.0 22 dddd f hbw    (1) where f is the test force in newton, d is the diameter of the ball indenter in millimeter, and d is the measured mean diameter of the indentation in millimeter. 3. brinell indentation profile measurement  in order to determine the actual contact diameter, it was necessary to obtain the cross-sectional profile of the indentation. this was accomplished using a contact stylus profilometer. the instrument’s lateral resolution is 0.125 µm and the vertical resolution is 0.0008 µm. the nominal radius of the diamond tip is 2 µm. the instrument’s auto-crown function can automatically align the diamond tip on the central point (the bottom or top point of the spherical surface), so that several parallel measurement sections can be determined around this point. at each measurement section, the diamond stylus traces on the surface with a traversing speed of 0.25 mm/s. the digitized profiles at different sections are collected for analyses. 4. finite element modeling   modeling of the brinell spherical indentation process was performed using a commercial fea package. taking advantage of axis-symmetry of the ball indenter and the test material, only cross sections of the ball and test material were modeled. the ball indenter was modeled as rigid. the mesh of the test material used axi-symmetric, four-node bilinear elements with the minimum element size of the specimen being 0.2 m, which is close to the lateral resolution from the profilometer indentation profile measurements as described above. the test material was modeled as having isotropic elastic-plastic behavior with power-law hardening of the normal form  = kn following the von-mises yield criterion and associative j2 flow theory. the linear elastic behavior uses young’s modulus e and poisson’s ratio = 0.3. from the fea modeling, the pile-up/sink-in behavior was systematically studied by adjusting various parameters including the material’s strain hardening exponent n and the ratio of young’s modulus to yield strength e/y. 5. results and discussion  5.1. indentation edge contact point from fea modeling   theoretically, the brinell hardness value should be the ratio of the indentation force to the surface area of indentation. according to international brinell hardness test method standards, the surface area of indentation is determined by measuring the diameter d of the resulting indentation after removal of the indentation force [14-15]. ideally, the surface area of indentation should be the contact area while under the indentation force. as in-situ measurement of indentation diameter is not practical, the diameter is measured after removing the indenter. however, the indentation shape changes after removing the indentation force, primarily because of the elastic recovery of the test material. from fea modeling, the indentation edge contact node point was determined at the maximum indentation force from model contact output and identified by the deformation geometry mesh. the corresponding mesh-node number and coordinate were identified, and by tracking the mesh contact node number, the position of the indentation edge contact point after removing the test force was determined. 5.2. indentation contact pressure and surface stress distribution  from fea model  when an indentation force is applied to a spherical indenter, an indentation contact pressure is induced on the contact surface while no contact pressure is induced on the surface not in contact with the indenter. figure 1 plots the normalized surface contact pressure, and the radial and circumferential stresses for typical pile-up and sink-in cases without friction. the x-axis is the indentation profile radial coordinate r normalized by the contact radius ac. the y-axes are the (a) contact pressure p(r), (b) radial stress r(r) and (c) circumferential stress (r) along the indentation profile normalized with respect to the contact pressure at the indentation center p(0). it can be seen from figure 1(a) that the contact pressure increases from p(0) at the center of the indentation (r = 0) to the maximum p(r) at r/ac  0.9 and then smoothly drops to the normalized pressure of 0.8 at the normalized radius near the contact edge for the sink-in case; while, for the pile-up case, the contact pressure is maximized at the center, smoothly decreases along the contact radius until r/ac  0.95, then increases slightly and drops again. for both cases, the contact pressure around the contact edge shows a quick sharp drop from a normalized value of over 0.6 to zero. as both cases are modeled as having a frictionless interface between the indenter and specimen, the contact pressure is one of the principal stresses on the surface. the remaining two stress components of radial (see figure 1(b) and circumferential (see figure 1(c)) stress exhibited a sharp transition at the contact edge region as well. within the contact area, both radial and circumferential stresses are compressive (lower than zero). outside the contact region, the radial stress shifts to tension for the sink-in case while remaining as compression after a sharp increment at the contact edge for the pile-up case (see figure 1(b)); the circumferential stresses showed tension for the sink-in case while changing from compression to tension and back to compression again at the contact edge for the pile-up case (see figure 1(c)). acta imeko | www.imeko.org  september 2014 | volume 3 | number 3 | 11  5.3. indentation edge contact characteristics  from figure 1, it can be seen that all of the surface pressure and stresses exhibit sharp changes at the contact edge. those sharp changes should generate corresponding sharp deformation changes between the contact and non-contact regions at the edge of the indentation profile. therefore, a sharp deformation change should occur at the contact edge. we define the slope angle  along the indentation profile as           r z r 1tan)( (2) where r and z are the indentation profile radial coordinate and depth coordinate, respectively. for a digitized indentation profile, the slope angle can be calculated from point to point on the indentation profile as            11 111tan ii ii i rr zz  (3) where i represents the data point number. we also defined the change of slope angle, or slope rate, ’, as r r      )(' (4) which can be digitalized as 11 11'      ii ii i rr   . (5) the types of materials for which pile-up or sink-in occurs have been examined by finite element simulation by analyzing load and depth sensing indentation data. in general, pile-up is greatest in materials with large e/y and little or no capacity for work hardening (i.e., “soft” metals that have been cold-worked prior to indentation). the ability to work-harden inhibits pileup because as material at the surface adjacent to the indenter hardens during deformation, it constrains the upward flow of material to the surface. sink-in predominates for materials with a work hardening value of n = 0.5. the cross-over from sink-in to pile-up should occur at n = 0.22 [5]. the ratio of the elastic modulus to yield stress, e/y, represents the relative amounts of elastic and plastic deformation. this parameter physically represents the reciprocal of the elastic strain at yielding and can therefore be used as a measure of the amount of deformation that is accommodated elastically during indentation. in the limit e/y = 0, contact is strictly elastic and dominated by sink-in from hertzian contact theory. at the other extreme, the limit e/y =  corresponds to rigid-plastic deformation or n = 0, for which there is extensive pile-up of material around the hardness impression. in this research, we selected strain hardening values of n = 0, 0.22 and 0.5 for studying the pile-up, intermediate and sink-in cases. figure 2 shows the normalized indentation profile z/h (figure 2(a) and figure 2(b)), its corresponding slope angle  (figure 2(c) and figure 2(d)) and the slope angle rate ’ profiles (figure 2(e) and figure 2(f)) as a function of relative radius r/a (a is the surface contact radius at the maximum test force) while under the test force (left column) and after removing the test force (right column) for the pile-up (n = 0) and sink-in (n = 0.5) cases and an additional intermediate case (n = 0.22) from fea modeling. in all the cases, e/y is 200 and the normalized indentation depth h/r is 0.2 at the maximum force. the crosses “×” in the figures indicate the indentation edge contact position. it can be seen that a typical curvature transition occurs at the contact edge region of the indentation profile while under the test force (see figure 2(a)) and after removing the test force (see figure 2(b)). however this transition point is not easily identified from the indentation profile. when we look at the slope angle profiles, the transition area stands out as a sharp slope angle change. under the test force as shown in figure 2(c), the slope angle increases from zero at the center of the indentation to the maximum near the contact edge and then drops sharply. the contact edge coincides with the highest slope angle drop. after removing the load, due to the elastic recovery which is unevenly distributed along the radial direction, the slope angle shows an increase curve to maximum and drop again. the contact edge point is still at the maximum slope angle change. to make the contact edge position clearer, we plotted the slope rate profiles as shown in figure 2(e) and figure 2(f). the slope rate maintains almost no change across the entire indentation cross-section except an obvious narrow downward spike at the contact boundary region. the contact boundary was determined to be at the negative peak of the slope rate function both under the test force and after removing the test force. 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 pile‐up (n = 0) r/ac p (r )/ p (0 ) e/y=200, =0 h/r=0.2 sink‐in (n = 0.5) (a) ‐1.0 ‐0.9 ‐0.8 ‐0.7 ‐0.6 ‐0.5 ‐0.4 ‐0.3 ‐0.2 ‐0.1 0.0 0.1 0.2 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 pile‐up (n = 0) r/ac  r( r) /p (0 ) e/y=200, =0 h/r=0.2 sink‐in (n = 0.5) (b) ‐0.8 ‐0.7 ‐0.6 ‐0.5 ‐0.4 ‐0.3 ‐0.2 ‐0.1 0.0 0.1 0.2 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 pile‐up (n = 0) r/ac  ( r) /p (0 ) e/y=200, =0 h/r=0.2 sink‐in (n = 0.5) (c) figure  1.  normalized  surface  contact  pressure  (a),  radial  stress  (b)  and circumferential stress (c) distributions for pile‐up and sink‐in cases.  acta imeko | www.imeko.org  september 2014 | volume 3 | number 3 | 12  figure 3 shows the normalized contact diameter difference between the loaded and unloaded conditions as a function of modulus yield stress ratio (e/y) for the pile-up/sink-in and intermediate cases under a normalized indentation depth of h/r = 0.2. it can be seen that the contact diameters enlarged after removing the indentation force because of the elastic recovery of the indented materials. since the smaller e/y represents a larger reciprocal of the elastic strain after plastic deformation, the increase in contact radius is larger on the e/y = 20 materials. as higher strain hardening contributes to a larger elastic strain ratio, there is a larger increase in the contact radius for higher strain hardening materials. the incremental increase in contact diameter 2ac relative to the surface contact radius 2a, after removing the indentation force, decreases from 3.2% to 0.77% for the high strain hardening materials (n = 0.5) and decreases from 2.8% to 0.1% for the materials without strain hardening (n = 0) when e/y increases from 20 to 1000. 0.000 0.005 0.010 0.015 0.020 0.025 0.030 0.035 10 100 1000 n = 0.5 n = 0.22 n = 0 e/ y n o rm a li ze d  c o n ta ct  d ia m e te r  d if fe re n ce 2 (a c_ u n lo a d e d ‐ a c_ lo a d e d )/ a figure 3. normalized contact diameter difference between the loaded and unloaded conditions as a function of modulus yield stress ratio (e/y) for the  materials  with  strain  hardening  n  of  0,  0.22  and  0.5  under  a  normalized  indentation depth of h/r = 0.2.    ‐1.0 ‐0.9 ‐0.8 ‐0.7 ‐0.6 ‐0.5 ‐0.4 ‐0.3 ‐0.2 ‐0.1 0.0 0.1 0.2 0.3 0.4 0 0.5 1 1.5 loaded r/a z/ h n = 0 n = 0.22 n=0.5 e/y = 200, = 0, h/r = 0.2 (a) ‐1.0 ‐0.9 ‐0.8 ‐0.7 ‐0.6 ‐0.5 ‐0.4 ‐0.3 ‐0.2 ‐0.1 0.0 0.1 0.2 0.3 0.4 0 0.5 1 1.5 unloaded r/a z/ h n = 0 n = 0.22 n = 0.5 e/y = 200, = 0, h/r = 0.2 (b) ‐20 ‐15 ‐10 ‐5 0 5 10 15 20 25 30 35 40 45 0 0.5 1 1.5 r/a  (d e g .) n = 0 n = 0.22 n = 0.5 e/y = 200, = 0, h/r = 0.2 (c) ‐20 ‐15 ‐10 ‐5 0 5 10 15 20 25 30 35 40 45 0 0.5 1 1.5 r/a  (d e g .) n = 0 n = 0.22 n = 0.5 e/y = 200,  = 0, h/r = 0.2 (d) ‐160 ‐140 ‐120 ‐100 ‐80 ‐60 ‐40 ‐20 0 20 0.8 0.9 1 1.1 1.2 r/a ' n = 0 n = 0.22 n = 0.5 e/y = 200, = 0, h/r = 0.2 (e) ‐160 ‐140 ‐120 ‐100 ‐80 ‐60 ‐40 ‐20 0 20 0.8 0.9 1 1.1 1.2 r/a ' n = 0 n = 0.22 n = 0.5 e/y = 200, = 0, h/r = 0.2 (f) figure 2. the indentation profile (a and b), its corresponding slope angle profile (c and d) and slope rate profile (e and f) on the test load and after removing  the load for the pile‐up (n = 0), sunk‐in (n = 0.5) in figure 1 and an additional intermediate (n = 0.22) cases.  acta imeko | www.imeko.org  september 2014 | volume 3 | number 3 | 13  5.4. a new method to determine the contact diameter from direct  measurement  to verify that the above technique can be applied to real brinell indentations, indentation experiments were done. the characteristics of the indentation profiles, the slope angle and slope angle rate profiles as measured with the profilometer display the same characteristics as the fea model although with considerable more noise on the slope angle and slope rate profile results due to surface roughness. from the above fea study in section 5.3, it can be seen that the trend in the position of the ball indentation contact boundary is consistent as the maximum slope angle change or the negative peak of slope rate ’ profile for various indentation materials and conditions. this is most obvious on the slope rate profile. therefore, we suggest a new method to determine the contact diameter from direct measurement of the surface profile of the diametrical cross-section of the brinell indentation. the slope angle  and slope rate ’ profiles are calculated from the indentation profile using eq.3 and 5 from which the indentation contact boundary position may be determined from the negative peak of the slope rate. the indentation diameter based on the contact boundary positions can then be calculated. applying this new method, we measured several experimental brinell indentation profiles and calculated diameters using this technique. figure 4 demonstrates an experimental indentation made in a 503 hbw 10/3000 material figure 4. (a) experimental indentation profile (black), its slope angle (red), filtered slope angle (green) and filtered change of slope angle profile (purple) with  the fea and optical microscope measured position; (b) left hand side detail and (c) right hand side detail of (a).  acta imeko | www.imeko.org  september 2014 | volume 3 | number 3 | 14  using a 10 mm diameter ball and 29,420 n (3000 kgf) applied force. the indentation profile was measured using a stylus profilometer instrument. its slope angle profile was calculated by eq. 3. to minimize the effect of measurement noise due to surface roughness of the indented material on the slope angle and slope rate determinations, a gaussian filter with an 8 m short cutoff was applied to the slope angle profile. the slope rate profile is based on the filtered slope angle profile. the indentation contact diameter position was chosen using our new method as shown in figure 4 dashed line. meanwhile, the indentation diameter was measured using the optical measuring system by a secondary calibration laboratory. the indentation boundary location was picked from the actual measured diameter value, which is also indicated in figure 4 as a dotted line. it can be seen that the locations of the edge of the indentation measured using our suggested method and using the optical measuring microscope are not the same. the optical measuring microscope measured the indentation diameter about 7 m smaller than the actual contact diameter, corresponding to a 2.6 hbw difference. 6. conclusions  the brinell indentation diameter was investigated using fea modeling of indentations on various indentation pile-up and sink-in conditions, including materials’ strain hardening and the ratio of young’s modulus to yield stress. the surface contact pressure and stress distribution exhibit sharp changes at the contact region. similarly, the indentation slope angle and slope rate profile show sharp changes at the contact region for various materials. for both the loaded and unloaded conditions, the contact edge point is always at the maximum slope angle change or the negative peak of the slope rate profile. the contact diameters enlarged after removing the indentation force because of the elastic recovery of the indented materials. the contact diameter increment after removing the indentation load is larger for materials with higher strain hardening and lower ratio of young’s modulus to yield stress under deeper indentation with no friction. a new method is proposed to determine the edge of a brinell indentation from surface profile measurements. fea modeling has shown that the indentation contact boundary can be determined after unloading from the negative peak of the slope rate profile. by applying this method to real brinell indentations, this method provides an effective way for the national metrology institutes to obtain the indentation diameter for various materials and indentation conditions without the biases of optical measurements and an undefined indentation edge. this should improve the agreement on the calibrated brinell indentation reference among the national metrology institutes, and therefore increase the agreement propagated down to the users. acknowledgement  the authors are grateful to brian renegar for the valuable profilometer scan indentation measurements. references  [1] h. chardler, hardness testing. asm international, 1999. [2] d. tabor, hardness of metals: oxford: clarendon press, 1951. [3] h. o'neill, hardness measurement of metals and alloys: chapman and hall ltd., 11 new fetter lane, london ec4., 1967. [4] j.r. matthews, indentation hardness and hot pressing, acta metallurgica 28 (1979) pp. 311-318. [5] r. hill, b. storakers, a.b. zdunek, a theoretical study of the brinell hardness test, proceedings of the royal society of london. series a, mathematical and physical sciences 423 (1989) pp.301-330. [6] j, alcala, a.c. barone, m. anglada, the influence of plastic hardening on surface deformation modes around vickers and spherical indents, acta materialia 48 (2000) pp.451-3464. [7] k, ai, l. dai, numerical study of pile-up in bulk metallic glass during spherical indentation, science in china series g: physics mechanics and astronomy 51 (2008) pp.379-386. [8] s.h. kim, b.w. lee, y. choi, d. kwon, quantitative determination of contact depth during spherical indentation of metallic materials a fem study, materials science and engineering a-structural materials properties microstructure and processing 415 (2006) pp.59-65. [9] y.t. cheng, c.m. cheng, effects of 'sinking in' and 'piling up' on estimating the contact area under load in indentation, philosophical magazine letters 78 (1998) pp.115-120. [10] h. habbab, b.g. mellor, s. syngellakis, post-yield characterisation of metals with significant pile-up through spherical indentations, acta materialia 54 (2006) pp.1965-1973. [11] g. barbato and s. desogus, problems in the measurement of vickers and brinell undentations, measurement 4 (1986) pp.137-147. [12] a. germak, k. herrmann, and s. low, traceability in hardness measurements: from the definition to industry, metrologia, 47 (2010) s59-s66. [13] l. ma, s. low, and j. song, "investigation of brinell indentation diameter from confocal microscope measurement and fea modeling”, in proc. of hardmeko 2007, nov. 19-21, 2007, tsukuba, japan. [14] standard test method for brinell hardness of metallic materials. [e10-01]. american society of testing and methods, 2001. [15] iso 6506-1:2005, metallic materials-brinell hardness test – part 1: test method. geneve, switzerland: international organization for standardization, 2005. [16] b. taljat, g.m. pharr, development of pile-up during spherical indentation of elastic-plastic solids, international journal of solids and structures 41 (2004) pp.3891-3904. digital heritage dissemination and the participatory storytelling project #izitravelsicilia: the case of the archaeological museum of syracuse (italy) acta imeko issn: 2221-870x october 2018, volume 7, number 3, 31 41 acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 31 digital heritage dissemination and the participatory storytelling project #izitravelsicilia: the case of the archaeological museum of syracuse (italy) elisa bonacini1, davide tanasi1, paolo trapani2 1 department of history, institute for digital exploration (idex), university of south florida. usa 2 independent researcher section: research paper keywords: crowdsourcing, 3d scanning, photo scanning, sicily, digital storytelling citation: elisa bonacini, davide tanasi, paolo trapani, digital heritage dissemination and the participatory storytelling project #izitravelsicilia: the case of the archaeological museum of syracuse (italy), acta imeko, vol. 7, no. 3, article 7, month 2018, identifier: imeko-acta-07 (2018)-03-07 section editor: egidio de benedetto, university of salento, italy received april 6, 2018; in final form september 5, 2018; published october 2018 copyright: © 2018 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: davide tanasi, dtanasi@usf.edu 1. introduction participatory and crowdsourcing approaches in the domain of cultural heritage have become a powerful trend through technological applications [1], [2] thanks to new languages that have developed through the enormous success of social media. these new languages have encouraged the realization and sharing of users’ digital content [3], collaborating in “the making of meaning and construction of memories” [4]. digital technologies, with the potential to offer a great deal of information and production quality, have a real innovative role both in the cultural and touristic domains [5]-[7]. scholars agree on the importance of the role of digital storytelling [8]-[11] and participatory and crowdsourcing community-based initiatives in enhancing local cultural and touristic attractions, as they offer audiences more freedom and information, especially with reference to context and memorable experiences [12]-[14]. as discussed elsewhere [15], participatory platforms such as izi.travel, are gaining an important role in supporting local institutions’ ability to enhance their own heritage and to involve users in cultural, touristic, and territorial marketing and promotion, by sharing textual and visual information that are considered authentic and trustworthy by consumers. such process stimulates and supports tourists in planning their experience in a certain region and enjoying cultural and artistic attractions during their stay [16], [17]. in this study, we explore the izi.travel web platform and app, as used in a pilot project in sicily, which has made the users’ abstract the aim of this paper is to demonstrate how the participatory and crowdsourcing project #izitravelsicilia could be a key to the dissemination of 3d models on cultural heritage. this project has been recognized as a best practice both in territorial digital marketing and in digital promotion and valorisation through storytelling and crowdsourcing culture. #izitravelsicilia has already involved thousands of people, producing more than 180 museum and tour audio guides. it has become a real model of participation in the cocreation of cultural values. in this paper we briefly present the project and the case of the archaeological museum ‘paolo orsi’ in syracuse (italy), as best practice on digital dissemination through platforms such as google street view and izi.travel itself, revealing how this platform has been already largely used for the dissemination both of existing 3d models and for new 3d models, such as those made by usf idex for the ‘paolo orsi’ museum. specifically, a collection of 26 ancient sculptures and artefacts has been virtualized via laser scanning and digital photogrammetry in a participatory experience of measurement science. the global dissemination of those 3d models through izi.travel’s platform and other alternatives platforms will represent a significant contribute to the digital accessibility of the museum of syracuse but also the starting point of a metrological 3d database of ancient sicilian sculpture generated entirely crowdsourcing. mailto:dtanasi@usf.edu https://izi.travel/en acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 32 experience with sicilian cultural heritage better overall through storytelling by native people, such as students, teachers, local experts, museum managers and scholars. by linking the storytelling approach to other digital sources already published online, such as virtual tours and 3d models, the platform lets users have a deeper understanding of some artefacts and monuments. this paper presents the case of the project known as #izitravelsicilia on social media, launched in may 2016 with the primary goal of filling the great gap in sicilian cultural heritage and tourist attractions’ use of digital communication and development. this participatory process has been able to involve, in two years, more than 3.200 people, from students to museum directors, transforming them into co-creators of more than 180 audio guides, thus promoting their heritage by themselves with the consequent reinforcement of the sense of belonging and local identity. among these 183 audio guides, the archaeological museum ‘paolo orsi’s audio guide is one of the more complex and information heavy, with 135 schedules organized in 9 collections. the whole museum is browsable via a specific virtual tour launched in 2015 as a pilot project on google maps as well as a 360-degree tour of a group of selected objects, while the most important sculptures are enhanced through 3d models linked to a specific collection on sketchfab. all these elements can play an important role by making the museum into a well-known and enjoyable cultural and tourist attraction [16]. writers agree that visual digital contents, such as 3d reconstruction, augmented reality, virtual reality, docu-films, fiction, cartoons and so on, allow story-centric narratives to enhance users’ understanding of both cultural heritage and historical processes that can build on the “human experience” [18]. after a brief introduction on the platforms involved in this interdisciplinary project (google street view, izi.travel and sketchfab), the authors will deepen the methods and approach used. 2. google maps and google street view google maps and its software street view are the most important tools google uses to map the world, improving both the way people use maps and scientific research using satellite imagery, such as geographic studies at a global scale on environmental changes and natural disasters [19]-[21]. google maps tools have become the fastest and most publicly available tool for archaeological researchers to acquire vertical perspectives on archaeological landscapes and to locate and identify ancient sites or reconstruct 3d models on earth [22][24]. google, of course, did not invent anything in the heritage field to provide immersive online panoramas with hyperlinks, but the novelty lies in having taken advantage of linking these technologies to the maps [25]. furthermore, there is a growing demand for realistic 3d models which could enable increased understanding and popularization of historical-artistic artefacts [26]: the quality of the 3d reconstructions of environments, monuments, museums galleries and objects has now reached very high levels of precision and many solutions are now available to create 3d models from panoramic images according to fast and low cost methods [27], [28]. 3. izi.travel: a platform for free audioguides the izi.travel platform (figure 1), founded in amsterdam in 2011 and launched in 2013, with over 3 million uses per month and an annual growth of 100%, has become the most popular free platform based on creativity and storytelling in the world for creating museum audio guides and itineraries [15]. this platform could also be considered as a digital key tool not only for on-site visits but also for planning itineraries and further studies. by operating on the izi.travel content management system interface (cms), both local institutions and users are encouraged to create and manage self-produced cultural digital content by telling their stories. everyone can freely enjoy every museum audio guides and city audio tours. a simple text can easily become multimedia, being available in audio mode, with photo galleries and videos (linked to youtube channels or uploaded by the content provider); it can also become hyper-textual, with the addition of links, making the visit of a museum or the discovery of a city more profound and enjoyable [5]-[7], [15]-[17]. by adding content to the map, the content provider can create georeferenced points of interest and trigger the area, which will be gps-activated. all the audio guides can be viewed remotely online and on mobile phones via the izi general app (available from app store, google play store and windows phone store) and by scanning the qr-codes in situ. the user interface is very simple: visitors can interact with the content over the whole story through navigation menus, preview images or, in the walking mode, directly through gps position. the app of a selected museum or tour can also be used in offline mode if downloaded via the download section of the general app. as shown in table 1 and in figure 2, izi.travel allows users to browse the worldwide audio guides in more than 2200 table 1. izi.travel web and app statistics (courtesy of izi.travel, october 2018). izi.travel web and app statistics destinations 2200 city tours 6800 museums 2200 stories 250.000 languages 54 countries 99 content providers (cp) registered 20.000 new cp registration per month 1000 app installs 3.200.000 monthly usage 4.500.000 monthly average usage app 135.000 monthly average usage web 140.000 yearly usage 30.000.000 figure 1. a screenshot of the izi.travel web-platform homepage. acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 33 museum guides, counting more than 250.000 stories, in 54 languages for a total of 99 countries. the success of this platform is certainly tied to the new awareness of museums and institutions about the need to adjust their language, including by experimenting with the use of digital storytelling. 4. sketchfab: a platform for 3d models the field of photogrammetry and computer vision techniques for the creation of 3d models of objects has been characterized in recent years by several innovations [29]: one of the most important among them is the exploitation of structure for motion (sfm) techniques [30]-[32]. the web is becoming the main channel of publication and dissemination of visual media, including 3d models, offering a high level of accuracy and ease of use. 3d models should be shared and make available to the public through comprehensive professional software or through specific online viewers [33]. sketchfab could be considered the most important dissemination platforms for 3d models. born in 2013, sketchfab is a global platform to publish, share and discover 3d, vr and ar content, which can also be purchased and sold. scholars consider sketchfab “the de-facto standard for publishing 3d content on the web” [34], with more than 2 million uploads. 5. research’s methods at the “paolo orsi” museum established with a royal decree as a national archaeological museum in 1878, the museum was first placed inside an historical palace on the island of ortigia and was directed by the famous archaeologist paolo orsi from 1895 to 1934. in 1988 the archaeological collection moved to a new building, in the garden of villa landolina, fittingly located in the part of the city including the neapolis park with its greek theatre, the altar of hieron, the roman amphitheatre and the area of the catacombs of san giovanni. the collection contains artefacts from the prehistoric, protohistoric, greek (sectors a, b and c on the first level), hellenistic, roman and christian periods (sector d and f; the e sector is about to open, with findings from sites in central-eastern sicily, such as centuripe, morgantina, tindari and so on). finally, the unique collection of coins and medals from the archaic to the medieval periods is located in the basement. with over 20.000 artefacts on display in about 9,000 m² of exhibition area, the archaeological museum ‘paolo orsi’ of syracuse is one of the most important and richest archaeological museums in italy. this paper aims to present three different approaches to the promotion and enhancement of the sicilian cultural heritage, which have been integrated on the izi.travel platform in order to offer users, through a single platform, digital content produced in recent years together with the “paolo orsi” museum in syracuse. the pilot project on google street view was designed by elisa bonacini. the #izitravelsicilia project was created and coordinated by elisa bonacini, while the 3d models project was realized by davide tanasi in collaboration with paolino trapani. 5.1. the ‘paolo orsi’ museum pilot project on google street view thanks to the agreement between the general direction of the regional department for cultural heritage and sicilian identity and the european coordination of google business photo, a pilot project on google street view was born in order to increase the realization and enhancement of sicilian cultural heritage by creating “augmented” virtual tours, through 360° virtual tours of objects displayed in museum cases and information provided by schedules [35]-[36]. the project, in the form of a street view virtual tour, was carried out in close collaboration with the staff of the museum. a large photographic survey undertaken by gianfranco guccione began in august 2014, with the aim of mapping all the areas of the first and second levels open to visitors. a total of 3.924 photographs to cover 327 360° virtual tours were taken using a mobile station consisting of a reflex camera with fisheye type camera lens on tripods with a panoramic head. the light in the different museum’s sectors required adjusting the brightness each time. in the winding path of the museum, windows would often reflect one another, and precautions were taken to avoid those refractions whenever possible. thanks to this project, the “paolo orsi” pilot project “is the first archaeological museum in the world needless to say, the first museum in sicily entirely browsable on google maps platforms with a virtual tour in all exhibition halls and 360° virtual tours with integration of captions and description of artworks” [35] (figure 3). the “paolo orsi” museum is entirely browsable in 360° in a traditional street view since august 2014, for the first floor and the second floor and are connected to each other by arrows. the novelty of the “paolo orsi” project consists in the fact that “the project provided the opportunity to carry out 360° virtual tours of some exposed archaeological finds, like an augmented virtual tour, certainly innovative compared to what has been previously seen on google’s platforms” [35]. the augmented virtual tour, where one can view all additional items (virtual tour of the objects, maps, captions, info etc.), is browsable. figure 3. a screenshot of the 360° virtual tour of one selected object from the “paolo orsi” museum virtual tour. figure 2. the audio guides dissemination on the izi.travel platform (courtesy of izi.travel, may 2018). https://sketchfab.com/ https://goo.gl/maps/oagnd8urp1h2 https://goo.gl/maps/vrpdfuppgwm2 http://www.regione.sicilia.it/beniculturali/museopaoloorsi/google-maps/museo-paolo-orsi-syracuse-primo-piano/index.php acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 34 5.2. #izi.travelsicilia: a participatory storytelling approach for cultural heritage dissemination a large-scale regional project was launched on the izi.travel platform on may 2016 thanks to an annual grant, financed by izi.travel itself, and to a general agreement between the regional department of cultural heritage and sicilian identity and the university of catania [37]. the project aimed to fill the gap in communication and cultural and touristic promotion of sicilian heritage and territories, under a single regional-level management, coordination and support. under this coordination, a network of storytellers was created, embracing the common idea to promote culture and territories through a bottom-up and disseminated process, involving many other regional and local authorities, institutions, cultural associations, schools, and universities (all the “heritage communities” cited by the faro convention) to guarantee the common right to cultural heritage [38]. the project was soon transformed into a participatory process, without distinction of prominence or importance of heritage sites, but with the common aim of enhancing them: at the beginning, only seven audio guides were available in the whole of sicily; now there are 183. digital storytelling has been the key focus of this project: in all the published audio guides, the recounting of history took place in a narrative mode. the storytelling approach adopted for #izitravelsicilia, well described elsewhere [37], aimed at explaining a monument, a place, a landscape or a work of art according to the concepts of knowledge, languages and creativity [39]. all the people (about 3,200) who were involved become “digital ciceroni”, or storytellers of their own heritage, ranging from elementary school children to high school and university students to the museum managers themselves. in some cases, the storytelling approach adopted for the project can be described as a sort of digital theatrical storytelling, where some storytellers interpreted stories and characters. one of the projects worth mentioning, for its high level of educational and inclusive content, is one of the three audio guides made for the archaeological park of selinunte and cave di cusa by the students of the comprehensive institute “lombardo radice pappalardo” in castelvetrano (trapani). students from all the primary school classes produced temples and metopes children’s guide (i templi e le metope guida per bambini), also published in english. in the meanwhile, all the secondary school classes worked on two audio tours: children from selinunte tell (i piccoli selinuntini raccontano) and little stonemasons tell the cusa quarries (i piccoli scalpellini raccontano le cave di cusa). the children, embodying greek children from selinunte, tell the cultural heritage of their town and the nearby cusa quarries from which they extracted the building material. students of 66 classes worked on this project, for about 900 children, followed by over 50 teachers. 66 children gave different voices to each board (in general, people who contributed to the creation of audio guides are always credited in the summaries and even in the boards, with their first and last name). among these voices, one belongs to a child suffering from selective mutism preventing her from speaking in the classroom. nevertheless, she succeeded in speaking at home pushed by her will to contribute to the project. the framework briefly provided here only summarizes the results so far achieved, described more deeply elsewhere [37]. thanks to this project, sicily has become one of the few regions in the world to experience such a great participatory, democratic, horizontal and co-creative process in such a figure 4. a screenshot of the “paolo orsi” museum audio guide on the izi.travel platform. coherent and coordinated way, with the only aim to promote and enhance shared cultural heritage. on the same platform, without distinction of ownership and size, small and large museums and different itineraries on the territory are all together described. so far, the collaboration with all the sicilian cultural heritage institutions has been extensive. in many cases, the narrative voices are those of the museum staff, who give their voice to tell the heritage they love and protect every day. one example is exactly that one of the “paolo orsi” museum audio guide (figure 4), where all the archaeologists working there introduce every section of the museum and every work of art, telling all of their underlying stories. as just said, the audio guide is organized into nine collections with 135 schedules inside them. this audio guide used the digital storytelling approach as a specific tool for the democratization of cultural content [9], [11], [39]-[40]. narratives of complex context, such as those of an archaeological museum, require complex work, made with museum staff, for extracting the universe of stories that the sites, phases, and objects themselves enclose. the archaeological museum’s staff became storytellers, always addressing the users, stimulating their curiosity, giving them questions which they soon answer, simplifying but not trivializing the concepts, the historical epochs, and the objects. they took for granted that users do not know anything about archaeological contexts, ancient colonies and prehistoric or historical phases: the multimedia audio guide can help them to understand and deepen their knowledge. 5.3. the dissemination of 3d reconstructions and virtual tours through #izi.travelsicilia the logic itself of the izi.travel multimedia platform favours ease of use and familiarity of approach, allowing the user to browse and view content simultaneously. contexts, monuments, works of art, characters and so on become themselves narrative, using visual and always evocative support, with the core idea being to fit hyper-texts with images, videos, and, in the case of archaeological contexts, reconstructive 3d videos or 3d models, such as in the case of “paolo orsi” audio guide that we are going to introduce. the real productive process involved a deep reworking of knowledge and scientific communication, choosing a descriptive, simple, and appealing style. the aim of this approach was “to https://izi.travel/it/7dc4-i-templi-e-le-metope-guida-per-bambini/it https://izi.travel/it/7dc4-i-templi-e-le-metope-guida-per-bambini/it https://izi.travel/it/02eb-i-piccoli-selinuntini-raccontano/it https://izi.travel/it/e39e-i-piccoli-scalpellini-raccontano-le-cave-di-cusa/it https://izi.travel/it/4d91-museo-archeologico-regionale-paolo-orsi/it acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 35 figure 5. a screenshot of the video on youtube with the polizzello 3d virtual reconstruction. find a solution how to engage visitors in a story by maintaining story-coherency and preserving an uninterrupted feeling of a story flow, while also adjusting the story to better match visitor characteristics and needs” [41] and, in addition, their different interests. an appealing and creative style was created, with the primary goal to make both the content and the stories as attractive as possible, while always respecting the relationship between creativity and reliability. the izi.travel platform has also been easily used to disseminate 3d content to better understand spatiality and to contribute to deepen storytelling of an archaeological context. virtual reconstructions of ancient monuments already on youtube have been used as visual insights for a better understanding of ancient architecture through 3d virtual reconstructions (e.g. in the case of the greek theatre and temples of syracuse, the greek-roman theatre and the amphitheatre of catania, the roman theatre of taormina, the ancient acropolis of the indigenous site of polizzello in central sicily (figure 5), or the temple of jupiter in agrigento, just to cite some examples) or 3d models of sites (such as 3d models of some rooms and details of the roman villa of durrueli, near agrigento: [42], [43] (figure 6). the izi.travel platform was also used to disseminate online virtual tours, such as that of the roman “achillean” baths in catania, those of some churches in piazza armerina, the google collection project on the agrigento valley of the temples and the google street view pilot project around the “paolo orsi” museum, which users can browse directly from the summary of the izi.travel audio guide. 5.4. the dissemination of 3d models: the “paolo orsi” museum in the specific case of the “paolo orsi” museum audio guide, a sketchfab collection of 3d models of selected artefacts were linked to their schedules. between 2010 and 2016, the academic staff of idex undertook an interdisciplinary educational program at the archaeological museum of syracuse in which numerous italian and american undergraduate students participated as part of onsite classes and internships. the program was fostered by university of catania’s archeomatica project, a pioneering collaborative research project on digital archaeology, which is still active. between 2010 and 2016, computer science undergraduate students from the department of mathematics and computer science for the university of catania participated in an internship on digital tools for cultural heritage at the museum of syracuse. subsequently, between 2014-2016 american undergraduate students from the college of global studies of arcadia university carried out a series of on-site classes and lab activities at the museum as part of the course “3d modelling for archaeology and cultural heritage”. this last activity was also part of the museum’s education program “progetto scuola-museo” in which italian students from local high schools were also involved. under the supervision of idex academic staff, throughout the six years of the program students employed several 3d techniques to virtualize 26 archaeological artefacts ideally representing the entire collection of the museum of syracuse from prehistory to the roman period, including nonclassical statues such as the sikel enthroned goddess from poggio dell’aquila and the kourotrophos of megara hyblaea and the egyptian statue of the scribe petamenofi. students were trained in the use of low cost 3d scanners and in digital photogrammetry and subsequently the use software for the processing of the 3d data. the project had both educational and research goals. with respect to education, digital technologies were used to connect students to ancient artefacts showing how profitable the study of such artefacts can be when producing 3d models and interacting with those digital replicas. the second goal was to test and compare three different and popular low-cost 3d scanners employing un-experienced users to make an assessment on uncertainty and repeatability. this assessment would have played an important role for a long-term alternative goal that of using the group of 3d models of statues produced as the starting point of an online metrological 3d database meant to be progressively populated via crowdsourcing. low cost devices largely used by students to virtualize small and medium size artefacts include the structure sensor produced by occipital (figure 7), and the microsoft kinect for xbox 360 (figure 8). both are hand held scanners that use infrared structured light for 3d estimation and are quite popular among archaeologists [44], [45]. specifically, they project an infrared grid of points whose deformation provides depth information. such scanners do not work well with sunlight due to solar radiation of infrared light that interferes with the grid pattern emitted by the scanner. hence, they are ideal to be used in an indoor environment. for 3d scanning more detailed and small artefacts, a low cost desktop device largely used in physical anthropology [46], such as the nextengine triangulation laser scanner was used with an accuracy of 0:1mm in macro mode (figure 9). figure 6. a screenshot of the video on youtube of the 3d model of the roman villa of durrueli. http://www.archeomatica.unict.it/index.php?lang=english&page=13 http://structure.io/ http://www.nextengine.com/ acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 36 in certain cases, digital photogrammetry has been also used, employing a 24-megapixel nikon d7100 with an 18-140mm lens, to create photorealistic textures, which were mapped onto the meshes. digital photogrammetry is becoming increasingly popular among field archaeologists and curators in museums for its low costs and high-quality results [47]. the software employed for the processing of the 3d scanning data has been primarily meshlab (figure 10), a powerful crossplatform open source mesh processor [48], while processing the data from digital photogrammetry was done with an educational licensed version of agisoft photoscan (figure 11). the students at the end of the activity carried out the preliminary processing of the data. subsequently, idex academic staff finalized all of the models. figure 10. processing phase with meshlab. figure 11. processing phase with agisoft photoscan. 6. results considering that students conducted the entire exercise of 3d scanning and photo scanning of the 26 artefacts with only basic training as part of an educational program in digitization of cultural heritage, the results obtained are satisfactory. although comparative assessments on performances and accuracy of structure, kinect and nextengine have been already undertaken [48, 49, 50], experiments were carried out virtualizing the same artefacts with the three devices and with the same lighting conditions. specifically, limestone, marble and terracotta sculpture were chosen for the test. afterwards, the results were evaluated using meshlab, comparing the number of vertices and faces to assess resolution and using meshlab’s measuring tool for metrological comparisons. structure and kinect turned out very effective in capturing statues with matt surfaces, especially at close range. using the results produced by the triangulation laser scanner nextengine as a reference, the occipital structure proved to have an accuracy of 0.5mm at 40 cm of range, while the microsoft kinect had a more variable accuracy of 2.5 to 5.8 mm at 50 cm. the lighting conditions inside of the museum, a combination of natural and artificial lights with spotlights on certain artefacts and shady areas in the corners of the rooms, endangered in many instances the quality of the textures to the extent that in eight cases it was not possible to reconstruct them, leaving the models un-textured. in order to create a more uniform light environment four 90 x 180 m reflecting panels were placed on site and paired up with a system of yongnuo wireless led lights. table 2 details item by item the technique used to virtualize the artefacts and the related technical features of their 3d models. with respect to quality of the meshes, it ranges from models with 60.624 faces to those with 7.686.926 faces. in general, structured light scanners such as the structure and kinect cannot produce very high quality meshes like a laser scanner such as the nextengine can. however, the overall outcomes are acceptable even considering that, for 3d printing purposes, a 3d model must be decimated to 100.000 faces anyway. all but seven 3d models (the maltese type pedestal cup, the juglet from sant’angelo muxaro, the young boy from achradina, the non-hellenic inscription from mendolito di adrano, the torso from achradina, the head of augustus and the, ‘dama flavia’) were linked on izi.travel. the links point to the digital platform of sketchfab, where a dedicated collection, here listed in table 3, has been created on the idex channel (table 3, figure 12). figure 7. structure sensor clipped onto an ipad. figure 8. microsoft kinect (360). figure 9. nextengine triangulation laser scanner. https://sketchfab.com/cvast/collections/museo-archeologico-regionale-di-siracusa-italy acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 37 thanks to izi.travel cms, links have been created directly on the selected schedules, inviting the user to discover the 3d model of the object. a new panel opens the sketchfab model and the user can directly browse the model via both the web and the mobile app. this is a new approach to view and communicate heritage and reveals how creative the relationships between cultural institutions and local communities can be by creating new cultural experiences for end-users and new forms of cultural dissemination. in the case of the lion gutter, the 3d model was linked to the euryalos castle audio guide, which is also available in spanish. in the case of the kouros of lentini, the 3d model was also linked to the corresponding schedule in the castello ursino civic museum in catania, where the head of this statue (named the biscari head, after its discoverer) is displayed. the 3d model of the biscari head has also been linked to the corresponding figure 13. a screenshot of the schedule about the golden rings from sant’angelo muxaro on izi.travel web-platform with the corresponding 3d model on sketchfab. table 2. list of the artefacts virtualized with related technique employed and technical features of the 3d models (v.: vertices, f.: faces,). object method 3d model features pre/proto-historic lithic panel from tomb 31 of castelluccio (inv. no. 8922) 3d scanning (kinect) v. 920.267 f. 1.823.188 un-textured lithic panel from tomb 34 of castelluccio (inv. no. 8971) 3d scanning (kinect) v. 801.572 f. 1.603.131 un-textured maltese type pedestal cup (inv. no. 11246) 3d scanning (nextengine) v. 192.733 f. 385.462, textured juglet from sant’angelo muxaro necropolis (inv. no. 46410) digital photogrammetry v. 108.677 f. 216.940 textured archaic kouros of megara hyblaea (inv. no. 49401) 3d scanning (kinect) v. 199.017 f. 398.037 textured kouros of leontinoi (inv. no. 26624) 3d scanning (structure) v. 289.269 f. 572.577, textured young boy from achradina (inv. no 706) 3d scanning (structure) v. 30.314 f. 60.624, textured goddess of grammichele (inv. no. 23166) 3d scanning (kinect) v. 133.570 f. 265.758 textured goddess from poggio dell’aquila (inv. no 14366) 3d scanning (structure) v. 258.608 f. 488.110, textured kourotrophos of megara hyblaea (inv. no. 53234) 3d scanning (kinect) v. 978.738 f. 1852081 textured egyptian statue of scribe petamenofi (inv. no. 288) digital photogrammetry v. 45.121 f. 900.00 textured golden ring with cow and calf from sant’angelo muxaro (inv. no. 45905) digital photogrammetry v. 979.865 f. 1.959.730 textured golden ring with a wolf from sant’angelo muxaro (inv. no. 46517) digital photogrammetry v. 447.938 f. 895.876 textured milestone from naxos (inv. no 104387) digital photogrammetry v. 2.509.615 f. 4.117.051 textured non-hellenic inscription from mendolito di adrano (inv. no. 96962) 3d scanning (nextengine) v. 462.004 f. 999.999 textured classical torso from archadina (inv. no 6422) 3d scanning (kinect) v. 145.514 f. 371.184 textured satyr-telamon from neapolis district (inv. no 916) 3d scanning (nextengine) v. 523.042 f. 1.046.080 un-textured lion gutter from euryalos castle (inv. no. 102283) digital photogrammetry v. 152.200 f. 275.091 textured hellenistic venus landolina (inv. no. 694) 3d scanning (kinect) v. 466.731 f. 402.623 textured asclepius from castello maniace (inv. no. 737) 3d scanning (nextengine) v. 3.870.723 f. 7.686.926 un-textured statue of the ‘fisherman’ (inv. no. 49364) digital photogrammetry v. 89.574 f. 81.147 textured head of priapus (inv. no. 97227) 3d scanning (nextengine) v. 613.601 f. 1.227.158 un-textured roman imperial head of augustus from centuripe (inv. no. 50698) 3d scanning (nextengine) v. 853.005 f. 1.693.660 un-textured ‘dama flavia’ (inv. no. 697) 3d scanning (nextengine) v. 176.806 f. 352.880 un-textured roman magistrate (inv. no. 698) 3d scanning (structure) v. 480.326 f. 960.664 un-textured sarcophagus of adelphia (inv. no 864) digital photogrammetry v. 891.657 f. 1.783.300 textured https://izi.travel/it/7431-castello-eurialo/it https://izi.travel/it/8282-museo-civico-castello-ursino/it https://izi.travel/it/8282-museo-civico-castello-ursino/it acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 38 figure 12. idex’s digital collection from the archaeological museum ‘paolo orsi’ of syracuse, on sketchfab. table 3. list of the 3d models and the direct links to idex’s digital collection of the archaeological museum of syracuse ‘paolo orsi’ on sketchfab. 3d model 3d model link on idex sketchfab collection lithic panel from tomb 31 of castelluccio https://skfb.ly/6xsgm lithic panel from tomb 34 of castelluccio https://skfb.ly/6xsgq maltese type pedestal cup https://skfb.ly/6xsgt juglet from sant’angelo muxaro necropolis https://skfb.ly/6xsge kouros of megara hyblaea https://skfb.ly/6wwve kouros of leontinoi https://skfb.ly/6wwvz young boy from achradina https://skfb.ly/6vg9r goddess of grammichele https://skfb.ly/6wwvv goddess from poggio dell’aquila https://skfb.ly/6wwvu kourotrophos of megara hyblaea https://skfb.ly/6wwvd egyptian statue of scribe petamenofi https://skfb.ly/6wxnc golden ring with cow and calf from sant’angelo muxaro https://skfb.ly/6xsht golden ring with a wolf from sant’angelo muxaro https://skfb.ly/6xshw milestone from naxos https://skfb.ly/6xsi7 non-hellenic inscription from mendolito di adrano https://skfb.ly/6xsgv torso from achradina https://skfb.ly/6wwvb satyr-telamon from neapolis district https://skfb.ly/6xsho lion gutter from euryalos castle https://skfb.ly/6wwvy venus landolina https://skfb.ly/6wwvc asclepius from castello maniace https://skfb.ly/6wwvw statue of the ‘fisherman’ https://skfb.ly/6wwvf head of priapus https://skfb.ly/6wwvx head of augustus from centuripe https://skfb.ly/6xshq ‘dama flavia’ https://skfb.ly/6xsgy roman magistrate https://skfb.ly/6xsh6 sarcophagus of adelphia https://skfb.ly/6xsgg https://sketchfab.com/cvast/collections/museo-archeologico-regionale-di-siracusa-italy https://skfb.ly/6xsgm https://skfb.ly/6xsgq https://skfb.ly/6xsgt https://skfb.ly/6xsge https://skfb.ly/6wwve https://skfb.ly/6wwvz https://skfb.ly/6vg9r https://skfb.ly/6wwvv https://skfb.ly/6wwvu https://skfb.ly/6wwvd https://skfb.ly/6wxnc https://skfb.ly/6xsht https://skfb.ly/6xshw https://skfb.ly/6xsi7 https://skfb.ly/6xsgv https://skfb.ly/6wwvb https://skfb.ly/6xsho https://skfb.ly/6wwvy https://skfb.ly/6wwvc https://skfb.ly/6wwvw https://skfb.ly/6wwvf https://skfb.ly/6wwvx https://skfb.ly/6xshq https://skfb.ly/6xsgy https://skfb.ly/6xsh6 https://skfb.ly/6xsgg acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 39 schedule on the “paolo orsi” museum audio guide, and a digital reconstruction of this statue has been created and linked on both the audio guides, thus allowing users to digitally access and understand this unique piece of archaic sculpture split between two different collections. for the first time, remote and mobile users can enjoy a piece of art, such as one of the golden rings from sant’angelo muxaro (figure 13), by browsing the 3d model and by observing the accurate work of the animals on the rings, as if the users were turning the rings between their fingers. these are only brief examples of how the izi.travel platform can be useful to cross-link digital resources, giving them much more visibility, to enhance an end-user’s full experience of artworks. some final feedback, which users directly gave to the audio guides on the platform (out of more than 180 comments), appear to appreciate this kind of digital cultural communication, by integrating storytelling with 3d models. furthermore, the “paolo orsi” museum audio guide is connected to many other resources on izi.travel itself, creating a digital network between all these existing digital resources. all the 3d models on the idex digital collection on sketchfab of the archaeological museum ‘paolo orsi’ of syracuse are public and accompanied by a minimal set of metadata. 7. discussion the participatory virtualization project of the ancient artefacts of the archaeological museum of syracuse proved the efficacy of using low cost scanners as fast and accurate tools to document cultural heritage with satisfactory accuracy and produce invaluable teaching tools and ready-to-disseminate high impact visual outcomes. the metrological survey via 3d scanners provided new data, which helped curators to correct previous erroneous measurements of certain objects in the museum records, and the online availability of the 3d models will certainly be beneficial for scholars interested by this aspect of the research and not able to have direct access to the objects in the museum. occipital structure and microsoft kinect proved to be extremely versatile, easy to use, precise 3d scanners, perfectly suited for participatory museum experience where curators could involve the public or student interns. the use of a custom-built photographic studio with wireless led light and reflective panels was instrumental to mitigate the light aberrations, which would have had a more negative impact on the textures. ultimately, the incorporation of the 3d models in the izi.travel platform guarantees a tremendous visibility and could possibly represent an alternative to the current offerings in terms of dissemination platforms and 3d web viewers. but in order to conceive such a twist to the base idea behind izi.travel platform it would be necessary to work out an important limitation represented by the current structural lack of funds for the #izi.travelsicilia imitative. in fact, what has been done so far has accomplished in extreme frugality, revealing, however, how such a model can be replicated in a sustainable way. we have tried, in many cases, to compensate for the lack of funds that can be used for translation of content by using the voluntary collaboration of native speakers or translators; however, we are well aware that the translation of all audio guides, at least into the english language, would guarantee a visibility to sicilian heritage the sole use of the italian language can never guarantee. the izitravel platform has been exploited, in this project, to its full potential, implementing storytelling with hypertext links, virtual tours (such as the project on google street view) and existing 3d models, already produced with other projects (such as the schhol project made with sketchfab), according to the principle of digital content reuse. the model created in sicily is, in fact, fully replicable elsewhere, if someone else would try to involve, in a coordinated and controlled way, the heritage communities of a territory. to apply a similar model, it is certainly necessary to start by involving the broadest range of stakeholders, such as local administrations and local communities, such as schools, universities, museum networks, and associations. 8. conclusions as shown in table 4, the #izi.travelsicilia project has already achieved significant results in terms of general visualization of content and cultural enjoyment of them. the framework here provided only summarizes the results achieved so far within sicilian cultural heritage and it can also demonstrate how to add “value to digital cultural heritage collection content” [51, 52]. the #izi.travelsicilia project, in general, has been able to show how izi.travel can be an effective tool to foster mass participation and collaborative processes. it has already transformed the sicilian cultural context by activating a collective, participatory, and co-creative experience to promote the common cultural heritage. those who participated in the project, from children to museum directors, have been engaged with their own stories and voices. they became digital witnesses of their own heritage, helping to make it known and enjoyable and, all together, they are already bridging the gap in sicilian cultural heritage’s use of digital technologies. the #izi.travelsicilia project provided a new model of collaborative and co-creative partnership giving public cultural institutions and local heritage communities the opportunity to relate to each other and aim at obtaining greater democratic participatory action. this should be read as the very first phase of an important process. the real goal is both to create more local collaborative table 4. #izi.travelsicilia statistics #izi.travelsicilia statistics destinations 52 city tours 94 museums 84 stories 3.000 content providers (cp) centrally managed 58 users involved in the project 3.200 videos created by users 40 videos of 3d reconstructions linked to youtube 25 3d models linked from sketchfab 52 hypertextual content (link to other sources, as web pages or pdf) 320 number of visualizations of all contents 493.000 audio guides or audio tours fully used 61.000 audio guides or audio tours downloaded and used offline 48.000 feedback for the audio guides 183 acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 40 and co-creative projects about sicilian heritage, and to expand participation in these projects and their dissemination, including the dissemination of types of digital cultural content, such as what has been done in collaboration with idex and its collection of 3d models of artefacts. finally, with respect to the metrological value of idex’s participatory virtualization project and to the next step that could be taken in the research agenda, a new method for the online dissemination of the 3d models will be experimented with, as currently sketchfab does not provide any measuring tools. for this reason, a new web platform will be created and hosted in an idex server with a dedicated web site where the 3dmodels will be visualized with a 3dhop based viewer [53], where users would also be able to use measuring and slicing tools. once this new platform is in place, the 3d content will be re-linked in the izi.travel platform. in the future, the group of 26 3d models could become the starting point of a crowdsourcing measurement project via 3d scanning of the archaeological collections of the museum of syracuse, after having established clear and precise protocols and guidelines, as an alternative to the more popular idea of the dissemination of digital content for simple cultural enjoyment. 9. acknowledgments thanks to mrs. giuseppina monterosso, from the archaeological museum “paolo orsi”, for 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[51] p. cignoni, m. callieri, m. corsini, m. dellepiane, g. ganovelli, g. ranzuglia, 2008, meshlab: an open-source mesh processing tool, in v. scarano, r. de chiara, u. erra (eds), eurographics italian chapter conference, 1-8. [52] t. owens, 2013, digital cultural heritage and the crowd, in curator: the museum journal, vol. 56, n. 1, 121-130. r. scopigno, m. callieri, m. dellepiane, f. ponchio, m. potenziani, 2017, delivering and using 3d models on the web: are we ready?, virtual archaeology review 8.17, 1-9. links izi.travel: https://izi.travel/en sketchfab: https://sketchfab.com “paolo orsi” museum, first floor: https://goo.gl/maps/oagnd8urp1h2, second floor: https://goo.gl/maps/vrpdfuppgwm2, augmented virtual tour: http://www.regione.sicilia.it/beniculturali/museopaoloorsi/google-maps/museo-paolo-orsi-syracuse-primo-piano/index.php temples and metopes children’s guide: https://izi.travel/it/7dc4-i-templi-e-le-metope-guida-per-bambini/it children from selinunte tell: https://izi.travel/it/02eb-i-piccoli-selinuntini-raccontano/it little stonemasons tell the cusa quarries: https://izi.travel/it/e39e-i-piccoli-scalpellini-raccontano-le-cave-di-cusa/it “paolo orsi” museum audio guide: https://izi.travel/it/4d91-museo-archeologico-regionale-paolo-orsi/it university of catania’s archeomatica project: http://www.archeomatica.unict.it/index.php?lang=english&page=13 structure sensor produced by occipital: http://structure.io nextengine triangulation laser scanner: http://www.nextengine.com digital collection of the archaeological museum of syracuse: https://sketchfab.com/cvast/collections/museo-archeologico-regionale-disiracusa-italy euryalos castle audio guide: https://izi.travel/it/7431-castello-eurialo/it castello ursino civic museum in catania: https://izi.travel/it/8282-museo-civico-castello-ursino/it https://izi.travel/en https://sketchfab.com/ https://goo.gl/maps/oagnd8urp1h2 https://goo.gl/maps/vrpdfuppgwm2 http://www.regione.sicilia.it/beniculturali/museopaoloorsi/google-maps/museo-paolo-orsi-siracusa-primo-piano/index.php https://izi.travel/it/7dc4-i-templi-e-le-metope-guida-per-bambini/it https://izi.travel/it/02eb-i-piccoli-selinuntini-raccontano/it https://izi.travel/it/e39e-i-piccoli-scalpellini-raccontano-le-cave-di-cusa/it https://izi.travel/it/4d91-museo-archeologico-regionale-paolo-orsi/it http://www.archeomatica.unict.it/index.php?lang=english&page=13 http://structure.io/ http://www.nextengine.com/ https://sketchfab.com/cvast/collections/museo-archeologico-regionale-di-siracusa-italy https://sketchfab.com/cvast/collections/museo-archeologico-regionale-di-siracusa-italy https://izi.travel/it/7431-castello-eurialo/it https://izi.travel/it/8282-museo-civico-castello-ursino/it acta imeko contacts acta imeko issn: 2221-870x march 2020, volume 9, number 1 acta imeko | www.imeko.org march 2020 | volume 9 | number 1 | 0 journal contacts about the journal acta imeko is an e-journal reporting on the contributions on the state and progress of the science and technology of measurement. the articles are mainly based on presentations presented at imeko workshops, symposia and congresses. the journal is published by imeko, the international measurement confederation. the issn, the international identifier for serials, is 2221-870x. editor‐in‐chief dušan agrež, slovenia founding editor‐in‐chief paul p. l. regtien, netherlands associate editor dirk röske, germany editorial board section editors (vol. 7 9) leopoldo angrisani, italy filippo attivissimo, italy eulalia balestieri, italy eric benoit, france paolo carbone, italy lorenzo ciani, italy catalin damian, romania pasquale daponte, italy luca de vito, italy luigi ferrigno, italy edoardo fiorucci, italy alistair forbes, united kingdom helena geirinhas ramos, portugal sabrina grassini, italy fernando janeiro, portugal konrad jedrzejewski, poland andy knott, united kingdom francesco lamonaca, italy massimo lazzaroni, italy fabio leccese, italy rosario morello, italy michele norgia, italy pedro miguel pinto ramos, portugal nicola pompeo, italy sergio rapuano, italy dirk röske, germany alexandru salceanu, romania constantin sarmasanu, romania lorenzo scalise, italy emiliano schena, italy enrico silva, italy krzysztof stepien, poland marco tarabini, italy yvan baudoin, belgium marcantonio catelani, italy lorenzo ciani, italy egidio de benedeto, italy alessandro depari, italy alessandro germak, italy sabrina grassini, italy konrad jędrzejewski, poland min-seok kim, korea fabio leccese, italy rosario morello, italy vilmos palfi, hungary nicola pasquino, italy franco pavese, italy jeerasak pitakarnnop, thailand alexandru salceanu, romania emiliano sisinni, italy jan saliga, slovakia jorge c. torres-guzman, mexico ian veldman, south africa rugkanawan wongpithayadisai, thailand claudia zoani, italy about imeko the international measurement confederation, imeko, is an international federation of actually 42 national member organisations individually concerned with the advancement of measurement technology. its fundamental objectives are the promotion of international interchange of scientific and technical information in the field of measurement, and the enhancement of international co-operation among scientists and engineers from research and industry. addresses principal contact prof. dušan agrež university of ljubljana faculty of electrical engineering tržaška 25, 1000 ljubljana, slovenia e-mail: dusan.agrez@fe.uni-lj.si acta imeko lorenzo ciani university of florence dipartimento di ingegneria dell'informazione (dinfo) via di s. marta, 3, 50139 florence, italy e-mail: lorenzo.ciani@unifi.it support contact dr. dirk röske physikalisch-technische bundesanstalt (ptb) bundesallee 100, 38116 braunschweig, germany e-mail: dirk.roeske@ptb.de mailto:dusan.agrez@fe.uni-lj.si mailto:lorenzo.ciani@unifi.it mailto:dirk.roeske@ptb.de introductory notes for the acta imeko special section on the imeko tc17 cosponsored international conference ismcr’2019 acta imeko issn: 2221-870x december 2019, volume 8, number 4, 1 2 acta imeko | www.imeko.org december 2019 | volume 8 | number 4 | 1 introductory notes for the acta imeko special section on the imeko tc17 co-sponsored international conference ismcr’2019 yvan baudoin1, yves dubucq2, frank e. schneider3, tim van langenhoven4 1 professor emeritus royal military academy, manager ici/erkc, www.ici-belgium.be 2 director of the international cbrne institute, belgium 3 deputy head of the cognitive mobile systems department (cms), fraunhofer institute for communication, information processing and ergonomics fkie 4 director of the royal higher institute for defence, belgium section: editorial citation: yvan baudoin, yves dubucq, frank e. schneider, tim van langenhoven, introductory notes for the acta imeko special section on the imeko tc17 cosponsored international conference ismcr’2019, acta imeko, vol. 8, no. 4, article 1, december 2019, identifier: imeko-acta-08 (2019)-04-01 editor: dušan agrež, university of ljubljana, slovenia received november 26, 2019; in final form november 26, 2019; published december 2019 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: yvan baudoin, e-mail: yvan.baudoin@ici-belgium.be distinguished readers, from 24 to 27 september 2018, on the ‘hainaut security’ site, located in bauffe (mons, close to chievre airport and casteau nato headquarters), the annual competitions were held to demonstrate the capabilities of mobile robots in realistic scenarios. in 2011, this competition, devoted exclusively on research and development, brought together six european teams focusing on applications related to civilian security missions, on the site of the belgian defence demining service, in a wooded area. in 2016, the austrian ministry of defence, supported by the nato eod centre of excellence, managed the elrob tests entrusted to twelve european teams, focusing on the essential military applications: recognition of potential dangers in urban sites, transport conveying of equipment, search and rescue of victims, detection of explosive devices and mine neutralization. 2018 sees the execution of these same scenarios to which has been added the detection of improvised explosive devices (terrorist threat) and the evacuation of victims, scenarios favoured by defence experts gathered in research groups of the organization 'science and technology' of nato. the elrob'2018 competition presented 16 teams of which five (nic instruments ltd-uk, telerob gmbh-ge, brokk ltd.-se, elp gmbh-ge, taut-at) have been tested on explosive device detection capabilities and seven competitors tested on rescue capabilities for victims (telerob gmbh-ge, fraunhofer & tno-ge-nl, bisg-fi, taut, brokk and elp, avrora-ru). the aim of these competitions is to promote the best advances in technology and to strengthen cooperation in research. priority is today given to monitoring and reconnaissance missions and to the specific examination of suspect areas or areas affected by cbrn-type contaminations: the recent example of measures to confirm the use or location of chemical weapons in syria, as well as the use of drones for the detection of mineinfested areas in croatia confirm the interest of these machines. it is now utopian to legislate on the prohibition of 'combat' or 'combat support' drones, as investments in this area are very much involved while the role of drones alongside fighter planes is already essential: the conflicts affected by the intensive use of bombing (and precise targeting by some drones) are unfortunately, since the second world war, pests in terms of civilian casualties: the balkans yesterday, afghanistan, syria, yemen, between others, emphasize this damage. the united states are obviously very involved in the development of drones of different categories (rover system, predator, mq-5b hunter ...) but the european countries like france, germany, italy and great britain also have invested and also invest in drones in the arms industry, supported by nato, the european defence agency or even the european commission (oriented towards the civil security of the european area). these investments are in billions and today pose many problems in terms of intrinsic safety of the vehicles used (their acta imeko | www.imeko.org december 2019 | volume 8 | number 4 | 2 autonomy, control-or loss of control-possible interference with air traffic, reliability of communications, etc.) and in the field of ethics (protection of privacy, for example) but also interdependence between man and machine. the elrob event has been doubled in 2018 by the organization of the 21st international symposium on robotics (ismcr’2018) under the auspices of the international measurement confederation imeko, and more particularly its technical committee (tc17) coordinator of research in robotics, chaired by japan, of which belgium is an active member in charge of the organization of this symposium. the international cbrne institute (located in belgium – www.ici-belgium.be), aware on the above threats, decided to host and co-organize the competition elrob and the ismcr’2018. focus on mobile robotics has attracted, beside the 16 speakers of the elrob competition also 27 contributors from 10 countries. the contributions were presented during two days, introduced by six key-note speakers, namely lieutenant colonel monet (france), dr kris kidd (total), prof e. m. gradetsky (russia), prof inami (secretary tc17, japan), dr. ir. j. pink (uk co-organizer of elrob’2018). the topics of the ismcr’2019 may be summarized as follows: e-training and simulations of robotics systems (4 papers) measurement and control with sensor systems (3 papers) intelligent mapping and navigation (5 papers) architecture of robotics systems (7 papers) artificial intelligence (2 papers) validation through state-of-the-art of mobile robotics systems (4 papers) we are delighted to provide the readers around the world the opportunity to access the recent advancements in robotics, mobile robotics in particular, especially in dynamic conditions. we are looking for new innovations to arise from this scientifically sound base. the here presented release is the result of hard but rewarding scientific work, with authors, reviewers, copy-editors, layout editors and, last but not least, the editor in chief, prof. dušan agrež, providing his support for all this period. yvan baudoin guest editor introductory notes for the acta imeko special section on the ‘2018 ieee international workshop on metrology for industry 4.0 & iot’ acta imeko issn: 2221-870x june 2019, volume 8, number 2, 1 2 acta imeko | www.imeko.org january 2014 | volume 8 | number 2 | 1 introductory notes for the acta imeko special section on the ‘2018 ieee international workshop on metrology for industry 4.0 & iot’ alessandro depari, emiliano sisinni department of information engineering, university of brescia, via branze 38 – 25123, brescia (italy) section: editorial citation: alessandro depari, emiliano sisinni, introductory notes for the acta imeko special section on the ‘2018 ieee international workshop on metrology for industry 4.0 & iot’, acta imeko, vol. 8, no. 2, article 1, june 2019, identifier: imeko-acta-08 (2019)-02-01 editor: dušan agrež, university of ljubljana, slovenia received june 20, 2019; in final form june 20, 2019; published june 2019 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding authors: alessandro depari, e-mail: alessandro.depari@unibs.it; emiliano sisinni, e-mail: emiliano.sisinni@unibs.it distinguished readers, the here finalized issue of acta imeko presents research works accepted for the special section devoted to the 2018 ieee international workshop on metrology for industry 4.0 & iot (metroind4.0&iot). the conference was hosted by the university of brescia, brescia, italy, and took place on april 1618. 2018. the workshop aimed to discuss the contributions, the challenges and the new opportunities offered by industry 4.0 and internet of things (iot) paradigms for the development of new measurement methods and apparatus. therefore, metroind4.0&iot was the ideal place where researchers and practitioners from industry, academia, government and standardization committee can meet to review and discuss most recent trends in the area of interest and share innovative research directions. the conference participants enjoyed the talks in the general sessions, complemented by those of the several special sessions focused on perception methods to enhance the role of the man in the loop, on embedded vision methods and systems for edgecomputing and iot applications, on smart measurement systems for on-line quality control, on synchronization for internet of things, on measurement systems and approaches for smart manufacturing, on measurement systems in the industrial iot era, and on standards and technologies for cybersecurity of iot and industry 4.0. out of the 49 presented papers, from 11 countries, after a review process with highly qualified referees, ten papers have been accepted for this issue. innovative approaches and application scenarios have been devised by the authors of the selected paper, as briefly resumed in the following. in the paper “a rams analysis for a precision scale-up configuration of the ‘smart street’ pilot site: an industry 4.0 case study”, the authors describe the results of a rams analysis in a smart cities scenario (in particular, a smart lighting solution named “smart street”). they combined reliability analysis with failure mode, effects, and criticality analysis in order to identify the best configuration allowing to scale up the system without incurring in marginally reliable configurations that instead prove to be extremely vulnerable from the point of view of operational availability. the work “applying heuristics to the selection and prioritization of security assessment items in software assessment: the case of iso/iec 27001”, focuses on security assessment heuristics. the proposed approach (named hcappsec) is flexible and allows the inclusion of security dimensions and properties. hcapp-sec is designed to support (i) the generation of high-coverage assessment designs, and (ii) the evaluation of security standards with respect to the coverage of security aspects, as demonstrated analysing assessment items of the well-known security standard iso/iec 27001. the work by castello et al. deals with measurement equipment based on synchro phasor technology for the monitoring of power systems. in particular, authors present an innovative phasor data concentrator (pdc) design based on a low-cost platform, to be used coupled with low-cost phasor measurement units (pmus). the proposed device is characterized through several experimental tests for performance evaluation, in terms of both time synchronization and capability of managing several pmu data streams, as well as evaluating the feasibility of some additional functionalities and control logics under different possible scenarios. avallone et al. consider the most relevant physical-layer parameters used by long term evolution (lte) standard to optimize a multiple input multiple output (mimo) system. an urban scenario is considered, taking into account actual signal mailto:alessandro.depari@unibs.it acta imeko | www.imeko.org january 2014 | volume 8 | number 2 | 2 propagation. the analysis of performance, carried out at different frequencies and bandwidths, allows to verify if standard prescriptions are satisfied or not. bellitti et al. describe an impedance-based measuring system aiming at monitoring bacterial growth in petri dishes, while they are inside an incubator of the wasplab® specimen processing platform. besides having reconfiguration capabilities, to comply with user necessities, it has additional features that foster its connectivity with the wasplab® and other devices, developing a structure compliant with the industry 4.0 model. experimental results show system correct operation both in an ad hoc setup and while working with the wasplab®. the paper by lamonaca et al. concerns structural health monitoring (shm), with particular care to the concept of exchanging information among nearby buildings, to increase the timeliness of the alerts due to civil infrastructures failures. the proposed approach is based on the internet-of-things paradigm (shm-iot) and it carries out both a localized monitoring on a single building (collecting information from several sensors) and a wider monitoring on a group of building. the hardware and software architectures of the proposed shm-iot are presented together with the time synchronization requirements; experiments have been performed to validate the proposed approach in real scenarios. a low power iot architecture for the monitoring of chemical emissions, to be deployed in industrial plants or public buildings, is presented in the paper by addabbo et al. each developed sensor node hosts a broad range of environmental and electrochemical gas sensor. sensor nodes are complemented with lora lpwan connectivity that allows wide area data transmission (about 3 km). a multilayer, modular topology has been adopted, combining the features of lora technology with shorter and larger range telecommunication channels, resulting in an iot framework that can be customized according to the physical and technical features of the deployment scenario. the paper by angrisani et al., suggests the use of an iotoriented platform, programmed in the labview environment and leveraging the tcp/ip protocol stack, as a viable and effective solution for implementing an automated testing equipment (ate). remote access capability makes the proposed solution an ideal training tool for technicians. in particular, the scalability, complexity, and responsiveness of the developed system have been discussed. the work “an iot framework for the assessment of indoor conditions and estimation of occupancy rates: results from a real case study” exploits the heterogeneity of iot architectures to provide aggregated information from different sensor data. in particular, carbon dioxide concentration is combined with temperature and humidity parameters for indoor air quality (iaq) evaluation in one of the buildings at the smart campus of the university of brescia. additionally, the same sensor data provide reliable information about the presence of students/users, paving the way to customized future services and management optimization procedures. in the paper “on the performance of opc ua and mqtt for data exchange between industrial plants and cloud servers”, two well-known protocols, widely used in both iot and industrial iot applications, i.e. the open platform communication-unified architecture (opc ua) and the message queuing telemetry transport (mqtt), are compared. in particular, time-related performance for sending/receiving data to/from cloud servers are analysed. the proposed measurement methodology has been applied to specific use cases, demonstrating that the opc ua complexity results in a slower data exchange with respect to mqtt. we are deeply honoured to give our contribution to the measurement science community and to act as guest editors for this innovative and challenging issue of acta imeko. we are delighted to provide the readers around the world the opportunity to access the recent advancements in measurements for iot and industry 4.0 scenarios and new opportunities offered by iot and industry 4.0 paradigms to instrumentation and measurement research fields. we are looking for new innovations to arise from this scientifically sound basis. the here presented release is the result of hard but rewarding scientific work, with authors, reviewers, copy-editors, layout editors and, last but not least, the editor in chief, prof. dušan agrež, providing his support for all this period. alessandro depari emiliano sisinni guest editors acta imeko, title acta imeko issn: 2221-870x june 2018, volume 7, number 2, 110-113 acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 110 angelo mosso: transduction and measurement of physiological signals mara fausone, marco galloni astut, archivio scientifico e tecnologico dell’università di torino, sistema museale di ateneo, c.so massimo d’azeglio, 52 10126 torino, italy section: technical note keywords: angelo mosso; physiology; kymograph; measures of physiological processes citation: mara fausone, marco galloni, angelo mosso: transduction and measurement of physiological signals, acta imeko, vol. 7, no. 2, article 19, june 2018, identifier: imeko-acta-07 (2018)-02-19 section editor: lorenzo ciani, university of florence, italy received february 11, 2018; in final form may 21, 2018; published june 2018 copyright: © 2018 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: mara fausone, e-mail: mara.fausone@unito.it 1. introduction the archivio scientifico e tecnologico dell’università di torino (astut) was founded in 1992 when some researchers realized that it was necessary to create an institution devoted to the preservation of the heritage of scientific instruments, witnessing the history of science of our university. in the university of turin, six centuries old, there are important historical archives that preserve a large quantity of books and documents, but nobody had thought of collecting the material heritage. in 1991, in the ancient rooms of the archivio di stato, record office of turin, an exhibition, named “strumenti ritrovati” [1], was organized: it highlighted many examples of instruments of the past related to the different scientific academic disciplines taught in our university. one year later astut was born. the choice of the name was considered fundamental, it would have been “archives” and not “museum”, an institution devoted to preserve, catalogue, study and bring out the instruments and the objects of science and technology that are under risk to disappear but with the consciousness that it would hardly ever become a real museum open to the public. we organise temporary exhibitions about different themes using the available materials, paying a special attention to right setting and using originals photographs and when it is possible we make instruments work again and realize videos to help people understanding how they actually work. we continue every day to collect instruments and other objects forsaken in the modern laboratory in order to take care of them as a “memory for the future”. in a not too far future they will be important as they will be studied to understand all the steps of the history of science and technology. we have more than 3,000 square meters of exhibition rooms and warehouses where we house thousands of specimens from the scientific and teaching history of the university since eighteenth century to yesterday (figures 1 and 2). in the second section we will discuss about professor angelo mosso and his studies and researches; in the section three we will explain the use of new transducers and the new instruments applied in different fields of the physiology. in the last section the evolution and the relevance of the studies of angelo mosso are highlighted. abstract professor angelo mosso (1846-1910) was an italian physiologist who worked in the university of turin in the second half of nineteenth century. his researches were devoted to record and measure vital and mental processes. he invented many instruments and the signals were recorded on blackened paper on kymograph. in his work mosso had the important help of the technician luigi corino, whose skill allowed to obtain very precise and functional instruments. in the storage rooms of astut we preserve many of these instruments together with original documents, pictures. acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 111 2. angelo mosso angelo mosso (figure 3) was born in 1846, he studied medicine and succeeded jacob moleschott (1822-1893) as professor of physiology in turin. moleschott was a materialistic philosopher rather than a physiologist and his researches were devoted especially to biochemistry. mosso after the academic degree in 1870 moved to florence and had the opportunity to attend the laboratory of moritz schiff (1823-1896) and then went to leipzig, where he worked with carl ludwig (1816-1895). in germany he learnt to use the kymograph (figure 4), a new instrument invented by ludwig to record biological phenomena. the kymograph consists of a drum that is covered by a smoked sheet of paper. it rotates at a constant speed moved by a clockwork mechanism so that a mobile fine stick can draw a line that represents the variations in time of physiological parameters. this instrument had a revolutionary importance: for the first time it was possible to obtain a graphic record of biological events. after his german period he spent some months in paris, where he met claude bernard (1813-1878) and jules marey (1830-1904) and began to use the tympanum of marey (figure 5). the tympanum is made of a metal cylinder, closed by an elastic membrane on a side. changes in pressure transmitted to the tympanum via a pneumatic or hydraulic system acted on the membrane and caused the movement of a linked stick or a pen drawing on the kymograph drum. these are the words used by mosso about this technique: “every heartbeat, every wheezing breath, tremor of the muscles, speed of blood circulation, speech, thought and perception can leave an indelible trace through the graphical method.” [2]. when mosso came back in italy he could use both kymograph and tympanum in his experimental researches. the original and innovative ideas of the physiologist could figure 1. one of the exhibition rooms. figure 2. one of the warehouses . figure 3. angelo mosso. figure 4. kymograph. acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 112 become actual instruments thanks to the quality of the work done by the technician luigi corino, who followed for many years mosso. we know that the ability in mechanical manufacture of this humble scientific craftsman was often important in order to obtain the best realization of the instruments and to improve their utilization and functionality. in our stores we have many instruments signed l. corino and many others are present in different scientific instruments collections not only in turin. 3. new transducers the availability of a reliable graphic recorder induced angelo mosso to invent various transducers of physiologic events. first of all there was the plethysmograph [3]: it was made of a glass cylinder into which the distal part of a limb could be introduced. from the cylinder full of water, the variations of volume of the limb were transferred with a hydraulic transmission to a glass pressure gauge with a mercury column which moved a pen on the paper of a kymograph. the recorded changes are caused by variations of local blood flow, that is influenced by emotional states and variations of other contemporary physiological activities. the inertia of the recording system was too high and mosso improved his instrument and built the hydrosphygmograph [4], based on mixed hydraulic and pneumatic transmission system (figure 6). with this tool the physiologist studied the movements of the brain in people with skull defects. he put a tympanum of marey in contact with the head skin where the lack of bone allowed to perceive the movements of blood vessels through the meninx, and via a pneumatic system these volume variations were recorded so that it was possible to find relationships between mental activity and cerebral circulation. mosso invented also a sphygmomanometer [5]: it was a mercury manometer, with a pen on the meniscus surface. the patient had to put middle and ring fingers of both hands into rubber glove fingers placed in four metal tubes. the space between the walls of the tubes and the glove fingers was filled with water which transmitted the pressure oscillations of the fingers arteries to the manometer via a hydraulic system. in 1896, pediatrician scipione riva-rocci (1863-1937) from turin modified mosso’s instrument and obtained the arm cuff sphygmomanometer which is still used at present. mosso focused his attention on the distribution of blood in the entire human body. to this purpose, he built the bascule [6], an oscillating bed (figure 7). a patient resting on this bed, being exactly balanced on his centre of gravity, slightly and continuously oscillated mainly in response to the respiratory movements, so that correlations between respiration and circulation could be identified through the simultaneous record of the blood flow in the foot, by means of a plethysmograph. later on, mosso tried to study the relationship between the distribution of the blood to the brain and different types of stimulation in the attempt to correlate neural activity and energy supply. mosso investigated also on muscle work and fatigue: for this purpose he created the ergograph [7] (figure 8). in this instrument an arm is held and locked in the right position while a string, with a weight of five, six kilos at the figure 5. marey tympanum. figure 8. ugolino mosso, angelo’s brother, and the ergograph. figure 6. hydrosphygmograph. figure 7. bascule. acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 113 end, is attached to the middle finger. the contractions and releases of the finger, following the regular movement of a metronome, are recorded on blackened paper with a nib on a kymograph, in this case the signal transduction is mechanical and it is possible to draw the curve of the fatigue. angelo mosso recorded these graphs, ergograms, submitting many people to such experiments in different physical, psychological and environmental conditions. as the researcher understood that there was a significant influence of the nervous component in the development of fatigue, the ergograph was modified and became the ponometer [8] (figure 9). in this instrument the lifted weigh could be automatically disconnected at the end of the contraction, reducing the mechanical work exerted by the muscle and so stressing the effect of the mental strain. 4. conclusions angelo mosso was a creative researcher, indeed he invented several instruments for the record of physiologic events, some of which were very sensitive as they were conceived for quite feeble events. the first problem was the technique for transferring the signal picked up by the sensor to the kymograph, and this was initially obtained with simple mechanical means, like in the ergograph. hydraulic and pneumatic methods were adopted in other instruments, using glass tube manometers or the marey tympanum. the weakness of the physiologic signals often needed amplification to draw a clearly visible graphic trace on the recording paper and this was obtained only applying a long and light stick, often made of bamboo wood, that operated as a third class lever and increased the original movement according to the length ratio of the two segments, the first between fulcrum and effort (e.g. the marey tympanum) and the second from the effort to the writing end. in these devices all mechanical variables, such as friction and viscosity, had to be taken into account as those factors could reduce the effectiveness of the system. for example, in the hydraulic transmission of the hydrosphygmograph the liquid was petroleum ether because of its low density [9]. electronic amplification of electric signals by means of thermionic tubes was not yet available when mosso was conducting his researches. indeed, the only application of electricity he mentioned dealt with the writing of a time-base line on the paper of the kymograph that was needed for the measure of physiologic events duration. electric contacts on the gears of a clockwork could activate an electromagnet, powered by daniell cells, if a signal in the range of seconds was necessary. if there was the need for a higher frequency, the vibrations of a diapason could be picked up with a magnet attached to one of the prongs and surrounded by an electric wire coil. the induced current was sent to a deprez’s signal, an electromagnet that moved a marking shaft. references [1] m. galloni (ed.) “strumenti ritrovati. materiali della ricerca scientifica in piemonte tra settecento e ottocento”, regione piemonte, torino, 1991. [2] a. mosso, “carlo ludwig”, nuova antologia. rivista di scienza, lettere ed arti vol. 57, 1895, pp. 650-671. [3] a. mosso “sopra un nuovo metodo per scrivere i movimenti dei vasi sanguigni nell’uomo”atti della reale accademia delle scienze di torino, vol. ix, 1875, pp. 21-81. [4] a. mosso “sulle variazioni locali del polso nell’antibraccio dell’uomo” atti della reale accademia delle scienze di torino, vol. xiii, 1877, pp. 34-78. [5] a. mosso “sphygmomanomètre pour mesurer la pression du sang chez l’homme”, archives italiennes de biologie, vol. xxiii, 1895, pp. 177-197. [6] a. mosso “applicazione della bilancia allo studio della circolazione sanguigna dell’uomo” atti della reale accademia dei lincei – memorie della classe di scienze fisiche, matematiche e naturali, vol. xix, 1884, pp. 534-535. [7] a. mosso “le leggi della fatica studiate nei muscoli dell’uomo” atti della reale accademia dei lincei – memorie della classe di scienze fisiche, matematiche e naturali, vol. v, 1888, pp. 409426. [8] a. mosso “le leggi della fatica studiate nei muscoli dell’uomo” atti della reale accademia dei lincei – memorie della classe di scienze fisiche, matematiche e naturali, vol. v, 1888, pp. 419421. [9] a. mosso “applicazione della bilancia allo studio della circolazione sanguigna dell’uomo” atti della reale accademia dei lincei – memorie della classe di scienze fisiche, matematiche e naturali, vol. xix, 1884, pp. 534-535. figure 9. ponometer. a method for the evaluation of the response of torque transducers to dynamic load profiles acta imeko issn: 2221-870x march 2019, volume 8, number 1, 13 18 acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 13 a method for the evaluation of the response of torque transducers to dynamic load profiles rafael s. oliveira1, renato r. machado1, h. lepikson2, thomas fröhlich3, rené theska3 1 instituto nacional de metrologia, qualidade e tecnologia inmetro, duque de caxias, brazil 2 federação das indústrias do estado da bahia fieb, salvador, brazil 3 technische universität ilmenau tu-ilmenau, ilmenau-th, germany section: research paper keywords: dynamic torque; torque metrology; high torque variation rate; torque transducer; angular acceleration citation: rafael s. oliveira, renato r. machado, herman lepikson, thomas fröhlich, rené theska, a method for the dynamic calibration of torque transducers using angular speed steps, acta imeko, vol. 8, no. 1, article 3, march 2019, identifier: imeko-acta-08 (2019)-01-03 editor: petri koponen, mikes metrology, finland received august 3, 2018; in final form november 16, 2018; published march 2019 copyright: © 2019 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: rafael s. oliveira, e-mail: rsoliveira@inmetro.gov.br 1. introduction the current traceability methods for torque transducers used in all applications are based on static proceedings and static standard systems. traditional calibration standards and guidelines, such as [1], define the reference value as a completely static one, preferably using a dead weight and lever-arm torque generation machine, accompanied by a large period of stabilization during the load sequences. however, in regimes in which it is important to measure torque during periods with high torque rates, there must be a demand for different patterns. in addition, most of these applications also use sensors under rotation conditions, which is also not considered in the traditional standard to a great degree. traceability alternatives must be found in order to cover these non-static measurement systems. the research on new methods of providing traceability to the torque quantity is growing [2]-[4], and the dynamic approach is also becoming a key area of the research. this paper makes significant contributions to this research field, and this proposal, first introduced in [5]-[8] and now reviewed in detail, fits into the dynamic metrology scenario. there are different interpretations of the definitions or statements concerning how dynamic can measurements be. 1.1. the dynamics of the proposal to understand the dynamic basis of the proposal, it is important to point out two main facts that delineate the extremes into which it fits. first approaches to the traceability of dynamic torque were presented in [9]-[9], where an oscillatory (sinusoidal) regime was applied to the measurement system through a frequency range sweep, followed by the frequency domain analysis of the data. this method evaluates both the amplitude and phase responses of the device under calibration. on the other hand, [1] presents the continuous calibration method as an alternative to the static one, which can reduce the total calibration time in 1/10. however, beyond the necessity of the equipment being able to detect fast readings, instead of stabilised ones, rapid changes in torque values cause unwanted effects on transducer behaviour, such as creep phenomena and high reversibility errors [11]. a critical torque rate of around 17 n∙m∙s-1 during incremental loading is found in [12], where the linearity deviation results are very disturbed. in [13], it is shown abstract the traceability of the torque quantity finds a gap when there is a regime with torque variation rates. the traditional calibration methods define the references to be on the static regime, with null torque rates. this paper presents a method for providing torque traceability to rotating sensors under high torque variation rates. the principle of applying acceleration pulses to rotating shafts, with mounted reference discs with known mass moments of inertia thereon, is presented herein, followed by a description of sequential proceedings for obtaining an analysis in the time domain. the development of the uncertainty budget is also discussed. demonstrative experimental data is used in order to ratify the principle and to maintain a qualitative approach to the theoretical description of these methods. the research gives a very good idea of the necessary different future approaches to guarantee traceability for mechanical quantities. mailto:rsoliveira@inmetro.gov.br acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 14 that the different digital filtering parameters and the rates of torque application directly influence the stability of the readings. these behaviours can be attributed to dynamic effects and cannot therefore be followed by a simple continuous quasi-static loading methodology. therefore, within these two extremes, one question remains: how can traceability be covered for loading torque profiles, with high rates of variation of quantity and keeping direct analysis to the time domain? the research presented in this paper keeps the analysis in the time domain, being interpreted and summarised as a next step in the generation of reference torque pulses (no oscillations) with high torque rates, together with the sensor's rotation regimes, when the continuous torque calibration proceeding becomes insufficient. 2. calibration method and description as already introduced in [2], the proposed method is based on the principle of applying reference dynamic torque (𝑇) to the transducer through the acceleration (�̇�) of a part with a known mass moment of inertia, mmi (𝜃), as calculated in equation (1). 𝑇 = 𝜃 ∙ �̇� (1) in a rotating shaft, the proposed dynamic regime corresponds to the acceleration pulse period between two angular speed steps. with the speed signal measured, acceleration data can be obtained through its differentiation. it is important to highlight that the start-ups are done with the whole system under rotation. therefore, there are ramps between speed steps, where the reference acceleration is extracted, and the speed is then returned for the previous step through a braking period. figure 1 shows an example of this regime in time, with a system comprising an electric motor, an encoder, and a torque transducer coupled with it (see configuration i in figure 3), running three consecutive acceleration ramps between speed steps. figure 2 shows a zoom-in view of the time axis, with the measured speed signal and the calculated acceleration pulse during the speed step period. this period of acceleration pulse corresponds to the possibility of applying high torque rates, and the development of strategies for reaching the torque traceability during this regime is the challenge considered in this research. broadly, the proposed method should: • identify the time dependency of the quantity; • prioritise the application of torque rates; • evaluate the differences between the values read in the transducer (calibration curves) and those generated in the reference; • evaluate susceptibility to different kinematic conditions; • occur throughout the range of application of torque; and • assess reproducibility in sequential loadings. 2.1. sequence of operation a sequence of operation is proposed in order to get a logical path to reach the intended torque profiles to better evaluate the sensor under different kinematic conditions. figure 3 shows the proposed mechanical system, including the driver (electric motor), the transducer, the encoder, and the inertial shaft. configuration i shows the initial assembly with the main, the constants, and the components of inertia in the shaft, which will be called the initial mass moment of inertia (𝜃𝑖 ). the inertial torque (𝑇𝑖𝑖 ) generated by these components under acceleration correspond to the measured torque (𝑇𝑡𝑖 ) in the transducer. this value is considered a tare value for the evaluation of the net measured torque. in the sequence of measurements and start-ups, configuration ii shows the inclusion of a reference mass moment of inertia (𝜃𝑟 ) to the main shaft. now, the assembly’s total inertia generates an inertial torque of (𝑇𝑖𝑖 + 𝑇𝑖𝑟 ), and that will correspond to a measured torque of (𝑇𝑡 ). the net measured figure 1. angular speed steps and measured torque pulses. figure 2. angular speed step with calculated angular acceleration pulse. figure 3. assembly with the sensor in line with different configurations of inertia. table 1. correspondence between the parameters and assembly configurations i and ii of figure 2 config. i config. ii mmi of the shaft 𝜃𝑖 𝜃𝑖 + 𝜃𝑟 generated inertial torque 𝑇𝑖𝑖 𝑇𝑖𝑖 + 𝑇𝑖𝑟 gross measured torque 𝑇𝑡𝑖 𝑇𝑡 net measured torque (𝑇𝑡𝑖 − 𝑇𝑡) 𝑇𝑡𝑙 acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 15 torque (𝑇𝑡𝑙 ) is the difference between the values measured in both configurations and corresponds to the evaluable inertial reference torque (𝑇𝑖𝑟 ). these correspondences between inertial configurations and torques can be found in table 1. figure 4 shows the sequence of the operation of this system in terms of obtaining the sequential loadings, varying the conditions of inertia and accelerations that can be applied to a certain assembly of motor, shaft, and transducer. from this sequence, we can see that once an inertial condition is set, sequential speed steps and accelerations are applied to that configuration (loops 1 and 2). thereafter, the inertial configuration is changed (loop 3), and the same sequence of acceleration and speeds must be applied. therefore, the measured data of speed and torque are then evaluated to find the corresponding dynamic situations, and the net values can be calculated. 2.2. methods of analysis two methods of analysis are proposed based on the relationship between the quantities involved. both are applied from the same pack of measured data using only offline signal processing, including the calculation of the acceleration data from the differentiation of the speed data. to help elucidate the proposed comparison methods, we use, to give the study a qualitative nature, a couple of real pieces of data, obtained from a set of experiments carried out during the research, together with an explanation and discussion of the method. the experimental assembly (figure 5) was quite similar to that which was described in figure 3, except for the fact that the encoder used for speed measurement was the slot type encoder included in the t12 torque transducer from hbm. the results of the driving of two inertial discs, named #d20 and #d40, under one acceleration profile are used here. the first method is called ‘direct calibration’ and uses equation (2)(a), expanded in (2)(b), to evaluate the error results e, comparing the average values of reference torque 𝑇𝑖𝑟 to the measured net torque value at the transducer 𝑇𝑡𝑙 , both synchronized and indexed in time intervals as i. 𝐸(𝑖) = �̅�𝑡𝑙(𝑖) − �̅�𝑖𝑟(𝑖) (2.a) 𝐸(𝑖) = (�̅�𝑡(𝑖) − �̅�𝑡𝑖(𝑖)) − (�̅̇�(𝑖) ∙ 𝜃𝑟) (2.b) figure 6 shows the 𝑇𝑡𝑙 and 𝑇𝑖𝑟 curves for each inertial configuration with discs, obtained from the averages of these parameters acquired in the runs inside the repetitions of ‘loop 1’ applied to the system and the corresponding results of e. both inertial configurations were started with the same speed step and same acceleration profile, so they use the same averaged tare torque value. from these graphical results, it is clear that there is a time dependency and tendency toward the error plots, even if they are different for the inertial configurations. the error for the loading side (up) of the curve is less than the error for the unloading side (bottom). due to the instability in the extremes of the pulse curves, where acceleration and inertial torque have almost zero values, what can be better noted in the acceleration curve of figure 2 is that it would cause huge instability in the error analysis. this phenomenon induces the methodology to require a limitation for figure 4. sequence of operation to test different inputs to the sensor. figure 5. experimental assembly for testing runs in the measuring shaft with the initial configuration (left) and with #d20 (right). figure 6. error result (blue dot), between �̅�𝑡𝑙(𝑖) (black line) and �̅�𝑖𝑟(𝑖) (red line) according to the method of ‘direct calibration’ in the inertial configurations of (up) #d20 and (bottom) #d40. acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 16 the studied range of torque within each comparison proceeding, corresponding to a pair of inertia and acceleration start-up parameters. the second method is called ‘indirect calibration’ and tends to be a simpler process, with the analysis based on the linearity of the proposed principle and the differences between the angular slopes of linear fitting curves. this method deals with the theoretical linear relationship between the reference acceleration and the torque value measured from the transducer. in order to keep the matter simple, data on 𝑇𝑡 and �̇� can be combined, and a linear fitting curve can be calculated, where there is now a new parameter known as ‘adjusted torque’ (𝑇𝑎𝑑𝑗 _𝑡), which is expressed in terms of �̇� (equation (3)) and passes through the origin (0,0). this method does not demand the synchronization of torque readings. 𝑇𝑎𝑑𝑗 _𝑡 = 𝛼 ∙ �̇� (3) the coefficients (𝛼) of these expressions are intended as comprising one for each inertial configuration under the same acceleration profile, including the initial configuration, also working here as a tare reference. figure 7 shows a graphical interpretation of the method, calculating the angular coefficients for the two inertial discs configurations, 𝛼#𝐷20 and 𝛼#𝐷40, of the shaft, calculated by means of the slope triangle method. the line with coefficient 𝛼𝑖𝑛𝑖 corresponds to the configuration without discs attached. once these coefficients are calculated, a net angular coefficient (𝜃#𝐷𝑥𝑥 ) can be calculated for each inertial configuration, and that value can be compared to the mmi of reference (𝜃𝑟#𝐷𝑥𝑥 ), resulting in another error parameter (𝐸𝜃𝑥𝑥 ), as shown in equation (4). 𝐸𝜃𝑥𝑥 = (𝛼#𝐷𝑥𝑥 − 𝛼𝑖𝑛𝑖 ) − 𝜃𝑟#𝐷𝑥𝑥 = 𝜃#𝐷𝑥𝑥 − 𝜃𝑟#𝐷𝑥𝑥 (4) another interesting parameter that can be measured based on this method is the linearity deviation between the 𝑇𝑎𝑑𝑗_𝑡 values (red line) and the measured 𝑇𝑡 values (grey dots), as shown in figure 8. this gives another idea of the behaviour of the sensor referring to the proposed physical principle. from this graph, representing two runs for #d20, it is possible to observe that there is a better convergence (linearity deviation surrounding the zero value) on the upper part of the line, above 1.5 n∙m, corresponding to the top of the pulse torque load curve. this refers to the necessity of determining a cut point in the torque domain (𝑇𝑐𝑝 ), restricting the analysis to a more stabilized region on the load regime. in the same way as the direct calibration error, this phenomenon also influences the evaluation of error, and table 2 shows the different values of relative 𝐸𝜃 for both inertial configurations with different torque cut points 𝑇𝑐𝑝 . there, it can be noted that the closer the 𝑇𝑐𝑝 is from the top, the better error results it achieves, but the narrower is the range analysed. figure 7. graphical demonstration of the evaluable angular coefficients. figure 8. linearity deviation for the configuration of #d20. figure 9. synchronized (indexed intervals) torque curves. table 2. evaluated error 𝐸𝜃 for different inertial configurations. inertial config. 𝑇𝑐𝑝 𝜃𝑠 𝜃𝑟 𝐸𝜃 / n·m /kg·m2 /kg·m2 #d20 2.0 0.02355 0.02489 5.4 % 1.5 0.02275 8.6 % #d40 4.0 0.04650 0.04957 6.2 % 3.5 0.04571 7.8 % acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 17 by means of these experiments, the physical principle is validated. figure 9 shows the torque curves 𝑇𝑡 obtained from the averages for the measurements carried out during the loops with both inertial configurations and the same tare curve. the green box identifies the initial zero level, and there is an oscillation around this considered zero value, after the individual tare proceeding of each curve. figure 10 shows the achieved torque rates at these configurations, which were calculated as the first differentiation of the net torque curves. these maximum rates of torque achieved in the experiments were ±10 n·m·s-1 and ±15 n·m·s-1 for the #d20 and #d40 inertial configurations respectively. 3. highlights concerning uncertainty the first approach to the evaluation of the uncertainty of measurement was presented in [2], with the identification of the basic components and their quantitative estimates. however, the development of the research demanded the increment and detailing of those components. concerning the direct calibration method, for example, figure 11 shows a good explanation of these developed components of uncertainty for the construction of 𝑈𝐸 , mainly divided into uncertainty for the measured torque 𝑈𝑇𝑡𝑙 and uncertainty of the reference inertial torque axis 𝑈𝑇𝑖𝑟 . from that diagram, we can highlight that for the measured torque side, there are contributions from both inertial configurations i and ii. each configuration involves the repeatability of the curves during loop 1 and after synchronization; the oscillation of initial zero levels (see figure 9); and the static calibration uncertainty for the analysed point/range. this last contribution is important because there is a link between static and dynamic results. for the inertial torque side, there are uncertainty contributions for the mass moment of inertia and acceleration. the mmi’s contributions are related to the measurement of mass and geometry. the acceleration involves contributions from the constancy of the time intervals in the speed signal differentiation process, which is a parameter directly related to the acquisition system, from the encoder resolution and its static calibration (once there is no system for the calibration of speed), and from the repeatability of the acceleration curves during the loops. an example of the behaviour of these uncertainty contributions can be seen in figure 12. three regions of the torque load curves are detached as the highest positive torque rate (left), the peak torque (middle), and the highest negative torque rate (right). the indexation references for these regions can be viewed in figure 10. it is possible, for instance, to evaluate the asymmetry between the components in relation to the central axis of the torque curve. the identification of the negative contribution of the component concerning the correlations between the uncertainties considered from the certificate of static calibration of the transducer is considered four times for each measured torque value, as identified as ‘static calibration’ in figure 11. the uncertainty of the error ( 𝑈𝐸 ) is expressed in units of torque and is presented as a confidence interval to be traced around the 𝐸 curve, what can be seen as the red and blue lines. figure 10. torque rate curves for both inertial configurations. figure 11. main uncertainty contributions for 𝑈𝐸. figure 12. composition of the uncertainty budget considering each contribution parameter of uncertainty in the three regions of the torque load curve: highest positive torque rate (left), peak torque, torque rate null (middle), and the highest negative torque rate (right). axis: indexation (x), partial relative contribution (left y) and error in n∙m (right y). acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 18 for the indirect calibration method, the main contributions of uncertainty will be about the method for calculating the linear fitting curve. 3.1. how to report the results the dynamic calibration proposal included two different methods of analysis, what can be interesting to be joint in a same test report. nevertheless, the report should contain all the parameters of interest, due to the vast possibility of combinations, such as the following suggestions: ▪ characterize the torque pulses of interest (peak torque, maximum torque rate, acceleration or deceleration, clockwise or anticlockwise); ▪ highlight the static uncertainties; ▪ report kinematic conditions: speed step, acceleration times, and reference mmis used; ▪ report 𝐸 and its uncertainty 𝑈𝐸 in the graphic mode or select the most interesting points in the load curve and report those parameters for each torque point in a table. figure 13 shows the torque load curve for the experiment with the #d20 inertia configuration as an example. in the graph, there is the average �̅�𝑡𝑙 and the confidence interval for the uncertainty 𝑈𝐸 . ▪ report 𝐸𝜃 together with the corresponding 𝑇𝑐𝑝, such as in table 2. 4. conclusion and outlook this paper has focused on the need to better evaluate, in the time domain, those regimes of torque with high variations of quantity. two calibration evaluation methods are presented. the results obtained from the direct calibration method showed the temporal dependency of errors, and the indirect method can evaluate hysteresis and linearity deviations from the physical principle adopted for the realization of the reference dynamic torque. pieces with known moments of inertia and accelerations profiles are combined to give different torque load profiles. the two evaluation methods are complementary, originate from the same measurement data, and can join the same ‘calibration report’. uncertainty of measurement must be part of a parallel discussion. the presented uncertainty results could be considered to evidence the interesting behaviour of the different contributions on the different periods of the torque load curve and the time domain. the capacity of a system to reach high torque rates depends on the capacity of the electric motor to accelerate the shaft within its limits of maximum acceleration torque. from now on, it is necessary to repeat the experiments under different conditions, revalidating the method and analysing the proposed assembly. references [1] din 51309: materials testing machines calibration of static torque measuring devices (german standard), din (2005). 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https://www.imeko.org/publications/tc3-2002/imeko-tc3-2002-016.pdf https://www.imeko.org/publications/tc3-2005/imeko-tc3-2005-035u.pdf https://www.imeko.org/publications/tc3-2005/imeko-tc3-2005-035u.pdf https://www.imeko.org/publications/tc3-2014/imeko-tc3-2014-011.pdf https://www.imeko.org/publications/tc3-2014/imeko-tc3-2014-011.pdf evaluation of the results of regional metrology organisation comparisons and national inter-laboratory comparisons for electrical quantities acta imeko issn: 2221-870x june 2020, volume 9, number 2, 18 24 acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 18 evaluation of the results of regional metrology organisation comparisons and national inter-laboratory comparisons for electrical quantities oleh velychko1, tetyana gordiyenko2, stanislav karpenko1 1 state enterprise “ukrmetrteststandard”, 4, metrolohychna str., 03143, kyiv, ukraine 2 odesa state academy of technical regulation and quality, 15, kovalska str., 65020, odesa, ukraine section: research paper keywords: comparison of standards; inter-laboratory comparison; calibration laboratory; measurement uncertainty citation: oleh velychko, tetyana gordiyenko, stanislav karpenko, evaluation of the results of regional metrology organisation comparisons and national inter-laboratory comparisons for electrical quantities, acta imeko, vol. 9, no. 2, article 4, june 2020, identifier: imeko-acta-09 (2020)-02-04 editor: alexandru salceanu, technical university of iasi, romania received december 26, 2019; in final form march 18, 2020; published june 2020 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: oleh velychko, e-mail: velychko@hotmail.com 1. introduction the global metrological traceability fully depends on the implementation of two mutual recognition agreements (mras): the international committee on weights and measures (cipm) [1] and the international laboratory accreditation cooperation (ilac) [2]. metrological traceability at the highest level is ensured by international comparisons of national metrology institutes (nmis) and designated institutes (dis) standards, and metrological traceability at lower levels is ensured by the calibration of working standards in nmis and dis or accredited calibration laboratories [3]-[7]. the results of international comparisons of standards of nmis and dis of different countries [8]-[12] are used to implement the provisions of the cipm mra. key comparisons are made by the cipm consultative committees and six regional metrology organizations (rmos) using agreed technical protocols for the participants. rmos make supplementary comparisons for those measurements that are not covered by key comparisons, consultative committees, or rmos. results of key and supplementary comparisons are published in a special key comparison database – that of the international bureau of weights and measures (bipm) [13]. the results of the calibration of working standards and measuring instruments, conducted by accredited calibration laboratories for accredited test laboratories [14], are used to implement the provisions of the ilac mra. inter-laboratory comparisons are widely used to confirm the technical competence of accredited calibration laboratories [15]-[21]. nowadays, ac voltage and electrical power measurements at industrial frequencies have practical importance, as they have become the main basis of the commercial relationships between electricity consumers and electricity suppliers. it should be noted that the main purpose of legal metrology is to control the measuring instruments that are used in commercial transactions and to ensure and guarantee the accuracy of the measurement results throughout the period of use under operating conditions within the limits of the allowed permissible errors. it is certain that inter-laboratory comparison reports [22] can provide information on where a calibration laboratory that participated in the comparison may need improvement. it should be noted that comparing different results can only be done abstract the global metrological traceability fully depends on the implementation of international mutual recognition agreements in field of metrology. the linked results of international comparisons of national standards for alternating current/direct current (ac/dc) voltage transfer difference measurements and electrical power measurements, and national inter-laboratory comparison ac/dc voltage transfer difference and electrical power measurements at industrial frequency was presented. the main goal of this inter-laboratory comparison was the assessment of calibration laboratories capabilities that perform calibration in ac/dc voltage transfer difference and electrical power measurements. the consistency of the data obtained using the en numbers and χ2 test was estimated. mailto:velychko@hotmail.com acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 19 correctly if the measurement uncertainty of the results is taken into account [4][14]. an important task in the technical confirmation of metrological traceability is to establish both a connection between the results of comparisons of national standards [23][25] and a linking of the calibration results of accredited calibration laboratories within the framework of national interlaboratory comparisons [7][26][27]. the basis for establishing such a connection should be provided by the nmis or dis. the procedure of the linking between key or supplementary comparison results and national inter-laboratory comparison results is described in section 2. the proposed procedure is applied to ac/dc-voltage transfer difference measurements (section 3) and ac power measurements (section 4). the general recommendations for the laboratories that participated in the inter-laboratory comparisons (ilcs) for ac/dc-voltage transfer difference measurements and ac power measurements are presented in section 5. 2. description of the method reference values, their measurement uncertainty, degrees of equivalence, and uncertainties for all participants are determined in the evaluation of the results of the rmo comparisons. in the data evaluation, rmo key comparisons are determined: transformed participant data and their uncertainties; degrees of equivalence, and measurement uncertainty for all participants, with the exception of the linking nmis and dis [14]. the following procedure is used in the evaluation of interlaboratory comparison data with calibration laboratory participation: assigned value and its measurement uncertainty; inter-laboratory differences – degrees of equivalence and their uncertainties; and data consistency characteristics. there are various options to establish an assigned value, particularly the measured value by the reference laboratory, which ensures metrological traceability to the national standard. when conducting an inter-laboratory comparison for calibration laboratories, it is necessary to ensure the stability of the reference sample [18]. to establish the linking between the results of international comparisons of standards and the results of inter-laboratory comparisons, the optimal is the participation of the nmi or di as a reference laboratory. the general approach used for the evaluation of rmo comparison data and inter-laboratory comparison data is provided in [7][26][27]. inter-laboratory differences or degrees of equivalence for the i-th calibration laboratory traditionally defined by: = , lab i lab i av d x x− (1) where xlab i is measured value for the i-th calibration laboratory, and xav is the assigned value, which is determined by the reference laboratory. the expanded uncertainty of uav measurements is determined by: , av av stab u k u x u x 2 2 = ( ) + ( ) (2) where k is the coverage factor (traditionally k=2); u(xav) is the standard uncertainty obtained during the calibration of the measuring instruments of the reference laboratory; u(xstab) is the standard uncertainty about the instability of the measuring instruments during the inter-laboratory comparison. the expanded uncertainty uav of the i-th calibration laboratory in the inter-laboratory comparison is determined by: 2 2 ( ) = ( ) + ( ). lad i av lab i u d k u x u x (3) the results of the inter-laboratory comparison are expressed in relation to the rmo comparison: = nmij nmij rv d x x− . for this purpose, the degrees of equivalence of the inter-laboratory comparison (indicated lab i d ) are corrected by a correction factor d, which is determined based on the results of the participant laboratory in both comparisons (nmij – lab1): 1 , nmij lab d d d= − (4) where nmij d refers to the degrees of equivalence of the nmi or di i in the rmo comparison; lab i d refers to the degrees of equivalence of lab1 (nmi i) in the inter-laboratory comparison, with the uncertainty: 2 2 1 ( ) ( ) ( ) 2 nmij lab u d k u d u d = +  (5) the corrected degrees of equivalence for the i-th laboratory participant in the inter-laboratory comparison with respect to linking it to the rmo comparison are estimated as: lab i lab i d d d = + (6) with the expanded uncertainty: 2 2 ( ) ( ) ( ). lab i lab i u d k u d u d = + (7) en numbers are calculated using this equation: 2 2 lab i n lab i av d e u u = − (8) where ulab i is the expanded uncertainty of a participant’s result. an inter-laboratory comparison result is satisfactory if |en| ≤ 1, showing the compatibility of the measurement results. in other words, if |en| > 1, the result of the inter-laboratory comparison is unsatisfactory. according to [10], on the basis of measurement results and associated uncertainties presented by the participants of rmo comparisons of national measuring standards, we need to calculate the value of the χ2 test. the same formula from [10] can be applied to the evaluation of the consistency of the results of inter-laboratory comparisons for calibration laboratories: 2 2 2 1 ( ) n lab i i lab i d u x  = =  (9) to check the consistency of inter-laboratory comparisons for calibration laboratories, a criterion value is used, which is calculated based on data provided by the calibration laboratories. the criterion value does not exceed the critical value χ2 for confidence level 0.95 and the number of degrees of freedom n-1 (n is the number of the calibration laboratory participants in the inter-laboratory comparison): χ 2 < χ 2 0.95(n -1) . (10) acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 20 3. results of the ac/dc voltage transfer difference the results of the euro-asian cooperation of national metrological institutions (coomet) coomet.em-s1 supplementary comparison (vniim1, russia, and umts, ukraine) [28] is expressed in terms of the reference value of the coomet.em-k6.a key comparison (vniim, umts, belgim, 1 for abbreviations, see end of the article. belarus, inm, romania) [29]. vniim (russia) took part both in coomet.em-k6.a and coomet.em-s1 comparisons therefore it can serve as a linking nmi for these two comparisons. the results from all participant laboratories in coomet.em-k6.a, coomet.em-s1, and national ilc1 (five participants) in terms of rvk6.a (1 khz, 20 khz, and 100 khz) are shown in table 1 [27]. dk6a is degrees of equivalence from coomet.em-k6.a, and ds1 is degrees of equivalence from coomet.em-s1; u(dk6a) is the nmi’s or di‘s expanded uncertainty from coomet.em-k6.a, u(ds1) is the nmi/di expanded uncertainty from coomet.em-s1, and u(dilc1) is the expanded uncertainty of the i-th calibration laboratory from ilc1. in accordance with the proposed procedure and using the data in table 1, the degrees of equivalence for all the participant laboratories in the coomet.em-k6.a and coomet.em-s1 comparisons as well as national ilc1 in terms of rvk6.a with the expanded uncertainty at frequencies of 1 khz, 20 khz, and 100 khz were calculated, as shown in figure 1–figure 3. umts took part both in the coomet.em-k6.a and coomet.em-s1 comparisons and national ilc1; therefore, it can serve as a linking laboratory (reference laboratory). the results of the estimation of the en numbers and χ2-test of all participant laboratories in coomet.em-k6.a and coomet.em-s1 comparisons and national ilc1 are shown in table 2. results for all nmis/dis and lab participants are satisfied except lab 4 (en = 1.04) at frequency 1 khz (require correction of the calibration procedure). 4. results for electrical power standards the results of coomet.em-s2 supplementary comparison (umts, belgim and bim, bulgaria) [30] expressed in terms of the reference value of euramet.em-k5.1 key comparison (12 participants) [31]. umts took part both in euramet.emk5.1 and coomet.em-s2 comparisons therefore it can serve as a linking nmi for these two comparisons. the different travelling standards were used in these comparisons. umts as a pilot laboratory organized and conducted the coomet.em-s2 supplementary comparison in power for power factors 1.0, 0.5 lag, 0.5 lead, ac voltage 120 v, ac current 5 a at frequencies of 50 hz and 53 hz [30]. the radial scheme was used. radian research rm 15-04 was selected as travelling standard for this comparison. rm 15-04 is precision single-phase electrical power meter, works on principles of digital processing of electrical current and voltage signals and well-suited for test applications that require multiple measurements with high accuracy and stability. in addition to its auto ranging capabilities, rm 15-04 features three summing current inputs which can be used to perform closed link testing. currently there are about twenty ukrainian accredited calibration laboratories, but only seven calibration laboratories took part was in ilc2 which organized by umts to maintain their accreditation. the main goal of ilc2 was the assessment of calibration laboratory capabilities that perform calibration in power measurement. this inter-laboratory comparison helped to verify first of all technical competence of the staff of participated calibration laboratories, their technical and calibration procedures, environmental conditions [22]. figure 1. degrees of equivalence of all laboratories for coomet.em-k6.a, coomet.em-s1 comparisons and ilc1 in terms of rvk6.a at frequency of 1 khz. figure 2. degrees of equivalence of all laboratories for coomet.em-k6.a, coomet.em-s1 comparisons and ilc1 in terms of rvk6.a at frequency of 20 khz figure 3. degrees of equivalence of all laboratories for coomet.em-k6.a, coomet.em-s1 comparisons and ilc1 in terms of rvk6.a at frequency of 100 khz. -35.00 -30.00 -25.00 -20.00 -15.00 -10.00 -5.00 0.00 5.00 10.00 15.00 20.00 25.00 30.00 vniim umts belgim inm umts* lab2 lab3 lab4 lab5 di / μv/v coomet.em-k6a, coomet.em-s1, and ilc1 ac/dc voltage transfer difference, 3 v, 1 khz degrees of equilence di and expanded uncertainty ui (k = 2) -30.00 -25.00 -20.00 -15.00 -10.00 -5.00 0.00 5.00 10.00 15.00 20.00 25.00 30.00 35.00 vniim umts belgim inm umts* lab2 lab3 lab4 lab5 di / μv/v coomet.em-k6a, coomet.em-s1, and ilc1 ac/dc voltage transfer difference, 3 v, 20 khz degrees of equilence di and expanded uncertainty ui (k = 2) -50.00 -40.00 -30.00 -20.00 -10.00 0.00 10.00 20.00 30.00 40.00 50.00 vniim umts belgim inm umts* lab2 lab3 lab4 lab5 di / μv/v coomet.em-k6a, coomet.em-s1, and ilc1 ac/dc voltage transfer difference, 3 v, 100 khz degrees of equilence di and expanded uncertainty ui (k = 2) acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 21 table 1. degrees of equivalence of laboratories in coomet.em-k6.a, coomet.em-s1 comparisons and ilc1 in terms of rvk6.a. lab dk6a, μv/v u(dk6a), μv/v ds1, μv/v u(ds1), μv/v dilc1, μv/v u(dilc1), μv/v d'k6a, μv/v u(d'k6a), μv/v 1 khz vniim -1.10 1.79 -0.50 1.50 -1.10 1.79 umts 0.00 4.04 0.00 4.04 belgim 4.10 23.94 4.10 23.94 inm 1.20 2.44 1.20 2.44 umts* (lab 1) -2.00 4.10 0.00 4.47 -2.60 4.47 lab 2 -26.70 59.00 -29.30 59.17 lab 3 6.20 6.10 3.60 7.56 lab 4 14.10 10.10 11.50 11.04 lab 5 -48.70 260.00 -51.30 260.04 20 khz vniim 0.48 2.11 -1.20 1.50 0.48 2.11 umts 0.38 3.99 0.38 3.99 belgim 11.98 28.94 11.98 28.94 inm -1.12 2.37 -1.12 2.37 umts* (lab 1) -1.50 4.20 0.00 4.50 0.18 4.70 lab 2 -42.00 65.00 -41.82 65.17 lab 3 17.40 19.20 17.58 19.77 lab 4 28.10 28.10 28.28 28.49 lab 5 68.20 3140.00 68.38 3140.00 100 khz vniim 1.81 2.06 -3.80 3.00 1.81 2.06 umts -3.19 7.67 -3.19 7.67 belgim 25.80 139.00 25.80 139.00 inm -5.99 11.50 -5.99 11.50 umts* (lab 1) -5.00 5.80 0.00 8.50 0.61 6.15 lab 2 -14.00 95.00 -13.39 95.20 lab 3 11.00 45.50 11.61 45.91 lab 4 10.20 78.70 10.81 78.94 lab 5 1419.00 3140.00 1419.61 3140.01 table 2. the results of estimation of en numbers and χ2-test for ilc1. lab en χ2 χ20.95(n – 1) 1 khz vniim 0.61 2.58 3.33 umts 0.00 belgim 0.17 inm 0.49 umts* (lab 1) 0.58 lab 2 0.50 lab 3 0.48 lab 4 1.04 lab 5 0.20 20 khz vniim 0.23 2.65 3.33 umts 0.10 belgim 0.41 inm 0.47 umts* (lab 1) 0.04 lab 2 0.64 lab 3 0.89 lab 4 0.99 lab 5 0.02 100 khz vniim 0.88 1.57 3.33 umts 0.42 belgim 0.19 inm 0.52 umts* (lab 1) 0.09 lab 2 0.14 lab 3 0.25 lab 4 0.14 lab 5 0.45 table 3. degrees of equivalence of laboratories in coomet.em-s2 comparison and ilc2 in terms of rvs2. lab ds2, μp/p u(ds2), μp/p dilc2, μp/p u(dilc2), μp/p d's2, μp/p u(d's2), μp/p power factor = 1.0 belgim 33.9 36.6 33.9 36.6 bim -3.0 9.8 -3.0 9.8 umts* (lab 1) 8.5 17.4 0.0 12.3 8.5 17.4 lab 2 6.0 82.2 -2.5 83.1 lab 3 566.0 150.5 557.5 151.0 lab 4 76.0 145.0 67.5 145.5 lab 5 46.0 104.5 37.5 105.2 lab 6 46.0 100.0 37.5 100.8 power factor = 0.5 lag belgim 60.4 59.2 60.4 59.2 bim -2.7 16.4 -2.7 16.4 umts* (lab 1) 29.3 45.7 0.0 13.0 29.3 45.7 lab 2 22.0 82.0 -7.3 83.0 lab 3 282.0 150.6 252.7 151.2 lab 4 112.0 152.0 82.7 152.6 lab 5 -18.0 101.1 -47.3 101.9 lab 6 -8.0 70.0 -37.3 71.2 power factor = 0.5 lead belgim -17.0 59.3 -17.0 59.3 bim 2.7 16.7 2.7 16.7 umts* (lab 1) -0.2 46.7 0.0 13.0 -0.2 46.7 lab 2 18.0 82.0 18.2 83.0 lab 3 308.0 150.6 308.2 151.2 lab 4 78.0 140.5 78.2 141.1 lab 5 -22.0 101.1 -21.8 101.9 lab 6 -12.0 70.0 -11.8 71.2 acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 22 ds2 is degrees of equivalence from coomet.em-s2; u(ds2) is nmi’s/di’s expanded uncertainty from coomet.em-s2, u(dilc2) is expanded uncertainty of i-th calibration laboratory from ilc2. in accordance with the proposed procedure and using the data in table 3, degrees of equivalence for all laboratories for coomet.em-s2 comparisons and national ilc2 in terms of rvs2 with the expanded uncertainty at frequency of 50 hz was calculated, which are shown on figure 4–figure 6. umts took part both in coomet.em-s2 comparison and national ilc2 therefore it can serve as a linking lab (reference laboratory). the results of estimation of en numbers and χ2-test of all participant laboratories in coomet.ems2 comparison and national ilc2 are shown in table 4. results for all nmi/di and lab participants are satisfied except lab 3 (en = 3.69 for power factor = 1.0, en = 1.67 for power factor = 0.5 lag, en = 2.04 for power factor = 0.5 lead). lab 3 result was excluded when calculating of χ2-test (require correction of the calibration procedure). 5. general recommendations for calibration laboratories the comparative analysis of the results of provided by the relevant calibration laboratories allow the following general recommendations to be drawn: lab1, lab2, lab3, and lab5 for ilc1 meet the established requirements (|en| ≤ 1) when calibrating the travelling standard at frequency of 1 khz confirming the qualification of the participated calibration laboratory, therefore do not require any corrections in their work; lab 4 for ilc1 only has an unsatisfactory result at a frequency of 1 khz (en = 1.04); therefore, it requires correction in the calibration procedure. lab1, lab2, lab3, lab4, and lab5 in ilc1 meet the established requirements (|en| ≤ 1) when calibrating the travelling standard at frequencies of 20 khz and 100 khz, confirming the qualification of the participated calibration laboratory; therefore, they do not require any corrections. lab1, lab2, lab4, lab5, and lab6 for ilc1 meet the established requirements of the criterion (|en| ≤ 1) when calibrating the travelling standard for 120 v, 5 a, power factor = 1.0, 0.5 lag, and 0.5 lead at a frequency of 50 hz, confirming the qualification of the participant calibration laboratory; therefore, they do not require any corrections. figure 4. degrees of equivalence of all laboratories for coomet.em-s2 comparisons and ilc2 in terms of rvs2 for power factor = 1.0. figure 5. degrees of equivalence of all laboratories for coomet.em-s2 comparisons and in ilc2 in terms of rvs2 for power factor = 0.5 lag. figure 6. degrees of equivalence of all laboratories for coomet.em-s2 comparisons and ilc2 in terms of rvs2 for power factor = 0.5 lead. table 4. the results of estimation of en numbers and χ2-test for ilc2. lab en χ2 χ20.95(n – 1) power factor = 1.0 belgim 0.93 1.67 2.73 bim 0.31 umts* (lab 1) 0.49 lab 2 0.03 lab 3 3.69 lab 4 0.46 lab 5 0.36 lab 6 0.37 power factor = 0.5 lag belgim 1.02 2.27 2.73 bim 0.16 umts* (lab 1) 0.64 lab 2 0.09 lab 3 1.67 lab 4 0.54 lab 5 0.46 lab 6 0.52 power factor = 0.5 lead belgim 0.29 0.54 2.73 bim 0.16 umts* (lab 1) 0.00 lab 2 0.22 lab 3 2.04 lab 4 0.55 lab 5 0.21 lab 6 0.17 -100.00 -75.00 -50.00 -25.00 0.00 25.00 50.00 75.00 100.00 belgim bim umts* lab2 lab3 lab4 lab5 lab6 di / μp/p coomet.em-s2 and ilc2 ac power, power factor 1.0, 50 hz degrees of equilence di and expanded uncertainty ui (k = 2) -100.00 -75.00 -50.00 -25.00 0.00 25.00 50.00 75.00 100.00 belgim bim umts* lab2 lab3 lab4 lab5 lab6 di / μp/p coomet.em-s2 and ilc2 ac power, power factor 0.5 lag, 50 hz degrees of equilence di and expanded uncertainty ui (k = 2) -100.00 -75.00 -50.00 -25.00 0.00 25.00 50.00 75.00 100.00 belgim bim umts* lab2 lab3 lab4 lab5 lab6 di / μp/p coomet.em-s2, and ilc2 ac power, power factor 0.5 lead, 50 hz degrees of equilence di and expanded uncertainty ui (k = 2) acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 23 concerning lab3 in ilc2, failures were only detected in the measurements for 120 v, 5 a, power factor = 1.0, 0.5 lag, and 0.5 lead at a frequency of 50 hz; therefore, it requires a serious correction in the calibration procedure. general recommendations for lab 4 in ilc1 and lab3 in ilc2 follow: to review the existing calibration procedures or develop new ones in accordance with the requirements of international standards for accredited laboratories; to reconsider the approaches to the evaluation of the measurement uncertainty in the applied calibration procedures, in accordance with which the evaluation of the expanded uncertainty was performed; and to systematically carry out technical training for laboratory staff to perform the calibration of measuring equipment and pay special attention to the provisions of international and national documents for the evaluation of measurement uncertainty. 6. conclusions the proposed procedure for linking the results of international comparisons of national standards and interlaboratory comparison results has been applied to ac/dc voltage transfer difference measurements and ac power measurements. the presented linked results showed good agreement between all participant laboratories. to check the consistency of the linked results, the en number and χ2 test were used. participation in inter-laboratory comparisons provides independent verification of a calibration laboratory’s competence and demonstrates to the public, customers, accreditation bodies, regulators, and laboratory management that procedures are under control and gives stakeholders technical confidence in the service which calibration laboratory provide. the positive results of calibration laboratories that participated in the inter-laboratory comparisons mean the metrological traceability to the nmi or di of accredited calibration laboratories through the periodical calibration of their standards. references [1] cipm, mutual recognition agreement, 1999. 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[28] g. p. telitchenko, supplementary bilateral comparison of the national ac/dc voltage transfer references between vniim (russia) and ukrmetrteststandard (ukraine) (coomet.em-s1), metrologia 54 (2017) p. 01004. https://www.bipm.org/utils/en/pdf/cipm-mra-2003.pdf https://www.bipm.org/utils/common/documents/jcgm/jcgm_200_2012.pdf https://www.bipm.org/utils/common/documents/jcgm/jcgm_200_2012.pdf https://www.bipm.org/utils/common/documents/jcgm/jcgm_100_2008_e.pdf https://www.bipm.org/utils/common/documents/jcgm/jcgm_100_2008_e.pdf https://ilac.org/publications-and-resources/ilac-policy-series/ http://kcdb.bipm.org/ acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 24 [29] o. velychko, yu. darmenko, final report on coomet key comparison of ac/dc voltage transfer references (coomet.em-k6.a), metrologia 53 (2016) p. 01011. [30] o. velychko, s. karpenko, v. gachok, a. akhmadov, final report on coomet 344/ua/05 supplementary comparison of 50/53 hz power (coomet.em-s2), metrologia 52 (2015) p. 01008. [31] h. çayci, final report on key comparison euramet.em-k5.1 (euramet project no. 687): comparison of 50/60 hz power, metrologia 48 (2011) p. 01009. abbreviations belgim belarusian state institute of metrology, belarus bim bulgarian institute of metrology, bulgaria inm national institute of metrology, romania umts state enterprise “ukrmetrteststandard”, ukraine vniim d. i. mendeleev institute for metrology, russia fast blood impedance measurements as quality indicators in the pre-analytical phase to prevent laboratory errors acta imeko issn: 2221-870x december 2018, volume 7, number 4, 26 32 acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 26 fast blood impedance measurements as quality indicators in the pre-analytical phase to prevent laboratory errors denise de zanet1,2, monica battiston2, elisabetta lombardi2, alessandro da ponte2, ruben specogna1, francesco trevisan1, antonio affanni1, mario mazzucato2 1 polytechnic department of engineering and architecture, university of udine, via delle scienze 206, 33100 udine (ud), italy 2 department of research and advanced tumors diagnostics, cro-irccs national cancer institute, via f. gallini 2, 33081 aviano (pn), italy section: research paper keywords: electrical impedance; measurements; blood components; laboratory errors; pre-analytical phase management citation: denise de zanet, monica battiston, elisabetta lombardi, alessandro da ponte, ruben specogna, francesco trevisan, antonio affanni, mario mazzucato, fast blood impedance measurements as quality indicators in the pre-analytical phase to prevent laboratory errors, acta imeko, vol. 7, no. 4, article 6, december 2018, identifier: imeko-acta-07 (2018)-04-06 editor: alexandru salceanu, "gheorghe asachi" technical university of iasi, romania received march 28, 2018; in final form may 31, 2018; published december 2018 copyright: © 2018 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: denise de zanet, e-mail: dezanet@cro.it 1. introduction blood is a biological fluid that circulates throughout the body and contains extensive information on the state of a person’s health and disease. it is composed of a conductive liquid (plasma) and a suspension of particles (red blood cells, white blood cells, and platelets). once blood coagulates, cells aggregate to form red clots, and the surrounding liquid becomes serum. when making decisions about diagnoses, necessary further investigations, or patient management, clinicians rely on the results of blood laboratory analyses. it is, thus, clearly necessary to ensure the reliability and the accuracy of these analyses in healthcare to guarantee that the laboratory tests accurately reflect the patient’s health state. a huge number of control systems have been developed and used in most clinical laboratories to maintain high quality in the analytical phase [1]. this is not the case for the pre-analytical phase. the majority of laboratory errors are made in this phase, which covers all processes from the time that a laboratory test request is made by a physician until the sample is ready for testing. the magnitude of the effect of these errors on patient care is significant, and the focus of healthcare institutions is the improvement of laboratory test quality, renewing the interest in pre-analytical processes. thus, the use of quality controls to prevent laboratory errors should be an essential activity for any healthcare laboratory [2]-[5]. unfortunately, despite that several methods and systems are currently used in clinics for the preparation of quality control samples [6], [7], in practice, they depend on high dimensions instrumentation and specific commercial kits, whose usage procedures require optimization. the most frequent reason for the inadequacy of a sample, such sample being able to cause analytical errors in laboratories, is represented by hemolysis (i.e., the rupture of red blood cells), giving rise to the release of intracellular content, such as abstract in clinical laboratories, the major proportion of errors regarding blood analyses occurs in the pre-analytical phase. pre-analytical conditions are key, necessary factors in maintaining the high quality of specimens, limiting day-to-day and batch variations, and guaranteeing the absolute reliability and accuracy of clinical results and related diagnoses. the quality of serum samples must be very high in order to avoid interferences due to hemolysis, thereby preventing measurement errors. in addition, the quality of the blood should always be fast monitored to identify inadequacies and guarantee their complete usability in transfusion procedures. in the near future, the solution could be to supply laboratories with smart and portable devices that are able to perform fast quality tests for every sample. electrical impedance has relevant potential in analyzing and monitoring blood quality. we propose a new, simple impedancebased biosensor that can perform accurate and efficient single and multi-frequency impedance measurements in the pre-analytical phase and to check the quality of blood samples using quantitative thresholds as useful indicators to ensure the reliability of results and thereby prevent laboratory errors. the proposed sensor allows for discriminating different blood components, identifying hemolysis in serum, evaluating blood quality, and rapidly quantifying its hematocrit. mailto:dezanet@cro.it acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 27 hemoglobin. free hemoglobin causes interferences in many dosage methods and colorimetric analysis and makes serum samples unsuitable for analysis. another inadequacy in a sample is that the serum contains minute white clots, i.e., clots without red cells that are made by a structural net (called fibrin) that forms during coagulation. limited presence of hemolysis or white clots is a sufficient basis for technical problems and measurement disruption [8], [9]. nevertheless, the quality of serum in the pre-analytical phase is usually measured only for samples that visibly show pale red hemolytic shades, while it should be rapidly and always tested for every sample, without exclusion, in order to assure the complete accuracy of laboratory analyses. in addition, quality control of every sample before laboratory analysis or transfusion is not always provided for blood that is collected from both healthy donors and patients. consequently, the identification of inadequacies and the prevention of measurement errors are not guaranteed. anomalies involving the temperature of the containers for the transport and storage of blood are frequent and common and can lead to variations in blood consistency and to its undesired coagulation. these problems should be identified and monitored by means of auxiliary investigations to adequately preserve the samples and to rapidly identify the opportune corrective actions. accordingly, changes in hematocrit value, i.e., the volume of red blood cells as a percentage of the whole blood volume, could represent a useful indicator of transport and storage conditions quality. thus, the quality of blood samples should be always quantified quickly to guarantee their complete compatibility and efficiency in transfusion procedures and to avoid anomalies and errors in laboratory analyses. electrical measurements represent a highly promising tool for analyzing blood. the relevance of the impedance measurement technique for characterizing biomedical human aspects has already been proposed in [10], [11]. blood consists of conductive plasma and suspended cells (especially red blood cells, but also white cells and platelets). cells are composed of an outer lipidic capacitive membrane, which contains an intracellular fluid with dielectric and resistive properties that are similar to plasma. damage to the membrane and, consequently, to its functions is lethal for the cell. ordinary circuit elements, such as resistors and capacitors, are considered as lumped-constant quantities, which have ideal properties. with the aim of modeling blood in terms of its components and dynamic behavior, ideal circuit elements may be inadequate in describing its electrical response. in addition, the interaction of blood with electrodes is too complex to be represented with the ideal response expected for a single electrochemical reaction. the interaction between a biological fluid and a metal electrode forms a narrow interface, known as a double layer, where the current flows as in a capacitive layer, but the current phase shift, in magnitude, is less than -90°. both the blood and the double layer are thus modeled using the constant phase element (cpe) [12] in the equivalent circuit. the cpe is considered a distributed impedance element, examining the imperfect responses of real systems whose impedance is defined as żcpe(ω) = 1/[ccpe(jω) ψ] [12]. the parameter ψ, with 0 ≤ ψ ≤ 1, is the phase shift or phase angle, while ccpe is the capacitance. when ψ = 0 the cpe resembles a pure resistor, instead, when ψ = 1, the cpe looks like an ideal capacitor. measurements for the characterization of whole blood dynamic behavior in artificial micro-channels have been presented using electrical impedance spectroscopy, as described in [13]-[17], by using an impedance meter and a low-cost sensor based on printed circuit board technology. with regard to the characterization of single blood components, this challenge is still open and has not yet been settled by a single multimeasurement sensor. here, we propose a new, simple, impedance-based sensing device. comparing and combining the data on impedance measurements, we are able to characterize and discriminate different blood components and to perform fast and accurate controls to test the quality of blood and serum samples, with the possibility of confirming or rejecting their usability in clinics. 2. materials and methods for each experiment, 3 ml of venous blood was collected into citrate as anticoagulant (hematocrit of 38 ± 3 %) from each of the healthy blood donors who participated in this study. informed written consent was obtained from the participants according to the declaration of helsinki and the dms of the italian ministry of health on november 2nd 2015 (concerning the quality and safety of blood and blood donors). the ethics committee cro-irccs aviano approved the study. afterwards, each vial of whole blood was divided into three parts with the aim of obtaining the materials demonstrated in figure 1a. one part of the whole blood (1 ml) was left untreated (whole blood shown in figure 1a). another part of the whole blood (1 ml) was centrifuged to obtain, as shown in figure 1b, plasma (a liquid substance with all the coagulation proteins, but without cells) and whole concentrated blood (red cells, white cells, and platelets, with a hematocrit of 60 ± 3 %). finally, for the last 1 ml, cacl2 (5 mm final concentration) was added to stimulate the coagulation process and to obtain serum (a liquid substance without cells and without coagulation proteins) and the red clot (red cells, white cells, platelets, and fibrin). the serum thus extracted can appear yellow (the serum in figure 1a) or it can show pale red shades (the serum with low or high hemolysis in figure 1a, c, and d), a visible indication of the presence of hemolysis due to damage to red blood cells. in addition, serum can sporadically contain fibrin white clots. if the serum expresses hemolysis, the white clot can contain fragments of damaged red blood cells, and it looks light red (the white clot with hemolysis in figure 1a). these anomalous situations, not always identifiable by eye, can lead to technical problems and measurement interferences if they are not rapidly recognized. figure 1. blood components characterized with impedance measurements. components disposed in plastic wells (a). whole blood centrifuged (b). blood coagulated and centrifuged (c, d). acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 28 each blood component was homogeneously disposed in a single well (200 μl of substance, well diameter: 16 mm) and their electrical impedance was measured by using a low-cost, small, and reproducible sensing module with a simple geometry (figure 2a). the sensing module appears as a capacitor with parallel planar surfaces made of copper (width: 7 mm, distance between surfaces: 4 mm). a polycarbonate cap supports the accurate positioning of the capacitor in the blood substance (as schematically shown figure 2a and figure 2b) and, consequently, maintains the stability of the measurements. the device was connected to a high-precision lcr meter [18] in order to perform the impedance measurement in the frequency range [100 hz, 2 mhz] with 20 logarithmic spaced steps. a drive voltage of 50 mv was chosen, considering the electrochemical properties of the materials, and open-short calibration was applied with the aim of reducing parasitic effects. the equivalent lumped circuit is presented in figure 2c. it allowed for a satisfactory correlation between the experimental data and the physical processes that govern the system. the double layer at the interface is represented by cpe1, characterized by capacitance c1 and phase shift ψ1, whose impedance is żcpe1(ω) = 1/[c1(jω) ψ1]. furthermore, according to the purely resistive behavior of the extracellular fluid and considering the necessity of representing different cellular and membrane conditions with a generalized and versatile circuit element, blood was modeled in terms of a parallel between the resistance and the cpe. the cpe2 models the cells with their membrane and is characterized by capacitance c2 and phase shift ψ2, and exhibits and impedance żcpe2(ω) = 1/[c2(jω) ψ2], while resistance r represents the fluid in which the cells are suspended. the blood of n = 20 different donors, equally distributed in terms of gender, was collected and analyzed, obtaining impedance signals varying with frequency and stable in time. in addition, the hematocrit was measured for all the blood samples with a commercial instrument that is commonly used in clinical laboratories. 3. results and discussions characterization of blood components impedance signals related to the different blood components of figure 1 are displayed in figure 3 as magnitude (left panel) and phase (right panel). for each material, the average signal, with its related standard deviation, is illustrated (n = 20). it is evidently possible to use this new prototype to discriminate blood components on the basis of their impedance magnitude and phase, which are representative of their resistance to the current flow and depend on the amount of the capacitive contribution due to the lipidic membrane (mostly of the red blood cells). in particular, observing the magnitude of impedance at the central frequencies in figure 3, from the bottom to the top, following a decrease of conductivity, we can observe in order: white clot, plasma, serum, white clot with hemolysis, serum with increasing levels of hemolysis, whole blood at different densities, and red clot. the properties of blood components, revealing that the electrical conductance decreases as the red blood cells concentration increases, can also be qualitatively appreciated from the phase diagrams of figure 3, where it is evident that all the blood components without red cells or containing damaged figure 2. sensing module for electrical impedance measurements. a polycarbonate cap is placed on each well, and the parallel copper conductive surfaces are immersed in the fluid substance (a, b). equivalent circuit (c). figure 3. electrical impedance signals displayed as magnitude (left panel) and phase (right panel). each blood component is represented in terms of mean and standard deviation (n = 20). acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 29 red cells have an impedance phase that is distinguishable from the impedance phase of samples with healthy red blood cells. in addition to the evident capability of characterizing different blood components with electrical impedance measurements, the most promising results (presented in figure 3) are shown by the curves of serum (pure or with hemolysis as well as those relative to the blood), specifically the combination that comprises whole blood, whole concentrated blood, and red clot. these results allow for associating and proposing a specific quantitative preanalytical threshold value for the rapid quality control of serum and whole blood samples. according to the impedance magnitude signals analyzed and shown here, we can thus confirm that serum and whole blood samples can be accepted for laboratory analyses and transfusions if their impedance magnitude levels at the central frequencies measured with our sensor do not exceed 700 ω and 3 kω, respectively. an interesting point that deserves attention and further experiments is represented by the white clot curves. in figure 3, white clots without hemolysis seem to be the most conductive material among those analyzed, which is because the fiber net is a highly conductive structure of fibrin, and there are no resistive cellular components inside. by using this approach, fibrin could be electrically described, and its formation could be identified and monitored in real time during the whole physiological process of coagulation. the discrimination of blood components through the impedance signals was confirmed and enriched by the inverse analysis performed using complex non-linear least square (cnls) fitting software. thanks to this software, it was possible to quantify each single lumped elements parameter introduced in figure 2c (c1, ψ1, r, c2, ψ2) with an uncertainty in the order of 2%. the numerical parameters estimated, related to the equivalent circuit of figure 2c, are displayed in table 1 and are expressed in terms of mean and standard deviation (n = 20). an important aspect of the electrical characterization of a system is the possibility to identify and associate equivalent circuit elements with specific physical processes occurring. in particular, the parameters of cpe1, related to the double layer, (c1, ψ1) appear to be almost equal among all the blood components analyzed. in fact, the double layer is common among all the blood components because it is associated with the interface interaction between the material and the metallic electrodes. for this reason, cpe1, representing a common electrical behavior, can be compensated with the aim of evidencing the specific electrical properties of each biological material, dealing with the specific electrical response of the material alone. the numerical results confirm that the cpe2 is directly related to the presence or the absence of cells (mostly red blood cells) and the possible breaking of their membrane. in particular, the values of the cpe2 parameters (c2, ψ2) and r are significantly different if there are cells (the higher the number or concentration, the higher the value), if cells are absent, or if there are cells with a damaged membrane (the hemolysis condition). in addition, the phase shift of cpe2, ψ2, measures exactly one when cells are absent or their membrane is damaged (blood plasma and serum), indicating that, in such cases, cpe2 behaves like an ideal capacitor, with a very low capacitance c2. interpolations of frequency points are shown in figure 4, for all the blood components analyzed, in a real-imaginary plot, without double-layer compensation (the left panel) and with compensation (the right panel), which highlights the differences between the samples on the basis of their intrinsic characteristics. imagining the natural extension of the compensated curves in figure 4 towards infinite frequencies, it is evident that they tend to blood plasma in agreement with the decrease in the capacitive behavior at high frequencies. all the results confirm and verify the physical reasonability of the equivalent circuit that has been chosen and is proposed herein. single-frequency measurements with the aim of obtaining more accurate and informative results, measurements are generally expected to be performed using a multi-frequency approach. as previously described, the electrical impedance of blood components was measured over a table 1. extracted lumped elements according to the equivalent circuit. the values of c1, ψ1 represent the common double-layer effect, while the specific intrinsic blood components behavior can be described through the values of c2, ψ2 and r. acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 30 large frequency range, from 100 hz to 2 mhz. observing the signals of impedance magnitude (shown in figure 3), we tried to identify the most ideal single frequency (among the frequencies used) in terms of accuracy and capability in discriminating the different blood components with resolution, with the purpose of improving the efficiency and performance of our measurements. figure 5, which follows the same color codes used in figure 3, shows the quantitative analysis that is used for extracting the optimal frequency in the range [100 hz, 2 mhz]. observing the impedance magnitude signals in figure 3, it is clear that the best frequency that is usable for a single-frequency measurement takes place in the central interval of frequencies, where the distance between signals seems to be greater than the distance at lower or higher frequencies. according to this observation, we quantified, for all the possible pairs of signals belonging to the adjacent blood components visible in figure 3, the difference between their percentages in terms of impedance magnitude, choosing the worst difference for representation in figure 5a the minimal distance between a blood component and its adjacent one. for example, to present the black line in figure 5a, the percentage differences were computed between each red clot signal and each signal of whole blood concentrated, choosing, in the end, the lowest among the signals thus quantified. at the same time, we examined the trend of impedance magnitude standard deviation for each blood component, expressing it as a percentage of its average value and illustrating it in figure 5b. according to the results shown in figure 5a and figure 5b, the optimal frequency is 104 hz because, at this frequency, all the blood components can be accurately and completely discriminated (in terms of impedance magnitude) from the adjacent ones, as well as in the worst possible case. figure 5c shows the discrimination of blood components at 104 hz. impedance magnitude and hematocrit hematocrit is the volume of red blood cells as a percentage of the whole blood volume, and its value can give information about the quality of blood transport and storage conditions. hematocrit is a crucial parameter that is used to identify and quantity density variations or aggregative states in blood samples in situations of alteration that make blood unsuitable for analysis and unsuitable for transfusion. the hematocrit of each whole blood sample was measured with a commercial instrument that is commonly used in clinical laboratories and was compared with the impedance magnitude of the same sample, measured at 104 hz. as shown in figure 6, by measuring the impedance with the proposed sensor, we are able to rapidly achieve the corresponding hematocrit value. in fact, the data appears highly correlated, with a pearson’s correlation coefficient r = 0.89, and the correlation can be linearly approximated by y = 62.9 x + 6.8, where y represents the impedance magnitude and x represents figure 4. electrical impedance signals displayed as real-imaginary plot, without (left panel) and with (right panel) the compensation of the double layer effect. each blood component is represented in terms of mean and standard deviation, both for the real and the imaginary part (n = 20). figure 5. optimal frequency quantification for single-frequency measurements. worst percentage differences in impedance magnitude between adjacent blood components (a). impedance magnitude standard deviation for each blood component, expressed as a percentage of the related average value (b). impedance magnitude at 104 hz, where each blood component is represented in terms of mean and standard deviation (n = 20). acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 31 the hematocrit values. the obtained results show that the proposed measuring system can be used to rapidly detect the blood hematocrit level. 4. conclusion strategies in the field of risk management and patient safety are aimed at the prevention, detection, and reduction of adverse events by means of analysis of measurable and controllable errors. in clinical laboratories, the majority of critical issues occur in the pre-analytical phase, especially in activities in which the human component is still decisive. information provided by clinical laboratories affects almost the totality of clinical decisions about diagnoses and treatments. thus, the current focus of healthcare institutions is the improvement of patient safety, with more attention given to the prevention of pre-analytical mistakes and laboratory errors. the improvement of pre-analytical processes currently constitutes a challenge in clinical laboratories. the most frequently encountered problems are serum samples with hemolysis (in which their alteration is not visible and can compromise analytical results) and blood samples that are unsuitable for transfusion because of inappropriate sampling and storage conditions. there are different means and techniques available today, but the journey towards complete and effective rationalization of these laboratories is still long. in this work, we presented a new simple impedance-based biosensor to perform fast impedance measurements in the preanalytical phase. different blood components were measured and characterized in terms of impedance magnitude and phase. in particular, observing the impedance magnitude signals of serum and whole blood, we identified the upper thresholds of 700 ω and 3 kω, respectively, as useful indicators for a fast check of the quality of the samples using our sensor. in addition, each blood component has been quantitatively described, estimating the parameters of the equivalent circuit proposed and finding some interesting and coherent correlations between the experimental results and the physical characteristics of the system investigated. all these results, together with the interpolation of the frequency points in the real-imaginary plot shown, have confirmed the adequacy of the equivalent circuit that has been chosen. the impedance analysis has also allowed for the quantitative identification of 104 hz as the optimal frequency for performing single-frequency measurements with greater efficiency and without loss of accuracy. finally, correlation between the impedance magnitude of the whole blood at 104 hz and the hematocrit percentage value was achieved and quantified. thanks to the impedance measurements and the electrical characterization of different components of blood samples, further optimization of costs, resources, and means of performing quality control (especially on serum and blood in the pre-analytical phase) has been proven as feasible. through static electrical impedance measurements and analysis, it is possible to carry out very fast and automated controls for every sample, thus reducing the risk of errors in laboratory processes and limiting the propagation of these errors directly to the patients’ health and safety. acknowledgements we are grateful to dr. a. steffan and dr. r. vettori for their valuable clinical and methodological suggestions and to e. savaris, c. buciol, e. bolzonaro, and s. bravin for their technical support. references [1] g. lippi, m. plebani, a. m. simundic, quality in laboratory diagnostics: from theory to practice, biochem. med. 20(2) (2010) pp. 126-130. [2] m. plebani, quality indicators to detect pre-analytical errors in laboratory testing, clin. biochem. rev. 33(3) (2012) pp. 85-88. [3] g. lippi, g. c. guidi, c. mattiuzzi, m. plebani, preanalytical variability: the dark side of the moon in laboratory testing, clin. chem. lab. med. 44(4) (2006) pp. 358-365. [4] p. capraio, p. plebani, process control reduces the laboratory turnaround time, clini. chem. lab. med. 40(4) (2002) pp. 421-422. [5] g. lima-oliveira, w. volanski, g. lippi, g. picheth, g. c. guidi, pre-analytical phase management: a review of the procedures from patient preparation to laboratory analysis, scand. j. clin. lab. invest. 77(3) (2017) pp. 153-163. [6] g. lippi, g. l. salvagno, n. blanckaert, d. giavarina, s. green, s. kitchen, v. palicka, a. j. vassault, m. plebani, multicenter evaluation of the hemolysis index in automated clinical chemistry systems, clini. chem. lab. med. 47(8) (2009) pp. 934-939. [7] g. lippi, m. plebani, e. j. favaloro, technological advances in the hemostasis laboratory, semin. thromb. hemost. 40(2) 2014 pp. 178-185. [8] k. c. gersh, c. nagaswami, j. w. weisel, fibrin network structure and clot mechanical properties are altered by incorporation of erythrocytes, thromb. hemost. 102(6) (2009) pp. 1169-1175. [9] p. bhattacharjee, d. bhattacharyya, an insight into the abnormal fibrin clots its pathophysiological roles, fibrinolysis and thrombolysis, krasimir kolev ed., 2014, isbn 978-953-51-12655. [10] k. f. lei, k. h. chen, p. h. tsui, n. m. tsang, real-time electrical impedimetric monitoring of blood coagulation process under temperature and hematocrit variations conducted in a microfluidic chip, plos one 8(10) (2013) p. e76243. [11] a. siegman, s. abboud, bioimpedance technique for monitoring cerebral artery stenosis in a 3d numerical model of the head, med. eng. phys. 34(8) (2012) pp. 1095-1100. [12] e. barsoukov, j. r. macdonald, impedance spectroscopy theory experiment and applications, wiley, new york, 2005, isbn 9780-471-64749-2. [13] a. affanni, r. specogna, f. trevisan, combined electro-optical imaging for the time evolution of white thrombus growth in figure 6. quantitative comparison between the impedance magnitude at 104 hz and the hematocrit measured in the laboratory, for whole blood samples. the data is highly correlated (pearson’s correlation coefficient r = 0.89) and the correlation is linearly represented by the equation y = 62.9 x + 6.8, where y represents the impedance magnitude and x represents the hematocrit values. acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 32 artificial capillaries, ieee trans. instrum. meas. 62(11) (2013) pp. 2954-2959. [14] a. affanni, r. specogna, f. trevisan, ex vivo time evolution of thrombus growth through optical and electrical impedance data fusion, j. phys. conf. ser. 459 (2013). [15] a. affanni, g. chiorboli, l. codecasa, m .r. cozzi, l. de marco, m. mazzucato, c. morandi, r. specogna, m. tartagni, f. trevisan, a novel inversion technique for imaging thrombus volume in microchannels fusing optical and impedance data, ieee trans. magn. 50(2) (2014) p. 6749262. [16] a. affanni, g. chiorboli, r. specogna, f. trevisan, uncertainty model of electro-optical thrombus growth estimation for early risk detection, measurement 79 (2016) pp. 260-266. [17] d. de zanet, m. battiston, e. lombardi, r. specogna, f. trevisan, l. de marco, a. affanni, m. mazzucato, impedance biosensor for real-time monitoring and prediction of thrombotic individual profile in flowing blood, plos one 12 (2017) p. e0184941. [18] agilent technologies, agilent e4980a precision lcr meter users guide, 2008. acta imeko, title acta imeko issn: 2221-870x december 2017, volume 6, number 4, 100-104 acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 100 a new system for the measurement of gripping force based on scattering m. becchetti1, r. marsili1, f. cannella2, a. garinei3 1dipartimento di ingegneria, università degli studi di perugia, perugia, 06125, italy 2istituto italiano di tecnologia, genova, 16163, italy 3dmii, università degli studi guglielmo marconi, roma, 00193, italy section: research paper keywords: frustrated total internal reflection (ftir); fingertip contact pressure; biomechanics citation: m. becchetti, r. marsili, f. cannella, a. garinei, a new system for the measure of gripping force based on the scattering, acta imeko, vol. 6, no. 4, article 16, december 2017, identifier: imeko-acta-06 (2017)-04-16 editor: paul regtien, measurement science consultancy, the netherlands received may 16, 2017; in final form december 01, 2017; published december 2017 copyright: © 2017 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: roberto marsili, e-mail: roberto.marsili@unipg.it 1. introduction the investigation of the mechanisms of human tactile perception represents a fundamental topic in haptics. the fingertip deformation is the basic mechanical action in the tactile perception, since tactile sensitivity depends on the tissue strain and hence on the contact area. different models are available to predict the behaviour of the fingertip in terms of contact area, deformation and pressure distribution [1], [2]. measurements are thus required both to determine the static fingertip area and force as input to the numerical models and to measure the area and the pressure necessary for numerical model validation. when measuring the contact pressure, the goal is to set up a measurement system with high sensitivity at low pressure, since, for specific applications, the typical pressure range of interest is from 0 to 50 kpa. the main objective of this work is the development of a measurement system, which allows to characterize the different aspects of the interaction finger contact surface, through the measurement of the contact area and of the pressure distribution, both on static and dynamic conditions. many sensors based on different physical principles have been proposed for the measurement of pressure at contact surface between two rigid or flexible bodies. some contact pressure measurement systems have been used for measuring pressure distribution at interfaces between objects, with application to comfort analysis and improvement of vehicle seats [3], or steering wheel [4], for various biomedical applications such as measurement of contact pressure between hand and handle of a tool in order to analyse vibration transmission to the hand [5], [6] or between foot and ground for plantar pressure analysis [7], [8]. in [9]-[11] a new measurement technique for the measurement of contact pressure distributions between bodies in contact is proposed, based on the measurement principle known as thermoelasticity. many applications have been developed [12]-[14] also in other fields such as robot technology (tactile sensors), for contact force mapping of mechanical parts. the film sensors used for those applications are based on piezo-polymers, capacitive abstract the measurement of contact pressure of human fingers is very important to understand human perceptual mechanisms, that is the main goal of most of the neuroscientific studies. it may also lead to a correct development of tactile devices and haptic systems, as they are intended to convey controllable and effective stimuli. in this work, an optical measurement system based on frustrated total internal reflection (ftir) is proposed for the measurement of the pressure distribution on the contact area between a human finger and a flat surface. the feasibility study performed shows that the tested sensor can be effectively used for the measurement of the fingertip contact pressure both on static and dynamic conditions. acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 101 sensors, conductive ink, resistive polymers sensing elements or magnetoresistivity [15]. however, many metrological problems remain unresolved, due to non-linearity, material's rheological behaviour, mounting surface curvature and shape, dynamic characteristics, etc. also frequency limitations can be due to the sensor itself or to the electronic processing and data acquisition system, when a matrix of many sensing elements is necessary. piezo-polymers have probably the best metrological characteristic but cannot measure very slow fluctuations or static pressure components. sensors based on conductive ink or resistive polymers showed from preliminary tests that metrological characteristics change with time and have large hysteresis. the capacitive sensing elements can be used to measure both dynamic and static pressure with acceptable linearity and hysteresis, and bendable sensors may be applied to the wearable health monitoring/sensing system or electronic skins, as in [16]. here an optical measurement system based on light scattering is developed and analysed. the outline of this paper is as follows. first, the design of an optimized measurement system based on frustrated total internal reflection (ftir) is described. then the results of specific tests performed on the measurement system are shown, to define its static and dynamic behaviour and to evaluate if the proposed measurement technique can be effectively used to measure human fingertip mechanical properties in terms of contact area and pressure distribution. 2. experimental background and setup the principle of the measurement system is the light reflection/refraction at the interface between two transparent media. when the light passes through an interface between media of different refractive indices, the path of travel changes (snell’s law). thus when a light source is installed at the edge of a thin glass plate and the incident angle ϑ1 exceeds the critical angle ϑcritical (figure 1a), the plate acts as a light guide. in this case (total internal reflection) the light cannot be seen when observing the plate from below the plate. if a reflecting media is pressed against the plate, the surface of the media will scatter light in the contact surface, being the media with a refractive index different from the refractive index of the air (figure 1b). by observing the scattered light, it is thus possible to view the shape of the contact surface. depending on the roughness of the contact surface, the contact area will increase with the contact pressure, thus the light intensity can be used to measure the contact pressure between the two media (figure 1c). methods based on white light refraction are used for the measurement of normal contact stress between 'soft' structures and a rigid planar surface, specifically for application to tire footprint analysis [17], [18]. these systems are optimized to measure tire footprint pressures and are not suitable for the measurement of low pressures. the design of the proposed measurement system was developed to permit the measurement of low pressures that characterize human fingertip contact. the roughness and the mechanical characteristics of the contact surface are the most critical parameters for the realization of an effective pressure measurement system based on ftir. these parameters must be carefully evaluated, because of their effects on the range and on the sensitivity of the measurement system. the measurement system setup is shown in figure 2: a sliding system realized through threaded rods permits to adjust the camera position depending on its optics; a 200 mm x 200 mm x 200 mm darkroom was realized to avoid interference of light from the external environment; a 150 mm x 150 mm x 10 mm glass plate was fixed on a square support with a green led strip installed around the perimeter. the main characteristics of the glass plate are shown in table 1. 3. system characterization 3.1. static characterization a first static analysis of the system was performed to characterize the behaviour of the sensor when a constant pressure is applied on it. for this purpose, the glass plate was covered with a thin membrane and fixed on a square support figure 1. principle of measurement [17]. (a) shows the lighting system, (b) the light scattering, (c) the detail of the contact area. table 1. glass plate characteristics. chemical glass composition (sio2) + oxides aspect transparent crystallinity amorphous refractive index from 1.458 to 1.86 nominal tensile strength 4 mpa nominal compression strength 1 gpa nominal bending strength 40 mpa figure 2. test bench for static calibration. acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 102 with a green led strip installed around the perimeter. a plexiglas frame was then realized to install the thin lattice membrane at the top of the glass plate: the membrane is stretched, thus there is a thin air gap (no contact) between the membrane and the glass plate in the starting configuration. the lattice membrane is talc-covered to increase the roughness of the contact surface and thus the sensitivity of the measurement system. different test setups can be achieved by varying the amount (thickness) of talc powder (estimated by weighing) and using talc with different granulometry. in the work, we want to develop a methodology for measuring the metrological performance of the sensor, which is obtained by calibration. problems like the uniformity of the overlay will be faced during prototype engineering, with a productive industrial process more accurate than a laboratory production. in addition to talcum, other materials can be used. figure 3 illustrates a typical result of the relation between thickness and pressure for the latex membrane used. from this result it is possible to see, in general, the nonlinear behavior of the latex membrane. however, in the pressure range of interest (0 – 50 kpa), as a first approximation, the curve pressure vs thickness reduction can be considered linear. for the static calibration, a reference pressure is applied to the sensor by the thin membrane, which is loaded by a constant pressure chamber, whose pressure is measured by a pressure sensor connected to a scandura pascal 100 system. in order to avoid pressure losses, the contact between the pressure chamber and the glass plate is realized through an o-ring for pneumatic applications (figure 2). the reference pressure varies from 0 pa to 100 kpa, that is the maximum pressure to avoid fracture of the glass plate. a high performance reflex digital camera (canon eos 1100d, with a 12.2 megapixel cmos sensor, fast shutter speed 1/4000 s), was used to take high resolution grey-level images. the camera was equipped with a 18-55mm f/3.5-5.6 lens: the distance from the camera to the glass plate was adjusted depending on the focal length of the camera. the camera parameters (iso settings, shutter speed, white balance) were adjusted to obtain maximum sensitivity for the specific setup (fnumber = 5.6, exposure time = 1/8 s, iso 400). the raw images (4272 x 2848 pixels) were acquired with the software canon eos utility, converted in .tiff format and thus post-processed with a matlab toolbox. images were acquired for each pressure step (5000 pa – 21 steps) in the calibration range. for a given pressure, the grey level of each pixel varies depending on the pixel location and on the local characteristics of the contact surface (roughness). figure 4 illustrates a typical static calibration diagram: abscissa refers to the reference pressure, estimated by a wika pascal 100, with range 0 – 2.5 bar, resolution 0.01 mbar, uncertainty 0.025 % f.s.; the ordinate refers to the grey level (range 0 ÷ 256) measured by the sensor. the calibration curve is obtained using the method of least squares. the uncertainty is less than 1 % full scale. after these first measurements, a measurement area was defined to reduce the dispersion due to the localized imperfections of the contact surfaces (figure 5). this is the most effective sensing area both for static and dynamic measurements. the optimal measurement area is a 20 × 20 mm box. 3.2. preliminary dynamic characterization then a dynamic characterization was performed by loading the sensor with a sinusoidal excitation applied through a rigid feeler pin (figure 6). the compression force, generated by the electrodynamic exciter, is measured by a load cell. the deformation of the glass plate was so small that its effect can be neglected. tests were performed using sine excitation in the linear range of the sensor (10 − 100 kpa): typical signals obtained are shown in figure 7. for the image processing specific software in labview has been developed (figure 8). figure 4. typical static calibration diagram for a pixel. figure 5. measurement area selection. figure 3. typical pressure sensor thickness reduction curve [19]. acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 103 the tests are conducted changing the excitation frequency, while keeping the level of force fixed. the ratio between system output (gray level) and force amplitude vs frequency is plotted in figure 9. these results allow establishing that a useful bandwidth is from 0 up to 10 hz with tolerable response decrease lower than 5 %. further tests are planned to determine the maximum measurable frequency using this system. 4. conclusions the analysis of the contact pressure between the finger and a contact surface is of remarkable interest in the field of biomedical design, to characterize human fingertip mechanical properties in terms of contact area, deformation and pressure distribution. in this work a measurement technique based on ftir is proposed for the analysis of contact pressure between the finger and a flat contact surface (glass plate). a ftir measurement system was designed and tested to evaluate the feasibility and effectiveness of this measurement technique to measure the contact area and the pressure necessary for numerical fingertip model validation. results showed that there are good assumptions to use the sensor for the measurement of the fingertip contact pressure both on static and dynamic conditions. consistency and repeatability observed at this first stage is very encouraging for the development of an effective fingertip pressure measurement system based on ftir. this will permit to analyse the behaviour of the finger for different conditions, and thus to validate numerical models not only in contact area and force, but also in pressure, that is suitable for investigating the fingertip deformation on static and dynamic conditions. plans for future work are to further explore new materials for a higher sensitivity and testing different configurations with a complete set of loading conditions. references [1] j. z. wu, r. g. dong, s. rakheja, a. w. schopper, and w. p. smutz, “a structural fingertip model for simulating of the biomechanics of tactile sensation,” med. eng. phys., mar. 2004, vol. 26, no. 2, pp. 165–175. [2] s. shimawaki, n. sakai, “quasi-static deformation analysis of a human finger using a three-dimensional finite element model figure 6. test bench for dynamic characterization. figure 7. time history output system and force.figure x. stamp. figure 8. image processing software interface. figure 9. time history output system and force. acta imeko | www.imeko.org december 2017 | volume 6 | number 4 | 104 constructed from ct images”, journal of environment and engineering, 2007, vol. 2, no. 1. [3] m. shiratori, m. ishida, “development of body pressure distribution measurement device”, jsae review, july 1992, vol. 13 n. 3. [4] r. marsili, a. garinei, “development of a new capacitive matrix for pressure distribution measurement” international journal of industrial ergonomics volume 44, issue 1, january 2014, pages 114-119 doi: 10.1016/j.ergon.2013.11.012. 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[9] m. becchetti, r. flori, r.. marsili, m. moretti, "comparison between digital image correlation and thermoelasticity for strain field analysis" (2010) 9th international conference on vibration measurements by laser and noncontact techniques and short course; ancona; italy; 22 25 june 2010 aip conference proceedings, volume 1253, pp. 233-240, doi=10.1063/1.3455462 issn: 0094243x isbn: 978073540802-9. [10] r. marsili, g. brustenga, m. moretti, j. pirisinu, g.l. rossi, “measurement on mechanical component by thermoelasticity”, journal applied mechanics and materials, isbn 0-87849-987-3 issn 1660-9336, vols 3-4, august 2005, pp 337-342, pp 411 – 416. scopus: 2-s2.0-33745170726. 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[18] a. n. gent, j. d. walter, “the pneumatic tire”, the national highway traffic safety administration, u.s. department of transportation, washington dc 20590, august 2005, pp 248250. [19] r. marsili, a. garinei, “thermoelastic stress analysis of the contact between a flat plate and a cylinder”, measurement: journal of the international measurement confederation, 52 (1), pp. 102-110 (2014) http://dx.doi.org/10.1016/j.measurement.2014.03.005 (2014). a new system for the measurement of gripping force based on scattering << /ascii85encodepages false /allowtransparency false /autopositionepsfiles true /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /error /compatibilitylevel 1.4 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.0000 /colorconversionstrategy /cmyk /dothumbnails false /embedallfonts true /embedopentype false /parseiccprofilesincomments true /embedjoboptions 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can be opened with acrobat and adobe reader 5.0 and later.) >> /namespace [ (adobe) (common) (1.0) ] /othernamespaces [ << /asreaderspreads false /cropimagestoframes true /errorcontrol /warnandcontinue /flattenerignorespreadoverrides false /includeguidesgrids false /includenonprinting false /includeslug false /namespace [ (adobe) (indesign) (4.0) ] /omitplacedbitmaps false /omitplacedeps false /omitplacedpdf false /simulateoverprint /legacy >> << /addbleedmarks false /addcolorbars false /addcropmarks false /addpageinfo false /addregmarks false /convertcolors /converttocmyk /destinationprofilename () /destinationprofileselector /documentcmyk /downsample16bitimages true /flattenerpreset << /presetselector /mediumresolution >> /formelements false /generatestructure false /includebookmarks false /includehyperlinks false /includeinteractive false /includelayers false /includeprofiles false /multimediahandling /useobjectsettings /namespace [ (adobe) (creativesuite) (2.0) ] /pdfxoutputintentprofileselector /documentcmyk /preserveediting true /untaggedcmykhandling /leaveuntagged /untaggedrgbhandling /usedocumentprofile /usedocumentbleed false >> ] >> setdistillerparams << /hwresolution [2400 2400] /pagesize [612.000 792.000] >> setpagedevice development of a tri-axial primary vibration calibration system acta imeko issn: 2221-870x march 2019, volume 8, number 1, 33 39 acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 33 development of a tri-axial primary vibration calibration system zhihua liu1, chenguang cai1, ming yang2, mei yu1 1 national institute of metrology, bei san huan dong lu 18, 100029 beijing, china 2 beijing university of chemical technology, bei san huan dong lu 15, 100029 beijing, china section: research paper keywords: primary calibration; spatial orbit; multi-exciter control; interferometry; tri-axial vibration exciter citation: zhihua liu, chenguang cai, ming yang, mei yu, development of a tri-axial primary vibration calibration system, acta imeko, vol. 8, no. 1, article 6, march 2019, identifier: imeko-acta-08(2019)-01-06 editor: maija ojanen-saloranta, vtt technical research centre of finland, national metrology institute mikes, finland received august 24, 2018; in final form october 25, 2018; published march 2019 copyright: © 2019imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was supported by special fund for scientific research in the public interest (201410009) and the national natural science foundation of china (no. 51605461). corresponding author: zhihua liu, e-mail: liuzhihua@nim.ac.cn 1. introduction multi-exciter vibration systems have the ability to accurately replicate the real-word realistic dynamic environment of multiaxis motions, which provides access to the simultaneous multicomponent calibration of motion transducers [1], [2]. following single-axial vibration exciters and planar testing systems, tri-axial vibration exciters that can generate spatial orbit vibration is an important development in the field of transducer calibration [3], [4], [5]. ptb set up a tri-axial vibration standard measuring device for the calibration of vibration transducers under multi-axial excitation [6], [7], [8]. the tri-axial vibration exciter can generate arbitrary motion, the linear combination of motion, along each of its three axes. iso 16063-31 presents an approach to the testing of transverse vibration sensitivity using a tri-axial vibration exciter [9]. the vibration direction in the x-y plane is changed every 30 ° in order to evaluate transverse sensitivity [10]. umeda proposed the sensitivity matrix calibration method of accelerometers using the tri-axial vibration generator and three laser interferometers. the tri-axial vibration exciter was employed to generate three independent motion vectors in order to calibrate all the sensitivities of a tri-axis accelerometer [11], [12], [13]. qualified spatial orbits, such as linear orbits and circular orbits, play an important role in transducer calibration. vibration control on a multi-exciter testing system is an essential prerequisite to generate accurate spatial orbit vibration. multiexciter vibration control deals not only with amplitude control (as in single-exciter vibration control), but also cross-coupling compensation and phase control [14], [15]. the linear iterative algorithm involving correction of the driving spectrum with the error spectrum and the system impedance was first introduced in [16], [17]. subsequently, the adaptive iterative algorithm was presented in [18], [19], according to which the system impedance keeps updating for system nonlinearity, and a variable adjustment gain is used for driving spectrum correction. the current vibration control approach concerns more amplitudes and phases for each individual exciter instead of the spatial orbit for abstract a tri-axial primary vibration calibration system has been set up at national institute of metrology for simultaneous calibration of motion transducers. the system is driven by three electrodynamics exciters that are mounted along the three orthogonal axes. the crosscoupling unit based on air bearing has been developed for force transference and motion guiding. spatial orbit vibration is composited from sine vibration components of the three orthogonal axes. the relationships between the shapes and orientations of the spatial orbits and the amplitudes and phases of the sine vibration components are discussed. multi-exciter vibration control for both crosscoupling compensation and the amplitude and phase control of the sine vibration components is also investigated. the tri-axial measuring system can simultaneously measure the three orthogonal vibration quantities based on the band-pass sampling method. the experiments show that a variety of spatial orbits can be generated by efficiently reducing the cross-coupling of the tri-axial vibration exciter and the magnitude, and the phase shift of the sensitivities of a tri-axis accelerometer can be determined. mailto:liuzhihua@nim.ac.cn acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 34 calibration purposes. issues concerning the nature of the relationship between spatial orbits and sine vibration components and how to achieve a certain spatial orbit still require further study. to achieve the multicomponent calibration of vibration transducers, the national institute of metrology in china has developed a tri-axial primary vibration calibration system that consists of a cross-coupling unit, a spatial orbit control system, and a primary tri-axial vibration measuring system. in order to reduce costs and improve performance, some new technologies have been used in this system. the paper is organized as follows: in section 2, a description of the tri-axial primary vibration calibration system is given. in section 3, spatial orbit generation by means of the tri-axial vibration exciter and multi-exciter vibration control are investigated. in section 4, experimental investigation of the performance of the tri-axial primary vibration calibration system and calibration use is presented. in the last section, conclusions of this paper are given. 2. system description the tri-axial primary vibration calibration system was designed to generate and measure spatial orbit vibration in the three orthogonal axes. as illustrated in figure 1, it consists of the tri-axial vibration exciter, the vibration control system, and the primary vibration measuring system. 2.1. the air-bearing cross-coupling unit the tri-axial vibration exciter is driven by three electrodynamic exciters that are mounted along the orthogonal axes relative to the floor. the air-bearing cross-coupling unit was developed for the force transference and motion guiding of the tri-orthogonal axial vibration [20], [21]. as illustrated in figure 2, the air-bearing cross coupling unit is composed of five sets of air bearings. the x-, y-, and z-axial force-transferring air bearings are used to transfer the vibration generated by the electrodynamic exciters. the xand y-axial motion-guiding air bearings help to guide the xand y-axial motions, respectively. as the transducer under test needs to be mounted on the top of the coupling unit, there is no space to install a z-axis motionguiding air bearing. besides, each of the air bearings do not prevent the motion along the other two orthogonal axial directions. commercial tri-axial vibration exciters commonly develop cross-coupling units by means of oil film bearings that need additional auxiliary equipment and may also heat the device under test. the air bearings have the advantages of being cheap, easy to maintain, and not thermic; therefore, they have been used in the tri-axial vibration system. the restrictions of the tri-axial vibration exciter are listed in table 1. 2.2. the primary vibration measuring system the spatial orbit vibration is measured by tri-orthogonal heterodyne interferometers, which are mounted on vibration isolation platforms. the interference signals of the three interferometers and the outputs of the transducer under test are sampled by a six-channel data acquisition system. usually, there are two sampling methods of dealing with interference signals whose central frequency is about 40 mhz. to fulfil nyquist’s theorem, a high-speed acquisition card to deal with the reference interference signal with a central frequency of 40 mhz is necessary. in this case, a large amount of data needs to be sampled and processed. the other way is to use an analogue mixer and low-pass filter to down-convert the signal frequency. however, the phase shift and inconsistency of the analogue devices influence the measuring results. as the frequency band of the reference signal of the interferometer in this application is lower than 2 mhz, the band-pass sampling method can be used to directly sample the reference signals with a sample rate that is less than 10 mhz, without any analogy device. according to the procedures presented in iso 16063-11 [3], three synchronized interference signals are demodulated to realize the primary measurement of the spatial orbit vibration. 2.3. the vibration control system the vibration control system was established based on the pxi system, and the control program was developed using labview software. two ni pxi-4461 acquisition cards were used to acquire the tri-axial acceleration feedback signals and to figure 1. the tri-axial vibration calibration system. figure 2. the air-bearing cross coupling unit. table 1. the restrictions of the tri-axial vibration exciter max. peak force 1000 n max. pay load 10 kg max. peak acceleration 20 m/s2 frequency range 5 hz to 1.6 khz air pressure required 4 bar to 8 bar cross motion under control < 1% acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 35 generate three channels’ analogue voltage outputs for the exciters’ amplifiers. multi-exciter vibration control deals not only with amplitude control (as with single-exciter vibration control), but also with cross-coupling compensation and phase control. the transfer function matrix should be determined by pre-exciting the system with multi-random signals in order to calculate the impedance matrix. the desired motion signal, the response, and the driving multi-exciter sine signals are both transformed into spectrum vectors. the driving spectrum is corrected according to the error spectrum and the impedance matrix during the control loop. the impedance matrix also keeps updated for system nonlinearity, and a variable adjustment gain is used for driving spectrum correction. the vibration control algorithm is implemented using the mathscript rt module. a detailed deduction of the spatial orbit control algorithm is presented in section 3. 3. spatial orbit vibration control 3.1. spatial orbit generation sinusoidal vibrations of the three axes with the same frequency (regardless of amplitudes and phases) are bound to result in a spatial elliptical orbit as depicted in figure 3. the shape and orientation of the spatial elliptical orbit depend on the sinusoidal vibration amplitudes and phases of the three axes. the orientation of the spatial ellipse can be expressed by a set of three unit vectors that are orthogonal to each other. as shown in figure 4(b), u and v are parallel with the major and minor axes of the spatial ellipse respectively. w is perpendicular to the spatial ellipse plane. the three unit vectors are as follows:  t321 uuu=u ,   t 321 vvv=v , vuw = . (1) from the theory of coordinate transformation, any spatial ellipse can be obtained from a standard ellipse in the x-y plane by the rotation of certain angles. figure 4(a) depicts a standard ellipse whose major and minor axes are parallel with the coordinate axes. we can establish a local coordinate frame with its origin located at the centre of the spatial ellipse and its coordinate axes being parallel with the unit vectors, as shown in figure 4(b). the coordinate transformation of the two ellipses can be achieved by the rotation matrix r consisting of the three unit vectors.  wvur = (2) the sinusoidal vibration components of the spatial elliptical orbit can be described in its local coordinate frame as ( ) ( ) ( )                 =           t t ta ta ta z y x     sin cos 00 0 0 ~ ~ ~ 2 1 (3) by left multiplying equation (3) by the rotation matrix r , we can calculate the sinusoidal vibration components in the global coordinate frame. ( ) ( ) ( )                           =           t t wvu wvu wvu ta ta ta z y x     sin cos 00 0 0 2 1 333 222 111 (4) let us rewrite equation (4) as ( ) ( ) ( )    += += += tvtuta tvtuta tvtuta z y x    sincos sincos sincos 2313 2212 2111 (5) then, the amplitudes and phases of the sinusoidal vibration components for a given spatial elliptical orbit are given by ( ) ( )221 2 11 ˆ  vua x += ,         −= − 11 211 tan    u v x (6) ( ) ( )222 2 12 ˆ  vuay += ,         −= − 12 221 tan    u v y (7) ( ) ( )223 2 13 ˆ  vuaz += ,         −= − 13 231 tan    u v z . (8) 3.2. multi-exciter vibration control in order to acquire the accurate amplitudes and phases of the sinusoidal vibration components, vibration control is performed on the multi-exciters. the linear model of the multi-input multioutput system is given by ( ) ( ) ( )fff dhc = (9) where ( )fd is the driving spectrum, ( )fc is the response spectrum, and ( )fh is the frequency response function. the error spectrum is given by subtracting the response spectrum ( )fc from the reference spectrum ( )fr , as ( )( ) ( ) ( ) ( ) ( ) ( )ffffff dhrcrdf −=−= . (10) the objective of multi-exciter vibration control is to search for an appropriate driving spectrum with minimal error spectrum, i.e. ( )( ) 0df =f . broyden’s method is a highly powerful t t y t z x x y z (a) (b) figure 3. spatial orbit generation. (a) time-domain waveform; (b) elliptical orbit. x y z 1 2 x y z x y z u v w (a) (b) figure 4. coordinate transformation process. (a) the standard ellipse; (b) a spatial ellipse. acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 36 alternative to solving the equations by maintaining approximations of the solution and the jacobian matrix as the iteration progresses [22]. if ( )fnd and ( )fna are the current approximate solution and jacobian matrix respectively, then the next approximate solution is given by ( ) ( ) ( ) ( ) ( )( )fffff nnnn cradd −−= − + 1 1 . (11) after the computation of ( )fn 1+d , ( )fna is updated to form ( )fn 1+a , and the construction of ( )fn 1+a is determined by broyden’s method as ( ) ( ) ( ) ( ) ( )( ) ( ) ( ) ( )ff ffff ff nn nnnn nn 1 t 1 t 111 1 ++ +++ + − += ss ssay aa , (12) where ( ) ( ) ( ) ( ) ( )( ) ( )( )fff fff nnn nnn dfdfy dds −= −= ++ ++ 11 11 (13) in order to avoid the inverse operation in equation (11), the sherman-morrison formula is applied in order to update the inverse matrix of the approximate jacobian matrix, which is also called the system impedance. ( ) ( ) ( ) ( ) ( )( ) ( ) ( ) ( ) ( ) ( )fff fffff ff nnn nnnnn nn 1 t 1 t 111 1 ++ +++ + − += yzs zsyzs zz (14) equation (11) is then rewritten as ( ) ( ) ( ) ( ) ( )( )ftttt nnnn crzdd −−=+1 . (15) in addition, an adjustment gain  is supplied in the iterative expression in the case of receiving an overshoot response due to the large driving spectrum. ( ) ( ) ( ) ( ) ( )( )ftfff nnnn crzdd −−=+ 1 (16) the norm of the error spectrum is used to judge how close the response is to the reference. ( )( ) ( ) ( )( ) ( ) ( )( )ffffff nnn 111 +++ −−= crcrd t (17) furthermore, the optimal adjustment gain can be obtained when the derivative of ( )( )tf n 1+d with reference to  is zero. ( ) ( ) ( ) ( )( )( ) ( ) ( )( ) ( ) ( ) ( ) ( )( )( ) ( ) ( ) ( ) ( )( )( )ffffffff ffffff nnnn nnn crzhcrzh crcrzh t t −− −−− = (18) in equation (18), the system’s frequency response function is unknown and should be estimated in terms of known inputs and responses. if a known value  is used for the adjustment gain adopted in equation (16), the driving spectrum is given by ( ) ( ) ( ) ( ) ( )( )fffff nnnn crzdd −−=+ 1ˆ . (19) the corresponding response spectrum is measured as ( )fn 1ˆ +c . by left multiplying equation (19) by the system’s frequency response function ( )fh , we can get ( ) ( ) ( ) ( ) ( ) ( )( )ffffff nnnn crzhcc −−=+ 1ˆ . (20) then, we rewrite equation (20) into the following form ( ) ( ) ( ) ( )( ) ( ) ( )( )ffffff nnnn cccrzh −−=− +1ˆ 1  . (21) the unknown expression in equation (18) can be eliminated by substituting equation (21) into equation (18) ( ) ( )( ) ( ) ( )( ) ( ) ( )( ) ( ) ( )( )ffff ffff nnnn nnn cccc crcc t t −− −− = ++ + 11 1 ˆˆ ˆ  . (22) the vibration control flow is illustrated by means of a block diagram, shown in figure 5. the frequency response function matrix of the multi-exciter vibration system is first experimentally identified. then, the initial system impedance is calculated by inverting the result. the reference spectrum and response spectrum are determined according to the desired waveforms and the measured response respectively. the vibration control consists of three main procedures: calculating the adjustment gain according to equation (22), updating the impedance matrix according to equation (14), and correcting the driving spectrum by means of the iterative algorithm expressed in equation (16). the driving signal is then generated according to the driving spectrum. reference spectrum matrix driven signal response signal frf identification initial impedance matrix response spectrum matrix iterative algorithm eq. (16) driven spectrum matrix desired waveformadjustment gain eq. (22) impedance matrix eq. (14) figure 5. vibration control flow. 4. experimental investigation the proposed control method was applied on the tri-axial vibration exciter. three accelerometers were fixed on the vibration table in order to observe the generated spatial orbit. the sensing axes of the feedback accelerometers were aligned with the three vibration exciters. the cross-coupling of the tri-axial vibration exciter was evaluated by sequentially applying single-axis vibration along its three axes (x, y, and z). at first, no vibration control was performed on the tri-axial vibration exciter, during which time only the exciter along the vibrating direction was active. the ratios of the cross-coupling motion to the principal axis motion are shown in figure 6. as shown in figure 6(a), the y-axial crosscoupling ratio was about 0.1 and was close to 0.2 at certain frequencies (such as 100 hz and 1600 hz). the x-axial crosscoupling ratio was close to 0.1 as shown in figure 6(b). the xaxial cross-coupling ratio in figure 6(c) exceeded 0.1 and reached 0.2 from frequencies above 100 hz and the y-axial crosscoupling ratio was close to 0.1. the cross-coupling motion of the tri-axial vibration exciter was so large that it was unsuitable for calibration purposes. acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 37 then, the proposed multi-exciter vibration control was performed on the tri-axial vibration exciter. all the three exciters were operational, among which the principal axis exciter provided the stimulus, and the other two were used for crosscoupling compensation. the ratio for the cross-coupling motion to the principal axis motion is shown in figure 7. as seen in figure 7, the ratio of the cross-coupling motion to the principal axis motion is less than 0.01, which is very small. comparing figure 7 with figure 6, we can conclude that the proposed multiexciter vibration control can efficiently reduce the cross-coupling of the tri-axial vibration exciter. in addition, three spatial linear orbits parallel to the coordinate axes are generated, as shown in figure 8(a). furthermore, four spatial circular orbits that rotate every 30 ° around the x-axis are generated, as shown in figure 8(b). the plots in figure 8 demonstrate that the proposed multi-exciter vibration control can generate spatial orbits for further calibration application. a tri-axis accelerometer (model: kistler 8766a50, sn: 2118860) was selected for calibration using the tri-axial primary vibration calibration system. the calibration was carried out from 5 hz to 1600 hz. the magnitude and phase lag of the sensitivities measured are shown in figure 9. the x-, y-, and zaxial magnitudes are 9.72 mv/(m/s2), 9.80 mv/(m/s2), and 10.13 mv/(m/s2) at the reference frequency 160 hz. the x-, y and z-axial phase shifts are 0.06 °, -0.29 °, and -0.62 °, respectively. the xand z-axial magnitudes of sensitivities vary slightly from 5 hz to 1600 hz. the y-axial magnitude of sensitivity, however, has several peaks and troughs. the x-, y and z-axial phase shifts of sensitivities ranges from -1.67 ° to -1.40 °, from 5 hz to 1600 hz. (a) (b) (c) figure 6. cross-coupling without vibration control. (a) x-axial main motion; (b) y-axial main motion; (c) z-axial main motion. (a) (b) (c) figure 7. cross-coupling with vibration control. (a) x-axial main motion; (b) yaxial main motion; (c) z-axial main motion. acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 38 5. conclusions the national institute of metrology in china has set up a triaxial primary vibration calibration system that consists of an air bearing-based tri-axial vibration exciter, a spatial orbit vibration control system, and a primary tri-axial vibration measuring system. multi-exciter vibration control for cross-coupling compensation and the amplitude and phase control of sine vibration components is proposed. the experimental results show that the proposed control method can efficiently reduce the cross-coupling of the tri-axial vibration exciter and can generate a variety of spatial orbits for further calibration purposes. acknowledgement the authors would like to thank thomas bruns and leonard klaus for their valuable guidance and help in the multicomponent vibration calibration. this work has been supported by the special fund for scientific research in the public interest (201410009) and national natural science foundation of china (no. 51605461). references [1] c. harman, m. b. pickel, multi-axis vibration reduces test time, evaluation engineering 45, 6 (2006), pp. 44-47. [2] w. e. whiteman, m. s. berman, fatigue failure results for multiaxial versus uniaxial stress screen vibration testing, shock and vibration 9 (2002), pp. 319-328. [3] iso 16063-11, methods for the calibration of vibration and shock pick-ups – part 11: primary vibration calibration by laser interferometry. [4] j. j. dosch, d. m. lally, automated testing of accelerometer transverse sensitivity, http://www.modalshop.com/techlibrary/jdosch%20transverse %20calibration.pdf. [5] r. d. sill, e. j. seller, accelerometer transverse sensitivity measurement using planar orbital motion, proc. of the 77th shock and vibration symposium, monterey, ca, usa, 2006, pp. 8-12. [6] h.-j. von martens, c. weissenborn, simultaneous multicomponent calibration a new research area in the field of vibration and shock, proc. of the 1st meeting of the consultative committee for acoustics, ultrasound and vibration. bureau international de poids et mesures, sèvres, france, 20 – 21 july 1999. 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(a) magnitude; (b) phase shift. http://www.modalshop.com/techlibrary/jdosch%20transverse%20calibration.pdf http://www.modalshop.com/techlibrary/jdosch%20transverse%20calibration.pdf acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 39 [10] t. usuda, h.-j. von martens, c. weissenborn, theoretical and experimental investigation of transverse sensitivity of accelerometers under multiaxial excitation, meas. sci. technol. 15 (2004), pp. 896–904. [11] a. umeda, m. onoe, k. sakata, t. fukushia, k. kanari, h. iioka, t. kobayashi, calibration of three-axis accelerometers using a three-dimensional vibration generator and three laser interferometers, sensors and actuators a 114 (2004) pp. 93–101. [12] iec 60747-14-4, semiconductor devicesdiscrete devicespart 14-4: semiconductor accelerometers, 2011. [13] t. tsuchiya, o. tabata, a. umeda, dynamic sensitivity matrix measurement for single-mass soi 3-axis accelerometer, proc. of the ieee 25th international conference on micro electro mechanical systems (mems), 29 january 2 february 2012, paris, france. [14] c. harman, historical development of high performance multiaxis vibration test systems, environmental engineering 17, 1 (2004), pp. 40-41. [15] r. c. stroud, g. a. hamma, m. a. underwood, et al., a review of multiaxis/multiexciter vibration technology, sound and vibration 30, 4 (1996), pp. 20-27. [16] g. a. hamma, s. smith, simulation of dynamic loads by multichannel digital control. aiaa 78, 511 (1978), pp. 3-6. [17] m. chen, d. wilson. the new tri-axial shock and vibration test system at hill air force base, journal of iest 41, 2 (1989), pp. 2732. [18] m. a. underwood, adaptive control method for multiexciter sine tests. us patent 5299459, 1994. [19] m. a. underwood, t. keller, recent system developments for multi-actuator vibration control, sound and vibration 35, 10 (2001), pp. 16-23. [20] h. aoki, s. tsutsumi, t. fukushima, et al., vibration testing apparatus with increased rigidity in static pressure bearing. us patent 5549005[p], 1996. [21] m. p. bulat, p. v. bulat, the history of the gas bearings theory development, world applied sciences journal 27, 7 (2013), pp. 893-897. [22] c. t. kelley, iterative methods for linear and nonlinear equations. north carolina state university, raleigh, nc, usa, 1995. laser cleaning of cu-based artefacts: laser/corrosion products interaction acta imeko issn: 2221-870x october 2018, volume 7, number 3, 104 110 acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 104 laser cleaning of cu-based artefacts: laser/corrosion products interaction elisabetta di francia1, ruth lahoz2, delphine neff3, emma angelini1, sabrina grassini1 1 dipartimento di scienza applicata e tecnologia, politecnico di torino, torino, italy 2 centro de química y materiales de aragón/csic – universidad de zaragoza, zaragoza, spain 3 lapa nimbe iramat, cea/cnrs, université paris saclay, gif/yvetteparis-saclay, france section: research paper keywords: laser cleaning; fibre laser; archaeological bronze artefacts; artificial corrosion layer citation: elisabetta di francia, ruth lahoz, delphine neff, emma angelini, sabrina grassini, laser cleaning of cu-based artefacts: laser-corrosion products interaction, acta imeko, vol. 7, no. 3, article 16, october 2018, identifier: imeko-acta-07 (2018)-03-16 section editor: egidio de benedetto, university of salento, italy received may 18, 2018; in final form september 14, 2018; published october 2018 copyright: © 2018 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: elisabetta di francia, elisabetta.difrancia@polito.it 1. introduction the cultural heritage field is deeply related to metrological issues because measurements are involved in the characterisation and conservation of the artefacts and in the evaluation of the aggressiveness of the environment, which surrounds them [1-3]. tangible cultural heritage is a fundamental witness of mankind history and has to be preserved for future generations, in all its multimateric assets, starting from proper cleaning procedures, to restoration and conservation in controlled environments. dealing with cleaning procedures, several studies may be found in literature on laser cleaning effects on artefacts, as natural stones, paintings, fossils and gilded metals [4-11]. proper ablation conditions allow to assess the higher efficiency and selectivity of laser cleaning, with respect to traditional mechanical or chemical treatments. however, in the case of ancient metallic artefacts, a systematic investigation of the laser radiation/corrosion products interaction during cleaning is missing. it has to be underlined the difficulty of this investigation due to the fact that the corrosion processes of archaeological cubased artefacts, buried in soil for hundreds of years, lead to the formation of complex corrosion products layers with a stratified microstructure in dependence of the soil chemical-physical properties and of the environmental conditions [12-14]. as well known, corrosion of cu-based artefacts gives rise to at least two different layers: an inner one mainly constituted by cuprite, cu2o, grown directly in contact with the substrate, is the socalled noble patina, which can protect the metallic surface from further interaction with aggressive agents; an outer one, constituted by different chemical compounds in dependence of the interaction with soil constituents (cl, p, si, fe, al, k, ca, s, abstract this study aims to develop a low invasive and selective laser cleaning procedure for the removal of reactive corrosion products on cubased artefacts without damage the substrate. in a preliminary step, laser cleaning was performed on two typologies of art ificially corroded copper reference samples. the effect of the variation of laser parameters as pulse duration and output power, was thus evaluated on an oxide layer, simulating a protective patina, and a hydroxychloride layer, simulating a reactive corrosion products layer to be removed. the optimized cleaning procedure was validated on an archaeological artefact, a bronze coin. morphological, microchemical and microstructural characterizations were performed by means of optical microscopy, confocal microscopy, field emission scanning electron microscopy, x-ray diffraction and raman spectroscopy, before and after laser cleaning. the experimental findings show that laser cleaning, in optimized conditions, can reduce the thickness of the hydroxychloride layers slightly affecting the oxide layers. the difference in the interaction with laser radiation of these two layers seems to be mainly related to the difference in grain size and porosity. notwithstanding these encouraging results, in order to define the real feasibility of the laser cleaning procedure, a further validation on real artefacts is mandatory due to the variation in thickness and composition of the corrosion products formed during long-lasting uncontrolled degradation processes. mailto:elisabetta.difrancia@polito.it acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 105 co2), oxygen and humidity. in the outer layers copper oxides and hydroxychlorides, sulphates and carbonates as malachite cu2(oh)2co3, brochantite cu4(oh)6(so)4, atacamite and paracatamite cu2(oh)3cl, etc., may be found. moreover, the presence of high concentration of chloride ions at the metal/corrosion layer interface leads to the formation of reactive cuprous chloride (nantokite, cucl), responsible of the cyclic copper corrosion processes well known as “bronze disease”. cuprous chloride exposed to humidity, cyclically reacts with oxygen and water, giving rise to the formation of the greenish cu2(oh)3cl (atacamite and its polymorphs) that in turn reacts with copper to form new cuprous chloride and water. copper, chlorine, oxygen and water are converted into cuprite, cu2o, and atacamite, cu2(oh)3cl, in a cyclical and continuous process that can damage the archaeological artefact. non-invasive cleaning treatments able to remove the reactive corrosion products, preserving the protective patina could help in stopping this dangerous process. in the ideal scenario of the laser ablation process, the laser radiation has to be absorbed by the corrosion products to be removed thanks to physical processes (plasma), while it has to be reflected by the metal. during the laser cleaning of ancient cubased artefacts, the laser parameters have to be optimised, in order to remove the reactive corrosion products and leave a thin layer of the protective patina onto the metallic substrate, as shown in the scheme of figure 1. however, to modelize the laser physical effect is extremely complex: different stratifications lead to different thermal effects, and consequently to different melting and evaporation behaviours which have to be understood and controlled in order to avoid any damage to the artefact, satisfying the conservation requirements highlighted by conservators and restorers. consequently, due to the high unpredictable inhomogeneity of the corrosion product layers on ancient cu-based artefacts, artificially corroded copper specimens were used, in this preliminary study, to assess the laser effects as a function of the corrosion products chemical composition [15-17] and to optimize the cleaning procedure [18]. as a matter of facts, the laser cleaning was performed on two typologies of artificially corroded copper reference samples. the effect of the variation of laser parameters as pulse duration and output power, was thus evaluated on an oxide layer, simulating a protective patina, and a hydroxychloride layer, simulating a reactive corrosion products layer to be removed. the optimized cleaning procedure was validated on an archaeological artefact, a bronze coin. 2. materials and methods laser cleaning treatments were performed by means of a near-ir (nir) yb:yag fibre laser on a set of cu reference samples artificially corroded, as described below. figure 2. the follis massenzio coin: recto (left), verso (right). the images were taken with a high-resolution digital camera. laser cleaning treatment, in optimized conditions, was carried out on a roman empire bronze coin, shown in figure 2, the follis massenzio coin coming from a private collection. 2.1. artificially corroded cu reference samples a set of cu reference samples (45x15x5 mm; cu 99.96 wt%) were polished with 500 to 4000-grid sic paper, rinsed in ethanol in ultrasonic bath for 5 min, and well dried. figure 1. metal and corrosion products behaviour under laser ablation: the laser ablates the reactive corrosion products (green) while the protective noble patina (brown) is preserved. table 1. scanning tests performed on the artificially corroded cu reference samples. laser parameters geometrical parameters test power, p in w fluence, f in j/cm2 pulse duration, tp in ns scan speed, vscan in mm/s interlining, dl in mm 1 0.23 1.63 4 300 0.015 2 0.28 1.98 4 300 0.015 3 0.42 2.97 4 300 0.015 4 0.52 3.68 4 300 0.015 5 0.73 5.16 4 300 0.015 6 0.21 1.49 8 300 0.015 7 0.25 1.77 8 300 0.015 8 0.43 3.04 8 300 0.015 9 0.58 4.10 8 300 0.015 10 0.77 5.45 8 300 0.015 acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 106 then, two different kinds of artificial corrosion layers were formed in order to mimic the presence of an oxide layer (simulating a protective patina) and a hydroxychloride layer (simulating a reactive corrosion products layer to be removed). the oxide layer was obtained electrochemically: an anodic treatment was performed in 0.1 m na2so4 solution at room temperature [19] by applying a potential of + 0.084 v (vs ag/agcl) for 16 h to the cu reference samples. the hydroxychloride layer was obtained by two months immersion at room temperature of the cu reference samples in 0.5 m nacl solution, neither stirred or aerated. 2.2. laser cleaning treatments optical fibre spectroscopy measurements, discussed in detail elsewhere [20], were performed to determine the spectral signature of corroded cu samples as a function of the laser wavelength. the reflectance measurements exhibited a signature of the corrosion products from ~600 to 900 nm. the highest gap in reflectance between the metallic substrate and the corrosion products layer is around 900 nm, thus allowing the optimisation of the cleaning treatment. a pumped diode q-switched yb:yag fibre laser (model easymark-20 from jeanologia), operating in the near-ir at a wavelength of 1064 nm was used to perform the laser cleaning treatments. pulses, with durations in the range 4 200 ns, were delivered by means of two-galvanic mirrors and focused with a f-theta lens with 160 mm of focal distance. the laser system is coupled via computer with ezcad 2.1 uni, a vector graphic editor, with a cad-like capability that enables users to perform rapid, precise and complex surface scanning treatments in a repeatable way. moreover, the vector graphic editor allows to control the laser parameters (e.g. output power, p, and pulse duration, tp) and the geometrical ones. thermal interactions (e.g. melting) instead of ablation of the surface layer can be avoided adjusting pulse overlap, preventing thermal incubation and over irradiation. the overlap in x-axis is determined by the spot size and the scanning speed vscan, while overlap in y-axis is defined by the spot size and the interlining dl. several laser cleaning tests, summarized in table 1, were performed by varying pulse duration and power, directly related to the fluence, the pulsed power per area. the geometric parameters (vscan, dl) were chosen to guarantee the scanning of all the surface; a spot diameter of 30 µm and an interlining of 15 µm were used to insure a sufficient overlap and to cover all the irradiated areas. 2.3. samples characterisations morphological, chemical and microstructural characterisations of the samples before and after laser cleaning, were performed both on the surface and on the cross-section. cross-sections were prepared by embedding the samples in specifix-20-struers resin and polishing them with 500 to 4000 sic paper and with 6 µm to 1 µm cloths. high resolution digital photographs were taken by a digital camera (4000×3000 pixels, panasonic lumix g2) equipped by a stand with a 3000 k lamp and by optical microscopy (olympus bx51 microscope equipped with a nikon eos camera) in bright and in dark modality. confocal microscopy profilers were acquired using a sensofar plµ 2300 microscope using a 20x objective. field emission scanning electron microscopy (fesem, zeiss, supra40) images were collected in secondary electron and in field emission modes varying the acceleration voltage from 1.5 kv to 15 kv. µxrd diffraction patterns were obtained by a rigaku rint/rapid instrument equipped with a cu target; diffractions were acquired with the x-ray source at 40 kv and 20 ma with a 100 µm collimator, in the range 10°-160° 2 the qualitative analyses were performed by rapid xrd software and the qualitative identification of the reflections were done by comparison with the panalytical x’pert highscore plus v3.0 software. µraman analyses were performed under a leica x50/0.85 microscope objective and the spectra were acquired with a renishaw invia equipped with a doubled nd:yag laser (532 nm). the laser power on the sample surface was fixed to about 500 µw in order to avoid the thermal transformation of the analysed phases. the experimental findings were compared with the spectra collected in a database composed from commercial or synthetic powders obtained in laboratory and by comparison to references of the literature [21]. diffuse reflectance measurements were performed by means of cary 5000 v1.12 (varian®) in the wavelength range 2201500 nm, to determine the absorption performance of the corrosion products layers. 3. results and discussion 3.1. laser cleaning of artificially corroded cu reference samples the analysis of all the laser cleaned reference samples reveals that the best results in terms of higher ablation rates with the minimum thermal interaction effect on both the oxide and the hydroxychloride layers, were obtained utilizing the experimental parameters of test 1, shown in table 1. they are characterised by: the shortest pulse duration tp, 4 ns, utilized also in tests 2, 3, 4, and 5, while tests 6, 7, 8, 9, and 10 had a pulse duration tp of 8 ns; the lowest power p of 0.23 w and the lowest fluence f of 1.63 j/cm2 among the first five tests. the results of the laser cleaning performed with the experimental parameters of test 1, obtained due to the highest irradiation values, were chosen as the ablation threshold values and are discussed in detail. figure 3 shows the confocal microscope images of the oxide and hydroxychloride corrosion layers at the interface between laser treated and non-treated areas. both the oxide and the hydroxychloride layers have a thickness in the range 35-40 µm. the average roughness and the standard deviation were calculated on 100,000 points on the confocal pictures. for the oxide layer, figure 3a, the surface of the non-treated area is characterised by a rough structure with a standard deviation of 1.96 µm; the laser cleaning removes about 1.6 µm of this oxide layer and reduces the surface roughness to about 1.5 µm. in the case of the hydroxychloride layer, figure 3b, the laser cleaning removes about 24 µm and reduces the roughness from 11.9 µm to 8.1 µm. figure 3. confocal microscope images of the oxide (a) and hydroxychloride (b) corrosion layers grown on copper reference samples, at the interface between laser treated and non-treated area. acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 107 figure 4 shows the high-resolution and fesem images of the oxide and hydroxychloride corrosion layers. from the highresolution images, a negligible colour change is observed, both for the oxide layer, (figures 4a, 4c), and for the hydroxychloride layer (figures 4e, 4g). on the contrary, the surface of the oxide layer before and after laser cleaning is characterised by a different macroscopic appearance, with a superficial melting of the oxide, that is evidenced by the fesem image of figure 4d, that if compared with the one of figure 4b highlights the modification of the laser treated surface. on the artificial hydroxychloride layer, the high-resolution images reveal a similar macroscopic appearance. on the fesem images of figures 4f, 4h, negligible changes in the corrosion layer morphology due to the laser treatment may be observed: the presence of crystals and their size, 5-10 µm, seem unaffected by the interaction with the laser radiation. figure 4. images of the artificial oxide and hydroxychloride layers grown on the cu reference samples, laser treated in the conditions of test 1 and nontreated; images collected by high resolution camera (a, c, e, g) and by fesem (b, d, f, h). figure 5. diffuse reflectance of the oxide (brown) and hydroxychloride (green) corrosion layers. the diffuse reflectance spectra of the two layers shown in figure 5, reveal that, at a wavelength of 1064 nm, the reflectance for the hydroxychloride layer is lower than the one of the oxide layer. this means that the ablation threshold is higher for cuprite, so this layer needs a higher energy to be removed. consequently, during laser cleaning reasonably the oxide layer is better preserved, while the undesired hydroxychloride layer is partially ablated. the different behaviour of the oxide and hydroxychloride layers may also be related to the different grain size and porosity of the corrosion products, as well as to the absorption mechanisms of the laser irradiance of each corrosion layer. as shown in the fesem images, the untreated oxide layer, is characterized by lower grain size and higher porosity; while the hydroxychloride layer shows higher grain size and less porosity. the laser light absorption entity is noteworthy influenced by grain size dimensions. with smaller grain sizes, light scatters on multiple surfaces (grain borders) and there is less penetration into the surface, originating lower reflectance values [22]. for the porosity, the behaviour is similar, the higher the porosity the higher the scattering phenomena. this may explain the surface morphology observed after laser treatment, shown in figure 4. figure 6. µxrd and µraman spectra collected on the treated (test-1) and non-treated surfaces and cross-sections of the artificial oxide corrosion layer grown on cu reference samples. (µxrd database: reference pattern-00-005-0667 cuprite, reference pattern-00-004-0836 copper, reference pattern-03-065-2309 copper oxide; µraman database: ruff id-120076 copper oxide; ruff id-050374-3 cuprite). acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 108 the surface of the oxide layer laser irradiated appears melted. laser absorption through the grain borders has started melting on those points. hydroxychloride layer, with high absorption, remains with the same morphology since its big crystals microstructure reduces significantly the scattering effects, especially on the surface. supporting this discussion, previous work on laser cleaning of hydroxychloride layers characterized by grain sizes of high dimension have shown that the previously reported laser irradiation values do not affect big crystals shape [23]. µxrd analyses have been performed on the surfaces of the oxide and hydroxychloride corrosion layers, while µraman acquisitions have been performed on the cross-sections of the non-treated and treated areas. the microchemical and microstructural characterisations show that the oxide layer created by the electrochemical treatment is mainly composed of cuprite (cu2o), with the presence of tenorite (cuo) grown directly in contact with the metal as highlighted by the diffraction patterns and raman peaks of both phases, figure 6. by comparing the µxrd patterns collected on the treated and non-treated areas, a slight difference in the microstructure is detectable: the spectra show an increase of the relative intensity of the diffraction patterns associated to metallic copper onto the laser treated surface (figure 5) probably associated to a reduction of the oxide layer thickness. instead, from µxrd analyses (figure 7), the surface of the hydroxychloride layer, created by immersion in the chloride solution, is mainly composed of copper chlorides, such as clinoatacamite (cu2cl(oh)3), and copper oxides (cuo, cu2o). also, on this case onto the laser treated surface a slight increase of the metallic copper associated to a reduction in the relative intensities of the chloride components has been detected. µraman analyses confirm the presence of clinoatacamite (cu2cl(oh)3) on both treated and non-treated areas (figure 7) [21]. the analyses carried out on the artificially corroded reference samples have highlighted that the selected laser parameters are able to reduce the thickness of the hydroxychloride layer, slightly affecting the oxide layer. the difference on the corrosion layerlaser interactions seems to be mainly related to the difference in grain size and porosity of the oxide and hydrochloride layers. all these results highlight also that no changes in the composition of the corrosion layers seem to be induced by the laser cleaning treatment. 3.2. laser cleaning on an archaeological artefact the laser cleaning was performed on a fragment of the bronze coin of figure 2. the bronze coin (81.95 wt% cu, 4.1 wt% sn, 11 wt% pb, 2.95 wt% ag), had a more uniform greenish corrosion layer on the verso side (observe) than on the recto side (reverse), therefore the observe side was submitted to laser cleaning. figure 8 shows the high-resolution images of the coin fragment surface before and after the laser cleaning with the experimental parameters of test 1. in this case too only a negligible change in colour may be macroscopically observed. the om image of figure 9 shows on the non-treated crosssection of the coin fragment, the presence of an external compact corrosion products layer, up to 200 µm thick, and a less compact inner layer interconnected with the bulk metal up to 800 μm thick is observed. since the coin is a real artefact, the corrosion products layer is not uniform, both in composition and thickness, however, the results here presented refer to the more representative areas. figure 8. high-resolution images of the coin fragment surface before (a) and after (b) the laser cleaning (test 1). figure 7. µxrd and raman spectra collected on the treated (test-1) and non-treated surfaces and cross-sections of the artificial hydroxychloride corrosion layer grown on cu reference samples. (µxrd database: reference pattern-00-005-0667 cuprite, reference pattern-00-004-0836 copper, reference pattern-03065-2309 copper oxide, reference pattern-01-086-1391 clinoatacamite; raman: [21] clinoatacamite). acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 109 due to these differences, several measurements were performed on the treated and non-treated areas in order to evaluate the effect of laser in removing the corrosion products. figure 10 shows the dark-field optical microscope images of non-treated and treated cross-section areas of the coin. on the laser treated areas, the surface structure of the coin described above is still present. moreover, it is possible to observe a reduction of the external corrosion product average thickness up to 30 µm, while the inner and less compact corrosion products layer has not been affected by the laser radiation. dealing with the chemical composition, the dark-field optical microscopy allows to associate different colours to different compositions: the red colour can be associated to the presence of cu+ compounds, while the green colour can be associated to the presence of cu++ compounds. the compact layer presents mainly cu+ compounds (e.g. copper oxide) in the part closer to the metal, while the upper part is characterised by the presence of cu++ compounds (e.g. hydroxycholoride polymorphs). from the microscopical and microstructural point of view, it seems that the laser treatment removed the copper chlorides in a quite controlled way, without damaging the metallic surface. figure 9. om bright-field (a,b), om dark-field (c ) images of the cross-section of the non-treated area of the bronze coin. figure 10. om dark-field images of the cross-sections of non-treated (left) and laser treated (right) coin areas. 4. conclusions the preliminary results reported on cu reference samples related with the laser cleaning of metallic artefacts pointed out interesting application perspectives, which motivate the need of future application tests on archaeological and historic metallic artefacts. the behaviour of two different artificial corrosion layers, an oxide layer and a hydroxychloride layer, formed on cu-based reference samples was analysed before and after laser cleaning. a pulsed near-ir laser was used in a q-switched regime and the laser parameters were fixed during the cleaning treatment of oxide and hydroxychloride artificial corrosion layers. then, a validation of the laser cleaning procedure was performed on an ancient bronze coin. after all the laser treatments performed on the reference samples, the optimal laser condition for both the artificial oxide and hydroxycloride corrosion layers were characterised by the lowest power and pulse duration (4 ns) and accepted as the ablation threshold for the unwanted layers. since the ancient corrosion products layer is thicker and more compact than the artificially tested ones, a reduction of the unwanted corrosion products was observed, even if not in a complete and homogenous way. moreover, the artificial corrosion layers and the archaeological corrosion layers seemed not altered in their compositions. in conclusion, the laser ablation can remove successfully and in a highly controlled way the corrosion layers without modifying the composition of the remaining corrosion products. however, a further validation on real artefacts is mandatory due to the variation in thickness and composition of the corrosion products formed during long-lasting uncontrolled degradation processes. acknowledgements the authors would like to thank the european federation of corrosion (efc) for supporting the raman measurement campaign in the frame of the eurocorr young scientist grant 2016. a special thanks to jacopo corsi for the bronze coin donation. references [1] l. lombardo, s. corbellini, m. parvis, a. elsayed, e. angelini, s. grassini, wireless sensor network for distributed environmental monitoring, ieee transactions on instrumentation and measurement, volume 67(5), 2018, pp. 12141222. 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[23] c. l. soto quintana, development and optimisation of protocols for surface cleaning of cultural heritage metals, erasmus mundus master thesis in archaeological materials science, 2016. acta imeko, title acta imeko issn: 2221-870x june 2018, volume 7, number 2, 0 acta imeko | www.imeko.org june 2018 | volume 7 | number 2| 0 journal contacts about the journal acta imeko is an e-journal reporting on the contributions on the state and progress of the science and technology of measurement. the articles are mainly based on presentations presented at imeko workshops, symposia and congresses. the journal is published by imeko, the international measurement confederation. the issn, the international identifier for serials, is 2221-870x. editor-in-chief dušan agrež, slovenia founding editor-in-chief paul p.l. regtien, netherlands associate editor dirk röske, germany editorial board section editors (vol. 6 and 7) leopoldo angrisani, italy filippo attivissimo, italy eulalia balestieri, italy eric benoit, france paolo carbone, italy lorenzo ciani, italy catalin damian, romania pasquale daponte, italy luca de vito, italy luigi ferrigno, italy edoardo fiorucci, italy alistair forbes, united kingdom helena geirinhas ramos, portugal sabrina grassini, italy fernando janeiro, portugal konrad jedrzejewski, poland andy knott, united kingdom francesco lamonaca, italy massimo lazzaroni, italy fabio leccese, italy rosario morello, italy michele norgia, italy pedro miguel pinto ramos, portugal nicola pompeo, italy sergio rapuano, italy dirk röske, germany alexandru salceanu, romania constantin sarmasanu, romania lorenzo scalise, italy emiliano schena, italy enrico silva, italy krzysztof stepien, poland marco tarabini, italy marcantonio catelani, italy paolo carbone, italy lorenzo ciani, italy egidio de benedeto, italy alessandro germak, italy sabrina grassini, italy konrad jędrzejewski, poland min-seok kim, korea fabio leccese, italy rosario morello, italy vilmos palfi, hungary nicola pasquino, italy franco pavese, italy paul regtien, the netherlands alexandru salceanu, romania jan saliga, slovakia jorge c. torres-guzman, mexico ian veldman, south africa claudia zoani, italy final editing rosario morello, university mediterranea of reggio calabria, italy about imeko the international measurement confederation, imeko, is an international federation of actually 42 national member organisations individually concerned with the advancement of measurement technology. its fundamental objectives are the promotion of international interchange of scientific and technical information in the field of measurement, and the enhancement of international co-operation among scientists and engineers from research and industry. addresses principal contact prof. dušan agrež university of ljubljana faculty of electrical engineering tržaška 25, 1000 ljubljana, slovenia email: dusan.agrez@fe.uni-lj.si acta imeko dr. rosario morello dep.t diies, university mediterranea of reggio calabria via graziella, 89122 reggio calabria, italy email: rosario.morello@unirc.it dr. sabrina grassini dept. of applied science and technology, politecnico di torino corso duca degli abruzzi 24, 10129 torino, italy email: sabrina.grassini@polito.it support contact dr. dirk röske physikalisch-technische bundesanstalt (ptb) bundesallee 100, 38116 braunschweig, germany email: dirk.roeske@ptb.de journal contacts acta imeko, title acta imeko issn: 2221-870x june 2018, volume 7, number 2, 32-38 acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 32 calibration standards for hydrophones and autonomous underwater recorders for frequencies below 1 khz: current activities of “unac-low” project alper biber1, ata can çorakçı1, alexander golick1, stephen robinson2, gary hayman2, justin ablitt2, salvador barrera-figueroa3, silvano buogo4, salvatore mauro4, fabrizio borsani5, salvatore curcuruto5, markus linné6, peter sigray6, per davidsson6 1tübi̇tak marmara research center, materials institute, underwater acoustics laboratory, 41470 gebze/kocaeli, turkey 2national physical laboratory (npl), teddington tw11 0lw, united kingdom 3dansk fundamental metrologi a/s (dfm), dk-2800 kongens lyngby, denmark 4consiglio nazionale delle ricerche, marine technology research institute (cnr-insean), via di vallerano 139, 00128 roma, italy 5national institute for environmental protection and research (ispra), via v. brancati 48, 00144 roma, italy 6totalförsvarets forksningsinstitut (foi), 164 90 stockholm, sweden section: research paper keywords: underwater acoustics; low-frequency hydrophone calibration; underwater noise recorders; empir citation: alper biber, ata can çorakçı, alexander golick, stephen robinson, gary hayman, justin ablitt, salvador barrera-figueroa, silvano buogo, salvatore mauro, fabrizio borsani, salvatore curcuruto, markus linné, peter sigray, per davidsson, calibration standards for hydrophones and autonomous underwater recorders for frequencies below 1 khz: current activities of “unac-low” project, acta imeko, vol. 7, no. 2, article 6, june 2018, identifier: imeko-acta-07 (2018)-02-06 section editor: fabio leccese, università degli studi di roma tre, italy received january 16, 2018; in final form april 3, 2018; published june 2018 copyright: © 2018 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: this work was supported by euramet empir programme under research potential funding scheme corresponding author: alper biber, e-mail: alper.biber@tubitak.gov.tr 1. introduction “unac-low” [1] is a european metrology project aimed at developing low-frequency calibration standards in underwater acoustics together with a coherent european strategy for long-term operation of developed capacities. in this field, traceability is currently not readily available across europe while only a limited number of institutions outside europe offer low-frequency calibration services for hydrophones. the abstract the project entitled “underwater acoustic calibration standards for frequencies below 1 khz” (“unac-low”), currently active within the european metrology programme for innovation and research (empir), is presented by describing its objectives and current activities. the project aims at developing the metrological capacity of the european union (eu) for the calibration of hydrophones and autonomous recording systems for the frequency range between 20 hz and 1 khz, for which traceability is presently not fully available. in this way, eu metrological capacities for absolute measurement of underwater sound will be improved, with a direct effect on the implementation of regulation and eu directives that require underwater acoustic measurements to be traceable. after having completed the initial project tasks regarding the review of existing methods and the design of the experimental setups, comparison measurements between the project partners are currently under way and their results will be validated and presented upon project end after first quarter of 2019. to ensure long-term operation of the calibration capabilities by each partner, a coherent eu metrology strategy for underwater acoustics will be developed as one of the main project outcomes. current activities include the implementation of the calibration setups developed in earlier stages of the project for both hydrophones and autonomous recorders. the methods that shall be used for hydrophones are the pressure method in a closed chamber and the standing wave tube method. for autonomous recorders, in addition to the above methods, calibrations will be performed using free-field methods in different open-water test sites possessing suitable characteristics for low frequency measurements. acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 33 project started in may 2016 and will last three years until april 2019. the six participants (see figure 1 and table 1) are from 5 eu member states, each with access to different european seas (see figure 1). the project is led by tübi̇tak marmara research center (mrc) of turkey, whose underwater acoustics laboratory of mrc-materials institute is assigned as a designated institute in the field of underwater acoustics for turkey. other partners are 2 national metrology institutes (npl, dfm) and 3 external partners (cnr, ispra, foi). the project consortium is supported by a number of entities that have expressed interest in the project, including metrology institutes and agencies, manufacturers and service providers. the project is co-funded by the european union’s horizon 2020 framework research programme and by the european association of national metrology institutes (euramet) through the european metrology programme for innovation and research (empir) [2]. the main project objective is to improve the european metrological capacity in underwater acoustic calibration for acoustic frequencies from 20 hz to 1 khz. to do so, traceable measurement capabilities will be developed to meet the need for calibration of hydrophones and autonomous recording systems, that are both used for measurement of underwater noise at sea. the project also aims at developing the scientific and technical research capabilities of the participating partners, providing an improved framework to underpin the demand for traceable measurement of sound in support of regulation and eu directives such as the marine strategy framework directive (msfd). in this field, traceability is currently lacking and the development of international standards for measurement of underwater ambient noise has been recently recommended in msfd guidance reports issued by the eu technical subgroup on noise [3]. the reliability and quality control of data is recently assuming key relevance for the long-term observation of marine parameters based on autonomous platforms, that often includes monitoring of underwater noise [4]. the need for absolute measurement of sound is therefore increasing as it is driven by concerns about the environmental impact of anthropogenic noise, so that not only oceanographic science but also industry is showing an increased interest in this field as regulatory compliance imposes more demanding requirements on monitoring instrumentation and methods. the frequency spectra of man-made noise sources of major concern are concentrated in the range between 20 hz and 1 khz, so a direct and urgent need for traceable calibration of hydrophones falls in this frequency range. international standards already exist [5], [6] that describe calibration methods for hydrophones below 1 khz that may potentially be adopted in the present project. among the institutions that currently offer calibration services world-wide in the frequency range of interest, only one is located within europe (project partner npl) providing calibration services for hydrophones below the khz range that are traceable to primary standards for air acoustics. among lowfrequency calibration services available outside europe, the primary calibration method with a coupler reciprocity chamber is routinely used in the usa and has recently been demonstrated for the complex (magnitude and phase) calibration of hydrophones between 1 hz and 2 khz [7]. regarding autonomous recorders, which combine hydrophones and acquisition and data storage capabilities, given the technology push provided by their development and their increasing commercial availability, and since no standards have been developed yet for their calibration, there is also an urgent need to develop traceable measurement capabilities that account for this category of devices. such capabilities are generally not simply based on the available calibration methods for hydrophones, since the latter are generally calibrated below the khz range using pressure methods that require fitting them air-tight into a specific closed volume, which may not be readily feasible for all recording systems given the wide variety of acoustic sensors they can employ. section 2 will describe the project objectives, and in section 3 an overview will be given of its organization. in section 4 the current status of ongoing activities will be given. finally, in the concluding section the project achievements obtained so far will be outlined. 2. project objectives the scientific and technical objectives of unac-low project are the following: 1. to develop traceable measurement capabilities for calibration of hydrophones and of autonomous recorders, for frequencies between 20 hz and 1 khz covering the 63 hz and 125 hz third-octave bands as required by the eu msfd guidelines; 2. to develop an individual strategy for each participant for long-term operation of the developed measurement capabilities including regulatory support, research collaborations, quality schemes and accreditation; figure 1. unac-low participants and their location within europe. table 1. details of unac-low participants. participant status: nmi = national metrology institute, di = designated institute, ext = external funded partner. acronym full name country status tubitak tübi̇tak marmara research center, materials institute, underwater acoustics laboratory turkey di npl national physical laboratory united kingdom nmi dfm dansk fundamental metrologi a/s denmark nmi cnr national research council of italy, marine technology research institute italy ext ispra national institute for environmental protection and research italy ext foi swedish defence research agency sweden ext acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 34 3. to contribute to the development of a coherent metrology strategy for europe within this field; 4. to significantly increase europe’s research capacity in this field. in the project, appropriate measurement methods for the calibration of hydrophones in the frequency range from 20 hz to 1 khz will be developed and proficiency will be gained in the implementation of such methods. upon project conclusion, the selected calibration methods will be implemented into new calibration systems that will be validated by comparison measurements between the project partners. through this work, traceability for low-frequency underwater acoustic measurements will be provided across the eu countries, with project partners offering calibration services from their established facilities to the overall market, including other countries within and outside the eu. the key acoustic performance characteristics of the recorders will also be determined through the developed methods. such characteristics include the self-noise of the hydrophone and system, and the hydrophone and system sensitivities. the project partners will each implement their newly established methods and services will begin to be offered to the stakeholder community. moreover, recommendations will be sent to technical standards committees including iso tc43 sc3 and iec tc87 for preparation of related standards. a long-term strategy for the operation of the calibration capability will be developed by each project partner. this will be done by establishing regulatory support and research collaborations, as well as adopting appropriate quality schemes and accreditation. to offer calibration services both to their own country, and to neighbouring countries from their established facilities in a coherent way, each partner will develop an individual strategy that will form part of a coherent metrology strategy for europe within this field, discussed and agreed within the euramet community of nmis/dis via the euramet tc-auv. 3. project description the experimental activities are arranged in two workpackages that proceed in parallel: workpackage 1 (wp1) deals with calibration of hydrophones and workpackage 2 (wp2) deals with calibration of autonomous recorders. both wp1 and wp2 are arranged in three sequential stages with corresponding tasks: one for the review and selection of existing devices and calibration methods, one for the design and preparation of calibration setup, and one for the execution of round-robin experiments and for validating their results. during the design and preparation task, each group of partners in wp1 and wp2 is free to use their choice of devices, either already available or to be rented or purchased, to implement their methods and to establish a calibration setup. the implemented methods are described in technical procedures and field reports, and will be included in final deliverables at the end of project. the autonomous recorders will be typical of the more common designs among the fairly large number of models (over 30, as reported in [8]) that are currently available on the market. the selected models will be capable of different configurations, either with or without a hydrophone fixed to the recorder body. the developed setups will be then employed in subsequent round-robin tasks with only one device, provided by the task leader, to be circulated among the participants. from the results, uncertainty budgets will be prepared and the total uncertainty will be calculated, with 1 db target uncertainty for hydrophones. comparison tests will also be conducted between the calibration setups and results will be used to estimate the equivalence of national measurement standards for hydrophones within the low frequency range between 20 hz and 1 khz. the project schedule also includes activities by all partners to ensure proper dissemination of results. as a further means for knowledge transfer, output of the project will be used to extend existing training courses already offered by some partners, and to create new courses in different european areas. 4. current project status the project has completed its first stages dealing with review and design of calibration methods, and is proceeding along its final operative stage. the first parts dealing with review of existing calibration methods and of available autonomous recorders have been completed with no delay. the following tasks regarding the design and preparation of calibration setups have been completed, with only some minor delay due to procurement of autonomous recorders, either to be purchased or rented according to each partner's choice. details of the calibration setups have been discussed among partners and are currently being implemented by participants. in the following paragraphs, the current status is given for activities related to hydrophones (wp1) and autonomous recorders (wp2). two secondary calibration methods for hydrophones have been selected, among those included in the iec existing standard [5]. such methods, the comparison method in a coupling chamber and the standing wave tube method, will be used for the experiments. regarding autonomous recorders, the above secondary methods will be adopted for pressure calibrations. in addition, for specific frequency ranges according to the characteristics of available open-water sites, free-field methods will be used: the three-transducer reciprocity primary method and the method of comparison with a reference hydrophone calibrated by pressure method. to evaluate other methods for frequencies below 1 khz, possibly also including primary calibration methods, npl is also currently involved in activities that aim at an extension of free-field calibration using signal modelling techniques [9], the absolute method by a laser pistonphone [10], [11], and the vibrating column method [12]. these studies, although mostly funded by other projects, are obtaining useful results that shall be compared with those of the present project. 4.1. calibration of hydrophones with a coupling chamber the first method for hydrophone calibration is realised by inserting the device under test and a calibrated reference receiver into a closed chamber together with a sound source, therefore exposing both receivers to the same acoustic pressure. the reference may be either a microphone or a hydrophone, depending on whether the chamber is air-filled or water-filled. as the acoustic frequency is increased, the pressure field inside the chamber becomes non-uniform, the reference and the device under test are subject to increasingly different pressure levels and the method is no longer accurate. this happens when the acoustic wavelength approaches the same order of magnitude of the chamber size: for 1 khz in air at standard conditions this length is about 30 cm. the upper frequency limit of the chamber can be extended in case when the chamber is filled by liquid in which the acoustic wavelength is a few times longer than in gas. coupling chamber has been designed acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 35 and produced where the source drives a small air cavity so that the acoustic pressure is the same for both the reference and the device under test. with this design, a standard microphone can be used as a reference and a miniature loudspeaker as a source (see figure 2 for details). using an air-filled regime, initial tests on the designed chamber have been performed between 20 hz and 1.2 khz using a bruel&kjær 8104 as a hydrophone under test, a calibrated bruel&kjær 4160 microphone as a reference, and a micro-speaker type cds-15118b-l100, used in cell phones and miniature headphones, as a sound source. the sound pressure of the source in air near its surface was estimated to be approximately 1 kpa for a typical radiated acoustic power of 0.1 w, under plane wave approximation. the first results confirmed a good signal-to-noise ratio and essentially flat response below 1 khz, with only some minor deviation below 50 hz due to electrical loading of the measurement instrumentation. figure 3 shows the first results of a test calibration made with the designed chamber in the frequency range from 20 hz to 1 khz. as an example of comparison with independent calibration data, calculations for 250 hz yielded a sensitivity of -206.65 db re 1v/µpa with estimated uncertainty of ±1 db, in agreement with reference sensitivity of (-206.2 ± 0.7) db obtained by free-field method at 1 khz. 4.2. calibration of hydrophones using a standing wave tube the second method for hydrophone calibration has been implemented by foi using a calibration unit (model c100, underwater science, research & development, inc., usa) which is based on the standing wave tube method [12]. this unit is made of a cylindrical cavity with an oscillating membrane mounted at the bottom for sound generation (see figure 4). the cavity is filled with water and the hydrophone is placed approximately at the tube center. as a device under test, a hti96 hydrophone was connected to a wildlife acoustics sm2m submersible recorder unit. hydrophone voltage readout was done after completing the measurement session from its internal sd memory card, using a matlab script to automatically detect signals for each frequency and their respective amplitudes. with this setup, early tests for frequencies between 100 hz and 1 khz were done by comparison with a b&k 8104 reference hydrophone. typical results show general good agreement to within 1 db with the factory calibration of the hti-96 equal to -164.5 db re 1v/µpa for frequencies from 100 hz up to 700 hz. unexpectedly large deviations, from 3 db up to 8 db, have been observed for frequencies between 800 hz and 1 khz, whose cause is currently under investigation. 4.3. review of autonomous recorders the first activities that dealt with autonomous recorders were focused on the review of their characteristics that are relevant for low frequency calibration. the overall configuration of autonomous underwater acoustic recorders is broadly consistent, while there are differences in their design. each device is usually made of a hydrophone, possibly with an integral preamplifier, connected to an electronics body containing an amplifier, analogue-todigital converter (adc), data storage media and batteries to power the unit. all recorders may be broadly categorised in two main categories according to their overall configuration: that is, whether the hydrophone cable is hard wired to the recorder body, or attached to it via a detachable electrical connector. each configuration type brings its own difficulties for low figure 3. preliminary results of pressure calibration of a b&k 8104 hydrophone using the coupling chamber shown in fig.2, compared with reference calibration point at 1 khz on the same device using free-field method. figure 2. pre-design layout of the coupling chamber for low-frequency calibration of hydrophones with the comparison method. the chamber dimensions are 54 mm length by 23 mm internal diameter. figure 4. overview of the setup for low frequency hydrophone calibration using the standing wave tube method. acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 36 frequency calibration. in the first configuration the recorder must be calibrated as one system while the hydrophone is attached to the device. this can pose particular calibration challenges for free-field calibration where sound waves interacting with the body may influence the measured sensitivity. if the hydrophone is not detachable from the body, a common feature with this configuration, low frequency pressure calibration may also be made logistically difficult because the entire body must be supported when inserting the hydrophone into a calibration chamber. the second configuration type offers the possibility to calibrate the hydrophone separately from the recorder body. in some respects, this simplifies the calibration because the influence of the recorder body on the performance is minimised. however, in this case the separate calibrations of the hydrophone and recorder must be combined to form the overall system sensitivity. in doing this, the overall system sensitivity may not just be the simple product of the hydrophone and recorder sensitivities, and care must be taken to take into account any electrical loading effects. for an autonomous recorder, the following characteristics need to be established in the frequency band of interest: • system sensitivity • system self-noise • system dynamic range • directional response • frequency response system sensitivity includes the preamplifier gain, which may take quite different values according to whether background noise or noise from a high-amplitude source is to be measured, in order to avoid signal degradation due to poor signal-to-noise ratio, clipping or nonlinearity. the electrical noise originating from the hydrophone and recording system is also defined as the system self-noise. the equivalent sound pressure noise-floor of a recorder is the lowest sound pressure amplitude that can faithfully be represented with the device. to achieve acceptable signal-tonoise ratios, the system self-noise (expressed as the equivalent bandwidth noise pressure level) should be ideally at least 10 db below this lowest signal level. it has been noted that not all the commercially available systems would be suitable for use in the measurement of very low level ambient noise in conditions near sea state zero or wenz minimum level [13]. the measuring system response also needs to be linear over the full dynamic range, requiring that the system sensitivity be constant over the full range of measurable sound pressure. systems with dynamic ranges in excess of 60 db are preferred for measurement of high amplitude impulsive sources. as the dynamic range is related to the number of levels of the adc, and accounting for a precision loss of 2 or 3 bits due to internal noise, this implies that the system should feature at least a 12bit adc. for a recorder with a hydrophone that is widely separated from the body, the usual requirement of omnidirectional response is generally satisfied for frequencies up to 20 khz. however, many of the commercially available devices, including the ones used in the project, consist of a hydrophone mounted either directly to the recorder body or close to it via a relatively short cable (see figure 5). the recorder body is typically an airfilled cylinder that can scatter the acoustic signal and cause perturbation of the response at kilohertz frequencies. it has been demonstrated that at kilohertz frequencies the recorder/hydrophone combination is not omnidirectional, and hence the sensitivity varies with angle of incidence, making the determination of the correct sensitivity challenging [14]. for this category of devices, since the direction of arrival of incident sound is normally not known, a significant directional response would therefore be a disqualifying characteristic. although autonomous systems may be simply required to have a flat frequency response in the frequency range of interest to within an accepted tolerance (such as a 2 db tolerance in iso fdis 18406), calibration of hydrophones and measuring systems would allow to correct for their variations in sensitivity with better accuracy. if the recorded data are already processed into one-third octave bands before the correction for hydrophone sensitivity is applied, and the hydrophone sensitivity is not flat, care should be taken since a constant value across the band cannot be assumed. a flat system sensitivity within the desired frequency range requires an adequate sampling rate of the adc. a flat response also yields a uniform phase response that enables to faithfully represent the acoustic signal when peak-sound pressure or the waveform shape are to be measured. the effect of the proximity of the recorder body may influence the system frequency response, due to a combination of the direct and reflected waves that causes interference. this problem is most acute for narrow-band signals received from a specific direction at kilohertz frequencies, and less severe for measurements of underwater sound in one third octave bands, where a degree of frequency averaging of the sensitivity will occur. figure 6 shows an example of early test calibrations performed on a number of various autonomous recorders by figure 6. typical examples of measured receiving sensitivities, in db re 1 v/µpa as a function of frequency, of commercially available autonomous recorders. each curve is for a different model. figure 5. detail of an autonomous recorder used for the calibrations, with the hydrophone and end cap removed from the main body showing the internal recording device. body dimensions are approximately 16 cm diameter by 80 cm length. acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 37 inserting their hydrophones in a closed chamber, therefore excluding all effects coming from the recorder body. the data show some correlation between the sensitivity value in the flat portion for higher frequencies and the cutoff frequency, approximately between 50 and 100 hz, below which a decay begins mainly due to electrical loading of the input stage of the preamplifier. 4.4. calibration methodologies for autonomous recorders the hydrophone included in an autonomous recorder may be calibrated independently using one of the methods described in previous sections only in case the hydrophone can be detached from the recorder. however, in this case the recorder system must also be calibrated by electrical signal injection and a separate procedure must be developed for this purpose. the two sensitivities must then be combined to provide the overall system sensitivity that may not just be the simple product of the hydrophone and recorder sensitivities due to electrical loading effects. these latter are typical of certain types of recorder which are compatible with a range of different hydrophone models. pressure calibration in an air-filled closed chamber as a method of low-frequency has already been demonstrated to be applicable for autonomous recorders in the frequency range from 20 hz to 315 hz [13] and has been selected to be adopted in the project. this method has the disadvantage of only exposing the hydrophone to the acoustic field, so any effects from the body of the recorder will be unknown and need to be evaluated using free-field methods. on the other hand, free-field conditions require a medium that is free of reflecting boundaries, which is very demanding to be realised in a laboratory tank below the kilohertz range [15]. the solution adopted by partners involved in this project is to use larger volumes of water for calibration, such as a lake or a reservoir, with water depths ranging from about 20 m to about 200 m, so that a much longer echo-free time is obtained for the measurements (see figure 7). at npl, an open-water facility is available to support freefield recorder calibration. the facility is a fully instrumented floating laboratory situated on a freshwater reservoir with 20 m depth, allowing free-field measurement down to around 200 hz. similarly, foi have a mobile measurement facility on the ice sheet that forms over lake hornavan, arjeplog, in lapland, sweden. the facility offers a maximum depth of 220 m and is covered by an ice cap of between 0.8 m and 1.0 m in march. the facility supports routine measurements down to 50 hz. cnr owns a facility with exclusive access to lake nemi near rome, a natural lake with about 1.5 km mean diameter and about 30 m depth. this site is routinely used for acoustic measurements on marine systems and offers a mild climate with little precipitation and wind for most time of the year, a background noise below sea state zero, and absence of any other man-made activity. to characterize the devices in conditions that are closer to those encountered in the field, measurements will also be performed by ispra in lake bracciano, another larger natural lake 30 km north of rome with about 8 km diameter and about 150 m depth, that also exhibits limited climate changes and moderate noise level produced by sporadic vessel traffic. similarly, sea trials will also be performed by tubitak in marmara sea. the above mentioned open-water sites will be used in round-robin experiments to be performed between march 2018 and november 2018. data measured using free-field methods in open-water sites can be contaminated by self-noise of the measuring system itself as well as by noise originating from the platform or method of deployment. the more common sources of platform and deployment noise that have been identified are: flow noise, cable strum (low frequency vibration due to currents), mechanical noise from debris and mooring cables, water surface heave and agitation. to minimise these noise components, proper guidelines and draft field reports are being produced for preparation, deployment and retrieval of autonomous recorders by the involved partners. calibration methods based on diffuse sound field shall finally be investigated. this method has been demonstrated in air by dfm for microphones [16] and recently in a laboratory water tank by vniiftri (russia) for a receiver attached to a towed body or recorder in one third-octave band levels [17], [18]. this approach accounts for the measured sound wave that is incident on the hydrophone as well as the measured sound that has been scattered off the recorder body. npl will investigate diffuse field methods in the npl facilities as part of the project. 5. conclusions the unac-low project, addressing the development of calibration standards for hydrophones and autonomous recorders for frequencies from 20 hz to 1 khz, has entered its main phase during which experiments will be performed by all partners either in the laboratory or in open water sites. experimental setups for both pressure calibration methods and free-field methods have been designed and implemented in specific and overlapping frequency ranges. preliminary tests using the air-filled coupling chamber designed for pressure method for hydrophones showed an agreement with reference calibration data obtained with free-field method. the figure 7. open-water test sites used for calibrating autonomous recorders using free-field methods: npl facility at wraysbury, uk (left), foi mobile facility at lake hornavan, sweden (center), cnr test site at lake nemi, italy (right). acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 38 experimental setups for calibration of autonomous recorders with pressure methods and free-field methods have been designed and the first experiments are currently under way. the compatibility of results by different methods and for different devices within stated uncertainties will be evaluated in roundrobin experiments to be executed by circulating one single device, either a reference hydrophone or a reference autonomous recorder, among participants in the respective project workpackages. the experiments will take place throughout year 2018 and results shall be available near project end after first quarter of year 2019. among project output, draft procedures and guidelines shall be produced describing the implemented calibration methods, and a common strategy for long-term sustainability of capabilities shall be defined, in agreement with requirements for the empir programme. project results shall be disseminated to the stakeholder community to ease the adoption of the developed capabilities within eu member states, for harmonised regulation and increased awareness towards preservation of the marine environment. acknowledgement the unac-low project is funded by the euramet empir programme under the research potential scheme with project code 15rpt02. references [1] project title: “underwater acoustic calibration standards for frequencies below 1 khz” (unac-low). web site: http://empir-unaclow.com/ [2] euramet empir web site: www.euramet.org/researchinnovation/research-empir/. [3] dekeling, r.p.a., tasker, m.l., et al monitoring guidance for underwater noise in european seas, parts i, ii & iii, jrc scientific and policy reports eur 26556-6558 en, publications office of the european union, luxembourg, 2014, http://publications.jrc.ec.europa.eu/repository/handle/1111111 11/30979. [4] toma, d.m., garcia benadì a., mànuel gonzàlez b.j., del rio fernandez j., systematic quality control for long term ocean observations and applications, acta imeko, vol. 5, no. 1, article 12, april 2016, identifier: imeko-acta-05 (2016)-0112. [5] iec60565:2006 underwater acoustics – hydrophones – calibration in the frequency range 0.01 hz to 1 mhz, international electrotechnical commission, geneva, 2006. [6] ansi/asa s1.20-2012 procedures for calibration of underwater electroacoustic transducers, american national standards institute, new york, 2012. 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[17] isaev a. e. and chernikov i.v. “laboratory calibration of an underwater sound receiver in the reverberation field of a noise signal”, acoustical physics, 61 (6), 699–706, 2015. [18] isaev a. and nikolaenko a. “the calibration of a receiver of the measurement of ambient underwater noise”, proceedings of internoise2016, hamburg, germany, 2016. http://publications.jrc.ec.europa.eu/repository/handle/111111111/30979 http://publications.jrc.ec.europa.eu/repository/handle/111111111/30979 calibration standards for hydrophones and autonomous underwater recorders for frequencies below 1 khz: current activities of “unac-low” project acta imeko, title acta imeko issn: 2221-870x september 2017, volume 6, number 3, 45-51 acta imeko | www.imeko.org september 2017 | volume 6 | number 3 | 45 validation of a low-cost wireless sensors node for museum environmental monitoring livio d’alvia1, eduardo palermo1, stefano rossi2, zaccaria del prete1 1department of mechanical and aerospace engineering, ‘sapienza’ university of rome, via eudossiana, 18, roma 00184, italy 2department of economics and management, industrial engineering, university of tuscia, viterbo, italy section: research paper keywords: sensors; ict; wireless sensor network; environmental measurement; museum air quality citation: livio d’alvia, eduardo palermo, stefano rossi, zaccaria del prete, validation of a low-cost wireless sensors node for museum environmental monitoring, acta imeko, vol. 6, no. 3, article 7, september 2017, identifier: imeko-acta-06 (2017)-03-07 section editor: sabrina grassini, politecnico di torino, italy received march 14, 2017; in final form july 29, 2017; published september 2017 copyright: © 2017 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: phd student grant of sapienza university supported this work corresponding author: livio d’alvia, e-mail: livio.dalvia@uniroma1.it 1. introduction degrading effects of manufactures in cultural heritage can be classified in: morphological, physical-mechanical, physicalchemical, and optical alterations. morphological alterations can involve: dimensional variation (i.e. expansion, torsion, etc.), loss of material and continuity (holes and cracks). physical-mechanical alterations determine a decrease of cohesion, adhesion, and elasticity. physical-chemical alterations cause a variation of porosity, hydrophilic and hydrophobic characteristic, etc. optical alterations influence visual parameters, such as colour, luminosity, etc. in indoor conservation, all these degrading effects are mostly driven by: (i) relative humidity and temperature; (ii) gaseous pollutants (o3, sox, nox, cox, h2s, nh3, hcl, etc.) and particulate matter; (iii) light intensity; (iv) air velocity and direction; (v) sound pressure and vibration [1]–[3]. environmental parameters monitoring is necessary to preserve materials, to identify causes of degradation, and to quantify their effects, as a function of time [4], [5]. however, the development of a fully inclusive and compact solution to precisely and punctually identify the effects induced by environmentaland anthropic-related factors is still an open challenge [6]. commercial solutions and wireless devices are cumbersome, bulky, and not-aesthetic when placed next to artefacts, and expensive. currently wireless sensor networks (wsn) have been employed in several application fields as in indoor/outdoor environmental monitoring or gas detection, both in healthcare and industrial contest [7]–[9]. application of wsn in a museum scenario has been so far restricted to the monitoring of temperature and relative humidity [10], [11]. the deployment of a wsn monitoring system presents valuable pros, such as: architecture scalability, the capability to integrate multiple and heterogeneous sensors on a single small node, the possibility to distribute a high abstract wireless sensor network (wsn) has been adopted in many contexts, such as healthcare or industry. in museum scenarios, wsn has been introduced for environmental monitoring, to control temperature and relative humidity. nowadays, the development of low-cost micro-scale sensing units, opened to new possibilities for wsn development, enlarging the set of feasible measurements to gaseous pollutant concentration, relative humidity, temperature, light intensity, air flow, or vibration. this work is part of the overarching goal of developing a low-cost and minimally invasive wsn designed for a museum scenario. the proposed wsn node, based on a zigbee standard, gathers signals provided by: a 9-axis mimu, a sensor for temperature and relative humidity, and three gas detection boards. in this paper, specifically, we present performances of the wsn node in detecting: structure tilt, vibrations, and daily cycle of humidity, temperature, and gas deposition. acta imeko | www.imeko.org september 2017 | volume 6 | number 3 | 46 number of wireless and low-cost measurement points in the exhibition areas. we decided to develop a novel, low-cost, wireless, scalable system, capable to control environmental parameters, as well as vibrations and deformation, temperature and humidity, gaseous pollutant, over a multi-stage research project, combining mems sensor boards and electrochemical-cells. in this paper we present a system that we have developed and validated for: a) detection of tilt and shock and define the frequency limit, b) acquisition of the daily cycle of environmental pollutant. in section 2 we will discuss the state of art of heritage monitoring and wireless networks applied in this context and the most significant environmental parameters and their effect. in section 3 we describe the architecture of our developed wsn node based on the ieee 802.15.04 and the experimental tests performed to validate the system. results of the research are presented and discussed in section 4. finally, in the concluding section, the major achievements are summarized. 2. state of art despite internal environmental parameters play an important role in the museums conservation, the monitoring activity has not benefited substantially of recent technological developments [12], [13]. several studies proposed limited solutions to evaluate the quality of expositive habitat, focusing especially on temperature and humidity measurements, and, sometimes, on gaseous pollutant, through a passive/diffusive traditional sampler. some authors [14]–[20] highlighted the importance of investigating the fluctuation of air temperature (t) and percentage relative humidity (%rh), identifying these parameters as determinant in the deterioration of collected artefacts. at the same time, they pointed out the difficulty to install sparse measurement stations preserving artworks appearance. in fact, many museums, art galleries and historical buildings in general are themselves masterpieces, making the installation of bulky monitoring devices inappropriate. in addition to temperature and relative humidity, several works [16], [21]–[27] presented methodologies to sample the inorganic or volatile organic compounds (nox, o3, sox, voc) and particulate matter. the first quantities are sampled through chemisorbing cartridges that require a subsequent analysis with ultraviolet-visible (uv-vis) spectrophotometry or ion chromatography. for particulate matter, controlled flow ratio samplers with filters that permit a detailed analysis of elemental concentrations with sem microscopy are used. all these analyses cannot be performed in ordinary survey due to excessive costs of equipment and post-process analysis. moreover, only the thermo-hygrometric analysis can be associated to a remote wireless network, while the other techniques require the intervention of specialized technicians/analysts. saraga et al. [25] discussed the application of measurement units for external environmental monitoring, including: (i) ultraviolet photometry automatic analyzers for o3 (en 14625:05); (ii) ultraviolet fluorescence for so2 (en 14212:05); (iii) chemiluminescence for nox (en 14211:05), and (iv) gravimetric measurement for pm10 (en 12341:99) and pm2.5 (en 14907:05). the proposed instrumentation has a dimension of 42.5 cm (w), 157.5 cm (h), 58.5 cm (d), occupying a volume that could be incompatible with museum applications and a total cost around 10.000 €. the indoor, low-cost, small scale solution we are developing is inspired to what is proposed by mead et al. [28] for external monitoring. they integrated in a single board: (i) an environmental parameter sensor (temperature, relative humidity, barometric pressure); (ii) an electrochemical cell to monitor gas pollutant; (iii) one optical particles counter (opc) to control the total particle matter; and (iv) an anemometer to analyze the wind direction and intensity, showing the real benefits related to low-cost sensing. building on their findings, here we propose a solution cut off for including indoor functionality, integrating inertial data to the environmental ones. such an effort completes the solutions proposed by [10], [11], where an original equipment manufacturer (oem)-wsn has been proposed, to monitor only temperature and humidity into a museum environment. e. angelini et al. [29] proposed a smart sensors board for microclimate monitoring; corbellini et al. [30] introduced a sensor network architecture developed for museum monitoring and f. diego et al. [31] propose a sensor network architecture to read physical parameters in the cultural heritage field. the aim of these works was mainly focused on the variation of temperature and relative humidity in a museum. in this research, we seek to integrate in a single board two mems sensors: one for tilt and vibration detection and one for environmental parameter (temperature and relative humidity) monitoring. in addition, three electrochemical cells are used to monitor pollutant concentrations. current cost is 500 € for the whole system with a dimension of 9.5 cm (w), 16.0 cm (h), 6.0 cm (d). 3. materials and methods 3.1. prototype description the α-device of the wsn node is composed by a transmitting/receiving zigbee unit, a computational unit based on a risc microcontroller avr atmega328p, and two development-boards with mems sensor units: (i) a bosh bmo055 9-axis mimu (magnetic inertial measurement unit), (ii) a bosh bne280 humidity, pressure, and temperature sensor, and an alphasense 3 input analogic front-end sensor board for no, no2 and so2 concentrations, as reported by the schematic in figure 1. all sensors are connected to the figure 1. architecture of the proposed wsn with microcontroller and analysed parameter. acta imeko | www.imeko.org september 2017 | volume 6 | number 3 | 47 microcontroller via an i2c serial communication bus. figure 2 shows the real prototype with highlighted the constituent block: (a) control unit, (b) power supply, (c) radio modules and (d) sensors unit plug-in. a) control unit the proposed wireless node is based on a microcontroller (mcu) atmega328p, a low power microcontroller with two spi serial interfaces, one programmable serial usart (used to interface the mcu with an external pc for programming) and other serial devices (e.g. radio device) and a twi serial bus used to communicate with the mems sensor board (encircled in black). b) power supply the radio modules and sensor board require a stable 3.3 v source and a mcu at a 5 v source. a lt7805 regulates the input voltage (5-38 v) to a 5 v output with standard deviation lower than 35 mv and a ld33 tension regulator converts 5 v to 3.3 v (cyan blue box). c) radio module the radio module is a transmitting/receiving zigbee unit ieee 802.15.4 in 2.4 ghz band with +3db output power and 250 kbit/s transmission. it is used in at commands directly connected to the usart of the atmega328p (grey box). d) plug-in for sensor boards the bno055 (highlighted in red) is a low-cost mems system in package (sip) that integrates a 3-axis 14-bit accelerometer, a 3-axis 16-bit gyro, a 3-axis geomagnetic sensor and a 32-bit cortex m0+ microcontroller for the data sensor fusion (quaternion and euler angles) [32]. the possibility for the experimenter to set-up the acceleration ranges and the lowpass filter bandwidths, permits to adapt the sensor to several applications such as the museum scenario where the vibrations are induced by visitors or a heating, ventilation and air conditioning (hvac) system. the bme280 (encircled in green) is a low-cost mems sip that combines a digital humidity, a pressure sensor, and a temperature sensor. the alphasense 3-afe 810-0019-03 model sensor board (circled in orange) is an analog front-end board integrating three different electrochemical cells. the board is a current (na) to voltage (mv) converter. the current represents the output of the sensors and it is given by the reduction/oxidation of the pollutant due the electrolyte reservoir inside the electrochemical cell that generates a flow of electrons. the support circuit in addition provides a low noise power supply voltage and a bias compensation signal specific for each gas sensor. the implemented sensors are: a no-a4 for nitric oxide and a sensitivity of 0.342 mv/ppb [33], a no2-a43f for nitrogen dioxide with a range of 0 to 20 ppm and a sensitivity of 0.197 mv/ppb [34], a so2-a4 sulphur dioxide with a range of 0 to 50 ppm [35], where ppm and ppb mean respectively part per million and part per billion. all sensors are chosen according to the oms guidelines [36] that indicate the maximum level for the pollution. the system is calibrated and certified by the producer to give to the user information needed to compensate both zero and sensitivity drift for each sensor. 3.2. experimental procedure the mimu sensor is employed for two monitoring activities: (a) tilt detection of wall due to fracture and/or deformation, and (b) shock detection. to assess the performance of the mimu in these two applications, two experimental setups have been designed: (i) the former including a rotating plate and (ii) the latter including an electrodynamic shaker. in our node prototype, tilt and shock will be detected by different processing data from the same inertial sensor. for this reason we set a cut-off frequency for the in-built settable lowpass filter at 250 hz, a high value for tilt but required for properly acquiring vibrations. the environmental sensors, for hydrothermal values and pollutant concentration, have been placed outdoor, for evaluating the system in proximity of a certified pollution monitoring system by arpa lazio (lat. 41.864194 °, lon. 12.469531 °). 3.2.1. tilt detection a servomotor controlled in closed loop by means of an angular encoder (sanmotion rs1a03aa) has been used to estimate the accuracy and the stability of the embedded accelerometers. specifically, the mimu was mounted on a vertical plate connected to the servomotor through a belt as shown in figure 3. the bno055 has been programmed by setting the internal low-pass second order filter to 2501 hz and the measurement range to ±16 g (same parameters were selected for vibration detection test). a labview program has been implemented to rotate the plate around the horizontal axis from 0° to 180° with a step of figure 3. plate with highlighted the θraw, θfusion, θref angles. figure 2. α-device pcb with microcontroller boxed in black, power supply in cyan, zigbee in grey, and bno055, bme280 and electrochemical cell respectively in red, green and orange box. acta imeko | www.imeko.org september 2017 | volume 6 | number 3 | 48 1° every 15 minutes, simulating tilt rotations induced by structural deformations. we acquired tilt angles measured by the encoder (θref), the roll angle (θraw) calculated directly by three acceleration components (a x, a y, a z), and the roll angle provided by the data fusion algorithm that is embedded into the sensor (θfusion). to validate the built-in sensor fusion algorithm, the accuracy of the accelerometers was estimated by means of the root mean square error (rmse) between the average values of the measured signals (θraw, θfusion) in the 15 minutes’ window, and each reference angle (θref), gathered every 15 minutes. the standard deviation of θraw was evaluated to estimate the noise of the accelerometers, an important parameter to avoid misdetection, due to long-term functioning of the sensor. 3.2.2. shock detection a vibration exciter type 4809 (bruel&kejar) has been used to provide known inputs to the sensor. we compared the output of the filtered (at 250 hz cut-off frequency) mimu output with a reference signal provided by a certified monoaxial accelerometer (bruel&kejar 4371 model.) both sensors have been placed on top of the vibration exciter as shown in figure 4. the test has been repeated for 8 different oscillation frequencies in the range 10-50 hz with a 5 hz step. this range of frequencies has been chosen to respect the nyquist condition, as the maximum sampling frequency in this prototype is limited by the usart transmission time of 9 ms. however, the selected range is compatible with other studies in the field [37]–[39]. a high accuracy waveform generator has imposed the sinusoidal motion, with a fixed peak-to-peak displacement amplitude (0.5 mm) for all frequencies. the described procedure has been repeated three times, by aligning each time a different mimu axis with the motion axis. accuracy has been evaluated by calculating the rmse between test and reference signals, normalized to the peak-to-peak value of the reference sensor (nrmse). 3.2.3. environmental parameter in the preliminary test, we decided to put the sensor system outdoor, in proximity of a certified monitoring system (arpa), during a five-days acquisition. to evaluate the accuracy of the sensor output in mutable meteorological and traffic conditions the system has been programmed with a sample period of 1 minute. the acquired data have been post-processed by calculating the moving averages of the outputs, with a one-hour step and an 8-hours window (8h-avarage). regarding gas concentrations, the temperature dependence is corrected in post-processing using the output of the embedded pt1000. corrected gas concentration values are calculated using the formula provided by the calibration certificate 4. results and discussions 4.1. tilt detection the relation between θref and θraw is shown in figure 5. the mimu accuracy for this measurement, evaluated though the rmse was equal to 0.3°. the sd of the mimu output was always lower than 0.4° in the 0-180° tilt range. figure 5 reports the test results in the 0-90° range. the rmse between θfusion. and θref was equal to 0.2° and the sd was always lower than 0.2° in the 0°-180° tilt range. this wide range was chosen to assess accuracy in the tilt angle measurement regardless of initial placement of the sensor, since mems accelerometers embedded into mimus can present different accuracy levels for each axis. figure 6 shows the comparison between θraw and the one provided by the internal sensor fusion algorithm θfusion. figure 6 highlights how the fusion algorithm output is less sensible to noise and, consequently, more stable over time, with a maximum sd of 0.2°. the embedded data fusion algorithm, based on a kalman filter, can filter noise; however, due to the slow dynamics of the phenomenon, the increased stability is not paid in terms of noise, as demonstrated by the rmse. 4.2. results of shock detection figure 7 shows the comparison between 𝒂𝑟𝑟𝑟 and 𝒂𝑦𝑡𝑟𝑡𝑡, in correspondence of an excitation frequency equal to 20 hz. figure 8 highlights the nrmse for the three axes over the entire excitation frequency range. from the analysis of both figures it clearly appears that phase shift between the figure 5. relation between θraw and θref between 0-90 °. figure 4. vibration exciter whit reference sensor (a) and mimu (b). figure 6. comparison between θraw and θfusion in 0-3°. acta imeko | www.imeko.org september 2017 | volume 6 | number 3 | 49 acceleration measured through the mimu and the one measured with the certified accelerometer is negligible, despite the difference in signal filtering. mimu uses an internal second order low pass filter, with undeclared parameters, while to the certified accelerometer signal we applied a second order butterworth digital filter, with a 250 hz cut-off frequency. the nrmse value was similar for the three axes, with a mean value of 0.35 for the x and z axis, and of 0.36 for the y axis. 4.3. environmental and pollution detection figure 9 reports the outdoor concentration of pollutant gasses in ppb in two chosen days, between 12:00 am of friday 02/10/17 to 12:00 am of sunday 02/12/17. in general, the pollutant gasses concentration is lower during the night than during daytime. values are compatible with the concentration values provided by the certified system [40], [41] and lower than the normalized day limits (140 ppb for so2, 100 ppb for no2 and no). the arpa system reported a maximum of no2 hourly concentration at 7pm for both 10 and 11 february 2017. these peak timings correspond with the ones in figure 8, even if the actual value measured with our wsn (37 ppb against 60 ppb from arpa system) is affected by the height difference. the maximum level of so2 concentration appears to be 1pm. so2 is an impurity compound of fossil fuel commonly used in buildings. the highest values of hourly sd were 0.7 ppb for no2, 0.2 ppb for no, 0.8 ppb for so2. figure 10 shows the day-night cycle of temperature and rh in the two chosen days, between 12:00 am of friday 02/10/17 to 12:00 am of sunday 02/12/17. as expected, the sensor shows an evident decrease in rh, in correspondence to an increase in temperature (ex. 12 am). maximum values of sd were 0.4 °c and 0.6 % for temperature and relative humidity, respectively. 5. conclusions we designed and analysed the performances of a low cost wireless sensor network node for the environmental monitoring of cultural heritage. at this stage of development, we focused on the assessment of the accuracy and stability of tilt angle and shock detection, measurement of gass concentration and thermo-hygrometric parameters. the tilt measurement demonstrated a good accuracy for the targeted application. the observed stability was more than acceptable, demonstrating the robustness of the solution as a function of time. the embedded data fusion algorithm demonstrated a good capability of filtering noise without losing responsiveness for this application. shock vibration detection demonstrated a stable behaviour over the chosen frequency range (10-50 hz), along the three different axes. figure 8 shows not negligible nrmse values (35 %) for frequencies over 20 hz, but they can be considered acceptable for the shock detection purpose. outdoor behaviour confirmed the expected inverse proportionality between temperature and rh. gas concentration sensors showed a trend over time comparable to data from the arpa system, with a time correspondence in peak values, even if a difference in average values was observable, due to a different height positioning (figure 9). despite the wide operating range (0 to 20 ppm) of the gas sensors, in general gas concentrations at street level are in the range of 20-200 ppb for sox and nox, according to the oms guideline. values found in this study are in line with expectations. alphasense’s electrochemical cells present limitation due to cross sensitivity to other chemical compounds, life time (2 years) and drift. however, the limited cost (ca. 50 € each) allows for a large diffusion of sensor’s nodes in a limited area. the β-device of the wsn will integrate light sensors and other pollutant sensors, such as co2, to provide a complete figure 7. example of acceleration signals acquired via the two systems. figure 8. normalized rmse (nrmse) for all axis and frequencies. figure 9. 8h-average gas concentration for so2, no2, no with 8h-sd. figure 10. 8h-average for temperature and relative humidity with sd. acta imeko | www.imeko.org september 2017 | volume 6 | number 3 | 50 description of the environmental parameters of interest in a museum scenario. in addition, a finest firmware will be implemented on the device, and a data buffer will be added to overcome the communication timing jitter and extend the shock detection frequency range. acknowledgement the 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[41] a. tittarelli, a. borgini, m. bertoldi, e. desaeger, a. ruprecht, r. stefanoni, g. tagliabue, p. contiero, and p. crosignani, “estimation of particle mass concentration in ambient air using a particle counter,” atmos. environ., vol. 42, no. 36, pp. 8543–8548, nov. 2008. validation of a low-cost wireless sensors node for museum environmental monitoring << /ascii85encodepages false /allowtransparency false /autopositionepsfiles true /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /error /compatibilitylevel 1.4 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.0000 /colorconversionstrategy /cmyk /dothumbnails false /embedallfonts true /embedopentype false /parseiccprofilesincomments 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can be opened with acrobat and adobe reader 5.0 and later.) >> /namespace [ (adobe) (common) (1.0) ] /othernamespaces [ << /asreaderspreads false /cropimagestoframes true /errorcontrol /warnandcontinue /flattenerignorespreadoverrides false /includeguidesgrids false /includenonprinting false /includeslug false /namespace [ (adobe) (indesign) (4.0) ] /omitplacedbitmaps false /omitplacedeps false /omitplacedpdf false /simulateoverprint /legacy >> << /addbleedmarks false /addcolorbars false /addcropmarks false /addpageinfo false /addregmarks false /convertcolors /converttocmyk /destinationprofilename () /destinationprofileselector /documentcmyk /downsample16bitimages true /flattenerpreset << /presetselector /mediumresolution >> /formelements false /generatestructure false /includebookmarks false /includehyperlinks false /includeinteractive false /includelayers false /includeprofiles false /multimediahandling /useobjectsettings /namespace [ (adobe) (creativesuite) (2.0) ] /pdfxoutputintentprofileselector /documentcmyk /preserveediting true /untaggedcmykhandling /leaveuntagged /untaggedrgbhandling /usedocumentprofile /usedocumentbleed false >> ] >> setdistillerparams << /hwresolution [2400 2400] /pagesize [612.000 792.000] >> setpagedevice smartphone-based drive-test methodology for the experimental characterisation of the physical layer and a performance assessment of lte networks in urban environments acta imeko issn: 2221-870x june 2019, volume 8, number 2, 28 34 acta imeko | www.imeko.org june 2019 | volume 8 | number 2 | 28 smartphone-based drive-test methodology for the experimental characterisation of the physical layer and a performance assessment of lte networks in urban environments stefano avallone1, nicola pasquino1, giorgio ventre1, stefania zinno1 1 dipartimento di ingegneria elettrica e delle tecnologie dell’informazione, università degli studi di napoli federico ii, via claudio 21, 80125 napoli, italy section: research paper keywords: measurements; mimo; lte physical layer; lte quality indicators; 4g; multivariate statistical analysis; experimental characterisation citation: stefano avallone, nicola pasquino, giorgio ventre, stefania zinno, smartphone-based drive-test methodology for the experimental characterisation of the physical layer and a performance assessment of lte networks in urban environments, acta imeko, vol. 8, no. 2, article 5, june 2019, identifier: imeko-acta-08 (2019)-02-05 section editor: emiliano sisinni, university of brescia, italy received july 15, 2018; in final form june 11, 2019; published june 2019 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: stefania zinno; e-mail: stefania.zinno@unina.it 1. introduction multiple input multiple output (mimo) technology has introduced a strong increase in performance among mobile systems. contemporary use of multiple antennae at both the transmitter and receiver sides of a mimo system [1] enables mobile radio systems to achieve substantial performance improvement in terms of transmission efficiency and power consumption. this improvement is thanks to a reliable procedure that provides countermeasures to poor transmissionpath conditions and give the system a broad spectral efficiency. however, mimo is an even broader technology than that, with multiple implementation starting from its basic principle [2]. the long-term evolution (lte) standard supports a great variety of mimo operating schemes suitable for different radio scenarios, and it is able to select schemes dynamically in order to properly address the relevant circumstances. spatial multiplexing (sm) is expected to give better robustness than transmit diversity (td). in fact, lte sm-mimo in indoor environments, outperforms td-mimo and single input multiple output systems because a single data stream transmitted by different antennae can overcome the adverse multipath propagation that is typical of such environments [3]. furthermore, in [4], the re-enactments demonstrate that the adoption of mimo is superior to the single input single output approach. in [5], a timeline from release 8 to the latest releases of multi-user mimo (mu-mimo) techniques is presented. the discussion therein deals with specific aspects of mimo implementations and related solutions for mu-mimo. in order to widely investigate lte and its mimo implementation, in [6], an enodeb is tested in a reverberation chamber, and an experimental campaign is carried out to investigate how power boosting affects enodeb performance in a multipath environment like wireless and vehicular communications. channel quality parameters are measured, ranging from the abstract long-term evolution (lte) architecture has recently been enhanced by means of the introduction of multi-antenna systems that make use of multiple input multiple output (mimo) technology, both at the base transceiver station (enodeb) and user equipment (ue). mimo can deploy transmission diversity to achieve more robust transmission in poor propagation conditions and spatial multiplexing to increase throughput and thus lte performance from an end-user perspective. the most recent version of the standard (release 10) makes up to eight antennae available for the downlink transmission channel. the paper focuses on the most relevant physical-layer parameters used by lte to optimise a mimo system. performance at different frequencies and bandwidths is investigated over a wide urban area in order to study the network behaviour in actual propagation conditions and to verify whether standard prescriptions are satisfied by a fully operational lte mobile communication system. mailto:stefania.zinno@unina.it acta imeko | www.imeko.org june 2019 | volume 8 | number 2 | 29 empty chamber condition with a highly rich multipath channel to a fully loaded condition in order to mitigate multipath. anechoic chamber testing is performed in [7]. mimo user equipment (ue) conditions, and performance is tested along with the implementation of plane-wave synthesis and prefading signal synthesis techniques. frequency domain packet scheduling (fdps) has a great influence on lte systems [8]. considering how hard it is to optimise the system, the easiest solution could be to efficiently assign time-frequency resources to ues by using a proportional fairness metric. one of the first proposals to fully address an lte mimo fair comparison of different mimo schemes is carried out in [9]. the analysis is performed with multiple spatial transmission and a unique signal-and-interference-tonoise ratio (sinr), both with the effect of packet scheduling included in the post-scheduling sinr. the results show that spectral efficiency can benefit significantly from the combined use of sm and fdps. in [10]’s experimental setup, an lte enodeb shows that the throughput at the lower layers can easily be computed if the preferred channel quality indicator (cqi) and a number of assigned resource blocks (rbs) are well known in advance. by means of simulations, sridhar et al. [11] confirm that a channel quality estimation with an inter carrier interference (ici) cancellation technique reduces complexity and execution time in mimo networks. at first, the adoption of a maximum likelihood detection scheme is used to suppress the interfering channels, and parallel interference cancellation method and decision statistical combining are adopted in order to cancel ici and improve data symbol detection. the estimation process for interference gives rise to a significant criticality in lte systems that use mu-mimo. [12] uses a framework for estimation together with a practical solution based on a suboptimal non-iterative method. the nonideal behaviour of mu-mimo precoding schemes due to poor channel knowledge is also investigated, and a few benefits are presented along with a proposed solution. in [13], several antennae are placed in a 2d array configuration in order to realise spatially separated transmission links to many mobile stations that adopt fd-mimo. the design and implementation of the 2d antenna array are also presented in detail therein. the authors of this current article have also contributed to the analysis of lte network performance using statistical methods in [14]. the aim of the research is to assess network performance when a fully operating mimo system is adopted, and to compare them to what is stated in the 3gpp standard. focusing on the downlink channel throughput, measurements are conducted with reference to key parameters for radio optimisation, like sinr assessed directly on the end-user side. the results of an extensive measurement campaign and system characterisation highlight that standard expectations are too high with respect to actual urban environment deployment of the system. the paper is organised as follows: in section 2, we describe mimo features and their implementation. in section 3, the experimental setup is presented, while results are discussed in section 4. finally, section 5 will then draw conclusions and present some considerations about ongoing research. 2. mimo: improving efficiency and throughput multi-antennae technologies for reception and transmission at both the enodeb and ue side have enabled high lte performance. mimo antennae can bring several potential benefits to mobile radio systems, including more reliable execution of system procedures under poor signal conditions, wider spectral efficiency, and increased data rates for mobile users. although higher performance can be achieved by resorting to multiple antennae and sm techniques, systems costs are higher when compared to single antenna-based systems, due to an increase in hardware costs and complexity. mimo systems also reduce the effects of fading in the propagation channel at the cost of advanced software requirements. since release 10 (also known as lte-advanced [15]), up to eight antennae are available for the downlink channel. at a basic level, however, the goal is to ensure an error rate of 10 % or fewer, allowing the ue to take advantage of the sm. 2.1. configurations mimo in lte can adopt the 2×2, 4×4, and 8×8 antenna configurations. it can also be configured, as mentioned above, in two modes, sm and td, as shown in figure 1. on the one hand, the aim of sm is to transmit multiple data streams with different content over different parallel channels in order to increase the data rate. it takes advantage of multipath propagation to create several independent transmission channels between the transmitter and receiver, which enables two or more different signal streams to be transmitted simultaneously. using proper coding and signal processing, information on the different streams can be extracted independently at the receiver side. this technique has led to an increase of the throughput available to a single user or allows multiplex data from multiple users. the maximum number of channels that can be multiplexed together is equal to the smaller of the number of antennae on both the transmitter and receiver sides. on the other hand, if a single user is taking advantage of a single information flow, then td can achieve spatial diversity performance with the use of multiple antennae at the transmitter side. several versions of the same information stream are sent to the receiver as a countermeasure to channel fading and to improve sinr. td techniques are adopted when a ue is experimenting the worst channel conditions. both closed loop and open loop are implemented in the lte spatial diversity operating modes. with open loop, spatial diversity is adopted blindly, while with the use of closed loop, control information is collected by the ues in order to properly weight the antennae. when su-mimo is adopted, the basic idea is to transmit a multiple codeword to a single user in the same time-frequency interval so that there is only one scheduled user per sub-band or channel. a codeword is defined as a coded transport block, but in a wider sense, we consider it as a single independent data figure 1. sm and td mimo. acta imeko | www.imeko.org june 2019 | volume 8 | number 2 | 30 stream. although a mu-mimo system can perform much better than a su-mimo one, we only focus on su-mimo in the following sections, because that is the system configuration adopted in our experimental campaign. 2.2. transmission modes in the downlink, lte, apart from mimo, offers fall-back technologies such as td or siso. it also supports different transmission modes (tms) for each individual ue. which mode is applied is determined by both the capability of the ue and the capabilities and engineering of the enodeb. the tm specifies how to process data transmissions. for example, siso is implemented in tm1, while td is identified with tm2. sm is introduced in tm3 and tm4, which implement open loop and closed loop transmission, respectively; the former applies blind spatial diversity, and the latter uses data collected by the ue to properly weight each antenna. good summaries of each tm can be found in table 7.2.3-0 in [16] and in table 1. when tm1 is in use, the system transmits one single layer from each antenna port. reference signals (rss) combined with data are transmitted, adopting the same antenna configuration. to demodulate the signal at the receiver side, only the rs related to that specific user is required. although many antennae are simultaneously transmitting, with this technique, a single ue is not aware of the actual number of parallel transmission and perceives data as it was transmitted over the single stream. to improve sinr and robustness of transmission, tm2 implements a classical mimo. all data is in fact, sent via multiple antennae equipped with different modulation schemes and frequency configurations. when tm2 is in use, only a single layer is mapped with a space frequency block code (sfbc) that relies on the alamouti code [17] and is transmitted from multiple antennae. when two antennas are adopted, a frequency-based version of sfbc is used, while with a combination of four antennae, a mix of sfbc and frequency switched transmit diversity is used. in the case of a ue moving at high speed, so when channel information is missing or when the channel rapidly changes, sm is implemented with tm3. when tm4 is implemented, each transmission is mapped to one or more layers. tm4 adopts a closed loop technique, which grants channel estimation at the receiver. 2.3. cqi reporting in lte networks, ues usually estimate channel state information and aggregate it into a channel status information (csi) report, simply analysing the network data retrieved from the physical layer measurement. csi comprises [16]: cqi; precoding matrix indicator (pmi); precoding type indicator (pti); and rank indication (ri). measurements should be conducted to determine the optimal cqis under different csi conditions. to properly tune modulation and coding scheme (mcs), ues are used to periodically transmit csi to the enodeb in the form of a cqi report, based on the measurements that are periodically carried out by mobiles. cqi values span from 0 to 15. the higher the cqi, the higher the order of modulation suggested for adoption by the ue. cqi can be related to the overall operating band (wideband cqi) or to a few selected sub-bands of interest (sub-band cqi). the wideband report provides a cqi value computed by the ue for the whole downlink bandwidth, while in the calculation of sub-band cqis, it is assumed that each transmission occurs only in the preferred bands. if a ue-selected sub-band feedback is chosen, as in the enodeb-configured one, both kinds of cqis are collected i.e. the sub-band cqi is the average value measured in the m selected sub-bands, each of a specific size. typically, the higher layer configured sub-band report provides the highest granularity [16]. each tm requests a distinct set of reporting. with tm4, implementing a closed loop technique, the ue sends a full report to the base station as a response about the channel conditions. a full report also deals with pmi configuration, which is periodically sent, and possible pre-coding matrices are shared between the receiver and the transmitter. tm2 and tm3, on the other side, require no feedback at all or less information from the actual user about the channel propagation conditions. 2.4. measurement capabilities for e-utra different quantities are measured at the physical layer, and in the following section, a brief review of the most significant ones is given. one key parameter is the received signal strength indicator (rssi), which is the total received power, thus including both serving and non-serving co-channel cells interference, adjacent channel interference, thermal noise and the noise generated within the receiving demodulator bandwidth. with reference to a cell-specific rs, sinr is defined as the linear average of the power contribution given by the resource element (re) divided by the linear average of the sum of noise and interference power within the same frequency bandwidth. then, for a specific cell, the reference signal received power (rsrp) is defined as the average power of the portion of frame table 1. tm features and main quantities reported. transmission mode features main quantities reported tm1 single transmitting antenna ue-selected sub-band and wideband cqi; higher-layer configured wide-band and sub-band cqi, no pmi. tm2 transmit diversity ue-selected sub-band and wideband cqi; higher-layer configured wide-band and sub-band cqi, no pmi. tm3 open loop sm with cyclic delay diversity (cdd) ue-selected sub-band and wideband cqi; higher-layer configured wide-band and sub-band cqi, no pmi. tm4 closed loop sm (feedback from the ue) wideband cqi per codeword and pmi for each sub-band; ue-selected sub-band and wideband cqi per codeword, and pmi; higher-layer configured wideband and sub-band cqi, and pmi. acta imeko | www.imeko.org june 2019 | volume 8 | number 2 | 31 that carries cell-specific rs within the considered bandwidth. expressed in dbm, it is calculated as: rsrp = rssi − 10 log(12 ⋅ 𝑁), (1) where n is the number of rbs of the lte's rssi measurement bandwidth. reference signal received quality (rsrq) is a cell-specific signal quality metric. it is defined in linear units as: rsrq = n  rsrp rssi (2) like rsrp, rsrq is mainly used to provide ranking among different cells as candidates to become the serving cell based on their signal quality, and it represents the quality of the received rs. 3. experimental setup the measurement campaign was conducted in an urban area of a city in southern italy by means of a drive-test experimental activity, as seen in figure 2, where the ue is placed inside a car that is driven around the investigated area to collect georeferenced measurements, which are then post-processed. the figure also shows that the serving cell (the one connected by a red arrow) is not always the nearest one; rather, it is the one that has the parameter values that best fit the standard requirements. such methodology has been already explored and used in [14], [18], [19], [23], [24]. the location of measurements cannot be indicated due to a non-disclosure agreement signed between the authors of this article and the mobile operator. the only available information is that data was collected over a coastal city, which therefore has both typical urban propagation and clutter signal due to the sea, which is known to affect propagation by surface reflection and may therefore increase interference significantly. during the measurement campaign, we intentionally did not analyse just one source of traffic. rather, we monitored a few hours of a typical day of normal smartphone usage (both traffic and voice data were collected), in the time interval between 8 am and 12 am, which are typically peak hours of traffic for a cellular network. the ue has neither a specific hardware or software configuration, nor has it been placed in a specific position within the car so as to mimic the typical position it would have during normal use i.e. it was placed randomly in the driver’s jacket pocket, on the passenger seat, or on the dashboard. assessment of the network performance was carried out using the nemo handy tool. the software provides automated results in a logbook format from ues’ raw measurement data. it acts like a live network monitoring tool, and for each event, it collects different datasets. in our experiment, due to the reduced number of samples from two of the four lte bands tested, data from only two ues are reported and analysed, each set at a specific system carrier frequency/bandwidth value of an lte frequency division duplex (fdd) system, namely: lte fdd 2100 band 1 (10 mhz bandwidth); lte fdd 800 band 20 (20 mhz bandwidth). hardware and software specifications are summarised in table 2. however, other system carrier frequencies have been investigated but are not discussed in the paper due to the lack of samples collected. the network under test implements a 2×2 mimo, as shown in figure 3. we refer the reader to the nemo analyzer specifications and features [20] for specific information on each event that the software can monitor. we limit our focus to the content provided for lte technology regarding cellmeas (shown in table 3), cqi, and carrier per interference (ci) events. the events and the related parameter values were monitored according to the 3gpp technical specification for evolved umts terrestrial random-access network (eutran) version 12 [21] (see section 2.4 for an explanation of the physical layer quantities), and measurements were recorded when samples of interests are received. cellmeas stores rssi in dbm, rsrp between 0 and 140 dbm, rsrq between 0 and 30 db; cqi explores: wideband and sub-band cqi for both codeword 0 and 1, in the discrete range between 0 and 15. ci figure 2. drive-test measurement campaign concept. table 2. system and smartphone configuration. system system frequency band samsung model nemo handy version lte fdd 800 mhz 20 s7-edge 2.62.337 lte fdd 2100 mhz 1 s5 2.62.337 table 3. nemo handy cellmeas event event id cell meas cellular system gsm, tetra, umts fdd, iden, umts td-scdma, lte fdd, lte tdd, cdmaone, cdma 1x, evdo, wlan, gan, wlan, wimax, amps, damps, namps record state always description recordings occur when the retrieved sample is -50 received from the ue. it is not mandatory that -100 all received samples are recorded and the current frequency of recording is about twice per second when the ue is in a connected state. separate measurement event logging is kept for each system. tool nemo outdoor, handy, autonomous, q figure 3. mimo 2×2 spatial multiplexing. acta imeko | www.imeko.org june 2019 | volume 8 | number 2 | 32 investigates rs sinr, whose calculation method can depend on the device hardware. it can reach up to 50 db. 4. mimo analysis in this section, we present a mimo analysis of the supervised operator. most of the time, both systems adopt tm4 with closed loop sm and tm2 with td. as already stated, tm4 is in use when channel conditions are favourable, so we expect that the physical layer parameters confirm that we are in good channel propagation condition. 4.1. physical layer quantities statistical analysis the experimental probability density function (epdf) corresponding to the physical layer parameters presented in section 2 are shown in figure 4 and figure 5 with respect to the different tms and codewords. for band 20, only the values measured at tx (codeword 0, 1) are collected, while for band 1, values are collected for both txs and rxs. for band 20 (figure 4), channel conditions are shown to be in a good range, while for band 1 (figure 5), we can clearly see a worsening of the rsrq values, possibly due to the poor propagation properties of the higher frequencies. the sinr pdf is missing in figure 5 due to a lack of samples. there also seems not to be much of a difference between tms and codewords, as shown in figure 4, besides a slight and statistically insignificant shift to the left in the sinr density peak value position and a significantly smaller variance value for tm2, unlike the expected behaviour. this finding can be explained by that mode, which is based on td, being specifically designed to enhance network performance in terms of sinr. in addition, it is apparent from the picture that rsrq, unlike the other quantities, has a smaller variance i.e. it is more stable in its mean, leading to the rs’s overall more stable quality. furthermore, the significant difference between rssi and rsrp, the former being about 25 db greater than the latter, shows that the network is affected by a large interfering signal. in figure 5, we can see that tm3 differs from the other two modes. as a matter of fact, while rsrq presents a less prominent bimodality, rsrp and rssi are slightly shifted to larger values, yet with a smaller variance and almost monomodal behaviour (less so for rsrp). in figure 6 and figure 7, the time samples of rsrp, rssi, rsrq, and sinr are shown. for both systems, when tm3 is in use, we can see fewer samples, matching that tm’s standard specifications. tm3 implements open-loop sm and is adopted when few retrievals by the user are required in order for the system to work (poor channel conditions). on the other hand, we can see almost continuous reporting of values when tm4 and tm2 are adopted. again, this fully respects the standard specifications, since tm4 implements closed-loop multiplexing, and tm2 respects standard expectations to inform the network about channel conditions. in accordance to figure 4 and figure 5, we can see that rsrp and rssi differ by a few tens of dbs, showing quite a large interference, possibly due to co-channel contributions caused by an unexpected larger area covered by the involved base stations as an effect of sea water, which acts like a reflector. the numbers of samples for each tm are compared in figure 4. rs-related epdf for each tm and codeword for band 20 (800 mhz). figure 5. rs-related epdf for each tm and codeword for band 1 (2100 mhz). figure 6. rs-related quantities time samples for band 20 (800 mhz). figure 7. rs-related quantities time samples for band 1 (2100 mhz). table 4. total usage time, number of samples, and fraction of total per tm. no. of samples (percent of total samples) system frequency total time [h] total samples tm2 tm3 tm4 800 mhz 4.43 142154 42388 (29.82 %) 3810 (2.68 %) 95956 (67.50 %) 2100 mhz 3.94 36724 2856 (7.78 %) 264 (0.72 %) 33604 (91.50 %) acta imeko | www.imeko.org june 2019 | volume 8 | number 2 | 33 table 4, where (reading from left to right) the total time of use and total number of samples of each system as well as the number of samples in each mode (together with the corresponding percentage over the total number of samples) are reported. values are clearly in accordance with what is graphically perceived from figure 6 and figure 7. 4.2. cqi report analysis in figure 8, the ue cqi samples are shown for band 20. in accordance with tm implementation and requirements, both sub-band cqi and wideband cqi are reported. samples are fewer and with much lower values for tm3, since ue adopts this tm rarely, and the report is richer for codeword 0. with tm4, the samples are heavily collected, mostly for wideband cqi, while with tm2, implementing td, sub-band cqi has a predominant role. in figure 9, referring to band 1, a massive wideband cqi report takes place, while no sub-band cqi samples are reported. even if for codeword 1 we recorded fewer samples (no one at all for tm2), we can appreciate a different behaviour in tm3, where lower variance wideband cqi samples are reported, and in tm4, where higher values are collected. we reiterate that when cqi values are higher, higher order modulations are suggested for use for ue transmissions. comparing the physical layer parameters shown in figure 6 and figure 7 with the related cqi values shown in figure 8 and figure 9, with better channel conditions, higher modulation orders are suggested through cqis, and the reporting performs as depicted in 3gpp standard—as we expected. we also stress that each vendor operates differently to calculate cqi, and 3gpp does not mandate how to compute it. in this experiment, however, if the enodeb follows the ue’s indications without overriding them, based on the outcome of both this section and the previous one, cqi appears to be computed on rsrq and sinr quantities. this indication could be truthful since, as stated, rsrq calculation indicates the overall quality of the received reference signal, and it seems to be the best parameter for calculating cqi. 5. conclusions in this paper, we provided the results of a smartphone-based experimental characterisation of lte network mimo performance in an urban environment through a drive-test methodology. with reference to the investigated physical layer parameters, it is evidenced that the lte architecture performs as expected by the relevant standard, although it is necessary to investigate the relationship between those parameters and some user-related ones, such as the network throughput. for each tm, we highlight that the expected behaviour is generally encountered and that the 3gpp requirements are mostly matched, despite the complex propagation scenario. the analysis of the physical layers quantities has led us to the conclusion that the evolution over time of rsrp, rssi, rsrq, and sinr is efficiently reported, respecting standard expectations to inform the network about channel conditions. tm4 reports all cqi mandatory parameters together with tm2. additionally, tm3 reports a dataset even if the standard does not mandate reporting any sort of information to enodebs. with respect to cqi reporting for different tms, the ueselected sub-band and wideband cqi are shown. with the information provided by physical layer quantities and with favourable channel conditions, higher order modulation is suggested through cqis, and the reporting performs as envisioned in the 3gpp standard—as we expected. finally, this paper provides a certainly prodromal and possibly mandatory analysis for future thorough investigation of the quantities in which the end-user is mostly interested, like bit (or block) error rate and throughput in the downlink channel. by the joint study of such end-user quantities and physical layer ones, it will be possible to determine network criticalities and, where possible, to actuate corrective action for better network performance. references [1] s.sesia, i.toufik, m.baker, lte the umts long term evolution: from theory to practice. 2nd ed., john wiley & sons, chichester, uk, 2011, isbn 978-0-470-66025-6. 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[24] s.avallone, n.pasquino, g.ventre, s.zinno, ‘experimental characterization of long term evolution multiple input multiple output performance in urban propagation scenarios’, proc. of the 2018 workshop on metrology for industry 4.0 and iot, 2018, brescia, italy, pp. 254-259. doi: 10.1109/metroi4.2018.8428318. using a qualitative and quantitative validation methodology to evaluate a drone detection system acta imeko issn: 2221-870x december 2019, volume 8, number 4, 20 27 acta imeko | www.imeko.org december 2019 | volume 8 | number 4 | 20 using a qualitative and quantitative validation methodology to evaluate a drone detection system daniela doroftei1, geert de cubber1 1 royal military academy, avenue de la renaissance 30, 1000 brussels, belgium section: research paper keywords: drone detection; validation methodology citation: daniela doroftei, geert de cubber, using a qualitative and quantitative validation methodology to evaluate a drone detection system, acta imeko, vol. 8, no. 4, article 5, december 2019, identifier: imeko-acta-08 (2019)-04-05 editor: yvan baudoin, international cbrne institute, belgium received november 23, 2018; in final form february 12, 2019; published december 2019 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was supported by the european union’s horizon 2020 research and innovation programme under grant agreement n700643 (safeshore). corresponding author: daniela doroftei, e-mail: liliana.doroftei@rma.ac.be 1. introduction 1.1. problem statement drones are gradually becoming commodities. this is a positive evolution, as these tools have many positive uses, and the affordability of the current systems means that new business opportunities arise. however, we cannot also be blind to the negative aspects that these novel tools may induce into our society. indeed, next to the many airspace infringements, where uneducated hobbyists inadvertently enter potentially dangerous airspace (e.g. near airports, manned aviation), we also see an increasing use of drone technology by criminals [1], [2]. most countries have now established rules for access to airspace by unmanned aerial vehicles/drones/remotely piloted aircraft systems (rpass). the challenge is now to enforce these rules, as law enforcers lack the means to automatically detect airspace infringements. indeed, something like a car traffic speed camera for the air does not really exist yet, but it seems to be an essential tool if one wants to ensure safe airspace operations for everyone. 1.2. previous work on drone detection and the scope of the safeshore project numerous commercial and non-commercial parties have noted this gap in the market and have started the development of drone detection systems [1]. there are two main difficulties related to the detection of drones. first, the crosssection/detection baseline for these systems is generally very limited, whatever sensing technology is used. indeed, drones have a small radar cross-section, a small acoustic signature (from a relevant distance), a small visual/infrared signature, they use common radio signal frequencies, etc. of course, it would be possible to make the detection methodologies extremely sensitive, but this then leads to a second difficulty: how can false positives be avoided? indeed, the signature of many drones is quite close to that of birds, so it is very difficult to filter out these false positives [3]. abstract now that the use of drones is becoming more common, the need to regulate the access to airspace for these systems is becoming more pressing. a necessary tool in order to do this is a means of detecting drones. numerous parties have started the development of such drone detection systems. a big problem with these systems is that the evaluation of the performance of drone detection systems is a difficult operation that requires the careful consideration of all technical and non-technical aspects of the system under test. indeed, weather conditions and small variations in the appearance of the targets can have a huge difference on the performance of the systems. in order to provide a fair evaluation, it is therefore paramount that a validation procedure that finds a compromise between the requirements of end users (who want tests to be performed in operational conditions) and platform developers (who want statistically relevant tests) is followed. therefore, we propose in this article a qualitative and quantitative validation methodology for drone detection systems. the proposed validation methodology seeks to find this compromise between operationally relevant benchmarking (by providing qualitative benchmarking under varying environmental conditions) and statistically relevant evaluation (by providing quantitative score sheets under strictly described conditions). mailto:liliana.doroftei@rma.ac.be acta imeko | www.imeko.org december 2019 | volume 8 | number 4 | 21 sensing modalities that can be used to solve drone detection problem are typically radar [4], acoustics [5], visual [6], ir [7] (thermal and short-wave), sensing of the radio spectrum [8], lidar [9], etc. however, as the problem is so difficult to solve in realistic operating conditions, most of the existing solutions rely on a mix of different sensing methodologies in order to solve the drone detection problem [1] and use a mix of traditional detection and tracking methodologies [10], [11] originating from computer vision to achieve multi-sensor tracking. as no satisfying solution to this problem currently exists, the european commission decided to fund the eu-h2020safeshore project [12], which serves as a case study for this paper. the main objective of the safeshore project is to cover existing gaps in coastal border surveillance, increasing internal security by preventing cross-border crime, such trafficking in human beings and the smuggling of drugs. it is designed to be integrated with existing systems and create a continuous detection line along the border. the safeshore solution for detecting small targets that are flying in low attitude is shown in figure 1 and consists of a 3d lidar that scans the sky and creates a virtual dome shield above the protected area. in order to improve the detection, safeshore integrated the 3d lidar with passive acoustic sensors, passive radio detection, and video analytics. all those technologies can be considered as low-cost green technologies (compared to the traditional radar systems). it is expected that a combination of orthogonal technologies, such as lidar, passive radio, and acoustic and video analytics, will become mandatory for future border control systems in environmentally sensitive areas. the safeshore objective is to demonstrate the detection capabilities in the missing detection gaps of other existing systems, such as coastal radars, thereby demonstrating the capability to detect mini-rpas along the shore and the sea or departing from civilian boats. important safeshore goals are to ensure the intelligent combination and fusion of information sources; to increase the situational awareness and better implementation of the european maritime security strategy based on the information exchange frameworks; and to ensure privacy of the data and conformity to internationally recognized ethical issues concerning the safety of the information and the equipment subject of the project. there are two problems with the evaluation of drone detection systems: 1) drone detection systems most often rely on complex data fusion and sensor data processing, which means that it is necessary to carefully control the test conditions in order to single out the limits of the system under test. 2) drone detection systems need to be operational 24/7 and under all weather conditions i.e. it is necessary to assess their performance within a wide range of conditions. clearly, both of these constraints are somewhat contradictory, and it is not easy to find a compromise between these two types of requirements. the objective is therefore to find a validation methodology that satisfies both the requests of the end users for qualitative operational validation of the system and the platform developers for a quantitative, statistically relevant validation. 1.3. previous work and a discussion on quantitative and qualitative operational validation historically, representatives of different (scientific) communities used quantitative and qualitative evaluation methodologies. quantitative approaches have therefore been, in this sense, the favourite among members of the hard sciences community, as such methodologies permit a generalisation to be made about a large population on the basis of a relatively small set of (representative) samples. given a set of initial or boundary conditions, they can help assess the influence of certain variables on a system under test. in principle, they also allow other researchers to validate the original findings by independently replicating the analysis. of course, this assumption holds valid only if one has sufficient control over the boundary conditions, which limits the usability of such approaches for near-realistic operations. by collecting and analysing data in numerical form, quantitative researchers argue that they are upholding research standards that are simultaneously empirically rigorous, impartial, and objective [13]. an example of a quantitative evaluation methodology in the field of robotics is the set of standardised test methods for explosive ordnance disposal and search and rescue robots, issued by the us national institute of standards and technology (nist) [14]. following these standard tests, test subjects need to perform a well-defined standardised basic action (e.g. drive a figure-eight pattern) a statistically relevant number of times in a closed and well-controlled environment. in the field of social sciences, on the other hand, qualitative evaluation methods [15] have much stronger support due to the fact that many of the variables studied in these research domains cannot meaningfully be represented by singular metrics. carefully designed qualitative evaluation methods [16] are able to assess the success or performance of a system under test in a more holistic manner than is possible by means of sheer quantitative methods. this also means that they are able to produce results that are much closer to the performance assessment of the system under test by the actual human system user. a disadvantage is that the absence of a controlled working environment and hard metrics implies that repeatability is no longer ensured. in general, these qualitative assessment methodologies are now based on a ‘story’ or ‘scenario’-based approach [16], as this method has proven to be the most useful to incorporate the views of the human stakeholders in the evaluation process. an example of a quantitative evaluation methodology in the field of robotics is the eurathlon challenge [17], [18]. following this evaluation model, test subjects need to perform a high-level task (e.g. rescue a victim) in an uncontrolled outdoor environment. it is clear that both these methodologies have their advantages and disadvantages. this has led in the recent years to the development of so-called ‘mixed methods’ [19], whereby researchers collect and analyse both quantitative and qualitative figure 1. safeshore concept sketch. acta imeko | www.imeko.org december 2019 | volume 8 | number 4 | 22 data within the same study. the integration of qualitative and quantitative data provides a richer and more comprehensive overview of the performance of the system under test, taking into consideration several viewpoints: those of the end user and those of the platform developer. obviously, mixed methods also have their disadvantages, as they render the evaluation process and the subsequent analysis process more difficult, so a cost-benefit analysis is required in order to assess whether the investment is merited. in general, it can be stated that this is mostly the case when the human-system interface is a crucial component of the system design, or when the user acceptance of the system is not straightforward (which also explains why these methods are popular within healthcare) [19]. an example of a mixed qualitative-qualitative evaluation methodology in the field of robotics is the framework proposed in [20], where the performance was assessed of a series of search and rescue robots that work in close collaboration with human search and rescue workers. in this study, the human operators were non-experts and were quite reluctant to accept these new tools in the beginning but were only convinced of the use of the system following the outcome of the mixed qualitative-quantitative evaluation [21]. the remainder of the article is organised as follows. in section 2, we discuss the proposed methodology from a conceptual point of view and compare it to state-of-the-art approaches. in section 3, we validate the methodology, taking as the evaluation of the safeshore drone detection system as a case study. finally, in the concluding section, the major lessons learned are summarised. 2. proposed methodology 2.1. methodology for gathering user requirements the proposed methodology is a derivative of the work performed in [21] but ported to the domain of maritime threat agent detection [22]. a first step in the development of the validation framework was the requirements analysis, for which we followed a step-wise approach: • the different end-user communities and stakeholders were identified. • the different end-user communities were approached by means of market studies and targeted interviews. • an early draft methodology proposal was compiled. • this draft document was extensively discussed with both end users (in this specific case, maritime border management agencies) at relevant events and with platform developers in order to come to target performance levels that were both operationally realistic from an end user’s point of view as well as from a platform developer’s point of view in terms of the required effort, resources, and stateof-the-art and physical constraints. • as safeshore focuses on drone detection for the protection of maritime borders, a number of operational validation scenarios were proposed in order to address major issues the maritime border security community is facing today. • for each of the validation scenarios, the target performance levels were proposed in discussion with end users and platform developers. • these validation scenarios and target performance levels were updated throughout the lifetime of the project and were continuously adapted for the inevitably continuously changing operational requirements, technological capabilities, and scientific state of the art. 2.2. concept overview two crucial aspects of obtaining realistic results from validation scenarios are that the scenarios should be as close as possible to operational reality and that the validation tests should be repeated enough so as to ensure statistical relevance. these two considerations are often in conflict with one another, as operational testing requires uncontrolled environments, whereas statistical relevance of results can only be obtained in controlled settings. within safeshore, we have aimed to strike a balance between both aspects by providing a qualitative and quantitative assessment of the safeshore system capabilities and by having multiple repeated experiments in realistic environments, following scenarios described by end users based on their needs and today’s practical maritime border security problems. the different components of the safeshore validation concept are: • a traceability matrix that clearly indicates what, for each validation scenario, are the relevant user requirements that need to be tested, allowing for the identification of how (by which validation scenario) each system requirement will be validated. this is important in order to keep track of the different user requirements and to make sure that, for each of the requirements, there is a validation scenario in place that ensures the verification of the attainment of the requirement. an example is shown in table 1. • a number of detailed scenarios, each related to maritime border security and safety. in total, safeshore considers 14 validation scenarios: five to be executed in belgium, three in israel, and six in romania. in this paper, we focus on those executed in belgium. each of these scenarios contains: – a capability score sheet, allowing for a qualitative assessment of the validation of the target performance table 1. excerpt of the safeshore traceability matrix acta imeko | www.imeko.org december 2019 | volume 8 | number 4 | 23 levels. these capability score sheets allow us to make a binary assessment (yes/no) as to whether one of the user or system requirements has been attained by the system or not. table 2 shows a small excerpt of these capability score sheets. three issues must be noted when analysing this figure: o this figure only represents a very small fraction of the actual capabilities tested. in reality, there were over 60 capabilities that were evaluated for each scenario. o it is not possible to incorporate the actual performance of the system (pass/not pass) in this article. indeed, as the safeshore system is intended to be an operational border protection system, such information is restricted. this also explains the lack of actual data points in table 3 and table 4. o it is important to link the evaluated capabilities with the user requirements, as shown in table 2, as this creates a link to the origin of the capability requirement. – template forms to be filled in during the validation tests, providing standardised information on the threat agents and the detection results. these template forms contain valuable environmental information, such as weather conditions and sea state. they also provide crucial information on the drones used as test agents: their visual/infrared/radiofrequency/acoustic/lidar signature, including ground truth timestamped gps tracks, which allows for a full quantitative evaluation of the precision of the detection results. these evaluation forms also provide a means of evaluating the humanmachine interface, as they gather information on the sample sizes for human verification, detection resolution, video framerates, etc. table 3 shows a small fraction of a template form for one of the scenarios. these forms were generally completed prior to the actual test for recording the boundary conditions. after the test, they were filled with data concerning the test results. this can be performed easily, as the safeshore command and control system automatically logs and records all system operations. – a score sheet for the different metrics (key performance indicators [kpis]), allowing for a quantitative assessment of the validation of the target performance levels. table 4 shows an example list of a few kpis that were measured during one of the tests. – detailed target performance levels for each of the measured metrics. for each of the kpis, three different levels of scoring were assessed in collaboration with the end users: – minimum acceptance level: performance below this level is not acceptable by the end users in operational conditions. anything above is considered workable. – goal level: this is the performance level anticipated by the end users. – breakthrough level: this is a performance level beyond initial expectations that end users would one day like to have. in the three right-hand columns of the table in table 4, the target performance levels were pre-filled by the end users. for security reasons, this data had to be removed from this article. for a typical scenario, between 30 and 50 performance metrics were recorded in this way and compared to the target performance levels. 2.3. conceptual differences between the proposed method and the state of the art table 5 gives an overview of a number of qualitative and quantitative (and mixed) evaluation methodologies. however, table 2. excerpt of the safeshore capability score sheets. table 3. excerpt of the safeshore trial metrics forms. table 4. excerpt of the target performance levels. acta imeko | www.imeko.org december 2019 | volume 8 | number 4 | 24 the purpose of this table and this section is not – as is usual in a research article – to argue why one or other method is the best one in all cases. indeed, each of these evaluation methodologies has its merits and value, and each could be the best choice for a given application. the purpose of the table is to better indicate for the reader the differentiating factors between the different evaluation methodologies such that an educated choice can be made between each of these options in a given case. therefore, we will first explain the terminology of the different criteria used in table 5: • repeatability: considers how well the performance evaluation results can be replicated by other researchers. quantitative methods are obviously much better in this regard than qualitative methods, with mixed methodologies somewhere in between. • statistical significance: considers from a statistical point of view whether the produced results are significant or not. quantitative methods are obviously much better in this than qualitative methods, with mixed methodologies somewhere in between. • potential for standardisation: considers whether the approach can lead to a widely adopted test method standard. this requires repeatability and statistical significance, so it is related to the previous two criteria. • realistic: considers whether the methodology takes into account realistic scenarios. here, quantitative methods often fail to reproduce realistic operational conditions, as they need to stick to well-constrained boundary conditions. • story-based: considers whether a story-based approach is followed in which end users’ perspectives are incorporated, which is mostly the case for qualitative and mixed approaches. • comprehensiveness: considers whether the evaluation results effectively showcase the performance of the system under test in a wide parameter domain, which is the strong point of the mixed-method approach. • cost: considers the cost of doing the test. here, quantitative methods excel, as they can generally keep the costs down, whereas qualitative and mixed approaches can be very expensive. • user involvement: considers the amount by which end users of the product under test are involved in the evaluation process. this is typically very high in mixedmethod approaches (and in many qualitative approaches, but less in [17]). it is related to the level of realism of the test as well as the cost. • methodological flexibility: considers the adaptability to different study designs. mixed-method approaches are typically capable of elucidating more information than can be obtained in only quantitative research and therefore excel in this domain. • interdisciplinary constraints: considers the requirement that a multi-disciplinary team is required to perform the evaluation. this is typically the case for the mixed-method approach. it has an impact on the comprehensiveness of the data as well as the cost. • various viewpoints: considers the capability to give a voice to study participants and to ensure that the study findings are grounded in user experiences. • complexity: considers the complexity of setting up the evaluation. this is obviously related to the cost as well, which is typically very high for the qualitative and mixedmethod approaches, as a realistic setting needs to be reproduced, and many stakeholders need to be involved. comparing the proposed methodology to the other mixed methods [23], [24] and the earlier evaluation method proposed by the authors [20] (from which the approach proposed in this study is derived), it becomes obvious how this proposed evaluation methodology has been geared towards the application at hand. indeed, it is important to note that all other mixed methods feature a higher level of user involvement than the proposed method. this is exactly due to the application domain: rao considers social investment as an application domain in [23], so stakeholder interaction is crucial. furthermore, in the healthcare domain [24], the primary actor is the patient, so the evaluation focused on this aspect. in [20], there was a great reluctance between the search and rescue workers to use the unmanned tools, so too here was user involvement (to foster user acceptance) crucial. for the safeshore case, the user involvement was lower on the priority list, as the tool to be developed was essentially an autonomous sensor system that raises alarms for border security agents from time to time. furthermore, there is table 5. overview and comparison of qualitative and quantitative evaluation methodologies. method winfield et al. [17] jacoff et al. [14] rao et al. [23] goldman et al. [24] doroftei et al. [20] doroftei et al. proposed method type of method qualitative quantitative mixed mixed mixed mixed repeatability low high medium medium medium medium statistical significance low high medium medium medium medium potential for standardisation low high low low low low realistic high low high high high high story-based yes no yes yes yes yes comprehensiveness medium medium high high high high cost very high low very high very high very high high user involvement low low very high very high high medium methodological flexibility low low high high high high interdisciplinary constraints low low high high high high various viewpoints no no yes yes yes yes complexity high low very high high very high high application field search and rescue robotics robotics social investment healthcare search and rescue robotics threat detection acta imeko | www.imeko.org december 2019 | volume 8 | number 4 | 25 no specific reluctance to use such technology, among these border security agents (even quite to the contrary). therefore, the user involvement aspect within the proposed evaluation methodology was lowered with respect to [20]. the advantage of this approach was that costs can be saved, as the complexity of the evaluation is somewhat reduced. 3. validation of the methodology 3.1. trial concept and execution as discussed above, five different trial scenarios related to maritime border security and safety were validated during the safeshore trial in belgium, which was the first in a series of three trial events of the project from which this validation methodology was applied. for this operational field test, 11 different drone platforms (rotary wing, fixed wing, systems made of different materials, very fast drones, slow drones, etc) were deployed during a twoweek measurement campaign in order to grasp different kinds of system capabilities as well as meteorological and operational conditions. for these series of tests, the belgian local police west coast acted as an end user, using the safeshore detector as an integrated part of their surveillance capability. this allowed us to validate the system performance in a near-operational context. figure 2 shows the safeshore prototype as it was installed on the beach in belgium for a period of two weeks. 3.2. trial results as this was the first of a series of three successive test campaigns, it was to be expected that the system would have some quirks and teething problems. the performance validation methodology was therefore essential in order to identify these issues and to indicate the causes for these problems. thanks to the proposed validation, at the end of each validation day, it was possible to provide an overview of the performance of the system, both from a qualitative as well as from a quantitative point of view. this analysis is shown in figure 3. as stated above, the safeshore validation trial in belgium lasted for two weeks, consisting of ten working days. figure 3 indicates for each working day the percentage of qualitative capabilities and quantitative metrics that were evaluated using the proposed methodology. based on this daily real-time update, daily debriefings between safeshore developers and safeshore end users could be held in order to discuss the possibilities of and deficiencies with the system. as such, an action plan could be set up on a daily basis in order to improve the performance of the system. due to this iterative review of the system, the performance of the safeshore system improved on a daily basis. at the end of the trial, the proposed validation methodology enabled us to provide a full overview of the performance of the system for all five scenarios, both from a qualitative as well as from a quantitative point of view. however, as this was the very first trial, it was not possible to sort out all problems with the system by the end of the trial period (which is why the 100 % mark was not reached, as shown in figure 3). based on the results of the validation method, a new action plan was therefore elaborated between end users and developers in order to improve figure 2. safeshore system as installed on the beach in belgium, by daniel orban acta imeko | www.imeko.org december 2019 | volume 8 | number 4 | 26 the performance of the system during the next trials in israel and romania. 4. conclusions in this paper, a validation methodology for evaluating complex systems was proposed, aiming to strike a balance between the rigorous, scientifically correct, and statistically relevant evaluation methodologies requested by platform developers in the iterative design stage on the one hand and the requirements of the end users on the other hand. the end users require field tests in operational conditions in order to evaluate the real-life performance of the system. the proposed methodology achieves this objective by incorporating and integrating qualitative and quantitative aspects in the validation process. the comparison of the proposed approach to state-of-the-art approaches showed that the proposed methodology has its value in reducing the cost related to the execution of qualitative and quantitative evaluation processes, while maintaining a realistic outlook and while staying close to operational conditions. the proposed methodology was tested on a drone detection system in the context of the eu-h2020 safeshore project and allowed the project participants (a heterogeneous mix of end users and platform developers) to improve the performance of the system on a daily basis during the operational field tests of the system, thereby proving the value of the proposed methodology. during the safeshore trial, the validation methodology proved to work as expected: the number of validated capabilities (both quantitative and qualitative) rose on a daily basis, as indicated clearly by figure 3. it is also clear from figure 3 that the proposed methodology achieves a balance between the qualitative and quantitative aspects of the validation. concerning future work, we will focus on improving the proposed methodology to better incorporate the different priorities among requirements. indeed, the current version does not take into consideration that some capabilities/metrics are more important than others (stemming from optional, desired, or mandatory requirements). this can sometimes lead to a twisted view on reality and will be improved in a future iteration. acknowledgement this work was supported by the european union horizon 2020 research and innovation programme under grant agreement n700643 (safeshore). references [1] u. franke, drone proliferation: a cause for concern?, isn articles, november 2014. 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[24] r. e. goldman, d. r. parker, j. brown, j. walker, c. b. eaton, j. m. borkan, recommendations for a mixed methods approach to evaluating the patient-centered medical home. ann. fam. med., 13(2) (2015) pp. 168-75. doi: 10.1370/afm.1765. http://www.sciencedirect.com/science/article/pii/s2352146517311043 http://www.sciencedirect.com/science/article/pii/s2352146517311043 indicators of reproducibility and suitability for assessing the quality of production services acta imeko issn: 2221-870x december 2021, volume 10, number 4, 54 61 acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 54 indicators of reproducibility and suitability for assessing the quality of production services oleksandr vasilevskyi1, maryna koval1, svetlana kravets2 1 vinnytsia national technical university, 95 khmelnitsky shose str., 21021 vinnytsia, ukraine 2 vinnytsia national agrarian university, 3 solnyschna st. , 21008 vinnytsia, ukraine section: research paper keywords: quality assessment; product quality; quality of manufacturing services; suitability index; reproducibility index; probability of defective product, production process citation: oleksandr vasilevskyi, maryna koval, svetlana kravets, indicators of reproducibility and suitability for assessing the quality of production services, acta imeko, vol. 10, no. 4, article 11, december 2021, identifier: imeko-acta-10 (2021)-04-11 section editor: francesco lamonaca, university of calabria, italy received april 10, 2020; in final form december 5, 2021; published december 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: oleksandr vasilevskyi, e-mail: o.vasilevskyi@gmail.com 1. introduction to ensure the competitiveness of enterprises products, manufacturers should apply a strategy of continuous improvement. for the implementation of a such strategy, product manufacturers or service providers need to constantly evaluate their products [1]. it is advisable to apply the methods recommended by the international standards of the iso / tr 18532, iso 13528 and iso / tr 22514 [2]-[5]. for the successful implementation of actions to continually improve the quality of products or services, it is necessary to monitor the sources of the production process deviations and their stability. in the conditions of competition for producers, not only the price of products or services should be important, but also the costs, that consumers will spend when using products (or services). therefore, the purpose of any manufacturer should be to continuously reduce the deviations of the production process parameters (ensuring the stability of the production process), and not only compliance with established requirements. the strategy of continuous improvement will reduce the costs associated with failures, and will increase the sustainability of enterprise development in a competitive environment. in addition, reducing deviations will reduce control costs or reduce the frequency of selective control. quantitative evaluation of deviations allows us to make conclusions about the suitability and conformity of the production process to the established requirements. for identification of deviations, the different methods, such as drawing up a flowchart and identifying inputs and outputs of a production process, using a causal diagram, etc. can be used. a number of international standards [1]-[7] recommend a variety of statistical methods that can be used to manage, control abstract a technique is proposed for estimating the probability of the possible appearance of defective products or the inconsistency of the production service on the basis of indexes of suitability and reproducibility of the production process. the index of reproduction is recommended to be calculated on the basis of the standard deviation, which can be established by the average span of the control map and the limits of the tolerance field. at the same time, the production process must necessarily be in a state of statistical controllability. the suitability index cannot be calculated on the basis of the control card characteristics, but it is evaluated solely on the basis of the total of the standard deviation, which is calculated by the bessel formula and the tolerance limits. the production process does not need to be statistically controllable. conclusions on the suitability or insufficiency of the products (production services) are based on the value of the indexes of reproduction and suitability, which can take values less than or greater than one. on the basis of these indexes of obtained values, the probability of production appearance or characteristics of the production process, that does not match the established requirements, is established. mailto:o.vasilevskyi@gmail.com acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 55 and improve the production process in order to analyze data and evaluate product quality indicators. therefore, the purpose of this article is to develop mathematical models for point estimation of reproducibility indices and the suitability of the production process to confirm its statistical stability, as well as to determine the fraction of the probability of defective products. the description of the mathematical models that can be used to evaluate the quality indexes on the basis of reproduction and suitability indexes is an actual scientific task, since many manufacturers of products do not understand their differences and consequently incorrectly interpret the obtained results. 2. materials and methods the first step requires to develop methods for evaluating the quality indexes of products (services) on the basis of the indexes of reproducibility and suitability of the production process, as well as to formulate the criteria for normalization of reproduction and suitability indexes for decision-making on conformity (sufficient, satisfactory, good) or noncompliance of product quality indicators or production services to the established requirements. the article reviews the methodology for calculating the reproducibility level of product production, depending on the input data set. depending on the specifics of production and the adopted quality management strategy, we may have a specific set of data to calculate the value of reproducibility. therefore, one unified methodology cannot be adopted because it may give erroneous results. the level of production reproducibility is a very important element in the process efficiency assessment process according to the deming cycle. based on the statistical calculations made and review of the methodologies included in the literature, the assessment approach was optimized depending on the possibility of obtaining data recorded during production. when analyzing the reproducibility of the production process, it is necessary (iso 5725:2002) [7]: a) establish all requirements of the production environment (e.g. temperature and humidity requirements); b) establish requirements for uncertainty of measurements; c) ensure an opportunity to analyze multifactor, multilevel aspects of the production process; d) establish and register the length of data receiving; e) determine the frequency of sample creation, as well as the start and end date of the data receiving; f) use a control card to control the production process [8]; g) to provide the state of statistical control of the production process. indicator of the reproducibility of a production process is a measure of its own change of the output characteristics of the production process, which is in a state of statistical control, and which enables to assess the ability of the process to maintain the output characteristics of the production process at the level of requirements set for it. this measure characterizes the variability that remains after eliminating all known causes. if the control of the production process is carried out using a control card, then the control card shows that the production process is in a controlled state [4], [8]-[10]. reproducibility of the production process is often estimated by the number of products, the characteristics of which are within the tolerance field. since the production process in a statistically controlled state can be described by the predicted distribution law, then the quantity of products which characteristics is beyond the tolerance field can be estimated. while the production process remains in a state of statistical control, the manufactured products have, on average, the same proportion of the defective products (products that do not match the established requirements). the actions of the production process management, which are aimed to reducing the deviations caused by accidental causes, will make it possible to improve the conformity of the production process with the requirements of the quality management system [10]. to do this, it is necessary: to determine the characteristics of the production process and the conditions of operation (if the conditions are changed, then new studies of the characteristics of the production process are necessary); to evaluate the parameters of short-term and long-term deviations in the form of percentages from full changes and to minimize them; maintain the stability of the production process and ensure its statistical control; to evaluate the own variability of the reproduction process; select the required parameter of the reproducibility of the production process. also, it is necessary to check the control card, the data of which had been used for statistical control, and the histogram data with all the established limits applied to it. in addition, it is necessary to check the normality of the distribution law by a valid criteria, for example, such as the anderson-darling criteria [10] or the κ2-criteria [4]. these criteria are effective in detecting the deviation of the law of distribution from normality on the distribution tails, since this area is important in the evaluation of the indexes of reproduction and suitability of the production process. also, an abnormal data explanation must be found and appropriate actions taken with the data to calculate the investigated parameter. exclusion of data allocated to others is unacceptable. such deviations can be very informative about the properties of the production process and should be investigated. reproducibility parameter of the production process may be a value that characterizes one or more properties of the distribution of the input characteristic in the conditions of the production process reproducibility. the general parameter of the distribution position is the mean (mathematical expectation) μ, but sometimes selective median х50% is used. for a normal distribution law, the best position parameter is the median. the best parameter characterizing the own variability of the production process is the standard deviation σ index of reproducibility of the production process. it is recommended to evaluate it according to the average magnitude r obtained by the control card, when the production process is stable and is in a state of statistical control �̂� = �̄� 𝑑2 , (1) where 𝑑2 is a constant corresponding to the sample size in the subgroup, its value is chosen from table 1 [9]. table 1. coefficients of the control card to estimate the standard deviation. sample size (n) d2 c4 2 1.128 0.7979 3 1.693 0.8862 4 2.059 0.9213 5 2.326 0.9400 6 2.534 0.9515 7 2.704 0.9594 8 2.847 0.9650 9 2.970 0.9693 10 3.078 0.9727 acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 56 if we use the average standard deviation for controlling deviations within a subgroup, which is determined by the data of the control card, then the own standard deviation of the production process can be estimated by the formula �̂� = �̄� 𝑐4 , (2) where �̄� is the average sampled standard deviation and 𝑐4 the constant corresponding to the sample size in the subgroup (n); its value is chosen from table 1 [9]. if for each subgroup it is possible to calculate the standard deviation of a subgroup, then a formula for assessing the standard deviation of the production process is recommended, which gives a more accurate estimate than formulas (1) and (2), which is described by formula �̂� = √ 1 𝑚 ∑ 𝑆𝑗 𝑚 𝑗=1 , (3) where 𝑆𝑗 is the sampled standard deviation of j-th subgroup; m is the number of subgroups with n observations in each subgroup. it is also necessary to distinguish between the standard deviation, that characterizes only short-term changes in the production process, and the standard deviation that characterizes the long-term changes in the production process. and the data received over a long period of time have bigger value of standard deviation due to a more significant change in the production process. in this case, it is recommended to use the symbol σt to denote the standard deviation the total (full) standard deviation. if the data are obtained from the observation of a production process that is not in a state of statistical control or if control cards have not been used, then formula (1) (3) should not be used to calculate the standard deviation, but it is necessary to apply the following formula: �̂�𝑡 = √ 1 𝑁 − 1 ∑(𝑥𝑖 − �̄�) 2 𝑁 𝑖=1 , (4) where 𝑁 is the total sample size, 𝑥𝑖 the i-th value in the sample, �̄� the average arithmetic mean. equation (4) should be used when the production process has a change in the average value due to the presence of a systematic error [11]-[13], which can not be excluded, and this variability must be taken into account with along the random variability. this variation parameter is also suitable for use in the calculation of the suitability indexes of the production process. with the normal distribution of the production process as an estimate of the reproducibility of the production process, an expression can be used �̄̄� ± 𝑧𝛼 ⋅ �̂�𝑡 , (5) where �̄̄� = 1 𝑚 ∑ �̄�𝑗 𝑚 𝑗=1 is the arithmetic average of several sample meanings; �̄�𝑗 the selective average of j-th subgroup; 𝑧𝛼 a quantile of a normalized normal distribution law. the choice of the value of 𝑧𝛼 depends on the used value of the reproducibility index of the production process in units of production of one million. typically, 𝑧𝛼 is assigned a value of 3, 4 or 5. if the reproducibility index of the production process matches the requirements, 𝑧𝛼 = 3 means an average of 2700 units of products per million beyond the requirements. similarly, 𝑧𝛼 = 4 means an average of 64 units of products per million that do not match the established requirements, and 𝑧𝛼 = 5 means an average of 0.6 such items per million. reproducibility indexes of the production process are the point of estimation of their reference values. using the reproducibility index of the production process allows us to characterize the state of the production process. the index of reproducibility of the production process is the ratio between the differences of the tolerance field to the length of reference interval с𝑝 = (𝑈 − 𝐿) (𝑋99,865% − 𝑋0,135%)⁄ , (6) where 𝐿 is thlower limit of the tolerance field; 𝑈 the upper limit of the tolerance field; 𝑋0,135% the lower limit of the reference interval defined as the quantile of distribution at the level of 0,135%; 𝑋99,865% the upper limit of the reference interval, which is defined as quantile of distribution at 99,865%. to estimate the index of reproducibility of the production process the reference interval 𝑇 = 𝑌2 − 𝑌1 is used [4], [5], which includes 99.73% of the production process characteristics values, which are in the state of statistical control. at the same time 0.135% of each side of the distribution law is cut off [13], [14]. this interval is recommended to apply even in the case of a nonnormal distribution law of the production process characteristics values [13]. for a normal distribution law, the length of the reference interval is six standard deviations (figure 1). control maps are usually used to evaluate reproducibility. if the control card shows weakened control lines or modified control lines, the actual standard deviation of the process will be greater than the standard deviation obtained from the control card with standard control lines. these features affect the reference interval, so it is important that the standard control lines were fixed in the evaluation of the reproducibility index of the production process. a reproducible process is a production process with a reference interval t smaller than the tolerance field (l, u) for a certain value, as shown in figure 1. it is also recommended by international standards [1], [3], [4] to use other indexes that characterize both the state and the figure 1. the length of the reference interval t and the lower l and the upper u limits of the tolerance field. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 57 variability of the production process, for example, the reproducibility index сpk. if this index is smaller than a given value, then it can be assumed that in the manufacturing process there is a high probability of occurrence of defective products, that is, the characteristic of the production process goes beyond the tolerance field (l, u). the reproducibility indexes сpk can be defined as the ratio of difference between the tolerance field and the production process value to the difference between the corresponding limits of the production process value and the parameter of the production process: с𝑃𝑘𝑈 = (𝑈 − 𝑋50%) (𝑋99,865% − 𝑋50%)⁄ , (7) с𝑃𝑘𝐿 = (𝑋50% − 𝐿) (𝑋50% − 𝑋0,135%)⁄ , (8) where 𝑋50% is the quantile of the distribution law of the production process at the level of 50%. these reproducibility indexes (сpkl) provide information about how tightly the characteristics are rouped around the centerline and whether product specification requirements may be violated. even if the value of the index ср, сpku is high, then the low values of the сpk indexes indicate that the production process is poorly concentrated around the central line, and the probability of the appearance of quality characteristics values beyond the established limits of the set requirements is high. if the observed values are distributed according to the normal distribution law, the length of the reference interval is equal to 6σ, and the reproducibility index can be estimated by expression с̂p = (𝑈 − 𝐿) (6�̂�)⁄ . (9) if the distribution of individual values is subject to the normal distribution law, then quantile 𝑋50% is equal to the mathematical expectation 𝜇, and the upper and lower indexes of the reproducibility сpk can be estimated from the expressions: с̂pku = (𝑈 − 𝜇) (3�̂�)⁄ , (10) с̂pkl = (𝜇 − 𝐿) (3�̂�)⁄ . (11) based on the evaluation of the lower с̂pkl and upper с̂pku reproducibility indexes, for the final evaluation of the reproducibility index сpk, it is necessary to assume a lower reproducibility index с̂pk = min(�̂�pkl, �̂�pku) . (12) in calculating the reproducibility index of the production process, it must be taken into account that the variability of the production process should correspond to the situation when the data has been obtained in a state of statistical control of the production process. 3. empirical results if the index of the reproducibility ср < 1 (or сpk < 1), then the upper u and the lower l limits of the tolerance field are inside the reference interval t of the production process this means that the production is not possible without a defect, and the production process is unsatisfactory (the probability of defective production is very high and may be higher than 0.27%). if the value ср = 1 (or сpk = 1), then the upper u and the lower l limits of the tolerance field coincide with the reference interval t of the production process. in this case, if the process is centered and the distribution of quality indicators obeys the normal law, then the possible lack of products is 0.27% (2700 defective products per 1 million manufactured goods). in this case, the production process is considered to be the minimum acceptable (satisfactory, it is recognized as reproducible). if the value of the reproducibility index ср > 1 (or сpk > 1), then the upper u and the lower l limits of the tolerance field are outside the reference interval t of the manufacturing process this means that production is possible without defects, and the production process is considered to be satisfactory. if the value сpk (or cp) lies within 1 < сpk < 1.33, then the probability of occurrence of defective products will be in the range from 0.006% to 0.27%. if the value of the index of reproducibility is greater than 1.33 (сpk > 1,33), then the probability of occurrence of defective products is less than 0.006%, and the production process is considered good. the appropriateness of the production process regarding the quality of products is the achieved distribution of results. the only important difference between the suitability and reproducibility of the production process is that for assessing the suitability of the production process there is no requirement for the presence in the production process of the state of statistical control and control cards. in the analysis of the suitability of the production process: a) all technical conditions, including requirements for the production environment, such as temperature and humidity requirements, must be established [8], [15]; b) requirements for uncertainty of measurements must be established [15]-[24]; c) an opportunity should be provided for the analysis of multi-factor and multilevel aspects of the production process; d) data must be obtained and registered within a specified time period; e) the frequency of sampling and the start and end of time of the data obtaining must match the requirements set by the quality management system [1]; f) the process may not be monitored by a control card; g) the process may be statistically uncontrolled, in particular, previously obtained data, which sequence is unknown, can be used to assess the suitability of the production process. the index of the suitability of the production process is a statistical indicator, which is determined by the output characteristic of the production process, which used to evaluate the production process, the location of which in the state of statistical control is not confirmed. the parameter of the suitability of the production process may be the quantities describing one or more properties of the quality characteristic distribution in terms of suitability. to estimate the suitability parameter, in contrast to the reproducibility parameter, under the normal distribution of the quality characteristic, we can only by expression (4). the index of the suitability of a production process is an index that reflects the stability of the production process to the specified field of tolerance. if the values of the parameters under study are distributed according to the normal distribution law, then the length of the reference interval is equal to 6 �̂�t [25], [26]. therefore, the value of the index of fitness pp can be calculated by expression 𝑃p = (𝑈 − 𝐿) (6 �̂�t)⁄ . (13) the upper ppku and lower ppkl indexes of the suitability of the production process can be estimated by the expressions: acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 58 𝑃pku = (𝑈 − �̄�) (3 �̂�𝑡 )⁄ , (14) 𝑃pkl = (�̄� − 𝐿) (3 �̂�𝑡 )⁄ . (15) the indexes of the suitability of the production process ppk is assumed to be equal to the lower of the two values of ppku and ppkl, i.e. ppk= min(ppku, ppkl). the lower the value of the index of suitability, the greater the probability of occurrence of defective products, and the production process will not match the established requirements. as follows from expressions (13) (15), the assessment of the indexes of suitability is similar to the evaluation of reproducibility indexes (6) (12). the difference in the evaluation of suitability indexes from the reproducibility indices is that the production process does not necessarily have to be statistically controlled, and the standard deviation that characterizes the best indicator of the suitability of the production process can not be calculated based on the parameters of the control card. the ppk suitability index characterizes the confirmed quality. if the production process is centered, that ppk, but when the process is shifted, the suitability index is shifted from its nominal value, and ppk = pp becomes less than pp index will only be the case when the goal is achieved with a minimum deviation from the average arithmetic value. in case of noncentration of the production process, the pp index can be adjusted by introducing a noncentral correction 𝑃𝑃𝑘 = (1 − 𝑘)𝑃𝑃 , (16) where k is a corrective coefficient that corresponds to the value of noncentration and is defined as the difference between the given reference value of the product characteristic and the average value of the production process parameter. if the process is centered, then k = 0 and ppk. if the process is shifted relative to a given reference value, then k increases and the index ppk = pp becomes smaller than the pp index. if the suitability indexes are equal to (ppk = pp), the production process is within the tolerance. if the index pp < 1, then this means that the production process has low accuracy and the production process is unsatisfactory. the using of statistical methods [3]-[7] during the regulation will not give the necessary effect. in this case, it is necessary to improve the accuracy of the production process by replacing (or/and repairing) technical equipment and ensuring the quality of measurements (unity of measurements and accuracy of measurements) [25]. if the ppk suitability index is in the range from 1 to 1,33 (1 ≤ ppk < 1,33), the production process has sufficient accuracy this means that the procedure for setting it up is correct. at the same time it is advisable to apply the acceptance control cards and to combine the procedure of manufacturing process regulation and the acceptance of products in one common procedure of the spc (statistical process control) [25]-[28]. if the index pp > 1,33, then the production process is considered to be good (with high potential accuracy). if pp > 1 and ppk < 1, then the production process is considered to have sufficient potential precision, but there are factors that shift the manufacturing process and remain unnoticed. in this case, it is recommended to use shuhart's control cards to identify factors that may result in displacement of the manufacturing process center. if the index pp > 1,66, then the production process is ideally configured [28]. the main properties of the normal distribution law, on which the calculation of the defect fraction is based, are shown in figure 2. as it follows from figure 2, in order to avoid a noticeable fraction of defective products or characteristics of the production process that deviates from the established requirements, the width of the tolerance field must be not less than 6 σ. estimation of the values of pt production process characteristic or the quality parameter of products that do not match the requirements under the normal distribution law can be found on the basis of the upper and lower parts of units that do not match the requirements: �̂�t = �̂�l + �̂�u = φ ( 𝐿 − �̄� �̂�𝑡 ) + φ ( �̄� − 𝑈 �̂�𝑡 ) , (17) �̂�l = 1 − φ (3 с̂pkl) , (18) �̂�u = 1 − φ (3 с̂pku ) , (19) where �̂�𝐿 is the assessment of the lower part of the units that do not match the requirements the part of units of the process or product characteristics distribution, which does not exceed the lower limit of the field of tolerance 𝐿; �̂�𝑈 is the assessment of the upper part of the units that do not match the requirements the part of units of the process or product characteristics distribution, that exceeds the upper limit of the tolerance field 𝑈; φ (*) the function of a normalized normal distribution law. in order to estimate the proportion of product units that do not match the requirements of the suitability of the production process, it is necessary to replace the reproducibility indexes and indexes of suitability in formulas (18) and (19), and thus common part of the values of the characteristics distribution of the production services (production process) that may go beyond the tolerance field. 4. measurement systems analysis measurement systems analysis (msa) [5], [29] manual presents guidelines for assessing the quality of a measurement system primarily used in the industrial world. three fundamental issues must be addressed when evaluating a measurement system: the measurement system must demonstrate adequate resolution to detect changes in product or process variation. figure 2. properties of the normal distribution law, on which the calculation of the defect fraction is based. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 59 typically, its applied that instrument discrimination should divide the tolerance (or process variation) into ten parts or more; the measurement system must be stable. under repeatability conditions, the measurement system variation is due to common causes only and not special causes; the statistical properties (errors) are consistent over the expected range and adequate for the purpose of measurement (product control or process control). the location characteristics are defined by bias (systematic error component of the measurement system), linearity (the change in bias over measuring range) and stability (the change in bias over time). repeatability and reproducibility are considered for variation characteristics (figure 1). gage repeatability and reproducibility standard-deviation (grr) is the combined estimate of measurement system repeatability and reproducibility. grr percentile calculation (%grr) when compared to the product tolerance (t), is calculated according to equation: %𝐺𝑅𝑅 = 6 ⋅ �̂�𝑡 𝑇 100 % , (20) where 𝑇 = 𝑌2 − 𝑌1 is the product tolerance. table 2 presents a summary of the acceptance criteria set out in the 4th edition of msa manual [25], [30]. the analysis of these parameters is relevant only if the measurement system is stable. for bias and linearity characteristics, they are required to be significantly equal to zero for a 95% confidence level, and the errors obtained in experiments should be less than the maximum permissible errors established for the instruments. this requirement considers that systematic errors should be practically non-existent, presenting only random errors. however, it is common for equipment to remain repetitive, but with typical systematic errors. but, when compared with the product tolerance, it represents a small portion. for the variation characteristic of a measurement system, the acceptance criteria is 10 % of the tolerance or variation of the production process. the change in this last revision of msa manual is that, depending on the application or the costs involved, values up to 30 % for %grr could be acceptable, but it must be approved by the client. the conformity assessment consists of verifying compliance with product tolerance, established between lower specification limit (lsl) and higher specification limit (usl). usually, decisions about compliance with the requirement depend on the measures obtained in the product inspection. since the decisions are made according measurement results, and these measurements are characterized by uncertainty, these decisions may be incorrect [30], [31]. such incorrect decisions are of two types: an item accepted as conforming may actually be nonconforming, and an item rejected as non-conforming may actually be conforming. according to iso 14253-1 [32], if no previous agreement has been made between the supplier and the customer, the principle behind the rules for proving conformity and nonconformity with specifications is the following: the measurement uncertainty always counts against the party who is providing the proof of conformity or nonconformity and therefore making the measurement. the supplier shall prove conformity with a specification. it is proved when the complete measurement result (including measurement uncertainty) falls within the tolerance zone of a workpiece characteristic (figure 3). and the customer shall prove nonconformity with a specification to could reprove a product. nonconformity with a specification is proved when the complete measurement result falls outside the tolerance zone. regions reduced or extended to the tolerance field are called guard bands u (figure 3). jcgm 106 [33] points out that the use of these guard bands (u) provides a way to limit the probability of making an incorrect conformance decision based on measurement information. jcgm 106 also lists the rule known as shared risk [31]. under this rule, the customer and supplier agree to accept as conforming (and reject otherwise) an item whose property has a measured value in the tolerance interval. in this case, producer and user share the consequences of incorrect decisions. and a limit for the measurement uncertainty must be established. in the msa manual, the parameter related to the measurement uncertainty is �̂�t, where 6 ⋅ �̂�t represents the variation of the measurement system to a confidence level of 99,73 %. in its most rigorous condition, %grr cannot be greater than 10% of tolerance and bias should be significantly equal to zero. if there is an agreement with the customer, %grr could reach 30% of tolerance. the standard-deviation of the real productive process �̂�rpp is obtained by equation (11), where �̂�t is the measured standarddeviation of the manufacturing process, �̂�mp is the standarddeviation of the measurement process �̂�rpp 2 = �̂�t 2 − �̂�mp 2 . (21) measurement systems must be adequate for the required measurement task, and statistical studies should be performed to assess the impact of measurement uncertainty on the manufacturing process. however, care must be taken not to disable appropriate measuring systems due to the use of overestimated acceptance criteria. 5. conclusions the quality of products or services is largely determined by the effectiveness of the quality management system at the enterprise and the proper organization of the production process. the quality management system, built in accordance with the principles of overall quality management, involves continuous improvement of the marketing activities of the table 2. acceptance criteria established in msa manual. characteristic acceptability criteria bias and linearity in general, significantly equal to zero and not exceed the maximum permissible error established by the gage calibration. %grr <10% tolerance (or process variation): generally considered to be an acceptable measurement system. 10–30% tolerance (or process variation): may be acceptable for some applications, but should be approved by the customer. figure 3. conformity with specification. acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 60 enterprise, improving the quality of products and ensuring of the needs of all interested parties, both customers and producers, through the establishment of appropriate management in the enterprise. the application of indexes of reproducibility and suitability of the production process in the system of quality control allows us to visually estimate the possibility of reducing the percentage of defective products by reducing and eliminating the effects of non-random causes of the production process parameters deviation (ensuring the stability of the production process), as well as reducing the impact of the random reasons leading to deviations of the production process parameters. this will allow timely warning and corrective actions that will enable them to find reserves for improving product quality, reduce financial costs for defective repair, and increase the competitiveness of the enterprise. the reproducibility index and suitability index are not related to the mean of the process. that is, when the entire distribution is shifted, the reproducibility index and the fitness index will not change, it will only respond to the variation of the spread or sweep. the reproducibility index and the suitability index will be equal to 1 when the variation of 6σ is equal to the tolerance. if the indices are greater than 1, then the range is less than the tolerance, if the indices are less than 1, then the variation is greater than the tolerance. it should be taken into account that since the indices are not related to the position of the mean, then when the mean is far beyond the tolerance, the value of the indices may be much greater than 1. the accuracy of the proposed estimation methodology of the reproducibility and suitability index, which includes the proposed mathematical apparatus, was estimated on the basis of the correctness index, the method of assessment of which is carried out in accordance with the international standard iso 57252:2002. in conditions requiring a high degree of accuracy (or not), acceptance criteria would be agreed directly between customers and suppliers. that is, it would be up to the client to specify items that require more stringent acceptance criteria. only in extraordinary situations, it would be up to the supplier to demonstrate that a measurement system with the highest measurement error still performs measurement task correctly, whether due to the high capability of the manufacturing process or due to the use of guard bands. references [1] iso, quality management systems fundamentals and vocabulary, iso 9000:2015 (2015). [2] iso/tr, guidance on the application of statistical methodsto quality and to industrial standardization, iso/tr 18532:2009 (2009). [3] iso, statistical methods in process management capability and performance part 2: process capability and performance of timedependent process models, iso 22514-2:2017 (2017). [4] iso/tr, statistical methods in process management capability and performance part 4: process capability estimates and performance measures, iso/tr 22514-4:2016 (2016). [5] iso, statistical methods for use in proficiency testing by interlaboratory comparisons, iso 13528:2015 (2015). [6] iso, statistical control. selective control on alternative grounds part 5: continuous sampling plan systems, indexed by quality assurance limits (mpy) for consecutive sampling of parties, iso 2859-5: 2009 (2009). [7] iso, accuracy (trueness and precision) of measurement methods and results part 2: basic method for the determination of repeatability and reproducibility of a standard measurement method, iso 5725-2:2002 (2002). 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[21] o. m. vasilevskyi, metrological characteristics of the torque measurement of electric motors, int. j. metrol. qual. eng. 8 (7) (2017), art. no. 7 doi: 10.1051/ijmqe/2017005 [22] alberto lavatelli, emanuele zappa, uncertainty in vision based modal analysis: probabilistic studies and experimental validation, acta imeko 5(4) (2016) pp.37-48. doi: 10.21014/acta_imeko.v5i4.426 https://doi.org/10.1049/iet-rsn.2018.5144 http://dx.doi.org/10.30525/978-9934-26-085-8-4 http://dx.doi.org/10.1016/j.ijar.2014.10.003 http://dx.doi.org/10.21014/acta_imeko.v10i3.1061 http://dx.doi.org/10.21014/acta_imeko.v10i2.810 https://doi.org/10.1007/978-3-642-04898-2_118 http://dx.doi.org/10.1016/j.ijar.2017.06.013 http://www.spcpress.com/pdf/djw189.pdf https://doi.org/10.1051/ijmqe/2017016 https://doi.org/10.1051/ijmqe/2014017 http://dx.doi.org/10.21014/acta_imeko.v4i3.259 https://doi.org/10.1117/12.2501576 https://doi.org/10.1051/ijmqe/2017005 http://dx.doi.org/10.21014/acta_imeko.v5i4.426 acta imeko | www.imeko.org december 2021 | volume 10 | number 4 | 61 [23] o. m. vasilevskyi, methods of determining the recalibration interval measurement tools based on the concept of uncertainty, technical electrodynamics 6 (2014) pp. 81-88. doi: 10.1051/ijmqe/2014017 [24] o. m. vasilevskyi, p. i. kulakov, i. a. dudatiev, v. m. didych, a. kotyra, b. suleimenov, a. assembay, a. kozbekova, vibration diagnostic system for evaluation of state interconnected electrical motors mechanical parameters, proc. spie 10445, photonics applications in astronomy, communications, industry, and high energy physics experiments, 104456c, 2017. doi: 10.1117/12.2280993 [25] s. beckert, w. paim, critical analysis of the acceptance criteria used in measurement systems evaluation, int. j. of metrol. qual. eng. 8 (23) (2017) pp. 1-9. doi: 10.1051/ijmqe/2017016 [26] m. beer, s. ferson, v. kreinovich, imprecise probabilities in engineering analyses, mech. syst. signal process 37 (1-2) (2013) pp. 4-29. doi: 10.1016/j.ymssp.2013.01.024 [27] o. vasilevskyi, p. kulakov, d. kompanets, o. lysenko, v. prysyazhnyuk, w. wójcik, d. baitussupov, a new approach to assessing the dynamic uncertainty of measuring devices, proc. spie 10808, photonics applications in astronomy, communications, industry, and high-energy physics experiments, 108082e, 2018. doi: 10.1117/12.2501578 [28] r. trishch, o. nechuiviter, k. dyadyura, o. vasilevskyi, i. tsykhanovska, m. yakovlev, qualimetric method of assessing risks of low quality products, mm science journal (2021) pp. 47694774. doi: 10.17973/mmsj.2021_10_2021030 [29] iso, guidelines for implementation of statistical process control (spc) – part 1: elements of spc, iso 11462-1:2001 (2001). 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[33] bipm, iec, ifcc, iso, iupac, iupap, oiml, evaluation of measurement data − the role of measurement uncertainty in conformity assessment, joint committee for guides in metrology, jcgm 106:2012 (2012). http://dx.doi.org/10.1051/ijmqe/2014017 https://doi.org/10.1117/12.2280993 https://doi.org/10.1051/ijmqe/2017016 https://doi.org/10.1016/j.ymssp.2013.01.024 https://doi.org/10.1117/12.2501578 http://doi.org./10.17973/mmsj.2021_10_2021030 http://dx.doi.org/10.1016/j.jclepro.2019.07.017 https://doi.org/10.1177%2f0899764016655620 iso 6789 under revision – proposals for calibration results of hand torque tools including measurement uncertainty acta imeko june 2014, volume 3, number 2, 23 – 27 www.imeko.org iso 6789 under revision – proposals for calibration results of hand torque tools including measurement uncertainty dirk röske physikalisch-technische bundesanstalt, bundesallee 100, 38116 braunschweig, germany section: research paper keywords: hand torque tools; torque wrenches; calibration; iso 6789:2003; uncertainty citation: dirk röske, iso 6789 under revision – proposals for calibration results of hand torque tools including measurement uncertainty, acta imeko, vol. 3, no. 2, article 7, june 2014, identifier: imeko-acta-03 (2014)-02-07 editor: paolo carbone, university of perugia received february 20th, 2013; in final form december 7th, 2013; published june 2014 copyright: © 2014 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: dirk röske, e-mail: dirk.roeske@ptb.de 1. introduction the iso 6789:2003 standard [1] is widely used as a guide for the calibration of torque wrenches and other hand-operated torque tools like screwdrivers indicating the applied torque, or setting torque tools. unfortunately, this standard does not contain a method or even a proposal for the uncertainty calculation of the measurement results obtained in these calibrations. on the other hand, according to the gum [2] it is necessary to report a complete measurement result including its uncertainty. this problem was the subject of an earlier work which was based on the previous issue of the standard [3] and did not take into account some important influences such as the connecting profile (often the square drive) and the point of force application (lever length, cross forces and bending moments). this paper describes a proposal on how the standard could be improved in order to become a complete calibration guide. methods for calculating calibration results and estimating their uncertainties will be presented. the paper will point out influencing quantities which have not been taken into account until now and will deal with some specialties of torque tool calibration devices. the subject matter of this calibration guide is also the focus of discussions within the deutscher kalibrierdienst (dkd) – the recently (re-)founded german board for technical cooperation between ptb and torque calibration laboratories. some conclusions resulting from these discussions are also mentioned in this paper. 2. general iso procedure the iso 6789:2003 standard deals with two types of hand torque tools: type i – indicating torque tools and type ii – setting torque tools (see figure 1). for both of these types various classes are defined, such as torque wrenches or screwdrivers utilizing a scale, a gauge or an electronic display. some of these devices have a housing, some have a fixed measuring point, others are adjustable. in general, the calibration procedure consists of one (type i) respectively five (type ii) pre-loadings with the maximum torque, without recording the values and a subsequent series of five (for two classes of type ii: ten) repeated applications of three different torques from the measurement range. the three torque values are defined by the standard as 20%, approximately 60% (depending on the value that can be adjusted on the scale) and 100% of the maximum (nominal) figure 1. setting torque wrench model 3360, [4]. abstract the iso 6789:2003 standard is widely used as a technical document for the calibrations of hand torque tools, but it does not require any statements about uncertainties of the calibration results. nevertheless, according to the gum it is necessary to report a complete measurement result including its uncertainty. in addition, according to iso/iec 17025:2005, calibration laboratories must take into account measurement uncertainties in the case that they make a conformity declaration in their calibration certificates. in this paper, a proposal is given on how to estimate measurement uncertainties of iso 6789 calibration results taking into account various influencing quantities. acta imeko | www.imeko.org june 2014 | volume 3 | number 2 | 23 torque of the tool. special conditions for the loading have to be fulfilled for setting torque tools: an increasing force rising to approximately 80% of the target value has to be applied first. then the force has to be constantly and slowly increased up to 100% of the target value within a time window of 0.5 s … 4 s. a calibration facility for setting torque tools must take this requirement into account. 3. calibration facilities there are some other important remarks to be made with respect to the calibration facilities. first of all, they are very diverse. we know, for example, facilities that utilize lever-mass systems and a supported lever with air bearing for generating a pure torque. the torque tool or torque wrench is connected to the measuring axis of the facility and fixed or supported on the opposite side near the handle. these facilities offer the smallest uncertainties. the majority of the calibration facilities use integrated torque transducers and they are calibrated with torque transfer wrenches. the torque is generated then by either rotating the torque transducer against the fixed or supported tool or by applying a force at the handle of the torque wrench perpendicular to the beam and the torque axis. these systems allow the torque application to be controlled with the help of the torque transducers used as references. this is often the more accurate and reliable procedure, but it cannot be applied to setting torque tools. in these cases the torque must be increased until the setting torque is reached. if a calibration facility and the measurement results obtained are to be considered, it must be clear whether the calibration procedure was controlled by the facility’s calibration torque or by the tool to be calibrated. another topic is related to the signals recorded by the data acquisition. most of the torque transducers used in the facilities are strain-gauge based and deliver nearly exclusively signals in mv/v or n·m. these can be set to zero (be tared) before starting the measurements or not. in addition, when the calibration procedure for measuring tools says that a certain torque has to be applied, then it is not always the best idea to adjust this value as well as possible. especially in such cases when the force is applied with a manually operated crank on a spindle, this can take a long time, even for a well-trained person. if the torque is as close to the target value as necessary, then the value can be recorded. sometimes it may be useful to calculate the result for the calibration torque using a regression function. 4. calibration results according to the existing standard iso 6789:2003 according to the current version of the standard (published in 2003), the relative deviation as in % must be calculated from as(%) = xa − xr xr ∙ 100 . (1) here xa is the torque indicated by the tool and xr is the reference torque, i.e. the calibration torque measured by the calibration facility. in the special case of type ii, classes c and f tools (without scale), the value of xa is calculated as the mean of the ten reference torques xr measured by the calibration facility. this value is then used in equation (1). as stated above, no uncertainty of the input or resulting values is given. there is just one requirement regarding the calibration facility: the maximum uncertainty of the indicated torque must be ± 1%. the conformity can be declared if the deviations are within the limit set by the standard (± 4%, or ± 6% for some classes). again, there is no measurement uncertainty taken into account in contradiction to the iec/iso 17025:2005 standard (“general requirements for the competence of testing and calibration laboratories”) which requires conformity statements to be made taking the uncertainty of measurement into account [5]. this is very important for accredited calibration laboratories. 5. proposed improvements there are a number of possible improvements to the iso 6789:2003 standard. all of them have been discussed within the torque committee of the dkd. calibration range: within the dkd committee it was proposed to choose the starting point of the tool’s measurement range as the first calibration point instead of 20% of the full scale. this would be more useful from the point of view of the tool’s application and is especially important for indicating torque tools which are used in a wide measuring range. calibration facility’s operation mode: calibration facilities usually utilize torque transducers or sensors of precision torque wrenches as sensing elements. these sensors are calibrated as single devices or as part of the whole facility. it is quite interesting that the iso 6789 standard does not offer the possibility that the torque is controlled by the calibration facility’s sensor but the tool under calibration has to be loaded with increasing force (at the end of the beam) until it indicates the corresponding torque. on the other hand, automated calibration procedures can be realized much more easily if the torque is controlled by the facility’s sensor and not by the tool. in addition, if the tool shows completely invalid values, then wrong torques can be applied. it is proposed to allow the torque application to be controlled by the calibration facility’s indication. this is hereinafter referred to as case a. in the case that the torque application is controlled by the tool, we propose calling this case b. of course, this discussion does not completely apply to type ii torque tools since their reaction (click) is essential for the calibration. designations: calibration certificates are sometimes not very clear with respect to the numbers shown in their tables. it would be very helpful to offer some unique designations for the quantities to be measured. taking into account different possible options, it is important to clearly distinguish between the indications recorded during the calibration: indications of the calibration facility and those of the tool, both can be gross or net (tared) values, the calibration facility’s operation mode: case a or case b (see above), and the calibration procedure, i.e. if the calibration torques are tuned in (by hand or electronic control) or if a subsequent interpolation is necessary. the designations given in tables 1 and 2 are proposed. calibration result: for the user of a measuring instrument it is sometimes of interest not only to know if the tool is within the specifications (conformity), but also to know the value of the deviation and the uncertainty of this value. especially with acta imeko | www.imeko.org june 2014 | volume 3 | number 2 | 24 indicating tools, the deviation found during the calibration can be compensated when the tools are used in an application. this is, of course, an additional effort, but the reward is a more accurate and reliable measurement. it is now proposed to calculate an additional calibration result for each torque step (and each direction of the torque, if necessary) as the mean of the five or ten single values found for this torque step during the calibration. according to the two operation modes, the calibration result is a value calculated from the indications of the tool (case a) 𝑌𝑖 = 𝑌�𝑀n,𝑖� = 1 𝑛 �𝑌a,𝑖,𝑗 𝑛 𝑗=1 , (2) or from the indications of the calibration facility (case b) 𝑀𝑖 = 1 𝑛 �𝑀a,𝑖,𝑗 𝑛 𝑗=1 . (3) it is important to understand that in both cases the calibration torque is defined by the calibration facility. the deviations fq,i are always calculated as differences between the torque value of the tool and that of the calibration facility, regardless of the facility’s operation mode. influencing quantities: it is known that many quantities can influence the calibration result of a torque tool and most of them are not mentioned in the standard. this is normally not a problem when dealing with mechanical torque tools with permissible deviations of 4% or 6%. but in the last few years electronic tools with claimed uncertainties of 1% have come onto the market. on the other hand, comparison measurements have shown that connecting parts, such as square drives, can have a significant influence, especially when the results are close to the permissible limits. these effects are mainly caused by the geometrical dimensions, manufacturing tolerances and material properties (hardness, heat treatment) of the connecting elements (often square drives or other types of profiles). sometimes these parts are worn out but there is no criterion saying that they must be replaced. the influence of the connecting profile bv can be measured using the following method: for a number of k = 1, 2, … m (for example, m = 4 for square drives) equally distributed over 360° positions of the profile, l = 1, 2, … o single measurements (for example, o = 10) have to be carried out in each position, i.e. for m = 4 and o = 10, in total 40 measurements. the applied torque should be the starting point of the tool’s measurement range with a nominal beam length defined by the middle of the handle. the parameter bv can then be calculated in case a from 𝑏v = max𝑘 � 1 𝑜 �𝑌a,𝑘,𝑙 𝑜 𝑙=1 � − min 𝑘 � 1 𝑜 �𝑌a,𝑘,𝑙 𝑜 𝑙=1 � , and (4) in case b from 𝑏v = max𝑘 � 1 𝑜 �𝑀a,𝑘,𝑙 𝑜 𝑙=1 � − min 𝑘 � 1 𝑜 �𝑀a,𝑘,𝑙 𝑜 𝑙=1 � . (5) another possible influence is the position of the force introduction point of torque wrenches, i.e. the beam length. for constant torque, an increasing beam length corresponds to a decreasing force. this effect is not applicable to screwdrivers. the influence of the beam length can be measured using the following method: at 60% of the nominal torque, l = 1, 2, … o single measurements (for example, o = 10) have to be carried out with nominal beam length (defined by the middle of the handle), with the beam length increased by 10 mm and with the mean beam length decreased by 10 mm, i.e. for o = 10, in total 30 measurements. the parameter bl can then be calculated in case a from 𝑏l = max �� 1 𝑜 �𝑌a,mean,𝑙 𝑜 𝑙=1 − 1 𝑜 �𝑌a,long,𝑙 𝑜 𝑙=1 � , � 1 𝑜 �𝑌a,mean,𝑙 𝑜 𝑙=1 − 1 𝑜 �𝑌a,short,𝑙 𝑜 𝑙=1 �� , and (6) in case b from 𝑏l = max �� 1 𝑜 �𝑀a,mean,𝑙 𝑜 𝑙=1 − 1 𝑜 �𝑀a,long,𝑙 𝑜 𝑙=1 � , � 1 𝑜 �𝑀a,mean,𝑙 𝑜 𝑙=1 − 1 𝑜 �𝑀a,short,𝑙 𝑜 𝑙=1 �� . (7) table 1. proposed designations for the indications of the calibration facility and the tool to be calibrated. designation description i ordinal number of the torque step (for example i = 1: 20 n·m, i = 2: 60 n·m, i = 3: 100 n·m, i = 4: -20 n·m and so on) j ordinal number of the single value measured for torque step i (j = 1, 2, … 5 , or j = 1, 2, … 10) j0,i,j zero signal indicated by the calibration facility prior to the measurement of value j for torque step i jm,i,j measurement signal indicated by the calibration facility during the measurement of value j for torque step i mk,i,j tared calibration torque in the measurement of value j for torque step i i0,i,j zero signal indicated by the tool under calibration prior to the measurement of value j for torque step i im measurement signal indicated by the tool under calibration during the measurement of value j for torque step i ym tared measuring value in the measurement of value j for torque step i table 2. proposed designations taking into account the facility’s operation mode and, if necessary, interpolation. designation description case a mn,i nominal calibration torque for torque step i (all j) ya,i,j the tool’s tared and interpolated measuring value for the measurement of the single value j for torque step i yi the tool’s calibration result for torque step i case b yn,i nominal calibration torque of the tool for torque step i (all j) ma,i,j tared and interpolated calibration torque for the measurement of the single value j for torque step i mi the facility’s calibration result for torque step i acta imeko | www.imeko.org june 2014 | volume 3 | number 2 | 25 here, “mean” means nominal beam length, “long” is nominal beam length increased by 10 mm, and “short” is nominal beam length decreased by 10 mm. many setting hand torque tools have a ratchet included. so the question arises of how the calibration results change if different positions of this ratchet are used. this effect is partly included in parameter bv if the ratchet is used to change the position of the connecting profile. otherwise an additional measurement with all possible positions of the ratchet should be considered, but this can be quite time-consuming. the author’s working group has started to collect such values and would be very grateful for any assistance. environmental influences play an important role in many calibrations. this is partly taken into account by the standard’s requirement that the temperature should be constant (± 1°c) within an interval of 18°c to 28°c. ambient humidity must be recorded and be lower than or equal 90% (relative humidity). this value is too high. one problem is that steel parts begin to rust when exposed to very high humidity. another problem may be the changing viscosity of lubricants inside mechanical tools thus affecting their mechanical properties. it is expected that changing temperatures have a similar effect. how can all these influences be treated? worn square drives or ratchets must be replaced. extreme temperatures and humidities should be avoided. thus, it is proposed to include at least the influence of the connecting parts’ geometry bv and the position of the force introduction point (beam length) bl. in order to keep the calibration effort at a justifiable level, it is not necessary to measure the corresponding parameters every time the tool is calibrated and for every single measuring instrument, however, typical values for the given type of tool should be used instead. we think it is sufficient to use an estimate for these parameters for a series of tools with the same design and make. the manufacturers are asked to provide these values. only in the case of worn out connectors, suspected damage of the tool, or other evidence of deviations, the complete measurement is useful. uncertainties: it is proposed to calculate the parameter repeatability bi from the five (or ten) single measurement values for each torque step as a span maximum – minimum: in case a from 𝑏𝑖 = 𝑏(𝑌𝑖) = max𝑘 �𝑌a,𝑖,𝑗�−min𝑘 �𝑌a,𝑖,𝑗� , and (8) in case b from 𝑏𝑖 = 𝑏(𝑀𝑖) = max𝑘 �𝑀a,𝑖,𝑗�−min𝑘 �𝑀a,𝑖,𝑗� . (9) together with bv and bl, another parameter must be taken into account: the resolution r of the tool, as defined in the standard. the influence of the interpolation fa can often be kept small enough to be neglected. from all these parameters, relative standard uncertainties are calculated according to table 3. together with the standard uncertainty of the calibration torque wm,i, the standard uncertainties wsv,i of the single measurement values can be determined from 𝑤sv,𝑖 = �𝑤m,𝑖 2 + 2 ∙ 𝑤𝑟2 + 𝑤v 2 + 𝑤l 2 . (10) the standard uncertainties wmv,i of the mean values calculated from (2) or (3) can be determined from 𝑤mv,𝑖 = �𝑤m,𝑖 2 + 2 ∙ 𝑤𝑟2 + 𝑤𝑏,𝑖 2 + 𝑤v 2 + 𝑤l 2 . (11) in (10) and (11) it was taken into account that the measurement value is often the result of two readings, one at zero and one at the load point. it was assumed that both readings are affected by the resolution of the tool’s setting or indication in the same manner. the resolution of the calibration device was not used in this formula directly since it had to be table 3. proposed distributions and corresponding standard uncertainties for important parameters. parameter, evaluation type and distribution function relative standard uncertainty w in % case a case b r type b, rectangular 𝑤r = �𝑟2� √3 100 𝑀n,𝑖 𝑤r = �𝑟2� √3 100 𝑌n,𝑖 bi type b, rectangular 𝑤𝑏,𝑖 = �𝑏𝑖2 � √3 100 𝑀n,𝑖 𝑤𝑏,𝑖 = �𝑏𝑖2 � √3 100 𝑌n,𝑖 bv type b, rectangular 𝑤v = �𝑏v2 � √3 100 𝑀n,𝑖 𝑤v = �𝑏v2 � √3 100 𝑌n,𝑖 bl type b, rectangular 𝑤l = �𝑏l2 � √3 100 𝑀n,𝑖 𝑤l = �𝑏l2 � √3 100 𝑌n,𝑖 figure 2. conformity evaluation using uncertainty. table 4. sample calibration data for case a. mn,i n·m ya,i,j in n·m j = 1 2 3 4 5 i = 1 20 20.2 20.1 20.3 20.3 20.1 2 60 60.4 60.5 60.6 60.4 60.6 3 100 101.0 101.5 102.0 102.5 103.0 table 5. calculated calibration results for case a. mn,i n·m in n·m yi fq,i bi bv bl r 20 20.2 0.20 0.20 0.20 0.10 0.10 60 60.5 0.50 0.20 0.20 0.10 0.10 100 102.0 2.00 2.00 0.20 0.10 0.10 table 6. calculated uncertainties for case a. mn,i n·m in % wm,i wb,i wv,i wl,i wr,i wmv,i w’i 20 0.05 0.289 0.289 0.144 0.144 0.481 1.963 60 0.05 0.096 0.096 0.048 0.048 0.167 1.168 100 0.05 0.577 0.058 0.029 0.029 0.585 3.169 #1 2 3 4 5 tcal tcal + 4% tcal 4% tcal = tmin , (60%, 100%) · tmax exp. rel. uncertainty of the single value → basis for conformity declaration → no conformity here } torque acta imeko | www.imeko.org june 2014 | volume 3 | number 2 | 26 considered when the uncertainty of the calibration torques wm,i was calculated. as usual, the expanded relative uncertainties can be found by multiplying standard uncertainties with the corresponding coverage factor k = 2. the expanded relative uncertainty of the single value wsv,i should be the basis for a conformity evaluation as shown in figure 2: the red line is the calibration torque, the dotted lines mark the permissible interval. the uncertainty bar of the first value lies partly outside the permissible range and no conformity can be declared. the relative deviation as defined in (1) now reads with the new designations introduced above in table 2 in case a as 𝑓q�𝑀n,𝑖� 𝑀n,𝑖 = 𝑓q,𝑖 𝑀n,𝑖 = 𝑌𝑖 − 𝑀n,𝑖 𝑀n,𝑖 , and (12) in case b as 𝑓q�𝑌n,𝑖� 𝑀𝑖 = 𝑓q,𝑖 𝑀𝑖 = 𝑌n,𝑖 − 𝑀𝑖 𝑀𝑖 . (13) due to the fact that this deviation is not a stochastic quantity but shows rather deterministic behaviour, it was not treated like an uncertainty. therefore, it cannot be combined with the other uncertainties under the square root sign in (10) or (11). for the calibration results, uncertainty intervals w’i will be calculated instead as the sum of the absolute value of the relative deviations and the expanded relative uncertainties. the result for case (a) is 𝑊′𝑖 = � 𝑓q�𝑀n,𝑖� 𝑀n,𝑖 � ∙ 100% + 𝑘 ∙ 𝑤mv,𝑖 , (14) and for case b it is 𝑊′𝑖 = � 𝑓q�𝑌n,𝑖� 𝑀𝑖 � ∙ 100% + 𝑘 ∙ 𝑤mv,𝑖 . (15) numerical examples case a – calibration controlled by the calibration facility’s indication (not applicable to type ii tools): case b – calibration controlled by the torque tool: the values and results given in tables 4 to 9 can be taken as reference data sets for the validation of calculations. 6. conclusions the improvements of the iso 6789 standard discussed here would allow this document to become a full calibration guide which is in accordance with the gum. the methods proposed in this paper take into account the most important and relevant influencing quantities which are known to date. references [1] iso 6789:2003, assembly tools for screws and nuts – hand torque tools – requirements and test methods for design conformance testing, quality conformance testing and recalibration procedure. [2] evaluation of measurement data guide to the expression of uncertainty in measurement, internet: http://www.bipm.org/ [3] l. g. bochese, “uncertainty of measurement in torque wrench calibration”, 2003 ncsl international workshop and symposium, tampa, florida, usa, august 17-21, 2003. [4] source: “http://de.wikipedia.org/w/index.php?title=datei: drehmomentschl%c3%bcssel_3360.jpg&filetimestamp=201202 25121256”, author: torquemaster. [5] iso/iec 17025:2005, general requirements for the competence of testing and calibration laboratories. table 7. sample calibration data for case b. yn,i n·m ma,i,j in n·m j = 1 2 3 4 5 i = 1 20 19.8 19.9 19.7 19.7 19.9 2 60 59.6 59.5 59.4 59.6 59.4 3 100 99.0 98.5 98.0 97.5 97.0 table 8. calculated calibration results for case b. yn,i n·m in n·m mi fq,i bi bv bl r 20 19.8 0.20 0.20 0.20 0.10 0.10 60 59.5 0.50 0.20 0.20 0.10 0.10 100 98.0 2.00 2.00 0.20 0.10 0.10 table 9. calculated uncertainties for case b. yn,i n·m in % wm,i wb,i wv,i wl,i wr,i wmv,i w’i 20 0.05 0.289 0.289 0.144 0.144 0.481 1.963 60 0.05 0.096 0.096 0.048 0.048 0.167 1.168 100 0.05 0.577 0.058 0.029 0.029 0.585 3.169 acta imeko | www.imeko.org june 2014 | volume 3 | number 2 | 27 iso 6789 under revision – proposals for calibration results of hand torque tools including measurement uncertainty a software tool for the semi-automatic segmentation of architectural 3d models with semantic annotation and web fruition acta imeko issn: 2221-870x october 2018, volume 7, number 3, 64 72 acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 64 a software tool for the semi-automatic segmentation of architectural 3d models with semantic annotation and web fruition giorgio de nunzio1 1 dipartimento di matematica e fisica “ennio de giorgi”, university of salento, lecce, italy section: research paper keywords: 3d models; semantic segmentation; photogrammetry; sfm; mvs; autocorrelation function citation: giorgio de nunzio, a software tool for the semi-automatic segmentation of architectural 3d models with semantic annotation and web fruition, acta imeko, vol. 7, no. 3, article 11, month year, identifier: imeko-acta-07 (2018)-03-11 section editor: egidio de benedetto, university of salento, italy received april 8, 2018; in final form august 23, 2018; published october 2018 copyright: © 2018 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: this work was partly supported by corvallis spa (padua – italy, http://www.corvallis.it) corresponding author: giorgio de nunzio, giorgio.denunzio@unisalento.it 1. introduction nowadays, an important part of architectural heritage documentation consists in 3d models of buildings, obtained by photogrammetric techniques (i.e. from sequences of digital photos processed by structure from motion, sfm, and multiview stereo, mvs, procedures) or range data (by laser scanning). these models can be stored in databases to be retrieved via the world wide web. professionals access this data for better cultural heritage (ch) comprehension, management and preservation, while tourists can take virtual tours before traveling, or use the models as an alternative to the usual souvenir photos. 3d digital representations can be semantically enriched by adding annotations (metadata), associated to the model as a whole or to its noteworthy elements. in order to partition a digital model into parts, a process known as segmentation is employed. when the parts are significant, i.e. they reflect a possible decomposition into standard architectural elements, and these elements are conveniently annotated, we obtain a semantic segmentation. as an example, an order [1] consists of three parts: the entablature, the columns, and the crepidoma or the pedestal; each component can be further subdivided, e.g. the entablature consists of the cornice, the frieze, and the architrave; the column is composed of the capital, the shaft, and may have a base, and so on. the partition process goes on until we obtain “atomic parts” which are the basic elements of classical architecture. each of these parts has a name, it may have material information, a history, details on the conservation status, etc. fruition within the semantic web requires semantics and annotations to be expressed in standard ways: metadata should be assigned by respecting field-specific ontologies. the standard formal ontology for ch is the cidoc conceptual reference abstract the thorough documentation of cultural heritage artifacts is a fundamental concern for management and preservation. in this context, the semantic segmentation and annotation of 3d models of historic buildings is an important modern topic. this work describes a software tool currently under development, for interactive and semi-automatic segmentation, characterization, and annotation of 3d models produced by photogrammetric surveys. the system includes some generic and well-known segmentation approaches, such as region growing and locally convex connected patches segmentation, but it also contains original code for specific semantic segmentation of parts of buildings, in particular straight stairs and circular-section columns. furthermore, a method for automatic wall-surface characterization is devoted to rusticated-ashlar detection, in view of masonry-unit segmentation. the software is modular, so allowing easy expandability. it also has tools for data encoding into formats ready for model fruition by web technologies. these results were partly obtained in collaboration with corvallis spa (padua – italy, http://www.corvallis.it). mailto:giorgio.denunzio@unisalento.it acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 65 model (crm) (http://www.cidoc-crm.org/) by the international council of museums (http://icom.museum), which provides definitions and a formal structure to describe the implicit and explicit concepts and relationships used in ch documentation. cidoc-crm extensions specific to architecture exist, e.g. crmba (http://www.cidoccrm.org/crmba/home-7), created to support building documentation. this article describes some results on the development of a software system for partial semiautomatic semantic segmentation, characterization, and annotation of building 3d models produced by photogrammetric surveys, and their fruition by web technologies. the paper extends the work reported in [2]. after the introduction, section 2 reports some details on segmentation, while section 3 gives a summary of annotation with web fruition. the concluding section sums up the main results and states some perspectives. 2. segmentation many algorithms were integrated in the software to obtain the segmentation of buildings into their salient elements, such as stairs, columns (subdivided into capital, shaft and base), walls, doors, windows. the software is written in c++ with the point cloud library, pcl, [3] and some matlab parts for rapid prototyping and testing. several techniques and algorithms address the problem of point cloud or mesh segmentation, such as region growing, model fitting, and machine learning approaches [4]. it is well known that no single method is able to manage all the segmentation goals, so the software was designed with a modular approach, i.e. as a framework in which the implementation of new techniques might be easy. we used some ideas from the literature as well as our own algorithms, consistently integrating them in the system. following this idea, a number of segmentation priorities were identified. before all, some standard and generic segmentation approaches were implemented, to give the user a sort of “swiss knife” to cope with different situations concerning object subdivision. then, specific algorithms were inserted for particular tasks. horizontal and vertical plane identification was achieved by ransac (random sample consensus) [5-8]: this step is preliminary to all the other segmentation tasks, because it allows the identification of walls and the floor, and the ceiling if present in the model. wall identification is important in order to prepare for the recognition of openings (doors and windows) [9], to characterize wall-surfaces (e.g. distinguishing flat from rusticated walls), and to define the interior and the exterior of buildings. stairs were identified by a generative approach, in which parametrically generated shapes are localized in the model by fitting procedures. circular-section columns were recognized by ground histogram calculation. as a complementary result, walls were annotated as flat or rusticated by a method based on the autocorrelation function, and rusticated wall segmentation into the elementary wall element is planned in the near future. the various segmentation processes are of course not completely automatic, because some parameters must be set in accordance with the particular model features and are not flawless. in the following paragraphs, some more details and some results will be given concerning the overall software structure (paragraph 2.1), the generic segmentation methods implemented (paragraph 2.2), the detection and localization of straight stairs and round cross-section columns (paragraphs 2.3 and 2.4 respectively), the method for wall surface characterization (paragraph 2.5). 2.1. overall software structure and user workflow the software is structured in a modular way, providing some general-use segmentation methods, to be tested on typical architectural 3d models built out of a photogrammetricreconstruction pipeline. the different methods will be definitely incorporated in the code based on the results gradually obtained during the tests. a picture of the software graphical user interface is shown in figure 1. most existing segmentation algorithms work on point clouds. this software allows segmentation of both point clouds and (textured) meshes: in the latter case, the mesh is transformed into a “sparse” point cloud by “baking” the texture on the mesh vertices. it is planned to convert also the mesh to a “dense” cloud, by dense sampling. the segmentation algorithms are then applied to the derived cloud. after segmentation, if a particular cluster has been chosen as the desired result of the segmentation procedure, before saving to disk it is converted back to textured mesh. the user workflow, to be adapted in various ways according to needs, is as follows: a. load a 3d model/scene to be segmented (e.g. for annotation, or for model cleaning). the scene may be composed of different disconnected parts, and the various connected components may have a finer structure to be partitioned (e.g., a column could be segmented into a shaft, a base and a capital). b. visually examine the 3d scene: thanks to the mouse-operated tools for space and object manipulation (panning, rotation, scaling, and viewpoint displacement), the scene is examined to identify the constituent parts of interest (and useless ones to be considered as noise, if present). c. use the segmentation tools to cluster points and identify/characterize/label the connected components ("objects"). d. manipulate the objects of interest, with a variety of selection/deselection/deletion/saving tools (mouse click to explore and select, select all, deselect all, reverse selection, delete selected, delete unselected, restore deleted, save selected objects to disk) figure 1. the software graphical user interface. acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 66 e. start again with the segmentation procedures on one or more of the objects produced in the preceding steps. f. if desired, add annotations to the objects. one can imagine working on one object at a time, eliminating the others, segmenting them into parts, and saving the individual parts with their respective semantic labels: afterwards, it will be possible to reconstruct the whole scene by merging the labels and collecting the meshes together. the procedure can be applied cyclically, selecting one or more clusters each time in order to interactively reach a useful and optimal segmentation. an "undo" mechanism has been implemented to recover the previous state of clustering when the one obtained is not optimal: in this way, it is possible to try new algorithms or different values of the parameters 2.2. general-purpose point-cloud segmentation at present, the following segmentation approaches, already available in pcl, have been tested and partially implemented in the software: euclidean cluster extraction1 (ece), conditional euclidean clustering2 (cec) region growing segmentation3 (rgs), min-cut segmentation4 (mcs), difference of normals based segmentation5 (dons), locally convex connected patches6 (lccp). the ece method implements a simple data clustering approach in a euclidean sense by using a 3d grid subdivision of the space with fixed width boxes (octree). this data structure is very fast to build and is useful for situations where a volumetric representation of the point cloud is needed, or the data in each octree leaf should be approximated with a different structure. in ece, the octree structure helps in the implementation of a flood-fill like clustering technique by efficiently determining each point nearest neighbours in a sphere with radius smaller that a given threshold. cec is similar to ece, with the advantage that the constraints for clustering are now customizable by the user. some disadvantages include: no initial seeding system and no overand under-segmentation control. rgs belongs to the same family; its purpose is to merge the points that are close enough in terms of a smoothness constraint computed by comparing the angles between point normals. the output is a set of clusters, where each cluster is a set of points that are considered a part of the same smooth surface. mcs is a well-known segmentation method [10] that creates a binary segmentation of the given input cloud. the algorithm divides the cloud into two sets: foreground and background points (points that are considered to belong to the object and those that are not). the algorithm is based on a graph representation of the data, in which the cloud points are the graph vertices, and edges connect the points to 1http://www.pointclouds.org/documentation/tutorials/clus ter_extraction.php 2http://pointclouds.org/documentation/tutorials/condition al_euclidean_clustering.php 3http://pointclouds.org/documentation/tutorials/region_gr owing_segmentation.php 4http://pointclouds.org/documentation/tutorials/min_cut_ segmentation.php 5http://pointclouds.org/documentation/tutorials/don_seg mentation.php 6http://docs.pointclouds.org/trunk/classpcl_1_1_l_c_c_p_ segmentation.html their nearest neighbours and to two special vertices called source and sink. weights are assigned to the edges (smoothness cost) and foreground/background penalties are calculated. finally, the search of the minimum cut is made and the cloud is divided into foreground and background. dons algorithm [11] performs a scale-based segmentation of the given input point cloud, finding points that belong to each cluster within the scale parameters given. it is a computationally efficient multi-scale approach, quite effective in the segmentation of scenes with a wide variation of scale. for each point in a point cloud, two unit point normals are estimated with different radii and the normalized (vector) difference of these point normals is calculated to define the don operator. the motivation behind don is the observation that surface normals estimated at any given radius reflect the underlying geometry of the surface at the scale of the support radius. finally, lccp [12] is an efficient unsupervised and model-less approach, which begins by breaking down the scene into a surface patch adjacency graph based on a voxel grid. the graph edges are then classified as convex or concave using a combination of simple criteria that work on the local patch geometry. in this way, the graph is divided into locally-connected convex subgraphs, which with great accuracy represent parts of the modelled object. in addition, a new depth-dependent voxel grid is used in order to address the decrease in cloud point density at large distances. this improves segmentation, allowing the use of fixed parameters for very different scenes. the algorithm is simple to implement and requires no training data, yet it produces results comparable to the state of the art of methods that incorporate high-level concepts such as classification, learning and model adaptation. the algorithm aims to separate parts of objects rather than selecting whole objects. in our software, a tab widget allows to choose between the various segmentation approaches, exposing the algorithm parameters to the user (see figure 2). in figure 3 some example of model segmentation are shown. even if segmentation is applied and performed to the point cloud, the result is then projected onto the original textured mesh, so for example figure 4 shows the statue separated from the support, saved as a textured mesh. figure 2. the control panel containing the tab widget that allows the choice of the segmentation algorithm and the assignment of parameter values. acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 67 2.3. detection and segmentation of straight stairs stairs detection is important in several different fields, such as multi-storey path finding for explorer robots venturing into buildings [13–15], as an aid for the visually impaired [16–18], and last but not least in the semantic segmentation of 3d models of architectural heritage buildings [19, 20]. algorithms developed for the first two cases are usually not directly applicable to semantic segmentation for ch, [15] because they generally work on organized point clouds derived from rgb-d data, while ch applications use unorganized clouds coming from photogrammetry or laser scanners. they may anyway suggest methods valid for unorganized point clouds too. the literature proposes diverse techniques. some start from edge point extraction. for example in [13] edge points are detected in rgb-d images by the concave hull algorithm available in the pcl framework, then classification relies on depth and geometric information. in paper [14], a staircase extraction algorithm based on super-voxels is proposed. in [15] the authors develop a graph-based detection method for point clouds, which first segments planar regions and extracts the stair tread and riser segments. with these segments, a dynamic graph model is initialized that is used to detect stairs. see [15] also for a good review of some previous approaches useful for unorganized point clouds, and for a discussion on plane-based and edge-based methods. in [16] walls, doors, stairs, and a residual generic class of obstacles on the floor are detected in rgb-d data; stairs are found by searching for points on planes at increasing height from the ground, with a given step height and a tolerance. in [17] depth maps, calculated from rgb-d or stereo data, are used to feed a classifier. in [18] the authors propose a staircase detection algorithm in rgb-d data, based on a support vector machine (svm): the hough transform is used to extract parallel lines in rgb frames so as to detect stairs candidates, and the depth frames are employed to classify the staircase candidates as upstairs, downstairs, and negatives (such as corridors). paper [19] comes from the ch context and develops an application specific to unorganized point clouds, by a generative modeling approach: parameterized object models are built by gml, the generative modeling language, and fitted to the point cloud (by subpart fitting without previous segmentation). in [20] an interactive framework to extract hi-level primitives (e.g. columns or staircases) from 3d models is presented, again based on a generative and fitting approach. the particular approach described in this paper is inspired by [19]. it is generative and ideally automatic. the large computational effort, necessary for 3d subpart fitting is avoided by a pre-processing step which reduces the fitting problem to 2d. for this purpose, the horizontal straight edges outlining the steps are detected and their mean points are arranged into a 2d pattern used for fitting. it is useful to introduce the method by first giving the appropriate terminology and by classifying the various types of stairs by geometrical considerations [21]. stairs are made of series of steps (flights) with landings at appropriate intervals. each step consists of a tread (the horizontal part, with its depth and width) and a riser (the vertical part between treads, with its height). we can distinguish: straight stairs (consisting of either one single flight or more than one flight with landings, with no change in direction; if there is a change in direction, we have parallel or angle stairs, such as quarter-turn stairs, half-turn or doggedlegged stairs, etc); circular stairs; spiral stairs, and others. sometimes only stairs without direction change are considered as true straight stairs. our algorithm only addresses straight stairs, with or without direction change, so that we can assume parallel stairs edges. figure 3. examples of subsequent segmentation operations. top: simple rgs was used to label the connected components with colour, thus separating the statue from the background. bottom: lccp allowed isolating the statue from its support. the white wireframe bounding boxes are used to select the various clusters prior to applying segmentation, preserving, deleting or saving commands. figure 4. the statue in figure 3, after saving it as a fully textured mesh. acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 68 as mentioned, a parametric generative model was chosen for its versatility. the problem complexity was reduced by projection to a 2d space before model fitting. as illustrated in detail afterwards, the algorithm first detects the most significant edge points in the cloud (step 1) and finds the straight edges by ransac (step 2); then the horizontal edges are preserved and clustered into groups of parallel segments (step 3); the clusters are projected to a plane perpendicular to the segments, so getting a 2d pattern of points representing the cluster, and modelling the stairs (step 4); finally a parameterized generative model of stairs is fitted to the point pattern (step 5), giving an accuracy measure and the model parameters; if the fit accuracy is satisfying, the position of the stairs in the original 3d world is found, and a bounding box is visualized in the cloud. in more detail: step 1: edge points are detected by the fast and precise method described in [22], where sharp edge features are discovered by analysing the eigenvalues of the covariance matrix that are defined by each point’s k-nearest neighbours. the c++/pcl code for edge detection is kindly shared by the authors.7 step 2: straight lines in the edge point cloud are detected by the ransac method. step 3: the edges found at step 2 are clustered by dbscan8 (density-based spatial clustering of applications with noise) [23], a density-based clustering algorithm which can potentially identify clusters of any shape in a data set containing noise and outliers. dbscan usually clustered the stairs edges very well, collecting in a same group almost all and only the detected edges pertaining to a single flight of stairs. other clustering algorithms were tested with far inferior results. the features used for clustering were the stair direction (one of the two non-zero direction cosines of the edges) and the three coordinates of the edge center. a larger weight was given to the direction feature because only strict parallel edges could belong to the same stairs, having decided to limit to straight stairs. step 4: for each cluster, a plane is now defined, normal to the cluster edges, and the edge centers are projected onto it, so giving a planar pattern of points representing the possible stairs. the coordinates of these points on the plane are defined by introducing a pair of basis vectors (u, v) (orthogonal and unit length). the definition of u and v is not unique, but a convenient way is the following. let n = (a, b, c) be the plane normal unit vector: in our case, a is the average x direction cosine of the edges in the cluster, b ~ 0 (y being the vertical axis, and the edges being horizontal), and c = sqrt(1 – a2). we can set: u = (c, 0, –a) v = n × u (1) where the × symbol represents cross product. we now project each edge center (xc, yc, zc) onto the (u, v) plane and calculate its (uc, vc) coordinates by dot product: uc = u  (xc, yc, zc) vc = v  (xc, yc, zc) (2) where the origin of the (u, v) coordinates is the world origin (0, 0, 0). each point ideally comes from an edge, so marking the transition from riser to tread, or from tread to riser. this pattern can contain noise (points due to incorrect edges) and may miss some points related to edges that were not found in step 2. step 5: a point pattern is generated as a stairs model (each point being the projection of a stairs edge), depending on the following stairs parameters: number of steps, riser height, tread depth, 3d stair position (i.e. the position of the centre of first edge of the lowest stairs step). the model is fitted to the patterns obtained in step 4, and the optimized parameters, as well as the value of the objective function as an accuracy measurement, are returned. these values are then automatically 7 https://github.com/denabazazian/edge_extraction 8 http://yaikhom.com/2015/09/04/implementing-thedbscan-clustering-algorithm.html figure 5. point-pattern fitting. the small circles are the real-world points obtained by projecting the edges centres (in an edge cluster) onto a plane perpendicular to the edges (step 4; u and v are the coordinates in the projection plane). there are some outliers (corresponding to erroneous edges), points are noisy, and some may be missing. the model is on the left. the fitted model perfectly reconstructs the stairs silhouette. figure 6. a case study. the stairs model (top-left picture) was downloaded from https://archive3d.net (model n290716) and converted to a point cloud by the cloudcompare software (http://www.cloudcompare.org/), the edge points were detected (top-right image), then the straight edges were located and the horizontal ones were clustered (bottom-left picture; lightgrey edges are those rejected as outliers). the straight stairs were then detected as explained in the text, and automatically highlighted with bounding boxes (bottom-right picture). acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 69 evaluated in order to accept or reject the cluster as representing stairs. the point fitting algorithm, which gave the best results in terms of robustness and accuracy, was coherent point drift (cpd) [24]. figure 5 shows an example of model matching. after stairs detection, the model consists of the following parameters: lower-step center coordinates, stairs direction parameter in the horizontal x-z plane, the number of steps, tread depth, riser height, and step width. the procedure was tested on synthetic and real-world models. step 5, in particular, was tested against synthetic point patterns, and the fitting procedure was stressed by adding noise to the ideal point position, by inserting points not corresponding to stairs edges, and by removing significant points. figure 6 shows the application of the algorithm to a synthetic case of study. 2.4. detection and segmentation of round cross-section columns the literature is not rich in papers concerning column detection in 3d scenes. for example, ef. [18] applies generative models also to columns and arcades, while in [25] round and rectangular cross-section columns are detected by ground histogram calculation. for the sake of performance, we chose the latter approach, which decreases the problem dimensionality from 3d to 2d, and decided to initially limit the scope to circular columns. the model point cloud was projected to the ground plane, and planar point density was evaluated, so obtaining ground histograms. the vertical structures left easily recognizable signatures in the point-density map, e.g. circular columns produced circular shapes that could be located by the circular hough transform (cht), [26] with subsequent column detection and segmentation. the columns were then partitioned into their constituent parts. the details follow. step 1: after down-sampling, the point cloud is projected to the ground plane. step 2: with the purpose of calculating point density in the plane, and finding the signature of vertical structures, an octree is built from the projected cloud, and box-searching is performed in each cell so as to measure each cell occupancy; an image is then built, containing one pixel per octree cell, with grey value proportional to the point density in that cell. step 3: cht is applied to the image created in step 2, to find the signatures of columns (circles) if present. step 4: the centers and radii of the circles give hints on the horizontal column location, but nothing is known about vertical position and size: therefore, a region, centered on each column center and slightly larger than the column diameter, is searched for a cylinder by ransac; looking for cylinders only in the regions located by the cht reduces false positives and speeds thing up. step 5: the columns are finally segmented into their constituent parts (capital, shaft and base) by convex decomposition [27]; this algorithm computes a hierarchical segmentation of the mesh triangles by applying a set of decimation operations to the mesh dual graph, guided by a cost function related to concavity: the generated segmentation is then exploited to construct an approximation of the original mesh by a set of convex surfaces; the generated convex hulls can be used as space filters to segment the original points of the point cloud. segmentation is not always accurate enough, causing small segments under the column base or in the architrave – beside the three main column parts – to be returned. to overcome this problem, the longest segment is identified and assigned to the shaft, and the two segments connected to it from above and from below are selected as the capital and the base respectively, disregarding the other parts if present. of course, segmentation quality is affected by the model quality. figure 7 shows the application to a model of the apollo theatre (lecce, italy) and to the gerrard hall (north carolina) both reconstructed by photogrammetry from sets of about 100 pictures each (gerrard hall photos are available at https://colmap.github.io/datasets.html). see the figure caption for details. 2.5. wall-surface characterization: flat vs rusticated walls this module is not a segmentation tool in itself. rather, it acts as an automatic classification/annotation instrument and as a pre-processing step for rusticated-ashlar masonry-unit segmentation. for this reason, it is associated with the wall segmentation module. figure 7. top (black background): the apollo theatre, in lecce (italy). the model is followed by the ground point density map (obtained by projecting the model to the ground plane), where cht (circular hough transform) was applied, which allows column detection: the column signatures are shown in red). a picture of the isolated columns follows. bottom (white background): the gerrard hall (north carolina) model, after column detection and segmentation into capital, shaft, and base (as convex component parts). the model is followed by the ground density map with cht applied. acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 70 two basic patterns used in stone masonry walls are rubble and ashlar masonry [28-31]. the former is made from stones with irregular sides (not at right angles to each other), while in the latter, the stone sides are dressed square (at right angles to each other). the front and back faces of the stone may be dressed or undressed. both rubble and ashlar masonry may be random or coursed, according to mortar joint regularity. another attribute of masonry is rustication, in which the visible faces of the dressed stones are raised with respect to the horizontal and usually the vertical joints, which may be rabbeted, chamfered, or bevelled. the joints are therefore emphasized. the stones may be left rough or worked in such a way as to give a strong textural effect. in particular, rusticated ashlar is a coursed and dressed stonework with edges and joints emphasized by rustication. rustication has been used since ancient times and was first described by serlio in his famous treatise [32]. one of the most known rusticated surfaces belongs to the palazzo dei diamanti (diamond palace) in ferrara, with its rusticated façade of diamond-shaped marble blocks. the purpose of this software module is to distinguish various types of stonewalls, in particular discriminating rusticated ashlar from “flat” masonry. from a mathematical point of view, a rusticated-ashlar masonry cloud point is a surface presenting some sort of periodicity. let the x and the y axis be respectively horizontal and vertical, then the joints parallel to the x axis are always rearward with respect to the stone face, and many times this is true also for the joints parallel to the y axis. as a consequence, periodicity appears in the y axis or in both. periodicity can be studied and characterized by a large number of tools, such as fourier analysis, the autocorrelation function, wavelets, and others. in this work, the fourier transform and the autocorrelation function were tried, and the latter proved simpler, effective, and more resistant to noise. this approach will be briefly described here, and some results will be given. the autocorrelation function (acf) for a real, continuous, univariate function f(x) is defined as: 𝑎(𝑡) = ∫ 𝑓(𝜏) 𝑓(𝑡 + 𝜏) 𝑑𝜏 +∞ −∞ (3) this definition, in the case of real, discrete, bivariate functions on a finite support such as 2d digital images i, becomes: 𝑎(𝑥, 𝑦) = ∑ ∑ 𝐼(𝑚, 𝑛) 𝐼(𝑥 + 𝑚, 𝑦 + 𝑛)𝑁𝑛=1 𝑀 𝑚=1 (4) the acf can be used as the basis of a texture measure in 2d images, because peaks in the autocorrelation function of a regular-texture image characterize the texture periodicity. compared with the peaks in the fourier spectrum, the peaks in the acf are usually more prominent and thus easier to find. moreover, the acf is also less disturbed by noise. the use of the acf for 2d-image texture characterization is wellestablished [33]. in the scope of this paper, the interest is not in 2d images, but in 3d point clouds of building walls. it is nonetheless very easy to convert our 3d problem to a 2d one, considering that a rusticated surface can usually be expressed as a (non-uniformly sampled) 2d function h = f(x, y) where h is the stone point elevation from a reference plane xy parallel to the wall, and the (x,y) coordinates locate the point projection to the plane. assigning a grey level to h, and applying (e.g. bilinear) 2d interpolation, the wall can be converted to an image and all the tools available for grey-level 2d images can be employed. this is totally different from taking a single photograph of the surface, which records colour and shadows but not directly 3d structure. so, in this work, the wall surface was converted to an image, the acf was calculated, and the horizontal and vertical profiles (1d sections) passing through the acf center were derived. these profiles respectively contain the information on x and y periodicity, if present, as coded in a certain number of peaks. the absence of periodicity gives only a central peak, while periodic signals give secondary peaks prominent from the background. these peaks are smaller than the central one but evident. in order to get rid of the slowly varying acf background, a 1d top-hat morphological operation is applied to the profiles, and the central peaks are measured (for intensity reference) and removed. finally, the height of the remaining peaks is measured. the ratio between the mean height of the latter peaks, and the central-peak height, is used as a feature to assess horizontal or vertical periodicities, which correspond to the presence of rusticated ashlar. if a profile only contains the central peak, 0 is assigned to the corresponding periodicity feature. large values (> 0.1) are a sign of periodicity. the result of periodicity assessment is finally associated as an annotation to the wall under exam. in figure 8 two cases are shown, a rusticated façade and a flat wall. in perspective, a similar approach can be used to estimate a representative tile of the rusticated ashlar masonry, or to study stone damage. a specific paper concerning method sensitivity, robustness to noise, and accuracy as a function of feature threshold, is in preparation. 3. semantic annotation and web fruition the system allows to insert annotations into the segmented model according to standard cidoc-crm entity e22 (manmade object) and property p46 (is composed of), for future compatibility with the semantic web. the models are then converted to the nexus file format [34, 35]. nexus, being a progressive-mesh format, allows smooth lod (level of detail) choice depending on the current view (i.e., model distance, framing, device power). with this approach, it is possible to initially display a model with the lowest lod and then let it gradually show more and more details, with effective real time visualization. as to web fruition, our http pages are dynamically built based on 3dhop [36] for real-time model drawing, and some javascript code for semantic annotation visualization by an information tree linked to the model: a click on the model hotspots, defined by the previously segmented architectural elements, allows selecting the corresponding tree node, and vice versa (see figure 9). acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 71 conclusions the paper presents a software framework for semantic segmentation and web fruition of cultural heritage 3d models. the software includes some standard general-purpose segmentation approaches available in pcl, and some specific semantic segmentation modules (in particular, straight stairs and circular-section columns). it also contains modules devoted to object characterization, such as the one currently under test, for the detection of rusticated ashlar masonry. in addition, the paper gives some details on the web system prototype. the software proved successful in a number of monument models and synthetic point clouds. nonetheless, the framework is still under development, with many segmentation modules relying on parameter tuning. the development will continue, with new modules added to the framework, and limitations hopefully relaxed. acknowledgement the results reported in this work were partly obtained in collaboration with corvallis spa (padua – italy, http://www.corvallis.it), who also financially supported the research. references [1] a. tzonis, l. lefaivre, classical architecture: the poetics of orders, 1987, the mit press. 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[12] s. c. stein, m. schoeler, j. papon and f. wörgötter, object partitioning using local convexity, proc. of 2014 ieee figure 8. two cases of wall periodicity measurement by the autocorrelation function, for rusticated ashlar masonry detection. top: a rusticated wall (interior of palazzo adorno – lecce, italy); bottom, a flat wall with noise (slight stone degradation). both cases show the point cloud (left) and the acf of the corresponding 2d grey-level image obtained from the point cloud (right). the rusticated wall gives well-defined peaks in the acf, related to periodicity, while the flat façade has none but the central one. in the former case, the value of the periodicity features calculated as described in the text is around 0.4, while in the latter case they are lower than 10-3. figure 9. the gerrard hall model inserted in the web system prototype, using 3dhop and some javascript code for tree management. hotspots were inserted at the column parts (top) and used as links to the semanticstructure tree (bottom). clicks on the model allow navigating the tree. http://ieeexplore.ieee.org/search/searchresult.jsp?searchwithin=%22authors%22:.qt.r.%20harrap.qt.&newsearch=true acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 72 conference on computer vision and pattern recognition, columbus, oh, 2014, pp. 304-311. 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[27] k. mamou, and f. ghorbel, a simple and efficient approach for 3d mesh approximate convex decomposition, proc. of 16th ieee international conference on image processing (icip), 2009. [28] n. davies, e. jokiniemi, dictionary of architecture and building construction, elsevier, 2008, isbn: 978-0-7506-8502-3. [29] f. d. k. ching, building constructions illustrated 5th ed., wiley, 2014 , isbn 978-1-118-45834-1, p. 5.33. [30] j. summerson, the classical language of architecture, revised and enlarged ed., 1980, thames and hudson, isbn 0-50020177-3, p.45–47, p. 58–59. [31] r. chitham, the classical orders of architecture, 2nd ed, elsevier, 2005, isbn 0 7506 61240, p. 190-192. [32] s. serlio, “libro quarto – regole generali di architettura di sebastiano serlio bolognese sopra le cinque regole degli edifici” xvi (1537). [33] h.-c. lin, l.-l. wang, s.-n. yang, extracting periodicity of a regular texture based on autocorrelation functions, pattern recognition letters 18 (1997) 433-443. 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[35] f. ponchio, m. dellepiane, fast decompression for web-based view-dependent 3d rendering, web3d 2015. proc. of the 20th international conference on 3d web technology, pp 199-207 [36] m. potenziani, m. callieri, m. dellepiane, m. corsini, f. ponchio, r. scopigno, 3dhop: 3d heritage online presenter, computers & graphics volume 52, november 2015, pages 129141. measurement applications in industry 4.0: the case of an iot-oriented platform for remote programming of automatic test equipment acta imeko issn: 2221-870x june 2019, volume 8, number 2, 62 69 acta imeko | www.imeko.org june 2019 | volume 8 | number 2 | 62 measurement applications in industry 4.0: the case of an iotoriented platform for remote programming of automatic test equipment leopoldo angrisani1, umberto cesaro1, mauro d’arco1, oscar tamburis2 1 university of naples federico ii, department of computer science and electrical engineering, via claudio 21, 80125 naples, italy 2 department of veterinary medicine and animal productions, via delpino 1, 80137, naples, italy section: research paper keywords: tcp/ip communication; multithreading; multi-core processors; client-server application; execution system citation: leopoldo angrisani, umberto cesaro, mauro d’arco, oscar tamburis, measurement applications in industry 4.0: the case of an iot-oriented platform for remote programming of automatic test equipment, acta imeko, vol. 8, no. 2, article 9, june 2019, identifier: imeko-acta-08 (2019)-02-09 section editor: emiliano sisinni, university of brescia, italy received july 27, 2018; in final form june 18, 2019; published june 2019 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: mauro d’arco, e-mail: darco@unina.it 1. introduction the term ‘industry 4.0’ refers to the fourth industrial revolution we are currently witnessing: the whole sphere of industrial production is in fact undergoing a comprehensive transformation that involves the merger of conventional industrial processes with digital technologies and the internet. such phenomenon is intended both (i) to improve existing technological capabilities by complementing them with valueadded features and (ii) to enable new ones by exploiting the cooperation between the most recent digital equipment and more traditional assets. a great impact on mass customisation, integration of value chains, and process efficiency has come from the merging of today’s sensor technology, interconnectivity, and data analysis capabilities. accordingly, new open collaboration and innovation paradigms are taking root, always seeking confidence in an efficient quality infrastructure, thus pushing towards relevant changes in societal and economic activities. companies are in fact not only reorganising their innovation processes by acquiring external information, knowledge, and skills, but they are also providing access to complementary assets outside their boundaries [1]. this situation causes, for instance, increasing deployment of remotely controlled strategic activities in several industrial plants. the same openness lies behind the iot technological paradigm, envisioned as a global network of physical devices connected to the internet [2]. on this basis, authorised users can access equipment from anywhere via internet in order to inspect data, make decisions, and drive operations [3]-[14]. despite the level of application, achieving an efficient quality infrastructure lies inevitably in the ability to obtain valid data based on high-precision measurements so that it is appropriate to include such a scenario in the category of ‘metroindustry 4.0’. in particular, the role of measurement science in the promotion of digital expansion and transformation is being intensively pushed forward by means of research programmes and by setting up new capacity groups. it is also targeted at the centre of the development of new strategies for engineering education 4.0 [15], [16]. its application in simulations and abstract a laboratory regarded as a site that collects iot devices, and which allows remote clients to use them as an automatic test equipment (ate) through a controller acting as service provider, is proposed herein. to assure efficiency and responsiveness, the controller is programmed as a multithreading system that takes advantage of multicore processors. the controller includes a server application that supports communication with clients by means of a tcp/ip protocol. it uses gpib bus functionalities to control the instruments of the local ate. it allows several clients to connect and interact with the specific resources of the laboratory. thanks to the availability of identical sets of resources and to the underlying multithreading philosophy, client requests are processed in tandem rather than according to a classical queuing approach. mailto:darco@unina.it acta imeko | www.imeko.org june 2019 | volume 8 | number 2 | 63 virtual measuring instruments is therefore counted among the newly identified focuses. accordingly, laboratories are upgradable to sites collecting iot, capable of offering value-added functionalities to the interacting subjects [16]-[24]. from a broader perspective, they can also be thought of as computing systems with access to physical measurement and control instruments as well as connectivity to the internet—or, in other words, cyberphysical systems (cpss) [25]. to this end, the present work introduces a proposal for the remote programming of automatic test equipment (ate) and the development of measurement and automation applications. different to several web-based applications, implemented nowadays by means of high-level technologies such as webservices, the proposed solution relies on basic tcp/ip data transfer mechanisms, not involving the hypertext transfer protocol (http) and related concepts [26][25]-[31]. in particular, it consists of a platform, designed as a labview application, which is initially conceived as a training tool for technicians involved in remote programming tasks. technicians usually work with hardware directly connected to some development kits or integrated into evaluation boards. less frequently, technicians are required to program remote instruments without the possibility of acquiring immediate visual feedback or conducting focused inspections on physical features of the instruments during a series of scheduled operations [33]-[37]. the platform user can ask the remote ate for the execution of measurement applications, as specific services, by programming and specifying a sequence of tasks for a given set of instruments. the server authenticates the connecting clients and interrogates them in order to obtain more details about their request. thereafter, it processes the request in order to decode the specific tasks in the list, and it then activates a selected set of resources that can include stimulus sources and data acquisition systems. each task is an action that either defines a list of settings for the addressed instrument or queries the transfer of measurement data. the user can control the timing execution of the tasks by introducing delays between each of them. several measurement applications typically require a source to stimulate a device under test and, after a controlled delay related to the responsiveness of the device, an observer to inspect its output. the server eventually sends to the client a string response containing the information that is relevant to the outcomes of the measurement process. by definition, remote laboratories are essential for the process of learning and assimilating scientific concepts. the proposed iot-oriented approach, specifically envisioned for application in an industry 4.0 environment, is particularly valuable in addressing responsivenessand complexity-related issues. in the following sections, the software architecture of the proposed platform is illustrated, and the code of the main threads is shown, defined by means of the labview graphical language. 2. proposed platform in order to provide a general picture of the macro activities performed by the proposed platform, a high-level unified modelling language (uml)-like activity diagram [38] is shown and annotated in figure 1. then, in order to illustrate each macro activity and provide details related to actual implementations, made in labview 7.0 se, in sufficient detail, screenshots of the block diagrams of the most important threads are also given. a general picture of the block diagram that illustrates the operation of the server machine can be found in appendix i. the server uses several parallel threads (named tcp connection thread, check-in thread, management thread, and check-out thread) to acquire, process and hand over client requests. it also uses n measurement threads, named gpib threads, to control the available measurement stations. in the proposed case study, only two ates have been used. the laboratory is made up of identical measurement stations, each one including an arbitrary waveform generator (awg) and a digital storage oscilloscope (dso), and it allows n concurrent clients to work in parallel. in brief, the tcp connection thread waits for a client connection and assigns an identifier to any new connection. the check-in thread communicates with newly connected clients and creates a cluster of data specifying the details of the client request. the management thread inspects the queue of active requests as well as the state of available ates. if a request is queued and an ate becomes available, it schedules the client request to the available ate and wakes up the gpib thread that is responsible for controlling that ate. gpib thread decodes and serves the client request; finally, it queues figure 1. uml-like activity diagram illustrating the macro activities performed by the proposed platform. the user requests the execution of measurement tasks via the remote server. the server authenticates the connecting clients, then it retrieves, queues and processes their request by activating a selected set of resources i.e. a specific ate. finally, the server sends a response containing the measurement results. acta imeko | www.imeko.org june 2019 | volume 8 | number 2 | 64 the results into a response string message. the response string is retrieved by the check-out thread and is sent to the client by means of tcp/ip functions. the threads exchange data by means of queues implemented with functional global variables (fgvs). queue structures based on fgv concepts can solely be accessed by means of the methods defined by the programmer and allow threads to exchange data in synchronized way. in the following section, the names of the main threads are written in bold, while the names of sub-vi and the variables are written between quotation marks. 2.1. tcp connection thread the thread is typically idle and wakes up on client connection. it is coherent to a typical programming template that uses a while loop inside which a ‘tcp wait on client connection’ function is cyclically executed, as shown in figure 2. the input data for this internal function, namely the listener reference number (green wire) and error info (pink wire), is obtained by a ‘tcp create listener’ function outside the loop. the function inside the loop waits indefinitely for a new client connection and returns a specific reference number for a newly connected client. the functional global variable ‘fgv refnum’ implements a queue, wherein both queueing and dequeueing operations can be performed by selecting a boolean control (true requests for de-queueing all data, false for queueing a single input). should any error occur in the waiting connections, the tcp connection thread halts the server operations. to this end, it quits the while loop and changes the value of the boolean variable ‘alt acq’, which acts as stopping signal for the check-in thread. to carry out the change in the variable that is defined in another thread, the variable is addressed through a property node. the error propagates outside the while loop through the pink wire, where it is handled by the labview standard error handler function. 2.2. check-in thread the basic operation cycle of the thread is demonstrated by the block diagram in figure 3. first, the queue implemented through the ‘fgv refnum’ variable, which contains the reference numbers of the clients connected to the server, is flushed. if the queue is not empty, then the state of the ‘tcp warden’ semaphore is acquired, and if the semaphore provides the necessary permission, sub-vi ‘acq request’ is invoked. the sub-vi is capable of preventing unknown users from hampering the platform by implementing an authentication mechanism. in particular, it acquires a short header message (8 bytes) from the client and scans it, looking first for a 32-bit integer, which should represent an authentication key. if the authentication key is absent, the client connection is immediately closed. otherwise, the message is scanned further in order to acquire a second 32-bit integer specifying the length of the string that details the client request. the string is subsequently acquired along with an additional read operation expecting as many bytes as are specified in the aforementioned header. the sub-vi ‘acq request’ returns the string to the calling thread i.e. check-in thread, which releases the semaphore after completion of the operation. it concatenates the string to the client identifier. the compound string is queued into the variable ‘fgv request’ and is passed to the management thread through the very same ‘fgv request’ variable. note that if the semaphore does not provide the necessary permission, no request is acquired, and the cycle terminates skipping the queueing operation. the repetition of the operation cycle of the check-in thread is delayed for a short time of 250 ms in order to reduce the usage of the central processor when shared with other threads. the cycle repeats indefinitely unless the value of the boolean variable ‘alt acq’, which is controlled by the tcp connection thread, becomes true. on the left side of figure 3, outside the while structure, some initialisation operations are performed. in particular, inspecting the aforementioned portion of the block diagram from the bottom up, one can find that the queue implemented by the variable ‘fgv refnum’ is initially flushed in order to start the tcp connection thread and check-in thread with an empty queue. furthermore, the value of the boolean control ‘alt acq’ is set to false, and a semaphore that is later utilised figure 2. block diagram of the tcp connection thread. figure 3. block diagram of the check-in thread. acta imeko | www.imeko.org june 2019 | volume 8 | number 2 | 65 by the check-in thread and check-out thread is created and named tcp warden. 2.3. management thread as shown in figure , the thread flushes the queue containing the client requests and updates a waiting list appending the requests to those already present in the list. if the waiting list is not empty, the thread looks for available ates by flushing the queue implemented by the ‘fgv availab’ variable to obtain the list of available ates, each one identified by an integer. then, it retrieves from the waiting list the same number of requests as the number of the available ates. the strings that detail the requests are individually signalled to identical instances of the gpib thread, each one controlling an ate of the available ones. note that if the number of available ates is greater than the number of waiting requests, the management thread inserts the identifiers of the ates that are not engaged into operations into the queue implemented by ‘fgv available’, even if they are available. on the left side of the block diagram, outside the ‘while structure’, the array of strings used as the waiting list is initialised to an empty array, and the queues implemented by means of ‘fgv request’ and ‘fgv availab’ are flushed; the latter is then initialised inserting the number identifying the available ates. the stopping criterion of the thread, as illustrated in figure , considers a set of conditions. the thread stops if the boolean control ‘alt acq’ becomes true, provided that the waiting list of the client requests is empty and all gpib threads are idle. to acquire the state of the gpib threads, the threads are interrogated through property nodes. the boolean value that results from the logic multiplication of the aforementioned conditions is also used to set the value of the boolean control ‘alt send’, which is defined in the check-out thread. note that like the check-in thread, the repetition of the operation cycle of the management thread is also delayed for a short time of 250 ms in order to reduce the usage of the central processor when it is shared with other threads. 2.4. gpib thread parallel gpib threads are responsible for the execution of multiple measurement processes. their software framework is identical, except for the addressed ate. each gpib thread is awakened from its idle state on the occurrence of an event, signalled by the management thread as soon as the latter finds a client request in the waiting list and an available ate. specifically, the management thread uses a property node to signal and contextually transfer the string containing the client request to a supplemental thread, shown in figure , which simply invokes the gpib thread and provides it with the string as an input parameter. once invoked, each gpib thread is temporarily locked to a single client request until the completion of the required operations. the block diagram of the prototype gpib thread is shown as an example in figure . the thread retrieves the client message, skipping the header 4 bytes that contain the reference to address the client in tcp/ip communication. it then flattens the remaining string in order to recover an array of clusters, each one specifying a measurement task. each measurement task is detailed in terms of: • an action upon the gpib bus (read/write); • an addressed instrument (dso/awg); • a waiting time before task execution; and • a string message. the string message contains either a command message (i.e. in figure 4. block diagram of the management thread: core operation. figure 5. block diagram of the management thread: stopping condition. figure 6. block diagram of the thread with the event structure that activates the gpib thread. acta imeko | www.imeko.org june 2019 | volume 8 | number 2 | 66 the case of a write action) or a byte-count specification (i.e. in the case of a read action). the function decodes the sequence of tasks and performs the required communication via the gpib interface for each of them. then, it assembles an array of string messages, each one expressing a response to a specific task. when all tasks contained in the client request have been completed, this array is flattened to a single string and inserted into the ‘fgv respons’ variable. finally, the measurement station is made available again by updating the variable ‘fgv availab’. 2.5. check-out thread the software for the thread is shown in the block diagram in figure . the thread flushes the queue implemented with the ‘fgv respons’ variable. for each string contained in the queue, it recovers the 4-byte header containing the client reference for use in tcp/ip communication, and it finally sends the measurement results contained in the remaining part of the string by invoking the sub-vi ‘send response’. the thread stops if the value of the boolean control ‘alt send’ is set to true by the management thread. the initialising operations performed by the functions visible on the left side of the block diagram outside the ‘while structure’ imply flushing the queue implemented by means of ‘fgv respons’ and setting the value of the boolean control ‘alt send’ as false. 3. user side the user of the platform is required to run a vi client application, the front panel of which is shown in figure . in the following demonstration, the front panel is configured by the user in order to program the execution of three sequential tasks according to the following aims [37]-[41]: 1. set awg, addressed by means of the boolean control ‘dso/awg’, to generate a triangular waveform on a high impedance load characterised by 1.850 khz fundamental frequency, 10 v peak-to-peak amplitude, and zero offset. to this end, the user chooses the action ‘write’ through the ‘read/write’ control, since they need to send the configuration data to the remote instrument. the value of the control ‘wait’ is set to 0 ms, requiring immediate execution. 2. set dso to acquire samples using 100 s main scale time/div (i.e. 2.5 msa/s sampling rate) and 1 v volt/div. dso is also pre-configured to transmit a portion of the acquired record. in particular, no header for the data block is required; channel 1 is the specified data source; singlebyte ascii encoding is demanded; a portion of the acquired record, delimited by means of start and stop values, is selected; the instrument is queried to send the figure 7. block diagram of gpib thread figure 8. block diagram of the check-out thread. figure 9. front panel of the client vi. acta imeko | www.imeko.org june 2019 | volume 8 | number 2 | 67 selected portion with the last command, which is terminated with a question mark. 3. collect the samples acquired by the remote dso, which is analysing the awg output. to this end, a byte-count value that is sufficiently larger than the expected number of bytes, to be exchanged through the gpib bus, is specified together with the option ‘read’. the front panel of the vi contains a waveform graph indicator showing the graphic of the waveform digitised by the dso present in the ate. the waveform is first retrieved by the local server through the gpib interface, and then by the remote client via tcp/ip communication. 4. discussion in order to effectively benefit from multithreading technology, high-level programming expertise is required both in the development and debugging phases. in particular, if the application uses the same subvi in different threads, the execution of the subvi must be re-entrant. for instance, at the debugging stage, when it is practical and useful to run multiple client applications on a single pc, the developer has to configure all client subvis as re-entrant subvis. in addition, race conditions due to simultaneous requests by multiple threads of shared resources, such as the gpib bus in the considered application, or asynchronous modifications of shared variables, must be prevented. fortunately, the gpib bus controller can avoid conflicts if instruments are identified by means of different addresses. on the other hand, the use of fgv has been effective to contrast race conditions issues [39]-[41]. nonetheless, to implement further optimisations, the programmer can configure the labview execution system, assigning different priorities (up to six priority levels) to different threads according to their strategic role. the standard execution adopts a separate thread to run user interface activities, while the main threads are divided into daughter threads, containing separable parts of the original processing code and organised into a circular queue. daughter threads can be retrieved from the queue manage by the execution system and run by different processors in multi-core architectures. when a thread needs to use a control, its execution is suspended, and responsibility is passed to the user interface thread, slowing down the application execution. more specific remarks are related to the scalability of the platform and to the communication latencies. in theory, each gpib remote controller can manage up to 15 ates, made up of a couple of instruments, using just the primary addresses. measurement tasks can be very time consuming, especially if they involve the download of arbitrary waveforms synthesised with up to 32768 samples and the retrieval of long data records acquired by dsos. situations like these can cause dangerous bottlenecks at the gpib bus level for a platform like the proposed one. if all ates are charged with heavy tasks, which results in a long occupancy rate for the gpib bus, the latencies observed on the client side will range from many seconds to even a few minutes. these latencies are unacceptable and are not contemplated in the proposed platform, which limits the tcp/ip communication operations to 25 s by means of a timeout control. the platform design could be developed either by taking into account statistics related to both the average time to serve a request and the average number of concurrent connections, or by deciding to always avoid disconnections due to time-out occurrences on the client side. in the first case, as described in the present work, a strategy that leaves room for waiting requests to be served can be admitted, accepting the risk for the connected clients to undergo time-outs. in the second case, the server has to deny the acceptance of the client’s request when all available ates are busy by sending them an invite-to-retry message. 5. conclusions metroindustry 4.0 represents the broad scenario in which the achievement of an efficient quality infrastructure lies inevitably on the ability to obtain valid data based on highprecision measurements. the role of measurement science and its application towards simulations and virtual measuring instruments is therefore counted among the newly identified focuses. starting from these premises, an iot-oriented platform realised by means of a labview application has been proposed. it uses basic tcp/ip data transfer mechanisms to allow clients to access and program remote ates so that they can be operated at a distance. the platform is essentially conceived as a training tool for technicians involved in remote instruments programming jobs within an industry 4.0 environment. the core software modules of the platform have been described in detail. an example of the operations of the platform has been briefly demonstrated. remarks related to the scalability, complexity, and responsiveness of the system have also been discussed. acknowledgements the authors thank a. tocchi and d. grillo for their help in setting up and testing the presented platform. appendix i a complete picture of all operations performed by the server machine is given in figure . the server includes the parallel threads: tcp connection thread, check-in thread, management thread, and check-out thread. it also uses further measurement threads, named gpib threads, to control the available measurement stations. the threads exchange data between each other by means of queues 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[42] bipm european association of metrology institutes, metrology for the digitalization of the economy and society, october 2017 [online] available: https://www.bipm.org (access date: 10/10/2018). an advanced gcode analyzer to predict build-time in am components acta imeko issn: 2221-870x december 2020, volume 9, number 4, 30 38 acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 30 an advanced gcode analyser for predicting the build time for additive manufacturing components luca di angelo1, paolo di stefano1, emanuele guardiani1 1 university of l’aquila, department of industrial and information engineering and economics, monteluco di roio, 67100 l’aquila, italy section: research paper keywords: build time estimation; manufacturing costs; process planning; additive manufacturing citation: luca di angelo, paolo di stefano, emanuele guardiani, an advanced gcode analyzer to predict build-time in am components, acta imeko, vol. 9, no. 4, article 5, december 2020, identifier: imeko-acta-09 (2020)-04-05 section editor: leopoldo angrisani, university of naples federico ii, italy received october 15, 2019; in final form january 17, 2020; published december 2020 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: emanuele guardiani, e-mail: emanuele.guardiani@graduate.univaq.it 1. introduction the use of additive manufacturing (am) technologies is growing day by day due to their various advantages [1]. for one, they allow for creating designed objects using new and innovative shapes, which could not be achieved via traditional manufacturing processes [2]. this is the case with the shape optimisation of structural components in which the unstressed material can be removed to reduce the weight [3], [4]. moreover, since the am processes are well integrated with computer-aided design (cad) instruments, the pre-processing for the physical creation of a given part requires less time compared to classical subtractive technologies. this property presents a key feature for companies that work in a competitive industry in which the reduction of time-to-market determines the competitivity of the company. however, am technologies are still expensive and the manufacturing process is significantly longer than that of the classical subtractive manufacturing technologies [5]. in a competitive market of am services, manufacturing costs must be estimated in a reliable way [6]-[8], which is why the accurate estimation of build times is crucial. a reliable quantification of build time also aids ascertainment of the deposition direction, which minimises the manufacturing costs [9]-[23]. in view of this, several studies have been conducted over the few last years that have led to two different strategies for estimating the build time. the first strategy involves performing a detailed analysis of the manufacturing activity and is regarded as the most reliable approach. meanwhile, the second strategy involves performing the build time estimation according to appropriately defined parametric functions in which the independent variables are a number of build-time driving factors. the detailed-analysis-based methods use complete information related to the geometry of the object and to the manufacturing process. in using this method, a highly accurate estimation of the build time could potentially be obtained. however, complete information is required for the evaluation, as is a detailed analysis of the manufacturing activity. for these reasons, the detailed analysis approach is computationally expensive, and more time is required for the build time estimation. meanwhile, while the parametric-based methods are less accurate than the first approach, they require fewer data as input and are less expensive in computation terms. the parametricbased methods make use of certain pertinent build time factors, which serve as the independent parameters used in the functions to evaluate the dependent variable, that is, the build time. these parameters can be computed through analysing the geometric abstract additive manufacturing is a technology for quickly fabricating physical models, functional prototypes, and small batches of parts by stacking two-dimensional layered features directly from computer-aided design data. one of the most important challenges in this sector relates to the capability to predict the build time in advance, since this is crucial to evaluating the production costs. in this paper, an accurate method for obtaining build-time is proposed. this method is based on an advanced gcode analyzer written in python following an object-oriented paradigm for scalability and maintainability. various examples are used to demonstrate the reliability of the algorithm, while its potential applications are also illustrated. mailto:emanuele.guardiani@graduate.univaq.it acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 31 model of the object to be manufactured (volume, bounding box, etc.). however, the most challenging aspect of this type of method lies in identifying these factors. the set of parameters used should take into account all the elements that affect build time, while they must also be mutually independent such that any cross-correlation is avoided. the parametric-based methods would appear to be more promising for many practical applications, such as in the case where an accurate prediction of the build time is required but a limited set of data describing the object is available. an example here is the budgeting process where the customer may not want to provide the full geometrical model of the object to the seller in order to protect their intellectual property but instead will provide a few parameters that affect cost [24]. moreover, build time estimation is a mandatory step for any optimisation method devoted to ascertaining the best build direction. it is for these reasons that the parametric approaches have been largely proposed as one element of far more complex methods devoted to ascertaining the optimal manufacturing build direction. in [25], a very simplified model is proposed, in which the build time is proportional to the number of layers in the sliced model. meanwhile, in [21], [23], [24], [26]-[28], more complex formulations of the build time are proposed, with the build time dependent on the volume of the object, the supports, and other geometrical features of the object. while they incorporate many more parameters, the limitation of these methods lies in the function describing the build time, which is linear. the relationship between build time and the attendant driving factors is highly complex and largely unknown. in order to take this issue into account, the use of an adaptive model based on grey modelling has been proposed [29], while an artificial neural network for identifying the relationship between the driving factors and the build time was proposed in [30]. here, the authors demonstrate that a highly accurate estimation of the build time can be obtained using these methods. however, the drawback of both approaches is that they require a large set of model samples. in order to provide a set of training samples, a number of factors driving build time and the actual build time must be examined. the accuracy of the estimation performed by the neural network, for example, increases with the number of test samples, meaning a large number of actual build time evaluations is required. clearly, this is neither economically nor temporally feasible. in order to define a significative set of samples, an accurate detailed-analysis-based method could be used in view of performing a less expansive evaluation of the build time in relation to a real experiment. such a detailed analysis can be operated by using specialised computer-aided engineering (cae) programs, such as those supplied with the am machine. an example here is simplify 3d, which is a 3d printing slicing software that controls every aspect of the printing process and also performs build time evaluations. alternatives include cura by ultimaker and mattercontrol by matterhackers, both of which are freeware. nevertheless, the build time estimations provided by this software differ from those performed using a fused deposition modelling (fdm) machine (german reprap x350). the comparison of the estimated build time with the real situation is reported in table 1; it involved using the three test cases shown in figure 1. the estimation performed was not accurate enough to be used for obtaining qualified training data for a neural network. the main reason for the evident mismatch in the aforementioned programs is that they do not consider certain process-related parameters of the machine in use, namely, ‘acceleration’ and ‘jerk’. the control strategy performed by certain machine firmware, such as the open-source reprap, uses acceleration strategies defined by the two parameters (jerk and acceleration) that widely affect build time and object quality. this is shown in figure 2, where two cubes of the same dimension were manufactured by a german reprap x350 using different acceleration parameters. in figure 2a a module of 300 mm/s2 was selected for the x-y axis, while in figure 2b, twice the value was used. a comparison between the two objects indicated that the quality of the corners of model (a) was significantly better than that of model (b). this was due to the different inertia forces, which affected the accuracy of the material deposition performances, especially on the corners. in order to overcome all the previously discussed limitations of the implemented detailed-analysis-based methods, a new method for build time evaluation is developed in this research. the approach followed was a relatively general purpose approach, meaning the method can be used in various am figure 1. reference models used for time comparison. table 1. comparison between actual build time and estimations provided by professional cae software for the models reported in figure 1. model software actual estimated error a simplify3d 1532 min 1344 min −14 % b cura 374 min 242 min −54 % c mattercontrol 326 min 286 min −14 % figure 2. comparison between two cubes based on the same nominal geometry and manufactured using a different acceleration value. acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 32 applications. the method involves performing gcode analysis, taking into account the most important process-related parameters. as such, a highly accurate estimation of the actual build time can be obtained. in the next sections, the method is explained in more detail. a number of test cases are used to test the proposed approach, with the results critically discussed. gcode gcode is the most widespread programming language used for giving instructions to a computer numerical control (cnc) machine. it consists of a series of textual instructions (word address) devised in accordance with the iso 6983 standard, which regulate the behaviour of the machine during the manufacturing process. this includes defining the speed of the axis, regulating the temperature of the extruder, and numerous other aspects. this set of commands is generally called the ‘part program’. during the ‘first era’ of automatic machines, a manual approach was adopted for generating the part program. here, a highly skilled operator could write the part program alone or, at least, with the aid of visual programming software. today, the preferred solution consists of an automatic approach based on the integration between the cad and cam systems (figure 3). following the designing of the object, the geometry is imported into the cam, where many of the process-related parameters are defined. with reference to am technologies, these parameters could be represented by the layer thickness, the typology of hatching, the speeds of the motors, and various other aspects. then, if required by the technology in use, the supports are also generated into the cam, typically using a graphical procedure. at the end of these steps, the cam generates the tool path and the post processor transforms all the previous information into a set of instructions, readable by the machine in use, which is the part program, and saved into a textual file that is finally sent to the machine. consequently, the part program defines many characteristics of the manufacturing process that will be followed by the machine. however, an analysis limited to the part program is generally not sufficient for establishing the kinematic behaviour of the machine. many of the attendant decisions are taken in realtime by the controller of the machine. this is the case for acceleration ramps, which are highly dependent on the machine in use. due to the axis inertia and/or torque availability of the motors, any cnc machine is subject to magnitude constraints on the accelerations. here, three different control strategies can be adopted. let us suppose that the deposition tool has to follow the path reported in blue in figure 4, where p1, p2, and p3 are three control points interpolated linearly. a first option consists of maintaining a fixed speed along the entire path. this condition represents the optimal solution in terms of time-saving, while a similar behaviour can be realised only theoretically due to the infinite acceleration module required to realise it. in reality, when a fixed speed is adopted (figure 5), a diverted path (green line of figure 4) is generated due to the impossibility of realising an infinite module acceleration. in order to avoid a similar behaviour that causes aleatory geometrical errors on the finite piece, another strategy, which is characterised by having a zero speed in correspondence with the control points (blue-dotted line in figure 5), can be used. in this case, linear acceleration ramps are generally adopted. this allows for obtaining the optimal positioning accuracy, since the theoretic path and the real path are the same. the drawback of this strategy lies in the increased build time. this issue has led to the formulation of a third approach, which represents a compromise between geometrical accuracy and build time. in this case, the control unit of the machine in use does not limit itself to executing the instructions provided by the gcode; rather it performs a ‘clever’ evaluation. in addition to the commands in the execution, various subsequent instructions are read and cached. then, through a vectorial comparison of the velocities, the acceleration ramps are computed in order to maintain the maximum feedrate possible (red-dotted line in figure 5). this control strategy is typically known as ‘look-ahead’ figure 3. automatic programming for generating the part program. figure 4. comparison of tool paths using different acceleration strategies. figure 5. speed comparison between constant speed, zero-velocity, and lookahead strategies. acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 33 and can be regarded as a multi-objective optimisation problem, where the geometrical accuracy conflicts with the reduction of the build time. many researchers have worked on this topic in recent years. since the related literature is wide and voluminous, the most important and recent methods are outlined here. in [29], it was proposed that each corner is smoothed by replacing a subset of the path that contains it with a conic ‘splice’ segment, deviating from the exact corner by no more than a prescribed tolerance. in [30], using a fine-interpolation parametric method in which the corners are replaced by arc curves is recommended, while in [31], [32] the b-splines are used to approximate the corners. meanwhile, in [33], a different kind of approach is used, whereby the control acts on the acceleration ramps instead of the tool paths. the presence of so many different approaches to the ‘look-ahead’ optimisation can lead to some confusion. this is certainly the case with fdm machines for which, as we verified in the previous section in the case of a german reprap x350, professional tools are not able to reconstruct the real kinematic behaviour of the machine, leading to a very rough estimation of the build time. in this paper, a comprehensive investigation of the behaviour of reprap devices is carried out. the kinematic laws are reconstructed and, lastly, we provide a script for estimating the build time with high accuracy that was written in python following an object-oriented paradigm for scalability and maintainability. 2. proposed method the proposed method is based on the analysis of the gcode, the role of which was previously described. specifically, all the instructions that relate to movement command were considered since these are largely responsible for defining the build time. other instructions that may contribute to the build time are the temperature commands that, for example, define the temperature of the printing bed. however, for this category of instructions, an exact evaluation of how they contribute to the build time is difficult to attain since they are largely dependent on the environmental conditions. moreover, their influence is generally negligible with regard to movement commands and they were thus not considered. the gcode movement commands are described in annex e of iso 6983, and will thus not be discussed in detail here. as illustrated in figure 3, as the gcode used for am applications is generated starting from a triangular mesh (.stl) that is subsequently sliced, most of the analytical information that defines the original geometry is lost during the data exchange. consequently, the simplest and most efficient way for generating the part program is to define numerous geometrical control points that belong to the model and to interpolate them linearly. as such, most of the movement instructions used for am applications are linear interpolations, introduced by the code g01. the algorithm evaluates each interpolation line using a regular expression matching. each movement is associated with its relative velocity and length. in figure 6, the interpolation lines i and i+1 of a generic gcode file are presented as an example. the length 𝑙𝑖 of path i-th, the nominal speed 𝑣1𝑖 that should be reached and the length of the extruded filament 𝑒𝑖 during the i-th step can be formulated as follows: 𝑙𝑖 = √(𝑒𝑒𝑒.𝑒𝑒𝑒 − 𝑎𝑎𝑎.𝑎𝑎𝑎) 2 + (𝑓𝑓𝑓.𝑓𝑓𝑓 − 𝑏𝑏𝑏.𝑏𝑏𝑏)2 𝑣1𝑖 = 𝑑𝑑𝑑𝑑 𝑒𝑖 = 𝑔.𝑔𝑔 for each i-th step, the build time is provided by the contribution of four terms: 𝑡𝑖 = 𝑡jerk𝑖 + 𝑡0𝑖 + 𝑡1𝑖 + 𝑡2𝑖 (1) where: 𝑡jerk𝑖 = jerk phase time during the i-th step 𝑡0𝑖= acceleration phase time during i-th step 𝑡1𝑖 = cruise phase time during the i-th step 𝑡2𝑖: deceleration phase time during i-th step. to evaluate these terms, the acceleration phase and the jerk phase need to be discussed and illustrated. the jerk phase is referred to as the ‘look-ahead’ strategy used by reprap machines. during this phase, an ‘instantaneous’ change of speed δ𝑣 is applied. this specific behaviour is made possible with the use of stepper motors. clearly, an ‘instantaneous’ change of velocity implies high values of acceleration, which means the value of δ𝑣 must be limited to avoid both vibration and a loss of steps from the motors. since the contribution of 𝑡jerk𝑖 is generally extremely small compared to the other three terms of formula (1), it is assumed to be negligible. 𝑡jerk𝑖 ≅ 0 (2) in order to proceed, for each step, a generic j-th degree of freedom that satisfies condition (3) is taken as the reference. 𝑎𝑖 𝑗 ≠ 0 𝑗 = 1,2,…,𝑁 (3) where 𝑎𝑖 𝑗 is the acceleration module of i-th step related to the j-th degree of freedom, and 𝑁 is the number of degrees of freedom of the machine. this assumption is justified by the synchronous behaviour that should be exhibited by the axis, that is, the axis should start and stop executing the command of a certain instruction line at the same moment. the following condition can then be evaluated: |𝑣2𝑖 𝑗 − 𝑣0𝑖+1 𝑗 | > δ𝑣𝑖 𝑗 (4) where 𝑣2𝑖 𝑗 is the end speed of step i-th related to the j-th degree of freedom, 𝑣0𝑖+1 𝑗 is the start speed of step (i+1)-th related to the j-th degree of freedom. here, 𝑣2𝑖 𝑗 and 𝑣0𝑖+1 𝑗 were initially computed with reference to the feedrate value reported in the gcode instructions and, generally, they are different from each other. this is due to the fact that from step i-th to step (i+1)-th, the vector 𝒗𝟏 could change in terms of direction and/or magnitude. if condition (4) is true, the end speed 𝑣2𝑖 𝑗 or/and the start speed 𝑣0𝑖+1 𝑗 need to be revaluated. in this case, two possibilities are defined. if 𝑣2𝑖 𝑗 ∙ 𝑣0𝑖+1 𝑗 > 0 (5) then only one of the two velocities requires updating: figure 6. example of two linear interpolation gcode instructions. i) g1 xaaa.aaa ybbb.bbb ec.cc fdddd i+1) g1 xeee.eee yfff.fff eg.gg acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 34 { 𝒗𝟐𝒊 ∗ = 𝑣0𝑖+1 𝑗 + δ𝑣𝑖 𝑗 𝑣2 𝑖 𝑗 𝒗𝟐𝒊 |𝑣2𝑖 𝑗 | > |𝑣0𝑖+1 𝑗 | 𝒗𝟎𝒊+𝟏 ∗ = 𝑣2𝑖 𝑗 + δ𝑣𝑖 𝑗 𝑣0 𝑖+1 𝑗 𝒗𝟎𝒊+𝟏 |𝑣2𝑖 𝑗 | < |𝑣0𝑖+1 𝑗 | (6) otherwise, both velocities are recalculated in the following way: { 𝒗𝟐𝒊 ∗ = δ𝑣𝑖 𝑗 𝑣2 𝑖 𝑗 + 𝑣0 𝑖+1 𝑗 𝒗𝟐𝒊 𝒗𝟎𝒊+𝟏 ∗ = δ𝑣𝑖 𝑗 𝑣2 𝑖 𝑗 + 𝑣0 𝑖+1 𝑗 𝒗𝟎𝒊+𝟏 (7) the physical significance of the jerk phase can be better understood with reference to figure 7 by assuming that the deposition tool is moving from point a to point b then onto c’ or c’’ while maintaining a constant feedrate. in the case of point c’, the variation in the direction of the velocity vector is small. therefore, instead of reducing the speed in point b to zero, only a limited deceleration is applied up until a certain speed value, the entity of which depends on the chosen jerk module (eq. 6). otherwise, when the deposition tool has to follow the path abc’’, the vectorial variation of the velocity is significant. consequently, both the end speed of path i-th 𝒗𝟐𝒊 and the start speed of the path (i+1)-th 𝒗𝟎𝒊+𝟏 are down-scaled (eq. 7). here, it is interesting to note that, in both cases, the speed at point b is never equal to zero, which affects the manufacturing process in terms of vibration, quality of deposition, and accuracy, which, by limiting the jerk parameter, can be maintained at acceptable values. furthermore, this compromise allows for obtaining a drastic reduction of the build time. once the jerk phase has defined the start speed 𝒗𝟎𝒊 and the end speed 𝒗𝟐𝒊 of the i-th step, the acceleration ramps can be computed. this type of machine generally makes use of linear acceleration ramps, which are the simplest to handle when a digital electronic device is utilised. assuming this to be at step ith, four different types of situation may occur due to the gcode’s reading. the first, which is shown in figure 8, represents the condition for which the nominal speed 𝑣1𝑖 𝑗 is reached along the j-th degree of freedom. the following quantities are thus defined as in equations 8 and 9: { 𝑡0𝑖 = |𝑣1𝑖 𝑗 | − |𝑣0𝑖 𝑗 | 𝑎 𝑖 𝑗 𝑠0𝑖 𝑗 = |𝑣0𝑖 𝑗 | 𝑡0𝑖 + 1 2 𝑎𝑖 𝑗 𝑡0𝑖 2 (8) { 𝑡2𝑖 = − |𝑣2𝑖 𝑗 | − |𝑣0𝑖 𝑗 | 𝑎 𝑖 𝑗 𝑠2𝑖 𝑗 = |𝑣1𝑖 𝑗 | 𝑡2𝑖 − 1 2 𝑎𝑖 𝑗 𝑡2𝑖 2 (9) if the following condition (10) becomes true, the behaviour illustrated in figure 8 is performed during the i-th step: 𝑠0𝑖 𝑗 + 𝑠2𝑖 𝑗 > 𝑠𝑖 𝑗 (10) in this case, 𝑡1𝑖 and 𝑠1 𝑗 can be computed as follows: { 𝑡1𝑖 = 𝑠1 𝑗 𝑣1 𝑖 𝑗 𝑠1 𝑗 = 𝑠𝑖 𝑗 − 𝑠0𝑖 𝑗 − 𝑠2𝑖 𝑗 (11) where 𝑠i j is total distance during the i-th step along the j-th degree of freedom, 𝑠0i j is the acceleration distance during the i-th step along the j-th degree of freedom, 𝑠1i j is cruise distance during the i-th step along the j-th degree of freedom, and 𝑠0i j is the deceleration distance during the i-th step along the j-th degree of freedom. other cases can occur when the nominal speed 𝒗𝟏𝒊 𝒋 along the j-th degree of freedom cannot be reached due to an insufficient path length 𝒔𝒊 𝒋 . in such a case, we can distinguish three different figure 7. during the jerk phase, the start speed and the end speed of the paths are ingeniously ‘overwritten’. figure 8. nominal speed reached during step i-th. figure 9. insufficient distance for acceleration; new end speed. acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 35 possibilities, the first of which is illustrated in figure 9 and occurs when condition (12) becomes true. { 𝑠0𝑖 𝑗 > 𝑠𝑖 𝑗 𝑣0𝑖 𝑗 < 𝑣2𝑖 𝑗 (12) { 𝑡0𝑖 = |𝑣2𝑖 𝑗 | − |𝑣0𝑖 𝑗 | 𝑎 𝑖 𝑗 𝑠0𝑖 𝑗 = |𝑣0𝑗𝑖|𝑡0𝑖 + 1 2 𝑎𝑖 𝑗 𝑡0𝑖 2 (13) in this case, a new end speed 𝑣2𝑖 𝑗 is necessary and, consequently, condition (4) must be re-evaluated between step ith and step (i+1)-th. this specific behaviour requires the use of a ‘closed-loop’ control. for each i-th step, updated evaluations can also be performed for the (i-1)-th and (i+1)-th steps, if required. for this reason, the step elaborated by the ‘look-ahead’ algorithm is a little further forward in relation to the step that the machine is realising. the new end speed 𝑣2𝑖 𝑗∗ and acceleration time 𝑡0𝑖 ∗ are evaluated in the following way: { 𝑣2𝑖 𝑗∗ = √2 𝑎 𝑖 𝑗 𝑠 𝑖 𝑗 + |𝑣0 𝑖 𝑗 | 𝒗𝟐𝒊 ∗ = 𝑣2𝑖 𝑗∗ 𝑣1 𝑖 𝑗 𝒗𝟐𝒊 𝑡0𝑖 ∗ = |𝑣2𝑖 𝑗∗ | − |𝑣0𝑖 𝑗 | 𝑎 𝑖 𝑗 (14) the opposite situation is illustrated in figure 10. in this case, there is insufficient space for decelerating and, consequently, the start speed 𝑣0𝑖 𝑗 needs to be updated. this occurs when condition (15) is verified: { 𝑠2𝑖 𝑗 > 𝑠𝑖 𝑗 𝑣0𝑖 𝑗 > 𝑣2𝑖 𝑗 (15) { 𝑡2𝑖 = |𝑣2𝑖 𝑗 | − |𝑣0𝑖 𝑗 | −𝑎 𝑖 𝑗 𝑠2 𝑗 = |𝑣0𝑗𝑖|𝑡2𝑖 − 1 2 𝑎𝑖 𝑗 𝑡2𝑖 2 (16) as such, much like in the previous case, the new start speed 𝑣0𝑖 𝑗∗ and deceleration time 𝑡2𝑖 ∗ can be formulated as follows: { 𝑣0𝑖 𝑗∗ = √2 𝑎 𝑖 𝑗 𝑠 𝑖 𝑗 + |𝑣2 𝑖 𝑗 | 𝒗𝟎𝒊 ∗ = 𝑣0𝑖 𝑗∗ 𝑣1 𝑖 𝑗 𝒗𝟎𝒊 𝑡2𝑖 ∗ = |𝑣2𝑖 𝑗 | − |𝑣0𝑖 𝑗∗ | 𝑎 𝑖 𝑗 (17) a last possible situation is presented in figure 11. in this case, the nominal speed 𝑣1𝑖 𝑗 is not reached but there is sufficient distance for both accelerating and decelerating. this behaviour is realised when condition (18) is verified: 𝑠0𝑖 𝑗 + 𝑠2𝑖 𝑗 > 𝑠𝑖 𝑗 (18) { 𝑡0𝑖 = |𝑣1𝑖 𝑗 | − |𝑣0𝑖 𝑗 | 𝑎 𝑖 𝑗 𝑠0𝑖 𝑗 = |𝑣0𝑖 𝑗 | 𝑡0𝑖 + 1 2 𝑎𝑖 𝑗 𝑡0𝑖 2 (19) { 𝑡2𝑖 = − |𝑣2𝑖 𝑗 | − |𝑣0𝑖 𝑗 | 𝑎 𝑖 𝑗 𝑠2𝑖 𝑗 = |𝑣1𝑖 𝑗 | 𝑡2𝑖 − 1 2 𝑎𝑖 𝑗 𝑡2𝑖 2 (20) in this case, a new cruise speed 𝑣1𝑖 𝑗∗ is defined: 𝑣1𝑖 𝑗∗ = √ 𝑣0 𝑖 𝑗2 + 𝑣2 𝑖 𝑗2 2 𝑎 𝑖 𝑗 + 𝑎 𝑖 𝑗 𝑠 𝑖 𝑗 (21) after calculating 𝑣1𝑖 𝑗∗ as described in equation (21), the terms 𝑡0𝑖 and 𝑡2𝑖 can finally be evaluated. { 𝑡0𝑖 = |𝑣1𝑖 𝑗∗ | − |𝑣0𝑖 𝑗 | 𝑎 𝑖 𝑗 𝑠0𝑖 𝑗 = |𝑣0𝑖 𝑗 | 𝑡0𝑖 + 1 2 𝑎𝑖 𝑗 𝑡0𝑖 2 (22) { 𝑡2𝑖 = − |𝑣2𝑖 𝑗 | − |𝑣1𝑖 𝑗∗ | 𝑎 𝑖 𝑗 𝑠2𝑖 𝑗 = |𝑣2𝑖 𝑗 | 𝑡2𝑖 + 1 2 𝑎𝑖 𝑗 𝑡2𝑖 2 (23) figure 10. insufficient space for deceleration; new start speed. figure 11. nominal speed not reached. sufficient space for accelerating and decelerating. acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 36 the previous relations and considerations were implemented into a python script, as illustrated in the flowchart presented in figure 12. at the initial stage, the software evaluates the total number of instructions contained in the part program. then, using a regular expression matching, the movement commands are identified and, for each, a corresponding speed and length is extracted and memorised. the next phases consist of applying the jerk and acceleration phases for each movement command in accordance with the relationships previously illustrated. at the end of the cycle, the total build time estimated by the algorithm is provided. 3. results in order to validate and refine the model, various comparisons were made between the theoretical build time and the actual time required by an fdm machine (german reprap x350). nine objects characterised by different topological and geometrical features were chosen for the experiment, as shown in figure 13. every part was physically realised to obtain the real value of the build time. different cae software (cura, mattercontrol and simplify3d) were used for generating the part program, with the estimations compared both with the actual build time and the estimation calculated by the proposed algorithm. since each cae software program could implement a different slicing strategy, the part program of the same part obtained using different automatic procedures cannot be compared, which is also the case if the same process parameters are used. therefore, for each test case, only one of the three cae software programs was used. table 2 lists the parameters used for the experiment. figure 12. algorithm flow-chart. table 2. process-related parameters used for the experiment. setting value acceleration 300 mm/min² (slow) 1000 mm/min² (rapid) jerk (δv) 18 mm/min layer thickness 0,1 mm maximum speed 100 mm/s (x-y) 10 mm/s (z) 100 mm/s (e) retract length 2 mm figure 13. models used for analysing the accuracy of the proposed algorithm. acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 37 the results of the tests are presented in table 3. here, it is clear that for each of the cae software programs, the error values cannot be ignored. specifically, for the cura platform, the average error for the three analysed test cases amounted to 54 % of the actual build time. a better situation was attained using simplify 3d, which returned an average error of 15 % in relation to the actual build time. however, such as error cannot be accepted in most of the applications. finally, it was confirmed that the optimal behaviour was obtained using mattercontrol. the average error for this software was 8 % of the actual build time. a similar value could be accepted in certain contexts, including the budgeting process. this notwithstanding, two aspects must be noted. first, the error for the test case f was equal to 14 % of the actual build time, which indicates that the error was not characterised by a well-known trend, meaning the level of confidence of the estimations is low. moreover, a parametric method such as that adopted in [34] can provide a better estimation of the actual build time while requiring significantly less time for the set-up. meanwhile, the valuation obtained using the proposed algorithm was highly satisfactory, with the error negligible (≤1 %) in all cases. in fact, for parts b, e and f, the estimation was equal to the actual build time. these results confirm that the proposed method can accurately evaluate the real behaviour of reprap machines. moreover, all the factors that systematically affect the build time were identified and taken into account. 4. conclusions in this paper, a new method for performing accurate estimations of build times was proposed. the resulting accuracy was made possible by an advanced analysis of the part program of the object to be manufactured, which was integrated with the information related to the control strategies. the kinematic behaviour of a cnc machine is partially defined by the part program since the tool movements are also determined by the control strategies implemented in the machine. this is the case with the so-called ‘look-ahead’ algorithm, which is implemented to control the machine movements with the aim of reducing the build time while maintaining the geometrical and dimensional quality of the final object. it was ascertained that the professional cae tools developed for am applications are not able to accurately evaluate the build time required for fabricating objects realised using additive technologies. here, the results appear largely negative since they were obtained from the software that performs the manufacturing process from the given geometric data (.stl file). therefore, with the aim of testing the procedure for determining the build time, the case study of reprap was taken as reference, since it presents a highly diffused controller for am machines. through detailed analysis of the reprap machines, the control strategies were identified and reproduced inside the proposed method. specifically, the method provides a mathematical formulation of the ‘look-ahead’ control strategy (‘jerk phase’). in order to compute the build time, a custom python application was developed. as outlined in the results section, the method provided a highly accurate estimation of the build time. for each of the nine test cases analysed, the error was less than 0.2 % of the actual build time, while in some cases, the estimated time was equal to the actual build time. one limitation of this method is that it requires the part program, which is only obtained following a more-or-less significant number of steps. the approach is thus fairly timeconsuming and would not be the optimal solution when a rapid evaluation is required or when some information is missing, such as specific parameters of the machinery in use (e.g. acceleration and jerk). in these cases, the optimal solution remains the parametric methods, for which the proposed method can present a powerful instrument for the set-up. further work is required 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https://doi.org/10.1007/s00170-011-3394-3 https://doi.org/10.1016/j.cad.2014.05.002 https://doi.org/10.1007/s00170-015-7776-9 https://doi.org/10.1007/s00170-018-2496-6 https://doi.org/10.1007/s00170-012-3924-7 a data-based approach for smart meter online calibration acta imeko issn: 2221-870x june 2020, volume 9, number 2, 32 37 acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 32 a data-based approach for online smart meter calibration fangxing liu1,2, chengbin liang2,3, qing he1,2 1 harbin institute of technology, harbin, p. r. china 2 national institute of metrology, beijing, p. r. china 3 college of electrical and information engineering, hunan university, changsha, p. r. china section: research paper keywords: smart meter; online calibration; meter error citation: fangxing liu, chengbin liang, qing he, a data-based approach for smart meter online calibration, acta imeko, vol. 9, no. 2, article 6, june 2020, identifier: imeko-acta-09 (2020)-02-06 editor: alexandru salceanu, technical university of iasi, romania received january 17, 2020; in final form march 24, 2020; published june 2020 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work is supported by the national key r&d program of china under grant no.2016yff0201201. corresponding author: qing he, e-mail: heqing@nim.ac.cn 1. introduction the smart meter is an important component in a smart grid. the performance and measurement accuracy of smart meters is directly related to power grid safety and energy trade settlement [1]. consequently, it is particularly important to determine the operation error state of smart meters [2]-[3]. a smart meter is not the same as a traditional electricity meter. in addition to the basic function of metering electricity, a smart meter has two-way communication and multiple measurement modules, such as the data transmission and anti-stealing module. it represents the direction of the development of the intelligent terminal of future energy-saving smart grids. the widespread popularity of smart energy meters enables the collection of a large amount of electricity data, such as energy consumption, voltage, and current data. in china, 460 million smart electricity meters have been installed and in us and uk, the deployed meter number is 70 million and 2.9 million [4]. with the increasing number of deployed smart meters, the verification of massive meters tends to be an important task for energy companies. as a significant indicator of a meter’s working status, a meter with the error that it exceeds the official specific threshold will be classified as an abnormal meter. at present, onsite and laboratory checking are two main ways in which power companies verify meter accuracy [5]. if a meter needs to be detected, the electrician should remove the meter from the site and install a new one, and they should then send the removed meter to professional agencies to test its metrological character. it is a time-consuming and costly work to detect millions of smart meters. many smart meters are designed to be used for more than ten years. in this way, an energy company can determine in which year the energy meters should be replaced. for example, china smart grid removes meters in the eighth year after the meter’s installation in order to reduce the testing costs. furthermore, some energy providers conduct random sampling to pick up parts of meters to test their performance. the staff carry instruments to conduct on-site field tests at the meter installation site. then the status of every meter can be inferred from the on-site test results. however, the disadvantage of these two existing methods is that they cannot verify the meter error of all meters. as a result, it is difficult to decide whether the smart meter should continue to be used or if it should be removed. advanced metering infrastructure (ami) is the basic component of a smart grid, which enables large-scale measurement data collection and transmissions in a power grid. with the rapid development of ami, an increasing amount of data can be acquired from a smart meter [6]-[8]. smart meter data analytics has become a popular research topic, saturated with the increasing volume of data collected by meters. many novel abstract the smart meter has been considered the key element in the smart grid. the current smart meter verification method mainly focuses on on-site and laboratory detection and fails to test all deployed meters. online calibration is a novel approach that calculates meter error by analyzing meter reading data. by presenting a comprehensive survey and basic model of online smart meter calibration, this study proposes a recursive algorithm in order to estimate meter error. concrete steps are subsequently illustrated. the following testing case shows the higher accuracy of the method. finally, some issues related to the application of online calibration are discussed. mailto:heqing@nim.ac.cn acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 33 methods and algorithms have been developed to analyse meter data in order to discover the internal information. energy data analysis includes consumer profiling, load forecasting, electricity pricing, identification of irregularities, metering, and real-time operations [9]-[12]. consumption data analytics is helpful for constructing a consumer behaviour model and forecasting consumer consumption levels. due to the emergence of ami and the broad application of enormous intelligent meters, it is possible to carry out more accurate load forecasting by using a large amount of household load data [13]-[15]. the energy pricing strategy can be updated according to consumption data analytics because the energy data shows the peak and troughs of daily electricity usage. abnormal data can be detected by mining the inner relationship among energy usage data. furthermore, data analytics is a powerful and effective method when implementing the monitoring the equipment in the power grid [16]. online calibration for smart meters is a novel method that computes meter errors by processing meter reading data. due to the increased digitisation of smart meters, multiple types of energy data can be stored and transmitted to energy data centres. those multiple types of data contain information about energy consumption, voltage, current, time, location, and installation condition. to analyse the data and estimate the meter error, online calibration is a new idea that can solve the problem of ineffective meter detection. by analysing the energy usage data, the errors of all meters in a low-voltage system can be calculated at the same time. then, a meter error that exceeds the threshold will be classified as an abnormal meter. there are few studies on online smart meter calibration. korhonen [17] introduced a method by which energy consumption data were used to evaluate meter error and energy loss. however, this method does not work well in a large energy loss rate system. in [18], kong proposed a double-parameter based least squares algorithm to estimate meter errors. in [19], an algorithm combined with cluster theory and regularization was proposed to calculate meter error. this method performs better when the system energy loss rate is low. in [20], peng adopted the monto carlo method to estimate combined meter error. in the relevant work on this topic, energy data analytics has been widely used in load forecasting and abnormal equipment detection. online calibration is a novel idea that estimates meter error by analysing meter measurement data. the traditional meter calibration process includes the verification of the initial installation of electricity meter; regular on-site and laboratory meter error testing; periodic replacement; and so on. these steps are extremely complex and costly when detecting a huge number of smart meters, while online calibration can mitigate this shortcoming well. in this article, a method is proposed for the online estimation of meter error. this method provides a theoretical basis for the online calibration of large-scale smart meters. the meter’s deployment structure and low-voltage system model are analysed. our meter error estimation process has three steps. first, we filter the abnormal data and cluster the data with similar features. then, an approach based on recursive least squares is adopted to estimate meter error. finally, we compare the estimated meter error with a threshold to determine an abnormal meter. 2. online calibration 2.1. low-voltage network smart meters are deployed in a low voltage network. as shown in figure 1, there is a transformer connecting the information centre and the smart meters. the energy meter installed under the low-voltage side records the consumption of the energy user. all data collected by the meter are transmitted to the information centre through the communication system. wireless, power line carrier, and rs485 are typical power information transmission systems in a power grid. the information centre collects meter data at a specific frequency, such as every fifteen minutes or one hour. the collected data contains information about the current, voltage, power, time, meter number, address, load rate, and average power factor of the energy grid. 2.2. online calibration model smart meters are topologically deployed in the low-voltage system, which means a master meter is installed in front of some submeters. the master meter has higher accuracy than the submeter and is considered as the standard meter in the system. we assume the master meter has been calibrated and the reading is accurate. electric energy passes through the master meter and then to all the power users, whose consumption is recorded by the submeters. the consumption value that the master meter records equals the sum usage value of the submeters in an ideal way. then, we have the equation: 0 1 ( ) ( ) n j j e i e i = = (1) where ej(i) is the true energy consumption on branch j recorded by submeter j, and e0(i) is the true energy consumption on all branches recorded by the master meter at the ith measurement. however, the meter has an error, and the meter reading is not the true energy consumption. furthermore, there are energy losses in the system due to the meter’s own consumption, connection, cabling, etc. to take this into account, equation (1) can be rewritten as: 0 1 0 ( ) ( ) ( ) 1 1 n j j j i i e i    = + = + +  (2) where j is the relative error of meter j, and j(i) is the reading of submeter j. 0(i) is the reading of the master meter. e(i) is the … 1 m 2m 1n m − n m 0 m … information centre transformer meter figure 1. low-voltage network. acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 34 energy losses in the system. after n measurements, we can write a matrix equation: t y  = (3) where: 1 2 1 2 1 2 0 01 0 2 0 0 0 (1) (1) (1) (2) (2) (2) ( ) ( ) ( ) (1)1 (1) 11+ (1) (2)1 (2)(2) 1+ 1 ( ) 1 ( ) ( ) 1+ 1 i i n n n n n e e y n n e n                           =          −   +           −     + = = =               −   +    (1) (2) ( ) y y y n      =            (4) the measurement number should not be less than the submeter number n, such that the relative error can be computed by solving the linear equation. the energy losses mainly consist of self-consumption and power line losses. the mathematic formulation of ei is as follows [21]: 1 ( ) ( ) ( ) ( ) t 100 n i j j e t u t e t p t t k =  = +  （ ） (5) where j p is the rated power of the smart meter. e(t) is the master meter reading, and u is the loss rate of the voltage. kp is a factor related to the voltage between the submeters and master meter. 2.3. recursive meter error estimation the advanced metering infrastructure enables the information centre to collect and store a large amount of data. the effective analysis of those meter data could enhance estimation accuracy. solving the n×n linear equation can result in an error estimation, but there is a limitation. it is an off-line method and only uses the n times measurement without considering other measurement data. the data quality of the meter reading strongly affects the solution’s accuracy because one abnormal data could change the matrix property and then lead to an unfavourable result – even no result. the recursive algorithm is an effective method of estimating the meter error by updating new data. the least squares method seeks to minimize the squares of the difference between the measured data and the fitted data, and the cost function is defined as follows: 2 1 ( ( ) ( ) ( )) n i j y i i i  = = − (6) j is minimised by calculating partial derivatives to each : ( ) 1 2 ( ) ( ) ( ) ( ( )) 0 n j ij j y i i i i    =  = − − =   (7) then,  in the classic least squares model is solved, and the error parameter is estimated as: 1ˆ ( ) t t y −  =    (8) to impose weights on the measurements with suitable weights  and the weighted least squares estimation for  is: 1ˆ ( ( ) ( ) ( )) ( ) ( ) ( ) t t n w n n n w n y n −  =    (9) where 1 2 0 0 0 0 0 0 n w w w w      =       (10) in the recursive weight least squares model, the new model (n+1) is calculated as the sum of the previous estimates. the recursive equation is written as follows: ˆ ˆ ˆ( 1) ( ) ( )( ( 1) ( 1) ( )) t n n n y n n n + =  + + − +  (11) where ˆ ( 1)n + is the new estimation parameter, and (n) is the update factor. the n+1 measurement is 1ˆ ˆ( 1) ( ( 1) ( 1) ( 1)) ( 1) ( 1) ( 1) t t n n w n n n w n y n −  + =  + +  +  + + + (12) using the exponential forgetting factor in the weight matrix and the estimation equation results in: ˆ ˆ ˆ( 1) ( ) ( 1) ( 1)( ( 1) ( 1) ( )) t n n p n n y n n n + =  + + + + − +  (13) where 1ˆ( 1) ( ( 1) ( 1) ( 1)) t p n n w n n − + =  + +  + (14) calculating p and the updating factor, the recursive weight least squares are: 1 ( ) ( ) ( 1)( ( 1) ( ) ( 1) ) t n p n n n p n n   − = +  +  + + (15) 1 ( 1) ( ( ) ( 1)) ( ) t p n i n n p n   + = − + (16) 2.4. the framework of error estimation in order to estimate meter error, abnormal data should be filtered at first. then, data with similar features should be processed and clustered. next, the linear equation should be constructed using the clustered data. finally, the equation should be solved in order to obtain the meter error estimator. figure 2 shows the framework of online error estimation: 1) loading and screening data: given the smart meter data and check each measurement ej(j). the measurement ∑ 𝐸𝑗 (𝑖) > 𝐸0(𝑖) 𝑛 𝑗=1 ,which means the sum of the submeters exceeds the sum of the master meters, will be classified as abnormal data. those data will be removed from the data set. 2) the meter self-consumption and online energy losses are calculated by equation (5). energy losses are subtracted from the master meter reading in order to reduce the energy influence on the meter error estimation. 3) data pre-processing: data is clustered using the kmeans algorithm in order to collect data with similar features [21]-[23]. a submeter matrix can affect the solution of the linear equation. the data sets with high correlation will lead to an ill-condition matrix that is difficult to solve. clustering data with similar characters into a group and pick up the data from different groups to build the matrix. this matrix is a low ill-condition matrix. acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 35 4) meter error estimation: the linear equation with the recursive algorithm in section 2.3 is solved. initialising (0) = 1, ∑ 𝑤(0) = 0, p(0) = i.  is a small constant. for an n submeters system, p(0) is an n  n matrix and (0) is a n1 vector. the meter error can be obtained through the solution. root mean square error (rmse) is a method of verifying the effectiveness of a calculation algorithm. the formula of rmse is: 2 1 1 ( ) n i i i rmse n   = = − (17) a smaller rmse value indicates a better estimation. 3. error estimation in order to verify the validity of the algorithm, we selected a dataset from a typical low voltage system in an urban area as the testing case. there are 123 electricity meters with one master meter and 122 submeters deployed at this site. meter measurement data were collected for three years, from 2014 to 2016. the master meter measured all the consumption at the transformer secondary, and all consumption data were recorded and sent to the energy centre at 0:00 every day. the dataset has no information about meter error, which we simulate. we randomly assign large meter errors to the submeters. according to the official smart meter regulation, a meter error above 2 % is classified as an abnormal meter. we start by screening and filtering the abnormal data, then we cluster the data in order to construct the meter data equation. the meter errors estimated by the recursive algorithm are shown in figure 3. meter errors are calculated once a day – at the same frequency as the data acquisition. the x-axis represents the number of measurements and the y-axis shows the meter error in percentage form. the error estimator varies greatly in the initial 200 measurements, then it seems to be a stable value after 500 measurements. it can be shown that three meters estimated errors exceeding 2 %. all the submeters’ estimated results are gathered to verify the effectiveness of the method, as shown in figure 4. it can be seen that most meter errors fall into an acceptable range, with an absolute value of error below 2 %. meters 36, 78, and 91 were abnormal meters, with errors 7.969 %, 6.537 %, and 5.871 %, respectively. the rmse of this estimation is 0.22 %, which means the deviation between the estimator and the true meter error is very small. this is an effective approach to the calculation of meter error. 4. discussion the basic idea of online calibration has been introduced in previous sections. it is an effective method in the energy system with a master deployed in front of submeters. however, the real low-voltage system is a complex energy network, and the meter’s data quality is not as good as we expected. there are several issues in online smart meter calibration application. 1) data collection in order to construct and solve the equation (3) successfully, energy consumption data from all meters should be collected. the more data collected, the more accurate the meter error estimation result will be. in some energy systems, the data are transmitted once a day, while meter data are collected every 15 minutes in other systems. in a 200-submeter system, the matrix requires at least 200 measurements. it takes a long time for a system to store the data if the data are collected only once a day. read meter data 1 (t),2(t),…….,n (t), screening and removi ng abnormal data energy loss es estimation pj(t)t, ei(t) data pre-processing determine cluster center number n and initial centers c and relation matri x v cal culate j(c,v) and update cluster center when j(c,v)< ,obtain final cluster center construct submeter matrix and initial (0),w(0),p(0) update (i),w(i),p(i) and estimate meter error obtain 1(i),2(i),3(i),…… ,n(i) cluster recursive estimation figure 2. framework of online error estimation. figure 4. meter error estimates. figure 3. estimate error from different measurements acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 36 data consistency is also an important consideration. in the framework of online calibration, the data from the master meter and submeters should be collected at the same time. in real data collection, some of the submeters fail to receive the data transmission signal on time and transmit the delayed data to the control centre. those nonsynchronous data adversely affect the validity of the online calibration model. data collection is an important foundation in remote calibration. this is an issue that analysts can hardly solve in a mathematical way. it is necessary to confirm the synchronization of measurement data collection. 2) energy system in a normal low-voltage system, a master meter is deployed in front of the submeters. the master meter is three-phase energy meter, while the submeters are single-phase energy meters. in the low-voltage system model, all submeters should be installed in the same phase lines so that the master meter energy consumption reading is equal to the sum of submeters. however, the real energy system may mix with the submeters and place them in different phases. two more submeters connected with each other in different phase branches will affect the energy conservation relationship. this will further significantly influence the equation solution and lead to a biased error estimator. 3) abnormal data abnormal data always exists in a power system. power theft and environmental changes will generate abnormal data. the stolen electricity in the system is only recorded by the master meter. so, those abnormal data will break the equality relationship and damage the meter error solution. electricity theft is a worldwide problem, and researchers have attempted to address the problem by analysing smart meter data. supervised learning methods, such as support vector machines (svm), are effective approaches to detect the abnormal data. they first train an abnormal data detector and then input the real data to classify and determine the bad data. a pre-processing of smart meter data will be a great help in the completion of the online error computing. 4) energy losses energy losses always exist in the energy network. most energy losses happen on the power line and the equipment selfconsumption. in the low voltage system, the smart meter selfconsume is fixed. the accuracy of meter error estimation declines with increasing energy losses online. the energy losses are difficult to estimate due to the lack of system parameters such as the length of power line from each master meter to submeter. the deviation of energy loss estimation will bring some issues to the remoting meter calibration. 5) analytical methods the solution to the equation and estimation of the meter error is one of the most effective methods of online calibration. however, the imperfect equal relationship in the equation will influence the equation solution. many algorithms have been proposed to improve the accuracy of meter error estimation on estimating meter error. new technologies involving computer science, particularly machine learning, will be powerful tools in meter data analysis. classification, regression, and deep learning methods are popular in smart grid data analytics. transfer learning and incremental learning have also been used in smart meter analytics. however, when introducing one algorithm to solve a problem, the meaning and physical implications thereof should be taken into consideration carefully. 5. conclusions this article explained the online calibration principle and proposed a recursive algorithm approach to estimate meter error. the case analysis showed the excellent performance of this method. online calibration is a promising metrological method, as are the development of big data acquisition systems and advanced data analytics techniques. online meter error estimation and calibration is an emerging research area, which has a broad application and great economic value. we hope this article can provide readers with a picture of and insights into the usage of meter data and online meter calibration. acknowledgement this work is supported by the national key r&d program of china under grant no.2016yff0201201. references [1] d. alahakoon, x. yu, smart electricity meter data intelligence for future energy systems: a survey, ieee transactions on industrial informatics 12 (2016) pp. 425-436. [2] j. hu, a. v. vasilakos, energy big data analytics and security: challenges and opportunities, ieee trans. smart grid 7 (2016), pp. 2423–2436 [3] y. kabalci, a survey on smart metering and smart grid communication, renewable and sustainable energy reviews 57 (2016) pp. 302-318. [4] y. wang, q. chen, t. hong, c. kang, review of smart meter data analytics: applications, methodologies, and challenges, ieee transactions on smart grid 10(2018), 3125-3148. [5] m. huang, y. shi, n. lin, z. m. lin, y. liu, y. s. he, common faults and treatment methods in the verification process of electric meter verification device, electr. eng. 2 (2018) pp. 89-91. [6] f. benzi, n. anglani, e. bassi, l. frosini, electricity smart meters interfacing the households, ieee transactions on industrial electronics 58 (2011) pp. 4487-4494. [7] a. bidram, a. davoudi, hierarchical structure of microgrids control system, ieee transactions on smart grid 3 (2012) pp. 1963-1976. [8] f. l. quilumba, w. lee, h. huang, d. y. wang, r. l. szabados, using smart meter data to improve the accuracy of intraday load forecasting considering customer behavior similarities, ieee transactions on smart grid 6 (2015) pp. 911-918. [9] y. wang, q. chen, d. gan, j. yang, d. s. kirschen, c. kang, deep learning-based socio-demographic information identification from smart meter data. ieee transactions on smart grid 10(2019) pp. 2593-2602. [10] a. k. marnerides, d. p. pezaros, h. kim, d. hutchison, internet traffic classification using energy time-frequency distributions, proc. of the ieee international conference on communications ieee, budapest, hungary, 9-13 june 2013. [11] x. tong, r. li, f. r. li, c. q. kang, cross-domain feature selection and coding for household energy behavior, energy 107 (2016) pp. 9-16. [12] a. jindal, a. dua, k. kaur, m. singh, n. kumar, s. mishra, decision tree and svm-based data analytics for theft detection in smart grid, ieee transactions on industrial informatics (2016) pp. 1005-1016. [13] j. bedi, d. toshniwal, empirical mode decomposition based deep learning for electricity demand forecasting, ieee access 6 (2018) pp. 49144-49156. [14] w. kong, z. y. dong, d. j. hill, f. luo, y. xu, short-term residential load forecasting based on resident behaviour learning, ieee transactions on power systems 33 (2018) pp. 1087-1088. [15] h. shi, m. xu, r. li, deep learning for household load forecasting: a novel pooling deep rnn, ieee transactions on smart grid 9 (2018) pp. 5271-5280. acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 37 [16] h. janetzko, f. stoffel, s. mittelstadt, d. a. keim, anomaly detection for visual analytics of power consumption data, computers & graphics 38 (2014) pp. 27-37. [17] a. korhonen, verification of energy meters using automatic meter reading data, master’s thesis, aalto university, espoo, finland, 2012. [18] x. kong, y. y. ma, x. zhao, y. li, y. x. teng, a recursive least squares method with double-parameter for online estimation of electric meter errors. energies, 12(805) (2019) pp.1-12. [19] f. liu, q. he, s. hu, l. wang, z. jia, estimation of smart meters errors using meter reading data, proc. of the conference on precision electromagnetic measurements (cpem), paris, france, 8-13 july 2018. [20] x. j. peng, l. wang, z. s. jia, x. w. wang, h. t. huang, l. j. liu, evaluation of digital energy meter error by monte carlo method. conference on precision electromagnetic measurements (cpem 2016), ottawa, canada, 10-15 july 2016. [21] h. tang, research on line loss analysis and loss reduction measures in low voltage substation. master’s thesis, southeast university, nanjing, china, 2017. [22] p. arora, s. varshney, analysis of k-means and k-medoids algorithm for big data, procedia computer science, 78 (2016) pp. 507-512. [23] a. k. jain, data clustering: 50 years beyond k-means, pattern recognition letters 31(2010) pp. 651-666. concepts to improve the quality of production plans using machine learning acta imeko issn: 2221-870x march 2020, volume 9, number 1, 32 39 acta imeko | www.imeko.org march 2020 | volume 9 | number 1 | 32 concepts for improving the quality of production plans using machine learning lukas lingitz1, wilfried sihn1,2 1 fraunhofer austria research gmbh, theresianumgasse 27, a-1040, vienna, austria 2 tu wien, institute of management science, theresianumgasse 7, a-1040, vienna, austria section: research paper keywords: production planning; planning quality; master data; prediction; machine learning citation: lukas lingitz, wilfried sihn, concepts to improve the quality of production plans using machine learning, acta imeko, vol. 9, no. 1, article 6, march 2020, identifier: imeko-acta-09 (2020)-01-06 editor: lorenzo ciani, university of florence, italy received november 6, 2019; in final form february 10, 2020; published march 2020 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was supported by the european union under the programme electronic component systems for european leadership (ecsel) eu, project: power semiconductor and electronics manufacturing 4.0 (semi40) (grant agreement no. 692466). corresponding author: lukas lingitz, e-mail: lukas.lingitz@fraunhofer.at fehler! linkreferenz ungültig. 1. introduction industry 4.0, cyber-physical production systems (cppss) [2], [3], and the industrial internet of things have a significant influence on production planning and control (ppc) today and will do so in the future. deploying cppss raises several challenges for industries, as addressed in [4]. these challenges include the extraction of knowledge from heterogeneous data sources; interoperations with production information systems; and new possibilities in terms of the changeability, adaptability, and reconfigurability of production systems. compared to traditional production planning, which is based on a generally static knowledge base, smart factories enable the collection and exchange of real-time information between products, machines, processes, operations [5], and systems. the application and exchange of data by the elements of a smart factory lead to an automated and decentralised production, which is an essential characteristic of industry 4.0 [6], [7]. however, there is a need to study how the different solutions – enabled by digitalisation – can support ppc and contribute to an increased corporate competitiveness [8]. planning quality (pq) is a commonly used term in industrial practice when discussing ppc. however, the term is not clearly defined in the scientific context. therefore, the authors give a proposal for a new definition of the term, trying to set a widely accepted standard. after defining pq, a case study is given, wherein the suggested pq has been increased through the application of machine learning (ml) for lead time prediction. lastly, the authors give an outlook of two novel approaches that will be the main subject of an ongoing research project. the paper is structured as follows. in section 2, the authors give a literature review concerning pq and the application of ml in the field of ppc. in section 3, a definition of the term pq is given. a case study from the semiconductor industry in section abstract there are always deviations between production planning and subsequent execution. furthermore, it has been found that the reliability of production plans and thus planning quality (pq) can drop down to 25 % in the first three days after plan creation [1]. these deviations are caused by uncertainties, such as inaccurate or insufficient planning data (including data quality and availability); inappropriate planning and control systems; and unforeseeable events. production planners therefore use buffers in the form of inventories or extended transitional periods to create possibilities for implementing corrective measures in production control. buffers, however, lead to increased coordination and control efforts as well as to negative effects, particularly on the inventory, throughput time, and capacity utilisation. the potential for more accurate planning remains largely unexploited. the objective of this paper is to investigate the possibilities of increasing planning quality. within a case study, the authors demonstrate how machine learning can be used to predict cycle times. furthermore, the increased accuracy compared to the current method is shown. based thereon, two approaches are presented, focusing on the reduction of gaps between the master data and predicted data used during the production planning process. moreover, further research needs are identified. mailto:lukas.lingitz@fraunhofer.at acta imeko | www.imeko.org march 2020 | volume 9 | number 1 | 33 4 demonstrates the accuracy of ml for cycle time prediction compared to the currently used static approach. section 5 relays the results of the case study and presents different approaches to how to improve pq by applying ml. in section 6, the authors discuss these approaches and identify further research needs. finally, a conclusion and outlook are given. 2. related literature literature on ppc approaches the phrase pq from different angles, considering several influencing factors. the first aspect focuses on classical logistical targets of ppc, such as flow times, due dates, setup costs, and product features. since accurate and high-quality planning data is one of the most important parts of a good production plan, master data management is an important aspect in our discussion as well [9]. these first two approaches do not deal with (planned or unforeseeable) changes or uncertainties of the production environment, which are typical elements of the paradigm shift towards industry 4.0 [10]–[12]. therefore, robustness and resilience consider such disturbances. a production plan is robust if its performance is guaranteed – even when facing events not known at the time of planning [13], [14]. resilience is, in contrast, the ability of a system to cope with changes of all kinds [15]. two efficient ways of dealing with uncertainty are the application of stochastic fuzzy models and the use of adaptive and cooperative approaches [16]. the overall objective of ppc is the creation of reliable production plans, so their realisation on the shop floor should be close to – or ideally the same as – the production plan as it was originally planned. the deviation between planning and reality on the shop floor increases up to 75 % after just three days in medium-sized mechanical engineering enterprises [1]. as shown in the literature review, it is desirable to have more reliable production plans. measuring the quality of the prediction, as an alternative, may reveal a potential for bridging the gap between planned and actual figures. besides, the success of a good production plan depends on the decision-making process itself. in the era of industry 4.0, the automation of decision-making processes and the level and way of human engagement are also essential topics [17], [18]. it can be concluded that there is no exact definition of pq. in this paper, a novel industry-oriented concept for measuring, evaluating, and improving pq will be developed. a general truth is that data does not bring any added value on its own, but in practice, domain-specific knowledge and algorithms are needed to extract useful information from heterogeneous and scalable data sources [19]. simple statistical analysis is often not sufficient, as it is time-consuming and often does not lead to the desired results. hence, automated data extraction and analytical methods are needed. together with the rise of data science as one of the most popular emerging research and application fields today, ml has gained increasingly high attention in the recent past. at the very beginning of the development of ml, the vast majority of papers were published in journals related to the topic of computer science. however, with increasing demands of computational capabilities and big data analytics, the area is growing, with far-reaching applications in diverse disciplines. nowadays, many different disciplines use ml algorithms, as experienced in 2012 [20]. the first ml applications in management science can be found in finance and marketing [21]. in 2009, choudhary et al. identified that the emerging application of ml in ppc has not been systematically explored [22]. however, in the following years, several research papers were published in the context of production management, focusing on applying ml to advanced planning and scheduling [23]; quality improvement; process monitoring; and defect analysis [24]. yet, researchers have not intensively focused on (sub-)topics relevant to ppc – such as flow time prediction, lot cycle time prediction, and lead time prediction; thus, the improvement potential has not been completely identified and maximised. the results of our literature review determine that the current trend in ppc is to employ ml-based simulation and optimisation algorithms. furthermore, it can be recognised that the focus of most of the analysed publications is either given to production scheduling (47 %) or other applications (33 %), while the prediction of planning relevant times is rarely considered (20 %), as shown in [19]. 3. planning quality – definition until now, there has been no uniform mathematical definition and understanding of the term pq in the scientific literature within the framework of ppc. therefore, we briefly explain and define what is meant by the term pq as we see it. pq is foreseen as a key indicator for the planner to assess the reliability of the production plan in the planning phase (time t) and to continuously improve the operational reliability of production plans in forthcoming phases (time t+n). in principle: • the pq is high if there are ideally no deviations or at least deviations within an acceptable range (which can vary depending on different industrial contexts) between the production plan created in advance and the actual execution. based upon this general statement, we need to derive some measurable values to determine exactly if the production plan and the execution show deviations or not. on a macroscopic level, we could say that pq is high if at least the calculated end date of the last production step of a production order from the production plan meets the actual end date. we can also break this assumption down for every operation. this would mean that pq is high if every operation/production step of the production order starts and ends on the designated day/shift/hour calculated during the plan creation. this leads to the conclusion that the times that are used for the plan creation need to be very accurate so that the start and end times of the setup, operation, transition, and waiting not only meet on average – if we look to a longer period of time – but also need to match the reality for every order in every situation (e.g. overand underload). this brings us to the next definition: • the pq is high if there are ideally no deviations or at least deviations within an acceptable range (which can vary depending on different industrial contexts) between the planning times created in advance and the actual times that result during the execution phase. there are many factors that could influence the actual times and cause deviations between the planning times and the actual times. one of the main problems is that we normally use static average values for the planning times. if we would use ml models with a high prediction accuracy and trained with confirmation data from the actual production system to predict the actual times, we could reformulate the statement from above: • the pq is high if there are ideally no deviations or at least deviations within an acceptable range (which can vary depending on different industrial contexts) between predicted times and actual times (e.g. lead time, setup time, and operation time). accurate dynamic prediction models acta imeko | www.imeko.org march 2020 | volume 9 | number 1 | 34 are required to continuously reduce the deviation between reality and prediction and • there are ideally no deviations between the static planning times used for the production plan creation and the predicted times or • instead of static planning times, the prediction model itself is used to create production plans that have a high pq. based on the general statement that pq measures the deviation between a production plan and the later execution, we now have two possibilities for creating production plans with higher pq. we achieve this by having accurate prediction models of the planning times on the one hand and by using these models during the planning phase on the other hand. the definition shows two methods. the first option is to adapt the planning times to match the predicted times. in this case, we assume that the planning times are still one static value, so we might adapt the times in several iterations until we reach a convergence point. alternatively, we can use the dynamic time models, which can be considered as a function with several variables as the input for the plan creation. the above definitions and statements give rise to the conclusion that there are two possible ways of creating production plans with high pq. these two methods are shown in two approaches that are described and discussed in sections 5 and 6 and are subject to further research within a funded research project. within the upcoming case study, the possibility of predicting planning times based on confirmation data is demonstrated. 4. case study on time prediction in this section, the authors give an example of the application of ml for time prediction. after an introduction to the related industrial sector of the use case partner and a general description of the production system, the approach and the results are given. based on the results, ideas of how to apply ml within the planning process are stated. 4.1. introduction as semiconductor industries offer a high availability of data for the production process, these industries take the lead when it comes to innovative applications in the area of industrial data science. furthermore, the market is highly competitive, and high productivity as well as short lead times (among others) are the key factors for success. most products in the semiconductor industry are built in layers, where the same or similar production steps are repeated in cycles to build integrated circuits on the layers. nevertheless, most production systems do not consist of rigidly linked machines. rather, they are made up of highly automated machines that perform specific operations organised in a job shop-type production system. the often broad product spectrum is driven by rapid product innovations and results in a high material flow complexity. therefore, constant sequencing is needed. within the research project, the goal has been to increase the pq of a semiconductor manufacturer. due to good data availability, the authors decided to apply ml techniques to build prediction models for lead time prediction that can subsequently be used to analyse the accuracy of the production plans. as the company had no simulation model of the plant and the authors did not build a model, the prediction models were solely built with readily available data from the shop-floor it systems (enterprise resource planning [erp] system and manufacturing execution system [mes]). 4.2. approach and results after collecting the process confirmation data from the mes, historical data about machines/equipment/work centres and customer-related information, features were generated based on the domain knowledge and experience of the process experts. furthermore, some features that were relevant from a production logistics point of view were added. many features that were created can be parametrised in a way such as ‘average lead time of the last x lots’, where x can be any positive integer number. the feature catalogue with the most important features can be found in [19]. according to the domain experts, the overall lead time (hereafter defined as the time span from the beginning of the first operation to the finish of the last operation of an order) is influenced by three main process steps. therefore, we started the analysis with these steps. after calculating the features, several regression methods were tested to predict the cycle time. for the evaluation of the accuracy, the normalised root mean squared error (nrmse) was chosen as the measurement index. the nrmse is the square root of the mean of the squares of the deviations between a predicted value and its actual regression dependent value divided by the range of the biggest and smallest regression dependent value for normalisation purposes. the normalisation makes different datasets and models comparable. the conclusion is that ensemble tree-based methods (bagged regression trees, random forests, and boosted regression trees) outperformed all tested regression algorithms. furthermore, these methods give quantitative feedback about the feature’s importance i.e. the importance of a variable on the prediction. after these experiments, the scope of the study was broadened (time frame, products, and process steps). as a result, the lead time prediction for 80 process steps was done. also for the broader scope random forest outperformed linear regression, based on different aspects (the need for data cleaning, the training time, and the accuracy of the model). to evaluate the advantages of the proposed approach compared to the current approach – creating production schedules with static target cycle times for every layer of every different product, updated in each quartal – the dynamic cycle time prediction was table 1. nrmse values for the different cycle time calculations. productid average ml number of lots 1 17.9 11.7 2228 2 12.5 12.7 1103 3 15.9 13.7 964 4 11.8 18.0 237 5 23.1 18.4 289 6 20.6 18.8 345 7 23.7 20.4 142 8 27.4 25.1 118 9 25.9 29.0 32 10 56.0 41.0 11 11 37.4 43.4 32 12 19.4 66.3 145 13 18.3 7 14 117.8 9 acta imeko | www.imeko.org march 2020 | volume 9 | number 1 | 35 done for each layer of the 14 most important products with the random forest algorithm. a comparison of the nrmse values is given in table 1 for the currently applied static planned cycle times (average) and for the predicted dynamic cycle times (ml). in most cases, random forest gives a more accurate prediction of the cycle time, as the nrmse values are mostly smaller, especially when the sample is bigger. this is of high relevance in industry, as the lead times, in general, are three to seven weeks long, and even small deviations between predicted and actual end dates, calculated based on the lead time, cause a delay in shipping of several days. however, this means that the prediction accuracy is sensitive to the amount of data, and under a certain limit, no cycle time prediction with ml could be done. the inclusion of products with a minimum of 200 samples (productids 1 to 5) leads to the mean nrmse values for the predicted dynamic cycle times (ml) of 15.5 % and 17 % for the static planned cycle times (average). to highlight the difference between the static and dynamic approach, the real cycle times (black dots) are compared with the currently used static (orange line – some outliers are not printed due to the scaling of the graph) and the predicted dynamic (green dots) cycle times for a given product and layer in figure 1. it must be mentioned that a rolling forecast was applied to capture trends. this means that the ml algorithm learns from the past 2000 observations and predicts the next 100 observations. therefore, for the first 2000 observations, no prediction is available. 4.3. outcome of the project for the deployment, an interactive web application was developed to support the whole cycle – data cleaning, outlier detection, feature generation, model training, and prediction. the whole process is depicted in figure 2. for the development of the web application, the authors used the free static programming language ‘r’ with the respective package ‘shiny’. the user can upload new data – exported from different it systems – in a specific predefined format. after uploading, automated data cleaning and an outlier detection process were executed. several algorithms prepared the data for the next step of feature generation. the planner can select a set of different predefined features (e.g. the average lead time of the last x lots, where x can be any positive integer number defined by the user, for more see [19]) and add those features to the dataset (1). some of the features are parameterisable, as explained above. the user can try different parameters and add the same feature with different selected parameters. in the next step, the user can train (2) different models. he is able to set a number of observations for the training and a number of predictions before the next training. for evaluation, he gets the nrsme (3) and can also see the predicted values and compare them to the currently used target values. based on the results, he can adapt the parameters of the features and select the best features (4) until he reaches an acceptable accuracy level. lastly, there is also the possibility of getting a prediction of a certain lot by entering the values of the features (5). an automated implementation of the developed prediction model for the current planning process is in progress, as the features used for training must also be calculated from the production plan itself. for this reason, new approaches need to be developed and researched further in order to be integrated into the application. what can be clearly seen is that even if the underlying planning data is updated regularly (in the current case, every three months), the cycle time is highly dependent on several influencing factors that cannot be depicted in a single, static planning value. numerous influencing factors can be exploited with the help of different ml methods. this knowledge – gained from the available dataset – can be used to improve the quality of ppc in cppss. 5. approach of the integration of machine learning with production planning in this section, the authors pick up the ideas from section 3 and discuss the two possible approaches mentioned above. in the first approach, the planning times are adapted to match the predicted times in several iterations. this approach is called the ‘evolutionary approach’. in the second approach, the dynamic time models are used directly for plan creation. this approach is called the ‘function-based approach’. 5.1. evolutionary approach the overall approach consists of six steps, numbered from 0 to 5, while steps 1 to 5 run in ongoing loops and are depicted in figure 3. it is assumed that in every iteration, pq increases. this iterative process aims for continuous improvement and is the reason why the authors named the approach ‘evolutionary’. the planning starts with the generation of an initial production plan, whereby the planning itself is carried out by the conventional planning system, e.g. the erp or mes of the company, and the planning data is extracted from the master data of the underlying system (steps 0 and 1). the production plan is the input for the second step. in step 2, ml models that have been trained by utilising historical data from the company beforehand are employed to predict different planning times, such as the cycle time, setup time, and operation time. in step 3, the deviations between the planned times and the forecasted figure 1. comparing the real and predicted cycle times of layer 1 with the currently used static cycle time. figure 2. shiny web application scheme. 2. training 3. test / evaluation ok? 4. feature selection erp mes r shiny 0. upload xls export xls no yes outputinput 1. feature generation 5. prediction acta imeko | www.imeko.org march 2020 | volume 9 | number 1 | 36 times and the impact thereof are evaluated. the results are summarised in a dashboard, showing the logistical fitness as proposed by lödding et al. [25], and the newly developed pq index. depending on the impact of the deviation, the quality gives feedback about the reliability of the production plan. in the fourth step, the planner decides whether the production plan meets the preferences and objectives of the company or not. if not, it is possible to change the planning related master data by using the support system, which provides the planner with some alternative proposals to change the data. the overall decision and therefore the responsibility stays with the human (i.e. a humanin-the-loop model) [18]). after the refinement and adjustment of the planning data, the planning can be re-initiated, and the cycle starts again. if the planner is satisfied with the key performance indicators (kpis) in step 4, he can activate the plan, thus transferring the planning related master data that was used to create the production plan to the planning systems. the innovative characteristics of the proposed evolutionary approach are as follows: • the provision of an evaluation method for the assessment of the production plans in step 3, • the creation of a dashboard to comprehensibly provide feedback to the planner through the visualisation of the main kpis, and • the establishment of a method for the proposition of more suitable planning data. 5.2. function-based approach the second approach differs – the planning cannot be carried out by the standard/conventional planning system. while in the evolutionary approach, the quality of the production plan continuously increases, in the function-based approach, pq is always set to high, and other logistical fitness factors of the plan are optimised iteratively, as shown in figure 4. step 0 is necessary for generating the prediction models and therefore must be done regularly. based upon these ml models of planning-related data, a novel planning method generates production plans. the novelty of the planning models lies within the usage of the dynamic set of functions (i.e. an extendable vector of possible functions) for the planning-related data and not a single value or set of values. the general approach seems as follows: orders are subsequently scheduled, always taking the current information of the production plan as information for the prediction models of the planning related data. some features for the prediction are not available at the time of planning, and therefore, these dynamic elements are predicted using different models. as there is no comparable approach in the field of research, this is the highest overall innovation within the approach. after planning, the evaluation of the logistical fitness follows. the logistical goal weighting of the company (e.g. an average lead time of six weeks and the timeliness of 87 % of all orders) models the benchmark that a production plan must achieve. since pq is already high, only these kpis must be checked. in step 3, the planner again decides whether the plan is acceptable or not. if not, they try another planning sequence. as a support function, the system offers several strategies (based on the order size, due date, costumer priorities, etc.). additionally, the planner can make manual changes. if the sequence is defined, they start the planning again. these steps are repeated until the plan is satisfactory. if so, the plan can be transferred/released to the erp or mes. in sum, the innovative characters of the proposed function-based approach can be specified as follows: • providing a novel planning method for dynamic times. • creating a recommendation method to propose changes in the planning sequence. 6. results and discussion the objective of both approaches is to increase pq in the sense of smaller deviations between planning and subsequent execution. the key is in the incorporation of uncertainty in the planning phase by carrying out production planning on the basis of dynamic instead of static time values. only with the use of dynamic time values (e.g. standard times for machining and setup) can the interdependencies of different influencing factors, which occur naturally, be depicted close to reality. these influencing factors can be well known, but most planning system do not offer the possibility of considering them in planning or scheduling activities. in the worst-case scenario, these influencing factors remain unknown or at least undetected. an example of the former case could be fluctuations in the machining and setup times per week and/or shift. this is commonly caused by different skill levels of employees or the actual machine or tools that execute the job. even though these effects are well known, most companies do not have the resources to levy and document all the effects so that a system would be able to process the information. still, this information is available in, for example, the confirmation data, and ml is able to quantify the impact to some extent. in order to be able to represent these dynamics as realistically figure 3. evolutionary approach. figure 4. function-based approach acta imeko | www.imeko.org march 2020 | volume 9 | number 1 | 37 as possible by using an ml algorithm, it is essential to ensure high data quality. in both approaches, the prediction provided by the ml algorithm can be as reliable and valid for the actual production system as the underlying historical data source that is used. for further discussion, we focus on different data sources for mes data. we distinguish between plant data acquisition (pda) and machine data acquisition (mda) data. mda data can generally be considered as more reliable than pda data, as the actual machine status is automatically captured. for example, a status like ‘no spindle rotation’ and ‘no malfunction’ indicate that the process has obviously been completed, and a time feedback via mda is done correctly. in the same case with feedback via pda, there can either be a time delay in the feedback of the machine operator or feedback is entirely missing. therefore, in the case of pda feedback, higher fluctuations and lower-quality training data are to be expected in comparison to mda data. however, this does not mean that the approach is only applicable for mda-generated data or data that is generated with a simulation model of the production system. longer observation periods and therefore a larger data source help to increase the reliability. in conclusion, it can be stated that the use of data that reflects reality is essential. to ensure that the machine status is mapped in production planning, it is also important to consider them as input features of the ml algorithm. depending on the input data of the machine status, the underlying causes should rather be questioned instead of a direct link between the change of the data and a change of the production plan. for example, following the assumption that the machining times of the machine increase over time, a variety of causes can be assumed, such as a change in the condition of the machine tool or the production machine (e.g. blunt tool or slow feed). in this case, it seems reasonable to invest resources to adjust the machine condition instead of generally increasing the machining times. it is important to provide the production planner with a decision support system in order to present decision options and their effects on the planning system. it seems conceivable to constantly increase the processing times due to a lack of investment in old machines or to restore the original machine condition. in the second case, the actual machining times should be significantly reduced. a major advantage of the function-oriented approach is the timeconsuming master data maintenance that is not required. instead, the approach generates benefits through the dynamic adjustments of the relevant values. however, it should be noted that the logic of the ml algorithm can only create a production plan on the basis of the current data situation. missing or defective data usually leads to a reduced pq. even if the condition of the machine worsens, immediate repair of the machines and the associated shortening of the processing times is generally not to be expected, nor is it appropriate. the permanent monitoring and visualisation of the machine status can alternatively be used to initiate measures to reduce production time. an example is the maintenance or change of the production parameters. as an ideal solution, the integration of a logic into the ml algorithm can be considered, which enables the examination of data from different data sources, machines, etc. the algorithm can assist in the interpretation and decision-making or, if required, perform appropriate weighting of individual data and states. by weighting, the effects of individual influences on production planning can be adapted, and implicit knowledge of planning can be represented to further increase pq. 6.1. evolutionary approach since this approach uses the existing production planning system, there are fewer interventions in the current existing system. this means that the existing systems do not become obsolete and can still be used to generate production plans. the evolutionary approach is meant to be an additional decision support system (dss) to the actual planning system. within this dss, a comparison is made between the planned times and the predicted times (cycle time, setup time, operating time, etc.). feedback to the planner is given in the form of various kpis and the pq index. furthermore, the planner gets feedback on why a certain time prediction is different from the planned time. based on the chosen ml method, ‘drivers’ for predicted times can be identified, and a note to the planner is given. since the system is an additional system, the implementation is expected to be easier and quicker. furthermore, the acceptance by the planners is expected to be higher, as the actual planning is still done by the existing system. the transparency and sovereignty of the planner about decision-making is an important factor for the acceptance of employees and proves a significant advantage of this approach. 6.2. function-based approach as the production plan is created directly using the time prediction models and these models get updated frequently, the quality is independent from the stored master data. furthermore, the ‘dynamic’ models are always updated automatically. the authors expect this approach to require a smaller number of iteration cycles because the system always uses the planning data that most likely depicts the later execution. the iterations are needed to meet the objectives for the logistical kpis. therefore, the number of iterations is expected to be similar to the current number of iterations, which are needed for the creation of a proper plan today. however, there are several open research questions that need to be answered. within the function-based approach, the planning algorithm chooses one order after the other. this means that the prediction only considers orders that have already been planned. however, subsequent orders have an impact on the features that are used for the prediction; for example, on the work in progress, when an order arrives at the same workstation, and the prior planned order is not finished. it is therefore crucial to derive correlations from planned orders and unplanned orders in the planning process. according to the authors, further research is needed to define a way in which the necessary features for the time prediction can be determined. the proposed approach is designed to replace the current planning system with a new planning algorithm. the planner has the option of weighting the kpis (timely delivery, lead time, stock, etc.) and thus manually adapting the priorities for production planning. since the pq index is always high, it is important to check the corresponding kpis and only implement the production plan when the kpis deliver sufficient results. a further advantage of this approach is that in contrast to the evolutionary approach, no adjustments of the master data have to be carried out. instead, a collection of the feedback data is necessary for training the ml algorithm. 6.3. evaluation of the approaches for the functionality and correct use of both approaches to increase pq, it is also important to determine when and which approach is applied. therefore, in addition to further developing the approaches, attention should be given to creating an evaluation method for the two approaches. this should clearly acta imeko | www.imeko.org march 2020 | volume 9 | number 1 | 38 demonstrate the advantages of each approach as well as the complexity of implementation. it should also be ensured that the right approach is always used depending on the application, the industry, and other influencing factors in order to achieve optimal results. 6.4. implementation in erp/mes in order to ensure that both approaches are fully functional and can be used separately from the implemented erp or mes solution, they have to be made accessible regardless of the platform that is used. different variants of erp and mes systems in the industry do not represent restrictions for the application of new approaches. therefore, the transfer of historical data for the training of the ml model is a general solution and must guarantee a data transfer at any time, but the training data can consist of individual characteristics depending on the application case. even if the accuracy of the production plan prediction increases with the amount of training data used, it is important not to select a disproportionately large amount of data. the horizon of the data also has an important influence. the use of long-term data ensures that the results are based on a long history, while the use of short-term data is suitable for the representation of outliers and random events. the storage of data duplicates in mes and erp has to be prevented as well. in each planning run, only the delta from old and new data is to be transferred, which guarantees speed advantages in the data transfer and keeps the transfer duration low. in fact, data transfers are not only possible for order confirmations but also for partial confirmations via mes or erp. according to the current status, however, only the historical data of a completed order (order confirmation) is used to forecast production plans. since the time horizon of the data used can influence the result of the prediction, it is also possible to differentiate between two variants of time-based use. in variant 1, the time horizon extends over the entirety of all training data, with all previous training data having an unknown influence on the results. in variant 2, the time horizon can be individually selected according to the principle of ongoing planning, which has advantages, for example, for the more accurate reproduction of short-term events. it is assumed that old data is less valid and can lead to distortions. in the further course of the research project, the advantages and disadvantages of different time horizons on the quality of production planning will be investigated. an essential basis for the successful introduction and use of these approaches in erp or mes is an analysis and optimisation of the existing process flows. a large part of the benefits that can be realised by the integration can hardly be evaluated in advance using quantitative criteria. the improvement of the internal production planning is an essential benefit of the presented solution, but its valuable support in the entire order processing is difficult to present. the benefit assessment in particular is considered problematic, since only parts of the achievable benefits can be quantitatively assessed in advance; for instance, adherence to delivery dates or error avoidance. another element that needs to be considered is the basic hardware requirements at the factory level that the constant use of ml algorithms in planning entails. a quantification of the required computing power is evaluated in the course of a research project. 7. conclusions and outlook during the work, a case study for an application of ml for time prediction was given, the term pq was defined, and two approaches to incorporate ml models for time prediction in the production planning process were discussed. the case study shows how ml can be used to predict cycle times based on the readily available data of a semiconductor manufacturer. the results show that the prediction is more accurate than the quarterly adapted static cycle time – the arithmetic average of cycle times of comparable lots within the past quarter (period of three months starting with january). the problem lies in the application of the trained models in the planning phase. therefore, two approaches of how to utilise these trained ml models during the plan creation are presented. the two approaches are the evolutionary approach and the function-based approach. in the first approach, new plans are created repeatedly, while the used planning times are adapted after each cycle. in addition, pq is measured by calculating the deviation between planning times and predicted times. if the deviation is within an acceptable level, the planning stops. the latter approach uses functions instead of static values for the planning times. therefore, there are no deviations between planning times and predicted times. still, several planning cycles are needed, as other logistical kpis need to be optimised. in the first place, an evaluation method for the two approaches should be developed. the function-based approach appears to be smarter due to the usage of a new planning method based on historical data, creating a production plan immediately instead of creating a production plan with master data and then comparing it with predicted data. however, both approaches have to be examined more closely in the further proceedings of the research before an exact assessment can take place. therefore, the authors successfully submitted a research proposal called ‘mlinppc’ (project number 877446) to the 32nd ‘produktion der zukunft’ call of the austrian funding agency ‘österreichische forschungsförderungs-gesellschaft mbh’. with the approval, the two proposed approaches will be implemented and tested together with two industrial companies. furthermore, the whole process, from data recording to pq evaluation, will be realised in order to assess both approaches. in fact, production plans are currently created from erp or mes data. the acceptance level of employees in the planning department depends strongly on the recognisability of deviations between the predicted and the classic (erp/mes) production plan. this is a clear advantage of the evolutionary approach. the confidence of the planner in a production plan that was created by an unknown planning logic could initially be low and thus restrict the implementation. therefore, in addition to the development of the algorithms themselves, further research must be carried out to enable the planner to trace the result of the algorithms. this is especially important during the deployment phase of the new approaches. therefore, it becomes clear that no matter which approach is chosen, the transparency of the approach must be consistent and comprehensible for the employee. only if the two approaches are accepted and trusted can a correct implementation be achieved and the pq in production planning increased, which will lead to an optimisation of logistics target values. acknowledgement the authors would like to acknowledge the financial support received within the eu project: power semiconductor and electronics manufacturing 4.0 (semi40), which is funded by the programme electronic component systems for european leadership (ecsel) (grant agreement no. 692466). acta imeko | www.imeko.org march 2020 | volume 9 | number 1 | 39 references [1] g. schuh (ed.), 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[25] h. lödding, handbook of manufacturing control – fundamentals, description, configuration, springer, berlin, heidelberg, 2013, 978-3-642-24458-2. maintainability improvement using allocation methods for railway system acta imeko issn: 2221-870x march 2020, volume 9, number 1, 10 17 acta imeko | www.imeko.org march 2020 | volume 9 | number 1 | 10 maintainability improvement using allocation methods for railway systems marcantonio catelani1, lorenzo ciani1, giulia guidi1, gabriele patrizi1 1 university of florence, department of information engineering (dinfo), via di s. marta 3, 50139, florence, italy section: research paper keywords: maintainability, allocation, maintenance, complex system, corrective maintenance downtime citation: marcantonio catelani, lorenzo ciani, giulia guidi, gabriele patrizi, maintainability improvement using allocation methods for railway systems, acta imeko, vol. 9, no. 1, article 3, march 2020, identifier: imeko-acta-09 (2020)-01-03 editor: lorenzo ciani, university of florence, italy received october 30, 2019; in final form january 14, 2020; published march 2020 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: giulia guidi, e-mail: giulia.guidi@unifi.it 1. introduction maintenance comprises all the techniques and management actions carried out during the useful period of a product in order to maintain it or restore it to a specific state with required functionality. maintainability measures the ease and speed with which the system can be restored after a failure occurs [1]-[7]. maintenance policies are divided into three categories: • corrective maintenance (cm) • preventive maintenance (pm) • condition-based maintenance (cbm) in cm, the maintenance task is actuated after a failure detection, whereas in pm, several periodic controls are implemented in order to avoid the presence of anomalies. cbm is similar to pm. it is focused on the prediction of a component degradation process and on the actions used to limit the wear of the component. the cbm technique is a maintenance strategy that uses certain procedures before the presence of product defeats, taking into account the operating context, the prediction of the failure effects, and the field data of similar products [8][10]. this work analyses how it is possible to achieve the maintainability improvement of complex systems using the maintainability allocation (ma) procedure. the main objective of the article is the study of the parameters included in the different ma techniques in order to identify the best procedure. in the initial design phases of a project, ma is performed. ma is a process used to move a maintainability requirement at the system level (generally demanded by customers) into product specifications relative to the subsystems and single components (the procedure of item selection performed by the designer). the purposes of ma techniques are: • identifying the maintainability requirements for subsystems and items in order to guarantee the system maintainability requirements and goals; and • focusing on maintainability complications during the design phase to ensure the assessment of requirements. ma is an essential and convenient process used in product design, generally because all the projects start with well-defined and clear goals, and maintainability, as an intrinsic product characteristic, is determined in the design phase. abstract an optimal maintenance policy is an essential condition of many industrial products in terms of saving resources and minimising operational costs and system downtime. some maintenance actions (e.g. corrective maintenance, preventive maintenance, and condition-based maintenance) are illustrated in the first part of this article. the article focuses on maintainability allocation techniques. four procedures are analysed (the failure rate-based allocation method; the trade-off of failure rate and design feature-based allocation method; fuzzy maintainability allocation based on interval analysis; and the time characteristic-based ma model). traditional procedures are characterised by several drawbacks; therefore, attention is given to the time characteristics-based method, which turned out to be the best and the most complete procedure because it does not have the limitations of the other methods. the last part of the article proposes a case study analysed using the techniques implemented in the maintainability allocation optimal method. two different cases are studied. they differ in the mean time to repair objective. initially, the requirement could vary inside a range, then, it is fixed to the value of six hours. mailto:giulia.guidi@unifi.it acta imeko | www.imeko.org march 2020 | volume 9 | number 1 | 11 optimising the allocation procedure by using a reasonable apportion diagram, it is possible to minimise the environmental impact and the necessary resources, making the ma procedure as cheap as possible [11]. since system failure is inevitable, quick system restorability is therefore the most important feature [11]. the main available methods for performing ma are the following: • the equivalent allocation method; • the failure rate-based allocation method; • the trade-off of failure rate and design feature-based allocation method; • the similar product maintainability data-based allocation method; and • fuzzy ma based on interval analysis. considering a system composed of n replaceable units, the mean corrective maintenance downtime �̅�ct (also known as mean time to repair [mttr]) is evaluated using the following equation [12][13]: �̅�ct = ∑ �̅�c𝑖𝜆𝑖 𝑛 𝑖=1 ∑ 𝜆𝑖 𝑛 𝑖=1 (1) where 𝜆𝑖 is the failure rate of the i-th element, and �̅�c𝑖 is the system active corrective or repair downtime when the i-th item fails. when the designers implement a ma procedure, it is mandatory that the mean corrective maintenance downtime assessed for each item satisfies equation 1. this issue could be solved using several different solutions, so the benefits and drawbacks of the proposed method must be considered in order to determine the most suitable result. usually, traditional allocation procedures neglect some important design factors (e.g. maintenance access channel). in this way, the mean corrective maintenance downtime allocated for every item could be not completely controlled, which is not allowed by the objectives of the allocation procedure [13]. in 2013, dong et al. [14] overcame the drawbacks of traditional ma procedures, proposing an advanced method based on time parameters. the following paragraph analyses three of the five previous methods. the equivalent allocation method and the similar product maintainability data-based allocation method were excluded because of their limited use. 2. failure rate-based allocation method this procedure allows an easy reconfiguration of the original system to optimise the components and subsystems’ maintainability. the equation provided from this method allows us to calculate the optimal time to repair (ttr) values to allocate for each item. the principle of this allocation method is that the repair time allocated to the unit with the high failure rate is short and vice versa. the hypothesis of this method is that the allocated/ predicted values of reliability metrics already exist. the mttr distributed to each unit is calculated as [14][15]: �̅�ct𝑖 = �̅�ct ∑ 𝜆𝑖 𝑛 𝑖=1 𝑛 𝜆𝑖 (2) where �̅�ct𝑖 is the mttr distributed to unit i, �̅�ct is the mttr of the system, 𝜆𝑖 is the failure rate of unit i, and n is the total number of unit types. not all units in an upgraded system need to be redesigned, because some of the units of the original system are adopted. assuming that an upgraded system is composed of n subsystems, among which l subsystems are the same as the starting system, the ma for the new layout follows this equation [14]: �̅�ct𝑗 = �̅�ct ∑ 𝜆𝑖 𝑛 𝑖=1 − ∑ 𝜆𝑖 �̅�ct𝑖 𝐿 𝑖=1 (𝑛 − 𝐿)𝜆𝑗 (3) where: • 𝑗 = 𝐿 + 1, … , 𝑛; • �̅�ct𝑗 is the mttr for newly designed subsystem j; • �̅�ct is the mttr of the upgraded system; • �̅�ct𝑖 is the mttr of original subsystem i; and • 𝜆𝑖 and 𝜆𝑗 are the failure rates of subsystems i and j, respectively. the allocated maintainability indicators �̅�ct𝑖 , based on the failure rates, are reasonable but may not be feasible. for example, one or several indicators could be extremely small and technically impossible to achieve. indicators must be adjusted in case they require extreme technical solutions or very high costs (from an economic, time, and manpower point of view). the various maintainability qualitative characteristics of the starting layout that influence repair time (e.g. complexity, accessibility, scalability, ease of replacement, and testability) must be considered, and a trade-off is the solution that determines the allocation results [16]-[18]. 3. trade-off of the failure rateand design feature-based allocation methods some relevant factors, such as complexity, accessibility, and testability, are transformed into weight coefficients when the allocation method, based on the trade-off of failure rate and the design features, is used [14]. the core structure of this method is similar to the previous one, but the introduction of weight factors offers more accuracy and allows the calculation of the importance of each factor within the ma procedure. the mttr allocated to each unit is calculated as: �̅�ct𝑖 = 𝛽𝑖 �̅�ct (4) where: • �̅�ct𝑖 is the mttr distributed to unit i; • �̅�ct is the mttr of the system; • 𝛽𝑖 = 𝜆 ̅𝑘𝑖 𝜆𝑖 �̅� is the weight coefficient of the repair time for the unit; • 𝜆̅ = ∑ 𝜆𝑖 𝑛 𝑖=1 𝑛 is the average failure rate of each unit; • �̅� = ∑ 𝑘𝑖 𝑛 𝑖=1 𝑛 is the average of each unit weight coefficient; and • 𝑘𝑖 = ∑ 𝑘𝑖𝑗 𝑚 𝑖=1 is the weight coefficient of factor j in unit i. the ma of the improved design subsystem is the following: �̅�ct𝑗 = �̅�ct ∑ 𝜆𝑖 𝑛 𝑖=1 − ∑ 𝜆𝑖 �̅�ct𝑖 𝐿 𝑖=1 𝜆𝑗 ∑ 𝑘𝑗 𝑛 𝑗=𝐿+1 𝑘𝑗 (5) where 𝑗 = 𝐿 + 1, . . . 𝑛. the requirement for the maintainability design should be clear when this procedure is used. the weight coefficients in this method are the indices of the influencing factors for the maintainability indicators of each unit [14]. acta imeko | www.imeko.org march 2020 | volume 9 | number 1 | 12 many influencing factors should be used following this method, but it would be unacceptable in the case that different factors are taken into account without considering the differences of their effects; in addition, the values of the weighted factors need to be given directly. it is difficult to use a real number to describe the values of the weighted factors, since a little mistake could produce a serious error in the allocation outcomes. for this reason, two statements are mandatory: the weight value of every influencing factor is uncertain, and for a certain subsystem, the values of the weighted factors are also uncertain. a piece of information associated with a single value is always incomplete, and it is subjected to errors that could make the results inaccurate. this problem is usually solved with an advanced method explained in section 4 [13]. 4. fuzzy maintainability allocation based on interval analysis fuzzy theory introduces the human mind logic to a binary logic system. human reasoning admits the possibility of switching from one concept to another gradually, while this is not possible in the boolean logic, where an element can only belong to a set (or not). fuzzy logic represents a deviation from the classic two-valued sets and logic. it uses ‘soft’ linguistic system variables (e.g. large, hot, and tall) and a continuous range of truth values in the interval [0,1], rather than strict binary decisions and assignments (‘true’ or ‘false’). fuzzy rule-based systems apply these features to solve many types of ‘real-world’ problems, especially where a system is difficult to model, controlled by human operators or in case ambiguity or vagueness is common [19]-[21]. before the allocation assessment, it is necessary to determine an index of maintainability. the most common maintainability parameters involve: mttr, mean active pm time, mean active corrective maintenance time, maximum corrective maintenance time and maintenance downtime. mttr is the fundamental metric, and it is widely used in repairable systems. it represents the average time required to repair a failed component or device. in other words, it is the total corrective maintenance time divided by the total number of corrective maintenance actions during a given period of time [22]-[24]. the index value should be determined after choosing the maintainability parameter. the following pieces of information are necessary to determine the maintainability index [13]: • usage requirement; • maintainability level of the similar products; and • techniques supposed to be used. furthermore, factors such as reliability, life cycle cost, and product development should also be taken into account. the allocation model is based on a fuzzy comprehensive evaluation, as follows [25][26]: �̃� = 𝑊°̃�̃� = [𝑤1 𝑤2 … 𝑤𝑚 ] [ 𝑟11 ⋯ 𝑟1𝑛 ⋮ ⋱ ⋮ 𝑟𝑚1 ⋯ 𝑟𝑚𝑛 ] (6) �̃� = [𝑏1 𝑏2 … 𝑏𝑛 ] (7) where: • �̃� is the weight vector of the ma’s influencing factors; • �̃� is the matrix of the subsystems’ influencing factors; • �̃� is the result of the maintainability comprehensive evaluation; • b1, b2, … bn express the relative values of the maintainability weight factor of each subsystem; • the symbol ° is the fuzzy composition operator; • m is the number of influencing factors; and • n is the number of subsystems [27]. in the fuzzy procedure, the ma process is as follows [13]: • step 1: synthetically calculate the original image �̃� by means of interval analysis and analytic hierarchy process (ahp); • step 2: calculate the mapping �̃�; and • step 3: draw image �̃� by means of fuzzy mapping. after determining the set of influencing factors of ma, it is necessary to determine the weight of each factor, and there are various methods of doing so. the most used are listed below: • the ahp could combine qualitative analysis and quantitative analysis. this combined method is widely used in the decision-making for and evaluation of complex systems, and the weight of each attribute is acquired by a pair-wise comparison matrix [28]-[30]; • the delphi method uses experts’ direct weighting. each of them gives a weight value by their experience, and then the average value is taken as the weight [31]. many methods can be used to determine the element of vector 𝐵,̃ and the maintainability allocation can be written as follows: �̅�ct𝑗 = 𝑏𝑗 ∑ 𝜆𝑗 𝑛 𝑗=1 ∑ 𝜆𝑗 𝑏𝑗 𝑛 𝑗=1 �̅�ct (8) 5. time characteristics-based maintainability allocation model for complex equipment usually, in company policies, various design departments monitor different items; therefore, the repair time must be divided into two categories [14][32]: • the common repair time 𝑇cmn is determined by the overall design or upper-level department, and it is not directly affected and controlled by product design at this stage. • the individual repair time 𝑇idv is the repair time determined by product design, such as assembly, changing, and adjustment time. considering that the formation mechanisms of the different types of maintenance time are different, the types of maintenance time are influenced and controlled by different product design departments. some influencing factors must be evaluated in order to assess the repair time of the system. the time characteristics-based ma model identifies two different allocation levels (high and low) and two different respective allocation procedures [33]. the high-level procedure considers a system with n subsystems, characterised by the respectively failure rates 𝜆1, 𝜆2, … 𝜆𝑛 . using equation 9, it is possible to evaluate the common repair time 𝑇cmn: 𝑇cmn = 𝑇p + 𝑇a + 𝑇r (9) where tp is the maintenance preparation time, ta is the approaching time, and tr is the final reassembly time [9]. acta imeko | www.imeko.org march 2020 | volume 9 | number 1 | 13 the method of determining tp is similar to the product ratio method. if the mttr of the designing product is �̅�ct, �̅�′ct and 𝑇′p are respectively the mttr and preparation time for similar products, the preparation time for designing the product is [34]: 𝑇p = �̅�ct �̅�′ct 𝑇′p (10) the common repair time is divided into three parts, and in a generic manufacturing application, the time intervals are the following: the time spent opening the flap of the access channel (𝑇a1), the time spent moving and accessing the cabin that requires repair (𝑇a2), and the time spent opening the fasteners and removing the obstacles near the replacement unit (𝑇a3). the approaching time 𝑇a is [14]: 𝑇a = 𝑇a1 + 𝑇a2 + 𝑇a3 (11) the approaching time is not equal to zero when the system shares the maintenance access channel; however, when each item has its own maintenance access channel, the approaching time belonging to the common repair time is zero. similar to the approaching time, the reassembly time is controlled by the overall sector or the upper-level design department, and it is affected by the same factors that affect the approaching time. thus, 𝑇r is: 𝑇r = 𝑇a (12) the individual repair time is calculated by eliminating the common repair time from the mttr of the whole system [34]. 𝑇idv = �̅�ct − 𝑇cmn (13) then, according to the failure rate-based allocation method, the repair time of each subsystem can be evaluated as follows: �̅�ct𝑖 = ∑ 𝜆𝑖 𝑛 𝑖=1 𝑛 𝜆𝑖 𝑇idv = 𝜆̅ 𝜆𝑖 𝑇idv (14) where �̅�ct𝑖 is the repair time allocated to subsystem i, 𝜆𝑖 is the failure rate of subsystem i, and 𝜆̅ = ∑ 𝜆𝑖 𝑛 𝑖=1 𝑛 is the average failure rate of all subsystems. for a system consisting of n subsystems, where l of them (𝐿 < 𝑛) are existing products, the maintainability indicators of the newly designed subsystems are allocated as follows [14]: �̅�ct𝑗 = 𝑇idv ∑ 𝜆𝑖 𝑛 𝑖=1 − ∑ 𝜆𝑖 𝐿 𝑖=1 �̅�ct𝑖 𝜆𝑗 (𝑛 − 𝐿) 𝑘𝑗 (15) isolation, disassembly, and replacement are included in the maintenance procedure for low-level items. the time needed for these three actions is included into the individual repair time. finally, considering the low-level components, the individual repair time is given by [14]: 𝑇ru−idv = �̅�ct𝑖 − 𝑇ru−cmn (16) where: • 𝑇ru−idv is the individual repair time allocated to the single replaceable unit (ru); • �̅�ct𝑖 is the time indicator allocated to subsystem i from the system level using the high-level product allocation model; and • 𝑇ru−cmn is the common repair time in this level. the equipment replacement procedure should be considered in the late development stage to improve the precision of allocation. when a failure is isolated to a single ru, the ru can be replaced individually to correct the fault. if the failure is isolated to an ru group and the rus in the group are irrelevant, the group can be considered as a single ru. the group in which the rus are replaced alternately is denoted as ruge. the group in which the rus are all replaced is denoted as ruga. the ma procedure involves the allocation of the system mttr to 𝑅𝑈1, … 𝑅𝑈𝑗 , 𝑅𝑈ge1 , … 𝑅𝑈ge𝑘 , 𝑅𝑈ga1 , … 𝑅𝑈ga𝑙 according to the failure and design features of each ru or ru group. considering an ru group composed by m single rus, the average number of replacements 𝑆𝑖 is: 𝑆𝑖 = [ 𝑚 + 1 2 ] (17) where the symbol [x] means the maximum integer not larger than x. in this study, six kinds of maintainability design features are considered: fault detection and isolation; maintenance channel; fasteners; internal assembly; replacement; and scalability. these features may be different among different products depending on specific circumstances. the repair time differs because different maintenance schemes are employed. in order to increase the reasonability and accuracy of the ma procedure, this method considers a new coefficient 𝛼𝑖 . there are three different cases [14]: • if the failure is isolated to a single ru, the correct coefficient is 𝛼𝑖 = 1; • if the failure is isolated to an ru group consisting of m replaced alternately 𝛼𝑖 = 𝑆𝑖 = [ 𝑚+1 2 ]; and • if the failure is isolated to an ru group consisting of r and all are replaced, the correct coefficient is 𝛼𝑖 = 𝑟; using the following equation, the repair time allocated to each ru can evaluated: �̅�ct𝑖 = 𝑘𝑖 ′ ∑ 𝜆𝑖 𝑗+𝑘+𝑙 𝑖=1 𝜆𝑖 ∑ 𝑘𝑖 ′𝑗+𝑘+𝑙 𝑖=1 𝑇ru−idv (18) where: • 𝑘𝑖 ′ = 𝛼𝑖 𝑘𝑖 = 𝛼𝑖 ∑ 𝑘𝑖𝑗 𝑚 𝑗=1 with m number of weight coefficients and 𝑘𝑖𝑗 weight coefficient of factor j in unit i; • 𝑖 = 1, … , 𝑗, 𝑗 + 1, … , 𝑗 + 𝑘, 𝑗 + 𝑘 + 1, … , 𝑗 + 𝑘 + 𝑙 for the reconfigured system, the maintainability indicators of the newly designed subsystems are allocated as follows: �̅�ct𝑗 = 𝑇ru−idv ∑ 𝜆𝑖 𝑗+𝑘+𝑙 𝑖=1 − ∑ 𝜆𝑖 �̅�ct𝑖 𝐿 𝑖=1 𝜆𝑖 ∑ 𝑘𝑗 ′𝑛 𝑗=𝐿+1 𝑘𝑗 ′ (19) where 𝑗 = 𝐿 + 1, … 𝑛. figure 1 highlights the main stages of the time characteristicsbased ma procedure. initially, the high-level procedure allocates the mttr to the ru group, meaning the subsystem level. in the second step, the mean corrective downtime of every single level replaceable unit (lru) is evaluated using the low-level method. 6. comparison of the methods ma is considered a vital process in the design phase of a new system, since the optimal allocation of maintainability indicators to the various subsystems of a complex system has many advantages. obviously, each ma procedure is required to satisfy the system maintainability objective. acta imeko | www.imeko.org march 2020 | volume 9 | number 1 | 14 once the allocation target is defined, it is mandatory to plan a series of system interventions, reconfigurations, and corrections in order to optimise maintenance activities, as well as reducing their implementation costs and their impact on society and the environment. in the previous sections, four ma methods were analysed and compared to bring out their characteristics, strengths, and weaknesses and to identify which is the optimal method for the application under analysis. the failure rate-based allocation method is a basic procedure for ma assessment. it is easy and fast to apply and is based on the resolution of elementary mathematic equations. in any case, an intrinsic limit is hidden in this method: the designer must know beforehand the overall system mttr and the failure rates of each single unit. this procedure is not widely used due to the lack of precision and accuracy because it does not consider the number of influencing factors, which deeply affects maintenance activities. the trade-off between the failure rateand design featurebased allocation methods solves the problems explained previously, but it also introduces some other issues. some influencing factors, particularly complexity, accessibility, and scalability, are taken into account using dedicated weight coefficients that reflect each role in the maintenance process; however, this solution has some disadvantages: it is necessary to know in advance a great number of parameters, and the final results profoundly vary if the evaluation is not accurate. another problem is the inability to differentiate the influencing factors depending on the nature of their effects, and this is not acceptable. the fuzzy ma based on interval analysis method was introduced because a weight factor, assigned as a real number, cannot fully describe the real scenario. in this procedure, a real number is replaced by an interval of real numbers in order to build a judgement matrix. the method combines fuzzy theory, interval analysis, testing data, and expert judgement. this ma procedure is characterised by a high implementation complexity and a significant waste of budget, resource, and time. therefore, attention is given to the last method, which turned out to be the best and the most complete because it exceeds the other methods’ limitations. indeed, the time characteristicsbased ma method is practical, quick and easy to implement, and provides satisfying results. the relationship between the project and the repair time is taken into account, and time is divided into common and individual repair time. the first type is deducted from the total system repair time, while the second is allocated to specific units using one of the traditional methods. the following pieces of information are required to apply this procedure: system structure and corresponding hierarchical levels, the information about the ru, and the overall system mttr (the goal of the allocation procedure). 7. case study redundancy increases system reliability leading to a fault tolerance design [35]-[38]. it is widely used in many technological fields, such as energy production [39]-[41] and transportation [42], [43]. in particular, redundancy is a mandatory design tool in railway applications, because the functional safety constraints demanded by the international railway standards [44]-[46] require that the system continues to perform its tasks also in case of failure, occasionally with less functionality, instead of failing completely [23][24][47]. a generic complex system, including redundant configurations, has been studied in order to analyse the time characteristics-based ma procedure (figure 2). the system under test is an electronic controller included in many safetyrelated systems used for railway signalling. in such systems, an optimised maintenance plan is able to reduce the overall operational costs, leading to a reduction in the waste of resources. the input of the ma procedure is the system’s mttr. in order to obtain the maximum system reliability, availability, and maintainability (ram) performance, in the first case, the system mttr could vary inside the interval (1 h 8 h). table 1 shows the failure rates of the components related to the reliability block diagram in figure 2. table 2 contains the input and output of the low-level allocation procedure. in order to distribute the mttr in the ru group level, the following equation is used: 𝑇cmn = 𝑇p + 𝑇a + 𝑇r = 2.5 h . (20) figure 2. reliability block diagram (rbd) of the system under test. figure 1. time characteristic-based ma method flow-chart acta imeko | www.imeko.org march 2020 | volume 9 | number 1 | 15 feedback and field data have been used to determine the time values. in compliance with equations 13 and 14, 𝑇idv = 1.5 h and �̅�ctsys = 𝑀𝑇𝑇𝑅sys = 4 h. the overall mttr is acceptable because it belongs to the objective mttr range (1 h 8 h). in order to allocate the mttr of the lru level, the objective is �̅�ctsys = 𝑀𝑇𝑇𝑅sys = 4 h. in compliance with equation 9, the common repair times are listed in table 2. based on equations 10 to 14, the mttr of the ru groups are listed in table 3. according to equation 16 and using the method described in paragraph iii, it is possible to calculate the mttr of each element (lru); the results are shown in table 4. a second case study (case b) has been analysed to investigate the possible changes of the procedural outcomes caused by some input changes. the objective of this study is to underline the relationship between the maintenance index allocated using this method and the overall mttr requirement of the control system under test used in railway applications. the overall mttr objective in case b is 6 hours, which is higher than the previous case. the results are reported in table 5. based on a comparison between table 4 and table 5, the high values of the mttr objective correspond with the high values of the single mttr parameters allocated to the items. figure 3 shows the differences between mttr in different cases. the component characterised by the highest mttr (i.e. the most critical item) is lru11 with an allocated mttr of 23 h 53 m. the items characterised by the lowest mttr (i.e the least critical equipment) are lru3, lru5, lru7, and lru9 with an allocated mttr of approximately 1 h. the values collected in table 5 result increased. the percentage deviations of the less critical items are more significant, and they surpass 100 % for lru3 and lru7. table 2. low-level allocation ru group lru code si αi ki-lru ki-ru k’i tru-cmn in h ru 1 lru 1 1 1 8 8 8 0 ru 2 lru 2 1 1 8 8 8 0 ru ge1 lru 3 lru 4 lru 5 lru 6 2 2 14 16 20 12 16 32 1.33 ru ge2 lru 7 lru 8 lru 9 lru 10 2 2 14 16 20 12 16 32 1.33 ru ge3 lru 11 lru 12 lru 13 2 2 16 12 12 12 24 1.5 ru 3 lru 14 1 1 8 8 8 0 ru 4 lru 15 1 1 8 8 8 0 ru 5 lru 16 1 1 8 8 8 0 table 3. mttr allocated to the ru groups. ru 1 ru 2 ru ge1 ru ge2 ru ge3 ru 14 ru 15 ru 16 3h22’ 6h18’ 3h13’ 3h13’ 10h26’ 8h17’ 1h53’ 8h17’ table 1. failure rates of the components included in the rbd of the control system under analysis ru group lru code λlru [fit] λru [fit] ru 1 lru 1 956.4 956.4 ru 2 lru 2 510 510 ru ge1 lru 3 lru 4 lru 5 lru 6 1422 704 1595 273 3994 ru ge2 lru 7 lru 8 lru 9 lru 10 1422 704 1595 273 3994 ru ge3 lru 11 lru 12 lru 13 140 392 392 924 ru 3 lru 14 387.7 387.7 ru 4 lru 15 1700 1700 ru 5 lru 16 387.7 387.7 table 4. ma procedure output for a complex system in case a. lru1 lru2 lru3 lru4 lru5 lru6 lru7 lru8 lru9 lru10 lru11 lru12 lru13 lru14 lru15 lru16 mttr i 3h22’ 6h18’ 1h12’ 2h46’ 1h31’ 5h20’ 1h12’ 2h46’ 1h31’ 5h20’ 23h35’ 6h19’ 6h19’ 8h17’ 1h53’ 8h17’ table 5. ma procedure output for complex system in case b. lru1 lru2 lru3 lru4 lru5 lru6 lru7 lru8 lru9 lru10 lru11 lru12 lru13 lru14 lru15 lru16 mttr i 5h22’ 9h27’ 2h30’ 4h45’ 2h37’ 9h11’ 2h30’ 4h45’ 2h37’ 9h11’ 36h29’ 9h46’ 9h46’ 12h26’ 2h50’ 12h26’ acta imeko | www.imeko.org march 2020 | volume 9 | number 1 | 16 figure 4 depicts the difference between the two cases relative to the maximum mttr value obtained in the first analysis (lru 11 ). the maximum deviation is associated with the most critical elements. conclusion initially, this article illustrated the different aspects of the maintenance policies by focusing on maintainability assessment. starting from a study of four allocation methods, this article proposed the time characteristics-based maintainability assessment technique as the optimal procedure, because it could weigh the influence of every item on the system parameters. maintainability assessment during the design stage represents one of the most important phases in making structural decisions and selecting the best items needed to complete the mission: this feature is a huge improvement in industrial applications because design can be based on a maintainability assessment. in particular, an electronic controller for a railway signalling system has been studied. in the initial part of the study, the objective was 𝑀𝑇𝑇𝑅sys ∈ [1 h − 8 h] and the obtained result was 𝑀𝑇𝑇𝑅sys = 4 h. then, the overall mttr has been increased up to 6 h. a comparison between the two cases highlights that by increasing the objective mttr, the maintainability indices allocated to the items increase. considering the most critical components, a high objective mttr allows a significant increase in the common repair time. references [1] mil-hdb-338b, electronic reliability design handbook. us 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[47] d. seneviratne, l. ciani, m. catelani, d. galar, smart maintenance and inspection of linear assets: an industry 4.0 approach, acta imeko 7(1) (2018) p. 50. thermographic and reflectographic imaging investigations on baroque paintings preserved at the chigi palace in ariccia acta imeko issn: 2221-870x march 2021, volume 10, number 1, 187 192 acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 187 thermographic and reflectographic imaging investigations on baroque paintings preserved at the chigi palace in ariccia sofia ceccarelli1, noemi orazi1, fulvio mercuri1, stefano paoloni1, ugo zammit1, francesco petrucci2 1 dipartimento di ingegneria industriale, università̀ degli studi di roma tor vergata, via del politecnico 1, 00133, rome, italy 2 palazzo chigi, piazza di corte 14, 00040, ariccia, italy section: research paper keywords: mid-infrared imaging; thermography; reflectography; hidden features; baroque paintings; chigi palace citation: sofia ceccarelli, noemi orazi, fulvio mercuri, stefano paoloni, ugo zammit, francesco petrucci, thermographic and reflectographic imaging investigations on baroque paintings preserved at the chigi palace in ariccia, acta imeko, vol. 10, no. 1, article 25, march 2021, identifier: imeko-acta10 (2021)-01-25 editor: ioan tudosa, university of sannio, italy received april 28, 2020; in final form february 2, 2021; published march 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: noemi orazi, noemi.orazi@uniroma2.it 1. introduction in the study of painted artefacts, important information can be obtained by displaying features hidden under the surface pictorial layers, such as underdrawings and pentimenti. since this kind of information is often not accessible to the naked eye, nondestructive techniques for the inner inspection of artworks are needed. for this purpose, optical methods based on the use of infrared (ir) radiation are nowadays considered effective tools for the analysis of subsurface features in cultural heritage (ch) [1], [2] items. among these methods, infrared reflectography (irr) is the most commonly adopted technique, primarily employing radiation in the near infrared range. in particular, near-ir reflectography (nir) provides information about the very first layers beneath the painted layer [3], [4], thus allowing the detection of underdrawings and subsurface elements by means of different types of imaging device [5]-[9]. all these approaches explore the artwork’s layered structure through ir images, which record with increasing resolution and, consequently, at increasing cost [10], [11]. the most interesting underlying elements of an artefact with a stratigraphic structure are often beneath multiple layers of painting and materials. in this respect, the use of mid-infrared range (3–5 m) imaging analyses makes it possible to increase the depth range of non-destructive investigations and, thus, to reach integrative and complementary information that would not be achievable using only nir techniques [12]. in recent years, imaging techniques operating in the midinfrared range have been employed on different kinds of painted artefact, such as wooden/canvas paintings [13] and illuminations [14], by using either pulsed thermography (pt) and/or midinfrared reflectography (mir). pt is based on the detection of the time-dependent distribution of ir radiation emitted from the sample surface following heating induced by the absorption of a short light pulse in the visible range (vis). this technique has proved to be able to characterise the main surface and subsurface elements of ch artefacts [15]-[17], providing information on both the conservative state and subsurface graphical and pictorial features, such as underdrawings and pentimenti [18]. similar items have also been investigated using mir, which collects the images immediately after the beginning of the illumination of the sample abstract in this work, two different mid-infrared imaging techniques operating in the 3–5 m spectral range are applied to the study of three paintings on canvas, dating back to the xvii century, preserved at the chigi palace in ariccia (italy). a combined approach based on the use of pulsed thermography and mid-infrared reflectography is proposed for the analysis of the ‘primavera’ by filippo lauri and mario nuzzi, the ‘ritratto di mario nuzzi che dipinge un vaso di fiori’ by giovanni maria morandi and mario nuzzi and the ‘ebbrezza di noè’ by andrea sacchi. the aim is to show how the integrated use of these techniques enables a depth-resolved investigation of the entire layered structure of the paintings, from the surface up to the canvas support. the complementarity of the presented results allows an evaluation of the conservative status of the support and the detection of graphical and pictorial features hidden beneath the surface layer, such as pentimenti, as these features are important for the historical and artistic characterisation of the artefact. mailto:noemi.orazi@uniroma2.it acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 188 with mid-infrared radiation, thus allowing a reduction in the number of stray effects on the reflectographic image caused by the mid-infrared radiation emitted from the heating of the sample. the novel approach proposed in this work is based on the combined use of the two mid-wave infrared imaging techniques, usually carried out by means of a separate setup, to obtain complementary information on the artwork. in this approach, the same ir camera is used to record both the thermograms and reflectograms corresponding to the same area of the painting. by so doing, the comparative analysis of the two recorded images allows a better understating of the features revealed by the adopted techniques, which can be further improved by the combined numerical processing of the images. in the framework of this adamo project [19], the proposed integrated use of pt and mir has been adopted for the depthresolved investigation of three paintings on canvas, dating from the xvii century and belonging to the baroque period, in order to detect subsurface features and structural elements. 2. the baroque paintings the investigated paintings are preserved at the chigi palace in ariccia (rome, italy), which is considered a primary example of italian baroque architecture. the palace was originally built for a rich, local family during the second half of the xvi century, and it was restored by gian lorenzo bernini for the chigi family between 1664 and 1672 [20]. the restoration of the palace was carried out with the aim of exhibiting the splendour and the importance of the chigi family, one of the leading families in the italian papal lineage. in this palace, several valuable paintings are preserved, mainly dating back to the xvii century. among these artworks, three paintings were investigated as part of this study: ‘ritratto di mario nuzzi che dipinge un vaso di fiori’ by giovanni maria morandi and mario nuzzi, ‘primavera’ by filippo lauri and mario nuzzi, and ‘ebbrezza di noè’ by andrea sacchi. 2.1. the portrait the ‘ritratto di mario nuzzi che dipinge un vaso di fiori’ (‘portrait of mario nuzzi painting a flower vase’) is one of the greatest masterpieces of the roman baroque because of its high pictorial quality and the peculiar combination of two pictorial genres (still life and portrait) realised by the two artists. this artwork was painted by mario nuzzi and giovanni maria morandi, who were responsible for the floral decoration and nuzzi’s portrait, respectively (figure 1). this painting is a typical example of baroque portraiture, and the artist, mario nuzzi, is represented in his atelier, in front of his easel during the creative process, with a melancholic look towards the viewer. in this painting, mario nuzzi, also called mario de’ fiori for his extraordinary talent for depicting flowers, achieved the highest level of quality in the representation of a single flower vase composition [21], [22]. the canvas is composed of two identical smaller pieces, vertically stitched together. this division was created in order to make it possible for the artists to work separately on the two different parts of the painting [23], [24]. 2.2. the spring allegory the ‘primavera’ (‘the spring’) painting was also realised by the collaboration of two artists: filippo lauri depicted the female figure and the cherubs, while mario nuzzi painted all the garlands and the floral elements (figure 2). this artwork belongs to the series ‘le quattro stagioni’ (‘four seasons’), nowadays preserved in ariccia’s chigi palace and considered one of the greatest examples of roman baroque painting. the entire series was commissioned by flavio chigi, nephew of pope alessandro vii [25], and executed between 1658 and 1659. the studied painting represents the allegory of spring, symbolising youth, depicted as a beautiful woman surrounded by six cherubs and several types of flower, while holding up a rose as a trophy. in this artwork, the two artists combined their own artistic skills, achieving an incredible harmonisation of the two talents [26]-[28]. 2.3. the drunkenness of noah the ‘ebbrezza di noe ̀’ (‘the drunkenness of noah’) (figure 3), preserved at the chigi palace, is one of the numerous replicas produced by andrea sacchi in the second half of the xvii century. the dating of the painting is uncertain, as is the possibility of establishing whether it is the original version or one of the many replicas [29], [30]. the large number of copies of this painting is due to the following reasons: the importance of this biblical story from genesis and the painter’s constant search for perfection in its artistic production. the painting represents a famous biblical scene in which noah is lying on a rock because of his drunkenness and, for this reason, he is being derided by cam, while the more modest and respectful brothers, sem and jafet, look away. this scene was figure 1. giovanni maria morandi and mario nuzzi, ‘ritratto di mario nuzzi che dipinge un vaso di fiori’, 1658–1659, oil on canvas, 195 × 265 cm, chigi palace, ariccia. figure 2. filippo lauri and mario nuzzi, ‘primavera’, 1659, oil on canvas, 150 × 250 cm, chigi palace, ariccia. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 189 represented extensively during the renaissance period, lasting until the xvii century, referring also to more pagan views, such as those relating to bacchanal models [31]. 3. methods among the mid-infrared imaging techniques, pt has become one of the most used methods for the investigation of cultural heritage [1], [32]. it is one of the so-called photothermal techniques, which have been widely used for the determination of the thermal transport properties of several kinds of material [33]-[38]. pt provides a sequence of ir images, referred to as thermograms [39]. the presence of a sub-surface feature leads to local modifications in the amount of emitted ir radiation and, hence, to the formation of contrast in the recorded ir images. in the case of semi-transparent media, such as paintings, the contrast in the thermograms originates in the differences in the local optical properties of the subsurface features compared to those of the surrounding material. more specifically, there are two main opto-thermal mechanisms that enable the buried features to be displayed. the first mechanism occurs when the features are located at a depth in the sample smaller than the penetration depth of the heating vis light. in this case, the different absorption properties in the visible range of the features with respect to the surroundings may result in a different local temperature increase and, hence, in the contrasted ir emission being recorded in the thermograms. however, when the depth of the subsurface element is too large to be reached by the incident light, the heating of the buried features can only be associated with the diffusion of the heat in the sample depth. under such circumstances, the contrast is mainly due to the different local values of the ir emissivity. in the mir technique, the images are obtained by directing mid-infrared radiation from a suitable source onto the sample. here, the image contrast originates primarily from the local difference in the reflection of the mid-infrared radiation [40]. for this reason, unlike pt, mir does not provide information about the depth of the detected features, and its application shows some limitations in the detection of features buried beneath layers that strongly diffuse mid-infrared radiation. the setup employed for the analysis of the paintings in the chigi palace is as follows. in the pt technique, the sample heating is induced by means of two 3 kw flash lamps oriented at 45° with respect to the surface of the investigated sample, delivering a few millisecond-long pulses. the emitted light from the lamps is filtered in order to eliminate the mid-infrared spectral component and, hence, to suppress any contributions to the recorded signal originating from reflections at the sample surface. in the mir technique, the sample illumination is performed using a continuous-wave halogen lamp positioned at about 2 m from the paintings. in both these techniques, the images are recorded by means of a cedip jade camera (320 × 240 pixel, insb focal plane array, 30 m pitch, 3.6–5.1 m wavelength range, noise equivalent temperature difference < 25 mk at 30 °c), positioned perpendicularly to the painting surface. image acquisition and processing are conducted using altair 5.50 software. 4. results in the following discussion, the thermographic results will be compared with those obtained by mid-infrared reflectography in the areas of the paintings under investigation. to facilitate the comparison, the thermograms are presented with an inverted grey palette so that the hotter elements will appear darker in the images. 4.1. giovanni maria morandi and mario nuzzi’s painting a series of pentimenti were detected in the ‘ritratto di mario nuzzi che dipinge un vaso di fiori’ by giovanni maria morandi and mario nuzzi. as an example, both the thermogram (figure 4b) and the mid-infrared reflectogram (figure 4c) reveal several changes to the initial concept of the painting. the arrows in the figures indicate a different hairstyle and head contouring with respect to the current appearance of the painting (figure 4a). a further element highlighted by the pt and mir investigations is shown in figure 5. the thermogram (figure 5b) and the mid-infrared reflectogram (figure 5c) both detect the stitching line of the two separate pieces that form the canvas figure 3. andrea sacchi, ‘ebbrezza di noè’, n.a., oil on canvas, 150 × 205 cm, chigi palace, ariccia. figure 4. the ‘ritratto di mario nuzzi che dipinge un vaso di fiori’, giovanni maria morandi and mario nuzzi, chigi palace, ariccia. (a) picture and corresponding (b) thermogram and (c) mid-infrared reflectogram in which the arrows indicate some variations in nuzzi’s hairstyle. figure 5. the ‘ritratto di mario nuzzi che dipinge un vaso di fiori’, giovanni maria morandi and mario nuzzi, chigi palace, ariccia. (a) picture and corresponding (b) thermogram and (c) mid-infrared reflectogram in which the arrows indicate the join between the two pieces of canvas. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 190 (indicated by the arrows in the figures), which has been reported in the literature [23]. it is worth noting that the pentimenti has only been detected on nuzzi’s head, thus evidencing the great attention shown by giovanni maria morandi to the realistic representation of nuzzi’s physiognomy. with the exception of the join between the two pieces of canvas, the thermographic results do not show any defects or inhomogeneities, thus indicating the painting’s good state of conservation. 4.2. filippo lauri and mario nuzzi’s painting in the ‘primavera’ by filippo lauri and mario nuzzi, several pentimenti were revealed, especially among the numerous cherubs. for instance, the mid-infrared reflectogram (figure 6c) shows several changes to the initial concept of the cherub’s physiognomic traits (indicated by the arrows), which are not visible in the thermogram (figure 6b), thus indicating a pentimento beneath the surface pictorial layer. important information has also been obtained by recovering the contrast of some of the darkened features, such as those concerning the cherub displayed in figure 7. in this case, pt allowed the recovery of the cherub’s physiognomic traits and the contouring of his entire head (arrow in figure 7b), while mir revealed some variations near the eyes, nose and mouth (dashed arrows in figure 7c). furthermore, interesting features came to light from the analysis of the flowers (figure 8). unlike the mir images, which do not show any relevant features, probably because of the presence of the highly reflective varnish layer (figure 8b), pt enabled the detection of several elements. in fact, the thermogram recorded just after the light pulse allowed the recovery of floral details (arrows in figure 8c) that are not clearly visible to the naked eye, while the pt image recorded at increasing delay times detected the canvas pattern (figure 8d). moreover, several canvas defects were revealed by pt and mir in the area of the woman’s hand (figure 9). both the thermogram (figure 9b) and the reflectogram (figure 9c) highlight the presence of repaired lacunas, as indicated by the arrows in the figures. the ir investigations of the ‘primavera’ painting allowed the detection of several changes to the initial painting concept. such changes mainly concern the representation of human figures, such as in the case of the cherubs. on the contrary, the floral elements seem to have been depicted by the artist with more confidence, as evidenced by the absence of clear pentimenti, and are actually covered in part by the dark tones of the painting. finally, unlike the results obtained in the portrait, here, the thermographic investigations enabled the detection of structural features, such as the repaired lacunas, indicating the occurrence of previous restorations of the painting. 4.3. andrea sacchi’s painting mid-infrared imaging investigations were carried out in order to characterise the ‘ebbrezza di noe ̀’ by andrea sacchi. in this painting, several pentimenti were revealed, such as the one reported in figure 10. in this case, the back of the figure shows numerous modifications, which can be observed both in the thermogram (figure 10b) and in the mid-infrared reflectogram (figure 10c). in both the images, the arrows indicate the sketch of a facial profile larger than the one that is visible (figure 10a). even in this case, some restored lacunas have been detected by pt (dashed circles in figure 10b). another pentimento was observed on noah’s foot (figure 11), as shown in the thermogram (figure 11b) and in the mid-infrared figure 6. the ‘primavera’, filippo lauri and mario nuzzi, chigi palace, ariccia. (a) picture of the cherub and (b) corresponding thermogram; (c) mid-infrared reflectogram in which the arrows indicate the pentimento. figure 7. the ‘primavera’, filippo lauri and mario nuzzi, chigi palace, ariccia. (a) picture of the cherub and corresponding (b) thermogram and (c) midinfrared reflectogram in which the arrows indicate the head contouring and some variations in the cherub’s nose and mouth. figure 8. the ‘primavera’, filippo lauri and mario nuzzi, chigi palace, ariccia. (a) picture of flowers and corresponding (b) mid-infrared reflectogram and thermograms recorded just after the light pulse (c) and at increasing delay times (d). figure 9. the ‘primavera’, filippo lauri and mario nuzzi, chigi palace, ariccia. (a) picture of the woman’s hand and corresponding (b) thermogram and (c) mid-infrared reflectogram in which the arrows indicate the restored lacunas. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 191 reflectogram (figure 11c). moreover, as in the other investigated paintings, the thermogram enabled a series of complementary pieces of information to be obtained, such as the presence of defects in the canvas support. in this case, the thermogram shows a series of dark areas (the most significant is indicated by the dashed arrow in figure 11b), probably corresponding to restored gaps or filled detachments, which, conversely, are barely visible in the reflectogram (figure 11c). important information enabling an understanding of the artistic technique followed for the execution of this painting was obtained from the analysis of the images recorded in the area of noah’s face (figure 12a). here, the thermogram reveals several dark spots (arrows in figure 12b), presumably corresponding to restorations of the painting’s surface. moreover, the lack of subsurface graphical features in the reflectogram corresponding to the hair and beard (arrow and circle in figure 12c) could suggest the absence of underdrawings in this area. the imaging investigation carried out on the painting by andrea sacchi reveals some pentimenti and small variations in the pictorial composition. such findings can be thought to be in agreement with the continuous improvements introduced by sacchi in the representation of this biblical story, but these variations do not indicate whether the chigi painting is one of original versions of the numerous copies produced by the artist. 5. conclusions in this work, an innovative approach based on the combined use of two mid-wave infrared imaging techniques is proposed. the results obtained in the study of three xvii century paintings on canvas, preserved at the chigi palace in ariccia, are presented. in particular, it has been shown how pulsed thermography and mid-infrared reflectography can be successfully used to investigate features lying beneath the surface pictorial layer and, consequently, to obtain complementary information about the stratigraphy of a painted artwork. the capability of these techniques for investigating the paintings over different depth ranges has allowed the identification of several pentimenti in all three paintings, such as the head in the portrait by mario nuzzi, the cherubs of ‘la primavera’ and the head of the man in the ‘ebbrezza di noè’. furthermore, thermography has enabled the characterisation of the canvas structure and, in particular, revealed the presence of repaired defects. in the case of the portrait by mario nuzzi, both techniques revealed the join between the two pieces of canvas. in ‘la primavera’, the thermography investigations have highlighted the presence of numerous lacunas in relation to the woman’s hand, presumably repaired in the last restoration. similar surface damage has been revealed by thermography in ‘ebbrezza di noè’ near the area of noah’s foot and face. in the latter, the combination of the two mid-infrared imaging techniques has allowed significant information to be gathered for a better understanding of the process involved in the realisation of the hair and beard. in fact, these two elements are clearly visible in the thermographic image while, on the contrary, they are barely visible in the corresponding reflectogram, probably because of the transparency of the pigment in the mid-infrared range. this is consistent with the hypothesis that the absence of underdrawings in relation to noah’s beard and hair may suggest that these elements were painted by the artist in the final stage. these results could indicate that the painting preserved at the chigi palace in ariccia is the original version of the numerous copies realised by the painter, andrea sacchi. in conclusion, the results obtained by the combined use of thermography and reflectography have proven to offer useful information allowing an evaluation of the paintings’ state of conservation and, in addition, new insights into the processes followed by the artists in the execution of such complex artworks. acknowledgement this study has been carried out within the framework of the adamo project, part of the activities of the centre of excellence of the district of technologies for culture of lazio region (dtc). the authors acknowledge the staff of palazzo chigi for their support during the analyses. references [1] d. gavrilov, r. g. maev, d. p. almond, a review of imaging methods in analysis of works of art: thermographic imaging method in art analysis, can. j. phys. 92 (2014), pp. 341-364. doi: 10.1139/cjp-2013-0128 [2] m. faries, analytical capabilities of infrared reflectography: an art historian’s perspective, in: scientific examination of art, modern techniques in conservation and analysis, the national academies press, washington d.c., 2003, pp. 87-104. doi: 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(a) picture of the figure from the back and corresponding (b) thermogram and (c) mid-infrared reflectogram, with arrows and circles indicating some changes in the facial profile and the restored lacunas, respectively. figure 11. the ‘ebbrezza di noè’ by andrea sacchi, chigi palace, ariccia. (a) picture of noah’s foot and corresponding (b) thermogram and (c) midinfrared reflectogram. the red arrows indicate some changes in position, and the dashed arrow in (b) indicates one of the restored gaps. figure 12. the ‘ebbrezza di noè’ by andrea sacchi, chigi palace, ariccia. 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https://doi.org/10.1016/j.optlaseng.2017.10.006 application of machine learning techniques and empirical mode decomposition for the classification of analog modulated signals acta imeko issn: 2221-870x june 2020, volume 9, number 2, 66 74 acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 66 application of machine learning techniques and empirical mode decomposition for the classification of analog modulated signals domenico luca carnì1, eulalia balestrieri2, ioan tudosa2, francesco lamonaca2 1 department of computer science, modeling, electronics and system, university of calabria, ponte p. bucci 41c, rende (cs), italy 2 department of engineering, university of sannio, corso garibaldi 107, 82100, benevento,italy section: research paper keywords: classification of analog modulated signals; empirical mode decomposition; machine learning technique) citation: domenico luca carnì, eulalia balestrieri, ioan tudosa, francesco lamonaca, application of machine learning techniques and empirical mode decomposition for the classification of analog modulated signals, acta imeko, vol. 9, no. 2, article 11, june 2020, identifier: imeko-acta-09 (2020)-02-11 editor: dušan agrež, university of ljubljana, slovenia received march 12, 2020; in final form may 30, 2020; published june 2020 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: eulalia balestrieri, e-mail: eubal@unisannio.it 1. introduction communication systems typically use different analog and digital modulation techniques to send information between two or more apparatuses. the monitoring and classification of analog communication signals play an important role in military and civilian applications regarding the monitoring of non-authorised transmitters; electronic surveillance; and cognitive radio and reconfigurable communication systems [1][2]. in some of these applications, after the detection of the presence of a modulated signal in the spectrum [3][4], the automatic classification of the modulation scheme without a priori information is needed for the demodulation of the signal. the classification methods proposed in the literature can be divided into two main groups: decision theoretic and statistical pattern recognition. in the first group, the classification is based on the information obtained by signal enveloping characteristics, statistical moments, and phase trend [5]-[7]. usually, the decision theoretic classifiers require empirical evaluation of thresholds that are compared with the estimated parameters of the signal or, in any case, some previous knowledge about the detected signal. it is noteworthy that the empirical thresholds are evaluated in specific operating conditions, and as a consequence, the effectiveness of the classifier cannot be warranted for new cases. finally, the classification performance of the decision theoretic methods typically decreases along with the increasing of the amplitude of the noise superimposed on the signal. in the second group, the classification is obtained by the pattern recognition of some features estimated in the time or frequency domain on the signal under investigation [1][2][8]-[16]. moreover, the classification performance of the statistical pattern recognition methods typically decreases along with the increasing abstract in this article, an automatic analog modulation classifier based on empirical mode decomposition and machine learning approaches (amc-em) is proposed. the amc-em operates without a priori information and can recognise typical analog modulation schemes: amplitude modulation, phase modulation, frequency modulation, and single sideband modulation. the amc-em uses empirical mode decomposition (emd) to evaluate the features of the signal for the successive classification by using machine learning (ml). in the design of the amc-em, the selection of the specific ml technique is performed by comparing, with numerical tests, the performance of the (i) support vector machine (svm), (ii) k-nearest neighbor classifier, and (iii) adaptive boosting, since they are commonly used in the field of signal classification. the tests have highlighted that the svm, specifically the quadratic svm, permits the best possible performance concerning classification accuracy, by considering different noise intensities superimposed on the signal. to assess the advantages of the proposal, a comparison with other classifiers available in the literature has been undertaken through numerical tests. finally, the amc-em is experimentally evaluated, and the experimental results agree with those of the simulation. mailto:eubal@unisannio.it acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 67 of the amplitude of the noise superimposed on the input signal due to the variability of the features. the automatic modulation scheme classification methods proposed in the literature therefore present important limitations due to the requirement to have some previous knowledge about the detected signal, the dependence of the classifier effectiveness on specific operating conditions, and the influence of the amplitude of the noise superimposed on the signal on the classifier performance. in this article, a new analog modulation classifier (amc) relying on the joint utilisation of empirical mode decomposition (emd) and machine learning (ml) techniques is proposed to overcome the limitations of the existing classification methods. the amc-em estimates a set of features based on the analysis of the signal under investigation and uses these features as an input for a traditional classifier based on opportunely trained ml. the features are extracted by the emd [17]-[24] of the input signal. the emd is chosen for the feature estimation because, differently to other decomposition methods based on fourier or wavelet, it does not need a basis defined a priori. so, the emd is suitable for non-linear and non-stationary signals that are typical in telecommunications. moreover, each component obtained by the emd is related to a different band of the signal, and as a consequence, it is influenced in a different way to the superimposed noise. this allows for the acquisition of some components that are influenced by the noise. concerning the traditional classifier, which is based on ml, different techniques are compared by numerical tests in order to establish the one that ensures the best classification accuracy in different operating conditions. in this article, the operating conditions that are taken into account are (i) the inaccurate carrier frequency estimation of the signal under examination and (ii) different levels of superimposed noise. the ml techniques taken into account are those that are the most commonly used in the field of signal classification: (i) the support vector machines (svms) [1][25] with linear, quadratic, and cubic kernels, (ii) the k-nearest neighbor (knn) classifier [25], and (iii) adaptive boosting [25]. the signal modulation schemes considered in this paper are amplitude modulation (am), phase modulation (pm), frequency modulation (fm), and single-sideband (ssb) modulation. the paper is organised as follows: in order to make the article self-contained and to declare the symbols used to define the signals, in section 2, basic knowledge about the analog modulation schemes is briefly reported. the emd and the ml techniques are summarised in section 3 and section 4, respectively. in section 5, the signal generation parameters and the features that require estimation are highlighted, and a comparison of the training results of different ml techniques is provided. in section 6, the amc-em is compared with other classifiers proposed in the literature by numerical tests, and its experimental evaluation is presented. in the last section, conclusions are drawn. 2. analog modulation techniques the analog modulation techniques codify the information that is to be transmitted in the amplitude, frequency, or phase modifications of the carrier signal, which is based on a sinewave characterised by the frequency fc. the general form of an analog modulated signal is expressed by the function: 𝑠(𝑡) = 𝑥(𝑡)cos⁡(2𝜋𝑓𝑐𝑡 + 𝑚(𝑡) + θ) (1) where x(t) and m(t) are respectively the amplitude and the phase modification codifying the information, and  is the initial phase. the parameters of this form are imposed according to the particular modulation technique. in the case of am, the information is codified only in the amplitude of the carrier signal, and no modification is introduced in its phase. then, equation (1) is modified as follows: 𝑠(𝑡) = [1 + 𝑖𝑚𝑥(𝑡)]cos⁡(2𝜋𝑓𝑐𝑡) (2) with im being the modulation index and x(t) the information that is to be transmitted. in the pm case, the information is codified as an instantaneous phase variation of the carrier signal, while the amplitude of the carrier signal remains constant. therefore, the modulated signal can be expressed as: 𝑠(𝑡) = cos⁡(2𝜋𝑓𝑐𝑡 + 𝑘𝑝𝑚(𝑡) + θ) (3) where m(t) is the information that is to be transmitted, and kp is the deviation constant. in the fm case, the information is encoded in the instantaneous frequency of the carrier signal. in particular, the deviation of the carrier frequency with respect to its centre frequency is proportional to the amplitude of the information that is to be transmitted. to obtain this result, the information is integrated with respect to time and is added to the carrier frequency. the resulting signal can be expressed as: 𝑠(𝑡) = cos⁡(2𝜋𝑓𝑐𝑡 + δ𝑓 ∫ 𝑚(𝜏) 𝑡 −∞ 𝑑𝜏) (4) where f is the frequency deviation. by considering m(t), bounded in the range ±1, f represents the maximum variation of the instantaneous frequency of the modulated signal with respect to the carrier frequency. the ssb modulation is a particular am whereby the bandwidth of the signal is reduced. to obtain this result, equation (1) is modified as follows: 𝑠(𝑡) = 𝑥(𝑡)cos⁡(2𝜋𝑓𝑐𝑡) ∓ �̂�(𝑡)sin(2𝜋𝑓𝑐𝑡) (5) where �̂�(𝑡) is the hilbert transform of x(t). 3. empirical mode decomposition the emd is the basis of the hilbert-huang transform [1724]. through the emd, a signal is decomposed in a set of functions called intrinsic mode functions (imfs) and in a monotone residue. the imfs are characterised by slow amplitude variations with respect to the oscillation period. differently to other decomposition approaches, such as fourier and wavelet, the emd does not have a prefixed basis; therefore, the emd allows for the acquisition of a highly efficient decomposition. the main drawback of the emd is the unpredictable number of the obtained imfs. as a consequence, it is necessary to establish a suitable fixed number of imfs to be taken into account for classification purposes. on the one hand, the acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 68 number of imfs must be sufficiently high so as to allow the evaluation of a sufficient number of features for the ml classifier. on the other hand, it must be sufficiently small so as to be suitable for all the kinds of signal modulation schemes considered in this study. in order to present the theoretical tools that justify the selection of this number and to make the article self-contained, the criteria used by the emd to evaluate each imf and the implemented algorithm are described here. each imf satisfies two conditions [26]: 1. the number of local maxima, local minima, and zero crossings are equal or differ at most by one in the observation interval. this condition ensures that the local maxima and minima are positive and negative, respectively. 2. the mean value of the envelope defined by the local maxima and the envelope defined by the local minima is zero – at any time instant. this ensures that the instantaneous frequency does not have unwanted fluctuation due to the asymmetric waveform. by considering that s(t) is decomposed in a finite number n of imf, the input signal can be expressed as: 𝑠(𝑡) = ∑ imfi(t) + 𝑟(𝑡) 𝑁 𝑖=1 (6) where r(t) is the residue. the steps of the emd algorithm are presented in figure 1, and they are explained as follows: a. detect the local extrema of the input signal s(t), as the samples that are larger than four of its neighbouring samples. b. fit through cubic splines the upper and lower extrema to obtain the upper and lower envelope. figure 1. flowchart of the emd algorithm. figure 2. trend of an ssb modulated signal and its first four imf components. acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 69 c. evaluate the mean value m1(t) of the upper and lower envelope point by point. d. define h1(t) = s(t) m1(t). e. if the two imf conditions are satisfied by h1(t), then this last one is an imf. if they are not thus satisfied, then repeat from step (a) by considering h1(t) as the input. subtract from the input signal s(t) all the evaluated imf. if the residue has at least two extrema, repeat the procedure from step (a) on the residue for the evaluation of another imf. 4. machine learning techniques the ml includes algorithms designed to learn some of the data features from a dataset automatically [27]-[29]. ml theory proposes different techniques that can be classified as supervised or unsupervised. in supervised ml algorithms, the learning process requires the class and the features of each sample used for training. once the algorithm learns from the training samples, it can be used on unclassified samples. conversely, unsupervised learning algorithms tries to learn from unclassified samples (used for training) how to classify them. this article is focused on the most commonly used supervised learning techniques for signal classification [25]: svms, the knn classifier, and adaptive boosting. the svm [1][25] is a supervised learning algorithm that determines the separation of hyperplanes for the classification of different classes of data. the criteria for determining the hyperplanes is typically the maximisation of the margin among the classes and the minimisation of the classification error. in the case that the input data are not linearly separable, it is possible to map the data in a higher-dimensional feature space by using a nonlinear svm (for example, quadratic or cubic ones). the svm can be advantageously used in the convex optimisation problem i.e. the classification. in fact, the svm provides a unique solution. moreover, the classification is performed in the input space by using a kernel function. this also provides an efficient computation in the case of a large set of features. with knn, the classification of the input data is based on the closest training example in the feature space [25]. a data is classified on the basis of the majority vote of its knns, where the number of neighbours, k, is selected a priori [25]. the neighbours are taken from a dataset with known classification, and the common value of k is 1, for which the allocation is made to the nearest neighbour. the main advantage of knn is the computation time, which is lower than other ml techniques. adaptive boosting is a ml approach that combines many relatively weak and inaccurate rules (classifiers) to create a highly accurate prediction [30]. iteratively, for each estimated classifier, a score is assigned and linearly combined with the other ones to obtain the final classifier. 5. signal generation and feature extraction the set of signals used to train the ml techniques includes both real and simulated signals. in particular, in the following section, the case of voice signals is considered as a practical example. the band of the signals is limited to 4 khz, which is the typical single voice-frequency transmission channel bandwidth, by means of a 9th-order lowpass butterworth filter. the real signals are obtained by modulating audio conversation. instead, the simulated signals, as suggested in [1], are generated by means of a first-order autoregressive process: 𝑦(𝑘𝑇𝑠) = 0.95⁡𝑦((𝑘 − 1)𝑇𝑠) + 𝑛(𝑘𝑇𝑠) (7) where ts is the sampling period, and n(kts) is a random gaussian generator. the sampling frequency for the signals is 80 khz. figure 3. trend of a noisy ssb modulated signal and its first four imf components. acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 70 both the real and the simulated signals are modulated by using the schemes described in section 2. in order to obtain the features to train the classifiers and to ensure that these features include the case of inaccurate evaluation of the carrier frequency, for each modulation technique, a set of 650 modulated signals is considered. the set comprises 50 real signals and 600 simulated signals. by directly considering the intermediate frequency band, the intermediate carrier frequency used to generate the signals varies from 5 khz to 5.5 khz. this carrier frequency range permits the acquisition of the resulting signals with a low sampling frequency. finally, the modulated set of signals is affected by arbitrary white gaussian noise with a signal-to-noise ratio (snr) included in the range [-5, 65] db. to establish the number of imfs for the training of the ml techniques, it is considered that the decomposition process preliminarily isolates the high-frequency mode, and successively, with each step representing a lower-frequency mode. in fact, in figure 2, the case of an ssb modulated signal and its first four decomposition imfs is shown. the decomposition can change in terms of its dependence on the level of noise superimposed on the signal. figure 3 shows the decomposition of the ssb modulated signal shown in figure 2 with superimposed noise. the noise makes the imf1 characterised by higher variability on its trend compared with the one shown in figure 2 because imf1 mainly represents the noise. conversely, the other imf components show a similar trend to the corresponding ones in figure 2. for this reason, for the training of the ml techniques, features extracted from the first three imf components of the modulated signal affected by different values of noise are considered. the features extracted from the imf for the training of the ml techniques are: • standard deviation: the second-order moment that measures the variability of the data around the mean value, defined as: σimfi = √ 1 n ∑(imfi(k) − imf̅̅ ̅̅ ̅i) 2 n k=1 (8) where n is the number of samples and imf1 is the mean value of the i-th imf component. • skewness: the third-order moment that represents the asymmetry degree of the distribution of the value around the mean value of the imf, defined as: simfi = 1 n ∑ ( imfi(k) − imf̅̅ ̅̅ ̅i σimfi ) 3n k=1 (9) • kurtosis: the fourth-order moment that describes the flatness of the imf distribution with respect to the normal distribution, defined as: kimfi = 1 n ∑ ( imfi(k) − imfi̅̅ ̅̅ ̅̅ σimfi ) 4n k=1 (10) moreover, in order to better identify the phase modulation, the utilised parameter is the standard deviation of the instantaneous frequency of the imf component, defined as: 𝜎𝐷𝑝imf𝑖 = [ 1 n (∑ angle(hilbert(imf𝑖)) 2 n k=1 ) −⁡…⁡( 1 n ∑ angle(hilbert(imf𝑖)) n k=1 ) 2 ] 1 2 (11) figure 4. mean value of the standard deviation for am, pm, fm, and ssb modulation from the first three imfs. figure 5. mean value of the skewness for the modulation am, pm, fm, and ssb from the first three imfs. acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 71 in order to reduce the number of parameters used in the classification permitting the correct classification, the mean value of each parameter versus the snr is considered. figure 4 shows the mean value of the standard deviation for the considered modulation schemes versus the snr in the case the carrier frequency is 5 khz. the figure highlights that the first three imfs determine different regions in the feature space for the classification of each modulation scheme. the trend of the parameters is due to the spectrum distribution of the input signal, of the noise, and to the band represented by the imf. the increasing of the noise level reduces the imf band that will include more noise than signal. naturally, these parameters are insufficient for the classification, particularly for a low value of snr. in fact, the separation between the modulations is small, and the fluctuation of the parameters can generate a misclassification. figure 5 shows the mean value of the skewness for the considered modulation schemes versus the snr. in this case, the trend of the skewness of imf1 for the different modulation schemes shows a different range of value, particularly for the am, with minimal influence from the noise. instead, the other components do not show the same characteristic; therefore, they are not considered as being between the parameters for the training and classification of the ml techniques. a similar analysis is performed for the mean value of the kurtosis of the imf of the modulated signals versus the snr showed in figure 6. in this case, the kurtosis value of imf1 shows a separation between the different modulation and is used for the classification. the other decompositions do not show a separation among the modulations, particularly for a low snr value. finally, figure 7 shows the mean value of dp for the modulation scheme under consideration versus the snr value. this figure highlights that all the three imfs furnish information that can be used for the classification. this parameter is particularly important in the case that a carrier frequency error is present on the signal under examination. then, the total number of features used in the ml algorithm is 12. 6. numerical and experimental tests the feature extraction and classification systems are implemented in the matlab environment. the number of samples used for the classification feature estimation is 20,000, which represents an observation time of 0.25 s [1]. the training set of signals is generated as previously described, and the confusion matrix for the considered ml techniques is shown in figure 8. the diagonal elements represent the modulated signals correctly classified. instead, the values outside the diagonal are the misclassified signals. as evidenced, the linear, quadratic, and cubic svms and the knn have an overall classification accuracy of 100 %. instead, the adaptive boosting method completely misclassifies the fm signals – recognised as pm. for this reason, in the successive analysis, the adaptive boosting ml technique is not considered. in order to verify the behaviour of the ml techniques with signals not included in the training set, for each modulation scheme, a test set of 200 signals is generated with a carrier frequency included in the range [5.0, 5.5] khz, and with the snr equal to 5 db. figure 9 shows the confusion matrix for these signals: for the quadratic svm, the overall classification accuracy continues to be 100 %; it decreases to 99.9 % for the linear and cubic svms and to 99.5 % for the knn. 6.1. comparison with other methods in order to assess the advantages of the proposed classifier, amc-em is compared with another classifier available in the literature [15] that is capable of classifying the same modulation schemes as the proposed one. figure 6. mean value of the kurtosis for the am, pm, fm, and ssb modulation from the first three imfs. figure 7. mean value of σdp for the modulation am, pm, fm, and ssb modulations from the first three imfs. acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 72 other available classifiers are specific to single modulation and fail in the case examined in this article [24]. the comparison is performed by numerical tests executed in the aforementioned operating conditions. the method proposed in [15] is based on artificial neural network (ann) for the pattern recognition of a set of features extracted from the modulated signal. the features used by the method proposed in [15] are: • the maximum of the squared fourier transform of the normalised signal amplitude; • the index of the spectrum symmetry around the carrier frequency; and • the standard deviation of the instantaneous phase. the first disadvantage of the classifier proposed in [15] with respect to the proposed amc-em is that it requires knowledge of the carrier frequency, and this information must be included in the training and in the test phases. this a priori requirement requires that the numerical tests are executed by considering a fixed value of the carrier frequency in order to compare the two methods. in the following section, a carrier frequency of 5.25 khz is selected. the same number of samples and sampling frequency are used to test both the classifiers. after the training of the network, 100 signals for each modulation scheme are classified by the two methods: the proposed one (i.e. the quadratic svm with features extracted from the emd) and the ann proposed in [15]. figure 10 shows the overall accuracy of the two methods versus the snr. if the snr of the signal is higher than 5 db, both methods obtain an accuracy of 100 %. for an snr lower or equal to 5 db, the ann decreases its accuracy with respect to the proposed method. 6.2. experimental tests in order to evaluate the performance of the proposed amcem experimentally, a specific test bed is designed (figure 11), figure 8. confusion matrix of the training set for the ml techniques taken into consideration. figure 9. confusion matrix of the classification results with signals not included in the training set. figure 10. overall accuracy, in percentage, for the proposed amc-em and for the classifier proposed in [15]. acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 73 allowing for the acquisition and analysis of the modulated signals in a wide-band spectrum [31]. the test bed is formed of: • a pc for the management of the instrumentation and the analysis of the acquired signals; • a pxi chassis, which includes: o an ni 5622 if digitiser from national instruments for the acquisition of the signals; and o a pxie-5641r transceiver for signal generation. suitable software developed in labview runs on the pc for the instrumentation setup and the acquisition of the signals. in particular, the labview software: • sets up the pxie-5641r to generate a signal with a carrier frequency of 305.25 khz, a selected modulation, and an established superimposed level of noise; • triggers the pxie-5641r to generate the signal; and • triggers the pxi 5622 to acquire the modulated signal, which is digitally down-converted to have a carrier frequency of 5.25 khz and is down-sampled to 80 khz. after the digital down conversion, the labview software executes the matlab code for the classification. in the tests, 100 signals are used for each modulation scheme. figure 12 shows the results of these experimental tests by varying the snr. in particular, 0, 5, and 10 db of snr have been considered. as shown in figure 12, the experimental results agree with the numerical ones. in particular, the robustness of the amc-em is also experimentally assessed for a low value of snr: at 0 db, there is a misevaluation of the fm signals of only 2 %. 7. conclusions in this article, a new automatic analog modulation classifier, the amc-em, was presented. the amc-em works without the a priori information of the input signals. it analyses the modulated signal by means of the emd to extract the features that are used by a suitable ml technique for the classification. selection of the specific ml technique is performed by comparing the performance of the most commonly used ones in the field of signal classification and the quadratic svm has showed the best performance. the amc-em has been characterised in simulation by considering signals corrupted by additive white gaussian noise, and it also shows very interesting results for low snr values. the usefulness of the proposed classifier with respect to the existing ones was assessed by comparing the amc-em with another classifier, available in the literature, based on ann. with respect to other classifiers presented in the literature, the ann can be used with different modulations, such as the amc-em. conversely, the other ones are suitable only for specific modulations. differently to amc-em, the ann classifier requires the a priori knowledge of the carrier frequency, while the amc-em operates in completely blind conditions. moreover, by testing the two classifiers in the same operating conditions, it is highlighted that the amc-em is more robust concerning the influence of noise. experimental tests have been carried out to evaluate the amc-em with real signals. the experimental results agree with the simulated ones, making amc-em a promising method for the automatic classification of the analog signal in typically noisy environments. references [1] a. sengur, multiclass least-squares support 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[31] l. angrisani, f. bonavolontà, a. liccardo, r. schiano lo moriello, l. ferrigno, m. laracca, g. miele, multi-channel simultaneous data acquisition through a compressive samplingbased approach, meas. j. int. meas. confed. 52(1) (2014) pp. 156172. acta imeko, title acta imeko issn: 2221-870x march 2018, volume 7, number 1, 0 acta imeko | www.imeko.org march 2018 | volume 7 | number 1| 0 journal contacts about the journal acta imeko is an e-journal reporting on the contributions on the state and progress of the science and technology of measurement. the articles are mainly based on presentations presented at imeko workshops, symposia and congresses. the journal is published by imeko, the international measurement confederation. the issn, the international identifier for serials, is 2221-870x. editor-in-chief dušan agrež, slovenia founding editor-in-chief paul p.l. regtien, netherlands associate editor dirk röske, germany editorial board section editors (vol. 6 and 7) leopoldo angrisani, italy filippo attivissimo, italy eulalia balestieri, italy eric benoit, france paolo carbone, italy lorenzo ciani, italy catalin damian, romania pasquale daponte, italy luca de vito, italy luigi ferrigno, italy edoardo fiorucci, italy alistair forbes, united kingdom helena geirinhas ramos, portugal sabrina grassini, italy fernando janeiro, portugal konrad jedrzejewski, poland andy knott, united kingdom francesco lamonaca, italy massimo lazzaroni, italy fabio leccese, italy rosario morello, italy michele norgia, italy pedro miguel pinto ramos, portugal nicola pompeo, italy sergio rapuano, italy dirk röske, germany alexandru salceanu, romania constantin sarmasanu, romania lorenzo scalise, italy emiliano schena, italy enrico silva, italy krzysztof stepien, poland marco tarabini, italy marcantonio catelani, italy paolo carbone, italy lorenzo ciani, italy egidio de benedeto, italy alessandro germak, italy sabrina grassini, italy konrad jędrzejewski, poland min-seok kim, korea fabio leccese, italy rosario morello, italy vilmos palfi, hungary nicola pasquino, italy franco pavese, italy paul regtien, the netherlands alexandru salceanu, romania jan saliga, slovakia jorge c. torres-guzman, mexico ian veldman, south africa claudia zoani, italy final editing rosario morello, university mediterranea of reggio calabria, italy about imeko the international measurement confederation, imeko, is an international federation of actually 42 national member organisations individually concerned with the advancement of measurement technology. its fundamental objectives are the promotion of international interchange of scientific and technical information in the field of measurement, and the enhancement of international co-operation among scientists and engineers from research and industry. addresses principal contact prof. dušan agrež university of ljubljana faculty of electrical engineering tržaška 25, 1000 ljubljana, slovenia email: dusan.agrez@fe.uni-lj.si acta imeko dr. rosario morello dep.t diies, university mediterranea of reggio calabria via graziella, 89122 reggio calabria, italy email: rosario.morello@unirc.it dr. sabrina grassini dept. of applied science and technology, politecnico di torino corso duca degli abruzzi 24, 10129 torino, italy email: sabrina.grassini@polito.it support contact dr. dirk röske physikalisch-technische bundesanstalt (ptb) bundesallee 100, 38116 braunschweig, germany email: dirk.roeske@ptb.de journal contacts << /ascii85encodepages false /allowtransparency false /autopositionepsfiles true /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /error /compatibilitylevel 1.4 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.0000 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/hrv (za stvaranje adobe pdf dokumenata najpogodnijih za visokokvalitetni ispis prije tiskanja koristite ove postavke. stvoreni pdf dokumenti mogu se otvoriti acrobat i adobe reader 5.0 i kasnijim verzijama.) /hun 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can be opened with acrobat and adobe reader 5.0 and later.) >> /namespace [ (adobe) (common) (1.0) ] /othernamespaces [ << /asreaderspreads false /cropimagestoframes true /errorcontrol /warnandcontinue /flattenerignorespreadoverrides false /includeguidesgrids false /includenonprinting false /includeslug false /namespace [ (adobe) (indesign) (4.0) ] /omitplacedbitmaps false /omitplacedeps false /omitplacedpdf false /simulateoverprint /legacy >> << /addbleedmarks false /addcolorbars false /addcropmarks false /addpageinfo false /addregmarks false /convertcolors /converttocmyk /destinationprofilename () /destinationprofileselector /documentcmyk /downsample16bitimages true /flattenerpreset << /presetselector /mediumresolution >> /formelements false /generatestructure false /includebookmarks false /includehyperlinks false /includeinteractive false /includelayers false /includeprofiles false /multimediahandling /useobjectsettings /namespace [ (adobe) (creativesuite) (2.0) ] /pdfxoutputintentprofileselector /documentcmyk /preserveediting true /untaggedcmykhandling /leaveuntagged /untaggedrgbhandling /usedocumentprofile /usedocumentbleed false >> ] >> setdistillerparams << /hwresolution [2400 2400] /pagesize [612.000 792.000] >> setpagedevice acta imeko, title acta imeko issn: 2221-870x december 2017, volume 6, number 4, 2-4 acta imeko | www.imeko.org december 2017 | volume 6 | number 4|2 introductory notes for the acta imeko special section on the “21st symposium on measurement of electrical quantities” and the “19th workshop on adc/dac modelling and testing” alexandru salceanu, vilmos pálfi ”gheorghe asachi” technical university of iasi, mangeron str. 67, 700050, iasi, romania budapest university of technology and economics, magyar tudósok krt. 2. i, 1117, budapest, hungary section: editorial citation: alexandru salceanu, vilmos pálfi, introductory notes for the acta imeko special section on the “21st symposium on measurement of electrical quantities” and the “19th workshop on adc/dac modelling and testing”, acta imeko, vol. 6, no. 4, article 1, december 2017, identifier: imeko-acta-06 (2017)-04-01 section editor: paul regtien, measurement science consultancy, the netherlands received: november 24, 2017; in final form: november 24, 2017; published december 2017 copyright: © 2017 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: alexandrusalceanu, e-mail: asalcean@tuiasi.ro 1. introduction number 4, volume 6 of acta imeko is submitted to your assessments and suggestions. this issue is centered on the 21st symposium on measurement of electrical quantities and 19th workshop on adc/dac modelling and testing which were held in the city of budapest, hungary on september 2016. the main objectives of these scientific tribute and events were to present the latest research, to stimulate information exchanges on current research dedicated to the measurement, monitoring and recording of electrical signals. since october till december 2016, the members of the imeko tc4 board identified 11 valuable contributions presented in the budapest events, for eventual publication in acta imeko. their authors were invited in december 2016 to submit extended and updated versions of their selected papers. all of the authors that gave positive responses to this challenge started sending their extended papers to the acta imeko online submission system early 2017. a significant number of representative reviewers proceeded in successive stages, their task of assessing the papers and submitting their recommendations. the final list of 8 published papers in this issue is a high-quality exhibition of the first-class papers submitted to our symposium. all the reviewers involved in the process gave their support aiming to further improve not only the shape but also the intrinsic content of the papers. we sincerely hope that all the contributions you will find in the next pages provide you up-to-date information of your interest, being an invitation to continue and develop your multilevel partnership with acta imeko. 2. about tc4 the key, vital interest and activity of imeko tc4 (measurement of electrical quantities) is the development of theoretical and practical research in the field of electrical and electronic measurements. this technical committee organizes each year, since its inception in 1984, specific symposia and workshops. the continued boost in general relevance and the inclusion of electrical and electronic tools for the measurement, monitoring and recording of electrical signals in our daily lives has significantly improved our activities and increased the world-wide addressability. the current members of tc4 are: dominique dallet, chairperson france pedro m. ramos, deputy chairperson -portugal alexandru salceanu, scientific secretary romania linus michaeli, past chairperson slovak republic pasquale daponte, past chairperson italy antonio manuel da cruz serra, past chairperson –portugal janusz mindykowski, past chairperson poland mario savino, honorary chairperson italy raul land estonia joaquin del rio fernández spain turgay özkan turkey umberto pogliano italy gelson rocha brazil jan saliga slovak republic dragana popovic renella-switzerland sergey g. semenchinsky russia yurij tesyk ukraine http://www.bme.hu/ acta imeko | www.imeko.org december 2017 | volume 6 | number 4|3 olfa kanoun germany dušan agrež slovenia victor i. didenko russia izzet kale united kingdom leo van biesen belgium laurent francis-belgium mihai cretu romania christian eugène – belgium sohair fakhry egypt vladimir haasz czech republic jouko halttunen finland peter händel sweden damir ilic croatia voicu groza canada elefterios kayafas greece wang xiaofei china he qing – china platon sovilj-serbia hyung kew lee –republic of korea 3. the articles the paper submitted by zsolt kemény et al (p.5) presents developed research focused on the dependence of work-surface resistance on ambient conditions and surface contamination in a live industrial environment, relying on an automated facility management. it is beneficial that the results have been taken without interfering with the working system, while using contactless and remote techniques. the model of surface resistance is based on data acquired during one year of various measurements. the paper presented by marco crescentini et al (p.17) is mainly an experimental research based on a solid theoretical foundation on the evaluation of the upper limit of hall sensors’ bandwidth. in the paper an innovative technique is developed, specially dedicated to estimating the response time of the hall sensor, proposing an equivalent electric model for its dynamic response. a very useful differentiation between the upper limit of the bandwidth determined by intrinsic capacitance and the more practical one imposed by the general input capacitance of the reading interface is sustained. the hardware and software implementation in labview of a low-cost multichannel interface designed for passive resistive sensors is detailed in the paper put forward by zivko kokolanski et al (p.25). the use of a monostable configuration for the multivibrator oscillator has the advantage of direct interfacing with fpgas or microcontrollers. the presented details of sensor interfacing and calibration techniques are convincing, being also supported by the practical realization of a prototype multichannel resistive sensor interface that could be a very suitable solution when a medium accuracy, low-cost and small-sized implementation is desirable. a compact size, high-precision amplifier for a strain-gauge transducer is here provided by andré schäfer (p.31). the argumentation for the use of inductive dividers (based on the ratio of the number of windings of the coil) versus resistive dividers to the realization of high precision amplifiers involved for primary calibration purposes is detailed. a compact dualchannel module is presented which can process and display multiple signals, with different requirements for accuracy and bandwidths. the very good time and temperature stability of the amplifiers is proved by the performed measurements. the general approach proposed by alexandru salceanu et al (p.37) is developed around an affordable determination of low frequency electric fields, from the perspective of sm enterprises, which have to comply with the restrictions imposed by directive 2013/35/eu. the mechanisms by which electric fields interact with humans are reviewed, their values and limitations being comparatively analyzed, while respecting the documentation elaborated by different specialized bodies. constructive details of two laboratory-made suitable e-field sensors are also described. the case study detailing a cumulative assessment methodology for evaluating the exposure to electric fields with different frequencies is complemented by modeling the electric field in the vicinity of an overhead high voltage line with specific configuration, based on free, open access software. the topic of improving the automated lung screening for acquiring depth information, without high exposure to x-rays, at affordable cost is dealt by bence tilk (p.46).a very good compromise between time, cost and exposure to ionizing radiations could be the digital tomosynthesis (dts). the paper presents solutions for solving the main problem of dts, the blurred slice image that makes it hard to automatically segment the lung areas, essential requirement for an efficient computer-aided diagnosis system. a combined approach is proposed by using snake methods and adjacent image segmentation information to improve lung screening and to provide robust solutions. three different model-free approaches are used to take advantage of each method on different areas of the lungs: intensity-based segmentation for center point identification, gradient-based segmentation for non-central points of the lung and snake for segment the outer side of the lung. the advantage of using more complex models is to gain information from adjacent images to improve the incorrectly segmented region on the non-central images. an improvement of the estimation of the signal components from random equivalent and non-coherent sampling measurements is provided by dušan agrež (p.54). a balance between theory and experiment is submitted, by demonstrating the behavior of estimations of the periodic signal parameters in the case of random sampling and under non-coherent sampling condition in a real measurement environment. aiming to achieve the lowest estimation errors, the non-parametric idft estimation of frequency is here used first for detection of the component frequency in both the parametric and nonparametric approach and complementary, the iterative parametric method has been used for other parameters when the sampling frequency is relatively lower than the nyquist frequency and the sampling procedure becomes pure random. simulations and experimental results demonstrated that the estimations of the components frequency, amplitude and phase are possible when the duty ratio of the random samples from the total number of samples in the non-coherent measurement interval is very low. in paper (p.61) lászló sujbert et al introduced an identification procedure for random independent, random block‐based and general markov model‐based data loss patterns. in this paper the authors proposed a quasi-inverse procedure, by comparison with the mostly encountered one: the data loss model can be identified by the fourier transform of the data availability indicator function. first the power spectral density of the indicator function was calculated and then a parametric system identification method has been used to get the spectral parameters. by comparing the measurement spectra to the model spectra, the model order selection and acta imeko | www.imeko.org december 2017 | volume 6 | number 4|4 verification can be also easily done. the developed method provides not only an estimation of the loss signal, but also makes an estimation of the parameters of the data loss model available. 4. conclusions it was a great honor for us to act as guest editors for an issue of acta imeko, both from the perspective of working for a high-profile scientific journal and also, of promoting tc4’s great tradition symposia, now reaching the 21st edition. one of us had previously the privilege of working as guest editor for a special issue of acta imeko in 2013, after the barcelona tc4 symposium. we have noted with satisfaction the increase in quality in general, mainly the higher flexibility of the electronic submission platform, the constructive exigencies of the reviewers’ team, elevated standards for copyediting, proofreading and layout editing services. but, first of all, we had to take notice of the superior quality of end-accepted articles for publication. a special appreciation for the two editors in chief, paul p.l. regtien (who, even if “honorary”, continues to be very active, prompt and involved) and paolo carbone, whom i felt completely devoted to this wonderful ascendant project called acta imeko. introductory notes for the acta imeko special section on the “21st symposium on measurement of electrical quantities” and the “19th workshop on adc/dac modelling and testing” << /ascii85encodepages false /allowtransparency false /autopositionepsfiles true /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /error /compatibilitylevel 1.4 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false 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/hrv (za stvaranje adobe pdf dokumenata najpogodnijih za visokokvalitetni ispis prije tiskanja koristite ove postavke. stvoreni pdf dokumenti mogu se otvoriti acrobat i adobe reader 5.0 i kasnijim verzijama.) /hun 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can be opened with acrobat and adobe reader 5.0 and later.) >> /namespace [ (adobe) (common) (1.0) ] /othernamespaces [ << /asreaderspreads false /cropimagestoframes true /errorcontrol /warnandcontinue /flattenerignorespreadoverrides false /includeguidesgrids false /includenonprinting false /includeslug false /namespace [ (adobe) (indesign) (4.0) ] /omitplacedbitmaps false /omitplacedeps false /omitplacedpdf false /simulateoverprint /legacy >> << /addbleedmarks false /addcolorbars false /addcropmarks false /addpageinfo false /addregmarks false /convertcolors /converttocmyk /destinationprofilename () /destinationprofileselector /documentcmyk /downsample16bitimages true /flattenerpreset << /presetselector /mediumresolution >> /formelements false /generatestructure false /includebookmarks false /includehyperlinks false /includeinteractive false /includelayers false /includeprofiles false /multimediahandling /useobjectsettings /namespace [ (adobe) (creativesuite) (2.0) ] /pdfxoutputintentprofileselector /documentcmyk /preserveediting true /untaggedcmykhandling /leaveuntagged /untaggedrgbhandling /usedocumentprofile /usedocumentbleed false >> ] >> setdistillerparams << /hwresolution [2400 2400] /pagesize [612.000 792.000] >> setpagedevice fault detection and diagnosis of historical vehicle engines using acoustic emission techniques acta imeko issn: 2221-870x march 2021, volume 10, number 1, 77 83 acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 77 fault detection and diagnosis of historical vehicle engines using acoustic emission techniques alejandro roda-buch1,2, emilie cornet1, guillaume rapp1, brice chalançon3, stefano mischler2, laura brambilla1 1 haute ecole arc conservation-restauration, hes-so university of applied sciences and arts western switzerland, espace de l’europe 11, ch-2000, neuchâtel, switzerland 2 ecole polytechnique fédérale de lausanne, epfl ch-1015, lausanne, switzerland 3 association de gestion du musée national de l’automobile, 188 av. de colmar, 68100 mulhouse, france section: research paper keywords: historical vehicles; acoustic emission; ndt; fault diagnosis citation: alejandro roda-buch, emilie cornet, guillaume rapp, brice chalançon, stefano mischler, laura brambilla, fault detection and diagnosis of historical vehicle engines using acoustic emission techniques, acta imeko, vol. 10, no. 1, article 11, march 2021, identifier: imeko-acta-10 (2021)-01-11 editor: eulalia balestrieri, university of sannio, italy received may 1, 2020; in final form september 26, 2020; published march 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was supported by hes-so rcdav funding, switzerland corresponding author: laura brambilla, e-mail: laura.brambilla@he-arc.ch 1. introduction the peculiarity of the objects constituting technical, industrial, and scientific cultural heritage lies in the presence of specific mechanisms. the functionality of these artefacts, i.e. the possibility of making them work, is an integral part of the objects themselves. the reactivation of these mechanisms, however, is often a challenge for the conservators, especially if the functioning has been stopped for a long period of time. in short, the presence of corrosive products, deposits, oxidation, and particles, as well as the scaling of oils or lubricants, can prevent the mechanisms from working properly or can even lead to their breakdown during reactivation. in addition, the reactivation procedure must be performed while respecting the material authenticity of the object, i.e. preserving the original parts of the mechanism as far as possible. among the objects that incorporate mechanisms, vehicles are especially complex due to of the number of parts and or subsystems involved in their functioning. given the aforementioned obstacles to the reactivation of historical engines, the choice of exhibition and maintenance of the vehicles in museums and collections is generally reduced to the following two options: i. static: the vehicle is exposed and its engine is never operated (on occasion, the engine is even removed from the vehicle for practical and safety reasons); ii. dynamic: the vehicle is used, or at least activated periodically. in order to be able to reactivate the mechanisms without damage while simultaneously preserving the cultural value and the original materials of the objects, the conservators require specific diagnostic tools for the detection of any malfunctions at a very early stage. in the field of historical vehicle conservation, there currently exists no reliable non-invasive technique for the diagnosis and monitoring of the engine’s functioning. therefore, the maintenance of old engines is extremely time consuming and abstract the reactivation of artefact mechanisms is always a challenge for conservators. non-invasive diagnostic techniques, applicable directly on the artifacts, allows for performing early-stage diagnostics and avoiding damage. the acoustic emission monitoring of historical vehicles (acume_hv) project represents the first use of acoustic emission (ae) as a non-invasive technique for the diagnostics of historical vehicles. the aim of this project is to develop an objective, human-independent method. this will help museum personnel to make decisions regarding the reactivation of historical vehicle engines using measurements and data analysis rather than merely personal experience. herein, we present the results of the first phase of the acume_hv project, which was focused on the development of a protocol for the use of ae during cold tests. mailto:laura.brambilla@he-arc.ch acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 78 requires personnel with specialised competency. indeed, the assessment of the correct functioning is generally left to the expertise of the specialists, who tend to largely base their evaluation on previous experience, using methods ranging from visual examination to listening to the sound of the engine. however, this approach is empirical, human-dependent, and has never been correctly standardised. as such, the use of non-invasive techniques, such as acoustic emission (ae) techniques [1], for the evaluation of the mechanisms’ condition could prevent damage occurring during the reactivation and could help in developing a monitoring protocol based on factual measurements. the acoustic emission monitoring of historical vehicles (acume_hv) project, led by he-arc cr, represents the first use of ae as a non-invasive technique for the diagnostics of historical vehicles, and was carried out in collaboration with the musée national de l’automobile de mulhouse (mnam) in france. the aim of this project is to develop an objective, human-independent method that will assist museum personnel with their decision making regarding the reactivation of historical vehicle engines using measurements and data analysis rather than personal experience alone. the use of the ae technique has already been applied to the diagnosis of faults during the operation of newly produced engines [2]-[10]. here, el-ghambry et al. [4] analysed the ae rms signals of diesel engines in the time domain in view of identifying – using the machine timing – machine fault conditions. the authors used selective time windowing of the ae signals and different statistical features and pattern recognition techniques to isolate and identify various fault conditions in reciprocating engines. meanwhile, douglas et al. [5] studied the interaction between piston rings and cylinder liners using ae for in-service engine monitoring and found that, for small high-speed direct injection (hsdi) engines, the ae activity was proportional to the piston speed and that the most likely ae source (among other contributions) was the boundary friction between the oil-control ring and the cylinder liner. various analytical models and predictive tools [11]-[19] have been developed through using the ae technique to characterise and model the friction and wear in sliding contacts under various lubricated conditions, including the scuffing phenomenon. meanwhile, in the field of cultural heritage, the ae technique has been used for monitoring the conservation state of heritage objects or for detecting insects in wooden musical instruments [20]-[22]. in addition, ae has been used for several years for the structural damage analysis and health monitoring of historical masonry buildings. of particular interest in this domain is the application of the gutenberg–richter (gbr) law to ae measurements in order to correlate the magnitude and the number of events within a certain time period [23]-[26]. despite the extended use of the ae technique in the field of modern engine diagnosis, the capacity of this technique for providing valuable diagnostic results remains a challenge in the field of cultural heritage. in short, the attendant limitations arise from the technical characteristics of the historical vehicles and their conservation state, mainly in the case of reactivation processes. the present work is organised in terms of five sections. in section 2, the methodology adopted for the acume_hv project to define a protocol for the reactivation of the historical vehicle engines is described before section 3 describes the measuring system and the set of tests carried out to follow the discussed methodology. in section 4, the results accounting for the non-uniform driving speed of the engine during the tests are presented and discussed as is a comparison between the tests to discriminate the effect of compression in the cylinders. finally, the main conclusions are summarised in section 5. 2. methodology a reliable diagnostic tool must have the capacity to capture signal features that can be correlated with the state of the system. the conservation/restoration procedure for reactivating an engine [27] begins with a visual inspection of the whole engine in view of evaluating the general condition of the individual components. at this stage, the liquids of the oil and cooling systems are changed and the accessorial components are repaired where necessary. prior to starting the engine, the next step involves assessing the functioning of the mechanisms. in order to achieve this, the engine is moved manually before an expert checks the engine’s operation, both visually and acoustically. this procedure is known as a ‘cold test’ and is performed without starting the engine to minimise the possible damage during the reactivation process. to ensure this condition, the combustion process is excluded from the evaluation, with only the mechanical displacements taken into account [27]. at this critical stage, the procedure developed during the acume_hv project will be introduced in view of replacing the human-dependent diagnosis, with the measurement of the ae signals generated by friction and/or impacts at different contact pairs of the engine mechanisms, such as crankshaft/connecting rod, connecting rod/piston, piston rings/cylinder liner, as well as inside the cam chain system. the airflow or air leakage at the valve inlets/outlets and between the piston-ring/cylinder-liner gaps can also be detected at this point. the following step of this new procedure involves obtaining the mechanical signature of the engine for different reactivating conditions (i.e. with/without cylinder compression). finally, the last step involves the use of statistical analysis techniques to extract specific features that will then be used to classify and correlate the operational state of the mechanical parts of the engine. first, the method was tested on an engine that was removed from the vehicle to ensure complete access to all the parts of the engine. this engine, known as a ‘bench engine’, was bought by the mnam as a spare for the possible reparation of certain vehicles in their collection. the engine was mounted on a test bench while keeping all of the cam system and pump mechanisms operative. the mechanical parts were manually operated at a relatively low rotating speed through turning the crankshaft via a handle. this procedure allowed for maintaining good control of the interplay between different parts, thus avoiding possible damage to the machine. 3. engine, measurement system and test sets the ae tests were performed on two renault ag1-type engines (figure 1), which are part of the collection held by the musée national de l’automobile de mulhouse. among the reasons for selecting this specific model were the easy access to most parts of the engine, even when assembled in the vehicle, the simplicity of the engine, which has only two cylinders and basic auxiliary systems, and the availability at mnan of three identical engines (ag1), two of which are still in working order [28]. the measurement equipment is shown in figure 2a, while a block diagram of the experimental set-up is schematised in acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 79 figure 2b. an ae system from vallen® was used to acquire the ae signals, which included a mb2-v1 chassis with four ae sensors and four parametric input channels. the four broadband ae sensors were vs900-m (between 100 and 900 khz) sensors, with the corresponding aep5 preamplifiers (+34 db) also included in the system. the sampling frequency of the ae signals was set to 2 mhz, while a 1-mhz low pass filter was used to reduce the noise from high frequencies. the crankshaft angular position was also measured with a full continuous 360 ° smart position sensor (vishay spectrol 601-1045; output signal 0– 5 v) at a sampling rate of 1.25 khz. to carry out this research, three different sets of tests were implemented. an initial set of measurements was performed to determine the optimal location of the ae sensors for obtaining representative signals from different ae sources [29], [30]. the locations selected for the following sets of tests were those presenting higher signal levels, lower signal-to-noise ratios, and the highest presence of events for the measured ae signals. figure 3 shows the final locations of the four ae sensors. here, two were placed on the cylinder block, the first (1) on the outer part of the first cylinder (the one closest to the front), and the second (2) on the opposite side close to the first cylinder valves. meanwhile, the remaining two sensors were placed on the crankcase, one (3) on the cover of the gears of the cam system, and the other (4) on the crankcase leg of the valve side. a second set of tests was then conducted to analyse the effect of the engine speed on the ae signal level. in this set, the spark plugs of both cylinders were removed to prevent air compression inside the cylinders and, consequently, to produce a smoother and more constant motion of the engine. here, different engine speeds were tested, ranging from 0.25 to 1.2 cycles per second (cps), with the engine activated manually using an external handle. finally, a third set of tests was performed to observe the influence of the compression of the air inside the cylinders on the ae signals. the results obtained with the spark plugs removed (set 2) were then compared with those obtained with the plugs left in (set 3). test sets 2 and 3 were also performed on two different engines to observe the main characteristics of the mechanical signature and the mechanical airflow signature of different motors of the same type, that is, the bench engine already used for test set 1 and the engine of the collection car, which remained mounted in the car. figure 1. renault ag1 (inv.2209) with its 2 cylinder engine (© mnam, 2016). figure 2. (a) vallen® measurement system of ae signals, (b) block diagram of the experimental set-up (© he-arc cr, 2018). figure 3. ae sensor location on the renault ag1 engine (© he-arc cr, 2019). acoustic emission sensor daq + conditioning pc + signal analysis techniques tested engine (b) acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 80 4. results in this section, the results for two specific studies are presented. the first study relates to the influence of the rotational speed of the engine on the ae feature and the second to the mechanical signature and the mechanical airflow signature of the two engines. the root means square of the ae signals (ae rms) was the feature used to extract the mechanical and air compression signature of the engine under cold-test conditions. the rms was calculated for blocks of signals with a rate of 10 ms. this feature is related to the ae signal energy generated by, for example, the sliding friction processes in contact pairs [6]. 4.1. engine speed influence in acoustic emission signal feature figure 4a shows the time change of the ae rms of the cylinder at sensor 1, figure 4b the angular position, and figure 4c the rotational speed of the crankshaft. in figure 4b, the crankshaft angular position is related to the thermodynamic cycle of the engine, which is comprised of 720 ° or two complete crankshaft revolutions. when the first piston was at the top dead centre (tdc) position, the rotation reached angular values of 0 °, 360 ° or 720 °. it can be seen that the test started slightly before the piston passed by the tdc before the crankshaft was rotated by four revolutions and its movement was extended for a little longer. this procedure guaranteed that two thermodynamic cycles were fully recorded. figure 4c was obtained using the time derivative of the curve in figure 4b. it was clear that there was a high correlation between the ae rms feature and the crankshaft angular position and, consequently, the relative motion of the piston to the cylinder. minimum ae rms values emerged at the tdc and the bottom dead centre (bdc) of both pistons where the piston speed was the lowest, while maximum values emerged at the mid position between the tdc and bdc where the piston speed was maximised. this indicates that the main source of the ae waves was the sliding friction between the piston rings and the cylinder liners. another outcome that must be highlighted is that the crankshaft rotational speed was not constant throughout the test (figure 4c), which was due to the way the motion was driven (i.e. manually) and to the variable resistance torque of the engine during its motion. a normalisation of the ae rms feature to a defined crankshaft rotational speed of 0.5 cps was performed in order to compare the ae rms feature for different crankshaft positions obtained at different speeds with both the same test and with different tests. this speed was selected since it can be easily reached in cold tests both with and without air compression inside the cylinders. to transform the ae rms from the measured signal to the normalised speed, a proportional relationship described by eq. 1 was applied at regular time intervals of 10 ms: 𝐴𝐸𝑟𝑚𝑠,𝑛𝑜𝑟𝑚 (𝑡) = 𝐴𝐸𝑟𝑚𝑠,𝑚𝑒𝑎𝑠 (𝑡) 0.5 𝑐𝑝𝑠 𝑛𝑚𝑒𝑎𝑠 (𝑡) (1) where aerms,norm(t) and aerms,meas(t) are the ae features normalised and measured, respectively, and nmeas(t) and 0.5 are the measured crankshaft rotational speed (in cps) and the normalised speed for the corresponding time interval, respectively. this transformation is based on the results obtained by douglas et al. [5], which indicated that the ae energy generated during the sliding friction between the piston rings and the cylinder liners is proportional to the relative speed between the solids in contact. this result was confirmed, for the range of speeds tested, by observing the ae rms from measured signals as a function of the speed at different crankshaft positions (see figure 5). (a) (b) (c) figure 4. time evolution of (a) ae rms signal of sensor 1, (b) crankshaft angular position, (c) crankshaft rotational speed. figure 5. ae rms signal of sensor 3 vs. crankshaft rotational speed at ±75 ° from tdc. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 81 4.2. acoustic emission signatures the normalised ae rms signals measured using cylinder sensor 1 in relation to the crankshaft angular position are plotted in figure 6 for the two different engines: (a) the bench engine and (b) the collection car engine. the crankshaft rotation evolved for two complete thermodynamic cycles (0 °–720 °; first cycle in blue and second cycle in red) corresponding to four full revolutions of the crankshaft. the tdc of cylinders 1 and 2 corresponded to a 0 ° and a 180 ° rotation, respectively. both pistons completed four back-and-forth reciprocating motions, while every pass through the tdc and the bdc corresponded to a change in speed direction, where the piston speed was null. at these points, the ae rms was close to zero, which confirmed that the main source of the ae events in the cylinder block was the sliding friction between the piston rings and the cylinder liner. the absence of air compression inside the cylinders prevented, in these cases, the generation of ae waves from airflows. the results are clearly reproducible for every engine when the normalised ae rms is taken into account. these results represent the mechanical signature (with an absence of air compression) of the engine’s condition. in fact, when comparing the ae rms evolution between the two engines, it became clear that the mechanical signature is univocally representative of the engine. the normalised ae rms signals of sensor 1 when the spark plugs were left in, meaning air was compressed inside the cylinders, are plotted in figure 7 for the two previously tested engines. two main differences can be observed for the results with (figure 7) and without (figure 6) spark plugs. first, there was a clear increase in ae rms level in the range of 90 °–180 ° and 630 °–720 °, which was associated with the compression phase of the second and first cylinder, respectively. second, various punctual spiky ae events that were related to the valves’ opening and closing operations emerged. the ae rms signals shown in figure 7 are the combination of the mechanical signature (figure 6) and the ae events from the airflows in the compression phases and the valve operation of the engine when the spark plugs were left in. a deeper analysis of the data shown in figure 7 allowed for the detection of a failure in the valve system of the collection engine (case b). the ae rms signals of the bench engine (figure 7a) during the compression phases of both cylinders (90 °–180 ° and 630 °–720 ° for the second and first cylinders, respectively) presented a similar evolution. on the contrary, for the collection engine (figure 7b), the signal evolution of the compression phase of the second cylinder (90 °–180 °) was significantly different (in both shape and level) from the signal of the compression phase of the first cylinder (630 °–720 °). after noting this divergent compression evolution, a specific test was performed to check the airtightness of the second cylinder, which revealed a fault in the seat of the intake valve, leading to a loss in compression of around 35 %. (a) (b) figure 6. normalised ae rms signals of sensor 1 with spark plugs removed: (a) bench engine, (b) collection engine. (a) (b) figure 7. normalised ae rms signals of sensor 1 with spark plugs left in: (a) bench engine, (b) collection engine. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 82 this malfunctioning behaviour could also be observed when analysing the ae rms signals of other channels. figure 8 shows the normalised ae rms evolution for the bench (case a) and collection (case b) engines. here, the results without (in blue) and with (in red) spark plugs are superimposed for each engine. on comparing the red results in both graphs, it was clear that while the signal evolution during the compression phase for the bench engine was quite similar, the signal of the collection engine presented a spike at around 90 ° (marked with arrows in figure 7b) during the compression phase of the second cylinder. this event was not observed either in the compression phase of the first cylinder of the collection engine or in the compression phases of the bench engine. 5. conclusions in this paper, a diagnostic tool based on ae measurements was developed to help conservation/restoration specialists in reactivating historical vehicle engines. this new tool will provide innovative and more objective diagnostic features that are free of human subjective evaluation. by performing cold tests at the initial reactivation stages to prevent any damage to the heritage artefacts, the mechanical signature of the engine’s condition can be obtained from appropriate ae measurements. in order to collect reliable ae signals for various types of engine, a preliminary study should be performed in view of determining the correct sensor positions, which will ensure that appropriate signatures for performing an accurate diagnosis of the engine’s condition will be obtained. when using ae rms features, the influence of the crankshaft rotational speed in the signal levels must be taken into account. in this work, for the range of speeds used in the tests, a proportional correlation was applied to normalise the ae rms signal. this normalisation procedure ensured good reproducibility of the obtained signals. the use of a controlled speed system for driving the engine motion or the use of other analytical techniques in the frequency or time-frequency domains could help to avoid the issue of non-constant rotational speeds. the preliminary results demonstrated that performing cold tests using this ae technique is a promising approach to detecting various malfunctions (e.g. air leakages in the valves), one that involves applying adequate data treatment procedures to the ae features obtained from the mechanical signatures of the engines. another interesting potential use for this technique is in the future maintenance procedures of the analysed engines. once a reliable signature of the actual working condition of an engine is registered, this diagnostic tool can be used to compare or analyse the evolution of the future engine signatures in relation to the reference signature. as such, a database of the engines of an existing collection could be created and stored for future comparisons. acknowledgements the authors wish to acknowledge hes-so rcdav for funding the acume_hv project, the musée nationale de l’automobile in mulhouse (france) for their material and technical support, and jean-charles peruchetti, lecturer at ensisa in mulhouse (france), for the development of the engine’s angular speed monitoring sensor. references [1] c. b. scruby, an introduction to acoustic emission, j. phys. e: sci. instrum. 20 (1987), pp. 946-953. doi: 10.1088/0022-3735/20/8/001 [2] s. delvecchio, p. bonfiglio, f. pompoli, vibro-acoustic condition monitoring of internal combustion engines: a critical review of existing techniques, mech. syst. and signal proc. 99 (2018), pp. 661-683. doi: 10.1016/j.ymssp.2017.06.033 [3] b.c. kaul, b. lawler, a. zahdeh, engine diagnostics using acoustic emissions sensors, sae int. j. engines 9(2) (2016), pp. 684-692. doi: 10.4271/2016-01-0639 [4] m. h. 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https://doi.org/10.1016/j.proeng.2017.09.439 https://doi.org/10.1109/mim.2018.8573586 maintenance and testing protocols in railway industry acta imeko issn: 2221-870x december 2020, volume 9, number 4, 4 12 acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 4 maintenance and testing protocols in the railway industry alessandro massaro1, emanuele cannella1, giovanni dipierro1, angelo galiano1, giovanni d’andrea2, giorgio malito2 1 dyrecta lab it research institute , via vescovo simplicio, conversano (ba), 70014, italy 2 medtec s.r.l., fraz. traversa n. 4, via j. f. kennedy n.28, rende (cs), italy section: research paper keywords: railway; protocol; maintenance; test; qr code citation: alessandro massaro, emanuele cannella, giovanni dipierro, angelo galiano, giovanni d’andrea, giorgio malito, maintenance and testing protocols in railway industry, acta imeko, vol. 9, no. 4, article 2, december 2020, identifier: imeko-acta-09 (2020)-04-02 section editor: leopoldo angrisani, university of naples federico ii, italy received september 23, 2019; in final form august 4, 2020; published december 2020 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: alessandro massaro, e-mail: alessandro.massaro@dyrecta.com 1. introduction according to the frascati manual guidelines [1], knowledge gain (kg) represents an important challenge in the field of industrial research and development (r&d). following industry 4.0 logic, kg can be improved through enabling technologies, such as the internet of things (iot) [2] and using digitised traceability [3] for the automation of processes [4]. specifically, in manufacturing control systems, traceability plays a crucial role in the monitoring of the state of specific components in a system [5]. as traceability technologies can be applied to industrial processes, barcode and/or qr code [6]-[12] technologies are useful for supply chain management [11], [12]. the traceability of processes is, therefore, an important part of the risk management process [13] and can also be applied to testing procedures [14]-[16], leading to a general improvement of working and testing protocols by means of iot-supporting technology upgrades and multiple data systems [17]-[18]. another important issue for the optimisation of manufacturing production is process mapping, which can be implemented by 4m (manpower, material, methodology, machine) charts [19][20], the pdca (plan-do-check-act) cycle [21]-[22] (according to iso 9001:2015), xm-r charts [23]-[24] and p-control charts [25]-[27]. according to recent literature, there are alternative approaches that facilitate process mapping, namely the enhanced dmaic (define, measure, analyse, improve, control) or edmaic model [28], machine learning oriented towards production quality [29] and artificial neural networks that enable predictive maintenance in industry 4.0 [30]. controlling sensors within production processes could be adopted to check robotic arm control and actuation if they are interconnected through an intelligent unit [31]. in this paper, we propose a traceable approach to manufacturing control systems in which the maintenance process is organised in a structured and systematic way. traceable and systematic processes make it possible to identify each phase and eventually take corrective action. this is particularly difficult if there is no structure or schedule for these activities, even if it may not appear relevant at first sight. focusing on an industry working with train-part processing [32] and testing as a case study, we apply the proposed protocols to the turning processes and pneumatic testing of train braking systems using digitised traceability (see scheme in figure 1). the proposed protocols are based on the concepts described in the state-of-the-art, which includes the new auto-adaptive intelligent control facilities and an actuation process improved by artificial intelligence (ai), as set out in figure 2. in particular, we will discuss a generic protocol usable for all manufacturing industries and another two specific protocols that relate to the case study presented, which concern the process and test mapping of turning-machine operations and braking tests, respectively. abstract this paper introduces new maintenance and testing protocols regarding processes in the railway industry. the first protocol is general and can be applied to industries working in similar production processes. the second and third protocols are more specific to the railway industry and concern the turning-machine production line and the pneumatic testing of train braking systems, respectively. this study has been conducted within the framework of industry research on the design of intelligent control and actuation systems suitable for auto-adaptive industry 5.0 facilities. mailto:alessandro.massaro@dyrecta.com acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 5 2. first protocol: generic procedure for part processing when the maintenance of a locomotive or a generic wagon is required, faulty components are generally disassembled, investigated and restored through the manufacturing process, mostly based on cutting-edge technologies. several procedures can be carried out for this purpose, such as turning, cutting, milling, reaming and polishing, depending on the type of renovation required. usually, a faulty component is either broken or damaged in terms of its functional characteristics. for example, a gear wheel can be worn, and the shape of the teeth may deviate from the original design, generating an inefficiency within the system where it is employed. the restoration of such a component could therefore be carried out on the profiling of the gear shape, e.g. by using a turning machine. the evaluation of the problem and the process for solving it are usually achieved through ‘operator experience’, producing a maintenance process that is strictly dependent on the operator and, thus, not repeatable. each operator in a specific workshop has different skills and degrees of experience in the field, leading to components that are restored and tested in several different ways. moreover, restoring the components to their original and defect-free condition is challenging. the different wear mechanisms of industrial components (adhesive, fatigue, etc.) and different stress and strain concentration mechanisms (asperity, dent, etc.) can irreversibly affect the overall amount of wear. therefore, the maintenance process restores components to their original functionality, but it may marginally affect the amount of wear of these components. the restoration process will therefore increase the amount of wear. after maintenance, the wear evolution and the maximum operating time of the component will be affected, which may change the effectiveness of the components over time. a possible solution to this, proposed by the present authors, is based on developing a traceable approach, where every single phase of the maintenance process is organised in a structured and systematic way, i.e. process mapping and test mapping. this can be clearly represented by a theoretical protocol, as shown in the flow chart in figure 3. in this case, the protocol is meant to be generally applied, independent of the type of component and operation. the main protocol is applied to the workpiece that requires maintenance. it becomes clear that the workpiece is not working correctly, and some critical issues are therefore investigated. first, a qr code reference would be fundamental in gaining insights into the history of the workpiece, namely, the life cycle of the component, the total number of maintenance operations it has undergone and the type of maintenance. the overall figure 1. study case [32]: (a) functional scheme of the layout designed for industrial traceability; (b) optical sensor system that estimates accuracy in turning processes; (c) schematic layout of the bench designed for the pneumatic testing of train braking systems. figure 2. intelligent control and actuation process for the generic working protocol in manufacturing processes. ai actuation sensors control feedback intelligent feedback process mapping and traceability acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 6 amount of wear of the component can be inferred based on an estimate of the operating working cycles and a model of the evolution of the amount of wear related to that specific component. the qr code has to be unique to that specific component. additionally, for predictive maintenance purposes, the qr code also indicates the life-prediction curve of the tool, plotted in terms of an index of quality (iq) as a function of the number of working cycles. an example of such a diagram is plotted in figure 4. it can be theoretically described by an exponential law that fits equation 1: 𝐼𝑄(𝑥) = e−𝜆𝑥, (1) where λ is a coefficient that determines the slope of the curve and x is the number of working cycles. the iq values are expected to vary from 0 to 1, indicating an end-life and a new component, respectively. generally, the iq value and its evolution within the working cycles are closely related to the overall amount of wear of the component. the scope of the maintenance is to restore the value of the iq as close as possible to the unit value. however, since the amount of wear cannot be restored to its original state, the time evolution of the iq value is expected to be significantly affected after the maintenance process. more specifically, this might lead to an increase in the slope of the curve (i.e. an increase in the value of t in equation (1), indicating a more rapid degradation of the component while working. this is shown in the theoretical diagram in figure 5. in summary, the qr code provides all the required historical data that could play a key role in defining the problem. indeed, the problem is, therefore, analysed and the activities scheduled in order to proceed with the maintenance process. in this phase, all the features and parameters of the workpiece, which are found to be out-of-tolerance, have to be corrected by applying manufacturing procedures, which, in the case of a metallic material, consist of several machines that mainly perform milling and turning. each process has its own characteristics, in terms of figure 3. macro protocol designed for general maintenance operations in the railway industry. figure 4. theoretical exponential model describing the residual lifetime of a workpiece as a function of a specific index of quality (iq). figure 5. theoretical exponential model describing the tool life curves predicted after maintenance in the case of a generic component. the iq is enhanced, as shown by the red-dotted curve and compared to the black one, but the workpiece is more sensitive and degrades faster. worn workpiece qr code problem definition machine operations measures/ tests analysis of the measured data updating the qr code • the workpiece is worn and requires maintenance • qr code data reading; • life cycle of the workpiece; • total number of maintenance operations; • type of maintenance operations; • estimation of the residual life cycle since the last maintenance process, based on an index of quality iq. • definition of the types and grade of the outof-tolerance; • definition of the activities to carry out for the restoring of the workpiece; • selection of the manufacturing machines to be employed; • prediction of the plot indicating the residual life time of the workpiece. • definition of the optimal process parameters; • in-line monitoring of the indicated performance parameters, if any. • measuring phase and data collection, useful for quality analysis; • decision on the metrological instruments and techniques to be employed. • verification of the workpiece outputs obtained from the maintenance operations; • control of tolerances and actions in case of out-of-tolerance; • definition of the updated life cycle diagram if results are intolerance. • update of the total number and types of maintenance operations; • update of the new life cycle diagram obtained for the estimation of the residual life. if out-of-tolerance, go back to the problem definition retroactive and actuation controls process mapping acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 7 parameters, that need to be correctly set and optimised. this will be analysed in depth in relation to turning in section 3. at the first attempt, based on the set parameters and the wear grade of the workpiece, a draft tool life curve after maintenance could be predicted. when machines and procedures are set, the process mapping suggests that the maintenance procedure be initiated. the process parameters are optimised by referring to the historical data and based on the requirements of the maintenance process. if possible, in this phase, specific outputs should be assumed to be the ‘fingerprints’ of the process. this approach has also been followed by other research concerning similar manufacturing engineering fields [32], [33]. these outputs may provide information on how best to conduct the restoration process, and, if the values measured are out-of-range, they can trigger proper actuation controls that enable the correction of the previously set process parameters, as suggested in figure 3. after the workpiece has been restored and fixed, proper tests should be selected and carried out to check the quality of the restoration. of course, these tests are made for the key functional features that are required to be improved/fixed. data is thus collected in order to be analysed. the data analysis is made by checking that the values of the key functional features are within the designed working intervals. if this is not the case, the flow chart indicates the necessity of reconsidering the problem definition and, therefore, proceeding with a second round as soon as all the functional values are measured within the tolerance range. this step is important for a systematic evaluation of a successful maintenance process, which gives an objective evaluation of the result without relying only on the operator’s experience. of course, it is fundamental that all the functional features are correctly identified in the phase definition. at the end of the process, the protocol suggests that the qr code be updated with the new data. a new life-cycle diagram is then built by following the description previously given. this data will be the starting point for the next phase of maintenance of a specific workpiece, given that each component is uniquely defined by a qr code. in sections 3 and 4, two specific cases in which this approach has been applied are described and the interconnection between the different protocols is also highlighted. 3. second protocol: turning machine control and actuation by following the general approach described in section 2, a study case is now discussed concerning its application to turning processes for renovating axisymmetric workpieces. the flow chart shown in figure 6 is made by adapting the theoretical approach of the diagram seen in figure 5. the first steps are the same and, therefore, start with the workpiece definition, identification of the key functional features and analysis of the historical data given by the unique qr code. to highlight the chart’s interconnections, the same colour (blue) shows those process steps that are mainly the same. conversely, the grey colour characterises those elements identified as specific to a turning operation. turning is a well-known machining process that is based on the usage of a horizontal lathe. usually, the processed workpiece problem figure 6. maintenance protocol adapted to turning operations. worn workpiece qr code problem definition: turning turning operations measures analysis of the measured data updating the qr code • the workpiece is worn and requires maintenance • qr code data reading; • life cycle of the workpiece; • total number of maintenance operations; • type of maintenance and operations; • estimation of the residual life cycle since the last maintenance process: theoretical exponential model, based on an index of quality – iq. • definition of the types and grade of the outof-tolerance; • the workpiece is axisymmetric and requires turning operations: e.g. threading, external or internal turning, reaming; • selection of the machines to be used, manual or cnc lathe; • prediction of the plot indicating the residual life time of the workpiece. • selection of the turning tool/s: e.g. shape, material; • setting of the main process parameters: depth of cut [mm], feed rate [mm/rev], spindle speed [rpm], type of lubricant; • tool wear conditions of the back rack angle. • in-line monitoring of the process outputs, e.g. turning forces, tool geometry. • functional features to be measured on the workpiece: e.g. diameters, lengths, surface roughness; • decision on the metrological instruments and techniques to be employed: e.g. laser and touch probe systems, surface analysis instruments, non-destructive techniques. • functional test of the workpiece and verification of the positive results from maintenance; • control of the functional tolerances and actions in case of out-of-tolerance; • definition of the updated life cycle diagram if results are intolerance. definition of the updated exponential model. • update of the total number and types of maintenance operations; • update of the new life cycle diagram obtained for the estimation of the residual life. if out-of-tolerance, go back to the problem definition process mapping retroactive controls, in case of cnc retroactive systems act by adjusting the process parameters setting, based on the measured process outputs. acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 8 has axisymmetric features. several operations can be done by turning depending on the tools used and the process approach, e.g. drilling, cutting, threading and polishing. the main characteristic of turning processes is that the workpiece is clamped at the machine spindle, which rotates at a set rotational speed. the cutting tool is thus making contact with the rotating workpiece (see figure 7), interacting with the workpiece and generating the so-called chip removal. a turning operation is correctly managed by setting four main process parameters: • depth of cut (mm) • spindle speed (rpm) • feed rate (mm/rev) • presence of lubricant/refrigerant. these parameters are extremely important for the optimisation of the output characteristics. by identifying the most critical outputs, i.e. geometrical and dimensional tolerances and surface roughness, those parameters can be related in order to apply an in-line process control. the optimisation of turning operations has been studied by several researchers [34]-[36]. the retroactive controls designed within the process mapping have the role of being actuated for the in-line correction of the process parameter values. as outputs to be used for the monitoring of the turning process, force and wear measurements are the most consistent. the force is measured by using a piezoelectric load cell mounted on the cutting tool. the tool wear is monitored by using vision systems, e.g. image vision technologies, laser systems or optical coordinate-measuring machines (cmms). the surface roughness of the workpiece is another key feature; however, the in-line monitoring of such parameters may be difficult to undertake. of course, other features may be involved in this analysis, but this would be strictly dependent on the functional properties of the tool being renovated. the support given by the actuation systems helps in terms of cycle time and maintenance efficiency. these elements represent the main limitation of the manual maintenance carried out only through operator experience. quality control can clearly only be done after the turning process to test the quality of the restoration process. as already described in the generic flow chart in figure 2, the key features are investigated to check the success of the maintenance operation, and the correct instrumentation has to be selected and employed. data is therefore analysed and actions have to be taken if there is an indication that any features are out-oftolerance, meaning that the process requires additional steps to ensure the component attains the desired geometry and functionality. figure 7. schematic drawing showing the functional principle of a turning operation. figure 8. maintenance protocol adapted to turning operations. acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 9 as a last step, each operation is recorded within the unique qr code with a definition of each of the main steps carried out and the generation of a new life-cycle curve, as shown in figure 5. 4. third protocol: braking system and pneumatic test of trains the two previous protocols, discussed and shown in figure 3 and figure 8, describe process mapping charts concerning the maintenance process steps for a railway workpiece. the following section proposes that the same scheme, as discussed in the introduction of this article and shown in figure 2, be adapted for a particular testing process carried out when checking the efficiency of locomotive brakes. the scheme is therefore referred to as the test mapping chart (see figure 8). the quality control of locomotive brakes is an important test for safety and efficiency reasons. as with the previous maintenance mapping charts, every process starts with an analysis of the qr code, which is unique and should include all the important information concerning the locomotive’s historical data, with particular reference to the brake tests. a typical braking system for locomotives consists of a pneumatic circuit enabling the mechanical pads to act on the rotating wheel. this system starts from the locomotive, and a circuit propagates the pneumatic force when the brakes are activated, as shown in figure 9. railway brakes have three principal roles: • reducing or stopping the speed of the train • balancing its weight while on slopes to maintain a constant speed • parking the train when stationary or at a station stop. the testing of the brake system can be done through an automated test setup. the pressure values in the brake pipe, braking cylinder and auxiliary reservoir are checked primarily to assess their effectiveness. these parameters are plotted as a function of time and compared to the design curves, which are determined under optimal conditions. the test system works using a pressure stabilisation approach. proper connectors make it possible to associate the locomotive being tested with the brake-check apparatus. when the locomotive is connected, the pressure of the test system is adjusted by increasing the pressure value in the brake system being tested. this makes it possible to check the functionality of the brake by measuring the time required to reach a certain pneumatic pressure and then release it. this method is consistent with the starting preamble shown in figure 2. after the test has been conducted, the system generates a report. as a sample, a case study of a test of an ordinary locomotive is shown in table 1. it is important to check these results to establish whether the obtained values lie within the functional intervals. if the data shows that the values are off design, action on the locomotive must be taken. the test protocol thus suggests the identification of the critical elements and, therefore, the need to proceed with the macro protocol shown for the maintenance of the railway elements (see figure 3). after maintenance, the locomotive should be checked again to test the proper functional and safety conditions of the entire braking system. conversely, if the results show that the values lie within the design intervals, the test should be considered as successfully completed. no action is therefore required for maintenance. this report is subsequently saved in the qr code for traceability reasons. in addition, the data obtained and the conditions observed should suggest and schedule the next round of tests. 5. conclusions this study dealt with the design and development of maintenance protocols for the railway industry. the novelty of this approach has been highlighted as the traceability of the maintenance operations in the railway structures, which are currently mostly performed without following a systematic approach. the fundamental concept starts with the idea of having a unique systematic, traceable and repeatable approach, which would address the problems associated with ‘human operator’ errors. the method described the qr code as a key element, which has proved to be important to keep track of the operations and the data related to a workpiece. the approach makes it possible to actuate corrections while processing in order to decrease the amount of time spent on the secondary repeated operations that are eventually required to ensure that the designed features fulfil their capabilities. the main protocol described a systematic practical approach where a specific component is treated in several phases. within the protocol, the existence of a retroactive system makes it possible to actively interact with the system along with the in-line monitoring process. as a consequence, time is saved from making corrections that should be done, instead, at a post-processing phase. the case study concerning the application of the main protocol to the turning operations for the maintenance of a workpiece demonstrated the applicability of this approach in terms of input parameters and actuation systems. through inline identification and by adjusting the most important parameters in turning, namely the depth of the cut, the spindle speed, the feed rate and the lubricant, it was possible to take an active control over the process outputs, which are the cutting force and the tool wear. the same main protocol was adapted to the testing procedures, as in the case of the braking tests for locomotives. each phase was systematically followed. during the testing, the pressures applied in the braking system were actively monitored and adjusted to successfully carry out the testing procedures. if the test report showed negative outputs, the control would proceed to the maintenance operations, following the required steps already set out. figure 9. schematic drawing showing the pneumatic system of a train brake and the pneumatic action while braking: (a) main air tank, (b) generic pipe, (c) brake cylinder, (d) brake wheel, (e) exhaust pressure system and (f) central braking controls and emergency button. acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 10 table 1. sample results obtained from a brake quality control made on an ordinary locomotive. type of control measured interval measuring unit results pressure of the main tank (a) 8 7.5 … 9 bar positive system pressure stabilised in the generic pipe (b) 5 5 ± 0.05 bar positive filling time up to 95 % of the maximum pressure of the brake cylinder (c) m brake (d) 18 … 30 s positive v brake (d) 3 3 … 5 s positive brake release to a pressure of 0.4 bar at the brake cylinder m brake (d) 40 … 60 s positive v brake (d) 16 15 … 20 s positive pressure decrease required to obtain full breaking (d) 1.5 1.5 ± 0.1 bar positive brake sensitivity (e) ok the brake must act within 1.2 s for a pressure decrease of 0.6 bar in 6 s positive brake non-sensitivity (e) ok the brake must not act for a general pressure decrease of 0.3 bar in 60 s positive high pressure stroke ok 6 bar for a total duration of 2 s. 6 … 5.2 in 1 s bar positive capacity air loss in the generic pipe (b) 0 ≤ 0.1 bar/min bar positive loss in the generic pipe (b) 0 ≤ 0.1 bar/5 min bar positive loss of the brake cylinder (c) 0 ≤ 0.1 bar/5 min with a pressure in the generic pipe at 0 bar and constant pressure of the auxiliary tank bar positive clear test of the bilateral command ok with the generic pipe at 5 bar the auxiliary tank has to reach 0 bar positive test of the emergency button (f) empty the generic pipe through interventions in each device (b) ok the pressure of the generic pipe reaches 0 bar. the pressure of the brake cylinder is maximum positive restore 5 the pressure of the generic pipe reaches up to 5 bar. the pressure of the brake cylinder is 0 bar positive testing of the brake command from each driving location design pressure on the generic pipe (b) when the air pressure of the main pipe (b) is stabilised, take the brake command (f) as ‘released’ 5 the pressure in the generic pipe is stable at 5 ± 0.05 bar positive when the air pressure of the main pipe (b) is stabilised, brake (f) with a subsequent release (c) 5 the pressure in the generic pipe again reaches 5 ± 0.05 bar positive times of pressure loss in the generic pipe (b) with the pressure of the generic pipe stable at 5 bar, enable a pressure decrease of the generic pipe up to 3.5 bar (b) 3 time interval between 3 s and 4 s with a pressure decrease in the generic pipe 5 bar … 3.5 bar s positive time of pressure increase in the generic pipe (b) with stabilised pressure in the generic pipe at 3.5 bar, increase the pressure inside up to 5 bar 8 time interval between 3 and 4 s for a pressure increase in the generic pipe (3.5 bar … 5 bar) s positive variability of braking increase and release (d) with a stabilised pressure of the generic pipe at 5 bar, decrease the pressure up to 5 bar with intermediate levels of 0.2 bar ok for each level, the pressure of the brake cylinder has to increase. with the pressure in the generic pipe at 3 bar, verify the stability of the pressure within the generic pipe for 1 min. variation should be <= 0.1 bar positive with a stabilised pressure in the generic pipe at 5 bar, starting from the generic pipe pressure at 3 bar, enable several pressure increases of 0.2 bar up to 4.7 bar ok for each level, the pressure of the brake cylinder has to decrease positive control of the direct brake (d) variability of the braking action and release (d) increase the pressure of the cylinder brake by 0.2 bar at each level up to the maximum ok for each level, the pressure of the brake cylinder has to increase positive decrease the pressure of the cylinder brake by 0.2 bar at each level down to 0 bar. ok for each level, the pressure of the brake cylinder has to decrease positive other pipe and valve efficiency ok positive manometer efficiency ok positive parking brake efficiency ok positive efficiency of dynamic braking, tested at a constant speed of 20 km/h on a planar and linear track 10 allowed braking length ≤ 20 m m acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 11 at the end of each process/test mapping, the qr code worked to keep track of these operations and maintain the traceability of the specific workpiece. the proposed protocols can also be applied to electrical improvements and maintenance in railway applications [38], [39]. furthermore, such protocols may be implemented in measuring systems, which are already being experimented with in the railway structure, [40] and for continuous monitoring [41]. acknowledgements this study has been developed in the framework of the italian projects ‘automazione dei processi e di test di misurazione orientati alla tracciabilità della lavorazione di pezzi speciali e delle attività di officina: ‘trainindustry 4.0’’ (automation of processes and measurement tests oriented towards the traceability of the processing of special parts and worker activities: ‘trainindustry 4.0'). the authors would like to thank the dyrecta lab researchers, g. birardi, b. boussahel, a. calicchio, d. carella, a. colonna, a. costanzo, v. custodero, a. d’accolti, l. d’alessandro, m. legrottaglie, a. leogrande, a. lombardi, a. lorusso, n. malfettone, s. f. massari, o. rizzo, d. d. romagno, n. savino, s. selicato, m. solazzo, m. m. sorbo, f. tarulli and v. vitti for their efforts. the authors would also like to thank medtec srl for the support provided on the working procedures. the paper is an extended version of an earlier paper [37]. references [1] frascati manual 2015: the measurement of scientific, technological and innovation activities guidelines for collecting and reporting data on research and experimental development. oecd (2015), isbn 978926423901-2 (pdf) doi: https://doi.org/10.1787/9789264239012-en [2] v. roblek, m. meško, a. krapež, a complex view of industry 4.0, sage open april-june 2016, pp. 1-11. 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factors influencing the distribution of maximum specific absorption rates in far field human exposure scenarios acta imeko issn: 2221-870x september 2020, volume 9, number 3, 59 64 acta imeko | www.imeko.org september 2020 | volume 9 | number 3 | 59 factors influencing the distribution of maximum specific absorption rates in far field human exposure scenarios ovidiu bejenaru1, catalin lazarescu1, marius valerian paulet1, alexandru salceanu1, marius vasile ursachianu1 1 ‘gheorghe asachi’ technical university of iasi, romania, faculty of electrical engineering section: research paper keywords: sar; human exposure; far field; cst simulation citation: ovidiu bejenaru, catalin lazarescu, marius valerian paulet, alexandru salceanu, marius vasile ursachianu, factors influencing the distribution of maximum specific absorption rates in far field human exposure scenarios, acta imeko, vol. 9, no. 3, article 10, september 2020, identifier: imeko-acta09 (2020)-03-10 editor: jan saliga, technical university of košice, slovakia received february 27, 2020; in final form march 23, 2020; published september 2020 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: alexandru salceanu, e-mail: asalcean@tuiasi.ro 1. general framework aiming to protect humans against different electromagnetic field (emf) sources, numerous countries around the world have already proposed and recommended various safety standards and regulations. the actual standards for human exposure entail, according to the selected frequency domain, totally different associated methods and measurements for defining the so-called basic restrictions, with either internal induced current density or internal electric field (up to 100 khz), localised specific absorption rate (sar; from 100 khz up to 10 ghz) or power density (from 10 ghz to 300 ghz) establishing the corresponding safety limits. the results presented in this article are mainly related to sar modelling and simulations; therefore, we refer to the 10 mhz– 10 ghz frequency range. in relation to this particular frequency range, if the distance between the emf source and human subject is greater than 5 metres, this can be considered a ‘far field’ approach. the body is exposed not only to the direct (uniform) plane wave but also to the reflected waves. in addition, it must be said that, although the incident wave is ‘uniform’, the way the energy is absorbed by the body is significantly non-uniform, due to both the different dielectric properties of the various tissues and organs and also the possible resonance phenomena that may occur. various international organisations are concerned with establishing measurable limits (considered non-dangerous) of exposure for both the general public and professional workers [1]-[7]. additionally, in the military field, there are further specific regulations regarding limiting exposure to non-ionising electromagnetic radiation [8], [9]. regarding the radio-frequency spectrum (and above), an important limit for the incident power density is 1.0 mw/cm2 [3]. this value should guarantee that, in any part of the exposed body, the basic restriction on the local sar (2 w / kg averaged over any 10 g of tissue in the head and trunk regions and 4 w / kg for the limbs) is never exceeded [1], [3]. abstract the biological and health effects of electromagnetic fields are a current concern. with the aim of evaluating these effects, it is essential to estimate the levels of human exposure to various types of electromagnetic sources within different environments. these assessments could be performed by determining the current density distributions inside organs or tissues, by modeling the different mechanisms of electromagnetic exposure or even by modelling the development of effects in living beings. everything should be done while considering various exposure scenarios. in this paper, we have determined, through simulations employing cst suite studio software, the specific absorption rates (sars) values for a human body, averaged from the values for different 10 grams of tissue in different areas of the body, in the case of its exposure to an electromagnetic field generated by a source placed in the far field domain, where the human body is located in an empty room inside a building. we have used our previously tested 3d human body model that has the geometry of an elliptical cylinder. the simulated sar values have been also compared with the reference levels accepted by the international commission on non-ionizing radiation protection and ieee standards for safety levels for both public and professional exposure. mailto:asalcean@tuiasi.ro acta imeko | www.imeko.org september 2020 | volume 9 | number 3 | 60 as computer processing speeds and techniques continue to significantly improve, sar determinations using numerical methods for different types of exposure can even be performed for realistic models of the human bodies. the human body can be 3d modelled with a geometry more or less detailed depending on the number of voxels and the electrical characteristics of the organs and tissues. some of the first basic models of the human body for the numerical simulation of power absorption were the rotational cylinder [10] for frequencies below 600 mhz, the prolate spheroid [11] for frequencies above 10 ghz and the multi-layered cylinder [12] for frequencies between these. there are significant differences between these simplified 3d models with uncomplicated geometries and quasi-complete (realistic) voxel models of the human body developed over many years by very powerful software companies (a well-known example being the hugo model [13]). one consideration of these 3d models is that they must allow scaling according to the very wide variety of human physical dimensions [14]. therefore, the number of voxels needed for a specific model could be extremely high, and ‘slice by slice’ processing of various images could involve a considerable amount of time and resources. a solution to this issue could be the use of simplified 3d models in such cases. different 3d electromagnetic simulation software programs offer very accurate and efficient computational solutions for electromagnetic design and analysis. electromagnetic simulation software allows engineers to efficiently investigate the electromagnetic properties of (sub)systems, computing the field distribution at specific points or zones of interest [15]-[17]. 2. background of our approach for real human being, the sar values will generally vary from those calculated using numerical methods applied to different simpler models (sphere, cylinder and rotational ellipsoid) of the human body. obviously, to accurately calculate the average sar at high frequencies, much more realistic models of the human body are strongly required. nevertheless, as an entry point to the study of any type of human exposure to non-radiating emfs, a simplified intermediary model could be accepted, as it would offer positive outcomes that could be considered in further approaches and developments. using numerical simulations provided by the cst studio suite software, we have determined the sar at the surface of the proposed 3d human body model. the emf source was located outside the building, while the subject is inside an empty room with concrete walls located on the ground floor. the distance between the source and subject is higher than 5m; consequently, the incident wave is plane (for the whole spectrum of interest) and the set-up should be considered ‘far field’. the simplified 3d model proposed and used in this study is considered to be a nude one (without clothes). it has been proven that the average sar is affected by the thickness of any worn clothes. some researchers have previously investigated this effect [5] using the multi-layered cylindrical model while also considering the introduced power loss [18]. 3. material and methods specific absorption rate is generally used to quantify the electromagnetic energy absorbed by the human body from different types of electromagnetic sources. different numerical methods are used for sar computations, but the most prominent numerical methods employed are finite-difference time-domain (fdtd) and finite integration technique (fit), [19], [20]. these methods consider the volume of study (the actual human model and the neighbouring space) as being composed of a very large number of cells identified by indexes (i, j, k), with the field components being defined at fixed locations. maxwell’s equations in the time domain are then discretised by using the finite-difference approximations for time and space derivatives. the time-stepping system resulting from the discretisation of the maxwell’s equations is solved by imposing the excitation field as a function of time with adequate initial and boundary conditions. for initial conditions, all the components of the electromagnetic field are equal to zero at the initial starting time step. in this paper, the incident electric field has been supposed to have the characteristics of a plane wave with time-harmonic behaviour (far field hypothesis). under these assumptions, the local sar at each cell is given by the equation (1): 𝑆𝐴𝑅(𝑖, 𝑗, 𝑘) = 𝜎(𝑖, 𝑗, 𝑘)[𝐸𝑥 2(𝑖, 𝑗, 𝑘) + 𝐸𝑦 2(𝑖, 𝑗, 𝑘) + 𝐸𝑧 2(𝑖, 𝑗, 𝑘)]/𝜌(𝑖, 𝑗, 𝑘) (1) where 𝜎(𝑖, 𝑗, 𝑘) and 𝜌(𝑖, 𝑗, 𝑘) are the space dependent conductivity and density, respectively, of the (i,j,k) voxel. in equation (1), the peak value of the electric field components has been considered. the skin tissue of the human body (mainly due to ‘skin’ effect but also due to its conductive properties) plays the role of ‘electromagnetic shield’ in protecting the inner organs against microwave exposure. this is the main reason for the fact that the human body model here proposed is covered with ‘skin material’ from the cst library. for instance, at 900 mhz, for skin, relative permittivity is 0.86, conductivity is 0.86 s/m and density is 1100 kg/m3, with no magnetic properties. the way sar varies depending on the electrical properties of the skin at different frequencies can be analysed using numerical methods, and the skin properties (as tissue) can be estimated at an analytical level using the debye equation. we have also considered the relative complex permittivity 𝜀�̇� of the skin (according to debye dispersion characteristics) with two relaxation time constants, 𝜏1 and 𝜏2 [21]. the dispersion equation is as follows: 𝜀�̇� = 𝜀𝑟 − 𝑗 𝜎 𝜔𝜀0 = 𝜀∞ + 𝜀𝑆1 − 𝜀∞ 1 + 𝑗𝜔𝜏1 + 𝜀𝑆2 − 𝜀∞ 1 + 𝑗𝜔𝜏2 , (2) where 𝜀𝑟 is the relative permittivity and σ is the conductivity. here 𝜀∞ is the relative permittivity at the highest frequency, while 𝜀𝑆1 + 𝜀𝑆2 − 𝜀∞ is the relative permittivity for dc. if these parameters can be estimated by matching the measured data for several specific frequencies, the electrical properties of skin tissue for these frequencies might be deducted. in our study, we have determined the sar values for 10 g of specific tissue, in the following referred to as “sar max (10 g)”. spatial mediation is applied, the highest value being retained. we have used a quasi-simplified homogeneous 3d model of the human body and a 900 mhz source (a representative frequency for the global system for mobile communications, gsm). the 3d model of the human body designed and used in this study is an updated version of a simplified 3d model previously used in [22]-[24]. for the design of the 3d human body model, we have used the cad interface from the ‘modeling’ section of the cst software. the developed 3d model has been ‘built’ from circular rotational bodies with the exception of the trunk part, acta imeko | www.imeko.org september 2020 | volume 9 | number 3 | 61 which uses an elliptic cylinder (see figure 1). this body shape design is much closer to reality than the variant used in other studies with a circular cross section, and it allows for different evaluations of field distribution and induced currents on the surface of the model. we have made these minor modifications to the shape and dimensions of the human body cross-sections to provide a more realistic approach. as can be seen, the height of our model is 175 cm, which is average for a male person in standing position. the computation background has been provided by ‘microwave studio’ module from the cst studio suite software. the far-field source is considered to be a linear polarised uniform plane wave with the incident electric field of ezinc parallel with the oz axis, as presented in figure 2. the plane wave irradiates parallel to the model from the minus direction of the y axis. the value of the incident electric field is considered to be 41.25 v/m for the general public exposure assessment and 90 v/m for the professional exposure simulation. these two distinctive values have been chosen in accordance with the acceptable limits for public and professional exposure established by the international commission on nonionizing radiation protection (icnirp). in accordance with our research interests, to determine the sar max (10 g) distribution on the surface layer of our 3d model, we have used the previously mentioned specific characteristics of human skin at 900 mhz. different exposure scenarios have been considered. sar values have been obtained for five simulations with the following distinct conditions: case a: the human body is directly positioned on the ground without (insulated) shoes on his feet (no shielding, no ground isolation; see figure 3.a); case b: there is a concrete floor (no shielding, partially isolated from the ground; see figure 3.b); case c: the human body is placed inside an airtight concrete room (good shielding; see figure 3.c); case d: the human body is inside the same room but with one closed window (shielding and partially shielded aperture; see figure 3.d); case e: the human body is inside the same room but with one open window (shielding with aperture; see figure 3.e). in all of the scenarios presented above, the 3d model of the human body is placed in the middle of the room in an unchanged position. the ground (soil) has also been modelled in the cad section of the cst studio suite software. its rectangular shape has been designed as a parallelepiped with dimensions 800 × 800 × 30 cm³ in ox, oy, oz directions. the soil material used was the ‘loamy’ type (dry) from the cst library materials. the walls, floor and ceiling of the room are simulated as being made from concrete with a minimum age of one year old; this material can also be found in the cst library. the room is a parallelepiped with dimensions 500 × 200 × 200 cm³ in ox, oy and oz directions, respectively. additionally, we have chosen the ‘lead glass’ window material from the cst library, as we were interested by the study of its refractive behaviour [25]. figure 2. direction of the incident plane wave toward the object under testing. figure 1. proposed 3d model of a human body. all dimensions are expressed in centimetres (cm). a) case a b) case b c) case c d) case d e) case e figure 3. different simulation scenarios for determining the sar max (10 g) distribution on the surface of our model. acta imeko | www.imeko.org september 2020 | volume 9 | number 3 | 62 furthermore, in the cst software, under the ‘simulation’ menu for background properties, the material type was set as ‘normal’ and the space as ‘air-filled’, air (or free space) having well-known electromagnetic properties. for the surrounding space, we have set a value of 15 cm for the lower x, upper x, lower y, upper y, lower z, upper z distances, applied in all directions. these distances extend the virtual surrounding box of our model. by default, the cst software considers the space surrounding a structure as being λ/8 cm in all directions. other approaches assume this space to be a quarter of a wavelength in each direction around the structure. we have extended the space around the structure in each direction with a length approximately equal to λ/2 (for a 900 mhz frequency, this means about 15 cm). the ‘boundary’ was set as ‘open’, with extra space in all directions. the frequency range (more precisely, fmin and fmax) was set according to the frequencies of interest. unjustified augmentation of the frequency domain can lead to a higher number of mesh cells, which would increase the simulation time accordingly. the frequency range has been set to 0 1.5 ghz in the simulation menu. before starting the simulation process to determine the sar max (10 g) values, the ‘time domain solver’ should be selected. to avoid a situation where the steady state energy criterion is not satisfied, we extended the maximum number of pulses from 20 to 50. this is because, for complex cases during the simulation process, using the time domain solver, the program cannot successfully complete the simulation process. when a larger maximum pulse number is used, the solver works properly, and the results display with acceptable errors. this is an ‘adaptive meshing type’ solution, which aims to save time and hardware resources. the mesh type is considered to be hexahedral, and the source type selected is ‘plane wave’. we can control the mesh density for our structure by increasing the number of lines per wavelength. the simulations were carried out with the default value of 10 lines per wavelength, while lower mesh limit was set by default to 7. these settings offer a good compromise to start with. considering the fact that a complete convergence study (where the number of lines per wavelength was increased from 10 to 40 for a better mesh structure) has been previously carried out in a recent study [26], in the present work, we will take into consideration the conclusions drawn regarding the accuracy of the final values of sar max (10 g) obtained in these previous simulations. 4. results and discussions a simulation process for the determination of the distribution of sar max (10 g) values on the surface of a 3d model of the human body has been performed for different scenarios (cases a–e, as previously defined). figure 4 presents these distributions. in all cases, we have set the mesh grid as 10 lines per wavelength. the external incident field has been set as 41.25 v/m (the upper accepted limit for exposure for the general public). table 1 summarises how exactly the sar max (10 g) values vary for different regions of human body surface for the various cases (a–e). table 2 provides the same data for when the incident electric field is 90 v/m, i.e. the corresponding maximum value for occupational exposure. the cursor tool option from the cst software interface was fixed as ‘field’ with the intention of searching the maximum values for sars per 10 grams of (skin) tissue on the surface of our model. a) case a b) case b c) case c d) case d e) case e figure 4. sar max (10 g) distribution on the surface of 3d model of the human body due to an external electric field ezinc = 41.25 v/m with 900 mhz frequency. figure 5. comparative sar max (10 g) values for public versus professional exposure: head region. table 1. sar max (10 g) values on different areas of the surface of the 3d model of the human body (10 lines per wavelength, external electric field ezinc = 41.25 v/m [public exposure limit] and 900 mhz frequency). location sar max (10 g) in w/kg case a case b case c case d case e head 8.31 · 10-2 7.01 · 10-2 4.33 · 10-2 6.04 · 10-2 5.17 · 10-2 trunk 2.17 · 10-2 2.45 · 10-2 1.07 · 10-2 1.87 · 10-2 1.33 · 10-2 limbs 1.71 · 10-1 6.45 · 10-2 4.80 · 10-2 5.59 · 10-2 4.73 · 10-2 table 2. sar max (10 g) values on different areas of the surface of the 3d model of the human body (10 lines per wavelength, external electric field ezinc = 90 v/m [professional exposure limit] and 900 mhz frequency). location sar max (10 g) in w/kg case a case b case c case d case e head 3.91 · 10-1 3.31 · 10-1 2.03 · 10-1 2.84 · 10-1 2.33 · 10-1 trunk 1.17 · 10-1 1.21 · 10-1 5.78 · 10-2 8.93 · 10-2 6.29 · 10-2 limbs 5.37 · 10-1 3.27 · 10-1 2.31 · 10-1 2.26 · 10-1 2.07 · 10-1 acta imeko | www.imeko.org september 2020 | volume 9 | number 3 | 63 extracted from these tables, more intuitive sar max (10 g) values comparing public and professional exposure are illustrated in figure 5 for the head area, figure 6 for the trunk and figure 7 for the limbs area. from this comparative analysis (general public versus professional) of the sar max (10 g) values recorded for significantly different areas of the surface of the proposed model, it can be seen that the values for professional exposure are 3.2 to 5.4 times higher. as expected, for the head and trunk areas, the sar highest values have been recorded in case a (open space, grounded) and the smallest values have been recorded in case c (airtight, concrete walls). somewhat surprisingly, for all three regions here studied (head, trunk, limbs), the exposure in the case of the open window is lower than in the case of the closed window (case e versus case d), even though it is assumed that ‘lead glass’ might have reflective properties. we suppose that a plausible explanation could be offered by the existence of the ‘aperture effect’ and some cumulative reflections. it should be noted significant variation of sar values exists between the two main types of scenario, i.e. scenarios with an open-air body (cases a and b) versus scenarios with a body inside a building whose walls have shielding properties, e.g. reinforced concrete (cases c–e). greater accuracy for the final sar max (10 g) values could be obtained by increasing the number of lines per wavelength in the mesh section of the cst software: in other words, using a finer mesh grid. taking into consideration the previous studies performed by authors on this topic [26], [27], [28], it can be concluded that the mesh grid influences the simulated sar values. more specifically, there is a 50 % increase in the simulated sar values when 30 lines per wavelength are used in place of 10 lines per wavelength. due to the positioning of the human model inside the enclosure, we can also assume the presence of the multiple reflection phenomenon due to the reflectivity of the walls; this phenomenon might have had an impact on the final sar values of the simulations. 5. conclusions we conducted a study assisted by cst software that aimed to simulate the average sar values for 10 g of tissue on different areas on the surface of a 3d model of a human body (developed by the authors). in the simulations performed, the human body model was located in the ‘far field’ region (the distance between source and ‘victim’ is longer than /(2 )). the incident value of the external electric field ezinc (along the oz axis) was specified as 41.25 v/m for simulations of general public exposure and 90 v/m for simulations of professional, controlled exposure. a frequency of 900 mhz was chosen, as it is the gsm mobile communication band. these values for the incident electric field have been selected as they are the maximum values admissible according to icnirp regulations. therefore, the present study could have relevance to different exposure scenarios. using a 3d elliptical cylindrical geometry for the human body, electromagnetic simulations have been performed using the microwave module of the cst studio suite software (published by simulia). our study has been carried out taking into account the two main possible scenarios of a human body in relation to a transmitting antenna, i.e. being in open space or being inside a room with walls that have shielding/attenuating properties. the effect produced by the presence of a window in these scenarios, open or not (with glass also having protective properties), has been also studied. the study was differentiated (even from the perspective of exposure risk) for the three main parts of the human body: head, trunk, and limbs. regarding the convergence of the sar max (10 g) values, this should be analysed in more details in a future study on effect of different positionings of the proposed model inside the enclosure. we have concluded that the sar max (10 g) values obtained in our simulations using the cst software for this type of far field exposure are considerably lower than the acceptable icnirp values, which is in decent agreement with ieee and eu standards for both types of exposure: residential and occupational. references [1] a. ahlbom, u. bergqvist, j. h. bernhardt, j. p. cesarini, l. a. court, m. grandolfo, m. hietanen, a. f. mckinlay, m. h. repacholi, d. h. sliney, j. a j stolwijk, m. l. swicord, l. d. szabo, m. taki, t. s. tenforde, h. p. jammet, r. matthes, international commission on non-ionizing radiation protection, guidelines for limiting exposure to time-varying 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https://doi.org/10.1109/15.293278 https://doi.org/10.1109/icepe.2018.8559927 https://doi.org/10.2528/pierm13012812 https://doi.org/10.1016/j.enganabound.2009.07.013 https://doi.org/10.1109/atee.2019.8725013 https://www.imeko.org/publications/tc4-2019/imeko-tc4-2019-038.pdf https://www.imeko.org/publications/tc4-2019/imeko-tc4-2019-038.pdf https://doi.org/10.1109/sielmen.2019.8905798 microsoft word article 6 85-729-2-le.doc acta imeko december 2013, volume 2, number 2, 28 – 33 www.imeko.org acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 28 evaluation of inconsistent data of key comparisons of measurement standards a. chunovkina, n. zviagin, n. burmistrova d.i.mendeleyev institute for metrology, 19, moskovsky pr. st petersburg, russia section: research paper keywords: measurement model, inconsistent data, metrological compatibility, measurement uncertainty, degree of equivalence. citation: a.chunovkina, n.zviagin, n.burmistrova, evaluation of inconsistent data of key comparisons of measurement standards, acta imeko, vol. 2, no. 2, article 6, december 2013, identifier: imeko-acta-02 (2013)-02-06 editor: paolo carbone, university of perugia received february 22th, 2013; in final form december 12th, 2013; published december 2013 copyright: © 2013 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: this work was done in the framework of the project “kompetenzdreieck optische mikrosysteme – spitzenforschung und innovation in den neuen ländern”, which is supported by the german ministry of education and research (fkz: 16sv5473). corresponding author: a. chunovkina, e-mail: a.g.chunovkina@vniim.ru 1. introduction since the signing of the cipm mra [1], many different approaches have been proposed for analysing key comparison data. the consistency of measurement data and the models applied is a crucial point in choosing a correct method of data evaluation. in general, all methods can be divided into two large groups (see figure 1). the first group comprises methods for evaluating consistent data. these methods do not require any additional information apart from measurement results and associated uncertainties provided by participants. as a rule, the key comparison reference value (kcrv) is calculated as a weighted mean [2-3], and a degree of equivalence of measurement standards is established as the deviation of a measurement result from the kcrv and associated uncertainty of this deviation in full agreement with the cipm mra. if detailed information about an uncertainty budget is available, the bias estimates for results of participating laboratories can be obtained [4]. these estimates should not be regarded as the degree of equivalence and no further corrections for systematic effects are implied. this additional information about systematic biases obtained from the joint evaluation of all comparison data can be used for improving a measurement procedure used in every laboratory. the second group comprises methods for evaluating inconsistent data. it should be stressed that all these methods are based on some additional assumptions [5-13], the validity of which for concrete comparison data should be analysed in every particular case of their usage. the present paper deals with evaluating inconsistent data of a cipm key comparison. an algorithm for calculating the kcrv and the degree of equivalence (does) is suggested. the paper is divided into 4 main sections. in section 2 some general consideration concerning the concept of equivalence of measurement standards is given. section 3 presents a brief analysis of different models and algorithms which are used for the inconsistent data evaluation. in section 4 the algorithm suggested is discussed. the application of the algorithm for analysing the ccqm-k5 data and the comparable analysis of this algorithm with other approaches are given in section 5 2. equivalence of measurement standards the cipm mra does not explicitly determine the concept “equivalence of measurement standards” and therefore different interpretations of the concept are available. the mra gives the explicit definition of a measure of equivalence, namely the degree of equivalence. according to the mra “the degree abstract this paper deals with evaluation of inconsistent data obtained in key comparisons of national measurement standards. the concept “equivalence of measurement standards” is discussed in the context of metrological compatibility of measurement results. applications of different methods for evaluating inconsistent data are briefly considered. an explicit practical approach is proposed for evaluating inconsistent data. it is illustrated by an analysis of the results of ccqm-k5. acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 29 of equivalence is taken to mean the degree to which these standards are consistent with reference values determined from the key comparisons and hence are consistent with one another. the degree of equivalence of each measurement standard is expressed quantitatively by two terms: it's deviation from the key comparison reference value and the uncertainty of this deviation (at a 95% level of confidence)”. thus, in the definition of a degree of equivalence the basic accent is made on the definition of the key comparison reference value (kcrv). before discussing methods for determining the kcrv the authors would like to clarify the following issues:  concept of the equivalence of measurement standards,  meaning of the kcrv,  equivalence of measurement standards in case of inconsistent data. the authors share statements [2-3] that the equivalence of measurement standards means the equivalence of measurement results obtained in national metrology institutes (nmi’s) participating in a key comparison. in this context the kcrv is understood as an estimate of a measurand based on measurement results provided by nmi’s. therefore, each nmi provides an estimate of the measurand and, if these data are consistent, the kcrv is calculated as a superior estimate of the same measurand. to our opinion, the equivalence of measurement standards (or of measurement results) can be defined as the metrological compatibility of the following set of measurement results: 1 1{( ) ( );( )}n n ref refx ,u ,..., x ,u x ,u . according to the vim3 [14], the metrological compatibility is “a property of a set of measurement results for a specified measurand, such that the absolute value of the difference of any pair of measured quantity values from two different measurements results is smaller than some chosen multiple of standard measurement uncertainty of that difference”. often, in case of a key comparison, for a confidence level of 95% the multiple equals to 2. so, the equivalence of measurement standards means that for any i and j the following equations are satisfied:  2i j i jx x u x x   and  refirefi xxuxx  2 . (1) the above equations are similar to the equation used in an ne criterion applied in the analysis of key comparison data. if there is no covariance between the results obtained in the i-th and j-th laboratory, then the uncertainty of the difference between these results is given by the formula      2 2 2i j i ju x x u x u x   . (2) in case when the measurement of each laboratory is realized independently of the other institutes’ measurements and the value is calculated via the weighted mean of measured values obtained by other laboratories, using the inverses of squares of the associated standard uncertainties as the weights, the uncertainty of the difference between a measurement result and a reference value is given by      2 2 2  i ref i refu x x u x u x . (3) for equivalent measurement results the degree of equivalence is expressed by two terms: i i refd x x  and  i refu x x . the authors would like to stress that the second term is even more informative than the first one, since it is to a great extent based on the uncertainty associated with the measurement results of a particular laboratory and characterises the dispersion of possible deviations of the measurement results of this laboratory from the reference value. from the above consideration the authors can conclude the following:  equivalence of measurement standards can be interpreted as the compatibility of a set of measurement results, which comprises the results obtained by nmi’s and the kcrv with an associated uncertainty  the kcrv is regarded as the estimate of a value of a specified measurand, which is based on consistent measurement results provided by laboratories  equivalence of measurement standards implies the consistency of corresponding data. it directly follows from equations (1). in general, there could be several reasons for the data inconsistency: instability or drift of a measurement standard, underestimating the measurement uncertainty by some participants or significant systematic hidden biases in some results. methods for analysing the key comparison data in the presence of a linear drift of a travelling standard is beyond the scope of this paper [6]. the stability of the travelling standard is assumed in this paper. in practice the travelling standard stability is investigated by a pilot laboratory before sending the travelling standard to other comparison participants. the pilot laboratory often performs repeated measurements during the time of comparison with the aim to check the stability of the travelling standard. actually both the underestimated measurement uncertainty and hidden systematic errors of values measured can be regarded as a result of the incorrect uncertainty evaluation. in order to separate these two reasons, it is required to get additional information about a meaningful value of measurement uncertainty. there are several approaches and a lot of algorithms for evaluating inconsistent data [5-13]. in the next section the authors will briefly discuss the models used in these approaches. 3. modeling this paper considers the evaluation of inconsistent data of key comparisons. the basic model applied for the key comparison data figure 1. two approaches for evaluating measurement data of key comparisons of national measurement standards. data and model consistent inconsistent estimates of bias (biases are overlooked in uncertainty budget) revision of declared uncertainties estimates of biases degree of equivalence, kcrv +additional information +additional assumptions revision of measurement data acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 30 analysis is as follows. each laboratory measures the same measurand x : ix x . (4) and provides the measurement result ix and associated standard uncertainty niui ,...,1,  . it is assumed that all laboratories have introduced corrections for known systematic biases and have provided a combined standard uncertainty. if model (4) and data  ii ux , are consistent, the conventional approach is applied [2-3]. in case of inconsistent data, the first that can be done is to form the largest consistent subset [3]. but sometimes, there could be difficulties in forming a single subset. a drawback of this approach is that for several laboratories (not included in the subset) the does are not established. if the number of these laboratories is significant, the key comparison actually has to be repeated. there are two possibilities to remove the data inconsistency. the first one is to propose a more complicated measurement model, and the other one is to modify the measurement uncertainties associated with the results provided by nmi’s. usually the following model is considered for processing inconsistent data:  i ix x b , (5) where ib is the laboratory effect. under the fixed effects model ib is treated as the laboratory systematic bias. estimation of , ix b requires additional information or some additional assumptions [10-12]. in [10] it is assumed that some laboratories measure without biases. actually only the number of these laboratories should be specified, the identity of the laboratories is not presumed to be known. this assumption and application of bayesian model averaging allows the estimates of the kcrv and those of the biases of the rest laboratories to be received. it should be noted that application of the fixed effect model for the key comparison data analysis is still an issue to be discussed because the relation between the does and biases estimates is not explicitly identified. another application of model (5) (random effect model) consists in treating ib as random variables [8, 13]. usually they are assumed to be normally distributed with the zero mean and variance 2 . the kcrv is calculated as the weighted mean, 2 2 2 2 1        i i ref i x u x u . (6) the estimate of 2 is based on measurement results provided by laboratories and can be obtained numerically. the does can be calculated using simulation in order to take into account the correlation between the measurement data and estimate of 2 . the authors would like to discuss possible interpretations of the biases ib in model (5) when it is applied for evaluating the kc data. they can describe the instability of the travelling standard. this instability is assumed to be significantly less than the measurement uncertainties declared by participants. so, usage of repeated measurements in a pilot laboratory seems more preferable for estimating 2 than the usage of all results provided by the participants. another interpretation of ib is considered in [8], where each laboratory underestimates the measurement uncertainty due to the fact that one and the same random error is hidden by all laboratories. actually, this means that the laboratories have to recalculate the measurement uncertainties during the comparison in order to take into account the additional uncertainty component 2 . in fact, it contradicts the cipm kc rules [15], so in practice any increase of measurement uncertainties is not applied. on the other hand, in similar cases the uncertainty associated with the kcrv is actually enlarged when it is taken as the uncertainty of the weighted mean (6). sometimes, when the measurement uncertainties seem to be underestimated by participants, the kcrv uncertainty is calculated as a sample standard deviation of the simple mean. it means that the uncertainties declared by participants are not used in establishing the kcrv and the associated uncertainty. therefore, in such cases a confirmation of the declared uncertainties by the results of key comparisons becomes questionable. the kc aims are to establish does and to provide a foundation for calibration and measurement capabilities of the nmi. if the measurement uncertainties are not confirmed in the kc, the nmi can enlarge the uncertainties presented in the calibration and measurement capabilities so that the measurement results can be consistent with the kcrv [1]. sometimes initial measurement results are mutually noncompatible because several participants have underestimated the measurement uncertainties rather than that a few results can be regarded as outliers. in such cases the increasing measurement uncertainties of these results seems to be reasonable. the procedure proposed below addresses exactly these cases. the idea is to increase the measurement uncertainties associated with several results in order to make the results compatible with each other and with the kcrv too. two questions arise:  to what extent the measurement uncertainties may be increased;  what uncertainties are to be enlarged . the dispersion of the measured values reported by the participants contains valuable information. it should be noted that one cannot rely on the declared measurement uncertainties because the measurement data are inconsistent. the authors would like to distinguish the data having significant systematic biases from those of which the uncertainties are underestimated. here the authors propose to use the criterion nz for constructing the homogeneous set of measured values. this set is indicated as reference group. the meaningful measurement uncertainty is calculated as sample standard deviation of values included in this set. the data beyond this group are regarded as having a significant systematic bias. nevertheless they can be consistent with the kcrv if the associated measurement uncertainties are large enough. otherwise these measurement results are not equivalent to the others and the does are not established for them. measurement uncertainties are enlarged only for those results from the reference group which are non-compatible with the kcrv determined initially as the weighted mean of all measurement results. 4. evaluation procedure the proposed method implies usage of the basic model (4). acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 31 all laboratories report measured values and standard measurement uncertainties {xi, ui}. step 1: analysis of metrological compatibility the evaluation starts with analysis of compatibility of the set of measurement results: 1 1{( ) ( ); ( )}n n ref refx ,u ,..., x ,u x ,u . initially the reference value is calculated as the weighted mean of measurement results provided by all laboratories: 2 2 ( ) 1 ( )    i i ref i x u x x u x ,   1 2 1 ( )         ref i u x u x . (7) if the measurement results are compatible with each other and with the kcrv the conventional approach can be applied for calculating the does. if the measurement results are not compatible one comes to step 2. step 2: determining the reference group the reference group comprises the measurement results which pass the criterion nz : 2   in x x z s , 1 1    m i i x x m ,  2 1 1 1      m i i s x x m . (8) the results with 1nz  , are included into the reference group; m ( )m n is the number of results in the reference group. step 3: extending the measurement uncertainties the sample standard deviation obtained using measured values from the reference group serves as recommended value of uncertainty for measurement results that are non-compatible with the reference value (step 1). after enlarging measurement uncertainties for several results the compatibility of a set of modified data is analysed. the kcrv is recalculated as the weighted mean of measurement results from the reference group. new weights are used because some measurement uncertainties have been changed. if compatibility with the reference value still has not been achieved for the measurement results from the reference group it might be reasonable not to calculate the kcrv. the results of kc can be reported by a matrix of pair wise degree of equivalence. 5. example the proposed approach is illustrated by its application for ccqm-k5 data evaluation. the initial data are shown in figure 2. the kcrv was calculated as the simple mean with associated uncertainty taken as sample standard deviation of the mean. all measurement results (except number 10) were used for calculation of the kcrv and associated uncertainty. the authors can conclude that the kcrv uncertainty is large compared with measurement uncertainties of some participants. it is explained by the fact that the kcrv uncertainty is not calculated using the measurement uncertainties reported by the participants. application of the conventional approach [2-3] reveals the inconsistence of measurement data. table 1 contains the values characterizing the metrological compatibility of measurement results, 2 2 i j i j x x u u   , and their compatibility with the kcrv, 2 2 i ref i ref x x u u   . if the measurement results are compatible, the corresponding values should be less than 1. result 10 can be identified as non-compatible with all others and with the kcrv (table 1). below the authors present an analysis of the ccqm.k-5 data using the approach proposed. the reference group comprises all measurement results except that of number 10. the sample standard deviation equals 0, 026u  . the expanded uncertainties associated with measurement results 1, 3, 5, 6, 8 and 10 were replaced by 2u . then the kcrv was recalculated. table 2 presents the compatibility of the modified measurement results. most of the data revised demonstrate the compatibility of measurement results, i.e., with each other and with the reference value. however, the result of laboratory number 10 is non-compatible with others. therefore, the degree of equivalence was not established for measurement results obtained in this laboratory. figure 3 presents the modified data after extending the measurement uncertainties for measurement results 1, 3, 5, 6, 8 and 10. the approach given is compared with a random effect model as well as with the method used in the ccqm k-5 report. comparison of different methods applied for the analysis of the ccqm.k-5 data is presented in table 3 and table 4. the approach proposed provides the smallest measurement uncertainty associated with the reference value. table 1. metrological pair-wise compatibility of the measurement results. the last column presents compatibility with the kcrv. the bold figures indicate the pairs of measurement results which are non-compatible with each other or the kcrv. 1 2 3 4 5 6 7 8 9 10 en 1 1.07 1.72 0.07 0.71 0.03 0.93 0.64 1.37 3.90 0.49 2 1.07 1.01 0.49 2.27 0.42 0.14 2.07 0.45 3.55 0.45 3 1.72 1.01 0.89 2.58 0.75 0.69 2.46 0.63 1.69 1.22 4 0.07 0.49 0.89 0.20 0.05 0.52 0.18 0.63 1.70 0.29 5 0.71 2.27 2.58 0.20 0.34 1.66 0.05 2.50 5.53 1.25 6 0.06 0.85 1.50 0.10 0.68 0.79 0.62 1.13 3.35 0.38 7 0.93 0.14 0.69 0.52 1.66 0.39 1.57 0.19 2.43 0.47 8 0.64 2.07 2.46 0.18 0.05 0.31 1.57 2.31 5.19 1.16 9 1.37 0.45 0.63 0.63 2.50 0.56 0.19 2.31 2.87 0.78 10 3.90 3.55 1.69 1.70 5.53 1.68 2.43 5.19 2.87 3.23 acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 32 the kcrv values inherent in these three methods are compatible. but the does significantly differ depending on the method used. the approach given provides the compatibility of the measurement results and reference value for all data except result 10. as stated in the ccqm.k-5 report the significant systematic bias was revealed in this result. the random effect model method and the method used in the ccqm.k-5 report do not provide mutually compatibility of the results. it is clearly seen by the fact that the corresponding does exceed the values of the associated expanded uncertainties. 6. conclusion the paper is devoted to a discussion of issues concerning the evaluation of inconsistent data of key comparisons of measurement standards. the approach proposed is based on interpreting the equivalence of measurement standards as the compatibility of a set of measurement results, which comprises the data provided by nmi’s and the reference value. the approach implies the extension of measurement uncertainties for several results provided by participants, which do not show the compatibility with the reference value. extension of measurement uncertainties during the processing of results is always a questionable point and requires a sound foundation. the authors consider that a significant argument is that the key comparisons are used to confirm the calibration and measurement capabilities of nmi’s. these capabilities should be in agreement with the kc results. figure 3. modified data. the error bar shows the expanded uncertainty. the solid line indicates the recalculated kcrv and the dashed lines indicate the expanded uncertainty associated with the kcrv. table 2. metrological compatibility of the modified measurement results. the uncertainties associated with measurement results 1, 3, 5, 6, 8 and 10 were extended. 1 2 3 4 5 6 7 8 9 10 en 1 0.50 0.76 0.06 0.24 0.03 0.53 0.23 0.68 1.47 0.52 2 0.50 0.54 0.49 0.83 0.46 0.14 0.82 0.45 1.50 0.03 3 0.76 0.54 0.74 1.01 0.73 0.43 0.99 0.35 0.71 0.57 4 0.06 0.49 0.74 0.16 0.08 0.52 0.15 0.63 1.37 0.50 5 0.24 0.83 1.01 0.16 0.27 0.84 0.01 1.00 1.71 0.87 6 0.03 0.46 0.73 0.08 0.27 0.50 0.26 0.64 1.44 0.48 7 0.53 0.14 0.43 0.52 0.84 0.50 0.83 0.19 1.32 0.17 8 0.23 0.82 0.99 0.15 0.01 0.26 0.83 0.99 1.70 0.85 9 0.68 0.45 0.35 0.63 1.00 0.64 0.19 0.99 1.30 0.69 10 1.47 1.50 0.71 1.37 1.71 1.44 1.32 1.70 1.30 1.58 table 3. comparison of the approaches applied for the analysis of the ccqm k-5 data. calculation kcrv. ccqm-k5 given approach random effect model kcrv uncertainty kcrv uncertainty kcrv uncertainty 1.513 0.023 1.5247 0.0083 1.5213 0.0243 table 4. comparison of the approaches applied for the analysis of the ccqm k-5 data. . calculation does. ccqm-k5 given approach random effect model di u(di) di u(di) di u(di) 1 -0.015 0.029 -0.0267 0.0514 -0.0233 0.0227 2 0.012 0.025 0.0003 0.0113 0.0037 0.0129 3 0.041 0.032 0.0293 0.0514 0.0328 0.0195 4 -0.020 0.067 -0.0317 0.0635 -0.0282 0.0602 5 -0.033 0.025 -0.0447 0.0514 -0.0413 0.0128 6 -0.013 0.031 -0.0247 0.0514 -0.0213 0.0189 7 0.016 0.033 0.0043 0.0246 0.0077 0.0262 8 -0.032 0.026 -0.0437 0.0514 -0.0402 0.0165 9 0.022 0.026 0.0103 0.0148 0.0137 0.0170 10 0.093 0.026 0.0813 0.0514 0.0848 0.0151 acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 33 otherwise, this nmi should modify the measurement uncertainties declared in its calibration and measurement capabilities (after comparison) to make them consistent with the results of the comparison. if the measurement data reported by kc participants show the non-compatibility and no outliers can be identified unambiguously, the extension of measurement uncertainties for several measurement results seems to be reasonable. advantages of the method proposed are the following. the kc data analysis results in obtaining a set of compatible measurement results. the results are compatible with each other and with the kcrv calculated using these results. the interpretation of measurement standard equivalence as the metrological compatibility of a set of measurement results is explicit. this interpretation corresponds to the practical usage of kc results as objective foundation for mutual recognition of measurement results obtained in the participating labs. the comparison of the procedure proposed with other approaches for evaluating inconsistent data is discussed and is illustrated by the analysis of ccqm.k-5. it is important to stress that there is no single method for analysing inconsistent data of kc’s. application of any method requires a preliminary consideration of the data reported by participants and a clear interpretation of a model to be chosen for a particular kc. references [1] cipm mra. bureau international des poids et mesures 1999. [2] cox m g “the evaluation of key comparison data: an introduction” 2002 metrologia 39, pp. 587-8. [3] cox m g “the evaluation of key comparison data” 2002 metrologia 39, pp. 589-95. [4] chunovkina a. g., elster c., lira i., woeger w. “analysis of key comparison data and laboratory biases”, 2008 metrologia 45, pp. 211-6. [5] lira i. “combining inconsistent data from interlaboratory comparisons”, 2007 metrologia 44, pp. 415-21. [6] zhang n f, liu h, sedransk n and strawderman w e “statistical analysis of key comparisons with linear trends” 2004 metrologia 41, pp. 231-7. [7] the authorsise k. and wöger w. “removing model and data non-conformity in measurement evaluation”, 2000,meas. sci. technol. 11, pp. 1649-58. [8] j.willink r. “statistical determination of a comparison reference value using hidden errors”, 2002 metrologia 39, pp. 343-54. [9] cox m. g. “the evaluation of key comparison data: determining the largest consistent subset”, 2007 metrologia 44, pp. 187-200. [10] elster c., toman b.“analysis of key comparisons: estimating laboratories’ biases by a fixed effects model using bayesian model averaging”,2010, metrologia 47, pp. 113-119. [11] white d r “on the analysis of measurement comparisons” 2004 metrologia 41, pp. 122-31. [12] sutton c m “analysis and linking of international measurement comparisons” 2004 metrologia 41, pp. 272-7. [13] toman b, possolo a “laboratory effects models for interlaboratory comparisons”, 2009 accreditation and quality assurance 14, pp. 553-563. [14] international vocabulary of metrology – basic and general concepts and associated terms, 3rd edition, 2008 version with minor corrections, bipm, jcgm 200, 2012. [15] cipm mra-d-05 measurement comparisons in the cipm mra (http://kcdb.bipm.org/). acta imeko contacts acta imeko issn: 2221-870x december 2019, volume 8, number 4 acta imeko | www.imeko.org december 2019 | volume 8 | number 4 | 0 journal contacts about the journal acta imeko is an e-journal reporting on the contributions on the state and progress of the science and technology of measurement. the articles are mainly based on presentations presented at imeko workshops, symposia and congresses. the journal is published by imeko, the international measurement confederation. the issn, the international identifier for serials, is 2221-870x. editor‐in‐chief dušan agrež, slovenia founding editor‐in‐chief paul p. l. regtien, netherlands associate editor dirk röske, germany editorial board section editors (vol. 7 and 8) leopoldo angrisani, italy filippo attivissimo, italy eulalia balestieri, italy eric benoit, france paolo carbone, italy lorenzo ciani, italy catalin damian, romania pasquale daponte, italy luca de vito, italy luigi ferrigno, italy edoardo fiorucci, italy alistair forbes, united kingdom helena geirinhas ramos, portugal sabrina grassini, italy fernando janeiro, portugal konrad jedrzejewski, poland andy knott, united kingdom francesco lamonaca, italy massimo lazzaroni, italy fabio leccese, italy rosario morello, italy michele norgia, italy pedro miguel pinto ramos, portugal nicola pompeo, italy sergio rapuano, italy dirk röske, germany alexandru salceanu, romania constantin sarmasanu, romania lorenzo scalise, italy emiliano schena, italy enrico silva, italy krzysztof stepien, poland marco tarabini, italy yvan baudoin, belgium marcantonio catelani, italy lorenzo ciani, italy egidio de benedeto, italy alessandro depari, italy alessandro germak, italy sabrina grassini, italy konrad jędrzejewski, poland min-seok kim, korea fabio leccese, italy rosario morello, italy vilmos palfi, hungary nicola pasquino, italy franco pavese, italy jeerasak pitakarnnop, thailand alexandru salceanu, romania emiliano sisinni, italy jan saliga, slovakia jorge c. torres-guzman, mexico ian veldman, south africa rugkanawan wongpithayadisai, thailand claudia zoani, italy about imeko the international measurement confederation, imeko, is an international federation of actually 42 national member organisations individually concerned with the advancement of measurement technology. its fundamental objectives are the promotion of international interchange of scientific and technical information in the field of measurement, and the enhancement of international co-operation among scientists and engineers from research and industry. addresses principal contact prof. dušan agrež university of ljubljana faculty of electrical engineering tržaška 25, 1000 ljubljana, slovenia e-mail: dusan.agrez@fe.uni-lj.si acta imeko yvan baudoin professor emeritus royal military academy manager international cbrne institute/erkc rue sart-dames-avelines, 8a, 86210 les bons villers, belgium e-mail: yvan.baudoin@ici-belgium.be support contact dr. dirk röske physikalisch-technische bundesanstalt (ptb) bundesallee 100, 38116 braunschweig, germany e-mail: dirk.roeske@ptb.de mailto:dusan.agrez@fe.uni-lj.si mailto:dusan.agrez@fe.uni-lj.si mailto:yvan.baudoin@ici-belgium.be mailto:yvan.baudoin@ici-belgium.be mailto:dirk.roeske@ptb.de mailto:dirk.roeske@ptb.de modelling and simulations for signal loss evaluation during sampling phase for thermoluminescence authenticity tests acta imeko issn: 2221-870x march 2021, volume 10, number 1, 150 154 acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 150 modelling and simulations for signal loss evaluation during sampling phase for thermoluminescence authenticity tests anna maria gueli1, martina pace1, stefania pasquale1, giuseppe politi1, giuseppe stella1, carlo trigona2 1 ph3dra labs, department of physics and astronomy “ettore majorana”, university of catania & infn-chnet sez ct, via santa sofia 64, 95123 catania, italy 2 department of electrical electronic and computer engineering, university of catania, viale andrea doria 6, 95125 catania, italy section: research paper keywords: thermoluminescence; drilling; electrical measurements; simulation; glow curve. citation: anna maria gueli, martina pace, stefania pasquale, giuseppe politi, giuseppe stella, carlo trigona, modelling and simulations for signal loss evaluation during sampling phase for thermoluminescence authenticity tests, acta imeko, vol. 10, no. 1, article 20, march 2021, identifier: imeko-acta10 (2021)-01-20 editor: carlo carobbi, university of florence, italy received may 15, 2020; in final form february 26, 2021; published march 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: stefania pasquale, e-mail: stefania.pasquale@ct.infn.it 1. introduction thermoluminescence testing (tl) has for many years been a routine methodology for dating and verifying the authenticity of terracotta works of art. the assumption of tl is that a material absorbs energy during exposure to ionising radiation, which is then stored. the stored energy is released as visible light when the material is heated. this is due to the particular properties of natural dosimeters such as quartz mineral contained in ancient terracotta [1]. the crystal lattice of quartz (sio2) often presents trace impurities (i.e. aluminium ions, alkali ions and hydroxyl groups) and maintains a large number of complex defect configurations (such as dislocations and vacancy clusters). the lattice imperfections act as electron trapping regions when these charge carriers are activated to move through the crystal following ionisation by alfa, beta and gamma rays existing in nature. when a material containing quartz is heated to temperatures up to 500– 600 °c, the binding between the imperfection and the electron may be upset, and the freed electron gives rise to photon emission during the return to its atomic site [1]. the heating of terracotta in a furnace resets tl accumulated by the quartz, and from this time on, tl begins increasing again until a new heating occurs. in the laboratory, the sample is reheated by a controlled heating process following a specific temperature rate (), and then the energy is released in the form of visible light (tl emission). the intensity of this photon emission (itl) as a function of the temperature is plotted in a glow curve that represents the most important result of tl measurements [1], [2]. the decay rate for an initial population (n) of electrons in traps of a single activation energy (depth) using the arrhenius equation according to the first-order randall–wilkins model [1] is 𝑑𝑛 𝑑𝑡 = −𝑛 𝑠 𝑒 − 𝐸 𝑘𝑇 (1) where n is the concentration of trapped carriers, e is the activation energy of the trap (ev), k is the boltzmann constant (ev·k−1) and t is the temperature of the sample (k). the parameter s can be estimated with the following equation [1]: abstract in authenticity tests using the thermoluminescence (tl) method, the sampling phase is fundamental to collecting an appropriate amount of powder for analysis. the powder is usually obtained by drilling in hidden and pertinently selected areas of an artefact. during the drilling, a local temperature increase can occur, and, because thermoluminescence emission is dependent on the heating rate, the authenticity test result could be invalidated due to underestimation of the signal intensity. in this work, the percentage of signal intensity loss is investigated through the combination of a dynamic electro-mechanical model and a typical tl glow curve simulation. after first modelling the drilling procedure to estimate the maximum temperature reached, the optimal parameters that should be used during the sampling phase are checked by simulations together with an evaluation of the correlated signal losses. mailto:stefania.pasquale@ct.infn.it acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 151 𝑠 = 𝛽 𝐸 𝑘 𝑇𝑚 2 𝑒 𝐸 𝑘 𝑇𝑚 2 (2) where  is the temperature rate (k/s) and tm is the temperature value related to maximum intensity of peak (k). a glow curve of quartz is originated by the convolution of several peaks [3]-[7]. according to many authors, the precise temperature value of these peaks depends on the origin of the quartz analysed [8]-[10]. this represents the most important result of tl measurement [1], [2] and depends upon the material of the sample under examination. the shape of each glow curve peak’s intensity (itl), as described by equation (3), can be approximated by an analytical expression based on first-order kinetics where im indicates the tl intensity of the peak at temperature tm [11]: 𝐼tl(𝑇) = 𝐼m 𝑒 {1+ 𝐸 𝑘 𝑇m ( 𝑇−𝑇m 𝑇m )−𝑒 [ 𝐸 𝑘 𝑇m ( 𝑇−𝑇m 𝑇m )] } (3) for high quartz-concentration terracotta samples, four peaks at tm = 110 °c, 230 °c, 325 °c and 375 °c have been individuated. the 325 °c and 375 °c peaks are commonly used in tl dating because of their stability [12]. in this work, we consider these temperature values in the context of the cultural heritage applications of tl. the 110 °c peak is not used directly in applications of tl in archaeomaterials because the traps and the electrons are unstable at environment temperatures, and thus the tl signal cannot build up over time. during the pottery's archaeological burial, the electrons trapped at imperfections, which are responsible for this tl peak, have long since escaped and decayed away under the effects of environment ground temperature [13]. this peak is used for pre-dose dating, which is a well-established experimental method for determining the dose in cases in which the itl signal of a terracotta sample is affected by spurious signal and anomalous fading [14], [15]. the itl peaks at tm =110 °c, 230 °c, 325 °c and 375 °c also play a fundamental role in authenticity tests performed by tl. authenticity tests are used to distinguish between old (original) and new (fake) ceramic art objects through different sets of itl measurement. these sets consist of heating at a constant rate (), recording the natural itl signals (natural glow curve) and then exposing the sample to a calibration dose of beta radiation, preferably at a level comparable to its natural dose. finally, the calibration curve is elaborated and compared with the natural glow curve [16]-[18]. data interpretation is a key phase, and many factors must be considered, above all the method of obtaining the sample to be authenticated. as stated above, the sample is collected in the form of powder by drilling at low speed in a hidden area of the artefact to be analysed. the use of drilling, which is necessary in order to speed up the procedure, has a main drawback of enhancing the local temperature and so possibly causing a diminishment of the itl signal due to its dependence on the heating rate. this is evident in equations (1)-(3). in particular, increasing the heating rate causes the intensity of the tl emission to decrease [19]-[23]. this underestimation can lead to erroneous results in terms of false negatives because the reduced itl signal from an authentic sample could be confused with noise, causing the artefact to be considered a fake. within the framework of measurement, tl is the object of several studies that seek to improve the capabilities of new architectures for automatic sensing and readout systems [24][28]. in order to assess the effect of drilling on the collection phase and thus on the results of authenticity tests, a multidisciplinary study is carried out and two studies have been proposed by the authors in [29] and [30]. gueli et al. [29] presented preliminary research based on a non-invasive and in situ measurement methodology that estimated the temperature reached during the sampling. in particular, different conditions such as bit diameter, drill speed and interval time were explored. this method makes it possible to estimate the thermal energy dissipated during drilling through the measurement of an output voltage across a known resistor. the results were obtained on a pottery sherd, and the proposed solution is suitable to estimate the variation of temperature [29]. in gueli et al. [30], the authors validated the indirect method through comparison with a standard direct method based on the acquisition of thermal images. the experimental data show a good agreement between both indirect and direct approaches, and an analysis regarding the role of the drill bits was pursued. the results presented in gueli et al. [30] fixed the basis for a more generalised model and a study of the effects of temperature on the tl authenticity tests. in this context, this paper addresses these specific points and improves the state of the art by presenting a more exhaustive model of the entire drill-based architecture that can simulate any loss of the tl signal due to heating during sampling. in particular, by using a dynamic electro-mechanical model, it is possible to simulate the drilling procedure and estimate the maximum temperature reached during sample collection by measuring an electrical quantity. the simulations can also help to identify the optimal parameters to be used for the sampling phase. furthermore, the effects of the temperature reached during drilling in terms of tl signal loss in the collected sample have been simulated on the basis of the typical peaks of quartz crystalline inclusions. 2. measurement architecture and model a contactless measurement methodology was used to estimate the temperature during the drilling of an archaeological pottery sherd that was the object of a tl authenticity test. figure 1 shows the equivalent circuit of the drill [31], [32]. this section will present the measurement approach that was used to estimate, through an indirect method and by using a model, the temperature dissipated during the drilling procedure. the left image of figure 1 represents the excitation of the dc electric machine, which creates the static magnetic field used to move the rotor (m) following the faraday–lenz principle. the image on the right represents the movable part composed of the rotor, the windings with resistance ri and the external dc voltage v. in the presence of this voltage, current i will flow through the circuit and the rotor will start to rotate. it should be noted that several losses occur in the system [33], and, in particular, the term ri will cause dissipation due to the figure 1. equivalent circuit of the drill. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 152 joule effect. by using the resistor r and by measuring the voltage vm, it is possible to estimate the temperature variation during drilling. the energy balance equation in the rotor can be written as: 𝑉 = 𝐾𝑛𝑟 𝛷 + 𝑅𝑖𝐼 (4) where k is a constant that depends on the construction parameters of the machine (length, diameter, number of poles, type of windings, etc.), nr is the rotor speed and φ is the excitation flux. by multiplying both members by the current i (see equation 5), it is possible to write the balance equation in terms of powers. in particular, as shown in equation (6), the electrical power corresponds to the sum of the mechanical power and the power dissipated by the joule effect. 𝑉 𝐼 = 𝐾 𝑛 𝛷 𝐼 + 𝑅𝑖 𝐼 2 (5) 𝑃𝑒 = 𝑃𝑀 + 𝑃𝐽 (6) the mechanical torque (cm), which is a consequence of the applied voltage v, is slightly smaller [31] than the electromotive torque (cem) due to leakages caused by friction and ventilation as well as leakages in the iron. this mechanical energy, as shown in equation (7), corresponds to the product of the excitation flux φ, the current i and the constant kc. 𝐶em ≅ 𝐶m = 𝑃𝑀 𝜔 = 𝐾 𝑛 𝜙 𝐼 𝜔 = 𝐾 𝑛 𝜙 𝐼 2 π 𝑛 60 = 𝐾𝑐 𝜙 𝐼 (7) the mechanical balance equation can be written as: 𝐶m = 𝐶r + 𝐵 𝜔 + 𝐽 d𝜔 d𝑡 (8) where cr is the resistive torque, b is the viscous coefficient, j is the moment of inertia, ω is the angular velocity of the rotor and t is the time. in the presence of a mechanical load such as the drilling procedure, thermal energy et will be dissipated. the hypothesis of the method proposed here is that the work of the resistive torque is proportional to the thermal energy dissipated during drilling thanks to the temperature difference t along the bit. et can also be expressed as a function of the measurand: 𝐸t = 𝛹 𝜆 𝑆 ∆𝑇 𝐿 ∆𝑡 (9) where ψ is an experimentally estimated coefficient that is able to take the leakages into the account and to compensate the hypothesis of the model, λ is the thermal conductivity of the bit material, s is the contact surface, l is the dissipation length, δt is a given time interval and δt is the physical quantity of interest. the indirect estimation of δt can be accomplished by inverting the model above and setting the thermal power in proportion to the electrical power: 𝐸t 𝛥𝑡 = 𝑉m 2 𝑅 (10) δt can thus be estimated as follows: ∆𝑇 ≈ 𝑉m 2 𝐿 𝛹 𝜆 𝑆 𝑅 (11) as can be observed, in the absence of a mechanical load, the energy et is equal to 0 j. it is worth noting that by using equation (11) it is possible to estimate the value of the temperature variation during drilling. in fact, this condition will change the value of the mechanical torque, see equation (8), and, consequently, the value of the absorbed current i, see equation (7), and, finally, the voltage vm. through a differential measurement of vm, it is possible to estimate the temperature difference δt along the bit. assuming that the initial temperature was room temperature, t evaluation makes it possible to obtain the maximum temperature reached by the bit during drilling. this last causes the thermal energy transfer to the collected sample. 3. glow-curve simulation the aforementioned method was validated through the comparison between electrical measurements and infrared (ir) thermography image acquisitions [30]. the correlation between the maximum temperature estimated by the voltage measurements and the related ir images showed that the accuracy of the model corresponds to about 11%. the method determined the maximum temperature and the time to achieve it during drilling with different drill bits. according to the method, once the maximum temperature value is reached, it remains constant. in the present study, the method is applied in three different experimental conditions with three metal bits with diameters equal to 2 mm, 3 mm and 5.5 mm. with these three configurations, called drill 2, drill 3 and drill 5, the maximum temperatures reached were 130 °c, 82 °c and 60 °c, respectively. using these values and applying the arrhenius relation, we looked for itl signal loss and then simulated the consequent glow curves based on the randall-wilkins model and the podgorsak approximation [11]. the arrhenius equation defines the probability per unit time of the release of an electron from the traps: 𝑝 = 𝑠 ∙ 𝑒 (− 𝐸 𝑘 𝑇 ) (12) where p is the probability per unit time and s is the frequency factor defined in equation (2); we consider this value to be 10-12 s-1. e is the trap depth (the energy needed to release an electron from the trap), k is the boltzmann’ constant (8.617 × 10-5 ev/k) and t the absolute temperature. table 1 shows the temperature values related to the maximum intensity of the quartz peaks (tm) and the corresponding values of the trap depths (e), which were used for the calculation of the tl intensity losses [34]. the glow-curve simulations have been performed using equations (1)–(3) with the parameters in table 1. a temperature range between 0–500 °c and a heating rate of 5 °c/s, corresponding to those typically used in a laboratory during an authenticity test, were considered. the experimental tl glow curve used as reference was recorded using these parameters on an automated risø reader model tl-da-10 equipped with an emi 9235qa table 1. parameters used to simulate the glow curves: tm is the temperature value related to maximum intensity of peaks; e is the value of the trap depths. tm (°c) e (ev) 110 0.891 230 1.170 325 1.400 375 1.510 acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 153 photomultiplier tube. tl signals were detected using corning 759 and schott bg-12 optical filters. 4. results and discussion in table 2, for each quartz peak (tm), the percentage of residual itl for the configurations drill 2 (130 °c), drill 3 (82 °c) and drill 5 (60 °c) is listed. the values are expressed in terms of the tl intensity in the standard dating procedure, in which the powder of the sample is obtained by grinding the sample at room temperature. these values are reported in figure 2; the decreasing of tl intensity as a function of drilling temperature is evident, and the effect is stronger for lower tl peak temperatures. the percentage of residual itl is significant for some peaks. the peak at 110 °c is very sensitive to temperature increase. in fact, at temperatures greater than 22 °c, the residual itl for this peak is 57% at 60 °c, 38% at 82 °c and 0 at 130 °c. this could have important consequences. for example, if a 2 mm diameter drill bit that reached a temperature of 130 °c was used during sampling, an artefact could be reputed as original despite it being a fake. concerning the peaks at 325 °c and 375 °c, two possible drawbacks could occur due to the low residual itl at high sampling temperature: a recent sample could be considered an original even if it was actually a fake, and, on the other hand, the expected age of an antique sample could be significantly underestimated. thanks to the glow curve simulation described in section 3, we can directly see the effect of the signal loss on the tl signal. in figure 3, we compare an experimental glow curve (no drill) used for normalisation with those obtained at the different sampling conditions shown in table 2 (drill 5, drill 3 and drill 2). the no drill condition is obtained by the deconvolution of an experimental glow curve generated during a standard dating procedure used as reference. 5. conclusions starting from an electrical and mechanical model, a method has been developed to apply during sampling for an authenticity test. by simulating the parameters at stake, this method makes it possible to evaluate the percentage of itl signal loss that could occur in tl measurements in the laboratory. the temperature reached during sampling could be sufficient to compromise the results of an authenticity test. the 5.5 mm drill bit seems to be the best for powder collection because the temperature achieved during drilling with this bit is not high enough to significantly affect the itl. although the actions performed during the collection phase depend on several factors, above all the intrinsic characteristics of the sample and the directives of the artefact owner in order to preserve the object, by our method it could be possible to determine the underestimation of itl in advance and thus make the opportune corrections during the data elaboration. our results indicate that, during sampling for an authenticity test, the size of the drill bit and the duration of powder collection must be marked because they can affect the tl test result. with this in mind, this research could be continued with further studies of drill sampling on ancient artefacts in order to endorse the method now proposed by recording and analysing experimental glow curves. references [1] m. j. aitken, thermoluminescence dating, academic press, london, 1985, isbn 0120463814. 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effectiveness test for e-training with the use of a dromader tele-operated vehicle igor ostrowski1, andrzej masłowski1 1 digital mobile robotics division/research and academic computer network/ national research institute, kolska 12, 01-045 warsaw, poland section: research paper keywords: operator training; mobile robots; simulator test; humanitarian demining citation: igor ostrowski, andrzej maslowski, simulator effectiveness test for e-training with the use dromader tele-operated vehicle, acta imeko, vol. 8, no. 4, article 10, december 2019, identifier: imeko-acta-08 (2019)-04-10 editor: yvan baudoin, international cbrne institute, belgium received dezember 5, 2018; in final form april 18, 2019; published december 2019 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: igor ostrowski, e-mail: iostrowski@wp.pl 1. introduction the detection and removal of landmines from infested fields has been a severe problem in recent decades, with vast political, social, and economic dimensions [1], [2]. in this context, the international scientific community has shown a broad interest in solving this problem from different points of view and using different procedures [3]-[7]. the anti-personnel mines, submunitions, and unexploded ordnance (uxo) could remain active for several years (in some cases, up to fifty years). they do not discriminate between military and civilian people, killing or injuring indiscriminately, including children and humanitarian aid workers [8], [9]. many types of mobile vehicles that could take onboard sensors and other equipment to displace them over a mineinfested area (e.g. wheeled vehicles, tracked vehicles, and even legged robots) have been proposed to perform demining tasks efficiently and safely. wheeled robots are the simplest, cheapest, and easiest to control [10], but tracked robots have an excellent ability to travel across almost any terrain. the need to develop mobile robots to operate in certain situations in which other humanitarian demining systems cannot operate properly or have low yield, have been investigated by several research centres, including the royal military academy (rma) and free university of brussels, belgium; the tokyo institute of technology, japan; the spanish national research council (csic), spain; and others. it is expected that diverse types of robots, differing in the kind of traction, load capacity, range, manipulation ability, and equipment with sensors are applied in the missions. a need to train a significant number of persons in the operation of these robots and to obtain a high proficiency in the operation thereof is particularly relevant in humanitarian demining for the sake of possible contact with explosive substances, creating a danger for operators, the population at large, and the environment. training tasks require many hours of exercises with different types of robots. conducting such training with the use of real robots would be unprofitable and probably unfeasible for technical and organisational reasons, including difficulties with the creation of all possible situations with which a robot operator must cope. the use of trainers, simulating robots’ behaviour in different situations and circumstances, would be a necessity. summarising the procedure for training an operator of demining platforms with a simulator can reduce the training time, as studied in [11]-[13]. among others, the unmanned ground vehicle (ugv) dromader used in humanitarian abstract this article presents the results of an effectiveness test of a simulator for the e-training operators of tele-operated vehicles with the use of a ugv dromader. the mobile platform dromader, its control panel, and the simulator system (hardware and software) are described. the results of the 3d modelling and construction of physical models are included. graphical models using 3dsmax software from a cad model and the mechanical part developed using vortex editor are discussed. two tests conducted with the participants divided into two groups is undertaken. the first operated the dromader robot without previous training on the simulator, the second went through simulator training before operating the robot. the trial was composed to test driving and the manipulation operators’ abilities. the results of effectiveness test are discussed. mailto:iostrowski@wp.pl acta imeko | www.imeko.org december 2019 | volume 8 | number 4 | 55 demining is very hard to operate and a long time for training operators is required. for this reason, the application of a simulator system is advisable. this would reduce the training time and the time for which the robot is used for training purposes. this article is organised as follows: we first discuss the results of the applications of the tools for 3d modelling and construction of physical models for a simulator dedicated to the e-training of demining robot dromader operators. graphical models are developed using 3dsmax software from a cad model. it is possible to perform straight conversion from the cad model to a graphical game-ready model, but the results are poor. the most serious issues were performance and texturing. graphical models had to be created from scratch. the mechanical part was developed using vortex editor. vortex editor is a tool that allows the creation of a mechanical system for the vortex physics universe. it is a type of cad software, which allows for creating projects, running test simulations, and tuning parameters without the need to export the model to a simulation framework. thereafter, the results of the effectiveness test of the simulator for e-training operators of tele-operated vehicles are shown. the test was conducted with the participants divided into two groups. the first group operated the dromader robot without previous training on the simulator. members of the second group went through simulator training before operating the robot. the trial involved two exercises. the first of these was devoted to testing the driving abilities of the operators. the task for the operator was to cover the distance in a short time and to not make any errors (such as deviations from the route or collisions with obstacles), which were scored negatively. the second exercise was devoted to testing the manoeuvre abilities, and the manipulation of heavy elements (concrete blocks) using the robot’s manipulator was the substance of the test. finally, some conclusions are offered in section 6. 2. ugv dromader the ugv dromader robot, shown in figure 1, was designed and built by the military university of technology, warsaw, poland [14]. the robot's structure is characterised by a segmented chassis connected using an articulated joint. the turning angle of the front segment in relation to the rear can reach up to 90 °. this provides high manoeuvrability, allowing for easy turns in confined spaces. in conjunction with the track drive system and dual-axis drive, it allowed the dromader to achieve very good off-road mobility. the rear section houses a 15kw combustion drive unit, while the front section houses the control system and several components of the platform's drive system. because of the desire to minimise the weight of the robot, the entire chassis of the robot is made of aluminium. the robot may be equipped with an experimental protection system against remotely detonated improvised explosive devices (ieds), which may be mounted in the rear section above the engine inside the chassis. using this configuration requires the detector and jammer antennas to be mounted in very specific locations to avoid interference and not to reduce their working area due to the chassis structure and other equipment mounted on the robot. the rear segment also contains a wi-fi transceiver antenna (2.4 ghz) for the remotecontrol system. the manipulator was mounted on the front section of the robot. part of its structure and location requirements was that, due to its transport position, it would not obscure the operator's field of view registered by the teleoperation system. the manipulator has three degrees of freedom allowing for the control of the effector's position in the vertical plane. due to weight limitations, the horizontal plane control is achieved by turning the entire front section instead of an additional control mechanism. the dromader is equipped with three cameras, as shown in figure 2. a typical gamepad serves as the control console for robot operation. the dromader is operated from an operation stand, with the use of camera views. the operation stand engaged during the validation trial is presented in figure 3. figure 1. the demining ugv dromader. figure 2. the dromader robot’s cameras marked as 1, 2, and 3. figure 3. the dromader’s operation stand. acta imeko | www.imeko.org december 2019 | volume 8 | number 4 | 56 the developed simulator tool is used for training dromader’s operators. the training stand with the simulator, engaged during the validation trial, is presented in figure 4. 3. ugv dromader simulator 3.1. simulation framework description the simulation system we developed is a universal tool for multi-robot operation. figure 5 shows the simulation system concept. the created software solution can be divided into two sub-projects. the first is the simulation server, which is a standalone binary package. this program runs a simulation server, which is responsible for physics resolution, multipleclient scene synchronisation, and monitoring the mission progress. the second is a library that allows for communication with the simulation server. this software is a dynamic linked library. it allows for separating the simulation and trainees' consoles, so the simulation process can be run on multiple machines robustly. another important advantage of the presented solution is the fact that the rendering scene with multiple cameras can be a heavy task for a computer, but it no longer affects physics simulation. the scene is synchronised by means of a mixed transmission control protocol (tcp) and user datagram protocol (udp) protocol. it allows the connection of up to ten clients to the system. for the physics simulation, the cm-labs vortex library is used. this library is a solution for serious games (games not mainly designed for pure entertainment), industry simulations (like cranes and excavators), and military training. it comes with a mature framework that allows for the development of a simulation of almost any type of vehicle. the framework also allows for the simulation of internal combustion engines, electric motors, a variety of transmission systems (including classical manual and hydrostatics transmissions), and nearly any kinematic layout. it is also possible to simulate complex structures, like robotics manipulators. moreover, some simulation situations can be scripted using python language. the library is proprietary and closed-source. it is well integrated with open scene graph (osg) [12], another library, which is an open-source, opengl-based graphics engine. it is a c++ library that is very often chosen in serious games and training systems. this library provides a number of useful tools, such as a hierarchical structure of the scene, a hierarchical structure of the graphical model with several types of relationships between model parts, graphical model loaders (including proprietary formats like like *.flt, *.cae, or *.fbx), and many levels of detail. it also supports opengl implementation from 1.1 up to 4.0, including opengl es. the next part of the simulation framework is the qt 5 library. this is a popular open-source c++ framework under the lesser general public license (lgpl). it is widely used in graphical user interface (gui) applications, but it provides many solutions for non-gui developers. in the project, qtxml and qtnetwork modules were heavily used. the last used library is boost.python. this library is part of the boost open-source set of c++ libraries. boost.python allows for the exposure of some dynamically allocated c++ objects in the python name-space. this allows for the creation of tools that can be used during the execution of simulated mission scenarios. for example, during mission scenario development, some triggers can be prepared. those triggers can cause a number of actions, such as adding penalty points and time measurement. because of the use of python language, compilation is not needed. the code is loaded and interpreted by the integrated python parser in runtime. the simulation client used only osg and qt libraries. it makes this solution gui-ready, multi-platform, and lightweight. it can be easily integrated with the operator or trainer console. this library deals with recreating the graphical scene in client software; rendering camera views; and sending and receiving data from and to robots. it can also receive some messages from the simulation server, which can be sent from the scenario's script, executed on the server's side. 3.2. the dromader robot simulation the project for the robot in the simulation system can be divided into two main parts. the first is the graphical model, and the second is the physical model. it is important that the second part is based on the created graphical model. graphical models are developed using 3dsmax software based on a cad model. it is possible to perform a straight conversion from the cad model to a graphical game-ready model, but the results are poor. the most serious issues were performance and texturing. the graphical models had to be created from scratch. the mechanical part was developed using vortex editor. vortex editor is a tool that allows the creation of a mechanical system for the vortex physics universe. it is a cad-like software, which allows for creating projects, running test simulations, and figure 4. the dromader training stand. figure 5. the simulation system concept. acta imeko | www.imeko.org december 2019 | volume 8 | number 4 | 57 tuning parameters without the need to export the model to the simulation framework. the graphical model is organised in a hierarchical structure, where every moveable part has its own node with its own coordinate system. during physics modelling, a number of properties were added. first, the collision geometries were created. the collision geometries are simple geometric primitives, like boxes and cylinders that interact with each other. collisions are not resolved between graphical meshes. these shapes are organised into parts. every collision shape has its own mass, inertia matrix, and materials. thereafter, constraints were created. these are objects that create a connection between two or more parts. it could be, for example, prismatic or hinge-pair. several parameters describe the constraints, like character (free, motorised, or locked), internal friction, damping, stiffness, motor, blockage, maximum force, and limits. in this fashion, hydraulics actuators were added to the simulation. the next robot-vehicle system was added to the physics model. there is a typical vehicle template, like cars (all wheel drive or rear wheel drive or front wheel drive with auto and manual transmissions), tracked vehicles with skidsteering (like excavators [hydrostatics torque division] or tanks [differential torque division]). the robot dromader is a unique vehicle solution, so a new template was created (figure 7). the template, after having been loaded to vortex editor, automatically creates complex structures like the awd truck. objects such as the engine, differentials, shafts, transmission system, driving constraints, and wheels with suspension are introduced and assigned proper relationships. the structure of the dromader robot is fairly simple. the engine is connected to the hydrostatics transmission and the ratio of every output motor can be set separately. the robot does not have differential steering (like an excavator), so the ratio for each track is the same. driving is realised using hinge constraints with a velocity actuator. the final layer of the physics model is logic, which does not have to be hard-coded in the application code. every robot’s object can communicate with python scripts. these scripts are an elegant way of implementing some logical dependencies. for example, the dromader robot has three driving modes. switching driving modes affects motor velocity but also reconfigures the hydrostatics system, connecting the motors as parallel; thus, a higher torque is generated. this function cannot be implemented directly but can be realised as a script that would change the hydrostatics system ratio in a proper way. finally, the vortex high level interface allows creating a universal interface for the vehicle model to communicate with the rest of the application. after loading the model into the final simulation application, it allows us to read and write the parameters from the c++ code. 3.3. environment the sample environment was created from point cloud. it is much easier to create a real location model using geodetic data figure 6. the physical model in vortex editor. figure 7. the robot dromader system template. figure 8(a). environment representation. figure 8(b). environment representation acta imeko | www.imeko.org december 2019 | volume 8 | number 4 | 58 than only using photos [19], [20]. the environment was also created using 3dsmax. the building’s floor plans were extracted from point cloud data, and an elevation map was created for the ground shape. vegetation was replaced with a simple billboard representation. the billboard is a graphics method used for creating planar shapes that always face the viewer's camera. this method is widely used for simple vegetation rendering [22], [23]. the scene was textured and finally imported to vortex editor, where the parameters of the materials were applied to the surface [24], [25]. the configuration is an object-oriented xml file that the simulation server loads during initialisation. these files contain a great deal of information, like the spawn position of the robots, the scene type, and the position of third-person view cameras. it allows us to setup or modify the simulation scene in a highly convenient way. furthermore, the xml file contains a script coded in python language, which allows us to interact with the simulation, so some events can be dynamically created. it can create many opportunities to develop life-like, interactive scenarios. 3.4. operator console the operator console, as mentioned above, was created using the simulation client library. the dromader operator's console is a handheld computer with a game-style controller. it communicates with the robot using a wireless connection. the operator can observe one camera and steer the robot and its manipulator using analogue sticks and buttons on the controller. operations like connection; disconnection; and turning the robot’s system on and off are realised by means of the touchscreen gui. due to a lack of access to the source code of the operator's console, the interface was recreated using our own framework. this program was successfully integrated with the simulation client library, and finally, a realistic operator's console was made. the separation of the simulation client library and the console’s code is a great advantage because it makes a simulator independent from the input and output devices used and it allows for integration with real equipment. a view on the simulator screen is presented in figure 9. 4. course of the validation trial the validation trial was conducted with 20 participants (military academy students) divided into two groups. the first group, comprising 11 persons, operated dromader without previous training on the simulator. the members of the second nine-person group participated in simulator training before operating the robot. the trial involved two exercises. the first of these was devoted to test driving the abilities of the operators. the route used in this exercise is presented in figure 10(a) and figure 10(b). the task for the operator was to cover the distance in a short time and not make errors (such as deviations from the figure 9. the operator console connected to the simulator server. figure 11. the exercise for testing the robot’s manipulation abilities. figure 10(a). the route for testing the driving ability of dromader (aerial view). figure 10(b). the route for testing the driving ability of dromader (ground view). acta imeko | www.imeko.org december 2019 | volume 8 | number 4 | 59 route or collisions with obstacles), which were scored negatively in accordance with [1]. the second exercise was devoted to test the robot’s operator ability to manipulate heavy elements (concrete blocks). the exercise, presented in figure 11, consisted of driving up to a certain block, localising it, grasping it (i), and then carrying it to a certain location (ii). errors were scored negatively. the block, being the manipulation subject, was located near a vertical wall. it represented an obstacle, like other concrete objects in the area. 5. results in both the manipulation and driving exercises were the results obtained by comparison of the results for the participants of the two groups i.e. with and without previous training on the simulator. in both exercises, the time taken for fulfilling the task and the number of errors made were measured. the global outcome in relation to the time was calculated in two ways, as an arithmetic mean and as a median, according to the following formulas: a) arithmetic mean (equation 1): 𝑥sr = ∑ 𝑥𝑖 𝑛 𝑛 𝑖=1 (1) and median (equation 2): 𝑚e = { 𝑥𝑛+1 2 for odd 𝑛 1 2 (𝑥𝑛 2 + 𝑥𝑛 2 +1 ) for even 𝑛 (2) where: n – number of participants; x – time taken to fulfil the task. the results obtained in the driving exercise are shown in table 1 and table 2. after rejecting the extreme values, the arithmetic mean and median adequately amounted to 232 and 230. after rejecting extreme values, the arithmetic mean and median adequately amounted to 306 and 294. the differences in percentages between outcomes of two groups were calculated for the achieved times according to the following formulas: ∆𝑥sr = 𝑥srnsz − 𝑥srsz 𝑥srnsz ∙ 100% (3) ∆𝑚e = 𝑚ensz − 𝑚esz 𝑚ensz ∙ 100% (4) the obtained values are as follows: ∆𝑥sr = 22 %, and for rejected extreme values 24 %, ∆𝑚e = 21 %, and for rejected extreme values 22 %. the differences in percentages ∆𝑒 between the outcomes of the two groups for errors made during the driving exercise, calculated in the same way, are as follows: ∆𝑒 = 29 % for the total number of errors, ∆𝑒 = 37 % for the maximum number of errors made by a single participant. the results obtained in the manipulation exercise are shown in table 3 and table 4. the differences in percentages between the outcomes of the two groups for the time taken to fulfil the task, calculated in the same way as for the driving exercise, are as follows: ∆𝑥sr = 44 %, ∆𝑚e = 42 %. 6. conclusions the test described in this paper confirms that using simulators in training robot operators is an effective way of reducing the cost of training and of allowing trainees to make errors with lower levels of stress. comparing our method with table 1. driving outcomes of participants trained on the simulator. participant time taken to fulfil the task, in s duration of training in min number of errors arithmetic mean xsresz in s median mesz in s 1 217 40 0 249 243 2 302 20 1 3 243 30 3 4 245 30 3 5 385 30 4 6 193 60 4 7 190 60 5 8 246 30 5 9 216 30 0 table 2. driving outcomes of participants not trained on the simulator. participant time taken to fulfil the task, in s duration of training in min number of errors arithmetic mean xsresz in s 1 280 3 319 308 2 377 4 3 324 4 4 348 8 5 260 4 6 308 3 7 262 2 8 455 7 9 254 0 10 247 0 11 398 8 table 3. the manipulation outcomes of the participants trained on the simulator. participant time taken to fulfil the task, in s duration of training in min number of errors arithmetic mean xsresz in s median mesz in s 1 207 40 1 190 192 2 175 20 3 3 231 30 0 4 143 30 0 5 233 30 3 6 174 60 1 7 192 60 3 8 165 30 0 9 192 30 1 acta imeko | www.imeko.org december 2019 | volume 8 | number 4 | 60 the use of real machines during the first lessons of robot use, our method is a much faster way of getting satisfactory results. our results were validated by determining how the simulator under consideration meets the requirements of its performance threshold. for the simulator, a triple-value threshold was set with three performance levels: • level u – unsuitable tool – less than 10 % difference between the groups’ outcomes; • level i – the tool needs improvement – difference between 10 % and 30 %; • level s – suitable tool – difference larger than 30 %. differences of outcomes in driving exercises are in the 21 % 24 % bracket for the time taken to fulfil the task and in the 29 % 37 % bracket for the number of errors made. differences of outcomes in manipulation exercises are in the 42 % 44 % bracket for the time taken to fulfil the task and reach 40 % for the number of errors made. considering that the time taken to fulfil the task of approach in demining missions is of secondary importance [4], it is justified to decide the level s obtained by the trainer-simulator as a result of the validation trial. references [1] y. baudoin, m. k. habib, i. doroftei mobile robotics systems for humanitarian demining and risky interventions, in: using robots in hazardous environments y. baudoin, m. k. habib (editors). woodhead publishing ltd, elsevier, uk, 2011, pp. 331. [2] y. baudoin, i. doroftei, tridem – a wheeled mobile robot for humanitarian mine clearance, proc. of the 6th iarp workshop on humanitarian demining (hudem), 24-26 april 2012, sibenik, croatia, pp. 93-98. [3] s. hirose, k. kato, quadruped walking robot to perform mine detection and removal task, proc. of the 1st international conference on climbing and walking robots, 1998, brussels, belgium, pp. 261-266. [4] p. gonzalez de santos, j. cobano, e. garcia, j. estremera, m. armada (2007) a six-legged robot-based system for humanitarian demining missions, mechatronics 17 (2007) pp. 417-430. [5] e. fukushima, m. freese, t. matsuzawa, t. aibara, s. hirose, humanitarian demining robot gryphon. current status and an objective evaluation, intl. journal on smart sensing and intelligent systems 1(3) (2008) pp.735-753. [6] r. fernández, h. montes, c. salinas, p. gonzález de santos, m. armada, design of a training tool for improving the use of hand-held detectors in humanitarian demining, industrial robot: an international journal 39(5) (2012) pp. 450-463. [7] h. montes, l. mena, r. fernandez, j. sarria, m. armada, inspection platform for applications in humanitarian demining, proc. of the 18th international conference on climbing and walking robots and the support technologies for mobile machines, 6-9 september 2015, hangzhou, china, pp. 446-453. [8] icbl, international campaign to ban landmines: 12th annual landmine monitor report, 2010. http://www.themonitor.org/media/1641811/landmine_monitor_2010_lowres .pdf [online, accessed: 14 june 2016] [9] icbl, international campaign to ban landmines: 17th annual landmine monitor report, 2015. http://www.themonitor.org/media/2152583/landmine-monitor2015_finalpdf.pdf [online, accessed 19 september 2016] [10] p. kopacek, l. silberbauer, a new locomotion concept for humanitarian demining robots, proc. of the 7th iarp international workshop on humanitarian demining, 28-30 march 2008, el cairo, egypt, pp. 32-36. [11] a. maslowski, training in military robotics and eod unmanned systems, in: conference and seminars, nato eod center of excellence publishing, trenčín, slovakia, 2014. [12] t. łuczynski, i. ostrowski, m. zoppi, j. bedkowski, a. masłowski, m. przybylko, p. wojcieszynska, m. szczepaniak, mobile demining devices modeling for the training purpose using vortex framework, proc. of the international conference on military engineering: problems and prospective, 23-25 april 2013, institute of military engineering publishing, wroclaw, poland, pp. 233-240. [13] a. kaczmarczyk, m. kacprzak, a. maslowski, muti-level simulation training of devices operators: an application to mobile robots operators, elektronika 11 (2009) [in polish]. [14] j. łopatka et al., intelligent communication supervising on the battlefield with the use ugv dromader, warsaw military university bulletin 62(3) (2013) pp. 13-25 [in polish]. [15] j. bedkowski, k. majek, i. ostrowski, p. musialik, a. maslowski, a. adamek, a. coelho, g. de cubber, methodology of training and support for urban search and rescue with robots, proc. of the 9th international conference on autonomic and autonomous systems, 2013, lisbon, portugal. [16] tiramisu consortium, training tools for the tiramisu project: annual report [online] http://www.fp7-tiramisu.eu/. [17] p. musialik, j. bedkowski, i. ostrowski, t. łuczyński, k. majek, a. maslowski, a. adamek, virtual environment modeling for the training of unmanned vehicle operators. air force academy publishing, deblin, poland, 2015. [18] j. bedkowski, a. maslowski, g. de cubber, real-time 3d localization and mapping for usar robotic application, industrial robot: an international journal 39(5) (2012). [19] p. b. silva, a. coelho, r. rodrigues, a. a. de sousa, a procedural geometry modeling api, proc. of the international conference on computer graphics theory and applications, grapp & ivapp 2012, 24-26 february 2012, rome, italy, scitepress. [20] p. b. silva, a. coelho, procedural modeling for realistic virtual worlds development, journal of virtual worlds research, vol. 4, no. 1, 2011. [21] d. jesus, a. coelho, c. rebelo, a. cardoso, a. a. sousa, a pipeline for procedural modelling from geospatial data. proc. eurographics 2012, pp. 9-10, 2012. [22] t. martins, p. b. silva, a. coelho, a. a. sousa, an urban ontology to generate collaborative virtual environments for municipal planning and management, proc. grapp 2012 -7th international conference in computer graphics theory and applications, pp. 507-510, 2012. [23] j. bedkowski, k. majek, a. nüchter, general purpose computing on graphics processing units for robotic table 4. manipulation outcomes of the participants not trained on the simulator. participant time taken to fulfil the task, in s duration of training in min number of errors arithmetic mean xsresz in s 1 305 4 341 334 2 414 2 3 283 4 4 334 1 5 383 1 6 294 1 7 316 2 8 415 5 9 249 1 10 403 2 11 354 0 http://www.the-monitor.org/media/1641811/landmine_monitor_2010_lowres.pdf http://www.the-monitor.org/media/1641811/landmine_monitor_2010_lowres.pdf http://www.the-monitor.org/media/1641811/landmine_monitor_2010_lowres.pdf http://www.the-monitor.org/media/2152583/landmine-monitor-2015_finalpdf.pdf http://www.the-monitor.org/media/2152583/landmine-monitor-2015_finalpdf.pdf http://www.the-monitor.org/media/2152583/landmine-monitor-2015_finalpdf.pdf http://www.fp7-tiramisu.eu/ acta imeko | www.imeko.org december 2019 | volume 8 | number 4 | 61 applications. journal of software engineering for robotics, vol 4, no 1(2013), pp. 22-33. [24] m. a. fischler, r. c. bolles, random sample consensus: a paradigm for model fitting with applications to image analysis and automated aartography, comm. of the acm 24 (6), june 1981, pp: 381–395, doi :10.1145/358669.358692. [25] s. schumacher, j. bohm, georeferencing of terrestrial laser scanner data for applications in architectural modelling, 3darch 2005: virtual reconstruction and visualization of complex architectures, mestre-venice, italy, 22-24 august 2005, universität stuttgart, 2005. [26] j. elseberg, d. borrmann, a. nüchter, efficient processing of large 3d point clouds. information, communication and automation technologies (icat), 2011 xxiii international symposium on. ieee, 2011. acta imeko, title acta imeko issn: 2221-870x march 2018, volume 7, number 1, 36-41 acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 36 analysis of the magnetic field in the presence of linear sub-surface cracks using ect dario j. pasadas, artur l. ribeiro, helena g. ramos instituto de telecomunicações and instituto superior técnico/universidade técnica de lisboa, av. rovisco pais 1, 1049-001 lisbon,portugal section: research paper keywords: non-destructive testing; eddy current testing; sub-surface cracks; aluminium citation: dario j. pasadas, artur l. ribeiro, helena g. ramos, analysis of the magnetic field in the presence of linear sub-surface cracks using ect, acta imeko, vol. 7, no. 1, article 7, march 2017, identifier: imeko-acta-07 (2017)-01-07 section editor: lorenzo ciani, university of florence, italy received september 29, 2017; in final form november 29, 2017; published march 2018 copyright: © 2018 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: dario j. pasadas e-mail: dpasadas@lx.it.pt 1. introduction eddy current testing (ect) is a widely used non-destructive testing (ndt) technique for the inspection of cracks in conductive materials [1], without damaging the structural integrity of those materials under inspection. this technique can be applied to several industries, such as aircraft structures [2] and petrochemical pipelines [3]. due to the complexity of the evaluation process, ect instruments with extended capabilities for ndt inspection of materials are actively researched [4], [5]. the ect technique is based on electromagnetic induction phenomena. it can be applied to detect and characterize surface and sub-surface cracks with a minimum preparation of the surfaces under inspection [6]-[8]. the eddy currents can be induced in the conductor by sinusoidal, pulsed or velocity excitation. a primary magnetic field is created by imposing a time-varying current in a coil. in the proximity of the coil to a metal, eddy currents are induced. a secondary magnetic field is created by the eddy currents which contains information about the state of the material. due to the nature of the electromagnetic phenomena, the main disadvantage of this technique is related to its limited sub-surface crack detection and characterization at deeper levels [9]. the eddy currents that flow in the metallic structure are sensitive to several factors, such as the material properties (electrical conductivity and magnetic permeability), the test frequency, the lift-off effect, the edge effect and the dimensions of the crack [10]-[11]. the flow of eddy currents in a conductor is maximum at the material surface and it is attenuated with increasing depth. for lower electrical conductivity or low magnetic permeability of non-ferromagnetic materials, the eddy current achieves deeper penetration. for a given material, the standard penetration depth (δ) is the depth at which the current density decreases to 1/e of the surface density amplitude [12], [13], when a uniform field is applied at the material surface. nowadays, tools like artificial neural networks or other classification methods based on automatic machine learning algorithms have been introduced in the research field to estimate the dimensions of cracks [14], [15], but they are still in development. in this kind of methods, the determination of features is essential for the success of the estimation of defect dimensions from the measured signals. for this purpose, it is essential to study the parameters that influence the penetration abstract this manuscript reports a study concerning the penetration of eddy currents in metallic non-ferromagnetic materials in the presence of linear sub-surface cracks. simulations were performed for a set of different lengths and depths of the crack using a sinusoidal excitation current with uniform magnetic field distribution. the perturbed magnetic field components (bx and bz) that occur due to the presence of cracks were obtained to understand the electromagnetic phenomena involved. the end and center of the crack were analyzed by the magnitude and the complex signature of the obtained magnetic field. the dependences between the length and depth of the crack and both magnetic field components were discussed. acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 37 depth of the electromagnetic field into a conductive material when surface or sub-surface cracks exist. hence, this paper presents simulation and experimental work to investigate the penetration of eddy currents in the material when a coil is excited by a sinusoidal current with uniform magnetic field distribution to evaluate sub-surface cracks. the tests were performed for linear cracks with different lengths and depths. 2. numerical model and conditions of test the simulations were computed for a 1050 aluminum alloy plate with 4 mm of thickness. linear cracks with fixed width of 1 mm, but different lengths and depths were studied. the simulations were performed using a 2d spatially invariant excitation field with sinusoidal time variation. the excitation, in simulation and experimentally, was applied by a planar coil at a fixed lift-off distance of 0.5 mm. the finite element method software comsol® multiphysics was used to evaluate the magnetic field components (bx and bz) in the proximity of the metallic plate containing sub-surface cracks. the physics used in this model was magnetic fields (ac/dc module) with a frequency domain study type. a parametric sweep of the length and depth of the sub-surface defects were used in this study. the excitation was performed by imposing a uniform surface electromotive force. the type of mesh elements used in this model was tetrahedral. to obtain the magnetic field components, a series of probing points along a straight line was added at 2.2 mm above the surface of the plate. a small air block, as shown in figure 1, was created surrounding the points where the magnetic field components were computed. the air block was meshed with high density elements in order to improve the accuracy of the results and minimize the error due to the interpolation. figure 1 depicts the defect, the air block and the measurement points used in the model. 3. eddy current distribution the eddy currents generated inside the aluminum plate were spatially uniform inside the restricted area of planes parallel to the surface, as depicted in figure 2(a), when no cracks are present. the eddy currents close outside the indicated region. in the proximity of a crack, the eddy current distribution is perturbed and the magnetic field produced by the eddy currents changes, as depicted in figure 2(b). the magnetic field is sensed by a magnetic sensor located in the proximity of the crack region. sensing coils can be used as sensing elements to detect cracks near the surface under inspection. however, their use as sensing element is insufficient to detect deep sub-surface cracks because the sensitivity of a coil is proportional to the operating frequency. other sensors, such as superconducting quantum interference devices (squid), hall effect, anisotropic magnetoresistors (amr) or giant magnetoresistors (gmr), have been introduced in eddy current systems giving better sensitivity to detect sub-surface cracks at deeper levels [16]. using these magnetic sensors, the perturbed magnetic field originated by the presence of a crack inside the metal is the measured quantity which is obtained to evaluate the size of the cracks. the measurements are obtained around the crack region above the surface of the metal under inspection. in the simulations, the perturbed magnetic field components (bx and bz) were obtained on the top of the aluminum plate in a straight line above the linear cracks region. the distance z considered between the measurement points and the sample plate was 2.2 mm. the bx component contains the excitation magnetic field (bext) and the perturbed magnetic field originated by the crack presence (bec), where the bext and the bec components are oppositely aligned vectors. the bz component is zero when the aluminum plate is without cracks, and non-zero when the plate contains cracks. thus, this means that the bz component contains only information about the perturbed magnetic field originated by the presence of a crack in the metallic plate. for the inspection of sub-surface cracks in metallic plates, an accurate choice of the operating frequency allows the primary magnetic field to penetrate more inside the conductor. for the 1050 aluminum alloy plate with 4 mm of thickness, a sinusoidal excitation was applied with an operating frequency ( f ) of 440 hz in order to allow the eddy currents achieve deeper penetration in the material. this frequency was chosen based on the standard penetration depth ( δ ) for this material, and computed using the following relation: 𝑓 = 1 𝛿2𝜇𝜇𝜇 (1) where the conductivity ( σ ) is 73.57 10× s/m and the permeability ( µ ) is 74 10π −× h/m. figure 1. – illustration of the defect, air block and series of probing points used in the model. figure 2. representation of the eddy current flow: (a) without defect presence; (b) with defect presence. acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 38 at this operating frequency, the standard penetration depth is 4 mm, which means that the flow of eddy currents at that depth is 1/e times the maximum current density at the material surface. for this value of the penetration depth in the aluminium plate under study, the chosen operating frequency and lift-off, the magnetic field response above the crack region is expected to be strongly dependent on the dimensions of the crack. taking this into account, in the next section, the two obtained magnetic field components (bx and bz) are presented and their dependence on the length and depth of the cracks are analysed. 4. results the simulations were performed for two crack depths (1 mm and 3 mm), and for fourteen different lengths (between 4 mm to 30 mm with an incremental step of 2 mm), as depicted in figure 3, in order to study the eddy current density depth profile when sub-surface cracks exist in conductive materials. the depth and the length of the crack are denoted by 𝑑 and 𝑙, respectively. the thickness (𝑡) of the aluminum plate and the standard penetration depth ( δ ) are equal to 4 mm. the simulations and the experimental tests were carried out for fixed excitation current amplitude of 200 ma. from the simulations results, figure 4 depicts the magnitude of the magnetic field components bx and bz obtained in a series of points in a straight line of 40 mm above the crack region. this straight line was placed in the region where maximum magnitude of the magnetic field perturbation is obtained in presence of a linear defect. the results show clearly the changes of the magnetic field response due to the variation of the dimensions of the crack. it is possible to say that both magnetic field components contain information about the size of the crack and both fields allow the estimation of the position of the cracks. for each crack size, it is possible to observe that the perturbed magnetic field bz contains two peaks at the positions of the crack ends. the center of the defect is located at the mid figure 3. cross section view in the x-z plane at the crack position. figure 4. simulated results of bx and bz obtained in a straight line above a set of different crack lengths (𝑙); (a) cracks depth equal to 1 mm; (b) cracks depth equal to 3 mm. acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 39 point between the peaks of the magnitude of bz. it is also possible to see that the magnitude of bz, at the crack ends, is directly related with the length and depth. for long sub-surface cracks with 1 mm of depth, the magnitude of bz tends to a constant value at the ends of the crack. if the depth of the crack increases, the same effect occurs but the magnitude of bz tends to a constant value for much longer cracks. concerning the magnitude of the perturbed magnetic field component bx, the obtained results indicate that the shape of the magnetic field response at the center of the crack changes when the length of the crack increases. in order to explain these phenomena, the peak value of bx and bz in function of the crack length were extracted and plotted in figure 5. figure 5(a) and figure 5(b) depict the peak values of bx and bz for a crack depth of 1 mm and of 3 mm, respectively. from figure 5(a), it can be seen that the peak value of the magnitude of bx increases with respect to the length of the crack up to 12 mm. this means that a lower eddy current density region exists above the centre region of the crack, which causes an increase in the resulting magnitude of the bx component (due to the decrease of bec). this effect is reversed for long sub-surface cracks. from figure 5(a), it can be seen that the peak value of the magnitude of bx decreases when the crack length increases beyond 12 mm (point of inversion). this occurs because the eddy currents that flow around the crack tend to flow above the crack. this means that the upper layer of the centre region of the crack becomes the path of lower resistance for the eddy currents. it can also be observed that the peak value of bz increases when the crack length increases up to 12 mm (knee point). beyond this crack length, the peak value of the magnitude of bz tends to a constant value. comparing bx and bz, it is possible to observe that the point of inversion of the peak value of bx curve matches approximately the knee point of the bz curve. taking this into account, it is possible to conclude that the amount of eddy currents that flow around/above the sub-surface cracks can be estimated from the obtained magnetic field by the length/depth ratio of the crack. however, it is important to mention that this ratio changes if the operating frequency changes. the measurement of just one component of the magnetic field is not enough to evaluate the length/depth of the crack if the peak values of the magnetic field components are considered. as an example, from figure 5(a), if the peak value of the bx component is equal to 9.66×10-4 t, it can be seen that the length of the crack has two solutions (8 mm and 18 mm). thus, to evaluate a crack in conductive materials both components of magnetic field (bx and bz) provide complementary information and both fields should be considered. from figure 5 (b), it is possible to observe that when the depth of the crack increases, the same effect occurs but the magnitude of bz tends to a constant value for cracks with longer length. still in the case of figure 5 (b), the bx component has two corresponding solutions when the length of the crack is equal to 10 mm or 28 mm. experimental results were obtained with similar conditions using a measurement system with an attached planar probe to measure the magnetic field in the presence of linear sub-surface cracks. the experimental setup is described in [17]. a planar coil was used to impose uniform current distribution in the aluminium plate. two gmr sensors were positioned in the figure 5. peak value of bx and bz with respect to the crack length; (a) for a crack depth of 1 mm; (b) for a crack depth of 3mm. acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 40 centre of the coil to measure the bx and bz components. figure 6 presents the components of the magnetic field (bx and bz) obtained experimentally for two linear sub-surface cracks with lengths of 12 mm and 8 mm at a fixed depth of 3 mm. comparing the experimental and the simulated results, it can be seen that the signatures of the magnitude of the magnetic field components (bx and bz) are similar. however, there is a mismatch between the two peak values of the bz component. this is due to the small misalignment between the sensor and the crack orientation during the scan. for the bx component, a baseline mismatch between the simulation and experimental results occurred because the gmr sensor was not exactly parallel to the excitation magnetic field in order to measure the strong bx component. this small misalignment of the sensor causes a decrease in the value of magnitude measured by the gmr sensor. from the simulations, the complex signatures of the magnetic field components were also obtained. figure 7 depicts the complex signatures (from bz) obtained crossing the center of all sub-surface defect length. the results are presented for two crack depths (1 mm and 3 mm). in figure 7, the simulated results show that the information regarding the length of the defect can be evaluated from the complex signature of the magnetic fields components bz. in both cases, the results show that the shape of the signature changes when defect length also changes. this means that the area of each signature could be considered as a feature and calculated to estimate the crack length. 5. conclusions simulations and experimental tests were performed on aluminum samples for eddy current studies. the penetrations of eddy currents in the material when a coil is excited by a sinusoidal current with uniform magnetic field distribution were studied and presented in this manuscript for sub-surface linear cracks. for a pre-defined excitation current amplitude and frequency, the eddy current penetration strongly depends on the dimensions of the crack. the results show that the peak values of the magnetic field components (bx and bz) contain information about the crack. however, the measurement of only one component of the magnetic field is insufficient to evaluate the length and depth of the crack. this is mainly because the eddy currents tend to flow (a) (b) figure 7. simulated results of the complex signatures of bz obtained in a straight line above linear cracks having various lengths (𝑙): (a) cracks depth equal to 1 mm; (b) cracks depth equal to 3 mm. (a) (b) figure 6. experimental results of bx and bz obtained in a straight line above the linear sub-surface crack at a fixed depth of 3 mm and two length of 8 mm and 12 mm: (a) bx magnetic field component ; (b) bz magnetic field component. acta imeko | www.imeko.org march 2018 | volume 7 | number 1 | 41 in the least resistance path, which has a dependence on the dimensions of the crack. this means that both magnetic field components (bx and bz) should be considered to evaluate cracks in metallic structures. concerning the obtained complex signatures of bz, the results showed that the shape of the signatures changes when the defect length changes. for a pre-defined excitation current amplitude and frequency, this feature can be useful to classify the length and depth of the sub-surface defect in a range of known geometries of the defect. acknowledgement this work was supported in part by project uid/eea/50008/2013 relim and the project fct ptdc/eei-elc/5811/2014. this support is gratefully acknowledged. references [1] j. g. martín, j. g. gil, e. v.-sánchez, “non-destructive techniques based on eddy current testing”, sensors, 2011, vol. 11, pp. 2525–2565. [2] g. yang, z. zeng, y. deng, x. liu, l. udpa, a. tamburrino, s.s. udpa, “3d ec-gmr sensor system for detection of subsurface defects at steel fastener sites”, ndt&e international, vol. 50, 2012, pp. 20-28. [3] m. coramik, y. ege, “discontinuity inspection in pipelines: a comparison review”, measurement, 2017, vol. 111, pp. 359-373. [4] d. pasadas, t. rocha, h. g. ramos, a. l. ribeiro, “evaluation of portable ect instruments with positioning capability”, measurement, january, 2012, vol. 45, no. 1, pp. 393-404. [5] d. e. aguiam, l. s. rosado, p. m. ramos, m. piedade, “heterodyning based portable instrument for eddy currents nondestructive testing”, measurement, september, 2015, vol. 45, pp. 146-157. [6] j. h. espina-hernánez, e. ramírez-pacheco, f. caleyo, j. a. pérez benitez, j. m. hallen, “rapid estimation of artificial nearside crack dimensions in aluminium using a gmr-based eddy current sensor”, ndt & e international, october, 2012, vol. 51, no. 1, pp. 94-100. [7] d. j. pasadas, a. l. ribeiro, h. g. ramos, t. j. rocha, “inspection of cracks in aluminum multilayer structures using planar ect probe and inversion problem”, ieee trans. instrum. meas., may, 2017, vol. 66, no. 5, pp. 920-927. [8] y-j kim, s-s lee, “eddy current probes of inclined coils for increased detectability of circumferential cracks in tubing”, ndt & e international, july, 2012, vol. 49, pp. 77-82. [9] b. p. c. rao, “practical eddy current testing”, alpha science international ltd, 2007. [10] j. garcía-martín, j. gómez-gil, e. vázquez-sánchez, “non destructive techniques based on eddy current testing”, sensors, 2011, vol. 11, pp. 2525-2565. [11] m. ricci, g. silipigni, l. ferrigno, m. laracca, i. d. adewalea, g. y. tian, “evaluation of the lift-off robustness of eddy current imaging techniques”, ndt & e international, january, 2017, vol. 85, pp. 43-52. [12] j. kral, r. smid, h. m. g. ramos, a. l. ribeiro, “the lift-off effect in eddy currents on thickness modeling and measurement”, ieee trans. on instr. and meas., 2013, vol. 62, no. 7, pp. 2043 2049. [13] a. bernieri, g. betta, l. ferrigno, m. laracca, “multi frequency ect method for defect depth estimation”, ieee sensors applications symposium, february, 2012. [14] a. bernieri, g. betta, l. ferrigno, m. laracca, s. mastrostefano. "multifrequency excitation and support vector machine regressor for ect defect characterization", ieee trans. instrum. meas., 2014, vol. 63, no. 5, pp. 1272-1280. [15] j. kim, g. yang, l. udpa, s. udpa, “classification of pulsed eddy current gmr data on aircraft structures”, ndt& e international, 2010, vol. 43, no. 2, pp.141-144. [16] h. g. ramos, a. l. ribeiro, “present and future impact of magnetic sensors in nde”, procedia engineering, 2014, vol. 86 pp. 406-419. [17] d. j. pasadas, a. l. ribeiro, t. rocha, h. g. ramos, “2d surface defect images applying tikhonov regularized inversion and ect”, ndt& e international, 2016, vol. 80 pp. 48-57. analysis of the magnetic field in the presence of linear sub-surface cracks using ect << /ascii85encodepages false /allowtransparency false /autopositionepsfiles true /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /error /compatibilitylevel 1.4 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.0000 /colorconversionstrategy /cmyk /dothumbnails false /embedallfonts true /embedopentype false /parseiccprofilesincomments true /embedjoboptions true /dscreportinglevel 0 /emitdscwarnings false /endpage -1 /imagememory 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/pdfxoutputintentprofileselector /documentcmyk /preserveediting true /untaggedcmykhandling /leaveuntagged /untaggedrgbhandling /usedocumentprofile /usedocumentbleed false >> ] >> setdistillerparams << /hwresolution [2400 2400] /pagesize [612.000 792.000] >> setpagedevice acta imeko contacts acta imeko issn: 2221-870x june 2019, volume 8, number 2, 0 acta imeko | www.imeko.org june 2019 | volume 8 | number 2 | 0 journal contacts about the journal acta imeko is an e-journal reporting on the contributions on the state and progress of the science and technology of measurement. the articles are mainly based on presentations presented at imeko workshops, symposia and congresses. the journal is published by imeko, the international measurement confederation. the issn, the international identifier for serials, is 2221-870x. editor‐in‐chief dušan agrež, slovenia founding editor‐in‐chief paul p. l. regtien, netherlands associate editor dirk röske, germany editorial board section editors (vol. 7 and 8) leopoldo angrisani, italy filippo attivissimo, italy eulalia balestieri, italy eric benoit, france paolo carbone, italy lorenzo ciani, italy catalin damian, romania pasquale daponte, italy luca de vito, italy luigi ferrigno, italy edoardo fiorucci, italy alistair forbes, united kingdom helena geirinhas ramos, portugal sabrina grassini, italy fernando janeiro, portugal konrad jedrzejewski, poland andy knott, united kingdom francesco lamonaca, italy massimo lazzaroni, italy fabio leccese, italy rosario morello, italy michele norgia, italy pedro miguel pinto ramos, portugal nicola pompeo, italy sergio rapuano, italy dirk röske, germany alexandru salceanu, romania constantin sarmasanu, romania lorenzo scalise, italy emiliano schena, italy enrico silva, italy krzysztof stepien, poland marco tarabini, italy marcantonio catelani, italy lorenzo ciani, italy egidio de benedeto, italy alessandro germak, italy sabrina grassini, italy konrad jędrzejewski, poland min-seok kim, korea fabio leccese, italy rosario morello, italy vilmos palfi, hungary nicola pasquino, italy franco pavese, italy alexandru salceanu, romania jan saliga, slovakia jorge c. torres-guzman, mexico ian veldman, south africa claudia zoani, italy alessandro depari, italy emiliano sisinni, italy about imeko the international measurement confederation, imeko, is an international federation of actually 41 national member organisations individually concerned with the advancement of measurement technology. its fundamental objectives are the promotion of international interchange of scientific and technical information in the field of measurement, and the enhancement of international co-operation among scientists and engineers from research and industry. addresses principal contact prof. dušan agrež university of ljubljana faculty of electrical engineering tržaška 25, 1000 ljubljana, slovenia e-mail: dusan.agrez@fe.uni-lj.si acta imeko alessandro depari, emiliano sisinni department of information engineering university of brescia via branze 38 – 25123, brescia, italy e-mail: alessandro.depari@unibs.it, emiliano.sisinni@unibs.it support contact dr. dirk röske physikalisch-technische bundesanstalt (ptb) bundesallee 100, 38116 braunschweig, germany e-mail: dirk.roeske@ptb.de mailto:dusan.agrez@fe.uni-lj.si mailto:alessandro.depari@unibs.it mailto:emiliano.sisinni@unibs.it mailto:dirk.roeske@ptb.de estimation of microwave resonant measurements uncertainty from uncalibrated data acta imeko issn: 2221-870x september 2020, volume 9, number 3, 47 52 acta imeko | www.imeko.org september 2020 | volume 9 | number 3 | 47 estimation of microwave resonant measurement uncertainty from uncalibrated data k. torokhtii1, a. alimenti1, n. pompeo1, e. silva1 1 department of engineering, roma tre university, via vito volterra, 62, 00146 roma, italy section: research paper keywords: dielectric resonator; microwaves; uncertainty citation: kostiantyn torokhtii, andrea alimenti, nicola pompeo, enrico silva, estimation of microwave resonant measurements uncertainty from uncalibrated data, acta imeko, vol. 9, no. 3, article 8, september 2020, identifier: imeko-acta-09 (2020)-03-08 editor: jan saliga, technical university of košice, slovakia received january 17, 2020; in final form april 21, 2020; published september 2020 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author k. torokhtii, e-mail: kostiantyn.torokhtii@uniroma3.it 1. introduction continuous interest in microwave measurements has led to the realisation of industrial grade methods for material characterisation. a wide range of material properties could be characterised at microwaves. one of the advantages of microwave techniques is their high sensitivity. they are widely used for the measurement of the complex permittivity of liquids [1] and solid dielectrics [2]. traditionally, microwave techniques have been used for the surface impedance (of 𝑍𝑆) measurements of conductors. in particular, the microwave dielectric resonator (dr) method is a standard for the measurements of 𝑍𝑆 of superconductors [3]. in connection with other techniques as dc measurements, the dr method can show a detailed picture of the microscopic properties of superconductors [4], [5]. in this article, we focus on the dr-based measurement technique. in recent years, vector network analysers (vnas) have become more accessible due to their low cost [6], [7]. however, the quality of the measurements relies heavily on the calibration procedure. in the case of low-end vnas, the importance of calibration becomes critical. in some cases, calibration becomes an impossible operation. one notable case is when the device under test (dut) is placed in a harsh or particularly difficult environment. a representative example is given by measurements of materials or devices at low, cryogenic temperatures [8], [9]. apart from the complexity of the cryogenic measurement system, one of the main problems originates from the part of the microwave line inside the cryostat, which cannot be calibrated using the standard procedure. here, the main role is taken by the absence of microwave standards that are capable of maintaining their characteristics in a hostile environment. one possible solution is to develop custom standards [10], but this often requires very advanced techniques and specific expertise. moreover, it is rather difficult to associate the appropriate uncertainty with the measured quantity. in this article, we extend the analysis of the effect of the uncalibrated measurements [11], [12] to a more complete generalised picture of the resonant parameters. we focus our attention on the resonant frequency (𝑓0) and quality factor (𝑄). here, we explore the impact of different vnas and fitting methods on the estimation of uncertainties on 𝑄 and 𝑓0. a key objective of the study is the determination of the uncertainty of 𝑄 and 𝑓0 for measurements where no calibration could be performed. for the measurements, we use the hakki-coleman (h-c) dielectric resonant system described in [11], [12]. 2. variable-q resonator cell in this study, we aim to evaluate the measurement uncertainty on 𝑄 and 𝑓0 when a microwave line, or a part thereof, cannot be calibrated, as in the cryogenic scenario outlined above. we designed an ‘open’ type h-c resonator, operating at room temperature. in our experiment, we used phase stable microwave cables with female-female connectors. we designed the dr cell abstract we present an extended study on the uncertainty in resonant measurements. the uncertainty of the resonant frequency and quality factor has been estimated. two different measurement systems and different fitting approaches are used. the effect of the use of uncalibrated resonant curves on uncertainty has been extensively studied. for the uncalibrated data, the systematic contribution to the uncertainty is determined. mailto:kostiantyn.torokhtii@uniroma3.it acta imeko | www.imeko.org september 2020 | volume 9 | number 3 | 48 to have the reference plane of the microwave cable connector near the edge of the cell. coupling was made by removable antennas inserted directly in the cable connector. with this design, we were able to perform the calibration up to the reference plane of the cable connectors. the dut is therefore the resonator cell only, as is desirable in an ideal setup. in this way, we are able to directly compare the uncalibrated measurements with the calibrated ones – without any unaccounted-for contribution and without the need for a deembedding procedure [13]. in figure 1, a sketch of the measurement cell is shown. the dielectric resonator is cylindrical and loaded with a sapphire puck, chosen because of its low dielectric losses and high permittivity. the latter allows us to concentrate the electromagnetic field within and near the dielectric puck. a mono-crystal sapphire puck with a diameter of (8.0 ± 0.1) mm and a height of (4.5 ± 0.1) mm was used. an anritsu 37269d vna and a r&s zva-40 vnas with phase-stable anritsu cables were used for measurements and connections. we are interested in an evaluation of the uncertainty in the measurements of the resonator parameters with different quality factors 𝑄. we then designed a system for measurements in a wide range of 𝑄 factors in the same conditions, which include, in particular, the frequency range of operation of the dr. in order to change the 𝑄 values of the resonator in a controlled way, we could substitute the bases using metals with different resistivities and change the position of the upper base, thereby changing the height of the corresponding gap of the dr. by fine tuning the latter, we were able to keep the resonant frequency within a chosen fixed range, (13.0 ± 0.5) ghz. 3. q-factor and resonant frequency extraction 𝑄 and 𝑓0 are important properties of a resonator. a good example is the resonant technique to measure the surface impedance of a material 𝑍s = 𝑅s + 𝑖𝑋s, where 𝑅s and 𝑋s are the surface resistance and surface reactance, respectively. by using this measurement technique, the quality of the measurements of the surface impedance by the resonant method depends directly on 𝑓0 and 𝑄. usually, the variation of the surface impedance with some external parameter is obtained experimentally, while the determination of the absolute value of 𝑍s is a more complex issue. the relationship between 𝑍s and resonant parameters is the following [14]: 𝛥𝑍s = 𝛥𝑅s + 𝑖𝛥𝑋s = 𝐺𝛥 1 𝑄 − 2𝑖𝐺 ∆𝑓0 𝑓0 + 𝑏𝑔, (1) where g is a constant called geometrical factor; the background ‘bg’ is the contribution of the resonator without the sample; and 𝛥𝑥 is the variation of the parameter 𝑥 with respect to the reference value. from equation (1), it can be seen that the uncertainty of 𝑍s depends on 𝑢(𝑄), 𝑢(𝑓0), and (𝑢(𝐺). the latter, usually < 1 %, can be obtained through electromagnetic simulations. the remaining uncertainties, 𝑢(𝑄) and 𝑢(𝑓0), are not readily available when no calibration is possible: hence the interest in the estimate of 𝑢(𝑄) and 𝑢(𝑓0) is evident. in a resonator working in transmission, 𝑓0 and 𝑄 can be obtained from the measurements of the response of a two-port microwave resonant system, described through a 2 × 2 scattering matrix [s] [15]. in particular, the most reliable determination of the resonant parameters could be obtained from the complexvalued transmission coefficient, 𝑆tr ( 𝑆12 or 𝑆21). while preliminary estimates for 𝑄 and 𝑓0 can be obtained by the socalled -3 db method, a more precise approach is to use an appropriate fit of the resonance curve. in [16], it was shown that a lorentzian curve can be used to fit the function |𝑆tr(𝑓)| 2 only in ‘ideal’ cases. the real resonator includes corrections connected to cross-coupling and phase contributions. the resonance curve can be described through the following equation (the fano resonance model [17]) with a phase correction: 𝑆tr(𝑓) = [ 𝑆tr(𝑓0) 1 − 2𝑖𝑄 𝑓 − 𝑓0 𝑓0 + 𝑆c] 𝑒 𝑖𝜑 , (2) where 𝑆tr(𝑓0) > 0 is the transmission 𝑆-parameter at the resonant frequency, and 𝑆c represents the contribution originated from the cross-coupling between the resonator ports and 𝑒 𝑖𝜑 , with 𝜑 = 𝛼 + 𝛽𝑓, taking into account the propagation phase delay along the line. there is a large number of methods for the extraction of 𝑓0 and 𝑄 from the experimental data [18]. a widely used method is the so-called circular fit approach [19], but it is also possible to use a complex-valued modification of the levenberg-marquardt algorithm to fit the complex 𝑆tr [20]. however, complex-valued fitting requires more calculation power and custom algorithms. additional complexities arise when the uncertainty of the fitting parameters must be estimated. from this point of view, the fit of the modulus of 𝑆tr remains more accessible due to already implemented algorithms in popular programming languages such as python and matlab [21]. using the fit of |𝑆tr(𝑓)|, one can implement a fitting procedure with cheap computers like the raspberry pi. |𝑆tr(𝑓)| is obtained from equation (2) as: |𝑆tr(𝑓)| 2 = 𝑆tr(𝑓0) 1 + 4𝑄2 ( 𝑓 − 𝑓0 𝑓0 ) 2 [𝑆tr(𝑓0) + 2re(𝑆c) + 4𝑄 𝑓 − 𝑓0 𝑓0 im(𝑆c)] + re2(𝑆c) + im 2(𝑆c ). (3) this fit requires only five independent parameters. since a vna allows the measurement of the 𝑆-parameters as complex quantities, a fit of the complex quantity 𝑆tr(𝑓) could figure 1. variable𝑄 resonant cell. acta imeko | www.imeko.org september 2020 | volume 9 | number 3 | 49 also be devised, yielding also the additional information concerning the phase correction 𝑒 𝑖𝜑 . hence, differently to [12], we additionally perform a complex fit of equation (2) and compare the results that can be obtained using both approaches. resorting to standard algorithms, which usually accept only real quantities in input, we use a specific approach to fit complex data and include experimental data uncertainty (𝑢(𝑆tr)). in order to do so, we define the objective function to be minimised by separating the real and imaginary parts of 𝑆tr – each with its experimental uncertainty. hence, each measured 𝑆tr curve, composed of n data points, becomes an array of 2 n data points – with the first half given by the real parts and the second by the imaginary parts. correspondingly, the fit function is given by the real and imaginary parts of equation (2) for the first and second half of the measurement data set, respectively. consequently, differently to the fit to equation (3), the fit is done with seven independent parameters. in summary, the uncertainties 𝑢(𝑄) and 𝑢(𝑓0) become functions of various parameters and conditions, such as the sparameters’ uncertainty, contributions from the fit algorithm (ℂfit), and the presence of the calibration (ℂcal) 𝑢(𝑄) = ℱ𝑄 (𝑢(𝑆tr), ℂfit, ℂcal) and 𝑢(𝑓0) = ℱ𝑓0(𝑢(𝑆tr), ℂfit, ℂcal ). 4. measurement procedure in this section, we present an extended elaboration of the measurement procedure presented in [11]. the aim is to complete the estimate of the uncertainty of the measurements of the characteristic parameters of a resonator for uncalibrated measurements, including the resonant frequency. we performed experimental measurements of the transmission coefficient 𝑆tr(𝑓) corresponding to a wide range of 𝑄 factors, namely 1500 – 9500, with very small variations of other controlled parameters of the resonant system. the interest is the comparison between the 𝑄 and 𝑓0 measurements obtained from the fit of 𝑆tr(𝑓) with and without the vna calibration. measurements were done using two different vnas (the anritsu 37269d and the r&s zva-40) to assess the robustness of the results among different instruments. indeed, this point is not trivial, since the uncalibrated curves contain contributions that also originate from the internal vna circuitry. in calibrated measurements, the vna circuitry nonidealities are transparently removed. firstly, we fixed the calibration in the narrow frequency band 12.5-13.5 ghz, which was chosen to accommodate all the resonant curves (with different 𝑄) requiring measurement with sufficient density of frequency points. measurements were performed using frequency sweeps with 1601 calibrated points with the anritsu 37269d vna (the maximum number of data points) and 32000 calibrated points with the r&s zva-40. the calibration was performed by the standard 12-term short-openload-thru (solt) procedure by the algorithms incorporated in both vnas. for the experimental measurements, we followed the anritsu 37269d and r&s zva-40 standard calibration guides. the series of resonance curves were then recorded, varying the 𝑄-factor in the range 1500 < 𝑄 < 9500. in order to obtain such a wide range of 𝑄 with the same setup, we changed the air gap and we used different metals for the bases, as we explained before. the gap, controlled by only one movable part of the resonator cell – the upper base (figure 1) – allowed us to tune the operating resonance frequency to be always within the frequency range of the calibration. for each fixed position of the upper base, two measurements were performed: one with and one without the calibration. in figure 2, an example of the measured calibrated and uncalibrated resonant curves is shown for anritsu 37269d (left panels of figure 2) and r&s zva-40 (right panels of figure 2). the difference is immediately appreciated, confirming the need to assess the quality of an uncalibrated measurement even for a resonant device (naïvely, one could expect that resonant systems are sufficiently narrowband such that they are almost insensitive to the calibration). 5. results a series of 𝑆tr(𝑓) resonant curves (~ 40) was recorded and fits by equation (1) (complex fit) and equation (2) (fit of the modulus) were applied to the uncalibrated and calibrated data. care was taken to maintain similar fit conditions. it should be noted that in our situation, in each curve, the measured points were taken at evenly spaced frequencies for all measurements. since 𝑄 changes significantly, without proper ‘normalizations’, the fits of the curves with low and high 𝑄 would not be in the same conditions concerning the density of information per interval. therefore, we proceeded as follows. firstly, we limited the span of each curve to 𝑁𝛥𝑓fwhm, where 𝛥𝑓fwhm is the full-width half-maximum frequency range, and 𝑁 > 1 is a proper multiplicative factor. in our case, 𝑁 was constrained by the available frequency range of the calibration and the requirement to be the same for all experimental curves. as a result, we chose 𝑁 = 4. the resulting curves, which have different frequency spans with evenly spaced frequency points, had different numbers of points in this step. each set was then trimmed uniformly in order to maintain the same amount of evenly spaced data points as much as possible. as a result, we obtained the final set of the resonant curves with 𝑄 in the range of 1500 − 9500, trimmed and with same information density, with the span 4𝛥𝑓fwhm, and with the number of data points near 300. we now discuss the result of the elaboration of this set of curves. figure 2. the measured resonant curves with and without calibration applied in the modulus (upper panels) and phase (lower panels). panel (a) anritsu vna. panel (b) r&s vna. acta imeko | www.imeko.org september 2020 | volume 9 | number 3 | 50 concerning the uncertainties, it should be noted that the uncertainty 𝑢(𝑆tr) could only be estimated for the calibrated curve [12]. in particular, for the anritsu 37269d, these estimations could be given by the exact uncertainty calculator – a proprietary software tool of anritsu [22] (although alternative tools like in [23] could be used). this tool automatically estimates the uncertainties for all 𝑆-parameters, taking into account the characteristics of the vna, cables, connectors, and calibration kit. for r&s zva-40, on the other hand, the uncertainty was obtained from the general specifications of the vna provided in the datasheets. hence, the uncertainty for r&s zva-40 is not accurately tailored to the measurements and should be considered as an over-estimated, worst-case figure. for both vnas, the estimation of 𝑢(𝑆tr) is provided in the form of dependencies of the uncertainties of 𝑢(|𝑆tr|) and 𝑢(arg(𝑆tr)) on |𝑆tr|. thus, separate uncertainties should be assigned for each measured point, yielding additional complications in the fit procedure. moreover, since the uncertainties are provided on the modulus and phase, the uncertainties on the real and imaginary parts of 𝑆tr needed for the complex fit were derived by the uncertainty propagation. in the calibrated conditions, the worst case of the measurement uncertainty on 𝑆tr was < 1.7 % for the modulus and < 0.5 % for the phase for anritsu 37269d. the worst-case measurement uncertainties for r&s zva-40 were < 2.4 % for the modulus and < 1.6 % for the phase. for the uncalibrated curves, zero uncertainty on 𝑆tr had to be used in the absence of the necessary information. we limited our study scope to already implemented solutions only. a widely used levenberg-marquardt [20] fit algorithm implementation in the python scypy package (curvefit) applied a minpack code to perform a reliable fit. the fit procedure used here provides a covariance matrix based on the numerically computed jacobian (𝐽) – based on the modified model function. the covariance matrix is 𝐶𝑜𝑣 = (𝑅𝑇 𝑅)−1, where 𝑅 = 𝑟𝑃, 𝑟 is the upper triangle of 𝐽, and 𝑃 is a permutation matrix (more details regarding fit algorithm can be found in [21], [24]). the square root of the diagonal elements of the 𝐶𝑜𝑣 matrix yields an estimate of the uncertainties of the fit parameters. in this way, the uncertainties of 𝑄 and 𝑓0, as fit parameters can be estimated. moreover, as a consequence of the non-application of the calibration, additional significant sources of uncertainty are expected. thus, we define as a measure of this uncertainty (for the uncalibrated measurements) the discrepancy between the resonant parameters as derived by the fitting of the calibrated and uncalibrated curves. the discrepancy of 𝑄 can be defined as the relative variation between 𝑄 obtained from calibrated (𝑄cal) and uncalibrated (𝑄uncal) data, 𝐷𝑄 = (𝑄uncal − 𝑄cal )/𝑄cal. for frequency 𝑓0, the discrepancy is defined in the same manner as 𝐷𝑓0 = (𝑓0,uncal − 𝑓0,cal)/𝑓0,cal, where 𝑓0,uncal and 𝑓0,cal are the resonant frequencies obtained by the fit of the uncalibrated and calibrated curves, respectively. in figure 3 and figure 4, we show the main results of the study with the aim of comparing the two fitting approaches. in order to fully exploit the available data, both the scattering coefficients 𝑆21 and 𝑆12, simultaneously measured for each resonator configuration, are fitted as 𝑆tr. indeed, although the resonator itself is a reciprocal device, the same does not hold true for the whole line due to the inevitable asymmetry of the lines between the resonator and vnas, so 𝑆21 and 𝑆12 are different. the uncertainties of the uncalibrated measurements (because in this case, 𝑢(𝑆tr) = 0 was taken) include only the contribution of the uncertainty arising from the curve fit process, as provided by the fit algorithms. this leads to a relatively low uncertainty for 𝑄 and 𝑓0. very approximately, based on the available number of measured curves, an estimation of the relative uncertainty of the experimental uncalibrated data gives median values for 𝑢(𝑓0)/𝑓0 between 0.05 ppm and 0.18 ppm and for 𝑢(𝑄)/𝑄 between 0.08 % and 0.17 % (see table 1). comparing the results with the two different fit approaches, i.e. of the complex 𝑆tr and of its modulus, we obtain similar levels of the uncertainties as provided figure 3. panel (a) relative uncertainty on calibrated 𝑄-factor vs 𝑄. panel (b) discrepancy 𝐷𝑄 vs 𝑄. figure 4. panel (a) relative uncertainty on calibrated 𝑓0 vs 𝑄. panel (b) discrepancy 𝐷𝑓0 vs 𝑄. acta imeko | www.imeko.org september 2020 | volume 9 | number 3 | 51 by the fit algorithms. moreover, since these are well below the error produced by the absence of the calibration, when dealing with uncalibrated measurements, the simpler fit of the modulus of 𝑆tr is certainly adequate and avoids unnecessary numerical complications and longer fit times. considering the calibrated data, the uncertainties of 𝑄 and 𝑓0 become more than two times higher than in the uncalibrated case because of the additional contribution of the known 𝑢(𝑆tr) ≠ 0. in the following discussion, we focus our attention on the calibrated data. in figure 3(a) and figure 4(a), the relative uncertainties (𝑢𝑟 ) of the 𝑄-factors and resonant frequencies 𝑓0 of the calibrated data are shown. comparing the results of the two fitting approaches, we observe that apart from providing the additional important information about the phase correction, the fit of the complex 𝑆tr data is in overall characterised by a ~ 1.5 times lower 𝑢𝑟 on the fitting parameters. from figure 3(a), it is clear that the maximum 𝑄-factor uncertainty reduces from 𝑢(𝑄)/𝑄 < 0.7 % for the fit of |𝑆tr| to 𝑢(𝑄)/𝑄 < 0.3 % for the fit of the complex 𝑆tr. additionally, it can be seen that 𝑢𝑟 (𝑄) is almost independent of the 𝑄-factor value. moreover, reasonably, 𝑢(𝑄)/𝑄 is also independent of the vna used for the measurement. concerning the relative uncertainty of 𝑓0, the fit of |𝑆tr| yields 𝑢(𝑓0)/𝑓0 < 1.5 ppm (figure 4(a)), which becomes lower than 0.5 ppm using the fit of the complex 𝑆tr. the comparison of the fit parameters between the calibrated and uncalibrated cases can be explained using the discrepancy parameter 𝐷, reported in figure 3(b) and figure 4(b) for both vnas used in measurement and for the two fitting approaches (black markers for |𝑆tr| and red markers for fit of 𝑆tr). firstly, since the absolute values of the discrepancy are, on average, larger than the uncertainties of the calibrated measurements, we can infer that they are indeed a good measure of the error due to the use of uncalibrated data. moreover, within the relatively large set of experimental curves (~ 40), it can be observed (figure 3(b) and figure 4(b) use the same legend as in the ‘a’ subfigures) that the discrepancy values have a balanced distribution around zero for both fit approaches. this feature calls for a closer investigation of the characteristic of this distribution, allowing us to go beyond a rough estimation of the error due to the use of uncalibrated curves as worst-case values only [10]. the latter yields, as an estimate of the maximum additional contribution to the measurement relative uncertainty, arising from the use of uncalibrated data (equal to 5.4 % for 𝑄 and to 16 ppm for 𝑓0), but we show below that the data supports a reduction of this figure. we thus gain further information by making use of the finding, as depicted in figure 3(b) and figure 4(b), that the discrepancy is independent of the measuring device (which is not obvious, since we are dealing with uncalibrated data). in the following section, we combine all the data from figure 3(b) and figure 4(b) for the observed discrepancies for data obtained with anritsu 37269d and r&s zva-40 in order to compare the fit approaches. analysing the statistical distribution of the 𝐷 values, reported in the histograms in figure 5(a) and figure 5(b) for 𝑄 and 𝑓0 respectively, it can be seen that they yield relatively symmetric shapes centred almost at zero. in particular, for the discrepancy in 𝑓0, its absolute mean value is 0.44 ppm, which is due to the fit of |𝑆tr| (black histogram) and 1.9 ppm for the 𝑆tr fit (red histogram), close to the upper value of the corresponding uncertainty of 𝑢(𝑓0)/𝑓0 in the calibrated data. the absolute mean values of the quality factor discrepancies are less than the uncertainty of 𝑢(𝑄)/𝑄 for the calibrated data: 0.38 % for the fit of |𝑆tr| and 0.16 % for the 𝑆tr fit. interestingly, for the two fitting approaches, the standard deviations of 𝐷 are similar: ~ 6 ppm and ~ 2.2 % for the discrepancies in 𝑓0 and 𝑄, respectively. it can be noted that the discrepancy distributions have a dispersion (as measured by the standard deviation) that is much larger than their centre values (given by the mean values). this allows us to obtain a refined estimation of the overall uncertainty by resorting to the standard deviations of the discrepancy distributions with respect to the above-mentioned worst-case values. 6. conclusions we performed an extended study of the measurement uncertainty on the quality factor and resonant frequency 𝑓0 of table 1. median values of the relative uncertainties of 𝑄 and 𝑓0 of experimental uncalibrated data. anritsu 37269d vna measurement r&s zva-40 vna measurement 𝒖(𝒇𝟎)/𝒇𝟎 𝒖(𝑸)/𝑸 𝒖(𝒇𝟎)/𝒇𝟎 𝒖(𝑸)/𝑸 fit by equation (3) (|𝑆tr|) 0.074 0.132 0.140 0.141 fit by equation (2) (𝑆tr) 0.05 0.083 0.183 0.174 figure 5. distribution of the discrepancy 𝐷q (panel a) and 𝐷𝑓0 (panel b). acta imeko | www.imeko.org september 2020 | volume 9 | number 3 | 52 the microwave resonator, which arises when the microwave line is not calibrated. by exploring a wide range of 𝑄, enabled by the implementation of an ad-hoc resonator cell, we compared the obtained results with calibrated and uncalibrated data. we used different vnas for data acquisition following the same procedure and different fit methods. after proper ‘normalisations’ of the datasets, in terms of frequency span and point number, we find that while the worst-case uncertainty in the calibrated curves is below 0.75 % for 𝑄 and below 1.5 ppm for 𝑓0, in the uncalibrated measurements, an additional contribution arises. by estimating this contribution from the discrepancy between the fit results of the calibrated and uncalibrated data, we obtain an experimental quantification of the additional contribution that is equal to 5.4 % for 𝑄 and 16 ppm for 𝑓0 as a maximum error. by combining all the data for the discrepancy, we can reduce these figures to 2.2 % and 6 ppm, respectively, which now should be intended as the standard uncertainties. these results provide a guide for the evaluation of the uncertainty contribution, which should be taken into account when the calibration of microwave lines similar to the one studied is not feasible. references 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planck constant; revised si; stipulation; metrology; metre convention citation: franco pavese, on the revised si, specifically on the numerical value of the planck constant, acta imeko, vol. 7, no. 4, article 15, december 2018, identifier: imeko-acta-07 (2018)-04-15 editor: alistair forbes, national physical laboratory, london, uk received october 2, 2018; in final form december 4, 2018; published december 2018 copyright: © 2018 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: franco pavese, e-mail: frpavese@gmail.com, frpavese@yahoo.com 1. introduction considering the acceptance of the cgpm of the cipm proposal for the revision of the si, this paper analyses the important results of the codata (short for codata-tgfc) 2017 adjustment reported in [1] (now incorporated in the draft resolution a for the 26th cgpm). study [2] supports the adjustment, providing a deeper analysis than [1] with further details on the method used. in particular, figure 2 in [2] shows the 2014-2017 data for the planck constant, h, used by codata. for convenience, table 1a reports here the codata adjustments since 2006 for h, while table 1b of the appendix reports the same data for the other three constants: e, k, and na. the ccu has basically terminated its task and the cipm prepared in its 106th 2017 meeting [3] drafts of documents to submit at the 26th meeting of the cgpm in february 2018 draft resolution a. regardless thereof, the intention of this paper is to consider the present information (see [4, 5] for an analysis of the previous 2014 drafts) with the aim of making a positive contribution to the scientific debate on some aspects of the revised si, a debate that certainly shall not stop, as is typical in science. the revision gives rise to some questions that it is the intention of this paper to outline. an appendix complements some contents of this paper. 2. analysis and comments 2.1. “exactness” of the stipulated data concerning papers [1] and [2], a reader informed on the si revision process will note that the number of digits now indicated by the codata for the stipulated constants is larger than was previously required, possibly with the exception of k. this is certainly due to the lowering of the experimental uncertainties— an outstanding ≈5 times since 2006. however, apparently, it is also due to a specific use of the original data and of the associated uncertainties. table 2 shows both the codata adjusted/stipulated numerical values and the “exactly known values.” the cipm preference expressed in [6] (and that of ccu) has been “for the minimum number of digits [of the stipulated value] for each defining constant h, e, k, and na of the revised si that yields consistency factors equal to [exactly] 1 within their uncertainties” (emphasis added). the codata stipulations [1, 2], presented here in the third column of table 2, were considered to match this preference, but there are different ways of treating digits with the same numerical values. should the “expanded uncertainty”, u, which is commonly advised to be used in experimental science, be considered for the stipulation of the numerical values? by using u, the uncertainties of the numerical values in the second column of table 2 would be multiplied by ≈2, and the above divergence would not change for any of the four constants if the “exactly known numbers” are abstract the results of the codata 2017 adjustment and the consequent stipulation of the numerical values of the constants are essential considerations concerning the consequences of the cgpm accepting the cipm proposal for the revision of the si. this paper raises (unresolved) questions on these results, specifically concerning the numerical value of the planck constant. mailto:frpavese@gmail.com mailto:frpavese@yahoo.com acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 97 not rounded. case. when rounded instead, k and na are “losing” one further digit. however, there is another principle, explicit among the cipm/ccu rules when they speak of “consistency factors,” which must also be respected in stipulation: the “continuity principle” [4, 5]. values of the “consistency factors” can be computed from the stipulated values reported in table 2 (obtained from the adjusted values in table 1) and are reported here in the equations below: [m(k)/(kg)rev]/1 = 1.000 000 001(10) [1.2 × 10–8] (1) [µ0/(h m–1)rev]/(4× 10–7) = 1.000 000 000 20(23) [2.3 × 10–10] (2) [m(12c)/(kg mol–1)rev]/0.012= 1.000 000 000 37(45) [4.5 × 10–10] (3) [ttpw/(k)rev]/273.16 = 1.000 000 01(37) [5.7 × 10–7]. (4) these factors, basically the same than those now included in cgpm resolution 1, are considered to correspond with the cipm’s indicated criteria (see the appendix for their exact meanings). the above equations are the direct consequence of using all codata stipulated digits, but they may conflict with the continuity principle (see an example in the appendix). the possible conflict is due to the fact that, rather obviously, one should not stipulate a number whose (last) digit(s) are not firmly confirmed by the experiments. could one stipulated number have, say, three digits more than the experimental uncertainty level? the fact that the uncertainty will eventually be dropped in stipulation is entirely irrelevant: uncertainty means that the digit(s) affected by it could presently be different from the stipulated one(s). a basic dilemma arises (apparently undetected, certainly unpublished and unresolved) for the constants in the above situation: (0) accepting constant stipulated values exceeding the present experimental accuracy, but preserving unit magnitude continuity or (1) requiring constant stipulated values to conform to present experimental accuracy, but inducing a unit magnitude discontinuity (small but significant). in fact, the present experimental results, despite their conspicuous five-time uncertainty lowering, still do not support the firm continuity of the units’ magnitudes. on the other hand, the codata 2017 analysis has been unable to take into account some more recent findings, e.g., on the bipm kibble balance [7–9], about possible systematic effects. to obtain the desired continuity, one is obliged to “guess” the last digit(s) affected by uncertainty (see more in the appendix). furthermore, a formal problem may arise from the fact that the codata, a scientific committee, has been required to use, in addition to the lsa adjustment, a second (implicit in [2]) criterion: that of continuity, implying the possible inclusion of stipulated digits affected by uncertainty (see more in the appendix about that implication and the “adjusting numerical values”). in fact, the codata was appointed by the cipm to make the stipulation proposal, despite that its mission would consist only of providing adjusted values and their uncertainties. however, the issue of continuity is strictly one of the most basic features of the metrology regulatory context and should not be considered a task included within the codata’s competence. confounding the two tasks could be improper in the si regulatory context. 2.2. inconsistency of data and the effect thereof another major issue has arisen since publication of the 2017 ccu document [10] concerning the evident inconsistency of some of the new 2017 data on the planck constant: “notes…that work is under way in nmis to understand the cause for the dispersion of the experimental determinations of the planck and avogadro constants…” this was also noted by the codata in [1-2]. nevertheless, the ccu concluded that “numerical values and uncertainties for the boltzmann constant and the avogadro constant provided by the codata task group on fundamental constants in their special least-squares adjustment of the experimental data provide a sufficient foundation to support the redefinition…” and recommended that the cipm should proceed for the 26th cgpm in 2018. the cipm did so in its 2017 meeting [3]. if data inconsistency is called, as it is by default, evidence of non-overlapping uncertainty intervals (for k = 1 in the case of the codata) for the data, figure 2 in [2] shows such a case for three portions of the 2017 data, which have been considered as such by codata. however, in [11], the conclusion does not confirm the lack of consistency, as the study is based on a different specific statistical tool. in that paper, musing on the data for the planck constant (one of the main reasons for the commonly agreed-on urgency of the si revision), a strong assertion is made: “the preliminary value of the answer to (i) is 10 parts in 109, and the discussion in the following sections will only reinforce the view that (ii) should be answered in the affirmative”, where the answers were “about (i) the relative uncertainty that the codata-tgfc will use to qualify the value recommended for h in the special adjustment of 2017, and about (ii) whether this relative uncertainty warrants the redefinition in the sense that it guarantees seamless continuity in the primary realizations of the unit of mass before and after the redefinition, to within an uncertainty that is comparable to the uncertainties prevailing in the current, routine dissemination of the ipk.” on the contrary, in [12], evidence of inconsistency comes, too, from the use of another analytical method for the same 2017 data on the planck constant. in this study, the use of a bayesian method, specifically including the three critical portions of the 2017 data, led to a further conclusion. while the codata 2017 adjusted value for h [1] basically results in a value that is equal to that of the nrc-17, from figures 3-4 in [12] (to be compared with the less clear figure 2 in [11]), the evidence shows a continuing trend in time towards higher values of h and points rather to the iac-17 value (which is inconsistent with the codata 2017 adjusted value [2]). that trend in time (see also table 1a) is still sufficiently significant in 2017 to permit the doubt that the codata result and the assertion in [11] are not confirmed by or are insufficiently based on the available data. in fact, in figures 3-4 of [12], an increase in the credible interval is also shown, another reason for cautiousness about the number of stipulated digits. these facts even led to the conclusion in [12] that “although nothing can be concluded about a possible future development of the codata values for the planck constant, their contingent change over the past decades does not encourage a redefinition of the kilogram at present.” the above are facts that cannot currently be modified until other data would become available. should no additional acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 98 experimental data be included and analyzed before the final decision, proposing “solutions” is risky: one should not feel sufficiently reassured by either the need to find the “right’” statistical test to get a comfortable result [11] or to merely resort to the method of making the uncertainty of such data uninfluential [1, 2]. 2.3. treatment of inconsistent data in [1], it is said (full citation in the appendix), “…to achieve consistency, multiplicative expansion factors were applied to the uncertainties…the uncertainties of these input data are multiplied by a factor of 1.7. with this expansion of the uncertainties of the eight data, five have relative standard uncertainties ur at or below 50 × 10–9, with two at or below 20 × 10–9…” the position is also indicated in [2], where it is specified, “it is note worthy that even after applying an expansion factor of 1.7 to the uncertainties of all…data, thereby bringing them into agreement, the relative uncertainties of the first five values of h…are, in parts in 109, only 15, 20, 23, 34, and 42, respectively.” the reported “uncertainty lowering” is strictly a feature of the consistency-checking lsa method, a possible weakness in cases like this one. in fact, one should not be particularly surprised by this effect, because it is certainly not the first time that the uncertainty of a constant is reduced thanks to the well-known interconnections that the lsa method establishes between all elements of the dataset. in the present case (a “special adjustment” not involving the usual full set of constants), it might merely indicate that the dispersion of the values for h, after having been assigned an uncertainty that is 1.7 times larger, has become almost irrelevant for the general consistency degree of the whole dataset. here, consistency has a different meaning with respect to the data consistency, discussed in [11, 12] by using specific statistical tools that are different from those of the codata. however, since here the uncertainty lowering is not reflected in the experimental findings, it should be considered as an lsa artefact, and the conclusions reported in [12] seem reasonable (see more in the appendix). basically, the lsa made the 2017 data uninfluential on the results, i.e., the data and results of the 2017 adjustment for h are in fact the same as the 2014 adjustment, because the same data is basically used. the lsa method of increasing discrepant data uncertainty to eliminate inconsistencies is common in metrology; however, it should be considered as a better-than-nothing solution, since the discrepancy could be, in reality, not due to an underestimation of the uncertainty but to a real bias of the provided value. in the present case, too, it does have some inconvenience. the fact that the uncertainty remains small after the uncertainty “expansion” is not necessarily good news when the main goal is actually the adjustment of the numerical value. the latter could become impaired by the small sensitivity of the constant’s subset with respect to the overall dataset. as a consequence, it may happen that the value is adjusted more or less than is correct (see more in the appendix). 3. conclusion: insufficiency of the present analyses of the 2017 final data concerning the 2017 adjusted values in [1], whose stipulations are indicated in [1, 2], in general, the presently available analyses should be considered insufficient concerning such an important subject matter. in particular, the codata 2017 value of h is not supported by all the presently published analyses. for all new constants, the stipulated values include digits that are not supported by experimental data. considering the extraordinary effort made by several nmis to supply more data in order to support the necessary decisions about the numerical values for assignment to the constants, one would have expected that deeper analyses should have been made available in support of the results of the 2017 codata adjustment and, beyond it, to the available dataset, which should ideally be increased. the importance of the result of the si revision (not only for metrologists, but for the entire scientific community) should have prompted a broad range of competent and independent analyses, using different methods. in this respect, analyses that are independent of those of codata should have been. the advantage would be to mitigate the yet unconsidered effect of the possibly biased values and reduction of uncertainty levels caused by the exclusive use of the lsa consistency check, thus leading to improved evidence concerning the digits needed and allowed for expressing the numerical values of the constants— the planck constant in particular. a combined “best value” and its associated uncertainty should be obtained by using several diverse methods (frequentist, bayesian, etc.), the lsa being one of them. this approach would then lead to higher confidence in the process of stipulation of “exact” numerical values. a deadline for the start of the validity of the revision farther than the present one, may 20, 2019, is suggested, to fix these issues, and with also their inclusion in the text of the 9th si brochure to be finalised by the cipm. references [1] d. b. newell, f. cabiati, j. fischer, k. fujii, s. g. karshenboim, h. s. margolis, e. de mirandes, p. j. mohr, f. nez, k. pachucki, t. j. quinn, b. n. taylor, m. wang, b. wood, z. h. zhang, the codata 2017 values of h, e, k, and na for the revision of the si, accepted 20 december 2017, metrologia 55 (2018) p. 29. see also: cgpm, resolution 1 of the 26th meeting, https://www.bipm.org/en/cgpm/db/26 (access date: 9 january 2019). [2] p. j. mohr, d. b. newell, b. n. taylor, e. tiesinga, data and analysis for the codata 2017 special fundamental constants adjustment for the revision of the si, metrologia 55 (2018) pp. 125-146. [3] cipm, decisions of the 106th cipm (october 2017), bipm, sèvres [online] available: https://www.bipm.org/en/committees/cipm/meeting/ 106.html (access date: 9 january 2019). [4] f. pavese, are the present measurement standards still valid after si redefinition? acqual 22 (2017) pp. 291-297. [5] f. pavese, the new si and the codata recommended values of the fundamental constants 2014 (arxiv:at-phys 1507.07956), following the ccu 2016 draft of the 9th si brochure and its 22nd meeting, a note on the 8th digit for stipulated {k}, arxiv:physics.data-an, 1512.03668v3, jun. 13, 2017. [6] cipm, decision cipm/105-15, 2016 [online] available: https://www.bipm.org/en/committees/cipm/meeting/105.html (access date: 9 january 2019). see also: ccu decisions 2016, bipm, sèvres, [online] available: https://www.bipm.org/cc/ allowed/22/ccu2016_7july2016 (access date: 9 january 2019). [7] bipm, improved understanding of kibble balance magnets, 2018 [online] available: https://www.bipm.org/en/news/fullstories/2018-01-kibble.html (access date: 9 january 2019). [8] s. li, f. bielsa, m. stock, a. kiss, h. fang, a permanent magnet system for kibble balances, metrologia 54 (2017) pp. 775-783 [9] s. li, f. bielsa, m. stock, a. kiss, h. fang, coil-current effect in kibble balances: analysis, measurement, and optimization, metrologia 55 (2018) pp. 75-83. https://www.bipm.org/en/cgpm/db/26 https://www.bipm.org/en/committees/cipm/meeting/%20106.html https://www.bipm.org/en/committees/cipm/meeting/%20106.html https://www.bipm.org/en/committees/cipm/meeting/105.html https://www.bipm.org/cc/%20allowed/22/ccu2016_7july2016 https://www.bipm.org/cc/%20allowed/22/ccu2016_7july2016 https://www.bipm.org/en/news/full-stories/2018-01-kibble.html https://www.bipm.org/en/news/full-stories/2018-01-kibble.html acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 99 [10] ccu, recommendation u1, on the possible redefinition of the kilogram, ampere, kelvin and mole in 2018, bipm, sèvres, 2017 [online] available: https://www.bipm.org/cc/alloweddocuments.jsp?cc=ccu (access date: 9 january 2019). [11] a. possolo, s. schlamminger, s. stoudt, j. r. pratt, evaluation of the accuracy, consistency and stability of measurements of the planck constant used in the redefinition of the international system of units, metrologia 55 (2018) pp. 29-37. [12] g. wübbeler, o. bodnar, c. elster, robust bayesian linear regression with application to an analysis of the codata values for the planck constant, metrologia 55 (2018) pp. 20-28. [13] p. j. mohr, d. b. newell, b. n. taylor, codata recommended values of the fundamental physical constants: 2014, arxiv:1507.07956v1, rev. modern physics 88 (2016) pp. 1-73. [14] e. tahl, naturalness and convention in the international system of units, measurement 116 (2018) pp. 631-643. appendix on the “exactness” of the stipulated data a) stipulation. let us compare the cipm preference, expressed in [6] and shared by the ccu, “for the minimum number of digits for each defining constant h, e, k, and na of the revised si that yields consistency factors equal to 1 within their uncertainties” (emphasis added) with the codata stipulations [1, 2] in table 2 of the main text of this paper. initially, these uncertainties were required by the cipm to be those transferred by the constants in table 1 to the stipulated values of the present si. then, they became those representing the state of the art. the reason for the latter preference comes from one of the fundamental principles on which the revised si is based, i.e., that the numerical stipulated values reported by the present-si should remain unaltered in the revised-si when the “continuity principle” is respected. in addition, let us also remember that “stipulation” is a convention agreed by consensus that rounds up (not truncates) a “best value” (in a statistical sense and at a specific time) into a “reference value” to be then taken as exact, thus characterized by the highest available reliability (confidence level/degree of belief) about its future stability in time. such a condition implies constraints in choosing the number of stipulated digits, e.g., it requires excluding any of those among the least significant ones that are affected by uncertainty. in fact, the criteria for stipulation cannot be independent on the resulting uncertainty of a chosen value based on experimental evidence, the only available evidence for deciding the number of stipulated (last) digits. b) in respect of the above cipm/ccu criterion: an example. according to the note in [5] concerning the boltzmann constant, the current reproducibility of a well-operated triple-point cell of water (tpw) is stated to be (k = 1) ≈ 10 µk (3.7 × 10–8 in relative units). thus, assuming the consistency factor of the new si with respect to the present one is exactly 1 for the nine-digit codata value 1.380 648 52(79) × 10–23 (2014: only a sevendigit exact value), that coefficient should not be lower than 0.999 999 964. by taking an eight-digit codata 2014 value, 1.380 6485 × 10–23, one gets 0.999 999 99, corresponding to a temperature unit mismatch of |4| µk. meanwhile, taking a seven-digit one (rounded), 1.380 649 × 10–23, one gets 1.000 000 35, corresponding to a temperature unit mismatch of |95| µk. however, one should also consider that the uncertainty provided by codata for the nine-digit numerical value corresponds to a temperature uncertainty of 156 µk on the ttpw due to the uncertainty on k. this means that the mismatch for the eightdigit format is actually |4|(156) µk. in 2017, the nine-digit codata value changed to 1.380 649 03(51) × 10–23, so that, by chance, the eight-digit and seven-digit cases actually coincide. c) the contrast between the continuity principle and the uncertainty of the experimental outcomes. should column “exact number” of table 2 in the main text of this paper be taken as correct in respect of the above considerations, the use of a seventh to tenth digit(s), depending on the constant, would not be justified, according to the 2017 codata expanded uncertainties, u, of the experimental outcomes and the 2017 adjustment: h (u ≈ 2.2  10–8), e (u ≈ 1.2  10–8), na (u ≈ 2.2  10–8), and k (u ≈ 1.5  10–6). this would mean that the correct consistency could be realized without invoking any non-exact digit, only for at least one order of magnitude less than is intended and indicated in equation (1) of the main text of this paper. d) inconsistent data and the treatment thereof. the full reference to [1] is: “to achieve consistency, multiplicative expansion factors were applied to the uncertainties of two subsets of input data corresponding to two adjusted constants for the 2017 special adjustment. the first subset consists of the eight input data for the planck and avogadro constants…relevant to the adjusted value of the planck constant. the uncertainties of these input data are multiplied by a factor of 1.7. with this expansion of the uncertainties of the eight data, five have relative standard uncertainties ur at or below 50 × 10–9, with two at or below 20 × 10–9, where the latter includes results from both the kibble balance and the x-ray crystal density (xrcd) methods.” in [2], it is also stated, “it is note worthy that even after applying an expansion factor of 1.7 to the uncertainties of all the kibble balance and xrcd data, thereby bringing them into agreement, the relative uncertainties of the first five values of h in table x are, in parts in 109, only 15, 20, 23, 34, and 42, respectively.” the fact that the uncertainty remains small after the uncertainty “expansion” is not necessarily good news. problems could possibly be revealed by analyzing the magnitude of the “adjusting numerical values” (anv), i.e., the values added to the original values in order to obtain the adjusted values. in a normal situation, most of the anvs are insignificant concerning the uncertainties associated with the adjusted values, but some anvs may show significant or even anomalously high values. analysis of the anvs should be mandatory. the anvs were never published by codata, although they are necessarily available as an outcome of the lsa method, being the adjustments. evidence of the difference between the “original” value and the “adjusted” value is reported in [2], where it is indicated: “as in past codata adjustments, the uncertainty of a theoretical expression is taken into account by including in the expression an additive correction 𝛿. each such 𝛿 is taken as an adjusted constant and is included as an input datum with initial value zero but with an uncertainty equal to that of its corresponding theoretical expression.” actually, a variable 𝛿 is associated with each single data piece in the set, with the initial value equal to zero, which is the “adjustment.” its value ≠ 0 is then optimized by the lsa procedure. these are anvs. the codata should publish a table of them, in which each outlying datum would appear with a larger 𝛿 with respect to non-outlying data. the existence of an “adjusting numerical value” is disputed concerning the application of the lsa to experimental values of the constants. it is true that in the case of a theoretically computed value in a “law,” that value acts as a fixed reference so that the “adjustment” clearly consists of an added 𝛿, computed as in [2]. however, the meaning of “adjustment,” a term consistently used in all past and present codata studies, https://www.bipm.org/cc/alloweddocuments.jsp?cc=ccu https://arxiv.org/abs/1507.07956 acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 100 necessarily implies that there is another numerical value (normally different) for any and each of the “adjusted” quantities in the case that the codata lsa method is not applied. it must be noted that those 𝛿 are the corrections that after stipulation of the numerical value of a constant, the laboratories will not necessarily have to apply to their published inconsistent numerical values in their standards when assessing that their national realization reproduces the stipulated value of the constant. these 𝛿 are the “best” values according to the lsa criteria for obtaining the “adjusted values,” which are a dataset that has, for each constant, a certain dispersion from which a representative value for each constant is then chosen. that is what is used for the correction. in the case of the experimental values of a quantity, the original ones are the published ones, each with an associated uncertainty: a statistical representative value (summary statistics) can be obtained from them, different for each statistical tool chosen, as found in the literature before 2017. none of them are considered an “adjustment” of another one. the lsa method is one of these tools; however, it is not strictly statistical. its characteristic is that it does not consider the quantities individually; rather, it also uses deterministic relationships between those quantities, linking them with each other with the aim of achieving a more robust evaluation of the quantity numerical values’ inter-consistency. that method results in a numerical value for each of the “adjusted” quantities that is usually different from that which is obtained by other means, being influenced by the abovementioned relationships with other quantities of the set, “relationships” here meaning “equations” (“observational equations” in table 7 [1]) regardless of (or in addition to) the dispersion of the experimental values. in fact, it happened in the past that the resulting codata value for some constants changed with respect to the previous “adjustment,” even in the absence of any new experimental determination for those constants in the time interval between the two adjustments. considering the basic dependence of the lsa results on the observational equations, one might see a possible similarity with some problems concerning “fundamental laws” recently indicated in [14]1: does the meaning of “adjustment” involve the formulation of these laws, thus also involving their mutual degree of consistency? 1 e.g., “the need to maintain simplicity in the expression of theoretical laws constrains the choice of metric conventions for the quantities appearing in these laws… law-based metric conventions are specified by reference to one or more scientific laws. intervals of a quantity are considered equal if and only if their equality is consistent with the specified laws being empirically adequate… fundamental theoretical laws, being idealized, do not provide accurate predictions by themselves… changes of metric convention may lead to a change in the mathematical form of laws in which affected kinds of quantity appear… shifting between non-equivalent metric conventions may also affect so-called ‘dimensionless’ constants… questions that are still without firm answers q1 codata support. how can the codata analysis be considered the single criterion for establishing the stipulated values of the four new constants when its value for the plank constant, at least, is not supported by all the existing analyses [11]? q2 validity in time of the stipulated values. how can the codata analysis be considered sufficient, e.g., by [10], when the future trend of the values of some constants cannot yet be considered stable [11] with sufficient confidence/degree of belief, on subsequent add-ons to the results database? q3 data inconsistency. should a deeper discussion than in [10, 11] on the present inconsistency of some data and on the best provisions for taking the issue into account be made available? if not, why not? q4 lsa versus other methods. how can the lsa method be considered the only formal tool for use when the 2017 evidence of data inconsistencies has demonstrated the existing difference between the use of a criterion for only the consistency testing of a given dataset and the use instead of more comprehensive criteria for establishing its precision? q5 stipulated number of digits. since “stipulation” means a conventional “best value” round-up to a “reference value,” then considered exact and fixed for future periods of time, should a stipulated numerical value ensure the future stability of all its digits at the highest possible confidence level/degree of belief? q6 use of expanded uncertainty. considering the importance of the result of the si revision, why is the use of expanded uncertainty (which is common practice in science for important issues) not considered mandatory for determining the stipulated values? q7 unit-magnitude continuity. for expanded uncertainties of experimental outcomes, should the fact that the consistency degree is at least one order of magnitude worse than aimed (thus possibly inducing a critical non-continuity degree for some of the unit magnitudes) require a deep, open discussion about possible provisions? q8 meaning of “adjustment.” since “adjusted” implies the existence of another (normally different) numerical value in the case that the lsa method is not applied, what is the reason why this term is used, if such “original” value cannot be identified? otherwise, how does its need arise from the original value or from the theoretical relationships between constants, and which 𝛿 is explicitly that resulting from the application of the lsa for each of the constants in the set? … by fixing the numerical value of a constant, one assumes that it is indeed constant. for example, in fixing the numerical value of the planck constant, one presupposes that the energy and frequency of a photon have a constant ratio. after all, constants are relations among quantities in scientific laws. postulating their constancy necessarily involves the assumption that some (or all) of the fundamental laws in which those constants feature are empirically adequate… proportionality assumption: the correct way of measuring the quantities of physics is such that the currently known fundamental laws of physics preserve their mathematical form… the direct incorporation of elements from fundamental theory into the definitions of measurement units carries with it clear advantage. if the above analysis is correct, this incorporation also generates an implicit tension between the need for long-lasting unit definitions and the need for testable fundamental laws.” acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 101 table 1a. change in numerical value and its uncertainty u of the codata adjustments 2006-2017, for the new constant k in the revised si definition. for e, k, and na in table 1b, see the appendix. constant codata numerical value * u / relative  107 change total shift planck h 1034 2006 6.626 0690 3.3/0.50 — 2010 6.626 0696 2.9/0.44 6  10–7 2014 6.626 070 04 a 0.8/0.12 4.4  10–7 10.4  10 –7 2017 6.626 070 15 a 0.7/0.11 1.1  10–7 11.5  10 –7 * the codata reports standard uncertainty u. the smaller case digits, obtained from the codata two-digit uncertainty format, are those affected by uncertainty. a the 2014 codata outcome is 6.626 070 040(81); therefore, the numerical value can be as low as 6.626 069 959, thus also affecting the preceding digit. similarly, for the 2017 one, the codata outcome is 6.626 070 150(69). consequently, the upper bound of the 2017 interval is 6.626 070 121, and the lower bound of the 2017 interval is 6.626 070 081, not significantly overlapping. table 1b. change in numerical value and its uncertainty u of the codata adjustments 2006-2017, for the new constants e, na, and k involved in the revised si definition. see table 1a for h in the main text. constant codata numerical value * u / relative  107 change total shift electron charge 2006 1.602 176 49 0.4/0.23 — 𝑒 × 1019 2010 1.602 176 57 0.35/0.19 8  10–8 2014 1.602 176 62 0.1/0.06 5  10–8 1.3  10 –7 2017 1.602 176 634 b 0.08/0.05 1.4  10–8 1.44  10 –7 avogadro 2006 6.022 1418 3.0/0.50 — 𝑁a × 10 −23 2010 6.022 1413 2.7/0.45 -5  10–7 2014 6.022 1408 c 0.7/0.12 -4  10–7 -9  10–7 2017 6.022 140 76 c 0.6/0.10 1  10–7 -8  10–7 boltzmann 2006 1.380 6504 d 24/17 — 𝑘 × 1023 2010 1.380 6488 d 13/9.4 -1.6  10–6 2014 1.380 6485 8/8.5 -0.3  10–6 -1.9  10 –6 2017 1.380 649 e 5/3.6 -0.5  10–6 -1.4  10–6 * the codata reports standard uncertainty u. the smaller case digits are uncertain, taken from the codata two-digit uncertainty format—except for boltzmann constant k, see notes c) and e). b the 2017 codata outcome is 1.602 176 6341(83); therefore, the numerical value can be as low as 1.602 176 6258 and as high as 1.602 176 6424, thus also affecting the preceding digit. c the 2014 codata outcome [13] is 6.022 140 857(74); therefore, the numerical value can be between 6.022 140 783 and 6.022 140 931, thus affecting the preceding digit. the analysis is similar for the 2017 one, where the codata outcome is 6.022 140 758(62). d two digits are shown because the rounding affects also the preceding digit. e the codata 2017 outcome is 1.380 649 03(51); thus, the rounding does not include uncertain digits (the only occurrence in the table). however, the numerical value can be as low as 1.380 648 50, thus in fact affecting the last digit. table 2. different ways of treating the digits with the same numerical values for the four constants. constant numerical value codata 2017 (k = 1) [1] stipulated a [1] exactly known number b h 1034 6.626 070 150(69), urel = 1.1  10–8 6.626 070 15, urel = 7.5  10–10 6.626 070 (1), urel = 2.3  10–8 e 1019 1.602 176 6341(93), urel = 0.6  10–8 1.602 176 634, urel = 3.1  10–10 1.602 176 6 (3), urel = 4.0  10–7 na 10–23 6.022 140 758(62), urel = 1.1  10–8 6.022 140 76, urel = 8.3  10–10 6.022 140 (8), urel = 1.3  10–7 k 1023 1.380 649 03(51), urel = 7.4  10–7 1.380 649, urel = 3.6  10–7 1.380 6 (5), urel = 3.5  10–5 a the smaller case digits in the proposed stipulations are used here for those affected by the uncertainty interval in the previous column. b here “exact” means unaffected by the original experimental uncertainty. the added smaller case digit in parentheses is not exact, being affected by the codata uncertainty interval indicated in the previous column: it is reported because the rounding up, different from truncation, would affect the exact numerical value of na and k. improvement of the realisation of the mass scale acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 4 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 4 6 improvement of the realisation of the mass scale z. zelenka1, s. alisic2, b. stoilkovska3, r. hanrahan4, i. kolozinsky5, g. popa6, d. pantić7, v. dikov8, j. zůda9, m. coenegrachts10, a. malengo11 1 bev bundesamt für eichund vermessungswesen ptp physikalisch-technischer prüfdienst, austria, zoltan.zelenka@bev.gv.at 2 institute of metrology of bosnia and herzegovina, bosnia and herzegovina, sejla.alisic@met.gov.ba 3 ministry of economy bureau of metrology, north macedonia, bianka.stoilkovska@bom.gov.mk 4 nsai national metrology laboratory, ireland, rory.hanrahan@nsai.ie 5 national scientific centre institute of metrology, ukraine, iren_kolozinsky@ukr.net 6 brml biroul roman de metrologie legala, inm institutul national de metrologie, romania, george.popa@inm.ro 7 ministarstvo privrede direkcija za mere i dragocene metale, serbia, pantic@dmdm.rs 8 bulgarian institute of metrology, bulgaria, v.dikov@bim.government.bg 9 cesky metrologicky institut, czech republic, jzuda@cmi.cz 10 fps economy, metrology division national standards (smd ens), belgium, mieke.coenegrachts@economie.fgov.be 11 istituto nazionale di ricerca metrologica, italy, a.malengo@inrim.it abstract: the project 19rpt02 “improvement of the realisation of the mass scale” (empir [1] call 2019 – energy, environment, normative and research potential) has just started. its aim is to improve the quality of one of the most important tasks in mass metrology, the realisation of the mass scale. after the new definition of the kilogram this technique is getting more important. keywords: mass realisation; kilogram; multiples and sub-multiples; subdivision and multiplication; oiml r111 [2] 1. introduction the overall aim is to improve calibration methods for the realisation of the mass scale. in the past, two euramet projects have tried to address the issues related to the realisation of the mass scale, but due to the complexity of the work, were unable to fully satisfy the requirements. the project includes the development of the design of the measurements; the improvement of the applied mathematical methods; provision of realistic algorithms and software tools for the realisation of the mass scale and the respective uncertainties. furthermore, a draft of a guide for the dissemination of the mass unit with respect to the new definition of kilogram is among the goals. the project partners are: cmi, czech republic; bev-ptp, austria; smd, belgium; imbih, bosnia and herzegovina; bim, bulgaria; nsai, ireland; inrim, italy; brml, romania; dmdm, serbia; me-bom, north macedonia and nsc-im, ukraine. figure 1: some weights to be calibrated against limited number of mass standards 2. importance of the work in the last years the mass metrology community was focusing on the definition and realisation of the unit of mass, the kilogram [3, 4]. on the other hand the other big challenge is the calibration of the other nominal values against a single mass standard was somewhat overlooked. a comprehensive analysis of the existing dissemination methods is not yet available. the used methods of dissemination of mass unit with some exceptions are so far based on a limited number of sources such as procedure developed and published by ptb [5, 6]. due to several factors such http://www.imeko.org/ mailto:zoltan.zelenka@bev.gv.at mailto:sejla.alisic@met.gov.ba mailto:bianka.stoilkovska@bom.gov.mk mailto:rory.hanrahan@nsai.ie mailto:iren_kolozinsky@ukr.net mailto:george.popa@inm.ro mailto:pantic@dmdm.rs mailto:v.dikov@bim.government.bg mailto:jzuda@cmi.cz mailto:mieke.coenegrachts@economie.fgov.be mailto:a.malengo@inrim.it acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 5 as involvement of robotic mass comparators, dimensional limitations of the inner chambers or weighing pans of the mass comparators or different composition of the weighing set, the methods shall be reshaped to the actual laboratory needs. some available publications are covering parts of the task [7 14]. despite the very recent new si, usually the 1 kg mass standard is still used as starting point of the dissemination process, as it was at the time when the definition of the kilogram was based on the international prototype. one of the outcomes of the new definition is that the uncertainty of the calibration of the one kilogram mass standards is increased. it forces a lot of national metrology institutes to enlarge their calibration measurement capabilities [15]. the most important scientific result of the proposed improvement in subdivision and multiplication techniques is that it could play an important role in preventing many national metrology institutes (nmis) from being forced to increase their calibration and measurement capabilities [15]. the specific objectives of the project are: a) to analyse the methods for realisation and dissemination of the mass scale including the impact from the recent redefinition of the kilogram; to create an appropriate methodology in order to optimise for different technical requirements and parameters (e.g. robustness, effectiveness, small uncertainty, properties of different weighing instruments, different types of weight sets, number of control weights or standards). b) to develop and implement calibration methods to realise, improve and maintain the mass scale. c) to develop advanced mathematical and statistical tools and software solutions to calculate the results from mass measurements and to evaluate the associated uncertainties (including correlations between standards and measurements and handling of outliers). d) to draft a euramet guideline for the realisation of the mass scale. e) for each participant, to develop an individual long-term strategy. 3. aimed scientific results improved designs of the measurements with guidance for choosing an appropriate design will be produced. it includes optimisation for robustness, effectiveness, uncertainty, taking into account the properties of different weighing instruments, different types of set of weights, the number of control weights and mass standards. second scientific results would be a reduction of the uncertainties via better mathematical methods. currently used mathematical solutions are usually limited to ordinary or weighted least squares methods. generally, these methods do not allow the use of multiple mass standards as input to the disseminations. this is probably the biggest opportunity to improve the realisation of the mass scale, because the different realisation of mass unit delivers calibration of different nominal values. recalling that in the past the one kilogram was the solely the starting point of the realisation of the mass scale. nowadays with kibble balances, the mass of weights with a wider range of nominal values can be determined (e.g. 50 g to 1 kg) while using other techniques (e.g. capacitive, electrostatic) small mass standards (e.g. 10 mg to 1 g) can be realised. all these calibrated mass standards shall be included as input into the calculation of the mass scale in order to reduce the uncertainties. 4. summary this article is to bring into attention the scientific potential of the empir project “realmass”, which is aiming the improvement of one of the most important techniques in mass metrology, the realisation of the mass scale. 5. acknowledgment this project has received funding from the empir programme co-financed by the participating states and from the european union’s horizon 2020 research and innovation programme. 6. references [1] european metrology programme for innovation and research (empir), euramet. online [accessed 20200817]: https://www.euramet.org/researchinnovation/research-empir/ [2] oiml recommendation r 111, weights of classes e1, e2, f1, f2, m1, m1-2, m2, m2-3 and m3. [3] euramet emrp jrp sib05 newkilo developing a practical means of disseminating the new kilogram. [4] euramet emrp jrp sib03 know realisation of the awaited definition of the kilogram resolving the discrepancies. [5] m. borys, r. schwartz, a. reichmuth, r. nater, fundamentals of mass determination, springer, 2012. [6] r. schwartz, m. borys, f. scholz, guide to mass determination with high accuracy, physikalischtechnische bundesanstalt braunschweig and berlin, wirtschaftsverlag nw, bremerhaven, 2007. [7] w. bich, “variances, covariances and restraints in mass metrology”, metrologia vol. 27, no. 3, pp. 111-116, 1990. [8] z. zelenka, “dissemination of the unit of mass in a fully automatic mass laboratory using subdivision”, proc. xix imeko world congress, lisbon, portugal, 6 – 11 september 2009. online [accessed http://www.imeko.org/ https://www.euramet.org/research-innovation/research-empir/ https://www.euramet.org/research-innovation/research-empir/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 6 20200817]: http://www.imeko2009.it.pt/papers/fp_123.pdf?w b48617274=736acbab [9] z. zelenka, “improvements on dissemination of the unit of mass in bev”, proc. xx imeko world congress, busan, republic of korea, 9 – 14 september 2012. online [accessed 20200817]: https://www.imeko.org/publications/wc2012/imeko-wc-2012-tc3-p7.pdf [10] z. zelenka, “robust subdivision for the dissemination of the unit of mass”, proc. xxi imeko world congress, prague, czech republic, 30 august – 4 september 2015. online [accessed 20200817]: https://www.imeko.org/publications/wc2015/imeko-wc-2015-tc3-095.pdf [11] z. zelenka, “estimation technique of the mass of drifting mass standards”, j. phys.: conf. ser. vol 1065, 042030, 2018. online [accessed 20200817]: http://iopscience.iop.org/article/10.1088/17426596/1065/4/042030 [12] m. grum, m. oblak, i. bajsić, m. perman, “subdivision of the unit of mass using weight support plates”, proc. xvii imeko world congress, dubrovnik, croatia, 22 – 27 june 2003. online [accessed 20200817]: https://www.imeko.org/publications/wc2003/pwc-2003-tc3-034.pdf [13] a. vâlcu, “improvement of the calibration process for class e1 weights using an adaptive subdivision method”, 4th int. conf. adaptive & self-adaptive systems & applications, 2012. [14] a. vâlcu, d. dinu, “subdivision method applied for oiml weights using an automatic comparator”, proc. xix imeko world congress, lisbon, portugal, 6 – 11 september 2009. online [accessed 20200817]: http://www.imeko2009.it.pt/papers/fp_149.pdf [15] calibration and measurement capabilities, available from the bipm key comparison database. online [accessed 20200817]: http://kcdb.bipm.org/appendixc/ http://www.imeko.org/ http://www.imeko2009.it.pt/papers/fp_123.pdf?wb48617274=736acbab http://www.imeko2009.it.pt/papers/fp_123.pdf?wb48617274=736acbab https://www.imeko.org/publications/wc-2012/imeko-wc-2012-tc3-p7.pdf https://www.imeko.org/publications/wc-2012/imeko-wc-2012-tc3-p7.pdf https://www.imeko.org/publications/wc-2015/imeko-wc-2015-tc3-095.pdf https://www.imeko.org/publications/wc-2015/imeko-wc-2015-tc3-095.pdf http://iopscience.iop.org/article/10.1088/1742-6596/1065/4/042030 http://iopscience.iop.org/article/10.1088/1742-6596/1065/4/042030 https://www.imeko.org/publications/wc-2003/pwc-2003-tc3-034.pdf https://www.imeko.org/publications/wc-2003/pwc-2003-tc3-034.pdf http://www.imeko2009.it.pt/papers/fp_149.pdf http://kcdb.bipm.org/appendixc/ acta imeko, title acta imeko issn: 2221-870x june 2020, volume 9, number 2, 1-2 acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 1 introductory notes for the acta imeko special issue on the “23rd symposium on measurement of electrical quantities” and “international workshop on metrology for agriculture and forestry 2019” alexandru salceanu1, mauro d’arco2, oscar tamburis3 1 ”gheorghe asachi” technical university of iasi, mangeron blvd. 67, 700050, iasi, romania 2 università di napoli federico ii dieti, via claudio 21, 80125 napoli, italy 3 università di napoli federico ii – dmvpa, via delpino 1, 80137 napoli, italy section: editorial citation: alexandru salceanu, mauro d’arco, oscar tamburis, introductory notes for the acta imeko special issue on the “23rd symposium on measurement of electrical quantities” and “international workshop on metrology for agriculture and forestry 2019”, acta imeko, vol. 9, no. 2, article 1, june 2020, identifier: imeko-acta-09 (2020)-02-01 editor: dušan agrež, university of ljubljana, slovenia received: june 24, 2020; in final form june 24, 2020; published june 2020 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding authors: alexandru salceanu, e-mail: asalcean@tuiasi.ro; mauro d’arco, e-mail: mauro.darco@unina.it 1. introductory notes on the “23rd symposium on measurement of electrical quantities’’ distinguished readers, this current number of acta imeko is mainly connected with the 23rd symposium on measurement of electrical quantities which has been held in the city of xi’an, people's republic of china on september 2019. one of the imeko most active technical committee (tc) is the no. 4, established since 1984, being mainly dedicated to the measurement of electrical quantities. the central objective of this scientific event was to present the latest achievements in the field of electrical measurements and to stimulate information exchanges on current researches. at the end after the review process the target of 17 papers with valuable content and elegant form imposed by our magazine have been successfully reached. of these papers, 8 have been selected for this issue of acta imeko, while the other 9 will be included in our very next release. we hope that each reader of this issue will find interesting and useful information, with a high degree of applicability. in the paper submitted by pavol dolinský et al, ”the ecg signal model based on the parametric description of the characteristic waves” there is developed a research focused on new ecg signal modelling based on geometrical properties of the signal. instead of artificial functions used in common ecg models, the proposed model is based on the modelling of real ecg signal divided into time segments, each segment being modelled by simple geometrical forms. the final ecg signal model is represented by the sequence of parameters of the base functions. parameter variation allows generation of different waveforms for each subsequent heartbeat without mixing pqrst waves order that allows representing the irregularities of the consecutive heartbeats. the paper presented by andrea mariscotti et al ”experimental characterization of pantograph arcs and transient conducted phenomena in dc railways” is centred on the depiction of this transient event quite common in electrified transportation systems. it is discussed the wide range of responses related to the substation and on-board filters and line resonances and antiresonances. there are analyses of the electrical characteristics of these transients and of the excited responses to define their typical spectral signatures in dc railways. in the assessment performed by oleh velychko et al, ”evaluation of results of rmo comparisons and inter-laboratory comparisons for electrical quantities” there are approached elements of the global metrological traceability from the perspective of the implementation of international mutual recognition agreements in the field of metrology. the linked results of international comparisons of national standards for the ac/dc voltage transfer difference measurements and electric power measurements and national inter-laboratory comparison ac/dc voltage transfer difference and electric power measurements at industrial frequency have been detailed and evaluated. ”uncertainty of the energy measurement function deriving from distortion power terms for a 16.7 hz railway” is a different but even complementary paper also presented by andrea mariscotti. the paper is an implementation of the en mailto:asalcean@tuiasi.ro acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 2 50463-2 energy measurement function, including the criteria for the significance of the measured and calculated terms and carries out a monte carlo analysis to assess the impact of harmonic power terms on the measured energy and its uncertainty. the following submission, ”a data-based approach for smart meter online calibration”, by fangxing liu et al. starts from the assertion that the smart meter should be considered as the key element of the smart grid. current smart meter verification method mainly focuses on in situ and laboratory detection and fails to test all the setup meters. the authors insist on the advantages of the ”online calibration” as a novel approach which calculates meter errors by analysing meter reading data. by presenting a comprehensive survey and basic model of smart meter online calibration, this paper proposes a recursive algorithm for estimating the meter accuracy. an internet of things (iot) prototype which consists of a data acquisition device wirelessly connected to internet via wifi is detailed in the paper ”a wi-fi iot prototype for ecg monitoring exploiting a novel compressed sensing method” developed by eulalia balestrieri et al. the proposed system performs a novel compressed sensing (cs) based method on ecg signal with the aim of reducing the amount of transmitted data, thus realizing an efficient way to increase the battery life of such devices. the evaluation of the reconstruction quality of the ecg signal in terms of percentage of root-mean-squared difference (𝑃𝑅𝐷) is reported for several compression ratios (𝐶𝑅𝑠). an overview of the recent research work dealing with the dac testing is been detailed in the paper ”research trends and challenges on dac testing” submitted by eulalia balestrieri et al. there are highlighted the trends and issues, also being provided useful information for the revision of the standard ieee std. 1658. main testing challenges the dacs include the reduction of the test time and cost, the measurement uncertainty computation and facing with the emerging built-in self-test (bist) solutions. ”a review of accurate phase measurement methods and instruments for sinewave signals” is performed for us by eulalia balestrieri et al. the phase measurement of sine-wave signals is important for several applications such as electric and electronic instrumentations, telecommunications and optical interferometry. the paper presents an overview of some high accurate phase measurement methods and instruments to highlight the characteristics in terms of measurement uncertainty of the main used ones, by taking into account a varying operative frequency range. it was a great honour for me to act as guest editor for this issue of acta imeko. i would like to express a special appreciation for our current editor in chief dušan agrež, who continues to the same high standards of competence and exigency, the tradition with which we have been already accustomed by his predecessors, paul regtien and paolo carbone. alexandru salceanu guest editor 2. introductory notes on the international workshop on ‘metrology for agriculture and forestry’ 2019 distinguished readers, the here finalized issue of acta imeko includes a first selection of scientific contributions that have been presented at the ieee international workshop on metrology for agriculture and forestry held in portici, naples on october 2019. as for the previous edition, the workshop was organized with the intent of creating an active and stimulating forum where academics, researchers and industry experts in the field of measurement and data processing techniques for agriculture, forestry and food could meet and share new advances and research results. as guest editors of this issue, we selected out of the 69 presented papers from several countries, the papers that appeared most suitable with the profile of acta imeko journal. a couple of papers has been considered to be included in this acta imeko issue. we would like to highlight, anyhow, that other contributions presented to the workshop and elected for technically extended versions are still under review for the future issues. the first paper included in the special section presents machine learning techniques aimed at measuring the soil moisture. the methodological approach exploits the meteorological data as inputs and compares the performance granted by three different machine learning techniques, all relying on artificial neural networks (anns). this is an exploratory work containing interesting proposals: the presented approach, in fact, uses free-access data that grant worldwide coverage, and seems to allow a realistic prediction of the soil moisture value, which can be useful for irrigation scheduling. the work addresses methods which deserve great attention by the measurement community since, due to the intervention of machine learning techniques, they are disrupting for the classical physical-model based measurement practice by suggesting a rethinking to some extents to the very concept of prediction induced by measurement results. the second paper selected contains detailed technical notes related to a “rugged” intelligent support system for precision farming, named evja. precision agriculture and livestock farming systems represent milestones in the contemporary innovation process. they are based on the exploitation of suitable technologies that help the implementation of data measurement and subsequent processing paradigms. the goals of these paradigms are the support to immediate decisions capable of assuring increased production, lowered consumption, and reduced environmental impact. in particular the work is focused on the crop production and highlights the details related to the decision support and predictive capabilities of the evja system. the decision support module represents the core part of the processing stage of the system and is introduced as a viable diagnostic system to assess the health of plants. also, the interfacing feasibility of the system that makes data and analytics easily available to the farmer is described. finally, we would like to show our gratification for having contributed to this issue of acta imeko journal as guest editors. we hope the readers could be inspired by the themes and proposals that have been elected and included in this special section related to innovations in metrology for agriculture and forestry. mauro d’arco, oscar tamburis guest editors template for an acta imeko event paper acta imeko issn: 2221-870x june 2018, volume 7, number 2, 1-2 acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 1 editorial to selected papers from the 1st imeko tc19 workshop on metrology for the sea fabio leccese “roma tre” university of rome, science department, via della vasca navale n.84, 00146 roma, italy section: editorial citation: fabio leccese, editorial to selected papers from the 1st imeko tc19 workshop on metrology for the sea, acta imeko, vol. 7, no. 2, article 1, june 2018, identifier: imeko-acta-07 (2018)-02-01 editor: dušan agrež, university of ljubljana, slovenia received june 4, 2018; in final form june 4, 2018; published june 2018 copyright: © 2018 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: fabio leccese, email: fabio.leccese@uniroma3.it dear reader, the sea has been the element that more than other represented for humanity the sense of unknown with its intrinsic luggage of fears and doubts, maybe representing the first barrier that humanity had to overcome in an organized way, thus representing the first challenge towards knowledge. because its complexity, the sea is studied from several points of view involving many research fields. the first edition of imeko tc19 workshop on metrology for the sea held in the prestigious and renowned “parthenope” university of naples on 11th-12th october 2017, represented a first meeting point between scientists coming from many scientific fields which given their vision of subjects dear to the measures both theoretical and practical. in fact, the concept of measurement uncertainty is faced in the particular operative context (high pressures, operative logistic problems, etc.) that, in some cases, stresses the instrumentation, which present technical characteristics extremely pushed to face the problems typical of the sea. in this perspective, the congress won the challenge to reach a first general objective to promote the scientific debate and the encounter of different sensibilities around the typical topics of the "measurements worlds" promoting a better and deeper comprehension and harmonization of them. the first edition of the congress gathered more than 100 papers by researchers of all over the world, whose almost 50 accepted and published. among this relevant number of contributions, the board of congress selected a few papers to be extended and submitted for the peer review of acta imeko journal. a first selection of the papers eligible for the special issue has been done from the congress board, then, the successive fundamental contribute of the journal reviewers to which my special thanks go, determined eight papers considered worthy of publication in this journal and which represent a synthesis of the scientific activity in this field. in the first paper, e. petritoli and f. leccese examine the development of an attitude and control system for a tailless autonomous underwater vehicle without movable control surfaces. for it, it is extremely interesting the auv manoeuvring system managed acting, with high accuracy, on the buoyancy system and on the centre of gravity displacement, which control is realized by an ad hoc control system called glider integrated control system. the device has a wide wing surface allows a smooth glide slope and an excellent operational range. in order to highlight their strengths and weaknesses, e. romano et al. deal a critical review of the main methods used for sampling, measuring and classifying marine sediments whose knowledge is useful in many research areas as sediment contamination, ecology of benthic communities, seismic studies, remote sensing surveys and beach nourishment. this revision shows as the highest accuracy of grain size data is obtained using flexible strategies and adopting suitable sampling devices and analytical instruments based on the characteristics of the study area and the specific aim of the survey. in order to improve absolute positioning provided in gnss context, s. gaglione et al. propose to jointly use well-known quality indicators like the signal-to-noise ratio and the satellite elevation angle with an additional indicator, the redundancy number. the benefits of using the proposed weighting schemes is demonstrated in urban canyon and with single (gps only) or multiple (gps/glonass) constellations. in fact, the position errors, using the weighting scheme based on redundancy numbers, are significantly reduced, for all the considered configurations. acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 2 f. arrichiello et al. present an overview of the recent and ongoing research activities of the italian interuniversity centre on integrated systems for the marine environment (isme) in the field of geophysical seismic survey. such activities, performed in the framework of the h2020 european project wimust, include the development of technologies and algorithms for autonomous surface crafts and autonomous underwater vehicle to perform geophysical seismic survey by mean of a team of robots towing streamers equipped with acoustic sensors. s. buogo et al. present the empir “unac-low” project, which aims at developing the european metrological capacity for the calibration of hydrophones and autonomous noise recording systems for frequencies below 1 khz. the project will allow improving the eu metrological capacities for absolute measurement of underwater sound, with a direct effect on the implementation of regulation and eu directives that require underwater acoustic measurements to be traceable. a coherent eu metrology strategy for underwater acoustics will be developed as one of the main project outcomes. in the sixth paper, r. marsili et al. show an interesting example of a new and fast solution for 3d modelling, based on the same principles of photogrammetry: the photo-modelling. the comparison between this and the metrological technique of structured light 3d scanning, provided by the creaform go scan 50 with metrological certification, is the aim of this paper, defining performances and verifying the potential of this innovative, simple and economical technique. in order to investigate the alignment error due to a poor geometry reconstruction many test have been performed. the results show different performances, in terms of deviation distributions and volumetric errors. g. d’onghia et al. show some practical applications of the memo (marine environment monitoring system) lander, with indications of relative advantages and limitations. memo is a baited lander equipped with 2 video cameras, a multi-parametric probe and a current meter. memo is able to explore and monitor marine ecosystems in the mediterranean sea for 24 consecutive hours survey even down to 1000 m in depth. during its activity, data on environmental parameters (depth, salinity, temperature, current) and related to bentho-pelagic species (small-scale distribution, size and behaviour), have been recorded. g. gaglianone et al. show the application of a particular geophysical method: the “geodive”, which allows surveying with high accuracy nearshore shallow waters with a low budget. the authors tested and tuned the “geodive” method allowed to map the submerged morphologies and to acquire highresolution optical images for further photogrammetric processing. by coupling the processing of underwater-acquired data obtained by a high definition action sport cam, with the direct surveys performed by underwater scuba operators, it was possible to perform some morphological and sedimentological measurements and observations on the experimental targets and obtain 3d high-resolution models even in low visibility conditions. the fishermen know that the sea is dangerous and the storm terrible, but they have never found these dangers sufficient reason for remaining ashore. (vincent van gogh). fabio leccese guest editor editorial to selected papers from the 1st imeko tc19 workshop on metrology for the sea dynamic measuring methods: a review acta imeko issn: 2221-870x march 2019, volume 8, number 1, 64 76 acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 64 dynamic measuring methods: a review aleksandr l. shestakov1 1 south ural state university, lenina, 76, chelyabinsk, russia section: research paper keywords: dynamic measurements; dynamic error analysis; restoration of a dynamically distorted signal; regularisation; evaluation of the dynamic characteristics of measuring equipment citation: aleksandr l. shestakov, dynamic measuring methods: a review, acta imeko, vol. 8, no. 1, article 10, march 2019, identifier: imeko-acta-08 (2019)-01-10 section editor: franco pavese, italy received march 23, 2018; in final form may 31, 2018; published march 2019 copyright: © 2019 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: the work was supported by act 211, government of the russian federation, contract № 02.a03.21.0011. corresponding author: aleksandr shestakov, e-mail: a.l.shestakov@susu.ru 1. the concept of dynamic measurements the term ‘dynamic measurements’ refers to measurements whereby the output signal is significantly different to the measuring input signal due to the inertia lag of the measuring equipment or measuring system. such situations occur when impulse or other quick-changing actions are measured. a sensor, or the whole test facility, can act as measuring equipment. a common example of such measurements is an impulse pressure measurement 𝑃(𝑡) (figure 1). as a rule, the value of the dynamic error is a time function. it is significantly different to the measurement uncertainty in static measurements. in dynamic measurements, errors of tens of per cents are common. 2. short historical background the pioneering work on dynamic measurement is the study of the french physicist cornu [1], published in 1887. in this work, he studied the motion of a galvanometer and associated the value of the damping factor with the physical parameters of the device. later on, the inventor of an oscilloscope, french engineer blondel [2], considered dynamic models of some of its modules (1893). also, the work of d. i. mendeleev [3], published in 1897 and focused on accurate weighing on the unsteady laboratory balance should be referred to the first academic papers. in this work the great russian chemist without using differential equations found formulas for accurate weighing in the context of the swing of laboratory balance. the work of a. n. krylov [4], wherein a device for measuring mine explosion pressure was examined, can be considered as a starting point in the development of the theory of dynamic measurement. measuring error was studied as a time function for the first time. v. a. granovskii had a key role in the development of the theory of dynamic measurement, and figure 1. dynamic measuring of impulse pressure: pi = measuring input signal, p0 = observed output signal abstract the paper presents an overview of the directions in the development of dynamic measurement methods. first, the concept of dynamic measurements and a short historical background are outlined. the author describes several types of dynamic characteristics of measuring equipment, which fully characterise their dynamic properties. the next part is devoted to an analysis of the types of dynamic errors and their corrections. in conclusion, an overview of the identification of dynamic characteristics is given. mailto:a.l.shestakov@susu.ru acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 65 in his book [5], he presented all the main parts of this theory: the determination of the dynamic characteristics of measuring equipment, analysis of dynamic errors, dynamic error correction, and issues of dynamic measurement uniformity. 3. dynamic characteristics of measuring equipment there are several types of the dynamic characteristics of measuring equipment, which fully characterise its dynamic properties. the transfer function of the measuring equipment is 𝑊(𝑝) = 𝑦(𝑝) 𝑢(𝑝) = 𝑝𝑚 + 𝑏𝑚−1𝑝 𝑚−1 + ⋯ + 𝑏1𝑝 + 𝑏0 𝑝𝑛 + 𝑎𝑛−1𝑝 𝑛−1 + ⋯ + 𝑎1𝑝 + 𝑎0 , (1) where 𝑢(𝑝) and 𝑦(𝑝) are laplace transforms of input and output signals 𝑚 < 𝑛, and its discrete analogue is: 𝑊(𝑧) = 𝑦(𝑧) 𝑢(𝑧) = 𝑧𝑙 + 𝑟𝑙−1𝑧 𝑙−1 + ⋯ + 𝑟1𝑧 + 𝑟0 𝑧𝑘 + 𝑞𝑘−1𝑧 𝑘−1 + ⋯ + 𝑞1𝑧 + 𝑞0 (2) 𝑧 is a delay operator. the frequency characteristics of the measuring equipment, amplitude-frequency and phasefrequency characteristics are illustrated in figure 2. the impulse transient function ℎ(𝑡) (figure 3) is a response of the measuring equipment to the 𝛿-impulse of the infinitely small duration, in terms of time and an infinitely large amplitude ∫ 𝛿 ∞ 0 (𝑡)𝑑𝑡 = 1 the transient function 𝑘(𝑡) (figure 4) is a response of the measuring equipment to the unit step input. all these characteristics fully characterise the dynamic properties of the measuring equipment, and from this perspective, they are equivalent. furthermore, there are methods for converting one characteristic into another. theoretical methods for defining dynamic characteristics have been quite fully developed in terms of identifying automatic control systems. moreover, there is no differential difference between the automatic control system and the dynamic measuring system. however, the dynamic characterisation of the measuring equipment is a complex problem. in the passport data provided by the sensor manufacturers, the presence of dynamic characteristics is still the exception rather than the norm. for complex dynamic measuring facilities, this process requires special experiments. 4. analysis of dynamic measuring errors after the information on dynamic characteristics of measuring equipment is obtained, the next question is: what error occurs when taking measurements in dynamic mode? we can single out several approaches to the analysis of dynamic measuring errors. for the first, the deterministic approach, a structural diagram of the dynamic error analysis can be represented in the following way (figure 5): the laplace transforms of dynamic errors take the form (𝑝) = 𝑢(𝑝)[𝑊(𝑝) − 1] (3) in fact, the dynamic error through the inverse laplace transform is measured as follows figure 2. amplitudeand phase-frequency characteristics of measuring equipment figure 3. impulse transient function of the measuring equipment. figure 4. transient function of the measuring equipment. acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 66 (𝑡) = 𝐿−1{𝑢(𝑝)[𝑊(𝑝) − 1]}, (4) and through the solution of an integral equation (𝑡) = ∫ ℎ ∞ 0 (𝑡 − 𝜏)𝑢(𝜏)𝑑𝑡 − 𝑢(𝑡), (5) where 𝑊(𝑝) and ℎ(𝑡 − 𝜏) are the transfer function and the impulse transient function of measuring equipment respectively. equation (4) and equation (5) show that the dynamic error of measurements is the time function and depends both on the dynamic properties of the measuring equipment and on the measuring signal. the analytical expression for the dynamic errors of the measuring step signal for a measuring transducer of arbitrary order was outlined by v. a. granovskii [5]. in a general case in which the input of the measuring system is affected by the input signal of the laplace transform, which can be represented as 𝑢(𝑝) = 𝑢1(𝑝) 𝑢2(𝑝)⁄ . the measuring system has the transfer function 𝑊(𝑝) = 𝑀(𝑝) 𝐷(𝑝)⁄ , where 𝑀(𝑝) and 𝐷(𝑝) are polynomials of 𝑚 and 𝑛 order, respectively, and 𝐾(0) = 𝑀(0) 𝐷(0)⁄ . the dynamic error, in steady-state conditions, takes the form (𝑡) = ∑ 𝑢1(𝜌𝑘 ) 𝑢′2(𝜌𝑘 ) 𝜈 𝑘=1 [ ∏ (1 − 𝜌𝑘 𝛾𝑖 )𝑚𝑖=1 ∏ (1 − 𝜌𝑘 𝜆𝑖 )𝑛𝑖=1 − 1] 𝑒 𝜌𝑘(𝑡) (6) where 𝜆1, … 𝜆𝑛 and 𝛾1, … 𝛾𝑚 are transfer function poles and zeros. 𝜌𝑘 is a pole of the laplace transform of the input signal 𝑢 [6]. in the same work, dependencies for determining the poles and zeros of the measuring system, providing the required maximum allowable errors to measure linear, parabolic, and exponential signals, are obtained. more complex signals can be approximated by a combination of simpler signals and taking into account nonzero initial conditions when they are joined. however, analytical dependencies are lengthy. the mathematical simulation method is more preferable for analysing dynamic errors in such a case. this approach is useful for the analysis of dynamic errors when signals of the known form are measured. [7] presents the expressions of the dynamic errors of the measuring system used when periodic square impulse and trapezoidal wave signals are measured. the transition function ℎ(𝑡) is used here as a characteristic of the dynamic properties of the measuring system. there is also an approach whereby a dynamic error is determined as an integral in some period of observing an integral square error. thus, [8] analyses the method for decreasing an order of the dynamic system based on the minimisation of a quadratic error criterion for a simplified system with regards to a high-order system. the error formation scheme is shown in figure 6. the method for determining a signal 𝑢0(𝑡) switching from +1 to –1 at the moments 𝑡1, 𝑡2, … 𝑡𝑟 (figure 7), maximising the square error on the interval [0, 𝑇], was suggested. the results of this work can be considered as a method for finding a signal that generates the maximum dynamic error of the measuring system. [9] considers a method for obtaining a switching input signal, limited in the value |𝑢(𝑡)| ≤ 𝑎, whose speed is also limited. |�̇�(𝑡)| ≤ 𝜃,  𝑡 ∈ [0, 𝑇] maximises the integral square error of the difference between the signals of the measuring system and its simplified dynamic model. the method requires the solution of a set of complicated integral-convolution equations. as the author notes, applying this method, it is difficult to receive a signal with more than 25 switching operations. [10] suggests a method for generating a switching signal on the basis of wavelet transform. the measured input signal has limitations in amplitude and the input change rate (figure 8). the method generates a switching signal that maximises the integral square error of the dynamic measuring system. the switching points are specified by the genetic algorithm. based on the approach by which the measured input signal (which provides the maximum integral square error of the dynamic measuring system) is generated, an attempt to create a hierarchy of dynamic measuring systems according to their dynamic accuracy was made in [11]. the diagram for determining dynamic accuracy is suggested in figure 9. here, 𝐾𝑥 (𝑝)and 𝐾𝑠 (𝑝) are the transfer functions of the figure 5. structural diagram of forming dynamic errors in the deterministic case. figure 6. block diagram for determining the integral square error. figure 7. input signal maximising the integral square error. figure 8. input signal maximising the integral square error. acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 67 measuring system and a standard system with the desired dynamic characteristics. the system error has two components. the coefficient 𝑐 = lim 𝑝→∞ [𝐾𝑥 (𝑝) − 𝐾𝑠 (𝑝)] (7) specifies a steady-state component of the error 𝑐𝑢(𝑡), and 𝑦(𝑡) = ∫ 𝑘(𝑡 − 𝜏)𝑢(𝜏) 𝑡 0 𝑑𝑡 (8) where 𝑘(𝑡 − 𝜏) is an impulse transient function of the difference between the systems 𝐾𝑥 (𝑝) − 𝐾𝑠 (𝑝). a criterion for the dynamic accuracy of the measuring system is proposed on the basis of the quadratic criterion 𝐼 = ∫ [𝑐𝑢(𝑡) + 𝑦(𝑡)] 𝑇 0 2 𝑑𝑡 (9) to use its maximum value max 𝐼 [ (𝑡)] = max 𝑡∈[0,𝑇] max 𝑢∈[𝑈] 𝐼{ (𝑡)[𝑢(𝑡), 𝑐, 𝑘(𝑡), 𝑇]} (10) the attempt to classify (create a hierarchy) dynamic measuring systems by dynamic errors is very useful. however, it has not been as successful as the establishment of measuring equipment in the case of static measurements by measurement uncertainty and accuracy classes. it is not clear what is considered as a standard measuring system. the situation is logical. as shown in [9], the measured signal 𝑢(𝑡), maximising the integral square error, is disruptive. furthermore, finding it is highly complicated, especially for high-order systems. the interval of integration should also be compared with the dynamic properties of the measuring system. nevertheless, the problem statement is promising. the third approach to the analysis of dynamic errors is based on consideration of the measured signal as random with the known spectral density. then, the structural diagram for estimating the measurement errors can be presented in the following form (figure 10). in figure 10, 𝑊(𝑗𝜔) is a frequency characteristic of the dynamic measuring system. 𝑆𝑢 (𝜔),   𝑆𝑛 (𝜔),   𝑆𝑤 (𝜔),   𝑆𝑦 (𝜔),  𝑆𝜀 (𝜔) are spectral densities of the input signal 𝑢(𝑡); input and output noise 𝑛(𝑡) and 𝑤(𝑡); the output signal 𝑦(𝑡), and the error (𝑡). the spectral densities, even the function in the form of fractional rational functions, can be represented as [12], [13] 𝑆𝑢 (𝜔) = 𝑏0 + 𝑏1𝜔 2 + ⋯ + 𝑏𝑚 𝜔 2𝑚 𝑎0 + 𝑎1 𝜔 2 + ⋯ + 𝑎𝑛 𝜔 2𝑚 . the typical characteristic of the spectral density of the measured input signal indicates that this signal always has a restricted frequency spectrum (figure 11). taking into account the mutual uncorrelatedness of signals 𝑢(𝑡), 𝑛(𝑡), and 𝑤(𝑡), the spectral density of the error takes the form 𝑆𝜀 (𝜔) = |1 − 𝑊(𝑗𝜔)| 2𝑆𝑢(𝜔) + |𝑊(𝑗𝜔)| 2𝑆𝑢(𝜔) + 𝑆𝑤(𝑗𝜔) (11) the average value of the squared error is calculated by the spectral density ̄2 = 1 2𝜋 ∫ 𝑆𝜀 (𝜔) ∞ −∞ 𝑑𝜔 . 5. dynamic error correction by this term, we refer to the decrease in dynamic measuring errors by mathematical processing of the measurement results using the knowledge of dynamic characteristics of the measuring system. there is also another name for this process: the restoration of a dynamically distorted signal. the first approach, which results in a number of methods for solving this problem, is based on the description of the process of dynamic measurements by the volterra equation of the first kind. the impulse transient function ℎ(𝑡 − 𝜏) is used as a dynamic characteristic of the measuring equipment (figure 12). the measured input signal 𝑢(𝑡) is connected with the observed output signal 𝑦(𝑡) by the integral equation 𝑦(𝑡) = ∫ ℎ(𝑡 − 𝜏) 𝑡 𝑇1 𝑢(𝜏)𝑑𝜏, (12) where 𝑇1 ≤ 𝑡 ≤ 𝑇2. in this case, 𝑇1 can be equal to 0 or –∞, and 𝑇2 = ∞. ℎ(𝑡 – 𝜏) is an impulse transient function of the measuring equipment. as the variable 𝑡 is time, the output signal cannot appear before the input one. that is why a requirement of physical realisability should be met: ℎ(𝑡 − 𝜏) = 0 in case of 𝑡 < 𝜏. figure 9. block diagram for determining dynamic errors of the system figure 10. structural diagram of the formation of dynamic errors in the case of a random signal. figure 11. typical characteristic of the spectral density of the measured signal. figure 12. block diagram of the measuring equipment represented by the impulse transient function. acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 68 to restore the measured signal 𝑢(𝑡), it is necessary to solve equation (12). the direct method for solving equation (12) is based on replacement of the integral by the sum [14]: ∑ 𝐴𝑗 ℎ𝑖𝑗 𝑢𝑗 = 𝑦𝑖 , 𝑖 𝑗=1 (13) are coefficients of the quadrature formula. there are many such formulas [15], [16]. however, a general feature of this approach is the impossibility of the direct determination of 𝑢1 for further development of a recurrent algorithm. to find it, it is necessary to differentiate 𝑦(𝑡) at the starting point 𝑢1 = 𝑦′(𝑇1) ℎ11 . (14) in the presence of noise in the output signal, this leads to a significant error. there are many possible methods of solving equation (12). in [17], it is suggested to approximate the output signal 𝑦(𝑡) and the impulse transient function ℎ(𝑡 – 𝜏) by a certain type of function. taking into account a damped oscillating nature of the studied processes, we can approximate 𝑦(𝑡) = 𝑒 −𝛽|𝑡| ⋅ cos 𝜔 𝑡 (15) ℎ(𝑡 – 𝜏) = 𝑒 −𝛼|𝑡−𝜏| ⋅ cos(𝜈(𝑡 − 𝜏)), (16) where 𝛽, 𝛼, 𝜔, 𝑣 are approximation parameters. then, equation (12) can be solved by simple numerical or even analytical methods. one method for solving equation (12) is based on the series expansion of the desired signal 𝑢(𝑡) by eigenfunctions 𝜑𝑖 (𝑡)of the impulse transient function ℎ(𝑡 – 𝜏) [18], which satisfies the equation ∫ ℎ(𝑡 − 𝜏) 𝑎 −𝑎 𝜑𝑖 (𝜏)𝑑𝜏 = 𝜆𝑖 𝜑𝑖 (𝑡), (17) where 𝜆𝑖 is an eigenvalue of the function ℎ(𝑡 – 𝜏). then, 𝑢(𝑡) is expressed in terms of the series sum, as follows: 𝑢(𝑡) = lim 𝑁→∞ ∑ 𝑆𝑖 𝜑𝑖 (𝑡) . 𝑁 𝑖=0 (18) the recurrent algorithm for calculating the measured signal is suggested in [19]: 𝑢𝑛+1(𝑡) = 𝑦(𝑡) + 𝑢𝑛 (𝑡) − ∫ ℎ(𝑡 − 𝜏) 𝑡 𝑇1 𝑢𝑛(𝜏)𝑑𝜏 , (19) where 𝑢0(𝑡) = 0; 𝑢1(𝑡) = 𝑦(𝑡). the common feature of the abovementioned methods is the high sensitivity of solving the volterra integral equation to noise, which is always present in the output signal of the measuring equipment. this is due to the incorrectness of the task in restoring the dynamically distorted signal. one of the first works, which has been devoted to the peculiar features of dynamic measurement, is the work of g. n. solopchenko [20]. in [21], the integral equation is solved taking into account the output noise of the measuring equipment. the integral equation (12) is approximated by a discrete equation, which in matrix form, is written as �̄� = 𝐻�̄� + �̄�, where �̄� = [𝑦(0), 𝑦(1), … , 𝑦(𝑀 – 1)]𝑇 �̄� = [𝑢(0), 𝑢(1), … , 𝑢(𝑁 – 1)]𝑇 �̄� = [𝑧(0), 𝑧(1), … , 𝑧(𝑀 – 1)]𝑇 . it is assumed that the output noise z of the measuring equipment is a gaussian probability distribution with the average 𝐸{�̄�} = 0 and the known covariance matrix 𝑅𝑧 = 𝐸{�̄��̄� 𝑇 }. minimising the mean-square error by the least squares method, the values of the measured signal takes the form �̄� = (𝐻𝑇 𝑅𝑧 – 1𝐻)– 1 𝐻𝑇 𝑅𝑧 – 1�̄� . the method is quite simple and takes into account the noise assuming it is gaussian. however, we note that the measurement error increases with time, and it does not quite solve the problem of noise control. when the dynamically distorted signal is restored, the noise present in the output signal is amplified. the desire to weaken the influence of this phenomenon has led to the development of methods for regularising the solution to equation (12). the work of a. n. tikhonov [22] has been highly valuable in the development of these methods. the principle of the method for producing regularised solutions of equation (12) is as follows. let the output signal of the measuring equipment be represented in the following form: 𝑦(𝑡) = 𝑦𝑇 (𝑡) + 𝜈(𝑡), (20) where 𝜈(𝑡) is measurement noise; 𝑦𝑇 (𝑡) is an output signal of the measuring equipment without noise at its output. applying the fourier transform to equation (12), reduced to the infinite limits of integration, we can write 𝑢(𝜔) ⋅ 𝐻(𝜔) = 𝑦𝑇 (𝜔) + 𝜈(𝜔), (21) where 𝑢(𝜔), 𝑦т(𝜔), 𝜈(𝜔) is the fourier transform of the corresponding signals, and 𝐻(𝜔) is the fourier transform of the impulse transient function ℎ(𝑡 – 𝜏). 𝐻(𝜔) characterises the frequency properties of the measuring equipment. then, 𝑢(𝜔) = 𝑢𝑇 (𝜔) + 𝜈(𝜔) 𝐻(𝜔) (22) where 𝑢𝑇 (𝜔) is the fourier transform of the accurate measured signal. the application of the inverse fourier transform to equation (22) gives the formula 𝑢(𝑡) = 𝑢𝑇 (𝑡) + 1 2𝜋 ∫ 𝜈(𝜔) 𝐻(𝜔) 𝑒 −𝑗𝜔𝑡 +∞ −∞ 𝑑𝜔, (23) in which the integral diverges, as a rule. the reason for that is the presence of high frequencies in the function 𝜈(𝜔) and the excess of the degree of a denominator under the degree of a numerator in н(𝜔). the idea of the regularisation method is the suppression of the influence of high frequencies 𝜔, multiplying the function 𝑦(𝜔) 𝐻(𝜔) by a stabilising factor𝑓(𝜔, 𝛼), which depends on the parameter 𝛼 and frequency 𝜔. a. n. tikhonov [22] proposes determining the stabilising factor as 𝑓(𝜔, 𝛼) = |𝐻(𝜔)|2 |𝐻(𝜔)|2 + 𝛼𝑀(𝜔) (24) where 𝑀(𝜔) is a given even function of 𝜔. with this in mind, we obtain the classes of the regularised solutions of integral equation (12): acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 69 𝑢𝛼 (𝑡) = 1 2𝜋 ∫ 𝐻(−𝜔) ⋅ 𝑦(𝜔) 𝐻(𝜔) ⋅ 𝐻(−𝜔) + 𝛼 ⋅ 𝑀(𝜔) +∞ −∞ 𝑒 −𝑗𝜔𝑡 𝑑𝜔 . (25) in this case, one of the possible variants of the polynomial 𝑀(𝜔) takes the form 𝑀(𝜔) = ∑ 𝑞𝑛 𝜔 2𝑛 𝑝 𝑛=0 (26) where 𝑞𝑛 is a given non-negative constant. it is suggested that the value of parameter 𝛼 should be found according to ‘discrepancy’. let us identify the limiting error 𝛿 of determining the output signal of the measuring equipment 𝑦(𝑡). let us define the ‘discrepancy’ of the approximate solution: 𝜌(𝛼) = ∫ [∫ ℎ(𝑡 − 𝜏)𝑢𝛼 (𝜏)𝑑𝜏 − 𝑦(𝑡) 𝑡 𝑜 ] +∞ −∞ 2 𝑑𝑡 . (27) we choose a monotonic sequence of numbers 𝛼0, 𝛼1, . . . , 𝛼𝑛 , e.g. a geometric progression. for each value of 𝛼𝑖 , the ‘discrepancy’ 𝜌(𝛼𝑖 ) is calculated. as the desired value of 𝛼, we take such 𝛼𝑘, for which the equality 𝜌(𝛼𝑘 ) = 𝛿 is fulfilled with the required accuracy. the approximate solution of this equation can be found with the help of other numerical methods; for example, newton methods [23], [24]. in [24], an optimal regularised solution of integral equation (12) is obtained when the additional information on the spectral densities of the output noise of the measuring equipment 𝑆𝜈 (𝜔) and the input signal 𝑈(𝜔) is available. this solution does not depend on parameter 𝛼  and has the form 𝑈𝑜𝑝𝑡 (𝑡) = 1 2𝜋 ∫ 𝐻(−𝜔) ⋅ 𝑦(𝜔) 𝐻(𝜔) ⋅ 𝐻(−𝜔) + 𝑆𝜈 (𝜔) 𝑈(𝜔) ⋅ 𝑒 −𝑗𝜔𝑡 +∞ −∞ 𝑑𝜔 . this result coincides with the result of applying the optimum filtration according to wiener [25], [26]. in many practical cases, the spectral density of noise of the output signal 𝑦(𝑡) can be determined according to the observation results. it is more difficult to specify the spectral density of the input signal 𝑢(𝑡). its evaluation can be obtained on the basis of a priori information about the measured signal. this complicates the task of the optimal restoration of the input signal of the measuring equipment. the impulse transient function used above is not the only characteristic that reflects the dynamic properties of the measuring equipment. the dynamic properties can also be defined by a differential equation with the corresponding coefficients; a transfer function; and a set of amplitudefrequency and phase-frequency characteristics [5]. the use of these characteristics enables the development of relevant methods for the restoration of a dynamically distorted signal or to correct dynamic errors. the use of differential equations enables the development of methods of dynamic measurements based on the results of automatic control theory. [27] presents a review of automatic control theory and the prospects of its application in measurement technology. in [28], the term ‘dynamics’ is considered from different angles in respect to the problems of metrology. the importance of studying measuring signals is noted. a classic description of dynamic systems on the basis of differential equations is given. the author pays attention to the related concepts of the properties and behaviours of systems. the concepts of the stability, stationary state, and drift of dynamic systems are also analysed in the work. the basis for applying the methods of automatic control theory to the correction of dynamic measuring errors is the similarity of structures of automatic control systems and dynamic measuring systems [32]. the classic structure of the automatic control system is illustrated in figure 13, which is a graphical representation of the system of differential equations. in figure 13, 𝑢 is an input signal of the system; 𝑥 is a state vector of the system; 𝑦 is an observation vector; and 𝐴, 𝐵, 𝐶, 𝐷 are the corresponding matrices. the required dynamic properties are provided by setting the coefficients of the feedback matrix 𝐾. in dynamic measuring systems, it is impossible to establish the feedback from the output to the input, since the input’s measured action is not available. that is why, in such systems, the possibility of reducing measurement errors is realised by means of a special filter at the output of a primary measuring transducer (sensor). in this case, the structural diagram of the dynamic measuring system is shown in figure 14. in figure 14, u is an input measured vector of the system; 𝑥𝑆, 𝑦𝑆 are state and observation vectors; 𝐴𝑆, 𝐵𝑆 , 𝐶𝑆, 𝐷𝑆 are corresponding matrices of the sensor; 𝑥𝑓 , 𝑦𝑓 are vectors of the state and observation of the filter; 𝐴𝑓 , 𝐵𝑓 , 𝐶𝑓 , 𝐷𝑓 are filter matrices of the corresponding dimension; and 𝐾 is a matrix for setting the coefficients to provide the required dynamic error. in this system, based on the similarity of the sensor and filter structures, the latter can be considered as a dynamic sensor model. therefore, dynamic measuring systems have been developed based on the models for creating automatic control systems outlined in [30]. a dynamic measuring system with modal control of dynamic characteristics has the following structure, as illustrated in figure 15. at this time, the transfer functions of sensor 𝑊𝑆 and a model of sensor 𝑊𝑀𝑆 are equal. if the output signals 𝑦𝑆 and 𝑦𝑀𝑆 are close to each other, then the input signals 𝑢 and 𝑢𝑀𝑆 will differ from each other slightly. consequently, the input signal of the model 𝑢𝑀𝑆 can be used to judge the input signal of the sensor 𝑢. this is the basic idea figure 13. structural diagram of the automatic control system. figure 14. structural diagram of the dynamic measuring system. acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 70 of the correcting the dynamic measuring error. in this system, the transfer function 𝑊𝐷𝑆(𝑝) = 𝑢𝑀𝑆(𝑝) 𝑢(𝑝)⁄ can be obtained with any given zero and poles. if 𝑊𝑆 (𝑝) = 𝑏𝑚 𝑝 𝑚 + 𝑏𝑚−1𝑝 𝑚−1 + ⋯ + 𝑏1𝑝 + 𝑏0 𝑝𝑛 + 𝑎𝑛−1𝑝 𝑛−1 + ⋯ + 𝑎1𝑝 + 𝑎0 , then the transfer function of the dynamic measuring system can be obtained by 𝑊𝐷𝑆 (𝑝) = (𝑏𝑚 – 𝑑𝑚 )𝑝 𝑚 + (𝑏𝑚−1– 𝑑𝑚−1)𝑝 𝑚−1 + ⋯ + (𝑏0– 𝑑0) 𝑝𝑛 + (𝑎𝑛−1– 𝐾𝑛−1)𝑝 𝑛−1 + ⋯ + (𝑎1– 𝐾1)𝑝 + (𝑎0– 𝐾0) where 𝑑0, 𝑑1, …, 𝑑𝑚 are configurable filter parameters 𝑊𝑓 , and 𝐾0, 𝐾1, . . . , 𝐾𝑛−1are configurable feedback vector parameters of 𝐾 [31]. the development of this method for the observed state coordinate vector is outlined in [31], [34]. if the spectral density of the measuring signal 𝑢, and the observed coordinates are known, then the structural diagram of this measuring system is shown in figure 16. if the spectral densities of the measuring signal 𝑆𝑢 (𝜔) and noises 𝑆𝑣2 1 (𝜔), … , 𝑆𝑣2 𝑚 (𝜔) in the channels of the observed state coordinates are known, then, in [33], the optimum individual characteristics of the correcting device that reduces the measuring error are obtained. dynamic measuring systems based on the iterative principle of the measuring signal recovery are given in [32], [34]. in the structure of such a measuring system, there are several sensor models. every subsequent model processes the measuring errors of the previous ones. the main property of such measuring systems is their low sensitivity to sensor noise and their ability to reduce dynamic error without the significant increase of the system bandwidth. the structural diagram of this measuring system is shown in figure 17 [29]. in [32], [35], dynamic measuring systems in which a sliding mode is implemented are observed and explained. the structures ensuring the stability of such systems are analysed. a combination of high dynamic accuracy and low sensitivity to sensor noise is shown. a structural diagram of a dynamic measuring system in sliding mode is given in figure 18. the presence of a relay element for a high-order system may cause instability of the system. to ensure stability a cascading structure of the measuring system is given in reference [32]. every subsequent cascade restores the measuring signal more accurately than the previous one. the results of the system modelling in a sliding mode with three cascades are shown in figure 19. in [32], [36], dynamic measuring systems are observed on the basis of neural network structures. the stability conditions of such systems are investigated therein. special means of ensuring the suppression of sensor noise are introduced to the system structures. the unique feature of neural network dynamic measuring systems is their ability to adapt to the type of such measuring signals and to reduce the measuring error in the multiple measuring of such signals. a feature of dynamic measuring systems developed on the basis of methods of automatic control theory is the possibility to obtain an additional channel for dynamic error estimation [32]. an error in an estimate of a dynamic error is comparable with a measuring error of the input signal of the measuring system. a structural diagram of the channel for dynamic error estimation is shown in figure 20. in [32], [37], intelligent dynamic measuring systems that can adapt their parameters to dynamic measuring errors are figure 15. structural diagram of a dynamic measuring system with modal control of dynamic characteristics. figure 16. structural diagram of a measuring system with the observed state coordinate vector. figure 17. structural diagram of an iterative dynamic measuring system. figure 18. structural diagram of a dynamic measuring system in sliding mode. acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 71 considered. measuring systems with parameter adaptation after the performance of measurements during the processing of their results are considered. measuring systems with adaptation of dynamic parameters in the process of measuring are also given. based on ideas of automatic control theory, [38] considers dynamic measuring systems from the standpoint of simultaneous measurement and observation. a structural diagram of this measuring system is shown in figure 21. here, 𝑃 is a dynamic process in the system; 𝑀𝑈 and 𝑀𝑌 are measurement processes 𝑢(𝑡) and 𝑦(𝑡) correspondingly; and 𝑥𝑜𝑏𝑠 (𝑡) is the observation process. the use of the observation process allows us to refine the model of the process p. this approach is mainly focused on multi-connected dynamic measuring systems, where u is the vector of the measured coordinates. its implementation is based on the use of the methods and results of automatic control theory, which are well developed at present [39]. this approach is promising both from the standpoint of the development of the general theory of dynamic measurements and in applications to dynamic measurements of the specific systems. another approach in dynamic measurements that considers the uncertainty of the dynamic model is based on the use of chaos theory. thus, in [40], compensation for the dynamic error is considered in the presence of uncertainty in the coefficients of the sensor dynamic model. the uncertainty of these coefficients is expressed on the basis of the method of polynomial chaos theory. the method also assumes the description of the input measured signal in an analytical way. this is its essential limitation. this method gives an estimate of the measurement error and takes into account noise at the sensor output. 6. identification of the dynamic characteristics of a measuring equipment concerning the identification of dynamic characteristics and methods of solving this problem, measuring equipment does not differ from automatic control systems. for automatic control systems, a great many identification methods have been developed [41]. however, these methods do not pay much attention to the problem of accuracy in the identification of dynamic characteristics. however, they are an important feature of measuring equipment. the approach of determining dynamic characteristics with an estimate of their accuracy was proposed by v. a. granovskii [5]. the determination of dynamic characteristics requires the formation of test signals (figure 22). on a number of test signals, he identified errors in the determination of the transient characteristics, impulse response, and frequency characteristics. he proposed a method for the multiple integration of differential equations of the measuring equipment, which helps to determine the coefficients of differential equations and errors in their determination for the input test signals of an arbitrary shape. he developed an adaptive method for the identification of dynamic characteristics. this method determines the transfer function by the subsequent complication of the transfer function, step by step, and identifies optimal coefficients at every step. based on the minimisation of integral quadratic error, this method makes use of five types of test signals: stepwise; exponential; and three impulse signals of triangular, cosine, and sinusoidal forms. the determination of impulse and transfer functions by means of statistical regularity is outlined in [42], [43]. in fact, determining the dynamic characteristics of measuring equipment is not an easy task, especially for multi-axis sensors and multichannel measuring systems. quite often, the test equipment is a complicated facility. in the vast majority of cases, the specification of requirements for the test equipment is made by means of the expert method, and the accuracy of the identification of dynamic characteristics is not estimated. there are several specific examples of the determination of dynamic characteristics, given below. in [44], the technology for the determination of dynamic characteristics for a six-component force-and-moment sensor is considered. eigen frequencies for each of the six coordinates are considered to be such characteristics. the study offers two methods of obtaining these frequencies. the computational method uses an ansys software simulation. the experimental method measures the frequency of natural oscillations after impulse force. in [45], the method used for the dynamic calibration of the force measurement devices is considered. in this case, a figure 20. structural diagram of a channel for dynamic error estimation. figure 19. results of modelling a tri-cascade dynamic measuring system in sliding mode. figure 21. structural diagram of the measuring process and observation. figure 22. structural diagram of the equipment used to identify dynamic characteristics. acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 72 standard force dynamometer and force dynamometer to be tested connected in a series through the load frame in the general structure are used. by exciting the oscillations of this structure, the frequency response of the force measuring device is defined, including the natural oscillation frequency. in [46], the experimental determination of the dynamic characteristics of the mechanical system is considered. based on measurements with accelerometers and interferometers, the amplitude-frequency and phase-frequency characteristics of the system along the axes are determined. frequency characteristics of noise in a mechanical system are also estimated. in [47], the influence of a dynamic effect on the process and accuracy of a flowmeter calibration is considered. the dynamic properties of flowmeters are given in the form of a delay in the signal for converting the measuring flow signal into an electrical signal. the study proposes that the model of dynamics is described by the equation of the first order. its parameters are determined in the process of calibration. this reduces the measurement uncertainty of the flowmeter. in [48], the calibration method of a torque measuring transducer operating in dynamic modes is considered. this method is based on an accurate estimate of mass angular acceleration with a known momentum of inertia. the researchers in [49], [50] consider the determination of the frequency characteristics of the kernel of the volterra equations for singleand multi-channel dynamic systems. this method is based on the use of the fourier transform when applying known harmonic (sinusoidal) signals to the input of the dynamic system. this method identifies the frequency characteristics of a dynamic system with non-linearities. this method is pointed to the application in dynamic measuring systems to measure the electrical signals in a broad band. the method of dynamic calibration of a high-pressure sensor based on a quasi- function is considered in [51]. this signal is given in the form of a half sine wave with the duration of several microseconds, µs. using the fast fourier transform of the input quasi -signal f(t) and the output signal y(t): 𝐹(𝜔) = fft[𝑓(𝑡)], 𝑌(𝜔) = fft[𝑦(𝑡)] and the normalised frequency response of a pressure sensor is determined experimentally 𝐻(𝜔) = | 𝑌(𝜔) 𝑌(0) 𝐹(𝜔) 𝐹(0) | . the quasi 𝛿-impulse is created by the bullet going through the hopkinson bar. the researchers in [52] consider the behaviour of a dynamic measuring system in the neighbourhood of a user-specified frequency. a method of approximation of a frequency response in the neighbourhood of the frequency by polynomials is proposed. the polynomial approximation takes into account the non-zero initial conditions of a dynamic system. however, in this case, it does not take into account disturbances and noise in the system, considering that at frequency response approximation, the input and output signals of the measuring system are known. the error of the polynomial approximation is also studied. this method is pointed to the analysis of the behaviour of a dynamic measuring system concerning the measurement of harmonic (sinusoidal) signals in the neighbourhood of a certain frequency. the creation of mathematical models of measuring equipment that adequately reflects the properties has made it possible to develop measuring equipment with optimal characteristics and to obtain higher measurement accuracy when processing measuring results. this approach is implemented in mathematical models, and software is created in npl. thus, in [53], one can see the application of the method of mathematical simulation to analyse dynamic measurements executed in npl. the first example demonstrates the mathematical model of a lock-in amplifier. the comparison between the operation of a lock-in amplifier in a real experiment and the results of the simulation reveals sources of additional error. the second example considers simulation of an underwater acoustics network of three hydrophones. the case is considered when one of the sensors has a smaller bandwidth than the others. the software helps to simulate the operation of this network under various conditions. the third example considers an imperfect clock with a quartz oscillator. the accuracy of the temporal data fit has a direct influence on dynamic error. based on the monte carlo method, this software is used in the simulation of the long-term behaviour of the timing device, taking into account long-term drift and jitter within the measurement data. the fourth example presents a shock tube as a means of generation of calibration pressure signals with a broad bandwidth. the effects in a shock tube are simulated on the basis of wave propagation in a shock tube. the simulation enables the determination of the optimal parameters of a shock tube. [81] is devoted to the development and application of a new open-source software package for the analysis of dynamic measurements. at the same time, in many publications, the question of the determination of dynamic characteristics and measurements in different problems is analysed. in [54], [55], the method of autoregressive analysis of the spectrum of the output signal is used to determine the dynamic characteristics of the pressure transducer (the transfer function in the frequency domain and the transient function in the time domain). in this case, the spectral analysis is undertaken on the basis of the fourier transform of the transducer response to a step change in the pressure signal. the results of the experimental studies show good correspondence between the characteristics of the obtained dynamic models and real transducers. a generator of dynamic pressure with a rotating valve is proposed in [56]. the methods of frequency response estimation and the speed of the pressure transducer are developed on the basis of an analysis of the transducer’s reaction to the generated signal as a series of rectangular pressure impulses with variable frequencies and amplitudes. experimental studies have shown good repeatability of the amplitude spectrum of the generated signal and the possibility to compare the frequency responses of two pressure transducers while simultaneously observing their output signals. in [57], a vibration pressure transducer with a frequency output is considered as the object of analysis. to determine the dynamic characteristics of a transducer, the method of parametric identification is used with a harmonic input action and a controlled average value and amplitude. when modelling the measuring system, the method of computational hydrodynamics is used. the results of the modelling allow us to reveal the effects during the operation of the given pressure transducer, which influence dynamic characteristics a great deal. acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 73 in [58]-[64], methods of developing the variable effects of different types of dynamic calibration of pressure transducers are considered. in [58] and [59], generators of impulse pressure in the form of a short sinusoid half-wave are presented. the principle of operation of the first thereof is the use of the system with drop mass, and the second is based on the use of the system with a hopkinson bar and a piston gauge. in [60] and [61], the system of calibration with the generator of a periodically varying pressure in the form of a double-acting pneumatic actuator is considered. methods of dynamic calibration that are based on the use of a shock tube as a generator of step-change pressure are studied in [62]-[64]. dynamic calibration with the use of one or another calibration action involves the determination of the dynamic characteristics of the resulting impulsive, amplitude-frequency, or transient functions of pressure transducers. an experimental method of the determination frequency response of a pressure sensor of the second order is considered in [64]. the experimental installation generates the input step signal. according to the transient process, the parameters of transfer function are determined, and the frequency characteristics are determined from them. the methods used for the dynamic calibration of different types of conditioning amplifiers are presented in [82]. the necessity for the dynamic measurement of pressure arises at low [65] and high [66] temperatures of gases and liquids [67]. there are also examples of the use of the kalman filter to determine the parameters of a dynamic measuring system. the researchers in [68] consider the method of dynamic weighing with letter scales. for the model of scales of the second order, the coefficients of the difference equations of weights and the weight of an object are determined with the kalman filter. the solution to dynamic measurement problems in specific applications is characterised by the great variety thereof and particular methods of resolving them. the correction of a dynamic error is undertaken on the basis of setting acceptable dynamic properties for a specific problem. [69] considers the correction of a dynamic error of an auxiliary wale heat flux sensor. this sensor is described by the differential equation of the first or second order. the correction of a dynamic error is performed by the inverse transfer function, the parameters of which are selected according to an acceptable dynamic error. there are studies wherein analysis of the dynamic properties of measuring equipment helps to develop the optimal structure of the measuring equipment. there are studies that found that the dynamic measuring equipment has nonlinearities. in [70], an approach to develop magnetic-field sensors using a nonlinear dynamic equation describing the operation of the sensors is considered. the basic dynamic equation is the following: 𝜏 𝑑𝑥 𝑑𝑡 = −𝑥 + tanh [ 𝑥 + ℎ(𝑡) 𝑇 ] , where 𝑥 is the strength of a magnetic field; ℎ(𝑡) is an external magnetic field; 𝜏 is the system time constant; and 𝑇 is the temperature. on the basis of an analysis of this equation (2), states of the sensor having minimum potential energy are identified. this essentially nonlinear differential equation is used to create an optimal sensor circuit. in [71], dynamic measurements of the composition of gases in chemical industry are considered. chemical sensors are characterised by inertia and have nonlinear statistical characteristics. the following inverse model of a dynamic measuring system is given. 𝑢[𝑛 − 𝑘] = ∑ 𝐵𝑗 (𝑞)𝑦[𝑛] 𝑛 𝑗=1 + 𝑐[𝑛] , where 𝐵𝑗 (𝑞) is a polynomial in the delay operator 𝑞 (𝑞 𝑢[𝑛] = 𝑢[𝑛 − 1]), 𝑢[𝑛] and 𝑦[𝑛] are output and input signals of the inverse system, respectively, and 𝑐[𝑛] is a residual term. the parameters of the model are determined by minimising the measuring error in the test experiments. the studies on this topic also give information about completely unexpected effects in dynamic measurements. in [72], four algorithms for the compensation for static measurement error in the operation of the measuring device in dynamic mode are considered. these algorithms are based on the linear statistical characteristics of the measuring device, with the following methods: the piecewise linear interpolation compensation algorithm, polynomial progressive compensation algorithm, improved polynomial progressive compensation algorithm, and artificial neural network compensation algorithm. there are no parameters of dynamic characteristics in the algorithms. testing them in dynamic mode has shown that they have quite certain frequency characteristics. in this case, the bandwidth of the first three algorithms is approximately the same, but the bandwidth of a neural algorithm is slightly larger. [73] considers the system of pipelines in the form of a set of concentrated masses described by the system of differential equations. the problem of determining the frequencies of the oscillations of the system is solved by the method of the theory of inverse problems solution, with boundary conditions at the ends of the pipeline. [74] is devoted to the use of the bayesian estimation problem to measure spatial coordinates of the objects. here, in the general case, a discrete variant of a nonlinear dynamic system is considered, in which a coordinate vector of the state to be measured is connected with a vector of the observed coordinates by the known model. the technique used for computing spatial coordinates is traditional and requires the knowledge of their probability density function and noise probability densities. [75] studies the rheological properties of drops by means of experimental installation with a capillary, piezo translator, and pressure sensor. a particular characteristic of the experimental installation is obtained. the results of the research are obtained on the basis of the fourier transform of pressure sensor signals accounting for the particular characteristic of the experimental installation. in [76], the properties of thermal electric transducers are studied by measuring their electrical resistance at various frequencies. the dynamic model of an integrated electrical resistance is given. the natural frequencies of this model’s poles provide information on the thermal electrical transducer. although the problem of restoring a dynamically distorted signal or correcting a dynamic progression is not solved here, the studies conducted are based on an analysis of the dynamic properties of the object. acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 74 the researchers in [77] propose a method of estimation of square error criterion for measuring the signals by an accelerometer with a dynamic model of the second order. it is shown therein that the maximum error occurs when measuring triangular and trapezoidal signals. for measuring signals of different forms, the methods of their approximation are given on the basis of a genetic algorithm to obtain the maximum quadratic measuring error of these signals. the authors suggest using this method to calibrate accelerometers in dynamic measurement. [78] considers the design of a dynamic compensator for a second-order temperature sensor. the compensator parameters of the second order are determined by the kalman filter adapted to evaluate the parameters of a dynamic compensator. the problem of reducing a dynamic error in the measurement of forces by the tri-component dynamometer of a micro-machine is considered in [79]. at the initial stage, the matrix of the frequency characteristics of the tri-component dynamometer is identified accounting for the cross links on the measured coordinates. compensation for a dynamic error is performed on the basis of multiplying the observed force vector by the inverse matrix of the frequency response. in the article, the problem of the regularity of the inverse matrix and suppression of the influence of high-frequency noise when it is reversed is not discussed. 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[82] l. klaus, t. bruns, h. volkers, calibration of bridge-, charge and voltage amplifiers for dynamic measurement applications, metrologia 52, 1 (2015), 72– 81. wos:000348119700011 http://apps.webofknowledge.com/daisyoneclicksearch.do?product=wos&search_mode=daisyoneclicksearch&colname=wos&sid=f3whqdxfsinsap6akgo&author_name=eichstadt,%20s&dais_id=1873439&excludeeventconfig=excludeiffromfullrecpage http://apps.webofknowledge.com/daisyoneclicksearch.do?product=wos&search_mode=daisyoneclicksearch&colname=wos&sid=f3whqdxfsinsap6akgo&author_name=elster,%20c&dais_id=168540&excludeeventconfig=excludeiffromfullrecpage http://apps.webofknowledge.com/daisyoneclicksearch.do?product=wos&search_mode=daisyoneclicksearch&colname=wos&sid=f3whqdxfsinsap6akgo&author_name=smith,%20im&dais_id=125538&excludeeventconfig=excludeiffromfullrecpage http://apps.webofknowledge.com/daisyoneclicksearch.do?product=wos&search_mode=daisyoneclicksearch&colname=wos&sid=f3whqdxfsinsap6akgo&author_name=esward,%20tj&dais_id=1381882&excludeeventconfig=excludeiffromfullrecpage http://apps.webofknowledge.com/full_record.do?product=wos&search_mode=generalsearch&qid=4&sid=f3whqdxfsinsap6akgo&page=3&doc=25 http://apps.webofknowledge.com/full_record.do?product=wos&search_mode=generalsearch&qid=4&sid=f3whqdxfsinsap6akgo&page=3&doc=25 javascript:; experimental research on mass determination of non-metal weights acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 23 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 23 26 experimental research on mass determination of non-metal weights m. hu, j. wang, c. cai, r. zhong, k. jiao, d. wu, p. cheng national institute of metrology, beijing, china, hmh@nim.ac.cn abstract: with the revision of the definition of the kilogram, more and more weights made of non-metal material, such as silicon spheres and pm 2.5 film, need to be determined with high accuracy. as conventional mass is commonly used in mass metrology, this paper discusses the mass-measuring process for silicon and pm 2.5 film. electrostatic eliminators are used to eliminate the electrostatic effects of non-metal material to improve the stability of mass measurement. due to the big difference of weights’ densities, the air buoyancy correction and its uncertainty evaluation are also conducted. keywords: mass determination; non-metal weights; uncertainty evaluation; silicon sphere 1. introduction there are more and more demands in mass metrology for mass measurement of non-metal material [1-4]. as conventional mass is commonly used in mass dissemination of mass metrology, according to oiml r111 [5], altitude and corresponding changes in air density can affect the measurement process when using the conventional mass of weights. the mass of test weight, 𝑚t, can be expressed by equation (1): 𝑚t = 𝑚r + (𝑉t − 𝑉r) × 𝜌a + ∆𝐼 × 𝑚cs ∆𝐼s⁄ . (1) in which 𝑚t and 𝑚r are the mass of test weight and reference weight; 𝑉t and 𝑉r are the volume of test weight and reference weight; 𝜌a is density of moist air; ∆𝐼 is indication difference of the balance; 𝑚cs is the conventional mass of the sensitivity weight; and ∆𝐼s is the indication difference of the balance when the sensitivity weight is put on the weighing pan. thus electrostatic effects and air buoyancy correction become the two most important aspects for the determination of mass object of non-metal material. to evaluate electrostatic effects and air buoyancy correction to the stability of weighing process of non-metal weights, a silicon sphere of 1 kg and pm 2.5 film are used to experimentally evaluate the standard deviation of weighing process. the pm 2.5 film is tested for its common use and need to be weighed with high accuracy in determination of air pollution. the air buoyancy correction is also investigated and the uncertainty evaluation is also conducted. 2. description of the work m-one mass comparator is used for mass determination below 1 kg. with 6 weighing positions, the electronic weighing range is (0 ~ 1.5) g and readability is 0.1 µg. according to air density calculation formula of cipm 2007, the density of moist air 𝜌a can be expressed as: 𝜌a = (𝑝𝑀a 𝑍𝑅𝑇⁄ ) × [1 − 𝑥v(1 − 𝑀v 𝑀a⁄ )] . (2) where: 𝑝 is the air pressure; 𝑀a is the molar mass of dry air; 𝑍 is the compressibility; 𝑅 is the molar gas constant; 𝑇 is the dynamic temperature using its-90; 𝑥v is the mole fraction of water vapour; and 𝑀v is the molar mass of water. figure 1: picture of new m-one prototype mass standard comparator the air density measuring system, developed by nim, china, can measure the pressure, temperature, relative humidity, and carbon dioxide content, as shown in figure 2. the mass comparator and pm 2.5 film are shown in figure 3. figure 2: pictures of air density measurement system http://www.imeko.org/ mailto:hmh@nim.ac.cn acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 24 table 1: uncertainty budget for the density of moist air source xi standard uncertainty value u(xi) sensitivity coefficient ci uncertainty contribution ui(ρa) pressure, 𝑝 1.2 pa 1 × 10-5 kg·m-3·pa-1 1.2 × 10-5 kg·m-3 temperature, 𝑡 0.007 k -4 × 10-3 kg·m-3·k-1 2.8 × 10-5 kg·m-3 relative humidity, ℎ𝑟 0.008 -9 × 10-3 kg·m-3 7.2 × 10-5 kg·m-3 carbon dioxide content, 𝑥co2 0.000 003 4 0.4 kg·m -3 1.3 × 10-6 kg·m-3 cipm 2007 formula, 𝑢f 22 × 10 -6 kg·m-3 combined uncertainty of air density u(ρa) 8.3 × 10-5 kg·m-3 u(ρa) (k = 2) 1.7 × 10 -4 kg·m-3 figure 3: pictures of mass comparator, stainless steel standard weights and pm 2.5 polypropylene film 3. results and discussion according to equation (2), the uncertainty of air density, 𝑢𝜌a , can be expressed as: 𝑢𝜌𝑎 = √ 𝑢f 2 + ( 𝜕𝜌a 𝜕𝑝 𝑢𝑝) 2 + ( 𝜕𝜌a 𝜕𝑡 𝑢𝑡 ) 2 +( 𝜕𝜌a 𝜕ℎ𝑟 𝑢ℎ𝑟 ) 2 + ( 𝜕𝜌a 𝜕𝑥co2 𝑢𝑥co2 ) 2 (3) in which: 𝑢f = 22 × 10 −6𝜌a , 𝜕𝜌a 𝜕𝑝 = 10−5𝜌a pa −1 𝜕𝜌a 𝜕𝑡 = −4 × 10−3𝜌a k −1 𝜕𝜌a 𝜕ℎ𝑟 = −9 × 10−3𝜌a , 𝜕𝜌a 𝜕 2co x = 0.4𝜌a the uncertainty budget for the density of moist air is shown in table 1. the density of stainless steel weights is around 7950 kg·m-3. the density of pm 2.5 film made of teflon is 2300 kg·m-3. according to equation (4), for the difference density and volume, the true mass of pm 2.5 film due to air buoyancy correction is around 59 μg, when comparing with stainless steel weights with density around 7950 kg·m-3. 𝑚ct = 𝑚cr + (𝑉t − 𝑉r) × (𝜌a − 𝜌0) ± ∆𝐼 × 𝑚cs ∆𝐼cs (4) in which 𝑚ct and 𝑚cr are the conventional mass values of the test and reference weights respectively, 𝜌0 is the reference air density value of 1.2 kg·m -3, and ∆𝐼cs is the change in indication of the balance due to the sensitivity weight. figure 4: volume difference of stainless steel weights and teflon ptfe film (pall) of 160 mg when the air density is around 1.1834 kg·m-3, the air buoyancy correction is around 0.8 μg. however, due to the air buoyancy effect, when measuring the mass of the non-metal material such as teflon film, the mass deviation of 6 μg is observed between the abba weighing cycles, which can be seen in figure 5. when measuring the mass of silicon sphere using stainless steel weights, same mass deviation is also be observed. for the 1 kg silicon sphere, the density of silicon is (2320 ~ 2340) kg·m-3, the air buoyancy for the true mass is around 370.795 mg. while measuring in air with density of 1.1834 kg·m-3, the air buoyancy correction for the conventional mass can be 5.1 mg, which means that the air density including the air temperature, relative humidity, air pressure and contents of co2 need to be determined with high accuracy for the air buoyancy correction. stainless steel sheet weights teflon membrane stainless steel weights pm 2.5 film xpr comparator http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 25 table 2: uncertainty budget for pm 2.5 teflon membrane source of uncertainty standard uncertainty sensitivity coefficient uncertainty contribution / mg standard uncertainty of average (�̅�) 0.001 mg 1 0.001 standard uncertainty of 160 mg standard 0.001 mg 1 0.001 volume of stainless steel weights 0.000 2 cm3 1.1834 mg·cm-3 0.000 237 volume of teflon 0.000 2 cm3 1.1834 mg·cm-3 0.000 237 air density 0.000 083 mg·cm-3 0.0494 cm3 0.000 004 1 balance linearity 0.000 000 7 mg 1 0.000 000 7 combined standard uncertainty 0.001 5 expanded uncertainty (k = 2) 0.003 0 figure 5: mass difference of teflon material between abba weighing cycles due to the air buoyancy correction as shown in figure 6, the electrostatic eliminator is used to decrease the influence of electrostatic effects. 11 abba weighing cycles are carried out between stainless steel weights and silicon sphere. figure 6: volume difference of different weights between stainless steel weights and silicon sphere weights figure 7: mass difference of silicon sphere between abba weighing cycles due to the air buoyancy correction as shown in figure 7, the mass difference between different weight cycles can be up to 13.5 μg which were carried out over a period of 48 hours in the laboratory. the uncertainty of air buoyancy correction due to volume difference between stainless steel weights and the non-metal material of pm 2.5 teflon film are listed in table 2. due to the high accuracy determination and relative small nominal mass value and volume difference between teflon membrane and stainless steel weights, the uncertainty contribution of air buoyancy correction is only 0.004 μg. the expanded uncertainty (k = 2) for the conventional mass of pm 2.5 teflon film is 3 μg. 4. conclusion for mass determination of non-metal weights, it is important to ensure the limitation of electrostatic effects. for that conventional mass is commonly used in mass metrology, this paper discusses about mass measuring process for silicon and pm 2.5 membrane. electrostatic eliminators are used to eliminate the electrostatic effects of non-metal material to improve the stability of mass measurement. due to the big difference of density of weights, the air density including the air temperature, relative humidity, air pressure and contents of co2 need to be determined with high accuracy for the air buoyancy correction, the air buoyancy correction and its uncertainty evaluation are also conducted. 5. acknowledgement this research is supported by national key research and development program for national quality infrastructure under grant 2017yff0205006. 6. references [1] m. gläser, m. borys, “precision mass measurements”, rep. prog. phys., vol. 72, no. 12, 126101, 2009. [2] m. borys, m. glaser, m. mecke, “mass determination of silicon spheres used for the avogadro project”, measurement, vol. 40, pp. 785-790, 2007. -25 -20 -15 -10 -5 0 5 10 15 20 25 1 2 3 4 5 6 mass difference between two abba weighing cycles μg 14.818 14.820 14.822 14.824 14.826 14.828 14.830 14.832 14.834 14.836 0 1 2 3 4 5 6 7 8 9 10 11 silicon sphere vs stainless steel mg electrostatic eliminator http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 26 [3] p. fuchs, k. marti, s. russi, “new instrument for the study of ‘the kg, mise en pratique’: first results on the correlation between the change in mass and surface chemical state”, metrologia, vol. 49, pp. 607-614, 2012. [4] k. lehrmann, d. knopf, f. härtig, “status of the realization and dissemination of the kilogram via silicon spheres”, cpem, 2018. [5] oiml r111-1, “weights of classes e1, e2, f1, f2, m1, m1-2, m2, m2-3, and m3”, 2004. http://www.imeko.org/ editorial to selected papers from 2019 ieee international workshop on “metrology for industry 4.0 and iot” acta imeko issn: 2221-870x december 2020, volume 9, number 4, 1 3 acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 1 editorial to selected papers from 2019 ieee international workshop on “metrology for industry 4.0 and iot” leopoldo angrisani1, francesco bonavolontà1 1 università di napoli federico ii, dept. of information technology and electrical engineering, via claudia 21, 80125 napoli, italy section: editorial citation: leopoldo angrisani, francesco bonavolontà, editorial to selected papers from 2019 ieee international workshop on “metrology for industry 4.0 and iot”, acta imeko, vol. 9, no. 4, article 1, december 2020, identifier: imeko-acta-09 (2020)-04-01 editor: dušan agrež, university of ljubljana, slovenia received: december 12, 2020; in final form december 12, 2020; published december 2020 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding authors: leopoldo angrisani, e-mail: angrisan@unina.it, francesco bonavolontà, e-mail: francesco.bonavolonta@unina.it dear reader, this issue of acta imeko is dedicated to papers that have been selected from those presented at “2019 ieee international workshop on metrology for industry 4.0 and iot” held in naples in june 2019. metroind4.0&iot aims to discuss the contributions both of the metrology for the development of industry 4.0 and iot and the new opportunities offered by industry 4.0 and iot for the development of new measurement methods and apparatus. metroind4.0&iot aims to gather people who work in developing instrumentation and measurement methods for industry 4.0 and iot. the central objective of this workshop was to present the latest achievements in the new technology for metrology-assisted production in industry 4.0 and iot, industry 4.0 and iot component measurement, sensors and associated signal conditioning for industry 4.0 and iot, and calibration methods for electronic test and measurement for industry 4.0 and iot. participants have had an excellent opportunity to meet top specialists from industry and academia all over the world and to enhance their international cooperation. twenty selected papers from metroind4.0&iot 2019 are presented in two sections (part 1 and part 2). part 1 – section editor: leopoldo angrisani the first section, edited by leopoldo angrisani, includes the following ten scientific contributions. the first paper by alessandro massaro et al, “maintenance and testing protocols in railway industry”, introduces new maintenance and testing protocols regarding processes in the context of railway industry. the first protocol is general and can be applied to industries working in similar production processes. the second and the third protocols are more specific to railway industry and concern the turning machine production line and the pneumatic test of the train braking system, respectively. the study has been performed within the framework of an industry research providing the design of intelligent control and actuation systems suitable for auto-adaptive industry 5.0 facilities. the second paper by luciano fratocchi and cristina di stefano, “industry 4.0 technologies and manufacturing back-shoring: an european perspective” , aims to shed new light on the role that industry 4.0 technologies may play as a driver or an enabling factor for companies which are evaluating rsd alternatives. a two-steps explorative approach is implemented. in the first step, a theoretical approach is followed by developing a structured literature review based on 115 scopus indexed journal articles. the second step of the methodology is based on empirical evidence belonging to european countries based on the univaq manufacturing reshoring dataset. collected data provide evidence of the growing interest of scholars in this issue; however, attention has been mainly focused on two single technologies, namely production automation and additive manufacturing. in the third paper, edited by susanna spinsante et al, “a field measurements based lora networks planning tool”, after comparing different path loss models based on a field measurement campaign of lora received signal strength indicator (rssi) values within a university campus, two main modifications of the lora simulator tool are implemented. they are aimed at improving its accuracy in the prediction of the number of sustainable nodes, according to the target data extraction rate set. simulations based on field measurements show that by an improved path loss evaluation, and using three gateways, the number of nodes could increase theoretically from about 100 to about 6000. in the fourth paper, edited by luca di angelo et al, “an advanced gcode analyser to predict build-time in am components”, an accurate method to predict in advance the build-time in additive manufacturing (am) technology is proposed. this method is based on an advanced gcode analyser written in python following an object-oriented paradigm for scalability and maintainability. some examples were used to demonstrate the mailto:angrisan@unina.it mailto:francesco.bonavolonta@unina.it acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 2 reliability of the algorithm and possible use-cases of it are illustrated. in the fifth paper, edited by mariorosario prist et al, “cyberphysical manufacturing systems: an architecture for sensors integration, production line simulation and cloud services”, following the paradigm of new smart factories, the main challenges of the cps complexity have been faced by developing: from one hand, a new modular, extensible and interoperable software architecture based on osgi framework for managing heterogeneous devices, simulation and hil prototype; on the other hand, a cloud computing to manage and analyse data, as well as to expose business data and services via external apis interfaces. in detail, a testing of a smart production line, before the production deployment, has been developed. in the sixth paper, edited by pierpaolo fucile et al, “strategies for the design of additively manufactured nanocomposite scaffolds for hard tissue regeneration”, a systematic study on the design of pcl/ha scaffolds for hard tissue regeneration was reported. in particular, 3d pcl/ha scaffolds were designed and analysed according to a strategy already reported for additive manufacturing of pcl scaffolds involving techniques based on extrusion/injection methods. the procedure was extended to pcl/ha considering that the difficulties in processing nanocomposite materials are usually greater than those found for the neat polymers. benefiting from all the results, as well as from the reverse engineering approach, the feasibility to design customized scaffolds for mandibular defect regeneration (i.e., ramus and symphysis) was reported. in the seventh paper, salvatore surdo et al, “acoustically-shaped laser: a machining tool for industry 4.0”, present a method to parallelize laser direct-write systems (ldws) by using acoustically shaped laser light. they use an acousto-optofluidic (aof) cavity to generate acoustic waves in a liquid causing periodic modulations of its refractive index. such acoustically controlled optical medium diffracts the incident laser beam into multiple beamlets that, operating in parallel, result in enhanced processing throughput. in addition, the beamlets can interfere mutually, generating an intensity pattern suitable for processing an entire area with a single irradiation. their results demonstrate the aof technology can broaden the usage of lasers as machine tools for industry 4.0. in the eight paper, edited by domenico solari, “novel concepts and strategies in skull base reconstruction after endoscopic endonasal surgery”, an insight into the development of novel strategies and devices for skull base defects was provided by integrating rheological/mechanical concepts, image capture and analysis techniques, cad-based approach and additive manufacturing. specifically, a systematic study on the design of multifunctional systems in the form of injectable tools and “solid” customized devices was reported in the current research. the focus was set on the importance of viscoelastic properties and flow behaviour of materials in the case of injectable systems, as well as on the potential to start from the geometry of skull base defects for designing additive manufactured closure devices with tailored properties (i.e., flexibility, strength) for skull base reconstruction after endoscopic endonasal surgery. the ninth paper, edited by nicola rocco et al, “additive manufacturing and technical strategies for improving outcomes in breast reconstructive surgery”, dealt with challenges, principles and methods to develop 3d additive manufactured structures in breast reconstructive surgery. specifically, the aim was to design 3d additive manufactured poly (ε-caprolactone) scaffolds with different architectures (i.e., lay-down patterns). preliminary mechanical and biological analyses showed the effect of the laydown pattern on the performances of the manufactured structures. the last paper of the part 1, edited by castrese di marino et al, “a new approach for an anthropocentric design of human-robot collaborative environment”, deals with collaborative robotics by highlighting the main issues linked to the interaction between humans and robots. a critical study of the in-force standards on human-robot interaction (hri) and the current principles of the workplace design for human-robot collaboration (hrc) are presented. the paper focuses on an anthropocentric paradigm, where the human becomes the core of the workplace in combination with the robot, and it presents a basis for designing workplaces through two key concepts: (i) the introduction of human and robot spaces as elementary spaces, (ii) the dynamic variations of the elementary spaces in shape, size and position. according to this approach, the limitation due to the safety-based approach, introduced by the standards, are overcome by positioning the human and the robot inside the workplace and managing their interaction through the elementary spaces. part 2 – section editor francesco bonavolontà the second section, edited by francesco bonavolontà, includes the remainder ten papers. the first paper, edited by marianne silva et al, “performance evaluation of a vehicular edge device for customer feedback in industry 4.0”, investigates a performance evaluation of an edge obd-ii device which collects data from vehicles in an autonomous way in order to provide customer feedback and tracking. different sets of obd-ii parameter ids (pids), responsiveness, driver behaviour and co2 pollution estimate were the metrics evaluated. for validation purposes, the results obtained from vehicles were compared with an obd-ii emulator which demonstrated the accuracy of experiments. the second paper, edited by giulio d’emilia et al, “uncertainty assessment for measurement and simulation in selective laser melting: a case study of an aerospace part”, analyses the additive manufacturing process slm with the aim to realize a complex piece for aerospace applications. in particular, the effect of the manufacturing process and of the following thermal treatments on the dimensions of the workpiece is evaluated. the study is based on a hybrid approach including the whole manufacturing process simulation by advanced software packages and dimensional measurements by coordinate measuring machine (cmm) of the realized pieces. enrico petroli and fabio leccese, in the third paper, “precise takagi-sugeno fuzzy logic system for uav longitudinal stability: an industry 4.0 case study for aerospace”, show how the architecture of the drone based on a "flying wing” allows discharging the complexity of the longitudinal stability equations (critical for that type of architecture). they used a very reliable simplification due to the fuzzy logic. their approximate method allows them to have a very "light" calculation effort at the price of a negligible error in terms of the size and dynamics of the vehicle so lowering the inescapable activities of the telecommunication segment that manages the departing and landing manoeuvres. in the fourth paper giovanni bucci et al, “an iot system based on powerline technology for pulse oximetry measurement”, propose the use of the powerline network as an alternative to the realization of iot systems. in particular, an application case of use an iot system for monitoring of pulse oximeter values in a hospital setting will be described and discussed. the novelty of the acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 3 proposed system is the adoption of the powerline communication in the medical environment. the fifth paper, edited by marco carratù et al, “smart wearable devices for human exposure vibration measurements on two-wheel vehicles”, discloses an original solution to allow the driver of motorcycles to know the exposure to the vibration during a ride using a lowcost wearable device (smartwatch). a suitable measurement system has been designed and tested on a real motorcycle. different indexes proposed in the literature are adopted for the comfort analysis. they are experimentally compared with the hand transmitted vibrations measured trough a fixed accelerometer according to show the feasibility of the proposed approach in typical application conditions. in the sixth paper, daniel chicayban bastos et al ,“on pseudorandom number generators”, present a new result which leads to recommend against the use of the nist sp 800-22 statistical test suite for testing random number generators, which has the practical impact for anyone using random number generators, thus anyone making direct or indirect use of computer sampling or simulations. the nist sp 800-22 statistical test suite, last revised in 2010, was designed to test and reject random number generators with poor randomness. being a very popular software package, researchers using this software package may be getting a false sense of security. the section “on the insufficiency of the nist sp 800-22 package” proves nist sp 800-22 software package is not correctly implemented and shows all the details necessary for reproducing the experiment. the seventh paper by andrea rega et al, “a sensor data fusionbased locating method for large-scale metrology”, tackles the problem of big-size object measurement proposing a method to correctly locate and track a handling device for reverse engineering systems. the method aims to improve accuracy of point cloud realignment tracking a reverse engineering system which jointly moves with a handling device. the proposed method is based on sensor data fusion of redundant sensors by means the kalman filter. the method was verified through hardware-in-the-loop simulation, using an arduino uno rev3 board programmed in matlab/simulink® environment. the results show that the sensor data fusion accuracy in better than the simulated single sensors. in the eight paper, r. la rosa et al, “an integrated circuit to null standby by using energy provided by mems sensors”, addressed a “zero-energy standby” solution which is able to supply the power requested by the measurement equipment to turn on appliance only when needed. in particular, an integrated circuit (ic) solution suitable to be used with mems scale transducers has been pursued. both the mems transducer and the integrated conditioning circuit have been simulated, designed realized and characterized, so that the “zero-energy standby” solution has been experimentally studied by using the realized devices. in the ninth paper, giovanna di pasquale et al, “geometrical and thermal influences on a bacterial cellulose based sensing element for acceleration measurements”, investigate the influence of the geometry and of the environmental temperature of bc based sensing elements. more specifically, the influences of such quantities on a previously investigated bc-based accelerometer are reported. an experimental campaign and the characterization of the proposed green device for several geometries and various temperatures has been addressed obtaining very intriguing results. the last tenth paper of the part 2, edited by michela borghetti et al, “validation of a modular and wearable system for tracking fingers movements”, shows a protocol for validating a new wearable system for tracking finger movements. the wearable system consists of two measuring modules worn on the thumb and on the index measuring flexion and extension of the proximal interphalangeal (pip) joint by a stretch sensor and rotation of the proximal phalanx (pp) by an inertial measurement unit. a marker-based opto-electronic system is used to validate the proposed device by capturing specific finger movements. four movements that simulate typical tasks and gestures, such as grasp and pinch, were specifically performed. the results are useful for the data interpretation when the system is adopted for monitoring finger movements and gestures. the proposed system equipped with imus and stretch sensors could be adopted for monitoring finger movement tasks in a variety of conditions, also in an industrial setting, for example, in evaluating workers’ tasks to improve their efficiency and safety and for virtual reality. it was a great honour for us to act as guest editors for this issue of acta imeko, both from the perspective of working for a high-profile scientific journal. we would like to sincerely thank all the authors for their valuable contributions, and we hope the readers could be inspired by the themes and proposals that have been elected and included in this special section related to innovations in metrology for industry 4.0 and iot. leopoldo angrisani, francesco bonavolontà guest editors acta imeko, title acta imeko issn: 2221-870x june 2018, volume 7, number 2, 16-23 acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 16 using local redundancy to improve gnss absolute positioning in harsh scenario antonio angrisano1, silvio del pizzo2, salvatore gaglione3, salvatore troisi3, mario vultaggio1 1g.fortunato university, benevento, italy 2inav geomatics solutions, massa lubrense (na), italy 3parthenope university, naples, italy section: research paper keywords: gnss; weighting schemes; redundancy matrix; local redundancy citation: antonio angrisano, silvio del pizzo, salvatore gaglione, salvatore troisi, mario vultaggio, using local redundancy to improve gnss absolute positioning in harsh scenario, acta imeko, vol. 7, no. 2, article 4, june 2018, identifier: imeko-acta-07 (2018)-02-04 section editor: fabio leccese, università degli studi di roma tre, italy received january 15, 2018; in final form march 26, 2018; published june 2018 copyright: © 2018 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: antonio angrisano, e-mail: : a.angrisano@unifortunato.eu 1. introduction in gnss context, scenarios characterized by bad signal quality and/or bad satellite geometry, usually due to natural or artificial obstacles, are considered harsh environments. mountainous or highly urbanized areas are examples of harsh environments. in urban scenarios, multipath interference and non-line-of-sight (nlos) phenomena typically affect the gnss signals. the multipath interference happens when a signal is received through multiple paths; the combination of multiple signals causes a distortion of the correlation function (between received and locally generated signals) and yields a range error up to several tens of meters [1]. the nlos reception happens when the direct signal is blocked and only reflected signals are received; this phenomenon is very common in urban scenario, yielding range errors of even km order [2]. the nlos and multipath phenomena are even more widespread in high sensitivity receivers, which are able to receive very weak signals. absolute positioning, also called single point positioning (spp), is the most common gnss operational mode. in fact, spp mode is usually adopted by mass-market receivers, which are mounted on tablet, smartphones, car navigation devices, uavs [3]-[5]; on these devices, spp is often combined with other information sources, such as maps, inertial sensors or vision systems [6], [7], [8]. the aforementioned phenomena are responsible for the presence of blunders among the measurements, which can cause large position error. two approaches are usually adopted to cope with this problem: abstract in gnss context, absolute positioning is a widespread operational mode, largely used in many field activities as automotive, aerospace and ships navigation. the functional model of absolute positioning, relating pseudorange measurements to unknowns, is well defined, while its stochastic model, defining the behavior of measurement error, is currently under investigation and is the core of this work. a realistic model of measurement error can be used to define a weighting scheme, able to reduce the positioning errors; a weighted approach is mainly necessary in signal-degraded scenario, where pseudorange accuracies are significantly different and equally weighting all the measurements would yield very large errors. the quality indicators, commonly adopted to define a pseudorange error model, are the signal-to-noise ratio and the satellite elevation angle. in this work, an additional indicator, the redundancy number, is introduced. the redundancy numbers are the diagonal elements of the redundancy matrix and they represent the degree of controllability of the measurements; a large value of the redundancy number corresponds to a well-controlled measurement, while a small one corresponds to a leverage observation, with a high potential to influence the solution. the benefits of using the proposed weighting schemes is demonstrated in urban canyon and with single (gps only) or multiple (gps/glonass) constellations. acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 17 • receiver autonomous integrity monitoring (raim) techniques, able to detect and reject blunders [9]-[11]; • robust estimation techniques, able to absorb the effects of the blunders [12]. in this work, it has been followed a different approach, consisting in defining a suitable weighting scheme, able to deweighting potentially dangerous measurements. a capillary survey has been carried out on the existing measurement weighting schemes, which are tested in urban environment; afterwards, an additional quality criterion is proposed in order to limit the effect of potential blunders. in literature, two quality indicators are typically associated to the gnss measurements, in order to differently weighting them: satellite elevation angle and carrier-to-noise ratio. satellite elevation (el) is considered a quality indicator of a measurement, because satellites at low elevation are usually noisier, due to typical behaviour of multipath, tropospheric and ionospheric errors [1]. the signal-to-noise ratio (s/n), or better the carrier-tonoise ratio (c/n0), is the ratio between the carrier power and the noise power per unit bandwidth and is usually expressed in decibel-hertz; c/n0 is a measure of signal strength, so it is a suitable indicator of measurement quality [10]. the el and c/n0 indicators are adoptable individually or synergistically to define a model of measurement error variance; a weighting scheme can be obtained, inverting the variances of the measurement errors. the positioning solution is differently influenced by each measurement of a dataset; indeed, some measurements, called leverage observations, have high potential to influence the solution, while others are less influent. if a leverage observation is affected by a gross error, a very large solution error could occur; for this reason, the leverage observations are critical. it can be demonstrated that the concept of leverage observation is related to the local redundancy [13], which is well represented by the diagonal elements of redundancy matrix, called redundancy numbers. in urban scenario, the satellite geometry is often weak and the presence of blunders is very common; in this conditions, a blunder on a leverage observation could strongly degrade the solution. in this work, an innovative weighting scheme, which takes into account the local redundancy information, is proposed and its effectiveness is shown in a very harsh environment like an urban canyon. the proposed scheme can be superimposed to the existing weighting strategies, which involve the indicators el and c/n0, and the benefits of this combined approach are demonstrated. the proposed weighting scheme is applied to gps only and to gps/glonass cases, demonstrating its usefulness in both configurations. below, the spp operational mode is briefly described; afterwards, the existing weighting schemes, based on el and/or c/n0, are described. then the redundancy matrix, whose diagonal elements are the redundancy numbers, is defined and is linked up to the concept of local redundancy. the possible contribution of the redundancy matrix to a weighting strategy is shown. at the end, the proposed weightings are applied to real data and the results are discussed. 2. single point positioning in spp, the functional model, defining the relationship between measurements and unknowns, is 𝒛 = 𝐻∆𝒙 + 𝜺 (1) where 𝒛 is the vector of measurements, defined as the difference between measured and computed (with a priori information) pseudorange, 𝐻 is the design matrix, ∆𝒙 is the state vector, containing the corrections to update the receiver coordinates and clock offset, ε is the measurement error vector. equation (1), also referred to as measurement model, is a linearized equation; the receiver coordinates and clock offset are obtained correcting the a priori information with the estimated state vector. the stochastic model, expressing the uncertainty of the measurement model, is: 𝐶 = � 𝜎12 ⋯ 0 ⋮ ⋱ ⋮ 0 ⋯ 𝜎𝑚2 � (2) where 𝐶 is the variance-covariance matrix of the measurement errors; 𝜎𝑖=1,…,𝑚 2 are the measurement error variances; 𝑚 is the number of measurements. the matrix 𝐶 is diagonal, under the usual assumption of independent measurement errors. the number of measurements is usually larger than the number of unknowns and the equation (1) is solved using weighted least squares (wls) method. wls solution is: ∆𝒙� = (𝐻𝑇𝑊𝐻)−1𝐻𝑇𝑊𝒛 (3) where 𝑊 is the weighting matrix, usually set equal to the inverse of 𝐶. 3. weighting schemes review most of weighting schemes in literature are based on c/n0, satellite elevation el or both; hence, the weighting strategies review is organized according to the considered quality indicators. 3.1. elevation-based weighting schemes the most widespread variance models for gps carrier phase, based on satellite elevation, depend on sin (𝐸𝐸) or sin2(𝐸𝐸) functions [14], [15]; despite being designed for carrier phase, they were simply adapted for pseudorange observation [16], [17] as shown below: 𝜎𝑃𝑃2 = 𝜎02 sin(𝐸𝐸) (4) 𝜎𝑃𝑃2 = 𝜎02 sin2(𝐸𝐸) (5) where σ02 is the pseudorange error variance when 𝐸𝐸 = 90°. slight modifications to the previously mentioned models are proposed in literature [18], maintaining the concept that low elevation satellites correspond to noisy measurements: 𝜎𝑃𝑃2 = 𝐴2 + 𝐵2 sin2(𝐸𝐸) (6) 𝜎𝑃𝑃2 = 𝜎02 tan2(𝐸𝐸 − 𝜃) (7) where 𝐴, 𝐵, 𝜃 are parameters to be defined according to the receiver and the operational scenario. acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 18 a more complex variance model, which takes into account the contributions from several measurement error sources, is defined in [19], [20]; it is designed for wide area augmentation system (waas) and it is shown below: 𝜎𝑃𝑃2 = 𝜎𝑈𝑈𝑃𝑈2 + 𝐹2(𝐸𝐸) ∙ 𝜎𝑈𝑈𝑈𝑈2 + 𝜎𝑆𝑆𝑃2 + 𝜎𝑚452 tan2(𝐸𝐸) + 𝜎𝑡𝑡𝑡2 sin2(𝐸𝐸) (8) where 𝜎𝑈𝑈𝑃𝑈 = 0.5 𝑚 is the variance of the supplied tropo-free and iono-free pseudorange correction, 𝜎𝑈𝑈𝑈𝑈 = 0.5 𝑚 is the variance of the vertical ionosphere correction, 𝐹 is the obliquity factor, projecting vertical ionospheric variance into receiver – satellite direction, 𝜎𝑆𝑆𝑃 = 0.22 𝑚 is the receiver noise variance, 𝜎𝑚45 = 0.22 𝑚 is the variance of the multipath contribution at 45 degrees, 𝜎𝑡𝑡𝑡 = 0.15 𝑚 is the variance of the vertical tropospheric delay estimate. the model (8) has been adapted to the case of gps spp in [21] as shown below: 𝜎𝑃𝑃2 = 𝜎𝑈𝑃𝐴2 + 𝜎𝑈𝐼𝐼𝐼2 + 𝜎𝑇𝑡𝐼𝑇𝐼2 + 𝜎𝑚𝑇2 (9) where 𝜎𝑈𝑃𝐴2 is the user range accuracy (ura) related to the satellite ephemeris and clock, broadcast in the gps navigation message, 𝜎𝑈𝐼𝐼𝐼2 is the accuracy related to ionosphere delay after klobouchar model application, 𝜎𝑇𝑡𝐼𝑇𝐼2 is the accuracy related to troposphere error after correction model application, 𝜎𝑚𝑇2 is the accuracy related to multipath error. 3.2. c/n0-based weighting schemes in [14], a variance model for carrier phase observation is proposed; it is re-used and adapted in [2] for pseudorange measurement. the model is exclusively based on c/n0 and has the following form: 𝜎𝑃𝑃2 = 𝑐 ∙ 10 − 𝐶/𝑆0 10 (10) where c is a constant, empirically defined. the model of hartinger and brunner [15] has been modified by wieser and brunner [22], introducing an additive term, and making the model parameters estimation in view of their application to carrier phase observable. kuusniemi et al. [23], [24] adapt the wieser and brunner [22] model to pseudorange, tuning the parameters according to actual measurement errors and used equipment; the obtained variance model is: 𝜎𝑃𝑃2 = 𝑎 + 𝑏 ∙ 10 − 𝐶/𝑆0 10 (11) where a and b are the model parameters. kuusniemi [10] has estimated the parameters of model (11) for different scenarios (lightly and heavily degraded signal environment). the hartinger and brunner [14] model has been the basis for brunner et al. (1999) to build a carrier phase variance model, called sigma-δ, whose expression is very similar to (10), but with at the exponent a correction term proportional to the difference between the measured and the predicted (according to satellite elevation) c/n0. li and wu [26] have modified the model (11), adopting euler’s number as base of the exponentiation and the difference between the measured c/n0 and its minimum threshold as exponent: 𝜎𝑃𝑃2 = 𝑎 + 𝑏 ∙ 𝑒𝑘[(𝐶/𝑆0)𝑚𝑚𝑚−𝐶/𝑆0] (12) the parameters of the model are a, b and k and are empirically determined. aminian [27] proposed an alternative expression for the pseudorange error variance without exponential form, as shown below: 𝜎𝑃𝑃2 = 𝜎02 𝐶/𝑁0(𝑍𝑍𝐼𝑖𝑡ℎ) 𝐶/𝑁0 (13) where 𝜎02 and 𝐶/𝑁0(𝑍𝑍𝐼𝑖𝑡ℎ) are respectively the theoretic variance and carrier-to-noise ratio in case of satellite at zenith and open-sky scenario. 3.3. weighting schemes based on both satellite elevation and c/n0 in literature, few weighting schemes, based on both satellite elevation and c/n0, are available. in [28] the following model is developed: 𝜎𝑃𝑃2 = � 1 sin2(𝐸𝐸) ᴦ, 𝑖𝑖 𝐶/𝑁0 < 𝑠1 1, 𝑖𝑖 𝐶/𝑁0 ≥ 𝑠1 (14) where ᴦ = 10− 𝐶/𝑆0−𝑠1 𝐵 � � 𝐴 10− 𝑠0−𝑠1 𝐵 − 1� 𝐶/𝑁0 − 𝑠1 𝑠0 − 𝑠1 + 1� (15) the term s1 is a threshold, used as comparison for the measured c/n0; if 𝐶/𝑁0 ≥ 𝑠1, the measurement is considered accurate and the corresponding weight is set to 1 (the maximum possible value for a weight), otherwise a complex formula is adopted for the weight. the parameters in (14) and (15) are determined empirically as 𝑠0 = 10, 𝐴 = 30, 𝐵 = 30. in [29] a model, based on both satellite elevation and c/n0, designed for land navigation, is defined: 𝜎𝑃𝑃2 = 𝑘 ∙ 10− 𝐶/𝑆0 10 sin2(𝐸𝐸) (16) the model (16) is clearly a fusion between the elevationbased model (5) and the c/n0-based model (10). the constant k is 1 if the signal is received in line-of-sight (los) and 2 or +∞ if the signal is received after reflection by obstacles surrounding the antenna (nlos); in [29], a fish-eye camera is used to distinguish los and nlos signals. in this work, a modified model (16) is used, where k is always equal to 1. 4. redundancy matrix the residuals 𝒗, defined as the difference between the actual measurements and their estimated values (𝒛�), are an indicator of the measurement mutual agreement [13]: 𝒗 = 𝒛 − 𝒛� = 𝒛 − 𝐻∆𝒙� (17) it can be simply demonstrated that the relationship between the measurement errors 𝜺 and the ls residuals, considering equally weighted measurements, is: 𝒗 = 𝑅𝜺 (18) the matrix 𝑅 is called “redundancy matrix” and has the following expression: 𝑅 = 𝐼 − 𝐻(𝐻𝑇𝐻)−1𝐻𝑇 (19) the matrix 𝑅 has 𝑚 rows and 𝑚 columns, its trace is equal to the total redundancy (or the degree of freedom) of the equation, that is (𝑚 − 𝑛). the i-th diagonal element of 𝑅, 𝑟𝑖 with 𝑖 = 1, … 𝑚, is called “redundancy number” and is the contribution of the i-th measurement to the total redundancy acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 19 [31]. the redundancy number assumes values between 0 and 1. small values of 𝑟𝑖 (near 0) correspond to measurements providing little contribution to total redundancy and so hardly controlled; on the other hand, approximately equal values of 𝑟𝑖 are desirable, meaning that every measurement is controllable. measurements with small 𝑟𝑖 values are leverage observations and have high potential to influence the solution; if a blunder or a large bias is present on a leverage observation, harmful effects can be evident on the positioning. in this context, de-weighting leverage observations could be a successful strategy, above all in environments, as urban canyon, characterized by frequent gross errors. in table 1, an example of gps geometry, in terms of satellite elevations (𝐸𝐸) and azimuths (𝐴𝐴), is shown; from 𝐸𝐸 and 𝐴𝐴, the design matrix can be computed and, consequently, the redundancy matrix 𝑅 is obtained. the diagonal elements of 𝑅 are the redundancy numbers. in figure 1, the sky-plot of the satellite geometry, detailed in table 1, is shown; all the listed satellites are considered, so the number of visible gps satellites is 10 and the pdop (position dilution of position) value is 1.59. the total redundancy of the measurement model is 6 (= 𝑚 − 𝑛), therefore the average redundancy number is 0.6. the sky-plot is accompanied by the values of the redundancy numbers and it is evident that all the values are near the average. in figure 2, the sky-plot of the satellite geometry, detailed in table 1, is shown, but satellites with prns 17, 19 and 25 are excluded; consequently, the number of visible gps satellites is 7 and the pdop value raises to 2.15. the total redundancy is 3, so the average redundancy number is about 0.43. the redundancy numbers are all around or above the average value, except the one corresponding to satellite 22, which is very low and so can be considered a leverage observation. from figure 2, it is evident a geometric interpretation of leverage observation, for gps spp, as a measurement corresponding to a satellite “isolated” with respect to the others. in case of multi-constellation approach, a redundancy number is null when a single satellite from a system is present. for instance, in a gps/glonass combination, if there is only one gps measurement and several glonass, the redundancy number corresponding to the unique gps observation is 0 and the observation is uncontrollable (because it is the only one able to estimate the clock offset to gps time). the same concept is valid for any gnss combination, including the european gnss, galileo [30], or the chinese one beidou. 5. contribution of redundancy matrix to weighting schemes the diagonal elements of the redundancy matrix, the redundancy numbers 𝑟𝑖, represent the local reliability of the measurement model and so the controllability of the measurements [31]. measurements with low 𝑟𝑖 are critical for the solution, because they can strongly influence it. for this reason, an effective weighting strategy should take into account the redundancy number, above all in scenarios with high probability of blunders. in [32] the redundancy number is used, together with several other parameters, as an indicator of the measurement quality for robust estimation. in [10] the extra-diagonal elements of the redundancy matrix are used to evaluate the correlation among the measurements, in order to avoid erroneous blunder rejections. recently, falco et al. [33] adopted a purely geometric strategy to identify measurements which could strongly influence the solution, in order to de-weight them. in this paper, the information contained in the redundancy matrix is used to correct the original weighting matrix, which could be defined according to one of the previously described methods. in general, the weight 𝑤𝑖, associated to the i-th measurement, is obtained inverting the pseudorange error variance 𝜎𝑃𝑃𝑖 2 . the contribution of a redundancy number to the weighting strategy could be introduced making 𝑤𝑖 proportional to 𝑟𝑖 as shown below: 𝑤𝑖 = ⎩ ⎨ ⎧ 𝑟𝑖 𝜎𝑃𝑃𝑖 2 , in case of redundant measurements 1 𝜎𝑃𝑃𝑖 2 , in case of lack of redundancy (20) the redundancy numbers are 0 in case of lack of redundancy and the scheme (20) takes into account this possibility. figure 2. sky-plot of gps satellite configuration; the total redundancy is 3. table 1. example of gps geometry, in terms of satellite elevation and azimuth. prn el deg az deg 2 12 30 3 24 50 5 45 80 6 21 119 9 65 130 11 30 170 17 81 190 19 20 250 22 19 282 25 30 348 figure 1. sky-plot of gps satellite configuration; the total redundancy is 6. acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 20 the strategy is tested on a multi-constellation static data collection, carried out in urban scenario, and demonstrated its efficiency in both gps only and gps/glonass cases, exhibiting significant improvements on positioning accuracy. the same strategy could be simply extended to gnss velocity estimation [34]. 6. test to demonstrate the benefits of including the redundancy number into a weighting scheme in difficult environment, a static data collection is carried out in urban scenario; specifically, the test location is the “centro direzionale di napoli” (cdn), a district of naples (italy) characterized by several skyscrapers. a specific point in cdn is surveyed with topographical methods, in order to determine its coordinates with mm order. the used equipment consists in a nvs receiver, a single frequency, double constellation (gps/glonass) and high-sensitivity device, connected to a patch antenna; the device is placed on the defined point to collect pseudorange measurements for some hours; the collected data are processed in single point to test several weighting schemes. in figure 3, the considered point (indicated as p) is shown and the urban canyon scenario is evident. the data collection was carried out on 1st july 2016; the nvs receiver was placed on p point for about 2 hours. in figure 4, the number of gps/glonass visible satellites and the corresponding pdop values during the session are shown. the number of gps visible satellites ranges between 0 and 8, with an average of 6.4. the maximum number of glonass visible satellites is 5, while the maximum number of combined gps/glonass satellites is 12. the gps solution availability, defined as the time percentage where the solution can be computed (at least 4 satellites should be visible), is about 99.3%, while the gps/glonass solution availability is about 99.5%. despite the urban canyon scenario, the solution availability is high, owing to the use of a high-sensitivity receiver, able to acquire very weak signals. the rapid oscillations of the number of visible satellites is due to frequent signal loss, typical of hostile environment. in gps only case, the pdop average value is about 2.7, with maximum and minimum respectively about 15 and 1.6; in gps/glonass case, minimum, maximum and mean values are respectively 1.5, 17 and 2.4. the maximum pdop value in gps/glonass case happens when gps satellites are less than four, so in that time window gps dop cannot be computed. 7. results several weighting strategies are described in section 2; among them, some techniques are chosen to process data collected in urban scenario, in order to compare their performance. the considered configurations, implementing gnss spp with a distinct weighting scheme, are: • the one using weights defined by formula (5), shortly indicated as elv1; • the one using weights defined by formula (9), shortly indicated as elv2; • the one using weights defined by formula (11), shortly indicated as cn01; • the one using weights defined by formula (13), shortly indicated as cn02; • the one using weights defined by formula (14), shortly indicated as elvcn01; • the one using weights defined by formula (16), shortly indicated as elvcn02. the previously mentioned configurations are compared with the baseline one, where all the measurements are equally weighted, shortly indicated as eqw. the figure of merits adopted for the comparison are the mean, rms and maximum errors of both horizontal and vertical components of position. in table 2 the configuration performance, in case of gps only, are summarized. figure 3. test location. figure 4. number of visible gps/glonass satellites and corresponding pdop during session. table 2. figure of merits obtained with gps only, using classic weighting methods. mean m rms m maximum m hor. vert. hor. vert. hor. vert. eqw 16,0 18,2 44,9 45,9 351,6 316,6 elv1 10,4 12,9 28,2 33,3 187,7 224,0 elv2 13,0 12,2 38,6 29,0 283,3 218,0 cn01 15,2 17,2 43,1 44,3 340,1 300,9 cn02 15,4 17,6 43,4 45,1 341,0 305,8 elvcn01 8,4 9,9 22,4 27,9 158,5 194,9 elvcn02 5,9 5,8 14,9 19,5 130,1 173,4 acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 21 the results obtained with the eqw configuration is clearly inaccurate; in fact, the errors, on both components and for each figure of merit, are very large owing to the frequent blunders in the dataset. the considered weighting schemes reduce drastically the errors; the models based uniquely on satellite elevation (elv1 and elv2) show better performance with respect to models based uniquely on carrier-to-noise ratio (cn01 and cn02). the methods which consider both quality indicators (elvcn01 and elvcn02) demonstrate significant improvements relatively to the other configurations. in particular, the model introduced by tay and marais [29], i.e. elvcn02 configuration, shows the best performance for the considered data session; indeed rms and mean errors are reduced, with respect to eqw configuration, between 58% and 68%, and the maximum horizontal and vertical errors are reduced respectively by 63% and 45%. the considered weighting schemes are augmented, using information embedded in the redundancy matrix as described in previous sections; the configurations using this information are shortly indicated as rdm. for instance, a configuration using the elevation-based model (5), augmented with redundancy matrix-based approach, is shortly indicated as elv1+rdm. in table 3, the performance of the configurations, obtained combining the considered classical weighting schemes with redundancy matrix information, are resumed. moreover, in the table, the percentage improvements, with respect to corresponding configurations without redundancy matrix use, are reported alongside the errors. it is evident that the introduction into the weighting schemes of information from redundancy matrix brings to significant improvements. the best configuration is elvcn02+rdm, characterized by horizontal mean, rms and maximum errors respectively about 4.5, 8.8 and 87.2 meters, a very good performance considering the strongly degraded scenario. the same analysis, carried out for gps only, is provided for gps/glonass multi-constellation case. in table 4 the gps/glonass configuration performance are summarized. the addition of glonass measurements to gps ones does not improve all the considered figure of merits, as glonass measurements are often very noisy; in detail, gps/glonass configurations have better performance in horizontal plane, but worse performance in vertical direction. the errors of eqw configuration are very large, because there are several blunders among the measurements, from both used systems; the glonass use allows a reduction in maximum horizontal of about 130 meters. as for gps only case, the weighting schemes based uniquely on satellite elevation (elv1 and elv2) demonstrate better performance than models based uniquely on carrier-tonoise ratio (cn01 and cn02); models exploiting both quality indicators show the best results. in table 5, the figure of merits of the gps/glonass configurations, using classical weighting schemes augmented with redundancy matrix information, are resumed; in the table, the percentage improvements, with respect to corresponding configurations without redundancy matrix use, are reported alongside the errors. the benefits of using redundancy matrix in weighting schemes are evident even in gps/glonass case; indeed, all the considered errors are strongly reduced. the best configuration is elvcn02+rdm, characterized by horizontal mean, rms and maximum errors respectively about 4.5, 8.1 and 73.6 meters, a very good performance considering the strongly degraded scenario and slightly lower with respect to gps best configuration. the use of information derived from redundancy matrix demonstrates very good performance for both gps only and gps/glonass cases. the error reductions are impressive for each considered configuration and figure of merit. in particular, the best configuration is elvcn02+rdm, using 𝐶/𝑁0, satellite elevation and local redundancy as parameters for weighting; the obtained performances are characterized by mean and rms horizontal errors respectively below 5 and 10 meters, and maximum horizontal error about 80 meters. the considered approach, taking into account the information contained into the redundancy matrix, demonstrates its effectiveness, owing to the ability to deweighting the geometrically dangerous measurements. table 4. figure of merits obtained with gps/glonass, using classic weighting methods. mean m rms m maximum m horiz. vert. horiz. vert. horiz. vert. eqw 14,6 28,0 33,5 62,3 212,9 343,2 elv1 8,7 19,0 17,9 40,4 112,0 205,7 elv2 12,7 12,7 37,4 29,7 268,9 217,7 cn01 13,7 26,2 31,8 59,2 196,5 335,1 cn02 14,0 26,9 32,3 60,5 202,0 339,0 elvcn01 7,1 14,4 14,4 32,0 97,5 178,9 elvcn02 5,3 8,4 10,4 21,2 92,4 162,0 table 5. figure of merits obtained with gps/glonass, using classic weighting methods, augmented with information from redundancy matrix. mean m rms m maximum m horiz. vert. horiz. vert. horiz. vert. eqw+rdm 13,5 (7,5%) 19,8 (29,4%) 32,9 (1,9%) 44,1 (29,1%) 236,6 (11,1%) 266,0 (22,5%) elv1+rdm 7,7 (12,3%) 12,1 (36,0%) 15,5 (13,2%) 25,9 (36,0%) 94,8 (15,3%) 153,1 (25,5%) elv2+rdm 11,0 (13,5%) 10,0 (21,1%) 29,5 (21,0%) 23,0 (22,5%) 212,0 (21,2%) 164,8 (24,3%) cn01+rdm 12,5 (8,8%) 18,0 (31,3%) 30,6 (3,9%) 41,1 (30,7%) 224,7 (14,4%) 242,2 (27,7%) cn02+rdm 12,8 (8,5%) 18,6 (30,7%) 31,2 (3,4%) 42,3 (30,1%) 224,3 (11,1%) 249,4 (26,4%) elvcn01+rdm 6,1 (14,1%) 8,6 (40,0%) 11,9 (17,6%) 19,6 (38,9%) 86,1 (11,7%) 122,6 (31,5%) elvcn02+rdm 4,5 (14,8%) 4,6 (45,4%) 8,1 (22,8%) 12,3 (41,7%) 73,6 (20,4%) 109,1 (32,7%) table 3. figure of merits obtained with gps only, using classic weighting methods, augmented with information from redundancy matrix. mean m rms m maximum m horiz. vert. horiz. vert. horiz. vert. eqw+rdm 14,0 (12,2%) 15,5 (14,9%) 36,4 (19,0%) 37,2 (18,9%) 276,5 (21,4%) 241,5 (23,7%) elv1+rdm 8,0 (22,9%) 9,0 (29,8%) 18,1 (35,7%) 22,8 (31,6%) 121,1 (35,5%) 154,7 (30,9%) elv2+rdm 10,6 (18,4%) 9,1 (25,3%) 27,8 (27,8%) 20,9 (27,9%) 186,2 (34,3%) 136,5 (37,4%) cn01+rdm 13,0 (14,4%) 14,2 (17,6%) 33,7 (21,7%) 34,7 (21,6%) 261,5 (23,1%) 228,1 (24,2%) cn02+rdm 13,3 (13,7%) 14,7 (16,6%) 34,4 (20,9%) 35,8 (20,6%) 261,7 (23,2%) 230,3 (24,7%) elvcn01+rdm 6,3 (24,7%) 6,3 (35,7%) 13,6 (39,4%) 17,6 (36,9%) 108,1 (31,8%) 125,0 (35,9%) elvcn02+rdm 4,5 (23,6%) 3,3 (43,8%) 8,8 (41,1%) 11,4 (41,7%) 87,2 ( 33,0%) 115,9 (33,2%) acta imeko | www.imeko.org june 2018 | volume 7 | number 2 | 22 8. conclusions in gnss context, some environments, such as urban canyons, are critical, owing to the likely presence of blunders among the measurements, which yields very large position errors. in these conditions, it is essential to adopt a suitable method to differently weight the measurements, in order to limit the blunder effects. in literature, several schemes exist, based on two quality indicators: satellite elevation and carrierto-noise ratio. in this paper, the most representative weighting schemes for gnss absolute positioning are tested in urban scenario and their performance are compared. the best results are obtained with the weighting strategies based on both the abovementioned quality indicators. the concept of local redundancy refers to the degree of controllability of the observations; for instance, a measurement with low local redundancy is difficultly controlled and, if affected by gross errors, it strongly influences the solution. the redundancy matrix relates the measurement errors to the residuals and its diagonal elements are the redundancy numbers, representing the local redundancy. in this work, a strategy to include the local redundancy into the existing weighting schemes is suggested. in the proposed approach, each weight value is set proportionally to the corresponding local redundancy number. in this way, the most influencing measurements (i.e. the leverage measurements) are de-weighted and their effects on the solution are limited. the considered weighting schemes are applied to process in spp a 2-hour dataset, collected in urban scenario; both gps only and multi-constellation gps/glonass configurations are analysed. the obtained results demonstrate the effectiveness of the proposed weighting strategy; indeed, the position errors, using the weighting scheme based on redundancy numbers, are significantly reduced for all the considered configurations. the best performances are attained with a weighting scheme, comprising the two classical quality indicators, satellite elevation and carrier-to-noise ratio, and the redundancy number; 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(2013). testing the test satellites: the galileo iov measurement accuracy. 2013 international conference on localization and gnss, icl-gnss 2013, art. no. 6577253. doi: 10.1109/icl-gnss.2013.6577253 [31] schaffrin, b. (1997). reliability measures for correlated observations. journal of surveying engineering, 123(3), 126–137. [32] wieser, a. (2001). a fuzzy system for robust estimation and quality assessment of gps data for real-time applications. proceedings of the 14th international technical meeting of the satellite division of the institute of navigation (ion gps 2001), salt lake city, ut, us. [33] falco, g., nicola, m. and falletti, e. (2016). constellation-aware method for computing the covariance matrix of gnss measurements. proceedings of the european navigation conference 2016, helsinki, finland. [34] freda, p., angrisano, a., gaglione, s., troisi, s. (2015). timedifferenced carrier phases technique for precise gnss velocity estimation. gps solutions, 19 (2), pp. 335-341. doi: 10.1007/s10291-014-0425-1. validation of a ptb force-balanced piston gauge primary pressure standard acta imeko issn: 2221-870x march 2021, volume 10, number 1, 271 276 acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 271 validation of a ptb force-balanced piston gauge primary pressure standard ahmed s. hashad1, wladimir sabuga2, sven ehlers2, thomas bock3 1 national institute for standards (nis), giza, egypt (guest researcher at ptb germany) 2 physikalisch-technische bundesanstalt (ptb), braunschweig, germany 3 physikalisch-technische bundesanstalt (ptb), berlin, germany section: research paper keywords: fpg; cross floating; effective area; pressure standard; experimental verification citation: ahmed s. hashad, wladimir sabuga, sven ehlers, thomas bock, validation of a ptb force-balanced piston gauge primary pressure standard, acta imeko, vol. 10, no. 1, article 1, march 2021, identifier: imeko-acta-10 (2021)-1-1 section editor: momoko kojima, nmij, japan received april 22, 2020; in final form july 23, 2020; published march 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was supported by the empir-participating countries within euramet and the european union and by the egyptian study mission. corresponding author: ahmed s. hashad, e-mail: ahmed_hashad84@hotmail.com 1. introduction force-balanced piston gauges (fpgs) present an important class of piston gauge that enables pressure measurement from a few pascals to several tens of kilopascals. in these instruments, the force of the pressures acting on the piston of a pistoncylinder assembly (pca) is measured by a high-accuracy balance. this force and the effective area (𝐴) of the pca can be used to calculate the difference in the pressures above and below the piston [1], [2]. at ptb, a force-balanced piston gauge manufactured by fluke calibration (usa), model fpg8601 [3], [4], was characterised as a primary pressure standard by determining 𝐴 of the pca from the pca’s dimensional properties [5], [6]. for this, a rarefied gas dynamics model was used to determine the pressure distribution of the gas flow inside the gap between the piston and the cylinder [7]-[9]. three experiments were then performed to validate the pca’s effective area and the fpg pressures by comparing them with the pressures of three different standards. these standards have various operation principles and pressure ranges, as elaborated on below. 2. experiments experiments that can be used for the metrological characterisation of an fpg are described in, for example, [3], [10]-[17]. in the present study, experiments were specified that allowed the verification of the metrological characteristics obtained by the theoretical calculations. in the following sections, these experiments are described in detail. in all the experiments, a very low-pressure controller (vlpc), produced by fluke calibration, was used to stabilise the fpg measurement and lubrication pressures, unless otherwise stated. abstract experimental methods using different pressure standards were applied to verify theoretical results obtained for the effective area of a piston-cylinder assembly (pca) and for pressures measured with a force-balanced piston gauge (fpg). the theoretical effective area was based on the pca’s dimensional properties defined by the diameter, straightness and roundness measurements of the piston and cylinder, derived from gas-flow modelling, using principles of rarefied gas dynamics and presented as two values, one for absolute and the other for gauge-pressure operation mode. both values have a relative standard uncertainty of 510-6. the experimental methods chosen were designed to cover the entire operating pressure range of the fpg from 3 pa to 15 kpa. comparisons of the fpg with three different ptb pressure standards operated in different pressure ranges – a pressure balance, a mercury manometer and a static expansion system – were performed using the cross-float method and by a direct comparison of the generated pressures. for the theoretical and experimental effective area, as well as for pressures generated by the fpg and the reference standards, all the results demonstrated full agreement within the expanded uncertainties of the standards. mailto:ahmed_hashad84@hotmail.com acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 272 2.1. effective area determination the fpg was calibrated against a ruska pressure balance, model 2465a (gas-piston gauge), equipped with a pca, serial no. tl1568, to determine the effective area of the fpg by the crossfloat method, as described in [17], [19]. the ruska pca has an effective area equal to 3.35666  (1 ± 5.810-6) cm2 (all uncertainties in this paper are given for 𝑘 = 1, unless otherwise specified). the effective area of this ruska pca is traceable to the state-of-the-art pressure balance that was used in the experiments on the redetermination of the boltzmann constant [20]-[22] and is the german national pressure standard in the 180 kpa to 7.5 mpa range of absolute pressure. both instruments, ruska and fpg, were connected via the setup shown in figure 1. a class e2 stainless-steel mass set was used to characterise the fpg balance [22]. a 10 torr capacitance diaphragm gauge (cdg) was placed in the measurement line to indicate the pressure difference between the two instruments. the pressures of both instruments were controlled to maintain the pressure difference measured with the cdg as close to zero as possible. the measurements were performed in gauge and absolute mode in the pressure ranges of 3 kpa to 15 kpa and 2 kpa to 15 kpa, respectively. the higher starting pressure point of 3 kpa in gauge mode was chosen due to the ruska pressure balance’s poor performance at lower pressures. during the measurement, valve v1, which separated both instruments, was closed, and valve v2 was opened. the pressure of the fpg was generated by the vlpc at the target pressure, and the variable pressure volume was used to generate the target pressure within the ruska pressure balance. the vlpc then finely adjusted the pressure within the fpg until the cdg indication was close to zero. finally, valve v1 was opened and valve v2 closed to allow a direct cross float between the fpg and the ruska pressure balance. from the experimental results, the effective area was calculated by means of equation (1), 𝐴 = 𝐹lc (𝑝ref − 𝑝res)[1 + (𝛼 + 𝛽)(𝑡 − 20)] , (1) in which 𝑝ref is the pressure of the reference standard, 𝑝res is the residual pressure on the lower side of the fpg pca, which is equal to zero in gauge mode, 𝛼 + 𝛽 is the thermal expansion coefficient of the fpg pca and 𝐹lc is the force measured by the balance and corrected for buoyancy and lubrication pressurechange effects, as set out in [24] and shown in equation (2), 𝐹lc = 𝑚cal (1 − 𝜌lub 𝜌m ) 𝑔 𝑁cal (𝑁 + 𝛿𝑁1 + 𝛿𝑁2 + 𝛿𝑁3), (2) where 𝑚cal is the true mass of the internal reference mass used for the balance’s internal calibration, 𝑁cal is the reading of the balance when 𝑚cal is loaded, 𝑁 is the indication of the balance resulting from the pressure difference, 𝛿𝑁1 is the indication correction due to the lubricant gas-pressure variation, 𝛿𝑁2 is the indication correction due to the drag-force change and 𝛿𝑁3 is the indication correction due to the atmospheric-pressure variation. the ruska reference pressure (𝑝r.ref) was calculated by means of equation (3), 𝑝r.ref = 𝑔 [𝑚𝑖 (1 − 𝜌amb 𝜌𝑖 ) + 𝑉 (𝜌l − 𝜌amb)] 𝐴r0[1 + (𝛼r + 𝛽r)(𝑡 − 20)] + 𝑝r.rest + 𝑔 ℎ (𝜌l − 𝜌amb), (3) where 𝑔 is the local gravity acceleration, 𝑚𝑖 and 𝜌𝑖 are the masses and their densities, respectively, 𝜌amb is the density of the ambient gas, 𝑉 is the piston’s additional volume (which is required as a correction due to the buoyancy produced by the pressure-transmitting medium) with density 𝜌l, 𝐴r0 is the effective area of the ruska pca, 𝛼r + 𝛽r is the thermal expansion coefficient of the ruska pca, 𝑝r.rest is the residual pressure in the ruska bell jar and ℎ is the height difference between the reference levels of the pressure balance and the fpg. 2.2. fpg vs mercury manometer in this experiment, the fpg was calibrated against the ptb mercury manometer (hgm), model 1025b made by schwien engineering (usa), which was modified at ptb and metrologically characterised, as described and validated in [25]-[26]. the design of this instrument is shown in figure 2. this manometer comprises two vessels filled with mercury and connected by a flexible tube (a) with the ability to change the height of one of the vessels mechanically. the change in height of the movable vessel is measured by means of a laser interferometer (b). the height levels of the mercury’s free surface in the vessels are controlled by capacitance measurements (c1 and c2). in the experiments, the fpg was (a) (b) figure 1. experimental setup for cross floating between fpg and ruska pressure balance: sketch of the whole setup (a) and life photo of the experiment (b). atm cdg flc pref m pres vlpc hi lovac. m k s v1v2 v3 v4 pv acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 273 connected to the hgm directly; this is demonstrated in figure 3 for measurements in gauge mode. a cdg was placed between the two instruments to measure the pressure difference when changing the pressure in each instrument. all connections were thermally isolated to minimise the effect of ambient temperature changes even though all measurements were performed in an air-conditioned environment with a temperature stability within 0.3 k. the measurements were carried out at a pressure range from 100 pa to 15 kpa in the gauge mode and from 1 kpa to 15 kpa in the absolute mode. in the gauge-pressure mode, the vlpc was used as usual. at the beginning, zero pressures were set in both instruments, and both instruments were zeroed. after zeroing, pressures were generated separately. when generating the pressures, valve v1, which separated both instruments, remained closed, and valve v2 was open, allowing the hgm to be connected to its pressure generator. thus, the target pressure was generated by the vlpc within the fpg and by the pressure generator within the hgm. the height of the movable vessel was adjusted to make capacitances c1 and c2 equal (see figure 2). valve v2 was then closed and, using the vlpc to finely adjust the pressure in the fpg, a close-tozero indication of the cdg was reached. next, bypath valve v1 was opened to establish a direct connection and a pressure equilibrium between both systems. when equilibrium was achieved and all relevant indications became stable, their values and all conditions were recorded. in the absolute-pressure mode, the vlpc was replaced with valve v4, which has a finely controlled opening and closing mechanism, behind which a turbopump was located (see figure 4). the lubrication pressure was equal to 40 kpa, allowing the measurement pressure inside the fpg to be calculated by means of equation (4), 𝑝hgm = 𝑔 ℎhg 𝜌hg + 𝑝m.rest − 𝑔 𝑙 𝜌𝑙 , (4) in which ℎhg is the height difference between the mercury levels in the two vessels, 𝑙 is the height difference between the reference level of the hgm and the mercury surface in the lower vessel, 𝜌hg is the density of mercury, 𝜌𝑙 is the density of the pressure-transmitting gas and 𝑝m.rest is the residual pressure in the upper vessel of the hgm. the fpg pressure was calculated by means of equation (5) with two additional components, containing the height difference ℎ between the pressure reference levels of the fpg and hgm and the residual pressure 𝑝res. on the reference side of the fpg, which appears only in the absolute-pressure mode, 𝑝fpg = 𝐹lc 𝐴 [1 + (𝛼 + 𝛽)(𝑡 − 20)] + 𝑝res. − 𝑔 ℎ 𝜌𝑙 . (5) 2.3. fpg vs static expansion system the static expansion system (ses) [28], [29], see figure 5, is a primary vacuum standard consisting of two stages of expansion, starting with two small volumes (vl1 and vl2) and two intermediate volumes (vl4 and vl5) and concluding with a figure 2. ptb mercury manometer (hgm) with its reference (-) and measurement (+) pressure lines and climatisation temperature sensors (x). (a) (b) figure 3. experimental setup for cross floating between the fpg and the hgm in gauge-pressure mode: sketch of the whole setup (a) and life photo of the experiment (b). figure 4. experimental setup for cross floating between fpg and hgm in absolute mode. c1 b a motor laser water thermostat pump x x + h c2 atm laser pref h pres vlpc hi lovac. cdg v1 v2 v3 v4 pv height controller pressure generator flc cdg pref h atm pres vlpc hi lovac. m k s v1 v2 v3 pv height controller pressure generator v4 flc l a s e r acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 274 calibration vessel (vl6) with a nominal volume of 100 litres. uuc is the unit under calibration. this set of volumes enables the realisation of the expansion ratios 𝑓𝑖 , which are presented in table 1. a quartz bourdon spiral (qbs) was used to control and measure the initial pressure value 𝑝fill when filling the first volume with the nitrogen gas in a range between 1 kpa and 100 kpa. the final pressures produced by ses 𝑝s.ref in the range of 10 mpa and 1 kpa can be calculated by means of equation (6), 𝑝s.ref = 𝑓𝑖 ′ 𝑝fill 𝑇1 𝑇0(1 + 𝑝fill 𝐵 (𝑅 𝑇0)⁄ ) , (6) where 𝑓𝑖 ′ is the corrected expansion ratio, taking the additional volume of the uuc into account, 𝑇0 is the initial temperature before and 𝑇1 the final temperature after the expansion, 𝐵 is the virial coefficient and 𝑅 is the gas constant. the ses was used to verify the fpg in the low-pressure range from 3 pa up to 300 pa. because of the humidity of the nitrogen used inside the fpg and in order to keep the expansion volume vl6 defined, a cdg, with a full range of 13.33 kpa and 1 mpa resolution, constantly separated the two systems, and its readings were taken into consideration in the comparison results. figure 6 shows the experimental setup for the comparison of the fpg with the ses in absolute-pressure mode, where the ses is sketched simply with only two volumes. the vlpc was used to keep the pressure difference measured by the cdg as close to zero as possible. initially, the qbs was used to set the pressure in vls1. valves v2 and v6 were then opened to expand the gas into the larger volume vls2 and the tubes connected to the left side of the cdg before v1. the total volume was measured precisely. the ses pressure value was calculated according to equation (6). 3. results the results of the effective area determination based on the fpg calibration against the ruska pressure balance, which was described in section 2.1, are presented in table 2 and figure 7 for both gauge and absolute-pressure operation modes. figure 5. design of ses. (a) (b) figure 6. experimental setup for comparison between fpg and ses: sketch of the whole setup (a) and life photo of the experiment (b). table 1. expansion ratios of the ses shown in figure 5. symbol expansion ratio f f1 𝑉𝐿1 𝑉𝐿1 + 𝑉𝐿3 + 𝑉𝐿6 9.173210-3 f2 𝑉𝐿2 𝑉𝐿2 + 𝑉𝐿3 + 𝑉𝐿6 7.423110-4 f3 𝑉𝐿1 𝑉𝐿1 + 𝑉𝐿3 + 𝑉𝐿4 + 𝑉𝐿5 9.197810-3 f4 𝑉𝐿2 𝑉𝐿2 + 𝑉𝐿3 + 𝑉𝐿4 + 𝑉𝐿5 7.437810-4 f5 𝑉𝐿5 𝑉𝐿5 + 𝑉𝐿6 9.17010-3 table 2. experimental effective area a and its standard uncertainty u(a) (𝑘 = 1) of fpg at nominal pressures p in gauge and absolute modes. p / kpa gauge mode absolute mode a / cm2 u(a) / cm2 a / cm2 u(a) / cm2 2 9.80592 4.810-4 3 9.80624 2.610-4 9.80601 2.510-4 5 9.80620 1.610-4 9.80600 1.810-4 6 9.80611 8.810-5 9.80602 1.810-4 8 9.80607 1.110-4 9.80601 1.510-4 10 9.80614 8.310-5 9.80601 9.410-5 11 9.80617 7.510-5 9.80606 1.010-4 13 9.80613 7.210-5 9.80598 9.410-5 15 9.80610 8.010-5 9.80599 8.710-5 vls1 vls2 qbs atm pres vlpc hi lovac. m k s cdg v1 v2 v3 v4 v5 v6 flc acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 275 in figure 7, these experimental results are shown together with the results of a theoretical determination of fpg’s effective area based on the dimensional properties of the pca, calculated using rarefied gas dynamics (rgd) methods and with a relative standard uncertainty of 410-6. figure 7 also shows the effective area reported by the manufacturer with a relative standard uncertainty of 1310-6. all experimental and theoretical effective areas agree well in that the experimental standard uncertainties are lower than typical fpg uncertainties [29]. the results of the pressure comparison between the fpg and the hgm are presented for gauge mode and absolute mode in figure 8 and figure 9, respectively. the pressure differences measured in gauge mode have a more pronounced scatter than in absolute mode since the gaugepressure measurements were affected by the instability of the atmospheric pressure. because the time constants of the fpg and the hgm differed considerably, the temporal reactions of the two instruments to the ambient pressure variations were significantly different, leading to the pressure differences observed in the experiment. in addition, the large volume of the tubes connecting the fpg to the hgm made it difficult to control the pressure during the measurements. despite this, almost all pressure differences are covered by the expanded uncertainty (𝑘 = 2) of the hgm. on the contrary, in absolute-pressure mode, the measurements were free of the ambient pressure effect, and the results were characterised by a much lower scatter of the pressure differences (see figure 9). the differences in the pressures measured with the fpg and those measured with the hgm are much smaller than the expanded uncertainty of the hgm [26], which demonstrates the consistency of the two pressure standards. in the lowest pressure range of the fpg operation (from 3 pa to 300 pa), consistency was verified via a comparison with the ses in the absolute-pressure mode. the results of the comparison of the two pressure standards in terms of the pressure differences and their uncertainties are shown in figure 10. above 30 pa, the pressure differences agree with the standard uncertainty (𝑘 = 1) of the ses. at lower pressures, the scatter of the pressure differences is greater than the standard uncertainty of the ses, but it is in agreement with the expanded uncertainty (𝑘 = 2) and allows conclusions to be drawn about the consistency of the two standards at 10 mpa. 4. conclusions for the theoretical and experimental effective area of the pca, as well as for pressures generated by the fpg and the three reference standards based on different operation principles (the pressure balance, the mercury manometer and the static expansion system), all the results demonstrate full agreement within the expanded uncertainties (𝑘 = 2) of the three standards. thus, the fpg, which has already been characterised as a primary pressure standard, is validated experimentally. for the pressure range from 3 pa to 15 kpa, the fpg standard uncertainty is presently estimated as 10 mpa + 610-6  𝑝. figure 7. experimental (exper.), rgd-calculated (theor.) and manufacturer’s (manuf.) effective areas of the fpg in absolute (abs.) and gauge (gauge) modes. figure 8. differences in gauge pressures measured with fpg and hgm. figure 9. differences in absolute pressures measured with fpg and hgm. figure 10. differences in absolute pressures measured with fpg and ses. 9,8054e-04 9,8056e-04 9,8058e-04 9,8060e-04 9,8062e-04 9,8064e-04 9,8066e-04 9,8068e-04 0 5 10 15 e ff e ct iv e a re a / m 2 pressure / kpa exper. gauge theor. gauge exper. abs. theor. abs. manuf. -0,5 -0,4 -0,3 -0,2 -0,1 0,0 0,1 0,2 0,3 0,4 0,5 0 5000 10000 15000 p re ss u re d if fe re n ce / p a pressure / pa press. diff. uncertainty k=2k = 2 -0,4 -0,3 -0,2 -0,1 0,0 0,1 0,2 0,3 0,4 0 5000 10000 15000 p re ss u re d if fe re n ce / p a pressure / pa 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takagi-sugeno; fuzzy logic; uav; stability; aerospace citation: enrico petritoli, fabio leccese, precise takagi-sugeno fuzzy logic system for uav longitudinal stability: an industry 4.0 case study for aerospace, acta imeko, vol. 9, no. 4, article 14, december 2020, identifier: imeko-acta-09 (2020)-04-14 section editor: francesco bonavolonta, university of naples federico ii, italy received october 14, 2019; in final form february 4, 2020; published december 2020 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: fabio leccese, e-mail: fabio.leccese@uniroma3.it 1. introduction the authors have been involved in the development of a fixed wing tailless drone (uav). we will examine the critical challenge of maintaining longitudinal stability that confronts aircraft with this design [1]-[3]. in the next sections, we will briefly illustrate the type of hardware that was chosen, and, after discussing the aerodynamic challenges, we will address the fuzzy logic resolution method that was used to simplify the calculations. in the current context of flight in which the ground, air and space segments must converse with each other, the ground segment, thanks to internet-ofthings (iot) and wi-fi technology, is taking on an increasingly important role for uavs during flight phases and landing via the base station [4]. we can therefore say that industry 4.0 is making inroads even into the extremely conservative and slow-to-change field of aerospace. this reluctance to change is, of course, due to a key aspect of both air and spaceflight: safety. it took decades of testing and certification to entrust an airliner to the autopilot, and even now, the cabin must be staffed by a human pilot. contrary to popular belief, safety is not less important in drones, even though there are no people on board. in the case examined in this paper, we entrust the control of longitudinal stability to an automatic system that, far from behaving like a simple machine, uses fuzzy logic to approach human behaviour, while still avoiding human weaknesses [5]. 1.1. state of the art the flying wing design (i.e. an aircraft without horizontal empennage) was among the first to appear in the world of aviation; however, it was soon found to be intrinsically unstable. this instability leads to a loss of control if the flight is disturbed by a gust or a manoeuvre. following the rapid development of aeronautical engineering in the early 1900s, the idea of tailless figure 1. rendering of the flying wing drone. abstract industry 4.0 is making inroads even into the field of aerospace, which is extremely conservative due to the safety concerns involved with the introduction of new technology. we used fuzzy logic to create a very reliable simplification and reduce the complexity of the critical longitudinal stability equations for a flying wing drone. our approximate method allowed us to have a very light calculation effort at the price of a negligible error in terms of the size and dynamics of the craft, thus reducing the work required by the telecommunication segment that manages the takeoff and landing manoeuvres. mailto:fabio.leccese@uniroma3.it acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 107 aircraft took hold in the world of gliders. in particular, the horten brothers developed an entire family of gliders of this type. this led to the study and use of special aerofoils optimised for this type of vehicle. after an uncertain start in the 1950s by northrop aviation (today northrop-grumman), the b-2 spirit arrived in the late 1990s, an aerodynamically mature aircraft from all points of view. in the design of uavs, the tailless formula was immediately successful, generating a series of models and strategies of use. generally, they are engaged in missions in which acrobatics are clearly not the main requirement; indeed, stability is an essential feature in their use as aerial photogrammetric platforms. due to the low drag, as has been noted, they are especially useful for surveillance tasks in temporally demanding missions. for this reason, the use of remote piloting is increasingly abandoned in favour of more tactical-operational autonomy. as a result, autonomous navigation systems that contain robust stability routines have been developed. within this context, our work is aimed at proposing increasingly fast and efficient calculation methods. 2. the sytem 2.1. the uav the design (see figure 1) of the drone is based on a flying wing: it is tailless with winglets at the wing tips and is energised by a pusher propeller. such designs have extremely low aerodynamic inducted resistance at the price of a strong criticality: poor longitudinal stability [6]-[8]. in order for the drone to be stable, it is necessary to manage the weight so that the centre of gravity is set lower than in a classically designed aircraft. furthermore, there is always the danger that it can enter an uncontrollable aerodynamic state outside the flight envelope and fall into a ‘dead leaf’ condition, which is completely uncontrollable and inevitably leads to the loss of the drone [9]-[11]. 2.2. the flight control system (fcs) the flight control system (fcs) is the heart of the navigation system: it manages the attitude of the craft, the data coming from the payload and all communications. in order to develop a fast processor for the fcs, our attention was focused immediately on two main problems: firstly, we needed to calculate large volumes of parallel data with a single device; and secondly, we needed an accurate evaluation of approximated stability functions in order to increase the calculation speed. as stated, the main tasks of the fcs are three: housekeeping (attitude control), data handling from the payload and telecommunications management. the innovation consists in not using three separate microprocessors for all functions but instead wrapping them all into one positively reprogrammable microprocessor. 2.3. the iot system in this section, we will examine uav navigation within the iot network (see figure 2). the uav receives position signals from the global navigation satellite system (gnss) constellation, processes them for navigation and then uses telemetry to transmit data to the tracking system [12]. the tracking system consists of a high gain antenna (used to extend the range) that collects the signals and sends them to the server. the server has the task of managing and sorting the signals and sending them to the operator [13]. on the landing strip, meanwhile, equipment is available that uses a camera to detect the state of the track and meteorological information, which will then be transmitted via wi-fi to the aircraft as it approaches the runway [14]. the advantage of this system is clear: non-essential flight data are not transmitted, keeping the communication systems clear. in europe, the ieee 802.15.2 standard assigns the frequency of 868.3 mhz to telemetry and telecommands (ttc) for uav. since the communication channel is limited, saturating it is extremely easy, regardless of the type of modulation chosen [15]. 2.4. fuzzy systems currently, fuzzy logic-based systems are among the most important applications of fuzzy logic in soft computing and applied mathematics, and they are widely used for solving control problems in all aspects of engineering [16]-[21]. the popularity of fuzzy logic-based systems is due to their ability to appropriately simulate human thinking, surpassing the limits of boolean logic and expressing the system in its full complexity. the takagi-sugeno model (t-s), represented by the fuzzy relation ‘if-then’, manages to describe a non-linear dynamic system with a linearised model that meets all the conditions set and has a negligible error rate that will be shown at the end of the paper [22]-[27]. the logical processes that lead to the formulation of the inference conditions are as follows: firstly, the logical process must undergo fuzzification, a modelling based on the rigorous application of fuzzy (not boolean) rules; and finally, the whole linear system must be defuzzified to return to the physical world [28]. 2.5. fuzzy systems algebra for clarity of exposition, we will briefly introduce the fuzzy algebra method that we used [29]-[34]. consider a fuzzy set 𝛷𝑖𝑗 composed of: { 𝕫1(𝑡) = 𝛷𝑖1 𝕫2(𝑡) = 𝛷𝑖2 𝕫3(𝑡) = 𝛷𝑖3………… 𝕫𝑝−1(𝑡) = 𝛷𝑖(𝑝−1) 𝕫𝑝(𝑡) = 𝛷𝑖𝑝 (1) for the model rule, 𝑖𝑡ℎ, we have a set of 𝑟 model rules, and then we have: 𝜁𝑣(𝑡) = input vector 𝑥𝑣(𝑡) = state vector figure 2. schematic diagram of the iot network for uav general control. acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 108 𝜉𝑣(𝑡) = output vector the complete rule now is: if 〈𝑧1(𝑡) = 𝛷𝑖1 and 𝕫2(𝑡) = 𝛷𝑖2 and… and 𝕫𝑝(𝑡) = 𝛷𝑖𝑝〉 (2) then { 𝑥𝑣(𝑡 + 1) = 𝒜𝑖𝑥𝑣(𝑡) + ℬ𝑖𝜁𝑣(𝑡) 𝑦(𝑡) = 𝒞𝑖𝑥𝑣(𝑡) with 𝑖 = 1,2,…𝑟 (3) where 𝒜𝑖 = ( 𝕘11 ⋯ 𝕘𝑝1 ⋮ ⋱ ⋮ 𝕘1𝑝 ⋯ 𝕘𝑝𝑝 ) . (4) 𝒜𝑖: is the matrix (square) with the real elements and 𝕫1(𝑡),𝕫2(𝑡),…,𝕫𝑝(𝑡) are known premise variables. each subsystem is represented by a linear equation: 𝒜𝑖𝑥𝑣(𝑡) + ℬ𝑖𝜁𝑣(𝑡) . (5) now for the model we have: 𝑥𝑣(𝑡 + 1) = ∑ 𝜍𝑖[𝕫(𝑡)]{𝒜𝑖𝑥(𝑡) + ℬ𝑖𝜁𝑣(𝑡)} 𝑟 𝑖=1 ∑ 𝜍𝑖[𝕫(𝑡)] 𝑟 𝑖=1 . (6) simplifying: 𝑥𝑣(𝑡 + 1) = ∑ℎ𝑖[𝕫(𝑡)]{𝒜𝑖𝑥(𝑡) + ℬ𝑖𝜁𝑣(𝑡)} 𝑟 𝑖=1 . (7) so, according to (6) 𝜉𝑣(𝑡) = ∑ 𝜍𝑖[𝕫(𝑡)]𝒞𝑖𝑥(𝑡) 𝑟 𝑖=1 ∑ 𝜍𝑖[𝕫(𝑡)] 𝑟 𝑖=1 . (8) then we have: 𝜉𝑣(𝑡) = ∑ℎ𝑖[𝕫(𝑡)] 𝑟 𝑖=1 𝒞𝑖𝑥(𝑡) (9) where 𝕫(𝑡) = [𝕫1(𝑡) ∙ 𝕫2(𝑡) ∙ ………… ∙ 𝕫𝑝(𝑡)] . (10) the expression of 𝜍𝑖 is: 𝜍𝑖[𝕫(𝑡)] = ∏𝛷𝑖𝑗[𝕫𝑗(𝑡)] 𝑝 𝑗=1 . (11) then: ℎ𝑖[𝕫(𝑡)] = 𝜍𝑖[𝕫(𝑡)] ∑ 𝜍𝑖[𝕫(𝑡)] 𝑟 𝑖=1 . (12) the membership of 𝕫𝑗(𝑡) in 𝛷𝑖𝑗 is: { ∑𝜍𝑖[𝕫(𝑡)] 𝑟 𝑖=1 > 0 𝜍𝑖[𝕫(𝑡)] ≥ 0,with 𝑖 = 1,2,…𝑟 . (13) we have { ∑ℎ𝑖[𝕫(𝑡)] 𝑟 𝑖=1 = 1 ℎ𝑖[𝕫(𝑡)] ≥ 0,with 𝑖 = 1,2,…𝑟 (14) for all t. 3. fuzzy stability 3.1. fuzzy model application for the longitudinal stability interval of the drone, we now have the following system: { �̇�𝑉 = 𝑣 𝐿w ≅ 1 2 𝑣2 𝔽 𝛼w (15) where 𝑣min < 𝑣 ≤ 𝑣max (16) 𝑣min = minimum allowed speed (= 𝑣stall) 𝑣max = maximum allowed speed according to the linearity interval. so (15) becomes: [ 𝑥1 𝑥2 ] = [ �̇�𝑉 𝐿w ] = [ 1 0 𝑣 𝔽 𝛼w 1 2 𝑣2 𝔽 ] ∙ [ 𝑣 𝛼w ] . (17) the fuzzy variables 𝑣 and 𝛼𝑤 are nonlinear terms in the expressions. posing z1 and z2 as premise variables that may be functions of state variables, [ 𝑧1 𝑧2 ] = [ 𝑣 𝔽 𝛼𝑤 1 2 𝑣2 𝔽 ] (18) we can express (17) as: [ 1 0 𝑣 𝔽 𝛼w 1 2 𝑣2 𝔽 ] = [ 1 0 𝑧1 𝑧2 ] . (19) now we should calculate the minimum and maximum of the parameters: { 𝑣 ∈ [𝑣stall 𝑣max] 𝛼w ∈ [−4 8] , in °. (20) therefore, expanding (19), we have the limits: { max 𝑧1 𝑣,𝛼w = 8 𝑣max 𝔽 min 𝑧1 𝑣,𝛼w = −4 𝑣stall 𝔽 max 𝑧2 𝑣,𝛼w = 1 2 𝑣max 2 𝔽 min 𝑧2 𝑣,𝛼w = 1 2 𝑣stall 2 𝔽 . (21) therefore 𝑣 and 𝛼w can be represented by membership functions m1, m2, n1 and n2 as follows: 𝑧1 = m1(𝑧1) ∙ (8 𝑣max 𝔽) + m2(𝑧1) ∙ (−4 𝑣stall 𝔽) (22) 𝑧2 = n1(𝑧2) ∙ ( 1 2 𝑣max 2 𝔽)+ n2(𝑧2) ∙ ( 1 2 𝑣stall 2 𝔽) (23) acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 109 where { m1(𝑧1) + m2(𝑧1) = 1 n1(𝑧2) + n2(𝑧2) = 1 (24) now we introduce the model rule. this is the liaison between real physical elements and the limits. #1: if z1 is ‘high’ and z2 is ‘big’ then �̇�𝑉 = 𝐴1 ∙ 𝑥𝑉 #2: if z1 is ‘high’ and z2 is ‘small’ then �̇�𝑉 = 𝐴2 ∙ 𝑥𝑉 #3: if z1 is ‘low’ and z2 is ‘big’ then �̇�𝑉 = 𝐴3 ∙ 𝑥𝑉 #4: if z1 is ‘low’ and z2 is ‘small’ then �̇�𝑉 = 𝐴4 ∙ 𝑥𝑉 where for the an parameter we have: 𝐴1 = [ 1 0 max 𝑧1 𝑧1 ∈ ℎ𝑖𝑔ℎ max 𝑧2 𝑧2 ∈ 𝑏𝑖𝑔 ] = [ 1 0 8 𝑣max 𝔽 1 2 𝑣max 2𝔽 ] (25) 𝐴2 = [ 1 0 max 𝑧1 𝑧1 ∈ ℎ𝑖𝑔ℎ max 𝑧2 𝑧2 ∈ 𝑠𝑚𝑎𝑙𝑙 ] = [ 1 0 8 𝜌 𝑣max 𝔽 1 2 𝑣stall 2 𝔽 ] (26) 𝐴3 = [ 1 0 max 𝑧1 𝑧1 ∈ 𝑙𝑜𝑤 max 𝑧2 𝑧12 ∈ 𝑏𝑖𝑔 ] = [ 1 0 −4 𝜌 𝑣stall 𝔽 1 2 𝑣max 2 𝔽 ] (27) 𝐴4 = [ 1 0 max 𝑧1 𝑧1 ∈ 𝑙𝑜𝑤 max 𝑧2 𝑧12 ∈ 𝑠𝑚𝑎𝑙𝑙 ] = [ 1 0 −4 𝑣stall 𝔽 1 2 𝑣stall 2 𝔽 ] (28) now, �̇�𝑉 can be derived from the defuzzification process as: �̇�𝑉 = ℎ1(𝑧) 𝐴1 ∙ 𝑥𝑣 + ℎ2(𝑧) 𝐴2 ∙ 𝑥𝑉 + ℎ3(𝑧) 𝐴3 ∙ 𝑥𝑀𝑉 + ℎ4(𝑧) 𝐴4 ∙ 𝑥𝑉 (29) in which { ℎ1(𝑧) = 𝑀1(𝑧1) × 𝑁1(𝑧2) ℎ2(𝑧) = 𝑀1(𝑧1) × 𝑁2(𝑧2) ℎ3(𝑧) = 𝑀2(𝑧1) × 𝑁1(𝑧2) ℎ4(𝑧) = 𝑀2(𝑧1) × 𝑁2(𝑧2) (30) this fuzzy model exactly represents the nonlinear system in the region [𝑣stall 𝑣max] × [−4° 8°] in the 𝑣,𝛼w space. as is evident in equations (29) and (30), the responses of the fuzzy model can exactly follow the responses of the original equations, which means the fuzzy model can exactly represent the original system in the pre-specified domains. that is to say, inside of the boundaries of 𝑣 and 𝛼w, the above approach can accurately represent the original system. the advantage of this method is immediate: the calculation effort is enormously decreased. in practice, everything is resolved through linear systems or simple multiplication between square matrices. 3.2. the simulation the simulation was performed with the combined matlab®/ simulink® tool, and the results were validated separately on a sample of 44 specific points. the difference between a classical solution and a fuzzy one is simple: a classical set contains elements that satisfy precise properties of membership, while a fuzzy set contains elements that satisfy imprecise properties of membership. figure 3 illustrates the fuzzification procedure for the problem, which may be defined as transforming the available data from a crisp set into a fuzzy set. basically, this operation translates accurate crisp input values into linguistic variables. for this, it is necessary to define the rules, the membership functions and the architecture. subsequently, thanks to the t-s method (as explained above), a mathematical result is reached. figure 4 illustrates the data evaluation process. initially, a rather large set of real data obtained from tests on real drones (about 13 gb) was produced. these were processed in parallel both with the fuzzy method and with the system method of equations. since we knew the solution, we could therefore establish the percentage of error introduced in each method. obviously and as expected, the second provides less approximate data but requires much more calculation. to determine the difference, each solution was associated with a marker providing the calculation time. at the end of the process, we added up the time data and found that, on average, the t-s method reduced calculation time by 83.7 % for each cycle, thus allowing for much faster physical feedback. this speed is paid for with greater error, as we will see in the next paragraph, but the error is always within an absolutely acceptable range for our navigation system. 3.3. the error figure 5 shows the results of our work, with the blue line representing the percentage error of the approximate solution. takagi-sugeno is traced with respect to the ‘exact’ solution that was calculated with recursive and time-consuming numerical methods. for completeness, we have also included the generated simulation of the completed aircraft (red line). the green line indicates the difference between the two. the excellent behaviour of the t-s approximation within the linearity interval figure 3. fuzzification process. figure 4. simulation block architecture. acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 110 −4 ° ≤ 𝛼𝑤 < 8 ° is immediately visible, although it decays rapidly outside this non-linear characteristic. 4. conclusions in a more dynamic context, the execution speed of the algorithms as well as the structure or shape of drones helps to make some critical manoeuvring phases more agile by drastically reducing the computational cost of the analysis (see figure 5). for this reason, we feel our work is justified. the drone used in our case study was a fixed-wing uav that required micro-tapping of all ancillary and non-viable systems. fuzzy logic was used to reduce the complexity of the longitudinal stability equations, which are critical for flying wings, and thus reduce the computational load on the fcs. our approximate calculation method allowed us to have a very light calculation effort at the price of a negligible error in terms of the size and dynamics of the craft. appendix a: the horten aerofoil we chose the horten (t/c = 11 %, f/c = 2 %) nonsymmetrical aerofoil. it behaves quite well aerodynamically in the expected speed range, with a stall preceded by buffeting at a reasonably low stall speed (see figure 6a and 6b). in order to move the aerodynamic centre backwards, a 30degree sweep angle was chosen, resulting in a cg that was not too far forward but provided a noticeable decrease in drag [35]. for reference, we will consider the aerofoil to which the lift force of the wing is applied (ideally). this aerofoil was originally designed and used by the horten brothers, who built a number of high-performance sailplanes during the 1930s and 1940s. their designs are legendary, although some of their published performance data are a little bit too good [36]. their planes were well known for their good handling characteristics, which could be of greater importance than the actual performance figures from our project [37]. a.1 the longitudinal stability for the balance of the forces around the y-axis we have (see figure 7): 𝑀cgw = 𝐿w cos(𝛼w − 𝑖w) ∙ (𝑥cg − 𝑥ac) + 𝐷w sin(𝛼w − 𝑖w) ∙ (𝑥cg − 𝑥ac) + 𝐿w sin(𝛼w − 𝑖w) ∙ (𝑧cg − 𝑧ac) + (−𝐷w)(𝛼w − 𝑖w) ∙ (𝑧cg − 𝑧ac) + 𝑀acw (31) where 𝑀cgw = moment of all the wing forces around the centre of gravity 𝐿w = lift force of the wing 𝐷𝑤 = drag force of the wing 𝛼w = angle between the main chord and the wind direction (angle of attack) 𝑖w = angle between the main chord and the x-axis 𝑥cg = distance of centre of gravity from the z-axis 𝑥ac = distance of aerodynamic centre of gravity from the z-axis 𝑧cg = distance of centre of gravity from the x-axis 𝑧ac = distance of aerodynamic centre from the x-axis 𝑀acw = moment of all the wing forces around the aerodynamic centre c.g.= centre of gravity a.c.= aerodynamic centre now we consider that if the angle of attack is reasonably small, the lift is much less than the drag: 𝐿w ≫ 𝐷w (32) and for small angles: 𝛼w − 𝑖w ≪ 1 (33) figure 5. error vs. angle of attack. the t-s solution is blue, the total simulation is red and the difference is green. a) b) figure 6. horten (t/c = 11 %, f/c = 2 %) aerofoil a) flow field and pressure gradient of conditions: speed = 60 km/h, re = 105 and αw = 0° and b) cl vs. cd behaviour. acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 111 so we have: sin(𝛼w − 𝑖w) ≈ 0 (34) and cos(𝛼w − 𝑖w) ≪ 1 . (35) for thin aerofoils and small angles, we have: 𝑧cg = 𝑧ac ≈ 0 (36) so equation (31) becomes 𝑀cgw = 𝐿w ∙ (𝑥cg − 𝑥ac) + 𝑀acw . (37) and the new stability conditions are: 𝑀cgw − 𝑀acw = 𝐿w ∙ (𝑥cg − 𝑥ac) . (38) so 𝐿w = 𝑀cgw − 𝑀acw 𝑥cg − 𝑥ac . (39) this equation represents the longitudinal stability of the craft. now we consider the cl vs. αw characteristics of the aerofoil. the expression for the lift l is: 𝐿w = 1 2 𝜌 𝑣2 𝑆 𝐶𝐿 . (40) applying this to (39), we have: 1 2 𝜌 𝑣2 𝑆 𝐶𝐿 = 𝑀cgw − 𝑀acw 𝑥cg − 𝑥ac (41) where 𝜌 = air density 𝑆 = wing surface 𝑣 = relative speed (in reference to the air) 𝐶𝐿 = coefficient of lift for the horten aerofoil, according to the characteristic 𝐶𝐿 vs. 𝛼w, the angle of attack in the interval [-4 °; +10 °], is linear (see the red zone of figure 8). we can linearise equation (40), and we have: 𝐿w = 1 2 𝜌 𝑣2 𝑆 𝑘 𝛼w . (42) posing: 𝔽 = 𝜌 𝑘 𝑆 (43) we have: 𝐿w = 1 2 𝑣2 𝔽 𝛼w . 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[36] yomg fan, j.-h. zhu, x.-y. meng, kai liu, x.-l. yang, intelligent method based coordinated integrated flight control of a tailless stovl, 8th world congress on intelligent control and automation, jinan, china, 2010, pp. 85-90. doi: https://doi.org/10.1109/wcica.2010.5553863 [37] r. murray-smith, t. a. johanson, operating regime approach to nonlinear modeling and control, in: multiple model approaches to modeling and control. r. murray-smith, t. johanson (editors). taylor & francis, uk, isbn: 0-7484-0595-x, pp. 3-72. https://doi.org/10.1109/icnc.2008.91 https://doi.org/10.1109/wcica.2006.1713068 https://doi.org/10.1109/wcica.2010.5553863 metrological characterization of a textile temperature sensor in archaeological usage acta imeko issn: 2221-870x march 2021, volume 10, number 1, 166 172 acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 166 metrological characterisation of a textile temperature sensor in archaeology andrea zanobini1 1 department of information engineering, university of florence, via santa marta, 3 florence, italy section: research paper keywords: resistance temperature detector; temperature sensor; textile industry; smart textiles; wearable systems; textile sensors; yarn; pressure sensor citation: andrea zanobini, metrological characterization of a textile temperature sensor in archaeological usage, acta imeko, vol. 10, no. 1, article 22, march 2021, identifier: imeko-acta-10 (2021)-01-22 editor: carlo carobbi, university of florence, italy received may 21, 2020; in final form february 26, 2021; published march 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: andrea zanobini, e-mail: andrea.zanobini@unifi.it 1. introduction ‘temperature and relative humidity are essential elements in museums and in collections care. get the conditions wrong and items could be found ruined. mould, pests, deterioration, and warping are just a few of the problems that can happen if these elements are not stable and controlled.’ [1] 1.1. humidity all organic materials degrade in humid conditions. plants and animals contain a high proportion of water, so it is unsurprising that their products also retain moisture. when materials absorb and retain moisture, they are described as hygroscopic. such materials can and will absorb or give off moisture until they reach a state of equilibrium with the air that surrounds them. these materials include: wood, paper, cotton, linen, wool, silk, parchment, leather, fur, feathers, ivory, bone and horn. when the surrounding air is very dry, organic materials will give off some of their moisture. they become brittle and may shrink, warp, split or crack. when the surrounding air is damp, the materials will absorb some of the moisture from the air. they may swell, cockle, warp, change shape or lose strength. dampness can also cause mould and fungal growth on organic materials. inorganic materials such as glass, ceramics, metals and minerals are also affected by high or low humidity. materials that have a natural salt content may suffer from efflorescence when the air is dry. the salts in deteriorated glass, porous ceramics and some geological material are carried to the surface by moisture (which may have entered the pores during a period of higher humidity). the moisture evaporates, and the salts crystallise on the surface. other effects on inorganic materials include: corrosion in metals, fading in dyes and pigments and pyrite decay in geological materials. if the humidity of the air changes frequently, hygroscopic materials will swell and shrink repeatedly. this causes internal stress and damage. this is particularly a problem in composite objects made up of materials with different rates of shrinkage. the expansion of one material may force changes in the dimensions of another, causing considerable tension and eventually damage. such damage can be observed in items like skins on drums and paintings on wooden panels. moisture can also start or speed up the damaging effect of air pollutants and other harmful substances on many museum items. 1.2. temperature objects themselves are rarely directly affected by temperature. fluctuating heat, however, can damage or compromise items in indirect ways. uncontrolled temperatures can cause changes in humidity, damaging sensitive objects through fluctuations in relative humidity (rh). this is the main reason for controlling temperature. other reasons have to do with the way that temperature changes can speed up chemical processes and biological activity, making certain materials expand and contract. this is particularly damaging for composite abstract this paper presents the study of a new generation textile temperature sensor in two different heated ovens. the first chamber was used to evaluate temperature and the second was used to evaluate both temperature and humidity. data acquisition systems based on labview and agilent were developed using thermocouples and pt100 sensors. the results show many metrological characteristics that prove that the sensor is a resistance temperature detector. mailto:andrea.zanobini@unifi.it https://www.museumsgalleriesscotland.org.uk/advice/collections/identifying-and-reducing-air-pollution/ acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 167 materials with parts that expand at different rates. additionally, high or low temperatures can affect the comfort of people working with or visiting collection items. moreover, temperature and humidity affect each other. the humidity of the air depends on the temperature. while one cubic metre of air holds 10 g of water at 10 °c, the same volume can hold over 30 g when the air is heated to 30 °c. measuring absolute humidity – the number of grammes of moisture in the air – doesn't mean much when monitoring a museum's environment. 10 g of water feels damp at 10 °c but will seem dry at 30 °c. instead, the environment in museums is measured in terms of relative humidity (rh). this is expressed as a percentage of the maximum amount of water the air can hold at that temperature. for example: • at 10 °c, 10 g of water is the maximum amount the air can hold, so the rh is 100 %. • at 30 °c, 10 g is about one-third of the maximum amount, therefore the rh is approximately 33 %. when the temperature changes, for instance after the sun has set, so does the rh. this principle is one of the most important factors in environmental control in a museum. 1.3. environmental recommendations the recommended temperature for museum items is 16 to 20 °c. moderate fluctuations of temperature between 10 and 20 °c are unlikely to adversely affect museum items. rooms below 16 °c become too uncomfortable for visitors, while anything below 10 °c can cause condensation and affect rh. temperatures above 20 °c will be too hot for guests and can accelerate degradation in museum objects. museum stores can be cooler than 16 °c as they are not frequented by visitors. it is important to remember that items will need to acclimatise gradually between storage and display. relative humidity should not drop below 40 % or rise above 70 %. relative humidity below 40 % can cause sensitive items to become dry and brittle. the maximum level is determined by the point where fungal growth begins, which is at an rh of at least 70 %. reducing the fluctuation of rh is also important. but these are just broad recommendations. some items and materials require more specifically controlled levels of relative humidity. for more information, read our guide to items and their ideal conditions. in a naturally ventilated building in sound condition, indoor conditions will respond to outdoor conditions. there are, however, mitigating factors that can affect the temperature and humidity inside a building, room or display case. 1.4. continuous monitoring with sensors as so many factors can affect the temperature and humidity in a museum, the environment must be regularly monitored to keep track of fluctuations. after 12 months of monitoring, it is possible to develop a good idea of the changing environmental conditions in a museum. the information gained from monitoring can be used to decide where and how to display sensitive items in collections and to determine investments in control equipment when necessary. when continually monitoring a museum, particular attention must be paid to regular, frequent fluctuations, which can cause significant damage to museum items. the changes take place slowly enough for the objects to adjust but fast enough to cause frequent movement, stress and fatigue in the material. occasional very rapid fluctuations, within one or two hours, will have a less damaging effect on items. very gradual fluctuations will give the items enough time to acclimatise slowly. some types of items are more affected by fluctuations than others. when transporting items to an area with different environmental conditions, items must be kept well packed and wrapped in acid-free tissue and a box or blanket. this will ensure that acclimatisation to the new conditions can take place gradually. maintaining stable humidity and temperature in a museum can use up a lot of energy. this is expensive and can also increase a building's negative environmental impact. although maintaining stability is essential for the care of museums, some strategies can be put in place to increase energy efficiency. a slightly more flexible approach to environmental control is to be taken, and it is important to look for ways to reduce artificial means of altering temperature and humidity. this may mean allowing a greater range of desired rh and temperature, but it can reduce carbon footprint. 2. textile sensors fabric-based wearable devices help achieve high levels of pervasive and distributed sensing and computing in social, environmental or industrial contexts, with little or no user intervention. in this paper, we study the metrological characterisation of these devices, which provide high sensitivity and high spatial resolution and are easy to manufacture. they do not disturb the wearer, which is a critical factor in complex and hazardous workplaces such as the military, rescue units and fire stations. furthermore, these soft sensors can easily be incorporated into automotive applications or used in educational laboratories and professional trainings to introduce smart textiles, piezoresistive materials, nano-composites or signal and image processing concepts. wearable computing refers to the concept of a computer that could be worn on the body. due to their ability to easily reach a wide audience in both consumer segments and professional markets and driven by the use of several key enabling technologies such as low cost sensors, miniaturised electronics and the development of cloud-based iot services, wearables are becoming a pervasive and cost-effective technology. common wearables are made of materials with electrical and mechanical characteristics that can restrict their use. textiles embedded with electronics are usually more flexible and stretchable and are manufactured in a much simpler way. furthermore, they do not disturb the wearer, which is a critical factor in complex and hazardous workplaces (e.g. the military, rescue units or fire stations). smart textiles are textile fabrics that incorporate nontextile elements to sense, compute, actuate or adapt to given situations [2]–[3]. different transduction strategies have been demonstrated for smart textiles exploiting the capability to sense for force, pressure, strain or thermal fields, but a promising class of smart textiles is piezo-resistive textile sensors made from carbon nanotubes or carbon black-based nano-composites, since they provide the best trade-off between the device’s electrical and mechanical properties and the production costs. this paper presents the findings of the metrological characterisation of piezo-resistive fabric-based cost-effective sensors with high sensitivity and spatial resolution that are suitable to be used in wearable computing to measure pressure and thermal fields. the textile compounds used in this study were developed by plug&wear (plug&wear srl, it, www.plugandwear.com). this textile technology has been validated in a few previous works and https://www.museumsgalleriesscotland.org.uk/advice/collections/temperature-and-humidity-in-museums/ acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 168 in a wide variety of applications such as monitoring vital functions. textile sensors are the future. they are fabrics that feature electronics and have interconnections woven into them [2], [3] allowing for physical flexibility. they have been used for typical and robot-assisted non-invasive endoscopy capsule positioning [4], [5], rehabilitation studies and gaming and sport activities [7] as well as to develop a multimodal sensing framework to support the development of social skills [8] and to enhance human–robot interaction in a smart environment [10], [10]. this paper is organised as follows: in section 2, the design of the conductive layers and the design of the electrodes for the textile matrix pressure sensors are introduced. in sections 3 and 4, a metrological investigation of sensor characteristics is presented, highlighting the effectiveness of the sensors and their capabilities in the presence of pressure and an external thermal field. piezo-resistive sensors can be made in sizes that cannot be achieved with other existing electronic manufacturing. conductive fibres are the key elements for building these sensors [3]. the soft sensor investigated in this work is able to sense pressure or an external thermal field and is a resistance temperature detector (rtd) [4], according to the builder. piezoresistive textile soft sensors are made from layers of knitted textile fabric that are sewn to form one unit, with the piezoresistive nano-material in the innermost layer [6]. knitted power supply lines and data buses are fabricated by alternating conductive and not conductive yarns on the textile sheets, which are in direct contact with the sensitive layer. the pressure sensing element, or sensel, is at the intersection of two conductive lines in different layers. in the sensor under investigation, data buses are interfaced through conventional connectors to external cables that connect to an arduino mega 2560 r31 through a custom breakout board [2], [3]. software developed in labview displays and processes the acquired signals. exploiting the variation of the resistivity of some material at different temperatures, an innovative all-fabric single-layer (rtd) sensor is presented. using this layered architecture, three types of fabricbased sensors that differ based on the conductive materials of the fabric will be considered: fabric a conductive lines are made of copper wire (100 _m in width) and insulating material is coated copper wire (112 _m in width); fabric b fabric is made of polyester yarns alternated with yarns of fibres metallised with silver; rtd fabric all-fabric single-layer rtd sensor. it exhibits very low thermal inertia and hysteresis. the mechanical properties of textile fabrics depend on their manufacturing and the material of the yarns. in knitted fabrics, loops can be easily stretched in different directions, and the fabric overall is highly stretchable within the constraints of the material the yarns are made of. in particular, replacing metal yarn with silveror carbon black-coated polyester threads leads to a more elastic and stretchable textile matrix and significantly reduces the thickness of the compound of the sensor, thus increasing its capacity to adapt its shape to the human body for wearable applications. the purpose of this study is to determine whether the sensor is an rtd by calculating the linearity of the textile sensor [4] and searching for a mathematical function similar to the theoretical model 𝜌(𝑇) = 𝜌0 ∙ [1 + 𝛼(𝑇 − 𝑇0)]. by using 𝑅 = 𝜌 ∙ 𝐿 𝑆⁄ , it is possible to use the resistive model 𝑅(𝑇) = 𝑅0 ∙ [1 + 𝛼(𝑇 − 𝑇0)]. first, the study examined the behaviour of temperature in an internal science oven situated in a reliability laboratory at the analytical cetace test laboratory in scandicci (firenze, italy). next, the behaviour of temperature and humidity was analysed in the second oven chamber. this document proposes to characterise the textile [6] temperature sensor made by plug&wear srl (firenze, italy). the temperature experiment [7] was divided into four stages. in the first step, all contact resistances were avoided, and the sensor was placed on an insulating material. the second step was aimed at implementing a measurement system by developing a specific software through labview. the third step consisted of data acquisition, and finally, the study results were determined. temperature acquisition was performed with an agilent software. the humidity experiment required the same stages. to measure temperature during both experiments, thermocouples and one pt100 [8], [10] were supported by the sensor, but data elaboration was performed using the pt100 because of its high accuracy. data were collected for linearity, hysteresis, sensibility, output range and input range. 3. methodology 3.1. resistance settings resistance was measured using the 4-wire method and a digital multimeter hp-34401a [10]. resistance acquisition was performed using an application of labview [12], as shown in figure 1. to connect the multimeter to a pc, a national instruments gpb-usb-nhi was used. 3.2. temperature settings temperature was measured with an agilent 34970a data acquisition [12], [14] using a pt100, as shown in figure 2. temperature acquisition was performed using the agilent benchlink data logger software. to interface a pc and the data acquisition, an agilent usb/gpib was employed. figure 1. code created in labview with 34401a drivers and front panel. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 169 3.3. first experiment and analysis method data were collected every 5 seconds throughout the testing period, both for temperature and for resistance. results were reworked using excel. the experiment setup is shown in figure 3. all the measurements were taken after allowing the instruments to warm up for about 30 minutes. the temperature was increased quickly [14] from −10 °c to 103 °c using a weiss sb11500 chamber. 3.4. second experiment and analysis method the temperature was increased by steps of 5 degrees centigrade from -10 °c to 80 °c using a challenge 1200 acs chamber, as shown in figure 4. once the temperature was exactly at the set point, data were collected cyclically every 5 seconds for a period of 15 minutes. the results were reworked using excel. the pt100 was positioned 5 millimetres from the sensor, as observable in figure 5. all the measurements were taken after allowing each instrument to warm up for about 30 minutes. 3.5. third experiment and analysis method humidity [16] was analysed by setting the temperature at 30 °c and varying the humidity from 30 % to 90 %, dividing the range into 4 steps. at the set point, data was collected every 5 seconds for a period of 30 minutes using the same tools and automated acquisition system as in the previous experiments. humidity was studied in a challenge 1200 acs oven chamber. the results were reworked using excel. 4. data analysis 4.1. first experiment data the aim of this study was to determine how the sensor responds to a rapid change in temperature. as seen in figure 6, resistance variation seems linear, but the resistance values are not correct because of a quick temperature overview. data have never stabilised. figure 2. temperature sensor and pt100. figure 3. first experiment setup. figure 4. challenge 1200 acs oven chamber. figure 5. pt 100 positioned 5mm from sensor. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 170 4.2. second experiment data this study was done to obtain a function and more accurate data than the previous study by increasing the sampling density. due to sensor linearity, sampling was done every 5 degrees centigrade in 19 steps. the results are shown in figure 7. data were analysed using the statistical method of linear regression and an additional excel tool. r = 0.8232 t + 193.98 characterises the sensor. sensor sensitivity [16] is defined as 𝑆 = 𝑑𝑦 𝑑𝑥 and so is equal to 0.8232 ω/°c. because of this result, the uncertainty of the measurement was calculated to understand its effect on the resistance values. type a uncertainty [17] was evaluated by statistical method due to the result of repeated measurements of resistance at the same temperature and is equal to 𝑢𝐴 = 𝑠 √𝑛⁄ , where s = standard deviation and n = samples acquired. type b uncertainty was evaluated using information obtained from the manufacturer’s manual and is equal to 𝑢𝐵 = 𝑎 √3⁄ , where 𝑎 = the accuracy of the instrument. total uncertainty is calculated as 𝑢𝑇 = √𝑢𝐴 2 + 𝑢𝐵 2. all of the results are shown in table 1 and figure 8. 4.3. third experiment data the objective of this investigation was to evaluate the impact of humidity [19] on resistance value. as with the prior measurements, uncertainty was calculated as shown in table 2. figure 9 shows the results. 5. conclusions the experiments produced the following metrological characteristics: linearity, sensitivity equal to 0.8232 ω/°c, measurement range from −10 °c to 81.5 °c, operating range from (185.88 ±0.02) ω to (260.96 ±0.02) ω, non-significant hysteresis. no significant variation of resistance values was obtained by introducing the factor of humidity. during the first experiment, the feasibility and repeatability of the method were demonstrated. during the second and third experiments, the figure 6. preliminary graph of resistance as a function of temperature, with all sampled points in blue. figure 7. final graph of resistance as a function of temperature, with all sampled points in blue and the linear regression line in black. table 1. results of resistance measurement. t in °c r in ω u in ω t in °c r in ω u in ω −10 185.88 ±0.02 35.7 223.24 ±0.02 −5 189.74 ±0.02 40.7 227.46 ±0.02 0 193.88 ±0.02 45.7 231.67 ±0.02 5 198.06 ±0.02 50.8 235.84 ±0.02 10 202.14 ±0.02 55.8 240.01 ±0.02 15.3 206.24 ±0.02 60.9 244.23 ±0.02 20.3 210.32 ±0.02 66 248.38 ±0.02 25.3 214.84 ±0.02 71.2 252.60 ±0.02 30.3 219.06 ±0.02 76.5 256.91 ±0.02 81.5 260.96 ±0.02 table 2. resistance measurement with humidity effects. resistance in ω humidity in % uncertainty in ω 219,37 30 ±0,02 219,36 50 ±0,02 219,36 70 ±0,02 219,50 90 ±0,02 figure 8. chart of all the resistance samples at the same temperature. figure 9. histogram representing a non-significant variation of resistance depending on humidity. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 171 reproducibility of the method was shown. the aim of this investigation was also to create a function able to take resistance values and turn them into temperature values, as shown in figure 10. the final function is t = 1.214 r – 235.62, an important result. questions could be raised regarding hysteresis due to the acquisition method. data were acquired during the phase in which the weiss sb11500 chamber was cooling to -10 °c. however, dense sampling was not done in certain steps. resistance values in descent are very similar to those that would be found using the function obtained during the second experiment. it can be suggested that additional hysteresis studies should be done in the future. recently, important studies on temperature effects in archaeology have been performed in our research group (the measurements and reliability laboratory of the information engineering department, university of florence). during the test for relative humidity at 90 %, the challenge 1200 acs oven chamber had problems maintaining the temperature at 30 °c. therefore, since the sensor is very sensitive to even small changes in temperature, it has been deduced that the increase of approximately 0.13 ω in resistance values is due to this factor. it would be interesting to increase the measurement and operating range to reach the edge temperatures of the oven chambers. also, all of the functions could be refined by making a broad sampling degree by degree. many studies have focused on this sensor with regard to pilling, aging, textile strain, salt fog testing, chemical sweat and many other topics. several future uses for textile sensors could be suggested to add to the one analysed in this paper, such as implementation in biomedical systems for measuring body temperature in infants or measuring the temperature of a surface or tube, especially in very tight spaces that cannot be accessed by thermal imaging cameras. acknowledgment the authors would like to thank ing. lorenzo quartini for his important contribution to the first draft of this article. references [1] museums galleries scotland, temperature and humidity in museums, online [accessed 25 march 2021] https://www.museumsgalleriesscotland.org.uk/advice/collection s/temperature-and-humidity-in-museums/ [2] m. stoppa, a. chiolerio, wearable electronics and smart textiles: a critical review, sensors 14(7) (2014), pp. 11957-11992. doi: 10.3390/s140711957 [3] r. zeitler, a new optical sensor for measuring the velocity of webs by correlation techniques, ieee transaction on instrumentation and measurement 47(1) (1998), pp. 289-292. doi: 10.1109/19.728836 [4] l. capineri, resistive sensors with smart textiles for wearable technology: from fabrication processes to integration with electronics, procedia engineering 87 (2014), pp. 724-727. doi: 10.1016/j.proeng.2014.11.748 [5] a. chen, j. zu, b. li, temperature compensation for hot-film crosswind sensors, ieee instrumentation and measurement technology conference, ottawa, canada, may 19-21, 1997, vol. 2, pp. 1086-1088. doi: 10.1109/imtc.1997.612368 [6] a. song, y. han, h. hu, j. li, a novel texture sensor for fabric texture measurement and classification, ieee transaction on instrumentation and measurement, 63(7) (2014), pp. 1739-1747. doi: 10.1109/tim.2013.2293812 [7] p. godts, d. dupont, d. leclercq, direct measurement of the latent heat of evaporation by flowmetric method, ieee transaction on instrumentation and measurement 54(6) (2005), pp. 2364-2369. doi: 10.1109/tim.2005.858554 [8] v. h. carvalho, p. j. cardoso, m. s. belsley, r. m. vasconcelos, f. o. soares, yarn hairiness characterization using two orthogonal directions, ieee transaction on instrumentation and measurement 58(3) (2009), pp. 594-601. doi: 10.1109/tim.2008.2005082 [9] w. nawrocki, measurement systems and sensors, artech house, london, 2016, isbn 13: 978-160807-932-2. [10] m. arai, k. yamazaw, high-temperature furnace controlled by a platinum resistance thermometer, sice 2004 annual conference 2, sapporo, japan, 4-6 august 2004, pp. 1172-1175. doi: 10.11499/sicep.2004.0_66_4 [11] agilent technologies\hp, hp 34401a multimeter, (2000). [12] l. wen, j. x. jin, x. y. chen, l. jiang, a universal labviewbased hts device measurement and control platform and verified through a smes system, ieee transaction on applied superconductivity 24(5) (2014), pp. 1-5. doi: 10.1109/tasc.2014.2349897 [13] agilent technologies, agilent 34970a data acquisition/switch unit family product overview 34970a 34972a, (2010). [14] b. saggin, m. tarabini, g. lanfranchi, a device for the skin– contact thermal resistance measurement, ieee transaction on instrumentation and measurement 61(2) (2012), pp. 489-495. doi: 10.1109/tim.2011.2169179 [15] d. ha, k. woo, s. meninger, t. xanthopoulos, e. crain, d. ham, time-domain cmos temperature sensors with dual delay-locked loops for microprocessor thermal monitoring, ieee transaction on very large integration (vlsi) systems 20(9) (2012), pp. 15901601. doi: 10.1109/tvlsi.2011.2161783 [16] m. d. husain, o. atalay, r. kennon, effect of strain and humidity on the performance of temperature sensing fabric, international journal of textile science 2(4) (2013), pp. 105-112. doi: 10.5923/j.textile.20130204.04 [17] a. martinez-ríos, g. anzueto-sanchez, r. selvas-aguilar, a. a. c. guzman, d. toral-acosta, v. guzman-ramos, v. m. duranramirez, j. a. guerrero-viramontes, c. a. calles-arriaga, high sensitivity fiber laser temperature sensor, ieee sensor journal 15(4) (2015), pp. 2399-2402. doi: 10.1109/jsen.2014.2377654 [18] j. r. taylor, an introduction to error analysis: the study of uncertainties in physical measurements, university science books, sausalito, california, 1997, isbn 978-0-935702-42-2. [19] r. bhunjun, r. w. vogt, sensor system for contactless and online moisture measurements, ieee transaction on instrumentation and measurement 50(11) (2010), pp. 3034-3040. doi: 10.1109/tim.2010.2046692 [20] a. proietti, f. leccese, m. caciotta, f. morresi, u. santamaria, c. malomo, a new dust sensor for cultural heritage applications based on image processing, sensors 14(6) (2014), pp. 9813-9832. doi: 10.3390/s140609813 figure 10. final graph of temperature as a function of resistance, with all sampled points in blue and the linear regression line in black. https://www.museumsgalleriesscotland.org.uk/advice/collections/temperature-and-humidity-in-museums/ https://www.museumsgalleriesscotland.org.uk/advice/collections/temperature-and-humidity-in-museums/ https://doi.org/10.3390/s140711957 https://doi.org/10.1109/19.728836 https://doi.org/10.1016/j.proeng.2014.11.748 https://doi.org/10.1109/imtc.1997.612368 https://doi.org/10.1109/tim.2013.2293812 https://doi.org/10.1109/tim.2005.858554 https://doi.org/10.1109/tim.2008.2005082 https://doi.org/10.11499/sicep.2004.0_66_4 https://doi.org/10.1109/tasc.2014.2349897 https://doi.org/10.1109/tim.2011.2169179 https://doi.org/10.1109/tvlsi.2011.2161783 https://doi.org/10.5923/j.textile.20130204.04 https://doi.org/10.1109/jsen.2014.2377654 https://doi.org/10.1109/tim.2010.2046692 https://doi.org/10.3390/s140609813 acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 172 [21] l. maiolo, f. maita, a. castiello, a. minotti, a. pecora, highly wearable wireless wristband for monitoring pilot cardiac activity and muscle fine movements, proc. of the 4th ieee international workshop on metrology for aerospace, metroaerospace 2017, padua, italy, 21-23 june 2017, art. no. 7999578, pp. 271-275. doi: 0.1109/metroaerospace.2017.7999578 [22] g. mattana, t. kinkeldei, d. leuenberger, c. ataman, j. j. ruan, f. molina-lopez, a. vásquez, g. nisato, g. tröster, d. briand , n. f. de rooij, woven temperature and humidity sensors on flexible plastic substrates for e-textile applications, ieee sensors journal 13(10) (2013), pp. 3901-3909. doi: 10.1109/jsen.2013.2257167 [23] l. m. castano, a. b. flatau, smart fabric sensors and e-textile technologies: a review, smart materials and structures 23(5) (2014), p. 053001. doi: 10.1088/0964-1726/23/5/053001 https://doi.org/0.1109/metroaerospace.2017.7999578 https://doi.org/10.1109/jsen.2013.2257167 https://doi.org/10.1088/0964-1726/23/5/053001 vibration compensation using dummy load-cell in dynamic mass measurement acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 75 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 75 78 vibration compensation using dummy load-cell in dynamic mass measurement y. yamakawa1, a. tsuzura2, t. yamazaki3 1 the university of tokyo, tokyo, japan, y-ymkw@iis.u-tokyo.ac.jp 2 tokyo denki university, saitama, japan 3 tokyo denki university, saitama, japan, yamazaki@mail.dendai.ac.jp abstract: checkweighers, that can measure the mass of a target object continuously while conveying the object on a belt conveyor, are affected by environmental vibration. as a result, accuracy of the mass measurement gets worse. to solve this problem, we have proposed a method that can eliminate the effect of the environmental vibration. this paper enhances the proposed method and improves the performance of the proposed method. keywords: checkweigher; mass measurement; load cell; vibration compensation; dummy load cell 1. introduction checkweighers are devices that can measure the mass of a target object continuously while conveying the object on a belt conveyor. the checkweighers has operating an active role in various fields such as food, medicine, and industrial products. recently, a dynamic mass measurement rather than a conventional static mass measurement has been focused on. some studies related to the dynamic mass measurement have been done actively. to achieve the dynamic mass measurement with high accuracy contributes to the continuous mass measurement at high speed. however, there exists a problem that a disturbance such as a floor vibration affects an accuracy of the dynamic mass measurement strongly. in fact, although load cell scales are often used to measure the mass of the object in the checkweigher, the measurement accuracy is susceptible to environmental vibration due to the structure of the load cells. in order to solve the problems described above, some researches have been conducted using a load cell up to the present date. kamimura et al. showed that the accuracy of mass measurement has improved by designing the fir filter that removes the environmental vibration for the load cell [1]. umemoto et al. proposed a notch filter that can remove only signals of specific frequency components, and the transfer speed is improved by applying it to the checkweigher [2]. sun et al. suggested a method to remove environmental vibration using signal processing by newly installing a load cell for vibration detection separately from the measurement load cell. in this method, the natural frequencies of the two load cells do not match (there exists a difference in dynamic characteristics), which may adversely affect the measurement accuracy. hence, by applying the principle of relative compensation, it has been shown to improve the accuracy of mass measurement [3]. in addition, it takes 40 ms to identify on-line in this method, and high-speed mass measurement is difficult. hence, the shortening of identification time was considered by the compensation of the difference in dynamic characteristics using a polynomial approximation model [4]. we have proposed emfc (electromagnetic force compensation) system for measuring the mass at high speed. moreover, we constructed the mathematical model of the emfc system, which can duplicate the dynamic behaviour with high accuracy [5]. using the mathematical model, we also proposed the reduction method for the effect of the floor vibration [6]. as the result, we successfully achieved the reduction of the effect of the floor vibration with the reduction rate of over 50 %. however, the proper mathematical model of the system is strongly required in this method. since the construction of the mathematical model is difficult and takes a time, it is desired that the model is not required. in order to solve the effect of the vibration and improve the accuracy of the mass measurement, we have also manufactured a vibration exciter using an electric cylinder combining a servomotor and a ball screw. it has been confirmed that this exciter can reproduce vibration of 0.1 hz to 10 hz. on this basis, we measured the response of the load cell when applying vibration with the manufactured exciter. in addition, we have proposed a vibration compensation method using dummy load cells [7]. http://www.imeko.org/ mailto:y-ymkw@iis.u-tokyo.ac.jp mailto:yamazaki@mail.dendai.ac.jp acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 76 the dummy load cell can be produced by using the 3d cad model of the actual measuring system. the characteristics (natural frequency and damping rate) of the dummy load cell are the same as the actual system. the dummy load cell is a measuring device that can detect only vibration components contained in the weighing load cell. in this paper, we redesign the dummy load cell and aim to improve the performance of the vibration compensation method. finally, we show the effectiveness of the proposed method through the actual experiments. 2. system of vibration experiments 2.1 vibration exciter the scale of the checkweigher used in this study is load cell type, the specifications of this device are as follows; • measuring range: 12 g – 3 000 g • scale: 0.1 g • selection accuracy: ±0.2 g a photograph of the vibration exciter is shown in figure 1. the vibration exciter is manufactured using an electric actuator (smc, ley32dnzc-100) and a servomotor (yaskawa electric, σ-ii), it is possible to control the motor by sending a position command from the host device (pc) to the servo amplifier and to vibrate in the vertical direction. as the software for measurement and control, labview was used. figure 1: photograph of vibration exciter 2.2 dummy load cell a dummy load cell shown in figure 2 is installed in the checkweigher, as shown in figure 3, for the purposes of the measurement of the floor vibration and the compensation of the vibration. the dummy load cell is required to be small because it must be placed inside the weighing conveyor. in this time, it was designed and manufactured to be less than half of the weighing load cell. the natural frequency and damping rate of the dummy load cell were adjusted using weights to coincide with those of the weighing load cell. the natural frequency and damping rate can be obtained from 3d cad model or the mathematical model [5] of the actual measuring system. figure 2: specification of dummy load cell figure 3: system of vibration compensation method 3. proposed method to reduce the effect of the floor vibration, we proposed a compensation method using the dummy load cell. the method is to obtain the difference of the outputs between the actual load cell for mass measurement and the dummy load cell. the block diagram of the proposed method is shown in figure 4. in figure 4, each element is defined as follows: • weighing lc: actual load cell for mass measurement • dummy lc b: dummy load cell (the new type) • ckt: bridge circuit • amp: amplifier of signal • lpf: low-pass filter • 𝐾𝐼𝑊: coefficient to convert from voltage to strain • 𝐾𝐵: coefficient to convert from voltage to strain • (2𝑏𝑊𝑡𝑊 2 𝐸) (3𝐿𝑊)⁄ : coefficient to convert from strain to force • (2𝑏𝐷𝑡𝐵 2 𝐸) (3𝐿𝐷)⁄ : coefficient to convert from strain to force • (𝑚 + 𝑀𝑊 ) (𝑚 + 𝑀𝐷 )⁄ : coefficient to compensate difference between force values of both load cells • 1 g : coefficient to convert from force to mass, g is the gravitational acceleration measuring part motor driver electric actuator base servo motor jig for load-cell load-cell for measurement unit: mm dummy load cell fixed jig for dummy load cell weighing load cell fixed jig for weighing load cell http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 77 figure 4: block diagram of vibration compensation table 1: qualitative evaluation of proposed method frequency in hz before compensation in g after compensation in g reduction rate in % 1 34.3 28.9 15.8 2 120 42.6 64.4 3 281 46.2 83.5 5 646 55.4 91.4 the values after the low-pass filter (lpf) in this figure calculated offline. first, the resistance change in weighing load cell is output as the voltage 𝑉𝑊 through the bridge circuit, and it was converted into strain according to the specification value (𝐾𝐼𝑊 = 1200 μs/v). the force was calculated using the relationship between force and strain at the load cell described by equation (1). 𝐹 = 2𝑏𝑡2𝐸 3𝐿 𝜀 , (1) where 𝑏 (in mm) is the length of the load cell, 𝑡 (in mm) is the thickness of the notch, 𝐿 (in mm) is the length between notches, and 𝐸 (in gpa) is young's modulus. from this relationship, the final measurement force 𝐹𝑊 can be obtained. in the dummy load cell, the voltage 𝑉𝐷 is output by the dynamic strain amplifier (tokyo measuring instruments lab. dc-97a). at this time, since the value of 4 gauge is output directly, the value is multiplied by 1/4 after the calculation of the low pass filter. the nominal force 𝐹𝐷𝑁 acting on the load cell can be obtained. each load cell is subjected to centrifugal force due to vibration, and these forces are formulated by equations (2) and (3). 𝐹𝑤 = (𝑚 + 𝑀𝑊 )𝐴𝜔 2 (2) 𝐹𝐷𝑁 = (𝑚 + 𝑀𝐷)𝐴𝜔 2 . (3) the measured value of the dummy load cell is compensated using equation (4) so that the force caused by the vibration is equal for both load cells. 𝐹𝐷 = (𝑚 + 𝑀𝑊 ) (𝑚 + 𝑀𝐷 ) 𝐹𝐷𝑁 , (4) where 𝐹𝑊 (in n) is the force applied to weighing load cell, 𝐹𝐷𝑁 (in n) is the force applied to dummy load cell [n], 𝐹𝐷 (in n) is the compensated force of dummy load cell, 𝑚 (in kg) is the mass of an object to be measured, 𝑀𝑊 (in kg) is the mass of weighing section in weighing load cell, 𝑀𝐷 (in kg) is the mass of weighing section in dummy load cell, 𝐴 (in mm) is the amplitude, 𝜔 (= 2𝜋𝑓, in rad/s) is the angular velocity and 𝑓 (in hz) is the excitation frequency. since the proposed method is simple, it is easy to implement on the actual system. in addition, since the dynamics of the dummy load cell is considered to be the same as one of the actual load cell for measuring the mass, the difference �̂� may become to be 0 theoretically. consequently, the effect of the vibration will be deleted successfully. to achieve this, accurate dummy load cell described in section 2.2 is required. however, if experiment for estimating the natural frequency and damping rate of the system is conducted and these parameters are obtained, the mathematical model of the system is not required. 4. experimental results in experiments, the weight with 0.1 kg was put on the conveyor. the amplitude was 0.5 mm and the input frequency of the vibration exciter was changed from 1 hz to 2 hz, 3 hz and 5 hz. the qualitative evaluation and the time series responses of all the experimental results are shown in table 1 and figure 5, respectively. the reduction rate is calculated by reduction rate = 𝑌 − 𝑋 𝑌 ×100 [%] where 𝑋 and 𝑌 are the estimated mass after compensation and the estimated mass before compensation, respectively. it can be seen from table 1 that the effect of the vibration in the mass measurement can be reduced. in particular, the reduction rate in the vibration of 5 hz is 91.4 % which was the maximum reduction rate. in this calculation, we used the average value of the experimental results. http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 78 figure 5: experimental results it can be also seen from figure 5 that the responses from the actual weighing load cell and the dummy load cell are almost the same. from these results, we found that the design of the dummy load cell is appropriate to duplicate the environmental vibration. we also found that the effect of the vibration can be deleted using the proposed vibration compensation method without the mathematical model of the system as in reference [7]. consequently, the effectiveness of the proposed method is clarified from the experimental results. the strong point of our method is that the proper mathematical model is not required to implement reduction method of the effect of the floor vibration. 5. conclusion to solve the effect of the vibration and improve the accuracy of the mass measurement, we manufactured the vibration exciter that can reproduce environmental vibration. comparing the response waveforms of the weighing load cell and the dummy load cell while exciting the checkweigher attached the dummy load cell, it was confirmed that the measurement of low frequency was also possible. moreover, it is shown that environmental vibration can be removed by the proposed compensation method using the output of dummy load cell. in the future, we plan to improve the detection sensitivity of small input signals. 6. references [1] k. kamimura, h. kanai, n. chubach, “design of optimum fir filter for eliminating mechanical noise in weight measurement with loadcell”, acoustical society of japan, vol. 51 no. 11, pp. 845-853, 1995. [2] t. umemoto, m. kamon, y. kagawa, “improvement of accuracy for continuous mass measurement in checkweighers with an adaptive notch filter”, trans. of the society of instrument and control engineers, vol. 47 no. 10, pp. 477-484, 2011. [3] j. sun, y. fujioka, t. ono, t. nagao, t. kohashi, “on the fast and high accurate mass measurement under the conditions of floor vibration. on the method of compensating the difference in dynamics without on-line identification”, japan society for precision engineering, vol. 64 no. 7, pp. 1029-1034, 1998. [4] j. sun, y. fujioka, t. ono, t. nagao, “on the fast and high accurate mass measurement under the conditions of floor vibration. on the method of compensating the difference in dynamics by using the approximate polynomial model”, japan society for precision engineering, vol. 65 no. 10, pp. 1450-1455, 1999. [5] y. yamakawa, t. yamazaki, “mathematical model of checkweigher with electromagnetic force balance system”, acta imeko, vol. 3, no. 2, pp. 9-13, 2014. [6] y. yamakawa, t. yamazaki, “reduction of the effect of floor vibrations in a checkweigher using an electromagnetic force balance system”, acta imeko, vol. 6, no. 2, pp. 65-69, 2017. [7] a. tsuzura, y. yamakawa, t. yamazaki, “vibration compensation for checkweigher using dummy load cell”, proc. of apmf, p. 80, 2019. (a) 1 hz weighing load cell after compensation dummy load cell weighing load cell after compensation dummy load cell weighing load cell after compensation dummy load cell weighing load cell after compensation dummy load cell time [s] e st im a te d m a ss [ g ] time [s] e st im a te d m a ss [ g ] time [s] e st im a te d m a ss [ g ] time [s] e st im a te d m a ss [ g ] (b) 2 hz (c) 3 hz (d) 5 hz http://www.imeko.org/ microsoft word article 7 69-728-2-le.docx acta imeko december 2013, volume 2, number 2, 34 – 40 www.imeko.org acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 34 gum conformity of software products a discussion from a software tester’s perspective norbert greif1, heike schrepf1 1 physikalisch-technische bundesanstalt, institute berlin, abbestraße 2-12, 10587 berlin, germany section: research paper keywords: measurement uncertainty; gum conformity; measurement software quality; software validation citation: norbert greif, heike schrepf, gum conformity of software products a discussion from a software tester’s perspective, acta imeko, vol. 2, no. 2, article 7, december 2013, identifier: imeko-acta-02 (2013)-02-07 editor: paolo carbone, university of perugia received february 12th, 2013; in final form november 5th, 2013; published december 2013 copyright: © 2013 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: information not available corresponding author: norbert greif, e-mail: norbert.greif@ptb.de 1. introduction an increasing number of software products that claim to offer a gum-compliant calculation of measurement uncertainties are available on the market. in order to ensure that these products perform the calculations in accordance with the gum [1], a specific validation of the software products with respect to the gum is necessary. additionally, to guarantee comparability of the measurement uncertainties calculated by different software products, a defined comparability of the software products themselves is required. consequently, a reusable, automated test environment has been developed which supports both a gum-oriented validation and gum-related comparisons of different software products by tracing back the product features to the rules and requirements of the gum (see figure 1). the paper presents the benefit of the test environment, but also the limitations of validation and product comparisons. to bridge the gap between the gum guideline and the required test specification, an analysis of the gum from the perspective of software testing is presented. this detailed analysis of the gum has uncovered some issues and inconsistencies within the gum. included are, for example, non-testable gum statements, alternative options of implementations of gum statements, and missing definitions. to ensure unambiguous implementations of the gum and corresponding explicit test specifications, these ambiguities of the gum have to be overcome or minimised (see figure 1). to be sound, first of all, the gum guideline had to be transformed into a formal specification. for the core clauses of the gum guideline, such a specification was already presented in [6]. in this paper, the underlying specification is not the focal point. instead of that, the problems of deriving the specification from the gum guideline are dealt with. for example, as outlined in section 3.3, the gum often allows several computations resulting in differing solutions. the concept of the paper is that each of these possible solutions is considered to be gum-compliant as long as the solution itself is computed correctly. thus, several differing nominal results are possible. in the paper, the tester’s procedures to deal with different results are described in the clauses called accepted solutions. 2. motivation and aim for several years, the authors have been involved in the evaluation of software products which implement the gum [3, 4, 5]. recently, three further software products were comparatively evaluated concerning gum conformity. during this work, special experience was gathered and an abstract this paper describes how to assess gum conformity of software products which claim to offer a gum-compliant calculation of measurement uncertainties. to bridge the gap between the gum guideline and the required test specification, an analysis of the gum from the perspective of software testing is presented. problems of testability and ambiguity of gum statements are analysed in detail. the benefit and the limits of the developed validation procedure and test environment are outlined. acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 35 implementation-oriented view on the gum has emerged. one aim of the paper is to outline this special experience. for example, the following fundamental questions have come up regarding an implementation of the gum:  completeness what does it mean when a software product claims to implement the gum? is the gum completely implementable? is it possible to reformulate each of the gum statements so that it can be represented as a computational step? is a pocket calculator already compliant to gum when it correctly implements only one formula such as the average of repeated observations (as described in gum 4.2.1)?  correctness what does it mean when a software product claims to be correct? is the product able to calculate the correct values? or is it able to calculate the correct values, round them with a correct rounding procedure, and display them with a correct number of digits?  compliance what does it mean when a software product claims to be compliant (conforming) to the gum? the gum guideline does not contain a conformity clause. thus, is it allowed to claim a conformity statement based on completeness and correctness? already these few questions lead to one of the core problems of both testing gum software and comparing gum test results: the need to trace back each computational step and each test result to a certain well-defined, well-understood and uniformly interpreted gum statement. consequently, traceability should be the precondition for the validation of a specific software product as well as the comparison of different products. to get repeatable, comparable, and traceable validation results, the questions mentioned above and some further queries have to be answered. the corresponding answers have an important impact on the set-up of the software test environment. for the software specification step in between, specific introduction and guidance is given in [6]. in summary, the objectives of the work are  to prove a gum-compliant calculation of measurement uncertainties (to prove “conformity” of gum-supporting software products with the gum guideline);  to provide comparability of measurement uncertainties calculated using different software products (useful for key comparisons);  to provide comparability of test results and of the software products themselves. to achieve these objectives, the main tasks are  to perform a detailed analysis of the gum from the software testing and software implementation point of view;  to trace back each computational step and each test result to a certain well-defined gum statement;  to develop a gum-oriented validation procedure;  to develop a reusable, automated test environment;  to support a gum-related comparison of different software products by tracing back the product features to the requirements of the gum. this paper describes the analysis of the gum (see section 3) and gives a short overview of the validation procedure and the test environment developed (see section 4). figure 1. basic task: bridging the gap between the ambiguous gum guideline with inconsistencies or missing definitions, and an explicit test specification. acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 36 3. analysis of the gum from the perspective of software testing in this main section of the paper, some problems of testability and ambiguity of gum statements are analysed in detail. these issues have a straight influence on the traceability and comparability of validation results belonging to different software products. in the following, the gum issues under discussion are classified according to these (non-disjunct) four main categories:  testability of gum statements non-testable statements are analysed with respect to additional context information.  strictness of gum statements diffuse statements with regard to the possibility to use alternative options are analysed.  ambiguity of gum statements ambiguous statements regarding informal wording are discussed.  specific problems missing gum statements, gum inconsistencies and the handling of calculation results not covered by the gum are analysed. for each problem, the necessary decisions to guarantee testability, the unambiguous definition of the validation procedure, and the direct consequences for the development of the test environment are derived (cf. the examples with accepted solutions). some solutions cannot be realised within an automated test environment. 3.1. testability of gum statements the gum includes a series of statements which are not testable, even in the core clauses 4 through 8. these statements require additional decisions or context information to guarantee the unambiguous definition of the test process. software packages should be able to ask for the necessary context information. this has not been the case for all software packages validated so far. example 1 (gum 4.2.1, gum 4.2.3): the gum describes the computation of an arithmetic mean for an observation series and allows the use of the computed mean as an estimator for the quantity’s value as long as certain preconditions are met. one of these preconditions is the repeatability of observations. being allocated a list of observation values, no software package is able to decide whether the required repeatability conditions have been met. if the repeatability condition is to be tested, context information is necessary. accepted solution 1: the package asks the user to check the conditions. accepted solution 2: the package always assumes certain repeatability conditions. the user manual points out the responsibility of the user. example 2 (gum 3.2.3, gum 3.2.4, gum 8.1): the gum states that systematic deviations have to be incorporated into the model equation in the form of correction terms. being allocated a model equation, no software package is able to decide whether the model equation is complete in this sense. accepted solution: the package always assumes completeness of the model equations. the user manual points out the responsibility of the user. example 3 (gum g.2.1): the gum states that the output quantity is approximately normally distributed if its variance is “much larger than …”. a software package is not able to compare two values in this informal way. accepted solution: the package offers all information necessary to decide on distribution of the output quantity to the user (distributions of all input quantities, their uncertainty contributions, the linearity of the model equation). the user should be able to decide whether the distribution of the output quantity may be understood as normal or is “unknown”. in the case “unknown”, the package should not calculate and report the expanded uncertainty of that output quantity. example 4 (gum 5.1.2, gum 5.1.5): the gum states that “higher terms (of the taylor series of the model function) must be negligible”. should a software package neglect a term when its value is 1/20, 1/100, or 1/1000 of the sum of the low-order terms? accepted solution 1: the software package calculates results for both, the standard gum case (first order of taylor series), and the sophisticated case (including higher order). if the results for uc(y) are equal after rounding and shortening, then the higher order terms are negligible. accepted solution 2: alternatively to solution 1, the package can check the linearity of the model equation as long as the results are the same as for solution 1. example 5 (gum f.1.2.1 a) and c)): the gum explains that the covariance of two input quantities may be treated as insignificant if certain conditions are met. the software package is not able to decide whether the required conditions have been met. accepted solution 1: the software package calculates both, the result with and without correlation. if the results for uc(y) are equal after rounding and shortening, then the correlation is negligible. accepted solution 2: the software package asks the user to check the correlation values. 3.2. strictness of gum statements the possibility to use alternative options requires additional decisions or assumptions to ensure testability. such options have to be exercised, for example, in case of the formulation of model equations, or in case of the evaluation of sensitivity coefficients. example 6 (gum 3.1.7, gum 4.1.1, gum 4.1.2): gum 3.1.7 mentions that the presented concept, although only discussed for scalars, is applicable to vector results, too. however, there is no further treatment. gum 4.1.1 presents the model relationship as an equation solved for the scalar output quantity. acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 37 gum 4.1.2 states that the relation between input quantities and output quantities are not necessarily an explicit functional relationship. instead, an algorithm or a computer program which is able to produce result values y for certain input values xi may be used. accepted solution: the model equation may be represented by an explicit functional relationship or not. it may be formed for scalars or for vector results. each variant is considered to be compliant. example 7 (gum 5.1.3, gum 5.1.4): gum 5.1.3 describes the sensitivity coefficients as partial derivatives of the output quantity with respect to the input quantities at the point of the estimates of the input quantity values. note 2 of the same section states that the partial derivatives may be calculated using common numerical methods. gum 5.1.4 allows the experimental determination of these sensitivity coefficients. accepted solution: sensitivity coefficients may be determined analytically, numerically, or experimentally. each variant is considered to be gum-compliant. example 8 (gum 4.1.4): gum 4.1.4 states, that the estimated value of the output quantity is calculated using the estimated values of the input quantities and the model equation. the following note says that the estimate of the output quantity may also be calculated as the average of several output values, each of them calculated from a set of input values and the model equation. accepted solution: the value y may be calculated as a function value or as an average of function values. each variant is considered to be gum-compliant. in case of linear models, the results do not differ. example 9 (gum g.4.1, note 1): gum g.4.1 describes the treatment of a degrees-of-freedom value calculated by the welch-satterthwaite formula. to derive the coverage factor, two different methods are allowed, interpolation or truncation (cf. figure 2). both computations result in significantly differing values. accepted solution: each variant is considered to be gumcompliant. 3.3. ambiguity of gum statements ambiguity is caused by informal gum wording. usually, the informal wording shall improve readability and comprehensibility of the gum. example 10 (gum 7.2.6, gum h): gum 7.2.6 explains that the uncertainty should be given with "at most" two significant digits. more digits are allowed to avoid rounding errors in subsequent calculations. gum annex h mostly uses two, in some cases only one digit for uncertainty values (h.3, h.5, h.6). what should the programmer of a gum package do regarding the question of digits? how should the tester of a gum package formulate the nominal output for a test case? accepted solution: the software package should use two digits by default. it should allow a manual adjustment if necessary. example 11 (gum 7.2.6): gum 7.2.6 states that "it may sometimes be appropriate" to round uncertainties up rather than to the nearest digit. two examples are given: a value like 10.47 should be better rounded up to 11 instead of rounding it to the nearest digit, i.e. 10. in another case, a value like 28.05 should be rounded to the nearest digit, i.e. 28, instead of rounding up to 29. the gum obviously uses a rounding principle that is describable as "rounding up or down with a fraction limit somewhere between 0.1 and 0.4, instead of 0.5 as is usual". since this is not formulated explicitly, each programmer is free to use rounding up or rounding to the nearest digit (and half up). accepted solution: concerning testing, the decision was made to expect rounding to the nearest digit (and half up). example 12 (gum g.6.6): gum g.6.6 explains that in certain cases one may use the coverage factor values of 2 (to get a level of confidence of nearly 95%) or 3 (to get nearly 99%). afterwards, the gum discusses that in these cases significant overand underestimations of the confidence interval may occur and that a better estimation may be necessary. the user is recommended to choose a better estimation if the approximation is not sufficient for his purposes. the question arises whether a gum package should use the (gum-compliant) approximation or the (gum-compliant) better estimation. accepted solution(s): the user decides on the kind of distribution of the result quantity using the information delivered by the software package (see example 3). if the result quantity may be considered normally distributed: accepted solution 1: the user delivers the level of confidence p and the package calculates the coverage factor t(ν) based on a tdistribution, or accepted solution 2: the user delivers the level of confidence p and the package calculates the coverage factor k based on a normal distribution, and it delivers the deviation between k and t(ν). in all other cases: accepted solution 1: the distribution is unknown; the package does not calculate the coverage factor. accepted solution 2: the distribution is known to the user; the user delivers the coverage factor. example 13 (gum f.2.3.3): gum f.2.3.3 discusses the case in which only a minimum and maximum value (and therefore the half width a) for an input quantity is available. figure 2. illustration for example 9 (alternative implementation options). acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 38 the suggestions for the uncertainty of this input quantity vary from a/√3 (for a uniform distribution assumption), to a/√6 (for a triangular distribution assumption), and to a/√9 (for a normal distribution assumption). gum s1 6.4.2.1 suggests to assume a uniform distribution, based on the principle of maximum entropy [2]. accepted solution: the package has to ask the user. he has to decide which assumption holds. 3.4. specific problems finally, some problems regarding missing gum statements, gum inconsistencies, and the handling of calculation results not covered by the gum are considered. example 14 (gum g.4): the problem of effective degrees of freedom of the output quantity is discussed in relation to the problem of the output quantity’s distribution and other aspects (central limit theorem). the given formula (g.2b) and the reference to section gum 5.1.3 suggest that the formula is valid for uncorrelated input quantities only, but this is not expressed explicitly or discussed in detail. in particular, there is no explicit prescription not to use formula (g.2b) in case of correlated input quantities. accepted solution: the calculation of a degree-of-freedom value for the output quantity in case of correlated input quantities is not considered to be compliant. a value may be given, but its calculation has to be documented, and it has to be marked as outside the gum scope. example 15 (gum 4.3.8, gum g, gum f): a topic which is discussed very roughly is the usage of input quantities with asymmetric distributions. in this case, gum statements consist of a single section in the main text (gum 4.3.8), a short discussion in annex g (gum g.5.3), and the discussion of a particular case in annex f (gum f.2.4.4). the question arises: how should the user deal with asymmetrically distributed input quantities? they cannot be omitted, since gum does not prohibit their use. accepted solution: the distributions of the input quantities do not influence the computation of the value of the output quantity y and the standard measurement uncertainty uc(y). displaying y and uc(y), and omitting u(y) is considered gum-compliant. example 16 (gum 6, gum g): the problem of how to evaluate the expanded uncertainty of an output quantity (which is in practice of greater interest than the standard uncertainty) is only briefly discussed. gum 6 suggests to use a coverage factor between 2 and 3, and mentions that the selection of a proper value depends on experience or, alternatively, on knowledge about the output quantity’s distribution. the details of this discussion take place in annex g. for testers, the question arises whether a software product is gum-compliant if it uses an arbitrary coverage factor between 2 and 3 ignoring the statements of annex g. accepted solution: the statements of annex g are considered relevant for achieving gum-compliance. example 17 (overall gum): it is common sense that a correlation matrix should be checked with respect to its being symmetric and non-negative definite. most of the gum packages allow the user to do these checks, but the definiteness is not discussed in the gum. accepted solution 1: the software package checks the nonnegative definiteness of the correlation matrix. accepted solution 2: the software package does not check the non-negative definiteness of the correlation matrix. instead of that, before the output of the standard measurement uncertainty uc(y) of the output quantity, the package checks that the expression for uc2(y) is non-negative. example 18 (overall gum): the experience from the gum packages that have been validated is that most of these packages compute confidence intervals for output quantities with rectangular distribution, effective degrees of freedom in case of correlated inputs, and confidence intervals for correlated output quantities, irrespective of the fact that the gum does not prescribe anything in these cases. accepted solution: because these calculation results are not covered by the gum, they do not belong to a validation of a package with respect to gum conformity. on the other hand, however, these results are important in practice. with regard to the test process, testing of these calculations is performed, but the corresponding test cases are marked as “outside gum conformity testing”. 4. overview of the test environment in this section of the paper, the test environment as it has been developed for the validation and gum-related comparison of software products is described very roughly. a schematic overview of the test environment is illustrated in figure 3. a detailed presentation is given in [5]. the following description is restricted to the overall understanding of the test concept and to some aspects which are of importance for the analysis of benefits and the problems mentioned above. implementation details are omitted. the objective to validate software products that implement the gum is best achieved by establishing a well-defined, gumoriented test process supported by a reliable technical test environment. the environment itself has to obey certain quality requirements, for example, correctness and completeness. especially, the test cases must be designed in a way that they generally fit for any gum-supporting software product under test. consequently, comparability of certain validation results and after all the comparability of the whole validation process has to be ensured. to meet these requirements, the test environment consists of the following components:  data model defining the structure of information necessary for uncertainty calculations and corresponding tests. main components of the model are the test case identification, the test purpose with classification (cf. figure 3) and gum reference, the inputs for the software under test, and the nominal outputs which are criteria for the package’s results.  set of universal test cases which do not contain any product-specific or technical information. the test case repository is implemented based on the data model. acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 39 each test case is represented by a separate file with a unique identifier.  test case converter which translates universal test cases into product-oriented specific ones. the converter needs information about the package to be tested, the underlying operating system, and the test tool which will be used, for example. depending on the software under test and the validation task, the converter has to filter the test cases.  several sets of product-oriented specific test cases, each of which belongs to a specific software package to be tested. these test cases contain, for example, package-specific commands, input values, buttons to push and menu items to select.  capture-replay test tools to operate the test cases and to repeat automatically the overall test process. the universal and package-specific test cases are arranged concerning a well-defined classification scheme. this classification hierarchy is based on the software quality characteristics defined in the international software standard iso/iec 25010 [7]. the respective position of a test case in the hierarchy corresponds to the purpose of the test. in this way, the classification scheme allows a certain control of completeness and traceability of the validation process. in accordance with figure 4, the main levels of the classification hierarchy are:  assignment of the test cases to the set of software quality characteristics according to the software standard iso/iec 25010 [7], for example, functionality, usability, and reliability.  subdivision of test cases into positive cases (prove that the gum is correctly implemented) and negative cases (prove that in case of the non-applicability of the gum no calculation is carried out).  specific subdivisions depending on the value for the first level. an example for the third classification level is closely connected with the software quality characteristic functionality (see figure 4). in this case, the classification hierarchy represents the detailed calculation steps needed to prove the conformity of the software packages to the core sections and formulas of the gum. the calculations are split into the following steps (branches of the classification hierarchy, see figure 4):  calculations of type a uncertainties (without correlation of inputs);  calculations of type b uncertainties (without correlation of inputs);  interpretation of model equations and calculation of sensitivity coefficients (scs in figure 4);  calculation of values, standard measurement uncertainties, and coverage intervals of output quantities without and with the correlation of input quantities;  calculation of the correlations between output quantities (vector results);  calculation of the examples from gum annex h. for each of these calculation steps, further classification levels depending on the degree of complexity of the test cases can be defined. normally, we use between five and nine classification levels. in addition to the quality characteristic functionality, the characteristics usability and reliability were used to design and implement test cases. in future, the characteristic efficiency might become relevant to include response time evaluations of monte carlo simulation engines. 5. conclusions a number of software packages which claim to implement the gum are on the market. however, they differ in functionality and the have deficiencies which are not obvious. thus, a validation of these packages with respect to the gum is necessary. the ptb test environment has been used successfully to validate and compare three different gum-supporting software packages. to bridge the gap between the gum guideline and the explicit test specification, a detailed analysis of the gum from a tester’s perspective and certain decisions regarding the test process (cf. the accepted solutions of the examples in section 3) figure 3. schematic overview of the test environment. acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 40 were necessary. based on the results of this analysis, unambiguous and detailed test cases could be developed. the benefits of the test environment and the validation procedure are:  general procedure usable for any gum-supporting software product;  automated and reusable process;  comparability of the validation procedure and, especially, of the validation results;  automated documentation process. however, there are also limitations in the validation procedure, and in the process of product comparison. the current procedure includes sections 5.5 to 5.8, and 6 of [2], but does not consider sections 5.9, 5.10, and 7 (monte carlo simulations), and does not regard the handling of complex numbers. the general limitation is, that several obstructive characteristics of gum statements (with regard to software testing), such as ambiguities, missing or inexact specifications/definitions, do restrict the applicability and the objectiveness of the test environment. thus, some of the accepted solutions cannot be realised within an automated test environment. concerning the software quality characteristics, up to now, the validation procedure does not include efficiency testing (e.g. duration of monte carlo simulations). in principle, the test environment is prepared to realise the extensions mentioned above. some extensions concerning monte carlo simulations and vector results are already under construction. the work reported reveal some problems regarding the objectives of testing gum-supporting software products and the corresponding gum statements. these problems, for example, gum inconsistencies or ambiguities, have to be minimised. directly, they concern the implementation of gum-supporting software products and the corresponding product validations. the further discussion of these problems would enhance the traceability of implementation and validation results to the gum and the comparability of uncertainty calculations performed by different software products. references [1] iso/iec guide 98-3:2008, uncertainty of measurement part 3: guide to the expression of uncertainty in measurement, 2008. [2] iso/iec guide 98-3:2008/suppl 1:2008, propagation of distributions using a monte carlo method, 2008. [3] n. greif, h. schrepf, d. richter, software validation in metrology: a case study for a gum-supporting software, measurement, volume 39, 2006, pp. 849-855. [4] n. greif, h. schrepf, validierung von software zur bestimmung von messunsicherheiten, vdi-berichte 1947, messunsicherheit praxisgerecht bestimmen, vdi, 2006, pp. 409-418. [5] n. greif, h. schrepf, v. hartmann, g. kilz, a test environment for gum conformity tests, physikalisch-technische bundesanstalt (ptb), braunschweig und berlin, ptb report, to appear, 2013. [6] m. g. cox, p. m. harris, i. m. smith, software specification for uncertainty evaluation, npl report ms 7, march, 2010. [7] iso/iec 25010:2011, systems and software engineering system and software quality requirements and evaluation (square) system and software quality models, 2011. figure 4. classification hierarchy of test cases (extract). performance evaluation of underwater image pre-processing algorithms for the improvement of multi-view 3d reconstruction acta imeko issn: 2221-870x september 2019, volume 8, number 3, 69 – 77 acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 69 performance evaluation of underwater image pre-processing algorithms for the improvement of multi-view 3d reconstruction alessandro gallo1, fabio bruno1, loris barbieri1, antonio lagudi1, maurizio muzzupappa1 1 department of mechanical, energy and management engineering (dimeg), university of calabria, via p. bucci 46c, 87036 rende, italy section: research paper keywords: 3d reconstruction; underwater cultural heritage; image enhancement; underwater imaging citation: alessandro gallo, fabio bruno, loris barbieri, antonio lagudi, maurizio muzzupappa, performance evaluation of underwater image pre-processing algorithms for the improvement of multi-view 3d reconstruction, acta imeko, vol. 8, no. 3, article 11, september 2019, identifier: imeko-acta-08 (2019)03-11 section editor: egidio de benedetto, university of salento, italy received november 12, 2018; in final form may 28, 2019; published september 2019 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: loris barbieri, e-mail: loris.barbieri@unical.it 1. introduction the 3d reconstruction of submerged structures or archaeological finds has achieved notable popularity in underwater cultural heritage (uch) preservation, as the method may allow for the exploration of sites located in inaccessible and hostile environments. in the last decade, techniques and tools for 3d reconstructions have been widely employed in the underwater archaeology field according to the guidelines of unesco, which suggest the in-situ preservation of underwater heritage [1]. among the different 3d imaging techniques that are suitable for underwater applications, photogrammetry represents a valid method of reconstructing 3d scenes from a set of images taken from different viewpoints [2]. the popularity of this technique is also due to the acquisition devices (still or a movie camera with appropriate waterproof casings) that are affordable and easy to use compared to dedicated devices like lidar, multi-beam, etc. [3]. furthermore, these devices can be handled by scuba divers or mounted on underwater robots [4]. unfortunately, imagebased acquisition suffers due to the poor environmental conditions. the depth of the water, flora, fauna, weather conditions, and sea currents are all factors that affect visibility, refraction, and lighting conditions. consequently, these factors limit underwater photogrammetry’s scope to close-range applications, and further efforts are required to improve the radiometric quality of the images. for these reasons, the enhancement of underwater images is still a necessary step in improving the accuracy of 3d reconstruction and creating realistic textures. this article presents a performance evaluation of underwater image pre-processing algorithms for the improvement of multiview 3d reconstruction. two existing colour enhancement models, i.e. ace (automatic colour equalisation) and pca (principal component analysis) algorithms, have been tested to compare their results with those provided by a new method based on histogram stretching and manual retouching (hist). to this end, an experimental campaign has been planned using design of experiment (doe) [5] criteria to investigate the factors that affect reconstruction accuracy. the experimental abstract 3d models of submerged structures and underwater archaeological finds are widely used in various and different applications, such as monitoring, analysis, dissemination, and inspection. underwater environments are characterised by poor visibility conditions and the presence of marine flora and fauna. consequently, the adoption of passive optical techniques for the 3d reconstruction of underwater scenarios is a highly challenging task. this article presents a performance analysis conducted on a multi-view technique that is commonly used in air in order to highlight its limits in the underwater environment and then provide guidelines for the accurate modelling of a submerged site in poor visibility conditions. a performance analysis has been performed by comparing different image enhancement algorithms, and the results have been adopted to reconstruct an area of 40 m2 at a depth of about 5 m at the underwater archaeological site of baiae (italy). mailto:loris.barbieri@unical.it acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 70 campaign has been carried out in the underwater archaeological site of baiae (naples, italy), where the seafloor, with a water depth ranging between 2.5 and 20 m, offers a particularly interesting environment, as it encompasses a submerged area of many hectares and presents a wide range of different architectural structures with a number of decorations that are still preserved. the research has been conducted in the context of the imareculture project [6], [7], [8], which aims to develop new tools and technologies for improving public awareness of uch. the article is organised as follows. section 2 presents related works about underwater image pre-processing methods. section 3 describes the image acquisition and pre-processing stage. in section 4, the results of the statistical analysis are detailed. section 5 presents the 3d reconstruction obtained by means of the enhanced images, and finally, conclusions are presented in section 6. 2. underwater image pre-processing algorithms underwater pictures generally suffer from light absorption, which causes some defects mostly on the red channel (the first component of the light spectrum that is absorbed), and this effect is already noticeable at only a few metres of depth. the pre-processing of underwater images can be conducted with two different approaches: image restoration techniques or image enhancement methods [9], [10]. image restoration techniques need some environmental parameters to be entered, such as scattering and attenuation coefficients, while image enhancement methods do not require a priori knowledge of the underwater environment. the physical effects of visibility degradation have been analysed in [11], showing that the degradation effects can be associated mainly with the partial polarisation of light. the developed algorithm is based on a couple of images taken through a polariser at different orientation, improving contrast and colour and doubling the underwater visibility range. the work of [12] presents an image restoration filter based on a simplified version of the jaffe-mcglamery underwater image formation model, which can be used for images with limited backscatter in diffuse lighting. the ace algorithm [13] is inspired by the human vision, which is able to adapt to highly variable lighting conditions, extracting visual information from the underwater environment [14]. the algorithm combines the patch white algorithm with the gray world algorithm, taking into account the spatial distribution of colour information. in the first stages of the ace method, chromatic data and pixels are processed and adjusted according to the information contained in the image. subsequently, colours in the output image are restored and enhanced [15]. different to the ace algorithm, which can adapt to widely varying lighting conditions and can extract visual information, in order to reduce the number of variables considerably while still retaining much of the information in the original dataset, it is possible to adopt the principal component analysis (pca) algorithm. pca is one of the most popular multivariate statistical techniques that analyses a data table representing observations described by several dependent variables and extracts the important information in the form of a set of new orthogonal variables called principal components. in this specific application, the pca algorithm allows us to extract a dominant colour of the image. hence, in most cases, the water colour also provides good results in term of colour enhancement. an automatic enhancement algorithm that does not require any correction parameter has been proposed in [16], where each source of errors is corrected sequentially. the first step removes the moiré effect, then a homomorphic or frequency filter is applied to equalise brightness and to enhance the contrast. regarding the acquisition noise, a wavelet denoising filter followed by an anisotropic filtering has been applied. finally, dynamic expansion is applied to increase contrast, followed by colour equalisation. the process is performed on one channel, specifically the ycbcr colour space, in order to optimise the computation time. even if this last step speeds up all the following processes avoiding the need to process each rgb channel each time, it is important to point out that the use of a homomorphic filter affects the geometry and could generate errors on the reconstructed scene. the effectiveness of the use of different colour spaces for the enhancement of underwater images has been demonstrated in [17], where a slide stretching algorithm has been used both on rgb and hsi colour spaces. after a contrast stretching on rgb colour space has been performed, the resulting images have been converted to hsi colour space and processed through saturation and intensity stretching in order to increase the true colour and solve the problem of lighting. the aim of underwater colour correction is not only to obtain better quality images, but also to improve the performance of feature extraction algorithms in terms of the detection of feature points. the effects of different image preprocessing methods on the performance of the surf (speeded up robust features) detector [18] have been investigated in [20], and the iace (image adaptive contrast enhancement) method has been proposed. in particular, the iace method enhances the intrinsic features in images, like corners, edges, and blobs, along with maintaining the relative contrast between the pixels. thanks to this capability, the iace method has proven better than other techniques, like histogram equalisation and multiscale retinex algorithm for color enhancement, in terms of the repeatability of their surf detector and the robustness and distinctiveness of their surf descriptor. different to previous works [20] that focus on the comparison of different image enhancement algorithms, with all other conditions being equal, this article presents a performance analysis based on a doe approach, which takes into account the main influential factors that affect 3d reconstruction quality. 3. experimentation the experiment was undertaken in the underwater archaeological site of baiae, which is located few kilometres north of naples (italy). the submerged environment of baiae is characterised by highly critical visibility conditions due to water turbidity and the heavy presence of flora and fauna. the area selected for the experimentation is the thermal room of ‘villa protiro’, with a size of 5 x 8 m at an average depth from the sea level of 5 m. the choice of this area is due to the presence of different building materials (bricks, mortar, tile floors, etc.) and a strong colonisation of various bio-fouling agents. because of the critical visibility conditions, 3d reconstruction techniques based on the multi-view stereo method are not sufficient for performing an accurate 3d reconstruction of the submerged archaeological area. the underwater images require, then, a preprocessing stage involving the adoption of image enhancement algorithms that could have a relevant or mediocre impact on the quality of the final 3d reconstructed model. acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 71 3.1. experimental setup the experimental setup consists of a camera, its underwater housing, and two underwater strobes. the camera is a nikon d7000 reflex device equipped with a cmos (complementary metal-oxide semiconductor) sensor size of 23.6 x 15.8 mm and a resolution of 4928 × 3264 pixels (16.2 effective megapixels), as well as a af-nikkor 20 mm lens. the underwater housing, manufactured by ikelite, is equipped with a spherical port. the flashguns are connected to the camera housing with a pair of articulated arms at a distance of 45 cm. the two strobes have been fixed at a distance of 5 cm behind the dome pointing outwards to illuminate the object with the ‘edge’ of the light beam. a calibration panel, produced by lastolite, has been used in the beginning of the survey to acquire a colour calibration image to perform in-situ white balance correction, while a digital depth gauge has been used to maintain a constant depth from the seabed. 3.2. image acquisition the photogrammetric survey of the submerged area has been carried out in two different dive sessions, in the north and south parts of the site. the survey has been carried out according to a standard aerial photography layout: the diver swims at a distance from the submerged structures of about 2.5 m, taking overlapping pictures along straight lines that cover the whole area in the north-south direction. another set of images has been acquired in the east-west direction. the occluded areas have been acquired using oblique photographs. at the end of the survey activity, the dataset included a total of around 700 images. 3.3. colour enhancement of underwater images the original images (or) have been then enhanced by means of three algorithms (ace, pca, hist) and corrected through an in-situ white balance correction procedure. the ace (automatic colour enhancement) algorithm proposed in [9] and the pca algorithm proposed in [21] have been adopted. the hist (histogram stretching and manual retouching) algorithm is a semiautomatic enhancement methodology that has been developed for this study. it is based on histogram stretching and a manual colour retouching procedure and has been implemented using batch actions in a graphics editor to rescue the maximum amount of information from a set of defective and noisy pictures. in particular, the hist method consists of the following three-step procedure: preliminary histogram stretching to improve the contrast; mixing of the colour channels to balance the missing information on the red channel; creation of a set of adjustment layers, including saturation enhancement for some missing hues, contrast masks, colour balancing, and equalising. in addition to the enhanced algorithms, the images have been processed by performing an in-situ white balance correction procedure (wb) performed by means of a lastolite waterproof panel. the following figure shows an original uncorrected image (figure 1a) and those enhanced with the wb procedure (figure 1b), ace (figure 1c), hist (figure 1d) and pca (figure 1e) algorithms. 3.4. design of the experimental campaign the experimental campaign has been planned according to the doe criteria with the purpose of identifying the most influential factors affecting the results of the 3d reconstruction in the underwater environment. particular attention has been given to the effect of the image enhancement methods on the camera orientation and the self-calibration bundle adjustment process. the measured data has been compared and analysed by means of standard statistical tools to verify if a particular factor (or a combination of factors) has an impact on a parameter with a certain confidence level. on the basis of the results of this analysis, it is possible to find the best combination of factors that should be used for an accurate and dense 3d reconstruction by using a multi-view stereo technique. 3.5. influencing factors the first step is the selection and identification of the influencing factors that could have an influence on the quality of a 3d reconstruction performed with a multi-view technique in the underwater environment. the influencing factors have been chosen among those that cannot be controlled in situ, such as the presence of marine organisms in motion and the level of turbidity of the water. furthermore, factors that can be considered as a direct consequence of others have been taken into account. for instance, focus settings depend on the distance from the subject, and the focal length can be set according to the required field of view and to the working distance. the factors selected for the experiment are reported in table 1. the first factor (en) is related to the original images (or) and to the colour enhancement algorithm (wb, hist, ace and pca) adopted to improve underwater images. the second factor refers to image resolution (pyr). the full resolution images have not been used in order to save computational time. the raw images (4928 x 3264 pixels) have been resized by means of the mitchell-netravali cubic filter [22] in order to create the following levels: level 1 for images of 2464 x 1632 pixels; level 2 for images of 1232 x 816 pixels; and level 3 for images of 616 x 408 pixels. the third factor is represented by the composite rgb image and its three r (red), g (green), and b (blue) components. this factor has been taken into consideration in order to investigate the influence of a single-colour channel on the reconstruction figure 1. sample original image (a) corrected with the in situ white balance measurement (b), enhanced with the ace method (c), hist method (d), and pca method (e). acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 72 quality with respect to the grayscale image obtained by combining the rgb components. in order to perform a quantitative analysis of the impact of camera layout on the processing results, the last influencing factor is referred to the type of image set (set). seven subsets have been created, which differ among each other according to the type of shot (aerial vs. oblique), working distance, and overlapping pictures. in particular, the first two sets include photos that have been taken with a standard aerial layout. the third set includes pictures characterised by high overlap and good visibility due to the reduced distance from the submerged structures. the fourth and fifth sets cover the outside masonry structures of the outer walls, while the sixth and seventh sets group oblique pictures with variable working distances. 3.6. measured parameters the 3d reconstruction quality has been evaluated by means of four different parameters: the mean number of extracted features; the percentage of matched features; the percentage of oriented cameras; and bundle adjustment mean re-projection error. the mean number of extracted features (𝑛𝑢𝑚�̅�) has been calculated according to the sift (scale invariant feature transform) operator [23] that consists of the following relationship: 𝑛𝑢𝑚�̅�𝐸𝑁,𝑃𝑌𝑅,𝐶𝐻,𝑆𝐸𝑇 = 𝑛𝑢𝑚𝐹𝐸𝑁,𝑃𝑌𝑅,𝐶𝐻,𝑆𝐸𝑇 𝑛𝑢𝑚𝐼𝑚𝑆𝐸𝑇 (1) where 𝑛𝑢𝑚𝐹 is the total number of extracted features for each configuration, and 𝑛𝑢𝑚𝐼𝑚 is the number of images included in each set. the percentage of matched features (𝑚𝑎𝑡𝑐ℎ𝑒𝑑𝐹%) and percentage of oriented cameras (𝑐𝑎𝑚%) parameters have been evaluated using bundler [24], through the following relationships: 𝑚𝑎𝑡𝑐ℎ𝑒𝑑𝐹% 𝐸𝑁,𝑃𝑌𝑅,𝐶𝐻,𝑆𝐸𝑇 = 𝑚𝑎𝑡𝑐ℎ𝑒𝑑𝐹𝐸𝑁,𝑃𝑌𝑅,𝐶𝐻,𝑆𝐸𝑇 𝑛𝑢𝐹𝐸𝑁,𝑌𝑅,𝐶𝐻,𝑆𝐸𝑇 (2) 𝑐𝑎𝑚% 𝐸𝑁,𝑃𝑌𝑅,𝐶𝐻,𝑆𝐸𝑇 = 𝑐𝑎𝑚𝐸𝑁,𝑃𝑌𝑅,𝐶𝐻,𝑆𝐸𝑇 𝑛𝑢𝑚𝐼𝑚𝑆𝐸𝑇 (3) where 𝑚𝑎𝑡𝑐ℎ𝑒𝑑𝐹 represents the number of matched 3d points in the sparse scene reconstruction resulting at the end of the bundle adjustment process, and 𝑐𝑎𝑚 is the number of oriented images for each configuration. the bundle adjustment mean re-projection error (available as output in the bundler log file and measured in pixels) is the result of a minimisation problem applied to the sum of distances between the projections of each track (a connected set of matching key points across multiple images) and its corresponding image features. 3.7. dataset generation as mentioned above in section 3.5, the whole dataset has been grouped into seven subsets according to: camera orientation; distance from the subject; pictures taken with flash; and the heavy presence of dark and bright areas. the grouping procedure has meant a selection and reduction in the number of images to 196 pictures. a matlab script has been programmed in order to manage the selected images and apply the different image enhancement algorithms. firstly, the enhancement methods (ace, pca, wb, hist) and the wb correction technique have been applied to the original full resolution images. secondly, the red, green, and blue colour components have been extracted only from images enhanced with wb correction, ace, and hist methods. mo action has been taken on images enhanced with the pca method because this method produces a single-channel output. lastly, all the images have been resized according to the pyramid levels. 4. statistical analysis table 2 shows the mean values of the measured parameters, described in section 3.6, computed for each influential factor. the data has been analysed by means of statistical instruments and have been compared by performing an anova test with a 95 % confidence level. the summary of the results is presented in table 3, in which the measured parameters have been computed from the main source only. the tukey post hoc test has been performed in order to find out significant differences between the groups of each influential factor. 4.1. mean extracted features figure 2 reports the mean values of the extracted features for all the factors of influence (summarised in table 2). the results show that the number of extracted points strongly depends on image resolution: a greater number of features is obtained from images with a higher resolution. in this regard, the data summarised in table 3 shows a statistically significant difference table 1. influential factors and related symbol and levels. influential factor symbol level colour enhancement method en or, hist, ace, pca, wb image pyramid level pyr 1, 2, 3 colour channel ch rgb, r, g, b image set set set 1, set 2,..., set 7 figure 2. the mean values of extracted features for all the influential factors. acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 73 among the three levels of the pyr factor, and this result shows that the number of features has a strong relationship with image resolution. nevertheless, the images at level 1 (four times more pixels than level 2), produces only three times more features than the images at level 2 and nine times more features than the images at level 3 (containing 16 times less pixels). the image enhancement methods ace and hist have returned the best results. on the contrary, the colour channel appears to be less influential than enhancement algorithms, because these last ones operate a mixing between the various colour channels in different ways. the results related to the factor ‘image set’ show a difference in behaviour within the seven datasets. in particular, the highest number of features is extracted from the images belonging to set 3, which includes pictures taken at a reduced distance from the subject. the first two sets are related to the same area: the second set has been acquired after having removed the sand that covered the tiled floor in order to improve the reliability of point detection. set 6 shows a lower number of features, as the oblique pictures have been taken from a greater distance, and the presence of the blue background is more evident. 4.2. percentage of matched features the most influential factor on the parameter ‘percentage of matched features’ (table 2) is the image enhancement algorithm used. as depicted in figure 3, it is noticeable that the hist algorithm allows for matching a higher number of features. the second factor in terms of influence is the colour channel: rgb images and the green channel allow for matching the maximum number of features. the results related to the factor ‘image pyramid level’ reflect the low influence deduced from the anova analysis (table 3), but it must be pointed out that resized images lead to a higher percentage of matched points. for all the three enhancement algorithms used in the experimentation, pyr levels 1 and 2 leads to a higher performance of the feature-matching algorithm. one of the reasons for this behaviour is the fact that by reducing the resolution, it is possible to find more robust features, as they are extracted from the more evident details. the images at level 3 have not shown good results due to the lack of reliable details. regarding the ‘image set’ factor, the matching of the images included in sets 5, 6, and 7 leads to poor results, since these are composed by oblique photographs only. the best results have table 2. mean values of the measured parameters computed for each influential factor. factors mean extracted features (1) % matched features (2) % oriented cameras (3) bundle adjustment mean re-projection error (pixels) en hist 5344.9 2.41 43.24 0.19 ace 6984.8 1.45 37.72 0.22 pca 1841.2 2.07 33.38 0.18 wb 2452.1 1.21 21.23 0.22 or 260.1 1.14 9.26 0.10 pyr 1 8806.0 1.61 37.22 0.24 2 2829.6 2.04 32.58 0.16 3 954.2 1.35 20.42 0.13 ch rgb 3848.9 2.20 35.98 0.18 r 4315.0 1.36 30.51 0.17 g 3300.1 2.19 34.82 0.18 b 5322.4 0.91 18.97 0.16 set 1 2813.7 1.71 28.44 0.14 2 4120.1 1.81 25.97 0.15 3 5986.0 2.39 48.46 0.13 4 3647.2 2.56 47.02 0.22 5 4654.6 0.96 22.62 0.24 6 3040.7 1.43 26.59 0.24 7 5022.6 0.98 20.90 0.22 table 3. summary of the results of the anova analysis. factors mean extracted features (1) % matched features (2) % oriented cameras (3) bundle adjustment mean re-projection error f p-value f p-value f p-value f p-value en 1065.41 0 21.07 0 91.08 0 9.17 0.0002 pyr 1457.10 0 5.93 0.005 20.61 0 13.82 0 ch 47.80 0 14.54 0 12.39 0 0.18 0.9129 set 92.79 0 7.76 0.0001 18.64 0 6.11 0.0002 acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 74 been obtained for sets 3 and 4, which include images taken with a reduced working distance and a greater picture overlap. 4.3. percentage of oriented cameras the image enhancement algorithm is the most influential factor on the parameter ‘percentage of oriented cameras’ (table 2). the performances obtained using each method are clearly better than the results obtained with the original images. for the latter, only 9 % of the cameras have been oriented, while about 43 %, 37 %, and 33 % of cameras have been successfully oriented while using the hist, ace, and pca enhancement methods, respectively. the second most influential factor is image resolution. by analysing the results in figure 4 and the outcomes of the tukey post hoc test, it is noticeable that there are no statistically relevant differences between the values for the first and second levels of the image pyramid. this means that it is possible to obtain the maximum number of oriented cameras with a lower resolution, also saving computational time. 4.4. bundle adjustment mean re-projection error the anova results (table 3) show that the most influential factor for the parameter ‘mean re-projection error’ is image resolution. if we consider the pixel size of the subsampled images and the mean distance from the subject of 2.5 m, the first and second pyramid levels have demonstrated errors measured on the ground of 0.29 and 0.38 mm, respectively. by analysing the results presented in table 2 and depicted in figure 5, it is noticeable that a higher error has been measured on the image sets containing oblique photographs only. the presence of the blue background in almost all the pictures reduces the accuracy of the bundle adjustment process. furthermore, the anova analysis results (table 3) reveal that there is not a statistically significant difference among the different colour channels (ch). 4.5. discussion the statistical analysis allowed for choosing the best combination of factors that should be used to perform the 3d reconstruction of the site. the accuracy of the sfm (structure from motion) procedure (namely the mean re-projection error) is mainly related to the camera network orientation. sets 1, 2 and 3 are characterised by convergent images with a high overlap, forming a more robust network. moreover, as reported in the previous section, both levels 1 and 2 led to errors below the acceptable value of 0.5 mm. for these reasons, it is possible to save computational time using subsampled images, which also result in a higher percentage of matched features. in fact, in the same conditions and varying only the image resolution, the average reconstruction time for the datasets taken into account in the study presents a saving time of 81 % for pyr2 compared to pyr1, and 92 % for pyr3 compared to pyr1. hist and ace methods considerably increase the performance of image matching. the first method returns better results in terms of the percentage of oriented cameras and matched features, increasing the performance by about 150 % and 50 %, respectively. the analysis of the effects of white balance correction has shown good results for the parameters ‘mean number of extracted features’ and ‘percentage of oriented cameras’. in particular, white balance corrected images have shown a higher number of extracted features and better performances compared with pca method. the images figure 4. mean values of the percentage of oriented cameras for all the influential factors. figure 3. the mean values of the percentage of the matched features for all the influential factors. figure 5. bundle adjustment mean re-projection error for all the influential factors. acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 75 obtained using the custom white balance adjustment lead to less stable results, since the correction is performed at the beginning of the survey. the results in terms of the number of oriented cameras have shown similar values for the first and second image pyramid levels. concerning the re-projection error, as described in section 4.4, in this case the best choice is also to use a lower resolution in order to save computational time without affecting reconstruction accuracy. regarding the colour channel, the data has demonstrated the better performance of the rgb images, particularly the green channel, which outperforms the results for other channels in terms of matched features and oriented images. considering the results related to the factor ‘image set’, it is evident that the highest number of features is extracted from the sequences of images that have been taken using a standard aerial photography layout, with a reduced distance to the subject. on the contrary, oblique pictures, where the presence of the blue background is more evident, returned poor results in terms of percentage of matched features and percentage of oriented cameras. 5. results the 3d reconstruction pipeline starts by performing the orientation of the whole dataset of 722 pictures by means of the bundler software [25]. in the first instance, the 3d reconstruction has been performed on the dataset composed by the original images. the image orientation process failed to orient all the pictures in a single block: the dataset has been divided into two non-overlapping groups, the north and south parts, which have been reconstructed separately. in particular, 384 images have been oriented for the north block and 116 for the south block. this failure is mainly due to the sandy seabed present in the central part of the room, which makes the extraction and matching of features difficult, as a consequence of the low contrast. furthermore, the lack of overlapping areas in the reconstructed model prevented the alignment of the two blocks. the results of the statistical analysis allow for choosing the best combination of factors to be used in order to improve the reconstruction process, represented by rgb images resized to 25 % (second pyramid level) enhanced with the hist method. the enhanced dataset has been processed with bundler, and a subset of 533 images related to the whole area has been aligned, allowing for the generation of a complete 3d point cloud without the need to register the different meshes (figure 6). this result shows that colour correction considerably improves the matching process. the data returned by bundler (camera positions and camera parameters computed by a self-calibration procedure) and the undistorted images have been processed with pmvs2 (patch based multi-view stereo) [26] in order to create a dense cloud of about 10 million points related to the whole site. this algorithm estimates the surface orientation while enforcing the local photometric consistency, which is important for obtaining accurate models for low-textured objects or for images affected by blur due to the turbidity in the underwater environment. furthermore, pmvs2 automatically rejects moving objects such as fishes and algae. the dense stereo matching algorithm implemented in pvms2 receives, as inputs, an undistorted set of images and the 3 × 4 camera projection matrix computed by bundler. the output is a coloured dense 3d point cloud. the pmvs2 parameters used to fine-tune the 3d reconstruction are the size of the correlation window and the level in the internal image pyramid used for the computation. in our experiment, a fixed correlation window with a size of 7 × 7 pixels was adopted, while the image resolution (image pyramid level) was chosen according to the results obtained through the variational analysis. moreover, image triplets instead of pairs were used to increase the robustness of the reconstruction. the 3d point cloud has been elaborated with meshlab tools. the first operation was the manual selection and deletion of unwanted areas and outliers caused by the presence of underwater flora and fauna and bad visibility conditions. then, a watertight surface with about 25 millions of triangles (figure 7) was obtained through the poisson surface reconstruction algorithm. the resulting surface has been subsequently decimated in a mesh of 6.5 million triangles and 3 million points in order to be handled more efficiently without losing details. since the camera orientation procedure has been carried out with an unknown scale factor, it is necessary to scale the model by selecting two points with a known distance. in this experimentation, a scale bar has been placed in the scene and reconstructed in order to evaluate the scale factor. the last step consists in the application of the texture on the 3d surface. colour information can be extracted directly from the coloured point cloud, but this method does not allow for the figure 6. results of the camera orientation process (enhanced pictures with hist method): sparse point cloud and 533 oriented cameras. figure 7. reconstructed surface. acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 76 creation of a high-quality texture, because its resolution depends on the point cloud density. moreover, since the enhancement procedure is often performed to improve the feature extraction process (by increasing the contrast without taking into account the fidelity of the colours – usually single-component or greyscale images are used), the colour information stored in the pixels cannot be used. since the camera positions are known, the texture mapping has been carried out by means the projection and blending of high-resolution images directly on the 3d surface. in particular, an image subset has been selected because the averaging among neighbourhood values during the blending on the images works better if a small overlapping area is present. this subset of images has been extracted from the images enhanced with the hist method, which also gave the best results in terms of texture quality. this is mainly due to the manual retouching step performed on a sample image and then exported to the whole dataset. the result of this procedure is a texture with a resolution comparable to the original images (figure 8). 6. conclusions this paper has presented a performance analysis, based on a doe approach, of the main influential factors that affect 3d reconstruction quality. the performance of three different colour enhancement algorithms, ace, pca, and hist, have been evaluated by using a variance analysis, including the effects of image resolution and colour channels. the results of the anova analysis show that the factors en (image enhancement method), pyr (image pyramid level), and set (image set) are influential, with a confidence level of 95 %, for all the parameters, while the results related to the factor ch (colour channel) have shown a limited influence, since each enhancement method performs mixing operations among the channels. the anova data allowed for choosing the best combination of factors to optimise the sfm bundle adjustment mean reprojection error, the number of extracted features, oriented cameras, and matched features, also taking the processing time into account. more precisely, the best combination is characterised by rgb images resized to 25 % and enhanced with the hist method, which returns more stable results. by using the results of the statistical analysis to correct and process the underwater images, it has been possible to align an unordered sequence of more than 500 images belonging to the entire site. on the contrary, the original images could not be used to align all the cameras. moreover, the corrected images allowed for creating a model mapped with a high-quality texture, comparable with original images in terms of resolution and with a fair colour balance, since the whole dataset shares the same colour statistics. even if these techniques have been used in other works related to underwater archaeology, this experiment represents a significant case study for verifying their robustness in the presence of strong turbidity and poor environmental conditions, providing useful guidelines for an accurate modelling of a submerged site. acknowledgement this work has been supported by the imareculture project that has received funding from the european union’s horizon 2020 research and innovation programme under grant agreement no. 727153. references [1] unesco, convention on the protection of the underwater cultural heritage, 2 november 2001, http://www.unesco.org. [2] g. telem, s. filin, photogrammetric modelling of underwater environments, isprs journal of photogrammetry and remote sensing 65, 5 (2010), pp. 433-444. [3] f. menna, p. agrafiotis, a. georgopoulos, state of the art and applications in archaeological underwater 3d recording and mapping, journal of cultural heritage 33 (2018) pp. 231-248. [4] f. bruno, a. lagudi, l. barbieri, d. rizzo, m. muzzupappa, l. de napoli, augmented reality visualization of scene depth for aiding rov pilots in underwater manipulation, ocean engineering 168c (2018) pp. 140-154. [5] d. c. montgomery, design and analysis of experiments, john wiley & sons, new york, 2017, isbn: 978-1-119-11347-8. [6] imareculture, http://www.imareculture.eu [7] f. bruno, a. lagudi, g. ritacco, j. cejka, p. kouril, f. liarokapis, p. agrafiotis, d. skarlatos, o. philpin-briscoe, e.c. poullis, development and integration of digital technologies addressed to raise awareness and access to european underwater cultural heritage, an overview of the h2020 imareculture project, proc. of the mts/ieee conference oceans’17, aberdeen, uk, 19-22 june, 2017. [8] d. skarlatos, p. agrafiotis, t. balogh, f. bruno, f. castro, b.d. petriaggi, s. demesticha, a. doulamis, p. drap, a. georgopoulos, ‘project imareculture: advanced vr, immersive serious games and augmented reality as tools to raise awareness and access to european underwater cultural heritage’, proc. of the international conference on cultural heritage, nicosia, cyprus, 1-5 november, 2016. [9] a. mahiddine, j. seinturier, d. peloso, j. m. boï, p. drap, d. merad, underwater image pre-processing for automated photogrammetry in high turbidity water, vsmm2012, 2012, pp. 189-194. [10] r. schettini, s. corchs, imaging for underwater archaeology, american journal of field archaeology 27, 3 (2000), pp. 319-328. [11] y. y. schechner, n. karpel, recovery of underwater visibility and structure by polarization analysis, ieee journal of oceanic engineering, 2005, 30(3), pp. 570-587. [12] e. trucco, a.t. olmos-antillon, self-tuning underwater image restoration, ieee journal of oceanic engineering 31, 2 (2006) pp. 511-519. [13] a. rizzi, c. gatta, from retinex to automatic color equalization: issues in developing a new algorithm for unsupervised color equalization, journal of electronic imaging 13 (2004) pp.75-84. [14] m. chambah, d. semani, a. renouf, p. courtellemont, a. rizzi, underwater color constancy: enhancement of automatic live fish recognition, proc. of the 16th annual symposium on electronic imaging, 2003, united states, 5293, pp. 157-169. figure 8. final textured 3d model. acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 77 [15] f. petit, traitement et analyse d’images couleur sous-marines: modèles physiques et représentation quaternionique, doctorat, sciences et ingénierie pour l'information, poitier, 2010. [16] s. bazeille, i. quidu, l. jaulin, j. p. malkasse, automatic underwater image pre-processing, cmm’06 caracterisation du milieu marin, 2006. [17] k. iqbal, r. abdul salam, a. osman, a. z. talib, underwater image enhancement using an integrated colour model, iaeng international journal of computer science 32, 2 (2007) pp.239244. [18] h. bay, a. ess, t. tuytelaars, l. van gool, speeded-up robust features (surf), comput. vis. image underst. 110 (2008) pp.346359. [19] r. kalia, k.-d. lee, b.v.r. samir, s.-k. je, w.-g. oh, an analysis of the effect of different image pre-processing techniques on the performance of surf: speeded up robust feature, proc. of the 17th korea-japan joint workshop on frontiers of computer vision (fcv), ulsan, south korea, 9-11 february 2011, pp.1-6. [20] m. mangeruga, f. bruno, m. cozza, p. agrafiotis, d. skarlatos, guidelines for underwater image enhancement based on benchmarking of different methods, remote sensing 10, 10 (2018) 1652, pp.1-27. [21] a. tonazzini, e. salerno, m. mochi, l. bedini, blind source separation techniques for detecting hidden texts and textures in document images, image analysis and recognition lecture notes in computer science 3212 (2004) pp. 241-248. [22] d. p. mitchell, a. n. netravali, reconstruction filters in computer graphics, computer graphics 22, 4 (1988) pp. 221-228. [23] d. g. lowe, distinctive image features from scale-invariant keypoints, international journal of computer vision 60, 2 (2004) pp. 91-110. [24] z. zhang, a flexible new technique for camera calibration, ieee transactions on pattern analysis and machine intelligence 22, 11 (2000), pp.1330-1334. [25] bundler software, http://www.cs.cornell.edu/~snavely/bundler [26] y. furukawa, j. ponce, accurate, dense, and robust multi-view stereopsis, ieee transactions on pattern analysis and machine intelligence 32, 8 (2010), pp. 1362-1376. creating a finite element mesh of non-periodic masonry from the measurement of its geometrical characteristics: a novel automated procedure acta imeko issn: 2221-870x march 2021, volume 10, number 1, 23 31 acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 23 creating a finite element mesh of non-periodic masonry from the measurement of its geometrical characteristics: a novel automated procedure simone tiberti1, gabriele milani1 1 department of architecture, built environment and construction engineering, technical university of milan, piazza leonardo da vinci 32, 20133 milan, italy section: research paper keywords: geometric measurement; non-periodic masonry; pixel strategy; voxel strategy; homogenisation citation: simone tiberti, gabriele milani, creating a finite element mesh of non-periodic masonry from the measurement of its geometrical characteristics: a novel automated procedure, acta imeko, vol. 10, no. 1, article 5, march 2021, identifier: imeko-acta-10 (2021)-01-05 editor: eulalia balestrieri, university of sannio, italy received march 31, 2020; in final form august 6, 2020; published march 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: simone tiberti, e-mail: simone.tiberti@polimi.it 1. introduction in the past 25 years, the technique known as homogenisation has emerged as one of the most reliable and effective tools for modelling the mechanical behaviour of masonry structures [1][9]. homogenisation is a meso-scale technique using the representative elementary volume (rev) as its base. the rev is the smallest measurable volume that contains all of the physical and geometrical characteristics required for a comprehensive representation of the material. two main modelling strategies are usually employed in applications concerning masonry: macromodelling and micro-modelling. the former considers masonry as a homogeneous material with parameters equivalent to those of the combined units and mortar [10]-[14], whereas the latter models the two constituents separately [15]-[17] and at times also considers the physical interfaces that separate them [18], [19]. homogenisation can serve as a satisfying intermediate technique: unlike the two other strategies, it does not require experimental tests or the separate modelling of units and mortar on a large scale (this can be done at the cell level, using the rev). however, the correct geometrical representation of the masonry bond is needed for homogenisation, which can become a limiting factor when considering non-periodic masonry bonds such as are commonly found in heritage buildings. in fact, both the creation of the actual masonry geometry for meshing purposes and the generation of a finite element mesh for a suitable representation of that geometry are two significant issues that have not been addressed sufficiently in the past. on a larger scale, historical masonry buildings present a complex geometry that must be correctly represented. two advanced automatic survey techniques can be employed in this regard: terrestrial laser scanning (tls) [20] and terrestrial photogrammetry [21]-[23]. these techniques are used to create finite element (fe) models from 3d point clouds [24]-[26] and have been widely applied to architectural heritage objects for purposes such as monitoring their structural health [27]. however, neither of these techniques can be used to create an fe mesh directly; instead, they are usually employed to measure the overall geometry or characterise the masonry bonds of an investigated heritage building. a review of the literature highlights the need for a simpler, faster procedure for generating abstract this paper presents an automated procedure that enables the creation of a finite element mesh directly from an image file representing a rasterised sketch of a masonry element. when used to create a 2d mesh with planar and rectangular elements, the procedure is called ‘pixel strategy’, and the creation of a 3d mesh with elements of solid brick is called ‘voxel strategy’. the homogenised in-plane and outof-plane failure surfaces of historical masonry cells that display a non-periodic arrangement of units can be extracted from the obtained finite element meshes. in our tests, these surfaces were consistent with the expected results, and their shapes suggest that the behavior of this type of masonry may range between orthotropic (if bed mortar joints are clearly noticeable) and quasi-isotropic (if some units spread over two or more masonry layers). acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 24 an fe mesh using only a digital camera with suitable resolution. the input would be simply a rasterised sketch of a real masonry element, which is easy to obtain using the image processing toolbox functions made available in the software matlab [28]. this procedure could then be implemented in a custombuilt script in matlab, which could be made widely available to researchers. this paper presents two techniques for creating an fe mesh directly from the rasterised picture of a masonry element (panel, wall or pillar): one is based on the so-called ‘pixel strategy’ and enables the creation of a 2d fe mesh; the other creates a 3d mesh using a ‘voxel strategy’. this second strategy is also extended to create a 3d fe mesh of a multi-leaf wall. the meshes obtained with these procedures are then used in numerical applications to extract homogenised in-plane and outof-plane failure surfaces, which act as macroscopic strength criteria for masonry revs. the paper is structured as follows: section 2 presents the automated strategies for creating the fe meshes of non-periodic masonry from the rasterised image file of the considered masonry element, for both the 2d and 3d meshes. section 3 presents the use of the 2d fe mesh to derive in-plane homogenised failure surfaces for non-periodic masonry. analogously, section 4 presents the use of the 3d fe mesh to derive out-of-plane homogenised failure surfaces for nonperiodic masonry, for both singleand multi-leaf walls. 2. automated strategies for creating finite element mesh of non-periodic masonry 2.1. pixel strategy for 2d finite element mesh a fast and effective procedure for the generation of an fe mesh directly from the rasterised image file of a real masonry element is presented in this section. a specific procedure called ‘pixel strategy’ was devised, named after the chosen procedure, in which a finite element is automatically created from one pixel of the considered rasterised image file. the rasterised image needs to be a greyscale or, preferably, a black-and-white sketch of the considered masonry bond. a dedicated matlab script enables the actual generation of the finite element mesh. taking as input only the real dimensions of the masonry element under consideration, the script first extracts the red-green-blue (rgb) triplet for each pixel, which is entered into an m×n×3 array (m and n are the number of pixels along the vertical and horizontal directions of the image). second, an m×n matrix containing only the red values of the rgb triplet is extracted from the larger array. the red value is actually used as a threshold for determining the physical nature of the pixel (i.e. if it pertains to mortar or to a unit). each pixel is subsequently considered to be the centroid of a planar, rectangular-shaped finite element, and the script provides it with a pair of xy coordinates that are calculated from the input global dimensions; the centre of the reference system is located at the centroid of the considered masonry element. finally, the xy coordinates of each finite element’s four adjoining nodes are determined. overall, three matrices are thus created: one is the so-called ‘node matrix’, which contains the xy coordinates and the id number of each (1) (2) figure 1. (1) original redrawing of masonry panel, with dimensions in cm (the black parts simply denote a different mortar used in the wall), (2) rasterised sketch, with dimensions in pixel. (1) (2) figure 2. (1) rasterised sketch of sample masonry panel, (2) resulting 2d finite element mesh. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 25 node (ordered first from top to bottom and then from left to right); the second is the so-called ‘element matrix’, which contains the id number of each finite element, the id numbers of its four nodes (listed in counterclockwise order starting from the top left corner), the xy coordinates of its centroid and finally its ‘material flag’, which is based on the red value of its rgb triplet. finally, the third matrix is the so-called ‘macro element matrix’, which contains the id number assigned to each unit (‘macro element’) in the masonry element and the xy coordinates of its centroid. it is important to determine the correct resolution of the chosen digital camera when capturing the source image. the resolution must be adequate to accurately characterise the mortar joints so that cracks within the mortar joints are represented in the final fe mesh. figure 1.1 shows the original redrawing of a masonry panel investigated in [29], whose dimensions are quite large (258 cm in length and 193 cm in height). figure 1.2 shows the rasterised sketch of the same panel, whose dimensions in pixel are 430 and 322 in length and height, respectively. the ratio between millimetres and pixels is 6:1 in this case. hence, the resolution of this image is almost equal to 0.14 megapixels, which is enough to represent a single mortar joint with at least two pixels. since most recent digital cameras are provided with a resolution of 20 megapixels, they should be more than sufficient for recording all of the possible geometrical features of a chosen masonry panel. figure 2 shows an example of the resulting finite element mesh for a sample masonry panel after the application of the pixel strategy compared to the original black-and-white rasterised source image of the panel itself. 2.2. voxel strategy for 3d finite element mesh as in the previous case, the 3d finite element mesh was created in matlab from the rasterised image file of a masonry element. here, the mesh generation was based on the so-called ‘voxel approach’, in which each 3d pixel (the ‘voxel’) is transformed into a finite element. as above, the voxels are provided with a material flag to indicate whether they belong to a masonry unit or a mortar joint, as determined by the red value of their related rgb triplet. because of this, units and mortar must be denoted by clearly distinguishable colours in the source image, which, again, must be a simple black-and-white or greyscale sketch of the considered masonry element. the element’s overall dimensions are set as input by the user and exploited to determine the xyz coordinates of its centroid according to a reference system originated at the centre of the test-window. this reference system is a permutation of the one created in the 2d case: here, the y-axis represents the horizontal axis of the test-window, the z-axis represents the vertical axis and the x-axis represents the transversal direction. solid brick elements are then generated from the centroid’s coordinates, and the number of elements over the transversal dimension can be set by the user. three matrices are eventually created, one listing the node ids and their coordinates (the ‘node matrix’), the second listing the finite element ids, those of their 8 adjoining nodes, the material flag and the coordinates of their centroid (the ‘element matrix’) and the third listing the macro element ids and the coordinates of their centroid (the ‘macro element matrix’). it is worth noting that the transversal layout of the considered masonry element can be derived either from the translation of its in-plane configuration or by using an ellipsoidal shape for the bricks/stones. in this latter case, the in-plane configuration of the considered masonry element denotes the mid-plane of the 3d fe mesh; the ellipsoidal shape is achieved by reducing the mid-plane surface of the bricks/stones so that their 3d shape resembles either a full ellipsoid or a truncated one. figure 3 (1) (2) (3) figure 3. (1) resulting 3d finite element mesh with extruded transversal layout, (2) resulting 3d finite element mesh with ellipsoidal stones, (3) resulting 3d finite element mesh for a multi-leaf wall. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 26 shows an example of the 3d meshes resulting from the application of this procedure to the sample test-window shown in figure 2.1, including meshes obtained by using transversal extrusion (figure 3.1) and the ellipsoidal shape of bricks/stones (figure 3.2). 2.3. voxel strategy for 3d finite element mesh of multi-leaf walls a further matlab function enables the creation of a 3d fe mesh for a multi-leaf wall. the procedure is basically the same as the one for the generation of a standard 3d fe mesh, but the inputs are different: the user must choose the number of wythes for the considered multi-leaf wall and is allowed to select a separate source image for each wythe. the choice of the transversal configuration is still enabled for each wythe. once this setup is chosen, the procedure runs in the same way as for the single-leaf 3d fe mesh. an example of the final result is shown in figure 3.3 for a three-leaf wall in which both of the outer wythes are generated from the same source image (the testwindow in figure 2.1) for the sake of simplicity. however, since multi-leaf masonry walls commonly display different bonds in the two outer wythes, the procedure allows the user to choose between two different rasterised sketches for the creation of the final fe mesh, as shown in figure 4 for another sample case. it must be noted that the physical layout of the inner layer must also be correctly represented because it may consist of mortar and/or stone chips. some transversal stones spanning the whole thickness of the wall may also be included; their presence must be carefully assessed. of course, the layout of the inner layer cannot be achieved by simply capturing the image with a digital camera; it requires the use of more advanced techniques such as radiography or ultrasonic inspection. 3. homogenised in-plane failure surfaces 3.1. 2d finite element mesh the 3d fe mesh created through the procedure named ‘pixel strategy’ was used as input in a separate matlab script that was devoted to the determination of homogenised in-plane failure surfaces, which represent in-plane macroscopic strength criteria for masonry elements. they result from the solution of an upper bound limit analysis problem that includes a homogenisation approach, formulated as a minimisation problem (a sub-class of linear programming problems) in standard form. the finite elements of the mesh are supposed to be rigid; therefore, dissipation can only occur across the interfaces of adjoining elements. a mohr-coulomb failure criterion with a cut-off in tension is used to address the velocity jumps across the interfaces between mortar elements or between a unit and a mortar element. the equality constraints for the minimisation problem come from the velocity jumps due to dissipation, the periodicity conditions on the sides of the considered masonry element (as required by the homogenisation approach) and from the normalisation of the dissipated external power (which is needed for finding a single solution in terms of deformed configuration at collapse). four different masonry revs are considered in this numerical application, and they are pictured in figure 5.1. the parameters of the mohr-coulomb criterion used in this application are listed in table 1. figure 5.2 shows the homogenised in-plane failure surfaces in the tension–tension range for the four considered masonry revs. it can be seen that most of them display a shape that suggests an orthotropic response under tensile loads, which is to be expected since the bed joints are clearly visible, despite a nonperiodic arrangement of the units. only case (b) displays a quasiisotropic response, due to the presence of units that span two layers of masonry. 3.2. 3d finite element mesh the 3d fe mesh created with the voxel strategy was similarly used to create homogenised in-plane failure surfaces as the results of another matlab script containing a minimisation problem. this script was modified to accept a 3d mesh as input instead of a 2d one. in this case, a mohr-coulomb failure criterion with a cut-off in both tension and compression was used to address the velocity jumps across the interfaces between (1) (2) figure 4. (1) 3d finite element mesh for a sample three-leaf masonry wall in which the two outer wythes have different bonds, (2) exploded view of the 3d finite element mesh highlighting the different bonds. table 1. parameters of mohr-coulomb criterion for the 2d numerical application. mechanical parameter value cohesion 0.15 mpa friction angle 30° tensile strength 0.1 mpa acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 27 mortar elements or between a unit and a mortar element. the parameters used in this application were those listed in table 1 with the addition of a compressive strength equal to 1.5 mpa. figure 6.1 shows the 3d fe mesh used in this application, which represents case (d) of the previous section. figure 6.2 shows the full homogenised in-plane failure surface for the considered case. in the compression–compression range, the resulting homogenised failure surface is limited by the value 1.5 mpa, which is equal to the considered compressive strength; for composite in-plane load conditions, the considered masonry rev displays an increase in its strength, as expected. figure 6.3 shows the failure modes for four uniaxial load conditions, which are all consistent with the expectations. horizontal tension (figure 6.3a) causes widespread cracks in the masonry testwindow, while vertical tension (figure 6.3b) creates horizontal cracks. horizontal compression (figure 6.3c) also causes horizontal cracks due to lateral expansion, whereas crushing of mortar is observed in the case of vertical compression (figure 6.3d). 4. homogenised out-of-plane failure surfaces 4.1. single-leaf wall finally, the 3d fe mesh created with the voxel strategy was used to generate homogenised out-of-plane failure surfaces as the results of yet another matlab script containing a minimisation problem. here, the goal was to investigate the collapse behaviour of non-periodic masonry bonds under outof-plane loads such as seismic actions. in this case, a mohr-coulomb failure criterion with a cut-off in both tension and compression was used to address the velocity jumps across the interfaces between mortar elements or between a unit and a mortar element. the parameters used in this application were once again those listed in table 1 as well as a compressive strength of 1.5 mpa. in this application, two 3d fe meshes were created for cases (b) and (c) of figure 4, whose (1) (2) figure 5. (1) four masonry revs used in the 2d numerical application, (2) comparison of the homogenised in-plane failure surfaces for the four cases. (1) (2) (3) figure 6. (1) 3d finite element mesh for case (d) of section 3.1, (2) full homogenised in-plane failure surface for the considered masonry rev with 3d fe mesh, (3) failure modes for four uniaxial load conditions applied to the 3d fe mesh. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 28 depiction is omitted for sake of brevity. the thickness of the created meshes was equal to 40 cm for case (b) and 15 cm for case (c). figure 7 shows the homogenised out-of-plane failure surfaces for the two considered cases. the aforementioned matlab script enables the extraction of two distinct homogenised out-of-plane failure surfaces, one dealing with the flexural collapse behaviour and defined in the mxx-myy plane and the other dealing with the torsional collapse behaviour and defined in the mxx-mxy plane. mxx is the vertical bending strength, myy is the horizontal bending strength and mxy is the torsional strength. in both surfaces, the collapse moments are normalised with respect to a horizontal bending strength of (ft·t2)/2. it is possible to observe that case (c) displays a greater resistance in terms of vertical and, to a lesser extent, torsional moments. figure 8 shows the failure modes related to case (c) extracted for three relevant out-of-plane load conditions, namely mxx, myy and mxy. they are all consistent with the expectations for a quasiregular masonry bond. specifically, the vertical bending moment mxx (figure 8.1) causes widespread cracks in the masonry testwindow, while the horizontal bending moment myy (figure 8.2) causes a single horizontal crack, as expected from the application of such an out-of-plane action. 4.2. multi-leaf wall as a final numerical application, the 3d mesh of a multi-leaf wall generated with the voxel strategy was used to assess the outof-plane collapse behaviour of a quasi-regular multi-leaf masonry wall. this was once again achieved by extracting homogenised out-of-plane failure surfaces that result from a modification of the out-of-plane matlab script described in the previous subsection. two different instances of the considered multi-leaf wall were created: the first is depicted in figure 9.1 and simulates the absence of mutual interaction between the three wythes (labelled ml1), whereas the second is depicted in figure 9.2 and simulates the presence of a few transversal bricks spanning the whole thickness of the considered multi-leaf wall (labelled ml2). the first instance represents a common case observed in real multi-leaf masonry walls, in which the outer wythes are not (1) (2) figure 7. (1) flexural homogenised out-of-plane failure surface for cases (b) and (c) of figure 5; (2) torsional homogenised out-of-plane failure surface for cases (b) and (c) of figure 5. (1) (2) (3) figure 8. failure modes for relevant out-of-plane load conditions applied to case (c): (1) application of mxx, (2) application of myy, (3) application of mxy. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 29 connected, and the inner layer consists of loose material such as stone chips. in each instance, the thickness of the created mesh was 36 cm. figure 10 presents the homogenised out-of-plane failure surfaces for the two instances of the considered multi-leaf wall. it can be observed that the instance that includes transversal bricks (ml2) shows larger homogenised failure surfaces than the instance without transversal bricks (ml1). this confirms the assumption that multi-leaf walls in which the outer wythes are somehow connected to each other (for example with transversal elements spanning the whole thickness) are more resistant to out-of-plane actions. in the case considered here, the beneficial effect of the transversal interconnection can be observed most clearly in terms of resistance to myy, which is doubled with respect to instance ml1. the resistance to mxx and mxy are also increased in instance ml2. finally, figure 11 shows the failure modes related to the two instances of the multi-leaf wall and extracted for the three relevant out-of-plane load conditions mxx, myy and mxy. specifically, figure 11a–c show the failure modes related to instance ml1, while figure 11d–f show the failure modes related to instance ml2. the stiffening effect created by the presence of transversal bricks is very clear for both mxx and mxy. furthermore, when myy is applied to the two instances, there is an apparent change in their failure modes; a single crack opens across a bed joint in the lower part of the multi-leaf wall in ml1, whereas there are several smaller cracks appearing across the wall in ml2. 5. conclusions a fast, automated procedure for the generation of a finite element mesh directly from the rasterised sketch of a masonry element is presented. this procedure is particularly suitable for the complex and irregular (non-periodic) masonry bonds that are often found in heritage buildings or archaeological sites. the rasterised sketch can be generated from a picture of a masonry element obtained with a digital camera with suitable resolution and matlab software. two procedures were devised to create the fe mesh. one, named ‘pixel strategy’, converts each pixel into a single finite element, allowing for the creation of a 2d fe (1) (2) figure 9. (1) 3d fe mesh for the instance ml1 of the considered multi-leaf wall, (2) 3d fe mesh for instance ml2 of the considered multi-leaf wall and its exploded view to highlight the presence of transversal bricks. (1) (2) figure 10. (1) flexural homogenised out-of-plane failure surface for the two instances of figure 9, (2) torsional homogenised out-of-plane failure surface for the two instances of figure 9. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 30 mesh consisting of planar, rectangular elements. the other, called ‘voxel strategy’, first transforms a 2d pixel into an analogous 3d entity or ‘voxel’, which is then converted into a single finite element, enabling the creation of a 3d fe mesh consisting of solid brick elements. the 2d or 3d fe meshes can then be employed as input for several numerical applications that involve finite element analyses. in this case, the numerical applications were undertaken to extract homogenised in-plane and out-of-plane failure surfaces of masonry elements, which represent macroscopic strength criteria for in-plane and out-ofplane load conditions. homogenised in-plane failure surfaces were derived for four masonry revs by using 2d fe meshes developed with the pixel strategy and, in one instance, a 3d fe mesh developed with the voxel strategy. the shapes of the various surfaces indicate that non-periodic masonry displays a response that is orthotropic when the bed joints are still distinctly visible but may become quasi-isotropic in the presence of units that vertically span two (or more) masonry layers. in addition, two different sets of homogenised out-of-plane failure surfaces were extracted for two of the masonry revs to assess in-plane collapse behaviour. relevant failure modes were also obtained for both the in-plane and out-of-plane cases. finally, homogenised out-of-plane failure surfaces were extracted for two instances of a quasi-regular multi-leaf masonry wall in order to assess the influence of the transversal interconnection between the outer wythes on the structural response to out-ofplane actions. future numerical applications will involve the implementation of the automated procedure described here in an integrated gui-based matlab app that will also include a tool to rasterise a picture of a real masonry element. the proposed gui should at least include the following features: • a button for uploading the image of the real masonry structural element • a window for displaying the uploaded image • a button for starting the rasterisation procedure. • a separate window for displaying the rasterised sketch of the source image • a button for creating the finite element mesh and a menu that enables the selection of either a 2d or 3d mesh as well as other components (dimensions of the masonry element, etc.) • a separate window for displaying the 2d fe mesh or the in-plane configuration of the 3d fe mesh. • a button for starting the homogenised limit analysis of the considered test-window and a menu for selecting either in-plane or out-of-plane analysis • a separate window for displaying the resulting in-plane or out-of-plane homogenised failure surfaces • a button for extracting the deformed shapes at 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www.imeko.org june 2018 | volume 7 | number 2| 39 point cloud processing techniques and image analysis comparisons for boat shapes measurements sofia catalucci1,2, roberto marsili1, michele moretti1, gianluca rossi1 1università degli studi di perugia, dipartimento d’ingegneria, via goffredo duranti 93, 06125 perugia, italy 2present address: university of nottingham, advanced manufacturing building, jubilee campus, ng81bb nottingham, united kingdom section: research paper keywords: photomodelling; metrology; point clouds; iterative closest point algorithm citation: sofia catalucci, roberto marsili, michele moretti, gianluca rossi, point cloud processing techniques and image analysis comparisons for boat shapes measurements, acta imeko, vol. 7, no. 2, article 7, june 2018, identifier: imeko-acta-07 (2018)-02-07 section editor: fabio leccese, università degli studi di roma tre, italy received january 17, 2018; in final form march 25, 2018; published june 2018 copyright: © 2018 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: this work was supported by the engineering department, università degli studi di perugia, italy corresponding author: sofia catalucci, e-mail: sofia.catalucci@gmail.com 1. introduction the design and modelling of a boat involve complex freeform geometric shapes, difficult to measure and survey with traditional metrology methods. scan data allow creating precise 3d models that can be used by naval designers and engineers to ensure the quality of interior and exterior construction, as well as for simulation and inspection purposes. for this purpose, the paper describes photomodelling technique, a recent and fast image processing and alignment method that leads to the reconstruction of three-dimensional models, starting from the simple acquisition of photographic images. close to photogrammetry, the result obtained is a 3d point cloud, a set of x,y,z space coordinates, first form of the object surveyed [1]. a point cloud can be identified as a pixel cloud, because of the direct relationship between photomodelling and photography: each pixel of an image corresponds to a point of the cloud, thus preserving the chromatic characteristics of the object surveyed [2]-[6]. 2. test performed object of the survey is a beneteau first 456/s boat (figure 1), 1984, with 3 cabin all with toilets, engine yanmar 55 cv, 14 m length. the measurements have been performed on the driving seat area, by using photomodelling technique and the creaform 3d scanner, placed on the same surface (figure 2). at first, the survey has been performed using the 3d scanning system (figure 3); target stickers have been applied randomly to facilitate the capture process, due to the auto similarity of the surface pattern. figure 1. the boat. abstract photomodelling is a new and fast solution for 3d modelling, based on the same principles of photogrammetry. the comparison between photomodelling and the metrological technique of structured light 3d scanning, provided by the creaform go scan 50 with metrological certification, is the aim of this paper, defining performances and verifying the potential of this innovative, simple and economical technique. acta imeko | www.imeko.org june 2018 | volume 7 | number 2| 40 the survey carried out by the photomodelling technique included a totality of 20 images, loaded in three different dedicated software: agisoft photoscan, visual sfm and autodesk remake. each instrument is different because of its time data processing, difficulty of use, accuracy and precision of results [7]. furthermore, the proposed applications include both open-source and commercial software. 3. data processing the 3d data processing is the same for each software: the first operation is the manipulation of the 3d point cloud. the next step is the creation of three-dimensional model, called “triangulation”: starting from the input data vertices, edges and faces are generated. the result obtained is a set of coordinates, which is converted into a polygonal surface [8]-[10]. with editing software as meshlab and geomagic studio, it is possible to perform manual editing of data, merging, scaling, aligning of different surfaces, creating a three-dimensional surface with the aid of different algorithms [11]-[13] (figure 4, figure 5, figure 6). 4. comparison after the alignment process, the research concerned the comparison between the results of photomodelling technique, identified as test, and a reference model. the surface reconstructed by the scanning system has been chosen as reference [14]-[16]. figure 4. surface 3d model and point cloud by agisoft photoscan. figure 5. surface 3d model and point cloud by visual sfm. figure 2. driving seat area. figure 3. boat surface scanning by creaform scango 50. acta imeko | www.imeko.org june 2018 | volume 7 | number 2| 41 the software chosen for the comparison is geomagic qualify. the comparison has been made using algorithms that provide variances and deviations between geometric entities in the space and it had as output 2d and 3d maps of such deviations. it was possible to derive a matrix in .csv format of the test and reference points spatial coordinates x,y,z and of the values of the mutual distances for each pair of points. after the manual and automatic alignment process, a spectrum of 15 intervals of values has been set: -10 and +10 mm deviation is the range of acceptability; the range between -1 and +1 mm is the optimum correspondence between the test and reference points. as it is clear from the various elaborate graphs, the best result is the comparison with the test model of agisoft photoscan (table 1, figure 7). the results obtained with visual sfm are close to the previous ones (table 1, figure 8). on the contrary, autodesk remake highlights the differences in terms of precision and accuracy of both the previously mentioned software programs, especially the second one. autodesk remake is a simple software, which can be used by every kind figure 6. surface 3d model by autodesk remake. table 1. deviation distributions. deviation distribution range agisoft photoscan number of points visual sfm number of points autodesk remake number of points n>10 1536 1550 10136 8.5<n<10 1463 1802 2956 7<n<8.5 2605 2400 2745 5.5<n<7 5926 4144 2526 4<n<5.5 13862 8631 3104 2.5<n<4 25336 40056 3367 1<n<2.5 41193 73095 3837 (-1)<n<1 59806 74819 4168 (-2.5)<n<(-1) 41812 28267 3118 (-4)<n<(-2.5) 20400 19176 3924 (-5.5)<n<(-4) 13607 10287 4520 (-7)<n<(-5.5) 10720 6002 5298 (-8.5)<n<(-7) 5914 5847 5873 (-10)<n<(-8.5) 2082 6623 4188 n<(-10) 3499 23918 13311 n>10 1536 1550 10136 8.5<n<10 1463 1802 2956 7<n<8.5 2605 2400 2745 5.5<n<7 5926 4144 2526 4<n<5.5 13862 8631 3104 2.5<n<4 25336 40056 3367 1<n<2.5 41193 73095 3837 (-1)<n<1 59806 74819 4168 max +/: 54.381 / -63.689 mm average +/: 2.652 / -3.080 mm mean value of the distance between test/reference models:-0.27303 mm standard deviation: 3.9393 mm mean value of all differences +/devstd: (3.6663,-4.2124) mm number of measured points: 249761 figure 7. maps of deviation distribution (agisoft photoscan). acta imeko | www.imeko.org june 2018 | volume 7 | number 2| 42 of users, and it does not elaborate significant results (table 1, figure 9). after the comparison, the next step is to upload the data into matlab, processing the data in histograms of frequency, absolute values of deviations and curves of probability density (table 2, figure 10). a "measure" is a range of values, acquired with the purpose of controlling a process, performing the calibration of an instrument or allowing the physical understanding of a partially known phenomenon [17]-[19]. binding to this assertion, to further support the experimentation, the same obtained 3d models have been imported into the multi-paradigm numerical computing environment matlab. through the elaboration of chromatic maps (figure 13, figure 14, figure 15), which identify the volumetric error distribution and the distance between the closest point-to-surfaces of the meshes composing the models, it has been possible to compare the three-dimensional surfaces through an algorithm. this algorithm is based on a modified function operating through the icp (iterative closest point) approach, which minimizes the distance between two dispersion of multidimensional points, i.e. the two point clouds to be registered and compared, respectively labelled as reference point cloud (rpc) and measured point cloud (mpc). the algorithm works through an iterative process and its goal is to minimize the value of a global goal function. in this case the global function as been developed as the global volumetric error (gve), calculated as the sum of volumes of the single prismatic element, as shown in figure 11. the approach in terms of volumes instead of the classic point-to-point distance gives an improved information: the value of gve takes into account the size of the triangulated meshes elements. flat areas with large mesh size have a larger weight than areas characterised by a small mesh size. this behaviour is characteristic of volumetric approaches. for each comparison, the algorithm has elaborated box plot statistical diagrams and volumetric error distribution graphs (table 3, figure 12). it is possible to notice that the volumetric error distribution max +/: 27.726 / -45.905 mm average +/: 7.499 / -7.730 mm mean value of the distance between test/reference models: -1.2971 mm standard deviation: 8.772 mm mean value of all differences +/devstd: (7.4748,-10.0691) mm number of measured points: 73071 figure 9. maps of deviation distribution (autodesk remake). max +/: 51.380 / -22.958 mm average +/: 2.271 / -4.750 mm mean value of the distance between test/reference models:-0.81653 mm standard deviation: 4.9616 mm mean value of all differences +/devstd:(4.1451,-5.7781) mm number of measured points: 306617 figure 8. maps of deviation distribution (visual sfm). table 2. total number of measured points. agisoft photoscan 249761 visual sfm 306617 autodesk remake 73071 figure 10. probability density diagram. acta imeko | www.imeko.org june 2018 | volume 7 | number 2| 43 is quite similar to the behaviour of point-to-point approach, due to the high regularity of mesh sizing of both reconstructed surfaces. 5. conclusions the paper presents the metrological characteristics of simple and economical photomodelling techniques for boat shape measurements. the surfaces generated by two different techniques as photomodelling and a structured light 3d scanner with metrological certification have been detected in order to investigate the alignment error due to a poor geometry reconstruction. the comparison was made between three photomodelling processes performed by the software agisoft photoscan, visual sfm and autodesk remake; the results show different performances, in terms of deviation distributions and volumetric errors. the registration of the measured point clouds and the analysis of the error distribution have been figure 12. volumetric error distribution and box plot: agisoft photoscan, visual sfm and autodesk remake. table 3. volumetric error distribution. agisoft photoscan mean=5.6638 mm3 std=19.5801 mm3 visual sfm mean=7.8655 mm3 std=10.4855 mm3 autodesk remake mean=54.5649 mm3 std=223.882 mm3 figure 11. triangular mesh distance (mm): icp function (iterative closest point). figure 13. chromatic maps: agisoft photoscan. acta imeko | www.imeko.org june 2018 | volume 7 | number 2| 44 carried out with the commercial software geomagic qualify. a tailored software in matlab environment has been realised in order to modify the target of registration algorithms through the global volumetric error calculation. references [1] l. de luca, “la fotomodellazione architettonica. 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[19] r. marsili, g.l. rossi, m. becchetti, r. flori, “stress and strain measurements by image correlation and thermoelasticity”, society for experimental mechanics sem annual conference and exposition on experimental and applied mechanics 2009, volume 1, 2009, pages 70-75; albuquerque, nm; united states; 1-4 june 2009; code 78716; isbn: 978-161567189-2; 978161567189-2. figure 14. chromatic maps: visual sfm. figure 15. chromatic maps: autodesk remake. http://dx.doi.org/10.1016/j.measurement.2014.03.005 different long-term characteristics of hydraulic pressure gauges under constant pressure applications acta imeko issn: 2221-870x september 2019, volume 8, number 3, 19 – 24 acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 19 different long-term characteristics of hydraulic pressure gauges under constant pressure applications hiroaki kajikawa1, tokihiko kobata1 1 national metrology institute of japan (nmij), aist, 1-1-1 umezono, tsukuba-city, ibaraki, japan section: research paper keywords: pressure gauge; long-term drift; pressure calibration; pressure standard citation: hiroaki kajikawa, tokihiko kobata, different long-term characteristics of hydraulic pressure gauges under constant pressure applications, acta imeko, vol. 8, no. 3, article 4, september 2019, identifier: imeko-acta-08 (2019)-03-04 editor: jeerasak pitakarnnop, nimt, thailand received august 30, 2018; in final form june 17, 2019; published september 2019 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: hiroaki kajikawa, e-mail: kajikawa.hiroaki@aist.go.jp 1. introduction long-term drift is one of the important characteristics of pressure gauges and should be evaluated when the gauges are used as the reference standard for pressure calibrations or the transfer standard for inter-laboratory comparisons [1]-[5]. pressure gauges used as reference gauges in calibration laboratories are calibrated periodically (once a year in most cases), and the long-term drift is then evaluated and considered as an uncertainty factor. when pressure gauges are used as transfer standards for international comparison or proficiency testing of calibration laboratories, the drift of the transfer standard needs to be corrected to appropriately compare the participants' results. the drift is evaluated by the pilot institute at both the beginning and the end of the circulation of the transfer standard. the reports of the international comparisons include the evaluation of the long-term drift of the transfer standard [6], [7]. in the above cases, pressure gauges of interest are always kept in stable condition, and no pressure is applied except when they are used in measurements. furthermore, the gauges are used with a fixed procedure according to wellrecognised protocols, such as stepwise calibration cycles. thus, the long-term drift of these gauges can be appropriately evaluated and corrected by periodic calibrations. in scientific and industrial fields, however, pressure gauges are used in various environment and pressure conditions. some pressure gauges are used under constant and stable pressures, and others are under periodically changing pressure conditions. such pressure gauges may show different long-term characteristics from those that are kept in normal atmospheric conditions most of the time. to obtain reliable pressure values with higher accuracy, long-term drift should be evaluated in similar pressure conditions as the actual usage. in geophysical research, for example, pressure gauges are used for monitoring seafloor pressures to detect the uplift or subsidence of submarine grounds [8]-[10]. currently, many precise pressure gauges are installed at seafloors around the japanese islands and other earthquake zones, known as the circum-pan-pacific earthquake belt. the pressure gauges installed at seafloors in the depth of several kilometres are abstract this article focuses on the long-term drifts of hydraulic pressure gauges, which are constantly subjected to high pressures. a target measurement pressure was consistently applied to pressure gauges for more than two months, and the gauges were occasionally calibrated against a reference pressure balance. the long-term drift has been evaluated based on the variations in calibration values with the elapsed time since the pressure application. we evaluated the long-term drift of three kinds of pressure gauges that are commonly used for calibrations and industrial measurements. the long-term drift differed depending on the type of sensing structure. although the gauges that had similar sensing structures showed similar drift when the gauges were pressurised due to atmospheric pressure, the amount of the drift depended on each product. in addition, the long-term drift was also affected significantly by the preliminary pressure conditions, even under the same pressure application. preliminary or ex-post evaluations are necessary for the respective gauges under similar pressure conditions. in practice, the drift of zero-reading is often the main contributory factor of the drift; therefore, the drift can be mitigated by tracing the zero-reading during the pressure applications. mailto:kajikawa.hiroaki@aist.go.jp acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 20 always subjected to high pressures of several tens of mpa. to detect subtle crustal movements from the pressure data, the effect of the long-term drift of the pressure gauges (as well as other influential factors in the seafloor pressure data such as daily tidal fluctuations and seasonal variations in the sea level) should be evaluated and mitigated. the long-term trends of the pressure gauge outputs have been comprehensively evaluated based on the pressure data obtained during actual seafloor measurements [11]. laboratory experiments of long-term drift were also conducted for some pressure gauges [12]. currently, the characteristics of the gauges used at several tens of mpa for months and years are required for this purpose. laboratory experiments for high-pressure gauges can provide the data without other fluctuating factors, and they clearly show the drift trend that is intrinsic to respective gauges. in this study, we have evaluated the long-term drift of pressure gauges, which are constantly subjected to high pressures for long periods of time, with the aim of suggesting appropriate methods to evaluate and reduce long-term drift [13]. this paper shows the drifts of three kinds of pressure gauges that are used for pressure calibrations and industrial pressure measurements. the measurement system and evaluation methods are explained in section 2. the long-term drifts of the three kinds of gauges are compared in two experiments in section 3. practical measures to reduce and mitigate the long-term drift are also discussed in section 3. finally, a brief summary is given in section 4. 2. experiment figure 1 shows the measurement system that has been used for the long-term drift evaluation. three kinds of pressure gauges have been evaluated in this study: quartz bourdon-type pressure transducers (paroscientific, inc.), foil stain pressure gauges (hbm), and clamped thin-film pressure gauges (wika). table 1 shows the detailed information of the test gauges, including the maximum measurement pressure and accuracy claimed by the manufacturers. the pressure applied to the test gauges was kept in enclosed tubing by closing a shut-off valve in the system. during the experiment, the pressure was maintained at the target pressure within 1 mpa, despite the pressure fluctuation following the variations in the surrounding temperature in the calibration room. an additional valve was also used as a variable volume to adjust the pressure in the enclosed space during pressure application. during the pressure application, the test gauges were calibrated against a reference pressure balance. first, the system (including the reference pressure balance) was pressurised to the test pressure by using a pressure controller and a hand pump, and then the shut-off valve was opened to apply the pressure controlled with the reference pressure balance. the outputs of the gauges were obtained after an appropriate waiting time for pressure stabilisation. at each test pressure p, the deviation of the averaged output of the test gauge (ip) from the standard pressure by the reference pressure balance (ps), dip, was calculated as dip = ip – ps. after the measurement, the shut-off valve was closed to maintain the pressure on the test gauges, and the pressure of the reference side was released to atmospheric pressure. the long-term drifts of the pressure gauges were evaluated by tracing the variations in dip according to time. 3. results and discussion 3.1. different long-term drifts under constant pressure figure 2(a) shows the one-year history of the pressure application for three pressure gauges under test: pt200-1, figure 1. measurement system for long-term drift evaluation. table 1. specifications of pressure gauges used for experiments of long-term drift under constant pressure. type full scale accuracy (of full scale) manufacture, model applied pressure pt200-1 quartz bourdon-type pressure gauge 207 mpa 0.01 % paroscientific, inc., 9000-30k 200 mpa, 100 mpa pt200-2 clamped thin-film pressure gauge 250 mpa 0.25 % wika, hp-2-s 200 mpa, 100 mpa pt200-3 foil stain pressure gauge 200 mpa 0.10 % hbm, 1-p3tcp 200 mpa, 100 mpa pt70-1 quartz bourdon-type pressure gauge 70 mpa 0.01 % paroscientific, inc., 9000-10k 70 mpa pt70-2 clamped thin-film pressure gauge 100 mpa 0.25 % wika, s-10 70 mpa 0 100 200 300 20 40 60 80 0 100 200 300 0 100 200 time [day] [k p a ] p re ss u re [m p a ] time [day] di100 d e v ia ti o n f ro m di100 di200 pt200−1 interval a(a) st a n d a rd (b) (79 days) interval b (113 days) interval c (95 days) figure 2. (a) history of pressure application to three pressure gauges pt2001, pt200-2, and pt200-3. (b) variation in the deviation of pressure indications from the standard, dip, for pt200-1. acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 21 pt200-2, and pt200-3. the three gauges were firstly pressurised from atmospheric pressure to 200 mpa and maintained at 200 mpa for 79 days (interval a). then, the pressure was decreased from 200 mpa to 100 mpa, which was maintained for 113 days (interval b). on the 192th day, the pressure was released to atmospheric pressure. after the pressure release for 78 days, the gauges were pressurised to 100 mpa on the 270th day, which was then maintained for 95 days (interval c). the calibration values of the three gauges were traced during the respective pressure applications. figure 2(b) shows the calibration values dip of pt200-1 during the experiment. the calibration value varies with time differently according to the applied pressure and previous pressure condition. figure 3 compares the long-term drift of the three pressure gauges. figure 3(a) shows the variations in the calibration values at 200 mpa, di200, with the elapsed time since the pressure application. we observed three different drift characteristics, mainly according to the difference in the sensing structures of the three gauges. the calibration values for pt200-1 rapidly decreased immediately after the constant pressure application. the value decreased by 9.2 kpa, 46 parts per million (ppm) relative to 200 mpa, in the first day. the amount of the variation is large compared with the short-time stability of the gauge and it reached almost half of the claimed accuracy. the change rate became smaller as time passed. the average change rate from the 20th to the 80th day was approximately -0.13 kpa/day, relatively -0.65 ppm/day. the total variation in di200 during the 80 days was 28 kpa, relatively 140 ppm, which exceeds the expanded (k=2) uncertainty of 12.5 kpa in the normal stepwise calibration. in contrast, the calibration values for pt200-2 increased with time. the variation in di200 during the 80 days was 40 kpa, relatively 200 ppm, which is larger than that for pt200-1. most of the variation was made during the first 20 days. it should be noted that pt200-2 showed larger data scattering than the other gauges. the standard deviation of the obtained 18 indications was typically 5 kpa. the value seems almost constant within a data scattering after around the 40th day from the pressure application. pt200-3 showed a much larger and arch-shaped drift. at first, the calibration value rapidly increased by 60 kpa in a few days, and it then turned to decrease at the rate of -0.49 kpa/day, relatively 2.4 ppm/day, after the 20th day. figure 3(b) shows the variation in di100 according to time for the same three gauges during interval c. the drift trends in di100 were similar to those in di200 for all the three pressure gauges. for pt200-1, di100 rapidly decreased at first, and then, the change rate became smaller. the total variation in di100 during 80 days was 13 kpa, which is almost half of the variation in di200. pt200-2 also showed the similar increasing drift trend, but the variation in di100 was relatively smaller and more scattered than that in di200. similarly, pt200-3 showed an archshaped drift at 100 mpa. the amount of the first increase in di100 was about 45 kpa during 15 days, which is almost half of the increase in di200. both di200 and di100 turned to decrease after the first increase. from the above comparison, the gauges having the same sensing structure showed the similar drift trend when the pressure is increased from atmospheric pressure. the amount of the variation depends largely on the applied pressure. the long-term drifts also depend on the previous pressure conditions. figure 4 compares the drifts in di100 with different previous pressure conditions. as shown in figure 2(a), the applied pressure was decreased from 200 mpa to 100 mpa at the beginning of interval b, while the pressure was increased from 0 mpa (atmospheric pressure) to 100 mpa at the beginning of interval c. in figure 4, the data in interval b and interval c is shown by closed and open circles, respectively. in figure 4(a), for pt200-1, di100 observed during interval b and that during interval c varied in the opposite direction with each other, although the amount of the variation during the 80 days was similar. the data during interval b, denoted by closed circles, showed irregular drift behaviour. after becoming almost flat at around the 50th day, di100 started to increase again. this two-step trend was not observed when the gauge was pressurised from atmospheric pressure, and is thought to be due to the hysteresis effect. a discontinuous step by 2.5 kpa was also observed on the 91st day, the reason for which remains unknown. for pt200-2, the two drifts were not symmetric. di100 in interval b kept decreasing with time in larger amounts. for pt200-3, the direction of the drift in di100 during interval b and that during interval c were opposite to each other immediately after the pressure change. after 20 days, however, di100 both during interval b and interval c decreased over time at an almost constant rate. the average drift rates from the 30th day to 80th day, in interval b and interval c, were similar: the drift rates were -0.71 kpa/day in interval b and -0.75 kpa/day in interval c. the previous pressure figure 3. variation in the calibration value according to elapsed time in days, under constant pressure application from atmospheric pressure: (a) 200 mpa during interval a in figure 2(a), (b) 100 mpa during interval c in figure 2(a). acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 22 condition did not affect the drift behaviour of pt200-3 after a month, and a similar drift trend was noted. as shown above, even under similar pressure conditions, the drift in the calibration values largely differs depending on the previous pressure condition. the effect of the previous condition depends on each pressure gauge. in some cases, preliminary application of a higher pressure makes the drift more complicated and makes correction of the drift difficult. 3.2. long-term drifts at measurement pressure and atmospheric pressure under constant pressure in the next experiment, we evaluated the variations in the calibration value with the elapsed time at 70 mpa and 0 mpa (atmospheric pressure) when the pressure of 70 mpa was constantly applied to two pressure gauges pt70-1 and pt70-2. in this experiment, we intermittently released the applied pressure for a short time after the measurement at 70 mpa. the calibration value at atmospheric pressure di0 were then calculated as the deviation of the output from the reference value obtained by the reference atmospheric pressure gauge. this procedure was also explained in [13]. figure 5 shows the variations in pressure gauge indications according to time after the pressure release. the horizontal axis shows the elapsed time after the pressure was decreased to atmospheric pressure, and oil overflowed from a release port. figure 5(a) shows the indications of pt70-1 together with the variations in atmospheric pressure. the indication of pt70-1 increased rapidly after the pressure release and approached a certain value, but atmospheric pressure was almost constant during that time. we used the averaged value during the last 3.5 minutes, as shown by a bidirectional arrow in figure 5(a), and then obtained di0. the averaged values obtained during the same time interval were used for all the measurement points in order to avoid the effect of the measurement timing on the long-term drift evaluation. in contrast, the indications of pt70-2 in figure 5(b) are almost constant within the data scattering, showing no time-dependent behaviour. after the measurement of di0, the test gauges were maintained at 70 mpa again. since the time span at atmospheric pressure was short, less than 20 minutes, these intermittent di0 measurements did not have effects on the results of the long-term drift. figure 6 shows the variations in the calibration value with the elapsed time at 70 mpa, di70, and at 0 mpa, di0. for pt70-1 in figure 6(a), the drift trend of di70 is similar to di200 of pt200-1, showing a similarity in the drift behaviour for a similar sensing structure. in addition, the variations in di70 and di0 are almost parallel to each other. although di70 decreased largely and nonlinearly immediately after the pressure application, the difference between di70 and di0 was maintained as constant during the 80-day experiment and only varied within approximately 0.2 kpa, relatively 3 ppm. in a previous study [13], a similar kind of gauge showed a similar result at 100 mpa. the drift of zero-reading is thought to be dominant for the drift under constant pressure application. therefore, if the calibration values di0 at 0 mpa (atmospheric pressure) were obtained during the constant pressure applications, most of the 0 2 4 6 8 10 12 14 16 0.1012 0.1016 0.1020 0.1024 0 2 4 6 8 10 12 14 16 −0.034 −0.032 −0.030 −0.028 elapsed time after pressure release [min.] indication of pt70−1 atmospheric pressure p re ss u re [ k p a ] p re ss u re [ k p a ] indication of pt70−2 (a) pt70−1 (b) pt70−2 elapsed time after pressure release [min.] figure 5. variations in pressure gauge indications according to elapsed time after pressure release: (a) pt70-1 together with atmospheric pressure, (b) pt70-2. figure 4. variation in the calibration value according to elapsed time at 100 mpa for (a) pt200-1, (b) pt200-2, and (c) pt200-3. data during interval b and interval c is shown by closed and open circles, respectively. acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 23 drift could be corrected by tracing di0 without calibration at the measurement pressure. for pt70-2, in figure 6(b), di70 slightly increased with time, which is also similar to the drift of pt200-2. in addition, the variation in (di70 di0) with the elapsed time was maintained as constant, as shown in figure 6(b), although the scattering in di70 and di0 is large. 3.3. discussion we observed three different drift characteristics depending on the kind of pressure gauges (sensing structures). pressure gauges of the same model showed the same drift trend, if no pressure had been preliminarily applied to the gauges. to summarise the drift characteristics under the constant application of pressure, quartz bourdon-type pressure transducers manufactured by paroscientific inc. showed a decreasing drift trend. the calibration value largely decreased at first, and then the drift rate became gradually smaller. in contrast, clamped thin-film gauges manufactured by wika showed an increasing trend. foil strain gauges manufactured by hbm showed an arch-shaped drift trend. the calibration value first increased with time and then turned to decrease. it must be noted that although the drift trend of the gauges is reproducible, the amount of the drift depends on individual gauges, and even then, the drift is not quantitatively predictable only from the experiments for the selected samples. for precise pressure measurements at the actual site, experiments are necessary for respective gauges under similar pressure conditions for a time period similar to the actual measurements, although such experiments take a long time and are also costly. one possible way to reduce and correct the long-term drift under pressurised conditions is preliminary pressure application. we observed significant and rapid variations in the calibration values immediately after the pressure application, but after a while, the drifts became smaller and showed almost constant variations. thus, if the gauges are preliminarily pressurised at the measurement pressure, the first large drift during the actual usage could be suppressed, and the drift could be more easily corrected. this idea was also demonstrated in [13] for similar quartz bourdon-type pressure transducers (paroscientific, inc.) at 100 mpa. from the first experiment in section 3.1, however, the preliminary applied pressure should be similar or very close to the measurement pressure, because the drift behaviour also depends on the preliminary pressure conditions. as demonstrated in figure 4(a), irregular drift behaviour was observed for pt200-1, when the gauge was kept at 100 mpa after being depressurised from 200 mpa. the application of different pressures may cause large hysteretic effects, leading to a more complicated drift. if the drift of zero-reading is dominant for the drift under the constant application of pressure, most of the drift can be corrected by tracing di0, as shown in figure 6, and the pressure can then be measured with a level of uncertainty similar to the normal calibrations. however, if the drift of the span is influential to the overall drift, the effectiveness of this method would be limited. we are continuously evaluating the effectiveness of this method for various gauges and under various conditions. 4. conclusions the long-term drifts of three kinds of pressure gauges under constant pressure applications have been investigated by tracing the calibration values totally for a year. in the first experiment, the drift behaviour of the three kinds of pressure gauges was compared at 200 mpa and 100 mpa. the drift behaviour was completely different across the three gauges, mainly due to the difference in structures in the pressuresensing element. the long-term drift also depends largely on preliminary pressure conditions even under the same pressure application. in the second experiment at 70 mpa, the calibration values at atmospheric pressure intermittently obtained during the pressure application showed a similar trend to those at the measurement pressure. practical methods of evaluating and mitigating the long-term drift under constant pressure were discussed. preliminary pressure application to the pressure gauge can reduce the amount of drift during actual usage. the preliminarily applied pressure should be similar or very close to the measurement pressure in order to avoid complicated hysteretic effects on the drift. in addition, the drift of zero-reading is the main contributory factor of the long-term drift in most cases; therefore, tracing the zero-reading during the pressure application would be highly useful for tracing and mitigating the drift. the findings and ideas will be applied to real problems in scientific and industrial fields, including pressure monitoring at seafloors. 0 20 40 60 80 −6 −4 −2 0 0 20 40 60 80 10.6 10.8 11.0 elapsed time [day] [k p a ] d i 7 0 − d i 0 [k p a ] elapsed time [day] di70 di0 v a ri a ti o n i n 2 ppm 20 ppm (a) pt70−1 c li b ra ti o n v a lu e 0 20 40 60 80 0 10 20 0 20 40 60 80 −30.0 −20.0 −10.0 elapsed time [day] [k p a ] d i 7 0 − d i 0 [k p a ] elapsed time [day] di70 di0 100 ppm 100 ppm (b) pt70−2 v a ri a ti o n i n c a li b ra ti o n v a lu e figure 6. variations in the calibration value according to elapsed time at 70 mpa (di70) and 0 mpa (di0): (a) pt70-1, (b) pt70-2. variation in (di70 di0) according to elapsed time is shown in the lower figures. acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 24 references [1] t. kobata, characterization of quartz bourdon-type highpressure transducers, metrologia 42 (2005) pp. s235-s238. [2] j. singh, a. kumar, n.d. sharma, a.k. bandyopadhyay, reliability and long term stability of a digital pressure gauge (dpg) used as a standard: a case study, mapan 26 (2011) pp. 115-124. [3] i. kocas, m. bergoglio, an investigation of quartz type pressure transducer behavior under continuous pressure conditions and metrological characterization, measurement 45 (2012) pp. 24862489. [4] m. kojima, t. kobata, k. fujii, long-term stability and zero drift of digital barometric pressure gauges, metrologia 52 (2015) pp. 262–266. [5] h. kajikawa, t. kobata, characterization and successive maintenance of transfer standard for apmp inter-laboratory comparison of hydraulic pressures, proc. of the sice annual conference, september 20-23, 2016, tsukuba, japan, pp. 782785. [6] a. p. miiller, g. cignolo, m. p. fitzgerald, m. p. perkin, final report on key comparison ccm.p-k5 in differential pressure from 1 pa to 1000 pa, metrologia 39 (2002) tech. suppl. 07002. [7] t. kobata, a. k. bandyopadhyay, k. moore, a. a. e. eltawil, s. y. woo, t. k. chan, w. jian, j. man, n. n. con, c. s. fatt, final report on key comparison apmp.m.p-k7 in hydraulic gauge pressure from 10 mpa to 100 mpa, metrologia 42 (2005) tech. suppl. 07006. [8] y. ito, r. hino, m. kido, h. fujimoto, y. osada, d. inazu, y. ohta, t. iinuma, m. ohzono, s. miura, m. mishina, k. suzuki, t. tsuji, j. ashi, episodic slow slip events in the japan subduction zone before the 2011 tohoku-oki earthquake, tectonophysics 600 (2013) pp. 14-26. [9] l. m. wallace, s. c. webb, y. ito, k. mochizuki, r. hino, s. henrys, s. y. schwartz, a. f. sheehan, slow slip near the trench at the hikurangi subduction zone, new zealand, science 352 (2016) pp.701-704. [10] e. araki, d. m. saffer, a. j. kopf, l. m. wallace, t. kimura y. machida, s. ide, e. davis, iodp expedition 365 shipboard scientists, recurring and triggered slow-slip events near the trench at the nankai trough subduction megathrust, science 356 (2017) pp. 1157-1160 [11] a. polster, m. fabian, h. villinger, ‘effective resolution and drift of paroscientific pressure sensors derived from long-term seafloor measurements,’ geochem. geophys. geosyst. 10 (2009) q08008. [12] r. b. wearn jr and n. g. larson, measurements of the sensitivities and drift of digiquartz pressure sensors, deep sea research 29 (1982) pp.111-134 [13] h. kajikawa, t. kobata, evaluation and correction for long-term drift of hydraulic pressure gauges monitoring stable and constant pressures, measurement 134 (2019) pp.33-39. microsoft word article 8 62-521-1-galley finale.doc acta imeko december 2013, volume 2, number 2, 41 – 47 www.imeko.org acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 41 a study of savitzky-golay filters for derivatives in primary shock calibration hideaki nozato1, thomas bruns2, henrik volkers2, akihiro oota1 1 national metrology institute of japan, umezono 1-1-1, 3058563 tsukuba ibaraki, japan 2 physikalisch-technische bundesanstalt, bundesallee 100, 38116 braunschweig, germany section: research paper keywords: shock; acceleration; infinite impulse response filter; savitzky-golay filter; derivative citation: hideaki nozato, thomas bruns, henrik volkers, akihiro oota , a study of savitzky-golay filters for derivatives in primary shock calibration, acta imeko, vol. 2, no. 2, article 3, december 2013, identifier: imeko-acta-02 (2013)-02-03 editor: paolo carbone, university of perugia received february 6th, 2013; in final form june 30th, 2013; published december 2013 copyright: © 2013 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: this work was supported by jsps (japan society for the promotion of science) international program for young researcher overseas visits. corresponding author: hideaki nozato, e-mail: hideaki.nozato@aist.go.jp 1. introduction the international standardization document iso 16063-13 [1] describes shock and complex sensitivities of accelerometers determined by shock calibration. for primary shock calibration, it is important to derive an almost undistorted acceleration waveform using laser interferometry and digital signal processing. a setup involving a homodyne laser interferometer requires a digital filtering process associated with a derivative filtering to calculate acceleration from the measured displacement. to obtain a smooth acceleration waveform, iso 16063-13 recommends the use of a 4th-order butterworth lowpass (4th bw) filter followed by a difference method applied twice in sequence. the 4th bw filter works to remove highfrequency noise from the photo detector output signal and may also be used to suppress the resonant frequency of mechanical parts such as the anvil [2]. conversely, a savitzky–golay (s–g) filter [3] can be applied to directly obtain a 2nd derivative with smoothing to produce a less distorted waveform of acceleration than the 4th bw filter. to evaluate their performance, numerical simulations were conducted for a homodyne setup using known analytical excitation functions. this paper reports the characteristics of acceleration measurements using the s–g filter. for the comparison, the validation of acceleration measurements using laser interferometry is also presented. 2. basic information 2.1. low amplitude shock figure 1 illustrates the assumed acceleration waveforms over time and their respective frequency domain spectra for lowshock. the low-shock with a duration of 0.5 ms has acceleration components with several metre per second square between 5 khz and 15 khz. in this case, an appropriate cutoff frequency for the 4th bw filter would be 10 khz. thus, consideration of the spectrum of acceleration is important in shock calibration. abstract this manuscript reports the result of an investigation into two types of digital filters for numerical differentiation of the displacement signal in the case of primary shock calibration of accelerometers. the first is a difference method with a 4th-order butterworth lowpass (4th bw) filter; the other is a savitzky–golay (s–g) filter, which applies a moving polynomial approximation. the computational comparison was applied to low and high amplitude shocks using known excitation functions. each sum of residuals was compared for the optimized conditions of the 4th bw and s–g filters. the results of computer simulation indicated that the s–g filters exhibited better performance for the derivatives than the 4th bw filters. in addition, an analytical comparison using experimental vibration data also indicated that the s–g filters exhibited better derivative characteristics than the 4th bw filters. acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 42 the low-shock is generated by a collision motion between two rigid bodies: a hammer and an anvil through a rubber pad. an accelerometer to be calibrated is attached on an edge surface of the anvil, and a low-shock motion with a shape approximating a half-sine squared function is induced to the accelerometer by the collision. typical peak acceleration ranges from 100 m/s2 to 10000 m/s2 with a duration of several ms [2, 4]. in the computer simulations, equation (1) was used to define the mathematical function for low-shock.          tata low 2sin)( , ( < t < 2) (1) the value of a is the peak shock acceleration. the low-shock waveform is given by the square of the sine function, and the duration is defined as . in the range 0 < t <  and 2 < t < 5, the low-shock waveform vanishes. 2.2. high amplitude shock the high-shock is based on elastic wave propagation inside a long thin bar and is induced to an accelerometer attached on an edge surface of the bar where a reflection of the elastic wave occurs [5]. the shape of the high-shock waveform is normally described by the 1st derivative of the gaussian velocity function [6]. in the computer simulations, the mathematical function for high-shock was defined by equation (2).                       2 2 2 10 2 5.0 10)(      t high e t eata , (0 < t < 20) (2) figure 2 presents the assumed acceleration waveforms over time and their respective frequency domain spectra for highshock. typical peak accelerations range from 10000 m/s2 to 100000 m/s2 with a duration of several dozen microsecond. the shock duration is defined as 4. the calculation range of acceleration is set at values from −10 to 10, such that the initial displacement becomes sufficiently small compared to the half-wavelength of the ne–he laser. 3. numerical simulation 3.1. simulation procedure figure 3 illustrates the basic procedure followed in the computer simulations. quadrature signals with 400 mvp-p, white noise with 10 mvp-p, and 16-bit quantization for ±16 v are typical specifications for the homodyne laser interferometer in nmij [2]. the displacement waveform is calculated by computer simulation from the original acceleration waveform. the typical quadrature signals of the p and s waves are generated in accordance with this displacement waveform [7], including the effects of white noise and 16-bit quantization. figure 4 presents a typical waveform of acceleration, displacement, and interferometric quadrature signals obtained for a low-shock with peak acceleration of 5000 m/s2 and duration of 0.5 ms. the identification stage starts by applying the heydemann correction [8] followed by an arctangent demodulation to obtain the reconstructed displacement waveform. decimation is applied for reduction of data amount. then, the acceleration waveform is reconstructed through the differentiation process using the 4th bw and s–g filters. thus, performance of the differentiation is investigated by computational comparison between the original and reconstructed accelerations. 3.2. 4th order butterworth filter figure 5 presents the calculated differences between the 4th bw (red line) and s–g (blue line) filters in the case of lowshock with a peak acceleration of 5000 m/s2 and a duration of 0.5 ms. the black line represents an assumed acceleration waveform. from this computational result, the assumed figure 1. computer simulations of low-shock waveforms and corresponding frequency spectra. figure 3. procedure of numerical simulation. figure 2. computer simulations of high-shock waveforms and corresponding frequency spectra. acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 43 waveform is generated with a sampling frequency of 50 mhz and the demodulated displacement is decimated to a sampling frequency of 10 mhz. the second and third panels in figure 5 are residuals and their spectrum compared with the assumed waveform, respectively. the cutoff frequency of the 4th bw filter is optimized to obtain the smallest residuals and is 10 khz (see figure 6). each minimal value depends on the duration. shock waveforms have broad frequency bandwidth, as illustrated in figures 1 and 2; the longer the duration, the narrower is the frequency bandwidth covered by the shock waveform. the s–g filter is also optimized with 7 orders and 1400 side points (see figure 9). the second panel in figure 5 implies that the distortion of the s–g filter is smaller than that of the 4th bw filter. in the third panel, the s–g filter exhibits smaller residuals in both frequency ranges (below 3 khz and beyond 10 khz in the spectrum) compared to those of the 4th bw filter. consequently, such a small deviation of waveform would be effective in obtaining more accurate results in terms of shock and complex sensitivities [9, 10]. figure 7 illustrates the dependence of the deviation of the assumed peak acceleration from the computed peak acceleration for the 4th bw filter on the selected cutoff frequency. this result indicates that the peak acceleration that figure 4. example of acceleration, displacement, and interferometric quadrature signals (p and s waves) in computer simulation of low-shock with peak acceleration of 5000 m/s2 and duration of 0.5 ms. figure 5. assumed and two computed acceleration waveforms using the 4th bw and s–g filters for low-shock with duration of 0.5 ms. deviation and spectrum between assumed and two optimized acceleration waveforms. 10-4 10-3 10-2 10-1 0 5000 10000 15000 20000 25000 30000 0.1 ms 0.5 ms 2.0 ms n or m al iz ed s um o f r es id ua ls (a rb .) cut-off frequency (hz) figure 6. dependence of the normalized sum of residuals on cutoff frequency for 3 different durations of low-shock. acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 44 can be measured by a laser interferometer is more accurate for longer durations. for a duration of 0.5 ms, a deviation of less than 0.1% corresponds to a cutoff frequency below 10 khz. according to figures 6 and 7, a cutoff frequency of 10 khz is suitable to obtain both a less distorted waveform and a more accurate peak acceleration, for low-shock durations of 0.5 ms. in order to more accurately measure the acceleration waveform using a homodyne laser interferometer, the authors recommend selecting a duration greater than 0.5 ms. however, it should be noted that the appropriate duration and cutoff frequency depend strongly on the mechanical resonance of each shock calibration machine [2]. figure 8 shows the computed differences between 4th bw (red line) and s–g (blue line) filters in cases of high-shock with peak acceleration of 100 km/s2 and duration of 160 s. additionally, this graph visually implies that a s–g filter with 7 orders and 440 side points can achieve smaller deviations of the acceleration waveform than can be achieved with the optimal 4th bw filter. 3.3. savitzky-golay filter figure 9 illustrates the dependence of the sum of residuals on the order and number of side points of the s–g filter for low-shock, peak acceleration of 5000 m/s2, and duration of 0.5 ms. the sum of residuals using the 4th bw filter is minimized by optimizing the cutoff frequency (10 khz) as shown in figure 6. the sum of residuals indicates that the s–g filter with 7 orders and 1400 side points is optimal under the chosen conditions. table 1 summarizes the optimal value of each cutoff frequency in 4th bw filters and side point in s–g filters. -1.0 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1.0 0 5000 10000 15000 20000 25000 30000 0.1 ms 0.5 ms 2.0 ms pe ak a cc el er at io n de vi at io n (% ) cut-off frequency (hz) figure 7. dependence of peak acceleration deviation on the cutoff frequency for low-shock and 3 different durations. table 1. optimal values of cutoff frequency in 4th bw filter and window width in s–g filter. peak [ms2] duration [ms] decimation [mhz] cut-off [hz] side points window [ms] 5000 0.1 50 21000 na 5000 0.2 50 16000 4500 0.18 5000 0.5 50 10000 na 5000 1.0 50 7000 na 5000 2.0 50 5000 na 5000 0.1 10 23000 600 0.12 5000 0.2 10 16000 900 0.18 5000 0.5 10 10000 1400 0.28 5000 1.0 10 7000 2000 0.40 5000 2.0 10 5000 2800 0.56 5000 0.1 1 22000 60 0.12 5000 0.2 1 17000 80 0.16 5000 0.5 1 10000 140 0.28 5000 1.0 1 6000 220 0.44 5000 2.0 1 4000 320 0.64 5000 0.1 0.5 24000 30 0.12 5000 0.2 0.5 15000 40 0.16 5000 0.5 0.5 9000 70 0.28 5000 1.0 0.5 6000 100 0.40 5000 2.0 0.5 4000 160 0.64 1000 0.1 10 14000 800 0.16 1000 0.2 10 10000 1300 0.26 1000 0.5 10 7000 2100 0.42 1000 1.0 10 5000 3000 0.60 1000 2.0 10 3000 4300 0.86 figure 8. assumed and two computed acceleration waveforms using the 4th bw and s–g filters for high-shock with duration of 160 s. deviation and spectrum between assumed and two optimized acceleration waveforms. acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 45 “window”— the numerical value given by equation (3) —refers to window widths for polynomial approximation of a s–g filter. accordingly, peak accelerations of 1000 m/s2 require lower optimal cutoff frequencies compared to those for 5000 m/s2. therefore, the effect of high-frequency noise becomes more pronounced in cases of low peak acceleration.   1000/)1points2(  decimationsidewindow (3) the relationship between “cut-off” and “window” for lowshock, on the basis of table 1, is plotted in figure 10. the closed circles represent peak acceleration of 5000 m/s2 for 4 different decimations from 0.5 mhz to 50 mhz. the open circles represent peak acceleration of 1000 m/s2. these results indicate that the relationship between optimal cutoff frequency and window width is independent of three types of variables: peak acceleration, duration, and decimation. correspondingly, figure 11 illustrates this relationship for low and high shocks. the open circles represent high-shock, which exhibits different waveforms from low-shock. the peak accelerations are 20 km/s2 and 100 km/s2, and the duration ranges from 60 s to 300 s. thus, the relationship indicates that a s–g filter works on lowand high-shocks purely as a low-pass filter (with differentiation). 3.4. calculation time the s–g filter is a kind of finite impulse response (fir) filter in which the calculation time depends on the side points. on the other hand, since the 4th bw filter is an infinite impulse response (iir) filter with less filter coefficients, its calculation time is generally shorter than that of the s–g filter. so, two conditions in table 2 were selected to evaluate the calculation time between the difference method with the 4th bw filter and the s–g filter. here, the software and operating system were labview 2009 and windows 7 with the central processing unit of an intel core i7-2620m at 2.7 ghz. then, the calculation time using the difference method with the 4th bw filter and s–g filter was 1.029 ms and 123.087 ms in the decimation of 10 mhz. also, that was 0.171 ms and 3.429 ms in the decimation of 0.5 mhz. information of the calculation time was obtained using the profile performance in tools. from the results, the s–g filter took longer time than the 4th bw filter, and each calculation time drastically decreased by the data reduction due to the decimation. 4. investigation using experimental data experimental data in shock and vibration calibrations were examined to indicate the applicability of s–g filters, even when the original waveform is unclear. fundamentally, it is difficult to discern the correct amplitude and frequency from the shock 10-4 10-3 10-2 10-1 0 1000 2000 3000 4000 5000 5th order 7th order 9th order 4th bw n or m al iz ed s um o f r es id ua ls (a rb .) side points figure 9. sum of residuals between optimized 4th bw filter and s–g filters of 3 different orders for low-shock and duration of 0.5 ms. table 2. conditions for comparison of calculation time between difference with 4th bw filter and s–g filter. peak [ms2] duration [ms] decimation [mhz] cut-off [hz] side points window [ms] 5000 0.5 10 10000 1400 0.28 5000 0.5 0.5 9000 70 0.28 10-2 10-1 100 103 104 105 5000 m/s2 1000 m/s2 w in do w w id th ( m s) cut-off frequency (hz) figure 10. relationship between optimal cutoff frequencies and window widths in low-shock. 10-2 10-1 100 103 104 105 low shock high shockw in do w w id th (m s) cut-off frequency (hz) figure 11. relationship between optimal cutoff frequencies and window widths in lowand high-shocks. figure 12. typical experimental waveforms for shock and vibration calibrations. acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 46 waveforms, as shown in figure 12. conversely, a fundamental frequency is known in case of vibration waveforms. therefore, in order to obtain a reference to the fundamental frequency from the vibration waveforms, a fast fourier transform was applied to the experimental data of the nmij high-frequency calibration system [11]. figure 13(a) presents an example of the displacement (black line) and its acceleration waveforms that are derived experimentally using the difference method with 4th bw (red line) and s–g (blue line) filters at the fundamental frequency of 5 khz. here, the two acceleration waveforms were derived from the displacement waveform. the cutoff frequency of the 4th bw filter was 10 khz and the side point of the s–g filter with a 7th-order polynomial was given according to the relationship illustrated in figure 11. thus, the displacement waveform becomes the standard against which to evaluate whether a s–g filter can differentiate more accurately than a 4th bw filter. figure 13(b) shows a deviation between nominal and differentiated acceleration waveforms using 4th bw and s–g filters. firstly, the nominal acceleration waveform )(td is derived by applying equation (4) to the displacement waveform. ftbftbbtd  2cos2sin)( 210  (4) where 0b , 1b and 2b are parameters for sine-approximation, and f is the fundamental frequency. next, the nominal acceleration waveform is calculated by the second derivative of equation (4). as figure 13(b) shows, the s–g filter seems to have smaller deviation for the derivative than the 4th bw filter. figure 14 represents the spectrum of the displacement and accelerations calculated on the basis of each waveform in figure 13(a). figure 15 illustrates the results of comparison between the 2nd derivative of the 4th bw filter and that of the s–g filter on the basis of this spectrum. the reference value dis (black circle) of the acceleration amplitude was calculated from the displacement waveform according to the following equation:   dfdis 22  (5) the parameter d is the displacement amplitude. the s–g filter is better than the 4th bw filter in two respects: the first is that the acceleration amplitude due to the s–g filter is closer to the reference value than that of the 4th bw filter over the entire range of cutoff frequencies (i.e., up to 50 khz); the other is that the total harmonic distortion of the s–g filter is smaller than that of the 4th bw filter. on this basis, it can be concluded that the 2nd derivative of the s–g filter performed better than that of the 4th bw filter in obtaining the acceleration waveform for shock calibration. figure 15. comparison of acceleration amplitude and total harmonic distortion between the 4th bw and s–g filters. figure 13. (a) displacement and acceleration waveforms using 4th bw and s–g filters. (b) deviation between nominal and differentiated acceleration waveforms using the 4th bw and s-g filters. 10-14 10-13 10-12 10-11 10-10 10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 100 101 102 103 103 104 105 displacement 4th bw s-g d is pl ac em en t ( m ) a cc el er at io n (m /s 2 ) frequency (hz) 5 khz figure 14. spectrum of displacement and accelerations calculated using the 4th bw and s–g filters. acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 47 5. summary an approach for the calculation of derivatives using a s–g filter to obtain appropriate waveforms of acceleration in low and high shocks was proposed and discussed on the basis of computer simulations. the obtained waveform was compared with waveforms that were differentiated and smoothed twice using a 4th bw filter according to the procedure proposed in [1]. a relationship was derived from each optimal residual summation between 4th bw and s–g filters with 7th orders; this relationship did not depend on the decimation, peak acceleration, duration, or waveform in cases of low or high shocks. the results imply that the s–g filter produces better derivative and low-pass characteristics than the 4th bw filter for both low and high shocks. an analytical comparison between the derivatives of the 4th bw and s–g filters was investigated using experimental vibration data; fast fourier transform was applied to the displacement and two acceleration waveforms of the 4th bw and s–g filters. this analytical comparison also indicated that the acceleration amplitudes of the s–g filter were closer to the reference values derived from the displacement than those of the 4th bw filter. moreover, the s–g filter was considered superior in terms of total harmonic distortion. these results could lead to a more accurate measurement of shock and complex sensitivity for primary calibration of accelerometers implemented according to [1]. acknowledgements the authors would like to thank the staff of the working groups “realization of acceleration” and “impact dynamics” of ptb for their helpful partnerships and constructive suggestions. this paper was drawn up on the basis of an international collaboration between ptb and nmij. this work was supported by the jsps (japan society for the promotion of science) international program for young researcher overseas visits. references [1] iso 16063-13:2001 methods for the calibration of vibration and shock transducers – part 13: primary shock calibration using laser interferometry. [2] h. nozato, t. usuda, a. oota, t. ishigami, calibration of vibration pick-ups with laser interferometry: part iv. development of a shock acceleration exciter and calibration system, measurement science and technology, vol. 21, no. 6 (2010), 065107. [3] a. savitzky, m.j.e. golay, smoothing and differentiation of data by simplified least squares procedures, analytical chemistry, vol. 36, no. 8 (1964) pp. 1627-1639. [4] y. huang, j. chen, h. ho, c. tu, c. chen, the set up of primary calibration system for shock acceleration in nml, measurement, vol. 45, no. 10 (2012) pp. 2383-2387. [5] r.m. devies, a critical study of the hopkinson pressure bar, philosophical transactions of the royal society a, vol. 240, no. 821 (1948) pp. 375-457. [6] k. ueda, a. umeda, h. imai, uncertainty evaluation of a primary shock calibration method for accelerometers, metrologia, vol. 37 (2000) pp. 187-197. [7] h. nozato, t. usuda, a. oota, t. ishigami, the methods for the calibration of vibration pick-ups by laser interferometry: part v. uncertainty evaluation on the ratio of transducer’s peak output value to peak input acceleration in shock calibration, measurement science and technology, vol. 22, no. 12 (2011), 125109. [8] p.l.m. heydemann, determination and correction of quadrature fringe measurement errors in interferometers, applied optics, vol. 20, no. 19 (1981), pp.3382-3384. [9] t. bruns, a. link, f. schmähling, h. nicklich, c. elster, calibration of acceleration using parameter identification – targeting a versatile new standard, proc. of xix imeko world congress, lisbon, portugal (2009), pp. 2485-2489. [10] a. link, a. täubner, w. wabinski, t. bruns, c. elster, calibration of accelerometers: determination of amplitude and phase response upon shock excitation, measurement science and technology, vol. 17, no. 7 (2006), pp. 1888-1894. [11] a. oota, t. usuda, h. nozato t. ishigami, development of primary calibration system for high frequency range up to 10 khz, proc. of imeko 20th tc3, 3rd tc16 and 1st tc22 international conference, merida, mexico (2007), id-103. planck-balance 1 (pb1) – a table-top kibble balance for masses from 1 mg to 1 kg – current status acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 47 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 47 52 planck-balance 1 (pb1) – a table-top kibble balance for masses from 1 mg to 1 kg – current status c. rothleitner1, n. rogge2, s. lin1, s. vasilyan2, d. knopf1, f. härtig1, t. fröhlich2 1 physikalisch-technische bundesanstalt (ptb), 38116 braunschweig, germany, christian.rothleitner@ptb.de 2 technische universität ilmenau, 98693 ilmenau, germany, norbert.rogge@tu-ilmenau.de abstract: the planck-balance 1 (pb1) has been developed in a collaboration between the physikalischtechnische bundesanstalt and the technische universität ilmenau. it is a small-size kibble balance aimed to calibrate e1 mass standards for a mass range from 1 mg to 1 kg. this paper presents its principle and some preliminary investigations. keywords: kibble balance; mass metrology; planck-balance; kilogram realisation; kilogram dissemination 1. introduction in mass metrology mass standards had been traced to the international prototype kilogram (ipk) for more than 130 years. this was done by comparing masses and building up the mass scale. after the re-definition of the si unit kilogram this way of calibration can be continued. what has changed is that the ipk has been replaced by a definition based on the planck constant and a corresponding realisation of the kilogram, e.g., a silicon-28-sphere [1] or a kibble balance [2], up to now also at a nominal value of 1 kg. the redefinition, however, gives more freedom in mass metrology. since the planck constant is valid for any mass value the kibble balance technology can be used to calibrate mass standards that differ from 1 kg too. several national metrology institutes are working on such a balance fehler! verweisquelle konnte nicht gefunden werden. [4], which is sometimes called “table-top kibble balance” or “micro kibble balance (µkb)”. the physikalisch-technische bundesanstalt (ptb) is developing, in collaboration with the technische universität ilmenau (tuil), two tabletop sized kibble balances, called planck-balance 1 (pb1) and planck-balance 2 (pb2), for industrial applications. details on the pb2 have already been published elsewhere [5]. in this paper the concept 1 mpe (maximum permissible error). the required measurement uncertainty is calculated as 1/6 of the mpe along with some technical details of pb1 will be described. 2. concept of planck-balance 1 pb1 is a kibble balance that works in two modes, as originally proposed by bryan kibble (see [2]). one mode, called force mode, where the weight of a mass under test is compared to an electromagnetic force, produced by a voice coil system. and a second mode, called velocity mode, where the force factor (see below) is determined. the difference of the planck-balance, however, with respect to its high-precision counterparts, lies in the field of application. the aimed use of pb is by industry, calibration laboratories, but also national metrology institutes that are interested in a system for the primary realisation of the new si kilogram but cannot afford or maintain a highly sophisticated self-developed kibble balance. therefore, the pb1 is aimed to meet the following specifications: use for mass calibrations over a mass range from 1 mg to 1 kg. relative measurement uncertainties according to e1 mpe 1 specified in oiml r 111-1 (→ 8.4 × 10-8 (k = 1) @ 1 kg). compact and easy to use, i.e. the mechanical part should be of comparable size to a conventional analytical mass balance. the use of the balance should be possible by staff members that do not necessarily have profound knowledge in electrical metrology. modular design: parts should be easily exchangeable or adaptable in order to reach the required specifications. as an example: the load cell should be exchangeable if another nominal mass value is desired. this allows for using the, basically, same setup for the whole mass range. as a basis a commercial load cell (sartorius secura 1103-1x, see figure 1) is used, which can be found in high-precision analytical mass balances. of the mass value under test. for 1 kg the mpe (e1) is 0.5 mg. http://www.imeko.org/ mailto:christian.rothleitner@ptb.de mailto:norbert.rogge@tu-ilmenau.de acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 48 the specified repeatability (standard deviation) of this load cell is 1 mg. nevertheless, the load cell can be replaced by another one, with improved repeatability or a more adequate one for another mass range. figure 1: the setup of pb1 consists of a commercial load cell and an additional external voice coil attached to it. the motion in the velocity mode is measured with a laser interferometer. for automated weighing a mass lifter is used. in contrast to pb2, where the load cell is used as a mechanical guiding system only, in pb1 the lever ratio is exploited additionally. as shown in the schematic of figure 2 the internal voice coil (3), which is fixed to the longer lever arm, is used to compensate the weight (5) sitting on the shorter lever arm. the ratio of the levers is about 1:30. the velocity, however, is measured on the (short) lever arm. the external voice coil (2) is used to oscillate the lever arm, which finally induces a voltage in the internal voice coil (3). figure 2: schematic of pb1: 1-load carrier, 2-external voice coil, 3-internal voice coil, 4-laser interferometer, 5-weight, 6-position sensitive device, 7-mirror. this allows using a smaller voice-coil system with a force factor 𝐵𝑙 of about 27 t·m (𝐵 denotes the magnetic flux density, 𝑙 the length of the coilwire in the 𝐵 -field). this lever ratio is approximately 𝐵𝑙: 𝑢ind 𝑣 = 1: 30 (1) where 𝑢ind is the induced voltage amplitude and 𝑣 the velocity amplitude with which the coil is moved in the magnetic field. this is to be understood that the velocity is measured on the lever arm where the weight is placed, while the voice coil is connected to the other (the longer) lever arm. in the force mode the electrical current is measured from the voltage drop at a precision resistor. when putting a mass of a factor 10 lower then also the voltage drop will be by a factor of 10 lower. this would lead to reduced accuracy in the voltage measurement. in order to circumvent this issue, the pb1 employs resistors of values 100 , 1 k, 10 k corresponding to the load ranges of 1 kg to 100 g, 100 g to 10 g, and 10 g to 1 mg, respectively. a custom made “switch box” has been developed in order to switch between the force and velocity mode, as well as between different resistor values. current measurements with pb1 are performed in air but can also be done in a high-vacuum environment as shown in figure 3. for the sake of size scaling, on top of pb1 a weight of 1 kg is visible in figure 3. figure 3: pb1 in a vacuum chamber. visible on top of pb1 is a 1 kg mass standard. 3. current investigations of planck-balance 1 a series of measurements were performed with pb1 to investigate performance and possible systematic errors in the force and velocity modes. the following gives some examples. 3.1. force mode in the force mode the repeatability has been investigated by doing automated repeated measurements over several hours. every single measurement consists of an abba-cycle (a-with weight; b-without weight). figure 4 shows a series of measurements conducted with a load of 1 kg. the 𝐵𝑙 was calculated with corrections for changes of the environmental conditions, such as temperature, pressure and humidity. a temperature correction was done to a nominal value of 23 °c, where the temperature coefficient was determined from a long-term measurement. the mass lifting has been automized by means of the above-mentioned homemade mass lifter. the vertical translation stage is http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 49 high-vacuum compatible, has a resolution of 4 nm, and a force range of up to 20 n. it is based on a piezo actuator to avoid magnetic bias, and has a travel range of 10 mm. the obtained relative repeatability (standard deviation) for a 1 kg mass standard lies in the order of 0.6 ppm, which is better than the stated specifications of the load cell of 1 mg. for curiosity the mass change was also performed by hand. in the case the repeatability was around 1 ppm, which is still in accordance with the specifications. for lower mass values the relative standard deviation decreases. so, for example, for a 10 g mass piece the standard deviation was in the order of 2 × 10-7 kg. a complete abba-cycle currently takes about 80 s but can be improved further. figure 4: repeated force mode measurement of a 1 kg mass standard. each circle indicates a single abba cycle. the relative standard deviation for this measurement is 0.6 × 10-6. the balance allows, in principle, a measurement of masses over a range from 1 mg to 1 kg with the same load cell. because of the limiting standard deviation, however, an accurate measurement of small masses is not expected in an appropriate time period. this is why the balance is designed in a modular way. if the focus lies on smaller masses, the load cell can be simply exchanged in order to meet the specifications. nevertheless, measurements with calibrated e1 mass standards were done with the same load cell (no other load cell has been used to date in this setup), which showed an increasing deviation from their calibrated value with decreasing mass value, but within the specified linearity of the balance of 2 mg. in these measurements the 𝐵𝑙 , obtained for each nominal mass was compared to the 𝐵𝑙 as obtained with a 1 kg mass standard. corrections were made only for temperature (all corrected to 23 °c using the same temperature coefficient) and coil current effect [2]. the coil current effect is compensated by adjusting the tare weight for each mass value in a way that the absolute value of the required current is equal for a and b, but different in sign. this, currently, is done by putting additional physical weights on the load carrier. for masses below 1 g this turns out to be difficult. therefore, in the future an ‘electromagnetic’ tare will be implemented. this will be done by using the external voice-coil that is used in the velocity mode, as is already implemented in pb2. a constant current flowing through the coil will generate a constant force, which, in turn, acts like an additional weight. this second voice coil is the one that is usually used in the velocity mode to move the balance arm. a) b) figure 5: measurement campaign of a 1 kg mass standard with pb1. the magnet shows a hysteresis. depending whether the temperature is rising or falling with time, the temperature coefficient will be different by several per cent. a) temperature measured in the magnet. b) the force factor bl, measured during the campaign. further investigations were done on the stability of the magnet’s temperature coefficient. this is important since the laboratory, where the pb1 is installed, is currently not air-conditioned. the absolute temperature can vary significantly with respect to the temperature coefficient. from long term (several hours) measurements it was noticed that the temperature coefficient changed whether the temperature is increasing or decreasing, as can be seen in figure 5 a) and b). thus, a hysteresis-like behaviour has been observed. the temperature coefficient can vary by several per cent, while its relative value has been determined to be about -3.3 × 10-4 k-1, which is a typical value for a smco magnet. this temperature change was not http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 50 forced by heating or cooling the room but was a mere coincidence that such a linear rising and falling of temperature happened during the measurement. it would be interesting to investigate this behaviour further, e.g. by placing the setup into a climate chamber and forcing temperature changes with different slopes. the equilibrium position is monitored by means of a dual photo diode, a slit and an led. the voltage output of the diode is measured with a 7.5 digits digital multimeter, which, after a calibration of the sensitivity curve, allows to resolve the position to several nanometres (the sensitivity factor is about 2.9 × 10-3 v·µm-1). in order to estimate the uncertainty to repositioning, the 𝐵𝑙 as a function of position has been measured (see figure 6). to this end the pid control was set to maintain the equilibrium at several nominal positions, and at each position a weighing of one kilogram was performed. due to the restoring force of the flexure hinges of the load cell, the electrical current values for mass on and mass off were not equal in their absolute value any longer. as this would give a bias in the result, for each position an adjustment of the tare weight was done. figure 6: measurement of the force factor 𝐵𝑙 as a function of the coil position. the relative gradient of 𝐵𝑙 at the position zero was determined to be of 1.9 × 10-3 v-1 (voltage output of the position sensor). with a positioning uncertainty of, say, 150 µv (currently our pid threshold level – but can be further reduced; this voltage corresponds to about 50 nm) the possible relative error becomes in the order of about 3 × 10-7. 3.2. velocity mode as practicing in pb2 also in pb1 the voice coil is driven sinusoidally, giving a relative position over time according to 𝑠(𝑡) = 𝑆0 + 𝑆 sin(ω𝑡 + φs) (2) where 𝑆0 denotes a constant offset and ω the angular frequency, ω = 2π𝑓sig, with the oscillation frequency 𝑓sig, and φs is the initial phase. the coil velocity is obtained as the derivative of equation (2) with respect to the time 𝑡 as 𝑣(𝑡) = ω𝑆 cos(ω𝑡 + φ𝑠 ) (3) this motion induces an ac voltage of 𝑢(𝑡) = 𝑈 cos(ω𝑡 + φu) (4) across the coil ends. φu is again an arbitrary initial phase. due to non-linearities, e.g. in the magnetic field and the mechanical system, higher harmonics will be excited along with the excitation frequency. by determining the amplitude of the first harmonic (fundamental note) of the induced voltage, as well as of the position signal, the force factor, 𝐵𝑙, can be calculated. thus, for the sake of simplicity, assuming the force factor 𝐵𝑙 to be a constant during the whole range of coil movement, 𝐵𝑙 can be calculated as 𝐵𝑙 = 𝑈 ω𝑆 = 𝑈 2π𝑓sig𝑆 = 𝑈 𝑣 (5) this equation, however, is still lacking the lever ratio. if this ratio is taken into account, we obtain equation (1). the signal frequency is usually around 4 hz, which is close to the eigenfrequency of the load cell, and the amplitude is of about 4.5 µm. the error in the amplitude estimation arising from higher harmonics can be compensated by either including the higher harmonics into the regression analysis, or by adjusting the data length to an integer number of sine wave cycles (coherent sampling) [6]. as the coil oscillates over a range with varying 𝐵𝑙 (see figure 6) the measured 𝐵𝑙 is a weighted mean of all 𝐵𝑙 s over this range. for an exactly linear 𝐵𝑙 over the oscillation range, the measured 𝐵𝑙 value corresponds to the point of zero oscillation, which is the same point to which the pid control directs (i.e. the set point) the coil during the force mode. the more non-linear the 𝐵𝑙 becomes, the more the calculated point deviates from the set point in the force mode. this deviation, however, can be estimated by means of the shape of the 𝐵𝑙 (see figure 6). as the induction happens in the voice coil attached to the longer lever arm, and the lever ratio is approximately 1:30, the range of motion is about 270 µm and this gives a non-negligible bias. in our case the relative offset is in the order of -2.2 × 10-5 but can be corrected with a remaining relative uncertainty of 15 % of the correction. figure 7 shows the results of 19 consecutive 𝐵𝑙 determinations in the velocity mode. the interferometer output and the digital multimeter (keysight 3458a) were sampled with 10 khz. each measurement took 10 s. no temperature corrections have been applied in this case, as the temperature deviation was negligible. http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 51 figure 7: repeated measurement of 𝐵𝑙 in the velocity mode. the relative standard deviation is 2.4 × 10-6. each dot is the result of a 10 s measurement with a sampling rate of 10 khz. with pb2 it was noticed that the measured 𝐵𝑙 was a function of the excitation frequency [7]. based on this experience for pb1 a possible frequency dependence has been investigated. thus, 20 different excitation frequencies were selected ranging from 1 hz to 10.5 hz with an increment of 0.5 hz. for each selected excitation frequency, the induced voltage and the displacement were measured in air with a sampling period of 30 s and a sampling frequency of 10 khz. after corrections of the air refractive index, aperture time of the 3458a, and temperature, the 𝐵𝑙 was determined from the amplitudes of the induced voltage and the displacement as given in equation (1). figure 8: a measurement of the force factor 𝐵𝑙 for different excitation frequencies. the results shown in figure 8 suggest a strong dependence of 𝐵𝑙 on the excitation frequency in the order of d𝐵𝑙 d𝑓⁄ = 0.28 t·m·hz-1 at 4 hz. this effect is assumed to originate from an abbe error, which is produced by a small tilting of the load carrier during up and down motion. as this slope is significantly higher than the one observed with pb2, the excitation coil and measuring coil of the pb1 set-up were exchanged (not physically, but the wiring, so that the internal voice coil now functioned as the actor and the external voice coil induced the voltage), and the above measurement was performed again. both voice coils have approximately the same parameters, hence the results allow a comparison. compared to the former results, the variation of 𝐵𝑙 over the excitation frequency is in the order of d𝐵𝑙 d𝑓⁄ = 3.3 × 10-2 t·m·hz-1 at 4 hz in this case. this supports the conjecture that the frequency dependence comes from an abbe error, since in the second measurement the lever ratio was not exploited. exploiting the lever ratio thus makes the setup more sensitive to frequency variations. an explanation for this difference is given in figure 9. the external voice coil (2) is closer situated to the mirror than the internal voice coil (3). thus, the abbe offset is bigger when the voltage is induced in the internal coil. the frequency dependence of 𝐵𝑙 can be explained as follows. due to the inertia of the lever the tilt angle of the mirror changes with changing frequency, as the lever bends different due to the inertial mass. since the mirror axis does not overlap with the axis of the coil, both parameters, the mirror motion and the induced voltage, will be subject to different amplitudes for different frequencies. for a correction of this bias the mechanical tilt angle must be determined, as well as the abbe offset, i.e. the distance of the measurement point of the laser interferometer to the axis of motion of the induction coil. this work is currently under investigation. figure 9: explanation of the abbe error in pb1: the external voice coil (2) has a smaller abbe offset (ao2) than the internal voice coil (3), which is (ao3). the arrow at the mirror indicates that the mirror can tilt during up and down motion. 4. summary the concept and the current focus of investigations of the pb1 are presented in this paper. these investigations include performance tests of the setup in the force and velocity mode. the relative standard deviations for the force and velocity mode were 0.6 ppm and 2.4 ppm, respectively. the temperature coefficient has been determined and a hysteresis was noticed that leads http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 52 to different values in the coefficient whether the temperature is rising or falling. the difference is in the order of several per cent in the investigated case. a frequency dependence in the velocity mode has been detected, which is orders bigger if the lever ratio is exploited. it is assumed that the reason for this effect is an abbe error due to a tilting of the load carrier. this problem requires further investigations. in principle, measurements can be performed over the whole range from 1 mg to 1 kg with the same load cell. the standard deviation, however, is a limiting factor. to circumvent this problem, the load cell can be exchanged in order to meet the specifications for the desired load range. this will be done in future investigations. acknowledgement the research of this project is funded via the programme “validierung des technologischen und gesellschaftlichen innovationspotenzials – vip+”, a programme of the german federal ministry of education and research (bmbf), and is managed by vdi/vde innovation + technik gmbh. the authors thank also j. kloß, l. günther and m. spoors for help in the measurements and j. schleichert for fruitful discussions and technical assistance. 5. references [1] d. knopf, t. wiedenhöfer, k. lehrmann, f. härtig, “a quantum of action on a scale? dissemination of the quantum based kilogram”, metrologia, vol. 56, no. 2, 024003, 2019. [2] i. a. robinson, s. schlamminger, “the watt or kibble balance: a technique for implementing the new si definition of the unit of mass”, metrologia, vol. 53, no. 5, a46, 2016. [3] l. chao, f. seifert, d. haddad, j. pratt, d. newell, s. schlamminger, “the performance of the kibbg1 tabletop kibble balance at nist” metrologia, vol. 57, no. 3, 035014, 2020. [4] i. a. robinson, j. berry, s. davidson, c. jarvis, “towards a simplified kibble balance to realise mass in the new si”, euspen’s 17th international conference & exhibition, hannover, may 2017. [5] c. rothleitner et al, “the planck-balance—using a fixed value of the planck constant to calibrate e1/e2-weights”, measurement science and technology vol. 29, no. 7, 2018. [6] s. lin, c. rothleitner, n. rogge, t. fröhlich, “influences on amplitude estimation using threeparameter sine fitting algorithm in the velocity mode of the planck-balance” acta imeko, issn: 2221-870x, accepted for publication, 2020. [7] n. rogge, c. rothleitner, s. lin, s. vasilyan, t. fröhlich, f. härtig, d. knopf, “error sources in the force mode of the “pb2” planck-balance”, in proc. of 24th imeko tc3 int. conf., cavtat-dubrovnik, croatia, 16-18 november 2020. http://www.imeko.org/ a new approach for an anthropocentric design of human-robot collaborative environment acta imeko issn: 2221-870x december 2020, volume 9, number 4, 80 87 acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 80 a new approach to the anthropocentric design of human– robot collaborative environments castrese di marino1, andrea rega2, ferdinando vitolo3, stanislao patalano3, antonio lanzotti3 1 department of management, information and production engineering, university of bergamo, bergamo, italy 2 department of neurosciences, reproductive and odontostomatological sciences, university of naples ‘federico ii’, naples, italy 3 fraunhofer j-lab ideas, deptartment of industrial engineering, university of naples ‘federico ii’, naples, italy section: research paper keywords: anthropocentric approach; human–robot collaboration; collaborative environment; graph theory; digraph citation: castrese di marino, andrea rega, ferdinando vitolo, stanislao patalano, antonio lanzotti, a new approach for an anthropocentric design of human-robot collaborative environments, acta imeko, vol. 9, no. 4, article 11, december 2020, identifier: imeko-acta-09 (2020)-04-11 section editor: leopoldo angrisani, university of naples federico ii, italy received october 31, 2019; in final form november 19, 2020; published december 2020 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: castrese di marino, e-mail: castrese.dimarino@unibg.it 1. introduction industry 4.0 pillars and technological improvements are pushing manufacturing towards hyper-automated production lines [1]-[3]. however, many production applications cannot be implemented without a human presence; many assembly processes need to be performed with human involvement [4], [5]. these applications, usually based on multi-stage processes [6], are often characterised by a sequence of humanand robot-led stages. indeed, many researchers assume humans to be the central actors in manufacturing plants [6]-[8] because they cannot be easily replaced by advanced technologies [6]. according to several researchers [9], [10], novel workplaces can achieve higher productive levels by combining (i) human capabilities (intelligence, flexibility and adaptability) and (ii) robot characteristics (strength, endurance and accuracy). moreover, the synergic collaboration of humans and robots leads to new opportunities in ergonomics, such as a reduction in bad postures [11] and an increasing perception of well-being during work activities [12]. therefore, a synergic combination of human and robotic skills should be considered as the key approach to improving manufacturing plants. currently, three kinds of workplace are used in manufacturing plants: (a) manual workplaces, where an operator performs all the tasks alone; (b) automatic workplaces, where robots autonomously perform all the tasks; and (c) collaborative workplaces, i.e. hybrid workplaces where humans and robots work together by performing common tasks, thus creating a human–robot collaboration (hrc). although workplaces (a) and (b) are common in manufacturing, workplace (c) is still not being implemented for a number of reasons, including safety, the difficulty of installation, suitability and reliability. industry still lacks confidence in this kind of workplace. however, hybrid workplace installations could have a large impact on (i) processes characterised by small-batch production abstract this paper deals with collaborative robotics by highlighting the main issues linked to the interaction between humans and robots. a critical study of the standards in force on human–robot interaction and the current principles on workplace design for human–robot collaboration (hrc) are presented. the paper focuses on an anthropocentric paradigm in which the human becomes the core of the workplace in combination with the robot, and it presents a basis for designing workplaces through two key concepts: (i) the introduction of human and robot spaces as elementary spaces and (ii) the dynamic variations of the elementary spaces in shape, size and position. according to this paradigm, the limitations of a safety-based approach, introduced by the standards, are overcome by positioning the human and the robot inside the workplace and managing their interaction through the elementary spaces. the introduced concepts, in combination with the safety prescriptions, have been organised by means of a multi-level graph for driving the hrc design phase. the collaborative workplace is separated into sublevels. the main elements of a collaborative workplace are identified and their relationships presented by means of digraphs. mailto:castrese.dimarino@unibg.it acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 81 (repetitive tasks, interaction with fixed objects), (ii) processes that are hard to automate (complex tasks, randomly positioned pieces, skill-based processes) and (iii) processes designed to relieve human effort (non-ergonomic and repetitive tasks, assembly support). of course, safety is the main issue that needs to be addressed. within the current standards, it is clear how difficult it might be to assure safety in hybrid workplaces; to address all the standards in this context would be challenging and, sometimes, only a few indicators are provided [13]-[15]. a suggestion to overcome this difficulty is the adoption of engineering design methods that identify applications and requirements linked to the contents of the standards. one such method is to use axiomatic design [16] or graph theory, which will be presented in this paper, to find the functional requirements and the design parameters or the connections between the different elements of the workplace. moreover, according to the standards, a human presence is usually recognised as an intrusion (see safety-based monitored stops [13]) to be avoided whenever possible, and the designing of a workplace is safety-based and techno-centric, with a preference for static workspaces. all the effort goes into limiting the human presence in the workplace. potential harm to humans is limited through hazard controls by means of protective stops, speed reduction and control system performance [14], [15]. all these precautions are mandatory requirements. in hybrid workplaces, the usual approach consists of designing the workplace starting with the robot. the human– robot interaction (collaborative operations) is then selected depending on the tasks, and the workplace spaces are defined (operative and collaborative space). finally, the operator is added to help the robot perform all the tasks that it cannot carry out alone because of the lack of technology. this is the so-called ‘techno-centric’ design, i.e. all aspects are focused on the robot. moreover, in the standards, there is no mention of a ‘human workspace’, a dedicated space where only the human presence is allowed. cognitive stress (mainly related to tracking the robot’s movements) is another important topic in this field, and little research has been done in this direction [17]. whenever a human is involved in these workplaces, ergonomic, cognitive and safety issues are the main areas of focus for the ‘human-centred design’. the evaluation of the operator’s stress levels [18] is a topic that needs to be studied to improve the quality of the work. indeed, hrc represents an opportunity to improve not only the production but even the working conditions of the operators, who are liable to resign from heavy and repetitive jobs. physical improvements need to be accompanied by mental improvements so that the operators can feel comfortable working with a robot. there is no trust or confidence in something they do not understand. in the literature, many researchers have studied different approaches [19], [20] for designing human–robot workplaces based on (i) the different modalities of interaction and (ii) workspaces characterised by access properties. in [18] and [19], the authors define three dynamic areas with selective access (safe, warning and unsafe areas) and the way the robot system should react to the human presence. in [20], more details about levels of interaction are provided, and the static and dynamic space concepts are introduced. they move from a safety-based approach to one based on interaction assessment. a ‘humancentred design’ [21] could be adopted to overcome the current framework. hence, a new prospective design based on an ‘anthropocentric design’ approach is proposed: humans and their interaction with the robot should be at the core of the workplace and design process. furthermore, with the large use of automated guided vehicles (agv) and intelligent guided vehicles, standards for collaborative workplaces must not only address static robots. the use of agvs is increasing. generally, they are used for the internal and external transport of material, but they could also be employed to automate processes, such as in the reverse engineering of large objects [22]. mobile robots (on rails or carried by autonomous systems) continuously move the reference frame and, therefore, workspaces are constantly changing. the definition of dynamic space is a very challenging topic [19], [23]. in this paper, the basis for an anthropocentric design is highlighted, focusing on two main topics: (i) the definition of elementary spaces for collaborative workplaces and (ii) the dynamic variations of elementary spaces in order to lead the designer towards the creation of a collaborative workplace and layout definition. moreover, a multi-level graph-based approach for the design of an hrc workplace is presented for driving the hrc design phase. in the rest of the paper, section 2 introduces the standards and literature definitions, section 3 presents the proposed spaces in hrc and section 4 describes how the proposed approach influences collaborative operations. in section 5, an approach using graph theory is presented, and, finally, the conclusions are set out in section 6. 2. background definitions the composition of the workplace, provided by the iso standards [13]-[15], is the following (figure 1): • ‘maximum space’ is the space that can be swept by the whole robot system, made up of the robot’s moving parts, the end effector and the workpiece. • ‘safeguarded space’ is the space limited by the safeguarding perimeter. • ‘restricted space’ is a part of the maximum space that is restricted by limiting devices, and it establishes a limit that cannot be exceeded. • ‘operational space’ is a part of the restricted space that can be used to perform all the actions commanded by the task programme. • ‘collaborative workspace’ is the space, within the operating space, where the whole robot system (robot arm, end effector and workpiece) and a human can figure 1. collaborative workspaces according to iso standards acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 82 concurrently perform tasks during production operations. furthermore, the collaborative operations [13] can be summarised as follow (figure 2): • safety-rated monitored stop (srms): the robot ceases all movement before a human enters the collaborative space; after the human has exited the collaborative workspace, the robot can resume its movements. • hand-guiding (hg): motion commands are transmitted to the robot system either directly by a human or through a hand-operated device; the task is carried out by manually actuating guiding devices. • speed and separation monitoring (ssm): the robot system and the operator may move concurrently in the collaborative workspace maintaining the protective separation distance for risk reduction; if the separation distance falls below the protective separation distance, the robot system stops; if the protective separation distance decreases or increases, the speed of the robot system decreases or increases accordingly. • power and force limiting (pfl): physical contact between the robot system (including the end effector and workpiece) and the human can occur intentionally or unintentionally. in this paper, human–robot interactions are classified based on two principles: workspace sharing and time sharing [18], [20]. they can be used to define a ‘collaborative environment’ in terms of a collaborative interaction between humans and robots: • workspace sharing: robots and humans sequentially perform their tasks sharing the same workspace at different times. • time sharing: robots and humans concurrently perform their tasks without sharing the workspace. time sharing and workspace sharing can only exist within the collaborative space. therefore, a collaborative environment can be enabled whenever a human and a robot concurrently perform their tasks in a common space (space and time sharing). 3. spaces in human–robot collaboration having defined the collaborative environment, in which the human and robot can work together, this proposal now introduces the two elementary spaces, each one dedicated to either the human or the robot, in order to provide a definite spatial collocation. interactions can occur by introducing a composed space through the combination of the elementary spaces (figure 3). 3.1. elementary spaces the elementary spaces, in which the human and robot can each work, are the following: • ‘human space’ (h) is a space dedicated to the human and includes all the equipment and the necessary space for humans to perform their tasks. • ‘robot space’ (r) is a space dedicated to the robot system and includes all the space necessary for the robot to execute its movements and perform its tasks. such spaces have their own features and properties that will be explained below. the combination of the elementary spaces leads to the composed spaces. 3.2. composed spaces composed spaces are made by the combination of the elementary spaces as follows (figure 3): • ‘collaborative space’ (c) is the dynamic intersection of the elementary spaces (robot space and human space). • ‘operational space’ (o) is the combination of the human space and robot space and represents the space strictly necessary to carry out all the operations, including the collaborative space. • ‘restricted perimeter’ (p) is a perimeter around the workplace; a violation leads to a protective stop on all the operations. • ‘safeguarded space’ (s) is the space delimited by the safeguarded devices. figure 2. collaborative operations within iso/ts 15066 (also cited in [1]) figure 3. significant spaces related to different human–robot interactions: h) human space, r) robot space, c) collaborative space, p) restricted perimeter, s) safeguarded space, o) operational space (h + r + c). acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 83 3.3. features of existing spaces by using sensors and cams, it is possible to dynamically change the elementary spaces in terms of shape, size and position: • position: related to the human and robot, • size: related to relative distance and speed, • shape: related to the direction of movement and task. therefore, the spaces are no longer static but can adapt dynamically to the task. the control system can modify them in accordance with the task sequence or in reaction to an unintended situation. space position is strictly related to the position of the human and robot during the task execution. size is affected by the separation distance and the relative speed between the human and robot at any one time. shape is affected by the relative direction of movement of the human and robot at any one time and in accordance with specific tasks. as human and robot spaces can change during a task, the collaborative space can change as well. starting with the space definitions and their properties, the collaborative operations and rules for enabling the human–robot interaction can be defined. 4. description of the human–robot interactions based on elementary spaces the current iso standards for hrc describe only the behaviour of the robot according to different scenarios. indeed, the four collaborative operations (section 2) represent a different degree of interaction between the human and robot (table 1): • srms: no interaction, • hd: exploitation, • ssm: coexistence, • pfl: collaboration. these collaborative operations can be used to better explain the different kinds of interaction in this proposed approach. therefore, according to the definitions of the elementary and composed spaces (sections 3.1 and 3.2), the collaborative operations in the standards can be reviewed and adapted to follow a task-oriented approach; the use of the proposed spaces is described below. it will then be made clear how the spacesbased approach can help to define the layout of the workplace leading to a collaborative environment. 4.1. safety-rated monitored stop in srms, there is no collaborative space or collaborative operations. the human can approach the workplace only when the robot is on stand-by mode. any violation of the robot space generally causes a protective stop of the robot system; the human is seen as an intruder. therefore, there is no real interaction between the human and robot and the safety is guaranteed by limiting the collaboration. the human space and robot space are, ideally, overlapping and alternatively activated (workspace sharing). there is no time sharing, and a protective stop occurs to enable the human to enter the workplace by switching between the human and the robot space. in this context, all the workplace features seem to belong to the robot. there is no space for the human operator. the layout definition is hindered by this lack of information since it is not clear to the designer where to place the human operator (even though srms may not consider the human presence). according to the proposed approach, the human and robot spaces are simultaneously active but disjointed. both human and robot have their own domain where they execute their tasks. an intersection of spaces is not permitted while working (top of figure 3), but the human presence is taken into account during the work and the layout design phase. this information helps the design process and allows for the optimisation of the provided spaces. 4.2. hand guiding in an hg operation, the robot can be seen as a tool in a human’s hands. there is no robot space or human space because they cannot execute their tasks separately; in this case, the operational space is all the space required to carry out the task. however, hand guiding is not an actual collaborative operation since the human carries out all the tasks by handling the robot as a tool. there are no access properties for the working places, but there are switching procedures in order to guarantee human safety and enable the interaction. although such operations are characterised by both workspace sharing and time sharing, it seems incorrect to consider it to be a collaborative operation because it is characterised by an ‘exploitation’ of the robot’s capabilities by the human (table 1). in this particular case, there is no actual robot space, even though this modality is rarely used alone but in combination with the automatic mode. the robot system switches to this modality in order to permit a human to take control of the robot. thus, this can be defined as the robot’s domain during the collaboration and is characterised by two sub-domains: (i) the autonomous domain (outside the collaborative workspace) and (ii) the guided-movement domain, corresponding to the collaborative workspace (bottom of figure 3). the designing of the spaces helps to define the layout because of the preliminary allocation of resources for the human and robot. 4.3. speed and separation monitoring in ssm, the human and robot can perform their tasks independently, sharing time and workspace. the separation distance between the human and robot needs to be monitored in real-time; a robot’s speed is adjusted according to the safety rules and depending on the distance [13]. as for srms, direct and indirect contacts are not permitted; there are different safety distance levels, and a breach of a robot’s nearest perimeter activates the robotic system’s protective stop. according to the current definition, the robotic system is the core of the workplace, where it can perform its own tasks, whereas the human operator can move around it, in an unspecified area, and without approaching within certain limits. with the proposed approach, based on elementary spaces, the designer can provide a human and a robot with their own space table 1. table of comparison iso standards degree of interaction proposed approach srms no interaction disjointed areas hg exploitation no robot area ssm coexistence intersection of areas – no contact pfl collaboration intersection of areas – contact allowed acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 84 where they can perform their tasks. during any interactions, such spaces can intersect and overlap, generating different configurations (the middle of figure 3). using dynamic elementary spaces, it is possible to improve the interaction between humans and robots by adapting the position, size and shape. 4.4. power and force limiting in pfl, direct and indirect contacts are allowed. all necessary active and passive safety features are applied so that humans and robots can carry out their tasks, side by side, in a synergic and dynamic way. this represents a higher level of interaction and, possibly, the most dangerous. although this operation represents a complete hrc, it is not widely used due to the high levels of the operators’ cognitive stress (i.e. not a predictable robot path) and the challenge of guaranteeing an appropriate level of safety (avoiding collisions and the balancing force). furthermore, the workplace design leans towards the robot system; the human operator can move around and work near the robot as a host. in a complete hrc, the layout design process should consider humans and robots as peers. to improve the pfl, both humans and robots should be efficiently accommodated by not limiting their capabilities through a restrictive safety-based design. therefore, a space dedicated to humans and another dedicated to robots should be clearly defined during the workplace design. such spaces can be combined in order to better perform all the tasks. the elementary spaces can intersect (collaborative space) and humans and robots can enter the collaborative space together to perform their tasks (bottom of figure 3). 4.5. interactions: iso vs elementary spaces a comparison between the current definition (iso standards) and the proposed approach can be made. table 1 summarises the innovations introduced by the elementary spaces. according to the iso standards, the collaborative operations represent a degree of interaction or a modality of operation (hg); thus, the designing process occurs around the robot. instead, by adopting the elementary spaces, the designing process is carried out around the human–robot pair and the focus is switched towards the human. therefore, the basis for the design of the workplace layout becomes the identification of the human and robot spaces. furthermore, since the design process is performed around the human–robot pair, their interaction is placed at the core of the workplace, enabling a collaborative environment. 5. workplace multi-level graph-based modelling workplace design involves a multitude of requirements, standard constraints, technological constraints and design parameters characterised by intertwined relations. all these elements result in a dense network of relationships that is quite difficult to track and respect in the design phase. this aspect is more significant in the context of anthropocentric design due to the larger number of relations related to human space. dealing with such a multitude of relations requires a decomposition approach able to deepen and build the relations step by step. patalano et al. [24] proposed a multi-level approach based on graphs in the design of complex products using a digraph to manage and track the component relations. graph theory [25] provides many efficient tools to record and manage a large amount of information as well as to track their relationships. it provides an abstraction from the real model preserving all dependencies and relations between the elements involved. graph theory is characterised by a set of nodes and edges (g={n,e}) [26], and it uses an array or matrix for data organisation. the most significant matrix is the adjacency matrix that highlights the connection between the nodes; the elements of 1 or 0 mean the row element is either related to the column element or not. direct graphs (digraphs) [27] were used in this paper. the nodes are connected by arrows that explain the relation between the two nearest nodes. figure 4. multi-level approach table 2. adjacency matrix for the hrc workplace at the second level. pl c e i ec m w a hi r ps d pl 0 0 0 0 0 0 1 1 0 0 1 0 c 0 0 0 0 0 0 1 1 1 0 1 1 e 0 0 0 0 0 0 0 0 1 0 0 1 i 0 0 0 0 0 0 1 0 1 1 0 1 ec 0 0 0 0 0 0 1 0 1 0 0 0 m 0 0 0 0 0 0 1 1 1 1 1 0 w 0 0 0 0 0 0 0 1 0 0 1 1 a 0 0 0 0 0 0 0 0 0 0 1 1 hi 0 0 0 0 0 0 1 1 0 0 1 1 r 0 0 0 0 0 0 1 1 1 0 1 1 ps 0 0 0 0 0 0 0 0 0 0 0 0 figure 5. second-level digraph representation for an hrc workplace; initial nodes are circled in red acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 85 using a multi-level graph-based approach, it is possible to easily organise, exploit and manipulate the various elements of the workplace, helping to keep track of them. indeed, graphs are used in several fields, such as networks of people, electronics, ergonomics and engineering, because of their versatility [28]. in the context of anthropocentric design, we have proposed three levels of detail (figure 4) in which nodes are associated with different elements depending on their level of belonging, and directed edges always represent the relationships. in this abstract model of a workplace, the characterisation of the workplace comes from the combination of its basic elements, identified as nodes [25], where human intervention stands out from the beginning of the design process as a central element of the network, emphasising its importance. thus, the human presence is no longer seen only as a constraint, for safety reasons, but, in conjunction with the robot, as the core of the workplace because of the interactions. the whole collaborative workplace is assumed to be a firstlevel node. the second level is composed of functional elements defined by a logical decomposition. such elements are called second-level nodes: 1. (pl) physical limit: physical limit of the cell 2. (c) clearance: minimum distances required 3. (e) ergonomics: ergonomic requirements 4. (i) interaction: modality of interaction 5. (ec) environmental conditions: ventilation, welding sparks, etc. 6. (m) material: material to be worked and tools 7. (w) workspaces: areas inside the workplace 8. (a) access: access routes and paths 9. (hi) human intervention: operator intervention 10. (r) robot: robot characteristics 11. (ps) perimeter safeguarding: workplace border 12. (d) devices: requirements of manual devices the adjacency matrix for the second level is represented in table 2 and its graphical representation is shown in figure 5. the first six elements are source nodes because they are containers of information that contribute to the definition of other nodes, but they are not influenced by any other nodes in the current representation. indeed, their information is derived from outside the matrix (standards, task analysis, design decisions or risk assessment). each node can be further broken down into third-level nodes in order to refine their knowledge content. for example, the ‘workspaces’ (w) node (node number 7 in the set of second-level nodes) can be broken down as follow: • human space (hs): space where operators can perform their tasks during the operations. • robot space (rs): space where the robot system (including the end effector and workpiece) can perform its tasks during the operations. • load/unload space (l/u): space required to load and unload supply material. • collaborative space (cs): space obtained from the intersection of the hs and rs spaces; within this space, contacts between robot and operator are allowed. • operational space (os): space required to perform the whole task; it is the combination of the human and robot space and contains the collaborative space. the connections between the third-level nodes are highlighted in table 3, and the graphical representation is shown in figure 6. it is easy to see that some are source nodes, whereas others are sink nodes. indeed, in this representation, the starting point is the load/unload space; the operational and collaborative workspaces are sink points and are the final steps in the decisionmaking process in this subgraph. this process should be replicated for all the second-level nodes in order to obtain a complete matrix that highlights the connections between all the nodes. the result is a matrix that can be filled according to the specific application and task. this tool’s great utility and versatility is due to its easy organisation and the possibility of using a set of existing or ad-hoc developed algorithms. 6. conclusions in this paper, the contents of the iso international standards for hrc workplaces are illustrated and the basis for an anthropocentric design approach as key to fully enabling human– robot interaction and the collaborative environment is highlighted. two main elements are tackled: (i) the introduction of two elementary spaces (human and robot spaces) and (ii) their dynamic variations in terms of their shape, size and position. furthermore, the human is assumed to be the key to enabling a real collaborative relationship. the identification of dedicated spaces for humans and robots can help to manage their interaction and to set them inside the workplace, overcoming the limitations connected to a safety-based approach, which has been introduced by the standards. therefore, the presented approach is oriented towards the layout design process thanks to the use of elementary spaces. the purpose is to enable a collaborative environment where humans and robots can work in a synergic way. the proposed approach can guide the design phase of collaborative workplaces because it satisfies both human needs, table 3. adjacency matrix for workspaces (level-three detail) workspaces (w) os cs hs rs l/u w o r k s p a c e s ( w ) os 0 0 0 0 0 cs 0 0 0 0 0 hs 1 1 0 0 0 rs 1 1 0 0 0 l/u 1 1 1 1 0 figure 6. third-level digraph representation for workspaces acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 86 through an ergonomic workplace design, and productive needs, because of the consideration given to humans and robots together. indeed, innovative workplaces need design approaches that move from a safety-based design to a collaboration-based design. in this new context, the human is considered to be an element that improves the productivity capacity instead of an intruder. in this scenario, it is necessary to manage the concepts introduced above together with the safety prescriptions contained in the standards. for this reason, a multi-level graph-based approach has been used in order to: (i) define the functional requirements linked to the design parameters and associated with hrc and (ii) manage the dense network of interdependencies to drive the design process. starting with the contents of the standards, the main elements of a collaborative workplace were identified. a decomposition from the first level was then carried out. for the second-level nodes, a categorisation of the elements that make up the workplace and their relationships was proposed. finally, an example of third-level decomposition presented the main areas that make up the layout. thus, human–robot collaboration can open interesting scenarios for the future of manufacturing, leading to a higher level of flexibility and customisation. as a result, in the near future, hrc will likely be widely used in private and public fields. future perspectives point to the validation of collaborative workspaces and the possibility of dynamically updating working areas by using tools and methods, such as machine-learning algorithms and digital twins. acknowledgement this study was developed with the economic support of miur (italian ministry of universities and research) under the remit of project ars01_00861, ‘integrated collaborative systems for smart factory – icosaf’. references [1] v. villani, f. pini, f. leali, c. secchi, survey on human–robot collaboration in industrial settings: safety, intuitive interfaces and applications, mechatronics 55 (2018) pp. 248-266. doi: https://doi.org/10.1016/j.mechatronics.2018.02.009 [2] c. labate, g. di gironimo, f. renno, plasma facing components: a conceptual design strategy for the first wall in fast tokamak, nuclear fusion 55 (2015) 113013. doi https://doi.org/10.1088/0029-5515/55/11/113013 [3] g. di gironimo, d. carfora, g. esposito, a. lanzotti, d. marzullo, m. siuko, concept design of the demo divertor cassette-to-vacuum vessel locking system adopting a systems engineering approach, fusion engineering and design 94(1) (2015) pp. 72-81. doi: https://doi.org/10.1016/j.fusengdes.2015.03.039 [4] g. di gironimo, a. lanzotti, d. marzullo, g. esposito, d. carfora, m. siuko, iterative and participative axiomatic design process in complex mechanical assemblies: case study on fusion engineering, international journal on interactive design and manufacturing 9 (2015) pp. 325-338. doi: https://doi.org/10.1007/s12008-015-0270-7 [5] s. patalano, a. lanzotti, d. m. del giudice, f. vitolo, s. gerbino, on the usability assessment of the graphical user interface related to a digital pattern software tool, international journal on interactive design and manufacturing 11(3) (2017) pp. 457-469. doi: https://doi.org/10.1007/s12008-015-0287-y [6] p. franciosa, a. palit, f. vitolo, d. ceglarek, rapid response diagnosis of multi-stage 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international workshop on robot and human interactive communication, berlin, germany, 2002, pp. 399-404. doi: https://doi.org/10.1109/roman.2002.1045655 [11] j. krüger, t. k. lien, a. verl, cooperation of human and machines in assembly lines, cirp annals – manufacturing technology 58 (2009) pp. 628-646. doi: https://doi.org/10.1016/j.cirp.2009.09.009 [12] d. bortot, m. born, k. bengler, directly or on detours? how should industrial robots approximate humans?, journal of experimental psychology 55 (2013) pp. 352-358. doi: https://doi.org/10.1109/hri.2013.6483515 [13] iso, iso/ts 15066: 2016: robots and robotic devices – collaborative robots, geneva, switzerland: international organization for standardization, 2016. 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[16] l. gualtieri, e. rauch, r. rojas, r. vidoni, d. t. matt, application of axiomatic design for the design of a safe collaborative human-robot assembly workplace, matec web of conferences, 223, edp sciences, 2018, 01003, 8 pages. doi: https://doi.org/10.1051/matecconf/201822301003 [17] j. o. oyekan, w. hutabarat, a. tiwari, r. grech, m. h. aung, m. p. mariani, l. lópez-dávalos, t. ricaud, s. singh, c. dupuis, the effectiveness of virtual environments in developing collaborative strategies between industrial robots and humans, robotics and computer-integrated manufacturing 55 (2019) pp. 41-54. doi: https://doi.org/10.1016/j.rcim.2018.07.006 [18] t. arai, r. kato, m. fujita, assessment of operator stress induced by robot collaboration in assembly, robotics and computerintegrated manufacturing 59 (2010) pp. 5-8. doi: https://doi.org/10.1016/j.cirp.2010.03.043 [19] g. michalos, s. makris, p. tsarouchi, t. guasch, d. kontovrakis, g. chryssolouris, design considerations for safe human-robot collaborative workplaces, procedia cirp 37 (2015) pp. 248-253. doi: https://doi.org/10.1016/j.procir.2015.08.014 [20] m. bdiwi, m. pfeifer, a. sterzing, a new strategy for ensuring human safety during various levels of interaction with industrial robots, cirp annals 66 (2017) pp. 453-456. doi: https://doi.org/10.1016/j.cirp.2017.04.009 [21] o. ogorodnikova, human weaknesses and strengths in collaboration with robots, periodica polytechnica, mechanical engineering 52 (2008) pp. 25-33. doi: https://doi.org/10.3311/pp.me.2008-1.05 [22] a. rega, s. patalano, f. vitolo, s. gerbino, a sensor data fusionbased locating method for reverse engineering scanning systems, ii workshop on metrology for industry 4.0 and iot (metroind4. https://doi.org/10.1016/j.mechatronics.2018.02.009 https://doi.org/10.1088/0029-5515/55/11/113013 https://doi.org/10.1016/j.fusengdes.2015.03.039 https://doi.org/10.1007/s12008-015-0270-7 https://doi.org/10.1007/s12008-015-0287-y https://doi.org/10.1016/j.procir.2017.01.035 https://doi.org/10.1155/2019/3146782 https://doi.org/10.1016/j.procir.2018.03.214 https://doi.org/10.1016/s0007-8506(07)60040-7 https://doi.org/10.1109/roman.2002.1045655 https://doi.org/10.1016/j.cirp.2009.09.009 https://doi.org/10.1109/hri.2013.6483515 https://doi.org/10.1051/matecconf/201822301003 https://doi.org/10.1016/j.rcim.2018.07.006 https://doi.org/10.1016/j.cirp.2010.03.043 https://doi.org/10.1016/j.procir.2015.08.014 https://doi.org/10.1016/j.cirp.2017.04.009 https://doi.org/10.3311/pp.me.2008-1.05 acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 87 0&iot), naples, italy, 2019, pp. 123-126. doi: https://doi.org/10.1109/metroi4.2019.8792864 [23] g. michalos, s. makris, j. spiliotopoulos, i. misios, p. tsarouchi, g. chryssolouris, robo-partner: seamless human-robot cooperation for intelligent, flexible and safe operations in the assembly factories of the future, conference on assembly technologies and systems, procedia cirp, dresden, germany, 13-14 november 2014, volume 23, pp. 71-76. doi: https://doi.org/10.1016/j.procir.2014.10.079 [24] s. patalano, f. vitolo, a. lanzotti, a digital pattern approach to 3d cad modelling of automotive car door assembly by using directed graphs, graph-based modelling in engineering, mechanisms and machine science 42 (2017) pp. 175-185. doi: https://doi.org/10.1007/978-3-319-39020-8_13 [25] dnarsingh deo, graph theory with applications to engineering and computer science, courier dover publications, englewood cliffs, new jersey, 2017, isbn 0486820815, 9780486820811, 496 pages. [26] m. koutrouli, e. karatzas, d. paez-espino, g. a. pavlopoulos, a guide to conquer the biological network era using graph theory, frontiers in bioengineering and biotechnology 8 (2020) article 34. doi: https://doi.org/10.3389/fbioe.2020.00034 [27] a. majeed, i. rauf, graph theory: a comprehensive survey about graph theory applications in computer science and social networks, inventions 5(1) (2020) article 10. doi: https://doi.org/10.3390/inventions5010010 [28] s. e. schaeffer, graph clustering, computer science review 1(1) (2007) pp. 27-64. doi: https://doi.org/10.1016/j.cosrev.2007.05.001 https://doi.org/10.1109/metroi4.2019.8792864 https://doi.org/10.1016/j.procir.2014.10.079 https://doi.org/10.1007/978-3-319-39020-8_13 https://doi.org/10.3389/fbioe.2020.00034 https://doi.org/10.3390/inventions5010010 https://doi.org/10.1016/j.cosrev.2007.05.001 research on the swing and vibration restraint of the 2 mn deadweight machine frame acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 97 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 97 99 research on the swing and vibration restraint of the 2 mn deadweight machine frame lin shuo1 1 fujian province institute of metrology, fuzhou, china, linshuo1001@126.com abstract: in this paper, the new 2 mn deadweight force standard machine of fjim (fujian metrology institute) with an optimal 20 kn frame with swing and vibration restraint is detailed. load transient response is considered in the design of column, and a mass damper is used to make the frame stable in 3 s. these methods help the machine reach the uncertainty it claimed. keywords: deadweight machine, vibration, damper, dynamic 1. introduction a new 2 mn deadweight force standard machine (dwm) was installed in 2018 at fuzhou, china, and as shown in figure 1 in the structural sketch. the measuring range of the machine is (20 ~ 2000) kn with a stated uncertainty of 0.002 % (k = 3). all the masses are made of stainless steel. this machine is the largest dwm in china while the maximum range was 1.1 mn before. it has finished the comparison with nimtt china and npl uk, and the results give confidence in the claimed uncertainty of the machine. figure 1: fjim’s 2 mn deadweight machine compared to ptb’s 2 mn machine [1], there are some significant differences and improvements. first is the lower limit of measurement; fjim’s is designed to 1 %, so the measurement range is from 20 kn to 2 mn to meet more types of calibration requirements. in order to achieve this goal, a three steps frame was used. the frame includes a 20 kn reverser, and two 10 kn columns joined by cones. they are able to be loaded independently. to minimise the effects of longitudinal vibration and lateral swinging, and to improve stability in the high accuracy measurement, the method of dynamic optimisation and the mass damper were used in the design. 2. optimisation of the reverser and the column the conventional design method tends to consider only von mises stress under static conditions [2]. for the purpose of weight reduction, the static stiffness is used to the maximum extent, then the dynamic stiffness might be not enough. longitudinal vibration will appear when the masses loading at high speed under low dynamic stiffness of structure, and force fluctuation acts on the load cell which being tested and then be reacted on the instrumentation. the whole frame can be viewed as a qualityspring-damper system. short response time and short stabilisation time is the optimisation goal. fem tool was used in the design of 2 mn dwm frame, and different mass states and loading speeds were set as constraints conditions. figure 2: fem analysis of reverser through the optimisation design, the maximum stress value of the beam under the inverter is http://www.imeko.org/ mailto:linshuo1001@126.com acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 98 obviously reduced, the stress distribution range span is reduced, the overall stress is more uniform, and the working state is more stable. figure 3: fem analysis of column local reinforcement was implemented on the triple column reverser. under the guidance of simulation analysis, a step-shaped hollow column with a weight of 10 kn was produced, with a maximum diameter of 245 mm. topological optimisation helps the structure to have higher dynamic stiffness when the damping coefficient of material is constant. the stress concentration coefficient was lowered by using arc transition angle at the same time. the results of transient analysis show the acceleration corresponding curve in figure 4, and the longitudinal vibration stopped within 1 s when 2 mn masses load simultaneously. figure 4: transient of longitude vibration 3. mass damper there are several factors that cause lateral swing including installation error, the eccentricity of the mass centroid and the eccentric or tilt on load cell being tested. all of the above factors will lead to the imbalance of the frame and column caused by the deviation of the centre of mass from the central axis during the loading and unloading process, which will ultimately affect the accuracy and stability of the machine. an adams (a multibody dynamics simulation software) model was built up for kinetic analysis and is shown in figure 5. different initial conditions, including starting angles, weights, and mass positions were analysed, while the time to stop swing is the goal of simulation. the unbalanced state and swinging will stop under the action of friction and air resistance. an angle of 0.5 % was set in analysis and it took 10.6 s until becoming stable at 2 mn. figure 5: factors of swing and adams model table 1: stabilisation period under different loads load / kn time / s 150 2.6 900 5.4 1500 6.2 2000 10.6 in the process of weight configuration, the position of the weight should be considered in the budget. for the same loading weight, different loading positions are selected to analyse the influence of loading positions on the tilt and swing of the central column. as the results in table 2 show, the closer the loading position is to the top joint, the smaller the vibration amplitude and the shorter the stabilisation time. table 2: stabilisation period under different position load / kn distance / mm time / s 150 1500 6.1 3000 9.4 5000 15.8 7000 19.5 although the tilt and swing of the column can be minimised by careful processing and installation, this phenomenon cannot be completely eliminated. therefore, it is necessary to design a structure to reduce the swing amplitude and shorten the stabilisation time [3]. the simulation results show that the stabilisation time t is negatively correlated with the suspension mass and the friction coefficient of the ball hinge, and positively correlated with the rod length. after several times of optimisation, the optimal result of mass damper was designed as a 115 kg mass with a 200 mm column, and a spherical hinge with friction coefficient of 1.3. the structure and the simulation result are shown in figure 6. obviously, the stabilisation time under 2 mn and 0.5 % tilt of frame is shortened to less than 2 s compared to 10 s without mass damper. http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 99 figure 6: mass damper and the simulation of swing 4. test result tests of static deformation and dynamic response was tested using an hbm 2 mn load cell, a vibration sensor, and several laser displacement sensors, which are shown in figure 7. figure 7: dynamic response test system and mass damper the maximum deformation of the frame under 2 mn is less than 5 mm, and the uneven deformation less than 0.1 mm/m. the longitudinal vibration stopped in 0.2 s which hardly reaction on the dmp-41 indicator. a 5 mm eccentric was set on the load cell, and the swing caused by misalignment stopped within 3 s. figure 8 shows the swing displacement at the end of the column, and the stability is restored after one period of vibration. figure 8: test of swing time and result 5. summary fjim’s 2 mn deadweight machine was optimally designed so that the first step of 20 kn can be reached. the paper presents the methods for guaranteeing high accuracy force measurement. the longitudinal vibration can be restrained by the structure of column, and the lateral swinging of frame was reduced by a mass damper. test results show that all those advantages are effective. with the help of these innovations, the machine gets the uncertainty of 0.002 % (k = 2) and with excellent stability. the accuracy has been verified by international comparison. it is of positive significance to the application of force measurement up to 2 mn. 6. references [1] m. peters, d. peschel, “ptb’s new 2 mn deadweight force standard machine”, in proc. of 19th imeko tc3 congress, february 2005. [2] n. vilajic, a. chijioke, r. seifarth, “stress analysis of conical contact joints in the nist 4.45 mn deadweight machine”, journal of research of the national institute of standards and technology, vol. 121, pp. 222-237, may 2016. [3] l-k. wang, w-x. shi, “an adaptive-passive retuning device for a pendulum tuned mass damper considering mass uncertainty and optimum frequency”, structural control and health monitoring, pp. 135-141, vol. 26, may 2019. http://www.imeko.org/ comparison and integration of techniques for knowledge and valorisation of the corsini throne in corsini gallery in roma acta imeko issn: 2221-870x march 2021, volume 10, number 1, 40 46 acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 40 comparison and integration of techniques for the study and valorisation of the corsini throne in corsini gallery in roma marialuisa mongelli1, giulia chellini2, silvio migliori1, antonio perozziello2, samuele pierattini3, marco puccini1, alessandro cosma4 1 enea high performance computing laboratory, lungotevere thaon di revel 76, 00196 rome, italy 2 enea diagnostic and metrology laboratory, via enrico fermi 45, 00044 frascati, rome, italy 3 enea high performance computing laboratory, via madonna del piano 10, 50019 sesto fiorentino, florence, italy 4 corsini barberini national gallery, via della lungara 10, 00165 rome, italy section: research paper keywords: 3d reconstruction; 3d matching; structured light scans; photogrammetry; structure from motion technique; technology transfer citation: marialuisa mongelli, giulia chellini, silvio migliori, antonio perozziello, samuele pierattini, marco puccini, alessandro cosma, comparison and integration of techniques for knowledge and valorisation of the corsini throne in corsini gallery in roma, acta imeko, vol. 10, no. 1, article 7, march 2021, identifier: imeko-acta-10 (2021)-01-07 editor: eulalia balestrieri, university of sannio, italy received april 20, 2020; in final form november 17, 2020; published march 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: marialuisa mongelli, e-mail: marialuisa.mongelli@enea.it 1. introduction following upon the paper presented at 2019 imeko tc-4 international conference on metrology for archaeology and cultural heritage [1], this work takes into account the developments made through participation in the ecodigit project, which aims to create a digital ecosystem for the cultural heritage of the lazio region [2]. the ecodigit project is one of the initiatives of the centre of excellence of the technological district for cultural heritage and activities (dtc) of lazio, which aims to aggregate and integrate expertise in the field of technologies for cultural heritage and activities. the project focused on the design of a middleware platform to facilitate the integration of new data sources and enable the publication and reuse of services for the enhancement and enjoyment of the cultural heritage of lazio. the results demonstrate the opportunities for sharing and enhancing cultural heritage provided by 3d models that can be extended and linked with studies and other information in a simple way. by adding metadata to the knowledge graph, it is possible to create linked information that can be queried and shown with the viewer in the same web environment, allowing the academic, the researcher and/or the simple user to access resources and carry on with their studies. the corsini throne was found between 1732 and 1734 during excavations prior to laying the foundation abstract in recent years, digital technologies for enhancement and use of cultural heritage items has grown considerably. multimedia, virtual and augmented reality and 3d reconstructions make it possible to bring the general public closer to an understanding of something that no longer exists or that is from a distant time. but digital tools can serve more than educational purposes. to date, digitisation has become above all an essential tool in most cultural heritage projects involving conservation, restoration, documentation and research. this article shows a process that integrates photogrammetry and structured light scans to obtain a 3d reconstruction of the corsini throne, preserved at the corsini gallery in rome for its exhibition using a web application combined with semantic representation of metadata following fair principles. the process began during the development of the weact3 project (acting together – technology for art, culture, tourism and territory) jointly signed by the civita association, and the national barberini and corsini galleries, collaborating in a partnership of several national and international enterprises. within ecodigit project, financed by lazio region, an automated web tool prototype was developed by enea. it is able to display 3d models with correlated scientific information to assist research activities and knowledge sharing. mailto:marialuisa.mongelli@enea.it acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 41 of the corsini family’s chapel in the basilica of s. giovanni in laterano. the decorations and typology of this artefact could be considered typical of etruscan funerary thrones, which were commonly made with bronze or terracotta. the use of marble and the place of the discovery, on the other hand, show the roman origins of this piece, making it unique in ancient sculpture. this, together with the events relating to its discovery, has led to a hypothesis regarding its function and role: that it is a symbol of the royal descent of one of the most important women of the etruscan plautii silvani family, urgulania, who married m. plautius vir praetorius in around 40 bc. the artefact is, therefore, a roman copy of an etruscan throne from the late republican age (late 5th century bc), intended as proof of urgulania’s royal lineage [3]. the back of the throne is divided into two parts delimited by a frame of ivy; the upper part shows soldiers, while the lower one depicts scenes of wild boar hunting. above a plant frieze at the base, figures portray scenes of sacrifice and struggle and a procession, the interpretation of which is still not clear. the corsini throne was investigated using both photogrammetry and a structured light scanner. the use of these two non-invasive technologies enabled the creation of two different 3d reconstructions. these reconstructions can be shared virtually, improving the visibility of the artwork and enriching the museum’s catalogue [4]. photogrammetric reconstruction is widely used to generate 3d models of both small artworks and monumental complexes of considerable size [5], [6]. starting with 2d digital images taken by a simple camera, a scaled 3d numerical model is generated using structure from motion (sfm) and computer vision algorithms. photogrammetry is non-invasive, low-cost, contactless, fast and easy to execute [7]. in comparison, structured light scanning is slower due to the greater size of the instrumentation and less immediate because of the use of the scanner; moreover, the software is less intuitive and requires a specialised operator (figure 1). it quickly creates highly accurate 3d scans and is suitable for projects involving small, delicate or fragile objects. geometries and colours are captured simultaneously, allowing an exact match of the 3d coordinates and their corresponding colour information. 2. information and communications technology infrastructure and services for cultural heritage enhancement enea’s information and communications technology (ict) infrastructure is a large system of services, software and computing resources for a wide range of use cases, including the emerging field of digital heritage technologies. in 2019, enea supplied its ict know-how and resources to the ecodigit project, which aims to design and develop a digital ecosystem for the cultural heritage of the lazio region. in particular, enea worked to make a reliable, automatic 3d visualisation tool named 3dhop [8] that would enable semantic interoperability between databases to allow the automatic generation of the visualisation in a web page. with this result, developed by enea, 3dhop has become a module that can be integrated with any type of structured database, as described above [9]. its automatic features, which are based on 3d model metadata, allow for fast deployment of tools like databases, supporting the use of 3d model visualisation. in particular, 3d models, thanks to their specific properties, provide a starting point for reading and studying the real work of art and monitoring its physical conditions [10]. metric comparisons between the 3d model and the work can be used to document, measure and study any type of variation that occurs in the physical structure of the artefact. photogrammetry makes it possible to repeat the measurements at regular intervals so as to allow the study of changes, thanks to its agile instrumentation, speed of execution and relatively short processing times. photogrammetric surveys use less expensive instrumentation than other techniques. in addition, the instruments are easier to transport, and the image processing is faster. this makes it possible to check the results almost in real time and, if necessary, to proceed with a new scan. figure 1. the visualisation tool based on 3dhop. figure 2. elaboration of 2d images in agisoft photoscan pro software. figure 3. processing of 2d images for 3d photogrammetric reconstruction: dense cloud and mesh. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 42 a 3d model obtained through photogrammetry has a good balance between quality and construction time. where necessary, it is possible to integrate the photogrammetric results with high resolution scans performed, for example, using a structured light scanner. this can be helpful when it is necessary to highlight details or elements that are not legible at a minuscule resolution. 3. photogrammetric reconstruction by sfm technique photogrammetric scanning was used to produce more than 500 images (5184 × 3456 pixel, 5 mb each) taken with a nikon d60 digital camera with zoom and focus held constant. starting from a set of 2d images, the digitisation process of 3d photogrammetric reconstruction consisted of a sequential semiautomatic procedure. post-processing, the images were obtained using the itacha virtual platform on eneagrid [11]. the 3d reconstruction was elaborated with the commercial software photoscan pro (figure 2) based on computer vision algorithms and supported by sfm and multiple view stereovision techniques. the software is available by remote access in enea’s ict infrastructure through the fast access to remote objects graphical interface, which makes it possible to use computer graphics tools, to exploit the computational resources offered through hpc cresco6 systems and to store images and photogrammetric reconstruction results in the afs and gpfs enea storage areas [12]. after the acquisition of images, the reconstruction procedure using photoscan pro software consisted of the following steps: a) matching and alignment; b) solving for the camera’s intrinsic and extrinsic orientation parameters; c) reconstructing dense surfaces; d) reconstructing polygons; e) texture mapping; f) surface editing. out of 455 images, 435 were successfully aligned and processed in photoscan. after the alignment phase, the sparse cloud was processed to obtain a dense cloud from which the polygon mesh was built (figure 3). thanks to the use of the hardware and software resources of the enea ict computing infrastructure, it was possible to obtain the 3d reconstruction of the corsini throne in about 200 minutes. finally, the texture of the 3d model was reconstructed, and the model was also manually scaled according to real measures, creating a correct structure in terms of geometry (figure 4). the result is a photorealistic and metrically correct 3d model (figure 6). speed and ease of processing allow photogrammetry to be used as a valid tool for monitoring the conservation status of an artwork: indeed, by comparing 3d models generated at different times, it is possible to monitor any structural changes [13]. 4. structured light scanner in addition to photogrammetric scanning, the corsini throne was also subjected to scanning using structured light. this non-invasive technology enables the creation of highresolution 3d data sets. the structured light system supplied by enea is a smartscan by aicon 3d systems used in combination with optocat software. this system uses a high-resolution scan performed by a portable structured light aicon smartscan 5m pixel system (figure 7) with an m-300 optic to automatically map the texture onto the final mesh. the instrument’s technical features are listed in table 1. the instrument projects a pattern of light and detects the deformation of this pattern on the object. the acquired images are processed by optocat, a software produced by aicon that allows the quality of each scan to be verified in real time and consecutive scans to be merged. optocat uses specific algorithms to automatically align new scans with preceding ones. if the scans do not overlap sufficiently, manual alignment through the identification of homologous markers is required. the pattern deformation induced by the surface of the object is acquired by a camera and exploited to calculate 3d coordinates. the process of 3d reconstruction by structured light involved a sequence of several steps. after calibrating the device, we proceeded to scan the object and acquire the images. due to the dimensions and the particular shape of the object, two separate scan phases were necessary. the first scan covered the seat of the throne, and the second covered its backrest. each 3d scan was automatically aligned with the previous one by the optocat software or, if necessary, by the operator using manually positioned markers. due the impossibility of moving and rotating the corsini throne, the equipment had to be moved around the throne a total of 360 °. after aligning these two scans, optocat elaborated the point cloud and built the mesh. table 1. aicon smartscan 5m pixel system. accuracy in μm ± 26 field of view size in mm 240 × 200 measuring depth in mm 150 (x,y) resolution limit in μm 100 (z) resolution limit in μm 5 (z) noise in μm ± 11 figure 4. real measures and scaled model according to real measures (1:1). acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 43 this software is able to create colourful textures; the mapping of the high-resolution texture can be realised through the internal images captured by the 3d scanner or with images acquired by external cameras. the 3d models thus obtained were subjected to a merging process in order to obtain a single and complete model. in the case of the corsini throne, it was preferable to study the object without texture because the texture interfered with the analysis of the bas-relief. the result is a very high-resolution model with a greater definition of detail than that obtained through photogrammetry (figure 5 – figure 8). for this reason, exporting the 3d model in ply format creates an extremely large file that requires advanced computational resources. thus, it has been split into two parts, base and back, halving the size. thanks to this solution, it is possible to manage the two ply files using even a low-performance pc. 5. unwrapping and integration techniques in agreement with the curator of the museum, one of the goals of the structured light survey was to obtain a 3d model that would make it easier to read the bas-reliefs on the back and base of the corsini throne. in fact, the significance of the complex iconography on the base as well as the throne’s function remains unclear, generating questions that continue to fuel scientific debate. facilitating the narrative reading of the bas-reliefs would make it possible to further enrich the work of the museum and to study this piece in greater depth. the relief was ‘unrolled’ in meshlab using the deformation filter and geometric cylindrical unwrapping (figure 9 figure 10) [14]. figure 5. detail of high-resolution 3d model of corsini throne. figure 6. 3d model of the corsini throne with texture. figure 7. structured light scanner instrument. figure 8. detail of the high-resolution 3d model. figure 9. unrolling of the base of the corsini throne. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 44 this made it possible to observe and study the scenes reproduced at the base of the throne from a horizontal and consecutive perspective. the geometry of the model produced through the photogrammetric process does not provide a sufficient level of detail to allow a similar iconographic reading (figure 11). for this reason, it was considered useful to generate a single model through the integration of the two techniques. in so doing, we could obtain a partially high-definition model that was particularly detailed in the area most difficult to study, the base of the corsini throne. at the same time, the file size would be significantly reduced, thus allowing it to be accessed on a pc with lower performance. the models produced by photogrammetry and structured light were merged within meshlab [15]. either the complete 3d photogrammetric model or the structured light model of the base of the throne can be opened in the software. the photogrammetric model, which has smaller dimensions, was complete in order to provide the amount of surface necessary for overlap. first, we proceeded with the orientation of the two models using the meshlab manipulator tool for rotation and translation (figure 12). then, in the alignment phase, 5 homologous points were identified on the two models (figure 13). this allowed a satisfactory overlap (figure 14). subsequently, the surface of the photogrammetric model of the base was eliminated because it had become superfluous. the high-resolution base allows detailed study of the relief. by combining it, through alignment, with the backrest reproduced using photogrammetry, it was possible to obtain a single model that was used less processing space and was more manipulable. the result (figure 15) is an exportable model of reduced size that is easy for an average computer to handle. moreover, a 3d model using smaller files would increase the space available for study materials for those who intend to deepen their knowledge of the artefact. finally, a model with these characteristics would be very useful, practical and innovative for educational and dissemination purposes. for example, it would be easily accessible on websites, apps or devices, making 3d exploration available to the general public. 6. conclusions the results obtained with two different technologies were compared for 3d shape accuracy, texture quality, digitisation and processing time and finally price. the results show that the structured light scanner provided the best results with regard to geometric structures. one of the strengths of photogrammetry is the speed with which information can be acquired and the possibility of using simple 2d photographs (which, however, must be taken in the best way, otherwise geometric information can be lost and the quality of the model will suffer). only software is required to start the elaboration process. a high-performance pc speeds up the process, but, in the case of more complex images, it is possible in photoscan to elaborate individual sections and then merge them to obtain a single 3d model. photogrammetry offers a very good balance in terms of portability, costs and quality. photogrammetric models can be figure 10. unrolling of the backrest on the base of the corsini throne. figure 11. detail of the relief on the unwrapped corsini throne. figure 12. orientation of the two models in meshlab using the manipulator tool. figure 13. detail of the high-resolution 3d model. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 45 successfully used in the divulgation and valorisation of artworks. when integrated within an app or website, they provide a different and more complete level of knowledge than traditional 2d images. due to the ict enea infrastructure, it was possible to upgrade 3dhop to include the new features described above. these make 3dhop a more interoperable tool, enabling the creation of a more complex microservices architecture to integrate the available tools with other services developed in the same way. this could lead to the creation of more resilient tools for cultural heritage assets managers. furthermore, the unrolled throne could be useful not only for a better visualisation of the figures but also for classification, as researchers and/or automated segmentation recognition algorithms could work from the information provided in the model. acknowledgement a special thanks to the staff of corsini gallery and civita association for their institutional support. references [1] m. mongelli, g. chellini, s. migliori, a. perozziello, s. pierattini, m. puccini, a. cosma, photogrammetry and structured light: comparison and integration of techniques in survey of the corsini throne at corsini gallery in rome, proc. of international conference of metrology for archaeology and cultural heritage (metroarcheo2019), florence, italy, 4 – 6 december 2019, pp. 166-171. online [accessed 12 march 2021] https://www.imeko.org/publications/tc4-archaeo2019/imeko-tc4-metroarchaeo-2019-31.pdf [2] online [accessed 12 march 2021] http://ecodigit.dtclazio.it/ [3] m. torelli, la sedia corsini, monumento della genealogia etrusca dei plautii, in: mélanges pierre lévêques, vol. 5, anthropologie et société. m. m. mactoux, e. geny (editors). université de franchecomté, besançon, 1991, isbn: 2-251-60429-9, pp. 355-367. online [accessed 12 march 2021] https://www.persee.fr/doc/ista_00000000_1990_ant_429_1_1305 [4] g. agugiaro, f. 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https://www.persee.fr/doc/ista_0000-0000_1990_ant_429_1_1305 https://www.persee.fr/doc/ista_0000-0000_1990_ant_429_1_1305 https://doi.org/10.5194/isprsarchives-xxxviii-5-w16-23-2011 https://doi.org/10.5194/isprsarchives-xl-5-w4-101-2015 https://doi.org/10.3390/heritage2010050 https://www.imeko.org/publications/tc4-archaeo-2017/imeko-tc4-archaeo-2017-044.pdf https://www.imeko.org/publications/tc4-archaeo-2017/imeko-tc4-archaeo-2017-044.pdf http://ecodigit.dtclazio.it/deliverable/ecodigit_d2_3.pdf http://ecodigit.dtclazio.it/deliverable/d3_4_proof_of_concept.pdf http://ecodigit.dtclazio.it/deliverable/d3_4_proof_of_concept.pdf http://ecodigit.dtclazio.it/deliverable/ecodigit_d4_3.pdf https://www.enea.it/it/seguici/pubblicazioni/pdf-volumi/v2015-cresco-2014.pdf https://www.enea.it/it/seguici/pubblicazioni/pdf-volumi/v2015-cresco-2014.pdf acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 46 [12] f. iannone, i. bellagamba, g. bracco, b. calosso, g. giovanetti, s. migliori, m. 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reconstruction for the sphinxes frieze at the trajan’s forum in rome, proc. of international conference of metrology for archaeology and cultural heritage (metroarcheo2018), cassino, italy, 22 – 24 october 2018, ieee catalogue number: cfp18o73-usb. https://www.academia.edu/39007806/structure_from_motion_sfm_technique_in_the_catacombs_of_priscilla_in_rome https://www.academia.edu/39007806/structure_from_motion_sfm_technique_in_the_catacombs_of_priscilla_in_rome https://www.academia.edu/4440499/unwrapping_highly_detailed_3d_meshes_of_rotationally_symmetric_man_made_objects https://www.academia.edu/4440499/unwrapping_highly_detailed_3d_meshes_of_rotationally_symmetric_man_made_objects https://www.academia.edu/4440499/unwrapping_highly_detailed_3d_meshes_of_rotationally_symmetric_man_made_objects a monitoring system based on phasor measurement units with variable reporting rates acta imeko issn: 2221-870x december 2018, volume 7, number 4, 62 69 acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 62 a monitoring system based on phasor measurement units with variable reporting rates paolo castello1, carlo muscas1, paolo attilio pegoraro1, sara sulis1 1 department of electrical and electronic engineering, university of cagliari, cagliari, italy section: research paper keywords: pmu; pdc; synchrophasor; cloud; wams; variable reporting rate citation: paolo castello, carlo muscas, paolo attilio pegoraro, sara sulis, a monitoring system based on phasor measurement units with variable reporting rates, acta imeko, vol. 7, no. 4, article 11, december 2018, identifier: imeko-acta-07 (2018)-04-11 editor: alexandru salceanu, "gheorghe asachi" technical university of iasi, romania received april 7, 2018; in final form july 3, 2018; published december 2018 copyright: © 2018 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: paolo castello, e-mail: paolo.castello@diee.unica.it 1. introduction wide-area monitoring systems (wamss) based on synchronized measurements are characterized by a hierarchical architecture composed of phasor measurement units (pmus) and different levels of phasor data concentrators (pdcs) [1]. the pmu is the “sensor” device that measures electrical quantities, such as voltage and current phasors, frequency, and rate of change of frequency (rocof), with an accurate time tag based on coordinated universal time (utc) and usually obtained from a gps receiver or through ieee 1588 (precision time protocol) synchronization. the role of the pdc is to collect and align measurements provided by the pmus and to forward this data to the next higher pdc level and, finally, to the control center [2] that is responsible for the evaluation of the overall state of the electric grid. the wams, based on synchronized measurements, was originally designed for transmission systems; however, since the introduction of the smart grid framework, the benefits of synchrophasor technology are moving also to distribution networks [3]–[7]. the use of pmus in the distribution network context represents a new challenge: standalone pmus and pdcs could be replaced by dedicated functionalities implemented in intelligent electronic devices (ieds) [8] or by existing measurement devices upgraded in order to build an internet of things (iot) network with synchrophasor functionality [9]. in a synchrophasor system suitable for distribution grids, several measurement devices are necessary, and in this new scenario, a classical hierarchical architecture might be inadequate, as it might be unable to manage many pmus and/or pmu-enabled instruments. one solution could be to replace the hierarchical structure of pdcs with a less expensive and rapidly scalable structure based on cloud computing technology. normally, in a wams transmission system, pmus send data at a constant rate of 50–60 frames per second (fps) to guarantee the monitoring of dynamic events [10]. however, the communication systems used by distribution system operators abstract classical wide-area monitoring systems (wamss) are characterized by a hierarchical architecture, which is composed of phasor measurement units (pmus) and different levels of phasor data concentrators (pdcs). the wams, based on synchrophasor technology, was originally designed for transmission systems; nevertheless, with the development of the smart grid paradigm, the benefits of this technology are being extended to distribution networks. normally, pmus send measurement data at a high and constant reporting rate to guarantee the monitoring of dynamic events in an electric transmission network. however, typical communication systems that are expected to be used by distribution system operators are generally shared and/or public. in this case, the bandwidth available among pmus and pdcs, or among the pdcs and the control center, is strictly dependent on the type of communication channel that is used and on the level of network traffic. in this context, a new transmission logic for the transfer of data between pmus and pdcs, based on the knowledge of the ongoing conditions of the electric grid, can be implemented. the strategy proposed in this paper is to increase the measurement reporting rate only when the electric system changes from a steady-state condition to a dynamic and potentially unsafe one, without modifying the overall accuracy of the pmu measurement process. the risk of losing important information related to a dynamic event is mitigated by sending to the pdc (after the event detection) measurements relating to the pre-trigger time interval in a burst of data packets that allow insight into the signal evolution. mailto:paolo.castello@diee.unica.it acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 63 (dsos) are expected to be shared and/or public [11] (alternative solutions based on a private cloud with a private communication channel may be too expensive for dsos). in this case, the bandwidth available for the devices involved is strictly dependent on the type of communication channel that is adopted [12] in this context, the present paper proposes a new logic aiming to manage the dynamic change in the reporting rate (rr) of a pmu. the proposal is based on the outcomes of [13], in which the concept of a pmu adapting its rr in order to increase it only when the electric system changes from a steady state to a dynamic and potentially unsafe condition was firstly presented. the pmu is intended to store all the measurement data at the maximum rate inside the pmu and to send, after a change detection, pretrigger information to upper levels. in this paper, a real architecture that can operate in tandem with commercial devices to manage the above paradigm is introduced. the risk of losing important information concerning the beginning of a dynamic event due to a low steady-state rr is mitigated by sending the information relating to the pre-trigger period to a standard compliant pdc as soon as possible with a suitable policy. the proposed architecture comprises a real pmu prototype that is able to send data with a variable rr without affecting the accuracy of the measurement process. the pdc responsible for receiving the data is a conventional open-source software pdc with a specific setup that is able to manage the variable data streams from different pmus. in this way, it is not necessary to implement a specific device to receive variable rates in the measurement architecture, i.e., the pmu prototype can operate in a pre-existing synchrophasor measurement architecture. to the knowledge of the authors, this proposal is the first example of a pmu designed with an adaptive functionality for the transmission of measurement data. the policies are tested under simulation and the whole architecture is experimentally validated in a laboratory setup. 2. the proposed architecture figure 1 shows the typical hierarchical architecture of a wams based on the synchrophasor technology described in the ieee standard c37.118.2-2011 [2], with the pmus acting as “sensor elements” of the monitored network at the bottom and two levels of pdcs (typically referred to as “local” and “corporate” concentrators) above. in this case, the pmus send their measurements over the communication network in data packets in compliance with the standard protocol [2] by reporting up to 50 fps for a system frequency of 50 hz. it is also possible to find commercial pmus with rrs higher than those required by the standard. at each level, the pdc builds a single output data stream from the time-aligned input streams. the synchrophasor data latency at pdc output and, usually, the time for processing incoming data increase in size following each step in the pdc hierarchy. the number of pmus required for the electric distribution network is expected to be higher with respect to the transmission grid, due to the large number of nodes that must be monitored. consequently, the number and size of data streams could become difficult to manage. figure 2 illustrates the architecture proposed for a distribution network’s synchrophasor system. the communication channel is shared and/or public. in figure 2, there is a single level of the pdc that acts as a gateway between the devices in the field and the cloud, where the measurements are processed depending on the application (state estimation for real-time control, but also offline applications as post-mortem analytical tools [14]). the pmu continuously produces and stores the data internally with the maximum rr available (e.g., h_rr = 50 fps) but, in the presence of a steady-state condition, it sends the data to the pdc with a lower rr (e.g., l_rr = 1 fps). it is important to recall that in the design of a pmu compliant with [1], the value of rr determines not only the number of measurements per second that is sent to the pdc, but also, for each compliance class and each nominal system frequency, the measurement performance of the device under specific test conditions. thus, in the proposed architecture, where the actual rr of the pmu is constant (and the high-rate measurements are continuously stored in a circular buffer of suitable capacity), the metrological characteristics of the device do not change with the output rr, which, on the contrary, can be modified depending on the operating condition of the electric grid. in the presence of a dynamic event detected in the input signals (e.g., a given threshold is exceeded), the pmu can activate the highest output rr and send the pre-trigger data to the pdc, flushing the buffer [13]. 2.1. data handling with variable reporting rate devices based on synchrophasor technology must handle measurements at least at the mandatory reporting rates of 10, 25, and 50 fps (for a system frequency of 50 hz). however, other reporting rates are permissible, i.e., lower and higher than the mandatory values. thus, in this scenario, an rr of 1 fps indicates the lowest rate considered. figure 3 shows different policies that can be adopted by a pmu to send the measurement data, and an event occurring at t = 1 s. case 0 represents the standard policy with a fixed rr. in particular, it represents an rr equal to 1 fps where every measurement is time tagged at the occurrence of the utc second. figure 1. diagram of the wams hierarchy based on pmus and different levels of pdcs with a fixed and constant reporting rate of 50 fps. figure 2. proposed architecture based on flexible pmus and a single level of pdcs, exploiting different (high and low) reporting rates. acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 64 case 1 adopts a dynamic transmission policy based on [9] where, at the occurrence of an event of interest (t = 1 s), the rr changes from the lowest rate (1 fps between 0 s and 1 s) to a higher rate (10 fps after t = 1 s). the rr of 10 fps is the lowest rr subject to the dynamic requirements of the standard [1] and, thus, can be considered as suitable to follow varying electrical signals. case 2 shows the policy adopted in this paper to also send the measurement data relating to the pre-event time interval [13]. the pmu sends the data with the lowest rr until a significant variation is detected in the monitored quantities (t = 1 s). after the detection, the pmu starts sending the stored non-real-time measurements relating to the pre-trigger period. the pre-trigger measurements are sent within a burst of data packets (nine packets in this case, corresponding to the nine non-transmitted measurements of the pre-event interval of 1 s) at 100 fps (figure 3, case 2: orange blocks between t = 1 s and t = 1.1 s). figure 3 shows a zoom of case 2 and represents the details of the transmission of measurements between 1 s and 1.1 s. after sending such data packets, the pmu returns to the real-time data transmission, but with an increased rr of 10 fps, chosen to better follow the detected dynamics of the ongoing event. it is important to highlight that even if the pre-trigger data is sent with a rr of 100 fps, the measurements are always relative to a data evaluation every 100 ms. indeed, the timestamps of the measurements are 100 ms from each other. this solution is a trade-off between continuously following the signal in real time and recovering pre-trigger data offline. in fact, it allows the pmu to send the pre-event data before the occurrence of a new realtime data packet and before the pdc realigns all the measurement data in a timely manner. figure 4 shows the time alignment operation of the pdc responsible for receiving the variable rr. the input data received from the pdc is not timealigned, and the time between the packets is not constant. the pdc of a classic wams expects to receive data at regular time intervals, and considering the latency characteristics of the network channel, after a given waiting time, it assumes the missing measurements are lost [15]. for this reason, in the proposed architecture, the waiting time is exploited in order to manage incoming data streams at different reporting rates without any ad hoc implementation. equation (1) shows the different latency contributions used to set the correct values of the waiting time: 𝑇𝑤𝑎𝑖𝑡 = 𝑇𝑞 + 𝑀𝑎𝑥 𝑃𝑀𝑈𝐿 + 𝑇𝑁𝐿 . (1) the pdc is set to wait the incoming data for a time that is equal to the maximum time allowed in the buffer implemented in the pmu (𝑇𝑞 corresponding to the pre-trigger data window), a time depending on the maximum measurement process of pmus (𝑀𝑎𝑥 𝑃𝑀𝑈𝐿 ), and the latency between the pmu prototype and the pdc (𝑇𝑁𝐿 , corresponding to network latency). for 𝑀𝑎𝑥 𝑃𝑀𝑈𝐿 , the value of the pmu reporting latency suggested in the synchrophasor standard for a rr of 10 fps and for the m class configuration is considered: 𝑀𝑎𝑥 𝑃𝑀𝑈𝐿 = 7/𝑅𝑅 (2) in this way, the data for a pre-trigger event is considered simply as time-delayed data packets from a pdc point of view. it is important to highlight that in order to allow the operation of the pdc, in ieee standard c37.118.2, a new configuration frame can be sent when a pmu changes the format of the data stream. the rr is specified in the reporting rates information and every change in the parameters of the pmu would require a new configuration frame before the change occurs. in this paper, in order to save the communication bandwidth and always considering the format of data frames as constant, the rate value in the configuration frames is set as the highest used. in this way, even if the actual rr is lower, the pdc considers all the missing data as delayed and as missing (at the end of the waiting time), without any loss of generality. to implement the variable rr policy, the architecture of a pmu needs to be enhanced with other technical solutions. figure 5 is a schematic representation of the proposed pmu prototype. the device, which uses the same measurement algorithm as [5], acquires the relevant signals by synchronizing the acquisition process in order to time tag every signal sample. the detection of the dynamic event is implemented as a user-selected threshold depending on the values of interest. in particular, considering the scope of this work, the monitored variable is the frequency deviation from the nominal value. when a dynamic condition is figure 3. different measurement reporting policies with a more detailed representation of the case with pre-trigger data. figure 4. pdc input and output data with the time-aligned operation concerning the pre-trigger data. figure 5. schematic representation of the pmu prototype. acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 65 detected, the highest output rr is activated, which also maintains high values when the monitored quantity returns under the threshold level for a programmable time so that bounces around the threshold can be avoided. a central element of the architecture is the circular buffer, where the measurements are stored at the maximum internal rr of 50 fps, maintaining the accuracy of the measurement, which is characteristic of this rr. 3. test setup in the following simulation tests and real scenarios obtained by means of software modules implemented in the labview environment, the performance of the proposed monitoring system in a dynamic event is shown. as a representative example of possible data management policies, the detection of a significant frequency deviation from the nominal value of 50 hz is addressed. to save a significant portion of the monitored phasors, the pmu is equipped with a circular buffer, which can host 1 s of data. first, simulation test cases will be reported using rrs of 1 fps and 50 fps for the analysis. then, in the real scenario, the measurement rrs configured in the prototype for the tests are 1 fps and 10 fps, while, as described above, a packet rate of 100 fps can be used in the transmission module. 3.1. simulation tests communication between the pmu and the pdc functionality is developed in the labview environment by means of independent virtual instruments. in the following steps, the pretrigger data is sent in a single data packet following the event detection. this technique requires a specific pdc functionality that is able to manage single data packets with the stored pretrigger information and the associated timestamps. the aim of the following tests is to illustrate the impact of the policies on the event reconstruction. 3.1.1. phase modulation test case the synchrophasor standard [1] suggests the use of a sinusoidal phase modulation signal to test the bandwidth of a pmu using different modulation frequencies and a modulation level of 𝑘𝑎 = 0.1 rad. in such case, the maximum absolute variation from the nominal system frequency is: 𝛥𝑓𝑚𝑎𝑥 = 𝑘𝑎 ∙ 𝑓𝑚, (3) where 𝑓𝑚 is the adopted modulation frequency. the highest 𝑓𝑚 suggested by the standard is 5 hz, which corresponds to 𝛥𝑓𝑚𝑎𝑥 = 0.5 hz. on the contrary, considering a constant 𝑓𝑚, the deviation from the nominal system frequency is proportional to the modulation level 𝑘𝑎, and 𝛥𝑓𝑚𝑎𝑥 varies with the modulation level. it is thus possible to impose a threshold for dynamic condition detection on the maximum frequency deviation. in the simulation tests, 𝛥𝑓 = 0.4 hz is used to discriminate a strong dynamic condition, and upper and lower frequency thresholds are defined as follows: 𝑓𝑡ℎ𝑟𝑒𝑠ℎ𝑜𝑙𝑑 = 𝑓𝑜 ± 𝛥𝑓 . (4) figure 6 shows the frequency of the adopted test signal as a function of time, where 𝑘𝑎 changes with time. at the beginning, the test signal is in a steady-state condition at the nominal frequency (𝑘𝑎 = 0, no modulation). after one second has elapsed, the test signal is phase modulated with a modulation frequency of 5 hz and two different levels of modulation factor. between 1 s and 2.2 s, the modulation factor is 𝑘𝑎 = 0.05 rad, which leads to a maximum frequency deviation that is equal to 𝛥𝑓𝑚𝑎𝑥 = 0.25 hz. after 2.2 s, a stronger dynamic condition occurs when 𝑘𝑎 becomes 0.1 rad. in this case, the maximum deviation from the nominal system frequency is 𝛥𝑓𝑚𝑎𝑥 = 0.5 hz, i.e., above the fixed threshold. as recalled above, in order to avoid continuous changes between high and low reporting rates, the highest value is maintained for 𝑇ℎ𝑜𝑙𝑑 seconds (10 s for all the tests) after the monitored parameter returns below the threshold. the aim is to light up the ongoing phenomenon and trigger a more accurate observation of its evolution. 3.1.2. frequency ramp test case a second simulated scenario is represented by a variation of the frequency, with different values of rocof. the frequency trend shown in figure 7 comprises: • a steady-state condition at the nominal system frequency between 0 s and 1 s, • a positive frequency ramp at a rate of 0.5 hz/s from 1 s to 2.2 s, and • a frequency ramp with negative rocof (−0.6 hz/s) from 2.2 s to the end of the simulation. during these events, the values of the frequency are explained as: 𝑓(𝑡) = 𝑓𝑠𝑡𝑎𝑟𝑡 + 𝑅𝑓 ∙ (𝑡 − 𝑡𝑠𝑡𝑎𝑟𝑡 ), (5) where 𝑓𝑠𝑡𝑎𝑟𝑡 and 𝑅𝑓 are the signal frequency at the ramp starting time 𝑡𝑠𝑡𝑎𝑟𝑡 and the frequency ramp rocof, respectively. to simulate a real signal acquired from the power system, a uniform white noise with a snr level of 50 db is added to the test signal. using the same threshold value applied in the dynamic conditions of the phase modulation test, the signal reaches the threshold level after 𝑡 = 1.78 s. this will be discussed in detail in the next section. figure 6. frequency of the test signal used for validating the proposed method based on the detection of the off-nominal frequency. figure 7. frequency variation with a positive and negative linear ramp with different values of rocof. acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 66 3.2. architecture of the real test scenario figure 8 illustrates the test setup that is used to verify the feasibility of the proposed approach in a real context. the calibrator omicron cmc 256plus provides accurate voltage signals of amplitude equal to 10 v. the generator is programmed to reproduce an event represented by a waveform with different levels of phase modulation (different 𝑘𝑎) and a common frequency modulation 𝑓𝑚 of 1.7 hz chosen in order to emphasize the behavior of the monitored frequency (figure 9). the signal generator is programmed to reproduce the waveform every minute, in a time-synchronized manner, so that the performance of the system prototype can be evaluated continually. the frequency trend shown in figure 9 comprises: • a steady-state condition at the nominal system frequency between 0 s and 10 s and between 50 s and 59 s, • a modulation factor of 𝑘𝑎 = 0.05 rad from 10 s to 20 s and from 40 s to 50 s, and • a modulation factor of 𝑘𝑎 = 0.1 rad from 10 s to 20 s and from 40 s to 50 s. the signal generator provides the test signal to the prototype implemented in a compact reconfigurable input and output (compactrio) device manufactured by national instruments, which adopts a real-time controller and a field programmable gate array (fpga). the compactrio is composed of three main modules, used to implement the prototype: 1. the i/o modules provide the signals of interest for the overall system. in this prototype, the ni-9215 data acquisition module, a 16-bit, ±10 v, 100 ksa/s simultaneous channel, is used to acquire the voltage from the signal generator. moreover, a gps receiver that is often used in time‐keeping applications due to its availability, low price, and high accuracy [16] is considered. the ni-9467 synchronization module is used to receive the accurate time reference provided by the utc to synchronize the overall system, with a time accuracy of ±100 ns. 2. the i/o fpga, in this case a ni 9113, is configured to manage the data acquired from the i/o modules and to maintain the relationship with the acquired signals and the absolute time reference. 3. the real-time controller, in this case a ni 9024, is responsible for computing the values of interest, verifying the level of the monitored quantities, storing the data in the buffer, and managing communication. the pmu prototype sends the data to a workstation with pdc functionality provided with the software openpdc from the grid protection alliance [17]. the openpdc software can manage the data provided by pmus in the field, and it is used in different scientific applications to enhance proposed functionalities, thanks to its open-source characteristic [18]-[20]. in this work, the openpdc receives data from two pmus: the proposed prototype with the variable rr and a commercial pmu set to the fixed rr of 1 fps. the commercial pmu is not connected to a specific signal source, because its role is only to send the synchronized data (which is continuously measured) and to test the correct process of the pdc, in terms of data collection and time alignment in the presence of an incoming fixed rr in addition to the variable rr stream. to maintain the accuracy of the data, the pdc is set to wait 𝑇𝑤𝑎𝑖𝑡 = 2 𝑠, considering the maximum dimension of the internal buffer of the pmu prototype (𝑇𝑞 = 1 𝑠), the pmu latency for the pmu configuration that is being tested (𝑀𝑎𝑥 𝑃𝑀𝑈𝐿 = 0.7 𝑠), and a reasonable value of network latency (𝑇𝑁𝐿 = 0.3 𝑠). in this way, the pdc can align the pretrigger data concerning the detected event correctly. this solution increases the overall communication latency of the system because the pdc waits for two seconds before forwarding the data to the cloud. nevertheless, this solution also allows for the use of the proposed transmission strategy in a preexiting measurement system. it is important to recall that the aim of this proposal is to save communication bandwidth by reducing, when unnecessary, the amount of data that is sent to the control center. in the proposed architecture, it is also possible to set the specific waiting time on a per-pdc basis, depending on the area that is being monitored and on the available devices. 4. test results 4.1. simulation results 4.1.1. phase modulation test case figure 10, figure 11, and figure 12 plot the frequency measurements received by the pdc relating to the test signals described in the previous section. the three different cases corresponding to the three different rr policies illustrated above are: • case 0: the frequency measurements sent by the pmu with a constant rr=1 fps, thus giving four measurements. • case 1: the frequency measurements sent with a step change in the rr around the event detection without pre-trigger data transmission (similar to [9]). • case 2: frequency measurements received by the pdc when the 1 s pre-trigger data is sent along with the variable rr. figure 8. representation of the test setup for the real scenario. figure 9. one minute of the frequency trend of the test signal produced by the signal generator. acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 67 in case 0, the measurements are received every second, and as also explained in the synchrophasor standard, the rr is insufficient in terms of its ability to follow the sinusoidal variation of the frequency (it is far below the nyquist frequency of the occurring dynamics). case 1 represents an adaptive rr, and the highest rr (50 fps) starts when the value of the frequency exceeds the imposed threshold (t = 2.22 s). in this case, it is possible to follow the dynamic event as it occurs, but it is not possible to understand how the frequency evolved to such a critical level that distinguishes the steady state from the dynamic condition. case 2 covers this gap, as the measurements relating to the pre-trigger period are sent to the pdc in the first packet following the event. the pdc aligns the data to show the evolution of the frequency before the event occurred. 4.1.2. frequency ramp test case the second simulation test scenario presents a steady-state signal at the system frequency combined with two linear frequency ramps with different levels of rocof, as described in section 3.1.2. figure 13, figure 14, and figure 15 show the measurement data received by the pdc from the pmu in accordance with the aforementioned three different rr policies. figure 13 represents the frequency monitored for 4 s, with the lowest rr considered (case 0). it is important to highlight that for rr lower than 10 fps, pmus are not subject to the dynamic requirements. therefore, their configuration and measurement algorithm can be very different from those that are suitable for following the signal changes, as is the case in linear frequency ramp conditions. the trend of the frequency measurements received by the pdc from a pmu with a variable rr (case 1) is shown in figure 14. to save communication bandwidth, the pmu can adapt between two rrs: 1 fps when the frequency is under the threshold and 50 fps after the threshold has been exceeded (t = 1.78 s). the policy of a variable rr highlights the dynamic event and saves the communication bandwidth in the synchrophasor system during the steady-state period, but it does not allow the whole trend of the event to be followed before the threshold level is reached. figure 10. case 0: data received from the pdc with a constant rr policy equal to 1 fps. figure 11. case 1: data received from the pdc with a variable rr policy. figure 12. case 2: data realigned by the pdc with the variable rr and pretrigger data. figure 13. case 0: data received from the pdc with a constant rr policy equal to 1 fps. figure 14. case 1: data received from the pdc with a variable rr policy. figure 15. case 2: data realigned by the pdc in case of variable rr and pretrigger data. acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 68 to overcome this issue, a pre-trigger policy should be applied. figure 15 shows the data that is received and aligned by the pdc from the pmu with variable rrs and pre-trigger data. in this case, it is possible to follow the trend of the signal whose frequency has exceeded the threshold. the data stored in the circular buffer covers 1 s at rr = 50 fps before the trigger event, which causes the highest rr to be activated (t = 0.8 s). such a buffer size allows a relevant retrospective interval while avoiding a backup packet transmission that is too large. moreover, it is interesting that in both case 1 and case 2, during the negative ramp period, the dynamics are followed also below the threshold limit, thanks to the holding period introduced to avoid the jerky evolution of the rr. 4.2. real scenario results the tests on the real scenario focus on the capability of the monitoring architecture to handle the variable rr and the retrospective data provided by the pmu prototype. the pdc collects and aligns the input data from the two pmus with different rates without receiving the configuration frames before the rate variation. this behavior is permitted by the pdc, considering all the data as delayed or missing depending on the elapsed time. figure 16 shows the graphic interface of openpdc manager, which was used to configure the input and output data streams managing options. the figure shows the synchronized data streams of the pmu prototype (pmu vr, variable rate) and of the commercial pmu (pmu b). in figure 16, the beginning and end of the frequency evaluation corresponding to the oneminute input signal loop are visible. the output data of the pdc is shown in figure 17, figure 18, and figure 19, which corresponds to the data collected from the workstation running the pdc software. figure 17 shows the pdc output data relating to the received stream of the pmu prototype for a complete loop of the test signal. the monitoring activity can be divided into two categories: the first, from 0 s to 19 s and from 48 s to 59 s, is characterized by a low monitoring rate, while the second, from 19 s to 48 s, is characterized by a higher reporting rate. in figure 18, the data handling operation of the pdc is represented in more detail. the frequency value highlighted by the red circle is the first frequency value overcoming a threshold limit of 50.15 hz (𝛥𝑓 = 0.15 hz), taken from [21] as a value used as trigger condition for the monitoring of frequency events. the pdc output appears to change the rr from 1 fps to 10 fps before the detection instant, in the highlighted area between 19 s and 20 s. this behavior is the consequence of the data alignment operation of the pdc, also performed on the pre-trigger measurements considering the correct time stamps. finally, figure 19 reports the frequency of the generated signal loop, monitored from 35 s to 59 s. the figure shows the change of rate, from 10 fps to 1 fps, ten seconds after the actual detection of the last frequency value over the imposed threshold, as the result of the hold-on policy. the rate of 10 fps allows for the decrease in the deviation from the nominal frequency up to the extinguishment of the electrical phenomenon to be followed. conclusion to bring the benefits of synchronized measurements from the transmission to the distribution level, the classic architecture of the wide area monitoring system needs to be adapted to the new context. in this proposal, a new data handling method for a synchrophasor system based on variable rr and pre-trigger data transmission has been illustrated using simulation and real test cases. to efficiently manage data transmission and to save bandwidth in the case of shared and/or public communication channels, the rr of the pmu is changed using a selectable threshold, driven by the dynamic events that are possible in the figure 16. the graphic interface of open pdc manager: pmu prototype (pmu vr) and commercial pmu (pmu b) are listed. figure 17. data realigned by the pdc in the case of variable rr and pre-trigger data. figure 18. data realigned by the pdc in the case of change of reporting rates and pre-trigger data. figure 19. detailed trend of data realigned by the pdc relating to the last part of the monitored frequency. acta imeko | www.imeko.org december 2018 | volume 7 | number 4 | 69 electric distribution systems. the possibility of losing significant information about the beginning of a dynamic event is mitigated by sending the measurements relating to the pre-trigger interval to the pdc so that the pdc can realign them and report a useful measurement context for different applications. examples of the application of the proposed solution have been discussed, thus proving how the use of such adaptive policies can help save bandwidth while preserving important information when dynamic events occur. policies that can follow both very fast events and slower evolutions have been defined. the pre-trigger data is always useful to complete the available information, because it is possible to analyze the event origin or early behavior without the risks of oversensitivity and thus being affected by negligible fluctuations. the pre-existent measurement systems, including commercial pmus, can easily adopt the proposed solution; thus, variable rrs can also be managed in the presence of hybrid infrastructures. acknowledgement the research activities described in this paper have been conducted in the context of the research and development project ‘cagliari2020’, partially funded by the italian university and research ministry (grant# miur_pon04a2_00381). references [1] ieee, “ieee standard for synchrophasor 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[21] z. w. li, o. samuelsson, r. garcia-valle, “frequency deviations and generation scheduling in the nordic system”, proc. of ieee trondheim powertech, june 19-23, 2011, trondheim, norway. https://www.nerc.com/comm/pc_reliability_guidelines_dl/reliability%20guideline%20-%20pmu%20placement.pdf https://www.nerc.com/comm/pc_reliability_guidelines_dl/reliability%20guideline%20-%20pmu%20placement.pdf https://cordis.europa.eu/project/rcn/106390/factsheet/en https://github.com/gridprotectionalliance/openpdc hierarchical data fusion architecture for autonomous systems acta imeko issn: 2221-870x december 2019, volume 8, number 4, 28 32 acta imeko | www.imeko.org december 2019 | volume 8 | number 4 | 28 hierarchical data fusion architecture for autonomous systems ivan ermolov1 1 ishlinsky institute for problems in mechanics of the russian academy of sciences, russia section: research paper keywords: data fusion; sensor fusion; unmanned vehicles; autonomous systems citation: ivan leonidovich ermolov, hierarchical data fusion architecture for autonomous systems, acta imeko, vol. 8, no. 4, article 6, december 2019, identifier: imeko-acta-08 (2019)-04-06 editor: yvan baudoin, international cbrne institute, belgium received november 23, 2018; in final form september 12, 2019; published december 2019 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was supported by russian ministry of science and higher education (project reg. no. aaaaа17-117021310384-9) corresponding author: ivan l. ermolov, e-mail: ermolov@ipmnet.ru 1. introduction autonomy is a feature that modern robotic systems severely lack. in robotics, autonomy can be understood as the ability of a robot to function for an extended period of time, over broad spaces, and in unpredictable environments without the need to collaborate with friendly objects or human operators. in [2], the topic of robotic autonomy was discussed. the need for an autonomous system is urgent for the following reasons: autonomous systems, as a rule, are more efficient than human-operated systems, the expansion of the application areas of robotic systems, the abandonment of involvement of high-skilled human operators, the possibility of using robots in groups [11], [12], wide usage of optimisation and complex algorithms, the abandonment of redundant components from robotic systems, minimisation of the mass and size of robots, decreasing time delays in robots' functioning, and predictability of robots' behaviour. it is postulated herein that in order to implement the autonomy of unmanned vehicles, it is efficient to increase their information supply and intelligence level. both of these elements are connected with data processing coming from sensors of the robot and the environment. due to the specifics of the sensors used in robots, it is necessary to use data fusion. the importance of data fusion is emerging for mobile systems in particular. 2. data fusion in robotics data fusion in itself has been widely known for centuries. humans widely use data fusion (fusion of the five senses, distribution and doubling of sensors, etc.). much interesting research has been undertaken on data fusion, especially in robotics [9], [13]. abstract autonomy is becoming a key issue concerning unmanned vehicles nowadays. an effective functioning of an unmanned system requests the processing of a large amount of data coming from sensors, onboard databases, etc. therefore, data fusion technology is a key technology used in autonomous systems. in order to systemise such data processing in autonomous systems, special so-called data fusion architectures are used (e.g. jdl, waterfall, boyd). however, some of those solutions have many restrictions. a goal of this study is to present a novel hierarchical data fusion architecture that can be used in autonomous systems. this architecture consists of five basic layers: identification of parameters, state identification, object type identification, situation identification, and task implementation identification. the proposed architecture has some advantages in comparison to those that are already in use. the author considers that the presented architecture has good visibility; intuitive understanding; the possibility of deep feedback usage; and good potential for automatic reconfiguration and self-learning. the developed data fusion architecture can be used for building complex data fusion systems on board of autonomous systems, of a group of unmanned vehicles, and even of systems of a higher hierarchy. mailto:ermolov@ipmnet.ru acta imeko | www.imeko.org december 2019 | volume 8 | number 4 | 29 data fusion [14], [15], [17] in itself can be defined as a process of information generalisation based on more than one source of information. it is important to note the following advantages of data fusion for robotists: it is cheaper to produce new information by developing software than by installing extra sensors, considerable energy savings because information processing requires less energy than the sensor hardware, minimisation of the autonomous systems’ mass and sizes, which is especially crucial for autonomous systems working in severe environments [3], decrease of wires and other interfaces, and thus, higher reliability, decrease of negative interference among autonomous systems’ components, the possibility of using lower performance (i.e. cheaper) sensors with a higher quality of information therefrom, compensation for the restricted working space and spectrum of sensors, transfer of the computational load from the human operator to the onboard control system, increased unification by the use of the same set of sensors for various functional tasks. the author’s proposal is outlined in figure 1 to distinguish the following cases of data fusion in robotics: time-based data fusion. while tracking the variation of some parameters in time sequences, it is possible to estimate other parameters of the autonomous system. another case of such data fusion is data filtering. for more details see [4]. reliability-based data fusion. while fusing data from several sensors with low-reliability characteristics, it is possible to get highly reliable information. space-based data fusion. by fusing information from several sensors, each one with a narrow working space (example in figure 2), we may get information on the large working space. another case of space-based data fusion is fusing data from dispensed sensors, which gives us new information. sensor-type data fusion. fusion of data from sensors measuring the same parameter but functioning on various principles gives information with a higher reliability factor (figure 3). time-based data fusion location-based data fusion sensor operating principal-based data fusion type of data-based data fusion reliability-based data fusion data fusion cases x(ti) x(tf)x(ti+1) vi(ti+1) vm v distance to an object acoustic emission level acoustic emission of object object’s image transition object’s velocity object’s color type of object: tractor figure 1. basic cases of data fusion in robotics. figure 2. space-based data fusion (fusion of ultrasonic data using the pioneer p3dx platform by adept mobilerobots). figure 3. navigation data from various types of sensors is fused directly onboard the 120/3 navigation system from perm instruments making company. acta imeko | www.imeko.org december 2019 | volume 8 | number 4 | 30 data-type data fusion. this type is used to produce information based on data of various types (e.g. fusing information from a video sensor with information from a laser sensor). such fusion is intensively used, especially for object recognition [7]. in sum, most of the data fusion cases in unmanned systems can be described by these fusion cases or by a combination thereof. 3. hierarchical data fusion architecture modern autonomous systems are extremely complex, with a large number of data flows [5]. in order to analyse the large number of data flows, it is necessary to adjust those data flows. this is known as data fusion architectures. there is a variety of such architectures: jdl, boyd model, laas, the omnibus model, the waterfall model, and more [4], [6], [16]. by the way, these architectures are also used for other applications (see e.g. [18], [19]). however, according to the author, these architectures lack some intuitiveness, logic, and flexibility. the jdl architecture has some restrictions [6]: most of the models were organised based on some specific data or information, e.g. it is difficult to combine a jdl model for other applications, the outlook of the model seems to be rather abstract, which creates an obstacle to its interpretation, this architecture does not correlate with some specific data processing algorithms, which impedes its implementation in real systems. concerning data fusion architecture that is mostly suitable for unmanned vehicles, one may formulate the following criteria for such architecture: data fusion architecture should have a hierarchical structure, as only a hierarchy allows for the control of largescale systems, data fusion architecture should demonstrate all processes in their hierarchy, be easily understandable, and even be intuitive for its user, such architecture should allow various feedback and counter-current data flows, in some cases, the architecture should permit data transfer, omitting some hierarchical layers. the author proposes the hierarchical data fusion architecture shown in figure 4. the layers' terms are as follows [8]: parameter – the predicate that describes, as a rule, the quantitative property of a single component of an autonomous system or of the environment. the input for this level usually comes directly from the sensors. mathematically, the parameter can be described as follows:  == += y k k pp ki z j j gp jii pagap 11 , (1) where ip – i-th parameter; gp jia – the coefficient considering the influence of the j-th sensor on the i-th parameter; gj – input from the j-th sensor; z – number of sensors in the system; pp kia – the coefficient considering the influence of the k-th parameter on the i-th parameter (in k=i it is predicate’s ‘inertia’); kp – the value of the k-th parameter; and y – the number of parameters in system. state – the predicate that describes quantitatively or relatively the property of the whole autonomous system or of the environment. the input for this predicate comes directly from the parameters or from the sensors. mathematically, it can be described as follows:  === ++= w h h os hi x k k ss ki y j j ps jii oasapas 111 , (2) where is – i-th state; ps jia – the coefficient considering the influence of the j-th parameter on the i-th state; ss kia – the coefficient considering the influence of the k-th state on the ith state; ho – h-th instruction; x – the number of states in the system; os hia – the coefficient considering the influence of the hth instruction on the i-th state; and w – the number of instructions in system. object type – the generalised identification of an object present in the environment, defined by its typical data and by its potential interaction with the autonomous system. mathematically, it can be described as follows: , 1 111 m i m i v h u l l cm lih mm hi x k k sm ki y j j pm jii camasapam  +++ ++++=   = === (3) where im – the i-th type of object; pm jia – the coefficient considering the influence of the j-th parameter on the identification of the i-th type of objects; sm kia – the coefficient tasks implementation identification level situations identification level object type identification level state identification level parameters identification level sensors decision making level actuating system motors humanoperator control system of higher level effectors environment data base figure 4. hierarchical data fusion architecture. acta imeko | www.imeko.org december 2019 | volume 8 | number 4 | 31 considering the influence of k-th state on the identification of the i-th type of objects; mm hia – the coefficient considering the influence of various other types of objects presented in the environment to the identification of the i-th object type; v – the number of object type predicates in the system; cm lia – the coefficient considering the influence of the l-th state identification of the i-th object type; lc – the l-th situation predicate; u – the number of situations in the system; m i – information from the database on the trends of the i-th type of objects’ presence; m i – information from tasks on the possibility of the i-th type of objects’ presence. situation – a generalised notion that describes the combination of interactions between the robot and the environment. mathematically, it can be described as follows: , 1 111 c i c i v h u l l cc lih mc hi x k k sc ki y j j pc jii camasapac  ++ ++++=   = === (4) where ic – the i-th situation, pc jia – the coefficient considering the influence of the j-th parameter on the identification of the ith situation; sc kia – the coefficient considering the influence of k-th state on the identification of the i-th situation; mc hia – the coefficient considering the influence of various types of objects presented on the identification of the i-th situation; cc lia – the coefficient considering the influence of the l-th situation on the identification of the i-th situation; c i – information from the database on the trends of the i-th situation; and c i – information from tasks on the possibility of the i-th situation. tasks – a set of situations in serial-parallel order. these situations must be achieved (implemented) by the robot in order for a task to be fulfilled. mathematically, it can be described as follows: t i u h q l l tt lih ct hi x k k st ki y j j pt jii tacasapat ++++=   = === 1 111 , (5) where it – the i-th task implementation; pt jia – the coefficient considering the influence of the j-th parameter on the identification of the i-th task implementation; st kia – the coefficient considering the influence of the k-th state on the identification of the i-th task implementation; ct hia – the coefficient considering the influence of the h-th situation on the identification of the i-th task implementation; tt lia – the coefficient considering the influence of the l-th task implementation on the identification of the i-th task implementation; q – the number of tasks in the system; and t i – information from the database on the trends of the i-th task implementation. the operation of the scheme is now explained. usage of a common data bus is also an important advantage as it allows for the free exchange of data and information among various levels. as explained above, data may flow to the next level and to higher levels directly. this also allows for the human operator to receive information on any predicate directly in a case of need. all the data is fused in a bottom-up direction. however, it is obvious that some simpler or more primitive systems may omit some of the higher levels. however, highly autonomous systems will involve all the levels. when needed, extra data is acquired from the database. the final output comes to the decision-making level, which sends instructions to actuators and sub-systems. it also may be built using the same approach. the proposed hierarchical data fusion architecture has the following advantages compared to other architectures: it reflects the hierarchy of the modern autonomous systems' structure, it has clear demonstrable and visual properties, with a high degree of intuition for the human operator, it allows the wide usage of various data flows and feedback, including transient flows, this structure can be effectively implemented with modern control methods [1], the structure's modularity allows various ready-to-use solutions to be transferred from one scheme to another. it is the author’s opinion that the proposed structure can also be widely used for the automated programming of complex data fusion systems and algorithms. 4. mathematical implementation the question of which mathematical methods are the most appropriate for data fusion has been discussed by many researchers, e.g. [9], [10]. however, as discussed in [20], data fusion deals with various information types and is used for different tasks. hence, different types of data fusion can be supported by different mathematical methods. in [20], a special classification was presented, stating the following: low-level data fusion is better supported by kalman filter, figure of merit, and gating techniques. mid-level data fusion is more efficiently implemented by bayesian decision theory, dempster-schafer evidential reasoning, and neural networks. high-level data fusion solutions are realised by expert systems and fuzzy logic. however, according to the author of this article, it would be better to find some universal method for various levels of data and sensor fusion. this can be done using the fuzzy cognitive maps method [21]. in fact, the fuzzy cognitive maps method has the following advantages: it has good demonstrable and visual properties, with a high degree of intuition for software engineers, the structure allows for the wide usage of various data flows and feedback, including transient flows, it can deal with noisy signals i.e. it has good filtering capacity, the structure's modularity allows various ready-to-use solutions to be transferred from one scheme to another. therefore, these properties almost coincide with the properties of the proposed hierarchical data fusion architecture presented in this paper. in [8], an example is given supporting this statement. acta imeko | www.imeko.org december 2019 | volume 8 | number 4 | 32 5. conclusions data fusion is an important technology implementing the high autonomy of an autonomous system. hierarchical data fusion architecture has good properties of modularity, visualisation, and hierarchy. it allows for the systematisation of data processing in the complex system of modern autonomous systems. acknowledgement this work was supported by the russian ministry of science and higher education (project reg. no. aaaa-а17117021310384-9). references [1] i. m. makarov, v. m. lokhin, intelligent control methods, physmatlit, moscow, 2001. [2] i. l. ermolov, robots' autonomy: its measures and how to increase it, mechantronics, automation, control 8 (2008). [3] a. a. fomichev, v. b. uspensky, k. y. stchastlivets, y. y. broslavets, a. b. koltchev, ‘data fusion in integrated navigation system with laser gyroscopes based on generalized kalman filter’, proc. of the international conference on integrated navigation systems, 26-28 may 2003, st. petersburg, russia. [4] v. i. gorodetsky, o. v. karsaev, v. v. samoylov, ‘data fusion in complex situations analysis and understanding’, proc. of the perspective systems and control tasks conference, 2008, dombai, russia. [5] i. l. ermolov, factors affecting ugvs spacious autonomy level, vestnik yufu 1 (2016). [6] w. elmenreich, sensor fusion in time-triggered systems, technischen universitat wien, wien, im oktober 2002. [7] s. l. zenkevich, a. a. minin, mapping by autonomous system with a lidar basing recurrent filtration method. mechantronics, automation, control 8 (2007). 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[12] s. manko, v. lokhin, m. romanov, ‘from autonomous robots to multiagent robotic systems’, proc. of icmt 2012 – the 16th international conference on mechatronics technology, 2012, tianjin, china, pp. 594-596 [13] w. elmenreich ‘a review on system architectures for sensor fusion applications’, in: software technologies for embedded and ubiquitous systems (lecture notes in computer science vol 4761). r. obermaisser, y. nah, p. puschner, f. j. rammig (editors). springer, berlin, heidelberg, 2007. [14] e. p. blasch, e. bossé, d. a. lambert, high-level information fusion management and system design, artech house publishers, norwood, ma, 2012. [15] m. e. liggins, j. llinas. d. l. hall, handbook of multisensor data fusion: theory and practice, crc press, 2009. [16] f. castanedo, a review of data fusion techniques, the scientific world journal 2013, article id 704504. [17] h. b. mitchell, multi-sensor data fusion – an introduction, springer-verlag, berlin, germany, 2007. [18] d. h. de la iglesia, g. villarrubia, j. f. de paz, j. bajo, multisensor information fusion for optimizing electric bicycle routes using a swarm intelligence algorithm, sensors 17(11) (2017) 2501. doi:10.3390/s17112501 [19] m. m. almasri, k. m. elleithy, ‘data fusion models in wsns: comparison and analysis’, proc. of the 2014 zone 1 conference of the american society for engineering education. [20] d. j. dailey, p. harn, p.-j. lin, its data fusion: final research report, research project t9903, washington state department of transportation, april 1996. [21] c. d. stylios, p. p. groumpos, modeling complex systems using fuzzy cognitive maps, ieee transactions on systems, man and cybernetics, january 2004. a discrete time domain approach on time delay estimation acta imeko issn: 2221-870x march 2019, volume 8, number 1, 77 86 acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 77 a discrete time domain approach on time delay estimation jorma kekalainen section: research paper keywords: time delay estimation; criterion function; index delay; cross-correlation; reliability checking citation: jorma kekalainen, a discrete time domain approach on time delay estimation, acta imeko, vol. 8, no. 1, article 11, march 2019, identifier: imekoacta-08 (2019)-01-11 section editor: konrad jedrzejewski, warsaw university of technology, poland received april 26, 2018; in final form february 19, 2019; published march 2019 copyright: © 2019 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: jorma kekalainen, e-mail: jormakekalainen@outlook.com 1. introduction time delay estimation plays a significant role in numerous engineering applications, specifically signal processing [1], [2]. the problem of estimating time delays between noisy signals has attracted considerable attention over the years due to the variety of applications in areas such as acoustics, sonar, radar, flow, and velocity measurements. the accurate estimation of time delay is often complicated by conditions of poor signal-to-noise ratio, a changing time delay, multipath propagation, and dispersion. various time delay estimation procedures have been proposed and implemented over the decades. the general goal has been to minimise the variance of the time delay estimates in the presence of uncorrelated noise at the receiving ends. for time delay estimation in linear systems, there are three basic operations involved: 1) filtration of input signals in the case of non-white signal and noise spectral characteristics in order to accentuate the frequency bands with good signal-to-noise ratio; 2) comparison (or correlation) of the two filtered waveforms in order to apply the detection by means of threshold; and 3) computation of time delay estimate. time delay estimation procedures based on cross correlation, smoothed coherence transforms, unit impulse response, and maximum likelihood estimates can generally be viewed as the inverse fourier transforms of normalised (weighted) crossspectral density functions that yield delay estimates in time domain terms. many of the methods proposed for time delay estimation have been shown to be related by means of the generalised cross-correlation (gcc) approach, which involves filtering the received signals and estimating the time delay as the time lag where the cross-correlation function of filtered signals is at its maximum [3]. the gcc function and the estimate of time delay can be expressed in the well-known form { 𝑅(𝜏) = f−1{𝑊(𝑓)𝐺12(𝑓)} 𝜏𝐷𝑒𝑙𝑎𝑦 = arg{max 𝜏 [𝑅(𝜏)]} (1) where f-1 is the inverse fourier transform, w(f) = h1(f)h2(f)* is a weighting function determined by the filter frequency responses (* is used to denote the complex conjugate), and g12 is the cross power density spectrum of the received sensor signals 1 and 2. the arg symbol denotes an argument operator, that picks the time lag or delay argument τ of r. cross-correlation is mainly suggested in applications in which some knowledge of time delays is required, such as the velocity measurement of solid or non-solid material flow (e.g. steel and paper strips; road and rail vehicles; and mixtures of solids, liquids, and gas) and range measurement by sonar and radar [4]-[10]. abstract a time delay estimation method based on the discrete time domain approach is introduced here. in this dual-channel time delay estimation model, the criterion function compares the time differences of time sequences between channels, not the magnitude values of time functions as in the conventional cross-correlation method. an estimation task is formulated as an extreme value problem in discrete index space. using the index delay giving extreme value to the criterion function, it is possible to find the best estimate for time delay distribution in the meaning of that criterion. using this method, the estimated delay distribution and criterion function are clearly separated. thus, there are no theoretical problems in the determination of the average time delay or velocity in the non-constant or changing time delay case as long as a sufficient statistical similarity (correlation) exists between channel signals. the theoretical values of several criterion functions and the probability of occurrence of an anomalous estimate with the crosscovariance criterion function are derived. a basic performance analysis of the estimation method is presented. some potential real-time supervision methods based on the use of criterion functions in the detection of the possible unreliability of the time delay estimate are outlined. mailto:jormakekalainen@outlook.com acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 78 if the thresholding device in the basic model of figure 1 is moved to the front of the cross-correlator, a simple correlation scheme is obtained [11]. threshold crossing can be used as a criterion to save data, which is then processed by correlation and delay estimation. this algorithm, which allows for fast polarity correlation, was proposed by henry [12]. henry's idea is based on the fact that information used by a polarity cross-correlator comprises the actual zero-crossing times of the signals [13]. henry noted that the zero crossings are randomly distributed in time; hence, the observation of these zero crossings gives data with a sufficient time resolution without the need for sampling the data at close intervals that were previously thought necessary to give an adequate resolution. if polarity information is replaced by crossing over times, then the time delay estimate is the time interval between the corresponding crossing over times on different channels. to find the crossing over times corresponding to each other in both channels, some criterion function is needed for the decisionmaking process involved in the detection of the time delay estimate. the aim of this paper is to present the basic idea and analysis of a method that utilises pre-processed time signals and some simple criterion functions in time delay estimation. 2. time difference distribution method 2.1 time differences and criterion functions to determine an estimate for the time delay between two axially separated sensors, time differences are formed using the same sensor arrangement as in conventional cross-correlation methods, and the utilisation of information included in the zerocrossing times of the signals or any time moment is actually based on some other detectable and randomly distributed characteristics (vortices in turbulent flow, gas bubbles, clumps of particles, macroscopic objects, surface patterns, injected chemical or radioisotope markers, etc.) in flow. as a result, two primary time sequences {t} and {t} are obtained from one channel (input) and from the other channel (output). of course, in any case, a sufficient statistical similarity (correlation) between channel signals with some time delay is an essential prerequisite for the successful utilisation of the method. the time difference distribution (tdd) method for solving time delay estimation problems is based on the use of any criterion function that is able to utilise that statistical similarity between the channel signals and to fix the time elements corresponding each other in sequences {t} and {t}. using the index delay that gives the extreme value (maximum or minimum value depending on the criterion type) to the criterion function, it is possible to select one collection of (t, t)-pairs and to calculate the best estimate for time delay distribution (j , j=1, ... ) within the meaning of that criterion. with the help of estimated time delay distribution, the velocity distribution (l/j , j=1, ... ) can be calculated when the distance (l) between the axially separated sensors is known. of course, sufficient statistical similarity (correlation) between channel signals sets an upper limit for the distance l. all suitable criterion functions with the previous time difference formulation of the time delay estimation problem have been inherently defined in the discrete (index) domain. next, mainly as an example, we present the simplest possible criterion functions. the theoretical counterparts of these functions are presented in section 2.2. the first criterion function is called the sample variance delay function êvd. as a criterion of the best estimate, we use the minimum value of the sample variance delay function min 𝑘 {�̂�𝑣𝑑(𝑘)} = min 𝑘 {⟨∆2(𝑖 + 𝑘,𝑖)⟩ − ⟨∆(𝑖 + 𝑘,𝑖)⟩2} (2) where (i+k,i) = t(i+k)-t(i), t(i+k) is the (i+k)th element of {t}, t(i) is the ith element of {t}, k is an index delay, and  denotes a sample mean. the second criterion function is called the sample average delay difference function êadd min 𝑘 {�̂�𝑎𝑑𝑑(𝑘)} = min 𝑘 {⟨|∆(𝑖 + 𝑘,𝑖) − ∆(𝑖 + 𝑘 − 1,𝑖 − 1)|⟩} (3) where  is the absolute value. the third criterion function is the sample variance delay difference function êvdd min 𝑘 {�̂�𝑣𝑑𝑑(𝑘)} = min 𝑘 { ⟨|∆(𝑖 + 𝑘,𝑖) − ∆(𝑖 + 𝑘 − 1,𝑖 − 1)|2⟩ −⟨|∆(𝑖 + 𝑘,𝑖) − ∆(𝑖 + 𝑘 − 1,𝑖 − 1)|⟩2 } . (4) the fourth criterion function is correlation coefficient êcor max 𝑘 {�̂�𝑐𝑜𝑟(𝑘)} = max 𝑘 { ⟨∆𝑇(𝑖 + 𝑘)∆𝑡(𝑖)⟩ − ⟨∆𝑇(𝑖 + 𝑘)⟩⟨∆𝑡(𝑖)⟩ √𝑉�̂�𝑟{∆𝑇}𝑉�̂�𝑟{∆𝑡} } (5) where 𝑉â𝑟{∆𝛵} = ⟨∆𝛵2(𝑖 + 𝑘)⟩ − ⟨∆𝛵(𝑖 + 𝑘)⟩2 𝑉â𝑟{∆𝑡} = ⟨∆𝑡2(𝑖)⟩ − ⟨∆𝑡(𝑖)⟩2 ∆𝑇(𝑖 + 𝑘) = 𝑇(𝑖 + 𝑘) − 𝑇(𝑖 + 𝑘 − 1) ∆𝑡(𝑖) = 𝑡(𝑖) − 𝑡(𝑖 − 1). as the denominator and product of averages t(i+k) and t(i) are constants independent of index delay k in stationary sequences, the maximisation of the correlation coefficient is equivalent to the maximisation of the crosscorrelation function max 𝑘 {�̂�𝑐𝑐(𝑘)} = max 𝑘 {⟨∆𝑇(𝑖 + 𝑘)∆𝑡(𝑖)⟩} (6) where êcc(k) is sample cross-correlation function of time intervals. the minimisation of the quadratic delay difference function min 𝑘 {�̂�𝑞𝑑𝑑(𝑘)} = min 𝑘 {⟨|∆𝑇(𝑖 + 𝑘) − ∆𝑡(𝑖)|2⟩} (7) in stationary sequences is also equivalent to the maximisation of the correlation coefficient and the cross-correlation function. 2.2 theoretical values of some criterion functions in the following, stationary fifo is assumed, meaning here that the detectable objects in the flow do not mix in the space between the sensors  undistorted delay distribution case. in fluid mechanics, a plug flow is a simple model of the velocity crosscorrelator threshold delay estimator h 1 (f) h (f) 2 figure 1. block diagram model for dual-channel delay estimation with the generalised correlation method. acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 79 profile of a fluid flowing in a pipe. in plug flow, the velocity of the fluid is assumed to be constant across any cross-section of the pipe perpendicular to the axis of the pipe. in an ideal plug flow, the fluid is not mixed in the axial direction in the space between the sensors, and any infinitesimal fluid plug element that enters the space between sensors at time t exits the space between the sensors at time t=t+, where  is the delay time of any plug between the sensors. the delay time distribution function is, therefore, the dirac delta function at . however, any real plug flow has a delay time distribution that is a ‘narrow’ pulse around the mean delay time. plug flow is a special type of previously assumed fifo flow. the expectation of the sample variance delay function êvd (in the stationary fifo case) for independent random variables t and  can be presented in the form 𝑒𝑣𝑑(𝑘) = 𝐸{�̂�𝑣𝑑} = 𝐸{⟨∆ 2(𝑖 + 𝑘,𝑖)⟩ − ⟨∆(𝑖 + 𝑘,𝑖)⟩2} = |𝑘 − 𝑘0|𝑉𝑎𝑟{∆𝑡} + 𝑉𝑎𝑟{𝜏} (8) where e is the expectation operator, var{t} is the variance of the time interval in the sequence {t}, var{} is the variance of time delay , and k0 is a fixed index delay that minimises the criterion function êvd. there is a minimum of evd 𝑀𝑖𝑛 𝑘=𝑘0 {𝑒𝑣𝑑(𝑘)} = 𝑉𝑎𝑟{𝜏} . (9) therefore, moving (k-k0) index units from the optimum index delay k0, the value of the criterion function increases by an amount of time interval variance weighted with the absolute value of (k-k0) index units, as we see in figure 2. the sample criterion function previously denoted as êadd can be presented in the form �̂�𝑎𝑑𝑑(𝑘) = ⟨|∆𝜏(|𝑘 − 𝑘0| + 𝑖) + ∆𝑡(|𝑘 − 𝑘0| + 𝑖) − ∆𝑡(𝑖)|⟩ ≥ ⟨|∆𝜏(|𝑘 − 𝑘0| + 𝑖)|⟩ (10) where the time delay difference is (s) = (s)-(s-1). equality in equation (10) is true if time interval t() between the sequential elements in sequence {t} is constant or index delay k=k0, which minimises the value of criterion function êadd. the theoretical value of êadd(k) is 𝑒𝑎𝑑𝑑(𝑘) = ∫ |𝑧|∫ 𝑓∆[∆𝑡](𝑧 − ∆𝜏)𝑓∆𝜏(∆𝜏)𝑑∆𝜏𝑑𝑧 ∞ −∞ ∞ −∞ (11) where 𝑓∆[∆𝑡](𝑧 − ∆𝜏) = ∫ 𝑓∆𝑡(∆𝑡)𝑓∆𝑡(𝑧 − ∆𝜏 − ∆𝑡)𝑑∆𝑡 ∞ −∞ , 𝑓∆𝜏(∆𝜏) = ∫ 𝑓𝜏(𝜏)𝑓𝜏(∆𝜏 − 𝜏)𝑑𝜏 ∞ −∞ , 𝑧 = ∆[∆𝑡] + ∆𝜏, f∆t is a probability density function of t, and f is a probability density function of  . the minimum value of eadd is min 𝑘=𝑘0 {𝑒𝑎𝑑𝑑(𝑘)} = ∫ |𝑧| ∞ −∞ ∫ 𝑓𝜏(𝜏)𝑓𝜏(𝑧 − 𝜏)𝑑𝜏𝑑𝑧 ∞ −∞ (12) where z = . figure 3 shows the coincidence of the theoretical and sample values of the criterion function êadd. the sample variance delay difference function êvdd can be presented in the form �̂�𝑣𝑑𝑑(𝑘) = ⟨|δ𝜏(|𝑘 − 𝑘0| + 𝑖) + δ𝑡(|𝑘 − 𝑘0| + 𝑖) − δ𝑡(𝑖)| 2⟩ − ⟨|δ𝜏(|𝑘 − 𝑘0| + 𝑖) + δ𝑡(|𝑘 − 𝑘0| + 𝑖) − δ𝑡(𝑖)|⟩ 2 . (13) the theoretic counterpart of êvdd with the previous notes is 𝑒𝑣𝑑𝑑(𝑘) = [∫ 𝑧2 ∞ −∞ ∫ 𝑓δ[δ𝑡](𝑧 − δ𝜏)𝑓δ𝜏(δ𝜏)𝑑δ𝜏𝑑𝑧 ∞ −∞ ] − [∫ |𝑧|∫ 𝑓δ[δ𝑡](𝑧 − δ𝜏)𝑓δ𝜏(δ𝜏)𝑑δ𝜏𝑑𝑧 ∞ −∞ ∞ −∞ ] 2 . (14) the minimum value of evdd is min 𝑘=𝑘0 {𝑒𝑣𝑑𝑑(𝑘)} = [∫ 𝑧2 ∞ −∞ ∫ 𝑓𝑧(𝜏)𝑓𝜏(𝑧 − 𝜏)𝑑𝜏𝑑𝑧 ∞ −∞ ] − [∫ |𝑧|∫ 𝑓𝑧(𝜏)𝑓𝜏(𝑧 − 𝜏)𝑑𝜏𝑑𝑧 ∞ −∞ ∞ −∞ ] 2 . (15) 4 6 8 10 12 14 6 7 8 9 10 11 12 13 14 15 16 index delay values of criterion function êvd figure 2. theoretical and sample values of the square root of the variance delay function êvd. all 11 theoretical values presented in this index delay window are combined with the line. sample distributions in the simulation experiments are approximately gaussian tn(30,5²) and n(300,5²). the sample size is 1000. theoretical distributions are exactly gaussian with the previous parameters. the bar denotes the variation range of the sample values in 30 experiments. 5 6 7 8 9 6 7 8 9 10 11 12 13 14 15 16 index delay values of criterion function êadd figure 3. theoretical and sample values of the average delay difference function êadd with the same statistics as in figure 2. theoretical values computed from the equation (11) and equation (12) are combined with the line. the bar denotes the variation range of the sample values in 30 experiments. acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 80 figure 4 shows the coincidence of the theoretical and sample values of the criterion function. the theoretical counterpart of the correlation coefficient can be presented in the form of normalised cross-covariance function 𝑒𝑐𝑜𝑟(𝑘) = 𝐶𝑜𝑣{δ𝑇(𝑖 + 𝑘),δ𝑡(𝑖)} √𝑉𝑎𝑟{δ𝑇(𝑖 + 𝑘)}𝑉𝑎𝑟{δ𝑡(𝑖)} . (16) the maximum value of ecor is max 𝑘=𝑘0 {𝑒𝑐𝑜𝑟(𝑘)} = √ 𝑉𝑎𝑟{δ𝑡} 𝑉𝑎𝑟{δ𝑡} + 2𝑉𝑎𝑟{𝜏} (17) if t and  are independent. equivalently. cross-correlation function can be presented in the form 𝑒𝑐𝑐(𝑘) = 𝑅δ𝑡δ𝑡(𝑘) + 𝑅δ𝑡δ𝜏(𝑘) (18) where rtt is the autocorrelation function of t, and rt is the cross-correlation function of t and . the maximum value of the cross-correlation criterion function for independent t and  is max 𝑘=𝑘0 {𝑒𝑐𝑐(𝑘)} = 𝑉𝑎𝑟{δ𝑡} + [𝐸{δ𝑡}] 2 (19) because e{t}0 here, it is convenient to work with the crosscovariance form of the criterion function 𝑒𝑐𝑜𝑣(𝑘) = 𝑒𝑐𝑐(𝑘) − [𝐸{δ𝑡}] 2 , (20) then max 𝑘=𝑘0 {𝑒𝑐𝑜𝑣(𝑘)} = 𝑉𝑎𝑟{δ𝑡} . (21) the quadratic delay difference function eqdd can be presented in the form 𝑒𝑞𝑑𝑑(𝑘) = 𝐸{⟨|δ𝑇(𝑖 + 𝑘) − δ𝑡(𝑖)| 2⟩} = 2[𝑉𝑎𝑟{δ𝑡} + 𝑉𝑎𝑟{𝜏}] (22) and the minimum value of the criterion function is min 𝑘=𝑘0 {𝑒𝑞𝑑𝑑(𝑘)} = 2𝑉𝑎𝑟{𝜏} (23) if t and  are independent. because the maximisation of ecor, ecc, and ecov is equivalent to the minimisation of eqdd in stationary systems, figure 5 only presents the theoretical and sample values of the criterion function ecor. theoretical values of the previous criterion functions experimentally approached by the sample criterion functions were derived, assuming stationary stochastic processes generating independent time differences and fifo-type order relation. 3. performance of time difference distribution method 3.1 probability of occurrence of an anomalous estimate predicting the performance of time delay estimation methods, it is often possible to set a measure of performance by establishing bounds on performance. for the time delay estimation problem, the cramer-rao lower bound (crlb) is commonly used as a performance standard. the crlb yields a lower bound on the variance of any unbiased time delay estimate as a function of the signal and noise power spectra and the coherent processing time [3], [14]-[16]. concerning cases of practical interest, there is a theorem that provides that the maximum likelihood estimate can be made arbitrary, close to the crlb, for sufficiently long observation times [17]. often, it is not possible to obtain observation times that are sufficiently long. then, random variations in the values of the criterion function can mask correct extreme values, and large estimation errors or so-called anomalous estimates may occur. therefore, the actual performance is much worse than the crlb predicted by the linear analysis for a given observation time and signal-to-noise ratio [18], [19]. let us estimate the reliability of the time delay estimate of this tdd method, with a finite observation time. the maximum type of the criterion function probability of the occurrence of an anomaly can be described as values of criterion function êvdd 4 5 6 7 6 7 8 9 10 11 12 13 14 15 16 index delay values of criterion function êvdd figure 4. theoretical and sample values of the square root of evdd with the same statistics as in figure 2. theoretical values computed from the equations (14) and (15) are combined with line. bar denotes a variation range of sample values in thirty experiments. -0,1 0,0 0,1 0,2 0,3 0,4 0,5 0,6 6 7 8 9 10 11 12 13 14 15 16 index delay values of criterion function êcor figure 5. theoretical and sample values of the criterion function ecor with the same statistics as in figure 2. theoretical values computed from equation (16) and equation (17) are combined with the line. the bar denotes the variation range of the sample values in 30 experiments. acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 81 𝑃𝑎 = ∫ 𝑓�̂�(𝑚)(𝑥) ∞ −∞ [ 1 − ∏ ∫ 𝑓�̂�(𝑗+𝑚)(𝑦)𝑑𝑦 𝑥 −∞ 𝜔 2 𝑗=− 𝜔 2 (𝑗≠0) ] 𝑑𝑥 (24) where fê(m)(x) and fê(j+m)(y) denote the density functions of independent random variables ê(m) possessing correct index delay, and ê(j+m) possessing incorrect index delay if j0. the range of criterion functions (delay windows) over which the extreme value is searched is +1 ( is an even number). the probability of occurrence of an anomaly with the maximum criterion function type with identical gaussian densities fê(k)(y) is in the normalised form 𝑃𝑎 = 1 − 1 √2𝜋 ∫ 𝑒 − 𝜃2 2 ∞ −∞ { 1 √2𝜋 ∫ 𝑒 − 𝑡2 2 𝑑𝑡 𝑧(𝜃) −∞ } 𝜔 𝑑𝜃 (25) and 𝑧(𝜃) = 𝜃√𝑉𝑎𝑟{�̂�(𝑚)} + 𝐸{�̂�(𝑚)} − 𝐸{�̂�(𝑘)} √𝑉𝑎𝑟{�̂�(𝑘)} (26) where k=j+mm. assumption of a gaussian distribution of ê() is reasonable according to the central limit theorem in view of the large number of random variables summed to produce criterion function ê(). thus, the quantities needed for densities of ê() are their means e{ê()} and variances var{ê()}. given the means and variances of the sample criterion function, the probability of occurrence of an anomaly can be numerically evaluated. as a special example of the maximum criterion function type is the cross-correlation function (or, in fact, the cross-covariance function) because it turns out that whenever the zero mean assumption is unrealistic (here e{t} = t0), it is more convenient to work with covariance functions than with correlation functions. assuming a gaussian distribution, and since côvxy() is the unbiased estimate of covxy() or e{côvxy()}=covxy(), the only other quantity needed for the density of covxy() is its variance. the variance of the sample cross-correlation function is given by van trees [17] as 𝑉𝑎𝑟{�̂�𝑥𝑦(𝑠)} ≈ 1 𝑛 ∑ [𝑅𝑥𝑥(𝑟)𝑅𝑦𝑦(𝑟) ∞ 𝑟=−∞ + 𝑅𝑥𝑦(𝑟 + 𝑠)𝑅𝑦𝑥(𝑟 − 𝑠)] (27) and the variance of sample cross-covariance function is 𝑉𝑎𝑟{𝐶�̂�𝑣𝑥𝑦(𝑠)} ≈ 1 𝑛 ∑ [ 𝐶𝑜𝑣𝑥𝑥(𝑟)𝐶𝑜𝑣𝑦𝑦(𝑟) +𝐶𝑜𝑣𝑥𝑦(𝑟 + 𝑠)𝐶𝑜𝑣𝑦𝑥(𝑟 − 𝑠) ] ∞ 𝑟=−∞ (28) now, the simple model considered here for pre-processed discrete-time signals received at two spatially separate sensors is given by { 𝑥(𝑖) = δ𝑡(𝑖) 𝑦(𝑖) = δ𝑇(𝑖) = δ𝑡(𝑖 − 𝑚) + δ𝜏(𝑖 − 𝑚) (29) and so { cov𝑥𝑥 (𝑟) = cov∆𝑡∆𝑡(𝑟) cov𝑦𝑦(𝑟) = cov∆𝑇∆𝑇(𝑟) = cov∆𝑡∆𝑡(𝑟) + cov∆𝜏∆𝜏(𝑟) cov𝑥𝑦(𝑟 + 𝑠) = cov∆𝑡∆𝑇(𝑟 + 𝑠) = cov∆𝑡∆𝑡(𝑟 + 𝑠 − 𝑚) cov𝑦𝑥(𝑟 − 𝑠) = cov∆𝑇∆𝑡(𝑟 − 𝑠) = cov∆𝑡∆𝑡(𝑟 − 𝑠 + 𝑚) (30) substituting equation (30) into equation (28) yields 𝑉𝑎𝑟{𝐶�̂�𝑣𝑥𝑦(𝑠)} ≈ 1 𝑛 ∑ { 𝐶𝑜𝑣δ𝑡δ𝑡(𝑟)[𝐶𝑜𝑣δ𝑡δ𝑡(𝑟) + 𝐶𝑜𝑣δ𝜏δ𝜏(𝑟)] +𝐶𝑜𝑣δ𝑡δ𝑡(𝑟 + 𝑠 − 𝑚)𝐶𝑜𝑣δ𝑡δ𝑡(𝑟 − 𝑠 + 𝑚) } ∞ 𝑟=−∞ . (31) suppose that s=m 𝑉𝑎𝑟{𝐶�̂�𝑣𝑥𝑦(𝑠 = 𝑚)} ≈ 1 𝑛 { 𝐶𝑜𝑣δ𝑡δ𝑡(0)[𝐶𝑜𝑣δ𝑡δ𝑡(0) + 𝐶𝑜𝑣δ𝜏δ𝜏(0)] +𝐶𝑜𝑣δ𝑡δ𝑡(0)𝐶𝑜𝑣δ𝑡δ𝑡(0) } ≈ 1 𝑛 [2𝑉𝑎𝑟{δ𝑡} + 𝑉𝑎𝑟{δ𝜏}]𝑉𝑎𝑟{δ𝑡}  (32) because { 𝐶𝑜𝑣δ𝑡δ𝑡(0) = 𝑉𝑎𝑟{δ𝑡} 𝐶𝑜𝑣δ𝜏δ𝜏(0) = 𝑉𝑎𝑟{δ𝜏} . (33) correspondingly, if sm, 𝑉𝑎𝑟{𝐶�̂�𝑣𝑥𝑦(𝑠 ≠ 𝑚)} ≈ 1 𝑛 ∑ { 𝐶𝑜𝑣δ𝑡δ𝑡 2 (𝑟) + 𝐶𝑜𝑣δ𝑡δ𝑡(𝑟)𝐶𝑜𝑣δ𝜏δ𝜏(𝑟) +𝐶𝑜𝑣δ𝑡δ𝑡(𝑟 + 𝑠 − 𝑚)𝐶𝑜𝑣δ𝑡δ𝑡(𝑟 − 𝑠 + 𝑚) } ∞ 𝑟=−∞ ≈ 1 𝑛 [𝐶𝑜𝑣δ𝑡δ𝑡 2 (0) + 𝐶𝑜𝑣δ𝑡δ𝑡(0)𝐶𝑜𝑣δ𝜏δ𝜏(0)] ≈ 1 𝑛 𝑉𝑎𝑟{δ𝑡}[𝑉𝑎𝑟{δ𝑡} + 𝑉𝑎𝑟{δ𝜏}] . (34) according to equation (26) and the previous discussion, we obtain for the cross-covariance criterion function 𝑧(𝜃) ≈ 𝜃√2( 𝜎δ𝑡 𝜎δ𝜏 ) 2 + 1 + ( 𝜎δ𝑡 𝜎δ𝜏 )√𝑛 √( 𝜎δ𝑡 𝜎δ𝜏 ) 2 + 1 = 𝜃√2[( 𝜎δ𝑡 𝜎𝜏 ) 2 + 1] + ( 𝜎δ𝑡 𝜎𝜏 )√𝑛 √( 𝜎δ𝑡 𝜎𝜏 ) 2 + 2 (35) where =2, ²=var{} and t²=var{t}. if we compare equation (25) and equation (35) to the results of chan, yansouni, and crozier in [20], we obtain formal equivalence between the signal-to-noise ratio (snr) in the conventional cross-correlation method and the (t/)-ratio or the (t/)-ratio in the tdd method. according to equation (25) and equation (35), pa is constant when (t/)n is constant, and (t/)²<<1. this means that the distance between two pa curves in the (t/)-scale with large sample sizes of n1(≫ 1) and n2(≫ 1) is approximately 10 log(n2/n1) in decibels, as we clearly see in figure 6, in which theoretical pa curves computed with equation (25) and equation (35), and the corresponding experimental relative frequencies of anomalous estimates are presented. for example, the probability curves with sample sizes of 1000 and 3000 correspond to each other with a 5 db shift (approximately) in respect to the (t/) acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 82 ratio, as we see in figure 6. each relative frequency point in figure 6 was obtained from a series of 60 simulation experiments with sample sizes of 300, 1000, and 3000 per experiment. this is in good agreement with the theory presented previously. 3.2 threshold effect it is well known that below a certain signal-to-noise ratio, the estimation variance increases rapidly and strongly compared to the theoretical variance predicted by linear and high signal-to noise ratio analysis. this is due to the threshold effect caused by anomalous estimates. of course, the crlb is no more accurate an estimate of performance for time delay estimators when the probability of occurrence of an anomaly is sufficiently increased from zero. several studies have been conducted to find a bound that is tighter than the crlb and that would predict performance more accurately in the case of small signal-to-noise ratios [18]-[23]. for example, it has been evidenced that the zivzakai lower bound (zzlb) developed in [21] is a tighter bound than crlb for time delay estimators. in fact, zzlb is very nearly the greatest lower bound for the dual-sensor delay estimation problem [22]. generally, the variance in the time delay estimate is characterised by three distinct regions [24]. if the probability of occurrence of an anomaly in the tdd method is increased and approaches /(+1), prior information (e.g. the known maximum delay) limits the variance. this is equivalent to a pure guess of the index delay in the selected delay window. obviously, then, we can simply assume a uniform distribution of anomalies between the limits of the delay window as ianniello does in [18]. if the probability of occurrence of an anomaly approaches zero, the variance of the time delay estimate approaches crlb. between these limits, there is a transition region from the prior information limit to the linear mode or crlb. the theoretical variance of the time delay estimate in the presence of anomalous estimates can be determined from ianniello’s correlator performance estimate (cpe) model [18], which is modified here to 𝑉𝑎𝑟{⟨𝜏⟩} = (1 − 𝑃𝑎)𝑉𝑎𝑟{⟨𝜏⟩}𝑚𝑖𝑛 + 𝑃𝑎 [(𝜔 + 1)𝐸{δ𝑡}]2 12 (36) where var{ }min is a variance of sample mean   below the threshold. in normalised form, the variance of   is 𝑉𝑎𝑟{< 𝜏 >}𝑛𝑜𝑟𝑚 = (1 − 𝑃𝑎)𝑉𝑎𝑟{< 𝜏 >}𝑚𝑖𝑛 [(𝜔 + 1)𝐸{δ𝑡}]2 12 + 𝑃𝑎 . (37) an interval between two var{< >}norm curves in (t/)scale with large sample sizes of n1 and n2 is (approximately) 10 log(n2/n1) in decibels. according to the previous discussion of constant pa values, pa-curves intersect the corresponding (same sample size) var{< >}min curves (linear parts of figure 7) with an approximately constant value of normalised var{< >} as long as (t/)²<<1 or (t/)²<<2, which necessarily means that the sample size is large because the constant threshold value of pa is much smaller than one. thus, the threshold variance, or threshold time delay error, is approximately constant independent of the sample size. the threshold (t/)-ratio, at which the variance of the time delay estimate begins to deviate from the minimum variance corresponding to crlb, is approximately proportional to 1/n or to –10 log(n) in decibels. these results are equivalent to the results of scarborough, tremblay, and carter [24], [25]. they observed that the value of variance at which zzlb begins to deviate from crlb remains essentially constant, independent of the coherent processing time, with a large bandwidth-time product. the threshold signal-to-noise ratio is approximately inversely proportional to the square root of the coherent processing time for the large bandwidth-time product. figure 7 demonstrates the sample sizes of 300, 1000, 3000, and 10000 for which the theoretical threshold variances stay essentially constant with large sample sizes. figure 7 shows that a sample size of 100 samples is not sufficiently large. the threshold value condition (t/) 2<<2 is strongly violated. sample variances and theoretical variances of < > correspond quite well. only the experimental threshold (t/)-ratio seems to be decreased in the order of 5 db compared to the theoretical threshold. clearly, this effect is caused by the small number of experiments. for example, the probability that at least one anomalous estimate occurs in the nearest neighbourhood of the threshold (10-6 in figure 7) with 60 experiments is 0.00006 assuming the experiment outcome ‘anomalous estimate’ follows the binomial distribution. the threshold effect has particular significance concerning coherent vs. incoherent signal processing for time delay estimation. incoherent processing is an attractive alternative to coherent processing in many time delay applications of conventional correlation techniques because it becomes necessary to compensate for the effect of variations in time delay (e.g. due time-varying time delay, source or receiver motion) for coherent processing times other than short ones [26], [27], [28]. in incoherent processing, the whole sample size is divided into sections. the sections are processed individually to obtain one estimate for each section. these time delay estimates are then averaged to obtain the final estimate at the end of the whole observation interval. the coherent processing sample size with incoherent processing is reduced to n/n samples for each of the pa values as a function of (t/) t/ (db) figure 6. probability curves and corresponding relative frequencies of occurrence of an anomalous estimate with =10 and sample sizes 300 (• • •), 1000 (* * *) and 3000 () samples. each relative frequency point was obtained from series of sixty simulation experiments with sample sizes of 300, 1000 and 3000 per experiment. 0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 -40 -35 -30 -25 -20 -15 -10 -5 0 acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 83 n sections, although the total observation sample size is still n samples. if the actual (t/)-ratio is larger than the (t/)n/n-ratio, which is the threshold (t/)-ratio for the coherent processing sample size of n/n samples, the performance of the incoherent processor (according to previous threshold discussions) coincides with the coherent processor and with small error performance. however, if ( 𝜎δ𝑡 𝜎𝜏 ) 𝑛 < ( 𝜎δ𝑡 𝜎𝜏 ) < ( 𝜎δ𝑡 𝜎𝜏 ) 𝑛 𝑁 (38) the incoherent processor exhibits significantly poorer performance than the coherent processor. if n/n is large, the threshold (t/)-ratio for the coherent processing sample size of n/n samples is ( 𝜎δ𝑡 𝜎𝜏 ) 𝑛 𝑁 ≈ ( 𝜎δ𝑡 𝜎𝜏 ) 𝑛 √𝑁 . (39) thus, significant performance gains could be obtained for small (t/)-ratios by increasing the coherent processing time. note that the analogy between the (t/)-ratio in tdd and the signal-to-noise ratio in gcc continues, as we can see that comparing equation (39) to the following equation holds for large bandwidth-time products [24] 𝑆𝑁𝑅𝑇 𝑁 ≈ 5𝑙𝑜𝑔(𝑁) + 𝑆𝑁𝑅𝑇 (db) (40) where snrt is the threshold snr for a coherent processing time of t, and snrt/n is the threshold snr for a coherent processing time of t /n. in the tdd method, it is not necessary to compensate for the effect of moderate time variations in the time delay because the constant index delay giving the extreme value to the criterion function also contains information on the variable time delay and determines the best estimate for time delay distribution within the meaning of that criterion. in the conventional correlation method with coherent processing, there is only a constant time delay argument, which maximises the cross-correlation function without noticing a variable time delay. of course, if the time delay is strongly and constantly changing, it may destroy a sufficient statistical similarity (correlation) between the time sequences in tdd with coherent processing so that only random noise and sensor error effects remain. thus, the cessation of the error propagation coherent processing time in tdd is also bound. then, a prerequisite for the successful utilisation of incoherent processing is the condition (t/)>(t/)n/n. 3.2 distortion of time delay distribution in flow systems disobeying the fifo order relation the theoretical values of the previous criterion functions were derived assuming stationary stochastic processes generating independent time differences and fifo-type order relations between sensors. if the flow system is not a fifo system, time delay distribution, coherently computed with the help of information on two time sequences obtained from two sensors a fixed distance apart, is a distorted version of true time delay distribution. such a situation may, for example, be in traffic applications (a multilane motorway with unlimited speed as a pathological example) where the distance between the sensors is sufficiently long that this relation t <<  is true and the delay time distribution of the vehicles is sufficiently wide that this relation  << t is not true, the consequence being that the time delay distribution detected by the measurement system is distorted (without identification of the vehicles and their arrival and departure times). the results of applying the theory presented in the previous sections are fully applicable if the distortion is small. obviously, the distortion is small if the following expression 𝑃0 = ∫ 𝑓δ𝑡(δ𝑡)∫ 𝑓δ𝜏(δ𝜏)𝑑δ𝜏𝑑δ𝑡 𝛥𝑡 −𝛥𝑡 ∞ −∞ (41) where 𝑓δ𝜏(δ𝜏) = ∫ 𝑓𝜏(𝜏)𝑓𝜏(δ𝜏 − 𝜏) ∞ −∞ 𝑑𝜏 is near to one. p0 describes the probability per sample that the fifo order relation is not violated. quite arbitrary, we could require that the condition of the small distortion is valid if p0 is in the order of 0.9 or more. to obtain a simple approximation, we replace ft with dirac's delta function d() 𝑓δ𝑡(δ𝑡) = 𝛿𝐷(δ𝑡 − 𝐸{δ𝑡}) . (42) then 𝑃0 = 𝐹δ𝜏(𝐸{δ𝑡}) − 𝐹δ𝜏(−𝐸{δ𝑡}) (43) where f() is a cumulative distribution function of random variable . assuming gaussian variable  equation (43) becomes the simple form 𝑃0 = 2𝑒𝑟𝑓 ( 𝜇δ𝑡 𝜎𝜏√2 ) (44) where erf() is the error function 𝑒𝑟𝑓(𝑥) = 1 √2𝜋 ∫ 𝑒 − 𝑦2 2 𝑑𝑦 𝑥 0 . now, the condition of the previously determined small distortion holds if µt/2.33. for gaussian variables t and  𝑃0 = 2 𝑒𝑟𝑓 ( 𝜇δ𝑡 √𝜎δ𝑡 2 + 2𝜎𝜏 2 ) (45) var{< >}norm (db ) t/ (db) figure 7. normalised theoretical and sample variances of <> as a function of the (t/)-ratio. the sample size is the parameter. the theoretical variance curves are numerically computed using equation (25), equation (35), and equation (37) with =10, t=30, and t=1. each sample variance is calculated from 60 simulation experiments using sample sizes n=100 (* * *), n=300 (• • •), and n=1000 (). -70 -60 -50 -40 -30 -20 -10 0 -30 -25 -20 -15 -10 -5 0 5 10 -70 -60 -50 -40 -30 -20 -10 0 -30 -25 -20 -15 -10 -5 0 5 10 n=10000 n=3000 n=1000 n=300 n=100 acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 84 and the condition of small distortion holds if 𝜇𝛥𝑡 √𝜎𝛥𝑡 2 + 2𝜎𝜏 2 ≥ 1.65 . as a measure of distortion, we could use the following expression 𝐷 = ∑𝑖∫ 𝑓𝛥𝑡(𝛥𝑡)∫ 𝑓𝛥𝜏(𝛥𝜏)𝑑𝛥𝜏 −𝑖𝛥𝑡 −(𝑖+1)𝛥𝑡 ∞ −∞ ∞ 𝑖=1 𝑑𝛥𝑡 . (46) d describes the average amount of fifo order violations per sample. for gaussian variables t and  equation (46) can be obtained after manipulation to form 𝐷 = ∑𝑖{𝑒𝑟𝑓[𝐴(𝑖 + 1)] − 𝑒𝑟𝑓[𝐴(𝑖)]} ∞ 𝑖=1 (47) where 𝐴(𝑖) = 𝑖𝜇δ𝑡 √𝑖𝜎δ𝑡 2 + 2𝜎𝜏 2 . in figure 8, we see that the condition for the small distortion holds if the distortion percentage (100 % ∙ d) is about 5 % or smaller. the expectation of the sample variance of the estimated time delay is reduced due to distortion 𝐸{𝑉�̂�𝑟{𝜏}} = 𝑃0𝑉𝑎𝑟{𝜏} (48) where p0 is obtained from equation (41). therefore, the previous condition for the small distortion means that the standard deviation of the time delay is underestimated at approximately 5 % or less if the idealised fifo model is used. in figure 8, the reduction of e{vâr{}} from its true value var{} with gaussian variables t and  as a function of /µt presents holding t as a constant. figure 8 shows rather good harmony between the experimental results and the theoretical curves computed from equation (45), equation (47), and equation (48). each sample variance and distortion percentage were computed from the experiment of 1000 time samples. of course, the reduced sample variance of the time delay due to distortion will bias the estimate of mean transport velocity. however, the estimate of mean transport delay is still unbiased because of the compensation mechanism due to the inherent symmetry of distortion effects. however, violation of the fifo order or mixing in the space between the sensors effectively reduces the sufficient statistical similarity (correlation) between the channel signals, which is an essential prerequisite for the successful utilisation of the method. therefore, a sufficient statistical similarity between channel signals sets such an upper limit for the distance between sensors that the previous condition for the small distortion cannot disobey much. 3.3 some discussions on reliability-checking possibilities because the tdd method is based on the utilisation of criterion functions, there are several simple ways, in smart measurement systems, of implementing automatic reliability checking or supervisory operations even in real time. the trivial checking operation is based on magnitude testing of the extreme value of the criterion function. if the extreme value exceeds or passes under some predetermined range, it is reasonable to doubt the reliability of results. of course, a great disadvantage of this simple checking method is the determination of the acceptable range of extreme values, which may be problematic. obviously, it is possible to utilise the theoretical relationship between the extreme value of the criterion function and the (t/)-ratio to fix the predetermined range. for example, choosing the lowest acceptable limit of (t/), we get the lowest acceptable maximum value of the criterion function and vice versa. fixing the sample size and index delay window, we get a clear probability measure pa for a criterion of reliability. for example, the probability of an anomalous estimate as a function of the maximum value of the normalised cross-covariance criterion function ecor is 𝑃𝑎 = 1 − 1 √2𝜋 ∫ 𝑒 − 𝜃2 2 ∞ −∞ { 1 √2𝜋 ∫ 𝑒 − 𝑡2 2 𝑑𝑡 𝑧(𝜃) −∞ } 𝜔 𝑑𝜃 , 𝑧(𝜃) = 𝜃√𝑀𝑎𝑥 𝑘 {𝑒𝑐𝑜𝑟(𝑘)} 2 + 1 + √𝑛𝑀𝑎𝑥 𝑘 {𝑒𝑐𝑜𝑟(𝑘)} . (49) figure 9 shows several pa-curves with some sample sizes. the second possibility concerning the evaluation unreliability of measurement results is to study the sensitivity of the criterion function in surroundings of selected index delay. obviously, we can think that a result is reliable if the extreme value differs sufficiently from nearby values. determination of a sufficient difference between the extreme value and other values of the criterion function is also problematic. a quantitative measure of the quality of the time delay estimator in the delay window of (+1) index values ( is even) could be �̂� = �̂�(𝑘0) − 1 𝜔 ∑ �̂�(𝑘0 + 𝑖) 𝜔 2 |𝑖|=1 √1 𝜔 ∑ [�̂�(𝑘0 + 𝑖) − 1 𝜔 ∑ �̂�(𝑘0 + 𝑖) 𝜔 2 |𝑖|=1 ] 2𝜔 2 |𝑖|=1 (50) where k0 is the estimated index delay. a similar quality measure (‘peakedness’) was presented by mars and van arragon using the concept of the average mutual amount of information in the estimation of time delay in nonlinear systems [29]. we can further expand this idea. for example, in the case of the cross-covariance criterion function, a comparison of p0 d 0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,4 1,6 1,8 2,0 2,2 d i s t o r t i o n e{vâr{}}/var{} /t figure 8. reduction of the detected variance of transport delay and the distortion of delay distribution with gaussian variables (t=30, t=5). each sample variance is computed from 1000 samples. theoretical curves are computed from equation (45), equation (47), and equation (48). black spots denote the variation range of the sample values in 20 simulation experiments. acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 85 equation (26), equation (35), and equation (50) shows that measure û is an estimate of 𝑢 = 𝐸{�̂�(𝑘0)} − 𝐸{�̂�(𝑘)} √𝑉𝑎𝑟{�̂�(𝑘)} ≈ √𝑛( 𝜎𝛥𝑡 𝜎𝜏 ) √( 𝜎𝛥𝑡 𝜎𝜏 ) 2 + 2 (51) where kk0. in this way, we can transform this measure of ‘peakedness’ to the probability of occurrence of an anomaly, substituting 𝑧(𝜃) = 𝜃√ 𝑢2 𝑛 + 1 + 𝑢 (52) into equation (49). the third possibility concerning supervision of reliability is more practical and is ready to use. this method utilises several different criterion functions at the same time. an agreement of criterion functions concerning index delay is the measure of reliability. the advantage of this method is that it is not necessary to determine any threshold values or ranges. simultaneous use of several different criterion functions in reliability checking is based on the presumption that it is very unlikely that all different criterion functions have the extreme value in the same incorrect index delay. this presumption seems to be correct, as we see in table 1, in which each figure describes the relative number of undetected errors calculated in 60 experiments. due to the error detection capability of combined criterion functions, the relative amount of the undetected error strongly decreases compared to the single reference criterion function (the main criterion function), which in table 1, is a cross-covariance function êcov. the fourth possible method for the reliability evaluation of results is also practical and ready to use in smart measurement systems. this method utilises coherent and incoherent processing at the same time, continually comparing the magnitudes of extreme values and detecting optimum index delay changes, which may reveal errors caused by strongly and constantly changing time delays or all types of sensor errors, which may destroy a sufficient statistical similarity (correlation) between time sequences. an agreement on the optimum index delay is the measure of reliability. the advantage of this method is that it is not necessary to determine any threshold values or ranges. 4. conclusion in this time delay estimation model, the criterion function compares the time differences of time sequences between channels, not the amplitudes of time functions as in the conventional cross-correlation method. using the index delay, which gives the extreme value to the criterion function, it is possible to calculate the best estimate for time delay distribution within the meaning of that criterion. with the help of estimated time delay distribution, the velocity distribution can be calculated when the distance between sensors is known. using this method, the estimated delay distribution and criterion function are clearly separated. there is no need to make symmetry assumptions for the criterion function or to assume the criterion function as an estimate of the time delay distribution. thus, there are no theoretical problems in the determination of average time delay or velocity in the non-constant or changing time delay case as long as a sufficient statistical similarity (correlation) exists between the channel signals. the theoretical values of several criterion functions and the probability of occurrence of an anomalous estimate with the cross-covariance criterion function are derived. the threshold effect caused by anomalous estimates, coherent vs. incoherent processing, and the distortion of estimated delay distribution are discussed. some possibilities concerning reliability supervision based on the use of criterion functions are outlined. potential applications of the time difference distribution method are the delay and velocity measurements in stochastic flow, or the transport processes and time delay estimation problems in radar, sonar, and wireless communication systems. the basis and an analysis of the estimation method using simple data were presented here. topics for additional studies could be, for example, favourable subjects of practical application and real-time reliability-checking methods based on simultaneous use of different criterion functions and parallel incoherent and coherent processing in smart measurement systems. references [1] g. c. carter, special issue on time delay estimation, ieee trans. acoust., speech, signal processing assp-29, 3 (1981). pa (db below 1) max{ecor} n=100 n=300 n=1000 n=3000 n=10000 figure 9. the probability of an anomalous estimate as a function of the maximum value of normalised cross-covariance criterion function computed by equation (49), and =10. table 1. the relative amount of undetected errors (anomalous estimates) as a function of (t/)-ratio and corresponding reference probability of an anomalous estimate with cross-covariance criterion function pacov. all distributions are approximately gaussian. each figure describing the relative amount of undetected error of criterion functions (ecov, eadd, evd) is calculated from 60 experiments. t/ in db pacov relative amount of undetected errors ecov only combination of eadd, ecov, and evd n = 100 n = 300 n = 300 ω = 10 n = 100 n = 300 n = 100 n = 300 0 -5 ≪ 0.001 0 0 0 0 -5 -10 0.030 0.02 0.05 0 0 -10 -15 0.296 0.27 0.38 0.02 0.07 -15 -20 0.606 0.57 0.62 0.13 0.05 -20 -25 0.769 0.73 0.72 0.08 0.10 -25 -30 0.841 0.87 0.85 0.10 0.08 -180 -160 -140 -120 -100 -80 -60 -40 -20 0 0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 acta imeko | www.imeko.org march 2019 | volume 8 | number 1 | 86 [2] j. s. bendat, a. g. piersol, engineering applications of correlation and spectral analysis, wiley-interscience, new york, 1980. [3] c. h. knapp, g. c. carter, the generalized correlation method for estimation of time delay, ieee trans. acoust., speech, signal processing assp-24, 4 (1976) pp. 320-327. [4] m. h. butterfield, g. f. bryant, j. dowsing, a new method of strip speed measurement using random waveform correlation, trans. soc. instrum. tech. 13 (1961) pp. 111-123. [5] t. idowaga, t. ono, a new method of vehicle-speed measurement using cross correlation, proc. of the vii imeko world congress, london, uk, 10 14 may 1976, institute of measurement and control, london pp. blv 218/1-9. [6] c. zimmer, h. ryser, h. meyr, an instrument for non contact speed measurement by image correlation, proc. of the vii imeko world congress, london, uk, 10 14 may 1976, institute of measurement and control, london, pp. blv 217/110. [7] h. kashiwagi, paper strip speed measurement using correlation techniques, trans. soc. instrument and contr. engrs., japan, 1968, pp. 4304-312. [8] m. s. beck, correlation in instruments: cross correlation flowmeters, j. phys. e: sci. instrum. 14 (1981) pp. 7-19. [9] j. w. arthur, j. f. dix, e. harland, j. w. widdowson, p. b. denyer, j. mavor, a. d. milne, large time-bandwidth product ccd correlators for sonar, iee conference publication 1979/180, pp. 62-68. [10] d. e. n. davies, signal processing for radar applications, iee conference publication no. 1979/180, pp. 145-150. [11] w. h. haas, c. s. lindquist, a synthesis of frequency domain filters for time delay estimation, ieee trans. acoust., speech, signal processing assp-29 (1981) pp. 540-548. [12] r. m. henry, an improved algorithm allowing fast on-line polarity correlation by microprocessor or minicomputer, ieee colloquium on correlation processing, may 1974, london, uk pp. 3/1-4. [13] l. r. morris, the role of zero crossings in speech recognition and processing, proc. of the ieee conf. speech comm. processing, boston, usa, 24 – 26 april 1972, pp. 446-450. [14] b. friedlander, on the cramer-rao bound for time delay and doppler estimation, ieee trans. inform. theory, vol. it-30, may 1984, pp. 575-580. [15] w. r. hahn, optimum signal processing for passive sonar range and bearing estimation, j. acoust. soc. amer. 58 (1975) pp. 201207. [16] r. l. kirlin, j. n. bradley, delay estimation simulations and a normalized comparison of published results, ieee trans. acoust., speech, signal processing assp-30 (1982) pp. 508-511. [17] h. l. van trees, detection, estimation, and modulation theory, part i, john wiley and sons, new york, 1968. [18] j. p. ianniello, time delay estimation via cross-correlation in the presence of large estimation errors, ieee trans. acoust., speech, signal processing assp-30 (1982) pp. 998-1003. [19] j. p. ianniello, large and small error performance limits for multipath time delay estimation, ieee trans. acoust., speech, signal processing assp-34 (1986) pp. 245-251. [20] y. t. chan, p. yansouni, s. crozier, time delay estimation with low signal to noise ratios, proc. of the ieee international conference on acoustics, speech, and signal processing icassp ‘84, 19 – 21 march 1984, san diego, usa, pp. 616-619. [21] a. weiss, e. weinstein, fundamental limitations in passive time delay estimation part i: narrow-band systems, ieee trans. acoust., speech, signal processing assp-31 (1983) pp. 472-486. [22] j. p. ianniello, e. weinstein, a. weiss, comparison of the zivzakai lower bound on time delay estimation with correlator performance, proc. of the ieee international conference on acoustics, speech, and signal processing icassp ‘83, boston, usa, 14-16 april 1983, pp. 875–878. [23] j. p. ianniello, comparison of the ziv-zakai lower bound on multipath time delay estimation with autocorrelator performance, icassp 84, proc. of the ieee international conference on acoustics, speech, and signal processing icassp ‘84, 19 – 21 march 1984, san diego, usa, pp. 620-623. [24] k. scarborough, r. j. tremblay, g. c. carter, performance predictions for coherent and incoherent processing techniques of time delay estimation, ieee trans. acoust., speech, signal processing assp-31 (1983) pp. 1191-1196. [25] k. scarborough, g. c. carter, r. j. tremblay, implications of threshold effects for coherent and incoherent processing techniques of time delay estimation, proc. of the ieee international conference on acoustics, speech, and signal processing icassp ‘84, 19 – 21 march 1984, san diego, usa, pp. 612-615. [26] c. h. knapp, g. c. carter, estimation of time delay in the presence of source or receiver motion, j. acoust. soc. amer. 61 (1977), pp. 1545-1549. [27] w. b. adams, j. p. kuhn, w. p. whyland, correlator compensation requirements for passive time delay estimation with moving source or receivers, ieee trans. acoust., speech, signal processing assp-28, 2 (1980), pp. 158-168. [28] j. p. kuhn, s. e. robison, d. w. winfield, time delay estimation techniques for moving sources, proc. of the 24th midwest symp. on circuits and systems, albuquerque, usa, 29 june 1981, pp. 6975. [29] n. j. i. mars, g. w. van arragon, time delay estimation in nonlinear systems, ieee trans. acoust., speech, signal processing assp-29 (1981) pp. 619-621. design and validation of an e-textile based wearable system for remote health monitoring acta imeko issn: 2221-870x june 2021, volume 10, number 2, 220 229 acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 220 design and validation of an e-textile-based wearable system for remote health monitoring armando coccia1,2, federica amitrano1,2, leandro donisi2,3, giuseppe cesarelli2,4, gaetano pagano2, mario cesarelli1,2, giovanni d’addio2 1 department of information technologies and electrical engineering, university of naples ‘federico ii’, naples, italy 2 scientific clinical institutes maugeri spa sb, pavia, italy 3 department of advanced biomedical sciences, university of naples ‘federico ii’, naples, italy 4 department of chemical, materials and production engineering, university of naples 'federico ii', naples, italy section: research paper keywords: wearable devices; e-textile; electrocardiography; m-health; internet of things citation: armando coccia, federica amitrano, leandro donisi, giuseppe cesarelli, gaetano pagano, mario cesarelli, giovanni d'addio, design and validation of an e-textile based wearable system for remote health monitoring, acta imeko, vol. 10, no. 2, article 30, june 2021, identifier: imeko-acta-10 (2021)-0230 section editor: ronald summers, penzance, uk received september 16, 2020; in final form: march 12, 2021; published: june 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: research partially supported by sweet – smart wearable e-textile based m-health system, a horizon h2020 project financed by the ministry of economic development of italy. corresponding author: mario cesarelli, e-mail: cesarell@unina.it 1. introduction in recent years, wearable technologies have aroused a great deal of interest, which is expected to further continue thanks to the rapid improvements in technology. the involvement of big companies, such as apple and google, has fostered the focus of research activities in this field with the aim of developing and distributing wearable products ready for various applications. idtechex (www.idtechex.com) described the wearable technology sector as a market with great opportunities in terms of expansion, one that is expected to reach 51.6 billion usd by 2022 with a compound annual growth rate of 15.5%. in fact, billions of wearable electronic products are already being sold each year, covering various different sectors of the market, including military and defence, space exploration, health and wellness, fashion and entertainment. healthcare remains one of the most interesting markets, and the advantages provided by wearable technologies can potentially establish significant cost reductions for healthcare systems. the use of these technologies is increasing in clinical environments, with holter systems used for electrocardiographic (ecg) or long-term blood pressure monitoring [1]-[2], wearable integrated systems used in polysomnographic monitoring [3], inertial measurement unit (imu)-based systems used (attached on the patient’s skin) to recognise and evaluate activity [4] or to assess postural and gait analysis [5]-[6], and a variety of other technologies being introduced. furthermore, wearable devices abstract the paper presents a new e-textile-based system, named sweet shirt, for the remote monitoring of biomedical signals. the system includes a textile sensing shirt, an electronic unit for data transmission, a custom-made android application for real-time signal visualisation and a software desktop for advanced digital signal processing. the device allows for the acquisition of electrocardiographic, bicep electromyographic and trunk acceleration signals. the sensors, electrodes, and bus structures are all integrated within the textile garment, without any discomfort for users. a wide-ranging set of algorithms for signal processing were also developed for use within the system, allowing clinicians to rapidly obtain a complete and schematic overview of a patient’s clinical status. the aim of this work was to present the design and development of the device and to provide a validation analysis of the electrocardiographic measurement and digital processing. the results demonstrate that the information contained in the signals recorded by the novel system is comparable to that obtained via a standard medical device commonly used in clinical environments. similarly encouraging results were obtained in the comparison of the variables derived from the signal processing. mailto:cesarell@unina.it acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 221 for health monitoring can be easily used by the patient in the domestic environment and, when integrated within a complete communication chain, they allow for smart remote monitoring with great benefits for both caregivers and patients. starting from the 1990s, the process of the miniaturisation of electronic components has allowed for considering the realisation of portable devices [7]. today, the size of electronic devices has shifted from the microto the nano-scale dimension, which has allowed for the realisation of minimally invasive monitoring devices that can be used by the patient in their daily activities. the focus has therefore shifted toward the concept of electronic textiles (e-textiles) or smart textiles. the term e-textile refers to a textile substrate that incorporates electronic elements that provide it with certain capabilities for sensing (biometric or external), communication (usually wireless), power transmission, and interconnection technologies while maintaining the ‘wearable’ capabilities much like any other garment. the advances in e-textile technologies have led to the development of comfortable wearable garments directly integrated in internet of things (iot) networks. many applications have been developed exploiting this background in the field of remote monitoring, with the aim of ensuring and increasing the patient’s comfort, quality of life and safety. nevertheless, almost all the attendant projects remain within the research field and are not intended for entry into the commercial market. here, the main barriers include the regulatory issues related to patient safety, privacy, and data management [8], [9], as well as the need for a certain degree of reliability in terms of device performance. a detailed review of the wearable systems for health monitoring introduced up to 2010 is provided in [10], with a dedicated section on textile-based devices. the field of ecg signal monitoring is one of the fields most covered by e-textile applications. pani et al. (2018) provided a complete survey on textile-electrode technologies for ecg monitoring [11], with all the examined prototypes exclusively used in the scientific research field with the aim of investigating the feasibility of this form of biosignal monitoring. a number of the prototypes are used as stand-alone devices to record ecg signals in a clinical environment, rather than as part of an integrated tele-monitoring system [12]-[13]. meanwhile, other works have presented remote tele-monitoring systems focused on collecting ecg signals and other important biosignals, such as those related to electromyography (emg) [14], breathing [15]-[17], accelerometery [12], [18], and galvanic skin response [13]. the systems presented in [13] also provide tools for off-line digital signal processing, gathering the principal parameters assessed from signals, including heart rate, blood pressure, respiratory rate, and activity classification. hexoskin (https://www.hexoskin.com/) is one of the leading e-textile based remote monitoring products that is currently commercially available. this product allows for the collection of ecg and accelerometric signals, as well as for heart and respiratory rate monitoring, heart rate variability analysis and activity intensity assessment. here, the attendant hardware is distributed along with compatible mobile apps for real-time signal visualisation, with specific software used for basic off-line data processing. in this manuscript, the aim is to present our prototypal system, which is based on an e-textile sensing shirt with the capacity to collect ecg, emg and accelerometric signals. the sensing hardware uses bluetooth low energy (ble) technology to transmit data to a connected smartphone, enabling real-time visualisation. in exploiting the internet network, the relevant data can be shared on a dedicated server, where they can be accessed and downloaded only by authorised healthcare professionals. raw signals can subsequently be processed using a custom-made matlab desktop graphical user interface (gui) to assess a wide-ranging set of synthetic parameters. the overall aim is to provide a complete system for healthcare remote monitoring based on a textile device. unlike many of the referenced applications, the proposed system allows for the simultaneous acquisition of ecg, emg and acceleration signals, with their digital processing producing an extremely large set of synthetic parameters that comprehensively reflect the patient’s clinical state. the innovative tool is represented by the custom-made platform that gathers a set of advanced signal-processing algorithms, collecting all the possible information from the signals. healthcare professionals can access and manage the information in a practical way, with the possibility of directly contacting the patient through the mobile application in the case of dangerous clinical conditions. the details of the prototype’s design and development are provided in the first subsection of the following section. meanwhile, a validation analysis of the system was also undertaken by performing comparative assessments with a standard device commonly used in clinical environments. in this work, the focus is on ecg monitoring, providing validation analysis for the raw signals collected via the prototype and for the processed data flowing from the digital signal processing. the rules followed in the validation analysis are presented in section 3, with the validation results presented in section 4. in section 5, we discuss the results, outlining the advantages, limitations and perspectives of the proposed technology. finally, in the concluding section, the major findings are summarised. figure 1. system architecture: 1) wearable sensing device, 2) electronic unit, 3) sweet app, 4) cloud, 5) sweet lab. https://www.hexoskin.com/ acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 222 2. materials and methods the main aim of this work was to present the novel wearable device, sweet shirt, for remote health monitoring, and to validate its performance in terms of the acquisition and analysis of electrocardiographic activity. in this section, we describe in detail the units that make up the novel system and the materials and methods used to perform the validation analysis. 2.1. the sweet shirt the sweet shirt is a wearable sensing device that allows for the acquisition of electrocardiographic, bicep electromyographic and trunk acceleration signals. it can be integrated within a complete system for remote healthcare purposes, as illustrated by the schematic shown in figure 1. the remote monitoring system includes an e-textile-based sensing sock for gait and postural assessments, as described in [19]. the wearable sensor unit allows for bio-signal acquisition when connected to the analogue front-end located in the electronic unit. this unit also contains a microcontroller and allows for data transmission through an integrated ble module. a custom-made android mobile application was developed to receive and visualise real-time signals on a smartphone, and to subsequently upload data on a dedicated web server. this server presents a restricted area that is exclusively accessible (following prior authentication) by authorised and appointed healthcare professionals, who can download, analyse and process the data using the custom-made matlab desktop software. in the following sections, the functional modules of the system are individually presented. 2.2. wearable sensing unit the wearable sensing unit is comprised of a commercial elastic t-shirt in which e-textile electrodes are integrated. a knit conductive fabric with a resistance of less than 0.03 ohm per cm in any direction across the textile was used to produce the electrodes. this fabric (adafruit inc. www.adafruit.com – product id: 1167) is plated with real silver, which gives it highly conductive properties. two 4 × 2-cm electrodes were integrated within the garment as sensing elements for electrocardiography processes, with two 2 × 2-cm electrodes placed on each shirt sleeve for electromyography acquisition and a 2 × 2-cm electrode integrated within the upper part of the chest as a ground electrode for all the biosignals. a conductive ribbon (5 mm in width; adafruit inc. product id: 1244) was then used to connect the electrodes to the output connectors of the wearable unit, represented by snap buttons placed in a pocket on the chest of the shirt. the conductive ribbon is made of woven conductive stainless-steel fibres, with a resistance of less than 0.1 ohm per cm. conductive traces sewn onto the shirt were covered by a non-conductive fabric to avoid contact with the skin. figure 2 shows a schematic of the wearable sensing unit, with the complete unit and the main details shown. 2.3. electronic unit the electronic unit is a compact module containing all the electric and electronic elements that allow for the acquisition, digitalisation and wireless transmission of the signals. we decided to develop a custom-made analogue front-end for the ecg and emg measurement in view of suitably dealing with the higher impedance caused by the fabric electrodes. the analogue front-end for ecg measurement comprises four principal stages: an instrumentational amplifier ina 118 from texas instruments, a high-pass passive filter with a cut-off frequency of 0.05 hz, an isolation stage designed with an opamp lm358 in voltage follower configuration, and a lowpass active filter with a cut-off frequency of 40 hz. the first filter is a first order high-pass cr passive filter, while the last stage is represented by a first-order active filter comprising an opamp lm324 in non-inverting configuration with a rc feedback. in terms of the emg analogue front-end, three principal stages were designed, with the first two similar to those used for the ecg analogue front-end but with the high-pass cut-off frequency set to 15 hz. the last stage is a precision rectifier circuit with the integration of a low-pass filter. the rectifier circuit comprises an opamp lm324, two diodes and a resistor on the feedback connection. this form of configuration is also known as super-diode configuration. meanwhile, a capacitor was added in parallel to the resistor to ensure this stage acts as a firstorder low-pass filter. the various components were chosen to set the filter cut-off frequency at 30 hz. the introduction of this rectifying stage was important as we are interested in the emg envelope signal for performing the subsequent processing operations. generally, an emg signal is sampled and then rectified in the digital domain; however, we preferred to rectify it in the analogue domain in order to use a lower sampling frequency. the digitalisation of emg signals requires a high sampling frequency, around 800–1,000 hz, since the highest spectral components are at around 400–500 hz. in contrast, an emg envelope requires a lower sampling rate since its main spectral information is at low frequencies. the use of a lower sampling rate facilitates the real-time transmission of the signal. moreover, using this configuration, the mobile application can provide the user with real-time emg envelope signals, without the use of a processing stage that would increase the complexity of the system and potentially introduce delays. the electronic board, flora 9-dof (adafruit inc.), which mounts the triaxial inertial module inemo lsm9ds0, was integrated within the electronic unit to acquire accelerometric signals, while a lilypad simblee™ ble board (sparkfun inc.), was used as the system control unit. this unit provides the digitalisation of the ecg and emg signals and is connected to the flora accelerometer through the serial i2c bus. the lilypad simblee board also allows for sending data via a ble protocol (or bluetooth 4.0), using the simblee™ bluetooth® smart module integrated on the shield. in fact, ble technology presents a perfect trade-off between energy consumption, latency, piconet size, and throughput [20]. the choice of using ble technology can also be regarded as a means of increasing the battery life of the device as much as possible. battery life is a central issue in the development of portable devices and, in this type of application, it is mostly influenced by the data transmission operations. indeed, ble is one of the most datasaving transmission protocols, while other solutions have been figure 2. sweet shirt sensing unit: a) internal view with textile electrodes and connections, b) external view. http://www.adafruit.com/ acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 223 proposed based on reducing the amount of data to be sent, using a compression method that does not degrade the signal quality [21], [22]. the control unit features were implemented through employing an arm® cortex m0 microcontroller that can be programmed using the arduino ide. the control unit was programmed to digitalise ecg and emg analogue signals and to receive digital data from the accelerometer. here, the ecg signal is digitalised with a sample rate of 200 hz, while the emg and accelerometric signals are acquired using a sample period of 15 ms (66.7 hz). all data are collected in 20-bytes-sized packets and are sent in real time via ble to the smartphone using the sweet app. the packet transfer rate was set to 66.7 hz, which was experimentally identified as the maximum rate supported by ble transmission without data loss. hence, each packet contains one sample from emg and triaxial acceleration signals and three successive ecg samples in accordance with their sampling rates. despite the fact that the sampling rates chosen for the ecg and emg signals were lower than those usually used, they were in line with the time resolution required by our target applications. in ecg digital processing, we are not interested in signal morphology but on heart rate analysis, which can be accurately performed with a lower sampling rate [23]. with regard to emg signalling, the envelope signal was extracted in the analogue domain such that it can be safely sampled using the chosen rate. all the modules that make up the electronic unit are powered by a 1,200 mah/3.7 v lithium battery placed on the back of the unit, which is enclosed in a 3d-printed plastic case (10 × 7.5 × 2 cm³). on the top part of the case, eight snap buttons were integrated to allow for connection to the wearable sensing unit, thus providing the input signals for the analogue front ends. figure 3 shows the internal electronic board and the complete unit. 2.4. the sweet app the sweet app is a custom-made application for mobile devices requiring an operating system of android 6.0 or higher and ble technology. the application allows the smartphone to communicate and receive data coming from the electronic unit, via the ble protocol. when the application is started, it is possible to associate and connect the wearable device using its media access control (mac) address. following this, the measurement session can commence, with the data transferred from the electronic unit to the mobile device, which allows for real-time signal plotting. at the end of the session, the data will be automatically saved in a ‘.csv’ file, which is stored locally and can be uploaded at any time to a dedicated web server. figure 4 shows the main frames of the app. 2.5. signal processing algorithms data from the web server can be accessed and downloaded only by authorised healthcare professionals. the custom-made matlab gui software, sweet lab, can be used to plot and post-process signals in order to achieve a huge set of synthetic parameters of clinical interest. in this work, we focus on ecg signal measurement and processing validation, and therefore, here, we only discuss ecg signal processing, while a number of algorithms for emg and acceleration signal processing were developed. the first step in ecg signal processing involves the detection of qrs complexes using an okada algorithm [24] for the assessment of the tachogram and the discrete series of rr intervals. the subsequent analysis is divided into seven frameworks, the first of which relates to the heart rate (hr) analysis, with the instantaneous hr assessed as the mean over four successive beats. from this series, the minimum, the maximum, the medium and the median hrs can be extracted and the tachycardia (hr > 110 bpm) and bradycardia (hr < 60 bpm) events subsequently searched and listed. the second framework is dedicated to the heart rate variability (hrv) analysis in terms of the time, frequency and figure 3. sweet shirt electronic unit: a) internal electronic unit, b) complete unit, external view. figure 4. sweet app main frames: a) login, b) unit connection and c) realtime signal visualisation. table 1. hrv time domain variables. statistical measures variable description sdnn in ms (standard deviation nn-intervals) standard deviation of normal-to-normal intervals (nn). sdnn reflects all cycles responsible for heart rate variation in time, thus representing the total variability. sdann in ms (standard deviation averaged nnintervals) standard deviation of the average nn intervals calculated over 5 min. sdnn is therefore a measure of the changes in heart rate due to cycles longer than 5 min. sdnni in ms (sdnn index) mean of sds of nn intervals, calculated over 5 min. rmssd in ms (root mean square of successive differences) square root of the mean of the squares of the successive differences between adjacent nn intervals. nn50 number of pairs of successive nns that differ by more than 50 ms. pnn50 in % proportion of nn50 divided by total number of nn intervals. geometrical measures variable description hrv ti (triangular index) area of the histogram distribution of rr intervals, normalised to the maximum value of the histogram. tinn base width of the rr intervals histogram. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 224 time–frequency domains. here, the beats are first classified in terms of normal, ectopic, premature ventricular contraction (pvc) and artifacts based on their timing before the rr series is edited to exclude any artifacts and any beat-to-beat intervals that are too short or too long. the new rr series is then processed in the time domain to extract the statistical and geometrical measures, as listed in table 1 [25]. the hrv is also assessed in the frequency domain by analysing how the power spectral density (psd) is distributed as a function of frequency. the psd presents three main components in terms of very low frequency (vlf), low frequency (lf) and high frequency (hf). the frequency peaks and the absolute and relative power values of each component are computed along with the lfhf ratio [26]. three different methods are provided by the software to compute the psd, namely, the welch periodogram [27], burg periodogram [28] and e lomb–scargle periodogram [29] methods. the same analyses are conducted on the windowed periodogram of the rr series to obtain a time–frequency domain analysis of the hrv variability. the third framework in the ecg processing relates to heart rate turbulence (hrt) analysis. this form of analysis presents a non-invasive method that explains the response of the heart to ventricular arrythmias [30] and is a good predictor of mortality following acute myocardial infarction [31]. two numerical parameters are assessed by the software to describe hrt: turbulence onset (to: to describe the initial acceleration in heart rate following pvc), and turbulence slope (ts: to reflect the subsequent deceleration of the sinus rhythm) [30]. meanwhile, the fourth framework provides a nonlinear analysis of ecg signals using four different approaches: sample entropy, detrended fluctuation analysis (dfa), poincaré plots and fractal dimension analysis (fda). here, sample entropy presents a nonlinear method for determining the complexity of a rr series, which is computed in terms of various values of k and is used for hrv analysis [32]. meanwhile, dfa is used to quantify the fractal properties of brief intervals of the tachogram signal [33], while a poincaré plot is a plot of rr intervals vs. the previous rr intervals used to quantify self-similarity. two numerical parameters are assessed in poincarè plot analysis: sd2 (the magnitude of the major axis of the ellipse fitting the data; represents the short-term variability) and sd1 (the magnitude of the minor axis of the ellipse; represents the long-term variability). finally, fda provides the measurement of the fractal dimension of the rr series assessed using a higuchi algorithm [34]. the fractal dimension is a useful indicator in cardiology since it assumes different values for different types of heart disease [35]. 3. validation analysis here, we present a validation analysis related to sweet shirt ecg signal acquisition and processing. in fact, three different type of analysis were conducted in order to address any possible unconformity in the measurement and/or processing phases managed by the new prototype. we first compared the rr intervals identified by the sweet shirt with those obtained via a reference device. following this, the similarity between the ecg signals obtained via the different devices was assessed. finally, comparative analysis was carried out to validate a specific subset of parameters derived from the sweet lab software signal processing. 3.1. experimental setup a three-channel digital holter recorder (oxford medilog fd5) was used as the reference for the ecg signal measurement. the device incorporates seven electrodes and operates with a sampling rate of 8,000 hz and a resolution of 15.5 bits. a healthy subject, aged 25, was equipped with the clinical holter device along with the prototypal wearable device, sweet shirt, for the ecg measurement (figure 6). here, the holter’s electrodes were placed on the subject’s thorax (figure 6a, b) in order to avoid any overlapping with the sweet shirt e-textile electrodes and to ensure the two ecg waveform were as similar as possible by means of visual analysis. the ecg acquisition time was set to 2 h. 3.2. digital processing and analysis the ecg signals from both measurement units were loaded in the matlab environment for pre-processing and analysis operations, with both signals passed through a notch digital filter to remove any 50 hz interference. the r peaks in the ecg signal from the sweet shirt were identified using the okada algorithm, while those in the holter ecg were automatically detected via its own software and could be loaded in the matlab environment. the first analysis was carried out to compare the rr intervals by means of passing–bablok (pb) regression. to achieve interval-to-interval correspondence, six rr values from the holter series were removed since they corresponded to a region of artefacts in the sweet ecg signal. following this, comparative analysis was performed using the matlab function for pb regression [36]. the waves for each beat were subsequently isolated to allow for a beat-to-beat morphology comparison. the cut-off point was chosen as the midpoint between two subsequent r peaks in order to cover the complete signal. we chose r peaks as fiducial points since no significant differences were found among the rr locations in the first analysis (see the results section). a set of a total of 6,968 corresponding beats were obtained for the analysis. the waveforms were then resampled on a normalised axis, with figure 5. ecg electrode configuration used for signal acquisition. figure 6. average beat waveform from the sweet shirt and the cut-off points (red vertical lines) used to isolate single waves. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 225 a common number of samples in order to allow for correlation analysis among the corresponding beats. the number of samples was chosen to equal the maximum number of samples found in a non-normalised beat. a resampling operation allows for avoiding any signal distortion in the normalising time axis. we also decided to individually analyse the three principal constituent waves, namely, the p-wave, the qrs complex and the t-wave. two cut-off points were set in the normalised time axis to divide the three single waves, which were selected to be the two stationary points between the three local maxima representing the single waves, as calculated based on the average beat waveform from the sweet shirt recording (figure 5). the complete beat and the single waveforms were rearranged in eight matrices (four for each device recording), with each column containing the signal corresponding to an occurred beat. correlation analysis for the waveforms was carried out using the matlab function, ‘corr’, which computes the linear correlation between each pair of columns in the input matrices. the diagonal elements of the output matrix hence represent the linear correlations between the corresponding waveforms recorded by the devices under examination. the ‘corr’ function also returns a matrix of p-values for testing the hypothesis of no correlation vs. the alternative hypothesis of a non-zero correlation. we finally compared a subset of parameters derived from our software to those provided by the commercial holter software in order to validate our signal processing algorithms. to this end, a further 2-h ecg recording was measured using a 68-year-old volunteer experiencing a pathological disorder (cardiopathic), with the same experimental setup as used previously. the two records were then windowed in terms of 24 five-minute segments, which were individually processed, carrying out a set of 24 measures for each record and for each parameter. the ecg signals were also windowed to enlarge the dataset for the comparison, and because five minutes is the recommended duration for short-term ecg analysis [25]. since holter software only provides hrv measures in the time and frequency domains, validation analysis was carried out on a subset of two representative parameters, one for each hrv field, which were computed by both systems, that is, the standard deviation of normal-to-normal beats (sdnn) for the time domain, and the ratio between lowand high-frequency spectral power (lf/hf ratio) for the frequency domain. the agreement between the measures was assessed using root mean square error (rmse), pb regression and bland–altman analysis. 4. results 4.1. rr interval comparison the rr series were compared using pb regression. this method was first proposed in 1983 as a method for testing the agreement between two sets of measurements obtained via different systems [37], [38]. here, the pb regression involved searching for a linear relationship between the measures from the two systems and the returns slope and offset of the fitting linear model. the systems could be considered as equivalent if the confidence intervals of slope and offset contained 1 and 0, respectively. table 2 shows the results of the pb regression for the rr intervals. 4.2. signal waveform comparison the ecg waveforms were compared using pearson’s linear correlation analysis. figure 7 shows the distribution of pearson’s correlation coefficients for the complete ecg waveform, the pwave, the qrs complex and the t-wave. high values of correlation were found for the ecg waveform (mean value ± standard deviation: 0.94 ± 0.07), qrs complex (0.96 ± 0.04) and t-wave (0.96 ± 0.09), while lower values were returned in the p-wave analysis (−0.19 ± 0.36). we assessed the quality of the correlation between each couple of beats using the following rules: (i) high correlation if |r| ≥ 0.7, (ii) moderate correlation if 0.3 ≤ |r| < 0.7 and (iii) low correlation when |r| < 0.3. table 3 shows a summary of the qualitative assessment of the correlation in terms of the percentage of beats, indicating high, moderate or low correlation. table 2. summary statistics and results of the pb regression analysis for the rr interval series. statistics mean (std) rr intervals from sweet shirt in ms 1032 (77.44) rr intervals from holter medilog darwin 1032 (77.41) pb regression mean confidence interval slope 1.00 1.00 to 1.00 offset in ms 0 0 to 0 figure 7. boxplot of pearson’s correlation coefficient for complete and single ecg waveforms. table 3. qualitative assessment of correlation for ecg waveforms. quality of correlation % of the entire set high moderate low p-wave 5.97*** 49.13** 44.90 qrs complex 99.92*** 0.04** 0.04 t-wave 98.87*** 0.82** 0.31 ecg waveform 98.82*** 0.88** 0.30 *** p-value < 0.001 ** p-value < 0.005 table 4. main statistics and rmse assessed for the hrv variables under examination. non-pathological subject holter (mean ± std) sweet (mean ± std) rmse sdnn in ms 63.2 ± 11.2 63.2 ± 11.2 0.184 lf/hf ratio [adim] 1.54 ± 0.846 1.53 ± 0.850 0.0561 pathological subject holter (mean ± std) sweet (mean ± std) rmse sdnn in ms 20.9 ± 6,15 19.8 ± 5.87 4.41 lf/hf ratio [adim] 3.64 ± 3,62 2.96 ± 2.92 1.97 acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 226 almost all ecg beats recorded by the prototypal device exhibited a high correlation with the corresponding waveforms obtained via the standard instrument, with a p-value excluding the hypothesis of null correlation between them. specifically, the qrs complex and t-wave were the most comparable components, while the p-waves mainly exhibited moderate or low correlation values. 4.3. signal processing algorithm validation the first approach to the analysis of the parameters generated by the signal processing algorithms involved assessing the rmses among the different sets of measures. table 4 shows the rmse values and the principal descriptive statistics of the datasets, which were divided according to subject. in the first section of table 4, the results from the nonpathological volunteer session are reported. in this case, the rmse values were extremely low for both parameters: ~0.3 % of the mean value for the sdnn and ~3.6 % of the mean value for the lf/hf ratio. however, different results were obtained with the pathological subject, with the rmse values greater in terms of both parameters: the sdnn presented a rmse of almost 20 % of the mean value, while the lf/hf ratio rmse was higher than 50 % of the mean. the analysis of agreement was then further investigated using pb regression and bland–altman analysis, with the attendant results presented in table 5. for each of the analysed parameters, the slope and offset from the pb regression are provided, along with their 95 % confidence interval (ci). across all the results, the slope values were close to 1 and their cis always included values of 1. similarly, the offset values were close to 0 in all analyses, with the cis always including 0 values. in terms of the pathological subject results, the cis were larger than the corresponding cis in the non-pathological subject, confirming a better agreement in the measurements derived from the recording involving the healthy volunteer. the bland–altman analysis results included some bias with the 95 % ci and the limits of agreement [loa]. in terms of the results from the non-pathological volunteer, the bias values were very close to 0, while both the bias cis and loa exhibited a low width and always included a 0 value. meanwhile, in terms of the results for the pathological subject, the bias values for the sdnn and the lf/hf ratio were higher, with a wider loa including 0. the bland–altman plots are presented in figure 8 and figure 9. while the differences between the methods were greater in terms of both parameters assessed using the pathological subject, they exhibited a random distribution, meaning no systematic or proportional error could be confirmed from this analysis. 5. discussion in the first analysis, we compared the rr intervals obtained via the two systems under examination by means of pb regression. the results (table 2) confirmed that the systems can be considered as equivalent in terms of the identification of the r peaks along the ecg signal as beat reference points. we then compared the signal waveforms by means of pearson’s correlation analysis. this assessment demonstrated that good agreement existed between the signals, particularly in terms of the qrs complex and t-wave, while less correspondence was found in the comparison of the p-waves (see figure 7 and table 3). figure 10 shows the averaged ecg waveforms recorded by the two systems. here, the p-waves were less visible in the holter signal than in the sweet shirt recording. this was due to the non-standard electrode placement used for the holter system (see figure 6), which was chosen to avoid the overlapping with the textile electrodes enclosed in the shirt. therefore, the lower agreement level with the p-waves can be attributed to the different electrode placements used, which is all but compulsory in a simultaneous recording. we can therefore affirm that the prototypal shirt has the capacity to clearly record an ecg signal that is comparable with those acquired by commonly used clinical portable devices. finally, we investigated the performances of the developed software in terms of signal processing. as shown in figure 8, table 4 and table 5, excellent results were achieved in the analysis of the parameters assessed using the non-pathological subject. the rmse for both parameters under examination was figure 8. bland–altman plots of the parameters for the non-pathological volunteer. the red lines represent the bias, while the blue dashed lines represent the loa. figure 9. bland–altman plots of the parameters for the pathological volunteer. the red lines represent the bias, while the blue dashed lines represent the loa. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 227 extremely low, as were the biases assessed via the bland–altman analysis. meanwhile, the pb analysis revealed that there was a regression line very close to the identity line, underlining a strict correspondence between the measurements from the two devices. however, lower agreement was found in the analysis involving the pathological subject. here, the rmse and bias values were higher (table 4) and the pb cis were wider (table 5), albeit that they still involved values that allowed for concluding that there was some agreement between the two methods. however, the bland–altman plots (see figure 9) did not exhibit any prevalent trend in the distribution of the differences, thus suggesting that no systematic or proportional differences existed between the measurement systems. based on these results, the lower agreement level in the parameters related to the pathological subject can be attributed to the greater presence of artefacts in the sweet shirt record, which was likely due to the weak adherence of the textile electrodes on the patient’s skin or the higher number of movements made by the subject during the recording session. the ecg signal from the sweet shirt was clearly visible in 94.66 % of the registration time, while the signal from the holter recorder did not present any artefacts. the presence of artefact regions will affect any signal processing results since the artefacts must be replaced by a specific number of normative rr intervals to ensure the continuity of the rr series. in this case, the results were further affected by the fact that they were averaged using a reduced window of 5 min. 6. conclusions in this paper, a new textile-sensor-based wearable device for the measurement and analysis of vital signals was developed and presented. the innovative features of the system rely on the multi-parametric approach in health monitoring and on the wideranging set of tools available for digital signal processing. in the development of the sensing unit, various sensors, electrodes, and bus structures were integrated within the textile garment, making it possible for the patient to perform normal daily activities without any discomfort while their clinical status is monitored by a specialist. the system includes a custom-based app for realtime visualisation of the acquired signals and a software desktop for off-line plotting and digital signal processing. in this work, we described the design of the device and provided a validation analysis related to ecg measurement and digital processing. here, encouraging results were achieved, indicating that reliable measurements can be obtained using our prototype wearable device, both in terms of ecg signal acquisition and further signal processing. in terms of possible improvements, the adherence of the electrodes must be increased to reduce motion artifacts interfering with the signal, which is, in our experience, the major issue encountered in this area, one that can seriously affect processing operations. acknowledgement the authors would like to thank eng. 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[36] a. padoan (2020). passing and bablok regression, matlab central file exchange. online [accessed 24 june 2020] https://www.mathworks.com/matlabcentral/fileexchange/2489 4-passing-and-bablok-regression [37] h. passing, w. bablok, a new biometrical procedure for testing the equality of measurements from two different analytical methods. application of linear regression procedures for method comparison studies in clinical chemistry, part i. clinical chemistry and laboratory medicine 21(11) (1983), pp. 709-720. doi: 10.1515/cclm.1983.21.11.709 [38] w. bablok, h. passing, application of statistical procedures in analytical instrument testing, journal of analytical methods in chemistry 7(2) (1985), pp. 74-79. doi: 10.1155/s1463924685000177 https://doi.org/10.1161/01.cir.101.1.47 https://doi.org/10.1016/0167-2789(88)90081-4 https://www.mathworks.com/matlabcentral/fileexchange/24894-passing-and-bablok-regression https://www.mathworks.com/matlabcentral/fileexchange/24894-passing-and-bablok-regression https://doi.org/10.1515/cclm.1983.21.11.709 https://doi.org/10.1155/s1463924685000177 uncertainty of the energy measurement function deriving from distortion power terms for a 16.7 hz railway acta imeko issn: 2221-870x june 2020, volume 9, number 2, 25 31 acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 25 uncertainty of the energy measurement function deriving from distortion power terms for a 16.7 hz railway andrea mariscotti1 1 astm sagl, via comacini 7, 6830 chiasso, switzerland section: research paper keywords: electric transportation systems; energy consumption; power quality; power system harmonics; rail transportation. citation: andrea mariscotti, uncertainty of the energy measurement function deriving from distortion power terms for a 16.7 hz railway, acta imeko, vol. 9, no. 2, article 5, june 2020, identifier: imeko-acta-09 (2020)-02-05 editor: alexandru salceanu, technical university of iasi, romania received december 31, 2019; in final form march 13, 2020; published june 2020 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was developed within the 16eng04 myrails project and supported by the empir programme (european union’s horizon 2020 research and innovation programme) and by the swiss state secretariat for education, research and innovation (seri) under contract number 17.00127. corresponding author: andrea mariscotti, e-mail: andrea.mariscotti@gmail.com 1. introduction it is generally recognized that reactive power and harmonic distortion are responsible for increased losses and disturbance in distribution systems [1]-[4], from which the many power quality (pq) standards, especially for lowand medium-voltage public and industrial networks. similarly, in electrified traction systems, reactive and distortion losses may occur in the traction line and at substations, possibly also impacting the feeding high-voltage network upstream. single-phase ac networks are known to have problems with unbalance, load fluctuation, and reactive power demand for the utility, a popular research subject in recent years [4]-[7]. harmonics and power transients may be sources of disturbance for many systems and equipment operating in a railway context next to a traction line [8]-[10] or connected to it, such as sensing and metering equipment [11]. in particular, susceptibility to harmonics has been considered in the recent years for power and energy metering equipment and in industrial and railway applications [12][13]. likewise, it is important to note that harmonics also contribute to active and distortion power terms relevant to an accurate estimate of power and energy consumption [14]-[20]. energy consumption and fair billing aspects have become increasingly relevant, considering the efforts to improve energy efficiency, especially for modern smart transportation systems [19][20] and the introduction of a complex normative for on-board energy metering [13]. metering is achieved by measuring the electrical quantities at the interface between the load and the network: the measured voltage and current at the sliding contact supported by the train pantograph, vp and ip, can be used for both pq [21]-[25] and energy metering [14]-[18] purposes. it is generally assumed that distortion components carry little active power and they are negligible compared to the active and reactive power carried by the fundamental component. however, it was demonstrated that for ac railways harmonic terms can carry a significant amount of active and reactive (or distortion) power and need suitable characterization [14]-[16]. the relevance of the pantograph harmonic power terms should be evaluated not only in an absolute perspective (comparing to the respective quantities calculated at the fundamental only in the en 50463-2 [13]), but also by taking into account the influence of the train operating conditions (acceleration, cruising, coasting, braking and standstill) and their rate of occurrence. in this way, the influence of the harmonic power terms on the overall exchanged energy (‘consumed’ and ‘regenerated’ energy, to use the terminology of the en 50463-2 [13]) can be estimated. the exchanged energy is estimated with abstract in electrified railways, harmonic active power terms can be significant in the order of the uncertainty required by the en 50463-2 standard for power and energy measurements in railways. nonactive power terms (encompassing reactive and distortion harmonic terms) are much more significant than the sole fundamental reactive power. this work considers the implementation of the en 504632 energy measurement function, including the criteria for the significance of the measured and calculated terms, and it carries out a monte carlo analysis to assess the impact of harmonic power terms on the measured energy and its uncertainty. acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 26 an energy measurement function (emf) implemented on board, whichh includes the data acquisition system and the voltage and current sensors (voltage measurement function [vmf] and current measurement function [cmf]) and the consequential energy calculation function (ecf), using vp and ip as an input. the en 50463-2 [13], when specifying the limits of accuracy, is clearly written in terms of nearly sinusoidal waveforms, with power and energy calculated only taking into account terms at the fundamental (see e.g. the indication of ‘maximum phase displacement at rated frequency’). harmonics are considered as a disturbance from which the immunity requirements, common to the en 50470-1 standard [12], are defined and subject to a considerable amount of research [26]-[28]. the economic impact of fractional energy saving with a magnitude commensurate to the observed variability caused by harmonic power terms is definitely worth better comprehension of the problem. as pointed out in [14]-[16], using waveforms measured during tests in some european railways [22], the magnitude of the identified harmonic power terms is indeed relevant for the uncertainty budget. it is the objective of this work to demonstrate that neglecting the harmonic power terms may lead to errors in the total energy estimate of the same order of the emf accuracy specification. it is evident that the ecf and emf uncertainty is influenced not only by the metrological characteristics of the vmf and cmf chains, where sensors and acquisition systems are located, but also by the adequate and comprehensive definition of the objective quantities (power and energy) and their relationship with the basic quantities (pantograph voltage and current and their spectra). 2. harmonic power quantities the quantities of the problem are briefly introduced and clarified here. ieee std. 1459 [29] has been selected as the most useful source of the definition of power terms. the total apparent power in nonsinusoidal conditions is obtained by the multiplication of the two voltage and current vectors, expressed as a fourier series in the time domain or, equivalently, as a vector of harmonic components in the frequency domain of order h referring to the fundamental pulsation , both with the information on amplitude vh and ih and phase αh and h (for the sake of compact notation, the pedix ‘p’ for ‘pantograph’ is dropped from the harmonic components, transformed vectors, and power terms). (1) (2) active and nonactive power terms may be conveniently separated using equations (4) and (5) of the time-referred nonactive power pq in [30]. in equation (3), the active power terms appear for the fundamental p1 and each harmonic ph. in equation (4), the expression for the nonactive power terms is more complex, including products with harmonics of different orders and with the fundamental: • the first term is the harmonic reactive power qh. • the second term is a set of mixed products that define distortion power dh and involve both fundamental and harmonics (decomposing the term further does not provide any useful information and reopens the discussion on the interpretation and the physical meaning of the terms). • the last two terms, beginning with v0 and i0, are the contributions in the presence of doc components, which in ac railways may be assumed to be zero, except during transients. (3) (4) en 50463-2 for ac railways requires the measurement of both active and reactive power in traction and braking conditions (identified in this work as the ‘output quantities’ of the standard). correspondingly, four energy terms are defined. although the definitions of such terms are given for the fundamental frequency, harmonics can be included in a straightforward manner: • active power terms at the fundamental p1 and harmonics ph can be readily summed, all contributing to the total active power pt and then to the energy defined for it. • nonactive power terms are considered in terms of absolute value to indicate the amount of power (and then current) exchanged, so we may sum (by the rooted sum of squares) all these reactive and distortion power terms to get an indication of the total nonactive power qt and the energy defined for it. these quantities will be calculated with a frequency domain approach, using the discrete fourier transform (dft). the dft is calculated on each 60 ms cycle of vp and ip (the fundamental being 50/3 hz). the calculation of power quantities based on time records accounting for a single cycle or less are described well in [37]. a standard tapering window may be used to limit the effects of spectral leakage, which in such applications is caused by two factors: i) vp and ip waveforms suffer from slow fluctuations caused by the load changes and the reaction of active loads, causing a slight low-frequency modulation effect so that a slight difference between the two ends of the record may occur and ii) the instantaneous frequency of the fundamental varies as a result of various types of instabilities [31][32]. the transformed harmonic spectra v and i (subscript ‘p’ has been removed to keep the notation compact) are then ‘cleaned’ to remove noise components from the analysis, ‘pruning’ the components against a threshold determined to select components with a significant harmonic active power, as clarified in section 3. the active and nonactive power terms and the energy quantities calculated from v and i are detailed and further discussed in the next section. 3. energy measurement function the emf is simply the calculation of cumulated power terms, either active or reactive power. the en 50463-2 requires that the terms for the traction and braking phases (with positive and negative input current ip , respectively) are calculated separately, thus resulting in four energy terms: eat, eab, ert, erb (exy with x = active, reactive and y = traction, braking). the calculation  −+= h hhp thvvv )sin(20  −+= h hhp thiii )sin(20  −−= h hhhha thivp )]22cos(1[cos    −+−+ +−−+ +−−=  h hh h hh m nm nmnm h hhhhq thvithiv tntmiv thivp )sin()sin( )sin()sin(2 )22sin(sin 00 acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 27 must comply with the en 50463-2 standard criteria, expressed for the fundamental rms of the quantities: • area 1: full accuracy is required for ip  10 % in , with in being the locomotive nominal current. this accuracy is specified as the maximum error of the calculated energy and depends on the class, variable between 0.2 % and 1 %; for vp  umin1 of en 50163 [33], the required accuracy is the same for area 2. • area 2: for ip between 1 % in and 10 % in, the required accuracy is relaxed by a factor of two. • area 3: for ip between 0.4 % in and 1 % in, the energy measurement is required but with an unspecified accuracy. • area 4: ip  0.4 % in – no energy measurement is actually required (in table 16 of en 50463-2, it is written ‘if energy is measured, no accuracy requirements’). consequently, no accuracy requirement is specified. based on these four ‘areas’, we calculate the energy quantities separately for each area (indicating the quantities with an appended index i = 1...4, such as exy,i ) in order to assess the respective uncertainty impact. the energy quantities are then aggregated by simplifying the index i in order to make an overall final energy estimate. going back to the definition of the power terms for the calculation of the energy, the definitions in section 2 are followed using the harmonic vectors v and i. the sum of the fundamental and harmonic active power terms (p1 and ph, h  2) gives the total active power pt . the sum of the reactive and distortion power terms (q1, qh and dh) gives the total reactive power qt. a threshold is used to prune the harmonic vectors, avoiding the issue of noise contributing to the sum and thus negatively influencing the estimate of uncertainty. the threshold is determined using the fundamental active power as a reference, as explained in [14], section ii.b: considering the 0.4 % in threshold of area 4, assuming that the maximum 40 harmonic terms of equal amplitude add to the harmonic active power, the selection criterion is ph  0.01 % p1. 4. uncertainty framework and monte carlo a monte carlo analysis is performed for the calculated energy terms described in section 4.1 using the measured spectra and the calculated active and non-active power terms described in section 4.2. as explained in section 4.3, the spectrum components are not independent, and their joint probability would be very difficult to determine. for this reason, replicated versions of the measured spectra are used, keeping the correlation between components typical of the real system and applying a random disturbance to the vector components similar to the one estimated based on the measured data. in other words, the available measured and selected cycles (about 2000) are augmented using the observed variability as a model. the energy is calculated with the emf in compliance with en 50463-2 using the same criteria and thresholds shown in section 3. 4.1. uncertainty of the emf the en 50463-2, in defining the reference conditions for the assessment of the metrological performance, indicates reference values for the influence quantities, among which table 2 of the standard lists the harmonic distortion of the vp and ip waveforms themselves. considering the immunity to harmonic distortion, with a permissible tolerance of 2 %, it is clear that the usual distortion of 16.7 hz railways is underestimated [22], with values up to 3 % being quite typical for the voltage and up to 8 % being typical for the current. conversely, the distorted waveforms used to test the energy meter immunity by far overestimate it: 10 % in voltage and 40 % in current for the 5th harmonic in the verification of influencing quantities, 10 % to 30 % of the 3rd, 5th, and 7th harmonic in the phase-fired waveform test, both specified in en 50463-2 [13]. the percentage error limits for the emf appear in table 3 of the en 50463-2 and amount to 1.5 % and 3 % for active and reactive energy: the accuracy class of the vmf, cmf, and ecf must be selected accordingly. en 50463-2, assuming that the errors  in the vmf, cmf, and ecf are independent, states that the error of the emf is the root-sum-square combination: (5) the vmf comes in four possible classes of amplitude accuracy: 0.2 %, 0.5 %, 0.75 %, and 1.0 % for umin1  vp  umax2; when umin2  vp  umin1, the values are doubled. correspondingly, the phase accuracy at the fundamental is 10, 20, 30, and 40 minutes for the first interval and 15, 30, 45, and 60 minutes for the second lower-voltage interval. similarly, for the cmf, four classes of accuracy are specified: the basic reference values are again 0.2 %, 0.5 %, 0.75 %, and 1.0 %, and they apply to 10 % in  ip  120 % in, but the interval of area 2 is then split into two subintervals, for 5 % in  ip  10 % in and 1 % in  ip  5 % in , the first with a doubled accuracy limit and the second with an increase of five times. the uncertainty for the active and reactive power terms at the fundamental is calculated explicitly in annex b of en 50463-2. the calculation is carried out with some simplifying assumptions (e.g. invoking the small phase difference between vp and ip vectors) in order to reduce the number of terms and thus the complexity of expressions. however, including the effect of harmonic power terms in the analytical calculation of uncertainty requires significant effort. not only is there an increase in the size of the expressions to replicate what has been done for the fundamental to the h harmonic terms, but cross-products between harmonics of different orders (resulting in the distortion power dh) must also be considered. then, the non-linearity introduced by the thresholds on the voltage and current for the definition of the four areas needs to be considered as well, although it is of marginal impact (it influences the results only for very small values of voltage and current, whereas most of the relevant values are expected in area 1). a monte carlo approach, instead, is able to cope with the complexity of the system. it should be noted that for the generation of the input patterns at each trial, attention must be given to correctly determine the correlation between the harmonic components of the same vector and between vp and ip. they are, in fact, correlated to the train operating conditions and to the intensity of the traction and braking effort. 4.2. experimental data the analysis is based on measurements performed along the swiss network (15 kv, 16.7 hz) [14][22][23]. the measurement system was implemented with a rogowski coil, a capacitive voltage divider on the locomotive roof next to the pantograph line, and a dewetron 16-bit digitizer, using a sampling rate of 50 ks/s. the system was protected by low-pass filters on each 222 ecfcmfvmfemf ++= acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 28 channel and overvoltage suppressors terminating on a solid effective local ground, represented by an equipotential metallic plate. the uncertainty of the digitizer (< 0.1 %, including one bit for over-range) is negligible in relation to the goal of the present analysis. the current and voltage sensors account for 3 % and 1.4 %, but repeatability is an order of magnitude better. the line (zurich-brig) was subject to intense traffic, so there are mixed harmonic terms of internal (the locomotive used for the measurements) and external (other trains in the same supply section) origin. the used loco was a siemens re460 in normal commercial service. the measured vp and ip waveforms are used to derive the harmonic spectra and calculate the harmonic power terms. these terms are the input to the evaluation of uncertainty. data are organized in the measured harmonic vectors h of index n (hn = [vn in]), each tagged with a given operating condition oc, traction or braking intensity int, and speed sp (the state vector sn = [ocn intn spn]). as anticipated, the harmonic components of each vector are not independent, as they are all tagged under the same state vector sn , and the probability is very high, such that in a similar state, they will replicate with similar amplitude and phase conditions. conversely, harmonic vectors associated with different operating conditions will show less similarity, as pointed out in [34], figure 10 – a graphical representation of such differences. an overview of the data behaviour using a persistency plot is given in figure 1. the voltage waveform is quite stable, with a peak-to-peak spread of 2 kv, so 10 % of the full range, visible at the top (and bottom) of the curve. the current intensity is evenly distributed between low values for coasting and standstill, and large values for intense traction and braking. figure 2 shows the distribution of the measured records in terms of the mentioned state vectors, showing the correlation between the speed and the flowing current, either absorbed (traction) or regenerated (braking). at a low speed, a small positive current corresponds to the initial acceleration at a standstill. at a high speed, further acceleration requires a large current, as indicated by the current intensity at about 300 a. the likely braking current, at a speed of 80 to 100 kmph, corresponds to initial deceleration from cruising at commercial speed. the measuring instrumentation can achieve the 0.2 % accuracy most stringent requirement of the en 50463-2 for the purpose of this analysis: the overall uncertainty voltage and current probes is of 1-3 % (k = 2), including the impact of installation that is predominant, but repeatability is more than an order of magnitude better [22]. 4.3. monte carlo method the components of the harmonic vectors are not independent: depending on the specific state condition, the internal components will align with a more or less precise pattern of emissions; additionally, external components will behave independently, depending on the state conditions of other trains or injected by the feeding network. when the input quantities are not independent, for the assessment of uncertainty they must be accompanied by the mutual probability between each pair of them, contained in a covariance matrix [35], sec. 5.2. this approach is complicated and, as anticipated, has few chances to work in practice, as the harmonic components have different origins. a different approach is thus followed here: all the harmonic components of the spectrum of one record can be applied at once, preserving the underlying correlation between them as resulting from the evidence of the measured data. randomness is ensured by treating each harmonic component as a random variable with a distribution estimated from the original data. the state vectors are clustered selecting a certain number of bins for each of the three components: b1 = 3 for the operating condition, distinguishing positive and negative current for tractioning and braking, and situations of null speed and low current for standstill; b2 = 16 for the effort intensity int (8 positive for tractioning and 8 negative for braking); b3 = 8 for the speed sp. the combination of three bins defines a cell. the set of n records is then reduced to m = b1 b2 b3, following the clustering of the state vector: each cell is linked to the original records, but is also loaded with the resulting relative frequency (given by the number of state vector components clustered under figure 1. time-amplitude distribution of synchronized 5-cycle snippets: larger intensity during braking can be recognized, as well as a displacement factor close to unity (clean zero-crossings of the current in correspondence of the voltage zero-crossings). figure 2. a histogram of the fundamental rms current and speed, showing standstill (almost 0 speed and 0 current), the wide range of braking at various speed, with intense braking when decelerating from a cruising speed of 120140 km/h), and intense accelerations (300 arms at 80-120 km/h) that last for short time intervals. acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 29 the same cell) and with the estimate of the probability distribution of amplitude and phase of each harmonic component. to this aim it is understood that the number of records clustered into a single cell must be sufficient to achieve statistical significance. as shown in figure 2, many cells are empty because a particular combination of oc, int, and sp is either unlikely or impossible, e.g. at standstill, the speed is 0, and the current is very low, supplying only the auxiliaries. furthermore, coasting and cruising are extremely rare at a low speed, only in the case of traffic problems. referring to the specific case shown in figure 2, only about 50 % of the cells contain data (about 200 samples), 20 % of which have prevailing amplitude (as indicated by the white and light coloured cells). with random sampling and an a priori determination of the monte carlo trials, the number of trials k for a given coverage probability p can be determined based on the guide of uncertainty in measurements [35]: (6) for p = 95 %, the minimum number of trials m = 2 · 105. 5. results the calculated emf values consist of four energy values: active energy during tractioning eat, active energy during braking eab, reactive energy during tractioning ert, and reactive energy during braking erb. correspondingly, four energy terms are calculated for the contribution of harmonic power terms, calculating the harmonic active power terms into eat and eab, and the nonactive harmonic power terms into ert and erb, quantifying their contribution by a ‘delta term’ with the same naming convention. in order to assess the relevance of the harmonic power terms, the focus is on the average impact on the respective energy term and on the estimated dispersion. the results are synthesized in table 1. the swiss system shows a peculiar behaviour whereby the active power in traction conditions would be lower if harmonics are included in the calculation. this occurs for braking, which features a higher harmonic distortion and has a reverse power flow. in addition, a peculiar behaviour of the 3rd harmonic (50 hz) should be also accounted for, being both a network supply harmonic and the result of the leakage of the onboard auxiliaries, which work at 50 hz and not at the traction fundamental (50/3 hz = 16.7 hz). in general, however, it is apparent that the harmonics in the swiss system contribute a non-negligible but moderate active power (in the range of 0.3 % to 1.26 % for the area 1 and area 2 figures, which are the most relevant, following the approach of the en 50463-2). these mean values are supported by the estimated standard deviations of the same order, showing that the distributions are moderately dispersed. what is relevant is the amount of nonactive power carried by the harmonic terms, which is an order of magnitude larger than the reactive power at the fundamental q1 for areas 2 to 4, and comparable to q1 in area 1. this implies that neglecting harmonic nonactive power largely underestimates the nonactive power flow, the current exchange, and power losses in the system. the nonactive power mean values have significant dispersion, indicating that there are several factors affecting their value and distribution: the vehicle’s overall operating condition; the traction and auxiliary converters’ operating points; the influence of the traction line impedance; and its resonant behaviour and that of other trains nearby in the same supply section. in addition, it should be noted that while rolling stock is limited in order for the displacement factor at the fundamental to be inductive, there is no such limit for harmonic terms, for which capacitive behaviour is also possible, with the consequence of network instability and resonances. 6. conclusions active harmonic power terms for the power and energy budget of railways are significant comparable to the required uncertainty and accuracy classes in the en 50463-2 standard. conversely, nonactive harmonic power terms are much more relevant and account for an exchange of nonactive power – several times – occurring at the fundamental, for which limits of input displacement factor are applicable. in the en 50463-2 standard, the assessment of uncertainty is carried out analytically for the active and reactive terms at the fundamental. however, the assessment of the relevance of harmonic power terms and the impact of their variability is much more complex and cannot be carried out analytically. in this work, the active and nonactive power terms above the fundamental of a 16.7 hz system were considered. the approach to the estimation of the uncertainty of the energy terms (otherwise defined at the fundamental per en 50463-2) is indirect: a monte carlo analysis is used, based on the observed distributions of the voltage and current harmonics at the pantograph, derived from the measured data. the distributions are derived from the datasets measured and collected during the testing days on the swiss network, taking into account the locomotive operating conditions, the speed, and the effort during tractioning and braking (called the ‘state vector’). the considered data thus form a snapshot of the rolling stock and network conditions for one of the busiest lines in switzerland. statistical distributions for feeding to the monte carlo analysis are derived using a hybrid approach: the distribution of each component is estimated based on the clustered data for each operating condition (traction/braking/standstill, power absorbed /regenerated, and speed), whereas the correlation between components is indicated by the average spectrum vectors for each operating condition and between operating conditions. the results confirm that the harmonic active power terms influence the emf and the calculated energy by an amount similar or slightly larger than the required uncertainty. the nonactive energy terms are much larger than that which is calculated on the reactive power at the fundamental, and they should be specifically addressed concerning their correct p k −  1 10 4 table 1. ratios of harmonic energy quantities to the corresponding ones at the fundamental, as average and standard deviations (exy with x = active, reactive and y = traction, braking). quantity area 1 area 2 area 3 area 4 eat/eat av. s.d. -0.0025 0.0016 -0.0155 0.0123 -0.0623 0.0463 0.0253 0.0620 eab/eab av. s.d. 0.0028 0.0013 0.0126 0.0141 0.1098 0.0973 0.1010 0.0976 ert/ert av. s.d. 1.5027 2.0311 40.388 354.06 8.3024 4.2682 10.6219 1.2378 erb/erb av. s.d. 0.9261 0.4377 8.7617 74.6449 10.2674 23.0604 39.0149 55.6961 acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 30 interpretation and management, as their values are well beyond any values that might be interpreted as emf ‘uncertainty’. acknowledgement the presented results have been developed in the 16eng04 myrails project and the european union’s horizon 2020 research and innovation program. this work was supported by the swiss state secretariat for education, research and innovation (seri) under contract number 17.00127. the opinions expressed and arguments employed herein do not necessarily reflect the official views of the swiss government. references [1] g. w. massey, estimation methods for power system harmonic effects on power distribution transformers, ieee trans. industry app. 30(2) (1994) pp. 485-489. 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[11] g. crotti, d. giordano, d. signorino, a. delle femine, d. gallo, c. landi, m. luiso, a. biancucci, l. donadio, monitoring energy and power quality onboard train, proc. of the 10th ieee intern. workshop on applied meas. for pow. sys., 25-27 sept., 2019, aachen, germany. [12] cenelec en 50470-1, electricity metering equipment (a.c.) – part 1: general requirements, tests and test conditions – metering equipment (class indexes a, b and c), 2018. [13] cenelec en 50463-2, railway applications – energy measurement on board trains, 2017. [14] a. mariscotti, behaviour of spectral active power terms for the swiss 15 kv 16.7 hz railway system, proc. of the 10th ieee intern. workshop on applied measurements for power systems, 25-27 sept., 2019, aachen, germany. [15] a. mariscotti, relevance of harmonic active power terms for energy consumption in some railway systems, proc. of the 24th imeko tc4 int. symp., 17-20 sept., 2019, xi’an, china. 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[19] g. hinrichs, j. hegarty, introduction of energy metering, settlement and billing at sbb, european railway review 22(1) (2016) [online]. available: https://www.globalrailwayreview.com/article/26308/introductio n-of-energy-metering-settlement-and-billing-at-sbb/ [20] a. gonzález-gil, r. palacin, p. batty, j. p. powell, a systems approach to reduce urban rail energy consumption, energy conversion and management 80 (2014) pp. 509-524. [21] a. delle femine, d. gallo, c. landi, m. luiso, discussion on dc and ac power quality assessment in railway traction supply systems, proc. of ieee intern. instrum. and meas. tech. conf., 20-23 may, 2019, auckland, new zealand. [22] a. mariscotti, direct measurement of power quality over railway networks with results of a 16.7 hz network, ieee trans. instrum. and meas. 60(5) (2011) pp. 1604-1612. [23] a. mariscotti, data sets of measured pantograph voltage and current of european ac railways, data in brief, 30 (2020). [24] h. j. kaleybar, h. m. kojabadi, m. brenna, f. foiadelli, s. s. fazel, a. rasi, an inclusive study and classification of harmonic phenomena in electric railway systems, proc. of the ieee intern. conf. on envir. and elec. eng. (eeeic), 11-14 june, 2019, genoa, italy. [25] cenelec en 50388, railway applications – power supply and rolling stock – technical criteria for the coordination between power supply (substation) and rolling stock to achieve interoperability, 2013. [26] a. olencki, p. mroz, testing of energy meters under three-phase determined and random nonsinusoidal conditions, metrol. and meas. syst. 21(2) (2014) pp. 217-232. [27] h. m. cheng, z. d. wang, q. x. cai, x. q. lu, y. x. gao, r. p. song, z. q. tian, x. x. mu, error analysis of the threephase electrical energy calculation method in the case of voltageloss failure, metrol. and meas. syst. 26(3) (2019) pp. 505-516. [28] a. cataliotti, v. cosentino, a. lipari, s. nuccio, metrological characterization and operating principle identification of static meters for reactive energy: an experimental approach under nonsinusoidal test conditions, ieee trans. instrum. and meas. 58(5) (2009) pp. 1427-1435. [29] ieee std. 1459, ieee standard definitions for the measurement of electric power quantities under sinusoidal, nonsinusoidal, balanced, or unbalanced condition, 2010. [30] p. v. barbaro, a. cataliotti, v. cosentino, s. nuccio, a novel approach based on nonactive power for the identification of disturbing loads in power systems, ieee trans. on power delivery 22(3) (2007) pp. 1782-1789. [31] a. mariscotti, d. slepicka, analysis of frequency stability of 16.7 hz railways, proc. of ieee intern. instrum. and meas. tech. conf., 10-12 may, 2011, hangzhou, china. [32] r. lapuh, a. mariscotti, performance of three algorithms for the fundamental frequency estimation against 16.7 hz railways data, proc. of ieee intern. instrum. and meas. tech. conf., 13-16 may, 2012, graz, austria. [33] cenelec en 50163, railway applications – supply voltages of traction systems, 2013 https://www.globalrailwayreview.com/article/26308/introduction-of-energy-metering-settlement-and-billing-at-sbb/ https://www.globalrailwayreview.com/article/26308/introduction-of-energy-metering-settlement-and-billing-at-sbb/ acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 31 [34] h. hu, y. shao, l. tang, j. ma, z. he, s. gao, overview of harmonic and resonance in railway electrification systems, ieee trans. on ind. app. 54(5) (2018) pp. 5227-5245. [35] bipm, evaluation of measurement data — guide to the expression of uncertainty in measurement, jcgm 100:2008. [36] bipm, evaluation of measurement data — supplement 1 to the guide to the expression of uncertainty in measurement — propagation of distributions using a monte carlo method, jcgm 101:2008. [37] j. štremfelj, d. agrež, estimation of the power quantities below one signal period using dft coefficients, ieee trans. instrum. and meas. 68(6) (2019) pp. 1696-1704. introduction to the acta imeko special issue on the ‘16th imeko tc10 conference: “testing, diagnostics & inspection as a comprehensive value chain for quality & safety”, 2019 acta imeko issn: 2221-870x march 2020, volume 9, number 1, 1 2 acta imeko | www.imeko.org march 2020 | volume 9 | number 1 | 1 introduction to the acta imeko special issue on the 16th imeko tc10 conference “testing, diagnostics & inspection as a comprehensive value chain for quality & safety”, 2019 zsolt jános viharos1,2 1 institute for computer science and control, h1111, kende u. 13-17., budapest, hungary 2 john von neumann university, h6000, izsáki u. 9., kecskemét, hungary section: editorial citation: zsolt jános viharos, introduction to the acta imeko special issue on the 16th imko tc10 conference “testing, diagnostics & inspection as a comprehensive value chain for quality & safety”, 2019, acta imeko, vol. 9, no. 1, article 1, march 2020, identifier: imekoacta-09 (2020)-01-01 editor: lorenzo ciani, university of florence, italy received: march 12, 2020; in final form march 12, 2020; published march 2020 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: zsolt jános viharos, e-mail: viharos.zsolt@sztaki.hu distinguished readers, the here presented issue of acta imeko makes public in open access form the selected highlights of the 16th imeko tc10 conference on “testing, diagnostics & inspection as a comprehensive value chain for quality & safety” held in berlin, germany, on september 3-4, 2019. publication authors from 14 countries contributed to the scientific excellence of the conference. as a result of the review process with highly committed referees, the best 7 papers were accepted for this issue as introduced in the next paragraphs. ciani et al. presented a wireless mesh network to implement a widely distributed condition monitoring system for a wind farm. the aim of the presented work is to propose an architecture able to identify possible incipient failures in the most critical turbine’s components. an extract of the reliability centred maintenance (rcm) for the wind turbine is proposed. using this approach, maintenance and tasks policies are managed by the diagnostic mesh network. the paper suggests a group of measurement quantities and relative appropriate sensors to monitor the failure modes identified through the rcm. finally, a warm standby architecture is designed for the root node, reliability with and without redundancy are compared in order to show the high increase of the reliability with the warm standby architecture. the proposed warm standby configuration activates the redundancy in case of failure guaranteeing, a probability of failure of the root node is lower than 0.01%. the paper of catelani et al. illustrates the different aspects of the maintenance policies, focusing on maintainability allocation (ma). this paper proposes as the optimal procedure the timecharacteristics based ma technique because of it could weigh the influence of every item on the system parameters. maintainability assessment during design stage represents one of the most important phases to take structural decisions and select the best items to complete the mission. in particular, an electronic controller for railway signalling system has been studied for the objective mean time to repair (mttr). in the initial part of the study the objective is and the obtained result is . the comparison between the two cases highlights that increasing the objective mttr the maintainability indices allocated to the items increase. considering the most critical components a high objective mttr provides a significant increase of the common repair time. martins et al. addresses the metrological quality of dimensional measurements based on images obtained from close circuit television (cctv) inspections in drain and sewer systems. in this type of indirect visual inspection, a significant number of absolute and relative dimensional quantities can be quantified for the performance analysis of the drain and sewer systems outside buildings. unfavourable environmental factors and conditions affect estimation of the quantities of interest and the quality of the recorded images (lighting, lack of reference points, geometric irregularities and subjective assessments, among others). this study contributes to improve the quality of the dimensional measurements by defining experimental procedures, applicable to the optical systems used in cctv inspection. the paper describes the european normative framework for these inspection activities and proposes the metrological characterization of the optical systems used, as well as, to establish a traceability chain. in the paper of paniti et al., the characterization of almn1mg1 and single point incremental forming (spif) of the same material with 0.22 mm initial thickness have been conducted by mailto:viharos.zsolt@sztaki.hu acta imeko | www.imeko.org march 2020 | volume 9 | number 1 | 2 applying a design of experiments using l9 orthogonal array of taguchi. initial numerical simulations gave an unacceptable computing time in case of spif, but the continuous bending under tension (cbt) simulations gave similar results for the examined foil as in other documented cases using thicker sheets. the monitoring of servo motor currents allowed the estimation of the forming forces and a new crack monitoring method based on light sensors was given too. lingitz and sihn reported concepts to improve the quality of production plans using machine learning solutions. it was found that the reliability of the production plans and thus the planning quality (pq) can drop down to 25% in the first three days after plan creation due to uncertainties, e.g. inaccurate or insufficient planning data. production planners therefore use buffers in the form of inventories or extended transitional periods to create possibilities for implementing corrective measures. buffers, however, lead to increased coordination and control effort and to negative effects, e.g. on inventory, throughput time and capacity utilization. within a case study the authors demonstrate how machine learning can be used to predict cycle times. furthermore, the increased accuracy compared to the current method is shown. based on this, two approaches are presented, focusing on the reduction of gaps between master data and predicted data used during the production planning process. cipolletta et al. appointed that in order to enhance levels of security and reliability of power systems, allowing for advanced remote diagnostics, merging units (mus) play a key role. some of the benefits are a more efficient transmission of electricity and a better integration with renewable energy systems. in their paper an implementation of a stand alone merging unit (samu), compliant with the iec 61850-9-2 standard and based on a low cost arm microcontroller, is described. it acquires two signals, one voltage and one current, and it sends the samples over the ethernet connection. the results from the metrological characterization of the samu are here presented, the system exhibits less than 0.02 % of error on the voltage measurement, less than 0.06 % on current and less than 0.08 % on power measurements. the added value of harmonization and consistency of laboratories’ accreditation to the development of the global market is appointed by ribeiro et al. accreditation plays a fundamental role in the economy on a fair trade basis and to achieve this objective, it is necessary to ensure that the application and assessment to the standard requirements of iso/iec 17025, which is the reference standard for laboratory accreditation. the collaboration of the laboratory community for this purpose is necessary and the role of entities such as ilac laboratory committee is particularly relevant. it is also known that “the conformity assessment community including accreditation bodies is a conservative community” what reinforces the importance and the need of the diagnosis provided by this survey. this approach has also enabled an independent evaluation of national accreditation bodie’s performance as a way of supporting the evolution of accreditation. the fields of the selected papers represent the great diversity and colourfulness of the imeko tc10 community around the theme testing, diagnostics & inspection that results in an exciting annual scientific conference series of our imeko confederation. for further details, please visit the tc10’s website: https://www.imeko.org/index.php/tc10-homepage. we hope you will have a fruitful reading of this issue. zsolt jános viharos chair of imeko tc10 on testing, diagnostics & inspection lorenzo ciani guest editors editorial to additional papers dear reader, two additional papers are closing the first issue of acta imeko in 2020. the paper submitted by mikhalieva et al. is to deal with the ability to determine and control the normalized concentrations using the experimentally obtained dependencies of the active and reactive constituents of conductivity in the wide frequency range on the composition of the multicomponent fluid and the impurity concentrations. a method for controlling the qualitative and quantitative composition in water (cooling liquids) is developed. it is based on the comparison of the measured and experimentally established active conductivity component. the developed electric method allows quantitative and qualitative estimation of the composition of the coolant on the contents of controlled components in the short time (up to 2 seconds) in a non-laboratory environment and to ensure uninterrupted operation of the equipment. yi su et al. present investigation of the two methods, double weighing in air and hydrostatic weighing, for the determination of the volume of weights in the range from 5 kg down to 1 g. they present the mathematical equations of both methods and show that monte-carlo simulation is a suitable way to determine the measurement uncertainties and overcome the difficulties in dealing with correlated variables. it was found that the measurement uncertainties of the two methods are comparable and that double-weighing in air is an efficient method for determining the volume of weights below 1 kg. hope you will have an interesting and a fruitful reading of this issue of acta imeko. dušan agrež, editor-in-chief of acta imeko euramet cg-18 – state-of-the-art calibration guideline for non-automatic weighing instruments acta imeko issn: 2221-870x september 2019, volume 8, number 3, 10 – 18 acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 10 euramet cg-18 – state-of-the-art calibration guideline for non-automatic weighing instruments klaus fritsch1 1 mettler-toledo gmbh, im langacher 44, 8606 greifensee, switzerland section: research paper keywords: calibration; non-automatic weighing instruments; minimum weight; quality citation: klaus fritsch, euramet cg-18 – state-of-the-art calibration guideline for non-automatic weighing instruments, acta imeko, vol. 8, no. 3, article 3, september 2019, identifier: imeko-acta-08 (2019)-03-03 editor: rugkanawan wongpithayadisai, nimt, thailand received august 3, 2018; in final form may 8, 2019; published september 2019 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: klaus fritsch, e-mail: klaus.fritsch@mt.com 1. introduction the calibration of non-automatic weighing instruments (nawis) is one of the key activities in quality control and production that must be carried out rigorously in order to accomplish the pre-defined quality attributes of the weighing process. concerning nawis, many national calibration guidelines exist, most of which are based on the concepts described in the guide to the expression of uncertainty in measurement (gum) [1]. while these guidelines are generally quite similar, they differ in their details, which makes it difficult if not impossible to develop and use one single nawi calibration guideline on a global level. there have been recent activities in the scientific community to address this issue by creating a harmonised approach to the calibration of nawis that is based on an internationally recognised calibration guideline. this process is driven by euramet in the form of its calibration guideline euramet cg-18 ‘calibration of nonautomatic weighing instruments’, which has become the most widespread guideline in this field on a global level [2]. in most european countries, it acts as a basis for accrediting calibration laboratories according to iso/iec 17025 [3] and was furthermore transposed into an interamerican metrology system (sim) guideline. thus, it is frequently applied in the americas [4]. however, it has been – until very recently – virtually unknown in asia. this is about to change, as the first calibration laboratories have been granted accreditation based on euramet cg-18 in japan, thailand, malaysia, singapore, india, and indonesia. it is expected that the guideline will become more widely applied in asia, thus becoming an industry standard that is used on a global level. the value of euramet cg-18 is furthermore substantiated, as it not only describes how to derive uncertainty at the time of calibration, but also how to estimate the so-called uncertainty of a weighing result, characterising the performance of the instrument during day-to-day usage. this approach has significant practical importance, as it facilitates the assessment of the equipment’s performance against user-specific weighing tolerance requirements. one of the most important implications of the approach is the definition of the so-called minimum weight and the safe weighing range. the concept of minimum weight was added to euramet cg-18 in a recent revision and describes the smallest amount of mass that needs to be weighed on the instrument to achieve a relative measurement uncertainty that is smaller than the user-specific weighing process tolerance requirement. weighing quantities that are much larger than the abstract this paper presents a state-of-the-art strategy for calibrating non-automatic weighing instruments (nawis), based on the international calibration guideline euramet cg-18. while existing national and legacy guidelines have only dealt with measurement uncertainty at the time of calibration, euramet cg-18 also provides advice for the so-called uncertainty of a weighing result, which describes the device performance during day-to-day usage. furthermore, as a consequence of the uncertainty of a weighing result, the concept of minimum weight is introduced. the minimum weight is the smallest amount of mass that must be weighed on the device while still adhering to a predefined relative weighing tolerance requirement. the concept of minimum weight is the most important practical and quality relevant implication of nawi calibration, as it is the prerequisite to achieve the necessary accuracy commensurate with the quality requirements of the weighing process. mailto:klaus.fritsch@mt.com acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 11 minimum weight – in the safe range of the instrument – ensures adherence to the weighing tolerance requirement. in that regard, euramet cg-18 serves as a standard for achieving and maintaining weighing results with the necessary accuracy as required by the quality attributes of the weighing process. it also supports compliance with applicable normative requirements as glp, gmp, usp, food regulations, or iso 9001. this paper is structured as follows: after this introduction, section 2 defines the most important terms related to calibration that are described and used. the next section details the calibration procedure of euramet cg-18, followed by a part that describes the estimation of the standard uncertainty of the selected calibration points, and a further part on the expansion of the standard uncertainty. section 6 provides information on the estimation of the uncertainty of a weighing result, followed by an introduction to the concept of minimum weight and safe weighing range. finally, in the last section the major conclusions of this article are summarised. 2. metrology terminology related to calibration 2.1. calibration calibration is one of the key activities that must be performed periodically when instruments are used for quality relevant measurements. internationally, there are many standards that stipulate this requirement, such as iso 9001, good laboratory practice (glp), and good manufacturing practice (gmp) regulations, or standards concerned with food safety. almost everybody working in quality control and quality assurance, either in the laboratory or in the production environment, is familiar with the applicable requirements stipulated in these documents. however, there is no common understanding concerning the definition, implementation, or specific activities that comprise calibration. let us therefore start by defining calibration. calibration is a set of activities carried out on a measuring instrument to understand its behaviour. this is done by establishing a relationship between known values (measurement standards) and the associated measured values (indications). the relationship consists of a deviation and its associated uncertainty. the international vocabulary of metrology (vim) [5] defines calibration as follows: ‘[an] operation that, under specified conditions, in a first step, establishes a relation between the quantity values with measurement uncertainties provided by measurement standards and corresponding indications with associated measurement uncertainties and, in a second step, uses this information to establish a relation for obtaining a measurement result from an indication.’ it is evident that the relationship between the known and the measured values can only be established if the associated measurement uncertainties are derived. unfortunately, in practice, there is a widespread misconception about calibration, as many users outside of the calibration laboratory do not consider measurement uncertainty when ‘calibrating’ an instrument. measurement uncertainty is defined in the gum [1] as ‘parameter, associated with the result of a measurement that characterizes the dispersion of the values that could reasonably be attributed to the measurand.’ in simple terms, measurement uncertainty describes how far away from the true value a measurement result might reasonably be. the measurement results and the estimated uncertainty are usually documented in calibration reports or certificates. the iso/iec 17025 standard specifies the general competency requirements for laboratories to carry out tests and/or calibrations. accreditation by an accreditation body is a formal process that certifies that a calibration laboratory is competent and fulfils the requirements stipulated by iso/iec 17025. it is appropriate to note, at this point, that there is some misconception in the field concerning what constitutes calibration from an accredited provider. accreditation is awarded across a fixed scope or range of capability for each discipline of calibration. accreditation is not a ‘carte blanche’ for carrying out all calibrations for an instrument user who requires the service of an accredited calibration laboratory. in short, being accredited to iso/iec17025 or a similar standard means that a provider has been found to be capable of working to an accepted standard, within a stated working range, as documented in his scope of accreditation at a given level of uncertainty. therefore, calibrations can only be considered to be ‘accredited’ when the calibration work falls within the stated range of a calibration organisation’s accredited scope. 2.2. adjustment before detailing how weighing instruments can be calibrated and their respective measurement uncertainty estimated, it is important to emphasise another misconception about calibration: besides being calibrated, an instrument can also be adjusted. adjustment is defined in the vim as follows: ‘set of operations carried out on a measuring system so that it provides prescribed indications corresponding to given values of a quantity to be measured.’ in other words, when adjusting an instrument, its indications are modified in such a way that they correspond, as far as possible, to the quantity values of the measurement standards applied. unfortunately, many users apply the words calibration and adjustment interchangeably, incorrectly, or even randomly. quite often, they talk about calibrating a weighing instrument when they mean adjusting it. the vim also emphasises this point thus: ‘adjustment of a measuring system should not be confused with calibration, which is a prerequisite for adjustment. after an adjustment of a measuring system, the measuring system must usually be recalibrated.’ this statement highlights another important aspect of calibration: before an instrument is adjusted, it must be first calibrated in order to understand and document its behaviour. this is specifically important in order not to break the traceability chain of the preceding measurements on the instrument. equally, after an adjustment, the instrument must usually be recalibrated. quite often, users talk about an ‘as found’ calibration (a calibration before any modification [adjustment] is carried out) and about an ‘as left’ calibration (a calibration after any necessary adjustment and/or repair has been carried out). 2.3. verification besides calibration, measuring instruments can also be verified. usually, instruments need to fulfil predefined requirements, quite frequently expressed as tolerances or maximum permissible errors (mpe). the vim defines verification as follows: ‘provision of objective evidence that a given item fulfils specified requirements.’ acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 12 while calibration only establishes the relationship between measurement standards and indications (‘how well the instrument performs’), verification assesses the instrument as to whether it meets specific requirements or not (‘does the instrument perform well enough?’). usually, the outcome of verification is a ‘pass’ or a ‘fail’, while calibration itself does not provide an assessment. however, in many cases, the calibration procedure is the starting point for a subsequent assessment of the results. therefore, it is common practice to document the assessment as an annex to the calibration certificate. tolerances or maximum permissible errors can come from a variety of sources. with respect to weighing instruments, usually the manufacturer issues specifications for each balance or scale model. international or national testing recommendations and handbooks for weighing instruments used for applications involving commercial transactions (such as oiml r76-1 [6] or nist handbook 44 [7]) define mpe. furthermore, industryspecific regulations (like usp general chapter 41 [8]) can define tolerances. however, even more importantly, the users need to specify weighing tolerances that assure that the instrument performs well enough that they are able to fulfil their specific process requirements. in respect to the weighing application, these are the most important tolerances, as they have a direct impact on the quality of the final product. 3. calibration procedure of euramet cg-18 usually, a repeatability test, a test for errors of indication, and an eccentricity test are performed to assess the performance of the weighing instrument. concerning assessment of the normal use of the instrument or evaluation of the performance under special conditions of use, euramet cg-18 allows for flexible execution of the tests; however, adherence to specific minimum requirements in the tests is stipulated as explained in the following paragraphs. 3.1. repeatability test usually, a test load of about 0.5·max to max is quite common (max refers to the maximum capacity of the instrument). however, this test load is often reduced for instruments for which the test load would amount to several thousand kilograms. for multiple range and multi-interval instruments, a load below and close to the capacity of the range/interval with the smallest scale interval d may be sufficient. a special value for the test load may be agreed where this is justified in view of a specific application of the instrument. an example would be weighing standards or samples on analytical and micro-balances for which the typical quantity that is weighed is at the low end of the measurement range. here, a small repeatability test load at the lower end of the weighing range may be agreed. the test load should, as far as possible, consist of one single body. the load must be applied at least three times when it weighs 100 kg or more and at least five times for smaller loads. note that it is general practice to exceed the minimum requirement of euramet cg-18 for analytical and micro-balances by carrying out ten repeated weighings in order to obtain a more robust characterisation of the instrument’s behaviour. this is done because for these instruments, repeatability is typically a very large contribution factor to the standard uncertainty. repeatability is quantified by calculating the standard deviation of the repeated measurements: 𝑠 = √ 1 𝑛 − 1 ∑(𝐼𝑖 − 𝐼)̅ 2 𝑛 𝑖=1 , (1) with n being the number of repeated weighings, 𝐼𝑖 representing the individual indications, and 𝐼 ̅ being the mean value of the indications. 3.2. test for errors of indication this test requires at least five test points, distributed fairly evenly over the weighing range of the instrument. note that zero is considered a test point. this is because measurement uncertainty can be allocated to zero. consequently, an error of indication test with four physical test loads, plus the zero, fulfils the minimum requirements of euramet cg-18. the individual errors of indications 𝐸𝑗 are calculated as 𝐸𝑗 = 𝐼𝑗 − 𝑚𝑟𝑒𝑓,𝑗 . (2) 𝐼𝑗 represents the individual indications of the error of indication test points. the reference value of mass 𝑚𝑟𝑒𝑓,𝑗 of the test loads is usually approximated to its nominal value, 𝑚𝑁,𝑗 , or its conventional value, 𝑚𝑐,𝑗. the conventional mass value of a body is defined in oiml d28 [9] as being equal to the mass of a standard that balances this body under conventionally chosen conditions, i.e. at the reference temperature of 20°c, the reference air density of 1.2 𝑘𝑔 𝑚3 , and the reference density of the standard of 8,000 𝑘𝑔 𝑚3 . 3.3. eccentricity test the test is carried out with a test load of about max/3 or higher. depending on the shape of the load receptor, the number of the test points might vary. for rectangular and round platforms, usually four positions and the centre are taken as test points (see figure 1). from the indications 𝐼𝑖 obtained in the different positions, the differences 𝛥𝐼𝑒𝑐𝑐,𝑖 are calculated as 𝛥𝐼𝑒𝑐𝑐,𝑖 = 𝐼𝑖 − 𝐼1 , (3) with 𝐼𝑖 representing the indications at the different positions of the load outside the centre and 𝐼1 being the centre indication. for estimation of uncertainty, the largest 𝛥𝐼𝑒𝑐𝑐,𝑖 (as an absolute value) is taken into account, which is abbreviated as |𝛥𝐼𝑒𝑐𝑐,𝑖 |𝑚𝑎𝑥 . figure 1. graphical indication of the eccentricity test points for various platform shapes (rectangular, circular, elongated). the round circles indicate the centre, while the intersection points of the diagonals determine the offcentre positions. acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 13 4. standard uncertainty for discrete values the first step in estimating the measurement uncertainty is deriving the so-called standard combined uncertainty for discrete values 𝑢𝑐 (𝐸). the basic formula for that step is 𝐸 = 𝐼 − 𝑚𝑟𝑒𝑓 , (4) with the related variance 𝑢𝑐 2(𝐸) = 𝑢2(𝐼) + 𝑢2(𝑚𝑟𝑒𝑓 ). (5) for better legibility, the index j was omitted. note that the above formulae apply for every error of indication test point, including zero. this means that for every error of indication test point, the associated standard uncertainty is derived individually. this is graphically indicated in figure 2. as can be seen from equation (5), the standard combined uncertainty for discrete values comprises of two main sources: 𝑢(𝐼), the standard uncertainty of the indication, and 𝑢(𝑚𝑟𝑒𝑓 ), the standard uncertainty of the reference mass. these two sources are also evident from the definition of calibration of the vim, which states that the quantity values provided by the measurement standards as well as the corresponding indications are associated with measurement uncertainties. 4.1. standard uncertainty of the indication the standard uncertainty of the indication comprises four individual contributions, which account for the rounding of the no-load indication; the rounding of the indication at load; and the repeatability and the eccentricity of the weighing instrument. 4.1.1. rounding of the no-load indication and the indication at load the rounding is taken into account twice, as any indication 𝐼 related to a test load is the difference between the indications at load, 𝐼𝐿 , and at no-load, 𝐼0. the uncertainty of rounding is derived based on the assumption of a rectangular distribution of the unrounded (analogue) measurement values. in other words, it is assumed that the probability of occurrence of any unrounded measurement value (which is not indicated on the instrument) is constant within the limits of ± 𝑑0 2 or ± 𝑑𝐿 2 , respectively, where 𝑑0 is the scale interval of the indication at zero load, and 𝑑𝐿 is the scale interval of the indication at load. the standard deviation of the rectangular distribution is 𝑑0 2√3 ≈ 0.29𝑑0 or 𝑑𝐿 2√3 ≈ 0.29𝑑𝐿 , leading to the variances 𝑑0 2 12 and 𝑑𝐿 2 12 , respectively. in case the scale intervals at no-load and at load are the same (abbreviated as d) the total variance accounting for the rounding of the indication would thus be derived as 𝑢𝑟𝑜𝑢𝑛𝑑𝑖𝑛𝑔 2 = 2 · 𝑑2 12 , leading to an uncertainty of 𝑢𝑟𝑜𝑢𝑛𝑑𝑖𝑛𝑔 = 𝑑 √6 ≈ 0.41𝑑. this value is also found in regulations (e.g. usp general chapter 41) and constitutes the lowest standard uncertainty of the indication that can be realised on a weighing instrument in the case that the standard deviation of the repeatability is zero and the uncertainty related to eccentricity is neglected. 4.1.2. repeatability normally, a single repeatability test is carried out during calibration as described in section 3.1. when estimating measurement uncertainty, the standard deviation s, as stated in equation (1), is considered as being representative of all indications of the instrument and is taken as the uncertainty contribution of repeatability for all error-of-indication test points. in the case that more than one repeatability test is carried out, euramet cg-18 describes specific rules on how to apply the standard deviations within the weighing range(s) of the instrument. these rules are not further detailed in this article. 4.1.3. eccentricity the standard uncertainty accounting for eccentricity, 𝑢𝑟𝑒𝑙 (𝛿𝐼𝑒𝑐𝑐 )𝐼, is approximated as a linear function of the indication 𝐼. the relative uncertainty 𝑢𝑟𝑒𝑙 (𝛿𝐼𝑒𝑐𝑐 ) is given as 𝑢𝑟𝑒𝑙 (𝛿𝐼𝑒𝑐𝑐 ) = |𝛥𝐼𝑒𝑐𝑐 ,𝑖|𝑚𝑎𝑥 2𝐿𝑒𝑐𝑐 √3 , (6) with |𝛥𝐼𝑒𝑐𝑐,𝑖 |𝑚𝑎𝑥 being the absolute value for the largest deviation between the corner indications and the centre indication, and 𝐿𝑒𝑐𝑐 being the test load applied. note that it is a well-established and accepted mathematical model assuming a linear relationship between the eccentric load deviation and the load applied (which is represented by the indication in the above formula). the actual behaviour of the eccentric load deviation might differ from a linear relationship based on the specific construction principle of the instrument; however, it is not common practice to assess this property at various measurement points. 4.2. standard uncertainty of the reference mass the standard uncertainty of the reference mass comprises of four individual contributions, which account for the maximum permissible error or uncertainty of the reference mass; air buoyancy; a possible drift of the reference mass since its last calibration; and convection effects due to a potential temperature difference between the reference mass and the instrument. 4.2.1. maximum permissible error/uncertainty of the reference mass either the maximum permissible error or the uncertainty of the reference mass is taken as a contribution to the standard combined uncertainty of the weighing instrument, depending on whether the nominal or the conventional value is selected as reference mass for the calculation of the individual errors of indication. figure 2. graphical indication of the error of indication test points with their respective measurement uncertainty from a calibration certificate (green circles for as found [before adjustment] and blue rhombi for as left [after adjustment] of the weighing instrument). the measurement uncertainty of the as left results is typically smaller than the measurement uncertainty of the as found results as the instrument is usually adjusted in-between the two calibrations in order to optimize its performance. acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 14 if the nominal value, 𝑚𝑁 , is selected, then the maximum permissible error of the weight, 𝑚𝑝𝑒, is taken into account, leading to the following standard uncertainty of the reference mass 𝑢(𝛿𝑚𝑐 ) = 𝑚𝑝𝑒 √3 . (7) international recommendations and specifications such as oiml r111-1 [10] or astm e617 [11] define maximum permissible errors for mass standards of various accuracy classes. if the conventional value, 𝑚𝑐 , is selected as reference mass, then the weight uncertainty 𝑈 and the respective coverage factor 𝑘 of the weight calibration certificate are taken into account, leading to 𝑢(𝛿𝑚𝑐 ) = 𝑈 𝑘 . (8) as both the abovementioned documents stipulate that the expanded uncertainty of the weight should not be larger than one third of the respective mpe, it is evident that the contribution of 𝑢(𝛿𝑚𝑐 ) can be minimised if 𝑚𝑐 is selected instead of 𝑚𝑁 . in the case that a test load consists of more than one standard weight, the standard uncertainties are added arithmetically, not by sum of squares, to account for the assumed correlations of the weights. 4.2.2. buoyancy in general, air buoyancy can be corrected if the density of the reference mass, 𝜌, and the air density at the time of calibration of the weighing instrument, 𝜌𝑎, are known. normally, however, the air density is not measured during the calibration of the weighing instrument, so an air buoyancy correction is not frequently carried out in practice. in these cases, the (unknown) buoyancy correction is taken into account as an intrinsic part of the buoyancy uncertainty. in this scenario, two cases must be distinguished: one in which the sensitivity of the instrument is adjusted immediately before calibration and the other in which the instrument is not adjusted before calibration. if the instrument is adjusted immediately before calibration, the respective buoyancy uncertainty is smaller, as the potential change in buoyancy due to different air densities at calibration and adjustment is removed. if the instrument is adjusted independent of the calibration, worst-case assumptions should be made regarding the potential air density variation between adjustment and calibration. if conformity of the standard weights with oiml r111-1 is established, recourse can be taken to section 10 of oiml r1111, which stipulates that the density of the weights should be close enough to the reference density of 8,000 𝑘𝑔 𝑚3 , so that a deviation of 10 % from the reference air density (1.2 𝑘𝑔 𝑚3 ) does not produce an error attributable to buoyancy that exceeds one quarter of the weight mpe. using this approach, the uncertainty due to buoyancy is estimated as one quarter of the uncertainty contribution of the weight itself, as expressed in equation (7). therefore, if the sensitivity of the instrument is adjusted immediately before calibration, the uncertainty due to air buoyancy can be derived as 𝑢(𝛿𝑚𝐵 ) ≈ 𝑚𝑝𝑒 4√3 , (9) where mpe is the maximum permissible error defined by oiml r111-1 for the selected weight. if the sensitivity of the instrument is not adjusted before calibration (thereby assuming a worst-case air density variation of 10 % of the reference density of air 𝜌0 between adjustment and calibration), then 𝑢(𝛿𝑚𝐵 ) ≈ 0.1 𝜌0 𝜌𝑐 𝑚𝑁 + 𝑚𝑝𝑒 4 √3 , (10) where 𝜌0 is the reference density of air, 1.2 𝑘𝑔 𝑚3 , and 𝜌𝑐 is the reference density of a standard weight, 8,000 𝑘𝑔 𝑚3 . the uncertainty due to buoyancy is typically in the order of a few ppm of the reference mass but strongly depends on whether the instrument was adjusted before calibration or not. when the instrument was not adjusted, the uncertainty due to buoyancy could be a factor of three or more higher compared to when the instrument was adjusted before calibration. equation (10) constitutes a highly conservative approach to estimating uncertainty due to buoyancy, and if there is evidence that air density variations are smaller than 0.1 𝜌0, this value should be substituted by a less conservative value. this is usually done by assuming realistic air pressure, temperature, and humidity variations at the installation location over the time of use of the weighing instrument, from which the respective air density variation can be calculated. 4.2.3. drift of the reference mass a possible drift of the reference mass 𝑚𝑐 since its last calibration can be assumed based on the difference in 𝑚𝑐 evident from the consecutive calibration certificates of the standard weights. the drift may be also estimated in view of the quality of the weights, as well as the frequency and care of their use, to a multiple (expressed by a factor 𝑘𝐷) of their expanded uncertainty 𝑈(𝛿𝑚𝑐 ). it is not advisable to apply a correction to the reference mass; rather, it includes the potential drift in the uncertainty instead: 𝑢(𝛿𝑚𝐷) = 𝑘𝐷𝑈(𝛿𝑚𝑐 ) √3 , (11) 𝑘𝐷 is usually chosen between one and three. for weights conforming to oiml r111-1 or astm e617, an upper limit 𝑚𝑝𝑒 √3 for the uncertainty contribution due to the drift of the reference mass can be applied, provided that subsequent weight calibrations confirm that the mpe of the applicable weight class is adhered to. 4.2.4. convection effects in the case that weights have been transported to the calibration site, they may not have the same temperature as the instrument and its environment. a temperature difference leads to a change in the apparent mass of the weights, 𝛥𝑚𝑐𝑜𝑛𝑣 , due to viscous friction at their surface induced by air flow originating from convection (figure 3). note that the term "apparent mass" is not a strict metrological term and not defined in oiml d28. in the context of the uncertainty of the reference mass, the apparent mass describes the fact that the gravitational force on the standard may be altered if a temperature difference between the standard and the environment in close vicinity to the instrument is present. if the calibration weight is warmer than the environment, the air that is in contact with the weight warms up and moves upward, and by means of viscous friction, its acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 15 apparent mass is reduced. if the weight is colder than the environment, the downward moving air increases its apparent mass. this effect should be taken into account either by allowing the weights to acclimatise to the extent that the remaining change 𝛥𝑚𝑐𝑜𝑛𝑣 is negligible in view of the uncertainty of the calibration or by considering the possible change of indication in the uncertainty budget. the effect may have to be considered for weights of a high accuracy class as oiml e2 or f1 weights. it is not advisable to apply a correction; rather, one should include the potential convection effects in the uncertainty instead: 𝑢(𝛿𝑚𝑐𝑜𝑛𝑣 ) = 𝛥𝑚𝑐𝑜𝑛𝑣 √3 . (12) appendix f of euramet cg-18 provides further information and tables for deriving 𝛥𝑚𝑐𝑜𝑛𝑣 . 4.3. summary – standard uncertainty for discrete values summarising the contributions as described above, the standard combined uncertainty for discrete values, 𝑢𝑐 (𝐸), can be derived as 𝑢𝑐 2(𝐸) = 𝑑0 2 12 + 𝑑𝐿 2 12 + 𝑠2(𝐼) + 𝑢𝑟𝑒𝑙 2 (𝛿𝐼𝑒𝑐𝑐 )𝐼 2 + 𝑢2(𝛿𝑚𝑐 ) + 𝑢 2(𝛿𝑚𝐵 ) + 𝑢 2(𝛿𝑚𝐷 ) + 𝑢2(𝛿𝑚𝑐𝑜𝑛𝑣 ) . (13) some of the factors contributing to the standard uncertainty might be relatively small, depending on the instrument and the accuracy class of the reference masses under consideration. it is within the responsibility and the competence of the calibration laboratory to neglect these contributions if appropriate. 5. expanded uncertainty at calibration after having derived the standard combined uncertainty at calibration, this uncertainty must be expanded by the coverage factor, 𝑘, which is chosen in a way such that the expanded uncertainty of measurement has a coverage probability of 95.45 %: 𝑈(𝐸) = 𝑘 𝑢𝑐 (𝐸) . (14) this means that the expanded uncertainty shall ensure that the true value, which is unknown, has a probability of at least 95.45 % within the interval ‘measured value ± expanded measurement uncertainty’. in the case that a normal (gaussian) distribution can be attributed to the error of indication and the standard uncertainty is sufficiently reliable, 𝑘 = 2 applies. normal distribution may be assumed where several uncertainty components, each derived from ‘well-behaved’ distributions (normal, rectangular, or similar) contribute to 𝑢𝑐 (𝐸) in comparable amounts. this is normally the case when calibrating nawis. sufficient reliability can be assumed, such as in the case that during a repeatability test, a load is applied no less than ten times. in the case that a normal distribution can be attributed to the error of indication but 𝑢𝑐 (𝐸) is not sufficiently reliable, then the coverage factor is determined mathematically according to the concepts described in appendix b of euramet cg-18 and gum. such a situation usually arises when a load is applied less than ten times during the repeatability test. due to the smaller number of the repeated weighings, the coverage factor is larger than two. the coverage factor depends on the effective degrees of freedom, 𝜈𝑒𝑓𝑓 , determined using the welch-satterthwaite formula: 𝜈𝑒𝑓𝑓 = 𝑢4(𝐸) ∑ 𝑢𝑖 4(𝐸) 𝜈𝑖 𝑁 𝑖=1 , (15) where 𝑢𝑖 (𝐸) refers to the contributions to the standard combined uncertainty, 𝑢𝑐 (𝐸), and 𝜈𝑖 represents the effective degrees of freedom of the standard uncertainty contribution, 𝑢𝑖 (𝐸). the coverage factor is subsequently calculated using the t-distribution or the student’s distribution. 6. uncertainty of a weighing result 6.1. interpretation of calibration data after calculating the expanded uncertainty, the calibration itself is formally completed. however, the data derived so far does not include the following information: 1) the behaviour of the instrument in between the selected error of indication test points; 2) the estimation of the measurement uncertainty in normal usage; and 3) the assessment of the instrument against specific requirements, such as weighing process tolerances or specifications. the calibration data offers information only for a limited amount of error-of-indication test points. selecting more errorof-indication test points does not resolve this problem, as this procedure still constitutes a limited picture of the real behaviour of the instrument and increases the time (and cost) of calibration. furthermore, calibration only assesses the behaviour of the instrument at the time of calibration, but not during normal usage. any potential influence that occurs before or after the calibration cannot be taken into account during the calibration itself. however, when assessing the instrument against specific requirements (such as weighing process tolerances or specifications), the normal usage should be taken into account as far as possible. euramet cg-18 offers detailed information on how to interpret calibration data, specifically concerning how to derive the so-called standard uncertainty of a weighing result, 𝑢𝑐 (𝑊). the standard uncertainty of a weighing result takes into account the normal usage of the instrument and allows for the estimation of the measurement uncertainty for any quantity of material that is placed on the weighing instrument during day-to-day use. the figure 3. the effect of convection, demonstrated with an erlenmeyer flask: in the left picture, the flask is warmer than the environment, leading to an upward convection, and in the right picture, the beaker is colder than the environment, leading to a downward convection. acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 16 uncertainty of a weighing result is frequently used for the assessment against weighing process requirements (tolerances); for example, in respect to minimum weight. this specific assessment is detailed later in this paper. the contributions to 𝑢𝑐 (𝑊) are usually either constant over the measurement range or depend linearly on the reading of the instrument, 𝑅, and thus may be grouped in two terms, 𝛼𝑊 2 and 𝛽𝑊 2 , 𝑢𝑐 2(𝑊) = 𝛼𝑊 2 + 𝛽𝑊 2 𝑅2 . (16) for multi-interval and multiple range instruments, the uncertainty of a weighing result is expressed per interval/range. note that the uncertainty of a weighing result is not covered by the accreditation as it is based on an interpretation of the calibration results and is not directly and exclusively derived from the measurement values of the instrument, which were taken during calibration. 6.2. expanded uncertainty of a weighing result the expanded uncertainty of a weighing result is determined by 𝑈(𝑊) = 𝑘𝑢𝑐 (𝑊) . (17) for 𝑈(𝑊), the coverage factor, 𝑘, is equal to two in most cases, even where the standard deviation, 𝑠, is obtained from only a few repeated weighings and/or where 𝑘 > 2 (at calibration) is stated in the calibration certificate. this is due to the large number of terms contributing to 𝑢𝑐 (𝑊). 6.3. errors included in uncertainty – the ‘global uncertainty’ it is common practice and could even be considered crucial to derive a ‘global uncertainty’, 𝑈𝑔𝑙 (𝑊), which includes the errors of indication such that no correction needs to be applied to the reading of a weighing result. in practice, applying corrections to the day-to-day work is inefficient and almost impossible. note that the concept of ‘global uncertainty’ as derived in euramet cg-18 is not a strict metrological concept, and the name might be misleading if not referenced properly. as explained above, the ‘global uncertainty’ is usually derived by adding the absolute of the error of indication to the expanded uncertainty of a weighing result. this ensures that the ‘global uncertainty’ associated with a specific weighing process that is carried out during normal usage of the instrument establishes a sufficiently high confidence level that the mass that is weighed lies within the interval ‘measured value ± global uncertainty’, without correction of the measured value. therefore, the weighing result in its final form can be expressed as 𝑊 = 𝑅 ± 𝑈𝑔𝑙 (𝑊) , (18) where r is the reading on the device, and 𝑈𝑔𝑙 (𝑊) is the ‘global uncertainty’. a common approach to calculating the global uncertainty is to arithmetically add the expanded uncertainty of a weighing result, 𝑈(𝑊), and the absolute value of the error of indication, 𝐸(𝑅), reflecting possible correlations between these two terms. 𝐸(𝑅) is quite often approximated by a linear equation: 𝐸(𝑅) = 𝑎1𝑅, with 𝑎1 being the linear regression coefficient, such that 𝑈𝑔𝑙 (𝑊) = 𝑘√𝛼𝑊 2 + 𝛽𝑊 2 𝑅2 + |𝑎1|𝑅 . (19) in order to facilitate the understanding and interpretation of the results, 𝑈(𝑊) = 𝑘√𝛼𝑊 2 + 𝛽𝑊 2 𝑅2, which is a rather complicated formula, is approximated by a linear equation, such that 𝑈𝑔𝑙 (𝑊) ≈ 𝑈(𝑊 = 0) + 𝑈(𝑊 = 𝑀𝑎𝑥) − 𝑈(𝑊 = 0) 𝑀𝑎𝑥 𝑅 + |𝑎1|𝑅 . (20) by introducing the parameters 𝛼𝑔𝑙 and 𝛽𝑔𝑙 , the ‘global uncertainty’ can be expressed as 𝑈𝑔𝑙 (𝑊) = 𝛼𝑔𝑙 + 𝛽𝑔𝑙 · 𝑅 (21) with the intercept 𝛼𝑔𝑙 (i.e. the uncertainty without load) and the slope 𝛽𝑔𝑙 (i.e. the parameter describing the increase of the uncertainty when larger loads are applied on the instrument). the corresponding formulae for multi-range and multi-interval instruments are derived accordingly. in the next section of this article, unless otherwise noted, uncertainty always refers to global uncertainty. the term ‘global’, the indexes ‘gl’, and the symbol ‘w’ are omitted to increase legibility. 7. minimum weight and safe weighing range due to its importance in practice, the concept of minimum weight was included in euramet cg-18 in a recent revision. ‘minimum weight’ is defined in the literature as ‘…the smallest sample quantity required for a weighment to just achieve a specified relative accuracy of weighing’ [12], [13]. it is general practice for users to define specific requirements for the performance of a weighing instrument (user requirement specifications). concerning accuracy, a requirement is normally expressed as an upper threshold for the relative measurement uncertainty 𝑈𝑟𝑒𝑙 = 𝑈/𝑚 that is acceptable for a specific weighing application, and it is referred to as weighing process accuracy or a weighing tolerance requirement. consequently, for a given relative tolerance requirement, tol, only weighments with a load m fulfilling the relative uncertainty inequality, 𝑈 𝑚 ≤ 𝑇𝑂𝐿, (22) comply with the respective user requirement. the limit value 𝑚𝑚𝑖𝑛 = 𝑈/𝑇𝑂𝐿, i.e. the smallest load that fulfils the requirement, is called minimum weight. figure 4 visualises the relative measurement uncertainty 𝑈𝑟𝑒𝑙 as a function of the applied load by using the model in equation (21). it is evident that the smaller the load is, the larger the relative measurement uncertainty becomes, and for quantities smaller than the minimum weight, the relative measurement uncertainty exceeds the weighing tolerance requirement. as the measurement uncertainty may be difficult to estimate due to environmental changes (vibrations, draughts, and operator influence) or specific influences of the weighing application (electrostatically charged samples, magnetic stirrers), a safety factor sf that is larger than one is usually applied, such that the tolerance requirement divided by the safety factor acts as the applied user requirement: 𝑈 𝑚 ≤ 𝑇𝑂𝐿 𝑆𝐹 . (23) acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 17 consequently, using equation (21), the minimum weight (including safety factor 𝑚𝑚𝑖𝑛,𝑆𝐹 ) can be calculated as follows: 𝑚𝑚𝑖𝑛,𝑆𝐹 = 𝛼 · 𝑆𝐹 𝑇𝑂𝐿 − 𝛽 · 𝑆𝐹 . (24) this calculation leads to the definition of the safe weighing range: the range of the instrument, where the user fulfils the weighing tolerance requirement and takes into account the defined safety factor. the lower boundary of the safe weighing range is given by 𝑚𝑚𝑖𝑛,𝑆𝐹 . as shown in figure 4, weighing quantities in the red region results in non-compliance with the tolerance requirement, while weighing quantities in the green region ensures the user requirement is fulfilled (safe weighing range). weighing quantities in the yellow area fulfils the tolerance requirement; however, the safety factor is not taken into account. the smallest quantity that the user wants to weigh on the device on a day-to-day basis is called the smallest net weight. with that, we can state: when the smallest net weight equals or is larger than the minimum weight (including the safety factor [i.e. when the smallest net weight is in the safe weighing range]), then the user requirement is fulfilled. the minimum weight and the safe weighing range refer to the net (sample) mass that is weighed on the instrument, i.e. the tare vessel mass must not be considered as fulfilling the tolerance requirement. therefore, minimum weight is frequently called ‘minimum sample weight’. it is important to note that for weighing small loads on analytical and micro-balances, the dominant factor that contributes to the uncertainty stems from repeatability. as for these laboratory applications, the minimum weight is typically a very small sample size compared to the capacity of the balance. the absolute uncertainty at the minimum weight can be approximated by 𝛼, and the increase of uncertainty with mass that is characterised by 𝛽 can be neglected. furthermore, 𝛼 mainly consists of the uncertainty contributions due to the rounding error of the indication and repeatability, with repeatability being the dominant factor for such types of instruments (the uncertainty due to eccentricity errors is usually very small compared to the uncertainty from rounding and repeatability). consequently, the minimum weight for applications when small loads are weighed on analytical and micro-balances (for a safety factor of 1) can be approximated as 𝑚𝑚𝑖𝑛 = 𝛼 𝑇𝑂𝐿 − 𝛽 ≈ 𝛼 𝑇𝑂𝐿 ≈ 𝑘 ∙ 𝑠 𝑇𝑂𝐿 . (25) an example of this simplified approach to minimum weight is presented in usp general chapter 41. in this regulation, the repeatability requirement is defined as: ‘repeatability is satisfactory if two times the standard deviation of the weighed value, divided by the desired smallest net weight, does not exceed 0.10 %.’ in other words, repeatability is considered the only significant contribution to the standard uncertainty. this approach is valid for the affected weighing applications, as typically analytical and micro-balances are used to weigh small quantities of samples or standards. in both usp general chapters 41 and 1251 [14], the concept of minimum weight as described above is included. taking the repeatability criterion of usp general chapter 41, the minimum weight is derived as 𝑚𝑚𝑖𝑛 = 𝑘 · 𝑠 𝑇𝑂𝐿 = 2 · 𝑠 0.10 % = 2000 · 𝑠 . (26) 8. conclusions the calibration of measuring instruments is among the most important activities within any quality management system. unfortunately, industry practices with respect to weighing instruments do not always appropriately reflect state-of-the-art concepts. the most evident shortcoming was the lack of a scientifically correct estimation of the measurement uncertainty, which is needed to assess whether the instrument under consideration fulfils the predefined process tolerances. the euramet cg-18 calibration guideline is the most widespread reference document that details the methodology of deriving the measurement uncertainty of non-automatic weighing instruments. it not only includes information concerning uncertainty at calibration, but it also includes information on the uncertainty of a weighing result that describes the performance of the instrument during day-to-day work. the guideline frequently serves as a basis for assessing an instrument against predefined tolerances. a critical consequence of calibration is the concept of the minimum weight. by determining the minimum weight, a user can assure compliance with their weighing requirements by weighing a sufficiently higher quantity of material than the minimum weight, expressed quantitatively by the safety factor. the minimum weight under consideration of the safety factor defines the lower boundary of the safe weighing range and weighing quantities of material within the safe weighing range ensures compliance with the required weighing process tolerance. in simple words, calibration and the subsequent interpretation of its data establishes the minimum weight and the safe weighing range, and it ensures that the user meets the applicable quality requirements. references [1] jcgm, ‘evaluation of measurement data – guide to the expression of uncertainty in measurement’, jcgm 100, 2008. [2] euramet, ‘guidelines on the calibration of non-automatic weighing instruments’, euramet cg-18, 4th ed., 2015. figure 4. safe weighing range for an industrial scale calculated from the calibration data and presented in an annex to a calibration certificate. the xaxis denotes the mass of the object applied on the weighing pan, while the yaxis denotes the associated relative measurement uncertainty in percentage. further to the required weighing tolerance of 1 %, the smallest net weight of 10.00 kg is the mass of the smallest sample that the user intends to weigh on the balance on a day-to-day basis. in this example, the smallest net weight is in the safe weighing range, i.e. it is larger than the minimum weight including the safety factor. thus, the calibration results have shown that the instrument fulfils the user requirement. acta imeko | www.imeko.org september 2019 | volume 8 | number 3 | 18 [3] iso/iec, ‘general requirements for the competence of testing and calibration laboratories’, iso/iec 17025, 2017. [4] sim, ‘guidelines on the calibration of non-automatic weighing instruments’, 2008. [5] jcgm, ‘international vocabulary of metrology – basic and general concepts and associated terms’, jcgm 200, 3rd ed., 2012. [6] oiml, ‘non-automatic weighing instruments, part 1: metrological and technical requirements – tests’, oiml r76-1, 2006. [7] nist, ‘specifications, tolerances, and other technical requirements for weighing and measuring devices’, nist handbook 44, 2018. [8] usp, ‘balances’, usp general chapter 41, us pharmacopeia usp42 – nf37, 2019. [9] oiml, ‘conventional value of the result of weighing in air’, oiml d28, 2004. [10] oiml, ‘weights of classes e1, e2, f1, f2, m1, m1–2, m2, m2–3 and m3, part 1: metrological and technical requirements’, oiml r1111, 2004. [11] astm, ‘standard specification for laboratory weights and precision mass standards’, astm e617, 2018. [12] r. nater, a. reichmuth, r. schwartz, m. borys, p. zervos, dictionary of weighing terms a guide to the terminology of weighing, springer berlin heidelberg, 2009, isbn 978-3-64202013-1. [13] k. fritsch, ‘gwp® the science-based global standard for efficient lifecycle management of weighing instruments’, ncsli, measure j. meas. sci. 8, 3 (2013) pp. 60-69. [14] usp, ‘weighing on an analytical balance’, usp general chapter 1251, us pharmacopeia usp42 – nf37, 2019. 9-24-5-ce acta imeko december 2011, issue 0, 1 www.imeko.org acta imeko | www.imeko.org december 2011 | issue 0 | 1 editorial paul p. l. regtien 1 1 measurement science consultancy, julia culpstraat 66, 7558jb hengelo, the netherlands keywords: acta imeko, editorial citation: paul p.l. regtien, editorial, acta imeko, no. 0, december 2011, p. 1, identifier: imeko-acta-00(2011)-01-01 editor: paul regtien, measurement science consultancy, the netherlands received december 28, 2011; in final form december 29, 2011; published december 30, 2011 copyright: © 2011 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: this work was supported by measurement science consultancy, the netherlands corresponding author: paul p. l. regtien, e-mail: paul@regtien.net the revival of acta imeko the first issue of the official publication of imeko was published in 1958, the year that imeko was founded. the title of this issue was: acta imeko 1958 ... proceedings of the international measurements conference ... budapest 240-30. xi. 1958 ... / [issued by the] hungarian sci. soc. for measurement & automation. [text in hungarian, german, eng. and fr.]. the issue contains the papers presented at the first imeko world congress, an event that is organized every three years. during many years, the contributions to imeko world congresses were published in acta imeko. later, the proceedings of the world congresses were issued by the organizers as proceedings, and the name acta imeko disappeared from these printed versions. the printed proceedings were rapidly replaced by proceedings on cdrom’s, and recently on memory sticks. the proceedings of the tc-events followed the same development. selected papers from world congresses and tc events were published in a newly established imeko journal “measurement” which first issue came out in 1983. this journal has been evolved gradually into an international journal (published by elsevier), with scientific articles not only from imeko events but from any scientist working in the field of measurement and instrumentation. the limited number of issues per year prevents publication of the many qualified contributions to imeko workshops and congresses. this is a major reason for imeko to start a new journal, obviously an electronic only journal, and named acta imeko as a modern successor of the original printed versions. paul regtien the proposed new si: consequences for mass metrology acta imeko june 2014, volume 3, number 2, 3 – 8 www.imeko.org the proposed new si: consequences for mass metrology roman schwartz, michael borys physikalisch-technische bundesanstalt, bundesallee 100, 38116 braunschweig, germany section: research paper keywords: new si; redefinition; kilogram; mass. citation: roman schwartz, michael borys, the proposed new si: consequences for mass metrology, acta imeko, vol. 3, no. 2, article 3, june 2014, identifier: imeko-acta-03 (2014)-02-03 editor: paolo carbone, university of perugia received february 14th, 2014; in final form may 25th, 2014; published june 2014 copyright: © 2014 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: (none reported) corresponding author: roman schwartz, e-mail: roman.schwartz@ptb.de 1. introduction of the seven base units of the si, only the kilogram is still defined in terms of a material artefact, namely the international prototype of the kilogram (ipk) kept at the bipm [1]. since the third verification of national prototypes of the kilogram (npk) against the ipk in the period 1989 to 1991 the stability of the ipk has been put into question, because the results of comparisons between the npks and the ipk show some divergence with time, the relative mass changes being in the order of 50 µg during a period of about 100 years. although the mass of the ipk is certainly not stable, neither could its drift yet be determined absolutely with sufficiently small uncertainty nor could it be clarified whether the observed mass changes are primarily due to a drift of the npks, or of the ipk, or of both. unknown changes in the mass unit also influence the electrical units, because the definition of the ampere is related to the kilogram, see figure 1. similarly, the definitions of the mole and candela also depend on the kilogram. in 1999 the cgpm therefore recommended that efforts be continued to refine experiments linking the unit of mass to fundamental constants with sufficiently high accuracy (some parts in 108), which preserves continuity in practical mass determination after a new definition of the kilogram is introduced. 2. proposed changes to the si at its 24th meeting in 2011, the cgpm adopted its resolution 1 "on the possible future revision of the international system of units, the si" [2] in which the cgpm takes note of the intention of the international committee for weights and measures (cipm) to redefine not only the kilogram, but all seven si base units, in terms of invariants of nature and to express all definitions uniformly. with this, the cgpm and the cipm clearly intend to revise the si with a view that it continues to meet the needs of science, technology, and commerce in the 21st century. the proposed changes to the si can be summarized as follows [3]. figure 1. the seven base units and their relationship in the current si. abstract at its 24th meeting in october 2011 the general conference on weights and measures (cgpm) adopted a resolution on the possible future revision of the international system of units (si). this paper provides an overview of the proposed changes to the si, the focus being on the proposed redefinition of the kilogram and possible consequences for mass metrology. acta imeko | www.imeko.org june 2014 | volume 3 | number 2 | 3 • keep the existing seven si base units, but define them all in terms of seven well-recognized fundamental or atomic constants, such as the planck constant h, see figure 2. • fix the values of all these constants to an exact number (with zero uncertainty), as is already the case for the speed of light in vacuum, c = 299 792 458 metre per second. • use "explicit-constant" formulations to express the definitions of all seven si base units in a uniform (but indirect) manner. • draw up specific "mise en pratique" (i.e. sets of instructions) for each base unit to explain how the units can be practically realized based on recommended top-level methods. the kilogram would be defined in terms of the planck constant h, the ampere in terms of the elementary charge e, the kelvin in terms of boltzmann’s constant k, and the mole in terms of the avogadro constant na. the second would still be defined in terms of the hyperfine splitting frequency of the ground state of the caesium 133 atom, ∆ν(133cs)hfs, but the formulation would be changed into an "explicit-constant" one, where the unit (here the second) would be defined indirectly by specifying explicitly an exact value for ∆ν(133cs), namely 9 192 631 770 hz. the same would hold for the metre (defined in terms of the speed of light in vacuum c), and the candela (defined in terms of the luminous efficacy kcd of monochromatic radiation of frequency 540 thz). the cgpm resolution 1 (2011) highlights the following advantages of such a "new si": • the uncertainties of all si electrical units together with the si values of the josephson and von klitzing constants kj and rk would be significantly reduced. • the kelvin would no longer be defined in terms of an intrinsic property of water that, while being an invariant of nature, in practice depends on the purity and isotopic composition of the water used. • the mole would no longer depend on the definition of the kilogram; this would emphasize the distinction between the quantities "amount of substance" and "mass". • the uncertainties of the values of many other important fundamental constants and energy conversion factors would be eliminated or greatly reduced. 3. redefinition of the kilogram in the present si, one kilogram is defined as exactly the mass of the ipk, see figure 3. according to draft chapter 2 of the 9th si brochure [4], the new explicit-constant definition of the kilogram would read: "the kilogram, kg, is the unit of mass; its magnitude is set by fixing the numerical value of the planck constant to be equal to exactly 6.626 06x × 10−34 when it is expressed in the unit s−1 m2 kg, which is equal to j s. " the exact value for x, which will be fixed by the latest codata adjustment at the time of the redefinition [5], requires further experimental effort to reach a sufficiently small relative measurement uncertainty in the order of 10-8. the consultative committee for mass and related quantities (ccm) is currently drafting a mise en pratique for the redefined kilogram. this will explain how the kilogram can be realized in the future by different primary methods (e.g. the “avogadro method”, also known as “x-ray crystal density (xrcd) method”, or the “watt balance method”) using primary mass standards, and how the npk or other secondary mass standards of national metrology institutes (nmi) can be linked to the primary mass standards and the planck constant h, see figures 4 and 5. compared with the redefinition of other base units the redefinition of the kilogram is the most critical one, for several reasons: • accurate weighings and mass determinations are of extraordinary importance in science, trade and industry, • there are partly very high demands on the accuracy of mass determinations. class e1 [7] accredited mass laboratories, for instance, keep reference standards with relative uncertainties between 2.5×10-8 and 5×10-8, • if the value for the planck constant h were fixed too early, there would be a risk of jumps in the order of 10-7 in mass values of precision mass standards, with consequences for the adjustment and verification of both class e1 and e2 weights according to oiml r 111 [7]. at its meeting in 2010 the ccm therefore recommended that the following conditions be met before the kilogram is redefined in terms of fundamental constants [8]: figure 2. definition of and relationship between the seven base units in the proposed new si. in the new si all base units will be defined in terms of fundamental or atomic constants. the changes to the current si, and the new relationships, are marked in red. the black arrows denote relationships that remain unchanged in the new si. figure 3. the ipk is kept at the bipm in sèvres. it still defines the unit of mass, the kilogram. its mass, mipk, is defined to be exactly 1 kg with zero uncertainty. in the proposed new si it will have non-zero uncertainty (source: bipm, sèvres, france). acta imeko | www.imeko.org june 2014 | volume 3 | number 2 | 4 1. at least three independent experiments, including work both from the watt balance and international avogadro coordination projects, yield values of the relevant constants with relative standard uncertainties not larger than 5×10-8. at least one of these results should have a relative standard uncertainty not larger than 2×10-8. 2. for each of the relevant constants, values provided by the different experiments should be consistent at the 95% level of confidence. 3. traceability of bipm prototypes to the international prototype of the kilogram should be confirmed. in addition, ccm recommendation g1 (2010) states the following: 4. the codata recommended values should be adopted for the relevant fundamental constants. 5. the associated codata relative standard uncertainties should be suitably considered when the initial uncertainty is assigned to the international prototype of the kilogram. 6. a pool of reference standards should be established at the bipm to facilitate the dissemination of the redefined kilogram. 7. the bipm and a sufficient number of national metrology institutes should continue to operate, develop or improve facilities or experiments that allow the realization of the kilogram to be maintained with a relative standard uncertainty not larger than 2×10-8. 8. the uncertainty component arising from the practical realization of the unit should be suitably taken into account. the above ccm recommendations, especially the first three, have not yet been met, but researchers are actively working to understand the differences in the experimental results and close the gaps. currently, only two experiments have achieved published relative uncertainties smaller than 5×10-8 (3.6×10-8 for the nist watt balance [9] and 3×10-8 for the international avogadro coordination experiments [10]). these results are discrepant by 1.7×10-7 as can be seen in figure 6. the discrepancy between the recently published result of the nrc watt balance [11] and that of the nist watt balance is even larger and amounts to about 2.6×10-7, see figure 6. 4. consequences for mass metrology while there is no doubt that a redefinition of the kilogram is desirable from the point of view of fundamental physics, the practical implications of a redefinition must be carefully considered beforehand in order to avoid negative consequences, not only for high-precision mass measurements, but also for the si in general. considering the extraordinary importance of weighing instruments, mass comparators and mass standards used in trade, industry and science, and also in daily life, the possible practical consequences and negative implications of a premature or even wrong decision could be tremendous. despite its limitations, the current definition of the kilogram has quite successfully guaranteed up to now, that all over the world high-precision mass standards and weights of accuracy classes e2, e1 and even better are calibrated and used in the global market without any problems. based on the cipm mutual recognition agreements (mra) [15] calibration certificates have meanwhile achieved a high degree of worldwide acceptance. a thorough examination of the realization, dissemination chain and uncertainty propagation for the redefined kilogram shows that [16, 17]: • if all the above ccm recommendations are closely observed and met, no serious changes in the calibration chain of mass standards will occur, including high-precision mass standards and weights of class e1, • however, even if the ccm recommendations are met, the uncertainty values in the "calibration and measurement capabilities" (cmcs) of nmis will increase by up to a factor of 2, at least in the range from 100 g to 10 kg, see figure 7, • if the ccm recommendations were not met, mass standards of high accuracy with a relative uncertainty smaller than or equal to 4×10-8, as presently offered by nmis, would no longer be available, and figure 5. existing primary methods for the realization of the new kilogram are the “avogadro method” (or “x-ray crystal density (xrcd) method”) and the “watt balance method”. left: view of a single crystal silicon sphere. the diameter of the sphere (about 90 mm), and hence, its volume is measured with a fizeau interferometer, which forms part of the xrcd method (source: ptb braunschweig, germany). right: view into the bipm watt balance [6]. figure 4. proposed future realization of the kilogram with primary methods that would link primary mass standards to the fundamental constant h, followed by the classical way of dissemination using the primary mass standards to calibrate secondary ones. acta imeko | www.imeko.org june 2014 | volume 3 | number 2 | 5 • there would be the risk that accredited calibration laboratories would no longer be able to calibrate class e1 weights according to oiml r 111; this would rather be the exclusive task of certain nmis, which have sufficiently accurate mass standards available. there is another risk of a premature redefinition if one or more of the above ccm conditions are not closely observed or are ignored. most likely the codata recommended values will be the basis for fixing the relevant fundamental constants, which is the planck constant h in case of the kilogram. figure 8 shows the long-term mass changes of platinum-iridium kilogram prototypes since 1889 [18]. this is contrasted with the values for h resulting from the codata adjustments since 1998 [5, 19]. the codata values jump within four years by about 1×10-7 which is a factor two worse than the assumed instability of the ipk during the past hundred years. it is obvious that jumps of such an order must at all be avoided for mass calibrations. the consequence for high-precision mass standards could be that the respective calibration certificates would have to be re-issued by the mass calibration laboratories responsible. figure 6. measurement results for the avogadro constant na with standard uncertainties (k = 1), represented as relative deviations from the codata 2006 value (na = 6.02214179(30)x10 23 mol-1). the results for the planck constant, h, (watt balance) and for kj (voltage balance) have been converted by means of the codata 2006 constants. explanations: "npl-07-h", for example, means: npl’s result in 2007 for a measurement of h. wgac: working group avogadro constant, iac: international avogadro coordination. currently, the three results with the smallest uncertainties are: (1) nist-07-h (ur = 3.6x10 -8), (2) iac-10-na (ur = 3.0x10 -8) and (3) nrc-12-h (ur = 6.5x10 -8). measurement results and codata values according to the data published in [5, 12, 13]. figure 7. calibration and measurement capabilities (cmcs) for mass standards in the range from 10 g to 100 kg of eight selected nmis [14] compared with different uncertainty limits for the realization of a redefined kilogram and relative standard uncertainties of the best primary density standards (see pointer "density"), for reference standards of class e1 calibration laboratories (see range indicated), and for class e1 weights (see pointer "class e1"). 0.0e+00 2.0e-08 4.0e-08 6.0e-08 8.0e-08 1.0e-07 0.01 0.1 1 10 100 u( m ) / m m / kg cem cms inrim kriss lne nist npl ptb density 2 × 10−8 3 × 10−8 1 × 10−7 ref. standards of class e1 cal. lab. class e1 1 × 10−8 } acta imeko | www.imeko.org june 2014 | volume 3 | number 2 | 6 in response to resolution 1 of the cgpm, the international organization for legal metrology (oiml) carried out an inquiry amongst its member states and its relevant technical committees in 2012, in order to explore in more detail the possible practical consequences of a revised si in general, and a redefined kilogram in particular. the results of that inquiry and the official statement of the oiml according to ciml resolution no. 25 (2012) [20] were recently published [19]. also in response to resolution 1 of the cgpm, the european weighing industry association (cecip), an important stakeholder in the field of mass measurement, submitted to the bipm a position paper on the revision of the si in 2012 [21]. the paper clearly addresses the potential negative impacts that a premature redefinition of the kilogram could have on precision mass measurements, but also on the si in general. in addition, cecip requests to improve the proposed new definitions, especially that of the kilogram, in order that the new si remains understandable for its users. 5. summary and conclusion this paper outlines the background and most important changes that are proposed to revise the present si. it also describes the current status of the redefinition of the kilogram, the relevant experiments, and the respective recommendations of the ccm that are intended to ensure the continuity of mass values before and after the redefinition of the kilogram at a level of about 2×10-8. it can be shown that if all ccm conditions and recommendations are closely observed and met, no serious changes in the calibration chain of mass standards would occur, although an increase of the best uncertainties of the cmc entries by up to a factor of two must be accepted. if the conditions of the ccm are not closely observed, however, especially if there is not enough patience to wait for the results of the very active experimental work which is ongoing across the globe to further improve the data for h and na as regards precision, uncertainty and consistency, there is a high risk that a premature redefinition of the kilogram could lead to significant jumps in the order of 10-7 in the mass values of high-precision mass standards having relative uncertainties as small as 4×10-8 and less. as a consequence, accredited industrial calibration laboratories could even loose their ability to calibrate class e1 weights according to oiml r 111. references [1] "on the possible future revision of the si", http://www.bipm.org/en/si/new_si/. [2] http://www.bipm.org/utils/en/pdf/24_cgpm_resolution_1.pdf. [3] http://www.bipm.org/en/si/new_si/what.html. [4] draft chapter 2 for si brochure, following redefinitions of the base units", ccu, 29 september 2010, http://www.bipm.org/ /utils/common/pdf/si_brochure_draft_ch2.pdf . [5] p. j. mohr, b. n. taylor and d. b. newell, “codata recommended values of the fundamental physical constants: 2010”, rev. mod. phys., vol. 84, no. 4, pp. 15271605, 2012. [6] http://www.bipm.org/en/scientific/elec/watt_balance/wb_bip m.html. [7] oiml r 111-1, "weights of classes e1, e2, f1, f2, m1, m1–2, m2, m2–3 and m3, part 1: metrological and technical requirements", ed. 2004, http://www.oiml.org/publications/r/r111-1-e04.pdf. [8] ccm recommendation g 1 (2010), “considerations on a new definition of the kilogram”, http://www.bipm.org/utils /common/pdf/ccm12.pdf#page=23. figure 8. mass changes of the six official copies (dashed lines) and national prototypes nos. 2 to 55 against the ipk since 1889 [18], compared with relative changes of codata values for the planck constant, h, since 1998 [5, 12, 13], where the codata 2006 value was (arbitrarily) chosen as a reference value. codata 1998 codata 2002 codata 2006 codata 2010 -100 -80 -60 -40 -20 0 20 40 60 80 100 0 20 40 60 80 100 120 p ar ts in 1 09 years since 1889 codata values for the planck constant: relative differences between codata results since 1998 and the codata 2006 value acta imeko | www.imeko.org june 2014 | volume 3 | number 2 | 7 http://www.bipm.org/utils/common/pdf/ccm12.pdf%23page=23 http://www.bipm.org/utils/common/pdf/ccm12.pdf%23page=23 [9] r. l. steiner, e. r. williams, r. liu and d. b. newell, "uncertainty improvements of the nist electronic kilogram", ieee trans. instrum. meas., vol. 56, pp. 592-596, 2007. [10] b. andreas, y. azuma, g. bartl, p. becker, h. bettin, m. borys, i. busch, m. gray, p. fuchs, k. fujii, h. fujimoto, e. kessler, m. krumrey, u. kuetgens, n. kuramoto, g. mana, p. manson, e. massa, s. mizushima, a. nicolaus, a. picard, a. pramann, o. rienitz, d. schiel, s. valkiers, a. waseda, "determination of the avogadro constant by counting the atoms in a 28si crystal", phys. rev. lett., vol. 106, 030801, 2011. [11] a. g. steele, j. meija, c. a. sanchez, l. yang, b. m. wood, r. e. sturgeon, z. mester and a. d. inglis, "reconciling planck constant determinations via watt balance and enriched-silicon measurements at nrc canada", metrologia, vol. 49, pp. l8–l10, 2012. [12] p. j. mohr and b. n. taylor, “codata recommended values of the fundamental physical constants: 2002”, rev. mod. phys., vol. 77, no. 1, pp. 1-107, 2005. [13] p. j. mohr, b. n. taylor and d. b. newell, “codata recommended values of the fundamental physical constants: 2006”, j. phys. chem. ref. data 37, pp. 1187–1284, 2008. [14] calibration and measurement capabilities – cmcs, http:// kcdb.bipm.or g/appendixc/. [15] cipm mutual recognition arrangement (mra): http://www.bipm.org/en/cipm-mra/. [16] m. gläser, m. borys, d. ratschko and r. schwartz, "redefinition of the kilogram and the impact on its future dissemination", metrologia, vol. 47, pp. 419–428, 2010. [17] r. davis, "proposed change to the definition of the kilogram: consequences for legal metrology", oiml bulletin, vol. lii, pp. 5-12, 2011. [18] g. girard, "the third periodic verification of national prototypes of the kilogram (1988–1992)", metrologia, vol. 31, pp. 317–336, 1994. [19] r. schwartz, p. richard, c. ehrlich., y. miki, “the proposed new si and consequences for legal metrology”, bulletin oiml, vol. liv, no.1, pp. 5-15, 2013. [20] http://www.oiml.org/download/docs/ciml/47_ciml_resolutions english.pdf. [21] cecip, position on possible future revision of the international system of units – si, http://www.bipm.org/ws/ccm/ mep_2012/allowed/november_2012/06a_cecip_position_o n_si_15_05_2012.pdf. acta imeko | www.imeko.org june 2014 | volume 3 | number 2 | 8 http://www.oiml.org/download/docs/ciml/47_ciml_resolutionsenglish.pdf http://www.oiml.org/download/docs/ciml/47_ciml_resolutionsenglish.pdf http://www.bipm.org/ws/ccm/%20mep_2012/allowed/november_2012/06a_cecip_position_on_si_15_05_2012.pdf http://www.bipm.org/ws/ccm/%20mep_2012/allowed/november_2012/06a_cecip_position_on_si_15_05_2012.pdf http://www.bipm.org/ws/ccm/%20mep_2012/allowed/november_2012/06a_cecip_position_on_si_15_05_2012.pdf the proposed new si: consequences for mass metrology torque standard comparison among provinces laboratories in china acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 184 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 184 188 torque standard comparison among provinces laboratories in china x. hu1, y. wang2, f. bao3, w. huang4 1,2,3 hubei institute of measurement and testing technology, wuhan, china, 392522537@qq.com 4 gtm china office, shanghai, china, william.huang@gtmchina.cn abstract: to estimate the situation of torque standard machines in different provinces’ laboratories within china, a torque standard comparison was carried out, for the first time. results were obtained and then analysed, with values of 𝐸n being calculated. the characteristics and the performance of the torque transfer standard transducer were indicated. we also discuss some problems during the comparison. keywords: torque comparison; province laboratory; torque transfer standard 1. introduction in recent years, more and more provinces’ laboratories have built torque standard machines. in china there are two traceability systems; one is to nim (national institutes of metrology of china) and the other to the military system of china (cimm). some laboratories derive traceability from nim and others from cimm. to estimate the situation of the province laboratories’ torque measuring capabilities, the hubei institute of metrology and testing technology piloted this comparison in 2019. in total, eight laboratories took part in this comparison. it covered the middle, eastern, western, and northern areas of china. it made sense to perform this comparison because no similar exercise had previously been carried out in this paper , we present the procedure, test results and analysis, performance of the torque transducer and some discussions. 2. equipment and machines used the torque transfer standards are two transducers: one of 100 n·m capacity and the other of 2 kn·m capacity. both are type dm-tn from gtm. these two transducers are equipped with etp (hydraulic brake) and couplings. all of them are fitted to the torque standard machines’ shafts for better alignment and performance. the torque standard machines at all laboratories were made by different machine manufacturers. to achieve the best accuracy of the measurements, we use the machine of 100 n·m or 2 kn·m. the test points are selected at 50 % and 100 % of the machine capacity. the amplifier 2026b has an accuracy of 0.005 %. 3. procedure the test procedure is according to the chinese verification guideline of torque standard machine: jjg 769. all laboratories were set to three groups according to the different machine manufacturers. the transfer route map is shown in figure 1. it makes a closed loop for each group. figure 1: route map of comparison the test was carried out in clockwise torque, including four points: 50 n·m, 100 n·m, 1 kn·m and 2 kn·m. the test points of 50 n·m and 100 n·m are with rotation at three positions: 0°, 120° and 240°. the 1 kn·m & 2 kn·m positions were not rotated, because the equipment of 100 n·m kit is light, but the equipment of 2 kn·m kit is too heavy. figure 2 is a comparison test steps chart of 100 n·m torque sensor. http://www.imeko.org/ mailto:392522537@qq.com mailto:william.huang@gtmchina.cn acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 185 figure 2: test steps for 100 n·m 4. test data 4.1 measurement deviation the measurement deviation is equal to the difference between the measurement value and the reference value divided by the reference value, expressed as a relative error. considering that the uncertainty of the measurement results of each laboratory is close (all around 0.04 %), the reference value is calculated by the arithmetic mean method. for example, in each comparison group, the pilot laboratory participated in two comparison measurements, and the reference laboratory participated in one. the average value of the above measurement results can be calculated to obtain the reference value. to ensure the comparability of the results, each comparison group has a reference value and an uncertainty, so we end up with three reference values, each with an uncertainty. table 1, table 2, and table 3 show the measurement deviation of the three comparison groups. table 1: measurement deviation, 1st group torque pilot lab a lab b lab c 50 n·m -0.012 % -0.020 % 0.043 % -0.010 % 100 n·m -0.014 % -0.017 % 0.044 % -0.013 % 1 kn·m 0.008 % ---0.001 % -0.026 % 2 kn·m 0.006 % --0.000 % -0.021 % table 2: measurement deviation, 2nd group torque pilot lab d 50 n·m 0.002 % -0.005 % 100 n·m -0.001 % 0.003 % 1 kn·m 0.008 % -0.016 % 2 kn·m 0.007 % -0.013 % table 3: measurement deviation, 3rd group torque pilot lab e lab f lab g 50 n·m -0.005 % -0.004 % -0.005 % 0.024 % 100 n·m -0.004 % -0.005 % -0.006 % 0.022 % 1 kn·m 0.004 % 0.005 % 0.001 % -0.014 % 2 kn·m 0.001 % 0.002 % 0.008 % -0.011 % 4.2 uncertainty table 4, table 5, and table 6 show the uncertainty of the comparative measurement results of each laboratory in the three comparison groups. the sources of uncertainty in each laboratory include extended uncertainty of the torque standard machine, measurement repeatability, azimuth error, resolution of the measuring instrument, zero error, alignment and temperature. here, the coverage factor k = 2. table 4: relative expanded uncertainty, 1st group torque expanded uncertainty / % pilot lab a lab b lab c reference value 50 n·m 0.040 0.040 0.050 0.036 0.021 100 n·m 0.040 0.040 0.051 0.036 0.021 1 kn·m 0.036 --0.049 0.034 0.023 2 kn·m 0.036 --0.049 0.034 0.023 table 5: relative expanded uncertainty, 2nd group torque expanded uncertainty / % pilot lab d reference value 50 n·m 0.038 0.034 0.025 100 n·m 0.038 0.034 0.025 1 kn·m 0.036 0.034 0.024 2 kn·m 0.036 0.034 0.024 table 6: relative expanded uncertainty, 3rd group torque expanded uncertainty / % pilot lab e lab f lab g reference value 50 n·m 0.036 0.044 0.040 0.047 0.021 100 n·m 0.036 0.044 0.040 0.046 0.021 1 kn·m 0.036 0.040 0.040 0.046 0.020 2 kn·m 0.036 0.040 0.040 0.046 0.020 4.3 normalized deviation value 𝑬n all the comparison test data were collected and analysed, and the normalized deviation value 𝐸n calculated as in equation (1). 𝐸n = 𝑥′−𝑥 √𝑈′ 2 +𝑈2 ,𝑘 = 2 . (1) where 𝑈′ is the extended uncertainty of a measuring point in a laboratory, and 𝑈 is the extended uncertainty of a reference value at a measurement point. the comparison criteria are: • |𝐸n| ≤ 1, satisfied • |𝐸n| > 1, not satisfied http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 186 figure 3, figure 4, and figure 5 show the histograms of three groups’ laboratories inner petals 𝐸n value. the pilot laboratory comparison data is involved in the calculation of the reference value and uncertainty in each petal. lab a, whose results are shown in figure 3, did not participate the comparison of 1 kn·m and 2 kn·m. in figure 3, figure 4, and figure 5, the 𝐸n values of each laboratory were less than 1, with satisfactory results. lab b is the only one which takes its traceability from cimm in all laboratories, resulting in larger 𝐸n values of 0.79 and 0.80 at torque levels of 50 n·m and 100 n·m respectively. all other laboratories, which are traceable to nim, show relatively smaller 𝐸n values. this shows that the difference of the traceability mechanism (the upper-level measurement technical institution) cause small difference in the torque standard machine’s test result, and the traceability to the same measurement technical institution can ensure the consistency of the torque standard machine value. figure 3: laboratory comparison results, 1st group figure 4: laboratory comparison results, 2nd group figure 5: laboratory comparison results, 3rd group 5. problem & discussion 5.1 stability test to verify the stability of the dm-tn/100 n·m and dm-tn/2 kn·m transducers, repeated sets of tests at comparison points of 50 n·m, 100 n·m, 1 kn·m and 2 kn·m were carried out. figure 6 and figure 7 show the stability test result of the two torque transducers mentioned above over a 10-month period. test data shown in the figure indicate that the fluctuations of the two torque transducers are less than 0.05 %. figure 6: stability test on transducer of 100 n·m figure 7: stability test on transducer of 2 kn·m 5.2 warming up transducers before each test, it is necessary to ensure that the torque transducer’s temperature is consistent and stable with the laboratory temperature. the pilot laboratory stipulates that the torque transducer should be placed in the laboratory for more than 12 hours. then connect the comparison transfer standard to the torque standard machine and connect the transducer to the instrument as a whole body to warm up. to verify the influence of the warm-up time on the performance of the torque transducers, the test was conducted with a small change in environmental conditions. the test result is shown in table 7. table 7: effect of warm-up time on measured data warmup time temperature / humidity output at 50 n·m / mv/v variation / % 0.5 h 18.4 °c / 41 % rh 0.998 930 -- 1 h 18.6 °c / 40 % rh 0.999 134 0.02 1.5 h 18.8 °c / 38 % rh 0.999 360 0.02 2 h 19.1 °c / 41 % rh 0.999 504 0.01 3 h 18.7 °c / 39 % rh 0.999 518 0.00 4 h 18.6 °c / 40 % rh 0.999 512 0.00 5.3 temperature change of transducers the humidity varies minimally during the test, while the temperature changes widely. the temperature decreases and rises rapidly in 2.5 h. it can be seen in the analysis data, compared to the initial test value, the ones after 2.5 h has changed by -0.03 %. that is to say, the rapid change of http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 187 temperature in a short time has a greater impact on the performance of the sensor, and it is not suitable for the temperature characteristic 0.01 %/10 k which is given by the torque sensor manufacturer. therefore, the temperature change needs special attention during the test of the high-accuracy torque sensor and ensuring the stability of the temperature is essential for accurate measurement, see table 8. table 8: effect of temperature on torque sensor time temperature / humidity output at 50 n·m 0 h 20.6 °c / 67 % rh 0.999 960 mv/v 0.5 h 20.1 °c / 65 % rh 0.999 952 mv/v 1 h 19.3 °c / 65 % rh 0.999 905 mv/v 1.5 h 18.5 °c / 63 % rh 0.999 817 mv/v 2 h 19.0 °c / 62 % rh 0.999 742 mv/v 2.5 h 19.6 °c / 61 % rh 0.999 634 mv/v 5.4 alignment effect due to manufacturing and installation errors, deformation after loading, and temperature changes, the drive shaft and driven shaft of the torque standard machine will cause changes in the relative position of the two shafts, and often cannot guarantee strict alignment. in order to ensure the correct transmission of the torque value and reduce the influence of processing and installation on the torque measurement results, we use the shaft locking device etp to connect the torque standard machine through the flexible coupling and the torque sensor, so as to ensure that the torque machine and torque sensor have good alignment. but can this connection form ensure accurate measurement even when the alignment is poor? a test ever did in a laboratory. the magnetic dial base is sucked on the movable shaft head, and the movable shaft head is rotated one week. the maximum change of the dial indicator reading is 0.82 mm. the movable end of the main shaft of the torque standard machine is adjusted to ensure that the alignment reaches 0.05 mm. table 9 is the detection data of the torque sensor before and after the adjustment of the torque standard machine. the alignment has a greater influence on the detection data of the torque sensor, and the difference between the two reaches nearly 0.03 %. this shows that the flexible coupling can improve the impact of the different shafts of the torque standard machine on the performance of the sensor, but this improvement is not large, that is, the flexible coupling cannot be expected to help the torque sensor achieve accurate measurement when the alignment of the torque standard machine is poor. table 9: effect of alignment condition on torque indication measuring point value before adjustment value after adjustment 50 n·m 0.999 987 mv/v 0.999 715 mv/v 100 n·m 2.000 237 mv/v 1.999 859 mv/v on the other hand, it is believed that the alignment condition of the torque standard machine can be determined by the corner test of the torque sensor. however, the author found through multiple sets of tests that before the adjustment of the alignment of the torque standard machine, the test data shows that the azimuth errors are less than 0.01 %, this shows that the test data of the rotation angle may not necessarily determine the alignment condition of the torque standard machine. 5.5 effect of connectors in this comparison, to discuss the role of torque connectors in the comparison, the pilot laboratory and laboratory b jointly discussed. laboratory b measures the 100 n·m sensor three times. the first two installation conditions are that the coupling connector and the shaft head of the torque machine are connected by another connector, as shown in figure 8; the third installation situation is that the coupling connector is directly connected to the shaft head of the torque machine is shown in figure 9, that is, there is no connecting piece at the third installation. figure 8: the first two installations http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 188 figure 9: the third installation the three measurements data are shown in table 10. this table shows that the time interval between the first and second measurements has reached more than 7 months. the installation condition of these two measurements is consistent, and the variation of the measurement data is within 0.02 %, which further confirms the good stability of the torque sensor. the third measurement only has one less connector than the previous two measurements, but the measurement data changes by about 0.01 %. this indicates that different connectors have little influence on the measurement data of torque measurement system (torque sensor, flexible coupling, hydraulic brake). table 10: three measurements of the 100 n·m sensor date temp. / humid. output at 50 n·m / mv/v change / % output at 100 n·m / mv/v change / % 18/01/19 20.0 ℃ / 62 % rh 1.000 054 --2.000 592 -- 09/09/19 22.5 ℃ / 52 % rh 1.000 129 0.007 2.000 935 0.017 16/09/19 22.4 ℃ / 45 % rh 1.000 048 -0.008 2.000 691 -0.012 6. summary test data indicated that all laboratories’ |𝐸n| values are smaller than 1. the difference between the measurement results of each laboratory and the reference value is within reasonable expectations, and the comparison results are acceptable. the mechanical alignment is very important. the temperature effects must be considered during the comparison test. the stability of the transducer is one of the keys. the results provide experience for the development of high-accuracy torque measurement in china. 7. references [1] f. meng, z. zhang, j. lin, “comparison between a reference torque standard machine and a deadweight torque standard machine to be used in torque calibration”, international journal of modern physics: conference series, vol. 24, 2013. [2] a. nishino, k. ogushi, k. ueda, “uncertainty evaluation of a 10 n·m dead weight torque standard machine and comparison with a 1 kn·m dead weight torque standard machine”, measurement, vol. 49, pp. 77-90, 2014. http://www.imeko.org/ a suitable geometrical model for the verification of knoop indenters with gal-indent optical system acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 216 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 216 220 a suitable geometrical model for the verification of knoop indenters with gal-indent optical system a. prato1, c. origlia1, a. germak1 1inrim – istituto nazionale di ricerca metrologica, torino, italy, a.prato@inrim.it abstract: iso 4545-2 and 4545-3 of knoop hardness tests require the geometrical verification of the indenter. inrim hardness laboratory developed a specific measuring system, commercialized by galileoltf as gal-indent optical system, which is used for the verification of vickers indenters. this system is able to measure the vertex angles of the indenter between two opposite faces, and the four quadrilateral base angles. using such quantities as inputs of a suitable geometrical model, the geometry of knoop indenters can be verified. this work deals with the description of the system and the geometrical model. keywords: knoop, hardness, optical system, indenter, gal-indent. 1. introduction knoop indenter is a rhombic-based pyramidal diamond that produces an elongated diamondshaped indent. the angles from the opposite edges at the vertex of the diamond pyramid of the indenter are 172.5° and 130°, and the ratio between long and short diagonals is approximately 7.11 to 1. this entails that the angles of the rhombic base are 164° and 16°, and the angles between the two opposite faces of the vertex are 129.57°. iso 4545-2 and 4545-3 [1,2] specify the requirements of the indenters with different tolerances. the second is more restrictive since it refers to the calibration of reference blocks. the tolerance for the angle of 172.5° is 0.1° in both documents, whereas for 130° the tolerances are 1° and 0.1°, respectively. furthermore, the angle between the axis of the diamond pyramid and the axis of the indenter holder (normal to the seating surface), namely tilt angle, shall not exceed 0.5° and 0.3°, respectively. the device used for the verification shall have a maximum expanded uncertainty of 0.07°. inrim hardness laboratory developed a specific measuring system, commercialized by the galileo-ltf as gal-indent optical system for the verification of the geometrical characteristics of vickers indenters [35]. this system is able to directly measure the main geometrical parameters of a vickers indenter required by the standard, i.e. the two vertex angles (nominally 136°) between two opposite faces and the four angles of the square base, both with an expanded uncertainty of 0.05°. using these measured quantities as input of a suitable geometrical model, the possibility to verify knoop indenters is investigated. this paper deals with the description of the system and the geometrical model. 2. the gal-indent optical system in inrim hardness laboratory a specific measuring system, commercialized by galileoltf as gal-indent optical system (figure 1), was developed and is currently used for the verification of vickers indenters. the system is also adopted by different national metrological institutes (nmi) and calibration laboratories around the world. the galileo-ltf gal-indent optical system is based on mirau interferometry. the geometrical characteristics assure that all mechanical parts are perfectly aligned and that the indenter-holder axis is perpendicular to the lens of the system. the knoop indenter is simultaneously rotated around the axis passing through the indenter vertex normal to the plane containing the indenter-holder axis and the optical lens axis, and around the indenter-holder axis, until a lateral face is parallel to the plane of the microscope lens by observing the interference fringes. these two rotations are measured by means of two angular encoders [6]. rotations around the indenter-holder axis represent the measurement of the angles between two consecutive faces, i.e. the quadrilateral base angles φ (two angles nominally 164°) and τ (two angles nominally 16°); whereas, rotations around the axis normal to the plane containing the indenter-holder axis and the optical lens axis represent the measurement of the supplementary angles for each lateral face ω (nominally 25.22°) from which the angles between two opposite faces 𝜃 (nominally 129.57°) are easily obtained [7]. such values, used as input in a suitable geometrical model, allow to evaluate the angles from the opposite edges at the vertex α and β, http://www.imeko.org/ mailto:a.prato@inrim.it acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 217 nominally equal to 172.5° and 130°, respectively, and the tilt angle δ, nominally equal to 0°. figure 1: the galileo-ltf gal-indent optical system. 3. the geometrical model 3.1. evaluation of the tilt angle the geometry of a real knoop indenter with four generic faces (a, b, c, d) is schematically depicted in figure 2. xyz and x'y'z' coordinate systems correspond, respectively, to the diagonals of the knoop indenter rhombic base, and to the optical reference system that is perpendicular to the perimeter of two opposite faces. therefore, the angle between xand y-axis is nominally 90°, whereas the angle σab between x'and y'-axis is nominally 164°. since the tilt angle δ is not exactly 0°, an angle γ between the projection of the pyramid vertex on z=0 plane and x'-axis appears. for each j-th face (j=a, b, c, d), the intersection between an optical reference axis (x'or y'-axis) and the base perimeter is identified by point hj, whereas the intersection with xand y-axis are identified by points sj and pj, respectively (thus sa≡sd, sb≡sc, pa≡pb, pc≡pd). the pyramid vertex v is arbitrally placed on z=1. a cross-section of a real knoop indenter along x'z' optical system plane is also shown in figure 3. the quadrilateral base angles φi and τi (i=1,2), nominally 164° and 16°, respectively, and the supplementary angles of each j-th lateral face (a, b, c, d) along x' and y'-axis, ωj, nominally (180°-129.57°)/2≈25.22°, are measured by means of the optical system previously described. from quadrilateral base angles measurements φi and τi (i=1,2), in order to take into account possible asymmetries of the rhombic-base, the mean angle ρj=hjosĵ between xy and x'y' reference systems, for each j-th indenter face, can be evaluated according to 𝜌𝑗 = (90 − 𝜑𝑗 2 ) + 𝜏𝑗 2 2 (1) where φa=φd= φ1, φb=φc= φ2, τa=τb= τ1, τc=τd= τ2. in this way, considering the mean value 𝜌 = ∑ 𝜌𝑗 /4 4 𝑗=1 among the four faces, the angle σab between x'and y'-axis can be obtained according to equation (2). 𝜎𝐴𝐵 = 180 − 2𝜌 (2) from the measurement of the supplementary angles ωj of each j-th lateral face, the two vertex angles 𝜃x' and 𝜃y' and the pyramid tilt angles δ x' and δ y', along x'and y'axis, can be calculated according to equations (3) and (4), respectively. 𝜃𝑥′ = 180 − (𝜔a + 𝜔c) 𝜃𝑦′ = 180 − (𝜔b + 𝜔d) (3) 𝛿𝑥′ = 𝜔a − 𝜔c 2 𝛿𝑦′ = 𝜔b − 𝜔d 2 (4) by decomposing the pyramid tilted axis vector v along non-orthogonal x'z' and y'z' planes, according to figure 4, equation (5) is derived. successively, implementing the equations of non-orthogonal systems (figure 5) and using equation (5), equations (6)-(8) and equations (9)-(10) can be derived. figure 2: 3-d schematic representation of a real knoop indenter rhombic-based pyramid http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 218 figure 3: cross-section of a real knoop indenter along x'z' optical reference system plane. figure 4: decomposition of the tilted pyramid axis vector v and the associated angles along x'and y'axis. figure 5: generic non-orthogonal reference system with relevant equations. ‖𝐯‖ cos 𝛿 = ‖𝐯𝒙′𝒛′ ‖ cos 𝛿x′ = (5) = ‖𝐯𝒚′𝒛′‖ cos 𝛿y′ ‖𝐯‖ sin 𝛿 cos 𝛾 = ‖𝐯‖ cos 𝛿 cos 𝛿𝑥′ sin 𝛿𝑥′ + +‖𝐯‖ cos 𝛿 cos 𝛿𝑦′ sin 𝛿𝑦′ cos 𝜎𝐴𝐵 (6) sin 𝛿 cos 𝛾 = cos 𝛿 cos 𝛿𝑥′ sin 𝛿𝑥′ + + cos 𝛿 cos 𝛿𝑦′ sin 𝛿𝑦′ cos 𝜎𝐴𝐵 (7) tan 𝛿 cos 𝛾 = tan 𝛿𝑥′ + tan 𝛿𝑦′ cos 𝜎𝐴𝐵 (8) ‖𝐯‖ sin 𝛿 sin 𝛾 = ‖𝐯𝐲′𝐳′‖ sin 𝛿𝑦′ sin 𝜎𝐴𝐵 = ‖𝐯‖ cos 𝛿 cos 𝛿𝑦′ sin 𝛿𝑦′ sin 𝜎𝐴𝐵 (9) tan 𝛿 sin 𝛾 = tan 𝛿𝑦′ sin 𝜎𝐴𝐵 (10) by performing the squared sum of equations (8) and (10), the total tilt angle δ can be obtained (see equations (11)-(13)): tan2 𝛿 cos2 𝛾 + tan2 𝛿 sin2 𝛾 = = tan2 𝛿𝑥′ + tan 2 𝛿𝑦′ cos 2 𝜎𝐴𝐵+ +2 tan 𝛿𝑥′ tan 𝛿𝑦′ cos 𝜎𝐴𝐵+ + tan2 𝛿𝑦′ sin 2 𝜎𝐴𝐵 (11) tan2 𝛿 = tan2 𝛿𝑥′ + tan 2 𝛿𝑦′ = = +2 tan 𝛿𝑥′ tan 𝛿𝑦′ cos 𝜎𝐴𝐵 (12) 𝛿 = arctan √ tan2 𝛿𝑥′ + tan 2 𝛿𝑦′ +2 tan 𝛿𝑥′ tan 𝛿𝑦′ cos(𝜎𝐴𝐵 ) (13) by performing the ratio between equations (10) and (8), the angle γ can be derived (see equations (14)-(15)): tan 𝛾 = tan 𝛿𝑦′ sin 𝜎𝐴𝐵 tan 𝛿𝑥′ + tan 𝛿𝑦′ cos 𝜎𝐴𝐵 (14) 𝛾 = arctan ( tan 𝛿𝑦′ sin 𝜎𝐴𝐵 tan 𝛿𝑥′ + tan 𝛿𝑦′ cos 𝜎𝐴𝐵 ) (15) 3.2. evaluation of the angles between the opposite edges at the vertex from the scheme of figure 2, reminding that the pyramid vertex v is placed on z=1, the vector of the tilted pyramid axis v referred to the xyz reference system can be written according to equation (16), where ρ is the mean angle between x'and x-axis (see equation (2)). http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 219 𝐯 = [ tan(𝛿) cos(𝛾 + 𝜌), tan(𝛿) sin(𝛾 + 𝜌), 1] (16) considering the triangle ohjv in figure 2 and figure 3 for each j-th indenter face (a, b, c, d), given that ohjv̂ = 𝜔𝑗, and implementing the law of sines, equation (17) is obtained: oh̅̅ ̅̅ 𝑗 = ‖𝐯‖ 𝑠𝑖𝑛 𝜃𝑗 2 𝑠𝑖𝑛 𝜔𝑗 (17) where θa=θc=θx' and θb=θd=θy'. in this way, equations (18) and (19) are derived. the sign of the vector components for the four faces follows the position on the xyz reference system as in figure 2. 𝐎𝐒𝒋 = ( oh̅̅ ̅̅ 𝑗 cos 𝜌𝑗 , 0,0) (18) 𝐎𝐏𝒋 = (0, oh̅̅ ̅̅ 𝑗 sin 𝜌𝑗 , 0) (19) considering the triangle ovsj, it is obtained that 𝑉𝑂𝑆�̂� = cos −1 𝐎𝐒𝒋 ∙ 𝐯 ‖𝐯‖‖𝐎𝐒𝒋‖ (20) again, by applying the law of sines to triangle ovsj, it is obtained that ‖𝐯‖ sin 𝑂𝑆𝑗�̂� = ‖𝐎𝐒𝒋‖ sin 𝑂𝑉𝑆�̂� = ‖𝐕𝐒𝒋‖ sin 𝑉𝑂𝑆�̂� (21) given that 𝑂𝑆𝑗�̂� = 180 − 𝑂𝑉𝑆�̂� − 𝑉𝑂𝑆�̂� and with some trigonometric calculations, equation (22) is obtained. sin 𝑂𝑆𝑗�̂� = sin(180 − 𝑂𝑉𝑆�̂� − 𝑉𝑂𝑆�̂�) = = sin 𝑂𝑉𝑆�̂� cos 𝑉𝑂𝑆�̂� + sin 𝑉𝑂𝑆�̂� cos 𝑂𝑉𝑆�̂� (22) in this way, combining equations (21) and (22), equations (23)-(25) are obtained: ‖𝐯‖ ‖𝐎𝐒𝒋‖ = sin 𝑂𝑆𝑗�̂� sin 𝑂𝑉𝑆�̂� (23) ‖𝐯‖ ‖𝐎𝐒𝒋‖ = = sin 𝑂𝑉𝑆�̂� cos 𝑉𝑂𝑆�̂� + sin 𝑉𝑂𝑆�̂� cos 𝑂𝑉𝑆�̂� sin 𝑂𝑉𝑆�̂� (24) ‖𝐯‖ ‖𝐎𝐒𝒋‖ = cos 𝑉𝑂𝑆�̂� + sin 𝑉𝑂𝑆�̂� tan 𝑂𝑉𝑆�̂� (25) and from equation (25), equation (26) is also obtained: 𝑂𝑉𝑆�̂� = tan −1 ‖𝐎𝐒𝒋‖ sin 𝑉𝑂𝑆�̂� ‖𝐯‖ − ‖𝐎𝐒𝒋‖ cos 𝑉𝑂𝑆�̂� (26) by applying the same calculations from equation (20) onward to triangle vopj, it is found that, 𝑂𝑉𝑃�̂� = tan −1 ‖𝐎𝐏𝒋‖ sin 𝑉𝑂𝑃�̂� ‖𝐯‖ − ‖𝐎𝐏𝒋‖ cos 𝑉𝑂𝑃�̂� (27) therefore, considering a single j-th indenter face, the angles between two opposite edges can be found according to: 𝛼𝑗 = 2 𝑂𝑉𝑃�̂� (28) 𝛽𝑗 = 2 𝑂𝑉𝑆�̂� (29) averaging the results obtained for each j-th indenter face, the angles between two opposite edges, nominally 172.5, equation (30), and 130°, equation (31), are finally obtained: 𝛼 = ∑ 𝛼𝑗 4 𝑖=1 4 (30) 𝛽 = ∑ 𝛽𝑗 4 𝑖=1 4 (31) 4. summary iso 4545-2 and 4545-3 of knoop hardness tests require the geometrical verification of the indenter. inrim hardness laboratory developed a specific measuring system, commercialized by galileoltf as gal-indent optical system, which is used for the verification of vickers indenters. this system is also adopted by other nmis and calibration laboratories around the world. it is able to measure the two vertex angles of the indenter between two opposite faces, and the four angles between two consecutive faces by means of two angular encoders mounted on the optical measuring system. using such quantities as inputs of the proposed geometrical model described in section 3, the geometry of knoop indenters, in particular the angles from the opposite edges at the vertex of knoop indenters, nominally 172.5° and 130°, and the angle between the axis of the diamond pyramid and the axis of the indenter holder, nominally 0°, can be verified. http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 220 5. references [1] iso 4545:2017-2 metallic materials — knoop hardness test — verification and calibration of testing machines. [2] iso 4545:2017-3 metallic materials — knoop hardness test — calibration of reference blocks [3] g. barbato, g. gori, “metrological references in hardness measurement: a necessary background for industrial quality assurance”, proc. of the 5th congreso nacional de metrologia industrial, zaragoza, spain, pp. 139-149, 1991. [4] g. barbato, s. desogus, “measurement of the spherical tip of rockwell indenters,” journal of testing and evaluation, vol. 16(4), pp. 369-374, 1988. [5] a. liguori, a. germak, g. gori, e. messina, “galindent: the reference metrological system for the verification of the geometrical characteristics of rockwell and vickers diamond indenters”, proc. of imeko tc3/tc5/tc20, pp. 365-371, 2002. [6] a. germak, a. liguori, c. origlia, “experience in the metrological characterization of primary hardness standard machines”, proc. of hardmeko 2007, tsukuba, japan, pp. 81-89, 2007. [7] a. prato, d. galliani, c. origlia, a. germak, “a correction method for vickers indenters squareness measurement due to the tilt of the pyramid axis”, measurement, vol. 140, pp. 565-571, 2019. http://www.imeko.org/ acta imeko  september 2014, volume 3, number 3, 3 – 8  www.imeko.org    acta imeko | www.imeko.org  september 2014 | volume 3 | number 3 | 3  proposed definition for the brinell hardness indentation edge  samuel low 1 , koichiro hattori 2 , alessandro germak 3 , andy knott 4   1  national institute of standards and technology, usa, 100 bureau dr., stop 8553, gaithersburg, md, 20899  2  national metrology institute of japan, japan, aist central 3 umezono 1‐1‐1, tsukuba, ibaraki 305‐8563  3  istituto nazionale di ricerca metrologica, italy, strada delle cacce 73, i‐10135, torino, italy  4  national physical laboratory, uk, hampton road, teddington, middlesex, tw11 0lw      section: research paper   keywords: brinell; contact; diameter; hardness; indentation  citation: samuel low, koichiro hattori, alessandro germak, andy knott, proposed definition for the brinell hardness indentation edge, acta imeko, vol. 3,  no. 3, article 3, september 2014, identifier: imeko‐acta‐03 (2014)‐03‐03  editor: paolo carbone, university of perugia   received february 13 th , 2013; in final form april 15 th , 2014; published september 2014  copyright: © 2014 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: (none reported)  corresponding author: samuel low, e‐mail: samuel.low@nist.gov    1. introduction  brinell hardness is an indentation test that has been in common usage by industry for well over 100 years. it is a simple test in concept in which a spherical ball indenter is forced under a specified test force into the test material, causing permanent indentation in the material. the brinell hardness value (hbw) is proportional to the test force divided by the surface area of the indentation. international test methods [1],[2] specify that the indentation surface area be determined based on the applied test force, the diameter of the ball indenter and by measuring the diameter of the indentation after the test force and indenter are removed. typically, the diameter measurement is made using an optical microscope measuring system. for most users of brinell hardness, the test procedure is indeed simple, providing a brinell hardness value with reasonable uncertainty levels for their needs in spite of the common use of very low-magnification microscopes to measure the indentation diameter. this is acceptable since many product specifications that require the brinell hardness test to determine compliance, often specify large ranges of acceptable values or simply specify maximum or minimum limits on the hardness value. for laboratories that calibrate brinell hardness reference blocks, as well as the primary national metrology institutes, the uncertainty in the hardness measurement needs to be minimized since they are at the first levels of the traceability chain in these measurements. it is well recognized that brinell hardness measurement accuracy is primarily related to measurement of the indentation diameter [3]. there are two main reasons for the significant contribution of error from the diameter measurement. the first is because of the many factors that influence a length measurement when using an optical microscope. the second reason is that the edge of a metallic indentation is curved and without a distinct physical edge that can be definitively observed with an optical microscope. this second reason is complicated by there not being an accepted physical definition of the edge of a brinell indentation. this paper proposes such a definition that has been demonstrated to be measurable. in section 2 we discuss the brinell hardness test procedure. section 3 discusses two choices of defining the indentation edge; either by a practical definition based on defining an optical measuring system or a physical definition based on a abstract  the industrial brinell hardness test has been in common use for over 100 years. the test is defined by standardized procedures stating  that the brinell hardness value is proportional to the test force divided by the surface area of the indentation. the test procedures  require that the surface area be determined by measuring the indentation diameter after removing the test force. this measurement  is usually made using an optical microscope, but without having a physical definition of the indentation edge. this paper proposes a  physical definition of the indentation edge such that the brinell indentation diameter can be unambiguously measured.  acta imeko | www.imeko.org  september 2014 | volume 3 | number 3 | 4  direct measurement of the indentation. in the next two sections, we describe our work in modeling brinell indentation and our experimental measurements of actual brinell indentations. in section 6 we compare brinell hardness measurements made using our technique with typical industrial measurements. finally, in the concluding section we propose physical definitions for the edge of a brinell hardness indentation and the brinell indentation itself. 2. brinell hardness test procedure  brinell hardness, as it is specified today, is measured by indenting a material normal to its surface with a hardmetal ball. a specified force is applied to the ball; the force is maintained for a specified time; the force is removed; and the surface area of the resulting indentation is determined, most often by optically measuring the diameter of its projected area. the brinell hardness value is calculated based on the applied force divided by the indentation surface area as: nindentatio of area surface force test constanthbw  (1)        22 2 102.0 dddd f hbw  (2) where f = test force (n) d = diameter of the ball indenter (mm) d = diameter of indentation (mm). from an idealistic point of view, it may be preferable to define the test as: a specified force is applied to a nondeformable ball; the force is maintained until the plastic flow of the indented material under load ceases; and then the contact surface area between the ball and test material is determined while under load. as suggested by meyer [4], it may also have been more appropriate to calculate a hardness value as a mean pressure based on the applied force divided by the projected contact surface area under load. this value, which increases with load, better represents the ball indentation behavior of a material where the resistance to penetration increases with increasing applied force. clearly this idealistic test procedure would not have been practical for general industrial use. brinell approximated the nondeformable ball by using a hardened steel ball which has now been replaced with a hardmetal (tungsten carbide) ball. brinell found that plastic flow was most rapid in the first 30 s of applying the maximum load, leading to his recommendation for a 30 s time application, now shortened to between 10 s and 15 s. more importantly, there was, and continues to be, no practical technique to measure the contact surface area while under load, due to the generally opaque nature of both indenter and test material (in recent years, methods for area measurement of transparent test materials under load have been investigated [5], as have measurements using transparent indenters [6]). brinell wanted a test that gives a constant value independent of load and found that basing the hardness value on the indentation surface area better exhibited this behavior as compared to the projected area [7]. the test procedure developed by brinell determines the contact surface area after removing the indenter and force by measuring the diameter of the projected area of the indentation and assuming that the unloaded indentation retains the shape of the ball indenter (a generally valid assumption considering the plastic deformation characteristics of most metals). the above discussion is given to point out that the brinell hardness test procedure is not a measurement of a physical property of a material. brinell hardness is an ordinal quantity prescribed by a test method procedure combining simultaneous and sequential measurements of force, length and time. the test was developed to provide industrial manufacturers with a tool to correlate a simple test result to a desired material property, such as strength or wearability. keeping this in mind, it is more appropriate to base a definition of brinell hardness on the brinell hardness test procedure rather than an ideal property measurement. 3. defining the indentation edge  one role of the working group on hardness of the consultative committee on mass and related quantities (ccm-wgh) of the international committee for weights and measures (cipm) is to develop definitions of the hardness tests for use by the world’s national metrology institutes (nmis). this includes the brinell hardness test. in november 2003, the ccm-wgh initiated a ccm brinell hardness key comparison (kc) [ccm.h-k2] between the world’s nmis that standardize brinell hardness. the kc concluded in 2004, and, in 2005, hattori (national metrology institute of japan) presented the initial results of the ccm.h-k2 brinell hardness key comparison [8] to the ccm-wgh members. two of the conclusions from the analysis of the kc data were:  the difference between institutes cannot be explained by the reported uncertainty from each institute in many cases. this means that the uncertainties reported from each institute are underestimated or that some uncontrolled parameter has an effect on the hbw measurements.  results of the diameter measurements on the reference indentation showed large differences between the institutes. high correlation was found between the results of reference indentation measurement and those of hardness measurement of their own indentation. the dispersion of the measurements within the institutes is much smaller than the difference between institutes. therefore, it can be concluded that the large difference between institutes was caused by certain things relating to the three-dimensional diameter measurement which is not defined either in the protocol or iso standard. these conclusions clearly pointed out the need for an improved definition of the brinell indentation edge, or a better defined indentation measurement procedure. this led to a discussion within the ccm-wgh of the effect of the numerical aperture (na) of optical microscopes. germak and origlia [9] have found that the effect of the na can be minimized when na > 0.2 or 0.3. it was subsequently found that the measurement differences between laboratories in the brinell kc could be reduced by correcting for the na. international brinell test method standards prescribe requirements on the various brinell hardness test parameters, such as for the application of force, capability of the indentation measuring system, etc., by stating permissible limits on parameter values. the goal of the ccm-wgh is to specifically define the test parameter values to minimize measurement differences between nmis, while usually not acta imeko | www.imeko.org  september 2014 | volume 3 | number 3 | 5  deviating from the procedure specified in the test method standards used by industry. for example, it is not the intent of the ccm-wgh to define brinell hardness as a measure of pressure, but rather to define the parameters of the brinell hardness test method. from the standpoint of ball indentation, a definition of the brinell hardness indentation edge should ideally be based on the surface area of contact between the ball indenter and the test material while the test force is applied. however, since there is no easily implemented method to determine the contact area while under the test force, the brinell test method specifies that the indentation is to be measured after unloading. the test method standards make the assumption that the indentation retains the shape of the ball, although, due to elastic recovery in metals, this clearly is not true. even after unloading, there is not a simple or quick method available to measure the surface area of contact, so the test methods specify that the contact area be estimated from a measurement of the diameter of the circular projected area of the indentation. although not explicitly required by the test methods, the diameter is usually measured with an instrument or system incorporating an optical microscope. at this time, the ccm-wgh is debating an appropriate definition for the brinell hardness indentation edge. two concepts have been discussed: (1) a practical definition based on defining requirements for the indentation measurement instruments and the measurement process [10]. (2) a physical definition based on the indenter/material contact boundary. 3.1. practical definition  a practical definition of the edge of a brinell indentation would be based on an observer’s perception of the indentation edge as the dark/light boundary when viewed with an optical microscope with defined parameters. it would not be based on an actual physical attribute of the indentation resulting from the indentation process except for how the shape of the indentation at the boundary zone reflects light back towards the microscope. it would be the measurement instrument and measurement process rather than the indentation edge that would be defined. at a minimum, a practical definition would require defining parameters for all aspects of the optical microscope measurement system for which typical variations would significantly contribute to the measurement accuracy. an advantage of defining the edge of a brinell indentation in this way is that the definition would mirror how industry estimates the indentation dimensions. there are many parameters and influences associated with using an optical microscope to measure the diameter of a brinell indentation, all contributing to variations in the measurement results. studies [3],[4],[11] have shown that these influences include light intensity, incident light direction, the numerical aperture of the lens, surface roughness and the operator’s subjective interpretation of the indentation edge. if each of the influence quantities can be optimized and clearly defined, then a definition of the brinell indentation could possibly be based on the characteristics of the measurement microscope, as well as the measurement procedure. for example, the ccm-wgh is currently proposing that an optical microscope having an na > 0.4 should be used when measuring brinell indentation diameters. there are several drawbacks to this type of definition. for example, there are very many designs of optical microscopes and illumination systems that would make it difficult to adequately define the requirements of all of these measurement systems. an operator’s subjective decision-making process of visually choosing the indentation edge would be difficult if not impossible to define. also, determining measurement uncertainty with respect to such a practical definition would be extremely challenging. these problems give doubt as to whether such a practical definition would provide the possibility of an unambiguous measurement. additionally, such a definition of specifying the parameters of an optical microscope would preclude the use of any other type of measurement technique to determine the indentation surface area. 3.2. physical definition  a physical definition of the brinell hardness indentation edge requires that a physical feature of the indentation can be observed and is measureable. for example, this could be the indenter/material contact boundary while under load. a second requirement is that the physical feature must be related to the indentation process, which in this case can only be the indenter/material contact boundary. lastly, the location of the physical feature must not significantly differ from the indentation edge as historically measured using an optical microscope. as previously discussed, it is currently impractical to measure the contact boundary while under load. in addition, the brinell test method specifies that the indentation is to be measured after unloading. to be able to define the edge of the brinell indentation with a physical definition, a physical feature must be present after unloading that can be observed and measured, and that is related to the indenter/material contact boundary. researchers at the national metrology institute of japan (nmij, japan), the national physical laboratory (npl, uk), the istituto nazionale di ricerca metrologica (inrim, italy) and national institute of standards and technology (nist, usa) have investigated the brinell indentation and have observed that the cross-sectional surface profile of a ball indentation exhibits a maximum change in gradient or slope at the edge region of a ball indentation (see figure 1). the questions to be answered are (1) whether the maximum change in slope is related to the indenter/material contact boundary that occurred while under load and (2) whether this is the same location that is commonly judged to be the indentation edge as observed with an optical microscope? if it is determined that these questions are true, then a physical definition of a brinell indentation is possible. 4. modelling brinell indentation  it is reasonable to assume that the indenter/material contact boundary on the test material occurring while under load will physically move after unloading due to the elastic recovery in the test material. unfortunately, the contact boundary cannot be observed with an optical microscope following the removal of the indenter. a study was conducted at nist to determine whether the location of the indenter/material contact boundary could be identified on the indentation surface after removing the applied test force. ma [12] used finite element modeling (fem) to identify the node location of the indenter/material contact boundary while under load, and then tracked the movement of acta imeko | www.imeko.org  september 2014 | volume 3 | number 3 | 6  the fem node during the unloading process. the magnitude of the movement of the contact boundary has been reported by ma [13] to vary depending on a material’s strain hardening, and ratio of modulus and yield stress. the results of the analysis showed that the location of the indenter/material contact boundary does in fact coincide with the maximum change in surface gradient or slope at the edge region of a ball indentation. ma also found that by plotting the change of the slope, the minimum peak of the curve is the point on the indentation surface at which the boundary of the contact between the indenter ball and test material occurred while the force was applied. an example of a minimum peak from measurements conducted at npl can be seen in the bottom two plots of figure 1 showing the gradient change at the left and right edges of a brinell indentation. additional fem work by ma [13] demonstrated that the correlation between the minimum peak of the slope change and the indenter/material contact boundary is consistent and independent of material parameters including the type of material, hardness level, and indentation edge pile-up or sink-in conditions. 5. measuring the indentation profile  in order for the edge of a brinell indentation to be defined as described above, the edge point must be measureable with sufficient resolution to be meaningful. surface profile measurements were conducted at nist to determine the practicality of determining the indentation diameter of brinell indentations from a cross-sectional profile measurement. it is not as straight-forward as it would seem. there are multiple issues that must be considered. two approaches for measuring the indentation crosssectional profile were investigated. the first technique was to measure across the diameter of the projected indentation area with a series of confocal microscope measurements oriented normal to the projected area of the indentation, then stitching together the images using software to obtain the full crosssectional profile. two significant problems were encountered when using this technique. the first was that the confocal microscopes that were available had difficulty in imaging larger surface angles at the indentation edge for deep indentations. unfortunately, that is the area of most interest. the second issue was in measuring larger indentations in which multiple images were needed to obtain the needed length resolution. the stitching software introduced dimensional errors due to the stitching process which multiplied with each additional image. these two issues alone prevented using these instruments to measure the indentation diameter. perhaps the confocal microscope technique can be viable with the use of an improved lens system that can resolve larger surface angles, and with the use of a traversing stage having an accurate displacement sensor to eliminate the need for stitching multiple images. the second technique used a contact stylus profilometer to measure the surface profile by making a linear trace across the indentation surface through the center point of the indentation. it is important that the measurement system has sufficient 2d resolution to adequately define the indentation edge. this is a similar technique as was used by npl to produce the measurement example given in figure 1. this technique proved to be successful; however, there are also issues that must be considered. figure 1. cross‐sectional profile of a brinell hbw 10/3000 indentation measured at npl using a stylus profilometer. the x‐axis (mm) is the relative distance  across the indentation profile. the indentation depth (black) is in mm; the corresponding gradient (red) is in mm/mm; and the gradient change (blue) is in  mm/mm per mm.  acta imeko | www.imeko.org  september 2014 | volume 3 | number 3 | 7  as compared to an fem model of an indentation, the surface of a real indentation is not perfectly smooth exhibiting roughness and imperfections due to material grain structure, inhomogeneity, etc. the effect of the surface roughness can be seen in figure 2, which shows nist surface profile measurements of the left and right edges of a ~270 hbw 10/3000 brinell indentation, as well as plots of the profile slope. it is interesting to note that the left and right edges are dissimilar in form. these surface irregularities are amplified when analyzing the slope of the indentation surface, and are amplified further when analyzing the change in slope producing significant variations or “noise” in the data. the effect of the surface roughness on the profile slope data is evident in figure 2, and tends to mask the maximum change in slope in the slope plots. in plotting the change-in-slope data, the magnitude of the noise produced by the surface roughness becomes too great to be usable. applying filters to the surface profile data can significantly reduce the noise levels in the data as shown by ma [13]. it is imperative that the measuring stylus traverse across the actual center of the indentation so that the true diameter is measured rather than an adjacent but shorter chord line. perfect alignment to the center is difficult and must be verified. this was accomplished for the nist measurements by making multiple parallel traces at known increments through the central region of the indentation and determining the resulting diameters. figure 3 illustrates the results of this technique showing that the true diameter is within the nine parallel traces spaced at 25 µm increments separation. the preliminary data shown in figures 2 and 3 are given only to illustrate the technique that is being investigated. a full uncertainty analysis is yet to be completed; however, the uncertainty based solely on the instrument measurements is estimated as being much smaller than ±1 µm (k = 2). the major contribution to the measurement uncertainty is due to determining the location of the maximum change in gradient. 6. comparison with optical microscopes  a final factor in deciding if it is appropriate for the brinell indentation edge to be defined by a physical definition is to determine how well measurements of the indentation diameter measured as discussed above compare with measurements using an optical microscope. nist collaborated with a commercial laboratory that produces brinell reference blocks asking them to measure two indentations for which the diameters had previously been determined based on the surface profiles. the commercial company measured the brinell indentation diameters with an optical microscope that uses a camera/image-analysis system to determine the projected area of the indentation. table 1 summarizes the results. although additional work is needed, preliminary comparisons indicate reasonable agreement between indentation diameters measured from the indentation profile and using optical microscopes. although a thorough uncertainty analysis has not yet been completed, the uncertainty (k = 2) of the indentation diameters measured from the indentation profile is estimated to be no greater than ±2 µm, and no greater than ±4 µm for the optical microscope measurements. it should be noted that for the higher hardness measurements (~500 hbw 10/3000), the difference between the profilometer and optical microscope measurements is not covered by the expanded uncertainties of the instruments. however, the uncertainty analyses are not based on the same criteria. in the case of the indentation diameter measured from the indentation profile, the uncertainty is with respect to resolving and measuring the maximum rate of change of gradient of the curved indentation surface profile. in the case of the optical microscope measurements, the uncertainty is based figure 2. nist surface profile measurements of the left and right edges of a ~270 hbw 10/3000 indentation, as well as plots of the profile slope.  figure 3. diameter measurements (solid circle) from nine parallel traces at  25 µm increments through the central region of the brinell indentation. also  shown  are  predicted  diameter  values  (cross)  based  on  a  circular indentation.  table  1.  comparison  of  brinell  indentation  diameter  values  measured  at  nist based on the indentation surface profile and values based on optical  measurements by a commercial calibration laboratory.  approximate hbw 10/3000 surface profile measurement optical microscope measurement measurement difference 270 3699 µm 3701 µm 2 µm (0.3 hbw) 500 2740 µm 2733 µm 7 µm (2.6 hbw) 3692 3693 3694 3695 3696 3697 3698 3699 3700 -125 -100 -75 -50 -25 0 25 50 75 100 125 d ia m e te r (u m ) profile offsets (um) measured diameters predicted diameters acta imeko | www.imeko.org  september 2014 | volume 3 | number 3 | 8  on measuring a flat glass reference standard, which may not be representative of measuring an actual indentation. hou et al. [14] detail complementary work investigating the measurement of low-force rockwell indentations by three different techniques, including both contact and non-contact methods, together with associated fe modelling. this work concludes that the 2d optical measurements overestimated the contact area, particularly when pile-up was present, but that the two 3d measurement methods were in good agreement. 7. conclusions and proposal  in cases where there is no distinct physical measurand that can be observed or directly measured for a test parameter, it may be reasonable to define the parameter in terms of a welldefined measurement system or measurement process as a practical definition. this would be the case for the smooth curving edge of a brinell indentation if there was no identifiable physical boundary that could be measured. however, we have shown that a location on the surface of a brinell indentation, related to the indenter/material contact boundary under load, can be identified and measured. this is a compelling argument for defining the brinell indentation edge by this physical definition. therefore we propose the following definition for the brinell hardness indentation: "the brinell hardness indentation is defined, after the force is removed, as the surface area of the material under test that made contact with the ball indenter during the force application process." the content of this paper and definition of the indentation lead to the following physical definition for the edge of a brinell hardness indentation: “the edge of a brinell hardness indentation is defined, after the force is removed, as the boundary of the surface area of the material under test that made contact with the ball indenter during the force application process, which is the point in any cross-sectional surface profile coplanar with the indentation axis at which the surface has its maximum rate of change of gradient when moving away from the center of the indentation.” although we recommend defining the brinell indentation edge as a physical definition, measuring the edge based on this definition is not a practical method for making routine brinell hardness measurements. currently, measuring and analyzing the indentation surface profile is a difficult and time consuming exercise. a promising application of the physical definition is for the calibration of reference standards of brinell indentations with certified diameter measurements for verifying optical microscope measuring systems. references  [1] iso 6506 parts 1, 2, 3 & 4 2005 metallic materials—brinell hardness test, geneva, international organization for standardization. [2] astm e10-10 2010 standard test method for brinell hardness of metallic materials, west conshohocken, pa, astm international. [3] g. barbato, s. desogus, “problems in the measurement of vickers and brinell indentations”, measurement, vol 4, no 4, pp137-147, oct-dec 1986. [4] e. meyer, “untersuchungen über härteprüfung und härte brinell methoden”, z. ver. deut. ing., 52, 1908. [5] m. sakai, n. hakiri, and t. miyajima, “instrumented indentation microscope: a powerful tool for the mechanical characterization in microscales”, j. mat. res. vol 21, 2006. [6] s. dvorak, k. wahl, i. singer, “in situ analysis of third body contributions to sliding friction of a pb–mo–s coating in dry and humid air”, tribology letters , vol 28, no 3, pp263-274, 2007. [7] v. lysaght, indentation hardness testing, reinhold publishing, new york, pp39-47, 1949. [8] k. hattori, g.w. bahng, “brinell key comparison ccm.h-k2”, final report, draft a, bipm, oct. 2005. [9] a. germak, c. origlia, “analysis and estimation of possible large systematic error in brinell hardness measurements”, proc. of hardmeko 2007, tsukuba, japan, 19-21 nov. 2007. [10] g. barbato, a. germak, “practical implications to the improvement of hardness scales definitions”, proc. of hardmeko 2004, hardness measurements theory and application in laboratories and industries, 11-12 november, 2004, washington, d.c., usa. [11] r. ellis, a. knott, k. herrmann, “verification of image analysis systems for measuring brinell indentations”, proc. of xviii imeko world congress, rio de janeiro, brazil, 2006. [12] l. ma et al., “investigation of brinell indentation diameter from confocal microscope measurement and fea modelling”, proc. of hardmeko 2007, tsukuba, japan, 70, 2007. [13] l. ma et al., “an approach to determining the brinell hardness indentation diameter based on contact position”, proc. of xx imeko 2012, busan, republic of korea, 2012. [14] x. hou, a. bushby and n. jennett, “direct measurement of surface shape for validation of indentation deformation and plasticity length-scale effects: a comparison of methods”, measurement science and technology, vol 21 no 11, 2010. torque transfer calibration system for 200 n·m and 2 kn·m, its control and characterization acta imeko june 2014, volume 3, number 2, 28 – 31 www.imeko.org torque transfer calibration system for 200 n·m and 2 kn·m, its control and characterization jesus galvan-mancilla, jorge c. torres-guzman centro nacional de metrología, cenam, km 4.5 carr. a los cues, queretaro, mexico. section: research paper keywords: torque; metrology; control system; characterization; torque transfer calibration system. citation: jesus galvan-mancilla, jorge c. torres-guzman, torque transfer calibration system for 200 n∙m and 2 kn∙m, its control and characterization, acta imeko, vol. 3, no. 2, article 8, june 2014, identifier: imeko-acta-03 (2014)-02-08 editor: paolo carbone, university of perugia received february 12th, 2013; in final form january 23rd, 2014; published june 2014 copyright: © 2014 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: this work was supported by conacyt, mexico in a project between sct metro mexico city and centro nacional de metrología, cenam, mexico corresponding author: jorge c. torres-guzman, e-mail: jorge.torres@cenam.mx 1. introduction normally, in a torque transfer calibration system the torque application is performed by manual operation. in this type of systems the torque load required is achieved using motor start and stop control buttons, which also applies for motor speed, applied torque load and motor spinning direction. this manual process required many man-hours for a calibration. cenam designed, built and put into operation an automated torque transfer calibration system for 200 n∙m and 2 kn∙m ranges for the collective transport system metro (mexico city). this paper presents the control system and the characterization of the torque transfer calibration system. 2. operation of the torque transfer calibration system the operation is based on the direct coupling of the calibration instrument to the standard transducer. for this purpose, flexible and rigid couplings are used and the whole system is supported by a ball bearing (figure 1) [1]. the manual operation is performed using buttons for start and stop of the motor, a 3 positions interrupter for spinning direction selection and 2 potentiometers, one to change the motor speed and the other to set the load. the whole process requires manual data acquisition. 2.1. calibration procedure firstly, the standard transducer and the calibration instrument are coupled and mounted on the system using all required accessories. using the manual control system, the measuring points are set. eight points are selected on the calibration instrument’s range (from 10% to 100%), all measurements are written down in a logbook. preloads, at 100% load, are applied (figure 2). figure 1. torque transfer calibration system as built. abstract for the dissemination of torque, medium accuracy transfer calibration systems are normally operated manually, originating high consumption of man-hours in the development of a calibration. this work presents the control system designed and implemented in a 200 n∙m and 2 kn∙m torque transfer calibration system and its characterization to evaluate the error and uncertainty of the system. the benefits of operating a medium accuracy system with an automatic control are also highlighted. acta imeko | www.imeko.org june 2014 | volume 3 | number 2 | 28 this procedure is applied with increasing and then decreasing load, for three different mounting positions of the calibrating instrument and in clockwise and anticlockwise directions. a total of 112 measurements are made for one calibration. since the process is manual the calibration is long and requires a large number of man-hours. this procedure is based in the din 51309:2005 materials testing machines – calibration of static torque measuring standard. 3. torque transfer calibration system automatization 3.1. control logics the control logics frame was designed as follows: 1) start the motor from 0% up to 100% of the calibrating instrument’s range. stop the motor. after 15 s to start the motor again in the opposite direction down to 0%. wait 15 s and repeat the cycle to perform 3 preloads. 2) start the motor and stop at 10% of the instrument’s range. wait 15 s and perform the measurement. continue in this way up to 100%. 3) the previous activities are repeated in descendent mode from 100% to 0%. 4) the metrologist rotates the calibration instrument to 120° and performs the previously described activities and does the same for 240°. 5) the same procedure is executed for clockwise and for anticlockwise directions. 3.2. equipment and material used software: windows vista, lab view version 2011, nidaq version 6.1. hardware: pc pentium, pc-dio 24 pnp card, interface card, pc – control system power card. 3.3. programming software the programming software used was lab view, as it is a versatile and easy to use program. it works in windows; uses blocks diagram notation and the graphic language “g”. see figure 3 for a view of the main screen arrangement. this type of programming software has proven to be particularly useful in similar applications, as can be seen in [2]. 3.4. control stage to develop the pc´s interface with the system, a digital input/output interface card pcdio24 pnp from national instruments was used. the card works in a virtual instrumentation programming environment (lab view) and a power stage. the card handles digital input and output; it has 3 ports (a, b and c) and 24 digital lines or 24 digital input and output channels. the ports can be configured as input or output, and is ttl logics compatible. for the card operation the ni-daq driver software of national instruments was used. the ni-daq is a function library called by the program during its interface operation. some built-in functions are: data acquisition, digital, input/output, counts/operation time. 3.5. control stage to develop the pc´s interface with the system, a digital input/output interface card pcdio24 pnp from national instruments was used. the card works in a virtual instrumentation programming environment (lab view) and a power stage. the card handles digital input and output; it has 3 ports (a, b and c) and 24 digital lines or 24 digital input and output channels. the ports can be configured as input or output, and is ttl logics compatible. for the card operation the ni-daq driver software of national instruments was used. the ni-daq is a function library called by the program during its interface operation. some built-in functions are: data acquisition, digital, input/output, counts/operation time. 3.6. pc control system interface card to perform the connection between the output of the pcdio 24 card (installed in the pc cpu) and the power stage, an interface card with an 8-pin connector was produced. pins 1 and 2 carry the electrical power supply (5 v dc) and ground (electrical supply for the computer), pins 3 to 8 are for the digital outputs. 3.7. power stage in the power stage, 2 electrical contacts 127 v/60 hz are integrated to the existing control system and are controlled by the power card. 3.8. control program the program follows the calibration procedure (2.1) and the control logics (3.1). the calibration starts with the preloads (3 preloads at maximum load). the automatic program performs the 3 first series at 0º position. the measuring range is divided figure 2. example of the calibration procedure at 0°, 120° and 240° mounting positions for classes 0.05 and 0.1 (din 51309:2005). figure 3. main screen of the control program. acta imeko | www.imeko.org june 2014 | volume 3 | number 2 | 29 into 8 points proportional to the operation time and measurements are taken in each point. the first reading is taken at 10% of the maximum range and the 8 points are evenly distributed up to 100% of the defined operation time. the program stops after the 3 first series to allow the rotation of the instrument under calibration to 120º and then continues according to the calibration procedure (2.1). the same series is repeated for 240º. the calibration process requires an open program, capable to perform the calibration in automatic mode but able to adjust it to fit all different measurement ranges, starting from 1 n∙m and up to 2 kn∙m. figure 4 presents a diagram of the control program. 4. torque transfer calibration system characterization the objective of this study is to evaluate the torque transfer calibration system for the reliable measurement of torque. the main part consists of the calibration of the whole system in situ, at normal operation conditions. 4.1. characterization for the characterization, it was necessary to perform various tests by means of reference torque transducers; in each test, an influence factor or metrological characteristic was isolated to determine its influence. the most important characteristics to be measured were a) torque stability, b) elapsed time between torques applied, c) repeatability, d) hysteresis, e) reproducibility, f) resolution (stability of reading) and g) combined expanded uncertainty of the whole system. the characterization of the torque transfer calibration system was performed by means of two torque standard transducers, one with a range of up to 200 n∙m and another with a range of up to 2 kn∙m. figures 5 and 6 show the results obtained from the analysis of the main factors of influence which contribute to the combined expanded uncertainty of the torque transfer calibration system (repeatability, reproducibility or position change, hysteresis or reversibility) and the error or deviation of the whole system. figure 6. factors of influence, transducer 2 kn∙m. figure 5. factors of influence, transducer 200 n∙m. figure 4. control program diagram. figure 7. error and uncertainty of the 200 n∙m torque transducer. acta imeko | www.imeko.org june 2014 | volume 3 | number 2 | 30 figure 5 presents the results of the measurements made for the case with the low range torque transducer (range up to 200 n∙m). figure 6 presents the results of the measurements made for the case with the high range torque transducer (range up to 2 kn∙m). in figures 7 and 8, the errors and uncertainties found in the torque transfer calibration system for each applied torque are shown: figure 7 for the 200 n∙m torque transducer and figure 8 for the 2 kn∙m torque transducer. 5. conclusions the control system and the results of the characterization brought a number of benefits to the operation and use of the torque transfer calibration system. some of the most important benefits of the work presented in this paper are: i. instrument’s calibration time considerable reduction. ii. minimum metrologist intervention during a calibration. iii. better control over the process influence variables. iv. better control on the data acquisition, ensuring better measurement quality. the characterization proved that the torque transfer calibration system is adequate for torque meters and medium accuracy torque transducers calibrations. acknowledgement we acknowledge conacyt mexico (the science and technology mexican board) for their financial support, since this project between sct metro and cenam was possible thanks to their support. references [i] jorge c. torres-guzman, eric e. torres de leon, fernando martinez-juarez, design, construction and start up of a 200 n∙m and 2 kn∙m torque transfer calibration system, xx imeko world congress, september, 2012, busan, rep. korea. [2] calixto morales, jorge c. torres-guzman, et. al., automation of the 1 kn∙m torque hybrid system standard, the 2006 international conference on dynamics, instrumentation and control, cdic-06, august, 2006, queretaro, mexico. figure 8. error and uncertainty of the 2 kn∙m torque transducer. acta imeko | www.imeko.org june 2014 | volume 3 | number 2 | 31 torque transfer calibration system for 200 n m and 2 kn m, its control and characterization mathematical model of checkweigher with electromagnetic force balance system acta imeko june 2014, volume 3, number 2, 9 – 13 www.imeko.org mathematical model of checkweigher with electromagnetic force balance system yuji yamakawa1, takanori yamazaki2 1 university of tokyo, hongo 7-3-1, bunkyo-ku, tokyo, japan 2 tokyo denki university, hatoyama, hiki-gun, saitama, japan section: research paper keywords: mass measurement; electromagnetic force balance system; dynamic behavior citation: yuji yamakawa, takanori yamazaki, mathematical model of checkweigher with electromagnetic force balance system, acta imeko, vol. 3, no. 2, article 4, june 2014, identifier: imeko-acta-03 (2014)-02-04 editor: paolo carbone, university of perugia received march 4th, 2013; in final form august 14th, 2013; published june 2014 copyright: © 2014 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: (none reported) corresponding author: yuji yamakawa, e-mail: yuji_yamakawa@ipc.i.u-tokyo.ac.jp 1. introduction recently, highly accurate mass measurement systems for packages moving along a high-speed conveyor belt —so-called “checkweighers”— have been increasing in importance in the food and manufacturing logistics industries. to achieve highspeed (continuous) measurement, packages should be moved in sequence. this means that the measuring time for each package is very short. in the near future, continuous mass measurement for 300 products per minute will be required. in general, there are two types of checkweigher systems, namely the load-cell type and the electromagnetic force compensation (emfc) type. in the load-cell system, the mass of an object can be measured by the deformation of the roberval mechanism when the mass to be measured is put on a weighing pan. although this type is applicable to a wide range of masses, it has difficulty achieving high accuracy. on the other hand, the emfc system keeps a balance with the displacement of the lever-linked roberval mechanism with electromagnetic force. then, the mass of the object can be measured by the driving (or feedback) current for the electromagnetic force. this type has achieved high accuracy by the null method, but is limited in its range of measurement. ono, w. g. lee, and kameoka have already clarified in detail the dynamic behavior of the load-cell system [1]-[4]. but the dynamic behavior of the emfc system has not yet been presented. our aim was to improve the performance of the checkweigher for better speed and accuracy. to do this, it was necessary to develop a control scheme for the emfc. we introduced a dynamic model of our proposed checkweigher using emfc in previous papers [5]. however, the validity of the model for the closed-loop system could not be confirmed. in this paper, the model of the measurement system is improved. then, the validity of this model is confirmed by a comparison with actual experimental results. 2. measurement system figure 1 shows the overall scheme of the checkweigher. only a measuring conveyor appears in the photograph; the feed conveyor is located to the left of the measuring conveyor and the sorter is located to the right. the products are moved by the feed conveyor, the mass of product measured by measuring conveyor and the product is removed by a sorter outside of the measurable range. the enlarged photograph of the mass measurement abstract our aim was to develop a high-speed and high-accuracy mass measurement system to be used with a conveyor belt (a checkweigher). the objective in this paper was to present a dynamic model of our proposed measurement system. the checkweigher with an electromagnetic force balance system was approximated using a spring-mass-damper system as the physical model, and the equation of motion was derived. the model parameters could be obtained from experimental data. finally, the validity of the proposed model was confirmed by comparison of the simulation results with the experimental responses. the dynamic model obtained offers practical and useful information to examine the control scheme and to achieve high-performance mass measurement. acta imeko | www.imeko.org june 2014 | volume 3 | number 2 | 9 mechanism in the checkweigher is shown in figure 2. the mass measurement system consists of a weighing platform, the roberval mechanism, the lever-linked roberval mechanism, the counter weight, the electromagnetic force actuator and the displacement sensor. by applying the roberval mechanism to the measurement mechanism, the mass of the product can be measured regardless of where the product is located on the weighing platform. the mass of the product is estimated from the current of the electromagnetic force actuator to control the lever displacement. the mass-measuring method can be explained as follows: 1. when the product of the mass m is put on the measurement system, this causes the displacement of the roberval mechanism. 2. the displacement of the roberval mechanism is magnified twenty times by the lever. 3. the magnified displacement can be measured by the displacement sensor. 4. the current is controlled so that the displacement of the lever is maintained at zero. 5. the current is measured and the current is converted to the estimated mass using a linear function. 3. modeling figure 3 and figure 4 show the physical model of the measurement system. the equation of motion about mass m can be obtained as follows: , (1) where m is the mass of the roberval mechanism, m is the mass of the product to be measured, c is the a damping coefficient, k is a spring constant, g is the acceleration of gravity, l (= 20 m/m) is the lever ratio, and x is the displacement of mass m. in addition, ml is the mass of the lever, xl is the displacement of mass ml, and f (= bli, where b is a density of magnetic flux, l a length of the coil, and i a current) is an electromagnetic force input to control the position xl of the mass ml. from eq. (1), the natural frequency is . from the experimental results, the natural frequency of the lever is 5 hz. thus, the spring constant of the measurement system can be given as follows: . (3) the damper coefficient can be adjusted so as to match the convergence rate in the responses of the lever. from eq. (1), the roverbal displacement at steady-state for the open-loop system (f = 0) can be described by: . (4) in addition, the lever displacement at steady state, which is the roverbal displacement magnified by the lever, can be obtained by the following equation: . (5) simulations of the measurement system can be performed using eq. (1)-(5). figure 5 shows a block diagram of the figure 1. overall of checkweigher. figure 2. photograph of roberval mechanism. figure 3. overall of measurement system. figure 4. physical model of measurement system. weighing platform counter weightroberval mechanism lever acta imeko | www.imeko.org june 2014 | volume 3 | number 2 | 10 measurement system. in simulations and experiments, the validity of the model can be confirmed by comparing the sensor outputs of the lever displacement (xl). 4. experimental and simulation results for step response in this section, the validity of the proposed model is confirmed by comparing the simulation result and the experimental results for open-loop and closed-loop systems. 4.1. open-loop system the validity of the proposed model was first examined by comparing the simulation and experimental results for the open-loop system. figure 6 depicts the experimental and simulation results for m = 0.01, 0.02, 0.05 and 0.1 kg. the red and blue lines indicate the experimental and simulation results, respectively. the startup operation is the time when the product of the mass m is put on the measurement system. when the time is 0.2 s, the product of the mass m is removed. at the same time, the figure 5. block diagram of measurement system. figure 6. comparison between experimental result and simulation result (open loop). acta imeko | www.imeko.org june 2014 | volume 3 | number 2 | 11 displacement xl is measured by the displacement sensor. in figure 6, the vertical axis shows the displacement sensor output of the lever displacement. it can be seen from these figures that the responses of the simulation were in excellent agreement with the experimental results. thus, the validity of the proposed model was confirmed for the open-loop system. although the amplitude of the response changes depending on the mass, the convergence time is not changed. thus, this measurement system can be assumed to be a linear system. namely, even if the controller is added to the measurement system, the response does not depend on the mass. in the next section, the validity of the proposed model for the closed-loop system was examined. 4.2. closed-loop system this section explains the experimental and simulation results with emfc. the electromagnetic force was controlled by a proportional-integral-differential (pid) control scheme. taking the actual circuit of the d action into account, the ideal d action could not be implemented. so, it is assumed that the d action is the approximated differential action. namely, the transfer function of the pid controller c(s) can be described by , (6) where kp is the proportional gain, ki is the integral gain, and kdn and kdd are the numerator and denominator coefficients of the differential gains, respectively. the control voltage uv(s) (laplace transform of uv(t)) can be adjusted as follows: , (7) where e(s) is laplace transform of e(t), and e(t) (= xlr − xl) is the error between the reference xlr and the displacement sensor output of the lever xl. the pid control can be performed by fpga (field programmable gate array) every 0.1 ms. figure 7 depicts the experimental and simulation results for m = 0.01, 0.02, 0.05 and 0.1 kg. the red and blue lines indicate the experimental and simulation results, respectively. the simulation and experimental conditions are the same as the open-loop system. responses such as the convergence time, the rise time and the settling time were almost the same for the simulation and experimental results. thus, the validity of the proposed emfc model was confirmed. we consider that the reasons for the modeling error were the frictional force in small displacements and the magnification mechanism of the lever. however, the peak value of the simulation result for m = 0.1 kg was different from that of the experimental result. moreover, a highfrequency response occurred in the experiments due to the system noise. figure 7. comparison between experimental result and simulation result (closed loop). acta imeko | www.imeko.org june 2014 | volume 3 | number 2 | 12 5. simulation results of floor vibration here, we discuss the effect of floor vibrations in the simulation and the experimental results. the floor vibration oscillates the base of the system. the vibration input applies to the same position in the block diagram (figure 5) as the mass of the object. in the experiment, a vibration exciter is used in order to oscillate the system. figure 8 shows the simulation result and the experimental result of floor vibration. the amplitude and the frequency of the vibration were set at xb = 1 mm and fb = 10 hz, respectively. the angular frequency ωb can be calculated by 2πfb. the mass mb of the base of the system was set at 0.175 kg. the acceleration input ab of the floor vibration was set at xb×ωb2. the red and blue lines indicate the experimental and simulation results, respectively. as can be seen from this result, exactly the same responses were obtained. in addition, figure 9 shows the results for the simulation and the experiment when the amplitude and frequency of the floor vibration were set at xb = 1.25 mm and fb = 10 hz, respectively. the red and blue lines indicate the experimental and simulation results, respectively. it can be seen from figure 9 that the same responses were obtained. the validity of the proposed model was thus confirmed for different amplitudes of vibration. furthermore, figure 10 shows the result of the simulation and the experiment in which the amplitude and the frequency of the floor vibration were set at xb = 1 mm and fb = 15 hz. the red and blue lines indicate the experimental and simulation results, respectively. the error between the simulation and the experimental result was about 1 g. when the frequency of the floor vibration increases by 1.5 times, the acceleration increases 2.25 (=1.52) times. thus, the output theoretically increases 2.25 times in the simulation. however, the output of the experimental result increased only about 1.75 times in this case. we consider that the reason was the mechanical limitation of the displacement in the mass measurement system in order to avoid the system breakdown. the simulation could not replicate the experimental results for this reason. as a result, we can evaluate the effect of the floor vibration by the proposed model. 6. conclusions in this paper, a dynamic model of an electromagnetic force compensation (emfc) system was constructed. the model was approximated by a spring-mass-damper system. the model parameters were identified by the experimental data for an open-loop response. then, the validation of the proposed model was examined for both open-loop and closed-loop systems. the simulation and experimental results for the open-loop system were exactly the same. the results for the closed-loop system were almost the same. as a result, the validity of the proposed emfc model could be confirmed. in addition, we confirmed the validity of the proposed model with simulations of floor vibrations. in the future, an entirely new control scheme for the electromagnetic force compensation can be implemented using the proposed model. this high-speed, high-accuracy mass measurement system will be applicable to many industrial fields. references [1] t. ono, “basic point of dynamic mass measurement”, proc. sice, pp. 43-44, 1999. [2] t. ono, “dynamic weighing of mass”, instrumentation and automation 12, no. 2, pp. 35, 1984. [3] w.g. lee et al., “development of speed and accuracy for mass measurements in checkweighers and conveyor belt scales”, proc. ismfm, pp. 23-28, 1994. [4] k. kameoka et al., “signal processing for checkweigher”, proc. apmf, pp. 122-128, 1996. [5] y. yamakawa and t. yamazaki, “mathematical model of checkweigher with electromagnetic force compensation”, proc. xx imeko world congress, tc3-o-19, 2012. figure 8. simulation result of floor vibration (amplitude:1 mm, frequency: 10 hz). figure 10. simulation result of floor vibration (amplitude:1 mm, frequency: 15 hz). figure 9. simulation result of floor vibration (amplitude:1.25 mm, frequency: 10 hz). acta imeko | www.imeko.org june 2014 | volume 3 | number 2 | 13 mathematical model of checkweigher with electromagnetic force balance system generation of static and dynamic small calibration forces and their measurements by electromagnetic force compensation balance acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 113 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 113 117 generation of static and dynamic small calibration forces and their measurements by electromagnetic force compensation balance s. vasilyan1, n. rogge1, e. manske1, t. fröhlich1 1 institute of process measurement and sensor technology, ilmenau, germany, suren.vasilyan@tu-ilmenau.de abstract: the paper presents some of the results of the static and dynamic force measurements at 100 nn to sub-10 µn ranges which are generated due the photon-momentum. the force sensor with resolution about 20 nn and operating in differential measurement mode is developed by two electromagnetic force compensation balances. in order to generate these calibration forces, cw lasers with different operational modes, power levels, and wavelengths are used. multi-reflection configuration of the laser beam inside the macroscopic cavity with highly reflective mirrors are used to test and variate the total amount of the forces. keywords: photon momentum, static and dynamic small calibration forces, high power laser, highly reflective mirrors 1. introduction in recent years, the use of photon momentum to determine the optical power of lasers or to generate precision/calibration small forces [1, 2, 3] has made important progress, especially for the measurements of optical laser power at kilowatt levels [4]. the measuring principle is based on the measurement of the force which is exerted due to the transfer of the photon momentum upon reflection of the radiant power from a highly reflective mirror. a measurement device developed by williams et al. [4] uses this measurement technique and achieves a relative expanded measurement uncertainty of 1.6 % for optical power levels between 1 kw and 50 kw. in the core of the device is a force sensor, consisting of a commercial off-theshelf electromagnetic force compensation (emfc) balance and a mirror with high reflectivity (r = 0.999 8 %) attached to it. vasilyan et al. [1, 2] carried out measurements with a device having similar components, however, here two force sensors adapted for differential measurements were used, by which the noise level has been reduced. here, for the stability considerations a low power laser system (around 1 w) was used in multireflection configuration to generate calibration forces at the currently existing lowest end of the small force standard typically referred to be from 10 nn up to 10 µn. despite the statistical error observed as an oversight between the measurements and simplified theoretical calculations, under the multi-reflection configuration the total measured force was amplified by at least an order of magnitude in comparison to the single reflection configuration. furthermore, with the usage of singleand multi-reflection configurations, a possible standard for the force calibration routine, or reversed, standard for the optical (laser) power calibration routine with direct and more simplified traceable chain to the recently renewed si base units [2, 5, 6] was already established. 2. description of the work in order to develop a proper methodology in using this effect of the photon momentum-generated small calibration forces, initially several test measurements have been carried out. the setup consists of two emfc balances operating in synchronous measurement modes such that the difference signal can be obtained with substantially reduced noise level. figure 1: set-up for measuring the generated small calibration forces using emfc balance, the supporting optical components and the reference optical power sensors. inset the photograph showing the lower parts of the emfc balances, mirrors and the reflection spots http://www.imeko.org/ mailto:suren.vasilyan@tu-ilmenau.de acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 114 the emfc balances are mounted independently from the common metrology frame and on each of them a highly reflective mirror is attached with total weight of about 70 g together with the adjustment mechanism (see scheme in figure 1). a conventional off-the-shelf highly reflective mirror with reflectivity 𝑅 above 0.995, universally used for lasers with different wavelengths (see table 1), has been used to create the macroscopic optical cavity where the multi-reflections of the laser beam are obtained. table 1: summary of the parameters of the laser (power 𝑃, power uncertainty 𝑈, wavelength 𝜆) and the mirror reflectivity coefficient 𝑅 used during different measurements laser 𝑹 𝑷 / w 𝑼 / % 𝝀 / nm 1 0.98 1 420 ± 10 0.999 2 0.05 2 1 450 ± 7 0.999 3 0.1 0.9 1 520 ± 2 0.995 4 2 11 < 1 532 ± 5 0.995 5 2.5 8.8 <1 532 ± 5 0.999 97 theoretically, at the single reflection of the laser beam with 1 w optical power from a perfectly reflecting mirror, a force of about 6.7 nn is generated. in the case of the multiple reflecting specular type of reflections, a force of about 220 nn can be created, see equations (1) and (2). 𝐹 = 𝑃 𝑐 (1 + 𝑅) (1) ∑ 𝐹𝑗 = 𝑛 𝑗=1 (1 + 𝑅) 𝑐 ∑ 𝑃𝑗 𝑛 𝑗=1 (2) where 𝐹 is the force in n, 𝑃 is the average laser power in w, 𝑐 is the speed of light in m/s, 𝑅 is the reflectivity coefficient of the mirror, and 𝑗 is an integer assigned to each of the 𝑛 reflections. during the measurements the photon momentum-generated forces due to specular type of multireflection are described in equation (3). 𝐹total = 𝐹1 − 𝐹2 = = [𝐹p1 + 𝐹err1] − [−𝐹p2 + 𝐹err2] (3) where 𝐹p1 and 𝐹p2 are the photon momentumgenerated forces acting on the emfc balances attached left and right mirrors from 2nd, 4th, 6th… and 1st, 3rd, 5th … reflections respectively, 𝐹err1 and 𝐹err2 are the noise measured by each emfc balance which are assumed to be the same (see figure 1). the parameters listed in table 1 were used during different measurement campaigns. the averaged laser power has been pre-set using varying methods. we have separated dynamically changing and statically applied laser powers in order to characterise the force measurement signals in relation to them, more specifically in the time or frequency domains. in the case of the static signals have been used either a shutter system or completely turning the lasers off. in the case of the dynamic signals the power of the laser has been varied by varying the gain current of the laser’s control unit using equation (4). 𝑃 ~ 𝐼𝑖 (𝑓𝑖 ) = ∆𝐼 sin(2𝜋𝑓𝑖 ∆𝑡𝑖 ) = = ∆𝐼 sin(2𝜋[𝑓0 + 𝑖 ∙ ∆𝑓] ∆𝑡𝑖 ) (4) where ∆𝐼 is the peak-to-peak amplitude of the applied gain current, within the frequency range 𝑓0 to 𝑓end, with ∆𝑓 separation between the grid points, and ∆𝑡𝑖 is the signal length. any other function type could be used, e.g. a square wave (see figure 2). figure 2: example of the measured forces as a) sine and b) square wave functions [2] 3. uncertainty assessment the photon momentum-based method, as mentioned, may directly relate the measure of the force, which is produced by the momentum transfer of the absorbed and re-emitted photons from a highly reflective mirror, with the measure of the optical power of a laser beam. the following simplified numerical example, based on equating the photon momentum-generated forces with the http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 115 gravitational force acting on a standard mass piece, gives an understanding of the advantages underlying this method. we should note here beforehand that as it is known from recent developments the mass pieces mainly at 1 kg and 500 g are already calibrated directly traceable to the quantum-based natural constants, more specifically it is traceable to the numerical value of the planck constant, however there are still ongoing developments to extend that calibration method of mass pieces for the range of smaller mass pieces (down to 1 mg) [6]. using the kibble balance apparatus, one may even provide calibration of the arbitrary values of mass pieces, therefore on the account of measuring the gravitational force acting to this standard mass pieces one may also provide static force calibration values with similar accuracy and precision. here we will rather provide our analysis on the basis of existing basic specifications of the measurement components that are well known/established and at the same time assumes the future speculations related to possible approaches of calibrating forces at smaller force levels. it is known that the optical power measurements at 1 kw level are typically made using reference standard thermal detectors with several percent of uncertainty, at the same time becoming a non-trivial technological task to implement since their measurement capability and accuracy strongly depend on the absorbance and heat capacity of the cavity used as sensor. in comparison to this, the 1 kw laser power after first reflection from the ultra-high reflective mirror with 0.999 95 reflectivity exerts a force of about 6.673 116 µn, whereas after 33 reflections it is 220.036 76 µn. one may use standard practices known from mass/force metrology to perform the force measurements at these levels. in the case that one uses a commercially available precision balance with 3 µg reproducibility (standard deviation) in measurements of the weight pieces, the equivalent force calculated with equation (2) is to be 29.4 nn assuming the value for the local gravitational acceleration of 9.812 503 m·s-2 at the site of the measurements. thus, in this case we can measure the optical power generated forces after first reflection with 0.440 % relative standard deviation and with 0.013 % after 33 reflections. however, since force standards at the nanonewton level have not yet been established, the implementation of a traceable optical power measurement using the photon momentum approach can only be verified in connection to the measurement capabilities of a certain class of apparatuses, e.g. with an electromagnetic force compensation emfc weighing balances down to 10 µn (with about 0.3 % relative measurement uncertainty) traceable from there down to about 10 nn only accounting for the reproducibility or in general the type b uncertainty evaluation (an alternative to be the development of an uniquely-designed custom-made instrument with well characterised and si-traceable calibration certificates whose performance could be verified by an independent party). generally, the balances are calibrated by employing the deadweight effect exerted on standard weight pieces in accordance with the equation 𝐹 = 𝑚𝑔. on the other hand, the optical power can be determined by combining this with equations (1) and (2) for single or multiple reflection cases yielding the simplified form given in equation (5). from which, by using the standard uncertainty propagation, we obtain equation (6). the value of the gravitational acceleration can typically be obtained to better than 0.2 ppm. the values of the reflectivity for the ultra-high reflective mirrors to our best knowledge can be estimated only indirectly by the measurement of the losses typically given as the optical transmission curve. in accordance to most of the datasheets provided by manufacturers it can be determined with 70 ppm uncertainty [9]. the uncertainty values of the mass vary dependent from the nominally used discrete set of standard mass piece (ref. figure 3) against which the nominally applied magnitude of the optical power should be compared. for this, one should follow at least the recommendations given in [8]. figure 3: expanded relative uncertainties and maximum permissible errors associated with weights of oiml r 111-1 class e1 [8] thus, if the measurements would be conducted at the 1 kw optical power level for the single reflection case the combination of uncertainty 𝑃 = 𝑐 𝑚𝑔 1 + 𝑅𝐿 (5) 𝑢(𝑃) 𝑃 = √( 𝑢(𝑚) 𝑚 ) 2 + ( 𝑢(𝑔) 𝑔 ) 2 + ( 𝑢(𝑅𝐿 ) 𝑅𝐿 ) 2 (6) http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 116 contributions using equation (6) leads to an estimated standard uncertainty of the optical power measurements of about 𝑢(𝑃)/𝑃 = 1 002 ppm; for the multi-reflection configuration with 33 reflections the value is 86 ppm. in figure 4 the full estimation curves of relative measurement uncertainties in the optical power obtained by equation (6) are presented. here the relative standard uncertainty is presented as a function of the nominal magnitude of the applied optical power in four different (1, 3, 21, 33) reflection configuration cases with interpolated data as an extended estimate for the power levels below 1 kw. the blue dashed line presents the lower limiting factor as a result of the reflectivity coefficient of the mirrors, the solid black line with 0.05 % notation highlights the typical limit of the relative standard uncertainties in measuring laser power at 10 w and above with conventional power meters [7]. figure 4: estimated relative standard uncertainty of the optical power realisation by equations (5) and (6) this method of establishing the traceability of the optical power measurements through the use of measurements of the photon momentum-generated forces and their further intercomparisons with the standards and procedures known from mass/force metrology is theoretically more direct and simplistic which may further improve and lower the measurement uncertainties at least in continuous wave optical power measurement. 4. results currently, we perform deliberately planned trial measurements in collaboration with division 4 (optics) ptb, braunschweig for establishing the traceability of the optical power measurements through the use of photon momentum-based force measurements and we compare the results with those performed by using a calibrated reference standard detector, thermopile and si-diode traceable to ptb’s primary standard for optical power (the cryogenic radiometer) as shown in schematics in given in figure 1. the optical power values were used to calculate theoretically the expected photon momentumgenerated forces for comparison with the data from the measurements. in this configuration we can directly compare the reference for the force measurements and the reference for the optical power measurements, towards the si-based traceable comparison (actually in future to be in reference to the planck constant) of the force/mass and laser power references. in figure 5 we show one typical set of static forces and the corresponding laser power measurements. figure 5: example of the static force measurements with a periodically applied laser power (10 s) in the case of 33 reflections. top measured force signal from each balance. middle the difference signal (a feed backward averaging filter was chosen for the last 15 bins to filter raw data). bottom input and output power from the raw data of the force measurements during the 33 reflections configuration (wavelength 532 nm, mirror reflectivity 0.999 95, input power 8.5 w, output power 8.3 w), a mean value of 1 873 nn is obtained with a relative combined standard deviation of 1.71 % (32 nn). as a comparison, the standard uncertainty of the optical power measurements using the calibrated reference standard detector was 0.3 %. furthermore, in this particular case, the mean value of the measured forces in comparison to the value calculated theoretically differs by 1.07 %, from all measurements this difference is within 3 %. with these measurements at 10 w level using well established methods from mass/force metrology we demonstrate that upon the increase of the input power entering to the optical cavity and the number of reflections, the resulting uncertainties of generated small force values or vice versa the measurements of the input optical power can markedly be reduced. thus, by collecting more experimental evidence at the edge of the small force measurements in static and dynamic regimes, improving the accuracy of the measurements, and http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 117 eliminating further the systematic errors associated with the measurements, then the photon momentum-based force generation can serve more evidently as a superior method for obtaining accurate measurement of small (calibration) forces below 10 µn through the traceable optical power measurements. thereafter, by virtue of the same relation, the photon momentum-based force measurements can be used to develop viable, accurate and absolute optical power detectors / sensors / power meters at higher ranges with direct traceability to si units. in accordance with the existing technical limits and instrumentation capabilities as well as the theoretical obtained relations, we can still improve the accuracy and the precision of the optical power and small force measurements by at least several orders of magnitude (ref. figure 4). 5. summary in this work we present photon momentumgenerated precision small force measurements in static and dynamic modes, in the range of forces below 10 μn and above 100 nn, using a force sensor with about 20 nn resolution. static (considered to be below 0.2 hz) and dynamic (ranging from 0.2 hz to 10 hz) modes of measurements are considered for different excitation signals with sine and square waveforms whose peak-to-peak amplitudes range from several hundred milliwatts up to several watts. we compare the measured forces in reference to the calculated values. the calculations of the theoretical data are made based on reference measurements of the optical power by calibrated reference standard detector, thermopile and si-diode traceable to ptb’s primary standard for optical power (the cryogenic radiometer). 6. acknowledgement this work is funded by dfg projektnummer 409476492. the authors are grateful to colleagues from institut für prozessmessund sensortechnik and collaborators from ptb division 4 for all fruitful discussions. 7. references [1] s. vasilyan, t. fröhlich, e. manske, “total momentum transfer produced by the photons of a multi-pass laser beam as an evident avenue for optical and mass metrology,” optics express, vol. 25, pp. 20798-20816, 2017. [2] e. manske, t. fröhlich, s. vasilyan, “photon momentum induced precision small forces: a static and dynamic check”, meas. sci. technol. vol. 30, 105004, 2019. [3] g. a. shaw, j. stirling, j. kramar et al., “comparison of electrostatic and photon pressure force references at the nanonewton level”, metrologia, vol. 56, 025002, 2019. [4] p. williams et al., “portable, high-accuracy, nonabsorbing laser power measurement at kilowatt levels by means of radiation pressure”, opt. express, vol. 25, pp. 4382–92, 2017. [5] p. a. williams et al., “meta-study of laser power calibrations ranging 20 orders of magnitude with traceability to the kilogram”, metrologia, vol. 57, 015001, 2020. [6] c. rothleitner, j. schleichert, l. günther et al., “the planck-balance – using a fixed value of the planck constant to calibrate e1/e2-weights,” meas. sci. technol., vol. 29, 074003, 2018. [7] https://www.ptb.de/cms/ptb/fachabteilungen/abt4/ kalib.html, pdf kalibrier-und messmöglichkeiten der abt. 4. [8] oiml r 111-1: weights of classes e1, e2, f1, f2, m1, m1-2, m2, m2-3 and m3 – part 1: metrological and technical requirements, international organization of legal metrology, 2004. [9] super mirror, 30 mm diameter, 532 nm wavelength, tfhsm-50c08-532, sigmakoki group. http://www.imeko.org/ https://www.ptb.de/cms/ptb/fachabteilungen/abt4/kalib.html https://www.ptb.de/cms/ptb/fachabteilungen/abt4/kalib.html study on short-term creep effect and hysteresis for the hbm z4a force transducer under compressive and tensile forces acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 137 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 137 142 study on short-term creep effect and hysteresis for the hbm z4a force transducer under compressive and tensile forces j. fidelus1, k. cybul2 1 gum, central office of measures, 00139 warsaw, poland, janusz.fidelus@gum.gov.pl 2 gum, central office of measures, 00139 warsaw, poland, kamil.cybul@gum.gov.pl abstract: the results of creep tests (for compressive and tensile forces) for the hbm z4a force transducer made in a force standard machine (fsm) working in the range from 1 kn to 55 kn in the central office of measures are described. studies have shown that the largest indications of the force transducer (given in electrical units, mv/v) in the first seconds of testing are most likely due to the mechanical properties of the materials from which the transducer is built. higher values of relative hysteresis error and relative measurement uncertainty were observed for the compressive forces. keywords: force; metrological traceability chain; creep; force transducer; calibration; hysteresis 1. introduction the currently-available traceability chain covers only uniaxial static forces, applied perfectly, at constant temperature, to test specimens that react in a purely axisymmetric manner. this is not how systems perform in practice, where there are additional effects due to short term creep, hysteresis, temperature, data synchronisation, instrumentation, alignment and non-axial force application [1]. the purpose of this work is to examine the effect of short-term creep and hysteresis for a selected force transducer subjected to compressive and tensile forces. 2. creep and hysteresis studies 2.1. experimental conditions calibrated object the force-proving instrument comprised the force transducer and measuring amplifier, both manufactured by hbm, germany, and the hbm catman software for data archiving and analysis. force transducer characteristics: • maximum capacity 50 kn • for compressive and tensile forces • type z4a • serial number 90330092 • temperature coefficient k = 0.000 015 °c-1 measuring amplifier characteristics: • type dmp41-t2 • serial number 819192401 • resolution 0.000 001 mv/v • channel 1 • filter 0.10 hz and 1 hz bessel, measuring range ±2.5 mv/v, excitation voltage 5 v environmental conditions ambient temperature for compressive forces: (22.8 to 23.7) °c ± 0.1 °c and tensile forces: (24.1 to 24.8) °c ± 0.1 °c. traceability calibration results of the force-proving instrument have been referred to the force reference standard maintained at the central office of measures through application of the force standard machine no. s03 (ws-55 kn), covering the force range from 1 kn up to 55 kn, with the uncertainty of applied force 0.01 % of the measured value. creep measurement (compressive and tensile forces) for the z4a force transducer of the hbm company with a measuring range up to 50 kn and sensitivity of 2 mv/v, was carried out at the gum’s fsm working in the range of 1 kn to 55 kn (ws-55 kn), see figure 1. due to the inability to determine the stiffness of the measuring system, the measurement of the indication of the force transducer was carried out electrically, in mv/v units, using the dmp41-t2 measuring amplifier of the hbm company (with a 0.1 hz bessel digital filter), which has the best accuracy class and resolution of 1 ppm in the measuring range of ± 2.5 mv/v. in order to verify the correctness of the results (the appropriate number of measuring points), the tests were also carried out using digital filters of higher frequencies. tests for compressive and tensile http://www.imeko.org/ mailto:janusz.fidelus@gum.gov.pl mailto:kamil.cybul@gum.gov.pl acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 138 forces were carried out in accordance with the pn-en iso 376:2011 standard [2]. a b c d figure 1: gum’s force reference standard maintained at the central office of measures through application of the force standard machine no. s03 (ws-55 kn), covering the force range from 1 kn up to 55 kn, with the uncertainty of applied force 0.01 % of the measured value: a) upper part: fsm with mounted force transducer prepared for compressive test; b) upper part: fsm with mounted force transducer prepared for tension test; c) fsm lower part; d) hbm z4a force transducer 2.2. creep study due to the elastic effect of the hbm z4a force transducer components (elastic material and strain gauges), there are slight changes in the output signal with constant force caused by creep. this effect is essential not only in long measurements, where the low creep value is very important, but also in shortterm measurements. creep error may be affected by, among others, design of the force transducer and elements of which it is built, used force transducer electric cable, measuring meter, measurement axis and construction of the reference station itself. with increasing the temperature, creep of steel and strain gauges increases. the indication creeping may also be caused by temperature gradients causing uneven thermal deformation of the elastic element. therefore, it is important to ensure that the temperature of the transducer and its surroundings during the measurement is the same and constant. in order to accurately measure creep, the number of measuring points in the first thirty seconds of the test after applying the maximum reference force and after removing the maximum reference force has been increased (compared to those applicable in the pn-en iso 376:2011 standard). the indication in mv/v was measured every 1 s in the first 30 s and every 60 s in the period from 60 s to 300 s. figure 2 shows a graph of force as a function of time in the direction of tensile and compressive forces in accordance to iso 376:2011 standard with creep test. after the initial load (3 × 60 s), the actual measurement was carried out. the light brown colour shows the tested creep ranges. the relative zero indication error is 0.001 % of the maximum reference force. the relative interpolation error was determined in accordance with the pn-en iso 376:2011 standard. figure 2: calibration of the z4a force transducer in the direction of tensile and compressive forces in accordance to iso 376:2011 standard with creep test. the light brown colour shows the tested creep ranges tensile forces the metrological characterization of the z4a force transducer for the tensile forces presents table 1. each time the creep was tested at the beginning and at the end of the measurement series. the creep measurement was carried out after the initial (preliminary) loads, after applying the maximum reference force (figure 2a and figure 3) and after removing the maximum reference force, which was maintained for 300 s (figure 2b and figure 4). figure 3 shows the indication of the z4a force transducer (in mv/v) after applying the maximum reference force fmax. the insert represents the creep in the first 10 s of the measurement. the error was determined in accordance to iso 376:2011standard. in the third second you can see the change in the direction of the plot. in the case of removing the force (after previously applying fmax for 90 s), you can see increasing of the creep (figure 4). the maximum creep error can be observed in the first three seconds. the relative interpolation error fc was determined using a cubic equation of deflection as a function of the calibration force f: 𝑋a = −3.996 42 × 10 −2 𝐹 − 3.1152 × 10−7 𝐹2 − 8.43 × 10−10 𝐹3 . (1) http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 139 table 1: metrological characteristic and measurement uncertainty of the z4a force transducer for the tensile forces. zero reading of the transducer: 0.000 938 mv/v taken after the last measuring series, without any loading fittings calibration force average value of deflection relative repeatability error relative reproducibility error relative creep error computed value of deflection relative interpolation error relative measurement uncertainty class f 𝑿r̅̅ ̅ b ' b c xa ƒc w kn mv/v % % % mv/v % % 1 -0.039 963 0.010 0.008 -0.039 965 -0.004 0.019 00 2 -0.079 930 0.005 0.008 -0.079 930 0.000 0.016 00 5 -0.199 832 0.001 0.013 -0.199 829 0.001 0.014 00 10 -0.399 677 0.000 0.008 -0.399 674 0.001 0.013 00 15 -0.599 536 0.000 0.005 -0.599 536 0.000 0.013 00 20 -0.799 414 0.000 0.004 -0.799 415 0.000 0.013 00 25 -0.999 311 0.000 0.003 -0.999 313 0.000 0.013 00 30 -1.199 228 0.000 0.003 -1.199 229 0.000 0.012 00 35 -1.399 166 0.000 0.005 -1.399 165 0.000 0.012 00 40 -1.599 122 0.000 0.005 -1.599 120 0.000 0.012 00 45 -1.799 099 0.000 0.005 -1.799 097 0.000 0.012 00 50 -1.999 092 0.000 0.005 0.002 -1.999 094 0.000 0.012 00 figure 3: graph of z4a force transducer indication given in electrical units (mv/v) in the time function (s) for measurements made after applying the maximum tension reference force. the insert represents the creep in the first 10 s of the measurement. for the sake of clarity, there are no error bars. figure 4: graph of z4a force transducer indication given in electrical units (mv/v) in the time function (s) for measurements made after removing the maximum tension reference force (90 s). the insert represents the creep in the first 10 s of the measurement coefficients of this function were determined from average values of deflection 𝑋r̅ using the method of least squares. the inverse function of the above is used to calculate force fa from the deflection x: 𝐹a = −25.022 394 𝑋 − 4.882 31 × 10 −3 𝑋2 + 3.271 76 × 10−4 𝑋3 (2) symbols and their meanings: 𝑋a = computed value of deflection of the force-proving instrument corresponding to any force 𝐹 𝐹 = calibration force 𝐹a = computed value of calibration force reproduced by the force-proving instrument, calculated for any value of deflection 𝑋 𝑋 = deflection with increasing force the relative uncertainty of measurement w, expressed as a percentage, is dependent on force f (in units of kn) and defined by equation (3). 𝑊 = 2 × (𝐶0 + 𝐶1 𝐹 ) (3) where: 𝐶0 = 5.868 × 10 −3 𝐶1 = 3.536 × 10 −3 http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 140 table 2: metrological characteristic and measurement uncertainty of the z4a force transducer for the compressive forces. zero reading of the transducer: -0.000 150 mv/v taken after the last measuring series, without any loading fittings calibration force average value of deflection relative repeatability error relative reproducibility error relative creep error computed value of deflection relative interpolation error relative measurement uncertainty class f 𝑿r̅̅ ̅ b ' b c xa ƒc w kn mv/v % % % mv/v % % 1 0.039 970 0.005 0.030 0.039 967 0.007 0.027 00 2 0.079 935 0.000 0.021 0.079 934 0.000 0.020 00 5 0.199 830 0.002 0.018 0.199 833 -0.001 0.016 00 10 0.399 658 0.001 0.012 0.399 657 0.000 0.014 00 15 0.599 475 0.001 0.010 0.599 474 0.000 0.014 00 20 0.799 283 0.001 0.007 0.799 284 0.000 0.013 00 25 0.999 090 0.001 0.006 0.999 090 0.000 0.013 00 30 1.198 894 0.000 0.006 1.198 892 0.000 0.013 00 35 1.398 692 0.000 0.006 1.398 692 0.000 0.013 00 40 1.598 489 0.000 0.006 1.598 491 0.000 0.013 00 45 1.798 289 0.000 0.006 1.798 290 0.000 0.013 00 50 1.998 092 0.000 0.005 0.001 1.998 091 0.000 0.013 00 compressive forces in the case of compression forces, the metrological characteristic of the z4a force transducer is included in table 2. as for the tensile forces, the relative interpolation error ƒc was determined using cubic equation of deflection as a function of the calibration force f: 𝑋a = −3.996 76 × 10 −2 𝐹 − 2.050 13 × 10−7 𝐹2 + 1.792 × 10−9 𝐹3 (4) while fa depends on the deflection x and is given by equation (5). 𝐹a = 25.020 267 𝑋 + 3.212 18 × 10 −3 𝑋2 − 7.025 67 × 10−4 𝑋3 . (5) the relative uncertainty of measurement w (in %) is dependent on force f (in units of kn) and defined by equation (3), with 𝐶0 = 6.114 × 10 −3 and 𝐶1 = 7.101 × 10 −3. the creep measurement was performed after the last measurement series in accordance with figure 2. figure 5 presents a graph of the force transducer indication dependence (given in electrical units, mv/v) as a function of time for measurements made after applying the maximum reference force, while figure 6 illustrates a similar relationship after removing the maximum reference force. for both measurements, a decrease in the relative creep error value as a function of time can be observed. this error is the largest in the first minute of testing. the higher indication values of the force transducer (given in mv/v) in the first seconds of testing are mainly due to the mechanical properties of the materials it is made of. studies have shown the difference in the relative creep error size for creep measurements after applying the maximum reference force, and the measurements after removing the maximum compressive reference force. higher (by order of magnitude) values of relative creep error occur after removing the maximum reference force. figure 5: graph of z4a force transducer indication given in electrical units (mv/v) in the time function (s) for measurements made after applying the maximum compressive reference force. the insert represents the creep in the first 35 s of the measurement 2.3. hysteresis study it is known that the indications of a transducer at a given force depend on its loading history. the measuring hysteresis of the z4a force transducer may include the following reasons: • hysteresis of an elastic element conditioned by internal friction, slips and permanent deformations of the material of the element • friction and play in mechanisms converting deformation to indication, i.e. in levers, bearings, etc. http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 141 • friction between the measuring elements, the transducer housing, seals and membranes and their fixing places • friction on boundary surfaces • elastic hysteresis of strain gauges, washers, glue and electric cables figure 6: graph of z4a force transducer indication given in electrical units (mv/v) in the time function (s) for measurements made after removing the maximum compressive reference force (90 s). the insert represents the creep in the first 35 s of the measurement during the calibration of the z4a transducer, measurement hysteresis errors were determined for each of the applied load, defined as the difference between the indications obtained with the same force, achieved with a load increasing from zero and decreasing from the upper limit of the measuring range (table 3). figure 7 shows the dependence of the relative hysteresis error and the relative measurement uncertainty (insert) on the calibrated force (compressive and tensile forces) for the z4a force transducer. in the range of up to 5 kn, higher values of relative hysteresis error and relative measurement uncertainty were recorded for the compressive forces, while in the range from 5 kn to 30 kn higher values of hysteresis errors were observed for the tensile forces. 3. summary the results of creep tests at maximum studied force and after force removal in accordance to iso 376:2011 for the hbm z4a force transducer made on the gum’s force reference standard machine up to 55 kn (ws-55 kn) no. s03, covering the force range from 1 kn up to 55 kn, with the uncertainty of applied force 0.01 % of the measured value were presented. the short-term creep effect of the hbm z4a force transducer was investigated. table 3: relative hysteresis error and measurement uncertainty of the z4a force transducer for tensile and compressive forces calibration force tensile forces compressive forces relative hysteresis error relative measurement uncertainty class relative hysteresis error relative measurement uncertainty class f v w v w kn % % % % 1 0.068 0.035 00 0.080 0.044 0,5 2 0.044 0.025 00 0.057 0.029 00 5 0.033 0.019 00 0.034 0.020 00 10 0.027 0.017 00 0.023 0.017 00 15 0.022 0.016 00 0.019 0.016 00 20 0.019 0.016 00 0.015 0.016 00 25 0.015 0.015 00 0.012 0.016 00 30 0.012 0.015 00 0.009 0.015 00 35 0.009 0.015 00 0.007 0.015 00 40 0.006 0.015 00 0.004 0.015 00 45 0.003 0.015 00 0.002 0.015 00 50 0.015 00 0.015 00 http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 142 figure 7: calibration force versus relative hysteresis error for the z4a force transducer after applying tensile and compressive forces. the insert represents the plot of the calibration force versus relative measurement uncertainty for the transducer after applying tensile and compressive forces studies have shown that the largest indications of the force transducer (given in electrical units, mv/v) in the first seconds of testing are most likely due to the mechanical properties of the materials from which the transducer is built. in the range of up to 5 kn, higher values of relative hysteresis error and relative measurement uncertainty were recorded for the compressive forces, while in the range from 5 kn to 30 kn higher values of errors and uncertainty were observed for the tensile forces. the performed calibration of the z4a force transducer in the measuring range from 1 kn to 50 kn for tensile forces confirmed its class 00 for both creep and hysteresis. in the future, similar studies are planned on other hbm z4/z4a transducers to see if they exhibit comparable behaviour. 4. acknowledgements this work has been carried out in the framework of the empir joint research project 18sib08 'force' with the title “comprehensive traceability for force metrology services”, comtraforce for 2019 – 2023. the empir is jointly funded by the empir participating countries within euramet and the european union. 5. references [1] website of the empir project “comprehensive traceability for force metrology services”, 18sib08 comtraforce: https://www.ptb.de/empir2019/comtraforce/project/o verview/ [2] pn-en iso 376:2011 standard “metallic materials – calibration of force-proving instruments used for the verification of uniaxial testing machines”. http://www.imeko.org/ https://www.ptb.de/empir2019/comtraforce/project/overview/ https://www.ptb.de/empir2019/comtraforce/project/overview/ acta imeko  september 2014, volume 3, number 3, 57 – 62  www.imeko.org    acta imeko | www.imeko.org  september 2014 | volume 3 | number 3 | 57  fuzzy scales for the measurement of color  eric benoit   listic, polytech annecy‐chambery, university of savoie, b.p. 80439, 74944 annecy le vieux cedex, france      section: research paper   keywords: measurement theory; color; colour; scale; distance; fuzzy subset; fuzzy scale  citation: eric benoit, fuzzy scales for the measurement of colors, acta imeko, vol. 3, no. 3, article 12, september 2014, identifier: imeko‐acta‐03 (2014)‐ 03‐12  editor: paolo carbone, university of perugia   received march 8 th , 2013; in final form march 24 th , 2014; published september 2014  copyright: © 2014 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: (none reported)  corresponding author: eric benoit, e‐mail: eric.benoit@univ‐savoie.fr    1. introduction  some quantities like color, odor or software complexity are usually measured with inappropriate scales. indeed, the theories chosen to abstract such quantities usually define an affine space to represent measurement values even if this choice is not justified. for example, colors are represented in many different colorimetric spaces like rgb, xyz, luv, lab, hsv and the transformation from one to each other is not always an affine transformation. we can conclude from this situation that the empirical space of colors doesn’t hold an affine structure and then cannot be represented by an affine space. conversely, the metric, defined with psychophysic experiments stays stable and is the most known relation on colors. the basis hypothesis of this paper is that the empirical space of some quantities manifestations, more specifically the color, can be represented by a non-affine abstract space that holds a metric. the determination of such metric depends on the theory used to perform calculus reasoning or decision, and on an abstract world where quantities are represented by their quantity value [1]. let us describe the full process. first the area of interest, i.e. the concrete world is identified. then the concrete objects and their associated quantities are selected. finally, a theory that is made of entities, axioms and theorems is chosen. experiments are then performed in order to obtain some observations of the quantity manifestations. the representations of the manifestations are named quantity values [2] and are expressed into a space which structure depends on the chosen theory. the choice of the theory is crucial and depends on the goal of the experiment. in the color area, the experiment goal can be a color based identification of chemical components. in this case the theory is defined on the area of molecular physics and color manifestations are represented by spectral energy distributions. the spectra are expressed as an ndimensional vector space. if the goal is to check the quality of a manufactured color, then the experiment is based on a theory of color vision and colors are expressed into a colorimetric space. 2. color vision representation  this paper, will focus on psychophysical aspects of colors. this means that color quantities are not considered exclusively into the context of physics but also into the context of human perception. from a pure physics based consideration, the color of an electromagnetic flow is defined by its spectral power distribution (spd). as for any distribution, a general definition is never obtained due to the necessity to define the spectral resolution. indeed the quantity that represents a color is a vector which length depends on the chosen resolution and on abstract  the color, with the particularity to be defined simultaneously as a physical quantity and as a psychophysical quantity, is one of the  concepts  that  can  link  hard  sciences  and  behavioural  sciences.  from  the  viewpoint  of  behavioural  sciences,  colors  are  basically  measured with nominal scales. in hard sciences, colors are measured with interval scales. our hypothesis is that the main relation that  must be preserved during a color measurement is a metric. we suggest then that colors must be measured with metrical scales. the  fuzzy metrical scale is preferred due to the possibility to define it like a nominal scale. acta imeko | www.imeko.org  september 2014 | volume 3 | number 3 | 58  the chosen range of the spectrum. we can see that even with a given theory, the goal of the experiment has a strong incidence on the representation of the measured quantities. as an example, the international commission on illumination (cie) specified that for color measurements of visible light the spectrum range is from 360 nm to 830 nm with 1nm resolution. this institution gave also a first approximation of human color perception with the definition of the tristimulus values xyz. the x, y and z values are obtained with 3 colorimetric observers x(), y() and z() (see (1) and figure 1) that approximate the spectral sensitivity of human photosensors [3]. (1) this representation of colors into a 3d space is justified by the fact that 2 colors represented with the same xyz tristimuli are considered as equal by any human. it is then easy to deduce that this color measurement is based on a nominal scale, i.e. on a scale that links an equivalence relation on empirical quantities with an equality on measured values. all colorimetric spaces are derived from the xyz tristimuli by the use of one-to-one transformations. as one-to-one transformations are admissible transformations for the nominal scales, this confirms that the color measurement scales are equivalent when they are considered as nominal scales. for a given color classification, color histogram methods do not depend on the colorimetric space and they use the classification as a nominal scale. most other methods need a metric on the colorimetric space or at least a similarity relation between colors [4]. in the first case, a metrical scale, i.e. a scale that preserves a metric, is needed. with its ability to preserve a similarity relation, a fuzzy nominal scale is a good candidate for the second case [5][6]. some methods use colorimetric spaces as affine spaces [7]. these last approaches are questionable when the different colorimetric spaces must be considered as equivalent representation spaces of the same quantity. from a formal point of view, a method must not depend on the choice of a representation space that actually depends on the choice of a scale. in this paper, we promote the hypothesis that the number of different colorimetric spaces proposed shows that the representation space of a scale for color measurement may have a metric but not necessary be an affine space. the preservation of distances is then the generic property of such scales known as metrical scales. the consequence is that any signal processing method needs to be defined on the basis of a distance. 3. color representation by lexical fuzzy subsets  fuzzy nominal scales where introduced in order to formalize an application to the measurement process of the description of a quantity by a fuzzy subset of symbols [8]. with these scales, the measured values are expressed in the representation space with fuzzy subsets of symbols, also called lexical fuzzy subsets (lfs). the measurement is split into a measurement from the set of manifestations to a numerical space x and into a mapping from x to a linguistic space. a mapping d called fuzzy description or simply description translates a numerical scalar or a vector into a lexical fuzzy subset. in the following example, a manifestation is represented by a scalar itself described by a lfs defined by its membership function on a lexical set s = {a,b,c,d}. building the fuzzy representation. 3.1 the following notation is used for the representation of fuzzy subsets: a fuzzy subset a on a set s is characterized by its membership function also denoted a: then a(c) is called the grade of membership of c to a. the grade of membership of a symbol a to the fuzzy description d(x) of a value x is then denoted d(x)(a). the grade of membership of a couple (x,y) to a relation r will be denoted by r(x, y), or by (x r y). in this paper, we restrict our study to the fuzzy nominal scales that respect: (2) such scales define a fuzzy equivalence relation between lfss like for example the simplest one: (3) this relation also known as similarity relation is a representation of a relation between manifestations. it respects the reflexivity condition, expressed by (2), and a weak version of the transitivity condition. in our case ~ is tl-transitive: (4) where (5) using fuzzy scales for color measurement is justified by the fact that a similarity relation between colors always exists even if such a relation is not clearly known. figure 1. the color matching functions x(), y() and z() as given by the  cie.  figure 2. example of a fuzzy description mapping a scalar into a lexical fuzzy  subset (lfs).  acta imeko | www.imeko.org  september 2014 | volume 3 | number 3 | 59  a fuzzy representation mechanism is defined by a fuzzy symbolism <x, s, r> where x is a numerical space, s is a lexical set and r is a fuzzy mapping from x to s. the fuzzy description d is then (6) on the other hand, the fuzzy meaning of a symbol s is defined by: (7) applying eq. (2) imposes the set family m(s) to be a fuzzy partition of the set x. (8) finally, given a numerical space and a lexical set, the fuzzy representation mechanism is simply defined by a fuzzy partition on the numerical space. definition of the numerical space 3.2 from an anthropocentric viewpoint, a color is fully defined by 3 coordinates in one of the standard colorimetric spaces. actually, several colorimetric spaces hold a coordinate for luminance, and two separate coordinates for the chromacity. in this paper we restrict the study to the measurement of the chromaticity. a color is then represented by an element of a chromatic plane. in this case, white, black and intermediate grey colors are represented by the same value. as for many applications, the ab chromatic plane defined as a projection of the lab space, is chosen for its closeness with human perception. definition of the lexical set  3.3 the numerical set can be used as lexical set where each item is a couple (a,b). in this trivial situation the fuzzy relation r is reduced to an isomorphism and the scale is no more fuzzy but is a classical two-dimensional ratio scale. this case gives the larger lexical set that can be used to represent the chromacity. smaller lexical sets can be defined by sub-sampling. for example the set of couples s={-10,...,10}x{-10,...,10}. the semantic of such lexical set stays close to the preceding one, and the syntax, i.e. the set of available relations, is derived from the syntax of the ratio scale. the smaller lexical sets are well known and hold three symbols: s = {green, red, blue} and s = {cyan, magenta, yellow}. in this case, using fuzzy subsets of symbols to represent colors fits in with the additive mixing or with the subtractive mixing of primary colors. a usual lexical set for the representation of colors is the set of the 8 colors of the rgb cube and of the affine transformations of the rgb cube: s = {green, yellow, red, purple, blue, cyan, black, white}. as we work only on the chomatic plane, we choose the symbol neutral for the linguistic representation of the values associated to white, black and the intermediate gray colors. the new lexical set is then defined as s = {green, yellow, red, purple, blue, cyan, neutral}. we propose also to use different symbols to represent real colors and colors that define the boundaries of the chromatic plane. finally we add the color orange to the set in order to obtain a lexical set more representative of the human feeling. a possible lexical set is then s = {full_green, full_orange, full_yellow, full_red, full_purple, full_blue, full_cyan, neutral, green, yellow, orange, red, purple, blue, cyan}. (9) definition of the fuzzy meaning 3.4 except for the trivial case of an infinite lexical set, the set of fuzzy meaning of symbols defines a fuzzy partition of the numerical space. two other simple cases are the definition of the fuzzy meaning of the lexical sets s = {green, red, blue} and s = {cyan, magenta, yellow}. assuming that each color x into the mapping of the rgb cube on the lab plane has obviously an rgb coordinate (xr,xg,xb), the fuzzy partition can be simply defined by the normalized rgb coordinates: (10) the same approach can be used to define the fuzzy meaning of the lexical set s = {cyan, magenta, yellow} with the cmy (cyan, magenta yellow) coordinates. these two fuzzy representations of colors are equivalent to a chromatic plane defined on the rgb space and to a chromatic plane defined on the cmy space, respectively. in this case, the fuzzy representation is useless. between these two extreme situations, the fuzzy representation of colors is useful when the lexical set has more than 3 symbols. in our approach, we suggest to use the preceding lexical set made of 15 symbols as defined in eq. (9). in order to define the meaning of each symbol, we first associate each symbol with a chromatic coordinate that characterizes this symbol. this coordinate is called the modal coordinate of the symbol. the fuzzy meaning is defined by a piecewise linear interpolation based on a triangulation of the chromatic plane. first a set of symbols and their modal coordinates are defined. then the plane is split into triangles such that vertices are modal coordinates. the meaning of a symbol is then defined as a fuzzy subset which membership function is equal to 1 for the modal coordinate of the symbol and equal to 0 for the modal coordinates of the other symbols. the membership of any figure  3.  projection  of  the  lighter  colors  of  the  rgb  cube  into  the  lab  chromatic space.    acta imeko | www.imeko.org  september 2014 | volume 3 | number 3 | 60  coordinate to a fuzzy meaning is then interpolated on the triangles. figure 3 shows a triangulation used to define the meaning of the lexical set on the ab chromatic plane. 4. fuzzy scales  within the formalism of the representative theory of measurement, the scale <x,s,r,~,=,(~,=)>, where r is a fuzzy relation and ~ is a similarity relation, is a fuzzy nominal scale. this scale preserves a similarity relation on x during the measurement process. actually, r is a morphism that links the empirical relational system <x,~> and the representational relational system <fs(s),~>, where fs(s) is the set of fuzzy subsets of s and ~ are relations that coincide with the equality = on the singletons of s. this means that the grade of membership of the couple ({a} ~ {b}) is equal to 1 when a = b: the similarity relation ~ on x, is associated with the similarity relation ~ on fs(s) (eq. 3). at this step, the similarity relation preserved by the measurement process allows to compare the colors in a small region of the chromatic space. in order to compare colors over a wider range, we need a stronger scale. a two-dimensional affine scale will be the solution applied in the case of a numeric scale, i.e. in the case of an infinite lexical set from the view point of this paper. we consider that the chromatic plane cannot be considered as an affine space. this hypothesis is first based on the multiplicity of color spaces that cannot be transformed from one each other by an affine transformation. in this case, the affine scale cannot be used to measure colors. another argument to confirm this hypothesis is that color perception highly depends on a context. as for any psychophysical human perception, such context is given by the human itself and leads to his history (see [9]) and subjectivity. the goal of the perception is also part of the context. another hypothesis is to consider that chromatic planes are metrizable spaces, i.e. spaces that can hold a metric. then it might be possible to build a scale that preserves a metric: a metrical scale. building a metrical scale from a fuzzy scale needs to define a distance d on the lexical set and to define a distance d’ between lexical fuzzy subsets that verifies:  the singleton coincidence: d’({a},{b}) = d(a,b),  the continuity property,  the precision property that imposes that the distance between two lfss must be a positive real number,  the consistency property that is usually verified by distances on crisp subsets. the transportation distance verifies all these properties [10]. it is computed as solution for a mass transportation problem [11] where the masses are membership degrees, the sources and the destinations are items of the lexical set and the unit cost from a source to a destination is given by the distance d on s [12]. this distance can be defined relatively to the goal of the measurement process or relatively to the application. if the distance on s is unavailable we propose to use the triangulation to compute a distance on the basis of the adjacency of the symbols provided by the graph of characteristic coordinates. 5. adaptation of the scale  according to the hypothesis that color perception is context dependent, it is necessary to adapt the scale to a given context. in this part, we propose to start from an initial knowledge materialized by a scale defined as a mean consensus about the meaning of colors. the adaptation, performed iteratively, is based on the identification of clusters of colors in the chromatic plane. at each iteration the clusters are identified relatively to the preceding scale. then clusters are used to update the scale. the crucial point of the scale definition is the location of the modal coordinates. the modal coordinates given as the initial knowledge are defined for general use and are not usable for specific cases. for example, a van gogh painting usually not fits with this generic scale. in this paper, we choose a painting as application example, because the used colors highly depend on the subjective perception of the artist. the proposal of this paper is to perform a fuzzy c-means clustering (fcm) to adapt the generic knowledge given by the initial modal coordinates. the idea is to fit each characteristic figure  4.  triangulation  that  defines  the  fuzzy  meaning  of  s  =  {full_green,  full_orange,  full_yellow,  full_red,  full_purple,  full_blue,  full_cyan,  neutral,  green,  orange,  yellow,  red,  purple,  blue,  cyan}.  the  letters  a,b,c,..  replace  full_green, full_orange, full_yellow, …    figure 5. the fuzzy meaning of the symbol orange (letter k) is a fuzzy subset  with a membership equal to 1 for the modal coordinate of orange and equal  to 0 for other modal coordinates.  acta imeko | www.imeko.org  september 2014 | volume 3 | number 3 | 61  point with the center of its closest cluster. the original fcm algorithm is based on the minimization of an objective function based on the computation of the euclidean distance between samples and cluster centers and cannot be directly used. indeed, as seen before, nothing can justify that the colorimetric space, or the space of lfss, holds an euclidean metric. in the original fcm algorithm a set of clusters is first defined. each cluster is randomly defined by a fuzzy subset of samples. at each iteration, the fcm algorithm computes the center of each cluster. then the membership degree of each sample to each cluster is re-evaluated relatively to its proximity to the associated cluster center. in our approach, we propose several adaptations to this process. let m be a set of samples in x. in the initial state, each cluster cs is identified by a symbol s and is defined by a fuzzy subset of x derived from the fuzzy description. (11) where (12) as the fuzzy equivalence relation that characterizes the scale defines a distance for short range lfss, eq. (11) can be simplified to: (13) the main difference with the standard fcm is the inclusion of a basic knowledge at the initial step of the algorithm. this knowledge can be considered as an average knowledge about the representation of colors. at each iteration, the cluster center is simply computed as the gravity center of the cluster. (14) the scale is then transformed such that the center of the cluster cs becomes the modal coordinate of the symbol s. as for the original algorithm the iterations stop when the changes reach a termination criterion. the  parameter, must be defined into [0,1]. it represents the inertia of the learning process. if  = 1, each iterated cluster includes only its modal point as unique sample, and the modal points never moves during the algorithm. if  = 0, then each iterated cluster can include new samples far from the modal point. the next figure shows the triangulation after the adaptation of the scale with this method. as the colors full_green, full_orange, full_yellow, full_red, full_purple, full_blue, full_cyan, neutral are synthetic colors defined by a norm, they are not supposed to be changed during the learning process. so the parameter  is set to 0 for these colors. 6. discussion  the information of color has the property on one side to be typically psychophysical information, on the other side to be acquired with accurate measuring instruments and then to be accurately represented in a numerical space. between the physical world, from where the color entities are issued, and the abstract human mental world, where they are defined and represented, is the sensitive world that is a partial perception of the physical world. color entities, like the orange color for example, cannot be considered as concrete physical entities of figure 6. van gogh painting as a context for color measurement.  figure  7.  color  histogram  of  the  painting  in  comparison  with  the  modal  coordinates of each symbol before the adaptation of the scale.  acta imeko | www.imeko.org  september 2014 | volume 3 | number 3 | 62  the concrete physical world. they are issued from a concrete entity, an electromagnetic spectrum, and appear into the sensitive world. the difficulty of color measurement is that the human sensitive world, i.e. the human perception of the concrete world, differs from the instrumental sensitive world, i.e. the instrumented perception of the world. the consequence is that the abstract worlds used to represent these sensitive worlds also differ. in particular, the structure of the colorimetric spaces of the instrumental abstract world is richer than the structure of the colorimetric spaces of the mental abstract world. indeed, each colorimetric space of the instrumental abstract world holds a metric. usually, but not necessarily, an euclidean metric. so it is legitimate to consider that the colorimetric space in the instrumental sensitive world also holds this metric. a colorimetric space of the abstract mental world is a metrizable space, but the associated metric is not defined. this fits with the general knowledge that a distance between color exists but cannot be precisely defined. within this context, the space of lexical fuzzy subsets gives an alternative to the usual numerical colorimetric spaces. indeed, this space is a metrizable space, and the metric depends on the goal of the color process and not on the sensitive world. furthermore, the distance is based on a fuzzy scale that can be adapted through a learning algorithm. 7. conclusion  the color measurement does not lead to a unique theory and needs a scale for each application, or more precisely for each context. we proposed in this paper to use scales that preserve a similarity relation or a metric. the fuzzy scales, with their ability to express the measurement values on a non affine space, give a good solution for color measurement. the counter part is the necessity to adapt the scale according to a context or a color process. this paper gives an algorithm to perform such adaptation. this adaptation can be compared with a calibration process where the calibration standards are color entities. finally the colorimetric space associated to a fuzzy scale has a structure closer to the human representation than classical colorimetric spaces. the goal of a measurement process is to obtain a consensual value to represent a unique quantity. this goal might be felt as contradictory with the approach presented in this paper. but actually it is not. indeed the linguistic representation of a psychological quantity is influenced by subjectivity and the usual scales are not adapted to reach a consensual value. using scales that can be adapted to a context or a human perception is a way to compensate the subjectivity and then to reach the initial goal of any measurement process. references  [1] l. finkelstein, representation by symbol systems as an extension of the concept of measurement, kybernetes, vol. 4, (1975) 215223. [2] jcgm/wg2, vim, international vocabulary of metrology — basic and general concepts and associated terms, ed. bipm, 2008. [3] h. pauli, proposed extension of the cie recommendation on “uniform color spaces, color difference equations, and metric color terms”, j. opt. soc. am. 66, (1976) 866-867. [4] m. stricker, m. orengo, similarity of color images, proc. spie 2420, storage and retrieval for image and video databases iii, san jose /ca, usa, 1995, 381-392. [5] e. benoit, l. foulloy, “towards fuzzy nominal scales”, measurement, issn 0263-2241, vol. 34, no. 1 (fundamental of measurement), (2003) 49-55. [6] e. benoit, l. foulloy, g. mauris, “fuzzy approaches for measurement”, handbook of measuring systems design, vol. 1, eds. peter sydenham and richard thorn, john wiley & sons, 2005, isbn 978-0470021439, 60-67. [7] t.a. ell, hypercomplex color affine filters, icip 2007, ieee international conference on image processing, 2007, 249-252. [8] zadeh l.a., quantitative fuzzy semantics, information sciences, vol. 3, (1971) 159-176. [9] e. ozgen, "language, learning and color perception", current directions in psychological science, vol. 13, no. 3 ,(2004) 95-98. [10] t. allevard, e. benoit, l. foulloy, “dynamic gesture recognition using signal processing based on fuzzy nominal scales”, measurement, no. 38, (2005) 303-312. [11] t. allevard, e. benoit, l. foulloy, “the transportation distance for fuzzy descriptions of measurement”, metrology and measurement systems, vol. xiv, no. 1/2007, (2007) 25-35. [12] s.t. rachev, l. rüschendorf, mass transportation problems, springer-verlag, new-york, 1998, isbn 978-0387983509. figure  8.  color  histogram  of  the  painting  in  comparison  with  the  modal  coordinates after the adaptation of the scale with the fcm like clustering (  = 1 for colors a to h,  = 0.5 for colors i to o).  study on alignment test based on astm e1012 and chinese guideline acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 159 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 159 162 study on alignment test based on astm e1012 and chinese guideline xiaobing zhao1, wansheng li2, minghui fei3, xiaolan zheng4, william huang5 1 wuxi institute of metrology and testing, wuxi, china, zxb@wxmtc.com 2 shandong institute of metrology, jinan, china, liwansheng@sdim.cn 3 wuxi institute of metrology and testing, wuxi, china, fmh@wxmtc.com 4 yunnan institute of measuring and testing technology, kunming, china, zhengxlan06@163.com 5 gtm china office, shanghai, china, william.huang@gtmchina.cn abstract this paper describes the study about alignment test on testing machine. it is based on the standards of astm e1012-14 (standard practice for verification of testing frame and specimen alignment under tensile and compressive axial force application) and chinese calibration guideline. the strain-gauge specimens of astm and reference extensometer of chinese guideline were both tested on one testing machine. the test results were analyzed and compared. the traceability about both methods were also discussed. meanwhile the amplifier was calibrated before test. it is the first time in china to performance this comparison test. keywords: alignment test, astm e1012, chinese alignment guideline 1. introduction the alignment on testing machine is very important to obtain the right test results of material. the standard of astm e1012-14 [1] and chinese guideline [2] [3] both descript how to test the alignment. but the methods are different, and the philosophy of traceability are also not same. to prove both methods reliable on testing the alignment at one testing machine, it is necessary to do a comparison test to see if there will be big tolerance by these two standards. therefore, two laboratories in china, both own both equipment of astm e1012 and chinese alignment guideline, decide to do the comparison test in 2019. iso 23788:2012 (metallic materials – verification of the alignment of fatigue testing machines) [4] also describes about alignment test, but it is very similar as astm e1012. the most difference between iso 23788 and astm e1012 is: there are total 12 pieces of strain-gauges on three levers at the transducer according to astm, but there are only 8 strain-gauges on two levers according to iso. the details and difference between astm e1012 and chinese guideline are: the specimen, the procedure, the calculation, the classification (deviation) and the traceability. they are described in section 2 to section 5. 2. the equipments according to astm e1012, there are four strain-gauges at same lever but different position on alignment specimen (so-call alignment transducer). total 12 or 8 pieces of strain-gauges on 3 or 2 levers at the transducer. as figure 1: figure 1: strain-gauge transducer the equipment to be performance alignment test consist: an alignment specimen with 12 pieces of strain-gauges, amplifier of 12 channels, the software. they are called alignment tool kit by the product from zwickroell, a germany testing machine manufacturer. as figure 2. the alignment specimen details (sharp, dimension) as figure 3. http://www.imeko.org/ mailto:zxb@wxmtc.com mailto:liwansheng@sdim.cn mailto:fmh@wxmtc.com mailto:zhengxlan06@163.com mailto:william.huang@gtmchina.cn acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 160 figure 2: alignment tool kit from zwickroell figure 3: alignment specimen the amplifier is type quantumx mx1615b. excitation voltage is 10v and full-bright are set in the software. 12 channels of this amplifier are used to be connected to the alignment specimen. according to chinese guideline jjg 475-2008: verification of electric testing machine, and jjg 556-2011：verifications of uniaxial fatigue testing machine [2] [3], the alignment kit is twins’ extensometers and specimens, as figure 4. figure 4: twins’ extensometers on specimen the twins’ extensometers mean there are two extensometers: one is attached on the left side of specimen and another is on the right side. the extensometer is used for measuring the elongation of the specimen during tensile test. theoretically the elongation of both sides of specimen should be same if the alignment of the testing machine is perfect. the specimen’s specification is descripted as in table 1 and figure 5. fmax of testing machine in kn diam.d in mm gauge length lo in mm l in mm material [100---600) 10 100 130 steel no.45 (30---100) 10 100 130 alloy steel table 1: the dimension of the specimen figure 5: the drawing of the specimen 3. procedure we use a zwickroell testing machine z050 to do test. the capacity of the machine force is 50kn. the classification of this testing machine is class 0.5. 3.1 according to astm e1012-14: 1. clamp the alignment transducer at the top grip, zero the signal, clamp the lower grip. apply a small force to see all signal ok and then remove the force. 2. increase the force and apply it to 4 %, 6 % and 8 % of the capacity of the testing machine, that means 2 kn, 3 kn, and 4 kn. record the measurement value. and calculate all the data automatic by software. (the original is to apply the force to 10%, 20% and 40% of the machine capacity, here we apply 4%, 6% and 8% of the machine capacity) 3. remove the force to zero. change the position of the alignment transducer from position 0° to 180° and repeat the step 2. 4. remove the force to zero. change the position from 180° back to 0°, and repeat step 2. 3.2 according to chinese guideline: 1. clamp the specimen on both grips of testing machine 2. apply the force of 0.5 % of the machine and attach the twin’s extensometers at the position of 1 and 3 on the specimen as figure 6. apply the force to 1 % of the machine and zero the signal of both extensometers. apply to the force to 10 % of the machine. that means the pre-apply. do it three times. 3. remove the force to zero. apply the force to 1 % of the machine and zero the signal of both extensometers. increase the force to 4 %, 6 % and 8 % of the machine. record all the data. (the original is to increase the force to 4% , 6%, 8% and 10% of the machine capacity, here we apply 4%, 6% and 8% of the machine capacity) http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 161 4. change the extensometers to positions 2 and position 4. same procedure as step 2 and 3. figure 6: the positions for twins’ extensometers attached 4. data analysisi according to astm e1012, the cylindrical strain-gauged alignment specimen use following equations:(as figure 7） figure 7: the equations of pb pb is the percent bending, which is the value of the alignment. from the test 3.1, all data were collected. here we show the data from the middle lever of the straingauge, as in table 2. in astm e1012, the original “record data points at 10%, 20% and 40% of the force transducer range or testing machine capacity”, but here we only apply the force to 4%, 6% and 8%, and the best value of pb is 10.7 at 4kn (8% of the machine capacity of 50k), because we try to make the same force apply as chinese calibration guideline. force position pb (%) pb （ave） 2 kn middle 0° 25.3 21.5 180° 16.89 0° 22.3 3 kn middle 0° 17.42 14.3 180° 11.62 0° 13.94 4 kn middle 0° 14.08 10.7 180° 8.98 0° 9.12 table 2: the data of the middle lever of the straingauge according to the chinese guideline, the value of e is the alignment specification of the testing machine. it uses following equations: (as figure 8） figure 8: the equations of e from the test 3.2, we collect the data of e as in table 3. force e (%) 2 kn 10.3 3 kn 7.3 4 kn 5.4 table 3: the data by test 3.2 test results of test 3.1 and test 3.2 are compared as in table 4. force pb (%) e (%) 4 % of machine 2 kn 21.5 10.3 6 % of machine 3 kn 14.3 7.3 8 % of machine 4 kn 10.7 5.4 table 4: the results compared http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 162 the pb results are almost exactly double the e results. but it should be no coincidence because the classification is different as discussion later. 5. discussion the classification of alignment by astm e1012: and chinese calibration guideline is different, as in table 5. astm e1012 classification pb (%) classification of testing machine e (%) 5 5 0.5 12 8 8 1 15 10 10 2 20 table 5: the different classification. the standard of astm only talks about the alignment classification, instead of the machine. the chinese guideline takes care the machine must fulfil the requirement of the alignment. the data from test 3.1 does not apply enough force on the strain-gauge specimen according to astm e1012. therefore, it cannot give decision that pb of 10.7 does not fulfil class 10. the data from 3.2 test shows that the machine full fill class 0.5, because the alignment of e is 10.3%, which is within the tolerance of 12%. additional topic need taken care is the traceability. in astm e1012, there mentioned: “8.1.1 calibration of strain-gaged alignment transducers is not required by this standard. traceable national standards do not generally exist for such calibrations. however, great care should be taken in the manufacture of strain-gage alignment transducers used for the determination of alignment” that means only the amplifier can be calibrated. we did calibration as in figure 9 for this. the standard signal is created by a device. the deviation between standard signal and amplifier (mv/v) is less than -0.3%. its fine. the details data as shown in table 6. standard amplifier mv/v amplifier μm/m deviation % 0 0.0003 0.4 0.25 0.2499 254.6 -0.04 0.5 0.4997 509 -0.06 0.75 0.7485 763 -0.20 1 0.9977 1017 -0.23 table 6: calibration data of amplifier. figure 9: the standard signal (above), and the amplifier to be calibrated (below) concerning the calibration on twins’ extensometers of chinese, its easy: calibrate the two extensometers one by one. each is done by a master extensometer. they can be traceability to the primary standard of length. that mean all the chinese alignment kit can be calibrated. 6. summary the main difference between the standard of astm e1012-14 and chinese calibration guideline about alignment are classification, tool kit hardware, and the force apply. astm is more accuracy on alignment specificity of testing machine. but there is no solution for the whole traceability. the method of chinese guideline is easier to handle and can be traceability. both methods are reliable to determine the alignment of the material testing machine. 7. references [1] astm e1012-14. standard practice for verification of testing frame and specimen alignment under tensile and compressive axial force application. [2] jjg 475-2008: verification of electric testing machine, [3] jjg 556-2011. verifications of uniaxial fatigue testing machine. [4] iso 23788:2012 (metallic materials – verification of the alignment of fatigue testing machines) http://www.imeko.org/ the impact of cleaning of weights before calibration acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 7 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 7 11 the impact of cleaning weights before calibration christian müller-schöll1 1 mettler-toledo international inc., greifensee, switzerland, christian.mueller-schoell@mt.com abstract: this paper investigates the necessity of cleaning weights before calibration. the conflict between cleaning a weight and breaking the history by cleaning is analyzed. experience from many hundreds of calibrations with measurements before and after cleaning is reported and analyzed. it is concluded, that weights should be cleaned in order to reach a stable state but should also be calibrated before and after cleaning to gain information about metrological traceability and thus the validity of the results that were produced with the weights. keywords: calibration; weights; cleaning; contamination; oiml r111-1; traceability 1. introduction cleaning of physical material measures for mass has been a topic since the making of the first kilogram standards in the 18th century. this paper details on the necessity and the effects of cleaning of industrial weights which are used for testing or calibration of weighing instruments. for this, a pool of some hundred calibrations of weights before and after cleaning are investigated. section 2 sets the background for the historically documented need for cleaning weights. section 3 treats the question if cleaning requires calibrations before and after, while section 4 raises the question of safety that is gained by cleaning. section 5 gives a practical example of a laboratory’s policy regarding cleaning of weights. in section 6 we detail the results of one specific weight, while in section 7 we shed light from different angles on the question of significance. in section 8 we present results of some hundred weights’ calibrations and in section 9 we investigate the effect of cleaning on metrological traceability. section 10 gives an overview of the conclusions. 2. background for cleaning weights from the beginning of modern mass metrology in the 18th century, metrologists have considered cleaning of mass artefacts: even the ipk, which served as the definition of the kilogram until 2019, assumed its exact mass only after a cleaning and washing procedure (french: “nettoyage-lavage”) which apparently removed as much as 15 µg from the artefacts at the bipm [1]. obviously, the artefacts reach a stable mass value only in a “clean” state whereas the “unclean” state is instable, unreliable and dependent on external influences like time since last cleaning, intensity, frequency, and kind of usage etc. still today, in oiml r111-1, the step “cleaning weights” is listed as a mandatory element in the procedure for the assessment of conformity and oiml states “it is important to clean weights before any measurements are made…” [2]. however, no specific method is prescribed. the calibration or verification laboratory may choose appropriate methods according to their expertise, competence and capabilities. although the definition of the kilogram has been freed from being dependent on an artefact in 2018, weight pieces are still an indispensable device for establishing a mass scale, for calibrating weights and for calibrating and testing weighing instruments. it is worth noting that immediately after cleaning the surfaces of weights are likely to be in an unstable state. therefore, it is necessary to allow for a certain stabilisation period (b.4.2 in [2]) whose length is dependent on the cleaning method and the accuracy class of the weights. the reason for cleaning is obviously the fact that weight pieces are frequently contaminated and that the “clean state” is the only reliable and reproducible state, while the “dirty state” is rather volatile. in fact, the mass of the dirty state can be reduced as unintentionally (by randomly wiping away dirt) as it can be raised (by touching or contaminating the weight). with a reliable and reproducible and non-abrasive cleaning method, it should be possible to bring a piece of stainless steel always to the same mass (within measurable reproducibility). 3. is cleaning an intervention? iso 17025 requires that calibration results shall be collected, “before and after any adjustment or repair, if available” [3]. the intention of this http://www.imeko.org/ mailto:christian.mueller-schoell@mt.com acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 8 requirement is obviously that an uninterrupted history of the device under calibration is available and the user of the object can use the information of the “as found” state in order to assess the appropriateness of handling, of assumed accuracy and stability and finally of calibration intervals. unfortunately, [3] limits the applicable cases in an exhaustive list to “adjustment” and “repair”. it is clear that a cleaning intervention on a weight might have the same effect as an adjustment intervention and thus should be considered in the same way. the strict application of the ideas of [3] together with the procedure given in [2] must lead to the conclusion that all weights have to be cleaned before calibration and all calibrations have to consist of a set of two full calibrations, one before and one after the cleaning. the according calibration certificate is then a so-called “as-found– as-left-certificate”. not collecting data of the state before cleaning would – strictly speaking – result in a “break in the history” of the measuring instrument. 4. safety considerations the fact that measurement instruments can carry over any kind of contamination from the environment they are used in, into the calibration laboratory, is frequently under-estimated, according to the view of the author. this applies for both hazardous contaminations as well as for “just dust and dirt” in the case of weight pieces. both kinds of contaminations mentioned above endanger the integrity of the calibration laboratory equipment, and, even more important, endanger the integrity of un-involved third parties’ equipment, when contamination is carried from an instrument onto the laboratory equipment and from there to third party customers’ instruments. 5. the policy of the swiss calibration lab of mettler toledo the laboratory of mettler-toledo gmbh in switzerland calibrates many thousands of weights per year. customers expect calibration values of a stable state of their weights. unfortunately, most customers are not willing to pay for the effort of a double calibration (before and after cleaning). as a compromise between the pure metrology of not changing instruments without additional calibration and the minimisation of the risk of contamination, and the request of low prices and stable and consistent calibration results, the policy generally applied for weight calibrations is as follows (and this policy is made known to the customer): • weights will be cleaned generally to protect the laboratory and un-involved parties’ artefacts. • e1 class weights will not be cleaned. • if a customer wants a calibration of the uncleaned state, the risk for the laboratory is estimated. in severe cases, the bottom surface of the weight is cleaned in order to protect the comparator balances from dirt. the customer may choose if the weight shall be cleaned and calibrated again. 6. effect of cleaning weights in contrast to most customers of the calibration laboratory, there is one big company which, as a general policy, requests the cleaning of weights together with a double calibration, before and after the cleaning. this company is in the pharmaceutical industry and is therefore strictly regulated. the management is used to issuing internal procedures and their staff is used to strictly following such procedures. we have monitored some of these calibrations over a period of approximately ten years. although the sop strictly advises the personnel not to touch the weights and use clean tools to handle the weights, these weights show clear signs of contamination. figure 1: typical calibration history (50 g, e2) blue (upper) line shows deviation from nominal before cleaning; red (lower) line are values after cleaning. uncertainty bars 95 %. figure 1 shows as an example a typical history of such a weight: the weight was first produced and calibrated in 2013 in a “clean” state. it was then recalibrated every two years with a calibration before (blue) and after (red) cleaning. in 2015 and 2017, the effect of cleaning is clearly visible: the values before cleaning are significantly higher. after cleaning, the values always come to the same stable level (within uncertainties). for the calibration of 2019, no contamination was detected: either the handling of the weight has changed significantly or the weight has not been used (this information was not available). the cleaning was done in a fully automated procedure with a heated bath with ultrasound cleaning using a regular, neutral industrial cleaner for stainless steel while constantly moving the weights. after cleaning, the weights are rinsed http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 9 multiple times in a cascade style process in filtered and de-ionised water and let stabilise and acclimatise according to [2]. the difference before and after cleaning of the same year is about -25 µg1 for the 50 g weight, but varies between recalibrations. this means that the cleaning procedure has removed contamination of as much as 25 µg mass from the weight of 50 g nominal mass. there are two proofs that the cleaning did not remove any metal material from the weight (as one could suspect) but did effectively remove only dirt: first, all the “clean” values are consistent within their uncertainties. second, we use the same type and material of weights in our calibration laboratory on robotic comparators as reference standard weights. for these weights, we do not observe any significant change in mass, neither over time nor before and after cleaning. we conclude, the weight pieces (would they be left un-touched) are stable with time and our cleaning procedure does not affect the pure metal mass of the weight pieces in any detectable way. 7. is the contamination significant? depending on the point of view, there are multiple answers to the question if such a contamination is significant. 7.1. significance considering calibrated values referring to figure 1, it is obvious that, since the uncertainty bars (95 %) of approximately 12 µg do not overlap, the difference in the values of 2015 and 2017 are significant. en-values amount up to -1.6 which, from the metrological view of the calibration laboratory clearly indicates that the values before and after cleaning are inconsistent and the amount of dirt removed therefore is significant. 7.2. significance if used “per accuracy class” one could also consider how the weights are used in the customer’s process: these weights are used for routine testing of balances. the requirements for the weights coming from the customer’s procedure is the adherence to class e2 according to [2]. if the en-value is calculated using the mpe value of 100 µg instead of calibration uncertainty, en amounts to no larger than -0.2. thus, it could be concluded that for the process of the application, the mass of contamination is insignificant since the weight value is allowed to vary within the class limits during its service life without affecting the balance test result. 1 for the mass of removed contaminants as well as for correspondingly calculated en-values, we maintain negative values in this publication. 7.3. significance for quality control interests the results are a clear sign that the instructions in the sop (“handle weights with clean tweezers or with clean gloves”) are not effective or not effectively executed. thus it can be concluded, that the mass of the contamination is significant. 7.4. significance if trends are considered should the balance testing results be analysed not only for passing or failing a limit test, but also for trends over time, then the contamination of the test weight might superpose the actual behaviour of the balance. should trends in the order of size of the contamination be relevant, then the contamination might be significant. (the one example given in section 6 might be considered irrelevant if the weights are used e.g. on a balance with a readability of 10 µg and the amount of contamination is only a small multiple of the readability.) the considerations above underpin the general rule that only the user of the measurement device can make the final decision on the relevance of the data for his specific application. 8. general view 8.1. en-values of a pool of calibrations in our laboratory, we have observed some hundred calibrations of a pharmaceutical company with values before and after cleaning. the general picture is as follows: figure 2: en-values vs. nominal mass as can be seen from figure 2, we found many envalues in the region between -1 and -2. as explained above, this is metrologically significant, but probably insignificant for the actual application. nevertheless, there is a wide variation and there were some results where the cleaning resulted in envalues greater than -3 and one even larger than -10 (this was a 5 g weight). it is unknown unfortunately, how the customer reacted to these results. there are also some positive en-values (the weight values appeared heavier after cleaning). http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 10 these are owed to the reproducibility of the procedures of the calibration laboratory when obviously no significant contamination was present. 8.2. absolute mass of contaminants another view on the pool of values is the absolute amount of contamination. figure 3 shows the washed off contaminants’ mass vs. the nominal mass of the pieces. figure 3: absolute mass of contaminants vs. nominal mass it is obvious that with increasing mass (or probably rather with increasing surface), the mass of contaminants rises: a 1 mg piece shows hardly any contamination, while large contaminations occur for the largest weights. furthermore, the more difficult to handle, the more contaminations occur. this holds especially true for weights of 2000 g and above. according to the impression of the author, this is owed to improper manual handling of the weights with improper (or even without) tools or inappropriate gloves. it is certainly not a metallurgical or corrosive surface effect since other weights of the same type under our own control behave absolutely stably (see section 6). 9. possible loss of traceability during a calibration, metrological traceability to the reference is established (definition 2.41 in [4]). during the service life of the calibration data (i.e. the calibration interval), traceability has to be maintained. this can be done by a combination of and not limited to: (1) intermediate checks using a check standard, (2) adding a component of uncertainty that accounts for assumed changes (e.g. 𝑢𝑖𝑛𝑠𝑡 in [2]) or (3) a predictive modelling of the behaviour of the instrument. at the end of the calibration interval, a calibration must be performed in order to confirm that traceability has persisted throughout the interval. a possible means of doing this is an entest where the value and uncertainty assumed during service life are compared to value and uncertainty of the confirming end-of-interval calibration. if the confirmation of traceability fails, an according investigation is mandatory (e.g. “nonconforming work” in [3]). possible consequences could be reconsidering the uncertainty assumptions or reconsidering the length of the calibration interval. it is obvious that for the example of weight pieces, the confirming calibration at the end of the interval must be done in a not-cleaned state, exactly in the state the weights were used until their last day in service. cleaning of the weight piece and the following calibration are then used to again establish traceability for the interval to come. 10. conclusions from these findings, we draw the following conclusions: • the topic of cleaning of weights has been introduced historically, but is still legitimate today. • from an analysis of many weights from a highly regulated environment and used by trained personnel, we showed that contaminations occur frequently, even when there are clear instructions (sops) which should prevent contaminations. • contaminations (in absolute units) tend to be large for weights of larger size. • only the user of the weights – with deep metrological knowledge of the process in which the weights are being used – can answer the question if the amount of contamination is significant or relevant. • oiml r111-1 neglects the topic of calibrating before and after cleaning. • iso 17025 is unclear (or incomplete) in listing cases in which a “before and after” calibration is mandatory and thus leaves the responsibility of the decision to the customer. • metrological traceability is endangered by improper handling. measures must be taken to maintain traceability during the calibration interval. • calibrations before and after cleaning (“as found” and “as left”) are inevitable to gain all information necessary for appropriate management of weights as a metrologically traceable measuring device and to confirm traceability of the past interval. 11. summary in this paper, we have presented the importance of cleaning weight pieces. calibrations should be performed before and after cleaning in order to obtain information about the behaviour of the devices, about the effects the usage has, about the appropriateness of the uncertainty and of the calibration interval. the information from both calibrations is indispensable for establishing, http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 11 maintaining and confirming metrological traceability. 12. references [1] m. stock et al., “calibration campaign against the international prototype of the kilogram in anticipation of the redefinition of the kilogram part i: comparison of the international prototype with its official copies”, metrologia, vol. 52, pp. 310-316, 2015. [2] oiml r111-1, organisation internationale de métrologie légale, paris, france, 2004. [3] iso/iec 17025, international organization for standardization, geneva, switzerland, 2017. [4] jcgm 200, international vocabulary of metrology — basic and general concepts and associated terms (vim), bipm et al., 2012. http://www.imeko.org/ gum’s rockwell hardness standard machines after modernization acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 240 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 240 246 gum’s rockwell hardness standard machines after modernisation j. d. fidelus1, m. kożuchowski2 1 gum, central office of measures, 00139 warsaw, poland, janusz.fidelus@gum.gov.pl 2 mericore, 02654 warsaw, poland, m.kozuchowski@mericore.com abstract: this article describes two twin deadweight-type rockwell hardness standard machines (hsms – hsm-s02 and hsm-nt) of gum after modernisation. a new control system for the station with database, a hydraulic pump, a displacement measuring system and an application enabling the operator to operate the measuring station are described. the adjustment process of the hsm-s02 with entering automatic compensation (correction) enabling the improvement of the measured result in real time is presented. keywords: rockwell hardness standard machine; deadweight-type machines; adjustment; industry 4.0, labview, database 1. introduction the gum hsms were designed and manufactured in the 1970s by ernst leitz gmbh wetzlar (germany). in order for the systems to work in a fully automatic mode and in accordance with industry 4.0 requirements [1], their modernisation was necessary. the proper and precise functioning of the hsms requires their accurate adjustment and calibration. although there is a precise procedure for calibrating the reference standard blocks, there is no such procedure for the hsms. 2. gum’s rockwell hardness standard machines in cooperation between gum (główny urząd miar/central office of measures) as the national metrology institute of poland for standardisation of hardness measurement and mericore, a concept has been developed for the automation and modernisation of two twin hsms scale a, b, c, d, e, f, g, h, k (hsm-s02) and scale n, t (hsm-nt), in accordance with pn-en iso 6508-3 [2, 3]. as part of the modernisation process, a new control system for the station, a hydraulic pump, a displacement measuring system and an application enabling the operator to operate the measuring station were made, see figure 1. figure 1: gum’s rockwell hardness standard machine composed of the following elements: 1. indenter 2. specimen support 3. preliminary load 4. hydraulic cylinder 5. weights 6. body of the hardness tester 7. electric drive of the pump actuator 8. monitor 9. electric control cabinet 10. controller's rack 11. hydraulic cylinder of the pump the station controller was built in accordance with industrial and laboratory standards, combining reliability and precision required in laboratory equipment. the use of the latest solutions made it possible to prepare the station to be networked in accordance with the idea of industry 4.0. the use of an industrial computer, a system of distributed islands for the acquisition of signals from sensors, lan communication with individual executive systems of the system, and a 22-inch, touch screen monitor creates an extremely efficient and convenient to use facility. the hydraulic pump uses an electrically controlled actuator to control the piston position with single micrometre resolution. this makes it possible to control the movement of the overlap of individual loads unheard of in commercial workstations. due to the fact that the modernised http://www.imeko.org/ mailto:janusz.fidelus@gum.gov.pl mailto:m.kozuchowski@mericore.com acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 241 stations date back to the 1970s, it was necessary to replace the original sensing of the mechanical systems. contact ends and switches were replaced by inductive proximity sensors which, thanks to the absence of moving parts and contacts, ensure failure-free operation even in the case of contamination or flooding with oil. depth displacement is measured using the renishaw resolute system, which consists of an optical encoder (head) and a stainless steel belt scale with laser-engraved absolute code (figure 2). the system used has a measurement resolution of 1 nm and an error of 3.5 µm per metre of scale length. when measuring displacements of 200 µm, the scale error is negligible. the head has its own electronics, which controls the optoelectronics of the reading but also monitors whether the setting of the optical axis to the scale is correct. if any deviation is detected, the measuring system sends information to the control application that the reading surfaces must be adjusted or cleaned. in combination with the digital readout transmission, this makes the system extremely resistant to interference and errors. figure 2: gum’s rockwell hardness standard machine equipped with the renishaw resolute system, which consists of an optical encoder (head) and a stainless steel belt scale with laser-engraved absolute code the software for the workplace control has been developed from scratch in the labview environment, which is dedicated to creating applications in laboratory and industrial applications. in particular, it is dedicated to the operation of measuring systems due to the implemented functions of hardware operation and data analysis and archiving capabilities. the control application enables the measurement to be carried out and also includes an equipment diagnostic module. thanks to it the user can easily check the correctness of operation of particular systems. in addition, the application will make it impossible to switch on the station or perform a measurement if it finds that any of the systems are not working properly or return values outside the accepted control limits. each station has its own database in which all data of orders placed as well as the results of conducted tests are stored. for the convenience of users, the database is accessible through a specially created program in labview using a web browser. this makes it possible to enter and modify data from any computer in the same network (figure 3). figure 3: a computer with dedicated software (control and device diagnostics application written in labview) for data analysis and archiving in accordance with the requirements of safety standards, the controller has been equipped with a certified safety controller and an emergency stop button, thanks to which the user can stop the hydraulic pump operation at any time, protecting himself or his equipment. resumption of the workstation is only possible after the appropriate safety system restart procedure. 3. hsm-s02 – force calibration and mechanical study 3.1. force calibration the force calibration of the machines using the force-proving instrument composed of the force transducer (type z3h2 r, hottinger) and the measuring amplifier (type dmp41-t2, hbm). data archiving and analysis was carried out, i.e. using hbm (catman) software. the exact description of the force calibration will be the subject of a separate work. http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 242 3.2. preliminary hardness measurements in order to verify the indications of the rockwell hardness standard machine (hsm-s02), a number of hardness blocks measurements were made for the hrc scale, which are shown in table 1. each sample was measured at five points and then the average value was determined, which was compared with the valued of the measured standard. based on the measurements carried out, a statistical analysis was performed to adjust the characteristics of the hsm-s02 to the hardness reference blocks. thanks to this, the automatic compensation (correction) entered into the system can correct the measurement result in real time. table 1. measurements of rockwell hardness c without compensation hardness blocks measurements (hrc) difference designation hrc 1 2 3 4 5 average uniformity mpa nrw 1214601.1006 21.62 22.95 22.80 22.93 22.86 23.00 22.91 0.20 1.29 mpa nrw 863401.0206 30.20 30.91 31.08 31.10 31.06 30.98 31.03 0.19 0.83 mpa nrw 6975302.0916 44.71 44.91 44.82 44.70 44.64 44.74 44.76 0.27 0.05 as 2271/98 55.90 56.39 56.22 56.29 55.9 56.08 56.18 0.49 0.28 mpa nrw 900809.0306 61.13 60.87 60.83 60.69 60.88 60.66 60.79 0.22 -0.34 mpa nrw 863905.0206 64.94 64.51 64.50 64.50 64.51 64.60 64.52 0.10 -0.42 first, a plot was made where measurement values were placed on the x axis, while the hardness values of the tested blocks were placed on the y axis. figure 3 presents a graph of this relationship. figure 3: the graph presents the relationship between the hardness values measured on the hsm-s02 and the nominal hardness values of the hrc blocks then, a trend line was determined which was finally fitted using a 3rd degree polynomial (r2 = 0.999), given in equation (1). 𝐻𝑅𝐶 = 0.00006 ∙ 𝑋3 − 0.0077 ∙ 𝑋2 + 1.3469 ∙ 𝑋 − 5.8 (1) where the x value is the raw result of the hardness measurement at the hsm-s02. after taking into account equation (1) in the calculations of the hrc result value from the measurements according to table 1, the relation between the deviations of the values calculated from the measurements and the nominal values was obtained (figure 4). as shown in figure 4, the distribution of deviation values is quite random and their values do not exceed ± 0.5 hrc. therefore, it was decided to compensate the remaining deviation values by creating a table of correction values in specific intervals. the results are shown in table 2. table 2: summary of final corrections interval number hrc hardness range correction accepted 1 0-30 -0.15 2 30-35 -0.1 3 35-40 +0.1 4 40-47.5 +0.2 5 47.5-50 +0.1 6 50-53.5 -0.1 7 53.5-58 -0.25 8 58-60 -0.1 9 60-62.5 +0.0 10 62.5-68 -0.3 figure 4: deviations of the measurement hrc values from the nominal values after applying the compensation equation 20 25 30 35 40 45 50 55 60 65 70 20 30 40 50 60 70 n o m in a l va lu e ( h r c ) measured value (hrc) -0,3 -0,2 -0,1 0 0,1 0,2 0,3 0,4 20 40 60 80 h r c v a lu e d if fe re n ce measurement points http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 243 in order to verify the correct operation of the hsm-s02 with the compensation equation and table of corrections, after 3 months from the first measurements summarised in table 1, a series of measurements were made on the same hardness blocks. measurement were made at 23 °c ± 0.5 °c using the same indenter. the results of the verification measurements are summarised in table 3. table 3. hardness measurements of hrc blocks with active compensation hardness blocks measurements (hrc) difference designation hrc 1 2 3 4 5 average uniformity mpa nrw 1214601.1006 21.62 21.50 21.51 21.36 21.53 21.55 21.49 0.19 -0.13 mpa nrw 863401.0206 30.20 30.25 30.21 30.19 30.07 30.28 30.20 0.21 0.00 mpa nrw 6975302.0916 44.71 44.99 44.75 44.84 44.80 44.77 44.83 0.24 0.12 as 2271/98 55.90 55.89 56.06 56.05 55.70 55.68 55.88 0.38 -0.02 mpa nrw 900809.0306 61.13 60.99 61.25 61.04 61.15 61.18 61.12 0.26 -0.01 mpa nrw 863905.0206 64.94 65.01 64.88 65.01 65.00 65.12 65.00 0.24 0.06 the results of the introduced compensation are shown in figure 5 and figure 6. figure 5 shows the deviations of the calculated values of the five measurements with and without active compensation, while figure 6 shows a comparison of the uniformity values of both measurement series. figure 5: difference of measured hardness value and hrc nominal hardness blocks value. comparison of average values from the measurement series without compensation (in blue) and with active compensation (in orange) figure 6: uniformity for individual measurement series (measurements without compensation – in blue; measurements with compensation – in orange) as shown in figure 5, measurement results with active compensation differ much less from the reference blocks than in a series of measurements without compensation. by reading the difference in table 3, the average hardness measurements of each block do not differ by more than 0.15 hrc from its nominal value. figure 6 summarises the values of measurement spread for measurement series made before and after the introduction of deviation compensation. in both cases, we can see that the scatter values for individual blocks for both measurement series are very similar and oscillate around 0.2-0.25 hrc. the exception are the uniformities when measuring the blocks with a value of 55.9 hrc, which shows higher uniformity (older generation block with higher uniformity of surface hardness). the final stage of verification of the correctness of the hsm-s02 stand and the compensation used was the performance of the third series of measurements on other blocks than those used in the first two tests with a maximum uncertainty of 0.4 hrc. table 4 summarises the results from the measurements of the third series. these measurements were carried out on the same day as series 2 to eliminate the effect of time on possible position instability. figure 7 shows the difference between the hrc value of the block and the average value of its five measurements. the measurements were performed with active compensation taking into account the table with corrections to maintain consistency with the second measurement series. analysis of the results showed that the values of uniformities and average of individual measurements are acceptable. the block 22.73 hrc is an exception. -1 -0,5 0 0,5 1 1,5 21,62 30,2 44,71 55,9 61,13 64,94 h r c d if fe re n ce hrc nominal value 0 0,1 0,2 0,3 0,4 0,5 0,6 21,62 30,2 44,71 55,9 61,13 64,94 h r c u n if o rm it y hrc nominal value http://www.imeko.org/ table 4. results of measurements of 3rd series made on additional blocks with a maximum measurement uncertainty of 0.4 hrc hardness blocks measurements (hrc) difference designation hrc 1 2 3 4 5 average uniformity 19776 d-k 22.73 21.42 21.52 21.25 21.33 21.47 21.40 0.27 -1.33 press-9794 35.80 36.07 35.96 35.90 35.99 35.91 35.97 0.16 0.17 19777 d-k 45.91 45.56 45.64 45.57 45.59 45.66 45.61 0.10 -0.30 as4322/85 50.50 50.76 51.00 50.81 51.01 50.95 50.91 0.25 0.41 as 60/98 61.00 60.73 60.64 60.54 60.92 60.75 60.72 0.38 -0.28 19778 d-k 65.86 65.52 65.49 65.49 65.65 65.57 65.54 0.16 -0.32 figure 7: the values of the difference between the mean value of the measurements and the value of the hardness block in the 3rd series. measurements with active compensation the average values of measurements for all blocks, except the block with the lowest hardness, are within 0.4 hrc according to the documentation of blocks. the average value of the measurements for the 22.73 hrc block measured on the hsm-s02 stand is 1.33 hrc too small in relation to its nominal value. compared with the measurements from the first series of the 21.62 hrc block, it can be seen that they were 1.29 hrc too high in relation to its nominal value. therefore (to check the uniformity of the soft 21.62 hrc block), a comparative study was conducted with the hsm-s01 stand, which uses the same type of indenters and is very similar in construction. the results for both blocks and stands (hsm-s01, hsm-s02) are presented in table 5. first, the values of uniformities were analysed. according to figure 8, the dispersion values are comparable and according to the data from table 5 for individual standards, they do not differ by more than 0.01 hrc, which can be considered a negligible value. in addition, the scatter values for both blocks are small, which suggests that both blocks have similar uniformity and the positions are repeatable and stable. figure 8: uniformities for comparable test results at hsm-s01 (in grey) and hsm-s02 (in black) stands the following conclusions can be drawn from the conducted tests: 1. it should be assumed that the block with a value of 21.62 hrc has a different actual value than the declared nominal value. 2. assuming no fatal error in the measurements at the hsm-s01 and hsm-s02 stands and theirstability on the basis of the uniformities shown in table 5 and figure 8, it is suggested to take the actual value of the hardness of the 21.62 hrc block as the average of the measurements on the hsm-s01 stand and measurements from the first series on the hsm-s02 stand without compensation, giving a calculated average difference of 1.062 hrc. 3. if it is assumed that the average value of the 21.62 hrc block measurements taken on the hsm-s02 stand in the first series without compensation should be corrected by the calculated average value from comparative measurements according to point 2, then the average value from the results of the block with the 22.73 hrc value is -0.269 hrc. this value is close to the value measured at the hsm-s01 stand. -1,5 -1,0 -0,5 0,0 0,5 22,73 35,80 45,91 50,50 61,00 65,86 h r c d if fe re n ce hrc nominal value 0 0,05 0,1 0,15 0,2 0,25 0,3 21,62 22,73 h r c d if fe re n ce hrc nominal value acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 245 after introducing a change in compensation and correction value, the current graph in figure 7 is shown in figure 9. final calibration and corrections on hsm-s02 (and hsm-s01) stands will be introduced after conducting bilateral comparative tests with the national institute of metrological research (inrim, italy) [4]. table 5. comparison of measurement results for the 21.62 hrc and 22.73 hrc blocks at hsm-s01 and hsm-s02 hardness blocks measurements (hrc) difference designation hrc 1 2 3 4 5 average uniformity hsm-s01 mpa nrw 1214601.1006 21.62 22.36 22.36 22.56 22.56 22.44 22.46 0.20 0.84 19776 d-k 22.73 22.64 22.56 22.60 22.48 22.76 22.61 0.28 -0.12 hsm-s02 mpa nrw 1214601.1006 21.62 21.50 21.51 21.36 21.53 21.55 21.49 0.19 -0.13 19776 d-k 22.73 21.42 21.52 21.25 21.33 21.47 21.40 0.27 -1.33 figure 9: measurement results of the 3rd series after updating the correction value for the 0-30 hrc range. measurements with active compensation 4. summary gum’s deadweight-type rockwell hardness standard machines were modernised in accordance with industry 4.0 requirements. at the stage of preparing the article, the stand was not yet ready to conduct the final research and determine the uncertainty budget. the stand was manufactured in 1975 and has been used sporadically in recent years, which raised concerns about its technical condition. after the modernisation, its stability and repeatability had to be determined and the developed algorithm to compensate for measurement deviations had to be tested. for this purpose, hardness blocks were used, which were measured under reproducible conditions with an interval of three months. the obtained measurements, which were used in the article, confirmed that the station’s mechanisms work properly, and the compensating algorithm allows correction of any deviations to the level of 0.05 hrc for the entire scale range. the presented way of the adjustment hsm-s02 with the introduction of automatic compensation (correction) allows to improve the measured result (hardness) in real time. on this basis, an uncertainty budget will be developed for the stand without using plate hardness standards. the compensation algorithm can be used to adjust the characteristics of the indenter or the system for measuring the displacement of the indenter if comparative measurements made with the laser interferometer showed its non-linearity or offset. it is planned to use the hsms (the hysteresis / rockwell hardness investigations) in the empir project with the acronym comtraforce. 5. acknowledgements modernisation works of the hsms were financed from funds for investment tasks for the gum in 2019. as a part this work had been carried out in the framework of the project with the title “comprehensive traceability for force metrology services”. the 18sib08 comtraforce project has received funding from the empir programme cofinanced by the participating states and from the european union’s horizon 2020 research and innovation programme. -0,4 -0,3 -0,2 -0,1 0 0,1 0,2 0,3 0,4 0,5 22,73 35,80 45,91 50,50 61,00 65,86 h r c d if fe re n ce hrc nominal value http://www.imeko.org/ https://msu.euramet.org/downloads/documents/empir_logo.jpg acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 246 6. references [1] klaus schwab, the fourth industrial revolution, 2016 https://luminariaz.files.wordpress.com/2017/11/th e-fourth-industrial-revolution-2016-21.pdf [2] en iso 6508-1/2/3: metallic materialsrockwell hardness. [3] j. d. fidelus, participation of the central office of measures in the european industrial project empir in the field of force, metrology and hallmarking the bulletin of the central office of measures, 1 (24) / 2020, 7-14. baztech. [4] a. germak, j. d. fidelus, c. origlia, bilateral comparison in rockwell c hardness scale between inrim and gum, ongoing study. http://www.imeko.org/ http://? http://? https://luminariaz.files.wordpress.com/2017/11/the-fourth-industrial-revolution-2016-21.pdf https://luminariaz.files.wordpress.com/2017/11/the-fourth-industrial-revolution-2016-21.pdf development of a combined xrf/xps surface-analysis system for the surface-layer quantification of 28si spheres acta imeko issn: 2221-870x march 2021, volume 10, number 1, 290 294 acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 290 development of a combined xrf/xps surface-analysis system for the surface-layer quantification of 28si spheres yu-hsin wu1, lin tsao1, jeng-yu chiu1, sheng-jui chen1, michael kolbe2, rolf fliegauf2, edyta beyer3, frank haertig3 1 center for measurement standards, industrial technology research institute, 30011 hsinchu, taiwan 2 physikalisch-technische bundesanstalt (ptb), 10587 berlin, germany 3 physikalisch-technische bundesanstalt (ptb), 38116 braunschweig, germany section: research paper keywords: xrcd; si sphere; xps; xrf citation: yu-hsin wu, lin tsao, jeng-yu chiu, sheng-jui chen, michael kolbe, rolf fliegauf, edyta beyer, frank haertig, development of a combined xrf/xps surface-analysis system for the surface-layer quantification of 28si spheres, acta imeko, vol. 10, no. 1, article 39, march 2021, identifier: imeko-acta10 (2021)-01-39 editor: koji ogushi, nmij, japan received may 15, 2020; in final form august 5, 2020; published march 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work is supported by the bureau of standards, metrology and inspection (bsmi), taiwan, republic of china corresponding author: yu-hsin wu, e-mail: yh.wu@itri.org.tw 1. introduction at the 26th meeting of the conférence générale des poids et mesures (cgpm) in 2018 [1], the definition of the unit of mass, the kilogram, was redefined by a fixed value of the planck constant h = 6.626 070 15×10-34 j s to replace the international prototype of the kilogram (ipk), and the new definition came into force on 20 may 2019 [2], [3]. there are two primary methods for realising the definition of the kilogram, the kibble balance, which compares electrical power to mechanical power [4]-[6] and the x-ray-crystal-density (xrcd) method [7]-[10]. with the transmission of the information and technology from physikalisch-technische bundesanstalt (ptb, germany), the center for measurement standards, industrial technology research institute in taiwan (cms/itri) adopted the xrcd method to realise the new kilogram definition and established the combined x-ray fluorescence (xrf)/x-ray photoelectron spectroscopy (xps) surface-analysis system to evaluate the surface-layer mass of a si sphere. in this paper, we report the surface-characterisation method and current progress, which includes the system assembly, vacuum-chamber design, hardware integration and the intended work on the construction of the xrf/xps surface-analysis system. 2. the surface-layer composition of the 28sienriched sphere based on the new plank constant, xrcd will be used to determine the mass of the si sphere to realise the new kilogram. the core mass mcore of the si sphere is determined by counting the number of si atoms inside a 28si-enriched sphere and abstract to achieve a new kilogram definition using the x-ray crystal density method, the center for measurement standards, industrial technology research institute in taiwan has established the combined xrf (x-ray fluorescence)/xps (x-ray photoelectron spectroscopy) surface analysis system for the quantitative surface-layer analysis of si spheres. the surface layer of a si sphere is composed primarily of an oxide layer, carbonaceous contamination and physisorbed/chemisorbed water. this newly combined instrument has been implemented to measure the xrf for the direct determination of the mass deposition of oxygen (ng/cm2) with a calibrated silicon drift detector and the xps for the ratio between the elements (o, si, c) and composition identification. these two complementary methods of x-ray metrology allow an accurate determination of the surface-layer mass of the si sphere. in this paper, the construction of a combined xrf/xps surface-analysis system is reported, including the surface characterisation method, the assembly of parts of the loadlock chamber and ultra-high-vacuum analysis chamber, the vacuum-system design, hardware integration and the intended research on surface-layer measurement. it is anticipated that the measured surface-layer mass will be combined with the core mass of the si sphere. .for mass dissemination. mailto:yh.wu@itri.org.tw acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 291 multiplying it by the si mean molar mass [11], [12]. however, an oxide layer and carbonaceous layer is formed on the sphere surface. the mass of the si sphere msphere should take the mass correction of the surface layer into account: 𝑚sphere = 𝑚core + 𝑚sl . (1) the surface-layer (sl) model (figure 1) for the si sphere is composed of an oxide layer with sio2 and suboxides (si2o, sio, si2o3) at the interface between the silicon crystal and carbonaceous contaminations (cox, c, hydrocarbon) as well as the physisorbed/chemisorbed water layer [13]-[15]. to identify the chemical elements and determine the mass present in the surface, an accurate evaluation of surface layers by xps and xrf spectrometry is conducted [16]. 3. surface characterisation 3.1. combined xrf/xps analysis for a surface layer the quantitative surface analysis of the silicon sphere combines the xrf and xps measurement techniques. xrf is used to quantify the mass deposition of oxygen on the si-sphere surface layer based on the calibrated sio2 reference samples, and the xps measurement provides information on the chemical state in the oxide and carbonaceous layers, which can be used for a surface analysis of the composition by stoichiometry. with the development of the combined xrf/xps surface-analysis system, both the xrf and xps measurements can be carried out using this integrated instrument. 3.2. x-ray fluorescence xrf is a typical method of elemental and chemical analysis. the characteristic x-ray is emitted from a material that has been excited by being bombarded with high-energy x-rays or gamma rays (figure 2). for si spheres, xrf is utilised to measure the surface density, i.e. the mass deposition (ng/cm2), of the oxygen present in the surface layer. since the in-house x-ray source is not stable enough for element quantification, a synchrotron radiationbased reference-free x-ray fluorescence analysis [17]-[20] is employed for the direct determination of the mass deposition of oxygen. with this technique, the mass deposition of oxygen from five sio2 reference samples with a nominal thickness ranging from 2 nm to 10 nm are measured at bessy ii, berlin. the combined xrf/xps surface-analysis system is used to measure the fluorescence intensities of oxygen and silicon to obtain the intensity ratio between o kα (525 ev) and si kα (1740 ev) from the reference samples. according to sherman’s equation, the correlation of the mass deposition in relation to its fluorescence radiation of o kα/(si kα) is given as 𝑚𝑑𝑂 = − 𝐺 𝜇tot 𝑙𝑛(1 − 𝐶0 𝑅osi 𝜇tot), (2) where mdo is the mass deposition of oxygen, c0 is the calibration factor, μtot is the total mass attenuation coefficient, g is the geometry factor and rosi is the intensity ratio of o kα to si kα. most of the si kα signal is from the si substrate. the ratio of o kα/(si kα) reflects the amount of change in the oxygen. the mass deposition of oxygen of the si sphere can be obtained by interpolation using the correlation curve with a ratio of o kα/(si kα) measured from the si sphere. 3.3. x-ray photoelectron spectroscopy xps is a surface-sensitive measurement technique. the photoelectrons escape from the sample because the x-ray energy irradiating the sample is larger than the binding energy of the elements. the kinetic energy of the photoelectrons is analysed by an electron spectrometer to identify the chemical elements and their binding states (chemical shift), as shown in figure 3. without the reference samples used for xps measurement, only ratios between si 2p, o 1s and c 1s are quantified according to the amount of the photoelectrons detected. in the surface layers, the relative atomic fraction ca between the elements is obtained by the normalised xps signal i’ja: 𝐼′𝑗𝐴 = 𝐼𝑗𝐴 𝜎𝑗𝐴 ∙ 𝜆(ke) ∙ 𝑇(ke) , (3) where t(ke) is the transmission function, σja is the photoelectric cross section of photoemission j of element a [21] and λ(ke) is the inelastic mean free path of the photoelectrons [22]. the atomic fraction can be expressed as follows: 𝐶𝐴 = 𝐼′𝑗𝐴 ∑ 𝐼′𝑗𝐴 . (4) the ratio between the elements measured by xps is proportional to the number of atoms of each element. with the mass deposition of oxygen determined by xrf, the number of oxygen atoms per unit area can be obtained; the absolute mass figure 1. model of the surface layer of the si sphere. figure 2. schematic diagram of x-ray fluorescence from an sio2 thin film with a thickness of several nanometres. figure 3. schematic diagram shows the photoelectrons escaping from the surface layer of a si sphere. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 292 depositions of si and c can be calculated by the mass deposition of oxygen in the surface layer based on xrf. the detailed xps scans of si 2p and c 1s give the ratios corresponding to different binding states. for example, there are several peaks in si 2p in the xps measurement. the main peak is from the silicon bulk, and the subpeaks are from sio2 and suboxides (si2o, sio and si2o3) in the interface. the distribution of the oxygen present in the silicon oxide and carbonaceous layers can be evaluated using a stoichiometric approach [23]. since hydrogen cannot be detected by xps, the mass deposition of hydrogen is estimated and addressed by the possible molecules in the carbonaceous layer. for chemisorbed water, the estimation of the mass deposition of hydrogen relies on the remaining oxygen and the silicon-hydroxyl group in the silicon-oxide layer [24]. 4. construction of xrf/xps surface-analysis system cms/itri established the combined xrf/xps surfaceanalysis system in 2019. this system is designed to carry out the xrf and xps measurement for si spheres. the construction work includes the assembly of parts of the load-lock chamber, the ultra-high-vacuum (uhv) analysis chamber, hardware integration and the design of the vacuum system. the combined xrf/xps surface-analysis system is shown in figure 4. the system is mainly composed of two chambers, the load-lock chamber and the uhv-analysis chamber. the si sphere is placed in the load-lock chamber and transferred into the uhv-analysis chamber for the surface-layer measurement. the incident radiation of the characteristic al kα fluorescence line is 1486.6 ev with an energy resolution of δe of 250 mev by quartz crystal using rowland geometry. the geometry between the incident radiation and the photoelectron detection channel (electron spectrometer) is fixed at the magic angle of 54.7°, with a pass energy of 80 ev for the spectrum survey and 40 ev for the detailed scan of each element. a bruker sdd (silicon drift detector) fluorescence detector with a windowless detecting area of 30 mm² is set up with respect to the incident radiation at angles of 45 ° out of plane and 15 ° in plane. the key component of this analysis system is the uhv five-axis manipulator to investigate the full si-sphere surface in different geometries inside the uhv-analysis chamber (figure 5). for the five-axis manipulator, two of the axes are linear motors moving along the vertical (axis ② ) and horizontal (axis ④ ) axes with a step resolution of 0.000061 mm and 0.0001 mm, respectively, to adjust the position of the si sphere to the centre of the chamber. the other three axes are composed of rotating motors. the motor at the lowest axis (axis ⑤) rotates around the centre of the uhv-analysis chamber with a step resolution of 0.0001 ° to change the angle of incidence on the si sphere. the upper two motors rotate around the centre of the si sphere to measure element distribution. the step resolutions of horizontal axis ① and vertical axis ③ are 0.00014 ° and 0.0001 °, respectively. when the measurement is underway, the chambers should be maintained in an ultra-high-vacuum environment with the pressure of the load-lock chamber at 10-5 pa and the uhvanalysis chamber (main chamber) at 10-7 pa. the design of the vacuum system is shown in figure 6. the load-lock chamber and main chamber are connected by a gate valve, the turbo molecular pumps are mounted on both chambers and connected to a root pump and an ionisation gauge and pirani gauge are used to monitor the pressure in both chambers. as a buffer to the main chamber, the load-lock chamber should be evacuated and refilled when changing the sample to be measured. an angled valve with a soft pump is used to avoid the diffusion of dust and particles in the load-lock chamber; the load-lock chamber will be filled with nitrogen before being opened. moreover, in order to avoid the window on the chamber being damaged by the high pressure of the nitrogen, a buffer volume is connected between the source of the nitrogen and the load-lock chamber. to maintain the pressure of the main chamber at 10-7 pa, except when using a turbo molecular pump, an ion pump is also used, and a residual-gas analyser (rga) is used to analyse the residual pressure in the main chamber. figure 4. the combined xrf/xps surface-analysis system. figure 5. schematic diagram (left) and the view (right) of the uhv five-axis manipulator. axes ① and ③ are rotated around the centre of the si sphere, axis ⑤ is rotated around the centre of the chamber and axes ② and ④ move linearly to adjust the position of the si sphere. figure 6. the vacuum-system design of the combined xrf/xps surfaceanalysis system. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 293 in addition, for the preparation of the xrf measurement, the response function and detector efficiency of the sdd has been calibrated in the energy range of 100 ev to 1850 ev [25]. the response function of the sdd is measured by changing the energy of the incident beam from monochromatised synchrotron radiation and will be convoluted with the theoretical bremsstrahlung and resonant raman scattering models [26]. to quantify the mass deposition of oxygen on the five sio2 reference samples with thicknesses ranging from 2 nm to 10 nm, the samples were measured in ptb’s laboratory at the bessy ii in berlin for synchrotron radiation. monochromatised synchrotron radiation at 684 ev photon energy was used to irradiate the sample. with the calibrated photodiode and sdd to collect the signal of the incident beam and fluorescence, the mass deposition of the oxygen can be directly determined. 5. conclusion and future work in conclusion, cms/itri has completed the construction work on the combined xrf/xps surface-analysis system, including the assembly of parts of the load-lock chamber, uhvanalysis chamber, vacuum-system design and hardware integration. it is anticipated that the system will measure the surface-layer mass of the si sphere periodically to maintain the primary mass standard via the xrcd method. to measure and characterise the surface-layer mass of a si sphere, any future work can be divided into two phases. in the first phase, the preprocessing work before starting the xrf and xps measurement of the si sphere should be completed. for this, the hardware and software control system must be integrated the light path is adjusted to pass through the centre of the chamber and the sample is aligned for the angle of incidence of the xrf and xps measurement, respectively. for the xrf measurement, the correlation of the mass deposition measured by ptb will be fitted with the fluorescence radiation of o kα/si kα by the xrf/xps surface-analysis system according to eqn (2). for the xps measurement, as one of the factors is intensity normalisation (eqn (3)), the transmission function t(ke) of the xps spectrometer must be estimated using the quantified peakarea approach method by measuring au 4f, au 4d, au 4p3/2, ag 3d, ag 3p3/2, cu 3p, cu 2p3/2, ge 3p and ge 2p3/2 standard peak areas from reference samples at a pass energy of 40 ev and 80 ev [27]. in the second phase, the expected goal is to provide the surface-layer mass and uncertainty of the si sphere. first, the xps or xrf measurement should repeat the scan of the si sphere several times to check for the angular setting liability of the uhv five-axis manipulator. the measured o/si fluorescence intensity of the si sphere is interpolated by the fitted calibration curve to obtain the mass deposition of oxygen present in the surface layer. the xps measurement is then carried out on the whole surface of the si sphere to calculate the ratio of photoelectrons between si, c and o. the uncertainty of the surface-layer mass will be evaluated from the source for the xrf measurement, including the calibrated mass deposition of the sio2 reference samples, the fitted o/si ratios and the reproducibility. to give the mass of the 28si-enriched sphere, the mass of the surface layer measured by the combined xrf/xps surfaceanalysis system should be monitored for its stability and combined with the mass from the sphere core. however, since the surface-layer measurement is carried out in a vacuum, the sorption correction for evaluating the mass difference of the mass standards transferred between in air and in vacuum have to be considered for 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acta imeko issn: 2221-870x june 2020, volume 9, number 2, 83 88 acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 83 introducing evja: a ‘rugged’ intelligent support system for precision farming niccolò loret1, antonio affinito2, giuliano bonanomi3 1 evja r&d, evja neaples, italy 2 evja cto, evja neaples, italy 3 department of agricultural sciences, federico ii university, neaples, italy section: technical note keywords: precision farming; internet of things; predictive models; bremia lactucae; hyaloperonospora parasitica; fusarium head blight. citation: niccolò loret, antonio affinito, giuliano bonanomi, introducing evja: a ‘rugged’ intelligent support system for precision farming, acta imeko, vol. 9, no. 2, article 13, june 2020, identifier: imeko-acta-09 (2020)-02-13 editor: mauro d'arco, university of naples federico ii, italy received february 28, 2020; in final form may 14, 2020; published june 2020 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: some of the research projects described in this article are funded by the iguess-med prima programme, funded under horizon 2020, the european union’s framework programme for research and innovation. corresponding author: niccolò loret, email: niccolo@evja.eu 1. introduction smart farming is revolutionising agriculture, helping farmers to increase the quantity and the quality of their products. when we refer to smart farming, we are talking about a vast array of systems, differing in terms of their technology and scope. however, at its core, smart farming consists of sensors managed by software that is aimed at making the farmers’ job more efficient and effective. some smart farming products focus on robotics, machine automation, location technology, or data analysis. when smart farming is based on iot systems, it is called precision farming [1][2][3][4]. precision farming follows a fourstep cycle that starts with the monitoring of the plants via sensors, followed by the diagnostics of the collected data and ending either with the decision-making of the farmer or with the activation of another system; for example, in automatic irrigation systems connected to the precision farming platform. the result is a more controlled crop cycle, with plant and weather conditions monitored metre by metre, and a more accurate intervention by the farmers, with action undertaken only when it is really needed. the advantages are significant: less pesticides and fertilisers are used; irrigation is more efficient; and the final product is healthier and more abundant. said results are achieved with minimum impact on the environment, leading to a win-win situation for the farmers, consumers, and the environment. we now introduce ‘evja: observe, prevent, improve’ (or just ‘evja’), a precision farming system of our design. evja is an intelligent support system that helps farmers optimise the usage of chemical products and water. by using iot and artificial intelligence, evja allows farmers to monitor their fields in real time, wherever they are. evja gathers data from a network of customisable sensor nodes (see figure 1) connected to servers abstract precision agriculture is a farming system based on the combination of detailed observations, measurement, and rapid response used to optimise energetic input to maximise crop production. precision agriculture uses a decision support system (dss) for optimising farm management. in this context, ‘evja: observe, prevent, improve’ (or just ‘evja’) is an intelligent support system used for precision agriculture. a vast set of data (temperature, relative humidity, deficit of vapour pressure, leaf wetness, solar radiation, carbon dioxide concentration, and soil moisture) is continuously collected, submitted to a local control unit, and processed through algorithms specifically developed for different crops. on the other hand, farmers can access evja from their pc and mobile devices, and they may monitor complex agronomic data analysis presented in a user-friendly interface. in this article, we show how evja works and how its output can be used to assess the health status of plants through a specific set of functions. moreover, we show the methodology utilised to develop useful predictive models based on this information. specifically, we describe a predictive algorithm that is capable of predicting the infection risks of downy mildew for baby leaves plantations and for fusarium ear blight of wheat. mailto:niccolo@evja.eu acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 84 where those data are processed. thanks to its self-calibrating agronomic models, evja helps farmers prevent plant diseases. evja is equipped with a simple and intuitive interface (see figure 2), which makes it very easy to use. it is available for every kind of crop and offers specific advanced features for greenhouse farms. we extensively describe the entire system in the following paragraphs, for the data collection process and data analysis. 2. state of the art iot has the potential to monitor irrigation and productivity, and the data gathered by iot sensors have the ability to provide information about the overall performance of the crops. existing monitoring systems use drones and weather stations, which are inappropriate for greenhouse crops. indeed, evja includes the first predictive algorithm for horticultural products in the european union, while the main direct competitors commercialise solutions that address generally all type of crops, without focus and verticalisation on specific crops and weak results. on the other hand, greenhouse monitoring systems are often designed for fully climate-controlled environments, closer in concept to a scientific laboratory than a farmhouse, while evja’s ‘rugged’ sensor nodes are designed to be handled roughly, in any kind of working conditions. the main features characterising the evja system are: • technological: the integration with advanced predictive models; • product: the bundling of hardware and software in a single solution, which allows for a seamless user experience; and • business: the high scalability of evja, which allows for targeting agricultural businesses anywhere in the world. indeed, compared to many other systems that are available, the evja hardware works not only with wifi or mobile coverage but also everywhere else due to the use of an innovative communication technology called a long-range (lora) network [5], which can fully operate with radio frequencies. furthermore, evja is a software, which is more flexible, more extendable, and more user-friendly than that of the majority of its peers. the evja system is based on a software as a service (saas) model, which offers an array of features, including real-time monitoring, forecasting, management, business intelligence, and social features like chat and media sharing. farmers can monitor and manage everything, in each field, directly from their desktop, tablet, or smartphones. they can mark every event, like an above-average harvest, and go through the history to see trends and correlations between such events and the key factors registered by the sensors. if a worker in the field spots a plant affected by a parasite, they can take a picture and share it with the agronomist in order to check the type of disease and take immediate action. the hardware consists of a device with sensors to be installed in fields in an intuitive ‘plug and play’ manner and does not require maintenance. the sensors collect all the relevant data, such as humidity, temperature, light, and soil status. the evja system is totally wireless. it does not need cables for its power supply nor for communication with other devices. it includes an electronic package containing a customisable processing unit and a wireless communication unit to communicate wirelessly with external systems. more specifically, evja’s hardware includes: • the base station (12 × 12 × 8 cm³), which has a robust waterproof enclosure and covers a homogeneous area of about 3 hectares; • the battery, which is recharged using the internal or external solar panel (23 × 16 × 2 cm³) – the rechargeable battery has a load of 6600 ma h, which ensures non-stop working time for a month; • sensors, which are used to measure temperature, leaf wetness, humidity, and solar radiation; and • mobile radio options – wifi and the lora network. 3. the evja system in detail the node’s core is a microcontroller powered by a battery charged by a solar panel. sensor probes can be easily attached to the device by simply screwing them into the bottom sockets. in the same way, sensor probes may be easily replaced. the basic evja functions need just temperature, humidity, leaf wetness, and solar radiation sensors. however, the entire system is easily customisable, and new sensors can be added depending on the user’s requests. as we will explain in detail later, the next evja release will be integrated with soil moisture sensors. in addition figure 1. the appearance of an evja sensor node. figure 2. evja’s simple and intuitive interface acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 85 to the agronomic observables, the system gathers data about the state of the system, such as gps, accelerometer data, the status of the battery, and gsm signal power. the data is collected by the sensors at every time interval (fixed by the system manager). at the end of the process, an antenna sends both agronomic and diagnostic data to the servers. the chosen data transmission protocol is very flexible, making use of 4g and wifi protocols. in the case of poor coverage, a customised radiofrequency network is deployed ad hoc using gateway to deploy a ‘star’ network or a mesh network of sensor nodes. in the case of a temporary lack of signal, the measurements are stored in a memory inside the control unit and are then communicated to the server when the signal is restored. the data stored in the servers are available to the users, who will always be able to access the required information through a simple and responsive interface (in figure 2) on their smartphone or laptop. logging into the interface, the user can see historical data in the form of simple graphics (as shown in figure 2), select the required time intervals, and check the reports about thresholds exceeded, parasites, and other useful information on plant health. 4. evja algorithms evja is equipped with several generic functions that are useful for defining plant status and needs, and it also has customdesigned predictive algorithms specifically designed to keep parasites under control. we give below some examples to better explain how the system works and how farmers can use it to rationalise their activities and spearhead resources. 4.1. functions using the evja interface, a farmer can check the conditions of a crop in real time. the system allows to fix thresholds for temperature, humidity, leaf wetness, solar radiation and other customisable observables (depending on the kind of sensors mounted on the device). in case those thresholds are exceeded, the system sends a warning email to the farmer. evja, however, does not just visualise those data. it also processes it in order to calculate the functions that are fundamental for depicting a clear picture of the plants’ health status, such as dew point, vapour pressure deficit (vpd), growing degree days (gdds), and evapotranspiration. the dew point is the temperature t dp, to which air must be cooled to become saturated with water vapour. dew point can be calculated through the formula: (1) where b = 18.678 and c = 257.14 °c. when the air temperature is close to the dew point, the air is saturated with moisture, plant perspiration will not evaporate, and droplets will form on leaves. dew point can also be called frost point if tdp < 0 °; in this case, the system warns the user about the possible damage to the crops. gdds is a parameter with the dimensions of a temperature, expressing the heat accumulation by plants and insects during their development. ambient temperature influences plants and pests’ rate of growth; therefore, when specific predictive models 1 one should bear in mind that gdds are defined by taking into account average daily temperatures, while the evja system can perform and store hundreds of measures each day. calculating some harmful insect outbreak probability or plant productivity are lacking, calculating the gdds can be a simple preliminary step in setting an indicator [6] [7]. a simple estimation of the gdds can be obtained by the formula . (2) where t is the time, n is the number of measures performed in one day,1 n is the total number of measures, and tm and tm are respectively the minimum and maximum temperatures defining the survival range of a living being. many calculation tricks, from trapezoid to monte carlo formulae for the numerical calculation of integrals, could make the approximation in equation (2) more reliable. however, since phenomenological tables are compiled by taking into account the rough approximation of the rectangles, it is always better to stick with the simpler gdds definition. for many species of insects, the time interval in which t < tm is called diapause; in that period, their development process basically stops, only to resume when weather conditions are more favourable. climate changes have recently affected the reliability of this indicator for pest control, because given the rising average temperatures in winter, many insects do not go through a diapause as they used to a few decades ago. moreover, a crop can be infested by many different pests, like aphids or non-diapausing insects. that is why evja’s gdds numbers should be used for pest control purposes only, with the careful assistance of an entomological specialist. much simpler is the use of the gdds indicator to predict the time left for a plant to develop and flourish: using weather forecast data, the system can estimate future heat accumulation and predict the period in which it is more likely that the plant will reach the gdds value related to their full development (by confronting the system with well-known phenomenological tables). in this way, farmers are assisted in planning their future activities, such as harvesting or determining the right amount of irrigation needed by their crops in the near future. the system will then help farmers to save water through other parameters. vpd is the difference between the vapour pressure within the leaves and that of the atmosphere: (3) where tl is the leaves’ temperature, and (ta, ua) are the temperature and humidity measured by the sensors. vpd is an indicator of the water flow within the plants, providing information on the efficiency of the inner steam pump in the plant’s stem. knowing both vpd and evapotranspiration et, the user can determine the actual plants’ need for water. evapotranspiration is estimated in evja through a simplified version of the penman-monteith equation [8][9]: (4) where s is a function characterising the role of the solar radiation, and kc is the crop coefficient (which depends on the plants’ growth stage). ( )       −      −             100 ln1 100 ln u t+c bt b t+c bt + u c =ut,t dp ( ) ( ) dtttt=t,tgdd mmm − mm t mi t<t< tt n       − 1 ( ) ( ) ( ) aaals u,tptp=ut,vpd − ( ) ( ) s0cscc ps,etk=ps,,ket acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 86 a clearer indication about whether is appropriate to irrigate a crop can be given by integrating some information about the actual soil moisture into the system. 4.2. soil moisture measurement and water saving water wastage in agriculture and excessive fertilisation are two important issues in present-day agriculture. problems related to the excessive and non-rational use of nitrogen fertilisers are related both to the accumulation of nitrates and nitrites in soil and plants as well as to the leaching of these nutrients to ground and surface water. while nitrates and nitrites in food are precursors of carcinogenic substances to humans, from an environmental point of view, a high concentration of these ions in water sources favours the phenomenon of eutrophication. management of water and nitrogen fertilisers in agriculture are strongly interconnected practices: the optimal absorption of fertilisers by plants depends mainly on temperature and soil moisture [10]. evja is being upgraded with a dynamic forecasting model that simulates the mineral nitrogen content in the soil within an integrated sensor-based irrigation system that provides data on atmospheric climatic conditions, integrated with soil moisture, soil temperature data, and weather forecasts. this will potentially be a key tool for high-tech agriculture aiming to reduce the adverse environmental impacts thereof. the system eliminates the difficulties in reading and interpreting data, facilitating the involvement of farmers in the field, who will receive real-time updates on soil water content and crop water needs directly on their mobile device, allowing for effective and efficient interventions. the system will acquire data from wireless soil moisture sensors (figure 3) to run computer simulations [11], which are validated through a chemical analysis of the soil in order to determine the actual nitrogen content in its different forms (total, organic, nitrate, and ammonium, which may be quantified through more advanced sensors such as those in [12][13]). 4.3. downy mildew predictive model since during the first years of evja’s life, our activity focused mostly on baby leaf crops, we studied a very reliable predictive model for downy mildew of lettuce (bremia lactucae) and rocket (hyaloperonospora parasitica). those oomycetes have a complex biological cycle [14][15] divided into many phases: a sporangia releases a large number of spores in the atmosphere, which eventually land on the plant’s leaves, where they develop (with the right climatic conditions) into zoospores. again, given the right climatic conditions, the surviving zoospores will then infect the plant. it can take some days for downy mildew to properly develop, but after some time, which can be estimated by taking into account the temperature, it can affect the entire crop, with devastating effects. we managed to parametrise this process, identifying the key conditions allowing each stage of the aforementioned cycle to take place. when the system identifies the ideal conditions for the zoospores to survive and penetrate the plants’ leaves, it calculates the approximate date of the manifestation of the symptoms (see figure 4) and sends a warning to the farmers. the warning can be visualised through an easy-to-read calendar (see figure 2), where the user can also add annotations, which are useful to us as feedback. that feedback is actually fundamental to us, since it allows us to validate the algorithm with real data, adjusting the parameters and the functions in close relationships with the farmers. 5. integration with satellite data as we have seen in the previous sections, evja algorithms work based on a wide variety of data through which one can sharply determine the health status of the plants from their photosynthetic metabolism on the leaves to the water draining efficiency of roots, but they also reliably predict the onset of diseases and future growth rate. those predictions tend to be correct for plants near to the control unit and less reliable the more we step away from it. the plants are constantly monitored by the system, with a measurement rate of around fifteen minutes, and alarms are triggered any time an observable (like temperature) exceeds a certain threshold. satellite information is, of course, inherently different from ground-gathered data: it allows us to monitor the crop in its entirety with a time interval between two measures of a few days, instead having localised data within minutes. satellites and sensors give, then, two different kinds of information on the crop status. the first one is very well localised and dense over time, while the second is available only at given moments and concerns the crop in its entirety with a certain rate of precision. the best solution is therefore not to rely on a single source of information; rather, they should be integrated using custom-made algorithms. figure 3. wireless soil moisture sensors to be integrated in the evja system. figure 4. ideal representation of the relationship between the ndvi and crop coefficient for a generic plantation. in this graphic, we modelized the behaviour of the crop coefficient over time for a fictitious plantation with kcmax = 1.1. acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 87 satellite images provide information on soil moisture only for the first 5 cm in depth, while soil moisture sensors planted in the ground provide data until 30 cm in depth. however, the number of sensors that one can plant into a crop is limited, and their data should be interpolated in order to approximate the water content of the soil in the points between the sensors. finding out the correlation between the soil water content observed by the satellite in the first 5 cm with the one measured by the sensors in the spots where they are present gives a good starting point for adjusting the interpolation algorithms in order to better generalise the measures of the sensors to the entire field, with a good rate of reliability. once that one gets a reliable grid that models the soil water content, it is possible to determine the crop water requirement, crossing over this information with evapotranspiration and vpd. in fact, while the reconstruction of the soil moisture in the first 30 cm of depth gives information about the roots’ efficiency and health, the vpd provides indirect knowledge of the dispersion of water from the leaves to the atmosphere. satellite data enhances the estimation of the crop’s complex evapotranspiration in a significant way. on the one hand, satellites record the heat flux from the ground, which is incorporated in the et0 term of equation (4). on the other hand, taking into account the normalised difference vegetation index (ndvi) [14] solves the problem of the identification of the correct value to assign to the kc coefficient. ndvi is defined as the reflection difference in the near-infrared (nir) inir and red spectrum ired divided by its total: 𝑁𝐷𝑉𝐼 = 𝐼nir − 𝐼red 𝐼nir + 𝐼red (5) chlorophyll strongly absorbs visible light, while the cell structure of leaves strongly reflects near-infrared light. the layer along the bottom of a plant causes these reflections. when a plant becomes dehydrated, sick, affected by disease, etc. this layer deteriorates, and the plant absorbs more of that near-infrared light rather than reflecting it. conversely, when near-infrared light hits a leaf on a healthy plant, it is reflected back. so, considering how nir varies compared to red light provides an accurate indication of chlorophyll, which correlates to plant health. ndvi is also related to the average leaf area of vegetables in a crop; therefore, studying its evolution gives direct insight into the plants’ growth stage. this information is of paramount importance to the evja system, since it allows us to determine the value of kc as (6) avoiding being forced to rely only on farmers’ estimates. satellite data are useful also for algorithms on plant disease outbreaks. in fact, those algorithms are highly reliable for plants that are close to the sensor node (for example, inside the same greenhouse), but they do not sufficient information about plants that may lie in a different area. it may happen, for instance, that while the climatic conditions all over the field are ideal for the development of a disease (e.g. peronospora bremia), but close to the control unit, the plants are just not wet enough or the temperature is too high for the zoospores to survive. in this case, the support system would not raise the alarm, and even if this is correct in theory, it would be ineffective. of course, most of the time, the data gathered by the sensor node represent the conditions all over the field quite well, but such issues are not uncommon. as for the water requirement model, through satellite data, the system would be able to split a crop in smaller sections and find out the relationships connecting the climatic conditions in different areas, using the sensor node as a landmark for extrapolating the other data. in this way, it may be possible to run the predictive algorithms in each cluster, obtaining indications about plant disease all over the crop, thereby enhancing the algorithms’ predictive power. 6. conclusions the evja system is currently working with top italian farmers and monitoring more than 500 hectares. using evja, farmers have been able to substantially reduce the number of chemical treatments required to hold off parasites and to save a large amount of water. moreover, such an intelligent management of chemicals and water saves important economic resources. in the near future, we plan to add many useful functions and algorithms to evja in order to improve the service quality we provide to the users: multi-spectral and hyper-spectral analysis to directly monitor plants’ health; intelligent insect traps to keep track of many dangerous species; and a novel predictive model for the fusarium graminearum fungus for adapting the evja system to work on outdoor crops (specifically cereals). the fusarium head blight is a harmful parasite, since it releases deoxynivalenol within kernels, which is toxic for humans [17]. we managed to refine an algorithm whose skeleton is similar to the downy mildew one, while also taking into account rainfall (whose intensity should be measured with state-of-the-art sensors [18][19]) and dishomogeneities in t and u. we are now in the testing stage, and we are obtaining some encouraging results in confronting the algorithm’s output with historical data. in case the chemical treatment of such disease has not been possible, one can act a posteriori, isolating the infected areas and preventing the infection from spreading to the unaffected wheat inside the warehouse where the product is stored. we are improving the evja system to calculate the percentage of infected ears in the different zones of the crop and to report on a map the areas in which the infected plants have exceeded the concentration threshold considered acceptable. using a geolocation system, evja should be able to suggest a smart path through the field in order to avoid the infected spots, telling the farmer to turn left or right, just like a common navigation system on a cell phone. both in the case of fusarium and downy mildew, as well as for every other predictive algorithm that will be implemented on the system, the most important thing for evja’s operation will be to endlessly evolve accordingly to the farmer’s feedback. this will allow us to adjust the functions’ and algorithms’ parameters in order to flexibly describe the situation on the fields and provide 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https://doi.org/10.1175/1520-0493(1972)100%3c0081:otaosh%3e2.3.co;2 https://doi.org/10.1016/j.agwat.2017.01.011 https://doi.org/10.1007/s11104-016-3039-2 https://doi.org/10.1016/j.measurement.2017.07.004 https://doi.org/10.1109/metroaerospace.2018.8453631 https://doi.org/10.1094/phyto-85-552 https://doi.org/10.1094/phyto.2004.94.4.396 https://ntrs.nasa.gov/search.jsp?r=19740022614 https://doi.org/10.1146/annurev.phyto.42.040803.140340 https://doi.org/10.1016/j.measurement.2020.107552 https://doi.org/10.1504/ijw.2018.095397 a colour-based image segmentation method for the measurement of masticatory performance in older adults acta imeko issn: 2221-870x june 2021, volume 10, number 2, 191 198 acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 191 a colour-based image segmentation method for the measurement of masticatory performance in older adults lorenzo scalise1, rachele napolitano1, lorenzo verdenelli1, susanna spinsante2, giorgio rappelli3 1 department of industrial engineering and mathematical sciences (diism), università politecnica delle marche, ancona, italy 2 department of information engineering (dii), università politecnica delle marche, ancona, italy 3 department of odontostomatologic and specialized clinical sciences (disco), università politecnica delle marche, ancona, italy section: research paper keywords: measurement of masticatory efficiency; chewing gum; colour image segmentation; k-means clustering citation: lorenzo scalise, rachele napolitano, lorenzo verdenelli, susanna spinsante, giorgio rappelli, a colour-based image segmentation method for the measurement of masticatory performance in older adults, acta imeko, vol. 10, no. 2, article 26, june 2021, identifier: imeko-acta-10 (2021)-02-26 section editor: yasuharu koike, tokyo institute of technology, japan received july 28, 2018; in final form june 9, 2021; published june 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: lorenzo scalise, e-mail: l.scalise@pm.univpm.it 1. introduction in the twenty-first century, the population aged 65 and over has increased, and forecasts confirm this to be a continuing trend. this is mainly due to a general increase in mean life expectancy and to a decrease in the birth rate (particularly in western countries) [1]. in addition, considerable socio-economic development over the last 50 years has contributed to a significant increase in the well-being of the population and the level of public health. quality of life depends largely on the prevention and control of disease, and one health condition that affects the quality of life is oral health. indeed, according to the world health organization [1], having a good state of oral health is about much more than just having healthy teeth; poor oral health is one of the most frequent causes of malnutrition, which can have a large impact on daily life. in particular, many activities can become difficult for older adults, such as eating adequately and satisfactorily, speaking fluently and correctly, smiling, laughing, showing one’s teeth without embarrassment and maintaining gratifying family and social relationships [2]. in a recent study [3], a questionnaire survey was conducted in order to provide a global overview of oral health conditions in older people. this survey indicated that incidence of oral disease is high in low-income countries, and the use of oral health services, widely used in developed countries, is low among the older population. in addition to socio-economic status and the regularity of dental visits, it is observed that dental caries and periodontal disease constitute a considerable public health problem in the majority of countries. in particular, the consequences of these oral problems that most frequently afflict the elderly are missing teeth, dry mouth and limitations in chewing ability. for these reasons, in the present study, the association between dental status and masticatory function, especially in elderly patients, was investigated in order to identify an impaired masticatory system and evaluate the appropriate clinical treatment [4]. the results indicated a direct relationship between masticatory function and the number of remaining abstract masticatory efficiency in older adults is an important parameter for the assessment of their oral health and quality of life. this study presents a measurement method based on the automatic segmentation of two-coloured chewing gum based on a k-means clustering algorithm. the solution proposed aims to quantify the mixed areas of colour in order to evaluate masticatory performance in different dental conditions. the samples were provided by ‘two-colour mixing’ tests, currently the most used technique for the evaluation of masticatory efficacy, because of its simplicity, low acquisition times and reduced cost. the image analysis results demonstrated a high discriminative power, providing results in an automatic manner and reducing errors caused by manual segmentation. this approach thus provides a feasible and robust solution for the segmentation of chewed samples. validation was carried out by means of a reference software, demonstrating a good correlation (r2 = 0.64) and the higher sensitivity of the proposed method (+75 %). tests on patients with different oral conditions demonstrated that the k-means segmentation method enabled the automatic classification of patients with different masticatory conditions, providing results in a shorter time period (20 chewing cycles instead of 50). mailto:l.scalise@pm.univpm.it acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 192 teeth. indeed, it was demonstrated that chewing difficulties increase as the number of posterior functional units decreases, when the remaining teeth are unevenly distributed or there are spaces between the teeth. in relation to the influence of prosthetic rehabilitation on masticatory function, it was observed that those with fewer teeth tended to have them replaced with a removable partial denture (rpd) with many artificial functional dental units [5]. subjects with fewer teeth reported greater masticatory difficulty than those with rpd. however, there is no certainty that replacing lost teeth with an rpd is efficient, as some studies have demonstrated. in fact, masticatory function is not necessarily better in patients with a minimum number of occlusal units (up to 20 teeth) and rpd. the replacement of lost posterior teeth with rpd is suggested when there are less than three pairs of posterior antagonist teeth that correspond to a number of teeth less than 20. for all these reasons, it is clear that dentists should implement all the measures at their disposal so that this category of patient can achieve and maintain good oral health. in particular, precise, repeatable and comparable measurement methods are sought for use with this class of patient. current methodologies there are a variety of methodologies describing masticatory function. in particular, there is the patient factor, which depends on a variety of personal and subjective habits. a subjective assessment is based on an individual’s self-perception of chewing ability and cannot provide a complete evaluation of chewing problems, oral conditions and prosthesis quality. this method is rather weak and is not a proven test. for this reason, subjective tests using questionnaires or personal interviews to determine masticatory ability are usually supplemented with scientifically objective tests to assess masticatory efficiency [6]. the combination of the two methods provides a wider evaluation of the chewing efficiency function [7], [8]. fragmenting tests with sieve analysis or optical scanning methods are regarded as the gold standard in assessing masticatory efficiency [9]. usually, these tests employ a standard number of peanuts or carrots as natural test foods or silicone cubes as artificial test foods; they are chewed for a determinate time or number of cycles and are then passed through sieves with different mesh diameters. the final degree of subdivisions of the chewed sample is evaluated by measuring the median particle size (d50) [10]. despite this technique being relatively accurate, it is not very practical; in fact, the particle size of the test foods is very small, and the mobility and sensitivity of the oral structures are reduced [11]. furthermore, the digital image processing software used in optical scanning methods is expensive and complicated to use. it is certainly not a method that can be used on an outpatient basis, as it requires high-precision laboratory instruments. alternative methods using two-coloured chewing gum are considered more practical for detecting changes in dental status. these techniques can provide both the degree of colour blending and the ability to form a bolus. these ‘two-colour mixing’ tests have a significant correlation with the sieving method, especially in patients with impaired masticatory function [12]. the advantage of using the two-colour mixing method in comparison with the sieving method lies in its simplicity of implementation, its lower acquisition times, the relatively cheap cost and the ease of recovery after chewing. liedberg et al. [13] evaluated the degree of blending of the two colours in different chewing cycles by means of a subjectively visual test. prinz computerised this process with an image processing approach using custom-made software [14]. in this study, ranking the different gum samples using image processing measurements was not found to be more accurate than subjective human evaluation. schimmel et al. [15], [16] also developed customised software, viewgum©, to determine the degree of colour mixing. this software calculates the hue value for each pixel in the pictures of semi-automatically segmented gum wafers and displays the standard deviation between two well-separated peaks on the hue axis [15]. the variance of the hue component in the hue, intensity and saturation (his) colour space was considered the measurement of mixing; a low variance indicates a greater colour mixture and, therefore, represents better masticatory performance, while a higher variance indicates inadequate mixing [16]. however, this software introduces errors due to an operator-guided procedure for the segmentation of the gum images and to the difficulty in distinguishing hue variation between the gum and the background colour, especially in well-mixed gum. indeed, extracted data are highly dependent on the accuracy of the segmentation process, which is an essential step in computer vision and automatic pattern recognition based on image analysis [17]. the aim of the present study was to propose an automatic colour-based segmentation method to separate mixed and unmixed colours of images derived from the application of a two-colour mixing test. from the different clustering methods described in the literature [18], ‘k-means clustering’ was adopted for its solid application history and its rather higher efficiency in image feature classification, which has also achieved very good results for unsupervised image datasets [19], [20]. as a result, this approach can provide an automatic classification of the colour areas of interest perfectly separated from background colours. within this context, the chewing performance of six older patients (> 65 years) with different dental status were evaluated. it has been shown that this segmentation method can discriminate between dental status perfectly and can provide quantitative data by means of computerised analysis using the best possible segmentation and minimising human interaction. 2. materials and methods 2.1. clinical protocol an experimental laboratory was used for the proposed measurement method, based on two-colour mixing tests, with the participation of six patients at the odontostomatologic clinic of the polytechnic university of marche (ancona). the study participants were elderly patients aged between 64 and 84 years (median age of 73.2 years). after providing informed consent for clinical treatment, the patients were subject to a general dental examination, in which medical and clinical data were collected. the instruments used for the clinical evaluation were a mirror, probes (curved and straight), tweezers and a periodontal probe. the pre-test check and the following outpatient clinic test were conducted between november 2016 and march 2017. personal data, lifestyle habits (use of tobacco, alcohol, stress, physical activity, nutrition), familial information, and physiological and pathological anamnesis were investigated. in addition, the following clinical data were evaluated and annotated in the folder: − missing teeth − root residue − decayed teeth − filled teeth acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 193 − mobile teeth − fixed partial dentures (fpd) − rpds − periodontal screening and recording (psr). a nutritional test to investigate eating habits was completed by a specialist doctor at the department of clinical and specialist dental sciences. the test used was the mini nutritional assessment (mna®), a well-known and largely accepted monitoring and evaluation tool designed to identify elderly patients at risk of malnutrition based on questions on their nutritional habits [21]. before starting the mna®, general information relating to the patient was entered in the appropriate space at the top of the form. the screening section (mna®-sf), consisting of six questions, was then completed. if the screening score was less than 11, an evaluation to determine the malnutrition index was carried out; if the score was > 23.5, the patient was considered to have a normal nutritional status, and no further action was necessary, while if the score was < 23.5, the patient was referred to a dietician or a nutritionist. 2.2. mixing test procedure after the pre-test clinical protocol had been completed, the test based on the proposed measurement method was performed. each specimen was prepared from two large sticks of bubble gum, one yellow and one pink (saliva did not affect the gum colours). strips 20 mm in length were cut from both colours and manually stuck together so that the test strip presented as 20 × 10 × 2 mm³ (figure 1-a). for all the participants, five pairs of chewing gum sticks were prepared, and five boluses for different chewing cycles were obtained (figure 1-b) for − 5 chewing strokes − 10 chewing strokes − 20 chewing strokes − 30 chewing strokes − 50 chewing strokes. each bolus was gently dabbed with a paper towel in order to eliminate any possible excess saliva and then flattened to form a 1-mm-thick wafer (figure 1-c). the total duration of the test was about 5 minutes. in the laboratory tests, an accurate digitisation of each sample was performed. the procedure was divided into two main phases: 1. an acquisition phase, in which the images of different chewed wafers were captured by means of a ccd camera (1,920 x 1,080 pixels) mounted on a smart phone (lg g2 d802). each wafer was placed on a dark background so that the different colours could be more easily separated. for all the images acquired, the camera resolution, type of illumination and camera-to-bolus distance remained fixed. 2. processing phases, necessary to classify an image into different colour groups. a pc (intel ® core ™ i76700hq, 2.6 ghz, 16.0 gb) with windows 10 home was used to process the acquired data, and matlab® r2017b software enabled the colour-based segmentation. 2.3. image segmentation method k-means clustering was the method proposed to process the images in order to obtain information on the colours in the mixed and unmixed areas of the chewing gum. this method separates pixels into regions according to their properties, such as colour, intensity or texture. for each region, called a cluster, it is possible to define k centroids, which are placed as far from each other as possible [19]. each point belonging to a given dataset is then assigned to the nearest centroid until no points are left. when all the points have been assigned to an initial group of centroids, the positions of the new k centroids are then recalculated as the barycentre of the cluster. after identifying these new k centroids, a new binding has to be established between the same dataset points and the nearest new centroid. as a result of this loop, it is possible to observe that the k centroids change their location systematically until they stop moving. in the present study, the optimal number of k clusters to be adopted was determined from the value that minimises the total intra-cluster variance [20]. this is equivalent to minimising the pairwise squared deviations of points in the same cluster: 𝐽 = ∑∑‖𝑥𝑖 − 𝑣𝑗‖ 2 𝑐𝑖 𝑗=1 𝑐 𝑖=1 (1) where xi is the set of data points, vj is the set of centres, ‖xi-vj‖ is the euclidean distance between xi and vj, ci is the number of data points in ith cluster and c is the number of cluster centres. in the framework of this study, the regions in each piece of chewed gum were grouped into a set of four classes according to colour: pink, yellow, mixed and background. the implemented algorithm was divided into the following steps: 1. ‘read two input images’: the first is the front of the chewed wafer (side a) and the second is the opposite side (side b). 2. ‘select a region of interest’: create bounding boxes around each wafer and crop the image. 3. ‘convert image from rgb to l*a*b colour space’: l* represents brightness, ranging from 0–100, while a* and b* represent the colour opponents, green–red and blue– yellow. this colour model encompasses the entire spectrum, including colours outside human vision. only a* and b* layers were used for the analysis to guarantee a) b) c) d) figure 1. five pairs of chewing gum sticks were used in this study: a) unchewed, b) bolus obtained from 5, 10, 20, 30 and 50 chewing cycles, from left to right, c) a flattened wafer and d) an image labelled by cluster index. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 194 uniform intensity values and preserve the integrity of the colour information. 4. ‘classify the colours in the a*b* space using k-means clustering’: this approach assumes each region has a location in space and partitions them into a k-number group of data (clusters) using the euclidean distance metric. in this way, the objects within each cluster are as close to each other as possible and as far from objects in other clusters as possible (figure 1-d). in this step, it is necessary to specify the number of k clusters to be partitioned and the number of times clustering using a new initial cluster centroid position should be repeated. in particular, the number of clusters is initially a userspecified parameter according to the number of clusters desired (in this case, four k clusters to partition the image into the four colour classes, as mentioned previously). then, each pixel is assigned to a centroid for each class, and this is repeated until the intra-cluster variance is minimised and no points change cluster. 5. ‘pixel labelling’: labelling every pixel in the image using the index corresponding to a cluster for every input object (figure 2). 6. ‘create segmented images’: separate objects using pixel labels. 7. ‘calculate area’: estimate the area in pixels of the objects in the k clusters. 8. ‘save segmented images’: save and show the images segmented by means of the colours. a scheme of the implemented algorithm is shown in figure 3. 2.4. subject population in our experimental study, six patients were selected and grouped according to their dental status (table 1): group 1: − pt. 1: n. teeth > 20 − pt. 2: n. teeth = 20 − pt. 3: n. teeth = 20 and rpd congruent condition group 2: − pt. 4: n. teeth < 20 and rpd congruent condition − pt. 5: n. teeth = 20 and rpd incongruent condition − pt. 6: n. teeth < 20 in this paper, we considered the first three patients to have a healthy dental status (group 1) and the other three to have a pathological dental condition (group 2). we used the ‘two-colour mixing’ test, mentioned above, to evaluate to what extent the masticatory performance had been reduced in patients with a minimum number of teeth. in addition, we assessed whether rpds allowed any significant improvement in masticatory performance in patients with a minimum number of teeth. 3. results the images of the samples (on both sides) collected from the subject population according to the described methodology were analysed by means of the k-means clustering system, providing the following results. 3.1. clustering results by means of the segmentation algorithm, it was possible to estimate the mixed and unmixed areas of the chewing gum. each colour area was returned as the sum of the partitioned pixels and, for each of the patients, the mixed fraction (mf) and unmixed fraction (uf) of the two colours at 5, 10, 20, 30 and 50 chewing cycles (figure 3) were calculated and normalised according to the following equations: 𝑀𝐹 = 𝑃𝑖𝑥𝑚,𝑎 + 𝑃𝑖𝑥𝑚,𝑏 𝑃𝑖𝑥𝑎𝑙𝑙 (2) a) b) c) d) figure 2. labelled clusters: a) pink, b) yellow, c) mixed and d) background. figure 3. block diagram of the implemented algorithm. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 195 𝑈𝐹 = (𝑃𝑖𝑥𝑦,𝑎 + 𝑃𝑖𝑥𝑦,𝑏) + (𝑃𝑖𝑥𝑝,𝑎 + 𝑃𝑖𝑥𝑝,𝑏) 𝑃𝑖𝑥𝑎𝑙𝑙 (3) where − pixm,a and pixm,b are pixels from each side of the mixed area, − pixp/y,a and pixp/y,b are pixels from each side of the pink or yellow area, − pixall is the total pixel area. the means and standard deviations of the mf for all the patients for each chewing cycle (30 samples considered) are summarised in table 2. it can be seen that the mf parameter increases with the increase in the number of masticatory cycles. the table also shows that the mf mean values at 20 chewing cycles indicate a very good agreement with the different dental status of patients: mf = 0.438 ± 0.214 (min: 0.126, max: 0.740). this is very important since it is possible to reduce the number of chewing cycles from 50 (typical in other studies [14], [22]-[25]) to 20 to measure mf parameters that are well correlated to masticatory efficiency. this value was used for the second series of experiments. in figure 4, the mf values obtained for each of the patients with different masticatory efficiency are reported as a function of the number of chewing cycles, and the linear regression analysis of the two groups of patients is reported. the first set of mf values (r2 = 0.920), relating to the three participants with a healthy dental status, is more sensitive to the increase in the number of masticatory cycles than the second (r2 = 0.91), which relates to the three patients with a diseased dental status. 3.2. masticatory performance the acquired data and their elaboration by means of the kmeans method were used to evaluate the differences in masticatory efficiency between the six selected patients. for this purpose, we assessed the masticatory performance (mp) as the uf of each patient. considering the high variability of the values between the participants at 20 chewing cycles due to the different dental status, for this evaluation we set the fixed number of cycles at n = 20 (figure 5). it is possible to observe how much lower the uf values are compared with the mp values reported for group 1 patients. 3.3. validity a comparison between the proposed segmentation algorithm and viewgum software, a commercial software used for the evaluation of the chewed gum [15], was performed to validate the obtained results. mp, considered as the measure of the unmixed colour and based on the results of both methods for the different chewing cycles for the six patients, is shown in figure 6. concerning the segmentation results obtained by viewgum, the degree of colour mixture was provided as the standard deviation of hue (sdhue) in the his cylindrical coordinate system after the selection of the gum surface [15]. in this study, the sdhue represents the uf measured by means of this commercial software. the normalised mp obtained by the uf data processed by the two methods is compared in figure 6. it is possible to observe that the proposed method is more sensitive (+75 %) to masticatory performance than the results obtained by the viewgum software. in figure 7, the uf of the six patients at 20 chewing cycles is calculated and compared with the sdhue value provided by the table 1. overview of styles and font sizes used in this template. n. teeth age chewing difficulty missing teeth decayed teeth filled teeth mobile teeth fpd rpd periodontitis remaining teeth tot. chewing teeth > 20 68 no 4 0 8 0 no no no 28 28 = 20 67 moderate 11 2 2 0 no no no 21 21 = 20 85 moderate 11 2 2 2 yes cong. yes 21 26 < 20 84 no 19 0 0 0 yes cong. no 13 30 = 20 68 no 12 1 4 0 yes incong. no 20 26 < 20 67 no 16 0 0 0 yes no no 16 16 table 2. means and standard deviations of mf for each number of chews. chewing cycles mf means s.d. 5 0.147 0.078 10 0.313 0.126 20 0.438 0.214 30 0.630 0.173 50 0.769 0.169 figure 4. mixed fraction (mf) for each patient for different chewing cycles and interpolating curves for group 1 (dotted line) and group 2 (continuous line). figure 5. uf calculated by the k-means method at 20 chewing cycles for the six patients. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 196 reference software at the same number of cycles. a higher correlation coefficient (r2 = 0.89) is reported for the k-means method than for the data provided by the reference software (r2 = 0.56). finally, in figure 8, the correlation between the sdhue obtained by the reference software and the uf data obtained by the k-means clustering method for the six selected patients for all the chewing cycles is reported. it is clear that the two indexes, calculated at 20 chewing cycles, are correlated (r2 = 0.64). 4. discussion 4.1. test food the use of these two gums for the test is appropriate to distinguish the different colour characteristics in the images and determine the mf and uf for each wafer. furthermore, it has been verified that the proposed algorithm can segment the foreground from the background perfectly using these image colours. a limitation is the difficulty of cutting a bolus to a thickness of 1 mm homogeneously to reduce the margin of error during the acquisition phase. in addition, the irregular shape of the wafers indicates the need for a mechanism that allows all samples to be flattened to the same thickness without the loss of information from inadequate flattening. 4.2. segmentation assessment using k-means clustering techniques for the evaluation, the mp for each wafer was evaluated by estimating the mixed and unmixed areas (mf and uf) of the chewing gum. this clustering system, using colour information, provides a high level of discrimination for the regions present in each image, reducing the errors resulting from manual segmentation. it is possible to see a significant increase in colour mixing depending on the number of chewing cycles, as demonstrated in previous studies [12], [15], [16]. the high variability between the mf shown in figure 4 resulting from different dental status can provide some indication of the mp of each patient. in particular, at 20 chewing cycles, it is possible to see that the mp decreases for the three patients with a diseased dental condition. 4.3. comparison with other clinical methods in this study, our colour segmentation method was compared with the previously described image segmentation provided by viewgum software. the higher discriminating power of the two mixed and unmixed colours was evident. linear regression analysis demonstrates that the k-means method (r2 = 0.931) can provide a better sensitivity compared with viewgum software results (r2 = 0.913). a pearson correlation between the two measures was very positive (r2 = 0.992, p = 0.02). it was observed that the uf varied over a larger range among the participants than sdhue, which lowered the correlation between the two measures. 4.4. clinical applicability we investigated the capability of this method for carrying out oral health surveys. the six selected patients in our study were recruited for their reduced dentition and their different oral conditions. the large variability in mf for each patient demonstrated that the proposed method provided a good evaluation of chewing function. in particular, the large standard deviations in the results demonstrated the high sensitivity of this image processing technique for different dental status, especially at 20 chewing cycles, which was consequently used as a reference for subsequent investigations. it is evident that chewing efficiency decreases in patients with 20 teeth, but it remains adequate. the reduction in mf is more pronounced in patients with less than 20 teeth and without rpd, according to naka [4]. however, the patients did not report masticatory difficulties. this is probably due to the small study sample, but we cannot underestimate the adaptability of the older population; they often believe that their difficulties are problems figure 6. comparison between the k-means and reference software (viewgum) data: masticatory efficiency vs chewing cycles. figure 7. comparison between the k-means and reference software data at 20 chewing cycles. figure 8. masticatory performance according to both methods. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 197 that must be compensated for and are accepted as the normal consequences of aging. rpds did not lead to significant improvements in the mf for patients with 20 teeth; indeed, we did not observe any great difference in patients with the same number of natural teeth but without rpd. we observed an evident difference in chewing efficiency between the patient with 20 teeth and adequate rpd and the patient with the same number of teeth but with incongruous rpd. this is probably due to the discomfort created by an incongruous prosthesis that negatively influences chewing efficiency despite having 20 teeth. in fact, masticatory function is better when partial dentures are well constructed, but most patients wear resin prostheses with problems in construction, which are badly adapted to the tissues. during mastication, the incongruity of the prosthesis is such as to reduce chewing efficiency to very low levels, but not lower than for patients with less than 20 teeth. however, prosthetic rehabilitation for patients with less than 20 teeth is fundamental since chewing efficiency improves substantially in the presence of adequate rpd. 5. conclusion in conclusion, this automatic segmentation method has been shown to be capable of quantifying the degree of colour mixing. the advantages of this method are that it is fast, robust and not too complicated to perform. it requires a priori specification of the number of cluster centres, which is often difficult to calculate. in this study, the number was selected by the user, who could change it if the segmentation was not satisfactory. however, our study has several limitations; the small number of enrolled patients cannot provide statistical information on the different dental conditions, and the results for other chewing gum colours have not been investigated, so these evaluations are valid only for the specific chewing-gum colours mentioned. however, as a pilot study, the results obtained can be easily combined with data in a larger study for further evaluation. acknowledgement the authors would like to thank prof. enrico primo tomasini for initiating the collaboration and the carmelo (center for advanced research on measurements for engineering and life optimization) research and development center for having instigated the research activity. references [1] world health organization (who), world report on ageing and health, 2015, 246 p. online [accessed 21 june 2021] https://apps.who.int/iris/handle/10665/186463?localeattribute=en& [2] m. chen, r. m. andersen, d. e. barmes, m. h. leclerq, c. s, lyttle, comparing oral health care systems: a second international collaborative study, who, 1997, 350 p. 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h. sato, s. sueda, t. shiozaki, m. kato, t. ohyama, validity and reliability of a newly developed method for evaluating masticatory function using discriminant analysis, journal of oral rehabilitation 30 (2003), pp. 146-151. doi: 10.1046/j.1365-2842.2003.01050.x [12] m. s. kaya, b. güçlü, m. schimmel, s. akyuz, two‐colour chewing gum mixing ability test for evaluating masticatory performance in children with mixed dentition: validity and reliability study, journal of oral rehabilitation 44(11) (2017), pp. 827-834. doi: 10.1111/joor.12548 [13] b. liedberg, o. ekberg, b. öwall, chewing and the dimension of the pharyngoesophageal segment, dysphagia 6 (1991), pp. 214218. doi: 10.1007/bf02493530 [14] j. f. prinz, quantitative evaluation of the effect of bolus size and number of chewing strokes on the intra‐oral mixing of a two‐ colour chewing gum, journal of oral rehabilitation 26 (1999), pp. 243-247. doi: 10.1046/j.1365-2842.1999.00362.x [15] d. j. halazonetis, m. schimmel, g. s. 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[21] e. holvoet, k. vanden wyngaert, a. h. van craenenbroeck, w. van biesen, s. eloot, the screening score of mini nutritional assessment (mna) is a useful routine screening tool for malnutrition risk in patients on maintenance dialysis, plos one 15 (2020), e0229722. doi: 10.1371/journal.pone.0229722 [22] v. anastassiadou, m. r. heath, the development of a simple objective test of mastication suitable for older people, using chewing gums, gerodontology 18 (2001), pp. 79-86. doi: 10.1111/j.1741-2358.2001.00079.x [23] r. manly, s. l. braley, masticatory performance and efficiency, journal of dental research 29 (1950), pp. 448-462. doi: 10.1177/00220345500290040701 [24] i. hayakawa, i. watanabe, h. shigezo, n. masanori, s. tetsuya, a simple method for evaluating masticatory performance using a colour-changeable chewing gum, international journal of prosthodontics 11 (1998), pp. 173-176. [25] k. hirano, s. hirano, h. iwao, the role of oral sensorimotor function in masticatory ability, journal of oral rehabilitation 31 (2004), pp. 199-205. doi: 10.1111/j.1365-2842.2004.01175.x https://doi.org/10.1371/journal.pone.0229722 https://doi.org/10.1111/j.1741-2358.2001.00079.x https://doi.org/10.1177/00220345500290040701 https://doi.org/10.1111/j.1365-2842.2004.01175.x integrated geophysical surveys for the knowledge of a monument of lecce: the sixteenth-century fortifications acta imeko issn: 2221-870x october 2018, volume 7, number 3, 73 78 acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 73 integrated geophysical surveys for the knowledge of a monument of lecce: the sixteenth-century fortifications giacomo di giacomo1, lara de giorgi1, immacolata ditaranto1, giovanni leucci1, ilaria miccoli1, g. scardozzi1 1 institute for archaeological and monumental heritage – national research council (ibam-cnr) c/o campus universitario, lecce, italy section: research paper keywords: archaeology; geophysics; measurements; ancient walls citation: giacomo di giacomo, lara de giorgi, immacolata ditaranto, giovanni leucci, ilaria miccoli, g. scardozzi, integrated geophysical surveys for the knowledge of a monument of lecce: the sixteenth-century fortifications, acta imeko, vol. 7, no. 3, article 12, october 2018, identifier: imeko-acta07 (2018)-03-12 section editor: sabrina grassini, politecnico di torino, italy received march 11, 2018; in final form august 16, 2018; published october 2018 copyright: © 2018 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited corresponding author: giovanni leucci, giovanni.leucci@cnr.it 1. introduction in 2014 and in 2015 integrated geophysical surveys were carried out outside the northern section of the sixteenth-century city walls of lecce by a team of researchers of the laboratory of geophysical applied to the archaeological and monumental heritage and the laboratory of ancient topography, archaeology and remote sensing of cnr-ibam in lecce. the investigations were carried out in cooperation with the chair of the medieval archaeology of the university of salento, responsible for archaeological excavations aimed at studying the ancient remains inside and outside the sixteenth-century city walls. the investigations were performed during the massive restoration works promoted by the municipality of lecce and aimed at enhancement of value of this section of the city walls, including the reconstruction of the original aspect of the fortifications and the external moat. the main goal of the geophysical surveys was to identify the large moat outside the city walls, excavated in the rocky bank (quite close to the surface) and filled with soil probably in the 18th century or at the beginning of the 19th century. in particular, the geophysical measurements were based on ground-penetrating radar (gpr) and electrical resistivity tomography (ert) techniques; in this paper, the main results of gpr prospecting are presented. first results were presented at the third international conference on metrology for archaeology [12]. geophysical survey allowed the researchers to determine the direction, width and depth of the sixteenth-century moat. hence, the excavation activities in the area immediately outside the fortifications were performed according to the results of the geophysical prospecting, which highlighted also other anomalies related to more ancient remains identified during the abstract within the project for the restoration of the northern section of the sixteenth-century city wall of lecce, cnr-ibam carried out geophysical surveys aimed at the study and the reconstruction of the fortifications system. in particular, the investigations focused on a wide band of land outside the two bastions at the northern far end of the fortifications. the integrated use of gpr and ert systems has allowed to detect anomalies related to the main ditch outside the city wall and to characterize the rocky bench in which it was excavated; at the time of the surveys, this ditch was still buried, and the results of the measurements have guided its excavations. moreover, the investigations have allowed to identify also other possible ditches or quarries and more ancient structures, which subsequent archaeological excavations have dated at the roman period. moreover, geophysical surveys highlighted the presence of modern walls, corresponding with the foundations of the stadium constructed in the first half of the 1900s close to the fortifications. so, geophysical investigations were very important since they allowed not only to direct the archaeological excavations, but also because they allowed to formulate some hypotheses on the ancient topography of the not excavated area. mailto:giovanni.leucci@cnr.it acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 74 archaeological excavations as a medieval dich and a roman road. at the same time, geophysical surveys have highlighted numerous anomalies related to the recent history of the investigated area, where in 1920s a stadium was built. lastly, the investigations were also very important since they allowed not only to direct the archaeological excavations, but also to formulate some hypotheses on the ancient topography of the not excavated area. 2. study area the study area is located at the northern end of the city walls of lecce built in sixteenth-century, partially retracing the previous fortifications of the messapian and medieval periods (figure 1). these works were encouraged by the king carlo v and executed by architect gian giacomo dell’acaya, starting from 1542. this intervention also included the adaptation of the northern end of the defensive system; in fact, this sector was perhaps initially constituted by a lance-shaped bastion, then replaced by a more effective polyorcetic solution consisting of an advanced defensive-work with double bastions tenaille-shaped (nowadays called bastions of s. francis) [1]. the imposing defensive system was completed by a deep moat, about which all traces have been lost due to a subsequent filling. therefore, the main goal of the geophysical investigations, in line with the project of recovery and valorisation of the city walls, was to identify the main moat of this sector of the defensive system and other possible anomalies that could contribute to increase the knowledge of the ancient topography of the area and its transformations over the centuries. at this regard and in order to correctly interpret the data obtained from geophysical surveys, historical maps and historical aerial photographs have been acquired and studied. the aim was to reconstruct the recent history of the investigated area and to document the presence of modern structures nowadays demolished (at this regard see also the previous researches carried out in lecce [2,3]). specifically, while in the “pianta della città di lecce” drawn by michele astuti in 1882 the area immediately outside the city walls appeared as an open space, in an aerial photo of 1947 it was partially occupied by modern edifices. in particular, two buildings were immediately to the north of the double bastions and the structures of the “campo polisportivo achille starace”, built in 1924, were in the area immediately to the west of this stretch of city walls. these structures were used until 1966; afterwards, they progressively dropped in disuse and finally were dismantled to use the area, in the last decades, as a parking. the investigated area involved a stretch of about 500 m of the city walls and a surface of approximately 1.8 hectares, which was entirely covered by the gpr measurements. profiles and time slices were positioned using a differential gps (dgps) system and georeferenced on the large-scale archaeological map of lecce. 3. geophysical measurements geophysical survey was undertaken in five areas using both ground-penetrating radar (gpr) and electrical resistivity tomography (ert) techniques. 3.1. gpr data analysis for gpr measurements a ris hi mod georadar system with dual band 600 mhz-200 mhz antennae was used. the gpr data have been gathered with transect with a variable distance (0.5m and 2.0m) between any two adjacent measurement lines, and the data have been processed by making use of the gprslice code [7]. particularly, the processing has consisted in zero timing, background removal, gain variable vs. figure 1. lecce, the northern side of the sixteenth-century city walls with the bastion of st. francis. in yellow the investigated area. figure 2. lecce, area of the st. francis bastions: gpr time slices (129-144 cm depth; 200 mhz) georeferenced on 2010 google earth satellite image. figure 3. lecce, area of the st. francis bastions: gpr profiles (600 mhz antenna above, 200mhz antenna below) acquired in a perpendicular way to the walls. acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 75 the depth and kirchhoff migration [8]. the dielectric constant has been retrieved from the diffraction hyperbolas [8]. the analysis of the gpr data pointed out (figure 3): the presence of an interface, denominated “base rock”, located between 3.0 m and 4.0 m in depth; the presence of numerous anomalies that represent a cut in the bed rock; such anomalies are due to the probable presence of the limit of the moat; other reflection events could be related to the probable presence of archaeological structures. in figures 2, 4 and 6, the horizontal slices at the respective estimated depths of 129-144 cm (figure 2), 147-162 cm (figure 4), and 135-170 cm (figure 6) are represented. they show the most interesting results among those gathered in the investigated areas. figure 5. lecce, sixteenth-century city fortifications. schematic georeferencing of gpr profiles: a, sixteenth-century moat; b, medieval moat. in green the bedrock. figure 6. lecce, sixteenth-century city fortifications. gpr time slices (135170 cm depth; 200 mhz antenna) georeferenced on 2015 google earth satellite image. a, anomalies related to the stands of old stadium; b, anomalies probably related to the building of changing room of the stadium. figure 4. lecce, sixteenth-century city fortifications: gpr time slices (147-162 cm depth; 600 mhz antenna) georeferenced in large scale map of lecce. in yellow the layout of the “campo polisportivo achille starace”. figure 7. lecce, ert measured area. acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 76 3.2. ert data analysis the surveyed area is approximately 12000 m2 (figure 7). in order to investigate below the whole area, special ert arrays were used. the electrodes are distributed in such a way as to surround the area [9, 10, 11]. a dipole–dipole equatorial-parallel array was used. initially, a 2d survey is conducted along each perpendicular line or transect. in the next step, the current electrodes remain at the end of one line, while the potential is moved along the line. then, the current electrodes move one electrode position and the potential electrodes move as previously described. the process is repeated until the current and potential electrodes cover the l geometry. this sequence of observations produces a series of apparent resistivity observations towards and beneath the central portion of the array (figure 7). resistivity data were collected using a syscal kid switch device (iris instruments, france) supporting 24 electrodes with two reels of 55 m long connecting cable and 5 m maximum separation between electrodes. the electrode separation for all arrays was 2 m. four profiles, denoted ert1, ert2, ert3 and ert4, were acquired (figure 7). a penetration depth of about 6 m was obtained. after the data acquisition process was performed, the apparent resistivity data were analyzed to identify abnormal measurements with a high standard deviation. this is related to the instrument that allow to perform a number of measurements (in this case 6) and to calculate the mean value with the associated standard deviation (figure 8). the investigated volume was computed using the software ertlab (http://www.geostudiastier.it), which uses a finite elements algorithm. the true resistivity model computed has an investigation depth of 6 m, which guarantees that the inverted true resistivity model is deeper than the expected features. in order to understand the foundation depth, the analysis of 2d ert data was performed. the 2d ert data analysis evidenced the presence of a heterogeneous subsurface with resistivity values ranging from 20 to 6000 ·m. moreover, it is possible to note the presence of (figure 9): areas denoted with ‘‘a’’, with resistivity values between 100 and 800 ·m; these values indicate the probable presence of the foundation walls; areas denoted with “line foundation”, with higher resistivity values (5000-6000 ·m) that could be related to the bedrock; areas denoted with “b”, with lower resistivity values (20-30 ·m) that could be related to degraded material used to fill the moat. for 3d ert data analysis several horizontal depth slices were produced in order to observe the extension at depth of the resistivity features. figure 9 shows the slices from 1.0 to 2.5 m in depth. first, it is possible to note the presence of a heterogeneous subsurface with resistivity values ranging from 1000 to 6000 m. moreover, it is possible to note the presence of an area delimited by dashed dark lines with resistivity values between 1000 and 4000  m; these values indicate the probable presence of areas where the moat was located. figure 11 shows the slices from 1.0 to 2.5 m in depth. first, it is possible to note the presence of a heterogeneous subsurface with resistivity values ranging from 1000 to 4000 ·m. in addition, it is possible to note the presence of an area delimited by dashed dark lines with resistivity values between 1000 and 3000 ·m; these values indicate the probable presence of areas where the moat was located. geophysical investigations have allowed us to identify numerous anomalies, some closely linked to the sixteenthcentury defensive system, while others are likely to be linked to more ancient structures. the data relating to the characterization of the bedrock on which the walls were built are also very important, especially for the areas outside the two bastions and south of the north-western one. in particular, these measurements highlighted that the surface of the rocky bank was irregular and partially integrated by the foundations of the walls. 4. the sixteenth-century moat the geophysical surveys clearly documented the sixteenthcentury moat that was excavated immediately outside the city walls, along the whole investigated stretch. the major anomalies related to the moat surrounding the fortifications have been highlighted thanks to the gpr profiles, which were carried out along the entire perimeter of the city walls and with an orientation orthogonal to the fortifications. figure 8. example of bad data. figure 9. lecce, 2d ert related to ert 2 profile. http://www.geostudiastier.it/ acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 77 the measurements documented a moat between 6-8 and 1015 m large, and between 4.50 and 6 m deep, excavated in the bedrock and characterized by vertical faces (figures 3-4); it was filled with soil and blocks. it is interesting to highlight that along l. leo road, in the stretch south of the north-eastern bastion, the moat has quite the same width of the modern road; so, the modern buildings that flanked the eastern side of the road are built immediately to the east of the edge of the moat. moreover, four gpr profiles measured in the area north of the bastions of st. francis, beyond the moat, have highlighted other anomalies maybe linked to an additional moat (as frequent in the sixteenth-century fortifications systems); this area is under bastioni square and was not excavated. 5. other archaeological remains a series of other interesting anomalies was documented by gpr surveys a short distance from the western sector of city walls, in correspondence with giaconia palace. these are roughly parallel to the city walls, well visible in particular with the 600 mhz antenna, at a depth of between 1.25 and 1.60 m (figure 4). the anomalies were hypothetically attributed to an ancient structure about 6-7 m wide; subsequent archaeological excavations have clarified that this evidence was a roman road, of glareata style and contained between two side walls built with large blocks. this road was immediately out of the city of lupiae (about the urban layout of lecce in the roman period see [4,5,6]) and a stretch of it, about 140 m long, was brought to light. in the same area, some gpr profiles (figure 5) documented anomalies linked to another moat cut in the bedrock, located a few meters to the west of the sixteenth-century moat; subsequent archaeological excavations have clarified that this evidence was a medieval moat, which was 4-5 m large and partially cut the roman road. it had a direction nnw-sse quite divergent by the roman road and the sixteenth-century city walls, and at its northern end it turned in east direction; two stretches, respectively 85 m (nnw-sse orientation) and 25 m (e-w orientation) were brought to light. lastly, again in this area west of the city walls, some gpr profiles have also highlighted other anomalies more to west of the medieval moat, in a not excavated area; they may be linked to another moat (perhaps pertaining to the sixteenth-century fortification system) or ancient quarrying activities for the extraction of construction stone materials. 6. modern structures lastly, in the western investigated area, gpr surveys documented also some structures, built in the 1920s using calcareous blocks and pertaining to the “campo polisportivo achille starace”; in particular, a stretch of the eastern stand of the stadium (figure 6, a) and the building of the dressing room (figure 6, b) were identified. these structures were abandoned in 1967 and afterwards demolished. 7. conclusions the investigations presented in this paper highlight the importance of non-invasive geophysical systems for the research on cultural heritage. geophysical prospecting, in fact, was able to identify numerous anomalies in the area of the northern city walls of lecce, which were very useful to direct the archaeological excavations. in particular, the geophysical investigations have identified (i) the sixteenth-century moat outside the city walls, and the presence of (ii) a further moat and (iii) a road, which subsequent archaeological excavations dated respectively to the medieval and roman periods. even some demolished structures of the stadium built in 1920s were identified. the use of gpr method also documented the characteristics of the bedrock, highlighting that its surface is irregular, and many detected geophysical anomalies are related to human activities. for the interpretation of gpr measurements, the positioning by dgps system and the georeferencing of profiles and time slices on the large-scale archaeological map of lecce were very important. also, the study of the historical maps and documents of the 19th century and first-half of 20th century was important for the geophysical data interpretation. in fact, only the managing of all the collected and available data allows the understanding of the relations between the documented evidence and the correctly interpretation of the collected the anomalies. about the last matter, obviously the verification through archaeological excavations, which allow precise interpretation and dating, was very important. references [1] v. cazzato, m.fagiolo, “lecce. architettura e storia urbana”, congedo, galatina, 2013. [2] g. di giacomo, l. de giorgi, i. ditaranto, g. leucci, i. miccoli, g. scardozzi, “integration of urban archaeology and geophysical prospecting: the research on the public monumental complexes figure 10. lecce, 3d ert related to ert1, ert2 and ert3 profiles. figure 11. lecce, 3d ert related to ert4 profile. acta imeko | www.imeko.org october 2018 | volume 7 | number 3 | 78 of lupiae (lecce)”, in proceedings of the 1st imeko international conference, 2015, pp. 521-526. [3] g. di giacomo, l. de giorgi, i. ditaranto, g. leucci, i. miccoli, g. scardozzi, the contribution of geophysical prospecting to the reconstruction of the ancient topography of the messapian necropoleis in lecce, in proceedings of the 1st imeko international conference, 2015, pp. 527-533). [4] l. giardino, per una definizione delle trasformazioni urbanistiche di un centro antico attraverso lo studio delle necropoli: il caso di lupiae, in studi di antichità, 7, pp. 137-203, galatina, 1994. [5] l. giardino, “lecce-lupiae”, in a. pranzo (ed.) salento. architetture antichi e siti archeologici. edizioni del grifo, lecce, 2008, pp. 53-56. [6] l. giardino, n. lonoce, le necropoli ad incinerazione di età romana di lecce e la forma urbana di lupiae, in m. lombardo, s. alessandrì (eds.), antiquitas. scritti di storia antica in onore di salvatore alessandrì. congedo, galatina, 2011, pp. 119-145. [7] d. goodman, gpr slice version 7.0 manual, 2013, available online at http://www.gprsurvey. [8] g. leucci, 2015. geofisica applicata all’archeologia e ai beni monumentali. dario faccovio editore palermo, pp 368. [9] g. chávez, a. tejero, m. a. alcantara, r. e. chavez, the ‘larray’, a tool to characterize a fracture pattern in an urban zone: extended abstracts of the 2011 near surface geophysics meeting, european section meeting (printed in cd), leicester, uk (2011). [10] d. argote-espino, a. tejero-andrade, g. cifuentes-nava, l. iriarte, s. farías, r. e. chávez, f. lópez, 3d electrical prospection in the archaeological site el pahñu, hidalgo state, central mexico: journal of archaeological science, 40 (2013), pp. 1213-1223. [11] a. tejero-andrade, g. cifuentes, r. e. chavez, a. lopezgonzalez, c. delgado-solorzano, 2015. land corner-arrays for 3d electrical resistivity tomography: an alternative for urban zones. near surf. geophys. 13, 355e367. [12] g. di giacomo, l. de giorgi, i. ditaranto, g. leucci, i. miccoli, g. scardozzi. 2017. geophysical surveys for the study and reconstruction of the sixteenth-century fortifications of lecce. proceedings of 3rd imeko international conference on metrology for archaeology and cultural heritage, 369-373, isbn: 978-92-990084-0-9. http://www.gprsurvey/ error sources in the force mode of the 'pb2' planck-balance acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 53 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 53 57 error sources in the force mode of the “pb2” planckbalance n. rogge1, c. rothleitner2, s. lin2, s. vasilyan1, t. fröhlich1, f. härtig2, d. knopf2 1 technische universität ilmenau, 98693 ilmenau, germany, norbert.rogge@tu-ilmenau.de 2 physikalisch-technische bundesanstalt (ptb), 38116 braunschweig, germany, christian.rothleitner@ptb.de abstract: the pb2 planck-balance is a table-top kibble balance, that is designed for the calibration of class e2 weights in a range of 1 mg up to 100 g. this work presents typical systematic errors which have to be considered during the calibration and will show results for measurements with small masses. keywords: kibble balance; measurement uncertainty; mass calibration; instrument adjustment 1. introduction kibble balance experiments were originally designed for the determination of planck constant and played an important role in the redefinition of the si unit kilogram, which entered into force in may 2019. today, they represent one of the two methods to achieve the lowest uncertainties for mass calibrations of 1 kg. nonetheless, the working principle of a kibble balance can also be used in smaller, less expensive and less complex table-top set-ups, when the user does not need to achieve higher accuracy. in the planck-balance (pb) project, two systems were developed in order to calibrate class e1 and e2 weights, where the system for the calibration of e2 is named “pb2” and is depicted in figure 1. figure 1: the pb2 set-up: balance and interferometer (right) and signal processing system (left). the calibration of weights is performed in two modes: velocity mode and force mode. in the velocity mode, the coil under test is moved sinusoidally relative to the magnetic field of the actuator with a frequency of typically 4 hz and an amplitude of 20 µm. from the ratio of coil velocity 𝑣 and induced voltage 𝑈ind the geometric factor 𝐵𝑙, which is the product of the magnetic flux density 𝐵 and the wire length 𝑙, can be determined. this factor can also be derived from the ratio of the gravitational force 𝐹g from a weight and the compensation current 𝐼 that is necessary to counterbalance the weight with an equal force generated by the actuator (electromagnetic force), which corresponds to the calibration with the use of a calibrated mass (force mode). this procedure of force equilibration is similar to the common calibration process of an electromagnetic force compensation (emfc) balance. in a kibble balance, both modes are combined in order to determine the mass of an unknown weight. however, this work that focuses on errors sources in the force mode, the results of the force mode measurements are presented as determinations of 𝐵𝑙 in order to be less dependent on the results of the velocity mode and to differentiate between the errors that occur in the different modes. similar to the existing emfc balances, the alignment along the gravitational acceleration g and air buoyancy has to be considered in the measurement during the force mode. due to this different calibration method compared to a calibration of the compensating actuator with mass standards, additional effects like the alignment along velocity mode trajectory, coil current effect and coil displacement need to be taken into account. 2. measurement set-up the pb2 system [1] consists of a commercially available emfc balance, an interferometer, a resistor and two digital multimeters (dmms) of type keysight 3458a-002. the dmms are regularly calibrated against a programmable josephson http://www.imeko.org/ mailto:norbert.rogge@tu-ilmenau.de mailto:christian.rothleitner@ptb.de acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 54 voltage standard that is available in pb2 laboratory, while the shunt resistor of type vishay vha-518 was calibrated at physikalisch-technische bundesanstalt (ptb). all these individual components form the pb2 system and its measurement infrastructure are based on a modular principle, so that upon necessity the procedures of replacements or the integration of additional devices are maximally simplified. for the purpose of characterisation of the system components, additional multimeters and current sources are available in the set-up that is shown in figure 1. an additional voice coil actuator (measurement actuator) that is attached to the load carrier of the emfc balance forming a mechanically rigid connection generates the compensation force in the force mode and is characterised in the velocity mode. the internal actuator (drive actuator) is utilised to excite the coil of the measurement actuator in the velocity mode and is connected to a current source of type hp 3245 in force mode in order to produce a constant force offset that is necessary for the reduction of the coil current effect that is described in section 3. furthermore, a mirror is attached to the load carrier, similarly forming a mechanically rigid connection, in order to determine the displacement that is used to calculate the velocity of the mirror and the measurement coil. for the sake of an accurate velocity and voltage measurement, a stable time base is required. the acquisition of voltage and displacement is triggered by a signal generator which is supplied with traceable time base by a gps-disciplined oven-stabilised quartz oscillator. figure 2: schematic of the pb2 set-up. in contrast to a normal usage of an emfc balance, the compensation force is generated by an actuator placed on the same side of the lever as the mass under test as shown in figure 2. in the pb2 set-up, this mass can be lifted and lowered remotely by an automated weight lifter to minimise disturbances caused by an operator of the device. furthermore, a vacuum compatible piezo-based linear motor of type n-310 by physik instrumente is utilized in the weight lifter system that allows the pb2 system to operate in vacuum. due to its working principle of a walking motion of alternatingly deflected piezo actuators, it does only generate mechanical noise during the active lifting and lowering process and not during the measurement when a compensation current is applied. additionally, it does not generate magnetic fields that could disturb the measurement actuator. the weighing pan is designed as a fork structure as shown in figure 3, while a fitting fork structure is used to lift the weight under test. the weight lifter is placed on a plate that can be adjusted in six degrees of freedom to allow a small gap between the forks without the risk of contact between them. this weight lifter design enables the investigation of a wide range of sizes and shapes of the mass standards including wire weights. figure 3: cad model of the weighing cell mechanics and the weighing pan with fork structure. the compensation current in the force mode is provided by a pid controller that is implemented on a digital signal processing system (dsp). the dsp is augmented by a power amplifier that drives the coil current of the measurement actuator in the force mode and a transimpedance amplifier circuit that converts the photo currents of the balance’s internal position sensor into a voltage signal that is utilized to indicate the successful compensation of the external force. since the dsp that is currently used, does not allow the use of an external clock signal, the closed loop frequency of the implemented control algorithm is measured by a frequency counter that is connected to the reference clock. this provides a traceable time base that is crucial for the velocity mode. since the measurements with the pb2 system are mostly performed in air, the air buoyancy of the weight needs to be corrected as described in oiml r 111-1. for this purpose, the density of the air has to be determined, which requires the measurement of air temperature, humidity and pressure. for this purpose, a calibrated climate module of type sartorius ycm20mc is included in the pb2 system. http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 55 3. coil current effect the necessary compensation current in the force mode is determined as the difference of the compensation currents of the loaded and the unloaded state of the balance. since the coil current affects the magnetic field of the actuator, this current difference depends on the absolute values of both compensation currents [2]. in order to decrease the error that is caused by the coil current effect, the lever of the emfc is balanced such, that the compensation currents in both loaded and unloaded states yield the same absolute value, but different polarity. this is ensured in the pb2 set-up by feeding a constant current 𝐼𝑖 through the internal actuator of the emfc balance. therefore, the balancing of the lever can be done quickly and automatically for any arbitrary value of the test mass. to adapt this internal offset current 𝐼𝑖 to the changing 𝐵𝑙 of the internal drive actuator, an adjustment procedure is performed regularly. a small current δ𝐼𝑖 is added to the previously used offset current 𝐼𝑖0 and for both currents through the drive actuator, the differences ∆𝐼𝑚 of the absolute values of the necessary compensation current in loaded (𝐼𝑚+) and unloaded (𝐼𝑚−) state are determined: ∆𝐼𝑚 = |𝐼𝑚+| − |𝐼𝑚−| . (1) from these measurements the new internal current 𝐼𝑖 is calculated by linear interpolation to obtain a vanishing ∆𝐼𝑚: 𝐼𝑖 = 𝐼𝑖0 + ∆𝐼𝑚(𝐼𝑖0)∙δ𝐼𝑖 ∆𝐼𝑚(𝐼𝑖0)−∆𝐼𝑚(𝐼𝑖0+δ𝐼𝑖) . (2) the procedure is repeated before a new set of force mode measurements is taken to compensate for the temperature sensitivity and the general drift of the drive actuator. 4. magnetic profile for the calibration of mass standards with a kibble balance, the 𝐵𝑙 that is determined in velocity mode is used to convert the current difference in loaded and unloaded state into a gravitational force. to achieve a low combined uncertainty, it needs to be ensured that the position of the coil in loaded and unloaded state is the same as the position for which the 𝐵𝑙 was determined in the velocity mode. assuming a given uncertainty of the coil positioning, the error that is caused by this effect can be reduced by adjusting the coil position to be in apex of the magnetic flux density of the magnet system. the current pb2 system does not provide the possibility to adjust the immersion depth of the coil and relies on the manufacturing tolerances of the voice coil actuator and the coil mounting. therefore, the actual profile was acquired experimentally by determining the geometric factor 𝐵𝑙 at different coil positions. as shown in figure 4, the apex was not exactly met and the 𝐵𝑙 changes with approximately 0.5 ppm/µm, which is sufficient for a comparison of the 𝐵𝑙 determined in force and velocity mode respectively, since the repeatability of the coil positioning is in the order of nanometres. the shown profile was determined in force mode while changing the set point position of the controller. to reduce a change of the profile due to the coil current effect, the symmetry of compensation currents was adjusted for each position considering the different restoring force of the differently displaced balance mechanics. figure 4: relative change of 𝐵𝑙 depending on coil position. 5. alignment in contrast to the calibration of a standard emfc balance, the alignment error cannot be compensated by having the same angle of misalignment during the calibration with mass standard and weighing process. therefore, the force measurement system needs to be aligned parallel to the vector of free-fall acceleration 𝑔 . for alignment purposes, mirror cuboids that possess a reflective coating on all sides are attached to the load carrier of the balance (figure 5). the angles between the surfaces of these optical alignment cuboids are measured and well known. in accordance to the calibration certificate they agree better than 21.8 µrad with a right angle. during the alignment process, a second interferometer is used to measure the orthogonal displacement of the side of the mirror cuboid, thus measuring the horizontal displacement, while the load carrier is driven up and down by an excitation of the lever. the interferometer beams need to be aligned to better than 0.4 mrad so that the interferometers work properly. from the ratio of horizontal to vertical displacement measured by the first interferometer, the trajectory of the guiding parallel lever system and its misalignment can be identified. http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 56 figure 5: schematic of the alignment axes with an alignment cuboid. a measurement of the horizontal displacement is shown in figure 6, which sums up a set of six repeated measurements of both horizontal axes in x and z-direction. the shown horizontal displacements correspond to an angular misalignment of 2.8 mrad and 2.2 mrad, respectively. the resulting cosine errors of 3.9 ppm and 2.4 ppm of the determination of the displacement amplitude are therefore well known and can be corrected during the data analysis. this procedure needs to be repeated every time the angle between the trajectory of the guided coil and the normal vector changes, which occurs when the balance or the mirrors are temporarily unscrewed during redesign and readjustment of the system. since this is done regularly in the recent devolvement process of pb2, there are no investigations on the long-term stability of these angles yet. an additional horizontal displacement of about maximum 6 nm occurs due to the parallel lever system guides the load carrier on a circular trajectory. the horizontal velocity caused by this effect, which generates an additional induced voltage, occurs mainly at the maximum and minimum of the sinusoidal motion and is close to zero when the vertical velocity is at its maximum and minimum. therefore, the voltage that is induced due to this horizontal motion contributes only a small error to the determination of the amplitude of the sinusoidal induced voltage caused by vertical motion and is rejected by the sine fitting of the data. figure 6: horizontal displacement of the load carrier. with an autocollimator and the reflective surface of a liquid pool, the normal vector of the top surface of the mirror cuboid can be adjusted relative to 𝑔. the water pool is only present during the adjustment procedure and regular checks of the mirror orientation. since the orientation of the trajectory relative to the mirror surface is known from the horizontal displacement, the trajectory adjustment follows from the adjustment of the top surface. 6. results the determination of 𝐵𝑙 in force and velocity mode was performed with several different e1 mass standards in a range from 1 mg to 50 g. the systematic deviation relative to the result with a calibrated 20 g standard as well as the statistical uncertainty over time period of 8 hours are shown in figure 7. the statistical uncertainty is composed of the relative experimental standard deviations of the mean of the determinations of 𝐵𝑙 in velocity mode and force mode. while the contribution to the statistical uncertainty of the velocity mode experiment is independent from the weight under test, the relative standard deviation increases for smaller mass values. the maximum permissible error (mpe) of e1 and e2 standard weights according to oiml r-111 is also shown for comparison. compared to the mpe of e2 standards, the results are quite satisfying for the higher nominal values. however, for the smallest standards, there are deviations that need to be identified in future investigations. http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 57 figure 7: deviation between force and velocity mode. 7. summary and outlook the main error sources in the force mode of the pb2 system were identified and a good agreement of force and velocity mode was achieved for most of the measurement range. future work will aim at identifying the deviations in small mass range and improving the statistical part of the uncertainty. this can be achieved by improving the controller hardware that is generating a significant fraction of the noise on the compensation current due to its resolution of 16 bit for adc and dac. a signal processing system based on a combination of microprocessors, a fpga chip and 24 bit delta-sigma adc is currently under development. in the range of masses greater than 50 g, the system performance is limited by nonlinear deformation phenomena that are subject to ongoing research. further investigations are also planned regarding the long-term stability of the mechanical system and its adjustment status. acknowledgement the research of this project is funded via the program “validierung des technologischen und gesellschaftlichen innovationspotenzials – vip+”, a program of the german federal ministry of education and research (bmbf), and is managed by vdi/vde innovation + technik gmbh. the authors thank also l. günther, j. schleichert und m. spoors for fruitful discussions. furthermore, the authors want to acknowledge the help of working group 1.11 of ptb for the calibration of mass standards and working group 2.61 for the calibration of standard resistors. 8. references [1] n. rogge, s. lin, c. rothleitner, s. vasilyan, “excitation frequency dependent deviations during the “velocity mode” of bl measurements in the planck-balance”, in proc. of 23rd imeko tc4 international symposium, xi’an, china, 2019. [2] s. li, f. bielsa, m. stock, a. kiss, h. fang, “coil-current effect in kibble balances: analysis, measurement, and optimization”, metrologia, vol. 55, pp. 75-83, 2018 http://www.imeko.org/ a sensor data fusion-based locating method for large-scale metrology acta imeko issn: 2221-870x december 2020, volume 9, number 4, 136 143 acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 136 a sensor data fusion-based locating method for large-scale metrology andrea rega1, ferdinando vitolo2, stanislao patalano2, salvatore gerbino3 1 department of neurosciences, reproductive and odontostomatological sciences, university of naples ‘federico ii’, naples italy 2 department of industrial engineering, university of naples ‘federico ii’, naples, italy 3 department of engineering, university of campania ‘luigi vanvitelli’, aversa, italy section: research paper keywords: large-scale metrology; sensor data fusion; position measurement; handling device; product design citation: andrea rega, ferdinando vitolo, stanislao patalano, salvatore gerbino, a sensor data fusion-based locating method for large-scale metrology, acta imeko, vol. 9, no. 4, article 18, december 2020, identifier: imeko-acta-09 (2020)-04-18 section editor: francesco bonavolonta, university of naples federico ii, italy received october 31, 2019; in final form may 15, 2020; published december 2020 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was developed with the economic support of miur (italian ministry of universities and research) corresponding author: andrea rega, e-mail: andrea.rega@unina.it 1. introduction the measurement of geometric deviations in the manufacturing of large-sized objects is a critical issue. variations are difficult to measure, and they have a significant impact on the quality of the assembly and the reliability of the final product. one of the most efficient ways to identify dimensional and geometric variations in this scenario is to compare the nominal product with a digitalisation of the real product. in order to achieve this objective, a reverse engineering (re) process has to be used. reverse engineering of geometric models [1] is widely used not only in the industrial field [2] but also in the fields of architecture [3], archaeology and cultural heritage [4], topography [5], food [6] and medicine [7]-[9]. this process aims to obtain a digital mock-up (dmu) [10] of the real products and components, playing a key role in their inspection [11]. the dmu of real objects is very useful for quality control, the detection of defects [12] and performance evaluation [12]-[16], and it is supported by virtual reality (vr) for virtual maintenance applications [17],[18]. in the context of large-sized objects, the digitalisation process requires multiple acquisitions from different locations followed by the realignment of the point clouds. the larger the object size, the higher the number of acquisitions required, and each acquisition is affected by a locating error. it follows that the realignment phase has a huge impact in this context. therefore, the point-cloud realignment step is crucial to control form errors [19]. there are three main steps in the point-cloud realignment process: (i) the operator realigns the point clouds by using common features among the acquisitions. this technique works well if the objects are rich in features, but the operator’s postprocessing phase is long; (ii) coded or non-coded markers/targets are placed on the product and in its surrounding space, and they are precisely recognised and used to find a abstract the measurement of geometric and dimensional variations in the context of large-sized products is a complex operation. one of the most efficient ways to identify deviations is by comparing the nominal object with a digitalisation of the real object through a reverse engineering process. the accurate digitalisation of large geometric models usually requires multiple acquisitions from different acquiring locations; the acquired point clouds must then be correctly aligned in the 3d digital environment. the identification of the exact scanning location is crucial to correctly realign point clouds and generate an accurate 3d cad model. to achieve this, an acquisition method based on the use of a handling device is proposed that enhances reverse engineering scanning systems and is able to self-locate. the present paper tackles the device’s locating problem by using sensor data fusion based on a kalman filter. the method was first simulated in a matlab environment; a prototype was then designed and developed using low-cost hardware. tests on the sensor data fusion have shown a locating accuracy better than that of each individual sensor. despite the low-cost hardware, the results are encouraging and open to future improvements. mailto:andrea.rega@unina.it acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 137 correspondence between the acquisitions. this method also requires user actions; (iii) the reference system of the acquisition device is tracked, enabling the automatic realignment of the point clouds without user actions. currently, the point-cloud realignment process is mostly manual, which takes a long time and requires skilled users. the market offers several acquisition devices with different operational characteristics in terms of speed and accuracy. the most common categories are based on point-by-point and fullfield technologies [20]-[22]. point-by-point technology entails high accuracy acquisitions but also high acquisition time. conversely, full-field systems acquire large and complex geometries quickly but with lower accuracy. the leica t-scan 5 is an example of a commercial hand-held scanner device. it is used for high-precision measurements in the industrial field. the scanner stands out for its ability to acquire a large number of points within an area, guaranteeing a good resolution [23]. the leica t-scan 5 needs an external absolute tracker to measure the device’s coordinates. maxshot 3d by creaform is a portable coordinate optical measuring system especially used for the acquisition of large parts. it is based on photogrammetry technology and it guarantees an accuracy of 0.025 mm/m for parts between 2 and 10 m long [24]. moreover, the device provides a real-time position display, helping the users and improving the efficiency of the process. however, the requirement for markers to be placed on the target model and the acquisition volume, together with the requirement for user action, are still limiting factors. therefore, these and similar commercial solutions restrict their application to environments where a human presence is allowed. the tracking of the acquisition device’s reference system could be a good strategy for overcoming the hand-held limitations and to acquire large and featureless geometries with a high level of accuracy. in the aeronautical field, for example, fuselage inspection could be significantly improved by increasingly using the comparison between the nominal dmu and the digitalised models. nevertheless, acquisition is a critical process because the object is large in size, it does not have enough different features and it is important to ensure there is no interaction with its surfaces. digitalisation could be performed by treating the fuselage as though it were a tube and acquiring it section by section by means of a scanning device able to move autonomously inside it along a path collinear with its axis. according to the exact location of the device reference system, the acquired section can then be aligned in the 3d digital environment. establishing the exact location of the device reference system in relation to the global reference system, for each acquisition, is still a challenge. a. paoli and a.v. razionale [25] tackle the measurement of large hulls by means of a six-axis robotic arm equipped with an optical scanner as an end effector. the robot can be translated onto a horizontal rail 12 m long and a vertical rail 3 m long. the robotic system, combined with an industrial laser station (leica tda 5006), ensures the alignment of the point clouds by tracking the position of the tool centre point (tcp) in the global reference frame. the need for a careful setting for the position of the laser station and the large size of the hulls are the major limitations of this system. moreover, it requires object movement to perform the full acquisition. barone et al. [26] propose a methodology to automatically align point clouds in a 3d environment. they have developed a system that integrates a full-field optical scanner and a stereo vision system. the stereo vision system tracks the optical scanner by means of retro-reflective infrared markers rigidly fixed to the scanner. however, the system operates within a limited working volume due to the tracking system. the performance of this system is linked to the visibility of the markers and to the camera view. hui du et al. [27] have developed a large-scale 3d scanning system that combines a six-axis robotic arm, a laser tracker and a binocular-structured light scanner. the robot, with the scanner as the end effector, moves through the planned discrete positions allowing the scanner to acquire the 3d point clouds of the corresponding regions. this system is more advanced than the one described in [25], but it still requires a laser station to track the scanner coordinates in the global reference system. jinlong shi et al. [28] propose a method to measure large-sized objects based on the integration of a 3d scanner with a laser rangefinder. the scanner is assembled on a robotic arm that is able to slide along a guide rail (figure 1). the partial sections of the target object are acquired consecutively by the scanner, which, together with the robotic arm, moves forward on the guide rail. the laser rangefinder, rigidly fixed on the scanner, locates its position in the global frame. the solutions described above have clear drawbacks: (i) the tracking space is limited and usually requires additional components (e.g. a laser station), (ii) all systems are characterised by their large size and (iii) they have been developed mainly to perform external shape acquisition. accordingly, the present paper proposes an acquisition method based on an autonomous, small-sized device able to selflocate. the aim of the device is to carry a scanning system for model digitalisation in order to enable an easier change of position and an inner surface acquisition. the device was first designed, and then its performance was simulated [29]. following this, a low-cost prototype was developed [30]. it addresses the locating problem by using sensor data fusion (sdf) [31] based on a kalman filter [32],[33]. the kalman filter is a very common method for addressing navigation problems [34]. in the context of autonomous systems, the filter is commonly used to combine sensor data in order to obtain a more accurate measurement compared to the original one from a single sensor [35]. the key idea is to develop a redundant measurement system to minimise the location error of the handling device, thus improving the accuracy of the digital form. furthermore, the handling device removes the need for a skilled operator, facilitates inner surface acquisition and makes the acquisition field potentially unlimited. figure 1. (a) hardware architecture proposed by jinlong shi et al. [28]; (b) experimental system. acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 138 in the rest of the paper, section 2 introduces the problem formulation and section 3 presents the locating method. section 4 then presents the results of the hardware-in-the-loop simulation of the proposed method, while section 5 presents the hardware of the developed prototype. the validation is described and discussed in section 6. finally, conclusions are given in section 7. 2. problem formulation in the context described above, the re system is composed of two subsystems: the 3d acquisition system and the handling device. the first moves together with the handling device during the acquisition process. thus, the acquisition system acquires the point clouds in its reference frame, and the handling device defines the relative position between the acquiring reference frame and the global reference. consequently, the final realignment quality is directly related to the self-locating accuracy of the proposed handling device. given an acquired point 𝑷, its location is related to the scanning reference 𝐎𝟏 by means of the p 1 vector. the location of the point 𝑷 is related to the global reference 𝐎0 by means of the p0 vector. the scanning reference 𝐎𝟏 is related to the global reference 𝐎0 by means of the 𝐨𝟏 𝟎 vector (figure 2). if 𝐑𝟏 𝟎 is the rotation matrix of the scanning reference related to the global reference, the position of point 𝑷 in relation to the global reference can be expressed as 𝑝0 = 𝑜1 0 + 𝑅1 0𝑝1. (1) considering movements in a cartesian plane in the direction of the y-axis, equation (1) can be written as follows: { 𝑝𝑥0 0 𝑝𝑦0 0 𝑝𝑧0 0 1 } = [ 0 −1 0 0 1 0 0 𝑜𝑦0 0 0 0 1 0 0 0 0 1 ] { 𝑞𝑥1 1 𝑝𝑦1 1 𝑝𝑧1 1 1 } , (2) where the homogeneous (4 x 4) transformation matrix refers to the scanning reference 𝐎𝟏 with respect to the global reference 𝐎0. the proposed method aims to accurately estimate 𝒐𝒚𝟎 𝟎 , which corresponds to the device’s displacement along the y-axis. 3. locating method the proposed method adopts the sdf, based on the kalman filter, to reduce the positioning error of the handling device using multiple and redundant measurements [24]. the proposed acquisition process is depicted in figure 3. it is composed of four steps: (s1) sub-task definition, which splits the main task by using evenly spaced movement steps, according to the scanner operation parameters. the computed step movement is assumed as a sub-task target; (s2) the actuator system is turned on until the sub-task’s target is reached; (s3) estimation of the current system location using sensor data fusion, which evaluates the locating error; (s4) corrective action to move towards the target. the locating method based on the sdf is the core of the proposed process: in step 3, two different kinds of sensor carry out a location measurement. the sensors return different values of the same quantity due to their difference in accuracy and different technology. the signals generated are fused to reduce the error of each individual measurement. in this specific case, the device is equipped with incremental encoders and an ultrasonic sensor. therefore, it is able to predict location using the odometry model and the encoders’ data, and then it updates the measurement by fusing the ultrasonic sensor’s data. 3.1. odometry model the re system position is represented in the global reference 𝐎0 by its central point 𝐎𝟏 with the cartesian coordinates (𝑥1,𝑡,𝑦1,𝑡) at time t. the local coordinates system 𝐎𝟏 is fixed to the device. therefore, the device’s location at step 𝑡 + 1 is described by the following odometrical model: 𝑥0,𝑡+1 = 𝑥0,𝑡 + 𝛿𝜌0,𝑡 ∙ cos𝜃0,𝑡 𝑦0,𝑡+1 = 𝑦0,𝑡 + 𝛿𝜌0,𝑡 ∙ sin𝜃0,𝑡 𝜃0,𝑡+1 = 𝜃0,𝑡 + 𝛿𝜃0,𝑡, (3) where 𝛿𝜌0,𝑡 is the movement length and 𝛿𝜃0,𝑡 the elementary rotation from state 𝑡 to 𝑡 + 1. assuming there is no wheel slippage, then figure 2. reference system. figure 3. proposed acquisition process. acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 139 ∆𝛿𝜌0,𝑡 = ∆𝑈𝐿𝑡 + ∆𝑈𝑅𝑡 2 ∆𝜃0,𝑡 = ∆𝑈𝐿𝑡 + ∆𝑈𝑅𝑡 𝑏 . (4) ∆𝑈𝐿𝑡 and ∆𝑈𝑅𝑡 are calculated using the right and left encoders, respectively. with reference to equation (2), and assuming a gaussian distribution of the measurement noise, the odometry model can be written in the following compact form: o1,𝑡+1 = 𝑓(o1,𝑡,𝛿𝜌0,𝑡) + 𝑛𝜌,𝑡, (5) where o1 = [0,𝑦1,𝑡] and 𝑛𝜌,𝑡 denote the system noise that models the uncertainties of the odometry model. 3.2. measurement model the measurement of the ultrasonic sensor is 𝑑𝑟, which is the distance between the marker with a known position (𝑥0,𝑚,𝑦0,𝑚) and the ultrasonic sensor’s central point. the measurement model is given by 𝑧𝑡 = [ 𝑥0,𝑚 − 𝑑cos𝜃0 𝑦0,𝑚 − 𝑑sin𝜃0 ], (6) where 𝑑 = 𝑑𝑟 + 𝐿 is the distance between the marker position and the central point of 𝐎𝟏 fixed to the re system. equation (6) defines the measurement model, which can be written in the following compact form (including noise): 𝑧𝑡 = ℎ(o1,𝑡) + 𝜈𝑡, (7) where 𝜈𝑡 is considered to be the zero-mean white noise. 3.3. position estimate at time 𝑡𝑖, the location measurements 𝑧1𝑖 and 𝑧2,𝑖 come from the encoder and ultrasonic sensor, respectively. the estimated location depends on the previous estimate at time 𝑡𝑖−1 and the new measurements, which are affected by an uncertainty expressed as the standard deviation 𝜎𝑧1 and 𝜎𝑧2. the predicted location 𝑦(𝑡𝑖) is expressed as follows: 𝑦0(𝑡𝑖) = �̂�0(𝑡𝑖−1) + 𝑧1,𝑖 𝜎𝑦0 2 (𝑡𝑖) = 𝜎𝑦0 2 (𝑡𝑖−1) + 𝜎𝑧1 2 . (8) in the updating stage, the ultrasonic sensor conducts the measurements, and, therefore, the estimated location �̂�(𝑡𝑖) is expressed as follows: �̂�0(𝑡𝑖) = 𝑦(𝑡𝑖) + 𝐾(𝑡𝑖)[𝑧2,𝑖 − 𝑦(𝑡𝑖)], (9) with 𝐾(𝑡𝑖) = 𝜎𝑦0 2 (𝑡𝑖) 𝜎𝑦0 2 (𝑡𝑖) + 𝜎𝑧2 2 ; 𝜎𝑦0 2 (𝑡𝑖) = 𝜎𝑦0 2 (𝑡𝑖−1) + 𝜎𝑧1 2 , (10) where 𝐾(𝑡𝑖) is the kalman gain. in the re context, the kalman filter improves the alignment of the point clouds by minimising the mean square error of the estimated locations. 4. simulated results a hardware-in-the-loop (hil) simulation has been used to test the proposed method. an arduino board was programmed in a matlab/simulink® environment, and random signals with gaussian distribution and a zero mean were used to simulate the sensors. the simulation-ending condition was set at 1000 mm, whereas the forward step was set at 100 mm. the mean of the generating values for 20 iterations is shown in table 1. figure 4 depicts the comparison of errors. the errors of the ultrasonic sensor are represented by the red line; the encoder errors, estimated as the difference between the average of the encoder’s signal and the target value, is represented by the blue line; the errors estimated by sensor data fusion are represented by the orange line. the estimated location is very close to the target. the average error obtained by means of the sensor data fusion method is lower than the errors from the single simulated sensors. the total measurement error has been reduced by up to ten times using sdf. the following section describes the prototype built to validate the results obtained from the hil simulation. 5. the handling device the structure of the prototype (figure 5) is composed of three main parts: the sensors, motors with a motor control and a microcontroller board. 5.1. sensors the measurement system used by the device to self-locate is composed of two incremental encoders and an ultrasonic sensor module. table 1: measurements by means of the hil simulation (mm). step encoders ultrasonic sensor sdf target 1 100,024 100,105 100,085 100,000 2 199,985 200,042 200,031 200,000 3 299,874 300,112 300,065 300,000 4 399,880 400,031 400,002 400,000 5 499,884 500,078 500,040 500,000 6 599,982 600,074 600,056 600,000 7 700,009 700,104 700,085 700,000 8 800,023 800,045 800,040 800,000 9 900,112 899,798 899,859 900,000 10 999,735 999,967 999,922 1000,000 figure 4. comparison between the errors of the generating signals (rotary and ultrasonic sensor) and the estimated position, using sensor data fusion. acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 140 the ultrasonic sensor module, hc-sr04 (see table 2), incorporates a transmitter, a receiver and a control circuit. it is mounted on the front of the device’s chassis and is used to measure the distance between itself and a reference fixed in a known position. the measurement is based on quantifying the reflection time of the ultrasonic waves (40 khz). thus, a piezoelectric transducer emits ultrasonic waves that are reflected when they detect an object. the reflection waves, called echoes, are bounced back to the receiver. the built-in microcontroller then calculates the time interval between the sending of the signals and the returning echoes in order to determine the distance to the reference. the incremental encoders are used to measure the angle rotation of the motor shafts. the linear displacement of the device is then calculated by means of a conversion based on the wheels’ radius and the gear ratio. the encoders mounted on the proposed device (see table 3) are composed of two main parts: a board with a double-channel hall-effect sensor and a six-pole magnetic disc. the magnetic disc is fitted directly onto the motor shaft and moves with it. the board, fixed in front of the magnet, senses the variation in the magnetic flow resulting from the spinning disc, providing 12 counts per revolution of the gearmotor shaft. 5.2. motors and motor control the handling device is equipped with two miniature highpower, 6 v brushed dc motors (see table 4). the motors are characterised by a rear shaft to mount the encoders. the cross section is 10 x 12 mm, and the output shaft is 9 mm long and 3 mm in diameter with a d shape. the dc motors are controlled by an adafruit motor shield v 2.3. the employed shield has a fully dedicated pwm driver chip onboard. all the motor and speed controls are handled by this chip through the 12 c protocol, leaving the arduino interrupt pins available to control the encoders. in fact, to drive multiple motors, only the sda and scl data pins and the gnd and vcc power pins are required. 5.3. microcontroller board the device’s equipment is controlled by an arduino uno rev3 board. it can interact with several commercial off-the-shelf (cots) components, offering various possibilities, with applications for educational projects [36], process control [37],[38], networked control [39] and in the robotic field [40],[41]. the selected board is based on the atmega328p microprocessor. it includes six analogue pins and 14 digital input/output pins, of which six can be used as pwm outputs. it includes a 16 mhz quartz crystal, a usb port for easy connectivity, a power jack, an icps header and a reset button. the control code of the device has been written in a matlab/simulink® environment. programming is made easier by the matlab/simulink® support package for arduino® hardware. it enables the use of matlab to programme the arduino board and to control the hardware using custom addon libraries directly from matlab. the device’s chassis was manufactured in acrylonitrile butadiene styrene (abs) through fused deposition modelling (fdm) technology. the chassis is composed of a bottom plate and a top plate. the control unit, composed of the arduino board and the motor-shield, is mounted on the top plate together with the ultrasonic sensor, which is placed on the front. the two motor assemblies, composed of an encoder, dc motor and wheel, are mounted on the bottom plate. two rail-wheels are table 2: hc-sr04 datasheet. property value operating voltage 5 v operating current < 15 ma operating frequency 40 khz max measuring distance 4500 mm min measuring distance 20 mm accuracy 3 mm measuring angle 15° trigger input signal trigger 10 µs pulse ttl echo output signal echo signal pwm ttl dimensions 45.5 x 20.5 x 15.3 mm table 3: rotary encoder datasheet. property value operating voltage 2.7 18 v count per revolution 12 weight (board + disc) 1 g dimension (board) 10.6 x 11.6 mm table 4: gearmotor datasheet. property value operating voltage 6 v weight 9.5 g shaft diameter 3 mm gear ratio 100:1 no load speed @6 v 320 rpm no load current @6 v 120 ma stall current @6 v 1600 ma stall torque @6 v 2.2 kg*cm figure 5. prototype of the device. acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 141 assembled in their housings on the front and on the rear of the bottom plate. the device is equipped with a separate power supply: a 9 v battery placed on the top plate to power the sensors and the control unit and a 6 v battery pack placed on the bottom plate to power the dc motors (figure 6). 6. validation the prototype was used to validate the simulated results. as with the simulation, we have set the step forward at 100 mm and the device stroke at a meter. the device reference has been set at the centre of the wheels’ axis. the ultrasonic sensor was calibrated taking the laboratory temperature of 25 °c into account and using the adjusted speed of sound. to evaluate the accuracy of the measurements, a calibrate rail has been used. measurement values for each step are shown in table 5. figure 7 depicts a comparison of the errors: the errors of the ultrasonic sensor are represented by the red line; the errors of the encoders, estimated as the difference between the average of the two encoders’ measurements and the target value, are represented by the blue line; and the errors estimated by sensor data fusion are represented by the orange line. the errors related to the ultrasonic sensor measurement display a high level of variation with an average error greater than 5 mm. the behaviour of the ultrasonic sensor is strongly influenced by the temperature. therefore, the fluctuations of the measurement are likely to be attributable to the uncontrolled temperature of the laboratory together with the low level of accuracy of the sensor. there are less errors related to the encoder measurement. however, the measurement value is usually higher than the value of the target. this behaviour is probably attributable to the sequential code of the s2 task. the average error is around 2 mm. the estimation obtained through sdf is very close to the target with an average error of less than 1 mm. therefore, the estimated location is more accurate than the measurement of the single sensor. the improvement in the estimate is related to the kalman-filter algorithm, which considers the unreliability of the ultrasonic sensor in terms of standard deviation. the result is a reliable location with a 0.1% error/meter. considering the accuracy of the sensors involved, the obtained results are certainly positive. figure 8 shows the results obtained from the hil simulation compared with the results obtained from the validation performed with the above-mentioned experimental setup. figure 6. components of the handling device: 1) adafruit motor shield; 2) arduino board; 3) spacers; 4) 9 v battery-mounting bracket; 5) 9 v battery; 6) hc-sr04; 7) breadboard; 8) breadboard support; 9) top plate; 10) 6 v battery-mounting bracket; 11) spacers; 12) 6 v battery pack; 13) gearmotormounting bracket; 14) rail-wheels support; 15) gearmotor; 16) wheel; 17) rail-wheel; 18) rotary encoder; 19) bottom plate. figure 7. comparison between the errors of position measurement (rotary encoder and ultrasonic sensor) and the final estimated position through sdf. table 5: measurement data (mm). step encoders ultrasonic sensor sdf target 1 100,531 98,725 100,486 100,000 2 200,419 200,788 200,393 200,000 3 300,243 303,646 300,451 300,000 4 400,164 403,644 400,654 400,000 5 499,884 499,939 500,330 500,000 6 599,614 594,985 599,494 600,000 7 700,494 686,141 698,659 700,000 8 801,895 787,756 798,493 800,000 9 903,932 890,588 899,212 900,000 10 1004,778 994,450 999,294 1000,000 figure 8. simulation errors versus validation errors. acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 142 the three diagrams provide a comparison for each measurement tool (encoder, ultrasonic sensor and sdf). the first diagram, relating to the encoder, depicts close values up to the seventh step. then, the sequential code, with an error source not considered in the simulation, causes the values to deviate. therefore, in future developments, the simulation will also take into consideration the recording time, which is the delay between the measurement time and the stopping time. the second diagram, concerning the ultrasonic sensor, shows different values due to the low level of accuracy of the sensor coupled with the uncontrolled temperature of the room. the third diagram shows the behaviour of the sdf. in both cases (simulation and validation), the estimates are close to the target. 7. conclusion the present work tackles the problem of large-sized object measurement by proposing a method to correctly locate and track a handling device for reverse engineering systems. the method aims to improve the accuracy of the point-cloud realignment by tracking a reverse engineering system that moves alongside a handling device. the proposed method is based on the sensor data fusion of redundant sensors by means of a kalman filter. the method was verified through a hardware-in-the-loop simulation, using an arduino uno rev3 board programmed in a matlab/simulink® environment. the results show that the sensor data fusion accuracy is better than that of the simulated single sensors. a prototype handling device was developed to validate the results obtained from the simulation. the prototype was equipped using low-cost hardware. a one-meter-long calibrate rail was used to test the prototype’s behaviour. the tests demonstrated an accuracy of 0.1% error/meter, which was better than the accuracy of the measurement sensors. the simulation and validation results are comparable since, in both cases, the accuracy obtained through sdf is better than that of the single sensor measurements. considering the use of low-cost hardware, the results are encouraging and show the possibility for future improvements. therefore, the next step is the development of a real-sized device integrated into a reverse engineering system. better results, in terms of accuracy and reliability, may be reached using more sensors and microcontrollers. furthermore, the development of a device able to perform planar movements would be useful for a large number of applications in the metrology field. acknowledgement the present 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https://doi.org/10.1109/cicsyn.2012.80 https://doi.org/10.1007/s12008-018-0482-8 https://doi.org/10.1016/j.fusengdes.2015.02.021 https://doi.org/10.1002/er.3750 https://doi.org/10.1016/s0890-6955(02)00030-5 https://doi.org/10.1016/j.ijmachtools.2005.01.028 https://www.hexagonmi.com/en-gb/products/3d-laser-scanners/leica-tscan-5 https://www.hexagonmi.com/en-gb/products/3d-laser-scanners/leica-tscan-5 https://www.creaform3d.com/en/metrology-solutions/optical-measuring-systems-maxshot-3d https://www.creaform3d.com/en/metrology-solutions/optical-measuring-systems-maxshot-3d https://doi.org/10.1016/j.rcim.2012.02.010 https://doi.org/10.1016/j.optlaseng.2011.10.019 https://doi.org/10.3390/s17122886 https://doi.org/10.1364/ao.54.002814 https://doi.org/10.1109/metroi4.2019.8792864 https://doi.org/10.1109/rem.2019.8744116 https://doi.org/10.1016/j.cirp.2009.09.008 https://doi.org/10.5081/jgps.2.1.42 https://doi.org/10.1109/ific.2000.859866 https://doi.org/10.1109/fie.2010.5673417 https://doi.org/10.1109/conielecomp.2012.6189895 https://ieeexplore.ieee.org/document/6240931/ https://doi.org/10.1109/tla.2012.6222571 https://doi.org/10.1109/tvt.2012.2203362 https://doi.org/10.1109/cicsyn.2012.80 enhancing the handling of standard substitution weights on a hydrostatic weighing apparatus acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 27 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 27 32 enhancing the handling of standard substitution weights on a hydrostatic weighing apparatus d. simões 1, a. furtado 2, j. pereira2, rui f. martins3 1faculdade de ciências e tecnologia, universidade nova de lisboa, caparica, portugal, dm.simoes@campus.fct.unl.pt 2 instituto português da qualidade (ipq), i.p., caparica, portugal, afurtado@ipq.pt 3 unidemi, faculdade de ciências e tecnologia, universidade nova de lisboa, caparica, portugal, rfspm@fct.unl.pt abstract: the hydrostatic weighing apparatus is a firstlevel method used to measure the density of solids and liquids. at the portuguese institute for quality the hydrostatic tests for the determination of the liquids’ density implies the multiple manual placement of a set of standard substitution weights on apparatus’ balance pan, leading to undesirable perturbations in the weighing environment. also, by this the eccentricity effects on the balance, contribute for an unwanted increase of the measurement uncertainty. to overcome these phenomena, an automated mechanism was designed and implemented, which allows the placing and recollection of the set of standard weights on the balance. to validate the new mechanism, tests to ultrapure water at 20 °c were executed before and after its implementation and the obtained results were compared. despite a 3-fold increase in density measurement uncertainty, mainly due to the exposure of the standard substitution weights to air convection currents, the execution of hydrostatic tests was improved, not only for the fact that the measurements are now carried out quicker, but also for the fact that the operator only needs to intervene once during the whole test. moreover, the smaller absolute deviation from the reference density value for ultrapure water at 20 °c results also in a normalised error 𝐸n lower than 1 (0.3), thus validating the designed automated mechanism for the handling of standard substitution weights. keywords: hydrostatic weighing; density of liquids; automated mechanism; substitution weights 1. introduction the hydrostatic weighing is the first-level measurement method used to determine the density of liquids. it is essentially based in the archimedes’ principle, which states that a body, partially or fully immersed in a fluid, is subject to an upward force named buoyancy, equal to the weight of the fluid displaced by the body. therefore, the loss in its apparent mass is the mass of fluid displaced by the body. in this method, the immersed body is commonly known as “sinker”. the buoyancy force, b, can be calculated as follows: 𝐵 = 𝜌l 𝑉s 𝑔 (1) where 𝜌l is the density of the liquid, in kg m -3, 𝑉s the volume of the sinker, in m3, and 𝑔 the gravitational acceleration, in m s-2. as the buoyancy force acts against the weight of the sinker, the resulting force measured by a balance to which it is connected results in a smaller value of mass when compared to the mass of the sinker when it is weighed in the air. this smaller value of mass is often named as the “apparent mass”. if the real mass 𝑚s and volume 𝑉s of the sinker are known, and its “apparent mass” 𝑚s a determined by hydrostatic weighing, the density of the liquid in which the sinker is immersed can be calculated according to equation (2). 𝜌l = 𝑚s − 𝑚s a 𝑉s (2) a measuring instrument or apparatus which uses a comparison method against standards allows a decrease in the uncertainty by eliminating environmental and instrumental variations. in the case of the hydrostatic weighing apparatus, the apparent mass of the sinker is compared against the mass of standard substitution weights. this procedure is named “substitution method”. therefore, the apparent mass of the sinker can be calculated according to equation 5 [1]. 𝑚s a = 𝑊w + (𝑊sread − 𝑊wread ) (1 − 𝜌a 8000 ) (3) in the previous equation, 𝑊w denotes the mass value of the used set of standard substitution weights, in kg; 𝑊sread and 𝑊wread denote the readings of the balance when loaded with the sinker and with the set of standard weights, in kg, respectively. the term (1 − 𝜌a 8000 ) represents the air http://www.imeko.org/ mailto:dm.simoes@campus.fct.unl.pt mailto:afurtado@ipq.pt mailto:rfspm@fct.unl.pt acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 28 buoyancy correction, where 𝜌a is the density of air, in kg m-3, in the weighing region. the mass value of the used set of standard weights can be calculated based on the parameters given by the respective calibration certificate and according to equation 4 [1]. 𝑊w = 𝑚w (1 − 𝜌a 𝜌w ) (4) where 𝑚w and 𝜌w correspond to the mass and the density values of the standard weights given by their calibration certificate, in kg and kg m-3, respectively. the density of air is calculated with the cipm2007 formula [2]. 2. materials and methods a hydrostatic weighing apparatus features essentially a balance, suitable for underfloor weighing, an underfloor mounted suspension, with a holder for the sinker, and a vessel which contains the test liquid (figure 1). a mechanism for ipq’s hydrostatic weighing apparatus was designed to allow the automated loading/unloading of the standard substitution weights on the balance (figure 2). this mechanism mainly consists of a fork system, vertically actuated by an electric linear axis, which picks and places a standard disc-shaped weight set on a holder, connected to the hook located underneath the balance. the standard weights are loaded on the fork by a qualified operator. it is needed only once before the density measurement. figure 1: complete hydrostatic weighing apparatus in use at ipq. figure 2: overview of the automated mechanism designed for the handling of standard substitution weights implemented in ipq’s hydrostatic weighing apparatus. the mechanism is located between the balance and the thermostatic bath. all the designed components of the mechanism were manufactured in aisi 316 austenitic stainless steel. the design and simulation of the mechanism were accomplished through the solid modelling computer-aided design (cad) and computer-aided engineering (cae) program solidworks® and solidworks simulation®, respectively. 2.1. holder for the standard substitution weights the holder for the standard substitution weights (figure 3) consists in a base, made of a 5 mm thick rectangular sheet, where the set of standard weights is placed. there are four 8.5 mm wide slots in the base which allow the passing of the tines of the fork when this element is picking or placing the weights on the holder. the holder sustains the weights through three 2.5 mm wide tines. in order to accommodate the standard weights, there are two 1.25 mm deep cavities at the base for the larger diameter discs (∅48 mm and ∅22 mm), with a 1.5 mm radial clearance between each cavity and disc. the holder has a wire structure welded to the base with a circular ∅2.5 mm diameter cross-section, having a hook at the top to connect this component to the balance and a ring at the bottom to link it to the suspension. http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 29 figure 3: cad model of the base of the holder for the standard substitution weights (at left) and end-product of the component (at right). to simulate the stresses and strains on the holder, a static load of 2.5 n was applied at the surface of the cavity which supports the larger diameter discs. this value corresponds to the mass of the silicon sphere when weighed in the air (approximately 234 g) with a safety factor applied. the effect of the gravitational acceleration was also taken into account (g = 9.81 m·s-2). figure 4 shows the static stress-strain analysis of the holder for the standard substitution weights. figure 4: stress (at left) and strain (at right) analyses of the holder for the standard substitution weights. the deformed shape is not at the real scale. to prevent the material from yield, the condition given by equation (5) must be verified: 𝜎v ≤ 𝜎max = 𝜎y 𝑆𝐹 (5) where 𝜎v is the maximum von mises stress, in mpa, determined by the finite element method (fem), 𝜎max is the maximum allowable stress of design, in mpa, 𝑆𝐹 is a safety factor of 𝑆𝐹=1.5 and 𝜎y is the yield stress (290 mpa). given that the maximum value of the von mises stress, as observed in figure 4, is approximately 𝜎v =20 mpa, the von mises stress is substantially lower than the maximum allowable design stress (𝜎v = 20 mpa ≪ 𝜎max = 193.33 mpa) , hence concluding that the holder operates in the elastic domain. given the required precision to the motion of the fork to allow its tines to pass through the slots of the holder in order to successfully exchange the standard weights between these two components, it is crucial that the displacements caused by the weight of the discs when supported on these elements are not excessive. this factor could potentially lead to an undesirable contact between the fork and the holder, preventing the correct operation of the mechanism. it was verified that the maximum displacement on the holder had a value of approximately 0.24 mm and is located on the bottom ring for suspension connection. this value does not compromise the functioning of the mechanism. 2.2. fork for the handling of standard substitution weights the fork for the handling of standard substitution weights (figure 5) consists of an arm, manufactured from a 5 mm thick rectangular sheet with 374.65 mm long. the end of the arm, where the standard weights are supported, has four tines with a width of 5.5 mm each, ensuring a total horizontal clearance of 3 mm between these tines and the existing slots in the holder for the standard weights. this same clearance is also verified between the tines of the holder and the slots in the fork. in order to compensate for any possible misalignment between the fork and the holder from their working positions due to the movement of the mechanism, the existence of these considerable clearances is mandatory. the fork is also provided with two cavities of similar dimensions to the ones designed for the holder of the standard weights. the fork is connected to the carriage of the electric linear axis through a rectangular sheet of dimensions 60 mm × 54 mm × 5 mm, welded to the arm. this fixing sheet is then fastened to the carriage of the linear axis through four m4 bolts. to reduce the deformations at the end of the fork due to the weight of the discs, a rib was welded at the bottom of this component to the arm and the fixing sheet. figure 5: cad model of the tip of the fork for the handling of standard substitution weights (at left) and the respective end-product (at right). figure 6 shows the stress-strain analysis of the entire mechanism (set consisted by the fork, a simplified geometry model of the electric linear axis and its respective fixing bars for fastening the mechanism to the supporting structure of the balance). http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 30 figure 6: stress (at left) and strain (at right) analyses of the entire mechanism, consisted by the fork, a simplified geometry model of the electric linear axis and its respective fixing bars. the deformed shape is not at the real scale. applying the previous methodology, the von mises stress is maximum on the rib of the fork with a value of approximately 9.4 mpa. according to the design criteria defined by equation (5), it is concluded that the fork is distant from yielding (𝜎v = 9.4 mpa ≪ 𝜎max = 193.33 mpa). the maximum of displacements is on the edge of the fork’s tines with a value of 0.36 mm. this is small enough to meet the required precision of the mechanism. 2.3. electric linear axis based on the reaction forces at the support of the fork, i.e., the fixing sheet, an electric linear axis from igus® (figure 7) was chosen for the mechanism for the handling of the standard weights. the linear axis’ carriage is driven by a lead-screw, coupled to a nema17 litze stepper motor, being the preferred option for the required precision. figure 7: cad model of the lead-screw driven electric linear axis from igus® (size 0630) (at left); real model of the electric linear axis (at right). the nema17 litze stepper motor of the linear axis is controlled through the tb6600 driver, connected to an arduino uno r3 board (figure 8). the arduino platform was chosen to program the stepper motor because it is an open-source software, having a wide integration with different programming languages, particularly with labview®. the stepper motor is powered by a 24 v dc power supply, shown as well in figure 8. figure 8: tb6600 stepper motor driver (at left), arduino uno r3 board (at the centre) and 24 v dc power supply (at right). to hold the electronic components in place, and protect the respective connections, these were installed in the interior of a proper aluminium housing (figure 9). figure 9: electric control circuit of the stepper motor mounted in the interior of the housing (at left), existing connections at the back of the housing (at the centre) and ventilation holes on the top of the housing (at right). the nema17 stepper motor of the electric linear axis was programmed using the linx interface for labview®. this interface is responsible to establish the connection between the arduino uno r3 board and the labview® software. the developed labview® program to control the stepper motor was integrated within an already existing program which controls the hydrostatic test, acquiring all the data from the measuring instruments. it is important to point out that the carriage of the electric linear axis always moves at a constant speed of 0.223 mm·s-1. it is essential that the actions of placement and removal the standard weight set by the fork are carried out at low speeds to minimise the oscillations in the holder and to prevent the weights from being dropped. 3. results and its discussion the validation of the mechanism for the handling of standard weights was performed through the carrying out of hydrostatic tests to ultrapure water (type 1), at 20 °c. the choice of this liquid was because ultrapure water is a density standard whose properties are well known at a certain temperature range [3]. the sample used in the tests was degassed using a vacuum oven. the results of density and its respective expanded uncertainty, obtained from the measurements to ultrapure water at 20 °c, both with the hydrostatic weighing apparatus pre and postoptimised, are given in table 1. the term “prehttp://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 31 optimisation” refers to the original apparatus, as it was, while the “post-optimisation” term refers to the improved apparatus with the enhancement of the handling of standard substitution weights. the reference density value for ultrapure water at this temperature, as stated by tanaka [3], is also listed in the same table. table 1: results of density values 𝜌𝐿 and respective expanded uncertainty 𝑈𝜌𝐿 (absolute and relative values) obtained from measurements with the hydrostatic weighing apparatus, pre and post-optimised, of an ultrapure water sample at 20 °c. the reference value observed by tanaka [3] is also shown. figure 10 illustrates the previous results in a scatter plot to provide for a better comparison between them. figure 10: scatter plot of the density of ultrapure water (type 1), at 20 °c, measured by the hydrostatic weighing (hw) apparatus pre and post-optimised and the respective value observed by tanaka [3]. the vertical bars represent the values of expanded uncertainty relative to each measured density value. another indicator which allows for a clearer comparison of results is the relative deviation in density 𝛿𝜌 , in ppm. considering the density value of ultrapure water, at 20 °c, observed by tanaka [3] as reference, the relative deviation of the obtained results by hydrostatic weighing from the reference value is given as follows: 𝛿𝜌 = | 𝜌hw − 𝜌ref. 𝜌ref. | × 106 (6) where 𝜌hw and 𝜌ref. are the density values of the liquid experimentally measured by hydrostatic weighing and determined by tanaka, respectively. the term |𝜌hw − 𝜌ref.| can be described as the absolute deviation in density, ∆𝜌. the validation procedure of a certain measuring method implies the calculation of the normalised error 𝐸n of a measurement result against a reference value. the results are satisfactory if the normalised error is less than or equal to 1, i.e., 𝐸n ≤ 1. assuming once again the density value for ultrapure water at 20 °c determined by tanaka [3] as reference, the normalised error yielded by the results obtained by hydrostatic weighing can be calculated according to equation (7). 𝐸n = |𝜌hw − 𝜌ref.| √𝑈𝜌hw 2 + 𝑈𝜌ref. 2 (7) in the equation above, 𝑈𝜌hw and 𝑈𝜌ref. are the expanded uncertainty values in the measurements by hydrostatic weighing and in the reference value, observed by tanaka [3], respectively. the absolute and relative deviations in density, of the obtained results by hydrostatic weighing to ultrapure water at 20 °c from the reference value observed by tanaka [3], both in the stages of pre and post-optimisation of the apparatus, are listed in table 2. the normalised error of the method is also shown. table 2: absolute, ∆𝜌 , and relative, 𝛿𝜌 , deviations in density, of the obtained results by hydrostatic weighing to ultrapure water (type 1) at 20 °c from the reference value observed by tanaka [3], both in the stages of pre and post-optimisation of the apparatus. the normalised error, 𝐸𝑛 , is also listed. comparing the results obtained in the two different stages, with the reference value suggested by tanaka [3], a smaller absolute deviation in the density of the liquid ∆𝜌, was verified in the stage of post-optimisation (0.004 kg m-3 (3.9 ppm)) relatively to the pre-optimisation (0.0126 kg m -3 (12.6 ppm)). these results yield in a normalised error 𝐸n of 2.9 for the stage of pre-optimisation of the apparatus and 0.3 for the stage of postoptimisation. the main factor which could have potentially led to this difference is the fact that the ultrapure water sample used, despite being the same at pre and postoptimisation tests, may varied in its gas saturation [3], due to different ambient conditions, and therefore resulting in different density deviations from the tanaka’s reference value. there are other possible causes which could lead to a more significant deviation in density from the reference value, such as the lack of knowledge of the chemical composition of the sample (in ultrapure water 𝝆𝐋 𝑼𝝆𝐋 / kg·m-3) /kg·m-3 / ppm preoptimisation 998.219 3 0.004 2 (k =1.97) 4.2 postoptimisation 998.211 0.013 (k =2.07) 13.2 tanaka 998.206 7 0.000 83 (k =2.00) 0.8 stage ∆𝝆 𝜹𝝆 𝑬𝐧 / kg·m-3 / ppm preoptimisation 0.0126 12.6 2.9 postoptimisation 0.004 3.9 0.3 http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 32 particular, the isotopic composition) [3]; or other unknown factors, such as mathematical distributions, heat or contamination of the sample. concerning the density expanded uncertainty of the liquid 𝑈𝜌l , a value approximately 3 times greater was verified in the hydrostatic tests after the optimisation of the apparatus (0.013 kg m-3) when compared against the value obtained from the preoptimisation stage (0.0042 kg m-3). this increase was mainly caused by a greater standard uncertainty in the repeatability of the readings of the balance 𝑢rep., in the stage of post-optimisation (2.6 × 10 -4 g) when compared against the pre-optimisation stage (5.0 × 10-5 g). this phenomenon was primarily due to the exposure of the holder and respective standard weights to convection currents and vibrations from the bath, disturbing the stability of the set and leading to unstable readings of the balance. another modification made to ipq’s hydrostatic weighing apparatus after its optimisation was the installation of the pressure, temperature and relative humidity sensors for the atmospheric air in the operating area of the mechanism for the handling of standard weights. these sensors were previously installed in the interior of the campanula which covers the balance, so they had to be moved to a location closer to where the standard weights are now placed in order to yield measurements with a higher reliability degree, resulting in a more precise calculation of the buoyancy force originated by the air. given that the operation region of the mechanism is exposed to the surrounding environment, a significant variation in the air conditions within this region may occur, leading to a greater dispersion in the measurements of this component, thus, resulting in a larger uncertainty in repeatability. nonetheless, a slightly smaller uncertainty in the measurements of the air was still verified in the stage of post-optimisation of the apparatus. 4. conclusion despite a 3-fold increase in density measurement uncertainty 𝑈𝜌𝐿 , from 0.0042 kg m -3 to 0.013 kg m-3, the smaller absolute deviation from the reference density value for ultrapure water at 20 °c results also in a satisfactory normalised error 𝐸n of 0.3, thus validating the designed automated mechanism for the handling of standard substitution weights. furthermore, the execution of hydrostatic tests was improved with the introduction of a mechanism for the handling of standard weights, not only by the fact that the measurements are now carried out quicker, but also that the operator only needs to intervene once during the whole test. a better stabilisation of the weighing environment (i.e., in air conditions) was also observed, leading to a slight decrease in the uncertainty in the density of the liquid. 5. acknowledgement this work was developed under the framework of the empir project “17rpt02-rholiq” which is carried out with funding of european union under the empir. the empir is jointly funded by the empir participating countries within euramet and the european union. 6. references [1] h. fehlauer, h. wolf, “density reference liquids certified by the physikalisch-technische bundesanstalt”, meas. sci. technol., vol. 17, pp. 2588–2592, 2006. [2] a. picard, r. s. davis, m. gläser, k. fujii, “revised formula for the density of moist air (cipm-2007)”, metrologia, vol. 45, pp. 149–155, 2008. [3] m. tanaka, g. girard, r. davis, a. peuto, n. bignell, “recommended table for the density of water between 0 °c and 40 °c based on recent experimental reports”, metrologia, vol. 38, pp. 301-309, 2001. http://www.imeko.org/ connecting volume effects on dynamics of pneumatic pressure measurement systems acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 315 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 315 318 connecting volume effects on dynamics of pneumatic pressure measurement systems j. kutin1, a. svete2 1 university of ljubljana, faculty of mechanical engineering, ljubljana, slovenia, joze.kutin@fs.uni-lj 2 university of ljubljana, faculty of mechanical engineering, ljubljana, slovenia, andrej.svete@fs.uni-lj abstract: the pressure sensors are often placed at a certain distance from the measured object. beside the properties of the transmission fluid, dynamic characteristics of such a measurement system depends on the geometry and dimensions of the connecting elements. so far made research works have shown that the internal volume of the pressure sensor can have a large influence on the dynamic response. this paper is focused on theoretical analysis of the effects of the sensor volume on characteristic parameters of both the frequency and the time response of the system under discussion with a gas medium. keywords: pressure measurement; pressure transmission; dynamic response; sensor volume; connecting tube 1. introduction the dynamic response of the pressure measurement system depends on dynamic characteristics of its constructional components. because the pressure sensors are often placed at a certain distance from the measured object, an additional pressure transmission system is formed in-between. the dynamic response of the pressure transmission depends on geometry, dimensions and stiffness of the connecting elements, as well as thermodynamic and transport properties of the transmission fluid. the most frequently studied physical model of the pressure transmission system consists from a straight cylindrical connecting tube and an internal volume of the sensor (figure 1). following the work of bergh and tijdeman [1] the authors of this paper derived one of the latest, most sophisticated versions of the linear mathematical model, which is applicable for both pneumatic and hydraulic pressure measurement systems, and considers viscous, thermal and compliance effects [2]. this improved mathematical model was also experimentally validated for the case of pneumatic pressure measurement systems [3]. figure 1: physical model of the pressure transmission system in some limited frequency ranges the dynamics of the pressure transmission system can be well described by low-order lumped-parameter mathematical models; e.g., the second-order model with two parameters (natural frequency, damping ratio) or the first-order model with one parameter (time constant) [4–7]. in [2] the authors derived analytical approximations of these lumped parameters by applying an asymptotic series expansion to the complete mathematical model. the paper is organized as follows. section 2 presents the mathematical model for the frequency and the step response of the pressure transmission system and its analytical approximations. section 3 discusses effects of dimensions on characteristic parameters of dynamic response and exposes potential directions for optimal design of the pressure transmission systems. section 4 shortly summarizes conclusions about the effects of the sensor volume. 2. mathematical model the presented mathematical model deals with the pressure transmission system that consists of the cylindrical connecting tube with radius rt and length lt and the cylindrical sensor volume with radius rv and length lv (figure 1). it is based on the following main assumptions: the internal radius of the tube is small enough in comparison with its length to neglect the end-flow effects, the dimensions of the sensor volume are small enough to neglect fluid-wave effects, and all the pressure variations in the system have small enough amplitudes for validity of the linear model. http://www.imeko.org/ mailto:joze.kutin@fs.uni-lj mailto:andrej.svete@fs.uni-lj acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 316 2.1. complete model the complex frequency response function of the pressure measurement system under discussion can be written as: 1 (i ) cosh sinh out v tin t v p g v np v n  = =  +   , (1) where vt = π rt 2lt is the internal volume of the tube, vv = πrv 2lv is the internal volume of the sensor,  is the propagation wavenumber, and nt and nv are the effective thermodynamic indexes for the gas in the tube and the sensor volume, respectively (see [2] for the complete expressions for , nt and nv, including the wall compliance effects). the time step response is calculated from the frequency response function by using the fourier transform, considering the rectangular pulse of the input pressure signal. the fourier transform of the rectangular pulse with the length tpulse is defined as: ( ) i i 1 e (i ) e d i pulset t in in p p t t −  −  − −  = =   . (2) the output signal in the frequency domain is determined as: (i ) (i ) (i ) out in p g p =   , (3) and the time response is calculated by its inverse fourier transform: i1 ( ) (i )e d 2 t out out p t p   − =     . (4) with the intention to improve the efficiency of calculations, the procedure presented by equations (2–4) is implemented in calculations in this paper by using the discrete fourier transform. the corresponding sampling frequency and the number of samples were adapted in a way to assure that errors in calculations of all presented response times are lower than 10-3. 2.2. analytical approximations the study in this paper is limited to the pressure transmission systems with the gas medium, where the wall compliance effects are usually not of much importance, so the presented analytical approximations taken from ref. [2] do not consider the compliance effects. for the connecting tubes with relatively small internal diameters (overdamped conditions) the pressure transmission system can be approximated with the first-order low-wavenumber model: 1 1 (i ) 1 i g  = +  , (5) which is defined with one parameter, i.e., the time constant : 2 2 2 8 1 2 t v t t l v c r v    = +     , (6) where c,  and  are the speed of sound, density and dynamic viscosity of gas, respectively. the amplitude frequency response of the first order system is defined as: ( ) 1 2 1 (i ) 1 g  = +  , (7) and the step response can be defined as: ( ) /,1 1 e t out p t −  = − . (8) for the connecting tubes with relatively large internal diameters (underdamped conditions) the pressure transmission system can be approximated with the second-order high-wavenumber model: 2 2 2 1 (i ) 1 2 i / / n n g  = +    −  , (9) which is defined with two parameters, i.e., the natural frequency n and the damping ratio : 1 1 2 n vt t c vl v  = + (10) ( ) 2 12 1 2 2 n t t v v t n t l v r c v   +     = + +         , (11) where t and v represent the thermal effects for the gas in the tube and the sensor volume, respectively: 1 pr t  −  = , 1 1 pr t v v v v r l l r    −  = +    , (12) where γ is the heat capacity ratio and pr is the prandtl number. for  << 1 the amplitude frequency response of the second order system can be approximated as: 2 2 2 1 (i ) 1 / n g   −  , (13) and the lower envelope of the oscillating step response can be approximated as: ( ),2 1 e n t out p t −   − . (14) http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 317 3. results 3.1. frequency bandwidth characteristics of the frequency response are analysed in terms of the frequency bandwidth, which is defined as the range of frequencies over which the relative amplitude dynamic errors do not exceed 10-2 or 1%. the bandwidth limit frequency flim is calculated as the minimum frequency, where the amplitude ratio reaches 0.99 or 1.01. figure 2 shows variations of the limit frequency for different dimensions of the pressure transmission system with dry air ( = 1.19 kg/m3, c = 343 m/s,  = 18.2 μpa s, γ = 1.4, pr = 0.71). (a) sensor volume vv = 1.77 cm3 (lv = 2.5 mm, rv = 15 mm) (b) sensor volume vv = 7.07 cm3 (lv = 10 mm, rv = 15 mm) figure 2: variations of the limit frequency with the tube radius for two different sensor volumes (tube length lt = 1 m, dry air) the dashed lines in figure 2 represent the results of the first-order low-wavenumber (lw1) and second-order high-wavenumber (hw2) approximation models. the lw1 limit frequency is determined using equation (7): ( ) 2 1 1 1 r (l) lim e= − +  , (15) which results in: ( ) 2 1 1 1 2 2 1 (l) (l) lim lim r f e  = = −   − . (16) the hw2 limit frequency is determined using equation (13): 2 2 1 1 1 / r(h) lim n e= + −   , (17) which results in: 1 1 2 2 1 (h) (h) lim n lim r f e   = = −   + . (18) the lw1 model predicts the limit frequency relatively well for thiner connecting tubes. the frequency bandwidth is inversely proportional to the time constant τ and, therefore, is inversely proportional to (1/2 + vv/vt). the hw2 model predicts the limit frequency relatively well for thicker connecting tubes. the frequency bandwidth is proportional to n and, therefore, is inversely proportional to (1/2 + vv/vt) 1/2. the frequency bandwidth is the largest in the intermediate region of validity of the lowwavenumber and high-wavenumber approximate solutions. for the presented simulation results the largest limit frequency is found at the tube radius of 0.63 mm for the smaller sensor volume (figure 2(a)) and at the tube radius of 0.74 mm for the larger sensor volume (figure 2(b)). the optimal tube radius that results in the largest frequency bandwidth can be well predicted by the intersection of both approximate models. 3.2. settling time characteristics of the step transient response are analysed in terms of the settling time, which is defined as the minimum time that the normalized response needs to fully settle within 0.99 and 1.01; i.e., the relative dynamic errors of the response do not exceed 10-2 or 1%. figure 3 shows variations of the settling time for different dimensions of the pressure transmission system. the dashed lines in figure 3 represent the results of the first-order low-wavenumber (lw1) and second-order high-wavenumber (hw2) approximation models. the lw1 settling time is determined using equation (8): ( ) 1 e 1 l set t r e − − = − , (19) which results in: ln (l) set r t e= − . (20) complete model lw1 approx. model hw2 approx. model 0.1 1 10 0.1 1 10 100 tube radius r t / mm l im it f re q u e n c y f li m / h z complete model lw1 approx. model hw2 approx. model 0.1 1 10 0.1 1 10 100 tube radius r t / mm l im it f re q u e n c y f li m / h z http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 318 (a) sensor volume vv = 1.77 cm3 (lv = 2.5 mm, rv = 15 mm) (b) sensor volume vv = 7.07 cm3 (lv = 10 mm, rv = 15 mm) figure 3: variations of the settling time with the tube radius for two different sensor volumes (tube length lt = 1 m, dry air) the hw2 settling time is determined using equation (14): ( ) 1 e 1 h n set t r e −  − = − , (21) which results in: 1 ln (h) set r n t e= −   . (22) the lw1 model predicts the settling time relatively well for thiner connecting tubes. the settling time is proportional to the time constant τ and, therefore, is proportional to (1/2 + vv/vt). the hw2 model predicts the settling time relatively well for thicker connecting tubes. the settling time is inversely proportional to nζ and, therefore, is proportional to: ( ) ( ) 5/ 4 1 21 1 1 1 2 v t(h) set vn t v t v v t v v   +        +  + +  ; (23) so if the thermal effects are neglected (v → 0, t → 0), the settling time is found proportional to (1/2 + vv/vt) 1/4. similar to the findings about the frequency bandwidth, the fastest response is found in the intermediate region of validity of the lowwavenumber and high-wavenumber approximate solutions. the smallest settling time occurs at a bit higher tube radius in comparison with the tube radius that results in the largest limit frequency (marked in figure 3 with the vertical dot line). 4. summary as could be expected, the dynamic response of the pressure transmission system is deteriorated with the increased sensor volume. these effects significantly depend on dimensions of the connecting tube. both the limit frequency of the frequency response and the settling time of the step response are influenced by the sensor volume in terms of the magnitude of (1/2 + vv/vt). therefore, the effects of the sensor volume can be neglected if vv << ½ vt. 5. references [1] h. bergh, h. tijdeman h, theoretical and experimental results for the dynamic response of pressure measuring systems. amsterdam: nat. aeroand astronautical res. inst., report nlr-tr f.238, 1965. [2] j. kutin, a. svete, “on the theory of the frequency response of gas and liquid pressure measurement systems with connecting tubes”, measurement science and technology, vol. 29, pp. 125108, november 2018. [3] a. svete, j. kutin, “experimental validation of an improved mathematical model for pneumatic pressure measurement systems with connecting tubes”, measurement science and technology, vol. 31, pp. 015101, october 2019. [4] e. o. doebelin, measurement systems: application and design. boston, ma: mcgrawhill, 2004. [5] s. a. whitmore, b. fox b, “improved accuracy, second-order response model for pressure sensing systems”, journal of aircraft, vol. 46, pp. 491–500, march–april 2009. [6] b. f. hall, t. povey, “a practical model for pressure probe system response estimation (with review of existing models)”, measurement science and technology, vol. 29, pp. 1–12, february 2018. [7] i. bajsić, j. kutin, t. žagar, “response time of a pressure measurement system with a connecting tube”, instrumentation science and technology, vol. 35, pp. 399–409, june 2007. complete model lw1 approx. model hw2 approx. model 0.1 1 10 0.01 0.1 1 10 tube radius r t / mm s e tt li n g t im e t se t / s complete model lw1 approx. model hw2 approx. model 0.1 1 10 0.01 0.1 1 10 tube radius r t / mm s e tt li n g t im e t se t / s http://www.imeko.org/ verification of radial displacement correction effect in instrumented indentation testing acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 265 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 265 269 verification of radial displacement correction effect in instrumented indentation testing y. tanaka1, y. seino2, k. hattori3 1 national metrology institute of japan, national institute of advanced industrial science and technology, tsukuba, japan, yukimi.tanaka@aist.go.jp 2 national metrology institute of japan, national institute of advanced industrial science and technology, tsukuba, japan, y.seino@aist.go.jp 3 national metrology institute of japan, national institute of advanced industrial science and technology, tsukuba, japan, hattori-k@aist.go.jp abstract: the radial displacement had been ignored in the analysis of unloading curve of instrumented indentation test. there are two representative methods of radial displacement correction, proposed by hay et al. [j. mater. res., 14, 2296, (1999)] and chudoba-jennett [j. phys. d, 41, 215407, (2008)]. in this study, we examine the effect of radial displacement corrections in finite element analysis and experiments. keywords: instrumented indentation testing, radial displacement 1. introduction instrumented indentation testing is a useful method to evaluate local mechanical properties such as the elastic modulus (e) and hardness (h). sneddon’s solution indicates the relationship between load (p) and displacement (h) in elastic half space [1]. after that, oliver and pharr showed the calculation elastic modulus and hardness in elasticplastic deformation, by following equations [2]: 𝐸𝑟 = 𝑆 (2𝑎)⁄ (1) 𝐻 = 𝑃𝑚 (𝜋𝑎 2)⁄ (2) where pm is the maximum load, s is the unloading stiffness at the maximum depth, and a is the contact radius. er is reduced modulus. however, it is pointed out that this method can cause errors in measuring elastic modulus and hardness due to ignoring radial displacement (ur) [3-4]. the correction factor of radial displacement is basically shown as γ = 1 + |ur/a|. there are two representative method of radial displacement correction, proposed by hay et al. [3] and chudoba-jennett [4]. hay et al. [3] pointed out that indentation by a rigid cone requires larger loads than those predicted by sneddon’s analysis due to the radial surface displacements. they proposed that the following correction: 𝐸𝑟_𝑐𝑜𝑟𝑟 = (1 𝛾𝐻⁄ ) ∙𝑆 (2𝑎)⁄ (3) 𝛾𝐻 = 1+ 1−2𝑣 4(1−𝑣) cot𝜃 (4) 𝛾𝐻 = 𝜋 𝜋 4 +0.1548cot𝜃 1−2𝑣 4(1−𝑣) [ 𝜋 2 −0.8312cot𝜃 1−2𝑣 4(1−𝑣) ] 2 (5) where a = (2hmtanθ)/π, θ is the indenter angle and v is the poisson’s ratio. eq.(4) is suitable for cubecorners, and eq.(5) is for indenters whose equivalent cone angles are 70.3°. chudoba and jennett [4] assumed elastic-plastic deformation and proposed the following correction factor based on the radial displacement at the contact edge: 𝛾𝐶 = 1+[(1−2𝑣)(1+𝑣)𝐻] (2𝐸)⁄ ∙ cos𝛼𝑟 (6) where αr assumes the residual angle between the face of the impression and the original surface plane. iso14577 recommends the correction by chudobajennett method [5]. in the iso, correcting indentation results for radial displacement increases the elastic modulus as given in following equation: 𝐸𝑟_𝑐𝑜𝑟𝑟 = 𝛾𝐶 ∙ 𝑆 (2𝑎)⁄ (7) where a is determined by real (unloaded) indenter shape. the contact radius satisfying s = 2era is the one during loading and smaller than after unloading. note that (90° − θ) is used instead of αr when γc is calculated in the iso, because it is difficult to obtain the residual angle from load-displacement data. the correction factors γh and γc are both γ > 1. hay’s correction decrease er as shown in eq.(3), whereas chudoba’s correction increase er as shown in eq.(7). these methods are inconsistent in the direction of correcting elastic modulus, although http://www.imeko.org/ mailto:yukimi.tanaka@aist.go.jp mailto:y.seino@aist.go.jp mailto:hattori-k@aist.go.jp acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 266 they both assume that the surface point is displaced inward under pressure. in actual experiment, the area function, ac, is defined by experimental data of the reference materials whose e is known, by following equation: 𝐴𝑐 = (𝜋 4⁄ ) ∙ (𝑆 𝐸𝑟⁄ ) 2 (8) fused silica (fs) is often used as reference material. hay et al. mentioned that eq.(8) led to an overestimation of ac by 20% when fs was used. on the other hand, chudoba and jennett indicated that ac determined by fs was underestimated by 13 % compared with the area function from direct measurement by afm. correction ways of area function are also different between hay’s and chudoba’s methods. in this study, the effectiveness of these methods is verified by actual experiments and simulation of finite element analysis (fea). in experiments, the materials whose elastic moduli are known are measured by indentation testing. the effects of radial displacement corrections are investigated by comparing to theoretical elastic modulus. in fea simulation, the contact radius with radial displacement is considered in fully elastic deformation and elastic-plastic deformation. then, we discuss the accurate correction method which can be applied to actual measurement. 2. methods 2.1. experiments to verify the effects of the correction methods, nanoindentation-tests were carried out by using imicro (nanomechanics) with a berkovich indenter. to calculate the elastic modulus, continuous stiffness measurements were used. measured materials were sapphire (al2o3), tungsten (w), aluminium (al), sic, gan, mgo, molybdenum (mo), and caf2. these materials have single crystal structure. area function, ac(hc), was determined by measuring data of fs (e = 72 gpa, v = 0.17) or bk7 glass (e = 83 gpa, v = 0.21) as reference materials. it is necessary to consider both radial displacement effects of reference and measured materials. the significant difference due to the radial displacement was found for fs [3-4]. fs and bk7 were used as reference materials to consider the difference between reference materials. 2.2. finite element analysis (fea) to confirm the validity of the correction methods, we simulated the fully elastic half-space and the elastic-plastic half-space by fea using comsol multiphysics. fully elastic deformation with a 70.3° rigid conical indenter was simulated with a twodimensional axisymmetric model. the examined materials have different poisson’s ratios (v = 0.0, 0.1, 0.2, 0.3, 0.4 and 0.5). the elastic modulus was fixed with 100 gpa. in addition, elastic-plastic deformation with a 70.3° rigid conical indenter was simulated with a two-dimensional axisymmetric model. the elastic-plastic materials were modelled by swift’s law. the examined materials have different work-hardening coefficients (n = 0.0, 0.2, and 0.4), poisson’s ratios (v = 0.1, 0.2, 0.3 and 0.4), and ratios between elastic modulus and yield stress (e/σy = 10, 25, 50, and 100). 3. results 3.1. experiments table 1 shows the elastic moduli with and without radial displacement correction by hay and chudoba methods in the experiments. corrected elastic moduli of measured materials depend on the radial displacement of both themselves and the reference material, because ac(hc) is corrected by the radial displacement of the reference material before applying the correction method to measured materials. in the case of using ac function from fs, most of corrected elastic moduli by chudoba method got closer to the theoretical values than that by hay method. however, in the ac function from bk7, elastic moduli of hay method approach the table 1: elastic moduli obtained from experiments. samples v theo. e experimental e, gpa ac(hc) from fs ac(hc) from bk7 no-corr. hay chudoba no-corr. hay chudoba sic 0.07 475 576 ± 17 567 ± 17 568 ± 18 554 ± 27 542 ± 26 554 ± 28 al2o3 0.16 422 483 ± 9 482 ± 9 473 ± 9 465 ± 13 461 ± 13 461 ± 14 gan 0.18 320 326 ± 3 326 ± 3 318 ± 3 314 ± 6 313 ± 6 311 ± 6 caf2 0.21 120 132 ± 3 133 ± 3 127 ± 3 127 ± 3 127 ± 3 124 ± 3 mgo 0.24 288 315 ± 5 319 ± 5 305 ± 5 304 ± 6 306 ± 7 298 ± 7 mo 0.28 332 338 ± 7 345 ± 7 324 ± 6 325 ± 7 329 ± 7 316 ± 7 w 0.28 409 411 ± 10 419 ± 10 394 ± 10 394 ± 10 399 ± 10 383 ± 10 al 0.36 68 70 ± 3 73 ± 3 67 ± 3 68 ± 3 70 ± 3 66 ± 3 average absolute difference from theo. 7.8 % 8.8 % 6.4 % 5.7 % 5.5 % 6.2 % values reported as mean ± standard deviation. http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 267 theoretical values than that of chudoba method. it is unclear that the effectiveness of hay and chudoba method only from experiments, thus, we conducted the simulation of fea. 3.2. fully elastic deformation by fea for verifying the way of correcting radial displacements, the indentation was simulated by fea. figure 1 shows the fea simulation results of fully elastic deformation. there are three ways to calculate contact radius, asn based on sneddon’s solution a = (2hmtanθ)/π, afea obtained from fea results, and ae calculated from a = s/(2er). in v = 0.5, asn, afea and ae were consistent each other because of ur = 0. however, they were not consistent in v = 0. the surface point is displaced inward, thus afea is smaller than asn. in contrast, ae is larger than asn, because the indentation load and stiffness of fea is higher than that of sneddon’s solution. as shown in figure 1b, ae and afea depend on poisson’s ratio but their tendencies are different. in addition, the ratio of the correction by hay and chudoba methods is also shown in figure 1b. note that pm/(πa 2) was used instead of h in chudoba’s factor because the fully elastic half space was assumed. these results suggest that hay’s method can be useful for ae estimation, and chudoba’s method can estimate afea. 3.3. elastic-plastic deformation by fea for verifying the way of correcting radial displacements in experiments, the elastic-plastic indentation was simulated by fea. note that hay’s method doesn’t assume elastic-plastic deformation. figure 2 shows the fea simulation results of elastic-plastic deformation. figure 2a shows the relationship between the amount of radial displacement and hf/hm, where hm is the maximum depth and hf is the final depth. ur/a are directly obtained from fea, ur indicates the difference between loading and unloading at r = a. fea results indicate that the radial displacement depends on the poisson’s ratio and the amount of elastic deformation. |ur/a| values calculated by chudobajennett method agreed with fea results, whereas the estimated values by hay et al. method which depends on the poisson’s ratio were different from fea values. this result suggest that chudobajennett method is suitable for estimating |ur/a| in the actual experiments. the figures 2b and 2c indicate the difference in contact radii calculated by three ways, ae, afea and aop which is obtained by hc = hm – ε(pm/s). when sink-in behaviour is easy to be occurred such as n = 0.4, the fea results were similar to those of fully elastic deformation. ae/aop is consistent with the correction factor of hay’s method whereas afea/aop is consistent with that of chudoba’s method. however, when pile-up behavior is easy to be occurred such as n = 0.0, the variation of ae/aop and afea/aop was large, because the estimation of hc and aop might be wrong. 4. discussion in fully elastic deformation, our results of fea simulation suggest afea < asn < ae. schwarzer indicated the contact radius considering radial displacement was smaller than that based on sneddon’s analysis, by using basic principles of linear elasticity [6]. in addition, hay et al. showed the load from fea simulation was larger than that from sneddon’s analysis, and the effective cone angle was larger than the original angle [3]. these previous studies are consistent with our results of fea. it is necessary to be careful not to confuse a in fea and a = s/(2er) which is used for calculating elastic modulus by eq. (1). the present results of fea suggest that hay’s method can obtain the contact area for calculating (a) scheme of fea simulation at v = 0.0 and v = 0.5 (b) difference in calculated contact radius figure 2: fea simulation results of fully elastic deformation considering radial displacement effects. asn: contact radius based on sneddon’s solution. afea: contact radius obtained from fea. ae: contact radius calculated by 𝑎𝐸 = 𝑆 (2𝐸𝑟)⁄ . 𝑎 𝐸 𝑎 𝑎 𝐸 𝑎 𝑣 = . 𝑣 = . l o a d displacement l o a d displacement fea results sneddon 𝑣 = . 𝑣 = . 𝑃 = 𝜋 2 𝐸𝑟𝑎 2 cot𝜃 𝑃 = 𝜋 2 𝐸𝑟𝑎𝐸 2 cot𝜃 0,8 0,9 1 1,1 1,2 0 0,1 0,2 0,3 0,4 0,5 poisson's ratio 𝑎e∕𝑎sn 𝑎fea∕𝑎snhay chudoba http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 268 elastic modulus by eq.(1), while chudoba’s method can obtain the apparent contact area during loading. therefore, eq.(3) of hay’s method seems to be suitable to obtain appropriate elastic modulus than eq.(7) of chudoba’s method. however, hay’s method considered the ideal boundary condition based on sneddon analysis (related to asn), although it should consider the actual boundary condition (related to afea). this suggests that both of hay’s and chudoba’s corrections should be taken into account. recent research shows the improved correction factor of hay’s method which is taken the boundary condition after the surface displaced inward into consideration in fully elastic deformation [7]. furthermore, the boundary condition in elastic-plastic deformation is more complex than that in fully elastic deformation, because the amount of radial displacement is varied with e/σy and v as shown in figure 2a. it can be disputable to consider the value of correction factor in elastic-plastic deformation. in addition, xu et al. suggested that γ-factor with berkovich indenter is almost independent of the e/σy ratio, whereas that with conical indenter depends on the e/σy [8]. if the γ-factors of reference and measured materials are the same, and it is unnecessary to correct the radial displacement effect. the further studies require to consider about the radial displacement effect of actual indentation testing using berkovich indenters. (a) the relationship between |ur/a| and hf/hm with different e/σy and n (b) difference in ae/aop with poisson’s ratio, work-hardening coefficients and e/σy ratio. aop: contact radius based on oliver-pharr method. ae: contact radius calculated by 𝑎𝐸 = 𝑆 (2𝐸𝑟)⁄ . (c) difference in afea/aop with poisson’s ratio, work-hardening coefficients and e/σy ratio. aop: contact radius based on oliver-pharr method. afea: contact radius obtained from fea. figure 2: fea simulation results of elastic-plastic deformation considering radial displacement effects. 0 2 4 6 8 10 12 0,2 0,4 0,6 0,8 1 |u r/ a |, % hf/hm hay chudoba fea v = 0.1 0 1 2 3 4 5 6 0,2 0,4 0,6 0,8 1 |u r/ a |, % hf/hm hay chudoba fea v = 0.4 0,8 1 1,2 1,4 0,1 0,2 0,3 0,4 𝑎 e ∕ 𝑎 o p poisson's ratio n = 0.0 0,8 1 1,2 1,4 0,1 0,2 0,3 0,4 poisson's ratio n = 0.2 0,8 1 1,2 1,4 0,1 0,2 0,3 0,4 poisson's ratio hay chudoba e/σy 10 25 50 100 n = 0.4 0,8 1 1,2 1,4 0,1 0,2 0,3 0,4 𝑎 f e a ∕ 𝑎 o p poisson's ratio n = 0.0 0,8 1 1,2 1,4 0,1 0,2 0,3 0,4 poisson's ratio n = 0.2 0,8 1 1,2 1,4 0,1 0,2 0,3 0,4 poisson's ratio e/σy 10 25 50 100 n = 0.4 hay chudoba http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 269 in actual experiments as shown in table 1, these limited results suggest that the radial displacement correction is small because the effect of difference in reference materials is larger than that of radial displacement. some studies pointed out that fused silica has densification effects, and higher elastic recovery than borosilicate glasses [9-10]. thus, the determination of area function also depends on characteristics of reference material. the cause for errors of elastic moduli measured by nanoindentation requires further investigation. 5. summary in this study, we examined the effect of radial displacement corrections using fea and experiments. hay’s method can obtain the contact area for calculating appropriate elastic modulus, whereas chudoba’s method can estimate the apparent contact area during loading. the further studies require to consider that the necessary correction method for obtaining appropriate elastic moduli and hardness values in actual indentation testing. 6. references [1] i. n. sneddon, “the relation between load and penetration in the axisymmetric boussinesq problem for a punch of arbitrary profile”, int. j. engng. sci., vol. 3, pp. 47-57, 1965. [2] w. c. oliver, g. m. pharr, “measurement of hardness and elastic modulus by instrumented indentation: advances in understanding and refinements to methodology”, j. mater. res., vol. 19, no. 1, pp. 3-20, 2004. [3] j. c. hay, a. bolshakov, g. m. pharr, “a critical the analysis of nanoindentation data”, j. mater. res., vol. 14, no. 6, pp. 2296-2305, 1999. [4] t. chudoba, n. m. jennett, “higher accuracy analysis of instrumented indentation data obtained with pointed indenters”, j. phys. d: appl. phys., vol. 41, pp. 215407, 2008. [5] iso 14577-1:2015, “instrumented indentation test for hardness and materials parameters – part 1 test method”. [6] n. schwarzer, “effect of lateral displacement on the surface stress distribution for cone and sphere contact”, phil. mag., vol. 86, pp. 5231-5237, 2006. [7] j. h. lee, g. m. pharr, y. gao, “corrections to the stiffness relationship in 3-sided and conical indentation problems”, int. j. solid. struct., vol. 166, pp. 154-166, 2019. [8] z.-h. xu, x. li, “effects of indenter geometry and material properties on the correction factor of sneddon’s relationship for nanoindentation of elastic and elastic-plastic materials”, acta mater., vol. 56, pp. 1399-1405, 2008. [9] k. r. gadelrab, f. a. bonilla, m. chiesa, “densification modelling of fused silica under nanoindentation”, journal of non-crystalline solids, vol. 358, pp. 392-398, 2012. [10] s. kasimuthumaniyan, a. a. reddy, n. m. a. krishnan, n. n. gosvami, “understanding the role of post-indentation recovery on the hardness of glasses: case of silica, borate, and borosilicate glasses”, journal of non-crystalline solids, vol. 534, pp. 119955, 2020. http://www.imeko.org/ first density comparison on viscoelastic samples by oscillation-type densimetry acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 79 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 79 84 first density comparison on viscoelastic samples by oscillation-type densimetry a. furtado1, j. pereira2, r. quendera3, m. schiebl4, e. lenard5, e. malejczyk 6, a. alic7, s. alisic8, j rauch9, f. lorenz10, a. bescupschii11, a. ciubara12, b. laky13, r. amsüss14 1,2,3 instituto português da qualidade (ipq), portugal, 1 afurtado@ipq.pt, 2 jpereira@ipq.pt, 3 rquendera@ipq.pt 4 physikalisch technischer prüfdienst des bundesamt für eich und vermessungswesen (bev-ptp), austria, markus.schiebl@bev.gv.at 5, 6 central office of measures (gum), poland, 5 elzbieta.lenard@gum.gov.pl, 6 ewa.malejczyk@gum.gov.pl 7, 8 institut za mjeriteljstvo bosne i hercegovine (imbih), bosnia and herzegovina, 7 azmir.alic@met.gov.ba, 8 sejla.alisic@met.gov.ba 9, 10 physikalisch technische bundesanstalt (ptb), germany, 9 juergen.rauch@ptb.de, 10 florenz@ptb.de 11, 12 i.p. institutul national de metrologie (inm), republic of moldova, 11 anatolii.bescupschii@inm.gov.md, 12 ana.curdov@inm.gov.md 13, 14 anton paar gmbh, austria, 13 barbara.laky@anton-paar.com, 14 robert.amsuess@anton-paar.com abstract: to provide a deeper insight about the damping effects produced by the viscoelasticity of nonnewtonian fluids during density measurements with oscillation-type density meters, and about how reference laboratories overcome these effects, an international comparison was performed, within the scope of the empir project 17rpt02 rholiq. the results of the comparison evidenced the possibility to measure density of viscoelastic samples by means of oscillation-type density meters with an uncertainty between 0.10 kg·m-3 and 0.25 kg·m-3. however, these instruments may be able to reach lower uncertainties if compared with higher precision density measurement methods such as hydrostatic weighing. keywords: density; viscoelasticity; oscillationtype densimetry: degree of equivalence; rholiq 1. introduction the measuring principle of an oscillation-type density meter is based on the law of harmonic oscillation [1]. in short, in these instruments, the measuring cell, that acts like a flexural oscillator, is filled with the fluid sample and is subjected to an oscillating force. the measuring cell oscillates at its own fundamental frequency, which is a function of the mass of the system. during the oscillation, the viscous component of a newtonian liquid causes, on one hand, a formation of a boundary layer which increases the inertial mass of the resonator and, on the other hand, a damping of the oscillation due to the wall shear stress acting on the resonator. consequently, these two parasitic effects lead to a lower resonance frequency of the measuring system and, thus, to an inaccurate density measurement. the detection of these influences is performed, in the instruments, by analysing the frequency of harmonics of the oscillation. so, these effects can be corrected by means of algorithms obtained during the calibration of the oscillator with viscous certified reference materials. these measuring instruments were proven to be able to measure density of newtonian fluids in a wide density, viscosity, and temperature ranges, with an uncertainty better than 0.001 kg·m-3 [2] when using adequate calibration methods, as the substitution method. density measurement uncertainty values between 0.010 kg·m-3 and 0.005 kg·m-3 can be achieved when regular calibration methods are used, i.e. by means of the used of reference liquids and calibration curves. however, most of the liquids measured by the end-users consist in viscoelastic materials, i.e. materials that, under a certain strain, show simultaneously viscous and elastic behaviour. the viscous portion behaves according to newton’s law of viscosity and the elastic portion to hooke’s law of elasticity. with the purpose of getting the first insight on how different laboratories handle the density measurements’ artefacts caused by the viscoelasticity of non-newtonian fluids when using oscillation-type density meters, an international comparison was performed, within the scope of the empir project 17rpt02 rholiq [3]. 2. description of the work this work presents the results of the first international comparison on density of viscoelastic http://www.imeko.org/ mailto:afurtado@ipq.pt mailto:jpereira@ipq.pt mailto:rquendera@ipq.pt mailto:markus.schiebl@bev.gv.at mailto:elzbieta.lenard@gum.gov.pl mailto:ewa.malejczyk@gum.gov.pl mailto:azmir.alic@met.gov.ba mailto:sejla.alisic@met.gov.ba mailto:juergen.rauch@ptb.de mailto:florenz@ptb.de mailto:anatolii.bescupschii@inm.gov.md mailto:ana.curdov@inm.gov.md mailto:barbara.laky@anton-paar.com mailto:robert.amsuess@anton-paar.com acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 80 samples by oscillation-type densimetry. this comparison involved a set of measurements performed with six oscillators, all borosilicate-made (dma 4500m, dma 5000, and dma 5000m all from anton paar), by five participants: five national metrology institutes (gum, poland; ipq, portugal; imbih, bosnia and herzegovina; ptb, germany and inm, moldova) and one density meter manufacturer (anton paar, austria). for this comparison, seven viscoelastic fluids were prepared and distributed among the participants: poly(vinyl alcohol) (pva) at 0.036 g·ml-1 and sodium borate (borax) at 0.0036 g·ml-1 (f1); carboxylpolymethylene (carbomer) solution at 0.15 cg·g-1 (f2); hydroxyethyl-cellulose at 0.5 cg·g-1 (f3); starch dispersion at 3 cg·g-1 (f4); poly(acrylamide-co-diallyldimethylammonium chloride) (p(aam-co-dadmac)) solution at 5.5 cg·g-1 (f5); commercial apple (f6) and grape juices (f7) (table 1). each participant followed their own density measuring procedure and have applied corrections to the density meters indications according to the calibration curve obtained with newtonian reference fluids. these curves allowed to describe the dependence of oscillation damping, in terms of density error, with viscosity of this type of fluids (with no elastic behaviour). calibration curves with non-newtonian liquids were not done as viscoelastic reference liquids are not available yet. therefore, none of the laboratories made corrections due to the elasticity of the samples tested in this comparison. the metrological traceability of density results of each participant is independent. all the measurements were performed at 20 ºc and at ambient pressure. the reference density value r (at 20 ºc) of each test fluid was determined by ipq and gum through gravimetric method with the use of pycnometers (table 1). these tests were performed by substitution weighing method and using the approach described in iso 2811-1 [4]. despite of its high uncertainty (when compared with the best uncertainty able to be achieved with oscillation-type density meters, i.e., 0.001 kg·m-3 [2]), the pycnometry method was chosen to be used as reference as it is a static density measurement method. in this way, the viscoelastic properties of the fluids are not expected to influence the accuracy and precision of these density measurements. hydrostatic weighing method could be used as reference alternative; however, this method was not yet studied with viscoelastic samples. this corresponds to one of the objectives of the rholiq project [3]. the mechanical properties of the samples, such as complex shear modulus 𝐺 ∗ = 𝐺 ′ + 𝑖𝐺′′, storage modulus 𝐺 ′ and loss modulus 𝐺′′ , under the oscillation frequency 𝜔 (equal to 2π𝑓 , where 𝑓 stands for frequency) produced during the density measurements in the oscillation-type density meters, usually in the oscillation frequency interval from 1715 hz to 1753 hz, were determined by oscillatory tests with a rheometer (haake mars iii, thermoscientific) using two different measuring geometries: a cone-plate (c35/2° ti l) for p(aamco-dadmac) and pva and borax samples and a concentric-cylinder (cc25 din ti) for the remaining samples. the loss or damping factor tan , was calculated as the quotient of the loss g’’ and the storage moduli g’. thus, this parameter gives the ratio between the viscous and the elastic portion of the viscoelastic deformation. table 1: codification of the test fluids (f#) and reference density values r, at 20 ºc and ambient pressure, and respective expanded uncertainty values ur (for k = 2) determined by gravimetric method with a pycnometer f# fluid r / kg·m-3 ur / kg·m-3 f1 pva and borax 1008.159 0.077 f2 carbomer 998.764 0.089 f3 hydroxyethyl-cellulose 999.645 0.070 f4 starch dispersion 1008.21 0.10 f5 p(aam-co-dadmac) 1012.139 0.086 f6 apple juice 1048.983 0.075 f7 grape juice 1068.107 0.071 the density values of the test fluids obtained by the oscillation-type density meters of each participant were compared against the reference density values r (obtained by gravimetric method using a pycnometer) by means of the normalised error en statistical analysis [5]. in addition, the mechanical properties of the tested fluids under such oscillation frequency 𝜔 was also correlated with the obtained density deviations. the measurement uncertainties were performed according to gum methodology [6]. 3. mechanical characterisation of viscoelastic samples the complex shear modulus, 𝐺 ∗ = 𝐺 ′ + 𝑖𝐺′′, of the viscoelastic samples were measured in the oscillation frequency range ω, from 0.6 hz to 628 hz, within linear viscoelastic region of the samples, by using a rheometer (haake mars iii, thermoscientific) with two different measuring geometries: a cone-plate (c35/2° ti l) for samples f1 and f5 and a concentric-cylinder (cc25 din ti) for the remaining samples. as the range of oscillation frequencies of interest (i.e. the ones produced by the density meter’s measuring cell) is higher than the frequency range able to be measured http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 81 by rheometry, an extrapolation methodology was used as described later. as demonstrated in figure 1, the storage 𝐺 ′ and the loss moduli 𝐺′′ of all seven samples closely follow an exponential frequency characteristic in the high frequency regime whereas, for low frequencies, considerable deviation from an exponential behaviour is observed. in rheology, viscoelastic materials can be modelled by determination of their response to a stimulus by an arbitrary combination of the properties of an ideal elastic, ideal viscous, and ideal malleable model materials. however, the behaviour modelled in this way still shows differences to the behaviour of real materials, even with complex model bodies. this has several reasons. the hookean and newtonian elements assume a linear relationship between stress and deformation, or deformation speed. however, there is often a non-linear relationship. with socalled non-newtonian fluids, effects such as dilatancy and structural viscosity occur, i.e. the viscosity is no longer a constant, but in turn depends on the rate of deformation. viscosity can even change with time while the deformation rate remains constant, which is called thixotropy or rheopexy. finally, the deformation behaviour also depends on external influences, especially temperature. thus, for the characterisation of the viscoelastic behaviour of the samples, a phenomenological approach was used. detailed analysis of the low-frequency dielectric relaxation reveals (figure 2 (bottom)) that the spectra in the low frequency regime of all samples can be better fitted to a modified power law behaviour given by: ln 𝐺 ′ = 𝐴′(ln 𝜔 − 𝐶′)𝑚′ + 𝐵′ (1) ln 𝐺′′ = 𝐴′′(ln 𝜔 − 𝐶′′)𝑚 ′′ + 𝐵′′ (2) with 0 ≤ 𝑚′ ≤ 1 and 0 ≤ 𝑚′′ ≤ 1. in order to obtain quantitative information of the complex viscosity at the resonance frequency of the oscillation-type density meter, the analysis of the complex shear modulus in the high-frequency regime was performed as follows. the storage 𝐺 ′ and the loss 𝐺 ′′ moduli of all seven samples closely follow an exponential frequency characteristic (figure 1 and figure 2 (top)). in order to prevent values with no physical meaning for the storage modulus 𝐺 ′ , which describes the elastic portion of the complex shear modulus, an upper limit of 𝐺s= 80 gpa, the shear modulus of steel, was considered for the fitting function. that means, that it was assumed, that the stiffness of all samples under test do not exceed the stiffness of steel. thus, the spectra in the high frequency regime for the storage modulus of all samples can be best fitted to: ln 𝐺 ′ = ln 𝐺s − 𝐵𝑒 −𝐶𝜔 (3) figure 1: storage 𝐺′ and loss moduli 𝐺′′ as a function of the oscillation frequency ω (2πf). the black dashed line is a guide to the eye which depicts an exponential behaviour at higher frequencies figure 2: fits of the storage modulus 𝐺 ′ , loss modulus 𝐺′′ , and damping factor tan  spectra against the oscillation frequency ω (2πf), according to equations (1) to (4), for the sample f1 http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 82 the loss modulus 𝐺 ′′ is not limited since it is governed by the viscous part of the material. the viscosity is defined by the relation between shear stress and the velocity gradient perpendicular to the deformation. hence, the spectra in the high frequency regime for the loss modulus of all samples can be best fitted to: ln 𝐺′′ = 𝐴𝜔 + 𝐶 (4) from the calculated values of 𝐺 ′ and 𝐺 ′′ , the viscous portion, ′ i.e. dynamic viscosity, and the elastic portion, ′′ , at the oscillation frequency produced in the oscillation-type density meter during the density measurement, were calculated according to, ′ = 𝐺′′ 𝜔 (5) and ′′ = 𝐺′ 𝜔 (6) the loss or damping factor tan  was calculated as the quotient of the loss and the storage modulus. thus, this parameter gives the ratio between the viscous and the elastic portion of the viscoelastic deformation. it is given by: tan  = 𝐺 ′′ 𝐺 ′ (7) if tan  > 1 the sample is in liquid state i.e. showing a behaviour of a viscoelastic liquid, for tan  < 1 the sample is in a solid state i.e. showing figure 3: damping factor tan  as a function of the oscillation frequency ω (2πf) for the seven samples tested a behaviour of viscoelastic solid, and for tan  = 1 a sol-gel transition will be met, i.e. having a viscoelastic behaviour showing 50:50 ratio of the viscous and elastic portions. from the analysis of the damping factor tan  (figure 3) the transition between the different frequency behaviour of the complex modulus may be related to the sol-gel transition of the material. however, discrepancies exist. the values of the storage 𝐺′ and loss 𝐺′′ moduli and damping factor tan  of the seven viscoelastic test fluids (f#) are given in table 2. in table 3 are shown the values of the dynamic ’ and “elastic” ’’ viscosities of the seven viscoelastic test fluids (f#) at oscillation frequency 𝜔 of the oscillation-type density meter obtained from the values of table 2 and equations (5) and (6). table 2: storage 𝐺′ and loss 𝐺′′ moduli and damping factor tan  of the seven viscoelastic test fluids (f#) at oscillation frequency 𝜔 of the oscillation-type density meter f# 𝝎 / hz 𝑮′ / pa 𝑮′′ / pa tan  f1 1746 5.4 × 106 9.9 × 105 0.18 f2 1750 2.1 × 108 1.6 × 1014 7 105 f3 1750 3.0 × 106 2.1 × 107 6.8 f4 1745 6.9 × 107 5.0 × 108 7.2 f5 1745 1.1 × 107 2.5 × 108 22 f6 1729 2.2 × 108 1.8 × 104 8.0·10-5 f7 1721 3.5 × 108 1.5 × 108 0.44 table 3: dynamic ’ and “elastic” ’’ viscosities of the seven viscoelastic test fluids (f#) at oscillation frequency 𝜔 of the oscillation-type density meter f# 𝝎 / hz ’ / pa·s ’’ / pa·s f1 1746 568 3088 f2 1750 9 × 1010 1.2 × 105 f3 1750 1.2 × 104 1727 f4 1745 2.9 × 105 4.0 × 104 f5 1745 1.4 × 105 6400 f6 1729 10.5 1.3 × 105 f7 1721 8.9 × 104 2.0 × 105 4. results of density measurements the assessment of density deviation values ´ obtained by each laboratory, and respective uncertainties u´, presented in table 3, was done by means of the normalised error en statistical analysis [5] and are summarised in table 4. the density values used as reference r are the ones obtained by gravimetric method and shown in table 1. the missing values of table 4 are due to some anomalies found in some of the samples. http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 83 table 3: summary of the density deviations ' and respective expanded uncertainties u´ (k = 2), obtained by six laboratories for the seven test fluids (f#) f# gum ipq imbih ´ u´ ´ u´ ´ u´ / 10-2 kg·m-3 f1 -2.3 7.8 2.3 9.4 -3.8 8.3 f2 -7.6 9.1 -7.2 9.1 f3 9.0 7.2 10.8 8.9 -30.2 20.8 f4 11.0 10.3 f5 -17.7 9.9 f6 3.6 7.7 1.3 7.8 -0.3 8.2 f7 4.6 7.2 5.8 7.4 -10.8 14.4 f# inm-md ptb anton paar ´ u´ ´ u´ ´ u´ / 10-2 kg·m-3 f1 -8.0 8.3 -8.9 7.8 -7.2 9.4 f2 -11.4 9.8 -13.4 9.1 -9.0 10.2 f3 7.1 7.7 3.7 7.1 9.1 8.4 f4 5.8 10.6 9.4 10.2 8.6 11.1 f5 -50.2 9.1 -18.6 8.8 5.9 21.9 f6 12.5 8.4 -1.1 7.7 f7 -4.4 7.7 -2.7 7.2 2.4 11.4 table 4: summary of the statistical analysis of the normalised error en obtained by the participant laboratories for each test fluid (f#) f# gum ipq imbih inm ptb anton paar en f1 0.30 0.25 0.46 0.96 1.14 0.77 f2 0.83 0.79 1.17 1.47 0.89 f3 1.26 1.21 1.45 0.92 0.52 1.08 f4 1.06 0.55 0.92 0.77 f5 1.79 5.52 2.11 0.27 f6 0.47 0.17 0.03 1.49 0.15 f7 0.63 0.78 0.75 0.57 0.38 0.21 the weighted density mean values 𝜌𝑊̅̅ ̅̅ (with respective expanded uncertainty u 𝜌𝑊̅̅ ̅̅ ) (table 5) and weighted relative density mean values 𝛿′𝜌𝑊̅̅ ̅̅ (with respective relative expanded uncertainties 𝑈𝛿′𝜌𝑊̅̅ ̅̅ ̅) (table 5), were calculated having in account only the satisfactory results (i.e., with en < 1) (table 6). for the calculation of the relative expanded uncertainties of the weighted relative density mean values 𝑈𝛿′𝜌𝑊̅̅ ̅̅ ̅ no correlation was considered among the values as each laboratory has its own traceability chain. this uncertainty also includes the contribution of the uncertainty due to the heterogeneity of the samples. table 6 demonstrates that, on average, 38 % of the measurements of these viscoelastic samples presented unsatisfactory measurement results, meaning that the laboratories are underestimating table 5: weighted density mean values 𝜌𝑊̅̅ ̅̅ , and respective expanded uncertainty u 𝜌𝑊̅̅ ̅̅ (k = 2) of unsatisfactory results f# 𝝆𝑾̅̅ ̅̅ / kg·m-3 u𝝆𝑾̅̅ ̅̅ / kg·m-3 f1 1008.12 0.19 f2 998.67 0.21 f3 999.70 0.10 f4 1008.29 0.18 f5 1012.20 0.22 f6 1048.99 0.16 f7 1068.09 0.24 table 6: summary of the weighted relative density deviation mean values 𝛿′𝜌𝑊̅̅ ̅̅ , respective relative expanded uncertainties 𝑈𝛿′𝜌𝑊̅̅ ̅̅ ̅ (k=2), and frequency f of unsatisfactory results (i.e., with en > 1) for each fluid (f#) f# 𝜹′𝝆𝑾̅̅ ̅̅ 𝑼𝜹′𝝆𝑾̅̅ ̅̅ ̅ f (en > 1) / % / % / % f1 -0.004 0.019 17 f2 -0.010 0.021 40 f3 0.005 0.010 67 f4 0.008 0.018 25 f5 0.006 0.022 80 f6 0.001 0.015 20 f7 -0.002 0.022 17 the uncertainty and/or the corrections applied to density indications to compensate measurement artefacts due to samples’ viscosity and elasticity are not sufficiently established due to the lack of previous knowledge on this subject. however, the establishment of a calibration curve for viscoelastic fluids, due the great number of possible variations on the viscous and elastic portion, is not an easy task to achieve. another point that should be taken into consideration is the stability and homogeneity of the samples, since they were not all measured at the same time by the laboratories and the long-term stability and the stability under transportation condition was not tested, and therefore not considered. in table 6, the high frequencies of unsatisfactory values (f (en > 1)) presented in fluids f2, f3 and f5 can be a proof of this. previous unpublished investigations, of newtonian liquids, showed a linear dependence of density deviations on the squared root of dynamic viscosity (√′) up to viscosity of ~0.3 pa·s reaching a plateau of maximum density deviation value achieved when the thickness of the fluid boundary layer was equal to the inner diameter of the oscillator cell, where the density deviation does not longer depends on the squared root of dynamic viscosity (√′). however, for the lowest value of dynamic viscosity of the tested viscoelastic samples is 10 pa·s, meaning that the mentioned plateau was already achieved, so considering the weighted relative density deviation mean values 𝛿′𝜌𝑊̅̅ ̅̅ http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 84 (table 6 and figure 4) can be seen that it was not found any dependence of density deviation with the squared root of dynamic viscosity (√′). the influence of viscoelastic properties of a fluid on the resonance frequency of the oscillation-type density meter is given by, on the one hand, that additional harmonic forces act on the oscillator due to the elastic portion of the shear modulus of the fluid. this leads to an apparent increase of the spring constant of the oscillator. on the other hand, the viscous portion of the shear modulus of the fluid increases the inert mass of the oscillator due to the movement of fluid layers in the boundary layer which is in phase with the oscillator. figure 4: weighted relative density mean values 𝛿′𝜌𝑊̅̅ ̅̅ and respective uncertainties (vertical bars), obtained for the seven tested viscoelastic fluids (f#), against the squared root of dynamic viscosity (√′) of the fluids. legend: s or l – sample at solid (s in red) or liquid (l in blue) state at the oscillation frequency produced in the oscillation-type density meter 5. summary this paper shows the results of the first international comparison on density of viscoelastic samples by oscillation-type densimetry, allowing to conclude about the degree of equivalence of density measurements of this type of samples with this measuring method, by experts on this metrology field. from the results it seems to be possible to measure density of viscoelastic samples by means of oscillation-type density meters with an uncertainty between 0.10 kg·m-3 and 0.25 kg·m-3. the lower limit of this uncertainty interval was limited by the uncertainty of the method used as reference (pycnometry with a measurement uncertainty of ~0.10 kg·m-3), meaning that these instruments may be able to reach lower uncertainties if compare with higher precision density measurement methods such as hydrostatic weighing. further investigations will be done on this topic under the scope of the empir project “17rpt02-rholiq”. this work was developed under the framework of the empir project “17rpt02-rholiq” which is carried out with funding of european union under the empir. the empir is jointly funded by the empir participating countries within euramet and the european union. 6. references [1] h. stabinger, “density measurement using modern oscillating transducers”, south yorkshire trading standards unit, sheffield, 1994. [2] h. schmidt, h. wolf, e. hassel, “a method to measure the density of seawater accurately to the level of 10−6”, metrologia, vol. 53, no. 2, p. 770, 2016. [3] a. furtado, j. pereira, m. schiebl et al., “establishing traceability for liquid density measurements in europe: 17rpt02-rholiq a new empir joint research project”, journal of physics: conference series, vol. 1065, no. 8, 082013, iop publishing, 2018. [4] iso 2811-1, “paints and varnishes determination of density part 1: pycnometer method”, 2016. [5] iso 13528, “statistical methods for use in proficiency testing by interlaboratory comparison”, 2015. [6] jcgm 100, “evaluation of measurement data – guide to the expression of uncertainty in measurement”, bipm, 2008. http://www.imeko.org/ validation of a modular and wearable system for tracking fingers movements acta imeko issn: 2221-870x december 2020, volume 9, number 4, 157 164 acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 157 validation of a modular and wearable system for tracking fingers movements michela borghetti1, paolo bellitti1, nicola francesco lopomo1, mauro serpelloni1, emilio sardini1 1 department of information engineering, university of brescia, via branze 38, 25123 brescia, italy section: research paper keywords: finger movement tracking; wearable module; inertial measurement unit; stretch sensor; industrial setting; data validation; opto-electronic systems citation: michela borghetti, paolo bellitti, nicola francesco lopomo, mauro serpelloni, emilio sardini, validation of a modular and wearable system for tracking fingers movements, acta imeko, vol. 9, no. 4, article 21, december 2020, identifier: imeko-acta-09 (2020)-04-21 section editor: francesco bonavolonta, university of naples federico ii, italy received november 8, 2019; in final form august 8, 2020; published december 2020 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was supported through grant prin 2015c37b25 by the italian ministry of instruction, university and research (miur) corresponding author: michela borghetti, e-mail: michela.borghetti@unibs.it 1. introduction in recent years, industry 4.0 has radically revolutionised manufacturing processes and industrial production [1]. the rapid increase in the level of automation and the introduction of information and communication technologies into the manufacturing world are enabling more efficient and more flexible production processes that can fabricate higher-quality goods at reduced cost and at high production rates [2]. in this context, industrial settings are transformed into smart factories; information about processes is shared in real time through the internet of things and cyber-physical production systems in order to improve efficiency and throughput as well as the quality of the final products [3], [4]. although one of the main principles of industry 4.0 is that the cyber-physical system should make decisions and perform tasks as autonomously as possible, the worker remains one of the most important resources inside the factory; in the era of digitalisation and data exchange, the supervision of workers’ actions inside the factory is crucial [5]. for example, manual tasks are often required to guarantee flexibility in the production process, especially in semi-automated or non-automated settings, and incorrect movements and action may significantly affect the production processes [6]. in other situations, workers control and supervise industrial robots to perform repetitive and exhausting tasks or for the inspection and maintenance of hazardous and extreme settings in plants [7]. a valuable way to interact with robots without physical interaction is the use of hand gestures [8]. an important key element in industry 4.0 are collaborative robots, which are specially designed to work together with humans. in this context, the workers’ movements and position should be monitored continuously to ensure the workers’ safety and to improve the collaboration and the production workflow [9]. finally, assembly tasks are based on the repetitive composition of different parts to produce the final product, and the quality of the product is affected by human errors, especially when the manual operations contain many steps abstract supervising manual operations performed by workers in industrial environments is crucial in a smart factory. indeed, the production of products with superior quality at higher throughput rates and reduced costs with the support of industry 4.0-enabling technologies is based on the strict control of all resources inside the factory, including workers. this paper shows a protocol for validating a new wearable system for tracking finger movements. the wearable system consists of two measuring modules worn on the thumb and index finger that measure flexion and extension of the proximal interphalangeal (pip) joint by a stretch sensor and rotation of the proximal phalanx (pp) by an inertial measurement unit. a marker-based opto-electronic system is used to validate the proposed device by capturing specific finger movements. four movements that simulate typical tasks and gestures, such as grasp and pinch, were specifically performed. the maximum root-mean-square error is 3.7 deg for the roll angle of pp. the resistance changes of the stretch sensors with respect to flexion and extension of the pip joint is 0.47 /deg. the results are useful for data interpretation when the system is adopted to monitor finger movements and gestures. mailto:michela.borghetti@unibs.it acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 158 with a specific order and many different operating objects [6], [10]. indeed, the efficiency of the operation depends on quickly finding the individual parts in the warehouse and on the degree of training enabling the worker to follow the specific assembly order effectively [11]. furthermore, musculoskeletal injuries known as repetitive motion disorders are the result of the combination of incorrect and repetitive movements in workshops and individual predispositions [12]. motion tracking systems are currently used in different applications, and they are an ongoing topic of research [13]-[15]. finger tracking systems are used for movement analysis during the execution of precise actions [16], for object design in combination with virtual reality [17], for rehabilitation purposes [18], [19] and for tracking hand gestures used with humanmachine interfaces [20]-[22]. finger tracking systems are mainly used in vision-based and contact-based systems [8]. a visionbased system consists of a set of cameras and image processing algorithms. mechanical constraints are reduced, especially in the case of markerless motion capture systems, and this guarantees good freedom of movement in the space. in order to increase accuracy, several active or passive markers should be placed on the hand while multiple cameras record the displacement of markers in order to capture all the movements [23]. optoelectronic systems are extensively used to measure human kinematics (e.g. vicon, bts, optitrack, etc.) [24]. in [25], an opto-electronic marker system was used to validate a glove equipped with 11 inertial measurement units. these systems require expensive and complex equipment, an accurate calibration algorithm and a confined space, such as a laboratory [26]. the illumination is a critical condition in order to properly capture properly all the segments [27]; moreover, waste and dust deposited on the lens may significantly affect the measurements. for all these reasons, vision-based systems are rarely adopted for industrial applications, in less-constrained settings or for continuous monitoring [28]. contact-based systems include wearable devices equipped with sensors and electronics for elaborating and transmitting measurement data to an external elaboration unit. they are basically composed of low-cost components and they can be easily used in many applications and environments. a wearable device suitable for industrial applications should be easily wearable and should comply with workspace safety rules. furthermore, the device should not introduce mechanical constrains that limit the task execution. in previous works [11] and [29], we proposed different measurement systems for tracking finger movement, composed of different wearable modules with embedded sensors and electronics. the electronics elaborate the measurement signals and transmit the elaborated data to a mobile device, which displays and collects the data by using a software specifically designed for this application. the wearable systems can be worn in a minimally invasive way, and we demonstrated that the system configurated with two measurement modules could be a valuable solution for many industrial applications by simulating tasks potentially performed by a worker in the industrial field. in this paper, we show the validation of the data collected by the wearable system proposed in [11] using a marker-based optoelectronic system. in section 2, we describe the measurement system. in section 3, the experimental setup and protocol used for the data validation are given, and in section 4, we show and discuss the experimental results. finally, in the concluding section, the major findings are summarised. 2. system description the proposed system (shown in figure 1) was designed to be modular, light, low-cost, battery powered and cable-free. it is composed of two sections: 1) a wearable system equipped with sensors and electronics for measuring finger motion and transmitting data and 2) a local device (a laptop) for collecting and displaying data measurements. the wearable system is configurable, and it is composed of wearable and independent measuring modules (shown in figure 2). the number of modules depends on the application and on the physical constraints. each module communicates with the laptop directly via bluetooth, reducing the number of wires and cables that could obstruct the finger movements. in this work, two modules were used to test and validate the system: one was worn on the thumb and the other on the index finger of the right hand. the module design allows the system to be worn on different fingers of both hands with few changes. the box (dimensions: 36 mm × 25 mm × 10 mm) and the ring that supports this box are made of polylactic acid, and they were made using fused deposition modelling technology for an easy and tailor-made fabrication of the module according to finger size. the box encloses the electronic board and the 40 mah polymer figure 1. measuring system for tracking the movements of the index finger and the thumb. in this example, the system is worn on the right hand (index finger and thumb), but it could be worn on the left hand without changes. the wearable system provides information about the flexion/extension of the pip joint by reading the resistance of the stretch sensors and about the orientation of the proximal phalanges through the imu’s output. module 1 module 2 module 2 module 1 wearable system local device acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 159 li-ion rechargeable battery (20 mm × 11 mm × 3 mm), for a total weight of 14 g. a more extended description of the module is reported in [11]. the finger movements are measured by an inertial measurement unit (imu) mounted on the electronic board and a piezoresistive sensor. the lms9ds1, produced by stmicroelectronics, is used as a 9 degrees of freedom imu, which incorporates a 3-axis accelerometer, a 3-axis gyroscope and a 3-axis magnetometer for the determination of the proximal phalanx (pp) orientation. the orientation of each sensor is shown in figure 3. the three-dimensional (3d) coordinate system of the accelerometer and gyroscope do not satisfy the right-hand rule. flexion and extension of the proximal interphalangeal (pip) joint is monitored by the stretch sensor sold by image scientific [30], which is placed over the dorsal side of the finger. the stretch sensor is made of a conductive rubber whose electrical resistance varies with the applied force and thus with the sensor elongation. the nominal cord resistance is 395 /cm in its unstressed state. one sensor end is anchored to the plastic ring, and the other one is fixed to an adjustable onewrap hook and loop fastener tied around the middle phalanx. the sensor length is determined by finger size. a voltage divider circuit included in the electronic board is used to convert the resistance change into a voltage measurement and thus into a digital signal. atmega328p is the microcontroller for the electronic board and supervises the measuring process, such as the communication with the laptop. once the communication with the laptop is established, the microcontroller collects the measurements from the sensors and sends them to the laptop every 45 ms. the resolution of the analogue–digital converter is 3.22 mv/lsb, which corresponds to a resolution in the measurement of sensor resistance of 0.86 ω/lsb. in order to reduce the power consumption, a bluetooth low-energy (ble) module, rn4871 from microchip, is used for wireless communication. the gatt roles (generic attribute profile) are used to exchange data in accordance with the ble protocol. the module can properly operate with the input voltage of a 3.7 v rechargeable battery thanks to the on-board low-dropout voltage regulator (tps71533, texas instruments), with an overall power consumption of 75 mw. on the laptop side, a labview virtual instrument developed for this specific application collects, stores and displays the data sent by the two modules on the same screen. 3. validation procedure 3.1. experimental setup a male subject wore the measuring system on the thumb and the index finger of his right hand. the imus were calibrated according to the standard calibration procedures (an example is reported in [31]). the relationship between the strain and the resistance change of the stretch sensor is reported in [29] and [32]. the calculated gauge factor was 4.73 and the linear correlation coefficient (r2) was equal to 0.98 in the range 0– 10 %. a marker-based opto-electronic system (dx400, bts bioengineering s.p.a) including 8 cameras acquiring at 100 fps and a dedicated marker-set was exploited to have a ground truth reference for the joint kinematics. the calibrated acquisition volume was approximatively 1.00 m x 1.00 m x 1.00 m. in these tests, the measurements acquired by the proposed system and by the opto-electronic system were post-processed to estimate the orientation and rotation of the fingers. 3.2. marker protocol a configuration of 11 markers was used, covering the palm of the hand, the proximal and distal phalanx of the thumb and the proximal and middle phalanx of the index finger (figure 4). in particular, three markers were located approximately at the ulna styloid process, the radius styloid process and the dorsum of the hand (i.e. the middle of the third metacarpal bone). the thumb was fit with 4 markers, three at the pp – exploiting the thumb board enclosure – and one at the tip of the distal phalanx. equivalently, 4 markers were used for the index finger: three markers at the pp – using the index board enclosure – and one on the distal joint of the middle phalanx. for both the static pose and each acquired movement, 3d marker coordinates were computed in every frame (100 fps) of the time sequence and used to calibrate the model and estimate joint kinematics. 3.3. coordinate frames and kinematic assessment in order to correctly decompose joint kinematics, during the static phase a specific coordinate frame was defined for each segment involved in the kinematic chain. in particular, the coordinate frame of the hand segment was defined as in figure 4: figure 2. coordinate frame and description of the measuring module to be worn on the finger. figure 3. electronic board including imu and electronic components for conditioning, elaborating and transmitting measurement data. the accelerometer and the gyroscope have a different orientation with respect to the magnetometer, and this aspect is considered in the data elaboration algorithm used to measure the phalanx orientation. x y z stretch sensor electronic board battery imu adjustable ring plastic ring top side bottom side ble module mcu ldo imu x y z z y x magnetometer accelerometer/ gyroscope acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 160 • x-axis is the direction identified by wrist flexion/extension axis (i.e. the axis connecting the markers placed on the ulnar and radial styloids). • z-axis is the normal axis identified by the hand plane defined by the 2 markers on the wrist and the marker on the dorsum of the hand • y-axis is the cross-product between the z-axis and y-axis for the hand. for both the thumb pp and the index finger pp, the markers placed on the board enclosures and the specific configuration were also used to define the anatomical coordinate systems: • y-axis direction is defined by the two markers placed on the longer side of the enclosure. • x-axis direction is defined by the markers on the shorter side. • z-axis is the cross-product between the x-axis and the yaxis. the anatomical coordinate frames for the distal phalanxes were then defined by starting from the board coordinate frames and shifting them by an appropriate quantity, taking into account the dimensions of both the boards, the support and the fingers. for the distal phalanx of the thumb and the middle phalanx of the index finger, the coordinate frames were defined by keeping the hand in a neutral posture and considering the coordinate frames aligned with the most proximal ones but moved along the y-axis by the length of the proximal segments themselves. once each coordinate frame was defined, the local position of each marker was defined. then, for the kinematic analysis, the position and orientation of the coordinate frames were estimated by minimising the local position of each marker with respect to its global position. minimisation was performed through leastsquares optimisation. the coordinate frame of moving imus was aligned as shown in figure 2, and it was aligned with the coordinate frame of the segments (index finger and thumb) detected by the optoelectronic system. the rotation matrix associated with the imu measurements was calculated after the acquisition by using the kalman filter algorithm implemented in sensor fusion and tracking toolbox (matlab r2019b). since the coordinate system of the accelerometer and of the gyroscope do not satisfy the right-hand rule, the measurements of acceleration in the xdirection were swapped. the orientation of the segments and of the imus were referred to the neutral posture: wrist, thumb and index finger were kept unflexed with the index finger and palm parallel to the ground. joint kinematics (i.e. angles) was then assessed by decomposing joint rotations following euler’s xyz notation (defined as roll, pitch, yaw). 3.4. experimental protocol during the experiment, the subject was seated in the chair and his forearm was supported by the table. considering the experimental setup and the final application of the proposed system in industrial settings, we selected four simple movements shown in figure 5. the selected movements involve the flexion/extension as well as the adduction/abduction of the fingers. using simple movements and poses is considered a valuable method to validate wearable systems for tracking finger movements [11], [33]. all the motions started with the hand in a neutral position (all fingers were kept extended and closed). in m1, the subject grasped and released a ball, while in m2, the subject closed his hand, simulating the grasp of a small object. in m3, the subject grasped and released the screwdriver handle. figure 4. marker protocol. figure 5. illustration of the movements involved in this study: m1 ball grasp; m2 virtual grasp; m3 screwdriver grasp; m4 pinch. figure 6. wrist flexion and extension for x-axis detection. on the left, the neutral posture is shown. palm markers index markers thumb markers x y z m1 m2 m3 m4 acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 161 finally, in m4, the subject executed the pinch trial. each movement was repeated five times. before starting the test, the subjected was asked to flex and extend his wrist (figure 6) in order to determine the x-axis of the hand segment coordinate frame. 4. results and discussion for the opto-electronic system, according to the calibration procedure and to the experimental setup, the median root-meansquare error (rmse) on the estimation of the single axis of the associated coordinate frame was 0.5 deg, 0.4 deg and 0.6 deg for x-axis, y-axis and z-axis, respectively. the elaborated measurements of the index finger movements collected during the m1 test are shown in figure 7 and figure 8. here, we analysed the postures of the index finger while the subject grasped and released the ball five times. the orientation of the pp of the index finger measured by the opto-electronic system and by the imu is shown in figure 7, where roll, pitch and yaw angles are the extrinsic rotation of the x-axis, y-axis and z-axis of the index pp frame relative to the index pp frame defined in the neutral posture. as expected, the main rotation is around the x-axis (of 30 deg maximum), whereas the yaw angle is limited; indeed, the metacarpophalangeal joint has only two degrees of freedom: flexion and extension and ulnar/radial deviation [31]. the summary of the results reported in table 1 shows the reliability of the imu measurements, which are comparable to the ones obtained by the opto-electronic system as highlighted by the rmse value. as shown in figure 8, the stretch sensor measures a resistance change of 30 % with respect to the resistance measured keeping the hand in the neutral posture, and this result is obtained when the opto-electronic system measures a roll angle of 60 deg for the middle phalanx. in this case, the roll angle was calculated with respect to the rotating coordinate frame of the pp, and thus it represents flexion and extension of the pip joint. the other two angles are not shown because their values are negligible; indeed, the pip joint has only one degree of freedom [34]. the resistance variation of the stretch sensor with respect to the roll angle figure 7. orientation of the pp of the index finger measured by the optoelectronic system (dashed line) and the imu (solid line) during m1 movement. figure 8. rotation of the pip joint of the index finger measured by the optoelectronic system (on the top) and by the stretch (on the bottom) during m1 movement. figure 9. rotation of a) pip joint of the index finger measured by optoelectronic system (x-axis) and by stretch (y-axis); b) metacarpophalangeal (mcp) joint of the index finger measured by opto-electronic system (x-axis) and by imu (y-axis). table 1. summary of results depicted in figure 7. for each orientation, span is the maximum range of motion measured by the opto-electronic system, rmse is the deviation of the measurements obtained by the imu from the ones obtained by the opto-electronic system, and nrmse relates rmse to span orientation span (deg) rmse (deg) nrmse (%) roll 41.4 1.7 4.1 pitch 17.1 1.9 11.1 yaw 10.9 2.7 24.8 acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 162 variation of the pip joint measured by the opto-electronic system is 0.45 /deg. this value is the slope of the line that combines the measurements of the stretch sensors with the ones of the opto-electronic system, and it is calculated through the linear least-squares method. the correlation of the two systems is 0.988 (pearson correlation), and thus sensor output is linear throughout the pip flexion/extension range with a negligible variation of the sensitivity, as shown in figure 9a). the line represents the average of five repetitions, and the shaded area represents the standard deviation. the relationship between the roll angle of the pp of the index finger obtained from imu measurements and the one obtained from the opto-electronic system is linear, as shown in figure 9b). a similar result is obtained for the pitch angle of the pp of the index finger. the measurements obtained by the proposed measuring system and by the opto-electronic system during all the tests m1, m2, m3 and m4 for the index finger are summarised in figure 10 and figure 11. for better viewing, only one repetition for each movement summarised in figure 5 is shown, since the results are similar for the entire duration of the test, as proved by the results of the m1 test in figure 7 and figure 8. the roll angles measured by the two systems are shown in figure 10, since the values of the other angles are negligible. depending on the type of movement, the range of motion of both the phalanges changes. as expected, the maximum rotation is obtained when the subject grasps the handle of the screwdriver (m3 test), while the minimum rotation is obtained during the m1 test. in all of the cases, the median rmse of roll angle is lower than 3.7 deg. the roll angles of the pip joint and the resistance of the stretch sensor are compared in figure 11 during one repetition of the movements m1, m2, m3 and m4. in these cases as well, the stretch sensor can be correlated with the flexion and extension of the pip joint. considering all of the tests, the resistance of the stretch sensor with respect to the roll angle of the pip joint measured by the opto-electronic system is 0.47 /deg, and it is similar in all the tests, as is the repeatability and the hysteresis. we verified that the correlations between the measurement systems are the same as shown in figure 9a) and in figure 9b) for all the movements. although the flexion of the pip joint is similar for movements m1 and m4, the combination of the measurements obtained by the imu and the stretch sensor makes it possible to distinguish these two different gestures, and thus many others. figure 10. comparison between the opto-electronic system (dashed line) and the imu (solid line) considering the rotation of the pp of the index finger. the roll angle represents the flexion and extension angle of the mpc joint during the following movements: m1 ball grasp; m2 virtual grasp; m3 screwdriver grasp; m4 pinch. an illustration of the movements is shown in figure 5. figure 11. comparison between the opto-electronic system (on the top) and the stretch sensor (on the bottom) considering the pip joint of the index finger. the roll angle represents the flexion and extension angle of the pip joint during one repetition of the following movements: m1 ball grasp; m2 virtual grasp; m3 screwdriver grasp; m4 pinch. an illustration of the movements is shown in figure 5. acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 163 5. conclusions median rmse in roll angles of the pp of the index finger between the imu and the opto-electronic marker system is between 1.7 deg and 3.7 deg. the smallest difference is observed when the subject grasps and releases a small ball, while the largest difference is observed during the pinch movement. the differences between the two systems could be partially explained by skin movements artifacts that cause the relative marker movements and that affect the estimation of the segment orientation. drift errors of the imus and possible distortions of the static magnetic field could equally affect the measurements and the estimation of the segments. finally, the decay of the resistance of the stretch sensor over time influences the measurements. all these sources of error are largely discussed in the literature [25]. the proposed system equipped with imus and stretch sensors could be adopted for monitoring finger movement tasks in a variety of conditions, including in an industrial setting. for example, wearable electronics for finger tracking are used to evaluating workers’ tasks to improve their efficiency and safety [35] and for virtual reality [36]. these devices are usually equipped with imus, bend sensors and piezoresistive sensors due to their weight, costs and dimensions [37]; the resulting wearable devices unobtrusively detect fingers movements while the workers perform their tasks and provide affordable measurements. in the previous work [11], the proposed system was successfully used to supervise a worker during assembly operations and to identify simple gestures for human–robot collaboration. the validation tests and the results reported in this paper need to be considered for the interpretation of the data when the proposed system is used to monitor finger movements and gestures. the proposed validation protocol is adopted for the evaluation of the system’s performance when the system is used to measure thumb and index finger movements. further implementations will be introduced to detect more complex movements of the hand. a gesture recognition approach based on a deep learning model will be considered for human– computer interaction through the proposed wearable system. references [1] m. hermann, t. pentek, b. otto, design principles for industry 4.0 scenarios, proc. 49th hawaii 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https://ulster-staging.pure.elsevier.com/files/11555412/sensordevices2015.pdf https://doi.org/10.1109/memea.2014.6860127 3d-printers dielectric materials characterization at microwave frequencies acta imeko issn: 2221-870x september 2020, volume 9, number 3, 26 32 acta imeko | www.imeko.org september 2020 | volume 9 | number 3 | 26 characterisation of dielectric 3d-printing materials at microwave frequencies andrea alimenti1, kostiantyn torokhtii1, nicola pompeo1, emanuele piuzzi2, enrico silva1 1 dipartimento di ingegneria, università degli studi roma tre, 00146 roma, italy 2 dipartimento di ingegneria dell’informazione, elettronica e telecomunicazioni, sapienza università di roma, roma, italy section: research paper keywords: microwave characterisation; loss tangent; 3d-printer materials; dielectric resonator citation: andrea alimenti, kostiantyn torokhtii, nicola pompeo, emanuele piuzzi, enrico silva, 3d-printers dielectric materials characterization at microwave frequencies, acta imeko, vol. 9, no. 3, article 5, september 2020, identifier: imeko-acta-09 (2020)-03-05 editor: jan saliga, technical university of košice, slovakia received january 17, 2020; in final form april 14, 2020; published september 2020 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: andrea alimenti, e-mail: andrea.alimenti@uniroma3.it 1. introduction in recent years, the fast development and improvement of 3d-printing technologies has had a significant impact on many areas of human activity [1]-[5]. printed materials are now utilised in high-frequency applications such as telecommunication technologies and microwave frequencies [6]-[12]. thus, a reliable and handy electromagnetic (e.m.) characterisation of these materials is increasingly required [13]. in this work, the microwave characterisation of plastic 3dprinting materials is performed using a resonant perturbative technique. the physical quantity under investigation is permittivity , which is defined as the quantity (in a more general sense, the tensor) that describes the proportionality between the electric displacement vector 𝑫 and the electric field strength vector 𝑬 in a medium, 𝑫 = 0𝜺 ⋅ 𝑬, with 0 being the vacuum permittivity. in the case of the materials studied here, after one accounts for the isotropic, homogeneous and linear limits, then 𝜺 = ̃ is a scalar quantity that does not depend on positioning. the scalar complex relative permittivity is defined as ̃ = ′ − i ′′, where the real part ′ is a measure of the energy storage properties of the medium, while the imaginary part ′′ is related to the e.m. losses, and i = √−1. since ̃ is a complex quantity, it is often represented on the complex plane, where the angle 𝛿 between ̃ and the real axis is known as a loss angle. thus, the ratio ′′ ′⁄ = tan𝛿 is called the loss tangent. in this work, a microwave (∼ 12.9 ghz) measurement method based on a resonant technique is proposed. the dielectric loaded resonator (dr) presented here was designed considering the e.m. properties of the dielectric materials usually used in 3d-printing and exploiting the possibility, given by 3dprinters, of shaping the samples in almost every desired shape. in the paper, an in-depth uncertainty analysis shows how the designed resonator is optimized to provide the best accuracy for this kind of material [14]. however, it is clear that its use is not limited only to 3d-printing plastics but to every kind of material electromagnetically and mechanically similar to them. moreover, to improve the repeatability of the measurement, the dr was specially designed to measure ̃ by placing the dielectric sample on one of its flat bases, removing the need to disassemble and reassemble the whole structure for each measurement and thus reducing the uncertainties involved. drs are well known for their high sensitivity [15] but also for poor measurement repeatability [16], [17], particularly in relation to resonant frequency. thus, a closed structure is especially useful for characterising samples as it reduces the systematic errors inevitably introduced by each mounting procedure. moreover, since it is not necessary to reassemble the resonator for each measurement, the measurement time is reduced. abstract 3d-printer materials are becoming increasingly appealing, especially for high frequency applications. as such, the electromagnetic characterisation of these materials is an important step in evaluating their applicability for new technological devices. we present a measurement method for complex permittivity evaluation based on a dielectric loaded resonator (dr). comparing the quality factor q of the dr with a disk-shaped sample placed on a dr base, with q obtained when the sample is substituted with an air gap, allows a reliable determination of the loss tangent. mailto:andrea.alimenti@uniroma3.it acta imeko | www.imeko.org september 2020 | volume 9 | number 3 | 27 a part of the volume of the resonating structure is substituted with the dielectric material under study. comparison of the changes induced by the insertion of the sample on the unloaded quality factor 𝑄 and the resonant frequency 𝑓0 can be used to evaluate the electric/magnetic properties of the sample. if the changes to the distribution of the e.m. field caused by the insertion of the sample are ‘small’, the resonant medium perturbation method [15] can be used. dielectric printed materials are already used for high frequency applications, and some works have explored their dielectric permittivity. of particular note is the result obtained in [18], where acrylonitrile butadiene styrene (abs) doped with different quantities of batio3 microparticles gave 2.6 < ′ < 8.7 and 0.005 < tan 𝛿 < 0.027, thus opening up the possibility of engineering these materials for specific uses. the measurements were performed at 15 ghz with a split post dielectric resonator obtaining 𝑢(tan 𝛿)/ tan 𝛿 ∼ 0.4 %. the split post resonator is a very sensitive measurement instrument but does have some critical issues related to its assembly procedure [15]. the resonator presented in this work is tuned to a similar frequency (∼ 12.9 ghz) and has a somewhat reduced sensitivity compared with the split post resonator but a much improved ease of operation – a useful feature in view of the routine measurements required. in sec. 2, the measurement method and system are presented. then, in sec. 3, a detailed uncertainties analysis is provided, while the results are shown in sec. 4. finally, conclusions are given in sec. 5, comparing the obtained results to those given by a standard waveguide transmission/reflection method and to other relevant scientific works [13], [18], [19]. 2. description of the method a special configuration of a hakki–coleman dielectric loaded resonator [20] was designed to guarantee enhanced measurement repeatability at room temperature. the two physical quantities that characterise the response of the resonator are the unloaded quality factor 𝑄 and the resonant frequency 𝑓0. 𝑄 is defined as 𝑄 = 𝜔0𝑊/𝑃, where 𝑊 is the energy stored into the resonator at the resonant angular frequency 𝜔0 = 2𝜋𝑓0 and 𝑃 is the power dissipated at the same frequency. thus, as will be shown below, one can obtain information about the dielectric losses of the material under study (i.e. tan𝛿) from the 𝑄 measurement. 𝑃 is the sum of all the power losses 𝑃 = 𝑃𝑆 + 𝑃ω + 𝑃𝑉 , where the subscripts 𝑆, ω, 𝑉 indicate the quantities related, respectively, to the sample, to the metal surfaces and to all the other dielectric materials inside the resonator volume. hence: 1 𝑄 = 𝑃𝑆 + 𝑃ω + 𝑃𝑉 𝜔0𝑊 = 1 𝑄𝑆 + 1 𝑄ω + 1 𝑄𝑉 , (1) with: 1 𝑄𝑆 = ∫ ε𝑆 ′′ε0|𝐄| 2d𝑉 𝑉𝑆 2𝑊 = [ ε𝑆 ′ ∫ ε0|𝐄| 2d𝑉 𝑉𝑆 2𝑊 ] ε𝑆 ′′ ε𝑆 ′ = 𝜂𝑆tan𝛿, (2) 1 𝑄ω = ∑ ∫ 𝑅𝑖|𝑯𝜏 | 2d𝑆 𝑆𝑖 2𝑊 𝑖 = ∑ 𝑅𝑖 𝐺𝑖𝑖 , (3) 1 𝑄𝑉 = ∫ 𝑉 ′′ 0|𝑬| 2𝑑𝑉 𝑉𝑉 2𝑊 = 𝜂𝑉 tan 𝛿𝑉 , (4) where 𝑬 is the electric field and 𝑯𝜏 is the magnetic field tangential to the 𝑖-th metallic surface 𝑆𝑖 with surface resistance 𝑅𝑖 and geometrical factor 𝐺𝑖 . 𝜂𝑆 and 𝜂𝑉 are the filling factors of the sample and of the dielectric elements inside the resonator, respectively. thus: 1 𝑄 = 𝜂𝑆 tan 𝛿𝑆 + ∑ 𝑅𝑖 𝐺𝑖 + 𝜂𝑉 tan 𝛿𝑉 𝑖 . (5) since 𝑊 and the field configuration depend on ′ of all the dielectric elements inside the resonator, both 𝜂 and 𝐺 are also functions of 𝑠 ′ . therefore, to evaluate tan 𝛿𝑠 of the dielectric sample placed in the resonator from eq. (5), 𝑠 ′ must be measured. to achieve this aim, one can exploit 𝑓0 of the resonator as a second measurand. however, it is known that the absolute value of 𝑓0 is strongly affected by many intrinsic (e.g. the electromagnetic properties of the elements inside the resonator) and extrinsic (e.g. temperature, pressure, humidity) factors, meaning it is very difficult to utilise it in practice. however, the variation δ𝑓0/𝑓0,𝑟𝑒𝑓 of 𝑓0, with respect to the reference value 𝑓0,𝑟𝑒𝑓 , is much more reliable due to the change of one or more parameters [15]. in this case, δ𝑓0, as caused by the insertion of the dielectric sample, is measured. then, with electromagnetic simulations, δ𝑓0 is evaluated as a function of 𝑆 ′ . thus, 𝑆 ′ is determined as the value at which the simulated δ𝑓0 coincides with the measured one. thus, 𝑆 ′ is evaluated with the aid of e.m. simulations of the resonator. after 𝑆 ′ is evaluated, the factors 𝜂 and 𝐺 in eq. (5) can be analytically or numerically (with simulators) calculated. following this, eq. (5) can be inverted to obtain tan 𝛿𝑆 from the 𝑄 measurements if all the 𝑅𝑖 and tan 𝛿𝑉 of the resonator are known from previous measurements or calibration procedures. it must be mentioned that the unloaded 𝑄 in eq. (5) differs in principle from the measured 𝑄𝑙 because of the coupling of the resonator with the external lines. however, with very small coupling (i.e. 𝑃𝑒𝑥𝑡 /𝑃 < 0.01, with 𝑃𝑒𝑥𝑡 being the losses in the external transmission lines), as in our set up, one has 𝑄𝑙 ∼ 𝑄 and 𝑢(𝑄) ∼ 𝑢(𝑄𝑙 ) [15]. the use of eq. (5) can give unacceptably large uncertainties since, at microwave frequencies, the accuracy with which all the quantities in eq. (5) are known is poor when compared to dc or low frequency measurements. in fact, in our case, 𝑅 = 92 mω with 𝑢(𝑅) 𝑅⁄ ∼15 % and tan 𝛿𝑉 = 4 ⋅ 10 −5 with 𝑢( tan 𝛿𝑉 )/ tan 𝛿𝑉 ∼ 50 %. in order to reduce the effects of these uncertainties on tan 𝛿𝑠, a perturbative approach is proposed here. the difference δ(𝑄−1) between the measured quality factors 𝑄𝑆 −1 and 𝑄𝐴 −1, obtained with the sample into the resonator (subscript s) or with a gap of air in its place (subscript a), respectively, can be written as: δ(𝑄−1) = 𝜂𝑆 tan 𝛿𝑆 + ∑ 𝑅𝑖 δ(𝐺𝑖 −1) + δ(𝜂𝑉 ) tan 𝛿𝑉 , 𝑖 (6) where it is clear that the smaller δ(𝐺𝑖 −1) and δ(𝜂𝑉 ) are, then the smaller their contributions to the uncertainties of 𝑅𝑖 and tan 𝛿𝑉 are. 2.1. measurement system and procedure the resonator used for this study is depicted in figure 1. the single sapphire crystal is a cylinder (height ℎ = (5.0 ± 0.1) mm, acta imeko | www.imeko.org september 2020 | volume 9 | number 3 | 28 diameter ⌀ = (8.0 ± 0.1) mm). a k-type coaxial transmission line, ending with coupling loops, is used to excite and sense (in transmission mode) the e.m. field configuration in the resonator. the dielectric samples are supported by a brass mask with a central hole ⌀ = (13.00 ± 0.01) mm and closed with a brass cap in order to prevent energy radiation, as depicted in figure 1. the dr is excited in the te011 mode, and thus, the 𝑬 field is oriented parallel to the bases of the resonator. it is important to underline the orientation of the 𝑬 field because the layered deposition techniques typical of most of the 3d-printers can generate anisotropic effects on the e.m. properties of the printed samples. a uniaxial anisotropy factor of almost 7 % was measured for ′ at 40 ghz for polylactide (pla) samples using the waveguide reflection method [19]. in the method presented here, the 𝑬 field is almost parallel to the deposition layers of the sample; thus, our results probe the direction along the layer deposition without significant mixing of the perpendicular component. the resonator transmission complex scattering parameter 𝑆12 , from which 𝑄 and 𝑓0 are evaluated, is measured with an anritsu 37269d vector network analyzer (vna) in the following way: the vna is calibrated using the short open load through (solt) method, and the 12-errors parameters are applied to the frequency range in which the measurements are performed; the transmission scattering parameter 𝑆12(𝑓) is acquired with 1601 points evenly distributed in a frequency range width 7δ𝑓−3𝑑𝐵 , where δ𝑓−3𝑑𝐵 is the width of the resonance curve at half power. each data point is averaged with 10 acquisitions to reduce the noise contribution; the absolute value of the acquired points |𝑆12(𝑓)|, which have the uncertainty 𝑢(𝑆21(𝑓)), given by the vna after the calibration [21], is fitted to the fano resonance curve [22], [23]: |𝑆12(𝑓)| = | 𝑆12(𝑓0) 1 + 2𝑖𝑄 𝑓 − 𝑓0 𝑓0 + 𝑆𝑐 | , (7) where the complex constant 𝑆𝑐 represents the crosscoupling contribution. for each resonance curve, 𝑄 and 𝑓0 are evaluated with their uncertainties 𝑢(𝑄), 𝑢(𝑓0). the uncertainties of the fitted parameters are obtained by standard statistical methods starting with the fitting residuals variance 𝜎𝑅 2 [24]; for each mounting, 10 resonance curves are acquired. then, the mean values of 𝑄 and 𝑓0 are evaluated with their standard deviation: 𝑢(𝑄) 𝑄⁄ ∼ 0.05 % and 𝑢(𝑓0 )/ 𝑓0 ∼ 1 ppm; for each sample, 5 mountings are performed, disassembling and resetting the sample in its position. then, the mean value of 𝑄 and 𝑓0 with their standard deviation are evaluated. the final uncertainties 𝑢(𝑄) 𝑄⁄ ∼ 1 % and 𝑢(𝑓0 )/𝑓0 ∼ 20 ppm exist mainly due to the repeatability of the assembly. 3. uncertainties analysis in this section, the behaviour of the measurement technique is explored for the whole sample parameter space in order to establish the best working condition and its boundaries as a function of 𝑠 ′, tan 𝛿𝑠 and sample thickness 𝑡. first, the sensitivity of the resonator to tan 𝛿𝑠 variations is analysed. the sensitivity is evaluated from eq. (5) as: 𝑐 = 𝜕𝑄 𝜕 tan 𝛿𝑆 = − 𝜂𝑆 (𝜂𝑆 tan 𝛿𝑆 + 𝑙𝑟 ) 2 = −𝜂𝑆 𝑄 2, (8) with 𝑙𝑟 = ∑ 𝑅𝑖 𝐺𝑖 +𝑖 𝜂𝑉 𝑡𝑎𝑛 𝛿𝑉, which, as a first approximation, in this analysis is assumed to be independent from the sample properties; in the small perturbation limit, the changes in the e.m. field configuration due to the sample are small and practically negligible, meaning the conduction/volume losses given by the resonator components do not change appreciably. in our case, 𝑙𝑟 ∼ 5000 = 𝑄𝐴 −1. in figure 2, |𝑐(tan 𝛿𝑠, 𝜂𝑠 )| is reported for 10−5 < tan 𝛿𝑠 < 10 0 and 𝜂𝑠 = {10 −4, 10−3, 10−2, 10−1}. one can notice a different log |𝑐| slope 𝑚 at high tan 𝛿𝑠 values (𝑚 = − 2) and at low tan 𝛿𝑠 (𝑚 = 0) in the log–log plot (figure 2). the 𝑚 = −2 behaviour is due to the losses inside the dielectric samples when the following hold: 𝜂𝑠 𝑡𝑎𝑛 𝛿𝑠 ≫ 𝑙𝑟 , and, from eq. (8), 𝑐 → − 𝜂𝑠 −1(tan 𝛿𝑠) −2. conversely, when 𝜂𝑠 tan 𝛿𝑠 ≪ 𝑙𝑟 , 𝑐 → − 𝜂𝑠 𝑙𝑟 −2 ; as such, 𝑐 is no longer dependent on tan 𝛿𝑠. as expected, the bigger 𝜂𝑠, the higher |𝑐|, as long as the sample losses are small. thus, for low tan 𝛿𝑠 samples, a higher 𝜂𝑠 is preferable, while at higher tan 𝛿𝑠, a lower 𝜂𝑠 gives better performances. at a fixed tan 𝛿𝑠 value, the maximum sensitivity is obtained at the crossover of |𝑐| (see figure 2); thus, 𝜂𝑠 = 𝜂𝑠,𝑜𝑝𝑡 = 𝑙𝑟 / tan 𝛿𝑠 and |𝑐|𝑚𝑎𝑥 = (4𝑙𝑟 tan 𝛿𝑠 ) −1. 𝜂𝑠,𝑜𝑝𝑡 is the optimum sample filling factor and gives the maximum sensitivity for 𝑄 measurements. as such, the geometry of the samples under figure 1. sketch of the dielectric loaded resonator (not to scale). figure 2. solid lines: the absolute value of resonator sensitivity |𝑐| as a function of the sample 𝑡𝑎𝑛 𝛿𝑠 and filling factor 𝜂𝑠. dotted line: the maximum sensitivity |𝑐|𝑚𝑎𝑥 reachable for every 𝑡𝑎𝑛 𝛿𝑠 value. acta imeko | www.imeko.org september 2020 | volume 9 | number 3 | 29 investigation can be adjusted in order to fulfil the requirement for 𝜂𝑠,𝑜𝑝𝑡 . in our case, this is the expected tan 𝛿𝑠 ∼ 10 −2 . so, from figure 2, 𝜂𝑠,𝑜𝑝𝑡 ∼ 10 −2. the loss tangent measurement uncertainty 𝑢(tan 𝛿𝑠) is evaluated as follows [20]: 𝑢2(tan 𝛿𝑆) = 1 𝜂𝑆 2 [(𝑢(δ(𝑄−1))) 2 + ∑(δ(𝐺 −1)𝑢(𝑅𝑖)) 𝑖 2 + ∑ (𝑅𝑖 𝑢(δ(𝐺 −1))) 2 + (tan 𝛿𝑉 𝑢(δ(𝜂𝑉 ))) 2 𝑖 + (δ(𝜂𝑉 )𝑢(tan 𝛿𝑉 )) 2 + (tan 𝛿𝑆𝑢(𝜂𝑆 )) 2], (9) with: 𝑢2(δ(𝑄−1)) = ( 𝑢(𝑄𝑆 ) 𝑄𝑆 2 ) 2 + ( 𝑢(𝑄𝐴) 𝑄𝐴 2 ) 2 , (10) 𝑢2(δ(𝐺 −1)) = ( 𝑢(𝐺𝑖,𝑆) 𝐺𝑖,𝑆 2 ) 2 + ( 𝑢(𝐺𝑖,𝐴) 𝐺𝑖,𝐴 2 ) 2 − 2𝑟𝐺 𝑢(𝐺𝑖,𝑆 )𝑢(𝐺𝑖,𝐴) 𝐺𝑖,𝑆 2 𝐺𝑖,𝐴 2 , (11) 𝑢2(δ(𝜂𝑉 )) = 𝑢 2(𝜂𝑉,𝑆) + 𝑢 2(𝜂𝑉,𝐴) − 2𝑟𝜂 𝑢(𝜂𝑉,𝑆)𝑢(𝜂𝑉,𝐴). (12) the correlation factors 𝑟𝐺 and 𝑟𝜂 are supposed to be almost 1 since the evaluation of 𝐺𝑖 and 𝜂𝑉 is performed with the same algorithm and with the same settings. conversely, the 𝑄 measurements are not strongly correlated, since the different mountings can introduce different uncorrelated error contributions. next, 𝑢(tan 𝛿𝑠) is explored in the space (1 < ′ < 10, 10−5 < tan 𝛿𝑠 < 10 0, 0.5 mm < 𝑡 < 2 mm) to establish the operative limits of this technique. 𝑢(tan 𝛿𝑠) is evaluated using eq. (9) with geometrical 𝐺 factors and filling 𝜂 factors obtained through e.m. simulations. using e.m. simulations, it has been verified that tan 𝛿𝑠 variations (in the studied space) do not alter the e.m. field configuration. therefore, for the evaluation of 𝐺 and 𝜂, tan 𝛿𝑠 is fixed (i.e. tan 𝛿𝑠 = 10 −2). both 𝑢(𝐺) and 𝑢(𝜂) are obtained with monte carlo e.m. simulations randomly varying all the physical dimensions and the e.m. properties of the materials from which the resonator is made in their uncertainty space [25]. it should be noted that 𝑢(𝑄)/𝑄 is ideally constant for every 𝑄 value if the measurement frequency span is kept proportional to 𝑓0/𝑄 and the number of points constant [26]. yet, because of the mounting repeatability limitation, the presence of other resonance modes and other unideal factors (e.g. a complex background signal on the transmission parameter and a crosscoupling contribution), 𝑢(𝑄) is somehow limited even at low 𝑄. as such, its absolute value is assumed constant, i.e. 𝑢(𝑄) ∼ 40. in figure 3, 𝑢(tan 𝛿𝑠)/ tan 𝛿𝑠 is evaluated in the plane ( ′, tan 𝛿), and it is reported for samples with 𝑡 = 1.5 mm and 𝑡 = 0.5 mm. 𝑢(tan 𝛿𝑠)/ tan 𝛿𝑠 strongly depends on the sample thickness 𝑡 and, thus, on 𝜂𝑠, as expected from figure 2. 𝑢(tan 𝛿𝑠) sharply increases with thinner samples, particularly at low tan 𝛿𝑠 values: with tan 𝛿𝑠 ∼ 10 −2 and 𝑠 ′ ∼ 2, 𝑢(tan 𝛿𝑠) tan 𝛿𝑠⁄ ∼ 100 % for a 0.5 mm thick sample, while 𝑢(tan 𝛿𝑠) tan 𝛿𝑠⁄ ∼ 10 % in the same conditions but 𝑡 =1.5 mm. in figure 3, the 𝑠 ′(tan 𝛿𝑠) curve corresponding to |𝑐|𝑚𝑎𝑥 is reported; it agrees to a fair extent with the lowest 𝑢(tan 𝛿𝑠) tan 𝛿𝑠⁄ level. once the maximum 𝑢(tan 𝛿𝑠) threshold level is fixed, taking what is shown in figure 3, the space ( ′, tan 𝛿) where the proposed technique can be reliably used is defined. however, two other limiting factors must be considered. the first is related to the impossibility to discriminate small δ𝑄 ∼ 𝑄𝐴 − 𝑄𝑆 variations because of the measurement noise. thus, where δ𝑄 < δ𝑄𝑚𝑖𝑛 , this technique is no more sensitive. in our case, with 𝑄𝐴 ∼ 5000, δ𝑄𝑚𝑖𝑛 ∼ 40. in figure 3, the δ𝑄𝑚𝑖𝑛 curve is reported; however, one can notice that it crosses the ( ′, tan 𝛿) plane where 𝑢(tan 𝛿𝑠 )/ tan 𝛿𝑠 > 100 %. the second limit is set where the losses of the sample are so high as to make 𝑄𝑠 too small to be reliably measured. due to the 𝑆12(𝑓) background, we set this minimum value min(𝑄𝑠 ) ∼ 100. in figure 3, the dotted line represents this limit: above this curve no tan 𝛿𝑠 measurements are possible. the presented technique can be easily optimized for the characterization of these materials. in fact, the position of the minimum of 𝑢(tan 𝛿𝑠)/ tan 𝛿𝑠 shown in figure 3 arises in the region of the expected values of 𝑠 ′ and tan 𝛿𝑠 of 3d-printer materials, and it can be tuned with 𝜂𝑠. figure 3. relative loss tangent uncertainty 𝑢(𝑡𝑎𝑛 𝛿𝑆 )/ 𝑡𝑎𝑛 𝛿𝑠 contour plot in the plane ( ′, tan 𝛿) for samples of thickness 𝑡 = 0.5 mm (upper panel) and 𝑡 = 1.5 mm (lower panel). the thicker solid line represents the points of maximum sensitivity |𝑐|𝑚𝑎𝑥 as evaluated from figure 2. the dashed line corresponds to the minimum quality factor appreciable variation 𝛥𝑄𝑚𝑖𝑛 = 𝑄𝐴 − 𝑄𝑆 ∼ 40 and the dotted line to the minimum evaluable 𝑄𝑆,𝑚𝑖𝑛 ∼ 100. acta imeko | www.imeko.org september 2020 | volume 9 | number 3 | 30 4. results and discussion the measurements were performed on four dielectric samples, which were of different thickness values 𝑡 as reported in table 1, made of a photopolymer material printed using the polyjettm deposition technique. the thicknesses and flatnesses of the samples were checked with a micrometre. the mean values 𝑡̅ and their standard deviation 𝜎𝑡 = √∑ (𝑡 − 𝑡̅) 2𝑁 𝑖=1 (𝑁 − 1)⁄ have been obtained by 𝑁 = 10 different measurements of the thickness by probing the surfaces of the samples. thus, 𝜎𝑡 can be read as a measure of the flatness of the samples. to achieve a sufficiently sensitive method of evaluating ′, the frequency repeatability of the setup needs to be reliable to allow the accurate measurement of the differences between 𝑓0,𝑆 (measured with the dielectric samples mounted) and 𝑓0,𝐴 (measured without the sample and leaving an air gap of the same sample thickness). this is achieved using 3d-printed rings prepared in the same way and of the same thickness as the dielectric samples. the data are presented in figure 4, from which one can then evaluate ε's = 2.9 ± 0.2. once 𝑠 ′ is estimated with its uncertainty, the geometrical and filling factors of the resonator components (with their uncertainties) are evaluated from the simulations as shown in the previous section. then, tan 𝛿𝑠 is evaluated through eq. (6) from the measured 𝛥(𝑄−1) = 𝑄𝑆 −1 − 𝑄𝐴 −1. the quality factors 𝑄 are reported in table 2. in figure 5, the measured tan 𝛿𝑠 is shown with the error bars evaluated with eq. (9) using the uncertainties for the measured quantities and simulated parameter shown previously. figure 5 shows the best estimate: tan 𝛿𝑆 = (1.8 ± 0.2) ⋅ 10 −2 as calculated with ′𝑆 = 2.9 ± 0.2. the evaluation of tan 𝛿𝑆 is performed taking as best value the centre point in the common confidence interval of all the experimental points (the light blue band in figure 5). then, the uncertainty is the half width of that common interval. one can note that the uncertainty bars rapidly increase in size when the sample thickness becomes small due to a lack of sensitivity, as expected from the analysis presented in sec. 3. this effect is present also in figure 4, where the simulated curves for small thickness values tend to coalesce. conversely, samples with a large thickness can cause e.m. field radiation from the structure, thereby significantly changing the resonant mode and adding further losses. the method presented here, in our geometry, is then most suitable for samples of thickness between 1 and 2 mm. 5. conclusions in this work, a dielectric loaded-resonator-based technique was developed and presented for the measurement of the complex permittivity ̃ of 3d-printing materials. we exploited the possibility of shaping the sample into appropriate shapes (disks, in our case) and the excellent frequency repeatability of our setup to reliably measure the quality factor 𝑄 and the resonance frequency 𝑓0 both with and without the sample loaded into the cavity. from the variations of 𝑄 and 𝑓0 given by the sample insertion, ̃ is obtained using the perturbation approach. the measurement technique performances were deeply analysed in terms of sensitivity and accuracy in the whole parameter space in order to establish the sample geometry, as a function of its e.m. properties, for the best measurement accuracy. the technique was tested by measuring photopolymer material printed using polyjettm deposition. with the presented technique here, it was obtained that ε's = 2.9 ± 0.2 and tan 𝛿𝑠 = 0.018 ± 0.002. the obtained result agrees well with other studies. in particular, using a combined technique, a broad band (1 mhz÷11 ghz) characterisation of 3d-printer materials was presented in [13]. the high frequency range (8.2 ghz÷11 ghz) figure 4. differences between the resonant frequencies 𝑓0,𝑆 (with the samples mounted) and 𝑓0,𝐴 (with the sample substituted by an air gap of the same thickness). the red dots are the experimental data and the blue lines the simulations results. table 1. the mean thickness 𝑡̅ of the samples and its standard deviation 𝜎𝑡 . 𝑺𝟏 𝑺𝟐 𝑺𝟑 𝑺𝟒 𝑡̅ (mm) 0.522 1.002 1.512 2.063 𝜎𝑡 (mm) 0.003 0.004 0.005 0.004 table 2. the inverse of the measured quality factors when the four samples, s1-s4, are inserted. 𝑄𝑆 −1and 𝑄𝐴 −1 refer to the measurement with the sample or an air gap of equal thickness, respectively. 𝑺𝟏 𝑺𝟐 𝑺𝟑 𝑺𝟒 𝑄𝑆 −1 ⋅ 104 2.15 2.48 3.06 4.25 𝑄𝐴 −1 ⋅ 104 1.95 1.96 1.98 2.00 figure 5. the measured loss tangent of the dielectric 3d-printed material 𝑡𝑎𝑛 𝛿𝑆. the error bars are evaluated with eq. (9) and the green area represents the confidence interval. 0 0.02 0.04 0 0.5 1 1.5 2 f 0 -measure '=2.0 '=2.2 '=2.4 '=2.6 '=2.8 '=3.0 '=3.2 '=3.4  f 0 ( g h z ) t (mm) 0 0.02 0.04 0.06 0 0.5 1 1.5 2 2.5 ta n  t (mm) tan s =0.018(2) acta imeko | www.imeko.org september 2020 | volume 9 | number 3 | 31 was studied through a waveguide in reflection mode, although an uncertainty study was lacking in this frequency range. the reported values at 11 ghz were 2.5 < ′ < 3.29 and 0.005 < tan 𝛿 < 0.037, perfectly in agreement with our results. in order to check the accuracy of the dr technique shown in this paper, ̃ of the studied materials was also measured with a standard reflection/transmission method. a wr90 waveguide was used with a pna network analyzer (model e8363c, agilent technologies) with the agilent 85071e software and the ‘nist precision’ method [27]. to perform this measurement, parallelepipeds of 22.8 ⋅ 10.1 mm2 and different thicknesses (4, 5, 6, 7, 8 mm) were printed. 𝑆 ′ ∼ 3.1 was measured by extrapolating the value at 12.9 ghz, and no significative sample variations can be seen (figure 6). this value compares well with the one obtained using the proposed dr technique and the 𝑓0 variation. then, the imaginary part 𝑠 ′′ ∼ 0.23 is obtained with the waveguide method, which yields tan 𝛿𝑠 ∼ 0.074 with a significative inter-sample scattering. this value is about 4 times higher than the one obtained with the dr method. however, it must be mentioned that the ‘nist precision’ method was developed to solve the accuracy problems of the nicholson–ross–weir technique (nrw) near the sample resonances [15], [27]. the ‘nist precision’ and the nrw give comparable results out of the resonances. it is well known that the nrw is not a reliable method for the ′′ evaluation of lowloss materials. in fact, the same measurement fixture was tested with a polytetrafluoroethylene sample obtaining 𝑃𝑇𝐹𝐸 ′′ ∼ 1.5 × 10−2. however, from the literature [28], this is expected to be about two orders of magnitude smaller. also, for materials with higher losses, nrw accuracy is limited; the comparison presented in [29] between the nrw and other methods shows some discrepancies, even at tan 𝛿 ∼ 10−2 values (e.g. with nylon samples), as in our case. thus, the disagreement between the dr tan 𝛿 measurement and that obtained in waveguide, which was somewhat expected, will be studied in further works. in summary, a new measurement technique was presented for the e.m. characterisation of dielectric materials, with interesting potential applicability to industrial fields due to its simple conceptual approach and high accuracy. references [1] n. hopkinson, p. dickens, emerging rapid manufacturing processes, in: rapid manufacturing; an industrial revolution for the digital age. n. hopkinson, r. hague, p. dickens (editors). wiley & sons ltd, chichester, 2006, isbn: 0-470-01613-2, pp. 55-80. [2] m. j. werkheiser, j. dunn, m. p. snyder, j. edmunson, k. cooper, m. m. johnston, 3d printing in zero-g iss technology demonstration, aiaa space 2014 conference and exposition, san diego, california, 2014, pp 2054-2078. 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[26] k. torokhtii, a. alimenti, n. pompeo, e. silva, uncertainty in uncalibrated microwave resonant measurements, proc. of the 23rd imeko tc4 international symposium electrical & electronic measurements promote industry 4.0, xi’an, china, 2019, pp. 98101. online [accessed 24 september 2020] https://www.imeko.org/publications/tc4-2019/imeko-tc42019-022.pdf [27] j. baker-jarvis, e. j. vanzura, w. a. kissick, improved technique for determining complex permittivity with the transmission/reflection method, ieee transactions on microwave theory and techniques 38 (1990) pp. 1096-1103. doi: https://doi.org/10.1109/22.57336 [28] j. krupka, measurements of the complex permittivity of low loss polymers at frequency range from 5 ghz to 50 ghz, ieee microwave and wireless components letters 26 (2016) pp. 464466. doi: https://doi.org/10.1109/lmwc.2016.2562640 [29] z. abbas, r. d. pollard, r. w. kelsall, complex permittivity measurements at ka-band using rectangular dielectric waveguide, ieee transactions on instrumentation and measurement 50, (2001) pp. 1334-1342. doi: https://doi.org/10.1109/19.963207 https://doi.org/10.1109/77.979858 https://doi.org/10.1007/s10948-006-0129-z https://doi.org/10.1038/srep22714 https://doi.org/10.1109/tmtt.1960.1124749 https://doi.org/10.1088/1742-6596/1065/5/052027 https://doi.org/10.1109/tmtt.2002.802324 https://doi.org/10.1109/i2mtc.2017.7969903 https://www.imeko.org/publications/tc4-2019/imeko-tc4-2019-022.pdf https://www.imeko.org/publications/tc4-2019/imeko-tc4-2019-022.pdf https://doi.org/10.1109/22.57336 https://doi.org/10.1109/lmwc.2016.2562640 https://doi.org/10.1109/19.963207 microsoft word article 3 82-698-1-ga.docx acta imeko december 2013, volume 2, number 2, 3 – 12 www.imeko.org acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 3 algorithm for a high precision contactless measurement system tran trung nguyen, arvid amthor, christoph ament system analysis group,ilmenau university of technology, p.o. box 100565, 98684 ilmenau, germany section: research paper keywords: optical measurement, multi laser tracker system, self-calibration citation: tran trung nguyen, arvid amthor, christoph ament, algorithm for a high precision contactless measurement system, acta imeko, vol. 2, no. 2, article 3, december 2013, identifier: imeko-acta-02 (2013)-02-03 editor: paolo carbone, university of perugia received february 19th, 2013; in final form september 9th, 2013; published december 2013 copyright: © 2013 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: this work was done in the framework of the project “kompetenzdreieck optische mikrosysteme – spitzenforschung und innovation in den neuen ländern”, which is supported by the german ministry of education and research (fkz: 16sv5473). corresponding author: tran trung nguyen, e-mail: tran-trung.nguyen@tu-ilmenau.de 1. introduction the accurate position of the tool centre point (tcp) during the production process is a crucial issue for efficient machine processing of complex tasks with smaller and smaller components and structures of steadily decreasing size. capacitive and inductive position sensors achieve a limited measurement resolution and/or a limited working range and need to be integrated in the process machine. as the sensors measure the tcp position indirectly subsequent measurement errors are induced. also all alignment errors of a measured serial kinematics deteriorate the measured position of the tcp. in contrast, optical sensors offer a high precision and a large working range. apart from that, optical sensors measure the tcp position directly and do not need to be integrated in the process machine. the preferred optical measurement device especially in high precision applications is the laser tracker. a laser tracker is an optical instrument which permits a contactless 3d measurement of a moving target. the device consists of a very precise laser interferometer and a beam deflection system. the development of laser trackers began in the 1980’s when lau et al. [7][8] used the laser tracking technique to determine the performance of a robot. this tracking system was implemented in a real-time system to identify the three-dimensional static as well as dynamic positioning accuracy of a robot end effector. the measuring volume of the described tracking system was approximately 3×3×3 m and a maximum speed of 300 mm/s was reached. in order to enhance the tracking speed parker and mayer developed an optical laser tracking system using a 2-axis rotational galvanometer scanner as beam deflection unit [14][15][16]. their system was used to measure the absolute position of a moving optical target which was mounted on a robot’s manipulator. the first who utilized a laser interferometer for a precise distance measurement were takatsuji et al. they presented an interferometer based laser tracking system called distance-only-measurement (dom). this system determined the position of a tracked target using the method of trilateration and consists of four single laser tracking units. using four systems detecting one target offers the advantage of a redundant measurement. thus, the position of the tracker units and the initial position of the target can be calculated from abstract this paper presents a self-calibration method for the multi laser tracker system (mlts) which can track a retroreflector mounted on a kinematical system (e.g. positioning stage, robot manipulator etc.). four laser trackers build up the mlts. in the first part of the study the required algorithms enabling the mlts to measure the position of the retroreflector are presented. the algorithms include the localization of the retroreflector, the communication between the laser trackers and the tracking controller as well as the calculation of the tool centre point (tcp) position. in the second part of this study a detailed analysis of the high precision self-calibration algorithm is carried out. a sensitivity analysis shows that the quality of the parameter estimation highly depends on the optimal arrangement of the mtls. acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 4 multiple position measurements. the resulting measurement error was about 40 microns with a target distance of one meter [18]. due to the high resolution of the interferometer the dom method is state of the art in non-contact high precision metrology. beside the achievable precision an additional advantage of the proposed method is the fact that the three-dimensional position of the reflector is detected directly and hence abbe’s principle is not violated. the crucial question concerning the trilateration is the determination of the absolute coordinates of the reflector and the position of each laser tracker given the initial measurement lengths. these parameters are calculated by self-calibration methods without the usage of a reference calibration device. in order to apply the self-calibration algorithm the number of free measurement points must be greater than the number of unknown parameters. this leads to an overconstrained system of equations, which can be solved using powerful numerical techniques. the authors of this contribution found that the quality of the self-calibration algorithm depends on various metrological arrangements of the measuring system e.g. the number of measurements, the working range, the distance of the laser tracker among each other and the measurement point. in [19] an optimal arrangement of four laser trackers is clarified. nevertheless, there is no mathematical description for the various arrangements of measurement points and the laser trackers available up to date and hence an optimal configuration for the self-calibration is not given. numerical simulations can (or will) be used to close this gap. the present contribution is organized as follows. within the framework of the “kompetenzdreieck optische mikrosystem (optimi)”, which is supported by the ministry of education and research (bmbf), the systems analysis group of ilmenau university of technology has developed a mlts for tracking a kinematical system [1][2][3][4][5][6] (see figure 1). the hardand software components of this experimental set-up is presented in the following section. section 3 describes the required tracking control algorithm of one laser tracking unit and section 4 presents the calculation of the tcp position using the trilateration method. finally, selected results of the parameter estimation are shown in order to increase the accuracy of the calibration method presented. the results are further concluded and a short outlook is given. 2. experimental set-up the set-up of a single laser tracker module is decomposed of the opto-electronic components, the supply electronic unit and the opto-electronic detection unit (figure 2). the opto-electronic components include a michelson interferometer and a galvanometer scanner. both components are fixed on a base panel. the homodyne interferometer uses a stabilized he-ne laser as beam source and realizes a position resolution on sub nanometer scale [9][10]. beside the michelson interferometer a four quadrant diode (4qd) unit is integrated. in this study the utilized 4qd has an active area of 5×5 mm. the four quadrants are separated by a gap, which amounts to 30 µm. this kind of diode is able to detect a misalignment between the interferometer and the retro-reflector. a corner cube mirror is used as a retro-reflector in this work [22]. the described opto-electronic components are arranged in such a way that the emitted beam hits the galvanometer scanner. the galvanometer scanner consists of two beam deflection units and the laser beam is reflected by both of them [1]. each deflection unit consists of a mirror, a torque transducer, a high precision angle sensor and an analogue servo closed loop control. the servo is used to control the angles and the current required for the actuators. the developed algorithms (localization, communication, tracking control and determination of the tcp position) are implemented in matlab/simulink® using c-code-s-functions. the simulink coder® is used to automatically generate c code, which is carried out by a modular rapid control prototyping system of dspace®. all algorithms work with a sample rate of 10 khz. figure 3 depicts the functionality of one laser tracker module in detail. the laser beam is transported to the interferometer via an optical fibre. inside the interferometer the laser beam hits the first polarizing beam splitter b1 and dividing the beam into a reference beam and a measuring beam. the reference beam then hits the fixed reference mirror and returns to the first beam splitter b1. simultaneously, the measuring beam passes the second beam splitter b2 and leaves the interferometer. starting from the interferometer the measurement beam hits the first mirror of the galvanometer scanner, which is rotating around the x-axis. the x-galvanometer scanner (x-scanner) deflects the measurement beam to the second mirror, which is rotating around the y-axis. the y-galvanometer scanner (yscanner) deflects the measurement beam again to the retrofigure 1. the developed mlts for tracking kinematics. figure 2. the developed tracker module. acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 5 reflector. the retro-reflector is fixed on the tcp of the kinematics tracked. it reflects the measurement beam back to the galvanometer scanner. the reflected beam returns to the interferometer via the galvanometer scanner and hits the second beam splitter b2. part of the reflected beam reaches the 4qd. this signal of the 4qd is used to track the target. the other part of the reflected beam passes through the second beam splitter b2 and is superimposed on the reference beam in the beam splitter b1. furthermore, the superimposed beam hits the opto-electronic detection unit. a resolution of 0.1 nm is achieved for the position detection [10]. 3. tracking algorithm 3.1. localization of the retro-reflector the proposed localization method is based on an archimedean spiral which is derived in polar coordinates. the spiral is defined by three parameters. the radius r0 is used to describe the maximum rotational angle of the galvanometer scanner in polar coordinates. the duration t0 defines the rotation of the laser beam from the origin of the spiral to the predefined radius r0. the number of rotations is given by the parameter ω0. to describe the archimedean spiral showed in figure 4 the following equations in polar coordinates are given for the x-axis and for the y-axis:     0 0 0 0 0 0 0 0 cos 2 sin 2 r x t t t t t r y t t t t t                   (1) due to the fact that the behaviour of the galvanometer scanner is specified in angle coordinates, the designed archimedean spiral has to be transformed from polar coordinates into angle coordinates. the rotation angles of the galvanometer scanner are calculated as follows:         arctan arctan x t t l y t t l                   (2) in equation (2) the parameter l is the distance between the laser tracker and the localization plane. the distance is predefined by 1 m. the angle θ(t) describes the rotation of the galvanometer scanner about the x-axis and the angle φ(t) describes the rotation of the galvanometer scanner about the y-axis. 3.2. communication of the mlts the proposed localization algorithm is used for a single laser tracker module. due to the fact that four tracker modules are used in an mlts, the developed algorithm is expanded by a communication channel between the four tracker modules. hence, the tracker modules are able to share the angle information and the retroreflector position with each other and this significantly accelerates the localization (all trackers have found the tcp) of the whole mlts. to realize an effective acceleration of the localization phase, the approximate (low precision) tcp position is required in a global coordinate system (see section 4.2). initially, all tracker modules are searching for the tcp. if at least two trackers find the tcp the triangulation method will be used to determine the tcp position. the tcp position is subsequently transferred to the untraced tracker modules and therefore they find the tcp immediately. it has to be mentioned that during localization the approximate position coordinates of all tracker modules are required. for this purpose a low precision calibration process based on the angles and distances estimates the coordinates of the four tracker modules. for detailed information on the tcp localization process the reader is referred to [14]. figure 3. the principle of the tracker module. figure 4. the designed spiral in cartesian coordinates. figure 5. an example of the communication protocol. x-scanner photodiode reference mirror 4qd b1 b2 laser beam y-scanner retroreflector optical fibre 0 0.5 1 -0.1 0 0.1 -0.1 -0.05 0 0.05 0.1 z-axis in mx-axis in m yax is in m t1 t3 t2 communication determination of the radial distance determination of the tool center point coordinate transformation receivertransceiver position-angletransformation ( , )   1 3,   ' '2 2,  acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 6 figure 5 shows an example of a communication sequence. initially, the tracker modules t3 as well as t1 have found the retroreflector. based on the radial distance between the two tracker modules and the retro-reflector as well as the angle information the position of the retro-reflector is computed in the spherical coordinates of e.g. t1. the position of the retroreflector is then transformed to the global coordinate system of mlts. in a final step, the global tcp position is transformed to the polar coordinates of the residual (still searching) tracker modules, in our example t2. the local position is now shared with t2 and t2 finds the tcp immediately. to combine the localization algorithm with the communication of the mlts a state machine showed in figure 6 was designed. this finite-state machine is used to model the switch behaviour of every tracker module in operation. the algorithm has three binary input variables (a, n, p) and two output variables (θx, φy). the output variables are the rotation angles of the tracker module and the input variables are:  a: communication mode active  n: tracker does not hit the retroreflector  p: localization mode active four finite-states (z00, z01, z10 and z11) are used during the localization:  z00: the localization mode is deactivated. the tracker module has no information about the position of the retroreflector.  z01: the localization mode is activated and the communication is deactivated. the tracker module searches the retroreflector using the proposed localization algorithm.  z10: a minimum number of two tracker modules hit the target center of the retroreflector. the communication mode is activated. using this additional information the other tracker modules are looking for the retroreflector in a defined small area around the communicated tcp position.  z11: all tracker modules have found the retroreflector. 3.3. tracking control in the case that the laser beam has found the retroreflector, the beam has to track it. for this purpose a feedback control algorithm has to be implemented. a model based control approach is one possibility to minimize the tracking error of the laser beam. the requirements in this case are stability, large bandwidth and robustness against external disturbances. the 4qd is used as a feedback sensor and to provide the displacement of the reflected beam as a x4qd and y4qd – signal, respectively. if the beam hits the target center, then the laser beam is found in the origin of the 4qd coordinates. therefore the x4qd-signal only affects the x-scanner and the y4qd-signal only affects the y-mirror. figure 7 shows the structure of the closed loop tracking control scheme for one axis. the motion of the tracking kinematics xretro in x-direction generates a 4qd signal (x4qd), which is handled as a dynamic disturbance. the error e between the set point w4qd and the disturbance signal x4qd is compensated by a pid controller, which generates an output, φfeedback. the output φdis is computed by the disturbance compensation unit gd in order to suppress the influence of the moving stage. the compensation unit gd reflects the inverse system dynamics of the galvanometer scanner gm and the beam path gb. thus, the disturbance compensation unit acts as a feed forward controller and reduces the tracking error of the laser beam, which tracks the positioning stage. the regulating variable φref is a combination of outputs φfeedback and φdis, and drives the motors of the beam deflection unit. first, the system model is introduced. the beam path describes the relationship between the laser beam from the mirror to the retroreflector and the rotation angle of the galvanometer scanner. moreover, the beam path model is a function of the absolute distance between the mirror and the retro-reflector. the beam path model is given as follows: beam b rad x g 2     (3) the dynamics of the galvanometer scanner can be approximated by a second order differential equation. the model parameters have to be identified using experimental data. the model can be expressed in laplace space as follows: figure 6. the state machine describing the performance of the localization algorithm. figure 7. the control scheme. p nap 01z 00z 10z 11z nap nap nap nap p nap 0x  0y  x xf  y yf  x xs xc    y ys yc    x xs  y ys  np nap np np nap np p p 4qdw re trox 4qdxe  beamxfeedback dis ref bg dg cg pid mg   scanner mirror beam path disturbance compensation acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 7 0 m 2 ref 0 1 2 b g a a s a s       (4) for detailed information regarding the system model the reader is referred to [1]. in the feedback path a digital pid controller is used to suppress the error e. the pid parameters are determined using the pole placement method. the pid controller for single-input and single-output systems can be described in laplace space as follows: 2 i p d c k k s k s g s    (5) the controller affects on the error e, which is the difference between the control variable x4qd and the set point value w4qd. the set point of the feedback controller is w4qd=0. in the case that the motion of the tracking kinematics is known, the path planning signals of the kinematics can be used to compensate the disturbance xretro. hence, the disturbance compensator gd (see figure 7) has to be taken into account. its dynamical behaviour can be approximated by a second order system and is expressed in laplace space: 2dis d 0 1 2 re tro g k k s k s x      (6) to determine the parameters of the pid controller a transfer function of the whole closed loop shown in figure 7 has to be found. the output of the transfer function is the control variable x4qd and the input is the motion of the retroreflector xretro. thus the transfer function can be expressed as follows: 4 qd d b m w re tro c b m x 1 2g g g g x 1 2g g g     (7) inserting equations (3), (4), (5) and (6) in equation (7) yields:            2 2 2 1 1 0 0 2 w 1 d 0 p3 2 i 2 2 2 s s a k s a k a ka g a k a k k s s s a a a                  (8) with: 0 rad4b   (9) in equation (8) the polynomial a(s) represents the numerator and b(s) describes the denominator of the transfer function. the transfer function decreases to zero for t  , consequently s  0:    ws 0 a s lim g 0 b s   (10) equation (10) shows clearly that the proposed model-based controller is able to track the target with a tracking error of zero. the dynamical behaviour of equation (10) depends on the parameters of the denominator b(s) as well as the controller parameters kp, kd and ki. to calculate the parameters of the denominator b(s) the following desired polynomial p(s) is used: 3 2 2 1 0p( s ) s p s p s p    (11) then the desired polynomial p(s) is equated with the denominator b(s) of equation (11) and the parameters of the pid controller are calculated as follows: 0 2 0 2 0 0 2 1 i p d 0 rad 0 rad 0 rad p a p a a p a a k , k , k 4b 4b 4b         (12) equation (12) shows that the parameters ki, kp and kd are not constant and change as a function of the absolute distance between the laser tracker and the retroreflector. due to the fact that the interferometers can only measure a relative distance, there is a need to determine the absolute distance. to solve this problem we have developed a triangulation method, which opens the possibility to determine the absolute distance with the precision needed by using at least two tracker modules [2] [3] [4]. the proposed control algorithm was experimentally validated by a kinematical system, which moved along one axis. figure 8a shows the sinusoidal motion of the tracked kinematics and figure 8b shows the resulting tracking error of the laser beam. the dynamical set-points of the kinematics were generated by an analytic fourth order path planning algorithm, which is able to plan motions with arbitrary initial and final velocities [10]. the kinematics carried out a sinusoidal motion with amplitude of 100 mm and a frequency of 0.4 hz. without disturbance compensation the tracking error of the laser beam is approximately 1 v at peak and a clear phase shift is observable. using the proposed model-based disturbance compensation the tracking error can be reduced by a factor of 2. moreover, the phase shift is not noticeable anymore and this shows the effectiveness of the presented model based control approach. 4. calculation of the tcp 4.1. calculation of the tcp for one tracker module after the implementation of the required control algorithms one tracker module is able to measure the position of the retroreflector, viz. the tcp position. in the case of one tracker module the determination of the tcp position is carried out using vector analysis. this operation requires a precise geometrical model of the beam path. figure 9 shows the geometric relation between laser beam and beam deflection system. the laser beam starts from point p0 and hits the x-scanner mirror in the point p1. the x-scanner mirror reflects the laser beam to the y-scanner mirror at point p2 and then the laser beam is reflected to the retroreflector at the tcp. to calculate the tcp the points p0, p1, p2 have to be known in a predefined coordinate system. the proposed coordinate system uses the rotation axes of the beam deflection system as orthogonal axes: t t 1 1 x 1 y 1 z 2 2 x 2 y 2 zv v ,v ,v ,v v ,v ,v          (13) first, the rotation axis of the y-scanner is used as the x-axis of the coordinate system. second, the z-axis is defined by the cross product of the rotation axes, which is perpendicular to the figure 8. the sinusoidal motion f = 0.4hz of the tracked kinematics a) and the tracking error b). 0 5 10 15 20 -200 0 200 time in s x r e tr o in m m 0 5 10 15 20 -2 0 2 time in s tr ac ki ng e rr o r in v pid controller pid controller with disturbance compensation a) b) acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 8 x-axis. finally, the y-axis is the cross product of xand z-axis. all axes are given as follows:      1 1 2 x v z v v y x z        (14) the laser beam is considered to be a straight line represented by a position and a direction vector. the direction vector after an arbitrary deflection can be computed as follows:  1 0 0r r 2 r n n       (15) the vector r0 is the direction vector of the laser beam before the deflection and the vector n is the normal vector of the utilized mirror. in the case that the vector rp0 is known, the direction vectors rp1, rp2 can be calculated using equation (15) (see figure 9). furthermore, the direction vectors are used to determine the reflection points p1, p2. the rotating mirror is treated as a plane described by a point on the surface and the according normal vector. the intersections p1 and p2 can be calculated as follows:  1 1 1 u p0 n p1 p0 rp0 rp0 n            (16)  2 2 2 u p1 n p2 p1 rp1 rp1 n            (17) the vectors n1, n2 are the normal vectors of the mirrors. the vectors u1 and u2 represent an arbitrary point on every mirror surface. a detailed description of the presented geometric model is shown in [14], [15]. in the case of a moving tcp the points p1 and p2 are variant due to the rotation of the mirrors. a normal vector of the mirror has to be updated online using the rotation axis vi = [vix viy viz] as well as the rotation angle : i in n , i=1,2  r (18) with       2 ix ix iy iz ix iz iy 2 ix iy iz iy iy iz ix 2 ix iz iy iy iz ix iz i i i v c v v v s v v v s v v v s v c v v v s , v v v s v v v s v c 1 cos , c cos , s sin , i 1,2                                   r (19) after the determination of the deflection points p1 and p2 the tcp can be calculated by using the straight line with an initial position at point p2 and a direction vector rp2. the tcp is given as follows:   tcp p2 rp2     (20) the parameter ℓ is the radial distance between the y-scanner mirror and the tcp and ℓ is the sum of the unknown initial length ℓ0 and the measured relative distance δℓ. the proposed calculation method of the tcp requires the determination of the unknown system parameters as well as the unknown initial length. after the calibration process the presented geometric model is used to correct the length measurement of the interferometer [14][15]. 4.2. high precision determination of the tcp position using the mlts the tcp position can be calculated by the relative length measurements of all interferometers. figure 10 shows the four laser trackers (t1, t2, t3 and t4) and the position of the retroreflector ptcp = [x, y, z] t in the global cartesian coordinate system. every tracker module has three location parameters ti = [xi, yi, zi] t and hence the number of the location parameters lead to a total of twelve location parameters describing the whole multi-laser tracking system. to reduce the number of the unknown location parameters from twelve to six and simplify the calculation of the tcp position, the geometric configuration in figure 10 is chosen. for detailed information regarding the choice of the global coordinate system the reader is referred to [17] [18]. the tracker t1 is located in the origin of the coordinate system and has position [0, 0, 0] t. the tracker t2 is located in the x-axis, the tracker t3 is located on the x-y-plane and the position of the tracker t4 is freely selectable. the parameter ℓi describes the radial distance of the four laser trackers and is defined as follows: i 0i i ,i 1 4      (21) the parameter ℓ0i represents the offset length (dead zone) after the initialization phase and the interferometer detects the relative radial distance δℓi. if the positions of all trackers and all figure 9. the geometric representation of the beam deflection system. 1t 2t 3t 4t y x z   2 4 3 1 tcpp figure 10. the position configuration of the mlts. p0 p1 p2 tcp rp0 rp1 rp2 x-scanner mirror 1 y-scanner mirror 2 2v 2k 2u 1u 2n 1k 1n 1v acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 9 distances ℓ0i are known, the tcp position can be determined by an approach called multilateration. the four spherical equations in the euclidian space are given by:  22 2 2 01 1x y z      (22)    2 22 22 02 2x x y z       (23)      2 2 223 3 03 3x x y y z        (24)        2 2 2 24 4 4 04 4x x y y z z         (25) after the insertion of equation (22) in equation (23), equation (24), equation (25) and using of the relation ℓi = ℓ0i + δℓi, the following linear system of equations can be defined [17]: 2 2 2 2 1 2 2 2 2 2 2 3 3 1 3 3 3 2 2 2 2 2 4 4 4 1 4 4 4 4 m x 0 0 x x 2 x y 0 y x y x y z z x y z                                         (26) it is possible to calculate the tcp if the matrix m can be inverted: 2 2 2 1 2 2 1 2 2 2 2 1 3 3 3 2 2 2 2 2 1 4 4 4 4 x x 1 y x y 2 z x y z                         m       (27) the inverted matrix m is given by: 2 1 3 2 3 3 4 3 4 4 2 4 2 3 4 3 4 4 1 0 0 x x 1 0 x y y x x y y 1 x z x y z y z z                        m (28) 4.3. estimation algorithm of the system parameters equation (25) can be used to prove the validity of the calculated tcp position. therefore, the location parameters x4, y4 and z4 as well as ℓ04 are needed. the offset length ℓ04 is calculated by substituting equation (27) in equation (25). the relative distances can be set to zero in order to determine ℓ04 which is a function of the location and the remaining offset length parameters:   with 0404 2 3 3 4 4 4 01 02 03 1 2 3 f x , x , y , x , y , z , , , 0              (29) to achieve a simplification the location parameters and the independent offset length parameters can be denoted as follows:     l 2 3 3 4 4 4 o 01 02 03 p x , x , y , x , y , z p , ,      (30) in order to calculate the tcp position very precisely the objective is the determination of the optimal parameter values. as the location parameters are static they can be estimated offline. for this purpose a pattern-search algorithm is used [23]. that method is rather time-consuming but has to be accomplished only once. in the case that there is no reference position of the kinematics the offset length parameters change with every shift of the initial position of the tcp. the strategy is to identify the location parameters in a previous step with a pattern-search algorithm. then, the offset length parameters are estimated in real time. as an optimization method for the offset length parameters the gauss-newton approach is applied. the gauss-newton approach is a specific modification of the newton method and convenient for nonlinear problems [20]. the calculation of the hessian is very complex and requires a lot of computing resources. hence, a quasi-newton approach is applied that does not require the calculation of the hessian. the objective function is based on equation (25) and is solved for δℓ4:  4c4 l o 1 2 3f p , p , , ,        (31) equation (31) is a function dependent on the offset length as well as the location parameters. the different distances can be interpreted as input signals. hence, the objective function results in the difference of the calculated and measured distance of tracker t4. to apply the estimation algorithm a definite number of measurements is needed leading to the residual function is defined as follows: with c i 4 i 4 ir i 1 n        (32) where n is the index of the measurement data. since the objective function is clarified the next step is the application of the gauss-newton algorithm. subsequently, the algorithm shall be outlined. the goal is to minimize the sum of squares of ri: min n 2 i i 1 r   (33) the gauss-newton approach is an iterative process and the current estimated parameters are modified by a correction term c:     with with k k 1 t t t 1 2 n p p c c r r r , r , , r        d d d  (34) the jacobian matrix d is determined by the derivatives of the residual function equation (32), i.e. the offset length parameters. every row correlates with a measurement value. the jacobian matrix reads as follows: 1 1 1 01,1 02,1 03,1 2 2 2 01,2 02,2 03,2 01, 01, 01, n n n n n n r r r r r r r r r                                                      d (35) due to the fact that the residuum changes continuously with the adjustment of the parameters the jacobian d has to be updated in every iteration step. to determine the end of the iteration a criterion is necessary. here, the norm of the difference of the modified (pk) and the current (pk-1) parameter vector is used. the calculation will be terminated as soon as the norm gets below a defined threshold, is e: 1k kp p e  (36) the calibration process runs online and automatically. it shall be depicted in the following. acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 10 after the measurement system is activated the control unit handles the initial procedures that include enabling the actuator and sensor units. if the target is localized by all laser trackers, the system is prepared for the calibration procedure. here, a cube dimensioned motion with 100×100×100 mm³ is executed. this space is marked with 27 grid points and at every point the positioning stage stops for a short period of time. the trajectory is quasi-static because at every grid point several measurement values are taken to generate an average value. after the cube trajectory is finished the measurement data is used to estimate the offset length parameters online. all 27 measurement points compose the residual function r and the jacobian d. when the termination criterion is reached the parameters are identified. then, they are written as fixed values in the control system and the multi laser tracker system is ready to measure the motion of the target. 4.4. influence of the initial tracker arrangement on the parameter estimation after the self-calibration, a precise estimation of all nine system parameters is available. the real values within the simulation are well known. to evaluate the quality of the estimation, the maximum difference between the known and the estimated parameters is calculated: realq max | p p |     first investigations have shown that the optimization algorithm is able to converge to the designated minimum. to analyse the influence of noise and local minima, the starting point for optimization is set into an area of ±50 mm around the real parameters. the measurement noise of the interferometers has a gaussian distribution with standard deviation of 1 µm. these assumptions match with the experimental setup. the choice of the boundary values has significant influence on the accuracy of the self-calibration algorithm. then, the number of measurement points (ag), the distance between the different trackers among each other (att), the size of the working range for the reference points (abtcp), the distance between the laser trackers and the measurement area (attcp) are investigated. in the simulation these configuration values are varied within the following intervals:  ag: [27, 64, 125, 256]  att in [m]: [0.2, 0.3, 0.5, 0.8]  abtcp in [m]: [0.3, 0.4, 0.5, 0.6, 0.7, 0.8]  attcp in [m]: [0.5,0.7, 0.9, 1.1, 1.3, 1.5, 1.7, 1.9] for ag and att there exist four variations of boundary values, six for abtcp and eight for attcp. the total number of calibration set-ups nc is the product of the configuration variations providing the different maximum combination options. nc is calculated as follows: cn ag att abtcp attcp 4 4 6 8 768         (37) for every combination the optimization is carried out and the quality q is determined. the current boundary and the optimization results are stored. for the evaluation the maximum aberration between the estimated parameters and the real parameters is compared with a threshold value of 50 µm. the combination is valid if the maximum aberration is below the threshold value. in the case that all combinations are valid, a maximum number of valid calibrations nmax can be defined as the quotient between the total number of the calibrations nc and the number of the used intervals for every boundary:  ag → nmax = 192  att → nmax = 192  abtcp → nmax = 128  attcp → nmax = 96 the results of a simulation without measurement noise are presented in figure 11. the number of valid calibrations is minimal below the maximum number of valid calibrations nmax. it can be stated that the utilized estimation algorithm is appropriate for the selected boundary values. figure 12 depicts the simulation results with measurement noise. it can clearly be seen that the choice of the boundaries influences the quality of the parameter estimation. only 508 out of 768 calibrations are below the threshold value. the boundary att shows the strongest sensitivity regarding a valid calibration. furthermore, a large distance between the trackers among each other significantly increases the number of the valid calibrations. a step-by-step rise of the attcp decreases the number of valid calibrations. in contrast, an increasing ag as well as an increasing abtcp rises the number of the valid calibrations only slightly. to show the effect of the boundary values, the threshold figure 11. variation of four boundary values and number of valid calibrations without measurement noise, threshold value = 50 µm. figure 12. variation of four boundary values and number of valid calibrations with measurement noise, threshold value = 50 µm. acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 11 value is changed in the simulation from 50 µm to 10 µm. in figure 13 the variation of the boundary values and the related valid calibrations is depicted. only 240 of 768 calibrations are below the threshold value. it can be seen that the number of successful optimizations varies extremely with the choice of the boundaries. the number of valid calibrations increases with the number of measurement points ag as well as with an increasing distance between the laser trackers att. the results further improve with a larger area of reference points. the best performance for attcp is reached for a minimal distance between the trackers and the measurement interval. further simulations showed that unequally spaced reference points have no negative impact on the estimation error as long as every direction in space is used for measurement. if points only lie in one or two spatial directions, one cannot act on the assumption that all effects of the parameters are gathered. in table 1 the estimated parameter values for a non-uniform distribution are shown, following the rules mentioned above. 5. conclusions in the first part of this contribution we present the developed multi laser tracker system and the required control algorithms allowing to track a highly dynamic moving target. the proposed control algorithms include the model based tracking control, the localization routines for finding the retro-reflector and the communication between the four laser trackers. in the second part of this work the authors present a method to determine the position of the tcp based on a multilateration algorithm. the key point in the calculation of the tcp is the determination of nine system parameters. the estimation of these parameters is highly dependent on the initial arrangement of the mtls. in order to evaluate the sensitivity of the parameter estimation various arrangements of the mlts were investigated. the presented simulation shows that the choice of the boundary values affects the estimation quality significantly. a sufficiently large distance between laser trackers among each other is required to achieve good results. furthermore, the reference points should be distributed in a large area and a minimal distance between the laser trackers and the measurement area is recommended. finally, a large number of measurement points ensures the convergence of the parameter estimation. acknowledgement this work was done within the framework of the project “kompetenzdreieck optische mikrosysteme – spitzenforschung und innovation in den neuen ländern”, which is supported by the german ministry of education and research (fkz: 16sv5473). the authors would also like to thank all colleagues, who offered help with the work presented. references [1] t. t. nguyen, q. t. nguyen, a. amthor, c. ament, “control of a multi laser tracker system used as a position feedback sensor”, proceedings of the iasted international conference on modelling, identification and control, asiamic (2010), phuket, thailand, pp. 132-137. 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[10] a. amthor, j. werner, a. lorenz, s. zschaeck, c. ament, “asymmetric motion profile planning for nanopositioning and nanomeasuring machines”, journal of systems and control engineering, vol. 224, no. 1, 2010. table 1. estimated parameters [mm]. parameter 1 2 3 2x 3x real 786.8714 692.2187 706.3521 300.0000 150.0000 diff. 0.0022 0.0032 0.0029 0.0005 0.0001 parameter 3y 4x 4y 4z real 160.0000 150.0000 50.0000 150.0000 diff. 0.0002 0.0005 0.0000 0.0008 figure 13. variation of four boundary values and number of valid calibrations with measurement noise, threshold value = 10 µm. acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 12 [11] a. amthor, s. zschaeck, c. ament, ”dynamical friction modelling and adaptive compensation on the nanometer scale”, 37th annual conference of the ieee industrial electronics society, proceedings, pp. 511 – 516, melbourne, 2011. 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[16] j.r. mayer, g. a. parker, “calibration and assessment of a laser based instrument for robot dynamic measurement”, 11th triennal world cong. int. measurement confederation (imeko), houston, tx, oct. 1988, vol.: theory, simulation, calibration, testing, education. [17] t. takatsuji, y. koseki, m. goto,t. kurosawa, “restriction on the arrangement of laser trackers in laser trilateration“, measurement science & technology, no.9, pp. 1357-1359, 1998. [18] t. takatsuji, m. goto, t. kurosawa, et al, “the first measurement of a three-dimensional coordinate by use of a laser tracking interferometer system based on trilateration”, measurement science & technology, no.9, pp. 38-41, 1998. [19] y. lin, g. zhang, “the optimal arrangement of four laser tracking interferometers in 3d coordinate measuring system based on multi-lateration”, int. symposium on virtual environments, human-computer interfaces, and measurement systems, lugano, switzerland, pp.138-143, 2003. [20] o. nelles, nonlinear system identification, springer verlag, berlin, 2001. [21] sios meßtechnik gmbh, http://www.sios.de, 27.11.2013. [22] edmund optics, http://www.edmundoptics.com, 27.11.2013. [23] themathworks, http://www.mathworks.de/de/help/gads/patternsearch.html, 27.11.2013. introductory notes for the acta imeko special issue on the “23rd symposium on measurement of electrical quantities and electronic measurements promote industry 4.0” – part ii acta imeko issn: 2221-870x september 2020, volume 9, number 3, 1-2 acta imeko | www.imeko.org september 2020 | volume 9 | number 3 | 1 introductory notes for the acta imeko special issue on the 23rd imeko tc4 international symposium “electrical and electronic measurements promote industry 4.0” – part ii ján šaliga technical university of košice, faculty of electrical engineering and informatics, letná 9, 042 00 košice, slovakia section: editorial citation: ján šaliga, introductory notes for the acta imeko special issue on the “23rd symposium on measurement of electrical quantities and electronic measurements promote industry 4.0” – part ii, acta imeko, vol. 9, no. 3, article 1, september 2020, identifier: imeko-acta-09 (2020)-03-01 editor: dušan agrež, university of ljubljana, slovenia received: september 25, 2020; in final form september 25, 2020; published september 2020 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding authors: ján šaliga, e-mail: jan.saliga@tuke.sk introductory notes on the “23rd symposium on measurement of electrical quantities’’ – part ii measurement is one of the oldest human activities and one which is the essential tool and need of any science, engineering projects or of industrial production. the ongoing revolution in industry known as industry 4.0 means transformation of traditional manufacturing practises to new, more effective with high quality of production based on the latest smart technology such as internet of thing and machine to machine communication. this transformation has not been possible without new measuring and test methods and instrumentation together with improved metrology and standardisation. nowadays nearly all data acquisition systems focused on any physical quantity convert the sensed quantity to an electrical quantity process in industrial systems using the sophistic digital signal processing. this is the reason why the measurement of electrical quantities including ad and da conversion and signal processing methods have been continually in the scope of scientists, researchers and engineers over all the world. the 23rd symposium on measurement of electrical quantities offered a chance to present the newest ideas and results of research to scientists in the field from all the word. the symposium was held in september 2019 in amazing historical town xi’an, china. the famous silk road, which began in the town xi’an was the ancient route linking the east and west, china and europe allowing not only trade and exchange of goods, but it was a mean for exchange of knowledge and experiences. therefore, the imeko tc-4 symposium in xi’an was a symbolic continuation of this history. the symposium addressed the work of a wide forum of experts across academia as well as industry and gave to the attenders an opportunity for presenting their latest research. imeko tc4 is one of imeko technical comities, which task and goal is to create an international platform for experts in the field of measurement of electrical quantities, emphasizing both theoretical and practical aspects of research in the field. the country members of imeko which have current representatives in the tc4 board are: belgium, brazil , bulgaria, canada, croatia, czech republic, egypt, estonia, finland, france, germany, greece, hungary, china, italy, poland, portugal, republic of, korea, romania, russia, serbia, slovakia, slovenia, spain, switzerland, turkey, ukraine, united kingdom. any new country is cordially welcome to join imeko tc4 activities. many papers dedicated to a range of problems and questions related to the measurement of electrical quantities which is under scope of imeko tc-4 technical committee were submitted to the symposium, and a number were accepted for the symposium and workshop. as it was mentioned in the previous issue of acta imeko v9.2, the member of imeko tc-4 board identified nineteen the most valuable papers and their authors were invited to write extended and updated version of their papers to be submitted for refereeing. by the end some seventeen papers were accepted. the first seven papers were published in the previous issue of acta imeko and the resting nine are being chosen for this issue. metrological aspects of measurements are closely bound with uncertainty analysis and improvement of measurement accuracy and precision as it is required for industry 4.0. the new definition of kilogram based on the fixed value of planck constant leads also to the improvement of reference standards in national metrological institutes, what is not possible without new electronic apparatus. haci ahmedov et al. present the new definition realized in national metrology institute in turkey via oscillating-magnet kibble balance. the procedure is less sensitive to environmental disturbances using michelson or mailto:jan.saliga@tuke.sk acta imeko | www.imeko.org september 2020 | volume 9 | number 3 | 2 fabry-perot interferometer to precisely determine the displacement. daniel belega et al. deal by the influence of disturbing signal component in the form of a small amplitude interharmonic component to the sine-wave parameters estimation returned by the classical interpolated discrete fourier transform algorithm. the derived expressions allow to analyse the impact of an interharmonic on the accuracies of frequency, amplitude, and phase estimators using the classical interpolated discrete fourier transform algorithm. the microwave material characterisation is in the centre of scope also of paper presented by andrea alimenti et al. the authors deal with a convenient measurement method for the complex permittivity evaluation based on a dielectric loaded resonator. the method was evaluated for 3d-printer materials that becoming increasingly more appealing also for high frequency applications. paper of haci ahmedov et al. is dedicated to the kibble balance apparatus as effective primary realisation for the new definition of kilogram. balance apparatus operating at national metrology institute of turkey is designed with a stationary coil and an oscillating magnet. this paper describes method and model based on external magnetic flux measurements to consider the effect of the field on realisation of kilogram. kibble balance implemented in the form of table-top version planck-balance is in the scope of paper presented by shan lin et al. authors analysed influence of different perturbations such as additive gaussian noise, time jitter etc. on amplitude accuracy estimation of a sinusoidal moving coil. the amplitude is calculated by three parameter fitting method. by optimizing measurement and data processing approach, the bias and standard deviation of the estimated amplitude can be effectively reduced. results of uncertainty investigation in the microwave field are presented by kostiantyn torokhtii et al., where they extended the analysis of the effect of the uncalibrated microwave measurements for material characterisation to have a more complete generalized picture of the resonant parameters. a key objective of the study is the determination of the uncertainty to be associated to the quality factor and resonant frequency for measurements where no calibration could be performed. influence of electromagnetic field on human being and its biological and health effects are becoming very important for human mankind with growing of em sources in our living environment. marius valerian paulet et al. performed a study of the relative position relevance of human torso to the plane of various overhead high voltage power lines towers. the simulations were performed for a few relative positions of the human trunk with respect to the plane of the transmission towers and for two common types of symmetric, doubled three-phase networks. the obtained results allow to formulate recommendations on the reduction of human exposure to magnetic fields. the biological and health effect of electromagnetic field is also in scope of the paper by ovidiu bejenaru at al. the authors studied influence of far field effects for the human body inside the building in an empty room. the paper presented by jakub svatos et al. describes a longterm measurement system for diagnostics of modern woodhouse, which is one of nowadays trends for our green sustainable future. the system allows acquiring all important parameters for evaluation building materials and construction of houses. the papers chosen for this special issue of acta imeko cover wide range of topics bound with measurement of electric quantities and i as the guest editor hope that each reader of this issue will find interesting and useful information in the papers. finally, i would like to thank all authors for their cooperation and improvements of the papers, the reviewers for highly professional reviews, notes and recommendations to the authors, and also to the editors of acta imeko dušan agrež and dirk röske for their help and valuable advices. ján šaliga guest editor editorial to additional papers dear reader, two additional papers are closing this third issue of acta imeko in 2020. the paper presented by giorgia bucci et al. is the last selected paper from the ieee international workshop on metrology for agriculture and forestry held in portici, naples on october 2019. it presents the topic of precision agriculture (pa), which offers the opportunity for farmers to improve both efficiency in managing resources and optimisation of process inputs, thus increasing their whole farm’s profitability. the economic aspect is undoubtedly one of the most important aspects to consider before adopting pats (pa technologies). in most of the cases, farmers are reluctant to introduce precision farming systems since the costs and uncertainty about the profitability and advantages need to be addressed. this study aims to explore how pats could affect the profitability of a representative italian farm specialising in the production of cereals, making this a case study. in detail, an economic analysis was applied to determine the profitability of the farm, which showed that the adoption of pat’s increased the yield of durum and soft wheat and significantly reduced the cost of mechanical operations and technical means. therefore, the potential gains from the adoption of pats challenges policymakers to design targeted interventions which could encourage their uptake. the philippine industrial technology development institute (itdi) pioneered the development of reference materials (rms) as part of its vision to establish metrological traceability and to support the rm needs of chemical testing laboratories in the country. food safety of philippine products were prioritised in this activity as these could affect the health of consumers as well as trade issues. the focus of this study presented by benilda ebarvia et al. was on contaminants like histamine and benzoic acid in canned tuna and mango juice, respectively. the materials were prepared and characterised, and they were found to be homogeneous and stable for at least six months. higher order methods like the isotope dilution technique, coupled with liquid chromatography–triple quadrupole mass spectrometry, were used to assign reference values to the rms, and the results were cross-checked using gravimetric techniques for highperformance liquid chromatography analysis. these steps resulted in rms for histamine and benzoic acid in philippine products, and itdi disseminated the measurement traceability to local laboratories through proficiency testing schemes organised and conducted for the analyses. on behalf of the editorial board i sincerely hope that you will enjoy the scientific content of this acta imeko issue. dušan agrež, editor-in-chief of acta imeko monte carlo determination of the uncertainty of effective area and deformation coefficient for a piston cylinder unit acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 338 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 338 342 monte carlo determination of the uncertainty of effective area and deformation coefficient for a piston cylinder unit c. wuethrich1, s. souiyam2 1 metas, bern, switzerland, christian.wuethrich@metas.ch 2 lpee-lnm, casablanca, morocco, souiyam@lpee.ma abstract: this paper describes the approach made to describe the uncertainty on the area at zero pressure (a0) and on the deformation coefficient (𝜆) of piston-cylinders used for pressure definition. we perform the monte-carlo simulation for an ordinary least squares (ols) as well as for a weighted least squares (wls) and a generalized least squares (gls). we also introduce an innovative technique to improve the results obtained by wls in order to get close to the quality obtained using gls. we discuss the different situations that guides to the best choice between ols, wls and gls. keywords: monte-carlo, uncertainty, pressure balance, piston-cylinder 1. introduction traditionally, pressure laboratories have an uncertainty on the effective area determined by cross floatation based on the uncertainties on the influence factors and the sensitivity coefficients. [13]. the uncertainty on the area at zero pressure (a0) and the deformation coefficient (𝜆) determined by least squares is not trivial to obtain by the technique of the sensitivity coefficients. the project euramet 1125 [4] described the results obtained by several national metrology institutes in europe for a set of data, simulating measurements affected by known uncertainties. alternative techniques have been recently proposed to improve the uncertainty [5-7]. we explore the uncertainty of the determination of effective area at zero pressure and deformation coefficient based on monte-carlo simulation of the calibration process and the determination of 𝜆 and a0 2. goal of this work metas and lpee-lnm performed recently a bilateral comparison of piston-cylinder units. we wanted not only to compare the effective area, at a given pressure, but also the values for a0 and 𝜆 obtained by least squares calculation on the set of measurements. a monte-carlo simulation is an efficient and mathematically correct way to assess the uncertainty of a value obtained using a least squares calculation. we are confronted to inhomogeneous uncertainties on the effective area depending of the pressure at which the effective area is measured, leading to heteroscedasticity on the residuals. we also have a correlation in the error of the mass used on each pressure balance as the mass used for the realisation of the first pressure step remains on the pressure balance for the second pressure step and so on for each successive pressure step. the whole set of mass is also calibrated against the same reference and the different pieces of charge are correlated to some degree. figure 1: setup used for the determination of the pistoncylinder effective area. the reference pressure balance, on the left, is working with gas and the pressure balance under calibration, on the right, is working with oil. 3. determination of a0 and lambda in pressure metrology we determine the effective area of a piston cylinder unit (pcu) at different pressure points by cross floatation [2-3]. based on these measurements we want to determine the effective area at zero pressure (a0) and the deformation coefficient (𝜆), in order to have a model of the deformation of the pcu under pressure. the area of the pcu at a given pressure is the given by: 𝐴(𝑝) = 𝐴0(1 + 𝜆𝑝) (1) http://www.imeko.org/ mailto:christian.wuethrich@metas.ch mailto:souiyam@lpee.ma acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 339 because we have only two free parameters to determine but have measurements performed at a larger number of pressure points it is convenient to use a least square approach to solve this equation. it is convenient to rewrite (1) the following way: 𝐴(𝑝) = 𝐴0 + 𝑏 ∙ 𝑝 (2) where b is in fact a0*𝜆. 3.1. least squares determination the model used for the relation between the set of measurement, the deformation coefficient and the effective area at zero pressure is as follow: ( 1 𝑝1 ⋮ ⋮ 1 𝑝𝑛 ) ∙ ( 𝐴0 𝑏 ) = ( 𝐴(𝑝1) ⋮ 𝐴(𝑝𝑛 ) ) + ( 𝜀1 ⋮ 𝜀𝑛 ) . (3) where the pi are the pressure steps at which we determined the effective area a(pi). the value a0 is the effective area at zero pressure and b is the increase of area per unit pressure due to deformation and these two values are the unknown of the system. the 𝜀i are the residual that we will minimise using the least square equation. we can rewrite the equation 2 using the following matrix definition: 𝑋𝐶 = 𝑌 + 𝐸 (4) we introduce a weighting matrix that multiplies the terms of the equation on the left and on the right. 𝑉 −1𝑋𝐶 = 𝑉 −1𝑌 (5) the matrix v is the weighing coefficient. v is an identity matrix in the case of ordinary least square (ols). v is a diagonal matrix with coefficients proportional to the uncertainty of a(pi) or to the square of the uncertainty of a(pi) in the case of the weighted least squares according to respectively the uncertainty (wls-u) or the uncertainty squared (wls-u2). v is the matrix of variance and covariance of the a(pi) in the case of generalised least squares (gls). for simplicity of writing we define the matrix w as v-1 we then solve the equation by multiplying by x´, the transposate of x on the left side: 𝑋′𝑉 −1𝑋𝐶 = 𝑋′𝑉 −1𝑌 (6) and finally we multiply by (x´v-1x)-1 on the left side and obtain the solution of the equation system: 𝐶 = (𝑋′𝑉 −1𝑋)−1 ∗ 𝑋′𝑉 −1𝑌 (7) 4. monte carlo simulation the monte-carlo simulation reproduces the errors obtained when performing the determination of the effective area of a piston-cylinder using the cross-floating technique. the effective area for a given pressure step is obtained by using the usual equation of effective area determination. in a first step we calculate the pressure generated by the reference pressure balance by the jth pressure step: 𝑝𝑗 = ∑ 𝑚𝑖 𝑔(1 − 𝜌𝑎 𝜌𝑚𝑖⁄ )𝑖 𝐴𝑅0(1 − 𝜆𝑝) (1 + (𝛼𝑝 + 𝛼𝑐 )(𝑡 − 𝑡𝑟 )) . (8) and then we calculate the effective area of the piston under test. 𝐴𝑝𝑗 = ∑ 𝑚𝑖𝑗 𝑔 (1 − 𝜌𝑎𝑗 𝜌𝑚𝑖𝑗⁄ )𝑖 𝑝𝑗 (1 + (𝛼𝑝 + 𝛼𝑐 )(𝑡 − 𝑡𝑟 )) . (9) in our model there is 9 pressure steps and we assume that the measurement is repeated 10 times in order to determine the effective area. a set of measurement comprises the 90 measurement resulting from the 9 pressure steps repeated ten times. we repeat the calculation to obtain 10'000 measurement sets in order to perform the uncertainty calculation on a0 and 𝜆 of the piston under calibration. the errors we generate in the monte-carlo simulation are supposed to depict as close as possible the reality of the metrological process. all influence factors have an error contribution considered constant in the whole set of measurements that do not change from one pressure step to the next one. some influence factors are considered to have an error contribution that does change at each pressure step. this is the case for influence factors that need a new measurement at each pressure step. figure 2: plot (blue stars) of the effective area obtained in the monte-carlo simulation. the red line is the average value of the effective area and the green curves are placed at plus and minus the standard deviation. 4.1. generation of the set of data as illustration of the numerical technique we simulate a piston of 2 bar/kg working with air used to calibrate a piston of 5 bar/kg working with oil. 0e00 1e072e06 4e06 6e06 8e06 1.2e07 1.4e07 1.6e07 1.962e-05 1.9616e-05 1.9618e-05 1.9622e-05 1.9624e-05 1.9617e-05 1.9619e-05 1.9621e-05 1.9623e-05 pressure (pa) a re a ( m 2 ) area versus pressure used in the simulation http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 340 we perform the calibration at 1 mpa, 2 mpa and then each 2 mpa up to 16 mpa. the values and respective uncertainties type a and b are given in table 1. we assume for all the uncertainties a normal distribution in our simulation. the simulation is performed using scilab 6.1.0 [9] on a small computer running a 64 bits operating system like linux or windows. the generation of the 10'000 sets of data, the resolution of the least squares equations for the different weighting schemes and some statistical calculations takes about 2 minutes. table 1: values and related uncertainties of type a and type b used in the simulation. influence factor abreviation value uncertainty type b uncertainty type a effective area reference piston a0,ref 4.905 × 10 -5 m2 9.64 × 10-10 m2 deformation coefficient reference 𝜆ref 7.0 × 10-13 pa-1 1.0 × 10-13 pa-1 effective area piston to calibrate a0,cal 1.962 × 10 -5 m2 def. coefficient piston to calibrate 𝜆cal 7.0 × 10 -13 pa-1 thermal expansion coeff. reference αref 9 ppm/°c 1 ppm/°c therm. exp. coeff. piston to calibrate αcal 9 ppm/°c 1 ppm/°c mass used on the reference piston mi,ref 5 kg 50 mg 10 kg 50 mg mass used on the piston to calibrate mi,cal 2.0 kg 20 mg 4.0 kg 20 mg height of the oil column δh 0 m 0.05 m 0.003 m temperature of the reference piston tref 22 °c 0.1 °c 0.1 °c temperature of the piston to calibrate tcal 22 °c 0.1 °c 0.1 °c density of the mass used on the ref. ρmiref 7950 kg/m 3 71 kg/m3 dens. of the mass used on the sample ρmical 7950 kg/m 3 71 kg/m3 density of air ρair 1.2 kg/m 3 0 kg/m3 0.005 kg/m3 5. determination of the weighting matrix the weighting matrix v used in the least square calculation according to equation 5 can be defined of different manners [8]. the simplest way is to take a diagonal matrix with 1. this is the situation of ordinary least squares determination (ols). 𝑣𝑖𝑗 = 0: 𝑖 ≠ 𝑗 , 𝑣𝑖𝑖 = 1 (10) traditionally people working on the determination of the parameters of pcu use a diagonal weighting matrix in which the values of the elements on the diagonal are related to the uncertainty of the effective area obtained at a given pressure step. (wls-u) 𝑣𝑖𝑗 = 0: 𝑖 ≠ 𝑗 , 𝑣𝑖𝑖 = 𝑢(𝐴(𝑝𝑖 )) (11) another solution consists in a weighting matrix in which the diagonal elements are determined by the square of the uncertainty of the effective area at a given pressure. (wls-u2) 𝑣𝑖𝑗 = 0: 𝑖 ≠ 𝑗 , 𝑣𝑖𝑖 = 𝑢 2(𝐴(𝑝𝑖 )) (12) finally we can determine the matrix v using the variance and covariance of the effective area at the different pressure steps. this approach is susceptible to provide the best uncertainties but is difficult to implement because the determination of the covariance can be challenging. in our situation we have a large set of simulated data through the monte-carlo calculation and are able to calculate the covariance based on the set of data at our disposal. (gls) 𝑣𝑖𝑗 = 𝑐𝑜𝑣 (𝐴(𝑝𝑖 ), 𝐴(𝑝𝑗 )) : 𝑖 ≠ 𝑗 , 𝑣𝑖𝑖 = 𝑢 2(𝐴(𝑝𝑖 )) (13) until now all the matrices we have considered are based on the uncertainties observed on the effective area of the pcu. in a recent publication [6], p. otal introduced the notion of matrix coefficients based on the uncertainty of ã(p) which is calculated assuming only the uncertainty contributions on the effective area that are pressure dependant and contribute to heteroscedasticity. in our model we take into account the contribution of the height of the oil column (δh) and the deformation coefficient of the reference pcu (𝜆ref). the plot of the ã(p) and their standard deviation is shown on fig. 3. we are able, based on the uncertainty of ã(p), to define the following matrices used in the least squares determination. http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 341 a weighted least squares based on the uncertainty of ã(p) as explained in [6] (wls-uã): 𝑣𝑖𝑗 = 0: 𝑖 ≠ 𝑗 , 𝑣𝑖𝑖 = 𝑢 (�̃�(𝑝𝑖 )) (14) a weighted least squares based on the square of the uncertainty of ã(p) (wls-u2ã): 𝑣𝑖𝑗 = 0: 𝑖 ≠ 𝑗 , 𝑣𝑖𝑖 = 𝑢 2 (�̃�(𝑝𝑖 )) (15) finally, we can also define a generalised least squares matrix based on ã(p) (gls-ã): 𝑣𝑖𝑗 = 𝑐𝑜𝑣 (�̃�(𝑝𝑖 ), �̃�(𝑝𝑗 )) : 𝑖 ≠ 𝑗 , 𝑣𝑖𝑖 = 𝑢 2 (�̃�(𝑝𝑖 )) (16) figure 3: plot (blue stars) of the effective area obtained in the monte-carlo simulation taking into account the uncertainties related to the oil column and the deformation coefficient. the red line is the average value of the effective area and the green curves are placed at plus and minus the standard deviation. it is interesting to note that the diagonal elements of the matrix are similar in the equation 12 and 13 as well as in equation 15 and 16. the coefficients of the diagonal of the matrix w are summarised in table 2 and show the relative importance given to the different measurements. table 2: elements wii used according to the different weighing schemes. a normalisation has been applied so that the larger element is equal to one. wls -u wls -u2 wls -uã wls -u2ã w11 0.395 0.157 0.086 0.008 w22 0.644 0.414 0.170 0.029 w33 0.861 0.743 0.325 0.106 w44 0.937 0.877 0.468 0.219 w55 0.968 0.938 0.600 0.361 w66 0.984 0.968 0.722 0.521 w77 0.992 0.984 0.830 0.689 w88 0.997 0.993 0.923 0.851 w99 1.000 1.000 1.000 1.000 5.1. uncertainties on a0 and lambda we applied the different weighing schemes to our set of data. we managed to obtain an average value for a0 and 𝜆 similar to the nominal value for all the weighting matrices demonstrating that the technique is correct. the standard deviation obtained on a0 and 𝜆 greatly depends on the weighting scheme selected. the results summarised on table 3 show that the gls techniques delivers the best uncertainties. weighting least squares based on the square of the uncertainty are more performant than those based on the uncertainty. the most interesting result is that the uncertainty on a0 and 𝜆 for the wls based on u2(ãp) is the closest from the results obtained with a gls approach. it is also interesting to note that the gls approach based on ã(p) delivers results similar with the gls based on a(p). table 3: values and respective uncertainties obtained for the area at zero pressure and the deformation coefficient for the pcu under calibration, for different least squares techniques. a0 𝜆 nominal value 1.962 × 10-5 m2 0.70 × 10-12 pa-1 u(ols) 36 ppm 2.20 × 10-12 pa-1 u(wls-u) 31 ppm 1.70 × 10-12 pa-1 u(wls-u2) 28 ppm 1.35 × 10-12 pa-1 u(gls) 20 ppm 0.36 × 10-12 pa-1 u(wls-uã) 28 ppm 1.21 × 10-12 pa-1 u(wls-u2ã) 24 ppm 0.74 × 10-12 pa-1 u(gls-ã) 20 ppm 0.35 × 10-12 pa-1 the plot of 𝜆 versus a0 for the different least square weighting matrices, depicted in fig. 4, shows the uncertainties and correlation obtained with the different least squares approximations. in order to assess the independence of the uncertainties on a0 and 𝜆, we calculated the correlation between the two parameters for the different weighing matrices used in the least square calculation. the results, presented on table 4, show that the correlation is relatively high when ols is used but can be decreased to almost negligible amount in the case of a gls approach. the correlation for the wls according to u2(ãp) is the best value if we exclude the gls. table 4: correlation between a0 and 𝜆 for the different weighing matrices tested in the simulation. weighing matrix correlation a0 𝜆 ols -0.83 wls-u -0.77 wls-u2 -0.71 gls -0.28 wls-uã -0.69 wls-u2ã -0.55 gls-ã -0.27 0e00 1e072e06 4e06 6e06 8e06 1.2e07 1.4e07 1.6e07 1.962e-05 1.9618e-05 1.9622e-05 1.9617e-05 1.9619e-05 1.9621e-05 1.9623e-05 1.96175e-05 1.96185e-05 1.96195e-05 1.96205e-05 1.96215e-05 1.96225e-05 pressure (pa) a re a ( m 2 ) area versus pressure used in the simulation http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 342 figure 4: plot of 𝜆 versus a0 for different least square calculation: from left to right, top:ols, wls-u , wls-u2 and gls, bottom: wls_uã, wls_u2ã, gls_ã. 6. summary we applied successfully a monte-carlo simulation to determine the uncertainty on a0 and 𝜆 determined by cross floating of pressure balances. our numerical simulation has been used to optimise the weighting matrix used in the determination of a0 and 𝜆. the raw data from the monte-carlo simulation have been used to determine the covariance between the effective areas determined at two different pressure. we have shown that an improvement of the weighting matrix proposed by otal and al. is able to provide an uncertainty close to what is obtained using a generalised least squares approach. the authors want to thank the ptb for providing this collaboration opportunity between metas and lpee-lnm through the project "promotion of quality-assurance capabilities and services in the maghreb to strengthen international trade". we want to thank dr. kilian marti for his help in correcting the manuscript. 7. references [1] dadson r s, lewis s l and peggs g n, the pressure balance: theory and practice (london: hmso) pp 19–72, 1982. [2] rmaéro 802-21, étalonnage et utilisation des balances manométriques—balance à application de masses [3] calibration of pressure balances, euramet cg-3, version 1.0 (03/2011) euramet e.v., technical committee for mass [4] i. morgado et al, "evaluation of effective float measurement with pressure balances", euramet project 1125, final report [5] sutton c m, fitzgerald m p and giardini w, a method of analysis for cross-floats between pressure balances, metrologia, vol. 42, pp. 212–215, 2005. [6] p. otal, c. yardin, modelling methods for pressure balance calibration, measurement science and technology, vol. 31, 034004, 2020. [7] v. ramnath, determination of pressure balance distortion coefficient and zero-pressure effective area uncertainties, int. j. metrol. qual. eng, vol. 2, pp. 101-119, 2011. [8] strutz and tilo, data fitting and uncertainty (a practical introduction to weighted least squares and beyond)., 2010 [9] https://www.scilab.org 1.962e-051.9618e-05 1.9622e-05 0e00 -5e-12 5e-12 area (m2) l a m b d a ( 1 /p a ) lambda vs a0 for ols 1.962e-051.9618e-05 1.9622e-05 0e00 -5e-12 5e-12 area (m2) l a m b d a ( 1 /p a ) lambda vs a0 for wls u(ap) 1.962e-051.9618e-05 1.9622e-05 0e00 -5e-12 5e-12 area (m2) l a m b d a ( 1 /p a ) lambda vs a0 for wls u2(ap) 1.962e-051.9618e-05 1.9622e-05 0e00 -5e-12 5e-12 area (m2) l a m b d a ( 1 /p a ) lambda vs a0 for gls ap 1.962e-051.9618e-05 1.9622e-05 0e00 -5e-12 5e-12 area (m2) l a m b d a ( 1 /p a ) lambda vs a0 for wls u(ãp) 1.962e-051.9618e-05 1.9622e-05 0e00 -5e-12 5e-12 area (m2) l a m b d a ( 1 /p a ) lambda vs a0 for wls u2(ãp) 1.962e-051.9618e-05 1.9622e-05 0e00 -5e-12 5e-12 area (m2) l a m b d a ( 1 /p a ) lambda vs a0 for gls ãp http://www.imeko.org/ https://www.scilab.org/ an invar-based fabry-perot cavity refractometer with a gallium fixed-point cell for assessment of pressure acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 293 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 293 298 an invar-based fabry-perot cavity refractometer with a gallium fixed-point cell for assessment of pressure i. silander1, c. forssén1,2, j. zakrisson1, m. zelan2, o. axner1 1 department of physics, umeå university, se-901 87 umeå, sweden, isak.silaner@umu.se, clayton.forssen@umu.se, johan.zakrisson@umu.se, ove.axner@umu.se 2 measurement technology, rise research institutes of sweden, se-501 15 borås, sweden, martin.zelan@rise.se abstract: an invar-based fabry-perot cavity refractometer equipped with an automated, miniaturized gallium fixed-point cell for assessment of pressure is presented. the use of an invar cavity spacer has previously demonstrated pressure assessments with sub-0.1 ppm precision. the fixed-point cell, whose design and implementation are presented here, provides a reference for temperature assessment of the gas inside the cavity with an uncertainty of 4 ppm. this opens up for a self-contained system for realization of the pascal with an accuracy in the low ppm range. this is an important step towards disseminating the pascal through fundamental principles. keywords: refractometry, fabry-perot cavity, temperature, gallium. 1. introduction with the recent revision of the si-system, alternative paths to realize pascal have become feasible. by measuring the refractivity and the temperature of a gas one can calculate its pressure by the use of the lorentz-lorenz equation and an equation of state [1]. such an instrument would not rely on any mechanical actuator; instead, if wellconstructed, its performance could depend solely on how well the molar refractivity and the gas temperature can be assessed [2]. we have previously developed an innovative method for refractometry, termed gas-modulation refractometry (gamor), that allows for a significant reduction of the influence of drifts and fluctuations of the system [3-5]. this has opened up for the use of non-conventional cavity-spacer materials that otherwise were considered unsuitable due to a high thermal expansion. a system realized around an invar-based dual fabry-perot cavity (dfpc) refractometer has been constructed and presented [6]. as discussed in detail in that work, in comparison to cavities based on conventional materials [zerodur® and ultra-low expansion glass (ule®)], such a system has several advantages, mainly: (1) a high ratio of thermal conductivity and volumetric heat capacity, which implies that the spacer will have smaller thermal gradients; (2) a high volumetric heat capacity, which reduces temperature fluctuations; (3) a young's modulus that gives a smaller pressureinduced deformation; (4) potentially a lower degree of he diffusivity and permeation; and (5) it can be machined in a standard metal workshop, whereby more complicated geometries can be created more swiftly to a low cost. for an empty cavity, the system could demonstrate an allan deviation of 0.03 mpa. when assessing pressure at 4303 pa, the system provided a minimum allan deviation of 0.34 mpa, which corresponds to a relative deviation (or a precision) of 0.08 ppm. however, the system, as realized in ref. [6], assessed temperature by the use of uncalibrated pt-100 sensors, which gave it a limited accuracy for pressure assessments. for a wider dissemination of the method, the temperature needs to be assessed with high accuracy and be traceable to a thermometric standard, preferably without any calibration chain. to realize such an instrumentation, we present here an invarbased dfpc refractometer that has been equipped with an automated temperature regulating and assessment system. similar to what previously has been demonstrated by deployment on the international space station (iss) [7], the latter employs an automated, miniaturized, custom-made fixed-point cell based upon the well-defined melting point of gallium. by the use of a thermocouple working close to zero temperature difference, the temperature of the cavity assembly is constantly assessed with respect to this melting point. the use of an invar-based dfpc facilitates, according to the first two points of above, i.e. (1) and (2), the temperature regulation of the cavity. referencing the temperature to a gallium fixed-point cell improves on the accuracy of the system. the combination of an invar-spacer, the gamor methodology, http://www.imeko.org/ mailto:isak.silaner@umu.se mailto:clayton.forssen@umu.se mailto:johan.zakrisson@umu.se mailto:ove.axner@umu.se mailto:martin.zelan@rise.se acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 294 and a ga fixed-point cell therefore provides a basis for a self-contained refractometer that in the future only needs a pure gas supply and a frequency reference to realize the pascal. 2. theory it has previously been shown that for a dfpc refractometer the refractivity can be expressed as [4,6] 11 1 f q n f   +  − = − + , (1) where f is the relative shift of the beat frequency given by 01 /f  , where, in turn, f is the shift of the beat frequency and 01  is the empty cavity frequency of the measurement laser, and 1 q represents 1 01 /q q , where 1 q is the change in mode number, 01 q , of the mode addressed in the empty measurement cavity when gas is let into the cavity.  is a normalized cavity deformation, defined as 0 ( / ) / ( 1)l l n − , where l and 0l are the pressure induced length deformation and the empty cavity length, respectively. for pressure up to atmospheric pressure, the gas density  can be calculated from the refractivity by use of the extended lorentz-lorenz equation, 1 2 ( 1)[1 ( 1)] 2 n r n b n a  − = − + − , (2) where r a and 1n b − are the molar dynamic polarizability and a series expansion coefficient, respectively. the latter is given by 2 (1 4 / ) / 6 r r b a− + , where, in turn, r b is the second refractivity virial coefficient in the lorentz-lorenz equation. up to atmospheric pressure, the pressure can be obtained from the density and temperature as [1 ( ) ]p rt b t   = + , (3) where r is the ideal gas constant, t is the temperature of the gas, and ( )b t  is the second density virial coefficient. if higher pressures are to be assessed, additional terms need to be added in the eqs. (2) and (3). 3. experimental setup 3.1. the invar-based dfpc-refractometer the invar-based dfpc-refractometer used in this work is, to a large extent, identical to the one presented in detail in a recent publication [6]. the heart of the system is the cavity spacer, which is machined from a rod of invar (see fig. 1). in the spacer, two holes for the measurement and reference cavities, and three holes for temperature probes (seen on the short end and on the top of the cavity assembly, respectively), have been drilled. to each of the cavities, a vacuum tube is connected in order to either supply or evacuate the cavity. to mount the cavity mirrors to the spacer the mirrors were placed in a machined inset with an o-ring above. to provide vacuum tight seals, plates were screwed into the spacer to press the mirrors, via the o-rings, onto the spacer. figure 1: the naked invar cavity assembly before being equipped with temperature probes and mounted inside the aluminium enclosure (the oven). the cavity spacer is placed in an aluminium enclosure (referred to as an oven) and supported by four sharp pegs at the points of minimum deflection. on top of the enclosure, four pneumatic vacuum valves, used to control the flow of gas via vacuum tubing to and from the cavities, are placed. a picture of the enclosure is shown in fig. 2. the enclosure does not only support the valves and fixates the cavity; it also plays a crucial role in the temperature measurement and stabilization system as a regulative oven. figure 2: the metal enclosure (the oven), containing the invar cavity. the valve assembly is attached to the temperature-controlled enclosure to reduce thermal gradients from the gas lines attached to the cavity. 3.2. temperature stabilization and assessment the temperature stabilization and assessment system, schematically illustrated in fig. 3, stabilizes the cavity “close” to the temperature corresponding to the melting-point of gallium ( gat = 302.9146 k), and then independently monitors the actual cavity temperature, which is considered an adequate representative of the gas temperature, t , by measuring the difference in temperature between a fixed-point cell and the cavity by the use of a type-t thermocouple. http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 295 figure 3: schematic illustration of the thermal management system. the temperatures of the cavity assembly (left) and the gallium fixed-point cell (right) are controlled by separate pid controllers. the temperature stabilization is performed in three stages. in the first, the measurement environment is heated to a temperature close to gat by heating the breadboard on which the enclosure rests by a heat mat. in the second stage, the temperature of the oven is measured with a pt-100 probe inserted in the bottom of the enclosure. this temperature reading is used to apply feedback to four peltier elements, placed under the oven, that regulate the oven temperature. in the third stage, the temperature of the cavity spacer, measured as the mean of three pt-100 probes that are inserted into the cavity spacer, cavt , is used to apply feedback to the set point of the oven servo stage. this feedback aims at keeping the spacer in thermal equilibration with the oven at (or close to) the ga melting point. the fixed-point cell consists of a ga sample (7n aldrich mkch6524) housed in a 15 ml falcon tube. the tube is mounted in a heating block that, in turn, is mounted on top of a peltier element. a probe assembly, which consists of a 3 mm folded kynar® (polyvinylidene fluoride, pvdf) tube heat shrunk around a pt-100 probe, is used to monitor the temperature of the gallium cell, cell t . the measurement junction of the thermocouple is immersed into the gallium. the temperature of the heating block, block t , is regulated to a temperature slightly above the meltingpoint of gallium, (ca. 50 mk) by applying feedback to the peltier element. whenever the temperature of the cell rises 15 mk above gat , the ga is considered melted and the cell is automatically reset by a refreezing procedure. in this, the temperature of the block is set to 25 °c. when the cell temperature reaches 25.1 °c, at which the gallium is considered to be frozen, the temperature of the block is again increased to a temperature slightly above gat . the length of the melting cycle of the ga depends on the temperature at which the heat block is held, but lasts typically 80 hours for a temperature of the heat block that is 50 mk above the ga melting point. the time for refreezing is typically less than 1 hour. the thermocouple, whose voltage is assessed by a nanovoltmeter (keithley, 2182a), is used to assess the temperature difference between the cavity and the gallium cell, tc t . when on the ga melting plateau, the temperature of the cavity measured by the thermocouple, tc t , is given by tc ga t t + . to reduce temperature overhearing, its reference junctions at the cavity are wound around the spacer a multitude of turns. 4. results 4.1. allan deviation of pressure assessments performed by the invar-based refractometer figure 4 provides a comparison between the invar-based system and a similar system based on a zerodur cavity [4]. before the ga fixed-point cell was implemented, for each cavity system, two measurement series were taken at dissimilar pressures; one when the system was evacuated, indicated in the form of allan deviations in fig. 4 by the magenta and yellow markers, and one at 4303 pa provided by a pressure balance, represented by the blue and red markers [6]. the data from the invar and zerodur cavities are presented by magenta and blue, and, yellow and red markers, respectively. figure 4: allan deviations of pressure assessments made by a previous zerodur-based system (red upper markers) and the invar-based system (blue), both at 4303 pa. the yellow and purple (lower) markers illustrate the corresponding empty cavity measurements. reproduced with modifications with permission from [6]. compared to the zerodur-based dfpc, the invarcavity based refractometer demonstrated an improved stability at both pressures, on all time scales. for the empty cavity assessments, the implementation of an invar-based dfpc provided a significantly improved white-noise behaviour. as is discussed in more detail in ref. [6], this is attributed to higher mechanical stability of the system. for the pressure assessment at 4303 pa, and for integration times between 400 and 1100 s, a performance with an allan deviation below 0.43 mpa (representing sub-0.1 ppm levels) could be demonstrated, attributed to the improved thermal http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 296 conditions of the invar material. for details about these responses, see ref. [6] in which a more rigorous scrutiny is presented. it should be noted though that, since the pt-100 sensors used at the time when the data shown in fig. 4 were taken had not been calibrated, the accuracy of the temperature assessment of the gas (and thereby the pressure) was, at that time, estimated to solely be slightly below 400 ppm [6]. 4.2. evaluation of the ga fixed-point-based temperature reference system to evaluate the performance of the gallium fixedpoint cell, a thorough estimation of its uncertainty, presented in table 1, was first performed. table 1: the estimated uncertainty of the gallium fixedpoint cavity temperature measurement setup. uncertainty components uncertainty (k = 2), mk type chemical impurities 0.011 b atmospheric pressure variations 0.062 b hydrostatic pressure correction 0.013 b temperature over-hearing to the block 0.014 a repeatability 0.225 a nanovoltmeter stability 1.26 b total: 1.2 the uncertainties due to chemical impurities, atmospheric pressure, and the hydrostatic pressure correction, were all, according to footnotes 1-3, estimated to be below 0.1 mk. to estimate the uncertainty due to temperature over-hearing, a series of melting curves were recorded while the system was stressed, i.e. the temperature of the block, the cavity, and the breadboard were deliberately changed in order to introduce overhearing effects. the only substantial overhearing was found between the block and the ga fixed-point cell. it was found that a change of 1 k of the temperature of the block influenced the thermocouple reading 1 mk. assuming an uncertainty in the assessment of the temperature of the block of 10 mk this will therefore solely give an uncertainty in the assessed temperature of 10 µk. to estimate the stability of the system, the temperature of the cavity measured by the thermocouple, tc t , was first compared with that measured by the pt-100 probes, cavt . the difference between the two is shown in fig. 5 for a single heating cycle. 1 gallium 7n sourced from aldrich lot mkch6524. 2 estimated from: variation of 10 kpa over a year [8] and ( /dt dp ) of -2.0 × 10-8 k/pa [9]. 3 estimated from: mounting point of probe and ( /dt dl ) of -1.2 × 103 k/m [9]. figure 5: the difference in cavity temperature measured with the thermocouple, tc t , and the pt-100 probes, cav t . the blue markers corresponds to the data evaluated. the dashed lines correspond to ± 2σ i.e. ± 220 µk. the stability of this assessment is given by a combination of the stability of the actual temperature of the gallium in the fixed-point cell, that of the thermocouple measurement, and that of the cavity temperature measured with pt-100 probes. to evaluate the influence of the latter, the stability of the resistance measurement module was monitored by the use of a standard resistor. figure 5 shows the ga melting cycle out of five that showed the highest resistance measurement stability. the part of the melting cycle during which temperature was assessed was taken as half of the melting plateau (the blue markers in fig. 5). its stability was estimated (as two standard deviations over the 13 h period) to 220 µk. since the assessment of the stability of the measurement module was found to represent 80 µk, the combined stability of the fixedpoint cell and thermocouple measurement can be assumed to be somewhat smaller than 220 µk. this result is in agreement with the expected melting curve of 7n ga [10]. it was found that the main contribution to the uncertainty came from the nanovoltmeter, which provided an uncertainty of 1.2 mk. since all other contributions to the uncertainty solely were fractions of this value, it also constitutes the total estimated uncertainty of the ga fixed-point cell based temperature measurement setup. this corresponds to a relative uncertainty of the assessment of temperature of 4 ppm. 4.3. a comparison between the two means to assess cavity temperature; the pt-100 sensors and the thermocouple system to demonstrate the performance of the temperature regulation and assessment system, a series of measurements was performed at a pressure of 11 620 pa (produced by a dead weight pressure balance) over 4 assuming an error of 10 mk in the assessed block temperature. correction formula 0.001 x ( block ga t t− ). 5 measurement 2×std over 13 h. 6 estimated from: accuracy: ± (40 ppm of reading + 4 ppm of range) (90 days). http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 297 a single melting cycle (~90 h). a measurement of various temperatures in the system is shown in fig. 6. the red, blue, and black curves represent the measured temperatures of the cavity measured by the pt-100 sensors, cavt , the thermocouple, tct , and the temperature of the fixed-point cell, cell t , all compared to the nominal melting point of ga, gat , respectively. figure 6: red curve: the temperature of the cavity assessed by the pt-100 sensor referenced to the nominal melting point of ga, i.e. cav ga t t− ; blue curve, the temperature of the cavity assessed by the thermocouple, referenced to the same entity, i.e. tc ga t t− ; black curve: the temperature of the ga fixed-point cell assessed by the pt-100 sensor referenced to ga t , i.e. cell ga t t− . while the black curve solely is used for monitoring the ga melting-cycle, the red is used to give feedback for the stabilization of the cavity temperature. drifts in the temperature assessment by the pt-100 sensors will therefore be transferred into the temperature of the cavity (the lack of drifts in the red curve indicates that the temperature regulation system works well). the blue curve represents the cavity temperature assessed by the thermocouple. as the uncertainty in this assessment (given by table 1) is smaller than its drift, the assessed tc t represents the actual temperature of the cavity. 4.4. assessment of the ga fixed-point referenced invar-based dfpc system to characterize the combined ga fixed-point cell and thermocouple system regarding its ability to assess pressure, a comparison between the pressures assessed by the refractometer using the two different means of temperature assessment was performed. the data taken during the melting cycle presented above were converted to refractivity by the use of eq. (1). in one of the cases, the pressure was assessed from this data by use of the temperature measured by the pt-100 sensors (the red curve in fig. 6) placed in the cavity spacer, providing cavt . in the other case, the pressure was assessed (from the same refractivity data) by use of the temperature assessed by the combined ga fixed-point cell and thermocouple system, i.e. tc t (the blue curve in fig. 6). the refractometry system and the piston gauge was exposed to 3600 gas filling-and-empting cycles according to the normal procedures of the gamor methodology [3,4,6]. each cycle lasted 100 s while the entire measurement campaign took 100 hours. the difference in pressure assessed by the refractometer and that set by the pressure balance, evaluated by the two different assessed temperatures, are presented in fig 7 (assessed as cavt , the lower black-and-red curve, and as tc t , the upper blackand-blue curve). figure 7: the pressure difference between the set pressure of the pressure balance and the pressures assessed by the refractometry evaluated using temperatures assessed by use of pt-100 probes (black-and-red curve), cav t , and the thermocouple referenced to the ga cell (black-and-blue curve), tc t . the dashed lines correspond to ± 2σ, which is ± 120 mpa for the pt-100 based measurements and ± 32 mpa for the thermocouple-based assessments. the coloured parts of the curves represent the centre half of the ga melting-plateau. evaluation of the system was made during this time period. the lowermost curve, representing assessment of the pressure difference by the use of the pt-100 sensors, was found to have a 2 -deviation over 40 h of 120 mpa, while the same measurement data, evaluated by use of the thermocouple system, was 32 mpa. for the pressure used (11 620 pa), this corresponds to stabilities of 10.4 and 2.8 ppm, respectively. this confirms the statement above that the temperature assessed by the combined ga fixed-point cell and thermocouple system provides the actual temperature of the cavity. 5. summary in summary, this work presents a dfpc-based refractometer made of invar with a temperature regulation and assessment system that is based on a regulation of the temperature of the cavity based on the cavity temperature assessed by use of three pt100 sensors, while the temperature of the cavity is assessed by the use of a thermocouple referenced to a ga fixed-point cell. it was first noted that an invarbased refractometer can provide a precision below 0.1 ppm for time scales between 400 and 1100 s. the temperature regulation and assessment system was then described and scrutinized in some detail. an estimation of the uncertainties in the temperature measurement system indicated that it provides a total http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 298 uncertainty of 1.2 mk (4 ppm), dominated by the stability of the nanovoltmeter used for assessment of the thermocouple voltage. this work has shown that the combination of invar spacer and ga fixed-point cell can provide a basis for a self-contained system that only needs a pure gas supply and accurate frequency references to realize the pascal. this is an important step towards the dissemination of the pascal through fundamental principles. this project (quantumpascal, 18sib04) has received funding from the empir programme cofinanced by the participating states and from the european union's horizon 2020 research and innovation programme. the authors also acknowledge support from the swedish research council (vr) (project no. 621-2015-04374); the umeå university industrial doctoral school (ids); the vinnova metrology programme (project nos. 2018-04570 and 2019-05029); and the kempe foundations (project no. 1823, u12). 6. references [1] p. f. egan, j. a. stone, j. h. hendricks, j. e. ricker, g. e. scace, g. f. strouse, “performance of a dual fabry–perot cavity refractometer,” opt. lett. vol. 40, 3945-3948, 2015. [2] p. f. egan, j. a. stone, j. e. ricker, j. h. hendricks, “comparison measurements of low-pressure between a laser refractometer ultrasonic manometer,” rev. sci. instrum., vol. 87, 053113, 2016. [3] i. silander, t. hausmaninger, m. zelan, o. axner, “gas modulation refractometry for high-precision assessment of pressure under non-temperaturestabilized conditions”, j. vac. sci. & technol. a, vol. 36, 03e105, 2018. [4] i. silander, t. hausmaninger, c. forssén, m. zelan, o. axner, ”gas equilibration gas modulation refractometry for assessment of pressure with subppm precision”, j. vac. sci. & technol. b, vol. 37, 042901, 2019. [5] o. axner, i. silander, c. forssén, j. zakrisson, m. zelan, "ability of gas modulation to reduce the pickup of fluctuations in refractometry," josa b, vol. 37, 1956-1965, 2020. [6] i. silander, c. forssén, j. zakrisson, m. zelan, o. axner, "invar-based refractometer for pressure assessments," opt. lett., vol. 45, 2652-2655, 2020. [7] topham, t., bingham, g., latvakoski, h. et al. observational study: microgravity testing of a phasechange reference on the international space station. npj microgravity, vol. 1, 15009, 2015. [8] retrieved from (in swedish) www.smhi.se/kunskapsbanken/meteorologi/lufttryc k-1.657. [9] “guide to the realization of the its-90”, www.bipm.org/en/committees/cc/cct/guideits90.html. [10] nakano, t., tamura, o. & sakurai, h. realization of the gallium triple point at nmij/aist. int j thermophys, vol. 29, 112–118, 2008. [11] j. zakrisson, i. silander, c. forssén, m. zelan, o. axner, “procedure for robust assessment of cavity deformation in fabry–pérot based refractometers” j. vac. sci. technol. b, vol. 38, 054202, 2020. http://www.imeko.org/ http://www.smhi.se/kunskapsbanken/meteorologi/lufttryck-1.657 http://www.smhi.se/kunskapsbanken/meteorologi/lufttryck-1.657 http://www.bipm.org/en/committees/cc/cct/guide-its90.html http://www.bipm.org/en/committees/cc/cct/guide-its90.html laboratory measurement system for pre-corroded sensors devoted to metallic artwork monitoring acta imeko issn: 2221-870x march 2021, volume 10, number 1, 209 216 acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 209 laboratory measurement system for pre-corroded sensors devoted to metallic artwork monitoring m. faifer1, s. goidanich2, c. laurano1, c. petiti2, s. toscani1, m. zanoni1 1 deib, politecnico di milano, piazza leonardo da vinci 32, 20133 milano (italy) 2 department of chemistry, materials and chemical engineering "giulio natta", politecnico di milano, piazza leonardo da vinci 32, 20133 milano (italy) section: research paper keywords: measuring system; corrosion; metallic artwork monitoring; heritage citation: marco faifer, sara goidanich, christian laurano, chiara petiti, sergio toscani, michele zanoni, laboratory measurement system for pre-corroded sensors devoted to metallic artwork monitoring, acta imeko, vol. 10, no. 1, article 28, march 2021, identifier: imeko-acta-10 (2021)-01-28 editor: ioan tudosa, university of sannio, italy received may 4, 2020; in final form november 23, 2020; published march 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: marco faifer, e-mail: marco.faifer@polimi.it 1. introduction the monitoring process plays a crucial role in the preservation of metallic artworks. in fact, whenever possible, the control of the various environmental parameters presents a critical step in the area of preventive conservation. here, the real-time monitoring of environmental corrosivity is a particularly important tool for the appropriate preservation of works of art. such an approach allows for not only checking whether the environment could be harmful to the artefacts but also for selecting the most appropriate conservation solutions. here, metals may be particularly sensitive to any increase in relative humidity (rh) or to the presence of specific pollutants in the atmosphere, such as low molecular weight carboxylic acids [1], [2]. however, while the monitoring of parameters such as humidity, temperature, and pollutants is a common practice [3][4], this does not necessarily provide specific information regarding the actual impact of the environment on the degradation rate of different materials. one possible solution is to expose both the object to be monitored and a sample made with the same alloy to the same environmental conditions. this approach also offers the opportunity to perform invasive measurements on the specimen without damaging the work of art [3]-[7]. another possible approach involves the employment of environmental corrosivity sensors [7]-[9]. while for other fields of application, various different types of sensors have been reported [10]-[12], few solutions have been specifically designed for the cultural heritage field [7], [13]-[15]. in fact, the majority of the sensors proposed thus far, as well as the european standard for the classification of the corrosivity of atmospheres [16], do not consider the effect of the presence of corrosion layers on the surface. this is of particular importance when dealing with historic surfaces, which generally present a fairly complex stratigraphy of corrosion layers that can significantly affect their corrosion behaviour [7], [9], [17]. the presence of hygroscopic products such as chlorides may significantly accelerate the corrosion processes, making the corrosion active abstract the monitoring of environmental corrosivity around works of cultural heritage is a key task in the field of both active and preventive conservation. in the case of metallic artworks, this task can be performed by means of coupons or sensors realised with the same materials as the artworks to be conserved. in this work, a measurement system for the development and testing of sensors for atmospheric corrosivity monitoring is presented. the metrological features of the measurement system operated in conjunction with a developed sensor are analysed. the sensor allows for considering the different corrosion behaviours due to the presence of corrosion layers on the object to be preserved. the first developed sensors are made of pre-corroded copper and their resistance is measured. the developed system allows for monitoring thickness loss of over 3 nm in the temperature range of 23 °c – 39 °c. the performed analysis demonstrated that the system presents an efficient laboratory setup for the development and characterisation of sensors for atmospheric corrosivity monitoring. mailto:marco.faifer@polimi.it acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 210 even at a low rh values or in the presence of specific pollutants [7], [9], [18], [19]. electric resistance (er) sensors are based on electrical resistance measurements that are performed periodically. the time-variation of er measurements can be correlated with the instantaneous corrosion rate. however, in order to fully represent how environmental conditions affect real works of art, an er-based sensor should be pre-corroded or pre-aged prior to exposure. therefore, in order to implement and optimise a more effective tool for the monitoring of the degradation effects, it became necessary to design and characterise a new setup for the realisation of pre-corroded sensors. in previous works, the authors developed a sensor that can be preliminarily corroded so as to emulate the actual state of conservation of the artwork to be monitored [9], [20]. the sensor is based on printed circuit board (pcb) technology and requires a measurement system performing an electrical resistance tracking over time. however, the optimisation of the sensor and the study of specific corrosion processes require a huge amount of laboratory endeavours supported by high accuracy measurements and controlled environments. in this paper, our developed measurement system and its metrological characterisation is presented. the system was designed to guarantee high measurement performance, flexibility, and modularity, as well as a good number of acquisition channels and ease of use. the metrological characterisation allows us to optimise the parameters of the measurement system so as to achieve the best performance. a complete metrological characterisation of the measurement system and the developed sensor is described. specifically, the effectiveness of the temperature compensation on pre-corroded and non-pre-corroded sensors is reported. moreover, the effect of the uncertainty of the measurement system on the evaluation of the corrosion of the sensor is also investigated. finally, an initial test of the corrosion monitoring via pre-corroded copperbased sensors in high humidity conditions is reported. 2. sensor developement the development of the sensor was driven by the aspects of cost, flexibility, exposed surface area, measurement system characteristics, and sensitivity. specifically, the developed sensor had to guarantee the following: • the high surface area is representative of actual corrosion conditions, • there is an appropriate distance between the traces to avoid the presence of short circuit bridges due to the formation of corrosion products, • there is the possibility of reconfiguration through changing the sensor resistance value, • it allows for performing a temperature compensation of the resistance variation due to thermal drift, • low cost of production. based on the above constraints, a pcb solution was adopted. here, the sensor was designed in four sections, two for each face of the pcb (figure 1), to allow for the analysis of different measurement techniques and for implementing temperature compensation. in this work, terminal e and e’ were connected to ensure a full pcb face was exposed to corrosion. meanwhile, one face was protected from corrosion and was adopted as the reference for the temperature compensation (reference resistance: rr), while the other face was subjected to corrosion (corrosion resistance: rc). the geometrical parameters of the sensor were selected as shown in figure 1. the tracks were made of copper to allow for the monitoring of copper-based objects, and had a rated thickness of 17 µm, meaning each face of the sensor had a rated value of 0.317 ω. the pcb substrate was made of fr4 and had a thickness of 1.6 mm. in the preliminary design phase, we considered different environmental conditions leading to different corrosion rates ranging from 100 µm/year (representing highly polluted and aggressive outdoor environments) to 5·10−3 µm/year (representing controlled environments with low levels of rh and pollutants). the corrosion rate (µm/year) was defined by assuming that the corrosion was uniform on the sensor surface. this means that by monitoring the thickness of the sensor, it should be possible to estimate the corrosion processes at the same time. now, let us consider the expression of the resistance in terms of resistivity ρ and the geometrical parameters: l l r s h    = = (1) where, l, h, δ are the total length, the width, and the thickness of the pcb tracks, respectively. here, there exists an inversely proportional relationship between the resistance of the sensor and its thickness. therefore, by monitoring the variation of the sensor resistance, it was possible to obtain information regarding the corrosion rate. moreover, it became clear that the choice of track thickness will directly affect the sensitivity and lifetime of the sensors. therefore, the design of the sensor was optimised in terms of lifetime and sensitivity for a specified level of corrosion rate, within a reasonable range of variation. by measuring the rr and rc, it was possible to compensate the thermal drift of the resistance’s values – which is due to the figure 1. schematic and photograph of the sensor layout. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 211 environment temperature variations – by computing the ratio between the two resistive values for a generic temperature t: ( )( ) ( )( ) 0 0 0 00 0 1 1 cc c c r rr r t tr r k r rr t t   + − = = = + − (2) where rc0 and rr0 are the corrosion and the reference resistance values at temperature t0, respectively, and α is the thermal coefficient of the conductive material of the pcb tracks. the assumption for a good compensation is that α is the same for both resistances. assuming that the corrosion acts only on the rc track and that it is uniform, it is possible to define the relative variation of the thickness between a generic time t and a reference time t0: ( ) ( ) ( ) ( ) 0 0 0 ( ) ( ) c c c c t t t t t t      −  = = (3) therefore, kc at the generic time t is independent of the temperature t and can be defined as follows: ( ) ( ) ( )( )0 1 c r r c rc c l h k t lh t t    = − (4) where (t) is the relative reduction of the track thickness, l, h, δ are the total length, width and thickness of the pcb tracks, respectively, and the subscripts r and c are the reference and corrosion resistances, respectively. by normalising kc(t) in terms of its value at the reference time t0, it is possible to estimate the variation of the thicknesses and hence the corrosion rate: ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) 0 0 0 1 1 rc c c r c r tk t r t c t k t r t r t t = = = − (5) moreover, the absolute thickness reduction can be computed, provided the initial track thickness δc(t0) is known: ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) 0 0 0 0 0 1 ( ) 1 1 c c c c r c r c t t t t c t r t r t t r t r t        = − = −  =       = −      (6) naturally, the minimum detectable variation of (t) strongly depends on the resolution and the accuracy of the instrument used to perform the resistance measurements. in fact, by considering the measurement uncertainty of the instrument and the measurement procedure, the uncertainty on the evaluation of (t) can be defined. as described in the next section, all resistances were measured using the same digital multi-meter (dmm), the measurement accuracy of which is defined by the common binomial equation: ( )( ) a r x ppm of reading y ppm of range=  + (7) this expression considers the effect of the noise and the nonideality of the instrument for non-correlated measurements. it is clear that, given the measurement process based on the ratio of the two measured resistances, the estimation of the measurement uncertainty must involve some consideration of the correlation between the measurements. in fact, the correlation reduces the uncertainty in the evaluation of c(t). for each resistor, the true value r is known only through a measurement process, while since the instrument could be affected by a gain error δh, the measurement model that relates the measured value rm to r is the following: ( )1mr h r r= +  +  (8) where: • δh is a zero mean random variable that models the calibration error of the multimeter. since it slowly changes in time, temporally close extractions are completely correlated (therefore, it is not reduced via repeated measures) while its correlation decreases when temporally distant extractions are considered, thus, due to the effect of the drifts, they can be regarded as independent. • δr is a random variable with a mean value of zero that models the impact of the instrument noise. the variance due to the noise σ2(r) can be estimated using a category a approach and/or the constant term of the binomial formula of the multimeter. the extractions can always be regarded as independent. by substituting eq. (8) in eq. (5), we can obtain the following: ( ) n c t d = (9) where the numerator and denominator of (9) can be written as follows: ( ) ( )( ) ( ) ( ) ( )( ) ( )0 0 0 1 1 c c r r r t h t r tn r t h t r t   + +  =     + +    (10) ( ) ( )( ) ( ) ( ) ( )( ) ( )0 0 0 1 1 r r c c d r t h t r t r t h t r t  =  + +       + +    (11) by neglecting the second-order products in eqs. (10) and (11), eq. (9) can be written as follows: ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) 0 0 0 0 0 0 0 0 1 r c c r c c r c r c r r r t r t r t c t h t h t r t r t r t r t r t h t h t r t r t r t r t   = +  +  + +    + −  −  − +  −   (12) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) 0 0 0 0 0 0 1 r c rc r c c r c r c r r t r t r tr t c t r t r t r t r t r t r t r t r t   = + +    − −   (13) meanwhile, the standard deviation of parameter c(t) can be computed as follows: ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) 2 22 2 0 0 0 0 1 1 1 1r c c r r c c r c r r t r t r t r t r t r t r t r t             = + + +                        (14) acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 212 where r can be evaluated via a category a approach. finally, the standard uncertainty on the thickness variation evaluation can be estimated as follows: ( ) ( ) ( )( ) ( ) ( ) ( ) 22 02 2 0 2 1 ( ) 1 ( ) c c t u t t c t c t c t        −   = − +         (15) 3. measurement setup in order to appropriately characterise the proposed sensor, a vi-based measurement system was developed (figure 2). as noted above, the measurement setup must guarantee good measurement performance, flexibility, and modularity, as well as a good number of acquisition channels and ease of use. a keithley 3706 was used for performing the required 4-wire resistance measurements. as such, no contact resistances were included in the resistance measurements. the keithley instrument was chosen since it provides a multi-channel measurement solution by means of switching cards, which means different pre-corroded sensors can be tested with the same highresolution dmm as part of a cost-effective solution. in fact, each switching card can perform 20 4-wire resistance measurements. the 4-wire configuration allows for compensating the additional resistances due to the cables, the switching card, and the connection. the keithley 3706 incorporates six slots, which means 120 resistance measurements can potentially be acquired with one dmm. in this work, only one switching card was connected. for each tested sensor, three resistance measures were required: the resistance of the track exposed to corrosion, the resistance of the track used as reference for the temperature compensation, and the resistance of the pt100 temperature sensor. given this requirement, the measuring system has the capacity to accommodate six sensors for characterisation. each single measure presented is the result of ten averaged measurements performed over five power line cycles (plcs). the minimum analysis time for each sensor was 18 s. these limits and configuration parameters were defined on the basis of the measuring system characterisation that is reported in the following section. for each measure, the mean value and its standard deviation were computed and subsequently saved on a log file. the graphical interface of the developed measuring system is presented in figure 3. the interface was made up of a configuration panel that allowed for setting the instrument parameters, as well as a measurement panel on which the measurements of the resistances over time were displayed. 4. measurement system characterisation in order to evaluate the accuracy and sensitivity of the measurement system, a series of tests were performed. first, the measurement noise of the system was evaluated by performing a measurement campaign on a standard resistance rs = 0.1  characterised by a low thermal coefficient and placed into a climatic chamber that guaranteed a temperature of 23.0 °c ± 0.2 °c. the temperature was monitored using a digital thermometer (dostman electronics model p655-log) with a pt100 sensor. meanwhile, a second channel was used for measuring the resistance of one face of the developed sensor placed in the same climatic chamber. the current was injected from terminal a and collected at terminal b (see figure 1), while the voltage drop was measured between terminals c and d. this allowed for evaluating the noise related to the sensor geometry and connections. the experimental setup is presented in figure 4. the dmm range was set to 1 ω, which was appropriate for both the standard resistance and the developed sensor, while the dry circuit and offset compensation features were enabled. the dmm used in a 4-wire resistance configuration allows for setting the measurement time in terms of number of plcs (nplc). moreover, the time between the closing operation of the switching card and the dmm activation can be controlled, with the intentional delay time td used to ensure that the resistance measurements are performed when the system is in a steady state condition. in order to optimise the dmm parameters, different tests were performed on the standard and the sensor resistances. a total of 100 measurements were performed for both channels, with a td ranging from 0 to 10 s and an nplc deemed as equal figure 2. the measurement system architecture. figure 3. the vi front panel. figure 4. measurement system characterisation setup. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 213 to 1, 5, or 10. the standard deviation was then computed for both the standard resistance rs and the sensor resistance rc in each test condition: ( )   2 avg 1 1 1 n n n s r r r n = = − −  (16) the results are presented in figure 5 and figure 6 for different td and nplc values. focusing on the standard deviation of rs, it was clear that when nplc = 1 was considered, the overall uncertainty was much higher than with nplc = 5 and nplc = 10 (approximatively double). meanwhile, when no td was considered, a higher standard deviation value was observed. therefore, it can be stated that the best trade-off between measurement time and uncertainty was obtained with nplc = 5, as was expected given the instrument’s datasheet [22]. in terms of the sensor resistance rc, higher standard deviation values were observed in all cases, while a similar trend was evident. thus, for each td, the best results were achieved with nplc = 5. in fact, when nplc = 10, that is, double the observation time, no significant improvements in terms of standard deviation were achieved. focusing on the data for nplc = 5, it was important to check whether the resistance measurements were performed in a steady state condition for each td. the attendant measurement results for the standard are presented in figure 7, while those for the sensor resistances are presented in figure 8. in terms of the standard resistance rs, it was clear that when no delay was set, there existed a transient effect that extended across around 10 measurement points. this transient effect could be mitigated by increasing the td. however, in terms of the sensor resistance rc, this behaviour was more evident and a td of at least 3 s had to be considered to prevent the emergence of the transient effects. these differences in dynamic behaviour can be attributed to the different impedance of the two channels. finally, it can be stated that the best trade off in terms of accuracy and time was achieved with td = 5 s and nplc = 5, with the standard deviation on the standard resistance reaching a value of 3.7‧10−6 ω and the sensor resistance reaching a value of 8.7‧10−6 ω under these conditions. this difference can be attributed to the design of the pcb sensor and to other environmental noise sources. as a final point on the measurement noise analysis, the standard uncertainty had to be determined in terms of the average value of rc across all 100 measurements: ( ) ( ) ( ) ,avg c a c c s r u r s r n = = , (17) which resulted in 8.7‧10−7 ω. 5. experimental results the proposed measurement system was developed to evaluate the sensitivity of the developed sensor to different environmental conditions. as noted above, one face of each sensor was protected from the corrosion using three layers of incralac, an acrylic-based protective coating, which ensured the temperature compensation as described in the previous section. to evaluate the efficiency of the proposed compensation, a measurement campaign was performed. specifically, four figure 5. standard deviation of rs in different test conditions. figure 6. standard deviation of sensor resistance in different test conditions. figure 7. measurements of rs for nplc=5 and different td values. figure 8. measurements of rc for nplc = 5 and different td values. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 214 sensors (s01, s02, s03, and s04) were tested. here, a layer of cuprite (cu2o) was realised on s01, while a chloride and sulphate-based patina was realised on s02, with the remaining two sensors not pre-corroded. all sensors were left to corrode at 98 % rh for around one month prior to the characterisation. here, the main aim was to produce artificial patinas with a composition that was representative of the corrosion products commonly found on the surfaces of cultural heritage objects. the cuprite patina was selected since it is generally the first corrosion product that forms on copper alloys [23]-[27]. meanwhile, the chloride and sulphate-based patina was selected to reproduce the surface conditions of highly unstable artefacts from a corrosion point of view [7]. before considering the performance of the developed sensor in the corrosion monitoring, it was crucial to verify the efficiency of the temperature compensation in different corrosion conditions of the sensors. therefore, the sensors were placed in a climatic chamber, with the rh kept below 20 % to ensure negligible corrosion rates [8]. a temperature profile was then set and monitored using the pt100. here, the sensors and the pt100 were carefully placed in the chamber so as to reduce the effects due to the thermal gradient during the warming up process. the temperature was changed from a minimum of 23 °c to a maximum of 39 °c, as shown in figure 9. due to the control system of the climatic chamber, the temperature profile was characterised by overshooting during the warming up phase and by a temperature ripple of around 1.2 °c with a frequency of 30 min during the phase of maintaining the temperature. the developed measurement system was used to monitor the sensors, s01, s02, s03, and s04, for around 320 h. for each sensor, the two resistive values, rc and rr, were measured. as expected, the temperature variation provoked a non-negligible change in sensor resistance (figure 10). as previously described, this effect can be compensated by computing the parameter c(t) according to eq. (5). this parameter allowed for evaluating the thickness reduction as described in eq. (6). 11 shows the (t) computed for each sensor, with the attendant fluctuation indicating that there was a temperature mismatch between the two faces of the sensors during the environment temperature variation. specifically, it was clear that the rapid temperature ripple of 1.2 °c caused a fluctuation in (t) of around 2 nm. in order to remove this effect, a moving average filtering of the data could be performed. based on the temperature oscillation, as well as some consideration of the sensor application, an average time window of 1 h was chosen. figure 12 presents the results of the filtering process, where it was clear that the temperature compensation depended on the sensors. overall, the experimental results demonstrated that the temperature compensation achieved a good performance. in figure 9. temperature over time. figure 10. sensor resistances over time. 11. the (t) over time for sensors s01, s02, s03 and s04. figure 12. the filtered (t) for sensors s01, s02, s03 and s04. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 215 fact, the fluctuation in (t) was in the order of 3 nm for a temperature variation of around 20 °c. this value was higher than the uncertainty that was due to the measurement system, which, assuming a sensor thickness uncertainty of 10 %, was in the order of 0.3 nm. this means that any corrosion producing a variation in (t) over 3 nm can be attributed to the corrosion effects in the considered temperature range for the sensors in question. sensors s01, s02, and s03 were used to evaluate the increase in corrosion rate that occurs when the rh is increased to extremely high values (rh ≥ 98 %, 25 ± 3 °c). as figure 13 shows, a sudden and relevant increase in corrosion rate was recorded across all the sensors as a consequence of the rh increase. the corrosion was calculated from the slope of the curves and, during the first two days, it was found to be around 5 μm/y for the sensor with the cuprite patina (s01), while a far more dramatic increase was observed up to a value of around 58 μm/y for the sensor with the chloride and sulphate-based patina (s02). the non-pre-corroded sensor (s03) exhibited the lowest corrosion rate, which, during the first two days, was around 0.2 μm/y. it was also clear that the corrosion rate tended to decrease with time (figure 13), which suggested a gradual stabilisation of the surface over time. the obtained results demonstrated that the sensors can track metal corrosion and that the acceleration of the corrosion rate is highly influenced by the presence and composition of the corrosion layers over the surface. 6. conclusions in this paper, the metrological characterisation of a measurement system designed for the development of sensors for the monitoring of the environmental corrosivity affecting copper-based artworks was presented. furthermore, a sensor that allows for consideration of the presence of corrosion layers on historic surfaces was reported. the measuring system is characterised by good flexibility and modularity, and allows for performing measurements on up to 40 sensors. according to the results of the preliminary tests, the system appears to be highly promising. specifically, the measurement uncertainty and the efficiency of the temperature compensation process was evaluated, with an accuracy of 3 nm within a temperature range of 23 °c – 39 °c demonstrated. this means that the proposed corrosion sensors could be used for heritage preservation. the proposed system was then used to evaluate the impact of precorrosion on the corrosion rate of copper-based artefacts providing relevant information. the proposed system can be regarded as a promising solution for the development and characterisation of sensors for the monitoring of copper-based cultural heritage. references [1] d. m. bastidas, m. criado, s. fajardo, v. m. la iglesia, e. cano, j.m. bastidas, copper deterioration: causes, diagnosis and risk minimisation, international materials 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monitoring exploiting a novel compressed sensing method acta imeko issn: 2221-870x june 2020, volume 9, number 2, 38 45 acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 38 a wi-fi internet-of-things prototype for ecg monitoring by exploiting a novel compressed sensing method eulalia balestrieri1, pasquale daponte1, luca de vito1, francesco picariello1, sergio rapuano1, ioan tudosa1 1 department of engineering, university of sannio, benevento, italy section: research paper keywords: internet of things; sampling; compressed sensing; ecg signal; embedded systems; data acquisition; energy consumption. citation: eulalia balestrieri, pasquale daponte, luca de vito, francesco picariello, sergio rapuano, ioan tudosa, a wi-fi internet-of-things prototype for ecg monitoring by exploiting a novel compressed sensing method, acta imeko, vol. 9, no. 2, article 7, june 2020, identifier: imeko-acta-09 (2020)-02-07 editor: alexandru salceanu, technical university of iasi, romania received february 7, 2020; in final form march 25, 2020; published june 2020 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: the study has been supported by the pon project ars01_00860 ambient-intelligent tele-monitoring and telemetry for incepting & catering over human sustainability – atticus – rna/cor 576347. corresponding author: francesco pciariello, e-mail: fpicariello@unisannio.it 1. introduction the concept of internet of things (iot) was proposed as new paradigm for machine-to-machine communication, whereby the things (i.e. smart sensors/actuators) are augmented with internet connectivity in order to (i) observe/actuate upon various physical quantities, (ii) collect information, (iii) transmit and receive, (iv) store, and (v) analyse the acquired data [1], [2]. nowadays, the iot concept is successfully applied in several measurement applications [1], such as smart cities, intelligent transportation [3], and even biomedicine [1][3]. biomedical systems adopting iot (i.e. thus forming internetconnected instrumentation for patient monitoring) are currently used in wearable health monitoring systems, thus allowing the implementation of personalised healthcare and telemedicine services [4]. patient mobility requires that many such biomedical wearables are battery powered and consequently connected health wearable instrumentation is strongly dependent on the energy costs of wireless internet communication [5][6]. to illustrate, bio-electrical signals like the electrocardiogram (ecg) are used for the identification of arrhythmia or irregular abnormalities [4][7]. this process of patient monitoring requires the transmission and storage of ecg records in the long term. in the literature, developments of iot prototypes for ecg signal monitoring [8][9] and digital signal processing algorithms for ecg signal quality improvement have been reported in the literature [10]-[20]. the actual research direction is motivated by the aim of developing biomedical wearables that exhibit lowpower consumption in order to prolong their battery life [21]. the research activity presented in this article is part of the project entitled ‘ambient-intelligent tele-monitoring and telemetry for incepting & catering over human sustainability’, atticus, supported by the italian ministry of education and research. the project aims to develop a smart wearable device for the abstract this article presents an internet-of-things prototype consisting of a data acquisition device wirelessly connected to the internet via wifi for continuous ecg monitoring. the proposed system performs a novel compressed sensing-based method on ecg signals, with the aim of reducing the amount of transmitted data and thus realizing an efficient way of increasing the battery life of such devices. for the assessment of the energy consumption of the device, an experimental setup is arranged, and its description is presented. the evaluation of the reconstruction quality of the ecg signal in terms of the percentage of the root-mean-squared difference is reported for several compression ratios. the obtained experimental results clearly demonstrate high energy efficiency and the usefulness of the wi-fi-based internet-of-things device adopting the considered compressed sensing method for the data compression of ecg signals. furthermore, it allows for the reduction of the energy consumption of the internet-of-things device by increasing the compression ratio without significantly degrading the quality of the reconstructed ecg signal. mailto:fpicariello@unisannio.it acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 39 monitoring of several vital parameters by adopting novel technologies (i.e. hardware and software) for minimising power consumption [4][21][22]. the general architecture of the atticus system (see figure 1) consists of [4] (i) a smart wearable device (s-wear), (ii) an ambient intelligence device (s-box), (iii) a decision support system (dss), and (iv) a monitoring station. in case of wearable devices, which are wirelessly connected to the internet, the main contribution to the overall power consumption is from the transmission of the acquired data (e.g. real-time ecg signals). in order to reduce this contribution, it is necessary to decrease the number of times that the wireless transceiver is transmitting data. to this end, several data compression algorithms can be adopted [16]. thus, a data compression method implemented on a wearable device should exhibit low complexity, due to the limited processing capabilities of the device and good quality in the reconstruction of the original signal. several data compression methods for ecg signals based on compressed sensing (cs) theory have been developed in the literature [5]-[7]. in [12], a novel method for the cs-based sampling of ecg signals for iot systems is proposed. a first experimental evaluation of the method on hardware is presented in [21]. this article aims to present the experimental results of the hardware implementation of the proposed cs method, which is tested on a wi-fi system-on-chip (soc) module, with the aim of experimentally assessing the method in terms of the energy consumption of the wearable device and reconstruction quality. the article is organised as follows. a brief presentation of the state of the art concerning iot-based patient monitoring systems adopting wi-fi data transmission is given in section 2. section 3 describes the adopted cs-based method. in section 4, the iot prototype is presented. the experimental setup and the obtained results (in terms of the signal reconstruction quality and energy consumption of the implemented prototype) are described in section 5. several conclusions and recommendations for future work are reported in section 6. 2. state of the art nowadays, the health status of patients and real-time services for emergency notification are possible due to the advancement of iot technologies [23][25]. thus, the biotelemetry of ecg data represents an important task that helps to assist the cardiac activity of monitored patients [26]. many studies have presented ideas and developments concerning remote cardiac monitoring systems, which could be used by even laypeople for multiple patient monitoring in a residential environment [23][26]. in [25], the authors describe a wi-fi-based ecg monitoring system using the concerto iot platform. the concerto family of microcontroller units (mcus) are multicore socs from texas instruments (ti). the proposal in [25] is built on a single chip ecg signal acquisition, a simplelink cc3000 wi-fi-based communication module, and a receiving peer – a smartphone. the low-energy feature of cc3000 is used for its advertising function, which sends messages for the establishment of the wifi connection. once the connection is established, a user datagram protocol (udp) packet containing 7680 bytes is sent to the smartphone. however, in [25], there is no experimental evaluation of the energy consumption of the proposed iot system for real-time ecg signal monitoring. in [27], an iot-based portable ecg monitoring system for smart healthcare is presented. the authors developed an ecg wearable device comprising an (i) ad8232 as an analog front end (afe) module for ecg signal acquisition, (ii) an arduino uno board for ecg sample transfer, and (iii) a raspberry pi 3b iot platform for wi-fi data communication, which receives the ecg samples from the arduino uno board and sends them to a wi-fi peer receiver i.e. a smartphone. a similar approach is developed and presented in [28]. in the present work, the authors use (i) a classical ecg amplifier circuit built on ad620an ic, (ii) an lpc2148 arm7-based microcontroller for the analog-todigital conversion of ecg signals, and (iii) a raspberry pi 3 iot platform for wi-fi data communication to a laptop computer. in [29], the design of seven lead ecg monitoring systems exploiting wi-fi data communication is presented. the proposed system is based on a sensing device comprising (i) an afe module for ecg signal acquisition, (ii) an stm32f103 microcontroller for data processing, and (iii) a wi-fi esp8266 module for data communication. another work presenting iot wi-fi-based real-time heart rate variability monitoring is [30]. the ecg afe is built using ad8232 ic, is connected to the ti msp430f5529 mcu, and sends data wirelessly by means of the ti cc3100 simplelink wifi booster pack evaluation board. the receiver is a laptop computer that receives the ecg data and streams it to an iot cloud platform called pubnub. other works dealing with iot-based ecg monitoring exploiting data communication using wi-fi networks include [31]-[35]. from the prior research that we considered, [23]-[35], none presents an analysis regarding the energy consumption of wi-fi data transmission. for wearable iot healthcare sensing devices, the instantaneous power consumption represents an important design parameter, allowing us to control the expectation of battery life. moreover, in the surveyed literature, the adopted processing steps for ecg signal acquisition and transmission is like the one presented in figure 2 (a), and as it was observed, there are no results presenting data compression and energy consumption measurements. in this work, the adopted processing steps for ecg signal acquisition and transmission is the one presented in figure 2 (b). thus, using a soc-type mcu that embeds the wi-fi transceiver will gives an advantage in energy consumption optimisation compared to the existing proposals in the literature, which deal with systems like the one in figure 2 (a). figure 1. the architecture of the atticus system [4]. acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 40 3. description of the cs-based method in this section, the implemented cs-based method for ecg compression in the proposed iot system is briefly described. this method was firstly presented in [22], and it is based on a sensing matrix that contains information that is strictly correlated to the power of the ecg signal in a single frame. usually, cs frameworks for the acquisition of ecg signals exploit sensing matrices, which are built up by using random sequences generated according to several probability density functions (e.g. bernoulli and gaussian) [12][13]. the reconstruction step, which operates on the compressed ecg samples, requires the definition of a dictionary matrix (e.g. symlet 4, daubechies 6, and mexican hat wavelets), whereby the ecg signal can be considered sparse (i.e. it can be represented with a small number of coefficients). in these cases, by considering the same dictionary matrix, the reconstruction quality of the ecg signal strictly depends on the adopted random sequence used for defining the sensing matrix. in order to overcome this limit, in [12], the sensing matrix is built up by using a deterministic sequence. moreover, this sequence depends on the power content of the ecg signal under observation. in this way, the information content (i.e. r-peak, qrs complex, and p-wave) on the compressed samples of the ecg signal is maximised, and then, the reconstruction quality is enhanced compared with classic approaches [12][13]. for one lead ecg signal compression, the adopted method performs the following steps (see figure 3) [21][22]: 1. a vector 𝒙 ∈ ℝ𝑁×1 of 𝑁 discrete samples, including at least one period of the ecg signal, is acquired. 2. based on this vector, the average value, 𝑥𝑎𝑣𝑔 , is calculated. 3. the absolute value of the point-by-point difference between the 𝒙 vector and its average value, 𝑥𝑎𝑣𝑔 , is performed, thus obtaining the vector 𝒙𝒂. 4. the vector 𝒙𝒂 is compared point-by-point with a fixed threshold value, 𝑥𝑡ℎ. 5. the vector 𝒑 ∈ ℝ𝑁×1 of 𝑁 binary values is built as follows: (i) if the element of 𝒙𝒂 is higher than (or equal to) 𝑥𝑡ℎ, the value 1 is inserted into the corresponding vector position of 𝒑, and (ii) if the element of 𝒙𝒂 is lower than 𝑥𝑡ℎ, the value 0 is inserted in the corresponding vector position of 𝒑. 6. each row of the sensing matrix 𝜱 ∈ ℝ𝑀×𝑁 is obtained by the circular shifting of the vector 𝒑𝑻, where ⋅𝑻 represents the transpose operation. the number of shifted samples is equal to the compression ratio, 𝐶𝑅 = 𝑁/𝑀, where 𝑀 is the number of compressed samples and corresponds to the number of 𝜱 rows. the circular shifting is performed in order to comply with the restricted isometry property and to reduce the coherence of the sensing matrix with the dictionary matrix, which are required for guaranteeing the signal reconstruction in cs theory [23]. 7. the 𝑀-compressed samples, which are contained in the vector 𝒚, are obtained by multiplying the sensing matrix 𝜱 by the vector 𝒙. the above operations do not require a large amount of memory and high-processing computational efforts. thus, they could be performed on low-cost microcontrollers. in order to construct the vector 𝒙, which represents an estimation of the original signal 𝒙, from the compressed samples 𝒚, the following steps are performed (see figure 4): 1. the dyadic mexican hat wavelet matrix, 𝜳 ∈ ℝ𝑁×𝑁+1, and the sensing matrix, 𝜱 are built. 2. the orthogonal matching pursuit (omp) algorithm is used for estimating the 𝑅 coefficients that represent the 𝒙 vector in the domain defined by the dyadic mexican hat wavelet, by solving the following minimisation problem: �̂� = 𝑎𝑟𝑔 𝑚𝑖𝑛 𝜶 ‖𝜶‖1, subject to 𝒚 = 𝜱 ⋅ 𝜳 ⋅ 𝒙, where �̂� is the vector containing the 𝑅 estimated coefficients. 3. by multiplying the estimated �̂� vector with the dyadic mexican hat wavelet matrix, 𝜳, the vector 𝒙 is estimated. the above reconstruction operations are performed on a processing platform that does not have energy consumption constraints and size limitations. 4. the implemented iot system prototype in this section, the architecture of the implemented iot system performing the proposed cs method is described. the figure 2. the processing steps of the ecg signal acquisition and transmission: (a) a generic overview of the proposals in the literature and (b) a generic overview of the adopted system in this work. figure 3. the processing steps of the adopted cs-based data compression. figure 4. the processing steps for the reconstruction of the ecg signals from the compressed data. x xavg xa      p   1 0  1) 2) 3) 4) 5) 6) xth xth 7) y  1)  ̂1 ̂ 2 ̂ n+1 omp algorithm   y   2)   ^ 1x x̂ 2 x̂ n  1 1  1  3)  ̂1 ̂ 2 ̂ n+1 n  n+1 acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 41 system consists of (i) an ecg data acquisition system, which embeds a wi-fi transceiver, (ii) a wi-fi router, and (iii) a laptop. the ecg data acquisition system acquires the ecg samples, performs the data compression for obtaining the vector 𝒚, and sends to the wi-fi router both vector 𝒚 and 𝒑 in a udp packet. the router collects the received data and transmits them to the laptop. the laptop receives the compressed samples 𝒚 and the vector 𝒑, and it executes the omp algorithm in order to reconstruct 𝒙. the reconstructed samples are shown to the user through a graphic interface. in the following section, the hardware and the firmware of the implemented ecg data acquisition system are described. furthermore, a labview application running on the laptop for data acquisition and reconstruction is developed. its main tasks are described thereafter. 4.1. ecg data acquisition system: hardware the ecg data acquisition system consists of a cc3200mod launchpad evaluation board [37], embedding the cc3200mod soc. this soc is a wireless microcontroller module that integrates an arm cortex-m4 core running at 80 mhz and a wifi network processor to completely offload the host processor along with an 802.11 b/g/n radio, baseband, and mac for secure wlan internet connections. the network processor supports station, wireless access point (wap), and wi-fi direct connection modes. moreover, it implements the ipv4 tcp/ip stack [36]. the microcontroller acquires the samples of the ecg signal by means of its embedded 12-bit analog-to-digital converter (adc) on the adc-ch2 pin. the adc works at a sampling rate of 500 hz and has an input voltage range of [0, 1.4] v [36]. the afe has not been designed in the implemented prototype, since its energy consumption is negligible in respect to the necessary power for wi-fi communication. in order to reduce the power consumption of the overall soc, both the cortex-m4 application processor (ap) and the networking processor (np) can work in different power states. the user program controls the power modes of the ap, which can be in the following five states: (i) active, (ii) sleep, (iii) lowpower deep sleep (lpds), and (iv) hibernate. during the active state, the ap executes the code at a 80 mhz clock rate. on the other hand, in sleep mode, the ap clocks are gated off, and the entire state of the device is retained. in that state, the ap can be configured to wake up by an internal fast timer or by activity from any general purposes input output (gpio) line. in lpds mode, the state information is lost, and only certain ap specific register configurations are retained. the ap can wake up from external events or by using an internal timer with a wake-up time of around 3 ms. the lowest power mode in which all digital logic is power-gated is the hibernate mode. the real-time clock (rtc) keeps running, and the ap supports wake up from an external event or from an rtc time expiry. in this state, the wake-up time is about 15 ms. the np can be active or in lpds mode. furthermore, when there is no network activity, the np sleeps most of the time and wakes up only for a beacon packet reception. in table 1, the current consumption in the different power states of both ap and np, according to the datasheet [36], are summarised. for the implemented prototype, the ap alternates between the active and the idle states, while the np works into the transmission (tx), reception (rx), and lpds states. 4.2. ecg data acquisition system: firmware a general overview of the firmware executed by the ecg data acquisition system is depicted in figure 5. in the beginning, the ap initialises the np and provides it with the service set identifier (ssid) and the password of the wap. the ap waits until the connection has been established between the np and the wap. a timer working at a frequency of 500 hz is initialised, and the interrupt signalling due to its overflow is enabled. two global variables are initialised to zero – count and length (see figure 5). the variable count contains the number of acquired ecg samples at any program instant, while the variable length contains the length of the data that will be sent to the router through a udp packet. the ap is in an idle state until the number of acquired samples, performed during the interrupt routine of the timer, is equal to 𝑁. vector 𝒑 is constructed according to the fixed threshold 𝑥𝑡ℎ and the vector 𝒙 containing the 𝑁 acquired samples. data compression is performed on vector 𝒙 according to sensing matrix 𝜱, which is a circulant matrix in which the first row is vector 𝒑𝑻. vector 𝒚 is evaluated as a multiplication between the sensing matrix and the vector 𝒙. both the 𝒚 and 𝒑 vectors are coded in a vector called payload. the variable length is incremented according to the length of the coded data. the data compression procedure is performed for each frame of 𝑁 ecg samples until the length of the vector containing the coded data achieves the maximum value of the payload length, max, allowed by the udp packet, corresponding to 1472 bytes. when the maximum payload length is reached, the udp packet is sent to the laptop through the wap. in order to reduce power consumption, due to the wi-fi transceiver, the np is always in the lpds state. it goes into the reception state for receiving the beacon packet sent by the wap, figure 5. a general overview of the firmware executed by the ecg data acquisition system. table 1: current consumption of the ap and np in the available states [23]. ap state np state current consumption active tx 278 ma rx 59 ma idle 15.3 ma sleep tx 275 ma rx 56 ma idle 12.2 ma lpds tx 272 ma rx 53 ma lpds 0.275 ma idle 0.875 ma hibernate hibernate 7 µa start np initialization timer initialization at 500 hz enabling timer interrupt count = 0, length = 0 connected to the wap? no yes count = n? no count = 0 power vector, p, evaluation data compression, y length = max? no sending the udp packet length = 0 length = length + + payload(y,p) yes yes main loop interrupt routine of the timer count = count +1 timer reset timer routine x(count) = adc reading board initialization acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 42 and for responding to it every 2 s. the number of transmitted udp packets depends on the 𝐶𝑅. of course, by increasing the 𝐶𝑅, the udp packets will be sent by the ecg data acquisition system more and more sporadically, and the entire energy consumption of the data acquisition system will then reduce. 4.3. laptop: software the task of the laptop is to collect compressed samples 𝒚 and power vector 𝒑 sent by the ecg data acquisition system and to perform the omp algorithm to reconstruct 𝒙. to this end, a labview application has been realised. in the beginning, the laptop waits for the reception of a packet containing max = 1472 bytes. when the packet is received, the vector of the compressed data, 𝒚, and vector 𝒑 are extracted. for each ecg frame of n samples, the length of 𝒚 is 2 ∙ 𝑀 bytes, and the length of 𝒑 is 𝑁/8 bytes. thus, according to the 𝐶𝑅, several frames of ecg signals are enclosed in a udp packet. for example, if 𝐶𝑅 = 4 and 𝑁 = 512, 𝑀 is 128. for each frame, 256 + 64 = 320 bytes are required, and in a udp packet, 4 frames are enclosed. by considering that the sampling frequency of each ecg sample is 500 hz, 4 frames of compressed ecg samples will be received by the laptop about every 4 s. vector 𝒙 is reconstructed according to the omp algorithm (see figure 4), which is performed in real time for each frame contained in a udp packet. vector 𝒙 is finally shown to the user. 5. experimental results and discussion an experimental setup has been implemented (see figure 6) for (i) assessing the quality of signal reconstruction at different 𝐶𝑅 values and (ii) measuring the energy consumption of the ecg data acquisition system for different values of 𝐶𝑅. in order to evaluate the quality of signal reconstruction, in the udp packet, 𝒚 and 𝒑 are enclosed as well as 𝒙. in that case, the setup consists of an ecg signal provided by the agilent 33220a arbitrary waveform generator connected to the adc-ch2 pin of the simplelink wi-fi cc3200 module launchpad [37]. the generated ecg signal has a high level of 950 mv and a low level of 100 mv, with a frequency of 1 hz, which corresponds to a heart rate of 60 bpm. the labview application reconstructs 𝒙, which is compared with the original ecg signal contained in 𝒙. as a figure of merit, the percentage of root-mean-squared difference (prd) is computed as follows: 𝑃𝑅𝐷 = ||𝐱 − �̂�||2 ||𝐱||2 × 100% . (1) this figure of merit is widely used by the scientific community for comparing cs algorithms in terms of the reconstruction quality of ecg signals [23]. the 𝑃𝑅𝐷 value is evaluated along an acquisition time of 1 minute of the ecg signal for the following 𝐶𝑅 values: {2, 4, 8}. those 𝐶𝑅s have been chosen according to the results obtained in [21], where it is reported that with the proposed method, a good reconstruction quality is obtained for 𝐶𝑅 ≤ 8. for the experimental assessment, threshold, 𝑥𝑡ℎ, has been chosen at 20 mv. in figure 7, a single frame reconstructed signal for each considered 𝐶𝑅 and the point-by-point absolute value of the difference between the estimates and the original signal are depicted. it is observable that the point-by-point differences increase along with the 𝐶𝑅. in particular, for 𝐶𝑅 = 4, the maximum difference is around 20 mv, while for 𝐶𝑅 = 8, it is 60 mv [see figure 7 (d) and (e)]. however, for all the considered 𝐶𝑅, the reconstructed signal approximates the original one well. for assessing the energy consumption of the ecg data acquisition system for several 𝐶𝑅 values, the experimental setup depicted in figure 7 is used. it consists of a power measurement system consisting of (i) a shunt resistor with a nominal value of 1.2 ω, (ii) the ni-daq 6036e, and (iii) a pc interfacing with the ni-daq. the shunt resistor is placed in a series on the power supply pin of the cc3200. its drop voltage, 𝑣𝐼 (𝑡), proportional to the current dissipated by the soc, is measured by using a differential input of the ni-daq, while the voltage measurements, 𝑣𝑃 (𝑡), are performed by means of a single-ended analog channel in the ni-daq. the current and voltage measurements are multiplied for measuring the instantaneous power consumption of the ecg data acquisition system. these values are acquired by the pc in continuous mode with a sampling frequency of 100 khz. figure 6. a general overview of the implemented experimental setup for assessing the quality of signal reconstruction and for measuring the energy consumption of the egc data acquisition system. acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 43 the voltage and current measurements have been calibrated against the value provided by the fluke 5500a calibrator. the current measurements have been calibrated by imposing current values from 0 ma up to 500 ma with a step of 50 ma. for each imposed current value, a type a uncertainty evaluation is performed on 105 measurements. the obtained expanded uncertainty is 0.4 ma by considering a coverage factor 𝑘 = 2. on the other hand, the voltage measurements have been calibrated within a range of 0 v up to 5 v, with a step of 0.5 v. for each of the imposed voltage values, the type a uncertainty assessment is performed on 105 measurements. the obtained expanded uncertainty is 0.3 mv, by considering a coverage factor 𝑘 = 2. according to the law of the propagation of uncertainty, the expanded uncertainty for the power measurements is 1.5 mw. in figure 8, the instantaneous power consumption of the ecg data acquisition system is depicted for a time window of 15 s in the case of no compression, implemented on the data acquisition system [figure 8 (a)], and data compression performed for 𝐶𝑅 = {2, 4, 8}, see figure 8 (b), (c), and (d), respectively. the measurements have been evaluated by considering for both np and ap, the lpds current consumption values reported in table 1. in figure 8, the power consumption due to the udp packet transmission and the packet due to the beacon response are marked. as expected, by increasing 𝐶𝑅, the number of times that the beacon response is sent and the udp packet is transmitted reduce. in particular, for 𝐶𝑅 = 8, it is observable that the beacon response is sent every 2 s and that the udp packets are transmitted about every 5 s. the power consumption measurements are taken for 4 s and, for this time period, energy consumption is estimated. those measurements are repeated 10 times, and the type a uncertainty assessment is performed. furthermore, for the considered 𝐶𝑅, 100 ecg frames of 1 s each are acquired, and the 𝑃𝑅𝐷 and its uncertainty are estimated. in table 2, the energy consumption and the 𝑃𝑅𝐷 values when compression is not performed and for the 𝐶𝑅 values of 2, 4, and 8 are reported. for the expression of the expanded uncertainties a coverage factor, 𝑘 = 2 has been considered. from this table, it can be noted that for 𝐶𝑅 = 2, there is no advantage in terms of energy consumption respect with the no compression case (the two energy consumption measurements are compatible). for the 𝐶𝑅 values of 4 and 8, the energy consumption decreases even if, as it was expected, the 𝑃𝑅𝐷 increase. however, the obtained 𝑃𝑅𝐷 for 𝐶𝑅 = 8 is lower than the 9 %, which is usually considered the maximum acceptable limit value for medical diagnosis [22][23]. 6. conclusions in this article, an iot prototype for continuous ecg monitoring that performs a novel cs-based method to reduce the amount of transmitted data was presented. the implemented prototype consists of a wearable device wirelessly connected to the internet via wi-fi, based on the cc3200 soc. the hardware of the implemented prototype and the firmware and software executed on the wearable device and the laptop, respectively, were described. a description of the experimental setup used for the assessment of the energy consumption of the device for several 𝐶𝑅 values and for the evaluation of the reconstruction quality in terms of 𝑃𝑅𝐷 was given. the obtained experimental results figure 7: single frame reconstructed signal for the following 𝐶𝑅 = {2, 4, 8}, a), b) and c), and the corresponding point-by-point absolute value of the difference between the estimates and the original signal, d), e), and f). table 2: energy consumption and prd when the compression is not performed and for 𝐶𝑅 values of 2, 4, and 8. energy consumption in j prd in % no compression (2.65 ± 0.09) − 𝐶𝑅 = 2 (2.70 ± 0.08) (0.94 ± 0.50) 𝐶𝑅 = 4 (2.49 ± 0.06) (1.21 ± 0.10) 𝐶𝑅 = 8 (2.32 ± 0.04) (4.61 ± 0.55) acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 44 demonstrate that the tested iot prototype, which implements the cs method presented in [22], allows for the reduction of the energy consumption of the wearable device by increasing the compression ratio, without degrading the quality of the reconstructed ecg signal. for 𝐶𝑅 = 8, the obtained energy consumption for 40 s is 2.32 j with respect to 2.65 j, obtained without compression, and the 𝑃𝑅𝐷 is 4.61 %. further work in this area should test the method by using ecg signals from a patient collected by an afe module. the implementation of an algorithm for the online evaluation of the threshold value, 𝑥𝑡ℎ, by considering patient specific ecg signals should be considered. acknowledgement the study has been supported by the pon project ars01_00860 ‘ambient-intelligent tele-monitoring and telemetry for incepting & catering over human sustainability’’ – atticus – 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[online]. available: www.ti.com/tool/tidc-cc3200modlaunchxl http://www.ti.com/tool/tidc-cc3200modlaunchxl from survey to semantic representation for cultural heritage: the 3d modelling of recurring architectural elements acta imeko issn: 2221-870x march 2021, volume 10, number 1, 98 108 acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 98 from survey to semantic representation for cultural heritage: the 3d modelling of recurring architectural elements valeria croce1, gabriella caroti1, andrea piemonte1, marco giorgio bevilacqua2 1 civil and industrial engineering department (dici)/university of pisa, largo lucio lazzarino 1, 56122 pisa, italy 2 department of energy, system, territory and construction engineering (destec)/univeristy of pisa, largo lucio lazzarino 1, 56122 pisa, italy section: research paper keywords: cultural heritage; 3d survey; h-bim; point cloud classification; calci charterhouse citation: valeria croce, gabriella caroti, andrea piemonte, marco giorgio bevilacqua, from survey to semantic representation for cultural heritage: the 3d modelling of recurring architectural elements, acta imeko, vol. 10, no. 1, article 13, march 2021, identifier: imeko-acta-10 (2021)-01-13 section editor: carlo carobbi, university of florence, italy received april 30, 2020; in final form september 30, 2020; published march 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: the work is partly funded by the vinci2019 project, promoted by the université franco-italienne. corresponding author: valeria croce, e-mail: valeria.croce@unifi.it 1. introduction three-dimensional models are increasingly being used for the graphic representation of cultural heritage items. the development of three-dimensional (3d) surveying and representation systems promoted the birth of new forms of documenting, disseminating and analysing cultural heritage objects in digital environments. reality-based acquisition systems provide dense and reliable 3d reconstructions of the object of study. laser scanning and photogrammetry are widely applied methodologies [1], [2] for capturing the basic 3d data to be used as input for the creation and development of digital models. while the acquisition of 3d survey data is a well-established practice, recent studies have explored the subsequent phase, which involves the processing and elaboration of raw data obtained from the survey. this representation phase, involving the reconstruction of the current or past form of a building based on survey acquisitions, requires a logical process. it is necessary to decode and organise the raw information obtained from laser scanning or photogrammetric survey in order to represent the morphological complexity of heritage buildings. it is necessary to understand the shape and geometry of the elements to be represented as well as their mutual relationships in order to build complete and intelligible representations [3]. only through this phase, which requires a logical and operational process of segmenting and classifying the raw data obtained from the survey, can an accurate and complete 3d model be reconstructed and used as a support for the documentation and analysis of the cultural heritage object. once the model has been reconstructed in a 3d environment, the geometric elements can be associated with semantic data, i.e. information about the historical asset. abstract the digitisation of cultural heritage objects paves the way for new approaches for the surveying and restitution of historical sites. with a view to the management of integrated programmes of documentation and conservation, the research is now focusing on the creation of information systems that link the digital representation of a building to semantic knowledge. through reference to an emblematic case study of the calci charterhouse, also known as pisa charterhouse, this contribution illustrates an approach to be followed for transforming 3d survey information derived from laser scanning and photogrammetric techniques into semantically enriched 3d models. the proposed approach is based on both the recognition, segmentation and classification of semantic/geometric elements and the manual mapping of non-uniform rational basis spline elements on the original raw point cloud. for this shape recognition process, reference to architectural treatises and the vocabulary of classical architecture is a key step. the created building components are finally imported into a heritage building information modelling environment, where they are enriched with semantic information related to historical knowledge, documentary sources and restoration activities. mailto:valeria.croce@unifi.it acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 99 it is thus possible to identify the following phases in the process that leads to the attainment of structured models from raw data (figure 1): i) acquisition of survey data through laser scanning, photogrammetry or a combination of both techniques; ii) first data processing, including point cloud registration, subsampling and noise removal; iii) segmentation and classification of architectural elements; iv) construction of the 3d model and semantic enrichment. the aim of this contribution is to show how these different phases of the process develop and support one another based on an examination of studies involving the recovery and conservation of cultural heritage objects. the construction of semantic models using heritage building information modelling (h-bim) technologies is also considered. this workflow is illustrated in detail through an emblematic case study of the calci charterhouse (certosa di calci), also known as pisa charterhouse, an extensive architectural complex dating back to the 14th century that has been the subject of an integrated restoration and maintenance plan. the paper is organised as follows: in section 2, the state of the art for the process of acquiring semantically rich 3d models from survey data is illustrated and discussed, with a focus on scan-to-bim acquisition systems. section 3 illustrates the materials and methods of the proposed approach, using the above-mentioned case study as an example. finally, section 4 presents concluding remarks and possible future developments of the work. 2. related work the use of digital technologies in cultural heritage documentation and preservation applications is increasingly gaining ground, thanks to the development of 3d surveying and modelling systems. laser scanning and photogrammetry make it possible to describe heritage objects by acquiring reliable data in the form of point clouds [4]. these data, however, are raw and not directly usable as they contain purely geometric or radiometric information. a data processing phase is thus necessary in order to transform the original point cloud into a geometric 3d model or a parametric object characterised by geometrical, topological and semantic attributes [5]. obviously, the appropriate processing method strongly depends on the type of model to be obtained. in addition to webbased approaches involving the on-line sharing of 3d models with respective semantic annotations [6]-[8], a technology that is becoming increasingly popular for cultural heritage documentation and dissemination is the generation of bim objects, which have typically been used in the field of new construction [9]. the procedure for the construction of h-bim objects from relevant data is a reverse engineering technique known as scanto-bim [10]. the basic approach, which is the same as for bim, is to create a digital database of the building, in which all of the information related to the study and documentation of the asset can be linked to a digital representation. by doing so, data can be organised, archived and updated at any time by the various professionals involved because the geometric representation of the object is associated with a semantic value. the first studies related to the application of bim techniques to cultural heritage date back to 2009 and are attributed to dore and murphy, from the school of surveying and construction management of the dublin institute of technology [11]-[13]. since these first studies, the complexities of the application of bim to cultural heritage have been highlighted. in historical buildings as opposed to new buildings, the transition to parametric modelling requires the performance of an architectural survey, the recognition and classification of characteristic elements and recurring architectural typologies and the interpretation of parts of the building that are not directly visible or inspectable. the transition from survey outputs to semantic models requires much more time and skill in reconstructions of heritage buildings than in traditional bim [14]-[16]. indeed, historical architectures are commonly characterised by complex geometries that are difficult to model: successive stratifications resulting from different temporal states, anthropogenic modifications and degradation or even loss of elements. in other words, built heritage has heterogeneity in terms of both geometry and semantics [17]. figure 1. general process for the creation of semantically rich models starting from surveys. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 100 recurrent elements and morphological units thus need to be identified and distinguished within this heterogeneity in terms of form [18], [19] and terminology [20]-[22] in order to obtain semantic representations. consequently, it is common to refer to classical architectural treatises [23]-[25] or to the construction of controlled vocabularies of cultural heritage [26], [27]. for the case of h-bim modelling, the result of the classification is the construction of a parametric family made up of similar building components [28], [29]. different methods exist for the construction of the parametric model based on the original point cloud. they can be distinguished according to the respective degree of automation. most automated procedures are founded on the construction of parametric surfaces on the basis of shape detection algorithms: several studies have employed the ransac algorithm [30]-[33] or other robust estimators derived from it [34]. moreover, many commercial software (e.g. edgewise) or plug-ins directly integrated in bim platforms (e.g. scan-to-bim and faro) are equipped with tools for the semi-automatic reconstruction of walls and floors and, in some cases, for recognising the position of the openings of the window elements. although semi-automatic labelling procedures have been given a great deal of attention in research, they are currently only applicable to the recognition of regular elements, such as slabs, walls and ceilings. they are still difficult to apply in the case of more complex architectural elements, which are often found in historic buildings. for this reason, the more widely used procedure in the field of cultural heritage is the manual mapping of three-dimensional geometries, starting from the point cloud: the construction of parametric elements is predicted [17], [29] through reference to historical and archival documentation and conceptual forms contained in treatises or architectural vocabularies [35], [36]. the modelling is thus performed by manually tracing the geometries on the point cloud [37]. this can be done directly on bim platforms, e.g. by importing the point cloud into autodesk revit [5], [19], [38], [39] or, in the case of elements with more complex geometry, by first moving to free-form modelling software. in the latter situation, non-uniform rational basis splines (nurbs) [39]-[41] or mesh models [17], [42] are created externally and imported into a bim environment at a later stage. the use of mesh within bim platforms, however, involves a significant increase in file size and is less compatible with the principles of bim parametric modelling [38]-[39], which is why this contribution focuses on modelling using nurbs. referring again to the diagram in figure 1, the current treatment of survey data, which includes segmentation and classification of the point cloud, is not yet standardised and may involve more or less automatic construction processes starting from the dense point cloud. in any case, once the model has been reconstructed, it can be enriched with qualitative information pertaining to the knowledge and study of the asset, which can be directly associated with 3d geometries [41]. in this context, the aim of the following contribution is to present a possible approach to the data processing phase, with the general objective of building a semantic model. for the complex being studied, such a model should be able to contain, within an overall bim perspective, the different multi-temporal data related to construction phases including transformations and modifications, original and degraded status and past and future maintenance or restoration interventions. 3. materials and methods 3.1. a pilot case study 3d modelling for cultural heritage and scan-to-bim modelling techniques in particular require the application of multidisciplinary knowledge for classifying, managing and sharing information [41], [43]. in this section, this principle is illustrated with reference to a pilot case study, which is that of the already mentioned calci charterhouse, located on the slopes of pisan mountains, a few kilometres from the italian city of pisa in tuscany. that monumental complex has been the subject of an integrated conservation and recovery plan funded by the university of pisa, which has involved many professionals in a variety of cognitive studies and research activities. the complex was founded in 1366 by carthusian monks at the behest of the archbishop of pisa, francesco moricotti. towards the end of the 16th century, some of the main rooms were decorated and frescoed. the structure then underwent changes and transformations during the 17th and 18th centuries, with the restructuring of some areas and the creation of additional decorations and frescoes. the complex became state property in 1866 and was later declared a national monument. since 1972, the spaces of the charterhouse have housed both the natural history museum, under the management of the university of pisa, and the national museum belonging to the tuscan museal pole, under the direction of the italian ministry of heritage and cultural activities and tourism (ministero per i beni e le attività culturali e per il turismo) [44], [45]. visiting the complex, it is possible to find the spaces that remain from its ancient use, articulated according to the principles of a monastic life: a large cloister overlooked by the cells of the choir monks, built according to a modular scheme; a church with a sacristy and chapels; a refectory; a guest house; a chapter house with an adjoining cloister; the dormitory of the lay brothers and adjoining spaces for agricultural activities and manual work. due to the large size of the architectural complex, surveys must be carried out using integrated acquisition techniques (section 3.2). the survey restitution is thus made on a larger or smaller scale depending on the required level of detail and representation. among the various environments surveyed, it is worth noting the grand ducal cloister, a large internal courtyard whose construction began in 1614 under the direction of the architect matteo nigetti, replacing a previous 14th century cloister. for its reconstruction, nigetti was inspired by the principles defined in the sixth book of the 16th century treatise entitled the idea of the universal architecture, by vincenzo scamozzi [46]. the cloister has a cistern in the centre and is closed on four sides. along the closing perimeter, there is a walkway on the first level, covered by cross vaults. the columns, surmounted by ionic capitals and resting on a continuous base, are made of pietra serena, a typical grey sandstone widely used in italy. the architecture of the grand ducal cloister, characterised by its classical style and the repetition of typological and stylistic elements, makes it a suitable case study for the creation of a parametric model as a basis for semantic enrichment. 3.2. integration of survey techniques the survey campaigns were carried out by the scientific and topographic applications for operative survey (astro) acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 101 laboratory of the university of pisa and have been ongoing since 2013. in these campaigns, integrated survey systems were needed to restore the complexity of the architectural organism, with the objective of providing the different groups involved in the study and conservation activities with reliable and metrically correct geometric information. together with traditional topographic surveys, laser scanning, ground-based photogrammetry and indoor and outdoor drone photogrammetry were applied. the vast size and scope of the charterhouse necessitated the use of these different techniques. for a detailed description of the different types of survey used, please refer to [47]; the choice of the type of technique depended on the level of detail and restitution required, as well as on the complexity of the environment to be surveyed. in general, laser scanning enriched the ground measurements for the accurate restitution of elevation walls, plans and sections of the complex at 1:100 scale and of some significant portions at 1:50 scale (figure 2 and figure 3). for the most valuable architectural and decorative elements and for the frescoed walls, whose documentation required a more detailed restitution scale, photogrammetric surveys were performed. this made it possible to complement the geometric precision of the laser scanner with the radiometric precision (in terms of colour) of a photographic campaign (figure 4 and figure 5). aerial photogrammetry using unmanned aerial vehicles was undertaken to represent parts that were not directly visible or inspectable, such as roofs, wells and underground tanks, as previously described in [48], [49]. the different acquisitions were all georeferenced with respect to a single reference system, called 'charterhouse reference system', which was set up through the construction of a network of benchmarks [47]. the integrated use of surveying techniques made it possible to create geometric and radiometric documentation of the building components at different scales of representation that can be used to provide information regarding: the current state, including the dimensions and arrangement of architectural elements; the mapping of materials; the state of degradation, e.g. the extension and depth of degradation mechanisms. the above mentioned procedure of operative survey is of paramount importance for the generation of 3d models that could be used by many experts for the collection, archiving and continuous updating of data related to the historical and documentary analysis of cultural heritage objects. 3.3. proposed methodology the proposed methodological approach applies to buildings whose structure can be reconducted to ideal and canonised shapes, often by referring to architectural treatises or construction rules. the method simplifies the process of transforming survey data into 3d informative models and is very effective when recurring architectural elements are present. it is based on the following steps: 1. point cloud optimisation, including scan registration, noise/obstacle removal and subsampling; 2. semantic segmentation and classification performed manually over the registered point cloud; figure 2. grand ducal cloister: portion of the laser-scanned point clouds. figure 3. registration of laser-scanned point clouds for the area of the grand ducal cloister. each sphere indicates a scan location. figure 4. spatial resection of images on a 3d point cloud for the photogrammetric processing of frescoed walls. figure 5. photogrammetric survey of a frescoed room. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 102 3. nurbs-based modelling of typical and recurring building components characterised by morphological complexity; 4. generation of a parametric model of the heritage object in a bim platform with nurbs-based elements imported as mass components; 5. semantic enrichment of the model, in which documentary or analytical sources are connected to the geometry of building components. in steps 2 and 3, reference is usually made to architectural treatises for element recognition, profile generation and shape modelling. in step 4, a nurbs-based reconstruction is used to bring the identified repetitive typological elements back to their ideal geometries. this approach offers several advantages for the creation of parametric geometries and for the improvement of the scan-to-bim workflow: elements can be created in a faster and more accurate manner by making use of a free-form modelling software, e.g. rhinoceros, by robert mcneel & associates. in comparison with the mesh-based methods illustrated in section 2, a greater control over construction parameters of the ideal geometry is ensured. in fact, the proposed approach allows a direct conversion of the created elements into parametric components that can be used in h-bim platforms with no loss of meaningful information. the different elements to be represented are modelled by making reference to an ideal geometry, that is the one defined by architectural treatises and building canons. this ideal geometry is a benchmark for all other elements with similar characteristics and is created through mathematical relationships in a unique way: parameter specifications and modelling commands are used to create the nurbs elements. in contrast to mesh-based methods, each ideal geometry can be easily managed, converted and integrated in a unique way [50]. this systematic and mathematical approach to digital heritage, based on the simplification of shapes and the reference to ideal geometries, presents an advantage in the parametric logic of hbim platforms, where series of smart objects, represented in 3d, are enriched with a large amount of technical information, related for example to geometry, materials, thickness, etc. [28], [40], [51]. the interoperability between nurbs modelling software and hbim platforms has also been verified [50], [52]. the reference to ideal shapes makes it possible to describe similar elements that are characterised by significant variations in terms of conservation states, e.g. medieval capitals or severely decayed and deformed architectural objects. in such cases, the information on the different conservation states can be attached to each single element to aid in the reconstruction of disparity and displacement maps between similar components [37]. 3.4. semantic segmentation and creation of parametric elements the construction of semantically rich models from survey data, as specified in section 2, requires a rational process in which the main building components are determined: in the multitude of detected points in the raw cloud, it is necessary to distinguish a finite number of parameters. the methodological approach proposed in section 3.3 is illustrated hereafter with reference to the area of the grand ducal cloister of the charterhouse. the laser scanning surveys of the area were used to demonstrate the suitability of the approach following the completion of the scan registration, point cloud subsampling and cleaning (including the removal of obstacles and disturbing elements). a video showing the original point cloud that was used for the creation of the parametric model is available at [53]. figure 6. semantic segmentation of the 3d point cloud. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 103 the case study is emblematic since the cloister has an architectural scheme that is common in religious complexes: it is made up of an open space, square or rectangular in shape, surrounded by vaulted corridors that open onto the central void with a series of arches and colonnades. it is therefore an area of the building characterised by the recurrence of basic elements, with typological characters, shapes and construction details intrinsically bound up with each other. in our case, for the recognition of the components, reference was made to architectural treatises and in particular to the already mentioned treatise the idea of the universal architecture, by vincenzo scamozzi [46], which inspired the original design of the cloister. with reference to the classification and proportions of the architectural orders illustrated by scamozzi, a manual semantic segmentation of the point cloud was performed. the identification and recognition of the main components thus followed the classification illustrated in figure 6. using this approach to the segmentation and subsequent classification of the cloud, each identified element could be modelled separately as a single building component. to parametrically reconstruct the classified typological elements, the steps are: 1. the creation of nurbs surfaces, mapped on the original point cloud; 2. the adoption of these surfaces as modelling components in bim-based software. the use of nurbs surfaces is of fundamental importance since it simplifies the reconstruction process: once the profile of the architectural component has been manually traced over the point cloud (with reference to the canons of architectural treatises) the surface can be built by extrusion, loft, revolution or sweep along this profile, as illustrated in figure 7 for a column shaft. the construction process was performed via the software rhinoceros and its integrated graphical algorithm editor grasshopper. figure 8 shows the nurbs surfaces for the base and the ionic capital of the column. the volutes of the ionic capital were reconstructed by following the procedure illustrated by the 16th century architect andrea palladio in his treatise the four books of architecture [54]. complex-shaped building components were created as nurbs elements in a rhinoceros environment and were only later imported into the bim platform revit, using the standard acis text (.sat) extension. the creation of a sat format file made it possible to use the created geometry as a mass component in the bim environment, thus making it possible to perform other calculations and cuts over the imported 3d element. using this procedure, it was possible to reconstruct a georeferenced h-bim model that presents the conceptual and parametric forms of the grand ducal cloister’s building components. in the 3d model, typological and recurring elements were originally represented in rhinoceros, using nurbs modelling, and then imported to revit. reference to architectural treatises enabled the reconstruction of those specific elements following a univocal procedure with established proportions and shapes, regardless of which person built the parametric component. the model thus constructed, as displayed in figure 9, provides the basis for the insertion of semantic information, i.e. documentary and analytical knowledge about the heritage asset. 3.5. information storage and update an information system based on the approach to 3d reconstruction described in section 3.4 is built in a bim environment to manage both the geometry and the semantics of the grand ducal cloister. the process is based on the connections between the parametric 3d representation of the building and the heterogeneous data derived from documentary and analytical sources (such as shape and dimensions and metric information). as for studies of the materials, the state of deterioration or possible interventions, a shared and univocal model of data insertion within the 3d model could be applied. the proposed scheme is illustrated in figure 10: information on ‘material’, ‘state of degradation’, ‘diagnosis’ and type of performed or needed ‘intervention’ is associated with each typological element. also, references to ‘documentary sources’, provided in the form of text files or images, can be linked to the element through the connection to a directory. following the same principle, the external 3d survey files that were initially used for the construction of the parametric forms can be linked to the model using the ‘data acquisition’ toolbox. all of the toolboxes are implemented in the revit platform as project parameters. thus, the semantic structuring can take place in two distinct ways: the information can simply be linked to the individual building components, or, in a more refined process, it can be connected to only a part of a component (figure 11). in the first case, the information is simply associated with a certain element in its totality; in the second case, however, the semantic data does not concern the whole element, but only a part of it. this happens, for example, in cases in which it is helpful to define a detailed map or annotate local characters, e.g. a phenomenon of superficial degradation that is not present along the whole extension of the element but concerns only a certain section. figure 7. creation of the column shaft by means of a loft operation applied to circular surfaces with the centre located on the shaft axis. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 104 in this situation, the annotation is inserted by means of a trick: reference is made to the construction of a mass component representing the degradation along the surface of the element. this operation is performed directly on revit and makes it possible to map 3d information onto the model. the database setting is structured in such a way that it is always possible to retrieve information about: the professional or scholar who entered the annotation; the time the information was entered; the period to which the information refers. with this expedient, the different temporal states of construction and evolution of the model can also be displayed. the information system thus built, if customised according to the heritage building’s needs, allows for the management, recovery and updating of metric and semantic data. in other words, multiple representations and their associated information can be organised around the same semantic description model. in the future, cross-referencing capabilities are expected to improve, making it possible to establish correlations between levels of information, e.g. between materials and degradation status, decay level and improper interventions or diagnosis and repair and restoration interventions. the digitally enriched model is flexible and dynamic: the semantic base can be upgraded if new information of a different nature or from another research domain needs to be added. for instance, the envisaged future work on correlation queries or even mechanical or physicochemical properties of a material linked to a typological element may require the database entries to be updated by entering additional fields and tags onto the database structure. different levels of database access and permissions (entry, modification of a field, read-only access, entry verification) are assigned to the different actors involved, based on their respective roles and responsibilities. this ensures greater control over the information entry and also makes it possible to keep track of the person who has modified, updated or added certain information within the 3d model. further advantages of the proposed approach include: the ability to track the time of updates and thus describe the evolution of a given annotation over time; information overlay, meaning the ability to handle multiple levels of information. different temporal states, for instance, can be displayed over the same 3d representation. in this multilayer information management system, it is also possible to overlap surface annotations (local components) with volume annotations (global components, describing a parametric element in its entirety). despite the huge potential of this information system, it is worth noting that the insertion of surface-type annotations within bim platforms remains cumbersome and time consuming, especially when compared to the insertion of volume-type annotations. in addition, bim commercial platforms were originally designed for new constructions, and their application to cultural heritage is still limited. such platforms typically allow the performance of volume-type annotations and are not designed to manage local annotations on the elements. the inclusion of localised information within a certain parametric component, which is necessary for the analysis of existing structures, is still required. figure 8. generation of capital and base elements of the column via nurbs. the modelling phase was guided by reference to architectural treatises, [55]. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 105 figure 9. recognition of ideal shapes over the point cloud for the creation of the 3d model. figure 10. proposal for the organisation of the information to be associated with parametric elements. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 106 indeed, limitations in the overlay of surface and volumetric annotations, in terms of file size, response time, functionality of the model, etc., have not yet been fully assessed in the current state of the research. these aspects are under study, and they will be the subject of future publications. further developments of information modelling systems to support cultural heritage documentation should therefore aim at improving the system’s ability to handle local annotations to describe e.g. a degraded part, the change or loss of material, or a decorated element. 4. conclusions and future work heritage studies involve a plurality of actors and resources and produce multiple and heterogeneous data sets. this leads to a broad range of disparate information and knowledge. to sort the different information in a logical way, it is increasingly essential to create digital systems localised within a 3d representation of the object of study in which different types of data can be entered, stored and displayed. this approach combines the acquisition, restitution and documentation phases of heritage studies. with the creation of semantically enriched 3d models, the qualitative information derived from studies of cultural heritage objects can be combined with quantitative information linked to the geometry of the represented elements and to their reciprocal spatial relations. to digitally represent heritage objects, however, a logical process of segmenting and classifying the existing elements is required. in pisa charterhouse, the size of the complex itself and the presence of multidisciplinary teams were the factors that determined the urgency of creating a shared system of information management and archiving. tackling these issues, this contribution presents a method of manual segmentation and classification starting from the raw 3d survey data. the method is based on the study of architectural canons and on the use of nurbs surfaces for the modelling of complex building components. the final goal is the reconstruction of an enriched 3d model, where information related to the knowledge, maintenance and recovery of the heritage building is suitably stored in a 3d digital environment. the presented approach uses a manual reconstruction of the components over the point cloud and the subsequent importation and handling of the geometries in an h-bim environment. future developments follow the automation approach of the scan-to-bim process via shape recognition algorithms. an interesting possible application could integrate modern machine learning and deep learning techniques in order to automate the classification of recurrent geometric elements and repetitive building components. although these techniques are already widely used in the geospatial field, only a few studies refer to their application in the cultural heritage sector [56], [57]. the implementation of such technologies could greatly aid in the construction of semantic models from laser scanning or photogrammetric survey data and improve the implementation of h-bim processes. acknowledgements the work presented in this paper was undertaken within the project ‘studi conoscitivi e ricerche per la conservazione e la valorizzazione del complesso della certosa di calci e dei suoi poli museali’ (cognitive studies and research aimed at the conservation and enhancement of the calci charterhouse complex and its museum poles), 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https://www.imeko.org/publications/tc4-archaeo-2019/imeko-tc4-metroarchaeo-2019-50.pdf https://doi.org/10.5194/isprs-archives-xlii-3-w4-137-2018 https://doi.org/10.5194/isprs-archives-xlii-2-w15-351-2019 https://doi.org/10.5194/isprs-archives-xliii-b2-2020-1381-2020 https://doi.org/10.1007/978-3-319-13695-0_16 https://doi.org/10.5194/isprs-archives-xlii-2-w15-413-2019 https://youtu.be/eniilal-dfu https://doi.org/10.3390/rs13030461 https://doi.org/10.3390/rs11070847 https://doi.org/10.5194/isprs-annals-iv-2-w1-119-2016 optical metrology applied in cctv inspection in drain and sewer systems acta imeko issn: 2221-870x march 2020, volume 9, number 1, 18 24 acta imeko | www.imeko.org march 2020 | volume 9 | number 1 | 18 optical metrology applied in cctv inspection in drain and sewer systems luís martins1, maria do céu almeida1, álvaro ribeiro1 1 lnec – national laboratory for civil engineering, avenida do brasil 101, 1700-066 lisbon, portugal section: research paper keywords: cctv inspection; drain; sewer; dimensional measurement citation: luís martins, maria do céu almeida, álvaro ribeiro, optical metrology applied in cctv inspection in drain and sewer systems, acta imeko, vol. 9, no. 1, article 4, march 2020, identifier: imeko-acta-09 (2020)-01-04 editor: lorenzo ciani, university of florence, italy received november 4, 2019; in final form january 30, 2020; published march 2020 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was supported by lnec – national laboratory for civil engineering, portugal. corresponding author: luís martins, e-mail: lfmartins@lnec.pt 1. introduction drain and sewer systems are integrated in urban areas, often large and complex. these networks are composed by multiple types of components, such as drains and sewers, manholes, gullies, combined sewer overflows, storage and retention tanks, pumping stations, among others. these systems operate essentially under gravity to convey wastewater and stormwater to a treatment works or receiving water. in the last decades, extensive work has been carried out by european standardization committees [1]–[3], aiming at the harmonization and improvement of strategic, policy and operational activities concerning drain and sewer systems. the established framework ambition is to contribute to fulfilment of strategic objectives of these services (including protection of public and occupational health and safety, environmental protection and sustainability). set principles and functional requirements detailed in the standards are related to three main lines of action: (i) investigation and assessment; (ii) design and construction; and (iii) management and control. the investigation and assessment of drain and sewer systems outside buildings [2] aims to establish an overview of their condition and performance and it can be defined as a sequential process, which includes: (i) purpose; (ii) scope; (iii) review of existing information; (iv) inventory update; and (v) hydraulic, environmental, structural and operational investigations. the results of these investigations allow determining the hydraulic performance, environmental impact, structural condition and operational deficiencies, supporting the comparison with the established performance requirements from which nonconformities can be found and management plans can be updated. the investigations are carried out using several sources of information, including external or internal inspection activities for the detection and characterization of anomalies which can negatively affect the performance of the drain or sewer system. close circuit television (cctv) inspection is a largely used visual abstract this paper addresses the metrological quality of dimensional measurements based on images obtained from cctv inspections in drain and sewer systems. in this type of indirect visual inspection, a significant number of absolute and relative dimensional quantities can be quantified, contributing to the characterization of the observations and, consequently, to the analysis of performance of drain and sewer systems outside buildings. unfavorable environmental factors and conditions within the drain or sewer components affect estimation of the quantities of interest and the quality of the recorded images (lighting, lack of reference points, geometric irregularities and subjective assessments, among others). quantification improvement of the dimensional quantities is a key objective to achieve a better assessment of components condition from these inspections. this study contributes to improve the quality of the dimensional measurements by defining experimental procedures, applicable to the optical systems used in cctv inspection. the paper describes the european normative framework for these inspection activities and proposes approaches aiming at increasing confidence in the dimensional measurements based on the metrological characterization of the optical systems used, as well as, to establish a traceability chain. results presented and discussed include the evaluation of measurement uncertainty. mailto:lfmartins@lnec.pt acta imeko | www.imeko.org march 2020 | volume 9 | number 1 | 19 inspection technique for non-man entry components. the use of a remotely controlled cctv camera is generally motivated by: (i) the need to inspect non-man entry components; (ii) safety issues, avoiding direct visual inspection by persons inside the drain and sewer systems; and (iii) faster and economic advantages, namely, when compared with quantitative methods such as laser scanning, ground piercing radar, sonar, infrared thermography and other available techniques [4]–[5]. this technique allows recording videos and individual images of the anomalies found in drains, sewers, manholes and inspection chambers. a standard european coding system has been developed [3], in line with existing national systems in member countries, facilitating common approaches and circulations of services within the eu. data on observations includes: (i) type (e.g. fissure, deformation, displaced joints, defective connections, roots, infiltration, settled deposits, attached deposits and other obstacles, subsidence, defects in manholes and inspection chambers, mechanical damage or chemical attack); (ii) characterisation; (iii) location (longitudinal and radial); and (iv) quantification. a significant number of absolute and relative dimensional quantities can be calculated, based on the recorded videos and images. since these values are determining the results of the investigations and prioritization for corrective actions [6], efforts towards reduction of systematic deviations and measurement uncertainties can be highly relevant. in parallel with technological developments, on-going research efforts include automated image processing techniques [7] allowing the reduction of human influence in image analysis (which is time-consuming, prone to human error and to subjectivity) and the increase of image quality, e.g., by noise reduction [8]–[10]. however, from a metrological perspective, a deeper discussion on the measurement accuracy of the calculated quantities based on this method is still needed [11]–[13]. in the following sections, this paper presents constrains found to achieve an acceptable accurate level on dimensional measurements in cctv inspections and proposes approaches for the improvement of the process and to the evaluation of measurement uncertainties. these approaches aim to reduce subjectivity in the image dimensional analysis and to increase reproducibility, using metrological characterization of the applied optical systems and its integration in a traceability chain. 2. dimensional measurements in cctv inspections according to en 13508-2:2003+a1:2011 standard [3], a number of quantities should be calculated and recorded to fully characterise the observation identified in the inspection. in the case of absolute dimensions measurements (e.g. in millimetres or in degrees), record of the quantities that can be required include: • width of fissures, cracks, connections, channels and sections; • height of connections and channels; • length of breaks, collapsed regions, intruding connections and sections; • maximum dimension of obstacles; • thickness of deposits; • depth of sediments, anomalies in walls; • longitudinal and radial displacement of joints; • level difference between coverage and surface; • curvature; • angular displacement at joints. in addition, relative quantities that can be required (expressed in percentage) include [3]: • dimensional reduction; • level relative to the diameter or vertical dimension; • reduction in effective cross section area; • intrusion length relative to the diameter or vertical dimension. examples of images obtained from a cctv camera (e.g. figure 1) in drain and sewer systems inspections are shown in figure 2 and figure 3, illustrating some common anomalies and the corresponding quantification parameters. figure 1. remotely controlled cctv camera used in drain and sewer systems inspections [14]. figure 2. inspection image showing dimensional reduction by deformation effect [14]. figure 3. inspection image showing fissure [14]. acta imeko | www.imeko.org march 2020 | volume 9 | number 1 | 20 as a rule, in this type of inspection, the camera’s longitudinal location in each observation is known and recorded with a recommended resolution of 100 mm. an operational speed in the range of 0.1 m/s to 0.2 m/s is usually adopted in order to ensure an adequate detection of observations. to improve the accuracy of the measurements, special attention was given to the cctv camera position inside the sewers or drains, thus reducing geometrical projection deviations. an empirical tolerance up to 10% is usually defined for the cctv camera vertical position (1/2 and 2/3 of the drain/sewer height, for a circular/regular or egg shape, respectively). lighting tests should be carried out before each inspection, using image quality testing charts such as the marconi resolution chart no. 1, allowing in situ qualitative evaluation of the recorded images, namely, the radiometric and spatial resolution as well as the geometrical distortion. although benefits arise from these operational practices, limitations on non-existence of reference points inside the component under inspection makes any dimensional measurement vulnerable to subjective image analysis. in the following section, approaches aiming at improving measurement accuracy are proposed, as well as, the comparability between measurements undertaken by different operators or in different moments in time. 3. proposed approaches a first approach, aiming at increasing the confidence in the calculated dimensional quantities carried out, is to ensure the traceability of the metrological characterization of the optical system – the cctv camera – used in drain or sewer inspection. this characterization includes both a geometrical component and a radiometric component, allowing a rigorous comparison between different cctv cameras available in commercial solutions and their corresponding suitability to the measurement environment. the main objective of the geometrical characterization is the quantification of cctv camera intrinsic parameters such as the focal distance, the principal point coordinates and the lens distortion coefficients, which are input quantities for the determination of accurate dimensions in the field-of-view. this task can be achieved in a laboratorial setup, using traceable reference dimensional patterns and applying known algorithms such as the dlt – direct linear transform [15], the tsai [16] and the zhang method [17]. the radiometric characterization can be carried out following the emva standard guidelines [18]–[19], allowing determining the cctv camera sensitivity, linearity, noise, dark current, spatial non-uniformity and defecting pixels. in addition to the cctv camera metrological characterization, the measurement model itself must be defined and used in the dimensional measurement uncertainty evaluation, following the gum framework [20]–[21]. if both the intrinsic (focal distance, principal point coordinates and lens distortion coefficients) and extrinsic parameters (camera position and orientation in the local or global coordinate system, acquired by dedicated instrumentation of the cctv camera) are known, the perspective camera model [22], shown in figure 4, can be used to determine the coordinates related to points of interest inside the sewer or drain. assuming, in a first approach to this problem, that the lens distortion does not have a significant impact on the dimensional accuracy when compared with other uncertainty components, the perspective camera model assumes a straight line between the interest point in global or local coordinate system (x, y, z), where z defines the camera movement direction, and in the image (x, y) coordinate systems, defining the so-called collinearity equations 𝑋 = 𝑋0 + (𝑍 − 𝑍0) ∙ 𝑟11∙(𝑥−𝑥0)+𝑟12∙(𝑦−𝑦0)−𝑟13∙𝑓 𝑟31∙(𝑥−𝑥0)+𝑟32∙(𝑦−𝑦0)−𝑟33∙𝑓 (1) 𝑌 = 𝑌0 + (𝑍 − 𝑍0) ∙ 𝑟21∙(𝑥−𝑥0)+𝑟22∙(𝑦−𝑦0)−𝑟23∙𝑓 𝑟31∙(𝑥−𝑥0)+𝑟32∙(𝑦−𝑦0)−𝑟33∙𝑓 (2) where: (i) intrinsic parameters – f is the focal distance and (x0, y0) are the principal point image coordinates; (ii) extrinsic parameters – (x0, y0, z0) are the camera’s global or local coordinates and rij are the rotation matrix elements defined by the camera’s orientation angles ω, φ, κ, given by 𝑟11 = cos 𝛷 ∙ cos 𝐾 (3) 𝑟12 = −cos 𝛷 ∙ sin𝐾 (4) 𝑟13 = sin 𝛷 (5) 𝑟21 = cos 𝛺 ∙ sin𝐾 + sin 𝛺 ∙ sin𝛷 ∙ cos 𝐾 (6) 𝑟22 = cos 𝛺 ∙ cos 𝐾 − sin𝛺 ∙ sin𝛷 ∙ sin𝐾 (7) 𝑟23 = −sin𝛺 ∙ cos 𝛷 (8) 𝑟31 = sin𝛺 ∙ sin𝐾 − cos 𝛺 ∙ sin 𝛷 ∙ cos𝐾 (9) 𝑟32 = sin𝛺 ∙ cos𝐾 + cos 𝛺 ∙ sin𝛷 ∙ sin𝐾 (10) 𝑟33 = cos 𝛺 ∙ cos 𝛷 (11) if the camera’s intrinsic and extrinsic parameters are not available, a less rigorous approach can be followed, assuming a parallel geometrical relation between the image plane and the cross-section plane in the drain or sewer and using an orthographic projection camera model [22] to define a scale coefficient, sc, between real dimension (in millimetres) and image dimension (in pixels). in the observed field-of-view, the only available dimensional reference is related to the height of the drain or sewer, h. therefore, the dimensional quantification of a certain observation must be supported by the determination of the scale coefficient relative an average cross-section plane in the drain or sewer which includes the quantity to be measured and where its height is also visible, as shown in figure 5. figure 4. schematic representation of the perspective camera model. acta imeko | www.imeko.org march 2020 | volume 9 | number 1 | 21 in this approach, the image dimensions (in pixels) related to the drain or sewer height in the average cross-section plane, ph, and to the observed anomaly, px, are obtained by image processing. therefore, the scale coefficient, expressed in millimetres per pixel, is obtained by 𝑆𝐶 = 𝐻 𝑝𝐻 (12) while the dimensional measurement of the observation, x, (in millimetres) is given by 𝑋 = 𝑆𝐶 ∙ 𝑝𝑋. (13) since these equations can be considered as linear models, the application of the uncertainty propagation law [20], results in the standard measurement uncertainty expressions for the scale coefficient, u(sc), and for the dimensional measurement of the anomaly u(x), respectively, 𝑢(𝑆𝐶) = √ 1 𝑝𝐻 2 ∙ 𝑢 2(𝐻) + 𝐻2 𝑝𝐻 4 ∙ 𝑢 2(𝑝𝐻) (14) 𝑢(𝑋) = √𝑝𝑋 2 ∙ 𝑢2(𝑆𝐶) + 𝑆𝐶2 ∙ 𝑢2(𝑝𝑋) (15) where u(h) is the standard uncertainty of the height of the drain or sewer and u(ph) and u(px) represent respectively the standard uncertainties of the image dimensions of ph and px. 4. results and discussion this section contains the approach considered for the evaluation of the dimensional measurement uncertainty related to the adoption of the perspective camera model or the orthographic projection camera model. the quantified uncertainty components and estimates are merely illustrative, reflecting values which are expected to have in a sewer or drain cctv inspection scenario. therefore, in a real case scenario, the presented probabilistic formulation and quantification must be confirmed and updated if required. in the study using the perspective camera model and considering the typical inspection scenario in drain and sewer system, a reduced focal distance lens is generally adopted, in order to have a large field-of-view. in this type of lens, distortion can cause the image geometrical deformation, thus affecting the accuracy of the performed dimensional measurements. distortion coefficients and other relevant camera intrinsic parameters such as the focal distance and the principal point coordinates can be obtained, for example, from the dlt algorithm [15] implemented by a matlab® toolbox [23] using images of traceable reference dimensional patterns. the lens distortion model [24] included in this toolbox [23] has radial distortion components, δxrad and δyrad, given by ∆𝑥rad = (𝑥 − 𝑢0) ∙ (𝑘1 ∙ 𝑟 2 + 𝑘2 ∙ 𝑟 4) (16) ∆𝑦rad = (𝑦 − 𝑣0) ∙ (𝑘1 ∙ 𝑟 2 + 𝑘2 ∙ 𝑟 4) (17) and tangential distortion components, expressed by ∆𝑥tan = 𝑝1 ∙ [𝑟 2 + 2 ∙ (𝑥 − 𝑢0) 2] + 2 ∙ 𝑝2 ∙ (𝑥 − 𝑢0) ∙ (𝑦 − 𝑣0)∆𝑥tan = 𝑝1 ∙ [𝑟 2 + 2 ∙ (𝑥 − 𝑢0) 2] + 2 ∙ 𝑝2 ∙ (𝑥 − 𝑢0) ∙ (𝑦 − 𝑣0) (17) ∆𝑦tan = 2 ∙ 𝑝1 ∙ (𝑥 − 𝑢0) ∙ (𝑦 − 𝑣0) + 𝑝2 ∙ [𝑟 2 + 2 ∙ (𝑦 − 𝑣0) 2] (18) where 𝑟 = √(𝑥 − 𝑢0) 2 + (𝑦 − 𝑣0) 2. (19) table 1 shows an example of the intrinsic parameters estimates and standard uncertainties obtained for the case of a camera with a 4 mm nominal focal distance lens and an image sensor with 480 x 640 squared pixels, considering a pixel linear dimension equal to 6.5 μm. high-order radial distortion coefficients were considered negligible. the standard uncertainty related to the image coordinates, resulting from the performed intrinsic parameterization, was equal to 0.04 pixel. in order to know the impact of distortion in the image coordinate measurement accuracy, a monte carlo method was used [21], considering the results shown in table 1 and the mathematical models given by expressions (16-19). the computational simulation algorithm was developed in matlab®, using the mersenne-twister pseudo-random number generator [25] and validated computational routines. 105 trials were used in order to achieve convergent solutions. figure 6 shows the estimates of the image variation (in pixels) due to the combined effect of radial and tangential distortions, while figure 7 presents the corresponding 95% expanded measurement uncertainty. as shown in figure 6 and figure 7, the distortion impact in the image coordinates is quite low. as expected, a higher distortion is observed in the extreme regions of the image, especially in the corners. the maximum distortion estimate is close to 0.050 pixel with a 95% expanded uncertainty of 0.001 pixel. table 1. results obtained from the camera intrinsic parameterization. input quantity symbol estimate standard uncertainty focal distance f 4.273 9 mm 0.001 6 mm principal point x-coordinate x0 302.72 pixel 0.20 pixel principal point y-coordinate y0 242.33 pixel 0.19 pixel 1st radial distortion coefficient k1 891.7∙10-12 pixel-2 2.5∙10-12 pixel-2 2nd radial distortion coefficient k2 96.6∙10-17 pixel-4 2.6∙10-17 pixel-4 1st tangential distortion coefficient p1 -6.5∙10-10 pixel-2 9.3∙10-10 pixel-2 2nd tangential distortion coefficient p2 12.0∙10-11 pixel-2 9.3∙10-11 pixel-2 figure 5. schematic representation of the measurement procedure in cctv camera images, based on the orthographic camera model. acta imeko | www.imeko.org march 2020 | volume 9 | number 1 | 22 these results allow the use of the collinearity equations (1-11) in the calculation of the dimensional measurement uncertainty, when considering the perspective camera model. with the exception of the distortion coefficients results shown in table 1, the remaining intrinsic parameters results were considered, however, three sets of unknown input variables are identified: (i) the camera’s orientation angles estimates and measurement uncertainties; (ii) the camera’s local coordinates and the location of the observation plane; (iii) the measurement uncertainty of the image coordinates. this situation led to the adoption of the following assumptions: (i) the camera’s orientation angles estimates and measurement uncertainties are available from the laboratorial parameterization using the matlab® toolbox [23] and can be used as a suitable approximation for an inspection scenario study; (ii) the camera is located in the local coordinate system origin (the centre of the drain or sewer cross-section), and the observation plane parallel to the xy plane is located 1 m away; the corresponding standard measurement uncertainties related to these estimates are considered equal and varying between 1 mm and 100 mm; (iii) the measurement uncertainty of the image coordinates is comprised between 0.005 pixel (best case scenario, which only includes the uncertainty components related to the parameterization and distortion effects) and 5 pixels (worst case scenario, accounting for additional uncertainty components related to illumination and manual selection of interest points). table 2 summarizes the probabilistic formulation of the collinearity equations input quantities, which was used in the dimensional measurement uncertainty evaluation. due to the non-linear and complex mathematical models related to perspective camera model (expressions 1 to 11), a monte carlo method [21] was again applied in the performed numerical simulations, in order to obtain the dispersion of values related to the x and y local coordinates. a 95% computational accuracy level lower than 1 mm was obtained. figure 8 shows the variation of the 95% expanded measurement uncertainty of the calculated x and y local coordinates, considering the adopted measurement uncertainty intervals related to the standard uncertainty of the camera, observation plane location and image coordinates. the results obtained for a camera and plane location standard uncertainty above 5 mm are not shown due to the high value of the obtained table 2. probabilistic formulation adopted for the input quantities related to the collinearity equations. input quantity symbol probability density function estimate standard uncertainty focal distance f gaussian gaussian gaussian gaussian gaussian gaussian gaussian gaussian gaussian gaussian gaussian gaussian gaussian gaussian 4.273 9 mm 0.001 6 mm principal point x-coordinate x0 302.72 pixel 0.20 pixel principal point y-coordinate y0 242.33 pixel 0.19 pixel camera orientation angles ω -0.006 7 rad 0.000 9 rad φ -0.004 5 rad 0.000 5 rad κ 0.003 5 rad 0.000 1 rad camera location in the global or local coordinate system x0 0 mm 1 mm – 100 mm y0 0 mm 1 mm – 100 mm z0 0 mm 1 mm – 100 mm observation plane location z 1 000 mm 1 mm – 100 mm image x-coordinate x 1 pixel – 480 pixels 0.005 pixel – 5 pixels image y-coordinate y 1 pixel – 640 pixels 0.005 pixel – 5 pixels figure 6. image distortion estimates in pixels. figure 7. image distortion 95% expanded uncertainties in pixels. figure 8. xy measurement uncertainties in the perspective camera model. acta imeko | www.imeko.org march 2020 | volume 9 | number 1 | 23 95% expanded uncertainty of the x and y coordinates (from 189 mm up to 263 mm, for a 100 mm standard uncertainty). the results show that a dimensional accuracy level lower than 10 mm can only be achieve for a camera and plane location standard uncertainty of 1 mm and an image coordinate standard uncertainty bellow 3 pixels. in a sensitivity point of view, the camera and plane standard uncertainty has a stronger contribution to the dimensional accuracy level, rather than the image coordinate measurement uncertainty. when compared with the global dimensions of the corresponding camera field-of-view (974 mm x 731 mm), the relative 95% expanded measurement uncertainty of the x and y coordinates, is comprised between 0.2% and 4.1%, for the results shown in figure 8. in the study using the orthographic projection camera model, in a first stage, the measurement uncertainty of the scale coefficient is exemplified, considering an observation scenario where the drain or sewer height in the average cross-section plane is assumed to be 1000 mm, corresponding to an image dimension of 500 pixels. the application of expression (12) originates an estimate equal to 2.00 mm/pixel. figure 9 illustrates the standard measurement uncertainty of the scale coefficient, considering different levels of measurement uncertainty related to the mentioned input quantities (from 1 mm up to 10 mm, and from 0.01 pixel up to 5 pixels). as seen in figure 9, the standard measurement uncertainty of the scale coefficient can assume, in the considered inspection scenario, a minimum value of 0.004 mm/pixel and a maximum value of 0.057 mm/pixel. these results can be propagated through expression (13) and combined with the previous considered standard uncertainty values of the dimensional measurement in the image (between 0.01 pixel and 5 pixels). figure 10 presents the 95% expanded relative uncertainty of dimensional measurements between 10 mm and 200 mm. in the worst case scenario, related to the scale coefficient with the highest measurement uncertainty (dashed lines in the plot shown in figure 10), the obtained dimensional measurement accuracy is always above 5%. better accuracy levels are possible, namely, in the case of the lowest measurement uncertainty of the scale coefficient, for standard uncertainties of 1.3 pixel (for dimensional measurements close to 100 mm) and 2.5 pixels (for dimensional measurements of 200 mm). 5. conclusions this paper presents the main contributions of a metrological perspective to the problem of the dimensional measurement accuracy in cctv inspections, indicating experimental procedures applied in the optical characterization and modelling of the used cameras, followed by the corresponding suitable measurement uncertainty calculation procedure. this study gives a first step towards the improvement and the calculation of measurement accuracy in this field of inspection, namely, for the dimensional measurements obtained indirectly from the recorded inspection images. measurement uncertainty analysis tools were developed and are now available to be used within the approaches presented and for the type of experimental input information, regarding estimates and related measurement uncertainties. however, the results obtained so far in this study, already indicate a high impact of the accuracy of the camera and plane locations for the perspective camera model. in the orthographic projection camera model, results show a strong influence of the image quality in the accuracy of the dimensional measurements. these calculation tools can also be used to quantify accuracy operational improvements, namely, due to changes in field lighting, image analysis, cctv camera selection and metrological characterization. future work will be focused on determining the uncertainty component related to the adoption of the orthographic projection camera model, which is considered an approximation of the perspective camera model, and development of guidelines for operators, taking into consideration characteristics of available technologies. acknowledgement the authors acknowledge the financial support provided by lnec – national laboratory for civil engineering. references [1] en 752:2017, drain and sewer systems outside buildings. sewer system management, cen – european committee for standardization, 2017. [2] en 13508-1:2012, investigation and assessment of drain and sewer systems outside buildings – part 1: general requirements, cen – european committee for standardization, 2012. figure 9. standard uncertainty of the scale coefficient in the studied inspection scenario. figure 10. 95% relative expanded uncertainty of the dimensional measurement. acta imeko | www.imeko.org march 2020 | volume 9 | number 1 | 24 [3] en 13508-2:2003+a1:2011, investigation and assessment of drain and sewer systems outside buildings – part 2: visual inspection coding system, cen – european committee for standardization, 2011. [4] j. m. makar, diagnostic techniques for sewer systems, j. infrastruct. syst. 5 (1999) pp. 69-78. [5] n. stanic, m. lepot, m. catieau, j. langeveld, f. clemens, a technology for sewer pipe inspection (part 1): design, calibration, corrections and potential application of a laser profiler, automation in construction 75 (2017) pp. 91-107. [6] r. a. fenner, approaches to sewer maintenance: a review, urban water 2 (2000) pp. 343-356. [7] t. c. su, m. d. yang, application of morphological segmentation to leaking defect application in sewer pipelines, sensors 14 (2014) pp. 8686-8704. [8] t. c. su, m. d. yang, n. f. pan, y. f. yang, systematic image quality assessment for sewer inspection, expert syst. appl. 38 (2011) pp. 1766-1776. [9] d. li, a. cong, s. guo, sewer damage detection from unbalanced cctv inspection using deep convolutional neural networks with hierarchical classification, automation in construction 10 (2019) pp. 199-208. [10] a. hawari, m, alamin, f. alkadour, m. elmasry, t. zayed, automated defect detection tool for closed circuit television (cctv) inspected sewer pipelines, automation in construction 89 (2018) pp. 99-109. [11] b. roghani, f. cherqui, m. ahmadi, p. le gauffre, m. tabesh, dealing with uncertainty in sewer condition assessment: impact on inspection programs, automation in construction 103 (2019) pp. 117-126. [12] t. p. pridmore, d. cooper, n. taylor, estimating camera orientation from vanishing point location during sewer surveys, automation in construction 5 (1997) pp. 407-419. [13] j. kannala, s. brandt, j. heikkilä, measuring and modelling sewer pipes from video, machine vision and applications 19 (2008) pp. 73-83. [14] p. henley, sewer condition – classification and training course, wrc, 2017. [15] j. heikkilä, o. silvén, “a four-step camera calibration procedure with implicit image correction”, conference on computer vision and pattern recognition – proceedings cvpr, 1997. [16] r. tsai, a versatile camera calibration technique for high-accuracy 3d machine vision metrology using off-the-shelf tv cameras and lenses, ieee journal of robotics and automation ra-3 4 (1987). [17] z. zhang, a flexible new technique for camera calibration, ieee transactions on pattern analysis and machine intelligence 22 11 (2000). [18] emva 1288:2010, emva standard 1288. standard for characterization of image sensors and cameras, european machine vision association, release 3.0, 2010. [19] m. rosenberger, c. zhang, p. votyakov, m. peibler, r. celestre, g. notni, emva 1288 camera characterisation and the influences of radiometric camera characteristics on geometric measurements, acta imeko 5, 4 (2016) pp. 81-87. [20] guide to the expression of uncertainty in measurements, iso – international organization for standardization, 1993, isbn 9267-10188-9. [21] evaluation of measurement data – supplement 1 to the “guide to the expression of uncertainty in measurements” – propagation of distributions using a monte carlo method, jcgm – joint committee for guides in metrology, 2008. [22] r. hartley, a. zisserman, multiple view geometry in computer vision. cambridge university press, new york, 2003, isbn 9780521-54051-3. [23] http://www.vision.caltech.edu/bouguetj/calib_doc/ [24] d. brown, “close-range camera calibration”, proc. of the symposium on close-range photogrammetry, 1971, illinois, usa, pp. 855-866. [25] m. matsumoto, t. nishimura, mersenne twister: a 623dimensionally equidistributed uniform pseudo-random number generator, acm transactions on modelling and computer simulation 8 1 (1998) pp. 3-30 http://www.vision.caltech.edu/bouguetj/calib_doc/ research on dispersion in indenter and reference block for rockwell hardness test acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 221 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 221 225 research on dispersion in indenter and reference block for rockwell hardness test takeshi sawa1 1 mitutoyo corporation, 1-20-1 sakado, takatsu-ku, kawasaki-shi, kanagawa-ken, 213-8533, japan takeshi_sawa@mitutoyo.co.jp abstract: in this report, the rockwell indenters and hardness reference blocks obtained from different three manufacturers were evaluated. and these results with the tolerances for the indirect verification prescribed in iso 6508-2 and astm e18 were compared, it has been suggested that the tolerances prescribed in astm e18 might be too strict in view of actual circumstances. keywords: rockwell indenter, hardness reference block, iso/iec 17025, iso standards, astm standards 1. introduction the importance of ensuring the traceability in measurement has been increased in these years, and accordingly, the spread of systems certifying tests and calibrations in conformity with iso/iec 17025[1] that can publicly give assurance has been promoted in a worldwide manner. in addition, the mutual recognition agreement (mra) that mutually certifies the conformity assessment results of own country as equivalent to another country is implemented among many accredited systems, and the accredited certificates are valid in any countries. it can be said that this system is extremely beneficial for globalization of manufacturing and industry. the hardness test is no exception, and the number of cases where the calibration of hardness reference blocks, indenters and hardness testing machines is requested to conform to iso/iec 17025 is increasing. it can be said that the supply source of indenters and hardness reference blocks that are used for calibration of hardness testing machines does not matter if the testing and calibration are performed in conformity with accredited systems system. however, a hardness value is industrial quantity and has no physical reference, therefore, the traceability system and uncertainty evaluation method for measurement values and calibration values are not completely standardized in the global aspect under present circumstances. such present circumstances may become a problem for calibration of hardness testing machines. we know that even differences in hardness values among national metrology institute level are a problem for calibration of actual hardness testing machines [2]. in this report, evaluating the dispersion of rockwell indenters and hardness reference blocks obtained from different three manufacturers have been conducted for the purpose of investigating the realistic circumstances. then, based on these results, the uncertainty of calibration in indirect verification of the hardness testing machine was estimated, and the consistency with the tolerance value prescribed in the standard was investigated. 2. experiment three pieces of rockwell diamond conical indenters, tungsten carbide ball indenters with 1.5875 mm in diameter and six types of hardness reference blocks from three manufacturers are obtained. the scales and hardness levels of the six hardness reference blocks were 30 hrc, 60 hrc, 90 hr15n, 30 hrbw, 90 hrbw and 35 hr 15tw. three manufacturers are hereinafter referred to as manufacturer a, manufacturer b, and manufacturer c. manufacturer a is a famous indenter and reference block manufacturer in europe. manufacturer b and manufacturer c are us manufacturers and a lot of their products are widespread in the united states. an accredited certificate is issued for all of these indenters and reference blocks to prove that calibration of them conforming to iso/iec 17025 has been conducted. sht-31, a rockwell hardness standard testing machine manufactured by mitutoyo corporation, was used for evaluation. the specifications and accuracy of each part of the testing machine is conformed to iso 6508-3: calibration of the hardness reference block [3]. moreover, the testing conditions such as time of loading the testing force and duration time were set according to this standard. http://www.imeko.org/ mailto:takeshi_sawa@mitutoyo.co.jp acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 222 2.1. results of evaluating indenters of each manufacturer testing on the same reference block with the use of obtained indenters and our indenter were conducted. our indenter, i.e. mitutoyo indenter is conformable to iso 6508-3, and is equivalent to an indenter classified in class a prescribed in astm e18[4]. the differences between the hardness values of the targeted indenters to be evaluated and the hardness value of our indenter were evaluated. figure 1 shows the differences of hardness values in each scale. the medians were regarded as the average of the differences of hardness values in each scale. the value pi shows a 95% confidence interval of dispersion calculated by multiplying the experimental standard deviation of the differences of hardness values in each scale by a coverage factor. here, the coverage factor was determined to be 2.31 on the basis of the student's t-distribution in consideration that the degree of freedom of data in each scale is 8. figure 1: difference from hardness value with mitutoyo indenter 2.2. results of evaluating obtained reference blocks of each manufacturer testing on the obtained reference blocks with the use mitutoyo indenter were conducted, and the measured hardness values were compared with the calibration values of the targeted hardness reference blocks to be evaluated. figure 2 shows the differences from the calibration values of the reference blocks in each scale. the medians were regarded as the average of the differences of hardness values in each scale. the value pb shows a 95% confidence interval of dispersion calculated by multiplying the experimental standard deviation of the differences of hardness values in each scale by a coverage factor. here, the coverage factor was determined to be 2.31 on the basis of the student's t-distribution in consideration that the degree of freedom of data in each scale is 8. figure 2: differences from the calibration values of the reference blocks 3. discussion 3.1. regarding results of evaluating indenters table 1 shows the pi value in fig. 1 and the tolerance of indenter prescribed in iso 6508-2[5] and astm e18. for diamond indenter, the difference from the reference indenter is prescribed in standard as tolerance. there are some pi values that are larger than the tolerance. this fact suggests that indenters whose tolerance exceeds the standards may be on the market. -2.0 -1.0 0.0 1.0 2.0 a b c a b c a b c d if fe r e n c e f r o m h a r d n e ss v a lu e w it h m it u to y o i n d e n te r [ h r ] 30 hrc 60 hrc 90 hr15n indenter manufacturer pi = 0.6 hrc pi = 0.7 hrc pi = 1.3 hr15n diamond indenter scales -2.0 -1.0 0.0 1.0 2.0 a b c a b c a b c d if fe r e n c e f r o m h a r d n e ss v a lu e w it h m it u to y o i n d e n te r [ h r ] 30 hrbw 90 hrbw 35 hr30tw indenter manufacturer pi = 1.1 hrbw pi = 0.4 hrbw pi = 0.4 hr30tw wc ball indenter scales -3.0 -2.0 -1.0 0.0 1.0 2.0 3.0 a b c a b c a b cd if fe r e n c e f r o m c a li b r a ti o n v a lu e o f r e fe r e n c e b lo c k [ h r ] 30 hrc 60 hrc 90 hr15n reference block manufacturer pb = 1.0 hrc pb = 0.8 hrc pb = 0.7 hr15n diamond indenter scales -3.0 -2.0 -1.0 0.0 1.0 2.0 3.0 a b c a b c a b cd if fe r e n c e f r o m c a li b r a ti o n v a lu e o f r e fe r e n c e b lo c k [ h r ] 30 hrbw 90 hrbw 35 hr30tw reference block manufacturer pb = 1.9 hrbw pb = 0.7 hrbw pb = 1.0 hr30tw wc ball indenter scales http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 223 for tungsten carbide ball indenter, the difference from the calibration value of reference block is in standard as tolerance. except for 30 hrbw, the pi value is sufficiently smaller than the tolerance of standard. however, this result is a difference from the value measured with the same testing machine and test conditions, and the uncertainty of the calibration value of reference block is considered to be larger than measured value. that is, the fact that the pi value is smaller is expected result. nevertheless, for 30 hrbw, the pi value exceeds the tolerance of astm standard. in addition, there is a bias of indenters among manufacturers. that is, for 30 hrbw, it is suggested that the use of indenter from another manufacturer may exceed the tolerance of astm standard. table 1: evaluation results of indenter and tolerance of standard 3.2. estimation of uncertainty for indirect verification using results of evaluating reference block. the disperse i.e. uncertainty of hardness values generated in the indirect verification of the testing machine was estimated. as factors for the uncertainty, we hypothesized the influences of the depth measuring device, difference of the indenter with respect to the reference indenter, the calibration value and uniformity of the hardness reference block, and machine hysteresis. it thought that the testing force, indentation time, and duration time of testing force can be ignored because these factors have little influence. the standard uncertainty for each factor was estimated based on the description in table 2. if a tolerance is different between the iso standards and astm standards, stricter tolerance was used for estimation. table 3 shows the uncertainty budget for each scale. table 2: hypothesized uncertainty factors 3.3. regarding consistency with the tolerance of the standard table 4 shows the tolerances in indirect verification prescribed in iso 6508-2, astm e18 and estimated expanded uncertainty. when the expanded uncertainty of the hardness value in indirect verification is compared with the tolerance of the standard, it is considered that the uncertainty is within the tolerance range of the iso standards in all the evaluated scales. with regard to the astm standards, on the other hand, the uncertainty may not be within the tolerance range except for 30 hrbw and 35 hr30tw through the indirect verification even with the use of indenters and reference blocks that are conformable with the standards. the tolerance of 30 hrbw in the astm standards was revised from ±1.5 hrbw to ±2.5 hrbw in 2019, and it is thought that review in light of present circumstances is conducted. however, even for 30 hrbw and 35 hr30tw, the margin for the tolerance is small, and the tolerance may not be appropriate. in this report, the estimated expanded uncertainty of the hardness value in indirect verification is the result of extracting only main uncertainty factors, and is obtained by using the smaller tolerance between two types of standards. in addition, experimental estimation of indenters and hardness reference blocks was conducted only about three manufacturers. thus, the actual uncertainty may be even larger. regardless of the above, when the dispersion is compared with the tolerance of the astm standards, the dispersion is equivalent to or scale tolerance (iso 6508-2) tolerance (astm e18) experimental result (p i value) 30 hrc 0.8 hrc 0.8 hrc 0.6 hrc 60 hrc 0.8 hrc 0.4 hrc 0.7 hrc 90 hr15n 0.8 hr15n 0.5 hr15n 1.3 hr15n 30 hrbw 4.0 hrbw 0.8 hrbw 1.1 hrbw 90 hrbw 2.0 hrbw 0.8 hrbw 0.4 hrbw 35 hr30tw 3.0 hr30tw 0.8 hr30tw 0.4 hr30tw uncertainty factors hypothesized variation of hardness values depth measuring device tolerances in verification of depth measuring device prescribed in iso 6508-2 and astm e18 diamond indenter tolerance of difference in hardness value from reference indenter at testing with the identical reference block prescribed in astm e18 ball indenter tolerance of difference in calibration value from reference block at testing reference block prescribed in astm e18 uniformity of reference block tolerance of difference between the maximum and minimum values in 5point measurement prescribed in standard for reference block, iso 6508-3 or astm e18 calibration value of reference block pb values in fig. 2 are hypothesized as expanded uncertainty. machine deformation although the tolerance is ±1.0 hr in iso 6508-2 or astm e18, it is hypothesized to be ±0.3 hr in view of actual circumstances. http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 224 exceeds the tolerance. especially regarding 60 hrc, the difference between the tolerance ±0.5 hrc of indirect verification prescribed in the astm standards and the dispersion of the estimated hardness values is large. this fact suggests that the tolerance of astm standards is not realistic. table 3: uncertainty budget for each scale table 4: tolerance of standards and estimated extended uncertainty in indirect verification 4. summary the following items including verification and evaluation were conducted and described in this report. • evaluation of the dispersion of indenters and hardness reference blocks which are in the market. • comparison of evaluation result and tolerances for indenter. • estimation of uncertainty of hardness value in indirect verification of the testing machine based on the evaluation result of the reference block. • verification of validity of the tolerance in the indirect verification prescribed in the iso standards and astm standards. by conducting the items above, and the following were found. • for diamond indenter, there is a possibility that the indenters exceed the tolerance of standard are in market. • for tungsten carbide ball indenters, there is a possibility that the difference may exceed the tolerance of standard for 30 hrbw if the indenter manufacturer changes. • it was suggested that the estimated dispersion of the hardness values generated in indirect verification fell under the tolerance range of the iso standards. • it was suggested that the estimated dispersion of the hardness values generated in indirect verification exceed the tolerance range of the astm standards, and in particular, the tolerance of astm standard for 60 hrc may be too strict. 5. conclusion a hardness value is not a physical quantity, but an industrial quantity obtained by a prescribed testing method. the fact is that testing machines and testing conditions vary within the prescribed tolerance range and relatively large dispersion of hardness values is observed even in a national standard level. if the fact that this is a destructive 30 hrc factor distribution divisor depth measuring 0.5 hrc rectanglar √3 0.289 hrc indenter 0.4 hrc rectanglar √3 0.231 hrc uniformity of block 0.35 hrc rectanglar √3×√5 0.090 hrc uncertainty of block 1.0 hrc normal 2 0.500 hrc machine hysteresis 0.3 hrc rectanglar √3 0.173 hrc 0.652 hrc 1.3 hrc 60 hrc factor distribution divisor depth measuring 0.5 hrc rectanglar √3 0.289 hrc indenter 0.4 hrc rectanglar √3 0.231 hrc uniformity of block 0.2 hrc rectanglar √3×√5 0.052 hrc uncertainty of block 0.8 hrc normal 2 0.400 hrc machine hysteresis 0.3 hrc rectanglar √3 0.173 hrc 0.574 hrc 1.1 hrc 90 hr15n factor distribution divisor depth measuring 0.5 hr15n rectanglar √3 0.289 hr15n indenter 0.5 hr15n rectanglar √3 0.289 hr15n uniformity of block 0.25 hr15n rectanglar √3×√5 0.065 hr15n uncertainty of block 0.7 hr15n normal 2 0.350 hr15n machine hysteresis 0.3 hr15n rectanglar √3 0.173 hr15n 0.569 hr15n 1.1 hr15n 30 hrbw factor distribution divisor depth measuring 0.5 hrbw rectanglar √3 0.289 hrbw indenter 0.8 hrbw rectanglar √3 0.462 hrbw uniformity of block 0.5 hrbw rectanglar √3×√5 0.129 hrbw uncertainty of block 1.9 hrbw normal 2 0.950 hrbw machine hysteresis 0.3 hrbw rectanglar √3 0.173 hrbw 1.116 hrbw 2.2 hrbw 90 hrbw factor distribution divisor depth measuring 0.5 hrbw rectanglar √3 0.289 hrbw indenter 0.8 hrbw rectanglar √3 0.462 hrbw uniformity of block 0.35 hrbw rectanglar √3×√5 0.090 hrbw uncertainty of block 0.7 hrbw normal 2 0.350 hrbw machine hysteresis 0.3 hrbw rectanglar √3 0.173 hrbw 0.676 hrbw 1.4 hrbw 35 hr30tw factor distribution divisor depth measuring 0.5 hr30twrectanglar √3 0.289 hr30tw indenter 0.8 hr30twrectanglar √3 0.462 hr30tw uniformity of block 0.5 hr30twrectanglar √3×√5 0.129 hr30tw uncertainty of block 1.0 hrbw normal 2 0.500 hr30tw machine hysteresis 0.3 hr30twrectanglar √3 0.173 hr30tw 0.770 hr30tw 1.5 hr30twexpanded uncertainty (k = 2) combined standard uncertainty expanded uncertainty (k = 2) combined standard uncertainty expanded uncertainty (k = 2) combined standard uncertainty expanded uncertainty (k = 2) combined standard uncertainty expanded uncertainty (k = 2) combined standard uncertainty expanded uncertainty (k = 2) combined standard uncertainty range (±) range (±) range (±) range (±) range (±) range (±) standard uncertainty standard uncertainty standard uncertainty standard uncertainty standard uncertainty standard uncertainty scale tolerance (iso 6508-2) tolerance (astm e18) analysis (expanded uncertainty) 30 hrc 1.5 hrc 1.0 hrc 1.3 hrc 60 hrc 1.5 hrc 0.5 hrc 1.1 hrc 90 hr15n 2.0 hr15n 0.7 hr15n 1.1 hr15n 30 hrbw 4.0 hrbw 2.5 hrbw 2.2 hrbw 90 hrbw 2.0 hrbw 1.0 hrbw 1.4 hrbw 35 hr30tw 3.0 hr30tw 1.5 hr30tw 1.5 hr30tw http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 225 test in which absolute reference does not exist is considered, it can be said that such dispersion is generated as a matter of course. on the other hand, under such circumstances, accredited systems and traceability are utilized. this type of issue may be a big problem that cannot be solved by company alone for manufacturers of hardness testing machines in each country. we hope that verification of consistency with public standards becomes emphasis for the sake of the industrial world. 6. references [1] iso/iec 17025:2017 general requirements for the competence of testing and calibration laboratories. [2] t. sawa, "current state of rockwell hardness in the world", proceedings of imeko world congress, belfast, uk, 2018. [3] iso 6508-3:2015 metallic materials rockwell hardness test: calibration of reference blocks. [4] astm e18-20 standard test methods for rockwell hardness of metallic materials. [5] iso 6508-2:2015 metallic materials rockwell hardness test: verification and calibration of testing machines. http://www.imeko.org/ acta imeko contacts acta imeko issn: 2221-870x june 2020, volume 9, number 2 acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 0 journal contacts about the journal acta imeko is an e-journal reporting on the contributions on the state and progress of the science and technology of measurement. the articles are mainly based on presentations presented at imeko workshops, symposia and congresses. the journal is published by imeko, the international measurement confederation. the issn, the international identifier for serials, is 2221-870x. editor‐in‐chief dušan agrež, slovenia founding editor‐in‐chief paul p. l. regtien, netherlands associate editor dirk röske, germany editorial board section editors (vol. 7 9) leopoldo angrisani, italy filippo attivissimo, italy eulalia balestieri, italy eric benoit, france paolo carbone, italy lorenzo ciani, italy catalin damian, romania pasquale daponte, italy luca de vito, italy luigi ferrigno, italy edoardo fiorucci, italy alistair forbes, united kingdom helena geirinhas ramos, portugal sabrina grassini, italy fernando janeiro, portugal konrad jedrzejewski, poland andy knott, united kingdom francesco lamonaca, italy massimo lazzaroni, italy fabio leccese, italy rosario morello, italy michele norgia, italy pedro miguel pinto ramos, portugal nicola pompeo, italy sergio rapuano, italy dirk röske, germany alexandru salceanu, romania constantin sarmasanu, romania lorenzo scalise, italy emiliano schena, italy enrico silva, italy krzysztof stepien, poland marco tarabini, italy yvan baudoin, belgium marcantonio catelani, italy lorenzo ciani, italy egidio de benedeto, italy alessandro depari, italy alessandro germak, italy sabrina grassini, italy konrad jędrzejewski, poland min-seok kim, korea fabio leccese, italy vilmos palfi, hungary franco pavese, italy jeerasak pitakarnnop, thailand alexandru salceanu, romania emiliano sisinni, italy jan saliga, slovakia jorge c. torres-guzman, mexico ian veldman, south africa rugkanawan wongpithayadisai, thailand claudia zoani, italy mauro d’arco, italy oscar tamburis, italy about imeko the international measurement confederation, imeko, is an international federation of actually 42 national member organisations individually concerned with the advancement of measurement technology. its fundamental objectives are the promotion of international interchange of scientific and technical information in the field of measurement, and the enhancement of international co-operation among scientists and engineers from research and industry. addresses principal contact prof. dušan agrež university of ljubljana faculty of electrical engineering tržaška 25, 1000 ljubljana, slovenia e-mail: dusan.agrez@fe.uni-lj.si acta imeko alexandru salceanu technical university of iasi str. prof.dr.mangeron 67, 700050 iasi, romania e-mail: asalcean@tuiasi.ro support contact dr. dirk röske physikalisch-technische bundesanstalt (ptb) bundesallee 100, 38116 braunschweig, germany e-mail: dirk.roeske@ptb.de mailto:dusan.agrez@fe.uni-lj.si mailto:asalcean@tuiasi.ro mailto:dirk.roeske@ptb.de improvement of enea laser-induced fluorescence prototypes: an intercalibration between a hyperspectral and a multispectral scanning system acta imeko issn: 2221-870x march 2021, volume 10, number 1, 70 76 acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 70 improvement of enea laser-induced fluorescence prototypes: an intercalibration between a hyperspectral and a multispectral scanning system maria federica caso1, luisa caneve1, valeria spizzichino1 1 enea – italian national agency for new technologies, energy and sustainable economic development – via enrico fermi 45, 00044 frascati, rome, italy section: research paper keywords: laser-induced fluorescence (lif); lif prototypes; intercalibration; cultural heritage analysis; sarcophagus lif analysis citation: maria federica caso, luisa caneve, valeria spizzichino, improvement of enea laser-induced fluorescence prototypes: an intercalibration between a hyperspectral and a multispectral scanning system, acta imeko, vol. 10, no. 1, article 10, march 2021, identifier: imeko-acta-10 (2021)-01-10 editor: eulalia balestrieri, university of sannio, italy received april 22, 2020; in final form october 13, 2020; published march 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: adamo regional project within dtc (technological district for cultural heritage) lazio det.reg. g08622 corresponding author: maria federica caso, e-mail: mariafederica.caso@enea.it 1. introduction over the last decades, spectral imaging technology, which was previously restricted to astrophysics, remote sensing and military applications [1], has been employed in cultural heritage, thanks to the development of infrared reflectography in the study of paintings [2], [3]. increases in the number of spectral bands due to improved imaging spectrometers and technical advancements have led to the emergence of hyperspectral imaging [4]. these spectral imaging systems, which are able to collect both spectral and spatial information, are non-invasive, and, for this reason, are particularly suitable for the diagnostic and digital imaging of artworks [5]. laser-induced fluorescence (lif) is a spectroscopic technique that has been widely used over the last 40 years for many scientific activities, such as conducting combustion diagnostics [6], [7], probing energy transfer processes in molecules and atoms [8], identifying gastrointestinal tumours [9]-[11] and melanoma skin cancers [12] and characterising cardiovascular tissue [13], [14]. lif spectroscopy detects the uv-vis radiations emitted by luminescent molecules on the target surface caused by allowed electronic transitions stimulated by uv laser. the emission spectrum can provide a large amount of both qualitative and quantitative information about the chemistry and molecular structure of a sample [15]. like many other laser-based diagnostic instruments [16], lif spectroscopy has been widely used in the cultural heritage field [17]-[18]. most biological and organic materials common to the field of cultural heritage [20] absorb deep uv, corresponding to their first electronic state, and, through fluorescence and/or phosphorescence, emit at longer wavelengths, generally greater than 280 nm [21]. one of the advantages of fluorescence and abstract laser-induced fluorescence (lif) is a well-recognised spectroscopic technique used in cultural heritage for non-destructive surface chemical analysis. it is particularly suitable for in situ analysis of delicate items such as artworks because it does not require any sample preparation or contact and can be used at a distance in situations where only optical access is available. recently, enea has developed two lif prototypes with multispectral (forlab) and hyperspectral (lifart) scanning systems that each return different types of results, making them necessary for and dependent on each other. forlab’s motorised optics enable the rapid acquisition of fluorescence maps and images of large surfaces in specific spectral wavelengths, while lifart returns complete fluorescence spectra, providing the complete spectral information of an object. in this paper, the intercalibration of the two systems is reported together with a data analysis of the calibration samples and a software that automatically corrects imaging data, taking forlab’s filter passband and optical efficiencies into account in order to make these two configurations as easy to compare as possible. the new correcting algorithm is also tested on lif measurements carried out on an egyptian casket and sarcophagus, obtaining higher quality fluorescence images. mailto:mariafederica.caso@enea.it acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 71 phosphorescence is that their spectra are generally independent of the excitation wavelength [22]-[23]. this technique is nondestructive and non-invasive, does not cause modifications or degradations of the chemical structure of targets and does not need any sample preparation. for this reason, it is particularly recommended for fragile and delicate artworks [25]. furthermore, lif instruments do not require contact and can be used at a distance or in situations in which only optical access is available while maintaining their high sensitivity. transportable instruments can be easily designed for in situ analysis, providing results in real time [26]. recently, enea has developed lif prototypes with hyperspectral (lifart) [27] and multispectral (forlab) [28] scanning systems that are capable of both fluorescence and reflectance imaging. they return different types of spatial and spectral information, making them necessary for and dependent on each other. in particular, forlab was designed to permit the rapid acquisition of fluorescence maps of large surfaces (up to 100 m2) in specific spectral wavelengths, allowing for faster acquisitions and providing results in real time. furthermore, a custom software was developed for this system for the automatic recognition of specific classes of materials. conversely, lifart returns complete fluorescence spectra, providing the complete spectral information of an object. the two lif systems described here are innovative prototypes, specifically designed for remote (entailing a distance up to some tens of meters) cultural heritage investigations. they possess complementary features for the fast and accurate characterisation of both large and small artworks thanks to their different configurations. these differences, however, have hampered the combined use of the two instruments, which has until now been quite laborious. to overcome this point, this work investigates the intercalibration of the two instruments, taking into account forlab’s filter passband and optical efficiencies. this intercalibration makes the data registered by the two instruments as easy to compare as possible. the numerical elaboration so obtained represents a significant step forward, creating an innovative tool for the rapid early diagnosis of large surfaces. in fact, the intercalibration described here makes it possible to combine the two enea prototypes, which dramatically improves the presentation and interpretation of results, thanks to the overlap and comparison of data and the expected increased readability of spectra. 2. instrumentation this paper focuses on two specific instruments developed by enea diagnostic and metrology laboratory: the hyperspectral lif scanning system (lifart) and the multispectral lif scanning system (forlab), whose detailed technical features have already been described [27], [28]. briefly, lifart can be used at a distance of up to 10 m from the target via a diode-pumped solid state laser emitting at 266 nm in the uv. a coaxial optical design is used to transmit the exciting radiation and receive the fluorescence signals from the investigated target. point scanning is accomplished by manipulating the last mirror using two highly accurate rotation servo motors. the practical scanning range is limited by the mounting assembly to approximately ±45 ° in both the horizontal and vertical axes. the fluorescence radiation is focused into a fibre-optic cable linked to a compact qe-pro spectrometer from ocean optics, allowing for hyperspectral imaging in the range of 250–900 nm with a 2.5 nm bandwidth. forlab is a multispectral system that enables the rapid collection of large images on several different spectra. the system is based on the use of motorised laser delivery optics to scan the whole surface to be analysed with a uv laser (excimer krf laser at 248 nm). the collecting subsystem, which is connected to an iccd camera (andor istar) with a large field of view, is fixed. the laser source is characterised by a high pulse repetition rate (up to 500 hz), which lowers the acquisition time. a filter wheel is mounted in front of the iccd in order to detect fluorescence images at 8 preselected spectral bands (table 1. features of forlab filters.table 1). both systems are able to acquire fluorescence and reflectance spectra with the laser switched on or off, respectively. fluorescence data can be processed to obtain fluorescence and false colour images for material mapping. active reflectance measurements are based on elastic backscattering of the incident light collected from the analysed surface in daylight or under a specific white light source. figure 1. fluorescence spectra of reference solutions. figure 2. reference samples. left: marble tile. right: earthenware tile. table 1. features of forlab filters. forlab filter wavelength transmission band 290 nm tavg > 75% 277.5 302.5 nm 315 nm tavg > 75% 307.5 322.5 nm 340 nm tavg > 75% 327 353 nm 376 nm tavg > 95% 366 386 nm 415 nm tavg > 90% 410 420 nm 445 nm tavg > 93% 435 455 nm 480 nm tavg > 92% 471.5 – 488.5 nm 530 nm tavg > 93% 521 539 nm acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 72 3. materials several different materials, including laboratory samples and real artworks, were used to calibrate and test the instruments in order to evaluate the correction algorithm. for the intercalibration of the two instruments, materials characterised by known fluorescence spectra covering the entire range between 270 and 700 nm were selected. in particular, solutions of phenylalanine (0.1 m), 4’, 6-diamidino-2-phenylindole (dapi, 5 mg/ml) and a mix of amino acids (tyrosine 0.017 m, tryptophan 0.018 m and phenylalanine 0.1 m), which are also responsible for protein fluorescence in organic media in paintings [29], were prepared, and their fluorescence spectra excited at 266 nm are presented in figure 1. additionally, a reference model (figure 2, left) simulating some real case studies was selected to mimic a sample that might be encountered during a field campaign. the sample was a carrara marble tile divided into six sections: s1) marble; s2) carnauba wax (vegetable wax); s3) phase wax (industrial wax); s4) beeswax; s5) beeswax and carnauba wax mixture; s6) beeswax and carnauba wax mixed with sienna pigment. these materials were chosen because their fluorescence spectra cover the whole wavelength range of interest. moreover, these materials have often been used as coatings in sculpture and architecture since antiquity, and sienna and ochre were frequently applied to give a chiaroscuro effect to artworks. in figure 3, the fluorescence spectra of the six sections obtained with an excitation at 266 nm are presented. another sample prepared in the laboratory, an earthenware tile (figure 2, right), was used to test the obtained intercalibration. using buon fresco or mezzo fresco techniques, this sample was treated with a lime–pozzolan mortar and a pigment and then covered with a modern surface coating. in particular, the four sections of the tile were composed of the following: a) white lead and rhodorsil rc-80; b) vine black and rhodorsil rc-80; c) green earth and paraloid b72; d) yellow ochre and rhodorsil rc-80. the calibration was also tested on a real ancient artwork. in particular, a previous study [30] on ancient egyptian artworks was chosen to validate the calculous for a third material, wood. the analysis of the fluorescence spectra of the casket and sarcophagus of peftjauyauyaset (xxvi din. -vii-vi sec. a.c.) from the archaeological museum of milan (figure 4) was chosen because of the discontinuity of the artwork’s surface. the spectra figure 3. fluorescence spectra of the six sections of marble tile. figure 4. egyptian casket and sarcophagus. figure 5. measurements of reference solutions with forlab (top) and lifart (bottom) equipped with both lasers. orange: krf at 248 nm; blue: nd:yag at 266 nm. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 73 successfully revealed the presence of non-homogeneous materials applied in previous restoration actions. moreover, the fully decorated surfaces provide an amplified chiaroscuro contrast that improves the image quality when the calibration is applied to the forlab results. 4. results and discussion 4.1. system intercalibration the reference solutions and the marble tile described in section 3 were utilised for the intercalibration. preliminary experiments involved measurements of the reference solutions only with both lifart (whole spectra) and forlab (8 filters at 290, 315, 340, 376, 415, 445, 480 and 530 nm) equipped with either krf emitting at 248 nm or nd:yag emitting at 266 nm in order to assess the different contributions of the laser sources and optical systems to the acquisition of the signal. in the forlab spectra, reported on the top half of figure 5, with the exclusion of the signal at 445 nm (probably due to a filter malfunctioning), the spectra present a fairly good correspondence when distinguished from the exciting source. this point will not be considered in further elaboration. phenylalanine shows significant discrepancies in the region above 350 nm because of very low emissions in that region. therefore, in this case, we only used signals in the region below 350 nm for the calibration. a similarly good spectra correspondence was observed in the lifart spectra shown in the bottom half of figure 5. major discrepancies in the spectra of phenylalanine were attributed to the high proximity of the laser source wavelength to that of the absorption of phenylalanine. fluorescence analyses for the intercalibration were carried out with both lifart and forlab on drops of the reference solutions placed on an aluminium surface as well as on a small square in the centre of the sections of the marble tile, and lifart and forlab spectral data registered in this area were mediated. the recorded data were processed to obtain intercalibration values that were as independent of the experimental measurement conditions as possible. first, the transmission bands of the forlab filters (reported in table 1) had to be considered. after this correction, both instruments’ data were normalised with the integral across the whole range (the sum of the intensity values corresponding to the 8 filters for forlab). second, to compare forlab and lifart data, only spectral data from lifart mediated within the passband of the filters used in forlab were considered. sections 1, 2 and 4 of the marble tile were preferred due to the high reproducibility of their measurements and their higher fluorescence intensity with respect to the others. the forlab and lifart fluorescence data for the reference solutions and sections s1, 2 and 4 of the tile were divided, and the resulting quotients, called calibration ratios, are presented in table 2. the blue values in table 2 were considered reliable because they had higher signal to noise ratios, as verifiable in figure 1 and figure 3. therefore, these values were used to calculate the final ratios. unfortunately, just one value was considered for filter 290 because only phenylalanine presented a peak in the 270–340 nm wavelength range. a confirmation of this assessment is provided in figure 6. the blue points indicate the highlighted blue values from table 2, and the red points indicate values that were not used in the calibration of the ratio calculous due to their distance from the blue points or their incoherence. the final calibration ratios, which were utilised for the intercalibration, were obtained by mediating the highlighted blue values. they are presented with their corresponding standard deviations in table 3. table 2. calibration ratios obtained from the reference solutions and sections s1, 2 and 4 of the tile. only blue values were considered in calculations. forlab filter wavelengths phenylalanine amino acids dapi s1 s2 s4 290 nm 9.32 75.77 2459.86 71.79 150.25 39.52 315 nm 7.17 28.11 2664.24 49.09 165.13 24.51 340 nm 16.33 15.58 3709.70 19.41 117.02 36.74 376 nm 53.38 11.54 2013.75 12.88 62.79 22.12 415 nm 50.10 8.96 410.70 16.59 29.05 16.75 445 nm 53.38 21.88 299.98 22.86 21.84 21.71 480 nm 27.36 31.88 31.45 6.90 10.67 7.62 530 nm 20.83 63.52 13.24 9.30 15.79 6.35 figure 6. calibration values: considered values in blue, excluded values in red. table 3. calibration ratios for every forlab filter. forlab filter wavelengths calibration ratios standard deviation 290 nm 9.32 / 315 nm 19.93 11.2 340 nm 22.01 10.0 376 nm 15.51 5.8 415 nm 17.84 8.3 445 nm 22.07 0.5 480 nm 19.31 12.1 530 nm 11.17 4.2 acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 74 4.2. application of calibration ratios to a laboratory sample the calibration ratios were applied to the 4 sections of the earthenware tile described before. as in the previous tile, a small square in the centre of every section was selected, and the corresponding lifart and forlab spectral data of this area were averaged. the forlab data before and after calibration were plotted with the lifart spectra to highlight the fairly good correspondence of lifart spectra and calibrated forlab values (figure 7). the calibrated values, although not perfectly matched with lifart ones, presented the same shapes and trends in the peaks and bands. 4.3. application of calibration ratios to a real case the calibration ratios were applied to a previous study [30] of the egyptian casket and sarcophagus described in section 3. the processed data confirmed the discontinuity of the surface materials and the identification of the modern coatings applied in an undocumented restoration action. forlab data provided evidence of some retouches not visible to the naked eye, and the application of lifart on these spots made it possible to identify zinc oxide, acrylic paint and more. very interesting results were obtained by the analysis of the external part of the casket and the interior part of the upper valve of the sarcophagus (figure 8), where the presence of canvas under the paints and acrylic coating was revealed on the legs of the goddess nut. forlab images at preselected wavelengths can be employed for the definition of signal ratios and the development of algorithms aimed at the identification of different substances. the application of calibration ratios to forlab fluorescence ratios made it possible not only to obtain more luminosity, contrast and definition in the images (figure 9) but also to emphasise the different treatments through the correction of the highlighted areas, making the data more reliable. the discontinuity of the surface materials was also evident on the long and short sides of figure 7. the earthenware tile fluorescence spectra. black: lifart fluorescence spectra; blue: forlab values calibrated; red: forlab values not calibrated. a–d refer to the four areas described in the materials section. figure 8. upper valve of the sarcophagus, long and short side of the casket. figure 9. forlab fluorescence ratios of the interior part of the upper valve of the sarcophagus. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 75 the casket (figure 10 and figure 11). in fact, the forlab processed data, confirmed by lifart analysis, showed the presence of acrylic consolidation material applied in several restorations over time. the calibration ratios considerably improved the quality of the forlab images (figure 11), making some fissures and several restorations stand out in the casket, particularly near the winged goddess nefti. furthermore, a more precise localisation of materials and retouches was obtained by applying the calibration ratios to all of the forlab measurements. 5. conclusions lif spectroscopic technique is used widely in the field of cultural heritage because it allows easy access to valuable information without damaging the specimens. enea diagnostic and metrology laboratory developed hyperspectral and a multispectral lif scanning systems, whose intercalibration provided a correction tool for imaging data. some reference solutions and a carrara marble tile treated with different coatings were employed to develop calibration ratios for the forlab system. these ratios were then applied to the fluorescence spectra of an earthenware tile covered with pigments and modern protective paints. the obtained results, although still imperfect, proved very encouraging and provided higher quality forlab images. moreover, the availability of an intercalibration protocol is expected to provide access to extrapolated spectra from forlab data. acknowledgement the authors would like to thank adamo regional project in the frame of dtc (technological district for cultural heritage) lazio det.reg. g08622 and e. antonelli and the consorzio croma laboratory for 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[28] v. spizzichino, l. caneve, f. colao, stand-off device for plastic debris recognition in post-blast scenarios, challenges 7(2) (2016) 23, 12 pp. doi: 10.3390/challe7020023 [29] a. nevin, s. cather, d. anglos, c. fotakis, laser-induced fluorescence analysis of protein-based binding media, in: lasers in the conservation of artworks, springer proceedings in physics 116. j. nimrichter, w. kautek, m. schreiner (editors). springer, cham, switzerland, 2007, isbn 978-3-540-72130-7, pp. 399-406. doi: 10.1007/978-3-540-72310-7_47 [30] l. caneve, v. spizzichino, e. antonelli, l. bertani, study of ancient egyptian artefacts by non-destructive laser based techniques, proc. of ieee international conference on metrology for archaeology and cultural heritage, cassino, italy, 22-24 oct 2018, pp. 379-383. doi: 10.1109/metroarchaeo43810.2018.9089811 https://doi.org/10.1002/lsm.10125 https://doi.org/10.1016/s1386-1425(97)00106-6 https://doi.org/10.1002/lsm.1900100305 https://doi.org/10.1081/asr-200054370 https://doi.org/10.2307/1506145 https://doi.org/10.4028/www.scientific.net/jnanor.8.47 https://doi.org/10.1016/s0003-2670(99)00827-2 https://doi.org/10.1016/j.culher.2016.09.005 https://doi.org/10.1021/bi00308a026 https://doi.org/10.1081/asr-100101229 https://doi.org/10.1063/1.3184102 https://doi.org/10.3390/challe7020023 https://doi.org/10.1007/978-3-540-72310-7_47 https://doi.org/10.1109/metroarchaeo43810.2018.9089811 self-calibration of the 1 mn deadweight force standard machine at inrim acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 100 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 100 105 self-calibration of the 1 mn deadweight force standard machine at inrim a. prato1, f. mazzoleni1, a. facello1, a. germak1 1 inrim – istituto nazionale di ricerca metrologica, torino, italy, a.prato@inrim.it abstract: the inrim 1 mn deadweight force standard machine (dfsm) was installed in 1995. it adopts a binary sequence of ten weights whose combinations generate forces up to 1 mn. the advantage of this system lies in the self-calibration of its weights. the procedure is based on the comparison between two forces generated by a single weight and by a group of smaller weights, nominally equal. after 25 years, a verification of the dfsm was performed. results are within the declared cmc limits, i.e. a relative expanded uncertainty of 2 × 10-5. keywords: deadweight force standard machine; self-calibration; uncertainty 1. introduction back in 1995, the advancement of technology and available resources allowed to design and install the 1 mn dfsm at inrim [1, 2]. the machine is able to generate known forces and is generally used as a reference for the calibration or verification of force transducers and load cells. the dfsm has a declared cmc with a relative expanded uncertainty of 2 × 10-5 [3]. twenty-five years after the installation, despite several positive international comparisons, it was necessary to carry out a verification of the machine. for this purpose, a self-calibration method was developed. measurements were performed between november 2018 and april 2019. this paper deals with the description of the self-calibration method and the analysis of experimental results. 2. description of the 1 mn deadweight force machine the 1 mn dfsm was designed to obtain a reference standard capable of keeping its metrological characteristics unchanged. it consists of three macro-components: a main frame that supports the entire structure, a loading frame, and a series of weights acting in the local gravitational field. the main frame consists of three columns anchored to the load-bearing structure arranged at 120° on a diameter of about 6 m (figure 1). figure 1: the 1 mn deadweight force standard machine the 1 mn dfsm adopts a binary sequence of ten weights (two 10 kn weights, three weights of 20 kn, 40 kn, and 80 kn, four weights of 160 kn, and one weight of 200 kn), consisting of stainless steel discs. each of them can be applied to the loading frame independently to the others. in this way, it is possible to generate a large number of force values from 10 kn up to a maximum capacity of 1 mn with steps of 10 kn, and to perform a self-calibration of the weights to check the stability of the standard without dismantling the whole system [4]. the selection of the weights is performed by an electro-pneumatic system, software controlled, and supported by an electric motor able to generate a constant load on the force transducer during weights replacement operations. the load is kept constant by a feedback system exploiting the deformation elasticity of the loading frame. 3. self-calibration procedure the self-calibration method is based on the comparison between two nominally equal forces alternatively generated by a single weight and by an equivalent group of smaller weights [5]. the reference force transducers adopted for the selfcalibration were a top-transfer hbm (z30a and z4a), with hbm dmp40 amplifier. the initial reference force was provided by the smallest weight, associated with a force of 10 kn (m10/1), which was previously calibrated against a reference mass http://www.imeko.org/ mailto:a.prato@inrim.it acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 101 standard before the installation of the machine. another advantage of this method is that it can consider any influences due to the behaviour of the whole structure under load since it directly compares the force vectors applied on the force transducer and not the force generated in centre of mass as in the standard method for calibrated force standard machines. for each force value (10 kn, 20 kn, 40 kn, 80 kn, 160 kn, 200 kn), two identical forces were alternately generated: one generated by the single nominal weight (measure b), the other by the sum of smaller weights of equivalent total force (measure a). the forces generated for each weight comparison are shown in table 1. table 1: weights used for the comparison weight single weight force / kn (measure b) combination of smaller weights with equivalent force (measure a) m10/1 10 m10/2 10 m10/1 m20 20 m10/1 + m10/2 m40 40 m10/1 + m10/2 + m20 m80 80 m10/1 + m10/2 + m20 + m40 m160/1 160 m160/4 m160/2 160 m160/4 m160/3 160 m160/4 m160/4 160 m10/1 + m10/2 + m20 + m40 + m80 m200 200 m160/4 + m40 a comparison scheme was then adopted to identify an abab…ba type sequence. this sequence was chosen since the most significant causes of disturbance are due to weights replacement operations and to creep effects of the force transducers. for each pair of equal forces, j = 10 measurement runs were performed. during each j-th run, i = 20 force values were acquired in a time interval of 20 s. 4. analysis of relative deviations once measurements were performed, data analysis was carried out for each force value. first, the average, corrected by subtracting the zero value, of the 𝑖-th temporal acquisition values of the 𝑗-th run, 𝐹𝑎,𝑖𝑗 and 𝐹𝑏,𝑖𝑗 , related to the two forces alternately generated and nominally equal ( 𝐹𝑎,𝑖𝑗 corresponds to the force generated by the reference 10 kn weight, m10/1, or by the sum of the smaller weights for forces greater than or equal to 20 kn, and 𝐹𝑏,𝑖𝑗 corresponds to the force generated by the single weight), was performed according to equation (1). the average of the temporal mean values 𝐹𝑎,𝑗̅̅ ̅̅ and 𝐹𝑏,𝑗̅̅̅̅̅ of the 10 runs is given by equation (2). 𝐹𝑎,𝑗̅̅ ̅̅ = ∑ 𝐹𝑎,𝑖𝑗 20 20 𝑖=1 𝐹𝑏,𝑗̅̅̅̅̅ = ∑ 𝐹𝑏,𝑖𝑗 20 20 𝑖=1 (1) �̅� = ∑ ( 𝐹𝑎,𝑗̅̅ ̅̅ + 𝐹𝑏,𝑗̅̅̅̅̅ 2 ) 10 10 𝑗=1 = ∑ 𝐹𝑎,𝑗̅̅ ̅̅ + 𝐹𝑏,𝑗̅̅̅̅̅ 20 10 𝑗=1 (2) to minimize creep effect, although the short time elapsed between two replacements (about 100 s), a processing scheme, based on the differences between a single value and the mean value between the previous and next measurement, was used. according to this scheme, the absolute deviations 𝑑𝑗, equation (3) and relative deviations 𝛿𝑗 , equation (4), were calculated for each force value 𝐹. 𝑑𝑎,𝑗 = 𝐹𝑎,𝑗̅̅ ̅̅ + 𝐹𝑎,𝑗+1̅̅ ̅̅ ̅̅ ̅̅ 2 − 𝐹𝑏,𝑗̅̅̅̅̅ 𝑑𝑏,𝑗 = 𝐹𝑎,𝑗̅̅ ̅̅ − 𝐹𝑏,𝑗̅̅̅̅̅ + 𝐹𝑏,𝑗+1̅̅ ̅̅ ̅̅ ̅̅ 2 (3) 𝛿𝑎,𝑗 = 𝑑𝑎,𝑗 �̅� 𝛿𝑏,𝑗 = 𝑑𝑏,𝑗 �̅� (4) in this way, it was possible to calculate the mean relative deviation 𝛿̅ according to equation (5). 𝛿̅ = ∑ 𝑑𝑎,𝑗 + 𝑑𝑏,𝑗 20 10 𝑗=1 �̅� = ∑ 𝛿𝑎,𝑗 + 𝛿𝑏,𝑗 20 10 𝑗=1 (5) to check the actual force values generated by the individual weights, it was necessary to add the mean relative deviations 𝛿̅ of the smaller weights used. in this way, the total mean relative deviations �̅� can be obtained. assuming that the force 𝐹m10/1 generated by the reference m10/1 weight is exactly 10 kn with relative expanded uncertainty of 2.82 × 10-6 [6], the mean relative deviation of the reference 10 kn weight (m10/1), 𝛿m̅10/1, is equal to zero, with an associated uncertainty equal to the uncertainty of the reference force 𝑈(𝛿m̅10/1) = 𝑈(𝐹m10/1) 𝐹m10/1⁄ =2.82 × 10 -6. in this way, equations (6) and (7) are obtained. http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 102 𝛿m̅10/1 = �̅�m10/1 = 𝐹m10/1 − 10 10 = 0 (6) 𝐹m10/1 10 = 1 + 𝛿m̅10/1 (7) as a consequence, the mean relative deviation of m10/2 weight, 𝛿m̅10/2, can be written as 𝛿m̅10/2 = 𝐹m10/1 − 𝐹m10/2 10 = = 1 + 𝛿m̅10/1 − 𝐹m10/2 10 (8) from which, the total relative deviation of weight m10/2, �̅�m10/2, is obtained: �̅�m10/2 = 𝐹m10/2 − 10 10 = 𝛿m̅10/1 − 𝛿m̅10/2 (9) by operating in the same way for weight m20, the following relations are obtained: 𝛿m̅20 = 𝐹m10/1 + 𝐹m10/2 − 𝐹m20 20 = = 1 2 𝐹m10/1 10 + 1 2 𝐹m10/2 10 − 𝐹m20 20 = = 1 + 𝛿m̅10/1 − 1 2 𝛿m̅10/2 − 𝐹m20 20 (10) �̅�m20 = 𝐹m20 − 20 20 = = 𝛿m̅10/1 − 1 2 𝛿m̅10/2 − 𝛿m̅20 (11) iterating the same procedure for all other weights, the following equations are obtained: 𝛿m̅40 = 𝐹m10/1 + 𝐹m10/2 + 𝐹m20 − 𝐹m40 40 (12) �̅�m40 = 𝐹m40 − 40 40 = = 𝛿m̅10/1 − 1 2 𝛿m̅10/2 − 1 2 𝛿m̅20 − 𝛿m̅40 (13) 𝛿m̅80 = = 𝐹m10/1 + 𝐹m10/2 + 𝐹m20 + 𝐹m40 − 𝐹m80 80 (14) �̅�m80 = 𝐹m80 − 80 80 = = 𝛿m̅10/1 − 1 2 𝛿m̅10 2 − 1 2 𝛿m̅20 − 1 2 𝛿m̅40 − −𝛿m̅80 (15) 𝛿m̅160/4 = 𝐹m10/1 + 𝐹m10/2 + 𝐹m20 160 + + 𝐹m40 + 𝐹m80 − 𝐹m160/4 160 (16) �̅�m160/4 = 𝐹m160/4 − 160 160 = = 𝛿m̅10/1 − 1 2 𝛿m̅10/2 − 1 2 𝛿m̅20 − 1 2 𝛿m̅40 − 1 2 𝛿m̅80 − 𝛿m̅160/4 (17) 𝛿m̅160/1 = 𝐹m160/1 − 𝐹m160/4 160 (18) �̅�m160/1 = 𝐹m160/1 − 160 160 = = 𝛿m̅10/1 − 1 2 𝛿m̅10/2 − 1 2 𝛿m̅20 − 1 2 𝛿m̅40 − − 1 2 𝛿m̅80 − 𝛿m̅160/4 + 𝛿m̅160/1 (19) 𝛿m̅160/2 = 𝐹m160/2 − 𝐹m160/4 160 (20) �̅�m160/2 = 𝐹m160/2 − 160 160 = = 𝛿m̅10/1 − 1 2 𝛿m̅10/2 − 1 2 𝛿m̅20 − 1 2 𝛿m̅40 − − 1 2 𝛿m̅80 − 𝛿m̅160/4 + 𝛿m̅160/2 (21) 𝛿m̅160/3 = 𝐹m160/3 − 𝐹m160/4 160 (22) �̅�m160/3 = 𝐹m160/3 − 160 160 = = 𝛿m̅10/1 − 1 2 𝛿m̅10/2 − 1 2 𝛿m̅20 − 1 2 𝛿m̅40 − − 1 2 𝛿m̅80 − 𝛿m̅160/4 + 𝛿m̅160/3 (23) 𝛿m̅200 = 𝐹m160/4 + 𝐹m40 − 𝐹m200 200 (24) �̅�m200 = 𝐹m200 − 200 200 = = 𝛿m̅10/1 − 1 2 𝛿m̅10/2 − 1 2 𝛿m̅20 − 3 5 𝛿m̅40 − − 2 5 𝛿m̅80 − 4 5 𝛿m̅160/4 − 𝛿m̅200 (25) http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 103 5. uncertainty assessment the uncertainty analysis was carried out according to gum-jcgm 100:2008 [7]. for each force value, the uncertainty analysis of the total mean relative deviation �̅� was carried out in two consecutive steps. in the first, the uncertainty associated to the mean relative deviations 𝛿̅ of the single weight comparisons was assessed, taking into account the maximum uncertainty among the time series of twenty measurements, corresponding to the different weights substitutions used for the evaluation of the relative deviations 𝛿𝑗 , and the reproducibility uncertainty contribution due to the ten measurement runs. in the second, by applying the law of propagation of errors, the expanded uncertainty, at a confidence level of 95 %, of the total mean relative deviations �̅� was evaluated. 5.1. uncertainty of mean relative deviations first, among the 𝑗-th relative deviations 𝛿𝑗, the one with maximum uncertainty due to repeatability of each force measurement of the three successive time series (𝐹𝑎,𝑗̅̅ ̅̅ , 𝐹𝑏,𝑗̅̅̅̅̅, 𝐹𝑎,𝑗+1̅̅ ̅̅ ̅̅ ̅̅ or 𝐹𝑏,𝑗̅̅̅̅̅, 𝐹𝑎,𝑗̅̅ ̅̅ , 𝐹𝑏,𝑗+1̅̅ ̅̅ ̅̅ ̅̅ ) was evaluated. once the 𝑗 -th relative deviation 𝛿𝑗with the maximum dispersione was identified, the uncertainty contribution due to the resolution of the hbm control unit was added for each force value. no uncertainty was associated to the value �̅�, since it was used only to calculate the relative difference from absolute measurements. by way of example, the detailed uncertainty budget for 𝛿𝑗 from the comparison between the 10 kn weights (m10/1 and m10/2) is shown in table 2. force values are expressed in mv/v. table 2: uncertainty budget of the j-th relative deviation 𝛿𝑗 with maximum dispersion from the comparison between the 10 kn weights (𝛿m̅10/2) variable xk u²(xk) ck uk²(ax) symbol value note 𝐹𝑎,𝑗̅̅ ̅̅ 0.999 494 res. 1.7e-14 0.5 4.2e-15 repeat. 4.9e-14 0.5 1.2e-14 𝐹𝑏,𝑗̅̅̅̅̅ 0.999 500 res. 1.7e-14 1.0 1.7e-14 repeat. 1.2e-12 1.0 1.2e-12 𝐹𝑎,𝑗+1̅̅ ̅̅ ̅̅ ̅ 0.999 499 res. 1.7e-14 0.5 4.2e-15 repeat. 1.6e-13 0.5 3.9e-14 �̅� 0.999 490 𝛿𝑎,𝑗 -3.33e-06 variance, u²(𝛿𝑎,𝑗) 1.3e-12 standard uncertainty, u(𝛿𝑎,𝑗) 1.1e-06 subsequently, the evaluation of the expanded uncertainty associated with the mean relative deviation u(𝛿̅) was performed, as shown in table 3. the variance of the relative deviation, u2( 𝛿𝑗 ), obtained from table 2 (in bold), was used as variance associated with the maximum standard deviation. as uncertainty contribution due to reproducibility, the standard deviation of the ten measurement runs 𝛿𝑎𝑗and 𝛿𝑏𝑗 was considered. the expanded uncertainty 𝑈(𝛿̅) of the mean relative deviations was calculated at a confidence level of 95 %, i.e. k = 2. for each force value, the same procedure was repeated by assessing reproducibility and maximum standard deviation of the temporal series. results of each weight comparison are shown in section 6. table 3: uncertainty budget of the mean relative deviation 𝛿̅ from the comparison between the 10 kn weights variable xk u²(xk) ck uk²(ax) symbol value note 𝛿̅ -2.49e-06 reprod. 2.0e-13 1.0 2.0e-13 max. 1.3e-12 1.0 1.3e-12 st. dev. 𝛿̅ -2.49e-06 variance, u²(𝛿̅) 1.5e-12 standard uncertainty, u(𝛿̅) 1.2e-06 5.2. uncertainty of total mean relative deviations by applying the law of propagation of errors to eqs. (6), (9), (11), (13), (15), (17), (19), (21), (23), and (25), using as input the expanded uncertainties associated with the mean relative deviations 𝑈(𝛿̅) evaluated according to section 5.1, the expanded uncertainties of the total relative deviations 𝑈(�̅�) are obtained. by way of example, the uncertainties for the m10/2, m20, and m40 are reported in equations (26) (28). 𝑈(�̅�m10/2) = = 2√( 𝑈(𝛿m̅10/1) 2 ) 2 + ( 𝑈(𝛿m̅10/2) 2 ) 2 (26) 𝑈(�̅�m20) = = 2 √ ( 𝑈(𝛿m̅10/1) 2 ) 2 + ( 1 2 ) 2 ( 𝑈(𝛿m̅10/2) 2 ) 2 + +( 𝑈(𝛿m̅20) 2 ) 2 (27) 𝑈(�̅�m40) = = 2 √ ( 𝑈(𝛿m̅10/1) 2 ) 2 + ( 1 2 ) 2 ( 𝑈(𝛿m̅10/2) 2 ) 2 + +( 1 2 ) 2 ( 𝑈(𝛿m̅20) 2 ) 2 + ( 𝑈(𝛿m̅40) 2 ) 2 (28) http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 104 6. experimental results 6.1. mean relative deviations �̅� overall, nine mean relative deviations 𝛿̅, were calculated, each referred to a particular force value (or weight comparison), according to table 1. summary of experimental results is shown in table 4. table 4: mean relative deviations weight f / kn �̅� / u(�̅�) / m10/1 10 0.00e+00 2.82e-06 m10/2 10 -2.49e-06 2.55e-06 m20 20 -1.23e-05 5.38e-06 m40 40 -4.46e-06 6.54e-07 m80 80 8.02e-06 3.92e-06 m160/1 160 -2.13e-06 8.09e-07 m160/2 160 -3.31e-07 3.86e-07 m160/3 160 -3.51e-06 6.15e-07 m160/4 160 -3.19e-06 6.36e-07 m200 200 2.03e-06 2.57e-07 6.2. total mean relative deviations �̅� using data of table 4 and equations (6) (28), the total mean relative deviations �̅� of all weights are obtained with the associated expanded uncertainties. results show values lower than the limits declared in cmc, i.e. within an expanded uncertainty of 2 × 10-5, as reported in table 5 and in figure 2. table 5: total mean relative deviations of the weights weight f / kn �̅� / u(�̅�) / m10/1 10 0.00e+00 2.82e-06 m10/2 10 2.49e-06 3.80e-06 m20 20 1.35e-05 6.21e-06 m40 40 1.18e-05 4.15e-06 m80 80 1.59e-06 5.68e-06 m160/1 160 6.65e-06 4.67e-06 m160/2 160 8.45e-06 4.62e-06 m160/3 160 5.27e-06 4.64e-06 m160/4 160 8.78e-06 4.60e-06 m200 200 7.36e-06 4.44e-06 6.3. total relative deviations �̅� of typical weight combinations used for the calibration of force transducers finally, the total mean relative deviations of different force combinations were assessed. in fact, during ordinary calibration procedures of force transducers, according to uni en iso 376:2011 [8], the 1 mn dfsm adopts predefined series of weights able to generate the required different force values. for this purpose, the total absolute deviations �̅� of each weight, previously shown, were combined in order to obtain the total relative deviations for each force generated by any combination of weights, with the associated expanded uncertainty calculated using the law of propagation of errors as in equations (26) (28). also in this case, results fall within the limits of the declared cmcs [3] as shown in the graph of figure 3. furthermore, 500 kn and 1 mn total relative deviations, which are around 7 ppm – 8 ppm, confirm results of force key comparison ccm.f-k3 [9]. figure 2: total mean relative deviations of the 1 mn dfsm weights http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 105 figure 3: total mean relative deviations of typical forces generated during ordinary calibration procedures 7. summary the verification of the inrim 1 mn deadweight force standard machine was performed with a selfcalibration procedure. the self-calibration method is based on the comparison between two nominally equal forces alternatively generated by a single weight and by an equivalent group of smaller weights. experimental measurements and the uncertainty budget assessment were performed. results are within a relative expanded uncertainty of 2 × 10-5, i.e. within the limits declared in the cmcs. in the future, the possibility to correct these deviations during ordinary calibration procedures will be investigated. 8. references [1] g. barbato, a. bray, f. franceschini, r. levi, f. trevissoi, “the new 1 mn dead-weight force standard machine: description of the installation procedure followed at the istituto di metrologia g. colonnetti”, in proc. of xiii imeko world congress, pp. 183-188, torino, italy, 1994. [2] a. germak, l. marzola, g. pratola, f. trevissoi, g. barbato, “rapporto tecnico interno r417 imgc”, technical report, torino, italy, 1995. [3] calibration and measurement capabilities (cmcs), bipm. online [accessed 20200824]: https://www.bipm.org/kcdb/ [4] g. barbato, a. germak, “rapporto tecnico r411”, technical report, inrim, torino, italy, 1995. [5] c. ferrero, c. marinari, e. martino, “force metrology in italy: the new 1 mn deadweight force standard”, in proc. of imeko 16th tc3 conference, taejon, korea, 14-18 september 1998. [6] g. barbato, f. trevissoi, p. iudici, “pesata delle masse di riferimento della macchina campione di forza da 1 mn”, technical report, inrim, torino, italy, 1995. [7] jcgm 100:2008, evaluation of measurement data guide to the expression of uncertainty in measurement, online [accessed 20200825]: https://www.iso.org/sites/jcgm/gum/jcgm100/c 045315e-html/c045315e.html?csnumber=50461 [8] iso 376:2011metallic materials calibration of force-proving instruments used for the verification of uniaxial testing machines, iso, 2011. [9] r. kumme, p. averlant, t. bartel, a. germak, a. knott, j. man, n. medina, a. ostrivnoy, y. park, d. röske, r. seifarth, m. wozniak, k. ueda, z. zhimin, “final report on the force key comparison ccm.f-k3”, metrologia, vol. 56, 07001, 2019. http://www.imeko.org/ https://www.bipm.org/kcdb/ https://www.iso.org/sites/jcgm/gum/jcgm100/c045315e-html/c045315e.html?csnumber=50461 https://www.iso.org/sites/jcgm/gum/jcgm100/c045315e-html/c045315e.html?csnumber=50461 metrological researches of national primary standard machine for leeb hardness scales acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 247 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 247 249 metrological researches of national primary standard machine for leeb hardness scales a. aslanyan1,2, e. aslanyan1, s. gavrilkin1, a. shchipunov1 1 vniiftri, mendeleevo, moscow region, russian federation, 2 andrey_aslanyan@vniiftri.ru abstract: the paper describes the study of metrological characteristics of the national primary standard machine for leeb hardness scales. the data on the contribution of various values to the extended uncertainty of hardness measurements are presented. keywords: leeb hardness; national primary standard machine; expanded uncertainty 1. introduction from 2016 to 2018, vniiftri improved the national primary standard machine for shore d hardness scales for metals [1]. it was improved by introducing into the standard machine devices that measure the hardness of metals on the leeb d and g scales. the modernization of the standard was necessary to ensure the traceability of hardness testers that implement the leeb method to the national primary standard of hardness. the paper presents the values of the main components of the expanded uncertainty of measurements on the national primary standard machine for leeb hardness scales. 2. description of the work the process of reproducing the hardness numbers on the leeb scales consists in the fall of a impact body with a diamond, ceramic or tungsten tip on a sample or hardness test block and in determining the ratio between the rebound and impact velocity of the impact body [2]. the hardness numbers on the leeb scales are determined by the formula 𝐻𝐿 = 1000 𝑣𝑟 𝑣𝑖 (1) where 𝑣𝑟 is a rebound velocity of impact body; 𝑣𝑖 is a impact velocity of impact body. the mass of impact body, radii of the tip of impact body, impact velocity, material of the tip are standardized for each leeb scale. the most common leeb scales in industry are leeb d and leeb g scales. requirements for these scales are presented in table 1, where r is a radius of the tip of impact body, m is a mass of impact body. table 1: requirements for leeb scales d and g requirements hld hlg m, g 5.45 ± 0.03 20.00±0.03 r, mm 1.500±0.003 2.500±0.003 𝑣𝑖 , m/s 2.0500±0.0025 3.000±0.005 devices for leeb hardness measuring consists of tubes with electromagnetic coils, impact bodies with built-in magnets, a mechanical device for resetting impact bodies. after faulting the impack body and its passage through the coil, an electromotive force (emf) is induced in it, which is fixed by an electronic unit. the ratio of maximum voltage after impact and before it is equal to the ratio of the impact velocity to the rebound velocity of the impact body. the electronic unit recalculates this ratio into hardness numbers on leeb scale. the expanded uncertainty of measurement of hardness values on the leeb scales affected by deviations from the nominal values of mass and radius of the tip of impact body, the impact velocity of impact body, the uncertainty of the measurement of the ratio of velocity at the time of rebound of the impact body at the moment of impact. when improving the primary standard machine, the traceability of all values that affect the reproduction of hardness numbers to the national primary machines is ensured: units of mass – kilogram, units of length-meter, units of time – second, units of voltage – volt. the masses of the impact bodies were measured using weights, and the radii of the tips of the impact bodies were measured using a profilometer. the impact velocity was measured using a laser triangulation displacement sensor. the displacement sensor was attached to the reference tube in a special holder. the impact body fell down the tube, the sensor, depending on the displacement of the impact body, issued an electrical signal with a frequency of 400 khz. the time intervals during which the impact body passed the measured displacement were measured by an http://www.imeko.org/ mailto:andrey_aslanyan@vniiftri.ru acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 248 oscilloscope. the oscilloscope also measured the electrical voltage on the sensor, which was converted into displacement using the calibration function. the resulting dependence of the displacement of the impact body on time determines the velocity of the fall of the impact body. in parallel, the oscilloscope measured the voltage on the electromagnetic coil through which the impact body flew. the moment of impact of the impact body on the hardness test block was determined using an oscilloscope at the point of transition of the "voltage time" curve in a straight line when the impact body flew through a tube with a coil. the standard uncertainty of measurement of displacement by the sensor was determined from the "voltage-displacement" calibration curve. the velocity of an impact body with a magnet passaging through an electromagnetic coil is proportional to the voltage induced on the coil. the ratio of the impact body velocity when it is impact to the impact body velocity when it is rebound is equal to the ratio of voltages at these times. the electrical voltages generated on the coil after passage through it the sides with the magnet were measured using an oscilloscope connected to the coil in parallel with the electronic unit. the sensitivity coefficients of the influencing quantities included in equation (1) were determined by differentiating equation (1) by the influencing quantity. the sensitivity coefficients of values not included in equation (1) were found experimentally. among the values not included in equation (1) are the radius of rounding of the impact body and the mass of the impact body. the sensitivity coefficients for these values were determined by a small change in only one parameter and determining the change in hardness. the sensitivity coefficient is equal to the ratio of the change in hardness to the change in this parameter. table 2: the sensitivity coefficients of influence quantities influencing values hardness level, hld sensitive coefficients mass of impact body 466 -21.3 hld/g 601 -18.7 hld/g 765 -9.4 hld/g radii of the tip of impact body 466 314.1 hld/mm 601 286.3 hld/mm 765 201.5 hld/mm impact velocity 466 -227.3 hld·s/m 601 -293.2 hld·s/m 765 -372.7 hld·s/m the ratio of the impact body velocity to the impact body velocity all range 1000 hld table 2 shows the sensitivity coefficients for values that affect the leeb d hardness measurements. table 3 shows the sensitivity coefficients for values that affect the leeb g hardness measurements. table 3: the sensitivity coefficients of influence quantities influencing values hardness level, hlg sensitive coefficients mass of impact body 426 -19.5 hlg/g 560 -14.3 hlg/g 636 -7.9 hlg/g radii of the tip of impact body 426 293.1 hlg/mm 560 245.3 hlg/mm 636 189 hlg/mm impact velocity 426 -142 hlg·s/m 560 -186.7 hlg·s/m 636 -211.7 hlg·s/m the ratio of the impact body velocity to the impact body velocity all range 1000 hlg table 4 shows the standard measurement uncertainty values that affect the reproduction of hardness numbers on the scales hld, hlg. table 4: the contribution of influencing values to the standard combined uncertainty of hardness measurements on the leeb scales influencing values hld hlg mass of impact body 0.06 0.09 radii of the tip of impact body 0.12 0.09 impact velocity 0.75 0.64 the ratio of the impact body velocity to the impact body velocity 1.29 1.22 the standard combined uncertainty of measurement on leeb hardness scale hld is no more than 1.5. the expanded uncertainty of measurements on the leeb hld scale with a coverage factor of 2 is no more than 3.0. the standard combined uncertainty of measurement on leeb hardness scale hlg is no more than 1.39, and the expanded uncertainty of measurements on the hlg scale with a coverage factor of 2 is no more than 2.8. 3. summary metrological researches of the national primary standard machine allowed us to estimate the expanded uncertainty of measurements on the leeb hld and hlg hardness scales. http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 249 4. references [1] e. g. aslanyan, m. v. balakhanov, n. s. gusyatinskaya, a. s. doinikov, v. i. kozlov, n. a. safarov, v. r. shlegel, measurement techniques, vol. 45, no. 3, pp. 223–227, 2002. [2] k. herrmann, hardness testing principles and applications. leeb method, published by asm international, novelty, usa, pp. 78–86, 2011. http://www.imeko.org/ application of µ-raman spectroscopy to the study of the corrosion products of archaeological coins acta imeko issn: 2221-870x march 2021, volume 10, number 1, 234 240 acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 234 application of -raman spectroscopy to the study of the corrosion products of archaeological coins tilde de caro1, emma angelini2, leila es sebar2 1 istituto per lo studio dei materiali nanostrutturati national research council (ismn-cnr), rome, italy 2 dipartimento di scienza applicata e tecnologia (disat), politecnico di torino, torino, italy section: research paper keywords: raman spectroscopy; bronze disease; corrosion, cultural heritage. citation: tilde de caro, leila es sebar, emma angelini, application of µ-raman spectroscopy to the study of the corrosion products of archaeological coins, acta imeko, vol. 10, no. 1, article 31, march 2021, identifier: imeko-acta-10 (2021)-01-31 editor: ioan tudosa, university of sannio, italy received may 25, 2020; in final form november 10, 2020; published march 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: tilde de caro, e-mail: tilde.decaro@cnr.it 1. introduction in the past few decades, a series of cutting-edge analytical techniques have found an ever-growing application in the cultural heritage conservation field, offering interesting insights into the provenance, history and fabrication methods of cultural heritage artefacts. among these techniques, raman spectroscopy has become a fundamental tool in conservation science, as it does not require sampling of the artefact under study and the analysis can be performed in museum galleries, storage facilities and conservation laboratories thanks to the use of portable instrumentation [1], [2]. the raman effect provides a quick and relatively straightforward molecular identification of a material under examination. a raman spectrum is something like a fingerprint that can be used to identify compounds against a database of standard spectra [3]. this paper deals with the application of micro-raman spectroscopy (μ-rs) for the analysis of the corrosion products of some metallic artefacts. μ-rs furnishes an identification of the corrosion products by determining the vibration modes and thus the bond vibrations in the structure [4]. micro-raman spectroscopy can be used to identify the mineralogical nature of micro-phases and discriminate between different polymorphs present in the corrosion products of the patina 5. furthermore, μ-rs has the advantage of being a fast and non-destructive technique, and, in backscattering geometry, it is especially suitable for analysing surfaces. due to its interesting features, μ-rs in addition to other techniques such as x-ray diffraction (xrd) and scanning electron microscopy coupled with energy dispersive x-ray spectroscopy (sem–eds) was utilised in order to investigate the chemical and structural nature of the corrosion products (i.e. the patina) grown on archaeological bronze artefacts. in particular, three unreadable coins from the phoenician–punic period taken from the tharros excavation site were studied. abstract in this paper, a study of the corrosion products formed on archaeological bronze artefacts excavated in tharros (sardinia, italy) is presented. the investigation was carried out by means of the combination of different analytical techniques, including optical microscopy, micro-raman spectroscopy (µ-rs), scanning electron microscopy coupled with energy dispersive x-ray spectroscopy and x-ray diffraction. the artefacts under study are three bronze coins from the phoenician–punic period that are deeply corroded due to the chloride-rich soil of the tharros excavation site. µ-raman spectroscopy was chosen to investigate the corroded surfaces of the artefacts because it is a non-destructive technique, it has high spatial resolution, and it makes it possible to discriminate between polymorphs and correlate colour and chemical composition. through µ-rs, it was possible to identify different mineralogical phases and different polymorphs, such as cuprite (cu2o), copper trihydroxychloride [cu2cl(oh)3] polymorphs, hydroxy lead chloride laurionite [pbcl(oh)] and calcium carbonate polymorph aragonite. the experimental findings highlight that micro-raman spectroscopy can be used to provide further knowledge regarding the environmental factors that may cause the degradation of archaeological bronzes in soil. mailto:tilde.decaro@cnr.it acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 235 the ancient city of tharros is located along the north-western coast of sardinia (italy) and was first established in the nuragic period. it is currently an archaeological site near the village of san giovanni di sinis, municipality of cabras, in the province of oristano. it is located on the southern shore of the sinis peninsula, which forms the northern cape of the bay of oristano, as shown in figure 1. the phoenicians populated the area during the 8th century bc, and then the city was governed by carthaginians until the roman conquest in the 3rd century bc 5], [6. the remains of the town include foundations of temples, roman baths, a roman castellum acquae, a phoenician–punic tophet and an artisan quarter. the town was the capital of the judicate of arborea until around 1070 ad, when it was abandoned in favour of oristano under the pressure of the arab incursions. during archaeological excavations carried out in tharros, a large amount of punic, roman and carthaginian bronze artefacts, such as coins, nails, rings and everyday tools, was found 7. due to the length of time they had been buried in the soil, most of the bronze artefacts were covered by a thick layer of corrosion products containing chlorides and were affected by bronze disease, an irreversible and nearly inexorable corrosion process that occurs when chlorides come into contact with bronze or other copper-based alloys. the degradation process induced by bronze disease is cyclic and self-sustaining and destroys the artefacts, transforming them into a greenish powder [8]-[10]. the main purpose of this study was to characterise, by means of a multi-analytical approach, the three coins shown in figure 2, in order to identify the corrosion products, to ascertain the actual state of conservation and provide recommendations for the selection of tailored conservation approaches [11] and to recommend materials to inhibit the degradation phenomena. in section 2, the different analytical techniques are presented. in particular, optical microscopy (om) and sem–eds were employed in order to investigate both the morphology of the corrosion products and their elemental composition. this combination provides the best results for the characterisation of corrosion layers on ancient metal objects and is valid for corrosion layers with thicknesses in the order of micrometres or more. micro-raman spectroscopy was performed to identify the main corrosion products present on the surfaces; it has high spatial resolution, and it can be used to discriminate between polymorphs and to correlate colour and chemical composition [10]. finally, xrd was carried out to determine micro-structural identification of the crystalline phases and to provide complementary information with respect to the previous techniques. in section 3, the results of the performed analyses are reported and discussed. finally, the conclusion section presents the major contributions of this study. 2. instruments and methods the study of the corrosion mechanisms and the characterisation of the layers of corrosion products on the coins were mainly performed by means of μ-rs, which proved to be a powerful technique for the identification of the different corrosion products. in the study of corrosion, μ-rs is widely employed to assess the composition of corrosion products in a non-destructive way. a monochromatic light, usually from a laser in the visible range, is employed to determine the vibration mode of the molecules and identify them based on the shift in energy. micro-raman analyses were performed at room temperature using a renishaw rm2000 equipped with a peltier-cooled charge-coupled device camera in conjunction with a leica optical microscope with 10 ×, 20 ×, 50 × and 100 × objectives. measurements were performed using the 50 × objective (laser spot diameter of about 1 μm) and the 785 nm line of a laser diode. two edge filters blocked the rayleigh-scattered light below 100 cm−1. for this reason, the study of ultra-low wavenumber raman spectra in the region < 100 cm−1 is overlooked. in order to avoid damaging the patina and to prevent the fluorescence from covering the raman signal, the laser power was lowered. no baseline was subtracted from the recorded spectra. the spectra obtained were compared with grams spectroscopy software and databases available in the literature [12]. moreover, an investigation of the chemical composition and morphology of the corrosion product layers was carried out by optical and electron microscopy and x-ray diffraction. an optical microscope uses visible light and a system of lenses to generate magnified images. om measurements can be easily performed without any sampling and can be used to study the morphology and distribution of corrosion products on the artefact surface, allowing for identification based on colour. sem uses a beam of accelerated electrons to scan an object’s surface and obtain higher magnification images. generally, the microscope is coupled with eds, which makes it possible to obtain information on the chemical composition of the sample. eds characterisation capabilities are due to the fundamental principle that each element has a unique atomic structure that appears as a unique set of peaks on its electromagnetic emission spectrum. figure 1. the archaeological site of tharros (sardinia, italy). figure 2. the phoenician–punic coins under study found in the archaeological site of tharros (sardinia, italy): (a) tht clo1; (b) tht clo2; (c) tht clo3. https://en.wikipedia.org/wiki/cabras,_italy https://en.wiktionary.org/wiki/inexorable https://en.wikipedia.org/wiki/corrosion acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 236 optical microscopy investigations were performed using a leica microscope m 125c equipped with a digital camera. sem and eds were carried out by a cambridge 360 scanning electron microscope equipped with a lab6 filament and with an inca 250 energy-dispersive spectrometer and a four-quadrant backscattered electron detector (bse). sem images were recorded in bse mode at an acceleration voltage of 20 kv and a working distance of 25 mm. micro-structural identification of the crystalline phases formed on the bronze coins was determined by xrd analysis by means of a siemens 5000 x-ray powder diffractometer using nifiltered cu kα radiation (λ 1.5418 å). angular values in the range of 20 °– 60 ° in additive mode, a step size of 0.05 ° and a sampling time of 2 s were the experimental parameters used for first data acquisition. 3. results and discussion the three phoenician–punic bronze coins selected for this study, whose visual appearance is shown in figure 2, were found in a sewer of the archaeological site of tharros. the coins, identified as tht clo1, tht clo2 and tht clo3, appeared to be poorly conserved and were analysed before any cleaning or preservation treatment. copper-based alloys are more vulnerable to attack by chloride compared to other corrosive species, especially when they are exposed to marine or coastal environments or buried in chlorideenriched soils. this is the situation experienced by the metallic artefacts from tharros. artefacts are mainly endangered by bronze disease when pitting corrosion develops in conditions of high relative humidity and chloride supply. moreover, phase segregation phenomena contribute to surface heterogeneity and induce local galvanic cells. at visual examination, the surface patinas of all the coins appear greenish and dusty, as shown in the om images of figure 3, figure 4 and figure 5, where the sem images accompanied by the energy-dispersive spectra and the xdr diffractograms are also reported. eds spectra reveal the presence of oxygen, copper and chlorine on the coins’ surfaces [13]. the xrd spectra of the patinas of the three archaeological artefacts highlight the presence of dangerous copper-containing corrosion products such as trihydroxychlorides [cu2cl(oh)3] [13]-[16]. furthermore, the om images show the presence of red ruby crystals that can be attributed to cuprite crystals mixed with a small amount of soil components and other corrosion products [16]. xrd spectra confirm the presence of cuprite (cu2o) mixed with small amounts of soil components including quartz (sio2) and calcite (caco3). after these characterisations, the coins’ patinas were analysed by means of -raman spectroscopy [16]-[18]. as evidenced in figure 6, the spectra confirm that the main aggressive agent is represented by chloride-containing species, which lead to the formation of copper chlorideand oxy-chloride-based corrosion products. the raman spectra allow differentiation between two different polymorphs of [cu2cl(oh)3]: clinoatacamite and atacamite [19]. the spectra of these two mineralogical species are very similar: the region where the difference can be observed is highlighted by a red line, while the vibrational assignments are listed in table 1. as explained above, bronze disease is a cyclic reaction that occurs when the archaeological artefact interacts with chlorides in the soil that, in the presence of moisture, produce light green powdery copper chloride [nantokite (cucl) and hydroxychlorides (atacamite and its polymorph clinoatacamite) [20]. the outermost green layer can be constituted by atacamite and its polymorphs, botallackite and clinoatacamite. thermodynamic data show that clinoatacamite is the most stable figure 3. coin tht clo1: om image (a), xrd spectrum (b), sem image (c) and eds spectrum (d). figure 4. coin tht clo2: om image (a), xrd spectrum (b), sem image (c) and eds spectrum (d). figure 5. coin tht clo3: om image (a), xrd spectrum (b), sem image (c) and eds spectrum (d). acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 237 phase of the three polymorphs, and botallackite, which forms first, is the least stable. therefore, when atacamite is found on an archaeological artefact, this indicates the process is in an intermediate stage, while clinoatacamite represents the completion of the sequence [21]. the nantokite phase, which is very unstable, is the first phase that forms, and it is found as a corrosion product only under dry environmental conditions. from this information, it can be inferred that some corrosion products, such as clinoatacamite, are stable, and therefore, the artefact once excavated will not encounter serious degradation processes. other products, on the contrary, such as atacamite, change their crystalline structure over time, increasing their volume, for example, and these processes can lead to rapid degradation of the artefact. the broad bands of the spectra suggest that these minerals have a disordered structure and are not well crystallised. the cuprite layer, evidenced in the raman spectra shown in figure 7 and table 2, should work as an electrolytic membrane allowing the transport of anions such as cl− and o2 − inside and cations such as cu+ outside. the large amounts of chlorine and oxygen in the interface metal/patina can be interpreted as an autocatalytic reaction that facilitates the oxidation of copper and the accumulation of chloride ions, allowing the formation of cuprite and copper chloride. it is interesting to observe that the crystal structure of cuprous oxide is still preserved after the incorporation of a small amount of other cations into the lattice. indeed, the passive layer of cu2o could incorporate up to about 2 at. % of sn into the lattice, forming a defective layer of copper oxide. the substitution of cu by sn in the cu2o phase is noticeable, as confirmed by the presence of the overtone peak at 218 cm−1. the increase in the intensity of the band indicates that cu2o is disordered and not well crystallised. furthermore, the presence of sn, which has replaced some cu atoms in the lattice structure, could be linked to a small red shift of the band at 640 cm−1 [22]. the raman spectrum, shown in figure 8 and table 3, highlights the presence of laurionite [pbcl(oh)], which is related table 1. results of vibrational spectra (*wavenumber/cm−1) and raman bands attribution of the polymorphs atacamite and clinoatacamite. atacamite cu2cl(oh)3 clinoatacamite cu2cl(oh)3 raman bands attribution 975* 928* hydroxyl deformation 910 893 845 820 800 580 cuo stretching vibration 512 513 451 444 cucl stretching vibration 424 354 365 239 259 200 169 147 142 ocuo bending modes 116 figure 6. raman spectra of copper trihydroxychlorides [cu2cl(oh)3] atacamite and clinoatacamite on the coins found at tharros. table 2. results of vibrational spectra (wavenumber/cm−1) and raman bands attribution of cuprite. cuprite (cu2o) raman bands attribution 637 raman symmetry allowed 529 raman symmetry allowed 416 overtone 218* overtone-defective cu2o* 149 raman symmetry allowed figure 7. raman spectra of cuprite (cu2o) on the coins found at tharros. the raman spectra collected in a cu‐rich area evidence the presence of poorly crystallised cuprite. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 238 to the interaction of lead in the alloy with chloride in the soil. laurionite is a mineral that is thermodynamically stable over a wide range of ph values and chloride concentrations [24], [24]. lead was commonly added to bronze in the ancient manufacture of artefacts characterised by low mechanical properties such as coins. indeed, an addition of lead up to 2 % improves the fluidity of the melted bronze alloy. figure 9 and table 4 show the raman spectra and bands attribution of aragonite, a polymorph of caco3, in the patina of coin tht clo3. for the calcium carbonate polymorphs, the stability and consequently the solubility increases as follows: amorphous calcium carbonate (acc) > vaterite > aragonite > calcite [26], [26]. despite the fact that aragonite is unstable at room temperature and atmospheric pressure, the presence of this mineral is not unusual on cu base alloy patinas found in marine and/or coastal environments. this can be due to the formation of microclimates on the surface of the metal with different ph levels and temperatures and the presence of mg2+ ions, which are abundant in sea water. moreover, previous studies have noted that the ratio of mg2+ to ca2+ ions in solutions could facilitate the formation of aragonite over calcite [28], [28]. the experimental findings of the adopted multi-analytical approach reveal the deep interaction between soil components and corrosion processes and products and also evidence the relevant presence of chlorides in the patina of the archaeological cu-based artefacts found in tharros. this latter occurrence is considered dangerous because it could induce the previously mentioned bronze disease, a cyclic and self-sustaining corrosion reaction of copper that would disfigure the artefact. these corrosion products have to be neutralised, and their harmful and irreversible actions have to be stopped in order to preserve the artefact [30]-[32]. several steps can be taken to both prevent and treat bronze disease: i) removing moisture from the artefacts by placing them in the oven on low heat in order to dry them out, with the unwanted possible effect of darkening the surfaces, ii) soaking the artefact in either distilled water or a solution of sodium carbonate and sodium bicarbonate, a non-permanent fix that will only halt the reaction until the cuprous chloride comes into contact with moisture in the air, iii) using corrosion inhibitors such as benzotriazole (c6h5n3), a highly carcinogenic complexing agent, or iiii) coating the artefact with a varnish, wax or resin to prevent the recurrence of corrosion; however, if the protective layer lacks adhesion and if the bronze disease hasn’t been completely eliminated, the metal will continue to corrode beneath the coating [33]-[36]. curtailing bronze disease is still an open problem, along with the question of whether humidity control could serve as an alternative method to reversible surface modifications of artefacts. an environmental condition in which relative humidity is maintained below 39 % is ideal for bronze storage and display. however, humidity control is costly and sometimes impractical in a display setting. consequently, the effects of bronze disease have to be controlled with due precaution, and careful periodic examination of artefacts has to be carried out. to this aim, microraman spectroscopy has been shown to be a powerful nondestructive tool to characterise the surfaces of archaeological artefacts. 4. conclusions by means of -raman spectroscopy, the micro-chemical structure of long-term corrosion products growing on bronze archaeological artefacts found at tharros was investigated. through the multi-analytical approach proposed in this study it was possible to identify the presence of cu (i) species such as cuprite (cu2o) and of copper trihydroxychlorides [cu2cl(oh)3] polymorphs. furthermore, the presence of the hydroxy lead chloride laurionite [pbcl(oh)] and of the calcium carbonate polymorph aragonite was detected. taking into account that the artefacts were found in a soil rich in chlorides due to the location table 3. results of vibrational spectra (wavenumber/cm-1) and raman bands attribution of laurionite. laurionite pbcl(oh) raman bands attribution 612 oh deformation modes 327 pbcl stretching vibration 279 figure 8. raman spectrum of laurionite [pbcl(oh)] on coin tht clo1. table 4. results of vibrational spectra (wavenumber/cm1) and raman bands attribution of aragonite. aragonite caco3 raman bands attribution 1084 c-o symmetric stretching in co32 682 c-o asymmetric bending mode in co32 280 caco32stretching 145 figure 9. raman spectrum of aragonite (caco3) on coin tht clo3. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 239 of the archaeological site along the western coast of sardinia, it was possible to correlate the corrosion phases to the chemical composition of both the artefacts and the burial context. the obtained results confirmed that raman spectroscopy is an effective, versatile and non-invasive technique that allows for the fast detection of mixed polymorphs in the outermost layers of the patina before and after the restoration of the artefact and also during the storage and exhibition period. this is of particular interest in the determination of focused conservation methods for these materials. in fact, raman data are an indispensable part of the multianalytical approach adopted for the study of the coins. the high spatial resolution makes it possible to obtain details regarding the physico–chemical interrelationships of mixed phases and corrosion products of a specific metallic artefact – a single micrometric-sized crystal may be analysed. this property is of particular interest in the corrosion field because most of the corrosion products are distributed unevenly on the surface of the artefacts corresponding to localised corrosion formed by holes or craters of micrometric depth. indeed, once the corrosion phases have been detected, precautions have to be taken in order to assure the long-lasting preservation of artefacts. according to the authors’ knowledge, to achieve this aim, the transformation of dangerous copper chloride or oxy-chlorides into some stable phase is the most suitable approach. it is worth noting that conservators have developed several procedures to neutralise and stop these degradation processes, ranging from inhibitors to coatings [37]. however, before using a new restoration product, it is necessary to know in advance the way the patina will respond once exposed to the new compounds. a corrosion-inhibiting product must stop the aggressive substances and at the same time preserve the colour and composition of the patina in order to avoid altering the appearance of the archaeological artefact. acknowledgement the authors gratefully acknowledge gianni chiozzini and claudio veroli (ismn-cnr) for their skilful technical assistance with the sem and xrd. references [1] f. casadio, c. daher, l. 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https://doi.org/10.1007/s11224-016-0792-z https://doi.org/10.1002/jrs.4037 https://doi.org/10.1039/b008302j https://doi.org/10.1016/s1386-1425(03)00054-4 https://doi.org/10.1063/1.4871900 https://doi.org/10.1073/pnas.1423898112 https://doi.org/10.1016/s1386-1425(02)00315-3 https://doi.org/10.1016/j.tsf.2016.12.024 https://doi.org/10.1179/019713690806046064 https://doi.org/10.1007/s00339-013-8171-8 https://doi.org/10.1002/sia.4841 https://doi.org/10.1140/epjp/i2018-12368-3 simplified measurements of the humidity coefficient of torque transducers in calibration laboratories acta imeko june 2014, volume 3, number 2, 32 – 38 www.imeko.org simplified measurements of the humidity coefficient of torque transducers in calibration laboratories andreas brüge physikalisch-technische bundesanstalt, bundesallee 100, 38116 braunschweig, germany section: research paper keywords: torque; transducer; sensitivity; humidity; measurement citation: andreas brüge, simplified measurements of the humidity coefficient of torque transducers in calibration laboratories, acta imeko, vol. 3, no. 2, article 9, june 2014, identifier: imeko-acta-03 (2014)-02-09 editor: paolo carbone, university of perugia received february 15th, 2013; in final form june 25th, 2013; published june 2014 copyright: © 2014 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: (none reported) corresponding author: andreas brüge, e-mail: andreas.bruege@ptb.de 1. introduction torque transducers are known to be influenced by humidity [1], [2]. due to different levels of sealing on the surface of the sensing elements of the transducers, the deviation of the sensitivity caused by a change of humidity can vary in amount and time constants. the usual methods to measure the effect are complicated and extensive. nevertheless, this information is essential to complement the measurement uncertainty budget of torque calibration. in calibration laboratories only few resources are available for such investigations. in order to enable these laboratories to test their transfer torque transducers at low cost, this paper describes simple methods to determine the humidity effect and demonstrates their equivalence to the extensive methods. in section 2 a theoretical description of the effect is given. in section 3 different possibilities to alter the humidity of torque transducers are discussed, followed by a description of how the humidity coefficient could be determined. here important sources of uncertainty are also analyzed. in section 5 the results of measurements on typical transducers are presented and in section 6 a summary is given. 2. humidity effect the influence of humidity is supposed to be due to the hygroscopic behaviour of the strain gauges and the adhesive layer which couples them to the elastic element. the change of their mechanical properties can affect the sensitivity of the strain gauge bridges as well as the time constants of their response, and a change in additional strain gauges for temperature compensation can cause a signal offset. a stepwise humidity change will not act instantaneously on the bridge but in an exponential way. the effective time behaviour of the humidity f in the bridge can be described as         −+=       − f1)( f0 τ t eaftf (1) with t: time f0: initial amount of humidity af: change of the humidity τf: time constant of the humidity. this causes time behaviour of the transducer sensitivity s as follows:         −+=+=       − s t easfssts τ1)()( s00 (2) with t: time s0: initial amount of sensitivity as: change of the sensitivity τs: time constant of the sensitivity. abstract several simple methods to test the humidity influence on the sensitivity of torque transducers are examined. their suitability for use in calibration laboratories and important influences are discussed. the equivalence of former determinations of the humidity effect with extensive methods to the simplified methods is demonstrated. acta imeko | www.imeko.org june 2014 | volume 3 | number 2 | 32 sensitivity in this paper is the signal of the transducer at nominal value in mv/v as is usual in the field of torque measurement. if the alteration of humidity does not penetrate all the elements of the transducer’s sensing element in the same velocity, transient effects on the sensitivity should be taken into account:         −=       − t t)( τ t eatt (3) with t: contribution of transient effect t: time at: change of the transient signal τt: time constant of the transient effect. a simulation of these functions shows the principal behaviour of them (figure 1). a separation of the contributions s and t to the total response is only possible if the initial sensitivity valid just before the humidity step starts of the transducer is known. the effects which affect this sensitivity depend on the method of applying the change of humidity and therefore are to be considered when a method should be recommended. the relative coefficient of the humidity effect on the transducer sensitivity ch can then be defined as: ( ) ( ) ( ) ( ))()(),( 1212 12 h tftftts tsts c − − = (4) a negative coefficient thus stands for an increasing sensitivity at decreasing humidity and vice versa. the unit of the relative humidity coefficient is 1/%rh, which means “per percent point of relative humidity”. in this work the value of the relative humidity is given in percent (%), and the value of a span of different levels of relative humidity as percent points of relative humidity (%rh). 3. variation of humidity 3.1. humidification of the laboratory the straightforward way to change humidity for the determination of the relative humidity coefficient ch is by using an air-conditioned laboratory room with a torque calibration facility inside. here a reference torque calibration facility is not suitable, for the humidity effects of the reference torque transducer and of the transducer under test would combine. therefore, a deadweight facility is necessary which is assumed to be independent of humidity. in this setup the entire room can be altered to different levels of humidity at a constant temperature. because of the high volume of an entire room, the alterations need a long time until stability is reached. for the same reason the stability of the humidity level is of high quality. the first observation of the humidity effect happened in this way, during a key comparison in laboratories of different humidity levels [1]. if there are more calibration facilities in the room, their results may be influenced by the humidity alteration too. therefore, other facilities might be stopped during the humidification, so the costs of the room humidification can be much higher than economically orientated laboratories can bear. 3.2. in-situ humidification in a chamber another possibility is to construct an air-conditioned chamber around the transducer under test mounted inside the calibration facility (figure 2). in such a chamber, the temperature and humidity of the air should be controllable in a stable or transient way as desired. it should provide low parasitic effects due to a mechanical bypass, the behaviour of the control unit, air flow and evaporationcooling. altogether the method is connected with high mechanical and electrical effort. when these difficulties have been overcome, the method potentially provides measurements at very stable humidity levels and with low uncertainty even in reference torque calibration facilities. some systematic investigations of the humidity effect were performed at the physikalisch-technische bundesanstalt (ptb) with such a figure 2. example of an air-conditioned chamber integrated into a setup of a force calibration facility. figure 1. simulation of the principal time behaviour of the humidity effect. acta imeko | www.imeko.org june 2014 | volume 3 | number 2 | 33 chamber [2] and serve as a reference for the simplified methods presented in this work. 3.3. in-situ humidification, local casing the preceding methods are possible for most national metrological institutes (nmis) but, for calibration laboratories, usually the effort is too high. a very simple and cheap version of a climate chamber used in this work is a casing of light acrylic (figure 3). inside the casing a wet cloth or a pad of silica gel produces a relative humidity of about 80% and 20% respectively, which is measured by a small environment logger placed within the casing near the transducer. the actual level of humidity depends on the relation between the water transmission capability of the cloth or the pad, respectively, and the air leakage of the casing. in order to get higher humidity steps and to ensure small spatial humidity gradients within the casing, the leakage should be minimized. since the casing spans both shaft ends of the transducer twisting up to several angular degrees while loaded to nominal torque, an effective seal is associated with a mechanical bypass between these shaft ends. additionally, the casing should feature a sealed cable feedthrough. together with the relative humidity of the laboratory being about 45% this in-situ humidification is able to generate three levels of humidity steps. because these levels are uncontrolled, a systematic investigation of the humidity effect with arbitrary humidity steps is not possible. nevertheless, the method is useful because it reaches humidity spans of more than 50%rh, whereas the ex-situ humidification described later allows up to 35%rh only. additionally, this method reliably includes the transient contribution t mentioned above, which is not the case for the ex-situ humidification. the possibility to observe both the initial and the final humidity level in a stable state is essential to overcome the transient effects. 3.4. ex-situ humidification often an external climate chamber is easier to construct than a version integrated in the calibration facility or alternatively a climate cabinet may be available in the laboratory. then the transducer under test can be humidified externally to a stable level, while the temperature stays constant at the laboratory value (figure 4). after a fast transit into the calibration facility, the measurement of the humidity effect takes place during the relaxation of the humidity impact within the sensing element to the laboratory level of humidity (figure 5). because of the measurement delay due to the relocation, an extrapolation of the sensitivity response towards the origin is necessary. the initial sensitivity cannot be measured directly in this procedure. despite this difficulty, the use of a climate cabinet is appropriate for systematic investigations using a great number of different humidity levels. another advantage of the ex-situ humidification is the possibility to use the calibration facility for current work while the transducer under test stays in the climate cabinet. due to long time constants of the humidity effect this can save several days of machine time. additionally, for testing torque wrenches, an in-situ humidification is nearly impossible, as a local casing would induce high mechanical bypass via the lever of the wrench. combining the simplicity of a light casing with the ex-situ method which avoids a mechanical bypass, torque wrenches can be humidified in a bag together with a wet cloth or silica gel, respectively, and an environment logger (figure 6). torque wrenches are usually mounted with square drives, so the figure 3. in-situ humidification of a torque transducer inside a provisional local casing. an environment logger is used for the determination of the development of humidity and temperature. figure 4. ex-situ humidification of a torque transducer in a climate cabinet. figure 5. response of transducer sensitivity at nominal load to different humidity deviations from laboratory level which are generated in a climate cabinet. acta imeko | www.imeko.org june 2014 | volume 3 | number 2 | 34 downtime of the measurement of the humidity effect is much shorter for them than for torque transducers. a variation of the in-situ method can help for transducers which cannot be transported to and mounted into the calibration facility safely and fast enough. here an undisturbed measurement of the initial state in laboratory ambient air without a casing is followed by a humidification using a temporary local casing. at the end of the humidification the casing is removed quickly and the relaxation to the initial sensitivity level can be observed free of mechanical bypass effects and transient effects. here the challenge is in the construction of a casing which can be removed fast enough to minimize the downtime. 4. measurement of the humidity coefficient 4.1. measurement of sensitivity the determination of the humidity coefficient calls for the measurement of the sensitivity of a torque transducer. two methods lend themselves to the implementation of these measurements in a reference torque calibration facility. firstly, with reference calibration facilities continuously loading and unloading delivers a characteristic curve of the transducer under test by interpolation methods [4]. in this way a sensitivity measurement with good averaging is possible within one minute. together with a preloading and a consecutive creep time, a sampling time of 5 minutes could be achieved. in addition, some important parameters of the characteristic curve are included: zero point, nonlinearity, hysteresis and return to zero. secondly, a stepwise loading and unloading is possible with reference calibration facilities or with deadweight facilities. usually only three steps at 0%, 50% and 100% of nominal load are applied to get both a fast measurement and a value of the hysteresis. to achieve a measurement stability like that obtained with the continuous method, it is necessary to average data during each load step. this leads to a sampling time of about 10 minutes. the first method is advantageous in investigations about the time behaviour of the humidity effect, where a fast measurement is crucial and the second method can help for measurements at transducers with high creep. as a third method a loading with a simple manually handled lever and a deadweight was tested but rejected. a tempting idea was to work with a steady torque generated by a fixed weight while modifying the humidity at the transducer under test within a local casing. then the response of the sensitivity would be available directly as the output signal of the transducer. however, because the signal wanted is superimposed by the zero signal which responds at least by the same amount to the humidity as the sensitivity itself, this method is not recommended. in this work, measurements of the sensitivity took place with continuous loading up to the nominal torque of the transducer under test, for clockwise torque only and in one mounting position only, which was reproduced carefully for all measurements compared. also to obtain a good comparability of conditions, all measurements were preceded by a preload up to nominal torque and a waiting time of 3 minutes (figure 7). after the waiting time, the load should pass through a value below the chosen virtual zero level in order to avoid hysteresis effects. performing a sequence of measurements with a maximum rate of six measurements per minute, the time behaviour of the humidity effect can be investigated with a good time resolution in comparison to the observed time constants of the effect. 4.2. transient signal contributions as mentioned in section 2, transient effects can occur during the change of humidity level. reactions of the local temperature due to humidity change are also possible. regardless of the reasons, an effect depending on the change of humidity is expected to generate a signal contribution like the function t in figure 1, and in the sum an exponential function with an abrupt linear beginning could result ('s+t’ in figure 1). in the measurements these curves are often found both at exsitu and in-situ humidification at the moment of a fast change in humidity. in contrast to effects of mechanical bypass the transient effect does not call for compensation. after the relaxation of the effect, the signal level is unchanged. thus, in the examination it is essential to regard equilibrium states only (figure 8). an analysis of the curve with the help of an exponential fitting function should take into account its linear start. 4.3. mechanical bypass as described in section 3.3 a well-sealed local casing breeds a mechanical bypass in the torque sensing transducer. the amount of torque deviation depends on the extent of friction of the gliding parts and on the stiffness of the deformed parts of figure 6. ex-situ humidification of a torque wrench in a bag. figure 7. achievement of the transducer sensitivity using the continuous calibration method. each measurement sequence consists of at least one preloading (yellow), a measurement loading (blue) and a three minute break between them. acta imeko | www.imeko.org june 2014 | volume 3 | number 2 | 35 the casing. consequently, the casing consists of a stiff cylinder of acrylic at the fixed end of the transducer and an overlapping blanket of flexible and gliding thin film at the measurement side. the acrylic cylinder is intended for carrying the mass of the environment logger and the wet cloth, or respectively, the gel pad. nevertheless, changing the mass load of the cylinder and manipulating its access panel may change the position relative to the blanket and thus cause a deviation in the mechanical bypass. for the casing used in this work, the effect reaches 0.015 n∙m. so for a 1000 n·m transducer, this contribution can be neglected, but for transducers of smaller ranges it can be considerable. the total amount of friction in the setup could be higher, but with an untouched casing, constant friction will not affect the run of the sensitivity curve. only additional friction due to manipulation at the casing appears in the measurements and has to be compensated for by appropriate shifting of the data points concerned (figure 8). although compensation is possible, it makes for additional uncertainty. therefore, careful handling should be performed to avoid any mechanical bypass which demands a well-trained operator. 4.4. zero signal a torque transducer exposed to a changing humidity level not only shows a response in the sensitivity but also in the zero signal. the reaction of the zero signal often dominates the sensitivity by a factor of more than 10 (figure 9). the determination of the sensitivity includes a taring of the transducer signal at a certain load with the zero signal achieved at the beginning of the loading process. therefore, not the entire amount of zero signal deviation affects the sensitivity measurement. only the deviation of the zero signal taking place during the loading process cannot be compensated by the taring and has an impact on the sensitivity measurement. the typical duration time for the continuous loading method is 30 seconds. with the stepwise loading, durations of 5 minutes were implemented. assuming a constant change of the zero signal in short times, the contribution of the zero signal change can be derived from the time behaviour and an approximative correction of the sensitivity is possible (figure 10). 5. results values of ch obtained for several state-of-the-art transducers with different nominal values are found to be about 5·10-6/%rh. this is close to the results of [3] where -4·10-6/%rh was found by changing the humidity using a chamber mounted into the calibration facility described in section 3.2. the effects described in this work cause uncertainties of the ch measurements of about 3·10-6/%rh. a dominating contribution is the uncertainty of the determination of the sensitivity. at ex-situ humidification the downtime during relocation requires an extrapolation of the sensitivity curve, so here the change of the sensitivity at the first two measurements indicates the uncertainty of the sensitivity. at in-situ setups the noise of the sensitivity measurements and the doubtful compensation of mechanical bypass comes to the fore. as ch is defined as relative to humidity steps, the relative uncertainty increases for decreasing humidity steps. because humidity steps above 40%rh are not accessible relative to the ambient humidity of the laboratory, a sequence of a wet cloth and a drying gel pad is necessary. with this advance the influence of the mechanical bypass is increased because the casing has to stay permanently and there is no measurement possible between stable states without the casing. the uncertainty of the humidity step measurement is estimated at about ±2%rh. this is possible with the environment logger, because only differential measurements are figure 8. sensitivity response of a transducer treated with humidity steps inside a local casing (blue dots). at the first step a pure transient effect occurs. the following two steps are influenced by the additional impact of friction due to a mechanical bypass in the casing. by appropriate compensation of the friction, the measurement can be corrected (red dots). figure 9. sensitivity response of a transducer experiencing a relaxation of a humidity step generated in a climate cabinet (blue dots). the zero signals are gained at the beginning of each corresponding torque loading sequence, which is necessary for the sensitivity measurement (red line). a correction of the influence of the variable zero signal delivers a sensitivity curve with a flatter beginning (green line). figure 10. influence of a changing zero signal on a measurement tared at the beginning of the loading. the contribution of the zero signal to the total signal can be estimated by a linear approximation of half of the zero signal increase during the time between two tarings. thereby, a corrected value of the full load signal (cross) can be calculated. acta imeko | www.imeko.org june 2014 | volume 3 | number 2 | 36 taken. the short-term change in temperature due to cooling effects by the humidity amounts to 0.1 k. with a typical relative temperature coefficient of torque transducers of 1·10-6/k, this effect can be neglected here. considering these limitations, measurements at a typical transfer torque transducer yield values of ch which are similar to former measurements [3] and are consistent for both methods of humidification used in this work (figure 11). the measurements at different humidity steps in consideration of the measurement uncertainty allow an interpretation both as linear and as nonlinear behaviour of ch. examining the absolute signal response to the humidity steps in spite of a relative response, the nonlinear character of the measurements becomes more evident (figure 12). because of blending measurements during increasing and decreasing humidity and because of performing humidity steps between humidity levels below and above laboratory equilibrium it is impossible to mark a reference point. nevertheless, the data points in figure 12 seem to be consistent in spite of the lack of a well defined working point. in the daily work of a laboratory humidity alterations of less than 20%rh are usual. in this range a possible nonlinearity of ch is negligible. for the determination of ch with the methods described in this paper, humidity steps of more than 30%rh are opportune. thereby, the values of ch are tending towards overestimation which is not harmful for the calculation of a robust uncertainty budget. 6. summary the simplified humidity tests described in this paper can deliver estimations of ch with uncertainties of 3·10-6/%rh which are small compared to the needs of calibration laboratories, where often providing information is sufficient if the coefficient’s range is 10-6/%rh or 10-5/%rh. with the help of these tests, the requirements for climate specifications of the air conditioning in the laboratories and for the transportation and storage conditions of transducers can be established properly. the results verify the equivalence of the discussed simplified humidification methods with the extensive method of former investigations within the uncertainty limits mentioned above. thereby, the calibration laboratories are able to determine humidity coefficients of their transducers at low costs in order to complement the corresponding uncertainty budgets. for example, the method of humidification in a local casing is possible with only a small set of cheap and everyday additional equipment as shown in figure 13 and figure 6. a survey of transducers of different types and ranges shows that for state-of-the-art transducers typical absolute values of ch are smaller than 10-5/%rh (figure 14) and thus compatible to the usual uncertainty budgets in calibration laboratories. transducers with unusual designs can vary in their humidity effect both in the time constant and the humidity coefficient. in this manner, transducers could exceed the uncertainty budget of a laboratory which combines small relative uncertainties of 2·10-4 with high limits of relative humidity alteration of more than 20%rh. one transducer type in the survey is a hermetically sealed transducer, which shows a humidity coefficient below the detection threshold. with the exception of this type, all transducers deliver a negative humidity coefficient. one transducer aged more than 15 years shows an absolute value of the humidity coefficient higher than 5·10-5/%rh. reasons might be either a design inapplicable in the sense of the humidity effect or a worn out sealing. the effect of a protective sealing on the strain gauges seems figure 11. values of ch obtained with different humidification methods at a typical transfer torque transducer (100 n∙m) demonstrating the equivalence of the methods. the run at different humidity steps suggests the nonlinear behaviour of ch. the error bars are calculated from the uncertainty of the measurements of respectively the humidity steps and the transducer sensitivity (see main text). figure 12. data of figure 11, calculated as absolute differences of sensitivities due to humidity steps showing a nonlinear increase with the amount of the humidity step. the error bars are calculated from the uncertainty of the measurements of respectively the humidity steps and the transducer sensitivity (see main text). figure 13: set of equipment necessary for the in-situ humidification of a torque transducer in a local casing as described in section 3.3. acta imeko | www.imeko.org june 2014 | volume 3 | number 2 | 37 to be restricted to the time constant of the sensitivity response to a humidity step. one example in the survey shows a very short time constant of half an hour which points to a weak sealing. nevertheless, its humidity coefficient is not higher than those of the other transducers with typical time constants of more than 8 hours. the time constant not only provides data about the quality of the strain gauge sealing but also gives important information about the downtime of a transducer, which has faced humidity conditions far from the laboratory specifications. to this effect, a transfer transducer intended for travelling could benefit from a weak strain gauge sealing, if this would not expand the humidity effect but shorten the time to reconstitute the humidity equilibrium. in an extended survey (figure 15) also hand torque tools were tested. especially the mechanical limiting version of hand torque tools – the click torque wrenches –work without strain gauges. here the humidity effect comes from the mechanical leverage system whose lubrication grease could be highly hygroscopic. although these wrenches are meant to work with high uncertainties of some percent, a humidity effect of more than 10-3 /%rh calls for restrictions of the humidity change during their operation. references [1] röske, d., “key comparisons in the field of torque measurement”, 19th international conference on force, mass and torque (imeko tc3): "theory and application in laboratories and industries", cairo, 19-23 february 2005, egypt, on cd-rom, file name: pdf\36.pdf. [2] röske, d.; mauersberger, d., “on the stability of measuring devices for torque key comparisons”, imeko xviii world congress and iv brazilian congress of metrology, "metrology for a sustainable development", rio de janeiro, 17-22 september 2006, brazil, on cd-rom, file name: 00181.pdf. [3] röske, d., “final report on the torque key comparison ccm.tk1: measurand torque: 0 n·m, 500 n·m, 1000 n·m”, metrologia, 46 (2009), tech. suppl., 07006, http://dx.doi.org/10.1088/0026-1394/46/1a/07002. [4] brüge, a., “fast torque calibrations using continuous procedures”, 18th conference on force, mass and torque, celle, germany, 24-26 september 2002. figure 15. extended survey of torque sensing instruments including hand torque tools. some of them, the click torque wrenches, yield humidity effect values of more than 10 -3 /%rh . this calls for consideration of the humidity effect, even with these apparently tough everyday tools. figure 14. survey of torque transducers of different types and ranges concerning the relative humidity coefficients and the time constants of the sensitivity response to a humidity step. transducers within the green area are compatible to the usual needs of calibration laboratories. acta imeko | www.imeko.org june 2014 | volume 3 | number 2 | 38 an internet-of-things system based on powerline technology for pulse oximetry measurements acta imeko issn: 2221-870x december 2020, volume 9, number 4, 114 120 acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 114 an internet-of-things system based on powerline technology for pulse oximetry measurements giovanni bucci1, fabrizio ciancetta1, edoardo fiorucci1, andrea fioravanti1, alberto prudenzi1 1 department of industrial engineering information and economics, g. gronchi 18, 67100, l’aquila, italy section: research paper keywords: iot; distributed measurement system; powerline communication; pulse oximetry; radio quiet zone citation: giovanni bucci, fabrizio ciancetta, edoardo fiorucci, andrea fioravanti, alberto prudenzi, an internet-of-things system based on powerline technology for pulse oximetry measurements s, acta imeko, vol. 9, no. 4, article 15, december 2020, identifier: imeko-acta-09 (2020)-04-15 section editor: francesco bonavolonta, university of naples federico ii, italy received october 14, 2019; in final form january 21, 2020; published december 2020 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: fabrizio ciancetta, e-mail: fabrizio.ciancetta@univaq.it 1. introduction the fourth industrial revolution (or industry 4.0) is currently underway, and new technologies are profoundly changing the way people live and work. in this new era, everything is connected. in every industrial, commercial and domestic sector, modern devices are integrated into data communication systems, creating both vertical and horizontal integrations. machine-tomachine (m2m) technology is an example of using horizontal integration, where different machines can communicate with each other using ad hoc transmission systems without any human interaction. among all the enabling technologies in the industry 4.0 paradigm, the internet of things (iot) is the most promising. the term iot describes an environment built with intelligent objects that can communicate data, access information from other sensors and share data on the internet. connections are no longer just between computers but between countless other digital objects. in this way, iot technology offers a new way to make links through the internet. the main manufacturing companies claim that, by 2025, more than 5 billion iot devices will be connected, and the advent of 5g, scheduled for 2022, will represent an epochal turning point [1]. much of the success or failure of a wireless system depends on the performance of the wireless connection. this includes the parameters, such as the operative range, a robust and repeatable connectivity and the data transfer rate. the multitude of iot technologies is characterised by a great heterogeneity of wireless communication and transport layers, such as 2g/3g, bluetooth/ble, 802.15.4, lora, lte, wi-fi, zigbee, z wave and many more, as depicted in figure 1. although all these technologies are currently used, the wi-fi network is the most widely adopted due to its use in industrial as well as domestic contexts. other networks require a special gateway to route data traffic over the internet and need an ad hoc network infrastructure to enable communication between a large number of devices. with the expansion of the wi-fi network, the density of installed devices operating at nearby frequencies has increased (for example, routers, access points, modems and antennas) as well as the interference between them. as a result, the signal-tonoise ratio decreases along with the quality of communication in terms of speed and reliability. moreover, there are no minimum radio frequency (rf) performance requirements that impose a consistent performance on wi-fi devices. the absence of these rf requirements for wi-fi devices can introduce inconsistencies and changes in the performance of the wi-fi network. abstract internet-of-things (iot) systems, covering any type of application, are becoming increasingly important. the most important network infrastructure used for the development of iot applications is the wi-fi network, as it is widespread and provides greater freedom during installation. there are cases where the cost of building the wi-fi infrastructure or the specific field of use do not justify the adoption of this network standard. the following study proposes the use of the powerline network as an alternative to wi-fi for the realisation of iot systems. an iot system for the monitoring of pulse oximeter values in a hospital setting will be described and discussed. the novelty of the proposed system is the adoption of powerline communication for the medical environment. mailto:fabrizio.ciancetta@univaq.it acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 115 in other situations, it is not possible to use any wireless communication network because, in that environment, it is necessary to guarantee low levels of radio frequency to ensure, for example, the correct functioning of vulnerable medical instruments. in these cases, the widely adopted solution is to use a wired ethernet network. if a wired network is not already available and is required, it is possible to opt for the use of powerline communication (plc) [2], [3]. this is a valid alternative mainly used to connect smart devices to the smart grid [4] or a smart city [5]. for example, in italy, the energy company, e-distribuzione, uses this consolidated technology to communicate with electric smart meters, such as its energy meters (second-generation energy meters), which carry out remote monitoring and/or remote control, in real time. powerline technology has many advantages, such as its accessibility from any electrical outlet, scalability and the absence of interference with nearby networks. its disadvantages may be related to the poor quality of the electrical system and the lower data rate [6]. in the hospital context, wi-fi has now replaced the wired ethernet. in addition, more and more machines, such as cardiac telemetry systems, real-time location systems and patient monitors, as well as hospital procedures, such as medical or drug test requests, nursing communication and guest internet services, also use these protocols to transmit sensitive data. implantable cardiac devices, such as pacemakers and defibrillators, are completely immune to wi-fi interference, even when operating at maximum power. the ansi/aami pc69 standard establishes the criteria for the electromagnetic immunity for pacemakers and the implantable cardioverter defibrillator (icd) in the frequency range between 450 mhz and 3 ghz [7]. in particular, the standard prescribes the electromagnetic compatibility tests that must be carried out at the different frequencies included in this range. assessments of potential critical interactions are continuously being researched in biomedical industries. in the clinical field, there have been errors in the formulation of diagnoses caused by electromagnetic interference and incorrect interpretations due to altered biometric measurements. for some pathologies, it might be useful to have sterile environments protected from wi-fi signals for patient care (radio quiet zones). in these cases, the only way to transmit data would be to connect the measuring devices using shielded ethernet lines or, even more easily, through the powerline. this paper describes an iot system based on plc suitable for use in cases where it is not possible to use the wi-fi network. the proposed system makes it possible both to read data remotely and to control the system using a double link transmission system. 2. powerline communication plc allows the transmission of data on higher frequency bands (tens of khz) compared to the lower frequency of 50 hz or 60 hz. this communication is limited by several factors: 1) signal attenuation at the frequencies of interest, 2) the presence of noise, 3) interference from electronic devices connected to the system and 4) variability in the time and frequency of the channel. however, this type of structure has some important advantages: the transmission network has already been implemented and widely adopted throughout the world, and it is possible to use the same channel to supply both electricity and data to users, thus reducing costs. plc can be divided into three different classes. the first is the ultra-narrow band (unb) frequency a few khz with a low bitrate (100 bps). distances of hundreds of kilometres can be reached, but, unfortunately, this is a proprietary technology. the second is the narrow band (nb) class operating at the vlf/lf/mf frequency bands (3–500 khz). this class can be further divided into two other subclasses characterised by the data transmission speed: a) ldr, a low kbps capacity and b) hdr, a maximum bitrate equal to 500 kbps. the third class is the broadband (bb) technology based on hf/vhf frequencies (1.8–250 mhz) with a data transmission rate of hundreds of mbps. in terms of the nb, in different regions of the world there are different frequency band assignments. the main organisations that regulate the use of frequency bands are: cenelec (european committee for electrotechnical standardisation), arib (association of radio industries and businesses), epri (electric power research institute) and the fcc (federal communications commission). the principle behind plcs involves combining the highfrequency data signal with a low-frequency waveform (50–60 hz). these signals are conveyed on the same cable and, at the end of the transmission, separated by appropriate filters. figure 1. data rate vs range of wireless technologies used in iot systems. figure 2. transmission methods used in powerlines. acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 116 substantial progress has been made with the application of signal modulation algorithms that are coded with a modulation that assigns different frequencies to different subcarriers, as shown in figure 2. the two types of signal are separated by filters using a phasing technique, which recognises the number of subcarriers as a function of frequency. particular attention must be paid to the choice of the type of modulation to be used to maximise the data rate and to ensure a strong noise rejection. 3. the proposed iot system based on plc in this paper an iot system based on plc technology is proposed. the whole system uses a star network topology [8] in which all powerline slave systems (psss) are connected to a master powerline system (mps) via the powerline network. as shown in figure 3, the pss consists of a pulse oximetry transducer connected to a nonin oem iii module. the module generates both red and infra-red signals and acquires and amplifies the output signals captured by a photodiode. the module is connected to a raspberry pi board via a uart link to transfer data to the powerline from the gridcomm module. the pss is connected to the mps using a gridcomm module configured as the master of the powerline network, as shown in figure 4, where the whole system is sketched. the master control system (mcs) uses labview-based software, which provides communication to all the psss present in the powerline network. the mcs allows the real-time display of all data as a function of time and provides local data processing with histograms. the data is stored in a mysql server-based database. the mcs incorporates a web server, developed on the apache/php platform, to publish all the data over the internet. in the following sections, the main parts of the proposed iot system, based on plc technology, are discussed in detail. 3.1. pulse oximetry the operating principle of pulse oximetry is that of spectrophotometry. in fact, the pulse oximeter is a small spectrophotometer in which the source, positioned on one arm of the clamp, emits light radiation at two wavelengths, in the red and infrared fields, at 660 nm and 940 nm, respectively. the red and infrared beams pass through the finger and all its tissues and structures up to the detector placed at the other arm of the clamp. during this phase, the light beams are absorbed by the oxygen-bound haemoglobin (oxyhaemoglobin or hbo2) and the unbound haemoglobin (hb). the oxyhaemoglobin is primarily absorbed in infrared light, while the unbound haemoglobin is mainly absorbed in red light. the pulse oximeter is able to evaluate oxygen saturation levels through the changing absorbance at each of the wavelengths. due to the operating principle on which pulse oximetry is based, it is very important that the probe is positioned on an area where i) there is a superficial circulation and ii) the light radiation can reach the pulse oximeter detector positioned on the opposite side of the light beam source. the pulse oximeter provides the oxygen saturation values as a percentage of haemoglobin linked to the latter: • values between 95% and 100% are generally considered normal, although a 100% oxygen saturation value may indicate the presence of hyperventilation. • values between 90% and 95%, however, are associated with a live hypo-oxygenation. • values below 90% indicate the presence of hypoxaemia, for which it is necessary to perform more in-depth analyses, such as a blood gas analysis. although pulse oximetry is widely used, it still has limits and does not allow for the accurate detection of oxygen saturation if the patient has certain conditions, pathological or not. in fact, if the patient has vasoconstriction, anaemia or methylene blue disease, the oxygen saturation measurement is altered, which is also the case if the patient is wearing coloured nail varnish or has voluntary or involuntary movements during the measurement. finally, it should be noted that pulse oximetry is able to determine the percentage of bound haemoglobin but does not discriminate which type of gas it is bound to. under normal conditions, the haemoglobin is bound to oxygen. when performing the pulse oximetry, it is therefore assumed that the bound haemoglobin is oxyhaemoglobin, which transports oxygen. in this research, a low cost and low power sensor [9], based on the pulse oximetry 8000aa, was used in conjunction with figure 3. the proposed powerline slave system (pss). figure 4. the proposed master powerline system (mps). pulse oximetry 8000aa o e m i ii m o d u le raspberry pi 3 g ri d c o m m g c 8 8 0 2 powerline uart uart master control system g ri d c o m m g c 8 8 0 2 powerline u a r t to u sb mysql server web service apache / php firewall gateway master powerline system internet acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 117 the nonin oem iii module to acquire data and send it to the processing unit built by raspberry pi 3 (see figure 3). 3.2. the powerline module the powerline module chosen for the development of the communication system is the gridcomm gc8802 (see figure 3). the characteristics of this module are the following: i) it has an orthogonal frequency-division multiple access (ofdma) transceiver with a 175 kbps maximum data rate, ii) a repeater mode to increase distance coverage, iii) a frequency range from 5 khz to 500 khz, iv) 3 qpsk, 3 bpsk or 1 bpsk modulation schemes with up to 18 levels of redundancy, v) configurability for fcc, arib and cenelec modes of operation, vi) a builtin afe and line-coupling circuit and vii) an operative distance of up to 3 km [10]. we decided to use this component since it has the following operating modes: i) simple broadcast broadcast communication from any node to a star network without routing or a repeater and without the need to use the master–slave mode; ii) point to point communication between two nodes without routing or a repeater and without the need to use the master–slave mode; iii) an automatic routing master that creates a network in which an auto-routing algorithm is present with addressing via a physical id and sets the device to master; iv) an auto-routing slave that creates a network in which an auto-routing algorithm is present with addressing via physical id and sets the device to slave; v) a broadcast routing master that creates a network using an algorithm that is a combination of broadcast and auto-routing without addressing by physical id and sets the device to master; vi) a broadcast routing slave that creates a network through an algorithm that is a combination of broadcast and auto-routing without addressing via physical id and sets the device to slave. for the proposed application, we adopted the broadcast routing master configuration for the mcs and the slave mode for the powerline network client (the pulse oximetry transducer with raspberry pi). the broadcast routing configuration is optimal because it has both the advantages of broadcast communication (from a device to a network of devices) and routing. as shown in figure 5, the automatic routing mode can increase the connection range of the system due to its multi-hop feature. 3.3. master control system the mcs, based on labview software, retrieves all the data provided by the psss. the software is organised by tabs. the first tab (figure 6) shows all the pulse oximeter signals related to time. both the instantaneous measure and the general trend over time are displayed. the second tab (figure 7) shows the data histograms, acquired from the pss. the third tab (figure 8) stores some information relating to the people being examined, and, in addition, makes it possible to save the configuration path and the pss request/response time (sampling time). on the right side of the tabs, it is possible to see clear information on the status of the people being monitored through the rgb leds. the leds can be green, orange and red depending on the pss value and the relative threshold. if the received value is less than the first threshold, the person is in the warning zone, and then the corresponding led turns orange. if the value falls below a critical value, the software changes the led colour to red and the system immediately sends an emergency e-mail. figure 5. auto-routing mode of module gc8802. figure 6. main panel with pulse oximetry signal vs time. figure 7. histogram panel. figure 8. setting panel. acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 118 3.4. database architecture all data acquired by the mcs is stored in a database server. the database schema for a patient is shown in figure 9. for each remote transducer, identified by the mac address, the distributed system collects all the measurement data under ‘measurement’ in the database. both the value transmitted by the pss and the timestamp are stored to be able to extract the pulse oximeter's trends. in the proposed system, the psss do not present any time synchronisation, otherwise the architecture of the pss would have to be more complex [11]. the timestamp is managed by the mcs, which uses the ntp protocol to keep its time reference synchronised. the data is stored on a mysql server, as shown in figure 4. 3.5. access to data access to data on the internet is guaranteed by an apache/php web server, which has been configured to use a soap-oriented web service. this web service was developed using the php nusoap class, which increases security by limiting access only to those authorised to manage personal data (privacy). it also provides a standard and multi-platform soapbased communication protocol for the development of systems for data exchange over the internet [12]]-[[15]. 4. data transmission and network architecture figure 10 shows the network architecture, with the mcs connected to the nodes (ppss) via the powerline network. this network is a common bus in which each device can initiate a communication. to make the data transmission robust, we decided to manage the clients in such a way that they have to wait for a request from the master; the communication is a request– response type. if the master does not receive a response after a timeout, it repeats the request up to three times, after which it stops communicating with the client. 4.1. data format and communication system the adopted communication protocol is via handshaking. for each request, the client responds with an ack or with data. the messages have the format shown in figure 11, where the function field defines the type of message and the measure field contains the measurement data. at the end, a crc field is used to check if any errors have occurred during the communication. 4.2. network considerations some parameters for the proposed system allow an extension of the network architecture and a reduction in data collisions. to avoid an excessive slowdown of the network, the mps is limited to 240 slave nodes, implemented by the gc8802 module. this is not a big limitation, as, if the application requires more than 240 slave nodes, it is possible to use multiple main nodes to extend the network. the mps involved in the creation of the global network and in the management of the related nodes will build different networks (see figure 12). in this case the network architecture does not change – the database server stores all the information on all the sensors present in the network via the mpss. the network architecture of the proposed system (shown in figure 10) allows for the management of all communication through the nodes since the requests are stored in a queue from which the mcs performs a sequential access, thus avoiding data collisions on the powerline network. if the network architecture adopted is similar to that of figure 12, the neighbouring nodes, which belong to different networks but communicate on the same electric line, could produce data collisions. in fact, if the network structure is extended, the mcss will not be coordinated and, therefore, could make simultaneous figure 9. database architecture. figure 10. network architecture of the proposed system. figure 11. master/slave communication system and data packet format. figure 12. network architecture with subnetwork. mcs pss3 pss2 pss4 pss1 network mysql server web service apache / php master powerline system pss6 pss7pss5 mcs 1 mysql server web service apache / php pss1.3 pss1.2 pss1.4 pss1.5pss1.1 mcs 2 pss2.3 pss2.2 pss2.4 pss2.5pss2.1 mcs 3 pss3.3 pss3.2 pss3.4 pss3.5pss3.1 network 1 network 2network 3 acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 119 transmission requests on the electric line. this problem can be solved by assigning to each network carrier frequencies different from those of the other networks. in this way, each network is a coherent system in which information is exchanged independently and orthogonally to avoid collision problems. another problem is the latency time, which increases as the number of hops needed to connect some remote nodes to the pms increases. in some borderline cases, the round-trip communication time may expire, generating a communication error. a balance has been made between containing the hop numbers, in order to increase network performance, and the consequent reduction in network extension. in our test, we obtained a good compromise by using a one-hop network topology. 5. data transmission considerations powerline technology offers a lot of freedom by using the electricity grid for data transmission, given its widespread use. however, dissemination is also a disadvantage in that the transmitted data is accessible to all. for this reason, the sensitive data transmission between master and client has been reinforced by a series of measures. the gc8802 module uses the ofdma plc transceiver, which has 18 independent channels capable of communicating over 54 carrier frequencies in a wide range from 5 khz to 500 khz. an appropriate choice of transmission parameters makes communication ‘non-standard’ and, therefore, difficult to access and attack by unauthorised persons. greater security can be achieved with communication encryption algorithms. this option was not implemented in this phase since the three-byte data packet would be penalised by the transmission of a long private key. 6. results of the proposed systems in order to test the performance of the proposed system, measurements have been carried out in order to evaluate the round-trip time and the data packet losses. the measurements have been performed using the platform presented in [6]. the platform models a medium-sized house with an area of 74 m2 into which electronic equipment can be plugged in order to study the behaviour of the plc under real conditions. the measurement setup reported in figure 13 has been used to study the performance of the plc module in the multi-hop transmission system. a schematic view of the whole measurement system is reported in figure 14. in particular, the measurement setup is composed of i) a gc8802 powerline modem used to perform the master/slave communication, ii) a uart to usb module in order to connect the modem to a pc, iii) a tektronix tds5054 used to acquire the round-trip time and evaluate the mean and standard deviation time and iv) labview software developed in order to simulate the master and slave behaviour, to implement the communication protocol and to check the frame. the obtained results have been reported in table 1, where, by increasing the hops, the frame error rate increases with the round-trip time. a good balance between performance and hop can be obtained using just one hop. in fact, even if the number of hops increases the network range, the communication between the two gc8802 modems is guaranteed for a distance of 3 km, as reported in the datasheet. for this reason, one hop can increase the operating range of the distributed sensors network without communication being figure 13. setup used to measure the performance of the proposed system. figure 14. platform test. table 1. results of the proposed system for different hop configurations. hop round-trip time (ms) packet loss mean rms error rate 0 1.7 0.6 1/500 1 4.4 1.6 1/300 2 13.6 2.9 1/100 uart usb gc8802 uart usb powerline powerline uart usb gc8802 uart usb standard electrical system socket socket tektronix tds5054 cha chb master slavecheck frame labview software acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 120 compromised by an increase in the error frame rate. using the results provided by these tests, a timeout of 10 ms has been adopted in the master/slave communication system, as reported in figure 11. 7. conclusions in this document a new iot system for pulse oximetry applications with powerline technology has been reported and discussed. the system is composed of a slave system that acquires and sends data through a powerline network and a master system that receives and displays data on a gui and saves them on a database server. the data can be accessed on the internet using a web service based on an apache/php architecture using the nusoap class. some measurements have been performed to study the 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https://www.sensorsportal.com/html/digest/june_2013/p_1241.pdf buck-based dmppt emulator: a helpful experimental demonstration unit acta imeko issn: 2221-870x june 2021, volume 10, number 2, 14 23 acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 14 buck-based dmppt emulator: a helpful experimental demonstration unit marco balato1, annalisa liccardo1, carlo petrarca1, francesco bonavolontà1 1 department of electrical and information technologies, university of naples federico ii via claudio 21, napoli (na), italy section: research paper keywords: distributed maximum power point tracking; mismatching; pv emulator citation: marco balato, annalisa liccardo, carlo petrarca, francesco bonavolontà, buck-based dmppt emulator: a helpful experimental demonstration unit, acta imeko, vol. 10, no. 2, article 4, june 2021, identifier: imeko-acta-10 (2021)-02-04 section editor: ciro spataro, university of palermo, italy received december 15, 2020; in final form may 1, 2021; published june 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: marco balato, e-mail: marco.balato@unina.it 1. introduction the world is currently in a dramatic climate crisis due to the growth of greenhouse gas emissions. to reduce climate change, a sustainable energy transition, oriented to the achievement of the carbon neutrality, is needed. in this direction, the total production of electricity, using renewable energy sources, represents the main goal. to increase the distributed green energy generation, photovoltaic (pv) systems are among the most promising renewable sources. in pv applications, the heaviest working conditions, not only from an energetic efficiency point of view [1]-[5], but also from a reliability one [6]-[8], are known as “mismatching conditions”. in particular, when mismatching conditions occur, due to clouds, shadows, dirtiness and/or different orientation of pv modules etc., the commonly used grid-connected pv systems, made of string of pv modules connected in parallel and feeding a central inverter (figure 1), are ineffective [1]-[8]. various maximum power point tracking (mppt) control techniques have been proposed in the literature to track the mpp and force the system to work at the mpp point, including high-performance mppt techniques [9]-[19], reconfiguration architectures and algorithms, [20]-[29] and distributed mppt (dmppt) approach [30]-[41]. among of all these techniques, the dmppt (figure 2) is by far the most promising solution to enhance the reduced energetic performance of mismatched pv systems [30]-[41]. differently from the centralized approach of figure 1, dmppt uses a mppt module dedicated dc/dc converter realizing the mppt for each pv module. nevertheless, there are still several factors limiting such performance, including the efficiency of the power stage, constraints imposed by the topology, the finite rating of silicon devices, atmospheric conditions, and the suboptimal value of string voltage [39]-[43]. the fact that many of these factors are not under our control represents a severe restriction in conducting experimental test activities when real dmppt pv systems are considered. what emerges is the necessity of the implementation of a dmppt emulator as a helpful tool to emulate the output i-v characteristics of many commercial pv modules with a dedicated buck dc/dc converter not only in controlled environment conditions (as it happens for common pv emulators [42]-[45]) but also with different currents rating of the switching devices. based on the abstract distributed control strategy represents the most promising solution to enhance the lackluster energetic performance of mismatched photovoltaic (pv) systems. moreover, many factors that contribute to such poor performance are still to be explored. to fully understand the advantages offered by the distributed maximum power point tracking (dmppt) approach, the implementation of a dmppt emulator is necessary. based on the above needs, this paper describes the realization and use of a buck-based dmppt emulator and shows its high flexibility and potential. the realized device is capable to emulate the output current vs. voltage (i-v) characteristics of many commercial pv modules with a dedicated buck dc/dc converter not only in controlled atmospheric conditions but also with different currents rating of the switching devices. the system implementation is based on a commercial power supply controlled by a low-cost arduino board. data acquisition is performed through a low-cost current and voltage sensor by using a multichannel board by national instruments. experimental results confirm the validity and potential of the proposed dmppt emulator. mailto:marco.balato@unina.it acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 15 above considerations, the current study describes the realization and use of a buck based dmppt emulator. the repeatability of experimental tests, the tighter control of climatic conditions, the narrowing of the gap between the physical dimensions of a pv plant and the space available in a university lab, the simplicity with which new algorithms can be tested, and the low maintenance costs are just some of the benefits offered by the emulator presented in the paper. the proposed buck based dmppt emulator consists of a commercial power supply controlled by a low-cost arduino board whose control strategy is based on a set of equations defining the mathematical model of a dmppt device. the present paper is organized as follows: the mathematical model of a single buck based dmppt unit is described in detail in section 2; section 3 highlights the still open issues regarding the dmppt approach; section 4 is dedicated to the design and description of the proposed buck based dmppt emulator; experiments and tests are presented in sections 5; finally, section 6 draws the conclusions. 2. mathematical model of a buck based pv unit in the following, the system shown in figure 3 will be considered and analysed. it is composed of a pv module equipped with its own buck dc/dc converter realizing the dmppt function. to simplify the readability, the considered system will be indicated with the acronym “b-pvu” that means buck based photovoltaic unit. in figure 3, ipv (vpv) and ib-pvu (vb-pvu) denote, respectively, currents (voltages) at the input and output ports of the buck dc/dc converter. moreover, the symbol ppv indicates the power extracted from the pv module, while the symbol pb-pvu indicates the power at the output of the converter. the pv module is described by a single diode model, in which: the current generator represents the photo-induced current, the diode d1 takes into account the effects at the silicon p–n junction of the pv cell; the series (rs) and parallel (rsh) resistances take into account the loss mechanisms taking place in the pv module due to metallic ribbon. in figure 4, the typical output static current vs. voltage (i-v) characteristics of a pv module (dashed line) and of a b-pvu (bold line) are reported, at constant irradiance (s) and temperature (t) values. losses occurring in the power stage of the buck converter (switching, conduction, and iron losses) and the settling time of the step response of a closed or open loop b-pvu are neglected. in addition, the mppt efficiency of the dmppt controllers is supposed to be equal to one (ηdmppt = 1). in these hypotheses, the output static i-v characteristic of a single b-pvu is marked by the presence of three different operating regions: best operating region (bor), and two worst operating regions (wor1 and wor2). the adjectives best and worst are not used randomly insofar as they allow to discriminate operating points with high efficiency with respect to the other ones. worst operating region 1 (wor1): in the wor1, defined for 0 v ≤ vb-pvu ≤ v1, the output i-v characteristic of the bpvu is flat and equal to: 𝐼b−pvu = 𝐼dsmax . (1) to explain the meaning of the voltage value v1 some preliminary considerations are necessary. since the buck converter is able to lower its output voltage vb-pvu with respect to the input voltage vpv, and by considering that, when the pv module is working in its mpp, it must be: 𝑉b−pvu ⋅ 𝐼b−pvu = 𝑃mpp , (2) where pmpp is mpp power. it is evident that, as long as the pv module operating point matches the mpp, the lower the output voltage the higher the output current. by indicating with idsmax the maximum allowed value of ib-pvu without harming any silicon device (i.e. power mosfets) we get: 𝑉1 = 𝑃mpp 𝐼dsmax . (3) figure 1. commonly used grid-connected photovoltaic (pv) system. figure 2. commonly used dmppt grid-connected photovoltaic (pv) system. figure 3. circuit model of b-pvu. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 16 therefore, v1 is the lower limit of the output voltage vb-pvu when the output power pb-pvu assumes its maximum value and it can be calculated by using (3) once idsmax is known. in practice, the information regarding the value of idsmax is included in the silicon devices’ datasheet. best operating region (bor): bor, defined for v1 ≤ vb-pvu ≤ vmpp (rbor-v = [v1, vmpp]) is described by a hyperbole of (4), where vmpp is the mpp voltage. 𝐼b−pvu = 𝑃mpp 𝑉b−pvu . (4) as shown in figure 4, the current range (rbor-i) associated to bor (optimal current range) is defined as follows: 𝑅bor−i = [𝐼mpp , 𝐼dsmax] . (5) worst operating region 2 (wor2): wor2 is defined for vmpp ≤ vb-pvu ≤ voc, where voc is the open circuit voltage that can be provided by the adopted pv module in the considered atmospheric conditions: 𝑉oc = 𝑉ocstc [1 + 𝛼v 100 (𝑇 − 𝑇stc)] , (6) where the vocstc ( tstc) is the open circuit voltage (temperature) in the standard test conditions (sstc = 1000 w/m2, tstc = 25°c) and 𝛼v is the voltage temperature coefficient. in such a region, the characteristic of the controlled pv module coincides with the pv module one. in particular, for vmpp ≤ vb-pvu ≤ voc, the dmppt controller forces the buck dc/dc converter to work with a duty cycle equal to one. in this condition the current ib-pvu is equal to: 𝐼b−pvu = 𝐼pv = 𝐼ph − 𝐼d − 𝐼rsh , (7) where iph is the photo-induced current, which, in accordance with (8), is linearly dependent on the irradiance level (s) and the pv module temperature (t), id is the current in diode d1 (9), and irsh is the shunt-resistor current (10): 𝐼ph = 𝐼scstc 𝑆 𝑆stc (1 + 𝛼i 100 (𝑇 − 𝑇stc)) (8) 𝐼d = 𝐼sat (𝑒 𝑉pv+𝑅s𝐼pv 𝑉t − 1) (9) 𝐼rsh = 𝑉pv + 𝑅s𝐼pv 𝑅sh , (10) where vt is the thermal voltage, 𝛼i is the current temperature coefficient and isat is the diode reverse bias saturation current (11): 𝐼sat = 𝐶 𝑇 3 𝑒 (− 𝐸gap 𝑘 𝑇 ) , (11) where k = 1.38 j/k is the boltzmann constant, egap is the band gap of the semiconductor material (in the following it is assumed egap = 1.124 ev), and c is the temperature coefficient [46]. the i-v output characteristics of a single b-pvu are strictly dependent on the irradiance and temperature levels (12). typical curves are shown in figure 5. 𝑇 = 𝑇ambient + 𝑁𝑂𝐶𝑇 − 20 800 𝑆 . (12) since in real environmental conditions the temperature usually changes quite slowly with respect to variation of the irradiance level occurring during the day, all subsequent results were obtained by considering a constant value of the pv module temperature equal to 57.5 °c (t = 57.5 °c) which corresponds to tambient = 25 °c. in figure 6, the i-v output characteristics of a single b-pvu, obtained by changing both the irradiance (s) and idsmax values are reported. from the figure 5 and figure 6 the non-stationarity behaviour of the bor emerges, especially when time-varying atmospheric conditions occur. in particular, the higher the value of irradiance s, the lower the amplitude of the optimal range, assessed in terms both of the current and the voltage. (rbor-i and rbor-i). inverse conditions occur for the value of idsmax. figure 4. pv module i-v characteristic and b-pvu i-v characteristic. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 17 3. open issues in the previous section it has been shown that, under nonstationary operating conditions, the best operating region (bor) of a single b-pvu, whether assessed in terms of the current or the voltage, has time-varying characteristics (figure 5 and figure 6). of course, the same considerations can be extended to more complex systems, which consist of many strings of b-pvus connected in parallel. without loss of generality, it is possible to consider, as a reference case, a pv system with a simple topology; in particular, a pv system, consisting of a single string of ns = 2 b-pvus (b-pvu1 and bpvu2), will be analysed in the following (figure 7). the standard test conditions (stc) parameters of the considered commercial pv module (sunmodule sw225 [47]), are reported in table 1. in figure 8, the output static power vs. voltage (p-v) characteristics of the reference system are reported in three different mismatching scenarios and with three different values of idsmax (case i, case ii and case iii). in such cases, as shown in figure 8, the adopted distributions of the irradiance values (s) characterizing the pv modules, the values of the maximum allowed current (idsmax) provided by the silicon devices and the value of the ambient temperature (tambient) are the following ones: [s] = ([1000; 800]) w/m2, idsmax = 8 a (case i), [s] = ([1000; 600]) w/m2, idsmax = 10 a (case ii), [s] = ([1000; 200]) w/m2, idsmax = 12 a (case iii), tambient = 25 °c. it is worth clarifying that the j-th component of vector [s] represents the constant irradiance value of the j-th pv module of the array (j=1, 2). in figure 8, the three different best operating regions, correspondig to the three different scenarios, are highlighted. figure 5. i-v characteristics of b-pvu. figure 6. i-v characteristics of b-pvu. table 1. solarworld sw 225 pv module electrical characteristics in stc, (sstc = 1000 w/m2; tsct = 25 °c). stc open circuit voltage 𝑉ocstc = 36.7 v stc short circuit current 𝐼scstc = 8.13 a stc maximum power point voltage 𝑉mppstc = 29.7 v stc maximum power point current 𝐼mppstc = 7.59 a voltage temperature coefficient 𝛼v = −0.34% / k current temperature coefficient 𝛼i = 0.034% / k nominal operating cell temperature 𝑁𝑂𝐶𝑇 = 46 °c acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 18 this means that, the energetic performances of pv systems, adopting the buck based dmppt approach, strongly depend on the value assumed by the string voltage (vstring), that, necessarily, must be contained in the bor. for such a reason, the string voltage must be considered as a variable to be controlled through a proper mppt algorithm that acts at the output of the entire pv system. this approach is known with the acronym hmppt that means hybrid mppt since it is based on the joint adoption of both central and distributed approaches. as a further example a more complex pv system, composed of a string of ns = 11 bpvus can be considered. the p-v characteristics are plotted in figure 9 and refer to the following parameters: [s] = ([1000; 1000; 1000; 1000; 800; 800; 800; 700; 700; 600; 600]) w/m2, idsmax = 8 a (case i), [s] = ([650; 650; 500; 500; 500; 500; 400; 400; 300; 300; 200]) w/m2, idsmax = 10 a (case ii), tambient = 25 °c. also in this case, for an optimal energetic performance, the bor must be continuously tracked. the above considerations have general validity since, as shown in [38]-[40], they do not depend on the size of pv system and/or on the type of the used dc/dc converters such as buck, boost and buck-boost. moreover, recent studies have highlighted the possibility to combine the dmppt and reconfiguration approaches as a powerful strategy for increasing the efficiency of pv systems [41]. this means that the opportunities offered by the dmppt approach are still to be explored. to fully understand their benefits, it is necessary to have access to a tool, able to emulate not only the i-v characteristic of a single b-pvu but also of a string of generic number of b-pvus, in controlled atmospheric conditions and with different values of idsmax. the possibility to swap the value of idsmax allows the emulation of many buck converters commercially available, which results in a consistent reduction in time and cost. it is expected that the proposed tool will give new impetus to the advance in pv applications since it represents a useful incentive for both development and testing of new mppt algorithms suitable for dmppt implementation. moreover, it is possible to experimentally validate the performances offered by the joint adoption of dmppt and reconfiguration approaches. the newly emerged concepts represent the main motivations that pushed the authors to the development of an experimental demonstration unit based on the buck dc/dc converter. the proposed emulator is unique as confirmed by the absence both in literature and on the market of an equivalent device, able to emulate the i-v characteristic of a dmppt unit based on the buck converter. 4. implementation of b-pvu emulator a block diagram of the proposed b-pvu emulator is shown in figure 10. it consists of three fundamental blocks: power block (pb), control block (cb) and acquisition block (ab). power block (pb): pb is realized by means of two commercial power supplies (kepco bop 100-4 [48]). one represents the power stage of the b-pvu emulator and is used as a current-controlled source whose output current ipvu(t) is regulated by means of a proper controller. the other power supply is used to obtain a controlled electronic load able to properly scan the i-v characteristics of the b-pvu emulator. the two power supplies are suitable to work in all four quadrants of the current–voltage plane. they are linear power supplies with two bipolar control channels (voltage or current mode), selectable and individually controllable by either front panel controls or remote signals. the input signal of the pb (sref (t)) is achieved based on the following equation: 𝑆𝑟𝑒𝑓 (𝑡) = 𝑆(𝑡) ∙ 𝐼𝐵−𝑃𝑉𝑈 (13) where s(t) represents the time varying irradiance value. concerning the value of ib-pvu, it is obtained according to (1), (4) and (7). control block (cb): cb consists of controlling and conditioning units. the embedded board “arduino mega 2560” is used as controlling unit. the “arduino mega 2560” is powered via a usb connection and provides 54 digital input/output pins (15 of which can be used as pwm outputs) and 16 analog inputs, and it can be programmed through arduino ide software [49]. figure 7. pv system under test. figure 8. p-v characteristics of a string of two b-pvus. figure 9. p-v characteristics of a string of eleven b-pvus. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 19 the analog input signals of the microcontroller, which are marked with an asterisk (figure 10), represent a scaled version of the corresponding signals vb-pvu(t), ib-pvu(t), vs(t) and idsmax. such scaling is necessary to adapt the electrical characteristics of such signals to the limited range [0,5] v of the microcontroller. regarding the conditioning units, they are divided in: (a) the input conditioning unit and (b) the output conditioning unit. the input condition unit consists of a voltage (current) sensor for sensing and adapting the pvu output voltage (vb-pvu(t)) (current ib-pvu(t)) to the maximum allowed input voltage (v*b-pvu(t) and i*b-pvu(t)) and a national instruments generation board (bnc2100 series connector blocks), which is used to reproduce the input signals v*s(t) and i*dsmax. to configure the data generation hardware (bnc-2100 series connector blocks) matlab data acquisition toolbox is used. the arduino’s input signals must fulfil the following equations: 𝑉𝐵−𝑃𝑉𝑈 ∗ (𝑡) = 5 ∙ 𝑉𝐵−𝑃𝑉𝑈 (𝑡) 𝑉𝑀𝐴𝑋 ∈ [0, 5]𝑉 (14) 𝐼𝐵−𝑃𝑉𝑈 ∗ (𝑡) = 5 ∙ 𝐼𝐵−𝑃𝑉𝑈 (𝑡) 𝐼𝑀𝐴𝑋 ∈ [0, 5]𝑉 (15) vs ∗(t) = 5 ∙ αs ∙ s(t) sstc ∈ [0, 5] v (16) 𝐼dsmax ∗ = 5 ∙ 𝐼dsmax 𝐼max ∈ [0, 5] v . (17) where vmax = 100 v and imax = 4 a are the maximum allowed values of the output voltage and current from the power unit: s = 1 v m2/w. the adopted current sensor is an “ina169 current sensor module”, which is a high-side, unipolar, current shunt monitor, that allows the measurement of continuous current up to 5 a. to reduce the pvu output voltage up to 10 times compared to the original, a voltage divider formed by two resistances of 220 kω and 11.5 kω, respectively, was adopted as a voltage sensor. at the end, the output conditioning unit consists of a digital analog converter (dac) “adafruit mcp4725”. acquisition block (ab): ab consists of a commercial national instruments multichannel usb data acquisition system (ni compactdaq with ni9215 modules characterized by 16bit resolution and maximum sampling frequency of 100 ks/s) that allows to back up the experimental data in matlab environment. the experimental setup was designed and built in the circuit laboratory of the university of naples federico ii and is shown in figure 11. in table 2, the electrical characteristics of the proposed bpvu emulator are reported. to verify the performances of the proposed emulator, a preliminary test activity on the system composed of a microcontroller “arduino mega 2560” and a dac “mcp4725” has been carried out. in particular, two different operating conditions (oc1 and oc2) have been chosen, corresponding to different mismatching scenarios. oc1 refers to the following parameters: s = 200 w/m2, tambient = 25 °c, and to two different values of the maximum allowed current (idsmax = 4 a and idsmax = 3 a, respectively). the oscilloscope (tektronix mdo 3034) screenshots shown in figure 10. block diagram of proposed b-pvu emulator. table 2. electrical characteristics of the proposed b-pvu emulator. maximum output current 𝐼b−pvumax = 5 a maximum output power 𝑃b−pvumax = 150 w maximum output voltage 𝑉b−pvumax = 36 v acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 20 figure 12 represent the (i-v) output characteristics of the emulator for idsmax = 4 a (turquoise curve) and for idsmax = 3 a (pink curve). the i-v characteristic of the proposed b-pvu emulator have been scanned by applying a periodic (frequency 1 hz) ramp signal (yellow curve of figure 12) at the microcontroller input v*b-pvu(t). the amplitude of the adopted ramp signal varies from 0 v to 1.66 v which corresponds to the b-pvu ramp voltage vb-pvu(t) ranging from 0 v to the open circuit voltage voc at t = 57.5 °c (voc = 33 v in (12). the comparison of the experimental characteristic obtained when idsmax = 4 a (turquoise curve) with the theoretical one (white curve) shown in figure 13 confirms the excellent capability of the emulator. the output characteristics have also been obtained in a different operating condition (oc2). it refers to the following parameters: tambient = 25 °c, idsmax = 3 a and to two different values of irradiance (s = 300 w/m2 and s = 200 w/m2, respectively). the oscilloscope screenshots of figure 14 represent the (i-v) characteristics when s = 300 w/m2 (turquoise curve) and s = 200 w/m2 (pink curve). to fully explore the potentialities of the proposed emulator, in figure 15, the time-domain behaviour of the signal s*ref, obtained by considering a pv system composed of a string of two b-pvus is reported. in such a case the adopted distribution of the irradiance values (s) characterizing the pv modules, the values of idsmax and the value of tambient are the following: [s]=([300; 200]) w/m2, idsmax = 4 a (turquoise curve), idsmax = 3 a (pink curve) and tambient = 25 °c. the amplitude of the ramp signal (yellow curve), differently from the previous cases, varies from 0 v to 3.32 v which corresponds to b-pvu ramp voltage vb-pvu(t) ranging from 0 to the open circuit voltage voc at t = 57.5 °c of the considered string (voc = 66 v). in figure 16 the experimental i-v curves (black lines), acquired at the output port of the power block, are reported together with the theoretical ones (white lines) when idsmax = 4 a. from the figures it can be observed that the experimental and theoretical curves are nearly superimposed, thus confirming the excellent capability of the proposed solution of emulating bpvu behavior. at the end, a montecarlo analysis has been performed on the model, considering the irradiance measurement affected by an uncertainty (gaussian probability density function) equal to the figure 11. experimental setup of proposed b-pvu emulator. figure 12. oscilloscope screenshot (v-i curve in operating condition oc1). figure 13. comparison between experimental and theoretical i-v curves: s = 200 w/m2, tambient = 25 °c, idsmax = 4 a. 0 5 10 15 20 25 30 35 voltage [v] 0.5 1 1.5 2 2.5 3 3.5 c u rr e n t [a ] experimental i-v curve theoretical i-v curve acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 21 5 % of the reading, according to pyranometer datasheet available on the market. moreover, a tolerance (uniform probability density function) equal to 3 % has been associated with the value of idsmax; this value has been set according to the power tolerance declared in the specifications of the pv module solarworld sw 225. for each quantity 10000 realizations have been drawn according to the considered pdfs. the corresponding output of the maximum power point current impp has been achieved; the corresponding experimental pdf is shown in figure 17. the observed mean value is equal to 7.60 a and the standard uncertainty is 0.38 a (about 5 %). in figure 18 the i-v characteristics of b-pvu are shown. in particular, the portion of i-v plain corresponding to the considered input quantities variability can be appreciated. 5. conclusion in this paper, a buck based dmppt emulator has been presented and discussed. a detailed study was carried out in order to clearly understand the set of equations on which the mathematical model of the b-pvu is based on. the proposed emulator was designed to reproduce the i-v characteristics at different values not only of the irradiance levels but also of the value of idsmax. moreover, the possibility to emulate a string of b-pvu is also provided. the high flexibility offered by the proposed solution allows to fully explore the performance of the dmppt approach in academic laboratories. moreover, the possibility to swap the value of idsmax allows the emulation of many buck converters, which results in a consistent reduction in time and cost. in particular, the proposed device represents a suitable compromise between time and cost insofar as the inexpensive choice to adopt a commercial power supply is compensated by the possibility to emulate the behaviour of many commercial devices. the proposed experimental results fully confirm the validity of the proposed emulator. figure 14. oscilloscope screenshot (v-i curve in operating condition oc2). figure 15. oscilloscope screenshot (v-i curve in a string of two b-pvus). figure 16. comparison between experimental and theoretical i-v curves: s = (300, 200) w/m2, tambient = 25 °c, idsmax = 4 a. figure 17. observed experimental pdf of impp in monte carlo analysis. figure 18. set of i-v characteristics obtained in monte carlo analysis. 0 10 20 30 40 50 60 70 voltage [v] 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 c u rr e n t [a ] experimental i-v curve theoretical i-v curve acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 22 references [1] n. d. kaushika, a. k. rai, an investigation of mismatch losses in solar photovoltaic cell networks, energy 325 (5) (2017), pp. 755– 759. doi: 10.1016/j.energy.2006.06.017 [2] c. w. a. baltus, j. a. eikelboom, r. j. c. van zolingen, analytical monitoring of losses in pv systems, proceedings of the 14th european photovoltaic solar energy conference, barcelona, spain, 1997, pp. 1547–1550. 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http://www.solarworld-usa.com/~/media/www/files/datasheets/sunmodule-plus/sunmodule-solar-panel-225-mono-ds.pdf http://www.solarworld-usa.com/~/media/www/files/datasheets/sunmodule-plus/sunmodule-solar-panel-225-mono-ds.pdf https://www.kepcopower.com/bop.htm https://www.arduino.cc/en/main/software performance evaluation of a vehicular edge device for customer feedback in industry 4.0 acta imeko issn: 2221-870x december 2020, volume 9, number 4, 88 95 acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 88 performance evaluation of a vehicular edge device for customer feedback in industry 4.0 marianne silva1, gabriel signoretti1, ivanovitch silva1, paolo ferrari2 1 federal university of rio grande do norte – postgraduate program in electrical and computer engineering (ppgeec), natal, brazil 2 university of brescia, brescia, italy section: research paper keywords: industry 4.0; vehicles; obd-ii; edge computing; pollution citation: marianne silva, gabriel signoretti, ivanovitch silva, paolo ferrari, performance evaluation of a vehicular edge device for customer feedback in industry 4.0, acta imeko, vol. 9, no. 4, article 12, december 2020, identifier: imeko-acta-09 (2020)-04-12 section editor: francesco bonavolonta, university of naples federico ii, italy received september 26, 2019; in final form december 1, 2019; published december 2020 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding authors: marianne silva, e-mail: dinizmarianne@edu.ufrn.br, ivanovitch silva, e-mail: ivan@imd.ufrn.br 1. introduction various industries have been undergoing several modifications since the advent of the internet of things (iot) concept, which is changing the way human beings interact with the world. this is accomplished through the information exchange between interconnected physical objects and platforms. these can communicate with each other as well as with users through smart sensors and software that transmit data through a given network [1], [2]. it essentially presents an emergent paradigm as it comprises hardware infrastructure, software, and services that enable physical objects, known as ‘things’, to be connected to the internet [3], [4]. due to the new logistical challenges faced within the industrial ecosystem, the concept of the industrial internet of things (iiot) has emerged, which is considered to be one of the main stimulus mechanisms of the current revolution in the field of industrial automation [5]]-[7]. this concept, in turn, relates to the notion of industry 4.0, a term introduced in 2011 as an initiative of the german government with a focus on improving the efficiency in the manufacturing industry [8], [9]. industry 4.0 disseminates how business objectives, intelligent algorithms, analytics, predictive technologies, and cyber-physical systems should be combined to establish a new way of thinking about production management and factory transformation [10]. furthermore, as [11]]-[13] note, industry 4.0 is aimed at developing intelligent products that inherently collect and store information throughout their lifecycle, providing answers that can be used to improve the development and production processes. it is notable that the various mechanisms that make up the industrial and commercial matrix are becoming more intelligent. here, the data generated at all levels of the production process can be used to improve product quality, flexibility and productivity [5], [9]. in this scenario, recent years have seen the automotive industry focus on the development of smart vehicles equipped abstract industry 4.0 is the term used to specify the current industrial revolution, not only from a technological point of view but also from economical, sociological and strategical points of view. the revolution involves several traditional economic sectors, as is the case with the industrial ecosystem. the main benefits are related to creating value during the entire product lifecycle and in terms of customer feedback, which is particularly relevant to the automotive industry. its disruptive diffusion is due to various enabling technologies, such as the internet of things (iot), and, as such, it is a vision rather than a technological step forward. thus, this paper investigates a performance evaluation of an edge obd-ii device, which collects data from vehicles in an autonomous way in order to provide customer feedback and tracking. the metrics evaluated were different sets of obd-ii parameter ids (pids), responsiveness, driver behaviour and co2 pollution estimates. the experiments were performed using three vehicles in urban and highway areas in the city of natal, brazil. for validation purposes, the results obtained from the vehicles were compared with an obd-ii emulator, which demonstrated the accuracy of the experiments. mailto:dinizmarianne@edu.ufrn.br mailto:ivan@imd.ufrn.br acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 89 with various sensors [14], with interfaces also available to automatically capture and extract digital information through the sensors and the communication protocols present in the cars themselves [3]. it is expected that the vast majority of vehicles manufactured up until 2035 will have the capacity for wireless communication [14], with the vehicles connected via various types of devices (e.g. sensors, smartphones, cameras), wireless communication protocols and media broadcast systems [15]. these advancements can be categorised as the development of a new paradigm, that is, the ioiv paradigm [16], [17]. as a result, the extracted data from the various vehicle systems can be used for interactions within a new processing and communication context, leading to a revolution in the way vehicles are used [3], [18], [19]. as such, a combination of the ioiv concept and industry 4.0 can enable a more efficient and accurate decision-making process for vehicle manufacturing organisations [20]. as industry 4.0 is focused on exchanging and collecting information throughout the entire product lifecycle [12], these results can be accomplished by promoting a feedback loop from the final users (the vehicle owners) and the manufacturers (i.e. a constructive return of the vehicle information to the industry). in order to enable services such as those previously mentioned, a number of solutions have been proposed in the existing literature [3], [18], [19], [21], which include the means to capture vehicular data within the context of the ioiv. these studies adopt a smartphone or a raspberry pi as a communication module alongside the vehicle's on-board diagnostics (obd-ii) system, a self-diagnosis system available on most vehicles currently in circulation [22] (figure 1a). in this work, the freematics one+ is adopted,1 which is a device that does not depend on a smartphone to communicate with the obd-ii interface, as shown in figure 1b. in fact, the device operates autonomously with access to the vehicle's engine control unit (ecu), making it possible to perform computations on the device itself before sending data to the cloud. the freematics one+ is an arduino-based programmable vehicle telematics prototyping platform in the form of an obd dongle. it plugs into a vehicle’s obd port and works as a standalone device with access to the vehicle’s ecu, the high-rate global navigation satellite system (gnss), the 9-dof (degrees of freedom) motion sensor and possibly a number of external sensors. the collected data can be processed in real time, stored in an internal flash or micro sd card and transmitted via bluetooth, wifi or 3g connections. the autonomous operating mode of the freematics one+ presents a direct benefit for the implementation of edge computing, a technology applied to devices that performs data processing and can ensure maximum security and reliability [23], [24], while it also provides the capacity of local storage and computing [23], meaning the device can operate even when not connected to the internet. due to these properties, an important factor that must be taken into account when using edge computing is the response time of the requests. given that some of the current applications require a response time of around 20 ms, a substantial delay can cause serious damage to the driver or the machine [25], [26]. within this context, this paper aims to evaluate the response time of the obd-ii parameter id (pid) requests and proposes a state machine to model the driver behaviour while evaluating the co2 emission estimation. all the data were collected via an 1 https://freematics.com/pages/products/freematics-one-plus/ experiment involving vehicles that were selected in the city of natal, brazil for convenience. the results indicated that the request time of the pids depends on each vehicle and not on the type or quantity of the pids themselves. the remainder of the paper is organized as follows. first, section 2 outlines the obd-ii system. section 3 then discusses the related works, while the experimental evaluation methodology is described in section 4. the results are provided in section 5, while section 6 outlines the issues with the validity of the study. finally, the conclusions and the future directions are provided in section 7. 2. obd-ii the obd-ii is a hardware that is connected to the vehicle's ecu, as shown in figure 2. it provides access to the status of the various vehicle subsystems, allowing the reading and transmission of the data measured by the sensors and actuators of the vehicle in real time [27]. this interface allows for connection to the vehicle through various different protocols. however, generally, each vehicle implements only one of these protocols. the most commonly used protocol is the controller area network (can) created by bosh in 1980. the main rationale behind its creation was to allow communication between different ecus. meanwhile, a variety of other widely used protocols exist, including sae j1850 pwm, sae j1850 vpw, iso 9141-2 and iso 14230 kwp2000 [28]. the obd-ii technology implements 10 operating modes, with each mode designed for a specific purpose. for every mode, there are distinct sets of available commands that are used to obtain data from the sensors and actuators of the vehicles [3]. here, the data is requested using codes known as parameter ids or pids. however, manufacturers are not required to implement all modes in their vehicles. therefore, prior to any data request, it is necessary to check which commands are supported by the vehicle. the data collection and analysis system (obd-ii) is plugged into the vehicle’s obd-ii port and is subsequently configured (through its internal connection or via smartphones) to send the recorded data to the cloud database through mobile networks. (a) traditional obd-ii (b) obd-ii freematics one+ figure 1. architecture overview. https://freematics.com/pages/products/freematics-one-plus/ acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 90 3. related works following a systematic review of the literature, several papers were identified that influenced the research carried out and, therefore, contributed to the development of the proposed solution. a fleet management system based on the obd-ii was implemented in [29]. here, an obd-ii reader was designed to measure the speed and mass air flow (maf) from which the distance and fuel consumption were also calculated. many tests were performed to assess the system functionalities, while none were related to the system performance. similarly, a customer feedback platform for the automotive industry within the context of industry 4.0 is described in [3]. such a platform is capable of collecting and analysing data from sensors available in the vehicles through an obd-ii scanner. this platform aims to assist in the management, prevention, and mitigation of different vehicular issues. what differs from the proposed solution is that, much like with the previously mentioned work, the response time of the pids requests was not evaluated. working along similar lines, a graph database solution to industry 4.0 was proposed in [30] to manage the large amount of data generated by the sensors disseminated inside and around the vehicle in the urban environment. a number of benefits were recognised through the geo-localisation of the vehicles and the identification of specific events. however, this solution differs from the proposed solution in that it only evaluates the performance of the vehicle based on the data collected. elsewhere, a platform was proposed in [22] with the aim of estimating the amount of co2 emissions based on the readings of the vehicles sensors in order to monitor vehicular pollution. however, it was not applied to the context of industry 4.0. furthermore, a mobile measurement system for urban pollution monitoring that can be installed on public ground transportation vehicles was proposed in [31]. this system includes several important features that make it suitable for the specific purpose, namely, the low cost, the reduced dimensions, the autonomous power supply, and the measurement strategy capable of minimising the measurement uncertainty. however, it differs from the proposed work as it does not make use of the obd-ii. in fact, it is clear that the obd-ii reading performance in terms of all sensors has yet to be evaluated, which means there remains a gap in the literature that must be explored to promote the development of new solutions incorporating obd-ii and ioiv technology. 4. experiment this section describes the methodology used to plan and execute the experiments. 4.1. goal definition the main objective of this paper was to evaluate the response time of the obd-ii pids requests. in addition, a state machine for modelling driver behaviour was proposed in order to clarify the results, while estimations of co2 emissions were also performed. 4.2. planning and evaluation design this subsection details all the evaluation design aspects. in terms of context selection, the evaluation targeted vehicles manufactured in brazil from 2010 on, since this was the year when the implementation of the obd-ii system in vehicles became mandatory. meanwhile, in terms of research context, the questions we are attempting to address are as follows: 1) what is the driver behaviour during the test?; 2) what is the time distribution for the request of all supported pids in the vehicles, considering both real and simulated scenarios?; 3) what is the performance of the request response time when considering the same set of pids for each vehicle?; 4) what is the difference between the response time of the pids requests in relation to the can?; and 5) what is the estimated co2 emission rate during the test? finally, in terms of sample selection, this was based on convenience and the availability of the drivers and specific vehicles, as described in table 1. 4.3. experimental design the experiment was designed in relation to a route with urban and highway sections in the city of natal, brazil. the pre-shift route was 30 km long and the drivers took the same route in two schedules (13 h and 16 h) on different days. the experiment was conducted according to these schedules in order to obtain more variability in the data, given that the traffic in the urban section at 16 h is heavier than at 13 h. 4.4. instrumentation the process was initiated with the configuration of the environment for the experiment and the planning of the data collection procedure. the settings were coded into the freematics one+ and were defined as follows: connection type to 3g, server hosting on the amazon web service, pid set request interval configured to 500 ms, and the interval for sending data to the server set to 5 s intervals. 4.5. execution the experiment was performed according to the state machine presented in figure 3. the initial state is with the engine off (‘start’). immediately after turning the vehicle on, the status of the state machine can switch to ‘stopped’ or ‘moving’. ultimately, the driver can step on the brake at certain points without stopping the vehicle, only reducing the speed. in addition, the driver can stop due to a semaphore, a traffic jam or figure 2. communication interface between a typical obd-ii device and a vehicle. table 1. availability of vehicles. model year motor fuel pids ford ka 2019 1.5 flexible 21 nissan kicks 2017 1.6 flexible 22 chevrolet onix 2015 1.4 flexible 22 acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 91 other similar situations. finally, the execution of the experiment is completed when the car is turned off. following this process, data was collected for around 180 km, with 30 km for each round of the experiment per vehicle. of these, 60 % was collected on urban roads and 40 % on highways. the experiments totalled (after the cleaning process) 1.4 mb (8,557 samples) for the ford ka, 370 kb (2,152 samples) for the nissan kicks and 1.7 mb (10,230 samples) for the chevrolet onix. obd-emulator mk22 hardware was adopted for the simulation scenario. the simulation period was configured to run for 60 minutes, which was the average time of the real experiments. the collected samples were approximately 9,678, 11,448, and 10,680 records, respectively, which corresponds to 1.6 mb, 1.9 mb and 1.8 mb. 5. results and discussion this section aims to answer the research questions previously proposed in section 4. as noted in subsection 4.3, the experiment established a default route to be performed by all the drivers in their respective vehicles. figure 4 shows the map of the first trip of each vehicle. it should be noted that the points are colour graded based on the speed of the vehicle at that specific time, red being slower and yellow being faster. it is also possible to see areas with large gaps in the readings, especially on the route performed by the nissan kicks, as can be observed in figure 4b. these gaps may have been caused by a poor internet connection, resulting in the data not being sent to the remote server (hosted on the amazon web service). in this specific case, the connection flaws were likely caused by the large rain precipitation at the time of the experiment, which may have interfered with the communication network. first, as figure 5 shows, it was possible to compare different driving styles (e.g. stopped ratio) to address our first question. in addition, the average values of five pids were also analysed: speed, revolutions per minute (rpm), engine load, throttle position and battery voltage. with these six variables, we built two radar plots (one for the routes performed at 13 h and one for those performed at 16 h) to compare the drivers and the routes. figure 5a summarises the results of the profiles of the drivers in relation to the route taken at 13 h. the drivers had virtually the same profile in terms of rpm and battery voltage. the stopped ratios for both the ford ka and chevrolet onix 2 https://freematics.com/products/freematics-obd-emulator-mk2/ vehicles were around six times larger than that for the nissan kicks. this clearly indicates the heavier traffic encountered by the drivers of the former. the experiment using the nissan kicks was carried out on a saturday, when the traffic is generally lighter than on weekdays. the throttle position (position of the accelerator pedal) indicated that the chevrolet onix's driver had a more aggressive driving style, while the speed of the nissan kicks' driver was faster due to the lighter traffic. the increased engine load presented by the ford ka is justified by the fact that it was a brand-new vehicle, which means the engine is still adjusting. in terms of the 16 h route shown in figure 5b, we obtained a profile similar to the one in figure 5a, albeit with one notable difference: the stopped ratio for both the chevrolet onix and the ford ka were almost triple the value of the previous observation. this clearly indicates the heavier traffic encountered by the drivers at that time compared to the earlier routes. meanwhile, there was no significant difference in the time for the nissan kicks to complete the route. this is due to the fact that the experiment using this car was, as noted above, carried out on a saturday with lighter traffic. regarding question 2, figure 6 describes the reading time required to collect all pids supported by the vehicles, namely, figure 3. state machine modelling the driver behaviour during the experiment. a) ford ka b) chevrolet onix c) nissan kicks figure 4. route performed by each vehicle. https://freematics.com/products/freematics-obd-emulator-mk2/ acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 92 maximum value, first quartile (q1), mean (red diamond), median (grey line), third quartile (q3), minimum value, outliers (grey diamond). according to table 1, each vehicle used in the experiments has a different set of supported pids. the ford ka had an average response time for all 21 pids of 630 ms. these results are around 44 % greater than the simulation performance (438 ms). as expected, the simulation scenario had a better result given that it did not consider internal delays in the vehicle bus or communication errors, etc. the same behaviour was also verified in tests using the nissan kicks (22 pids, 39 % ratio) and the chevrolet onix (22 pids, 19 % ratio). a further interesting result obtained from figure 6 is related to the dispersion of the response time in order to collect all pids. the interquartile ranges (iqr) for the ford ka, nissan kicks and chevrolet onix vehicles were very close: a) real scenario – 7 ms, 7 ms, 4 ms, b) simulation scenario – 35 ms, 36 ms, 35 ms. the poorer result for the simulation scenario was due to the additional delay imposed by the serial connection between simulator and computer. these results demonstrated the response times involved a small variation and that they were concentrated around the mean. it is important to stress that during the experiments, transient communication failures occurred between the freematics one+ and the vehicles. these delays generated a number of outliers in the results, while these were mainly related to the nissan kicks. in addition to the average reading time of the pids, the standard deviation for each car and for the simulator was also investigated. the obtained results indicate that the standard deviation of the nissan kicks was the largest at 27.9 ms. this implies that the reading time of the pids for the nissan kicks varied more between each sample than those for the ford ka and the chevrolet onix, which were 7.9 ms and 3.6 ms, respectively. here, the reading times of each sample were closer to the respective mean for both cars. with these values, we calculated the minimum and maximum reading times of all pids per car and obtained the amplitude. as expected, the results exhibited the same trend as with the standard deviation values. the reading time amplitude for the nissan kicks, ford ka, and chevrolet vehicles was 145 ms, 83 ms, and 29 ms, respectively. in contrast, the standard deviation for the simulator was around 15.2 ms. in addition, the simulator presented a reading time amplitude for the pids of the ford ka of 42 ms, while for the nissan kicks and the chevrolet onix, we obtained a value of 43 ms. a further point that must be highlighted is related to the pid/server request interval of 500 ms and 5 s, respectively. the experiments verified that it was only possible to achieve the metric of the pid request interval in the simulator. for clarity, let us look at the specific case of the nissan kicks, which had the longest pid request time of around 638 ms. as such, only eight packets could be sent to the server in the 5 s interval range. however, it was possible to send 10 packets in the same interval range with the simulator. regarding question 3, we analysed the data response time for only the intersection of the pids supported by the ford ka, nissan kicks and chevrolet onix vehicles. this intersection resulted in a list of 17 pids. figure 7 shows the cumulative distribution of response time for this set of pids. even with using the intersection of the pids, the response times for the real scenarios differed and were greater than those achieved by the simulator. the response times obtained for the ford ka, the nissan kicks and the chevrolet onix were 513 ms, 492 ms and 421 ms respectively. as expected, the simulation obtained the lowest response time (356 ms). meanwhile, the average response rate was 30 ms for the ford ka, 28 ms for the nissan kicks and 24 ms for the chevrolet onix. the average simulator response time was around 20 ms. hence, it can be concluded that the a) 13 hour route b) 16 hour route figure 5. driver behaviour modelling by route. figure 6. response time distribution for all supported pids. figure 7. cumulative distribution of response time for the pid set intersection among the ford ka, nissan kicks and chevrolet onix vehicles. acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 93 response time of the pids depends on each vehicle and not on the type or quantity of the pids. to address question 4, we aimed to investigate whether or not there was a difference in the reading time of the pids with respect to the can. here, we verified that the can for all the cars is iso15765-4 can11/500, while it was observed that there were different response times even though all cars operate under the same protocol. as such, the can protocol may not be a relevant factor for the response time. finally, to address question 5, we analysed the co2 emission rate estimates. to perform the required calculation, it is necessary to ascertain the amount of air that enters the engine at any given time. this value can be obtained directly by the maf pid. however, this sensor is not available in some vehicles. in such cases, the value can still be estimated using the approach used in [26]. the approach makes use of the following variables: p, which represents the pressure in the combustion chamber and can be obtained through the intake manifold absolute pressure (map) sensor in the unit kpa; v, which is the total volume of the vehicle's cylinder capacity in litres or cm³; r, which is the ideal gas constant with a value of approximately 8.3145 j/(mol·k); t, which is the intake air temperature and can be obtained by the absolute temperature (iat) sensor in k; air molar mass, which is a constant value equal to 28.87 g/mol (in the following equations, the air molar mass will be abbreviated to mma); and rpm, which is the engine revolutions per minute obtained by the obd-ii. in addition to these variables, the volumetric efficiency (ve) is required, which is the ratio of the air-fuel mixture volume that each cylinder admits in relation to the nominal volumetric capacity of the cylinder. thus, the calculation of mass air flow (maf) can be written as in eq. (1), which corresponds to a value equivalent to that obtained directly by the maf sensor when it is available. 𝑀𝐴𝐹 = 𝑃 × 𝑉 × 𝑉𝐸 × 𝑅𝑃𝑀 × 𝑚𝑚𝑎 120 × 𝑅 × 𝑇 × 1000 (1) after obtaining the maf value, the estimate of the 𝐶𝑂2 emission rate emitted by the vehicle could be calculated. for this, it was first necessary to calculate the fuel volume (𝑉fuel) according to eq. (2): 𝑉fuel = 𝑀𝐴𝐹 𝐴𝐹𝑅 × 𝑑𝑒𝑛𝑠𝑖𝑡𝑦 (2) the values of the constants used in eq. (2) vary depending on the type of fuel used, as described in table 2. finally, eq. (3) could be used to calculate the estimated 𝐶𝑂2 emission rate of the vehicle. in this formula, the vfuel value is multiplied by the carbon dioxide mass generated after burning one litre of the fuel in question (𝐶𝑂2 pl) 𝐶𝑂2 ( 𝑔 𝑠 ) = 𝑉fuel × 𝐶𝑂2 𝑃𝐿 . (3) with these equations at hand, it was possible to perform embedded processing in the freematics one+ to obtain the results in real time. as stated above, not all vehicles have the same set of sensors and, as such, may lack the maf of the map. therefore, to validate the maf estimation using the other vehicle sensors, we used data from the chevrolet onix, as it was the only vehicle analysed that had both sensors (maf and map). the comparison between the real maf sensor reading and the maf estimation results can be seen in figure 8. in addition, figure 9 shows the comparison between the emission rate calculated directly from the maf sensor and that calculated with the estimated maf. here, it is clear that the results are very close to each other, as was the case for the comparison presented in the previous figure. finally, figure 10 allows us to identify the places with the highest incidence of co2 emission during the routes. the darkest points represent the places where the emission rate was higher. these analyses can contribute to identifying the emission indices presented by the vehicles during normal use and can help verify whether they are equivalent to those issued by the automakers. 6. limitations during the use of the obd-ii (freematics one+), the chosen route incorporated urban areas as well as highways. given that, it was noted that in certain sections of the route, the hardware did not capture the information related to the geolocation of the table 2. fuel conversion factors. fuel 𝐂𝐎𝟐 per liter air-fuel ratio (afr) density (𝓟) gasoline 2310 g/l 14.7:1 737 g/l diesel 2660 g/l 14.6:1 850 g/l ethanol 1510 g/l 9.0:1 789 g/l figure 8. maf estimate. figure 9. co2 pollution estimate. acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 94 vehicles. this was due to the lack of gprs or 3g signal, making it impossible to send the data to the cloud. in addition, further difficulties were encountered during the implementation of the application used in the study, including how, since it was not a controlled environment (as in the simulator), the connection was prone to errors, which is a factor that must be addressed. 7. validity issues the threats to the validity of the present study are as follows: a. internal validity, driver behaviour: each driver has a unique driving profile, which may have affected the total time needed to complete the route. b. external validity, connection: connection failures can cause communication error with the server, resulting in a timeout that could have led to a loss of data. a further external threat relates to the varying traffic conditions, which could have increased the time needed to complete the route. c. construction validity, appropriate instrumentation: the vehicles were evaluated on different days, since it was not intended to make any type of comparison regarding the day of the week, only a verification of the performance of the obd-ii. 8. conclusions the aim of this paper was to conduct a performance evaluation of an edge obd-ii device (freematics one+). the main requirements were driver behaviour modelling, pid request interval evaluation and co2 emission estimation. for this, experiments were performed in terms of both real and simulation scenarios. the experiments involving the real vehicles present a differential contribution to the existing literature since they have the potential to highlight important issues within a customer feedback context (e.g. bottlenecking, delays, amount of data, profiles) which is a core part of industry 4.0. the first requirement (driving profile) was satisfied using the proposed state machine, where, with the use of only five pids, aggressive and/or moderate behaviours were identified. the results related to the response time of the pids demonstrated that this depends on the specifications of each vehicle and not on the type or quantity of the pids required. in fact, this aspect is crucial to determining the amount of data that will be used in a given customer feedback vehicular application. in terms of the final, complementary, requirement (co2 emission estimation), it was possible to identify the pollution and maf estimations for vehicles that do not support the sensor. future work should include investigating the dependability issues in order to address the faults of the obd-ii device and the communication issues with the cloud infrastructure. in addition, the following should be included: increasing the sample size, analysing why no data was written to the sd card at the point that it failed to communicate with the internet, implementing a data compression algorithm (such as the ‘swinging door’ algorithm), identifying the optimal request time for pids, and making use of big data and machine learning techniques to identify patterns in the collected data. acknowledgements this study was financed in part by the coordenação de aperfeiçoamento de pessoal de nível superior brasil (capes) finance code 001, with grateful 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element analysis for the lectotype of the large electronic truck scale structure acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 58 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 58 62 finite element analysis for the lectotype of the large electronic truck scale structure wei tieping1, zhou xingyang2, yang xiaoxiang3, yao jinhui4 1 school of mechanical and automotive engineering, fujian university of technology, fuzhou, china, tpwei@fjut.edu.cn 2 school of mechanical and automotive engineering, fujian university of technology, fuzhou, china, 1224122843@qq.com 3 school of mechanical engineering and automation, fuzhou university, fuzhou, china, yangxx@fzu.edu.cn 4 fujian province institute of metrology, fuzhou, china, yaojinhui2000@126.com abstract: the stiffness of truck scales is the main influence factor on its measurement accuracy. three kinds of scs-100t large electronic truck scales are different in size arrangement of u-beam. based on the finite element method, the models of the truck scales were established. the stiffness and strength of the scales were compared on account of the analysis results. the results show that u-beam arrangement of the low at both ends but high in the middle structure for the truck scale not only can save material but also meet the requirement of stiffness. simulation analysis provides the theoretical basis for the structural design of truck scales. keywords: electronic truck scale; u-beam; stiffness; finite element analysis 1. introduction at present, the structures of the electronic truck scale mainly include three kinds, including i-section steel, channel-section steel and u-section steel. the study found that u-section steel layout structure of the electronic truck scale was considered to be the most compressive capacity and the highest material utilization rate [1, 2]. traditionally, the truck scale structure is usually simplified into a simply supporting beam in order to analyse the stiffness of the truck scale structure [3-5]. however, there is a large error in this simplification method. therefore, to make the calculation results more consistent with the actual model, many studies have been carried out by establishing a complete three-dimensional finite element model of the scale. the truck scale structure of u-beam structure was optimised, and the optimised mass ratio was reduced by 6.7 % [6]. the truck scale body structure of section steel was replaced with cold-bending groove made of steel plate, which finally made lighter weight, less consumables and its strength and stiffness requirements in mechanical properties can be achieved [7]. in addition, the strength index should be used as an important checking index when the scs series truck scales was overloaded by simulation analysis [8]. also, the deformation of single scales in scs series 120 t truck scales was studied under different loading modes with the finite element method [9]. however, the size of ushaped beam arrangement has a great influence on the stiffness of the scale body, which has not been seen in the literature. in this paper, the scale body of scs-100t ubeam structure is taken as the research object, and the three u-beam size arrangement modes are simulated and calculated to explore the influence of size arrangement mode on the stiffness. furthermore, the optimal arrangement mode is determined to provide theoretical basis for the transformation of truck scale without weight verification [10, 11]. 2. finite element model of scale structure 2.1. model simplification and mesh generation the scs-100t u-beam truck scale body structure produced by a company was taken as the research object, as shown in figure 1. generally, a truck scale mainly consists of the load bearing device (scale platform), the weighing display instrument, the weighing sensor, connecting parts, the limit device and junction boxes. first, when the weighing object or the truck is placed on the scale, its gravity is transferred to the weighing sensors through the scale under the action of gravity, which leads to the deformation of the elastomer of the weighing sensor. then, the strain gauge bridge attached to the elastomer is out of balance and it will output an electrical signal that is proportional to the http://www.imeko.org/ mailto:tpwei@fjut.edu.cn mailto:1224122843@qq.com mailto:yangxx@fzu.edu.cn mailto:yaojinhui2000@126.com acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 59 weight value. after that, the electrical signal is amplified by a linear amplifier and conversed to digital signal with a/d conversion. finally, the weight signal is processed by the microprocessor of the meter and the weight data is displayed directly. figure 1: schematic diagram of the truck scale structure the layout of three different u-beam size is shown in figure 2. figure 2(a) shows that the scale body is designed with equal height layout and the height of the beam is h. figure 2(b) shows that the scale body is designed with the layout of high intermediate and low at two ends. figure 2(c) shows that the scale body is designed with the layout of low intermediate and high at two ends. (a) equal height layout (b) high intermediate and low at two ends layout (c) low intermediate and high at two ends layout figure 2: cross section diagrams of different u-beam size arrangement of the scale body this study mainly analyses the platform stiffness of different u-beam sizes. because there is little significant influence of weighing sensors and junction box on the stiffness, it can be ignored. the linear elastic isotropic material q345 is selected for all components in the structure. its elastic modulus is 206 gpa and poisson’s ratio is 0.28. in addition, ansa software is a powerful cae pre-processing software, which has good efficiency and quality in geometric model and finite element mesh processing. compared with other similar software, ansa software has no limit on the size of the model and can read faster. therefore, the finite element mesh model of the truck scale structure was established with ansa. the finite element model of the scale structure with u-beam of equal height is shown in figure 3. solid 185 element is used as the element type of model. the solid 185 element is used to construct a three-dimensional solid structure. the element is defined by eight nodes, each of which has three degrees of freedom translated along the xyz direction. the element has super elasticity, stress toughening, creep, large deformation and large deformation capacity. the element can also simulate almost incompressible elastoplastic materials and completely incompressible super elastic materials using a mixed mode. in the whole model, the longest side length of the cell grid shall not exceed 15 mm, and the shortest is 5 mm and the numbers of elements and nodes are 377 369 and 701 895 respectively. figure 3: the finite element model of scale structure 2.2. boundary conditions and loading conditions boundary conditions in actual working conditions, the structure of the scale body is contacted with the ground through the weighing sensor, and there is no other attachment between the weighing sensor and the ground. therefore, in the finite element model, the connection between seven load cells and the ground can be simplified as constraining y-direction displacement of load cells’ base, while the other one as constraining all degrees of freedom. loading conditions according to the standard gb/t 7723-2008 (this standard is a revision of gb/t 7723-2002 fixed electronic scale, based on oiml r76 nonautomatic weighing apparatus (2006e) recommended by international legal metrological organization) fixed electronic truck scale on the maximum relative deformation of the large weighing machine, the calibration must be carried out using the load of 40 t acting on the middle of each weighing platform for a scale with a maximum scale of 100 t. the loading area is 2.6 m wide and it can be considered as evenly distributed force. boundary constraints and loading conditions are shown in figure 4. http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 60 figure 4: schematic diagram of boundary and loading conditions 3. result of simulation the y-direction displacement distribution cloud diagrams of three different u-beam scales are shown in figures 5(a), 5(b) and 5(c) respectively. as can be seen from the figures, the main force area of the truck scale is in the middle, and the maximum displacement of the three scales occurs in the centre of the main scale. the maximum displacements values in the y direction of the u-beam scales with equal height layout (as shown in figure 5 (a)), the u-beam scales with high intermediate and low at two ends layout (as shown in figure 5 (b)) and the u-beam scales with low intermediate and high at two ends layout (as shown in figure 5 (c)) are 7.027 mm, 7.413 mm and 8.125 mm, respectively, which all occur in the middle panel of the first scale body. (a) equal height layout (b) high intermediate and low at two ends layout (c) low intermediate and high at two ends layout figure 5: the y-direction displacement cloud diagrams of the scale structure with different u-beam the stress distribution cloud diagrams of three different u-beam scales are shown in figure 6, figure 7 and figure 8, respectively. it can be seen from the figures that the maximum von mises stress value of the three scales occurs at the joint of the first scale body and the second scale body. the maximum von mises stress values of the three types of scales are in the order from large to small as following: high intermediate and low at two ends layout, low intermediate and high at two ends layout, equal height layout. the maximum stress values are 1221.6 mpa, 1188.6 mpa, and 1100.1 mpa, respectively. in addition, the maximum stress value positions occur at the joints between the first weighing body and the second weighing body. http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 61 (a) the stress cloud diagram of the whole scale structure (b) stress clouds of the u-beam figure 6: the von mises equivalent stress cloud diagram of the u-beam scale with equal height layout (a) the stress cloud diagram of the whole scale structure (b) stress clouds of the u-beam figure 7: the von mises equivalent stress cloud diagram of the u-beam scale with high intermediate and low at two ends layout (a) the stress cloud diagram of the whole scale structure (b) stress clouds of the u-beam figure 8: the von mises equivalent stress cloud diagram of the u-beam scale with low intermediate and high at two ends layout 4. analysis and discussion combining with the analysis from figure 5 to figure 8, it can be seen that: (1) the maximum stress value lies at the joint of the scale body’s edge, and the stress concentration area at the compression http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 62 point of the u-beam is very small, so it will not affect the strength of the scale body structure; (2) the y-direction displacement of the u-beam with high intermediate and low at two ends layout is larger than that of the u-beam with equal height layout, but the difference is very small. in addition, the scale body is more evenly loaded, and the deflection of the scale body is within the scope of the regulation standard. therefore, reducing the size of u-beam at both ends can improve the stress state of the structure and achieve the purpose of saving material and cost; and (3) for the u-beam with low intermediate and high at two ends layout, because of reducing in the size of the two u type beam which is mainly loaded, the deflection of the scale increases. according to the regulation gb/t 77232008 fixed electronic truck scale, the u-beam scale with low intermediate and high at two ends layout does not meet the stiffness requirements (the maximum deformation of the weighing body in use shall not exceed 1/600 of the length of the weighing platform, and the corresponding displacement shall not exceed 8 mm). 5. conclusion based on finite element method, the influence of the strength and stiffness of three scales with different u-beam size layout was analysed in this paper. according to the simulation results, the following conclusions and recommendations can be obtained as following: (1) the structural stiffness of the three scales is in the order from large to small as following: equal height layout, high intermediate and low at two ends layout, low intermediate and high at two ends layout. (2) the middle part of the scale structure is the main deformation area which is under the maximum load. the scale structure of the u-beam with low intermediate and high at two ends layout is not reasonable and it does not meet the rigidity requirements. (3) the scale body of the u-beam with high intermediate and low at two ends layout not only needs less material and is more uniform in the stress distribution, but also can meet the requirements of strength and stiffness of the scale structure. 6. references [1] wang jinqi, “bearing structure with welded beam of u type of section for electronic weighing apparatus of trucks”, port operation, no. 3, pp. 37-37, july 2004. [2] jiang binsong, “analysis of prestress force of yieldable u-steel support”, china mining magazine, vol. 7, no. 2, pp. 43-46, april 1998. 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[11] wei tieping, yang xiaoxiang, yao jinhui, xu hang, “the additional side force on the force transducer in the combinatorial load cell”, advanced materials research, vol. 541, pp. 13271332, 2014. http://www.imeko.org/ acta imeko contacts acta imeko issn: 2221-870x september 2020, volume 9, number 3 acta imeko | www.imeko.org september 2020 | volume 9 | number 3 | 0 journal contacts about the journal acta imeko is an e-journal reporting on the contributions on the state and progress of the science and technology of measurement. the articles are mainly based on presentations presented at imeko workshops, symposia and congresses. the journal is published by imeko, the international measurement confederation. the issn, the international identifier for serials, is 2221-870x. editor‐in‐chief dušan agrež, slovenia founding editor‐in‐chief paul p. l. regtien, netherlands associate editor dirk röske, germany editorial board section editors (vol. 7 9) leopoldo angrisani, italy filippo attivissimo, italy eulalia balestieri, italy eric benoit, france paolo carbone, italy lorenzo ciani, italy catalin damian, romania pasquale daponte, italy luca de vito, italy luigi ferrigno, italy edoardo fiorucci, italy alistair forbes, united kingdom helena geirinhas ramos, portugal sabrina grassini, italy fernando janeiro, portugal konrad jedrzejewski, poland andy knott, united kingdom francesco lamonaca, italy massimo lazzaroni, italy fabio leccese, italy rosario morello, italy michele norgia, italy pedro miguel pinto ramos, portugal nicola pompeo, italy sergio rapuano, italy dirk röske, germany alexandru salceanu, romania constantin sarmasanu, romania lorenzo scalise, italy emiliano schena, italy enrico silva, italy krzysztof stepien, poland marco tarabini, italy yvan baudoin, belgium marcantonio catelani, italy lorenzo ciani, italy egidio de benedeto, italy alessandro depari, italy alessandro germak, italy sabrina grassini, italy konrad jędrzejewski, poland min-seok kim, korea fabio leccese, italy vilmos palfi, hungary franco pavese, italy jeerasak pitakarnnop, thailand alexandru salceanu, romania emiliano sisinni, italy jan saliga, slovakia jorge c. torres-guzman, mexico ian veldman, south africa rugkanawan wongpithayadisai, thailand claudia zoani, italy mauro d’arco, italy oscar tamburis, italy about imeko the international measurement confederation, imeko, is an international federation of actually 42 national member organisations individually concerned with the advancement of measurement technology. its fundamental objectives are the promotion of international interchange of scientific and technical information in the field of measurement, and the enhancement of international co-operation among scientists and engineers from research and industry. addresses principal contact prof. dušan agrež university of ljubljana faculty of electrical engineering tržaška 25, 1000 ljubljana, slovenia e-mail: dusan.agrez@fe.uni-lj.si acta imeko prof. ján šaliga technical university of košice faculty of electrical engineering and informatics letná 9, 042 00 košice, slovakia e-mail: jan.saliga@tuke.sk support contact dr. dirk röske physikalisch-technische bundesanstalt (ptb) bundesallee 100, 38116 braunschweig, germany e-mail: dirk.roeske@ptb.de mailto:dusan.agrez@fe.uni-lj.si mailto:jan.saliga@tuke.sk mailto:dirk.roeske@ptb.de nmisa’s new 5 kn deadweight force standard machine acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 85 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 85 87 nmisa’s new 5 kn deadweight force standard machine s. dlamini1, m. sibisi1, j. matosse1, b. aydemir2, c. vatan2, h. dizdar2 1 national metrology institute of south africa (nmisa), tshwane, south africa, sdlamini@nmisa.org 2 tübi̇tak national metrology institute (tubitak ume), gebze, kocaeli, turkey abstract: this paper describes nmisa’s new 5 kn deadweight force standard machine. the machine has the capability to generate force in the range 50 n to 5 kn. the machine uncertainty is conservatively estimated at 0.008 % across the entire measuring range. the weights are adjusted according to the gravity value measured in the laboratory. this machine is used for the calibration of force transducers in tension and compression. keywords: deadweight; force standard machine 1. introduction the national metrology institute of south africa (nmisa) has acquired a new deadweight force standard machine (fsm). this is part of an ongoing project to improve calibration measurement capabilities (cmc) in the lower force ranges. currently, in the lower force ranges, the nmisa force laboratory can only realise and disseminate the unit of force in the range 100 n to 1 kn with a cmc of 0.03 %. the newly acquired fsm will realise and disseminate the unit of force in the range 50 n to 5 kn with an improved cmc of 0.008 % across its entire measurement range. this improved capability means that nmisa can provide traceability to the calibration industry in south africa and neighbouring countries with the lowest uncertainty. in this paper, the evaluation of the machine using a set of force transfer standards is discussed. a bilateral interlaboratory comparison (ilc) between nmisa and tubitak ume of turkey was agreed upon. the results of the ilc are used to support the claimed machine uncertainty. 2. machine characteristics the machine’s working principle is based on a stack of deadweights generating forces ranging from 50 n to 5 kn. the mass stack configuration of the machine is shown in table 1. this mass stack configuration can cover 500 n, 1 kn, 2 kn and 5 kn force capacities with ten steps for each capacity. table 1: mass stack configuration 10 mass pieces, each generating 50 n 7 mass pieces, each generating 100 n 4 mass pieces, each generating 200 n 6 mass pieces, each generating 500 n the mass pieces were designed and adjusted to generate nominal newton values using the local measurement of gravitational acceleration. this gravity value of (9.786 098 5 ± 0.000 000 5) m∙s-2 was measured by south africa’s council for geoscience and geological survey. the gravity value was also confirmed by the nmisa kibble balance team using their standard gravimeter. the 5 kn deadweight fsm, shown in figure 1, can be operated both manually and automatically. figure 1: the 5 kn deadweight fsm at nmisa http://www.imeko.org/ mailto:sdlamini@nmisa.org acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 86 3. machine evaluation the machine was evaluated through an ilc between nmisa and tubitak ume, using the equipment detailed in table 2. table 2: details of the transfer artefacts device manufacturer model serial no. force transducer hbm z30a (1 kn) 213313018 readout hbm dmp41-t2 831891701 bridge calibration unit hbm bn100 001 the force transfer standard was calibrated using the force standard machines detailed in table 3. table 3: details of the fsms used institute fsm capabilities type capacity cmc tubitak ume deadweight 11 kn 0.002 % nmisa deadweight 5 kn 0.008 % the measurements were performed following a protocol drafted jointly by nmisa and tubitak ume. the artefact was first measured at nmisa then couriered to tubitak ume for their measurements and thereafter back to nmisa for the final round of measurements. the transducer was installed into the calibration machine and plugged into the amplifier and energised for a minimum period of 12 hours before ilc measurements were taken. the dmp41 was used with the following settings: absolute (abs), 0.22 hz bessel filter, 5 v excitation voltage and ± 2.5 mv/v measuring range. the measurements were carried out at laboratory conditions of (21 ± 1) ˚c for temperature and (45 ± 10) % rh for relative humidity. the ilc artefact was measured at five positions relative to the axis of the machine (0°, 90°, 180°, 270° and 360°). two measurement runs were applied at each position to show repeatability. measurement runs were carried out with increasing force only in compression. two minutes was the time allowed to achieve stability following loading and unloading of each force step. only measurements from 40 % of the artefact’s capacity were considered for analysis. the mean values and expanded measurement uncertainties were calculated for each of the measurements carried out first at nmisa (round 1), then at tubitak ume, and finally at nmisa (round 2), and are summarised in table 4 and table 5. the relative deviation is calculated using equation (1). rel. dev. = 𝑋nmisa − 𝑋tubitak ume 𝑋tubitak ume (1) where 𝑋nmisa (round 1 or round 2) and 𝑋tubitak ume are the average deflection at each applied force for all positions. table 4: deviation in the ilc measurements of nmisa from tubitak ume (reference) force / n mean deflection 𝑿 / mv/v relative deviation from tubitak ume tubitak ume nmisa round 1 nmisa round 2 nmisa round 1 nmisa round 2 400 0.800 515 0.800 458 0.800 494 -7.1e-05 -2.6e-05 600 1.200 719 1.200 675 1.200 743 -3.7e-05 2.0e-05 800 1.600 927 1.600 880 1.600 966 -3.0e-05 2.4e-05 1 000 2.001 122 2.001 070 2.001 190 -2.6e-05 3.4e-05 figure 2: relative deviation between nmisa and tubitak ume the relative deviations between nmisa and tubitak ume are less than 8 × 10-5 for the range investigated. furthermore, the normalised error (𝐸n ) values are calculated for the ilc and used as a way of assessing measurement equivalence with respect to the uncertainty. table 5: deviation and the relative expanded uncertainties of measurements for nmisa and tubitak ume. force / n relative deviation from tubitak exp. uncertainty u (k = 2) utubitak ume / % unmisa round 1 / % unmisa round 2 / % nmisa round 1 nmisa round 2 400 -7.1e-05 -2.6e-05 0.008 0.009 0.009 600 -3.7e-05 2.0e-05 0.008 0.009 0.010 800 -3.0e-05 2.4e-05 0.007 0.010 0.008 1 000 -2.6e-05 3.4e-05 0.006 0.010 0.008 equation (2) is used to calculate the 𝐸n values. 𝐸n = 𝑋nmisa − 𝑋tubitak ume 𝑋tubitak ume√𝑈nmisa 2 + 𝑈tubitak ume 2 (2) -8e-05 -6e-05 -4e-05 -2e-05 0e+00 2e-05 4e-05 400 600 800 1000 rel. dev. applied load / n nmisa round 2 nmisa round 1 http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 87 where 𝑋nmisa and 𝑋tubitak ume are the average deflection at each applied force for all positions and 𝑈nmisa and 𝑈tubitak ume are the relative expanded uncertainties of measurements for nmisa and tubitak ume respectively. figure 3: ilc results comparison (normalised error 𝐸n) as depicted in figure 3, 𝐸n values are below one across the ilc force range. this indicates good agreement in measurements with respect to the uncertainties. to further support the stated ilc results between nmisa and tubitak ume, the nmisa force laboratory used a set of calibrated force transfer standards to evaluate the capabilities of the 5 kn deadweight fsm. the transfer standards consisted of high accuracy force transducers (with capacities of 500 n, 1 kn, 2 kn and 5 kn) which were used to verify the machine across the measurement range. the transfer standards were first calibrated according to iso 376 [1] at metas in switzerland using deadweight machines. they were then calibrated again at nmisa according to iso 376 using the 5 kn deadweight fsm and results compared to determine the degree of equivalence. the relative deviations between the average deflections obtained by nmisa and metas were calculated throughout the range and are given in table 6. repeatability and reproducibility for the measurements carried out at metas are almost of the same order of magnitude as those obtained at nmisa. the normalised error values were also calculated to determine equivalence, with respect to the measurement uncertainty between the calibration results of nmisa and metas. the results are depicted in figure 4. the normalised error values are less than one throughout the measurement range thus showing good agreement in calibration results between nmisa and metas. table 6: relative deviation from metas, repeatability (b) and reproducibility (b’) comparison between nmisa and metas calibrations transducer capacity applied force / n rel. dev. metas nmisa b / % b' / % b / % b' / % 500 n 50 -4.2e-05 0.000 0.001 0.002 0.004 250 7.6e-06 0.000 0.001 0.001 0.032 500 7.0e-06 0.000 0.001 0.001 0.010 1 kn 700 1.4e-06 0.000 0.001 0.000 0.002 900 3.3e-06 0.000 0.001 0.000 0.002 1 000 4.5e-06 0.000 0.001 0.000 0.002 2 kn 1 400 -1.4e-05 0.001 0.000 0.001 0.002 1 800 -1.6e-05 0.001 0.000 0.001 0.001 2 000 -1.6e-05 0.001 0.000 0.001 0.001 5 kn 3 000 -4.5e-05 0.001 0.004 0.000 0.001 4 000 -4.8e-05 0.001 0.004 0.000 0.001 5 000 -5.1e-05 0.001 0.003 0.000 0.001 figure 4: results comparison (normalised error 𝐸n) 4. summary the normalised error values for the ilc measurements carried out between nmisa and tubitak ume using their deadweight fsms are less than one for the entire comparison range. the maximum relative deviation observed for the ilc is 7 × 10-5 (0.007 %). there is also good agreement between the results of calibrations conducted according to iso 376 by metas and nmisa. the claimed uncertainty of 0.008 % is therefore acceptable for the entire operational range of the 5 kn deadweight fsm. 5. references [1] iso 376:2011, metallic materials – calibration of force proving instruments used for the verification of uniaxial testing machines, international organization for standardization, 2011. http://www.imeko.org/ ume kibble balance displacement measurement procedure acta imeko issn: 2221-870x september 2020, volume 9, number 3, 11 16 acta imeko | www.imeko.org september 2020 | volume 9 | number 3 | 11 a ume kibble balance displacement measurement procedure haci ahmedov1, mehmet çelik1, recep orhan1, beste korutlu1, ramiz hamid1, ersoy şahin1 1 tübi̇tak national metrology institute (ume), kocaeli, turkey section: research paper keywords: kibble balance; planck constant; interferometer; si unit; resolution uncertainty citation: haci ahmedov, mehmet çelik, recep orhan, beste korutlu, ramiz hamid, ersoy sahin, a ume kibble balance displacement measurement procedure, acta imeko, vol. 9, no. 3, article 3, september 2020, identifier: imeko-acta-09 (2020)-03-03 editor: jan saliga, technical university of košice, slovakia received january 9, 2020; in final form april 1, 2020; published september 2020 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was supported by tübi̇tak national metrology institute, turkey. corresponding author: haci ahmedov, e-mail: haji.ahmadov@tubitak.gov.tr 1. introduction on 16 november 2018, a crucial decision on the redefinition of the kilogram unit was made according to the votes of the member states of the international bureau of weights and measures (bipm) – at the 26th meeting of the general conference on weights and measures (cgpm) held in versailles, france. based on the affirmative vote, effective from 20 may 2019, the planck constant with a fixed value of ℎ = 6.626 070 15 × 10−34 j s is used in the new definition of the kilogram, thereby ensuring a uniform, globally accessible, longterm stable unit for mass measurements rather than a physical artefact with limited access and quantitative limitations – namely the international prototype of the kilogram (ipk). the new definition of the kilogram can be realised by means of the kibble balance experiment [1], which compares electrical power to mechanical power with the help of two macroscopic electrical quantum phenomena: the quantum hall effect [2] and the josephson effect [3]. different construction and operating modes in kibble balance experiments (see [4] and the citations therein) are favourable for gaining a thorough understanding of possible systematic errors. the tübi̇tak national metrology institute (ume) of turkey contributes to the ongoing worldwide scientific work on the redefinition of the kilogram, with an oscillating magnet kibble balance experiment, in which its novel dynamical measurement procedure makes the system less sensitive to environmental disturbances compared to the traditional kibble balance experiments [5]-[7]. all existing kibble balances consist of a coil suspended from a balance embedded in a strong magnetic field created by a permanent magnetic circuit. the relative motion between the coil and the magnetic circuit induces faraday’s voltage across the ends of the coil. the precise measurements of the displacement and the voltage are vital in order to achieve the required total relative uncertainty of 2 × 10−8 in the kilogram realization experiments with kibble balance systems. this article focuses on the displacement measurements. in the kibble balance abstract the redefinition of the kilogram in terms of planck constant came into effect on 20 may 2019. the national metrology institute of turkey (ume) realised the new definition by means of the oscillating magnet kibble balance. the novel dynamical measurement procedure developed for kibble balance in turkey has the advantage of being less sensitive to environmental disturbances compared to the traditional kibble balance experiments. precise displacement measurements are performed either with michelson or fabry-perot interferometers in worldwide kibble balances. moreover, most of them operate in a global vacuum. a commercial michelson interferometer has been used in ume’s kibble balance experiment. in this article, we determine the contribution of ultra-small oscillations to the planck constant by taking simultaneous displacement measurements on two back-to-back mirrors attached to the piezoelectric transducer, undergoing an oscillatory motion with the michelson and fabry-perot interferometers. the following novel measurement procedure makes such measurements possible in a regular laboratory environment. otherwise, the experiment needs to be performed in a global vacuum. this is why we were required to investigate the resolution performances of these devices in laboratory conditions. as the expected relative uncertainty in the redefinition of kilogram is above the resolution uncertainties of both interferometers, we may conclude that a commercial michelson interferometer will serve our purposes in our route to the redefinition of a kilogram by means of local vacuum. mailto:haji.ahmadov@tubitak.gov.tr acta imeko | www.imeko.org september 2020 | volume 9 | number 3 | 12 experiments, laser interferometers, e.g. the michelson interferometer (mi) [8]-[14] and the fabry-perot interferometer (fpi) [15] are used to measure the displacement between the magnetic circuit and the coil (see table 2 in [4] and the citations therein). in the mi, the laser beam is split into two perpendicular arms, the reference arm and the measurement arm, and recombines at the beam splitter after being reflected by the reference and measurement reflectors, respectively. the phase differences between the two beams are influenced by tiny variations in environmental changes, as they follow different optical paths. the fpi, on the other hand, includes only one arm with two parallel mirrors aligned to form an optical resonator. as it features a common optical path, the fpi is less sensitive to environmental disturbances than the mi [16]. the factors determining the accuracy, linearity, and repeatability of highresolution displacement measurements are thoroughly investigated in [17]. in traditional kibble balances, extreme precautions are taken for eliminating the environmental disturbances in the displacement measurements between the moving coil and the stationary magnet such that the entire system (all mechanical components, including the weighing cell; the balance pan and connecting components, coils, and magnet; and the interferometer) is covered by a global vacuum of less than 0.1 pa. hence, the stability of the refractive index along the paths of the laser beams is ensured. in moving-coil kibble balance experiments, a global vacuum is the only possibility for precise displacement measurements with interferometer optics. one disadvantage of this design is that the vacuum complicates the experiment. firstly, all components in the experimental setup should be compatible with the vacuum. the second challenge is the dissemination of a mass unit. since the mass unit is realised in a vacuum but disseminated in the air, one should consider the sorption effects on the reference mass calibrated in the kibble balance experiment with a vacuum enclosure before using it for the dissemination of the kilogram [18]-[23]. a more direct method for disseminating the mass unit realised in a vacuum has been developed at nist, where the magnetic suspension mass comparator (msmc) is used to perform direct comparison of a mass artefact in a vacuum to the one in the air [24]. however, one needs a transportable vacuum component to transfer the mass artefact from the nist kibble balance to the mscs. in this article, we apply the dynamical measurement procedure developed for the ume oscillating magnet kibble balance experiment on interferometric displacement measurements performed by the mi and fpi simultaneously. we are mainly concerned with the measurement capabilities of the mi for ultra-small displacements, where environmental disturbances are common. the fpi is used to justify the ultrasmall displacement measurements taken by the mi. the results indicate that the measurement precision of the mi is immune to environmental disturbances thereon provided that the aforementioned measurement procedure is followed. this allows us to use a commercial miniature plane mirror mi with a compact sensor head for the precision displacement measurements in the ume oscillating magnet kibble balance. moreover, in the ume kibble balance experiment, as the moving piece is the magnetic circuit rather than the coil, one may introduce a local vacuum (covering only the mirror on the magnet together with its tilt mechanism) on the mechanically decoupled magnetic circuit. currently, a local vacuum is under construction for ensuring stability of the refractive index. it is important to note that it is the stationary coil setup of the ume kibble balance and the short duration in the measurement procedure that allow the use of a local vacuum instead of a global one. in section ii, we introduce the novel measurement procedure developed for the ume kibble balance experiment. in section iii, we present the results obtained with the mi and fpi simultaneously. finally, in the concluding section, the major advantages of the ume kibble balance are summarised. 2. measurement procedure the ume oscillating magnet kibble balance consists of a stationary coil suspended from the handler of the mass comparator and a mass standard of 1 kg on the handler. the coil is placed in the air gap of an oscillating magnetic circuit along the direction of the gravitational acceleration. ampere’s force law and faraday’s law of induction play central roles in this configuration such that the gravitational force on the mass standard is counter balanced by the lorentz force 𝐹, created on the coil carrying a dc current 𝐽, through radially outward magnetic flux density. the oscillating magnetic circuit with the velocity 𝑢(𝑡) induces an ac faraday’s voltage 𝑉(𝑡) across the ends of the coil. combining these two laws and assuming the velocity 𝑢(𝑡) is directed along the direction of gravitational acceleration (no horizontal or angular motions), one may write ℎ ℎc 𝑉(𝑡) = 𝐺 𝑢(𝑡), (1) where ℎ is the actual planck constant, ℎc is the consensus value of the planck constant, as announced in the 26th cgpm meeting. the ratio ℎ/ℎc on the left hand side of equation (1) appears due to the fact that the voltage and resistance are measured based on the josephson and klitzing constants, which are determined by using the consensus value of the planck constant. 𝐺 = 𝐹/𝐽 is the geometrical factor, which depends on the structure of the magnetic flux density and the geometry of the coil. in the context of this study, we may neglect the inhomogeneity of the geometrical factor in the vertical direction. following this assumption, the measurement procedure followed in the ume kibble balance yields ℎ ℎc = 𝐺 {𝑧|𝑊}, (2) where 𝑧 = ∫ 𝑢 𝑑𝑡 is the displacement of the magnet, 𝑊 = ∫ 𝑉 𝑑𝑡 is the magnetic flux passing through the coil, and {𝑧|𝑊} = 1 2𝑁 ∑ 𝑧𝑘+1−𝑧𝑘 𝑊𝑘+1−𝑊𝑘 2𝑁 𝑘=1 , (3) represents the averaging of the displacement over the magnetic flux (see [6] for details). a multiple of the fundamental period of oscillation has been chosen as the integration time of the kibble balance experiment. we divide the integration time into 2𝑁 halfcycles, denoted by 𝑘. one half-cycle is the region between the consecutive displacement extrema. for the 𝑘th half-cycle, 𝑧𝑘 is the preceding extrema, while 𝑧𝑘+1 is the succeeding one, as shown in figure 1. the corresponding 𝑊𝑘 and 𝑊𝑘+1 values are taken at the displacement extremum points. using equations (1) and (2), we obtain ℎ(𝑚) ℎ = {𝑧 (𝑚)|𝑧}, (4) acta imeko | www.imeko.org september 2020 | volume 9 | number 3 | 13 where the parameters with the superscript (𝑚) are the measured values, and those without the superscript are the actual values of the corresponding parameters. the ratio of the measured planck constant to the actual one is equal to the averaging of the measured displacement over the actual displacement. an ideal displacement measurement device would make this ratio equal to one. any deviations from this value represent the inaccuracies of the measurement device. as we are interested in determining the displacement measurement uncertainties, in deriving equation (4) we assume that the measurement uncertainties on all other contributing parameters (i.e. electrical current, voltage, mass, and gravity) are sufficiently small for the realisation of the kilogram. the resolution of a measurement instrument is an essential property as it describes the smallest measurable quantity. in this article, we investigate the reading performance of the commercial mi used in the ume kibble balance experiment. we use a sinusoidal reference signal with an ultra-small amplitude. as a reference signal 𝑧, we use the oscillation produced by the piezoelectric transducer (pzt). the high stability of pzt allows us to describe the reference signal by only three parameters: fundamental oscillation frequency 𝑓, oscillation amplitude 𝐴, and phase 𝜑, such that 𝑧(𝑡) = 𝐴 cos(2π𝑓𝑡 + 𝜑). (5) the fundamental oscillation frequency 𝑓 and the phase 𝜑 are obtained by the signal processing followed in the next section. the oscillation amplitude 𝐴 is chosen to be close to the resolution of the mi. since we neglect uncertainty over the full scale, we do not need to measure the amplitude 𝐴 of the ultrasmall oscillations exactly. we may rescale it to a desired value. in our analysis, we rescale the oscillation amplitude of pzt to be equal to that of the magnetic circuit in the ume kibble balance experiment. in this case, equation (4) defines the contribution of ultra-small amplitude oscillations to the planck constant, and its uncertainty defines the resolution uncertainty. in the rest of the article, we refer to the contribution of ultra-small amplitude oscillations to the planck constant simply as ‘ultra-small oscillation contributions’. 3. results and discussions the experimental setup consists of a sios ae sp 2000e miniature plane mirror mi and a ume-made fpi [25], [26], as shown in figure 2. two back-to-back mirrors attached to the pzt operate in an oscillatory motion. the frequency and amplitude of this motion is supplied by the ac voltage applied on the pzt by the function generator. the reading performance of mi is tested by using a reference signal with an amplitude of about 10 pm. this value is given in the specifications of the device as the resolution. in order to achieve such ultra-small amplitudes, in-line attenuators are used at the output of the function generator. the oscillations of about 1 hz are measured by fpi and mi simultaneously for a time interval of 500 s. the oscillation frequency is chosen to be equal to the 1 hz oscillation frequency of magnetic circuit in ume kibble balance experiment [5]-[7]. a keysight 33512b two-channel arbitrary waveform generator is used for the synchronisation of the fpi and mi data samplings of 50 hz. the displacement data for mi (𝑧𝑛 m) and fpi (𝑧𝑛 fp) are shown in figure 3. the signal discretization index 𝑛 runs from 1 to 𝑁 = 25000. in order to estimate the fundamental oscillation frequency 𝑓 and the phase 𝜑 of the reference signal encoded in these curves, it is figure 1. synchronised data of the displacement and magnetic flux. a small portion of the data is presented for better illustration of the half-cycles. figure 2. a schematic diagram of the experimental setup. figure 3. displacement data measured by fpi (cyan) and mi (magenta). for illustration purposes, only a portion of the data is shown. figure 4. the low-pass filtered data of fpi (cyan) and mi (magenta). for illustration purposes, only a portion of the data is shown. 0 0.5 1 1.5 2 2.5 3 3.5 4 -1000 -500 0 500 1000 time [s] d is p la c e m e n t [ m ] -0.2 -0.1 0 0.1 0.2 m a g n e ti c f lu x [ v .s ] k=1 k=2 k z k z k+1 z w 0 10 20 30 40 50 -0.5 0 0.5 1 1.5 2 2.5 3 time [s] d is p la c e m e n t [n m ] fabry-perot michelson 0 10 20 30 40 50 -0.5 0 0.5 1 1.5 2 2.5 3 time [s] d is p la c e m e n t [n m ] fabry-perot michelson acta imeko | www.imeko.org september 2020 | volume 9 | number 3 | 14 useful to decompose the data into two parts as digitally bandpass filtered data and its residual. digitally band-pass filtered data �̃�𝑛 m(fp) is of the form �̃�𝑛 m(fp) = 1 2𝑙 + 1 ∑ 𝑧𝑛+𝑠 m(fp) 𝑙 𝑠=−𝑙 , (6) where 2𝑙 + 1 is obtained by taking the ratio of the oscillation frequency to the sampling frequency and 𝑛 = 𝑙 + 1, ⋯ , 𝑁 − 𝑙. the digitally band-pass filtered data given in figure 4 describes the low-frequency processes that are caused mainly by environmental disturbances. the four parameter sine-fit (fpsf) algorithm [27] is performed in order to determine the amplitude, frequency, and phase of the residual data measured by mi and fpi. the results are summarised in table 1. the residual fpi and mi data and the signal reconstructed by the fpsf algorithm are given in figure 5 and figure 6, respectively. the findings of the analysis indicate that the parameters estimated by the fpsf algorithm on the mi and fpi residual data are quite close to each other (table 1). this is a strong indication of the consistency between the interferometric measurements taken by mi and fpi. the differences occur due to the low signal-to-noise ratio of about −20 db in the residual data. it is reasonable to assume that the actual phase and frequency values are in close proximity to the obtained ones for the reference signal. to establish the resolution performance of the interferometers for ultra-small oscillations, we divide the raw data 𝑧mi(fpi) of 500 s into five independent measurement sets of 100 s and represent each set of displacement data by 𝑧𝑠 mi(fpi) where 𝑠 = 1, ⋯ ,5. the ratio ℎ𝑠 mi(fpi) ℎ⁄ is determined for each set using equation (4). the mean value of these sets defines the contribution of the ultra-small amplitude oscillations to the planck constant, and the standard deviation defines the resolution uncertainty. as summarised in table 1, the interferometric measurements of the ultra-small oscillations by mi and fpi are close to each other. therefore, it is reasonable to expect ratio ℎ𝑠 mi ℎ⁄ to be close to ratio ℎ𝑠 fpi ℎ⁄ . consequently, one may perform the following optimisation procedure for estimating the parameters of the reference signal. as a first step, an optimisation function 𝐹(𝑓, 𝜑) = ∑ ( ℎ𝑠 fpi ℎ − ℎ𝑠 mi ℎ ) 25 𝑠=1 , (7) is introduced. then, using the optimisation function, 𝐹(𝑓, 𝜑), a 100 × 100 matrix is constructed in light of the reconstructed signals, by increments of 0.00001 hz for the frequency range from 0.99950 hz to 1.00050 hz and by increments of 0.01 rad for the phase range from 0.65 rad to 1.65 rad. the estimated values for the reference signal are 𝑓 = 1.00023 hz and 𝜑 = 0.95 rad as they are the numerical values that make the minimum difference in equation (7). figure 7 summarises the ultra-small oscillation contributions obtained for each set 𝑠. the resolution uncertainties for the fpi and mi and the corresponding mean values determining the ultra-small oscillation contributions are summarised in table 2. it is important to emphasise that even though the optimization procedure makes the results of mi and fpi agree better with each other, the mean value and the figure 5. the residual data (cyan) measured by fpi and the estimation of the signal reconstructed by the fpsf algorithm performed on the residual data of fpi (black). for illustration purposes, only a portion of the data is given. figure 6. the residual data (magenta) measured by mi and the estimation of the reconstructed signal by fpsf algorithm performed on the residual data of mi (black). for illustration purposes, only a portion of the data is given. table 1. the frequency, amplitude, and phase obtained by the fpsf algorithm applied on the fpi and mi residual data. parameters fpi mi difference frequency (hz) 1.00036 0.99987 0.00049 amplitude (pm) 8.5 8.1 0.4 phase (rad) 1.17 1.10 0.07 table 2. the mean values for the ultra-small oscillation contributions and resolution uncertainties for fpi and mi. parameters fpi mi mean 6.9 × 10−9 7.2 × 10−9 standard deviation 2.2 × 10−9 1.4 × 10−9 figure 7. the contribution to hs mi(fpi) h⁄ obtained by simultaneous measurements with mi (circles in magenta) and fpi (stars in cyan). 0 10 20 30 40 50 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 time [s] d is p la c e m e n t [n m ] 0 10 20 30 40 50 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 0.2 time [s] d is p la c e m e n t [n m ] 1 2 3 4 5 3 4 5 6 7 8 9 10 x 10 -9 measurement number u lt ra s m a ll o s c il la ti o n c o n tr ib u ti o n s fabry-perot michelson acta imeko | www.imeko.org september 2020 | volume 9 | number 3 | 15 uncertainties obtained without the optimisation procedure are still below 10−8. in most of the existing kibble balance experiments, the system operates within a global vacuum to ensure the stability of the index of refraction along the paths of the laser beams that measure the displacements of the moving coil with respect to the stationary magnet. the interlaced mechanical construction of these experiments restricts the vacuum to being global, as it is not possible to keep the coil itself within a separate vacuum. on the other hand, the ume oscillating magnet kibble balance experiment can operate within a local vacuum. there are two fundamental reasons for this advantage. the first one is that in the ume kibble balance experiment, the moving piece is the magnetic circuit – not the coil. as the magnetic circuit is not connected to the handler of the mass comparator, it could be treated as a separate piece. hence, a local vacuum could be installed directly on the mechanically decoupled magnetic circuit. in figure 8, the experimental setup with a local vacuum is shown, and the coil suspended from the handler of the mass comparator, the surrounding magnetic circuit, and the local vacuum placed on the magnet are indicated. for the purpose of better illustration, the middle section of the magnet where the coil resides is shown transparently. the second reason is that in displacement measurements, the uncertainty contribution due to the environmental conditions (fluctuation in the index of refraction and elongation of materials) are proportional to the measurement time. as the oscillation frequency of the magnetic circuit in the ume kibble balance experiment might be increased, the time it takes for peak-to-peak displacements decreases. in such a short duration of time, the changes in the environmental conditions are sufficiently small, as it is shown in this article that a local vacuum stands as a practical application. currently, the vacuum chamber to be placed on the magnetic circuit is under construction (figure 9). it is fixed on the horizontal plate such that the vertical displacement of the magnet is measured in a medium with a constant index of refraction with a magnitude of one. the bellows between the surface of the magnet and the bottom of the table are placed in order to introduce movement of the magnetic circuit into the vacuum. in the final setup, we plan to use a sios differential interferometer for measuring the vertical displacements of the magnetic circuit whereby one of the beams is directed towards the flat mirror in the bottom of the handler, from which the coil is suspended and the second one is directed towards the upper surface of the cubic mirror within the local vacuum. the reason for using the cubic mirror is that the six degrees of freedom (three translations and three rotations) in the motion of the magnetic circuit could be traced via the cubic mirror. we plan to use a sios double-beam interferometer and a sios triple beam interferometer for measuring the horizontal and angular displacements of the magnetic circuit via the reflected beams of the two coincident side faces of the corner cube. in figure 9, we show only the differential interferometer for the purpose of better illustration. 4. conclusions in this study, we applied the dynamical measurement procedure developed for the ume oscillating magnet kibble balance to interferometric displacement measurements performed simultaneously by mi and fpi to test the resolution performance of mi. the data analysis yields mean values for the ultra-small oscillation contributions of 6.9 × 10−9 and 7.2 × 10−9, and resolution uncertainties of 2.2 × 10−9 and 1.4 × 10−9, for fpi and mi, respectively. as the relative uncertainty for the redefinition experiments of the kilogram is expected to be 2 × 10−8, the ultra-small oscillation contribution and the resolution uncertainty obtained for the commercial miniature plane mirror mi will serve our purposes without the need to take extreme precautions for eliminating the environmental disturbances on the displacement measurements. the parasitic effects are eliminated simply by performing the dynamical measurement procedure developed for the ume kibble balance experiment. this feature of measurement and the mechanically decoupled nature of the magnetic circuit allow us to use a local vacuum around the cubic mirror attached to the oscillating magnet as opposed to most of the existing kibble balance experiments, in which a global vacuum is used. currently, the local vacuum that we plan to use in the ume kibble balance is under construction. figure 8. the ume kibble balance experiment. figure 9. the local vacuum chamber in the ume kibble balance experiment. acta imeko | www.imeko.org september 2020 | volume 9 | number 3 | 16 references [1] b. p. kibble, i. a. robinson, feasibility study for a moving coil apparatus to relate the electrical and mechanical si units, technical report des 40, npl, 1977. 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https://doi.org/10.1088/0026-1394/49/4/455 https://doi.org/10.1109/19.893254 microsoft word article 13 jena paper 1.docx acta imeko  august 2013, volume 2, number 1, 56 – 60  www.imeko.org    acta imeko | www.imeko.org  august 2013 | volume 2 | number 1 | 56  new education strategy in quality measurement technique  with image processing technologies ‐ chances, applications  and realisation  maik rosenberger, mathias schellhorn, gerhard linß  technische universität ilmenau, gustav kirchhoff platz 2, 98693 ilmenau, germany      keywords: quality assurance; control chart; image processing; spring inspection; statistical process control  citation: maik rosenberger, mathias schellhorn, gerhard linß, “new education strategy in quality measurement technique with image processing  technologies ‐ chances, applications and realization”, acta imeko, vol. 2, no. 1, article 13, august 2013, identifier: imeko‐acta‐02(2013)‐01‐13  editors: paolo carbone, university of perugia, italy; gerhard linß, ilmenau university of technology, germany  received march 27 th , 2013; in final form april 4 th , 2013; published august 2013  copyright: © 2013 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: this work was supported by federal ministry for education and research (bmbf), germany  corresponding author: maik rosenberger, e‐mail: maik.rosenberger@tu‐ilmenau.de    1. introduction  the stability of the production process and whose control technologies, is one of the major topics for the quality of the product. therefore fast measurement technique plays an important role. actually more than 80% of quality assurance applications are realized with image processing so the annual report of the german vdma association in 2010 [1]. for 2011 the prognosis is an additional increase and an extension to 100% inspection “current standard practice in the pharmaceutical industry and in areas such as automobile safety components will become a prerequisite for industrial production as a whole in the future: 100% quality control rather than random checks – in other words, each and every stage of production will be subject to comprehensive documentation and traceability” [2]. so there is a need for the combination of quality assurance and image processing. on the ilmenau university of technology a new way to educate both of them had been started. as one of the first steps to communicate that, the name of the department quality assurance was changed into quality assurance and industrial image processing. furthermore in the new publication quality management for engineers (qualitätsmanagement für ingenieure) a separate chapter for image processing was added in this new publication [3]. after the funding in 1990 two parallel research topics were initiated by the head of the department. so there actual there is a big database and experiences in both research areas, in image processing as well as in quality management. in the first chapter a short introduction of the new publication (qualitätsmanagement für ingenieure) was given, followed by a more detailed look into the statistical process control in section 2. chapter 3 deals with two special applications where both technologies are combined together. at the end of the paper a strategy was presented which outlines how a teaching model can be, and how it was applied in the department of quality assurance and industrial image processing [3]. abstract  according  to  the  publication  “new  education  strategy  in  quality  measurement  technique  with  image  processing  technologies”  presented on the 14 th  joint international imeko tc1+tc7+tc13 symposium, in this paper further issues were discussed. so the theory  of quality charts especially with the use of data sets generated with camera sensors are enlarged in this publication. furthermore an  additional application, developed in the department quality assurance and industrial image processing was added to this issue. the  main focus of this paper lays on the realization, how to combine image processing with classical quality assurance methods. to gain  the importance of this trial two industrial applications were used to describe the problem. mostly the technical realization of sensor  systems and data processing is completely separated to quality inspection tasks. to close this gap special trainings as well as special  parts in the lectures were developed and structured. in sum it ends to one deepening direction, opto techniques in the consecutive  master ‐ bachelor program optronic on the ilmenau university of technology. acta imeko | www.imeko.org  august 2013 | volume 2 | number 1 | 57  2. quality assurance versus quality management  quality management is the overall process in a company or an institution to monitoring the quality level. quality assurance means to manufacture material and immaterial products and processes in specified quality, to control and constantly improve processes in specified quality [1, 2, 3, 4]. so quality assurance was the technical part of the quality management. to train quality assurance and give the students the possibility to get a better understanding, the main book [3], according to the given lectures and three training books were published. 3. image processing in the field of quality  assurance   to point out the strong need of the combination of image processing and quality assurance, in the following chapters an example for two industrial applications is given. the methods and the knowledge to realize quality control elements like quality charts, quality loops etc. are contained in the new book. these procedures and the knowledge were applied in this example. 3.1. quality control loops  in analogy to technical control loops therefore quality control loops with quality controlled system, quality controller and quality control element must be developed. quality control loops (figure 1) are defined as closed technological-organizational action sequence in a process for the production of a quality product. with view on the whole production process, quality control loops must be divided into small quality control loops and large quality control loops [3, 5]. small quality control loops are used for current control in the production process by instantaneous influencing control on the individual manufacturing steps. they exert direct influence on quality criteria during their production. they include direct statistical process control (spc) units in the form of quality control charts. as quality reference for example the desired value and the range of tolerance of a quality criterion are consulted [6]. the overall quality control loop includes all activities from the idea of the product up to the line production process. they are used for subsequent inspection and quality confirmation of the production process by delayed influence. in the application, presented in this paper, the small quality control loop is applied [6]. 3.2. statistical process control with quality control charts  statistical process control is a continuous accompanying monitoring of the manufacturing processes by collection of all characteristic numbers relevant for the product quality. spc supplies the base data for the recognition of weak points and thus the condition for the constant improvement of the respective processes. spc developed from the quality control charts technique. quality control charts are one of the oldest tools in the quality management and an important aid to the quality control. the quality control chart is a form for graphically representing of measured values taken up by sequential samples (figure 2). they are used for the purpose of the quality control in comparison with warning and/or control limits [7]. the main objective is to recognize promptly error developments for regulative intervention. examples of quality control charts are shewhart quality control chart, average value quality control chart and median quality control chart. for the definition of the interference and warning limits of quality rule maps the following error bands are used:  α = 1%  99% error band  control limit  α = 5%  95% error band  warning limit in the 99% error band covers at least 99% of the values of the characteristic. with the fact it is improbable that with an unchanged process values arise, which lie outside of this range. that means, if the values lie within the control and warning limits, the process can be continued without intervention. a violation of the warning limits leads to a monitoring of the process with increased attention. if the values fall outside the control borders, intervention must take place into the process, in order to guarantee quality of the product. in addition the causes for the change of the process must be examined. for the successful implementation of spc four conditions must be fulfilled: data integrity, data traceability, identify critical process parameters and real-time capability [6]. data integrity means that the measurements must be accurate, so that fluctuations during the process are recognized surely. the traceability to the respective process and product must be secured, so that interferences in the correct place are accomplished. beyond that the production steps must be recognized, which have a significant influence on product quality. each intervention into a current process means a interference. the reduction of unnecessary interventions into the process is a goal of the spc. the presented inline figure 1. components and structure of quality control loops [3].  figure 2. structure of a quality control chart (ucl = upper control limit (or  upper action limit); lcl = lower control limit (or lower action limit); uwl  = upper warning limit; lwl = lower warning limit) [3].  acta imeko | www.imeko.org  august 2013 | volume 2 | number 1 | 58  measuring system on basis of image processing technology fulfils all these conditions [6]. 3.3. application for a statistical process control using quality  assurance and image processing technologies in the quality  control of springs  in the presented application, the quality control chart represents the quality controller in the quality control loop (figure 2, figure 5). the set points for the spring production are the length and the diameter. the drives of the production machine and its mechanic system represent the control path. the entry values for the quality control chart deliver an image processing system. so the control loop is closed. troubles during the control path can be, for example, defective wire or abrasion of the mechanical forming tools. the novel approach in this application is that the ranges of the quality control chart directly controls the spring forming process. therefore an upper warning limit and a lower warning limit were defined. also an upper control limit and a lower control limit were to be used for the control of the process. beyond that, the software offers additional support to the operator. in a typical measuring scenario is assumed humans know the item under test and its situation exactly. in the field of technical recognition the algorithm must be able to detect type and orientation of the unit under test. for humans it is relatively easy to differentiate on a single view cylindrical, single-conical or double-conical springs (figure 4). the different spring types cover different geometrical boundary conditions, which must be monitored during the production process. the teach-in process of these conditions for inspection is however more difficult for the operator to realize. that leads to the fact, that a manual setting-up of the individual values, depending on the type of spring, increases the danger of operator errors. in this new approach of software the detection and the classification of the spring type works automatically. the operator only has a one button solution to set up the whole production system. the manual configuration of area of interest (aoi) for the measurement is replaced by the new algorithm. so the operator errors and the teach-in time for new geometric spring forms can be minimized. another novel algorithm in the software realizes automatic tracking of the measuring points. that means, during the production process the spring can begin to swing and has different orientations. this leads to not deterministically assignable deviations of the spring position of two following springs during the production process (figure 3). from it results, that the use of static aoi is not possible in such high-dynamic processes. after the machine configuration the software is able to make a good or bad divisor identification based on the set points from the small quality loop. furthermore the software generates new setup values for the forming tools during the runtime of the machine. in case of a bad divisor identification a sorting device is steered, in order to segregate the incorrect spring. the software for statistical process control was integrated directly into the machine control software (figure 6). algorithms compare the quality controlled variable determined by the measuring system (geometrical dimension) with the quality reference (nominal dimension and tolerance). in the case of a quality offset the drive control is readjusted. different control algorithms can be realized. 3.4. application for quality inspection of mechanical tools  in the first example the use of quality control charts and the combination of image processing and quality assurance in a bad or good decision application was presented. another important task is the qualitative measurement of tools or parts during or after the use. therefore image processing is also a well suited method. in the department a special hardware and software solution for the quality inspection of tool cutting edges (tool presetter) were developed. the tool is connected to a rotary axis which is combined with a rotary encoder. so the current angle of the tool can be determined in reference to the figure 3. images of swinging springs (left: horizontally; middle: downward  swinging; right: upward swinging) [7].  figure 4. different spring types left: cylindrical spring; middle: single conical  spring; right: double‐conical spring).  figure 5. small quality control loop using quality control charts for spc.  figure  6.  machine‐integrated  image  processing  software  with  analysed  spring and quality control plan.  acta imeko | www.imeko.org  august 2013 | volume 2 | number 1 | 59  basement. two linear axes also with integrated linear encoders are carrying an image system which is connected with the basement. with the three references axes it is possible to calibrate the image system in reference to a defined reference point. all axes are manual movable by the operator. after an additional calibration of the camera system the operator is able to measure the cutting edges on the tool. during this research several students were embedded in this work. that is one of the options during the lectures and practical tests they have to absolve. so they helping the researchers to solve mechanical tasks like how to arrange the linear bearings, where to install the electrical power supply, electrical tasks like which pc is an optimal solution for this application, as well as testing the devices and programming little parts for the image processing. the result of the joint research is depicted in the figure 8. on the left upper corner of the left picture the camera system combined with an integrated incident light is illustrated. in the middle of the picture, the telecentric range of the lens, the device under test is placed. beside the measurement system, a picture captured with this system is presented. the interesting area for the quality inspection shows the diameter and the edges in the near of this range. with this system uncertainty of measurement of less than 1 micron can be achieved. after finishing this demonstrator in software as well as in hardware the option for statistical investigation is given. figure 7 shows a part of a screenshot during the measuring task. the red circle is the best fit into the interesting radius of this tool. from there it is also possible to calculate the angles which are embedded within the green lines and the blue line. also the angle between the red line and the blue line is interesting for the sample geometry and can be determined with this system. for the measurement of the features a complex software architecture were build, which is possible to handle such tasks very fast and also save the results in a database. after the measurement data processing, two options are availible to continue with the quality assurance. one option is to send the current data like cutting angle, linear offset etc. via a special interface directly to the computer numerical controlled manufacturing machine. so the control system of the manufacturing centre is able to correct offset errors after the tool is attached to tool changer. this way is similar to statistical process control. the other option saves the quality features of the same tools in the incoming goods inspection, to get information long term stability of the tool manufacturer quality. once more this additional example shows the importance of the interaction between quality assurance and image processing. all the results of the development and research for that industrial application are directly fed back to the lectures image processing part i and image processing part ii as well as to the quality management lectures. furthermore a few students got the chance to absolve an internship in local companies which working on his type of imaging system. 4. how to teach the new strategy  first of all the students have to absolve the lectures concerning to quality management. in this part quality figure  7.  cutting  edge  inspection  with  measurement  overlay  back illuminated.  figure 8. image processing system in a tool presetter (left), device under  test in the tool presetter (right) [8].  figure 9. flow chart education steps in a combined strategy in image  processing and quality assurance [9].    acta imeko | www.imeko.org  august 2013 | volume 2 | number 1 | 60  assurance, technical reliability and quality administration are contained. simultaneously they have to pass the lectures in industrial image processing part i and part ii. if they have done it successfully, they begin with the practical training. it is divided in the basic quality assurance skills training and basic image processing skills training (figure 9). during the practical training the students get the chance to benefit from actual research projects and realized solutions like the application in the given examples. to learn the theoretical knowledge after the lectures the new publications support them additionally. afterwards they have the possibility to get deeper in that field of science, absolving their bachelor, master or phd degree in the department [9]. 5. chances and realisation  important aspects, teaching this philosophy in the industrial quality management lectures, are that the students have to understand almost the complete operation of complex imaging systems. moreover students have to understand and learn how to apply the approach of a closed loop control system combining quality assurance with image processing. that leads to the capability to analyse, understand and finding bugs in inside of these systems. summarized, the global system oriented working; developing and thinking will be trained with this approach. mentioned in chapter four students have the chance to absolve three different degrees in the field of study optronic, deepening direction opto techniques. with this field of study the alumnus are well educated for the industrial reality. they have the option to work as a quality systems engineer as well as an application or development engineer in the field of image processing. local industry requests for alumnus with this education increases in the last time. the study course started in the winter semester 2005. up to now a few master alumni received their degree in the last three years. the students which done their master thesis in industry or were supported by the department of quality assurance and industrial image processing are actually employed and had no problems to get good job offers. the outlook for the coming years according to the requirements of a 100% quality inspection [2] leads to a high demand of specialists in this application area. that shows that this approach is a good way to combine these technologies and give students good chances on the job market. 6. conclusion  industrial image processing in combination with quality assurance is a powerful method to monitor the quality of industrial processes. in the first given example a quality control chart combined with a small control loop is used to steering the spring production. the data therefore were captured with an imaging system. capturing a cutting edge tool and determining the geometric features were discussed in the second industrial example application. these applications gains and shows the need of a combined education in the field of imaging processing and quality assurance. the students which pass the lectures in the department of quality assurance and industrial image processing, gets lectures of quality assurance as well as image processing. so there are able to understand the interdisciplinary working principles of both techniques. the issues in the vdma reference [2] and the demand of a 100% quality inspection in industrial applications will be a gaining factor for the establishment of this education strategy. students which receive a degree will became the makers of industrial quality inspection in the coming years. references  [1] p. schwarzkopf, “der markt für industrielle bildverarbeitung in deutschland”, industrial vision days 2010, 2010, powerpoint presentation. [2] vdma machine vision group within the robotics + automation association, “key technology for automation solutions machine vision 2011/12 applications – products – suppliers”, machine vision, volume i, 2012, pp. 6-9. [3] g. linss,“qualitätsmanagement für ingenieure”, hanser fachbuchverlag, munich, 2011, 978-3446-41784-7. [4] a. weckenmann, “verkettete qualitätsregelkreise auf der basis systemgerechter prozessund schnittstellenbewertung”, sonderforschungsprojekt 396, teilprojekt c2, lehrstuhl qualitätsmanagement und fertigungsmesstechnik der friedrichalexander-universität erlangen-nürnberg. [5] p. profos, t. pfeifer, “handbuch der industriellen messtechnik”, r. oldenbourg verlag, munich, 1994, 3-48622537-5. [6] w. a. levinson, f. tumbelty, “spc essentials and productivity improvement: a manufacturing approach”, asqc quality press, volume i, 1997, p. 97. [7] h.-p. jungen, “werkzeuge und statistische methoden für das qualitätsmanagement”, vienna, 1993, pp. 4-6. [8] schellhorn m., rosenberger m., güttner s., linß g., “bildverarbeitungstechnologien für die qualitätssicherung in der produktion”, in:vielfalt qualität – tendenzen im qualitätsmanagement bericht zur jahrestagung 2012, volume i , 2012, pp. 3-17; [9] g. linss , m. rosenberger, m. schellhorn , p. werner, s. lübbecke, “new education strategy in quality measurement technique with image processing technologies”, joint international imeko tc1+tc7+tc13 symposium 2011, jena, germany, 2011, urn:nbn:de:gbv:ilm1-2011imeko-040:9. multi-component measurement of grinding force during high speed internal thread grinding acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 163 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 163 167 multi-component measurement of grinding force during high speed internal thread grinding z. zhang1, b. li2, w. shi1, y. huang1 1 beijing institute of precise mechatronics and controls, beijing, china, jimzzj050@sina.com 2 changcheng institute of metrology and measurement, beijing, china, libo@cimm.com.cn abstract: experiments of internal thread grinding and flat groove grinding show that grinding efficiency and quality can be significantly improved by increasing the wheel speed. according to the characteristics of internal thread grinding and flat groove grinding, the grinding force measurement experiment is designed. the equivalent substitution method is used to compare with the grinding force per unit area in surface grinding test under the same conditions. the experimental results show that the measurement error of grinding force per unit area is less than 25 %. keywords: high speed internal thread grinding; grinding force; multi-component force measurement; machining detection; online measurement 1. introduction the results show that the surface quality of a workpiece can be effectively improved by increasing the linear speed of the grinding wheel. when the linear speed of the grinding wheel reaches 80 m/s, the surface quality of workpiece can reach a roughness of ra = 0.4 µm. in a certain range of grinding force, the surface quality will not be reduced while the feed speed of workpiece is increased, that is to say, the machining time is shortened and the work efficiency is improved. the existing literature mainly focus on high-speed grinding experiments of the plane and the outer circle, and the grinding force is taken as the index to verify the correctness of the high-speed grinding mechanism [1-2]. however, the experimental data of other types of high-speed grinding, such as internal thread and flat groove grinding, are less, so it is difficult to provide more comprehensive data support for studying the mechanism and theory of high-speed grinding and the calculation of grinding force per unit area. the general method for measuring grinding force is to select a three-component measuring instrument as a sensor and to connect the workpiece, the three-component measuring instrument and the machine tool. the grinding force generated by the grinding wheel and the workpiece is decomposed into axial force, tangential force and normal force through the three-component dynamometer [3]. the difficulty of measuring the grinding force of internal threads is that after the lead of the internal thread is determined, the workpiece needs to keep rotating with respect to the machine tool at a certain angular speed, which will greatly interfere with the normal force and tangential force. the linear speed of the grinding wheel is limited by the inner diameter of the nut, so it is difficult to enlarge the diameter of the grinding wheel. it is difficult to keep the relative stability between the grinding wheel and the workpiece because of the vibration of the supporting rod caused by the increase of the rotating speed. moreover, the contact area between the grinding wheel and the workpiece is very small in internal thread grinding and flat groove grinding, which leads to the decrease of grinding force amplitude, which makes it difficult to distinguish the interference amplitude caused by machine tool vibration, grinding fluid impact and other factors, and reduces the signal-to-noise ratio. all these factors reduce the accuracy of the grinding force measurement. in order to measure the internal thread grinding force as accurately as possible, the grinding force of internal thread grinding is measured by a six-dimensional torque sensor and a torque sensor respectively, and the grinding force of flat groove grinding is measured by a three-component dynamometer. by using the equivalent substitution method, the contact models of instantaneous grinding wheels for surface grinding, internal thread grinding, external thread grinding, and flat groove grinding are established respectively. the instantaneous contact area and the equivalent radius of grinding wheel for surface grinding by internal thread grinding, external thread grinding, and flat groove grinding are calculated respectively, and the grinding force per unit area is calculated on the basis of these models. http://www.imeko.org/ mailto:jimzzj050@sina.com mailto:libo@cimm.com.cn acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 164 2. description of the work 2.1. measuring method the grinding force in internal thread grinding is measured by a six-dimensional torque sensor and a standard torque sensor respectively (we just used one axis of the six-dimensional torque sensor so that is able to compared with the standard torque sensor), and the grinding force in flat groove grinding is measured by a three-component dynamometer. in the measurement test of each sensor, the cutting line speed of the grinding wheel is 80 m/s, the wheel speed is 40 000 rpm, the grinding wheel 𝑟1 is 20 mm, and the tooth profile half angle 𝜃 is 45°; two kinds of head frame speeds are selected, which are 1 rpm and 2 rpm respectively, and the inner radius 𝑟2 of the workpiece is 50 mm; the feed 𝑝 is 0.02 mm and this is performed five times until the grinding depth reaches 0.1 mm, and ten groups of data are obtained. a total of thirty groups of data were obtained from the three kinds of sensors. grinding force measurement method based on six-axis torque sensors the experimental arrangement for the grinding force measurement using a six-axis torque sensor is shown in figure 1. the maximum outer diameter of the grinding wheel is 𝑑1 (𝑑1 = 2𝑟1), and the half angle of tooth profile is 𝜃 ; the workpiece to be ground is a cup-shaped wheel with an inner diameter of 𝑑2 ( 𝑑2 = 2𝑟2 ) and the workpiece is fixed to the sensor by a screw; after the sensor is fixed to the tooling through the screw, it is installed in the chuck of the machine tool (ensuring that the radial runout of the workpiece is not greater than 0.002 mm). the sensor is sensitive to the grinding torque 𝑀1 generated in the high-speed grinding process of the internal thread, and enters the data acquisition system through the data line as the original data of the test. the 𝑛 is number of data points in a cycle. by calculating the average grinding torque 𝑀1̅̅ ̅̅ and dividing by 𝑟2 , the tangential grinding force 𝐹1 measured by six-axis torque sensor can be obtained: 𝑀1̅̅ ̅̅ = ∑ 𝑀𝑖 𝑛 𝑖=1 𝑛⁄ , 𝐹1 = 𝑀1̅̅ ̅̅ 𝑟2⁄ (1) grinding force measurement method based on standard torque sensor the experimental arrangement for the grinding force measurement using a standard torque sensor is shown in figure 2. the grinding wheel with the maximum outer diameter of 𝑑1 (𝑑1 = 2𝑟1) and half angle of tooth profile of 𝜃 and cup-shaped wheel shaped workpiece with inner diameter of 𝑑2 (𝑑2 = 2𝑟2) are connected to the torque sensor by a flat key, and the axial displacement is limited by bolts; the sensor is installed in the chuck of the figure 1: experimental assembly for measurement based on six-axis torque sensors machine tool through the flat key (ensuring that the radial runout of the workpiece is not greater than 0.002 mm), and the bottom is fixed to the base using screws. the axial elastic strain cell of the sensor collects the grinding torque 𝑀2 generated during the high-speed grinding of the internal thread, which enters the data acquisition system through the data line as the original data of the test. by calculating the average grinding torque 𝑀2̅̅ ̅̅ and dividing by 𝑟2, the tangential grinding force 𝐹2 measured by the standard torque sensor can be obtained: 𝑀2̅̅ ̅̅ = ∑ 𝑀𝑖 𝑛 𝑖=1 𝑛⁄ , 𝐹2 = 𝑀2̅̅ ̅̅ 𝑟2⁄ (2) figure 2: experimental assembly for measurement based on standard torque sensor grinding force measurement method based on three component dynamometers the experimental arrangement for the grinding force measurement using a three-component dynamometer is shown in figure 3. the maximum outer diameter of the grinding wheel is 𝑑1 http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 165 (𝑑1 = 2𝑟1), and the half angle of the tooth profile is 𝜃 . the force sensor is a three-component dynamometer (9256c2, kistler). the workpiece to be ground is a circular flat plate. the threecomponent dynamometer is fixed between the base and the workpiece by screws (the parallelism between the workpiece and the plane datum of the machine tool is not greater than 0.003 mm). when the grinding wheel moves parallel to the surface of the workpiece, its moving speed is equivalent to the relative speed between the workpiece and the grinding wheel during internal thread grinding. the forces 𝐹𝑥 and 𝐹𝑦 on the surface of the parallel workpiece are collected by the three-component dynamometer, and the resultant force 𝐹he is obtained by point-by-point geometric addition. by calculating the average grinding force 𝐹he̅̅ ̅̅ in the grinding process, the tangential grinding force 𝐹3 measured by the dynamometer can be obtained: 𝐹he = √𝐹𝑥 2 + 𝐹𝑦 2, 𝐹3 = 𝐹he̅̅ ̅̅ = ∑ 𝐹he𝑖 𝑛 𝑖=1 𝑛⁄ (3) figure 3: experimental assembly for measurement based on three component dynamometers 2.2. grinding contact model and grinding force per unit area calculation grinding contact model the instantaneous contact models of flat grinding, flat groove grinding, and internal thread grinding are shown in figure 4. among them, the workpiece surface of flat grinding and groove grinding is plane, and the workpiece surface of internal thread grinding is cylindrical; the grinding wheel of flat groove grinding and internal thread grinding is a disc wheel with tooth half angle of 𝜃, and flat grinding wheel is a cylindrical wheel. for the flat grinding process, the contact between the grinding wheel and the workpiece is shown in figure 5. it can be seen that the contact area is related to the wheel width 𝑘, feed rate 𝑝 and grinding wheel radius 𝑟1. if the contact area is 𝑆p, then: 𝑙1 = √𝑟1 2 − (𝑟1 − 𝑎𝑝) 2 (4) 𝑆p = 2𝑘𝑟1 sin −1(𝑙1 𝑟1⁄ ) . (5) figure 4: the instantaneous contact models of flat grinding, flat groove grinding and internal thread grinding figure 5: shape of contact area in flat grinding process figure 6: shape of contact area in flat groove grinding process http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 166 for the flat groove grinding process, the instantaneous contact between the grinding wheel and the workpiece is shown in figure 6. as opposed to flat grinding, the grinding wheel used in flat groove grinding is no longer a cylinder, but a symmetrical cone. therefore, the relationship between the wheel width 𝑘 and the feed rate 𝑝 and the tooth profile half angle 𝜃 of the grinding wheel is as follows: 𝑘 = 𝑝 tan 𝜃 . (6) if the contact area is 𝑆c, then: 𝑆c = 𝑟1 2 (𝛼 − cos 𝛼 sin 𝛼) cos 𝜃⁄ , (7) where 𝛼 is the angle corresponding to the arc length of the grinding wheel contacting the workpiece in the flat groove grinding. the relationship between the angle and the feed rate 𝑝 and the grinding wheel radius 𝑟1 is as follows: 𝛼 = cos−1[(𝑟1 − 𝑎𝑝) 𝑟1⁄ ] . (8) for the internal thread grinding process, the contact between the grinding wheel and the workpiece is shown in figure 7. figure 7: shape of contact area in internal thread grinding process compared with the flat groove grinding, the grinding wheel profile used in internal thread grinding is the same, but the shape of the workpiece is changed from a plane to a circular arc surface, so the contact area is related to the inner radius 𝑟2 of the workpiece. when the contact area of internal thread grinding is 𝑆𝑛, then: 𝑆𝑛 = [𝑎𝑐 − 𝑟2 2 sin−1 ( 𝑐 𝑟2 ) + 𝑟1 2 sin−1 ( 𝑐 𝑟1 )] cos 𝜃 , (9) where 𝑎 is the distance between grinding wheel axis and workpiece axis: 𝑎 = 𝑟2 − 𝑟1 + 𝑝 , (10) 𝑏 is the tangential distance from the contact arc of grinding wheel to the plane of grinding wheel and workpiece axis: 𝑏 = (𝑎2 + 𝑟1 2 − 𝑟2 2) 2𝑎⁄ (11) and 𝑐 is the normal distance from the contact arc of grinding wheel to the plane of grinding wheel and workpiece axis: 𝑐 = √𝑟1 2 − 𝑏2 . (12) calculation of grinding force per unit area given the radius of the grinding wheel 𝑟1 , the inner radius of the workpiece 𝑟2, the half angle of tooth profile 𝜃 , and the feed 𝑝, the unit grinding force 𝐹�̅� (𝑖 =1 to 3) can be obtained by dividing the tangential grinding force 𝐹�̅� ( 𝑖 = 1 to 3) by the surface area formula of the corresponding contact model: 𝐹1̅ = 𝐹1 𝑆n , 𝐹2̅̅ ̅ = 𝐹2 𝑆n , 𝐹3̅̅ ̅ = 𝐹3 𝑆c . (13) 2.3. test data and results measurement environment and processing parameters measurement environment: temperature is 22 °c ± 1 °c, relative humidity is 56 % ± 10 %, atmospheric pressure is local atmospheric pressure. processing parameters: grinding wheel speed is 40 000 rpm, two kinds of head frame motor speed are selected, which are 1 rpm and 2 rpm respectively; each thread line is fed five times, each feed 𝑝 is 0.02 mm, grinding depth is 0.1 mm; grinding wheel radius 𝑟1 is 20 mm, profile half angle 𝜃 is 45°, thread lead is 3 mm; workpiece inner radius 𝑟2 is 50 mm, material is gcr15, inner diameter is 100 mm, length is 50 mm. the cylindricity of the outer circle is 0.001 mm. the coaxiality of the inner hole is 0.002 mm. main parameters of sensors table 1: parameters of sensors sensor type range resolution sensitivity linearity / % fso six-axis torque 1.5 n·m 7.5 mn·m 0.1 pc/n 0.5 torque sensor 1.5 n·m 50 mn·m 0.1 pc/n 0.5 kistler 9256c2 25 n 0.1 n -24.79 pc/n 0.04 statistical tables of results by comparing table 2 and table 3, it can be seen that the grinding force per unit area based on six-dimensional torque sensor and torque sensor has little difference, and the maximum relative difference is 17.4 %; the maximum relative difference between the results from the torque http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 167 sensors detailed in table 2 and table 3 and the three-component sensor detailed in table 4 are 21.3 % and 21.7 % respectively. at the same time, it can be seen that with the increase of cutting depth, the grinding force increases, while the grinding force per unit area decreases gradually; when the head frame speed increases, both the grinding force and the grinding force per unit area increase. table 2: grinding force test results using six-axis torque sensor head frame speed / rpm cut number cut depth / μm measured grinding force / n grinding force per unit area / n/mm2 1 1 20 0.0248 0.57 2 40 0.0259 0.21 3 60 0.0356 0.16 4 80 0.0481 0.14 5 100 0.0565 0.12 2 1 20 0.053 1.21 2 40 0.0742 0.60 3 60 0.0876 0.39 4 80 0.1212 0.35 5 100 0.1224 0.25 table 3: grinding force test results using standard torque sensor head frame speed / rpm cut number cut depth / μm measured grinding force / n grinding force per unit area / n/mm2 1 1 20 0.0264 0.60 2 40 0.0272 0.22 3 60 0.0392 0.17 4 80 0.0429 0.12 5 100 0.052 0.10 2 1 20 0.057 1.30 2 40 0.072 0.58 3 60 0.093 0.41 4 80 0.1203 0.34 5 100 0.1411 0.29 table 4: grinding force test results using three-component force sensor head frame speed / rpm cut number cut depth / μm measured grinding force / n grinding force per unit area / n/mm2 1 1 20 0.035 0.63 2 40 0.041 0.26 3 60 0.052 0.24 4 80 0.061 0.21 5 100 0.071 0.18 2 1 20 0.061 0.80 2 40 0.096 0.55 3 60 0.105 0.39 4 80 0.151 0.35 5 100 0.150 0.24 3. summary through three kinds of grinding force measurement experiment, the grinding force per unit area under the same working conditions was measured. the results show relative agreement to within 25 %. with an increase of grinding depth, the grinding force increases while the grinding force per unit area decreases. 4. references [1] z.-r. pang, j.-s. wang, c.-h. li, g.-q. cai, “merits and key technologies of ultra-high speed grinding”, machinery design & manufacture, no. 4, pp. 160-162, 2007. [2] g.-l. song, y.-d. gong, g.-q. cai et al, “ultrahigh speed grinding and its application”, aviation precision manufacturing technology, vol. 36, no. 3, pp. 16-20, 2000. [3] z.-x. zhou, d.-w. zhou, c. mao, j. yang, “experimental study of the contact length in surface grinding”, journal of hunan university (natural sciences), vol. 36, no. 2, pp. 27-31, 2009. http://www.imeko.org/ practical investigation for the concept of a serial-type build-up force measurement system acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 143 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 143 149 practical investigation for the concept of a serial-type build-up force measurement system m. abdulhakim1, saher r. hassan2, f. tegtmeier3 1 national institute of standards (nis), giza, egypt, moh_mech2007@yahoo.com 2 physikalisch-technische bundesanstalt (ptb), braunschweig, germany, saher.r.hassan@ptb.de 3 physikalisch-technische bundesanstalt (ptb), braunschweig, germany, falk.tegtmeier@ptb.de abstract: this paper introduces a proposed design for the first serial-type build-up system (s-bus). the work includes the design and implementation of two s-bus prototypes, the metrological characteristics of the prototypes were evaluated. the evaluation results prove the validity of the system practically. the paper also discusses the key differences between the parallel build-up system (p-bus) and the s-bus, and what makes it important to study the s-bus concept. keywords: build-up system; meganewton; force traceability 1. introduction all force measuring instruments take their traceability from force standard machines. the deadweight force standard machines (d-fsm) are the most precise machines for force generation but these types of machines have limited capacities due to their high manufacturing cost furthermore their huge size at high capacity [1]. the force amplification machines replace the deadweight machines at higher force ranges, these machines still have problems such as manufacturing cost and complex technology requirements [2]. currently, the comparator type force standard machines which use a bus as a reference are considered the optimum solution for traceability issue at high force level (meganewton range). the calibration is carried out by comparing the response of the force measuring unit under calibration with the response of the standard one (bus) [3]. there are many machines worldwide work based on this principle [4] [5] [6]. furthermore, the bus individually can work as a transfer standard [7] [8]. the p-bus consists of three or more identical force transducers arranged mechanically in parallel. based on three-transducer based p-bus (figure 1 and figure 2), it is found that each force transducer carries one-third of the load acting on the system. the p-bus has advantages such as small size relative to a force standard machine at the same range, capable of measuring high forces more economically, and the most important is that it is used to transfer the traceability from smaller force standard machines to larger force measuring systems. figure 1: a schematic drawing of the p-bus figure 2: p-bus-top view (without upper plate) to allow the transducers within a bus to carry an equal share of force, the system shall include a force introduction plate with enough stiffness to distribute the load on the force transducers. this arrangement has some disadvantages as the size becomes huge at high rated forces also the mechanical stability and stiffness of the overall system become a big challenge. in 2013 the european association of national metrology institutes (euramet) launched a project entitled http://www.imeko.org/ mailto:moh_mech2007@yahoo.com mailto:saher.r.hassan@ptb.de mailto:falk.tegtmeier@ptb.de acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 144 “force traceability within the meganewton range” under designation emrp sib63 [9], the main objective of this project is to extend the traceability of force measurements up to 50 mn and to give users new procedures and technical guidelines on the use of high force measurement devices [10]. the work of the project was split into six work packages. the main objective of work package wp1 was to develop transfer standards (based on bus) for forces up to 50 mn with the lowest possible uncertainty, also all expected influences on measurement uncertainties had been studied during this project [9]. indeed, there were great works that had been done during this project, many designs for p-bus were built. most of the uncertainty sources were discussed. some innovative adaptation parts were designed and tested to investigate their intended effect. now there is enough knowledge about the p-bus and how to use it. 2. s-bus concept force transducers are available in many design configurations; tension rods, compression strut, bending beams, shear beams etc [11]. bending-ring force transducers, also known as pancake design, are one of those widely known for their good metrological characteristics. if this type is modified to allow many units to stand above each other, this will help in the design of the target s-bus as shown in figure 3. using the proposed design, it is expected that the total load will be distributed on the force measuring units equally. one benefit of this design is the measuring units have the same loading axis. under this condition, the bending moments and cross forces will nearly vanish. there is no need for the upper force introduction plate as in the pbus. the geometry of the proposed system is simpler than the p-bus. two finite element models were developed to check the validity of the proposed design. the first model represents a single transducer loaded with a 10 kn axial compressive force while the second model represents three transducers above each other loaded with 30 kn axial compressive force (three times the load in the previous model). the related stress and strain results prove the validity of the design, where the stress and strain values are nearly the same (figure 4 and figure 5). this demonstrates that every unit in the second model is still subjected to just 10 kn compressive force, also this proves that the stiffness of the second model was increased to three times the stiffness of the first model. the trick of the design is the three units contact each other at the inner hub as shown in figure 5, this allows them to deflect as a single unit and to work together to carry higher force since their stiffness increased by three due to this arrangement. for the first model, the maximum von-mises stress is 527 mpa the maximum equivalent strain is 2.261 × 10-3 and the maximum axial deflection is 0.13 mm. for the second model, the maximum von-mises stress is 549 mpa the maximum equivalent strain is 2.377 × 10-3 and the maximum axial deflection is 0.14 mm. this preliminary study shows that the s-bus concept is valid theoretically from the point of load distribution. to test the concept practically a 20 kn and a 30 kn s-bus prototypes were designed and implemented then tested on the ptb 20 kn and 100 kn (d-fsm). figure 3: the schematic drawing of the proposed design of the s-bus max. v.m stress = 527 mpa figure 4: fe results for single transducer model (10 kn force) figure 5: fe results for s-bus model (30 kn force) http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 145 3. major differences between p-bus and s-bus a comparison between p-bus and s-bus was prepared before building the s-bus prototypes (table 1). the purpose of this survey is to determine the factors/parameters that were found to affect the p-bus behaviour based on the knowledge previously gathered through the sib63 project. it’s expected that those factors/parameters will take more interest during the study of the s-bus. table 1: comparison between s-bus and p-bus parameter p-bu system s-bu system bus structure: • force introduction plate. • base plate. • loading button. • overall size. ▪ it is found that the stiffness of the adaptation parts has a significant effect on the system behavior, the more stiff parts are the best behavior the system can give, but this criterion has a limitation where the weight is the opposite face for the stiffness. for the 50 mn p-bu system, the introduction plate weighs 900 kg [10]. at high rated capacities the overall dimensions of the system become relatively large. ▪ the s-bus does not include force introduction plates or other adaptation parts are required. ▪ the overall size is very compact compared to a same capacity p-bus. influences related to structure: • bending moment. • cross forces ▪ these influences are consequences of elastic deformation for the adaptation parts, especially the force introduction plate, to reduce these effects some work had been done during the sib63 project such as: ▪ there is a patented design for a force introduction plate that has been created and studied to nearly eliminate these effects [12]. ▪ for 15 mn ukrainian p-bus, the force introduction plate had been optimized to keep the elastic deformation as low as possible to avoid cross forces and bending moments [7]. ▪ it is expected a very small effect as similar to bending-ring transducers. ▪ the transducers can be enhanced using special load cups to improve their compensation for any structure related influences [12]. type of force transducer used: ▪ standard columns type or bending-ring type can be used. ▪ the bending-ring transducers show the best behavior in the range up to 1 mn [10]. ▪ there is strong evidence that the strain cylinders are more suitable than bending-ring transducers for use in the p-bus at higher rated loads as they can usually be assembled in a more compact arrangement [12]. ▪ only a bending-ring type can be used. ▪ future studies could be carried out to benefit from its design at a high level may be by building the transducer from ground plates. no. of force transducers: ▪ for ptb 50 mn, maximum no. is 5 transducers. ▪ for gtm 5.4 mn, maximum no. is 9 transducers [13] ▪ still under investigation. bus capacity ▪ there is a 60 mn p-bu system available right now in fjim, china [14]. ▪ still under investigation application of force on force transducers ▪ during the individual calibration – the load comes directly from a fsm into the transducer. ▪ during the bus calibration – the load shifted through adaptation parts. ▪ singledirectly from fsm. ▪ bussame load axis (no load shift) metrological characteristics: • reproducibility. • creep and creep recovery. • hysteresis • indication deviation ▪ the influences of different structures on the uncertainty are more significant than expected [15]. ▪ the reproducibility of some p-bus’s is better than the reproducibility of a single transducer. ▪ it’s found that the creep of the p-bus is sensitive to the bus structure while the creep recovery is not affected by the structure [16]. ▪ the number of contact surfaces between single adaptation parts increases the hysteresis effect [12]. ▪ the bu system deviation is affected by all system stiffness, the larger the system stiffness the lower the indication deviation is [12]. ▪ the characteristics of the system are still under investigation, but it is expected to be good due to the similarity of the system to bending-ring force transducers. http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 146 4. s-bus prototype design a 30 kn capacity was chosen to build the first sbus prototype which consists of three force measuring units. it is expected that each unit will carry one-third of the total applied load. each force measuring unit is designed in the form of a bending ring elastic body with 8 strain gauges installed to it at pre-determined locations. the elastic element design process began by proposing a preliminary design, then this design developed step by step to reach the point at which the proposed "model" design would meet the key design criterion. the main design criterion is achieved when a local average normal strain with a value of 1000 µm/m has resulted in the strain gauge pre-determined position, this criterion has to be achieved at a safe stress level relative to the material allowable stress limit. if it's possible to minimize the maximum stress corresponding to 1000 µm/m strain value, this will improve the linearity and hysteresis of the measuring units [17]. the material usedthe material chosen to build the elastic element is the 30crnimo8 (mat. no.1.6580) spring steel. this material is well known in the force transducers manufacturing field due to its good mechanical properties e.g. stiffness and yield strength. strain gauge selectiona linear strain gauge with a 3 mm length measuring grid and 350 ώ resistance was chosen to measure the strain. the grid carrier dimensions are 7.3×4.5 mm; this is the area where the mean strain is measured. 5. s-bus signal generation and processing the signal of the s-bus is generated as illustrated in figure 6. the s-bus signal is displayed in the force unit (n). the s-bus signal fs is the sum of the three force transducers signals (fis, 1, fis,2, fis,3), also these three signals are in force unit (n). in order to obtain a force transducer signal in a force unit, a polynomial equation should be obtained by calibrating the force transducer then fitting its calibration data. using the polynomial equation, the force transducer response (mv/v) could be converted into a force unit (n). the three transducers were calibrated individually according to iso 376:2011 [18], then three third-order polynomial equations were obtained. finally, the signal of the s-bus is compared to the force standard machine load step fls to determine the real-single indication deviation of the s-bus dl. the calibration and evaluation of the s-bus are carried out according to the procedure mentioned in [10] and [15]. 6. experimental setup and calibration the ptb 20 kn and 100 kn d-fsms were used to calibrate the three transducers and the whole sbus. each machine has a relative expanded uncertainty of 2 × 10-5. the transducers were calibrated individually one by one using the 20 kn d-fsm (figure 7). once the individual calibrations were finished, two transducers were assembled to build a 20 kn s-bus, this system was calibrated on the 20 kn d-fsm then its calibration results were analysed according to procedures mentioned in [10] and [15]. finally, the three transducers are assembled to build a 30 kn s-bus, this system was calibrated as a single unit on the 100 kn d-fsm (figure 8). as an additional step, each transducer is dismounted and calibrated individually for the second time to determine the stability of the transducers and also to determine any negative influences coming from the loading of the three transducers above each other in the s-bus assembly. figure 6: signal generation from the real acting force to the measured sum force [10] figure 7: calibration of a 10 kn transducer on the 20 kn ptb d-fsm http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 147 figure 8: calibration of the 30 kn s-bus on the 100 kn ptb d-fsm 7. calibration results and analysis each transducer consists of three parts, for example, transducer no 1 consists of the base no. 0, loading button no. 4, and the elastic element no. 1 (figure 3). the calibration results for the three transducers are shown in table 2. each transducer is designed to a capacity of 10 kn, but the calibration was stepped up to 8 kn only for safety purposes. as shown in table 2, the transducer no. 1 gives the best behaviour where its calibration results prove class 00 according to iso 376 starting from 10 %, while the transducer no. 3 proves class 0.5 from 10 % and the transducer no. 2 proves class 0.5 from 20 %. once each transducer was calibrated individually, two of them (no. 1 and no. 2) are assembled to build a 20 kn s-bus, this system was calibrated up to 16 kn only. the results are collected and analysed according to the procedure mentioned previously. the calibration results are presented in table 3. figure 9 shows the behaviour of real-indication deviation dl along the 20 kn s-bus range. indeed, these results are better than expected, the indication deviation values are better than 2.7 × 10-4 and the relative expanded uncertainty of the system doesn’t exceed 3.2 × 10-4. finally, the three transducers are assembled above each other, as in figure 8, constituting a 30 kn s-bus. the system was calibrated on the 100 kn ptb d-fsm up to 25 kn only, the calibration results are presented in table 4. the behaviour of real-indication deviation resulted along the whole range is shown in figure 10. it’s highly appreciated for the first prototype to prove the validity of the concept and to achieve this level of precision. the maximum indication deviation was increased to 6.6 × 10-4, this may be because of the excess deformation resulting from the increase in the applied force although this point should take a special interest in any future studies especially when adding more units. the transducers’ signals are collected using the hbm dmp41 measuring amplifier which has a resolution of 0.000 001 mv/v and an excitation voltage of 5 v (figure 8). table 2: the calibration results for the three transducers (individually calibrated) load step fls transducer no. 1 transducer no. 2 transducer no. 3 mean response �̅�𝒓 �̅�𝒓 �̅�𝒓 kn mv/v mv/v mv/v 1 0.314 184 0.307 153 0.307 189 2 0.628 532 0.614 343 0.614 557 3 0.942 957 0.921 610 0.922 052 4 1.257 456 1.228 957 1.229 683 5 1.572 027 1.536 387 1.537 440 6 1.886 651 1.843 877 1.845 308 7 2.201 320 2.151 426 2.153 274 8 2.515 998 2.459 013 2.461 315 table 3: the calibration results of 20 kn s-bus load step indication deviation calibration result fls dl w(dl) fs w(fs) kn % % kn % 2 -0.024 0.011 2.000 0.032 4 0.005 0.011 4.000 0.030 5 0.012 0.011 5.000 0.030 6 0.016 0.011 6.001 0.029 8 0.015 0.010 8.001 0.029 10 0.018 0.010 10.002 0.028 12 0.019 0.010 12.002 0.028 14 0.022 0.010 14.003 0.028 15 0.025 0.010 15.004 0.028 16 0.027 0.010 16.004 0.028 where w(fs) is the relative expanded uncertainty of the sum force, dl is the real indication deviation w(dl) is the relative expanded uncertainty associated with dl. figure 9: the 20 kn s-bus indication deviation -0,04 -0,03 -0,02 -0,01 0,00 0,01 0,02 0,03 0,04 0 2 4 6 8 10 12 14 16 18 in d ic a ti o n d e v ia ti o n d l / % applied force fls / kn http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 148 table 4: the calibration results of 30 kn s-bus. figure 10: the 30 kn s-bus indication deviation 8. summary the work involved in this study introduces a new term to the community of force metrology. achieving the concept of serial type build-up system is the aim of this study. the work involves numerical then experimental validation for the proposed s-bus design. two s-bus’s prototypes were implemented during this study, the first based on two force transducers with a 20 kn capacity and the second based on three force transducers with a 30 kn capacity. the evaluation results were better than expected. the deviation of the 20 kn s-bus is below 0.027 % while for the 30 kn s-bus the deviation did not exceed 0.066 %. 9. acknowledgment the research team would like to acknowledge that without the help of ptb staff, particularly mr a. schmidt and mr f. ries, this work could not be completed within the scheduled time plan. 10. references [1] m. abdulhakim, design and metrological study of a 10 mn build-up force standard machine, cairo university, 2014. [2] d. kang, h. song, j. lee, y. park, j. kim, traceability of large force standards in korea, in proc. xvii imeko world congress, dubrovnik, croatia, 22-27 june 2003. [3] s. heamawatanachai, k. chaemthet, n. sumyong, c. amornsakun, analytical study on the uncertainty of load cells calibrated with deadweight-force-standard machine and forcecomparator machine, 2nd tsme int. conf. mech. eng., krabi, thailand, 2011. [4] j. a. robles carbonell, j. a. fernández garcía, j. l. robles verdecia, enlargement of the force capability at cem: development of a 10 mn hydraulic force standard machine, 20th imeko tc3 int. conf., pp. 148–154, 2007. [5] b. çal, s. fank, h. özbay, e. pelit, c. kuzu, b. aydemir, 3 mn hydraulic type build-up force standard machine installed at national metrology institute (ume), in proc. xvii imeko world congress, pp. 301-304, dubrovnik, croatia, 22-27 june 2003. [6] q. li, j. yao, h. xu, h. li, t. y. li, 10 mn buildup force standard machine developed in china, proc. 21st imeko tc3 conf., pp. 37–40, pattaya, thailand, november 2010. [7] m. wagner, a. tsiporenko, f. tegtmeier, development of a 15 mn build-up system for calibrations in ukraine’s heavy industry, proc. xxi imeko world congress, prague, czech republic, 2015. [8] j. cruz, v. prodonoff, j. guerhard, research, development and characterization of a build-up system prototype up to 600 kn, proc. 17th imeko tc3 conf., pp. 36–44, istanbul, turkey, september 2001. [9] r. kumme, f. tegtmeier, d. röske, a. barthel, a. germak, p. averlant, force traceability within the meganewton range, proc. 22nd imeko tc3 conf., pp. 135–136, cape town, south africa, 2014. [10] f. tegtmeier, m. wagner, r. kumme, investigation of transfer standards in the highest range up to 50 mn within emrp project sib 63, proc. xxi imeko world congr., prague, czech republic, 2015. [11] hbm, the route to measurement transducers, hbm company, 2008. [12] f. tegtmeier, d. röske, w. liang, practical applications of an enhanced uncertainty model for build-up systems, 23rd imeko tc3 conf., helsinki, finland, 2017. [13] g. schulder, u. kolwinski, d. schwind, a new design of a 5.4 mn build-up system, proc. xviii imeko world congress, rio de janeiro, brazil, 2006. [14] w. liang, f. tegtmeier, j. yao, h. xu, force comparison between fjim and ptb up to 16.5 mn, 23rd imeko tc3 conf., helsinki, finland, 2017. [15] m. wagner, f. l. tegtmeier, processing and evaluation of build-up system measurement data, proc. xxi imeko world congress, prague, czech republic, 2015. [16] w. liang, x. yang, j. yao, f. tegtmeier, investigation the creep and creep recovery behavior of build-up systems, 23rd imeko tc3 conf., helsinki, finland, 2017. -0,02 0,00 0,02 0,04 0,06 0,08 0 2 4 6 8 10 12 14 16 18 20 22 24 26 in d ic a ti o n d e v ia ti o n d l / % applied force fls / kn load step indication deviation calibration result fls dl w(dl) fs w(fs) kn % % kn % 2 0.000 0.010 2.000 0.027 4 0.029 0.009 4.001 0.025 6 0.040 0.009 6.002 0.024 8 0.044 0.009 8.003 0.024 10 0.049 0.009 10.005 0.024 12 0.053 0.009 12.006 0.023 14 0.056 0.009 14.008 0.023 16 0.061 0.009 16.010 0.023 18 0.063 0.009 18.011 0.023 20 0.065 0.009 20.013 0.023 25 0.066 0.009 25.017 0.023 http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 149 [17] t. kleckers, radial shear beams used as high precision transfere transducers, proc. 21st imeko tc3 conf., pattaya, thailand, november 2010. [18] iso 376, metallic materials calibration of force proving instruments used for the verification of uniaxial testing machines.” international organization for standardization (iso), 2011. http://www.imeko.org/ the testing evaluation of several digital torque wrenches by using a torque wrench tester acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 189 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 189 193 the testing evaluation of several digital torque wrenches by using a torque wrench tester k. ogushi1 1 national metrology institute of japan, aist, tsukuba, japan, kji.ogushi@aist.go.jp abstract: hand torque wrenches (htws) with electrical indicating devices (type i, class b defined in iso 6789-1:2017; called “digital torque wrenches”) manufactured by several manufacturers were tested and evaluated. the digital torque wrench is the most accurate in all types of htws in general. the reference standard was a torque wrench tester (twt) with a rated capacity of 50 n∙m. this twt has been calibrated by using a traceable reference torque wrench (rtw). testing results are described with some characteristics. keywords: torque; standard; torque wrench; torque wrench tester; testing evaluation 1. introduction although hand torque tools (htts) are just tools which fasten elements such as bolts, nuts, and screws, they also have a function of measuring devices which indicate the fastening torque or give out a kind of signal when achieving a pre-set torque. htts include hand torque wrenches (htws) and hand torque screwdrivers (htss). iso 6789 [1][2], which is an international standard of the requirement for htts, has been revised in 2017. it has been separated into two documentations. part 1 describes the testing method and the conformity assessment of htts, whereas part 2 describes the calibration method and evaluation of uncertainties. the author supposes that the former is for both manufacturers and customers, and the latter is mainly for manufacturers and calibration laboratories. there are some excellent investigation results concerned with htws. röske proposed the evaluation method of measurement uncertainty for the htws calibrated according to iso 6789:2003 [3]. brüge investigated the measurement uncertainty of setting type htws when they were used as transfer devices for the comparisons or proficiency tests [4]. bangi et al. showed a simple uncertainty evaluation method for the calibration results of htws in action [5]. khaled et al. proposed a new calibration procedure for setting type htws to reduce the measurement uncertainty differentiating from iso 6789-1:2017 [6]. whereas, there might not be detailed investigations for electrical indicating htws manufactured by different manufacturers so far. iso 6789-1:2017 classifies htts as follows: a) indicating torque tools (type i): 1) class a: htw, torsion or flexion bar; 2) class b: htw, rigid housing, with scale or dial or display; 3) class c: htw, rigid housing and electronic measurement; 4) class d: hts, with scale or dial or display; 5) class e: hts, with electronic measurement. b) setting torque tools (type ii): 1) class a: htw, adjustable, graduated or with display; 2) class b: htw, fixed adjustment; 3) class c: htw, adjustable, non-graduated; 4) class d: hts, adjustable, graduated or with display; 5) class e: hts, fixed adjustment; 6) class f: hts, adjustable, non-graduated; 7) class g: htw, flexion bar, adjustable, graduated. in japan, some customers criticize that conformity assessments are not correct because more significant deviations occur than accuracies declared by manufacturers themselves, particularly in lower capacities of htws. then, the national metrology institute of japan (nmij), in the national institute of advanced industrial science and technology (aist) examined testing of htws with electrical indicating devices (type i, class c defined in iso 6789-1:2017; called “digital torque wrenches: “dtw”), manufactured by several manufacturers. the dtw is the most accurate in all types of htws in general (around 1 %, as a relative accuracy declared by manufacturers). the reference standard was a torque wrench tester (twt) with a rated capacity of 50 n∙m. this twt has been calibrated by using a reference torque wrench (rtw). the rtw has also been calibrated by using a torque standard machine (tsm) at nmij. some characteristic results are described. http://www.imeko.org/ mailto:kji.ogushi@aist.go.jp acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 190 2. experimental method 2.1. experimental apparatus the specifications of dtws which were tested in this research are shown in table 1. those specifications are declared by manufacturers on their catalogues. the dtws were categorized into two groups, which were group i (rated capacities of 20 n∙m to 34 n∙m) and group ii (rated capacities of 50 n∙m to 135 n∙m). a photograph of these dtws is shown in figure 1. effective lever lengths and sizes of the square drive are different depending on manufacturers. all torque wrenches show their “accuracies” on the catalogues. the definitions are generally unclear, although a few manufacturers define the accuracies as “the relative deviations from the reference values plus one digit.” it would be better to recognize the accuracy as the “maximum permissible relative deviation,” which is defined in iso 6789-1:2017. figure 1: digital torque wrenches (dtws) used for the testing and evaluation table 1: specifications of digital torque wrenches a twt of dote50n4, manufactured by tohnichi mfg. co., ltd., was used as a reference standard for these tests. a photograph of the twt is shown in fig. 2. the rated capacity and resolution of the twt are 50 n∙m and 0.005 n∙m. a maximum effective length is 410 mm. the accuracy declared on the catalogue is ±1 % + 1 digit. nevertheless, the uncertainty of calibration of the twt was evaluated by using a reference torque wrench (rtw). the rtw is the combination of tts/100nm (as a torque transducer in the shape of torque wrench) and mgcplus with ml38 (as an indicator/amplifier), as shown in fig. 3. an appearance of the calibration of twt by using rtw is also shown in fig. 4. the rtw was calibrated by using a torque standard machine (tsm) at nmij as well. therefore, the traceability of measurement was utterly established in the series of experiments. relative expanded uncertainties of the rtw calibration by using the tsm were as follows for clockwise (cw) and counterclockwise (ccw) torques: cw: 0.045 % (5 n∙m ~ 100 n∙m) ccw: 0.056 % (-5 n∙m ~ -100 n∙m) relative expanded uncertainties of the twt calibration by using the rtw were as follows: cw: 0.38 % (5 n∙m ~ 50 n∙m) ccw: 0.51 % (-5 n∙m ~ -50 n∙m) atech1fs240n gek-030-r2 t2dt30h cem20n3x10d exacta 2-25 atech2f100bn gek-060 -r3 t3dt60h cem50n3x12d group i min max 1 atech1fs240n snap-on inc. --30 2 gek-030-r2 kyoto tool co., ltd. 6 30 3 t2dt30h tone co., ltd. 6 30 4 exacta 2-25 sturtevant richmont --34 5 cem20n3x10d tohnichi mfg. co., ltd. 4 20 square resodrive lution / mm / n・m cw ccw 1 atech1fs240n snap-on inc. 6.3 0.01 2 3 2 gek-030-r2 kyoto tool co., ltd. 6.3 0.02 4 4 3 t2dt30h tone co., ltd. 6.3 0.01 3 3 4 exacta 2-25 sturtevant richmont 10 0.01* 3) 1 1 5 cem20n3x10d tohnichi mfg. co., ltd. 10 0.02 1 2) 1 2) group ii min max 6 atech2f100bn snap-on inc. --135 7 gek-060-r3 kyoto tool co., ltd. 12 60 8 t3dt60h tone co., ltd. 12 60 9 cem50n3x12d tohnichi mfg. co., ltd. 10 50 square resodrive lution / mm / n・m cw ccw 6 atech2f100bn snap-on inc. 10 0.1 2 3 7 gek-060-r3 kyoto tool co., ltd. 10 0.05 3 3 8 t3dt60h tone co., ltd. 10 0.01 3 3 9 cem50n3x12d tohnichi mfg. co., ltd. 10 0.05 1 2) 1 2) *1) some of definitions of accuracy are unclear. *2) ±1 % + 1 digit. *3) "0.005" in the case of less than 10 n・m. type manufacturer accuracy* 1) / ±% effective length mm 237 150 type manufacturer meas. range n・m 160 317 217 length n・m mm type manufacturer meas. range effective 347 150 165 / ±% 254 type manufacturer accuracy* 1) http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 191 figure 2: the torque wrench tester (twt) used as a reference standard figure 3: the reference torque wrench (rtw) used for the calibration of the torque wrench tester (twt) figure 4: the calibration of the twt using the rtw as a reference standard the reference [7] and [8] describe the detailed procedures of calibrations and uncertainty evaluations for the rtw and twt, respectively. the uncertainty of the calibration for the twt includes the uncertainty contribution due to the indicating error in the case of the display with the unit of torque. then, the indicated values of the twt were not corrected in the testing of htws afterward. the maximum values of indicating errors were 0.12 % for cw and 0.13 % for ccw. the normative requirement that “the maximum measurement error of the torque measurement device shall not exceed 1/4 of the claimed maximum permissible relative deviation of the torque tool at each target value.” in iso 6789-1:2017, was satisfied as the maximum permissible relative deviation is 4 % in this experiment [1]. 2.2. experimental procedure in the testing of htws, the effective lever lengths set out in the twt were adjusted according to the specification of each htw. besides, both the horizontal positions of htw and twt were adjusted by using just a level. the testing was conducted according to the procedure written in iso 6789-1:2017. here, the testing was separated to cw and ccw. one preloading was carried out by the maximum torque of that testing beforehand. five times at 6 n·m, five times at 12 n·m, and five times at 20 n·m were loaded out for the group i. five times at 12 n·m, five times at 30 n·m, and five times at 50 n·m were loaded out for the group ii. the torque steps were a bit different from the requirement of iso 6789-1:2017 (at the lowest specified torque value, at approx. 60 %, and finally at 100 % of the maximum torque). therefore, the experimental results cannot be directly compared to the maximum permissible relative deviation according to the iso document. the indications of the twt was read out when the readings of htws strictly coincided with the target values. the another consecutive five times loading was conducted after complete unloading to zero when overloading accidentally occurred. besides, the author conducted an extra testing at the maximum step of ccw torque, changing the effective length of the lever to 5 mm longer than setting average length. 3. results and discussion 3.1. relative deviation figures 5 and 6 show relative deviations of the twt indications from the indication values of dtws for group i and ii, respectively. the symbols of dtw-a to dtw -e, and dtw-k to dtw-n are just random. they are not related to the order in table 1. it was found that almost all dtws tested had a small repeatability with simple five measurements at each torque step. there was a little more extensive repeatability at the +6 n·m step of dtwa and at the +12 n·m step of dtw-l. dote50n4 tts/100nm http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 192 figure 5: testing results for group i figure 6: testing results for group ii the quantities of relative deviations depended on the type of dtws. one was nearly zero deviations, whereas the other was more than 2 % deviations. the reason for such different deviations probably comes from the different pathways of the traceability, different mechanisms of dtws, or both. all relative deviations were within the accuracy, declared in the catalogues of manufacturers in either case. the maximum permissible relative deviation for dtws is 4 %, according to iso 6789-1:2017. then, the maximum measurement error of twt should be equal or less than 1 %. however, many manufacturers insist that the accuracy of dtws of less than 4 %, e.g., 1 %, 2 %, or 3 %. in such cases, the maximum measurement error of twt should also be much smaller than 1 %. in the strictest case, 0.25 % of the measurement error is required. the value of error is quite difficult to be achieved, especially at the lower range of torque values (5 n·m or smaller). 3.2. influence of changing the effective length at the maximum step of ccw torque (-20 n·m for group i and -50 n·m for group ii), the effective lengths (loading points) of the lever were changed to 5 mm longer than original effective lengths. the relative deviations of average values for five measurements are shown in figs. 7 and 8. figure 7: relative deviations when changing the position of loading point to 5 mm outward in the case of group i figure 8: relative deviations when changing the position of loading point to 5 mm outward in the case of group ii the uncertainty contribution due to the variation of the force loading point should be considered for the total uncertainty of htts according to iso 6789-2:2017 [2] (the concept of the uncertainty of “calibration” but not “testing results” are introduced in part 2 of this standard). the variation value should be obtained by measuring ten times at 10 mm on either side of the center of the handhold position or the marked loading point for the lower limit torque values. although the measurement method in this experiment was a bit different from iso 6789-2:2017, we could obtain the tendency of this effect. it was found that there were quite large deviations in some dtws from figs. 7 and 8. only 5 mm change of the lever length caused more than 3 % deviations. it might be ascribable to the difference of structure inside of each dtw. there are some types of structure in dtws as follows, for example: (1) measuring the shear strain at the twisting axis, (2) measuring the bending strain of an elastic hinge rod inside, and (3) measuring the bending strain of an elastic hinge rod but putting an additional pivot and shortened the hinge length. -3.0 -2.0 -1.0 0.0 1.0 2.0 3.0 -20 -15 -10 -5 0 5 10 15 20 r e la ti v e d e v ia ti o n s / % torque / n·m a b c d e -3.0 -2.0 -1.0 0.0 1.0 2.0 3.0 -50 -40 -30 -20 -10 0 10 20 30 40 50 r e la ti v e d e v ia ti o n s / % torque / n·m k l m n -4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 a b c d e r e la ti v e d e v ia ti o n w it h c h a n g in g th e l o a d in g p o in t / % type of digital torque wrench -4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 k l m n r e la ti v e d e v ia ti o n w it h c h a n g in g th e l o a d in g p o in t / % type of digital torque wrench http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 193 those differences of structure might affect the influence of changing the effective length of the lever. finally, the handhold shapes of dtws possibly affect the influence of lever length change. dtws tested in this study have various shapes at the handhold grip (loading point) such as rounded, inclined, grooved, or protruded ones. 4. conclusion the author examined the testing of hand torque wrenches (htws) with electrical indicating devices (type i, class c defined in iso 6789-1:2017; called “digital torque wrenches: dtw”), manufactured by several manufacturers. the torque wrench tester (twt) was used for the reference standard, which was calibrated traceable to the national torque standard. all relative deviations were found to be within the accuracy declared in the catalogues of manufacturers. however, there were quite large deviations in some dtws when the effective lengths (loading points) of the lever were changed to 5 mm longer than original effective lengths. the structure problem of dtws might affect the total uncertainty of calibration when evaluated by using iso 6789-2:2017. the author is going to examine the testing results of dtws with different twts which have different pathways of traceability, as the next stage. acknowledgment the author appreciates mr. sakae kimura, who is the technical staff in nmij, for his contribution to data acquiring, photographing, and processing. references [1] iso 6789-1, assembly tools for screws and nuts hand torque tools – part 1: requirements and methods for design conformance testing and quality conformance testing: minimum requirements for declaration of conformance, international standard organization, 2017. [2] iso 6789-2, assembly tools for screws and nuts hand torque tools – part 2: requirements for calibration and determination of measurement uncertainty, international standard organization, 2017. [3] d. röske, iso 6789:2003 calibration results of hand torque tools with measurement uncertainty – some proposals, proc. xx imeko world congress, busan, rp. korea, 9 12 september 2012. online [accessed 20200727]: https://www.imeko.org/publications/wc2012/imeko-wc-2012-tc3-o34.pdf [4] a. brüge, operating conditions for transfer clicktorque wrenches, proc. imeko 22nd tc3, 15th tc5 and 3rd tc 22 int. conf., cape town, rp. south africa, 3 6 february 2014. online [accessed 20200727]: https://www.imeko.org/publications/tc32014/imeko-tc3-2014-017.pdf [5] j. o. bangi, s. m. maranga, s. p. nganga and s. m. mutuli, torque wrench calibration and uncertainty of measurement, proc. imeko 22nd tc3, 15th tc5 and 3rd tc 22 intr. conf., cape town, rp. south africa, 3 6 february 2014. online [accessed 20200727]: https://www.imeko.org/publications/tc32014/imeko-tc3-2014-030.pdf [6] k. m. khaled and s. m. osman, improving the new iso 6789:2017 for setting torque tools – proposal, measurement, 112, 150-156, 2017. [7] k. ogushi, a. nishino, k. maeda and k. ueda, range expansion of the reference torque wrench calibration service to 5 kn m at nmij, measurement, 45-5, 1200-1209, 2012. [8] k. ogushi, a. nishino, k. maeda and k. ueda, calibration of a torque wrench tester using a reference torque wrench, proc. sice annual conference，411-416, tokyo, japan, 2011. http://www.imeko.org/ https://www.imeko.org/publications/wc-2012/imeko-wc-2012-tc3-o34.pdf https://www.imeko.org/publications/wc-2012/imeko-wc-2012-tc3-o34.pdf https://www.imeko.org/publications/tc3-2014/imeko-tc3-2014-017.pdf https://www.imeko.org/publications/tc3-2014/imeko-tc3-2014-017.pdf https://www.imeko.org/publications/tc3-2014/imeko-tc3-2014-030.pdf https://www.imeko.org/publications/tc3-2014/imeko-tc3-2014-030.pdf in-situ dynamic force calibration using impact hammers acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 118 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 118 123 in-situ dynamic force calibration using impact hammers m. kobusch1, l. klaus2 physikalisch-technische bundesanstalt (ptb), bundesallee 100, 38116 braunschweig, germany, 1 michael.kobusch@ptb.de, 2 leonard.klaus@ptb.de abstract: this paper presents experimental investigations of in-situ dynamic force calibrations in which an impact hammer provides the dynamic reference force. here, the force transducer to be calibrated remains in the original mechanical structure of the force measurement application to which calibration shock forces are applied in a suitable way. numerous experiments with different force transducer set-ups and different impact hammer configurations were conducted to validate this insitu calibration method. the paper describes the analysis of the measurement data and presents the force transfer functions obtained. finally, these dynamic calibration results are discussed. keywords: shock force; dynamic calibration; transfer function; modal hammer 1. introduction while the measurement of dynamic forces plays an important role in many areas of industry, the traceability of such forces is predominantly based on static calibrations, as documentary standards and generally accepted guidelines for dynamic calibrations rarely exist or are still in their infancy [1, 2]. research on the dynamic calibration of force transducers has been conducted by several national metrology institutes in recent years. the main objective of this research is the establishment of calibration facilities in which sinusoidal or pulseshaped force excitations are used. although these facilities are suited to characterize a force transducer dynamically, the calibration results obtained by means of different excitation methods or calibration set-ups can often only be transferred to each other, or later to a specific application, via suitable mechanical modelling [3]. for this purpose, corresponding decisive dynamic parameters must be determined that result from the given mass and elasticity distribution of the force transducer and the calibration set-up connected to it. to avoid the difficulties and challenges of dynamic force traceability associated with a transducer dynamically calibrated in an external device, researchers from the american national metrology institute nist proposed in-situ calibration by means of a dynamically calibrated impact hammer used as a force reference [4]. furthermore, an application of a dynamically uncalibrated impact hammer to characterize the dynamic behaviour of a force transducer array was published in [5]. impact hammers (or modal hammers) are widely used in industry and science for the modal testing of mechanical structures. these hammers incorporate a piezoelectric force sensor that measures the force pulses applied. interchangeable hammer tips and mass extenders are used to vary the length and amplitude, and thus the spectral content, of the pulses. further information on the theory and practice of dynamic measurements and modal analysis by means of impact hammers can be found in [6, 7]. such an in-situ calibration has a significant advantage: the force transducer to be calibrated remains unchanged in its original mechanical set-up as used for the dynamic measurement application. possible changes in the dynamic system behaviour resulting from mounting and dismounting the force transducer (e.g. due to variations of contact stiffness, damping, component positioning or orientation) are avoided. in addition, no complex modelling or compensation of parasitic inertial force components are required. the in-situ calibration should be performed using the same amplifier and data acquisition system, configurations, filter settings and sampling rates that will be used later for dynamic force measurements. otherwise, the frequency responses of the different measuring chains must be known and taken into account accordingly. the in-situ dynamic force calibration provides calibration results in the frequency domain – the frequency response function which, in principle, allows traceability to other dynamic signal forms such as sine forces, step forces or arbitrary force pulses. in the following, experimental tests of the in-situ dynamic calibration method proposed are presented in which the measurement set-up uses a similar mechanical arrangement of a force transducer http://www.imeko.org/ mailto:michael.kobusch@ptb.de mailto:leonard.klaus@ptb.de acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 119 mounted at its base and loaded at its top by a load mass to which the shock force is applied. the use of different impact hammers as well as different measurement set-ups is simulated by varying the hammer configuration and changing the load mass of the transducer, respectively. this mechanical configuration is typical for many impact force measurement applications in industry such as component tests and crash tests. 2. hammer calibration the in-situ dynamic force calibration method proposed is a secondary calibration method in which the reference force is provided by a dynamically calibrated impact hammer. in contrast to the negative force step calibration method used in [4], calibrations at ptb are carried out on a conventional hammer calibration set-up that provides traceability by measuring the inertial force of a reference mass that has been hit by the impact hammer to be calibrated. the calibration device shown in figure 1 uses a string-suspended cylindrical mass body (e.g. 1 kg) that is struck by the hammer. the inertial force of this mass body is measured by an accelerometer and is compared to the hammer’s output signal. in order to simulate the curved movement of manually executed hammer strokes and to achieve a defined impact trajectory that can be reproduced well, the hammer is elastically clamped in a holder mounted on a pendulum. all measurements described in this paper were carried out with a kistler 9726a200001 impact hammer connected to an iepe signal conditioner (pcb 482c series) that forms part of the measuring chain considered. the hammer that is specified with a head mass of 500 g was equipped with different hammer tips and an optional mass extender (250 g) screwed onto the hammer head. all components of the force reference (acceleration sensor, charge amplifier, reference mass) were calibrated and traced to si units. both calibration signals were sampled by means of a national instruments pxi-5922 data acquisition card. whereas the calibration result commonly used in the time domain is the sensitivity defined as the pulse peak ratio, this paper will focus on calibration results in the frequency domain, i.e. the complex frequency response (sensitivity) in magnitude and phase. however, only magnitude responses are presented in the following, as these have the most practical importance in dynamic force measurement. typical pulse signals of the impact hammer calibration are visualized in figure 2 for two different hammer tips. all signals were passed through a 4th order 20 khz bessel low-pass filter to suppress disturbances caused by modal vibrations of the acceleration sensor or the reference mass observed above 50 khz. the acceleration signal obtained with the steel tip still shows some remaining ripples. the pulse width weakly depends on the pulse magnitude [8] due to the nonlinear contact stiffness of the hammer tip and is about 0.2 ms (steel tip) or 0.65 ms (plastic tip) using a reference mass of 1 kg. both calibration signals show very similar gaussian-like pulse shapes. figure 1: impact hammer calibration device at ptb the magnitude response of four impact hammer configurations – applying two different hammer tips with and without a mass extender – is visualized in figure 3; diagram (a) shows the response in the lower frequency range up to a frequency of 2 khz, (b) up to 10 khz. each curve represents the weighted spectral average of pulses with different shock force magnitudes and is given with its respective weighted standard deviation. the weighting is based on the power spectral density of the reference signal. the number of samples ranges from 12 to 33 and is indicated in brackets. at a sample length of 15 ms, the frequency resolution of the discrete fourier transform (dft) amounts to 67 hz. impact hammer reference mass pendulum accelerometer strings holder ____________________________________________________ 1 commercial devices are identified in this paper only to adequately specify the experimental set-up. such identification does not imply recommendation by ptb, nor does it imply that the equipment identified is necessarily the best available for the purpose. http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 120 the magnitude responses are almost constant at lower frequencies and the four configurations differ by less than 2.5 % up to a frequency of 2 khz. at higher frequencies, the curves clearly decrease, and measurements with the plastic tip demonstrate apparent ripples of unknown origin. in general, hammer calibrations with the metal tip or the mass extender yield slightly higher values. above 2 khz, the weighted standard deviation strongly increases for measurements with the plastic tip, which indicates an insufficient frequency content for large pulse widths. the increased deviations at the lowest frequency bins are due to power line noise. the coherence function is commonly used as a measure of the correlation between two signals in the frequency domain. figure 4 shows the coherence of the measurements presented. the values decrease substantially at higher frequencies, especially when measuring with the plastic tip. in this case, the usable frequency range of the hammer calibration data is limited to a few kilohertz, whereas the steel tip can be used up to 10 khz. figure 2: calibration signals of impact hammer (blue) and acceleration sensor (orange) obtained with hammer tips made of steel (top) and plastic (bottom). in order to estimate the measurement uncertainties of the impact hammer calibration, further research is needed to understand the observed behaviour. possible influences are elastic modes of the hammer or the reference mass. however, the lowest modal resonance of the 1 kg reference mass is at about 30 khz [8]. figure 3: magnitude response of the impact hammer for four hammer configurations: (a) frequency range up to 2 khz, (b) up to 10 khz. each curve shows the weighted mean with the weighted standard deviation. the number of samples is given in brackets. figure 4: coherence between the two calibration signals for four hammer configurations; each curve shows the arithmetic mean. the sample size is given in brackets. the rather flat sensitivity of the tested impact hammer seems to validate its use as a dynamic force reference in the lower kilohertz range. even if only the static calibration or a single-frequency calibration values were used, the potential dynamic errors would be quite small for many dynamic applications. in the following section, the suitability as a dynamic force standard for in-situ calibrations is experimentally investigated. 0 0.2 0.4 0.6 0.8 s ig n a l in a rb . u n it time in ms impact hammer accelerometer 0 0.2 0.4 0.6 0.8 s ig n a l in a rb . u n it time in ms 0.22 0.23 0.24 0 500 1000 1500 2000 m a g n it u d e i n m v /n frequency in hz(a) 0.1 0.15 0.2 0.25 0.3 0 2000 4000 6000 8000 10000 m a g n it u d e i n m v /n frequency in hz steel tip (16) steel tip, mass extender (12) plastic tip (31) plastic tip, mass extender (33) (b) 0.8 0.9 1 0 2000 4000 6000 8000 10000 c o h e re n ce frequency in hz steel tip (16) steel tip, mass extender (12) plastic tip (31) plastic tip, mass extender (33) http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 121 3. in-situ dynamic force calibration experiments the principal arrangement of the in-situ dynamic force calibration set-up is illustrated in figure 5. the force transducer to be calibrated (signal uft(t)) and the reference force sensor (signal uref(t)) incorporated in the impact hammer are marked in blue. the force transducer is vertically mounted on a heavy base plate by means of an adapter; its upper section consists of a load adapter and an optional load mass fixed in place by screws. the hammer is operated by hand in such a way that the hammer tip hits the load adapter as centrally and vertically as possible. photographs of the respective mechanical components of the experiments are shown in figures 6 and 7. figure 5: illustration of the in-situ dynamic force calibration. figure 6: components of the calibration experiments: hbm u3/10 kn force transducer (1) with load adapter (2) and two optional load mass discs (3) plus mounting screws (4); kistler 9726a20000 impact hammer (5) with two hammer tips (6) and optional mass extender (7). the in-situ dynamic calibration set-up can be modelled as a mass-spring-damper system; in the simplest approach with one degree of freedom, it is attached to a rigid base and its mass is struck by the hammer. the model mass consists of the applied load components plus the internal upper part of the force transducer, and the spring represents the transducer’s axial stiffness. the larger the mass, the lower the system’s fundamental resonant frequency. the tests were carried out using a small strain gauge force transducer (hbm u3 / 10 kn) mounted to a steel mass cube of 10 kg as the base plate, which rested freely on the foundation below via a plastic tray. this mass is the reaction mass of the 20 kn impact force calibration facility at ptb, thus simplifying future comparison calibrations. the force transducer is equipped with flange ends on both sides and features an integrated lateral force compensation, which is advisable for manual hammer blows where the impact position and force direction are not well defined. the hammer force was introduced to the upper flange end of the force transducer via a screwed-on load adapter with a spherical load button. to simulate mechanically different in-situ dynamic calibration set-ups, the top end of the force transducer was equipped with different loads. stacking the load adapter with two optional mass discs plus mounting bolts yielded four values determined by weighing: 135 g, 237 g, 446 g and 551 g. figure 7: in-situ dynamic force calibration using a load mass of 135 g and a steel hammer tip. 1 5 2 4 3 6 7 http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 122 all tests were performed with a single impact hammer; however, in a practical sense, the use of different hammer tips and an optional mass extension simulated the application of different hammers. for each combination of a force transducer set-up and a hammer configuration, hammer strokes of varying force magnitude were measured and analysed. figure 8 presents typical pulse signals in the time domain for four experimental configurations. the upper row was obtained with 135 g (the smallest external load mass), the bottom row with 446 g. the signals achieved with the steel hammer tip are shown on the left and those achieved with the plastic tip are shown on the right. in each example, the hammer peak force was in the range of 1.5 kn. figure 8: measurement signals of the in-situ dynamic force calibration for four experimental configurations: hbm u3/10 kn force transducer with a load of 135 g (top row) and 446 g (bottom row); hammer strokes with steel tip (left) and plastic tip (right). these examples clearly demonstrate that the pulse shape and the peak force of the impact hammer and the force transducer can deviate substantially, i.e. the comparison of peak values is not useful. the fundamental resonance is excited by the hammer strokes and the force transducer measures a ringing whose vibration amplitude depends on the specific configuration and impact conditions. the following diagrams present the magnitude response for different configurations of the calibration set-up and the impact hammer. each curve displays averaged data from about 40 measurements. the pulse peak values of the impact hammer ranged from about 0.3 kn to 2.5 kn. figure 9 demonstrates the effect of the variation of the load mass on the magnitude response showing measurements obtained with the steel hammer tip. the upper diagram (a) visualizes the data up to a frequency of 4 khz on a logarithmic axis scale, the magnified view (b) up to 2 khz on a linear scale. the three curves start at similar magnitudes – the force transducer is specified with a static sensitivity of 0.5 mv/n – and gradually rise to a dominant resonance, whose frequency strongly depends on the applied load mass. this resonance is described by the above-mentioned model with one degree of freedom. the magnified view shows that the magnitude responses also differ at lower frequencies. the kink at the lowest frequency bins is caused by the rather elastic coupling of the 10 kg base plate with the calibration set-up to the foundation below. this dynamic behaviour was confirmed by modelling the magnitude response function using a multi-degrees-of-freedom model that considered an elastic base coupling of the calibration set-up. figure 9: magnitude response for three different force calibration set-ups: load masses 135 g, 237 g, 551 g; impact hammer with steel tip; (a) resonance peaks in the frequency range up to 4 khz; (b) magnified view of the lower frequency range up to 2 khz. the influence of the impact hammer on the magnitude response of the calibration set-up is shown in figure 10 for an example load mass of 0 0.5 1 1.5 s ig n a l in a rb . u n it time in ms 0 0.5 1 1.5 s ig n a l v o lt a g e in a .u . time in ms 0 0.5 1 1.5 s ig n a l in a rb . u n it time in ms 0 0.5 1 1.5 s ig n a l v o lt a g e in a .u . time in ms impact hammer force transducer 0.1 1 10 100 0 1000 2000 3000 4000 m a g n it u d e i n m v /n frequency in hz kistler 9726a20000 without mass extender 135 g 237 g 551 g (a) 0.4 0.5 0.6 0.7 0.8 0 500 1000 1500 2000 m a g n it u d e i n m v /n frequency in hz kistler 9726a20000 without mass extender 135 g 237 g 551 g (b) http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 123 237 g. the measurements were obtained with both hammer tips, both with and without a mass extender. as can be seen, the four responses agree very well. the agreement of calibration results obtained with different hammer configurations and experimentally tested on different calibration setups allows the conclusion that the in-situ calibration method proposed is well suited for dynamic force calibrations, at least for mechanical arrangements similar to those investigated in this work. figure 10: magnitude response for different hammer configurations: steel/plastic hammer tip; with/without mass extender; load mass 237 g. 4. summary and outlook this paper presents experimental investigations of a dynamic in-situ force calibration method recently proposed by nist. this secondary calibration method uses a dynamically calibrated impact hammer to provide the dynamic reference force. a dynamic in-situ calibration has a considerable advantage: the force transducer to be calibrated remains in its original mechanical structure for the subsequent dynamic force measurement; furthermore, the many difficulties and challenges associated with an external dynamic calibration can be avoided. the in-situ calibration measurements are preferably performed with the same measuring electronics and the same settings as are later used for dynamic force measurements, so that further corrections do not have to be made. although all experiments presented in this paper were carried out using a single force transducer and a single calibration hammer, numerous variations of the mechanical structures of the calibration set-up and the force generation allow a generalised statement on the usability of the calibration method investigated. experiments on different calibration set-ups prove that calibration results obtained with different hammer configurations agree well; it can thus be concluded that impact hammers are suited to provide a dynamic in-situ force calibration. the calibration results expressed in the frequency domain will in principle provide traceability to any dynamic signal shape within the considered bandwidth; i.e. the in-situ calibration by means of an impact hammer is not restricted to shock force applications. future work will focus on the estimation of the measurement uncertainties of the hammer calibration as well as the in-situ force calibration. in this regard, some issues already addressed above will be further investigated to understand the observed dynamic behaviour. these activities may include alterations of the impact hammer calibration set-up using other reference masses or sensors, the modification of the measuring chain and the data analysis procedures, comparisons of guided and manually operated hammer strokes, the analysis of phase responses and modelling using multi-degreeof-freedom systems. 5. acknowledgement the authors gratefully acknowledge ms yang peng for her collaboration in performing the experimental measurements presented in this paper. 6. references [1] guideline dkd-r 3-10 sheet 1, “dynamic calibration of uniaxial force measuring devices and testing machines (basic principles)”, june 2017. doi: https://doi.org/10.7795/550.20190507ben [2] iso 4965, “metallic materials dynamic force calibration for uniaxial fatigue testing”, july 2012. [3] m. kobusch, s. eichstädt, “a case study in modelbased dynamic calibration of small strain gauge force transducers”, acta imeko 6 (2017) 1, pp. 312, doi: http://dx.doi.org/10.21014/acta_imeko.v6i1.433 [4] n. vlajic, a. chijioke, “traceable calibration and demonstration of a portable dynamic force transfer standard”, metrologia, vol. 54, 2017, pp. 83-98, doi: https://doi.org/10.1088/1681-7575/aa75da [5] e. van de bunt, j. dekker, f. jaouën, “determining the dynamic behaviour of force panels for measuring wave impacts on model structures”, proc. of the 6th international conference on advanced model measurement technology for the maritime industry amt'19, rome, italy, 2019. [6] agilent, “the fundamentals of modal testing”, agilent application note 243-3, agilent technologies, usa, 2000. [7] iso 7626-5, “mechanical vibration and shock experimental determination of mechanical mobility part 5: measurements using impact excitation with an exciter which is not attached to the structure”, 2019. [8] m. kobusch, l. klaus, l. muñiz mendoza, “investigation of impact hammer calibrations”, in proc. of imeko 23rd tc3, 13th tc5 and 4th tc22 international conference, helsinki, finland, 2017. 0.4 0.5 0.6 0.7 0.8 0 500 1000 1500 2000 m a g n it u d e i n m v /n frequency in hz calibration set-up with 237 g load mass steel ti p steel ti p, mass extender plas tic tip plas tic tip, mas s extender http://www.imeko.org/ https://doi.org/10.7795/550.20190507ben http://dx.doi.org/10.21014/acta_imeko.v6i1.433 https://doi.org/10.1088/1681-7575/aa75da partial differentiation of air density in mass metrology acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 33 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 33 36 partial differentiation of air density in mass metrology m. l. win1, t. sanponpute2, b. suktat3 1 national institute of metrology (myanmar), yangon, myanmar, marlarwin99@gmail.com 2 national institute of metrology (thailand), pathumthani, thailand, tassanai@nimt.or.th 3 consultant, pathumthani, thailand, bunjobs@hotmail.com abstract: there are four major uncertainty components to be considered when performing mass comparisons. they are uncertainties of weighing process, reference weight used, air buoyancy, and mass comparator. the systematic effect of air buoyancy can be greatly reduced if the air density and the densities of the test and reference weights are known. this paper will emphasis on the uncertainty due to air buoyancy correction only. to calculate the uncertainty of air density correction, partial derivatives of temperature, barometric pressure and humidity must be performed. in this paper, two methods for partial differentiation of air density components are discussed. keywords: air buoyancy; air density; partial differentiation of air density; august-rochemagnus approximation equation 1. introduction in mass metrology, there are many important influence quantities have to be taken into account. these include thermal stabilisation time, magnetic susceptibility, density of standard and test weights, environmental conditions, weighing instrument, weighing cycles etc. according to oiml r111-1[1, equation c.6.5-1], the combined standard uncertainty for mass comparison process is calculated using equation (1). 𝑢c(𝑚ct) = √𝑢w 2 (∆𝑚c̅̅ ̅̅ ̅̅ ) + 𝑢 2(𝑚cr) + 𝑢b 2 + 𝑢ba 2 (1) where: 𝑢c(𝑚ct) = combined standard uncertainty of the conventional mass of the test weight 𝑢w(δ𝑚c̅̅ ̅̅ ̅̅ ) = uncertainty of the weighing process 𝑢(𝑚cr) = uncertainty of the reference weight 𝑢b = uncertainty of the air buoyancy correction 𝑢ba = uncertainty of the balance 2. the systematic effects of air density when highly accurate mass comparisons are performed under the laboratory conditions, buoyant force that acts upon weights, depending on their volume and the air density, can cause significant systematic error to the measurement results. the systematic effects of air buoyancy can be corrected if the air density is known. the air density is a function of various influence quantities such as temperature, barometric pressure and humidity. to correct for the effect of air density, oiml r111-1 provides the following equation [1, eq. 10.2-1]: 𝑚ct = 𝑚cr(1 + 𝐶) + ∆𝑚c̅̅ ̅̅ (2) with [1, eq. 10.2-2]: 𝐶 = (𝜌a − 𝜌0) [ 𝜌t − 𝜌r 𝜌r𝜌t ] (3) where: 𝑚ct = conventional mass of the test weight 𝑚cr = conventional mass of the reference weight ∆𝑚c̅̅ ̅̅ = average weighing difference observed between test and reference weight 𝐶 = correction factor for air buoyancy 𝜌a = density of moist air 𝜌0 = density of air as a reference value equal to 1.2 kg·m-3 𝜌t = density of the test weight 𝜌r = density of the reference weight the uncertainty of air buoyancy correction is calculated according to oiml r111-1, as follows [1, equation c.6.3-1]: 𝑢b 2 = [𝑚cr 𝜌r − 𝜌t 𝜌r𝜌t 𝑢(𝜌a)] 2 + [𝑚cr(𝜌a − 𝜌0)] 2 𝑢2(𝜌t) 𝜌t 4 + 𝑚cr 2 (𝜌a − 𝜌0)[(𝜌a − 𝜌0) − 2(𝜌al − 𝜌0)] 𝑢2(𝜌r) 𝜌r 4 (4) where: 𝑢b = uncertainty of the air buoyancy correction 𝑚cr = conventional mass of the reference weight 𝜌r = density of the reference weight http://www.imeko.org/ mailto:marlarwin99@gmail.com mailto:tassanai@nimt.or.th mailto:bunjobs@hotmail.com acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 34 𝜌t = density of the test weight 𝑢(𝜌a) = uncertainty of density of moist air 𝜌a = density of moist air 𝜌0 = density of the air as a reference value equal to 1.2 kg·m-3 𝑢(𝜌t) = uncertainty of density of the test weight 𝜌al = air density during the (previous) calibration of the reference weight 𝑢(𝜌r) = uncertainty of density of reference weight 3. uncertainty of air density for air density 𝜌a , oiml r111 provides an approximation formula [1, equation e.3-1] given as equation (5). 𝜌a = 0.34848 𝑝 − 0.009 ℎ𝑟 × exp (0.061 𝑡) 273.15 + 𝑡 (5) where: 𝜌a = air density, in kg·m -3 𝑝 = air pressure, in mbar or hpa ℎ𝑟 = relative humidity, in % 𝑡 = air temperature, in c equation (5) has a relative uncertainty of 2 × 10-4 in the ranges 900 hpa < 𝑝 < 1100 hpa, 10 °c < 𝑡 < 30 °c and ℎ𝑟 < 80 %. under reference conditions of 𝑝 = 1013.25 hpa, 𝑡 = 20 °c, and ℎ𝑟 = 50 %, equation (5) gives an air density of 1.199 294 kg·m-3. the variance of air density is obtained from equation (6) [1, equation c.6.3-3]. 𝑢2(𝜌a) = 𝑢f 2 + [ 𝜕𝜌a 𝜕𝑝 𝑢𝑝] 2 + [ 𝜕𝜌a 𝜕𝑡 𝑢𝑡 ] 2 + [ 𝜕𝜌a 𝜕ℎ𝑟 𝑢ℎ𝑟 ] 2 (6) under the same reference conditions, the following numerical values for sensitivity coefficients are given: • 𝑢f = [uncertainty of the formula used] (for cipm formula 𝑢f = 10 –4 𝜌a ) • 𝜕𝜌a 𝜕𝑝 = 10−5𝜌a pa −1 • 𝜕𝜌a 𝜕𝑡 = −3.4 × 10−3𝜌a k −1 • 𝜕𝜌a 𝜕ℎ𝑟 = −10−2𝜌a however, if we calculate the three sensitivity coefficients by partial differentiation of equation (5), we obtain: 𝜕𝜌a 𝜕𝑝 = 0.34848 273.15 + 𝑡 𝜌a (7) = 0.34848 293.15 𝜌a = 10 −5 𝜌a pa −1 𝜕𝜌a 𝜕ℎ𝑟 = −0.009 × exp (0.061𝑡) 273.15 + 𝑡 𝜌a (8) = −0.009 × exp (0.061 × 20) 273.15 + 20 𝜌a = −10 −2 𝜌a 𝜕𝜌a 𝜕𝑡 = − 0.34848 𝑝 (273.15 + 𝑡)2 − (9) 0.009ℎ𝑟 [(273.15 + 𝑡) 𝑑 𝑒 0.061𝑡 𝑑𝑡 − 𝑒0.061𝑡 𝑑(273.15+𝑡) 𝑑𝑡 ] (273.15 + 𝑡)2 𝜌a = − 0.34848 𝑝 (273.15 + 𝑡)2 − 0.009(50)𝑒 0.061𝑡[293.15 × 0.061 − 1] (273.15 + 𝑡)2 𝜌a = − 0.34848 × 1013.15 (273.15 + 20)2 − 0.009(50)𝑒 0.061×20[293.15 × 0.061 − 1] (273.15 + 20)2 𝜌a = −4.4 × 10−3𝜌a k −1 from the above calculations, we found that sensitivity coefficient 𝜕𝜌a 𝜕𝑝 and 𝜕𝜌a 𝜕ℎ𝑟 values are exactly the same as oiml r111 values, but not the value of 𝜕𝜌a 𝜕𝑡 . therefore, we try to consider how to obtain the same 𝜕𝜌a 𝜕𝑡 value as provided by oiml r111. according to the august-roche-magnus approximation equation [4, equation 4], 𝑅𝐻 = 100 × 𝑒 17.625 𝑇𝐷 243.04 + 𝑇𝐷 𝑒 17.625 𝑡 243.04 + 𝑡 (10) the changes in temperature 𝑡 will also cause the relative humidity 𝑅𝐻 to change, if the dew point temperature 𝑇𝐷 is kept constant. in this case, we need to consider not only the changes in temperature but also the correlation between temperature and relative humidity according to august-rochemagnus approximation equation. in this equation, there are three variables, namely temperature 𝑡 , relative humidity 𝑅𝐻 and dew point temperature 𝑇𝐷. the dew point temperature can be calculated by using august-roche-magnus approximation equation [4, equation 5] as follows: 𝑇𝐷 = 243.04 ln ( 𝑅𝐻 100 ) + ( 17.625 𝑡 243.04+𝑡 ) (17.625 − ln ( 𝑅𝐻 100 ) − 17.625 𝑡 (243.04+𝑡) ) (11) http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 35 substituting in values of 𝑅𝐻 = 50 %, 𝑡 = 20 °c and 𝑝 = 101 325 pa, we get: 𝑇𝐷 = 9.261 ℃ (12) if we keep 𝑇𝐷 constant at 9.261 °c while the temperature changes from; 𝑡 = 20 °c to 𝑡 = 21 °c, from equation (11), we get: for 𝑡 = 20 c: 𝑅𝐻 = 100 × 𝑒 17.625×9.261 243.04+9.261 𝑒 17.625×20 243.04+20 = 50 % (13) for 𝑡 = 21 c: 𝑅𝐻 = 100 × 𝑒 17.625×9.261 243.04+9.261 𝑒 17.625×21 243.04+21 = 47 % (14) from the above examples, the change of temperature from 20 c to 21 c can cause the relative humidity to change from 50 %rh to 47 %rh. thus the air density also changes to: 𝜌a1 = 0.34848 × 1013.25 − 0.009(47)e0.061×21 273.15 + 21 (15) = 1.195 221 kg ∙ m−3 and thus, from equations (5) and (15): 𝜕𝜌𝑎 𝜕𝑡 = 𝜌𝑎1 − 𝜌0 𝜌0 = 1.195221 − 1.199294 1.199294 (16) = −3.4 × 10−3𝜌𝑎 k −1 this value of 𝜕𝜌𝑎 𝜕𝑡 is now exactly the same as the value provided in oiml r111. therefore, we need to consider the correlation between temperature and humidity when we perform partial differentiation of temperature against the air density. note: in the august-roche-magnus approximation equation, there are three variables, namely temperature 𝑡 , relative humidity 𝑅𝐻 and dew point temperature 𝑇𝐷. for our purposes, dew point temperature 𝑇𝐷 is considered constant while the temperature 𝑡 is changing. in this case any changes in temperature will also cause the relative humidity to change in the opposite direction. otherwise, we would not get the same value of 𝜕𝜌𝑎 𝜕𝑡 as that provided in oiml r111. 4. discussion relative humidity is the ratio, usually expressed in %, of the partial pressure of water vapour to the saturation (equilibrium) vapour pressure of water at a given temperature. relative humidity depends on the temperature and pressure of the system of interest. as the air’s temperature increases, it can hold more water molecules, decreasing its relative humidity. when temperatures drop, relative humidity increases. this is because colder air does not require as much moisture to become saturated as warmer air. 100 % relative humidity of the air occurs when the air temperature is the same as the dew point value. table 1 shows the relationship between temperature and relative humidity. the slope and correlation between these two variables are shown in figure 1. in this figure, it shows the correlation coefficient close to -1.0. correlation coefficient is used to measure the strength of the relationship between two variables. the range of values for correlation coefficient is -1.0 to 1.0. in other words, the values cannot exceed 1.0 nor be less than -1.0. table 1: relationship between temperature and relative humidity temperature / °c relative humidity / %rh 16 64.3 17 60.3 18 56.6 19 53.2 20 50.0 21 47.0 22 44.2 23 41.6 24 39.2 figure 1: correlation between temperature and relative humidity 5. summary in mass comparison uncertainty evaluation, we need to consider all contributions that are of significant. among them, one of the important uncertainty sources is air buoyancy correction. for air buoyancy correction, air density is the important parameter. air density depends on the environmental conditions such as temperature, barometric pressure and humidity. the uncertainty of air density 𝑢(𝜌a) is calculated from the standard uncertainty of air pressure 𝑢(𝑝), temperature 𝑢(𝑡), and relative humidity 𝑢(𝑟ℎ) , together with their y = -3,128x + 113,28 r² = 0,9938 30 40 50 60 70 80 15 16 17 18 19 20 21 22 23 24 25 rh / % temperature / °c http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 36 sensitivity coefficients, the partial derivatives of 𝜕𝜌a 𝜕𝑝 , 𝜕𝜌a 𝜕𝑡 , and 𝜕𝜌a 𝜕ℎ𝑟 . however, when we use the ordinary partial differentiation, the derived value of 𝜕𝜌a 𝜕𝑡 is not same as the value given in oiml r111. so, we need to consider the correlation between temperature and humidity when we perform partial differentiation of temperature against the air density. we use august-roche-magnus approximation equation to determine the correlation between temperature and relative humidity. finally, we can obtain the same value for 𝜕𝜌a 𝜕𝑡 as given by oiml r111. 6. acknowledgement the authors would like to thank ptb (physikalisch-technische bundesanstalt) and its international cooperation project “strengthening the quality infrastructure in myanmar”, which supports and enables us to make this publication possible. 7. references [1] oiml r111 – weights of classes e1, e2, f1, f2, m1, m2, m3 metrological and technical requirement, 2004. [2] a. picard, r. s. davis, m. gläser, k. fujii, “revised formula for the density of moist air (cipm2007)”, metrologia, vol. 45, 2008, pp. 149155. doi: http://dx.doi.org/10.1088/0026-1394/45/2/004 [3] guide to the expression of uncertainty in measurement, iso, 1995. [4] thomas k. thiis, ingunn burud, andreas flø, dimitrios kraniotis, stergiani charisi, petter stefansson, monitoring and simulation of diurnal surface conditions of a wooden façade, procedia environmental sciences 38, 217, pp. 331-339. doi: https://doi.org/10.1016/j.proenv.2017.03.088 http://www.imeko.org/ http://dx.doi.org/10.1088/0026-1394/45/2/004 https://doi.org/10.1016/j.proenv.2017.03.088 development of special measuring tool for trace of micro force value acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 270 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 270 273 development of special measuring tool for trace of micro force value li yang1, shi wei2, tao jizeng3 1 changcheng insitiute of metrology & measurement, beijing, china, ly304cimm@126.com 2 changcheng insitiute of metrology & measurement, beijing, china, swlltm@126.com 3 changcheng insitiute of metrology & measurement, beijing, china, tjz＿75@163.com abstract: this paper introduces the development of the special traceability measuring tool for micro force value with the test force range of 0.009807 n ~ 9.807 n(1 gf~1 kgf) and the accuracy of better than ± 0.1%, including the selection of the electromagnetic balance module, the design of the hardware structure, and the compilation of the software program. finally, the special traceability measuring tool integrating the test measurement, automatic data collection and automatic data processing is realized, and the test verification result is given 。 it provides a method to solve the problem of trace the small force value of existing micro vickers hardness tester and knoop hardness tester. keywords: micro force, measuring tool, traceability, micro vickers hardness, knoop hardness 1. introduction with the development of modern material surface engineering, such as physical vapor deposition, magnetron sputtering, ion implantation, surface modification, thermal spraying, nitriding and carburizing, etc., the thickness of micro vickers hardness and knoop hardness test samples or the surface modified layer on the sample surface is getting thinner and thinner, and the hardness test force value is getting lower and lower, reaching 0.009807 n. however, the minimum measuring range of the existing force measuring instrument is 1 n. according to the principle of one tenth use, the minimum measurement can be 0.1 n, which can not meet the existing requirements. therefore, it is very necessary and urgent to develop the special traceability measuring tool with the lower limit of 0.009807 n. 2. project design in this paper, the special traceability measuring tool for micro force value is developed, which has the force value range of 0.009807 n ~ 9.807 n(1 gf~1 kgf) and the accuracy of better than ± 0.1%. it selects the electromagnetic balance module as the core, and carries on the package design to it, has realized the test measurement, the data automatic collection, the data automatic processing as a whole special traceability measurement tool, specifically includes, the electromagnetic balance module selection design, the hardware structure design, and the software programming three parts. the output value of the balance is the mass. the relationship between the force value and the mass is shown in formula (1). 𝐹 = 𝑚𝑔 (1 − 𝜌1 𝜌2 ) (1) in the formula: f force value , n; m mass , kg; g the local acceleration of gravity in the laboratory , m/s2; 𝜌1 -air density , kg/m 3; 𝜌2 -weight density , kg/m 3. since the density of the weight is far greater than that of the air, 𝜌1 𝜌2 is negligible. 2.1 selection and design of electromagnetic balance module according to the research, referring to the test requirements, working space of most hardness tester and other factors, by selecting the range, accuracy, geometric dimension and other indicators of electromagnetic balance module, the two electromagnetic balance modules of sartorius are finally selected, their models are wza224-n and wza1203-n, as shown in figure 1, and their technical indicators are shown in table 1. a) wza224-n b) wza1203-n figure 1 electromagnetic balance modules http://www.imeko.org/ mailto:ly304cimm@126.com mailto:swlltm@126.com mailto:tjz75@163.com http://www.sartorius.com.cn/typo3temp/pics/wo0163d1-wh_4e082b197d.jpg acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 271 table 1 technical indicators of lectromagnetic balance modules model wza224-n wza1203-n weighing capacity(g) 220 1200 readability(mg) 0.1 1 maximum deviation(mg) ±1 ±5 standard deviation determined(mg) 0.1 1 maximum linearity(mg) ±0.2 ±2 physical dimension(mm) 181ⅹ67ⅹ73 200ⅹ67ⅹ78 although the electromagnetic balance module with the smallest geometric size is selected on the premise of meeting the requirements of technical specifications, its length and width meet the requirements of micro vickers hardness tester in our laboratory. because the size of the micro vickers hardness tester in our laboratory is 200 mm ⅹ 280 mm. but its height is still higher than the working space height of most hardness machines, including those in our laboratory. therefore, it is necessary to reform the selected electromagnetic balance module, remove its own measuring head and replace it with a thinner force measuring base. because the selected balance module is a mature commodity, it has a certain stiffness, and because the indenters used in micro vickers hardness test and knoop hardness test are diamond indenters, it is necessary to set a ceramic force measuring unit in the center of the force measuring seat to ensure its stiffness in use and reduce the impact on the test results. the designed balance module has good stiffness, which can reduce the influence of the deformation of the worktable on it in the use process. the accuracy of the design and operation is ± 0.01%, and the influence of the table deformation can be ignoredthe design drawing of electromagnetic balance module transformation is shown in figure 2. figure 2 design drawing of electromagnetic balance module transformation 2.2 hardware structure design the special measuring tool for trace of micro force value is often carried out, so it needs to be easy to carry, simple to make mistakes, and anti-collision and other functions. therefore, hardware structure design should be carried out on the basis of electromagnetic force balance module. the hardware structure design diagram is shown in figure 3. figure 3 hardware structure design drawing after completion, the special measuring tools for trace the micro force value include the box with pull rod, industrial computer, mouse, power supply, two modified electromagnetic balance modules and their corresponding control board, level meter, leveling nut, etc. the box with pull rod is made of aluminum alloy with light weight, high strength and easy to carry and move. the box is filled with thick sponge to protect internal parts. the industrial computer is used for operation test, collection and processing of test data. the level meter and leveling nut are used to ensure the level of the device during the test. the wza224-n module and wza1203-n module are used for test force the control module is used to control the electromagnetic balance module, and the trunking and mouse are used to link and operate. 2.3 programming and design of software in order to reduce the error of manual calculation, the special traceability measurement tool for micro force value is designed and compiled with supporting automatic processing software, which adopts automatic reading, automatic calculation and other functions. figure 4 automatic processing software interface http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 272 as shown in figure 4, the automatic processing software is divided into four areas, namely: a）serial port setting area the serial port can be set in this area for communication with the electromagnetic balance module. it is written in c# language, using standard serial port communication protocol and sbi communication protocol based on te series electronic balance module. the communication steps are as follows: create serial port: serialport sp1 = new serialport(); open the serial port: sp1. open(); read serial data: sp1. readline(); close the serial port: sp1. close(); as shown in figure 5, analyze the data according to the sbi protocol of the electromagnetic balance module, take the first to the 14th bit of data, remove the useless space, and display the data on the interface. figure 5 sbi communication protocol of electromagnetic balance module b）data configuration area in this area, we can select the reference standards, model and measurement standard value, and have the function of clearing before the test. reference standards include iso 6507 metallic materials – vickers hardness test, astm e 92 standard test methods for vickers hardness and knoop hardness of metallic materials, astm e 384 standard test method for microindentation hardness of materials, and jjg 148 verification regulation of vickers hardness reference blocks, and jjg 151 verification regulation of metallic vickers hardness testers. refer to the protocol of electromagnetic balance module, as shown in figure 6, set the equipment reset function, and the command format is as follows: sp1.writeline("esc\rv\r\n"); figure 6 protocol of electromagnetic balance module c) data collection area this area is used to display the collected and processed data. d) operation button area buttons in this area are used for software operation, including value taking, backward, clear, save and exit. after data acquisition, when the "value taking" button is pressed, the software will intercept the current test value and save it to the data acquisition area. the backward button deletes the last data in the acquisition area. the clear button clears all data in the data collection area. the save button saves the data in the data collection area to the record report. the exit button will push out of the system physical drawing of development of the special traceability measuring tool for micro force value, as shown in figure 7. figure 7 physical drawing of development of the special traceability measuring tool for micro force value 2.4 test verification in order to verify whether the special measuring tool for tracing of micro force value meets the required technical indicators, the test is carried out. see table 2 for the standard measuring instruments, and see table 3 for test results. table 2 standard measuring instruments name model accuracy level number standard weights 1mg～1g e2 1 sets standard weights 1g～6kg e2 1 sets it can be seen from table 3 that the accuracy and repeatability of the development of the special traceability measuring tool for micro force value are 0.005% and 0.003%, meeting the design and use requirements. http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 273 table 3 test results standard value (g) test value (g) error (%) repeatability (%) 1.0000 1.0000 0.000% 0.001% 2.0000 2.0001 0.005% 0.000% 5.0000 5.0002 0.004% 0.001% 10.0000 10.0000 0.000% 0.001% 20.0000 20.0002 0.001% 0.002% 50.0000 50.0000 0.000% 0.000% 100.0000 100.0009 0.001% 0.001% 200.0000 200.0012 0.001% 0.000% 300.000 300.001 0.000% 0.002% 500.000 500.000 0.000% 0.000% 1000.000 1000.001 0.000% 0.003% 3. summary this paper introduces the development of a special measuring tool for micro force traceability, which has a measuring force range of 0.009807 n ~ 9.807 n (1 gf ~ 1 kgf), and the accuracy is better than ± 0.1%. it includes the selection of electromagnetic balance module, the design of hardware structure, the determination of measurement accuracy, and the compilation of software program. finally, a special traceability measuring tool which integrates measurement, automatic data acquisition and data processing is realized, and the experimental verification results are given. at the same time, it has been well applied in our laboratory. the special inspection tool for trace of micro force provides a new method to solve the problem of micro force traceability of micro vickers hardness tester and knoop hardness tester. the next step is to develop a small force traceability tool with smaller geometry for most micro vickers and knoop hardness machines. 4. references [1] iso/tc 164, iso 6507: metallic materials-vickers hardness test, 2018. [2] astm e92 standard test methods for vickers hardness and knoop hardness of metallic materials, 2017. [3] astm e384 standard test method for microindentation hardness of materials, 2017 [4] jjg 148 verification regulation of vickers hardness reference blocks, 2006. [5] jjg 151 verification regulation of metallic vickers hardness testers, 2006. [6] liu maochun. li qi, c # program development case class. new: tsinghua university press, ch14, pp. 299-362, 2018. http://www.imeko.org/ preliminary study of an ancient earthquake-proof construction technique monitoring by an innovative structural health monitoring system acta imeko issn: 2221-870x march 2021, volume 10, number 1, 47 56 acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 47 preliminary study of an ancient earthquake-proof construction technique monitoring via an innovative structural health monitoring system carmelo scuro1, domenico l. carnì2, francesco lamonaca2, renato s. olivito3, gabriele milani4 1 department of physics, university of calabria, 87036 rende (cs), italy 2 dimes department of informatics modelling electronics and systems science, university of calabria, 87036 rende (cs), italy 3 dinci – department of civil engeneering, university of calabria, 87036 rende (cs), italy 4 department of architecture, built environment and construction engineering, technical university of milan, 20133 milan, italy section: research paper keywords: anti-seismic technique; experimental test; numerical analysis; structural health monitoring systems; measurement accuracy citation: carmelo scuro, domenico luca carnì, francesco lamonaca, renato sante olivito, gabriele milani, preliminary study of an ancient earthquake-proof construction technique monitoring by an innovative structural health monitoring system, acta imeko, vol. 10, no. 1, article 8, march 2021, identifier: imekoacta-10 (2021)-01-08 editor: eulalia balestrieri, university of sannio, italy received april 22, 2020; in final form june 16, 2020; published march 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: carmelo scuro, e-mail: carmelo.scuro@unical.it 1. introduction in the safeguarding and prevention of the possible collapse of structures of historical interest, the collection of information relating to the quality of the construction materials, the environmental actions, the propensity for an alteration of a single component or of the structure as a whole due to a loss of material strength over time or accident-related events is of fundamental importance [1]-[3]. structural health monitoring (shm) systems guarantee the characterisation, identification, and detection of the evolution of damage for different types of structures [4]. in the field of structural monitoring, shm systems consist of two main parts: a data logger that allows for the transmission and recording of information, and the acquisition system, which is used for evaluating the different physical quantities [5]. specifically, the main information provided by the shm system includes temperature, displacement, strain, acceleration, tensile stress, and compressive strength data [6]. generally, shm systems are placed in structures in a non-invasive manner, using a reversible process incorporating sensors. here, the sensors are placed in highly precise control points of the structure in order to minimise unnecessary costs and information acquisition [1], [7]. following a check, the useful information acquired by the sensors are used in mathematical numerical models of the structures to determine the evolution of any possible damage and safety issues [8]. in recent years, structural monitoring systems have been improved by applying the internet of things (iot) paradigm. this is because in this paradigm, each node is capable of processing, detecting, and transmitting data. the information is transmitted through an internet connection and stored in a cloud before being processed by distributed systems functionalised via a big data paradigm [9]. safeguarding new discoveries related to construction techniques is the ultimate aim of researchers, both in terms of structural engineering and the field of historic heritage, where the abstract the historical and cultural heritage analysis of the italian territory is of primary importance since this region is one of the richest in the world and can enrich our knowledge in different fields. in fact, in the field of structural engineering, a new discovery was made in calabria, in the south of italy. by investigating various architectural treatises related to earthquake-proof constructions, new knowledge was gained through analysing buildings constructed with fictile tubules bricks. among them is an unprecedented anti-seismic construction technique patented by pasquale frezza, which has been widely used in southern calabria. to prevent the collapse of the attendant structures, an innovative method for monitoring and obtaining the mechanical properties of these structures in real time while minimising the measurement uncertainty is proposed in this paper. mailto:carmelo.scuro@unical.it acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 48 structures must be preserved from damage and collapse for future generations. the monitoring of historical buildings in seismic areas is a prevalent issue in italy due to the richness of the nation’s historical cultural heritage, which cannot be investigated through invasive tests. typically, the general shm of churches or palaces is evaluated via dynamic analyses. this type of monitoring allows for the mechanical parameters functionalised for future restoration activities to be obtained, and for the interaction between the static action and the control of possible changes in behaviour to be evaluated. provided the construction techniques are known and the types of masonry are well-defined, it is possible to define an interaction model of the structure. however, in the case of an unknown structure or a new structure with no available information, this is not possible [10][12]. most of the historic anti-seismic constructions of calabria were built using rubble stone and brick masonry [13] and were frequently composed of various layers with little or no connection between them and built using various raw materials (bricks, stone or fictile tubules [14], [15]) and largely poor mortars. this paper presents the results of the total and partial destructive tests carried out on two replicas of historic masonry specimens constructed in accordance with the patent of the engineer, pasquale frezza. a preliminary study was carried out in relation to the use of an shm system placed on the ancient antiseismic structures that appeared from the beginning of the 1900s [16]. specifically, the paper is focused on earthquake-proof wooden frame structures made with common brick and fictile tubules [17]. in the shm system design, particular importance was afforded to the definition of the mathematical model of the structure for the calculation of the structural solicitations in accordance with the italian technical standards. the possibility of having a well-defined numerical model allows for real-time analysis based on the displacement and stress data recorded during the monitoring. this has been carried out by various authors, not only in terms of new structures such as bridges [18] but also in terms of the study and construction of large-scale monitoring systems for urban centres [19]-[21]. the remainder of the paper is organised as follows. sections 1 and 2 outline the theory of structural mechanics used in order to calculate the shear strength in the specimens before an experimental test is introduced and the results explained. in section 3, the numerical model obtained via finite element abaqus software is proposed before the experimental testbed is described and the measurement accuracy determined in sections 4 and 5 and the conclusions drawn in section 6. 2. criteria for the shear strength prediction of piers various simplified models are proposed in the relevant literature in relation to describing the damage that may occur in masonry piers. these models are based on the (inaccurate) evaluation of the local/mean stress generated by forces applied on pre-identified points/sections of the pier [22]. the most frequent failure modes that occur in piers are rocking/crushing, bed joint sliding, and diagonal cracking. for the diagonal cracking failure mode, two models are generally adopted to describe it [23], [24]. these models are used to describe different types of masonry and often provide different strength values, which is the case with the mann and müller model and the turnšek and čačovič model. in the former model, the limit strength domain is defined by means of ‘local’type parameters relating to the individual materials that comprise the masonry (mortar and bricks), such as the cohesion and friction coefficient in the mortar joints and the tensile strength of the block of masonry under analysis [23]. in general, these two types of parameter are obtained experimentally, and the individuation of the cohesion and friction coefficients can prove difficult for specific masonry, such as that used in fictile tubules. meanwhile, in the turnšek and čačovič model, the domain is defined through a single parameter of the material, i.e. the tensile strength of the masonry, which is generally determined via a diagonal compression test [24]. the model proposed by mann and müller describes the masonry as a composite material and considers the development of the cracks, separately, along the brick and mortar joints, while that proposed by turnšek and čačovič considers the masonry as an equivalent isotropic material and indistinctly describes the development of damage along the principal stress directions [25]. in studying the area of diagonal cracking, the above two models are generally recognised as the two main types. however, the application of the mann and müller model to a masonry wall made with bricks and fictile tubules can prove difficult. the model is based on two main hypotheses: (i) the mechanical properties of the head joints are negligible, and in the case in question, the bricks have a square section; (ii) bricks are much stiffer than mortar joints, but the tubules are hollow and have a cylindrical conformation, and this hypothesis cannot be satisfied. for these reasons, it was decided to use turnšek and čačovič’s model, which, as noted, considers the masonry as an isotropic material. here, the reference stress σc and the maximum principal stress acting at the centre of the pier σi must not exceed the tensile strength of the masonry ft. the latter parameter is assumed to be constant in any loading direction (isotropic limit stress domain) [26]. this is possible since, in previous works, the authors carried out a homogenisation of the materials from which the wall was made [27]. here, the maximum principal stress at the centre of the pier was calculated using the following equation: ( )        = + +     2 2 1 2 2 y y i dk (1) where 𝜏̅ and 𝜎𝑦̅̅ ̅ are respectively the mean shear and the normal stresses acting on the cross section of the pier and k1d is the ratio of the shear stress at the centre of the pier to the mean shear figure 1. the failure criteria of turnšek and čačovič’s model for masonry piers, represented in the resolved stress plane [21]. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 49 stress [24]. the failure criteria for masonry piers, which are represented in the resolved stress plane, are shown in figure 1. in the new italian technical standards (document c8.7.1.3.1.1 7/2019) turnšek and čačovič’s formulation is simplified in order to obtain the shear strength value of the masonry pier. figure 2 shows the adopted schematisation of the masonry block. the normal and tangential stress was calculated at point a using the following equations:  =  0 n l t (2)  =  0 t l t (3) where t and n are respectively the shear and normal force applied to the masonry block and l and k are the length and thickness, respectively, of the masonry. the stress state at point a is represented by mohr's circle in figure 3, as characterised by centre c and radius r, which are expressed by (4) and (5):  = 0 2 c (4)     = +    2 20 2 r (5) the principal tensile stress that occurs in the masonry could be obtained using the following equation:       = − = − +    2 20 0 2 2 t c r (6) squaring (6) and simplifying it, we obtain:    = −  2 2 0t t (7) if σt (tensile stress in the masonry) is equal to ft, the tensile strength of the masonry is imposed in terms of a limit domain of turnšek and čačovič’s formulation and τ = b∙ τ0, (7) becomes:   = + 00 1 tf b t (8) multiplying the area of the section, the shear strength of the masonry pier could be obtained:          = + = +  0 0 0 0 1.5 1 1 1.5 t d s d f l t l t v b t b (9) where vs is the shear strength of the masonry pier obtained for exceeding the limit domain, ft is the tensile strength of the masonry, b is a coefficient that takes into account the slenderness of the element and is the ratio between the height and length of the pier, with its value in the range of 1–1.5, and τ0d is the reference shear strength. 3. experimental campaign the experimental campaign was conducted in the laboratories of the university of calabria. using the same construction process, two masonry walls were fabricated following the patent of the engineer, pasquale frezza [28]. the first was tested using a diagonal compression test and was stopped after the specimen had collapsed. the second specimen was tested in the same way but was first damaged and then repaired using a basalt fibre reinforced cementitious matrix (b-frcm). following this, the test was repeated until the collapse of the reinforced masonry wall. 3.1. preparation of the specimens the dimensions of the two specimen walls were 60 × 60 × 15 cm3. the external timber frame was constructed in a carpentry workshop and consisted of four poplar beams with a cross section of 15 × 5 cm2 (poplar wood is a typical building material used for constructions in calabria). the connection between any contiguous beams was ensured by cutting their extremities to produce half-lap fastenings before attaching them using four iron bolts. after the outer timber frame was built, the inner masonry wall was created. here, the bricks used had a square cross section of 5.5 cm per side and a height of 13 cm. meanwhile, the fictile tubules were new nomenclating fictile tubules (nfts) [27], since these are produced in a pottery industry using faster, more innovative techniques before being refined on the potter’s wheel. the tubules had the same height as the bricks (13 cm) and a thickness of 6 mm (figure 4, figure 5). the mortar had a compressive strength of around 2.5 mpa, corresponding to that labelled as m2.5 bastarda according to table 11.10.iv of the 2008 italian building code. this specific mortar consists of one part cement, two parts lime mortar, and nine parts sand. the specimen wall was completed by casting a 2-cm-thick layer of mortar to cover the tips of the fictile tubules (figure 6). figure 2. adopted schematisation of the masonry block. figure 3. tensile state at point a. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 50 3.2. diagonal compressive test on the first specimen the diagonal compressive test was carried out by rotating the specimen wall by 45 ° before two steel caps were placed on the specimen, one at the bottom for support and the other at the top for uniformly distributing the load. the speed of the test was 0.5 mm/s. two pairs of transducers were applied to the masonry specimen to further monitor the relative displacements. here, the transducers labelled lvdt0 measured the vertical displacements between two bricks, while those labelled lvdt1 measured the horizontal displacements between two fictile tubules (figure 7). another transducer with the same characteristics as those used for calculating the displacements of the masonry was positioned near the upper part of the load cell in order to measure the lowering of the cross member of the test machine. the load displacement diagram is presented in figure 8. here, following a linear elastic branch up to 80 % of the maximum load, the elastic modulus decreased until reaching a plateau, where the peak load was attained and was found to be 51.48 kn. at this load value, the displacement of the load cell, as recorded by the third transducer, was 11.526 mm. the hardening branch was most likely due to the friction between the mortar and the fictile tubules elements, as well as to the overall compressive stress state of the specimen wall. the shear strength of the wall ss was evaluated using (10), as was suggested in [29]:  = 0.707 s n p s a (10) where p is the peak load and an is the net area, which was evaluated using (11): +  =    2 n w h a t n (11) where w and h are the width and the height of the specimen, respectively (here considered as the dimensions of the masonry figure 4. arrangement of bricks and nfts in the timber frame. figure 5. mortar casting. figure 6. creation of the upper layer of mortar. figure 7. set up of the test. figure 8. load displacement diagram. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 51 inner core, both = 50 cm), t is the thickness of the specimen (15 cm), and n is a coefficient related to the rate of voids in the specimen (here considered to be 0.6, where 1 corresponds to a wall with no voids). the shear strength corresponding to the peak load was 0.81 mpa. figure 9 shows the load vs. displacement diagrams recorded via transducers lvdt0 and lvdt1 during the test. 3.3. diagonal compressive test on the second specimen the same test set up was used to analyse specimen number 2. in this case, the test was stopped at the formation of the first damage and when the diagram arrived at the end of a linear elastic branch. the recorded peak load was 46.77 kn (figure 10). the shear strength of the wall ss was evaluated using eq. 9, with the value corresponding to the peak loads = 0.73 mpa. when damage began to develop in the specimen, the test was stopped, and the specimen was repaired and reinforced. three b-frcm strips of 100 mm were placed at the side of the wall featuring a 2-cm layer of mortar (figure 11). the b-frcm strips were chosen since compatibility with the support was ensured and since they are characterised by a tensile strength and a young’s modulus of 134 mpa and 19 gpa, respectively, while those for basalt fibres are generally 301.5 mpa and 16 gpa [11]. in order to measure the strains on the strips, two strain gauges were applied to the central strip in relation to the fibres perpendicular to the cracks. following this, the test was repeated on the reinforced specimen until it collapsed. the maximum value of the load recorded during the reinforcement test was 66.48 kn (figure 12). at this load value, a failure of the b-frcm reinforcement occurred [30], which was due to the debonding following the cohesive failure of the substrate (figure 13). to obtain the value of the strain that occurred in the reinforced basalt fibre during the test, two strain gauges were applied to the central part of the diagonal b-frcm. a strain gauge is a device commonly used to obtain the strain on a specimen subjected to different types of stress, such as compression or tensile stress. the strain gauges used here were the most common type and consisted of an insulating flexible backing that exhibited a figure 9. load vs. displacement diagrams of the transducers for specimen 1. figure 10. load vs. displacement diagrams of transducers for unreinforced specimen 2. figure 11. specimen 2 before and after reinforcement. figure 12. load vs. displacement diagrams of the transducers for reinforcement of specimen 2. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 52 metallic foil pattern. the gauge was positioned on the b-frcm strip using a cyanoacrylate adhesive. when basalt fibres are deformed, the foil is deformed, causing the electrical resistance to change, with this change, generally measured using a wheatstone bridge, related to the strain imposed by the quantity known as the gauge factor. figure 14 shows the shear stress vs. strain diagram obtained during the test conducted on the reinforced specimen. here, the shear stress was evaluated using eq. 9 and was found to be 1.04 mpa. the average value of the strain recorded during the test by the two strain gauges was 3.25 [μm/m] e-3. this value was in line with that prescribed by the manufacturer of the fibre, that is, a range of 3–3.5 [μm/m] e-3. 4. numerical model the principal goals in the creation of the numerical model were repeating the diagonal compressive test and simulating the behaviour of the structures in question. here, the commercial computer-aided design software, rhinoceros, was used to create a simplified geometrical model of the specimen. the outer timber frame, the common bricks, and the fictile tubules were modelled separately in terms of a solid composed of a surface characterised by a normal vector facing out [31]. following this, the single part was assembled and imported into the commercial finite element software, abaqus, where the analysis was conducted. the common bricks and the timber frame were meshed using hexahedral c3d8 finite elements (classified as c3d8 in abaqus), while tetrahedral elements (c3d4) were used for the fictile tubules and the inner mortar. the final model created was composed of a total of 47,093 nodes and 165,710 elements (1,120 c3d8 and 164,590 c3d4) [14]. to calculate the behaviour of the relevant bonds, a tie constraint was imposed between the mortar and the fictile tubules and bricks, while a general contact was imposed to simulate the interaction between the timber frame and the mortar, returning a friction coefficient of 0.8 [32]. the materials of the common bricks and the timber frame were modelled in terms of linear elastic materials. specifically, the orthotropy of the wood was not considered but was imposed as an isotropic material to simplify the final model and to reduce the computational burden [33]. this was possible because, during the experimental test, the timber frame experienced damage after the mortar did and did not undergo collapse. in fact, the fictile tubules and the mortar were simulated using the concrete damaged plasticity (cdp) material model available in the abaqus library. originally, the cdp model was created to simulate the behaviour of concrete under fairly low confining pressures and, given the damage that develops here, the model’s general formulation means it can be used in the simulation of a broader range of materials [34]-[36]. table 1 lists the young’s modulus values and the poisson’s ratios for the four materials considered. meanwhile, table 2 and table 3 present the tensile constitutive law values for fictile tubules and mortar, respectively, as expressed in terms of displacements and stresses. the ultimate displacement of the materials was imposed with a high value in order to successfully complete the analysis without affecting the structural response of the materials. a damage parameter of 0.99 was set for both the mortar and the clay of the tubules in proximity to a plastic displacement of 0.03 mm. the comparison between the numerical and experimental load displacement diagrams is shown in figure 15. the numerical analysis satisfactorily estimated the linear elastic branch of the graph, but the peak load attained was 8 % higher than the experimental load (55.54 vs. 51.48 kn). in addition, the numerical analysis was halted for an ultimate displacement that figure 13. collapse of the reinforcement strip. figure 14. shear stress vs. strain diagram. table 1. young’s modulus and poisson’s ratio values. material young’s modulus in mpa poisson’s ratio wood 270 0.00 clay (bricks) 1290 0.15 clay (tubules) 4800 0.15 mortar 275 0.20 table 2. tensile constitutive law values for mortar. tensile stress in mpa plastic displacement in mm 0.4200 0.00 0.0005 0.03 0.0005 6.50 table 3. tensile constitutive law values for tubule clay. tensile stress in mpa plastic displacement in mm 2.7500 0.000 0.0005 0.058 0.0005 10.75 acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 53 was close to that related to the load peak recorded during the experimental test. in fact, the displacement obtained by the numerical model was 11.700 mm, while the experimental displacement was 11.526 mm. the damage in the numerical model corresponded to the experimental outcome, while more cracks were present at the top, as were four single cracks originating at the mid-section of each timber beam. given the overall fair correspondence in terms of crack patterns, it can be assumed that these four single cracks appeared only for numerical reasons, meaning they played no significant role in the overall response of the numerical model (figure 16). 5. experimental testbed and measurement accuracy two different acquisition systems (ass) were used to monitor the displacements in the case of the first specimen, and the displacement and the strain of the basalt fibre in the case of the second specimen. the first as was composed of the following: three linear variable differential transformers (lvdts) [37] used for monitoring the horizontal and vertical displacement and connected to a data acquisition system (daq, spider-8), which transmitted the data to a computer [38]. meanwhile, the second as was composed of two lvdts and two strain gauges, which were connected to a daq 5100b micro-measurement system. the second daq allowed for the real-time acquisition of two different types of data, which were transmitted to a computer such that they could be processed in real time, while the lvdts used to monitor the displacement in the masonry wall were the wa-t 50 mm type and were procured by hbm. the wa-t displacement transducers were of a probe type and incorporated figure 15. shear stress vs. strain diagram comparison. figure 16. comparison of crack development in the back and front side of the numerical model and the experimental specimen wall. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 54 an active quarter bridge circuit based on the differential inductor principle. the bridge was also directly integrated in the sensor to create a full bridge circuit for easy connection of the wa-t transducer to the daq. the linearity variation in relation to the nominal value was ±1 %, while the range of displacement that can be measured using this type of lvdts is 0–50 mm [39]. meanwhile, the spider 8 daq was operated by an individual operator, who ensured a simultaneous measurement acquisition, a high sampling rate at a 16-bit resolution, and selectable digital filters, while the linearity variation in relation to the nominal value was ±0.05 %. system 5000 model 5100b scanners acquire test data within 1 ms from up to 1,200 channels at scan intervals as short as 0.02 s. this ensures both obtaining more accurate test results and having the ability to capture data under static loading conditions immediately before the failure of the structure being monitored. the strain gauge cards integrated included a built-in bridge completion for quarter and half bridges and a constant voltage power supply for 0.0, 0.5, 1.0, 2.0, 5.0, and 10.0 vdc bridge excitation. the a/d converter featured a 16-bit successive approximation converter with a total conversion time per reading of 40 μs [40]. the 5100b operates with 1 ms per scan, with 50 complete scans per s the typical usage. concurrent scanning was adopted for all scanners, with the input channels in each scanner scanned sequentially at 0.04-ms intervals and stored in random access memory within a 1 ms window. the strain gauges used were the pfl-10-1 type, which are polyester linear gauges with mild steel compensation, specifically, a foil strain gauge with a polyester resin backing. this type of strain gauge features 2-cm pre-attached lead wires, a strain limit of 2 % (20,000 μs), an operational temperature of 20 °c–80 °c, and a nominal resistance of 120 ω. the main problem that emerged during the second test on the second specimen was the return of the lvdt data. as indicated by the elastic branch of the load vs. displacement diagram presented in figure 12, there was a disturbance caused by a noise that was due to the daq. meanwhile, the data acquired by the channels dedicated to the strain gauges did not present this problem. as such, a spline fitness function was implemented to minimise the effect of the noise, thus preserving the linearity of the load vs. displacement graphs to obtain new diagrams, which are presented in figure 17. here, a cubic spline interpolation algorithm was used on the acquired data, with the spline function smoothing the random trend due to the noise, which allowed for highlighting the load vs. displacement trend. the linearity variation in relation to the nominal value of the daq was ± 0.05 %, and, as such, was negligible in the evaluation of the measurement accuracy with respect to the sensor linear characteristics. however, in terms of the linearity variation in relation to the nominal value for the lvdts, this was ±0.500 mm [39], [40]. this value had to be reduced by dividing it by √3 and identifying a range of ±0.288 mm [41]. 6. conclusions the principal aim of this paper was to conduct preliminary research into the reliability and monitoring under dynamic loads of the technique patented by pasquale frezza. this type of antiseismic construction consists of masonry walls built with bricks and fictile tubules, arranged in a staggered and alternating manner within a timber wooden frame. for the first specimen, which was tested until collapse occurred, the experimental diagonal compression tests obtained a shear strength of 0.81 mpa, while the lowering displacement of the steel plate of the testing machine was 11.526 mm. the second masonry wall was subjected to diagonal compression tests in two distinct phases. here, the specimen was first damaged and the test stopped when the first cracks appeared before the specimen was repaired using b-frcm and the test repeated until collapse. the shear strengths obtained here were 0.73 mpa and 1.04 mpa, respectively. these results indicate how, following the strengthening of the specimen with b-frcm, the shear strength underwent an increase of 43 % in relation to the unreinforced case, which highlights the importance of fibre reinforcement for increasing the load-bearing capacity in this type of structure. however, both values were higher than those generally obtained for common masonry walls, while the cracks that formed were consistent with those expected in this type of test. figure 18 shows the location of the cracks developed during the testing of the first specimen following the attainment of the maximum load. on both sides of the masonry wall, cracks developed through the mortar, parallel both to each other and to the direction of the load application, while they barely affected the fictile tubules. the numerical simulation demonstrated good agreement in terms of the load displacement diagrams, albeit with some minor discrepancies, while the crack development largely mirrored that obtained in the experimental test. in addition, the simulation of the damage in the wall obtained via the numerical model largely corresponded to the experimental results, while more cracks appeared at the top as well as four single cracks originating at the middle section of each timber beam. overall, the model was highly similar to the actual situation, with the four small cracks noted above appearing in the back of the wall. the displacement of the upper plate of the testing machine obtained via the numerical model was compared with the experimental results to evaluate their compatibility. the linearity variation of the lvdt measurement in relation to the nominal values was ±0.288 mm, while in the experimental case, the measured displacement was 11.526 ± 0.866 mm and the cover coefficient = 3. the displacement estimated by the numerical model was compatible with that obtained via the experimental tests (11.700 mm). figure 17. load vs. displacement diagrams following the application of the fitness function. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 55 the proposed as was composed of various lvdts, which were used for monitoring the horizontal and vertical displacement, and several strain gauges, used for monitoring the strain. the transducers were connected to a spider-8 daq, which transmitted the data to a computer. all the data acquired were then processed in real time using a spline fitness function in order to remove the disturbance caused by a noise resulting from the 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[40] a. katsuki, t. sajima, h. murakami, a. m. hazrat, o. ohnishi, k. akashi, development of a laser-guiding-type deep small-sized hole-measurement system: measurement accuracy, precision engineering 63 (2020), pp.18-32. doi: 10.1016/j.precisioneng.2019.12.012 [41] s. shi, b. muralikrishnan, v. lee, d. sawyer, methods to improve the dimensional measurement accuracy of a motion tracking system, optics and lasers in engineering 130 (2020), p.106092. doi: 10.1016/j.optlaseng.2020.106092 https://doi.org/10.1016/j.conbuildmat.2009.11.001 https://doi.org/10.1063/1.5012367 https://doi.org/10.1520/e0519_e0519m-15 https://doi.org/10.1016/j.prostr.2018.11.059 https://10.0.3.248/j.autcon.2005.10.010 https://doi.org/10.1155/2016/2749720 https://doi.org/10.1016/j.engstruct.2016.02.048 https://doi.org/10.1016/0008-8846(76)90007-7 http://resolver.tudelft.nl/uuid:0a401939-2938-4f9d-a395-b6a6652b2cd9 http://resolver.tudelft.nl/uuid:0a401939-2938-4f9d-a395-b6a6652b2cd9 https://www.hbm.com/en/3059/wa-t-inductive-displacement-transducer-probe/ https://www.hbm.com/en/3059/wa-t-inductive-displacement-transducer-probe/ https://www.hbm.com/en/6251/product-literature-archive/ https://doi.org/10.1016/j.precisioneng.2019.12.012 https://doi.org/10.1016/j.optlaseng.2020.106092 a review of accurate phase measurement methods and instruments for sinewave signals acta imeko issn: 2221-870x june 2020, volume 9, number 2, 52 58 acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 52 a review of accurate phase measurement methods and instruments for sinewave signals eulalia balestrieri1, luca de vito1, francesco picariello1, sergio rapuano1, ioan tudosa1 1 department of engineering, university of sannio, benevento, italy section: research paper keywords: phase measurement; waveform recorders; wide frequency range; measurement uncertainty. citation: eulalia balestrieri, luca de vito, francesco picariello, sergio rapuano, ioan tudosa, a review of accurate phase measurement methods and instruments for sinewave signals, acta imeko, vol. 9, no. 2, article 9, june 2020, identifier: imeko-acta-09 (2020)-02-09 editor: alexandru salceanu, technical university of iasi, romania received march 13, 2020; in final form april 23, 2020; published june 2020 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: eulalia balestrieri, e-mail: eubal@unisannio.it 1. introduction phase measurement is crucial important information for characterising a sinewave signal. according to [1], ‘the phase is the time (instant) for a given signal value on a waveform cycle’. usually, the phase is an expression of the relative time difference between two corresponding features (peaks or zero crossings) of two waveforms with the same frequency [1]. in the case of a comparison between two waveforms, the phase difference or phase angle is typically expressed in degrees as a number greater than -180 ° and is less than or equal to +180 ° [1]. moreover, phase can be sometimes expressed in radians, where one radian of a phase equals approximately 57.3 ° [1]. phase measurements are carried out in several applications for indirectly measuring strain, acceleration, and power quality as well as for characterising circuits and components in the frequency domain. for example, in the case of optical interferometry, the desired measurand (e.g. strain, temperature, and surface deformation) is usually encoded in the phase of the signal obtained from the interference of two optical waves at the same frequency [2]. in radar applications, instead, the distance and the direction of a target, relative to the antenna, is estimated based on the phase of the electromagnetic wave reflected by the target itself [3]. in those and other indirect measurements, the uncertainty of the phase measurement has a crucial role in determining the measurement uncertainty of the derived quantities (e.g. strain and temperature). several electric and electronic instrumentation applications are particularly interested in the phase spectrum as the target measurand. for example, for monitoring the state of an electric power transmission network, the most widely used instrument is the phasor measurement unit (pmu). an important task of a pmu is measuring the phase of each harmonic component of the voltage signal from the power supply network [4]. the phase spectrum is needed also for the estimation of the frequency response of a generic electrical circuit (device under test [dut]). in this case, the phase delay between a sinewave signal that is output by the dut and the same sinewave that is input into the dut is measured, [6]. a dut that is particularly interesting from a research point of view is the waveform recorder (e.g. digital oscilloscope) for which the frequency response is essential in compensating for the systematic effects of the instrument on the voltage abstract the phase measurement of sinewave signals is important in several applications, such as electric and electronic instrumentation; telecommunications; and optical interferometry. the uncertainty of the phase measurement has an essential role in ensuring the suitable performance of the devices and systems used by the relevant application. some highly accurate phase measurement methods have been developed and implemented in different instrument types that are currently available on the market or have been proposed in the scientific literature, each capable of covering very different frequency ranges. this article presents an overview of these methods and instruments in order to highlight the characteristics in terms of the measurement uncertainty of the main methods and instruments that are used, by taking into account a varying operative frequency range. the standard deviations considered in the surveyed literature are used to identify a phase measurement method that is capable of covering a large high-frequency range, simultaneously maintaining a low value of measurement uncertainty, as requested by some applications (like waveform recorder frequency response testing). mailto:eubal@unisannio.it acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 53 measurements [6]-[11]. the two main reasons for such interest are (i) the fact that it is a one-port dut with an analog input and a digital output that cannot be easily and accurately compared to the input and (ii) the increasingly wide bandwidth of such duts, which makes it difficult to scan the whole phase spectrum. in [14], the authors proposed a method for phase delay measurement between the digitised output and the analog input of a waveform recorder, working up to 20 khz. the authors declared that the method can be used for calibrating pmus and digital low-power instrument transformers. however, the proposed method [6] is not designed for calibrating the current oscilloscopes available on the market working above 100 ghz in of analog bandwidth. although for the measurement of the frequency response of a high-frequency oscilloscope, the most commonly used technique is based on the evaluation of the pulse response of the oscilloscope under test, when moving to high frequencies, this technique does not maintain high accuracy. in that case, the most suitable technique is the sinewave sweep [8]. knowing the phase of sinewave input into the waveform recorder, it is possible to estimate the phase response of the instrument for each input sinewave frequency [8], thereby providing the phase spectrum response of the waveform recorder. to that end, it is necessary to measure the phase of the sinewave output by the source generator relative to a reference signal that is used for synchronising the clocks of the waveform recorder and the generator. in all the cases mentioned above, the role of the measurement uncertainty associated with the phase measurement is essential for assuring the target uncertainty of the whole measurement system. as quoted above, the methods and instruments developed to carry out high-accuracy phase measurements differ – in the output domain (time or frequency), the measurement frequency range, and the type of signal considered (electric or optical). the phase measurement methods and instruments are sometimes compared in terms of their capability of satisfying a specific application requirement, like the bandwidth of analysis, the measurement resolution, or the waveform shape. a comparison based on uncertainty, independent from the application, is currently missing. in this article, therefore, a review of the main methods and instruments currently available on the market or described in the literature dealing with the phase measurement of sinusoidal signals is given, focusing on their measurement uncertainty performance and the covered frequency range. the presented analysis can be useful for helping researchers to find the method or the instrument they need by looking at the uncertainty and to pave the way to the design of a phase measurement method that is capable of covering a wide frequency range, maintaining low measurement uncertainty requirements. this article is organised as follows. firstly, the main phase measurement methods are described along with the instruments implementing them that are available on the market. second, a comparison of the instruments previously described is carried out in terms of the expanded uncertainty for the phase measurement of the sinewave signals. finally, conclusions are drawn. 2. phase measurement methods and instruments currently, different methods exist for measuring the phase delay of a sinusoidal signal relative to a reference signal. most of the methods that have been proposed in the literature or in the instrumentation market as providing highly accurate phase measurements can be grouped into three main classes (see table 1): (i) the event-counting-based method, (ii) the modulationbased method, and (iii) the sampling-based method. the first class provides the phase measurement as the time delay between two signals with the same frequency carried out by means of an event-counting technique. the instrument implementing this method is the universal counter, and the operating frequency order of magnitude usually ranges from 10-3 hz to 108 hz. the modulation-based methods provide the phase delay introduced by a dut, using a frequency mixer to multiply the sinewave under test with a signal from a reference oscillator. this method is used by three different kinds of instruments: the vector signal analyser (vna), the phase-to-voltage converter (pvc), and the interferometer. both the vna and pvc operate on electric signals, while the interferometer on optical signals. their output domain is the frequency one. the frequency ranges covered by the three instruments are different; for example, from 103 hz to 1011 hz for the vna, from 10 hz to 103 hz for the pvc, and from 1011 hz to 1012 hz for the interferometer. finally, the sampling-based methods are based on sampling the input sinewave using a reference clock, digitising it and processing it with algorithms that estimate the unknown phase from the sample record. a typical instrument implementing a sampling-based method is the pmu. the operating frequency range, in this case, is from 10 hz to 102 hz . in the following subsections, the three classes of phase measurement methods along with the typical instruments implementing them are described, highlighting their main advantages and limitations. although in time and frequency metrology, the phase difference is usually stated in units of time [1], in this article, its value and measurement uncertainty are expressed in degrees, since this is the way they are expressed in the datasheets and in the considered scientific articles. however, table 1. classification of the main phase measurement methods belonging to the event-counting-based, modulation-based, and sampling-based method class available in the literature and commercial instruments. method instrument frequency range in hz type of measurement type of signal output domain min. max. event-counting-based universal counter 10−3 108 phase delay between two isofrequential signals electric time modulation-based vector network analyser (vna) 103 1011 phase delay introduced by a device under test (dut) electric frequency phase-to-voltage converter (pvc) 10 103 interferometer 1011 1012 optical sampling-based phasor measurement unit (pmu) 10 102 phase of a sinewave signal according to a reference clock electric time acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 54 the degrees can be easily converted in units of time, considering that the time interval for 1 ° of the phase is inversely proportional to the frequency. therefore, if the frequency of a signal is given by f, then the time tdeg (in seconds) corresponding to 1 ° of the phase is 𝑡deg = 1 (360 𝑓) = 𝑇 360⁄⁄ , [1] where t is the period of the signal. 2.1. event-counting-based methods the methods based on event counting provide phase delay measurements between two isofrequential electrical signals. in particular, the two signals are converted into square waves using a schmitt trigger circuit. the first positive-going or negative-going transition of one square wave is used as a start signal of a counter. the stop signal is generated at the first positive-going or negative-going transition of the other square wave. the counter measures the number of transitions of an internal clock signal between the start and the stop events (see figure 1). based on the number of clock signal transitions and the period of the internal clock, the time delay is measured. then, by measuring the frequency of the signals under test, the phase delay is estimated. an example of a time interval counter for measuring phase delay between two atomic clocks is proposed in [12]. this counter has an internal clock frequency of 400 mhz, which corresponds to an absolute time resolution of 2.5 ns. an example of high-performance universal counter that is available on the market [13] has a frequency range from 1 mhz up to 350 mhz with a time resolution of 20 ps, and an uncertainty value in the order of 0.1 ° from 1 mhz up to 100 khz. for frequency values between 100 khz and 350 mhz, the uncertainty value as a quadratic function of frequency varies [13]. the main disadvantages of the counting-based method are (i) that the noise on the input signals causes time-varying transition occurrence instants of the square waves generated by the schmitt trigger circuits and (ii) that the measurement accuracy is mainly affected by the time resolution and stability of the reference internal clock. 2.2. modulation-based methods the modulation-based methods for phase measurement use a frequency mixer to multiply the sinewave under test with a signal from a reference oscillator. the mixer output is filtered by a low-pass or band-pass filter and is subsequently acquired with an analog-to-digital converter (adc). the following instruments exploit this method: (i) the vna, (ii) the pvc circuit, and (iii) interferometers. the vna is the most widely used instrument for characterising the frequency response of circuits or systems in terms of phase and magnitude spectrum. in the vna measurements, a sinewave signal generated internally stimulates the dut, and two or more receivers are used for measuring the phase and amplitude of the incident and reflected waves at the dut input and output ports. the vna has, as a reference oscillator, one local oscillator (lo) per port. each receiver typically has two mixers, one to multiply the lo output with the incident signal and the other to multiply the lo output with the received (reflected or transmitted) signal. an adc acquires the incident, reflected, or transmitted wave amplitude. the adcs are synchronised, and the acquired samples are used for estimating the phase delays between the reflected and incident waves. this instrument can be used for measuring the phase delay of a dut with at least one input and one output port, which is not the case for signal generators or waveform recorders, which have only one input port. vnas, however, can be used to measure the complex input impedance of a waveform recorder input port. in [14], the measurement specifications of a commercial vna are reported. this instrument exhibits an uncertainty value in the order of 0.01 ° for a frequency range of 900 hz to 120 ghz. the pvc is based on the multiplication of the sinewave under test with a sinewave of the same frequency. the resultant signal has a dc component that is proportional to the phase delay between the sinewaves. in the case of a phase delay that is equal to 90 °, the output dc component is ideally equal to zero. it can be used for measuring the phase delay between two sinewave signals or the phase delay introduced by a system with at least two ports. in [15], the authors present a high-performance automatic fully-analog pvc circuit for the phase shift measurements of periodic waveforms based on that method, working in a frequency range of around 100 hz. in particular, the pvc has been tested for measuring the phase of electrical signals, and the authors assessed the measurement uncertainty to be in the order of 0.001 °. apart from the narrow frequency range, the main drawback of pvcs is that the amplitude of the dc component is affected by the difference between the frequencies of both signals used as mixer inputs. for this reason, a phased-locked-loop (pll) circuit is used to control the frequency of the reference source signal so that it is nominally equal to the frequency of the sinewave under test. of course, the measurement accuracy depends on the pll stability [16]. at terahertz frequencies, the modulation-based method is often implemented by means of interference between optical and electrical signals [17]. in this case, the aim can be to measure the delay introduced by a sample material on a reference signal. the systems used for performing these measurements (figure 2) are (i) the conventional self-heterodyne system and (ii) the balanced self-heterodyne system. for both systems, a continuous electrical thz wave is generated by means of an optical-to-electrical conversion, also referred to as photomixing [18]. in particular, two frequency-detuned free-running lasers operating at two different frequencies, f1 and f2, are combined to produce a beat note at the frequency f2-f1. the frequency of laser1 is coherently shifted (by fs) with an optical frequency shifter (fs) so that the radio-frequency (rf) and optical lo figure 1. the principle of operation of the event-counting-based method, considering the positive-going transition to define the start and stop signals of the count. acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 55 signals have the same frequency/phase noise at the different average frequencies, realising the self-heterodyne detection [18][19] (figure 2a). the rf frequency can be tuned by tuning the frequency of one of the lasers – without changing the if frequency (fs) [19]. the rf signal is transmitted to the sample under test and the provided output mixed with the reference optical beat signal, lo. the phase and the amplitude of the output signal provided by the mixer can be measured simultaneously by means of a twophase lock-in amplifier [20]. in the self-heterodyne system, the frequency fluctuation of the rf signal and that of the optical lo signal has the same property; thus, the frequency fluctuations of the free-running lasers are cancelled out by means of the mixing operation [21]. however, the conventional self-heterodyne system is affected by phase instabilities or phase drift due to the optical phase fluctuations imposed in the optical fibers. in the balanced self-heterodyne system, this problem is resolved by dividing the thz wave into two paths (figure 2b). each amplitude and phase detected by two mixers is measured by a two-channel dual-phase lock-in amplifier. after the detection, the common phase noises are subtracted [17][21]. in this case, the authors of [17] identified an uncertainty in the phase measurements in the order of 1 ° at 300 ghz. 2.3. sampling-based methods the phase measurement methods based on sampling techniques have in common the direct digitisation of the signal from the source under test without modulating it. digital signal processing algorithms are then used to estimate the unknown phase from the sample record. in particular, for estimating the phase of a sinewave signal, a sine fit can be performed on the acquired samples. to that end, the adc used for sampling and digitisation must have sufficient amplitude resolution, such that the quantisation noise is lower than the noise from the measurement circuit. an example of an instrument based on the sampling method is the pmu. in [4], the authors present a pmu for the measurement of phasors in three-phase power distribution networks. the proposed pmu consists of an adc for acquiring four analog signals, the three monitored electrical inputs (voltages or currents) and the synchronisation signal provided by a gps receiver. then, the measurements of the amplitude and phase of the harmonic components in the three-phase system are provided by means of a fourier analysis of the acquired samples. such a method requires a significant number of samples to be acquired, in the order of 103, to achieve accurate (less than 0.01 % of actual) amplitude and phase measurements. for this reason, this method is useful for the accurate measurement of the phase of sinewaves at low frequencies (order of 100 hz). in [22], the authors report uncertainty in the order of 0.0001 ° at 50 hz. a small number of samples can be used by applying interpolation methods for pure sinewave signals. for example, if the sinewave frequency is known, it is possible to use a threeparameter sine fit [23] to estimate the phase of the signal. conversely, if the frequency is unknown, a four-parameter leastsquares sine fit can be used. in all the above cases, the measurement uncertainty is affected by (i) the noise on the signal under test, (ii) the aperture uncertainty of the sampling circuit, and (iii) the jitter on the sampling clock. the equivalent-time sampling techniques, which are used in some high bandwidth oscilloscopes, acquire each sample of a periodic waveform at a given delay (instant) after a trigger signal. a sample is acquired by the oscilloscope if the trigger signal exceeds a user-defined threshold. one sample per waveform instant is acquired per trigger event, and many samples per instant and many instants per waveform are necessary to capture a waveform. as reported in [8], the jitter in the time-base circuitry and between the trigger event and each acquired sample (i.e. trigger jitter [24]) can affect the timing and shape of the displayed waveform. the jitter trigger effectively acts to impose a low-pass filter on the acquired waveform. furthermore, if the input signal, which is used for generating the trigger event, has a slow slew rate (low frequency signal) or a low signal-to-noise ratio (snr), the trigger jitter increases [27]. 2.4. application limits of the event-counting-, modulation-, and sampling-based methods the main limitations of the described phase measurement methods can be summarised as follows: (i) the phase measurement is obtained using a reference signal that has the same frequency of the sinewave signal (the event-counting-based and modulation-based methods), and/or (ii) the phase measurement is provided for a limited frequency range (interferometric and sampling methods), and/or a huge number of signal samples per period is required (sampling methods) [4][22]. figure 2. configurations of the (a) conventional and (b) balanced self-heterodyne system. fs = optical frequency shifter and o/e = optical-to-electrical converter [17]. nodes 1 and 3 are optical splitters, and nodes 2, 4 and 5 are optical combiners. acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 56 3. phase measurement uncertainty comparison in this section, a comparison of the instruments described in the previous section in terms of the expanded uncertainty for the phase measurement of the sinewave signal is proposed. in the following section, the analysis of the data from the comparison is described first, and a discussion on the obtained results is then provided. 3.1. data analysis whenever a commercial instrument is available to measure phase, the comparison is carried out using the instrument characteristics that are stated by the manufacturer. when the references are technical papers, the standard deviations of the published results are used. five instruments or methods are compared: one, a universal counter, belongs to the event-counting-based methods; three, a pvc, an interferometric technique, and a vna, belong to the modulation-based methods; and one, an adc-based technique, belongs to the sampling-based methods. in particular, the instruments or methods considered are: (i) a universal counter [13] with a frequency range of 1 mhz to 350 mhz, (ii) the pvc, working at very low frequencies, 7 hz to 770 hz [15], (iii) an interferometric technique at frequencies in the order of thz [15], (iv) the technique based on adc and three-parameter sine-fit [22], used at very low frequencies (order of hz), and (v) a vna, working in the frequency range of 500 hz to 125 ghz [14]. for the expression of the type b combined standard uncertainty values, 𝑢φ, related to the universal counter, the following formula provided by the manufacturer has been used [13]: 𝑢φ = [3 ⋅ √2(𝑇ss 2 + 𝑇e 2) + 𝑇lte + 𝑠𝑘𝑒𝑤 + 2 ⋅ 𝑇accuracy] ⋅ 𝑓src ⋅ 360 [°], (1) where tss is the timing resolution of a start/stop measurement event, te is the threshold error due to the input signal random jitter, tlte is the timing error due to the threshold level, 𝑠𝑘𝑒𝑤 is the additional time error if two channels are used for a measurement, taccuracy is the measurement error between two points in time, and fsrc is the frequency of the signal under test, which is a sinewave. according to the keysight 53200a series rf/universal frequency counter/timers datasheet [12], the following values have been associated with each component of the uncertainty equation: tss = 20 ps, te = 0 ps, skew = 50 ps, taccuracy = 100 ps. furthermore, tlte has been evaluated according to the equation: 𝑇lte = 𝑇lse−start 𝑆𝑅−𝑠𝑡𝑎𝑟𝑡 + 𝑇lse−stop 𝑆𝑅−𝑠𝑡𝑜𝑝 , (2) where tlse-x is the threshold setting error given by: 𝑇lse−x = 0.002 ⋅ 𝑠𝑒𝑡𝑡𝑖𝑛𝑔 + 0.001 ⋅ 𝑟𝑎𝑛𝑔𝑒, (3) where, setting = 0 v and range = 5 v, and 𝑆𝑅−𝑥 = 2 ⋅ 𝜋 ⋅ 𝑓𝑠𝑟𝑐 ⋅ 𝑉0−to−pk, with 𝑉0−to−pk = 2 v. the uncertainties have been evaluated in the measurement frequency range of the universal counter, from 1 mhz up to 350 mhz and have been plotted in orange in figure 3. the pvc system considered in the comparison, described in [15], has been designed for working in the frequency range of a few mhz to 100 khz. furthermore, the experimental results, in terms of standard uncertainty on phase measurements of 0.01 °, figure 3. performance comparison in terms of the expanded uncertainty of the instruments available on the market for the measurement of the phase of sinewave signals. acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 57 0.001 °, and 0.0001 °, at the frequency values of 7.7 hz, 77 hz, and 770 hz, respectively, are reported and plotted in yellow in figure 3. the modulation method based on the interferometric technique at thz frequencies for characterising sample materials, described in [15], is aimed at measuring the phase delay introduced by the sample material, which is driven by an electrical signal. the proposed prototype exhibits an expanded uncertainty on phase measurements of 0.37 ° at 300 ghz [15], reported with the purple dot in figure 3. the technique based on sine fitting for phase measurements, described in [22], has an expanded uncertainty value of 0.0001 ° at 52.1 hz, considering 38 acquired samples and a snr of 75 db, resulting from the monte carlo analysis carried out by the authors of [22] (green dot in figure 3). the vna operating over the frequency range of 500 hz up to 125 ghz, according to the datasheet, and considering a power at the input port that minimises the phase uncertainty, provides an expanded phase uncertainty from 900 hz to 120 ghz of about 0.03 ° [14], as plotted in cyan in figure 3. 3.2. discussion by looking at figure 3 and table 2, the universal counter keeps the same amount of measurement uncertainty, higher than the other methods, until the frequency does not exceed the 105 hz, from which a rapid increase of the uncertainty values can be observed. this trend is due to the limitations of the universal counter performance at high frequencies caused by the time resolution of 100 ps of the internal clock, and at low frequencies by the trigger jitter due to the low slope of the sinewave signal affected by noise. the vna exhibits a lower measurement uncertainty amplitude than the universal counter, maintaining the same uncertainty value at the higher frequencies. moreover, the frequency range is higher than the universal counter one. the pvc works in a frequency range that is smaller than the two previous instruments. it can be seen, however, that the uncertainty amplitude quickly decreases approaching the upper end of the frequency range to values lower than the previous instruments. the interferometer operates at a higher frequency than the other instruments; however, the proposed prototype considered in the comparison can work within a narrow frequency range [17] and provides an uncertainty that is much higher than the vna one. the implementation of an interferometer operating on a wide range of frequency represents, in fact, is still an unsolved problem. the three-parameter sine fitting for phase measurement reaches the lower uncertainty amplitude of this comparison; however, this result is obtained at the unique frequency of 52.1 hz. in general, it must be noted that the instruments working within narrow frequency ranges or at a specific frequency exhibit the lowest measurement uncertainty (such as the adc threeparameter sine fit, pvc, and interferometry). on the other hand, the universal counter and the vna work within wide frequency ranges (lower and higher frequencies, respectively), but they exhibit higher uncertainties than narrow-band instruments. a single method that is able to work from few mhz to a few ghz with low uncertainty has not been published yet. 4. conclusions several current applications require the measurement of the phase of sinewave signals, assuring high accuracy. various methods and instruments have been developed for this purpose. however, it is not possible to find a method that is capable of covering a wide range of frequencies while also maintaining a low value of measurement uncertainty that can be used to develop a universal and highly accurate phase measurement instrument. in this article, an overview of the main methods used to measure the phase of a sinusoidal signal aimed at assuring high accuracy among the results is presented. three main classes of methods have been identified, together with examples of their implementation in instruments available on the market as well as the scientific literature’s proposals and prototypes. then, a comparison of the instruments described in terms of the expanded uncertainty for the phase measurement of sinewave signal has been carried out. each of the considered cases requires an individual approach and to take into account the available features based on the technical data of the instrumentation used and the relevant measurement conditions for evaluation purposes. to that end, some useful information can be found in the comparison results reported in the paper. this information can help in (i) finding the right method or instrument for a specific application by looking at the target uncertainty, (ii) designing new methods to improve phase measurement accuracy, and (iii) improving the existing ones in the covered frequency range while overcoming drawbacks as the requirement of isofrequential signals or the huge number of signal samples. table 2. performance comparison in terms of expanded uncertainty of the instruments available on the literature for the measurement of the phase of sinewave signals. method or instrument frequency range expanded uncertainty phase measurement method class keysight 53200a series rf/universal frequency counter/timers 1 mhz – 350 mhz 0.3° from 1 mhz to 100 khz 1.5° at 10 mhz 12.3° at 100 mhz event-counting-based pvc 7 hz – 770 hz 0.01° at 7.7 hz 0.001° at 77 hz, 0.0001° at 770 hz modulation-based interferometric technique 300 ghz 0.37° modulation-based vna 900 hz – 120 ghz 0.03° modulation-based adc 3-parameter sine fit 52.1 hz 0.0001° sampled-based acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 58 acknowledgement the research presented in this paper has been supported from grant 70nanb15h167 ‘a phase measurement system for calibrating electroshock weapon (pme)’ awarded by the national institute of standards and technology (nist), usa. the authors would like to thank prof. pasquale daponte for his useful suggestions during all the phases of this research. references [1] national institute of standards and technology (nist), time and frequency from a to z. [online]. available: https://www.nist.gov/pml/time-and-frequencydivision/popular-links/time-frequency-z/time-and-frequency-zp#phase [2] k. falaggis, d. p. towers, c. e. towers, phase measurement through sinusoidal excitation with application to multi-wavelength interferometry, j. of optics a: pure and applied optics 11(5) (2009) pp. 1-11. [3] a. schicht, k. huber, a. ziroff, m. willsch, l. p. schmidt, absolute phase-based distance measurement for industrial monitoring systems, ieee sensor journal 9(9) (2009) pp. 10071013. [4] a. carta, n. locci, c. muscas, a pmu for the measurement of synchronized harmonic phasors in three-phase distribution network, ieee trans. on instrumentation and measurement 58(10) (2009) pp. 3723-3720. [5] t. y. wu, y. l. lu, error analysis and uncertainty estimation for a millimeter-wave phase-shift measurement system at 325ghz, measurement 59 (2015) pp. 198-204. 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[online]. available: https://www.keysight.com [14] keysight technologies, n5291a 900 hz to 120 ghz pna mmwave system, datasheet. [online]. available: https://www.keysight.com. [15] a. de marcellis, g. ferri, e. palange, a fully analog high performance automatic system for phase measurement of electrical and optical signals, ieee trans. on instrumentation and measurement 64(4) (2015) pp. 1043-1054. [16] d. owen, good practice guide to phase noise measurement, national physical laboratory, may 2004, issn 1368-6550. [17] s. hisatake, y. koda, r. nakamura, n. hamada, t. nagatsuma, terahertz balanced self-heterodyne spectrometer with snrlimited phase-measurement sensitivity, optics express 23(20) (2015) pp. 26689-26695. [18] s. hisatake, g. kitahara, k. ajito, y. fukada, n. yoshimoto, t. nagatsuma, phase-sensitive terahertz self-heterodyne system based on photodiode and low-temperature-grown gaas photoconductor at 1.55 μm, ieee sensors journal 13(1) (2013) pp. 31-36. [19] s. hisatake, j. kim, k. ajito, t. nagatsuma, self-heterodyne spectrometer using uni-traveling-carrier photodiodes for terahertz-wave generators and optoelectronic mixers, journal of lightwave technology 32(20) (2014) pp. 3683-3689. [20] t. nagatsuma, s. hisatake, m. fujita, hai huy nguyen pham, k. tsuruda,s. kuwano, j. terada, millimeter-wave and terahertzwave applications enabled by photonics, ieee journal of quantum electronics 52(1) (2016) pp. 1-12. [21] t. nagatsumaet, s. hisatake, hai huy nguyen pham, photonics for millimeter-wave and terahertz sensing and measurement, ieice transactions on electronics e99–c(22) (2016) pp. 173180. [22] r. lapuh, phase estimation of asynchronously sampled signal using interpolated three-parameter sinewave fit technique, proc. of the ieee international instrumentation and measurement technology conference (i2mtc), 3-6 may, 2010, austin, tx, pp. 82-86. [23] ieee standard for digitizing waveform recorders, ieee std 1057-2007, 2008. [online]. available: https://www.ieee.org [24] e. balestrieri, f. picariello, s. rapuano, i. tudosa, review on jitter terminology and definitions, measurement 145 (2019) pp. 264273. [25] e. balestrieri, p. daponte, l. de vito, s. rapuano, jitter and its relatives: a critical overview, proc. of the ieee instrumentation and measurement technology conference, 2013, pp. 1141-1146. [26] understanding key high-performance oscilloscope specifications. [online]. available: https://www.tektronix.com/oscilloscopes [27] a. andreev, h. klingbeil, d. lens, phase calibration of synchrotron rf signals, proc. of 8th international particle accelerator conference (ipac2017), 2017, copenhagen, denmark, pp. 3945-3947. https://www.nist.gov/pml/time-and-frequency-division/popular-links/time-frequency-z/time-and-frequency-z-p#phase https://www.nist.gov/pml/time-and-frequency-division/popular-links/time-frequency-z/time-and-frequency-z-p#phase https://www.nist.gov/pml/time-and-frequency-division/popular-links/time-frequency-z/time-and-frequency-z-p#phase https://www.keysight.com/ https://www.keysight.com/ https://www.ieee.org/ https://www.tektronix.com/oscilloscopes design and development of high load brinell hardness standard machine at ume acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 226 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 226 229 design and development of high load brinell hardness standard machine at ume c. kuzu1, e. pelit2 1 ume, kocaeli, turkey, cihan.kuzu@tubitak.gov.tr 2 ume, kocaeli, turkey, ercan.pelit@tubitak.gov.tr abstract: after successful implementation of two hardness standardizing machines at tse (turkish standards institution) in rockwell and brinell-vickers hardness scales separately and deciding to combine them in one machine with a newer design and install it in hardness laboratory of ume (national metrology institute of republic of turkey) it was planned to design and develop a deadweight type brinell hardness standard machine at ume for high load scales within the scope of an internally funded project. in this paper the design of high load brinell hardness standard machine with dead weight force application system realized by ume hardness laboratory is explained. keywords: hardness, brinell, hardness standard machine 1. introduction with the demand of the turkish industry and in turn accredited calibration and testing laboratories, and after successful implementation of two hardness standardizing machines at tse in rockwell and brinell-vickers hardness scales separately, a high load brinell hardness standard machine had been decided for first time to be designed and established at ume to provide traceability in the field of brinell hardness measurements covering the range 187.5 kgf – 3000 kgf. in this new design the force application system was considered to comprise mass stacks realizing force under the gravitational acceleration and a frame to transfer the realized force to the tip of the indenter, a very well known deadweight force application principle. the frame was supposed to generate the 187.5 kgf, which corresponds to the hbw 2.5/187.5 scale. there are 12 mass stacks to be added to the frame for realization of the other brinell scales up to 3000 kgf. an indentation measurement system will be needed for diameter length measurement for brinell hardness. the indentation measurement system is not in the scope of this project, the system present in hardness laboratory will be used. as a future planning a new indentation measurement system with a better performance is under development. the testing cycle is managed by making use of a load cell (force transducer) to which the whole force application system; the frame and the mass stacks as a whole, is mounted. the force application time and dwell duration are sensed by measuring the force on the load cell instantaneously during load application and calculating the change in force values with respect to the nominal (initial/reference) values belonging to the scale, then force – time relationship is plotted. figure 1: cad of the high load brinell hardness standard machine http://www.imeko.org/ mailto:cihan.kuzu@tubitak.gov.tr mailto:ercan.pelit@tubitak.gov.tr acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 227 the indenter approach, force application and removal speeds will be adjusted by calibrating the main servomotor (m1) which will be used for realization of the measurement cycle and force application rate is measured via the change in the load cell value in time. there is a second servo motor on the bottom side of the machine used to make automatic selection of the masses (scales) and with this design the machine will be completely automatic in terms of every step to realize the measurement cycle in brinell scales indicated in the table 1. the cad of the machine is given in figure 1 and drawing in figure 2. figure 2: high load brinell hardness standard machine 2. theory and scope the theory of brinell hardness measurement, as a well-known method, comprises realisation of indentation and measurement of the diameter of the indentation to be used for calculation of the brinell hardness number by making use of the empirical equation eqn. 1. this method is completely used to measure mechanical property of material in line with the hardness definition, that is, the resistance which a material shows against deformation. in this paper the design of the machine used to realize the indentation with the highest accuracy and stability is explained. under these circumstances, regarding the theory, the following components are in the scope of this project: ➢ force application with the highest accuracy ➢ measurement cycle application precisely ➢ indenter to realize the deformation a certain amount of force is applied on a defined shape and material of indenter through a predefined application cycle comprising force application speed and time durations where the force is kept on the indenter applied and shown in figure 3. figure 3: realization of brinell hardness indentation after application of the measurement cycle, the realized indentation size, the diameter, is measured by some other device as shown in figure 4. then the measured diameter is used to calculate the brinell hardness by making use of eqn. 1. figure 4: measurement of brinell hardness indentation (1) there are 21 brinell hardness scales defined by the iso hardness standard (iso 6506-1:2014) [1] constituted with different combination of indenters and force values applied. the scales to be realized on this machine are highlighted in the table 1. table 1: brinell hardness scales to be realized http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 228 3. design of the machine the machine body was designed to be rigid and sturdy to minimize side effects during force application and realization of indentation, and every component constituting brinell hardness is considered to be with the highest accuracy to attain the best outcome quantity hardness. 3.1. body of the machine it is very important to have a rigid and strong enough body that will not affect the measurement results. for this reason, the machine body are designed to comprise the minimum number of parts and each having precise dimensional measurements and tight tolerances. see the cad of the machine body in figure 5. figure 5: high load brinell hardness standard machine 3.2. force application system to realize the force with the highest accuracy and stability, deadweight type force application principle is preferred. it comprises mass stacks made up of stainless steel and a frame to constitute the first load and transfer the other loads to the tip of the indenter. the frame is adjusted to apply 187.5 kgf and the mass stacks will be in additional order to constitute the other force values and scales. the other mass stacks are 62.5 kgf and 12 x 250 kgf to constitute the 250 kgf, 500 kgf, 750 kgf, 1000 kgf, 1500 kgf and 3000 kgf load steps including the frame itself. to prevent the frame from any pendulum and rotational motions during load application which will affect penetration performance of the indenter, it is guided by two air bearings at the two ends, both are working with (4-6) bar air pressure. the whole assembly of the masses and the frame are held by a high accurate force measurement sensor to control the testing cycle to measurement of the change in force in time and record the force application dwell time. the masses are designed to occupy the smallest size for the smallest possible height and lateral sizes of the machine possibly locate it in hardness laboratory of tubitak ume. the masses are also planned to be with high accuracy since they will be used in recalibration of the force sensor equipped onto the machine and may be used for calibration or intermediate check of the force measurement devices used in calibration/verification hardness testing/calibration machines. the mass stacks are also guided while seating on each other and prevent any of the masses from possible rotation in time as loading and unloading is performed. to do so, some additional apparatus are mounted to the mass stacks at three points, like the corners of an equilateral triangle, and the masses are seated on top of each other. the 12 mass stacks and the frame constituting the high load brinell scales are shown in figure 6. figure 6: force application system of high load brinell hardness standard machine 3.3. measurement cycle the measurement cycle is realized with application of force with a predefined indenter approach speeds and force application rates with respect to time. to figure out the measurement cycle of force application, a force measurement device is equipped onto the machine and data from this device is recorded. the force application time and duration of the force (load) application is measured by the change in force value of the force sensor http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 229 with respect to the nominal (initial) load value. it is also possible to divide the force application speed (rate) into two different region for two different application rate. any percent of the load to divide the regions and any force application rate for each region is possible. in this case it is avoided to make possible overshooting due to the possible final load application speed effect. the approach speed of the indenter (or any other speed) is another parameter needs to be controlled. this is possible in two ways: the first a displacement measurement sensor can be equipped onto the machine and real time data can be collected and manipulated. the second way is to calibrate the motor speed by making use of the high accurate displacement measurement sensor and the speeds are set into the software without any direct measurement. both option are available on this machine. the possible testing cycle of force application are given in figure 7. figure 7: brinell hardness indentation realization cycle 3.4. automation of the machine the machine is equipped with two servomotor, one force measurement sensor and optionally one displacement measurement sensor as the main component of the automation. the main servomotor (m1) is realizing the indentation by moving the force application system upand-down, the secondary servomotor (m2) is used to make the relevant mass stack to realize the relevant scale, load cell besides the measurement cycle is used to sense loading-unloading, parking etc.. there will be enough limit switches, ambient conditions measurement sensors and air pressure measurement gauge. it will be tried to perform closed loop control during load application to fix the load increase time in accordance with the load size and material properties like hardness. 3.5. performance of the machine in this paper the design of the machine is explained in detail, both in terms of mechanics and automation with hardware and software regarding the measurement cycle. the most important property of this machine is the accuracy and stability in force application method and the design of the system to realize precise and repeatable measurement cycle. as a full automatic dead weight force application system, after the implementation of this machine the force uncertainty is expected to be less than 5.10-5 relative, that value may go down to 2.10-5 relative. the time uncertainty in measurement cycle is expected to be less than 0.25 s. the uncertainties of the force application will be assessed and the results will be published accordingly after installation of the machine in ume hardness laboratory. 4. summary at the end of the work explained above; • a deadweight type high load brinell hardness standard machine is designed for first time at ume. • it was designed to comprise the brinell hardness scale components with the highest accuracy to constitute the highest accurate indentation realization. • the scale selection and the cycle of realization of indentation are all possible automatically. acknowledgements the author would like to thank mr. çetin doğan, mr. murat karabi̇naoğlu and mr. gökhan öner from ume, the national metrology institute of turkey, for their valuable contributions during the work has been done in this project of ume hardness laboratory. 5. references [1] en iso, 6506-1. metallic materials – brinell hardness test – part1: test method, 2014. [2] en iso, 6506-3. metallic materials – brinell hardness test – part3: calibration of reference blocks, 2014. http://www.imeko.org/ neural network models for soil moisture forecasting from remote sensed measurements acta imeko issn: 2221-870x june 2020, volume 9, number 2, 59 65 acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 59 neural network models for soil moisture forecasting from remotely sensed measurements andrea marini1, loris francesco termite2,3, alberto garinei1,4, marcello marconi1,4, lorenzo biondi1,4 1 idea-re s.r.l., via cornelia, 498, 00166, rome, italy 2 radarmeteo s.r.l., via iv novembre 119, 35020, due carrare (pd), italy 3 agrosit s.r.l., via briganti 75, 06127, perugia, italy 4 department of sustainability engineering, guglielmo marconi university, via plinio 44, 00193, rome, italy section: research paper keywords: soil water index modelling; machine learning; artificial neural network; lstm; anfis. citation: andrea marini, loris francesco termite, alberto garinei, marcello marconi, lorenzo biondi, neural network models for soil moisture forecasting from remote sensed measurements, acta imeko, vol. 9, no. 2, article 10, june 2020, identifier: imeko-acta-09 (2020)-02-10 editor: oscar tamburis, university of naples federico ii, italy received march 5, 2020; in final form june 22, 2020; published june 2020 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: the study presented in this paper is part of the watch-decision support system for sustainable water management in agriculture under climate change project financed by regione del veneto por fesr 2014-2020, italy. corresponding author: andrea marini, e-mail: amarini@idea-re.eu 1. introduction knowledge of soil moisture (sm) is fundamental in several scientific fields, such as rainfall-runoff modelling, landslide forecasting, soil nutrient cycling processes, drought monitoring, and agriculture [1]. in particular, the importance of having clear control over sm in agriculture is obvious. this has become even greater in recent years due to the climate changes that are increasingly affecting our world. in mid-latitude zones, the effects are visible as both long drought periods (and consequent yield reduction) or extremely intense and localised rainfall events, which could not only ruin a harvest but also lead to crisis when flooding risk occurs. in such a context, it is imperative to properly schedule irrigation practices, which rely on sm as a key variable determining the actual need for water of the crops. nevertheless, managing irrigation systems could be complicated, especially if irrigation is managed through the help of channels deriving from bigger water bodies, like the po valley in italy. generally, land reclamation authorities are responsible for the management of wide networks of interconnected channels, whose flow is regulated by the movement of inline or lateral gates. generally, a gate is open in order to move irrigation water towards a specific area, and it is closed in order to direct the flow elsewhere. traditionally, gates are open or closed manually, but in recent years, an automation process has started, using the introduction of remotely controlled actuators for moving the gates [2][3]. however, planning the manoeuvres still remains a complex task. decision-makers often receive water requests from different stakeholders and must deal with conflicting objectives [4][5]. sometimes, the schedule of gate-opening and irrigation planning is inconsistent with the actual crop need or with the forthcoming weather conditions. in this context, a decision support system (dss) could be helpful. dsss are information systems supporting decision-making processes, usually including a software component, and are emerging as powerful tools in several water management contexts [6][7]. according to power’s classification [8], it is proper to consider abstract machine learning techniques are employed to describe the temporal behavior of soil moisture using meteorological data as inputs. three different artificial neural network models, a feedforward multi-layer perceptron, a long-short term memory and the adaptive network-based fuzzy inference system, are trained and their results are compared. the soil moisture is expressed in terms of soil water index, derived from satellite retrievals, with the last known value also being used as input. the results are promising as the proposed methodology relies on free-access data with a worldwide coverage, allowing to easily estimate the forthcoming soil moisture. the knowledge of the expected value of this variable could be extremely useful for irrigation scheduling and it is the basis of decision support systems to efficiently manage water resources in agriculture. mailto:amarini@idea-re.eu acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 60 data-driven, knowledge-driven, and model-driven dsss (or a mix of the three) for use in irrigation management and scheduling. the common feature of dsss in the hydrological context is the collection of in-situ and remotely sensed variables describing the past and current status of the physical processes, and their elaboration aims to forecast their evolution so that decisionmakers can act promptly. in this way, real innovative irrigation ‘smart grids’ can be realised. in a cultivated area, knowledge of sm may give an indication of crop status and help decisionmakers decide whether to schedule irrigation or not. thus, monitoring the values of sm and forecasting its evolution are among the primary features that should be implemented in a dss aimed at optimally managing irrigation. sm can be monitored in several ways. the most efficient one is by direct in-situ measurements through specific sensor devices, such as time domain reflectometry (tdr) [9] or frequency domain reflectometry (fdr) probes [10]. however, at the time of writing, the actual diffusion of these sensors is quite limited, and this does not allow a reasonably uniform cover of the variable in extended areas [11]. paying the price of the lower accuracy and spatial resolution of the measurements, an alternative way of deriving water content information is by means of satellite data, thanks to recently developed earth observation programmes [12]. the great advantage in using satellite data is that they are easily accessible and also provide worldwide coverage. moreover, the knowledge of the forthcoming sm evolution could improve irrigation scheduling, e.g. avoiding water supply if the sm is estimated to increase because of weather variables. the temporal dynamic of sm can be estimated through physical, conceptual, or data-driven models [13]. the latter type is used in this study to carry out an exploratory quantitative analysis on the temporal behaviour of sm in agricultural fields. the use of machine learning has become widespread in many different contexts, ranging from medical [14] to industrial [15] applications. in the hydrological and environmental fields, machine learning is widely used to predict the future behaviour of several relevant variables [16]. in this study, using machine learning techniques, three artificial neural network (ann) models are trained to predict sm; subsequently, they are tested, and their results are compared. among the models’ inputs, remotely sensed data are used as a measure of the water content in soil. the main objective of this article is to develop a model that describes sm evolution in terms of meteorological data, which are clearly among the key factors affecting it. this will allow the forecasting of the future behaviour of the water content in the fields; thus, it will possibly represent the basis of a dss for water management in irrigation. 2. materials and methods a specific case study is considered by selecting a limited geographic area in the italian region of veneto. in particular, the area taken into account is located between the cities of venice and padua, next to the venetian lagoon (figure 1). this can be considered as a strategic location for the analysis, since it has traditionally been devoted to agriculture and is furnished with a wide and diffused water grid used for irrigation and land reclamation. the proposed approach makes use of remotely sensed sm data retrieved from copernicus, the earth observation program developed by the european union and european space agency. in particular, the sm, which is the target variable to be modelled, is described through the soil water index (swi), which provides an estimate of water content at various depths in the soil and is computed based on satellite measurements of the surface soil moisture (ssm) [17]. ssm and, consequently, swi are expressed as relative soil moisture, i.e. percentage of saturation. swi data can be accessed freely at the copernicus global land service [18], which is part of the copernicus programme. the copernicus swi data for europe have a 1 km resolution and are based on ssm data from sentinel-1/c-band synthetic-aperture radar (sar) and metop/ascat sensors [19], [20]. swi allows for the control of sm at different depths through parameter t: characteristic time length. this parameter describes the temporal dynamics of the water flux below the surface: increasing values of t correspond to deeper soil layers. the great advantage in using swi is that it requires fixing only one parameter, namely t, which is usually calibrated by means of comparison with probe measurements at the soil depth of interest [21]. copernicus global land service provides swi values for eight different characteristic time lengths, varying from 1 to 100. the value of the characteristic time length that was taken into account in this study is t = 15. of course, the same approach presented here can be equally repeated for other choices of t. the meteorological data taken into account in the analysis are the daily measurements of rainfall; minimum and maximum temperatures; average relative humidity; and wind speed. these data are measured by both ground-based and remote meters (weather radars) and are provided by radarmeteo. the meteorological data are available for the years 2015-2018, so the analysis is focused on this time period. the selected methodology involves using machine learning techniques, specifically ann models, to estimate the future evolution of swi given the available meteorological information. the last observed swi value, which, due to processing time, is figure 1. study area. acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 61 assumed to be available four days after the observation, is also used as input. thus, at time t the last available swi value is assumed to be 𝑆𝑊𝐼𝑡−4. the target is the expected difference, δswi, between the present-day swi and the last observed swi value. table 1 summarises the symbolism used for the models’ variables. three ann models have been taken into account in this study: a multi-layer perceptron (mlp), a long short-term memory (lstm) network [22], and an adaptive network-based fuzzy inference system (anfis) model [23]. a single hidden layer structure is selected to implement the mlp (figure 2). the number of units in the hidden layer is 40. the mlp inputs are 𝑅𝑡, 𝑅𝑡−1, 𝑅𝑡−2, 𝑅𝑡−3, 𝑅𝑡−4, 𝑇𝑡, 𝑇𝑡−4,𝑡−1, 𝑅𝐻𝑡, 𝑅𝐻𝑡−4,𝑡−1, 𝑤𝑠𝑡 , 𝑤𝑠𝑡−4,𝑡−1, 𝑆𝑊𝐼𝑡−4. note that the meteorological variables on day t are also used as inputs of the model. in real predictive applications, forecasted values have to be used for such inputs. the lstm network belongs to the wider class of recurrent neural networks (rnns), i.e. those with feedback connections that allow information to persist over the data sequence. this characteristic makes them naturally suited for modelling time series datasets. the second model taken into account in this study is an ann with one hidden layer formed by 20 lstm cells (figure 3). the inputs used in this setup are 𝑅𝑡, 𝑇𝑡 𝑚𝑖𝑛 , 𝑇𝑡 𝑚𝑎𝑥 , 𝑅𝐻𝑡, 𝑤𝑠𝑡 , 𝑆𝑊𝐼𝑡−4. in this case, it is sufficient to pass the current values of the variables as inputs, since the model is already capable – by itself – of keeping the relevant past information for the predictions. anfis is a particular type of ann used to implement fuzzy inference systems (fiss), i.e. inference rules based on the fuzzy logic concept [24][25]. anfis has a fixed structure of five layers corresponding to the five fis steps (input variables fuzzification, antecedent values combination, implication, aggregation of the consequents, and defuzzification of the output variable). since the number of anfis parameters requiring optimization grows exponentially with the number of inputs, the available meteorological data have been aggregated differently with respect to the two previous considered models in order to keep the number of inputs small. thus, the inputs used in the anfis model are 𝑅𝑡−4,𝑡 σ , 𝑇𝑡−4,𝑡−1, 𝑅𝐻𝑡−4,𝑡−1, 𝑤𝑠𝑡−4,𝑡−1, 𝑆𝑊𝐼𝑡−4. a schematic of the anfis network is depicted in figure 4. for each anfis input, three membership functions defined in equation (1) are used: table 1. input symbols. symbol description 𝑅𝑡 rainfall on day t 𝑇𝑡 min, 𝑇𝑡 max minimum and maximum temperatures on day t 𝑇𝑡 mean temperature on day t (a) 𝑅𝐻𝑡 mean relative humidity on day t 𝑤𝑠𝑡 mean wind speed on day t 𝑆𝑊𝐼𝑡 soil water index on day t 𝑋𝑡1,𝑡2 mean of variable x over days t1 ÷ t2 𝑋𝑡1,𝑡2 σ cumulative value of variable x over days t1 ÷ t2 (a) the mean temperature 𝑇𝑡 has been computed as the arithmetic mean of the minimum and the maximum daily temperatures. figure 2. scheme of the mlp network. figure 3. scheme of the lstm network. figure 4. scheme of the anfis network. acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 62 𝑓memb(𝑥; 𝜇, 𝜎, 𝜈, 𝛼) = exp [ − |𝑥 − 𝜇|𝜈 𝜎 2 1 1 + 𝑒−𝛼 𝑥 ] (1) each membership function has four free parameters, μ, σ, ν, and α, and can be seen as a deformation of the gaussian. indeed, setting ν = 2 and α = 0 yields the usual gaussian function, with mean μ and variance σ2. the parameter ν allows for different peak shapes, while the sigmoid factor (1 + 𝑒−𝛼 𝑥)−1 allows for skewness. it is customary in machine learning applications to split the available dataset into three subsets: the training, the validation, and the test sets. the training set, which usually comprises the most data, is used to feed the learning algorithm. the validation set is used for the selection of the model’s hyperparameters or for comparing models. the test set is only used at the end to evaluate the performance of the selected model (observing how well the model works when applied to unseen data). in this analysis, in order to exploit as much data as possible for training, the data has been split only into training and validation sets, disregarding the test set. in practice, the validation set has been used both for the model selection and for the final assessment of the selected model. though this is not optimal from a statistical point of view, this strategy is quite commonly used when the amount of available data is quite limited. for all the three models, data from january 2015 to december 2017 are used for training, and data from january to december 2018 are used for validation. as is usual in such modelling, the dataset has been normalised in order to facilitate the training procedure. the normalisation has been performed in such a way that all the variables have zero mean and unitary standard deviation, i.e. by computing the standard scores. the mean squared error of the output variable has been used as loss function regularisation strategies have been exploited in order to keep possible overfitting issues under control. for the mlp and lstmnetworks the dropout technique has been used [26]. instead, in the anfis model, an l2 regularisation penalty for the weights of the consequent part of the if-then rules has been added to the loss function. the training has been performed by minimising this loss function by means of adam optimisation [27]. the training procedure has been carried out 100 times for all the considered models, and the values of the parameters have been set to the ones giving the least value of the mean squared error for the validation set. the models have been evaluated by means of root mean square error (rmse) and the nash-sutcliffe efficiency (nse) index computed for the validation data. 3. results at the end of the training phase, the best mlp, lstm, and anfis models have been selected according to the abovementioned criteria, and a deeper analysis of the results has been carried out. figure 5 shows the scatter plots of the predicted vs. observed δswi for the three selected models. the blue dots refer to the training set, and the orange ones to the validation set. from these plots, a good agreement of the predictions can be observed when the actual δswi is in the intermediate region (approximately between −7 and 7). conversely, the predictions seem to be less accurate in the correspondence of the remaining external regions of the observed δswi, where the points deviate more markedly figure 5. scatter plots of the predicted vs. observed δswi for the three models. acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 63 from the perfect forecast line: in these regions, the models tend to underestimate the amplitude of the real δswi. it can be clearly observed that this effect is sharper for the mlp and the anfis models than for the lstm. a possible explanation of this issue may be related to the small number of occurrences of such events in the available dataset. note, however, that the difficulty of correctly reproducing extreme observations is also a known problem in machine learning modelling [28]. the plots in figure 6 show the temporal behaviour of the δswi predicted by the mlp, lstm, and anfis models on the validation set, along with the corresponding observed values. figure 6. predicted and observed δswi for the validation set. acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 64 the evaluation metrics, nse and rmse, are shown in table 2 for both the training and validation data. in particular, the metrics for the validation set are the most interesting ones for an assessment of the models. the results in table 2 shows that among the three models taken into account, the one that has the best performance, both in terms of the least rmse and higher nse, is the lstm one. the anfis model slightly outperforms the mlp. it is interesting to note that the rmse on the validation set and the rmse on the training set are quite similar. this is quite surprising, since one usually expects a worsening of the performance metrics on the validation set compared to that of the training set. indeed, for the anfis model, the validation rmse is even better than the training rmse. this ‘strange’ behaviour is probably due to the presence of a higher number of outliers in the training set with respect to the validation set. 4. conclusions this article presented an approach for modelling sm using machine learning techniques. sm is expressed in terms of swi retrieved from satellite measurements. these products are released on the copernicus website with a time delay of two days after observation. to be cautious, it has been assumed that the last available swi value dates back to four days earlier. thus, the variation of swi in a four-day period has been modelled through three different network-based models: a single hidden layer feedforward mlp, an lstm network, and an anfis. meteorological data, comprising rainfall, temperature, relative humidity, and wind speed records, have been used as inputs in both models along with the last known swi value. in the qualitative analysis, the temporal behaviours of the predicted and the observed δswi appear to correlate well. yet, at the quantitative level, the agreement is not always satisfying, especially when the observed value lies in the tail of the δswi distribution. however, when modelling a complex phenomenon using only a limited fraction of the actual set of variables influencing it, it is essential for the measured data to be extremely accurate and numerous. on the contrary, in this research, the available dataset spans over just four years (only three were used for training), and it could be that it has been insufficiently extended so as to guarantee an adequate generalisation capability to the models. moreover, while they have been extremely useful thanks to their worldwide coverage and daily frequency, the swi data obviously do not have the same accuracy as in-situ measurements. finally, due to lack of historical records, all the variables influencing the system status related to human interventions, e.g. irrigation supplies, have been neglected. the comparison of the results of the models that have been taken into account indicates that the lstm model outperforms the other two selected anns. the output of the models, i.e. δswi, may be added to the last known swi value, 𝑆𝑊𝐼𝑡−4, in order to obtain the predicted swi, swipred, which is what the decision-makers are eventually interested in. the time series of the observed swi and the lstm swipred are shown in figure 7. the results of the research are encouraging, as they show that in the absence of networks of in-situ sensors, satellite-derived measurements of sm can be forecasted by machine learning models, using simple meteorological data as inputs. this could be extremely useful in managing water resources, since sm is a fundamental variable to be considered when supplying irrigation. in principle, the proposed approach can be applied in all those settings where measurements from different sources over large areas (whether ground-based or remote sensing), meteorological data, and any quantitative information need to be processed to provide synthetic outputs for the final user. the exploitation of artificial intelligence techniques allows for modelling complex and highly nonlinear processes. furthermore, this factor makes it possible to avoid the use of those parameters that would be necessary in physical modelling and would require extensive field campaigns to characterise the study area or should be estimated during model calibration. for example, no information about the hydraulic and pedologic characteristics of the soil is provided to the ann models presented in this study. one limitation of the proposed approach is that it allows for the modelling of sm on a scale that is larger than the typical plot table 2. model results. training validation nse rmse nse rmse mlp 0.590 2.579 0.397 2.599 lstm 0.746 2.027 0.606 2.113 anfis 0.558 2.677 0.468 2.440 figure 7. swi time series reconstructed from the lstm model. acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 65 dimension. the resolution of this approach is indeed set by that of the satellite data, namely 1 km. however, this resolution is sufficiently high so as to provide a decision support tool for the authorities that are responsible for irrigation scheduling over large areas. moreover, even if the spatial resolution of this kind of model is coarse compared to the typical field dimensions, valuable information can be applied also on a smaller scale. indeed, it has been proven that sm follows a similar behaviour at global and local scales, and this allows for the downscaling of satellite measurements of this variable [29]. acknowledgement the authors are grateful to acque risorgive consorzio di bonifica (www.acquerisorgive.it) for providing the information and the data of the water channels system it manages. we also thank l. brocca (research institute for geo-hydrological protection irpi, italian national research council) for the useful discussion and the advice on the use of 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[29] l. brocca, t. tullo, f. melone, t. moramarco, r. morbidelli, catchment scale soil moisture spatial–temporal variability, journal of hydrology 422 (2012) pp. 63-75. https://land.copernicus.eu/global/products/swi https://land.copernicus.eu/global/sites/cgls.vito.be/files/products/cglops1_atbd_swi1km-v1_i1.20.pdf https://land.copernicus.eu/global/sites/cgls.vito.be/files/products/cglops1_atbd_swi1km-v1_i1.20.pdf fringe projection profilometry for recovering 2.5d shape of ancient coins acta imeko issn: 2221-870x march 2021, volume 10, number 1, 142 149 acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 142 fringe-projection profilometry for recovering 2.5d shape of ancient coins giuseppe schirripa spagnolo1, lorenzo cozzella1, fabio leccese2 1 università degli studi roma tre, dipartimento di matematica e fisica, rome, italy 2 università degli studi roma tre, dipartimento di scienze l, rome, italy section: research paper keywords: temporal phase unwrapping; profilometry; cultural heritage; 2.5d reconstruction citation: giuseppe schirripa spagnolo, lorenzo cozzella, fabio leccese, fringe projection profilometry for recovering 2.5d shape of ancient coins, acta imeko, vol. 10, no. 1, article 19, march 2021, identifier: imeko-acta-10 (2021)-01-19 section editor: carlo carobbi, university of florence, italy received may 14, 2020; in final form august 5, 2020; published march 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: fabio leccese, e-mail: fabio.leccese@uniroma3.it 1. introduction given its importance in many fields, ranging from quality control to medicine, from robotics to solid modelling, surveys of surface profiles by means of non-contact optical systems is the subject of extensive studies [1]: • in the field of cultural heritage, 3d acquisition is used primarily for documentation, comparative studies [2], [3], remote examination [4], [5] and degradation monitoring [6] as well as to create virtual museums [7, 8]. • in medicine, 3d scanners are useful for obtaining accurate anthropometric body segment parameters for the biomechanical analysis of human motion [9] and for the accurate acquisition of patient dentition [10]. • in the field of forensics, 3d reconstructions are helpful for the analysis of writing [11], [12], the virtual reconstruction of crime scenes [13], firearm examination proficiency tests [14] and face recognition [15], [16] • in quality control, 3d scanning can be used for production control and tool wear analysis [17]-[20]. furthermore, it can be used for underwater 3d reconstruction [21], [22]. in the field of cultural heritage conservation, both the determination of surface finish (surface texture) and its variations over time are important. in fact, in order to establish the actual vulnerability of cultural assets, one of the most important parameters to determine is the loss of material or, more generally, the determination of microstructural changes in the exposed surface (currently, for these determinations, indirect methods are used). therefore, the objective of the present research is to develop a non-contact optoelectronic system capable of analysing, at high resolution, the surface profile of artistic finds. the basic methodology to be used for the development of the system is the one known as grating projection or structured light [23]-[25]. this method, supported by optoelectronic signalprocessing techniques, allows the development of a system characterised by its high precision, small footprint and relatively low production cost. 2. the range finder projected-fringe profilometry is one of the most effective methods of measuring the 2.5d surface profiles of rough abstract the form of relief is undoubtedly one of the most topical subjects in the field of cultural heritage. physical access to historic and artistic products can be limited by a number of factors. for example, access to collections of ancient coins is difficult, especially for students. indeed, coin digital archives containing high-quality three-dimensional models and that can be accessed remotely are of great interest. the use of projected fringes for the measurement of surface profiles is a well-developed technique. in this paper, we present a surfaceprofile measurement system for small cultural heritage objects where it is important not only to detect the shape with a high degree of accuracy but also to capture and archive any signs of ageing. the equipment presented in this paper is simple, reliable and cheap. in addition, some examples are presented to demonstrate the potential of the proposed scheme for recovering 2.5d shapes of cultural heritage objects. mailto:fabio.leccese@uniroma3.it acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 143 engineering surfaces [26]-[28]. the fringe can be generated by projecting parallel light through optical grating slides with various projecting patterns, such as sinusoidal-like modulation. the traditional phase-shifting method, which involves physically moving a grating or a reference mirror, has two problems, namely, inefficiency due to mechanical movement and unavoidable phase-shifting errors incurred during the phaseshifting process. therefore, a digital micro-mirror device has been proposed to generate flexible structured-light patterns with highly precise phase shifting. this paper presents a surface-profile measurement method for micro-components based on the fringe-projection, phaseshifting technique and temporal phase unwrapping. linear sinusoidal fringe patterns are projected onto an artwork surface by a digital fringe projector. the projected sinusoidal fringes will be deformed by surface height variations. by detecting the deformed sinusoidal fringes in combination with phase stepping and temporal phase unwrapping, high-precision 2.5d surface measurements can be efficiently acquired. as shown in figure 1, the system is a telecentric phaseshifting projected fringe profilometry tool for small objects [29]. our tool consists of three main modules: a digital fringe projector, a charged-coupled device (ccd) camera and a computer. telecentric projection is realised by means of a commercially available digital light processing video projector (1920 × 1200 resolution), in which the projection lens is replaced by a telecentric lens. the optical telecentric imaging consists of a b/w ccd camera with a resolution of 1280 × 1020 and a telecentric lens. the measurement field size is about 120 mm × 100 mm and the depth of field reaches 10 mm, which is usually sufficient to measure the depth of a small, complex 3d object (i.e. an ancient coin). the optical axis of the projector is perpendicular to the reference plane and intersects with that of the camera at the reference plane. the line connecting the exit pupil of the projector and the entrance pupil of the camera is parallel to the reference plane. figure 2 shows the constructed system. structured light with sinusoidal fringes is projected onto the object surface. due to the depth variation of the surface, the structured light is phase modulated, leading to a deformed spatial carrier fringe pattern in which the topographic information of the object surface has been encoded. without loss of generality, it is assumed that the fringes are in the x–y plane and perpendicular to the y-axis. the optical geometry of the phase-measuring techniques is shown in figure 3. the intensity distribution of the fringe-pattern image can be expressed as [30]-[33] 𝐼(𝑥, 𝑦) = 𝐼0(𝑥, 𝑦) + 𝑀(𝑥, 𝑦) cos [ 2𝜋 𝑑 𝑦 + 𝜑(𝑥, 𝑦)], (1) where (𝑥, 𝑦) denotes an arbitrary point in the image, 𝐼0(𝑥, 𝑦) is the background intensity, 𝑀(𝑥, 𝑦) is the intensity modulation amplitude, 𝑑 = 𝑝0 𝑐𝑜𝑠𝜃 is the period of the fringe seen by the ccd camera on the reference plane and 𝜑(𝑥, 𝑦) is a phase change related to the 2.5d profile of the measured object. in other figure 1. basic setup for measuring surface profiles with projected fringes. figure 2. the layout of the fringe-projection system. figure 3. optical geometry for fringe projection. in the realised system, we have θ  45°; field of view x  100 mm, y  120 mm; max z  10 mm; minimum period of projected fringe 𝑝0  0.5 mm; minimum period of the fringe seen by the ccd 𝑑  1.0 mm. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 144 words, phase 𝜑(𝑥, 𝑦) contains the information about the surface height/depth variations. the relationship between the phase distribution and the profile information 𝑧(𝑥, 𝑦) of the measured object can be achieved by [34]-[37] 1 𝑧(𝑥, 𝑦) = 𝑎(𝑥, 𝑦) + 𝑏(𝑥, 𝑦) 1 𝜑(𝑥, 𝑦) + 𝑐(𝑥, 𝑦) 1 𝜑2(𝑥, 𝑦) , (2) where 𝑎(𝑥, 𝑦), 𝑏(𝑥, 𝑦) and 𝑐(𝑥, 𝑦) are the parameters related to the measurement system, which can be determined by a calibration process in advance [38]-[40]. to calculate the system constants 𝑎(𝑥, 𝑦), 𝑏(𝑥, 𝑦) and 𝑐(𝑥, 𝑦), we use the least-squares method. as shown in figure 4, during the calibration procedure, the reference plane is shifted to 𝑛 position along the direction of the object height. each new position has height 𝑧𝑛 with respect to the zero position (𝑧0). to determine the system constants 𝑎(𝑥, 𝑦), 𝑏(𝑥, 𝑦) and 𝑐(𝑥, 𝑦), we define the error function as 𝜀(𝑥, 𝑦) = ∑ { 1 𝑧(𝑥, 𝑦) − [𝑎(𝑥, 𝑦) + 𝑏(𝑥, 𝑦) ∙ 1 𝜑𝑛(𝑥, 𝑦) 𝑁 𝑛=1 + 𝑏(𝑥, 𝑦) ∙ 1 𝜑𝑛 2(𝑥, 𝑦) ]} 2 , (3) where 𝑁 is the number of shifts of the standard reference plane. in the least-squares sense, the system constants 𝑎(𝑥, 𝑦), 𝑏(𝑥, 𝑦) and 𝑐(𝑥, 𝑦) can be directly estimated by minimising the sum of square 𝜀(𝑥, 𝑦). the minimum error is at the point where the partial derivatives of 𝜀(𝑥, 𝑦) with respect to 𝑎(𝑥, 𝑦), 𝑏(𝑥, 𝑦) and 𝑐(𝑥, 𝑦) are all zero. therefore, we have [35] [ 𝑎(𝑥, 𝑦) 𝑏(𝑥, 𝑦) 𝑐(𝑥, 𝑦) ] = [ 𝑁 ∑ 1 𝑧𝑛(𝑥, 𝑦) 𝑁 1 ∑ 1 𝑧𝑛 2(𝑥, 𝑦) 𝑁 1 ∑ 1 𝑧𝑛(𝑥, 𝑦) 𝑁 1 ∑ 1 𝑧𝑛 2(𝑥, 𝑦) 𝑁 1 ∑ 1 𝑧𝑛 3(𝑥, 𝑦) 𝑁 1 ∑ 1 𝑧𝑛 2(𝑥, 𝑦) 𝑁 1 ∑ 1 𝑧𝑛 2(𝑥, 𝑦) 𝑁 1 ∑ 1 𝑧𝑛 4(𝑥, 𝑦) 𝑁 1 ] −1 x [ ∑∙ 1 𝜑𝑛(𝑥, 𝑦) 𝑁 1 ∑ 1 𝑧𝑛(𝑥, 𝑦) ∙ 1 𝜑𝑛(𝑥, 𝑦) 𝑁 1 ∑ 1 𝑧𝑛 2(𝑥, 𝑦) 𝑁 1 ∙ 1 𝜑𝑛(𝑥, 𝑦)] . (4) thus, if the phase map of 𝑁 planes and the relative positions 𝑧𝑛 are known, the system constants can be obtained from equation 4. for the phase calculation, it is possible to use the fast fourier transform algorithm [41]-[43] or phase-shifting technique [23]. generally, phase 𝜙(𝑥, 𝑦) = 2𝜋 ∙ 𝑦 𝑑 + 𝜑(𝑥, 𝑦) can be retrieved using the phase-shifting method. with this technique, a set of phase-shifted sinusoidal fringe patterns is projected onto the object’s surface. the intensity values, captured by the ccd camera, can be expressed as [44]-[47] 𝐼𝑛(𝑥, 𝑦) = 𝐼0(𝑥, 𝑦) + 𝑀(𝑥, 𝑦) cos [𝜙(𝑥, 𝑦) + 𝑛 − 1 𝑁 2𝜋] , (5) where 𝑛 represents the phase-shift index 𝑛 = 1 , 2 , … , 𝑁 . the values of phase 𝜙(𝑥, 𝑦) can be calculated by 𝜙(𝑥, 𝑦) = − tan−1 [ ∑ 𝐼𝑛(𝑥, 𝑦) sin ( 𝑛 − 1 𝑁 2𝜋)𝑁𝑛=1 ∑ 𝐼𝑛(𝑥, 𝑦)cos ( 𝑛 − 1 𝑁 2𝜋)𝑁𝑛=1 ] . (6) since there are three unknowns, 𝐼0(𝑥, 𝑦), 𝑀(𝑥, 𝑦) and 𝜙(𝑥, 𝑦), in equation 5, at least three images, 𝐼1(𝑥, 𝑦), 𝐼2(𝑥, 𝑦) and 𝐼3(𝑥, 𝑦), should be used to enable the calculation of 𝜙(𝑥, 𝑦). in equation 5, we considered the intensity of the sinusoidal fringes. however, the nonlinearity of the luminance caused by the gamma effect of the digital camera and projector introduces significant harmonics that lead to errors in the phase determination [48]-[51]. considering the harmonics, and indicating with 𝐻 the highest significant harmonic order of the captured fringes, equation 5 can be rewritten as [52] 𝐼𝑛(𝑥, 𝑦) = 𝐼0(𝑥, 𝑦)+ ∑ 𝑀𝑘(𝑥, 𝑦) cos [𝑘 (𝜙(𝑥, 𝑦)+ 𝑛-1 𝑁 2𝜋)] 𝐻 𝑘=1 (7) equation (7) involves (𝐻 + 2) unknowns: 𝐼0(𝑥, 𝑦), 𝑀1(𝑥, 𝑦), … , 𝑀𝑘(𝑥, 𝑦), … , 𝑀𝐻(𝑥, 𝑦) and 𝜙(𝑥, 𝑦); consequently, 𝑁 = 𝐻 + 2 phase-shifted fringe patterns are required to solve these unknowns. therefore, if we consider a set of 𝑁 = 𝐻 + 2 uniform phase-shifted sinusoidal fringe patterns, the values of the phase can be retrieved by means of 𝜙𝑤(𝑥, 𝑦) = − tan−1 [ ∑ 𝐼𝑛(𝑥, 𝑦) sin ( 𝑛 − 1 𝐻 + 2 2𝜋)𝐻+2𝑛=1 ∑ 𝐼𝑛(𝑥, 𝑦)cos ( 𝑛 − 1 𝐻 + 2 2𝜋)𝐻+2𝑛=1 ] . (8) equation 8 indicates that an 𝐻 + 2 uniform phase-shifted scheme can be employed to retrieve the phase accurately from the fringe patterns with nonlinear harmonics up to the 𝐻th order. generally, the highest significant harmonic order, in practice, is below eight. figure 4. the calibration process of the system constant. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 145 commonly, phase 𝜙(𝑥, 𝑦) can be distributed over an interval greater than 2𝜋. unfortunately, the arctan function, using the numerator and denominator signs, returns values of 𝜙𝑤(𝑥, 𝑦) only in the range from −𝜋 to +𝜋. in other words, the phase is rolled up in a 2𝜋 interval. superscript 𝑤 denotes a wrapped phase value. to recover the correct phase value, it is necessary to use an unwrapping phase procedure. this results in an unwrapped phase map, which is given by equation (9): 𝜙𝑤(𝑥, 𝑦) = 𝜙(𝑥, 𝑦) + 2𝜋 ∙ 𝑞(𝑥, 𝑦) , (9) where 𝑞(𝑥, 𝑦 ), the number of turns, is an integer that may be positive or negative. 3. phase unwrapping phase unwrapping is the restoration of the original phase values of an image, given the observed wrapped values. while, theoretically, this problem can be easily solved, this is not the case for ‘experimental’ wrapped phases. indeed, when a complex object is measured, abrupt and irregular changes in the measured surface may result in local shadows. with the use of computer-generated fringe patterns, however, the phase in the projected fringe pattern can be easily programmed. therefore, temporal phase unwrapping techniques can be used for recovered phase maps [53]-[55]. the temporal phase combines phase shifting with a change in fringe pitch (change in wavelength or distance between fringes). the strength of the method is that the problem with phase rolling never occurs and that it results in an absolute phase measurement. temporal phase unwrapping offers several potential advantages over the spatial method [56]: • errors do not propagate to the whole phase map • fringe order errors can be easily detected and filtered out ‘a posteriori’ since they are multiples of 2𝜋 • it does not require user intervention to indicate specimen edges or to provide the phase value at a particular point • it is very fast and does not require additional memory • the implementation is extremely simple. however, temporal phase unwrapping has the disadvantage of working well only with the hypothesis of static objects. in other words, throughout the process, it is critical that the measured quantities be of temporal invariance. the idea of temporal phase unwrapping is to use a sequence of fringe maps with a change in fringe pitch (see figure 5). in other words, frequency repeats single phase shifting 𝑀 times with periods of the fringe at 𝑑1, 𝑑2, … , 𝑑𝑡, … , 𝑑𝑀. in the fringe pattern, the relative phases are 𝜙(𝑥, 𝑦; 1), 𝜙(𝑥, 𝑦; 2), … , 𝜙(𝑥, 𝑦; 𝑡), … , 𝜙(𝑥, 𝑦; 𝑀). for subsequent time values (𝑡 = 1, 2, 4, . . ., 𝑀), the number of fringes is also set equal to 𝑡 so that the phase range increases to (−𝑡 ∙ 𝜋, +𝑡 ∙ 𝑛). there is a linear relationship between the phase and the number of the projected fringe. if one phase jump is projected, we have one fringe of wrapped phase map; when projecting two fringes, the wrapped phase map contains exactly two phase jumps. in temporal phase unwrapping, the phase difference is calculated from successive frames. this technique is inherently robust. wrapped phase measurements for each used fringe period are acquired sequentially in time. by means of equation 8, for each 𝑡, we obtain a wrapped phase. initially, a single fringe (𝑡 = 1) is projected so that the fringe phase range is only from −𝜋 to +𝜋. therefore, 𝜙𝑢(𝑥, 𝑦; 1) = 𝜙𝑤(𝑥, 𝑦; 1) . (10) for subsequent time values (𝑡 = 2, 4, . . ., 𝑀), the number of fringes is also set equal to 𝑡 so that the phase range increases to (−𝑡 ∙ 𝜋, +𝑡 ∙ 𝑛). with this procedure, for each 𝑡 value, 𝐻 + 2 phase-stepped images are acquired. the unwrapped phase distributions of the higher frequency fringes can be readily calculated without a general phase unwrapping. the algorithm can be expressed as follows [55], [56]: 𝜙𝑢(𝑥, 𝑦; 𝑡) = 𝜙𝑤(𝑥, 𝑦; 𝑡) + 𝑁𝐼𝑁𝑇 [ 𝜙𝑢(𝑥, 𝑦; 𝑡 − 1) − 𝜙𝑤(𝑥, 𝑦; 𝑡) 𝜋 ] ∙ 2𝜋 , (11) where 𝑁𝐼𝑁𝑇(. ) denotes rounding to the nearest integer. using an iterative process, the unwrapped phase can be computed by summing the wrapped phase difference between the successive phase maps. figure 6 shows an example of a sequence of fringe maps achieved with our system. figure 5. sequence of fringe maps projected onto an object surface. for each 𝑡 value, 𝐻 + 2 phase-stepped images are acquired. figure 6. example of fringe maps used in our system. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 146 in the presence of noise, as generally occurs, a better approach to the estimation of the unwrapped phase is a fitting, in the least-squares sense of the line defined by equation 9, to the experimental data [57]: �̂�𝑢(𝑥, 𝑦) = ∑ [𝑡 ∙ 𝜙𝑡 𝑢(𝑥, 𝑦; 𝑡)]𝑀𝑡=1 ∑ 𝑡2𝑀𝑡=1 . (12) since the algorithm involves only one single governing equation, it is very simple and easy to use. 4. experimental results and discussion first, some validation experiments were conducted. figure 7 shows the recovered 2.5d plot from a metallic specimen (2.5 cm × 2.5 cm) with a step of 50 μm in height. the specimen was realised by means of a grinding machine. a comparison between the conventional direct contact measurement and the optical results shows that the maximum discrepancy is about 2 μm. subsequently, we performed a test on a telephone token. figure 8 shows the recovered 2.5d plot of an italian telephone token, in use in italy from 1959 until 2001. dimensional measurements of the throat profile were obtained to check the quality of the 2.5d reconstruction. an example of a profile is shown in figure 9. again, the discrepancies between the optical measurement and direct contact measurement are less than 2 μm. considering the characteristics of our system, it is well suited to 2.5d coin reconstruction. in general, coins can be analysed in two and three dimensions. the advantage of 3d coin data is that it allows a more detailed and reliable analysis due to an exact description of the coin’s surface, which is a great benefit for any automated classification or recognition task [59]-[61]. to check the usability of the instrumentation for the 3d survey, we surveyed the surface topography of some coins. we initially acquired the relief of an old italian 10 lire coin used before 2000. figure 10 shows the recovered 2.5d plot; it can be noted that the three-dimensional details are correctly reconstructed. finally, the surface topography of a new zealand one-cent piece is shown in figure 11. on the reverse of the coin, a fern leaf is represented. in the 2.5d plot, all the details are correctly reconstructed. 5. pipeline for 3d colour model for a correct and accurate acquisition of the surface, it is essential that the colour characteristics are also acquired. in a typical 3d data elaboration pipeline, to obtain a complete 3d model of the acquired object from the raw data, the following steps should be followed: • acquisition of 2d colour information [62], [63]. • 3d geometric data acquisition. • data integration. • model conversion. figure 7. three-dimensional reconstruction of specimen with 50 μm calibrated steps. figure 8. recovered 2.5d italian telephone token (diameter ø = 24 mm). figure 9. depth profile plot of the telephone token throat. figure 10. recovered 3d plot of a coin: ‘10 italian lire’ (ear of corn; coin diameter ø = 23.3 mm). figure 11. recovered 2.5d plot of a coin: 1 cent, new zealand (standard circulation 1967–1985; ‘fern leaf’ on the reverse; coin diameter ø = 17.4 mm). acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 147 to test the usability of the proposed technique in the construction of a virtual gallery of ancient artifacts, we have digitised some ancient roman coins. figure 12 shows the data-processing pipeline steps used to obtain a complete 3d model of a roman sestertius of agrippina senior, who was born in 15 b.c. and died in exile in a.d. 33. this commemorative coin, bearing her name and portrait, was struck by her son, the emperor caligula (a.d. 37–41). figure 12(a) shows the colour photos of the coins (obverse and reverse), figure 12(b) provides the shared view of the geometry with surface details and figure 12(c) depicts the colour rendering of the coin. it is important to note that our system is only capable of a correct 2.5d reconstruction. therefore, the reverse side of the coin has not been reconstructed. 6. conclusions projected fringes for the measurement of surface profiles are extensively used in 2.5d optical metrology. however, low-cost, commercially available projectors are susceptible to gamma nonlinearity and a lack of depth of field, which distorts the fringe patterns and introduces significant errors in the phase measurements. in this work, we have presented a simple optical method that can be applied to the measurement of micro-surface profiles. in the proposed setup, the number of phase shifts applied in the projected grating can be changed via the user interface, depending on the accuracy required. the proposed device has the potential of the traditional fringe projection technique but with more versatility, offering the freedom to choose and control the fringe period. experiments using real samples have shown that the 2.5d profiling measured using the proposed system is efficient and effective. furthermore, the method relies on very simple and cheap equipment. the proposed system uses only consumer-available technology (a video projector and a digital camera) and can be achieved for less than $ 1,000. in other words, for less than $ 1,000, it is possible to achieve a system with precision and accuracy that is comparable to much more complex and expensive systems (> $ 5,000). references [1] g. sansoni, m. trebeschi, f. docchio, 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https://doi.org/10.1117/12.840203 https://doi.org/10.1364/josa.57.001105 https://doi.org/10.1109/isccsp.2012.6217832 methodological issues of metrological analysis of planned medieval towns and villages acta imeko issn: 2221-870x march 2021, volume 10, number 1, 217 223 acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 217 methodological issues of metrological analysis of planned medieval towns and villages maria legut-pintal1, anna kubicka2 1 university of wrocław, institute of archaeology, ul. szewska 48, 50-137 wrocław, poland 2 warsaw university of technology, department of geodesy and cartography, plac politechniki 1, 00-661 warszawa, poland section: research paper keywords: modular analysis; unit of measurement; lower silesia; parcellation; cosine quantogram citation: anna maria kubicka, maria legut-pintal, methodological issues of metrological analysis of planned medieval towns and villages, acta imeko, vol. 10, no. 1, article 29, march 2021, identifier: imeko-acta-10 (2021)-01-29 editor: ioan tudosa, university of sannio, italy received may 15, 2020; in final form november 13, 2020; published march 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: the paper presents the preliminary results of a project funded by the polish national science centre, no. 2020/39/d/hs3/01549. corresponding author: maria legut-pintal, e-mail: maria.legut-pintal@uwr.edu.pl 1. introduction imagine you are a surveyor, hired to plan a new town in the middle of nowhere using only a rope and a stick. is it possible that the effects of your work will still be visible after hundreds of years? the issue of surveying and dividing the area inside the walls of a medieval town has been the subject of academic debate [1][4]. research on the reconstruction of the primary layout of medieval towns in central europe has a long tradition [5]-[7]. this debate has mainly been stimulated by historians, architects and urban planners; however, a few archaeologists have also influenced the discussion. central europe from the 12th to the 16th century was a place of urban and rural revolution. as a result of colonisation, hundreds of towns and thousands of villages were newly established. the ‘regular’ town was a result of the implementation of german law on spatial dimensions [8]. towns with market squares and an orthogonal grid of streets appeared at the beginning of the 13th century in silesia, a region in today's poland. these silesian towns had a regular gridded urban pattern, which made them very distinct in the cultural setting of central europe. urban pattern analysis, using the modular method, has been conducted for numerous silesian towns. these studies have resulted in a better understanding of the medieval cityscape; for example, the exact sizes of primary plots have been estimated. during this period, some improvements in the layout of towns were made. the main observed tendency was that, in the 13th century, the city grid was ‘optimised’. the layout of the town centre evolved from a wide market street to a rectangular or square market the width of the plot decreased, the blocks of buildings were shortened, and the size and proportions of market squares became standardised [9]. the developed model of a silesian regular town spread further, to be used in neighbouring lands. in addition, the findings of archaeological excavations have extended the knowledge of the planning of medieval towns [10], [11]. it is worth noting that, in many cases, archaeological abstract in this paper, we illustrate problems related to the application of two methods used for the reconstruction of the parcellation of medieval towns: modular analysis and the cosine quantogram. we propose the usage of the cosine quantogram, which is rarely used to study urban layout, for the identification of units of measurement in regular medieval towns. based on two examples from the region of silesia, poland, namysłów and dzierżoniów, we discuss the usefulness of both methods in the reconstruction of the original measuring system and town layout. for the first time, we have applied these two methods to the study of the layout of a regular village. despite the limitations, such as the level of precision in the construction of a medieval town’s layout at the time of its foundation, later changes to the division of plots and the known inaccuracy of modern maps, it seems that combining both methods allows the determination of the module used for the planning of the original urban layout. the application of the cosine quantogram can be useful for improving the results of modular analysis. nevertheless, the results of both methods should be verified against archaeological results and historical research. mailto:maria.legut-pintal@uwr.edu.pl acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 218 discoveries have confirmed the theoretical consideration of metrological analysis. metrological analyses related to urban and rural planning are based on the assumption that in the case of regular settlements founded on previously uninhabited areas (lat. in cruda radice), the arranged space was specified in the contract between the locator and the founder (territorial lord king, duke, knight, bishop, etc.); the area, the number of plots and their sizes were measured in the field applying algorithms. despite subsequent changes, in many cases, in the division of plots and their development, it is possible to reconstruct the original ‘ideal’ plan. decoding the original intention of the planner is the starting point for analysing subsequent changes in urban or rural space and studying their causes. in this paper, we will discuss the potential of metrological methods and their limitations in the study of regular towns and villages based on selected examples from the silesian region. we aim to address the following questions: were medieval towns planned with mathematical precision? and if so, is it possible after all these years, to reconstruct the idea of medieval urban planning based on modern cadastral plans and terrain surveys? is it then possible to apply the methodology of urban analysis to regular villages? 2. methods in this paper we combine two methods of analysis, the modular analysis of a town layout and the cosine quantogram algorithm, which are applied to a series of measurements of a town. these methods represent two different approaches. in the first, the base unit of measurement is considered as known, from written sources, for example, and is well established. in the second method, the base unit is unknown and determined using statistical methods. 2.1. modular analysis the method used for the spatial analysis of an historic town’s plan consists of applying information from historical, iconographic, archaeological and architectural sources to the modern geodetic plan to reconstruct the transformation of the original city morphology [12]. this approach also includes the use of metrological methods. modular analysis or the meteorological–geometric analysis method assumes the existence of a module that can be found in the city structure (e.g. dimensions of the market square and blocks of buildings or the width of streets). among the lengths of measurement of basic objects in a town’s plan, a pattern of ‘round’ multiplications (50, 100, 150) of the base-unit (foot) length is sought to establish which measurement system was used by medieval surveyors for planning. having established a base measurement system, the whole plan is fitted into a rope-length grid, recalculating the lengths according to the multiplication of the rope lengths. however, grid adjustments are heavily burdened with errors in approximations related to the irregularity of the city and the fact that the original starting point from which it was measured is unknown. it is uncertain whether rope-length grid was commonly used as a too. while in some cases the rope-length grid was certainly used to determine the city's area and mark out the market square, in other cases the determination of the main axes of the town might have be sufficient. to reconstruct the original width of urban plots, a simple method is used. it consists of summing up the length of facades of neighbouring plots converted to the basic foot unit. its advantage is in enabling the partial elimination of border shifts between neighbours resulting, for instance, from the possibility of erecting a firewall on the neighbour’s plot or hereditary divisions. however, the main problem is often an arbitrary determination of the size of the basic unit. the modular analysis method has been widely criticised for its susceptibility to errors resulting from inaccurate measurements made on historical plans and a dependence on the arbitrarily selected basic length unit. thanks to new methods of analysis, in particular the possibility of measuring using geodetic ground plans or the use of gis software to apply information from various sources, the modular analysis method has returned to favour. nevertheless, the allegation that the underlying units of measurement are arbitrary has still not been resolved. 2.2. cosine quantogram the cosine quantogram is a method for reconstructing the smallest basic unit used in the design and construction process. the cosine quantogram algorithm was developed by d. g. kendall to detect a quantum of unknown size from a set of data [13]. a statistical model of a cosine quantogram has successfully been implemented for the analysis of architectural sites of mediterranean culture, studies on the layout of early medieval anglo-saxon settlements [14] and even in the meteorological analysis of architectural units in the incan architecture of machu picchu [15]. known examples of the method's use suggest that it could also be applied to the analyses of medieval regular cities. in the case of medieval town planning, a data set for metrological analysis is composed of parcel dimensions. each parcel dimension can be described as an integer m, multiplied by the basic unit q plus an error ɛ (epsilon); the error might be the result of inaccuracy in the layout of the medieval city as well as modern methods of measurement: 𝑋𝑖 = 𝑀𝑖𝑞 + ɛ𝑖 . (1) in the equation, ɛ, which is significantly smaller than q, is analysed and then the formula is applied to calculate an amount that clusters around q (2). the value of q, which maximises the formula within a given range, is the one with the highest probability of being a quantum: 𝑓(𝑞) = √ 2 𝑁 ∙ ∑ cos ( 2 π ɛ𝑖 𝑞 ) 𝑛 𝑖=1 , (2) where n is the sample size and the set of building dimensions. the term √2 𝑁⁄ adds the dependence of the cosine quantogram result to the sample size. the main problem of the application of the cosine quantogram to urban studies is the small size of the sample. we can usually obtain only a dozen, sometimes a few dozen, measurements. to ensure that the estimated quantum is neither evaluated by chance nor dominated by outlier measurements, we have applied the monte carlo bootstrap method, used to construct confidence intervals, to estimate the most likely quantum value range. assuming a 5% significance level, if the quantum estimation for the original data falls within the confidence interval, the risk that the quantum is the product of chance can be interpreted as very low. 3. case studies for an evaluation of the described methods, we selected two regular towns from silesia, namysłów and dzierżoniów, acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 219 founded in the advanced phase of colonisation. both towns were founded in cruda radice, so no previous settlements affected their shape. in both towns, architectural and archaeological excavation confirmed the existence of medieval masonry from buildings, whose presence can provide information on the position of medieval plot boundaries. to check whether the methods used for town studies can also be applied to village research, we chose for our analysis one of the most regular silesian villages, mnichowice. the main question was, if the boundaries between the plots had never been fixed by walls, but only by wooden fences, would it be possible to decipher the original division units? 3.1. namysłów namysłów was founded, according to written sources, around 1250 [16]. the layout of the town is quite well preserved. small changes to block layouts are connected with the existence of wooden construction and arcades around the market square as well as the introduction of some new developments after the damage caused by world war ii. nevertheless, the northern frontage of the market square was fully built from bricks in the 14th – 15th century, which froze the medieval boundaries of plots. this allows the measurements determined using the cadastral plans of the town in the field and their accuracy to be verified by measuring the façades of the tenement houses. first, we verified the results of previous studies on namysłów parcellation [15]. the quantum was estimated based on the series of archive data on plot dimensions that had previously been analysed using the neighbouring plot method. the cosine quantogram function provides clear results in relation to the estimated quantum from the archive data set, 0.25 m (figure 1a), which does not match the results of the neighbouring plot method, proposed by [17], in which the module of 0.31 m was re-established in the urban reconstruction of namysłów. this first step in the study of namysłów reveals that the two different methods of measurement analysis of the same data achieve two different results. therefore, in the next step of the module search in relation to regular town planning, we applied cosine quantogram analysis to the set of measurements collected for this study in the field from namysłów’s main square. the estimated value of the quantum from 16 field measurements was 0.155 m (figure 1b), which is half of the foot standard established from written sources 0.313 m, the socalled renish foot. the maximum of the function shows the smallest unit present in the set of measurements; therefore, we should interpret the renish foot standard as the module used in namysłów’s medieval parcellation of tenant houses in the main square. the final results of both methods of measurement analysis showed the same results in the search for modules in namysłów’s main square. however, the results of the first method could not be confirmed by the second method, the cosine quantogram. the difference in results from the same data was influenced by the accuracy to which the actual measurement values were rounded, which made it impossible to obtain the same results with different methods. nevertheless, the application of the cosine quantogram proved that, in the case of namysłów’s buildings, the division of the medieval plot is still preserved. in the second step, with an already confirmed unit of measurement, the 0.31 m renish foot, we applied modular analysis to the town's plan (figure 2). we built the modular grids based on three different units used in medieval poland: 44.5 m, based on a 0.296 m foot (green lines); 43.2 m, based on a foot of figure 1. a) quantum estimation from the archive data set for namysłów; b) quantum estimation from the data set collected during a field survey in namysłów. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 220 0.288 m (kulm/flemish foot; blue lines); and 46.5 m, based on a foot of 0.31 m (renish foot, red lines). we then compared the results. the third step was to fit a grid to the crucial elements of the medieval town. our reference for fitting the grid included the location and orientation of the most important elements of the town, such as the main frontage of the blocks (the west frontage) and the very well-preserved north frontage of the main square. the grid borders should finish within the line of fortification and be adjusted to the main elements of the town. the differences between the matching of the individual grids were not large, and it was possible to say that most elements from the preserved layout of the city fit into a rope grid with a side of 46.5 m (foot of 0.31 m). using the standard unit of the renish foot, the market has dimensions of 3 × 1.5 ropes, and the area of the whole regularly planned town probably fit into a grid 6 × 9 ropes. the results from applying three different methods of town planning analysis demonstrate the uniformity of the urban space in which the same unit of measurement (the renish foot) was used to divide housing and plan the grid of the whole medieval town. however, the internal divisions of the blocks seem to be independent of the rope grid, which marks the main elements and the city border. 3.2. dzierżoniów the founding of dzierżoniów (ger. reichenbach), took place around 1260 [17]. the organisation of the city centre is based on the model of a cross, with a centrally located market square and streets intersecting in the centre. the course of the walls surrounding the city is almost round. as with the previous case, we started the analysis of dzierżoniów’s plan with a metrological analysis of block parcellation. in this case, there are no similar studies with which to compare the results. the data set for the parcellation measurement came from the cadastral plans of the eastern wing of the market square. however, archaeological surveys conducted on the eastern block document the significant transformation of the earlier buildings. for this reason, we used a reconstruction of the width of each parcel for analysis, which is still visible in the modern town plan. the reconstruction of the potential medieval plot division is based on the results of the modular analysis and archaeological survey as well as an analysis of the preserved boundaries. we verified the existence of a potential quantum in a reconstructed model of the town (without later divisions of plots). the maximum value of the quantum was 0.272 m (figure 3). the bootstrap simulation also confirmed the figure 2. namysłów: matching different grid sizes to the town plan. green lines module 44.5 m, blues lines module 43.2 m, red lines module 46.5 m, red dots location of town gates. figure 3. quantum estimation for dzierżoniów’s data. data set was based on a reconstructed plot division of the eastern frontage. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 221 results of 0.272 m as the best candidate for the basic unit of measurement (figure 4). however, the value differed from that expected by 0.016 m. this error does not seem large if we consider that the measurements were taken from a plan with an accuracy of about 0.10 m on the plot borders. even if the quantum value obtained as a result of the distorted measurements is not identical to that expected, the quantum can be considered an indication of the unit used at a specific time period. in this example, the value of the quantum of 0.272 m is closer to the polish foot (0.288 m) than the renish (0.313 m). we presume that more precise measurements made in the field can bring the quantum value closer to the original unit of measurement. in the second step in the study of medieval town planning based on modular analysis and calculating the width of neighbouring plots, we tried fitting the rope-length grids into the town plan, in the same way as with namysłów (figure 5). the most regular part of the town was the market, with dimensions of around 2 × 3 ropes. although the western and eastern fronts fit into the grid well, the southern and northern frontages did not. seemingly regular streets appeared to diverge after the imposition of the grid, which is to a certain extent the result of uneven terrain and the town's location on a hill. it seems that the area of the medieval town fit into a square of 10 × 10 ropes. the most probable rope length used was that of 43.2 m (blue lines), which corresponds to a foot of 0.288 m. 3.3. mnichowice mnichowice is a small village in the district of bralin, in kępno county, greater poland province. this area historically belonged to silesia. the first mention of the village dates to 1220, at which time it was the property of the augustinian monastery in wrocław. the reorganisation of village according to germanlaw is confirmed in a document from duke henry iv dated 25 february 1276, and it is probably after this date that the village was transformed into a regular angerdorf shape. the angerdorftype villages, with oblong village greens and two rectangular blocks of farmsteads on either side, are typical for german-law villages in the lowlands, especially in saxony, silesia, bohemia and pomerania [18]. the ground plan of mnichowice’s intravilan (built-up area) is very regular, almost rectangular, and its border has been strengthened along the roads. the village green was affected by transformations in the modern period; it was partially built up, and plot owners have attached part of the square to their farmsteads. the original border of the village green has partly been preserved by the location of rural houses, which have generally been built in the same place since the middle ages. the ideal concept of mnichowice village consisted of a rectangular village green with a width of one rope (approx. 43.2 m) and two rows of farmsteads (a 2.5 rope width approx. 106 figure 4. dzierżoniów: the bootstrap confidence interval for the quantum is 0.272 m. figure 5. dzierżoniów: matching different rope-length grids to the town plan. green lines module 44.5 m, blues lines module 43.2 m, red lines module 46.5 m, blue dots location of town gates. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 222 m). the reconstructed area of the intravilan was 6 × 18 ropes, which is 0.75 of a lesser lan (ger. kleine hube an old unit of area used in lands of german colonisation). although it is not proven that the blocks of buildings were to be divided into equal plots, it seems that in this case a certain module was used (figure 6). the width of the plots ranged between 34 m and 37 m. in one case, the width of the plot was double and was 72.59 m. the maximum value of the quantum from the measurements of the plot widths was 36.29 m (figure 7). on this basis, we can conclude that the division into plots was independent of the distribution of the general layout of the village, and a rope of 43.2 m was not used. instead, the length of the whole intravilan was measured using a different module to plot division. for the plot division, the value of about 36 m was equal to the so-called ‘small’ rope; 125 feet was used to delineate the parcellation. small ropes are not confirmed in written sources from silesia, but they were in use in neighbouring lesser poland [19]. in order to measure the integer number of equal plots in the whole length of the village, it is necessary to find the least common multiple of both moduli. if we consider the use of two types of rope, 150 feet and 125 feet, the common multiple would be 750 feet. mnichowice’s intravilan is about 2,690 feet, and this length is not an integer multiplication of this module. this caused irregularities in plot division, visible at both ends of the village. the usage of different modules of division may also be explained by the idea that the designation of internal roads shortened the road from the centre of the village to the fields. 4. discussion studies on the reconstruction of the layout of medieval towns face many limitations related to the subject of research and the methods used. the first group of limitations is caused by inaccuracies made at the time the settlement was founded, resulting from, for example, human error, terrain conditions and the imperfection of tools used during delineation, such as ropes and rods. even in the most regular towns, only the central area, near the market square, is regular, and the further from the market, the more irregularities there are. this is caused by the gradual process of the building of towns, which took decades. the second group of limitations is the result of numerous transformations in the original layout. in many cases, central european small towns were developed primarily with wooden buildings, which did not perpetuate the original boundaries, including the arcades that occupied space in the streets or the market square. even the early introduction of masonry buildings did not preserve the original boundaries, as the law allowed the construction of frontier walls using a neighbour’s plot. numerous distortions in the original arrangement of blocks of buildings have also been lost as a result of divisions and the linking of neighbouring plots. in addition, fortifications built even a hundred years after the town's founding usually enclosed the town within an oval border, cutting off the rope-length grid and intersecting adjacent plots. the third group of limitations is connected with inaccuracies in the modern plans used for analysis. the accuracy of large-scale maps is about 0.10 m. this value is rather large when we think of determining units of length of 0.27–0.35 m. even taking accurate measurements in the field does not guarantee that the original boundaries of the plot will be captured because the boundaries of the façades of individual objects are not the same as the plot boundary. only in a few examples of silesian towns are the boundaries of plots confirmed by archaeological excavation (e.g. wrocław). the fourth group of limitations is related to the specification of methods. one of the aims of the research is to establish relatively small differences between the basic measurements of each system of town planning. in the case of the cosine quantogram method, the main risks and disadvantages are related to having a sufficient quantity of measurements with credible values. it is only in the towns of silesia and lesser poland that known systems of measurement exist in which basic values varied. depending on the time period, location, role and foundation of figure 6. mnichowice: current plot division (blue line) and reconstruction of an ideal plan of the village core (grey). figure 7. quantum estimation from the data set of block parcellation in mnichowice village. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 223 the town, we can expect differences in the module value. in some cases, we have written sources in which the standard measurements were mentioned, but in the vast majority of cases, only analysis can provide information about the measurement used to plan the layout of the town. 5. conclusion the results presented above indicate that the application of metrological methods in urban history studies has many limitations and requires a holistic and interdisciplinary approach. despite these limitations, it seems that the use of the cosine quantogram method in combination with traditional modular analysis may provide some new opportunities. in some cases, as in namysłów, confirmation of the basic unit may be possible. in others, it may indicate the most likely basic unit. the example of the village of mnichowice confirms that the cosine quantogram may also be used to establish larger modules (rope), not only small units (foot). in further studies, we also aim to apply the methodology we have adopted to regular settlements founded in cruda radice in western europe, for example, in france and italy. the appearance of stone houses there, built shortly after the founding process, should affect the stabilisation of the measured boundaries in the city structure; therefore, the results obtained in the measurement analysis should be more accurate. references [1] h. keller, die ostdeutsche kolonialstadt des 13. jahrhunderts und ihre südländischen vorbilder, f. steiner, wiesbaden, 1979, isbn -10: 3515032460. [2] w. boerejfin, the foundation, planning and building of new towns in the 13th and 14th centuries in europe: an architecturalhistorical research into urban form and its creation, amsterdam institute for humanities research, amsterdam, 2010, isbn 9789090251578. [3] k. humper, m. schenk, entdeckung der mittelalterlichen stadtplanung. das ende vom mythos der gewachsenen stadt, theiss, stuttgart, 2001, isbn-10: 3806214646. [4] u. untermann, strassen, areae, stadtmauern, mittelalterliche stadtplanung im licht der archäologie, in: städteplanungplanungsstädte. b. fritzsche, hj. gilomen, m. stercken (editors). chronos, zürich, 2006, isbn 10: 3034007620, pp. 39-50. [5] t. zagrodzki, regularny plan miasta średniowiecznego a limitacja miernicza, studia wczesnośredniowieczne 5 (1962), issn 00817031. [6] t. kozaczewski, wielkość i program budowy miasta średniowiecznego, wydawn. politechniki wrocławskiej, wrocław, 1972. [7] j. pudełko, próba pomiarowej metody badania planów niektórych miast średniowiecznych w oparciu o zagadnienie działki, kwartalnik architektury i urbanistyki 9 (1964) pp. 3-28. [8] g. köster, c. link, faszination stadt: die urbanisierung europas im mittelalter und das magdeburger recht, sandstein kommunikation, magdeburg, 2020, isbn -10: 3954984539. [9] r. eysymontt, kod genetyczny miasta. średniowieczne miasta lokacyjne dolnego śląska na tle urbanistyki europejskiej, via nova, wrocław, 2009, isbn 978-83-60544-556. [10] c. buśko, stan badań nad parcelą mieszczańską w średniowiecznych miastach śląskich, kwartalnik historii kultury materialnej 43 (1995) pp. 343-350. [11] j. piekalski, centrum średniowiecznego miasta jako problem badawczy archeologa, wratislavia antiqua 2 (2000) pp. 11-15. [12] t. zarębska, badania historyczno-urbanistyczne metodą analiz przestrzennych, kwartalnik historii kultury materialnej 43 (1995) pp. 15-32. [13] d. g. kendall, hunting quanta, philosophical transactions of the royal society of london, series a, mathematical and physical science 276 (1974) pp. 231-266. [14] j. blair, modules for anglo-saxon constructions. appendix to grid-planning in anglo-saxon settlements: the short perch and the four-perch module, anglo-saxon studies in archaeology and history 18 (2013) pp. 55-57. [15] a. kubicka, the metrological research of the machu picchu site. application of a cosine quantogram method for 3d laser data, acta imeko 6 (2017) 3, 52-56. doi: 10.21014/acta_imeko.v6i3.460 [16] namysłów, in: atlas historyczny miast polskich, vol. 4 śląsk nr. 11, m. młynarska-kaletynowa (editor). iae pan, wrocław, 2015, isbn 978-83-63760-58-8, pp. 53-59. [17] m. m. wańkowicz, rola działki miejskiej w procesie lokacyjnym na przykładzie miasta dzierżoniowa, wiadomości konserwatorskie 20 (2006) pp. 53-59. [18] m. hardt, formen und wege der hochmittelalterlichen siedlungsgründung, in: ostsiedlung und landesausbau in sachsen. die kührener urkunde von 1154 und ihr historisches umfeld. e. bünz (editor). leipzig, 2008, isbn-10: 3865831656, pp.143-160. [19] b. krasnowolski, lokacyjne układy urbanistyczne na obszarze ziemi krakowskiej w xiii i xiv wieku. cz. 1, miasta ziemi krakowskiej, chronologia procesów osadniczych i typologia układów urbanistycznych, wydawnictwo naukowe akademii pedagogicznej, kraków, 2004, isbn 83-7271-276-x, p. 138. http://dx.doi.org/10.21014/acta_imeko.v6i3.460 acta imeko issn: 2221-870x december 2020, volume 9, number 4 acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 0 journal contacts about the journal acta imeko is an e-journal reporting on the contributions on the state and progress of the science and technology of measurement. the articles are mainly based on presentations presented at imeko workshops, symposia and congresses. the journal is published by imeko, the international measurement confederation. the issn, the international identifier for serials, is 2221-870x. editor‐in‐chief dušan agrež, slovenia founding editor‐in‐chief paul p. l. regtien, netherlands associate editor dirk röske, germany editorial board section editors (vol. 7 9) leopoldo angrisani, italy filippo attivissimo, italy eulalia balestieri, italy eric benoit, france paolo carbone, italy lorenzo ciani, italy catalin damian, romania pasquale daponte, italy luca de vito, italy luigi ferrigno, italy edoardo fiorucci, italy alistair forbes, united kingdom helena geirinhas ramos, portugal sabrina grassini, italy fernando janeiro, portugal konrad jedrzejewski, poland andy knott, united kingdom francesco lamonaca, italy massimo lazzaroni, italy fabio leccese, italy rosario morello, italy michele norgia, italy pedro miguel pinto ramos, portugal nicola pompeo, italy sergio rapuano, italy dirk röske, germany alexandru salceanu, romania constantin sarmasanu, romania lorenzo scalise, italy emiliano schena, italy enrico silva, italy krzysztof stepien, poland marco tarabini, italy yvan baudoin, belgium marcantonio catelani, italy lorenzo ciani, italy egidio de benedeto, italy alessandro depari, italy alessandro germak, italy konrad jędrzejewski, poland min-seok kim, korea vilmos palfi, hungary franco pavese, italy jeerasak pitakarnnop, thailand emiliano sisinni, italy jan saliga, slovakia jorge c. torres-guzman, mexico ian veldman, south africa rugkanawan wongpithayadisai, thailand claudia zoani, italy mauro d’arco, italy oscar tamburis, italy francesco bonavolonta, italy about imeko the international measurement confederation, imeko, is an international federation of actually 42 national member organisations individually concerned with the advancement of measurement technology. its fundamental objectives are the promotion of international interchange of scientific and technical information in the field of measurement, and the enhancement of international co-operation among scientists and engineers from research and industry. addresses principal contact prof. dušan agrež university of ljubljana faculty of electrical engineering tržaška 25, 1000 ljubljana, slovenia e-mail: dusan.agrez@fe.uni-lj.si acta imeko prof. leopoldo angrisani university of naples "federico ii" dept. of information technology and electrical engineering via claudio 21, 80125 napoli, italy e-mail: angrisan@unina.it support contact dr. dirk röske physikalisch-technische bundesanstalt (ptb) bundesallee 100, 38116 braunschweig, germany e-mail: dirk.roeske@ptb.de mailto:dusan.agrez@fe.uni-lj.si mailto:angrisan@unina.it mailto:dirk.roeske@ptb.de strategies for the design of additively manufactured nanocomposite scaffolds for hard tissue regeneration acta imeko issn: 2221-870x december 2020, volume 9, number 4, 53 59 acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 53 strategies for the design of additively manufactured nanocomposite scaffolds for hard tissue regeneration pierpaolo fucile1, ilaria onofrio2, ida papallo1, vito gallicchio2, andrea rega2, vincenzo d’antò2, giovanni improta3, roberto de santis4, antonio gloria4, teresa russo4 1 department of advanced biomedical sciences, university of naples federico ii, 80131 naples, italy 2 department of neurosciences, reproductive and odontostomatological sciences, university of naples federico ii, 80131 naples, italy 3 department of public health, university of naples federico ii, 80131 naples, italy 4 institute of polymers, composites and biomaterials – national research council of italy, 80125 naples, italy section: research paper keywords: additive manufacturing; reverse engineering; scaffold design and analysis; design of experiments; nanocomposites citation: pierpaolo fucile, ilaria onofrio, ida papallo, vito gallicchio, andrea rega, vincenzo d'antò, giovanni improta, roberto de santis, antonio gloria, teresa russo, strategies for the design of additively manufactured nanocomposite scaffolds for hard tissue regeneration, acta imeko, vol. 9, no. 4, article 7, december 2020, identifier: imeko-acta-09 (2020)-04-07 section editor: leopoldo angrisani, university of naples federico ii, italy received october 31, 2019; in final form february 12, 2020; published december 2020 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding authors: pierpaolo fucile, e-mail: pierpaolo.fucile@unina.it, ilaria onofrio, e-mail: ilaria.onofrio@unina.it 1. introduction bone is capable of healing and remodelling itself except in the case of defects that exceed a critical size [1], [2]. autografts are generally considered the gold standard for bone reconstruction. even though it is reported that there is no risk of device rejection or disease transmission, many complications arise due to poor availability, prolonged hospitalisation, donor-site morbidity and pain, and high risk of infection and haematoma [1]-[3]. for this reason, allografts may be seen as an alternative to autografts but their clinical applications are strongly limited by the risk of pathogenic disease transmission as well as low integration with native tissues [4]-[9]. the use of metallic or ceramic man-made devices may be considered to be an interesting alternative system since these devices can immediately act as a mechanical support, providing the structural stability that is generally needed for the bonehealing process. however, the use of metallic devices may lead to the risk of bone resorption and fracture as a consequence of the low torsion of the great mismatch between the mechanical properties of the implant and the bone. alternatively, the brittleness of ceramic devices with high osteointegration and osteoinduction properties clearly limits their use [4], [9]-[16]. thus, for many years, novel approaches based on the combination of scaffolds with cells and/or biomolecules have been gaining importance as an intriguing strategy with which to overcome the above reported drawbacks [16]. the great challenge should be the design of a suitable biomechanical environment for cell growth and the consequent new tissue formation. over previous years, different combinations of scaffold design strategies, materials, biomolecules, and cells have been abstract additive manufacturing represents a powerful tool for the direct fabrication of lightweight and porous structures with tuneable properties. in this study, a fused deposition modelling/3d fibre deposition technique was considered for designing 3d nanocomposite scaffolds with specific architectures and tailored biological, mechanical, and mass transport properties. 3d poly(ε-caprolactone) (pcl)/hydroxyapatite (ha) nanocomposite scaffolds were designed for bone tissue engineering. an optimisation design strategy for the additive manufacturing processes based on extrusion/injection methods was at first extended to the development of the pcl/ha scaffolds. further insight into the effect of the process parameters on the mechanical properties and morphological features of the nanocomposite scaffolds was provided. the nanocomposite structures were analysed at different levels, and the possibility of designing 3d customised scaffolds for mandibular defect regeneration (i.e., symphysis and ramus) was also reported. mailto:pierpaolo.fucile@unina.it mailto:ilaria.onofrio@unina.it acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 54 widely investigated in an attempt to promote an effective interaction with the native tissue [17], [18]. in the field of tissue engineering, poly(ε-caprolactone) (pcl), which is an aliphatic polyester, represents one of the most commonly used biodegradable polymers, due to its interesting processability, biodegradation rate, and high thermal and chemical stability [19]-[21]. the development of ceramic materials with a composition similar to the bone mineral phase, such as hydroxyapatite (ha) and tricalcium phosphate, has potentially led to the possibility of improving both the bioactivity and the mechanical properties of the neat polymeric scaffolds [22]-[24]. with regard to bone tissue regeneration, pcl/ha nanocomposite scaffolds with tailored architectures and mechanical and mass transport properties have already been fabricated by additive manufacturing techniques (i.e., fused deposition modelling/3d fibre deposition) that allow the development of customised structures as well as the control of pore geometry and spatial distribution [16]. nevertheless, it is well known that the process and instrument parameters play an important role in determining the mechanical properties and morphological features of additively manufactured scaffolds. in this context, the influence of the process parameters on these characteristics has previously been studied in the case of 3d additively manufactured pcl scaffolds obtained through a bioextruder [25]. in particular, great efforts were devoted to the study of the effect of the deposition velocity (dv), screw rotation velocity (srv), slice thickness (st), and process temperature (pt) to find the best set of parameters for the fabrication of pcl scaffolds with enhanced properties and reproducibility [25]. in the processing of nanocomposite materials, the difficulties are usually greater than those encountered for the neat polymers. accordingly, taking into account the previously obtained results for the neat pcl structures [25], in this study, an optimisation design strategy for additive manufacturing processes based on extrusion/injection methods was first employed to develop pcl/ha nanocomposite scaffolds for hard tissue regeneration. the nanocomposite scaffolds were analysed at different levels, and examples of strategies for the development of customised scaffolds were reported. 2. materials and methods pcl/ha nanocomposite pellets were first developed and then processed through the fused deposition modelling (fdm)/3d fibre deposition technique. specifically, pcl (mw = 65000; sigma-aldrich, st. louis, mo) pellets were dissolved in tetrahydrofuran (thf; sigmaaldrich, st. louis, mo) while stirring at room temperature. ha nanoparticles and, then, ethanol was added to the solution. a pcl/ha weight ratio (w/w) of 90/10 was considered, and an ultrasonic bath (branson 1510 mt, danbury, ct) was employed for the dispersion of the nanoparticles in the pcl/thf solution. pcl/ha nanocomposite pellets were processed using a bioextruder [25] to fabricate 3d scaffolds (length l of 7.0 mm, width w of 7.0 mm, and height h0 of 8.0 mm), characterised by a 0/90° lay-down pattern. a nozzle with an inner diameter of 400 µm was used to extrude/inject the material. the nanocomposite fibres/filaments were deposited according to the selected sequence of stacking (i.e., lay-down pattern). a fibre spacing (i.e., filament distance [fd]) of 1,000 µm was used. the pcl/ha scaffolds were manufactured using three different values of the st, dv, srv, and pt, as shown in table 1. with regard to the fabrication of the devices, one parameter was varied iteratively, while maintaining the other three as constant parameters. the morphology of the scaffolds was investigated by scanning electron microscopy, focusing on the filament diameter, strand distance (centre-to-centre distance), and layer thickness. the mechanical behaviour of the 3d scaffolds was appropriately analysed. compression tests on the 3d scaffolds and nanoindentation analyses on the scaffold fibres were performed to assess the effect of the inclusion of ha nanoparticles on the mechanical behaviour and local surface properties. in particular, mechanical compression tests were carried out on the fabricated 3d pcl/ha scaffolds. the structures were tested at a cross-head speed of 1 mm/min up to a strain of 0.4 mm/mm, using an instron 5566 testing system. the ‘apparent’ stress σ and strain ε were calculated as reported below [16], [25]: 𝜎 = 𝐹 𝐿 ∙ 𝑊 (1) 𝜀 = ∆𝐻 𝐻0 (2) with f representing the force measured by the load cell, whereas ∆h represents the height variation of the device. the slope of the initial linear portion of the stress–strain curve was considered to determine the compressive modulus. for each set of parameters (table 1), five specimens were mechanically tested nanoindentation analyses were performed using the nanotest platform (micromaterials, u.k.) in a specific load range (1–5 mn). a diamond pyramid-shaped berkovich-type indenter tip was employed. the trapezoidal load functions characterised by the specific values for the load hold periods (i.e., 20 s) and the loading–unloading rates (i.e., 300 µn/s) were considered. using the oliver and pharr method, the hardness values were evaluated from the load-depth curves. the hardness (h) was calculated as follows: 𝐻 = 𝐿max 𝐴c (3) where ac and lmax are the projected contact area and the applied peak load, respectively. the biological performances of the fabricated pcl/ha scaffolds were assessed to analyse the effect of the nanoparticle inclusion. briefly, pcl/ha scaffolds were prepared for cell seeding following a reported protocol [16]. pcl and pcl/ha scaffolds table 1. manufacturing parameters: slice thickness (st), deposition velocity (dv), screw rotation velocity (srv) and process temperature (pt). st (µm) dv (mm/s) srv (rpm) pt (°c) 350 8 20 120 380 10 25 130 400 12 30 140 acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 55 were seeded with bone marrow-derived human mesenchymal stem cells (hmscs) using 1 × 104 cells/sample. the cell viability was evaluated at different time points using the alamar blue assay (abd serotec ltd,uk). the cell adhesion and spreading were analysed at different time points using confocal laser scanning microscopy (clsm) and rhodamine phalloidin staining. the image j software was employed, and a shape factor was introduced to analyse the clsm images of the cell-scaffold constructs [23]. the shape factor was calculated as follows: 𝜙 = 4𝜋𝐴 𝑃2 (4) with a and p representing the area of a cell and the perimeter, respectively. considering that circular objects are characterised by the greatest area-to-perimeter ratio, a shape factor of 1 represents a perfect circle. thin thread-like objects have the lowest shape factor, which approaches zero [23]. an example of the design and production process of customised pcl/ha nanocomposite scaffolds for mandibular defect regeneration (i.e., symphysis and ramus) was also reported, integrating different methodologies and approaches (material synthesis/preparation, reverse engineering, and additive manufacturing), as seen in figure 1. computed tomography (ct) was performed to acquire the image and, consequently, the shape and size of a human mandible. the obtained point clouds were appropriately processed. rapidform software and materialise magics were used for the reconstruction of the 3d model. 3. results the roles of reverse engineering [26]-[29], computer-aided design, and finite element analysis [30]-[34] have been frequently stressed in the literature. in addition, over previous years, the advances in methodologies and design strategies have pushed the research towards the development of novel structures for different fields of application [35]-[41]. the functional behaviour of 3d additive manufactured scaffolds is clearly dependent on the geometric and architectural features, as well as on the pore spatial distribution. concerning the development of additive manufactured scaffolds, many studies have already demonstrated the possibility of properly tailoring the road width (rw) by varying the instrument and process parameters at a fixed nozzle size [25]. in the current study, a nozzle with an inner diameter of 400 µm was employed to manufacture the pcl/ha nanocomposite scaffolds. in particular, an already considered approach to develop additive manufactured pcl scaffolds [25] was used. the manufacturing parameters were selected to obtain a value of the rw that was equivalent to the inner nozzle diameter (400 µm), attempting to reduce the fabrication time and to maintain the highest reproducibility without significant alteration of the structural stability of the devices. the results obtained from the experimental analyses evidenced the influence of the investigated parameters (pt, srv, dv, and st) on the flow behaviour of the material, which clearly resulted in changes in terms of the rw. such variations provided the 3d scaffolds with different morphological and mechanical features. the obtained stress–strain curves (figure 2) were similar to those found for the 3d additive manufactured scaffolds [16],[25]. the temperature was initially varied (120 °c, 130 °c, and 140 °c) at fixed values of the srv (30 rpm), dv (10 mm/s), and st (400 µm). when the temperature increased from 120 °c to 140 °c, an increase of the rw was evident (table 2). conversely, the porosity values decreased (table 2) due to a reduction in the pore height and the pore width (lg and fg, respectively; data not reported). the findings confirmed the effect of the pt on the morphological features. in particular, a thickening of the filament and a decrease of the scaffold porosity were found with the pt increasing from 120 °c to 140 °c. table 2. rw, porosity, compressive modulus, and maximum stress of 3d pcl/ha scaffolds achieved for different pt values (dv = 10 mm/s, srv = 30 rpm and st = 400 µm). the data are reported as mean value ± standard deviation. pt (°c) rw (µm) porosity (%) compressive modulus (mpa) maximum stress (mpa) 120 449 ± 5 60.0 ± 1.2 120.1 ± 11.4 13.5 ± 1.4 130 477 ± 7 55.0 ± 1.2 124.1 ± 12.3 14.8 ± 1.9 140 506 ± 5 50.9 ± 1.2 130.2 ± 14.0 13.3 ± 2.2 figure 1. design and production process of customised pcl/ha scaffolds for hard tissue regeneration. figure 2. typical results from compression tests. stress-strain curves for additive manufactured pcl/ha scaffolds with specific lay-down pattern and geometric features, tested up to a strain of 0.4 mm/mm. acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 56 in terms of the mechanical properties, the values of the compressive modulus and the maximum stress are shown in table 2. although differences in terms of the rw and porosity were observed at different pt values, the results suggested that both the compressive modulus and the maximum stress were not greatly affected if the pt was increased above 120 °c. the influence of the dv on the morphological and mechanical features was investigated by varying the dv (8, 10, and 12 mm/s) at fixed values of the srv (30 rpm), st (400 µm) and pt (120 °c). when the dv increased from 8 to 12 mm/s, the filament thinning provided an increase in the pore width and the scaffold porosity (table 3). table 3 suggests how the dv may influence both the compressive modulus and the maximum stress of the pcl/ha scaffolds. specifically, a high value of the dv should negatively affect the mechanical performances of the additive manufactured pcl/ha scaffolds. accordingly, in the fast deposition process, the filaments were too stretched, leading to a decrease of the rw and an increase of the porosity. in terms of the compressive modulus and the maximum stress, table 3 shows that the lowest values were obtained for a dv of 12 mm/s. the srv clearly influences the amount of the extruded/deposited material. to study the effect of the srv on the scaffold characteristics, the srv was varied (20, 25, and 30 rpm) at fixed values of the dv (10 mm/s), st (400 µm) and pt (120 °c). an increase of the rw was evident with the srv increasing from 20 to 30 rpm due to the higher amount of extruded material. however, higher srv values led to a decrease in the pore width (fg), the pore height (lg), and the porosity (table 4). as a consequence of the higher srv values, the thickening of the filament provided the pcl/ha structures with a lower porosity. the results reported in table 4 indicate that the highest values of the compressive modulus and the maximum stress were found for an srv of 30 rpm. the st also plays an important role in determining the filament diameter, pore width, pore height, and porosity, thus providing the possibility of tailoring the scaffold characteristics [25]. intuitively, a decrease of the st values generates a compression of the filaments in the adjacent layers and, hence, a change in the geometry of the filaments, which become more elliptical. this effect causes an increase of the filament diameter, together with a reduction of the pore width and porosity. as the st decreased from 400 to 350 µm, a decrease of the pore height was obviously obtained, as well as an increment of the filament diameter and, consequently, a reduction of the pore width and porosity (table 5). for this reason, the st would influence the mechanical properties. however, table 5 reports the values of the compressive modulus and the maximum stress. the current analysis was focused on the assessment of the optimal set of process/instrument parameters for the fabrication of pcl/ha nanocomposite scaffolds with minimal fabrication time avoiding to compromise the structural integrity and reproducibility features. accordingly, all the parameters were selected to find a value of the rw that was equivalent to the inner diameter of the employed nozzle (400 µm). as frequently stressed in the literature [25], additive manufacturing techniques based on extrusion/injection methods (i.e., fused deposition modelling/3d fibre deposition) enable the possibility of controlling the architectures, pore size, and distribution, consequently leading to the development of 3d scaffolds with tailored mechanical and mass transport properties. in this context, the present study analysed the effects of the pt, dv, srv, and st on the rw, which influence the pore size and porosity, as well as the compressive modulus and the maximum. the interaction of different pairs of parameters on the investigated features of the pcl/ha scaffolds (i.e., rw, compressive modulus, and maximum stress) is reported in figure 3–figure 5. for example, figure 3 illustrates the interaction plot for the rw, reporting the combined effects (pt and dv, pt and srv, pt and st) on the rw. similarly, the interaction plots for the maximum stress and the compressive modulus are shown in figure 4 and 5, respectively. table 6 summarises the optimised set of parameters for the manufacturing of 3d pcl/ha (90/10 w/w) scaffolds characterised by a pre-defined value of the rw, pore size, and porosity, without compromising the structural and mechanical characteristics. the results from the nanoindentation tests on the pcl fibres provided values of hardness ranging from 0.43 ± 0.03 gpa to 0.26 ± 0.02 gpa in the investigated load range (figure 6). table 3. rw, porosity, compressive modulus, and maximum stress of 3d pcl/ha scaffolds achieved for different dv values (pt = 120 °c, srv = 30 rpm and st = 400 µm). the data are reported as mean value ± standard deviation. dv (mm/s) rw (µm) porosity (%) compressive modulus (mpa) maximum stress (mpa) 8 479 ± 6 55.6 ± 1.3 128.1 ± 12.0 12.5 ± 1.3 10 449 ± 5 60.0 ± 1.2 120.1 ± 11.4 13.5 ± 1.4 12 431 ± 5 64.2 ± 1.3 91.5 ± 9.2 9.1 ± 0.9 table 4. rw, porosity, compressive modulus, and maximum stress of 3d pcl/ha scaffolds achieved for different srv values (pt = 120 °c, dv = 10 mm/s and st = 400 µm). the data are reported as mean value ± standard deviation. srv (rpm) rw (µm) porosity (%) compressive modulus (mpa) maximum stress (mpa) 20 381 ± 6 71.3 ± 1.7 65.6 ± 6.8 9.2 ± 0.8 25 430 ± 5 63.7 ± 1.4 81.3 ± 8.9 9.7 ± 0.8 30 449 ± 5 60.0 ± 1.2 120.1 ± 11.4 13.5 ± 1.4 table 5. rw, porosity, compressive modulus, and maximum stress of 3d pcl/ha scaffolds achieved for different st values (pt = 120 °c, dv = 10 mm/s and srv = 30 rpm). the data are reported as mean value ± standard deviation. st (µm) rw (µm) porosity (%) compressive modulus (mpa) maximum stress (mpa) 350 489 ± 6 46.8 ± 1.3 137.1 ± 13.8 15.2 ± 1.4 380 469 ± 6 53.3 ± 1.2 130.3 ± 13.1 14.4 ± 1.3 400 449 ± 5 60.0 ± 1.2 120.1 ± 11.4 13.5 ± 1.4 acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 57 the inclusion of ha nanoparticles led to an increase in the compressive modulus and the fibre hardness. in vitro biological tests were performed to evaluate the influence of the inorganic nanoparticles on the behaviour of the hmscs. typical results obtained from the alamar blue assay are reported in figure 7. with regard to the biological performances, the alamar blue assay was employed to assess the cell viability and proliferation. this method is based on a redox reaction that occurs in the cell mitochondria. the percentage of alamar blue reduction is related to the number of viable cells. specifically, a significant increase of alamar blue reduction was found over time, indicating that the hmscs could survive and proliferate throughout the scaffolds. although no differences were observed between the pcl/ha scaffolds and the pcl structures at day 1, the inclusion of ha significantly enhanced the cell viability/proliferation at day 3 and day 7 (figure 7). the clsm images and cell-shape factor further allowed investigation of the cell adhesion and spreading. the shape factor significantly decreased over time for both types of cell-laden constructs, and the typical values are reported in figure 8. figure 3. interaction plot for rw (µm). figure 4. interaction plot for maximum stress (mpa). figure 5. interaction plot for the compressive modulus (mpa). figure 6. results obtained from nanoindentation tests on the pcl fibres. hardness as a function of the applied load. the data are reported as the mean value, and the error bar represents the standard deviation. figure 7. percentage of alamar blue reduction for the pcl and the pcl/ha scaffolds at day 1, day 3, and day 7. the data are reported as the mean value, and the error bar represents the standard deviation. table 6. optimised set of parameters for the fabrication of pcl/ha (90/10 w/w) scaffolds through a bioextruder system. pt (°c) dv (mm/s) srv (rpm) st (µm) 120 10 30 400 figure 8. values of the shape factor obtained from the clsm images of the hmscs on the pcl and pcl/ha nanocomposite scaffolds. the data are reported as the mean value, and the error bar represents the standard deviation. acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 58 in comparison to the neat pcl scaffolds, at day 3 and day 7, a lower shape factor was achieved for the pcl/ha nanocomposite structures, even though at day 1 similar values were found for the two types of cell-laden constructs. it is also worth noting that a reduction of the shape factor should suggest better cell adhesion and spreading, since the lower the shape factor, the more elongated the cell [16]. these results confirmed the effect of the ha inclusion in enhancing cell adhesion. furthermore, considering the obtained results, the reverse engineering approach was also employed to develop customised and functional nanocomposite scaffolds for mandibular defect regeneration (i.e., symphysis and ramus; figure 9). 4. conclusions a systematic study on the design of pcl/ha scaffolds for hard tissue regeneration was reported in the current research. in particular, 3d pcl/ha scaffolds were designed and analysed according to a strategy already reported for additive manufacturing of pcl scaffolds, involving techniques based on extrusion/injection methods. the procedure was extended to pcl/ha, considering that the difficulties in processing nanocomposite materials are usually greater than those found for the neat polymers. the neat pcl scaffolds represented the starting point in the design process, and the results from different analyses were briefly summarised. the reported design strategy also aimed to stress the potential of tailoring the performances of 3d additive manufactured scaffolds through an appropriate material–design combination. similar stress–strain curves were achieved for the polymeric and the nanocomposite scaffolds, even if differences were obtained in terms of the mechanical properties. the inclusion of ha nanoparticles would enhance both the biological and mechanical performances of the pcl scaffolds. furthermore, the obtained findings demonstrated that the dv and the srv were the parameters with the highest impact on the investigated features (i.e., the rw, the compressive modulus, and the maximum stress). benefiting from all the results, as well as from the reverse engineering approach, the feasibility of designing customised scaffolds for mandibular defect regeneration (i.e., ramus and symphysis) was also reported. references [1] j. lee, m. m. farag, e. k. park, j. lim, h. yun, a simultaneous process of 3d magnesium phosphate scaffold fabrication and bioactive substance loading for hard tissue regeneration, material science and engineering: c 36(1) (2014), pp. 252-260. doi: https://doi.org/10.1016/j.msec.2013.12.007 [2] p. lichte, h. c. pape, t. 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december 2020, volume 9, number 5, 319 324 2d flow model for calculating effective area of piston-cylinder units w. sabuga1, a. s. hashad2, a, s. ehlers3 1 physikalisch-technische bundesanstalt (ptb), braunschweig, germany, wladimir.sabuga@ptb.de 2 national institute for standards (nis), giza, egypt, a guest researcher at ptb, ahmed_hashad84@hotmail.com 3 physikalisch-technische bundesanstalt (ptb), braunschweig, germany, sven.ehlers@ptb.de abstract: a 2d flow model is described for calculation of the effective area (𝐴) of pressure-measuring pistoncylinder units (pcu) based on their dimensional properties. with the 2d model, the uncertainty contribution associated with pcu's axial nonsymmetry can be eliminated and the uncertainty of 𝐴 can be reduced. the 2d model is applied to several primary pcus operated in absolute and gauge pressure modes with different pressuretransmitting media. the benefit of the 2d model in dependence on pcu's geometrical perfectness is discussed. keywords: pressure balance; effective area; dimensional data; flow model 1. introduction determination of 𝐴 from pcu's dimensional properties has become the main method applied by primary laboratories to realise the pressure unit by means of pressure balances. the uncertainty of this realisation can significantly depend on pcu's geometrical imperfectness, i.e. deviations of the piston and cylinder bore from a perfect cylindrical shape. axial variations of the piston-cylinder radii can be considered by applying 1d flow models, e.g. such as proposed by dadson et al. [1]. with the 1d flow models, tangential variations of the pcu radii are addressed by calculating 𝐴 values for different angular directions and taking their mean and standard deviation. the later can be a significant uncertainty factor in the case of axially nonsymmetrical pcus. with a 2d flow model, a calculation of 𝐴 becomes possible which takes into account variations of pcu radii in both, tangential and axial directions. herewith, a single 𝐴 value for the whole pcu is obtained, and the uncertainty contribution due to pcu's axial non-symmetry is removed. 2. theory in the first approximation, 𝐴 of a pcu can be taken as a mean cross-sectional area of the piston and cylinder bore. however, due to variability of piston and cylinder bore radii, 𝑟 = 𝑟(𝑧, ) and 𝑅 = 𝑅(𝑧, ) , along with pcu's axial and tangential coordinates 𝑧 and  , the uncertainty of such a determination is relatively high. variation of 𝑟 and 𝑅 with 𝑧 can be addressed in the 𝐴 determination by modelling the fluid flow in the axial direction. 2.1. 1d flow model in this model, the pcu is considered as axisymmetric with a gap profile corresponding to a selected tangential coordinate . for this particular , in the most general case, the effective area can be calculated by [1]: 𝐴 = π𝑟0 2 [1 + ℎ0 𝑟0 + 1 𝑟0(𝑝1−𝑝2) ∫ (𝑝 − 𝑙 0 𝑝2) d(𝑟+𝑅) d𝑧 d𝑧], (1) where 𝑟0 = 𝑟(0, ) , ℎ0 = ℎ(0, ) , ℎ = ℎ(𝑧, ) = 𝑅(𝑧, ) − 𝑟(𝑧, ), and 𝑝 = 𝑝(𝑧, ) is pressure distribution along the pcu gap having length 𝑙, and 𝑝1 and 𝑝2 are pressures at the gap entrance (𝑧 = 0) and exit (𝑧 = 𝑙). within the 1d flow model, for a selected , pressure distribution 𝑝(𝑧) depends on gap width for this  only, on boundary conditions 𝑝1 and 𝑝2 , as well as on density  = (𝑝) and viscosity  = (𝑝) of the pressure-transmitting medium in the pcu. based on the continuity equation for a 1d flow, in [1], equations for 𝑝(𝑧) are derived for incompressible media (typically liquids at moderate pressures) as 𝑝(𝑧) = 𝑝1 − (𝑝1 − 𝑝2) ∫ d𝑧 ℎ3⁄ 𝑧 0 ∫ d𝑧 ℎ3⁄ 𝑙 0 , (2) and for media having pressure-proportional density (ideal gas) as http://www.imeko.org/ mailto:wladimir.sabuga@ptb.de mailto:ahmed_hashad84@hotmail.com mailto:sven.ehlers@ptb.de acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 320 𝑝(𝑧) = [𝑝1 2 − (𝑝1 2 − 𝑝2 2) ∫ d𝑧 ℎ3⁄ 𝑧 0 ∫ d𝑧 ℎ3⁄ 𝑙 0 ] 0.5 , (3) both cases for media of constant viscosity. for both cases, expressions for 𝐴 of oil and of gas pressure balances operated in absolute mode are presented in [1]. when operating a pcu at low pressures, at which molecular properties of the gas require consideration, the pressure distribution can be determined by equation 𝑝(𝑧) = 𝑝1 − (𝑝1 − 𝑝2) ∫ d𝑧 (ℎ2𝐺𝑝)⁄ 𝑧 0 ∫ d𝑧 (ℎ2𝐺𝑝)⁄ 𝑙 0 , (4) in which 𝐺𝑝 is the poiseuille coefficient calculated by methods of the rarefied gas dynamics (rgd) [2]. having calculated 𝐴 for each , the effective area of the whole pcu (𝐴1d) is taken as the mean and its uncertainty due to pcu's axial nonsymmetry (𝑢ans) as the standard deviation of 𝐴. 2.2. 2d flow model continuity equation for a flow in a 2d gap built by two motionless surfaces is given by equation 𝜕 𝜕𝑧 (ℎ3 𝜌  𝜕𝑝 𝜕𝑧 ) + 𝜕 𝜕𝑡 (ℎ3 𝜌  𝜕𝑝 𝜕𝑡 ) = 0, (5) in which 𝑡 designates the tangential coordinate, 𝑡 = 𝑟, where  is angular coordinate [1]. for an incompressible medium with a pressureindependent viscosity, 𝜌(𝑝) = const, (𝑝) = const, it writes as 𝜕2𝑝 𝜕𝑧2 + 𝜕2𝑝 𝜕𝑡 2 + 3 ℎ 𝜕ℎ 𝜕𝑧 𝜕𝑝 𝜕𝑧 + 3 ℎ 𝜕ℎ 𝜕𝑡 𝜕𝑝 𝜕𝑡 = 0, (6) and for a compressible medium which obeys the ideal gas law, 𝜌(𝑝)~𝑝, (𝑝) = const, as 𝜕2𝑝2 𝜕𝑧2 + 𝜕2𝑝2 𝜕𝑡 2 + 3 ℎ 𝜕ℎ 𝜕𝑧 𝜕𝑝2 𝜕𝑧 + 3 ℎ 𝜕ℎ 𝜕𝑡 𝜕𝑝2 𝜕𝑡 = 0. (7) for 2d pressure distribution 𝑝(𝑧, ), the following boundary conditions apply: 𝑝(0, ) = 𝑝1, 𝑝(𝑙, ) = 𝑝2, 𝑝(𝑧, 0) = 𝑝(𝑧, 2) . using these boundary conditions, equations (6) and (7) can be solved numerically by the finite difference method (fdm) as described in [3]. with 𝑝(𝑧, ) calculated within the 2d flow model, the effective area (𝐴2d) is obtained by 𝐴2d = 1 2 ∫ 𝑟0 2 [1 + ℎ0 𝑟0 + 1 𝑟0(𝑝1−𝑝2) ∫ (𝑝 − 𝑙 0 2 0 𝑝2) d(𝑟+𝑅) d𝑧 d𝑧] d. (8) essentially, 𝐴2d involves the 2d pressure distribution for the whole axially non-symmetric pcu and, consequently, is free of 𝑢ans existing in the case of the 1d flow modelling. 3. piston-cylinder units totally 7 pcus of different type concerning their size and shape, pressure-transmitting medium and operation mode were studied. all these pcus were manufactured by dh instruments, later fluke calibration, and made of cobaltor nickel-bound tungsten carbide. they were characterised dimensionally, are traceable to length standards and thus allow the realisation of the pressure unit as primary pressure standards. 3.1. oil-operated 10 mpa pcus three specially designed pcus of 5 cm2 nominal effective area are used at ptb as primary standards for the range 10 mpa of gauge oil pressure [4]. they have serial nos. 278, 279, 280 and are operated with di-ethyl-hexyl-sebacate (dehs) as pressuretransmitting liquid in a modified harwood twin pressure balance with a maximum load of 500 kg on each piston. the effective area ratios of any two pcus could be determined from cross-float experiments with a relative standard uncertainty of about 210-6. in [4], the effective areas of the pcus were determined from their dimensional properties by the dadson theory for the 1d flow using eqs. (1) and (2), assuming dehs as an incompressible fluid with a constant viscosity. the theoretical 𝐴 of the pcus agreed with the experimental 𝐴 ratios within (-5.7 to 3.8)10-6. the relative standard uncertainty of the dimensional 𝐴 varied between (4.2 and 5.5)10-6, with 𝑢ans = (1.1 – 3.8)10 -6. this shows a potential for an uncertainty reduction by applying the 2d flow model. 3.2. gas-operated 700 kpa pcus three pcus with serial nos. 1159, 1162, 1163 have a nominal effective area of 20 cm2. they were created to be used for pressure measurements in the experiments on the determination of the boltzmann constant by the dielectric-constant gas thermometry (dcgt) [3], [5]-[8]. they are operated in two pressure balance platforms, manufactured by fluke calibration, equipped with 150 kg mass sets and automated mass handlers, which allow automated cross-float measurements in absolute and gauge mode [5]. the pcus have a cylinder-floating design with a double thickness of the cylinder walls, 8 mm, compared with that of the commercial 20 cm2 pcus. herewith, the pcu distortion coefficient was reduced, and cylinder roundness was improved. based on the 1d flow model, the effective area of these pcus was determined with a relative standard uncertainty, 𝑢(𝐴)= (0.8 – 1.2)10-6, which included uncertainty contributions due to pcus axial non-symmetry. in the dcgt experiments, these pcas were operated in absolute pressure mode with helium as a pressure-transmitting medium, but their http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 321 theoretical and experimental characterisation was performed also in gauge mode and with nitrogen as well. widespread results for 𝐴 of these pcus at different conditions were obtained in [3], which include calculation by the 2d flow model in gauge mode based on pcus' roundness data. with the 2d model, 𝑢(𝐴) could be reduced to (0.5 to 0.8)10-6. in the present work, the analysis is extended to the absolute mode, is performed at several pressures and is based on both, straightness and roundness data of the pcus as explained later. 3.3. gas-operated 15 kpa pcu the pcu with the serial no. 183 has a nominal effective area of 9.8 cm2 and is used in a forcebalanced piston gauge, manufactured by fluke calibration and known as fpg8601, capable of measuring absolute and gauge pressures in the range 1 pa to 15 kpa [9]. the cylinder has a bi-conical bore and two holes in the middle of its length, which are used for piston suspension and gas feed for the pcu lubrication. the lubricant gas flows through the tapered annular gap between the piston and cylinder which centres the piston in the cylinder bore, thus preventing any contact between the piston and the cylinder without piston rotation. the absolute pressure of the lubrication gas has a constant value of 140 kpa in gauge mode and 40 kpa in absolute mode. in [10] and [11], the fpg was characterised as a primary pressure standard using pcu's dimensional properties. in that analyses, the 1d flow model was applied with the pressure distribution calculated using rgd methods. in [10], 𝑢(𝐴) was equal to (9.8 and 9.4)10-6 in absolute and gauge mode, respectively, and was dominated by 𝑢ans being of up to 8.810 -6 in dependence on pressure and operation mode. this high 𝑢ans is explained by pcu's big length of about 100 mm, cylinder's biconical shape and a complicated manufacture procedure. the high 𝑢ans indicates a possibility for an uncertainty improvement by its eliminating. handling the pcu as axisymmetric, 𝑢(𝐴)  1.3110-6 was obtained in [11] where uncertainty contributions of the pcu diameters and the rgd model were considered. 4. pcus dimensional properties dimensional properties of the pistons and cylinders were determined from measurements of their diameters, straightness and roundness by different measurement techniques and devices. the straightness measurements were performed for 8 generatrix traces separated by 45°. roundness measurements were done in sections equidistantly distributed by 4 mm for the 5 cm2 and 20 cm2 pcus, and by 7 mm for the 10 cm2 pcu of the fpg. the diameters, straightness and roundness deviations of the oil-operated 5 cm2 pcus were measured with standard uncertainties (𝑘 = 1) of (25, 15 and 7.5) nm, respectively [12]. a procedure for linking the diameters and form deviation data was used as described in [12]. the uncertainty of the 3d data produced by the linking contributed 410-6 to the relative standard uncertainty of 𝐴 [4]. figure 1: piston and cylinder radii of pcus 278, oil, 5 cm2 (a), 1162, gas, 20 cm2 (b), and 183, gas, 10 cm2 (c), along 8 generatrix traces. in the dimensional measurements of the gasoperated 20 cm2 pcus, several instruments were involved [13], but the lowest uncertainties were achieved with a modified marform mfu8 device for form deviations and a reference length comparator komf [14] for diameter measurements, with piston and cylinder diameters measured with the standard uncertainties of 5 nm and 10 nm, respectively. for linking the diameter and form deviation data, a least-squares (ls) procedure was applied described in [15]. finally, the radii of the 12491600 12491800 12492000 12492200 12492400 12492600 12492800 12493000 12493200 -22 -18 -14 -10 -6 -2 2 6 10 14 18 22 r a d iu s / n m z / mm 0 45 90 135 180 225 270 315 (a) 24983700 24983800 24983900 24984000 24984100 24984200 24984300 24984400 24984500 -22 -18 -14 -10 -6 -2 2 6 10 14 18 22 r a d iu s / n m z / mm 0 45 90 135 180 225 270 315 (b) 17665000 17666000 17667000 17668000 17669000 17670000 17671000 17672000 -50 -40 -30 -20 -10 0 10 20 30 40 50 r a d iu s / n m z / mm 0 45 90 135 180 225 270 315 (c) http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 322 piston and cylinders were determined with standard uncertainties from 8 nm to 19 nm, which contributed (0.5 to 0.8)10-6 to 𝑢(𝐴) [6]. for the gas-operated pcu of the fpg8601, the diameters and the form deviations were all measured by the mfu8 device with the standard uncertainty of diameters of 25 nm. after applying the ls procedure to link the diameter and form deviations data [15], the piston and cylinder radii were obtained with the standard uncertainties of 45 nm and 31 nm, respectively. considering the uncertainties of the piston and cylinder radii as noncorrelated, the respective contribution to the relative uncertainty of 𝐴 was estimated as 3.210-6 [10]. figure 1 shows gap profiles between piston and cylinder for the 3 pcus of different types. the mean gap widths of the 20 cm2 (gas) and 5 cm2 (oil) pcus are of about 0.3 µm and 0.8 µm, respectively. the gap width of the 10 cm2 pcu of fpg is much bigger and varies from about 1.5 µm to 5 µm. also, the fpg pcu shows much bigger variations of the cylinder radius with angle than the classical 20 cm2 and 5 cm2 pcus. this explains the high uncertainty contribution of 𝑢ans reported in [10]. for getting an optimal accuracy of a 2d pressure distribution by a numerical integration of equations (6) and (7), dimensional data should have a similar density distribution in both axial and tangential directions. the original dimensional data obtained from the straightness measurements has a high density in the axial direction, typically 10 mm-1, but only (0.05 to 0.1) mm-1 in the tangential direction. vice versa, the roundness measurement data has a high density in the tangential direction, (2.3 to 4.6) mm-1, but only (0.14 to 0.25) mm-1 in the axial direction. additional dimensional data were generated by a linear interpolation of the straightness radii with the angle step of 1 ° and of the roundness radii with the axial step of 0.1 mm. these data sets had similar density distributions of (2.3 to 10) mm-1 in both directions. finally, two data sets with the same structure were obtained based on the straightness and roundness data, and, using them, all calculations were performed two times. 5. modelling conditions for the classical, oil-operated 5 cm2 and gasoperated 20 cm2 pcus, the calculations were performed at 10 % and 100 % of the pcus' pressure operation range, for the 20 cm2 pcus in both gauge and absolute pressure mode. as a boundary condition at the pcu exit, 𝑝2 = 100 kpa in gauge mode and 𝑝2 = 0.1 pa in absolute mode was used. for the 10 cm2 pcu of the fpg, separate modelling of the pcu's upper and lower parts was performed, because, due to the lubrication pressure in the middle of the pcu, each part has its own boundary conditions [10]. as 𝑝1, 40 kpa was taken for the absolute and 140 kpa for the gauge mode in the modelling of both pcu parts. as 𝑝2, 0.5 pa was figure 2: relative difference of 1d and 2d pressure distributions in the gap of pcus 278, oil, gauge mode, p1 = 10 mpa (a); 1162, gas, absolute mode, p1 = 700 kpa (b); and upper part of pcu 183, gas, absolute mode, p1 = 40 kpa, p2 = 0.5 pa (c), all based on straightness data. taken for the absolute and 100 kpa for the gauge mode in the modelling of the lower pcu part. for the upper part, 𝑝2 was taken as 1 pa and 15 kpa for the absolute mode as well as 100001 pa and 115 kpa in the gauge mode, which corresponds to the minimum and maximum pressure measured by the fpg in the two pressure modes. first, 1d flow pressure distribution 𝑝(𝑧) was calculated for a pcu gap profile of each  = 0, 1, …, 359 °. the calculation was done using equation (2) for the oil-operated and (3) for gas-operated (a) (b) (c) http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 323 pcus. then, taking this 1d flow pressure distribution as a starting condition, a 2d pressure distribution 𝑝(𝑧, ) was searched iteratively by the fdm. the iterations were repeated until the maximum pressure difference at any point (𝑧, ) in two subsequent iterations became smaller than 10−7 × (𝑝1 − 𝑝2). 6. results results of the 2d pressure distribution calculations for 3 pcus of different types are presented in figure 2 in relation to the 1d pressure distribution. they show deviations of the pressure distribution along 360 generatrix lines of the 2d flow model from the pressure distribution of the 1d flow model obtained for each gap profile. the difference between the 1d and 2d pressure distributions is bigger in the case of compressible fluid (gas) and the absolute pressure mode (figure 2, b and c) than in the case of incompressible fluid (oil) and gauge mode (figure 2, a). the difference is bigger for the upper part of the fpg pcu 183 (c) than for the classical pcu 1162 (b) because the first is stronger axially nonsymmetric than the second one. moreover, the maximum of the pressure distributions' deviation in the case of the fpg pcu is shifted to the gap exit (𝑧 → 𝑙), evidently, because of the pronounced gap narrowing towards the gap exit. results of 𝐴 calculation for the oil-operated pcus are shown in table 1 and for the gas-operated pcus in table 2. the values of 𝐴 and 𝑢ans in the tables are those obtained for the maximum difference of 𝑝1 and 𝑝2 , but the results at smaller 𝑝1 − 𝑝2 were very close to the values in the tables. the variation of the values was taken into account when calculating mean 𝐴 and 𝑢ans. for the 1d flow model, 𝑢ans was calculated as explained in section 2.1. for the 2d model, 𝑢ans was defined as the difference of 𝐴 values obtained by the 1d and 2d flow models. 1d 𝑢ans varies between (0.83 and 3.33)10-6, and, thus, for pcu 280 is comparable with the uncertainty of the dimensional measurements. by applying the 2d model, 𝑢ans is reduced to below 0.0710-6, and the combined relative standard uncertainties become lower than those of the 1d flow model. for all 3 pcus, they are equal to 410-6 and are slightly lower than reported in [4]. for gas-operated pcus, the results on 𝑢ans obtained for the gauge mode were similar with those for the absolute mode. for the 20 cm2 pcus, 1d 𝑢ans varies between (0.49 and 1.1)10 -6, and therefore is comparable with the dimensional uncertainties. by applying the 2d model, 𝑢ans is reduced to values below 0.0310-6, which agrees with the results previously obtained for these pcus in gauge mode using r data in [3]. finally, the combined relative standard uncertainties are essentially lower with the 2d flow than with the 1d flow model. for the fpg pcu 183, whose dimensional irregularities are much bigger than those of the 20 cm2 pcus, 1d 𝑢ans strongly dominates the uncertainty budget with a contribution of about 1010-6. by applying the 2d model it is reduced to 0.1510-6, which significantly reduces the combined uncertainty to about 3.210-6. in [10], a standard uncertainty of 9.810-6 was reported for the absolute mode which was affected by the axial non-symmetry of the s data used in the calculation. table 1: effective area (𝐴) of oil-operated pcus 278, 279 and 280 at gauge pressure 𝑝1 − 𝑝2 = 10 mpa, calculated on the basis of straightness (s) and roundness (r) data using 1d and 2d flow models; uncertainties due to pcus' axial nonsymmetry (𝑢ans ), dimensional data ( 𝑢dim ), variation of 𝐴 with pressure and choice of s or r data, and combined uncertainty (𝑢comb), all uncertainties being relative standard ones in 10 -6. 〈𝐴〉 and 〈𝑢ans〉 are means of 𝐴 and 𝑢ans at 𝑝1 − 𝑝2 = (1 and 10) mpa. data, model 278 279 280 𝑨 / mm2 𝒖𝐚𝐧𝐬 𝑨 / mm 2 𝒖𝐚𝐧𝐬 𝑨 / mm 2 𝒖𝐚𝐧𝐬 s 1d 490.2620 1.15 490.2634 0.59 490.2648 3.07 2d 490.2620 0.02 490.2634 0.01 490.2648 0.02 r 1d 490.2618 1.18 490.2633 1.07 490.2647 3.59 2d 490.2618 0.09 490.2633 0.01 490.2647 0.01 〈𝐴〉 / mm2 490.2619 490.2633 490.2648 𝑢dim 4.00 4.00 4.00 𝐴 0.13 0.08 0.04 〈𝑢ans〉 1d 1.16 0.83 3.33 2d 0.07 0.01 0.01 𝒖𝐜𝐨𝐦𝐛 1d 4.17 4.09 5.20 2d 4.00 4.00 4.00 http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 324 table 2: effective area of gas-operated pcus at pressures 𝑝1 = 700 kpa and 𝑝2 = 0.1 pa for pcus 1159, 1162 and 1163, and 𝑝1 = 40 kpa and 𝑝2 = 1 pa for the upper part of pcu 183. 〈𝐴〉 and 〈𝑢ans 〉 present results at 𝑝1 = (70 and 700 kpa) for pcus 1159, 1162 and 1163, as well as d 𝑝2 = 1 pa and 15 kpa for pcu 183. all designations are as in table 1. data, model 1159 1162 1163 183, upper part 𝑨 / mm2 𝒖𝐚𝐧𝐬 𝑨 / mm 2 𝒖𝐚𝐧𝐬 𝑨 / mm 2 𝒖𝐚𝐧𝐬 𝑨 / mm 2 𝒖𝐚𝐧𝐬 s 1d 1961.0094 0.67 1961.0044 1.03 1961.0423 0.35 980.6067 9.52 2d 1961.0094 0.00 1961.0043 0.04 1961.0422 0.03 980.6068 0.09 r 1d 1961.0089 0.82 1961.0033 1.20 1961.0411 0.63 980.6063 11.50 2d 1961.0089 0.00 1961.0033 0.02 1961.0411 0.01 980.6065 0.14 〈𝐴〉 / mm2 1961.0092 1961.0038 1961.0417 980.6067 𝑢dim 0.75 0.50 0.65 3.20 𝐴 0.08 0.16 0.17 0.27 〈𝑢ans〉 1d 0.75 1.12 0.49 10.04 2d 0.00 0.03 0.02 0.15 𝒖𝐜𝐨𝐦𝐛 1d 1.06 1.23 0.83 10.54 2d 0.75 0.52 0.67 3.21 7. summary by applying a 2d flow model, the effective area of 3 oil and 4 gas pcus used as primary standards for gauge and absolute pressures was determined. with this 2d flow model, the uncertainty contribution due to axial non-symmetry of the pcus' dimensional properties was eliminated and, finally, the combined uncertainty reduced by 2 % to 70 %. 8. references [1] r. s. dadson, s. l. lewis, g. n. peggs, the pressure balance – theory and practice, london, hmso, 1982. [2] w. sabuga, f. sharipov, t. priruenrom, “determination of the effective area of pistoncylinder assemblies using a rarefied gas flow model”, ptb-mitteilungen, vol. 121, no. 3, pp. 260-262, 2011. [3] t. priruenrom, “development of pressure balances for absolute pressure measurement in gases up to 7 mpa”, doctoral thesis, clausthal university of technology, germany, 2011. [4] j. jäger, w. sabuga, d. wassmann, “pistoncylinder assemblies of 5 cm 2 cross-sectional area used in an oil-operated primary pressure balance standard for the 10 mpa range”, metrologia, vol. 36, pp. 541-544, 1999. [5] w. sabuga, t. priruenrom, r. haines, m. bair, “design and evaluation of pressure balances with 110–6 uncertainty for the boltzmann constant project”, ptb-mitteilungen, vol. 121, no. 3, pp. 256-259, 2011. [6] w. sabuga w., “pressure measurements in gas media up to 7.5 mpa for the boltzmann constant redetermination”, ptb-mitteilungen, vol. 121, no. 3, pp. 247-255, 2011. [7] t. zandt, w. sabuga, c. gaiser, b. fellmuth, “measurement of pressures up to 7 mpa applying pressure balances for dielectric-constant gas thermometry”, metrologia, vol. 52, pp. s305–s313, 2015. [8] c. gaiser, et al., “final determination of the boltzmann constant by dielectric-constant gas thermometry”, metrologia, vol. 54, pp. 280-289, 2017. [9] p. delajoud, m. girard, “a force balanced piston gauge for very low gauge and absolute pressure”, proc. of 10th intern. metrology congress, st louis, france, 22-25 october 2001. [10] a. s. hashad, s. ehlers, o. jusko, w. sabuga, “characterization of a force-balanced piston gauge as a primary pressure standard”, measurement, vol. 131, pp. 723-729, 2019. [11] s. naris, et al., “computation of the effective area and associated uncertainties of non-rotating piston gauges fpg and frs”, metrologia, vol. 56, pp. 110, 2019. [12] o. jusko, h. bosse, f. lüdicke, “high precision 3d-calibration of cylindrical standards”, in advanced mathematical tools in metrology iii, edited by p. ciarlini, m. g. cox, f. pavese and d. richter, singapore, world scientific publishing company, 1997, pp. 186-194. [13] o. jusko, et al., “dimensional calibration techniques for pressure for pressure balances to be used in the new determination of the boltzmann constant”, congresso internacional de metrologia mecanica: anais, i-cimmec, infosim – informative bulletin of the interamerican metrology system – oas, august 2009, pp. 14-23. [14] m. neugebauer, et al., “a new comparator for measurement of diameter and form of cylinders, spheres and cubes under clean-room conditions”, meas. sci. technol., vol. 8, pp. 849-856, 1997. [15] w. sabuga, t. priruenrom, “an approach to the evaluation of dimensional measurements on pressure-measuring piston-cylinder assemblies”, proc. of 3rd conf. on pressure measurement imeko tc16, merida, mexico, 2007, http://www.imeko.org/publications/tc162007/imeko-tc16-2007-074u.pdf. http://www.imeko.org/ http://www.imeko.org/publications/tc16-2007/imeko-tc16-2007-074u.pdf http://www.imeko.org/publications/tc16-2007/imeko-tc16-2007-074u.pdf basic design of a new multicomponent standard machine and proposal for a definition of the nominal loads acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 88 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 88 90 basic design of a new multicomponent standard machine and proposal for a definition of the nominal loads d. schwind1, m. eller2, u. kolwinski3 gtm testing and metrology gmbh, bickenbach, germany 1 daniel.schwind@gtm-gmbh.com, 2 martin.eller@gtm-gmbh.com, 3 ulrich.kolwinski@gtm-gmbh.com abstract: the increasing demand in industry for calibrated multicomponent measuring, especially with mixed loads, supports the necessary development for the next level of multicomponent standard machines. our next step is a flexible machine, which ensures an automatic force and moment generation for realistic load situations. the multicomponent measuring in realistic load situations improves test results, assures comparability to application and reduces time expenses in calibration. this paper deals with definition of the capacity of the main components of the new machine type and its possibilities for realistic applications. keywords: multicomponent calibration; multicomponent standard machine; hexapod; stewart platform 1. introduction multicomponent measurements are getting more and more important in industrial applications [1]. many applications are integrated in numerous test benches in automotive, aviation, aerospace, railway and robotic industries. the customers need application-oriented calibrations of their multicomponent force transducers. a lot of applications include eccentric loads, levers and introduce mixed loads in testing. it is necessary to perform practice-oriented, flexible and traceable multicomponent calibrations taking into account the vector properties and using self-developed procedures [2]. therefore, gtm decided to develop a new fully automatic multicomponent standard machine, see figure 1. 2. description the multicomponent standard machine consists of a machine frame, including three columns, fixed in an upper crosshead and a lower crosshead. a 3-point spindle drive allows an adjustment of the working unit in vertical direction. the machine has a weight of approximately 20 t and a construction height of 4.5 m up to 6.5 m, if the working unit is adjusted in the upper end position. the force and moment generation is represented by a hexapod drive unit [3], installed at the working unit. the load measurement is represented by a traceable multicomponent measuring platform. it is installed in the bottom of the machine frame. it allows measuring the force and moments in all six degrees of freedom (dof). the test space has a usable base area of 1.5 m × 1.5 m and a height of up to 2 m. figure 1: multicomponent standard machine http://www.imeko.org/ mailto:daniel.schwind@gtm-gmbh.com mailto:martin.eller@gtm-gmbh.com mailto:ulrich.kolwinski@gtm-gmbh.com acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 89 3. calibration equpment 3.1. multicomponent measurement platform the reference system is able to measure the mixed loads simultaneously, see figure 2. the device under calibration (duc), e.g. a multicomponent transducer, or any multicomponent measuring system, is fixed on the upper plate of the reference system without backlash through a fixing plate. calibration results have to be transformed from the coordinate system of the measuring platform to the coordinate system of the duc. it is important to know the geometric properties of the reference system as well as of the duc, to precisely determine the introduced forces and moments. figure 2: multicomponent reference system 3.2. hexapod the loads are generated by a gtm-developed hexapod, see figure 3. it consists of six podes powered by electromechanical actuators. the podes and joints are working without backlash. this unit is able to generate mixed loads. the main adapter which can be disassembled is mounted in the centre of the movable platform of the hexapod. figure 3: hexapod 4. mounting and calibration procedure the duc is mounted on the fixing plate, which is bounded with the upper plate of the multicomponent reference system. subsequently the hexapod drive unit moves down until the mounting position is reached. an exact adjustment is achieved through an additional manual control and an adapter. the gtm-developed adapter insures the connection between the duc and the movable platform of the hexapod unit without backlash. the force introduction point is selectable for a realistic measurement with an automatically and a manual mode. 5. realistic force and moment introduction there are requirements to calibrate multicomponent force transducers in the same way they are used in real life. a realistic calibration ensured by additional components may be levers to define the introduction point in an eccentric way. therefore, it is possible to calibrate a duc in the same way as it is used by the customer. further this simulation increases the accuracy and plausibility of the values of the measurement. an important property represents the zero-point passage during the measurement. in addition to this, it does not require to turn and remount the duc during the process. the levers can be mounted on the movable hexapod plate in different eccentric positions, see figure 4. the force transmission is insured by an additional adapter, which can be mounted eccentrically at the movable hexapod plate. the eccentricity ranges from 0 mm to 700 mm. figure 4: hexapod with an eccentric mounted lever http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 90 both adapters are constructed from two parts. the first component is fixed in the hexapod drive unit. if necessary, it can be exchanged. the corresponding component is mounted on the duc. during calibration the two components are connected to transmit the hexapod generated loads to the duc. 6. nominal loads of the calibration equipment the hexapod-generated loads can be measured by two additional multicomponent reference systems (mrs) optionally mounted on the main reference system hidden in the bottom. the advantage here is an optimization of the measuring accuracy for lower loads. table 1: force and moment values maximum load fx fy fz mx my mz kn kn kn kn·m kn·m kn·m hexapod drive unit (centre) 200 200 500 50 50 50 mrs 1 200 200 500 160 160 160 mrs 2 40 40 70 10 10 10 mrs 3 4 4 10 1.25 1.25 1.25 7. defining the capacity of a multicomponent standard machine at the beginning of a new era of multicomponent standard machines, we should determine how to define the nominal loads. the specification must give a clear statement about the measuring range and the measurement uncertainty, as it is necessary to declare it in the scope of an accreditation certificate or in cmc (calibration and measurement capabilities) uncertainty statements e.g. of bipm. additionally the technical data about the capacity must be transparent for the customer, if the multicomponent standard machine is suitable for a special duc. it must be apparent if the machine meets the calibration requirements. based on the following definition, it is possible to describe the capacity of this machine type. 7.1. specification of force generation by the drive unit an important fact of the multicomponent standard machine described in this paper is that the multicomponent reference system is able to handle much higher moment as the drive unit is able to introduce directly without using a lever. therefore the capacity of the multicomponent standard machine should be specified by the possible loads of the drive unit. 7.2. specification by maximum single forces and moments possible definitions are dealing with mixed-load or single load specification. the disadvantage of a mixed-load specification is the missing information about the maximum forces and moments the multicomponent standard machine is able to handle. the most useful definition is to give the maximum single load specification, generated in the centre of the hexapod. in this definition the information about the possible maximum capacity is given. in addition to this, it is important to know the eccentric possible load introduction points relating to their maximum single-load properties. 8. conclusion a new gtm-developed multicomponent standard machine is built up in 2020. it allows measuring mixed loads simultaneously. an advantageous feature represents the zero-point passage and a realistic force introduction. the very promising option to define the machine capacity is to specify the maximum loads acting at the introduction point. 9. references [1] d. schwind, “multicomponent-transducers: definition, construction types and calibration considerations”, proc. imeko tc3, tc5 and tc22 conference, pattaya, thailand, 22 – 25 november 2010. online [accessed 20200818]: https://www.imeko.org/publications/tc32010/imeko-tc3-2010-006.pdf [2] d. schwind, h. raabe, “a new calibration procedure for multicomponent transducers”, proc. xx imeko world congress, busan, republic of korea, 9 – 14 september 2012. online [accessed 20200818]: https://www.imeko.org/publications/wc2012/imeko-wc-2012-tc3-o25.pdf [3] d. röske, “metrological characterization of a hexapod for multi-component calibration device”, proc. xvii imeko world congress, dubrovnik, croatia, 22 – 27 june 2003. online [accessed 20200818]: https://www.imeko.org/publications/wc2003/pwc-2003-tc3-021.pdf http://www.imeko.org/ https://www.imeko.org/publications/tc3-2010/imeko-tc3-2010-006.pdf https://www.imeko.org/publications/tc3-2010/imeko-tc3-2010-006.pdf https://www.imeko.org/publications/wc-2012/imeko-wc-2012-tc3-o25.pdf https://www.imeko.org/publications/wc-2012/imeko-wc-2012-tc3-o25.pdf https://www.imeko.org/publications/wc-2003/pwc-2003-tc3-021.pdf https://www.imeko.org/publications/wc-2003/pwc-2003-tc3-021.pdf on pseudorandom number generators acta imeko issn: 2221-870x december 2020, volume 9, number 4, 128 135 acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 128 on pseudorandom number generators daniel chicayban bastos1, luis antonio brasil kowada1, raphael c. s. machado1,2 1 instituto de computação, universidade federal fluminense, brasil 2 inmetro instituto nacional de metrologia, qualidade e tecnologia, brasil section: research paper keywords: randomness; random number generator; true random number generator; pseudorandom number generator; statistical tests; testu01; nist sp 800-22; random sequence; state-of-the-art; crush citation: daniel chicayban bastos, luis antonio brasil kowada, raphael c. s. machado, on pseudorandom number generators, acta imeko, vol. 9, no. 4, article 17, december 2020, identifier: imeko-acta-09 (2020)-04-17 section editor: francesco bonavolonta, university of naples federico ii, italy received october 30, 2019; in final form may 15, 2020; published december 2020 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this research was partially funded by the shcdciber project. corresponding author: daniel chicayban bastos, e-mail: dbastos@id.uff.br 1. introduction unfortunately, the list of past incidents involving bad random number generation is not too modest. bad randomness has been with us for as long as random number generation has been in use. perhaps the oldest catastrophe is randu, from ibm’s system/370, used in the 60s. ‘[the] very name randu is enough to bring dismay into the eyes and stomachs of many computer scientists! [...] [it] fails most three-dimensional criteria for randomness, and it should never have been used.’ [1] in 1996, netscape communications failed to properly seed their random number generator during ssl handshaking: they used the current timestamp and the browser’s pid and ppid. in unix systems, pid means process identifier and ppid is the process parent’s pid. in the 32-bit linux kernel version 2.5.68, every pid is an integer between 1 and 32767. in 64-bit systems, the value can get up to 222, approximately 4.2 million [2]. the seed per se was computed by the md5 hash function, but since an adversary could have a precise measurement of the current timestamp and the universe of possible pid numbers was not large, it was possible to considerably reduce the set of possible seeds available to the generator. while netscape thought they had 128 bits of security, they in fact had 47 bits [3]. in 2003, taiwan launched a project offering its citizens a smart card with which they could authenticate themselves with the government, file taxes, etc. rsa keys were generated by the cards using built-in hardware random number generators advertised as having passed fips 140-2 level 2 certification [4]. ‘on some of these smart cards, unfortunately, the randomnumber generators used for key generation are fatally flawed and have generated real certificates containing keys that provide no security whatsoever.’ as a result, a total of 184 distinct secret keys were found out of more than two million 1024-bit rsa keys downloaded from taiwan’s national key repository [5]. in 2008, a vulnerability in openssl on debian-based operating systems was caused by ‘a random number generator that [produced] predictable numbers, [making] it easier for remote attackers to conduct brute force guessing attacks against cryptographic keys’ [6]. in 2012, a survey of tls and ssh servers was performed [7]. the entire ipv4 space was scanned, providing a macroscopic view of the universe of keys on the internet. unfortunately, many servers were powered by malfunctioning random number abstract statistical sampling and simulations produced by algorithms require fast random number generators; however, true random number generators are often too slow for the purpose, so pseudorandom number generators are usually more suitable. but choosing and using a pseudorandom number generator is no simple task; most pseudorandom number generators fail statistical tests. default pseudorandom number generators offered by programming languages usually do not offer sufficient statistical properties. testing random number generators so as to choose one for a project is essential to know its limitations and decide whether the choice fits the project’s objectives. however, this study presents a reproducible experiment that demonstrates that, despite all the contributions it made when it was first published, the popular nist sp 800-22 statistical test suite as implemented in the software package is inadequate for testing generators. mailto:dbastos@id.uff.br acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 129 generators. about 5.8 million distinct tls certificates and 6.2 million ssh distinct keys were analysed from about 10.2 million hosts. it was found that 5.57 % of the tls servers and 9.60 % of the ssh servers shared keys with at least one other server. among tls servers, at least 5.23 % were using default keys generated by the manufacturer that had never been changed by the user. it seems some 0.34 % generated the same keys as one or more hosts due to malfunctioning random number generators. as a result, about 64,000 (0.50 %) tls private rsa keys and about 108,000 (1.06 %) ssh private rsa keys were factored by exploiting the fact that some of these keys shared a common factor with at least one other host due to entropy problems in random number generation. as technology adoption advances, incidents become more frequent. in 2013, a component of android responsible for generating secure random numbers contained a weakness that rendered all android wallets generated until then vulnerable to theft [8], [9], [10]. in 2015, a flaw in freebsd’s kernel made ssh keys and keys generated by openssl vulnerable due to the possible predictability of a random number generator [11]. it is not absurd to assume that, in the same way that smart phones use the same libraries as servers and desktop systems, embedded systems and others will use the same or similar versions of these software due to their often low resource demands, generating more security concerns as stable implementations of verified software might be changed to fit in with the requirements of more constrained systems. 2. terminology there are at least two types of random number generators, those called true random number generators (trngs) and those called pseudorandom number generators (prngs). the former is usually associated with a physical mechanism that produces randomness by way of a physical process ‘such as the timing between successive events in atomic decay’ [12]. a pseudorandom number generator is often an arithmetical procedure performed by a machine based on initial, hopefully random, information called a seed. if a pseudorandom number generator has enough desirable properties that it could be recommended for cryptographic applications, then the acronym csprng is often used, meaning computationally secure pseudorandom number generator. 3. what is a random sequence? looking at probability theory textbooks, one sees they require the concept of randomness, but most expositions carefully dodge the difficulty of precisely defining what a random sequence is, which is required for the definition of the term ‘probability’. instead of making absolute assertions, the theory concerns itself with telling how much probability should be attached to statements involving events. in other words, the objective is to quantify, measure and compute, not to give meaning [1]. from the perspective of a formalist, this is not unusual, for pure mathematics is mostly concerned with the form of statements, not with their content, a view of pure mathematics that has been remarkably described by bertrand russell [13]. pure mathematics consists entirely of assertions to the effect that, if such and such a proposition is true of anything, then such and such another proposition is true of that thing. it is essential not to discuss whether the first proposition is really true, and not to mention what the anything is, of which it is supposed to be true. [...] thus mathematics may be defined as the subject in which we never know what we are talking about, nor whether what we are saying is true. one should recognize, however, that mathematical logic is not able to capture the whole of mathematics, as has been clear since the advent of gödel’s theorems. (for a precise definition of ‘capture’, see section 4.6, page 35 of peter smith’s ‘an introduction to gödel’s theorems’, cambridge university press, 2007, isbn: 978-0-521-85784-0.) in the context of probability theory, if one has a random sequence, it can be used to draw samples from a population. given these random samples, then ‘such and such’ deductions can be made. ‘it is essential’ not to discuss whether the sequence with which one began is really random. it is by hypothesis. and, finally, it is essential not to discuss what probability really is, since that would prompt us to discuss what randomness is. (the issue is discussed at length by von mises in ‘probability, truth and statistics.’ richard von mises, 1957. dover publications, inc., 2nd edition, 1981. isbn: 0-486-24214-5. on page 24, von mises writes that ‘[t]he term “probability” will be reserved for the limiting value of the relative frequency in a true collective which satisfies the condition of randomness. the only question is how to describe this condition exactly enough to be able to give a sufficiently precise definition of a collective.’ on page 12, he defines collective as ‘a sequence of uniform events or processes which differ by certain observable attributes’. for example, ‘all the throws of dice made in the course of a game form a collective wherein the attribute of the single event is the number of points thrown.’) however, if a probability is measured as a number, it can then be compared. for example, one can assert that a probability 𝑥 is greater than a probability 𝑦, which is astoundingly useful. sequences that are ∞-distributed have been given serious consideration as candidates for a definition of a random sequence. to explain what ∞-distributivity is, it will help us to consider the particular case of binary sequences. a binary sequence is considered ∞-distributed if it is 𝑘-distributed for all natural numbers 𝑘. intuitively, a 𝑘-distributed binary sequence is one in which the probability of a certain 𝑘-digit binary string appearing in the sequence is the same as any other. in other words, the sequence’s probability distribution is uniform for 𝑘digit binary strings. in more precise terms, a binary sequence 𝑋𝑛 is 𝑘-distributed for a certain 𝑘 if pr(𝑋𝑛 𝑋𝑛+1 … 𝑋𝑛+𝑘−1 = 𝑥1𝑥2 … 𝑥𝑘 ) = 1/2 𝑘 for all binary 𝑘-digit numbers 𝑥1𝑥2 … 𝑥𝑘. for example, a binary 1-distributed sequence must satisfy pr(𝑋𝑛 = 0) = 1/2 as well as pr(𝑋1 = 1) = 1/2. one such sequence would be 0, 1, 0, 1, …, since pr(𝑋𝑛 = 0) is the limit of the sequence 1, 1/2, 2/3, 2/4, …, which converges to 1/2 [1]. another example is 0, 0, 1, 1, 0, 0, 1, 1, … for a binary sequence to be 2distributed, it would have to satisfy pr(𝑋𝑛 𝑋𝑛+1 = 00) = 1/4, pr(𝑋𝑛 𝑋𝑛+1 = 01) = 1/4, pr(𝑋𝑛 𝑋𝑛+1 = 10) = 1/4, pr(𝑋𝑛 𝑋𝑛+1 = 11) = 1/4. acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 130 one can check that the sequence 0, 0, 1, 1, 0, 0, 1, 1, … is also 2distributed, but it is not 3-distributed. it is not 3-distributed because pr(𝑋𝑛𝑋𝑛+1𝑋𝑛+2 = 000) = 0 when it should be 1/8. this suggests that for every periodic sequence, there is a natural number 𝑘 such that the sequence is not 𝑘-distributed. indeed, every periodic sequence of period 𝑝 is not 𝑝-distributed [14]. a periodic 3-distributed binary sequence is not easily guessed, but one can check that the sequence 0,0,0,1, 0,0,0,1, 1,1,0,1, 1,1,0,1, 0,0,0,1, … is indeed 3-distributed [1]. an algorithm is not limited to producing periodic sequences – for example, an algorithm that produces the digits of 𝜋 does not produce a periodic sequence – so the limitation of periodic sequences is no challenge to the idea that ∞-distributivity defines randomness. in fact, one of the formidable results of ∞distributed sequences is that they can be produced by algorithms: one such algorithm was given in 1965 [14]. the weakness in taking the notion of ∞-distributivity alone as a definition for a random sequence appears when one considers the subsequences of an ∞-distributed sequence. if such sequences were random, it would be expected that any subsequence of a random sequence would also be random, but this does not always happen with ∞-distributed sequences. given an ∞-distributed binary sequence 𝑋𝑛, one can construct a new sequence 𝑌𝑛 = 𝑋𝑛 except that 𝑌𝑛2 = 0 for every index 𝑛. clearly, 𝑌𝑛 is not random because, by construction, 𝑌0, 𝑌1, 𝑌4, 𝑌9, … , 𝑌𝑛2 = 0, yet 𝑌𝑛 is still ∞-distributed because setting squared elements to zero does not significantly change the probabilities required in the definition of 𝑘-distributivity [1]. that is, ∞-distributivity alone is too weak a definition. an apparently adequate definition is reached by making suitable restrictions to the rules governing which subsequences must be ∞-distributed. that is, not all subsequences of an ∞-distributed sequence 𝑋𝑛 must be ∞-distributed for 𝑋𝑛 to be qualified as random [1]. ‘the secret is to restrict the subsequences so that they could be defined by a person who does not look at’ 𝑋𝑛 ‘before deciding whether or not it is to be in the subsequence’ [1]. to date, there does not seem to be any objection to this strategy. 4. what is a pseudorandom number generator? since algorithms cannot compute random sequences [15], they are left with at most producing pseudorandom number sequences displaying the desired statistical properties. a pseudorandom number generator, therefore, is an arithmetical procedure that produces a sequence of numbers that one hopes will pass sufficient statistical tests and thus appear random. it can be as simple as a function 𝑓(𝑥𝑛 ) = 𝑥𝑛−1 2 + 1 mod 𝑁, for some fixed natural number 𝑁 or as complex as donald knuth’s ‘superrandom’ number generator [1], shown as an illustration of how complicated algorithms do not necessarily provide any more randomness. (the function 𝑓(𝑥𝑛 ) = 𝑥𝑛−1 2 + 1 mod 𝑁 is typically used in a method of integer factorisation known as pollard’s rho. one interesting fact about the method is that, while polynomials like 𝑓 do not have good statistical properties, the average number of steps taken by the procedure would considerably increase if a truly random sequence were used instead [16]. sometimes true randomness is not desired.) for illustration purposes, let us look at what a simple pseudorandom number generator looks like in the c programming language. uint32_t y = 2463534242u; /* the seed */ uint32_t xorshift(void) { y = y ^ (y << 13); y = y ^ (y >> 17); y = y ^ (y << 5); return y; } this is george marsaglia’s xorshift generator of 32 bits [17]. the variable y is a global variable in the xorshift procedure. the letter u at the end of the seed is just an indicator that that number is an unsigned integer. what this procedure does is multiply the arbitrarily set initial value 2463534242, the seed, to the number 213 and add the result to y; that is, it adds the product to the initial value. the multiplication is done in fast computer arithmetic, namely, shifting y to the left by 13 bits because, in base 2 arithmetic, shifting a number to the left means adding zeros to the right of the number, which is the same as multiplying it by a power of 2. for example, by taking the number 5, which is 101 in base 2, and multiplying it by 2, one gets 10, which is 1010 in base 2. the second step divides y by 217 and adds the result to y. the last step is similar. all such computations are reduced modulo 232. (ansi x3.159-1989 asserts in section 3.1.2.5 that ‘[a] computation involving unsigned operands can never overflow, because a result that cannot be represented by the resulting unsigned integer type is reduced modulo the number that is one greater than the largest value that can be represented by the resulting unsigned integer type’.) looking at the numbers produced by this generator, they appear random to the naked eye, but the sequence does not pass even a modest contemporary battery of statistical tests. 5. desirable properties of generators true random number generators have several disadvantages compared to a good pseudorandom number generator. for example, they are slower, more cumbersome to install and run, more costly and unable to reproduce the same sequence twice. (reproducing the same sequence is important for repeating simulations and testing applications.) but a pseudorandom number generator does need a good seed, which true random number generators can provide [12]. when choosing a pseudorandom number generator, one must know what to look for. some of the properties one can find in pseudorandom number generators, to name a few, are good statistical properties, good mathematical foundations, lack of predictability, cryptographic security, efficient time and space performance, small code size, a sufficiently long period and uniformity [18]. in the context of computer-generated randomness, good statistical properties are effectively what is meant by ‘random’ [18]. mathematical foundations allow us to be sure a pseudorandom number generator has some desirable property, such as its period, which is defined as the length of the sequence of random numbers the generator can produce before needing to repeat itself. having a long period is surely desirable. uniformity is a property closely related to the period. after the generator has output all its period, each number produced should occur the same number of times, otherwise it is not uniform. if it is not uniform, it is biased. uniformity alone, without a long acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 131 period, is certainly not desirable. consider what happens as a uniform generator is consumed. as the end of its period draws near, its uniformity effectively allows one to predict more and more of its output, since all output must occur the same number of times [18]. for example, let us consider a case where we can show that a generator must lack uniformity in its output. consider a generator with 𝑏 bits of state, but where one of the 2𝑏 possible states is never used, (perhaps because the implementation must avoid an all-bits-are-zero state). the missing state would leave the generator with a period of 2𝑏 − 1. by the pigeonhole principle, we can immediately know that it cannot uniformly output 2𝑏 unique 𝑏-bit values. so, the period of a generator cannot be too short, lest it repeat itself while in use, which makes it statistically unsound. a large internal state implies the possibility of a longer period because it allows for more distinct states to be represented. yet, in terms of period size, more is not always better. for example, if one is to choose between generators with period sizes of 2128 and 2256, one should notice that it would take billions of years to exhaust the period of the 2128 generator, so picking the generator with period 2256 does not bring a relevant advantage. ‘even a period as “small” as 256 would take a single cpu core more than two years to iterate through at one number per nanosecond.’ [18] another valuable property is unpredictability. ‘a die would hardly seem random if, when i’ve rolled a five, a six, and a three, you can tell me that my next roll will be a one.’ [18] still, pseudorandom number generators are deterministic, and their behaviour is completely determined by their input: they produce the same sequence given the same input. so, their randomness is only apparent to an observer who does not know their initial conditions. though the deterministic nature of pseudorandom number generators might seem more like a weakness than a strength, it is valuable for reproducing the same sequence multiple times, which is required in a number of applications, from simulations and games to the mere testing of programs. to repeat a sequence generated by a pseudorandom number generator, one needs only save its initial conditions, usually just the seed for the produced sequence. to repeat a sequence from a true random number generator, the entire produced sequence would have to be saved. it is not immediately obvious that a procedure computed by a machine can be unpredictable, but some pseudorandom number generators output a number while keeping another one hidden from the user. the hidden information is called the pseudorandom number generator’s internal state. predicting the pseudorandom number generator entails knowing the internal state. unpredictability is important for applications concerned with security because predicting a pseudorandom number generator allows for various types of attacks, including denial of service [19]. if a pseudorandom number generator leaks internal state information at each output, an adversary is able to little by little infer the complete internal state, at which point the generator becomes completely predictable, at least from that point in the sequence on, which is a flaw of mersenne twister [20]. predictability can be considered in two directions: forwards and backwards. a generator is said to be invertible if, once its internal state is known, the random numbers it generated previously can be recovered. being non-invertible is vital for applications that generate cryptographic keys; if the generator is invertible and its internal state is exposed at some point in time, adversaries will be able to recover all previously generated keys. so, cryptographically secure pseudorandom number generators are not invertible. although some applications may not be designed with cryptography in mind, it is prudent to pick the safest generator that a project can afford [18]. [because] we cannot always know the future contexts in which our code will be used, it seems wise for all applications to avoid generators that make discovering their entire internal state completely trivial. speed is another important property, particularly when considering low resource systems. an application that is too dependent on a random number generator will be as slow as the generator used. applications running in low-resource hardware will likely trade other properties for speed and space. many generators with good statistical properties are slow, but there are some generators that have relatively good time performance while showing acceptable statistical properties. for example, xorshift* 64/32 [17] has good performance and good statistical properties [18], although it is not safe for cryptographic applications. most generator implementations will take just a constant amount of memory to store their state, but considering the strict constraints some applications face, the size of these constants should also lead programmers to choose one over another. space is also related to speed: considering all other things to be equal, a generator that is able to keep its internal state completely within a processor register should outperform a competitor that needs many more bytes of internal state to be kept in main memory [18]. there are also the space constraints of code size. such space is most likely a constant, but constants do matter for applications running in low-resource hardware. the longer the code, the more likely it will include programming errors. such errors can be particularly difficult to detect in the context of random number generators [18]. from […] experience, i can say that implementation errors in a random number generator are challenging because they can be subtle, causing a drop in overall quality of the generator without entirely breaking it. another desirable property is seekability, the ability of a generator to skip ahead or jump back in the sequence. since pseudorandom number generators are cyclic, by skipping a sufficient number of elements, one can get back to the starting number, meaning that the ability to seek ahead also implies the ability to seek backwards. computationally secure pseudorandom number generators are designed not to be seekable, as it is not desirable to let an adversary read the sequence backwards, discovering which numbers might have been used in the past. 6. statistical hypothesis testing statistical theory allows us to posit a hypothesis 𝐻0 about a random number generator and devise tests to provide empirical evidence of the validity of 𝐻0. these tests, in turn, either give us more confidence in the hypothesis 𝐻0 or leads us to reject it. a acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 132 statistical test for a random number generator is defined by a random variable 𝑋 whose distribution under 𝐻0 can be well approximated. when 𝑋 takes the value 𝑥, define 𝑝𝑅 = pr(𝑋 ≥ 𝑥 | 𝐻0) and 𝑝𝐿 = pr(𝑋 ≤ 𝑥 | 𝐻0) as the left and right 𝑝-value, respectively. such 𝑝-values measure how likely it is to find a certain sample of the random number generator given 𝐻0 is true. if it turns out that very unlikely samples occur from the random number generator, that is then strong evidence the hypothesis 𝐻0 is not true. in fact, when testing random number generators, if either the right or left 𝑝-value is extremely close to zero, then 𝐻0 should be rejected. if a suspicious 𝑝-value is obtained, say near 10−2 or 10−3, one can repeat the particular test a few more times, perhaps with a larger sample size, in the hope that more tests will clarify the result [12]. in the context of testing for randomness, 𝐻0 is usually taken to mean that the sequence is random. for each specific test, a rule must be derived that allows one to reject or not to reject 𝐻0. taking 𝐻0 to mean that the sequence generated is random, the test produces a statistic with a certain probability distribution of possible values. this probability distribution must be determined by mathematical methods. from this distribution, a critical value is chosen such that a critical region in the set of possible values is determined. the statistic is then computed from the sample and compared to the critical value. if the statistic falls in the critical region, 𝐻0 is rejected; that is, one concludes the sequence produced by the generator is not random. otherwise, 𝐻0 is not rejected. if the generator produces a random sequence, then the computed statistic will have a very low probability of falling in the critical region, and if such an event occurs systematically, it provides strong evidence that the sequence is not random as assumed in 𝐻0. although the probability for such an event may be very low, it is not null. incorrectly classifying a sequence produced by a generator as not random is called a type i error. much worse would be if 𝐻0 is not rejected when the sequence produced by the generator is not random, an error that is called type ii. the probability of type i error is usually denoted by 𝛼 and is called the level of significance of the test. the probability of type ii error is usually denoted by 𝛽. the value of 𝛼 can be arbitrarily chosen; that is, if a specific probability of type i error is desired, say 1 %, one can set 𝛼 = 0.01 for the specific test. doing the same for type ii error is not so easy. recall that the probability distribution for the statistic produced by the test was determined assuming the generator does indeed produce a random sequence, that is, assuming 𝐻0 is true. in the case of type ii error, 𝐻0 is not true, so the probability distribution of the statistic test is not known. unless this probability distribution is known, 𝛽 is not a fixed value because there is an infinite number of ways that a sequence can be non-random. each different way determines a different 𝛽. 7. the state-of-the-art in statistical tests under the framework of hypothesis testing, a series of tests can be devised to analyse samples of the random number generator. there is no maximum number of tests one can apply to a random number generator, and there is no maximum number of tests a random number generator can pass that will prove it to be truly random. it is also not possible to build an algorithmic random number generator that passes all statistical tests [12]. nonetheless, the more tests one applies to a random number generator, the more confident one becomes of its quality. perhaps the first battery of tests was devised by donald knuth in 1969 [1]. in 1996, given the insufficiency of knuth’s tests, george marsaglia published diehard [21]. to supersede marsaglia’s tests, nist, in the united states, published its own battery [22] in the year 2000, with its latest revision in 2010. robert brown published dieharder in 2004. in 2007, pierre l’ecuyer and richard simard published testu01, a c library with which c programmers can implement and test random number generators [23]: … empirical testing of random number generators is very important, and yet no comprehensive, flexible, state-of-theart software is available for that, aside from the one we are now introducing. the aim of the testu01 library is to provide a general and extensive set of software tools for statistical testing of random number generators. it implements a larger variety of tests than any other available competing library we know. [...] testu01 was developed and refined during the past 15 years and beta versions have been available over the internet for a few years already. it will be maintained and updated on a regular basis in the future. testu01’s results were ‘sobering’ [14] for many ‘respectable’ and well-known random number generators [18]: [pierre l’ecuyer and richard simard] made a very significant contribution to the world of random-number– generator testing when they created the testu01 statistical test suite. other suites, such as [diehard], had existed previously, but testu01 (which included a large number of previously independently published tests, and applied them at scale) vastly increased the scope and thoroughness of the testing process. the library comes with three predefined test batteries: smallcrush, the small one, crush, the medium-sized one and bigcrush. smallcrush is the quickest, and it should finish in under a minute on most modern desktop computers. crush can take a few hours, and bigcrush takes many hours or perhaps a day. as for alternatives to testu01, two other packages are competitors: practrand 0.94 [24] and gjrand 4.2.1 [25], but neither has been formally published. 8. a note on using the testu01 library an inconvenience of testu01 is that it is restricted to the c programming language. it is a c library, after all; it cannot run unless a programmer writes a program that takes advantage of the library. besides, given that testu01 is written in c, it would not be straightforward to use it from another programming language, as one would have to know how to access a c library from within the chosen programming language. this inconvenience has been mitigated by crush [26], [27], a program capable of testing a random number generator against any of the three testu01 batteries, given the data is available on a file on a disk or can be produced at run time. for example, suppose one would like to test one’s local /dev/urandom against the largest testu01 battery. it suffices to say to the shell: %crush --battery big --name xyz < /dev/urandom ... % acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 133 similarly, if one has a program p that can produce its allegedly random data to the standard output in binary format, then crush can test such data against the small testu01 library with a command such as: %./p | crush --battery small --name my-prng ... % due to the facilities of a unix-like system1, crush eliminates the need to use the c programming language to take advantage of testu01’s default batteries. 9. a note on default random number generators if one is writing a new application that needs a random number generator, one should not just use random number generators offered by the system or by the chosen programming language. most programming languages have adopted flawed generators. java, for example, offers the package java.util.random, which is based on the pseudorandom number generator drand48. it failed five tests in smallcrush in less than a minute [27]. the default pseudorandom number generator in both python and php is mt19937, mersenne twister [20]. it passes smallcrush, but actually fails the linear complexity test, which is not included in testu01 even though it is a quick test to run and could have been part of the small battery. the number 19937 in mersenne twister’s name is due to its huge period of size 219937 − 1. despite having been a promising pseudorandom number generator, mt19937 can be totally predicted after collecting a sample of size 624 [18]. in c++, besides mt19937, the standard library also offers minstd and ranlux24, two well-known generators, but minstd fails 9 tests out of 15 of the small battery, and ranlux is not much better [23]. c++ does offer, however, ranlux48, which passes bigcrush and can be used by applications that can afford the higher cost of such generator. exceptionally, some programming languages offer good random number generators as the default option. for example, the default pseudorandom number generator in the racket programming language from the lisp family is pierre l’ecuyer’s mrg32k3a [28], which passes bigcrush [23]. for applications that require cryptography, a well-known computationally secure pseudorandom number generator is based on the stream cipher chacha20 [29]. chacha20 has replaced rc4 in openbsd starting at version 5.4, in netbsd in version 7.0 and replaced sha-1 in the linux kernel since version 4.8. these events present evidence that chacha20 is currently well regarded. 10. on the insufficiency of the nist sp 800-22 suite notwithstanding the ‘sobering’ results of testu01 [23], [18], it is not hard to find publications ignoring it [30], [31], [32] while giving attention to the software package provided by nist sp 800-22. enough flaws of this test suite have been previously reported [33]-[37], but we will now present one more result 1 notice windows is sufficiently unix-like for the purposes of running crush, and a win32 binary is available on crush’s homepage at https://bit.ly/319bg0h. 2 in practrand version 0.94, the implementation is found in src/rngs/other/fibonacci.cpp. regarding the insufficiency of the nist sp 800-22 statistical test suite implementation. it is known that the fibonacci sequence is not satisfactorily random to operate as a random number generator, but a ‘much better’ variation was proposed in 1958 by g. j. mitchell and d. p. moore, though it was never published. using an output of 32bit integers, let us call mm32 this pseudorandom number generator defined by the sequence 𝑋𝑛 = (𝑋𝑛−24 + 𝑋𝑛−55) mod 𝑚 where 𝑛 ≥ 55, 𝑚 is even, and 𝑋0, 𝑋1, … , 𝑋54 are arbitrary integers not all even. the constants 24 and 55 were chosen so that the least significant bits of the sequence, that is the sequence 𝑋𝑛 mod 2, will have a period of length 2 55 − 1, implying the sequence 𝑋𝑛 must also have a period of the same length [1]. despite mm32’s period length, ‘it is difficult to recommend [it] wholeheartedly [because] there is still very little theory to prove that [it does or does not] have desirable randomness properties; essentially all we know for sure is that the period is very long, and this is not enough.’ [1] that is, from a theoretical perspective, very little is known about mm32, but, assuming testu01 has a correct implementation of the statistical tests included in its batteries and practrand implements mm32 correctly2, statistical evidence suggests mm32 does not have desirable randomness properties. setting mm32 with an initial value of 0 in practrand’s implementation and submitting it to the battery smallcrush in testu01, the battery reports that mm32 fails the gap test [1] and the weight distribution test [38] after consuming approximately 6.7 gibibits3 from the generator in less than 10 seconds on a certain system. practrand reports that mm32 fails the binary rank test [39], among other failures, after consuming 2 gibibits from the generator in less than 5 seconds, and gjrand reports mm32 also fails the binary rank test consistently, among other failures, after consuming 8 gibibits from the generator in less than 15 seconds. nevertheless, the nist sp 800-22 statistical test suite does not notice anything unusual with mm32 after consuming a total of 8 gibibits from the generator, that is, after consuming 32 samples of 256 mebibits4 each in over 15 hours5. could the nist sp 800-22 statistical test suite reject mm32 by considering larger samples? it was found that 32 gibibytes of memory are not enough to give the nist sp 800-22 statistical test suite a sample size of 2 gibibits. when the size was reduced to 1 gibibit, the software received a recurrent unix sigsegv signal. in other words, it crashes at this sample length. the same crash can be reproduced with various small sequence lengths, such as 1031 and many smaller values. also, on sample lengths of sizes such as 256 mebibits, the nist sp 800-22 statistical test suite is not able to properly calculate a 𝑝-value for all of its tests with its default parameters. it was not easy to make sense of the 𝑝-values produced by the overlapping template matchings test on sample lengths such as 256 mebibits for any generator tested. regarding comparisons, notice each battery in each software package uses a different strategy, configurable in different ways, which makes comparison rather difficult. for example, despite the fact that smallcrush consumed a total of approximately 3 one gibibit is 230 bits. 4 one mebibit is 220 bits. 5 a more efficient implementation of the nist sp 800-22 statistical test suite has been reported [40], but its implementation could not be located. https://bit.ly/319bg0h acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 134 6.7 gibibits from mm32, each test individually consumed far less. for example, the weight distribution test used a sample length of 200,000 and took less than a second to run. with a generator producing random numbers at run time, the library by default decides not to restart the generator as it moves from one test to another. 11. conclusions choosing a random number generator is no simple task. it should not be underestimated. default pseudorandom number generators offered by popular programming languages usually do not offer enough statistical properties. it was argued that the nist sp 800-22 statistical test suite, as implemented in the software package and last revised in 2010, is inadequate for testing random number generators. with crush [26], [27], testing a random number generator against the state-of-the-art in statistical tests is a trivial matter. references [1] d. knuth, the art of computer programming, volume 2, 3rd edition, addison-wesley, boston, 1997, isbn: 978-0-201-896848. [2] linux programmer’s manual, 2017, see ‘man 5 proc’. [3] i. goldberg, d. wagner, randomness and the netscape browser, dr dobb’s journal-software tools for the professional programmer 21(1) (1996) pp. 66-71. [4] national institute of standards and technology (nist), security requirements for cryptographic modules, federal information processing standards publication (fips pub) 140-2 (may 2001). online [accessed 29 october 2020]. https://goo.gl/a0sze [5] d. j. bernstein, yun-an chang, chen-mou cheng, li-ping chou, n. heninger, t. lange, n. van someren, ‘factoring rsa keys from certified smart cards: coppersmith in the wild’, in: advances in cryptology – asiacrypt 2013. k. sako, p. sarkar (editors). springer, berlin, heidelberg, 2013, isbn 978-3-642-42044-3, pp. 341-360. [6] a debian weak key vulnerability. cve-2008-0166 (2008). [7] n. heninger, z. durumeric, e. wustrow, j. a. halderman, mining your ps and qs: detection of widespread weak keys in network devices, 21st usenix security symposium 2012, bellevue, usa, pp. 205-220. [8] bitcoin.org, android security vulnerability alert notice, 11 august 2013. online [accessed 29 october 2020]. https://goo.gl/zk1hpm [9] k. michaelis, c. meyer, j. schwenk, randomly failed! the state of randomness in current java implementations, in: topics in cryptology – ct-rsa 2013. lncs, vol. 7779. e. dawson (editor). springer, heidelberg, 2013, 978-3-642-36095-4_9, pp. 129-144. [10] s. h. kim, d. han, d. h. lee, predictability of android openssl’s pseudo random number generator, proceedings of the acm sigsac conference on computer & communications security, acm, 2013, berlin, germany, pp. 659-668. [11] j.-m. gurney, urgent: rng broken for the last four months (2015). online [accessed 29 october 2020]. https://goo.gl/ktqhd5 [12] p. l’ecuyer, ‘random number generation’, in: handbook of computational statistics. j. gentle, w. k. härdle, y. mori (editors). springer berlin, heidelberg, 2012, isbn 978-3-64221550-3, pp. 35-71. [13] b. russell, mysticism and logic and other essays, 2nd edition, george allen & unwin ltd, london, 1917. [14] d. knuth, construction of a random sequence, bit 5 (1965), pp. 246-250. [15] m. sipser, introduction to the theory of computation, 3rd international edition, cengage learning, 2013, isbn 978-1133-18781-3. [16] d. chicayban bastos, uma versão quântica do algoritmo rô de pollard (master’s thesis), universidade federal fluminense, niterói, 2019. [17] g. marsaglia, xorshift rngs, journal of statistical software 8(14) (2003), pp. 1-6. [18] m. e. o’neill, pcg: a family of simple fast space-efficient statistically good algorithms for random number generation, technical report hmc-cs-2014-0905, harvey mudd college, claremont, ca, 2014. [19] s. a. crosby, d. s. wallach, denial of service via algorithmic complexity attacks, usenix security symposium, 2003, washington, dc, usa, pp. 29-44. [20] m. matsumoto, t. nishimura, mersenne twister: a 623dimensionally equidistributed uniform pseudo-random number generator, acmt transactions on modeling and computer simulation (tomacs) 8(1) (1998) pp. 3-30. [21] g. marsaglia, diehard, a battery of tests for random number generators, cd-rom, 1996. [22] l. e. bassham, a. l. rukhin, j. soto, j. r. nechvatal, m. e. smid, s. d. leigh, m. levenson, m. vangel, n. a. heckert, d. l. banks, a statistical test suite for random and pseudorandom number generators for cryptographic applications, nist, special publication 800-22 rev 1a, 2010. [23] p. l’ecuyer, r. simard, testu01: a c library for empirical testing of random number generators, acm transactions on mathematical software (toms) 33(4) (2007) p. 22. [24] c. doty-humphrey, practrand, 2018. online [accessed 29 october 2020]. https://goo.gl/hwu9g5 [25] g. johnson, gjrand, 2014. online [accessed 29 october 2020]. https://goo.gl/2axrwu [26] d. chicayban bastos, l. a. brasil kowada, r. c. s. machado, measuring randomness in iot products. ii workshop on metrology for industry 4.0 and iot, 2019, naples, italy, pp. 466470. [27] d. chicayban bastos, l. a. brasil kowada, medindo a qualidade de geradores de números aleatórios, iv workshop sobre regulação, avaliação da conformidade, testes e padrões de segurança, 2019, campinas, brazil. [28] p. l’ecuyer, r. simard, e. jack chen, w. d. kelton, an objectoriented random-number package with many long streams and substreams, operations research 50(6) (2002) pp. 1073-1075. [29] d. j. bernstein, chacha, a variant of salsa20, workshop record of sasc 8 (2008) pp. 3-5. [30] k. hirano, t. yamazaki, s. morikatsu, h. okumura, h. aida, a. uchida, s. yoshimori, k. yoshimura, t. harayama, p. davis, fast random bit generation with bandwidth-enhanced chaos in semiconductor lasers, opt. express 18 (2010) pp. 5512-5524. [31] m. a. zidan, a. g. radwan, k. n. salama, random number generation based on digital differential chaos, ieee 54th international midwest symposium on circuits and systems (mwscas), 2011, seoul, south korea, pp. 1-4. doi: https://doi.org/10.1109/mwscas.2011.6026266 [32] m. stipcevic, ç. k. koç, ‘true random number generators’, in: open problems in mathematics and computational science. ç. k. koç (editor). springer, 2014, isbn 978-3-319-10682-3, pp. 275-315. [33] s. zhu, y. ma, j. lin, j. zhuang, j. jing, ‘more powerful and reliable second-level statistical randomness tests for nist sp 80022’, in: advances in cryptology, asiacrypt 2016. lecture notes in computer science, vol 10031. j. cheon, t. takagi (editors). springer, berlin, heidelberg, 2016, isbn 978-3-66253886-9, pp. 307-329. [34] song-ju kim, k. umeno, a. hasegawa, corrections of the nist statistical test suite for randomness, arxiv preprint nlin/0401040 (2004). https://goo.gl/a0sze https://goo.gl/zk1hpm https://goo.gl/ktqhd5 https://goo.gl/hwu9g5 https://goo.gl/2axrwu https://doi.org/10.1109/mwscas.2011.6026266 acta imeko | www.imeko.org december 2020 | volume 9 | number 4 | 135 [35] f. pareschi, r. rovatti, g. setti, on statistical tests for randomness included in the nist sp800-22 test suite and based on the binomial distribution, ieee transactions on information forensics and security 7(2) (2012) pp. 491-505. [36] k. hamano, the distribution of the spectrum for the discrete fourier transform test included in sp800-22, ieice transactions on fundamentals of electronics, communications and computer sciences 88(1) (2005) pp. 67-73. [37] k. hamano, t. kaneko, correction of overlapping template matching test included in nist randomness test suite, ieice transactions on fundamentals of electronics, communications and computer sciences 90(9) (2007) pp. 1788-1792. [38] m. matsumoto, y. kurita, twisted gfsr generators, acm transactions on modeling and computer simulation (tomacs) 2(3) (1992) pp. 179-194. [39] g. marsaglia l.-h. tsay, matrices and the structure of random number sequences, linear algebra and its applications 67 (1985) pp. 147-156. [40] a. suciu, k. marton, i. nagy, i. pinca, byte-oriented efficient implementation of the nist statistical test suite. international conference on automation, quality and testing, robotics, 2010, cluj-napoca, romania, pp. 1-6. doi: https://doi.org/10.1109/aqtr.2010.5520837 https://doi.org/10.1109/aqtr.2010.5520837 intercomparison between deadweight machines in china and uk up to 1 mn acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 106 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 106 108 intercomparison between deadweight machines in china and uk up to 1 mn lin shuo1, a. knott2, chi hui1, li shixin3, tang yun4 1 fujian province institute of metrology, fuzhou, china, linshuo1001@126.com 2 national physical laboratory, teddington, united kingdom, andy.knott@npl.co.uk 3 tianjin institute of metrological supervision and testing, tianjin, china, 13920372559@163.com 4 national institute of measurement and testing technology, chengdu, china, tangy@nimtt.com abstract: this paper describes a tripartite comparison at forces of 250 kn, 500 kn, and 1 mn between deadweight force standard machines located at fjim (china), nimtt (china), and npl (uk). two different transfer standards were used, and the results demonstrated good agreement between the three machines, with 𝐸𝑛 values of significantly lower magnitude than one. keywords: deadweight machine, comparison, uncertainty, 𝑬𝒏 value 1. introduction in order to investigate the agreement between the large force standards established by china and the united kingdom (uk), an intercomparison was carried out from september to november 2019. the new 2 mn deadweight machine (dwm) established in 2018 by fjim has a stated uncertainty of 0.002 % (k = 3) and is shown in figure 1. figure 1: 2 mn dwm at fjim in order to give support to the uncertainty claims, it was agreed that an international comparison with other machines of suitable capacity and uncertainty should be performed. nimtt and npl, the laboratories responsible for maintaining china’s and the uk’s national force standards at the 1 mn level, agreed to participate in such a comparison, using machines of the following specifications: ⚫ 1 mn dwm at nimtt, with a stated uncertainty of 0.002 % (k = 3) ⚫ 1.2 mn dwm at npl, with a stated uncertainty of 0.001 % (k = 2) the three test machines are shown in figure 2. the transfer standards used were a 500 kn top z4a load cell and a 1 mn c18 load cell (class 00 [1]), both manufactured by hbm. different hbm indicators (dmp40 and dmp41) were used at the three laboratories, with a single hbm bn100a bridge calibration unit employed to correct the deflections to nominal mv/v values. figure 2: tests at fjim, nimtt, and npl 2. comparison method the comparison method was based on that used for cipm force key comparisons [2], but with a single rotation of the transducer (from 0° to 360°) rather than two rotations (from 0° to 720°). the tests were first carried out by fjim (a1), then by nimtt (b1), then by npl (b2), and finally again by fjim (a2), in a single loop comparison. the start of the loading cycle is shown in figure 3. in order to minimise the effects of creep, each test was carried out in accordance with a strictly-timed loading profile, including the three preloads which were always performed at the start of each test and a preload after each rotation of the transducer in the machine. one value of deflection was obtained at each of six orientations, and the mean deflection was calculated as the mean of these six deflections. http://www.imeko.org/ mailto:linshuo1001@126.com mailto:andy.knott@npl.co.uk mailto:13920372559@163.com mailto:tangy@nimtt.com acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 107 figure 3: loading cycles for the two load cells before the comparison, the effect of temperature on transducer sensitivity was determined. the reference temperature of the comparison is 20 ºc, and each transducer was calibrated at temperatures of 15 ºc, 20 ºc, and 25 ºc, to obtain its temperature sensitivity characteristics. the period between the a1 and a2 tests was about two months, and the transducer drift was calculated to the day, assuming a linear trend. these two factors were used in the normalization of test results to make them comparable. 3. test results 3.1. measured values the measurement results are summarised in table 1 and table 2. in the tables, all effects resulting from indicator sensitivity, temperature, and transducer drift have been corrected for. table 1: results obtained at fjim and nmitt transfer standard force / kn fjim mean deflection / mv/v nimtt mean deflection / mv/v top z4a 250 0.999 821 0.999 834 500 1.999 952 1.999 978 c18 500 1.000 706 1.000 682 1 000 2.001 870 2.001 804 table 2: results obtained at fjim and npl transfer standard force / kn fjim mean deflection / mv/v npl mean deflection / mv/v top z4a 250 0.999 813 0.999 833 500 1.999 935 1.999 942 c18 500 1.000 698 1.000 677 1 000 2.001 854 2.001 812 3.2. uncertainty analysis for the calculation of the 𝐸𝑛 value, the correct uncertainty contributions obtained at the three laboratories must be included in the analysis [3]. the 𝐸𝑛 value is defined by equation (1). 𝐸𝑛 = 𝑥1 − 𝑥2 √𝑈2(𝑥1) + 𝑈 2(𝑥2) , (1) where 𝑥𝑖 is the mean deflection at a given laboratory and 𝑈(𝑥𝑖 ) is its expanded uncertainty (k = 2). it is therefore imperative that the correct uncertainty contributions are included in the analysis the following sections detail the uncertainty components which have been considered in this exercise and explain how each has been dealt with in the analysis. the uncertainty components that have been considered in this exercise include: the applied force; transducer drift; instrumentation; repeatability; reproducibility; resolution; and temperature. uncertainty of applied force this component is simply the claimed expanded uncertainty of the force generated by the machine, divided by the relevant value of k. drift of transducer sensitivity because the quality of the comparison is dependent upon the three measurements made during each loop, the stability of each transducer’s sensitivity is critical. the a1 and a2 calibrations of the transducers at fjim provide results of the drift in the transducer sensitivity throughout the exercise, and this drift is contributed into the fjim uncertainty value as a component. instrumentation it was assumed that the voltage ratios generated by the bn100a remained constant throughout the intercomparison, to enable corrections to be made, http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 108 but the uncertainty associated with this assumption is included in the budget. repeatability the repeatability of the transducer is calculated at the 0° orientation in each test. some of this variation may be due to the repeatability of the calibration force, but some will also be due to the transducer performance. the repeatability is estimated as a rectangular distribution, with a width equal to the spread of deflections. reproducibility the reproducibility of the transducer is defined as the variation in deflection at the six different orientations in a single test. the reproducibility is incorporated into each laboratory’s uncertainty budget as the standard deviation of these six deflections, divided by square root of six (the number of values used to calculate this standard deviation). resolution the resolution of the indicator (0.000 001 mv/v for both dmp40 and dmp 41 indicators) is incorporated twice into each deflection uncertainty estimation, once for the zero value and once for the value with the force applied. this is equivalent to a single triangular distribution of width 0.000 002 mv/v. temperature the calibrations at the three laboratories were carried out at different temperatures from 20.3 ºc to 21.4 ºc at fjim, 21.0 ºc at nimtt, and from 20.6 ºc to 20.9 ºc at npl. the temperature sensitivities of the transducers have been accurately determined by tests, assuming a rectangular distribution and using the measured temperature difference for each set of tests as the half-width of the distribution. 3.3. comparison results the associated 𝐸𝑛 values of tests are plotted in figure 4, while figure 5 shows the relative deviations between fjim and the other two laboratories and their relative expanded uncertainties. figure 4 shows that, of the eight points at which the two pairs of machines were compared, all of the resulting 𝐸𝑛 values are significantly smaller than one, giving confidence in the claimed uncertainties of the machines at these values. figure 5 shows that the relative deviations of the two pairs of machines is less than 3 × 10-5, with an expanded uncertainty of less than 5 × 10-5 at a confidence level of approximately 95 % (k = 2). it also demonstrates that the agreement between npl and nimtt is consistently within 1 × 10-5. figure 4: plot of 𝐸𝑛 ratio values figure 5: relative deviations from fjim 4. summary the results of a tripartite comparison of force standards between fjim, nimtt, and npl have been detailed, and provide evidence to support the uncertainty claims of the three laboratories. this comparison is of very positive significance for verifying the consistency of force between china and the uk, and to enhancing the confidence of those who depend on these standards in their respective countries, and providing useful suggestions for future research. 5. references [1] iso 376:2011 metallic materials calibration of force-proving instruments used for the verification of uniaxial testing machines, 2011. [2] w. vincke et al., “final report on force key comparison ccm .f-k2.a and ccm .f-k2.b (50 kn and 100 kn)”, metrologia, vol. 49, 07002, technical supplement 2012. [3] a. knott et al., “comparison of force standards between nis (egypt) and npl (united kingdom)”, proc. of xviii imeko world congress, rio de janeiro, brazil, 17 – 22 september 2006. http://www.imeko.org/ ccm pilot study overview: geometrical measurement of the rockwell diamond indenter acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 250 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 250 255 ccm pilot study overview: geometrical measurement of the rockwell diamond indenter r. r. machado1, s. low2, a. germak3 1 inmetro, duque de caxias, brazil rrmachado@inmetro.gov.br 2 nist, gaithersburg, usa, samuel.low@nist.gov 3 inrim, torino, italy, a.germak@inrim.it abstract: this paper describes an overview of the capability of the nmis that participated on the ccm pilot study measurement systems, conducted by the cipm/ccm/working group on hardness, to characterize the rockwell hardness diamond indenter geometry, by measuring the included cone angle, the straightness of the generatrix, the spherical tip radius, the deviation of the local radius and the tilt angle. nine nmis took part in this study: inmetro (brazil); inrim (italy); kriss (south korea); nim/pr (china); nimt (thailand); nist (usa); ptb (germany); tubitak ume (turkey); vniiftri (russia), where inmetro (brazil) served as pilot laboratory. keywords: hardness, diamond indenter, geometry indenter, pilot study, cipm/ccm/wgh 1. introduction international hardness measurement equivalence between the national metrology institutes (nmis) is the role of the consultative committee for mass and related quantities (ccm) working group on hardness (wgh) within the international committee of weights and measures (cipm) [1]. there have been numerous discussions among the members of the ccm-wgh concerning the difficulty in obtaining calibration-grade rockwell hardness diamond indenters having the correct shape within geometric tolerances as specified in international standards. there are two main reasons for this difficulty. the primary reason is that the crystallographic structure of diamond makes it extremely difficult to grind and polish to the required rockwell indenter shape. the rockwell indenter shape is a cone having a 120° included angle and a spherical tip radius of 0.2 mm. secondly, the accurate measurement of a rockwell indenter’s geometrical parameters is a difficult and often time-consuming process due to the conical and spherical indenter geometries and the optical transparency and high reflectivity of the polished diamond. therefore, the ccm-wgh agreed to carry out a pilot study (ps) on the measurement of rockwell hardness diamond indenter geometries in which the laboratories of national metrology institutes (nmis) should participate. inmetro (brazil) was designated as the pilot laboratory. the pilot study was conducted from december 2011 until january 2015, with all but the repeat measurements of the pilot laboratory completed by march 2013. 2. principle of the pilot study the purpose of this ccm-ps was to document how the nmis are measuring, analyzing and reporting results of their indenter geometry calibrations. this study was intended to reveal current nmi calibration capabilities and any measurement issues, before conducting a ccm key comparison that will establish measurement equivalence between the nmis engaged in the metrological characterization of diamond indenters. 2.1. test artifacts (measurand) three rockwell hardness diamond indenters designated as indenters 104, 122 and 130 (as engraved on them) were measured for specific geometry parameters by the participating laboratories. these three indenters were chosen for the pilot study from an inventory of indenters previously measured and provided by nist/usa. they were circulated among the laboratories with the following characteristics: one indenter was believed to have a near-nominal shape while the other two were less accurate but within permissible limits defined by iso 6508-3 [2]. figure 1 gives the approximate overall dimensions of them indenters. 2.2. format of the comparison the participating laboratories conducted dimensional measurements of specified geometric parameters on each of the three rockwell diamond indenters using their usual procedures for qualifying them, and according to the capacity of their measurement systems. those laboratories that did not http://www.imeko.org/ mailto:rrmachado@inmetro.gov.br mailto:samuel.low@nist.gov mailto:a.germak@inrim.it acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 251 have their own procedures to qualify rockwell diamond indenters were instructed to follow the requirements defined in iso 6508 part 3 [2]. deviations from the procedures established in this standard were to be reported. figure 1: approximate dimensions of the indenters the dimensional measurements were intended to illustrate measurement biases among nmis and measurement differences between different types of measuring instruments. figure 2 shows the geometry of a rockwell diamond indenter, according to the following parameters and limits defined in the iso 6508-3 [2]. • included cone angle of the diamond (). limits from the mean value: (120 ± 0.1)°. • radius of the spherical tip of the diamond (r). limits from the mean value: (0.200 ± 0.005) mm. • profile deviation of the local radius from the least square mean radius value. limit: max 0.002 mm. • straightness of the generatrix of the diamond cone, adjacent to the blend of the spherical and conical parts. limit from the mean deviation: max 0.000 5 mm over a minimum length of 0.4 mm. • angle between the axis of the diamond cone and the axis normal to the seating surface. limit: max 0.3°. figure 2: illustration of the geometries of the rockwell diamond indenter 2.3. indenter calibration systems a rockwell diamond indenter calibration system should be capable of making the dimensional measurements specified in iso 6508-3 [2]. for measurement systems that make axial-plane measurements, if possible, the diamond shape measurements should be made at the 8 section locations as illustrated in figure 3. measurement position 1 is in the axial plane of the indenter that is parallel with the flat on the indenter shaft. position 2 is in the axial plane rotated 45° clockwise around the indenter axis from position 1 as the indenter tip points towards the operator. the remaining 6 measurements should be made similarly at 45° rotations. figure 3: illustration of the measurement sections of the rockwell diamond indenter. the figure on the left is oriented with diamond tip pointing towards to the operator [3] 3. method of analysis two methods of analysis were used to determine the dispersion of all data points obtained by the labs. the first one was the weighted mean method, which is one of the most used methods in comparisons, based on the reproducibility between the labs (standard deviation of the mean). the second one was based on the nist consensus builder (nicob), which is consistent with the combination of measurement results obtained independently by different laboratories or measurement methods [4, 5]. 3.1. weighted mean method one of the methods used to analyze the results of the comparison and determine the dispersion of the data points obtained by the labs was the weighted mean method, equation (1). �̅� = ∑ 𝑥𝑖×𝑤𝑖 𝑛 𝑖 ∑ 𝑤𝑖 𝑛 𝑖 , where (1) �̅� weighted mean 𝑥𝑖 measured values provided by the nmi participant 𝑤𝑖 weight 𝑤𝑖 = 1 𝑢𝑖 2 , where (2) 𝑢𝑖 standard uncertainty of 𝑥𝑖. from the above method of analysis, the normalized errors from the weighted mean (en.w), equation (3), were calculated, and the respective graphics were plotted (clause 4). 𝐸𝑛. 𝑤 = (𝑥𝑖−�̅�) √𝑈𝑖 2+𝑈𝑟𝑒𝑓 2 , where (3) 𝐸𝑛. 𝑤 normalized error from the weighted mean 𝑥𝑖 measured mean values provided by the nmi participants �̅� weighted mean value 𝑈𝑖 expanded uncertainty provided by the nmi participants (k = 2) 𝑈𝑟𝑒𝑓 reference value of the expanded uncertainty from weighted mean. 14.27 11.10 9.52 6.3373 max 9.65 5.80 dimensions in mm flat http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 252 3.2. nicob method the other method used to analyze the results of the comparison, and to determine the dispersion of the obtained data points by the labs was the method based on the nist consensus builder (nicob) methodology. this methodology is consistent with the combination of measurement results obtained independently by different laboratories or measurement methods [4, 5]. among the three procedures available in the nicob method, the dersimonian-laird (dl) seemed to be the most suitable for this case, and it was the procedure applied to this analysis. this procedure uses equation (4) as a weighted mean. �̅�𝑫𝑳 = ∑ 𝒙𝒊×𝒘𝒊.𝑫𝑳 𝒏 𝒊 ∑ 𝒘𝒊.𝑫𝑳 𝒏 𝒊 , where (2) �̅�𝑫𝑳 dersimonian-laird weighted mean 𝒙𝒊 measured values provided by the nmi participant 𝒘𝒊.𝑫𝑳 dersimonian-laird weight 𝒘𝒊.𝑫𝑳 = 𝟏 𝒖𝒊 𝟐+𝝉𝟐 , where (5) 𝒖𝒊 𝟐 uncertainty associated with the value measured by the nmis 𝝉𝟐 indication of the heterogeneity of the measured results from the above method of analysis, the normalized errors from the dersimonian-laird weighted mean (en.dl), equation (6), were then calculated, and the respective graphics were plotted (clause 4). 𝑬𝒏. 𝑫𝑳 = (𝐱𝒊−�̅�𝑫𝑳) √𝑼𝒊 𝟐+𝑼𝒓𝑫𝑳 𝟐 , where3 (3) 𝐸𝑛. 𝐷𝐿 normalized error form dersimonianlaird weighted mean 𝑥𝑖 measured mean values provided by the nmi participants �̅�𝐷𝐿 dersimonian-laird weighted mean value 𝑈𝑖 expanded uncertainty provided by the nmi participants (k = 2) 𝑈𝑟𝐷𝐿 reference value of the expanded uncertainty from dl weighted mean 4. results some nmis provided more than one measurement result for the same parameter based on measuring different areas or windows of the indenter surface. consequently, for each window, different values were obtained for the same indenter. the windows considered in this report, were those around the window values described by john song et al. [6]. they said that for an ideally shaped rockwell indenter, as specified in the standards [7] and [2], i.e., 200 μm tip radius blending with a 120° cone angle in a true tangential manner, the window sizes must be ± 100 μm for the tip radius calibration; and ± (100 to 450) μm along the x-axis for the left and right contributions to the cone angle calibration, and the sizes must be specified in the data analysis. the choice of the measurement window is especially important for calculation of least squares radius and profile deviation in the center and cone flank straightness in the left and right. furthermore, the blend area of the tip radius and cone angle must be accounted for when choosing window positions. beyond these considerations, john song et al. [6], said that the blend point can change depending on the actual cone angle and tip radius. in light of these deviations, both the astm and iso standards specify that the straightness of the cone flank is measured “adjacent to the blend” which leaves some flexibility in the choice of the size and position of the windows on the flanks. 4.1. results of the weighted mean ( �̅�), dersimonian-laird weighted mean ( �̅�𝑫𝑳 ) and their expanded uncertainties in the coming clauses, we present the results of the mean values measured by the nmi participants and their respective measurement uncertainties for each parameter. for convenience, the graphics from figure 4 to figure 8 are showing only the results related to the weighted mean. 4.1.1. included cone angle the nmis measurement results related to the geometric parameter “included cone angle” are shown in figure 4 for the three indenters. in general, it can be seen in this figure good agreement between the nmi results. figure 4: the weighted mean values and the respective uncertainties (error bars) of the included cone angle for all indenters and the maximum limits specified by iso 65083 the only exceptions were due to the larger uncertainty provided by nmis “f’ and “j”, according to the error bars. we note that nmi “g” would fail this parameter for indenters 104 and 122 while all other nmis would pass them. http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 253 4.1.2. radius of the spherical tip figure 5 presents the data related to the geometric parameter “radius of the spherical tip”. it is possible to see a very good dispersion among the obtained results for the three indenters, measured by all nmi participants. however, for indenters 122 and 130, some nmis would have passed the radius geometry of the two indenters while others would have failed their compliance with iso 6508-3. also, nmi “e” reported a larger uncertainty than the other nmis, mainly when measuring the indenters 104 and 130. figure 5: the weighted mean values and the respective uncertainties (error bars) of the radius of the spherical tip for all indenters 4.1.3. maximum profile deviation of the radius from the true radius figure 6 presents graphics related to the geometric parameter “max profile deviation” for the indenters 104, 122 and 130. it is noted that only five nmis reported this measurement indicating an expansion of measurement capabilities may be needed at some nmis. figure 6: the weighted mean values and the respective uncertainties (error bars) of the max profile deviation for all indenters two of the nmi’s results varied from the other three with one nmi's results for one indenter exceeding the iso 6508-3 maximum limit, possibly due to measuring different window sizes and/or higher levels of measurement uncertainty. 4.1.4. straightness of the diamond cone generatrix figure 7 presents graphics related to the geometric parameter “straightness of the generatrix” for all three measurands. it is possible to notice in this figure there is a large variation in the reported measurement uncertainties, however these uncertainties do not explain the measurement differences. furthermore, almost half of the nmis would have failed the three indenters for exceeding the limit specified by iso 6508-3 for this parameter. a contribution to these measurement differences may be due to choosing a measurement window that includes part of the tip-radius curvature in the blend area which would negatively affect the measurement. it is also noted that only seven nmis reported this measurement, again indicating an expansion of measurement capabilities may be needed at some nmis. figure 7: the weighted mean values and the respective uncertainties (error bars) of the straightness of the generatrix for all indenters 4.1.5. angle between the axis of the diamond cone and the axis normal to the seating surface figure 8 presents graphics related to the geometric parameter “angle between axes” for the indenters 104, 122 and 130. it is possible to see in these graphics that, with the exception of nmi “g”, which stayed very far beyond the mean values, all other nmis stayed around the average values. figure 8: the weighted mean values and the respective uncertainties (error bars) of the angle between axes for all indenters http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 254 4.2. results of the normalized errors (en) this section reports the normalized errors calculated for the measurement results of the geometric parameters of the three indenters measured by the nmi participants of this pilot study. 4.2.1. included cone angle the behavior of en, calculated for all nmis related to the parameter “included cone angle”, is described in figure 9. it is possible to see in this figure that the influence caused by the two different methods of analysis were small. however, almost half of nmi’s measurements did not agree well with the mean values for one or more of the indenters. it is curious that, for some nmis, the measurement of this parameter on one indenter is outside the agreement with the other nmis while the same measurements on the other two indenters are in better agreement. this could possibly be explained by different blend locations on the indenters and choices of window sizes. figure 9: en graphics related to the weighted mean and dl mean values of the included cone angle for all indenters 4.2.2. radius of the spherical tip figure 10 is related to en calculated for the parameter radius of the spherical tip. in this figure, it is possible to see that the results of all nmis are reasonably consistent no matter the method of analysis considered. figure 10: en graphics of the weighted mean and dl mean values of the radius tip for all indenters as with the measurement of cone angle, several nmi’s measurements of this parameter on one indenter did not agree well with the mean measurement values while their measurements of the other indenters were consistent with the other nmis. again, this could possibly be explained by different blend locations on the indenters and choices of measurement windows. 4.2.3. maximum profile deviation of the radius from the true radius the following figure 11, related to the parameter max profile deviation of the radius shows the behavior of en, calculated for the five nmis that provided the results of this parameter. it is possible to see in these graphics that the nmi “f” came outside the en tolerance for almost all indenters measured, with the exception for the measurement of the indenter 130, but only when the dl weighted mean method of analysis is considered. figure 11: en graphics related to the weighted mean and dl mean values of the profile deviation for all indenters 4.2.4. straightness of the diamond cone generatrix in figure 12, which is related to the parameter “straightness of the generatrix of the diamond cone”, it is easy to see the dependence of en behavior calculated for all nmis on the two different methods used for the analyses. figure 12: en graphics related to the weighted mean and dl mean values of the straightness of the generatrix for all indenters http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 255 the dl analysis provided significantly improved results for the nmis “a”, “b”, “d”, “f” and “h” that had reported smaller uncertainties, and all measurements analyzed by the dl method were within the en limits except for only two cases. the dl analysis is clearly an improvement over the weighted-mean analysis for these nmi results, and it may provide a better representation of equivalence when there is a large variation in measurement uncertainties. 4.2.5. angle between the axis of the diamond cone and the axis normal to the seating surface figure 13 is related to en calculated for the parameter “angle between axes.” as with the parameter, “straightness of the generatrix of the diamond cone”, for many of the nmis the dl analysis provided significantly improved results compared with the weighted-mean method. however, many nmi measurements continued to fall outside the en limits for some or all indenters. figure 13: en graphics related to the weighted mean and dl mean values of the angle between axes for all indenters 5. discussions this comparison included nine participating labs from national metrology institutes, of europe, asia and the americas, where they had to measure the same geometric parameters of three indenters. however, no reference value and thus no uncertainty of a reference value were assigned. each lab obtained an average value of the same number of measurements, for each parameter, and reported the mean values with their respective measurement uncertainties. considering the measurement results and the plotted graphics (clause 4), this pilot study indicated several issues that the participating nmis should address in order to fully characterize a rockwell hardness diamond indenter, and the ccm-wgh should address in developing the test protocol for a future key comparison. these include: • some nmis have difficulty making measurements of some geometric parameters. this is evident from the examples of nmis having very different results than the other nmis. • some nmis need to expand their capabilities to measure all pertinent parameters. there were several nmis that did not report certain parameter measurement results. • measurement uncertainties are being underestimated and/or the same quantity (i.e., area) is not being measured. there were many examples where the reported uncertainties could not account for the measurement differences. this may not have resulted solely from poor estimations of uncertainties but may also be the result of not measuring the same geometric area. • improved definitions or instructions of what to measure are needed. the result of nmis using different window sizes may have been the primary cause of differences in the measurements related to the tip radius and cone angle, particularly when the radius/cone blend area was included in the measurements. • the nicob dl data analysis may provide improved evidence of measurement equivalence as compared to the weighted-mean analysis. the consequence of not addressing these issues is that some nmis may pass an indenter parameter’s incorrect geometry as specified by the consensus test methods of iso and astm, while other nmis may properly fail it, and vice versa. this has the potential of increasing differences between national rockwell hardness standards. 6. references [1] https://www.bipm.org/en/committees/cc/wg/ccmwgh.html [2] iso 6508-3:2005 metallic materials rockwell hardness test part 3: calibration of reference blocks (scales a, b, c, d, e, f, g, h, k, n, t), international organisation for standardisation, geneva, switzerland. [3] j. f. song, f. f. rudder, jr., t. v. vorburger, and j. h. smith. microform calibration uncertainties of rockwell diamond indenters. j. res. natl. inst. stand. technol. 1995 sep-oct; 100(5): 543–561. [4] nist consensus builder user’s manual. amanda koepke, thomas lafarge, antonio possolo, blaza toman. statistical engineering division information technology laboratory. national institute of standards and technology, gaithersburg, md, 2017. [5] amanda koepke et al. 2017 metrologia 54 s34. https://doi.org/10.1088/1681-7575/aa6c0e [6] j. song, s. low, a. zheng and p. gu. geometrical measurements of nist srm rockwell hardness diamond indenters. in: imeko 2010 tc3, tc5 and tc22 conferences metrology in modern context. november 22−25, 2010, pattaya, chonburi, thailand. [7] astm e18-20, standard test methods for rockwell hardness of metallic materials, astm international, west conshohocken, pa, 2020, www.astm.org http://www.imeko.org/ https://www.bipm.org/en/committees/cc/wg/ccm-wgh.html https://www.bipm.org/en/committees/cc/wg/ccm-wgh.html investigating the transverse motion of a pneumatic shock exciter using two different anvil mounting configurations acta imeko issn: 2221-870x june 2021, volume 10, number 2, 204 208 acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 204 investigating the transverse motion of a pneumatic shock exciter using two different anvil mounting configurations christiaan s. veldman1 1 national metrology institute of south africa, csir campus, brummeria, pretoria, 0184, south africa section: research paper keywords: transverse motion; shock calibration; shock excitation; accelerometer; pneumatic citation: christiaan smith veldman, investigating the transverse motion of a pneumatic shock exciter using two different anvil mounting configurations, acta imeko, vol. 10, no. 2, article 28, june 2021, identifier: imeko-acta-10 (2021)-02-28 section editor: gustavo ripper, inmetro, brazil received october 1, 2020; in final form february 15, 2021; published june 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: ian c. s. veldman, e-mail: csveldman@nmisa.org 1. introduction shock measurements [1], [2] have been performed internationally for a long time. in meeting the requirement to provide traceability for the international system of units (si), national metrology institutes (nmis) and commercial companies have developed various calibration procedures utilising different shock exciters (se) [3]-[9]. the operating principal of the se depends largely on the intended peakacceleration level. for low-level shocks, acceleration levels of ≈ 10 m/s², the ‘drop ball’ or pendulum method is used. for medium-level shocks, acceleration levels in the range of 50 m/s² to 10 km/s², a hammer-and-anvil method is employed (rigid body movement of an anvil), using a loaded spring or pneumatic system as the force applicator [10], [11]. for high-shock levels, acceleration levels up to 100 km/s2, a hopkinson bar (shock propagation inside a long, thin bar) is used as the shock exciter [8], [10], [11]. as part of its programme of work, the international organization for standardization’s technical committee (iso/tc) 108 developed parts 13 and 22 in the iso 16063 series of standards [10], [11]. part 13 of the standard specifies primary calibration procedures to determine the shock sensitivity of accelerometers, while part 22 of the standard specifies secondary calibration procedures to determine the shock sensitivity of accelerometers by comparison with a reference transducer. these procedures have been implemented by various nmis [3], [5], [6], [9]. the procedures relating to part 13, the implementation thereof and the nmis’ calibration measurement capabilities were validated during the first official international shock comparison, ccauv.v-k4 [12]. a prominent component in the estimation of the standard uncertainty (uom) in accelerometer calibrations is the effect of transverse motion (tm) [13]-[16]. as we are dealing with mechanical systems, this component of the uom cannot be reduced to zero. in practice, the tm uncertainty contribution is a result of two factors: 1) the physical tms 2) the transverse sensitivity of the accelerometer being calibrated [17]-[21]. as a result, care is taken to reduce the tm of the exciter as much as possible. in the case of the rigid body movement of an anvil system, this is generally achieved by using air-bearing guides. in this research paper, the author investigates two anvil support configurations implemented in a commercial pneumatic se system. in section 2, the system in question is described, outlining the two mounting configurations and the reduced tm envisaged by the implementation of the proposed changes. section 3 deals with the methods and instrumentation used to perform the tm measurements, which are then reported and discussed in section 4. section 3 also describes two different abstract this paper describes novel design changes to the accelerometer mounting support of a commercial pneumatic shock exciter, with the aim of reducing the transverse motion the accelerometer is subjected to during shock excitation. the author describes the mounting support supplied by the manufacturer, the design changes made and the measurement data to compare the transfer motions recorded using two different mounting designs. mailto:csveldman@nmisa.org acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 205 methodologies for considering the results. the author then draws some conclusions in section 5. 2. anvil mounting design the accelerometer shock sensitivity calibration system implemented at south africa’s nmi, nmisa, utilises a spektra se-201 pneumatic exciter. this exciter uses two anvil systems to cover the complete manufacturer’s specified acceleration range from 50 m/s2 to 100 km/s2. an air-bearing anvil unit is used for the acceleration sub-range from 50 m/s2 to 2,500 m/s2. for the high-shock (hs) acceleration range, a lightweight aluminium anvil system is used, covering the acceleration range from 2 km/s2 to 100 km/s². for hs, the accelerometer is mounted onto the anvil using a mounting stud. this accelerometer/anvil system forms the rigid body (rb) and is suspended (held in place) using a rubber band (o-ring) in a longitudinal configuration (mount 1). the o-ring position is shown graphically as the light blue line in figure 1. the o-ring is hooked around two aluminium pillars mounted on either side of the exciter-system base plate. the pillars have a groove to keep the o-ring in place. this arrangement keeps the rb gently at rest, waiting for the hammer strike. in the present study, we define this axial line as the y axis. as can be seen in figure 1, this arrangement results in two major and opposite tension forces, ts,1.1 and ts,1.2. there also exist two opposite tension forces, ts,1.3 and ts,1.4, which are much smaller than, and perpendicular to, ts,1.1 and ts,1.2. when the dominant forces are in line with the y axis, this longitudinal configuration will restrain movement along the y axis, with substantially more freedom of movement (less constraint) along the x axis. mount 2, a novel, inexpensive design modification for the mounting configuration, was implemented in this study. the red line in figure 1. illustrates the configuration for mount 2. the configuration results in four, almost equal tension forces. two opposing tension forces, ts,2.1 and ts,2.2, are perpendicular to ts,2.3 and ts,2.4. the resulting net force action on the rb results in a more uniform and balanced constraint in both x and y directions. the process for modifying the se high-shock mount from mount 1 to mount 2 is very simple and very low cost. the supplied o-ring is unhooked from the two suspension pillars and replaced with an o-ring with a larger circumference, ø ≈ 100 mm. the hs top safety plate is secured using four allen cap screws and four 20-mm-long spacers with a diameter of 12 mm. the four allen cap screws are removed one by one, and the o-ring is hooked over the aluminium spacer. once the oring is anchored around all four mounting posts supporting the top plate, the rb is held in position by four almost equal length and perpendicular rubber springs (a section of an o-ring) in an ‘x’ configuration, as shown in figure 2. these four sections of the o-ring create almost equal and opposite perpendicular forces, which are applied to the rb at rest and in motion, reducing the resulting tm. to ensure that the o-ring stays in place (does not move upward because of the shocks), the aluminium spacers were replaced with spacers containing a groove, into which the o-ring was slotted. these spacers (figure 3) were designed using freecad© software and manufactured using 3d-printing technology. figure 1. a pictorial representation, showing force vectors, of the two o-ring design layouts investigated. blue: original design, s1. red: new design, s2. figure 2. the new design layout without the triaxial accelerometer for clarity. figure 3. example of a 3d-printed support pillar to replace the aluminium pillar. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 206 3. transverse motion measurements the tm (acceleration along the x and y axes) was measured using a bi-axial accelerometer, while the principal acceleration (z axis) was measured using laser interferometry. the measurement setup is shown in figure 4. 3.1. bi-axial accelerometer a bi-axial (x–y) accelerometer was constructed using a 15 x 15 x 15 mm aluminium block. two endevco model 7259-100 iepe accelerometers were stud mounted perpendicular to each other to measure acceleration in the x and y axes. these accelerometers have a nominal sensitivity of 10 mv/(m/s2) over a wide frequency range, typically 5 hz to 30 khz, with a specified peak acceleration of 500 m/s2. even though the tm was to be measured for shock levels of up to 40 km/s², the expected peak tm did not exceed the specified acceleration range of the accelerometer. the construction of the bi-axial accelerometer was completed with the attachment of an m5 allen cap screw with a nut on the opposing side of each accelerometer. these allen cap screws with nuts were added to allow for a centre of gravity (cg) adjustment to the bi-axial accelerometer. the cg was adjusted to be as close to the centre of the aluminium cube as possible. 3.2. measurement methodology measurement data were collected using a national instruments pxi unit (ni-pxi). the pxi unit was fitted with three dual channel data acquisition units (daq). one daq sampled the interferometer i and q signals used for measuring the shock peak acceleration. the second daq was used to sample the output signal of the xand y-axis accelerometers, used to measure the transverse acceleration, while the third daq was used to trigger (start) each sampling event. the sampling of the three daq units was synchronised using the ni nitclk system, which enabled the data sampling to be time synchronised. the calculated average of a set of five measurements was taken as the measurement result for each measurement point (acceleration level). the resulting transverse acceleration was calculated as 𝑎𝑟 = √𝑎𝑋 2 + 𝑎𝑌 2 , (1) where ar is the resulting transverse acceleration, ax the measured acceleration in the x direction and ay the measured acceleration in the y direction. an example of a measured transverse acceleration is shown in figure 5. the tm reported and evaluated was the peak transverse acceleration relative to the peak acceleration of interest, the peak acceleration of interest being the peak acceleration along the z axis. the relative tm was calculated as 𝑎𝑇 = 𝑎𝑟 𝑎𝑍 , (2) where at is the relative transverse acceleration (rta), ar is the resulting tm, calculated using (1), and az is the peak acceleration along the z axis. measurements were performed over the acceleration range of 5 km/s2 to 40 km/s². the measurement results revealed a time delay between the reference acceleration (measured by the laser interferometer) and the tm (at) peak. the time delay between the two peaks can be figure 4. measurement configuration showing the two accelerometers to measure acceleration in x and y and the laser spot to measure acceleration in z in the mount 1 configuration. figure 5. example of a measured tm during shock measurements, showing ax, ay and the resulting ar. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 207 seen in figure 6. in view of this, two different peak rta evaluation methodologies were considered and reported on: 1) time-delayed peak transverse acceleration (at), which is the peak tm measured during the complete sampling time, 2) transverse acceleration at the time of the reference acceleration peak, which is the transverse acceleration level at the time (instance) of the z-axis acceleration. 4. measurement results the rta was measured at 5, 10, 20 and 40 km/s². measurements were performed using both mounting support designs described in section 2, mount 1 and mount 2. furthermore, the rta was determined at the two instances in time described in section 3.2. time instance one at the rta peak and time instance two synchronised with the z-axis peak, hence referred to as the @ ref peak. the results recorded for the two different mounts under investigation, as calculated using @ ref, are reported in table 1, with the results recorded for the two different mounts calculated at the rta peak reported in table 2. for accelerometer sensitivity calibration, the metrologist is concerned with the tm severity in the presence of an accelerometer’s transverse sensitivity. during rectilinear excitation, it can be assumed that tm is present while the exciter is in motion. although this is true during the shock excitation described in this research paper, the metrologist is only concerned with the rta the accelerometer is subjected to during the time/period of the shock pulse. the data reported in table 1 reveals that the implementation of mount 2 marginally improves the rta compared to mount 1, with the rta improvement being more significant at lower shock acceleration levels. at 5 km/s2, the rta is reduced by about 42 %, which translates into the same percentage reduction in the rta uncertainty contribution. as the shock level increases from 5 km/s2 to 40 km/s2, the rta reduces from 1.9 % to 0.6 % using mount 1, while the rta reduces from 1.1 % to 0.5 % using mount 2. the data reported supports the prediction of reduced tm through the novel anvil suspension design change. in consideration of the data reported in table 2, similar trends in the rta are noted between the two different mountings investigated. the rta tends to reduce as the shock acceleration level increases. it is noted that the rta using mount 2 (‘x’ configuration) is consistently lower than when using mount 1, with the largest reduction measured at 5 km/s2. however, it is necessary to consider the data carefully within the conditions of the data assessment. as pointed out earlier, as the shock level increases, the time difference between the z-axis shock peak and the rta peak increases; that is, the rta peak is reached some time after the principal axis acceleration peak. for shocks with peak acceleration levels above 10 km/s2, the rta peak occurred after the end of the sampling time. the aim of the investigation was not to determine the actual peak tm, but the tm that influences the uncertainty in shock sensitivity calibration. 5. conclusions the performance with respect to the resulting tm of two different mounting configurations used to mount/support the anvil of a pneumatic se were investigated. two different instances of peak tm were also considered, first, at the instance figure 6. shock (left vertical axis) and transverse acceleration (right vertical axis) time series. table 1. relative tm measurement results at the time of the z-axis peak. peak acceleration at mount 1 @ ref peak at mount 2 @ ref peak (km/s²) (%) (%) 5 1.9 1.1 10 1.4 1.2 20 0.9 0.8 40 0.6 0.5 table 2. relative tm measurement results at the time of the rta peak. peak acceleration at mount 1 rta peak at mount 2 rta peak (km/s²) (%) (%) 5 3.4 1.8 10 2.6 2.0 20 1.8 1.2 40 1.6 1.5 acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 208 in time when the peak acceleration occurs, and second, when the resulting tm (ar) peak occurs, which is generally some time after the principal axis acceleration peak. in terms of the influence of rta as an uncertainty contributor, the author focused on the results from the first instance. the results indicate that the configuration in mount 2 provides a reduction in tm of almost 50 % at shocks of 5 km/s2. the performance gain using mount 2 reduces as the shock acceleration increases to almost 0 for peak shock levels above 40 km/s2. the benefit (with possible reduced expanded uncertainty of measurement) of implementing the proposed mounting configuration is worth the minimal mechanical changes required to implement it. it should be noted that with this mounting configuration, mounting the accelerometer onto the anvil is a little more time consuming than the manufacturer’s original configuration. references [1] iso 16063-1, methods for the calibration of vibration and shock transducers part 11: basic concepts. 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http://www.modalshop.com/techlibrary/jdosch%20transverse%20calibration.pdf http://www.modalshop.com/techlibrary/jdosch%20transverse%20calibration.pdf geometric morphometrics reveal relationship between cut-mark morphology and cutting tools acta imeko issn: 2221-870x march 2021, volume 10, number 1, 109 113 acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 109 geometric morphometrics reveal relationship between cutmark morphology and cutting tools francesco boschin1, erika moretti1, jacopo crezzini1, simona arrighi2 1 dipartimento di scienze fisiche, della terra e dell’ambiente, ur ecologia preistorica/ università degli studi di siena, via laterina 8, 53100 siena, italy 2 dipartimento di beni culturali/ università di bologna, via degli ariani 1 48121, ravenna, italy section: research paper keywords: geometrics morphometrics; 3d microscopy; taphonomy; bone surface modifications; lithic tools citation: francesco boschin, erika moretti, jacopo crezzini, simona arrighi, geometric morphometrics reveal relationship between cut-mark morphology and cutting tools, acta imeko, vol. 10, no. 1, article 14, march 2021, identifier: imeko-acta-10 (2021)-01-14 editor: carlo carobbi, university of florence, italy received april 30, 2020; in final form october 4, 2020; published march 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: simona arrighi, e-mail: s.arrighi@hotmail.com 1. introduction the application of 3d imaging in taphonomy has increased in recent years. these studies are often aimed at analysing bonesurface modifications (bsm), such as butchering marks, gnawing marks or modifications due to trampling and abrasion caused by sediments [1]-[6]. the aim of such taphonomic studies is to better understand hominin behaviour through time [7]-[10]. at the university of siena, 3d imaging has been applied to the study of bsms since 2009 using a hirox kh-7700 digital microscope [2]-[4], [9], [11], [12]. a first pilot study, inspired by results achieved by colleagues [1], focused on the distinction between the cut-mark cross sections (i.e. elongated striations on the bone surfaces) produced with different tools (metal knives and retouched/unretouched flint implements). the obtained results [2] demonstrated how a morphometrical approach can be useful to characterise and study cross sections of bsms from archaeological sites. in addition, it demonstrated that the analysis of only one median cross section per mark can be enough to separate different sets of striae. further research has been carried out to understand how specific tools and actions could influence the morphology and morphometry of cut-mark cross sections [3], [4]. in particular, it has been observed that the same ‘category’ of lithic implement (for instance, an unretouched flint flake) can leave different traces when used for different tasks, such as for butchery activities or for the production of engraved art objects on flat bones [4]. at the same time, other research groups have begun to use geometric morphometrics to distinguish different types of bsms, achieving interesting results [7], [13]-[16]. in this paper, we use a geometric morphometric approach to analyse the cross sections of two sets of incisions, produced in two previous experimental works, in order to evaluate the cross-sectional variability of the traces produced by similar lithic implements (two burins) [4] and by an unretouched flint flake [2]. the aim of this contribution is to link the characteristics of the grooves to those of the cutting edges of the tools used. abstract the analysis of bone-surface modifications (bsm), such as butchering marks, is necessary to better understand how the exploitation of animal resources by past hominins influenced their biological and cultural evolution. in this paper, we try to quantify to what extent the depth of the cut marks influences the shape of their cross sections. this is of crucial importance for a valid interpretation of the shape data collected on archaeological bsms. two groups of slicing cut-mark cross sections were experimentally produced with two flint burins on a defleshed cattle innominate, and a set of butchering marks were produced with an unretouched flint flake. these were analysed by means of 3d microscopy and geometric morphometrics. the resulting sets of striae show different depths and different crosssectional shapes. shallower cross sections display less steep walls and, consequently, a wider opening angle. when the characteristics of the burin cutting edges were investigated, it was clear that the difference in shape between the two groups of striations was probably a function of the way in which the tool penetrated the bone. these results are taphonomically relevant since similar differences in crosssectional shapes have been found in marks produced with different tools. mailto:s.arrighi@hotmail.com acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 110 2. materials and methods in this work we analysed slicing cut marks produced with two burins and an unretouched flint flake. the three experimental lithic implements were made from the same raw material (flint from the gargano promontory, south-eastern italy). a set of three striations was produced with each burin on the flat, defleshed surface of a cattle innominate. since the production of the striations was the goal of this experiment, we maintained control over the type of active edge that inflicted the marks on the bone, a trihedral (figure 1 a). furthermore, in this paper we take into consideration an additional set of 22 striations produced in 2010 during a butchery experiment, carried out on a cattle autopodium (metapodial and phalanges), using an unretouched flint flake [2]. the autopodium was fresh, with all the soft tissues still attached to the bones. the aim of this experiment was the butchering itself, while the production of cut marks should be considered a collateral effect. this means that the operator had no control over the exact portion of the tool’s active edge that inflicted the marks. this set of striations is similar to those found in archaeology due to the lack of information on the tool used by the butcherer. the active edge of the burins and all the cut marks were scanned by means of a hirox kh-7700 digital microscope, equipped with an mxg-10c body, an ol-140ii lens and an ad-10s directional lighting adapter [2], [4], [17]. angle α was measured on the 3d model of the active edge of each burin (figure 1 a). three cross sections were analysed per mark produced with the burins (taken respectively at 25 %, 50 % and 75 % of the mark’s length), while only the cross section taken at 50 % of the mark's length was available for the striations inflicted with the flint flake. the depth of cut (dc) [1], [2] was measured on each profile (figure 1 b) and seven landmarks were placed on each cross section, as described in [10] (figure 1 c), using tpsutil (v. 1.58) and tpsdig (v. 2.17) software [18], [19]. the raw coordinates of the landmarks were imported into morphoj software (v. 1.8) [20]. after a procrustes fit and the generation of a covariance matrix, a principal component analysis was performed on the dataset. 3. results the grooves produced on the bone by the burins show some differences in relation to what are usually referred to as ‘slicing cut marks’ (as defined by greenfield [21]); the starting and ending points are sometimes abrupt. this is because the operator produced grooves of a prearranged length and the applied force did not change significantly from the starting point to the median section and the ending. for this reason, the morphological characteristics of the starting/ending points of the grooves are not important for this study. an example of how two grooves start and end is shown in figure 2. the grooves produced with the two burins show a significant difference in dc (mann–whitney u test, p = 0.006). deeper cuts were inflicted using burin 2, with dc ranging between 58.7 μm and 86.9 μm. in comparison, the dc of the marks produced with burin 1 are between 26.6 μm and 78.7 μm (figure 3). having used the same kind of tool, produced with the same raw material, and with the tools being applied on the same surface, this difference must be due to the difference in force adopted by the operator, as has already been observed during other tests [3]. geometric morphometric analysis is able to discriminate between the cut marks produced with the two burins. in particular, the two groups of cut marks, burin 1 (b1) and burin 2 (b2), are differently distributed along principal component 2 (pc2), which describes 31.5 % of the sample’s variance. the values of group b2 are significantly higher than those of group b1 (mann–whitney u test, p = 0.0005), indicating deeper cross sections with steeper walls (figure 4a). the difference in shape between the two groups can be easily seen with the procrustes analysis shown in figure 4b and 4c. whereas pc1 is not able to distinguish the two groups, it describes 54.8 % of the sample’s variance, and it is better related to the symmetry of the cross sections. figure 1. a: 3d image of the active edge of the burins; b: depth of the cut measured in the cross sections; c: landmarks placed on the cross sections. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 111 in terms of the analysis of the 3d models of the active edges, which were used to inflict the cut marks, it emerges that angle α is generally wider towards the top of the edge. in b1, the angle between the two surfaces that form the cutting edge is 119.6° ± 1.3°, if only the first 30 μm are considered, 96.9° ± 0.7°, if we consider a depth of 60 μm, and 82.3° ± 0.8°, if we consider a depth of 85 μm. these values are respectively 120.6° ± 0.7°, 109.8° ± 0.8° and 98° ± 0.5° in b2. figure 5a shows more clearly how the angle changes when different depths of penetration of the tools’ edge into the bone tissue are taken into consideration. this could imply that the general shape of the cross sections of a cut mark depends on the penetration of the cutting edge into the bone tissue (as exemplified in figure 5b). both principal components show a positive and significant linear correlation with dc: the p-value is 0.0001 for pc1 and 0.0004 for pc2. geometric morphometrics revealed that the grooves inflicted with the unretouched flint flake exhibited considerably greater variability in shape than the striations produced with the burins. these grooves are described by three main principal figure 2. a, b: starting and ending points of a groove produced with burin 1; c, d: starting and ending points of a groove produced with burin 2. dotted lines indicate the abrupt edges of the groove; arrows indicate the direction of the hand movement. figure 3. depth of cross sections (dc) according to which burin was used (b1: burin 1; b2: burin 2). figure 4. a: pca performed on the covariance matrix after a procrustes superimposition. b: procrustes analysis on the ‘burin 1’ group. c: procrustes analysis on the ‘burin 2’ group. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 112 components, pc1, pc2 and pc3 (which accounted respectively for 41.7 %, 34.4 % and 17.5 % of the variability). pc1 could be interpreted as a function of the symmetry of the cross section and pc2 as a function of the presence of slopes or ancillary striations on the slopes of the cross sections. pc3 could be interpreted as a function of the depth and is related to the open angle of the cross sections (shallower cuts with wider open angles vs deeper cuts with smaller open angles); it is only pc3 that shows a significant (and positive) correlation with dc (p = 0.01). finally, a principal component analysis was performed by combining the two experiments. in doing so, we considered only the cross section taken at 50 % of the mark’s length for all the striations. in this case, the variability is still described by three main principal components. of these, pc2 and pc3, which together describe 54.3 % of the sample’s variability (pc2 accounted for 35.1 % and pc3 accounted for 19.2 %), are related to the dc (pc2: p = 0.002; pc3: p = 0.02). the interpretation of these two components is the same as that for pc2 and pc3 in the previous analysis (shape of the slopes and open angle, figure 6). 4. discussion and conclusion data presented in this paper demonstrate how the shape of the mark’s cross sections can depend on the level of penetration of the cutting edge into the bone tissue. it also highlights the importance of combining shape data from geometric morphometrics with linear measurements (here the dc). despite the small sample size, the results of our analysis show that the depth of a striation can influence the shape of its cross sections. this relationship occurs both in cut marks produced with a formal tool (a burin) and with an unretouched flint flake. the active edge of a burin is a trihedral, and we have demonstrated that its level of penetration has a great influence on the open angle it forms (figure 5b). the unretouched flint flake has an elongated active edge that can be used as a blade. this edge is composed of several functional units (elongated parts and small trihedral portions), which are responsible for the greater variability of the cross-section shape. the more a functional unit penetrates into the bone tissue, the more its structure will characterise the shape of the striations. this is the reason, apart from the wider open angle, shallower striations are more symmetrical and show more regular slopes (figure 6). the results obtained on the set of striations produced with the flint flake confirm that the correlation between shape and depth of the cross sections is valid, whatever type of functional unit of a cutting edge is used. this is an important result because we do not usually know the type of lithic implement used to produce cut marks identified in an archaeological sample. the relationship between dc and the shape of the cross-section slopes was not recognised in the cut marks produced with burins. this is probably due to the very regular shape of the active edges of these tools. finally, it has to be emphasised that some of the differences in shape found in the marks analysed in this work (shallow profiles with wide open angles vs deep profiles with narrow open angles) are similar to the differences in shape found by other authors in cuts inflicted using tools produced with different raw materials [7]. a more in-depth analysis of tool cutting edges should be carried out in order to understand their variability at a microscopic level and to identify any differences that depend on figure 5.: an example of how angle α changes according to the penetration of the cutting edge into the bone tissue. dotted line: cutting edge. red lines: angle α when only 20 μm of flint penetrates into the bone tissue (left) and when 80 μm of flint penetrates into the bone tissue (right). b: schematic sketch explaining the possible influence of the cutting edge on the mark’s cross section, depending on the intensity of the penetration. figure 6.: variability of shape of the slopes and opening angles described by pc2 and pc3, combining cut marks produced by burins and cut marks produced by the unretouched flake. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 113 the raw material used, the nature of the cutting edge (i.e. the specific functional part of the tool) or the presence/absence of a retouch. since the study of marks on bones is of primary importance for the reconstruction of hominin behaviour in the past, e.g. [7], it is necessary to understand how the 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[20] c. p. klingenberg, morphoj: an integrated software package for geometric morphometrics, mol. ecol. resour. 11 (2011), pp. 353357. doi: 10.1111/j.1755-0998.2010.02924.x [21] a. greenfield, the origin of metallurgy in the central balkans based on the analysis of cut marks on animal bones, environ. archaeol. 5 (2000), pp. 93-106. doi: 10.1006/jasc.1998.0348 https://doi.org/10.1016/j.jas.2007.10.018 https://doi.org/10.1002/oa.1272 https://doi.org/10.1016/j.quaint.2013.12.056 https://doi.org/10.14237/ebl.6.1.2015.349 https://doi.org/10.1016/j.quaint.2018.12.019 https://www.imeko.org/publications/tc4-archaeo-2019/imeko-tc4-metroarchaeo-2019-109.pdf https://www.imeko.org/publications/tc4-archaeo-2019/imeko-tc4-metroarchaeo-2019-109.pdf https://doi.org/10.1111/bor.12224 https://doi.org/10.1371/journal.pone.0182127 https://doi.org/10.1007/s12520-018-0674y https://doi.org/10.1007/s12520-016-0401-5 https://doi.org/10.1016/j.quascirev.2018.11.021 https://doi.org/10.1016/j.jasrep.2019.101871 https://doi.org/10.1016/j.jas.2015.08.006 https://doi.org/10.1111/jmi.12575 https://doi.org/10.1016/j.jasrep.2017.05.043 https://doi.org/10.1371/journal.pone.0194324 https://doi.org/10.1371/journal.pone.0198292 https://doi.org/10.1111/j.1755-0998.2010.02924.x http://doi.org/10.1006/jasc.1998.0348 development and metrological characterisation of the new lne 500 n·m deadweight torque standard machine acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 168 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 168 172 development and metrological characterisation of the new lne 500 nm deadweight torque standard machine c. duflon1, p. averlant2 lne, paris, france, 1 carole.duflon@lne.fr, 2 philippe.averlant@lne.fr abstract: this paper describes the development and metrological characterisation of the new lne 500 nm deadweight torque standard machine. this machine is integrated into the lne 5 knm deadweight torque standard machine. new masses and a mass change platform have been developed and added to the 5 knm machine. after this, lne realised the metrological characterisation of the 500 nm machine. the determination of the uncertainties of this torque standard machine and the comparison measurements with the lne 50 nm deadweight torque standard machine and with cem, the national metrology institute of spain, are described in this paper. the results show that the estimated uncertainties are satisfactory because they are a factor of about four times smaller than those of the old standard machines. keywords: torque standard machine 1 introduction the development of the torque standard machine of 500 nm is part of a wider scheme which aims at updating french metrological references in torque metrology. the laboratoire national de métrologie et d’essais (lne) is conducting this project which involves the development of several machines: 5 nm; 50 nm; 500 nm; and 5000 nm. the development and the metrological characterisation of the 5 nm, 50 nm and 5000 nm torque deadweight standard machine have already been made and presented in articles [1][2][3][4]. the next sections introduce, for the 500 nm torque standard machine, the development of the deadweights, the principle of the mass change platform (see figure 1), the determination of the uncertainties and the comparison measurements carried out with the lne 50 nm deadweight torque standard machine and with the national metrology institute of spain (cem) 1 knm deadweight torque standard machine. after this comparison a euramet comparison will be done. this euramet comparison will be used to support our bipm calibration and measurement capabilities change. figure 1: 500 nm and 5 knm machines mass change platform weightstacks: 5 kn and 500 n http://www.imeko.org/ mailto:carole.duflon@lne.fr file:///c:/users/ajk1/appdata/local/microsoft/windows/inetcache/content.outlook/zr5yrj3g/philippe.averlant@lne.fr acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 169 2 development of the 500 nm machine the 500 nm deadweight torque standard machine is integrated into the lne 5 knm deadweight torque standard machine. new masses and a mass change platform have been developed and added to the 5 knm machine (see figure 1). the next sections describe these new parts. the lever arm and the air bearing are described in detail in the article [2]. 2.1 the weightstack figure 2: 500 nm weightstack two weightstacks (see figure 2) were manufactured to be placed on both extremities of the lever arm to apply torques either on the left or the right side. these stacks have a height and an arm interface identical to the weightstack of the 5 knm machine so that the machine can operate in the same way. they are made of a series of non-magnetic stainless steel disks that are sequentially hung from each other depending on the height of the weight carrier. each stack is composed of the same series of the following 22 discs: 10 discs each creating 10 n; 2 discs each creating 20 n; 2 discs each creating 10 n; 2 discs each creating 20 n; 6 discs each creating 50 n. this particular layout enables loadings to be conducted with intervals of 10 nm, 20 nm or 50 nm, by increasing or decreasing load values over ten steps with only one motorisation. this allows us to respect the international practices regarding calibration of torque transducers with a capacity of 100 nm, 200 nm and 500 nm. 2.2 the mass change platform the mechanism for carrying out the mass chain change consists of a change platform (see figure 3), on each side of the arm, placed on the rear of the machine the mass change platform is equipped with a slide and a pivot for translation and rotation. movements are motorised. the translational plate, allowing the approach of the masses in position 5 kn or 500 n, is comb-shaped and must be inserted in the basket support plate. then the existing basket motors are used to lift the masses. figure 3: mass change platform and weightstacks mass change platform weightstack 5 kn weightstack 500 n basket support plate http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 170 3 evaluation of uncertainty the uncertainty of the torque t applied by the 500 nm machine has been estimated including the uncertainty contribution of the deadweight force, of the length and due to sensitivity mobility of the lever (see table 1). concerning the deadweight force, the uncertainties have been estimated as usual and same components than for the 5 knm range of the machine (see [4]) have been taken into account. as there is only one lever used for both the 5 knm and 500 nm ranges, we use the same calibration results obtained on a three-dimensional measuring machine. so components link to length are exactly the same as for 5 knm range (see [4]). the sensitivity and mobility of the lever arm associated with the air bearing have been estimated in the same way as for the 5 knm range: lever imbalances have been measured using a small capacity torque transducer with a very small resolution. additional to all those components, a last uncertainty contribution is taken into account. doing interlaboratory comparison, deviations could be observed between two torque calibration machines. in force and torque field, sometimes, table 1: uncertainty budget of the lne 500 nm torque standard machine uncertainty component standard uncertainty / nm force mass calibration measurement truenessa drift (based on calibration uncertainty) earth’s gravitational field air buoyancy 2.7 × 10-6 × t 1.0 × 10-6 × t 1.5 × 10-6 × t 1.0 × 10-6 × t 3.1 × 10-7 × t 1.7 × 10-6 × t length calibration measurement truenessa temperature drift (based on calibration uncertainty) force position deformation and inclination 2.0 × 10-5 × t 1.2 × 10-5 × t 6.1 × 10-6 × t 4.9 × 10-6 × t 1.2 × 10-5 × t 6.7 × 10-6 × t 2.9 × 10-7 × t sensitivity and mobility 7.1 × 10-4 + 9.4 × 10-7 × t torque transmission 1.6 × 10-3 + 1.9 × 10-5 × t a due to the use of nominal value instead of measured value those deviations are bigger than the combined uncertainty, means that there are significant, or that the uncertainties have been underestimated. to avoid this, we enlarge the uncertainty, adding a component based on comparison results. we call this component “torque transmission” because the mechanical way to apply the torque to the device could explain the deviation results. for the 500 nm range, this component is estimated using the deviation between the measurements obtained by transducers calibrated by the lne 500 nm, lne 50 nm and cem machines. the comparison of measurements is presented in the next chapter. the expanded (k = 2) uncertainty 𝑈𝑇 of the torque (𝑇 , in nm) applied by the lne 500 nm machine is obtained by combining these components: 𝑈𝑇 = 3 mn∙m + (5.0 × 10 −5 × 𝑇) n∙m (1) 4 comparison measurements the comparison has been made with transducers calibrated by lne and cem. cem used their 1 knm torque standard machine. the expanded uncertainty (k = 2) on the applied torque t (in nm) is equal to (2.0 × 10−5 × t) nm. each torquemeter was calibrated twice in lne’s 500 nm torque standard machine, firstly before sending it to cem and secondly after receiving it back, thereby establishing the drift. for torque from 10 n·m to 50 nm an internal lne comparison with the 50 nm torque standard machine has been made. the expanded uncertainty (k = 2) on the applied torque t (in nm) is equal to 0.5 mnm + (5.0 × 10-5 × t) nm. the comparison took place over a few days so only one calibration was made on the 500 nm machine. for these comparisons we used five transfer torquemeters. table 2 shows the measuring bridges used and the measuring steps realised with the set of torquemeters. table 2: measuring ranges realised during the comparisons transfer torquemeter measuring bridges steps (n·m) raute tt1 50 n·m hbm dk38 10, 20, 30, 40, 50 gtm dm-tn 50 n·m hbm dk38 10, 20, 30, 40, 50 raute tt1 100 n·m hbm ml38 10, 20, 30, 40, 50, 100 raute tt1 500 n·m hbm ml38 50, 100, 150, 200, 250, 300, 400, 500 gtm dm-tn 500 n·m hbm ml38 50, 100, 150, 200, 250, 300, 400, 500 the calibrations were performed in both clockwise and anticlockwise modes. http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 171 the calibration of each torquemeter has been realised by performing, in order, the following operations: • application of three preloads at the nominal value of the torquemeter held for thirty seconds and with a rest period of thirty seconds between each preload. • application of four series of increasing charges and three series of decreasing charges without return at zero charge between each step. the measurements are taken after about thirty seconds. • rotation of the torquemeter around its axis (3 different angles). preload at the maximum load of the sensor after each rotation figure 4 shows, for each torque step, the weighted average of differences between the cem and lne 50 nm and the lne 500 nm machines. the vertical bars represent the uncertainty (k = 2) of the lne 500 nm torque standard machine. figure 4: weighted average of difference = (results cem and lne 50 nm results lne 500 nm) ± ulne500 for each step, the difference is less than the uncertainty, so it is not significant. maximum differences are taken into account in the uncertainty budget as the torque transmission component. 5 conclusion the metrological characterisation of the lne torque standard machine of 500 nm gave an expanded (k = 2) uncertainty equal to 3 mn·m + (5.0 × 10-5 × t) n·m. the comparison performed with the lne 50 nm machine and the cem 1 knm machine showed that this uncertainty is quite justified. the results show that the estimated uncertainties are satisfactory because they are a factor of about four times smaller than those of the old standard machines. a euramet comparison will be done to support our bipm calibration and measurement capabilities change. 6 references [1] p. averlant, a. gosset, “development of the new lne 50 n·m deadweight torque standard machine”, in proc. of 20th imeko tc3 int. conf., merida, mexico, 27 – 30 november 2007. online [accessed 20201126]: https://www.imeko.org/publications/tc32007/imeko-tc3-2007-097u.pdf [2] p. averlant, p. lacipiere, j.-m. davis, “development of the new lne 5 kn·m deadweight torque standard machine”, in proc. of joint imeko http://www.imeko.org/ https://www.imeko.org/publications/tc3-2007/imeko-tc3-2007-097u.pdf https://www.imeko.org/publications/tc3-2007/imeko-tc3-2007-097u.pdf acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 172 22nd tc3, 12th tc5 and 3rd tc 22 int. conf., cape town, south africa, 3 5 february 2014. online [accessed 20201126]: https://www.imeko.org/publications/tc32014/imeko-tc3-2014-001.pdf [3] c. duflon, p. averlant, “qualification métrologique des nouveaux bancs de référence de couple de 5 n·m et de 50 n·m du lne”, revue française de métrologie, vol. 2014-2, no. 34, doi: 10.1051/rfm/2014007, 2014. online [accessed 20201126]: https://metrologiefrancaise.lne.fr/sites/default/files/media/document/ rfm34-1402-duflon.pdf [4] p. averlant, c. duflon, “metrological characterisation of the 5 kn·m torque standard machine of lne”, in proc. of 12th imeko world congress, belfast, uk, 3 6 september 2018. online [accessed 20201126]: https://iopscience.iop.org/article/10.1088/17426596/1065/4/042044/pdf http://www.imeko.org/ https://www.imeko.org/publications/tc3-2014/imeko-tc3-2014-001.pdf https://www.imeko.org/publications/tc3-2014/imeko-tc3-2014-001.pdf https://metrologie-francaise.lne.fr/sites/default/files/media/document/rfm34-1402-duflon.pdf https://metrologie-francaise.lne.fr/sites/default/files/media/document/rfm34-1402-duflon.pdf https://metrologie-francaise.lne.fr/sites/default/files/media/document/rfm34-1402-duflon.pdf https://iopscience.iop.org/article/10.1088/1742-6596/1065/4/042044/pdf https://iopscience.iop.org/article/10.1088/1742-6596/1065/4/042044/pdf design and characterization of a stand alone merging unit acta imeko issn: 2221-870x march 2020, volume 9, number 1, 40 48 acta imeko | www.imeko.org march2020 | volume 9 | number 1 | 40 design and characterisation of a stand-alone merging unit giuliano cipolletta1, antonio delle femine1, daniele gallo1, carmine landi1, mario luiso1 1 dept. of engineering, university of campania "luigi vanvitelli", 81031 aversa (ce), italy section: research paper keywords: power system measurement; power system diagnostics; stand-alone merging unit; time synchronisation; analogue-to-digital converter citation: giuliano cipolletta, antonio delle femine, daniele gallo, carmine landi, mario luiso, design and characterization of a stand alone merging unit, acta imeko, vol. 9, no. 1, article 7, march 2020, identifier: imeko-acta-09 (2020)-01-07 editor: lorenzo ciani, university of florence, italy received november 11, 2019; in final form february 6, 2020; published march 2020 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was part of 17ind06 future grid ii project, which is jointly funded by the empir participating countries within euramet and the european union. corresponding author: a. dellle femine, e-mail: antonio.dellefemine@unicampania.it 1. introduction in recent years in the electricity supply industry, there has been a growing awareness of the need to re-invent europe's electricity networks in order to meet the demands of 21st-century customers. this justifies the growing interest in smart grids [1], [2], and it increases, in turn, the interest in smart substation development. according to [3], merging units (mus) play a crucial role in the design of substation communication and automation systems. substation automation, using the iec 61850 suite of protocols, is an established reality for the purpose of enabling fast-acting protection and control application. digital substations (dss) are electrical substations where operations are managed between distributed intelligent electronic devices (ieds) at different levels of automation, interconnected by communication networks. for a reliable information exchange among devices at different levels, data accuracy is needed not only for amplitude but in time as well. the stand-alone merging unit (samu) [4] acts as digital interface of the instrument transformers. it converts to digital the analogue input signals from instrument transformers (inductive or low power i.e. lpit) and provides a time-coherent combination of the current and voltage samples, which are used to create a data stream of sampled values (svs). svs are used for transmitting digitised instantaneous values of power system quantities, mainly primary voltages and currents of one or multiple phases, to protection relays and bay controllers through either ethernet or optical communication channels based on the iec 61850-9-2 protocol. iec 61850-9-2 handles this publication of svs over the ethernet, which are sent to microprocessor-based ied. the benefits of digital process buses include a reduced complexity of current and voltage instrument transformers’ (cts and vts) secondary cabling and a simplified connection for centralised substation automation functions, such as recording disturbances and monitoring power quality. the presented work is part of the empir 17ind06 future grid ii research project. one objective of this project is to develop metrological grade samus and traceable time synchronisation techniques. special care has to be taken to have accurate time synchronisation between units. multiple devices will have their local independent clock, which cannot be used as time reference for the published data. otherwise, samples coming from abstract merging units (mus) play a key role in enhancing the levels of security and the reliability of power systems, allowing for advanced remote diagnostics. some of the benefits are a more efficient transmission of electricity and a better integration with renewable energy systems. in this article, an implementation of a stand-alone merging unit (samu), compliant with the iec 61850-9-2 standard and based on a low-cost arm microcontroller, is described. it acquires two signals, one voltage and one current, and it sends the samples over the ethernet connection. a high-resolution analogue-to-digital converter (adc), synchronised to the universal time coordinated (utc) through a global positioning system (gps) disciplined oscillator, is used. the opportune insulation and conditioning stage have been designed. several tests have been performed, varying amplitude, frequency, and phase of the input signals, in order to evaluate the metrological performance of the proposed samu and they are here discussed. mailto:antonio.dellefemine@unicampania.it acta imeko | www.imeko.org march2020 | volume 9 | number 1 | 41 different devices are not suitable for reconstructing the power network state [5], [6]. therefore, svs need to be accurately time-stamped, providing synchronisation information to put in the relation data from different ieds placed in different locations of the smart grid. for this reason, local clocks must be synchronised to each other via a universal source. the standard [7] states that to ensure the deterministic operation of critical functions in the automation system, a precise time distribution and clock synchronisation in electrical grids with an accuracy of 1 µs must be used. the precision time protocol (ptp) [8], a network-based time synchronisation standard that can achieve clock accuracy in the sub-microsecond range, is suggested as time distribution mechanism. such a synchronisation method implies considerable efforts by microcontroller firmware to generate an analogue-todigital converter adc clock phase locked to the absolute time. in fact, continuous calibration of the internal microcontroller clock must be performed, with a proper closed-loop digital control system that follow the reference imposed by the ptp input. to avoid this computational overhead and to achieve higher synchronisation accuracy, the proposed samu instead makes use of a global positioning system (gps) disciplined oscillator (gpsdo) that allows long-term stability and does not need firmware calibration; the long-term stability relies on the high accuracy of gps caesium references. the adopted solution allows for synchronisation for each sample with a higher time resolution. in the following, section 2 describes the hardware implementation of the proposed samu, whereas section 3 focuses on the embedded measurement firmware. section 4 shows experimental results from the metrological characterisation. finally, section 5 draws the conclusions. 2. hardware architecture the block diagram of the proposed samu [9] is shown in figure 1.figure 1. block diagram of the proposed samu. it is based on a microcontroller, opportunely interfaced with a gpsdo and an adc with proper designed conditioning circuits. the various functional blocks are explained in the following subsections. 2.1. timing circuits the connor-winfield fts125-ctv-010.0m was adopted as the gpsdo. it produces a 1 pulse per second (pps) signal from the gps timing receiver and generates both a 10 mhz cmoslevel square wave and a 10 mhz sine wave output from a lowjitter voltage controlled temperature compensated crystal oscillator (vctcxo). it accomplishes the task of keeping the system synchronised to absolute time. in fact, it adjusts the frequency of the vctcxo to be an integer multiple of the pps and the relative phase, wherein the vctcxo is adjusted to have 10 million oscillations in the pps period, with the first pulse having the rising edge coincident with the pps pulse. 2.2. processing and coordination the 10 mhz square wave is used as reference clock for the stm32f767zi, an arm-cortex m7 microcontroller with 2 mb flash, 512 kb of sram, a clock frequency of 216 mhz, a l1 cache, an adaptive real-time (art) accelerator, and a digital signal processor (dsp) with a floating point unit (fpu). the microcontroller is interfaced via serial peripheral interfaces (spi) with the external adc max11960, which is a differential successive approximation register (sar) adc with a resolution of 20 bits, a maximum sampling frequency of 1 mhz, a dual simultaneous sampling, a signal-to-noise ratio (snr) of 99 db, total harmonic distortion (thd) of -123 db, and a differential non linearity of ± 1 lsb. the adc has two independent spis with a shared clock, while the two spis of the microcontroller have two independent serial clock signals. thus, in order to avoid electrical problems, one clock has been used for both channels. 2.3. data acquisition stage data acquisition is performed in accordance with [3], using a sampling frequency of 12.8 khz. thus, the microcontroller supplies a 12.8 khz sampling clock to the adc in order to have 256 samples for each rated period of the fundamental component (i.e. 50 hz). the device has a differential ± 3 v input range, being the adc reference voltage 𝑉ref = 3 v. thus, the signal conditioning stage produces a differential input signal centred around a common-mode dc voltage of 𝑉ref 2⁄ . a proper conditioning stage has been developed and it is presented in subsection 2.5. the conditioning circuits reproduce signals for voltage and current channels that lay within the operating range of the adc. once the signals are scaled down, accurately referred in time, they are sampled and converted to digital, and the microcontroller must transmit the svs. 2.4. communication regarding the communication hardware interface, stm32f767zi features a 10/100 mbit/s ethernet media access controller (emac) peripheral that consists of a mac 802.3 controller. it complies with both the media independent interface (mii) and the reduced media independent interface (rmii) to interact with the physical layer and supports ethernet frame time-stamping, as described in [8]. furthermore, it has a dedicated direct memory access (dma) controller that interfaces with the core and memories through the advanced high-performance bus (ahb) master and slave interfaces. the ahb master interface controls data transfers, while the ahb slave interface accesses the control and status registers (csrs) space. the transmit first-in-first-out (fifo) buffers the data read from the system memory by the dma, before the transmission by the media access control (mac). similarly, the receive fifo stores the received ethernet frames until they are transferred to the system memory by the dma. 2.5. signal conditioning stage in order to build up a complete samu, a voltage and current transducing stage has been realised. in figure 2, an electric figure 1. block diagram of the proposed samu. acta imeko | www.imeko.org march2020 | volume 9 | number 1 | 42 scheme of the proposed combined transducer and conditioning section is shown. the circuit fulfils two important tasks. the first is to guarantee electrical insulation in order to preserve the safety of workers and the integrity of instrumentations. the commonmode voltage between inputs of voltage transducers and the ground may be very dangerous. the second task is to adapt the signal levels and characteristics to the input of the digital conversion stage. the nominal input voltage and current of the proposed samu are 110 v and 5 a, root mean square (rms) value, respectively. these amplitude levels must be reduced to the range from – 3 v to 3 v (i.e. the adc input range). moreover, the adc works with fully differential inputs, so the signal must be converted from single ended to differential. a magnetic insulation has been achieved by adopting, for both current and voltage channels, two zero-flux transformers. they consist of two commercial closed-loop compensated halleffect transducers, one for voltage and one for current channel. obviously, the number of primary windings for the voltage transducer is higher than for the current. moreover, for the voltage side, a primary resistor (rin) is needed and it is put in series in order to limit the primary current. on the secondary of both voltage and current transducers, a passive network was used to maintain the highest possible linearity. proper burden resistors (rma and rmb) are chosen to tune the desired gain. the symmetric polarisation network obtained via identical rp resistors is needed to obtain differential voltage signals from the current output of the two transducers. note that the two measurement transducers have independent power supply circuits, both insulated with respect to the main supply and, of course, with respect to the 0 v reference of the adc. this is necessary in order to obtain differential signalling with the simple passive network. this network is supplied by the reference voltage vref of the adc, and the current flow in the measurement resistor rm produces a voltage drop, which is symmetrically split between ch+ and ch nodes with respect to the 0 v reference of the adc, producing two equal signals with opposite phases. it is important to highlight that all the adopted resistors consist of precision metal film-leaded resistors that have a very good overall stability, thanks to an extremely low temperature coefficient of resistance (tcr) of ± 2 ppm/k and a very tight tolerance in the range of ± 0.01 % to ± 0.25 %. the passive resistive divider networks, obtained with the series rp, rm, and rp, again produce an offset voltage for both channels equal to: 𝑉off = 𝑉ref 𝑅m 2 𝑅p+𝑅m , (1) this offset voltage has been maintained as reasonably low, choosing rp 2 order of magnitude higher than rm. by calculating equation (1), 𝑉off becomes almost 0.5% of 𝑉ref. this offset and the other offset due to the transducer feedback amplifier have been measured and digitally compensated by means of microcontroller’s firmware. 3. embedded measurement firmware the microcontroller was programmed using standard c language. the firmware implements the opportune drivers for all the described hardware devices and performs their coordination. starting from the disciplined 10 mhz clock, the 12.8 khz sampling has to be produced for the adc. there are various possible solutions for this problem. most of them, however, involve firmware latency. the only one that completely avoids this issue is the use of the 10 mhz clock as the main oscillator for the entire microcontroller. in this way, all the internal clocks of the microcontroller are locked in frequency and phase with the absolute time, and it is possible to construct the time base for the adc using only internal hardware devices, without involving execution time latency due to firmware routines. in particular, the time base for the adc has been built figure 2. voltage and current combined transducers. acta imeko | www.imeko.org march2020 | volume 9 | number 1 | 43 by making use of an internal timer as a key peripheral. in summary, the 10 mhz clock reference signal from gpsdo has been used as an external source and is given to the internal microcontroller phase locked loop (pll) circuit, which allows an increase in the frequency of up to 216 mhz. the timer peripheral acts as a divider, automatically controlling the external pulse width modulation (pwm). this line is used to clock the adc. to maintain a high level of accuracy, interrupt service routines (isrs) have been avoided, and the timer is configured to control the output in the hardware, without latency. figure 3 shows a block diagram of a sample clock generation. the system clock (sysclk) is obtained by equation (2) and is equal to 216 mhz: 𝑆𝑌𝑆clk = 𝐻𝑆𝐸 · 𝑃𝐿𝐿𝑀 · 𝑃𝐿𝐿𝑁 𝑃𝐿𝐿𝑃 . (2) the timer accurately divides the system clock by 16875 in order to generate a 12.8 khz square wave using the pwm output. the standard pwm mode is programmed with the auto-reload register (arr) and the capture compare register (ccr) to define the period and the duty cycle respectively. note that two different timers have been used for the two different channels. moreover, controlling the duty cycle and the polarity of the sample clock signal, it is possible to control the sampling instant position with respect to the 1-pps signal. consequently, it is possible to fine tune the absolute phase of the two channels within one sample range (i.e. 2∙π/256 rad at the chosen sampling frequency at 50 hz with a resolution of 2∙π∙50/216 µrad, thanks to the timer frequency of 216 mhz). as mentioned in section 1, the aim of the presented work is to realise an accurate samu. according to [10], the requested accuracy for the highest time performance class (i.e. t5) must be ± 1 µs. the use of gpsdo offers significant advantages in terms of synchronisation accuracy. moreover, the microcontroller firmware receives and parses national marine electronics association (nmea) sentences from the gps receiver to obtain a time stamp. since the samu must associate a time stamp with each sample, a hard real-time mechanism has been implemented in order to identify the precise instant in which a one-second frame starts, using a pps signal external interrupt. obviously, this synchronisation is needed only on the first pps frame at start up. from this moment on, the system proceeds to count up each sample time and follows the eventual corrections carried on by gpsdo. the firmware checks this correction process, monitoring the difference between internal counters (locked on disciplined 10 mhz from do) and nmea time strings (locked on pps signal from gps system). this difference must converge in the interval of ± 200 ns within a maximum time (which is used to detect time out condition). the microcontroller is responsible for iec61850-9-2 communication via ethernet as well. to fulfil this task, the integration between the lightweight internet protocol (lwip) stack and libiec61850 [11] (an open-source implementation of the standard) were realised. this integration was conducted through a berkley software distribution (bsd) socket interface figure 3. clock configuration. figure 4. adc samples collection via spi. acta imeko | www.imeko.org march2020 | volume 9 | number 1 | 44 in the lwip stack. iec 61850-9-2 is based on a raw socket, not fully supported by lwip. it was necessary to extend that support, opportunely modifying the whole library code. a producer-consumer design pattern was chosen for the data acquisition from the external adc. the samples reading is achieved by means of the event-based state machine shown in figure 4. the transition between the states is interrupt-based. in this way, the microcontroller can perform other tasks, like sending the sample frames through the ethernet. once the timer is enabled and started in pwm mode, a 12800 hz square wave synchronised with gps is produced and used as a conversion starter for both max11960 channels a and b in order to have a simultaneous sampling between the voltage and current signals. the rollover timer isr was used to start the state machine; from this moment on, it evolves by interrupts from the serial peripheral interface (spi) used for data transfer from the adc. the microcontroller firmware has been used also to compensate all the systematic errors of the proposed samu. the gain and the offset of the whole measurement chain for both channels, measured during the metrological characterisation of the proposed instrument, have been compensated. moreover, the absolute phase error of each channel was determined and compensated at 50 hz, acting, as previously described, on the duty cycle of the pwm timer output, which tunes the sampling instant position with respect to the 1-pps edge. 4. experimental test several tests were conducted on the realised instrument in order to fully characterise its metrological performance. in the following subsections, the more relevant test setups with related experimental results are explained. in particular, the linearity of the system was evaluated by dynamic testing of the adc, but the procedure also involved the whole chain, thus performing the tests at various amplitude levels. synchronisation accuracy was evaluated by comparing a reference clock signal with the one produced by the adopted gpsdo, but it also involved the whole chain again, adopting a pmu calibration facility. 4.1. adc dynamic test the block diagram of the measurement setup for the adc dynamic test is shown in figure 5 [12]. in particular, the effective number of bit (enob) of the adc was evaluated. the dynamic adc test method requires a high spectral purity test signal. this special signal must have a distortion of at least 10 db lower than that of the tested adc. for a 20-bit adc, a signal to noise and distortion ratio (sinad) of 120 db is required. moreover, the samu is a device that should measure signals at power frequency, so the frequency of the test signal should be of 50/60 hz. it is very difficult to achieve such spectral purity, particularly at a low frequency, because it is very difficult to build a good linear filter at low frequencies. for these reasons, the test was conducted at 1 khz. for this test, with reference to figure 5, a national instruments pci extension for instrumentation (ni pxi) chassis, housing the ni-5422 (16 bit, ± 12 v, 200 mhz arbitrary waveform generator, awg) and the ni-5922 (24 bit, ± 5 v, 500 khz data acquisition board, daq), were used. the pxi chassis was provided with a 10 mhz external clock source from the gpsdo. both boards have been configured to use the pxi clock as a reference clock in order to ensure coherent sampling between ni-5422, ni-5922, and the proposed reference samu under test. a sinusoidal signal with an amplitude equal to the adc input range and frequency of 1 khz were used as the test signal and were directly supplied (excluding conditioning stages) to both the samu adc and to the daq. in figure 6, the magnitude spectrum of the signal generated by the awg is reported, which reaches a sinad of 76.7 db. using the daq as a reference device, a closed-loop compensation was implemented in order to enhance the spectral purity of the test signal. the magnitude spectrum of the sine figure 5. dynamic adc test setup. figure 6. magnitude spectrum of the test signal obtained with awg only. figure 7. magnitude spectrum of the compensated test signal. acta imeko | www.imeko.org march2020 | volume 9 | number 1 | 45 wave obtained with this technique reported in figure 7. note that the amplitude of the harmonic components is reduced by an order of magnitude, and consequently, the sinad increases to 90.5 db. nevertheless, as we mentioned before, this is not enough to test a 20-bit adc. to further enhance the spectral purity of the signal, a passive linear filtering stage was added to the measurement setup. the electric scheme of the filter and its magnitude and phase frequency response are shown in figure 8 and figure 9, respectively. this is a narrow band-pass filter properly designed with a resonance frequency of 1 khz and realised adopting only tantalum capacitors and air-cored inductors in order to fulfil the high linearity requirements [13]. adopting both a compensation technique and analogue filtering to enhance spectral purity, a test signal with a sinad of almost 104 db (measured without the parallel connection of the samu adc) has been obtained. figure 12 shows the magnitude spectrum of this signal. obviously, this is not enough, but it is quite good for the preliminary characterisation of the proposed samu with a design validation purpose. the software for the measuring setup was developed in labview on windows operating system (os). unfortunately, windows has poor support for raw sockets. for this reason, the measurement results, i.e. the sampled values of the samu, are sent via ethernet to an iec 61850-9-2/tcp (transmission control protocol) bridge (realised using [11]) and subsequently to the pxi controller. however, when parallel connecting the samu adc, the spectral purity deteriorates significantly. in fact, as depicted in figure 10, which shows the magnitude spectrum of the sv, the spectral purity becomes lower as the sinad drops to about 88.5 db. it shows the presence of the same harmonics of the test signal, but there is an important presence of an undesired 50 hz spectral component and its second harmonic (100 hz) uncorrelated with the input signal. this presence, together with a load effect that slightly reduces the amplitude of the test signal, makes the sinad decrease. it is important to highlight that the noise floor of the svs (figure 10) is comparable to that which is obtained using the daq board (figure 12). figure 8. electric scheme of the band pass resonant passive filter. figure 9. magnitude and phase response of the filter. figure 10. magnitude spectrum of the sampled values. figure 11. magnitude spectrum of the sampled values with 0 v at the input of the adc. figure 12. magnitude spectrum of the test signal at the output of the filter. acta imeko | www.imeko.org march2020 | volume 9 | number 1 | 46 moreover, it is important to highlight that there is an undesired spectral component at exactly 3200 hz, which is not present in the input signal. this spurious spectral component is uncorrelated with the input signal. to prove that consideration, another test has been conducted, putting zero volts at the input of the adc (with a short circuit). as shown from figure 11, the undesired tone is still present, located at the frequency fd = 3.2 khz. investigations concerning this issue (changing the sampling frequency and/or changing the clock signal amplitude and frequency) led to the realisation that the disturbance is due to capacitive coupling with the 10 mhz clock signal. this last frequency is aliased according to equation (3): 𝑓𝑑 = |𝑓clock ± 𝑘 ∙ 𝑓s| (3) where 𝑓clock is the clock signal at 10 mhz, and 𝑓s is the sampling frequency of 12.8 khz. choosing k = 781 the 𝑓𝑑 result exactly equals 3.2 khz. despite its small amplitude, the spurious component is considerably higher than the noise floor. further efforts to enhance the performance of the anti-aliasing filter at the input of the adc are in progress. it is important to highlight that this problem is due to the choice of sampling frequency. by choosing a frequency submultiple of 10 mhz, the spectral component will fall at 0 hz and could be compensated as a further contribution to the offset. however, the sampling frequency is not an arbitrary value; rather, it will be established by a future standard focused on samus. 4.2. synchronisation test in order to evaluate the time synchronisation accuracy of the proposed samu, other experimental tests were conducted. an agilent 53230a universal frequency counter was used to determine the conversion starter stability. the experimental results show that a standard deviation of 10 µhz for the sampling frequency was obtained. furthermore, the maximum error of the sampling period, measured over a 10-minute observation time, is about 200 ns [10]. another test was conducted comparing the pps signal from a reference clock to that coming from the adopted gpsdo. a fluke 910r atomic clock, which exhibits a maximum jitter on the 1-pps output (when locked to gps) less than 60 ns relative to utc, was used as a reference. figure 13 shows the distribution of the delay between the 1-pps signal from the gpsdo and that coming from the reference clock. the test duration was 960 s. the maximum error is lower than 200 ns and the mean is -5.4 ns. 4.3. amplitude test other tests were conducted in order to evaluate the amplitude linearity and relative phase accuracy of the proposed samu. for these tests, a fluke 6105a electrical power calibrator was used (see figure 14) to generate a signal typical of power systems in sinusoidal conditions. for all the tests, 31 iterations (according with gum [14]) having 2 s duration were carried out. the standard uncertainty was calculated for the 31 iterations and added quadratically to the uncertainty of type b of the calibrator in the corresponding amplitude range. the measurements’ expanded uncertainty, with a coverage factor 3, are reported in the error bars of the subsequent figures. it is noteworthy that for the first test set, the voltage amplitude was varied in the range of 80 % to 120 % of the nominal value (110 v), and the current amplitude was varied in the range of 60 % to 120 % of the nominal value (5 a). the voltage rms, the current rms, and active power [15] were calculated for the acquired data, as shown in figure 15. the current has a dominant influence on the error in the measurements of active power. the adc and the conditioning network are symmetric, so the simple conclusion is that to further enhance the performance of the samu, the current transducer must be enhanced. figure 13. distribution of the jitter on the 1-pps output of gpsdo. figure 14. amplitude test setup. figure 15. ratio error on rms voltage, rms current and active power, varying the amplitude level. acta imeko | www.imeko.org march2020 | volume 9 | number 1 | 47 4.4. absolute phase test further experimental tests to evaluate the synchronisation error involving the whole measurement chain, from the transducers to the digital sampled values, were conducted by means of fluke 6135a/pmu-cal. figure 16 shows the block diagram of the measurement setup. this special instrumentation has been designed for testing the phasor measurement units (pmus), instruments devoted to synchrophasor measurement [16]. to this end, pmu-cal is a calibrator that can generate signals accurately, referenced in phase to the absolute time. it exhibits a phase standard uncertainty of 32 µrad. taking advantage of this characteristic, it has been possible to evaluate the phase error of the proposed samu with reference to the absolute time in steady state conditions. in figure 17, the absolute phase error of the proposed instrument for both the voltage and current are reported, varying the input frequency in the range of 45 hz to 55 hz. 31 iterations having a 2 s duration were also performed for these tests. the standard uncertainty (evaluated with type a method) was calculated for the 31 iterations and was quadratically added to the uncertainty contribution (type b method) of the pmu-cal. the expanded combined uncertainty (level of confidence 99 %) is reported in the error bars. note that the phase error is very small at 50 hz as a result of the compensation of the systematic error obtained by time shifting the sampling instant with respect to absolute time. of course, this kind of time domain compensation works very good only at one frequency. however, whereas the performance shown is quite good in the range of interest, it is noteworthy that almost all the measurements are compatible, which is due mainly to the uncertainty of the reference instrument. 5. conclusions this paper discussed the implementation of a reference samu based on an arm microcontroller. it also included an external high-resolution adc, a conditioning stage, a vt and a ct as well as a gpsdo. great care was given to the realisation of the insulation and conditioning stages. experimental results of the evaluation of the enob and the stability of the time base have also been presented. some results coming from a comprehensive metrological characterisation have also been reported here. the system exhibits deviations lower than 0.02 %, 0.06 % and 0.08 %, respectively on voltage, current and power measurement. the authors are currently working at two aspects of the realized samu: 1) improving the accuracy of the current channel, in order to reach the same accuracy exhibited by the voltage channel; 2) reducing the interferences, on the measured signals, induced by the capacitive coupling with the 10 mhz clock and other external radiated or conducted disturbances. acknowledgement the work presented in this paper was funded by the empir 17ind06 future grid ii project, which is jointly funded by the empir participating countries within euramet and the european union. references [1] g. crotti, d. gallo, d. giordano, c. landi, m. luiso, m. modarres, m. zucca, frequency compliance of mv voltage sensors for smart grid application, ieee sensors journal, vol. 17, no. 23, pp. 7621-7629, dec.1, 1 2017. doi: https://doi.org/10.1109/jsen.2017.2726116 [2] d. gallo, c. landi, m. luiso, e. fiorucci, g. bucci, f. ciancetta, realization and characterization of an electronic instrument transducer for mv networks with fiber optic insulation, wseas transactions on power systems, issn: 1790-5060, eissn: 2224-350x, issue 1, volume 8, pp. 45-56, january 2013. [3] iec en 61850-9-2:2011, communication network and systems for power utility automation – part 9-2: specific communication service mapping (scsm) – sampled values over iso/iec 88023, 2011. [4] m. augustoni, a. mortara, a calibration setup for iec 61850-92 devices, ieee transactions on instrumentation and measurement, 66(6) (2017) pp. 1124-1130. [5] g. crotti, d. gallo, d. giordano, c. landi, m. luiso, industrial comparator for smart grid sensor calibration, in ieee sensors journal, vol. 17, no. 23, pp. 7784-7793, dec.1, 1 2017. doi: https://doi.org/10.1109/jsen.2017.2724299 [6] g. crotti, a. delle femine, d. gallo, d. giordano, c. landi, m. luiso, measurement of the absolute phase error of digitizers, ieee transactions on instrumentation and measurement, 68(6) (2019) pp. 1724-1731. [7] iec en 61850-9-3:2016, communication networks and systems for power utility automation – part 9-3: precision time protocol profile for power utility automation, 2016. [8] ieee std c37.238-2017 (revision of ieee std c37.238-2011), ieee standard profile for use of ieee 1588 precision time protocol in power system applications, 2017, pp. 1-42. figure 16. absolute phase test setup. figure 17. absolute phase error in varying the power frequency. https://doi.org/10.1109/jsen.2017.2726116 https://doi.org/10.1109/jsen.2017.2724299 acta imeko | www.imeko.org march2020 | volume 9 | number 1 | 48 [9] g. cipolletta, a. delle femine, d. gallo, c. landi, m. luiso, design approach for a stand alone merging unit, proc. of the 16th imeko tc10 conference, 3-4 sept., 2019, berlin, germany. [10] iec en 61850-5:2004, communication networks and systems in substations – part 5: communication requirements for functions and devices models, 2014. [11] official repository for libiec61850, the open-source library for the iec 61850 protocols [online] http://libiec61850.com/libiec61850 [12] v. papez, s. papezova, highly pure signal generators, proc. of the 9th international conference on telecommunication in modern satellite, cable, and broadcasting services, nis, 2009, pp. 526-529. [13] v. papez, s. papezova, frequency filters with high linearity design and used devices, proc. of the xix symposium electromagnetic phenomena in nonlinear circuits, 2006, maribor, slovenia, isbn 83921340-1-x, pp. 121-122. [14] jcgm100:2008 – gum 1995 with minor corrections. published by the jcgm in the name of the bipm, iec, ifcc, ilac, iso, iupac, iupap, and oiml. [15] ieee standard definitions for the measurement of electric power quantities under sinusoidal, nonsinusoidal, balanced, or unbalanced conditions, ieee std 1459-2010 (revision of ieee std 1459-2000), 19 march 2010. [16] ieee/iec international standard – measuring relays and protection equipment part 118-1: synchrophasor for power systems – measurements’, in iec/ieee 60255-118-1:2018, pp. 1-78, 19 dec. 2018. http://libiec61850.com/libiec61850 research trends and challenges on dac testing acta imeko issn: 2221-870x june 2020, volume 9, number 2, 46 51 acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 46 research trends and challenges in testing digital-to-analog converters eulalia balestrieri1, pasquale daponte1, luca de vito1, francesco picariello1, sergio rapuano1, ioan tudosa1 1 department of engineering, university of sannio, benevento, italy section: research paper keywords: dac; test; ieee 1658 standard; non-linearity; uncertainty. citation: eulalia balestrieri, pasquale daponte, luca de vito, francesco picariello, sergio rapuano, ioan tudosa, research trends and challenges on dac testing, acta imeko, vol. 9, no. 2, article 8, june 2020, identifier: imeko-acta-09 (2020)-02-08 editor: alexandru salceanu, technical university of iasi, romania received february 7, 2020; in final form march 18, 2020; published june 2020 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: eulalia balestrieri, e-mail: eubal@unisannio.it 1. introduction digital-to-analog converters (dacs) perform an essential function in signal processing, since they allow circuits operating on digital signals to be connected with circuits operating on analog signals. the number of applications relying on the dac function is continuously increasing, including display electronics, motor control, software defined radio, data acquisition systems, optoelectronics, and test/measurement equipment. dacs can also be components of larger circuits; for example, in successive-approximation analog-to-digital converters (adcs). on the other hand, advanced high-speed dacs can embed digital signal processing and conditioning functionalities, such as complex modulation; digital filters and interpolators; and numerically controlled oscillators [1]. new dac architectures and techniques continue to be developed in order to meet specific application requirements, demanding higher and higher dac performance and assessing it more stringently. therefore, dac specification comparison and assessment are complicated by the variety of requirements, underlying conditions, and dac architectures. to bring order and clarity to the field in dac terms and definitions as well as to provide proper dac test methods, ieee standard 1658 has been published in 2012 [2], and it is currently under revision. previous work has been developed in the past on dac testing research directions [3]. however, in recent years, dac architectures and application contexts have continuously and rapidly evolved. consequently, new challenges and points of interest have arisen, leading to the need for an update to ieee standard 1658, as well. to provide useful information on the standard’s update process, a brief review of the latest research on dac testing has been presented in [4]. this study is an extended version of [4], which aims to highlight the prevailing issues and needs in the field that should be considered in the revision of ieee standard 1658. in the first section, the dac research trends are briefly introduced. then, focus is given to some recent research contributions, organised in four subsections, dealing with dac static and dynamic testing; abstract today, a broad range of different applications relies on digital-to-analog converters. different digital-to-analog converter architectures have been developed over the years, and several different specifications exist for quantifying their effective performance, the essential information to verify the requirement fulfilment, and the digital-to-analog converter’s suitability for the specific application. as a result, testing digital-to-analog converters has assumed and continues to assume increasing importance. the main testing challenges include the reduction of the testing time and costs; the measurement uncertainty computation; and emerging built-in self-test solutions. to clarify digital-to-analog converter terms, definitions, and test methods, ieee standard 1658 has been developed and is currently undergoing revision. to highlight the trends and issues that provide useful information for the revision of the standard, this article presents an overview of the recent research work dealing with digital-to-analog converter testing. mailto:eubal@unisannio.it acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 47 figures of merit; and measurement uncertainty. finally, conclusions are drawn. 2. dac research trends much of the scientific literature addressed to data converters continues to be devoted to adcs. an overview of the gap that exists between the number of adc and dac research contributions is given in [5]: ‘adc papers in the circuits’ conferences often outnumber the dac papers by 3:1 or more’. concerning dac research, recent contributions have mainly focused on architecture design topics for optimising performance according to different application requirements. many design strategies have been developed for dealing with dac error sources [6], like dac modelling [7]-[9] and calibration methods [10]-[12]. there are fewer scientific works that instead focus on testing dacs. the topics addressed by the researchers in this field concerning dac static testing are mainly devoted to the reduction of the test time and costs, also by means of the development of efficient dac built-in self-test (bist) solutions operating in system-on-chip (soc) and system-inpackage (sip) environments. regarding, instead, the dac dynamic testing, research has focused on overcoming the difficulties experienced in the characterisation of some specific dac parameters. hand in hand with dac technological development, research interests have been directed towards dac figures of merit in order to analyse and overcome the limitations of the conventional ones; to propose new ones that better reflect the needs of the specific applications; and to define the requirements for the figure of merit of interest depending on the specific application or reference standard. finally, even more research work has focused on the lack of measurement uncertainty estimation procedures. in the following subsections, some research contributions belonging to the abovementioned dac testing topics are briefly presented. 2.1. dac static testing dac static testing is aimed at characterising the converter response to dc or low-frequency input signals. typical dac static specifications are the gain, offset, integral non-linearity (inl), and differential non-linearity (dnl). with the increasing demand for high-performing dacs along with increasing resolutions, the required dac testing time increases exponentially, hand in hand with the test cost. this is especially true in the case of dac static linearity testing, since it traditionally requires a long time and is very expensive. the testing method and algorithm introduced in [13] form a solution for accurate dac linearity testing with reduced test time and costs. the proposed algorithm comes from the consideration that the number of independent error sources is, in most cases, much smaller than the number of dac codes examined in the linearity test. therefore, it is possible to carry out linearity testing with much fewer samples, which saves the test time. in particular, the number of unknowns that must be evaluated is decreased in [13] by means of a segmented nonparametric model for the inl curve of the dac. moreover, the proposed method involves the use of an onboard digitiser for dac output measurement in place of a highly accurate digital voltmeter, resulting in less test time per sample. this test time reduction has the direct consequence of reducing the test costs, too. additionally, the measurement error introduced due to the on-board digitiser is removed in the proposed algorithm, relaxing the stringent linearity requirement on the measurement device, allowing it to be orders of magnitude less linear than the dac under test, implying further cost savings. the proposed method can be applied to dac architectures inherently segmented, as binary weighted, r-2r and mdac, but it is not valid for string or thermometer-coded architectures [13]. other proposals for overcoming the requirements for high performance testing equipment and for avoiding the direct transfer of analog signals to external instruments are based on bist approaches. one example is the low-speed bist scheme for testing dac static parameters presented in [14]. the proposed approach is based on an under-sampling technique. the dac analog signal is modulated by the two sinusoidal carriers and then converted into a low-speed pulse stream. the relationship between the nonlinearity of the dac and the width of pulse stream can be further derived. moreover, the required operational frequency of the testing equipment used in the proposed scheme is much lower than that of dac [14]. in figure 1, the scheme for the proposed method along with the signals involved are shown. in particular, the bist approach relies on the use of a sinusoidal carrier generator, two comparators, a logic control unit, a time-todigital converter (tdc), and an all-pass filter acting as a delay element. the all-pass filter provides the sinusoidal carrier with a phase delay td [figure 1 (b)]. the comparators modulate the dac (c) (d) figure 1. (a) the scheme of the proposed method with the dual undersampling technique. (b) the relative signals. (c) the shadow area in figure 1 (b) of the linear case. (d) the shadow area in figure 1 (b) of the nonlinear case [14]. acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 48 output signal by both carriers, resulting in a narrow phase difference between two digital streams that represent the nonlinearity variation of the dac output signal. the logic unit provides the phase difference of the two digital streams, and the tdc converts these pulse widths into readable binary codes. in figure 1 (c) and figure 1 (d), the shadow area of figure 1 (b) is enlarged to show the linear and the non-linear cases, respectively. thanks to the very short duration between adjacent sample points, the sinusoidal carrier can be treated as the combination of piece-wise linear segments. as shown in figure 1 (d), in the presence of non-linearities, the displacement of the cross point of the carrier and dac output signal appears on the phase difference δw. the method was validated on an 8-bit 200ms/s dac, and it could be interesting to investigate the results obtained on dacs with greater resolution and speed. the bist structure for data converter static testing proposed in [15] focuses on the common case in which an adc and dac pair is embedded in a soc. by testing both the adc and dac converters simultaneously, the resulting test time can be decreased compared to other bist approaches (i.e. the loopback test). moreover, the hardware overhead is reduced compared to that of the standalone bist by sharing additional bist circuitry for testing the converters. in figure 2, the proposed bist scheme is shown. a counter is used for transition detection in adc testing and as a test pattern generator in dac testing. the ramp generator is used as a test input generator for adc testing and as a voltage reference for dac testing. for simultaneous adc and dac testing, the timing for each test must be arranged in order to share the use of the counter and the ramp generator. the proposed method is applicable to adc and dac with the same 1 lsb length and with the same or a different resolution. in this last case, the sampling frequencies of the two converters must be chosen to have the same time ticks [15]. 2.2. dac dynamic testing dac dynamic testing is aimed at characterising the converter response as the frequency and amplitude of the input signal’s variance. specifications describing the dac dynamic performance include the settling time, glitch, spurious-free dynamic range (sfdr), total harmonic distortion (thd), signal-to-noise ratio (snr), signal-to-noise-and-distortion ratio (sinad), effective number of bits (enob). dacs are currently used in a wide number of different applications, each with its own sensitivities to different dynamic specifications. consequently, research work on dac testing can be focused on a particular specification of interest. the settling time of the output signal is one of the most important parameters characterising high-speed dacs. however, it is also challenging to test the parameter accurately. a method that aims to automatically measure the settling time of high-speed dacs with a mid-high resolution has been proposed and experimentally investigated in [16]-[18]. in particular, in order to perform the automatic measurement of the settling times of 12-bit dacs (or more), the dac output signal is converted to digital form, and digital signal processing is performed [16][17]. the algorithms proposed in [16][17] show the possibility of measuring the settling time of the high-speed dacs with up to 16-bit resolution [18]. moreover, by using those algorithms, it is possible to reduce the level of 1/f and white noise by 100 times without distorting the measurement signal itself. another relevant specification for high-speed and highaccuracy dacs is represented by glitches. the dac glitches can actually compromise the overall converter spectral performance, making it unsuitable for specific applications. unfortunately, different definitions and testing methods exist, making misinterpretations of the real dac performance possible. an experimental analysis of the different dac glitch definitions and measurement methods proposed in the literature and adopted by manufacturers is provided in [19]. in particular, experimental investigation into the reproducibility of the results that can be achieved on the dac by varying the definitions and the testing methods has focused on (i) the applicability of each glitch definition and testing method; (ii) the required computational complexity; and (iii) the possibility of automating the area measurement procedure, excluding the need for operator supervision. preliminary results have shown that the restriction of the oscilloscope bandwidth greatly influences the estimation of the glitch values. an analysis of the numerical integration methods that can be applied to compute the glitch has been focused on the rectangle methods (the right and left corner; maximum and minimum corner; and the midpoint), the trapezoid, and simpson’s methods, as shown in figure 3. this analysis has also highlighted in this case the influence of noise on the results. moreover, the number of samples considered for the numerical integration greatly affects the confidence of results, representing a critical problem since in the case of very small and fast glitches, only a few samples can be captured. further research work is needed for comparing different definitions and testing procedures for dac glitches by analysing different converters. in addition, the glitch waveform dependence from the used measurement equipment should be investigated [19]. 2.3. dac figures of merit dac technological developments have led the research to focus on the dac figures of merit in order to introduce new ones that are capable of overcoming the conventional figures of merit limitations as well as to find their relationships with other relevant parameters, or specific requirements for the particular application or reference standard. figure 2. the bist scheme proposed in [15]. acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 49 dac performance is typically described by using figures of merit, focusing on the expected overall linear behaviour [20]. however, considering the internal structure and the electronic components embedded in the dac can provide a more reliable description of the system as well as a better explanation of the nature of the deviations observed at the dac output [20]. to achieve this target, a dac system model and a new figure of merit are presented in [20]. in particular, the proposed model consists of two blocks: the first includes a set of individual current (voltages) sources followed by a switching array, the second represents a summation step. the proposed dac model has been carried out in order to incorporate deviations at sources and switches as well the quantisation noise in the first block and deviations, which refer to the summation circuit in the second block [20]. a non-linearity assessment procedure is then introduced based on a generalised hammerstein network, including a prominent linear term associated with the input, additive, order-dependent, distortion terms, and an additive filtered noise term [20]. in this way, dac performance can be carried out by separately taking into account both the deviations at the input sources and switches as well as the nonidealities in the posterior summation circuit, allowing for the analysis of the cross influence between these two effects, too [20]. the novel figure of merit, named the system residue, is introduced in [20] to describe the full-scale dac response along the full input frequency span. this new dac parameter is able to separate the contributions of the different blocks in the system, such as the dac’s inner structure nonlinear deviations and memory effects, in such a way that it is easy to interpret by simple inspection [20]. in wideband communication systems, the half-rate interleaved topology is often employed in nyquist currentsteering dacs in order to obtain an ultra-high sampling rate. for this kind of dac, the duty cycle error is a main contributor to performance degradation [21]. however, little research has focused on this parameter [21]. to investigate the duty cycle distortion under the influence of limited output bandwidth in interleaved nyquist dacs, a closedform expression of the parameter’s signal-to-distortion ratio (sdr) based on the heaviside function is introduced in [21]. in particular, the sdr has two components, the former intrinsically caused by duty cycle error and the latter by limited bandwidth. in order to verify the sdr’s mathematical derivation, some simulations have been carried out in [21] by means of a model of a 6-bit 25gs/s dac (based on the half-rate architecture), with a 50 ω terminal load. the simulation results are discussed within the 1st-nyquist band, and the limited bandwidth is determined by the output time constant τ, depending on the output parasitic capacitance [21]. in figure 4, the simulated and calculated sdr changes with duty cycle error, varying from 0 to 10 %, are shown with the different normalised input signal frequencies (fin) in mind, from low to the 1st-nyquist bandwidth and a fixed value for the output time constant. it is evident the great influence of the duty cycle error on the dynamic performance, proven by the sdr degradation (>10db), which corresponds to a 1 % dutycycle error [21]. the distortion caused by the duty cycle has been shown to worsen as the input frequency rises, and the worst case is near the nyquist input frequency due to the impact of the limited bandwidth. sdr can therefore be used to evaluate the performance of interleaved half-rate dacs [21]. dac phase noise is a crucial parameter for all the systems that require high-purity rf signals [22][23]; for example, in radar applications [24]. however, from the analysis of numerous dac datasheets, it can be derived that the phase noise as well as amplitude noise performance have been poorly documented [22][23]. therefore, a method of measuring both the dac amplitude and phase noise by means of a phase noise analyser was proposed in [22][23]. the proposed method is described to have the advantage of requiring relaxed noise specifications for the phase noise analyser, by a factor of at least 20 db, with respect to the noise of the device under test. this can be obtained thanks to the rf amplification of the noise sidebands and the possibility of measuring the amplitude noise through a phase noise analyser, not including this measurement feature. the same measurements can be performed by using only an amplitude noise analyser (i.e. a power-detector diode followed by a fft analyser), without the phase noise measurement capability [22][23]. in addition to the phase noise in the frequency domain, the focus of this research is also on its relative in the time domain, the jitter [25]-[30]. research works have included the modelling of its effects for particular dac architectures [25][26] and the definition of the parameter requirements and the impacts thereof in specific applications [29][30]. 2.4. measurement uncertainty currently, evaluation of the measurement uncertainty has not yet been included in any dac standard. the measurement figure 3. the numerical integration methods analysed in [19]. figure 4. simulated and calculated sdr changes with the duty cycle error at output time constant τ = 0.2ts [21]. acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 50 uncertainty is a quantitative indication of the quality of the measurement and is essential for comparing results coming from different sources or with reference values given in specifications or standards. although adc research works dealing with the measurement uncertainty have been developed [31]-[35], the same was not found for dacs. very few papers have been published in this field. an example can be found in [36]: an evaluation of the measurement uncertainty of time domain dynamic parameters, as the dac rise time and fall time relying on a bootstrap technique according to the ‘guide to the expression of uncertainty in measurement’ [37] was presented. the proposed approach allows us to define confidence intervals from short acquisition records without needing hypotheses about the measurand distribution, in an easy and fast way, to ensure that the method can be also applicable in an industrial environment. however, since the approach deals only with the rise time and fall time, it would be useful to extend its application to other dac parameters. a new dac testing method together with its analysis from the uncertainty point of view is provided in [38][39]. the test is based on the comparison of a time-varying saw-tooth signal generated by the converter under test with a reference signal by a fast comparator. the reference signal is the superposition of the dc voltage measured by a precise dc voltmeter and slow dithering voltage with a known amplitude. the comparator detects the sequence of the dac control codes, which determine the dac nonlinearities. the proposed method has been validated both by simulations and experiments in a real setup under different working conditions. in figure 5, the simulation of the combined dependence of the uncertainty on record length (considering different numbers of samples l as well as on the amplitude of dithering w) is shown. simulation results show that by increasing amplitude w, the uncertainty grows approximately linearly, with the record length with the highest impact on the final uncertainty [38][39]. in particular, by increasing the number of samples, the uncertainty can be decreased, but at the cost of a longer measuring time. consequently, a compromise between the required uncertainties and the acceptable measuring time depending on user needs has to be made [39]. 3. conclusions today, an extremely large and growing number of new and rapidly evolving applications rely on dac functionality and performance. data converter design and testing are strictly related to the stringent performance specifications required by their reference application, also continuing to keep pace with technological advances. in the article, a brief overview of the latest research on dac testing has been presented in order to provide worthwhile information for the revision of the ieee standard dealing with the terminology and test methods of dacs. most of the relevant research has focused on dac architecture design strategies aimed at achieving the best performance in the specific application context. even fewer research contributions can be found concerning dac testing. recent research in this particular field has mainly aimed at providing mostly static testing methods requiring reduced time and cost, also paying attention to the situation in which the dac operates in a soc or in an sip. another source of the research interest is the testing of some specific dynamic parameters, like settling time and glitch, due to the challenging measurement maintaining suitable accuracy of the first one for high-speed dacs as well as the ambiguity in the developed definition and testing methods of the second one. new dac figures of merit capable of answering to some specific needs of the target applications are also objects of the relevant research. the opportunity of changing the conventional dac figures of merit and the definitions of their requirements to comply with the existing reference standards or emerging application needs have also sparked this research interest. unfortunately, measurement uncertainty evaluation has turned out to be an overlooked argument for dac testing. however, information about this parameter is essential in the evaluation of the effectiveness of the proposed dac testing methods and in comparing dac specifications. there is still a great deal of research to do on this specific aspect. references [1] tektronix, overcoming rf signal generation challenges with new dac technologies, white paper, 2017. [2] ieee standard for terminology and test methods of digital-toanalog converter devices, ieee std 1658-2011, 2012, pp. 1-126. [3] e. balestrieri, p. daponte, s. rapuano, recent developments on dac modelling, testing and standardization, measurement 39(3) (2006) pp. 258-266. [4] e. balestrieri, p. daponte, l. de vito, f. picariello, s. rapuano, i. tudosa, dac testing: recent research directions, proc. of the 23rd imeko tc4 international symposium electrical & electronic measurements: promote industry 4.0, 17-20 sept., 2019, xi’an, china, pp. 223-227. [5] d. h. robertson, problems and solutions: how applications drive data converters (and how changing data converter technology influences system architecture), ieee solid-state circuits magazine 7(3) (2015) pp. 47-57. [6] l. f. rahman, f. a. rudham, m. b. i. reaz, m. marufuzzaman, the evolution of digital to analog converter, proc. of the international conference on advances in electrical, electronic and systems engineering (icaees), 2016, putrajaya, malaysia, pp. 151-154. [7] d. hamidovic, j. markovic, t. buckel, p. preyler, m. huemer, a. springer, modeling of an iq rf-dac with error-free loswitching, proc. of the ieee international symposium on circuits and systems (iscas), 2018, florence, italy, pp. 1-5. [8] s. kim, k. lee, m. lee, modeling random clock jitter effect of high-speed current-steering nrz and rz dac, ieee transactions on circuits and systems i: regular papers 65(9) (2018) pp. 2832-2841. figure 5. dependence of the standard deviation of difference between the inl characteristics on the amplitude of dithering w and different numbers of samples l [39]. acta imeko | www.imeko.org june 2020 | volume 9 | number 2 | 51 [9] l. lai, x. li, y. fu, y. liu, h. yang, demystifying and mitigating code-dependent switching distortions in current-steering dacs, ieee transactions on circuits and systems i: regular papers 66(1) (2019) pp. 68-81. [10] s. xu, j. w. lee, a digital technique for calibrating and correcting nonlinearities in current-steering dacs, proc. of the ieee international symposium on circuits and systems (iscas), 2018, florence, italy, pp. 1-5. 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[23] c. e. calosso, a. c. cárdenas olaya, e. rubiola, phase-noise and amplitude-noise measurement of dacs and ddss, ieee transactions on ultrasonics, ferroelectrics, and frequency control 67(2) (2020) pp. 431-439. [24] p. delos, j. liner, improved dac phase noise measurements enable ultra low phase noise dds applications, analog dialogue 51 (2017). [25] s. kim, k. lee, m. lee, modeling random clock jitter effect of high-speed current-steering nrz and rz dac, ieee transactions on circuits and systems i: regular papers 65(9) (2018) pp. 2832-2841. [26] s. trampitsch, m. kalcher, d. gruber, m. lunglmayr, m. huemer, modeling non-idealities of capacitive rf-dacs with a switched state-space model, proc. of the 2019 ieee international symposium on circuits and systems (iscas), 2019, sapporo, japan, pp. 1-5. [27] e. balestrieri, p. daponte, l. de vito, s. rapuano, jitter and its relatives: a critical overview, proc. of the 2013 ieee international instrumentation and measurement technology conference (i2mtc), 2013, minneapolis, mn, pp. 1141-1146. [28] e. balestrieri, f. picariello, s. rapuano, i. tudosa, review on jitter terminology and definitions, measurement 145 (2019) pp. 264273. [29] s. almonacil, p. jennevé, p. ramantanis, p. layec, dac jitter requirements for high speed optical networks, proc. of the 2018 european conference on optical communication (ecoc), 2018, rome, italy, pp. 1-3. [30] h. xin k. zhang, y. fun, m. zhang, z. guo, h. he, w. hu, quantization noise and sampling jitter impact of adc/dac in multi-ifof based mobile fronthaul, proc. of the 2017 16th international conference on optical communications and networks (icocn), 2017, wuzhen, china, pp. 1-3. [31] p. arpaia, c. baccigalupi, m. martino, metrological characterization of high-performance delta-sigma adcs: a case study of cern ds-22, proc. of the ieee international instrumentation and measurement technology conference (i2mtc), 2018, houston, tx, pp. 1-6. [32] e. balestrieri, l. de vito, s. rapuano, d. slepicka, estimating the uncertainty in the frequency domain characterization of digitizing waveform recorders, ieee transactions on instrumentation and measurement 61(6) (2012) pp. 1613-1624. [33] e. balestrieri, m. catelani, l. ciani, s. rapuano, a. zanobini, word error rate measurement uncertainty estimation in digitizing waveform recorders, measurement 46(1) (2013) pp. 572-581. [34] a. baccigalupi, m. d’arco, a. liccardo, parameters and methods for adcs testing compliant with the guide to the expression of uncertainty in measurements, ieee transactions on instrumentation and measurement 66(3) (2017) pp. 424-431. [35] e. balestrieri, p. daponte, l. de vito, s. rapuano, jitter and its relatives: a critical overview, ieee international instrumentation and measurement technology conference (i2mtc), 2013, minneapolis, mn, pp. 1141-1146. [36] e. balestrieri, m. catelani, l. ciani, s. rapuano, a. zanobini, uncertainty evaluation of dac time response parameters, proc. of the international workshop on adc modelling, testing and data converter analysis and design and ieee adc forum, 30 jun.-1 july, 2011, orvieto, italy, pp. 278-281. [37] bipm, iec, ifcc, iso, iupac, iupap, and oiml, jcgm 100, guide to the expression of uncertainty in measurement, 2008. [38] m. sekerák, l. michaeli, j. liptak, j. šaliga, new concept for dac testing under dynamic condition by the comparison with reference voltage, proc. of the 22nd international conference radioelektronika, 2012, brno, czech republic. [39] l. michaeli, j. šaliga, m. sekerák, j. lipták, uncertainty of the dynamic dac testing by dc voltage with superimposed dithering signal, proc. of measurement 2013: the 9th international conference, 2013, smolenice, slovakia, pp. 47-50. mineral diagnostics: sem-eds monte carlo strategy for optimised measurements of ultrathin fragments in cultural heritage studies acta imeko issn: 2221-870x march 2021, volume 10, number 1, 193 200 acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 193 mineral diagnostics: sem-eds monte carlo strategy for optimised measurements of ultrathin fragments in cultural heritage studies daniele moro1, gianfranco ulian1, giovanni valdrè1 1 centro di ricerca interdisciplinare di biomineralogia, cristallografia e biomateriali, dipartimento di scienze biologiche, geologiche e ambientali, “alma mater studiorum” università di bologna, piazza di porta san donato 1, 40126 bologna, italy section: research paper keywords: monte carlo method; sem-eds microanalysis; glass fragments; metal fragments citation: daniele moro, gianfranco ulian, giovanni valdrè, mineral diagnostics: sem-eds monte carlo strategy for optimised measurements of ultrathin fragments in cultural heritage studies, acta imeko, vol. 10, no. 1, article 26, march 2021, identifier: imeko-acta-10 (2021)-01-26 editor: ioan tudosa, university of sannio, italy received april 28, 2020; in final form november 11, 2020; published march 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding authors: giovanni valdrè, e-mail: giovanni.valdre@unibo.it 1. introduction morphological, chemical, structural and other types of analyses, which are of paramount importance in the conservation and restoration of cultural heritage artefacts, are often faced with the impossibility of sampling a suitable quantity of material to apply conventional, well-established procedures of analysis. in these cases, where a very small amount of material is available, microand nano-scale methodologies are of fundamental importance to provide precise and accurate results for the characterisation, and hence, conservation and restoration, of the artefact. among the different advanced experimental and theoretical methodologies with microand sub-micrometric scale capability for the characterisation of natural or synthetic materials [1]-[8], scanning electron microscopy (sem), coupled with energy dispersive x-ray spectrometry (eds) with si(li) detectors or silicon drift detectors (sdd), is a very powerful and widely used microanalytical technique to study both the local morphology and chemical composition of materials found in the cultural heritage field [9]-[13], even when their dimensions are on the microand nanometre scale. this is typical of ceramics, lithotypes and glass fragments as well as ultrathin metal and alloy layers and fragments. however, in the latter, and particularly whenever a quantitative assessment is desired, the sem-eds parameter setup and the eds detector-to-sample configurations and arrangements have to be optimised because, if not set appropriately, the electron and x-ray scattering in the ultrathin materials, and even in those adjacent, could generate systematic errors. these problems are related to the elastic/inelastic scattering of energetic electrons in the finite size (mass) of the fragment, which is also affected by the mean atomic number. as a consequence, the eds spectrum x-ray intensity is mainly influenced by the ratio between the fragment size and the extension of the electron path in the material. in order to avoid, abstract the availability of minute quantities of sampling material is often an issue in the context of cultural heritage and archaeology due, for instance, to the value of the sample, its uniqueness or the small amount of residual material which testify the original form of the art to be restored. in this context, electron-excited energy-dispersive x-ray spectrometry (eds) performed in a scanning electron microscope (sem) has proven to be a primary methodology for analysing minute quantities of material thanks to its morphological and microanalytical capability. however, when dealing with microand sub-micrometre specimens, as can be the case in ultrathin glass and metal fragments, several effects resulting from the physics and operational settings of the measurement must be considered to avoid quantification errors. in this paper, a detailed study of the effects of microand nanometric-sized glass and gold-alloy fragments on semeds microanalysis is presented. monte carlo simulations of different kinds of elongated glass fragment, with a square section and a thickness of 0.1 to 10 µm, and of some gold alloys demonstrated a strong influence in terms of the fragment size and operational conditions (beam energy, detector position, etc.). this work can be used to devise an appropriate and optimised measurement strategy. mailto:giovanni.valdre@unibo.it acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 194 or at least control, these phenomena when a quantitative analysis of ultrathin fragments is desired, the sem-eds operational conditions (e.g. beam energy, etc.) have to be optimised to design specific analytical strategies, which consider both the chemical composition, the size of the specimen and the detector configuration. in particular, a systematic study of the optimal instrumental settings is required to achieve precise, accurate and consistent results. in this study, a monte carlo simulation [14] was proposed as an effective methodology to assess the appropriate strategy for an accurate quantitative sem-eds microanalysis of ultrathin samples. the monte carlo method was applied to simulate the transport of energetic electrons and x-ray generation and detection in two different types of ultrathin sample. to show the potential of the method, we chose two typical case studies from cultural heritage and archaeology. the first case study presents an investigation of ultrathin and elongated glass fragments with a square section. in the second case, ultrathin metal layers, such as those used in the glass tesserae of mosaics, were simulated and studied. all the simulations were carried out modelling realistic experimental conditions, investigating the effects related to the composition and thickness of the glass and metal fragments, the physics of electron beam-sample interaction (e.g. energy of the electron beam, elastic/inelastic scattering of electrons, threshold levels for element ionisation) and the sem-eds chamber setups (e.g. detector azimuthal and take-off angles, position of the electron beam on the sample). the results of this study are of interest to researchers in the field of cultural heritage and archaeology because of the potential for using sem-eds micro-nanoanalysis to obtain detailed, precise and accurate quantitative data from ultrathin fragments and objects. the presented monte carlo-based approach is a general, consistent and accurate strategy applicable whenever the ultrathin size of the investigated specimen is comparable to or less than the volume of the electron scattering, for example, of micron size. in the field of cultural heritage and archaeology, this is often the case, for instance, in ultrathin pigment layers, alteration and contamination layers, oxidised metal surfaces, superficial protective treatments and microand nanometric dishomogeneity. in section 2, the physics of the monte carlo model is introduced together with the specific sem-eds settings used in this study. in section 3, the results of the monte carlo investigation of two relevant case studies (ultrathin glass fragments and metal layers) are presented and discussed. finally, section 4 reports the main conclusions of this study. 2. monte carlo model the sem monte carlo method is an efficient solution for dealing with a detailed study of the scattering of electrons and the generation, absorption and fluorescence of x-rays in different kinds of material. this method was employed here to model the generation and transport of electrons and the generation and transport of continuum [15]-[18] and characteristic x-ray photons, also considering secondary fluorescence, in ultrathin structures and to model a realistic eds x-ray detector, with the aim of achieving an optimised and accurate quantitative sem-eds x-ray microanalysis. the trajectories of electrons were modelled taking into account elastic scattering and continuous energy loss (continuous slowing down approximation) [19]. czyzewski et al.’s mott cross section [20], jablonski et al.’s mott scattering cross section [21] and a basic screened rutherford model [22] were considered for the elastic scattering. energy loss was modelled through an empirical modification of the bethe equation for energy loss (joy–luo expression [23]), and the bote and salvat expression [24] was employed for the ionisation cross section. the mass absorption coefficients are those of chantler et al. [25], whereas the fluorescence yields are tabulated experimental values [26]. the physics of x-ray secondary fluorescence generation is modelled by propagating the emitted primary, bremsstrahlung and characteristic x-ray photons from the generation point in a random direction (from a uniform distribution) for a distance computed by the mean free path for photoionisation, until absorption by photoionisation, or until they exit the material [27]. in the case of a photoionisation event, relaxation follows with the related probabilities of characteristic x-ray emissions. the electron beam was modelled with a gaussian profile and focussed onto three-dimensional models of the glass and metal fragments. the eds spectra were obtained simulating a realistic si(li) eds detector, with a moxtek ap 3.3 ultrathin polymer window, a resolution of 130 ev (fwhm at mn kα), a sampleto-detector distance of 45 mm and an elevation angle of 40°, which are typical measurements in sem-eds microanalysis. after the spectra generation, the background was subtracted, the intensities of the characteristic peaks were integrated and the resulting values analysed against fragment size, beam energy, position of the electron beam with respect to the fragment edges and orientation of the long axis of the fragment with respect to the detector position (azimuthal angle). specimen coating by conductive layers were not considered. 3. results and discussion in general, for the microanalytical characterisation of the composition of very thin material, the requirements of the quantitative sem-eds approach need special care since several phenomena can influence the analytical results. this is particularly evident with ultrathin materials whose dimensions are close to or below that of the electron penetration volume. the interaction volume of energetic electrons and the x-ray generation volume in conventional x-ray microanalysis (i.e. 15– 30 kev) extend for several μm3 in many materials. for ultrathin materials, it is expected that the penetration of primary electrons extends further than the fragment size, with a variable percentage of electrons exiting the fragment before exciting x-rays (finite size effect). in turn, they may excite the adjacent materials or even the substrate, generating x-rays not related to the intended material. furthermore, the typical standard reference samples used in conventional quantitative microanalysis are bulk materials (i.e. of infinite size with respect to the scattering volume of electrons); therefore, more x-rays are generated in a standard reference sample than an ultrathin fragment. accordingly, the ratio of the x-ray intensity emitted by the unknown fragment to that of a massive sample (k-ratio) is subject to a decrement. these phenomena must be carefully taken in account to improve the precision and accuracy of the analysis. 3.1. case study 1: glass fragments to investigate the various effects of different sem-eds measurement conditions on the electron-excited x-ray microanalysis of ultrathin glass samples, three different types of glass of varying thickness and composition were considered: (a) silica-iron-magnesium glass, (b) sodium-bearing glass and (c) calcium-bearing glass (see table 1 for the specific compositions, reported in oxides). three-dimensional glass fragments were acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 195 modelled with a square section in the x–z plane and an elongated shape in the third dimension, deposited above a massive substrate of carbon. for each model, the thickness and lateral size of the fragment were varied between 0.1 and 10 μm to investigate the effects of size reduction, whereas electron beam energies of 5, 15 and 25 kev were employed. it was found that several effects should be carefully considered when the excitation volume approaches the size of the fragment under analysis, and the experimental setting must be accurately defined accordingly. figure 1a and b show, as an example, a two-dimensional view of the electron trajectories in the case of a 15 kev (a) and 5 kev (b) electron beam (vertical green line), respectively, focussed on the top of a sodium-bearing glass fragment (yellow square line), type b of table 1. the beam is centred with respect to the fragment edges. the fragment has a square section, a thickness of 1 μm and is deposited onto a carbon substrate (horizontal white line). in this case, working at 15 kev (figure 1a) does not prevent the electrons from escaping from the fragment towards the surrounding material. primary electrons scatter inside the whole fragment volume (yellow trajectories), even scattering into the carbon substrate (white trajectories) underneath and from the sides of the fragment towards the substrate (blue trajectories) or escaping from each different type of material (red ones). primary electrons penetrate the substrate for more than 1 μm, laterally spreading for several micrometres. reducing the beam energy to 5 kev (figure 1b) deeply affects the electron scattering volume, which reduces to about 4 x 10-2 μm3, completely confined within the glass material and without any contribution from the substrate. furthermore, it is worth noting that, compared to the 5 kev electron beam, when working at 15 kev, the electrons also laterally scatter from the fragment sides (blue and red trajectories). in the case of an embedded fragment, at 15 kev, these last electrons excite x-ray generation in the material adjacent to the glass fragment. figure 1. simulated electron trajectories for a 1-μm-thick glass fragment of composition sio2 53%, fe2o3 35%, na2o 8%, mgo 4% (type b in table 1) and beam energies of 15 kev (a) and 5 kev (b), under the experimental conditions detailed in section 2. electron trajectory colour: green – electron beam, yellow – inside the fragment, white – inside the substrate, blue – escaping from the fragment sides towards the substrate, red – scattering outside both the fragment and substrate. table 1. oxide composition (weight %) of the glass fragments. type sio2 fe2o3 na2o cao mgo a 53 40 7 b 53 35 8 4 c 62 2 14 22 figure 2. integrated x-ray intensity of si kα (a) and mg kα (b) emission lines versus electron beam energy, simulated for type-a glass fragments with a square section and thicknesses between 0.1 and 10 μm. legend in (a). acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 196 the eds spectra, obtained by the specific modelled si(li) detector and sem chamber setup, were background subtracted, and the intensity of the relevant x-ray peaks was integrated. these values were also compared with those obtained for a bulk glass reference standard. as an example, the integrated intensity (counts) of some x-ray emission lines as a function of the energy of the electron beam (5, 15 and 25 kev), for thicknesses of 0.1, 0.2, 0.5, 1.0, 2.0, 5.0 and 10.0 μm, is shown in figure 2, figure 3 and figure 4, in relation to a type-a, -b and -c glass fragment, respectively. it is worth noting that the integrated intensity trend of the si kα emission line (1.740 kev) for the type-a glass, as opposed to the beam energy (figure 2a), is highly variable with regard to fragment thickness, and it has a general non-linear trend. the xray emission of a 10-μm-thick fragment (blue dashed line and square marker) is comparable with that of the bulk material for each simulated energy of the electron beam. with the decrease in glass-fragment thickness and lateral size, a strong reduction in the integrated intensity was observed, with a different thickness threshold and non-linear trend for each electron-beam energy value. at 25 kev, a reduction in fragment thickness to 5 μm is figure 3. integrated x-ray intensity of fe kα (a), fe kα (b), fe l (c), mg kα (d) and o kα (e) emission lines versus electron beam energy, simulated for type-b glass fragments with a square section and thicknesses between 0.1 and 10 μm. legend in (b). acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 197 enough to induce a deviation in integrated intensity from that of the bulk standard. for this thickness value, the reduced absorption effect prevails over that of reduced mass (size), and an increase in x-ray counts measured by the detector is observed. it should be noted that ‘soft’ x-rays, which are subject to higher absorption, suffer a greater reduced x-ray absorption effect [28]. for thicknesses below 5 μm, the x-ray counts are strongly reduced with respect to that of the bulk material. at 15 kev, the detected x-ray counts start reducing below 2 μm, whereas the same effect occurs below 0.5 μm at 5 kev. figure 2b shows the results obtained for the integrated intensity of the mg kα emission line (1.254 kev) for the type-a glass fragment. similar considerations made for si kα can be extended to the general trend of this atomic species, although with different variations specific to mg kα. in particular, a higher reduced absorption effect is observed for the less energetic mg kα line compared with si kα (see also the counts at 15 kev for the 2-μm-thick fragment, cyan dash-dot line and star marker). the integrated intensity for fe kα (6.400 kev), fe kβ (7.059 kev), fe l (lα + lβ + lγ; 0.712 kev), mg kα (1.254 kev) and figure 4. integrated x-ray intensity of fe kα (a), fe kα (b), fe l (c), si kα (d) and o kα (e) emission lines versus electron beam energy, simulated for type-c glass fragments with a square section and thicknesses between 0.1 and 10 μm. legend in (b). acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 198 o kα (0.523 kev) in the type-b glass fragment is presented in figure 3. fe kα (figure 3a) and kβ (figure 3b) have a comparable non-linear trend, variable with the fragment thickness. the x-ray emission of 5and 10-μm-thick fragments (magenta dash-dot line and diamond marker, blue dashed line and square marker, respectively) are superimposed and comparable with that of a bulk material for each simulated energy of the electron beam. however, a strong non-linear reduction in the integrated intensity was observed as a function of the fragment size below 5 μm at 25 kev and 2 μm at 15 kev, whereas at 5 kev, these emission lines could not be excited. the lower energy emission lines of fe l, mg kα and o kα (figure 3c, d and e), particularly fe l and o kα, presented a marked increase in x-ray intensity before the reduction threshold due to a higher contribution of the reduced absorption effect with respect to that of reduced mass. a specific trend was observed for each x-ray line, with intensity reduction thresholds at 5 μm (25 kev), 2 μm (15 kev) and 0.5 μm (5 kev). only the fe l line revealed different thresholds: 2 μm (25 kev), 1 μm (15 kev) and 0.5 μm (5 kev). figure 4 shows, for the type-c glass fragment, the integrated intensity of fe kα (6.400 kev), fe kβ (7.059 kev), fe l (lα + lβ + lγ; 0.712 kev), si kα (1.740 kev) and o kα (0.523 kev), providing further details on the effects of different chemical compositions on the trend of the integrated x-ray intensities. the fe kα (figure 4a) and fe kβ (figure 4b) emission lines have trends close to those of the type-b glass fragment (figure 3a, b). the si kα x-ray emission (figure 4d) for the 5and 10-μm-thick fragments (magenta dash-dot line and diamond marker, blue dashed line and square marker, respectively) shows negligible deviations from that of a bulk material for each simulated energy of the electron beam, and no significant predominance of the reduced x-ray absorption effect was observed. in addition, for the less energetic x-ray lines, fe l (figure 4c) and o kα (figure 4e), a specific trend was observed with marked differences from that of the type-b glass (figure 3c, e), with no significant predominance of the reduced x-ray absorption effect. 3.2. case study 2: ultrathin metal layers an ultrathin gold alloy (au, ag and cu) was selected and placed adjacent to extended natron glass supports (with a mass density of 2.5 g cm-3) to simulate a composite reproducing the structure of a mosaic tesserae [29]. a natron-type glass is a sodalime-silica glass found in ancient mosaic tesserae and characterised by low magnesium, potassium and phosphorous content [9], [10]. two gold alloys were simulated varying the silver and copper weight %, as shown in table 2. two thickness values of the gold layer were simulated, corresponding to realistic thick (1 μm) and ultrathin (200 nm) gold layers found in ancient mosaics, as explained by conventi et al. and neri et al. [9], [10]. furthermore, to investigate the effects of the metal-layer thickness on the chemical quantification, the same simulation approach was applied to bulk standards of the alloys with ‘infinite’ sizes for electron scattering effects. reference calibration curves were obtained from the integrated intensities and compared with those of the elements of the metal layers. figure 5. images of the distribution of the detected x-rays (side view) for ag lα1 in gold leaf 1 μm (b, e) and 200 nm (c, f) thick, with the detector oriented in the same direction as the leaf extension (b, c), as shown in the schematic diagram (a), or at 90° (e, f), as shown in the schematic diagram (d), for a non-centred 25 kev electron beam. the images depict a 3d distribution projected onto a 2d plane. table 2. composition (weight %) and mass density of the gold alloys of the sub-μm-thick layers. au ag cu mass density in g/cm3 97.0 2.7 0.3 19.1 93.0 6.4 0.6 18.5 acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 199 the eds microanalysis of the gold alloy could include a not negligible unreal contribution from x-ray fluorescence, produced when a bremsstrahlung or characteristic primary x-ray photoionises core/shell electrons of elements present in the surrounding materials but not in the gold alloy. when the gold layer is ultrathin and elements, even in trace, are present in the adjacent material (in our simulation, the glass of the mosaic tesserae), x-ray fluorescence from the surrounding material should be considered for a correct quantification of the metal layer composition, otherwise incorrect results are obtained, with elements present in the glass being assigned to the gold alloy. in fact, it should be considered that secondary fluorescence can be generated in a material outside the electron scattering volume because the mean free path of energetic x-rays overrides electron spreading in most materials. in the present simulations, the secondary fluorescence contribution was considered. the results provided evidence that for a sem-eds microanalysis of a 1-μm-thick gold layer performed using operational settings such as a 25 kev beam positioned in the middle of the gold layer, when the sample is oriented so that the layer’s long axis is aligned with the detector, the resulting x-ray intensity value is very close to the reference calibration curves. conversely, the same operational conditions, when applied to the microanalysis of a 200-nm-thick gold layer, would have a strong negative effect on the accuracy of the quantitative measurement, with a calculated deviation (error) from the real composition of about 33% for silver and 53% for copper [29]. as a different case study, the effect of a non-ideally centred electron beam with respect to an ultrathin layer, a tilted layer or a layer that was not homogeneously prepared (figure 5) was also evaluated. the eds detector was oriented in axis with (figure 5a) or at 90° (figure 5d) to the gold layer, and the layer thickness varied between 1 μm and 200 nm. for the sake of this example, figure 5 demonstrates the distribution of the detected x-rays for ag lα1 in a gold leaf 1-μm thick (figure 5b, e) and 200 nm thick (figure 5c, f), with the detector oriented in the same direction as the leaf extension (figure 5b, c) or at 90° (figure 5e, f), for a noncentred 25 kev electron beam. in figure 5b, the considered operational settings cause a reduction in x-ray generation volume inside the layer with a consequent non-linear decrease in detected x-rays from the gold alloy and important contributions from the elements in the adjacent glass layer (cartellina). the reduction in gold-layer thickness to 200 nm further increases these effects, decreasing the detected ag x-rays with respect to a bulk reference sample and adding to the measured spectrum further spurious contributions from the glass layer present on the left (support glass). it should also be noted that the orientation of the gold leaf with respect to the detector azimuthal angle affects the distribution of the detected x-ray emissions. in contrast, as shown in figure 5e and figure 5f, a configuration with the detector at 90°, i.e. positioned in the direction of the cartellina glass, extends the distribution of detected x-rays inside the gold alloy in a non-symmetrical manner. indeed, figure 5e and figure 5f show an increase along the z-axis (depth) of the xray volume revealed by the detector, in both the 1-μm-thick and 200-nm-thick layers. this effect is due to the different detector position with respect to the gold alloy/glass edges, which causes a variation in the x-ray absorption path from the generation point to the detector. more x-rays will pass through the glass layer, a less dense material and with lighter elements than the gold alloy. it should be noted that the detected x-ray signal, even if the electron beam is focussed on the gold alloy, would present x-ray lines from the glass composition. in this regard, it should be considered that the interaction volume of electrons, and the associated x-ray generation, is highly dependent on material composition and electron-beam energy. the penetration of energetic electrons is influenced by elastic and inelastic scattering events inside the specific material that can generate a lateral extension of electron trajectories from hundreds of nm to several μm. to optimise the quantitative sem-eds microanalysis of an ultrathin, 200-nm-thick gold layer, as in the case of a gold-leaf mosaic tesserae, the monte carlo simulation results suggest a decrease in beam energy to 7.5 kev. under this condition, the xray signal generation is confined within the ultrathin gold fragment, which therefore behaves as a massive material. 4. conclusions in the quantitative sem-eds microanalysis of ultrathin fragments and objects of interest in cultural heritage and archaeology, the volume of electron scattering could be comparable to the size of such fragments. the present study showed that, in these cases, the interplay between the experimental settings, operational parameters and ultrathin fragment sizes can produce multiple and complex effects that strongly impact the intensity of the detected x-rays and, hence, the exact quantification. several potential error sources can affect the quantitative sem-eds x-ray microanalysis of ultrathin samples, such as glass fragments and metal layers, which may have negative consequences on indirect outcomes (e.g. artefact dating, historical records, geographical origin and geological setting). the simulation results demonstrated that quantification errors may be affected by sample size, chemical composition, the energy of the electron beam and several other experimental settings, with a specific trend for each analysed element. for this reason, for ultrathin fragments, monte carlo simulations should be adopted to identify potential error sources and devise the experimental settings necessary to improve and optimise 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https://doi.org/10.6028/nist.nsrds.64 https://www.govinfo.gov/content/pkg/govpub-c13-9342d46f6953c5f6a1f9a6ebc0bcc059/pdf/govpub-c13-9342d46f6953c5f6a1f9a6ebc0bcc059.pdf https://www.govinfo.gov/content/pkg/govpub-c13-9342d46f6953c5f6a1f9a6ebc0bcc059/pdf/govpub-c13-9342d46f6953c5f6a1f9a6ebc0bcc059.pdf https://www.govinfo.gov/content/pkg/govpub-c13-9342d46f6953c5f6a1f9a6ebc0bcc059/pdf/govpub-c13-9342d46f6953c5f6a1f9a6ebc0bcc059.pdf https://doi.org/10.1002/sca.4950110404 https://doi.org/10.1103/physreva.77.042701 http://physics.nist.gov/ffast https://doi.org/10.2172/10121422 https://doi.org/10.1017/s1431927617000307 https://doi.org/10.6028/jres.107.047 https://doi.org/10.1016/j.measurement.2018.07.025 recent advances in fabry-perot-based refractometry utilizing gas modulation for assessment of pressure acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 299 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 299 304 recent advances in fabry-perot-based refractometry utilizing gas modulation for assessment of pressure m. zelan1, i. silander2, c. forssén1,2, j. zakrisson2, o. axner2 1 measurement technology, rise research institutes of sweden, se-501 15 borås, sweden, martin.zelan@rise.se 2 department of physics, umeå university, se-901 87 umeå, sweden, isak.silaner@umu.se, clayton.forssen@umu.se, johan.zakrisson@umu.se, ove.axner@umu.se abstract: gas modulation refractometry (gamor) is a methodology that can mitigate the influence of fluctuations and drifts in fabry-pérot cavity–based refractometry. this paper presents a thorough description of its principles, what it enables, and its applicability. an overview of the latest results is presented, including the realization of a system based upon a cavity spacer made of invar that allows for detection of n2 with sub-ppm precision, and a characterization procedure that allows for assessment of n2 with an accuracy at low-ppm levels. keywords: refractometry, gamor, pressure. 1. introduction fabry-pérot cavity (fpc) based refractometry is a sensitive technique for assessment of gas refractivity, density, pressure, and flow using laser technology. by measuring the refractivity, n-1, where n is the index of refraction, and the temperature of a gas, the pressure can be calculated by the use of the lorentzlorenz equation and an equation of state [1]. recent works have indicated that the technique also has the potential to replace current pressure standards, in particular in the 1 pa to 100 kpa range [2]. such a realization of the pascal would not depend on any mechanical actuator but instead directly measure the gas density, potentially decreasing uncertainties and shortening calibration chains. with the revision of the si-system in may 2019, in which the boltzmann constant was given a fixed value [3], the performance would, in principle, be limited only by the accuracies of quantum calculations of gas parameters and assessments of the deformation of the cavity material and gas temperature. the technique is built on the fact that refractive index constitutes the ratio of the optical length under two conditions; without and with gas in the measurement cavity, here referred to as 0l and l 0 ( )nl= , respectively. in practice, the refractive index is assessed by measuring the change in frequency of a laser, locked to a cavity mode, when gas with a known refractivity is let into the cavity. besides offering the possibility of outperforming conventional technologies, fpc-based refractometry has several other advantages. due to the high accuracy of frequency measurements, and the fact that no moving parts are needed, the assessments can be done swiftly without any manual oversight. moreover, since the measurement principle is based on assessing a change in frequency, there is no fundamental limit of the dynamical working range of the technique. in practice, its lower limit is given by the resolution of the frequency measurement, while the upper limit is set by the mechanical properties of the cavity and the vacuum system. the fact that fpcbased refractometry can work with high precision over such a large pressure range, which currently requires several different techniques, makes it very versatile and unique. the conventional means of assessing refractivity by the use of fpc-based refractometry is to assess 0l and l in two separate assessments. under ideal conditions, under which no physical parameter of the system changes with time, it is possible to envision a procedure in which first, once and for all, 0l is assessed with no gas in the cavity, denoted a reference measurement. based on this, the refractivity (and thereby the pressure) can then be assessed by measurements of the optical length of the cavity under other conditions. however, in practice, this is not trivial since the cavity spacer material is subjected to deformation due to gas pressure, thermal expansion, aging, relaxations, and diffusion of gas into the material that changes its length in unpredicted ways. for example, a change in length of a 30 cm long cavity of solely 1 pm (a fraction of the “size” of an atom) corresponds to an error in the assessment of pressure (for n2) of 1 mpa. this severely restricts the performance and applicability of fpc-based refractometry to such an extent that its performance is in practice limited by the stability of the cavity length. http://www.imeko.org/ mailto:martin.zelan@rise.se mailto:isak.silaner@umu.se mailto:clayton.forssen@umu.se mailto:johan.zakrisson@umu.se mailto:ove.axner@umu.se acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 300 the conventional way of minimizing these limitations is to construct the fpc of low thermal expansion glass, e.g. ultra-low expansion glass (ule®) or zerodur®, place the refractometer in a highly temperature stabilized environment (combined gas and vacuum chamber), and let the system relax and equilibrate for long time periods after each gas filling/emptying. however, this is a cumbersome effort that increases the complexity of the systems as well as limits the use of the technology outside well-controlled laboratories. to reduce the constraints of such requirements, we decided to develop a methodology that could reduce the effect of fluctuations and drifts of the length of the cavity on the assessments. the methodology, which is built on a regularly alteration of the amount of gas in an fpc, was originally referred to as drift free (df) or fast switching (fs) dual fpc (dfpc) based refractometry, but is nowadays referred to as gas modulation refractometry (gamor) [4-9]. this paper provides a simple and comprehensible description of this methodology. it also gives an overview of the recent technological achievements and presents results that have followed from the latest development of the methodology. 2. the principles of gas modulation refractometry gamor is based on the same fundamental principle as ordinary (unmodulated) refractometry; it measures the change in refractive index between two situations; a reference measurement without gas, and a measurement with gas, in the measurement cavity. figure 1: illustration of the gamor-principle. upper panel: the pressure in the measurement cavity during a cycle. lower panel: illustration of the measurement of the corresponding shift in frequency (dashed-blue curve). by taking two reference values in rapid succession (red x) and making a linear interpolation between them (red line), 1 in particular fluctuations that appear on time scales of the measurement cycle or longer. linear drifts in the system can be eliminated (providing the solid curve). reproduced with permission from ref. [4]. however, in gamor, the amount of gas in the measurement cavity is modulated to allow for a large number of repeated measurements. this creates two cornerstones that gamor is built upon: (i) the first one is based on the principle that the influence of drifts and fluctuations1[9], in particular those thermally induced and those from aging, can be significantly (orders of magnitude) reduced by the performance of the frequent reference assessments in an empty cavity (marked with red crosses in fig. 1); (ii) the second one encompasses an interpolation procedure in which the response of the empty cavity system can be estimated at all times of the filled cavity assessment (marked with the red line in fig. 1). by this, the methodology can eliminate additional fluctuations as well as all remaining influences of the linear parts of the drifts. this allows the technique to benefit, also at long time scales, from the high precision fpc-based refractometry has at short time scales. 3. means of evaluation of refractivity, molar density, and pressure gamor has so far been based on dfpc systems and is hence governed by the same basic equations as unmodulated dfpc-refractometry. it has been found convenient to express the refractivity of the gas under scrutiny as a function of the shifts of the beat frequency between the two cavities, f , and the mode numbers of the modes addressed in the measurement cavity, 1q , when the latter is filled with (or emptied of) gas, as 11 1 f q n f   +  − = − + , (1) where f is the shift of the relative beat frequency given by 01/f  , where 01 is the empty cavity frequency of the measurement laser, and 1q is a shorthand notation for 1 01/q q , where 01q is the number of the mode addressed in the empty measurement cavity.  is a normalized relative cavity deformation, defined as 0( / ) / ( 1)l l n − , where l and 0l are the pressure induced length deformation and the empty cavity length, respectively [4, 5]. for pressure up to atmospheric pressure, the gas density  can be calculated from the refractivity by use of the extended lorentz-lorenz equation, 1 2 ( 1)[1 ( 1)] 2 n r n b n a  − = − + − , (2) http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 301 where ra and 1nb − are the molar dynamic polarizability and a series expansion coefficient, respectively. the latter is given by 2 (1 4 / ) / 6 r r b a− + , where, in turn, rb is the second refractivity virial coefficient in the lorentz-lorenz equation [5, 10]. up to atmospheric pressure, the pressure can be obtained from the density and temperature as [1 ( ) ]p rt b t   = + , (3) where r is the ideal gas constant, t is the temperature of the gas, and ( )b t is the second density virial coefficient [5, 10]. if higher pressures are to be assessed, additional higher order terms need to be added in the eqs. (2) and (3). it is of importance to note that eq. (1) shows that the refractivity (and thereby the molar density and pressure) can be assessed without any monitoring or assessment of 0l . there is neither any need to explicitly assess the free-spectral range (fsr). the reason no knowledge of 0l is needed is due to the use of frequent reference assessments, which repeatedly monitor the combined status of the two empty cavities, as well as the fact that  can be assessed experimentally [11]. the reason no assessment of the fsr is needed is that 1q is not expressed in terms of the fsr, but instead in the mode number of the cavity mode addressed, 01q . this is an integer that can be assessed uniquely (i.e. with no uncertainty) by ensuring that the assessed refractivity is a continuous function when the measurement laser is making a (controlled) mode hop [11]. 4. typical set-ups over the years, a variety of gamor instrumentation and procedures have been developed [4-7, 11]. while the first versions utilized a simplistic design and were constructed to carry out proof-of-principle demonstrations [4], the later realizations have been gradually upgraded to improve on their performance, firstly precision [5, 6] and lately accuracy [7, 11]. to visualize the developments during the last few years, fig. 2 displays a collection of pictures of some gamor-based refractometers. common for all systems is that they have been based on dfpc systems with fsrs of around 1 ghz and finesse values of 104. each cavity is probed by an erdoped fiber laser emitting light within the c34 communication channel, i.e., around 1.55 µm, that is coupled into an acousto-optic modulator (aom) for fast locking feedback and an electro-optic modulator (eom) modulated at 12.5 mhz for pound-drever-hall locking. for the assessment of pressure, the measurement of the gas temperature is crucial. in most gamor instrumentation, the gas has been monitored by pt-100 sensors placed in holes drilled into the cavity spacer. however, from our experience, even when the sensors have been properly calibrated, they constitute a limiting figure 2: pictures of the gamor refractometer-system during various stages of the development. the top pictures show one of the earliest versions using a non-temperature stabilizes zerodur-cavity and plenty of free-space optics. the middle images show a later version of the same system, now placed in a box for temperature stabilization and using less free-space optics and more fiber-optics. the bottom pictures show the most recent version of the gamor-system consisting of an invar-cavity encapsulated in an aluminium oven. factor in terms of accuracy. therefore, as is described in some detail in an accompanying presentation [7], we have recently constructed a ga fixed-point cell that, by the use of a thermocouple working close to zero temperature difference, allows us to assess the temperature of the fpc, and hence the gas inside it, at low ppm-levels. 5. results and progress 5.1. experimental demonstration of the ability of gamor to reduce drifts an example of the powerfulness of the gamor methodology in reducing the influence of drifts is shown in fig. 3 [4]. the upper blue curve in panel (a) http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 302 figure 3: illustration of the gamor-principle. upper panel: a 24 h long series of measurement of an empty measurement cavity evaluated by two different means: the lowermost (red) curve — without gas modulation, and the almost fully horizontal (black) curve — by use of the gamor methodology (both left axis). the uppermost curve (blue, right axis) represents the temperature. lower panel: a zoom in of the first hour section of the data taken with the gamor methodology. note the differences in scales. reproduced with permission from [4]. (right axis) shows the temperature drift over 24 hours while the lower red curve (left axis) shows the corresponding drift of a conventional non-temperaturestabilized dfpc refractometry signal (expressed in units of pressure) from an empty cavity. the horizontal black line is not, as it might appear, the time axis, but is, in fact, the actual corresponding gamor signal. panel (b) shows an enlargement of a part of this data. note the more than three orders of magnitude differences in scales. this illustrates the ability of the gamor methodology to significantly reduce (in practice eliminate) the drifts that limit conventional dfpc-based refractometry [4]. 5.2. investigation of the capability of gamor to reduce fluctuations in a recent work [9], we have provided a thorough explication of the capability of gamor to significantly reduce the effect of fluctuations. it explicates, in terms of the length of the modulation cycle, to which extent the modulation procedure can reduce the influence of fluctuations. the upper panel in fig. 4 illustrates the maximum fraction of specific fourier components of fluctuations an unmodulated refractometry system (in which the time between the filledand emptycavity measurements, umt , is 10 5 s) picks-up as a function of its fourier frequency, ff . the lower panel displays the same entity when the detection system utilizes the first cornerstone of the gamor methodology, (i), i.e. a frequent reference assessment (using a modulation cycle of 100 s). a comparison of the two shows that the repeated reference assessment is capable of reducing the influence of fluctuations figure 4: illustration of the ability of the gamorprinciple to reduce fluctuations. the maximum fraction of specific components of fluctuations the system picks-up as a function of its fourier frequency in terms of their normalized rms values for two different lengths of the gas filling-and-emptying cycle. panel (a) represents the case with unmodulated detection with a gas filling-andemptying time, um t , of 10 5 s while panel (b) illustrates the situation with gas modulated detection, utilizing a gas modulation period, mod t , of 10 2 s. in both cases, an averaging time, avgt , of 10 s has been assumed. with fourier frequencies below mod1 / ( )t significantly [those below 1 / ( )umt three orders of magnitude]. as is shown in an upcoming work, the gamor methodology will also significantly reduce the influence of drifts. the first cornerstone of the methodology (i) will convert linear drift to small amounts of inaccuracy. the interpolation part of the methodology, cornerstone (ii), will then eliminate this methodology-induced inaccuracy. it will additionally reduce the low frequency components; in the case considered, the influence of fluctuations with low fourier frequencies [below 1 / ( )umt ] will be reduced up to six orders of magnitude. this makes gamor virtually insensitive to both slow fluctuations and drifts of the lengths of the cavities. 5.3. use of alternative cavity spacer materials since gamor has the ability to reduce the influence of fluctuations and drift, it allows for the use of non-conventional cavity spacer material. as is described in [6, 7], we have recently developed a refractometer based on a cavity spacer made of invar, which is a material that has a number of advantages over ule® and zerodur®, mainly: (1) a higher ratio of thermal conductivity to volumetric heat capacity; (2) a higher volumetric heat capacity; (3) a higher young's modulus; (4) presumably a lower degree of he diffusivity; and (5) it can be machined in a metal workshop, whereby more complicated geometries can be created more swiftly at a lower cost. using a dead weight piston balance to provide a reference pressure, we have recently demonstrated http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 303 that such a system can outperform a similar one made of zerodur®. as is shown in fig. 5, the system could, for an empty cavity, demonstrate a white noise response up to 104 s with a minimum deviation (defined as the allan deviation) of 0.03 mpa. when set to assess pressure at 4303 pa, the system could provide (for measurement times of 103 s, representing 10 measurement cycles) a minimum allan deviation of 0.34 mpa, which corresponds to a relative deviation of 0.08 ppm. these results, which are a few times better than those obtained using a zerodurbased cavity, can, to a large extent, be attributed to the advantageous thermal properties of invar. figure 5: allan deviations of pressure assessments made by the previous zerodur-based system (red upper markers) and the novel invar-based system (blue), both at 4303 pa. the yellow and purple (lower) markers illustrate empty cavity measurements using the same two systems, respectively. the figure is reproduced and modified with permission from [6]. 5.4. procedure for assessing the pressure induced cavity deformation to assess pressure with adequate accuracy, a fpc-based refractometer system has to be characterized with respect to its pressure induced cavity deformation. to address this, we have, inspired by the work of egan et al [12], recently developed a procedure for robust assessment of cavity deformation in fp-based refractometers that provides extraordinarily small amounts of uncertainty [11]. the procedure is not only insensitive to any systematic error in the set pressure, but also not influenced by a systematically incorrect assessment of the gas temperature. furthermore, it is also insensitive to leakages of gas with the same refractivity as that addressed and outgassing processes. the procedure is based on assessing the difference between the pressure assessed by the uncharacterized refractometer and a supplied reference pressure at a series of pressures for two gases with dissimilar refractivity (he and n2). by fitting linear curves to these responses, their slopes are used as a basis for the assessment of the pressure-induced deformation of the cavity. the procedure is also capable of providing an accurate assessment of the sum of systematic errors of the temperature assessment, the molar polarizabilities used, and the set pressure. it is predicted that when a high-precision (subppm) refractometer is characterized, the uncertainty in the assessed deformation will contribute to the uncertainty in the assessment of pressure of n2 with solely a fraction of its uncertainty of the molar polarizability, presently ca. 1 ppm. this is a keyfinding for the further development of refractometry in general and gamor in particular, since it implies that cavity deformation no longer will be a limiting factor in fp based assessments of pressure of n2. 5.5. assessment of gas temperature as was alluded to above, to assess the gas temperature accurately, we have recently designed, constructed, and installed a gallium fixed-point cell based temperature reference system. the system can provide a stable temperature up to 60 hours. by operating the dfpc-system as close as possible to the temperature of the melting-point of ga, and measure the remaining temperature difference between the dfpc-cavity and the ga system with a type-t thermocouple, we can assess the temperature of the cavity with an accuracy below 5 ppm. the design and preliminary results are presented in detail in an accompanying work [7]. 5.6. dynamic range since narrow linewidth lasers have a restricted tuning range, we have, in the realization of our gamor systems, implemented a mode jumping procedure by which the laser automatically jumps to a neighbouring mode if the shift of the mode addressed is larger than the tunability of the laser (which typically is a few fsrs of the cavity). this implies that there is, in practice, no upper limit for its dynamic range! since the resolution is in the tens of µpa range [6, 7], given by stochastic noise in the system, and assessments have so far been demonstrated up to the tens of kpa range [5], a gamor based refractometry system is expected to have an extraordinary dynamic range (possibly up to 9 orders of magnitude). 5.7. a transportable refractometer gamor has also opened up for a simplified realization of the experimental set-up and measurement environment. as is reported in an accompanying work [8], the reduced constrains have allowed for the construction of a transportable refractometer that is to be used for various future scientific endeavours, including a european circlecomparison. 6. summary and conclusion this paper presents an overview of the current status of fabry-perot-cavity (fpc) based gas modulation refractometry (gamor) for pressure assessment. the basis of the gamor technique, including its most prominent advantages of reducing http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 304 the influence of fluctuations and drifts of the length of the cavity spacer, have been explicitly explained. the paper summarizes our most recent advances. one such is the use of cavity spacer material with advantageous properties compare to conventional spacer materials. in particular, we present results from a system with a fpc made of invar. when assessing pressure at 4303 pa, the system could provide a minimum allan deviation of 0.34 mpa, which corresponds to a relative deviation of 0.08 ppm. these results, which are a few times better than those previously obtained using a zerodur-based cavity, can, to a large extent, be attributed to the advantageous thermal properties of invar. furthermore, a procedure for assessing the pressure induced cavity deformation with such an accuracy that it does not contribute to the accuracy of the assessment of pressure with more than 1 ppm [11], as well as a temperature measurement system based on the melting point of ga [7], providing accuracy of below 5 ppm, are shortly summarized. this implies that neither cavity deformation, nor gas temperature, are the main limiting factors in fp based assessments of pressure of n2. instead, the limiting factor is the knowledge of its molar refractivity, which presently is 7 ppm [13]. the paper also explicitly emphasizes the potentially extraordinary large dynamical range of the technology that is made possible with automatic mode-jumping. the paper finally notes that the gamor methodology allows for the realization of transportable instrumentation. a first such instrument has been constructed [8] and an improved version is under construction. this project (quantumpascal, 18sib04) has received funding from the empir programme cofinanced by the participating states and from the european union's horizon 2020 research and innovation programme. the authors acknowledge support from the swedish research council (vr) (project no. 621-2015-04374); the umeå university industrial doctoral school (ids); the vinnova metrology programme (project nos. 2018-04570 and 2019-05029); and the kempe foundations (project no. 1823, u12). 7. references [1] p. f. egan, j. a. stone, j. h. hendricks, j. e. ricker, g. e. scace, g. f. strouse, performance of a dual fabry-perot cavity refractometer, opt. lett., vol. 40, pp. 3945-3948, 2015. [2] k. jousten, j. hendricks, d. barker, k. douglas, s. eckel, p. egan, j. fedchak, j. flugge, c. gaiser, d. olson, j. ricker, t. rubin, w. sabuga, j. scherschligt, r. schodel, u. sterr, j. stone, g. strouse, perspectives for a new realization of the pascal by optical methods, metrologia, vol. 54, pp. s146-s161, 2017. [3] m. stock, r. davis, e. de mirandés, m. j. t. milton, the revision of the si—the result of three decades of progress in metrology, metrologia, vol. 56, p. 049502, 2019. [4] i. silander, t. hausmaninger, m. zelan, o. axner, gas modulation refractometry for high-precision assessment of pressure under non-temperaturestabilized conditions, j. vac. sci. technol. a, vol. 36, p. 03e105, 2018. [5] i. silander, t. hausmaninger, c. forssén, m. zelan, o. axner, gas equilibration gas modulation refractometry for assessment of pressure with subppm precision, j. vac. sci. technol. b, vol. 37, p. 042901, 2019. [6] i. silander, c. forssén. j. zakrisson, m. zelan, o. axner, invar-based refractometer for pressure assessments, opt. lett., vol. 45, pp. 2652-2656, 2020. [7] i. silander, c. forssén, j. zakrisson, m. zelan, o. axner, an invar-based fabry-perot cavity refractometer with a gallium fixed-point cell for assessment of pressure, acta imeko 9 (2020) 5. [8] c. forssén, i. silander, d. szabo, g. jönsson, m. bjerling, t. hausmaninger, o. axner, m. zelan transportable refractometer for assessment of pressure in the kpa range with ppm level precision, acta imeko, 2020. [9] o. axner, i. silander, c. forssén, j. zakrisson, m. zelan, the ability of gas modulation to reduce the pick-up of fluctuations in refractometry, josa b, vol. 37, pp. 1956-1965, 2020. [10] a. d. buckingham, c. graham, density dependence of refractivity of gases, p. roy. soc. lond. a mat., vol. 337, pp. 275-291, 1974. [11] j. zakrisson, i. silander, c. forssén, m. zelan, o. axner, procedure for robust characterization of fabry perot based refractometer, j. vac. sci. technol. b, vol 38, p. 054202, 2020 [12] p. f. egan, j. a. stone, j. k. scherschligt, and a. h. harvey, measured relationship between thermodynamic pressure and refractivity for six candidate gases in laser barometry, j. vac. sci. technol. a37, 031603 (2019) [13] p. f. egan, j. a. stone, j. e. ricker, j. h. hendricks, comparison measurements of low-pressure between a laser refractometer and ultrasonic manometer, rev. sci. instrum. vol. 87, p. 053113, 2016. http://www.imeko.org/ provenance study of the limestone used in the construction and restoration of the batalha monastery (portugal) acta imeko issn: 2221-870x march 2021, volume 10, number 1, 122 128 acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 122 provenance study of the limestone used in the construction and restoration of the batalha monastery (portugal) yufan ding1, jose mirao1,2, pedro redol3, luis dias1, patricia moita1,2, emma angelini4, sabrina grassini4, nicola schiavon1 1 hercules laboratory, university of évora, largo marquês de marialva 8, 7000 évora, portugal 2 department of geosciences, university of évora, rua romão ramalho, 7000 évora, portugal 3 direcao-geral do patrimonio cultural, mosteiro da batalha, batalha, portugal 4 politecnico di torino, corso duca degli abruzzi 24, 10129 torino, to, italy section: research paper keywords: limestone; batalha monastery; archaeometry; provenance citation: yufan ding, jose mirao, pedro redol, luis dias, patricia moita, emma angelini, sabrina grassini, nicola schiavon, provenance study of the limestone used in the construction and restoration of the batalha monastery (portugal), acta imeko, vol. 10, no. 1, article 16, march 2021, identifier: imeko-acta10 (2021)-01-16 editor: carlo carobbi, university of florence, italy received may 6, 2020; in final form september 8, 2020; published march 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was supported by european union’s horizon 2020 research and innovation programme under the marie skłodowska-curie grant agreement no. 766311. corresponding author: yufan ding, e-mail: yding@uevora.pt 1. introduction the monastery of batalha (figure 1), known in portugal as ‘mosteiro de santa maria da vitoria’, was constructed to commemorate victory over castile, the main kingdom of present-day spain. it is considered an architectural masterpiece, demonstrating a perfect mixture of the gothic and manueline styles. initial construction began in 1386 and took two centuries to complete. in 1840, after the monastery had been nearly abandoned due to the suppression of religious orders in 1834, a first restoration programme began [1]. the batalha monastery has been added to the list of world heritage sites by unesco. over a century has passed since the last large-scale restoration was completed. the batalha monastery shows a high degree of stone decay, mainly due to bio-deterioration processes, which are figure 1. monastery of batalha abstract to assess the provenance of the limestones used in the construction and restoration of the batalha monastery in central portugal, stone samples collected from the monument and from five limestone quarries in the region surrounding the building were investigated by energy-dispersive x-ray fluorescence spectroscopy (ed-xrf), powder x-ray diffractometry (pxrd) and thermogravimetric analysis (tga). ca-sr binary diagrams from the ed-xrf result indicated the source of the samples collected from different parts of the monastery. thinsection observation supplemented the petrographic evidence for this identification. pxrd and tga were also used to acquire information on the mineral and chemical composition of the stones. preliminary results suggest that the monastery baluster was made of stone from the valinho do rei or reguengo do fetal quarries, whereas part of the church railing, the north-aisle eaves arch and royal cloister were made with stone from the pidiogo or cabeço do roxo quarries. mailto:yding@uevora.pt acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 123 known to play an ever-increasing role in stone decay both in urban and rural environments [2]. in order to plan an appropriate conservation strategy for the protection of the monastery, it is necessary to obtain petro-geochemical data from the stone used in its construction and successive restoration interventions [3]. there are a number of techniques that can be used for the provenance study of limestones. these include physical analytical methods, such as the uniaxial compressive strength and ultrasonic pulse velocity tests [4], optical microscopy (om) and scanning electron microscopy (sem) for petrographic and textural analysis [5], loss-on-ignition or thermogravimetric analysis (tga) to measure the weight percent of caco3, energy dispersive x-ray fluorescence spectroscopy (ed-xrf) to determine the chemistry, acid insoluble residue to determine the noncarbonate fraction present in the sample and x-ray diffractometry (xrd) to determine the mineralogical composition [6]. in addition, isotopic analyses and electron paramagnetic resonance can also be used to assess geochemical similarities or differences [7]. according to aires-barros [8], the two original limestone quarries used for the construction of batalha monastery in the 15th century were the pidiogo and valinho do rei quarries, located 8 km and 7.5 km, respectively, to the east of the monastery [7]. soares et al. indicated that four quarries served as the main source of building stone for the restoration during the 19th century: a) the reguengo do fetal quarry provided the stone from 1840 to the mid-1880s; b) from 1854 until the end of the restoration activities, the carvalhos quarry was mainly used; c) in the last decade of the 18th century, the cabeço do roxo and the outeiro de sebastião quarries were also supplying materials [10]. despite the somewhat scarce historical documentary evidence, no detailed petrographic/geochemical study has ever been carried out to test the accuracy of the assignment of these historical quarries to the supply of the carbonate stones for the building and the restoration of the monastery. in order to contribute to this open discussion, a multi-analytical approach has been adopted for the first time in this study, combining a) a sample collection of selected stones from the monastery, b) a field survey to identify the location of and collect samples from the ancient quarries mentioned in the literature and c) petrographic and geochemical analyses using tga, xrd, om and sem + ed-xrf. 2. methods and materials 2.1. field investigation and sample collection with special permission from the direção-geral do patrimonio cultural and the mosteiro da batalha authorities, 12 pieces of detached stone fragments were collected for destructive or non-destructive characterisation, depending on the agreement with the monastery. the collected stone fragments came from various parts of the monastery, as labelled in figure 2. the location of the pidiogo and valinho do rei quarries were indicated in a document released by batalha municipality [10]. the approximate location of reguengo do fetal, cabeço do roxo and outeiro de sebastião was recorded in the book ‘o restauro do mosteiro da batalha’ [9], although their exact location had been lost and only rediscovered by the authors of the current study. however, despite considerable efforts, it was not possible to find one restoration quarry, carvalhos, because it had been renamed during the last century. field trips were made to five quarries (figure 3), and the latitude and longitude were recorded with the help of google earth. dozens of samples were collected and marked with their corresponding origin, of which 18 samples were from valinho do rei and pidiogo, believed to be the original quarries used in the construction of the monastery in the 15th and 16th centuries, and 16 samples were from reguengo do fetal, cabeço do roxo and outeiro de sebastião, used as the restoration quarries in the 19th century. 2.2. thin-section petrography thin sections of the stone samples were obtained using the following procedure: stones were cut into cuboids with a crosssectional area of 2 cm × 3 cm, and the cross-sectional surface was polished with 220# sandpaper and 400# and 1000# sic powder, with water added sequentially. the polished stone figure 2. locations of samples collected from batalha monastery: a. west gallery of the royal cloister, outside the 3rd window (ground-floor level); b. royal cloister roof top (roof level); c. church north-aisle eaves arch (roof level); d. church roof railing (roof level); e. church south carved baluster (roof level). figure 3. photo records of field investigation to quarries: a. pidiogo (39°39'15.7"n 8°44'27.9"w), b. valinho do rei (39°39'32.5"n 8°44'58.1"w), c. reguengo do fetal (39°38'43.64"n 8°45'16.19"w), d. cabeço do roxo (39°35'39.84"n 8°51'27.19"w), e. outeiro de sebastião (39°35'38.89"n 8°51'27.49"w), f. a portion of the stone samples taken from quarries. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 124 surface was glued to the glass slide with epoxy resin and epoxy hardener mixed at the weight ratio of 2:0.9. after the epoxy glue had set, the stones were cut and ground to a thickness of 0.1 ~ 0.2 mm, and polished using 400# and 1000# sic powder with water until the thickness of the stone section reached 0.025 mm. thin sections were observed by om using a leica dm2500p. 2.3. powder x-ray diffraction (pxrd) the characterisation was carried out using a bruker d8 discover x-ray diffractometer, with a cukα source and operating at 40 kv and 40 ma. scans were run from 3 ° to 75 ° 2θ, with a 0.05 o 2θ step and a measuring time of 1 s/step per point. the diffrac.eva software package (bruker/axs gmbh, germany) and the pdf-2 database files (icdd, denver, usa) software with the pdf-2 mineralogical database were utilised to interpret the xrd patterns. the semiquantification was carried out using the reference intensity ratio method of hubbard et al. [11]. the specimens used to conduct the characterisation were stone powders hand milled in an agate mortar. 2.4. thermogravimetric analysis (tga) the tga analysis was carried out using a tg-dta netzsch sta 449f3 jupiter. the temperature of the sample was increased at a heating rate of 10 °c /min from 40 °c to 1,000 °c, under an n2 protective atmosphere, while the mass of the powdered sample was monitored against time and temperature. tga applied to stone allows the quantification of minerals that are decomposed by temperature, such as minerals with structural water content or carbonates. thermal decomposition with gaseous evolution can be detected by measuring the weight change, thus, by using stoichiometry, the content of the corresponding composition can be determined. 2.5. x-ray fluorescence spectroscopy (xrf) xrf analyses were performed with a benchtop edxrf bruker s2 puma, using a methodology similar to the one adopted by georgiou et al. [12]. quantifications were obtained using a regression method with 19 standard reference materials [12]. spectra elements 2.0 software was utilised for acquisition and data processing, reporting the final oxide/element (na2o, mgo, al2o3, sio2, p2o5, so3, k2o, cao, tio2, mno, feo) concentration and the instrumental statistical error. two sample preparation methods were used: (1) 1.2 g of sample powder were fused with 12 g of flux (li-tetraborate) on a claisse leneo to form fused beads; (2) 10 g of sample powder were compressed with 1 g of wax (n,n'-dioctadecanoylethylenediamine) on a specac manual hydraulic press to form pellets. 3. results and discussion 3.1. thin-section petrography the micrographs of the thin sections are presented in figure 4, in which oolites, calcite crystals and fossils may be clearly distinguished. based on the classification scheme introduced by flügel for identifying microfossils [14] and classifying carbonate grains in microfacies studies [15], [16], and referring to previous biostratigraphy of carbonate succession [17], [18], table 1 summarises the petrography features of the samples. the limestone samples from the reguengo do fetal and cabeço do roxo quarries and the limestone samples from the batalha monastery roof top, eaves arch and church baluster show oolites of a relatively uniform size and with compacted dispersion. however, the valinho do rei samples show clear differences in carbonate clasts; there are peloids with small diameters (0.1 mm on average) and ooids that are ten times larger a b c d e f g h i j k figure 4. om photos of stone samples from a. pidogo quarry; b. valinho do rei quarry; c. reguengo do fetal quarry; d. cabeço do roxo quarry; e. outreiro do sebastião quarry; f. batalha monastery, royal cloister; g. batalha monastery roof top; h. batalha monastery eaves arch; i. batalha monastery church railing 1; j. batalha monastery church railing 2; k. batalha monastery church baluster. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 125 (1 mm). the outreiro de sebastião quarry, the batalha monastery royal cloister and church railing 1 contain abundant marine microfossils. large-sized sparite crystals (up to 0.5 ~ 1 mm) can be identified as cement between oolites, alongside or inside the fossils, some of which was recrystallised from mud micrites (figure 4c). according to the dunham classification, all these stones can be classified as grain supported [19]. the presence of sparry calcitic cements implies the deposition occurred in a high energy environment. foraminifera in the samples belonged mostly to four genera: miliolid, kurnubia, trocolina and haddonia. these foraminifera are found in inner shallow-water platforms from the late jurassic and early and middle eocene, indicating the sedimentary environment and the age of these limestones [14]. this deduction matches that of the ‘carta geológica de portugal’ [20], with the pidiogo, valinho do rei and reguengo do fetal quarries showing limestone of bathonian age and cabeço do roxo and outreiro de sebastião of callovian age, both of which are in the middle– late jurassic period. the features in table 2 and described in the above paragraph were summarised from the thin-section photos, which can be found as supplementary data to this article. 3.2. x-ray diffraction figure 5 shows the xrd scan of all the stone powders from the quarry and monastery samples. the results indicate that all the samples have the same mineral composition: calcite (caco3), magnesium calcite ((mg0.03ca0.97)co3) and quartz (sio2). semiquantitative analysis provided the content of each mineral; the content of calcite in these limestones is over 95 % (caco3 60 ~ 70 % and mg0.03ca0.97co3 25 ~ 35 %), while the content of quartz is around or lower than 1 %. for the small peaks, the minerals indicated are whewellite (cac2o4•h2o), sodium phosphate (napo3), nitratine (nano3) and pyrophosphite (k2cap2o7). however, due to the weak intensity of these peaks (less than 0.5 %) and the possible overlapping of the major mineral peaks, this identification is inconclusive. 3.3. thermogravimetric analysis figure 6 shows the weight loss of one sample according to temperature increase. a mass loss in the temperature range of 600 °c–800 °c was observed, indicating the decomposition reaction of calcite [21]: caco3 → cao + co2. the differential table 1. petrographic features of limestone thin sections. sample name oolite morphology and dimension calcite crystalline fossils pidiogo ooids 0.2 ~ 0.5 mm peloids ~ 0.1 mm micrites and sparites of 0.01 ~ 0.1 mm between oolites, pseudo spars can reach 0.5 ~ 0.8 mm. gymnocodiacean algae, trilobite, foraminifera, gastropod, ostracod, intraclast valinho do rei ooids 0.4 ~ 1 mm peloids 0.05 ~ 0.1 mm mostly small sparites and micrites between peloids, spars of 0.1 ~ 0.2 mm between large ooids and bioclasts. foraminifera (trocholina, kurnubia, miliolina), intraclast, brachiopod, styliolinid, bivavle reguengo do fetal ooids 0.2 ~ 0.5 mm intraclasts 0.8 ~ 1 mm mostly sparites < 0.05 mm and micrites, spars of 0.2 ~ 0.5 mm are commonly seen alongside and inside fossils. ostracod, foraminifera (miliolid, alveospta, haddonia), trilobite, echinoderms, brachiopod cabeço do roxo ooids 0.2 ~ 0.4 mm mostly spars < 0.06 mm and micrites, very few spars of 0.2 ~ 0.3 mm. wood, gymnocodiacean algae, foraminifera outeiro de sebastião ooids 0.2 ~ 0.5 mm some micrites, rich sparites of 0.01 ~ 0.1 mm, spars of 0.2 ~ 0.4 mm are commonly seen between ooids and fossils. pseudodonezella, foraminifera (kurnubia), trilobite, echinoderm batalha monastery, royal cloister ooids 0.2 ~ 0.5 mm intraclasts > 1 mm small sparites inside fossils, big spars > 0.5 mm are commonly seen outside fossils and between ooids. gastropod, gymnocodiacean algae, bivalve, foraminifera (miliolid), intraclast batalha monastery, roof top ooids 0.2 ~ 0.5 mm mostly small sparites and micrites between ooids and inside fossils. foraminifera batalha monastery, eaves arch ooids 0.2 ~ 0.5 mm intraclasts > 1.2 mm mostly small sparites and micrites between ooids, very few spars of ~ 0.2 mm. foraminifera (trocholina alpina, miliolina), echinoderms, intraclast, trilobite batalha monastery, church railing 1 ooids 0.2 ~ 0.5 mm mostly small sparites and micrites, a few spars reach 0.5 mm. gymnocodiacean algae, foraminifera (lepidorbitoides), trilobite, bivalve, echinoderm batalha monastery, church railing 2 ooids 0.2 ~ 0.5 mm peloids 0.1 ~ 0.15 mm middle-sized spars of 0.1 ~ 0.5 mm are widely distributed between ooids, some small sparites and micrites can be seen inside fossils. salpingoporella istriana, foraminifera, trilobite batalha monastery, church balustrade ooids 0.2 ~ 0.4 mm mostly sparites < 0.1 mm between ooids and inside fossils. wood, gymnocodiacean algae, foraminifera figure 5. xrd results of the stone samples. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 126 of the thermal gravity curve showed the reaction to be single stepped. there is no mass change in other temperature ranges, demonstrating no absorbed water, gypsum, portlandite or muscovite in these stones. as seen in table 2, the calculated results show that the proportion of calcite in all the samples reaches 97.2 wt% ~ 99.2 wt%. it should be noted that the molar mass of caco3 was used in the calculation, thus the actual proportion of calcite would be lower than this number; this minor inaccuracy comes from the mg in magnesium calcite. slight differences could be seen by comparing the curves of all the samples. for example, limestone from the valinho do rei quarry showed obvious weight change after the temperature reached 600 °c, while for the sample from outreiro de sebastião, it was after 630 °c. a feasible thermodynamic explanation for this phenomenon is that a larger particle size causes the activation energy to be higher, thus consuming more energy to initiate the decomposition process [22]. grain size can also affect the reaction speed; limestone, with a small grain size, has a higher thermal decomposition rate [23]. this is in accordance with thinsection observation, as the major component of the valinho do rei limestone is characterised by micritic cement between small peloids, while the outreiro de sebastião limestone has larger ooids, more fossils and larger recrystallised sparite cements. 3.4. x-ray fluorescence analysis figure 7 illustrates the relative intensity of each element in the limestone samples. besides calcium, other elements detected in the limestone were silicon, phosphorous, sulphur, strontium, yttrium, zirconium, niobium, barium, thorium, lead, uranium and chlorine. the overall ratio of these elements is rather homogeneously distributed, with the exception of the batalha monastery baluster sample, which shows an apparent higher intensity of p, s and cl, and the naturally weathered valinho do rei stone, with the highest counts of cl. this could be explained by the effect of salt weathering [4]. in addition, there is a noticeable disparity in the strontium peak intensity between some samples. with the same coordination number and similar ion radius, sr2+ may substitute ca2+ ions in the calcite crystals. stones formed in the same geological environment would have similar degrees of such a substitution [24], [25]. from the ca-sr element alignment scatter plot (figure ), it can be seen that the plots that represent the batalha monastery baluster, valinho do rey and reguengo do fetal are similar, suggesting a similar slope of casr concentration of around 6.0 × 103. however, limestones from the batalha monastery royal cloister, eaves arch and church railing 2 have similar ca-sr ratios to the pidiogo and cabeço do roxo quarries of around 3.7 × 103. the plots for the batalha monastery roof top (ca/sr ratio 2.8 × 103) and church railing 1 (ca/sr ratio 2.3 × 103) are not similar to any existing quarry samples; a possible explanation is that these stones were from the lost carvalhos quarry. 4. conclusions a combined multi-analytical approach was used to establish the quarries of provenance for the batalha monastery. since considerable historical evidence can be drawn from the study of material provenance, these techniques have provided a powerful tool for the collection and analysis of valuable information of special interest to conservation research. the petrographic study confirmed that stones from the batalha monastery and the quarries that were used for construction and restoration are all oolitic limestones, dating back to the middle–late jurassic period and deposited on inner shallow-water platforms at the edge of the continental shelf. xrd and tga showed a high content of calcite (> 97 wt%) and a small amount of quartz (≤ 1 wt%). furthermore, to correlate each quarry with the different parts of the monastery, a ca-sr correlation chart was developed based on the ed-xrf result, suggesting that the monastery baluster was made of stone from the valinho do rei or reguengo do fetal quarries, while part of the church railing, the north-aisle eaves arch and royal cloister were constructed from stones from the pidiogo or cabeço do roxo quarries. these analytical results confirm the quarry-source attribution for the batalha building table 2. calcite proportion from tga calculation sample weight loss (wt%) calcite (wt%) pidiogo 43.93 99.9 valinho do rei 43.58 99.1 reguengo do fetal 43.85 99.7 cabeço do roxo 43.24 98.3 outeiro de sebastião 43.23 98.3 batalha monastery, royal cloister 42.77 97.2 batalha monastery roof top 43.48 98.9 batalha monastery eaves arch 43.39 98.6 batalha monastery church railing 1 43.21 98.2 batalha monastery church railing 2 43.64 99.2 batalha monastery church baluster 43.28 98.4 figure 6. tga curve of stone from the batalha monastery baluster. figure 7. xrf elemental results. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 127 materials found in the historical documents of the batalha monastery. for possible conservation and restoration in the future, this research can provide a reference for the selection of stones in order to obtain equivalent physicochemical and mechanical properties to maintain the consistency and integrity of the historical monument. to complement the current study, inductively coupled plasma-mass spectroscopy (icp-ms) analyses will be conducted to obtain minor and trace element data on samples from both the monastery and the quarries identified in this study to further confirm the results [26]. in future restoration interventions, finite element analysis could also be used to investigate the mechanical behaviour, which can take into account the different states of preservation of the original materials and those used during restoration [27], [28]. acknowledgement the research presented in this paper was carried out mainly using data collected at universidade de évora, politecnico di torino and direcao-geral do patrimonio cultural, as part of h2020-msca-itn-2017, ed-archmat (esr1). this project has received funding from the european union’s horizon 2020 research and innovation programme under the marie skłodowska-curie grant agreement no. 766311. references [1] j. c. v. da silva, the monastery of batalha, scala books, london, 2007, isbn 978-1857593822. 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uncertainty; level of confidence; control errors; activity of ions; expanded uncertainty citation: oleksandr vasilevskyi, assessing the level of confidence for expressing extended uncertainty through control errors on the example of a model of a means of measuring ion activity, acta imeko, vol. 10, no. 2, article 27, june 2021, identifier: imeko-acta-10 (2021)-02-27 section editor: maik rosenberger, ilmenau university of technology, germany received march 31, 2020; in final form march 14, 2021; published june 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution and reproduction in any medium, provided the original author and source are credited. corresponding author: oleksandr vasilevskyi, e-mail: o.vasilevskyi@gmail.com 1. introduction there is a need to develop methods for estimating the uncertainty of dynamic measurements that meet international requirements for evaluating and expressing the quality of measurements, which is a topical scientific task in the field of metrology. international standards support the use of combined uncertainty uc(y) as a parameter for quantifying the uncertainty of the measurement result [1]-[4]. although uc(y) is widely used to express uncertainty, in some cases, such as in trade, industry, regulatory acts or issues of health and safety, it is advisable to further specify a measure of uncertainty that determines the interval for the measurement result. this additional measure of uncertainty, which corresponds to interval estimation, is called expanded uncertainty. it is therefore relevant to determine a particular measurement technique to evaluate the trust level for calculating expanded measurement uncertainty. the purpose of the article is to develop a mathematical apparatus for estimating the confidence level in calculating the expanded uncertainty of measurements of ion activity, taking into account the manufacturer's (developer's) and consumer’s control errors, which will make it possible to establish the interval around the measurement result within which the majority of the distribution of values can be attributed to the measured value. 2. main materials of the research in the literature [1]-[9], only partial consideration is given to methods of establishing a trust level for calculating the expanded uncertainty of measurement. a mathematical apparatus that would allow a reasonable confidence level in the uncertainty measurement is not described. it is therefore advisable to propose and describe a methodology for estimating trust level based on the control errors of the manufacturer and the consumer, which will make it possible to establish the value of the coefficient of coverage k for the calculation of the expanded uncertainty of measurement. as an example, the developed means was used to measure the activity of ions. the confidence level for calculating the expanded uncertainty is proposed based on the control errors of the manufacturer and the consumer using the formula 𝐷 = 1 − 𝑃𝑛 = 1 − (𝛼 + 𝛽), (1) where 𝛼 represents the control errors of the manufacturer, 𝛽 represents the control errors of the consumer and 𝑃𝑛 represents the total value of control errors. the procedure for determining the confidence level is described on the basis of the combined uncertainty of the results abstract a method for estimating the level of confidence when determining the coverage factor based on control errors is proposed, using the example of measurements of ion activity. using information on tolerances and uncertainty, it is possible to establish a reasonable interval around the measurement result, within which most of the values that can be justified are assigned to the measured value. mailto:o.vasilevskyi@gmail.com acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 200 of reusable measurements of ion activity using a measuring instrument built on the principle of converting voltage to frequency [10], [11], which is described by the following transformation equation: 𝑁u/f = 𝑈pow 𝑓0 𝜏 (𝑈0 − 𝛼t(273.15 + 𝑡) 𝑛𝑎 𝑝𝑋𝑖 ) 𝑘2 (2) where рхі is the activity of the ions; 𝛼t is the temperature coefficient of steepness, equal to 198.4 × 10–3 °c–1; t is the temperature of the medium under investigation, k2 is the gain scaling factor (da1 and da2 in figure 1), upow is the value of the reference voltage of the voltage converter at a frequency of 10 v; τ = r c is the time constant of the voltage converter at a frequency that is used to set the full scale output frequency range (r = 1 kω, с = 47 µf), f0 is the frequency of the quartz resonator microcontroller (20 мhz), u0 is the standard potential of the reference electrode at the initial isopotential point and na is the charge of the ith ion [11]. the electrical functional circuit of the ion activity measuring device based on the conversion of voltage to frequency is shown in figure 1. the general law of uncertainty when measuring ion activity depends on many factors, such as the activity of interfering ions due to the limited properties of ion-selective electrodes or the presence of measurement error due to temperature, zero drift, instability of the power supply, etc., and it can be difficult to identify which factor is dominant [10]. this allows us to adopt the law of distribution for the centred value of the error when measuring the normal activity of ions, which we describe with the expression 𝑝(𝑝𝑋) = 1 𝑢𝑐 (𝑝𝑋)√2𝜋 e − (𝑝𝑋−𝑝�̄�)2 2 𝑢𝑐 2(𝑝𝑋) (3) where 𝑝�̅� is the estimated value of ion activity and uc(px) is the combined uncertainty of the measurement of ion activity. the value of the combined uncertainty of ion activity measurement when using a measuring device built on the basis of the conversion of voltage to frequency is calculated by the formula 𝑢𝑐 2(px) = 𝑢𝐴 2(𝑝𝑋) + 𝑢𝑐𝐵 2 , (4) 𝑢𝑐𝐵 2 = ∑ 𝑐𝑖 2𝑢𝑖 2(𝑝𝑋) 𝑁 𝑖=1 + +2 ∑ ∑ 𝑐𝑖 𝑐𝑗 𝑟(𝑝𝑋, 𝑡)𝑢𝑖 (𝑝𝑋)𝑢𝑗 (𝑡) 𝑁 𝑖=2 𝑁−1 𝑗=1 , (5) where ua(px) is the evaluation of type a standard uncertainty, ui(px) represents components of type b uncertainty for measurements of ion activity with the respective sensitivity coefficients сi, uj(t) represents components of type b uncertainty for measurements of temperature with the corresponding sensitivity coefficients cj and r(px, t) is the correlation coefficient between ion activity (px) and temperature (t) [10]-[14]. taking the transformation equation (2) into account, the type b combined uncertainty is determined by the formula 𝑢𝑐𝐵1 2 = ( 𝜕𝑁u/f 𝜕𝑝𝑋𝑖 ) 2 [𝑢𝐵𝐸 2 + 𝑢𝐵𝑈/𝐹 2 ] + (6) + ( 𝜕𝑁u/f 𝜕𝑈0 ) 2 𝑢𝐵𝛩𝑈0 2 + ( 𝜕𝑁u/f 𝜕𝑡 ) 2 𝑢𝐵𝑃 2 + + ( 𝜕𝑁u/f 𝜕𝑈𝑝𝑜𝑤 ) 2 𝑢𝐵𝑈pow 2 , 𝑢𝑐𝐵 2 = 𝑢𝑐𝐵1 2 + 2 𝜕𝑁u/f 𝜕𝑝𝑋𝑖 𝑢𝑐𝐵1 𝜕𝑁𝑡u/f 𝜕𝑡 𝑢𝑐𝐵𝑡 × × ∑ (𝑡𝑖𝑙 − 𝑡𝑖 )(𝑝𝑋𝑗𝑙 − 𝑝𝑋𝑗 ) 𝑛𝑖𝑗 𝑙=1 √∑ (𝑡𝑖𝑙 − 𝑡𝑖 ) 2𝑛𝑖𝑗 𝑙=1 ∑ (𝑝𝑋𝑗𝑙 − 𝑝𝑋𝑗 ) 2𝑛𝑖𝑗 𝑙=1 (7) where 𝑁 𝑡𝑈/𝐹 = 4 𝑈pow 𝑓0 𝜏 𝑘1 𝐸𝑠 𝛼𝑡 𝑡 is the equation of transformation for measurements of temperature (figure 1) [10], es = і r0 is the sensor supply voltage (r0 is the sensor resistance at a temperature of 0 °с) and k1 is the gain scaling factor (da3 in figure 1), 𝜕𝑁u/f 𝜕𝑝𝑋𝑖 = 𝛼𝑡 (273.15 + 𝑡) 𝑈pow 𝑓0 𝜏 [𝑈0 − 𝛼𝑡 (273.15 + 𝑡) 𝑛𝑎 𝑝𝑋𝑖 ] 2 𝑘2𝑛𝑎 = 12.98 𝑝𝑋 −1 is the sensitivity factor for ion activity at a temperature of 25 °с, 𝜕𝑁u/f 𝜕𝑈0 = − 𝑈pow 𝑓0 𝜏 [𝑈0 − 𝛼𝑡 (273.15 + 𝑡) 𝑛𝑎 𝑝𝑋𝑖 ] 2 𝑘2 = −0.22 v−1 is the coefficient of the voltage sensitivity of the standard potential of the reference electrode, 𝜕𝑁u/f 𝜕𝑡 = 𝛼𝑡 𝑝𝑋𝑖 𝑈pow 𝑓0 𝜏 [𝑈0 − 𝛼𝑡 (273.15 + 𝑡) 𝑛𝑎 𝑝𝑋𝑖 ] 2 𝑘2𝑛𝑎 = 0.3 °c−1 is the coefficient of sensitivity at an additional measured temperature, 𝜕𝑁u/f 𝜕𝑈pow = 𝑓0𝜏 (𝑈0 − 𝛼𝑡 (273.15 + 𝑡) 𝑛𝑎 𝑝𝑋𝑖 ) 𝑘2 = −9.08 v−1 is the coefficient of the voltage sensitivity of the stable power supply source of the voltage-to-frequency converter (vfc), nij is the number of pairwise measurements of temperature and ion activity, 𝑢𝐵𝐸 = 𝛾 𝑝𝑋max 100%√3 −3 px is the type b uncertainty caused by the presence of a primary electrode with a low accuracy class ( = 0.7 %), 𝑢𝐵𝛩𝑈0 = 𝛩𝑈0 √3 = 0.95 mv is the type b uncertainty caused by the instability (𝛩𝑈0 = 1.65 mv) of the standard potential of the reference electrode, 𝑢в𝑝 = 𝛥t ubiasmax 𝑈pout√3 ≈ 2.89 ⋅ 10−3 °с is the type b uncertainty caused by the existence of a bias voltage (ubiasmax 𝑈pout⁄ = 0.2·10 –3) of the operational amplifier when the temperature deviates (𝛥𝑡 = 5 °с) from the nominal value, 𝑢𝐵𝑈pow = 𝛩𝑈𝑃 √3 ≈ 1.16 mv is the type b uncertainty caused by instability in the reference voltage power supply source (𝛩𝑈𝑃 = 2 mv) and 𝑢𝐵vfc = 𝛿nonl𝑝𝑋max 100%√3 −6 px is the type b uncertainty caused by the nonlinearity (𝛿nonl) of the voltage converter (ad650) [10]-[14]. the combined uncertainty values are calculated in [10] and [14], and the maximum uncertainty value for measuring the acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 201 activity of phosphate ions is 17.21 × 10–3 px in the range of measurements from 6 to 0.3 px. a compatible two-dimensional confidence level density when measuring the activity of ions, taking into account the allowable deviation of the measurement error ε, which is established by the consumer, is described by the expression 𝑝(𝑝𝑋, ) = 1 2 π ⋅ 𝑢𝑐 (𝑝𝑋) ⋅ 𝑢 e − (𝑝𝑋−𝑝�̄�)2 2 𝑢𝑐 2(𝑝𝑋) − (δ )2 2 𝑢𝜀 2 . (8) in most practical cases, as noted in [9] and [15], the admissible deviation ε of the parameter controlled by the user, depending on the combined uncertainty of the measurement results uc(y), is determined by the formula [%] = 6 𝑢𝑐 (𝑦) 𝑑 100 = 6 𝑢𝑐 (𝑦) 𝑋max − 𝑋min 100 , (9) where d is the width of the tolerance field determined by the values of the upper xmax and lower xmin range of measurement for the physical quantity. by substituting the values of the maximum combined uncertainty when measuring ion activity uc(px) = 17.21·10–3 px and the values of the upper (xmax = 6 px) and the lower (xmin = 0.3 px) ranges of the measurement limit in equation (9), we obtain the value of the permissible error set by the consumer, which is ε = 1.81 %. the uncertainty uε of the tolerance field of the controlled parameter at which the result can be considered reliable in practice is recommended to be equal to 6uc(y) [9], [13], [15]-[18]. thus, uε = 6 × uc(y) = 6 × 17.21·10–3 = 103.26·10–3 px. taking into account expression (8), the manufacturer’s control errors α are estimated by the formula 𝛼 = ∫ [∫ e − (δ𝑝𝑋)2 2 𝑢𝑐 2(𝑝𝑋) − (δ )2 2 𝑢𝜀 2 2 π ⋅ 𝑢𝑐 (𝑝𝑋) ⋅ 𝑢 𝑋𝑚𝑖𝑛 −∞ + 𝑋𝑚𝑎𝑥 𝑋𝑚𝑖𝑛 + ∫ e − (δ𝑝𝑋)2 2 𝑢𝑐 2(𝑝𝑋) − (δ )2 2 𝑢𝜀 2 2 π ⋅ 𝑢𝑐 (𝑝𝑋) ⋅ 𝑢 𝑑𝛥 ∞ 𝑋𝑚𝑎𝑥 ] (10) and the consumer’s control errors 𝛽 are estimated by the formula 𝛽 = ∫ ∫ e − (δ𝑝𝑋)2 2 𝑢𝑐 2(𝑝𝑋) − (δ )2 2 𝑢2 2 π ⋅ 𝑢𝑐 (𝑝𝑋) ⋅ 𝑢 𝑑δ𝑝𝑋𝑑δ + 𝑋max−∆𝑝𝑋 𝑋min+∆𝑝𝑋 𝑋min −∞ + ∫ ∫ e − (δ𝑝𝑋)2 2 𝑢𝑐 2(𝑝𝑋) − (δ )2 2 𝑢2 2 π ⋅ 𝑢𝑐 (𝑝𝑋) ⋅ 𝑢 𝑑δ𝑝𝑋𝑑δ 𝑋max−∆𝑝𝑋 𝑋min+∆𝑝𝑋 ∞ 𝑋max . (11) the admission field is the value of the measured value δpx. in this case, this is the activity of the px ions, which is defined by the formula rs 232 pc rd td mcu msp430f123 ca1 ca0 19 20 vcc vss 2 4 xout xin 5 6 ta0 ta1 ta2 22 23 24 utxd urxd 15 16 rst 7 + 5 v p2.0 p2.1 8 9 20 мгц vss vdd + 5 v 2 9 5 h clk di h t 1 6 1 1 4 3 p3.0 p3.1 11 12 p3.3 p3.1 p3.2 p3.3 13 14c6 c7 dd4 1 dd5 px u 3 2 4 + 5 v 7 6 100m lmc 6001 5 v 3 2 4 + 5 v 7 ad820 6 5 v 18k 6k 0 1 2 3 а0 а1 аgnd 100k 100n 3 4 7 6 u/f ad650 2 1 8 5 + v in r ct ct f out lc vcc vss 561кп1 + 5 v y da2 da4 dd2 r1 r8 r9 r3 r6 r12 r 10 c1 c3 t ut 3 2 4 + 5 v 7 ad820 6 r2 3 4 7 6 u/f ad650 2 1 8 5 + v in r ct ct f out lc vcc vss + 5 v dd1 r7 r5 r11 c2 c4 r4 r5 in gnd 1171 cп42 2 1 3 dd2 r13 c8 r6 dd3 da1 da3 c1 c5 figure 1. the functional electrical circuit of the device for measuring ion activity. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 202 δ𝑝𝑋 = 𝑝�̄� 100 𝛿max , (12) where 𝑝�̅� is the estimation of the ion activity in the upper measurement range, 𝛿max is the maximum relative measurement error (for measurements of ion activity, this value is 0.7 % in the range of measurements from 6 to 0.3 px). consequently, when calculating the control errors of the manufacturer and the consumer, the field of admission ∆px can be calculated by (12), which, with the corresponding numerical values, is equal to 2.1·10–3 px. by substituting calculated tolerances in the formula for estimating the control errors of the manufacturer (10) and the consumer (11) and solving them using the maple 12 package, we obtain the following numerical values: 𝛼 = 0.24·10–67 (𝛼 ≈ 0), and 𝛽 = 0.00196. the total value of the control errors is pn = 0.00196, and the confidence level for calculating the expanded uncertainty of the measurement, according to formula (1) will be equal to d = 1 − pn = 1 − 0.00196 = 0.998. figure 2 illustrates the change in the manufacturer’s and consumer’s control errors depending on the parameter μ, which is equal to uε/uc(px). this parameter establishes the relationship between the permissible uncertainty of uε, which is specified by the consumer (via normative documents), and the combined uncertainty of uc(px), which is set by the manufacturer. the figure shows uε < uc(px), uε ≈ uc(px) and uε > uc(px). thus, based on the constructed characteristic of the change in reliability (figure 2), which is obtained by calculating the consumer and manufacturer’s control errors, a range can be calculated within which the majority of the distribution of values that can be attributed to the measured value is likely to be located, depending on the value of the accepted tolerance for the monitored parameter. as can be seen from figure 2, with a tolerance of six combined uncertainties, the confidence level is 99.8 %. given the level of confidence based on control errors, the expanded uncertainty can be calculated from the formula 𝑈 = 𝑘𝑝 ⋅ 𝑢𝑐 (𝑦) = 𝑘99.8 ⋅ 17.21 ⋅ 10 −3 , (13) where kp is the coverage ratio for the established confidence level (p = d), which is taken from the student's table. thus, with 30 degrees of freedom and a confidence probability of 99.8 %, the coverage factor kp is 3.385, and with 10 degrees of freedom and a probability of 99.8 %, it is 4.14. accordingly, if these coverage factor values are entered into (13), the expanded uncertainty will be u99.8 = 71.25·10-3 px for 10 degrees of freedom and u99.8 = 58.26·10–3 px for 30 degrees of freedom. 3. conclusions the work describes the characteristics of the change in the manufacturer’s and consumer’s control errors, on the basis of which a general characteristic of the change in the control errors that occurs during the measurement of ion activity is obtained. using this characterisation, it is possible to determine graphically the confidence level within which most of the distribution of values obtained by measuring the activity of ions is probably located. the obtained nomograph makes it possible to determine with a high probability the confidence level at a given tolerance for the controlled parameter to determine the coverage factor in calculating the extended uncertainty of ion activity measurement. the described approach for determining confidence probability on the basis of control errors can be applied to any type of measurements, provided a separate calculation is performed to characterise the control errors of the manufacturer of the measuring instrument and the consumer (i.e. errors of the first and second kind). figure 2. characteristics of the change of control errors depending on the parameter μ. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 203 references [1] bipm, iec, ifcc, iso, iupac, iupap, oiml, evaluation of measurement data – guide to the expression of uncertainty in measurement, joint committee for guides in metrology, bureau international des poids et mesures jcgm 100:2008 (2008). 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volume calibration using a comparison method with a transfer leak flow rate acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 343 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 343 348 volume calibration using a comparison method with a transfer leak flow rate f. boineau1, m. d. plimmer1, e. mahé1 1 laboratoire commun de métrologie du lne-cnam, 1 rue gaston boissier, paris, france, frederic.boineau@lne.fr abstract: this paper describes a volume comparison method carried out under a fine/rough vacuum using a small transfer flow rate from a capillary leak artefact. this method is suitable for volumes of vessels with complex shapes such as tubing arrangements equipped with various pneumatic parts (valves, gauges…), sample volumes, etc. the calibration, based on pressure rise measurements performed with a constant-volume flowmeter set-up, exhibits a standard relative uncertainty between 0.03 % and 2 % for volumes ranging from 1 to 3000 cm3, which is only about one order of magnitude higher than capabilities of the gravimetric volume calibration. keywords: volume calibration; vacuum; constant-volume flowmeter; pressure rise measurements; capillary leak rate 1. introduction the most accurate way to determine the volume contained by a vessel is the so-called “gravimetric volume calibration” method [1], which consists in weighing the vessel, empty and then filled with a liquid (usually water) and calculating the volume from the mass and density of the liquid added. as long as the vessel is transparent or can be filled with confidence so that no air bubbles remain inside, this primary method can provide a relative standard uncertainty in the 10-5 range. the comparison method presented here is suited to volumes either incompatible with liquids or else with internal shapes such that prevent complete filling. it is derived from the pressure rise method implemented in the laboratoire commun de métrologie (lcm lne-cnam) constant-volume flowmeter [2] used to calibrate leak rates in the range from 1×10-7 to 1×10-2 pa·m3·s-1. the leak rate is deduced from the measured pressure rise rate and the total volume into which the gas flows. in this flowmeter, a standard volume, calibrated using the gravimetric method, is connected in series with the internal volume of the tubing which links the leak artefact to the pressure sensor, called the dead volume. this latter may be determined prior to the leak rate measurement. setina and erjavec have studied a similar arrangement to calibrate the volume of chambers in a set-up under high vacuum [3]. in the present work, the standard and unknown volumes are connected in parallel. the volume calibration using a comparison method with a transfer leak flow rate was originally developed for the needs of companies owning sample volumes in which some gas is trapped for a further analysis. as this kind of artefact is sometimes incompatible with the gravimetric volume calibration and has to be functionally tight to a certain extent, the comparison method is well suited. 2. principle the principle is developed first for the arrangement where the standard and unknown volumes are connected in parallel. the necessary determination of the dead volume, corresponding to the connection in series of this dead volume and the standard volume, is presented on the same basis. 2.1. volumes connected in parallel a steady gas throughput 𝑞 from a capillary leak artefact is used. in a given volume v, at a constant temperature t, the pressure rise rate ∆𝑝 ∆𝑡⁄ produced by the leak rate 𝑞 is such that: 𝑞(𝑇) = 𝑉 ∆𝑝 ∆𝑡 . (1) figure 1: calibration set-up, volumes connected in parallel. the comparison method consists in directing the gas flow successively into a standard volume 𝑉𝑠𝑡−𝑥 q leak artefact unknown volume vx standard volume vst-x pressure gauge 1 2 p http://www.imeko.org/ mailto:frederic.boineau@lne.fr acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 344 (phase 1) and an unknown one called 𝑉𝑥 (phase 2), placed in a parallel arrangement (figure 1). the pressure is recorded over time during these phases, between two thresholds 𝑝𝑚𝑖𝑛 and 𝑝𝑚𝑎𝑥 (figure 2). it takes times ∆𝑡𝑠𝑡 and ∆𝑡𝑥 for the pressure to rise from 𝑝𝑚𝑖𝑛 to 𝑝𝑚𝑎𝑥 in volumes 𝑉𝑠𝑡 and 𝑉𝑥 respectively. assuming a constant temperature and no distortion of the walls of the volumes during measurements, the equality of 𝑞(𝑇) in 𝑉𝑠𝑡 then 𝑉𝑥 leads via equation (1) to: 𝑉𝑠𝑡−𝑥 [ 𝑝𝑚𝑎𝑥−𝑝𝑚𝑖𝑛 ∆𝑡𝑠𝑡 ] = 𝑉𝑥 [ 𝑝𝑚𝑎𝑥−𝑝𝑚𝑖𝑛 ∆𝑡𝑥 ] , (2) figure 2: successive pressure recordings in 𝑉𝑠𝑡 and 𝑉𝑥 . one can see from equation (2) that 𝑉𝑥 is determined from the value of 𝑉𝑠𝑡 (which can be calibrated with the gravimetric method) and from time interval measurements ∆𝑡𝑠𝑡 and ∆𝑡𝑥 . this means that only the repeatability of the pressure gauge can influence measurements, whether the gauge is calibrated or not. however, when the method is put into practice, it is much faster to record the pressure without caring about thresholds and to determine the pressure rise rate over time �̇� afterwards, using a least squares method. equation (1) can be then rewritten as: 𝑞(𝑇) = 𝑉�̇� , (3) and finally, from equation (3), the unknown volume is related to pressure rise rates via: 𝑉𝑥 = 𝑉𝑠𝑡−𝑥 �̇�𝑠𝑡−𝑥 �̇�𝑥 . (4) 2.2. volumes connected in series similarly to 2.1, equations can be established for two volumes 𝑉𝑑 and 𝑉𝑠𝑡−𝑑 connected in series, where 𝑉𝑑 corresponds to the dead volume and 𝑉𝑠𝑡−𝑑 a standard volume. the calibration set-up for determining 𝑉𝑑 is shown figure 3. the gas flows first in the volume composed of 𝑉𝑑 and 𝑉𝑠𝑡−𝑑 (phase 1), then the valve vvst-d is closed and the gas flow in 𝑉𝑑 only. corresponding pressure rise rates �̇�𝑠𝑡−𝑑 and �̇�𝑑 are determined and 𝑉𝑑 can be expressed as: 𝑉𝑑 = 𝑉𝑠𝑡−𝑑 �̇�𝑠𝑡−𝑑 �̇�𝑑−�̇�𝑠𝑡−𝑑 . (5) figure 3: calibration set-up, volumes connected in series. 2.3. calibration process equations introducing the ratios 𝐾𝑑 and 𝐾𝑥 written as: 𝐾𝑑 = �̇�𝑠𝑡−𝑑 �̇�𝑑−�̇�𝑠𝑡−𝑑 (≡ 𝑉𝑑 𝑉𝑠𝑡−𝑑 ) , (6) and: 𝐾𝑥 = �̇�𝑠𝑡 �̇�𝑥 (≡ 𝑉𝑥+𝑉𝑑 𝑉𝑠𝑡−𝑥+𝑉𝑑 ), (7) leads to expressions for 𝑉𝑑 and 𝑉𝑥 as follows: 𝑉𝑑 = 𝑉𝑠𝑡−𝑑 𝐾𝑑 , (8) and: 𝑉𝑥 = 𝐾𝑥 𝑉𝑠𝑡−𝑥 + (𝐾𝑥 − 1)𝑉𝑑 . (9) presentation of the measured quantities in a ratio form highlights the fact that the pressure gauge correction can be cancelled if the pressure response is linear or has been linearised previously. ratio determinations are thus independent of the pressure gauge drift over time. from equation (9), one can see that when 𝐾𝑥 tends towards unity (in other words when the values of 𝑉𝑥 and 𝑉𝑠𝑡 lie close to each other), the contribution of the uncertainty of 𝑉𝑑 tends towards zero. 3. influence parameters assuming the gas is perfect in the working pressure range, the calibration method relies mainly on the stability of the gas flow rate and a constant and identical gas temperature in the different volumes, during pressure rise rate measurements. degassing of internal walls of the volumes is supposed to be negligible. 3.1. temperature replacing the throughput (equation (1)) by the molar flow rate 𝑞𝜈 [4], we obtain for a constant gas temperature 𝑇: 𝑞𝜈 = 𝑉 ∆𝑝 ∆𝑡 1 𝑅𝑇 (10) where 𝑅 is the molar gas constant. let us suppose that 𝑇𝑥 and 𝑇𝑠𝑡−𝑥 are gas temperatures during measurements of �̇�𝑥 and �̇�𝑠𝑡−𝑥 respectively. if the temperature difference is: ∆𝑇𝑥 = 𝑇𝑥 − 𝑇𝑠𝑡−𝑥 (11) p t pmin pmax δtst δtx q leak artefact p dead volume vd standard volume vst-d vvst-d 2 1 pressure gauge http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 345 the relative difference 𝜀𝑥 is: 𝜀𝑥 = ∆𝑇𝑥 𝑇𝑥 . (12) if 𝜀𝑥 ≪ 1, the calculation leads to a ratio 𝐾𝑥 (∆𝑇𝑥 ) which takes into account the temperature difference: 𝐾𝑥 (∆𝑇𝑥 ) = 𝐾𝑥 (1 + 𝜀𝑥 ) . (13) a similar calculation can be performed with the dead volume measurement. indices “𝑑” and “𝑠𝑡 − 𝑑” are used instead of “𝑥” and “𝑠𝑡 − 𝑥”. we then have: ∆𝑇𝑑 = 𝑇𝑑 − 𝑇𝑠𝑡−𝑑, (14) and: 𝜀𝑑 = ∆𝑇𝑑 𝑇𝑠𝑡−𝑑 . (15) assuming 𝜀𝑑 ≪ 1 , the ratio 𝐾𝑑 (∆𝑇𝑑 ) which finally takes into account the temperature difference is: 𝐾𝑑 (∆𝑇𝑑 ) = 𝐾𝑑 1+𝜀𝑑 1−𝐾𝑑×𝜀𝑑 . (16) the other issue when measuring the pressure rise rate ∆𝑝 ∆𝑡⁄ in a closed volume is the temperature variation rate ∆𝑇 ∆𝑡⁄ .a thermal perturbation term (∆𝑝 ∆𝑡⁄ )𝑡ℎ arises which can be linked to ∆𝑇 ∆𝑡⁄ by deriving the perfect gas law: ( ∆𝑝 ∆𝑡 ) 𝑡ℎ = 𝑝 𝑇 ∆𝑇 ∆𝑡 . (17) this perturbation can be experimentally quantified if the flow rate 𝑞 is isolated from the pressure gauge and the volumes (during 𝑉𝑑 and 𝑉𝑥 determinations) so that the residual pressure rate can be measured. 3.2. flow rate stability in this work, a metal capillary leak was used to generate the gas flow rate. the downstream pressure of the capillary 𝑝𝑑 is the working pressure in the flowmeter. the upstream pressure 𝑝𝑢 is controlled by means of a pressure controller. the leak conductance 𝐶 depends on 𝑝𝑢 and the leak temperature 𝑇𝑙 via a temperature coefficient which absolute value is about 0.1-0.3 %·k-1 [5]. the generated molar flow rate 𝑞𝜈 can be written as: 𝑞𝜈 = 𝐶(𝑝𝑢, 𝑇0)(𝑝𝑢 − 𝑝𝑑)𝛼(𝑇𝑙 − 𝑇0) 1 𝑅𝑇𝑙 (18) where 𝑇0 is an arbitrary reference temperature. for given temperature 𝑇𝑙 and controlled upstream pressure 𝑝𝑢 , the flow rate depends on the downstream pressure stability. assuming 𝑝𝑢𝑚 ≫ 𝑝𝑑𝑚, we obtain: 𝑑𝑞𝜈 𝑞𝜈 ≅ −𝑑𝑝𝑑 𝑝𝑢 . (19) this equation helps one choose the optimal leak conductance such that the flow rate stability is compatible with the targeted uncertainty. 3.3. ratio of pressure rise rates stability measuring the pressure rise rates successively in the standard volume and the calibrated volume takes a few minutes during which the whole calibration system is likely to drift. in particular, the ambient temperature variation affects the flow rate stability (equation (18)). we illustrate in figure 4 a variation of the pressure rise rate in the standard volume 𝑉𝑠𝑡−𝑥 as a function of time. figure 4: interpolation of pressure rise rate measurements in the standard volume. the pressure rise rate in the standard volume increased from �̇�𝑠𝑡1 to �̇�𝑠𝑡2 between 𝑡𝑠𝑡1 and 𝑡𝑠𝑡2 . we evaluate what it should have been at the time 𝑡𝑥 when �̇�𝑥 was measured, by the simple interpolation formula: �̇�𝑠𝑡−𝑥 = �̇�𝑠𝑡1 + (�̇�𝑠𝑡2 − �̇�𝑠𝑡1) 𝑡𝑥−𝑡𝑠𝑡1 𝑡𝑠𝑡2−𝑡𝑠𝑡1 , (20) and use this value to determine the ratio 𝐾𝑥 . 4. experimental set-up the volume calibration set-up is implemented with our constant-volume flowmeter facility shown figure 5. figure 5: experimental set-up using the constant-volume flowmeter. cdg1: capacitance diaphragm gauge 690a, 100 pa full range (mks instruments, united states); cdg2: capacitance diaphragm gauge 220a, 10 kpa full range (mks instruments); cl: capillary leak (inficon, germany); p-cont: pressure controller dpi515 (druck, germany); pvp: primary vacuum pump (vane pump); vxx: bellows valves (flowlink, france); pt100-x and pt100-stx: pt100 thermometers. ̇ 1 2 ̇ 1 ̇ 2 ̇ − http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 346 this flowmeter determines flow rates from the pressure rise rate measurement in a known volume [2]. it is composed of volumes of different capacities, two capacitance diaphragm gauges (cdg) connected in parallel to the tube leading the gas flow rate. a suitable combination of the standard volume and cdg is used upon the flow 𝑞𝜈 to be calibrated. volumes and tubing are placed in an insulated box (63×42×23 cm³) to ensure better temperature stability. 5. procedure and uncertainty preliminary observations with different configurations of volume sizes allowed defining a suitable procedure and guiding the uncertainty analysis. 5.1. preliminary measurements for leak calibrations, the pressure is usually recorded for one minute with a rate of one measurement per second. the pressure rise rate is then determined by means of a linear least squares modelling. for volume calibrations, one needs to close and open valves and also pump down the volumes, for different purposes: to switch from one volume to another, to set the pressure to a value compatible with the pressure gauge range or to proceed with a residual pressure rate recording. these operations create pressure jumps and consequently temperature disturbances which take some time to stabilise. the induced transition phase is shown figure 6 for a pressure rise rate measured in a total volume (dead volume and standard volume) of about 590 cm3. the stabilisation in this example was reached in about four minutes. however, this stabilisation phase has a variable duration which depends for instance on the volume size or the pressure jump magnitude. during a calibration, this phase should be mastered to achieve the targeted uncertainty. figure 6: transition phase of the measured pressure rise rate after switching from one volume to another. the relative experimental standard deviation of the modelled pressure rise rate is about 8·10-5 for these measurements. figure 7: pressure variation rate determined from the recording of pressure without flow. residual pressure rise rate measurements were performed, over an hour or more, in well-sealed 304l stainless steel volumes to evaluate the range of temperature variation rate (degassing and absorption being assumed negligible) from equation (17) and its contribution on the uncertainty. the graph of figure 7 shows the residual pressure variation rate �̇�𝑠𝑡−𝑟𝑒𝑠 determined from the recording of the pressure over time in a closed total volume of 360 cm3. one can see that �̇�𝑠𝑡−𝑟𝑒𝑠 changes sign in a short time corresponding to a measurement sequence. the mean slope �̇�𝑠𝑡−𝑟𝑒𝑠̅̅ ̅̅ ̅̅ ̅̅ ̅ over an hour is estimated to be 5.5×10-7 pa·m3·s-1, which corresponds to a temperature variation rate of 0.25 mk·min-1 (equation (17)). we generally obtain an agreement of better than 15 % between temperature rates estimated from residual pressure rates and those estimated with pt100 sensors. typical absolute temperature rates in the isolation box placed in our air-conditioned room are below 1.0 mk·min-1. 5.2. procedure preliminary observations gave guidance to determine the ratio 𝐾𝑥 (or 𝐾𝑑). a sequence of three measurements of �̇�𝑥 (or �̇�𝑑 ) is performed between two sequences of three measurements of �̇�𝑠𝑡−𝑥 (or �̇�𝑠𝑡−𝑑 ) in order to apply equation (20). the pressure rise rate is obtained from the pressure recording over one minute (see § 5.1). a stabilisation time (see figure 6) before each measurement sequence of �̇� is respected such that the relative difference between two successive determined values of �̇� lies below 0.02 %. an alternation of four sequences of �̇�𝑠𝑡−𝑥(or �̇�𝑠𝑡−𝑑 ) determinations and three sequences of �̇�𝑥(or �̇�𝑑 ) determinations allow us to obtain three determinations of 𝐾𝑥 (or 𝐾𝑑) and thus three values of 𝑉𝑥 (or 𝑉𝑑). a measurement of the residual pressure rate is performed in each volume to check whether it lies in the typical range expected given the maximum temperature rate of 1.0 mk·min-1 (see equation (17)). rates are determined from pressure recording of at least two minutes. a residual pressure rate �̇�𝑥−𝑟𝑒𝑠 in the calibrated volume 𝑉𝑥 significantly http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 347 above the typical observed values would indicate some degassing (or absorption) of the volume walls which then should be subtracted from the determined values of �̇�𝑥 . 5.3. uncertainty budget the uncertainty budget is drawn using data from the calibration of a 100 cm3 volume identified as v100 (used for the comparison with the gravimetric volume calibration method presented § 6). experimental data are found in table 1. uncertainties in 𝑉𝑑 and 𝑉𝑥 are calculated using the variances propagation law from equations (8) and (9), including the repeatabilities estimated with the experimental deviations 𝑠(𝑉𝑑 ) and 𝑠(𝑉𝑥 ): 𝑢(𝑉𝑑) 𝑉𝑑 = √[ 𝑠(𝑉𝑑) 𝑉𝑑 ] 2 + [ 𝑢[𝐾𝑑(∆𝑇𝑑)] 𝐾𝑑 ] 2 + [ 𝑢(𝑉𝑠𝑡−𝑑) 𝑉𝑠𝑡−𝑑 ] 2 , (21) and: 𝑢(𝑉𝑥 ) = √ [𝑠(𝑉𝑥)] 2 + [𝐾𝑥 (∆𝑇𝑥 )𝑢(𝑉𝑠𝑡−𝑥 )] 2 + [(𝐾𝑥 (∆𝑇𝑥 ) − 1)𝑢(𝑉𝑑 )] 2 + [(𝑉𝑠𝑡−𝑥 + 𝑉𝑑 )𝑢(𝐾𝑥 (∆𝑇𝑥 ))] 2 . (22) table 1: quantities measured or used for the calibration of a 100 cm3 volume. quantity value 𝑞𝜈 4,9×10 -9 mol·s-1 (1,2×10-5 pa·m3·s-1 à 20 °c) 𝑝𝑢 700 kpa 𝑝𝑑 36 pa < 𝑝𝑑 < 44 pa 𝑉𝑑 59.9 cm 3 𝑉𝑠𝑡−𝑑 (𝑢𝑉𝑠𝑡−𝑑 ) 296.991 (0.076) cm 3 𝑉𝑥 100.1 cm 3 𝑉𝑠𝑡−𝑥 (𝑢𝑉𝑠𝑡−𝑥 ) 76.103 (0.019) cm 3 𝐾𝑑 0.201 𝐾𝑥 1.18 �̇�𝑑 0.19 pa·s -1 �̇�𝑠𝑡−𝑑 0.032 pa·s -1 �̇�𝑥 0.072 pa·s -1 �̇�𝑠𝑡−𝑥 0.085 pa·s -1 𝑇 20.5 °c ∆𝑇 ∆𝑡⁄ 0.19 mk·min-1 ∆𝑇𝑥 -0.01 k 𝑠(𝑉𝑑 ) 0.031 cm 3 𝑠(𝑉𝑥 ) 0.021 cm 3 thus, first we have to calculate the ratios 𝐾𝑑 and 𝐾𝑥 uncertainties from equations (6), (7), (13) and (16). prior to this calculation, we recall the following statements: the method used for the calibration cancels the error of the gauge so there is no systematic uncertainty component for the ratios; the temperature variation rate induces in the short term a residual pressure rate which can be regarded as a noise (see figure 7) on the pressure rise rate measured in the presence of 𝑞𝜈 ; the relative variation of 𝑞𝜈 due to 𝑝𝑑 variation (table 1) is at the maximum 1.1×10-5 (equation (19)) and can be neglected; the procedure described in § 3.3 allows to contain drifts of the whole calibration system. moreover, the experimental standard deviations respectively of the ratios 𝑠(𝐾𝑥 ) (or 𝑠(𝐾𝑑 )) and the volumes 𝑠(𝑉𝑥 ) (or 𝑠(𝑉𝑑 )) are fully correlated. finally, the remaining uncertainty components of the ratios are those related to temperature differences ∆𝑇𝑥 and ∆𝑇𝑑 (see § 3). ∆𝑇𝑥 is measured with two pt100 sensors placed on the volume walls. ∆𝑇𝑑 is too hard to measure since the dead volume is somewhat spatially extended. consequently, it was decided to consider ∆𝑇𝑥 and ∆𝑇𝑑 as non-applied corrections with ∆𝑇𝑑 arbitrary fixed at 0.1 k in the experimental conditions and ∆𝑇𝑥 as it is estimated with a pt100, with a minimum value of 0.02 k. the variance propagation law thus gives [6] for uncertainties 𝑢[𝐾𝑑 (∆𝑇𝑑 )] and 𝑢[𝐾𝑥 (∆𝑇𝑥 )]: 𝑢[𝐾𝑑(∆𝑇𝑑)] 𝐾𝑑 = (1 + 𝑉𝑑 𝑉𝑠𝑡−𝑑 ) 𝑠(𝐾𝑑) 𝐾𝑑 + 1 2 |1 − 1+𝜀𝑑 1−𝐾𝑑×𝜀𝑑 | , (23) and: 𝑢[𝐾𝑥(∆𝑇𝑥)] 𝐾𝑥 = 𝑠(𝐾𝑥) 𝐾𝑥 + 1 2 |1 + 𝜀𝑥 |. (24) even if 𝑠(𝐾𝑑 ) is neglected, one should pay attention to the choice of 𝑉𝑠𝑡−𝑑value regarding the sensitivity coefficient in equation (23). standard volumes (sample cylinders of 304l stainless steel) are connected to the input port of a bellows valve vl214 from flowlink (france) which has a dead volume of about 0.681 cm3. uncertainties of the standard volumes given in table 1 include the calibration uncertainty of the gravimetric method and that of the dead volume of the bellow valve 𝑢𝑏𝑒𝑙. the latter was estimated to be 0.012 cm3 from a study of the manufacturer drawings and several calibrations with the gravimetric method performed on valves of the same type randomly selected. as the volume v100 was also connected to a bellows valve of the flowmeter to be calibrated, 𝑢𝑏𝑒𝑙 was quadratically summed with 𝑢(𝑉𝑥 ) of equation (22). we eventually estimated 𝑢(v100) =0.032 cm 3 (0.031 % in relative value). 6. comparison with the gravimetric method a series of six volume artefacts of 1.5 cm3, 10 cm3, 100 cm3, 300 cm3, 500 cm3 and 2500 cm3 were calibrated to characterise the comparison method with a transfer leak flow rate, in the range 1 to 3000 cm3. the reference value is given by the gravimetric calibration method. table 2 summarises the comparison results: 𝑉𝑛𝑜𝑚 is the nominal value of the calibrated volume, 𝑑 is the difference between http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 348 𝑉𝑞 determined with the studied calibration method and 𝑉𝑔 determined with the gravimetric method: 𝑑 = 𝑉𝑞 − 𝑉𝑔. (25) 𝑢(𝑉𝑞 ) is the standard uncertainty of 𝑉𝑞 and 𝐸𝑛 is the normalised error 𝐸𝑛 = 𝑑 √(2𝑢(𝑉𝑞)) 2 +(2𝑢(𝑉𝑔)) 2 . (26) table 2: comparison results. 𝑉𝑛𝑜𝑚 (𝑉𝑠𝑡−𝑥 ) 𝑑 𝑢(𝑉𝑞 )(𝑢(𝑉𝑞 ) 𝑉𝑞⁄ ) 𝐸𝑛 cm3 cm3 cm3 1.5 (25) 0.032 0.020 (1.3 %) 0.77 10 (25) 0.015 0.024 (0.23 %) 0.28 100 (75) 0.006 0.032 (0.032 %) 0.08 300 (500) 0.010 0.096 (0.032 %) 0.04 500 (300) 0.004 0.15 (0.030 %) 0.01 2500 (500) 1.3 1.0 (0.041 %) 0.66 the nominal value 𝑉𝑠𝑡−𝑥, amongst the available standard volume, which was used for the calibration, is indicated together with 𝑉𝑛𝑜𝑚. the volumes of 300 and 500 cm3 are actually standard volumes which were cross compared. comparison results are satisfactory for all the calibrated volumes with an absolute value of the normalised error less than one. the higher standard uncertainty is for the smaller volumes. it is estimated to be about 2 % for a 1 cm3 volume. 7. summary a volume calibration method implemented using the lcm-lne-cnam constant-volume flowmeter has been characterised. successive measurements of pressure rise rates, due to a steady leak rate, are performed successively in a standard volume and in an unknown volume. from the value of the standard volume and the ratio of pressure rise rates, the unknown volume can be determined. special care taken in measurement and data analysis has led to a standard relative uncertainty between 0.03 % and 2 % for volumes ranging from 1 to 3000 cm3, roughly an order of magnitude higher than capabilities of the gravimetric volume calibration. 8. references [1] euramet e.v., technical committee for flow, guidelines on the determination of uncertainty in gravimetric volume calibration, calibration guide cg-19, version 2.1, 03/2012. [2] p. otal, f. boineau, j.-c. legras, etalonnage des fuites d’hélium: méthodes de référence et dissémination de 10-10 pa·m3·s-1 à 10-2 pa·m3·s-1, revue française de métrologie, vol. 1, n° 17, pp. 3– 9, 2009 (in french). [3] j. setina, b. erjavec, volume determination of a vacuum vessel by pressure rise method, xix imeko world congress, september 6–11, 2009, lisbon, portugal. [4] k. jousten et al., final report of key comparison ccm.p-k12 for very low helium flow rates (leak rates), metrologia, vol. 50, no 1a, p. 07001, 2013, doi: https://doi.org/10.1088/0026-1394/50/1a/07001 [5] d. mari et al., « realization, characterization and measurements of standard leak artefacts », measurement, vol. 61, p. 249–256, 2015, doi: https://doi.org/10.1016/j.measurement.2014.10.045 [6] afnor, “evaluation of measurement data – guide to the expression of uncertainty in measurement”, jcgm 100, paris, afnor, 2008, 121 p. http://www.imeko.org/ https://doi.org/10.1088/0026-1394/50/1a/07001 https://doi.org/10.1016/j.measurement.2014.10.045 investigation of the repeatability of a cleaning method for silicon spheres by gravimetric measurements acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 12 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 12 16 investigation of the repeatability of a cleaning method for silicon spheres by gravimetric measurements m. mecke1, m. borys2, e. beyer3 1-2 physikalisch-technische bundesanstalt (ptb), bundesallee 100, 38116 braunschweig, germany 1 michael.mecke@ptb.de, 2 michael.borys@ptb.de, 3 physikalisch-technische bundesanstalt (ptb), abbestraße 2-12, 10587 berlin, germany edyta.beyer@ptb.de abstract: after a short description of the metrological relevance of silicon spheres and the applied cleaning method, mass measurements of silicon spheres in vacuum and the evaluation of the results are outlined. the results show that the mass of silicon spheres in vacuum stabilises in the microgram range within two to three days after cleaning and that the repeatability of the applied cleaning method can be characterised by a standard deviation in the order of two micrograms. keywords: new si; kilogram; silicon spheres; cleaning method; repeatability; mass stability 1. introduction in the past, silicon spheres were used as primary density standards and for the determination of the avogadro and the planck constants with the smallest uncertainties [1], [2]. since the introduction of the revised si on 20 may 2019, the kilogram is no longer defined as the mass of the international prototype of the kilogram, but is instead derived from a fixed numerical value for the planck constant [3]-[5]. one of the two independent primary methods which allow a realisation of the definition of the kilogram with relative uncertainties of a few parts in 108 is the x-ray-crystal-density (xrcd) method. the silicon spheres which are currently used for the practical realisation of the definition of the mass unit are primary mass standards and are produced from nearly perfect isotopically enriched 28si crystals. relevant influences on the mass of these spheres must be considered for the realisation, maintenance and dissemination of the mass unit [4]. knowledge about their mass stability is required for uncertainty analysis, drift corrections and the determination of appropriate realisation intervals. complementary to 28si spheres, spheres produced from natural silicon (natsi) can be used for the maintenance and dissemination of the kilogram. since the mass stability of silicon spheres is, in addition to the growth of the oxide layer [6], influenced by the sorption of water and carbonaceous layers on the surface of the spheres, appropriate cleaning methods have been investigated and were successfully applied in the past [7]-[11]. in order to characterise the repeatability of the cleaning method used and to obtain reliable data for the assessment of its uncertainty contribution, extensive tests were performed at ptb in the past. the experimental design, results and conclusions are presented in the following sections. 2. gravimetric measurements six test spheres with the identifiers si15-12, sisc01_a, sisc01_b, sisc01_c, sisc01_d and sisc01_e were used for the investigation of the repeatability of the cleaning method. this method starts with rinsing a sphere with distilled water, followed by a manual washing with a soap solution made of deconex op 163 (2 %) and distilled water. the sphere is then placed under a stream of distilled water, rinsed with ethanol and finally dried with a cleaned microfibre cloth [11]. a silicon sphere with the identifier sm14 was selected as a mass reference. the mass stability of this sphere is known from previous measurements and was checked in the course of the measurements by an additional reference sphere named si12-06. the mass of the spheres was compared by means of a mass comparator, type sartorius ccl1007, in a pressure range from 5 × 10-5 hpa to 5 × 10-6 hpa. this mass comparator has a nominal load of 1 kg, achieves a standard deviation in vacuum of less than 0.1 µg and is equipped with a vacuum transfer system (see figure 1), which allows the loading and unloading of the mass comparator without breaking the vacuum. the mass measurements comprised two series, one in april/may 2017 with two to three cleanings and measurements of each sphere and a second one http://www.imeko.org/ mailto:michael.mecke@ptb.de mailto:michael.borys@ptb.de mailto:edyta.beyer@ptb.de acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 13 in september/october 2017 with four to five cleanings and measurements of each sphere. all the spheres were cleaned before the two measurement series and loaded into the mass comparator in air within two hours after completion of the cleaning procedure. the mass comparator was then evacuated and the mass comparisons were performed in vacuum over a period of three to fourteen days. after the measurements, the spheres were transferred under vacuum conditions (in a sealed container) to a surface analysis system, transferred to air by means of a vacuum transfer system, cleaned, loaded into the vacuum transfer system of the mass comparator again (within two hours after completion of the cleaning procedure), and were then transferred under vacuum into the mass comparator. after the completion of the first series and the beginning of the second, i.e. between the end of may and the beginning of september, all the test spheres were stored in their transport containers in air. (a) (b) figure 1: (a) mass comparator used for the gravimetric measurements (type sartorius ccl1007) in a pressure tight enclosure with a vacuum transfer system, (b) silicon sphere on the loading arm of the vacuum transfer system 3. results and discussion each sphere was cleaned and measured between six and eight times (42 cleanings and mass measurements in total). similar to the mass of kilogram prototypes, which are made of a platinumiridium alloy, and the mass of stainless steel weights, the mass of silicon spheres changes according to a non-linear function of time after applying a cleaning procedure with solvents. this non-linear behaviour can be described by the model function [12], [13]: 𝑚(𝑡) = 𝑚𝜏 (1 − 𝑒 − (𝑡−𝑡0) 𝜏 ) + 𝑏(𝑡 − 𝑡0) + 𝑚0 (1) with the mass contribution of the non-linear term 𝑚𝜏, the time constant 𝜏, the slope of the linear term 𝑏 and the mass of the sphere 𝑚0 at the time of the cleaning 𝑡0. in order to get an estimate of the time constant 𝜏 , least-squares adjustments of the parameters of the model equation (1) with measured data for the different test spheres were performed. examples of the time dependence of the mass values of the test spheres are given in figure 2. in order to allow a comparison of the different results in this figure, the data were shifted by the time of the cleaning 𝑡0 and the mass of the sphere at the time of the cleaning 𝑚0 . as a result of the least-squares adjustment, the time constant 𝜏 was determined to be 1.3 days with a standard deviation of 0.5 days and a stability of the mass of the spheres in the microgram range two to three days after cleaning. therefore, the results representing the mass of the spheres between two to three days after cleaning were chosen for the evaluation of the repeatability in order to ensure a good comparability, i.e. no corrections by means of equation (1) were applied. the results of the different spheres shown in figure 2 cover a period of seven to fourteen days. in comparison to the linear term, the non-linear term of equation (1) dominates during this time. therefore, an estimate of the slope 𝑏 of the linear term is not reliable and considered to be not significant regarding its influence on the mass difference measured two to three days after cleaning. however, for the slope 𝑏 a rough estimate of 35 ng/d with a standard deviation of 21 ng/d was determined from the measured data. before the first, between the first and the second and after the second series, the mass of the reference spheres sm14 and si12-06 in vacuum was determined traceable to the national prototype of the kilogram no. 52 in air. a mass stability within ±0.7 µg and ±0.9 µg was observed for the sphere sm14 and si12-06, respectively, with a standard uncertainty of 2.4 µg. the mass stability of the reference sphere sm14 in the course of the measurements in vacuum was determined to be within ±0.30(22) µg in comparison to the second reference sphere si12-06 for both, series 1 and 2. http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 14 for each test sphere and each measurement, the mass difference to the mean value of the measured mass with respect to the reference sphere (sm14) was determined and listed in table 1. a diagram of the results is given in figure 3. the curves illustrate the mass variation of the spheres after different cleanings. the observed difference between the minimum and the maximum deviation from the mean mass value of a single sphere ranges from 1.6 µg to 4.9 µg. the maximum deviation from the mean value observed for all spheres and cleanings amounts to 3.0 µg. by means of a linear regression, a measure for the mass change of each test sphere from one cleaning to the next was estimated. the slope of the mass values observed for the different spheres varies between 0.26 µg and −0.72 µg with a mean of −0.25 µg and a standard deviation of 0.32 µg. figure 2: examples of the measured time dependence of the mass of the test spheres in vacuum after applying the cleaning procedure in air at the time t = t0. all spheres were cleaned before the measurements and loaded into the mass comparator in air within two hours after completion of the cleaning procedure. the last figure in parentheses behind the identifier of the sphere in the legend indicates the number of the measurement in the case that more than one curve is shown for a sphere. table 1: results of the measured mass differences between the test spheres and the reference sphere (sm14). in order to allow a better comparability, the individual mass differences of a sphere are given with respect to their observed mean value (𝑚) and the time since their first cleaning. si15-12 sisc01_a sisc01_b sisc01_c sisc01_d sisc01_e time in d (𝒎𝒊 − 𝒎) in µg time in d (𝒎𝒊 − 𝒎) in µg time in d (𝒎𝒊 − 𝒎) in µg time in d (𝒎𝒊 − 𝒎) in µg time in d (𝒎𝒊 − 𝒎) in µg time in d (𝒎𝒊 − 𝒎) in µg 2.3 1.15 2.4 1.80 2.4 -0.44 1.7 0.67 1.8 0.44 2.6 -0.94 9.1 -0.60 15.5 3.04 15.6 3.03 15.7 1.19 15.8 -0.21 11.9 0.15 21.3 0.15 21.4 -0.26 23.5 0.05 24.7 0.94 134.0 -0.12 128.0 -0.36 134.4 0.63 134.5 -0.71 134.6 -0.60 133.9 0.08 145.7 0.99 140.8 0.45 140.2 0.17 141.5 -0.55 142.3 -0.81 137.6 0.21 153.8 -0.63 147.9 -0.22 148.4 0.28 149.2 -1.41 151.4 -0.20 143.7 -0.57 165.7 -0.48 161.8 0.92 156.1 -0.40 156.2 -1.90 157.1 -1.04 152.6 -1.07 168.2 -1.39 156.4 -1.46 0 0.5 1 1.5 2 2.5 3 0 2 4 6 8 10 12 14 (m m 0 ) / µ g (t t0) / d si15-12 (1) si15-12 (2) sisc01_a (1) sisc01_a (2) sisc01_b (1) sisc01_b (2) sisc01_c sisc01_d sisc01_e (1) sisc01_e (2) http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 15 figure 3: diagram of the data given in table 1 for the measured mass differences between the test spheres and the reference sphere (sm14). the individual mass differences of a sphere are given with respect to their observed mean value (𝑚) as a function of the time since the first cleaning. table 2: standard deviation of the observed mass variations after cleaning given for the individual test spheres and as a pooled standard deviation of all measurements. standard deviation in µg pooled standard deviation in µg si15-12 sisc01_a sisc01_b sisc01_c sisc01_d sisc01_e 0.8 1.8 1.4 1.0 0.6 0.7 1.1 the difference between the mean mass values of series 1 and series 2 amounts to 0.9 µg, which may be caused by a mass change of the reference sphere due to the second cleaning. since this value is in the order of magnitude of the uncertainties of the measured mass differences, which are estimated to be about one microgram (standard uncertainty (k = 1), influence of the repeatability of loading and unloading considered, but without influence of the cleaning process), this is not considered to be a proof. therefore, a correction based on the mass difference between mean values of the series was not applied. in order to provide a measure for the mass variations after the cleanings, the standard deviation of the results was calculated for the individual test spheres and for all the measurements (see table 2). the observed standard deviations are smaller than two micrograms, i.e. the standard deviations are in the order of magnitude of the uncertainties of the measured mass differences. 4. summary and conclusions the cleaning method applied at ptb for the cleaning of silicon spheres was investigated by gravimetric measurements in vacuum. the results show that the mass of silicon spheres in vacuum changes according to a non-linear function of time after applying the cleaning procedure in air. the non-linear behaviour was described by a non-linear model function. based on a least-squares adjustment, the time constant of this function was estimated to be 1.3 days with a standard deviation of 0.5 days. the stability of the mass of the spheres was observed to be in the microgram range two to three days after cleaning. the repeatability of the cleaning method applied at ptb can be characterised by a standard deviation in the order of two micrograms. it can be concluded that this uncertainty contribution is not significant in comparison to the present uncertainty of the realisation of the new definition of the kilogram by means of silicon spheres. 5. acknowledgements the authors would like to thank all their colleagues who were involved in the manufacture of the spheres in the workshop for ultra-precision machining at ptb. -3 -2 -1 0 1 2 3 4 -14 0 14 28 42 56 70 84 98 112 126 140 154 168 d if fe re n c e t o m e a n / µ g time since 1st cleaning / d si15-12 sisc01_a sisc01_b sisc01_c sisc01_d sisc01_e http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 16 6. references [1] b. andreas et al., “determination of the avogadro constant by counting the atoms in a 28si crystal”, phys. rev. lett., vol. 106, 030801-1-4, 2011. [2] p. j. mohr, d. b. newell, b. n. taylor, e. tiesinga, “data and analysis for the codata 2017 special fundamental constants adjustment”, metrologia, vol. 55, pp. 125–146, 2018. [3] resolutions of the 26th meeting of the general conference of weights and measures (cgpm), versailles, 13-16 november 2018. online [accessed 20201029]: https://www.bipm.org/utils/common/pdf/cgpm2018/26th-cgpm-resolutions.pdf [4] mise en pratique for the definition of the kilogram in the new si. online [accessed 20201029]: https://www.bipm.org/utils/en/pdf/si-mep/siapp2-kilogram.pdf [5] bureau international des poids et mesures: the international system of units (si), 9th si brochure, 2019. online [accessed 20201029]: https://www.bipm.org/utils/common/pdf/sibrochure/si-brochure-9-en.pdf [6] m. borys, m. mecke, u. kuetgens, i. busch, m. krumrey, p. fuchs, k. marti, h. bettin, “the growth of the oxide layer on silicon spheres and its influence on their mass stability”, proc. joint imeko international tc3, tc5 and tc22 conference, cape town, republic of south africa, 6 pages, 2014. [7] m. p. seah, s. j. spencer, “ultrathin sio2 on si. i. quantifying and removing carbonaceous contamination”, j. vac. sci. technol. a, vol. 21, no. 2, pp. 345-352, march/april 2003. [8] h. bettin, d. schiel, m. vogtmann, h. niemann, “cleaning of silicon density standards”, in proc. xix imeko world congress, pp. 179-181, 2009. [9] a. picard, p. barat, m. borys, m. firlus, s. mizushima, “state-of-the-art mass determination of 28si spheres for the avogadro project”, metrologia, vol. 48, pp. s112-s119, 2011. [10] n. kuramoto, k. fujii, a. nicolaus, g. bartl, m. gray, p. manson, w. giardini, “diameter comparison of a silicon sphere for the international avogadro coordination project”, ieee transactions on instrumentation and measurement, vol. 60, no. 7, pp. 2615-2620, july 2011. [11] j. w. chung, v. görlitz, m. vogtmann, e. beyer, f. härtig, “the ptb cleaning procedure for silicon spheres”, physikalisch-technische bundesanstalt, braunschweig, 2016. online [accessed 20201029]: https://www.ptb.de/cms/ptb/fachabteilungen/abt1/f b-11/si-kg.html [12] m. gläser, “time dependence of the mass of kilogram prototypes after cleaning”, personal communication, braunschweig, 2003. [13] k. marti, p. fuchs, s. russi, “traceability of mass ii: a study of procedures and materials”, metrologia, vol. 52 pp. 89-103, 2015. http://www.imeko.org/ https://www.bipm.org/utils/common/pdf/cgpm-2018/26th-cgpm-resolutions.pdf https://www.bipm.org/utils/common/pdf/cgpm-2018/26th-cgpm-resolutions.pdf https://www.bipm.org/utils/common/pdf/si-brochure/si-brochure-9-en.pdf https://www.bipm.org/utils/common/pdf/si-brochure/si-brochure-9-en.pdf https://www.ptb.de/cms/ptb/fachabteilungen/abt1/fb-11/si-kg.html https://www.ptb.de/cms/ptb/fachabteilungen/abt1/fb-11/si-kg.html vibration analysis of the oscillation support of column load cells in low speed axle-group weigh-in-motion system acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 63 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 63 68 vibration analysis of the oscillation support of column load cells in low speed axle-group weigh-in-motion system lai zhengchuang1,2, yang xiaoxiang1,3, yao jinhui1,2 1 college of mechanical engineering and automation, fuzhou university, fuzhou, china, lzcfjjl@126.com 2 fujian key laboratory of force measurement (fujian institute of metrology), fuzhou, china, lzcfjjl@126.com 3 quanzhou normal university, quanzhou, china, yangxx@fzu.edu.cn abstract: under the condition of dynamic weighing, the support types of the column load cells can be divided into elastic support and oscillation support. the weighing system with oscillation support shows significant vibration characteristics, which affects the weighing accuracy and the fatigue life of the load sensor. in this paper, the dynamic characteristics of the oscillation supporting column load cell in low speed axle-group weigh-in-motion system are analysed. it is found that the restoring force of the oscillation support is approximately proportional to the oscillation angle and the applied vertical load. the dynamic equation of the oscillation support vibration of the column load cells established by means of d’ alembert principle. the numerical calculations of the dynamic response of the weighing system with oscillation support are carried out in the free state and dynamic weighing state respectively. the factors affecting the amplitude and recovery time of the support vibration are obtained. this study provides a reference for the analysis of the dynamic weighing accuracy. keywords: vibration and wave; oscillation support; vibration analysis; dynamic weighing 1. introduction column load cell is an important component of the axle group dynamic truck scale. under the condition of dynamic weighing, the support types of the column load sensors can be divided into elastic support and oscillation support. the weighing system with oscillation support shows significant vibration characteristics, which affects the weighing accuracy and the fatigue life of the load cells [1] [2]. in this paper, the force analysis is carried out for the weighing process of the oscillation support column load cell. in the dynamic weighing system, the low speed axle group weigh-in-motion (lswim) system is used as the research object to analyse the axle load conditions. the dynamic response of the weighing system is obtained through dynamic system model and simulation. 2. oscillated stability analysis of the load cell oscillated support column load cell consists of strained elastomer and an upper and lower indenter. the lower indenter is fixed on the foundation and is in point-surface contact with the strained elastomer. the upper and lower ends of the elastomer are spherical and can roll on the upper and lower indenters. the upper indenter is fixedly connected to the weighing platform, the bottom surface is in point-to-point contact with the strained elastomer, and its movement state is shown in figure 1. when the elastomer of the load cell is purely rolled, the lower indenter is fixed, and the upper indenter is translated in parallel with the transient of the contact point of the elastomer. let point a and point b be the centre of the spherical surface before and after the lower ball end of the elastomer rolls, and point c be the centroid of the elastomer. according to the geometric relationship of the dimensions in the figure 1 (a) and (b), among them has the same indenter and elastomer. (a) (b) figure 1: oscillation support of the load cell, (a) oscillation position change of the load cell, (b) force analysis of the load cell http://www.imeko.org/ mailto:lzcfjjl@126.com mailto:lzcfjjl@126.com mailto:yangxx@fzu.edu.cn acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 64 in figure 1, 𝛼 is the inclination angle of the sensor strain elastomer axis, ℎ is the height of the sensor strain elastomer along the axis, 𝛽 is the inclination angle of the connection point between the ball head and the upper indenter and the rolling point, 𝑅 is the radius of the ball head. 𝑆 is the horizontal displacement of the axis of the upper indenter. 𝑆1 is the rolling displacement of the contact point between the ball head and the lower head. 𝑆2 is the horizontal movement distance of the contact point between the ball head and the upper indenter. for a stable and balanced oscillated support, the vertical force of the external load 𝐹𝑌 on the elastomer at this time can be decomposed into a radial force 𝐹𝑛 and a tangential force 𝐹𝜏 along the rolling support point o of the elastomer, where the tangential force 𝐹𝜏 provides the restoring force promotes structural stability. when the system is oscillating slightly, tangential force can be expressed as follows: 𝐹𝜏 = 𝐹𝑌 sin 𝛽 ≈ (2𝑅 − ℎ)𝐹𝑌 ℎ 𝛼 (1) it can be known that the restoring force of the micro-oscillated support is approximately proportional to the oscillated angle and the vertical force applied. 3. dynamic model of oscillated support vibration low speed axle-group weigh-in-motion system has four column load cells as carriers. when the vehicle only travels along the longitudinal axis of the weighing platform, the scale body only vibrates in the longitudinal direction of the weighing platform, regardless of the lateral vibration of the weighing platform, and the movement form of the scale body is a single degree of freedom system. it can be seen from equation (1) that its swing stiffness is approximately proportional to the external load and offset angle of each sensor. the dynamic model of its oscillation support vibration is shown in figure 2. let 𝑀𝑂𝑓𝑖 (𝑖 = 1, 2, 3, 4) be the rolling friction couple of the elastomer and the lower pressure head. during the rolling process, the rolling friction resistance moment is approximately equal to the maximum rolling friction resistance moment 𝑀max, which is the vertical pressure 𝐹𝑂𝑌𝑖 on the head bearing point is proportional to the size. let the proportionality constant be 𝛿. 𝑀𝑂𝑓𝑖 ≈ 𝑀max = 𝛿𝐹𝑂𝑌𝑖 (2) the force applied to the weighing platform by the external load is simplified to the lateral force 𝐹𝐺𝑆, the vertical force 𝐹𝐺 and the moment 𝑀𝐺𝑓 . the interaction between the weighing platform and the elastomer of the sensor can be regarded as a simply supported plate structure with point-to-surface contact. at the contact point 𝐾, the elastomer is only affected by the vertical force 𝐹𝑁𝑖 of the weighing platform, the horizontal friction force 𝐹𝑋𝑖 and the rolling friction couple 𝑀𝐾𝑓𝑖. we can also obtain: 𝑀𝐾𝑓𝑖 ≈ 𝛿𝐹𝑁𝑖 (3) (a) (b) figure 2: dynamic model of vibration state of oscillation support, (a) four-point support dynamic truck scale loaded model, (b) force analysis model of column load sensor the swing angle and direction of each column load cells are always consistent with the weighing platform. the total stiffness of the weighing system is equal to the sum of the swing stiffness of each parallel column load sensor. in the static state with the same swing angle, the stiffness of each column load cell is proportional to the vertical force. letting the scale factor be 𝐶, we obtain: 𝑘𝑖 (𝛼, 𝐹𝑁𝑖 ) 𝐹𝑁𝑖 = 𝐶 (𝑖 = 1, 2, 3, 4) (4) 𝑘 = ∑ 𝑘𝑖 (𝛼, 𝐹𝑁𝑖 ) 4 𝑖=1 = ∑ 𝐶𝐹𝑁𝑖 4 𝑖=1 = 𝐶𝐹𝑁 (5) it can be seen from formula (5) that when the vehicle is traveling on the weighing platform, although the force acting on each column load cell is transient, the support vibration of each sensor is synchronised with the vibration of the weighing platform, and the sum of the external forces received by the sensors remains unchanged. taking four column load sensors as the research object, the column load cell takes the torque balance to the http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 65 support point 𝑂 by the lagrange’s d’alembert principle, and the other three load sensors are superimposed to balance the torque at each support point. after the equation, the dynamic control equation of the oscillation support vibration of the weighing system can be obtained: 𝑀𝐾𝑓 + (𝑀𝑔 + 𝐹𝐼2 sin 𝛽 + 𝐹𝐺 ) ∙ 𝑂𝐻 + (𝐹𝐼2 cos 𝛽 − 𝐹𝐺𝑆) ∙ 𝐻𝐾 + 𝑀𝑂𝑓 + 4𝑀𝐼 + 4𝐺 ∙ 𝑂𝐶 sin 𝛽 + 4𝐹𝐼1 ∙ 𝑂𝐶 = 0 (6) where: 𝑀𝐼 = 𝐽�̈�; 𝐹𝐼1 = 𝑚 ∙ 𝑂𝐶�̈�; 𝐹𝐼2 = 𝑀 ∙ 𝑂𝐾�̈� 𝐺 = 𝑚𝑔; 𝑀𝐾𝑓 = 𝛿(𝑀𝑔 + 𝐹𝐼2 sin 𝛽 + 𝐹𝐺 ) 𝑀𝑂𝑓 = 𝛿[𝑀𝑔 + 4𝐺 + (4𝐹𝐼1 + 𝐹𝐼2) sin 𝛽 + 𝐹𝐺 ] in these equations, 𝑚 is the mass of the elastomer, 𝑀 is the mass of the weighing platform, and 𝐽 is the rotational inertia of the elastomer. according to the frictional moment of friction between the ball head and the upper and lower pressure heads, it always hinders the relative rolling of the elastomer, and the direction is opposite to the direction of the elastic angular speed of the elastomer. static friction does not do work during pure rolling. similar to the dissipation effect of coulomb damping on the elastic potential energy of a spring oscillator, free vibration will gradually stabilize near the equilibrium position. substituting the relevant parameters into equation (6), the vibration control equation of the system oscillation support is obtained: 2𝛿 [(1 + 2�̂�)𝑔 + 2(1 + �̂�) ∙ �̂� ∙ �̂̈� ∙ sin 𝛽 + 𝐹�̂� ] + (𝑔 + 𝐹�̂� ) ∙ 𝑙 − 𝐹𝐺�̂� ∙ �̂� + 4𝐽 ∙ �̂̈� + 4�̂� ∙ 𝑔 ∙ �̂� sin 𝛽 + 2[(𝑙 ∙ sin 𝛽 + �̂� cos 𝛽) + 2�̂� ∙ �̂�] ∙ �̂� ∙ �̂̈� = 0 (�̂̇� > 0) (7) among them: �̂� = 𝑚 𝑀 ; �̂� = 𝑂𝐶 𝑅 ; ℎ̂ = ℎ 𝑅 ; 𝛿 = 𝛿 𝑅 ; �̂� = 𝜔0𝑡 𝑔 = 𝑔 𝑅𝜔0 2 ; 𝐽 = 𝐽 𝑀𝑅2 ; 𝑙 = 𝑂𝐻 𝑅 ; �̂� = 𝐻𝐾 𝑅 𝐹�̂� = 𝐹𝐺 𝑀𝑅𝜔0 2 ; 𝐹𝐺�̂� = 𝐹𝐺𝑆 𝑀𝑅𝜔0 2 ; �̂̈� = �̈� 𝜔0 2 ; �̂̈� = �̈� 𝜔0 2 4. analysis of axle load conditions of vehicles 4.1 analysis of axle loading conditions of vehicles running on the road suppose that the vehicle is driven by the rear axle. the axle type is composed of the front axle and the rear axle installed with two axles, and 𝐹𝐺2 = 𝐹𝐺3 . the axle load and wheelbase are shown in figure 3. it is assumed that when the vehicle is driving on the road surface, the front and rear wheels accelerate pure rolling motion under the action of the driving force. the driving wheel is subjected to driving torque, backward traction reaction force, vertical downward axle load, ground friction and rolling resistance. the traction force of the driven wheel is set forward, and the driven wheel is not affected by the driving torque. taking each axis as the research object, considering the tires belonging to a single axis as a whole, suppose the direction of each axis force and the direction of acceleration are shown in figure 4 (a), (b), and the motion parameters of the two drive wheels are consistent, according to the direction of the static friction force of the pure rolling wheel in the figure, the magnitude of the static friction force is determined by the differential equation of motion. (10𝑚𝑉 + 𝑀𝑉 )𝑎𝐶 = 2𝐹𝑆2 − 𝐹𝑆1 (8) 2𝐽𝑉 �̈� = −𝑀𝑓1 + 𝐹𝑆1𝑅𝑉 (9) 𝑎𝐶 = 𝑅𝑉 �̈� (10) among them: 𝑀𝑓1 = 𝛿 ′𝐹𝐺1; 𝑀𝑓2 = 𝛿 ′𝐹𝐺2 𝑀𝑇 = 𝑀2 + 𝑀3 = 2𝑀2; 𝐹𝑛1 + 𝐹𝑛2 + 𝐹𝑛3 = 𝑀𝑉 𝑔 where 𝑀𝑉 is the body mass, 𝑚𝑉 is the mass of a single tire, 𝐹 is the traction force of the car to the front axle, 𝐽𝑉 is the tire moment of inertia, 𝛿 ′ is the rolling friction coefficient between the wheel and the weighing platform, 𝑅𝑉 is the tire radius, 𝑀𝑇 is the total driving torque of the rear axle, �̈� is the tire rolling angle acceleration. figure 3: schematic diagram of axle load and wheelbase of truck http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 66 (a) (b) (c) figure 4: schematic diagram of axle load and wheelbase of truck, (a) rear axle drive wheel, (b) front axle driven wheel, (c) front axle rolls at a constant speed we can obtain from equations (8) to (10): 𝐹𝑆1 = 𝑀𝑓1 𝑅𝑉 + 2𝐽𝑉 𝑎𝐶 𝑅𝑉 2 (11) 𝐹𝑆2 = 𝑀𝑓1 2𝑅𝑉 + (5𝑚𝑉 + 0.5𝑀𝑉 )𝑎𝐶 + 𝐽𝑉 𝑎𝐶 𝑅𝑉 2 (12) similarly, for the pure rolling state of uniform motion, as shown in figure 4(c), the driving force is equal to the static friction force, that is 𝐹 = 𝐹𝑆1 , which can be obtained from the balance relationship: 𝐹𝑆1 = 2𝐹𝑆2 = 𝑀𝑓1 𝑅𝑉 (13) 4.2 analysis of axle loading conditions of vehicles running on weighing platform in order to determine the external disturbance force of the forced vibration of the column load cell oscillation support, the axle group type dynamic truck scale weighing system is selected as the research object. the four-point supported weighing platform performs variable speed movement under the impact of the vehicle, taking the vehicle's driving direction as the positive direction, and when the tire of a certain axis of the vehicle rolls purely on the weighing platform, the angular acceleration of its wheels is: �̈�′ = 𝑎𝐶 − �̈� 𝑅𝑉 (14) the relationship between the tire rolling angle acceleration and the wheel centre translation acceleration of the other axles traveling on the road surface is the same as equation (10). in the same way, by balancing equations for each axis, the magnitude of the static friction force on each axis when the vehicle passes through the weighing platform can be obtained separately. 5. numeric calculations in order to obtain the vibration characteristics of the swing load of the column load sensor, the response of the column load cell is selected as the research object under the dynamic weighing of the axle group dynamic truck scale. the free vibration of the oscillation support of the weighing system and the forced vibration when the vehicle is under dynamic load are selected for simulation when there is no vehicle running. 5.1. numerical simulation of free vibration considering that the ball radius of the elastomer affects the weighing accuracy of the eccentric load of the column load sensor, only the influence of the adjustable elastomer height on the vibration state of the oscillation support is analysed here. in order to eliminate other influencing factors, the oscillation support is set as free vibration, which is not affected by the vehicle load, that is, 𝐹𝐺 , 𝐹𝐺𝑆, and 𝑀𝐺𝑓 are all zero. select the load cell parameters of the column load cell c16a c3 operating manual with a maximum range of 20 t. suppose the rolling friction coefficient of the elastomer and the upper and lower indenters is according to the coefficient between the steel wheel and the rail given by theoretical mechanics, and the relevant parameters are shown in table 1. due to 𝑀 being much larger than 𝑚, when the angle 𝛼 is small, the mass and rotational inertia of the elastomer of the load cell can be ignored, and sin 𝛼 ≈ 𝛼, cos 𝛼 ≈ cos 𝛽 = 1. it can be seen from figure 5 that as the height of the elastomer decreases, the time for the swing load vibration of the column load cell to reciprocate for one week decreases, the amplitude increases, and the vibration attenuation time also increases. table 1: free vibration parameters of oscillation support parameter value unit 𝑚 2.5 kg 𝑅 130 mm ℎ 150 mm 𝛿 0.05 mm 𝑀 4860 kg 𝐽 4.59×10-3 kg·m2 figure 5: numerical simulation curve of free vibration of oscillation support http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 67 5.2. numerical simulation of forced vibration of oscillation support in low speed weighing when forced to vibrate, suppose the weighing system is in equilibrium, and the initial angular displacement 𝛼0 and initial angular velocity are both zero. the dongfeng dfl1250a6 cargo truck (type 1+5) is selected as the rear-wheel drive vehicle with axle load. the rear axle is a dual-axle mounted side by side. the axle load and wheelbase are shown in figure 3. the wheel tires are radial wide rim tires and tires. the specification is 10.00r20, and the tire radius is 𝑅𝑉 = 527 mm. the rolling friction coefficient between the wheel and the scale surface is taken as the middle value 𝛿 ′ = 6 mm, and other relevant parameters are shown in table 2. table 2: force vibration parameters of oscillation support parameter value unit 𝑚𝑉 70 kg 𝐽𝑉 7.56 kg·m 2 𝐹𝐺1 68 000 n 𝐹𝐺2 89 000 n 𝐹𝐺3 89 000 n 𝑙1 4 350 mm 𝑙2 1 300 mm assuming that the vehicle drives through the weighing platform at a constant speed of 𝑣 = 2 m·s-1, 𝑎𝐶 = 0 m·s -2, the axle load and horizontal friction force of each axis of the vehicle acting on the weighing platform at different time periods are almost constant. the following only solves the axle load driving on the weighing platform, and the angular acceleration of the wheels driving on the weighing platform can be obtained from equation (14): �̈�′ = − �̈� 𝑅𝑉 (15) from the static friction formula (13) and formula (15) of constant speed rolling, we can obtain static friction force when the front axle travels on the weighing platform: 𝐹𝑆1 ′ = 𝐹′ = 𝑀𝑓1 𝑅𝑉 − 2𝐽𝑉 �̈� 𝑅𝑉 2 (16) when the front axle is on the weighing platform, the static friction of the rear axle is: 𝐹𝑆2 ′ = 𝐹𝑆3 ′ = 𝐹′ 2 (17) the length of the weighing platform is 5 250 mm, and the driving process of the vehicle on the weighing platform can be obtained according to the vehicle speed and the wheelbase. when the vehicle is just on the scale, set 𝑡 = 0 s, then only the front axle of the weighing platform applies load on the weighing platform, and the time interval until the second axis applies the load is: 𝑡 = 𝑙1 𝑣 = 4.35 2 = 2.175 s (18) the total vertical force applied to the weighing platform 𝐹𝐺 = 𝐹𝐺1, the total friction force is given by equation (16) 𝐹𝐺𝑆 = 𝐹𝑆1 , and rolling friction couple 𝑀𝐺𝑓 = 𝛿 ′𝐹𝐺1. similarly, the change process of the axle load applied to the weighing platform in the remaining time period can be obtained, as shown in table 3. table 3: loading conditions of weighing platform in different time periods time / s 𝑭𝑮 𝑭𝑮𝑺 𝑴𝑮𝒇 0.000 2.175 𝐹𝐺1 𝐹𝑆1 ′ 𝛿 ′𝐹𝐺1 2.175 2.625 𝐹𝐺1 + 𝐹𝐺2 𝐹𝑆1 ′ −𝐹𝑆2 ′ 𝛿 ′(𝐹𝐺1 + 𝐹𝐺2) 2.625 2.825 𝐹𝐺2 −�̅�𝑆2 𝛿 ′𝐹𝐺2 2.825 4.800 𝐹𝐺2 + 𝐹𝐺3 −�̅�𝑆2 − �̅�𝑆3 𝛿 ′(𝐹𝐺2 + 𝐹𝐺3) 4.800 5.450 𝐹𝐺3 −�̅�𝑆3 𝛿 ′𝐹𝐺3 > 5.450 0 0 0 similarly, considering 𝑀 is much larger than 𝑚, when the angle 𝛼 is small, the mass and moment of inertia of the sensor elastomer can be ignored. substituting the above given parameters, the initial angular displacement 𝛼0 and initial angular velocity of the oscillation support vibration are both zero, and the forced vibration solution of the car at a constant speed is shown in figure 6. it can be seen from figure 6 that with the increase of the shaft load, the more significant the vibration of the column load cell oscillation support, the faster the reciprocating motion, the greater the amplitude, and the longer the vibration attenuation time. it can be seen from figure 7 that increasing the flatness of the weighing platform surface and reducing the rolling friction couple to reduce the horizontal static friction between the wheel and the weighing platform can significantly reduce the amplitude of the oscillation support vibration, because the horizontal static friction is much less than the vertical to gravity, the time of one round of reciprocation is almost unchanged, and the decay time of vibration is also shorter. http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 68 figure 6: numerical simulation curve of forced vibration at constant speed driving figure 7: numerical simulation curve of forced vibration at acceleration state 6. summary the oscillation support column load cell shows vibration characteristics during dynamic weighing, which affects the stability and accuracy of weighing, and even affects the fatigue life of the load cell. the conclusions of this study can be used to analyse the dynamic process of the vibration of the load cell oscillation support and the influence of the weight accuracy, and can provide a basis for the corresponding vibration control or vibration reduction design. it can be concluded that: (1) the supporting vibration restoring force is approximately proportional to the vibration angular displacement and load capacity. (2) the recovery time of the oscillation support vibration of the weighing system decreases as the height of the elastomer decreases, the amplitude increases as the height of the elastomer decreases, and the vibration attenuation time increases as the height of the elastomer decreases. the noise interference frequency generated by the horizontal shaking of the scale body will affect the weighing data processing. under the condition of weighing accuracy, the height of the elastomer can be appropriately increased to reduce the vibration swing rate and amplitude. (3) the vibration stiffness of the horizontal support of the weighing platform is related to the magnitude of the load on the weighing platform, and has nothing to do with the distribution of the load. with the increase of the axial load, the vibration recovery time decreases, the amplitude increases, the more significant the vibration, the longer the vibration attenuation time. in general, the limit device is used to limit and consume the vibration of the weighing platform. (4) under the condition of anti-skid, the flatness of the weighing platform surface should be improved, and the rolling friction resistance should be reduced to reduce the horizontal static friction between the wheel and the weighing platform. the amplitude of the oscillation support vibration can be significantly reduced, and the vibration recovery time is almost change, and the faster the vibration decay. 7. references [1] d. rys, j. judycki, p. jaskula, “analysis of effect of overloaded vehicles on fatigue life of flexible pavements based on weigh in motion (wim) data”, international journal of pavement engineering, vol. 17, no. 8, pp. 716-726, 2016. [2] xianzhi dai, “an vibration energy harvester with broadband and frequency-doubling characteristics based on rotary pendulums”, sensors and actuators a: physical, vol. 241, pp. 161-168, 2016. [3] m. c. ciornei, s. alaci, f. c. ciornei et al, “a method for the determination of the coefficient of rolling friction using cycloidal pendulum”, iop conference series: materials science and engineering, vol. 227, no. 1, pp. 12-27, 2017. [4] i. gilavdary, s. mekid, n. riznookaya, “microslippage effects in pre-rolling induced by a disturbed and undisturbed pendulum with spherical supports”, proceedings of the institution of mechanical engineers part j journal of engineering tribology, vol. 228, no. 1, pp. 46-52, 2014. http://www.imeko.org/ design and development of a coaxial cryogenic probe for precision measurements of the quantum hall effect in the ac regime acta imeko issn: 2221-870x june 2021, volume 10, number 2, 24 29 acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 24 design and development of a coaxial cryogenic probe for precision measurements of the quantum hall effect in the ac regime martina marzano1, ngoc thanh mai tran1,2, vincenzo d'elia1, danilo serazio1, emanuele enrico1, massimo ortolano2,1, klaus pierz3, jan kučera4, luca callegaro1 1 inrim istituto nazionale di ricerca metrologica, strada delle cacce 91, 10135, turin, italy 2 politecnico di torino, corso duca degli abruzzi 24, 10129, turin, italy 3 ptb physikalisch-technische bundesanstalt, bundesallee 100, 38116 braunschweig, germany 4 cmi czech metrology institute, okružní 31, 638 00 brno, czech republic section: research paper keywords: quantum hall effect; metrology; impedance; graphene, cryogenic probe citation: martina marzano, ngoc thanh mai tran, vincenzo d'elia, danilo serazio, emanuele enrico, massimo ortolano, klaus pierz, jan kucera, luca callegaro, design and development of a coaxial cryogenic probe for precision measurements of the quantum hall effect in the ac regime, acta imeko, vol. 10, no. 2, article 5, june 2021, identifier: imeko-acta-10 (2021)-02-05 section editor: ciro spataro, university of palermo, italy received december 20, 2020; in final form april 26, 2021; published june 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: giqs: graphene impedance quantum standard is a joint research project, code 18sib07, of the european metrology programme for innovation and research (empir). this project received funding from the european metrology programme for innovation and research (empir) co-financed by the participating states and from the european union's horizon 2020 research and innovation programme. corresponding author: martina marzano, e-mail: m.marzano@inrim.it 1. introduction in the revised international system of units (si) the units of electrical impedance (ohm, henry, farad) can be realised from the quantised hall resistance rh = rk/i, where rk = h/e2 = 25812.807459 3045… ω, the von klitzing constant, is an exact value [1] and i is a small integer (typically, i = 2). the aim of the project giqs: graphene impedance quantum standard (see acknowledgments) is to develop and make available an affordable and easy-to-operate impedance standard exploiting the quantum hall effect (qhe) in graphene. graphene devices are of strong interest for the realisation of electrical units since they display the qhe at lower magnetic fields (below 5 t) and higher temperatures (several k) than semiconductor devices, such as the well-established gaas ones [2]-[5]. the operating conditions can thus be achieved with simpler and less expensive cryogenic systems. nevertheless, the direct measurement of the quantised resistance in the ac regime requires careful considerations about device wiring and shielding [6]-[8], to minimise the effects of stray parameters which can alter the apparent resistance of the device from the quantised value [7]. to date, the direct traceability of capacitance to the qhe has been implemented with a coaxial transformer quadrature bridge, using two independent qhe devices in a twin probe [9]-[11]. the development of high-accuracy digital impedance bridges [12], [13] opens the possibility for simplified implementations. among the goals of the giqs project is the development of abstract the quantum hall effect is the basis for the realisation of the resistance and impedance units in the international system of units since 2019. this paper describes a cryogenic probe that allows to set graphene hall devices in quantisation conditions in a helium bath (4.2 k) and magnetic fields up to 6 t, to perform precision measurements in the ac regime with impedance bridges. the probe has a full coaxial wiring, isolated from the probe structure, and holds the device in a to-8 socket. first, characterization experiments are reported on a gaas device, showing quantisation at 5.5 t. in the ac regime, multiple-series connections will be employed to minimize the residual error, quantified by electrical modelling of the probe. mailto:m.marzano@inrim.it acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 25 impedance standards based on electronic fully-digital impedance bridges [8], [14]-[16] associated to individual qhe devices operating in a simple cryogenic environment. the following describes the realisation of a cryogenic environment for qhe devices, which includes a coaxial cryogenic probe and a superconducting magnet of small size. the environment is suitable to perform qhe experiments at liquid helium temperature of 4.2 k, and for an applied magnetic induction up to 6 t, adequate for graphene qhe devices. the probe will be employed with a new fully-digital coaxial impedance bridge developed in [16]. the bridge is optimised to perform rc comparisons with a 1:1 magnitude ratio, with a comparison uncertainty around 10-7. in combination with the probe, it will allow the calibration of a capacitance standard in terms of rh and therefore the realisation of the unit of capacitance, the farad. the expected target relative uncertainty is 2 × 10-7. 1 the probe frame was fabricated by graphensic ab, sweden, according to inrim specifications. in the following, the probe is described and tested in the dc regime, using a novel gaas sample that shows quantization plateaus for a magnetic induction (5.5 t for the i = 2 plateau) lower than typical devices employed in metrology [7]. the results of the measurement are reported. when employed in the ac regime, the parasitic parameters of the device, the sample holder and the probe will shift the apparent resistance, measured at the end of the coaxial wiring, with respect to the quantized value. the minimization of this shift with the implementation of the multiple-series connection scheme is discussed. 2. the probe a simplified diagram of the probe assembly1 is shown in figure 1. the small size of the probe allows the insertion in a standard dewar having a 50 mm port, such as transport dewars. 2.1. superconducting magnet the probe supports a superconducting magnet (see figure 2) that can reach a field up to 6 t at 4.2 k when energized with a dc current of approximately 65 a. the field homogeneity is 0.1% over a 10 mm length. the magnet includes a superconducting thermal switch, which allows its operation in persistent mode. 2.2. probe insert the probe holds a sliding insert. the bottom side of the insert (figure 3), which enters the magnet bore, mounts a to-8 socket connected with nine coaxial wires2. of these nine wires, eight are intended to be used on the current and voltage terminals of the device; the 9th wire is available for driving a section of the double shield (see below). samples can be mounted on special to-8 sample holders (figure 4) implementing a double-shielding technique [17] that minimises the frequency dependence of the quantised hall resistance caused by unwanted stray capacitances. the socket is wired with the inner conductors of the coaxial cables; the outer conductors are connected to a common node very close to the sample holder. the coaxial wiring (figure 5) is terminated on a connection box at the top of the insert (figure 6) with isolated british post 2 lakeshore ultra miniature coaxial cable, type sc, teflon insulation. the wire has an outer diameter of 1 mm; the inner conductor has a series resistance per unit length of 0.282 ω m-1 at room temperature; the inner to outer capacitance per unit length is 154 pf m-1. figure 1. a computer rendering of the cryomagnetic probe assembly: (left) side view; (right) front view, showing the magnet bore. figure 2. the probe cryomagnet, showing the yellow winding insulation. a header (white) was added to ease the insertion in the cryostat. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 26 office multiple unit steerable array (bpo musa) coaxial connectors. often employed in impedance metrology, bpomusa connectors exhibit a good performance also in the dc regime. the whole coaxial network composed of the device and the wiring is completely isolated from the probe. 2.3. operation figure 7 shows the probe in operation. it is inserted into a 60 l liquid helium dewar. the magnet is energised with a dc high-current power supply (cryogenics ps 120a). a manual current-reversing switch allows to reverse the magnetic field polarity. the superconducting switch in parallel with the magnet is driven by a dc laboratory power supply, which is turned off to activate the persistent mode of operation. the persistent mode maximises field stability and minimizes helium consumption, since the magnet wires are unloaded, and can be used when performing precision measurements on a qhe plateau. 3. testing a test of the probe was made by performing measurement in the dc regime on a gaas sample. 3.1. gaas sample the investigated sample, p151-24, was fabricated at the physikalisch-technische bundesanstalt (ptb) facilities. details of the process can be found in [18]. p151 is a 500 µm semi-insulating gaas wafer on which a gaas-algaas heterostructure was grown by molecular beam epitaxy (mbe). the wafer was cleaved in rectangles of 7 mm × 3 mm, and 8 contacts (two current and six voltage contacts) were made by tin ball annealing. the contacts have a resistance of about 10 mω. p151 samples achieve a mobility of 58.5 t-1 and a carrier concentration in the two-dimensional electron gas n = 2.67 × 10-15 m-2. this concentration corresponds to a i = 2 quantum hall plateau at the magnetic induction b = nh/ie = 5.5 t, where h is the planck constant and e the electron charge. such magnetic induction can be compared with the typical induction required by ubiquitous lep samples [19], figure 3. the bottom side of the insert, which slides into the cryomagnet bore. (left) diagram of the drum that supports the sample holder and hosts the 9 coaxial connections. (right) photo of the assembled probe: the brass shield (right) has been removed to show the to-8 socket, which hosts a gaas device on its to-8 holder, and the 9 coaxial electrical lines. figure 4. to-8 sample holders, implementing the double-shielding technique [8], [17]. after the bonding of the quantum hall device onto the holder (right), the shielding cap (left) slids into the socket. figure 5. schematic diagram of the coaxial connections of the probe. nine coaxial connections are available, fully isolated from the probe metal bulk. the outer conductors of each line are joined together on the sample holder (pin 6). figure 6. the probe connection box. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 27 of about 9 t. tests at t = 2.2 k and i = 39 µa show full quantization to parts in 109, and a critical current density of 6 × 10-2 a m-1 (30 µa over 0.5 mm) at t = 1.2 k [18]. at t = 4.2 k, i = 77 µa, the relative deviation from the exact quantization is about -0.6 × 10-6. the sample is mounted on an unshielded to-8 holder with soldered pt wires. 3.2. measurements the device is driven by a purpose-built, isolated and batteryoperated dc current source. the source can be manually operated to deliver the current values i = 0 µa, ± 5 µa, ± 20 µa, ± 50 µa and ± 100 µa, and includes a fast protection circuit in case of device thermal runaway. the dc voltage on selected contacts is measured with a two-channel nanovoltmeter (agilent 34420a). figure 8 reports the outcome of the experiment. figure 8(a) is the graph of the hall resistance rh(b) versus the applied magnetic field b, measured with a current i = 20 µa. figure 8(b) shows the corresponding longitudinal resistance rxx(b). curves obtained with increasing and decreasing b are shown; a little hysteresis, related to the sweep rate (about 0.6 t min-1) can be appreciated. increasing the current up to i = 50 µa leads to similar results (not reported). in figure 8(a) quantum hall plateaux corresponding to filling factors i = 2 and i = 4 can be easily identified, and higher-index plateaux can be appreciated. figure 8(b) shows the corresponding shubnikov-de haas oscillations. on the i = 2 plateau, rh is flat over a range of about 0.2 t; the corresponding rxx is lower than 50 mω. the outcome of the experiment is consistent with the expectations on a gaas device. 4. ac quantum hall effect in the dc regime the quantum hall resistance is defined as a four-terminal (4t) resistance [20]. if the 4t definition is applied (no current is drawn from the voltage terminals by the measurement setup) the cable errors are due to the wiring parasitic conductances, which can be made negligible with adequate isolation. in the ac regime, the four terminal-pair (4tp) impedance standard definition is the most accurate [20, 21]. 4tp impedance definition is however impractical for the quantum hall resistance. since the output impedance of the voltage terminals of a qhe device is of the order of rh, the parasitic admittances of the cables give rise to very large errors at both the current and the voltage terminal-pairs. attempts to solve the problem by triaxial connections and active guards were not completely successful [22]. the peculiarity of the quantized hall state as a circuit element [23] allows to exploit the so-called multiple-series connections [24]. such connections redefine the quantum hall resistance as a twoterminal resistance (in the dc regime) or a two terminal-pair (2tp) impedance (in ac) by keeping the magnitude of the cable correction errors to very low values. in the dc regime, the behaviour of multiple-series connections has been extensively considered [24], and dedicated modelling tools for the electrical analysis of the connections are available [25]-[27]. in the ac regime, multiple-series connections have been also implemented, although in a limited number of setups [3, 6-8]. the connections schematics are shown in figure 9. for the case n = 1, no multiple-series schematic is employed, and the device is simply connected as a 2tp impedance with two coaxial leads. figure 7. overall view of the measurement setup for dc characterization. on the left, the liquid helium dewar, with the coaxial probe inserted. on the right, the rack of electronic instrumentation. (a) rh(b) (b) rxx(b) figure 8. plot of the measured values of rh and rxx versus the magnetic field b, for an applied current i = 20 µa. blue line: increasing magnetic field. red line: decreasing magnetic field. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 28 the case n = 2 and n = 3 are called double series and triple series, respectively. cases with n>3 can be conceived but are not analysed here. the electrical modelling of multiple-series connections in the ac regime is much more complex than the dc case, and yet to be fully developed. here, we apply an approximate model due to schurr et al. [28]. as can be seen from figure 10, each coaxial wiring is modelled as a t network with series impedance zw (mainly due to the series resistance of the inner conductor, which can be relatively high – 1 ω or greater – in cryogenic coaxial leads) and a parallel admittance yw (due to the cable capacitance and loss); a contact resistance rc models the bonding and the device junctions. the quantity of interest to be estimated is the relative deviation δzh caused by the connections 𝛿𝑍h = 𝑍h − 𝑅h 𝑅h , (1) where rh is the quantized hall resistance, and zh is the apparent two terminal-pair (2tp) impedance as seen by a measuring instrument at the extremes of the connections outside the cryostat. the prediction of the model is 𝛿𝑍h ≈ [ 𝑍w𝑌w 2 + ( 𝑅c + 𝑅w 𝑅h ) 𝑛 ] 2 . (2) the quantities zw and yw can be estimated from the cable manufacturer specifications: in the following, the calculation is done for a 1.7 m long lakeshore ultra miniature coaxial cable and a 0.3 m long rg58 coaxial cable. the quantity rc is strongly dependent on the individual device employed in the experiment, and to some extent it can also vary for the same device in different cooling processes. we therefore computed the outcome of equation 2 for two extreme cases, namely rc = 10 mω and rc = 10 ω. table 1 summarizes the result of the calculations performed at the frequency f = 1541 hz, which is of particular interest for the realisation of the farad unit from the quantized resistance [16]. it can be appreciated that, even for the case of a high contact resistance, a triple-series connection reduces the measurement error to about one part in 108. the triple-series connection will be thus employed in the experiment. the residual error can be corrected if accurate measurements of the stray parameters are available. 5. conclusions the test shows that the probe can be employed to reach the quantization condition in hall devices, and sensitive dc measurements can be performed. the probe is ready to be employed with a fully-digital coaxial impedance bridge designed for the calibration of a capacitance standard in terms of rh with an uncertainty of a few parts in 107. in combination with the probe, the bridge is therefore suitable for the realisation of the unit of capacitance, the farad. the effects of stray parameters will be minimized by exploiting the triple-series connection technique, which reduces the connection errors to around one part in 108. the probe is intended to be used with graphene single devices and arrays [29], to be developed in the frame of the giqs project. in the meantime, the gaas device here investigated will allow to perform first tests of the bridge in operating conditions. measurements in the ac regime will be performed with a fully-digital bridge [16], using the triple-connection series; the approximate electrical modelling of the connections reported in the paper predicts a maximum connection error of a few parts in 108. a more accurate modelling of such error, in progress, will allow to perform a correction of the measurement reading. table 1. calculation of the real part of the relative error δzh occurring when measuring a qhe device by using the n-series connections given in figure 9, for the frequency f = 1541 hz. the calculation is given for two different contact resistance values. re[δzh] n rc = 10 mω rc = 10 ω 1 +1.0 × 10-4 +1.7 × 10-3 2 -5.6 × 10-9 +1.4 × 10-6 3 -1.1 × 10-8 -9.5 × 10-9 figure 9. n-series connection of a qhe device. (top) n = 1, no multiple-series connection: the device is connected as a 2tp impedance by two coaxial leads. (middle) n = 2, double-series connection. (low) n = 3, triple-series connection. figure 10 electrical modelling of one of the connections to the qhe device. in the ac regime, each connection is modelled as a t network with series impedance zw and a parallel admittance yw. the contact resistance rc models the bonding and the device junctions. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 29 acknowledgement giqs: graphene impedance quantum standard is a joint research project, code 18sib07, of the european metrology programme for innovation and research (empir). this project received funding from the european metrology programme for innovation and research (empir) co-financed by the participating states and from the european union's horizon 2020 research and innovation programme. regular updates about the project are posted on the project webpage, ptb.de/empir2019/giqs/home/ and a linkedin group, linkedin.com/groups/8824119. lc and vde thank cristina cassiago and enrico gasparotto, inrim, for helping with a first 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[1]. when examining optimization, it is necessary to separate static processes, in which the property is time invariant, and dynamic processes, in which the property is time variant. taguchi et al. [2] proposed an online quality control procedure for dynamic processes by which the checking interval and adjustment limit are economically optimized. (the checking interval corresponds to the calibration interval for the control of a measuring device. the calibration does not contain the adjustment of the measuring device in this case.) however, the procedure is approximately derived, so that the optimization is not precisely given even when the property is in a randomwalking process, which is the simplest stochastic process. adams and woodall [3] confirmed the property of taguchi’s procedure and proposed a more accurate optimization procedure for random-walking processes. srivastava and wu subsequently reported a simpler solution than that of adams and woodall [4], followed by an improved version [5]. box and kramer [6] reported a procedure applicable to a more complex type of stochastic process. however, there has been little discussion of the adjustment uncertainty and time lag in the previous reports. with regard to the adjustment uncertainty, when a process is adjusted by replacing some parts, it is uncertain whether the idealistic property is realized. environmental or human factors can be sources of an adjustment uncertainty. the time lag refers to the difference between the time at which an item is produced or employed and the time at which the item is checked. this was dealt with in taguchi’s procedure in taguchi et al. [2]. on the other hand, no time lag was discussed in the above studies, so the effect is unclear even in a random-walking process. of course, there is some degree of time lag in a realistic process. clarifying the effects of these two parameters on optimization is, therefore, a prerequisite in order to apply the optimization procedure to a realistic process. abstract taguchi’s online quality control aims at the optimization of the checking interval and adjustment limit of a process from the economic point of view. (the checking interval corresponds to the calibration interval for the control of a measuring device.) the present study provides a mathematical formalism and an efficient computation method to take adjustment uncertainty and time lag into consideration in the optimization of a random-walking property. in cases where either the checking cost or the adjustment cost is free, the approximate analytical solution is given. qualitatively, a larger uncertainty of adjustment yields a shorter optimized checking interval and a larger adjustment limit. on the other hand, a larger time lag yields a smaller checking interval and adjustment limit. information of approximate equations and a data set useful for efficient computation is provided. acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 14 in the present study, the effects of the adjustment uncertainty and time lag on optimization are expressed mathematically and an algorithm to optimize the checking interval and adjustment limit is given. moreover, a case in which the cost of checking is free, which seems interesting in terms of reality, as well as a case in which the cost of adjustment is free, are investigated here. in actuality, analytical solutions can be given for these cases. in the present study, only simple random-walking processes are discussed. processes with a shift due to assignable causes, e.g., lorenzen and vance [7], banerjee and rahim [8], nayebpour and woodall [9], surtihadi and raghavachar [10], and wu et al. [11], are not within the scope of our interest here. measurement uncertainty in checking is also ignored. when measurement errors are constant, the effect of a measurement uncertainty can be assessed as in taguchi’s procedure described in taguchi et al. [12]. although cusum or ewma, which are presented in zhang [13], reynolds and stoumbos [14, 15], and other reports, might be useful to attenuate random measurement errors, the application of these techniques raises complexity in the optimization procedure. the discussion of processes with measurement uncertainty will be a future task. this paper is organized as follows: in section 2, the symbols employed in this paper are listed. section 3 gives the mathematical formalism of the optimization problem including the effects of the adjustment uncertainty and time lag. sections 4 and 5 show the numerical procedure for the optimization and its application, respectively. section 6 provides analytical solutions for cases in which the checking cost or the adjustment cost is free. the contents of the paper are briefly summarized in section 7. appendix 1 gives an explanation of the derivation of the equation in the main text. information of approximate equations and a data set useful for efficient computation is provided in appendix 2. appendix 3 explains the derivation of the equation as well as appendix 1. a data set useful for efficient computation is also provided as an online appendix. 2. symbols t: time elapsed since the last adjustment. x = xt: difference from the targeted value at time t. t: checking interval. d: adjustment limit. i: number of checkings since the last adjustment. e: error in an adjustment (x0 = e). : population standard deviation of e. t: time lag, meaning the time lapse between t = kt and the adjustment time. b: normalized brownian motion ((db)2 = dt). : diffusion coefficient. e[.]: operator for expectation with respect to both b and e. k: number satisfying |xkt| > d. cq: loss (= cost of poor quality) during t = 0 ~ kt. cl: loss during time lag (t = kt ~ kt + t). cc: cost of a single checking. ca: cost of a single adjustment. ct: total cost during t = 0 ~ kt+t. l: expected cost per unit time. t*: optimum checking interval. d*: optimum adjustment limit. l*: optimum expected cost per unit time. : dimensionless adjustment limit equal to d2/2t. : dimensionless standard deviation of adjustment error equal to /d. f(, ): dimensionless expectation of time between adjustments equal to 2/d2·e[kt]. g(, ): dimensionless expectation of integration of xt2 with t during t = 0 ~ kt equal to 2/d·e[∫ kt0 xt2dt] . h(, ): dimensionless expectation of square of measured value in the last checking equal to e[xkt2]/d2. n: iteration number in computation procedure. n: maximum iteration number in computation procedure. r0: random variable derived from the normal distribution with a mean of 0 and a variance of 2. r: random variable derived from the normal distribution with a mean of 0 and a variance of 1. d0: tolerance limit. l0: cost per time when the property is out of the tolerance limit. d: tentative adjustment limit for the parameter determination. t: tentative checking interval for the parameter determination. arl(d, t): average run length with d and t. p: approximate conditional probability such that |xit| ≤  on the condition that |x(i-1)t| ≤  (1 ≤ i ≤ k－1). 3. mathematical formalism since there is an uncertainty in adjustment, the difference from the targeted value, x, at t = 0 is not 0 but e, which is derived from the normal distribution with a mean of 0 and a variance of 2. it is assumed that x is along with a random walk for t > 0. in other words, the variation of x, dx, is given by the following stochastic differential equation: dbdx  . (1) b is a normalized brownian motion such that (db)2 = dt. note that it is not necessary for t to be actual time. in a case where the property varies with the number of produced items, t can be the number of items. the difference, x, is checked at t = it. when |xit| ≤ d, the process is not adjusted. after the k-th checking satisfying |xkt| > d, the process is stopped and the difference is adjusted. although the magnitude of the difference at t = kt, xkt, is found to be larger than d, the process cannot be adjusted at t = kt but at t = kt + t because of the existence of the time lag, t. this is a process realized by a brownian motion, b. examples of a process without and with the adjustment error and the time lag are shown in figure 1. a loss during an interval is expressed by the integration of the loss function proportional to the square of the difference, x2, as applied by srivastava and wu [4, 5]. hence, the loss during t = 0 ~ kt is given as follows:  kt dtxqc 0 2 q . (2) the loss during the time lag, t = kt ~ kt+t, is also given as follows:    tkt kt dtxqc 2l . (3) the total cost during t = 0 ~ kt+t, ct, is given as ct = cq + cl＋kcc + ca. what is minimized in the optimization is the expectation of cost per the expectation of time, l = e[ct]/ e[kt+t]. it should be noted that this expectation operator, e[.], does not mean the expectation only with respect to b but also with respect to e. acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 15 the expectation of the loss during the time lag, i.e., t = kt ~ kt + t, is given as     2222l 2 t q txqedtxqece kt tkt kt          . (4) see appendix 1 for a detailed derivation of this equation. thus, l can be expressed as follows:           tkte c t c ktet q txqedtxqe tkte ce l kt kt            ac2220 2 t 2  . (5) due to the time lag, the above equation is more complicated than the corresponding equation (22) in box and kramer [6]. l is the function of the combination of (d, t), and the optimization means the determination of the combination of (d*, t*) by which l is at its minimum. 4. computation d and t are the parameters of l, and it is necessary to compute equation (5) to obtain the optimized combination of (d*, t*). the calculation of the three expectations e[kt], e[∫ kt0 xt2dt], and e[xkt2] is a computationally heavy task and it is not realistic to directly conduct the numerical integration involving these expectations every time d and/or t changes in the optimization procedure. to avoid this, the key is to transform these three expectations as follows:      , 2 2 f d kte  , (6)    , 2 4 0 2 g d dtxe kt        , (7)    ,22 hdxe kt  , (8) where  = d2/2t and  = /d are dimensionless numbers considered as the dimensionless adjustment limit and the dimensionless standard deviation of the adjustment error, respectively. once f, g, and h are computed for the various combinations of (, ) and references are prepared with tables or approximate equations, the burden of computation of the expectations is considerably reduced by avoiding direct numerical integration. in the present study, a computation procedure entailing the following five steps was conducted to obtain the values of f, g, and h with n = 1000: 1) set  and . t = 1/. n = 0. 2) n = n + 1. derive r0 from the normal distribution with a mean of 0 and a variance of 2 where x0 = r0. 3) repeat to derive r from the normal distribution with a mean of 0 and a variance of 1 where xjt = x(j-1)t + √t·r until |xjt| > 1 4) k = j. fn = kt, gn = ∑ 1 0   k i {(t/3)·(xit2+ x(i+1)t2+ xit·x(i+1)t) +2t2/6} and hn = xkt2. go to 2) until n = n. 5) f(x,) = (1/n)·∑ 10   k i fn, g(x,) = (1/n)·∑ 1 0   k i gn, and h(x,) = (1/n)·∑ 10   k i hn. see srivastava and wu [4] (the third equation on p. 2152) for the calculation of gn in step 4). although in the case of  = 0, approximate equations for f and g (or those with minor mathematical differences) have been given in previous studies such as adams and woodall [3], srivastava and wu [4, 5], box and kramer [6], and box and jenkins [16], f and g with  = 0 are also recalculated in this study. the approximate equations for f, g and h are provided in appendix 2. the approximate equations are applicable for 0.1 <  < 10 and 0 <  < 1. moreover, since the utilization of tables and second-order interpolation seems to be helpful in obtaining more precise values for f, g, and h than approximate equations with a small number of parameters, tables were made for the range of 0.001 <  < 1000 and 0 <  < 1 with smoothing of the computed results. the information on the reference tables for log(f), log(g), and log(h) are also provided in appendix 2. once the (a) (b) figure 1. examples of a random-walking process (a) without and (b) with the adjustment error, e, and time lag, t. the open circles denote the starting points and the filled circles denote the checking points. acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 16 values of f, g, and h are obtained, l can be derived from equations (5)–(8) as follows:         tfd c t c f d tqthqdg d q l                       , , 2 1 ,, 2 2 a c 2 2 222 2 4 . (9) based on this equation, we can obtain (d*, t*) satisfying both ∂l/∂d|d = d* = 0 and ∂l/∂t|t= t* = 0 by an appropriate numerical method, such as newton's method. 5. exemplification 5.1. calculation conditions the example in subsection 23.3 in taguchi et al. [12] and modified cases are employed in the exemplification in the present study. although this example is basically for the property control of products, the mathematical formalism is the same as that in the property control of a measurement device. it should be noted that the symbols in this study are different from those in taguchi et al. [12]. since the parameters of q and  are not employed in the original example, it seems preferable to touch on the derivation method of them here. according to taguchi et al. [12], the parameter q can be determined as follows: 2 0 0 d l q  , (10) where d0 is the tolerance limit for the property, and l0 is the cost per time when the property is out of the tolerance limit. although,  = 0 is the prerequisite in the parameter determination of  in taguchi et al. [12], the determination method can be modified for the case of  > 0 as follows:     tdarl d , 22 2  , (11) where d and t is the tentative adjustment limit and checking interval for the parameter determination respectively, and arl(d, t) is the average run length with d and t. however, this parameter determination is appropriate only when t << d2/2. to determine , it might be generally applicable to employ the relationship between the variation in time, (ti－tj), and the variation in difference, (xi－xj), that is e[(xi－xj)2] =  |ti－tj|. the calculation conditions are summarized in table 1. case 1 is the same example as that in subsection 23.3 in taguchi et al. [12], while cases 2 and 3 are examples emphasizing the adjustment uncertainty and time lag, respectively. the minimizations of equation (9) for all the cases are conducted with f, g and h obtained from the reference tables and secondorder interpolation. the information on the reference tables for log(f), log(g), and log(h) are provided in appendix 2. 5.2. calculation results the calculation results are also summarized in table 1. the optimized combination for case 1 is (d*, t*) = (2.98 m, 288 units). this differs from the optimized combination of (3.80 m, 201 units) by taguchi’s procedure in taguchi et al. [12]. this result shows the necessity of taking the randomwalking behaviour of the property into consideration for accurate optimization. on the other hand, adams and woodall’s procedure using equation (4.2) in adams and woodall [3] gives the optimized combination of (2.91 m, 275 units). this implies that our procedure is basically the same as that of adams and woodall, because the slight gap depends on the marginal difference in the mathematical expression and the accuracy of the approximate equation. comparing the results of case 2 with case 1, the effect of the adjustment uncertainty can be confirmed. qualitatively, the larger the adjustment uncertainty is, the larger the optimized adjustment limit is and the smaller the optimized checking interval is. it is interesting that the larger adjustment uncertainty derives a larger adjustment limit, because the larger adjustment limit basically means looser control. the optimized cost per time, l*, becomes larger with a larger adjustment uncertainty, so it can be said that the adjustment uncertainty has a negative table 1. calculation conditions and calculation results. case 1 case 2 case 3 calculation conditions  /m·unit-1/2 0.144 q /$·m-2 0.003556 cc /$ 1.5 ca /$ 12  /m 0 1.0 0 t /unit 1 1 50 calculation results d* /m 2.98 3.14 2.85 t* /unit 288 278 281 l* /$·unit-1 0.0342 0.0356 0.0361 acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 17 impact in terms of the long-term cost. comparing the results of case 3 with case 1, the effect of the time lag can be confirmed. qualitatively, the larger the time lag is, the smaller both the optimized adjustment limit and checking interval are. the trend of the optimized adjustment limit differs from that of case 2. in terms of the long-term cost, the time lag has a negative impact as does the adjustment uncertainty. figure 2 shows the contours of the cost per time, l, for cases 1 and 2. the qualitative features in both cases are basically the same. it can be seen that the gradient of the variation of l is small in a certain direction. in other words, when making d larger and t smaller or d smaller and t larger from (d*, t*), the variation of cost is relatively insignificant. when certain conditions are imposed on the combination of (d, t), the utilization of a contour might be helpful to determine the optimized combination. 6. analytical solutions for specific cases 6.1. case of cc = 0 when the checking cost is essentially zero (or simply pushed aside), the difference, x, will be constantly monitored. this realistically means the total inspection of products or the everyday checking of a measuring device. although this case has been discussed in previous studies such as adams and woodall [3], srivastava and wu [4, 5], and box and kramer [6], the effects of the adjustment uncertainty and the time lag have not yet been clarified. on the condition of  << d, calculation of two of the expectations, e[kt] and e[∫ kt0 xt2dt], can be replaced by solving the corresponding differential equations as shown in øksendal [17]. the solutions of the corresponding differential equations are easily obtained: e[kt] = (d2－2)/2 and e[∫ kt0 xt2dt] = (d4 －34)/62. since e[xkt2] = d2 is obvious, l in equation (5) can be given as follows:   td ctqtqddq l    222 a 2242244 2 1 3 6 1   . (12) on the additional condition of t << e[kt] and t << d2/2, d* satisfying ∂l/∂d|d = d* = 0 can be approximately given as follows: t q c d a          22 212 * 6   . (13) this is the exact solution in the case of  = 0 and t = 0. moreover, it is worth noting that in the case of  = 0 and t = 0, the distribution of x is an isosceles triangle with a range from x = － d to x = d. as suggested in the numerical exemplification in section 5, equation (13) also implies that the adjustment uncertainty makes the optimized adjustment limit large. 6.2. case of ca = 0 according to miyagawa [18], when |xit| ≤ , the adjustment makes the variance of x larger. this gives rise to the phenomenon called hunting. adjusting the process after every instance of checking is not the best means of reducing the variance and the cost per time, even when the adjustment cost is zero or pushed aside. to prevent hunting, the process should be adjusted only after the checking when |xit| > . thus, when the adjustment cost is zero or pushed aside, the optimized adjustment limit, d*, is not 0 but . concerning the optimized checking interval, t*, although srivastava and wu [4] reported an analytical equation for this case when  = 0 and t = 0, the effects of the adjustment uncertainty and time lag have not yet been clarified. when 2 << 2t, the conditional probability such that |xit| ≤  (i = 1, 2, …, k－1) on the condition that |x(i-1)t| ≤  is considered almost constant irrespective of i. if the conditional probability is set as p (p << 1), l in equation (5) can be approximated as follows: (a) (b) figure 2. contours of the cost per time, l, with the calculation conditions of (a) case 1 and (b) case 2 in table 1. acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 18     t p t p c t q p ttq p tq l          1 12112 c22 222   . (14) see appendix 3 for a detailed derivation of this equation. when t << t, it is easily found that the value of p is not significant to l because p << 1. on the condition that 2 << 2t and t << t, t* satisfying ∂l/∂t|t= t* = 0 can be derived approximately as follows: t q c t  2 c* 2  . (15) it might be unexpected that  does not have an effect on the determination of t* according to equation (15). this is the exact solution in the case of  = 0 and t = 0. 7. summary this paper offers a theoretical formalism and an efficient numerical computation algorithm for taking the adjustment uncertainty and time lag into consideration in an online quality control procedure with a random-walking process model. although the adjustment uncertainty and time lag have been disregarded in most of the previous studies, these two parameters can have an impact on a realistic process. qualitatively, it can be said that a larger adjustment uncertainty derives a larger optimized adjustment limit and a smaller optimized checking interval, while a larger time lag derives a smaller optimized adjustment limit and checking interval. references [1] j.b. keats, e. castillo, e.v. collani, e.m. saniga, economic modeling for statistical process control, j. qual. tech. 29 (1997) pp.144–147. 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[9] m.r. nayebpour, w.h. woodall, an analysis of taguchi's online quality monitoring procedures for attributes, technometrics, 35 (1993) pp. 53–60. [10] j. surtihadi, m. raghavachar, exact economic design of x charts for general time in-control distributions, int. j. prod. res. 32 (1994) pp. 2287-2302. [11] z. wu, m. shamsuzzaman, e. s. pan, optimization design of control charts based on taguchi’s loss function and random process shifts, int. j. prod. res. 42 (2004) pp. 379–390. [12] g. taguchi, s. chowdhury, y. wu, taguchi’s quality engineering handbook, john wiley & sons, inc, new jersey, 2004, isbn 0-47-141334-8. [13] n.f. zhang, a statistical control chart for stationary process data, technometrics 40 (1998) pp. 24–38. [14] m.r. reynolds jr., z.g. stoumbos, should exponentially weighted moving average and cumulative sum charts be used with shewhart limits?, technometrics 47 (2005) pp. 409–424. [15] m.r. reynolds jr., z.g. stoumbos, comparisons of some exponentially weighted moving average control charts for monitoring the process mean and variance, technometrics 48 (2006) pp. 550–567. [16] g.e.p. box, g.m. jenkins, “further contributions to adaptive quality control: simultaneous estimation of dynamics: non-zero costs”, in: bulletin of the international statistical institute, proceedings of the 34th session ottawa 1963, h. segal (editor), university of toronto press, toronto, 1964, pp. 943–974. [17] b.k. øksendal, stochastic differential equations: an introduction with applications (in japanese), springer japan, tokyo, 1999, isbn 4-43-170804-9. [18] m. miyagawa, technology for getting quality–what the taguchi method has brought us (in japanese). union of japanese scientists and engineers (juse), tokyo, 2006, isbn 4-81-710339-6. appendix 1: derivation of equation (4) the expectation of cl, e[cl], can be expressed as follows:     2 l 2 , kt t kt kt t kt t ktkt e c e q x dt qe q x y dt              (a1) where yt is the stochastic process satisfying y0 = 0 and dyt = db. equation (a1) can be decomposed into two terms as follows:   2 2l 2 2 0 , kt t kt t kt t ktkt kt t kt t e c qe x dt qe y dt qe x t qe y dt                       (a2) because e[∫ tktkt  xkt·ytdt] = e[xkt]·e[∫ t 0 ytdt] and both e[xkt] and e[∫ t0 ytdt] are 0. as found from the equation for gn in step 4) of section 4 in the main manuscript,   2 2 2 2 2 0 0 2 2 2 2 2 3 6 . 3 6 2 kt t tkt t t t e y dt t t e y y y y t t t e y                              (a3) note that y0 = 0 and e[yt2] =  t. substituting equation (a3) into equation (a2) yields     222l 2 t q txqece kt   . (a4) this is equation (4). appendix 2: approximate equations and reference tables the approximate equations for f, g and h are provided in table a1. it should be noted that the approximate equations are applicable only for the range of 0.1 <  < 10 and 0 <  < 1. the reference tables for log(f), log(g), and log(h) for the range of 0.001 <  < 1000 and 0 <  < 1 are provided in the online appendix at http://acta.imeko.org/index.php/acta-imeko/ article/view/imeko-acta-02%20%282013%29-02-04/198. in the calculation in the main manuscript, the reference tables and second-order interpolation are employed. acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 19 appendix 3: derivation of equation (14) computation of the three expectations e[kt], e[xkt2], and e[∫ kt0 xt2dt] is the key to the derivation of equation (14). as described in the main manuscript, when 2 << 2t, the probability that |xit| ≤  (i = 1, 2, …, k－1) on the condition that |x(i-1)t| ≤  is considered to be almost constant irrespective of i. that probability is set as p (p << 1). the probability such that |xit| ≤  (i = 1, 2, …, k－1) and |xkt| >  is therefore given as pk － 1·(1 － p). hence, e[kt] can be calculated as follows:     p t ppktkte k k      1 1][ 1 1 . (a5) regarding e[xkt2], the distribution of xkt is almost approximated to the normal distribution with a mean of 0 and a variance of 2t, but it should be kept in mind that the magnitude of xkt is larger than . let the variance of the value that is derived from the normal distribution with a mean of 0 and a variance of 2t and the magnitude of that is larger than  be p2. it can be said that 2t ≈ p2p + e[xkt2](1－p). since the first term on the right-hand side is marginal compared with the second term, the following expression can be derived: p t xe kt   1 ][ 2 2  . (a6) as found from the equation for gn in step (4) of section 4 in the main manuscript, e[∫ kt0 xt2dt] is given as the sum of e[t/3·(xit2+ x(i+1)t2+ xit·x(i+1)t)+2t2/6] for i = 1, …, k－1. because xit2 < 2 (i = 1, …, k－1) and 2 << 2t, e[t/3·(xit 2+ x(i+1)t2 + xit·x(i+1)t)] is negligibly smaller than 2t2/6 except when i = k － 1. e[xkt2] cannot be ignored but should be assessed by equation (a6). thus, e[∫ kt0 xt2dt] can be approximated as follows:       2 2 2 2 0 2 2 2 2 2 3 6 . 3 1 6 2 1 kt kt t t e x dt e x e k e ktt t t p p                       (a7) substituting equations (a5) to (a7) into equation (5) results in equation (14). table a1. the approximate equations for f(,), g(,), and h(,). these approximate equations are applicable only for the range 0.1 <  < 10 and 0 <  < 1.                  4 0 4 0 lnexp, i j ji ijff  fij j = 0 j = 1 j = 2 j = 3 j = 4 i = 0 1.028 －2.386×10－2 －1.785×10－1 －8.825×10－3 3.794×10－2 i = 1 －4.683×10－1 －2.215×10－3 －2.583×10－1 1.462×10－1 －2.548×10－2 i = 2 9.580×10－2 5.409×10－3 －8.584×10－2 7.445×10－2 －2.008×10－2 i = 3 －2.487×10－3 －6.615×10－4 1.406×10－2 －1.429×10－2 5.733×10－3 i = 4 －2.071×10－3 －5.527×10－4 6.702×10－3 －7.450×10－3 2.685×10－3                  4 0 4 0 lnexp, i j ji ijgg  gij j = 0 j = 1 j = 2 j = 3 j = 4 i = 0 6.610×10－1 －1.503×10－2 2.771×10－1 5.225×10－2 －2.334×10－2 i = 1 －1.135 －2.939×10－4 －1.889×10－1 1.703×10－1 －3.026×10－2 i = 2 2.050×10－1 4.571×10－3 －1.114×10－1 1.167×10－2 2.088×10－2 i = 3 1.051×10－3 1.463×10－3 1.341×10－2 －2.623×10－2 1.073×10－2 i = 4 －4.765×10－3 1.966×10－4 7.269×10－3 －3.900×10－3 1.812×10－4                  4 0 4 0 lnexp, i j ji ijhh  hij j = 0 j = 1 j = 2 j = 3 j = 4 i = 0 1.028 －1.308×10－2 1.257×10－1 9.406×10－2 －2.560×10－2 i = 1 －4.678×10－1 －1.008×10－2 －8.802×10－2 1.961×10－1 －5.909×10－2 i = 2 9.608×10－2 －8.025×10－3 －2.901×10－2 3.029×10－2 －2.293×10－3 i = 3 －2.540×10－3 6.767×10－4 4.578×10－3 －7.100×10－3 2.066×10－3 i = 4 －2.113×10－3 1.066×10－3 －1.137×10－4 9.414×10－4 －1.133×10－3 investigations on nonlinear deformation phenomena in a force calibration system acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 150 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 150 155 investigations on nonlinear deformation phenomena in a force calibration system y. wang1, n. rogge2, s. vasilyan2, t. fröhlich2 1 tianjin university, tianjin 300072, china, 3016202030@tju.edu.cn 2 technische universität ilmenau, 98693 ilmenau, germany, norbert.rogge@tu-ilmenau.de abstract: previous investigations show, in force calibration and measurement systems that are based on electromagnetic force compensation (emfc), deformations of the flexure hinges can cause significant contributions to the uncertainty of the system. a simulink model of the planck-balance 2 (pb2) is established in matlab according to the cad model. the results are compared to simplified analytical models and the results obtained from measurement. keywords: planck-balance; emfc balance; mechanical nonlinearities; mechanical simulation; force calibration 1. introduction emfc balances are state of the art for weighing systems with low uncertainties. however, they can also be modified and used as force measurement systems. a recent example is the calibration of laser power by the measurement of the photon momentum [1]. another application, where a modified emfc load cell is used, is the planckbalance (pb). it is a table top kibble balance that was developed in cooperation between the physikalisch-technische bundesanstalt (ptb) and the technische universität ilmenau. in contrast to a standard emfc balance, an interferometer and a second voice coil actuator are attached to the load carrier. the additional actuator is calibrated in the velocity mode and utilized in the force mode to compensate the gravitational force of the test weight [2]. the problem that is addressed in this work will be exemplary shown for the pb2 system. this system is the version of the planck-balance that aims to achieve uncertainties suitable for calibration class e2 mass standards in the range from 1 mg to 100 g. in the force mode of the current pb2 system, the gravitational force is compensated by an external voice coil actuator that is attached to the load carrier of the system, while the position that indicates a force equilibrium is generated by the internal position sensor of the balance. preliminary investigations showed that the there is a different displacement on the load carrier side of the lever depending on the load that is placed onto it. as expected, these displacements are smaller than in the normal use of the emfc balance where the compensation force is generated at the position sensor side of the lever but, as shown in figure 1, they can be nonlinear towards the applied load. figure 1: difference of load carrier displacement for load changes of different masses the shown difference of the displacement between loaded and unloaded state is measured by the interferometer of the pb2 system, where the error bars represent the standard deviation of the repeated determinations of this difference. the models that are presented in this work are used to identify how deformations of the mechanical system contribute to the observed nonlinear behaviour. furthermore, an improved understanding of the effects will help to improve the design of the system and the correction of errors caused by the deformation of the mechanical subsystem. http://www.imeko.org/ mailto:3016202030@tju.edu.cn mailto:norbert.rogge@tu-ilmenau.de acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 151 2. model of the balance in order to investigate the nonlinear relation between mass and load carrier displacement, a rigid body model based on the one described in [3] is set up and modified. the model focuses on the investigation of geometrical nonlinearities, which are able to cause the observed deformation effects. the mechanical model is extended by the electronic hardware and the pid algorithm, which is used to balance the transmission lever after a change of the external mass on the weighing pan (see figure 2). the model is mainly made up of five sections: mechanical system, position sensor, pid controller, u/i converter and voice coil actuator. figure 2: main components of the balance model 2.1. mechanical subsystem the mechanical system is based on the cad model and mechanical drawings of the device. figure 3: simulink model of the mechanical subsystem the design of the mechanical system module is shown in figure 3. the simulation environment is ideal. the value of the acceleration of gravity g is taken from a previous measurement, and the effects of temperature, humidity or air damping in the environment are not considered. the damping of planck-balance during the measurement mainly comes from the following three sources: the friction of the flexure hinge, air friction and electromagnetic damping of the coil. compared to the other two types of damping, the air friction damping has a relatively small effect and can be ignored. the electromagnetic damping will be discussed later. the damping caused by the friction force in the flexure hinge is corrected by an empirical value obtained from fitting the experimental data to the simulation result. the position sensor is placed at the end of the transmission lever in the planck-balance mechanical system, in order to detect the displacement of the transmission lever. its characteristics are obtained from experimental data and a saturation is introduced to the model to represent the mechanical end stops. 2.2. pid controller the parameter setting of this module mainly refers to the ziegler-nichols method [4]. at first, set both the integral parameter ki and the differential parameter kd to zero. then adjust the proportional parameter kp from small to large until the stable oscillation occurs. read the kp value at this time and record it as ku. record the oscillation period as tu. then calculate the theoretical parameter values of kp, ki and kd according to the ziegler-nichols method parameter table. finally, adjust the parameters based on the simulation results. 2.3. voice coil actuator figure 4: simulink model for voice coil actuator the simulink model of the voice coil actuator is shown in figure 4. in terms of the working principle, the module is a voice coil actuator, which can convert the current signal input into an electromagnetic force. the output is returned to the load carrier in the mechanical system module to achieve a closed loop of the entire system. the module consists of two parts: coil and permanent magnet. the coil moves in the magnetic field generated by the permanent magnet, and the force generated by the coil actuator compensates for the gravitational force of the load carrier and the weight to be measured, which makes the pb2 system balance. the force on the electromagnetic coil is the lorentz force, which is the force generated by the interaction of the current flowing through the electromagnetic coil in the magnetic field. at the same time, when the coil moves in the magnetic field, a dynamic electromotive force is also generated inside the coil. http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 152 𝐹 = 𝐼 · 𝑙 × 𝐵. (1) 𝑉 = 𝑢 · 𝑙 × 𝐵. (2) f – lorentz force i – current flowing through the solenoid l – voice coil winding length b – magnetic induction v – dynamic electromotive force u – velocity of coil the parameter k needs to be set. k is the cross product of the coil winding length l and the magnetic field magnetic induction b, and the unit is n/a or v/(m/s). in this model, the value of k is 189 n/a. as for the internal actuator, the value of k changes to -50 n/a. another parameter that plays an important role during the motion of the coil is the translational damping coefficient. it is determined as 9 n/(m/s) by fitting the model behaviour to the experimental data. the simulink model of the voltage/current converter (u/i converter) is equivalent to a 500 ω resistor. when the input voltage signal u is within the set saturation voltage range, the output current signal can be equivalent to the current signal passing through the 500 ω resistor. the purpose of setting the saturation value of the voltage signal is to prevent the output current signal from being too large, which will cause a large deflection of the planck-balance module. this setting improves the stability of the simulation model. 3. result of simulation based on the analysis of the working principle of planck-balance, the simulink model of planckbalance is established in matlab according to the cad model, including the mechanical structure subsystem, position sensor, u/i converter, electromagnetic coil actuator and pid controller. the simulation model can be used to study the movement of the model in several non-ideal situations. 3.1. ideal condition the simulation model can accurately represent measurement process of the planck-balance. under ideal simulation conditions, a measurement cycle of 20 s is used. in the first half of the cycle, the weighing pan is unloaded. in the second half of the cycle, the weighting pan is loaded. the movement of the load carrier is observed and the output voltage of the pid controller is recorded. the displacement of the position sensor oscillates when the load alternates to unload and then quickly returns to zero. the output voltage of the pid controller is stable at unload and load respectively. the voltage difference between the two is proportional to the mass of the weight. the load carrier displacement for loaded and unloaded state of the balance is shown in figure 5. similar to the observed behaviour of the real pb2 system, both states show a different displacement, but the difference is smaller than the measured one. figure 5: displacement of load carrier 3.2. bending of the transmission lever in the actual measurement process, the transmission lever will bend due to the influence of gravity. in order to study the influence of this deformation on the measurement, in this section, the transmission lever will be bent by applying an external force. then, the movement of the load carrier is observed and recorded. 3.2.1. applying external force on one side of the balance the simple model shown in figure 6 describes the working state of the balance. g represents the gravity of the weight to be tested placed on the load tray, and f represents the force exerted on the load carrier. the default direction is the same as the direction of gravity. the right side of the balance model represents the position sensor. when the left side of the balance is forced to move, the right side of the balance will produce a corresponding reverse movement. at the same time, the position sensor converts the detected position offset signal into a voltage signal and inputs it to the pid controller. at the same time, the output voltage of the pid controller generates via the voltage-current converter (not shown in the simple model) an output current signal that enters the external voice coil actuator. so, the coil generates an upward electromagnetic force to balance the weight and the additional applied force. finally, the position sensor is kept in the zero plane and the balance is balanced. figure 6: a simple model of the pb2 (model a) a different model is shown in figure 7, an internal coil actuator is used. the torque generated http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 153 by the actuator acts on the end of the transmission lever. figure 7: model with internal actuator (model b) figure 8 shows a simpler planck-balance model. the end of the transmission lever is fixed by a mechanical method to keep it in zero plane all the time. figure 8: fixed transmission lever (model c) in the simulation, the external force f applied by the facility to the load carrier is 5 n, and the direction is opposite to the direction of the weight g. the three models were used for six sets of experiments. the weights used in the six sets of simulation were 10 g, 20 g, 50 g, 60 g, 70 g and 100 g. each set of simulation measures ten cycles of data, each cycle lasting 20 s. in the first half of the cycle, the weighing pan is unloaded. in the second half of the cycle, the weighing pan is loaded and is subjected to an external vertical upward force. figure 9: difference of displacement in model a/b/c as shown in figure 9, the red line represents the simulation result of model a. it is not difficult to find that the extension line of the red line segment crosses the (0,0) point, which means that under this condition, the displacement of the load carrier is proportional to the mass of the weight. model b shows only the difference that all data has a constant offset from the data of the classic model a. the blue line represents the simulation result of model c, which is almost the same as the result of model b. within the acceptable range of measurement error, it can be considered that fixing the end of the transmission lever by mechanical method and using the internal actuator has the same effect on the mechanical deformation of the measuring system. this is one of the reasons why the external electromagnetic coil driver is selected in the actual measurement process. the use of the external actuator can make less deformation of the mechanical structure during the measurement process and can be closer to the ideal state. it can reduce measurement errors, improve measurement capabilities and resolution. 3.2.2. apply external force on both sides of the balance figure 10: schematic of the external force applied at both ends of the balance in order to make the transmission lever have a bigger deformation, two forces in the same direction to both ends of the transmission lever are applied. one force is applied to the centre of gravity of the load carrier, and the other force is applied to the end of the transmission lever. the force applied by the facility to the load carrier is flc, and the force applied to the end of the transmission lever is fps. the direction of these two forces is the same as the direction of gravity, and flc = 3.75 ·fps. (3) figure 11: difference of displacement of load carrier when external force is applied at both ends http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 154 the simulation results are shown in figure 11. the abscissa indicates the mass of the measurement weight in grams. the ordinate is the displacement of the load carrier in nanometres. the red line represents the result of the simulink model when flc = 3.75 n and fps = 1 n; the green one represents flc = 37.5 n and fps = 10 n; the blue one represents flc = 75 n and fps = 20 n. the measurement results show that when the transmission lever bends due to the force on both sides, the movement of the load carrier will show nonlinearity, and the greater the degree of bending, the more prominent the nonlinear behaviour. however, even when the force is increased to values of flc = 75 n and fps = 20 n, the maximum external displacement of the load carrier still retains at about 3 nm, smaller than observed in measurement. therefore, it is considered that the bending of the transmission lever is not the main reason for the non-linear deformation in the force calibration system. figure 12: coordinate system at the centre of the weighing pan 3.3. off-centre load in the actual measurement process, the weight is picked up by the experimenter with tweezers or a mechanical arm and placed on weighing pan. in this process, there is no guarantee that the weights are placed exactly in the centre of the pan, which may cause the nonlinear displacement of the load carrier during the measurement. in order to study the influence of this phenomenon, as shown in figure 12, a plane coordinate system is established on the surface of the weighing pan, and the x-axis and yaxis are set according to the illustrated direction. in the simulation results that are shown in this section, the independent variable is the position of the weight. the object of observation is the displacement difference between load and unload. as in the experiment, this displacement is obtained at a position that is offset from centre of gravity of the load carrier since the beam of the interferometer cannot be pointed there due to design restrictions. therefore, the tilt of the load carrier due to a deformation of the mechanical system has an important role in the described phenomena. 3.3.1. y-axis offset figure 13: the weight is offset in y-axis direction as shown in figure 13, when the weight is placed in the centre of the weighing pan, the gravity g is equal to the force f generated by the external actuator. 𝐹 = 𝐺. (4) when the position of the weight is shifted in the y-axis direction, the gravity of weight is still g. the difference is that the equivalent force arm is l', where l'= l − δy, and the support force provided by the actuator at this time is f', according to the principle of lever balance. 𝐺 · 𝐿′ = 𝐹′ · 𝐿. (5) 𝐺 · (𝐿 − 𝛥𝑦) = 𝐹′ · 𝐿. (6) figure 14: simple spring model of load carrier in section 3.2, we discussed that when the right side of the balance is fixed, the left side can still balance the weight of the weight through its own deformation without external force. therefore, the right side of the balance can be considered as a simple spring model. as shown in figure 14, set the spring coefficient of the model to c. then the support force fc = c·δz is generated when the load carrier is displaced in the z-axis direction. for the force balance of the load carrier on the z-axis. 𝐺 + 𝐹𝐶 = 𝐹′. (7) so ∆𝑍 = −(𝐺 · ∆𝑦)/(𝐿 · 𝐶). (8) it can be obtained from equation (8) that in the above model, the additional displacement of the load carrier will be in accordance with the weight g of the weight and the deviation δy, where l and c are constants. the gravitational force g and the http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 155 deviation δy are used as variables to observe the movement of the load carrier during the measurement process. figure 15 shows the displacement of load carrier in different deviation δy. it was generated with the simulink model which does not show measured data. the results verify the rationality of the simple spring model as well as the correctness of equation (8) in an intuitive and simple way. the slope g/(l×c) can be identified from figure 15, then the value of c can be determined. figure 15: displacement of the load carrier when the weight is offset in y-axis direction 3.3.2. x-axis offset figure 16: displacement of the load carrier when the weight is offset in x-axis direction as shown in figure 16, the offset of the weight position on the x-axis will also cause additional displacement of the load carrier, but it is much smaller than the situation when y-axis is offset. therefore, it is considered that when the weights are offset on the x-axis and the y-axis at the same time, the y-axis offset is the main reason for the additional displacement of the load carrier. 4. summary the investigations show that nonlinear deformations can occur in emfc balances. a rigid body model based on previous work about modelling of the dynamic characteristics of the same balance is proposed in order to investigate the contribution of geometrical nonlinearities to the observed effects. the simulation results show that the weighing pan will have additional displacement under several conditions such as bending of the transmission lever, eccentric position of the weight, which influence the relation between mass and load carrier displacement that shows nonlinear pattern. however, these contributions above are much smaller than the deviation in the experiment. these factors are responsible for the nonlinear deformation in a force calibration system, but not the main reason. therefore, the basic factor which causes a dominant performance in the deviation phenomenon still needs further investigation. 5. references [1] e. manske, t. fröhlich, s. vasilyan, “photon momentum induced precision small forces: a static and dynamic check”, measurement science and technology, vol. 30, 105004, 2019. [2] c. rothleitner et al., “the planck-balance using a fixed value of the planck constant to calibrate e1/e2-weights”, measurement science and technology, vol. 29, no. 7, 2018. [3] h. bai, n. rogge, c. rothleitner, t. fröhlich, “model based correction of motion deviations in the planck-balance”, gma/itg-fachtagung sensoren und messsysteme, vol. 20, pp. 554-560, 2019. [4] j. g. ziegler, n. b. nichols “optimum setting for automatic controllers”, trans. asme, vol. 64, pp. 759-768, 1942. http://www.imeko.org/ design of sorption and buoyancy artefacts made of silicon acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 37 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 37 41 design of sorption and buoyancy artefacts made of silicon k. lehrmann1, r. tutsch2, f. härtig1 1 physikalisch-technische-bundesanstalt, braunschweig, germany, katharina.lehrmann@ptb.de 2 technical university braunschweig, institute of production metrology, braunschweig, germany, r.tutsch@tu-bs.de abstract: the design of sorption and buoyancy artefacts made of silicon will be shown. with these artefacts, also known as transfer standards, high-precision calibrations can be performed under atmospheric conditions. they are used to systematically determine and correct deviations in air density and deposits on the surface, such as water and hydrocarbons. this development has been of importance since the unit kilogram can be realized by monocrystalline spheres made of isotope enriched silicon in line with the new definition of the si in 2019. the special constraints and the complete geometric design parameters for sorption and buoyancy artefacts will be presented. keywords: sorption artefact; buoyancy artefact; kilogram; revised si 1. introduction since the redefinition of the si in 2019, silicon spheres came into focus in mass metrology. the best-known spheres have a nominal weight of one kilogram, and an uncertainty of 1.1 × 10-8 kg. they are made of highly enriched monocrystalline silicon of isotope 28 (28si) [1]. however, their use is very limited due to the rare availability of the material, its sophisticated manufacturing process and the very high price. the excellent behaviour of 28si spheres, such as mass stability and easy handling, makes it interesting to develop also spheres and transfer standards made of natural silicon (natsi) [2]. one disadvantage of silicon spheres is the important difference of density ρ between silicon 2 329 kg·m-3, national mass standards made of platinum-iridium alloy 21 530 kg·m-3 or conventional mass standards made of steel 8 000 kg·m-3 [3]. in a first order these variations lead to distinct volume and surface areas when different standards are used during a calibration process. the disruptive environmental influences can be prevented by using mass comparators operating in vacuum. an alternative method is the use of sorption and buoyancy artefacts [4]. both types are also called transfer standards. they allow systematic effects resulting from environmental conditions to be easily corrected. however, their design and handling conditions must be determined carefully in order to provide an expected standard deviation for mass in the range of 2 × 10-8. so far, only a few transfer standards made of natural silicon with nominal mass of 1 kg, that are comparable to silicon spheres, are manufactured and investigated [5]. 2. objectives of new transfer standards and constraints the main objective is the development of transfer standards that allow high accurate calibration of mass standards under environmental conditions without vacuum comparator. the technical application shall be explained, its optimized design parameters as well as the operation conditions for sorption and buoyancy artefacts made of natural monocrystalline silicon shall be determined. for the construction of transfer standards, the following manufacturing and handling requirements must be fulfilled, sorted by priority: ▪ similar nominal mass as the reference ▪ similar material and surface quality as the reference, natural monocrystalline silicon without amorphous surfaces ▪ nominal surface ratios from sorption artefacts to reference sphere should ideally be designed as integers (this supports the clear assessment of the sorption effects), see figure 1 ▪ equal nominal surface area for all buoyancy and duplex artefacts ▪ considering geometric limitation of the measuring chambers within typical mass comparators, correlated parameters: total height and radius of the transfer standards ▪ practice-oriented assembly, taking into account the geometric mounting conditions http://www.imeko.org/ mailto:katharina.lehrmann@ptb.de mailto:r.tutsch@tu-bs.de acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 38 of the comparators, this requires a special design of the base of sorption, buoyancy and inlay artefacts ▪ easy demounting of the sorption and inlay artefacts for efficient cleaning ▪ statically determined storage by frictional locking of the sorption artefacts discs to be stacked ▪ high tilting stability of the sorption artefact discs particular focus is given to the development of what is referred to as a duplex transfer standard that embodies the functionality of both a sorption and a buoyancy artefact. a further highlight will be another special transfer standard, the inlay buoyancy artefact. due to an enclosed tungsten core, this standard has a large volume difference to other buoyancy bodies, which contributes directly to the determination of the air density. 3. design of new transfer standards mass changes of weights caused by deposits on their surfaces or by buoyancy effects can be described by at least two transfer standards. figure 1 outlines systematically different variants of sorption and buoyancy artefacts. all transfer standards have the same nominal mass and closely similar surface properties as the reference standard, which is defined by a 1 kg silicon sphere. figure 1: schematic illustration of ratios of different sorption artefacts (left), buoyancy artefacts (right), duplex artefact (middle, red) sorption artefacts depositions such as physically absorbed water or hydrocarbons can be determined by sorption artefacts. in figure 1 these are the two transfer standards on the left. the geometric disc design increases the surface area while maintaining mass, density and volume. in this context, it is important to retain the volume so that a consistent buoyancy influences on the set of sorption artefacts is granted. on the other hand, the nominal surface area ratios of the sorption artefacts are designed as possible integer multiples of the reference to permit rapid assessments. the systematic error of the mass due to deposits on the surface areas can be determined by comparing measurements in air and vacuum, and the knowledge of the well-known surface area of the sorption artefacts used. the surface difference was systematically designed and calculated for a set of sorption artefacts consisting of two or seven discs and a cylinder. to simplify the loading of the comparators, the ground cylinder of the sorption artefact with seven discs was designed with a larger height than the discs on top of it. buoyancy artefacts buoyancy effects caused by density differences between mass standards and air can be ascertained by buoyancy artefacts. in figure 1 these are the two transfer standards on the right. key characteristics of these are the variations in volume and density while remaining the same mass and surface area. since the surface area and surface properties of the transfer standards and the reference should be identical, the difference in density can only be created by forming a cavity or a denser core. the hollow chamber can be vacuumed gas-tight or filled with a gas. in this case the buoyancy artefact is referred to as a hollow cylinder. a new approach is to increase the buoyancy difference by using a transfer standard with a core made of a denser material than that of the reference. this inlay artefact has a smaller volume compared to other transfer standards. the air density used to calculate the buoyancy correction is determined from measurements of the mass differences between the buoyancy artefacts in air and in vacuum. it is obtained from the difference (air to vacuum) of the difference (of the masses). duplex artefact there is another exceptional transfer standard, which is referred to as duplex artefact and illustrated in figure 1 in the centre. it may be used as both a sorption and a buoyancy artefact. because of the geometry selected it is also known as dumbbell artefact. the individual components of the transfer standards made of natural monocrystalline silicon are finished with a defined rotating chamfer to prevent chipping. http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 39 4. statically dismountable coupling a key issue for handling transfer standards is the accurate and careful cleaning of the surfaces and interspaces. the construction of both, the sorption and the inlay artefacts as separable components ensures this overall cleaning in order to avoid unpredictable mass differences between the artefacts due to contamination. in this context, it was essential to determine the contact areas as best as possible and to keep the discs easy to be assembled and statically positioned. in addition, it was important to achieve a maximum tilting stability of the stack of discs. up to now, two construction principles are best known for sorption artefacts: 1. artefacts with fixed shafts. although this construction principle results in a stable connection, a serious risk applies that the shaft will be scratched during the assembly process. in consequence the surface area behaviour of the silicon will change and will become more sensitive to impurities that access into micro scratches. on top of this, the cleaning of the gaps is very time-consuming. 2. construction in the form of disc stacks with discs placed on loose wires. however, putting the wires on the slippery silicon surface during the stacking process of the individual discs is very time-consuming. this technique also carries the risk that the stack may slip away or even collapse during handling and weighing process. for the purpose of managing the risks stated above, a new and suitable construction has been realized with a combination of spherical distance pieces. figure 2 shows the arrangement of the socalled coupling with spherical distance pieces. it shows the six green marked spheres on the top of the lower disc. the disc located above rests with its three red marked spheres on it. the resulting sixpoint contact is statically determined. the quasipoint-like area coverage will be minimal, so that it is negligible for further calculations. in experiments with different arrangements of the spherical distance pieces, this model was identified to be the easiest to assemble. likewise, the inclination stability with > 20° was also demonstrated experimentally. this tilt stability is more than sufficiently high to handle a stack of discs safely. the spheres were manufactured from the same material as the remaining transfer standards and therefore have the same sorption properties. their diameter of 4 mm was manufactured with a manufacturing tolerance of 1 μm and a form deviation of λ/10 which are not relevant for further calculations [6]. figure 2: top view of the location of the spherical distance pieces of a gap between two discs. green marked spheres are fixed on the lower disc, red labelled spherical distance pieces are located on the upper disc. as the radius of the coupling spheres decreases, the risk of plastic deformation or even chipping increases. to prove the stability, the limit value for the elastic deformation was evaluated. for this purpose, an analytical calculation [7] was carried out. table 1 lists the material properties of the material used in order to calculate the maximum stress of the contact points. since the silicon supplier itself could not provide any information on further material properties, the values according to [8] were used as a reference. the maximum load acting on a contact point of a sphere is assumed to be 0.333 kg. this static load can theoretically occur when a transfer standard is tilted. in this pessimistic view only three spheres are in point contact. this assumption can be regarded as an additional safety factor for following stability investigation. table 1: analysis of contact mechanics for coupling parameter value mass of contact point, in kg 0.333 acceleration of gravity, in m·s-2 9.81 radius of spherical distance piece, in m 0.002 elastic modulus, in kg·m-1·s-2 131 × 109 poisson’s ratio 0.221 the homogeneous and presumably isotropic raw material of the silicon used and the validity of hooke’s law fulfil the preconditions of the calculation for the hertzian pressure [9]. the effective radius r resulting from the two identical individual radii rsph of the coupling spheres is calculated according to equation (1). http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 40 𝑟 = 1 ( 1 𝑟sph + 1 𝑟sph ) (1) using this effective radius, the compressive stress can then be evaluated with the known mass of contact point 𝑚, poisson’s ratio 𝑣 and young’s modulus 𝐸, according to equation (2). 𝜎 = 1 𝜋 √ 1.5 ∙ 𝑚 ∙ 𝑔 ∙ 𝐸2 𝑟2(1 − 𝑣2)2 3 (2) the calculation results in a compressive stress of 1.443 × 109 pa. this is a factor of approximately 0.45 smaller than the maximum allowed compressive stress of 3.2 × 109 pa which is given in literature [10]. resulting from the observation above, the selected spherical spacers ensure sufficient stability and thus serve ideally as coupling components. equation (3) describes the expected deformation. 𝜔0 = √ 2.25 ∙ (1 − 𝑣2)2 ∙ (𝑚 ∙ 𝑔)2 𝐸2 ∙ 𝑟 3 (3) the technically also interesting value 𝜔0 for flattening is calculated to be 1.082 × 10-6 m. compared to the sphere radius, this leads to a negligible deformation of less than 0.2 %. 5. mathematical approach as previously pointed out in section 2, several constraints must be regarded for the determination of the geometric parameters of the transfer standards. above all, it is particularly important to achieve a mass of one kilogram in the range of  100 mg, as the established mass comparators are configured most accurately for this measuring range. with all constraints being taken into account, the following correlated variables for all transfer standards: mass, volume, surface area ratios between the transfer standards, number of discs as well as diameter and height of the discs/cylinders must be determined. due to the manufacturing process, the individual discs are finished with chamfers. consequently, this leads to a further change in mass and surface area for the discs/cylinders, which must also be factored into the calculation. in the case of transfer standards consisting of mounted discs, the mass, volume and surface area corrections caused by the spherical distance pieces for the coupling must also be incorporated. an exceptional case is the inlay artefact, as it is made of two different materials. in this application, the different densities are included in the complex calculations. depending on the type of transfer standard a minimum of seven up to a maximum of ten correlated parameter must be determined. the interesting geometrical parameters for the transfer standards are determined by the use of a highly sophisticated multi fitting algorithm. therefore, a newton’s multi parameter iteration method [7] in combination with a self-developed iteration method was used. the calculations were carried out in mathematica [11]. the complex equations are not in focus of this paper. 6. results based on the requirements discussed in section 3, transfer standards of monocrystalline silicon with a nominal mass of 1 kg were designed. for the sorption artefacts, this resulted in two types, a sorption artefact stack with seven and a stack with two discs, each with one cylinder. following this, two variants were also calculated for the buoyancy artefacts. the hollow cylinder is evacuated in this configuration. the inlay artefact consists of two discs and a cylinder which embodies a core of tungsten. in addition, the parameters for a duplex artefact were also developed, which can be used both as a sorption and a buoyancy artefact. the geometric parameters for the transfer standards are summarized in table 2. the density of the silicon used was specified as 2 328.8 kg·m-3 [12]. the calculations were based on a leg dimension of 0.5 mm and a bevel angle of 45° for all chamfers. the radius of the coupling spheres was defined as 2 mm. the chamber of the hollow cylinder was assumed to be evacuated. the inlay body was designed for a tungsten core with a density of 19 250 kg·m-3 [13]. the most important results considering surface area and volume required for new designed sorption and buoyancy transfer standards made of silicon are presented in table 3. according to internal investigations made by ptb, the crystallographic orientation of the silicon bodies has no significant effect for the corrections. http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 41 table 2: geometric parameters of different transfer standards. values are given in metres. so r p ti o n a r te fa c t n 8 so r p ti o n a r te fa c t n 3 d u p le x a r te fa c t in la y a r te fa c t h o ll o w a r te fa c t disc radius 0.042 3 to 0.043 4 0.043 4 0.045 0.034 disc height 0.008 to 0.015 5 0.024 2 0.025 5 0.030 radius cylinder 0.03 0.034 0.045 height cylinder 0.037 0.031 0.102 radius inlay core/ hollow chamber 0.016 9 0.030 height inlay core/ hollow chamber 0.015 2 0.077 table 3: example results for set of different transfer standards. sorption artefacts with seven discs (n8) and two discs (n3). transfer standard surface area a / m² volume v / m³ density ρ / kg·m-3 sorption artefact n8 0.110 101 0.000 429 406 2 328.8 sorption artefact n3 0.055 050 3 0.000 429 406 2 328.8 duplex artefact 0.041 287 7 0.000 429 406 2 328.8 inlay artefact 0.041 287 7 0.000 330 603 3 024.78 hollow artefact 0.041 287 7 0.000 646 366 1 547.11 7. summary and outlook the design strategy and related parameters for sorption and buoyancy transfer standards are explained. main focus is given on the geometrical design of the individual five artefacts. mathematical multi parameter fit algorithms allow to determine the individual parameters considering the limiting conditions such as measurement volume, weight of the objects, object density, volume, surface area, couplings of the sorption artefacts and others. in order to provide user friendly transfer standards, a dismountable, highly reproduceable coupling was developed that allows the disc stack of the sorption and inlay artefacts to be fixed with minimum risk of surface damage and sufficient inclination stability. next steps are the manufacturing of the transfer standards and the determination of the actual geometrical parameter in order to calculate the effective correction parameters including a solid measurement uncertainty budget. finally, the results will be validated by real measurements taken on mass comparators in vacuum and in air. 8. references [1] b. wood, h. bettin, “the planck constant for the definition and realization of the kilogram”, annalen der physik, vol. 531, 1800308, 2019. [2] d. knopf, t. wiedenhöfer, k. lehrmann, f. härtig, “a quantum of action on a scale? dissemination of the quantum based kilogram”, metrologia, vol. 56, no. 2, 024003, 2019. [3] m. gläser, r. schwartz, m. mecke, “experimental determination of air density using a 1 kg mass comparator in vacuum”, metrologia, vol. 28, no. 1, pp. 45-50, 1991. [4] r. schwartz, “precision determination of adsorption layers on stainless steel mass standards by mass comparison and ellipsometry: part i: adsorption isotherms in air”, metrologia vol. 31, pp. 117-128, 1994. [5] u. darmaa, j. w. chung, s. lee, s. n. park, “determination of adsorption layers on silicon sorption artifacts using mass comparison”, proc. imeko world congress, busan, rep. of korea, 9 – 12 september 2012. online [accessed 20200812]: https://www.imeko.org/publications/wc2012/imeko-wc-2012-tc3-o3.pdf [6] correspondence with u. schmidt, j. hauser gmbh & co. kg, 9 july 2020. [7] i. n. bronstein, k. a. semendjajew, taschenbuch der mathematik. ch. 7.1, p. 782, edition 19, harri deutsch thun, isbn 3871444928, 1980. [8] korth kristalle gmbh, material properties of silicon (si), august 2019. [9] h. dubbel, w. beitz, k.-h. küttner, taschenbuch für den maschinenbau. edition 14, springer-verlag, isbn 3540094229, 1981. [10] azom.com, “a background to silicon and its applications”. online [accessed 20200709]: https://www.azom.com/properties.aspx?articleid= 599 [11] mathematica, version 12.0. online [accessed 20200108]: www.wolfram.com/mathematica/. [12] ptb internal density determination by hydrostatic weighing at 20 °c, may 2020. [13] n. greenwood, a. earnshaw, chemie der elemente, p. 1291, edition 1, vch, weinheim, isbn 3527261699, 1988. http://www.imeko.org/ https://www.imeko.org/publications/wc-2012/imeko-wc-2012-tc3-o3.pdf https://www.imeko.org/publications/wc-2012/imeko-wc-2012-tc3-o3.pdf https://www.azom.com/properties.aspx?articleid=599 https://www.azom.com/properties.aspx?articleid=599 http://www.wolfram.com/mathematica/ on the regression of sensitivity characteristics of torque transducers acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 194 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 194 199 on the regression of sensitivity characteristics of torque transducers a. brüge1 1 department 1.2, physikalisch-technische bundesanstalt, braunschweig, germany, andreas.bruege@ptb.de abstract: a new method to estimate the influence of regression on the measurement uncertainty for the calibration of torque transducers will be presented in the following. this method is based on monte carlo simulations and on sampling calibration results. in some cases, the uncertainty contributions of regression thus obtained are considerably higher than those yielded by estimates according to widely accepted methods. the differences between the methods will be discussed and suggestions for simplified estimates within the scope of the new method will be made. keywords: regression; torque; calibration; measurement uncertainty; monte carlo simulation 1. introduction calibration results of torque transducers are usually documented in the form of discrete measurement points. these discrete points are then used to calculate a sensitivity characteristic curve by means of a cubic polynomial function, so that users can apply traceability also between those values. in widely accepted methods ([1], [2], [3]), the associated uncertainty contribution is estimated by means of a global characteristic value 𝑢𝑓a,std , assuming a triangular distribution. the conditions justifying such an estimation have not yet been investigated. the introduction of such conditions was mainly motivated by the necessity of keeping the calculation of measurement uncertainties straightforward for users in calibration laboratories. due to the fact that analytical approaches require considerable effort (see article by d. röske in these proceedings), systematic investigations have, to date, only been carried out either based on linear regressions or by means of simulation methods ([4], [5], [6]). as a rule, simulations provide results that are specific to a particular application. the special conditions of torque calibration have not yet been sufficiently investigated for this reason. in this paper, the influence of regression on the measurement uncertainty will therefore be examined by means of the monte carlo method (mcm); the results will then be discussed based on real measurement data. 2. monte carlo simulation the dependence of the output signals of strain gauge torque transducers on the load torque is essentially linear. since the nonlinear contributions are smaller than the linear ones by several orders of magnitude, characteristic curves are mostly represented as linearity deviations over the torque and can be modelled very conveniently by means of a cubic polynomial (figure 1, green line). the uncertainties associated with each of the data points are used in the mcm to shift the data points accordingly. in the regression, these uncertainties lead to different polynomial functions. in figure 1, two examples of these polynomials are shown (yellow and orange lines) which are based on the impact of all uncertainty bars in positive and negative directions respectively. figure 1: linearity deviations of a simulated series of measured values (blue dots) over a torque range to which uncertainties (blue bars) are assigned. a cubic regression of the dots provides the green curve. taking into account the uncertainties, other regression curves are also possible, two of which are shown as examples (yellow and orange). mcm allows the behaviour of the polynomials to be investigated in detail over a wide range of parameter values. synthetic measurements as data points are defined by the parameters of linearity deviation l, distortion d, noise n, and the relative -1.5e-03 -1.0e-03 -5.0e-04 0.0e+00 5.0e-04 1.0e-03 1.5e-03 0 50 100 l in e a ri ty d e v . i n m v /v torque load in % meas. model regress+ regresshttp://www.imeko.org/ mailto:andreas.bruege@ptb.de acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 195 position of the inflection point pi/nv (position of inflection point / nominal value). the data points are generated with a cubic polynomial in such a way that their maximum deviation from the linear course is given by l. with a normal distribution with the standard deviation d, the points, which until now have been ideally located on the regression curve, are randomly shifted individually. the simulated measurement curve obtained in this way is now varied randomly within the mcm by shifting the measurement points individually from the distorted centre position generated with d, using a normal distribution with the standard deviation n. the position of the inflection point pi can easily be calculated from the second and third order parameters 𝑐2 and 𝑐3 of the cubic polynomial: pi = 𝑐2 (3 𝑐3)⁄ . (1) for the simulation, the model function is adapted to the conditions of real transducers in certain fields of application by means of typical parameter combinations, as shown in figure 2. the parameters are obtained by sampling a large number of calibrations (n  3000). from the results obtained by means of mcm, it is possible to gain findings concerning the regression of characteristic curves of transducers. figure 2: model parameters in mv/v for the simulation of the regression of characteristic curves with a nominal signal of 2 mv/v in typical fields of application (see the text in section 4). the values show the expected values (horizontal lines) and the standard deviations (bars) obtained for a sampling of calibrations. values for the combined standard uncertainty of the calibrations at the 10 % data point 𝑢cert,0.1 are also indicated for comparison purposes. an analysis of the correlation shows that the measurement uncertainty contributions of interest are mainly influenced by noise n. for the standard deviation of the regression deviation 𝑆(𝑓a) , this yields a sensitivity of approximately 4. the systematic contribution of the regression deviation 𝑢𝑓a also depends on the distortion d. the linearity and the position of the inflection point do not play a significant role (figure 3). in the investigated range of values, the influences of n and d are strictly linear. the results of simulations using input parameters with defined values are therefore easily scalable to other input values. figure 3: correlation of measurement uncertainty contributions as output quantities of the mcm simulation of the regression with the model parameters. the variations of the reference points entering into the simulations 𝑢st are measured in accordance with the noise n which is composed of nonlinearity contributions of the amplifier [7], of its resolution and of the signal disturbance in the measurement chain. although nonlinearity leads to contributions with beta distribution, the simulations have shown that in realistic scenarios, the contributions of other components with a gaussian distribution prevail, so that a gaussian distribution may be assumed. it was possible to confirm this assumption empirically, based on 5 × 104 real data sets. the uncertainty of the regression function 𝑢r must be taken into account when assessing stepped calibration processes between the reference points. this uncertainty is obtained by determining the uncertainty of the polynomial coefficients that define the regression function. for cubic polynomials without an absolute term, simulation between the coefficients of different orders yields normalized covariances with absolute values near 1: 𝑐𝑜𝑣norm(𝑋𝑖 , 𝑋𝑗 ) = [ −0.9600 0.9028 −0.9600 −0.9870 0.9028 −0.9870 ]. (2) when calculating 𝑢r , the covariances may therefore not be neglected. 3. contributions to the measurement uncertainty 3.1. user's scenarios in current standardization procedures, the influence of the regression enters into the measurement uncertainty in a generalized manner via a plausible estimation. this method does not take the fact into consideration as to whether the user would like to use the regression at all. this paper therefore suggests considering these parameters separately. when traceability is required at the reference points only (scenario a), the regression and its contribution to the measurement 1.0e-07 1.0e-06 1.0e-05 1.0e-04 1.0e-03 v a lu e in m v /v tt tw at it estim. l estim. p l d n u_cert,0.1 http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 196 uncertainty are dispensable. only when values between the reference points are also required (scenario b) must this uncertainty contribution be taken into account. in addition, instead of the generalized estimation of the regression influence, this paper breaks down the contributions into the individual contributions that are made accessible by the mcm simulation: the systematic contribution 𝑢𝑓a , the random contribution 𝑢s𝑓a , and the contribution that is attributed to the model 𝑢r. typical curves of these different contributions in the torque range compared to the estimation according to accepted procedures 𝑢𝑓a,std have shown that 𝑢r cannot be used as the sole contribution to the uncertainty due to regression (figure 4). instead, 𝑢s𝑓a contains both this contribution and the interaction of the regression with the uncertainty of the reference points, so that these two contributions cannot both enter into the measurement uncertainty budget together. figure 4: curves of the measurement uncertainty contributions from the regression for maximum simulation parameters in accordance with the data sampling (n = 2 × 10-4 mv/v) compared to the contribution for 𝑓a in accordance with din 51309. the simulations have also shown that the estimates as used in accepted procedures are systematically below the estimations that are determined with the new procedure. 3.2. best estimator as long as the standard uncertainty of the systematic fraction 𝑢𝑓a is not larger than the uncertainty of the reference points 𝑢st , it can be assumed that the reference points, in principle, follow a cubic polynomial curve which therefore represents the “true characteristic curve”. the deviations of the reference points from this curve must then be understood as noise, and, due to its averaging action, the regression is the best estimator for the characteristic curve (estimator l). this corresponds the practice of using the average as the estimator of the expected value for quantities with a gaussian distribution. for the ratio 𝑒 = 𝑢𝑓a 𝑢st > 1⁄ , the assumption of a purely cubic characteristic curve is no longer justified, and the reference points represent the best available estimator for the sensitivity of the transducer (estimator p). for practical reasons, the individual values are combined into a geometric sum across all torque steps for this test. the combined total measurement uncertainty 𝑢c is calculated differently, depending on the user’s scenario (a, b) and on the best estimator for the sensitivity (l, p). all contributions to the measurement uncertainty that are not dealt with in this paper are summarized as 𝑢rest . since the sampled calibrations were carried out earlier than 2018, the contribution of the sawtooth effect usaw [7] must be added to their uncertainties 𝑢′c,std 2 . the fact that for torque values between the reference points, the curve of 𝑢𝑓a and 𝑢s𝑓a can only be determined by interpolation must additionally be taken into account for scenario b. the interpolation deviations 𝑢int,s𝑓a and 𝑢int,𝑓a can be determined at the data points, and an upper estimation with a constant absolute value can be found across the entire torque range (see section 5.2). from these deviations, the absolute value 𝑢int is calculated by geometric addition. 𝑢c,std 2 = 𝑢′ c,std 2 + 𝑢saw 2 (3) 𝑢rest 2 = 𝑢c,std 2 − 𝑢𝑓a 2 − 𝑢r 2 − 𝑢a 2 (4) 𝑢c,a,l 2 = 𝑢rest 2 − 𝑢saw 2 + 𝑢r 2 (5) 𝑢c,b,l 2 = 𝑢rest 2 − 𝑢saw 2 + 𝑢r 2 + 𝑢int 2 (6) 𝑢c,a,p 2 = 𝑢rest 2 + 𝑢r 2 + 𝑢a 2 (7) 𝑢c,b,p 2 = 𝑢rest 2 − 𝑢saw 2 + 𝑢𝑓a 2 + 𝑢s𝑓a 2 + 𝑢int 2 (8) = 𝑢rest 2 − 𝑢saw 2 + ( 𝑓a 2 ) 2 +𝑆(𝑓a) 2 + 𝑢int 2 (9) 4. empirical results real calibration results were analysed correspondingly to the evaluation for the synthetic measurement data. first, the data were classified into categories corresponding to the field of application of the torque transducers and exhibited different sample sizes (tt: 1466 transfer transducers, tw: 100 transfer wrenches, at: 117 application transducers, it: 208 industrial transducers). although this classification is arbitrary and does not provide any clear delimitation, it is nevertheless significantly reflected in the results. in addition, apart from the 0,0e+00 5,0e-05 1,0e-04 1,5e-04 2,0e-04 2,5e-04 3,0e-04 3,5e-04 0 20 40 60 80 100 u in m v /v torque load in % u_fa_din u_fa,syst u_s(fa) u_r http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 197 application categories, the data were also arranged according to the best estimator (1550 l, 341 p). when plotting across the interpolation uncertainty, it turned out that the categories tt and tw could mainly be treated with the estimator l (figure 5). however, there are also many cases where 𝑒 > 1. for the categories at and it, the distribution is not clear. the correct approach for the sensitivity estimator must therefore be determined individually for each transducer by determining 𝑒 . if 𝑒 is not known, then only the estimator p is justified. figure 5: ratio 𝑒 of the regression deviation 𝑢𝑓𝑎 to the uncertainty of the reference points 𝑢st against the interpolation uncertainty 𝑢int . the geometric sums across all reference points were considered. the data are categorized by field of application of the transducers. for each category, the distributions are indicated on either side. the combined standard measurement uncertainties determined in accordance with equations (5) to (8) by varying the data points by means of mcm and normalized to 2 mv/v are compared in the following with the corresponding standard measurement uncertainty obtained by means of the accepted procedures: 𝑢𝑐,std 2 = 𝑢rest 2 + 𝑢𝑓a 2 + 𝑢r 2 + 𝑢a 2 . (10) the assessment of the calibration data in scenario a for estimator l shows that the measurement uncertainty in accordance with the accepted procedures is higher than with the new procedure. here, the new procedure thus leads to lower measurement uncertainties, in particular for transducers with measurement uncertainties of less than 1 × 10-5 mv/v (figure 6, top). for scenario a and estimator p, the accepted procedure is compatible with the new procedure (figure 6, centre). in this setting, underestimations remain smaller than a factor of 2. figure 6: compared to the combined standard measurement uncertainty normalized to 2 mv/v from the sampling of real calibration data for the estimator l and scenario a, the combined standard measurement uncertainty in accordance with accepted procedures is an upper estimate (top). for the estimator p and scenario a, the combined standard uncertainty is a suitable estimate (centre), and for the estimator p and scenario b, it is an underestimate (bottom). the suitable data points that are plotted here for control purposes for the estimator l are unremarkable. in the case of scenario b with estimator p, underestimations of up to a factor of 4 occur with the accepted procedures. these underestimations are encountered over the whole uncertainty scale and are therefore not correlated with the quality of the transducers. thus, it is always recommendable to analyse the uncertainty contribution of the regression in more depth if 𝑒 > 1 (i.e. in the event of a large regression deviation and small uncertainty of the reference points) and if the calibration result between the reference points is needed. http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 198 5. simplified estimation calculating the measurement uncertainty contributions from the use of the regression with mcm requires increased computational effort, which may impede the use of the procedure in a calibration laboratory. it is therefore desirable to find practicable estimation procedures that work without mcm. 5.1. estimating 𝒖s𝒇a and 𝒖𝐑 with boundary values to determine the standard deviations of the residuals 𝑆(𝑓a) and of the polynomial coefficients of the regression function 𝑆(𝑎i) , using mcm requires numerous variations of the reference points within the limits of their measurement uncertainty and the associated regressions to be calculated. simplified estimations 𝑆 ′ of these standard deviations are obtained by calculating only selected variations of the reference points which take the boundary values of the regression functions into account. for this purpose, each of the reference points is varied with the maximum absolute values 𝑁 of the associated uncertainties and the standard deviation of the residuals and of the polynomial coefficients, respectively, is calculated. to estimate the standard deviation of the residuals, the reference points are varied alternately with +𝑁 and −𝑁, the regression is performed for each of them, and the standard deviation of these two states is calculated: 𝑆 ′(𝑓a) = 𝑆(𝑓a,+−n, 𝑓a,−+n) (11) for the simplified estimation of 𝑆 ′(𝑎i) , the reference points are continuously varied with +𝑁 and −𝑁, respectively, the regression is performed for each of them, and the standard deviations of the resulting polynomial coefficients are calculated. the standard uncertainty for the entire torque range can then be calculated based on these coefficients: 𝑆 ′(𝑎i) = 𝑆(𝑎i,+𝑁 , 𝑎i,−𝑁 ) (12) compared with the corresponding mcm calculations, the simplified calculations generally turn out to be upper estimates in real measurement data (figure 7). also in simulations, the simplified calculation leads to combined uncertainties which are slightly higher than combined measurement uncertainties when using the mcm (figure 8). 5.2. constant estimation as shown by the mcm investigations with synthetic signals, the random contributions 𝑆(𝑓a) and 𝑢r primarily depend on the uncertainty of the reference points 𝑁 (see figure 3). it therefore suggests itself to estimate the combination of the two contributions 𝑢p,c using this quantity. compared to the full mcm calculation, the simulation provides a reliable upper estimation in a contribution 𝑢p,c ≈ 2𝑁 that is constant across the torque range. the constant estimation thus yields a performance similar to that of the estimation of the boundary values. however, its disadvantage is that it leads to a slight overestimation towards the ends of the torque range. figure 7: relationships between simplified calculations of 𝑢𝑅 ′ and 𝑢𝑆𝑓a ′ and mcm calculations of 𝑢𝑅 and 𝑢s𝑓a , for real measurement data. both ratios remain generally above 1, so the simplified calculations are upper estimates. figure 8: simulated curves of the combined measurement uncertainties for regression with mcm (𝑢p,c), with boundary value estimation (𝑢p,c'), and with the estimation 2𝑁 compared to 𝑢𝑓a in accordance with din 51309. the simulation parameters correspond to the maxima of the values obtained by sampling real measurements. 5.3. interpolation uncertainties of 𝒖𝒇a and 𝒖s𝒇a the contributions to the measurement uncertainty of the residuals 𝑢𝑓a and of the variance of the residuals 𝑢s𝑓a may only be calculated at the reference points. in between the reference points, they have to be interpolated by means of a cubic regression (see section 3). the resulting uncertainty is yielded from the residuals of these regressions, for which a constant absolute value can be found as an upper estimation by means of its maxima in the torque range. in the sampling, 𝑢s𝑓a,int appears to be most of the time considerably smaller than 𝑢s𝑓a (figure 9, top) and does not represent a dominant contribution in the overall uncertainty budget. in contrast to this, the value of 𝑢𝑓a,int is similar to that of 𝑢𝑓a, which is 1,0e-06 1,0e-05 1,0e-04 1,0e-03 0 20 40 60 80 100 u in m v /v torque load in % u_fa_din u_p,c 2·n u_p,c' http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 199 most of the time smaller than 1 × 10-5 for calibrations in accordance with estimator l and thus does not compromise the limits of the overall measurement uncertainty. contributions for calibrations in accordance with estimator p range up to 1 × 10-4 and are therefore not negligible (figure 9, bottom). figure 9: interpolation uncertainty for 𝑆(𝑓a) as upper estimation in the entire torque range, calculated as the maximum of the residuals of a cubic fit to the curve of 𝑆(𝑓a), plotted against the value 𝑆(𝑓a) at the first load step (top). interpolation uncertainty for 𝑓a as the upper estimation in the entire measuring range, calculated as the maximum of the residuals of a cubic fit to the curve of 𝑢𝑓a, plotted against the value 𝑢𝑓a at the first load step (bottom). 6. summary estimating the contribution of the regression to the measurement uncertainty as described in accepted procedures is not always appropriate according to the results of mcm simulations and to the assessment of numerous real calibrations. if the systematic uncertainty of the residuals is larger than the standard uncertainty of the reference points and if the calibration results between the reference points are needed, then the measurement uncertainty obtained by means of accepted procedures may be underestimated by more than a factor of 4. a detailed measurement uncertainty analysis can take the requirements of the user of a calibration into account (whether the calibration values at the reference points only or those between the reference points are needed) in different cases. whether the reference points or the regression must be considered the best estimator for the characteristic curve of the transducer can be calculated in specific approaches. the new procedure avoids underestimation, but also involves high analytical effort. simplified procedures, either in the form of the estimation of the boundary values or of a constant upper estimation, can reproduce the results of the suggested procedure with reasonable effort. 7. references [1] din 51309 – materials testing machines – calibration of static torque measuring devices, 2005. [2] “statische kalibrierung von anzeigenden drehmomentschlüsseln” (in german): guideline dkd-r 3-7, 2018. fehler! linkreferenz ungültig. [3] “statische kalibrierung von kalibriereinrichtungen für drehmomentschraubwerkzeuge” (in german): guideline dkd-r 3-8, 2018. fehler! linkreferenz ungültig. [4] r. r. cordero, p. roth, “on two methods to evaluate the uncertainty of derivatives calculated from polynomials fitted to experimental data”, metrologia, vol. 42, pp. 39-44, 2005. [5] a. silva ribeiro, j. alves e sousa, c. oliveira costa, m. pimenta de castro, “uncertainty propagation in multi-stage measurements using linear regression analysis and monte carlo simulation” in proc. of the xviii imeko world congress, rio de janeiro, brazil, 17-22 september 2006. [6] m. schalles, m. hohmann, “einsatz von montecarlo-methoden zur bestimmung der kennlinienunsicherheit” (in german), vdi-berichte nr. 2319, pp. 151-162, 2017. [7] m. f. beug, h. moser, a. kölling, “bridge amplifier linearity investigation with a cascaded inductive voltage divider setup”, in proc. of the xxii imeko world congress, belfast, united kingdom, 3-6 september 2018. http://www.imeko.org/ a combined 3d surveying, xrf and raman in-situ investigation of the conversion of st paul painting (mdina, malta) by mattia preti acta imeko issn: 2221-870x march 2021, volume 10, number 1, 173 179 acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 173 a combined 3d surveying, xrf and raman in-situ investigation of the conversion of st paul painting (mdina, malta) by mattia preti sebastiano d’amico1, valentina venuti2, emanuele colica1, vincenza crupi3, giuseppe paladini2, sante guido4, giuseppe mantella5, domenico majolino2 1 department of geosciences, university of malta, msida campus, msd 2080, malta 2 department of mathematical and computer sciences, physical sciences and earth sciences, university of messina, viale ferdinando stagno d’alcontres 31, i-98166 messina, italy 3 department of chemical, biological, pharmaceutical and environmental sciences, university of messina, viale ferdinando stagno d’alcontres 31, i-98166 messina, italy 4 department of literature and philosophy, university of trento, via tommaso gar, 14, i-38122 trento, italy 5 giuseppe mantella restauro opere d’arte, circonvallazione paparo 25, isca sullo ionio, (cz) 88060, italy section: research paper keywords: ‘the conversion of st paul’ painting; photogrammetry; xrf; raman; pigment identification citation: sebastiano d'amico, valentina venuti, emanuele colica, vincenza crupi, giuseppe paladini, sante guido, giuseppe mantella, domenico majolino, a combined 3d surveying, xrf and raman in-situ investigation of the conversion of st paul painting (mdina, malta) by mattia preti, acta imeko, vol. 10, no. 1, article 23, march 2021, identifier: imeko-acta-10 (2021)-01-23 editor: ioan tudosa, university of sannio, italy received april 28, 2020; in final form november 4, 2020; published march 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: valentina venuti, e-mail: vvenuti@unime.it 1. introduction in recent years, the use of scientific methods to help with the restoration of artifacts has attracted a great deal of interest [1][4]. here, as testified by the increasing number of recent publications related to three-dimensional (3d) documentation and reconstruction [5], the use of software and various techniques to reconstruct digital models of the artifacts has grown significantly in view of providing useful tools for conservation and cultural heritage documentation, as well as the integration of scientific analysis and results. specifically, the possibility of generating highly accurate and detailed 2d/3d digital models from existing imagery presents a great opportunity for analysis, with various applicable tools available. meanwhile, x-ray fluorescence (xrf) spectroscopy represents a wellestablished, rapid and simple to use non-destructive method that allows for determining the elemental composition of the pigmenting agents of decorated surfaces [6]-[9]. this technique is often used in conjunction with another versatile technique, namely, raman spectroscopy, to achieve the molecular composition of pigments in a non-destructive manner, which is abstract this paper presents the results of three different approaches applied to the newly-restored titular painting entitled the conversion of st paul, the main altarpiece in the mdina cathedral in malta. this large, dramatic painting is the work of the baroque artist, mattia preti, known as il cavaliere calabrese. here, we focus on the results of the digital photogrammetric survey that adopts image-based approaches for 2d/3d model reconstruction. the model was used to quantify important features of the painting as well extensions of the areas restored. in addition, portable x-ray fluorescence and raman spectroscopies were used to non-destructively identify the nature of the painting materials, at the elemental and molecular spatial scales, respectively, with the ultimate goal of reconstructing the colour palette of the artist. the 3d model developed here could be applied to other paintings of preti to conduct comparisons between different measurements in the paintings, with the main goal of clarifying the technique used by the artist. this information, along with the characterisation of the materials used, is crucial for the reconstruction of the historical–geographical context of the artwork, since specific pigmenting agents and media tend to represent the stylistic expression of an artist or an epoque. mailto:vvenuti@unime.it acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 174 mandatory, especially when dealing with pigments of similar atomic composition and/or mixtures of pigments that are difficult to disentangle. the combined use of these two techniques allows for addressing various questions related to provenance, dating, and the manufacturing technology behind a variety of findings [10]. furthermore, in the case of large valuable paintings held in public collections, any form of sampling is discouraged by the curators and conservators. as such, an in-situ approach that requires the spectrometric investigation to be extended into the historical/artistic context is strongly recommended. in this paper, we present the main results obtained via three different approaches, namely, digital photogrammetry, portable xrf spectroscopy, and raman spectroscopy, all of which were used to help with the restoration of the titular painting entitled the conversion of st paul, which is located above the main altar at st paul’s cathedral in mdina, malta (figure 1) [11]. the author of this grand canvas, which measures 533 × 310 cm, was mattia preti, known as il cavaliere calabrese. preti worked on this masterpiece in 1682 having been commissioned to execute the painting by the bishop of malta, the catalan miguel jeronimo de molina y aragonès, a member of the knights of the order of malta, whose coat-of-arms is painted on the column of the righthand side of the painting. the conversion of st paul relates to the series of miracles and mysteries pertaining to malta’s st paul’s. in fact, this was the subject of a major cycle of paintings executed by mattia preti towards the end of his life during the last decades of the 17th century. given its size, the painting can be regarded as the masterpiece among the work completed by preti during his long stay in the maltese archipelago. it describes the moment when the young roman soldier, saulo, was struck blind by the thunderbolt vision of christ when on his way to damasco, whereupon he fell from his horse (event never actually mentioned in the bible) and remained blind for three days. the recent restoration of the conversion of st paul indicated that it was single-handedly painted by the master, without the collaboration of any of his assistants. the original painting is perfectly rectangular and was created by joining three pieces of canvas of equal size. the painting also features two enlargements, the first of which measures around 5 cm in width with the colours painted directly onto the canvas without a preparatory underlay made of oils, globigerina powder, and ochre. meanwhile, the second enlargement is dated to the first part of the xviii century and sees the addition of a new strip of canvas with a height of 50 cm that follows the profile of the new marble frame. the whole surface was covered by a thick layer of extremely greasy yellow/brown paint that altered the entire chromatic relationship. during the cleaning phase, the original testaferrata coat-of-arms, which had been hidden when the painting underwent a lengthening process, was revealed. consolidation was carried out and several gaps were repaired with stucco, while retouching was performed and the surface was given a transparent protective film. motivated by the historical/artistic value of the restoration, we decided to deepen the investigation of the painting using portable equipment, combining a high-performance 3d laser scanning system for surveying with the aforementioned noninvasive xrf and raman spectroscopic techniques. here, the first technique was used for reconstruction purposes, while the latter two techniques were used for pigment characterisation, with the ultimate aim of obtaining useful information regarding certain questions that remain unaddressed, which are largely related to the execution technique and to the dating of the maltese period in which the painting was produced. 2. data aquisition and processing the various modern technological solutions offer great opportunities to obtain complete geomatic surveys in various environments [12] and in the field of cultural heritage [2], [13], [14]. here, photogrammetry can be defined as a scientific pursuit aimed at obtaining reliable information regarding the spatial properties of land surfaces and objects, without the need for physical contact [15]. it presents a relatively new technique for the accurate digital capturing of 3d objects and surfaces, and has become increasingly popular within the conservator community, figure 1. a) workflow adopted to obtain the final digital model for the conversion of st paul painting. b) the painting reconstructed using digital technologies and processing. all points analysed via xrf and/or raman spectroscopy are displayed. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 175 often adopted in the field of cultural heritage as a set of new tools for archaeologists and experts working in this area. the method has the great advantage of having the capacity to capture, store, process, share, visualise and annotate 2d/3d objects. figure 1a presents the workflow adopted to obtain the final digital model for the conversion of st paul painting, which consisted of several phases of processing. the first phase (steps 1 and 2) involved the camera capturing and alignment, which allowed for correctly placing the images in relation to each other and/or calculating the exact position in the real space. it is at this stage that the software generates a sparse point cloud. meanwhile, the second phase (step 3) involved the generation of a dense point cloud based on the estimated camera positions. if required, the dense cloud of points can be modified prior to the generation of the 3d mesh model. the third phase (step 4) involved the reconstruction of a 3d polygonal mesh representing the object surface, as based on the dense point cloud. following the mesh geometry generation and reconstruction, the model was textured and used for orthomosaic generation. figure 1b shows the final digital model of the painting and the location of the pigments analysed via xrf and raman spectroscopy. here, the xrf measurements were carried out using a portable xrf alpha 4000 analyser (innovex-x system), which allows for the detection of chemical elements with an atomic number (z) between phosphorus and lead and is equipped with a ta anode x-ray tube as the source and a si pin diode (active area of 170 mm2) as the detector. the instrument was operated in soil mode, with a compton normalisation algorithm designed for achieving the lowest possible limit of detection (lod) (trace concentrations, ppm levels) for soil and bulk samples. the soil mode was used with the ‘environmental’ elements suite, which includes the following elements: p, s, cl, k, ca, ti, cr, mn, fe, co, ni, cu, zn, as, se, br, rb, sr, zr, mo, ag, cd, sn, sb, i, ba, au, hg, pb. for each sample, two sequential tests were performed, the first with operating conditions of 40 kv and 7 µa and the second with conditions of 15 kv and 5 µa, for a total spectrum collection time of 120 s. the instrument was controlled by a hewlett-packard ipaq pocket pc, which was also used for the data storage. the calibration was performed using a soil light element analysis program (leap) ii and was verified using alloy certified reference materials produced by analytical reference materials international. to obtain better statistics, a fluorescence signal was collected for 60 s per run. for all the investigated samples, the lines detected at around 8.15 and 9.34 kev were attributed to the l and l transitions of the ta anode. meanwhile, the raman spectra were collected using a portable ‘btr 111 mini-ramtm’ (b&w tek, usa) spectrometer with an excitation wavelength of 785 nm (diode laser) and a maximum laser power of 280 mw at the excitation port, and a chargecoupled device (ccd) detector (thermoelectric cooled, te). here, the laser output power can be continuously adjusted by maximising the signal-to-noise ratio while minimizing the integration time, while the system is equipped with a fibre optic interface for convenient sampling. the excitation laser emits from the probe’s end, where the raman signal is then collected from the sample. the spot size was 85 μm at a working distance of 5.90 mm. the maximum power at the samples was around 55 mw. all the spectra were registered in the wavenumber range of 60–3,150 cm−1 by using an acquisition time of 40 s and a resolution of 8 cm−1 while accumulating several scans for each spectrum in order to improve the signal-to-noise ratio. each spectrum was processed by subtracting the blank spectrum, while a smoothing process was performed using bwspec 3.27 software. meanwhile, the assignment of the raman vibrational bands, which was aimed at identifying the used materials, was performed with reference to the relevant databases and libraries [16], [17]. 3. results and discussion the digital model of the painting allowed for obtaining high precision measurements of the artifact as a whole as well as of selected areas of interest. figure 2 and figure 3 show examples of the measurements of selected areas as well as the distances between selected points. specifically, figure 3 shows details of the coat-of-arms in the painting, where two coat-of-arms can be clearly observed. the lower one was added at a later stage when the painting was slightly enlarged to fit a new frame, while the one above, which was previously hidden, was recovered during the restoration works. using the digital model, we were able to map and measure two parts of both coat-of-arms, highlighting the difference in shape and size between them with high precision, that is, with a spatial resolution of the order of millimetres, which is much lower than that of traditional analogical methods. it is worth noting that photogrammetry is an extremely useful tool both for the pre-restoration phase and for further studies and investigations. in fact, such an approach could help historians and conservators to plan any necessary interventions, or to study details of the painting while using the digital model, and, as such, without touching or directly interacting with the artifact. furthermore, the digital model can be used for mapping purposes, as was the case in this study, by inserting certain points of measurements in the model. finally, it is also possible to create a digital repository of historical and scientific information that could be easily consulted remotely. both these tools and the figure 2. details of the digital model of the painting, which highlights the high definition of the model. the table below presents some of the measurements taken from it. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 176 digital model could be combined with modern vision equipment to bring the painting into the virtual reality context, which has endless applications and uses. with the aim of identifying the pigmenting agents used for the different colours and tonalities of the conversion of st paul painting, seven points were selected for xrf and/or raman analyses (see figure 1) with some consideration of the short time available for the restoration and for the exhibition preparation. table 1 presents the entire set of performed measurements as well as the elemental and molecular composition, obtained via the xrf and raman techniques, respectively. meanwhile, figure 4 presents the xrf spectra collected for the red cloth of the lying soldier (point 1, figure 4a), the white band of the soldier standing upright (point 4, figure 4b), the blue area of christ's mantle (point 5, figure 4c), and the black fur of the horse (point 7, figure 4d) as examples. the xrf analysis for all the investigated red/reddish areas (points 1, 2, 3, and 6, see figure 4a for the representative spectrum) indicated a hg-based red pigment, likely cinnabar (hgs), which was used for the vestments, coat-of-arms, and carnations. traces of iron (fe) were also detected, suggesting the use of a small amount of red ochre (fe2o3). pigmenting agents were likely combined with a white pb-based composite, possibly lead white ([pbco3]2·pb[oh]2), to obtain a lighter tonality. the relative intensity of the peaks corresponding to the aforementioned elements differed depending on the desired shades. with regard to the white area (point 4, figure 4b), the observation of the high peaks associated with the lα (around 10.54 kev) and l (around 12.60 kev) transition lines of lead (pb) indicated the use of a pb-based white pigmenting agent, possibly lead white, largely responsible for the white coloration. despite its toxicity, this white pigment was widely used in panel paintings due to its good covering potential and brilliant white colour. in terms of the xrf spectrum of the blue area (point 5, figure 4c), no characterising lines were detected. here, the traces of fe, manganese (mn) and zinc (zn) could be due to the impurities that gradually emerged, which could support the notion of the use of an organic compound that cannot be detected via xrf since its characterising elements are lighter than those revealed by this technique. based on a comparison with findings in the existing literature [18], the application of indigo (c16h10n2o2) for the blue coloration would appear to be the most reasonable hypothesis, certainly given that the presence of this natural blue pigmenting agent has previously been identified in other religious paintings produced by preti during his later activity in malta [19]. here, it is worth noting that the presence of barium (ba) could suggest the use of barium sulphate (baso4), i.e. ‘barium white’, a synthetic compound used as a pigment, extender and filler since the early part of the 19th century [20], thus indicating that figure 3. example of measured areas and lengths of two parts of the coat-ofarms depicted in the painting. 0 3 6 9 12 15 0 2 4 6 8 0 3 6 9 12 15 0 2 4 6 8 10 12 0 3 6 9 12 15 0 2 4 6 8 10 0 3 6 9 12 15 0 2 4 6 8 s k, k s k, k s k, k pb k as k fe k ca k (a) ta l ar k pb l ta l ca k ti k fe k s k, k ar k hg l in te n s it y energy (kev) hg l ar k ar k ca k ca k fe k fe k ta l (b) ta l as k pb l in te n s it y energy (kev) pb l ar k ar k fe k ta k ar k ar k ca k ca k (d) ba l (c) in te n s it y energy (kev) ba l ta k fe k as k pb l pb l pb l in te n s it y energy (kev) ca k ca k fe k fe k pb l as k ta l ta l figure 4. xrf spectra collected (a) on the red cloth of the lying soldier (point 1), (b) on the white band of the soldier standing upright (point 4), (c) on the blue area of christ's mantle (point 5), and (d) on the black fur of the horse (point 7). table 1. elemental and molecular phases of the selected areas of the conversion of st paul, detected via xrf and raman spectroscopy, respectively. the key elements responsible for the analysed pigments are marked in bold, while the minor and trace elements are reported in brackets. analysed point description of the analysed area xrf elements detected raman phase 1 red cloth of the lying soldier s, pb, ca, hg, (cl, as, k, fe, se, sb) cinnabar, iron oxides, calcite 2 red bull of the coat-of-arms located at the bottom right s, pb, cl, as, fe, ca, k, hg, (sn, cd) cinnabar, iron oxides 3 red robe of the soldier standing upright s, pb, cl, as, fe, ca, k, hg, se, (mn, rb) cinnabar, iron oxides, calcite 4 white band of the soldier standing upright s, pb, ca, cl, (as, k, fe, co, zn) lead white, calcite 5 blue area of christ's mantle s, pb, ca, cl, ba, (as, fe, mn, zn, sr) fluorescence 6 reddish area of the incarnate of the cheeks of the little angel s, pb, cl, as, fe, ca, k, hg, se, (cd, mn, zn, rb) cinnabar, iron oxides, calcite 7 black fur of the horse s, ca, pb, cl, (k, as, fe, ba, zn, mn, cu) carbon black acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 177 modern paints were applied to the panel surface during previous restorations. finally, in the case of the black area (point 7, figure 4d), the high peaks of calcium (ca), together with the observation of fe and mn, allowed us to hypothesise about the use of an organic black pigment, based on ca, one that cannot be confirmed by xrf due to the low z-value of the characteristic elements, combined with a mixture of fe-based and mn-based pigments, such as natural and/or burned umber (fe2o3 + mno2 + nh2o + si + al2o3). in addition, it is worth noting that an important presence of ca, sulphur (s), and pb was observed in all the investigated areas. given their presence, these substances were likely related to the preparatory layer, suggesting a ca-based preparation ground that was possibly calcium carbonate (caco3) and/or sulphate (caso4). in terms of the detection of pb, its presence in every analysed point supports the hypothesis regarding the employment of a pb-based imprimation, likely made of lead white, could also have been used as a dryer for the binders. with regard to the raman measurements, we must first focus on the results of the analysis performed on points 2, 3 and 6 of the painting (figure 5), that is, the areas of the red bull of the coat-of-arms located at the bottom right (a), the red robe of the soldier standing upright (b), and the reddish area of the incarnate of the cheeks of the little angel (c), respectively. since the raman spectrum obtained for point 1 was highly similar to those reported in figure 5, it is not shown. as is clear, the spectra indicate the presence of low frequency peaks respectively centred at around 252, 289, and 345 cm−1, which could be attributed to the presence of the cinnabar [21] likely responsible for the intense red colour. cinnabar is a type of red mercury ore that was generally mixed with an equal amount of burning sulphur to create an expensive red paint used since antique times for cosmetic and decorative purposes. meanwhile, the band at around 611 cm−1 can be attributed to the presence of iron oxides, while the bands at around 944, 1,315, and 1,404 cm-1 can be ascribed to the presence of organic compounds derived, presumably, from the use of ligands of natural origin. finally, as figure 5b shows, a 1,083 cm−1 band indicative of the presence of calcite (caco3) was observed, which was likely related to the preparatory layer or to lightening purposes. the investigation of the white pigment was carried out in terms of point 4 of the painting (figure 6), that is, an area of the white band of the soldier standing upright. the spectrum revealed the presence of a strong and sharp raman band at around 1,050 cm−1, which is characteristic of the lead white (2pbco3·pb[oh]2) [22] that represents the main white pigment of european oil on canvas paintings. in fact, as reported in a previous study [23], the presence of this band indicated the degradation state of this pigment. meanwhile, the bands at around 946, 1,304, and 1,404 cm−1 may have been related to the presence of organic components. as with the red pigmented areas, the presence of calcite was indicated by its typical contribution at around 1,084 cm−1. this can be reasonably ascribed to the preparation, even if the combination of calcite with other white pigments, such as lead white, was often used by artists to modify the rheological and optical properties of their paints [24]. with regard to the blue area of christ's mantle (point 5), the raman spectrum obtained using the portable instrumentation was completely masked by huge fluorescence, which prevented the molecular identification of the painting materials. finally, the broad bands detected at around 1,331 and 1,581 cm−1 in the raman spectrum for the black fur of the horse (point 7, figure 7) can be ascribed to the d (disorder) and g (graphitic) bands of amorphous carbon. this occurrence indicates the employment of a carbon-based black pigment, the accurate identification of which via the raman technique remains fairly difficult. we now want to focus on a highly interesting point that surfaced from the entire set of spectroscopic results. in short, the 500 1000 1500 2000 500 1000 1500 2000 500 1000 1500 2000 (a) 348 287 in te n si ty ( a rb . u n its ) raman shift (cm-1) 611 946 253 1320 1404 (b) 611 1083 343 286 250 in te n si ty ( a rb . u n its ) raman shift (cm-1) 1315 1404 944 (c) in te n si ty ( a rb . u n its ) raman shift (cm-1) 253 294 344 611 943 1310 1404 figure 5. raman spectra recorded for three representative red/reddish pigmented areas of the conversion of st paul painting. (a) red bull of the coat-of-arms located at the bottom right (point 2), (b) red robe of the soldier standing upright (point 3), (c) reddish area of the incarnate of the cheeks of the little angel (point 6). 500 1000 1500 2000 1084 946 in te n s it y ( a rb . u n it s ) raman shift (cm-1) 1050 1404 1304 figure 6. raman spectrum recorded on a point (point 4) representative of a white pigmented area of the conversion of st paul. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 178 observation of ca in the xrf elemental composition of all the investigated points, along with the observation of calcite in most of the analysed pigmented areas, allow us to hypothesise about the use of typical maltese globigerina limestone for the preparation of the painting [25]. while we are aware that further petrographical, geochemical and physical analyses are required to unambiguously identify this limestone, the reported information can provide valuable support to ascribing the dating of the painting to the maltese period of this famous calabrian painter. 4. conclusions in this paper, a non-invasive, in-situ investigation was conducted in relation to the conversion of st paul titular painting by the calabrian artist and knight, mattia preti. the study was specifically aimed at demonstrating the potentialities of the combined use of 2d/3d photogrammetric surveys and spectroscopy (xrf and raman) techniques to, on the one hand, achieve a reconstruction, and on the other, achieve, at different spatial domains, the identification and characterisation of the pigments used for this wonderful painting, which can be seen at st paul’s cathedral in mdina, malta. the aforementioned methodologies were successfully applied as part of the scientific analysis carried out prior to, during and following the completion of the recent restoration of the painting. it was demonstrated that the use of photogrammetry in this context can play a key role in the creation of potential bases for research, analysis, and scientific investigations. furthermore, it allows for obtaining a scaled digital model that can be shared among curators and restaurateurs in order to study the artifact and all its features in detail. combining the findings obtained at the elemental and molecular levels via xrf and raman analyses, respectively, we were able to obtain valuable information regarding the artist’s palette. specifically, the main results included that cinnabar and iron oxides were observed in the red/reddish pigments, lead white in the white pigment, and carbon black in the black pigment. furthermore, it was reasonably hypothesised that calcite was used for the preparatory layer of the painting, which allowed us to formulate a hypothesis regarding the dating of the conversion of st paul painting to preti’s maltese period. this combined approach appears promising, not only for research, preservation and restoration activities but also for improving the promotion, dissemination and accessibility in the field of cultural heritage. this research also serves as a proof of concept and the next goal will be to delve deeper into the investigation of the various other paintings produced by mattia preti. acknowledgements the authors thank monsignor charles scicluna, archibishop of malta, monsignor edgar vella, director of mdina cathedral museum, and monsignor anton cassar, archpriest mdina cathedral. we are also grateful to the staff of the cathedral who allowed us to carry out the measurements, even beyond the normal opening hours. this paper was partially supported by the joint r&d bilater project entitled ‘noninvasive investigations for enhancing the knowledge and the valorization of the cultural heritage’, funded by the university of malta and the national research council of italy (biennal programme 2018-2019). references [1] v. crupi, s. d'amico, l. denaro, p. donato, d. majolino, g. paladini, r. persico, m. saccone, c. sansotta, g.v. spagnolo, v. venuti, mobile spectroscopy in archaeometry: some case study, j. spectrosc. 2018 (2018), article id 8295291. doi: 10.1155/2018/8295291 [2] v. venuti, b. fazzari, v. crupi, d. majolino, g. paladini, g. morabito, g. certo, s. lamberto, l. giacobbe, in situ diagnostic analysis of the xviii century madonna della lettera panel painting (messina, italy), spectrochim. acta a mol. biomol. spectrosc. 228 (2020), article id 117822. doi: 10.1016/j.saa.2019.117822 [3] d. fontana, m.f. alberghina, r. barraco, s. basile, l. tranchina, m. brai, a. gueli, s.o. troja, historical pigments characterization by quantitative x-ray fluorescence, j. cult. herit. 15 (2014) pp. 266-274. doi: 10.1016/j.culher.2013.07.001 [4] g. burrafato, m. calabrese, a. cosentino, a.m. gueli, s.o. troja, a. zuccarello, coloraman project: raman and fluorescence spectroscopy of oil, tempera and fresco paint pigments, j. raman spectrosc. 35 (2004) pp. 879-886. doi: 10.1002/jrs.1229 [5] f. hassani, documentation of cultural heritage; 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[21] p. dawson, the vibrational spectrum of α-mercuric sulphide, spectrochim. acta a 28 (1972) pp. 2305-2310. doi: 10.1016/0584-8539(72)80210-1 [22] p.c. gutiérrez-neiraa, f. agulló-rueda, a. climent-fonta, c. garridod, raman spectroscopy analysis of pigments on diego velázquez paintings, vib. spectrosc. 69 (2013) pp. 13-20. doi: 10.1016/j.vibspec.2013.09.007 [23] v.s.f. muralha, c. miguel, m.j. melo, micro-raman study of medieval cistercian 12–13th century manuscripts: santa maria de alcobaça, portugal, j. raman spectrosc. 43 (2012) pp. 1737-1746. doi: 10.1002/jrs.4065 [24] l. de viguerie, h. glanville, g. ducouret, p. jacquemot, p.a. dangb, p. walter, re-interpretation of the old masters' practices through optical and rheological investigation: the presence of calcite réinterprétation des pratiques des maîtres anciens par étude optique et rhéologique: la présence de calcite, c. r. phys. 19 (2018) pp. 543-552. doi: 10.1016/j.crhy.2018.11.003 [25] d. hradil, j. hradilová, g. lanterna, m. galeotti, k. holcová, v. jaques, p. bezdička, clay and alunite-rich materials in painting grounds of prominent italian masters – caravaggio and mattia preti, appl. clay sci. 185 (2020), article id 105412. doi: 10.1016/j.clay.2019.105412 https://doi.org/10.1016/j.jasrep.2018.08.028 https://doi.org/10.7423/xjenza.2017.2.04 https://doi.org/10.1109/iwagpr.2017.7996085 https://doi.org/10.1002/nsg.12046 https://doi.org/10.1515/9783110417104-003 https://doi.org/10.1016/s1386-1425(97)00140-6 https://doi.org/10.1016/0584-8539(72)80210-1 https://doi.org/10.1016/j.vibspec.2013.09.007 https://doi.org/10.1002/jrs.4065 https://doi.org/10.1016/j.crhy.2018.11.003 https://doi.org/10.1016/j.clay.2019.105412 a new design of rockwell-brinell-vickers hardness standard machine at ume acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 230 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 230 234 a new design of rockwell-brinell-vickers hardness standard machine at ume c. kuzu1, e. pelit2, i̇. meral3 1 ume, kocaeli, turkey, cihan.kuzu@tubitak.gov.tr 2 ume, kocaeli, turkey, ercan.pelit@tubitak.gov.tr 3 ume, kocaeli, turkey, ilker.meral@tubitak.gov.tr abstract: after successful implementation of two hardness standardizing machines at tse (turkish standards institution) in rockwell and brinell-vickers hardness scales separately, it became unavoidable to combine them in one machine with a newer design and install it in hardness laboratory of ume (national metrology institute of republic of turkey) within the scope of an internally funded project. in this paper the new design of the rockwell-brinellvickers hardness standard machine with deadweight force application principle and a laser interferometer optic system as depth measurement unit for rockwell hardness scales designed by ume hardness laboratory is explained. keywords: hardness, rockwell, brinell, vickers, hardness standard machine. 1. introduction the activity of realization and development of primary rockwell hardness standard machine working in the same manner was made at istituto nazionale di ricerca metrologica (inrim, national metrology institute of italy) in the 70 years. with the cooperation of inrim and a company called ltf s.p.a. in italy, they together provided such machines to themselves and some other nmis. ume also had designed and installed its own primary rockwell hardness standard machine in ume hardness laboratory in 2009 in the same manner with some modifications and a new machine for tse in 2016 with a better design and more modifications. after successful implementation of two hardness standardizing machines at tse and with the demand and the necessity of renewing ume hardness standard machines in rockwell, brinell and vickers hardness scales, it became an important and unavoidable issue to combine them in one machine with a newer design and install it in hardness laboratory of ume within the scope of an internally funded project. in this project it is aimed at making use of the final design made for the tse, overcoming the small defects of this design, improving some parts in terms of functioning, metrological properties and usage, and covering the missing hardness scales such as hv3, hbw2.5/6.25 etc. in this new design the force application system was considered to comprise mass stacks realizing force under the gravitational acceleration and a newly designed frame to transfer the realized force to the tip of the indenter, to be working in the most straight forward way of deadweight force application principle. this time the mass stacks are also mentioned to be redesigned to be handled easily, during mounting/demounting onto the machine, calibration, repair/replace etc. the mechanical parts are completely renewed, aiming at more stringent, stable and reproducible machine. figure 1: cad of the rockwell-brinell-vickers hardness standard machine http://www.imeko.org/ mailto:cihan.kuzu@tubitak.gov.tr mailto:ercan.pelit@tubitak.gov.tr mailto:ilker.meral@tubitak.gov.tr acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 231 a laser interferometer optic system is integrated to the machine as an indentation depth measurement unit. the depth measurement system is composed of laser source, linear interferometer and suitable optics mounted on top of the indenter, supposed to be in contact with the indenter and realize the same displacement as the indenter. in this way it will be easy to figure out the path through which the indenter is passing inside the material during realization of the hardness measurement. the testing cycle is managed by making use of a load cell to which the whole force application system is mounted. the force application durations are sensed by measuring the force change on the load cell instantaneously during load application and comparing the measured force values with the nominal values belonging to the scale. the indenter approach, force application and removal speeds will be adjusted and controlled by the laser interferometer system to control the servo motor (m1) which will be used for realization of the measurement cycle and force application rate is measured via the change in the load cell value in time. there is a second servo motor on the bottom side of the machine used to make automatic selection of the masses (scales) and with this design the machine will be completely automatic in terms of every step to realize the measurement cycle in rockwell, brinell and vickers hardness scales indicated in the table 1 and 2. the cad of the machine is given in figure 1 and drawing in figure 2. figure 2: rockwell-brinell-vickers hardness standard machine 2. theory and scope the theory of rockwell, brinell and vickers hardness measurement, as well-known methods, are explained in detail in the relevant iso hardness standards [1, 2, 3]. in all of the three scales the hardness measurement is mainly based on the realisation of indentation and measurement of the size of the indentation such as the depth, the diameter and the diagonal length, to be used for calculation of the rockwell, brinell and vickers hardness numbers, respectively, by making use of the empirical equation eqn. 1, eqn. 2 and eqn. 3. these methods are completely used to measure mechanical property of material in line with the hardness definition, that is, the resistance which a material shows against deformation. in this paper the design of the machine used to realize the indentation with the highest accuracy and sensitivity in brinell and vickers scales and including depth of indentation in rockwell scales is explained. under these circumstances, regarding the theory, the following components are in the scope of this project: ➢ force application with the highest accuracy ➢ measurement cycle application precisely ➢ indenter to realize the deformation ➢ indentation depth measurement for rockwell a certain amount of force is applied on a defined shape and material of indenter through a predefined application cycle comprising force application speed and time durations where the force is kept on the indenter applied and shown in figure 3. figure 3: realization of rockwell, brinell and vickers hardness indentations (left to right) after application of the measurement cycle, the realized indentation size, the depth is measured simultaneously; the diameter and the diagonal length are measured by some other device as shown in figure 4. then the measured size of indentation is used to calculate the rockwell, brinell and vickers hardness values by making use of eqn. 1 [1], eqn. 2 [2] and eqn. 3 [3], respectively. figure 4: measurement of rockwell, brinell and vickers hardness indentation size http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 232 𝐻𝑅 = 𝑁 − ℎ 𝑆 (1) 𝐻𝐵𝑊 = 0.102 × 2𝐹 𝜋𝐷2(1 − √1 − 𝑑2 𝐷2 ) (2) 𝐻𝑉 = 0.1891 × 𝐹 𝑑2 (3) where, hr : rockwell hardness hbw : brinell hardness hv : vickers hardness f : force / n h : indentation depth / mm d : indentation diameter/diagonal length / mm d : ball diameter / mm n, s : constant there are 15 rockwell, 21 brinell and 22 vickers hardness scales defined by the iso hardness standards, iso 6508-1:2016 [1], iso 6506-1:2014 [2] and iso 6507-1:2018 [3] constituted with different combination of indenters and force values applied. all rockwell hardness scales, some of the brinell and some of the vickers hardness scales are realized on this machine. the scales to be realized in brinell and vickers are highlighted in the table 1 and table 2. table 1: brinell hardness scales to be realized table 2: vickers hardness scales to be realized 3. design of the machine the machine body, every component constituting rockwell, brinell and vickers hardness scales, equipment used for measurements to control all parameters are considered to be with the highest accuracy to attain the most accurate outcome quantity hardness value. beside the metrological properties, the functionality, easiness to use, stability and confidentiality are the main properties aimed at attaining and the design were made accordingly. additionally production quality of the parts also play an important role in this design. 3.1. body of the machine it is very important to have a rigid and sturdy enough body that will not affect the measurement results. for this reason, the machine body is designed to comprise the minimum number of parts with each having precise dimensional measurements and tight tolerances. all of the parts are produced with cast ironing with small dimensional tolerances at the places connecting to the other working parts. see the cad of the machine body in figure 5. figure 5: rockwell-brinell-vickers hardness standard machine http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 233 3.2. force application system to realize the force with the highest accuracy and stability, deadweight type force application principle is preferred. it comprises mass stacks made up of stainless steel and a small frame to constitute the preload in superficial rockwell hardness and hv3 in vickers hardness and transfer the other loads to the tip of the indenter. the frame is adjusted to apply 3 kgf and the mass stacks will be in additional order to constitute the other force values and scales. the other mass stacks are 12 kgf, 2x15 kgf, 8 kgf, 40 kgf (2x20) and 50 kgf (in three pieces) to constitute the rockwell scales and some brinell and vickers scales. to prevent this small frame from any pendulum and rotational motions during load application which will affect penetration performance of the indenter, it is guided by two air bearings at the two ends, both are working with (4-6) bar air pressure. the whole assembly of the masses and the frame are held by a high accurate force measurement sensor to control the testing cycle through measurement of the change in force in time and record the force application dwell time. the masses are planned to be with high accuracy since they will be used in recalibration of the force sensor equipped onto the machine and may be used for calibration or intermediate check of the force measurement devices used in calibration/verification of hardness testing/calibration machines. the mass stacks are also guided while seating on each other. the six additional mass stacks and the small frame constituting the preload for superficial rockwell scales are shown in figure 6. figure 6: cad of the force application system of rockwell-brinell-vickers hardness standard machine 3.3. indentation measurement system for indentation depth measurement, a laser interferometer optic system is equipped onto the machine. the linear interferometer is placed on an adjustable plate on the machine and a corner cube which is supposed to move as far as the indenter moves is placed onto the indenter and fastened to the indenter. by this manner, movement of the indenter is recorded by automation of the system and depth – time graph is plotted. the target cycle expected to be measured is given in figure 7. figure 7: expected depth of indentation measurement in rockwell hardness (defined by the ccm wgh) 3.4. measurement cycle the measurement cycle is realized with application of force with a predefined indenter approach speeds and force application rates with respect to time. to figure out the measurement cycle of force application, a force measurement device is equipped onto the machine and data from this device is recorded and plotted as in figure 8 and 9. for indentation depth and speed of the indenter measurement in rockwell hardness scales a laser interferometer optic system is equipped on the machine. the speed of the indenter is measured by making use of the data taken from the laser interferometer. with the help of the speed information taken from the laser it is possible to characterize any test cycle for the material or sometime, depending on the hardness scale it is possible to realize a closed loop control. in this case the application speed is possible to correct and change to motor (m1) speed accordingly. figure 8: rockwell hardness measurement cycle http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 234 figure 9: brinell-vickers hardness measurement cycle 3.5. automation of the machine the machine is equipped with two servomotor, one force measurement sensor and optionally one displacement measurement sensor as the main component of the automation. the main servomotor (m1) is realizing the indentation by moving the force application system up-and-down, the secondary servomotor (m2) is used to make the relevant mass stacks to realize the relevant scale, load cell besides the measurement cycle is used to sense loadingunloading, parking etc. there will be enough limit switches, ambient conditions measurement sensors and air pressure measurement gauge. we will try to perform closed loop control during load application to fix the load increase time in accordance with the load size and material properties like hardness. 3.6. performance of the machine in this paper the design of the machine is explained in detail, both in terms of mechanics and automation with hardware and software regarding the measurement cycle and measurement principle. the most important property of this machine is the accuracy and stability in force application method and the design of the system to realize precise and repeatable measurement cycle. as a full automatic dead weight force application system with laser interferometer, after the implementation of this machine the force uncertainty is expected to be less than 5.10-5 relative, that value may go down to 2.10-5 relative. the time uncertainty in measurement cycle is expected to be less than 0.25 s. and the depth measurement uncertainty is expected to be 30 nm about. the uncertainties of the machine including its performance as a whole will be assessed and the results will be published accordingly after installation of the machine in ume hardness laboratory. 4. summary at the end of the work explained above; • a dead-weight type hardness standard machine is designed to realize rockwell, brinell and vickers hardness scales in one machine. • we have designed a new rockwell-brinellvickers hardness standard machine in accordance with relevant iso hardness standards [1], [2], [3], [4], [5], [6], with better metrological specifications. • the scale selection, realization of measurement cycle and other functions are all possible automatically. acknowledgements the author would like to thank mr. çetin doğan, mr. murat karabi̇naoğlu and mr. gökhan öner from ume, the national metrology institute of turkey, for their valuable contributions during the work has been done in this project of ume hardness laboratory. 5. references [1] en iso, 6508-1. metallic materials – rockwell hardness test – part1: test method, 2016. [2] en iso, 6508-3. metallic materials – rockwell hardness test – part3: calibration of reference blocks, 2015. [3] en iso, 6506-1. metallic materials – brinell hardness test – part1: test method, 2014. [4] en iso, 6506-3. metallic materials – brinell hardness test – part3: calibration of reference blocks, 2014. [5] en iso, 6507-1. metallic materials – vickers hardness test – part1: test method, 2018. [6] en iso, 6507-3. metallic materials – vickers hardness test – part3: calibration of reference blocks, 2018. http://www.imeko.org/ calibration of cyclic force with inertial force correction to a fatigue testing machine acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 124 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 124 128 calibration of cyclic force with inertial force correction to a fatigue testing machine tian feng1, yin xiao1, li bo1 1 changcheng institute of metrology and measurement, beijing, china, tfpu@sina.com abstract: the accuracy between a dynamic force and a static force applied on a specimen by a fatigue machine is usually not the same. by establishing physical vibration models of fatigue machines, it is concluded that the error of a cyclic force is mainly caused by the inertial force of the vibration mass between the machine sensor and the specimen. after the inertial force is exactly corrected, the force displayed on the machine would be consistent with the real force on the specimen. a standard dynamic force calibration sensor (dfcs) with an inertial force correction method has been used to do calibration of fatigue testing machines in this paper. compared with the replica test-piece method, the two calibration results are close to each other. keywords: fatigue machine; calibration; dynamic force; inertial force; correction 1. introduction the purpose of cyclic force calibration is to make the indication force of a machine sensor consistent with the actual force loaded on the specimen. replica test-piece method in iso 4965-1 and astm e467-08 has been widely used in fatigue machine calibration [1, 2]. in this method, there are some strict limitations of a calibration device regarding to stiffness and mass. because standard force sensors are widely used in various fields, they are expected to be used to calibrate fatigue machines, even though they usually don’t have the same stiffness and mass with a real specimen. by analysing vibration models of some fatigue machines, it is concluded that some types of machines do not need to have the same stiffness of a dfcs as the real specimen. it also suggests that although use a calibration device giving different mass and stiffness from that of a real specimen, doing inertial force correction would lead to the same result to the replica test-piece. the difference between the results given by the two methods is within 1 %. 2. analysis of the inertial force correction model the dynamic error of a cyclic force generally comes from two aspects: the performance of the machine force measuring system and the influence of the inertial force. the cyclic force error caused by the performance of a machine force measuring system could be avoided or eliminated by selecting a force measuring system with a better dynamic performance. the error caused by an inertial force cannot be avoided and eliminated, it could only be induced as little as possible or corrected as accurately as possible. the difference between a machine sensor force and a specimen real force is due to the fact that the machine sensor and the specimen are not in the same position. there is a component mass that cannot be ignored between them, and the connection stiffness between them is limited rather than infinite. during a dynamic test, the mass of the component between them has an acceleration, so that the inertial force is generated. for the reason that the inertial force does not act on the sensor and the specimen symmetrically, an error of the cyclic force is produced. if the exact value of the inertial force influence is known, the actual force loaded on a specimen could be derived, or calibration to a testing machine force could be done. an analysis of the vibration system model of a fatigue testing machine is needed to determine the influence of an inertial force. local modelling concerned with the force calibration is carried out. the local vibration segment contains all the parts between the testing machine sensor and the specimen. only the mass and stiffness within this segment are considered, while those beyond this segment are ignored. in this case, any fatigue testing machine could be analysed simply as a two-degree-of-freedom vibration model, as shown in figure 1 [3]. http://www.imeko.org/ mailto:tfpu@sina.com acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 125 figure 1: local vibration model of fatigue testing machines according to d’alembert’s principle, the two differential equations of the vibration system are: 𝑚1�̈�1 + 𝑘a(𝑥1 − 𝑥2) = 𝐹0 sin 𝜔𝑡 𝑚2�̈�2 + 𝑘b𝑥2 − 𝑘a(𝑥1 − 𝑥2) = 0 (1) the inertial force 𝐹i is as follows: 𝐹i = 𝑘bδ𝑥2 − 𝑘aδ𝑥1 = 𝑚2δ𝑥2𝜔 2 (2) the relative inertial force 𝐹rel,i is as follows: 𝐹rel,i = 𝐹i 𝐹b = 𝑚2δ𝑥2𝜔 2 𝑘bδ𝑥2 = 𝑚2𝜔 2 𝑘b (3) where: 𝑚1, 𝑚2 the equivalent mass at positions a and b, in kg; 𝑘a , 𝑘b the stiffness at positions a and b, in n·m-1; 𝑥1, 𝑥2 the instantaneous displacements of the elastic elements at positions a and b, in mm; δ𝑥1, δ𝑥2 the instantaneous deformation of the elastic elements at positions a and b, in mm; 𝜔 the angular frequency of a fatigue testing machine, in rad·s-1; 𝐹a, 𝐹b the real force between positions a and b, in kn; 𝐹i the inertial force between positions a and b, in kn. equation (3) above indicates that a relative inertial force is associated with 𝑚2 and 𝑘b. figure 1 illustrates two positions, position a and b. position a is far from the motionless end, on the contrary, position b is close to the motionless end. correspondingly, there are usually two configurations of fatigue machines. one is that the machine sensor lies at position a and the test specimen lies at position b – type 1 configuration. the other one is exactly the opposite, that is the test specimen lies at position a and the machine sensor lies at position b – type 2 configuration. for type 1 configuration that a is the machine sensor and b is the test specimen, both the stiffness and mass of a specimen are the influence factors of an inertial force. the mass and stiffness of every different specimen have to be measured, and the correction of the inertial force induced by dfcs, which has different mass and stiffness from the real specimen, has to be done. the function between the real force loaded on the specimen and the machine sensor force is expressed by equation (4), whereas the function between the dfcs force and the machine sensor force is expressed by equation (5). 𝐹a1 = 𝐹b1 (1 − 𝑚2𝜔 2 𝑘b ) (4) 𝐹a2 = 𝐹b2 (1 − 𝑚2 ′ 𝜔′ 2 𝑘b ′ ) = 𝐹b2 [1 − (𝑚2 + δ𝑚)(𝜔 + δ𝜔) 2 𝑘b + δ𝑘 ] = 𝐹b2 [1 − 𝑚2𝜔 2 𝑘b − 𝑚2(2𝜔δ𝜔 + δ𝜔 2) 𝑘b − (δ𝑚𝑘b − 𝑚2δ𝑘)(𝜔 + δ𝜔) 2 𝑘b(𝑘b + δ𝑘) ] (5) in equation (5), δ𝑚 is the increment of 𝑚2 after the real specimen is replaced by a dfcs in calibration. similarly, δ𝑘 is the increment of 𝑘b , and δ𝜔 is the increment of 𝜔 . where for nonresonant fatigue machine, δ𝜔 = 0. from equation (4), the dynamic error of a machine force obtained by a replica test piece is as follows: 𝛿 = 𝐹a1 − 𝐹b1 𝐹b1 = − 𝑚2𝜔 2 𝑘b (6) from equation (5), the dynamic error of a machine force obtained by a dfcs is as follows. note that equation (7) is equivalent to equation (6). 𝛿 = 1 𝐹b2 [ 𝐹a2 − 𝐹b2 + 𝐹b2 𝑚2(2𝜔δ𝜔 + δ𝜔 2) 𝑘b +𝐹b2 (δ𝑚𝑘b − 𝑚2δ𝑘)(𝜔 + δ𝜔) 2 𝑘b(𝑘b + δ𝑘) ] (7) for type 2 configuration that a is a test specimen and b is a machine sensor, 𝐹a is the real force of the specimen, and 𝐹b is the machine sensor force. because the specimen stiffness is 𝑘a instead of 𝑘b in this case, it is no longer the influence factor of the inertial force, given in equation (3). therefore, for this type of machine, the dfcs does not have to have the same stiffness as the real specimen. however, the correction of the inertial force could still be done using equation (7), where δ𝑘 = 0. http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 126 the inertial force 𝐹i could also be expressed as a function of the acceleration as follows: 𝐹i = 𝑘bδ𝑥2 − 𝑘aδ𝑥1 = 𝑚2δ𝑥2𝜔 2 = 𝑚𝑎 (8) equations (4) and (5) can be expressed equivalently as equation (10) and (11). where in equation (11), δ𝑎 is the acceleration increment of 𝑚2 after the real specimen is replaced by a dfcs. 𝐹a1 = 𝐹b1 − 𝑚2𝑎 (9) 𝐹a2 = 𝐹b2 − 𝑚2 ′ 𝑎′ = 𝐹b2 − (𝑚2 + δ𝑚)(𝑎 + δ𝑎) = 𝐹b2 − 𝑚2𝑎 − δ𝑚(𝑎 + δ𝑎) − 𝑚2δ𝑎 (10) equations (6) and (7) can be expressed equivalently as equation (12) and (13). 𝛿 = 𝐹a1 − 𝐹b1 𝐹b1 = − 𝑚2𝑎 𝐹b1 (11) 𝛿 = 1 𝐹b1 [𝐹a2 + δ𝑚(𝑎 + δ𝑎) + 𝑚2δ𝑎 − 𝐹b2] (12) for type 2 configuration of non-resonant fatigue machine, when 𝐹b1 = 𝐹b2 in equation (13), δ𝑎 = 0. in a cyclic force calibration, the real specimen is replaced with a dfcs. thus, the extra inertial force induced by the difference of the mass and stiffness of the dfcs from the real specimen should be corrected. to do the correction to the cyclic force, the first step is to measure 𝑚2 and 𝑘b, then the second step is to measure δ𝑚 and δ𝑘 for type 1 configuration, or only δ𝑚 for type 2 configuration. similarly, for acceleration method correction, the first step is to measure 𝑚2 and 𝑎, and the second step is to measure δ𝑚 and δ𝑎 for type 1 configuration, or only δ𝑚 for type 2 configuration. 3. experiments and discussion 3.1. composition of the calibration system a cyclic force calibration system with the inertial force correction function consists of a dynamic force measurement subsystem, an inertial force correction subsystem, an alignment measurement subsystem, and a data acquisition and processing subsystem. the dynamic force measurement subsystem includes a dfcs subsystem and a replica test-piece subsystem. a set of dfcs consists of some standard dynamic force sensors with different force ranges, as shown in figure 2. the inertial force correction subsystem includes an acceleration measurement subsystem and a stiffness/displacement measurement subsystem, as shown in figure 3. figure 2: dfcs figure 3:the inertial force correction subsystem 3.2. methodology and procedure of tests tests were done in an electromagnetic resonance fatigue machine with controlled force, as shown in figure 4. there is an inertial mass compensation function for this machine, but this function is not used in tests so that the inertial force compensation function of the dfcs could be verified. acceleration transducer grip a dfcs grip b machine sensor figure 4: calibration to an electromagnetic resonance fatigue testing machine with a dfcs firstly, use a replica test piece for calibration, and secondly, use a dfcs. when a dfcs is being used, a stiffness method or an acceleration method for additional inertial force correction to the dfcs are simultaneously used. the dfcs is connected to the fatigue testing machine with proper adapters, and the acceleration transducer is installed on the vibration mass between the dfcs and the testing machine sensor. set up the dynamic test force of the fatigue testing machine and start the machine. when http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 127 the dynamic test force reaches the target value and the sine wave is stable, collect the peak and valley values of 50 cycle waveforms, and record the test frequency and the acceleration value. the displacement data are collected at the same time with the former static force calibration, and then the stiffness value is calculated. 3.3. data of tests the calibration data is shown in table 1 and table 2. the calculations for the relative error of the cyclic force are given in equations (6), (7), (12), and (13) and the calculation parameters are shown in table 3. the sample rate is 5 khz, bessel filter frequency is 500 hz. table 1: calibration data of the replica test piece data of the replica test piece test frequency / hz peak force of the machine / kn valley force of the machine / kn amplitude of the machine force / kn peak force of the test piece / kn valley force of the test piece / kn amplitude of the test piece force / kn error of the machine amplitude / % difference from dfcs / % test 1 110.3 300.01 99.99 100.01 301.80 99.56 101.12 -1.10 -0.41 test 2 110.2 99.95 -100.05 100.00 102.02 -101.14 101.58 -1.55 -0.26 test 3 110.3 300.01 99.99 100.01 301.80 99.56 101.12 -1.10 -0.63 test 4 110.3 99.95 -100.05 100.00 102.02 -101.14 101.58 -1.55 -0.46 table 2: calibration data of the dfcs data of the dfcs test frequency 𝒇 / hz peak force of the machine / kn valley force of the machine / kn amplitude of the machine force / kn peak force of the dfcs / kn valley force of the dfcs / kn amplitude of the dfcs force / kn error of the machine amplitude (uncorrected) / % error of the machine amplitude (corrected) / % test 1 97.2 300.00 99.98 100.01 302.73 97.66 102.54 -2.46 -1.51 test 2 97.0 99.97 -100.04 100.01 102.94 -102.71 102.82 -2.74 -1.81 test 3 97.2 300.00 99.98 100.01 302.73 97.66 102.54 -2.46 -1.73 test 4 97.0 99.97 -100.04 100.01 102.94 -102.71 102.82 -2.74 -2.01 table 3: calculation parameters in additional inertial force correction of the dfcs calculation parameters of the stiffness method correction 𝒎𝟐 / kg 𝚫𝒎 / kg 𝒌 b / (n·m-1) 𝚫𝒌 / (n·m-1) 𝝎 / (rad·s-1) 𝚫𝝎 / (rad·s-1) test 1 35 6 3.1e+08 -7.0e+07 693.0 -82.3 test 2 35 6 3.1e+08 -7.0e+07 692.4 -82.9 calculation parameters of the acceleration method correction 𝒎𝟐 / kg 𝚫𝒎 / kg 𝒂 / (m·s-2) 𝜟𝒂 / (m·s-2) test 3 35 6 43.8 12.6 test 4 35 6 43.8 12.6 3.4. discussion the analysis of data according to the test results, the differences between the dfcs results with inertial force corrections and the replica test-piece results are mostly within 0.5 %. the differences of individual comparison data exceed 0.5 %, which might be caused by the inconsistency between the real measurement position and the ideal measurement position centre of equivalent inertial mass in the acceleration measurement. further studies about the acceleration distribution along the axis of the applied force and the acceleration measurement method are needed. the necessity of the dfcs method the calibration method of a replica test-piece has been used according to iso 4965-1. replica testpieces are made of one or more test-pieces to be tested, which are bonded with strain gauges. replica test-pieces are used in dynamic force calibration of fatigue machines after they are calibrated on force standard machines. because the calibration state is highly consistent with the testing state and there is no additional inertial force caused by external factors, a high accurate calibration result could be http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 128 achieved using the replica test-piece calibration method. but the disadvantages of this method are as follows: (1) different replica test-pieces are needed to be made for different fatigue machines. different replica test-pieces are also needed to be made for some same specification fatigue machines to be calibrated, only because the materials and specifications of the test-pieces of each fatigue machine are different. therefore, the production cost of the replica test-pieces is high; (2) because the maximum bearing force of the replica test-piece made of the actual test pieces to be tested on a fatigue machine usually could not cover the full range of the fatigue machine force, the maximum force of the dynamic force range of the fatigue machine could not be calibrated by its own replica test-piece of the fatigue machine; (3) because of the factors of the replica test-piece caused by the strain gauge bonding process, it is difficult to evaluate the uncertainty of the replica test-piece itself. the advantages of the dfcs calibration method are as follows: (1) the whole dynamic force range of a fatigue machine could be calibrated with a proper dfcs; (2) a dfcs could be reused, and could be used for many different fatigue machines. so it costs low in use and is easy to be promoted; (3) the technology of force sensor is mature, and its own uncertainty could be accurately evaluated for a qualified force sensor; (4) the dfcs method is not only suitable for a typical metallic material fatigue testing machine, but also for some other types of fatigue testing machines and for actuators with single or combined use. the disadvantage of dfcs method is that some additional inertial force influences are induced. in order to overcome this shortcoming, it is necessary to give the inertial force compensation method. there are two typical types of fatigue machines, which are the resonant fatigue machine and the nonresonant fatigue machine. for each type machine, there are two kinds of configurations, which are type 1 and type 2 configuration. so there are mainly 4 kinds of machines widely used so far. the above are the types and structures of the fatigue testing machines. aiming at the influence of inertial force always existing in fatigue machines, there are two methods of correcting the inertial force ,which is the stiffness method and the acceleration method. they are two correction methods which could be selected to use in a calibration. 4. summary (1) using a replica test piece that has the same stiffness and mass with the real specimen gives an accurate result of a cyclic force calibration because no other inertial force is induced except the one by the real specimen. (2) it would be best if the dfcs has exactly the same mass and stiffness as the real specimen, so that no additional inertial force is generated, and no correction is needed. (3) it is not necessary to strictly limit the mass and stiffness of the dfcs to be consistent with the real specimen. by accurately measuring mass and stiffness or acceleration to correct additional inertial force, the same accurate calibration results could be obtained by using a dfcs different from the real specimen. the difference between the dfcs and the replica test-piece methods is within ±1 %. (4) with an appropriate adaptor, a dfcs could be reused in the calibration for different fatigue machines. further studies about the acceleration distribution along the axis of the applied force and the acceleration measurement method are needed. 5. references [1] iso 4965-1, “metallic materials dynamic force calibration for uniaxial fatigue testing part 1 testing systems”, iso, switzerland, 2012. [2] astm e467-08, “standard practice for verification of constant amplitude dynamic forces in an axial fatigue testing system”, astm international, usa, 2014. [3] d. georgakopoulos, j. m. williams, a. knott, t. esward, p.s. wright, “dynamic calibration of fatigue testing machines”, in proc. of british electromagnetic measurements conference, teddington, uk, 14-17 november 2005. online [accessed 20201105]: http://eprintspublications.npl.co.uk/3317/1/bemc 2005-11.pdf http://www.imeko.org/ http://eprintspublications.npl.co.uk/3317/1/bemc2005-11.pdf http://eprintspublications.npl.co.uk/3317/1/bemc2005-11.pdf rockwell hardness unmanned precision metering calibration device acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 274 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 274 280 rockwell hardness unmanned precision metering calibration device shi wei1, zhang kailin2, tao jizeng1, li yang 1 1 beijing changcheng institute of metrology & measurement, beijing, china, swlltm@126.com 2 beijing university of aeronautics and astronautics, beijing, china, 13811357882@163.com 1 beijing changcheng institute of metrology & measurement, beijing, china, taojz_75@163.com 1 beijing changcheng institute of metrology & measurement, beijing, china, 978827741@qq.com abstract: based on the concept and technology of precision metering and intelligent metering, this research combines high-precision force loading technology, indentation measurement technology, automatic control technology and robot technology. an unmanned precise measurement and calibration device for rockwell hardness is designed and developed. the design scheme and research results are given. the test results show a high measurement accuracy of the device, and it has obvious advantages in the standardization and consistency of the test process. the completion of this device has made a great breakthrough in the work of hardness measurement and calibration and provided a good direction for the development of other parameter unmanned precision measurement and calibration. keywords: hardness measurement, rockwell hardness, unmanned, calibration device 1. introduction rockwell hardness is an index to determine the hardness value by the depth of plastic deformation of indentation. rockwell hardness is widely used in aerospace, automotive, marine, instrumentation and other manufacturing industries. almost 70% of raw materials and parts require rockwell hardness test, and the accuracy and efficiency of the hardness test results have always been the goal pursued by hardness measurement personnel[1]. at present, the degree of automation of rockwell hardness standard machine is relatively low. in the process of pressing the rockwell blocks, the replacement of different rockwell hardness blocks and the transformation of different indentation positions on each hardness block are all manually operated, which seriously affects the completion rate of the measurement task. at present, automatic measurement of single indentation point has been realized in hardness measurement, including automatic force loading and automatic indentation measurement. but there is no multi-indentation point, multi-hardness block automatic measurement device. based on the load sensor and differential sensor, liang weichao [2] realizes the load and depth measurement of the force through the pid algorithm and carries out the automatic control of rockwell hardness detection. hruz [3] et al. used particle swarm optimization to automatically measure the hardness of samples with poor surface quality. at present, the one-key operation of hardness measurement force value loading and indentation measurement has been achieved. balogh [4] et al. studied the combination of the manipulator arm system and soil hardness measuring instrument to realize the automatic measurement of soil hardness. with the development of mechanical automation technology, the automatic feeding technology using mechanical arm has gradually matured. therefore, the use of mechanical arm to replace manual movement of the dot position and manual loading and unloading of the hardness block can not only improve the efficiency of measurement calibration, but also improve the accuracy of measurement due to the reduction of human involvement. with the development of precision measurement and intelligent technology, this research combines the advantages of the two technologies well and is the first to apply the manipulator to the rockwell hardness standard device. it also realizes the unmanned precision measurement calibration of the rockwell hardness blocks. it can not only implement the automatic sample of multiple rockwell hardness blocks, but also automatically complete the automatic transformation of different pressure points of each rockwell hardness block, which would dramatically increase automation, free up manpower and resources, improve the comprehensive technical ability of rockwell hardness machine and the overall quality of the system. http://www.imeko.org/ mailto:swlltm@126.com mailto:13811357882@163.com mailto:taojz_75@163.com mailto:978827741@qq.com acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 275 2. device design and development 2.1. overall technical solution the overall technical scheme is shown in figure 1. the overall structure of the device is mainly composed of intelligent loading laser rockwell hardness measurement system, robot automatic feeding system and interactive control system. the intelligent loading laser rockwell hardness measurement system includes force loading system and indentation depth measurement system. the robot feeding system consists of the robot body, the grab, the sample loading system and the secondary positioning system. interactive control system includes computer control system, robot control system and interactive communication between them. figure 1: overall technical solution the rockwell hardness automatic calibration device designed in this paper can grasp and move hardness blocks of different shapes including rectangle, circle and ring smoothly and accurately. the precision of the manipulator to move the hardness block is ± 0.1 mm. the mass of the workpiece can reach up to 1 kg. therefore, this device can realize the automatic grasping and moving of all rockwell hardness blocks. rockwell hardness measurements with rulers of hra, hrb and hrc can be achieved. 2.2. subsystem technical solution 2.2.1 intelligent loading laser rockwell hardness measurement system the intelligent loading laser rockwell hardness measurement system is shown in figure 2, including two parts. the first is the intelligent force loading system, which uses a motor to drive the screw to drive the weight to rise and fall to load and unload the test force, so as to obtain the accurate test force value. the whole process of force application is monitored and fed back through a high-precision force sensor to achieve intelligent control in the whole process. the second is the indentation depth measurement device. the laser michelson heterodyne interference system installed above the spindle follows the displacement movement of the spindle and measures the depth of indentation in real time. the whole test process realizes one-key automatic operation. figure 2: overall structure diagram of intelligent loading laser rockwell hardness standard device figure 2: overall structure diagram of intelligent loading laser rockwell hardness standard device figure 4: scheme diagram of dual-frequency heterodyne laser interferometer system figure 4: scheme diagram of dual-frequency heterodyne laser interferometer system figure 3: rockwell hardness measuring device figure 3: rockwell hardness measuring device pc motor driver servo motor shock absorbing device ball screw damper weights of all levels weight change bracket mechanism motormotor driverforce selection control panel intelligent force loading system indenter sample main shaft force sensorconditioning amplifiera/d noncontact indentation measurement system heterodyne laser interferometer device intelligent control panel laser signal usb mirror amp6 dedicated cable demodulation system powerdemodulation system power 220vac 50hz 220vac 50hz photoelectric converter powerphotoelectric converter power speed output (analog)speed output (analog) pc (displacement output) environmental parameters speed demodulation system speed demodulation system demodulation part bncbnc optical part dual-frequency laser interferometer for measuring length displacement signal output displacement signal output displacement signal output (digital)displacement signal output (digital) laser powerlaser power http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 276 the structure diagram of force loading system is shown in figure 3. when the motor starts to work, it drives the synchronous ruler belt, reducer and ball screw to rotate, and the screw mother moves downward in a straight line. the weight of the small hanging and connecting parts on the motor is loaded onto the main shaft to realize the initial load. the screw mother continues to move downward in a straight line, and automatically controls the variable load motor set to release the first-level weight. the main shaft is loaded with the weight of the large crane and its connectors and the first-level weights to realize the first-level loading. the screw mother continues to move downward in a straight line, and the second weight of the variable load motor set is automatically controlled. the second level weight is loaded onto the main shaft to realize the second level loading; the loading speed of the whole loading process is controlled by the sensor's feedback to the loading force value. the laser indentation depth measurement system adopts the principle of dual-frequency heterodyne laser interferometer. the schematic diagram of the system scheme is shown in figure 4. the dualfrequency length measuring laser interferometer takes the optical head and demodulation system as a whole, takes the plane mirror as the cooperative measuring target, and directly outputs the displacement digital signal to the computer. the computer collects the environment temperature and humidity and air pressure, compensates the displacement automatically by software, displays the displacement measurement result directly, and gives the real-time curve of displacement. speed demodulation system as an independent function module, an extension of the system function, can directly obtain the speed output (analogy signal). there was no direct mathematical model for rockwell hardness test. according to various factors affecting hardness measurement, the following functions could be considered: ),,,,,,( 00 lttrfffhrc = (1) where: hrc -rockwell hardness value; f0、f -initial test force, total test force; r、α-conical angle of the head, radius of arc; t0、t -retention time of initial test force and total test force; l residual indentation depth. the gum uncertainty evaluation method was used to evaluate the uncertainty of rockwell hardness. according to the requirements of the international rockwell hardness definition, the uncertainty propagation coefficient of each component was determined. the influencing factors included: initial test force, total test force, indenter angle, indenter radius, initial test time, main test time and the error of the depth measurement system. deviations included test deviations and deviations from standard gauges. the standard uncertainty of each factor was obtained through the uncertainty synthesis of the deviation of each factor, as shown in table 1. the uncertainty propagation coefficient of each component defined by the international definition could be used to calculate the uncertainty component of each influencing factor to hardness. since each component was independent and unrelated, the uncertainty of synthetic standard could be calculated as follows: = )()( 222 ii xch  (2) the uncertainty of the synthetic standard of the device in this study was: hrcxch iic 098.0)()( 22 ==   (3) when k =2, the extended uncertainty u of the automatic sample loading hardness measuring device was: hrchku c 2.0)( ==  (4) 2.2.2 automatic feeding system of robot nachimz04-01 series is selected as the robot body, and the appearance diagram is shown in figure 5. it is small and compact, free of installation conditions, and can be adapted to any installation conditions; the cable of hollow wrist can avoid interference and improve the reliability of arm wiring. it has a smooth body, which reduces the concave and convex feeling of the robot surface and makes it easier to clean. it has fast action table 1: summary table of standard uncertainties influence symbol/unit error/um uncertainty coefficient u2 initial forch f0/n 0.03 0.016 0.17 7.40e-06 total force f/n 0.09 0.051 -0.08 1.66e-05 indenter angle α/° 0.09 0.05 0.36 3.24e-04 indenter radius r/μm 1.4 0.80 0.1 6.40e-03 initial time t0/s 0.5 0.29 0.016 2.15e-05 total time t/s 0.3 0.17 -0.05 7.23e-05 deep measurement l/μm / 0.05 -1 2.50e-03 http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 277 performance, which is the fastest robot of the same level, which is helpful to improve the production efficiency. it can support various applications, such as providing vision sensors, force sensors, additional shafts and other specifications. figure 5: mz04-01 robot body appearance diagram the robot gripper is composed of a grab driver and a grab: 1)fetch driver: the design of fetch gripper of the robot needs to grasp and move different shapes of hard blocks smoothly and accurately. according to the known conditions: the maximum workpiece quality is considered as 0.5 kgf, and workpiece size is biggest ф 60 mm * 10 mm. the inner diameter clamping method is adopted, and the clamping point position is calculated as 92.5 mm. when the pressure is 0.6 mpa, the clamping force of the inner diameter is 60 n and the friction coefficient is 0.25. considering the impact of acceleration, the actual clamping force is calculated to be 2.72 times of the required clamping force by taking into account acceleration and the time to accelerate to the maximum velocity of 600 mm/s is 0.5 s. therefore, it is more appropriate to adopt mhkl2-25d largestroke pneumatic clamping jaw of smc as handling drive, and its appearance diagram is shown in figure 6. it adopts a rigid wedge structure, so there is no lateral swing in the direction of travel after holding the workpiece. due to the built-in air claw speed regulating needle valve, the close speed of air claw can be adjusted; combined with sliding bearing guide, the running rigidity and smoothness of the gripper are increased. the repeatability can reach ± 0.01 mm. through the closed dust cover can prevent dust, water and other into the main body, but also can avoid the dust and grease produced inside the air claw flying[5]. figure 6: mhkl2-25d appearance diagram 2) in order not to damage the operating surface of the hardness block, the robot gripper adopts 2a12 hard aluminium block as the main material. with the surface anodizing treatment, the weight of the gripper is greatly reduced without reducing the strength of the gripper. schematic diagram of gripper is shown in figure 7. figure 7: schematic diagram of gripper the material placement system is used for placing samples to be tested and samples that have been tested. to ensure that the hardness block is easy to hold, the test sample block should be placed vertically as shown in figure 8. the manual feeding platform adopts a 4040 aluminium profile as the main structure. workpiece tray is made of nylon 1010 as the base material. the tray can carry 42 workpieces (3×14) at a time, which can achieve zero scratch on the workpieces. the control system is loaded on the main structure of the material placement system[6], which greatly reduces the occupied space of the equipment and improves the operability. figure 8: material placement system http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 278 the secondary positioning device is shown in figure 9. the smc large-stroke mhl2-20d1 translation-cylinder is adopted as the secondary positioning unit. secondary position correction can be made for different workpiece products to improve the flexibility of equipment. the robot gripper grabs the hardness block and places it on the platform of the secondary positioning device. the clamp of the secondary positioning device clamps the hardness block to measure the shape and size of the hardness block. at the same time, when the position of the hardness block is deviated, the centre of gravity of the hardness block can be adjusted by clamping to compensate the position error of the hardness block. after measuring the shape and size of the hardness block and compensating the position, the hardness block shall be pressed according to the set product specification and path. figure 9: secondary positioning device 2.2.3 interactive control system the principle block diagram of the computer control system is shown in figure 10. it is mainly used to control the reset and automatic variable load of weights, the automatic loading and unloading of test force, the automatic measurement of indentation depth, the automatic processing and operation of data, and the automatic determination of qualified and unqualified results. after the computer control system receives the reset signal, the control motor drives the loading mechanism to reset. during this period, the control system constantly detects the on-off state of the reset state line. when it reaches the reset state, the motor stops moving. according to the selected test force, the control system drives the corresponding weight corner mechanism to complete the automatic transformation of loading weights, and then the control system waits for the next command. after the control system receives the command to start the test, the computer automatically completes the loading, delay, main load loading, delay, main load unloading and initial load unloading of the initial load test force through controlling the motor with power loading mechanism. during this period, the control system collects the force sensor signal and the spindle running position and state signal in real time, controls the different speed of the motor and determines the reading time of the indentation depth of rockwell hardness according to the different state of the signal[7]. at the same time, the curve of force value changing with time is plotted. the displacement signals of the laser interferometer are collected at two moments after the initial test force loading and the main test force unloading to complete the calculation of parameters such as rockwell hardness. at the same time, the curve of the indentation depth of rockwell hardness changing with time is drawn. c o m p u te r c o n tr o l s y ste m driver force-loading mechanism actuating motor acquisition card i/o card i/o card acquisition card conditioning amplifier force sensor condition line weight angle mechanism laser interferometer robot figure 10: schematic diagram of computer control system the computer and robot information interaction system requires coordination and cooperation in order to realize automatic grasping of hardness block, automatic pressing the indentation point, measurement and determination under the overall control of the computer. its interaction flow and interaction signals are shown in figure 11. the interaction system between computer and robot communicates by means of six pulse signals, as shown in the figure, the first, third, fourth and fifth signals are the impulse signals sent by computer to robot, and the second and sixth signals are the impulse signals fed back by robot to computer, which are controlled by computer through pci3232i/o card. c o m p u te r c o n tro lle d sy ste m r o b o t c o n tro l sy ste m first signal: start signal third way signal: single point completion signal fifth signal: hardness block unqualified signal fourth signal: hardness block qualified signal sixth signal: the completion signal of robot feeding second signal: robot ready, signal over figure 11: interaction signals between computer and robot http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 279 2.3 research results this paper has realized the development of an unmanned precision measurement and calibration device for rockwell hardness, as shown in figure 12. the research results have the following characteristics: 1) the precision measurement of rockwell hardness is realized by using intelligent laser standard device to calibrate the hardness value; 2) using robot hands instead of human hands, which realizes the automatic grasping and sampling of multi-sample hardness blocks; 3) by using qualified material plate and unqualified material plate, the automatic determination and classification of the calibration results of the calibrated hardness block is realized; 4) the device realizes automatic unmanned measurement calibration of rockwell hardness throughout the test process; 5) different experimental sample blocks (round blocks and squares) can be measured, and different dot trajectories can be set. figure12: unmanned precision measurement and calibration device for rockwell hardness 3. experimental results and analysis rockwell hardness unmanned precision metering calibration device realized automatic metering calibration of 42 hardness blocks at a time. 12 hardness blocks with different hardness values were selected for the test. according to iso6508 metallic materials-rockwell hardness test[8], the original manual method was used to manually measure 6 points of each hardness block. finally, the standard value and uniformity of the hardness block were obtained. after the measurement was completed, the 12 hardness blocks were put into the material placement system. set measuring scale and dot track to realize the automatic measurement of hardness block. similarly, each hardness block was measured for 6 times (the first point is excluded) to obtain the hardness value and uniformity measured automatically. the test data were shown in table 2. in table 2, the standard value and the standard uniformity were the measured results of manual sample placement, while the test value and the test uniformity were the measured results of unmanned automatic sample placement. the hardness value deviation and the uniformity deviation were respectively compared. it could be seen from table 2 that the deviation of uniformity between automatic measurement and manual measurement was 0.1hrc at most. it proved that the unmanned calibration device has good repeatability and stability, and could reflect the uniformity of hardness block objectively and truly. the maximum deviation between the hardness value measured automatically and the standard value measured manually was 0.1hrc. it was shown that the unmanned calibration device of rockwell hardness in this paper has high accuracy, so it could completely replace the standard device of manual measurement. table 2: test record results hardness block number standard values（ hrc） standard value uniformity (hrc) test value (hrc) test value uniformity (hrc) hardness value deviation （hrc） uniformity deviation (hrc) r2007-101 61.2 0.1 61.1 0.2 -0.1 0.1 r2007-102 61.0 0.1 61.1 0.1 0.1 0.0 r2007-103 60.5 0.2 60.6 0.3 0.1 0.1 r2007-104 61.3 0.2 61.3 0.1 0.0 -0.1 r2007-105 63.1 0.2 63.2 0.2 0.1 0.0 r2007-106 63.5 0.2 63.6 0.3 0.1 0.1 r2007-107 40.8 0.2 40.8 0.3 0.0 0.1 r2007-108 41.8 0.3 41.7 0.4 -0.1 0.1 r2007-109 44.5 0.4 44.4 0.5 -0.1 0.1 r2007-110 41.9 0.4 41.8 0.4 -0.1 0.0 r2007-111 43.6 0.2 43.5 0.3 -0.1 0.1 r2007-112 42.0 0.5 42.0 0.5 0.0 0.0 http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 280 the completion of rockwell hardness unmanned precision measurement calibration device truly realized the unmanned precision measurement calibration of rockwell hardness, which has the following significance: 1) breakthrough in system control: it could automatically filter whether the hardness block was qualified or not, and automatically saved file parameters, etc. it could be continuously used for five hours without human intervention, and truly realized the unmanned automatic calibration of hardness blocks. it completely liberated the human resources and greatly improved the work efficiency; 2) breakthrough in calibration specification: the pressure position changed of a single hardness block in rockwell hardness automatic metering and calibration device was completed automatically by the robot hand through the set program. the position of the dot was completely in accordance with the spiral of archimedes, and the position of the marking point of the hardness block was further standardized； 3) during the calibration process, the system would automatically screen the hardness block for uniformity. the whole working procedure was orderly, the operation flow was simple and standard, and the device runed stably and reliably the device perfectly combined the precision measurement technology with the intelligent measurement technology. it has not only achieved a major breakthrough in hardness measurement, but also provided a good direction for the development and development of other hardness precision measurement calibration. 4. references [1] esteban broitman, indentation hardness measurements at macro-, micro-, and nanoscale: a critical overview. springerlink, vol.65, no.23, pp.1-18, 2017. [2] liang weichao, application of hardness detection based on automatic control system. engineering and test, pp. 45-46, 2016. [3] m. hruz, j. siroky, d. manas, particle swarm optimization for automatic hardness measurement. chemicke listy. vol.106, pp.434-437, 2012. [4] zs. balogh, f. bordás. sz. bérczi, manipulator arms and measurements with them on hunveyor college lander: soil hardness measurements in the test-terrain surrounding the lander. lunar and planetary science. vol.33, 2002. [5] hu zhigang, control system design for precise positioning of manipulator handling materials. experimental technology and management. vol.31, no.6, pp.93-96, 2014. [6] liu xu, wei xin, miao jie, et al. design of an automatic loading and unloading device for a stamping machine. mechanical engineering and automation. vol.4, pp.115-116, 2019. [7] yan j. h., guo x., liu y. b., et al. design and kinematics analysis of a modular manipulator. journal of harbin institute of technology. vol.47, no.1, pp.20-25, 2015. [8] iso 6508-1 metallic materials – rockwell hardness test – part 1: test method. http://www.imeko.org/ https://apps-webofknowledge-com-443.e2.buaa.edu.cn/outboundservice.do?sid=6adv4wy8qun7t4v8gkj&mode=rrcauthorrecordservice&action=go&product=wos&daisids=3127226 https://apps-webofknowledge-com-443.e2.buaa.edu.cn/outboundservice.do?sid=6adv4wy8qun7t4v8gkj&mode=rrcauthorrecordservice&action=go&product=wos&daisids=852286 https://apps-webofknowledge-com-443.e2.buaa.edu.cn/outboundservice.do?sid=6adv4wy8qun7t4v8gkj&mode=rrcauthorrecordservice&action=go&product=wos&daisids=1070986 in-situ multi-analytical study of ongoing corrosion processes on bronze artworks exposed outdoors acta imeko issn: 2221-870x march 2021, volume 10, number 1, 241 249 acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 241 in-situ multi-analytical study of ongoing corrosion processes on bronze artworks exposed outdoors leila es sebar1, leonardo iannucci1, caterina gori2, alessandro re3, marco parvis4, emma angelini1, sabrina grassini1 1 department of applied science and technology, politecnico di torino, italy 2 curator of the gori art collection, fattoria di celle, santomano, pistoia, italy 3 physics department, università di torino and infn, sezione di torino, italy 4 department of electronics and telecommunication, politecnico di torino, italy section: research paper keywords: in-situ measurements; raman spectroscopy; electrochemical impedance spectroscopy; atmospheric corrosion citation: leila es sebar, leonardo iannucci, caterina gori, alessandro re, marco parvis, emma angelini, sabrina grassini, in-situ multi-analytical study of ongoing corrosion processes on bronze artworks exposed outdoors, acta imeko, vol. 10, no. 1, article 32, march 2021, identifier: imeko-acta-10 (2021)-0132 editor: ioan tudosa, university of sannio, italy received may 25, 2020; in final form october 20, 2020; published march 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: leila es sebar, e-mail: leila.essebar@polito.it 1. introduction conservation prevents or slows the deterioration of cultural heritage by controlling the environment and the artwork’s structure to maintain it in as unchanged a state as possible. according to the latest definitions, conservation includes preventive conservation, remedial conservation and restoration. all measures and actions in this field should respect the significance and the physical properties of the cultural heritage item. at the 15th triennial conference (new delhi, september 2008) the icom-cc (international council of museums committee for conservation) defined the term ‘preventive conservation’ as ‘all measures and actions aimed at avoiding and minimizing future deterioration or loss. they are carried out within the context or on the surroundings of an item, but more often a group of items, whatever their age and condition. these measures and actions are indirect – they do not interfere with the materials and structures of the items. they do not modify their appearance.’ [1] indeed, the deterioration and loss of our cultural heritage are inevitable processes. however, it is possible to slow these processes down and to preserve artworks for as long as possible if the right methodologies and preventive conservation projects are implemented. when dealing with heritage metallic artefacts, particular care has to be taken because of the strong interaction between these objects and their surrounding environment. many authors [2]-[4] have discussed the strategies and methodologies necessary to abstract this paper presents a long-term in-situ campaign to monitor contemporary bronze statuary exposed outdoors. the case study relates to the characterisation of three sculptures belonging to the gori art collection, located in the fattoria di celle: ‘cavaliere’ and ‘miracolo – composizione’ by marino marini and ‘due forme o due ombre n°2’ by luciano minguzzi. the overall conservation state of the sculptures was investigated by means of a multi-analytical and non-invasive approach, involving different techniques. three-dimensional photogrammetry was performed to fully document the artworks. the chemical and microstructural features of the corrosion patinas were then characterised through x-ray fluorescence and raman spectroscopy. in addition, the stability and the protective effectiveness of the corrosion products were assessed by electrochemical impedance spectroscopy. thanks to the combined use of these specific techniques, the information extracted through the different analyses could be correlated with each other and with the exposure conditions. the different corrosion products were identified as being primarily copper sulphates and phosphates, and they were correlated with the different microclimate conditions related to their location on the statues. the information gathered from the presented multi-analytical approach represents the fundamental knowledge required to develop a tailored conservation project to assure the long-lasting preservation of these artworks. mailto:leila.essebar@polito.it acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 242 investigate the degradation mechanisms and corrosion products in the cultural heritage field, as all measurements should be noninvasive and non-destructive. in this study, phenomena and products related to the atmospheric corrosion of bronze artefacts are investigated there are many research projects with the overall aim of investigating this issue with a focus on atmospheric (rural, urban, marine, etc.) [5] and exposure conditions, appearance and morphology of the corrosion patinas by means of different analytical techniques. this paper presents a multi-analytical approach applied to the assessment of the conservation state of artistic installations and sculptures exposed outdoors. different and complementary analytical techniques were required to gain an overall characterisation of the artefacts and an assessment of their conservation state, together with the documentation of any change in the aesthetical appearance directly related to material degradation during the exposure time. this is a fundamental aspect of the definition of projects that assures the long-lasting preservation of artworks. the conservation of bronze artefacts exposed outdoors is influenced by the alloy composition and the chemical, microstructural and morphological features of the corrosion products formed on the metallic surface due to the interaction with the atmosphere, as well as by environmental and microclimate parameters. three-dimensional (3d) photogrammetry, raman spectroscopy (rs), portable x-ray fluorescence (p-xrf) and electrochemical impedance spectroscopy (eis) were exploited for an in-situ monitoring campaign. only by combining the information derived from all these techniques is it possible to reach a deeper understanding of the artwork’s conservation state. rs and p-xrf were exploited to identify which corrosion products were present and to characterise their chemical and microstructural features. eis was conducted to evaluate the electrochemical stability and the protective effectiveness of the corrosion product layers. photogrammetry allows the creation of a 3d model of an item, documenting geometrical, textural and colour features, all of which can change over time due to exposure to the outdoor atmosphere. these techniques are proven to be suitable for the cultural heritage field [6]-[14]. in this paper, the possibilities offered by portable instruments for in-situ and non-invasive measurements are presented. the bronze sculptures under study are part of a private modern art collection, the gori art collection (located in fattoria di celle, pistoia, italy). the investigated artefacts are all exposed to the same rural environment, with similar exposure conditions, and they were all cast in the same period (the second part of the 20th century). even if the items are of recent realisation, the information regarding the production techniques and the conservation state are not easy to find. therefore, a monitoring campaign was initiated in 2018, which is still in progress, to assess the atmospheric corrosion phenomena affecting the artworks as a function of the time of exposure to the environment with the final aim of developing tailored preservation procedures. in this paper, the results obtained for three sculptures are reported: ‘cavaliere’ (1957–58) and ‘miracolo composizione’ (1956–57) by the artist marino marini, and ‘due forme o due ombre n°2’ (1969) by luciano minguzzi. the paper is structured as follows. in section 2, the gori art collection sculptures investigated in this study are presented together with the applied analytical methodologies. in section 3, the results obtained are reported and discussed. finally, in section 4, the major results are outlined and conclusions drawn. 2. material and methods the monitoring campaign began in collaboration with the curators of the gori art collection in november 2018, and it has mainly been devoted to the assessment of the conservation state of the bronze sculptures exposed outdoors. the campaign began with the study of the ‘cavaliere’ sculpture created by the italian artist marino marini (figure 1a). from december 2019, the artworks ‘miracolo composizione’ by marino marini (figure 1b) and ‘due forme o due ombre n°2’ by luciano minguzzi (figure 1c) were included in the monitoring project, thus the preliminary results obtained so far for these two artworks are also presented. 2.1. artefacts gori art collection the bronze sculptures that are the subject of the present study belong to a private collection of modern art, the gori art collection, located in fattoria di celle (pistoia, italy). the collection was founded in 1950 in prato by the two owners, giuliano and pina gori. many artists contributed with their works, and soon, the collection became too large for the original location and the artworks had to be moved to a more suitable place. therefore, in 1970, a new phase for the collection began, after its transfer to the fattoria di celle, a large estate situated in santomato (pistoia, italy). the collection continued to grow, adopting a site-specific art programme in which the sculptures a) b) c) figure 1. the artworks of the gori art collection under study: a) ‘cavaliere’ and b) ‘miracolo composizione’ by marino marini; c) ‘due forme o due ombre n°2’ by luciano minguzzi. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 243 and the multi-material installations were directly connected to the surrounding parkland and landscape of the fattoria di celle, which has become an integral part of the collection. currently, it is composed of 80 site-specific artworks and installations created by artists from all over the world [15]. the majority of the pieces in the collection are permanently installed in the gardens of the fattoria di celle, in some cases in direct contact with soil or water. the artworks are connected to the space that surrounds them, therefore no protection from the environment is present. as a consequence, there are many corrosion phenomena present, which can affect these artworks. in this paper, only some of the bronze artworks under study will be presented, in particular, the in-situ monitoring of the ‘cavaliere’ sculpture, created by the italian artist marino marini. the sculpture was part of the exhibition hosted by the fondation maeght in saint-paul-de-vence (france) in 2012 [16], [17]. marino marini (1901–1980) was an italian sculptor, who developed his work through several themes: equestrian statues, the divinities, portraits and circus figures. he drew on traditions of etruscan and northern european sculpture in developing these themes. his aim was to develop mythical images by interpreting classical themes in light of modern concerns and techniques. the sculpture under study is part of the collection, and it is located in the villa’s garden, in contact with the soil, in an area surrounded by trees and near a fountain. the second of marino marini’s sculptures is an object of characterisation, the ‘miracolo composizione’, found by biancalucia maglione and confirmed with the fondazione marini of pistoia in december 2015 [17]. the third item of study is ‘due forme o due ombre n°2’ by luciano minguzzi (1911–2004), an italian sculptor whose products ranged from a series of vigorous plastic works featuring acrobats, contortionists and children at play to concentration camps, gas chambers and other war-related themes. all the artworks under study are placed in a similar context, on the ground and close to trees and bushes. even though all the artefacts are recently produced, there is a lack of information about the materials employed by the artists in their creation. moreover, information regarding the production techniques is limited. different areas of the sculptures were investigated after a first visual inspection. the areas were selected on the basis of the exposure conditions and the colour of the corrosion patina. some of the corrosion patinas under study are shown in figure 2. as can be observed, black (g), light green (f), light blue-green (a, b, e), bright dark-green (h), green (c) and blue (d) corrosion patinas are present on the sculptures’ surfaces. those indicated as a), b) and e) were detected on the bases of the sculptures, while the patinas labelled as c), d), f) and g) were mainly found on unsheltered surfaces, such as the side of the sculptures. finally, the area indicated as h) has the aspect characteristic of patinas present on unsheltered surfaces. hereinafter, the analysis points on the ‘cavaliere’ sculpture will be labelled as ‘cav’, on ‘miracolo composizione’ as ‘mir’ and on ‘due forme o due ombre n°2’ as ‘due’. in addition, all measurements will be identified according to their position on the artworks: those performed on the base will be labelled as ‘_b’, on the lateral unsheltered points as ‘_u’ and on sheltered points as ‘_s’. finally, the single measurement will be identified by adding a two-digit sequential number. 2.2. multi-analytical investigation in order to characterise the corrosion processes that are affecting the bronze sculptures, identify the corrosion products and assess the overall conservation state, an in-situ multianalytical and non-invasive approach was adopted. indeed, the artworks cannot be moved, and techniques that require sampling are not suitable for artefacts with an artistic value. first, a 3d photogrammetry survey was carried out to create a complete set of documentation for the artworks. the chemical and microstructural features of the corrosion patinas were then characterised by means of xrf and rs. finally, the conservation state of the bronze sculptures was investigated using eis in order to assess the stability and the protective capability of the corrosion patinas on the artefact’s surface. 2.2.1. photogrammetry photogrammetry is a technique that has become increasingly widespread in the cultural heritage field. documentation is a fundamental aspect of a monitoring campaign, and thanks to this technique, it is possible to store a 3d model with information about the colour, geometry and texture of an artefact. the photogrammetry measurements were performed using a nikon d3100 camera and af-s dx zoom-nikkor 1855 mm f/3.5-5.6g ed lens. image acquisition conditions were set before starting in order to obtain sharp and clean images. in particular, the lens focal length was fixed at 18 mm, with an aperture of f/6.3, an exposure time of 1/125 and an iso sensitivity equal to 400. the image acquisition was performed at different heights, circling around the artefact and also taking close-ups. finally, for all the sculptures, a picture with a colour chart in view was taken to ensure accurate colour reproduction. the images were obtained and saved in raw format. micmac (multi-images correspondances, méthodes automatiques de correélation), the open-source program developed by the french national geographic institute (ign), was employed [18]. the workflow described below was followed. first, the acquired images were inspected to eliminate those that did not fulfil the required quality. next, the selected images were aligned, and the sparse cloud, dense cloud and mesh were constructed to set the medium quality level. subsequently, the model was cut in figure 2. view of some investigated bronze surfaces. a) due_b 01; b) due_b 02; c) due_u 01; d) due_u 03; e) cav_b 01; f) cav_u 05; g) cav_u 07; h) cav_s 01. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 244 order to contain only the region of the sculpture that was of interest, and the model was then constructed and the texture applied to the highest quality. 2.2.2. x-ray fluorescence the portable x-ray fluorescence spectrometer is an important tool for the in-situ investigation of metallic artworks. indeed, by means of xrf, it is possible to perform a completely noninvasive and non-destructive element identification on an artefact’s surface. a bruker tracer 5i analyser was employed. the instrument is equipped with a proprietary 20 mm2 silicon drift detector and a rhodium (rh) excitation source. for the analysis, the voltage and current were set to the values of 30 kv and 14 μa, respectively, and the 3 mm collimator was mounted. the collected spectra were processed using the artax spectra (8.0.0.476) software. a bayesian deconvolution of the collected spectra was performed after elemental interference (pile up and escape peaks) and background correction. a net counts rate for each element was then obtained. the principal component analysis (pca) was used with the final aim of recognising different patterns on the net counts rate and, thus, discriminating between the alloys of the different sculptures. the pre-processing of the data involved, first, a logarithmic scaling, by which the net count rate value x is replaced by log(x). then, the mean centering and variance scaling were applied; for each specific element, the mean value was subtracted and divided by the standard deviation [19]. 2.2.3. raman spectroscopy in order to perform the chemical and microstructural characterisation of the corrosion products, raman spectroscopy measurements were conducted. specifically, the portable iraman plus (bwtek) was employed. this instrument is equipped with a green excitation laser (532 nm), a spectrometer (bws465-532s) that works in the range of 150 to 4,200 cm−1, with a resolution of 7.3 cm−1, using a ccd sensor with a cooling system. the analytical conditions were a laser power of 6 mw, an integration time of 30 s and 3 repetitions in each area of analysis. the analysed surface did not undergo any cleaning procedures. the acquired raman spectra were processed using the pca by means of a python script. the pre-processing, as is commonly done for spectral analyses, involved the steps described below. first, only the region of interest of the spectrum, where peaks related to the corrosion mineralogical phases are found (between 200 cm−1 and 1,300 cm1), was selected. the baseline was then removed by means of asymmetric least squares smoothing. a savitzky-golay filter was successively applied using a window length of 15 cm−1 and fitting the original spectrum with a secondorder polynomial. a standard normal variate transformation was then carried out. finally, the obtained spectrum was processed in order to find the principal components [20]. 2.2.4. electrochemical impedance spectroscopy eis was the selected technique for both the evaluation of the protective effectiveness of the corrosion patinas present on the figure 3. views of the 3d models created for the three artworks. some of the points of analysis are indicated. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 245 bronze artefacts and for the monitoring of their stability as a function of the exposure time in relation to the aggressivity of the environment [21]-[23]. the experimental setup exploited in this study is composed of a portable electrochemical interface (ivium-compactstat.e 10800) and a measuring probe suitable for non-invasive measurements. the probe is a two-electrode cell, with a pt wire as a counter electrode and the bronze artefact as the working electrode. the probe can be fixed to the metal surface by means of an adhesive closed-cell polyurethane disc and then easily removed, as described in [24], [25]. this particular configuration allows eis measurements to be performed in situ, where the artefacts are stored, with a non-invasive approach [26]. as a consequence, it is possible to relate the information regarding the corrosion behaviour of the material exposed to the surrounding environment. the impedance measurements were performed by applying an alternating voltage in the range of 10 to 100 mv, in the frequency range of 0.01 hz to 100 khz; na2so4 0.1 m solution was employed as an electrolyte. 3. results and discussion in this section, the results obtained in the long-term monitoring campaign are presented. 3.1. photogrammetry photogrammetry is a technology recently introduced into the cultural heritage field, and it presents the possibility of achieving many useful goals. one example is the possibility of sharing and communicating art using visual presentations, both for museum and educational purposes. furthermore, a 3d model can be used as a support for restoration or for the creation of physical replicas. in addition, digital 3d models can be a resource for cataloguing information about the characterisation of the shape, colour and texture of artefacts and for supporting diagnostics. these last aspects are the ones stressed in this study. indeed, photogrammetry was applied to the documentation of the conservation state of the sculpture and to store information relating to the investigated areas. in figure 3, the digital 3d models constructed within this monitoring project are shown. it is worth noting that the models are a photorealistic reproduction of the works of art. an example of how 3d models can be a tool for supporting a monitoring campaign is shown in figure 3. all the information regarding the analysed area was organised and integrated into the model itself. therefore, it became an accessible resource, useful during the campaign, for the data processing and for sharing knowledge with curators in a more efficient way than with traditional 2d images. figure 4. score and loading plots of the first three components (pc1-pc2, pc1-pc3 and pc2-pc3) of xrf data. table 1. xrf results obtained from ‘cavaliere’ (indicated as cav) and ‘due forme o due ombre n°2’ (indicated as due) expressed as counts per second. cu pb zn sn ca ni s al k si p due_u 03 590004 8162 26142 170 1888 10 153 136 due_u 02 458367 4236 43790 183 824 10 129 28 due_u 01 589645 4598 21856 158 1190 10 127 16 due_b 02 506225 8578 45229 213 1554 10 89 98 due_b 01 539622 9391 35579 242 1709 10 136 60 cav_s 10 515908 17757 22252 619 359 366 579 127 19 19 cav_s 09 526733 20171 28701 1244 459 744 531 138 28 7 cav_s 08 523469 12352 29929 1200 384 896 396 127 29 20 cav_s 07 453529 13579 20405 1009 359 425 479 98 6 15 cav_s 06 538036 9741 24153 347 256 460 500 124 13 36 cav_u 03 569577 5159 23296 12 48 596 402 139 20 32 15 cav_u 02 550258 5252 22204 17 67 568 448 152 33 22 27 cav_u 01 377554 25165 36945 2655 938 1259 353 96 1 12 52 cav_b 03 387317 32342 29512 234 533 597 325 111 93 38 57 cav_b 02 451979 28305 27094 381 509 643 348 142 77 55 83 cav_b 01 335434 28098 13535 709 523 389 173 93 68 33 30 acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 246 this approach can also be used to display the results of the monitoring project, as was done in [27]-[30]. in addition, the final model can be uploaded onto online platforms, such as the collection website, with engaging video sequences or as interactive models [31]. 3.2. xrf results in order to investigate the elemental composition of the artefacts that are the subjects of this study, x-ray fluorescence spectroscopy was used. the xrf superficial element analysis is useful for investigating the composition of the corrosion products and the alloy used by the artist. xrf analyses were carried out on both the ‘cavaliere’ and ‘due forme o due ombre n°2’ sculptures. thanks to the xrf analysis, it was possible to identify all the elements, as shown in table 1, together with the net intensities obtained through bayesian deconvolution. since it was not possible to use a suitable calibration due to the presence of the corrosion patina, the analysis performed had to be semiquantitative. therefore, the reported quantities should be interpreted as a relative concentration of elements for each investigated area. the obtained elements list defines the material of the sculptures as bronze composed mainly of cu, sn, pb and zn, in accordance with modern bronze composition [32]. the presence of ca, si, k, p and al can be attributed to soil or environmental contamination. in order to interpret the large amount of collected data, a pca analysis was performed. the results obtained after the pca data processing (section 2.2.2) of both sculptures are summarised in figure 4 as biplots, with both the loadings and the scores for the two selected components (pc1-pc2, pc1-pc3, pc2-pc3) shown in parallel. data are reported with different colours and markers based on the sculpture and on the analysed area; red was assigned to the ‘cavaliere’ work of art, while green to ‘due forme o due ombre n°2’. different markers were used to discriminate between the areas where the analysis was performed, as described in the previous section. as is evident from the pc1-pc2 plot (figure 4), data are divided into two main clusters, one for each sculpture. this behaviour is mainly due to the pc1 values, which were positive for the points related to the ‘due forme o due ombre n°2’ and negative for the other sculpture. this evidence can be related to a major difference in the alloy composition, represented by the nickel and zinc content. moreover, looking at the pc1-pc3 plot, it is possible to see that the measurements relating to the ‘cavaliere’ base (cav_b) are in a separate cluster from the others, suggesting that the base and the sculpture are made of two different alloys. indeed, from a further examination of the ‘cavaliere’ score points in the pc2pc3 plot, it is possible to observe that the points of the sculpture base are separated from the others and that they present a pb and p enrichment (the latter presumably due to soil contamination). 3.3. raman spectroscopy the identification of the raman spectra was carried out taking advantage of the information derived from the xrf spectroscopy and by comparison with references from the literature [33]. from a visual inspection of the collected spectra, it was possible to identify some recurring patterns typical of copper corrosion products. given the fact that the surfaces were not cleaned before the analysis, the spectra presented noisy figure 5. score plots of the first three components (pc1-pc2, pc1-pc3 and pc2-pc3) calculated from raman spectroscopy measurements. percent variance captured by each pc is reported in parenthesis along each axis. figure 6. representative raman spectra collected. a) cav_s 03, antlerite; b) mir_u 03 brochantite, pseudomalachite; c) cav_u 03 brochantite, kipushite and silicates; d) due_b 01 silicates. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 247 signals (due to fluorescence) and many minor peaks, which are not always easy to identify. moreover, the monitoring campaign resulted in the collection of a large number of spectra. therefore, pca, an unsupervised multivariate analysis, was applied in order to identify the main patterns present in the spectra. the obtained scores of the three principal components are plotted in figure 5. data are shown with markers of different shapes and colours; different colours were assigned to each sculpture (blue for ‘miracolo composizione’, red for ‘cavaliere’ and green for ‘due forme o due ombre n°2’), while different shapes were chosen to represent the category of the investigated area, as done previously. from the pc1-pc2 plot in figure 5, it can be seen that the measurements are grouped in four main clusters. the first group is composed of the ‘cav_s’ points, with positive pc1 values and negative pc2 values. the ‘miracolo composizione’ measurements belong to a separate cluster, with positive pc1 and pc2 values. there is a well-defined group composed of points with negative pc1 and pc2 values, while the remaining points can be grouped as having a positive pc2 value. in figure 6, a representative spectrum is reported for each of the four groups. the first group (figure 6a, cav_s) was identified on the basis of the four major bands at 416 cm−1, 482 cm1, 988 cm1 and 1,076 cm−1, which are associated with antlerite, a basic copper sulphate (cu₃(oh)₄so₄). with regard to the second (figure 6b, mir_u 03), the major peak assignments, 474 cm−1, 615 cm−1, 906 cm−1, 974 cm−1 and 1,104 cm−1, allow its identification as brochantite (cu4so4(oh)6). moreover, some of the peaks (974 cm−1 and the shoulder at 997 cm−1, 800 cm−1 and 605 cm−1) could be related to the presence of copper phosphates, such as pseudomalachite. the third typology (figure 6c, cav_u 03) was identified as a mixture of different mineralogical phases. the bands at 421 cm−1, 478 cm−1 and 974 cm−1 can be assigned to brochantite; the 478 cm−1, 723 cm−1, 856 cm−1,974 cm−1 and 1,001 cm−1 vibration bands are typical of hydroxy-phosphates and can be assigned to kipushite (rruff id-r060826 [34]), while 1,152 cm−1 is characteristic of silicate vibration modes [35]. finally, the fourth spectrum (figure 6d, due_b 01) vibration modes, 958 cm−1, 1,000 cm−1, 1,152 cm−1 and 1,287 cm−1, are typical of silicates [35]. given these results, it is possible to confirm the presence of different corrosion mineralogical phases, such as copper sulphates, i.e. brochantite and antlerite, and copper phosphates, i.e. kipushite and pseudomalachite (the presence of phosphorus was also detected by xrf measurements). the presence of silicates was observed, mainly as a contaminant present on the sculpture surfaces. furthermore, it is possible to correlate the corrosion products with their location and exposure conditions, in accordance with the model proposed by [35]-[38]. indeed, in all the analysed areas located on the side of the sculpture, i.e. on rain-exposed surfaces, the presence of brochantite was evident. this kind of soluble compound can be leached by rains, thus creating a cyclic corrosion mechanism linked to the frequency with which the surface is exposed to rain. a phase such as brochantite can be more stable in sheltered areas, but only in the 4–6 ph range. if the acidity of the water film present on the surface increases and the ph is lowered to values below 4 [33],[37], the precipitation of antlerite is observed. in this study, antlerite was identified in the areas referred to as ‘_s’, which relates to the sculptures’ sheltered surfaces, where there is a low water evaporation rate. 3.4. electrochemical impedance spectroscopy eis measurements were performed in two different campaigns, the first in november 2018 and the second in april 2019. eis is a powerful tool that allows researchers to investigate the corrosion mechanisms affecting metal surfaces and the stability of the superficial oxide and corrosion product layers in a specific environment. thus, impedance measurements were performed in order to correlate the chemical composition and morphology of the corrosion product layers with their electrochemical behaviour and to assess the protective effectiveness of the different corrosion patinas. the results are summarised in figure 7 as nyquist plots. looking at the eis spectra acquired during the november 2018 campaign, it is possible to observe higher impedance values for point cav_s01, which was in a sheltered position, compared to points cav_u04 and cav_u05, which were in an unsheltered position on the statue. the different electrochemical behaviours can be correlated with the different thicknesses of the patinas, depending on their exposure conditions. indeed, corrosion products present on unsheltered surfaces are more likely to be washed away by rain, reducing their thickness. it is worth noting that by also using raman spectroscopy, it was possible to identify two different behaviours on the basis of the location. as discussed in the previous section, rs experimental findings highlighted the presence of antlerite only in correspondence to sheltered positions, while brochantite is the main corrosion product detected in the unsheltered areas. thus, correlating the information obtained using these two techniques allows the ongoing corrosion mechanism to be explained. patina figure 7. eis nyquist plots recorded for the ‘cavaliere’ sculpture. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 248 dissolution in pristine rain is more detrimental in unsheltered areas, where brochantite is present, and leads to a decrease in thickness and change in the degree of porosity. therefore, in these rain-exposed areas, lower impedance values indicate a lower protective effectiveness of the corrosion patina. however, sheltered areas where antlerite is present exhibit higher impedance values because patina dissolution by rain is less effective, and its thickness is thus higher. the high heterogeneity of the corrosion patinas was further highlighted in the april 2019 campaign. points cav_u04, cav_u06 and cav_u07 exhibited similar impedance spectra, reaching impedance modulus values in the order of 105 ohm∙cm2. at the same time, point cav_u05, which demonstrated a behaviour similar to cav_u04 in the previous campaign, exhibited an impedance modulus more than one order of magnitude lower, confirming the preferential dissolution of brochantite in some positions. despite the presence of different behaviours, it is possible to conclude that in all the investigated points, the patina exhibited a good protective effectiveness for the underlying bronze substrate. the relationship between the different positions and the corresponding impedance spectrum should be further investigated in order to reach a deeper understanding of the corrosion behaviour of the patinas grown on the different points and the effect of exposure to environmental conditions. 4. conclusions the in-situ multi-analytical and non-invasive approach presented in this paper has proven to be a very interesting and effective methodology for the conservation state assessment of artefacts. this approach could be applied to different case studies, related to multi-material artefacts, allowing a continuous monitoring and documentation of the conservation state of the artefacts. moreover, the importance of the use of portable instruments lies in the possibility of conducting the measurements in situ, thus avoiding any sampling and being able to assess the degradation of the material directly in contact with the environment to which the artwork is exposed. the combination of techniques such as photogrammetry, raman spectroscopy, x-ray fluorescence spectroscopy and electrochemical impedance spectroscopy can be used to provide information regarding the overall conservation state of an artefact. indeed, it allowed the identification of which corrosion products were present and the characterisation of their chemical and microstructural features, together with their electrochemical stability and their correlation with exposure conditions and the surrounding environment. moreover, with this approach, it is possible to document all the analysis performed on the artworks and to integrate this information into a virtual 3d model, which can be stored and shared with curators and conservators. in addition, it was possible to identify copper sulphates as brochantite and antlerite and copper phosphate as kipushite and pseudomalachite. furthermore, the corrosion products detected were also correlated with their location on the sculptures and with their exposure conditions, and the difference between the sheltered and unsheltered surfaces was analysed. the information collected through the multi-analytical monitoring campaign presented in this paper is essential for establishing tailored preventive conservation strategies. moreover, the identification of the corrosion products present can help to identify if conservation treatments, such as cleaning, passivation or coating, are necessary. acknowledgement the authors would like to acknowledge dr. giuliano gori, owner of the gori art collection, for allowing the present investigation in the fattoria di celle and for the useful discussions and continuous collaboration. references [1] icom-cc, terminology to characterize the conservation of tangible cultural heritage. online [accessed 12 march 2021] www.icom-cc.org/242/about/terminology-for-conservation [2] l. robbiola, j. m. blengino, c. fiaud, morphology and mechanisms of formation of natural patinas on archaeological cusn alloys, corrosion science 40 (1998) pp. 2083-2111. doi: 10.1016/s0010-938x(98)00096-1 [3] f. gallese, g. laguzzi, l. luvidi, v. ferrari, s. takacs, g. venturi pagani cesa, comparative investigation into the corrosion of different bronze alloys suitable for outdoor sculptures, corrosion science vol. 50 issue 4, april 2008, pp. 954-961. doi: 10.1016/j.corsci.2007.11.015 [4] t. e. graedel, k. nassau, j. p. franey, copper patinas 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[36] robbiola, l., fiaud, c., s. pennec, new model of out-door bronze corrosion and its implications for conservation, in: icom committee for conservation tenth triennial meeting, 2, (1993), pp.796-802. online [accessed 12 march 2021] https://hal.archives-ouvertes.fr/hal-00975704 [37] l. robbiola, l. p. hurtel, new contribution to the study of corrosion mechanisms of outdoor bronzes. characterization of the corroding surfaces of rodin’s bronzes, memoires et etudes scientifiques de la revue de metallurgie 88 (1991), pp. 809-823. online [accessed 12 march 2021] https://hal.archives-ouvertes.fr/hal-00538986 [38] v. hayez, j. guillaume, a. hubin, h. terryn, micro-raman spectroscopy for the study of corrosion products on copper alloys: setting up of a reference database and studying works of art, journal of raman spectroscopy 35 (2004), pp. 732-738. doi: 10.1002/jrs.1194 https://doi.org/10.4995/var.2017.6321 http://www.goricoll.it/ https://doi.org/10.1186/s40965-017-0027-2 https://doi.org/10.1007/s10008-009-0902-6 https://doi.org/10.1016/j.measurement.2018.04.041 https://www.imeko.org/publications/tc4-archaeo-2019/imeko-tc4-metroarchaeo-2019-107.pdf https://www.imeko.org/publications/tc4-archaeo-2019/imeko-tc4-metroarchaeo-2019-107.pdf https://doi.org/10.1016/j.measurement.2016.07.014 http://dx.doi.org/10.21014/acta_imeko.v7i3.586 https://doi.org/10.21014/acta_imeko.v7i3.584 https://doi.org/10.1016/j.microc.2015.07.008 https://doi.org/10.1016/s1386-1425(02)00315-3 https://doi.org/10.1515/9783110417104-003 https://hal.archives-ouvertes.fr/hal-00975704 https://hal.archives-ouvertes.fr/hal-00538986 https://doi.org/10.1002/jrs.1194 microsoft word article 10 81-488-1-galley finale.docx acta imeko december 2013, volume 2, number 2, 56 – 60 www.imeko.org acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 56 the influence of source impedance on charge amplifiers henrik volkers, thomas bruns physikalisch-technische bundesanstalt, bundesallee 100, 38116 braunschweig, germany section: research paper keywords: charge amplifier, calibration citation: henrik volkers, thomas bruns, the influence of source impedance on charge amplifiers, acta imeko, vol. 2, no. 2, article 10, december 2013, identifier: imeko-acta-02 (2013)-02-10 editor: paolo carbone, university of perugia received february 15th, 2013; in final form november 13th, 2013; published december 2013 copyright: © 2013 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding:this work has received funding from the european union on the basis of decision no 912/2009/ec corresponding author:henrik volkers, e-mail: henrik.volkers@ptb.de 1. introduction since its invention in 1950 by kistler [1], the calibration of charge amplifiers for piezoelectric sensors is usually performed with a setup similar to figure 1. the sensor is replaced by a voltage source generating ug and a standard capacitor of a well known value cc. by assuming ui to be negligible, paralleled (cable) capacities cp are ignored and the input charge is qc = qi = ug·cc, resulting in the following formula for the complex transfer function: cg a uq )( cu u s  (1) calibrations at ptb and accredited laboratories of charge amplifiers performed with different standard capacitors cc in a range from 10 pf to 2000 pf showed up significant systematic differences of suq with increasing frequency. this led to the general conclusion that at higher frequencies, the burden voltage ui could no longer be ignored and the total source impedance of the sensor or calibration setup including cables as seen by the charge amplifier has to be regarded. while this effect is known for field measurements mainly caused by extended cable length capacity [2], the impact to laboratory calibrations had been overseen and has led to the following investigation. 2. measured deviations the sensitivity suq of six laboratory grade charge amplifiers of different types were measured with varying source impedances. for these amplifiers, figure 2 shows the relative amplitude and absolute phase deviation taking a calibration with a 100 pf standard capacitor as a reference and sourcing with an impedance of 1920 pf, quite well representing an endevco type 2270 transducer including the connecting cable. by using a 1000 pf standard capacitor for calibration, this systematic deviation still will be about halved. figure 1. schematic of a charge amplifier calibration setup. cc cp qc qp qi ug ui ua hf(ω) rfb a(ω) abstract this contribution discusses the influence of the source impedance on the complex sensitivity of a charge amplifier (ca). during calibration of a ca with varying source impedances, deviations at higher frequencies were observed, which if not properly taken into account may generate systematic errors beyond the limits of the measurement uncertainty budget. the contribution discusses a model to describe the effect as well as an extension to the ca calibration procedures which allow to quantify and correct the effect. acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 57 the sensitivity to the source impedance of a ca is mainly determined by the first amplifier stage, its feedback network and the surrounding input protection circuit. 3. modeling the charge amplifier for further analyses the charge amplifier with its unknown circuit details is regarded as a black box with a complex, frequency dependent input impedance zi(ω) and charge coupled base sensitivity s0(ω) as shown in figure 3. s0(ω) can be interpreted as the transfer function of the charge amplifier when driven by an ideal charge source. the output voltage is ioa )( qsu   (2) and the input burden voltage iiiii )(j)( qzqzu    (3) for i j ii ˆ   teqq , kirchhoff’s laws leads to pic qqq  , (4) cigc )( cuuq  , (5) pip cuq  , (6) with (5),(6) in (4): )( pcicgi ccucuq  . (7) from the ”traditional” calibration equation (1) we know: )(uq a cg s u cu  . (8) with the total source impedance cs = cc+cp and including (2), (3) and (8) in (7) this results in si 0 uq j)(1 )( )( cz s s      . (9) for an ideal charge amplifier with an input impedance of zi = 0 ω, the source impedance cs would have no influence on the resulting sensitivity suq. one important conclusion from (9) is that for different calibrations or sensor setups with equal total source impedance cs, the resulting transfer functions suq will be equal, too. for a given sensor cable impedance a charge amplifier calibration setup can be matched by adding an appropriate cp for a given (smaller) calibration capacitor cc. 4. compensation results to determine the characteristic values of the base sensitivity s0(ω) and input impedance zi(ω) of a charge amplifier, formula (9) is rewritten in the form s 0 i 0uq )( )( j )( 1 )( 1 c s z ss      , (10) which allows a linear complex fit for the measured suq with varying cs. cs was composed of three different calibrated standard capacitors cc1,2,3 of 10 pf, 100 pf and 1000 pf (genrad type 1404-a,b,c) and a variable capacitor genrad type 1422-d connected in parallel to the cable providing an adjustable capacitance cp in the range of about 270 pf to 1400 pf. the resulting total source impedance was measured by shortcutting ug and measuring cs at the connector to the amplifier using an hp4274a lcr meter. suq was measured using a pxi-system with an ni-pxi 5422 16 bit signal generator applying sine signals and a two channel ni-pxi-5922 24 bit digitizer with an ni-pxi-5900 differential figure 2. systematic relative amplitude error and absolute phase error of the sensitivity suq(ω) for different charge amplifiers if calibrated with cscal = 100 pf+120 pf and sourced with an impedance of cssen = 1920pf. the two lower graphs show the same data with different scaling. the legend indicates brand and type of the investigated ca, however the results represent individual devices and might not be generally representative for these types of cas. figure 3. simplified charge amplifier model. -1,0% -0,5% 0,0% 0,5% 1,0% re la tiv e am p lit ud e d ev ia tio n 0,1 khz 1 khz 10 khz 100 khz -2,0 -1,5 -1,0 -0,5 0,0 bk2525 bk2650 en2710 en2775 pcb443 bk2635 frequency p ha se d ev ia tio n in d eg re e -5,0% -3,0% -1,0% 1,0% 3,0% 5,0% re la tiv e am pl itu de d ev ia tio n 0khz 10khz 20khz 30khz 40khz 50khz -14,0 -12,0 -10,0 -8,0 -6,0 -4,0 -2,0 0,0 frequency p h as e d ev ia tio n in d eg re e cc cp qc qp qi ug ui ua zi qi·s0 acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 58 preamplifier to simultaneously capture ug and ua. each measurement was taken twice with swapped input channels to cancel differences in the channel amplification and group delay. the major remaining uncertainties in this setup are the nonlinearities of the pxi-5922/5900 at a single frequency for ratio measurements u(ua/ug) < 60 ppm ([3],[4]), the uncertainty of the standard capacitors u(cc1) < 50 ppm, u(cc2,3) < 20 ppm and the charge amplifier noise. the overall expanded uncertainty is estimated to be less than 200 ppm. the uncertainty of the total source impedance measurement u(cs) is less critical and the impact to the sensitivity uncertainty is about 2 orders smaller than u(cc). an uncertainty of u(cs) ≤ 0.5% is still sufficient for an u(suq) ≤ 200ppm. for each frequency, the reciprocal measured complex suq is split into a real and an imaginary part and a linear least square fit with the total source impedance cs as the independent variable was applied. figure 4 shows the reciprocal real and imaginary parts of suq(ω) for one single amplifier. each line represents 8 measurements at one frequency. the relative mean squared errors of the fits are smaller than 10-5 indicating the validity of the proposed model. two transfer functions suq(ω) were measured with nearly the same total source impedance of cs=1300 pf, but one used the cc1=10 pf and the second used the cc3=1000 pf. figure 5 shows the resulting suq differences of the amplifier most sensitive to source impedance variations. the increase to 50 khz indicates a slight mismatch of cs of about 3 pf. for the bk2635 and pcb443 amplifiers, no differences larger than the standard deviation of the measurements (s ≤ 5·10-5) were observed and are another proof of equation (9). the amplitude and phase of the complex input impedance zi(ω) in ω for the cas investigated are shown in figure 6. while the ideal charge amplifier would have an input impedance of zi = 0 ω, the real-world amplifiers investigated have input impedances ranging from 45 ω up to 500 ω. figure 5. deviation of two calibrations with cc1=10 pf and cc3=1000 pf where the total source impedance is matched to cs=1300(3) pf. figure 6. amplitude and phase of the complex input impedance zi(ω) of six different charge amplifiers. figure 7. amplitude and phase deviations of suq(ω) after compensation, all cas investigated, 280 pf ≤ cs ≤ 2300 pf. figure 4. the real and imaginary inverse of suq(ω) for various source impedances cs. 0,1 khz 1 khz 10 khz 100 khz -100 -50 0 50 100 150 200 250 frequency re la tiv e de lta s in 1 e6 10 100 1000 |z | i n o hm 0,1 khz 1 khz 10 khz 100 khz -80 -60 -40 -20 0 20 40 bk2525 bk2650 en2710 en2775 pcb443 bk2635 frequency p ha se z in d eg re e -150 -100 -50 0 50 100 150 re l a m pl itu de d ev ia tio n af te r co m pe ns at io n in 1 e6 0,1 khz 1 khz 10 khz 100 khz -0,04 -0,03 -0,02 -0,01 0,00 0,01 0,02 0,03 0,04 frequency p ha se d ev ia tio n af te r co m pe ns at io n in d eg re e 0,09 0,092 0,094 0,096 0,098 0,1 r e {1 /s _u q} in p f/ m v 0 500 1000 1500 2000 2500 -0,01 0 0,01 0,02 0,03 0,04 0,05 0,06 100 hz 160 hz 330 hz 560 hz 820 hz 1 k hz 1,6 k hz 3,3 k hz 5,6 k hz 8,2 k hz 10 k hz 11 k hz 15 k hz 20 k hz 25 k hz 30 k hz 40 k hz 50 k hz c_s in pf im {1 /s _u q} in p f/ m v acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 59 with s0(ω), zi(ω) and the known source impedance, the influence of the source impedance to the transfer function suq(ω) can be compensated by applying formula (9). figure 7 shows the remaining deviations after compensation of all 6 cas with source impedances from 280 pf to 2300 pf in a frequency range from 100 hz to 50 khz, 648 measurements in total. the values underlay the conservative estimation of a u(suq(ω)) < 2·10-4. 5. impact on sensor calibration to validate the model approach of the charge amplifier for sensor calibration, we measured an endevco 2270 bb accelerometer on a se09 shaker with four different cas. the acceleration was measured with two heterodyne interferometers. acceleration, output voltage amplitude and phase were determined by applying a sine approximation method (method 3 in iso 16063-11). the frequency ranged from 100 hz to 40 khz at an acceleration amplitude of â = 100 m/s². the accelerometer mounting and the interferometer alignments were only performed once at the beginning of the measurement campaign. the only variable was the exchange of the charge amplifiers. the four cas were calibrated with source impedances built of a cc = 100 pf and four cp = {148, 806, 1585, 2143} pf to determine zi and s0. the transfer functions suq(ω) were then calculated according to formula (9) for a total source capacity of cssen = 1740(2) pf measured at 10 khz with 1 v excitation. the complex mean of all four measured sensor sensitivities sqa1,2,3,4(ω) was taken as the reference and figure 8 shows the resulting deviations. in addition, figure 9 shows the systematic deviations that would occur without compensation and a ‘common’ cc = 100 pf charge amplifier calibration. at higher frequencies this systematic deviation, if not properly taken into account, may exceed the limits of the measurement uncertainty budget. note the different scaling in the vertical axes. the uncertainties marked are determined as follows:     2222120 ² xxxxx suuuku   , (11) u0â = 3.62×10-4, u1â = 2.23×10-9, u2â = 2.39×10-13, u0p = 2.10×10-2, u1p = 3.18×10-7, u2p = 1.91×10-11, coverage factor k = 2 and the standard deviation sx of the measurement. 6. conclusions the influence of source impedance on charge amplifiers can be measured and explained with high confidence by the model shown in figure 3 and the methods proposed. by characterising the charge amplifier with its base sensitivity s0(ω) and input impedance zi(ω) it is now possible to compensate this influence. for calibrations of charged-based sensors with lowest possible uncertainty, the charge amplifier as the key linking element to the data acquisition system is usually calibrated before and after the sensor calibration. to avoid systematic deviations, these charge amplifier calibrations should be performed with a source impedance matching the sensor impedance. now, this procedure has become a common practice in our laboratory at ptb. acknowledgement the research leading to these results has received funding from the european union on the basis of decision no 912/2009/ec. figure 8. amplitude and phase deviations of sqa(ω) after ca compensation, taking the compensated complex mean as reference. figure 9. amplitude and phase deviations of sqa(ω) with a cs=248 pf (cc=100 pf) ca calibration without compensation, taking the compensated complex mean as reference. 100 1000 10000 -0,6 -0,4 -0,2 0 0,2 0,4 0,6 s_qa_pcb s_qa_2635 s_qa_2525 s_qa_2650 +u_pcb +u_2635 +u_2525 +u_2650 -u_pcb -u_2635 -u_2525 -u_2650 frequency in hz p h as e de vi at io n in d eg re e -0,6% -0,4% -0,2% 0,0% 0,2% 0,4% 0,6% re l. am p lit u de d ev ia tio n 100 1000 10000 -12 -10 -8 -6 -4 -2 0 2 s_qa_pcb s_qa_2635 s_qa_2525 s_qa_2650 +u_pcb +u_2635 +u_2525 +u_2650 -u_pcb -u_2635 -u_2525 -u_2650 frequency in hz p ha se d ev ia tio n in d eg re e -4% -3% -2% -1% 0% 1% 2% re l. am pl itu de d ev ia tio n acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 60 references [1] schweizerische eidgenossenschaft, eidgenössisches amt für geistiges eigentum, ch267431 (a) 1950-03-31, messverstärker zur messung elektrischer ladungen, http://worldwide.espacenet.com/publicationdetails/biblio?ft =d&date=19500331&db=epodoc&locale=de_ep&cc=c h&nr=267431a&kc=a&nd=4. [2] d. pennington, charge amplifier applications, instruments & control systems, endevco, 1965, http://www.endevco.com/resources/tp_pdf/tp224.pdf. [3] f. overney, a. rufenacht, p.p. braun, b. jeanneret, p.s. wright. characterization of metrological grade analog-to-digital converters using a programmable josephson voltage standard, ieee trans. instrum. meas., vol. 60, no. 7, 2011, pp. 2172-2177. [4] g. rietveld, c. kramer, e. houtzager, o. kristensen, d. zhao, c. de leffe and t. lippert, characterization of a wideband digitizer for power measurements up to 1 mhz, conference on precision electromagnetic measurements, daejeon, korea, 2010, pp. 247-248. preliminary results of euramet rockwell comparison between inrim and ume (euramet.m.h-s1.a.b.c) acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 256 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 256 260 preliminary results of euramet rockwell comparison between inrim and ume (euramet.m.h-s1.a.b.c) c. kuzu1, a. germak2, c. origlia3, e. pelit4 1 ume, kocaeli, turkey, cihan.kuzu@tubitak.gov.tr 2 inrim, torino, italy, a.germak@inrim.it 3 inrim, torino, italy, c.origlia@inrim.it 4 ume, kocaeli, turkey, ercan.pelit@tubitak.gov.tr abstract: a bilateral supplementary comparison between inrim (national metrology institute of italy) and ume (national metrology institute of turkey) had been decided to be organized in the field of hardness metrology to determine the consistency of the national hardness standards in both countries realizing rockwell hardness measurements in accordance with iso 6508-1:2016 [1] and iso 6508-3:2015 [2] standards. in this paper the procedure and preliminary measurement results of the bilateral euramet supplementary comparison between the two laboratories are explained. keywords: hardness, rockwell, comparison, supplementary 1. introduction a bilateral supplementary comparison between inrim (istituto nazionale di ricerca metrologica) and ume (tübi̇tak ulusal metroloji enstitüsü) was carried out in the field of hardness metrology to determine the consistency of the national hardness standards used in both countries realizing rockwell hardness measurements in accordance with the iso 6508-1:2016 [1] and the iso 65083:2015 [2] standards, including the new definition of rockwell hardness scales accepted by the cipm/ccm wgh [3]. the most important and the most widely used rockwell hardness scales such as hra, hrbw and hrc constitute the scope of the comparison which was piloted by inrim. the blocks used in the comparison as transfer standards were provided by ume. each nmi measured five hardness levels for hrc and four hardness levels for hra and hrbw scales on the same transfer standards, using both their own and common indenters which also were provided by ume by providing its own indenters. measurements were carried out first by ume as the provider of the blocks, then by the pilot laboratory (pl) inrim, then again by ume for checking the stability of the transfer standards. the nmis were requested to realize the traceability of each component constituting the hardness scales on their national standards to the base si units and constitute their uncertainty budgets before the comparison measurements. 2. transfer standards used in the comparison in the bilateral comparison a total of three sets of hardness reference blocks were used; one set for each hardness scale of hra, hrbw and hrc, separately. five blocks for hrc scale and four blocks for each of hra and hrbw scales, a total of 13 hardness reference blocks were used. the blocks are placed in a wooden protective case during transportation with their anti-rusting paper and original cover provided by their producer. some information related to the hardness reference blocks used in the comparison and their pictures are given below. table 1: hardness reference blocks for hra scale no hardness value serial number producer 1 27 hra 668-385 yamamoto 2 46 hra 666-752 yamamoto 3 66 hra 293-733 yamamoto 4 87 hra 284-385 yamamoto table 2: hardness reference blocks for hrbw scale no hardness value serial number producer 1 32 hrbw 668-326 yamamoto 2 53 hrbw 661-029 yamamoto 3 73 hrbw 666-251 yamamoto 4 95 hrbw 666-850 yamamoto http://www.imeko.org/ mailto:cihan.kuzu@tubitak.gov.tr mailto:a.germak@inrim.it mailto:c.origlia@inrim.it mailto:ercan.pelit@tubitak.gov.tr acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 257 table 3: hardness reference blocks for hrc scale no hardness value serial number producer 1 25 hrc 492-096 yamamoto 2 36 hrc 489-678 yamamoto 3 45 hrc 369-719 yamamoto 4 57 hrc 454-774 yamamoto 5 67 hrc 486-336 yamamoto the pictures of the transfer standards used in the comparison with their nominal hardness values and serial numbers are given below. their surfaces were cleaned with soft material before and after measurements when it is needed. s/n: 668-385, 27 hra s/n: 666-752, 46 hra s/n: 293-733, 66 hra s/n: 284-385, 87 hra figure 1: transfer standards for hra scale s/n: 668-326, 32 hrbw s/n: 661-029, 53 hrbw s/n: 666-251, 73 hrbw s/n: 666-850, 95 hrbw figure 2: transfer standards for hrbw scale s/n: 492-096, 25 hrc s/n: 489-678, 36 hrc s/n: 369-719, 45 hrc s/n: 454-774, 57 hrc s/n: 486-336, 67 hrc figure 3: transfer standards for hrc scale 3. comparison procedure it was requested that each participant shall assure that the national standards to be used in the comparison at least were in accordance with the iso 6508-1:2016 [1] and the iso 6508-3:2015 [2] standards. under these circumstances the components to be calibrated/verified are as follows; • force (preliminary load, total load, preliminary load after removal of the additional load). • testing cycle (in accordance with the new definition of hrc scale accepted by the ccm wgh [3] or at least in accordance with the iso 6508-1:2016 [1] and the iso 6508-3:2015 [2] standards). • indenter geometrical parameters (in accordance with the iso 6508-3:2015 [2] standard). • depth measurement system (in accordance with the iso 6508-3:2015 [2] standard). beside management of calibration of the components given above, the blocks were placed in the laboratory one day before the measurements for temperature equilibrium. the measurement steps realized during the comparison measurement are as follows; • before starting the measurements, it was assured that the standardizing machines were working properly in accordance with their design parameters and the relevant iso standards’ requirements. http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 258 • it was assured that the anvil where the blocks are seated on and both surfaces of the reference blocks were clean. • it was assured that the relevant scale and related indenter and other requirements were mounted/selected etc. • the ambient temperature was recorded. • two sets of 7 measurements (with the two indenters, one common and one own) uniformly distributed on the surface of the block were made and recorded on the data form. • the ambient temperature was recorded. seven measurements were carried out: the first two were discarded in order to eliminate the effects coming from adaptation of the machine and the blocks. in the calculations the last five indentations were taken into consideration. the first two indentations were recorded but not used in the calculations. 4. reference values of influence parameters in the calibration of hardness reference blocks the calibration laboratories realize this work in accordance with some internationally defined standards or accepted definitions. to perform measurements under the same or very similar conditions by the participants it has been significant to agree on reference values of the influence parameters and testing cycles and realize them as much as possible to have more comparable reference systems. table 4: reference values for rockwell hardness scales hra, hrbw and hrc in this comparison the reference values were chosen according to the hrc definition decided by the cipm/ccm wgh [3]. deviations from these values were taken into the uncertainty calculations. table 4 includes the reference values and measurement cycle used in the hra, hrbw and hrc measurements and figure 4 shows the measurement cycle for hra, hrbw and hrc. figure 4: identification of the hra, hrbw and hrc measurement cycle [3] 5. elaboration of the results as its being a supplementary comparison, there is no possibility to assign a reference value linked to an internationally accepted reference. in line with this situation, each laboratory recorded the measurement results and uncertainty values in the data sheet provided in the annex of the technical protocol for each hardness reference block. the pilot laboratory was responsible for collecting the measurement data from the participants, compiling, elaboration and preparing the reports. the measurement results were used to compute the comparison reference value (crv), degrees of equivalence (doe) with the crv and en ratios. the calculation is shown in following steps and made by making use of the following equations. • calculation of the comparison reference value (crv) the pilot laboratory determined crv by calculating the weighted mean of measurements of the two participants (𝑥𝑟𝑒𝑓 ) by making use of the following equation, 𝑥𝑟𝑒𝑓 = 𝑥1 𝑢 2(𝑥1)⁄ + ⋯+ 𝑥𝑛 𝑢 2(𝑥𝑛)⁄ 1 𝑢2(𝑥1)⁄ + ⋯+ 1 𝑢 2(𝑥𝑛)⁄ (1) • the uncertainty of the crv ( 𝑥𝑟𝑒𝑓 ) was calculated by the following expression, • 1 𝑢2(𝑥𝑟𝑒𝑓) = 1 𝑢2(𝑥1) + ⋯+ 1 𝑢2(𝑥𝑛) (2) where, 𝑥𝑖 : measured value of participating institute, i (i = 1, 2, ..., n) 𝑢(𝑥𝑖) : standard uncertainty of 𝑥𝑖 http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 259 𝑢(𝑥𝑟𝑒𝑓) : standard uncertainty of 𝑥𝑟𝑒𝑓 • the deviation from the crv is calculated by, 𝑑𝑖 = 𝑥𝑖 − 𝑥𝑟𝑒𝑓 (3) • the uncertainty of this deviation at a 95% level of confidence is, 𝑈(𝑑𝑖) = 𝑘 ∙ 𝑢(𝑑𝑖) (4) where u(di ) is calculated by, 𝑢2(𝑑𝑖) = 𝑢 2(𝑥𝑖) − 𝑢 2(𝑥𝑟𝑒𝑓) (5) and 𝑘 = 2. • the coefficient en is the equivalence between the measurements of participating institutes, is calculated as given below, 𝐸𝑛 = 𝑥𝑖 − 𝑥𝑟𝑒𝑓 √𝑈2(𝑥𝑖) − 𝑈 2(𝑥𝑟𝑒𝑓) (6) where, 𝑈(𝑥𝑖) = 𝑘 ∙ 𝑢(𝑥𝑖) (7) 𝑈(𝑥𝑟𝑒𝑓) = 𝑘 ∙ 𝑢(𝑥𝑟𝑒𝑓) (8) • the xi is equivalent with xref (crv) at 95% confidence level, if |𝐸n| ≤ 1, 6. comparison results in this comparison the degree of equivalence of each participant with respect to the crv was calculated. this calculation comprises calculation the deviation of each participant from the crv and the associated uncertainty of this deviation. also the en ratios were also calculated for each hardness scale and level. the two participants declare consistent uncertainty values and measurements results are in a significant consistency with each other. below you can see the tabular and graphical interpretation of the measurement results and calculations. table 5: doe of inrim and ume (wrt. crv) – hra – common indenter table 6: doe of inrim and ume (wrt. crv) – hrbw – common indenter table 7: doe of inrim and ume (wrt. crv) – hrc – common indenter table 8: doe of inrim and ume (wrt. crv) – hra – own indenter table 9: doe of inrim and ume (wrt. crv) – hrbw – own indenter table 10: doe of inrim and ume (wrt. crv) – hrc – own indenter in regard to the calculations shown in the tables above the graphical representations are as shown below. figure 5: deviations of inrim and ume values from the crv with the associated expanded uncertainty – hra – common indenter http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 260 figure 6: deviations of inrim and ume values from the crv with the associated expanded uncertainty – hrbw – common indenter figure 7: deviations of inrim and ume values from the crv with the associated expanded uncertainty – hrc – common indenter figure 8: deviations of inrim and ume values from the crv with the associated expanded uncertainty – hra – own indenter figure 9: deviations of inrim and ume values from the crv with the associated expanded uncertainty – hrbw – own indenter figure 10: deviations of inrim and ume values from the crv with the associated expanded uncertainty – hrc – own indenter 7. summary at the end of the bilateral comparison between inrim and ume in the most widely used rockwell hardness scales hra, hrbw and hrc is completed without any unexpected phenomena in any stage of it. the participating institutes declared similar uncertainty values and there was a significant consistency between the measured values of the transfer standards. the comparison reference values (crvs), the deviation value of inrim and ume from the crv and their uncertainties, dinrim, dume, ud-inrim, ud-ume, and en ratios were calculated and shown in tables 5 to 10 and figures 5 to 10 in the comparison results. in some cases, the degrees of equivalence for the results obtained by the common indenter is greater than the degree of equivalence for the results obtained by the own indenters. this is probably because of a bias in our machine in that scale/range but still within the uncertainty declared. the degrees of equivalence show a significant consistency between inrim and ume hardness standards in hra, hrbw and hrc rockwell hardness scales and this comparison is supporting the present and possible new cmc submissions. 8. references [1] en iso 6508–1:2016, metallic materials–rockwell hardness test–part1: test method (scales a, b, c, d, e, f, g, h, k, n, t). [2] en iso 6508–3:2015, metallic materials–rockwell hardness test part3: calibration of reference blocks (scales a, b, c, d, e, f, g, h, k, n, t). [3] https://www.bipm.org/wg/ccm/ccmwgh/allowed/international_definitions/hrc_defi nition.pdf [4] http://www.ltf.it/en/prodotti.php?b=3&c=636&p=3 361 [5] euramet/cg–16/v.02, guidelines on the estimation of uncertainty in hardness measurements, 2011. [6] jcgm 100:2008, evaluation of measurement data– guide to the expression of uncertainty in measurement. http://www.imeko.org/ https://www.bipm.org/wg/ccm/ccm-wgh/allowed/international_definitions/hrc_definition.pdf https://www.bipm.org/wg/ccm/ccm-wgh/allowed/international_definitions/hrc_definition.pdf https://www.bipm.org/wg/ccm/ccm-wgh/allowed/international_definitions/hrc_definition.pdf http://www.ltf.it/en/prodotti.php?b=3&c=636&p=3361 http://www.ltf.it/en/prodotti.php?b=3&c=636&p=3361 characterization of spinning rotor gauge-3 using orific flow system and static expansion system acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 325 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 325 328 characterization of spinning rotor gauge-3 using orific flow system and static expansion system ashok kumar1,2,a, vikas n. thakur1,2b, harish kumar1,c 1 csir-national physical laboratory, dr. k. s. krishnan marg, delhi 110012, india 2 academy of scientific and innovative research (acsir), ghaziabad-201002, india a ashok553@nplindia.org, b vikasnthakur92@gmail.com, c harish@nplindia.org abstract: this article elucidates the calibration of newly procured spinning rotor gauge (srg 3) from mks instruments, usa using primary vacuum standards: orifice flow system (ofs) and static expansion system (ses) established at national physical laboratory, india (npli) in the range of 10-4 pa to 1 pa and further compared with manufacturer reported value which was calibrated by transfer standard of national institute of standards & technology (nist). the key parameters to calculate the pressure measured by srg is the accommodation coefficients. the accommodation coefficients for n2 gas obtained using ofs, ses, and calibration report form nist usa (srg2) are 0.957, 0.961, and 0.954 respectively. keywords: vacuum; orifice flow system; static expansion system; pressure; spinning rotor gauge 1. introduction as a national metrology institute, npli has the responsibility of maintaining the various primary vacuum standards and disseminate the traceability down to secondary and reference standards. barometric pressure and vacuum metrology section maintained the pressure standards in the range of 1 micro pa to 360 kpa. among these standards, this section has two primary vacuum standards ofs and ses which measure the pressure in the range of 0.1 pa to 1 µpa and 0.05 pa to 10 pa, respectively with associate uncertainty 2% and 0.4% of the pressure reading at k=2. the vacuum primary standards have participated in the international key comparisons [1,2] and found the reputations at the global level. there are many other transfer standards like resonant silicon gauge (rsg), old spinning rotor gauge (srg 2), capacitance diaphragm gauge (cdgs), etc. which are frequently calibrated by ses and ofs. ses and ofs are two primary pressure standards which are used to generate pressures very accurately and sitting at the top of traceability chain in vacuum metrology in india. there are some defense laboratories, research institutions and accredited laboratories which also possess cdgs and srgs as transfer or secondary standards which need to be calibrated time to time using npli primary vacuum standards. the low precision ionization gauges were used as transfer standard for inter-comparisons before high precision srgs and cdgs came into the existence. many intercomparisons were performed previously using these two vacuum standards i.e. ofs and ses [3,4]. recently this section bought srg 3 which has pressure measuring range of 1 pa to 10-4 pa. comsa et. al first invented the spinning rotor gauge in vacuum and high vacuum range with unmatched accuracy among the existing transfer standards, almost similar to the uncertainty of ofs [5]. srg is most commonly used as transfer vacuum standard since for the first time it was used as transfer standard by messer et. al. [6]. in 1989 and 2014, the international key comparisons were performed using two srgs [7]. mks has designed and developed the third generation srg and named it srg3 which is more robust and stable in high vacuum range. the npli studied the accommodation factors of srg3 in details. for the same purpose, npli has calibrated the srg3 using both the primary standards (ofs and ses) and further compared with the value given by srg2 (transfer standard of nist) 2. apparatus with characterization details static expansion system consists of two large chambers of different volumes of 𝑉𝑠 = 24 l and 𝑉𝐿 = 72 l. the gauge under calibration (guc) connected to the port of the chamber of volume 72 l. schematic diagram of ses is shown in figure 1. there are two larger chambers as discussed above are mechanically connected with three small chambers of volume 𝑣1 = 25 ml, 𝑣2 = 25 ml and 𝑣 = 343 ml between the valves 1 and 2, 4 and 5 and http://www.imeko.org/ mailto:ashok553@nplindia.org mailto:%20b%20vikasnthakur92@gmail.com mailto:%20c%20harish@nplindia.org acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 326 2 and 3, respectively as shown in figure 1. the large chamber 𝑉𝐿 connected to a turbomolecular pump having automated pneumatic gate valve to control and release the generate pressure. the chamber 𝑉𝑠 was pumped through a separate pumping system. prior to any measurement, the whole system was backed at 200 ℃ and minimum 10-8 pa for several days while system connected to pumping stations/systems. the pressures were generated by following the expansion equation: 𝑣2 → 𝑣2 + 𝑉𝐿 . the principle of working of ses is the static expansion of gas. the gas is first enclosed in the smaller volume i.e. 25 ml, then it is allowed to enter the larger volume i.e. 72 l to expand under nearly perfect isothermal conditions. the pressure in the larger volume was calculated using the gas equation given by equation (1), 𝑃𝐿 = 𝑃𝑠 𝑇𝐿 𝑅𝑇𝑠(1 + 𝑃𝑠𝐵 ′′) (1) where 𝑅 = 𝑉𝑠+𝑉𝐿 𝑉𝑠 is the ratio of final volume to initial volume, 𝑃𝑠 and 𝑃𝐿 are the initial and final pressure in smaller volume and larger volume respectively, 𝑉𝑠 and 𝑉𝐿 are the volumes of smaller and larger chambers respectively, 𝑇𝑠 and 𝑇𝐿 are the temperature values in the smaller and larger chambers respectively and 𝐵′′ = 𝐵/𝑇𝑅𝑚 is the second virial coefficient, 𝑅𝑚 is the molar gas constant. ses has the pressure measuring range of 0.05 pa to 10 pa with the expanded uncertainty of 0.4% of pressure reading at k = 2. orifice flow system is another vacuums standard which is used to calibrate the vacuum gauges in the molecular flow region i.e. the molecular mean free path is larger than the dimension of the vessel. ofs consists of two chambers of equal volume connected by a cylindrical path. the schematic diagram of the ofs is shown in figure 2(a). both the chambers are of equal diameter of 30 cm. the gauges under test are to be connected to the upper chamber. the residual gas analyzer was connected to the upper chamber to check the purity of the gas in the chamber. the configuration of the experimental setup consists of two spheres to have the maximum isotropy of the molecular distribution of maxwellian velocity distribution. the chambers were pumped out by following the same procedure as used explained by hojo et. al [9]. the turbomolecular pump is used for pumping the gas inside the chamber. a titanium sublimation pump (tsp) was also connected to the lower chamber. before the calibration start, the chambers were baked for several days by heating tapes and an ultimum vacuum of 10-10 pa was maintained. the temperatures were measured by the platinum resistance thermometers (prts) connected five different points in the system. the average of the temperatures in the calibration sphere was used as the reference temperature. at the centre of the cylinder path a disk is placed made up of copper known as orifice denoted by ‘c’. the magnified view of orifice is shown in figure 2(b). the orifice has the diameter of 10 mm. the edges of the aperture are prepared figure 1: schematic of static expansion system having chambers of volume 72 l and 24 l. [8]. http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 327 figure 2: (a) schematic diagram of the orifice flow system [8], tsp – titanium sublimation pump, bag – nude bayard alpert gauges, ms – mass spectrometer, corifice (b) magnified view of the orifice. in such a manner that the thickness of the disk at the periphery of the orifice nearly approached to zero. the working principle of the ofs is based on the flow of gas in molecular flow regions through the orifice of known conductance (c) connected across two chambers [10]. if q is the flow rate of the gas throughout then the pressure upstream is given by equation (2) 𝑃 = 𝑄 𝐶 (1 + 𝐶 𝑆𝑃 ) , (2) where p is the pressure upstream of the orifice or the pressure in the calibration chamber and sp is the pumping speed available in the chamber below the orifice. the srg 3 consists of ball–tube assembly using which vacuum is to be measured. the ball spins using a controlled electronics mechanism with the frequency of nearly 450 hz. while spinning the ball slows down due to the gas friction and falls down with the gas pressure. a sensing head is mounted on the ball–tube housing to keep the ball immobilized and collect the deceleration of ball due to change in pressure inside the chamber. according to the basic pressure measurement method of srg 3, the relation between gas pressure p and the speed of the ball is given by equation (3) [11] 𝑃 = (− 𝑑𝜔 𝑑𝜔𝑡 ) 𝑎𝜌 10𝜎 √ 8𝜋𝑅𝑚 𝑇 𝑀 − [𝑅𝐷] . (3) here, 𝑎 and 𝜌 are the radius and density of the srg rotor, respectively. m and t are the molecular mass of the gas and temperature inside the chamber, rd is the residual drag, i.e., the deceleration of the rotor, and − 𝑑𝜔 𝑑𝜔𝑡 is the relative deceleration rate of the sphere per unit time and has units of s-1, σ is the accommodation coefficient, and rm is molecular gas constant, 8.314x103 (nm/kmol . k). 3. results & discussion the srg3 ball was placed in their respective tubular housing. since the comparison did not involve transportation of the ball/tube assemblies to other laboratories, no need was felt of using the transport device for fixing the balls in the tubular housings during transportation. however, the ball/tube assembly was fitted with a mini ultra-high vacuum (uhv) right angle valve so that the rotors could be isolated from exposure to air during venting of the chamber for shifting the ball/tube assemblies from one calibration system to the other and back. precautions were taken to keep the sensing head mounted on the tubular housing so as to keep the ball immobilized because of permanent magnets of the head, while transferring the ball/tube assembly from one system to the other. the srg3 was first calibrated on the ses, in the pressure range of 0.1 – 1 pa, by following the method of ses i.e. successive expansion of the gas from one chamber to another. later, it was calibrated using ofs in the range of 0.001 – 0.01 http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 328 pa. the key parameter to compare, calibrate, and characterize any srg is accommodation coefficients. the accommodation coefficient is obtained by the intercept of the plot of the calibration factor – pressure measured by the vacuum standards (i.e. ses and ofs), where calibration factor is given by the ratio of the pressures measured by gauge under calibration (i.e. srg3) and vacuum standards. as srg3 has measuring range of 10-4 pa to 1 pa, therefore, srg3 was calibrated by both ses and ofs for vacuum and high vacuum range, respectively. to calculate the value of accommodation coefficients, the calibration factor for ses and ofs are plotted as a function of pressure in figure 3 and figure 4, respectively. the y-intercept (at zero pressure) of the graphs in fig. 3 and fig. 4 give the accommodation coefficients of the srg3 measured by ses and ofs, respectively. the values of accommodation coefficients calculated using ses, ofs and srg2 (mks, nist) are given by table 1. the values of accommodation coefficients given in table 1 show the maximum standard deviation of ≈0.003. figure 3: calibration factor of srg3 as a function of pressure using the ses. figure 4: calibration factor of srg3 as a function of pressure using the ofs. table 1: accommodation coefficients obtained by different vacuum standards. s. no. standard accommodation coefficient 1 ses 0.961 2 ofs 0.957 3 srg2 (mks, nist) 0.954 4. summary the values of the accommodation coefficients were successfully calculated and the values obtained from ses and ofs were further compared with the value obtained from srg3 (mks, nist). it is found that these three values of accommodation coefficients having the maximum standard deviation of ≈ 0.003. 5. references [1] d.a. olson, p.j. abbott, k. jousten, f.j. redgrave, p. mohan, s.s. hong, final report of key comparison ccm.p-k3: absolute pressure measurements in gas from 3 × 10 −6 pa to 9 × 10 −4 pa, metrologia. 47, 07004, 2010. [2] k. jousten, k. arai, u. becker, o. bodnar, f. boineau, j.a. fedchak, v. gorobey, w. jian, d. mari, p. mohan, j. setina, b. toman, m. vičar, y.h. yan, final report of key comparison ccm.pk12 for very low helium flow rates (leak rates), metrologia 50, 07001, 2013. [3] g. reich, c. meinke, comparison between static and dynamic calibration methods for ionization gauges, vacuum 17(3), 164, 1967. [4] k.f. poulter, a. calcatelli, p.s. choumoff, b. iapteff, g. messer, g. grosse, intercomparison of vacuum standards of countries within the european community in the range 8×10 −5 to 8×10 −2 pa., j. vac. sci. technol. 17, 679–687, 1980. [5] g. comsa, j. k. fremerey, and b. lindenau, in proceedings of the 8th international vacuum congress, cannes, le vide, 1980. [6] g. messer, p. röhl, g. grosse, w. jitschin, high vacuum measured by the spinning rotor gauge: status report of the bureau international des poids et mesures intercomparison, j. vac. sci. technol. a vacuum, surfaces, film. 5, 2440–2443, 1987. [7] j.k.n. sharma, p. mohan, w. jitschin, p. röhl, intercomparison of vacuum standards between the physikalisch technische bundesanstalt (germany) and the national physical laboratory (india) using two spinning rotor gauges, j. vac. sci. technol. a vacuum, surfaces, film. 7(4), 2788-2793, 1989. [8] j.k.n. sharma, p. mohan, d.r. sharma, comparison of two primary pressure standards using spinning rotor gauges, j. vac. sci. technol. a vacuum, surfaces, film. 8(2), 941-947, 1990. [9] h. hojo, m. ono, and k. nakayama, in proceedings of the 7th interna· tional vacuum congress and 3rd international conference solid surfaces. vienna, 1977 (berger and sohne, horne, austria), p. 117, 1977. [10] p. mohan, vacuum gauge calibration at the npl (india) using orifice flow method, vacuum 51(1), 69-74, 1998. [11] p. mohan and h. kumar, static expansion primary vacuum standard — part 2: characterization of two spinning rotor gauges, mapan—j. metrol. soc. india. 24, 111–118, 2009. http://www.imeko.org/ journal contacts acta imeko issn: 2221-870x december 2020, volume 9, number 5 acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 0 journal contacts about the journal acta imeko is an e-journal reporting on the contributions on the state and progress of the science and technology of measurement. the articles are mainly based on presentations presented at imeko workshops, symposia and congresses. the journal is published by imeko, the international measurement confederation. the issn, the international identifier for serials, is 2221-870x. editor‐in‐chief dušan agrež, slovenia founding editor‐in‐chief paul p. l. regtien, netherlands associate editor dirk röske, germany editorial board editors for this special edition leopoldo angrisani, italy filippo attivissimo, italy eulalia balestieri, italy eric benoit, france paolo carbone, italy lorenzo ciani, italy catalin damian, romania pasquale daponte, italy luca de vito, italy luigi ferrigno, italy edoardo fiorucci, italy alistair forbes, united kingdom helena geirinhas ramos, portugal sabrina grassini, italy fernando janeiro, portugal konrad jedrzejewski, poland andy knott, united kingdom francesco lamonaca, italy massimo lazzaroni, italy fabio leccese, italy rosario morello, italy michele norgia, italy pedro miguel pinto ramos, portugal nicola pompeo, italy sergio rapuano, italy dirk röske, germany alexandru salceanu, romania constantin sarmasanu, romania lorenzo scalise, italy emiliano schena, italy enrico silva, italy krzysztof stepien, poland marco tarabini, italy davor zvizdić, crotia lovorka grgec-bermanec, crotia andy knott, united kingdom dirk röske, germany renato r. machado, brazil jay h. hendricks, usa dominik pražák, czechia gustavo p ripper, brasil about imeko the international measurement confederation, imeko, is an international federation of actually 42 national member organisations individually concerned with the advancement of measurement technology. its fundamental objectives are the promotion of international interchange of scientific and technical information in the field of measurement, and the enhancement of international co-operation among scientists and engineers from research and industry. addresses principal contact prof. dušan agrež university of ljubljana faculty of electrical engineering tržaška 25, 1000 ljubljana, slovenia e-mail: dusan.agrez@fe.uni-lj.si acta imeko prof. davor zvizdić university of zagreb faculty of mechanical engineering ivana lucica 5, zagreb cr 10000, croatia e-mail: davor.zvizdic@fsb.hr support contact dr. dirk röske physikalisch-technische bundesanstalt (ptb) bundesallee 100, 38116 braunschweig, germany e-mail: dirk.roeske@ptb.de http://www.imeko.org/ mailto:dusan.agrez@fe.uni-lj.si mailto:davor.zvizdic@fsb.hr mailto:dirk.roeske@ptb.de influences of groove-type casing treatment on the tip leakage flow behaviour in linear compressor cascades at different tip clearances: influence of single groove at mid-chord acta imeko issn: 2221-870x march 2021, volume 10, number 1, 277 283 acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 277 influences of groove-type casing treatment on the tip leakage flow behaviour in linear compressor cascades at different tip clearances: influence of single groove at mid-chord masanao kaneko1 1 schoool of science and engineering, tokyo denki university, saitama, japan section: research paper keywords: gas turbine; compressor; tip leakage flow; casing treatment; circumferential groove citation: masanao kaneko, influences of groove-type casing treatment on the tip leakage flow behaviour in linear compressor cascades at different tip clearances: influence of single groove at mid-chord, acta imeko, vol. 10, no. 1, article 37, march 2021, identifier: imeko-acta-10 (2021)-01-37 editor: momoko kojima, nmij, japan received may 15, 2020; in final form july 30, 2020; published march 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: masanao kaneko, e-mail: kaneko@mail.dendai.ac.jp 1. introduction demand for the improvements in aviation and industrial gas turbine performances in terms of, for example, efficiency has been exponentially increasing due to the worsening global environmental problems and the concern for the depletion of fossil fuels. in light of these social demands, various studies related to an axial-compressor-constituting gas turbine have been carried out to specifically increase the efficiency and the pressure ratio per single stage and to extend the stable operating flow range. in the aforementioned type of gas turbine, an open impeller is generally used as the rotor impeller. here, clearances are inevitably present between the rotating blade tips and the stationary shroud casing wall. as a consequence, flow is generated through the tip clearance due to the pressure difference between the blade pressure and the suction surfaces, which subsequently affects the loss generation significantly through the generation of the tip leakage vortex [1]-[4]. moreover, the interface between the tip leakage flow and the main flow is shifted toward the upstream as the flow rate decreases. when this interface reaches the cascade inlet, the blockage due to the tip leakage flow influences the inception of the rotating stall, which is regarded as one of the factors that restricts the operating flow range [4], [5]. a circumferential groove casing treatment (cgct) is one of the techniques used to improve a compressor’s aerodynamic performance. the cgct controls the tip leakage flow behaviour through the single or multi groove installed on the shroud casing wall and this consequently shifts the minimum stable flow rate toward a lower flow rate. the influences of the groove location and the number of grooves on the expansion effect related to the stable operating range have been investigated by numerous researchers [6]-[14]. however, while it has been reported that the cgct reduces the efficiency at certain operating points [6], this mechanism has not been comprehensively investigated. abstract in this study, the influences of the single groove installed at the mid-chord – which is known to have a large expansion effect on the stable operating flow range of low-speed axial compressors – on the flow behaviour and the loss generation in a linear compressor cascade were investigated numerically at different tip clearances. reynolds-averaged navier–stokes simulations of the incompressible flow in the test cascade were performed, with the computed results clarifying the following remarkable phenomena, which are common to both small and large tip-clearance cases. the single groove locally weakens the tip leakage flow by the decrease in the blade loading and the streaming of the flow near the blade pressure side into the groove, consequently reducing the distance between the tip leakage vortex and the blade suction surface. meanwhile, although the groove decreases the loss due to the tip leakage vortex generated from the blade leading edge, the loss generation in the entire cascade passage is almost the same as that in the cascade without the groove due to the additional loss generation resulting from the presence of the groove. mailto:kaneko@mail.dendai.ac.jp acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 278 meanwhile, the axial-compressor-constituting gas turbine is generally composed of several pairs of rotors and stators, which are referred to as ‘stages’. in this type of compressor, the blade height is gradually reduced toward the final stage. as a result, in the rear stage, since the tip clearance size for the rotor blade height increases in relation to that in the front stage, the influence of the tip leakage flow on the aerodynamic performance is increased [7], [15]-[18]. therefore, to intensify the improvement effect of the cgct on the aerodynamic performance of the rear compressor stage, clarifying the influence of the tip clearance size on the flow phenomena generated by the cgct is also crucial. the aim of this study was to ascertain how to maximise the expansion effect of the cgct on the stable operating flow range of the rear compressor stage without a trade-off such as the efficiency penalty. as part of a preliminary study, the influences of the single groove installed at the mid-chord – which is known to have a large expansion effect on the stable operating flow range of low-speed axial compressors [8] – on the flow behaviour and the loss generation in a linear compressor cascade were investigated at different tip clearances. in the following 2 and 3 sections, the test cascade and the computational method and condition used in this study will be explained, respectively. the influences of groove at small and large tip clearance cases will be discussed in the 4 section and the conclusions derived from this study will be summarised in the final section. 2. test cascade figure 1 shows the configuration of the test compressor cascade, with its specifications provided in figure 2 and table 1. this test cascade is a linear compressor cascade that incorporates two-dimensional blades characterised by the blade tip shape of the ge rotor b section blade and by varied tip clearances. an experimental investigation of the tip leakage flow behaviour in this cascade was conducted under subsonic flow conditions at georgia institute of technology [19]. in the present study, the flows in the two types of cascade were analysed numerically at different tip clearances. the first cascade has a smooth wall as the tip side endwall (sw), while the other has an endwall in which a single groove is installed (gw). the tip clearance sizes were set to 4.2 mm (1.65 % of c) and 1.27 mm (0.5 % of c). the groove shape is shown in figure 3 and was designed with reference to [8]. 3. numerical analysis 3.1. computational method in this study, the computations were performed using the open source computational fluid dynamics (cfd) code openfoam. the flow in the cascade was assumed to be a steady-state incompressible flow and the reynolds-averaged navier–stokes (rans) equation was used as the governing equation in order to clarify the time-averaged effect of the groove on the flow in the compressor. meanwhile, the simple algorithm was adopted as the solution algorithm, while the convection term was estimated using the muscl scheme while considering both the convergence of the computation and the accuracy of the computed results. the low-reynolds-type sst k-ω model [20], which is well known to accurately predict the flow in turbomachinery [12], was chosen as the turbulence model. 3.2. computational condition in order to reduce the computational resources, the computational domain was set to the tip side endwall from the midspan in the single passage. the inlet and outlet boundary planes were arranged at 1c upstream of the cascade inlet and 2c downstream of the exit, respectively, while the computational grids were generated using the hand c-type structured grids. the total number of cells of the sw was approximately 5.5 million, while that of the gw was approximately 6.8 million. the computational grid used in this study is shown in figure 4. figure 1. the test cascade. table 1. test cascade specifications. chord c 254 mm axial chord ca 139 mm pitch s 236 mm span hb 249.8 mm inlet blade angle (measured from axial direction) β1 65.1 deg. stagger angle (measured from axial direction) φ 56.9 deg. solidity c/s 1.08 figure 2. configuration of the test cascade. figure 3. the groove shape. suction surface leading edge trailing edge s β1 vy vx vn φ ca v1 axial α1 inlet boundary 50% axial chord 2 7 % a x ia l c h o rd 13.5% axial chord l e a d in g e d g e t ra il in g e d g e blade tip side endwall groove acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 279 in terms of boundary conditions, the inlet velocity v1 and the inlet flow angle α1 were applied uniformly at the inlet boundary. with reference to the experimental conditions described in [19], the v1 and the α1 measured from the axial direction were set to 25.5 m/s and 65.1 deg., respectively. meanwhile, the free-stream boundary was used at the outlet boundary, while the cyclic boundary condition was applied at the pitchwise boundary plane. in order to reveal the basic flow phenomena generated by the single groove, the motion of the tip side endwall relative to the blade row was not considered in this study. 3.3. the y+ distribution and the validation of the computed results figure 5 shows the dimensionless wall distance y+ distribution on the blade surface and the sw, while figure 6 shows the axialwise distribution of the static pressure coefficient cps on the blade surface at midspan. in the latter figure, the x/ca is the axialwise length normalised by the blade axial chord length and takes 0 at the leading edge (le) and 1.0 at the trailing edge (te). the cps can be defined by the following equation: 𝐶𝑝𝑠 = 𝑃𝑠 − 𝑃𝑠1 𝑃𝑡1 − 𝑃𝑠1 (1) where ps is the static pressure, and ps1 and pt1 are the crosssectional mass averaged values of the static and the total pressures at x/ca = −1.3, respectively. in order to accurately predict the flow field using the lowreynolds-type turbulence model, it is recommended that the dimensionless wall distances y+ on the wall surfaces are set to less than 1. in terms of my computed results, the y+ became less than 1 on almost all of the wall surfaces, as shown in figure 5. meanwhile, following a comparison between the computed results and the experimental data, the quantitative differences were found to be small, as shown in figure 6. however, the computed results well captured the qualitative tendency observed in the experimental data. therefore, it can be stated that the computed results accurately predicted the flow behaviour in the test cascade. 4. results and discussion in this section, the investigation into the influences of the single groove on the flow behaviour and the loss generation in the cascade is described in terms of both the small and the large tip-clearance cases. figure 7 shows the axialwise distribution of the static pressure coefficient cps on the blade surface near the blade tip, while figure 8 presents the streamlines with the origin near the midgap at the blade suction side in the tip clearance. in the latter figure, the streamline colours are varied depending on the location of the origins (upstream of groove: red; groove location: black; downstream of groove: blue), while ss and ps indicate the blade suction and pressure surfaces, respectively. meanwhile, figure 9 shows the trajectory of the tip leakage vortex core generated from the blade leading edge, while figure 10 indicates the axialwise and pitchwise momentums ψx * and ψy * of the tip leakage flow through the surface stc, which is the blade suction surface extended from the blade tip to the tip side endwall. the ψx * and ψy * are defined by the following equations and indicate the non-dimensional axialwise and the pitchwise momentums per unit length at each axial location, respectively [23]: 𝛹𝑥 ∗ = ∫ 𝛹𝑥 endwall blade tip 𝑑𝑧 𝑆1⁄ (2) 𝛹𝑦 ∗ = ∫ 𝛹𝑦 endwall blade tip 𝑑𝑧 𝑆1⁄ (3) where ψx is ρvnvx/ρv1v1 and ψy is ρvnvy/ρv1v1. as shown in figure 2, vn is the velocity component normal to the stc, and vx and vy are the axialwise and pitchwise velocity components, respectively, while ρ is the density and s1 is the cross-sectional area at the cascade inlet. figure 11shows the flow rate qg of the tip leakage flow through surface sg, which was normalised by the figure 4. the computational grid. figure 5. the y+ distribution on the wall surface. figure 6. blade surface static pressure distribution at midspan. blade tip midspan suction surface leading edge blade tip midspan pressure surface trailing edge -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 c p s x/ca exp[22] ● cfd ― le te acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 280 flow rate through the single passage, while figure 12 presents the spanwise velocity vz, which was normalised by the v1, distribution on the interface between the groove and the bladeto-blade passage. meanwhile, figure 13 indicates the total pressure loss coefficient cpt distribution on the planes normal to the blade chord line and the streamlines with the origin around the high loss regions (lr). the cpt can be defined by the following equation: 𝐶𝑝𝑡 = 𝑃𝑡1 − 𝑃𝑡 𝑃𝑡1 (4) where pt is the total pressure. table 2 presents the mixed-out averaged total pressure loss coefficient cpt2,m downstream of the cascade. the cpt2,m can be defined as follows: 𝐶𝑝𝑡2,𝑚 = 𝑃𝑡1 − 𝑃𝑡2,𝑚 𝑃𝑡1 , (5) where pt2,m is the mixed-out averaged total pressure at x/ca = 1.3. the mixed-out value was obtained according to the procedure proposed by prassad [23]. 4.1. small tip clearance in this section, i discuss the influences of the groove on the flow behaviour and loss generation in the cascade with a small tip clearance. in the sw, the flow through the tip clearance due to the pressure difference between the blade pressure and the suction surfaces generated the leakage vortex from the blade leading edge (tlv), as shown in figure 7 and figure 8a. this vortex was also generated in the gw, as shown in figure 8b. however, around the groove location, the distance between the tlv core and the blade suction surface was reduced compared to that in the sw, as shown in figure 9. the distance between the tlv core and the blade suction surface depends not only on the momentum of the main flow but also on that of the tip leakage flow. thus, the influence of the groove on the momentum of the tip leakage flow was investigated. here, the negative ψx* and ψy* around the groove location were increased compared to those in the sw, as shown in figure 10, and this phenomenon was deemed to reduce the distance between the tlv core and the blade suction surface. the increase in ψx* and ψy* were believed to be due to the fact that the blade loading near the blade tip decreased around the groove location, which consequently reduced the qg compared to that in the sw, as shown in figure 7 and figure 11. moreover, as figure 12a shows, the flow around the blade pressure side flowed into the groove, and this phenomenon then further decreased the qg. the influence of the groove on the tlv behaviour can be summarised as follows. first, the groove weakened the flow indicated by the black streamlines in the sw (figure 8a) via the decrease in the blade loading around the groove location and the flowing of the flow near the blade pressure side into the groove then it inclined its flow direction toward the main flow direction. figure 7. blade surface static pressure distribution near the blade tip. (a) sw (small tip clearance) (b) gw (small tip clearance) (c) sw (large tip clearance) (d) gw (large tip clearance) figure 8. behaviour of tip leakage flow. figure 9. trajectory of tlv core. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 281 as a result, the distance between the tlv core and the blade suction surface was reduced. next, the influence of the single groove on the loss generation was investigated. in the sw, the high loss region was distributed around the trajectory of the tlv core, as shown in figure 13a, with a similar phenomenon also observed in the gw. however, downstream from the groove location, the high loss region due to the tlv was slightly reduced compared to that in the sw, as is clear from a comparison between figure 13a and figure 13b. here, as figure 13b shows, the separation bubble generated on the blade tip surface flowed along the downstream sidewall of the groove (black streamlines) and consequently generated the high loss region lr2. moreover, the blade loading near the blade tip suddenly increased downstream the groove, as shown in figure 7. this phenomenon produced the additional tip leakage vortex and consequently generated the high loss region lr3 (purple streamlines in figure 13b). while the loss generation due to the tlv was reduced in the gw, the cpt2,m value was almost the same as that in the sw, as shown in table 2. this was believed to be due to the generation of the lr2 and the lr3. 4.2. large tip clearance the influence of the groove on the flow behaviour and the loss generation at the large tip clearance was investigated in terms of comparing it with the influence at the small tip clearance. in the sw, the tlv was also observed in the large tip clearance case, as shown in figure 8c. however, toward the downstream of the cascade, the distance between the tlv core and the blade suction surface was further increased compared to that in the small tip clearance case, as shown in figure 9. (a) axialwise (b) pitchwise figure 10. momentum components of tip leakage flow. figure 11. flow rate through sg. (a) gw (small tip clearance) (b) gw (large tip clearance) figure 12. spanwise velocity distribution on interface between groove and blade-to-blade passage. -0.0016 -0.0014 -0.0012 -0.0010 -0.0008 -0.0006 -0.0004 -0.0002 0.0000 0.0002 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 ψ x * x/ca groovele te -0.0070 -0.0060 -0.0050 -0.0040 -0.0030 -0.0020 -0.0010 0.0000 0.0010 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 ψ x * x/ca groovele te -0.0016 -0.0014 -0.0012 -0.0010 -0.0008 -0.0006 -0.0004 -0.0002 0.0000 0.0002 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 ψ y * x/ca groovele te -0.0070 -0.0060 -0.0050 -0.0040 -0.0030 -0.0020 -0.0010 0.0000 0.0010 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 ψ y * x/ca groovele te 0.00 0.10 0.20 0.30 0.40 0.50 0 1 q g [% ] i [deg.]tip clearance small large sg groove tip clearance small large sw gw -0.40 -0.20 0.00 0.20 0.40 0.60 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 c p s x/ca b 0deg ct 0deg b s 0 ct s 0 -0.40 -0.20 0.00 0.20 0.40 0.60 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 c p s x/ca b 0deg ct 0deg b s 0 ct s 0 -0.40 -0.20 0.00 0.20 0.40 0.60 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 c p s x/ca b 0deg ct 0deg b s 0 ct s 0 -0.40 -0.20 0.00 0.20 0.40 0.60 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 c p s x/ca b 0deg ct 0deg b s 0 ct s 0 small tip clearance large tip clearance sw ● gw ● acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 282 in the large tip clearance case, the tlv was also generated in the gw, as shown in figure 8d. around the groove location, the single groove reduced the distance between the tlv core and the blade suction surface compared to that in the sw, as shown in figure 9. the comparison of the momentum components of the tip leakage flow and the qg in the gw with those in the sw revealed qualitatively similar tendencies to those in the small tip clearance, as shown in figure 10 and figure 11. moreover, in the gw with the large tip clearance, the blade loading around the groove location decreased compared to that in the sw and the flow near the blade pressure side flowed into the groove, as shown in figure 7 and figure 12b. therefore, much like in the small tip clearance case, the single groove locally increased the negative axialwise and pitchwise momentum components of the tip leakage flow via the reduction of the blade loading and the flowing of the flow near the blade pressure side into the groove. consequently, the groove reduced the distance between the tlv core and the blade suction surface. in terms of the loss generation in the sw, as figure 13a and figure 13c show, the high loss region also distributed around the trajectory of the tlv core in the large tip clearance case. meanwhile, the cpt2,m in this case was higher than that in the small tip clearance case, as shown in table 2. this was believed to be due to the fact that the loss generation caused by the tlv increased according to the increase in tip clearance. a comparison of the gw and the sw in the large tip clearance case revealed that the single groove decreased the loss generation due to the tlv downstream the groove but generated the lr2 and the lr3 in the same manner as in the small tip clearance case, as shown in figure 13c and figure 13d. as a consequence, the cpt2,m in the gw exhibited a similar value to that in the sw in the large tip clearance case, as shown in table 2. therefore, the influence of the single groove on the loss generation in the large tip clearance case was believed to be almost the same as that in the small tip clearance case. (a) sw (small tip clearance) (b) gw (sarge tip clearance) (c) sw (large tip clearance) (d) gw (large tip clearance) figure 13. total pressure loss coefficient distribution. table 2. mixed-out averaged total pressure loss. tip clearance sw (×10 -4 ) gw (×10 -4 ) 100×(gw-sw)/sw in % small 172.999 172.995 -0.002 large 173.398 173.380 -0.011 acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 283 5. conclusions in this study, the influence of the single groove on the flow behaviour and the loss generation in the linear compressor cascade was investigated in terms of both small and large tip clearance cases. the conclusions derived from the study can be summarised as follows: 1) in the cascade with the smooth tip side endwall, the tip leakage vortex was generally generated from the blade leading edge in both the small and the large tip clearance cases. this vortex generated the loss, and this was increased according to the increase in tip clearance size. 2) the single groove installed at the mid-chord locally weakened the tip leakage flow via the reduction of the blade loading and the flowing of the flow near the blade pressure side into the groove, consequently reducing the distance between the tip leakage vortex and the blade suction surface. moreover, the groove had the effect of decreasing the loss due to the tip leakage vortex generated from the blade leading edge. however, the loss generation in the entire cascade passage was almost the same as that in the cascade without the groove due to the additional loss generation resulting from the presence of the groove. these phenomena were widely observed in both the small and the large tip clearance cases. references [1] m. inoue, m. kuroumaru, structure of tip clearance flow in an isolated axial compressor rotor, asme j. turbomach 111(3) (1989) pp. 250-256. doi: 10.1115/1.3262263 [2] j. a. storer, n. a. cumpsty, tip leakage flow in axial compressors, asme j. turbomach. 113(2) (1991) pp. 252-259. doi: 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[23] f. r. menter, two-equation eddy-viscosity turbulence models for engineering applications, aiaa-journal 32(8) (1994) pp.269-289. doi: 10.2514/3.12149 [24] m. kaneko, influence of incidence angle on behavior of tip leakage flow in linear compressor cascade ，turbomachinery 47(10) (2019) pp. 27-34 (in japanese). doi: 10.11458/tsj.47.10_603 [25] h. chen, x. huang, k. shi, s. fu, m. ross, m., bennington, j. d. cameron, s. c. morris, s. mcnulty, a. wadia, a computational fluid dynamics study of circumferential groove casing treatment in a transonic axial compressor, asme j. turbomach. 136(3) (2013) p. 031003. doi: 10.1115/1.4024651 [26] a. prassad, calculation of the mixed-out state in turbomachine flows, asme j. turbomach. 127(3) (2005) pp. 564-572. doi: 10.1115/1.1928289 https://doi.org/10.1115/1.3262263 https://doi.org/10.1115/1.2929095 https://doi.org/10.1115/92-gt-215 https://doi.org/10.1115/92-gt-432 https://doi.org/10.1115/1.2929458 https://doi.org/10.1115/1.2841339 https://doi.org/10.1115/1.4042250 https://doi.org/10.1115/1.2750674 https://doi.org/10.1115/1.4037822 https://doi.org/10.1155/2017/4631751 https://doi.org/10.1115/1.4033472 https://doi.org/10.1115/1.4000569 https://doi.org/10.1115/1.4002986 https://doi.org/10.1115/1.2008970 https://doi.org/10.2514/1.b36181 https://doi.org/10.1115/gt2014-26859 https://doi.org/10.1115/1.4003000 https://doi.org/10.1115/1.4038319 https://doi.org/10.1115/1.4035521 https://doi.org/10.1115/1.4007547 https://doi.org/10.1115/1.1791643 https://doi.org/10.2514/3.12149 https://doi.org/10.11458/tsj.47.10_603 https://doi.org/10.1115/1.4024651 https://doi.org/10.1115/1.1928289 acta imeko  september 2014, volume 3, number 3, 33 – 37  www.imeko.org    acta imeko | www.imeko.org  september 2014 | volume 3 | number 3 | 33  comparison of three types of dry electrodes for  electroencephalography   patrique fiedler 1 , jens haueisen 1 , dunja jannek 1 , stefan griebel 2 , lena zentner 2 , filipe vaz 3  and carlos  fonseca 4    1  institute of biomedical engineering and informatics, ilmenau university of technology, gustav‐kirchhoff str. 2, 98693 ilmenau, germany   2  department of mechanism technology, ilmenau university of technology, max‐planck‐ring 12, 98693 ilmenau, germany   3  centro de física, universidade do minho, campus de azurém, 4800 guimarães, portugal  4  universidade do porto, faculdade de engenharia, demm, rua roberto frias, s/n, 4200‐465 porto, portugal and seg‐cemuc – dme,        university of coimbra, rua luís reis santos, 3030‐788 coimbra, portugal      section: research paper   keywords: electroencephalography; biomedical electrode; electric potential; eeg cap   citation: patrique fiedler, jens haueisen, dunja jannek, stefan griebel, lena zentner, filipe vaz and carlos fonseca, comparison of three types of dry  electrodes for electroencephalography, acta imeko, vol. 3, no. 3, article 8, september 2014, identifier: imeko‐acta‐03 (2014)‐03‐08  editor: paolo carbone, university of perugia   received march 27 th , 2013; in final form june 2 nd , 2014; published september 2014  copyright: © 2014 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: this work was supported by german ministry of science (3ipt605a) and thuringia ministry of science (2012fg 0014, tna x‐1/2012).  corresponding author: jens haueisen, e‐mail: jens.haueisen@tu‐ilmenau.de    1. introduction  electroencephalography (eeg) is a technique mainly used in the neurosciences and clinical neurological routine for detecting potential changes at the surface of the head, caused by electric activity inside the brain. the state of the art is characterized by the placement of 21 to 256 wet (gelled) silver/silver-chloride (ag/agcl) electrodes on the scalp, typically with the help of an eeg cap. the preparation procedure is lengthy (up to one hour) and requires well trained staff (typically medical technical assistants), and wear time is limited by the stability of the electrolytes (gels) [1], [2]. new application areas of eeg are in the field of brain-computer interfaces (bci) or ambient assisted living (aal). bcis might enable handicapped persons to interact with their environment, based on measured brain signals [3], [4]. for such applications, specific eeg systems are needed, which may be applied by non-experts in a simple and fast way. recently, the first eeg systems based on dry electrodes for bcis were developed, which, however, included only a very limited number of channels [5], [6]. abstract  a potential new area of routine application for electroencephalography (eeg)  is the brain‐computer  interface, which might enable  disabled people to interact with their environment, based on measured brain signals. however, conventional electroencephalography  is  not  suitable  here  due  to  limitations  arising  from  complicated,  time‐consuming  and  error‐prone  preparation.  recently,  several  approaches for dry electrodes have been proposed. our aim is the comparison and assessment of three types of dry electrodes and  standard  wet  silver/silver‐chloride  electrodes  for  eeg  signal  acquisition.  we  developed  novel  eeg  electrodes  with  titanium  and  polyurethane  as  base  materials,  which  were  coated  with  nanometer  sized  titanium‐nitride  films.  furthermore  gold  multi‐pin  electrodes  were  arranged  on  printed  circuit  boards.  the  results  of  the  comparison  of  these  electrodes  with  conventional  wet  silver/silver‐chloride  electrodes  in  terms  of  electrode  impedances  are  presented,  as  well  as  open  circuit  potentials  and  biosignal  measurements.  impedances  were  significantly  higher  for  all  dry  electrode  types  compared  to  wet  electrodes,  but  still  within  the  measurement  range  of  today’s  standard  biosignal  amplifiers.  it  was  found  that  the  novel  dry  titanium  and  polyurethane  based  electrodes show biosignal quality equivalent to conventional electrodes. in conclusion, the novel dry electrodes seem to be suitable  for application in brain‐machine interfaces.   acta imeko | www.imeko.org  september 2014 | volume 3 | number 3 | 34  our general aim is to develop a novel eeg system for home use with a high number of channels based on passive dry electrodes. several approaches were developed for manufacturing the dry electrodes, including titanium (ti) and polyurethane (pu) pin electrodes coated with titanium nitride (tin) [7], [8]. titanium nitride is chosen as coating material owing to its biocompatibility and chemical stability [9]. the aim of this paper is to compare the novel dry electrodes with standard wet silver/silver-chloride electrodes, in terms of electrochemical characteristics and signal acquisition performance. in addition to the tin-coated electrodes, we also compare gold (au) multi-pin electrodes recently proposed by other groups [10]. a comparative assessment of the different technologies concludes the study. 2. methods  2.1. electrodes  the basic design requirements for dry contact electrodes are sufficient hair layer penetration, biocompatibility, electrochemical stability, and signal quality comparable to standard wet electrodes. furthermore, our aim is to provide long term applicability, patient comfort, a cap ensuring sufficient electrode adduction, compatibility with conventional biosignal amplifiers, as well as ease of use and speed of preparation. based on these criteria, three main types of electrodes were designed: (i) titanium pin electrodes, (ii) polyurethane multi-pin electrodes, and (iii) gold multi-pin electrodes. figure 1 shows the three types of electrodes. the titanium pin electrodes and polyurethane multi-pin electrodes were coated with titanium nitride. the manufactured gold multi-pin electrodes are based on gold coated electronic precision brass pins soldered to a common epoxy baseplate. table 1 details the design parameters of the electrodes depicted in figure 1. note the different contact areas in row 4 of table 1. 2.2. electrochemical characterization  electrode impedance characterization was performed for a frequency range of 5 hz to 10 khz using a hewlett packard 4192a lf impedance analyzer (hewlett packard company, palo alto, usa). we used a standard four-point measurement setup in 0.9% nacl solution. open circuit potentials were measured in a 0.9% nacl solution with an agilent 34401a multimeter (agilent technologies, santa clara, usa), where an ag/agcl electrode served as reference (150 minutes after immersion into nacl solution). 2.3. eeg signal acquisition  for eeg measurements, the electrodes were positioned according to the international 10-20 system [11]. patient ground electrodes (always wet ag/agcl) were placed at the afz position (figure 2). all measurements were performed with a set of ag/agcl electrodes adjacent (distance approx. 2.5 cm) to the dry contact electrodes (figure 2), at two frontal and two occipital positions. for the ag/agcl electrodes reference measurement, two sets of ag/agcl electrodes were placed adjacent to each other (last column in table 2). test and reference signals were simultaneously recorded using two commercial amplifiers (refa, ant b.v., enschede, the netherlands), with common average reference and a selected sampling rate of 512 samples/s. for all eeg tests, the room temperature was 22 °c and the relative air humidity was 35%. the electrode-skin interfacial impedance was recorded preliminary to the eeg tests, using the corresponding function of the eeg amplifier. three types of eeg activity were recorded in a healthy volunteer: resting state, alpha activity, and visual evoked potentials (vep). resting state and alpha activity are spontaneous eeg signals while the vep is an averaged signal figure  2.  bipolar  measurement  setup,  showing  the  selected  electrode  positions. the stimulation screen was used for the visual evoked potential  measurements.  patient  ground:  afz;  bipolar  measurements:  o1‐fp1  and  o2‐fp2.  table 1. electrode design parameters  ti/tin  pu/tin  au  ag/agcl  effective pin diameter (mm)  1.5 1.5 0.5  n.a. effective pin height (mm)  2.5 6 3  n.a. pin number  1 24 15  n.a. approx. electrode contact surface (mm2)  3.5 85.1 6  42.5 approx. electrode weight (g)  0.05 0.73 2.21  0.61 figure  1.  (a)  titanium  single  pin  electrode,  (b)  polyurethane  multi‐pin  electrode,  (c)  gold  multi‐pin  electrode,  and  (d)  conventional  ag/agcl  ring electrode. the pin shape allows hair  layer penetration and causes a small indentation in the human scalp, which leads to an increased contact surface as well as reduced relative movements.   acta imeko | www.imeko.org  september 2014 | volume 3 | number 3 | 35  built to 250 single trials. thus, temporal and frequency characteristics of the spontaneous and evoked activity are different. all signals were recorded using asa software (ant b.v., enschede, the netherlands), and the analysis was performed using matlab (the mathworks inc., natick, usa). data recorded using two sets of conventional ag/agcl ring-shaped electrodes (ant b.v., enschede, the netherlands) served as reference. 3. results  figure 3 shows the electrochemical impedance spectra of all four types of electrodes (mean over 10 measurements). the ag/agcl electrodes exhibited the lowest absolute values of the impedance, followed by the gold multi-pin electrodes, the titanium pin electrodes, and the polyurethane electrodes. for the gold multi-pin and the titanium pin electrodes, significant capacitive behaviour was observed, as demonstrated by the high impedances at low frequencies and lower impedances at higher frequencies. for the phase responses, all electrodes showed a mainly decreasing phase shift with increasing frequency, corresponding to a capacitive behaviour. the ag/agcl electrodes showed the closest results to the mainly resistive behaviour, followed by the polyurethane pin electrodes, the gold multi-pin and the titanium pin electrodes. however, due to measurement equipment restrictions, the polyurethane pin electrodes were not measured over the entire frequency range, which makes the values not comparable for the very low frequencies. a stable and reproducible open circuit potential is important to avoid amplifier saturation problems, and strong signal drift limiting low frequency bandwidth. the mean open circuit potential (figure 4) for the ag/agcl electrodes was 221.8 mv and the most stable in terms of mean std (2.1 mv). mean values of 610.8 mv ± 439.4 mv, 360.3 mv ± 63.3 mv and 422.2 mv ± 56.7 mv, were measured for the gold multi-pin electrodes, titanium pin electrodes and for the polyurethane pin electrodes, respectively. figure 5 presents the measured impedances at the volunteer’s head, when 4 electrodes of the same kind were placed at the recording positions. the values for all dry electrodes were considerably higher than those for the ag/agcl electrodes. the dry electrodes showed also larger impedance variations. the results of the eeg measurements are reported in table 2. between the two simultaneously recorded eeg sets, figure 4. open circuit potentials of the four types of electrodes recorded over 150 minutes after immersion into 0.9% nacl solution. solid and dotted lines represent mean and std of 5 measurements, respectively.  figure  3.  impedance  spectra  of  the  four  types  of  electrodes:  (a)  absolute value  and  (b)  phase  of  the  impedance.  solid  and  dotted  lines  represent mean and std of 10 measurements, respectively.   figure  5.  measured  impedances  for  a  test  set  of  four  electrodes  of  each  type. all values are in kω.   figure  6.  overlay  plots  for  channel  o2  of  different  eeg  recordings  for  all  electrode types: a) vep test results in time domain, and b) power spectral  density of 10 seconds of eeg containing alpha activity.   acta imeko | www.imeko.org  september 2014 | volume 3 | number 3 | 36  the root mean square deviation (rmsd) and the correlation coefficients corresponding to 30 seconds of data were computed, respectively, indicating important magnitude and morphological differences. the rmsd between the different electrodes, for the various signal types, showed similar values for all tested electrodes, except for the gold multi-pin electrodes at the vep measurement. as expected, the correlation is higher for the vep, followed by the alpha activity. the lowest correlation is found for the resting state eeg. according to figure 6, similar results are also visible in overlay plots, for time and frequency domain. the main vep components are exhibited in the recordings of all electrode types. the visible amplitude and latency variations are likely caused by slight variations of the electrode positions. in the psd plot the alpha activity is clearly visible as a peak at approximately 11 hz in all recordings. the gold multi-pin electrodes exhibit an increased drift, represented by higher psd values for frequencies below 2 hz. 4. discussion  the most relevant result from the above reported experiments consists in the fact that all the proposed types of dry electrodes allowed the recording of standard eeg. the signal traces and spectra were very similar to the ones of the simultaneously recorded ag/agcl electrode signals (fig.5). the differences in the rmsd in table 2 were found to be similar for the different electrode types. this indicates that a main contribution to the rmsd stems from the spatial distance between the two compared sets of electrodes, and not from the materials differences. exceptions are the vep measurements with the gold multi-pin electrodes, where both the rmsd and the correlations showed deviating values. this can be explained by the difficulty in placing these electrodes at the back of the head, where a stronger curvature of the head makes it more difficult to achieve a good and reliable pin/scalp contact (compare also the length of the pins in figure 1). at the same time, this region shows the most prominent mapping features in the vep according to the activated brain areas. the correlation values are also small for the resting state eeg (row four in table 2). this can be explained by the fact that resting state eeg is highly variable in both time and spatial distribution. hence, the spatial distances between the test and reference electrodes was too high to provide an adequate spatial sampling [7]. table 3 summarizes the criteria for the comparison and the assessment of the different types of dry electrodes. all dry electrodes are easy and fast to apply and, according with bibliographic information, biocompatible (the printed circuit boards for the gold pins might be coated with biocompatible material). due to the pin-like design, hair layer interfusion is achieved with all types of electrodes. constant and sufficient electrode pressure on the skin is achievable for all types of electrodes, although patient comfort varies considerably. in this study, we considered conventional textile eeg caps for the pu/tin and the gold multi-pin electrodes, while a novel actuator-based cap was used for the ti/tin pins. the average contact pressure was similar for both cap types. considering the small size and adduction requirements, the novel dry electrodes enable high density multichannel electrode arrangements. these dense multichannel setups provide crucial spatial information about the brain activity, hence allowing for brain source reconstruction procedures in current [12] and new areas of application [3], [4]. compared to wet ag/agcl electrodes, the bandwidth of dry electrodes is reduced. while ag/agcl electrodes can record potential differences starting from dc, dry electrodes, due to their capacitive behaviour at low frequencies, might not be used below about 0.5 hz. the lower frequency limit depends on the table 3. criteria for comparison and assessment of dry electrodes.  criteria  ti/tin  pu/tin  au  dry, passive, fast and easy application yes yes yes biocompatible materials  yes yes pins only electrode shape for hair layer interfusion  yes yes yes cap system tested  adduction actuator textile textile signal quality equivalent to ag/agcl  0.5 – 40 hz 0.5 – 40 hz  3 – 40 hz biosignal amplifier compatibility  high impedance yes ocp fluctuations patient comfort  max. 1 h > 1.5 h max. 1 h table 2. mean rmsd and correlation of the eeg data.   eeg test  ti/tin  pu/tin  au  ag/agcl  r m s d   (µ v )  resting eeg  6.9 ± 3.4 5.3 ± 0.6 6.1 ± 2.0  4.6 ± 2.1 alpha activity  4.2 ± 1.8 4.4 ± 0.2 3.6 ± 0.6  3.9 ± 2.0 vep  0.6 ± 0.7 0.7 ± 0.3 1.9 ± 0.5  0.7 ± 0.4       c o rr e la ti o n   (% )  resting eeg  24 ± 19 59 ± 22 25 ± 13  58 ± 17 alpha activity  76 ± 9 73 ± 9 73 ± 9  85 ± 2 vep  92 ± 10 95 ± 4 74 ± 22  94 ± 2 acta imeko | www.imeko.org  september 2014 | volume 3 | number 3 | 37  type of electrode and is still to be optimized. in principle, all types of dry electrodes considered here are compatible with state of the art biosignal amplifiers. the relatively high impedances might, however, in some recording situations, cause artefacts. to circumvent such problems online artefact detection may be suitable [13]. the electrode-skin impedance characteristics of the different electrodes are considerably influenced by their actual contact surface. the contact surface given in table 1 was calculated based on the geometry of the pin tips. during application, variable contact surfaces occur depending on further parameters including hair interfusion, hair density and contact pressure. hence, further investigations should address the variability of these influencing parameters in a larger group of volunteers. the influence of each parameter will be quantified separately with a sensitivity analysis in a future study. there are several further differences for the dry electrode types. the price is lowest for the gold multi-pin electrodes and highest for the titanium pin electrodes. more important for multichannel eeg applications, where up to 256 electrodes are applied, is the weight of the electrodes. here, the gold multi-pin electrodes exhibit the highest weight, followed by the titanium pin electrodes (when taking 24 pins per electrode into account). on the other hand, the polyurethane electrodes weigh only about 100 g for 256 electrodes. furthermore, the mechanical flexibility of the substrate contributes to increased patient comfort while, at the same time, maintaining contact reliability (table 3 last line). this renders the polyurethane electrodes very suitable for the application in long-term measurements. 5. conclusions  we conclude that polyurethane based electrodes coated with titanium nitride are suitable for recording eeg in applications like brain-computer interfaces. references  [1] a. searle and l. kirkup, “a direct comparison of wet, dry and insulating bioelectric recording electrodes,” physiological measurement, vol. 21, no. 2, pp. 271-283, 2000. [2] p. tallgren, s. vanhatalo, k. kaila, j. voipio, “evaluation of commercially available electrodes and gels for recording of slow eeg potentials,” clinical neurophysiology, vol. 116, no. 4, pp. 799-806, 2005. [3] j.e. huggins, p.a. wren, k.l.gruis, “what would braincomputer interface users want? opinions and priorities of potential users with amyotrophic lateral sclerosis,” amyotrophic lateral sclerosis, vol 12, no. 5, pp. 318-24, 2011. [4] j.r. wolpaw, “brain-computer interface research comes of age: traditional assumptions meet emerging realities,” journal of motor behavior, vol. 42 no. 6, pp. 351-3, 2010. [5] t.o. zander, m. lehne, k. ihme, s. jatzev, j. correia, c. kothe, b. picht, f. nijboer, “a dry eeg-system for scientific research and brain-computer interfaces,” frontiers in neuroscience, vol 5, article 53, 2011. [6] c. grozea, c.d. voinescu, s. fazli, “bristle-sensors--low-cost flexible passive dry eeg electrodes for neurofeedback and bci applications,” journal of neural engineering, vol. 8, no. 2, article 025008, 2011. [7] p. fiedler, l.t. cunha, p. pedrosa, s. brodkorb, c. fonseca, f. vaz, j. haueisen, “novel tinx-based biosignal electrodes for electroencephalography,” measurement science and technology, vol. 22, article 124007 (7pp), 2011. [8] p. pedrosa, e. alves, f.n.p. barradas, p. fiedler, j. haueisen, f. vaz, c. fonseca, “tinx coated polycarbonate for bio-electrode applications,” corrosion science. 56:49-57, 2012. [9] l. t. cunha, p. pedrosa, c. j. tavares, e. alves, f. vaz, c. fonseca, “the role of composition, morphology and crystalline structure in the electrochemical behaviour of tinx thin films for dry electrode sensor materials,” electrochimica acta, vol. 55, no. 1, pp. 59-67, 2009. [10] y. m. chi, y.-t. wang, y. wang, c. maier, t.-p. jung, g. cauwenberghs, “dry and noncontact eeg sensors for mobile brain-computer interfaces,” ieee transactions on neural systems and rehabilitation engineering, vol. 20, no. 2, pp. 228-235, 2012. [11] g. h. klem, h. o. luders, h. h. jasper, c. elger, “the tentwenty electrode system of the international federation. the international federation of clinical neurophysiology,” electroencephalography and clinical neurophysiology, vol. 52 (suppl.), pp. 3-6, 1999. [12] s. klee, d. link, p. bessler, j. haueisen, “optoelectrophysiological stimulation of the human eye using fundus-controlled silent substitution technique,” journal of biomedical optics, vol. 16, no. 1, 015002, 2011. [13] u. graichen, r. eichardt, p. fiedler, d. strohmeier, j. haueisen, “sphara a generalized spatial fourier analysis for multisensor systems with non-uniformly arranged sensors: application to eeg”, plos one, submitted, 2013. analysis of mechanical properties for the key force transfer components of 2 mn deadweight force standard machine acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 91 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 91 96 analysis of mechanical properties for the key force transfer components of 2 mn deadweight force standard machine wei tieping1, guo jinquan2, lai zhengchuang3, lin shuo4, yang xiaoxiang5 1 school of mechanical and automotive engineering, fujian university of technology, fuzhou, china, tpwei@fjut.edu.cn 2 school of mechanical engineering and automation, fuzhou university, fuzhou, china, maxguo@163.com 3 fujian province institute of metrology, fuzhou, china, lzcfjjl@126.com 4 fujian province institute of metrology, fuzhou, china, linshuo1001@126.com 5 school of mechanical engineering and automation, fuzhou university, fuzhou, china, yangxx@fzu.edu.cn abstract: the good mechanical properties of deadweight force standard machine (dwm) are the basis to ensure the accuracy of its measurement. aiming at the development of 2 mn dwm, this paper carries out simulation analysis on the key components of the force value transfer process. the analysis results provide constructive suggestions for the development of 2 mn dwm. keywords: uncertainty; 2 mn deadweight force standard machine (dwm); finite element method; mechanical properties 1. introduction with the development of economy and industrial technology, people's demand for force measurement with high precision and large range becomes more and more obvious. at present, a large number of researchers and funds have been invested in developing the force standard (base) machine as the highest standard for force traceability, and a series of static machines with different ranges (50 n ~ 4.45 mn) and measuring accuracy have been developed, including 1 mn (urel = 0.002 %, k = 2) in china, 1 mn (urel = 0.002 %, k = 2) in inrim/italy, 1 mn (urel = 0.002 %, k = 2) in npl/india, 1.2 mn (urel = 0.002 %, k = 2) in npl/uk, 2 mn/1 mn (urel = 0.002 %, k = 2) in ptb/germany, 4.45 mn in nist/usa. the theoretical value of uncertainty of dwm measurement is 1 × 10-5, but the actual value is only 5 × 10-5 [1]. there are many factors affecting the measurement uncertainty of dwm, usually including the local acceleration of gravity, weight material density, uncertainty of weight mass, the density of air quality, the swing of the central boom and beam tilt (the uncertainty of dwm and the force transducer structures) [2-7]. also, it includes the influence of tidal level effect on gravitation, the influence of air conditioning airflow, the influence of gravitation between weights, humidity, temperature, hardness of the pad, and so on [8]. among these factors, some factors contribute more weight to the evaluation of measurement results. some influencing factors contribute little to the evaluation of the measurement results, but they also have great influence on the measurement stability of the static crane. among them, the strength, stiffness and stability of dwm are the most basic and the most presupposition factors. for structure design of 2 mn dwm in fujian province institute of metrology, its key value transfer components including the main body frame structure, the weight installation platform lifting mechanism, the structure of the inverter and the central derrick were simulated based on finite element method and the improved mechanical property solutions were put forward, which can provide constructive suggestions for the successful design of 2 mn dwm. 2. the operating principle of 2 mn dwm the structure diagram of 2 mn dwm is shown in figure 1. the suspension system of dwm is made of an inverter and a central derrick. the gravity value of the hanging system is 40 kn. there are a total of 39 pieces of weights, which are independently added by the motors [3]. this configuration can be used to load the internal force values of (10 kn ~ 1 mn) and (20 kn ~ 2 mn) in the range increasing by the multiple of 5 kn. before dwm works, the inverter and the central derrick are all placed on the rack. at the same time, the weight is placed in the v-shaped block of the transmission device through the weight handle. http://www.imeko.org/ mailto:tpwei@fjut.edu.cn mailto:maxguo@163.com mailto:lzcfjjl@126.com mailto:linshuo1001@126.com mailto:yangxx@fzu.edu.cn acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 92 while the calibration is in progress, the force transducer is placed on the moving beam to ensure that the central axis of the force transducer coincides with the central axis of the hanging system. the beam is driven upward through the roller screw by the motor until the hanging system is fully supported to reach stability by the force transducer. at this point, the hanging system is in a free state. after that, the transfer device will transmit the required weight downward until it is completely pressed on the weight tray. then, the force value of the weight will be transmitted to the force transducer through the hanging system. at this point, the hanging system is in the loading state. after the verification, the servo motor reverses direction and the transfer device drives the weight to move upward until the weight is completely off the weight tray. then, the moving beam moves downward until dwm returns to the initial state. figure 1: structural diagram of 2 mn dwm 3. simulation analysis for the key force transfer components based on the above analysis of the working principle of 2 mn dwm, it can be known that the main frame structure, the central derrick, the inverter structure and the lifting mechanism of the weight installation platform are all the key loadbearing components. the mechanical properties of these parts directly influence the transfer effect of the value in the standard machine, which determines the measurement performance of the standard machine. the following sections will conduct finite element modelling and simulation of the key components of the design one by one. the analysis results will provide constructive suggestions for the development of 2 mn dwm. 3.1. the main frame structures the main frame of the 2 mn dwm is used for placing weights, providing static support for weights and fixing the motor that drives weights to move up and down. the main frame structure is made of the upper and lower supports, the nine columns in the middle and every three columns form a group. five fixed bars distributed uniformly at the top and bottom are used to connect the columns to improve the stability of the structure, as shown in figure 2. the height of the main frame reaches 10 m, and the weight placement mechanism fixed on it has a maximum load bearing weight gravity of 2 mn. therefore, its strength and stiffness should be analysed emphatically. in addition, the security of the connection between the weight placement mechanism and the main frame is also a key consideration. considering the feasibility of the numerical simulation and the reliability of the analysis results, the simulation analysis can be divided into two parts: (1) the overall analysis: the strength and stiffness of the main structure are analysed, and the connection between the weight placement mechanism and the main frame is not considered; (2) the local analysis: the connection between the placement mechanism of weights and the main frame is considered, and the deformation of the main frame structure is not considered. figure 2: schematic diagram of the main frame structure (1) simplification of mechanical model as shown in figure 3, the supporting structure for placing weights is simplified to a t-shaped bracket for the overall analysis. it is assumed to be a rigid body, which is regarded as a whole with the main frame. the load of 2 mn acts on the t-shaped bracket according to the distribution position of weights. as shown in figure 4, the force value transfer mechanism of the connection between the weight placement mechanism and the main frame structure is comprehensively considered for the local analysis. on the premise of not affecting the calculation results, the deformation of the connecting column with the weight placement mechanism is ignored. therefore, only a section of the column structure is intercepted for modelling. the connection between the belt and the column is a fixed connection. http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 93 figure 3: simplified schematic diagram of the overall analysis structure figure 4: simplified schematic diagram of the local analysis structure (2) result analysis as shown in figure 5, the maximum von mises stress value of the overall analysis structure is 3.99 mpa at the connection position between the lower support and the column. the maximum total displacement value is 0.043 mm at the middle position of the column. therefore, there are sufficient strength and stiffness for the overall frame structure to meet the measurement accuracy of dwm. as shown in figure 6, the maximum von mises stress value of the local analysis structure is 96.61 mpa at the edge of the hole where the uppermost screw contacts the rack, which is under the action of the maximum weight block (15-ton weight). the maximum total displacement value is 0.076 mm at the outer edge of the v-shaped block. figure 5: stress cloud diagram of the overall frame structure figure 6: stress cloud diagram of weight placement mechanism the maximum von mises stress value of the weight placement mechanism and its connection screws are far less than the yield limits of the corresponding materials, which indicates that the structure is able to meet sufficient strength and stiffness for the accurate measurement of dwm. in addition, the modal analysis of the first six orders of the main frame shows that the frequency of the first three orders is about 10 hz, and the frequency of the second three orders is about 25 hz, which is relatively small. therefore, it is suggested to avoid these basic frequencies in practice to prevent resonance of the structure. 3.2. the central derrick (1) simplification of mechanical model on the one hand, the structure size of the central derrick will affect the project installation. on the other hand, there are many ways to combine weights at the check points. in addition, the strength and stiffness of the central derrick directly affect the measurement performance of dwm. therefore, it is of great importance to design the reasonable structure size for the central derrick. here, the simulation results of the derrick with the dimensions of equal section and stepped structure are compared, which can provide theoretical basis for the structural design of the central derrick. as shown in figure 7, the central derrick structure designed is composed of two round rods with equal sections in figure 7 (top), while the central derrick structure designed is composed of five round rods with variable sections in figure 7 (bottom). both are made of 2cr13. all levels of derrick are connected by threads and connecting rods. each stage of derrick is installed with an equal size ring. in addition, the main function of the ring is to support the weight tray and transfer weight gravity to the hanger. http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 94 figure 7: structure design scheme of the central derrick, top: two round rods with equal sections, bottom: five round rods with variable sections (2) result analysis the analysis results of the two design schemes are shown in figure 8 and figure 9. compared with the equal-section central derrick, the von mises stress value of the variable-section central derrick decreases by 29.99 %. the von mises stress value is far smaller than the yield limit of the material and the structure obtains better strength. in addition, the total displacement value for the variable-section central derrick is reduced by 31.45 %, and the structure has better stiffness. therefore, the variable-section central derrick is more in line with the actual high precision measurement requirements of dwm. in addition, the weight should be lowered to the weight tray slowly during loading and returned to the lifting platform immediately after verification, which can reduce the creep effect of force fluctuation on the central derrick. at the same time, it is easier for the multiple levels of variable-section derrick to install. therefore, the structural design of multi-level central derrick with variable crosssection is more in line with the actual situation. figure 8: stress cloud diagram of hanging structure with equal diameter central derrick figure 9: stress cloud diagram of the hanging structure in the classification central derrick 3.3. the inverter structure (1) mechanical model the inverter is the key load-bearing component, and its initial design structure and the structure after topology optimization are shown in figure 10. it is very important to ensure that the structure of the inverter is subjected to uniform and stable load in the verification process. the structure consists of an upper beam, three support rods and a lower beam. furthermore, the support rods are fixed to the top frame of the force standard device. in the tensile state, the lower beam of the inverter can bear the force up to 2 mn, which is transmitted to the fixed platform through the support rods. then, the tension sensor between the upper beam and the fixed platform is subjected to the weight gravity. in the pressure state, the lower beam of the inverter is subjected to the force up to 2 mn, which is transmitted to the upper beam through the support rods. then, the compression sensor located between the upper beam and the fixed platform is begin to be verified. here, the structure of the initial design is simulated and analysed, and the size of the initial structure is topologically optimized to obtain the optimized structure. meanwhile, the analysis results of the two are compared. figure 10: the inverter structure, left: initial design, right: after topological optimization (2) result analysis figure 11 and figure 12 are the overall displacement cloud maps of the two structures before and after optimization, respectively. it can be seen from the figures that the maximum von mises stress value of the inverter structure at the http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 95 contact point between the lower beam and the bearing decreases from 472 mpa to 176 mpa, which is far less than the yield limit of the material and could meet the strength requirement. the maximum overall displacement value of the inverter structure at the contact point between the lower beam and the hanger decreases from 2.6 mm to 1.6 mm. in addition, the modal analysis of the first six orders of the main frame is carried out, and it is found that the frequency of the first three orders after optimization is around 10 hz, which is one order larger than that the structure before optimization. while the frequencies of the second three orders before and after optimization are around 100 hz with little change. as can be seen from the above, the maximum von mises stress value of the lower beam of the optimized inverter is obviously reduced. furthermore, the stress distribution range of the optimized structure is smaller and the overall force is more uniform. therefore, the optimized inverter is able to obtain a more stable working state. figure 11: analysis results of the initial design structure of the inverter, left: initial design, right: after topological optimization figure 12: analysis results of the optimization structure of the inverter, left: stress cloud diagram, right: displacement cloud diagram 3.4. lifting mechanism of weight placing platform the lifting mechanism of the weight placing platform is mainly used in the loading and unloading of dwm. also, it is used for auxiliary lifting in the maintenance and repair, which is convenient for vertical transportation of personnel and rack appliances. as shown in figure 13 (left), the screw rod and the screw nut constitute a screw pair, and the motor drives the screw rod to rotate and drives the lift platform connected with the screw nut to rise and fall. the maximum placement weight of 2 mn dwm is 15 t supported by three screw rods in a 120° distribution. considering the high lifting height of the placing platform, the slender support bar and the high safety requirement, the mechanical analysis of the structure safety must be carried out. figure 13: schematic diagram of lifting platform mechanism for placing weight, left: without guide bars, right: with guide bars (1) mechanical model the simplified model mainly considers the deformation, strength and buckling of the whole lifting mechanism of the platform, so the connection problem of the detail structure such as the strength of screw thread is not considered. without affecting the simulation results, the platform lifting mechanism is simplified as following: 1) screw nonwork state cannot rotate, and lift platform by selflocking stays at a certain height. the top of the screw constraint torus only limits horizontal displacement, and six degrees of freedom of the bottom of the torus are all fixed ignoring the screw thread; 2) the railing of the outer ring of the lift platform is not considered, and the railing of the platform is much smaller than its own weight. therefore, only the weight of the lifting platform itself is considered; 3) the disassembled plate meets the strength requirement, and it can be considered as a rigid plate for the simulation model as shown in figure 14. figure 14 simplified schematic diagram of mechanism model of lifting platform for weight, left: without guide bars, right: with guide bars (2) result analysis as shown in figure 15 (left), the maximum von mises stress value of the mechanism without guide bars is 72.89 mpa located at the triangular floor plate under the disassembly end supported by the platform lifting mechanism. the maximum total displacement value is 1.55 mm located in the middle and outer edge of the decorative pattern board. as shown in figure 15 (right), the maximum von mises stress value of the mechanism with guide http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 96 bars is 66.6 mpa at the same location as the mechanism without guide bar. the maximum total displacement value is 1.35 mm at the same location as the mechanism without guide bars. figure 15 finite element analysis results of platform lifting mechanism, left: without guide bars, right: with guide bars according to the mechanical design manual, the allowable deflection of the shaft is 0.0003 l = 2.16 mm, l is the length of the bar. as we known in the figure 15, the maximum total displacement value of the lead rod is 1.01 mm without guide bars and 1.11 mm with the guide bars. therefore, it can be seen that the screw rods of the two kinds of structure meet the deflection requirements. at the same time, according to the euler formula of critical load, the screw rod is determined to be a large flexibility rod. furthermore, it is found that the first critical load is much larger than the actual maximum applied load according to the stability calculation. therefore, the mechanism meets the stability requirement. in addition, the lifting mechanism of the weight placing platform in the middle position is also in the extreme working condition, so the corresponding calculation and analysis should be carried out. the results show that the strength, stiffness and stability of the structure meet the requirements. here, because of limited space of the paper, it is not elaborated. figure 16: 2 mn dwm of fujian province institute of metrology in china 4. 2 mn dwm development result the research results of this paper have provided a useful theoretical reference and basis for the successful development of 2 mn dwm by fujian province institute of metrology. as the largest dwm in china, its uncertainty is better than 0.002 %. at the same time, its performance is comparable to that of 2 mn dwm in german ptb. compared with the british npl 1.2 mn dwm, its consistency is better than 0.002 %. 5. conclusion based on the finite element method, the key structure components in the force value transmission process of 2 mn dwm designed by fujian province institute of metrology have been simulated. the constructive suggestions are provided for its development. furthermore, the measurement characteristics of the 2 mn dwm will be further studied in the future. 6. references [1] d. i. kang, b. d. lim, “determination of the averaging time for precision calibration of a build-up system in a deadweight force machine”, measurement, vol. 19, pp. 217-227, november 1996. [2] a. lindau, r. kumme, a. heiker, “investigation in the local gravity field of a force laboratory of ptb”, vdi berichte, no. 1685, pp. 589-598, november 2002. [3] r. kumme, f. tegtmeier, d. röske, a. barthel, a. germak, p. averlant, “force traceability within the meganewton range”, in proc. of imeko 22nd tc3, 15th tc5 and 3rd tc 22 international conferences, 2014. [4] c. ferrero, “the measurement of parasitic components in national force standard machines”, measurement, vol. 8, no. 2, pp. 66-76, june 1990. [5] a. bray, g. barbato, f. franceschini, e. xhomo, “rotational and end effects: a model for uncertainty evaluation in force measurements by means of dynamometers”, measurement, vol. 17, no. 4, pp. 279-286, july 1996. [6] y. k. park, d. i. kang, j. h. kim, m. s. kim, “reduction of deadweight pendulum motion and its conformation by using a build-up system”, journal of mechanical science and technology, vol. 23, no. 11, pp. 2956-2963, august 2009. [7] a. abu-sinna, y. k. park, d. i. kang, m. s. kim, “the influence of loading frame stiffness on loadcell–deadweight force machine interaction”, measurement, vol 42, no. 6, pp. 830-835, july 2009. [8] d. röske, “gravitational forces between weights in force standard machines–simplified analytical and numerical approaches”, journal of physics: conference series., vol 1065, no. 4, pp. 142012, september 2018. http://www.imeko.org/ study on calibration method of hydraulic bolt stretcher acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 156 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 156 158 study on calibration method of hydraulic bolt stretcher t. fang1, y. jia1, h. xu1, h. zhang1, w. liu1 1 liaoning province institute of measurement, shenyang, china, 757573199@qq.com abstract: bolt hydraulic tensioner is ideal equipment for bolt fastening and disassembly. compared with traditional method, it has incomparable advantages. bolt hydraulic tensioner has been widely used in wind power, nuclear power, aerospace, steel, petrochemical and other major fields. in this paper, two methods of calibrating the hydraulic stretcher are proposed, and the problems that should be paid attention to in the process of calibrating the stretcher are analyzed in detail, it can be used for reference in the calibration of hydraulic stretcher. keywords: bolt hydraulic stretcher; calibration method; pressure gauge; standard dynamometer; fitting equation 1. introduction the bolt hydraulic tensioner is the ideal equipment for bolt fastening and disassembly, the hydraulic tensioner has the incomparable advantage of the traditional hammer hitting spanner or the method of using the hydraulic spanner. first, the bolt is only subjected to the tensile force while working, eliminating the consumption of friction and screw torque, the tensile force is determined by the elongation, tensile strength and yield coefficient of the material. working through the high-pressure oil pump to provide tensile force for the bolt, in the elastic deformation and slight plastic deformation of the bolt will be stretched, so that the diameter of the bolt smaller, so that the nut more easy to tighten or loosen, do not lose the bolt and nut. second, the bolt tensioner on the connector can ensure the connector axial force uniform, high reliability of the connection, can work multiple bolts at the same time, so as to ensure the synchronization of bolt pretightening force. third, bolt hydraulic tensioner is small and flexible, greatly reducing the occupancy rate of space. because of these advantages, hydraulic stretcher has been widely used in wind power, nuclear power, aerospace, steel industry, heavy machinery, petrochemical and other major fields, especially in the situation of serious environmental pollution or restricted working space, the hydraulic stretcher plays an irreplaceable role, as figure 1. figure 1: application of stretcher in wind power and nuclear power industry 2. calibration method whether the tensioner can provide accurate pretightening force is closely related to whether the bolt hydraulic tensioner calibration method is correct or not. at present, most of the bolt hydraulic tensioner calibration work is completed by the manufacturer in china, the authenticity, accuracy and reliability of the data cannot be certified and approved by the authority. there are few domestic literatures on the calibration method of the bolt stretcher, and few studies on the calibration device of the bolt stretcher. how to solving the problem of bolt tensioner calibration is the most important work in the development of bolt retightening technology. http://www.imeko.org/ mailto:757573199@qq.com acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 157 at present, there is no technical regulation for bolt hydraulic tensioner in our country. the calibration is generally referred to [ 1 ] , but this standard is only limited to the general technical index, the discussion on the testing method and testing procedure of the hydraulic stretcher is not only limited but also not completely suitable for the calibration of the instrument. especially in recent years the number of domestic manufacturers of the equipment is increasing, the use of such equipment is also rapidly increasing, the performance is uneven, so product quality is extremely important. if the calibration procedure could be better specified, the calibration of the bolt stretcher would be improved. such procedural changes might include: determine whether to meet the requirements of technical indicators, measuring instruments and auxiliary equipment, how to select and use standards, so that the calibration results will make enterprises and users feel more secure. based on many years of experience of bolt tensioner calibration, the authors summarize a set of hydraulic tensioner calibration device. the calibration device of hydraulic stretcher is usually composed of high-pressure pump, high-pressure hose, pressure gauge, standard dynamometer or special working dynamometer (testing machine, force standard machine). calibration methods are mainly divided into two methods, the difference between the two methods is the way to obtain the tensile force of the tensioner: 1. collection of tensile force by a special working dynamometer: the output force is exerted by a high-pressure pump through a series connection of a hydraulic stretcher with a special working dynamometer (a testing machine or a force standard machine), read the readings of special working dynamometer (testing machine, force standard machine) and pressure gauge, and establish the corresponding table of their relationship. 2. the tension force is collected by the standard dynamometer: the hydraulic tensioner is connected with the standard dynamometer mainly by the method of reaction force, the output force is exerted by the high pressure pump, the readings of the standard dynamometer and the pressure gauge are read, and the corresponding tables of the relationship between the two are established, as figure 2. comparing the two methods, the first method needs the purchase of special working force measuring machines (testing machines, force standard machines) which meet the requirements. the accuracy depends on the accuracy of testing machines and force standard machines and the measuring range needs to reach 3 mn. the second method requires only a standard dynamometer and related reaction equipment to meet the requirements, the investment is small, and the metrological and verification departments have it, it is easy to use the second method to develop the bolt hydraulic stretcher. figure 2: schematic diagram of standard dynamometer calibration stretcher 3. points to note in the tensioner calibration process 3.1. pressure gauge selection pressure gauges are common measuring instruments. the selection of pressure gauges should be based on the operational requirements and on the premise of meeting the technological requirements. comprehensive consideration should be given in the principle of economy and practicality, to achieve a reasonable choice of accuracy, range, type. accuracy grade: the theoretical error of the tensioner is not more than 3 %, so in theory the pressure gauge with accuracy grade not less than 1.6 can be selected, it is recommended to use a precision pressure gauge of not less than 0.4 grades. the reasons for the analysis are as follows. the data obtained by using the two pressure gauges that have been certified are shown in tables 1 and 2. table 1: calibration results using class 1.6 pressure gauges indicator reading / mpa force / kn interpolation value / mpa interpolation error / % 10 47.1 10.43 4.4 20 90.9 19.18 -4.1 40 197.2 40.54 1.4 60 292.6 59.85 -0.3 80 392.6 80.24 0.4 100 486.6 99.55 -0.5 120 586.4 120.19 0.2 http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 158 table 2: calibration results using class 0.4 pressure gauges indicator reading / mpa force / kn interpolation value / mpa interpolation error / % 10 48.1 9.91 -0.9 20 97.9 19.99 -0.1 40 198.1 40.28 0.7 60 293.7 59.77 -0.4 80 393.8 80.28 0.4 100 487.7 99.59 -0.5 120 587.3 120.18 0.2 as can be seen from tables 1 and 2, the interpolation error of 10 mpa in table 1 is 4.4 %, and that of 40 mpa is 1.4 %. the interpolation error of 20 mpa has a sudden change, while the data in table 2 are all less than 1 %. the same inspection qualified two pressure gauges, but the accuracy level is different, 1.6 pressure gauges in less than 1/3 of the range. the occurred beating data are not accurate and cannot be used. in order to ensure the accuracy of the measurement, the minimum working pressure should not be less than 1/3 of the measuring range and a high accuracy pressure gauge should be used. therefore, it is recommended to use a precision pressure gauge of 0.4 grade. also of special concern is that the equipment manufacturers do not generally sell the tensioner pressure gauge, tensioner use reference to the theoretical value provided by the manufacturer table or after calibration certificate metrology verification department. through many experiments, it is found that the final value of load is not the same even if different gauges with the same accuracy grade are used to calibrate the same tensioner. therefore, the authors recommend that the manufacturer of the tensioner should equip each tensioner with one or even two pressure gauges. one should be used for work, and when one of the pressure gauges is damaged, the other should be used as a backup pressure gauge, both pressure gauges are calibrated during the tensioner calibration to improve accuracy, safety, and sudden failure of the tensioner. 3.2. standard dynamometer selection standard dynamometers below 1 mn are recommended to be used at accuracy grade 0.1 or above, and those above 1 mn are recommended to be used at level 0.3 or above. nowadays, standard dynamometers are developing rapidly; most of the standard dynamometers can reach the accuracy grade recommended by the authors, which is helpful to improve the accuracy of the tensioner calibration. 3.3. high-pressure pump selection recommend the use of pneumatic hydraulic pump, pneumatic hydraulic pump with a safe and reliable, high output pressure, easy to operate, continuous start-stop, unrestricted and other advantages, especially suitable for the tensioner calibration. 4. data processing on data processing, it is recommended not only to give the indicator process value corresponding to the load value, it is also best to give the two corresponding quadratic curve fitting equations, the advantage of the fitting equation is that the corresponding output value of any input point in the measuring range can be obtained. it is convenient for users to face different types of bolts or to develop new products; the calibration certificate issued by liaoning institute will be accompanied by the corresponding calibration equations and charts of the instrument and equipment. the fitting equation and chart are shown in figure 3. figure 3: fitting equation of tensioner 5. summary through the introduction of bolt hydraulic stretcher and its application field, this paper puts forward two methods to calibrate the stretcher, analyzes the problems needing attention in the process of calibration, and gives a detailed discussion and recommendation, it can be used for reference in the calibration of bolt hydraulic tensioner. 6. references [1] jjg 621-2012, verification regulation of hydraulic jacks, aqsiq, china, 2012. y = 5e-06x2 + 0.2013x + 0.2238 0 20 40 60 80 100 120 140 0 200 400 600 pressure / mpa force value / kn http://www.imeko.org/ traceable calibration of pressure based water level data loggers acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 349 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 349 352 traceable calibration of pressure based water level data loggers l. grgec bermanec1,2, i. matas1,3, d. zvizdić1,4 1 university of zagreb, faculty of mechanical engineering and naval architecture, laboratory for process measurement, zagreb, croatia, 2 lovorka.grgec@fsb.hr, 3 ivan.matas@fsb.hr, 4 davor.zvizdic@fsb.hr abstract: this paper investigates accuracy, influence of temperature variations and time drift of water level data loggers, operating on a principle of hydrostatic pressure measurement. the laboratory setup for calibration of such devices, equipped with selfdesigned pressure chamber and commercial climatic chamber is presented. calibration procedure for logger’s built-in pressure transducer, with ensured traceability to the primary standards of pressure and temperature, is proposed. influence of temperature on the pressure reading was examined by conducting calibration at 10 °c, 15 °c and 25 °c. keywords: water level data logger; pressure; calibration setup; traceability 1. introduction demand for monitoring of groundwater and surface water levels is growing in oceanology, hydrology, water resource management and many other fields. water level data loggers are mostly used for investigation of water resources such as lakes, tidal areas or wells. scarcity of drinking water emphasizes the need for accurate groundwater level reading. also, ongoing climate changes cause water level increase, due to melting of ice caps, which confirms the importance of precise water level monitoring. in order to provide accurate water level reading, hydrostatic pressure has to be measured correctly and ambient temperature has to be monitored. built-in absolute pressure sensors should be calibrated against traceable pressure standards. however, industry-wide standardized method for that is lacking[1]. this paper provides the calibration setup for pressure transducer, incorporated into the water level logger that cannot be directly connected to pressure system and examines the dependency of the pressure reading on the temperature of the surrounding medium. 2. methods many commercially available water level data loggers that operate on principle of hydrostatic pressure measurement, have built-in piezoresistive pressure transducer as a measuring unit. such sensors, exhibit high sensitivity but have significant temperature dependency of pressure reading[2]. different techniques for temperature compensation of piezoresistive sensor can be found in the literature[3],[4]. this influence of temperature should also be taken into account when evaluating measurement uncertainty of calibration. therefore, the calibration setup was placed inside the climatic chamber, allowing the examination of temperature dependency of the calibrated logger to be done as well. in this study, a commercial water level data logger with operating range between 0 and 9 meters of water depth, was used. pressure measuring range of its built-in piezoresistive sensor is between 0 and 2000 mbar absolute. as it is mostly used for measuring water depth at sea level, the calibration range was decided to be from 1000 to 2000 mbar absolute. the calibration was performed at 10, 15 and 25 °c, since the logger is mostly used at that temperature range. 2.1. calibration setup since the logger cannot be directly connected to pressure system, it was necessary to design and produce a pressure chamber (figure 1) that can withstand a pressure up to 2500 mbar. figure 1: pressure chamber http://www.imeko.org/ mailto:lovorka.grgec@fsb.hr mailto:ivan.matas@fsb.hr mailto:davor.zvizdic@fsb.hr acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 350 other construction demands were that it could accommodate up to three loggers (l=150mm, ø 25 mm) at the same time and fit into the climatic chamber (figure 2) so that the calibration could be performed under different temperatures. also, the volume of the chamber, should be as small as possible to minimize pressure fluctuations and time for stabilization. the trade-off had to be made between number of loggers, that can be calibrated at once, and time for stabilization of conditions (pressure and temperature) inside the chamber. figure 2: pressure chamber inside the climatic chamber pressure chamber was constructed from zinc coated pipe and two threaded cast iron caps, one having a standardised pressure inlet, for the connection to the pressure system. threads were properly sealed, and a leak test was conducted, prior to calibration. as no leakage occurred at maximum pressure, chamber showed appropriateness to be used as mean for calibration of water level data logger. the calibration setup (figure 3 and figure 3) consisted of pressure chamber, climatic chamber, pressure standard, in form of pressure transducer (with operating range of 0 mbar to 1000 mbar gauge) connected to pressure calibrator (indication), barometric pressure logger and standard platinum resistance thermometer (pt-100). all of the equipment was calibrated prior to setup, with ensured traceability as laboratory for process measurement is national standard laboratory for pressure, temperature and humidity. figure 3: calibration setup – scheme 2.2. calibration procedure the calibration procedure was built upon standardised procedures and included 11 pressure points (0, 100, 200, 300, 400, 500, 600, 700, 800, 900, 1000 mbar gauge) in three sequences (two increasing and one decreasing)[5]. barometric pressure was measured with reference barometer in every calibration point. temperature was monitored with standard platinum resistance thermometer (pt-100) and logger’s built-in temperature sensor (thermistor). to ensure traceability, pressure transducer was first calibrated against gas pressure balance, characterized in [6] and then connected to pressure system. after the ambient temperature in the climatic chamber has stabilized, for every measurement point, the procedure was following. pressure was generated in the system using a hand pump. after achieving and stabilization of the desirable pressure point on the standard, water level logger recorded three pressure readings and the barometric pressure was manually logged. after completing all three calibrations, readout of the logger was performed with its software. water level data is calculated from the pressure readings when the input of fluid density and barometric reference is given. 3. results and mesurement uncertainty evaluation after analysis of raw data and determination of measurement uncertainty, the results showed that logger did exhibit certain change in pressure reading due to change of temperature. at 15 °c deviation from the pressure standard was the lowest of all temperatures, with the maximum absolute value of 0.9 mbar. a slight increase of deviation occurred at 25 °c, as seen from the figure 4. maximum deviation from the standard at 25 °c was 1.2 mbar. in both cases deviation stayed within raw pressure measurement accuracy stated in the logger’s calibration certificate (0.3 % fs or 6.3 mbar). figure 4: deviation from the pressure standard at 15 °c and 25 °c at 10 °c logger exceeded accuracy limits (dashed lines) as can be seen from figure 5. the measurement uncertainty was also significantly greater than at 15 °c and 25 °c, due to contribution of repeatability and hysteresis. deviations of the logger’s reading http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 351 with associated measurement uncertainties are shown in table 2. figure 5: deviation from the pressure standard at 10 °c logger’s built-in thermistor indicated deviation from the platinum resistance thermometer. greatest deviation, close to 0.4 °c occurred at lowest temperature, and decreased towards the highest temperature point, as can be seen from table 1. main contribution to the temperature measurement uncertainty is from the temperature gradients in the climatic chamber. table 1: comparison of temperature readings temperature standard (pt-100), °c temperature measured by logger, °c measurement uncertainty k = 2, °c 10.31 10.7 2.00 15.15 15.3 2.00 25.15 25.1 2.00 4. discussion the results at 15 °c and 25 °c indicated the deviation of logger’s pressure reading with temperature, although its calibration certificate states complete temperature compensation in the range from 0 °c to 40 °c. this confirmed the need for examination of temperature dependency of piezoresistive transducers prior to calibration. at 10 °c, logger’s reading did not show congruency with results at 15 °c and 25 °c. this can be attributed to sudden temperature change from room temperature with not enough time for stabilization. according to manufacturer, sudden temperature changes should be avoided. also, lower operating limit of climatic chamber is 10 °c, so the stabilization of temperature in the climatic, as well as in pressure chamber is much longer at that temperature. thus, large discrepancy between results at 10 °c and at other two temperatures, can be attributed to unsteady thermal conditions during measurements. this emphasized the importance thermal stability before calibration of such devices, for conducting low uncertainty measurements. these results were not considered for further uncertainty evaluation. from the measurements at 15 °c and 25 °c, contribution of temperature dependency of pressure reading to measurement uncertainty can be calculated. largest difference between deviation at 15 °c and at 25 °c was taken as the maximum value of error due to influence of temperature on the pressure sensor. example of measurement uncertainty budget, built upon[5], for further calibrations of such loggers using the presented laboratory setup is shown in table 3. it is visible that, after the uncertainty of pressure standard, temperature influence on the pressure reading makes second largest contribution to the overall calibration uncertainty. the proposal for examination of temperature influence on pressure reading would be to check the deviation in just few calibration points (eq. lowest, middle and highest pressure), at lowest and highest operating temperature of logger. from that data, influence of the temperature on the given logger can be estimated, without the conduction of the complete calibration procedure at two temperatures. table 2: deviation of the logger from pressure standard applied pressure, mbar at 10 °c at 15 °c at 25 °c deviation, mbar u (k = 2), mbar deviation, mbar u (k = 2), mbar deviation, mbar u (k = 2), mbar 1000.0 -8.7 8.92 -0.2 0.61 0.6 0.51 1100.0 -9.1 9.91 -0.3 0.51 0.5 0.52 1200.0 -8.5 11.02 -0.2 0.55 0.6 0.51 1300.0 -7.4 13.67 0.1 0.52 0.6 0.53 1400.0 -5.3 17.44 0.2 0.53 0.9 0.52 1500.0 -3.8 19.35 0.5 0.56 1.2 0.53 1600.0 -2.8 18.37 0.4 0.52 1.2 0.52 1700.0 -2.4 17.22 0.5 0.51 1.2 0.52 1800.0 -2.1 15.12 0.2 0.59 1.0 0.52 1900.0 -2.4 13.80 -0.2 0.52 0.4 0.53 2000.0 -0.8 13.57 -0.9 0.53 -0.4 0.57 http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 352 table 3: uncertainty budget for calibration of water level data logger at pressure of 1200 mbar absolute influence quantity estimate, mbar variability interval (2 a), mbar probability distribution divisor sensitivity coefficient contribution to the standard uncertainty, mbar2 pressure standard 200.0 0.500 normal 2 1 0.06250 barometric reference 1013.1 0.086 normal 2 1 0.00185 resolution 0 0.1 rectangular √3 1 0.00083 hysteresis 0 0.3 rectangular √3 1 0.00926 repeatability 0 0.2 rectangular √3 1 0.00232 temperature 0 0.8 rectangular √3 1 0.05333 u 0.36068 u(k = 2) 0.72137 5. conclusion calibration setup and procedure for calibration of water level data loggers with built-in hydrostatic pressure sensor was presented. calibration procedure is based on standardised calibration methods for pressure gauges. the laboratory calibration setup is additionally equipped with self-designed pressure chamber and therefore suitable for devices that cannot be directly connected to pressure system. by placing the whole setup inside the climatic chamber, the presented setup can be used for evaluation of temperature effect on pressure reading. traceability of calibration is ensured to the primary standards of pressure and temperature, as the laboratory for process measurement is holder of national standards of respected quantities. the results of calibration at different temperatures showed increase in deviation from the standard due to increase of temperature for the tested logger, which confirmed the need for examination of temperature influence. also, sudden temperature changes can lead to instability of pressure reading, so stable thermal conditions should be achieved prior to calibration. uncertainty budget for future calibration of water level data loggers, that includes temperature influence on pressure reading, is proposed. the presented setup and procedure have shown applicability as standardised method for calibration of water level data loggers. 6. acknowledgement the authors would like to thank company d.i.i.v. ltd. and its chairman donat petricioli for provision of the water level data loggers for the purpose of this research. 7. references [1] j. p. r. sorensen and a. s. butcher, “water level monitoring pressure transducersua need for industry-wide standards,” ground water monitoring and remediation, vol. 31, no. 4, pp. 56– 62, 2011, doi: 10.1111/j.1745-6592.2011.01346.x. [2] s. s. kumar and b. d. pant, “design principles and considerations for the ‘ideal’ silicon piezoresistive pressure sensor: a focused review,” microsystem technologies, 2014, doi: 10.1007/s00542-014-22157. [3] f. reverter, g. horak, v. bilas, and m. gasulla, “novel and low-cost temperature compensation technique for piezoresistive pressure sensors,” in 19th imeko world congress 2009, 2009. [4] m. aryafar, m. hamedi, and m. m. ganjeh, “a novel temperature compensated piezoresistive pressure sensor,” measurement: journal of the international measurement confederation, 2015, doi: 10.1016/j.measurement.2014.11.032. [5] n. m. aykurt altintas, wladimir sabuga, oliver ott, yasin durgut, pierre otal, janez šetina, “guidelines on the calibration of electromechanical and mechanical manometers guidelines on the calibration of electromechanical and mechanical manometers,” vol. 0, no. 17. euramet e.v., 2019. [6] l. grgec bermanec, m. katic, and d. zvizdic, “characterization of gas pressure balances at lpm,” measurement: journal of the international measurement confederation, 2019, doi: 10.1016/j.measurement.2018.10.070. http://www.imeko.org/ towards an improved helium-based refratometer for pressure measurements acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 305 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 305 309 towards an improved helium-based refratometer for pressure measurements z. silvestri1, d. bentouati2, p. otal3 and j-p. wallerand4 1 lcm lne/cnam, 1 rue gaston boissier, paris, france zaccaria.silvestri@cnam.fr 2 lcm lne/cnam, 1 rue gaston boissier, paris, france djilali.bentouati@lne.fr 3 lcm lne/cnam, 1 rue gaston boissier, paris, france pierre.otal@lne.fr 4 lcm lne/cnam, 1 rue gaston boissier, paris, france jean-pierre.wallerand@cnam.fr abstract: this paper describes a quantum realization of the pascal based a helium absolute refractometer at 532 nm. the short-term stability in pressure is ±2 mpa with a resolution in pressure of this new optical pressure standard is better than 1 mpa with temperature stability about 1 mk. a new design of the refractometer is presented to have better temperature stability and accurate temperature measurements to reach uncertainties in pressure better than current conventional methods. keywords: refractometry, fabry-perot, pressure measurement, quantum pascal 1. introduction for many decades and without any significant expected improvement, primary absolute pressure measurements in the range 1 pa to 100 kpa are mainly based on liquid column manometers, gas-operated pressure balance or static expansions systems. the relative uncertainty in cmcs is about 10-6 at 100 kpa and about 10-4 at 1 pa. at lcm, over the past few decades, the best realizations of the pascal, see figure 1, have been obtained using two different standards. first one in the range from 10 kpa to 500 kpa, an automatic absolute pressure balance apx50 equipped with 50 mm diameter piston-cylinder units is used. for the realization and the dissemination of the pressure scale, in the range 0.5 pa to 10 kpa is achieved with a force-balanced piston gauge (fpg). for example, the extended relative uncertainty is 9 × 10-6 at 100 kpa and 6 × 10-4 at 1 kpa. below 500 pa, the relative uncertainty increases sharply. there is a special need to reduce relative uncertainties below 100 kpa with, if possible, a single instrument in a wide range of pressures below atmospheric pressure. the consultative committee for masses and associated quantities (ccm) and the euramet technical committee for masses and associated quantities (tc-m) have also identified the needs and limitations of conventional methods. with the new international system of units (si), the scientific community is exploring new avenues directly linked to fundamental laws and or constants of physics. thus, for pascal, a standard could be based on optical or electrical measurements such as the refractive index or the permittivity of a gas, certain parameters of which can be calculated ab initio (with helium for example). in this context, it is necessary to continue the scientific and technological development of these methods to extend the measurement range and significantly the uncertainties of conventional methods. figure 1: current pressure relative uncertainties at lcm. in the range 10 kpa to 500 kpa, an automatic absolute pressure balance apx50 and in the range 0.5 pa to 10 kpa a force-balanced piston gauge (fpg). with the new international system of units (si), many developments are now in progress for the realization of a “quantum pascal” by means of methods such as superconductive microwave resonators [1], rayleigh scattering, multi-reflection interferometry [2], gas phase thermometry and absorption spectroscopy methods [3] as well as fabry-pérot (fp) cavity refractometers [4]. in the field of optical methods based on fabrypérot, the national institute of standards and technology (nist) develop since 2004 an absolute refractometer based on a double fabry-pérot cavity in ule using he-ne lasers. nist claims an extended http://www.imeko.org/ mailto:zaccaria.silvestri@cnam.fr mailto:djilali.bentouati@lne.fr mailto:pierre.otal@lne.fr mailto:jean-pierre.wallerand@cnam.fr acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 306 relative uncertainty ur(p) = 18 × 10 -6 at 100 kpa and 4 × 10-3 at 1 pa [5], [6]. rise (rise research institutes of sweden) and umeå university in sweden also develop from 2000’s, this type of refractometer (double fp cavity in zerodur®) using an original method of pressure modulation (gamor) to limit temperature effects on the cavity [7], [8]. nim (national institute of metrology of china) in china, nmij (national metrology institute of japan) in japan [9] and ptb (physikalisch-technische bundesanstalt) in germany [10] start the development of such instruments. french metrology is currently developing a quantum realization of the pascal based on a helium absolute refractometer at 532 nm [11]: the optical path travelled by a laser beam in a single fabry-perot cavity is measured either with helium gas or under vacuum. the ratio between these two optical paths, with appropriate corrections, gives the refractivity of the helium. by comparison with an ab-initio determination of helium refractive index and an accurate measurement of the gas temperature, we determine the absolute gas pressure inside the refractometer using lorentz-lorenz equation. other gases can be used, after determining their atomic parameters by comparison with helium. this paper describes shortly the current experimental set-up and the latest results on the performance of the refractometer for pressure measurements in the range 100 pa to 100 kpa. we also present an improved version to insure the best stability in temperature, a limited temperature gradient and accurate temperature measurements with an uncertainty lower than 1 mk. 2. experimental set-up 2.1. general view the experimental set-up was formerly used as an absolute refractometer of air and helium [12] for the determination of refractive index with a standard uncertainty of 10-8. it is mainly composed of a large vacuum chamber that hosts the fabry-perot cavity and two frequency doubled nd:yag lasers at 532 nm: one is servo-controlled on transition iodine and second one is servo-controlled on the fp cavity. a frequency beat is performed between the two lasers to measure the frequency of second laser. the fabry-perot cavity in figure 2 is a planoconcave optical resonator which spacer is made of zerodur® of 100 mm-long and 25 mm-diameter. it is placed inside a copper and dural cylinders with a temperature regulated system using thermofoils. the assembly is then placed in a 10l vacuum enclosure. gas temperature is measured with 4 wires 100  rtd sensors along the fpm cavity. figure 2: cross-section of the vacuum chamber with a fabry-perot cavity. fabry-perot resonator composed of two silica mirrors and a spacer made in zerodur®. 2.2. methods to pressure determination pressure 𝑃 can be determined using two methods (figure 3). in a first approach, refractometer can be used in absolute way: starting from vacuum as reference and measuring frequency variation 𝜈 of the servo-controlled laser on the cavity between vacuum and pressure to measure. frequency variation allows to determine refractive index 𝑛 variation of the gas and therefore pressure variation if the thermodynamic temperature 𝑇 is accurately measured. figure 3: schematic of the two methods (absolute and relative) to determine a pressure using a refractometer. 𝑃 is a pressure, 𝑛 is the refractive index et 𝜈 the frequency. 𝑟𝑒𝑓 index indicates the reference state of the parameters and 𝐻𝑒 index indicates the measurement state. second way consist to use the refractometer in relative mode. in this case, reference state is a gas near the atmospheric pressure. this pressure is measured with a conventional method (uncertainty better than 10-5). then, frequency variation of the laser is measured between this reference pressure and pressures up to 100 pa. it is not necessary to measure thermodynamic temperature of the gas but just only the temperature difference. in this case, the traceability of the pressure is given by the pressure balance for the absolute value and by atomic gas parameters for pressure variation. 3. main results to achieve the final objectives in terms of pressure uncertainties (5×10-4 at 1 pa, 10-5 à 1 kpa), ageing and mechanical deformation of the fp cavity http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 307 are two parameters which limit the uncertainty in relative mode [11]. in the case of absolute use, the thermodynamic temperature of the gas is also a limiting factor. we need to measure the temperature helium gas with an uncertainty lower than 1 mk and to maintain inside the refractometer a pressure with a stability lower than 0,01 pa over several tens of seconds. 3.1. ageing, xxx and uncertainties the uncertainty of the ageing of the cavity is 1 × 10−11/day . this value leads to a pressure variation of 3 mpa for n2 and 20 mpa for he. it is necessary to improve the uncertainty on this parameter by a factor greater than three for pressure below 100 pa by using a pressure balance. for the thermal expansion coefficient, a relative uncertainty about 4% is enough if the temperature of the cavity can be reproduced at 10 mk. these two conditions are already met today with a relative uncertainty of 0,15 mpa for n2 and 1,2 mpa for he. the uncertainty of 5×10-15 obtained for the pressure-induced deformation for the absolute mode (vacuum as a reference) do not ensure the required uncertainty. in the case of the relative mode measurements, the uncertainty is sufficient. however, recent changes to the cavity mirrors have significantly improved the uncertainty on the measurement of the free spatial range (about 200 hz in vacuum) and should improve this uncertainty component. 3.2. temperature stability and measurement of the temperature to stabilize the temperature in the fp cavity, we therefore set up a temperature regulation system at ±1 mk (completely autonomous system based on arduino). for that, to the dural cylinder, we are glued two thermofoils in the opposite side. the copper cylinder allows to create a heat shield. finally, the entire enclosure was confined in an expanded polystyrene box. the temperature is measured inside the cavity by 4 pt100 rtd probes (4-wire measurement) distributed over the entire cavity. the temperature stability inside the cavity is about 2 mk but the temperature gradient along the 100 mm of the cavity is around 5 mk. long-term stability is also not guaranteed due to the excessive volume of the vacuum enclosure. with the pt100 probes, it is not possible to measure thermodynamic temperature better than 10 mk. for an absolute pressure measurement with a relative uncertainty of 10-5 an uncertainty of less than 3 mk is required (whatever the pressure to be measured). it is therefore necessary to improve the uncertainty on the value of the gas temperature by temperature probes and its location. if the refractometer is used in relative mode, the only constraint is that the temperature between the reference state and the measurement state is the same to within 3 mk. 3.3. pressure stability in previous works [11], we were limited by the stability of the pressure due to the use of the commercial pressure regulator. the pressure resolution was then about 0.2 pa at the atmospheric pressure. to reduce this pressure stability, we have installed and tested a dhi pg7607 pressure balance associated with a 20 cm² piston-cylinder whose pressure expanded uncertainty is 0.20 pa + 8.0 × 10−6𝑝 this has led to reach the target pressure stability in figure 4 and then allows us to determine the pressure by means of the refractometer with a stability of ±2 mpa over 20 min. figure 4: pressure variation measured by the refractometer over 1200 s. we can note that this excellent result is only possible in the short term because, it is not possible to guarantee stability in pressure and temperature more than 20 min. 4. improved design of the refractometer to resolve the main issues of the current refractometer, we have designed a new set-up still using a helium absolute refractometer at 532 nm. this design has the advantage to be more compact (20 times smaller in volume) to avoid temperature gradient and temperature drift. figure 5 presents a project design of the improved set-up: a 50 mmsquared single fabry-perot resonator (two silica mirrors and a spacer made in zerodur®) and an enclosure in copper working under vacuum and gas. we also will use encapsulated high quality pt25 probe used at gallium point to have an uncertainty lower than 1 mk. in the longer term, a fp cavity made of a more stable material than zerodur® can be used. several publications report a ceramic, nexceratm (krosaki harima), which has an aging almost 5 times lower than zerodur® [13]. http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 308 (a) (b) figure 5: project design of our new quantum pascal realization set-up. (a) 50 mm single fabry-perot (fp) resonator composed of two silica mirror and a spacer made in zerodur®. (b). cross section of the copper enclosure including the fp resonator. simulations of the pressure-induced deformation have been performed on with new design of the fabry-pérot cavity using ansys workbench rxx academic version. we use the same methodology presented in [14]. the pressure-normalized deformation value is δ𝐿 𝐿 𝑃 = −6.5810 × 10−12 pa−1. the simulated value for the current fp cavity presented in this paper is δ𝐿 𝐿 𝑃 = −6.3899 × 10−12 pa−1. 5. summary this paper describes a quantum realization of the pascal based a helium absolute refractometer at 532 nm. the short-term stability in pressure is ±2 mpa with a resolution in pressure of this new optical pressure standard is better than 1 mpa with temperature stability about 1 mk. pressure can be determined using an absolute method or a relative method. aging and pressure-induced deformation of the fp are the main contributions of the uncertainty in the relative mode. in the case of absolute mode, the thermodynamic temperature of the gas is also a limiting factor. for that, a new design of the refractometer is presented to have better temperature stability and accurate temperature measurements to reach uncertainties in pressure better than current conventional methods. improvements (performance, uncertainties, etc.) are mainly linked to the objectives of the "quantumpascal" empir project. 6. acknowledgements this work is a part of quantumpascal project (18sib04) funded through the european metrology programme for innovation and research (empir). 7. references [1] p. gambette, r. m. gavioso, d. m. ripa, m. d. plimmer, et l. pitre, « towards a quantum standard for absolute pressure measurements in the range 200 pa to 20 kpa based on a superconducting microwave cavity », in 2018 conference on precision electromagnetic measurements (cpem 2018), paris, 2018, , doi: 10.1109/cpem.2018.8500912. [2] d. mari, m. pisani, et m. zucco, « towards the realization of an optical pressure standard », measurement, vol. 132, p. 402‑407, 2019, doi: 10.1016/j.measurement.2018.09.069. [3] k. o. douglass et d. a. olson, « towards a standard for the dynamic measurement of pressure based on laser absorption spectroscopy », metrologia, vol. 53, no 3, p. s96‑s106, 2016, doi:10.1088/00261394/53/3/s96. [4] k. jousten et al., « perspectives for a new realization of the pascal by optical methods », metrologia, vol. 54, no 6, p. s146‑s161, 2017, doi:10.1088/16817575/aa8a4d. [5] p. f. egan, j. a. stone, j. k. scherschligt, et a. h. harvey, « measured relationship between thermodynamic pressure and refractivity for six candidate gases in laser barometry », journal of vacuum science & technology a, vol. 37, no 3, p. 031603, 2019, doi:10.1116/1.5092185. [6] p. f. egan, j. a. stone, j. e. ricker, et j. h. hendricks, « comparison measurements of lowpressure between a laser refractometer and ultrasonic manometer », review of scientific instruments, vol. 87, no 5, p. 053113, 2016, doi: 10.1063/1.4949504. [7] o. axner, i. silander, c. forssén, y. guo, et m. zelan, « gas modulation refractometry (gamor) – on its ability to eliminate the influence of drifts », p. 15. [8] i. silander, t. hausmaninger, c. forssén, m. zelan, et o. axner, « gas equilibration gas modulation refractometry for assessment of pressure with subppm precision », journal of vacuum science & technology b, vol. 37, no 4, p. 042901, 2019, doi: 10.1116/1.5090860. [9] y. takei, k. arai, h. yoshida, y. bitou, s. telada, et t. kobata, « development of an optical pressure measurement system using an external cavity diode laser with a wide tunable frequency range », measurement, vol. 151, p. 107090, 2020, doi: 10.1016/j.measurement.2019.107090. [10] y. yang et t. rubin, « simulation of pressure induced length change of an optical cavity used for optical pressure standard », journal of physics: http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 309 conference series, vol. 1065, p. 162003, 2018, doi: 10.1088/1742-6596/1065/16/162003. [11] z. silvestri, f. boineau, p. otal, et j.-p. wallerand, « helium-based refractometry for pressure measurements in the range 1–100 kpa », in 2018 conference on precision electromagnetic measurements (cpem 2018), 2018, p. 1‑2, doi: 10.1109/cpem.2018.8501259. [12] t. badr, s. azouigui, j. p. wallerand, et p. juncar, « absolute refractometry using helium », in conference on precision electromagnetic measurements (cpem), 2010, p. 506–507, 2017. [13] c. j. kwong, m. g. hansen, j. sugawara, et s. schiller, « characterization of the long-term dimensional stability of a nexcera block using the optical resonator technique », measurement science and technology, vol. 29, no 7, p. 075011, 2018, doi: 10.1088/1361-6501/aac3b0. [14] j. zakrisson et al., « siimulation of pressure-induced cavity deformation the 18sib04 quantumpascal project », présenté à imeko 24th tc3, 14th tc5, 6th tc16 and 5th tc22 international conference, cavtatdubrovnik, croatia, nov. 2020. http://www.imeko.org/ metrological characterisation of current transformers calibration unit for accurate measurement acta imeko issn: 2221-870x june 2021, volume 10, number 2, 6 13 acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 6 metrological characterisation of current transformers calibration unit for accurate measurement valentyn isaiev1, oleh velychko1 1 se "ukrmetrteststandard", 4 metrologichna, 03143 kyiv, ukraine section: research paper keywords: ratio error; phase displacement; measurement uncertainty; calibration; current transformer citation: valentyn isaiev, oleh velychko, metrological characterisation of current transformers calibration unit for accurate measurement, acta imeko, vol. 10, no. 2, article 3, june 2021, identifier: imeko-acta-10 (2021)-02-03 section editor: ciro spataro, university of palermo, italy received december 1, 2020; in final form april 14, 2021; published june 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: valentyn isaiev, e-mail: black2001w@gmail.com 1. introduction the energy sector deals with electricity accounting for accurate settlements and monitoring the dynamic events for power quality assurance. the measuring instruments for phasor measurement [1], current conversion [2], [3] including current transformers (ct), are used all over the world. the accuracy of measuring instruments is steadily increasing and researchers continue to find the approaches for minimising the errors of equipment, in particular those of instrument transformers [4]. cts have often an accuracy class of 0.2s or 0.5s according to the iec standard and contribute to the accuracy of electricity accounting [5]. almost all countries have calibration laboratories that perform ct calibration, but not every laboratory is capable of calibrating a precision transformer with a measurement uncertainty of 50 μa/a, and less. for example, europe's leading ptb (germany) laboratory declared measurement uncertainty in on-site calibrating the isolating cts slightly less than 40 µa/a [6]. a sampling current ratio bridge for the calibration of cts was developed at vsl (netherlands) laboratory. the expanded uncertainty, declared by the developer, does not exceed 5 µa/a in magnitude and 5 µrad in phase [7]. the researchers have also offered alternative approaches to determining the metrological characteristics of instrument transformers, such as the application of a quasi-balance technique using the virtual instrument [8], or low cost efficient digitiser [9], or a method based on the low-voltage reciprocity principle [10], or a calculation of the errors based on the excitation table [11]. 2. overview and purpose a traditional technique of comparison with the reference ct using means of comparing remains predominant in the practice of most laboratories. the means of comparing the currents of both a working standard and a device under test (dut) is an important component of the ct calibration system. the contributions of such devices used by national metrology institutes can be observed in the uncertainty budgets presented in the final supplementary comparison reports. in particular, the measurement uncertainties of means of comparing associated with ptb and fgup "uniim" (russia) are approximately 0.5 abstract the manuscript presents a method for the metrological characterisation of the commercial ac comparators used to calibrate current transformers. the theoretical basis for simulating the difference between two almost identical currents has been outlined, as well as the mathematical models for both a ratio error and a phase displacement has been derived. the measurement setup, consisting of conventional measuring instruments, has been described with a detailed presentation of its parameters. the sources of uncertainty have been distinguished and analysed with determining the current phase shift which led to a significant increase of relative measurement uncertainty. the simulation of measurement results was yielded in two ways: physically using a method presented and virtually using a monte carlo method. the second method confirmed that evaluating the measurement uncertainty through derived sensitivity coefficients is correct enough. the simulation results in the range from 1 to 1200 parts per million for both ratio error and phase displacement motivated the use of a comparator characterised through the proposed method for accurate measurement, especially for very low errors. mailto:black2001w@gmail.com acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 7 part per million (ppm) [12]. the lowest contribution of the ptb’s bridge has a level of several hundredths of µa/a [13]. however, such a level of measurement uncertainty is reached only by some national metrology institutes, providing special conditions for calibration. the measurement uncertainty associated with the calibration of the current transformer bridge of gum (poland) is 5 µa/a in ratio error (re) and 14.5 µrad in phase displacement (pd) [14]. se "ukrmetrteststandard", as a conformity assessment body, deals with both calibration and certification of measuring instruments, in particular cts. based on the available information, it can be argued that in today’s calibration activities, ct manufacturers, testing labs, energy companies, and other related enterprises remain using commercially available means of comparing. when calibrating cts with an accuracy class of 0.2s, the contribution to the total measurement uncertainty, obtained via the admissible tolerance, from the use of the measuring bridge can be more than 90 % of the contribution of all the influencing variables when determining the re value of 480 µa/a [15]. the design features and factors that affect the accuracy of one of the commercial automatic test devices are described in a developer article. in particular, it is stated that the accuracy of such a measuring instrument should not exceed 1 percent of the measurement result [16]. according to the user's guide of the ca507 comparator produced in ukraine, the minimum intrinsic uncertainty is 2 µa/a when measuring re. the analogous figure for pd measurement is 8.73 µrad. the system for calibrating an automatic transformer test set using a current source with an operational amplifier was developed at the cmi (czech republic) [17]. the method, developed in tubitak ume (turkey), intended for determining the errors of the ct test sets allows characterising these tools with a total uncertainty of 10 ppm [18]. the creation of a convenient method for accurate metrological characterisation of comparison means for two almost identical alternating currents is the main task of the research work. evaluating the uncertainty of measurements during its calibration using a derived mathematical model is also an objective of the paper. to characterise the contribution of the work and to determine the place in the body of the instrumentation and measurement it should be noted that the mathematical models for determining the reference values of re and pd allow checking the accuracy of ct calibration unit (i.e., comparator of two approximately equal currents) in an alternative way using the suitable measurement setup. according to the obtained models, it is easy to calculate the contribution of each input quantity and to evaluate an expanded uncertainty both in determining re and in determining pd. 3. theoretical basis 3.1. definition of current transformer errors according to world practice, it is customary to express the imperfection of a scale conversion of the ct using two characteristics: the current re and the pd [19]. the current re (εi) following the iec standard is the ratio of the product of the і2 secondary current amplitude and the кт transformation ratio of ct to the amplitude of the і1 primary current, i.e.: ( )2 1 1ε =100i tk i i i  − . (1) when the current flows successively through the primary windings of the dut and reference transformer in the primary measuring circuit during calibration, one can consider the equation ( ) 1100=x s t x sk i i iε ε−   − , (2) where ix and is are the secondary currents of dut and working standard, respectively. since the error is usually obtained as a subtraction of the reference reading from the dut reading and divided by the reference reading, the primary current should be expressed through the secondary current of a reference transformer. in this case, it is possible to express the re of the dut through the ratio of secondary currents. to do this, one should obtain the expression of primary current і1 from equation (1) for the working standard: ( )1 1 100t s si k i ε=  + . (3) assuming the current re of the reference transformer is zero (the case of an ideal transformer), one can transform the expression (2) to ( )100 1=x x si iε  − . (4) for further analysis, it is more convenient to express the re in relative units (µa/a); therefore, factor 100 will be removed from the expression (4). the pd (δφ) between the φ2 initial phase of the sinusoidal secondary current and the φ1 initial phase of the primary current is the second component of the current scaling characteristic: 12φ φ φ = − . (5) when the same current flows in the primary circuit of both the dut and the working standard connected in series, the initial phase is the same for both devices, i.e.: 1 12 2 2 2= +x s x x s s x sφ φ φ φ φ φ φ φ − − − = − . (6) if the pd of the reference transformer is zero (for an ideal transformer) then the pd between the primary and secondary dut currents will be defined as the phase shift of the dut secondary current relative to the secondary current of the reference transformer. 3.2. simulation of current difference the comparator produced in ukraine is structured in such a way that its design has a measuring circuit where the current phasor is formed. this phasor is a result of subtracting two currents, i.e. dut current and working standard current. this measuring circuit converts the specified current phasor and the output current phasor of the reference transformer into voltage drop during its passing through the certain measuring shunt. furthermore, the measurement information is converted into digital codes by the analog-to-digital converter to calculate the ratio of the current difference phasor to the phasor of the reference transformer current. in the typical operation of a comparator, there is a comparison of two currents. the current difference between two currents id flows through the certain input circuit and the current is of the reference transformer flows through the other measuring circuit. the dut current ix is absent inside the device and the comparator indicates the value of the re and pd. let us consider the case where the dut current exceeds the amplitude of the reference transformer current, as shown in figure 1. the angle β is the angle between the phasors of id and acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 8 is currents. to determine the characteristics of the interrelation between the dut current and reference transformer current, it is necessary to determine the unknown elements of the triangle in figure 1. the ac segment is determined as: cc= osa di β . (7) the bc segment is determined as: sc= inb di β . (8) the magnitude of the ix phasor is present in expression (4). since this secondary current is not present in the circuit during simulation, its value must be expressed using expressions (7) and (8) through the equation: ( ) ( ) 2 2 22 2 2 2 2 2 2 2 2 2 2 = ac bc sin + + sin cos cos cos +cos x s s d d s s d d d s s d d i i i i i i i i i i i i i i β β β β β β + + = +  +  = +      = +    . (9) substituting the expression (9) into equation (4), the following expression can be obtained to determine re: 2 2 1 2 1= cosx d s d si i i iε β+   + − . (10) it should be mentioned that when considering the variant of the interrelation of id and is phasors, in case of excess in the amplitude of the current is, the sign before 2 changes to the opposite in expression (10). regarding the pd, it is determined from the ratio of the sides of the triangle in figure 1: ( ) ( )tan =bc +acx siφ . (11) the use of an inverse trigonometric function can define the expression for pd determination: ( )=atan sin cosx d s di i iφ β β   +    . (12) when considering the variant of the interrelation of id and is phasors, in case of excess in the amplitude of the current is, no change occurs in expression (12). since in this case, it is necessary to consider a right-angled triangle with a known angle of (π β). 4. realisation of method 4.1. measurement setup description the suggested technique allows calibrating the ct calibration unit by using the measuring scheme presented in figure 2. let us consider the elements of the measuring scheme for determining the relative current difference. a precision ct (t) should be used as a stable ac current scaling element to calibrate the ac comparator. the secondary current is estimated to be smaller than a percent (or one-tenth of a percent) of the primary current. it means that ct secondary current simulates current difference id between dut and reference transformer. to simulate the low current difference, it may be necessary to introduce another stable ct for cascading into the measurement scheme in figure 2. thus, it is possible to achieve simulated error values of about 1 µa/a, and even less. the elements of the measuring circuit are connected in such a way that id creates a voltage drop at the left p4834 resistance decade box as well as at the right measuring shunt of the ca507 comparator. a precision ac voltmeter is used to estimate the id value through ohm's law: =d rdb rdbi u r , (13) where urbd is a voltage drop caused by current id that flows through the left p4834 resistance decade box; rrbd is a true value of the left p4834 resistance decade box. to account for the branching of the current between the left p4834 resistance decade box and the precision voltmeter input circuit, the value of id was also adjusted by the entry of the kim branching factor. the current flowing in the reference transformer circuit can be estimated using the ca507 built-in ammeter with worse uncertainty. if there is a need to decrease the total measurement uncertainty in the calibration of the ac comparator, a precision ammeter can be connected in series or the output voltage of the fluke a40 precision shunt (r1) can be measured with a precision ac voltmeter. the contribution of using the ca507 comparator built-in ammeter to the combined standard uncertainty will be estimated below. the ca507 right measuring shunt is an integral part of the calibrated comparator design and is used for extracting information on the difference between two input currents. the ct primary current flows through the ca507 internal ammeter, and also creates a voltage drop at both r1 current shunt and ca507 left shunt. the last shunt is used in the comparator design for extracting information on the current is of the reference transformer. a dual-channel clarke-hess 6000a phase meter is needed to determine the angle β between the currents flowing through the fluke a40 current shunt and the left p4834 resistance decade box. both the right p4834 resistance decade box and the p5025 capacitance box are connected in parallel to shift the angle β. it should be noted that the phase-shifting unit is only needed when simulating the pd, and may be absent during the simulation of the re. instead, it is possible to find the relation between the capacitance and the resistance of the phase-shifting unit when the angle β is close to zero. figure 1. interrelation between currents. figure 2. the electrical scheme of ac comparator calibration. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 9 4.2. measurement setup characteristics according to the measurement scheme presented in figure 2, a selection of the appropriate parameters of the circuit elements allowed us to simulate the re and pd in the range of values from 1 ppm to 1200 ppm. table 1 summarises the estimates of the input quantities for which the re values were obtained. it should be mentioned that the measurement setup allows us to vary the input quantities over a wide range and simulate a large number of re values. besides, the smallest measurement uncertainty can be achieved by setting the angle β close to 0° or 180º, and the worst uncertainty will be if the angle β will be 90º. table 2 presents the estimates of the input quantities for which the pd values were obtained. the measurement setup allows us also to change the input quantities similarly to simulate the pd values. in this case, the smallest relative uncertainty of measurements can be achieved by setting the angle β close to 90 or 270º, and the worst uncertainty will be when the angle β will be 0 or 180º. the measurement uncertainty, which the set of measuring instruments used can provide, is also a characteristic parameter of the measurement setup. models (10) and (12) describe the dependence of simulated res and pds on variables whose uncertainty affects the total uncertainty of measurements of simulated quantities. table 3 presents the simplified uncertainty budget for the re of 32 µa/a and shows that the contribution of each input quantity for a given re simulation point is comparable to each other. table 4 presents a simplified uncertainty budget for the simulated pd of 241 µrad and shows that the contribution of each input quantity for a given pd simulation point is comparable to each other, as well as the previous case. 5. analysis of measurement uncertainty 5.1. general points according to gum 1995 [20], to determine the sensitivity coefficients of the input quantities, one must take the first partial derivatives for each input quantity. a detailed description of obtaining the sensitivity coefficients of the input quantities for the mathematical model (10) is given in the previous work [21]. as for the model (12), the expressions for determining the sensitivity coefficients are given in [22]. to verify the correctness of the expressions obtained, the ms excel software was used to calculate the sensitivity coefficients c(xi) as the ratio of the increase of the transformed mathematical models with a substitution of current id according to expression (13) to the small increase of each variable separately by the general expression: ( ) ( )i i iс x f x x=   , (14) where δf(xi) is a small increase of the corresponding mathematical model; δxi is a small increase in the chosen input quantity. the values obtained by the expression (14) were equal to the values calculated by the derived analytical expressions. one of the factors in a distortion of the results of sinusoidal current measurements is the shape of the curve. during the study, there were doubts about providing a sinusoidal curve in the secondary winding of a current-scaling transformer t. that is because in simulating the transformer errors, the secondary winding of this transformer was loaded with resistance in the range from 1 kω to 60 kω. to verify the curve of the sine wave at the terminals of the p4834 decade box, a voltage form was monitored using an oscilloscope since the secondary current flowed through the resistance decade box. the observation result is presented in figure 3 in a form of a photograph. in figure 3 it can be seen that the secondary current of the transformer t (bottom left), which was more than 10 µa, flowed through the resistance decade box (top left). the comparator display (top right) shows that the current in the transformer primary winding was approximately 1 a. the resistance value was 400 kω and the voltage at the terminals of the resistance decade box was controlled by a digital oscilloscope (bottom table 1. estimates of quantities for setting re. current phase shift (°) resistance decade box voltage (v) working standard circuit current (a) resistance decade box value (kω) current branching factor re (µa/a) 0.112 17.25 2.97 5 1.005 1167 0.125 6.029 3.04 5 1.005 399 0.131 1.8784 2.99 20 1.02 32.0 0.136 0.7636 3.035 20 1.02 12.8 0.145 0.709 2.99 40 1.04 6.17 0.501 0.2526 3.02 60 1.06 1.47 table 2. estimates of quantities for setting pd. current phase shift (°) resistance decade box voltage (v) working standard circuit current (a) resistance decade box value (kω) current branching factor pd (µrad) 139.82 4.103 1.035 2.2 1.0022 1167 175.00 21.26 1.41 2.2 1.0022 603 119.48 0.9949 2.995 1.2 1.0012 241 119.04 1.482 3.065 3.2 1.0032 133 173.52 0.3139 2.975 1.2 1.0012 10.0 173.36 0.1766 3.055 3.2 1.0032 2.1 table 3. characteristic of re measurement uncertainty. source of uncertainty standard uncertainty estimate sensitivity coefficient contribution to combined standard uncertainty kim 2.0∙10-3 3.1∙10-5 6.2∙10-8 urdb 5.6∙10-3 v 1.7∙10-5 v-1 9.6∙10-8 rrdb 100 ω 1.6∙10-9 ω -1 1.6∙10-7 is 0.01 a 1.1∙10-5 a-1 1.1∙10-7 β 7.9∙10-3 2.4∙10-7 1.9∙10-9 re estimate (µa/a) expanded uncertainty (µa/a) 32 0.45 (k =2) table 4. characteristic of pd measurement uncertainty. source of uncertainty standard uncertainty estimate sensitivity coefficient contribution to combined standard uncertainty kim 2.0∙10-3 2.4∙10-4 4.8∙10-7 urdb 3.0∙10-3 v 2.4∙10-4 v-1 7.2∙10-7 rrdb 6 ω 2.0∙10-7 ω -1 1.2∙10-6 is 0.01 a 8.1∙10-5 a-1 8.1∙10-7 β 7.9∙10-3 1.4∙10-4 1.1∙10-6 pd estimate (µrad) expanded uncertainty (µrad) 241 4.1 (k =2) acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 10 right). the display of the oscilloscope shows that the voltage form did not differ from the sine wave. thus, a load of several tens or hundreds of kω on the secondary winding with a current of less than 0.1% of a rated current did not distort the shape of the secondary current. there was only an increase in the phase shift of the secondary current relative to the primary and the change in the simulated characteristics. in a previous work, the p4834 decade box was characterised regarding the voltage phase shift at the terminals of this device relative to the flowing current [23]. both the dependence of the phase shift on the set resistance and the corresponding measurement uncertainty was determined. it was found that the effect of the phase shift relative to the measured value, especially for the current phase shift of about 180º, was noticeable. this was reflected in the fact that the ratio of expanded measurement uncertainty to the result of pd measurement increased markedly. in contrast, with a phase shift between currents that differed from 180º by more than 10º no increase in relative uncertainty was observed. 5.2. ratio error measurement uncertainty continuing the analysis, one can distinguish certain characteristic points when the simulated errors acquire values for which the measurement uncertainty far exceeds the error itself. this is due to the presence of a trigonometric component in both mathematical models. to find the characteristic points, the interrelation between input quantities was selected in a such position when the re measurement result was 18 µa/a. a series of the re values at fixed values of all input quantities except for the current phase shift were modeled. the corresponding measurement uncertainty was also evaluated. the results of this simulation are presented graphically in figure 4. according to the model (10), figure 4 shows that the re (blue curve) acquires minimum value when the current phase shift gets the value of 90º. however, the absolute value of measurement uncertainty varies slightly over the whole range. the corresponding relative uncertainty of the measurements (red curve) in the current phase shift range from 0° to about 60º and from 120° to 180º remained at almost one level of about 2 percent despite the measured value in the simulation varied several dozen times. with a significant decrease in the simulated error, the percentage ratio of the almost constant measurement uncertainty to the specified error increased significantly, going to infinity at a point of 90º. 5.3. phase displacement measurement uncertainty to find the characteristic points in the pd simulation, the interrelation between the input quantities was chosen in such a position when the measurement result was 603 µrad. several pd values were also simulated as well as the corresponding measurement uncertainty was estimated. the results of this simulation are presented graphically in figure 5. according to the model described by equation (12), figure 5 shows that the pd (blue curve) acquired minimal value when a current phase shift of 0° or 180º was achieved. however, the absolute measurement uncertainty was almost the same since the measured quantities remained constant, only the phase shift between currents changed. the intrinsic uncertainty of the phase meter also remained almost constant, and the change of phase shift only affected the sensitivity coefficients. since the measurand in the simulation varied 2-3 times in the current phase figure 3. observation of the shape of the voltage curve at the terminals of the p4834 decade box. figure 4. simulation results of the ratio error depending on the current phase shift. figure 5. simulation results of the phase displacement depending on the current phase shift. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 11 shift range from 30° to 150º, the corresponding relative measurement uncertainty (red curve) varied accordingly, reaching a minimum of about 2 percent. nevertheless, with a significant decrease in the simulated error, the percentage ratio of the almost constant measurement uncertainty to the specified error increased significantly, going to infinity at points of 0 and 180º. the phenomena analysed above can be illustrated in a vector form as shown in figure 6. option a) of figure 6 shows that at a current phase shift close to 90° or -90º when the small value of the difference between the іх and іs currents occurs, the whole error accounts for the pd (the amplitude of the іх and іs currents will be the same). instead, option b) illustrates the opposite case where pd is absent and the whole current difference is related to the re. such points occur when the current phase shift is 0° or 180º. 6. simulating measurement results 6.1. simulation using method proposed in table 5 and table 6, the results of simulating the errors of ct, using the scheme depicted in figure 2, are presented. according to the specification of the ca507 comparator, the intrinsic uncertainty when measuring re is determined in percentage by the formula ( )0.005 0.0002 0.0001reu =   + +   . (15) as can be seen from expression (15), the minimum uncertainty value cannot be less than 2 µa/a. according to the specification of the ca507 comparator, the intrinsic uncertainty in pd measurement is determined in minutes by the formula ( )0.005 0.03 0.7 15pdu =   + +   . (16) as can be seen in expression (16), the minimum uncertainty value cannot be less than 8.73 µrad. in table 6, the intrinsic uncertainty of ca507 is evaluated as 21.2 when measuring 605 µrad, and it exceeds the uncertainty when measuring 1132 µrad. one can see a clear upward trend in this characteristic with increasing measured value in formula (16). however, the interrelation between the re and pd at the simulated measurement point led to an increase in intrinsic uncertainty due to the sufficiently strong effect of the amplitude component portion [24]. figure 7 shows that the measurement uncertainty, evaluated according to gum 1995 [20] for measured res less than 100 µa/a, has a large margin regarding test uncertainty ratio. however, for the error above 200 µa/a, measurement uncertainty increases markedly under conditions without additional measures such as stabilisation of the supply of the measuring circuit, determination of the voltmeter input impedance, etc. in figure 7, it could also be seen that the difference between the comparator readout and the reference value increases with the increase of the simulated error value. 6.2. simulation using monte carlo method to verify the correctness of the measurement uncertainty evaluation for the method proposed we decided to apply the monte carlo method during the simulation of the measurement results. the simulation was performed within the uncertainty of each input quantity (a case for re of 32 ppm is presented in table 3, and for pd of 241 ppm in table 4). the simulation was yielded using the ms excel software by generating the values with randbetween function within the specified uncertainty intervals of the input quantities of both the model (10) for re and the model (12) for pd, the number of error value observations was set to 30, and the number of the values generated was set to 10. the values of the simulation results, which differed as much as possible from values reported in table 5 and table 6, are given in table 7 for re and in table 8 for pd, as well as the average deviation. compared with the data obtained using the method proposed, the results obtained by the monte carlo method have a maximum deviation which did not exceed the uncertainty limitations specified above in table 5 for re and in table 6 for pd. no simulated re and pd values were obtained that would exceed the limits of the measurement uncertainty evaluated. table 5. comparison of re measuring data using ca507 comparator and calculated results. current (a) re (µa/a) re measurement uncertainty (µa/a) comparator readout reference value comparator readout reference value 2.975 1154 1167 8.3 9.2 2.960 393 399 3.9 3.1 2.993 31.3 32.03 2.2 0.45 3.0356 12.5 12.84 2.1 0.21 2.991 6.1 6.17 2.0 0.16 3.022 0.6 1.47 2.0 0.11 figure 6. interrelation between current phasors in simulating errors of current transformer. figure 7. the difference in values obtained with evaluated measurement uncertainties. table 6. comparison of pd measuring data using ca507 comparator and calculated results. current (a) pd (µrad) pd measurement uncertainty (µrad) comparator readout reference value comparator readout reference value 3.051 1132 1167 16 26 2.974 605 603 21 12 3.065 238 241.1 10 4.1 2.995 130.7 132.6 9.5 2.3 1.417 10,2 10.0 8.9 1.1 1.035 2.3 2.11 8.8 0.29 acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 12 moreover, the average values of error estimates are an order of magnitude less than the measurement uncertainty evaluated. thus, the authors believe that the use of gum 1995 to determine the expanded uncertainty for the method proposed allows us to evaluate this characteristic correctly enough. 7. discussion one of the tasks of the calibration laboratory is to provide measurement uncertainty in the reference values 4 times less than that provided by the dut. this paper proposes a method that allows achieving a measurement uncertainty of several tenths of µa/a when calibrating a commercial ct calibration unit under ordinary laboratory conditions. the calibration procedure described in the paper [25] involves reaching the condition when the error current is in phase with the standard current when determining the re. to determine the pd, the shift of the error current by 90 degrees relative to the current through the standard current measurement circuit must be achieved. it should be noted that the proposed method allows characterising the commercial ac comparators without the requirement of in-phase currents or a 90° angle shift due to applying the precision phase meter as stated in section 4.1. as described in sections 5.2 and 5.3, the proposed procedure can be applied for determining the errors of ac comparators in a wide range of changes in the current phase shift. in the method described in [26] and [18], which has been mentioned in section 2, an increment of simulated errors is limited to the discreteness of the pc-controlled three-phase power source, and a step of changing the ratio of turns of the electronically-compensated current comparator corresponds to 0.5 %. the use of a phase-shifting unit allowed the simulation of ct errors of almost any value using the method proposed. due to the use of laboratory ct as a source, it is possible to generate the stable values of simulated errors, and cascading two such cts allow us to simulate very small errors. in table 5 and table 6 of section 6.1, one can see that achieving the measurement uncertainty less than 1 ppm in simulating the lowest error values is possible by the method presented. moreover, the proposed analytical expressions make it easy to calculate both the magnitude of the reference values and the associated measurement uncertainty. the simulation results by the monte carlo method, which are shown in table 7 and table 8, provide grounds to consider the evaluation of the measurement uncertainty to be correct. there are a large number of commercial comparators, and the user could check the accuracy of such a device using conventional precision measuring instruments through the proposed method. the measurement setup was tested several times using ca507 comparators. also, the results can be extended to other types of comparators of almost identical currents, such as aitts (india) or hgqa-c (china), which were characterised by the method that differs from the described one by the use of an oscilloscope for determining the phase shift angle and has somewhat worse mathematical processing [21]. however, some issues need to be addressed. the analysis of the uncertainty budget (regarding table 1 – table 4 of section 4.2) gives grounds for claiming that the total uncertainty can be significantly reduced also in the simulation of ct errors of about 1000 µa/a when additional measures are applied. the clarification of measurement uncertainty of a current (the use of precision ammeter), branching factor (the rigorous estimation of the input impedance of a voltmeter) can lead to a reduction of the total uncertainty of measurements. the relationship between the amplitude and angular components of the simulated current difference has a great influence on the uncertainty estimation. the smaller the amplitude portion the smaller the measurement uncertainty of the angular component and vice versa. this is a direction for refining the measurement results. the application of advanced technologies of data acquisition by replacement of voltmeter, phase meter by high-speed sample measuring devices with the subsequent automatic calculation of both reference values of errors and uncertainty of measurements should be also the direction of further improvement. 8. conclusions the presented measurement setup can be applied to simulate the errors of cts in a wide range of values with a small step of changing the simulated characteristics. the mathematical models derived for the determination of re and pd in simulating these characteristics made it possible to obtain reference values for the characterisation of the ct calibration unit. the measurement uncertainties evaluated in simulating the re and pd allowed us to assume that the proposed method is advisable to apply in verifying the calibration unit for the laboratory cts, especially in the range from 1 to 1000 µa/a (µrad). the analysis of uncertainty sources allowed determining the value of the current phase shift between currents (0°, 90°, 180°, and 270º) which leads to a rapid increase of the total relative measurement uncertainty. in determining the re, the formation of the ratio of the currents, at which the pd goes to zero, allows minimising the contribution of the current phase shift. references [1] p. castello, c. muscas, p. a. pegoraro, s sulis, a monitoring system based on phasor measurement units with variable reporting rates, acta imeko 7(4) (2018), pp. 62-69. doi: 10.21014/acta_imeko.v7i4.585 [2] m. crescentini, m. marchesi, a. romani, m. tartagni, p. a. traverso, bandwidth limits in hall effect‐based current sensors, table 7. results of simulating re using monte carlo method. re (µa/a) expanded uncertainty (µa/a) maximum deviation (µa/a) average deviation (µa/a) 1167 9.2 8.5 0.98 399 3.1 -2.6 0.46 32 0.45 -0.38 -0.03 12.8 0.21 -0.11 0.02 6.17 0.16 0.08 -0.01 table 8. results of simulating pd using monte carlo method. pd (µrad) expanded uncertainty (µrad) maximum deviation (µrad) average deviation (µrad) 1167 26 23 -2.8 603 12 9.6 0.87 241 4.0 -3.1 0.43 10.0 1.1 -0.92 -0.12 2.11 0.29 0.25 0.05 http://dx.doi.org/10.21014/acta_imeko.v7i4.585 acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 13 acta imeko 6(4) (2017), pp. 17-24. doi: 10.21014/acta_imeko.v6i4.478 [3] v. nováková zachovalová, m. šíra, p. bednář, s. mašláň, new generation of cage‐type current shunts developed using model analysis, acta imeko 4(3) (2015), pp. 59-64. doi: 10.21014/acta_imeko.v4i3.250 [4] m. cundeva‐blajer, application of optimization methods for improved electrical metrology, acta imeko 5(3) (2016), pp. 915. doi: 10.21014/acta_imeko.v5i3.378 [5] iec 61869-2:2012, instrument transformers – part 2: additional requirements for current transformers, 2012, p. 144. 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[20] jcgm 100:2008//gum 1995 with minor corrections, evaluation of measurement data – guide to the expression of uncertainty in measurement, 2008. online [accessed 06 june 2021]. https://www.bipm.org/utils/common/documents/jcgm/jcgm _100_2008_e.pdf [21] v. isaiev, method of reference values defining for calibration of two alternating currents comparator with using oscilloscope, world science 2(4 (32)) (2018), pp. 42-49. [22] v. isaiev, o. velychko, precise low-cost method for checking accuracy of current transformers calibration unit, proc. of 24th imeko tc-4 international symposium, palermo, italy, 2020, pp. 406-410. [23] v. isaiev, s. nosko, power frequency characterization of resistance decade box for calibrating ac comparator, international journal of scientific and engineering research 10(10) (2019), pp. 1450-1456. [24] v. isaiev, o. velychko, y. anokhin, comparator effect on equivalence of results of calibrating current transformers, easterneuropean journal of enterprise technologies 5(101) (2019), pp. 6-15. doi: 10.15587/1729-4061.2019.177415 [25] k. draxler, j. hlaváček, r. styblíková, calibration of instrument current transformer test sets, proc. of 2019 international conference on applied electronics (ae), pilsen, czech republic, 2019, pp. 1-4. doi: 10.23919/ae.2019.8866993 [26] h. çaycı, a complex current ratio devıce for the calibration of current transformer test sets, metrology and measurement systems 18(1) (2011), pp. 159-164. doi: 10.2478/v10178-011-0015-2 http://dx.doi.org/10.21014/acta_imeko.v6i4.478 http://dx.doi.org/10.21014/acta_imeko.v4i3.250 http://dx.doi.org/10.21014/acta_imeko.v5i3.378 https://doi.org/10.1007/s12647-009-0008-8 https://doi.org/10.1109/cpem.2014.6898232 https://doi.org/10.1109/imtc.2006.328442 https://doi.org/10.1109/amps.2018.8494842 https://doi.org/10.1109/iciea.2013.6566445 http://userequip.com/files/specs/6031/ct-analyzer_user%20manual.pdf http://userequip.com/files/specs/6031/ct-analyzer_user%20manual.pdf https://doi.org/10.1088/0026-1394/51/1a/01013 https://doi.org/10.1109/tim.2017.2648918 https://doi.org/10.1088/0026-1394/46/1a/01005 https://doi.org/10.5194/jsss-7-339-2018 https://doi.org/10.1109/tim.1983.4315033 https://www.imeko.org/publications/tc4-2008/imeko-tc4-2008-010.pdf https://www.imeko.org/publications/tc4-2008/imeko-tc4-2008-010.pdf https://www.imeko.org/publications/tc19-2010/imeko-tc19-2010-125.pdf https://www.imeko.org/publications/tc19-2010/imeko-tc19-2010-125.pdf https://www.bipm.org/utils/common/documents/jcgm/jcgm_100_2008_e.pdf https://www.bipm.org/utils/common/documents/jcgm/jcgm_100_2008_e.pdf https://dx.doi.org/10.15587/1729-4061.2019.177415 https://doi.org/10.23919/ae.2019.8866993 https://doi.org/10.2478/v10178-011-0015-2 analysis of the measurement uncertainty of a new measurement flexure calibration set-up acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 173 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 173 178 analysis of the measurement uncertainty of a new measurement flexure calibration set-up k. geva1, h. kahmann2, c. schlegel3, r. kumme4 1, 2, 3, 4 physikalisch-technische bundesanstalt, braunschweig, germany 1 kai.geva@ptb.de, 2 holger.kahmann@ptb.de, 3 christian.schlegel@ptb.de, 4 rolf.kumme@ptb.de abstract: a measurement flexure calibration set-up is presented in this paper. the measurement flexures under test are part of a new 5 mn·m standard torque machine measurement system at the ptb. the calibration set-up can create transversal forces up to 200 n and bending moments up to 100 n·m and respectively up to 150 n·m torque moments simultaneously. the measurement uncertainty budget of the set-up is investigated in a theoretical analysis. keywords: measurement uncertainty budget; bending moment; transversal force; multicomponent measurement 1. introduction at the ptb a new 5 mn·m standard torque machine (stm) is built to calibrate torque transducers [1]. the 5 mn·m stm consists of an actuator side for generating calibration moments (torque as well as bending moments) and a measurement side for measuring the applied load. both sides have a lever to which a torque transducer under test is flange-mounted. the measurement side contains six measurement flexures (mf) to receive the lever forces axially. there are two types of mf: two mf which measure torque (tmmf) and four mf which measure bending moments (bmmf). the bmmf’s tangential forces and torque moments share around 3 % of the overall 5 mn·m torque moment. due to mf’s displacement, parasitic forces and moments are inevitable. the knowledge of these moments and forces are essential to define the overall measurement uncertainty of the standard torque machine. to achieve a torque measurement uncertainty below 0.5 % it is necessary to evaluate the mf in a special calibration set-up. the calibration is designed to provide a measurement with an uncertainty lower than 1 %. the following paper explains the set-up to calibrate the mf and analyses the expected measurement uncertainty. finally, critical influences on the measurement uncertainty must be identified and concepts or measurements must be defined to reduce key measurement uncertainty influences. figure 1: 5 mn·m standard torque machine (stm) 2. measurement flexure calibration set-up 2.1. load scenario of bmmf in 5 mn·m stm table 1: forces on bmmf at 5 mn·m stm (feanalysis) 𝑭𝒙 in n 𝑭𝒚 in n 𝑭𝒛 in n bm 1 175 -5 -10 624 bm 2 175 5 10 624 bm 3 176 6 10 614 bm 4 176 -6 -10 614 table 2: moments on bmmf at 5 mn·m stm (feanalysis) 𝑴𝒙 in n·m 𝑴𝒚 in n·m 𝑴𝒛 in n·m bm 1 68 -73 -128 bm 2 -68 -73 -128 bm 3 68 74 -113 bm 4 -68 74 -113 the mf calibration scenario of the set-up must match the applied load combination occurring in the 5 mn·m stm. a fe-analysis of the measurement side under the load of 5 mn·m was performed to analyse the load scenario within the machine. table 1 and table 2 show the load applied on to the bmmf. all transversal forces are around 176 n and they are tangential from the lever’s point of view. http://www.imeko.org/ mailto:kai.geva@ptb.de mailto:holger.kahmann@ptb.de mailto:christian.schlegel@ptb.de mailto:rolf.kumme@ptb.de acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 174 the moment generated by these tangential forces add up to the overall torque applied to the measurement lever of the 5 mn·m stm and must be considered in the uncertainty budget of the 5 mn·m stm. the axial force is almost 100 times higher than the transversal force. therefore, the transversal force cannot be measured by strain gauges but must be determined by measuring the displacement with an interferometer. the rigidity can be derived and used for calculating the forces and moments applied on bmmf when measuring the displacement in the 5 mn·m stm during calibration. the fe-analysis indicates there is a constant ratio between applied tangential force and accompanying bending moment and torque moment respectively at all load steps. bending moment divided by transversal force is 0.56 and torque divided by transversal force is 0.73. thus, the lever arms’ lengths should meet these ratios to simplify the calibration procedure. two load scenarios are applied at the calibration set-up. load scenario 1 (ls1) is a combined result of a transversal force and a bending moment (bm). load scenario 2 (ls2) is characterized by a transversal force which generates a torque moment (tm). 2.2. calibration set-up figure 2 and figure 3 depict the calibration set-up for bmmf in both load scenarios. in both scenarios load is applied on top of each measurement flexure where the mf is connected to the measurement lever. there, a cantilever for each scenario is attached to an adapter at the mf. the ls1 cantilever consists of a horizontal and a vertical part. at the end of the vertical cantilever part the force introduction (fi bm) is aligned to the mf middle. figure 2: bmmf calibration set-up for ls1 figure 3: bmmf calibration set-up for ls2 there, the metal band (mb) is attached to a clamp and is aligned parallel to the mf top flange where the reference pivot point (pp) lies. the ls2 cantilever has only a horizontal part. at the end of the cantilever there is a shank supported by roll bearings the force is introduced to (fi tm). figure 4: definition of γ figure 5: definition of δ http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 175 the force is created via calibrated mass disks. for force application a thin metal band of 80 µm thickness is used for determining the position of force application and to avoid bending moments. as the measurement flexures are erected vertically the gravitational force of the mass stack must be redirected by a pulley. for reasons of metal band alignment, the pulley is placed on a z-cross table. to compensate displacement in x-direction a linear bushing (lb) is installed on a bar. also, the metal band can rotate with minimal friction on the linear bushing. a mass stack up to 200 n is chosen to cover the full calibration range. the disks are laid manually onto a hanger which applies its force onto another clamp attached to the metal band. 3. measurement uncertainty budget 3.1. measurement function table 3 lists all the uncertainty influence parameter that are necessary to characterise the measurement uncertainty of the calibration set-up. there are two model equations to characterise the calibration load all influence parameters are derived from. the parameters will be analysed later. the model equation for the force vector at fi is �⃗�𝑖 = ( sin 𝛾 cos 𝛾 cos 𝛿 sin 𝛿 ) ⋅ 𝐹𝑖 . (1) the absolute value of the calibration force 𝐹𝑖 is 𝐹𝑖 = (𝑚𝑖,𝑑 + 𝑚hg + 𝑚hk + 𝑚vb + 𝑚bo) ⋅ 𝑔loc ⋅ (1 − 𝜌a 𝜌𝑚 ) + 𝛥𝐹pm − 𝛥𝐹r,lb − 𝛥𝐹r,tc . (2) the model equation for the moment vector at pp is �⃗⃗⃗� = ( 𝑙0,𝑥 𝑙0,𝑦 𝑙0,𝑧 ) × �⃗�𝑖 . (3) for ls1 a bending moment (bm) is applied 𝑀𝑥,𝑖 = (𝑙bm,0,𝑦 ⋅ sin 𝛿bm − 𝑙bm,0,𝑧 ⋅ cos 𝛾bm cos 𝛿bm) ⋅ 𝐹𝑖 + 𝑀𝑥,0 . (4) for ls2 a torque moment (tm) is applied 𝑀𝑧,𝑖 = (𝑙tm,0,𝑥 ⋅ cos 𝛾tm cos 𝛿tm − 𝑙tm,0,𝑧 ⋅ sin 𝛾tm) ⋅ 𝐹𝑖 . (5) 3.2. influence quantities the influences are described in the following section. finally, the total measurement uncertainty budget for the transversal force, the bending moment and the torque is presented. table 3: list of uncertainty influence parameters parameter description 𝐹𝑖 transversal force for calibration 𝑀𝑥,𝑖 bending moment (bm) for ls1 calibration 𝑀𝑥,0,𝑖 bending moment (bm) due to weight of ls1 cantilever 𝑀𝑧,𝑖 torque moment (tm) for ls2 calibration 𝑀𝑧,tt,𝑖 torque moment (tm) for ls2 calibration, measured by a torque transducer 𝛾 vertical metal band inclination 𝛿 horizontal metal band inclination 𝑙bm,0/𝑙tm,0 ls1/ ls2 lever arm 𝑚𝑖,𝑑 mass disk per load step 𝑖 𝑚hg hanger mass 𝑚hk adapter hook mass 𝑚vb vertical band mass 𝑚bo band overlap mass 𝑔loc local gravity 𝜌a air density 𝜌𝑚 mass density δ𝐹pm force disturbance caused by pendulum motion δ𝐹r,lb force disturbance caused by linear bushing friction δ𝐹r,tc force disturbance caused by tm roll bearing friction (only ls2) mass disks and density the laboratory for solid mechanics at ptb provides calibrated disks for special force calibration procedures. in this case, it consists of a set of 5 n, 10 n and 20 n disks (𝑚𝑖,𝑑) which can be stapled manually onto a hanger 𝑚hg. furthermore, a small hook adapter 𝑚hk and the metal band 𝑚vb must be considered. the uncertainty of these adapting elements is unknown and therefore estimated much higher than the other elements. table 4 lists all mass disks with their mass, uncertainty and density. table 4: list of mass disks for 200 n mass disk mass in g density in 𝐤𝐠/𝐦𝟑 hanger 𝑚hg 1019.248 ± 0.005 7950 ± 140 mass 20 n 2038.518 ± 0.002 7927 ± 0.3 mass 10 n 1019.261 ± 0.001 7927 ± 0.3 mass 5 n 509.630 ± 0.005 7927 ± 0.3 metal band 𝑚vb 1.639 ± 0.010 7850 ± 140 adapter hook 𝑚hk 145.092 ± 0.050 2850 ± 140 band overlap the band overlap happens when load is applied onto the hanger. the thin metal band will be deformed and get longer. there will be an overlap http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 176 reaching over the pulley, resulting in additional mass as calibration load. the additional mass depends on the applied mass and is calculated by 𝑚bo = 𝜌mb ⋅ 𝐴mb ⋅ 𝛥𝑙bo . (6) inserting hooke’s law, it yields 𝑚bo = 𝜌mb ⋅ 𝑙fi−py ⋅ 𝐹𝑖 𝐸mb . (7) the band overlap mass is shown in table 5. table 5: additional mass caused by band overlap load step mass in mg 𝑚bo,50 n 10 ± 5 𝑚bo,100 n 20 ± 10 𝑚bo,150 n 30 ± 15 𝑚bo,200 n 40 ± 20 local gravity the local gravity is measured by the leibnizuniversity hannover at ptb. the calibration set-up will be placed near the 5 mn·m stm in a new building, so the local gravity is not measured yet. for this analysis it is satisfactory to use the measured gravity in [2] at the 1 mn-nme 𝑔loc = 9.812 516 ± 5 ⋅ 10 −6 m/s2 . (8) air density the air density 𝜌𝐴 is calculated by 𝜌a = 0.34848 ⋅ 𝑝a − 0.009024 ⋅ ℎa ⋅ 𝑒 0.0612⋅𝑇a 273.15 + 𝑇a . (9) at the ptb solid mechanics laboratory the air temperature 𝑇a is (21 ± 0.2) °c. the average air pressure 𝑝a is (1013 ± 2) hpa and humidity ℎa is (42 ± 5) %. pendulum motion the mass disks are placed manually onto the hanger which may cause swinging movements of the whole mass stack. to minimise pendulum motion, the hanger is clamped at the hanger bars while loading and unloading the mass disks. after the load step changes, the system must return to a balance where the signal caused by pendulum motion is less than 0.1 n. friction there are two components which cause friction. the pulley contains a z-cross table which carries a shaft. the shaft conducts a linear bushing. the bushing is shelled by a sleeve where the metal band is positioned. the linear bushing allows the movement of the metal band when loaded with low friction. the standard linear bushing from bosch rexroth with a borehole diameter of 30 mm has a breakaway force of 6 n. the second component effects only ls2 load scenario. the torque cantilever uses a metal band clamp that is attached to a shaft which rotates with the help of angular contact ball bearings. schaeffler provides the software tool bearinx easy friction to estimate the friction. the first bearing yields a friction moment of 0.038 n·m and the second of 0.046 n·m. the shaft diameter is 17 mm thus the overall breakaway force is 4.942 n. both friction forces must be measured in different set-ups. the cross table can be replaced by a tension force transducer. in this case an hbm z30a ptb standard force transducer will be used featuring an uncertainty less than 3 × 10-4. when measuring the tension at the point where the force introduction is meant to be, the working force will be reduced by the friction of the linear bushing. as the weight force of the calibrated mass is well-known, the friction can be measured for all load steps up to 100 n. a variance of the linear bushing friction force is expected and must be tested experimentally but will not be quantified in this consideration. a similar approach will be performed to assess the roll bearings at the tm cantilever. instead of using a force transducer a torque transducer is placed between cantilever adapter and bmmf top flange. it is fair to say that a common commercial flange torque transducer up to 100 n·m provides an uncertainty less than 1 × 10-4. comparing theoretically introduced torsion with the output of the interposed torque transducer reveals the friction of the roll bearing 𝛥𝐹r,tc,𝑖 = 𝐹tc,𝑖 − 𝑀rtt,𝑖 𝑙tm,𝑖 . (10) at all load steps the uncertainty for the so measured friction is less than 1 %. bm cantilever weight moment the bm cantilever has a weight of 10.193 kg and a mass centre at a distance of 0.113 m from pp, thus leading to a bending moment offset at all load steps of -11.291 n·m. various materials for different components, unknown density distribution and a complicated process to weigh the bm cantilever make an alternative way for estimation necessary. between the bm cantilever adapter and the bmmf top flange a force transducer equipped with an additional bending moment measurement channel can be installed. since the ptb lacks a test facility for bending moment, the measuring axis needs to be calibrated at the same bmmf calibration test stand. one will see later that calibrating bending moments lower than 20 n·m with a measurement uncertainty of 8 % are achievable. a variance to the friction is expected but cannot be estimated in this paper satisfactorily and therefore will not be quantified. the maximum http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 177 systematic deviation due to friction must be obtained experimentally. thermal expansion coefficient the cantilever is built from item-profiles made of an aluminium alloy. the expansion coefficient for aluminium is 24 × 10-6 k-1 with an uncertainty of 4.8 × 10-6 k-1 [3]. 3.3. geometrical characterisation the lever arm lengths 𝑙bm,𝑦,0 and 𝑙bm,𝑧,0 for bending moments and 𝑙tm,𝑥,0 and 𝑙tm,𝑦,0 for torque moments must be defined. furthermore, the orientation of the applied transversal force vector must be characterised by the angles 𝛾 and 𝛿 for both load scenarios at all load steps. it is planned to use an at960 lr absolute tracker from leica to measure the geometrical elements. the uncertainty due to the different measurements performed by the tracker are estimated according to hernla [3]. for each uncertainty influence quantity, he provides calculation tables based on technical specification parameters of the investigated coordinate measuring machine (cmm). especially the mpe (maximum permissible error) is used to quantify influences 𝑀𝑃𝐸 = 𝐴 + 𝐿 𝐾 , (11) where a is a constant and 𝐾 determines the uncertainty increase per length. the mpe specification is valid only for leica’s red ring reflectors which will be used strictly for the investigated measurements. for the used tracker, leica states 𝐴 is 15 µm and 𝐾 is 166.7 m/µm. hernla [3] uses two different model equations to estimate the uncertainty for the two specific tasks needed for measuring the mentioned geometry elements. the first equation describes the uncertainty of distance measurements 𝐿 = |𝑋1 − 𝑊1 ⋅ 𝑘1 − 𝛥𝑅𝑇1 − 𝑋2 + 𝑊2 ⋅ 𝑘2 − 𝛥𝑅𝑇2 | − 𝛥𝐷c + 𝛥𝐿kmg + 𝛥𝐿𝑇 , (12) where 𝑋1 and 𝑋2 are the coordinates for the elements measured. 𝑊1 and 𝑊2 are inclinations which need to be respected if the geometry element evaluated does not lie in the mass centre of the measurement points. 𝑘1 and 𝑘2 contain information about the coverage of the measure point range compared to the whole element measured. δ𝑅𝑇1 and δ𝑅𝑇2 are the deviation of the probe radius during calibration and are considered if a surface is measured. leica states the radius uncertainty of the reflector up to 2.5 µm. δ𝐷c is the deviation of the optical centre position which is less than 3 µm for a red ring reflector. δ𝐿kmg is the geometrical deviation of the cmm for position measuring. δ𝐿𝑇 is the deviation due to temperature for a cmm scale and for the measurement object. as the used tracker is temperature compensated and the length variation due to temperature is already considered, the term can be ignored here. for angle measurements he uses a second equation 𝐸𝑂 = 𝑊𝐸 ⋅ 𝑘𝐸 − 𝑊𝐵 ⋅ 𝑘𝐵 + 𝛥𝐸kmg , (13) where 𝑊𝐸 and 𝑊𝐵 are the inclinations two elements. 𝑘𝐸 and 𝑘𝐵 is again the coverage factor. δ𝐸kmg is the geometrical deviation of the cmm for inclination. both equations list all possible uncertainty influences on the length or angle measurement but depending on the task not all quantities must be considered. for each influence quantity the uncertainty is measured by 𝑢𝑖 = 𝑠𝑖 ⋅ 𝑏𝑖 ⋅ 𝑐𝑖 (14) with 𝑠𝑖 as standard deviation, 𝑏𝑖 as element parameter and 𝑐𝑖 as sensitivity coefficient. for all elements 𝑋𝑖 , 𝑊𝑖 and δ𝑅𝑇𝑖 hernla [3] suggests for conventional cmm to expect a deviation in the magnitude of 𝐴/3. however, in the following calculation the full mpe-specification tolerance equation (11) because a mobile tracker is used instead of a conventional cmm. the mpe for a tracker measuring distance of 1 m yields 21 µm. δ𝐿kmg is approximated by 𝑎𝛥𝐿kmg = 1 𝐾 ⋅ √𝐿2 + 𝑙2 (15) with 𝐿 from the mpe specification and 𝑙 as the biggest length of one of the geometrical elements. δ𝐸kmg is approximated by hernla’s [3] parallelism and rectangularity formula 𝑎𝛥𝐸kmg = 2𝐿 𝐾 (16) for a conservative estimation due to small expected angles. the factor 𝑏𝑖 contains the information how many measurement points are taken, how they are distributed over the measured element and what kind of geometry is extracted from the measurement. based on best-fit calculation of geometrical elements hernla [3] provides a calculation table for the most common measurement types. an fe-analysis was performed to obtain the displacements of all cantilevers and the pulley of the calibration set-up. from that, an estimation is made, how distances and angle change due to load application and presented in table 6. 3.4. measurement uncertainty budget the measurement uncertainty budgets are presented for transversal force, bending moment and torque at a nominal load of 50 n and 200 n in table 7. the force uncertainty budgets refer to the force in ls2 where the friction share is higher. the measurement uncertainty was calculated with the gum workbench [4]. http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 178 table 6: estimated measurement quantity for geometrical element measurand unit 0 n 50 n 100 n 150 n 200 n 𝑙bm,𝑦,0 mm 336.2 336.34 336.50 336.66 336.82 𝑙bm,𝑧,0 mm 563 562.96 562.91 562.86 562.81 𝑙tm,𝑥,0 mm 730 729.997 729.996 729.996 729.995 𝑙tm,𝑦,0 mm 0 0.249 0.492 0.735 0.978 𝛾bm ° 0 0.013 0.033 0.051 0.069 𝛾tm ° 0 0.001 0.002 0.002 0.004 𝛿bm and 𝛿tm are not listed for the estimated value at all load steps is zero degrees. table 7: measurement uncertainty budget (mub) for 𝐹50 n, 𝑀𝑥,50 n and 𝑀𝑧,50 n influence quantity index 𝑭tm,50 n index 𝑭tm,200 n index 𝑴𝒙,𝟓𝟎 n index 𝑴𝒙,𝟐𝟎𝟎 n index 𝑴𝒛,𝟓𝟎 n index 𝑴𝒛,𝟐𝟎𝟎 n 𝑚𝑑,𝑖 39.4 % 41.9 % 5.7 % 7.0 % 31.0 % 32.7 % δ𝐹pm 3.4 % 0.2 % 0.5 % 2.7 % 0.1 % δ𝐹r,tc 57.2 % 58.0 % 45.0 % 45.2 % 𝛾bm/tm 0.7 % 0.7 % 4.5 % 4.6 % 𝛿bm/tm 92.3 % 92.3 % 16.8 % 17.4 % 𝑀𝑥,0 0.8 % 0.0 % 𝑚bo,𝑖 , 𝑔loc, 𝜌𝑚, 𝑝a, ℎa, 𝑇a, 𝑙bm,𝑦,0,50 n, δ𝐹r,lb , 𝑙bm,𝑧,0,50 n, 𝑙tm,𝑥,0,50 n, 𝑙tm,𝑦,0,50 n and 𝛼w are not listed because they are less significant in this context. table 8: measurement uncertainty for each load step load step 𝑭𝑩𝑴 in n 𝒖𝑭𝑩𝑴 in n 𝑭𝑻𝑴 in n 𝒖𝑭𝑻𝑴 in n 𝑴𝒙 in n·m 𝒖𝑴𝒙 in n·m 𝑴𝒛 in n·m 𝒖𝑴𝒛 in n·m 50 n 45.5 ±0.4 (±0.9 %) 40.5 ±0.6 (±1.5 %) 14.0 ±0.9 (±6.4 %) 28.7 ±0.5 (±1.7 %) 100 n 95.5 ±0.9 (±0.9 %) 90.5 ±1.4 (±1.5 %) 41.8 ±1.9 (±4.5 %) 64.0 ±1.1 (±1.7 %) 150 n 145.5 ±1.4 (±1.0 %) 140.5 ±2.1 (±1.5 %) 69.7 ±2.9 (±4.2 %) 99.4 ±1.7 (±1.7 %) 200 n 195.5 ±1.9 (±1.0 %) 190.5 ±2.9 (±1.5 %) 97.5 ±3.9 (±4.0 %) 134.8 ±2.3 (±1.7 %) 4. conclusions the measurement uncertainty for transversal force is in both load scenarios less than 1.5 %. 𝑀𝑥 has a maximum measurement uncertainty at 50 n with 6.4 % whereas the measurement uncertainty of 𝑀𝑧 is 1.7 % at all load steps. this means, the aimed uncertainty less than 1 % is not achieved yet. in table 8 the mub of 𝑀𝑥 is mainly dominated by 𝛿bm/tm. it must be considered to use a more sophisticated model equation to describe the measurement uncertainty of the inclination measurements. the uncertainty of 𝑀𝑧 depends greatly on friction and the mass calibration uncertainty. thus, it must be investigated if friction measurement can be improved. 5. summary the calibration set-up for calibrating transversal force, bending moments and torque was presented. the measurement uncertainty budget of the calibration load was investigated. the characterisation of the applied force and the measurement of geometrical elements were characterised. this paper showed that the key factors are mass, friction and geometrical elements measurements whose uncertainty must be reduced either by experimental investigation or more accurate model equations. 6. references [1] h. kahmann, c. schlegel, r. kumme, d. röske, “principle and design of a 5 mn·m torque standard machine”, in proc. of 23rd imeko tc3 conference, helsinki, finland, 2017. [2] a. lindau, r. kumme, a. heiker, “investigation in the local gravity field of a force laboratory of ptb”, in proc. of 18th imeko tc3 conference, celle, germany, 2002. [3] m. hernla, “messunsicherheit bei koordinatenmessungen – abschätzung der aufgabenspezifischen messunsicherheit mit hilfe von berechnungstabellen”, band 78, expert-verlag gmbh. [4] metrodata gmbh, manual for gum workbench. http://www.imeko.org/ performance test of cantilever transfer standard used for afm tip calibration based on electromagnetic compensation acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 129 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 129 132 performance test of cantilever transfer standard used for afm tip calibration based on electromagnetic compensation j. jiang1, h. zhou2, y. zhang3, s. wu4, z. zhang5, g. hu6 1, 3, 4, 5, 6 laboratory of force and torque, division of mechanics and acoustics, national institute of metrology, china, 1 jiangjl@nim.ac.cn, 3 zhangye@nim.ac.cn, 4 wushi@nim.ac.cn, 5 zhiminzhang@nim.ac.cn, 6 hugang@nim.ac.cn 2 laboratory of photoelectric detection and image processing, school of precision instrument and opto-electronic engineering, tianjin university, china, huiyan9519@tju.edu.cn abstract: this paper describes stiffness measurement of cantilever transfer standards used for atomic force microscopy (afm) tip calibration based on electromagnetic compensation. the transfer standard of cantilever is designed and manufactured based on the bulk fabrication of soi wafers. the measure range of the transfer standard covers from 0.04 n/m to 16 n/m. the series of transfer standard is designed for the calibration test of the cantilever used in afm, along with the test apparatus specifically designed. the relative uncertainty of the stiffness is smaller than 2.4 % (k = 2). keywords: cantilever; stiffness; small force; electromagnetic compensation 1. introduction atomic force microscopy (afm) can be utilised to observe the matters under very small dimension with high sense of the force below 10-6 n [1-2]. the precise measurement depends on the spring constant of the cantilever and the displacement of the tip measured by positioning detector. in order to obtain the spring constant of the afm tip, highaccuracy measurements of si traceable small forces below millinewton have been realized by several nmis [3-11]. besides the small force standard, the standard cantilever also needs to be designed and manufactured so as to realize the stiffness measurement of afm tip without being measured directly on the small force standard. in this paper the design and the measurement of series of the cantilever transfer standard were presented with the small force standard based on electromagnetic compensation. the transfer standard was designed with a series of lengths for covering a wide range of stiffness and manufactured based on the bulk fabrication of soi wafers. the uncertainty analysis of the stiffness measurement was also conducted. the stiffness of the transfer standard can be obtained with relative extended uncertainty of 2.4 % (k = 2). along with the design of the test apparatus, the series of transfer standard can be used for the afm tip calibration by pushing the cantilever to the transfer standard. 2. description of the test and the results the measurement of the series of the transfer standard was carried out on the measuring system based on the electromagnetic compensation balance. the cantilever tip can be precisely loaded to the weighing part, driven by a piezoelectric moving stage with a distance of 100 μm, along with the observation of the contact of tip and the loading part in different directions. mounting to the clamp specially used in afm, the tip can be loaded very similar to the working condition in the afm. the structure of the measuring system is shown in figure 1. figure 1: the structure of the measuring system in figure 2, instead of using the electromagnetic compensation balance, the transfer standard cantilever is applied to the measuring system. it is http://www.imeko.org/ mailto:jiangjl@nim.ac.cn mailto:zhangye@nim.ac.cn mailto:wushi@nim.ac.cn mailto:zhiminzhang@nim.ac.cn mailto:hugang@nim.ac.cn mailto:huiyan9519@tju.edu.cn acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 130 calibrated before assembly. this structure is still under construction. so with the help of this measuring system the transfer standard cantilever will be able to be utilised to calibrate the afm tips without the complicated balance system. figure 2: design of the transfer standard test system the transfer standard of cantilever is designed and manufactured based on the mems fabrication of soi wafers. the effective lengths of the transfer standard are l0 = 8 mm, 7 mm, 6 mm, 5 mm, 4 mm, 3 mm, 2 mm, and 1 mm. the width of the transfer standard is w = 500 μm. the nominal thickness is t = 0.01 mm. this parameter is greatly influenced by the soi wafer selected before the manufacture. the nominal stiffness of the transfer standard is (0.041 ~ 21.125) n/m. in the test all the cantilevers were loaded in a cycle with 11 steps of force, repeated at least 10 times. in each step the loading was maintained for 30 s to obtain the stable force. the temperature range is 21 ℃ to 22 ℃. figure 3: design of the transfer standard and loading zone of transfer standard on the head of the tip the loading part is assembled with conical diamond probe tip used in nano-indentation with a tip diameter of 1 μm and the angle of 60º. this structure is for the test of the transfer standard. the typical test procedure is shown in figure 4. figure 4: the test process of one loading cycle comparing to the theoretical design of the transfer standard cantilever, the actual stiffness is lower. according to the comparison shown in figure 5, 10 % 20 % loss of the stiffness can be observed during the tests. especially at the low range of the stiffness, the difference between the designed and the actual stiffness is more obvious, causing an exponential factor smaller than 1 in figure 5 (bottom). figure 5: difference between designed and the measured value of the stiffness (top: fitted analysis of the stiffness, bottom: deviation from the designed stiffness) 3. uncertainty analysis the uncertainty analysis for the different type of the cantilevers was presented in references [12] and 0.5mm 0.01 mm 0 30 60 90 120 150 180 210 240 270 300 330 20 21 22 23 24 25 26 27 28 29 30 d e fo rm a ti o n (  m ) time （s） single loop of loading cycle 0 30 60 90 120 150 180 210 240 270 300 330 0 10 20 30 40 50 60 70 80 90 100 f o rc e (  n ) deformation force y = 16.77x-2.969 r² = 0.9991 y = 21.125x-3 r² = 1 0.01 0.1 1 10 100 0 1 2 3 4 5 6 7 8 9 s ti ff n e ss （ n /m ） nominal length l0 （mm） test results design fitting-test results fitting -design y = 0.8193x0.9894 r² = 0.9991 0.01 0.1 1 10 100 0.01 0.1 1 10 100 a c tu a l st if fn e ss （ n /m ） designed stiffness (n/m) http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 131 [13]. here the uncertainty of the stiffness measurement for the transfer standard due to the repeatability 𝑢1 is estimated by the standard deviation of the average value of measurement results in 10 cycles. the worst condition for the transfer standard is 0.97 × 10-2 throughout the whole test. the other aspect regarding to uncertainty are originated from force and displacement measurements, non-linearity of calibration curves, orientation of the cantilevers, location accuracy of the measurement point and the stiffness of the measurement chain. the uncertainty due to the force measurement is estimated in [12]. the uncertainty originated from force 𝑢2 is attributed to the uncertainty at the lower limit of the force. the displacements of the nano-positioning stage were measured by a laser interferometer over 100 μm with 20 steps of 5 μm. the standard uncertainty of the measurement results was 68.2 nm. the relative uncertainty due to the displacement measurement overall the whole range within the measurement, 𝑢3, is determined as 0.68 × 10 -4. derived from linear fit of force and corresponding displacement data, the residual of the linear fit is another source of the uncertainty, 𝑢4 , which is no more than 0.5 × 10-2. the stiffness of the cantilever is proportional to young’s modulus and the factors of cantilever dimensions. therefore, the location of the contact point between the cantilever and the tip of the weighing pan should be considered. for the pin shown in figure 3, the radius of the contact area is 3.3 µm. assuming that k = 2.58 (gaussian distribution), the relative standard uncertainty due to location accuracy 𝑢5 can be given by: 𝑢5 = 1 2.58 𝛿𝐿 𝐿 (1) where 𝛿𝐿 is the maximum position deviation from the ideal location. so 𝑢5 is calculated to be 1.28 × 10-3, where 𝛿𝐿 = 3.3 μm, 𝐿 = 1000 μm. the stiffness of the measurement chain 𝑘b was determined to be 1099.1 n/m [12]. assuming a rectangular distribution, the relative standard uncertainty due to the stiffness of measurement chain 𝑢6 is given by: 𝑢6 = 1 √3 | 𝑘m 𝑘b − 𝑘m | 𝛿𝑘b 𝑘b (2) where 𝛿𝑘b = 120 n/m is the maximum deviation of the stiffness of the measurement chain 𝑘b, and 𝑘m is the stiffness of the cantilever in each measurement. in the process of clamping the cantilever, we should also ensure that the balance probe and the central axis of the cantilever are in exact contact. thus, the torque of the cantilever is 0 and the error caused by the torsion of the cantilever is eliminated. however, in the actual operation process, it is difficult to ensure that the probe is just at the position of the central axis. it is known that the diameter of the probe used is 20 μm, the young’s modulus is 1140 gpa, and the poisson’s ratio is 0.07. the cantilever young’s modulus 𝐸 is about 150 gpa, its poisson’s ratio 𝜇 is about 0.22, and its shear modulus 𝐺 is given by: 𝐺 = 𝐸 2(1 + 𝜇) (3) when the probe just aligns with the image of the cross centre of the cantilever, the probe is now on the cantilever boundary line, from which it can be seen that its displacement from the central axis of the cantilever is about 240 μm. according to the knowledge of material mechanics, its torsion angle 𝜙 is given by: 𝜙 = 12𝑇𝑙 𝐺𝑤𝑡3 (4) the torque 𝑇 = 𝐹𝑤/2, where 𝐹 is the force on the cantilever under the current position. considering the maximum force 𝐹 of 200 μn in the test of the cantilever with lowest stiffness, along with the length of the cross in the loading area of the cantilever, the width 𝑤 cannot be larger than 15 μm, deflection angle 𝜙 = 0.032 rad, so the maximum uncertainty caused by torque angle is 0.13 × 10-3. the influence can be omitted. the combined standard uncertainty 𝑢kc is determined by: 𝑢kc = √∑ 𝑢𝑖 2 6 𝑖=1 2 (5) the relative standard uncertainty is less than 1.2 %. 4. conclusion the cantilever transfer standard was fabricated based on bulk process and its stiffness has been measured based on small force standard based on electromagnetic compensation. the relative uncertainty of the stiffness is smaller than 2.4 % (k = 2). 5. acknowledgement this research is sponsored by national key r&d program of china no.2018yff0212401. 6. references [1] g. binning, c. f. quate, c. gerber, “atomic force microsope”, phys. rev. lett., vol. 56, pp. 930-933, 1986. http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 132 [2] n. a. burnham, x. chen, c. s. hodges, et al, “comparison of calibration methods for atomicforce microscopy cantilevers”, nanotechnology vol. 14, pp. 1-6, 2003. [3] j. e. sader, j. w. m. chon, p. mulvaney, “calibration of rectangular atomic force microscope cantilevers”, review of scientific instruments vol. 70, pp. 3967-3969, 1999. [4] b. ohler, “cantilever spring constant calibration using laser doppler vibrometry”, review of scientific instruments, vol. 66, pp. 4583-4587, 1995. [5] s. j. chen, s. s. pan, “a force measurement system based on an electrostatic sensing and actuating technique for calibrating force in a micronewton range with a resolution of nanonewton scale”, meas. sci. technol. vol. 22 pp. 045104, 2011. [6] v. nesterov, “facility and methods for the measurement of micro and nano forces in the range below 10-5 n with a resolution of 10-12 n (development concept)”, meas. sci. technol. vol. 18, pp. 360-366, 2007. [7] m. s. kim, j. h. choi, et al, “accurate determination of spring constant of atomic force microscope cantilevers and comparison with other methods”, measurement, vol. 43 pp. 520-526, 2010. [8] m. s. kim, j. pratt, u. brand, c. w. jones, “report on the first international comparison of small force facilities: a pilot study at the micronewton level”, metrologia, vol. 49, pp. 70-81, 2012. [9] c. a. clifford, m. p. seah, “the determination of atomic force microscope cantilever spring constants via dimensional methods for nanomechanical analysis”, nanotechnology, vol. 16, pp. 1666-1680, 2005. [10] j. m. neumeister, w. a. ducker, “lateral normal and longitudinal spring constants of atomic force microscopy cantilevers”, rev. sci. instrum., vol. 65, pp. 2527-2531, 1994. [11] m. s. kim, j. h. choi, j. h. kim, “si-traceable determination of spring constants of various atomic force microscope cantilever with a small uncertainty of 1 %”, meas. sci. technol., vol. 18, pp.3351-3358, 2013. [12] g. hu, j. jiang, z. zhang, y. zhang, u. brand, m. s. kim, “investigation of a small force standard with the mass based method”, acta imeko, vol. 6, no. 2, pp.13‐20, 2017. [13] g. hu, l. song, f. meng, w. zhang, z. zhang, y. zhang, y. zheng, “research and development of small force standards at nim”, measurement of mass, force and torque (apmf 2013) international journal of modern physics: conference series, vol. 24, pp. 1360020-1-9, 2013. [14] j. wang, p. fuchs, s. russi, et al, “uncertainty evaluation for system of weighing equations for the determination of microgram weights”, ieee transactions on instrumentation and measurements, vol. 64, pp. 2272-2279, 2015. http://www.imeko.org/ validation of combinatorial evaluation of strain-gauge amplifier linearity acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 205 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 205 209 validation of combinatorial evaluation of strain-gauge amplifier linearity m. hiti1 1 slovenian national building and civil engineering institute (zag), ljubljana, slovenia, miha.hiti@zag.si abstract: this paper describes a validation of a combinatorial calibration technique based calibration procedure for strain-gauge amplifier calibration. the deviation between two strain-gauge amplifiers at calibration using the combinatorial technique is compared to the deviation between the same two amplifiers at calibration on a force calibration machine as a transducer-amplifier chain under unchanged conditions, serving as an amplifier linearity comparator. this enables the validation to be confirmed with a suitable expanded measurement uncertainty of 3 nv/v. keywords: strain-gauge amplifier; calibration; combinatorial technique; validation 1. introduction the calibration of strain-gauge amplifiers (bridge amplifiers) used in measuring chains for force, torque and pressure measurements is potentially subjected to large measurement uncertainties when calibrated with traditional voltage ratio (mv/v) standards – bridge standards. in previous publication [1] we presented an alternative calibration procedure which reduces the calibration uncertainty to a more acceptable level by calibrating the bridge amplifier at a single reference point with a traditional bridge standard and additionally performing linearity evaluation between the relative 0 mv/v and the calibrated reference point using the zag lincheck combinatorial system [2]. this procedure has already been offered to customers as a calibration service within iso/iec 17025 accredited scope with an expanded relative measurement uncertainty of 3 nv/v or 0.003 % (whichever is higher) in the range from 0.05 mv/v to 2.5 mv/v for positive and negative voltage ratios, covering the typically required range, figure 1. one of the requirements at assessment of a calibration laboratory’s competence regarding accredited services is the validation of calibration procedures. while the combinatorial procedure has been reproduced also by other laboratories to verify the linearity of amplifiers with comparable results [3], the validation of the procedure by comparison with a typical best-available calibrated voltage ratio standards is not sufficient. the measurement uncertainty of the latter exceeds the potentially achievable measurement uncertainty of the combinatorial calibration procedure even after the best available calibration procedure for bridge standard calibration has been improved, reducing the expanded uncertainty for bridge standard calibration from 10 nv/v to 5 nv/v (for k = 2) [4]. lower uncertainties for linearity evaluation were not possible until recently, when a cascaded inductive voltage divider setup was presented for bridge amplifier linearity evaluation, achieving expanded uncertainty of 2 nv/v [5] and offering 107 steps within the ±5 mv/v range. figure 1: expanded measurement uncertainty for bridge amplifier calibration with traditional bridge standards calibration and combinatorial procedure (left) and expressed as relative expanded uncertainty (right) http://www.imeko.org/ mailto:miha.hiti@zag.si acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 206 a direct comparison of the combinatorial procedure with the newly presented unique cascaded inductive voltage divider setup could provide the reference needed for the validation, but in this paper we present a different approach: to use a force calibration machine as a relative standard for voltage ratio linearity comparison. force calibration machines are widely available at nmis and calibration laboratories and offer expanded measurement uncertainties of generated force down to 0.001 %. when calibration of a single force transducer is repeated with different bridge amplifiers under unchanged conditions, the amplifier linearity could be compared by analysing the calibration results, and thus acting as a comparator for relative bridge linearity evaluation. 2. description of the work previous investigation of calibration of the same transducer with different amplifiers has shown that the agreement between comparable amplifiers of the same type and supply voltage can be within 0.003 % [6], figure 2. figure 2: results of deviation of transducer calibration with different amplifiers compared to an hbm dmp41 (amplifier sensitivity errors corrected) in these investigations, the same force transducer (1 kn) was calibrated with different amplifiers: 225 hz carrier frequency type amplifiers hbm dmp41, hbm ml38b and hbm dk38, 4.8 k hz carrier frequency type amplifier hbm ml55b and a dc excitation type amplifier hbm ml10b. each of these amplifiers was sequentially connected to the same transducer and measurements were performed in the same 1 kn dead-weight force calibration machine (fcm) with < 0.002 % reproducibility (the accredited cmc of the force calibration machine is 0.005 %, but this value includes many uncertainty contributions which are irrelevant in the case of comparative measurements). the transducer and the force calibration machine remained in unchanged configuration, as the aim of this investigation was to compare the effect of different amplifiers on the calibration result. measurements with one ml38b amplifier were done at the beginning of the investigation, at the end of the investigation, and between measurements with other amplifiers to verify the stability and reproducibility of the measurement setup and measurement procedure itself. before each measurement series, each amplifier was calibrated by a calibrated bridge standard hbm k3608 with expanded measurement uncertainty of 20 nv/v for the 225 hz carrier frequency, 330 nv/v for 4.8 khz carrier frequency, and 200 nv/v for dc excitation voltage. the deviation of resulting characteristics in figure 2 shows the indication deviation of each amplifier from a reference characteristic in this case the characteristic of dmp41 amplifier. but these measurements offered also the possibility to compare the linearity of the amplifiers, as the relevant information can be extracted from the results. results suggested that the amplifier linearity could be very good, as can typically also be observed by calibration with voltage ratio standards, albeit with a high measurement uncertainty (> 20 nv/v), and the much lower relative uncertainty of the force measurement setup could help to prove these assumption. figure 3: measurement setup with 1 kn force transducer positioned in 1 kn force calibration machine with hbm dmp41 amplifier and hbm mgcplus system with ml38b amplifier. a hbm k3608 bridge standard and a zag lincheck system for combinatorial calibration are also shown. based on these results the measurements with the 1 kn hbm z30a-top transducer in the 1 kn dead-weight force calibration machine were repeated for the hbm dmp41 and hbm ml38b amplifiers under more stringent conditions with the aim to investigate the comparability of linearity of these amplifiers, figure 3. http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 207 figure 4: deviation of two different amplifiers from linear characteristics (0 mv/v – 2 mv/v) for the same force transducer in the same fcm – results normalized to 2 mv/v for the best possible reproducibility of the force steps, the force transducer was positioned in a single rotational position, its position was not changed during the whole process, and the loading frame of the force calibration machine (the first 10 n weight) was never unloaded to keep the force introduction as repeatable as possible. this way most uncertainty sources arising from the force calibration machine and the force transducer have been significantly reduced or eliminated, such as the effects of machine and transducer alignment, reproducibility of force transducer in different transducer rotations and the effect of change in load application. by eliminating most uncertainty sources, the uncertainty of force results mainly from the repeatability of the relative force steps. the contributions of mass and gravitational field only influence the process as relative terms during the short measurement period and their exact absolute values do not even have to be known. to reduce the effect of force transducer creep, the force calibration machine was controlled in automatic mode with constant time intervals between each measurement and the same pre-defined loading profile for all measurements. both amplifiers were constantly powered on and a dummy load was used to simulate the transducer load when the actual 1 kn force transducer was occupied by the other amplifier, to keep the operating conditions of the amplifiers as constant as possible. amplifier parameters were set to the same values on both amplifiers: 5 v excitation voltage, 0.1 hz bessel filter settings. the selected time interval between measurements was 60 s. the final measurements were done sequentially within two hours. measurements were performed in the range from 10 n to 990 n in an ab-b-a sequence (dmp41-ml38b-ml38bdmp41). the same amplifiers were later calibrated also with the zag lincheck system for combinatorial calibration. 3. results the results of calibration of the 1 kn transducer with the dmp41 and the ml38b are shown in figure 4, where the characteristic sensitivities are normalized to 2 mv/v. the data obtained from the calibration of the amplifiers in the fcm includes also the transducer nonlinearity contribution, which is in this case unknown, so the nonlinearity of the amplifier cannot be simply extracted from the results. however, the difference in nonlinearity results can be used as a reference for the comparison as it should depend mainly on the amplifier, if other system parameters are kept constant. it can be seen in figure 4 that both measured dmp41 series and both ml38b series are in good agreement, with the largest deviation between all series being 3 nv/v (at 1.2 mv/v output ratio). the deviation between average ml38b and average dmp41 characteristics is shown in figure 5 as absolute and relative deviation. figure 5: deviation between average ml38b and average dmp41 as absolute deviation (left) and relative deviation (right). standard uncertainty of the deviation is also shown with error bars (contributions of repeatability and resolution). http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 208 figure 6: linearity evaluation of deviation of two different amplifiers from linear characteristics (0 mv/v to 2 mv/v) with the zag lincheck system. results adjusted at 2 mv/v. the result suggests that the linearity of the amplifiers agrees for the most part, with deviations of less than 2 × 10-5. for the uncertainty attributed to the measured deviation, contributions of the repeatability of two series and contributions of amplifier resolution were taken into account. it is within 2 nv/v or up to 5 × 10-5 for the lowest ratios. immediately after the linearity measurements on the force calibration machine, both amplifiers were also evaluated with the zag lincheck system employing the combinatorial method. the amplifier parameters were the same as during measurements on the force calibration machine, but the evaluated range was in this case up to 2.5 mv/v, fehler! verweisquelle konnte nicht gefunden werden.. the resulting characteristics are shown with standard uncertainty of nonlinearity determination of 0.9 nv/v for dmp41 and 1.2 nv/v for ml38b. the nonlinearity results of both amplifiers have been adjusted to a 2 mv/v reference point. for comparison with linearity results obtained by evaluation of the amplifiers on the force calibration machine, the difference between ml38b and dmp41 results was calculated, shown in figure 7 as absolute and relative difference. the determined nonlinearity difference of the two amplifiers using combinatorial method up to 2 mv/v is within 2 nv/v, below 2 × 10-6 if expressed as relative deviation, with standard uncertainty up to 7.5 × 10-5. figure 7: deviation between ml38b and dmp41 as absolute deviation (left) and relative deviation (right) measured by employing combinatorial method (zag lincheck). standard uncertainty of the deviation is also shown with error bars. figure 8: comparison of nonlinearity deviation between ml38b and dmp41 determined in a fcm (fcm) and by combinatorial method (comb). standard uncertainty of the deviation is shown with error bars. http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 209 the comparison of both methods of linearity evaluation is shown in figure 8. the figure shows the deviation in nonlinearity difference of both amplifiers, once obtained from calibration in the fcm and once obtained by combinatorial calibration. by comparing the results from both methods, it can be seen that the two methods agree to within their respective standard uncertainties. 4. discussion the evaluation of the linearity of amplifiers with different methods produced comparable results. as the combinatorial method has already been demonstrated to produce comparable results to traditional linearity evaluation with bridge standards, with large uncertainty of the latter, the additional linearity comparison via force calibration machine confirms the suitability of the combinatorial calibration procedure for amplifier calibration. the expanded measurement uncertainty of the comparison was 3 nv/v. this confirmation suggests, that it is possible to realize the linearity evaluation of 225 hz carrier frequency amplifiers with the zag lincheck system to within 2 nv/v expanded measurement uncertainty. at the same time it also confirms the comparability of linearity determination by transducer calibration in a fcm if an amplifier with known linearity is available. in combination with force calibration machines offering low expanded uncertainties of generated force, the aim is to further improve the amplifier calibration to be able to subtract the amplifier from the calibration result of the force transducer under calibration, specifying only the transducer characteristics. 5. summary this paper presents an example of a procedure to validate the combinatorial evaluation procedure for strain-gauge amplifier linearity evaluation. by using a single force calibration machine and a single force transducer with various bridge amplifiers, the force calibration system acts as a comparator to compare the linearity of amplifiers. the lower uncertainty of generated force enables better measurement uncertainties than offered by traditional bridge calibrators and is in line with the requirements for the combinatorial system (3 nv/v). the proposed procedure is suitable for the confirmation of performance of the combinatorial calibration procedure and should enable the validation and application of the procedure in iso/iec 17025 accredited calibration laboratories. 6. references [1] m. hiti, “reducing the uncertainty of strain gauge amplifier calibration”, acta imeko, vol. 6, no. 4, pp. 69-74, 2017. online [accessed 20200821]: https://acta.imeko.org/index.php/actaimeko/article/view/imeko-acta06%20%282017%29-04-11/pdf [2] m. hiti, “resistor network for linearity check of voltage ratio meters by combinatorial technique”, meas. sci technol., vol. 26, no. 5, 2015. [3] m. m. schäck: “long term proven and optimized high-precision 225 hz carrier frequency technology in a modern and universal data acquisition system”, proc. imeko 23rd tc3, 13th tc5 and 4th tc22 international conference, helsinki, finland, 30 may to 1 june 2017. [4] m. f. beug, a. kölling, h. moser, “a new calibration transformer and measurement setup for bridge standard calibrations up to 5 khz”, ieee transactions on instrumentation and measurement, vol. 66, no. 6, pp. 1531-1538, june 2017. [5] m. f. beug, h. moser, a. kölling: “bridge amplifier linearity investigation with a cascaded inductive voltage divider setup”, j. phys.: conf. ser. 1065 042010, 2018. [6] m. hiti: “analysis of exchanging of measuring amplifiers on force transducer calibration results”, erk 2017, portorož, slovenia, 25 26 september 2017. http://www.imeko.org/ https://acta.imeko.org/index.php/acta-imeko/article/view/imeko-acta-06%20%282017%29-04-11/pdf https://acta.imeko.org/index.php/acta-imeko/article/view/imeko-acta-06%20%282017%29-04-11/pdf https://acta.imeko.org/index.php/acta-imeko/article/view/imeko-acta-06%20%282017%29-04-11/pdf analysis on influence of static calibration on the axle-group weigh-in-motion system accuracy acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 69 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 69 74 analysis on influence of static calibration on the axle-group weigh-in-motion system accuracy lai zhengchuang1,2, yang xiaoxiang1,3, yao jinhui1,2 1 college of mechanical engineering and automation, fuzhou university, fuzhou, china, lzcfjjl@126.com 2 fujian key laboratory of force measurement (fujian institute of metrology), fuzhou, china, lzcfjjl@126.com 3 quanzhou normal university, quanzhou, china, yangxx@fzu.edu.cn abstract: the axle-group weigh-in-motion system has two functions: static weighing and dynamic weighing. according to the weighing model, the accuracy of dynamic weighing is affected by the static performance. this paper analyses the size of various factors affecting the static performance, such as sensor tilt installation, platform deformation, platform tilt installation, and these errors will lead to sensor swing, bearing head tilt, gravity line of action and sensor axis direction is not consistent, thus affecting the static weighing accuracy. however static calibration is the best way to reduce or even eliminate the above errors. the dynamic truck scale of different manufacturers with or without static calibration is used in the test process. the results show that the dynamic performance index can meet the requirements only after the static calibration is used. keywords: static calibration; axle-group weigh-in-motion; dynamic weighing; weighing accuracy 1. introduction the axle-group weigh-in-motion system consists of static truck scale platform and dynamic weighing technology, mainly including load carrier (scale platform), strain type weighing sensor, junction box and weighing instrument [1]. its quality model can be expressed as follows: 𝑦(𝑡) = 𝑊 + ∑ 𝐴𝑖 sin(𝑊𝑖𝑡 + 𝜗𝑖)𝑖 (1) where 𝑊 is the weighing value and 𝐴𝑖, 𝑊𝑖 and 𝜗𝑖 are the amplitude, frequency and phase of interference signal. 𝑊 is the basis of dynamic weighing data and determines the basic weighing accuracy. its model is expressed as: 𝑊 = 𝑘𝑛𝑣, 𝑊𝑠 = 𝑘𝑛𝑣 ∙ (∑𝜔𝑖𝑚𝑖 𝑁 𝑖=1 ) (2) where 𝑁 is the number of support points, 𝑚𝑖 is the load value (kg) allocated to each load cell in the static mode and 𝜔𝑖 the weight coefficient allocated to each load cell. 𝑛𝑣 is the running speed and 𝑘𝑛𝑣 is the quality conversion coefficient between the static part and the dynamic part based on the data processing algorithm in the 𝑛th running speed. 𝑊𝑠 is the static weighing data and also the only parameter that can control the error in the calibration process of the above model, which plays an important role in the accuracy of the dynamic weighing instrument. therefore, the static weighing performance needs to be guaranteed in the calibration. according to the full-bridge method of tension and compression elastic elements, theoretically the output strain value of the sensor is only related to the magnitude of the axial force 𝐹𝑛 along the strain direction of the elastic body axis, and is not related to the tangential force and the additional bending moment generated by the tangential force 𝐹𝜏 [2]. the relative error of the measurement is: 𝑒 = 𝐹𝑛 − 𝐹𝑁 𝐹𝑁 = cos(𝛼1 + 𝛼2 + 𝛼3) − 1 (𝑖 = 1,2,3,4) (3) where 𝛼1 is the inclination angle of the upper indenter, 𝛼2 is the inclination angle of the sensor strain elastomer axis and 𝛼3 is the angle between the external force direction and the axis of the upper indenter. 2. analysis of the factors influencing the accuracy of static weighing 2.1. the impact of the tilt installation of the column load cell on the weighing accuracy the axle group dynamic truck scale supported by four points is selected as the research object. when the supported sensors have different angles of http://www.imeko.org/ mailto:lzcfjjl@126.com mailto:lzcfjjl@126.com mailto:yangxx@fzu.edu.cn acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 70 rotation, the weighing platform has a horizontal frictional force on the sensors, as shown in figure 1. the sensor strains the elastomer to roll purely between the upper and lower indenters, taking the contact point when the elastomer is not deflected as the reference point, and when the elastomer deflects by angle 𝛼21 , the horizontal displacement of the upper indenter can be expressed as follows. 𝑆21 = 2𝑅𝛼21 − (2𝑅 − ℎ)sin𝛼21 (4) where 𝛼21 is the inclination of the axis of the strained elastomer and 𝑆21 is the horizontal displacement of the contact point caused by the deflection of the elastomer. meanwhile, there is the following geometric relationship: 𝑂𝐻 = (2𝑅 − ℎ)sin𝛼21 (5) 𝐻𝐾 = 2𝑅 − (2𝑅 − ℎ)cos𝛼21 (6) assume that the two sensors on the left have equal installation angles and the two sensors on the right are equal. when the deflection angle of the elastomer is small, the rolling friction resistance is less than the maximum rolling friction resistance couple, and the weighing platform does not produce translation, so the friction force of the weighing platform against the elastomer is zero. when the inclination of the elastomer is greater than the critical angle, the weighing platform has a tendency to move in translation under the action of the restoring force, and the weighing platform generates friction against the elastomer. suppose the initial angular displacements during installation are 𝛼01 and 𝛼02, respectively, and the state after loading is shown in figure 1. figure 1: supporting structure diagram of tilt installation of column load cell taking the force direction shown in figure 1 as the positive direction. according to the moment balance relationship between the elastomer and the support point o, we can obtain: 𝐹𝑁1(2𝑅 − ℎ)sin𝛼21 − 2𝑀𝑓1 − 𝐹𝑆1[2𝑅 − (2𝑅 − ℎ)cos𝛼21] = 0 (7) 𝐹𝑁2(2𝑅 − ℎ)sin𝛼22 − 2𝑀𝑓2 − 𝐹𝑆2[2𝑅 − (2𝑅 − ℎ)cos𝛼22] = 0 (8) 𝐹𝑆1 = 𝐹𝑆2 (9) with 𝐹𝑁1 = 0.25𝑀𝑔 + 0.5𝐹(𝑙 − 𝑎) 𝑙⁄ and 𝐹𝑁2 = 0.25𝑀𝑔 + 0.5𝐹𝑎 𝑙⁄ . when the angle is small, cos𝛼21 = cos𝛼22 ≈ 1. the deflection of the elastomer causes the horizontal displacement of the elastomer and the left and right reference points of the weighing platform to be equal. 𝑆21 − 𝑆01 = −(𝑆22 − 𝑆02) (10) 𝑆01 and 𝑆02 are the horizontal displacement of the reference point caused by the sensor deflection during initial installation. when the initial installation tilt angle is satisfied: (𝑅 − 0.5ℎ)sin𝛼01 ≤ 𝛿 and (𝑅 − 0.5ℎ)sin𝛼02 ≤ 𝛿 , the torque on the elastomer is not enough to overcome the rolling friction couple, and the weighing platform does not produce friction. when the inclination angle of the sensor on one side is greater than the critical angle, the elastomer has a rolling tendency, and the weighing platform generates horizontal frictional force. the value and direction of the rolling friction couple on both sides of the elastomer depend on the rolling tendency. suppose that the inclination angle of the right elastomer satisfies: (𝑅 − 0.5ℎ)sin𝛼02 > 𝛿 , and the load is continuously increased, so that the rolling friction of the elastic bodies on both sides reaches the maximum rolling frictional couple, that is, 𝑀𝑓2 = 𝛿𝐹𝑁2. when the torque is greater than the maximum rolling friction couple, the elastomer will turn to the left until a new equilibrium state is formed. at this time, the rolling friction of the elastic bodies on both sides is equal to the maximum rolling friction couple. from equations (7)-(9), we can obtain: 𝐹𝑆2 = 2𝐹𝑁2[(𝑅 − 0.5ℎ)sin𝛼22 − 𝛿] ℎ (11) (𝑅 − 0.5ℎ)(𝐹𝑁1 sin𝛼21 − 𝐹𝑁2 sin𝛼22)+ 𝛿(𝐹𝑁2 + 𝐹𝑁1) = 0 (12) (2𝑅 − ℎ)(sin𝛼21 + sin𝛼22 − sin𝛼01 − sin𝛼02) = 2𝑅(𝛼21 + 𝛼22 − 𝛼01 − 𝛼02) (13) we can obtain the value of rotation angle of elastomer 𝛼21 and 𝛼22 by simultaneous equations (12) and (13). according to the above inference, we found that the rotation angle of the elastomer has nothing to do with their respective inclination angles before being loaded but is related to the sum value of the inclination angles. during the loading process, the sum of the inclination angles remains unchanged. http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 71 when the sensor elastomer generates the abovementioned rotation angle, the relative error magnitude of the weighing result of each sensor can be obtained by equation (3). after loading, the inclination of the elastomer will not restore the state before loading and will affect the subsequent weighing. in addition, the presence of friction also reduces the weighing value. 2.2. deformation of the weighing platform affect on the weighing accuracy if the rigidity of the weighing platform is small, and the external load is large, the weighing platform will produce excessive deformation. this deformation will cause the angular displacement of the elastomer and affect the weighing accuracy, as shown in figure 2. figure 2: schematic diagram of load-bearing deformation of weighing platform when the weighing platform is loading, the distance between the contact points of the elastic bodies on both sides changes due to the deformation of the weighing platform, and the elastic bodies on both sides will deflect in the opposite direction. when the rotation reaches equilibrium, the contact point of the elastomer reaches the maximum rolling friction couple, as shown in figure 3. according to the loading and deformation process figure 3: loading model of column load cell when the weighing platform is deformed of the weighing platform, the direction of the rolling friction of the elastomer is the opposite direction when the figure is 0 < 𝑎 < 𝑙 2⁄ , and the direction is the same as the figure when 𝑙 2⁄ < 𝑎 < 𝑙. according to the moment balance relationship of the elastomer to the fulcrum o, we can obtain: 𝐹𝑁1[(2𝑅 − ℎ)sin𝛼21 + 𝑅sin𝛼11]− 2𝑀𝑓1 − 𝐹𝑆1ℎ = 0 (14) 𝐹𝑁2[(2𝑅 − ℎ)sin𝛼22 + 𝑅sin𝛼12]− 2𝑀𝑓2 − 𝐹𝑆2ℎ = 0 (15) 𝐹𝑆1 = 𝐹𝑆2 (16) the bending deformation of the neutral layer of the weighing platform reduces the horizontal distance of the neutral axis position of the original support point cross section by 𝑠1 + 𝑠2. according to the displacement relationship of the initial contact point, a supplementary equation is obtained. 2𝑅(𝛼21 + 𝛼22)− (2𝑅 − ℎ)(sin𝛼21 + sin𝛼22) = (ℎ2 + ℎ3)(sin𝛼11 + sin𝛼12)− (𝑠1 + 𝑠2) (17) from equations (14)-(17), the relationship between the angle of the elastomer and the magnitude and position of the load can be obtained. treat the weighing platform as a simply supported beam supported on both sides. ignore the initial deformation of the weighing platform and the rolling friction couple of the elastomer. { 𝛿𝑚 = 𝐹𝑎(2𝑎𝑙 − 𝑎2)3 2⁄ 9√3𝑙𝐸𝐼 (0 ≤ 𝑎 ≤ 𝑙 2 ) 𝛿𝑚 = 𝐹(𝑙 − 𝑎)(𝑙2 − 𝑎2)3 2⁄ 9√3𝑙𝐸𝐼 ( 𝑙 2 ≤ 𝑎 ≤ 𝑙) (18) { 𝛼11 = 𝐹(𝑙 − 𝑎)(2𝑎𝑙 − 𝑎2) 6𝑙𝐸𝐼 (0 ≤ 𝑎 ≤ 𝑙 2 ) 𝛼11 = 𝐹(𝑙 − 𝑎)(𝑙2 − 𝑎2) 6𝑙𝐸𝐼 ( 𝑙 2 ≤ 𝑎 ≤ 𝑙) (19) { 𝛼12 = 𝐹𝑎(2𝑎𝑙 − 𝑎2) 6𝑙𝐸𝐼 (0 ≤ 𝑎 ≤ 𝑙 2 ) 𝛼12 = 𝐹𝑎(𝑙2 − 𝑎2) 6𝑙𝐸𝐼 ( 𝑙 2 ≤ 𝑎 ≤ 𝑙) (20) { 𝑥 = 𝑙 − √ 𝑙2 − 𝑎2 3 (0 ≤ 𝑎 ≤ 𝑙 2 ) 𝑥 = √ 𝑙2 − (𝑙 − 𝑎)2 3 ( 𝑙 2 ≤ 𝑎 ≤ 𝑙) (21) 𝜃𝑚 = arcsin 𝛿𝑚 𝑥 (22) http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 72 𝑠1 = 𝑥(1 − cos𝜃𝑚) (23) 𝑠2 = (𝑙 − 𝑥)(1 − cos𝜃𝑚) (24) where 𝛿𝑚 is the maximum deflection of the weighing platform, 𝐸𝐼 is the bending stiffness of the weighing platform beam, 𝑙 is the supporting length of the weighing platform, 𝑎 is the horizontal distance from the load to the left end support point, 𝑥 is the horizontal distance from the position of 𝛿𝑚 to the left end support point, 𝑠1 is the left side horizontal displacement of the neutral layer at the cross section of the supporting point, 𝑠2 is the horizontal displacement of the neutral layer at the cross section of the right supporting point. as can be seen from figure 4, when the load begins to move away from the support point, the inclination angle of the upper head increases, and decreases slowly when approaching the centre, and quickly decreases to zero after passing the centre. figure 4: curve of inclination of upper indenter on left elastomer figure 5: curve of left elastomer inclination as can be seen from figure 5, when the load begins to move away from the support end, the elastomer of the support end deflects to the right by a small angle first, and then deflects to the left. the extreme point is on the other side of the midpoint, and then decrease to zero. at the same time, the greater the load, the greater the tilt angle of the upper head and the outward rotation angle of the sensor elastomer. when the sensor elastomer generates the above-mentioned rotation angle, the relative error magnitude of the weighing result of each sensor can be obtained by equation (3). 2.3. the influence of tilting installation of weighing platform on the weighing accuracy when the weighing platform is installed obliquely, the gravity of the weighing platform to the column load sensor will deviate from the axis direction of the strained elastomer. due to the rolling friction between the strained elastomer and the upper and lower indenters, the strained elastomer will change from a static state to a rolling state as the tilt angle of the weighing platform increases, as shown in figure 6. figure 6: weighing platform tilt installation when the tilt angle of the weighing platform exceeds the critical angle, the moment of the weighing platform against the gravity of the sensor exceeds the maximum rolling friction of the elastomer, and the elastomer produces a tilt angle. the tilt angle of the elastomer is approximately linearly related to the tilt angle of the weighing platform. under the same tilt angle of the weighing platform, the greater the height of the elastomer, the smaller the radius of the ball head, and the greater the tilt angle of the elastomer. when the sensor elastomer produces the above-mentioned rotation angle, the relative error magnitude of the sensor weighing result can be obtained from equation (3). in addition, the elevation difference of the sensor installation and the non-linear characteristics of the sensor itself have an influence on the result of the weighing. 3. the calibration situation of axle group wim system in section 2, we analysed the main factors affecting the weighing accuracy, such as tilt installation of the column load cell, stiffness and tilting installation of the weighing platform, the elevation difference of the sensor installation and the non-linear characteristics of the sensor itself. these influencing factors will cause the output of the wim system to show obvious nonlinearity, as shown in figure 7. line 1 is the ideal output of wim, line 2 is the output after static calibration, line 3 is the output before static calibration. according to equations (1) and (2), 𝑊𝑠 is the static weighing value, which affect the dynamic output directly. therefore, it is necessary to carry out static calibration to ensure static accuracy, in order to ensure the results of dynamic weighing. http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 73 real weight o ut pu t o f w im 0 1 2 3 figure 7: principle of static calibration influence on dynamic accuracy 4. experiment in order to verify the effect of static calibration on the dynamic weighing results. three different manufacturers’ axle group weigh-in-motion instruments (factory a, factory b, factory c) were selected to be calibrated by means of three methods respectively. first method: without any static calibration method (no calibration). second method: only using the reference vehicle calibrate the total weight of the tested weighing instrument (single-point calibration). third method: the static calibration of full scale and full performance are carried out by load measurement apparatus according to regulations (static full-scale calibration), as shown in figure 8. figure 8: static calibration test figure 9: dynamic weighing test after static calibration, we chose 2-axle rigid, 3-axle rigid, 4-axle articulated trailer and 6-axle articulated trailer at a speed of 5 km/h to test the dynamic weighing result, as shown in figure 9. none single-point static full-scale -1.0% -0.5% 0.0% 0.5% 1.0% 1.5% 2.0% 2.5% 3.0% t o ta l v e h ic le w e ig h in g e rr o r calibration method 2-axle truck 3-axle truck 4-axle truck 6-axle truck (a) none single-point static full-scale -5.0% -4.0% -3.0% -2.0% -1.0% 0.0% 1.0% t o ta l v e h ic le w e ig h in g e rr o r calibration method 2-axle truck 3-axle truck 4-axle truck 6-axle truck (b) none single-point static full-scale -3.0% -2.0% -1.0% 0.0% 1.0% 2.0% 3.0% 4.0% 5.0% t o ta l v e h ic le w e ig h in g e rr o r calibration method 2-axle truck 3-axle truck 4-axle truck 6-axle truck (c) figure 10: test results of dynamic weighing; (a) test result of factory a, (b) test result of factory b, (c) test result of factory c the testing result of the three manufacturers’ axle group weigh-in-motion are shown in figure 10 (a), (b) and (c). it can be seen from figure 10, only when the static measurement performance fully meets the requirements of medium accuracy truck scales, the error of the dynamic part is (+0.1 ~ +0.4) %, it can meet the legal error of http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 74 ±0.5 %. if any index of partial load or weighing fails in the static measurement performance, the error of the dynamic part is (-5.0 ~ +4.2) %, which far exceeds the legal maximum allowable error of ±0.5 %, and cannot meet the level 1 dynamic road vehicle requirements for automatic weighing instruments. 5. summary there are many factors that affect the static weighing accuracy of the axle group weigh-inmotion system, including sensor tilt, scale platform deformation, scale platform tilt sensor height difference and other factors, and the static weighing data is the decisive data of the dynamic weighing accuracy. when dynamic verification is carried out, the static performance calibration must be carried out first. the test data show that the accuracy of dynamic weighing can meet the error requirements of regulations only when all indexes of static performance meet the requirements. 6. references [1] r. bajwa, e. coleri, r. rajagopal, p. varaiya, c. flores, “development of a cost-effective wireless vibration weigh-in-motion system to estimate axle weights of trucks”, computer-aided civil and infrastructure engineering, vol. 32, no. 6, pp. 443-457, 2017. [2] j. richardson, s. jones, a. brown, e. o’brien, d. hajializadeh, “on the use of bridge weigh-inmotion for overweight truck enforcement”, international journal of heavy vehicle systems, vol. 21, no. 2, pp. 83-104, 2014. [3] a. t. papagiannakis, “calibration of weigh-inmotion systems through dynamic vehicle simulation”, journal of testing and evaluation, vol. 25, no. 2, pp. 197-204, 1997. [4] u. sonmez, n. sverdlova, r. tallon, d. j. klinikowski, d. streit, “static calibration methodology for weigh-in-motion systems”, international journal of heavy vehicle systems, vol. 7, no. 2-3, pp. 191-204, 2000. [5] p. lou, h. nassif, d. su, p. truban, “effect of overweight trucks on bridge deck deterioration based on weigh-in-motion data”, transportation research record, vol. 2592, pp. 86-97, 2016. http://www.imeko.org/ digital strategies for the valorisation of archival heritage acta imeko issn: 2221-870x march 2021, volume 10, number 1, 224 233 acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 224 digital strategies for the valorisation of archival heritage roberta spallone1, giulia bertola2, francesca ronco3 1 politecnico di torino, department of architecture and design, viale mattioli 39, 10125, torino, italy 2 modlab arch, viale mattioli 39, 10125, torino, italy 3 modlab design, corso settembrini 178, 10135 torino, italy section: research paper keywords: architectural archives; structure from motion; 3d modelling; bim modelling; augmented reality citation: roberta spallone, giulia bertola, francesca ronco, digital strategies for the valorisation of archival heritage, acta imeko, vol. 10, no. 1, article 30, march 2021, identifier: imeko-acta-10 (2021)-01-30 editor: ioan tudosa, university of sannio, italy received may 15, 2020; in final form november 6, 2020; published march 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: roberta spallone, e-mail: roberta.spallone@polito.it 1. introduction the considerable amount of materials, predominantly design drawings, held in 20th century architectural archives are among the cultural heritage objects currently in need of conservation, valorisation and enhancement. many international scholars are involved in the interpretation, fruition and sharing of archival materials with the aim of deepening the knowledge of contemporary masters and architectural movements. the safeguard of these archives depends largely on the discovery, analysis, knowledge and interpretation of their value as strongholds of cultural heritage. architectural archives were established relatively recently and given particular emphasis only from the end of the 1970s onwards. today, in the digital era, they are the object of new investigations. in accordance with the opinion of aleardi, we believe that the future of these archives lies in the creation of new forms of documentation for the architectural objects of which they are the narration. aleardi further observes that, since archival documents frequently represent architectural objects that no longer exist, those that were imagined but never created or those that are distant, they therefore cannot convey the same power as direct, kinaesthetic experience [1]. since the beginning of the 21st century, the digitisation of the physical documents kept in the archives has become one of the best practices for preservation, as it reduces the direct manipulation of the sources, and communication, as it enables remote viewing of the original materials. digitisation has been performed with information and communications technology (ict) tools, which have enabled the low cost scanning of documents and the sharing of sources to a wider public. nevertheless, the simple digitisation of archival sources seems insufficient for transmitting the same degree of knowledge and meaning that is possible through on-site consultation, especially when it comes to design drawings and physical models. in the case of drawings, the reduction in the scale and resolution of digital documents to enable consultation through screens smaller than the originals compromises the analysis and interpretation. to this is added the loss of the three-dimensional, material and tactile qualities of physical models. moreover, the knowledge gained from the analysis of archival documents still needs to be interpreted, represented and communicated. contemporary architectural archives could be defined as being on the borderline of cultural heritage: in fact, they abstract 20th century pre-digital architectural archives are a current topic of interest for scholars involved in historical studies as well as those studying conservation, valorisation and communication. these vast archival heritages could be enhanced by the methodologies, techniques and tools offered by the digital revolution. the present proposal demonstrates this potential through the application of structure from motion techniques to a physical scale model that represents the turin horse racing designed by carlo mollino. moreover, the paper presents a comparison of different software, methodologies and modelling tools and suggests a design for virtual and on-site communication strategies. these strategies fuel new interrelations of knowledge and ideas, through which an archive can become a place of convergence between the real and the virtual. mailto:roberta.spallone@polito.it acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 225 represent both documental and cultural heritage, so that they could be considered a hybrid of the archive and the museum [2]. since the late 1970s, such institutions have been coordinated by international organisations including the international confederation of architectural museums (icam) and the international council on archives (ica). these organisations have created guidelines for architectural archives that focus on the preservation and sharing of knowledge, the management of which has been guaranteed thanks to specific international protocols regarding cataloguing, digitisation, preservation and fruition. their first objective was the clear and univocal definition of an architectural document as every kind of documental material related to the history, theory and practice of architecture and related fields, independent from its support and physical characters, created or received by public or private authorities during their activity and gathered independently from its origin [3]. this definition establishes the great complexity of materials and interpretations related to architectural documents, including not only architectural drawings but also other kinds of materials (e.g. physical models, photographs, correspondence, structural calculations, costs and materials estimations, audio and video records) that could be differently interpreted by a potentially limitless public as historical proofs, technical documents or works of art. as irace stated, the peculiarity of architectural archives consists in the purpose of their design, which almost always tends to project the dimensions of the document into the more heroic dimensions of a monument [4]. ict enables methods of visualisation that are impossible in an ordinary encounter with a document, such as the integration of fragments or the 3d rendering of a drawing. in short, ict tools exponentially heighten the capabilities of analysis, research, study and even communication. contemporary research on architectural archives looks to multidisciplinary and crossdisciplinary approaches that could bring analysis, interpretation, communication and sharing into the digital era. design & cultural heritage, animated archive, a fairly recent book edited by fulvio irace and graziella leyla ciagà, is a collection of several proposals of this sort [5]. among them, leyla ciagà’s digital platform prototype, which aimed to dynamically relate existing artefacts with the archives that document their design development, stood out. the prototype set up by the scholar had two main aims in relation to two sets of users: first, sharing knowledge about 20th century architecture whose value and quality is often ignored in order to build support for preservation actions among a broader age demographic; and second, offering scholars a useful research tool in order to incentivise and inspire more in-depth studies performed directly on archival documents [6]. this work is a fundamental source of inspiration for our research. in section 2, we will present the case study and previous research. in section 3, we will analyse some case studies that provide background knowledge for the present work. in section 4, the structure from motion (sfm) methodology is applied to the physical model, and the object of this work is discussed. section 5 describes the technique for generating a digital model from a physical document. section 6 develops the scan-to-building information modelling (scan-to-bim) process, highlighting the challenges encountered in this case study. section 7 underlines the potentialities of bim for collecting and exchanging data and augmented reality to enhance the user’s experience of the archive. 2. enhancing archival architectural heritage objects through digital tools using the continuous innovations of digital modelling tools, a field of study aimed at the enhancement of archival architectural heritage objects is developing new and reconstructed interpretations of unbuilt projects or demolished buildings. twentieth century archives from the pre-digital era contain many different materials – not only drawings, which, as mentioned, form the basis of this project’s activities, but also technical reports, correspondence, pictures, photomontages, scale models and audio and video recordings. the value of the archives lies in the significance of these items, which provide proof of a phase in the design process that ended with the construction or remained on paper. this is the case for carlo mollino’s archive, which since 1973 has been kept in the archivi della biblioteca centrale di architettura ‘roberto gabetti’, politecnico di torino, fondo carlo mollino. among the more than 130 architectural designs collected in the archive, only about twenty were fully realised, and many of those do not exist today due to thoughtless demolitions [7]. several designs by mollino were recently digitally reconstructed as part of a master’s thesis undertaken in the school of architecture at the politecnico di torino and supervised by roberta spallone and sergio pace. another recent study created a unique collection of digital models of the most interesting and documented works, applying the latest low-cost technologies for user interaction and the integration of 3d modelling tools with publishing and online sharing systems. the prototyped platform, ‘digital interactive mollino’ (dimo), is experimental and replicable [8]. frequently, mollino paired his designs with physical scale models, which were documented in photographs and photomontages but not preserved. the models allowed him to visualise the physical appearance of the building within its environmental context. among mollino’s designs, the turin horse racing (figure 1) was a youthful masterpiece that was built figure 1. mollino, sketch of turin horse racing (archivi della biblioteca centrale di architettura ‘roberto gabetti’, in the following bca, politecnico di torino. fondo carlo mollino). acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 226 in 1937 and demolished in 1960 due to the urban transformation related to the celebration of the centenary of the unification of italy. it was later represented by a wooden scale model (figure 2) that was presumably realised in the 1970s or 80s and is preserved in the biblioteca centrale di architettura. the availability of ontologically different representations of this project made it an interesting case for testing a new development of the dimo prototyped platform. indeed, in this particular case, it was possible to create a circular flow of knowledge between the physical archival heritage objects and the digital interpretations. the application of sfm to the wooden model enabled the creation of a 3d digital model that could be compared with a second digital model generated from the original drawings. moreover, either the physical model or the drawings could provide the basis for marker-less experiences of augmented reality that link to other archival materials, point clouds and interpretative digital models. the archive could become a place for the exchange of knowledge and experiences. at the same time, the website could contain links to a selection of archival documents and interactive repositories, such as sketchfab® or 3dhop®, enhancing the fruition of the archive and encouraging its use. the present phase of the work is focused on the application of sfm to the physical model and the creation of a 3d digital model of the horse racing. 3. interpreting and sharing architectural heritage: the digitisation of physical models scale models and maquettes evoke a high level of craftsmanship, crystalise ideas and anticipate the future. they aid the architect in ‘knowing the beauty of a building, whose idea he just conceived, before even starting its construction’ [9]. the wooden model in this case represents the threedimensional historic memory of a structure that has been demolished. hence, it both looks backward and anticipates the future in the sense that it could be used for the crucial purpose of reconstruction, helping an architect bring the building back to life. therefore, the digitisation of physical models assumes even more importance when applied to entities that are about to be built or that do not exist anymore, as in the proposed case. a combination of photogrammetric and laser scanning techniques are commonly used for surveys aiming at the production of integrated and accurate 3d representations. both techniques are particularly useful for cultural heritage applications because they don’t require direct contact with the object. with the transition from analogical to digital culture, the transmission of data has radically changed both in principal and in method. approaches to acquisition and analysis have transformed in kind [10]. the ability to digitise a wooden model, visualise it in a real setting and compare it with historical sources represents a new approach for the history of architecture and also for the process of the improvement of cultural heritage. this was the case for a 2011 study of the masieri memoria building [11] in which photogrammetry and laser scanning were used to obtain models that were as precise as possible. when compared, the models were nearly overlapping. digitising techniques were also used to give viewers a sense of the realised building. a model of a city was realised using municipal cartography (plans) and scans acquired by laser scanner (to capture heights). using the camera match utility of 3d max studio®, a procedure similar to a backward resection was applied using the city model as the context. another interesting case illustrating a combination of different techniques is the survey of a maquette by antonio da sangallo at the basilica di san pietro carried out in 2007 [12]. the survey was made using a laser scanner technique focusing on both the internal and the external views. the model in this case became a means for the evaluation of the overall design – its spatiality, its geometry (which used control cross-sections to define the main generatrix and directrix) and its lighting characteristics. thanks to the great accuracy of the model, it was possible to analyse its individual elements and compare them with the main classical treatises. the maquette was also used to generate a virtual model through a process of reverse modelling. using the model, a video was created from a human point of view to highlight the spatial characteristics of the building and point out some limitations of sangallo’s design, confirming some of michelangelo’s doubts about lighting. this use of both virtual and reconstructed models introduces some interesting reflections about these different tools. the possibility of constructing a physical model (which, in some cases, is easier to construct and manipulate than a virtual model) in order to develop a digital model (which is more suitable for controlling dimensions and making drawings) therefore represents a particularly interesting track for the exploration of new approaches. this type of practice can benefit from numerous interactions between the analogue and digital models [13]. bim technologies are also used to create digital models. a 2019 study used collection information modelling (cim) to realise a 3d database of mobile heritage based on the collection of the museo egizio in turin [14]. although many of the objects explored in this study were smaller than a maquette, it is interesting to consider how museums can utilise achieved digital models for multiple purposes – for example, to monitor their collections and offer users (visitors) the possibility of exploring objects, even those that cannot be put on display due to a lack of space. the efficiency and precision of digital instruments along with their interactivity, manipulability and suitability for multimedia applications are essential elements for a sustainable reproduction. in all of the analysed studies, there was a need to strengthen the interaction and integration between different methods of figure 2. wooden model of turin horse racing (archivi bca, politecnico di torino. fondo carlo mollino). acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 227 representation from the most traditional to the most innovative (while maintaining a focus on accurate representation) and between the real structure, the model and the representation. the analytic and interpretative approaches to modelling that were presented in case studies could be applied to the turin horse racing maquette in order to enrich its significance as a cultural heritage item and its relevance as an object of scientific study. 4. applying structure from motion to physical models today, digital and numeric models are gradually taking the place of maquettes, in particular the handmade wooden ones. a wooden model does exist in the case of the turin horse racing, even though the building does not. through modern 3d representations and surveying technologies (digital photogrammetry, in this case) we can insert this building into the current context. by applying translating processes to the realised maquette and its digitisation, it is possible to re-create reality in a specific context and engage in dialogue with a specific public. automated photogrammetry makes it possible to transform maquettes into digital models. this process is not yet common: typical applications concern whole buildings or small objects, not medium-sized things. different sizes of objects and different accuracy requirements lead to the selection of different instruments and methods. the common use of photogrammetry is not only due to the availability of technical tools and the fact that they are less expensive than the equivalent tools for laser scanning. analysing this choice from a more conceptual point of view, photogrammetry and the subsequent photo modelling guarantee the construction of a model that directly transforms visible elements into the digital space. the obtained model thus provides a realistic representation of what we see. this technique is especially effective in situations in which the interpretation of geometry is not required. photogrammetry can also be performed in order to obtain three-dimensional information. photo modelling is a highly realistic and efficient tool for describing the surface characteristics of objects and for creating 3d models that are verifiable and navigable from infinite points of view. this feature is interesting for the process of surveying maquettes. even though maquettes are often characterised by a single material, photo modelling enables the restitution of their materiality and highlights small variations in the treatment of the surface. finally, thanks to its digital nature, photo modelling can make use of media techniques that connect images to stories by presenting a sequence of information that can be experienced as a narrative, while flanking the narrative with real data [15]. the wooden model of turin horse racing used in the case study (figure 3) has a 1:50 scale and dimensions of 176.5 (length) × 196.5 (width) × 20 cm (height), so it can be considered neither big nor small. therefore, as photogrammetry was used exclusively, it was important to take a sufficient number of photos from different points of view and with good illumination in order to obtain as precise a model as possible. following the rules of photogrammetry, the model was photographed in its entirety, making three turns around it at three different heights and taking photographs at each human step to allow a rich reconstruction using structure from motion (figure 4). the object always remained at the centre of a hypothetical sphere, and the camera was always oriented towards the centre. this made the virtual reconstruction easier, especially the texturing process. designing the surveying plan for the data acquisition required the identification of site conditions and constraints in order to determine the optimal locations for the camera and lights. the onsite survey conditions for the model of turin horse racing were quite ideal. the maquette was totally accessible from all sides and quite accessible from the top. the camera could be positioned at a sufficient hight to capture the entire maquette, with the exception of the zenithal point of view. we used four adjustable tripods with four 4000 k lights, two of them paired with reflecting umbrellas and the other two located in soft boxes to spread the light and make it uniform. the model was also equipped with markers positioned near the perimeter (to make the orientation phases and the positioning of the picture easier). in the followed scheme, converging shots were taken all around the model at three different heights and different camera angles. the creation of deep shadows, which could cause difficulties in the collimation, orientation and design phases, was avoided. figure 5 shows a graph of image residual values, figure 3. orthographic image of the 3d model generated via automated sfm photogrammetry. figure 4. schematic shooting plan. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 228 representing the average vectors of the reprojection error for the pixels in the corresponding cells. averaging was done across all the images in the calibration group and all the pixels inside the cell. higher values are marked in red and lower ones in cyan. during the survey, some measurements were taken to scale the wooden model. the scale transformation was then validated using other measurements. the photos were taken from three different heights, so we obtained three ground sample distance (gsd) values and determined their median value (table 1). the camera distance is also expressed as a median value because the wooden model is three-dimensional and the shooting was done by moving around it. although the acquisition protocols can be used for any kind of digital camera, image resolution obviously affects the quality of the final restitution. we chose to use a canon 1300d with an 18–55 mm lens and an 18 mp, 5184x3456 pixel aps-c (1.6x) sensor. we manually took 390 photos in the jpg format using an 18 mm focal length. this choice was based on the dimensions of the surveyed object, which were quite large compared to the size of the room, and the space available for walking around the model with the tripod. the tripod was always placed along the perimeter of the model. the exposure parameters, such as aperture, shutter speed and iso settings, were set so as to achieve the highest image quality and sharpness. considering the good lighting conditions, we fixed the iso parameter at the value of 100 in order to prevent any type of image noise. the aperture setting was fixed at f/8 in order to have a wide focus and facilitate image matching. since these two parameters were fixed, we chose to vary the shutter speed. the use of the tripod made it possible to use long exposure times without affecting the image quality. after our fieldwork, which lasted just a few hours, we processed the data using the metashape® environment. we then analysed the sharpness and focus quality of the images using the estimate image quality function before aligning the frames using align photos. 5. from physical to automated mesh models a good combination of multiple scan positions (figure 6) made it possible to create a complete and realistic point cloud of the detected object. the workflow was completely automatic both in regard to the orientation of the images and the generation and reconstruction of the model. this condition led to an optimisation of processing times, ensuring good performance of the machine/software complex [15]. once the photos were uploaded to metashape®, the following steps were taken taken (see also table 2): 1. photo alignment. this is the most important phase of the entire photogrammetric process. in this phase, the software aligns the photographs with each other, calculating their position in space and reconstructing the so-called gripping geometry. accuracy is one of the parameters that control the photo alignment procedure. higher accuracy settings generate more accurate camera position estimates and ensure that the software will work with the original photo size. considering the research purpose of this case study and the very sharp pictures that were taken, the highest accuracy setting was selected, despite the additional time this required. next, through a process of geometric triangulation, the software calculated the position of the elements present in the photographs in space. the quality of this alignment determines the quality of the final 3d model. the result of this phase was a sparse point cloud (figure 7). 2. processing time optimisation. before generating the dense cloud, it was necessary to clean the sparse cloud of points defined as ‘disturbing’ and choose depth filtering values based on the parameters of average quality and moderate depth. the depth filtering mode makes it possible to work on abnormal values caused by noise and blurred images. dot groups are filtered and deleted so that they no longer appear on the final model. metashape® implements different algorithms for different situations: the filtering is aggressive if the detection area does not contain small significant details and mild if there are small details that are important for the reconstructed scene. moderate filtering generates an intermediate result between the previous two, or the filtering can be disabled, which is not recommended because the resulting cloud may be too noisy. for the present case study, mild filtering was selected based on the large quantity of images, so that the processing times would be reasonable and the files could be handled in other programs and used for other purposes. the mild algorithm is also particularly suitable if there are small details in the model that are important to retain during the scene reconstruction phases, as was true for our case. an object can be described in more or less detail depending on the distance between the points used to describe it. the denser a cloud is, the figure 5. image residuals for canon eos 1300d (18 mm). table 1. calculation of ground sample distance. set of photos number of aligned photos camera distance in m gsd in mm/pixel set 1 208 1.6 0.38 set 2 92 1.2 0.30 set 3 90 1.1 0.27 median value 0.32 figure 6. photogrammetric survey: multiple image acquisition. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 229 more it is able to represent the shape of the object, the surface or the detected space in a manner that is faithful to reality. 3. building the dense cloud. in this phase, a dense cloud was constructed using dense image matching algorithms. metashape® made it possible to generate and display a dense point cloud model (figure 8) whose quality was set to assess the detail and geometric accuracy resulting from the reconstruction. in this case, the work was performed using a medium quality reconstruction setting. although the images were not used with their original resolution, the result was satisfactory, considering the reasonable processing times and the easily manageable .ply file that was obtained. 4. mesh extraction. once the dense point cloud had been constructed, it was possible to create the mesh by connecting the points in the cloud into triangles that represent surfaces. the extraction was based on the construction of region-by-region correspondences according to the subdivision of the geometry. a polygonal model based on the newly created dense cloud was generated (figure 9). 5. building texture. this process improved the definition of the 3d representation of the work under investigation. textures that had been obtained from the acquired images were applied to the meshes, generating a texturised model (figure 10). a texture size value of 4096 was chosen. this value indicates the width and height of the texture in pixels and determines the number of texture files to export. exporting textures to multiple files allows for higher texture resolution in the final model, while exporting the texture to a single file may fail due to ram value limitations. the generic mapping mode was chosen to create a uniform texture. the mosaic setting was chosen for the blending mode in order to pick the sharpest photos and create the most accurate result. 6. verification of measures. the last step was the metric control and scaling of the model, that is, the correct metric dimensions were assigned to the model in order to make precise measurements on it. while acknowledging the potential of this work and the opportunities that derive from it, it is nevertheless necessary to highlight and take into consideration its limitations. namely, the automatic construction of the mesh model did not include the interpretation of the building’s geometries and discretised its shape through an interpolated surface. all of the operations oriented to the extraction of characteristic sections and to the generation of the model will therefore be investigated further while recognising the limitations of the approach, which inevitably presents only the external surface of the analysed object. 6. from point cloud to bim model we decided to develop the model analysis using revit® software, which allowed us to consider its geometrical characteristics and transform it into a sort of collector of associated data and information [16], [17]. table 2. processing parameters. sparse cloud points 257,976 alignment parameters accuracy high reconstruction parameters quality medium depth filtering mild dense cloud points 9,485,212 reconstruction parameters quality medium depth filtering mild mesh faces 1,216,208 reconstruction parameters quality medium depth filtering mild general mapping mode generic blending mode mosaic texture size 4,096 × 4,096 figure 7. the sparse cloud model. figure 8. the dense cloud model. figure 9. the mesh model. figure 10. the texturised model. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 230 therefore, the point cloud was saved from metashape® as a .pts file, imported into recap® and saved as a .rcp file that could be opened with revit® (figure 11). the formats in which the cloud was exported had to allow the transmission and the storage of information for a 3d object composed of points arranged in space with specific geometric coordinates and also needed to tie these points to rgb colorimetric information. after importing the point cloud, we identified the best and most useful work plans (figure 11) for the construction of the bim model from the acquired 3d model: six vertical plane sections (four elevations, a longitudinal and a cross section) and horizontal plans, all drawn using the planes parallel to the default views. then we constructed the grid (figure 11) by intercepting, thanks to revit® object snaps, the most significant points of the point cloud. these axes are a useful trace for the construction of all the elements that will compose the model during the subsequent phases of the work. next, we identified the main elements of the wooden model (e.g. walls, floors, ceilings, curtain wall), including everything that was necessary for the construction of the model. in particular, the main volumes were identified. once the main axes and levels were defined, the geometries were modelled. modelling with the in-place mass command makes it possible to create families whose elements (e.g. edges, vertices, surfaces and volumes) can be modified in space without particular constraints. we decided to work on each level of the building and trace the point cloud with a closed outline. once all of the various silhouettes were constructed, we created conceptual masses and local families through the extrusion of previously drawn sections (figure 12). this procedure was applied to all the main portions of the wooden model in order to define a single volume for each of them. figure 11. autodesk revit 2021®: insertion of the point cloud. site level: insertion of the grid and section; south elevation: insertion of levels. figure 12. autodesk revit 2021®. 3d view: overlapping conceptual masses and point cloud; 3d view: conceptual masses of the wooden model overlapping archival drawings, conceptual masses and point cloud. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 231 these objects, made up of shapes, surfaces and volumes, will be the guide for the subsequent, more detailed phases of the model elaboration. in the case of archive projects like the turin horse racing maquette, it may be useful to think of the bim model as an information collector. a bim model creates database fields into which information can be inserted, and other fields can be added if necessary, such as, in this case, the one related to the archive drawings as well as fields for the geometry (masses) and survey data (point cloud), see figure 12. 7. interpretation, sharing and communication using geometric and bim models and augmented reality experiences in the present case study, the wooden model is one of the first, if not the first, interpretative representations of the turin horse racing. as far as we learned from the heirs of the company that built the wooden model (falco & boulanger s.a.s., moro esculapio), it was likely made between the 1970s and 1980s, i.e. after the demolition of the building. we can therefore imagine that the model was constructed from the same iconographic material that guided the creation of the digital 3d model. mollino’s turin horse racing was immediately recognised as a masterpiece by contemporary critics: in the early 1940s giuseppe pagano, armando melis and gio ponti featured the building in the most famous journals of the time [18]-[20]. therefore, the documentary iconographic materials range from the original sketches, diagrams, technical drawings and perspective views to the drawings prepared by mollino for publishing and the photographs and photomontage taken after construction and prepared by mollino and riccardo moncalvo (figure 13-figure 16). a bim model could be effectively used to collect a selection of these materials along with the journal articles and the texturised model obtained through sfm survey. moreover, a bim model created from the most reliable and consistent original drawings would document a design phase that could be interesting to compare with the mesh model obtained by photo modelling. another possible application could be a process of reverse engineering from the point cloud acquisition of the wooden model to bim modelling with the aim of determining the design phase expressed in the wooden model and comparing this with the digital model realised from design drawings. the final step of this experience broadens the scope of augmented reality (ar) applications from the archival design drawings to the wooden maquette. this process applies the results of previous experiences [22][24] involving the recognition and tracking of two and threedimensional objects in ar applications to the collected data. recent developments in this field relate to the technology available on widespread mobile devices (e.g. tablets and smartphones) that enable real-time 3d scanning. software development kits like vuforia, wikitude and arkit feature realtime 3d object recognition and marker-less tracking. furthermore, they are supported by many app programming environments, including unity, which is one of the most advanced and widespread cross-platform game engines. 8. conclusions the ability to digitise a wooden model, display it in a real context and compare it with other designs highlights the potential of new digital tools and invites a new approach to the history of architecture and the treatment of cultural heritage objects. by creating an array of reproductions and representations, we can assign an important and objective/metric role to the wooden model as a part of the design process. due to its experimental nature, this project is susceptible to changes and improvements in the various phases (from the survey through the outputs until musealisation). in the survey phase of photogrammetry, it could be interesting to use an endoscopic camera or a compact camera figure 13. mollino, technical ink perspective drawing of turin horse racing (archivi bca, politecnico di torino. fondo carlo mollino). figure 14. reconstructive digital model of turin horse racing in the urban context of the time (modelling: florida canaj). figure 15. mollino, technical ink drawings of turin horse racing (archivi bca, politecnico di torino. fondo carlo mollino). acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 232 (smaller than a reflex) to take pictures of difficult-to-access areas such as courtyards. in the processing phase, considering the nature of point clouds as unstructured entities lacking topology or semantic discretisation and, in contrast, the nature of bim, which requires ‘smart’ relationships among components, some progress could be made to ensure that the derived parametric geometry better fits the surveyed reality-based geometry. dedicated plug-ins, such as greenspider for revit [24], could be a good choice for the translation of points into native reference snaps, which would make it possible to model elements with much more precision than can be achieved by tracing. the output phase is probably the richest one: the use of different techniques of representation and the overlapping of different documents (e.g. historical photos, photos of the maquette and original drawings) could be very interesting (figure 15). this ‘mixed technique’ envisages a relationship between the virtual reality of the model reconstruction and the physical reality of the material of the studied object. these interconnections between different realities can lead to greater insights and more precise analyses. the application of digital technologies to a cultural heritage object allowed us, in this case, to obtain digital copies of a wooden model that can be published in a virtual museum and can also provide scholars with free access to an object that is difficult to handle and to move. this experience pre-figures new virtual links between archival materials and digital interpretations, in which the archive could become a place of convergence between the real and the virtual. although bim was developed for new buildings, we’ve seen that the typical bim approach, which involves the gathering of data about components within semantic relationships, can be successfully applied to the modelling of historical architecture. bim is the perfect tool for the creation of virtual and interactive archives linked to digital models, and it represents an intelligent and useful tool for the digital collection and preservation of archival documents. the envisaged integrated and digitised archives provide a space in which drawings, documents and real and digital models can be made available to different categories of users and adapted to their different needs. the presented case is an experimental first step of a more complex study that aims to create a sort of digital model museum in which to insert all of the historical information that is useful for telling the story of the evolution of the artefact over time, providing users with live interactive and immersive experiences of virtual and augmented reality through their personal devices. this paper is the result of a research project carried out by the authors. roberta spallone wrote sections 2 and 7. giulia bertola wrote sections 5 and 6. francesca ronco wrote sections 3 and 4. the introduction and conclusions were written by roberta spallone, giulia bertola and francesca ronco. references [1] a. aleardi, un futuro per gli archivi di architettura, in: verso un archivio digitale dell'opera di maestri del xx secolo, architettura disegno modello. p. albisinni, l. de carlo (editors). gangemi, roma, 2011, isbn 9788849220988, pp. 11-18. [2] r. spallone, f. paluan, digital representation techniques to interpret, communicate, and share 20th c. architectural archives, in: handbook of research on emerging technologies for digital preservation and information modeling. a. ippolito, m. cigola (editors). igi global, hershey pa, 2017, isbn 9781522506805, pp. 356-383. [3] r. domenichini, a. tonicello, il disegno di architettura. guida alla descrizione, il poligrafo, padova, 2004, isbn 9788871152653. [4] f. irace, the animated archive, in: design & cultural heritage, archivio animato/animated archive. f. irace, g. leyla ciagà (editors). electa, milano, 2013, isbn 9789610501060, pp. 10-14. [5] f. irace, g. leyla ciagà, (editors), design & cultural heritage, archivio animato/animated archive, electa, milano, 2013, isbn 9789610501060. [6] g. leyla ciagà, modern heritage in the web, in: design & cultural heritage, archivio animato/animated archive. f. irace, g. leyla ciagà (editors). electa, milano, 2013, isbn 9789610501060, pp. 30-42. [7] r. spallone, digital reconstruction of demolished architectural masterpieces, 3d modeling, and animation: the case study of turin horse racing by mollino, in: handbook of research on emerging digital tools for architectural surveying, modeling, and representation. s. brusaporci (editor). igi global, hershey, pennsylvania, 2015, isbn 9781466683792, pp. 476-509. [8] r. spallone, f. carota, digital interactive mollino, a collection of 3d models from carlo mollino design drawings, in: putting tradition into practice: heritage, place and design. g. amoruso (editor). springer international publishing ag, cham, isbn 9783319579368, pp. 607-617. [9] f. guerra, p. vernier, surveying and comparing the arco dei gavi and its historical wooden maquette, geoinformatics fce ctu 6, (2011), pp. 338-345. doi: 10.14311/gi.6.42 [10] a. rossi, p. m. cabezos bernal, il valore paradigmatico del grande plastico di pompei, disegnare idee immagini 55 (2017), pp. 62-71. [11] a. adami, c. balletti, f. guerra, from wooden maquettes to digital model: virtual reconstruction of a design path, proceeding xxiii international symposium cipa, ottowa, canada, 28 august – 01 september 2011, pp. 12-16. [12] m. docci, la basilica vaticana non costruita: l'opera di antonio da sangallo, disegnare idee immagini 34 (2007), pp. 24-35. [13] l. de luca, la fotomodellazione architettonica. rilievo, modellazione, rappresentazione di edifici a partire da fotografie, flaccovio dario, palermo, 2011, isbn 88-579-0070-3. [14] m. lo turco, f. rinaudo, p. piumatti, m. calvano, a. spreafico, g. patrucco, the digitisation of museum collection for research, management and enhancement of tangible and intangible heritage, 2018 3rd digital heritage international congress, 24th international conference on virtual system & multimedia, san francisco, usa, 26 – 30 october 2018, pp. 540–543. doi: 10.1109/digitalheritage.2018.8810128 figure 16. mollino and moncalvo, photomontage of turin horse racing (archivi bca, politecnico di torino. fondo carlo mollino). http://www.amazon.it/s?_encoding=utf8&field-author=p.%20albisinni&search-alias=stripbooks https://www.zvab.com/products/isbn/9788871152653?cm_sp=bdp-_-isbn13-_-plp https://doi.org/10.14311/gi.6.42 https://doi.org/10.1109/digitalheritage.2018.8810128 acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 233 [15] f. bianconi, m. filippucci, s. catalucci, line and points: critical analysis of evolution of archaeological survey in forty years of experiences in umbria, disegnarecon 10(19) (2017), pp. 18285961. [16] v. barrile, a. fotia, g. candela, e. bernardo, geomatics techniques for cultural heritage dissemination in augmented reality: bronzi di riace case study, heritage 2 (2019), pp. 22432254. doi: 10.3390/heritage2030136 [17] f. bianconi, m. filippucci, g. amoruso, m. bertinelli, from the integrated survey of historic settlements to the pattern book within the bim, international archives of the photogrammetry, remote sensing and spatial information sciences, xlii-2/w9 (2019), pp. 87-92. doi: 10.5194/isprs-archives-xlii-2-w9-135-2019 [18] g. moglia, f. m ugliotti, a. osello, m. dellosta, bim, virtual and augmented reality for citizen involvement in the evaluation of the urban transformation, uid 2016, 38° convegno internazionale dei docenti delle discipline della rappresentazione, florence, italy, 15 – 17 september 2016, pp. 1197-1200. [19] g. pagano, la nuova sede della società ippica torinese, costruzioni-casabella 157 (1941), pp. 14-17. [20] a. melis, la sede della società ippica a torino, l’architettura italiana xxxvi(2) (1941), pp. 45-62. [21] g. ponti, il vetro e l'architettura dello sport, in stile, maggiogiugno, 1941. [22] v. palma, m. lo turco, r. spallone, m. vitali, augmented iconography: ar applications to the fortified turin in the theatrum sabaudiae, in: defensive architecture of the mediterranean vol. ix. a. marotta, r. spallone (editors). politecnico di torino, torino, 2018, isbn 9788885745124, pp. 1053-1060. [23] v. palma, r. spallone, m. vitali, digital interactive baroque atria in turin: a project aimed to sharing and enhancing cultural heritage, in: earth 2018. a. luigini (editor). springer nature, cham, 2019, isbn 9783030122393, pp. 314-325. [24] v. palma, r. spallone, m. vitali, augmented turin baroque atria: ar experiences for enhancing cultural heritage, international archives of the photogrammetry, remote sensing and spatial information sciences xlii/w9 (2019), pp. 557-564. doi: 10.5194/isprs-archives-xlii-2-w9-557-2019 [25] s. garagnani, a. m. manferdini, parametric accuracy: building information modeling process applied to the cultural heritage preservation, international archives of the photogrammetry, remote sensing and spatial information sciences xl 5/w1 (2013), pp. 87-92. doi: 10.5194/isprsarchives-xl-5-w1-87-2013 https://doi.org/10.3390/heritage2030136 https://doi.org/10.5194/isprs-archives-xlii-2-w9-135-2019 http://hdl.handle.net/11583/2713061 http://hdl.handle.net/11583/2713061 http://hdl.handle.net/11583/2713061 https://doi.org/10.5194/isprs-archives-xlii-2-w9-557-2019 https://doi.org/10.5194/isprsarchives-xl-5-w1-87-2013 design of fully automatic calibration machine for reference block based on machine vision acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 235 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 235 239 design of fully automatic calibration machine for reference block based on machine vision xiang ren1, jinyu lu2, zecheng tao3 1 metrology of he nan institute, zhengzhou, china, 243553773@qq.com 2 metrology of he nan institute, zhengzhou, china, jinyu_lu@126.com 3 innovative testing technology co., ltd., kunshan, china, tzc@cxtest.com abstract: in order to solve the problems of the existing standard brinell hardness machine such as low automation, the time-consuming indentation measurement and the human influence in indentation measurement, a calibration machine for reference block was designed. it shows that this machine can reduce the influence of the temperature difference between the hardness block and the measurement platform; it eliminates the time for the indentation measuring system to find the indentation and juxtapose it with the centre of the field of view and implements fully brinell hardness automatic calibration. keywords: automatic indentation measuring system; machine vision technology 1. introduction the brinell hardness test method is the simplest and fastest method in the mechanical property test of materials. it is one of the main methods for determining the mechanical properties of materials, inspecting product quality, determining reasonable heat treatment specifications and machining processes [1]. as a standard calibration machine which must have the function of reproducing and storing the unit value, and its reliability has special significance for the stable measurement performance which required by the standard device [2]. among the existing reference block calibration machines, they basically adopts the loading method of the dead weight, which can ensure the loading accuracy during the measurement process, but most of them are produced of the 1970s.the loading system has the disadvantages of complicated wiring, large volume, single function and high failure rate, which brings many deficiencies and inconveniences to the hardness calibration work [3-5]. in the step of indentation measurement, the optical microscope is usually used, then people manually find the indentations and read the diameters of them, finally calculate the brinell hardness value. these operations will probably involve personal measurement errors in results, and different operators might get different results for the same indentation. moreover, measurement result may be different even when the same operator measures the same indentation multiple times. during the process, the operator is easy to get eye fatigue and it is more time-consuming to find smaller indentation points and the low efficiency is also the shortcoming of this traditional method [6][7]. existing indentation measuring instruments include tool microscopes, image measuring instruments, and contact and non-contact geometry measuring instruments based on mechanical vision technology; first of all, these instruments are not dedicated brinell hardness indentation measuring equipment, combining with the test force part of the machine it is difficult to realize automatic measurement; secondly, when measuring the indentation, the instruments need to move the reference block from the calibration machine to its measurement platform, this will inevitably introduce the influence of temperature [8]. furthermore, when the diameter of the indentation is small, the instruments will be more timeconsuming when searching for the indentation. at present, a new round of revolutions, such as machine vision, intelligent control, new energy and new materials, is emerging around the world. the rapid development of measurement and control technology and machine vision technology has made brinell hardness measurement surely move towards automation [9][10]. based on these two http://www.imeko.org/ mailto:243553773@qq.com mailto:jinyu_lu@126.com mailto:tzc@cxtest.com acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 236 technologies and combining iso6506-3:2014 and iso6506-2:2017 developed a calibration machine as shown in figure 1. figure 1: calibration machine for reference block the calibration machine is mainly composed of three parts: loading part, measuring part and twodimensional bearing platform. the loading part mainly includes the servo motor, reducer, lead screw, guide rod, measuring platform and upper and lower plates; the measuring part mainly consists of the force transducer group, laser rangefinder, indentation measurement system which are installed on the measuring platform and lead screw grating. 2. design of the automatic indentation measurement system 2.1. determine the parameters of core components the indentation measurement system based on mechanical vision is mainly composed of industrial cameras, telecentric lenses, and light sources. the selection of industrial cameras and telecentric lenses is mainly determined based on the measured feature size and calibration accuracy requirements. the main parameters of industrial cameras are resolution and sensor size; the main parameters of telecentric lenses are resolution, working distance, field of view area and magnification. the optimal selection of the indentation system is that the size of the detected feature is equal to its field of view area, which is enlarged by the telecentric lens magnification to the size of the target surface of the industrial camera. in this case, the camera pixels will be fully used to characterize the measured indentation, thereby reducing the cost of the calibration device. according to the resolution calculation formula (1) of the industrial camera, the requirements of the detected feature and the measurement accuracy must be obtained. 𝑅 = 𝐿max 𝐿min × 𝑝 (1) with 𝑅 – resolution; 𝐿max detected feature; 𝐿min – accuracy; p it indicates that several pixels are used to characterize the measurement accuracy. (if the entire image is regarded as a periodic signal with a minimum feature size, according to nyquist sampling law, the signal must be sampled at least 2 points per cycle to fully reproduce the signal.) the measured feature size is determined according to iso6506-1:2014 7.4, the test force should be chosen so that the diameter of the indentation, d, lies between the values 0.24 d and 0.6 d [11], as there are four types of hard metal ball diameters: 10 mm, 5 mm, 2.5 mm, and 1 mm, the corresponding indentation diameter ranges are shown in table 1. iso6506-3:2014 4.6 shows the requirements for the measurement accuracy of the indentation measurement system in table 1, [12]. combine the indentation diameter range with the corresponding accuracy requirements to calculate the requirements for the resolution of industrial cameras. table 1: indentation range and performance requirements of indentation measurement system hard metal ball diameter (mm) indentation range (mm) diameter of indentation (mm) performance (mm) 10 2.4 ~6 d ≥ 2.5 ± 0.0020 5 1.2 ~3 1 ≤ d ＜ 2.5 ± 0.0010 2.5 0.6 ~1.5 d ＜ 1 ± 0.0005 1 0.24 ~0.6 2.2. configuration of automatic indentation measure system because the range of indentation diameter is wide and the measure performance of different ranges is also different, if using a fixedmagnification telecentric lens with an ultra-highpixel industrial camera can realize the automatic measurement of various indentations. but in this industry camera, electric linear zoom lens and lights http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 237 configuration the indentation measurement system needs to ensure the complete shoot of the full maximum indentation diameter (6 mm), at the same time, it must also meet the measure accuracy requirement (0.0005 mm) of the minimum indentation diameter. when 2 pixels are used to characterize the measurement accuracy, according to the resolution calculation formula, r is equal to 24000, the camera resolution would be 24,000 × 24,000 about 576 million resolution. the camera cost will be extremely high. if one camera is used with multiple telecentric lenses, the hardware configuration cannot achieve automatic measurement of the full diameter range. the use of the electric linear zoom lens will reduce the camera cost and achieve automatic measurement of the full diameter range. firstly, the lens can be automatically adjusted to the specified magnification. when the lens enlarges a small indentation, the field of view will be reduced as well, so that the pixels can fully characterize the indentation. the automatic indentation measuring system uses an electric linear zoom lens with built-in coaxial blue light and its lens field of view can be adjusted to 6 mm, 2.5 mm and 1 mm, so each field of view can ensure the largest indentation can be sampled. at the same measurement accuracy condition, the larger indentation feature requires the higher resolution of the camera, so the camera resolution must meet the maximum indentation measuring requirement. when the indentation diameter range is between 2.5 mm and 6 mm, the indentation measurement accuracy is required to be 0.002 mm. when 2 pixels are used to characterize the measurement accuracy, calculated as follows according to the resolution formula (1): 𝑅 = 𝐿max 𝐿min × 𝑝 𝑅 = 6𝑚𝑚 0.002𝑚𝑚 × 2 𝑅 = 6000 . at this time, the maximum feature of the indentation is equal to the field of view. based on the above conditions, the pixel requirement in one direction of the camera is calculated to be 6000, since the detected indentation is a diameter, the number of pixels required in the other direction is also 6000, then ideally, the resolution of 36 million can meet the requirements of measurement accuracy. however, there are few industrial camera sensor on the market, generally rectangular with an aspect ratio of 1.33, such as 12.8 × 9.6, 6.4 × 4.8, etc. therefore, in order to facilitate camera selection, when the number of vertical pixels is 6000, the number of horizontal pixels is 6000 × 1.33 is about 8000, then the actual camera resolution selection is about 50 million. according to the above calculation process combined with the three field of view diameter and measurement accuracy requirements to calculate the corresponding resolution requirements, as shown in table 2. table 2: requirement of camera resolution diameter of indentation (mm) performance (mm) required camera resolution d ≥ 2.5 ± 0.0020 6000 1 ≤ d ＜ 2.5 ± 0.0010 5000 d ＜ 1 ± 0.0005 4000 according to table 2 it can be found that when the measurement accuracy of the indentation is 0.002 mm and the field of view area is 6 mm, the requirement of the camera resolution is the highest. it is concluded that the camera with about 50 million pixels and equipped with electric linear zoom lens can meet the automatically measurement and accuracy requirements. 3. design the force part of the calibration machine the test force of this calibration machine is provided by high-precision force transducers, according to the iso 6506-1: 2014-7.3, and the requirement in iso 6506-3: 2014-4.4, selects iso 376 class 0.03 high-precision force transducers and range of 30 kn, 3 kn and 300 n to be installed on the measurement platform. when the test condition is selected, the corresponding force transducer will move to the loading position, this process is implemented by the linear guide rail, slider, screw rod, grating and servo motor on the measurement platform. when the force transducer at the loading position, the axis of the force transducer coincides with the axis of the loading screw to avoid the generation of lateral force, and the loading and unloading of the test force is then completed through the measurement and control system and the loading mechanism. the two-dimensional bearing platform is composed of steel plate, screw, grating ruler, linear guide, slider and servo motor. the platform is precisely controlled by software to move the reference block. and write the requirements of http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 238 iso6506-3:2014 7.8 indentation selection into the software, so as to improve the efficiency of loading point selection and make better use of the test area of the reference block. figure 2: main parts for moving the force transducer and two-dimensional bearing platform 4. automatic calibration procedure 4.1. the key point of keeping indentation measurement accuracy during the calibration procedure the principle of machine vision image measurement is to map the length in the world coordinate system to the pixels in the camera coordinate system in combination with the imaging system model. the better reproduction of the calibration conditions is most conducive to ensuring the accuracy of machine vision indentation measurement. before the indentation measure system calibration, the camera should be installed vertically on the measurement platform to reduce the error caused by perspective distortion. the rest influence factors such as lens distortion and tangential distortion are caused by the hardware itself. once the system calibration is completed, they are not easy to change during measurement. the calibration process is to map the real length to the indentation measurement system pixel and find its work distance. the work distance of the indentation test system is the most important condition to ensure accuracy, because the change of the distance will directly affect the mapping relationship between pixel points and world coordinates. in order to ensure the reproducibility of the work distance, the laser rangefinder, the grating of monitoring the lead screw combined with advanced calibration procedures to ensure this condition. 4.2. reference block calibration procedure the first step is to place the reference block on the two-dimensional bearing platform, and then select the test force and light source according to the test condition. the second step is to determine the longitudinal sample point of the indentation measurement system, due to the inconsistency of the reference block thickness and it has the surface problem of straightness and parallelism, so the indentation measurement system must also move longitudinally to ensure the reproducibility of the calibration distance. to solve the above problem, the hardware configuration and calibration methods are as follows: a. the servo motor on the measuring platform moves the laser rangefinder to the horizontal working position through the linear guide, slider, screw and grating. in the working position, the optical point of the laser rangefinder coincides with the axis of the lead screw. b. then the laser rangefinder combined the thickness of the reference block preloading point with work distance of indentation measurement system find the longitudinal sample point, and the grating which monitors the lead screw records this longitudinal position. this position will be the longitudinal sample position of indentation measurement system, and then the measurement platform rises back to the initial position. the third step, the force transducer corresponding to the test force is moved to the horizontal working position. in the working position, the axis of the force transducer coincides with the axis of the lead screw to avoid the generation of lateral force, and then the test force is applied through the measurement and control system and the loading mechanism. after completing the above operations, the measurement platform rises back to the initial position. the fourth step: first, the indentation measurement system is moved to the horizontal working position. at this position, the centre of the lens coincides with the axis of the lead screw, and then the software combine the lead screw with its grating to move the indentation measurement system to the longitudinal sample position recorded in the second step b. in the fifth step, the software processes the sampled image and calculates the hardness value. after the work is completed, the measurement linear guide rail slider grating measurement platform guide rod screw linear guide rail slider servo motor screw reference block linear guide rail slider http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 239 platform and each measurement component are restored to the initial position, and then the next test point is automatically or manually selected according to the requirements by the twodimensional platform. 5. summary this calibration machine has the following advantages: first, it completes the automatic calibration of the brinell standard block through an advanced calibration program combined with the measurement and control system and the automatic indentation measurement system. second, due to the application of test force and the indentation are both on the two-dimensional pressure platform, this reduces the influence of the temperature by placing a certain number of reference blocks on the two-dimensional bearing platform in advance. third, according to the calibration procedure, the laser rangefinder, force transducer, and indentation measurement system will be moved to the same horizontal working position by the measurement platform during work, which ensures that the indentation is located in the centre of the field of view when sampling indentation, thus saving the time of finding the indentation and juxtapose it with the centre of the field of view so this improves the calibration efficiency. when other geometry measuring instruments based on mechanical vision technology measured the reference block, the indentation is formed. therefore, it can only use the surface other than the indentation to determine the working distance of the instrument and the reference block also has problems with straightness, parallelism and surface roughness, so it will affect the measurement results to a certain extent. compared to the instruments, this calibration machine’s work distance is obtained before the indentation formed. therefore, the influence of the working distance on the measurement result can be reduced to a certain extent. 6. references [1] yongchang huang, cheng zhou, tao chen, song li, guiping qiu “development of brinell hardness indentation automatic measuring device based on high definition digital image analysis”, measurement technique, pp. 10-14, july 2019. [2] wei zhang, xin luo, jiachun lin, “automatic control system for standard brinell hardness machine based on plc”, journal of mechanical & electrical engineering, pp. 211-212, feb 2020. [3] jizeng tao, wei shi, jinying li, zhongyan wu, “application of piezoelectric ceramics to the force loading system of the brinell hardness standard machine”, measurement & control technology, pp. 72-73, nov 2011. [4] yanan yu, wei shi, wu zeng, jizeng tao, ming xu, jinying li, “development of electric brinell hardness standard device”, china metrology, pp. 72-74, sep 2010. [5] jintao le, “development status and direction for the hardness testing machine manufacturing industry in china”, physical testing and chemical analysis (part a: physical testing), pp. 379-381, june 2017. [6] jizeng tao, wei shi, jinying li, “the brinell hardness measurement device based on ccd and grating ruler”, metrology & measurement technology, pp. 18-20, june 2011. [7] wutingting di, “computer measurement system of brinell hardness indentation ”, digital technology and application, pp. 48+51, nov 2016. [8] manlong chen, “the fast calibration method for image measuring instrument”, modern manufacturing engineering, pp. 126-130+140, aug 2017. [9] weiguang lu, guoyuan wang，hailiang wang, zheng cai “a brief talk on geometric measurement technology and its application in industrial manufacturing industry”, aviation precision manufacturing technology, pp. 43-46, may 2019. [10] yongchang huang, wei huang, bin chen, yu bai, mingtao feng, feiyu zhao, “application of brinell hardness indentation automatic measuring device in verification”, metrology & measurement technology, pp. 114-117, jan 2016. [11] iso 6506-1:metallic materials –brinell hardness test–part 1: test method[s]. switzerland gevena: international standardization organization,2014 [12] iso 6506-3:metallic materials –brinell hardness test–part 3: calibration of reference blocks[s]. switzerland gevena: international standardization organization,2014 http://www.imeko.org/ mathematic modeling of a new inlay buoyancy artefact acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 42 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 42 46 mathematic modeling of a new inlay buoyancy artefact k. lehrmann1, f. härtig1, r. tutsch2 1 physikalisch-technische bundesanstalt, braunschweig, germany, katharina.lehrmann@ptb.de 2 technical university braunschweig, institute of production metrology, braunschweig, germany, r.tutsch@tu-bs.de abstract: the novel buoyancy artefact concept for the use in mass metrology enables the determination and correction of systematic air buoyancy effects caused by measurements outside a vacuum. in contrast to other buoyancy artefacts with a nominal mass of 1 kg, the new artefact, called inlay artefact, has a smaller volume resulting from an enclosed tungsten core. the theoretical design of the new artefact consists of dismountable discs with spherical distance pieces and a cylinder. this work focuses on the design and mathematical model considering limiting factors caused by the handling and the geometry of the comparators used. keywords: buoyancy artefact; kilogram; silicon; inlay artefact; revised si 1. introduction highly accurate mass calibrations require high sophisticated mass comparators which are operated in vacuum. outside a vacuum the measurements are being strongly influenced by environmental conditions, such as buoyancy effects. this is particularly evident, if mass standards of the same nominal value made of different materials such as steel, platinum-iridium, or silicon are calibrated on the basis of a substitution method or compared against each other. in this case buoyancy effects significantly influence the findings because of the resulting different volumes [1]. one way to correct the buoyancy effects is to determine the air densities by using what is known as buoyancy artefacts calibrated by volume [2]. most accurate results for the determination of air densities are achievable with a relative uncertainty in the order of magnitude of 5 × 10-5 [3]. typically, aside the same nominal weight such artefacts have nearly the same surface areas and surface properties but different volumes. the greater the volume difference of buoyancy artefacts used, the more reliably the air density can be determined. a design is presented in order to achieve a large volume difference between buoyancy artefacts used. 2. design of a new kind of buoyancy artefact figure 1 shows a sketch of the proposed new buoyancy artefact. it consists of several discs made of monocrystalline silicon and one cylinder made of silicon with a tungsten core enclosed. the entire standard is referred to as an “inlay artefact”. according to the principal approach of buoyancy artefacts, the nominal mass of the artefact 𝑀ref is 1 kg and its nominal surface area must be identical to the surface area of the other buoyancy artefacts 𝐴ref used. due to the tungsten core the effective density of the artefact is much higher than the density of the buoyancy artefacts made of pure silicon. this leads to a smaller volume and thus, to less buoyancy. distance pieces, which are part of a statically determined coupling, allow the stackable and demountable individual parts of the artefact to be dismantled for cleaning purposes. figure 1: exemplary figure of an inlay buoyancy artefact with two discs (top and center disc) and a cylinder with enclosed tungsten core for modeling the new buoyancy artefact, the following variable input parameters apply: − the surface area ratio between discs and the cylinder − the number of discs − the diameter of the discs − the diameter of the cylinder http://www.imeko.org/ mailto:katharina.lehrmann@ptb.de mailto:r.tutsch@tu-bs.de acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 43 for practical handling the radius of the discs and the cylinder should be equivalent. to achieve the specific surface areas, the surface area ratio of discs to cylinder should be at least two. first investigations have shown that the required surface area can be manufactured by three identical discs made of pure silicon. in order to provide enough volume to include a tungsten core the size of the ground disc have to be enlarged. from now on the ground disc was named cylinder (silicon disc with tungsten core). an additional estimation shows that the resulting surface area could be realized by a minimum of two further discs. further requirements for the new buoyancy artefact are shown in table 1 and depend on the geometric specifications of the comparators used, practical handling and the considerations of tilting stability. table 1: settings for numerical iteration parameter range max. total height ℎmax ≤ 105 mm tilt stability 20 mm ≤ 𝑟dsc, 𝑟cyl ≤ 45 mm number of discs 2 – 5 the silicon cylinder and the plug enclosing the inlay core are detachably bonded to each other according to the specifications of conical tapered ground joints [4] as used for gas-tight silicon oxide compounds. for all calculations, the density of silicon is assumed to be 2 328.8 kg/m³ [5] and for tungsten is assumed to be 19 250.0 kg/m³ [6]. 3. mathematic modeling of a new kind of buoyancy artefact for the sophisticated calculation of the inlay artefact, a number of limiting conditions have to be considered. for example, the surface area must correspond to a specified reference surface area and at the same time the mass must correspond to a specified nominal mass. in addition to the basic cylindrical geometry, the geometries of the spherical distance pieces and chamfers have to be taken into account for constructive reasons. based on all these complex and correlated specifications, the radii and heights of the discs and the cylinder must be precisely dimensioned. in this section, first the calculations for the surface areas and then the masses are outlined. the corrections caused by chamfers are addressed comprehensively in section 4, while corrections relating to distance pieces are explained in section 5. in the following, this refers to all subsequent calculations that correlate with chamfers and distance pieces. 3.1. calculation of the surface areas for cylinder and discs the pure surface area 𝐴dsc of all discs, without any corrections for chamfers and distance pieces, depends on the given surface area of the reference 𝐴ref, and the surface area ratio of discs to cylinder, denoted 𝑟𝑎𝑡𝑖𝑜. this results in: 𝐴dsc = 𝐴ref 𝑟𝑎𝑡𝑖𝑜 𝑟𝑎𝑡𝑖𝑜 + 1 − 𝐴corr,dsc,cal + 𝐴corr,dsc,cha (1) and includes the correction for chamfers 𝐴corr,dsc,cha, and the correction for distance pieces 𝐴corr,dsc,cal. the surface of the cylindrical part is obtained as: 𝐴cyl = 𝐴ref 1 𝑟𝑎𝑡𝑖𝑜 + 1 − 𝐴corr,cyl,cal + 𝐴corr,cyl,cha (2) including the reference surface 𝐴ref , surface area ratio of discs to cylinder 𝑟𝑎𝑡𝑖𝑜 and the correction for distance pieces 𝐴corr,cyl,cal as well as the correction for chamfers 𝐴corr,cyl,cha. the height of a disc ℎdsc is calculated from: ℎdsc = 𝐴dsc 𝑛dsc − 2 π ∙ 𝑟dsc 2 2 π ∙ 𝑟dsc (3) with number of discs 𝑛dsc and radius of the disc(s) 𝑟dsc. the height ℎcyl of a cylinder with radius 𝑟cyl can be calculated as: ℎcyl = 𝐴cyl − 2 π ∙ 𝑟cyl 2 2 π ∙ 𝑟cyl (4) with the surface area of the cylinder 𝐴cyl calculated in equation (2). 3.2. calculation of masses of cylinder and discs the calculation of the total mass shall be composed of the calculation of the mass of the individual discs 𝑀dsc and the mass of the cylinder 𝑀cyl . the discs are designed on the basis of the previously determined heights and radii. the total mass of the cylinder 𝑀cyl consisting of the silicon shell and the inlay core of tungsten is calculated, knowing the reference mass 𝑀ref and the mass of all discs 𝑀dsc, by means of: 𝑀cyl = 𝑀ref − 𝑀dsc (5) the further steps are simplified if only cylindrical geometries are used for calculations. in order to obtain these principles, the masses of the distance pieces and chamfers are extracted from the surrounding silicon cylinder. the pure cylinder mass 𝑀cyl,pure is given by: 𝑀cyl,pure = 𝑀cyl − 𝑀corr,cal + 𝑀corr,cha (6) http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 44 as previously stated, the corrections for the distance pieces 𝑀corr,cal and chamfers 𝑀corr,cha will be shown in sections 4 and 5. having the volume of the cylinder 𝑉cyl,si,pure , the mass of the tungsten core 𝑀cyl,w , can be described by: 𝑀cyl,w = 𝑀cyl,pure− 𝜌si∙𝑉cyl,si,pure 𝜌w−𝜌si ∙ 𝜌w (7) including the corresponding densities of silicon 𝜌si and tungsten 𝜌w. according to: 𝑉cyl,si,pure = 𝑉cyl,si − 𝑉corr,cyl,cal + 𝑉corr,cyl,cha (8) the value of the reduced volume 𝑉cyl,si,pure of the silicon cylinder considers the volumetric corrections influenced by both, the spherical distance pieces 𝑉corr,cyl,cal and the chamfer 𝑉corr,cyl,cha. for reasons of symmetry and to simplify the calculation elegantly, the geometric proportions of the enclosing silicon cylinder 𝑟cyl , ℎcyl and the internal tungsten core are assumed to be identical, so that: 𝑟cyl ℎcyl = 𝑟cyl,w ℎcyl,w (9) both the radius 𝑟cyl,w of the inlay cylinder, given by: 𝑟cyl,w = √ 𝑉cyl,w π 𝑟cyl ℎcyl 3 (10) and the height of the inlay cylinder ℎcyl,w, given by: ℎcyl,w = 𝑟cyl,w ∙ 𝑟cyl ℎcyl (11) are calculated from the fixed ratio of the geometry of the silicon cylinder for reasons of symmetry. the effective density of the inlay artefact 𝜌ily is calculated as: 𝜌ily = 𝑀dsc + 𝑀cyl,si + 𝑀cyl,w 𝑉dsc + 𝑉cyl (12) derived from the mass of the discs and the mass of the cylinder with the inlay tungsten artefact. 4. correction of chamfer to correct the chamfers, volume, mass and surface area differences must be determined in comparison with the uncorrected discs and cylinder. the adjustment is classified by cylinder and chamfer. figure 2 shows a section through a cylinder or disc with a circumferential chamfer. according to the mathematical description in section 2, the radii 𝑟dsc or 𝑟cyl must be substituted for the variable 𝑟. the ring surface 𝐴rng, the lateral surface 𝐴shl of the uncorrected cylinder and the resulting chamfer surface 𝐴cha are shown under the chamfer angle α. figure 2: schematic cross section through a cylindrical object with straight chamfers 4.1. calculation of the surface areas for chamfer the correction 𝐴corr,cha depends on the surface of the cylinder shell surface area 𝐴shl , the ring surface area of the disc 𝐴rng and the surface area of the chamfer 𝐴cha. as a result: 𝐴corr,cha = (𝐴rng + 𝐴shl − 𝐴cha). (13) the surface area of an annular ring of the top face of a disc can be expressed as: 𝐴rng = π (𝑟 2 − (𝑟 − 𝑎 tan 𝛼 ) 2 ) (14) the surface area of a disc of solid material without chamfer can be calculated from: 𝐴shl = 2 π ∙ 𝑟 ∙ 𝑎 (15) for the surface correction of a chamfer the general approach for calculating rotational symmetric bodies [7]: 𝐴cha = 2 π ∫ 𝑓(𝑧) √1 + (𝑓 ′(𝑧)) 2 𝑑𝑧 𝑎 0 (16) is chosen. herein the function 𝑓(𝑧) describes the mathematical cross section of the rotatory body. 𝑓(𝑧) = 𝑟 − 𝑧 ∙ tan 𝛼 (17) hence, the correction of the chamfer surface area results in: 𝐴cha = 2 π ∫ (𝑟 − 𝑧 ∙ tan 𝛼) ∙ √1 + (tan 𝛼) 2 𝑑𝑧 𝑎 0 (18) after resolving the integral, the final correction for all chamfers is: 𝐴corr,cha = 𝑛dsc ∙ 𝑛cha ∙ ( −2 π √1 + (tan 𝛼)2 ∙ (𝑎 ∙ 𝑟 − 𝑎2 2 ) + 2 π ∙ 𝑟 ∙ 𝑎 + π (𝑟2 − (𝑟 − 𝑎 tan 𝛼 ) 2 ) ) (19) http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 45 4.2. calculation of volume and mass of chamfers the mathematical theory results from the approach to calculate a quarter torus. the general formula for the volume of a rotational symmetric object is [7]: 𝑉 = π ∫ (𝑓(𝑧)) 2 𝑑𝑧 𝑎 0 (20) the resulting correction for volume of chamfers is described by the uncorrected cylinder volume 𝑉cyl subtracted by the chamfer volume correction 𝑉corr,cha which results in: 𝑉cha = 𝑉cyl − 𝑉corr,cha (21) the detailed relation of this correction reads: 𝑉corr,cha = π ∫ 𝑟2 𝑑𝑧 𝑎 0 − π ∫ (𝑟 − 𝑧 ∙ tan 𝛼)2𝑑𝑧 𝑎 0 (22) after solving the integrals, the final volume correction 𝑉corr,cha becomes: 𝑉corr,cha = 𝑛cha ∙ (π 𝑟 2𝑎 − π (𝑟 2 ∙ 𝑎 + 𝑎3 3 ∙ (tan 𝛼)2 − 𝑟 ∙ 𝑎2 ∙ tan 𝛼)) (23) assuming the known density 𝜌si and the determined correction 𝑉corr,cha, the adjustment for the mass of the chamfers is: 𝑀corr,cha = 𝜌si ∙ 𝑉corr,cha (24) 5. correction of distance pieces the distance pieces enlarge the mass and surface area of the entire inlay artefact. hence, corrective calculations are necessary. distance pieces are placed on the top and bottom of the discs and the cylinder in relation to each other as shown in figure 3. distance pieces can be attached firmly to a disc in various methods, for example with a bond connection or high-vacuum compatible adhesive. figure 3: illustration of gap with spherical distance pieces (silicon spheres, 𝑟 = 2 mm, 𝛽 = 45°) in this context, only the upper half of the distance pieces must be considered for the correction of mass and surface area. table 2 lists the relation between the number of distance pieces and the disc type or cylinder. good mechanic design points out that a six-point mounting is the most stable stacking method for statically determined bearings. this avoids wiggling or instability of couplings. the selected stacking method requires exactly nine distance pieces per gap. this results in six distance pieces which are fixed on the top side of the cylinder. in addition, for any center disc, there are six distance pieces on the bottom side, and three further distance pieces on the top side. finally, there are three distance pieces left for the top disc. table 2: the total number of distance pieces per top disc, centre disc or cylinder and numbers for variable 𝑛, where 𝑛 depends on the number of total discs type of discs/cylinder number of distance pieces (𝒌) numbers for variable (𝒏) top disc 3 2 center disc(s) 9 𝑛dsc cylinder 6 2 5.1. calculation of the surface areas for spherical distance pieces the surface area correction 𝐴corr,cpl derives from the spherical surface area 𝐴cpl. however, the surface area of the cylinder 𝐴cir which is covered by the base of the hemisphere must be subtracted as: 𝐴corr,cpl = 𝐴cpl − 𝐴cir (25) using the relevant parameters regarding the number of distance pieces 𝑘 the correction becomes: 𝐴corr,cpl = 𝑘 2 ∙ 4 π ∙ 𝑟cpl 2 − k ∙ π ∙ 𝑟cpl 2 (26) 5.2. calculation of volume and mass of spherical distance pieces the volume correction through all distance pieces is made considering the number of discs 𝑛. polished silicon spheres with a radius of 2 mm serve as distance pieces. depending on the type of disc or cylinder, different numbers of the spherical distance pieces 𝑘 must be inserted into the calculations. this results in the equation for the volume correction 𝑉corr,cpl according to: 𝑉corr,cpl = (𝑛 − 1) ∙ 𝑘 2 ∙ 4 3 π ∙ 𝑟cpl 3 (27) knowing the density 𝜌si and the calculated correction 𝑉corr,cpl leads to the adjustment for the mass of the distance pieces 𝑀corr,cpl as: 𝑀corr,cpl = 𝜌si ∙ 𝑉corr,cpl (28) http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 46 6. results the parameters for height and radius of the discs and the cylinder are determined numerically by a minimizing fit algorithm (newtonian method) provided by mathematica [8]. the solution fulfilling all constraints can be achieved, if discs and the cylinder have an identical diameter and a minimum number of discs is used. this results in a volume difference of approximately 315 cm³ between the new inlay artefact and a silicon buoyancy hollow cylinder with a volume of approximately 646 cm³ [1]. to sum up, 1690 iteration runs were evaluated. table 3 shows one solution for the fitted values for the designed inlay artefact, which fulfills all constraints. table 3: model parameters for an inlay artefact input parameters value unit mass 1 kg ratio 2 radius of disc(s) 0.034 m radius of cylinder 0.034 m chamfer angle 45 ° chamfer length 0.000 5 m fitted parameters value unit mass of tungsten inlay 0.261 42 kg mass of silicon 0.738 58 kg number of discs 2 total height 0.096 689 8 m height of one disc 0.030 187 1 m height of cylinder 0.030 658 8 m height of tungsten core 0.015 204 4 m radius of tungsten core 0.016 861 4 m radial wall thickness si cylinder 0.017 138 6 m axial wall thickness si cylinder 0.007 727 2 m results value unit volume 0.000 330 60 m³ effective density of new inlay artefact 3024.78 kg/m³ 7. summary the aim of this theoretical design study of a new inlay buoyancy artefact was to create a buoyancy artefact of nominal mass 1 kg with a small volume compared to other buoyancy artefacts. the small volume of the artefact, resulting in a particularly low air buoyancy was achieved by a combination of discs and a cylinder made of silicon, with a tungsten core inside. in contrast to conventional artefacts made of stainless steel, the newly designed artefact can be efficiently and gently cleaned the same way as the reference, like a silicon sphere. special focus is given on the correction of chamfer and spherical distance pieces. a version of the inlay artefact is exemplarily calculated and illustrated. the theoretical design study for parameters, i.e. radius and height of the silicon parts and the tungsten core, were determined by numerous constraints using a variation method and could be a basis for future manufacturing. 8. references [1] k. lehrmann, r. tutsch, f. härtig, “design of sorption and buoyancy artefacts made of silicon”, imeko, cavtat-dubrovnik, 2020, not yet published. [2] d. knopf, th. wiedenhöfer, k. lehrmann, f. härtig, “a quantum of action on a scale? dissemination of the quantum based kilogram”, metrologia 56 024003, 2019. [3] m. kochsiek, m. gläser (eds.), “comprehensive mass metrology”, isbn 3-527-29614-x, p. 255, vch, berlin, 2000. [4] din 12242-1, laboratory glassware; interchangeable conical ground joints, dimensions, tolerances, 1980. [5] ptb internal density determination by hydrostatic weighing at 20 °c, may 2020. [6] n. greenwood, a. earnshaw, chemie der elemente, edition 1, vch, weinheim, isbn 3527-26169-9, s. 1291, 1988. [7] t. ahrens et al., mathematik, ch. 11 heidelberg, isbn 978-3-8274-1758-9, 2008. [8] www.wolfram.com/mathematica/ (2020-05-08). http://www.imeko.org/ http://www.wolfram.com/mathematica/ the use of image and laser scanner survey archives for cultural heritage 3d modelling and change analysis acta imeko issn: 2221-870x march 2021, volume 10, number 1, 114 121 acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 114 the use of image and laser scanner survey archives for cultural heritage 3d modelling and change analysis gabriella caroti1, isabel martínez-espejo zaragoza1, andrea piemonte1 1 civil and industrial engineering department (dici)/university of pisa, largo lucio lazzarino 1, 56122 pisa, italy section: research paper keywords: cultural heritage; photogrammetry; laser scanner; bim citation: isabel martinez espejo zaragoza, gabriella caroti, andrea piemonte, the use of image and laser scanner survey archives for cultural heritage 3d modelling and change analysis, acta imeko, vol. 10, no. 1, article 15, march 2021, identifier: imeko-acta-10 (2021)-01-15 section editor: carlo carobbi, university of florence, italy received april 30, 2020; in final form august 24, 2020; published march 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was partially funded by pra 2017/12 university of pisa project corresponding author: isabel martínez-espejo zaragoza, e-mail: isabel.zaragoza@unipi.it 1. introduction surveying and documenting cultural heritage is essential for its protection and sustainable management [1]. in the last decades, new instruments and innovative surveying methodologies have brought fresh data and insights to the field of cultural heritage. these methodologies are at once innovative, simplified, user-friendly and challenging for researchers. in addition to an understanding of the methodologies and techniques, these investigations require the full knowledge of a cultural heritage object, including its original concept, the timeline of any modification, its current conditions, the causes of the decay and their historical contextualisation, etc. for this purpose, surveys provide valuable support in the investigation of historical sources, whether bibliographical, documental or iconographical, and can help in maintaining the consistency of geometry, materials and build. photogrammetry methodologies play a prominent role due to the availability of a vast array of both historical and current photographical images of cultural heritage. such an array is of foremost importance, as it provides information that is valuable for preliminary investigations, e.g. evidence of past interventions that are difficult to trace or a presentation of a building’s current condition [2]. historical photogrammetry (hp) has been used in several cultural heritage investigations in order to assess whether additional information can be provided by referring these images to new 3d survey models and to evaluate the potential offered by this kind of operation [3]-[8]. laser scanning, both terrestrial and airborne, is possibly the most important surveying technology developed over the last 20 years. in this time span, its use as a means of producing dense point clouds for documenting, mapping and multi-scale viewing purposes has evolved, and it is now a standard approach [9]-[12]. moreover, classical photogrammetry is regularly integrated with other types of geomatic investigation. abstract cultural heritage studies often require the analysis of buildings that have undergone several changes and alterations during their lifetime. this often implies the loss of architectural elements or the construction of new elements, which both change the characteristics of the former buildings. the recovery of lost elements or structures through virtual reconstruction is of paramount importance in both scientific and cultural applications. novel procedures in surveying and photogrammetric processing including historical photogrammetry and historical terrestrial laser scanning offer powerful tools that enable the extraction of geometric information from historical documentation such as archival images. this paper presents the integration of a metric 3d model with information present in archival surveys of lost architectural volumes. the methodology implies the availability of historical plans representing the survey object at scales consistent with uav surveys and featuring shared elements. the methodology used to frame these plans in the reference system of the uav survey for an open source gis environment is also described as well as the accuracy checks. finally, the procedure followed for the virtual reconstruction of the fortezza in a bim environment, which produced a model derived from the integration of historic and current data, is described. mailto:isabel.zaragoza@unipi.it acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 115 the use of laser scanning in cultural heritage surveying and documenting for over 20 years has in turn led to a large archive of real-scale dense point clouds, which provides support for architectural research and historical cataloguing and makes it possible to perform multi-temporal comparisons and investigations. as with hp, data processing methodologies for historical terrestrial laser scanning (htls) also present some difficulties. first, htls data usually come in proprietary formats, which are not readily convertible to current standards. in addition, several htls data sets only provide 3d coordinates and reflectance values and lack any colour information, which can pose additional problems during manual point registration, which is necessary in cases where the cloud-matching algorithms are unsuccessful, e.g. due to insufficient overlap between adjacent scans. the lack of colour information could further complicate the detection of tie points for the orientation of historical images. finally, the density and precision of htls clouds reflect the technological limitations of the time periods in which the original images were created. the aim of the current study is to define a methodology for the recovery of htls and hp data in order to generate 3d models of survey objects to support multi-temporal investigations of the geometry of objects that have been subjected to alterations, modifications and changes of use over time. the investigation focused on two case studies: a section of the medieval urban walls by porta san zeno (st. zeno’s gate) in pisa and the fortezza vecchia (old fortress) site in livorno, italy. porta san zeno underwent major restoration and improvement activities starting in 2012. preliminary surveys, however, started in 2010 and provided historical tls, photogrammetry and photographic data that was used for the present investigation. more recent photographic data reporting the results of the restoration and improvement interventions was also used. the fortezza vecchia suffered several volume losses due to restoration interventions both during and after world war ii. in this case, archive images were used to create a metric 3d reconstruction of the palazzo di cosimo de’ medici. 2. materials 2.1. urban walls in pisa porta san zeno, one of the medieval gates that provided access to the city of pisa, is located near its namesake church (figure 1, top). its current setting dates back to 1935, when it was reopened in order to alleviate urban vehicular traffic by providing a route towards ss12 and sp2 (via del brennero and via calcesana). the defensive complex includes a round arch complemented by a lowered arch curtain built in verruca stone, following the pisa custom, and supported by rectangular pillars, also in verruca stone, topped with plain capitals. in the intrados, the stone rings, which originally provided support for the gates’ hinges, are still visible. the face in which the arch is set is made of asciano breccia that has been neatly cut into rectangular ashlars. it was built between 1156 and 1158 as a closure for a wall section joining two existing sections along the north-east direction by st. zeno’s abbey. the wall section including the gate was later involved in the reinforcement of urban defences following the defeat suffered by pisa in the battle of the meloria in 1284. during this time, the wall was widened and a moat was dug along its outer side. the section of urban walls between the gates of lucca and st. zeno retains its original inner and outer brackets [13]. 2.1.1. 2010 survey campaign in 2010, astro laboratory carried out a surveying campaign in order to document the current status of the wall sections around the gate. the surveys employed two different techniques: a tls survey using a riegl lms-z420i laser scanner and a photogrammetric survey using both a semi-metric rollei d7 digital camera and an amateur digital nikon d40x camera (figure 1, bottom). the riegl lms-z420i laser scanner automatically associates laser scans with high-resolution images collected by a calibrated camera installed in a dedicated socket on top of the device. it has a precision of 10 mm in up to a 50 m range (1σ @ 50 m range under riegl’s test conditions). the integrated camera used in the survey was a nikon d70 (figure 1, bottom) digital camera fitted with a fixed focus (f = 20 mm) lens, whose other features included a 6.1 mp, 23.7 × 15.6 mm2 sensor and a 2000 px × 3008 px image size. the rollei d7, which was used in the photogrammetry survey, had a 28 mm fixed focus (f = 7 mm, 1:2.8) lens on a 35 mm camera with a 5 mp, 8.8 × 6.6 mm sensor and a 2552 px × 1920 px image size. the nikon d40x had a variable focus (f = 18 – 55 mm) lens, a 10.2 mp, 23.6 × 15.8 mm2 sensor and a 3872 px × 2592 px image size. these images were intended for photographic straightening and therefore have no or very poor mutual overlap. since no photogrammetric modelling was planned, the focal length was also inconsistent between images collected with the nikon d40x camera. the nikon d70 also collected some context images of the arch as supporting documentation. 2.1.2. 2019 survey campaign figure 1. photogrammetric point cloud from historic images (top) and cameras used for collecting historic images (bottom). acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 116 the 2019 survey campaign carried out by astro laboratory integrated tls with structure from motion (sfm)and multiview stereo (mvs)-based photogrammetry (figure 2, top). the instrument used for the tls survey was a leica c10 scanstation with sub-centimetre precision and an on-board camera. photographs were collected by a nikon d750 digital camera (figure 2, bottom) fitted with a fixed focus (f = 50 mm) lens at an average range of about 10 m, resulting in a ground sampling distance of about 1.2 mm. a topographical survey performed with a leica tcrp 1201+ total station provided the coordinates of the ground control points (gcps) and therefore a shared reference system for tls and photogrammetry, as well as the ability to scale the latter. 2.2. the fortezza vecchia the fortezza vecchia in livorno, tuscany, is the last of an array of fortifications designed by architects giuliano and antonio da sangallo between 1488 and 1519, with which they experimented and improved upon the modern outline of fortifications with corner bastions. the fortezza vecchia complex included a preexisting fortification, known as quadratura dei pisani, that was built around the second half of the 14th century to strengthen an existing medieval keep. pre-existing structures as well as the site itself, which is surrounded by the sea, deeply affected the building process, resulting in several anomalies and departures from the ideal regular form pursued by renaissance military architecture [14]. 2.2.1. archive documentation historic documents referring to fortezza vecchia include a set of plans dating from 1669 to 1676 [15] that show the interior layout and the design of public and residential spaces. a further set of plans, dating from the 18th to the 20th century tracks multiple changes in the intended use of the complex after it ceased to serve as a military fortress [16]. photographs predating wwii show the size of the complex and the huge barracks located in the large squares of the fortress. early 20th century images mostly show global views of the complex. both the 18th and 19th century plans and the 20th century images record ongoing construction constraints due to lack of space. 2.2.2. current survey the astro laboratory status quo reference survey was made available by the north tyrrhenian sea port system authority within the framework of the 2017 pra research project ‘tuscany’s renaissance architectures: case studies between historical investigation, survey and structural analysis’, funded by the university of pisa. this survey of the fortress was based on uav-borne imagery, which enabled the reconstruction of a 3d model of the exterior. in addition, the uav-borne imagery was used to detect homologous points for the orientation of archive images based on unchanged architectural details. a fixed focus (f = 20 mm – 35 mm format equivalent) camera with a 12.4 mp (image size 4000 px × 3000 px) 1/2.3" cmos sensor set onto a dji fc330 uav was used to collect a total of 110 images, which were divided into two sets based on their flight levels relative to the top of the walls (40 m and 60 m). 3. methods and results 3.1. urban walls in pisa the data sets described above yielded two models. in the first case, all of the images collected by the cameras in the 2010 survey were processed using sfm and mvs to generate a 3d photogrammetric model (figure 1, top). orientation with automatic tie-point detection was successfully performed on 77 out of 92 images collected with the three cameras. the photogrammetric alignment for this model presented some problems due to both different image resolutions and quality and insufficient overlap, since the photographs were intended for the production of photoplans. in addition, the model has poorly defined sections and minor morphological issues resulting from variable focal lengths, pixel (px) sizes and image sizes. for these reasons, manual input was required for about 35 tie points in order to assist the photogrammetry software in correctly matching homologous points. the precision of the resulting model was assessed using check points (cps), the coordinates of which were derived from the tls survey. tls provided the coordinates of some of the gcps used for the photogrammetric processing (8 points). selecting another 10 control points for alignment checking yielded a mean error of 2.8 px / 5 cm. this model represents the survey object prior to any restoration intervention, showing the volumes and the decay. the 2019 survey generated a further high-precision photogrammetry model, representing the survey object after the restoration (figure 2, top). the comparison between these models provides important documentation of the changes and makes it possible to analyse variations in volumes and geometry and determine the impact of the restoration and the extant of structural decay. the 2010 model reveals several pathologies, such as the lack of crowning on the majority of the battlement, geometric disruptions and fractures and the presence of biotic film and figure 2. (top) photogrammetric model from 2019 survey. (bottom) nikon d750 camera. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 117 shrubs growing on the walls, breaking up the building materials and affecting the optimal structural conditions. moreover, the 2019 model makes it possible to track the geometrical changes that have accrued over time, such as the integration of volumes that modified previous shapes and the addition of external elements allowing for safe pedestrian movement. the removal of biotic film and the restoration of fractures can also be observed, along with the ongoing growth of shrubs and other plants continuing their disruptive action on joints. the two clouds have been compared in order to visually detect the morphological changes that have occurred over time (figure 3). 3.2. the fortezza vecchia after the second world war, the palazzo di cosimo was razed in some areas, while in others the walls suffered partial damage. a virtual reconstruction of the building using building information modelling (bim) software, in this case revit, was made to enable the study and historical documentation of these changes. the reference period for the reconstruction is the early 20th century, before the war and the partial destruction of the building. historical archive photographs that date back to this period are available. the virtual reconstruction is based on three key elements: ancient plans designed in the 18th century, photographs taken between the late 1800s and the early 1900s and the 3d model of the current situation obtained by the photogrammetric processing of airborne imagery. 3.2.1. measures obtained from the historic plans the 18th century plans of the palazzo were scanned at a resolution of 300 dpi (6370 px × 4632 px and 53.93 × 39.22 cm²). the image included a graphic scale indicated on the florentine coat of arms. in order to define the scale of the representation and therefore the real size of a single pixel, some linear measurements obtained from the historical plan were compared with those obtained from the photogrammetric model. from this comparison, it was determined that a pixel corresponds to a size of about 2 cm. a new graphic scale in metres was therefore inserted onto the image (figure 4). in order to be able to use these scans in the definition of the 3d model, it was necessary to georeference them and to minimise deformation errors as much as possible. georeferencing of the historical plan was performed in the qgis environment using a first-degree polynomial geometric transformation, upon which it was resampled using the nearest neighbour algorithm. to determine the transformation parameters, ten recognisable points were identified both in the paper plan and in the orthophotograph obtained from the photogrammetric survey (figure 5). six of these points were used as gcps to calculate the transformation parameters, and four were used as cps. the residuals for the gcps, which give an indication of the accuracy, were on average 42 pixels, while the residuals for the cps, which indicate precision, were on average 45 pixels. these results agree in terms of their order of magnitude, indicating that the solution provided is acceptable for the transformation. as stated above, the scanned plan's pixel size in metric units is equal to about 2 cm, so, based on the mean pixel error, the plan is accurate to within 84 cm and precise to within 90 cm. considering this type of historical plan, its state of conservation and the deformations that the support may have had over time as well as the graphical and metrical accuracy of the archive surveys, the authors believe that the achieved results of the transformation were adequate for the subsequent work of integrating and verifying the data obtained from the photogrammetric processing of the archival images. 3.2.2. 3d points definition through historical images figure 3. comparison between 2010 and 2019 photogrammetric models. figure 4. eighteenth century plan of the palazzo di cosimo. figure 5. gcps and cps on 18th century plan. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 118 in this case, the images had not been collected by digital cameras with known features; instead, analogue archival images were used. processing analogue photographs, especially older ones, is more complex than processing digital ones due to the quality of the photographs as well as issues related to digitisation and technical equipment. however, it is even more difficult to process materials that were produced from different sources of uncertain origin and unknown parameters, especially if those images are of poor quality and include deformations [4]. when using archival images, much of the information that is needed for correct photogrammetric processing is lacking, so it is not possible to achieve homogeneous geometric precision or generate complete 3d models. nevertheless, in the case of the fortezza vecchia, it was possible to reconstruct lost volumes. for this purpose, however, in addition to strictly following geomatics guidelines, an in-depth historic–architectural investigation of the study object was required to fully comprehend and better exploit the available images. while the photogrammetric processing of these images did not allow for automated point cloud generation, it did provide geometric references for subsequent processing. points can be measured in separate images and subsequently calculated by ray intersection using orientation parameters calculated by bundle adjustment. although seemingly straightforward, this step in fact poses some practical issues. due to differences in image scale, radiometric and geometric resolution, shooting position, lighting conditions, etc., it is often quite difficult to detect the same point in two different images. coordinate precision for the resulting points is related to the lower precision of pairs of image coordinates and the conditions of ray intersections [17]. the photogrammetry project included the set of uav-borne images and five archival images dating to the 1930s. as already stated, the different features of the archive images and their shooting geometry, as well as major scene modifications, prevented the automatic detection of tie points, which were therefore detected manually. this was carried out with the following steps: 1) areas that have not changed or undergone major restoration since the 1930s were selected based on historical research; 2) architectural elements within these areas were surveyed in order to keep sighting errors on the images as low as possible; 3) points providing the most homogeneous layout were identified on both the images and the 3d model. a total of 52 points were selected and sighted (figure 6, top). half of the points were on images collected along the north– south direction and half were on images collected in the east– west direction. the large number of points was necessary for figure 6. tie points on the 3d model (top) and on the archival image (bottom). acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 119 bundle adjustment in order to reduce sighting errors caused by the poor resolution of the archival images as much as possible. a further 20 control points were selected for alignment checking. a comparison of the 3d coordinates of the cps with those of the photogrammetric model resulted in a mean error of 1.3 px / 12 cm. the integration of the uav-derived 3d model of the fortress with on-site analysis made it possible to outline the main walls of some buildings, such as those bounding the cortile del castello vecchio, while other areas, particularly on the west and south sides, were not so well-defined. identifying the points from the archive images (figure 6, bottom) in the uav-derived 3d model yielded the external outline of the south and west perimeter walls as well as most eaves and some window sills. the easiest way to obtain the 3d model of the vertical walls would have been using photogrammetry starting from ancient photos. a similar method was used in modelling porta san zeno. however, as previously stated, it was not possible to automatically obtain a dense point cloud from these images due to poor image quality and resolution. since the orientation parameters of these images were known, it was instead possible to use them to obtain the 3d coordinates of significant points (windows, doors, wall heights, etc.). those 3d coordinates were then exported to a cad environment and subsequently to revit. historical photogrammetry, therefore, makes it possible to obtain metric information that is unavailable in other graphic archival documentation. these findings enabled the creation of a 2d outline for the entire complex, which is in turn essential for subsequent 3d reconstruction. calculated elevations of lost buildings along with numerical information from historical documents provide a useful support for a possible reconstruction of the interior that could also take advantage of extant building portions (figure 7). 3.2.3. 3d virtual reconstruction the palazzo di cosimo de’ medici was partially reconstructed using autodesk’s revit bim software. for this purpose, the first step involved using autodesk’s recap software to export the point cloud of the status quo model in revit. a local reference system was also created, as per revit requirements, based on the wgs84 reference system of the photogrammetry model. this step enabled the use of autodesk’s autocad software to trace the plans of the three levels of the building, which were previously processed in qgis as stated in 3.2.1. next, in revit, the point cloud model provided the basis for the reconstruction of the first level of the building, corresponding to the ground level plan of the palazzo. in order to retrieve the elevation information for the other levels, horizontal sections of the point cloud were checked against 3d points detected on historic images. upon calculation of the height of the different levels, these were defined in revit and the matching plans were imported. this information was fed into revit to reconstruct the walls. the next step involved windows and roofs. while the 2d positions of the windows were extracted from the historical plans, assessing their height required photogrammetric information. using revit, different window families and sizes were then defined for each window type. to reconstruct the roofs, the slope was defined by detecting relevant points on historical images, which also supplied visual clues as to the roof type, e.g. gable or hip roofs. bim modelling also takes the building materials into consideration, but in this case, the poor quality of the historic images prevented the determination of this information. however, future integrations involving different areas of expertise such as archaeology or architectural history could be undertaken to address this question. any ambiguous or illegible information was filtered out of the modelling process. only sources that effectively cleared a given reliability threshold were used in the generation of the bim model. 4. conclusions tls is a well-established surveying methodology that provides an effective means of representing historical and architectural heritage items. a simple comparison between tls surveys carried out at a 9-year interval highlights the powerful evolution this technology has undergone in this time span. the new approach to photogrammetry based on sfm and mvs algorithms has led to major changes in the field of geomatics, as it is possible to collect high-density colour 3d point clouds that are quite similar in size to those obtained using tls. it is, however, obvious that the achievement of such results requires adherence to specific guidelines when planning and performing the photographic shoots. historic images, including those that predate any changes or those from archives, can provide important information for either the 3d reconstruction of lost architectural volumes or the achievement of a fuller understanding of the status of a building during a specific period. it is not usually possible to automatically generate point clouds for the survey objects during the photogrammetric processing of these images, particularly for images with lower quality and poorer geometry. even when this is possible, homogeneous geometric accuracy is not ensured. with regard to the fortezza vecchia, further processing yielded geometric information that, although it was not always sufficient for a full 3d reconstruction, did provide some references for the subsequent integration of other iconography. in the case of porta san zeno, which had a more consistent, natively digital image set, a 3d model was generated, providing a more detailed figure 7. uav-borne orthophotograph overlaid with the plan of the palazzo di cosimo de’ medici. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 120 documentation of the pre-restoration status quo. in both cases, the methodology shows great potential for the expanded documentation of historical architectural objects. 3d survey products are in ever-rising demand among the managers of built heritage. in this context, the ability to recover data surveyed in the past in view of the generation of 3d models is of the highest importance. the final models could be integrated with the status quo model by means of photogrammetric tie points for possible fruition on vr/ar platforms. acknowledgements financial support from the university of pisa under programme pra 2017, project ‘architetture toscane rinascimentali: casi studio fra indagine storica, rilievo e analisi strutturale’ is gratefully acknowledged. the collaboration of the port system authority of the northern tyrrhenian sea is gratefully acknowledged. references [1] b. k. means, 3d recording, documentation and management of cultural heritage, hist. archaeol. 51 (2017) pp. 582-583. doi: 10.1007/s41636-017-0055-x [2] p. grussenmeyer, o. al khalil, from metric image archives to point cloud reconstruction: case study of the great mosque of aleppo in syria, isprs int. arch. photogramm. remote sens. spat. inf. sci. xlii-2/w5 (2017) pp. 295-301. doi: 10.5194/isprs-archives-xlii-2-w5-295-2017 [3] m. g. bevilacqua, g. caroti, i. martínez-espejo zaragoza, a. piemonte, orientation of archive images on 3d digital models of painted vaults: an interesting tool for restorers, appl. geomatics 10 (2018) pp. 385-398. doi: 10.1007/s12518-018-0212-8 [4] m. g. bevilacqua, g. caroti, a. piemonte, d. ulivieri, reconstruction of lost architectural volumes by integration of photogrammetry from archive imagery with 3d models of the status quo, isprs int. arch. photogramm. remote sens. spat. inf. sci. xlii-2/w9 (2019) pp. 119-125. doi: 10.5194/isprs-archives-xlii-2-w9-119-2019 [5] g. bitelli, m. dellapasqua, v. a. girelli, s. sbaraglia, m. a. tinia, historical photogrammetry and terrestrial laser scanning for the 3d virtual reconstruction of destroyed structures: a case study in italy, isprs int. arch. photogramm. remote sens. spat. inf. sci. xlii-5/w1 (2017) pp. 113-119. doi: 10.5194/isprs-archives-xlii-5-w1-113-2017 [6] g. bitelli, v. girelli, m. marziali, a. zanutta, use of historical images for the documentation and the metrical study of cultural heritage by means of digital photogrammetric techniques, isprs arch. xxxvi-5/c5 (2007) p. 6. [7] a. grun, f. remondino, l. zhang, photogrammetric reconstruction of the great buddha of bamiyan, afghanistan, photogramm. rec. 19 (2004) pp. 177-199. doi: 10.1111/j.0031-868x.2004.00278.x [8] f. maiwald, t. vietze, d. schneider, f. henze, s. münster, f. niebling, photogrammetric analysis of historical image repositories for virtual reconstruction in the field of digital humanities, isprs int. arch. photogramm. remote sens. spat. inf. sci. xlii-2/w3 (2017) pp. 447-452. doi: 10.5194/isprs-archives-xlii-2-w3-447-2017 [9] a. cardaci, g. m. roberti, a. versaci, from the continuous to the discrete model: a laser scanning application to conservation projects, isprs int. arch. photogramm. remote sens. spat. inf. sci. xxxviii-5 (2012) pp. 437-444. doi: 10.5194/isprsarchives-xxxviii-5-w16-437-2011 [10] g. caroti, i. martínez-espejo zaragoza, a. piemonte, range and image based modelling: a way for frescoed vault texturing figure 8. virtual 3d model with and without the palazzo di cosimo reconstruction. https://doi.org/10.1007/s41636-017-0055-x https://doi.org/10.5194/isprs-archives-xlii-2-w5-295-2017 https://doi.org/10.1007/s12518-018-0212-8 https://doi.org/10.5194/isprs-archives-xlii-2-w9-119-2019 https://doi.org/10.5194/isprs-archives-xlii-5-w1-113-2017 https://doi.org/10.1111/j.0031-868x.2004.00278.x https://doi.org/10.5194/isprs-archives-xlii-2-w3-447-2017 https://doi.org/10.5194/isprsarchives-xxxviii-5-w16-437-2011 acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 121 optimization, isprs int. arch. photogramm. remote sens. spat. inf. sci. xl-5/w4 (2015) pp. 285-290. doi: 10.5194/isprsarchives-xl-5-w4-285-2015 [11] m. bevilacqua, g. caroti, i. martínez-espejo zaragoza, a. piemonte, frescoed vaults: accuracy controlled simplified methodology for planar development of three-dimensional textured models, remote sens. 8 (2016) p. 239. doi: 10.3390/rs8030239 [12] m. c. spreafico, l. perotti, f. cervi, m. bacenetti, g. bitelli, v. a. girelli, e. mandanici, m. a. tini, l. borgatti, terrestrial remote sensing techniques to complement conventional geomechanical surveys for the assessment of landslide hazard: the san leo case study (italy), eur. j. remote sens. 48 (2015) pp. 639-660. doi: 10.5721/eujrs20154835 [13] a. zampieri (editor), le mura di pisa. percorsi, pacini editore, pisa, 2014, isbn-10: 8863157731. [14] d. ulivieri, fortezza vecchia in livorno, nexus netw. j. 16 (2014) pp. 675-697. doi: 10.1007/s00004-014-0203-y [15] vv. aa., livorno: progetto e storia di una città tra il 1500 e il 1600, nistri-lischi e pacini, pisa, 1980. [16] g. piancastelli, p. nencini, la fortezza vecchia: difesa e simbolo della città di livorno, silvana editoriale, milano, 1995. [17] a. wiedemann, m. hemmleb, j. albertz, reconstruction of historical buildings based on images from the meydenbauer archives, int. arch. photogramm. remote sens. xxxiii-p (2000) pp. 887-893. https://doi.org/10.5194/isprsarchives-xl-5-w4-285-2015 https://doi.org/10.3390/rs8030239 https://doi.org/10.5721/eujrs20154835 https://doi.org/10.1007/s00004-014-0203-y introductory notes for the acta imeko tc3, tc5, tc16 and tc22 special issue 2020 acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 1 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 1 3 introductory notes for the acta imeko tc3, tc5, tc16, and tc22 special issue 2020 andy knott1, dirk röske2, renato reis machado3, jay hendricks4, dominik pražák5, davor zvizdić6, lovorka grgec bermanec6, gustavo ripper3 1 national physical laboratory (npl), united kingdom 2 physikalisch-technische bundesanstalt (ptb), germany 3 national institute of metrology, quality and technology (inmetro), brazil 4 national institute of standards and technology (nist), usa 5 czech metrology institute (cmi), czech republic 6 faculty of mechanical engineering and naval architecture, university of zagreb, croatia distinguished readers, the joint imeko tc3, tc5, tc16, and tc22 international conference was first scheduled to take place in cavtat-dubrovnik, croatia, in may 2020. when the coronavirus appeared earlier this year, the organisers decided to postpone the conference to november 2020, hoping that the occurrence of infections could be reduced to a minimum by autumn. due to the continuing spread of the virus which turned into a pandemic, the conference was again postponed, this time to 2022, as the xxiii imeko world congress was already scheduled for the year 2021. however, more than 100 extended abstracts had already been submitted to the conference and peer-reviewed, with about 80 scientific papers ready for publication by november 2020. the chairs of the international programme committee therefore decided to offer the authors the possibility to publish these papers in a special issue of acta imeko and nearly all authors have agreed. technical committee 3 of imeko (imeko tc3 – “measurement of force, mass and torque”) was established as a technical committee for the "measurement of force and mass" in 1967. considering the growing importance and new developments in the field of torque measurement, the name of the tc changed to "measurement of force, mass and torque" in 1998. the main topics include: mass, force, torque, and dynamic measurement of these quantities. technical committee 5 of imeko (imeko tc5 – “hardness measurement”) was established in 1973. tc5 provides a forum for the dissemination of measuring techniques and principles, and discussion about problems in connection with these techniques and principles, related to the physical quantities of hardness: rockwell, brinell, vickers, instrumented indentation and leeb hardness scales, as well as new definitions for hardness scales. technical committee 16 of imeko (imeko tc16 – “pressure and vacuum measurement”) was established in 1986. its main topics include: quantum realisations of the pascal, pressure standards, vacuum standards, mass spectrometry and primary leaks, pressure and vacuum sensors, dynamic pressure measurements, and recent, novel, and new advances in pressure and vacuum technology. technical committee 22 of imeko (imeko tc22 – “vibration measurement”) was founded in 2005. its main topics include: vibration sensors, vibration measuring devices, single and multi-axial vibration, dynamics of interacting systems, and dynamics of acceleration-related quantities. imeko technical committees tc3, tc5, tc16, and tc22 have a long history of joint events in many combinations and forms. this is to be expected because the topics of the four tcs are interrelated and often depend on each other’s achievable uncertainties and developments. those events and related publications have always had a significant impact on international interchange of scientific and technical information and the enhancement of international co-operation among scientists and engineers from research and industry in the imeko tc3, tc5, tc16, and tc22 fields of activity. acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 2 the papers in this issue appear in the order given by the tc numbers with two general papers at the end. the layout of the papers differs slightly from the usual layout of articles in regular issues of acta imeko. this reflects the fact that the papers had been submitted to a conference using the “conference layout”. we have just replaced the header by that of the journal on the first page of each article and the footer on each page. we also needed to adjust the page margins to keep the structure of the papers. we are convinced that these format changes have no influence on the scientific content of the articles. and here is what to expect on the next 415 pages: papers related to imeko tc3 measurement of force, mass and torque there are eleven papers in the general area of mass standards – the first nine cover a wide range of subjects related to mass laboratory activities including cleaning, simultaneous mass and volume determination, weighing of non-metal weights, weight handling, air buoyancy effects, and buoyancy artefact development (pp. 4-46), while the final two relate to current kibble balance projects (pp. 47-57). these are followed by four papers related to weighing applications – one analyses the structure of a truck scale (pp. 58-62), two look at weighing-in-motion systems (pp. 63-74), and the fourth covers vibration compensation in dynamic measurements (pp. 75-78). the one paper in the density area looks at a comparison of viscoelastic samples (pp. 79-84). within the force standards topic, there are six papers related to the design, analysis, calibration of, and comparison between force standard machines (pp. 85-108), two papers on microforce measurement (pp. 109-117), and a further two papers on dynamic force (pp. 118-128). in the area of force applications, there are five papers that look at different aspects of the performance of force measurement systems (pp. 129-155), followed by three papers that consider more industrial applications, including bolt stretcher calibration, alignment tests, and thread-grinding force measurement (pp. 156-167). the six torque papers comprise two looking at torque standard machines, two dealing with calibration procedures and comparison results, and the final two evaluating torque wrenches and transducer sensitivity (pp. 168-199). the tc3 contribution is completed by three papers that look at the instrumentation used for force transducers, in both the static and dynamic regimes (pp. 200-215). papers related to imeko tc5 hardness measurement the papers submitted to tc5 include contributions related to indenter geometries (pp. 216-225), standard machines (pp. 226-249), and comparisons (pp. 250-260). in addition, there are other contributions concerning microscope image resolution (pp. 261-264), the instrumented indentation test unloading curve (pp. 265-269), traceability of small force for vickers calibration (pp. 270-273), and an unmanned precise measurement device for rockwell hardness measurements (pp. 274-280). papers related to imeko tc16 pressure and vacuum measurement tc16 is pleased to present papers on the current challenges and recent advances in pressure and vacuum metrology. first, advances in the world-wide effort to establish a quantum-based realisation of the pascal through refractive index are presented in five papers (pp. 281-309). under the new si, adopted in may 2019, the quantum-based pascal has benefited from redefinitions of the kelvin, now traceable to the boltzmann constant, and the kilogram, now traceable to the planck constant. because refractometry depends only on the number-density of the gas and the gas species, it is realised as a primary standard when the refractive index of the gas is derived from quantum chemistry calculations and pressure-induced effects of laser cavity distortions are also independently known. challenges in putting the quantum pascal into practice abound and these papers address the latest efforts to do so. the second theme of the collection is advances in addressing the challenging need for primary dynamic pressure standards. two papers on dynamic pressure cover both the issue of developing primary dynamic pressures, and the development of dynamic pressure sensors (pp. 310-318). finally, while the first two topics address innovation in pressure and vacuum metrology, the third and final theme deals with recent advances in long-established, tried and trusted methods. it would be a mistake to assume that the traditional methods of classical pressure balances, liquid-column manometry, and static expansion techniques have no room for improvement. this final section, eight papers in all, addresses recent acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 3 developments and advances in these important classical techniques (pp. 319-356). tc16 is confident that you will enjoy the entire collection of papers covering these three major and important areas of: 1) the quantum pascal, 2) dynamic pressure, and 3) classical pressure and vacuum metrology. papers related to imeko tc22 vibration measurement the papers submitted to imeko tc22 include two contributions related to laser vibrometer calibration using optical methods (pp. 357-364) and four related to low-frequency vibration measurements (pp. 365-382). in addition, there are single contributions covering primary shock calibration using a fast linear drive (pp. 383387), simultaneous multi-axis vibration calibration (pp. 388-393), calibration of digital angular rate sensors (pp. 394-400), and interlaboratory vibration comparison results (pp. 401-406). general papers there are two papers in this category. the first one provides a methodology for estimating the dependence of strain gauge force and torque transducers on the ambient temperature (pp. 407-413). this effect is usually investigated in special chambers in which the transducer is exposed to various temperatures within a given range while the machine is operated under unchanged laboratory conditions. the application of standard uncertainty calculation methods to the linear regression of measurement results using the least-squares approach was investigated. the regression analysis of the results allows the temperature coefficient to be determined, including an uncertainty analysis. the second paper deals with accuracy as a concept that is widely used in metrology (pp. 414-418). difficulties regarding present definitions are explained, present-day use of the concept is explained, the question of whether accuracy is a qualitative concept is discussed, and finally pre-conceptual ideas and a proposal for a new wording of a sustainable definition for accuracy are presented. enjoy reading! new microforce generating machine using electromagnetic force acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 109 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 109 112 new microforce generating machine using electromagnetic force junfang zhu1, toshiyuki hayashi1, atsuhiro nishino1, koji ogushi1 1 national metrology institute of japan (nmij), aist, tsukuba, japan, zhu-junfang@aist.go.jp abstract: this paper reports a new microforce generating machine under development at nmij, aist. we proposed a new microforce generating method by referring to the principle of the kibble balance experiment and planned to apply it to a rotary type machine. microforce in the micronewton and millinewton range can precisely be generated using this newly developed machine. keywords: microforce, electromagnetic force, kibble balance, rotary type machine, force standard 1. introduction microforce measurements in the micronewton and millinewton range have been applied in many industries and research fields, and the reliability of these measurements becomes essential, for example, for the evaluation of material mechanical property using an atomic force microscope (afm). therefore, several national metrology institutes (nmis) have developed various facilities, including microforce comparators that use the principle of the electromagnetic balance, to establish the traceability of microforce [1–3]. the national metrology institute of japan (nmij), national institute of advanced industrial science and technology (aist), has developed a 2 n deadweight type force standard machine that uses gravity force and can extend the japanese force standard down to 10 mn [4]. the above facilities all need to be traceable to mass in the sub-gram and milligram range. however, there is a limitation due to factors such as manufacturing technology and calibration of small weights. by referring to the principle of kibble balance experiment [5, 6], and an electromagnetic force type torque standard machine developed in nmij [7], we proposed a new microforce generating method using electromagnetic force and planned to apply it to a rotary type machine. 2. principle of proposed method as in the kibble balance experiment, there are two modes in measurement: static mode and figure 1: principle of the proposed method dynamic mode, as shown in fig. 1. a rotating coordinate system was applied. in the static mode, when an electric current i flows through a rectangular coil with a rotation axis shown in the broken red line in a magnetic field of a magnetic flux density b, an electromagnetic force f is generated as: 𝐹 = 𝐵𝑙𝐼 or 𝐹 = −𝐵𝑙𝐼 , (1) where l is the length of the rectangular coil parallel to the rotation axis. these two forces are in opposite directions, and their magnitudes are equal. therefore, a torque t is generated around the b f r l d i b f f m i r i  f f t f m b i u   t (a) static mode (b) dynamic mode rotation axis rotation axis http://www.imeko.org/ mailto:zhu-junfang@aist.go.jp acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 110 rotation axis of the coil indicated by the broken red line as: 𝑇 = 𝑁𝐵𝐴𝐼 cos𝜃 , (2) where n is the number of turns of the rectangular coil, a (= ld) is the area of the rectangle coil. if an arm with a length r is attached from the center of the coil in rotation axis, a force f m can be generalized and expressed as: f m = 𝑇/𝑟 = 𝑁𝐵𝐴𝐼 cos𝜃/𝑟 . (3) in the dynamic mode, when the rectangular coil is rotated with the rotation axis indicated by the broken red line to in the magnetic field at a constant angular velocity , an induced electromotive force u is generated as: 𝑈 = 𝑁𝐵𝐴𝜔 sin𝜔𝑡 . (4) when the magnetic flux lines and the rectangular coil are parallel in both static and dynamic modes, f m and u reach their maximum values f mmax and u max . by considering the 𝑁𝐵𝐴 is a value associated with the machine developed, f mmax can be expressed as: f mmax = 𝑈max𝐼/(𝜔𝑟) . (5) therefore, f mmax is obtained from measuring u max and  in the dynamic mode, i in the static mode and r. 3. design figure 2 shows an overview of the new machine in developing. it mainly consists of a servo motor, a rotary encoder, a pair of neodymium magnets fixed to a yoke, a rectangular coil, a balancing arm, a selfdeveloped aerostatic bearing, a loading frame, counterweights, a laser displacement sensor, and a lifting mechanism of the force transducer. a force transducer to be calibrated is installed on top of an exchangeable x, y, θ stage. more details of main mechanisms are described in the following sections. servo motor rotary encoder aerostatic bearing rectangular coil neodymium magnets laser displacement sensor (head) lifting mechanism force transducer balancing arm x, y, θ stage figure 2: overview of the new machine loading frame counterweights figure 3: arrangement of the magnets and the coil coupling servo motor rotary encoder drive shaft rectangular coil balancing arm aerostatic bearing neodymium magnets connecting shaft rotation axis http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 111 3.1. neodymium magnets and coil figure 3 shows the arrangement of the magnets and the coil viewed from the top. the neodymium magnets can be rotated with the rotation axis by connecting to the servo motor through a drive shaft and a coupling. the ring of the rotary encoder is mounted coaxially on the driveshaft to monitor the angle of the neodymium magnets. the coil is fixed to the balancing arm through a connecting shaft inserted in the aerostatic bearing. in fact, in the dynamic mode, the neodymium magnets are rotated by the servo motor, and the rectangular coil is fixed in an equilibrium position to induce the electromotive force u. we visually confirm the rectangular coil parallel to the neodymium magnets before the measuring of the dynamic mode. after measuring, a specific angular position of the neodymium magnets in u max can be defined. currently, the neodymium magnets have already been manufactured, the distribution of the magnetic flux density was evaluated by the manufacturer, and the average value of the magnetic field where the coil passes was estimated to be approximately 350 mt by the manufacturer's test report. we also have determined the area of the rectangular coil a, and the number of turns n is adjusted by referring to the estimated relationship between the magnitude of force generated f mmax and the magnitude of current i. 3.2. force transmission mechanism figure 4 shows an overview of the force transmission mechanism viewed from the front. the loading frame is linked to the balancing arm by a five-μm-thick metal band to act as an elastic hinge and can apply a compressive or tensile force on a force transducer. counterweights are set on the other end of the balancing arm and used to cancel out the force acting on the loading frame by monitor the equilibrium position of the balancing arm using a high precision laser displacement sensor. similar to the previous study [4], the aerostatic bearing is adopted as a high sensitivity less friction fulcrum. the length r indicating distance between fulcrum and force action point will be precisely measured and traceable to length standard. since the two ends of the balancing arm are processed into round ends with a radius of 250 mm centered on the fulcrum, it is considered that r does not change even if the balancing arm is rotated a certain angle. therefore, in the static mode, the neodymium magnets are fixed in the specific angular position, and the rectangular coil is free. when an electric current i flows through a rectangular coil, the torque t is generated, and the force f mmax can be applied to the force transducer in compressive calibration, as shown in fig. 4. 3.3. force transducer lifting mechanism again, the previous study showed the contact point between the loading frame and the force transducer under calibration has to be maintained during calibration irrespective of the height and stiffness of the transducer [4]. figure 5 shows the force transducer lifting mechanism newly developed with improvements by referring to the arrangement of the previous study [4]. this mechanism consists of a stepper motor, a coupling, a precision ball screw, four pairs of linear shafts and figure 4: force transmission mechanism balancing arm counterweights metal band metal band loading frame r loading jig fmmax t laser displacement sensor (head) round end round end installation table force transducer x, y, θ stage stepper motor coupling ball screw linear shaft linear bush figure 5: force transducers lifting mechanism http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 112 linear bushes, an installation table, and some connection parts. the force transducer to be calibrated is installed on top of an x, y, θ stage mounted on the installation table, and it is driven vertically by a stepper motor via a precision ball screw with a resolution of 1 μm. we can manually adjust the horizontal position and the rotation angle of the vertical axis of the force transducer in the x, y, and θ stages. however, in many cases of microforce transducers, the displacements of their elastic body in the rated capacity is small in several micrometer order; a lifting mechanism with the resolution in nanometer (nm) order is necessary in the future. 4. summary we proposed a new method for generating microforces by referring to the principle of kibble balance experiment. based on the technique, we are developing a newly rotary type machine. we plan to verify our proposed method after the machine completed. in the future, we expect to realize a microforce standard using the new microforce generating machine in nmij. 5. acknowledgments this work was supported by jsps kakenhi grant number jp18k45678. 6. references [1] c. schlegel, o. slanina, g. haucke, r. kumme, “construction of a standard force machine for the range of 100 μn-200 mn”, measurement vol. 45, pp. 2388–2392, 2012. [2] j. h. kim, y. k. park, m. s. kim, j. h. choi, and d. i. kang, “development of force standard machine in the milli-newton range based on electromagnetically compensated balance”, proceedings of asia-pacific symposium on measurement of mass, force and torque (apmf 2015), pp. 4–7, oct. 27–30, 2015, seoul, south korea, 2015. [3] g. hu, j. jiang, z. zhang, y. zhang, u. brand, and m. s. kim, “investigation of a small force standard with the mass based method”, acta imeko, vol. 6, no. 2, pp.13–20, 2017. [4] j. zhu, t. hayashi, a. nishino, and k. ogushi, “development of a 2 n dead-weight type force standard machine”, measurement, vol. 154, 107463 (6 pp), 2020. [5] b. p. kibble, “a measurement of the gyromagnetic ratio of the proton by the strong field method”, atomic masses and fundamental constants 5, pp. 545-551 1976. [6] b. p. kibble, i. a. robinson, and j. h. belliss, “a realisation of the si watt by the npl moving-coil balance”, metrologia, vol. 27.4, pp. 173-192, 1990. [7] a. nishino, k. ueda, and k. fujii, “design of a new torque standard machine based on a torque generation method using electromagnetic force”, meas. sci. technol., vol. 28, 025005 (11 pp), 2017. http://www.imeko.org/ simultaneous determination of mass and volume of a set of weights in group weighing acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 17 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 17 22 simultaneous determination of mass and volume of a set of weights in group weighing ch. wuethrich1, k. marti2 1 federal institute of metrology metas, switzerland, christian.wuethrich@metas.ch 2 federal institute of metrology metas, switzerland, kilian.marti@metas.ch abstract: this work introduces a new technique for the determination of mass and volume of a set of weights based on closed series (group weighing). a traditional closed series is repeated at two different air densities. a least squares set of equations, involving two lagrange multipliers, is used to determine the mass and the volume of each weight simultaneously with a traceability on the mass and the volume of the reference weight. keywords: double weighing; closed series; group weighing; volume determination 1. introduction the determination of mass in air is always strongly affected by air buoyancy. traditionally, the volume of weights is determined by hydrostatic weighing and used for the correction of air buoyancy. recently, several authors used double weighing in air of different density in order to determine the mass and the volume of an unknown artefact [1, 2]. the determination of the sub-multiples and the multiples of the kilogram still requires, in order to have an independent definition of the mass at nmi level, the volume determination through hydrostatic weighing [3]. we propose a technique of closed series in which the volume is an unknown of the system and is determined simultaneously with the mass by a set of double weighings in air of different densities. 2. theoretical foundations 2.1. traditional closed series a closed series is traditionally defined by a weighing scheme (design matrix) 𝑿 which is given as a combination of 0, -1 or +1 depending on whether or not the weight is involved in the comparison and whether the weight is considered to be a positive or a negative contribution to the weighing equation. the matrix 𝑿 has p columns, corresponding to the number of weights involved in the process and n rows, representing the number of weighing operations. it is possible to determine the mass values of the weights if p = n but it is better, in order to verify the consistency of the measurements, to have redundancy with n > p. this way, the determination of the mass values is realized by calculating the least squares of the overdetermined system of equation by applying the lagrange multiplier method described by kochsiek et al. [4]. this technique is widely spread in the determination of the conventional value of the mass. 2.2. closed series with known volumes and air buoyancy correction in order to determine the true mass, the matrix 𝑿 is converted to �̂� which incorporates a correction for the air buoyancy of each weight at each air density. we define the elements of �̂� in the following way: �̂�𝑖,𝑗 = 𝑥𝑖,𝑗 (1− 𝜌𝑖 𝐷𝑗 ) (1) where 𝜌𝑖 is the air density during weighing i and 𝐷𝑗 is the material density of weight j [5]. 2.3. closed series with unknown masses and unknown volumes in our innovative technique, the volumes of the weights are considered as unknowns of the system. we write the equation incorporating the buoyancy correction given by the unknown volumes. we repeat the weighing scheme 𝑿 and use it at two different nominal air densities. each individual air density is defined by its value 𝜌1 to 𝜌2𝑛. we define the new matrix 𝒁 with 2p columns and 2n rows. in matrix form we can write ( 𝑧1,1 ⋯ 𝑧1,𝑝 ⋮ ⋱ ⋮ 𝑧𝑛,1 ⋯ 𝑧𝑛,𝑝 ) = ( 𝑧𝑛+1,1 ⋯ 𝑧𝑛+1,𝑝 ⋮ ⋱ ⋮ 𝑧2𝑛,1 ⋯ 𝑧2𝑛,𝑝 ) = 𝑋 (2) and ( 𝑧1,𝑝+1 ⋯ 𝑧1,2𝑝 ⋮ ⋱ ⋮ 𝑧2𝑛,𝑝+1 ⋯ 𝑧2𝑛,2𝑝 ) = ⋯ … = −( 𝜌1 ⋯ 0 ⋮ ⋱ ⋮ 0 ⋯ 𝜌2𝑛 ) ( 𝑧1,1 ⋯ 𝑧1,𝑝 ⋮ ⋱ ⋮ 𝑧2𝑛,1 ⋯ 𝑧2𝑛,𝑝 ) (3) http://www.imeko.org/ mailto:christian.wuethrich@metas.ch mailto:kilian.marti@metas.ch acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 18 the unknown values of mass mi and volume vi of the weight i are expressed as �⃗� ∶= (𝑚1 …𝑚p𝑣1 …𝑣p) 𝑇 (4) where m1 and v1 are the mass and volume of the reference weight. the weighing process is given by 𝑍 ∙ �⃗� = �⃗� + 𝑒 (5) where �⃗� = (𝑦1 …𝑦2𝑛) 𝑇 is the weighing differences in the individual comparisons and 𝑒 = (𝑒1 …𝑒2𝑛) 𝑇 is the error associated with the weighing. the normal equation is given by 𝑍𝑇𝑍 ∙ �⃗� = 𝑍𝑇�⃗� + 𝑍𝑇𝑒 (6) to solve this redundant system of equations, we apply the lagrange multiplier method with two supplementary equations based on the multipliers 𝜆1 and 𝜆2 to fix both the mass and the volume of the reference weight. we add two columns and two rows of zeros and ones to 𝒁𝑻𝒁 and 𝒁𝑻. the matrices and vectors become 𝑍𝑇𝑍 = ⋯ = ( 𝜔1,1 … … … 𝜔1,𝑝 … … … 𝜔1,2𝑝 1 0 ⋮ ⋮ ⋮ 0 0 ⋮ ⋱ ⋮ ⋱ ⋮ ⋮ ⋮ ⋮ ⋮ ⋮ ⋮ ⋮ 𝜔𝑝,1 … … … 𝜔𝑝,𝑝 … … … 𝜔𝑝,2𝑝 0 0 ⋮ ⋮ ⋮ 0 1 ⋮ ⋱ ⋮ ⋱ ⋮ 0 0 ⋮ ⋮ ⋮ ⋮ ⋮ 𝜔2𝑝,1 … … … 𝜔2𝑝,𝑝 … … … 𝜔2𝑝,2𝑝 0 0 1 0 … … 0 0 0 … 0 0 0 0 0 … … 0 1 0 … 0 0 0) (7) 𝑍𝑇 = ( 𝜑1,1 … 𝜑1,𝑛 … 𝜑1,2𝑛 0 0 ⋮ ⋱ ⋮ ⋱ ⋮ ⋮ ⋮ 𝜑𝑝,1 … 𝜑𝑝,𝑛 … 𝜑𝑝,2𝑛 0 0 ⋮ ⋱ ⋮ ⋱ ⋮ ⋮ ⋮ 𝜑2𝑝,1 … 𝜑2𝑝,𝑛 … 𝜑2𝑝,2𝑛 0 0 0 … 0 … 0 1 0 0 … 0 … 0 0 1) (8) �⃗� = (𝑚1 …𝑚p𝑣1 …𝑣p 𝜆1 𝜆2) 𝑇 (9) �⃗� = (𝑦1 …𝑦2𝑛 𝑚𝑟 𝑣𝑟) 𝑇 (10) the solution matrix is given by 𝐿𝑆 ∶= (𝑍 𝑇𝑍)−1𝑍𝑇. (11) finally, the estimated values for the mass and the volume are 〈𝛾〉 = 𝐿𝑆�⃗�. (12) 2.4. monte carlo simulation we used a monte carlo simulation to estimate the measurement uncertainties of the mass and the volume of the weights. for this reason, we vary the input quantities which are the measured weighing difference from the balance, the air density, and the mass and the volume of the reference weight. furthermore, we assumed the input quantities to be normally distributed 𝑋~𝒩(𝜇,𝜎2). (13) for the mean value, µ, and the standard deviation, σ, we use the average value, �̅� , and the standard deviation, s, from the measurements. we assume no correlation between mass and volume of the 1 kg reference mass. after each variation, we re-calculate the mass and volume of the weights according to eq. (12). each re-calculation can be regarded as a random experiment, which is either real or, as in this case, numerically performed by a computer simulation. the number of re-calculations (iterations) is usually n = 10 000. the values for the mass and the volume of the weights as well as their standard uncertainties are given by the mean and standard deviation of the simulated values �̅�𝑗 = 1 𝑁𝐼𝑡𝑒𝑟 ∑ 𝛾𝑗,𝑝 𝑁𝑖𝑡𝑒𝑟 𝑝=1 (14) 𝑠𝛾𝑗 = √ 1 𝑁𝐼𝑡𝑒𝑟 − 1 ∑(𝛾𝑗,𝑝 − �̅�𝑗) 𝑁𝑖𝑡𝑒𝑟 𝑝=1 (15) where j represents the weight and p is the index of the iteration. 3. measurement procedure we determined the mass and the volume of a set of oiml weights from 10 g to 5 kg through double weighing measurements in three different mass comparators (see section 4). the first double weighing was carried out at about 950 hpa (ρair = 1.13 kg/m 3), the second at about 750 hpa (ρair = 0.90 kg/m 3). the volumes of the weights were additionally measured by hydrostatic weighing in our volume comparators (see section 4). the weighing schemes to cover the selected range of weights are as follows. ( −1 0 0 0 0 1 0 0 0 0 −1 1 0 0 0 0 1 0 0 0 0 −1 0 0 0 0 1 0 0 0 0 −1 0 1 1 0 0 0 1 0 0 −1 1 0 0 0 0 1 0 1 0 0 0 −1 0 0 0 0 1 0 0 0 0 −1 0 0 0 1 0 0 0 0 −1 0 0 0 0 1 0 0 0 0 0 −1 1 0 0 0 0 1 0 0 −1 0 1 0 0 0 0 1 0 0 0 0 −1 0 0 0 0 1) ( 𝑚100g 𝑚50g 𝑚20∗g 𝑚20g 𝑚10g 𝑚100𝑔 ′ 𝑚50𝑔 ′ 𝑚20∗𝑔 ′ 𝑚20𝑔 ′ 𝑚10𝑔 ′ ) (16) http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 19 ( −1 1 0 0 0 1 0 0 0 0 −1 0 0 0 1 0 0 0 0 −1 0 1 1 0 0 1 0 0 −1 1 0 0 0 1 0 1 0 0 0 −1 0 0 0 1 0 0 0 0 −1 0 0 1 0 0 0 0 1 −1 0 0 0 0 0 0 0 0 −1 1 0 0 0 1 0 0 0 0 −1 0 0 0 1) ( 𝑚1𝑘g 𝑚500g 𝑚200∗g 𝑚200g 𝑚100g 𝑚500g ′ 𝑚200∗g ′ 𝑚200g ′ 𝑚100g ′ ) (17) ( 0 0 0 0 −1 0 0 0 0 1 1 1 1 0 −1 0 0 0 1 0 0 0 0 −1 0 0 0 0 1 0 1 0 1 −1 0 0 0 1 0 0 0 1 0 −1 0 1 1 0 0 0 0 0 −1 0 0 0 0 1 0 0 0 0 −1 0 0 0 1 0 0 0 0 1 −1 0 0 0 0 0 0 0 1 0 −1 0 0 1 0 0 0 0 −1 0 0 0 0 1 0 0 0 0) ( 𝑚1𝑘g 𝑚2∗kg 𝑚2kg 𝑚5kg 𝑚10kg 𝑚1kg ′ 𝑚2∗kg ′ 𝑚2kg ′ 𝑚5kg ′ 𝑚10kg ′ ) (18) auxiliary weights are denoted by m'. they are necessary to stack individual weights on each other so that their sum corresponds to the nominal mass of the reference weight. 4. instrument setup we describe the technical specifications of our instruments for the double weighing measurements and the hydrostatic measurements. 4.1. mass comparators with constant air density to perform the double weighing measurements in different air densities, we used different mass comparators from mettler toledo: an at106, a mone, and an at10005 (table 1). all comparators are enclosed in an air-tight chamber in which the air density can be changed by varying the air pressure (figure 1). table 1: mass comparators with constant air density in the mass laboratory in metas. readability, repeatability, maximum load and weighing range of each comparator are shown at106 mone at10005 read. 1.0 µg 0.1 µg 10 µg repeat. 1.5 µg 0.5 µg 20 µg max load 111 g 1 001.5 g 10 011 g range 5g – 100 g 100 g – 1 kg 1 kg – 10 kg to change the air pressure we use a membrane pump (813.3, knf neuberger). the pressure, temperature and humidity of the air inside the chamber are recorded during the measurements. the pressure is measured by a druck dpi 141 precision pressure indicator with a range of (800 1150) hpa and a resolution of 0.01 hpa; and a fluke rpm4 with a range of (0 280) mpa and a resolution of 1 ppm. the temperature is determined with an accuracy of 0.01° c by means of 10 kω normal resistors (fluke), precision thermistors (ysi inc.), and a digital multimeter (keithley 2010). the humidity is measured by a hygrometer (hygrolab 2, rotronic ag) with a resolution of 1 %rh between (40 – 60) %rh. 4.2. volume comparators for the hydrostatic weighing, we used two different comparators from mettler-toledo: an at1005 for the volume comparator vk1, and a pr10003 for the volume comparator vk10 (table 2). the suspension is connected to the comparator and carries both the test weight and the reference weight (vk1) or the test weight only (vk10). the suspension is immersed in pure water (elga labwater, purelab). by means of the well-known water density, the volume of the test weight is determined. details can be found elsewhere [6]. table 2: volume comparators for hydrostatic weighing in the mass laboratory in metas at1005 pr10003 read. 10 µg 1 mg repeat. 20 µg 2 mg max load 1009 g 10100 g range ≤ 1 kg ≤ 10 kg 5. validation with a set of weights we measured a set of oiml weights ranging from 10 g to 5 kg in double weighing measurements according to the weighing schemes in section 3. we calculated the masses and the volumes according to the methods presented in sections 2.3 and 2.4. the volumes of the weights obtained from the double weighing measurements and from the hydrostatic measurements are summarised in table 3 with the expanded measurement uncertainties in brackets. we observed that the measurement uncertainty of the volume depends on the nominal mass of the weight. below 1 kg the uncertainty of the volume is smaller in the double weighing than in the hydrostatic weighing; above 1 kg the reverse is true. the relative measurement uncertainty of the volume depends on the nominal mass as well (figure 2). for the double weighing method the correlation between the relative measurement uncertainty and the nominal mass is negative below 1 kg (-0.60) and negative above 1 kg (-0.35). for the hydrostatic method the correlation is negative below 1 kg (-0.45) and negative above 1 kg (-0.31). note that weights smaller than 1 kg were measured on vk1 whereas weights larger than 1 kg were measured on vk10, which explains the step in the relative measurement uncertainty above 1 kg. http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 20 table 3: calculated volumes and expanded measurement uncertainties (k = 2) obtained from the double weighing and the hydrostatic weighing measurements nominal volume / cm³ mass double weighing water 10 g 1.2488(3) 1.2491(7) 20 g 2.5005(4) 2.5010(8) 20 g* 2.5004(3) 2.5004(2) 50 g 6.2444(6) 6.2450(6) 100 g 12.4889(5) 12.4903(8) 200 g 24.9838(5) 24.9833(6) 200 g* 24.9835(6) 24.9832(11) 500 g 62.4473(11) 62.4460(14) 1 kg 124.8739(20) 124.8739(20) 2 kg 249.8089(207) 249.8062(96) 2 kg* 249.8389(417) 249.8013(96) 5 kg 624.2566(615) 624.2107(106) figure 2: expanded relative uncertainty of the volume of the weights obtained from double weighing and hydrostatic weighing measurements for the hydrostatic weighing, the true (or conventional) mass of a weight is required to calculate the weight's volume. therefore, we had determined the mass of each weight on our comparators beforehand in a direct comparison to a reference weight. the results of the true mass deviation from the nominal mass value are summarised in table 4. additionally, in order to check the goodness of the equation matrices at the two pressures, the true mass deviations from the nominal mass values have also been calculated, from the first (950 hpa) and from the second (750 hpa) double weighing measurements and are shown in table 4 as well. the expanded measurement uncertainties are given in brackets and were calculated by using equation (15) on the basis of a monte carlo simulation. all measurement uncertainties refer to the uncertainty of mass and volume of the 1 kg reference weight (15-0-1kg), which were u(m) = 17 µg (k = 2) and u(v) = 2 mm³ (k = 2). note that these uncertainties were established before the introduction of the new definition of the kilogram. table 4: calculated true mass deviations from nominal mass values and expanded measurement uncertainties (k = 2) obtained from the 1st and 2nd double weighing (dbw) and from the hydrostatic weighing nominal mass mass deviation from nominal mass / mg 1st dbw 2nd dbw previous hydrostatic weighing 10 g 0.015(1) 0.015(1) 0.014(3) 20 g 0.016(1) 0.016(1) 0.004(3) 20 g* 0.040(1) 0.040(1) 0.032(2) 50 g 0.011(1) 0.011(1) -0.004(4) 100 g 0.052(2) 0.052(2) 0.031(2) 200 g 0.072(3) 0.072(3) 0.049(12) 200 g* 0.040(4) 0.040(3) 0.023(12) 500 g 0.088(9) 0.088(9) 0.045(38) 1 kg 0.117(17) 0.117(17) 0.048(13) 2 kg 0.591(43) 0.587(42) 0.435(600) 2 kg* -0.145(58) -0.148(57) -0.340(600) 5 kg 1.960(110) 1.952(108) 1.568(1500) 6 8 10 2 4 6 8 100 2 4 6 8 1000 u (v )/ v ( p p m ) 1 5 -0 -1 0 g 1 5 -0 -2 0 g 1 5 -1 -2 0 g 1 5 -0 -5 0 g 1 5 -0 -1 0 0 g 1 5 -0 -2 0 0 g 1 5 -1 -2 0 0 g 1 5 -0 -5 0 0 g 1 5 -0 -1 k g 1 5 -0 -2 k g 1 5 -1 -2 k g 1 5 -0 -5 k g air water figure 1: mass comparators in air-tight enclosures in metas http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 21 on the basis of the monte carlo simulation we analysed the correlation between the estimated values of the mass and the volume. for this reason, we calculated the standardised form (z-score), 𝑍 = (𝑋 − 𝜇)/𝜎, of the simulated values, x, of the mass and the volume, respectively. 𝑍𝑚𝑎𝑠𝑠,𝑗(𝑝) = 𝑚𝑗(𝑝)− �̅�𝑗 𝑠𝑚,𝑗 𝑍𝑣𝑜𝑙𝑢𝑚𝑒,𝑗(𝑝) = 𝑉𝑗(𝑝) −�̅�𝑗 𝑠𝑉,𝑗 (19) figure 3 shows the standardised variables of the mass and the volume calculated by a monte carlo simulation for the weights from 10 g to 5 kg. we observed that the correlation between the estimated values for mass and volume depends on the nominal mass of the weight. hence, we calculated the correlation coefficient 𝐶𝑜𝑟(𝑥,𝑦) = ∑(𝑥 − �̅�)(𝑦 − �̅�) √∑(𝑥 − �̅�)2(𝑦 − �̅�)2 (20) between 𝑍𝑚𝑎𝑠𝑠,𝑗 and 𝑍𝑣𝑜𝑙𝑢𝑚𝑒,𝑗 . the correlation coefficient is the smallest for the 1 kg weight and increases as the nominal mass decreases. above 1 kg, the correlation coefficient is significantly larger and seems to be independent of the nominal mass (table 5). table 5: correlation between the error on mass and density for the different values of mass nominal mass 𝑪𝒐𝒓(𝒙,𝒚) nominal mass 𝑪𝒐𝒓(𝒙,𝒚) 10 g 0.83 200 g* 0.07 20 g 0.74 500 g 0.01 20 g* 0.66 1 kg 0.00 50 g 0.52 2 kg 0.54 100 g 0.09 2 kg* 0.78 200 g 0.04 5 kg 0.58 to compare the calculated volumes from the double weighing and from the hydrostatic weighing measurements with each other, we calculated the degree of equivalence (doe) according to cox [7]. the doe is in good agreement between the two methods in the range from 10 g to 5 kg, except for the 100 g weight (figure 4). figure 4: degree of equivalence of the calculated volumes of the weights between the two measurement methods double weighing and hydrostatic weighing. error bars represent expanded uncertainty k = 2. top: 10 g to 1 kg on vk1; bottom: 2 kg to 5 kg on vk10 6. discussion the double weighing method offers several advantages over hydrostatic weighing: the maintenance is less complex, it is less time consuming and is easier to perform the measurements, -0.002 -0.001 0.000 0.001 0.002 d e g re e o f e q u iv a le n c e ( c m ³) 1 5 -0 -1 0 g 1 5 -0 -2 0 g 1 5 -1 -2 0 g 1 5 -0 -5 0 g 1 5 -0 -1 0 0 g 1 5 -0 -2 0 0 g 1 5 -1 -2 0 0 g 1 5 -0 -5 0 0 g 1 5 -0 -1 k g vk1 air water -0.08 -0.04 0.00 0.04 0.08 d e g re e o f e q u iv a le n c e ( c m ³) 1 5 -0 -2 k g 1 5 -1 -2 k g 1 5 -0 -5 k g vk10 air water figure 3: standardised variables of the calculated values for the mass and the volume of the test weights. the values come from a monte-carlo simulation with n = 10 000 iterations http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 22 the calculations are simpler, and it does not contaminate the surface of the weights. the achieved measurement uncertainties are similar to those of the hydrostatic weighing, and even better for small weights. the discrepancy in the calculated volume of the 100 g weight between the double weighing and the hydrostatic weighing remains unclear but may be related to an error made at the time of hydrostatic weighing. the volume value obtained from the double weighing measurements was confirmed by repeating the double weighing measurements on the moneplus comparator, which is similar to the mone but can also be used in vacuum. the difference between the two measured volumes for the 100 g weight was only 10 ppm and was thus well within the measurement uncertainty (not shown in figure 4). the pairs of weights with the same nominal mass, i.e. 20 g, 200 g and 2 kg, do not have the same measurement uncertainties on their respective volumes. the reason for this is the following: the dispersion of the measured weighing values in the hydrostatic weighing of 15-0-20g and 15-1-200g is larger than the dispersion of the measured weighing values of their counterparts 15-1-20g and 15-0-200g, respectively. the difference between the volume uncertainties of 15-0-2kg and 15-1-2kg in the double weighing measurements is more difficult to find out because these uncertainties originate from a closed series of measurements (group weighing). on the basis of the minimum and maximum limits for density of class e1 weights [8], we calculated the equivalent values for volume (table 6). table 6: minimum and maximum limits for density and volume of class e1 weights according to oiml r 111-1 nominal mass density / kg·m-³ volume / cm³ min max min max 10 g 7740 8280 1.208 1.292 20 g 7840 8170 2.448 2.551 20 g* 7840 8170 2.448 2.551 50 g 7920 8080 6.188 6.313 100 g 7934 8067 12.396 12.604 200 g 24.792 25.208 200 g* 24.792 25.208 500 g 61.981 63.020 1 kg 123.962 126.040 2 kg 247.924 252.080 2 kg* 247.924 252.080 5 kg 619.809 630.199 the results in table 3 show that the volumes of our weights, determined by double weighing and hydrostatic weighing, are all within the minimum and maximum limits given by oiml r 111-1. 7. summary we are able to achieve the full dissemination of multiples and submultiples of a set of weights by using group weighings in two different air densities. the measurements are traceable to the mass and to the volume of the reference weight. the technique is promising as there is no contamination from a liquid, and it enables measuring the full range of mass values covered by the constant air density comparators of a laboratory. this technique is well suited for the dissemination of the mass and the volume based on the new definition of the kilogram using a silicon sphere whose mass and volume are known with high precision. 8. references [1] m. t. clarkson, r. s. davis, c. m. sutton, j. coarasa, “determination of volumes of mass standards by weighings in air”, metrologia, vol. 38, no. 1, pp. 1724, 2001. [2] a. malengo, w. bich, “simultaneous determination of mass and volume of a standard by weighings in air”, metrologia, vol. 49, no. 3, pp. 289-293, 2012. [3] k. h. chang, y. j. lee, “hydrostatic weighing at kriss”, metrologia, vol. 41, no. 2, pp. s95–s99, march 2004. [4] m. kochsiek, m. gläser, massebestimmung. vch verlagsgesellschaft mbh, isbn 3-527-29352-3, 1997. [5] k. marti, “dissemination of the kilogram at metas: extended method of lagrange multipliers for air buoyancy correction”, in proc. of imeko 23rd tc3 int. conf., helsinki, finland, 2017. [6] y. su, k. marti, ch. wuethrich, “the determination of the volume of weights in the range of 1 g – 5 kg: a comparison of hydrostatic weighing and double weighing in air using the monte carlo simulation” acta imeko vol. 9, no. 1, pp. 61-68, march 2020. [7] m. g. cox, “the evaluation of key comparison data”, metrologia, vol. 39, pp. 589-595, 2002. [8] oiml r 111-1, “weights of classes e1, e2, f1, f2, m1, m1–2, m2, m2–3 and m3 part 1: metrological and technical requirements”, oiml tc 9/sc 3, 2004. http://www.imeko.org/ measure by measure, they touched the heaven acta imeko issn: 2221-870x march 2021, volume 10, number 1, 265 270 acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 265 measure by measure, they touched heaven luisa spairani1 1 gruppo astrofili eporediesi, c.so vercelli 444, 10015 ivrea, italy section: research paper keywords: women computers; imeko; pickering; cannon; leavitt; fleming; maury citation: luisa spairani, measure by measure, they touched the heaven, acta imeko, vol. 10, no. 1, article 35, march 2021, identifier: imeko-acta10 (2021)-01-35 editor: ioan tudosa, university of sannio, italy received april 28, 2020; in final form november 19, 2020; published march 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: luisa spairani, e-mail: luisa.spairani@netsurf.it 1. introduction today, why do only women work in labs where electronic boards are prototyped? the reason is the precision that women generally apply – the same precision that was required from women computers for astronomical measures. in the early 1900s, women computers at harvard observatory went beyond performing calculations and discovered a new method for measuring the distances of the stars and to classify them. this method is still used today. the harvard computers used cepheids as distance markers. the important feature that allows a cepheid variable to be used for distance measurements is that its period is directly related to its luminosity. this relation makes it possible to determine how much brighter than the sun the star is. the discovery of the connection between these variable stars and distance passed through several steps: distances in astrophysics are notoriously difficult to calculate. visual astronomical measurements introduced subjective uncertainties. photography overcame the problem of subjectivity by creating an enormous quantity of photos to analyse. women computers executed calculations, and measure by measure, they unveiled new laws that expanded the universe. women computers were not only active in the united states. there were female contributions in at least two italian observatories during the period taken into consideration here (the end of the nineteenth century and the beginning of the twentieth). 2. distances in astrophysics distances in astrophysics are notoriously difficult to calculate. it is possible to use geometric methods to determine the distance of objects that are in the proximity of the solar system, i.e. within a distance of about 150 light-years from us. beyond that, it is impossible to use any simple method to calculate distances. and this was the situation in astronomical research at the beginning of the last century. many new objects had been discovered, but without knowing their distances, it was impossible to put them into any stellar systems model. at that time, before 1900, it was not known that we live in a galaxy called the milky way and that many other galaxies exist. abstract the measure of distances is a recurring theme in astrophysics. the interpretation of the light coming from a luminous object in the sky can be very different depending on the distance of the object. two stars or galaxies may each have a different real brightness, although they may look similar. the correct measures were determined by women computers a century ago. special mention is due to williamina fleming, who supervised an observatory for 30 years and worked on the first system to classify stars by spectrum. antonia maury helped locate the first double star and developed a new star classification system. henrietta leavitt determined a law to calculate stellar distances. the most famous of the harvard computers was annie jump cannon. an expert in photography, she catalogued over 350,000 stars and expanded the classification system used today, but it was henrietta leavitt who left an indelible mark by discovering a law for the determination of stellar distances. in the same period, italian women computers began to collaborate in observatories, but their tracks are obfuscated. mailto:luisa.spairani@netsurf.it acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 266 in 1912, henrietta swan leavitt, an american woman computer, elegantly removed this large handicap with a very important discovery: she found a way to determine the real brightness of a particular type of star. yet henrietta was not the only lady of the stars. the extent of this discovery can be evaluated by connecting three different elements: variable stars, astronomical photography and women computers. 3. the cepheids for the calculation of the distance of nearby stars, the geometric method consists in observing the star on two occasions, six months apart from each other (so that the earth is in two opposite positions with respect to the sun). the star, compared to the background composed of other more distant stars, will be in two different positions, and from the measure of the subtended angle, it is possible to derive the distance. but can we measure the remote stars that are in the background or those stars whose brightness varies? the apparent brightness of a star is how bright it seems when viewed from the earth, but a large, bright star can appear dim if it is a long way from the earth, and a dim star can appear bright if it is close to the earth. therefore, the apparent magnitude has no bearing on the distance from the earth. to give an accurate measurement of the brightness of a star, an absolute magnitude scale is needed. moreover, not all stars have a constant brightness over time: many of them show a variable brightness (the causes of variability are many). in 1784, during the time of messier, a very young british amateur astronomer, john goodricke, discovered that the delta star of the constellation cepheus was a variable star with a brightness that oscillated in a very regular manner. in fact, this star reaches a maximum brightness and then fades until it reaches a minimum; then the brightness begins to grow again until it reaches its maximum and so on, like a clock. this particular type of variable star was called a cepheid in honour of this constellation. cepheids are very numerous, and above all, they are scattered everywhere; their period of change in brightness varies from a few hours to a few days and up to about two months. we now know that these variations are due to perturbations in the inner part of these stars and constitute a temporary phase of their life. until the mid-nineteenth century, every measure was meticulously performed based on the astronomer's observational abilities but with an unavoidable component of subjectivity, until it was possible to take photographs and thus have a database available to work with. 4. astronomical photography between 1847 and 1852, the photographer john adams whipple and the astronomer william cranch bond, at the time director of harvard college observatory, used the observatory's large refracting telescope to produce images of the moon of considerable detail and clarity. on july 16, 1850, whipple and bond made the first daguerreotype of a star: vega. it was the beginning of astronomical photography. although glass-plate photography was less sensitive than the eye, it had the advantage of having memory: what the eye does not see in a tenth of a second, it will never see; meanwhile, the photons that arrive at a given moment on the plate add their action to those that came before. that's why photography can employ long exposure times, which allow for the capture of more information. in 1872, the first photograph of the spectrum of a star was obtained, and the study of stellar spectra opened a new frontier. thus began the enormous work that edward pickering (1846– 1919) undertook at harvard college observatory (hco) in the united states. in this work, large prisms were used to disperse light and obtain the spectra of many stars at once. the observatory organised a systematic observation of the brightest stars in both the southern and the northern hemispheres. a large amount of data was collected even by today’s standards, and the harvard computers carried out much of the work of cataloguing and analysing that data. these computers were several dozen women who were hired to carry out the most tedious tasks of astronomical data analysis and paid about half as much as their male counterparts [1]. in 1928, astronomers worked on the spectra of 250,000 stars thanks to the measurements of women computers! 5. women computers as doctor and harvard professor edward clarke wrote in his 1873 book sex in education, ‘a woman’s body could only handle a limited number of developmental tasks at one time—that girls who spent too much energy developing their minds during puberty would end up with undeveloped or diseased reproductive systems.’ for two centuries, the computers were mental workers who could efficiently grind logarithms. a large percentage of them were women. the male scientists of that time considered creative mathematics to be beyond feminine abilities but found women perfect for this type of numerical ‘embroidery’. calculation times were measured in ‘girl hours’: a complex calculation could even require ‘kilo-girl-hours’. most human computers left no record of their personal life. one of the first was nicole-reine lepaute, the wife of a watchmaker of the king of france. in 1750, together with two male colleagues, she foresaw the return of halley's comet in 1758 after an absence of 76 years. the estimation was incorrect by just one month. human computers learned to execute complex tasks. someone cross-checked to screen errors, and a human computer called the comparator verified the work and searched for discrepancies. as late as 1940, a giant mathematical table project was active, employing more than 300 human computers, half of whom used only paper and pencil. in 1952, ibm began selling its electronic computer model 701 for scientific activities. by 1960, calculating machines performed almost all numerical calculations. returning to the harvard computers, the most remarkable woman computer was miss henrietta swan leavitt (1868–1921). the daughter of a congregationalist minister, she graduated from radcliffe college and was hired as a computer at hco in 1880. her job was to compare photographic plates of the magellanic clouds to detect small differences in the brightness of the stars. now we know these entities are neighbouring galaxies, the companions of our milky way. at that time, no one was quite sure what they were. bent over the plates in an observatory laboratory, miss leavitt found the model that led to the answer. she discovered a way to make measurements beyond our galaxy and begin to map the universe [2]. leavitt determined that some stars have a consistent brightness no matter where they are located, making it easy to figure out their distance from earth. the visual brightness of a star depends on its size or its distance from the earth: stars with similar apparent magnitudes http://www.gutenberg.org/files/18504/18504-h/18504-h.htm acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 267 could be at different distances. but leavitt had also noticed that variable stars emit pulses of light and that the true brightness of a star can be measured by the speed of its pulses. brighter stars flash more slowly. by comparing the length of time between pulses with the apparent brightness of the star, one can estimate how far away it is. the astronomers were impressed. ‘what a variable star “friend” miss leavitt is. one can’t keep up with the roll of the new discoveries’, wrote a princeton astronomer in a letter to his boss. although she was praised for her excellent work, miss leavitt earned neither a promotion nor any privilege to pursue her research. she quietly continued her human computer activities, remaining single and living a life appropriate for a distinguished bostonian young lady. even her emotions about her important discovery remain a mystery; she left no diaries or letters. yet her words witness her commitment. ‘the discovery of variable stars, at this observatory and elsewhere, has progressed so rapidly during the last five years, that the difficulty of keeping pace in observing and discussing them has become very great. in the study of distribution now in progress here, the actual time devoted to the search for new variables is small, but thorough observation requires much time, while the discussion of results may be prolonged almost indefinitely a remarkable relation between the brightness of these [cepheid] variables and the length of their periods will be noticed. since the [cepheid] variables are probably at nearly the same distance from the earth, their periods are apparently associated with their actual emission of light, as determined by their mass, density, and surface brightness. it is hoped that systematic study of the light changes of all the variables, nearly two thousand in number, in the two magellanic clouds may soon be undertaken at this observatory.’ other women computers left written testimonies that sometimes expressed their frustration because they were not allowed to explore the deeper implications of what they had found. of course, not all human computers were women. some were men, mostly young and looking for a stable job: they were mathematicians, not scientists. but it was the women computers who left a legacy. harvard computers became known as ‘the pickering harem’ [3]. this expression illustrates the valuation of their work. more than 80 women worked for pickering at the harvard observatory, putting in six-day weeks poring over photographs and earning 25 to 50 cents an hour (half what a man would have been paid). the daily work was largely clerical. some women would reduce the photographs, taking into account things like atmospheric refraction, to render the image as clear and unadulterated as possible. others would classify the stars by comparing the photographs to known catalogues. others catalogued the photographs themselves, making careful notes of each image’s date of exposure and the region of the sky. the notes were then meticulously copied into tables, which included the star’s location in the sky and its magnitude. in 1901, the director of yale observatory, william elkin, said: ‘i am thoroughly in favour of employing women as measurers and computers. not only are women available at smaller salaries than are men, but for routine work they have important advantages. men are more likely to grow impatient after the novelty of the work has worn off and would be harder to retain for that reason.’ many of the women were bright and inquisitive and quickly expanded their roles. williamina paton stevens fleming (1857– 1911) [4], who headed the group, discovered the horsehead nebula [5]. it is worth mentioning that she first worked as a maid in the home of professor pickering after being abandoned, pregnant, by her husband. williamina wrote: ‘a large nebulosity extending nearly south from zeta orionis for about 60 minutes. more intense and well marked on the following side, with a semicircular indentation 5 minutes in diameter 30 minutes from zeta.’ (annals of the harvard college observatory, 18 (1890) p. 116) ‘in the astrophotographic building of the observatory, 12 women, including myself, are engaged in the care of the photographs…. from day to day my duties at the observatory are so nearly alike that there will be little to describe outside ordinary routine work of measurement, examination of photographs, and of work involved in the reduction of these observations.’ ‘before lunch i found time to examine a few southern spectrum plates and marked a fourth type star and a gaseous nebula, both probably known. later in the afternoon i noted a few more interesting objects, among these two fourth type stars, one gaseous nebula, and several bright line stars. some of these may be new.’ figure 1. periods of variable stars published by harvard college observatory circular 173 edward c. pickering, march 3, 1912. figure 2. harvard computers, courtesy of harvard–smithsonian center for astrophysics’ wolbach library. https://en.wikipedia.org/wiki/cepheid_variable#history https://en.wikipedia.org/wiki/cepheid_variable#history https://en.wikipedia.org/wiki/magellanic_clouds http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1890anhar..18..113p&amp;data_type=pdf_high&amp;whole_paper=yes&amp;type=printer&amp;filetype=.pdf acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 268 the entries also provide insight into her dedication and the sheer workload. ‘looking after the numerous pieces of routine work which have to be kept progressing, searching for confirmation of objects discovered elsewhere, attending to scientific correspondence, getting material in form for publication, etc, has consumed so much of my time during the past four years that little is left for the particular investigations in which i am especially interested,’ she wrote. williamina expresses the same frustrations as current astronomers – that project management consumes more time than research. ‘the director, however, says that my time employed in the above work is of more value to the observatory so i have delegated my measures of variables etc to miss leland and miss breslin. i hope, however, to be able soon to finish the measures of the out of focus plates and to get well settled down to my general classification of faint spectra for the new draper catalogue.” a few, like miss leavitt, looked up at the stars and turned the job of human computer into something much greater. many other women at hco made history through astronomical research. in particular, annie jump cannon (1863– 1941), who was originally hired to do mathematical calculations by hand and examine photographs, was able to make a great contribution in the field of stellar classification. other women that deserve mention are antonia maury (1896–1952) and cecilia payne-gaposchkin (1900–1979). they discovered hundreds of variable stars and many novae; these last were revealed through analysis of the enlarged emission lines of their spectrum [6], [7], [8], [9]. they introduced a new classification of stellar spectra that included most of the stars and was related to the stars’ physical properties. in 1912, an extraordinary year, annie cannon described the sequence of spectral types in the article ‘classification of 1477 stars by means of their photographic spectra’ [10]; for mnemonic use, she invented the phrase ‘oh be a fine girl, kiss me’. in substance, this mnemonic is still in use today, o b a f g k m. angelo secchi, a jesuit priest and a scientist (1818–1878), first distinguished four main spectral types of stars. at hco in the 1880s, during the compilation of the henry draper catalogue of stars [11], more types were distinguished and were designated by letters in an alphabetic sequence from a to p according to the strength of their hydrogen absorption spectral lines. the modern cannon system eliminated many classes and reordered the remaining ones, creating a sequence dependent on the effective surface temperature of the stars. the classes are still in use: o: 30,000 – 60,000 k blue stars b: 10,000 – 30,000 k blue-white stars a: 7,500 – 10,000 k white-stars f: 6,000 – 7,500 k yellow-white stars g: 5,000 – 6,000 k yellow stars (like our sun) k: 3,500 – 5,000 k light orange stars m: < 3,500 k orange stars since its creation, 3 new classes have been added: n, r and s, which are darker orange and colder. 5.1. the law of leavitt the absolute magnitude (m, also called absolute luminosity) is the apparent magnitude (m) that an object would have if it were at a distance of 10 parsecs (32.616 light-years) or 3 × 1014 kilometres from the observer. leavitt’s law expresses the dependence of the absolute magnitude on the apparent one and presents the logarithm of the distance: 𝑀 = 𝑚 + 5 − 5 log (𝑑) , (1) where m is the apparent magnitude and d is the real distance of the star expressed in parsecs. leavitt’s discovery made it possible to calculate distances between 100 and 10 million light-years, thus paving the way for the fundamental work of astronomers hubble and shapley, which revolutionised our knowledge of the galaxy and the universe, while the period–luminosity relationship identified in the cepheids represents the foundation of our current extragalactic distance scale. throughout her career, leavitt discovered over 2,400 variable stars and provided the basis for scientists to further research and learn more about the cosmos. leavitt continued to work at harvard college observatory until 1921, when she died of cancer at the age of 53. perhaps in part because of this premature death, her name did not receive the recognition she deserved in the scientific community of the time. she was proposed as a nobel prize winner four years after her death by a committee member who did not know of her untimely end, so the award went to the figure 3. annie cannon with plate, courtesy of harvard–smithsonian center for astrophysics’ wolbach library. figure 4. ‘periods of 25 variable stars in the small magellanic cloud’ harvard college observatory circular, vol. 173. from wikimedia commons. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 269 astronomer harlow shapley, head of the harvard college observatory (1921–1952). the importance of her law was immediately appreciated, however. a year after the discovery, the danish astronomer ejnar hertzsprung (1873–1967) determined the distances of some cepheids in the proximity of our milky way using geometric methods. (today we know that this measure also depends on some factors that were unknown at that time. for example, we realised later that the measure depends in some way on the quantity of heavy elements in the stars.) within the next 5 years, shapley applied leavitt’s law to globular clusters and was able to produce the first map of the milky way. he made a model of a vast system of stars – about a hundred billion, structured in a disc – the centre of which is far from our solar system. shapley had created a whole new chapter in astronomy, and this was only possible because of leavitt's crucial discovery. since then, astronomers have devised a series of further relationships between the properties of different astronomical objects (supernovae and hii regions, i.e. regions of interstellar atomic hydrogen that is ionised, just to name a few) that can be used to determine even greater distances of objects in places where cepheids are difficult to locate. the period–luminosity relationship of cepheids, however, remains the least uncertain method of determining distances. in 1924, astronomer edwin p. hubble (1889–1953), using the new 254 cm diameter mount wilson telescope (at that time the largest in the world), was able to resolve the outer parts of the andromeda nebula and discover dozens of cepheids. based on the (very reasonable) hypothesis that the cepheids of the andromeda nebula behave like those of our galaxy, it was finally possible to calculate the distance of the nebula. the result made the astronomers pale: the nebula is between 700,000 and 1,000,000 light-years away, which, for those times, was an immense distance. furthermore, the application of leavitt's law allowed for the classification of celestial phenomena previously seen as spots in the night sky and provided substantial evidence that the universe is expanding. 5.2. leavitt’s legacy one of the key projects of the hubble space telescope, launched more than a decade ago, was the identification of the cepheid variable stars in the virgo cluster (the nearest galaxy cluster in the milky way, at a distance of about 50 million lightyears) to determine their distance more precisely. cepheids are the most useful stars in the sky; they are the ‘standard candles’. even today, a century later, thanks to the fundamental contribution of henrietta leavitt, cepheid variable stars remain a reference in astronomical research for the estimation of cosmological distances. 6. italian women computers during the same period as the harvard computers, in the young italian nation, women made important contributions at the observatory of turin and the vatican observatory. 6.1. at the turin observatory the observatory of turin, founded in 1759, moved from the roofs of palazzo madama in the town up to the hill in 1913. as early as 1904, women astronomers were contributing. ‘... the faculty of mathematics of turin is attended by quite a few young ladies, some of whom have a real aptitude for those severe studies. if then the post of assistant to the chairs of the first two years were given by competition, this would easily be won by one of these doctors. now, would it be prudent to put as a teaching assistant, for example in descriptive geometry and corresponding drawing, a young girl among 180 students?’ this question came from father giovanni boccardi, the director of the astronomical observatory of turin at the beginning of the twentieth century. he hired women computers. their work consisted of the reduction of observational data. moreover, the staff also began to run low, even before the first world war, beginning in 1911 with the war in tripolitania. the director of the observatory noted the exodus of assistant astronomers called to military service, so, in the shortage of trained professionals and funds, boccardi availed himself of the help of undergraduate women. luisa viriglio was on staff from 1904 to 1906. the volunteer assistants received a much lower salary than those at higher levels. despite the short amount of time she spent working at palazzo madama, viriglio holds another record in addition to being the first salaried astronomer. she was also the first italian female astronomer to have a publication in her name. other mathematicians employed in the period were ernesta fasciotti and giovanna greggi. the ‘essays of popular astronomy’, scientific bulletins published by the astronomical society urania, first appeared in january 1911. the years 1914 and 1915 even saw the simultaneous presence of three female graduate assistants in mathematics. teresa castelli and tiziana comi flanked the work of dr greggi. tiziana comi and jeannette mongini carried out dissemination activities. in 1919–1920, corinna (or carolina) gualfredo was listed among the staff as a custodian but also as an astronomer. all that remains of them are their names and surnames, graduation marks and salary records as well as the numbers they calculated patiently by hand. but we do not even have a portrait in an ancient photograph from the beginning of the century [12]. 6.2. at the specola vaticana in the early decades of the twentieth century, four nuns from the suore di maria bambina institute contributed to a catalogue of 400 thousand stars, which is still in the preserve of international stellar ‘cartography’. their names are emilia ponzoni, regina colombo, concetta finardi and luigia panzeri. the story begins in the late nineteenth century, in paris. the french capital hosted astronomers from all over the world with a prestigious goal: to create the largest chart of the sky in order to worthily represent the celestial vault. the vatican also took part, led by pope leo xiii, who was passionate about stars. representing the pope was the priest francesco denza, who made an agreement with the jesuit john hagen, from the united states. the latter confided to the italian the need for helpers who knew how to ‘read’ the celestial coordinates and therefore the positions of the stars. don francesco passed the work into the hands of four nuns employed at the vatican observatory who have remained anonymous until recently. the nuns learned the art of observing the stars and within a few years became experts in the starry world. they learned to manoeuvre the telescope and to extricate themselves from complicated mathematical calculations; they were women computers. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 270 the famous carte du ciel was concluded in 1966 and included the identification of five million stars, many of which were reported for the first time by four silent and devoted roman nuns [13]. 7. at the end fictional literature introduced computational devices when women computers were leading the field. ‘the machine stops’ (1909) by e. m. forster is a short story about the role of technology in our lives, and twentieth century fiction was full of this topic in the works of heinlein, asimov, van vogt and many others. the success and the limits of manual computing were a source of inspiration for writers of the positivism period. camille flammarion, who began his career as a human computer, mentioned computer devices in his book the planet mars. did the human computers’ scientific efforts influence the vision of the future? we cannot learn how to solve specific scientific problems from history, but we can come to better understand our contemporary science and frame it within its social context. ‘by the serious attempt to put ourselves back into the intellectual situation of the ancient thinkers, far less experienced as regards the actual behaviour of the nature, but also very often much less biased, we may regain from them the freedom of thought’ [14]. acknowledgement i would like to thank dr antonella bianchi (biblioteque bocconi university) for helping me with the presentation of this paper at the 2019 imeko tc4 international conference on metrology for archaeology and cultural heritage in florence, italy, december 4-6, 2019. i would also like to thank drs f. bonoli, i. chinnici, d. randazzo and v. zanini of the national institute for astrophysics [15] for their support with research about female italian astronomers. references [1] l. spairani, henrietta leavitt e la misura dell’universo, le stelle x (2011), pp. 68-73. [2] d. sobel, the glass universe, viking, new york, 2016, isbn 9780698148697, p. 9. [3] n. geiling, the women who mapped the universe and still couldn’t get any respect, smithsonian magazine, sept. 18 (2013). online [accessed 19 march 2021] https://www.smithsonianmag.com/history/the-women-whomapped-the-universe-and-still-couldnt-get-any-respect9287444/#0blccaahh [4] harvard university library – open collection programs, women working, 1800-1930. online [accessed 19 march 2021] http://ocp.hul.harvard.edu/ww/fleming.html [5] a. mcgrath the first computer: williamina fleming and the horsehead nebula, galactic gazette july 3 (2017). online [accessed 19 march 2021] https://web.archive.org/web/20190929142207/http:/altbibl.io/ gazette/the-first-computer-williamina-fleming-and-thehorsehead-nebula/ [6] wolbach library, historical figure index, antonia maury, online [accessed 19 march 2021] https://library.cfa.harvard.edu/phaedra/maury https://library.cfa.harvard.edu/redshift-spectroscopy. [7] c. p. gaposchkin, on the physical condition of the supernovae, proceedings of the national academy of sciences 22(6) (1936), pp. 332-336. [8] f. l. whipple, c. p. gaposchkin, on the bright line spectrum of nova herculis, proceedings of the national academy of sciences 22(4) (1936), pp. 195-200. [9] j. j. o'connor, e. f. robertson, cecilia helena paynegaposchkin, university of st. andrews. online [accessed 19 march 2021] http://www-groups.dcs.stand.ac.uk/history/biographies/payne-gaposchkin.html [10] a. j. cannon, classification of 1477 stars by means of their photographic spectra, harvard obs. annals 56 (1912), pp. 65-114, in: cannon, annie jump, 1863–1941. papers of annie jump cannon: an inventory, harvard university archives. online [accessed 19 march 2021] https://web.archive.org/web/20100714141345/http:/oasis.lib.h arvard.edu/oasis/deliver/~hua12001 [11] the draper catalogue of stellar spectra photographed with the 8inch bache telescope as a part of the henry draper memorial, cambridge university press, cambridge, 1890. online [accessed 19 march 2021] https://archive.org/details/drapercatalogueo00harvrich/page/n 8 [12] g. bernardi, a. vecchiato, the advent of female astronomers at turin observatory, the journal of astronomical history and heritage 21(1) (2018), pp. 13-28. online [accessed 19 march 2021] https://www.academia.edu/36686097/the_advent_of_f emale_astronomers_at_turin_observatory [13] s. sesti, l. moro, scienziate nel tempo. più di 100 biografie, ledizioni ledipublishing, milano, 2018, isbn 9788867057733. [14] e. schrödinger, nature and the greeks and science and humanism, cambridge university press, cambridge, 1996 (1954) isbn-13 978-0521575508. [15] inaf, polvere di stelle. online [accessed 19 march 2021] http://www.beniculturali.inaf.it/ https://en.wikipedia.org/wiki/international_standard_book_number https://en.wikipedia.org/wiki/special:booksources/9780698148697 file:///c:/firenze/smithsonianmag.com file:///c:/firenze/smithsonianmag.com https://www.smithsonianmag.com/history/the-women-who-mapped-the-universe-and-still-couldnt-get-any-respect-9287444/#0blccaahh https://www.smithsonianmag.com/history/the-women-who-mapped-the-universe-and-still-couldnt-get-any-respect-9287444/#0blccaahh https://www.smithsonianmag.com/history/the-women-who-mapped-the-universe-and-still-couldnt-get-any-respect-9287444/#0blccaahh http://ocp.hul.harvard.edu/ww/fleming.html https://web.archive.org/web/20190929142207/http:/altbibl.io/gazette/the-first-computer-williamina-fleming-and-the-horsehead-nebula/ https://web.archive.org/web/20190929142207/http:/altbibl.io/gazette/the-first-computer-williamina-fleming-and-the-horsehead-nebula/ https://web.archive.org/web/20190929142207/http:/altbibl.io/gazette/the-first-computer-williamina-fleming-and-the-horsehead-nebula/ https://library.cfa.harvard.edu/phaedra/maury https://library.cfa.harvard.edu/redshift-spectroscopy http://www-groups.dcs.st-and.ac.uk/history/biographies/payne-gaposchkin.html http://www-groups.dcs.st-and.ac.uk/history/biographies/payne-gaposchkin.html https://web.archive.org/web/20100714141345/http:/oasis.lib.harvard.edu/oasis/deliver/~hua12001 https://web.archive.org/web/20100714141345/http:/oasis.lib.harvard.edu/oasis/deliver/~hua12001 https://archive.org/details/drapercatalogueo00harvrich/page/n8 https://archive.org/details/drapercatalogueo00harvrich/page/n8 https://www.academia.edu/36686097/the_advent_of_female_astronomers_at_turin_observatory https://www.academia.edu/36686097/the_advent_of_female_astronomers_at_turin_observatory http://www.beniculturali.inaf.it/contatti/ establishment of air piston gauge as primary pressure standard at csir-national physical laboratory india acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 329 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 329 333 establishment of air piston gauge as primary pressure standard at csir-national physical laboratory india vikas n. thakur1,2a, sanjay yadav1,2b, ashok kumar1,2c 1 csir-national physical laboratory, dr. k. s. krishnan marg, delhi 110012, india 2 academy of scientific and innovative research (acsir), ghaziabad-201002, india a vikasnthakur92@gmail.com, b syadav@nplindia.org, c ashok553@nplindia.org abstract: the air piston gauge (apg) was established at csir-national physical laboratory, india (npli) since 2000. later the same piston-cylinder(p-c) assembly was calibrated in nist usa; however, it was never published for metrology communities. as per international protocol, the establishment of the apg as a primary standard, the effective area of pc assembly, and masses must be directly traceable to si units. the first time we have calculated the effective area and associated uncertainty of p-c assembly using dimension and mass metrology, traceability to the si units, i.e., meter and kilogram. to realize the apg as primary pressure standards, we have calculated the effective area of p-c assembly of apg directly from dimension metrology, which is further supported by various other methods. the effective area values obtained in the pressure range of 6.5 – 360 kpa lie in the range of 3.356729 – 3.357248 cm2 due to uncertainty limitation in the measurement of dimension of internal diameter of cylinder. the expected values of the effective area which are also measured from cross-float technique against ultrasonic interferometer manometer (uim), primary pressure standards. the accuracy in effective area measurement is possible only when the resolution in the internal radius of the cylinder should at least be up to 5th decimal order and the uncertainty is 80 nm. the expanded uncertainty was measured nearly 11 ppm at k = 2 by considering the uncertainty in internal radii of cylinder and radii of piston around 80 nm. keywords: uncertainty; air piston gauge; pressure; primary standard 1. introduction the most accurate pressure standard all over world is mercury based manometer i.e. ultrasonic interferometer manometer (uim). due to toxic nature of mercury and complications in handling the electronics and working of the whole uim most of the national metrology institutes (nmis) have decommissioned the uim. recently national institute of standards and technology (nist), usa also decommissioned the uim; however, npli still maintained the same and recently participated in apmp.m.p-k4 key comparisons. since most of the nmis have banned mercury based products including uim, now focus has shifted to apg with precision and high accuracy measurement in the radii/effective area of p-c assembly. various nmis have been developing a piston-cylinder (p-c) assembly as a primary pressure standard to replace the ultrasonic interferometer manometer. the p-c assembly can be converted into primary pressure standards using calibrated mass and dimensions form mass and dimension metrology, respectively. the npli has functional primary pressure standards, uim, which has the relative expanded uncertainty of 7.2 ppm at k = 2 in the pressure range of 1 pa – 130 kpa established in 1982 and maintained since then [1]. recently, nist usa has been developed laser interferometer based optical interferometer manometer (oim) primary pressure standards [2– 4]. to meet the requirement of mercury-free equipment, most of the nmis has developed p-c assembly based primary pressure standards to realize the pascal and maintain the traceability chain. the apgs are easy to handle, portable in nature, and very accurate pressure measurement over a wide range of pneumatic and hydraulic pressure, i.e., pa to gpa. in this article, our focus will be on the calculation of the effective area of p-c assembly using rarefaction gas dynamics and dimension metrology. to calculate the effective area, the theory of rarefaction gas dynamics has been used. a formula was derived for calculating the effective area, which is a combined form of viscous and free-molecular flow of gas in a medium [5–8]. the nist usa, and ptb germany have discarded the dadson formula to calculate the effective area of p-c assembly and opted to use the rarefaction gas dynamics method. http://www.imeko.org/ mailto:syadav@nplindia.org acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 330 here we report for the values of the effective area along with the uncertainties of the apg, model: ruska 2465a-754 using dimension metrology and rarefaction gas dynamics theory in the range of 6.5 – 360 kpa. 2. effective area calculation in this method, we have taken the dimensional measurements of p-c assembly like diameters of piston and cylinder at various angles in the range of 0ᵒ 360ᵒ along with the roundness and straightness at different heights. since the roundness and straightness are very low and within their uncertainty limits. therefore, the p-c assembly was considered to be ideally straight and circular. figure 1 illustrates the schematic diagram of the p-c assembly for calculating the effective area using the dimension of p-c assembly. figure 1 explains the cross-section of the p-c cylinder assembly. horizontal dash lines: pq and gh in figure 1 are the extreme ends of engagement length ‘l’ which are indicated by x = 0 and x = l. 𝑥 is a vertical axis coordinate in the range of [0, 𝑙]. vertical dash lines: ab is the ideal line for cylinder, and cd and ef are the ideal lines for the piston. the distance between dashed lines tu and ef is ideal radius of the piston (𝑟𝑜). the ideal crevice size is given by ℎ𝑜. the ideal radius of the internal surface of the cylinder is given by 𝑅𝑜 = 𝑟𝑜 + ℎ𝑜. solid curves: rs and tu are the imaginary curves (shapes in 3d) of the internal surface of the cylinder (on the left side) and piston (on the right side), respectively. the effective area at given temperature (𝐴𝑒 ) was calculated. since in theoretical calculation i.e. rgd, zero pressure effective area and 𝐴𝑒 are having negligible or no difference. figure 1: 2d cross-section image was drawn for the detailed explanation about the physical parameters. the formula to calculate effective area (𝐴0) of p-c assembly given by equation (1) has three contributions: (1) a1 is due to the force acting on the piston due to the net pressure, i.e. (δp = p1-p2), where p1 is the applied pressure at the bottom of the piston and p2 is the ambient pressure (given by equation (1a)), (2) a2 is due to the drag force due to the pressure flow in the gap between the piston and cylinder (given by equation (1b))and (3) a3 represents the area affected due to the variation of the piston radius (given by equation (1c)). 𝐴0 = 𝐴1 + 𝐴2 + 𝐴3 (1) 𝐴1 = 𝜋(𝑟2(𝑥 = 0)𝑃1 − 𝑟 2(𝑥 = 𝑙)𝑃2) 𝑃1 − 𝑃2 (1a) 𝐴2 = − 𝜋 (𝑃1 − 𝑃2) ∫ 𝑟(𝑥)ℎ(𝑥) 𝑑𝑃 𝑑𝑥 𝑑𝑥 𝑙 0 (1b) 𝐴3 = 2𝜋 (𝑃1 − 𝑃2) ∫ 𝑟(𝑥)𝑃(𝑥) 𝑑𝑟(𝑥) 𝑑𝑥 𝑑𝑥 𝑙 0 , (1c) where, 𝑃(𝑥) is the pressure distribution along the clearance/gap between the piston and cylinder, ℎ(𝑥) = 𝑟(𝑥) − 𝑅(𝑥) is the radial clearances between piston and cylinder at 𝑥 = 0 and along with the radial axis, respectively, 𝑟𝑜 and 𝑅𝑜 are the ideal radii of the piston and cylinder at 𝑥 = 0 and 𝑟 and 𝑅 are radii of piston and cylinder, respectively with varying engagement length (x). to calculate a2 and a3, pressure distribution has to be calculated http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 331 using the theories developed in the references3,4. the gas flow rate in the gap between the piston and cylinder is given by equation (2), 𝐺𝑎𝑠 𝑓𝑙𝑜𝑤 𝑟𝑎𝑡𝑒 = − 2𝜋𝑟𝑜 𝑣𝑚𝑝𝑠 𝐺𝑃 (𝛿)ℎ 2(𝑥) 𝑑𝑃 𝑑𝑥 (2) where 𝐺𝑃 (𝛿) is poiseuille’s coefficient which is function of rarefaction parameter (𝛿 = 𝑃(𝑥)ℎ(𝑥)/ (𝜂𝑣𝑚𝑝𝑠)) 3,4, where 𝜂 is the coefficient of viscosity of gas i.e. 17.72 µpa sec for n2 [9] and 𝑣𝑚𝑝𝑠 = √2𝑅𝑔𝑇 𝑚⁄ is the most probable speed, 𝑅𝑔 is the universal gas constant, 𝑚 is the molecular mass of the gas used and 𝑇 is the temperature inside apg. 𝛿(𝑥) is dependent on the pressure in the crevice 𝑃(𝑥). the pressure distribution is calculated using iteration method by varying crevice size ( ℎ(𝑥) ). after calculating pressure distribution and crevice size, rarefaction parameter was calculated. the rarefaction parameter was further used to calculate poisuelle's coefficient ( 𝐺𝑃 (𝛿) .by considering crevice as planar channel using kinetic boltzmann equation3,4 in case of the diffuse scattering of molecules on the crevice wall, the poiseuille coefficient is given as 𝐺𝑝(𝛿) = 3.869 + 1 − 𝛼𝑡 𝛼𝑡 𝑓𝑜𝑟 𝛿 < 2.13 × 10−3 (3a) 𝐺𝑝(𝛿) = − ln 𝛿 √𝜋 + 0.376 − (1.77 ln 𝛿 + 0.584)𝛿 + 2.12 𝛿 2 + 1 − 𝛼𝑡 𝛼𝑡 𝑓𝑜𝑟 2.13 × 10−3 < 𝛿 < 0.45 (3b) 𝐺𝑝(𝛿) = 𝛿 6 + 𝜎𝑝 + 0.61 𝛿 0.7 − 0.24 𝛿 + 1 − 𝛼𝑡 𝛼𝑡 𝑓𝑜𝑟 𝛿 > 0.45 (3c) where 𝜎𝑝 = 1.018 is the slip coefficient [11]. in real sense, n2 gas does not have the complete accommodation, but its tangential component of accommodation coefficients is given by 𝛼𝑡 = 0.9 [10]. thus, 𝐺𝑝(𝛿) should be corrected as 𝐺𝑝(𝛼𝑡 ) = 𝐺𝑝(1) + 1.772 1 − 𝛼𝑡 𝛼𝑡 (3d) after getting poiseuille’s coefficient and crevice size we get the pressure distribution in crevice given by equation (4) 𝑃(𝑥) = 𝑃1 − (𝑃1 − 𝑃2) ∫ (ℎ(𝑥)2𝐺𝑃 (𝛿)) −1𝑑𝑥 𝑥 0 ∫ (ℎ(𝑥)2𝐺𝑃 (𝛿)) −1𝑑𝑥 𝑙 0 (4) this pressure distribution is used to calculate the effective area using equation (1) to (1c) with varying applied pressure in the range of 6.5 kpa to 360 kpa with the associated uncertainty. 3. results & discussion the radii of the piston and cylinder are given in table 1 along with their associated uncertainty. based on dimension metrology data, rgd theory was used to calculate the effective area values of pc assembly pressure range of apg and obtained 3.356729 – 3.357248 cm2 with varying internal radius of cylinder within the uncertainty limit given by dimensional metrology i.e. 700 nm (shown in figure 2). the effective area found in cross float method considered to be the expected values of effective area. while varying the value of the internal diameter of the cylinder, the expected value of effective area i.e. 3.356770 – 3.356780 cm2 was found which is at internal diameter = 10.3380 mm (shown in figure 2(b)). this value of internal radius of cylinder is 650 nm away from the value given by the dimensional metrology given in table 1. table 1: radii of the piston and internal surface of the cylinder @ 23 ℃. part of apg radius (mm) uncertainty (nm) piston 10.33561 80 internal surface of the cylinder 10.3386 700 the expected values of effective area was found at internal diameter of cylinder (r) = 10.3380 mm within uncertainty limits of 80 nm. the values of effective area using r = 10.3380 mm is shown in figure 2(b) which are in the range of the effective area values obtained since last 20 years from crossfloat against ultrasonic interferometer manometer (uim) [11]. figure 2(b) also shows that the effective area values can be achieved with high accuracy only when atleast the radius of the internal surface of the cylinder has the resolution upto 5th decimal place an uncertainty should be within 80 nm. then the zero pressure effective area was also calculated using rgd which is the value at constant temperature and pressure. the zero pressure effective area values obtained from rgd and crossfloat against uim are 3.356775 cm2 and 3.356772 cm2, respectively. the standard deviation between these two values of zero pressure effective area is 2 × 10−6 𝑐𝑚2. the uncertainty in the measurement of zero pressure effective area was calculated using equation (4) by considering 80 nm uncertainty in both piston and cylinder radii. http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 332 figure 2: (a) effective area values with varying radius of internal surface of cylinder with in its uncertainty limits, (b) the region bounded in figure 2(a) is shown as magnified image. the expanded relative uncertainty in the zero pressure effective area was calculated to be 11 ppm of the reading at k = 2. 𝑢(𝐴0) 𝐴0 = √𝑢(𝑟0) 2 + 𝑢(𝑅0) 2 2 𝑟𝑎𝑣𝑜 (5) where 𝑟𝑎𝑣𝑜 is the average radius of the piston and internal surface cylinder radii. 4. summary the effective area values of apg obtained using rgd is in the range of 3.356729 – 3.357248 cm2 with varying internal cylinder radius within its uncertainty limit i.e. 700 nm in the complete pressure range of 6.5 – 360 kpa. but the expected values of the effective area i.e. 3.356770 – 3.356780 cm2 were obtained at 10.33797 mm to 10.33801 mm internal cylinder radius within the uncertainty limit less than 80 nm. the accuracy and low uncertainty is possible only when the resolution in the internal radius of the cylinder should have atleast be upto 5th decimal and the uncertainty is comparable to uncertainty of pistion i.e. 80 nm in the present case. the expanded relative uncertainty in zero pressure effective area was also calculated by considering 80 nm as the uncertainty in both internal cylinder and piston radii. the expanded uncertainty of 11 ppm at k = 2 was calculated in the zero pressure effective area. the value of expanded uncertainty obtained from rgd in zero pressure effective area is comparable to that of the cross-floating against uim. the zero pressure effective area values obtained from cross float against uim and rgd have standard deviation of 𝟐 × 𝟏𝟎−𝟔 𝒄𝒎𝟐. 5. acknowledgements authors would like to thank dr. felix sharipov and dr. y. yang, for scientific guidance to undersatnd the rgd model. 6. references [1] a. kumar; v. n. thakur; r. sharma; h. kumar; omprakash; s. yadav, uncertainty evaluation and phase variation of ultrasonic interferometer manometer: a primary pressure and vacuum standard, vacuum. 165 (2019) 232–238. https://doi.org/10.1016/j.vacuum.2019.04.023. [2] k. jousten, j. hendricks, d. barker, k. douglas, s. eckel, p. egan, j. fedchak, j. flügge, c. gaiser, d. olson, j. ricker, t. rubin, w. sabuga, j. scherschligt, r. schödel, u. sterr, j. stone, g. strouse, perspectives for a new realization of the pascal by optical methods, metrologia. 54 (2017) s146–s161. https://doi.org/10.1088/1681-7575/aa8a4d. [3] y. takei, k. arai, h. yoshida, y. bitou, s. telada, t. kobata, development of an optical pressure measurement system using an external cavity diode laser with a wide tunable frequency range, measurement. 151 (2020) 107090. https://doi.org/10.1016/j.measurement.2019.10709 0. [4] p.f. egan, j.a. stone, j.h. hendricks, j.e. ricker, g.e. scace, g.f. strouse, performance of a dual fabry–perot cavity refractometer, opt. lett. 40 (2015) 3945. https://doi.org/10.1364/ol.40.003945. [5] felix sharipov, rarefied gas dynamics: fundamentals for research and practice, wiley, berlin, 2016. [6] f. sharipov, y. yang, j.e. ricker, j.h. hendricks, primary pressure standard based on piston-cylinder assemblies. calculation of effective cross sectional area based on rarefied gas dynamics, metrologia. 53 (2016) 1177–1184. https://doi.org/10.1088/0026-1394/53/5/1177. [7] f. sharipov, v. seleznev, data on internal rarefied gas flows, j. phys. chem. ref. data. 27 (1998) 657–706. https://doi.org/10.1063/1.556019. [8] w. sabuga; f. sharipov; and t. priruenrom, determination of the effective area of pistoncylinder assemblies using rarefied gas flow model, in: ptb-mitteilungen 5th ccm int. conf. press. vac. metrol., 2011: pp. 260–2. [9] w.a. cole, w.a. wakeham, the viscosity of nitrogen, oxygen, and their binary mixtures in http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 333 the limit of zero density, j. phys. chem. ref. data. 14 (1985) 209–226. https://doi.org/10.1063/1.555748. [10] f. sharipov, application of the cercignani-lampis scattering kernel to calculations of rarefield gas flows. i. plane flow between two parallel plates, eur. j. mech. b/fluids. 21 (2002) 113–123. https://doi.org/10.1016/s0997-7546(01)01160-8. [11] vikas n. thakur; r. sharma; h.kumar; omprakash; d. a. vijaykumar; s. yadav; a. kumar, on long-term stability of an air piston gauge maintained at national physical laboratory, india, vacuum. 176 (2020) 109357. https://doi.org/10.1016/j.vacuum.2020.109357. http://www.imeko.org/ non-invasive diagnostic investigation at the bishop's palace of frascati: an integrated approach acta imeko issn: 2221-870x march 2021, volume 10, number 1, 180 186 acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 180 non-invasive diagnostic investigation at the bishop’s palace of frascati: an integrated approach luisa caneve1, francesco colao1, massimo francucci1, massimiliano guarneri1, marialuisa mongelli2, valeria spizzichino1 1 enea diagnostic and metrology laboratory, via enrico fermi 45, 00044 frascati, italy 2 enea high performance computing laboratory, lungotevere thaon di revel 76, 00196 rome, italy section: research paper keywords: non-invasive techniques; lif; 3d reconstruction; photogrammetry citation: luisa caneve, francesco colao, massimo francucci, massimiliano guarneri, marialuisa mongelli, valeria spizzichino, non-invasive diagnostic investigation at the bishop's palace of frascati: an integrated approach, acta imeko, vol. 10, no. 1, article 24, march 2021, identifier: imeko-acta-10 (2021)01-24 editor: ioan tudosa, university of sannio, italy received april 28, 2020; in final form november 12, 2020; published march 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was supported by adamo project, funded by lazio region for technological cultural district (dtc-lazio) corresponding author: luisa caneve, e-mail: luisa.caneve@enea.it 1. introduction in the frame of the project focusing on analysis, diagnosis and monitoring for cultural heritage conservation and restoration (adamo) financed by lazio region for the district of cultural technology (dtc lazio), a measurement campaign was undertaken at the bishop’s palace of frascati, an ancient fortress that is now the home of the episcopal tuscolana diocese. the measurements were performed to assess the status of previous restoration work. following the requests of the palace’s caretaker, particular attention was devoted to the investigation of the so-called stufetta room the ground floor of the palace (figure 1), and the landscape room on the first floor, which offers a view of the garden. an integrated investigation approach was adopted involving different non-invasive technologies and instruments. a laserinduced fluorescence (lif) system developed at enea’s diagnostic and metrology laboratory and previously applied as a diagnostic tool in cultural heritage investigations was used. lif is a suitable tool for the characterisation of valuable and unmovable targets due to its unique properties as a nondestructive, non-invasive remote technique using transportable or portable instruments that can provide first results in real time without sampling [1], [2]. lif spectroscopy has already been successfully applied in archaeological sites, providing useful information to restorers [3], [3]. the validity of the technique as a diagnostic tool for artworks of different materials, such as paintings, ceramics, stonework and textiles, has also been demonstrated [4]-[7]. lif-based systems are able to acquire fluorescence spectra and generate multispectral images that can be used to create a component material map of the investigated surface in order to identify and characterise materials of interest abstract artistic surfaces at the bishop’s palace of frascati were investigated using an integrated approach involving different non-invasive diagnostic techniques. a novel remote methodology aimed at the detection and location of materials from previous restoration actions as well as the monitoring of degradation processes is proposed. in this novel approach, a laser-induced fluorescence (lif) scanning system was used in synergy with a red-green-blue imaging topological radar 3d laser scanner and the structure from motion technique for 3d photogrammetric reconstruction. the spectral characteristics of the obtained lif images permitted us to identify restoration materials and bio-deteriorated areas even when they were not visible to the naked eye. the superimposition of lif and 3d images allowed for an optimised localisation of the areas of interest. the periodic repetition of this integrated approach could be used as a tool to monitor degradation processes as it could identify structural and chemical changes that occur in these areas of interest. the presented results could support the conservation, study and dissemination of cultural heritage items, keeping in mind that integration with information obtained through other innovative and standard analytical techniques is always fundamental. mailto:luisa.caneve@enea.it acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 181 [9], [9]. in this study, this technology was used in synergy with other non-invasive techniques that can localise bio-deteriorated areas and added restoration materials in order to support and integrate the experimental results. in particular, the structure from motion (sfm) method [10] was used to obtain a 3d photogrammetric reconstruction of the painted vault of the stufetta room in a very short time with accurate structure in terms of geometry and texture. this technique is commonly applied in the study of cultural heritage assets [11]. along with these instruments, a red-green-blue imaging topological radar (rgb-itr) 3d laser scanner prototype was used to digitalise the landscape room. this 3d laser scanner has already been employed for remote diagnostics of cultural heritage [12], [14]. its ability to measure in situ remotely along with its complete non-invasiveness eliminate the use of scaffolds, reducing the time and cost of the analysis. the obtained results highlighted the localised presence of different materials resulting from retouching or consolidating processes, even in areas where differences are not appreciable by the naked eye. deterioration phenomena, mainly due to environmental humidity, were also localised by the system in places where they were not clearly evident, suggesting the possibility of early detection of damage. the available techniques for characterising and monitoring biodegradation [15], [15] can be supported by the presented approach. the combined use of the different instruments offers several advantages. in particular, it is possible to overlay the lif spectral maps onto the 3d photogrammetric model obtained at a shortto-medium range of distance and onto the 3d laser coloured model obtained by the rgb-itr scanner. the system can also localise the areas on the surface that have submitted to degradation actions perfectly even in low light conditions or at great distances. the results of the proposed integrated approach can be of great usefulness for site conservation. the ability to detect and localise early signs of damage before they are clearly visible using non-invasive and rapid techniques can ensure that interventions and restoration actions are performed in time. moreover, the periodic repetition of this kind of investigation involving the combined results of different applied techniques regarding possible changes in the areas of interest, such as changes in the dimensions of bio-deteriorated areas or chemical degradation of the restoration materials, could be used to monitor degradation processes. 2. experimental setup 2.1. laser-induced fluorescence system the system applied in this work for fluorescence measurements was the enea prototype lif line scanning system, in which a cylindrical lens in the optical path modifies the shape of the laser beam on the target from a point to a line. this system, shown in figure 2 during the measurements at the palace, is described in detail in a previously published paper [16]. the improvement of the optical system together with the new rotating mechanical assembly have made possible a considerable reduction of the scan time (1.5×5 m2 scanned in less than 2 minutes at 25 m of distance from the target) in comparison to similar prototype systems developed previously [17], making this instrument particularly suitable for the analysis of large areas. using motorised optics controlled by software, the laser beam, working in this case at the wavelength of 266 nm, has a repetition rate of 20 hz, a pulse duration of 10 ns and 1.2 mj of energy for carrying out surface scans. the iccd camera has a 1024×1024 pixel detector. the lif system’s horizontal field of view is 6 degrees, and due to the coupling of optics and iccd, only half of the available pixels are useful for collecting the fluorescence signal. as a consequence, the horizontal spatial resolution of the system is 1 mm when calculated at 5 m of distance from the target. a labview program allows the user to set experimental parameters, define the scene to be scanned, control the different components of the system, acquire data and perform preliminary data processing quickly. moreover, the vertical spatial resolution can be set by the software by defining the number of rows necessary for the scan in order to cover the whole selected area. figure 1. stufetta vault at the bishop’s palace of frascati. figure 2. lif system operating at the bishop’s palace of frascati. laser beam line visible on the left wall. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 182 the measurement time is affected by the value of this parameter, and it can change depending on the specific requirements. the registration of all data related to both the system configuration and the acquisition parameters guarantees the measurement’s repeatability once the placement of the instrument is fixed. moreover, the lif scanning system can be used with the laser switched off to collect reflectance images upon the availability of an intense standard light source that can be used as a spatial reference on the target surface. the whole fluorescence spectrum in the 270–900 nm range for every examined point with a spectral resolution of 2.5 nm and fluorescence images of the scanned surface are generated by the system. the fluorescence images can be elaborated and reconstructed in false colour by using the three most intense detected bands, corresponding to the main features, as red, green and blue channels (rgb), respectively. 2.2. photogrammetry photogrammetry is a tested technique in the documentation of cultural heritage artefacts, and it has already been applied as a useful tool for cultural heritage surveys [18], [19]. 3d photogrammetric reconstruction using the sfm method is done to create three-dimensional models in a very fast and contactless way from the acquisition of two-dimensional digital images in the form of points cloud and/or polygonal mesh in order to fix their correct geometric data in terms of shape, size and spatial position. the sfm technique is based on the theoretical principles of optics, descriptive geometry fundamentals and, especially, inverse perspective theory: the scene reconstruction as well as the camera position and orientation are solved automatically by the software, using complex algorithms. one of the principal obstacles for the use of this technique is the large demands placed on hardware and software resources for image processing, data analysis and data storage, but the ability to access the computational resources offered by the enea computational infrastructure, especially the computational centre for research on complex systems (cresco) high performance computing (hpc) infrastructure, allowed us to overcome this problem. the photogrammetric scan of the stufetta tunnel vault is reported in figure 3 and was carried out by a canon eos 550d reflex digital camera with a resolution of 18 mpixel, a focal length of 18 mm and a 22.3×14.9 mm cmos sensor. the 115 2d 7.5 mb digital images were postprocessed remotely using the commercial code photoscan pro based on computer vision algorithms and sfm and multiple view stereovision (mvs) techniques. this was done via internet, through it@cha virtual lab, which is completely dedicated to cultural heritage applications. it@cha was developed to enable users to access graphic codes, process digital images and produce 3d spatial data on the web using the hardware and software capabilities of the enea cresco hpc infrastructure, which is distributed over 6 geographical sites in italy. in addition, the cresco system can process the large datasets required for many 3d reconstructions by providing computing facilities, storage resources and 3d data rendering tools [20]. thanks to the use of the hardware and software resources of the enea computing infrastructure, it was possible to obtain the 3d reconstruction of the vault at scale in a very short time with an accurate structure in terms of geometry and texture. 2.3. red-green-blue imaging topological radar laser scanner the rgb-itr, shown in figure 4, is a light detection and ranging (lidar) scanner based on the amplitude modulation of laser beams and the use of the lock-in technique for estimations of both colour and structure information [21]. two pieces of software designed for the system, scansystem and itranalyzer, allow the user to set the scan parameters, calibrate data, perform post-processing analyses and generate 3d models [22]. this instrument can be used at any time without being influenced by external factors such as the variability of the ambient light. differently from other laser scanners, the rgb-itr system positions the beam on the surface using a tv-like raster, which ensures the same resolution on the lateral walls and the top. the entire digitalisation was performed by placing the scanner in the middle of the room and rotating the optical head until the entire field of view was covered by the laser beams. in this particular case, the laser scanner was placed approximately 5 m away from the target; in general, it can operate from a distance of up to 35 m. the resulting spot size of the 3 super-imposed laser beams was 0.3 mm, while the spatial resolution was set at 0.5 mm as a good compromise between the scanning time and the image figure 3. 3d photogrammetric reconstruction of the stufetta tunnel vault. figure 4. rgb-itr laser scanner operating in the landscape room. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 183 resolution. the itr scanner is based on a polar coordinate system, so its spatial resolution is dependent on the distance between the instrument and the target. in the presented case study, the acquisition was made at an average distance of 5 m, and the angular point-to-point resolution of the motorised mirror was set at 2–4 mdeg. the average point-to-point spatial resolution was about 0.3–0.5 mm. a complete dissertation presenting a comparison of the image resolutions of standard commercial scanner cameras and rgb-itr as well as comparisons of their data resolution and accuracy can be found in a published paper [23]. during the acquisition phase, a linear calibration procedure was completed that consisted of pointing the laser beam at a calibrated white target from several distances; the resulting curves were used during the post-process phase to normalise the colour channel values. the landscape room, which is about 10 × 10 × 3 m3, was digitalised in 4 days without needing to wait for optimal ambient light; the sun provided light during the day and artificial lights were used during the night. 3. results the 3d scale model of the stufetta tunnel vault obtained by photogrammetry makes it possible to focus on particular details in order to support and integrate the measurements and experimental results of other non-destructive testing techniques [24]. after the photogrammetric measurements were completed, the lif scanning system was applied to the investigation of the stufetta vault. in figure 5, the false colour lif image, reconstructed at 300, 400 and 500 nm, is reported on the left. the relative photogrammetric image, with the measured area outlined by the yellow line, is reported as a reference on the right. as can be observed, some details not appreciable by the naked eye can be seen in the lif image. in particular, the presence of a different material in the area of the angel’s eyes is evident in the blue of the lif false colour image. by analysing the lif spectra resulting from these points, a narrow, intense emission band can be observed at 380 nm (figure 6, lower spectrum). this band is ascribable to the presence of restoration materials probably containing zinc oxide, as suggested by the database developed in laboratory for cultural heritage materials in similar experimental conditions. particular areas in the angel’s chest also stand out in the fluorescence image; in this case, however, the relative spectra figure 5. false colour lif image reconstructed at 300, 400 and 500 nm on the left; photogrammetric image with measured area in the yellow line on the right. figure 6. lif spectra related to areas with evidence of biodeterioration (upper spectrum) and restoration materials (lower spectrum). figure 7. photogrammetric details of the lunette at 1:1 scale. figure 8. partial 3d model reconstruction of the landscape room obtained by rgb-itr scanner. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 184 are the result of the superimposition of many different emission bands, implying the presence of a restoration material of different composition with a low concentration of zinc oxide. a similar material was also found in other parts of the analysed surface. the presence of biodeterioration was also detected by the measurements. a grid of cell size equal to 0.05 m was defined in the lunette above the angel in the photogrammetric model in order to monitor the areas affected by humidity over time (figure 7). lif measurements make it possible to detect the presence of biodeterioration not only in those areas where it is visible by the naked eye but also in some areas where it is not visible at all. lif spectra data of this kind of degradation is characterised by an emission band at 550 nm (figure 6, upper spectrum) that is absent in the lif spectra of non-deteriorated areas. this result, which makes it possible to discriminate between deteriorated and non-deteriorated areas even when the biodegradation is not visible, suggests the possibility of using lif analysis for preventive monitoring to reduce the eventual induced damage. the landscape room was also investigated. in the analysis of the stufetta tunnel vault, the lif maps were referenced to the 3d model, while the rgb-itr laser scanner was used in the landscape room. a partial 3d model reconstruction of the room is reported in figure 8. in this case, the lif system did not detect any degradation or evidence of restoration on the painted walls, and the emission bands in all the collected spectra could be ascribed to cellulose, which reflects the textile substrate. in figure 9 (top), the false colour lif image of a detail on the painted left wall is shown next to the rgb-itr image as an example. in contrast, the lif results for the painted wooden fire screen in the room highlighted the discontinuity of the surface materials. in figure 9 (bottom), the rgb-itr image is shown on the left with the measurement area outlined, and the 380/450 nm fluorescence image of that area is reported on the right. as can be observed, added materials that are probably the result of restoration actions and/or degradation processes were once again localised by lif analysis even though in some areas they are not visible by the naked eye. the possibility of overlaying the lif images onto the rgbitr 3d models in order to obtain some additional information was tested. to reference lif data on a 3d point cloud, 4 or more pairs of common points are selected on the respective datasets, and standard algorithms are used to find the best affine transformation. systematic error that arises during point positioning is attributed to the different resolutions between the two datasets: in this case, the rgb-itr acquired the surface points with a spatial resolution of about 0.3 mm, while the lif data was obtained with a spatial resolution of 1 mm. automatic registration algorithms were not adopted because the correction of non-optical aberrations on the lif dataset was introduced at this stage. in figure 10, the superimposition of the false colour lif image, in blue, onto the rgb-itr image of the same analysed area of the landscape room is reported. no evidence of localised restoration materials or bio-degradation was revealed in this case, as affirmed previously. moreover, a post-processing analysis of the rgb-itr 3d model was developed in order to identify areas of interest and evaluate eventual modifications based on images collected in previous periods [25]. the monitoring of the degradation processes could be provided in this way by repeating the measurements at different times. 4. conclusions the validity of the integrated approach developed in this work as a non-invasive diagnostic tool for cultural heritage has been demonstrated by the obtained results. useful information can be obtained quickly by the synergistic application of the employed different techniques. the ability to localise areas in which biodeterioration processes are occurring or restoration materials have been applied in previous conservation actions has been verified, including for large surfaces. the collected data attested to the good quality, in some cases, of the restoration actions performed in previous years at the bishop’s palace. the selected materials and techniques proved to be perfectly suitable for some retouches and integrations. however, degradation processes due figure 9. top: rgb-itr image with measured area in black line (left) and corresponding false colour lif image (right). bottom: fire screen rgb-itr image, with measured area in yellow line (left) and corresponding lif 380/450 nm image (right). figure 10. superimposition of lif image (in blue) onto the investigated area in the rgb-itr model of the landscape room. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 185 to environmental humidity have been highlighted by the investigation. the possibility of preventive monitoring by the application of the presented approach to reduce eventual induced damage has been raised. work is in progress to optimise the process of integrating the data from the different techniques. acknowledgement this research was supported by the adamo project financed by lazio region for technological 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[19] h. m. yilmaz, m. yakar, s. a. gulec, o. n. dulgerler, importance of digital close-range photogrammetry in documentation of cultural heritage, j. cult. herit. 8 (2007), pp. 428-433. doi: 10.1016/j.culher.2007.07.004 [20] j. mccarthy, multi-image photogrammetry as a practical tool for cultural heritage survey and community engagement, j. archaeol. sci. 43 (2014), pp. 175-185. doi: 10.1016/j.jas.2014.01.010 [21] g. ponti et al., the role of medium size facilities in the hpc ecosystem: the case of the new cresco4 cluster integrated in the eneagrid infrastructure, proc. of international conference on high performance computing and simulation, hpcs 2014, bologna, italy, 21 – 25 july 2014, pp. 1030-1033. doi: 10.1109/hpcsim.2014.6903807 [22] r. ricci, l. de dominicis, m. ferri de collibus, g. fornetti, m. guarneri, m. nuvoli, m. francucci, rgb-itr: an amplitudemodulated 3d colour laser scanner for cultural heritage applications, proc. of international conference on laser in the conservation of artworks (lacona viii), sibiu, romania, 2125 september 2009. [23] a. danielis, m. guarneri, m. francucci, m. ferri de collibus, g. fornetti, a. mencattini, a quadratic model with nonpolynomial terms for remote colorimetric calibration of 3d laser scanner data based on piecewise cubic hermite polynomials, mathematical problems in engineering 2015 (2015) art. 606948. doi: 10.1155/2015/606948 [24] m. guarneri, s. ceccarelli, m. ferri de collibus, m. francucci1 m. ciaffi, multi-wavelengths 3d laser scanning for pigment and structural studies on the frescoed ceiling the triumph of divine https://doi.org/10.1364/ao.40.006111 https://doi.org/10.1016/j.culher.2010.06.003 https://doi.org/10.1016/j.ibiod.2009.03.006 https://doi.org/10.1016/s1296-2074(00)00166-7 https://doi.org/10.1366/0003702963904863 https://doi.org/10.1016/j.microc.2015.12.009 https://doi.org/10.1155/2006/57934 https://doi.org/10.1063/1.3631813 https://doi.org/10.1364/oe.21.014736 https://doi.org/10.1016/j.culher.2008.04.008 https://doi.org/10.1016/j.geomorph.2012.08.021 https://doi.org/10.5194/isprsarchives-xxxviii-5-w16-285-2011 https://doi.org/10.1007/s11082-017-0952-4 https://doi.org/10.1155/2012/512902 https://doi.org/10.1007/s13213-014-0956-2 https://doi.org/10.1007/s00253-013-5283-1 https://doi.org/10.1007/s00253-013-5283-1 https://doi.org/10.1016/j.culher.2007.07.004 https://doi.org/10.1016/j.jas.2014.01.010 https://doi.org/10.1109/hpcsim.2014.6903807 https://doi.org/10.1155/2015/606948 acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 186 providence, the international archives of the photogrammetry, remote sensing and spatial information sciences xlii-2/w15 (2019), pp. 549-554. doi: 10.5194/isprs-archives-xlii-2-w15-549-2019 [25] m. mongelli, i. bellagamba, a. perozziello, s. pierattini, s. migliori, a. quintiliani, g. bracco, a. tatì, p. calicchia, photogrammetric survey to support non destructive tests at st. alexander catacombs in rome, proc. of metroarcheo2018, cassino, italy, 22 – 24 october 2018, pp. 308-313. doi: 10.1109/metroarchaeo43810.2018.13634 [26] l. caneve, m. guarneri, a. lai, v. spizzichino, s. ceccarelli, b. mazzei, non-destructive laser based techniques for biodegradation analysis in cultural heritage, ndt&e international 104 (2019), pp. 108-113. doi: 10.1016/j.ndteint.2019.03.007 https://doi.org/10.5194/isprs-archives-xlii-2-w15-549-2019 https://doi.org/10.1109/metroarchaeo43810.2018.13634 https://doi.org/10.1016/j.ndteint.2019.03.007 simulation of pressure-induced cavity deformation – the 18sib04 quantumpascal empir project acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 281 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 281 286 simulation of pressure-induced cavity deformation – the 18sib04 quantumpascal empir project j. zakrisson1, i. silander1, c. forssén1, z. silvestri2, d. mari3, s. pasqualin3, a. kussicke4, p. asbahr4, t. rubin4, o. axner1 1 department of physics, umeå university, se-901 87 umeå, sweden, johan.zakrisson@umu.se, isak.silaner@umu.se, clayton.forssen@umu.se, ove.axner@umu.se 2 lne-cnam, 61 rue du landy, 93120 la plaine saint-denis, france, zaccaria.silvestri@cnam.fr 3 inrim, national institute of metrological research, strada delle cacce, 91, 10135 torino, italy, d.mari@inrim.it, s.pasqualin@inrim.it 4 physikalisch-technische bundesanstalt (ptb), abbestr. 2-12, 10587 berlin, germany, andre.kussicke@ptb.de, patrick.asbahr@ptb.de, tom.rubin@ptb.de abstract: the 18sib04 quantumpascal empir project aims for development of photon-based standards that can replace primary standards of the si unit of pressure, the pascal. in this project, four partners simulated the pressure-induced deformation of a given fabry-pérot cavity, using various versions of two types of software, comsol multiphysics® and ansys workbench. it was demonstrated that, for a given geometry and set of material parameters, simulations of the deformation could be performed by the various partners with such small discrepancies that methodological mistakes of the simulation procedures will solely contribute to a sub-ppm uncertainty in the assessments of refractivity of n2. keywords: empir; quantumpascal; fem; pressure; refractometry; deformation. 1. introduction the current realization of the pascal is based on piston gauges and liquid manometers [1, 2]. their performance have remained basically unchanged during the last few decades and, especially for pressures below 100 kpa, they suffer from practical and environmental limitations [3, 4]. the 18sib04 quantumpascal empir project – “towards quantum-based realizations of the pascal” – aims to develop photon-based standards for pressure to address these limitations [5]. one of the techniques addressed is fabry-pérot (fp) cavity (fpc) based refractometry, which is a sensitive technique for assessment of gas refractivity, density, pressure, and flows by the use of laser 1 umeå university (umu), sweden; conservatoire national des arts et métiers (cnam), france; national institute of metrological research (inrim), italy; and technology. by measuring the refractivity, n-1, where n is the index of refraction, and the temperature of a gas, the pressure can be calculated by the use of the lorentz-lorenz equation and an equation of state. with the implementation of the refined sisystem in may 2019, the uncertainty in the boltzmann constant was eliminated [6]. this promises primary measurements limited only by the uncertainty in quantum calculations of molar polarizabilities and some virial coefficients, and in the assessment of axial pressure-induced cavity deformation and gas temperature. this means that, in the long term, such primary standards could provide fast and accurate pressure measurements at a fraction of the present cost. one means to assess the cavity deformation is to simulate it by the use of finite element methods (fem) [7]. to eliminate the risk of methodological errors in the set-up of the cavity and the execution of the simulation, the first activity (a1.1.1) in the aforementioned project was devoted to verifying that the simulation procedures used by the four partners that address the pressure induced cavity deformation in fpc-based refractometry1, here anonymized as partners a, b, c, and d, were appropriate. this was done by corroborating that they all could predict the same axial pressureinduced cavity deformation of a given fpc (provided by one of the partners) with a given set of material parameters. note that the activity presented in this work was not devoted to an assessment of to which extent the simulated deformation agrees with respect to a previously performed experimental assessment. physikalisch-technische bundesanstalt (ptb), germany. http://www.imeko.org/ mailto:johan.zakrisson@umu.se mailto:isak.silaner@umu.se mailto:clayton.forssen@umu.se mailto:ove.axner@umu.se mailto:zaccaria.silvestri@cnam.fr mailto:d.mari@inrim.it mailto:s.pasqualin@inrim.it mailto:andre.kussicke@ptb.de mailto:patrick.asbahr@ptb.de mailto:tom.rubin@ptb.de acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 282 instead, using the verified modelling procedure, each partner will, in a subsequent project activity (a1.1.2), model the deformation of a fpc they have access to in their own laboratory, whose deformation they also will experimentally characterize (a1.1.3). since all partners have fpcs with dissimilar geometries (sizes and spacer materials) and mirror mountings, the concept of accuracy will, by this procedure, at a later stage of the project, be scrutinized from a broad point of view. it was agreed on that the work should be performed in the following way: after a first round of simulations, performed individually by each of the four partners, the results were to be compared. if discrepancies appeared, their causes were to be identified and resolved. this was to be continued until no significant inconsistencies remained. when consensus regarding the pressure-induced cavity deformation had been reached, it was considered that the set-ups of the cavities and the executions of the simulations did not incorporate any methodological inconsistencies. 2. description of the cavity the cavity considered was a previously constructed and characterized cavity from cnam [8]. the spacer, made of zerodur®, schematically illustrated in figure 1, is 100 mm long with an inner and outer diameter of 34 and 56 mm, respectively. on each side, 15 mm thick mirrors, made of fused silica, with a diameter of 50 mm, are mounted to the cavity spacer by optical contacting. figure 1: a 3d model of the simulated fp cavity. to simplify the modelling, the cavity was considered to have flat mirrors. it was agreed on that the material parameters used in the simulations should be those presented in table 1. table 1. material properties used by all partners in the simulations. entity spacer material (zerodur®) mirror substrate (fused silica) young´s modulus (gpa) 90.3 73 poisson ratio 0.24 0.155 density (kg/m3) 2530 2195 3. simulation preocedure 3.1. software used to ensure that the simulation results were not software dependent the software used in the simulations were, at the start of the project, not only chosen by the partners themselves, it was also emphasised that not all partners should use the same type or version of software. three partners, a, b, and d, utilized various versions of comsol multiphysics®, viz. the versions 5.3a/5.5, 5.4, and 5.5, respectively, together with the structural mechanics module, while two of the partners, b and c, used ansys workbench – academic student 2019 r2 with a static structural toolbox (partner b thus used both types of software). 3.2. set-up of the cavity to create the 3d model of the cavity the partners a and b used the built-in design module in comsol multiphysics®. partner c utilized the designmodeler ansys workbench module to import cad file created in autodesk fusion 360. partner d employed the cad import module to import 3d models originally created in autodesk inventor professional 2020. both the material of the cavity spacer and the mirrors were modelled using a linear-elastic model. the cavity spacer and the mirrors were considered to be unified, which means that they were seen and treated as one joint solid object. to simulate the application of a pressure onto the cavity, a boundary load was applied to all external surfaces. to decrease computational work, the partners b, c, and d, utilized (three) symmetry planes and simulated solely 1/8 of the cavity. by using such planes, any rigid body motion of the cavity could automatically be eliminated, which facilitated interpretation of the results. the partner that simulated the full cavity, a, used prescribed displacement (or roller) constraints in the symmetry planes to restrict any rigid body movement of the cavity. partner a performed (a set of) simulations of the cavity deformation based on modelling of the (entire) cavity using 1 × 104 to 1.7 × 106 mesh elements. among the partners that simulated 1/8 of the cavity, to describe the modelled part of the cavity, partner b used 1.9 × 105 mesh elements in comsol multiphysics® and 1.7 × 102 to 2.9 × 106 mesh elements in ansys workbench, partner c utilized 3.6 × 102 to 2.1 × 104 elements, while partner d employed 1.2 × 104 to 3.2 × 106 elements. the relevant meshing parameters for the finest mesh used in the simulation by each partner and for each type of software are summarized in table 2. http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 283 table 2. meshing parameters for the finest mesh used in the simulation by each partner. partner a comsol b comsol b ansys c ansys d comsol number of mesh elements 1 735 324 190 000 2 948 108 21 061 3 168 777 maximum element size (m) 1.2 × 10-3 1.3 × 10-3 1 × 10-3 2.3 × 10-3 5 × 10-4 minimum element size (m) 9 × 10-6 1.3 × 10-5 2 × 10-4 2.8 × 10-4 5 × 10-6 growth rate 1.2 1.3 1.2 1.2 1.2 curvature factor 0.25 0.2 0.27 0.2 an example of the meshing of the cavity (provided by partner d) is presented in figure 2. figure 2. an example of a meshing of the cavity shown in a cross-sectional view. 4. results 4.1. typical strain in a pressurized cavity a typical pressure-induced deformation of the cavity addressed, created by partner d, is presented in figure 3. the colours on the cross section surface represent the pressure-normalized longitudinal repositioning of the cavity material (along the optical axis) relative to the centre of the cavity, i.e., , where is the axial repositioning relative the centre of the cavity and is the pressure. figure 3. cross section of the cavity. the colour on the cross section surface represents pressure-normalized longitudinal repositioning of the cavity material relative to the centre of the cavity, , in units of m pa-1. the semi-disc in the lower left corner represents the same entity for half of the reflective surface of one of the mirrors. this figure shows that the longitudinal deformation of the cavity is dominated by a (close-tolinear) deformation (compression) of the cavity spacer and that the bending of the mirrors plays a minor role. the simulations of the cavity deformation were then quantized in terms of a pressure-normalized strain of the cavity, i.e. the relative pressureinduced change in length of the cavity for a given pressure, defined as , where is the change in length between the centre points of the reflective surface of the two mirrors induced by the presence of gas and is the length of the undeformed cavity (i.e., for p = 0 pa). 4.2. first round of simulations the first round of simulations, based on independent work by each partner, predicted a pressure-normalized strain ranging from -5.4 × 1012 to -7.85 × 10-12 pa-1, with a mean of -6.49 × 10-12 pa-1 and a ( ) confidence interval of ±13%. this spread was significantly larger than considered adequate. a scrutiny revealed that it was attributed to methodological errors in the setup of the cavity and the execution of the simulations. this led to a revision process in which all partners independently reviewed, in detail, their set-up and execution processes. during this process, the causes for the large spread in results were successively identified. in all cases, they were found to originate from incorrect or inappropriately applied input parameters, boundary conditions, or procedures ascribed to misunderstandings about how the pertinent program interpreted or utilized the various input parameters. when the various misunderstandings were identified, they were remedied. 4.3. final set of simulations in the final set of simulations, when no methodological set-up or execution errors remained, the partners made systematic investigations of the influence of the number of mesh elements on the predicted pressure-induced deformation. such a study is shown in figure 4. it was found that there is a clear dependence of the pressure-induced strain on the number of mesh elements. for partner d, for example, which performed simulations using 1.2 × 104 to 3.2 × 106 mesh elements, the normalized pressure induced /l p l p /l p ( / ) /l l p l l 2 x  http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 284 figure 4. simulated normalized pressure-induced strain from the four partners. note that the “plus” makers indicate the use of comsol multiphysics® while the “square” markers denote the use of ansys workbench. moreover, partner a simulated the entire cavity spacer, while the other partners modelled only 1/8 of it. deformation ranged from 6.3888 × 10-12 to 6.3903 × 10-12 pa-1. the simulations performed by the other partners showed a similar behaviour. this implies that it is in general advisory to perform an analysis of the dependence of the strain as a function of the number of mesh elements, and choose an appropriate number of mesh elements for the type of simulation performed. however, it was also found that the pressureinduced strain levels off asymptotically with the number of mesh elements. this implies that the simulations with the largest number of mesh points are least dependent of the number of mesh points used and are thereby assumed to be those with highest accuracy. on the other hand, the difference in simulated deformation performed by partner d when the two largest numbers of mesh points were used (1.2 and 3.2 × 106 elements, respectively) is only 2 × 10-16 pa-1, which, for n2, corresponds to a relative uncertainty in the assessed refractivity of solely 0.075 ppm. this implies that simulations with an excessive number of mesh elements may not necessarily be needed figure 4 shows that the pressure-induced strain ranged, for each partner (simulated using the largest number of mesh points), from -6.3899 × 1012 to -6.3908 × 10-12 pa-1, with a mean of -6.3902 × 10-12 pa-1 and a relative 95% confidence interval ( ) of ± 3 × 10-16 pa-1, corresponding to ± 50 ppm (parts-per-million). since the simulations were performed under slightly different conditions, with somewhat dissimilar procedures, and this confidence interval corresponds, for n2, to a relative uncertainty in the assessed refractivity of 0.1 ppm, this was found adequate. the results are, for clarity, summarized in table 3. table 3. summary of the simulated pressurenormalized cavity deformation using the finest meshing by each partner and software. partner – software used pressure-normalized deformation: in 10-12 pa-1 a – comsol -6.3900 b – comsol -6.3899 b – ansys -6.3908 c – ansys -6.3900 d – comsol -6.3903 average value -6.3902 standard error of the mean ( ) 1.5 × 10-4 95% ( ) confidence interval 2.9 × 10-4 5. discussion 5.1. technicalities it can be noticed that the simulations performed on the entire cavity (by partner a) in virtually all cases predicted a deformation that is smaller than those addressing 1/8 of the cavity. however, when the entire cavity is simulated, there are roughly eight times more mesh elements than what would be if 1/8 of the cavity would be simulated. by recalculating the numbers of mesh elements used in the simulations of 1/8 of the cavity to those for the entire cavity, the results of the 1/8 of the cavity conform to the simulations of the entire cavity. from this we conclude that using symmetry planes can, for this cavity, be seen as appropriate. similarly, although there is a tendency that the simulations made using the ansys workbench software (by partner b and c) consistently predict larger deformations than those made on the comsol multiphysics® software (for comparable number of mesh elements), the difference in simulated deformation performed by the two types of software when the largest numbers of mesh points were used (partner b, ansys, 2.9 × 106 elements, and partner d, comsol, 3.2 × 106 elements, respectively), is only 5 × 10-16 pa-1, which, for n2, corresponds to a relative uncertainty in the assessed refractivity of solely 0.17 ppm. this indicates that when a large number of mesh elements are being used, the difference between the two types of software is insignificant in comparison to several other types of uncertainties, e.g., those from dynamic molar polarizabilities. it is also possible that some other meshing parameters (or settings) than those identified in this report can affect the simulations. however, no study of such phenomena was addressed in this comparison. 2 x  ( / ) /l l p x  2 x  http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 285 5.2. consensus as was alluded to above, the simulations performed by the various partners, utilizing dissimilar software and set-up strategies, did finally, for a given geometry and set of material parameters, provide a relative 95% ( ) confidence interval of ± 3 × 10-16 pa-1. for the fp cavity addressed, this corresponds, for n2, to a relative uncertainty in the assessed refractivity solely of 0.1 ppm, which, for the pertinent quantumpascal empir project, is considered adequate. 5.3. accuracy the deformation of this cavity has previously been experimentally assessed to (6.030 ± 0.005) × 10-12 pa-1 [8]. the simulated values differ systematically 6 % from this. since the accuracy of the simulations is given by the accuracy of both the cavity spacer material parameters and the geometrical quantities, and none of these where considered having any uncertainty in this work (see e.g. table 1), it is possible that this discrepancy can, to a certain extent, be explained by uncertainties in these entities. it is also possible that the accuracy is affected by features not included in the simulations, e.g. the mounting of the cavity, gravity, or aging. although such effects were not a part of the a1.1.1 task in the 18sib04 quantumpascal empir project under which this study was performed, the influence of the former of these was investigated by one of the partners (a) who showed that the mounting of the cavity indeed could affect the final deformation. it is also plausible that the deviations between the simulated and the experimentally assessed values can be attributed to yet unidentified experimental processes or features. it is finally worth to note that for cavities with more complex design (e.g. with more intricate mirror mounting), the accuracy is likely to be adversely affected by the fact that the actual cavity geometry will be less accurately known. 6. summary and conclusion the ability of four partners of the 18sib04 quantumpascal empir project to simulate, by fem-simulations, the pressure-induced cavity deformation of a given fp cavity has been assessed. when setting up the simulations, it was found that considerable care must be taken regarding the input parameters and boundary conditions since any minor inaccuracies in these could lead to non-negligible errors in the simulations. in the absence of methodological inconsistencies, and for a given geometry and set of material parameters, adequate agreement between the simulations made by the various partners was reached. it was found that the use of different types of software did not significantly affect the results. on the other hand, a certain dependence on the number of mesh elements used was found. it was found that the remaining discrepancies were small, with a confidence interval solely of ± 50 ppm. since this spread in the predicted cavity deformation corresponds to an uncertainty in assessed refractivity of n2 and he solely of 0.1 and 0.9 ppm, respectively, this was considered adequate for the project. although good consensus among the various partners regarding simulated pressure-induced strain of the cavity considered was reached, a comparison with a previous experimentally assessed deformation assessment indicated a 6 % deviation. considering that the most probable values of all material parameter were used in the simulations, this indicates that it may be unlikely that simulations by themselves can provide lowppm accuracy in assessment of refractivity. however, since accuracy and compliance with experimental data were not an issue of the a1.1.1 activity under which this work was performed, no further investigations of the causes for the observed discrepancy between simulated and experimental pressure-induced strain were performed; it was considered sufficient to demonstrate consensus regarding set up and execution of the fem software used for the simulations. instead, the upcoming activities in the project, in which each partner will model the deformation of a fpc they have access to in their own laboratory whose deformation they also will experimentally characterize, will assess to which extent simulations are a feasible means to assess cavity deformation. this project (quantumpascal, 18sib04) has received funding from the empir programme cofinanced by the participating states and from the european union's horizon 2020 research and innovation programme; the swedish research council (vr) (project no. 621-2015-04374); the umeå university industrial doctoral school (ids); and the kempe foundations (project no. 1823, u12). 7. references [1] c.r. tilford, "3 and a half centuries later the modern-art of liquid-column manometry", metrologia, vol. 30, pp. 545-552, 1994. [2] s. semenoja and m. rantanen, "comparisons to establish a force-balanced piston gauge and a spinning rotor gauge as the new measurement standards of mikes", vacuum, vol. 73, pp. 269274, 2004. [3] j. h. hendricks, d. a. olson, “1-15,000 pa absolute mode comparisons between the nist 2 x  2 http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 286 ultrasonic interferometer manometers and nonrotating force-balanced piston gauges, measurement, vol. 43, pp. 664-674, 2010. [4] j. ricker, j. hendricks, t. bock, p. dominik, t. kobata, j. torres, i. sadkovskaya, “final report on the key comparison ccm.p-k4.2012 in absolute pressure from 1 pa to 10 kpa, metrologia, vol. 54, p. 07002, 2017. [5] physikalisch-technische bundesanstalt (ptb), 18sib04 quantumpascal, https://www.ptb.de/empir2019/quantumpascal/ho me/ , (accessed: 18.09.2020). [6] m. stock, r. davis, e. de mirandés, and m. j. t. milton, “the revision of the si—the result of three decades of progress in metrology”, metrologia, vol. 56, pp. 049502, 2019. [7] t. rubin, y. yang, “simulation of pressure induced length change of an optical cavity used for optical pressure standard”, journal of physics: conf. series, vol. 1065, p. 162003, 2018. [8] z. silvestri, f. boineau, p. otal and j. wallerand, "helium-based refractometry for pressure measurements in the range 1–100 kpa," in ieee (ed.), 2018 conference on precision electromagnetic measurements, (ieee, new york, ny 10017, usa, 2018). http://www.imeko.org/ https://www.ptb.de/empir2019/quantumpascal/home/ https://www.ptb.de/empir2019/quantumpascal/home/ raman investigation of corrosion products on roman copper-based artefacts acta imeko issn: 2221-870x march 2021, volume 10, number 1, 129 135 acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 129 raman investigation of corrosion products on roman copperbased artefacts leila es sebar1, leonardo iannucci1, yuval goren2, peter fabian2, emma angelini1, sabrina grassini1 1 department of applied science and technology, politecnico di torino, torino, italy 2 department of bible, archaeology and ancient near east, ben gurion university of the negev, beer-sheva, israel section: research paper keywords: raman spectroscopy; pca; corrosion; archaeological artefacts citation: leila es sebar, leonardo iannucci, yuval goren, peter fabian, emma angelini, sabrina grassini, raman investigation of corrosion products on roman copper-based artefacts, acta imeko, vol. 10, no. 1, article 17, march 2021, identifier: imeko-acta-10 (2021)-01-17 editor: carlo carobbi, university of florence, italy received may 7, 2020; in final form august 5, 2020; published march 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this work was supported by the project for the internationalisation of research of the politecnico di torino (italy) in cooperation with the ben gurion university of the negev (israel). corresponding author: leonardo iannucci, e-mail: leonardo.iannucci@polito.it 1. introduction alloy chemical composition, microstructural features and corrosion layer stratification provide archaeologists, researchers and conservators with the scientific evidence necessary for the dating, authentication and identification of archaeological metal findings. thus, the understanding of the complex corrosion mechanisms occurring on copper (cu)-based alloys during burial is a fundamental prerequisite for their long-lasting conservation and safeguarding. the corrosion of cu-based archaeological artefacts buried in soil for long periods depends on many variables related to the microclimatic conditions and soil chemical–physical properties. in fact, soil is a very complex environment and many different parameters, such as geological and hydrological factors and soil chemical composition, can affect the degradation mechanisms of the objects. the formation of patinas during burial can be ascribed to oxygen and carbon dioxide, moisture content, temperature, ph and salt content [1], [2]. moreover, water in the soil acts as an electrolyte on the surface of the buried object, and, in the case of cu-based artefacts, a minimum ph value is required to induce the formation of stable corrosion product layers [3]. a precipitation–dissolution mechanism forms part of the degradation process, which can be explained by considering the dissolution of copper, which is kinetically controlled by the mass transport of this element from the alloy to the environment, and the diffusion of the aggressive ions towards the alloy interphase. anionic control, in the presence of chloride ions, produces higher volume change and less coherent corrosion layers, whereas cationic processes arise in the diffusion of metallic components and become the rate-determining step, resulting in compact corrosion layers [4], [5]. due to the complexity of both the patina structure and the degradation mechanisms, several analytical techniques and statistical data analyses are employed to collect reliable information on cu-based artefacts. among them, raman spectroscopy has found a wide application in the cultural heritage field because it is non-destructive and allows measurements to be performed in situ using portable instrumentation [6], [7]. in the last decades, different studies have analysed raman spectra acquired from copper minerals, providing important knowledge to characterise patinas related to copper corrosion [8], [9]. this abstract this paper illustrates a case study related to the characterisation of corrosion products present on recently excavated artefacts. the archaeological findings, from the rakafot 54 site (beer-sheva, israel), consist of 23 coins and a pendant, all dating back to the roman period. raman spectroscopy was used to identify the corrosion products that compose the patina covering the objects. to facilitate and support their identification, spectra were then processed using principal components analysis. this chemometric technique allowed the identification of two main compounds, classified as atacamite and clinoatacamite, which formed the main components of the patinas. the results of this investigation can help in assessing the conservation state of artefacts and defining the correct restoration strategy. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 130 has allowed researchers to use raman spectroscopy in different investigations in order to identify the compounds present in the patinas of archaeological findings [10]-[11][12]. this kind of study has great relevance for assessing the artefacts’ conservation state and, thus, proposing the most appropriate restoration strategy [13]. an additional tool that is often used by researchers involved in spectrometric analyses is multivariate statistics and, in particular, principal components analysis (pca). this chemometric technique allows the user to find characteristic patterns in the acquired spectra and, therefore, to discriminate between the different components that belong to the system under study. it is possible to find various case studies in the literature that have taken advantage of this technique in different contexts [14]-[16]. this study deals with a non-invasive raman investigation on bronze artefacts from the rakafot 54 archaeological site in israel. the proposed measuring approach is appropriate as a preliminary investigation, which could be performed in the field, to collect information about the stability and the conservation state of the artefacts immediately after excavation, thus leading to the development of tailored preservation strategies for the storage of these items. in section 2, details of the israeli bronze artefacts and the experimental methodology are presented, together with the model for pca analysis and data matrix decomposition. section 3 deals with the presentation and discussion of the results of the raman spectroscopy measurements and the pca data processing. finally, in the concluding section, the major achievements are summarised. 2. materials and methods in this section, the archaeological artefacts under investigation are described as well as the main methodological issues and the data analysis performed. 2.1. archaeological artefacts under study the artefacts that are the subject of this study were excavated in the rakafot 54 archaeological site near beer-sheva, israel, close to the border between judea and nabataea, involving a settlement dating back to the roman period. the site dates from the first century ad (second temple) to the bar kochba revolt against rome in 135 ad. the excavations were carried out by peter fabian, department of bible, archaeology and ancient near east, ben gurion university of the negev (beer-sheva, israel), and daniel varga, israel antiquities authority. during the excavation, several artefacts and dozens of bronze coins were found, most of them belonging to the roman procurators' period (6–66 ad). of these, a set of 24 artefacts, 23 coins and a pendant were selected for the investigation of the figure 1. images of the 24 analysed artefacts from the rakafot 54 archaeological site, labelled with their identifier code. each photograph represents an area of 3.5 cm x 3.5 cm. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 131 corrosion products. figure 1 shows the high-resolution images of the artefacts and their identifier codes. under visual observation, most of the artefacts appeared to be covered by a thick layer of corrosion products that were rather non-uniform. brownish or dark-green patinas were predominantly present on the surface, while some areas had a light-green colour; soil traces were also found on the surface. 2.2. raman spectroscopy superficial corrosion products were analysed using raman spectroscopy, a technique widely used in the characterisation of metallic artefacts. measurements were performed using the bwtek i-raman plus spectrometer. this is a portable instrument equipped with an optical-fibre probe to analyse specific areas of the sample and a high quantum efficiency ccd array detector to guarantee a good signal-to-noise ratio in the acquired spectra. a laser wavelength of 532 nm was used for all measurements, which were carried out in the range between 150 cm−1 and 4,200 cm−1 with a resolution of 7.3 cm−1. after optimising the measurement parameters on reference samples, as described in [17], spectra were acquired using a laser power of 6 mw, with 10 repetitions and an integration time of 20 s, in order to avoid any modification or degradation being induced in the material by the laser radiation. as the samples under study were not flat but had irregular shapes and high surface roughness, measurements were carried out by gradually changing the distance between the probe and the analysed artefact to focus the laser beam and have the best signal-to-noise ratio in the acquired spectrum. 2.3. data analysis pca was used to recognise the different patterns present in the acquired raman spectra and to support the identification of corrosion products on the samples. pca is a powerful tool that allows the reduction of the dimensionality of a dataset, keeping only those variables that account primarily for data variance [18]. thus, it transposes all the vectors representing each of the acquired spectra into a new space that has a dimension equal to the number of significant components evaluated through pca. in matrix notation, it is possible to express the original spectra as follows: 𝑋 = 𝑇 ∙ 𝑃𝑇 + 𝐸 , (1) where, if we have i samples and j points for each measurement, x is the original data matrix (with a dimension of i x j), p is the loading matrix (with a dimension of j x k, where k is the number of significant components), which form the eigenvectors representing the new space, t is the score matrix (with a dimension of i x k), which form the eigenvalues derived from the x matrix decomposition, and e is the residual matrix, also known as the error matrix, which contains the variance burden not explained by the pca model [19]. in the present study, pc analysis was carried out on the acquired raman spectra using a python script. first, preprocessing was performed, as is usually required for this kind of data [20]. the analysed spectrum was limited to the interval of interest between 150 cm−1 and 1,150 cm−1, where the peaks related to the copper corrosion products are present [21]. baseline removal was performed using asymmetric least square smoothing, as described in [22]. a savitzky-golay filter was then applied [23] using the savgol_filter function from the scipy library [24]; the original spectrum was processed using a window length of 15 cm−1 and fitted with a 2nd order polynomial. after that, standard normal variate transformation was performed [25] using the following calculation for each measurement point: 𝑦𝑆𝑁𝑉 = 𝑦 − �̅� 𝑠𝑡𝑑 , (2) where 𝑦𝑆𝑁𝑉 is the variable value after transformation, 𝑦 is the original variable, �̅� is the mean value in the original spectrum and std is the standard deviation. the results of the pre-processing operations can be seen in figure 2, where the results obtained for one of the spectra is presented. as can be seen, only the wavenumber range from 150 cm−1 to 1,150 cm−1 is selected. the broad background caused by fluorescence is then removed, the spectrum is filtered to improve the signal-to-noise ratio and, finally, normalisation is performed. original data matrix decomposition was performed using the pca package from the sklearn.decomposition module [26]; in this way, eigenvectors and scores are directly computed. the pca model was improved by eliminating possible outliers, which generally proved to be noisy measurements, considering two parameters, leverage and root mean square deviation (rmsd). the former is defined as the diagonals of the ‘hat matrix’ h: 𝐻 = 𝑇(𝑇𝑇 𝑇)−1𝑇𝑇 , (3) where t is again the score matrix. all measurements with a leverage higher than three times the median value were discarded from the model construction and analysed separately [27]. moreover, rmsd, which quantifies the difference between the acquired spectrum and the same spectrum reproduced by the pc model, was calculated as follows: 𝑅𝑀𝑆𝐷 = √ ∑ (𝑋 − 𝑇𝑃𝑇 )2 𝐽 𝑖=1 𝐽 , (4) where j is the number of points for each measurement, x is the original data matrix, t is the score matrix and p is the loading matrix. model optimisation was carried out until all the reproduced spectra had an rmsd lower than 30% of the initial standard deviation. after applying the obtained pca model to the spectra, it was possible to compute the scores related to the most important components. similarity between spectra was evaluated using figure 2. top: original raman spectrum acquired on sample 9883. bottom: same spectrum in the restricted range of interest between 150 cm−1 and 1,150 cm−1 after pre-processing and normalisation. the yellow line represents the computed baseline in the original spectrum. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 132 hierarchical clustering. this analysis was performed using the agglomerative clustering method in the sklearn.cluster module; euclidean metric was used to compute the linkage. in figure 3, a flow chart summarises the steps involved in pca processing. 3. results and discussion a preliminary study by peters et al. [28] analysed the chemical composition of the artefacts under investigation by means of xray fluorescence. this study revealed that most of the artefacts were produced from a copper-lead-tin alloy, with a lead content that, in some cases, reached 14 wt%. starting from this basis, the present investigation tried to identify the specific corrosion products that characterise the artefacts by means of raman spectroscopy. gentle brushing without any solvent, to avoid surface modification or the dissolution of the corrosion products, was the only cleaning operation that was performed before the raman measurements were taken, as soil traces can affect their acquisition by causing large fluorescence signals in the spectrum. the acquisition of raman spectra was not straightforward, as the shape and conservation state of the artefacts were not optimal. as mentioned above, due to the irregular shape, the focusing of the laser beam was performed by manually adjusting the distance between the spectrophotometer probe and the sample in order to achieve optimal conditions. moreover, despite the preliminary cleaning, soil traces caused a broad fluorescence signal in the central part of the spectrum, between 1,200 cm−1 and 3,000 cm−1, as can be seen in figure 2 for sample 9883. a good signal-to-noise ratio could be obtained only when brightgreen patinas were analysed, while measurements could not be performed on the brownish patinas, possibly associated with cuprite [28]. this could be due to the low thickness of these layers or to the excitation wavelength that was used in this study; only a 532 nm laser light was employed. for all 24 artefacts, at least five raman spectra were acquired for different areas, and in all cases, it was possible to find at least one area giving a good raman signal. taking into account the characteristic peaks for copper corrosion products [21], the analysis was limited to the wavenumber range between 150 cm−1 and 1,150 cm−1. after performing the pre-processing operations, as described in section 2.3, the resulting spectrum appears as shown in figure 2 for sample 9883. from a visual comparison of the different measurements, it was possible to distinguish two patterns that were predominant in most of the spectra. the first was characterised by four intense peaks at 505 cm−1, 814 cm−1, 905 cm−1 and 972 cm−1, with the peak at 814 cm−1 having a shoulder at a higher wavelength. the second pattern exhibited an intense peak at 505 cm−1 and then another four, respectively, at 796 cm−1, 888 cm−1, 921 cm−1 and 967 cm−1. these two patterns could be associated with the characteristic spectra of two copper hydroxy-chlorides (cu2cl(oh)3), atacamite and its polymorph, clinoatacamite. due to the presence of several minor peaks and the noise related to the non-ideal acquisition conditions, data were processed also using pca, an unsupervised multivariate analysis. using this chemometric technique, it is possible to classify different measurements, identifying their characteristic patterns. the pca model was built progressively, removing those spectra that were identified as outliers by the algorithm (see section 2.3 for further details on outlier detection). all these measurements had been previously classified as noisy acquisitions, and some of them were dubiously identified as atacamite. after model optimisation, all measurements fell into the desired leverage and rmsd range (leverage below three times the median value and rmsd lower than 30% of the initial variance). the cumulative explained variance trend can be observed in figure 4. the first three components represent an overall variance of about 85.5% respectively, 59.7%, 19.8% and 6.0%, which is a satisfactory level. taking also into account that the fourth component captures only 4.2% of the total variance, it figure 3. flow-chart summarising the steps involved in pca processing. figure 4. cumulative explained variance for the first five components, as obtained from the pca model. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 133 was decided to use only the first three components to carry out the analysis. using components representing a low percentage of the initial variance can have adverse effects because they would generally fit the measurement noise and not the important parts of the spectrum. the result of the pc analysis is presented in figure 5, which plots the scores of the three principal components two at a time; spectra that appear close in these graphs are similar to each other. as can be seen, measurements group together in two clusters, and the main discriminant is the pc1 score value, which is positive in one case and negative or with low positive values in the other. this behaviour is clearly visible in the pc1-pc2 and pc1-pc3 plots, while cluster formation is not evident in the pc2pc3 plot. four samples belong to the first group, namely 6151, 8103, 8299 and 8599, while 6015, 6058, 6081, 6101, 8260, 8261, 8591, 8603, 8893, 9024, 9483, 9803, 9830 and 9883 are part of the second. the absence of clusters in the pc2-pc3 plot can be explained by considering the low variance captured by the third component (only 6.0%) and the pc3-loading trend (figure 6). in fact, this plot does not contain many significant features, only a peak at about 977 cm−1, which can be found in the atacamite spectrum, and a minimum in the region around 198 cm−1. presumably, this component is used in the model in linear combination with the main two components only to improve the fitting of the most important peaks found in the other two components. the reason for the discrimination in separated clusters can be found by looking at the loadings associated with the first two components (figure 6). in the pc1 loading, peaks associated with atacamite (811 cm−1 and 904 cm−1) are in the negative range, so a negative score is needed in order to fit the original spectrum. at the same time, peaks associated with clinoatacamite (for example, at 884 cm−1) are in the positive range. pc2 loading exhibits a different trend; here, it is possible to find a peak in the negative range (506 cm−1), which can be seen both in the atacamite and clinoatacamite spectrum. however, peaks associated with clinoatacamite (885 cm−1 and 922 cm−1) are in the negative range, thus justifying the reason for the negative scores in the samples of the first group, and an intense peak at 970 cm−1 associated with atacamite is present. therefore, the attribution of both positive and negative scores for atacamite samples in the second component can be explained by their features being in both ranges. moreover, in samples with a negative pc2 score, the feature at around 970 cm−1 in the original spectrum can be reproduced by peaks at 962 cm−1 and 977 cm−1 in the pc1 and pc3 loading. the goodness of fit for the two example spectra can be evaluated by looking at figure 7: on top, a spectrum identified as atacamite is presented, while at the bottom, one associated with clinoatacamite is shown. the yellow line, representing the model, was computed as the product between the scores and the loading matrix: 𝑋𝑀𝑂𝐷 = 𝑇 ∙ 𝑃 𝑇 . (5) it correctly fits the black dots that represent the measured spectrum. notably, the most important peaks in both spectra are correctly fitted by the model, and the residual error is low in the whole range. thus, it is possible to state that the corrosion patina of the artefacts under study is mainly composed of copper hydroxychlorides, specifically atacamite and clinoatacamite. this agrees with the preliminary studies, which indicated the presence of chlorides in the excavation soil [28]. the presence of only two kinds of corrosion product suggests the existence of quite uniform conditions in the soil where the artefacts were buried, although, as discussed above, performing raman spectroscopy with a different laser wavelength might reveal other compounds. figure 5. score plots of the first three components (pc1-pc2, pc1-pc3 and pc2-pc3). percent variance captured by each pc is reported in parenthesis along each axis. figure 6. loadings associated with the first three components in pca. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 134 atacamite and its polymorph clinoatacamite are generally considered as the final product of copper corrosion in environments containing chlorides. first, a layer of cuprite (cu2o) grows on the metal; then, in the presence of water and chloride ions, cuprous chloride is formed (nantokite, cucl), followed by copper hydroxychlorides. for this reason, conservation conditions of the investigated artefacts can be considered quite stable, even if chlorides are present. finally, the morphology of the grown patina should also be taken in account. only if this layer is sufficiently dense and continuous can it hinder the diffusion of the reactive compounds, otherwise the cyclic reactions related to copper corrosion would continue until the mineralisation of the metal is complete. 4. conclusions a non-invasive characterisation of bronze artefacts was carried out using raman spectroscopy. using multivariate analysis (pca), it was possible to identify the main corrosion products present in the artefacts’ patina, namely atacamite and clinoatacamite. these results, coupled with the information from the soil characterisation, provides a deeper insight into the artefacts’ corrosion processes that occur during long-term exposure in a predominantly dry environment containing chlorides. moreover, they can be used by restorers and conservators to define the most appropriate restoration strategy for these artefacts. acknowledgement this research was performed in the frame of a joint project for the internationalisation of research between politecnico di torino (italy) and the ben gurion university of the negev, (israel). it was financially supported by compagnia di san paolo (torino, italy) in 2019. references [1] a. g. nord, e. mattsson, k. tronner, factors influencing the long-term corrosion of bronze artefacts in soil, protection of metals 41 (2005), pp. 309-316. doi: 10.1007/s11124-005-0045-9 [2] m. c. bernard, s. joiret, understanding corrosion of ancient metals for the conservation of cultural heritage, electrochimica acta 54 (2009), pp. 5199-5205. doi: 10.1016/j.electacta.2009.01.036 [3] d. scott, bronze disease: a review of some chemical problems and the role of relative humidity, journal of the american institute for conservation 29 (1990), pp. 193-206. doi: 10.1179/019713690806046064 [4] l. robbiola, j-m. blengino, c. fiaud, morphology and mechanisms of formation of natural patinas on archaeological cusn alloys, corrosion science 40 (1998), pp. 2083-2111. doi: 10.1016/s0010-938x(98)00096-1 [5] r. francis, the corrosion of copper and its alloys: a practical guide for engineers, national association of corrosion engineers, houston, 2009, isbn 9781575902257. [6] p. vandenabeele, h. g. m. edwards, l. moens, a decade of raman spectroscopy in art and archaeology, chemical reviews 107 (2006), pp. 675-686. doi: 10.1021/cr068036i [7] o. berger, p. b. yersinc, j-b. yersinc, c. hartmannd, e. hildbrand, v. hubert, k. hunger, m. ramstein, m. wörle, applications of micro-raman spectroscopy in cultural heritage examples from the laboratory for conservation research of the collections centre of the swiss national museums, chimia 62 (2008), pp. 882-886. doi: 10.2533/chimia.2008.882 [8] r. l. frost, w. martens, j. t. kloprogge, p. a. williams, raman spectroscopy of the basic copper chloride minerals atacamite and paratacamite: implications for the study of copper, brass and bronze objects of archaeological significance, journal of raman spectroscopy 33 (2002), pp. 801-806. doi: 10.1002/jrs.921 [9] g. bertolotti, d. bersani, p. p. lottici, m. alesiani, t. malcherek, j. schlüter, micro-raman study of copper hydroxychlorides and other corrosion products of bronze samples mimicking archaeological coins, analytical and bioanalytical chemistry 402 (2012), pp. 1451-1457. doi: 10.1007/s00216-011-5268-9 [10] a. inberg, d. ashkenazi, m. cohen, n. iddan, d. cvikel, corrosion products and microstructure of copper alloy coins from the byzantine-period ma'agan mikhael b shipwreck, israel, microchemical journal 143 (2018), pp. 400-409. doi: 10.1016/j.microc.2018.08.033 [11] o. papadopoulou, p. vassiliou, s. grassini, e. angelini, v. gouda, soil-induced corrosion of ancient roman brass a case study, materials and corrosion 67 (2016), pp. 160-169. doi: 10.1002/maco.201408115 [12] m. veneranda, j. aramendia, o. gomez, s. fdez-ortiz de vallejuelo, l. garcia, i. garcia-camino, k. castro, a. azkarate, j. m. madariaga, characterization of archaeometallurgical artefacts by means of portable raman systems: corrosion mechanisms influenced by marine aerosol, journal of raman spectroscopy 48 (2017), pp. 258-266. doi: 10.1002/jrs.4997 [13] l. iannucci, j. f. ríos-rojas, e. angelini, m. parvis, s. grassini, electrochemical characterization of innovative hybrid coatings for metallic artefacts, epj plus 133 (2018), pp. 1-7. doi: 10.1140/epjp/i2018-12368-3 [14] i. marcaida, m. maguregui, h. morillas, n. prieto‑taboada, m. veneranda, s. fdez-ortiz de vallejuelo, a. martellone, b. de nigris, m. osanna, j. m. madariaga, in situ non‑invasive multianalytical methodology to characterize mosaic tesserae from the house of gilded cupids, pompeii, heritage science 7 (2019), pp. 1-11. doi: 10.1186/s40494-019-0246-1 [15] s. carlesi, m. ricci, c. cucci, c. lofrumento, m. picollo, m. becucci, multivariate analysis of combined reflectance ft-nir and micro-raman spectra on oil-paint models, microchemical journal 124 (2016), pp. 703-711. doi: 10.1016/j.microc.2015.10.023 figure 7. top: sample 9883 (identified as atacamite). bottom: sample 8103 (identified as clinoatacamite). black dots are the measured spectra, while the result from the pca model is represented by the yellow line. https://doi.org/10.1007/s11124-005-0045-9 https://doi.org/10.1016/j.electacta.2009.01.036 https://doi.org/10.1179/019713690806046064 https://doi.org/10.1016/s0010-938x(98)00096-1 https://doi.org/10.1021/cr068036i https://doi.org/10.2533/chimia.2008.882 https://doi.org/10.1002/jrs.921 https://doi.org/10.1007/s00216-011-5268-9 https://doi.org/10.1016/j.microc.2018.08.033 https://doi.org/10.1002/maco.201408115 https://doi.org/10.1002/jrs.4997 https://doi.org/10.1140/epjp/i2018-12368-3 https://doi.org/10.1186/s40494-019-0246-1 https://doi.org/10.1016/j.microc.2015.10.023 acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 135 [16] a. sarmiento, m. pérez-alonso, m. olivares, k. castro, i. martínez-arkarazo, l. a. fernández, j. m. madariaga, classification and identification of organic binding media in artworks by means of fourier transform infrared spectroscopy and principal component analysis, analytical and bioanalytical chemistry 399 (2011), pp. 3601-3611. doi: 10.1007/s00216-011-4677-0 [17] l. es sebar, l. iannucci, y. goren, p. fabian, e. angelini, s. grassini, non-invasive characterization of ancient cu-based coins using raman spectroscopy, proc. of 2019 imeko tc4 international conference on metrology for archaeology and cultural heritage, florence, italy, 4 – 6 december 2019, pp. 389394. online [accessed 26 march 2021] https://www.imeko.org/publications/tc4-archaeo2019/imeko-tc4-metroarchaeo-2019-77.pdf [18] j. n. miller, j. c. miller, statistics and chemometrics for analytical chemistry, pearson, harlow, 2010, isbn 978-0-273-73042-2. 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diagonal length; diffraction limits; resolution; uncertainty of measurement 1. introduction consider establishing a national primary vickers hardness standard where any reference materials are not available, the error in diagonal length measurement is one of the most significant sources of measurement uncertainty as well as testing force, testing cycle, uniformity of reference blocks or repeatability of measurement. although the iso standard [1] requires to consider “both the resolution of the length measurement indicating instrument and the optical resolution of the measuring microscope”, most of discussions are focused on the indicating instrument (e. g., the discussion in the ea committee) and quite few studies concerned the resolving power of the microscope [2]. in reality, the same size of indentation is measured with different magnification or different numerical aperture depending to the design of the measuring device each calibration laboratory used. in this paper, possible errors caused by the design of an optical microscope are discussed. 2. microscope design factores and their effects on measurement typical design factors of a microscope are the objective magnification and its numerical aperture. usually, those are given as specifications of an objective lens and users cannot change those numbers by themselves. here, the effects of those design factors are briefly reviewed. 2.1 magnification the magnification directly affects to the resolution of readings whichever it is detected by a camera or a human eye. 2.2 numerical aperture the numerical aperture (na) is the index related to the half-angle of the cone of the light that can enter the objective q . na = n×sinq (1) where n is the index of refraction ( n = 1 in air). it determines the resolving power d , i. e., diffraction limit, by following equation [3] d = 0.61× l na , (2) where l is the wavelength of light. when the visible light is considered, l = 550 nmis often used. the na also affects the depth of focus (dof) of a microscope by following equation. dof = nl 2 × na 2 (3) when a microscope makes focus on a sample, the distance between the objective and the sample can be varied within dof. from a simple geometrical consideration, an image will expand by the difference of the distance and the maximum value is estimated as follows. e = dof × tanq . (4) 3. experiment in order to investigate the effects of microscope design factors, a series of measurement of vickers http://www.imeko.org/ mailto:satoshi.takagi@aist.go.jp mailto:yukimi.tanaka@aist.go.jp mailto:y.seino@aist.go.jp acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 262 indentations was carried out. photos of indentations on a hardness reference blocks were taken by an olympus bxfm optical microscope. in order to discuss effects of magnification and na separately, single objective lens of 50× magnification was used. this microscope system is infinity-corrected, i. e., the beam path is parallel between the objective and tube lenses (figure 1). by setting a diaphragm into this parallel beam section, its pupil diameter, therefore its na, can be changed without changing the objective magnification. images were taken in a format of 2592×1944 pixels and diagonal lengths of indentation were measured on a computer screen. the size of image was calibrated by taking an image of calibrated stage micrometer. the pixel size is 0.096 µm. in this experiment, indentation of 200 hv 1, 600 hv 10 and 900 hv 10 on hardness reference blocks were chosen. the typical diagonal lengths of indentations are 96 µm, 176 µm and 144 µm, respectively, which fit the field of view with the same 50× objective lens. those indentations were observed in five different conditions of optical system shown in table 1. to evaluate the repeatability of focusing, a displacement transducer, mitutoyo lgh-110, with 0.01 µm resolution was attached on the microscope column and the position of the microscope tube was recorded corresponding to every shot of image. 4. results figure 2 shows the difference of visibility of observed indentation with different na setting. with larger na, the image contrast is higher and the details of sample’s geometrical features can be seen clearer. on the other hand, with smaller na, the contrast is lower because the light intensity collected by the microscope is weaker. in addition, the inside of the indentation is blacked out and only the perimeter of indentation can be observed. the figure 1: infinity-corrected microscope with a replace able diaphragm sample surface objective lens diaphragm tube lens eyepiece camera table 1: experimental conditions pupil diameter, mm na resolving power δ, µm dof, µm maxium expansion ε, µm 3.6 3.0 2.5 1.9 1.3 0.50 0.42 0.35 0.26 0.18 0.7 0.8 1.0 1.3 1.9 0.4 0.5 0.6 0.8 1.1 0.6 0.7 0.8 1.1 1.6 figure 2: images of an indentation taken in dif ferent na setting (600 hv 10). na = 0.50 na = 0.42 na = 0.35 na = 0.26 na = 0.18 http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 263 most important fact is that the image got blurred when smaller na is chosen. it makes difficult to determine the exact position of the indentation corner and increases the uncertainty of diagonal length measurement. two diagonals were measured for an indentation, i. e., longitudinal length, d1 and lateral length, d2. conceptually, those two diagonal lengths should be same. however, in this experiment (and maybe in the most of other practical cases) they are not always identical. to eliminate the effect of measuring orientation and evaluate the uncertainty of measurement associated with the optical system only, whole set of data were analyzed by two-factor analysis of variance (anova) with replication. the results were shown in tables 2 to 4. they show that the effect of na is highly significant in all indentations. at the same time the effect of measuring orientation cannot be ignored for 600 hv 10 and 900 hv 10 indentations. therefore, following analysis are based on the measurement variation excluding the effect of measuring orientation. the standard deviation of error is 0.34 µm, 0.25 µm and 0.21 µm for 200 hv 1, 600 hv 10 and 900 hv 10 indentations, respectively. measured average diagonal lengths in different na are shown in figure 3. it indicates the diagonal length get longer as na decreases and this same trend is confirmed for all three indentations. it is not easy to explain the relationship between measured diagonal length and na, it seems that there is an effect of blurred images to overestimate the diagonal length. even though a theoretical model of this error has not been established, when a linear equation is fitted for the data, the sensitivity will be -0.13 to -0.20 µm per 0.1 unit of na. the repeatability of focusing is shown in figure 4. there is a clear trend that the repeatability gets better as na increases. it indicates that the position of the microscope tube distributed in the range almost same with expected depth of focus of the microscope. table 2: anova of the diagonal length measurement for 200 hv 1 indentation source s s d f m s fo na orientation error 2.795 0.115 4.996 4 1 44 0.70 0.11 0.11 6.2** 1.0 total 7.905 49 ** significant at 99 % confidence level table 3: anova of the diagonal length measurement for 600 hv 10 indentation source s s d f m s fo na orientation error 3.802 1.874 2.670 4 1 44 0.95 1.87 0.06 15.7** 30.9** total 8.347 49 ** significant at 99 % confidence level table 4: anova of the diagonal length measurement for 900 hv 10 indentation source s s d f m s fo na orientation error 1.465 0.294 2.014 4 1 44 0.37 0.29 0.05 8.0** 6.4* total 3.773 49 * significant at 95 % confidence level ** significant at 99 % confidence level 0.1 140.5 141.0 92.0 91.5 91.0 d ia g o n a l le n g th d , µ m 0.2 0.3 0.4 0.5 0.6 numerical aperture figure 3: diagonal length measured with dif ferent numerical aperture 173.0 173.5 174.0 174.5 200 hv 1 900 hv 10 600 hv 10 0.1 12 8 4 0 f o c u s re p e a ta b il it y , µ m 0.2 0.3 0.4 0.5 0.6 numerical aperture figure 4: repeatability of the microscope focussing 16 expected depth of focus 200 hv 1 600 hv 10 900 hv 10 range standard deviation http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 264 as described in the previous section, the variation of the distance between the sample and the objective lens causes the expansion of the image and it can affect the diagonal length measurement. figure 5 shows the repeatability of diagonal length readings (standard deviation and range) as well as the resolving power of microscope and the maximum expansion of image. the effects of the resolving power and the image expansion due to the uncertain focusing are similar and cannot be separated in this experiment. however, the variation of the diagonal length measurement seems to distribute within those two criteria. 5. discussions the iso 6507-2 describes the uncertainty estimation of the calibration results of hardness testing system. in the calibration of the diagonal measuring system, it requires to include the uncertainty due to the resolution of the measuring system. however, it is not clear what the resolution exactly means. in the experiment in this paper, the image resolution is 0.096 µm. the primary standard vickers hardness machine in our laboratory, mitutoyo sht-41, has a digital counter on its measuring system and it can detect the minimum of 0.006 µm of length with 40× objective. the results shown in this paper suggest the measurand distributes within the range of the resolving power of microscope which can be predicted through rayleigh’s criterion. as a result, the repeatability of measurement was 0.21 µm to 0.34 µm. it is much larger than the image resolution or the minimum count of the instrument. in addition, the result shows the diagonal length itself can vary with na of the objective lens. the na of microscope objective is typically ranging 0.2 to 0.9, according to its magnification and working distance. our result suggests the evaluated diagonal length can vary in the range of 0.9 µm to 1.4 µm, which is the amount of bias we cannot ignore. further consideration is expected in the guideline to estimate the uncertainty of measurement of a vickers hardness testing machine. 6. conclutions from a series of experiments of the diagonal length measurement of vickers indentation, the authors had following findings. the na of objective should have larger as possible. in general, it requires larger magnification. such objective brings higher resolving power and narrower dof. both are contributing to reduce the repeatability of readings. lower resolving power makes an image blurred and expanded, so that the measured diagonal length will be larger. the discussion in the paper is about the resolving power of optical system but not the image resolution and the former is much larger than the latter. it is recommended to consider the resolving power of the optical system is included in every uncertainty budget of vickers hardness standard. 7. references [1] iso 6507-2: 2018. [2] l. brice, f. davis, a. crawshaw, npl report cmam 87 (2003) [3] l. levi, applied optics, a guide to optical system design, vol. 1, john wiley & sons (1968) new york. (c) 900 hv 10 0.1 2.0 1.5 1.0 0.5 0 r e p e a ta b il it y , µ m 0.2 0.3 0.4 0.5 0.6 numerical aperture 2.5 range standard deviation resolving power maximum expansion (b) 600 hv 10 0.1 2.0 1.5 1.0 0.5 0 r e p e a ta b il it y , µ m 0.2 0.3 0.4 0.5 0.6 numerical aperture 2.5 range standard deviation resolving power maximum expansion (a) 200 hv 1 0.1 2.0 1.5 1.0 0.5 0 r e p e a ta b il it y , µ m 0.2 0.3 0.4 0.5 0.6 numerical aperture 2.5 range standard deviation resolving power maximum expansion figure 5: repeatability of diagonal length measurement in different nas http://www.imeko.org/ metrology for intraocular pressure measurements acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 353 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 353 356 metrology for intraocular pressure measurements d. pražák1, r. ziółkowski2, d. rosu3, m. schiebl4, j. rybář5, p. pavlásek5,6, e. sınır7, f. pluháček8 1 čmi, brno, czechia, dprazak@cmi.cz 2 gum, warszawa, poland, robert.ziolkowski@gum.gov.pl 3 ptb, berlin, germany, dana.rosu@ptb.de 4 ptp-bev, wien, austria, markus.schiebl@bev.gv.at 5 slovak university of technology, bratislava, slovakia, jan.rybar@stuba.sk 6 smú, bratislava, slovakia, pavlasek@smu.gov.sk 7 tübi̇tak ume, kocaeli, turkey, ekrem.sinir@tubitak.gov.tr 8 palacký university olomouc, olomouc, czechia, frantisek.pluhacek@upol.cz abstract: the world health organization reports glaucoma as the second leading cause of blindness and the leading cause of irreversible blindness. intraocular hypertension is the most relevant and only treatable risk factor for the disease. non-invasive intraocular-pressure (iop) measurements, carried out using eye tonometers, are used to screen for the intraocular hypertension. a correct measurement of the iop is essential in the prevention and the fight against glaucoma. the authors present their work towards ensuring on one side the accuracy of iop measurements for contact and non-contact tonometers in the central european region and obtaining a harmonization of the requirements for iop metrology at national level. keywords: eye tonometer; intraocular pressure; medical metrology; metrological traceability 1. introduction in addition to the european regulations for the medical devices with a measuring function entering the market, some european countries (e.g. czech republic and germany) have their own national regulations to ensure the proper performance of the eye-tonometers in operation. these are implemented through the periodic metrological checks performed by national metrology institutes (nmis) and private or governmental verification offices. however, most european nmis are not able to ensure a correct metrological traceability in this field. to overcome this, the project “developing research capabilities for traceable intraocular pressure measurements” (intense) was jointly initiated by the nmis of austria (ptp-bev), czech republic (čmi), germany (ptb), poland (gum), slovakia (smú) and turkey (tübi̇tak ume), together with palacký university in olomouc (upol) and slovak technical university in bratislava (stu) within the framework of the european metrology programme for innovation and research (empir). 2. project objectives 2.1. competence centre one of the project’s aims was to establish a competence centre for the iop metrology at the location of čmi that would not only benefit czech costumers, but other european nmis and stakeholders as well. this required the establishment of the necessary research capabilities for the iop measurements in the physiological and pathophysiological range of 10 mmhg – 80 mmhg (1.333 kpa – 10.67 kpa) at the čmi. 2.2. emerging techniques another objective of the project (and of the competence centre after the end of the project) was further development of these capabilities and keeping the pace with the technological progress in this field. the currently existing state-of-the-art in the iop metrology would be further advanced to emerging ophthalmological measurands (which can be additionally determined using the modern eye tonometers, such as e.g. corneal thickness) and to the iop measurements under non-standard conditions (e.g. incumbent patients). the external scientific expertise from university partners was crucial to achieve this goal, therefore the strong cooperation between nmis and universities initiated within the project will continue during the existence of the centre. 2.3. extending the centre on a mid to long term basis, the competence centre would make its expertise in the iop metrology accessible to a broad range of countries. this will be achieved by expanding the smart http://www.imeko.org/ mailto:dprazak@cmi.cz mailto:robert.ziolkowski@gum.gov.pl mailto:dana.rosu@ptb.de mailto:markus.schiebl@bev.gv.at mailto:jan.rybar@stuba.sk mailto:pavlasek@smu.gov.sk mailto:ekrem.sinir@tubitak.gov.tr mailto:frantisek.pluhacek@upol.cz acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 354 specialisation concept (ssc), geographically beyond central europe and thematically towards other relevant quantities in the area of the medical device metrology. 3. project results 3.1. competence centre at the beginning of the project, ptb was the only nmi able to provide the state-of-the-art in the field of iop metrology. this expertise was transferred during the project to čmi and represented a critical step in the impending establishment of the competent centre for iop metrology. all the planned advanced trainings of the čmi personnel by the experts of the ptb were successfully accomplished and the needed technical expertise was gained. the efficacy of these trainings was assessed and confirmed by two external experts in november 2018. in addition, a rigorous quality control system was established at cmi for all procedures related to the iop measurements. an external audit of the tonometry related sections of the quality system of cmi was successfully completed in march 2020. this ensures the qualification of the traceable measurement and research capabilities jointly developed at cmi for the iop measurements using common tonometer types in the physiological and pathophysiological range of 10 – 80 mmhg. consequently, the foundation of the competence centre in iop metrology was laid and clear steps in the establishment of this centre were taken. an important part was a bilateral comparison in the iop between čmi and stu. the laboratory standards involved were two clinically tested non-contact (airpuff) eye tonometers and both the set of the silicone eyes and an artificial model eye served as the transfer standard. the report of this successful comparison is available at the project’s website [1]. the centre also offered the first trainings to the project partners and stakeholders. the first of these trainings took place in the mid of march 2020, but the series was then interrupted due to outbreak of covid-19 epidemic. these trainings will be resumed after the travel restrictions will be lifted. the best practice guidelines developed during the project can also be found at [1]. moreover, the consortium prepared a draft of a new oiml international recommendation on “ophthalmic instruments – non-contact tonometers.” it is in the process of being approved and after that it will complement the existing oiml r 145 “ophthalmic instruments – impression and applanation tonometers.” 3.2. emerging techniques this part was started with a thorough literature search. firstly, the factors that significantly influence the iop measurements in the standard and non-standard conditions and for the most widely used eye-tonometers were identified. in general, it was found that the iop readings are mostly affected by corneal thickness and biometrical properties such as hysteresis and corneal rigidity. secondly, the effects of different parameters (including the nonstandard measurement conditions) on the iop measurements and uncertainties were summarized. especially the short-term physiological stress factors on the iop were studied. it was found that such factors can affect the iop values immediately before the measurement and significantly distort the results. both final reports concerning these findings are also available at [1]. clinical studies focused on the influence of hypoxia, head and body position on the iop values were performed and the results of these studies were published [2-3]. from a clinical view, the most important seems to be change of position and short-term physical activity, which induce immediate clinically significant (i.e. > 2 mmhg) iop changes. it also seems, that supine position corresponds with higher iop readings. other effects such as short-term hypoxia or maximal activity cause clinically insignificant mean changes of iop, although individual fluctuations may exceed the safe range and may be a risk factor especially for glaucoma patients. the incidence of discussed parameters is limited to a small time interval after their termination, usually within 20 min. based on these results, it can be recommended to keep at least 20 min rest before the iop measurement. the final reports [1] also present that coffee and energy drinks as well as water intake can change iop. thus, the measured subjects should avoid drinking of coffee or energy drinks in day of measurement or higher amount of water at least one hour before the measurement. a virtual digital model of the eye cornea was created at stu. then a real mechanical model (artificial eye) corresponding to the virtual model was constructed for the experimental verifications [4-9]. stu used this artificial eye as one of the transfer-standards in the mentioned above eyetonometry comparison. in a separate study, upol collected and evaluated the iop data using rebound and non-contact tonometers accompanied with the respective cornea properties data [10]. according to the previous literature review, the results indicate that the measurement of intraocular pressure using rebound and non-contact tonometers is influenced by corneal hysteresis and corneal thickness. this observation may also indirectly reflect the effect of corneal rigidity, which is correlated with the thickness. if some of these parameters are markedly out of the normal range, the iop reading can be distorted. moreover, corneal hysteresis significantly influences mutual differences between both http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 355 measurement techniques. thus, the measurement of corneal thickness, hysteresis or rigidity together with intraocular pressure could help to judge the clinical validity of iop readings. 3.3. extending the centre one important aspect for the establishment, functioning and broadening of the competence centre was the investigation and full understanding of the eu and the relevant national legislations [11]. the consortium evaluated the content of the medical devices regulation and discussed it during a workshop in berlin in october 2019 (some presentations are available at [12]). a workshop focused on the ssc was organised by gum in warszawa in august 2019 (some presentations are available at [1]). in addition, the consortium openly addressed the impact of the changes introduced by the new si to the medical and metrological communities [13]. all these findings and the feed-back obtained during the discussions at the mentioned above workshops were incorporated into “white paper on the smart specialisation concept developed to ensure a coordinated and optimised central european approach towards iop metrology” [1]. this document summarizes the principles of the iop metrology ssc and the commitments of the competence centre at čmi. from the beginning it was envisaged that the ssc would be extended in future geographically beyond the central europe and thematically beyond the iop metrology. the first step towards broadening the cooperation network established for the field of iop metrology within the ssc, was taken by adding blood pressure instruments to the priority list of the competence centre and geographically expanding it to other european countries (portugal, slovenia, ireland and bosnia and herzegovina) [14]. based on the all mentioned above experiences, “strategic plan for a paneuropean centre on medical device metrology was sketched and can be found at [1]. 4. summary and outlook the competence and training centre for iop metrology at čmi, established within the project, will continue providing services after the end of the project. these services will be permanently improved and upgraded according to the advancement of the technology and will ensure and maintain an up-to-date iop-metrology infrastructure for all interested european nmis and international stakeholders. the real mechanical eye model, prepared within the project, will be used for practical simulation of influence of different corneal factors to verify and specify their relationship with iop reading. based on the final reports summarising effects of corneal factors as well as physiological stress factors on iop, the new studies are planned. the obtained data and findings will also be published to share with professional public. 5. acknowledgement this work was funded through the european metrology programme of innovation and research (empir) project 16rpt03 intense. the empir initiative is cofounded by eu horizon 2020 programme and the empir participating states. 6. references [1] http://intense.cmi.cz [2] e. najmanová, f. pluháček, m. botek, j. krejčí, j. jarošová “intraocular pressure response to shortterm extreme normobaric hypoxia exposure”, frontiers in endocrinology, 2019. doi: https://doi.org/10.3389/fendo.2018.00785 [3] e. najmanová, f. pluháček, m. haklová “intraocular pressure response affected by changing of sitting and supine positions”, acta ophthalmologica, 2019. doi: https://doi.org/10.1111/aos.14267 [4] j. rybář, m. chytil, s. ďuriš, b. hučko, f. maukš, p. pavlásek, “use of suitable materials such as artificial cornea on eye model for calibration of optical tonometers”, aip conference proceedings, vol. 2029, 020067, 2018. doi: https://doi.org/10.1063/1.5066529 [5] p. pavlásek, et al. “development of new calibration standard for noncontact tonometers”, in proc. of 11th cisp-bmei 2018 beijing. doi: https://doi.org/10.1109/cisp-bmei.2018.8633148 [6] b. hučko, s. l. ferková, s. ďuriš, j. rybář, p. pavlásek, “glaucoma vs. biomechanical properties of cornea”, strojnícky časopis – journal of mechanical engineering, vol. 69, no. 2, pp. 111116, 2019. doi: https://doi.org/10.2478/scjme-20190021 [7] p. pavlásek, j. rybář, m. chytil, s. ďuriš, j. palenčár, b. hučko, “european project on developing research capabilities for traceable intraocular pressure measurements”, zdravotníctvo a sociálna práca – health and social work, vol. 14, no. 1, pp. 30-36, 2019. http://zdravotnictvoasocialnapraca.sk/wpcontent/uploads/zasp_cislo_1_2019_sk.pdf [8] p. pavlásek, et al. “developments in non-contact eye tonometer calibration”, in proc. of ieee i2mtc 2019 auckland. doi: https://doi.org/10.1109/i2mtc.2019.8827028 [9] b. hučko, l. kučera, s. ďuriš, p. pavlásek, j. rybář, j. hodál, “modelling of cornea applanation when measuring eye pressure”, lecture notes in mechanical engineering, pp. 287-294, 2020. doi: https://doi.org/10.1007/978-3-030-33146-7_33 [10] f. pluháček, a. unzeitigová, k. marešová, j. rybář, “the influence of the cornea on intraocular pressure measurement by icare pro and ora”, czech and slovak ophtalmology, vol. 75, no. 3, http://www.imeko.org/ http://intense.cmi.cz/ https://doi.org/10.3389/fendo.2018.00785 https://doi.org/10.1111/aos.14267 https://doi.org/10.1063/1.5066529 https://doi.org/10.1109/cisp-bmei.2018.8633148 https://doi.org/10.2478/scjme-20190021 http://zdravotnictvoasocialnapraca.sk/wp-content/uploads/zasp_cislo_1_2019_sk.pdf http://zdravotnictvoasocialnapraca.sk/wp-content/uploads/zasp_cislo_1_2019_sk.pdf https://doi.org/10.1109/i2mtc.2019.8827028 https://doi.org/10.1007/978-3-030-33146-7_33 acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 356 pp. 111-118, 2019. doi: https://doi.org/10.31348/2019/3/1 [11] e. sınır, et al. “towards the harmonization of medical metrology traceability in europe: an impact case study through activities in turkey and empir project intense”, in proc. of ieee memea 2019 conference istanbul. doi: https://doi.org/10.1109/memea.2019.8802141 [12] https://medmet2019.ptb.de [13] d. pražák, v. sedlák, e. sınır, f. pluháček “changing the status of mmhg”, accreditation and quality assurance, 2019. doi: https://doi.org/10.1007/s00769-019-01414-7 [14] https://www.adossig-empir.eu http://www.imeko.org/ https://doi.org/10.31348/2019/3/1 https://doi.org/10.1109/memea.2019.8802141 https://medmet2019.ptb.de/ https://doi.org/10.1007/s00769-019-01414-7 https://www.adossig-empir.eu/ characteristics improvement of pressure transfer standard using a silicon resonant sensor acta imeko issn: 2221-870x march 2021, volume 10, number 1, 284 289 acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 284 characteristics improvement of pressure transfer standard using a silicon resonant sensor hideaki yamashita1, hirokazu nagashima1, hideki yamada1 1 yokogawa test & measurement corporation, 4-9-8 myojin-cho, hachioji-shi, tokyo 192-8566, japan section: research paper keywords: transfer standard; international comparison; digital manometer; silicon resonant sensor; resonant silicon gauge citation: hideaki yamashita, hirokazu nagashima, hideki yamada, characteristics improvement of pressure transfer standard using a silicon resonant sensor, acta imeko, vol. 10, no. 1, article 38, march 2021, identifier: imeko-acta-10 (2021)-01-38 editor: momoko kojima, nmij, japan received may 15, 2020; in final form july 31, 2020; published march 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: hideaki yamashita, e-mail: h.yamashita@yokogawa.com 1. introduction the conclusion of the mutual recognition arrangement of the comité international des poids et mesures (cipm-mra) has promoted barrier-free trade in the globalised economy. the cipm-mra provides for the mutual recognition of the equivalence of national standards, with the equivalence ensured via international comparisons among the national metrology institutes (nmis) [1], [2]. the international comparison is commonly carried out using a transfer standard, which is circulated to the nmis in each country, with each nmi calibrating the transfer standard in accordance with their own national standards. the calibration results are then compared among the nmis participating in the international comparison and the equivalence of the national standards is confirmed. in the international comparison of pressure measurement, a pressure balance is mainly used as the transfer standard [3]. however, a pressure balance is large, heavy, and extremely precise, meaning it is not easy to transfer the instrument as the transfer standard from one country to another. as such, the nmis, including the national metrology institute of japan (nmij) and the advanced industrial science and technology (aist) institute, took notice of a digital manometer that is smaller, lighter, more portable, and easier to handle than the pressure balance. since the international comparison usually takes a year or longer, the transfer standard must demonstrate high stability. while the digital manometer provides less stability than the pressure balance, the nmis have established a method to compensate for this shortfall. here, the reliability of the transfer standard is ensured via redundancy using multiple digital manometers. the long-term stability of the digital manometer is compensated by correcting the calibration values using the results of long-term stability evaluations [3]. these methods have made it possible to use the digital manometer for the transfer standard, and a number of international comparisons using these transfer standards have already been conducted. the digital manometers and pressure sensors developed by the japanese electric company, yokogawa, have been used for such transfer standards for international comparisons [4]-[6]. a pressure sensor is a digital manometer without a display and without operation keys. yokogawa provides nmis with various abstract in the field of pressure measurement, numerous interlaboratory comparisons are carried out among national metrology institutes (nmis) using a pressure transfer standard to verify the degrees of equivalence. here, the yokogawa electric corporation has been producing a series of digital manometers using a silicon resonant sensor developed independently. this sensor demonstrates excellent long-term stability and has thus been adopted as the pressure transfer standard by many nmis and has been subsequently well received. the pressure transfer standard is known as the resonant silicon gauge (rsg) among nmis. from december 2016, the national metrology institute of japan (nmij), the advanced industrial science and technology (aist) institute, and yokogawa initiated a collaborative research with the aim of improving the characteristics of the rsgs and developing a portable transfer standard using a new silicon resonant sensor. the new rsg was adjusted using a standard device calibrated by either nmij or yokogawa. the measurement values of the standard device were corrected with the calibration results and used as the standard values for adjustment of the new rsg. the linearity of the new rsg adjusted via the proposed method was improved compared with that of a conventional rsg. mailto:h.yamashita@yokogawa.com acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 285 pressure sensors, including 1 kpa differential pressure, 10 kpa differential pressure and 100 kpa absolute pressure models under the model name 265381/z. meanwhile, the 265381/z is equipped with yokogawa's silicon resonant sensor [7]-[9], with this type of manometer known as a resonant silicon gauge (rsg) among the nmis [10]. since the nmis requested that the supply of the rsg be continued, the development of a new rsg has been initiated. the sensor structure of the new rsg is different from that of the conventional rsg, which requires the evaluation of basic performance (linearity, hysteresis, and repeatability). as such, yokogawa initiated a collaborative research in conjunction with nmij/aist, which involves national standards and precise evaluation techniques, to improve the basic performance and characteristics of the new rsg. 2. silicon resonant sensor yokogawa’s digital manometers are equipped with the silicon resonant sensor developed by the company itself [7]-[9], which is regarded as an rsg. this section provides an overview of this silicon resonant sensor. the diaphragm, resonator and vacuum chamber covering the resonator, which constitute the silicon resonant sensor, are all formed from single crystal silicon using semiconductor processing technology. two resonators are placed at positions where tension and compressive forces are generated on the diaphragm. the resonators are then excited by a magnetic circuit, while since they are located in the vacuum chamber, any vibration attenuation is suppressed, and a high q factor can be obtained. when the silicon diaphragm is deformed by the application of pressure, the resonators are distorted and their resonant frequencies change [7]. the frequencies of the two resonators are then detected and the difference is converted into a pressure value. in addition, the rsg output value is corrected using correction factors calculated from the relationship between the applied pressure and the frequencies of the resonators. the silicon resonant sensor has excellent reproducibility and exhibits only slight secular change since its pressure detection part is formed of single crystal silicon [7], which is an elastic material. this characteristic contributes to the achievement of high stability. figure 1 shows the calibration results of a 10-kpa differential pressure rsg, which was equipped with yokogawa’s silicon resonant sensor (10-kpa rsg), in relation to the company’s double pressure balances. the range of calibration values over a two-year period was within 0.06 pa, while the long-term stability required for a transfer standard demonstrated good results. however, inflection points were observed on the calibration curves [11]. the yokogawa 10-kpa rsg, which has been used as a transfer standard, has the same tendency. the stability and hysteresis, which are crucial elements of the basic performance, are determined by the structure of the sensor in terms of, for example, the detection part of the silicon resonant sensor and the diaphragm of the pressure detector section. meanwhile, the linearity can be improved using the adjustment method. with all this in mind, we developed a new rsg and worked on the confirmation of the basic performance and the improvement in linearity via adjustment. 3. collaborative research with nmij/aist the aim is to develop an rsg that can be used for a transfer standard for international comparison, one that can be continuously supplied. in order to confirm the basic performance of the rsg, development using national standards is necessary. in 2016, yokogawa initiated a collaborative research alongside with the nmij/aist, which has enabled an evaluation based on national standards. an rsg corresponding to national standards or an rsg directly calibrated by these standards was used as the standard device for adjustment. the rsg measured value was corrected via a calibration value and was subsequently used as a standard value. repeated simulations were then conducted using the standard value and the frequency data of the resonators and attempted to optimise the pressure points used for the adjustment. the effect of the adjustment was confirmed by directly comparing the adjusted rsg with the national standards. the development was carried out according to the following procedure: 1) yokogawa produce an rsg (referred to as rsg #a). 2) calibrate the rsg #a (the 10 kpa by yokogawa, the others by nmij/aist). 3) yokogawa feed back the result of 2), correct the output of rsg #a and use it as the standard device to newly adjust another rsg (referred to as rsg #b). 4) nmij/aist evaluate the rsg #b adjusted in 3) to confirm the effect of the adjustment. meanwhile, the following outlines yokogawa’s procedure for adjusting the rsgs: 1) place the rsg to be adjusted in a temperature chamber and the standard device outside the temperature chamber. 2) apply pressure from a pressure controller to the rsg and the standard device. 3) obtain the frequencies of the resonators of the rsg and the pressure measurements of the standard device by changing the temperature and pressure. 4) the coefficients of the approximation formula calculated from the acquired data are written to the internal memory of the rsg as a correction factor. in this research, multiple rsgs were adjusted at the same time. this is the same method as that used in the manufacturing process for yokogawa’s digital manometers. being able to use the existing method to adjust rsgs without requiring a special procedure and without the need for a large amount of time or expenditure is of great benefit to the continuous provision of rsgs. figure 2 shows the developed prototype of the 10-kpa rsg. while it has input ports on the front, it features no display or operation keys. the acquisition of the pressure values and of various settings is performed through usb communication with figure 1. calibration results for the 10-kpa rsg #a from 2014 to 2016. -1,0 -0,8 -0,6 -0,4 -0,2 0,0 100 1000 10000 (r s g s ta n d a rd ) /p a pressure /pa oct-14 apr-15 oct-15 apr-16 nov-16 20 ppm acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 286 a pc, with a usb port provided on the rear. the external appearance is the same regardless of the pressure range. in this collaborative research, we developed rsgs characterised by a 10 kpa differential pressure, 200 kpa gauge pressure, 1,000 kpa gauge pressure, 3,500 kpa gauge pressure and 130 kpa absolute pressure. the nmij/aist evaluated certain characteristics of the rsgs, including the linearity and stability. the following sections introduce the characteristics of the rsgs as obtained via the evaluations. 4. resonant silicon gauge characteristics 4.1. calibration results and linearity of the 10-kpa rsg the 10-kpa rsg #b was adjusted using the 10-kpa rsg #a shown in figure 1 as the standard device. for the evaluation, calibration was performed via direct comparison with the generated differential pressure using double pressure balances. ten points of measurement pressure were selected in the range of 1 pa–10 kpa, while the line pressure was 100 kpa. the 'ababa’ method [12], [13] was used for the differential pressure generation sequencing, with the measurement performed during the pressure ascending processes. the pressure was ascended five times and measurement was carried out once for each. the average of the measured values was then used as the calibration pressure value. figure 3 shows the calibration results of two units of rsg #b. the calibration data shown in the figures hereafter – with the exception of those shown in figure 8 – were provided by nmij/aist. the two units were calibrated at the same time and the maximum deviation in the nmij/aist pressure standards was 0.07 pa. the linearity of the rsg #b was greatly improved in comparison to the rsg #a, with the maximum difference between the two units 0.03 pa. it was confirmed that this difference could be obtained by simultaneously adjusting multiple rsgs in combination with the pressure controller and standard device. in fact, it is possible to supply rsgs with small variations in characteristics when multiple rsgs are required to improve the reliability of the transfer standard. the rsg #b was adjusted using the rsg #a calibrated by yokogawa as the standard device. while the rsg #a correlated with the national standards, it lay deep in the attendant hierarchy. therefore, the rsg #c was adjusted using the rsg #b calibrated in terms of the national standards as the standard device. the calibration results of the rsgs #b and #c obtained using the nmij/aist pressure standards were then compared with the regression lines, and linearity comparisons were made, with the results shown in figure 4. the rsg #c exhibited further improved linearity compared to the rsg #b. 4.2. shortand long-term stability of the 10-kpa rsg figure 5 shows each of the measurement results for the five pressure ascending processes, as described in subsection 4.1., which confirmed the short-term stability of the 10-kpa rsg during the calibration process in terms of the repeatability. even in the measurement results at 10 kpa, which had the largest variation, the standard deviation was only 0.028 pa, which was 3 ppm or less with respect to the full scale. figure 6 shows the results of two calibrations performed 10 months apart to confirm the long-term stability, with the calibration results aligning within 0.04 pa. it was confirmed that the stability was excellent, both in the short-term and in the longterm, through the evaluation performed by nmij/aist, where the procedure was highly controlled. figure 2. photograph of the prototype 10-kpa rsg. figure 3. calibration results for the 10-kpa rsgs #b1 and #b2. the error bars refer to expanded (k = 2) uncertainties. figure 4. linearity deviations from a regression line of the 10-kpa rsgs #b and #c. the rsg #c was adjusted using the rsg #b as the standard. figure 5. short-term stability of the 10-kpa rsg #b. -0,15 -0,10 -0,05 0,00 0,05 0,10 0,15 0,20 0,25 1 10 100 1000 10000 (r s g s ta n d a rd ) /p a pressure /pa rsg #b1 rsg #b2 5 ppm -0,08 -0,06 -0,04 -0,02 0 0,02 0,04 0,06 0,08 100 1000 10000 li n e a ri ty d e v ia ti o n fr o m r e g re ss io n l in e / p a pressure /pa rsg #b rsg #c 2 ppm -0,04 -0,02 0,00 0,02 0,04 0,06 0,08 0,10 0,12 1 10 100 1000 10000 (r s g s ta n d a rd ) /p a pressure /pa 1cy. 2cy. 3cy. 4cy. 5cy. 2 ppm 115 mm acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 287 4.3. influence of line pressure and temperature of the 10-kpa rsg while the line pressure of a differential pressure gauge at the time of calibration is usually 100 kpa, the gauge may be used with the line pressure set to vacuum in the international comparison of absolute pressure ranges [5], [10]. in this case, as the line pressure differed from that at the time of calibration by 100 kpa, it was necessary to check the influence of any changes in line pressure in advance [11], [13]. the rsg #b was calibrated with line pressures of 75, 100 and 125 kpa, and the influence per line pressure of 100 kpa was estimated from the calibration results. figure 7 shows the change in calibration results when the line pressure was altered by 100 kpa. a change in the span was observed, with the amount of change in the calibration results 0.6 pa at 10 kpa. the rsg #b was adjusted such that the zero point was not affected by the line pressure, while the span was not adjusted. there is the possibility that the line pressure dependence of the span can be corrected by changing the line pressure and acquiring specific span data, and then including it in the adjustment parameters. furthermore, since the characteristics are linear, it may also be possible to make corrections using regression lines. these approaches should be examined in future research. there exists a transfer standard using a temperaturecontrolled enclosure to reduce the influence of the ambient temperature [5], [10]. if the temperature characteristics of an rsg that is incorporated in the transfer standard are improved, the influence of the temperature can be minimised with the use of enclosure, while there is also the possibility that the enclosure can be omitted. the correction factor of the rsg #b was calculated using its adjustment parameters that were obtained in the range of −10 °c–50 °c. figure 8 shows the results of the measurement of the inputoutput characteristics at certain temperatures carried out by yokogawa, with deviations from 23 °c. in the range of −10 °c– 50 °c, the maximum temperature coefficient was 0.03 pa/°c. however, while the adjustment range was set to −10 °c–50 °c in this research, there is no such exact temperature range in the international comparison environment [5]. it is believed to be possible to reduce the influence of the temperature by optimising the adjustment temperature range. 4.4. characteristics of other pressure range rsgs the international comparison of pressure measurement is performed over a wide pressure range [3], [14]. this section reports the characteristics of the 130-kpa absolute pressure rsg (130-kpa abs rsg) and the 3,500-kpa gauge pressure rsg figure 6. long-term stability of the 10-kpa rsg #b. figure 7. influence of the line pressure on the calibration results for the 10kpa rsg #b. change in calibration results per 100 kpa of line pressure. figure 8. influence of temperature variation on output of the 10-kpa rsg #b. (a) (b) figure 9. calibration results for the 130-kpa abs rsg (a) #a and (b) #b. the error bars refer to expanded (k = 2) uncertainties. -0,06 -0,04 -0,02 0,00 0,02 0,04 0,06 0,08 0,10 1 10 100 1000 10000 (r s g s ta n d a rd ) /p a pressure /pa mar-17 jan-18 2 ppm -0,1 0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0 2 4 6 8 10 c h a n g e i n c a li b ra ti o n r e su lt /( p a /1 0 0 k p a ) pressure /kpa 10 ppm -0,8 -0,6 -0,4 -0,2 0,0 0,2 0,4 0,6 0,8 0 2 4 6 8 10 d e v ia ti o n f ro m 2 3 ° c / p a pressure /kpa -10 °c 6 °c 23 °c 37 °c 50 °c 20 ppm -15 -10 -5 0 5 0 20 40 60 80 100 120 140 (r s g s ta n d a rd ) /p a pressure /kpa 38.5 ppm -10 -5 0 5 10 0 20 40 60 80 100 120 140 (r s g s ta n d a rd ) /p a pressure /kpa 38.5 ppm calib. curve descend. ascend. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 288 (3,500-kpa rsg), which were developed in view of the expansion of rsgs into ranges other than 10 kpa. an adjustment was made using a pressure controller and a standard device following the procedure used for the 10-kpa rsg, with the rsg #a calibrated by nmij/aist used as the standard device. for the evaluation of the adjusted rsg #b, calibration was performed by direct comparison with the nmij/aist pressure balance. the measurements were carried out in each of three calibration cycles with the pressure both ascending and descending. the average of the measured values was then used as the calibration pressure value. figure 9 and figure 10 show the calibration results for the standard device rsg #a and the adjusted device rsg #b. here, the deviation from the national standards could be maintained at within 10 ppm of the full scale. specifically, a large effect was observed in improving the linearity of the 130-kpa abs rsg. this indicates that the method using a standard device calibrated by the national standards for adjustment is effective for rsgs of various pressure ranges. figure 11 shows the input-output characteristics in three cycles for the 130-kpa abs rsg #b and the 3,500-kpa rsg #b. here, the short-term stability was confirmed. the standard deviations of the three measurements both the #b rsgs were 0.5 pa and 4.0 pa, respectively, and the relative value of standard deviation with respect to the full scale was equivalent to that of the 10-kpa rsg. it was also confirmed that the reproducibility of repeated measurements was excellent. the adjustment method also proved to be effective in all the rsgs, including the 10-kpa rsg. therefore, the method was applied to the production of standard devices for the new digital manometer, which was labelled mt300 and features an array of ranges [15]. 5. conclusions through the collaborative research carried out in conjunction with nmij/aist, we attempted to improve the characteristics of the rsgs used as the transfer standards for international comparison. the developed rsgs were evaluated by nmij/aist through direct comparison with the national standards. the method through which the standard device was corrected via the calibration value and used for adjustment purposes was adopted to improve the linearity. as described in subsection 4.1., for the 10-kpa rsg, the linearity with respect to the national standards was greatly improved and adjustments with a slight difference between the two units were possible. meanwhile, in subsection 4.2, the unit’s excellent stability was confirmed in both the short-term and long-term. the performance of the transfer standard is expected to be improved by replacing the conventional rsg with the new rsg. furthermore, the line pressure and the temperature dependence described in subsection 4.3 can be expected to be improved by optimising the adjustment conditions, an approach that should be examined and evaluated in future research. as described in subsection 4.4., the adjustment method proved to be effective through the evaluation of the 10-kpa rsg as applied to the rsgs of other ranges. a deviation of within 10 ppm of the full scale pertaining to the national standards was achieved through making adjustments using the standard device directly calibrated in terms of the national standards. we believe it is possible to use rsgs in international comparisons covering a wide pressure range. the transfer standard using a digital manometer has improved the efficiency of international comparisons related to pressure measurement. the use of digital manometers in the area of international comparisons is expected to increase in the future. as such, digital manometers that are compact and lightweight and which have excellent characteristics will be indispensable, and yokogawa has the accumulated rsg technology to achieve this. we will continue to contribute to the advancement of pressure measurement in both japan and overseas through developments that include the characteristics improvement and range expansion of rsgs. figure 10. calibration results for the 3,500-kpa rsgs #a and #b. the error bars refer to expanded (k = 2) uncertainties. (a) (b) figure 11. short-term stability of (a) the 130-kpa abs rsg #b and (b) the 3,500-kpa rsg #b. -150 -100 -50 0 50 100 0 500 1000 1500 2000 2500 3000 3500 (r s g s ta n d a rd ) /p a pressure /kpa 14.3 ppm -2,0 -1,5 -1,0 -0,5 0,0 0,5 1,0 1,5 2,0 0 20 40 60 80 100 120 140 (r s g s ta n d a rd ) /p a pressure /kpa 3.8 ppm ascending descending -40 -30 -20 -10 0 10 0 500 1000 1500 2000 2500 3000 3500 (r s g s ta n d a rd ) /p a pressure /kpa 2.9 ppm ascending descending ascend. descend. calib. curve 1 cycle 2 cycle 3 cycle rsg #b rsg #a acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 289 acknowledgements the authors would like to thank the members of the nmij/aist pressure and vacuum standards group for arranging the evaluation, compiling the results, and providing helpful advice in the collaborative research. references [1] national institute of advanced industrial science and technology, center for quality management of metrology, international cooperation office website. online [accessed 13 may 2020] https://unit.aist.go.jp/qualmanmet/nmijico/en/comp/ [2] ministry of economy, trade and industry. online (in japanese) [accessed 13 may 2020] https://www.meti.go.jp/shingikai/keiryogyoseishin/pdf/g50913 a46j.pdf [3] t. kobata, m. kojima, h. kajikawa, improvement of reliability in pressure measurements and international mutual recognition incorporation of industrial digital pressure gauges to the national metrology system, synthesiology english edition 4 (2012), pp. 212-226. online [accessed 29 march 2021] https://www.aist.go.jp/pdf/aist_e/synthesiology_e/vol4_no4/v ol04_04_p212_p226.pdf [4] t. kobata, m. kojima, k. saitou, m. fitzgerald, d. jack, c. sutton, final report on key comparison apmp.m.p-k5 in differential pressure from 1 pa to 5000 pa, metrologia 44 tech. suppl. 07001 (2007), 52 pp. online [accessed 23 march 2021] iopscience.iop.org/article/10.1088/0026-1394/44/1a/07001 [5] j. ricker, j. hendricks, t. bock, p. dominik, t. kobata, j. torres, i. sadkovskaya, final report on the key comparison ccm.pk4.2012 in absolute pressure from 1 pa to 10 kpa, metrologia 54 tech. suppl. 07002 (2017), 37 pp. online [accessed 23 march 2021] iopscience.iop.org/article/10.1088/0026-1394/54/1a/07002 [6] j. hendricks, j. ricker, d. olson, protocol ccm international key comparison in absolute pressure (1 pa to 10 kpa) ccm.pk4.2012, the bipm key comparison database. online [accessed 13 may 2020] https://www.bipm.org/kcdb/comparison/doc/download/1194 /ccm.p-k4.2012_technical_protocol.pdf [7] k. ikeda, h. kuwayama, t. kobayashi, t. watanabe, t. yoshida, t. nishikawa, k. harada, silicon pressure sensor integrates resonant strain gauge on diaphragm, sensors and actuators a: physical 21 (1990), pp. 146-150. doi: 10.1016/0924-4247(90)85028-3 [8] t. saigusa, s. gotoh, h. kuwayama, m. yamagata, dpharp series electronic differential pressure transmitters, yokogawa technical report english edition 15 (1992), pp. 30-37. [9] t. ishikawa, t. odohira, m. nikkuni, e. koyama, t. tsumagari, r. asada, new dpharp ejx series pressure and differential pressure transmitters, yokogawa technical report english edition 37 (2004), pp. 9-14. online [accessed 29 march 2021] https://www.yokogawa.com/about/researchdevelopment/rd_te_report/rd_tr_report_a00037/ [10] j. hendricks, a. miiller, development of a new high-stability transfer standard based on resonant silicon gauges for the range 100 pa to 130 kpa, metrologia 44 (2007), pp. 171-176. doi: 10.1088/0026-1394/44/3/002 [11] m. kojima, t. kobata, characterisation of differential pressure gauges focusing on calibration curve and line pressure dependency, proc. of the 29th sensing forum (2012), pp. 9-13. (in japanese). [12] m. kojima, t. kobata, k. saitou, m. hirata, development of small differential pressure standard using double pressure balances, metrologia 42 (2005), pp. 227-230. doi: 10.1088/0026-1394/42/6/s18 [13] m. kojima, k. saitou, t. kobata, study on calibration procedure for differential pressure transducers, proc. of the imeko 20th tc3, 3rd tc16 and 1st tc22 international conference cultivating metrological knowledge, merida, mexico, 27 – 30 november 2007. online [accessed 13 may 2020] https://www.imeko.org/publications/tc16-2007/imekotc16-2007-044u.pdf [14] m. kojima, t. kobata, calibration system and traceability of low pressure and vacuum standards in japan, journal of the vacuum society of japan 59 (2016), pp. 352-359. (in japanese). [15] yokogawa test & measurement corporation, brochure for the mt300 digital manometer, 2020. online [accessed 23 march 2021] https://tmi.yokogawa.com/solutions/products/other-testmeasurement-instruments/manometers/mt300-digitalmanometer/ https://unit.aist.go.jp/qualmanmet/nmijico/en/comp/ https://www.meti.go.jp/shingikai/keiryogyoseishin/pdf/g50913a46j.pdf https://www.meti.go.jp/shingikai/keiryogyoseishin/pdf/g50913a46j.pdf https://www.aist.go.jp/pdf/aist_e/synthesiology_e/vol4_no4/vol04_04_p212_p226.pdf https://www.aist.go.jp/pdf/aist_e/synthesiology_e/vol4_no4/vol04_04_p212_p226.pdf https://iopscience.iop.org/article/10.1088/0026-1394/44/1a/07001 https://iopscience.iop.org/article/10.1088/0026-1394/54/1a/07002 https://www.bipm.org/kcdb/comparison/doc/download/1194/ccm.p-k4.2012_technical_protocol.pdf https://www.bipm.org/kcdb/comparison/doc/download/1194/ccm.p-k4.2012_technical_protocol.pdf https://doi.org/10.1016/0924-4247(90)85028-3 https://www.yokogawa.com/about/research-development/rd_te_report/rd_tr_report_a00037/ https://www.yokogawa.com/about/research-development/rd_te_report/rd_tr_report_a00037/ https://doi.org/10.1088/0026-1394/44/3/002 https://doi.org/10.1088/0026-1394/42/6/s18 https://www.imeko.org/publications/tc16-2007/imeko-tc16-2007-044u.pdf https://www.imeko.org/publications/tc16-2007/imeko-tc16-2007-044u.pdf https://tmi.yokogawa.com/solutions/products/other-test-measurement-instruments/manometers/mt300-digital-manometer/ https://tmi.yokogawa.com/solutions/products/other-test-measurement-instruments/manometers/mt300-digital-manometer/ https://tmi.yokogawa.com/solutions/products/other-test-measurement-instruments/manometers/mt300-digital-manometer/ sensor-based mobile robot for harsh environments: functionalities, energy consumption analysis and characterisation acta imeko issn: 2221-870x june 2021, volume 10, number 2, 209 219 acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 209 sensor-based mobile robot for harsh environments: functionalities, energy consumption analysis and characterisation roberto de fazio1, dany mpoi katamba2, aimè lay ekuakille1, miguel joseph ferreira2, simon kidiamboko2, nicola ivan giannoccaro1, ramiro velazquez3, paolo visconti1 1 department of innovation engineering, university of salento, lecce 73100, italy 2 department of electronics, institut supérieur de techniques appliquées ista university, kinshasa, dr congo 3 faculty of engineering, universidad panamericana, aguascalientes 20290, mexico section: research paper keywords: mobile robots; photovoltaic system; electronic control; energy consumption measurements; sensors; instrumentation; autonomous vehicles citation: roberto de fazio, dany mpoi katamba, aimè lay ekuakille, miguel joseph ferreira, simon kidiamboko, nicola ivan giannoccaro, ramiro velazquez, paolo visconti, sensor-based mobile robot for harsh environments: functionalities, energy consumption analysis and characterisation, acta imeko, vol. 10, no. 2, article 29, june 2021, identifier: imeko-acta-10 (2021)-02-29 section editor: francesco lamonaca, university of calabria, italy received august 6, 2020; in final form march 17, 2021; published june 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: paolo visconti, e-mail: paolo.visconti@unisalento.it 1. introduction the safety of human beings working in harsh environments is a theme of major concern for workers, entrepreneurs and policymakers. ‘harsh environment’ here signifies a hostile environment for human beings, whether indoors or outdoors. interest in autonomous mobile robots is increasing day by day; a brief overview of such applications makes clear their paramount importance. in hazardous industries, like those involving chemicals, oil and steel, many risks threaten human safety and the quality of goods. in the cracking processes used in the chemical and petrochemical industries, for instance, individual and group protections are necessary to lower exposure to gaseous and odorous components containing volatile organic compounds [1], [2]. in oil production sites, such as refineries, liquid, solid and gaseous materials present high risks for persons and goods. in the latter case, flammable materials can start fires, so protection devices are needed to prevent combustion. the steel industry also faces the above risks and an increased probability of accidents due to a direct and indirect combination of diverse materials and dangerous situations. protective masks with special filters are used to address some of these hazards [3]. in outdoor sites, robotic vehicles are of interest for safety, as illustrated above, but also for security, as, for example, in cases where there is a risk of explosions. security forces use mobile robots to monitor and penetrate hostile areas instead of employing humans [4]. in hospital pavilions and wards, levels of exposure are controlled to protect patients and healthcare abstract mobile robots and rovers play an important role in many industrial applications. under certain constraints, they are suitable in harsh environments and conditions in which protracted human activity is not safe or permitted. in many circumstances, mechanical aspects and electrical consumption need to be optimized for autonomous and wheeled mobile robots. the paper illustrates the design of a semicustom wheeled mobile robot with high-efficiency monoor polycrystalline photovoltaic panel on the roof that supports the lithium ion batteries during particular tasks (e.g. navigating rough terrain, obstacles or steep paths) to extend the robot’s autonomy. an electronic controller was designed, and data acquisition related to power consumption performed using a specific experimental setup. the robot can detect parameters such as temperature, humidity, concentrations of toxic gas species and the presence of flames, making it particularly suitable for contaminated environments or industrial plants. for this aim, the mobile robot was equipped with a wide range of commercial sensors and a global positioning system receiver to track its position. in addition, using an hc-06 bluetooth transceiver, the robot receives commands and instructions, and sends the acquired data to the developed iotool smartphone application, where they are displayed to be analysed by user. mailto:paolo.visconti@unisalento.it acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 210 workers from risks, such as covid-19 infection [5] or toxic gas/water leaks [6]. in this context, mobile robots designed for the automotive sector can be used to perform inspections and monitoring and to work as remote actuators [7]. for autonomous mobile robots, energy consumption is a truly basic issue; an analysis and design related to this aspect is also illustrated in the manuscript. photovoltaic (pv) systems are becoming an important support in many applications; for instance, in agriculture they are used to monitor environment and soil parameters [8], predict floods [9] and optimise irrigation cycles and crop growth [10], [11]. moreover, pv systems are useful for robotic vehicle applications. some examples of mobile robots powered by solar energy are present in the scientific literature: [12] presented a light (500 g, including the controller) robotic system made of lego bricks and powered by 32 small 0.08 w solar panels that carried an additional weight of only 180 g. in [13], a vehicle powered by solar energy that can be utilised to monitor the status of unstable transmission control protocol/internet protocol networks is presented with the aim of reducing downtime for the network itself. the robot is able to follow a path defined by a track line using infrared (ir) sensors and is powered by solar energy; its energy autonomy and utilisation in terms of network management are discussed. in [14], the design of another solar-powered robotic vehicle is proposed, including a solar tracking system to increase the robot’s energy autonomy together with two batteries, one directly recharged by the pv panel and one that provides energy to the robotic vehicle during the charging phase of the first battery or in case of the total absence of solar radiation. the switching system between the two batteries consists of two selectors driven by the microcontroller using a pulse-width modulation (pwm) signal. the latest generations of robotic vehicles used for space exploration make use of solar power systems: energy for the rover sojourner was generated via a small pv panel. rechargeable batteries were used for the first time in a space mission in the mars exploration rovers, where the need for greater operational autonomy in the opportunity and spirit rovers was solved by using very large solar panels. aspects that could be improved for future generations of exploration vehicles include the efficiency of the pv materials used to produce cells and the possibility of tracking maximum light intensity. for example, [15] proposed the design of a mechanism capable of tracking maximum light intensity independently from rover mobility. the mechanism was applied to a commercial robotic platform (vanter) provided with a pv panel and a pack of two lithium polymer (lipo) batteries continuously recharged by the pv system. an interesting review paper discussed the relationship between solar cells and robotics by analysing existing literature related to the solar cells applied to different robotic applications [16]. the authors showed that improvements in the two fields (i.e. solar cell production and robotics) contribute to better living and a cleaner environment through the renewable energy provided by solar cells and the automatic work provided by robotics. a recent quantitative analysis of the power contribution of solar cells in a mobile robot is reported in [17], which analysed the behaviour of a simple two-wheel differential-driven mobile robot. the main controller of the solar-powered robot is an arduino mega 2560 that receives input data from two position sensors and controls the robot’s movements by driving the direct current (dc) motors. a 1.5 w/12 v solar panel is used to charge the supercapacitor bank that supplies power to the robot. the solar-powered robot is also equipped with a voltage sensor to monitor the amount of charge continuously injected into the capacitor bank. the robot moves from one station to three others, simulating the stations in a factory scenario. supercapacitor charging was conducted for five days to detect any fluctuation due to external factors, such as weather conditions. the highest voltage value obtained in a day of charging was 4.96 v, with about 100,000 lux of irradiance received by solar cells while the robot moved, discharging the batteries during its task. another recent contribution discussed the design and implementation of a dual-axis tracker for pv panels able to provide additional power and thereby extend the operating time of a cleaning robot [18]. the dual-axis solar tracker worked by using four light sensors to compare the light intensity in order to determine the direction of incoming light. the reported experimental results showed that the highest output voltage reached 6.10 v for light intensity values from 110,800 up to 116,500 lux when the pv panel was orthogonal to the position of the sun, with a tolerance of 0–10 degrees. the average time to charge the battery from 2.5 v to 3.7 v using the pv panel was about 94 minutes. an additional pv panel permitted a longer operation time (an increase of 7 minutes and 17 seconds) for the cleaning robot. in the present paper, a specific robotic vehicle was designed and realised, and the related energy consumption was determined and minimised through several procedures. the heart of the system is an arduino r3 controller that drives four dc motors (model mot-03696, manufactured by dagurobot) and supervises a bluetooth hc-06 module and a sensing unit. we used experimental tests to quantify the contribution of each pv panel by considering changes in different parameters, such as the kinematics of the robot, the ambient temperature, the pv material (monocrystalline silicon [mono-si] or polycrystalline silicon [poly-si]), etc., in order to provide important and novel information for the future development of solar-powered robotic vehicles. the developed mobile robot was intended to detect the parameters in harsh environments where operating conditions are too dangerous for human intervention. the proposed solution makes it possible to safely inspect dangerous areas and determine if conditions are appropriate for subsequent human intervention. the robot is equipped with different sensors (e.g. to detect dangerous and harmful gases, flames and temperature) and a global positioning system (gps) receiver to inspect the considered area and track the vehicle’s position. in this context, several applicative scenarios can be identified, such as the inspection of empty tanks that previously contained chemicals, fuels or cereals or environments in which fires or gas leaks have occurred. the mobile robot includes an hc-06 bluetooth transceiver to transmit the acquired data to the developed iotool application, where the data are displayed and analysed, and receive instructions related to the operation of the mobile robot. thanks to the iotool application, a smartphone can act as an iot gateway, enabling the mobile robot to operate in remote mode as well as in outdoor areas where internet access points are not available. the paper is organised as follows: section 2 describes the design of the mechanical and electrical units of the mobile robot and introduces the fundamental equations used to estimate the energy requirements; section 3 reports the results related to the acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 211 robot’s power consumption, taking into account the contribution of the auxiliary solar energy harvesting system, which employed two different pv panel technologies. an analysis of the performances of the two commercial pv panels as a function of incident irradiance and temperature is reported. moreover, the sensors and devices equipping the robot and the developed iotool applications are introduced and fully described. finally, the conclusions are summarised in section 4. 2. mobile robot design and energy analysis 2.1. mechanical realisation and energy consumption evaluation the autonomous wheeled mobile robot was assembled partly with commercial components and partly with other devices adapted for the research; hence, it is a semi-custom vehicle. the energy consumption model for the mobile robot is divided into three components that are related to the following parts: the sensing system, the control system and the motion system. the energy consumption es of the sensing system is obtained by multiplying the electrical power consumption ps by the time interval t, as reported in equation (1) 𝐸𝑆 = 𝑃𝑆 ∙ δ𝑡 . (1) the energy consumption of the sensing system depends on the robot’s speed. fast sensor data transmission allows the robot to quickly monitor the surrounding environment. the speed of the mobile robot can range from low to high; if it is moving slowly and not working constantly, the sensors can wait for the robot to move and then detect the parameters of interest again. similarly, when the robot is in a standby state, there is no need to monitor its surroundings, so the sensors are set to sleep mode. when the robot reaches its maximum speed, the surrounding environment changes very quickly; thus, the sensors need to work continuously. equation (2) illustrates the overall energy consumption of the robot’s sensing system, which is proportional to the speed: 𝐸𝑆 = 1 𝑉𝑚𝑎𝑥 ∙ ∫(𝑣 ∙𝑃𝑆)𝑑𝑡 (2) where vmax is the maximum speed of the mobile robot. obviously, this method allows us to drastically reduce the energy consumption of the sensing system in comparison to a scenario in which the sensors work continuously. the energy consumption of a robot may be predicted by a statistical model that only depends on the robot’s path [19]; the authors in [19] proposed a probabilistic, data-driven approach to estimating a mobile robot's energy consumption on a set of trajectories. in this approach, the robot was treated as a black box, thereby removing the reliance on often unavailable system characteristics. they measured the consumption on the traversed routes directly and utilised features derived from publicly available maps to extrapolate energy consumption for real-world routes and then predict consumption for untravelled roads. the variance in the proposed statistical model is due to exogenous latent factors, such as traffic and weather conditions. in detail, the energy model derived from [19] and proposed in [20] is divided into the following three parts: energy consumption in the standby state, energy consumption when the wheeled robot first starts to move and, finally, energy consumption when the robot is running smoothly. the model can be stated as follows: 𝐸𝑐𝑜𝑛𝑡𝑟𝑜𝑙 = { 𝐸𝑠𝑡𝑎𝑛𝑑𝑏𝑦 = 𝑃𝑠𝑡𝑎𝑛𝑑𝑏𝑦 ∙ ∆𝑡 𝐸𝑠𝑡𝑎𝑟𝑡𝑢𝑝 = ∫(φ ∙ δv +( 𝑡2 10 ) + 𝐸𝑠𝑡𝑎𝑏𝑙𝑒 = ∫ (𝑃𝑠𝑡𝑎𝑛𝑑𝑏𝑦 + 𝑡 2)𝑑𝑡 𝑃𝑠𝑡𝑎𝑛𝑑𝑏𝑦) (3) where pstandby represents the power of the control sections (with the robot stopped), φ is the starting factor that determines the energy demand for the controller during the robot’s startup phases, δv is the rate of change at the current moment and t is the time when the robot starts moving. it is determined that the control system only accepts the sensors’ signals during the standby phase, so the power is constant. the 𝑡2 terms model the power consumption of the robot’s control system (dissipated as thermal power) as a function of the execution time, both in the startup (𝐸𝑠𝑡𝑎𝑟𝑡𝑢𝑝) and steady (𝐸𝑠𝑡𝑎𝑏𝑙𝑒) conditions. from a dimensional point of view, this term is implicitly weighted by the power consumption/time parameter expressed in w/s2, which relates to the control board. concerning the motion system, energy consumption can be divided into four parts: traction energy consumption, the increase of kinetic energy, friction energy dissipation and energy dissipated in thermal form (see (4)). as reported above, the robot’s motion can be divided into standby, startup and stable operation modes; in the startup mode, an instantaneous pulse sends the start signal to the electric motors. when the robot moves, it enters the stable operation mode: 𝐸𝑚𝑜𝑡𝑖𝑜𝑛 = ∫𝑃𝑚𝑜𝑡𝑖𝑜𝑛 𝑑𝑡 = 𝐸𝑘 + 𝐸𝑓 + 𝐸𝑒 + 𝐸𝑚 (4) where emotion is the energy consumed to attain and sustain the robot’s motion, ek is the kinetic energy (depending on the robot’s mass m and velocity v), ef is the energy dissipated due to friction during the movement of the robot (depending on the friction coefficient between the wheels and the ground), ee is the energy dissipated as heat in the armatures of the motors and em is the mechanical dissipation caused by overcoming the friction torque in the actuator. a simulation presented in [20] demonstrated that, in the classical condition of the robot’s movement, the most important energy contributions are given by ek and em. thus, once the geometry and kinematics of the mobile robot are fixed, it is possible to preliminarily evaluate the energy and power requested for the motion. the main aim of the research is to study a method to optimise wheeled autonomous and mobile robots in terms of energy consumption. in this context, it was decided to design a semicustom vehicle starting from a partly commercial robot and completing its construction by adding the control electronics, a sensing system with low-power sensors and an auxiliary battery recharged by the pv panel. reverse engineering (re) was adopted in the robot’s design [21], [22]; re is a common scientific/technical activity used to reprocess existing items or products. unlike forward engineering, re is based on the modification of a fully or partially existing system; the reverse engineer must not only provide a plan for the actual realisation of the system but also a plan for the transformations and justifications for their application. given this concept, we studied the available mobile robot and related items, and we used a dedicated software, solidworks (developed by dassault systèmes) [23], to redesign the whole vehicle and the additional parts that were used to complete the system. these additional sections are the following: acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 212 the hosting structure for the pv panel, the case for the atmega328 microcontroller [24], the sensor chassis and an additional electronic control board. figure 1 illustrates the different conceptual steps of the re vehicle design, in which the outcomes were updated to serve the purposes of the research, and figure 2 shows a photo of the assembled sensor-based vehicle moving on the laboratory floor. the model created using solidworks considered the components' mass characteristics and allowed a preliminary check on any structural problems in the mobile base or accumulations of mechanical stress in particular points of the structure. the subsequent implementation of the mobile robot made it possible to test the device's dynamic behaviour directly in the field. from the perspective of mechanical construction, all spare parts were assembled to reduce vibrations to a minimum. further mechanical vibrations are a source of energy loss. 2.2. functional and electronic schemes and experimental setup in order to quantify the power consumption, it is necessary to understand the real power demands of the different units involved in operating the robotic vehicle. figure 3 shows the main components and units of the designed sensor-based vehicle controlled by the arduino uno board based on an atmega 328 microcontroller. in detail, the four dc motors allow movement of the wheels through the l298n full-bridge motor-driving module (manufactured by stmicroelectronics). the heart of the system is the arduino r3 acquisition and processing board, which also controls the hc-06 bluetooth module and the power supply sections of the system. bluetooth technology was chosen for this application for two reasons: the ease of interfacing with smart devices (e.g. pcs, tablets or smartwatches), which are all equipped with bluetooth transceivers to exchange data and commands, and the suitability of the coverage range (<100 m) for the potential applications of the proposed system, such as inspecting tanks or pipelines and verifying the safety of environments in the presence of dangerous gas leaks. standard bluetooth was preferred over bluetooth low energy due to its compatibility with the software development tools. the mobile robot consumes 9.3 w of power (at maximum speed but without obstacles or energy loss), which are provided by an on-board battery pack consisting of two 9,900 mah lithium ion (li-ion) batteries (the blue cylindrical packages in figure 6) connected in parallel. these batteries (manufactured by gtf, model tr18650, 3.7 v nominal voltage) ensure an autonomy of more than five hours in the worst-case condition, corresponding to continuous current absorption of 2.5–2.6 a [25]. however, if additional power is required, in case of friction and/or complicated paths, a parallel solar energy harvesting system was installed to automatically boost the robot when needed, as described below. advances in the manufacturing of low-cost, high-performance solar panels and the availability of efficient and integrated electronic conditioning systems have enabled the simple integration of solar harvesting technology in iot or robotic applications [26]-[28]. given the small size of the vehicle’s roof, a 1.8 w/5 v monocrystalline solar panel (manufactured by shenzen portable electronic technology c. a) b) c) figure 1. conceptual re design using solidworks: a) basic vehicle chassis, b) over-lapping plate hosting the electronics and rechargeable batteries and c) full 3d vehicle including ir and ultrasonic (us) sensors. figure 2. assembled vehicle with the solar panel installed on the roof. figure 3. functional flowchart of the different sections of the designed mobile robot, all supervised by arduino and connected to the power supply unit. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 213 ltd, model petc-se1.8) and a 2 w/6 v polycrystalline solar panel (manufactured by solidal, model 130 110-3) were selected for the experiment. in particular, the energy harvesting system includes a cn3791-based solar charger (manufactured by consonance electronics) and a pwm switch-mode battery charger controller designed for solar harvesting applications; it can charge the battery in constant-current or constant-voltage mode, ensuring a regulated voltage equal to 4.2 v ± 1 % (figure 4). furthermore, the cn3791 solar charger features a wide input voltage range (4.5 ÷ 28 v), a maximum output current of 4 a and a maximum power point tracking function to optimise the power extraction from the solar panel. the cn3791-based board is equipped with two distinct outputs that are connected in parallel, allowing the battery to be connected to the first output and the electric load to the second. the charge scavenged by the solar panel is stored in a 4,000 mah single-cell lipo battery (model 905165, manufactured by ehao tech. co., ltd), connected in parallel to the mobile robot’s battery pack. the path from the auxiliary battery is dynamically enabled using a mosfet switch (model ir630, manufactured by stmicroelectronics), based on the overall current absorbed by the mobile robot, which is detected by an acs712 current sensor (manufacture by allegro microsystems) installed along the main power supply line (figure 4). the arduino uno main board acquires the signal from the sensor, and if the overall current absorbed is higher than 2.5 a, for instance when the mobile robot moves over rough ground, the auxiliary power supply path from the energy harvesting system is enabled. the arduino uno is a rapid prototyping board based on the atmega 328p microcontroller; it has a wide range of peripherals, including 20 digital input/output pins, of which six can be set as pwm outputs and six as analogue channels, and an integrated 10-bit analogue-to-digital converter. furthermore, the microcontroller integrates a single universal asynchronous receiver-transmitter (uart) interface and two-wire interface and serial peripheral interface (spi) modules, enabling easy interfacing with sensors and other external devices. figure 5 shows a block diagram of the designed system with the atmega 328 microcontroller that supervises all the vehicle systems. the pv-charged auxiliary and primary batteries are connected in parallel, when needed, to feed the whole system. when the required power becomes greater than 9.25 w, which corresponds to a current of 2.5 a (3.7 v provided by batteries), the auxiliary battery is connected to increase the available current and extend the lifetime of the primary batteries. the sensing system includes sensors for different applications; for path tracking, us and ir sensors are employed to control the robot’s motion [29]-[31]. thus, sensor-based path tracking allows the mobile robot to be guided during its movement from one point to another. comparing different pv panels (i.e. mono-si and poly-si) is important because changes in solar irradiance affect the energy harvesting system’s ability to provide power. a power contribution of 20 % from the pv system is crucial for the robot’s operation, but this contribution depends on the solar irradiance value and the panel’s temperature. the experimental setup, shown in figure 6, included the following main components: 1) the wheeled robot under measurement to determine its power consumption, 2) a hall effect sensor, 3) national instruments’ ni mydaq, 4) a pc displaying the acquired current signal and 5) a 220 v–15 v converter (5). a current hall effect sensor was used to measure the current absorbed by the tracking and environmental sensors and by the four dc motors; in addition, the overall power consumption of the mobile robot was monitored by measuring the current values absorbed directly from the battery pack. for this purpose, a custom smartphone application was developed using mit app inventor to control the robot’s movements, manage the sensing system as a function of the specific application and reduce the robot’s power consumption. the experimental tests were carried out at ista university’s mechatronics laboratory (kinshasa, dr congo). the wheeled robot, controlled by a mobile application, moved over the laboratory’s ceramic floor, travelling forward for 6 m at a constant speed of 1.5 m/s. therefore, the robot took a bit more than 4 s (including the time required to achieve the final velocity) to travel the fixed distance. energy consumption was measured during this time. to verify the impact of the pv panels, the vehicle was used outside under different solar radiation conditions. the obtained experimental results are reported in the next section. 3. system testing and experimental results the objective of this section is to illustrate the results of the experiments carried out in the laboratory and outdoors. the latter condition was important in order to test the behavioural a) b) c) figure 4. a) schematic representation of the auxiliary solar harvesting system used to boost the mobile robot when more power is required; b) the cn3791based board and c) the acs712 current sensor. figure 5. detailed functional block diagram of the power, control and driving systems of the mobile robot, with the microcontroller as the core component. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 214 capabilities of the wheeled vehicle when working outdoors and in stress conditions (namely, during operating modes with high current absorption by the motors, such as when the wheels are blocked due to an obstacle on the path or the terrain is rough). the outdoor experiments were also done to highlight the differences between the two types of pv panels able to supply 20 % of the total energy amount required by the mobile robot during diurnal activities (but obviously not during night operations). the experiments aimed to verify the dependencies between the trend of the absorbed current and the operating conditions under stress suffered by the robot due to any obstacles along the path or sudden changes in trajectory that cause speed variations. it should be noted that the average speed of the wheeled robot was about 1.5 m/s. it was set up to advance and reverse after a certain time, and our tests were carried out on a surface that allowed the wheels to turn freely. although the mobile robot travelled at a constant speed, figure 7a shows a variation in current over time because the robot moved back and forth; during the change of direction, an abrupt braking occurred, which affected current consumption. specifically, figure 7a depicts the trend of mean current absorbed by the entire mobile robot (the blue dashed curve), including the contributions of the motors and the acquisition and driving sections, and the mean current absorbed by the mobile robot’s four motors alone (the black curve). the electronic acquisition and driving units draw a mean current of about 100 ma, as detailed below. another acs712 current sensor was placed under the motor driver and connected to the acquisition board to measure the overall current absorbed by the motors. subsequently, as a stress test for both mechanical capability and energy consumption, the wheels were blocked and then released after a short time, which caused the current absorbed by the motors to rapidly increase until reaching a maximum value of 2.8–2.9 a (figure 7b). this figure also illustrates the trend of current absorption by the whole robot (the blue dashed curve). the maximum current is guaranteed by the supporting action of the auxiliary battery powered by the pv panel, which acts when the measured current exceeds 2.5 a. the red dashed line in figure 7b indicates the threshold current value (i.e. 2.5 a), which triggers the intervention of the auxiliary battery charged by the solar harvesting section. this intervention was activated after about 3.6 seconds and continued until the end of the test (10 seconds) (figure 7b). furthermore, several indoor tests were performed using different constant speeds (from 0.5 ms-1 up to 1.5 ms-1) and straight paths. when the robot’s speed was reduced, a proportionally lower current value was absorbed from the battery; for instance, at a speed of 0.5 ms-1, the robot’s motors drew an overall current of 0.63 a, while a constant velocity of 1.5 ms-1 correlated to a current absorption of 1.52 a on a smooth path. the developed robot implements two firmware strategies for tuning the speed as a function of the battery level in order to extend the battery’s lifetime. we developed a dual regulation mechanism; a remote control system allows the smartphone application to halve the set speed regardless of the battery status. we also implemented an automatic control mechanism that halves the robot’s speed when the battery voltage falls below 3.75 v, corresponding to 40 % of residual battery capacity. after the indoor tests, several outdoor experiments were performed in order to verify the contribution of each pv panel according to their technology (mono-si and poly-si) [32]. the pv panels, detailed in section 2.2, were tested to determine their contributions as a function of the solar irradiance and the panel’s temperature during the wheeled robot’s movements. all outdoor experiments were carried out in kinshasa (dr congo) during january, february and march 2020, as these are the warmest months in that area. figure 8 shows the average hourly temperature (the purple line) over three months (january, february and march 2020, respectively), with purple areas indicating the 25th to 75th and 10th to 90th percentile bands [33]. the thin dotted line is the average hourly perceived temperature. civil twilight and night are indicated by the shaded overlays. figure 8 demonstrates the repeatability of the trend of the average temperature during the day, with values varying from 23.3 °c to 30 °c in january, from 23.3 °c to 30.5 °c in february figure 6. experimental setup for the energy consumption analysis, with a dedicated acquisition architecture and measurement system. a) b) figure 7. current absorbed by batteries as a function of time during indoor tests: a) normal conditions; b) stress condition due to blocked wheels. the dotted red line indicates the threshold current value that activates the pv system’s supporting action. 0 0,5 1 1,5 2 2,5 3 1 2 3 4 5 6 7 8 9 10 a b so rb e d c u rr e n t in a time in s i_motors i_robot 0 0,5 1 1,5 2 2,5 3 3,5 1 2 3 4 5 6 7 8 9 10 a b so rb e d c u rr e n t in a time in s i_motors i_robot acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 215 and from 23.3 °c to 31.1 °c in march, representing a very small variation of the maximum temperature value in the considered months. the experimental validation was carried out by measuring the generated power during the robot’s outdoor navigation (using the same conditions as in the indoor tests) during the day as a function of the incident solar irradiance, which was detected by a digital pyranometer (model ht204, manufactured by ht instruments). the aforementioned quantities were acquired every 15 minutes, starting at 10 am, for a total temporal range of 5 hours. the characterisation results are shown in figure 9 and 10 for the mono-si and poly-si panels, respectively. in addition, the temperature of the panels was measured with an ir thermometer (model dem100, manufactured by velleman), and a first approximation was modelled, showing an increasing linear trend with the incident irradiance (figure 9 and figure 10). the measured power trends (the blue lines) have been compared with the ideal ones (the dotted red lines), which were obtained by assuming a constant conversion efficiency (i.e. 19 % for the monocrystalline panel [34] and 16 % for the polycrystalline one [35]) for high irradiance (> 300 w/m2/37,974 lux) and taking into account the parasitic effects emerging for low irradiance levels (e.g. shunt resistance). as can be seen for both solar panels, when the solar irradiance (and therefore the panel temperature) increases, the measured power increases, but the increase is less than in the ideal curve due to the conversion efficiency reduction, since the thermal excitation of electrons dominates the properties of the active material. however, the two typologies of solar panels (mono-si and poly-si) exhibit slightly different behaviours. up to 35 °c, both panels show the same trend, but at higher temperatures, the delivered power increases much less for the poly-si panel. this is due to the fact that the active material in the poly-si panel contains multiple crystals that prevent charge carriers from travelling a long distance before recombining (i.e. short diffusion length), leading to losses due to high temperatures, which mainly concerns the back of the panel. this loss increases the temperature within the robot's electronic components, thus inducing thermal drifts. to avoid this problem, a gap was created between the panel and the part immediately below it, where the electronic section is located, to allow natural cooling from the airflow induced by the robot’s movement [36]. this aspect is crucial, since the impact of temperature is a constraint for the use of remotely-controlled autonomous wheeled robots in long-term outdoor operations. three main factors influence the efficiency of pv panels: solar irradiance, technology and temperature. the exposure angle to the solar light and humidity are also important. the temperature of the electronic section was monitored using a pt100 sensor located near this section, which was used to stop the mobile robot if a temperature higher than 65–70 °c was detected [37]. the robot was constructed with the aim of detecting parameters in harsh environments where human intervention could be dangerous. in this context, several applicative scenarios can be identified, such as the inspection of empty tanks that previously contained chemicals, fuels or cereals or environments in which fires or gas leaks have occurred. the mobile robot is equipped with several sensors able to verify the absence of dangers to human health in the considered environment, thus allowing the subsequent intervention of a human operator [38]. the mobile robot includes a multi-directional flame sensor constituted by five-directional ir photo-transistors ( equal to 760 ÷ 1100 nm) that ensure a wide detection range (greater than 120 degrees); each transistor is arranged in common emitter configuration, with the output provided directly (analogue output) or digitalised using an lm393 comparator (manufactured by texas instruments) that compares the analogue voltage with a) b) c) figure 8. average daily environmental temperature in kinshasa during a) january 2020, b) february 2020 and c) march 2020. figure 9. trend of the maximum power provided by the 1.8 w mono-si pv panel (model petc-se1.8) as a function of solar irradiance and a linear model of the increase of the panel’s temperature induced by the incident radiation. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 216 a set voltage threshold. the five digital outputs are connected to five general-purpose input/output pins on the arduino board to detect fire and discern its location (figure 11a). furthermore, the robot is equipped with two gas sensors, namely a ze07-co module (manufactured by winsen inc.) and a ccs811 breakout board (manufactured by ams). the ze07co is constituted by a me2-co sensor with a conditioning/acquisition board based on a potentiostatic circuit (figure 11b). the me2-co sensor is a two-terminal electrochemical sensor, featuring high precision, low consumption, a wide linearity range and low cross-sensitivity with respect to other gaseous species; it has a wide detection range (from 0 to 1,000 ppm) and high sensitivity (0.015 ± 0.005 µa/ppm). the on-board acquisition section of the ze07-co module is based on an stm32f040f4 microcontroller (manufactured by stmicroelectronics), which acquires and elaborates the analogue signal provided by the conditioning section, making it available in the correct form to the master microcontroller via the uart interface. the ccs811 digital gas sensor is an ultra-low-power metal oxide (mox) gas sensor that detects the equivalent co2 (eco2) and the concentration of total volatile organic compounds (tvocs) (figure 11c) [39]. through intelligent algorithms applied to current and voltage values provided by the mox sensor, the gas concentration values are made available to a master microcontroller unit (mcu) employing the i2c interface. in addition, the sensor has three low-power modes optimised to extend the autonomy of portable applications. the ccs811 features a wide detection range (0 ÷ 1187 ppb for tvoc and 400 ÷ 8192 ppm for co2), high sensitivity, low power consumption and high robustness to external agents (i.e. high humidity and temperature as well as the presence of dust), making it suitable for the considered application. a neo-6m gps receiver (manufactured by u-blox) has been installed on the mobile robotic vehicle to provide the user with information about the vehicle’s position (figure 11d); it is very compact (16 mm × 12.2 mm × 2.4 mm) and features different power and memory modalities, which makes it ideal for applications in which there are space constraints or the power is supplied by batteries. the module interfaces with the arduino uno board using uart serial communication and provides gps coordinates via national marine electronics association messages. finally, the sensing system includes a k-type thermocouple (temperature detection range 0 ÷ 1024 °c) interfaced with a conditioning and acquisition board based on the max6675 ic (manufactured by maxim integrated), making the temperature data available to the host mcu by spi (figure 11e). the thermocouple was placed on the external housing of the robot to detect environmental temperature [40]. in figure 12, a schematic representation of the developed sensing and communication section installed on the mobile sensor-based robot is shown. the sensing system is enabled by an external command sent by the smartphone application via bluetooth communication; then, the sensing system takes 10 minutes to warm up the ccs811 gas sensor, allowing it to reach the optimal operating temperature. afterwards, using the hc-06 bluetooth module, the mobile robot transmits a packet containing appropriately arranged data every 60 seconds to the developed iotool smartphone application, which displays the incoming data and shares them with the ibm watson cloud, where they can be remotely visualised and analysed. through the iotool application, the smartphone acts as an iot gateway, providing internet access to the developed application and enabling the use of the mobile robot in areas where no access points are available (outdoors and/or in remote zones). the acquired data are shared figure 10. trend of the maximum power provided by the 2 w poly-si solar panel (model 130 110-3) as a function of solar irradiance and a linear model of the increase of the panel’s temperature induced by the incident radiation. a) b) c) d) e) figure 11. set of sensors and devices equipping the designed mobile robot: a) multi-directional fire sensor, b) winsen ze07-co gas sensor module, c) ccs811 gas sensing board, d) neo-6m gps module and e) max6675 module with k-type thermocouple. figure 12. graphical representation of the designed sensing system equipping the mobile vehicle; the acquired data and alarm conditions are synchronised with a smartphone acting as an iot gateway. acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 217 with a cloud platform, using the message queue telemetry transport protocol in javascript object notation (json) to create packets, which allows remote monitoring, storing and processing. thus, a remote operator can access the acquired data in real time, enabling better coordination of rescue, monitoring or maintenance activities. as described above, the developed mobile robot can support remediation operations in cramped places, such as tanks, pipelines and drains, or harsh environments, such as a room where there has been a fire or a dangerous gas leak. a remote supervisor’s availability enables, for instance, more rapid deployment of rescue teams or additional equipment in the presence of an abnormal co concentration or the resumption of a fire inside an environment. in addition, the iotool application allows the robot to receive remote commands via json packets; for instance, the robot could be signalled to optimise its power consumption or reduce the motors’ speed. furthermore, the application allows users to configure trigger and action events to send alarm messages via short message service and/or email if an abnormal value is detected for a monitored parameter [40], [41]. finally, in figure 13, two screenshots of the developed iotool application are shown, caught during a test of the robot’s sensing system; the temporal trends of the interest parameters (co, co2, tvoc and temperature) are plotted for the user. the absorbed current values measured on the different electronic units included in the acquisition, driving and communication systems are summarised in table 1. these were determined for each unit’s power supply line using the same experimental setup and methodology previously employed to measure the total robot’s power consumption. the overall current consumption of the mobile robot’s acquisition and communication system was about 70.4 ma during data acquisition and reached peak values of 78.9 ma during the communication phase, which, however, corresponds to less than 1 % of the robot’s operating time. furthermore, the driving system, constituted by an l298n motor driver, statically consumed 32.0 ma, representing the main contribution to the absorbed current. the sensing system can be disabled through another command sent by bluetooth communication, which acts on a mosfet switch placed in series to the supply line of the co sensor module, temperature sensor and flame detector and sets the gps receiver and the ccs811 gas sensor to low-power modality, obtaining a drastic reduction of the absorbed current (as low as dozens of µa). 4. conclusions the paper has presented the outcomes of a conceptual approach using re to design autonomous wheeled robots for monitoring harsh environments. energy consumption was a major focus, and the robot was equipped with a combination of embedded batteries and a solar energy harvesting section, including two different typologies of pv panels. the paper outlines the design of a wheeled sensor-based robot by taking into account the energy consumption of various components and the energy loss induced by the vehicle’s operation. using a re process and solidworks software, the whole robot was redesigned. after the integration of the solar harvesting system, which provided approximately 20 % of the total power required by the vehicle, experimental tests were carried out, taking into account friction and the loss of mechanical energy due to other displacement difficulties. tests were performed in two specific conditions: normal operating mode (figure 7a) and stress conditions simulating wheel blockages or rough paths, which both involve greater current demands (figure 7b). the pv panel plays a key role in these latter conditions, assuming that solar energy is available; two different pv panels (mono-si and poly-si) were alternatively mounted on the roof of the robotic vehicle for the outdoor tests. mono-si solar panels are usually of premium quality, with higher efficiencies and sleeker aesthetics. since the material used to produce solar cells is single-crystal silicon, the electrons have more room to move; thus, mono-si panels are more efficient than poly-si panels. the latter are made by melting many fragments of silicon to create the wafers for the pv panel. these panels are also referred to as ‘many-crystal’ or ‘multi-crystalline’ silicon. since there are many crystals in each cell, the electrons have less freedom to move; this results in lower efficiency than that typical of the mono-si panels [42]. an electronic conditioning component was developed for the pv harvesting system that recharges a 4,000 mah auxiliary liion battery and dynamically manages its activation based on the robot’s current absorption. the power trends for the two typologies of pv panels as a function of solar irradiance and panel temperature are shown in figure 9 and figure 10. as foreseen, the maximum power was close to 2 w; however, beyond 35 °c, a decrease of the harvested electrical power was observed, especially for the poly-si panel. figure 13. screenshot of the iotool application during experiments performed on the mobile robot’s sensing system, showing the temporal trends of the interest parameters. table 1. summary of the power consumption of the different units included in the acquisition, driving and communication systems of the mobile robot. component current consumption arduino uno board 24.2 ma hc-06 bluetooth module 8.5 ma (communication mode) 0.5 ma (sleep mode) ccs811 tvoc/eco2 gas sensor 30.4 ma (active mode) 18 µa (sleep mode) neo-6m gps receiver 8.2 ma (average value) multi-directional flame sensor 2.2 ma ze07-co module 4.7 ma k-type thermocouple + max6675 module 0.7 ma l298n motor driver 32.0 ma (quiescent current) acta imeko | www.imeko.org june 2021 | volume 10 | number 2 | 218 finally, a sensing system was designed and installed in the vehicle to allow the monitoring of harsh environments in which human intervention would be too dangerous. it includes a wide set of sensors (for flames, toxic gas species and temperature) and devices (a gps receiver and a bluetooth module) to detect the environmental parameters and track the vehicle’s position. furthermore, an hc-06 bluetooth module was used to transmit the acquired data to the developed iotool mobile application, where they can be displayed and analysed by the user. acknowledgement the authors gratefully thank the members of the mechatronics laboratory at ista university (kinshasa, dr congo) and moise avoci ugwiri of the university of salerno (fisciano, italy) for their technical support during the experiments. the research is part of a cooperation between ista university and the university of salento supervised by prof. a. lay-ekuakille. references [1] t. addabbo, a. fort, m. mugnaini, l. parri, s. parrino, a. pozzebon, v. vignoli, a low-power iot architecture for the monitoring of chemical emissions, acta imeko 8(2) (2019), pp. 53-61. doi: 10.21014/acta_imeko.v8i2.642 [2] a. bernieri, d. capriglione, l. ferrigno, m. laracca, design of an efficient mobile measurement system for urban pollution monitoring, acta imeko 1(1) (2012), pp. 77-84. doi: 10.21014/acta_imeko.v1i1.27 [3] a. lay-ekuakille, s. ikezawa, m. mugnaini, r. morello, detection of specific macro and micropollutants in air monitoring, review of methods and techniques, measurement 98(1) (2017), pp. 49-59. doi: 10.1016/j.measurement.2016.10.055 [4] a. p. plonski, j. vander hook, v. isler, environment and solar map construction for solar-powered mobile systems, ieee transactions on robotics 32(1) (2016), pp. 70-82. doi: 10.1109/tro.2015.2501924 [5] who, diseases, coronavirus disease (covid-19). online [accessed 21 june 2021] https://www.who.int/emergencies/diseases/novel-coronavirus2019. 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https://doi.org/10.1007/978-3-540-75997-3_300 https://doi.org/10.1016/j.measurement.2015.08.024 https://components101.com/sensors/pt100-rtd-temperature-sensor https://components101.com/sensors/pt100-rtd-temperature-sensor https://doi.org/10.3390/en13092161 https://doi.org/10.1049/iet-smt.2018.5664 https://doi.org/10.1109/jsen.2017.2669529 http://dx.doi.org/10.21014/acta_imeko.v7i4.675 http://dx.doi.org/10.21014/acta_imeko.v7i4.599 new study on force transducer’s temperature behaviour acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 133 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 133 136 new study on force transducer’s temperature behaviour kui gan1, hong jiang2, hongjiang chen3, hao zhang4, william huang5 1 hunan institute of metrology and test, changsha, china, fancygk@163.com 2 shanghai institute of quality inspection and technical research, shanghai, china, jianghong@sqi.org.cn 3 hunan institute of metrology and test, changsha, china, 31862083@qq.com 4 hunan institute of metrology and test, changsha, china, 664074026@qq.com 5 gtm china office, shanghai, china, william.huang@gtmchina.cn abstract: this paper describes a new study about the temperature behaviour of force transducers. a special force transducer with a pt100 for temperature measuring was developed by gtm. the curve of heating was created, and test data indicates the time for attaining the stable temperature. meanwhile the different sensitivity of the transducer under different temperatures was obtained, thus the temperature effect on characteristic value per 10 k (so-called tkc) was calculated. at the end, the correction of force transducer at different temperatures was made by the tkc factor of temperature. keywords: force transducer; temperature measurement; temperature effect 1. introduction as is well known, the transducer temperature is not exactly the same as the room temperature during calibration. iso 376:2011 [1] states that “sufficient time shall be allowed for the force-proving instrument to attain a stable temperature”, but there is no guarantee that the temperature of the transducer’s whole body has reached room temperature, even after several hours. normally the temperature of the surface of the transducer will then be the same as room temperature, but the temperature inside the transducer is unknown. additionally, every manufacturer of transducers provides data sheets specifying tkc (temperature effect on characteristic value per 10 k) and tk0 (temperature effect on zero signal per 10 k) in their catalogues or instruction manuals. however, it is not easy for the end user to know if this is true or if something may change after several years’ usage of the transducer. in the past some similar studies [2, 3] have been done and the papers also describe the temperature behaviour of the transducer. they measured the temperature of the chamber or the laboratory, but the temperature of the transducer was never measured directly. all the data of temperature mentioned were the room temperature. therefore, it makes sense to do a new study with special design of the transducer with temperature measurement on its body. 2. equipment a special transfer standard force transducer was designed and manufactured by gtm based on the type ktn-d. a pt100 sensor was attached inside this transducer on the elastomer, as shown in figure 1. figure 1: pt100 inside the transducer the transducer has two measurement circuits: one is for force (fz) and the other is for temperature (pt100), as in figure 2. figure 2: force transducer with pt100 inside http://www.imeko.org/ mailto:fancygk@163.com mailto:jianghong@sqi.org.cn mailto:31862083@qq.com mailto:664074026@qq.com mailto:william.huang@gtmchina.cn acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 134 this is a transfer standard force transducer of 300 kn capacity, class 00 according to iso 376. we use a 300 kn deadweight machine with urel ≤ 0.005 % (k = 2) in the force laboratory. the amplifier is dmp41 for force and a monitor for pt100. the uncertainty of the temperature measurement is urel = 0.5 °c (k = 2). 3. procedure we have done two calibration tests, with the transducer and amplifier in the same room. 3.1. temperature measurement only without load 1. at the beginning, the room temperature is around 15 °c. increase the temperature to 20 °c using the room’s air conditioning. record the values of both the pt100 and the thermometer every hour for nine hours. figure 3: temperature measurement 2. at the beginning, the room temperature is around 15 °c. increase the temperature to 25 °c using the room’s air conditioning. record the values of both the pt100 and the thermometer every hour for nine hours. 3.2. calibration test with load 3.2.1. test from 15 °c to 25 °c 1. under temperature of 15 °c, calibrate the transducer according to iso 376, but without change position. that means no rotation position. pre-load up to 300 kn. three calibration series, and the calibration steps are: 30 kn, 50 kn, 100 kn, 150 kn, 200 kn, 250 kn and 300 kn. the holding time of each step is 30 s. 2. under temperature of 25 °c, test the transducer, according to iso 376, but without change position. that means no rotation position. pre-load up to 300 kn. three calibration series, and the calibration steps are: 30 kn, 50 kn, 100 kn, 150 kn, 200 kn, 250 kn and 300 kn. the holding time of each step is 30 s. 3.2.2. test from 10 °c to 20 °c 1. under temperature of 10 °c, test the transducer according to iso 376, but without change position. that means no rotation position. pre-load up to 300 kn. three calibration series, and the calibration steps are: 30 kn, 50 kn, 100 kn, 150 kn, 200 kn, 250 kn and 300 kn. the holding time of each step is 30 s. 2. under temperature of 20 °c, test the transducer according to iso 376, but without change position. that means no rotation position. pre-load up to 300 kn. three calibration series, and the calibration steps are: 30 kn, 50 kn, 100 kn, 150 kn, 200 kn, 250 kn and 300 kn. the holding time of each step is 30 s. collect all the data. 4. data analysis the data from test 3.1 is plotted in figure 4 and figure 5. figure 4: temperature measurement 1 of 3.1 figure 5: temperature measurement 2 of 3.1 both curves show that it takes around eight hours for the transducer body (pt100) to reach room temperature (rt). the data from test 3.2.1 is given in table 1 and table 2, including the repeatability of the transducer (b’). table 1: test at 15 °c (pt100 = 15.24 °c) f run 1 run 2 run 3 mean b' kn mv/v mv/v mv/v mv/v % 30 0.200 076 0.200 074 0.200 067 0.200 072 0.004 50 0.333 453 0.333 453 0.333 450 0.333 452 0.001 100 0.666 888 0.666 883 0.666 882 0.666 884 0.001 150 1.000 302 1.000 298 1.000 288 1.000 296 0.001 200 1.333 701 1.333 690 1.333 680 1.333 690 0.002 250 1.667 053 1.667 045 1.667 040 1.667 046 0.001 300 2.000 347 2.000 333 2.000 325 2.000 335 0.001 http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 135 table 2: test at 25 °c (pt100 = 25.35 °c) f run 1 run 2 run 3 mean b' kn mv/v mv/v mv/v mv/v % 30 0.200 105 0.200 111 0.200 111 0.200 109 0.003 50 0.333 514 0.333 535 0.333 526 0.333 525 0.006 100 0.667 014 0.667 050 0.667 038 0.667 034 0.005 150 1.000 502 1.000 549 1.000 533 1.000 528 0.005 200 1.333 968 1.334 027 1.334 005 1.334 000 0.004 250 1.667 377 1.667 439 1.667 410 1.667 409 0.004 300 2.000 722 2.000 792 2.000 760 2.000 758 0.003 this transducer was made according to class 00 of iso 376. the repeatability in class 00 is b’ < 0.025 %. we can see that b’ at 15 °c is ≤ 0.004 %, and b’ at 25 °c is ≤ 0.006 %, meeting class 00 of iso 376. according to the chinese calibration guideline jjg 144 [4], the repeatability is 0.01 % and 0.03 % (class 0.01 and class 0.03). the data of table 1 and table 2 are compared, thus tkc was calculated as in table 3. table 3: calculation of tkc f mv/v mv/v tkc kn ave at 15.24 °c ave at 25.35 °c % / 10 k 30 0.200 072 0.200 109 0.018 50 0.333 452 0.333 525 0.022 100 0.666 884 0.667 034 0.022 150 1.000 296 1.000 528 0.023 200 1.333 690 1.334 005 0.023 250 1.667 046 1.667 409 0.022 300 2.000 335 2.000 758 0.021 tkc is one specificity of the transducer with strain-gauge technology. every manufacturer provides this data in the product catalogue. this transducer of ktn-d also gives the value of tkc = 0.02 % / 10 k in the datasheet. now we can see the value by the test is 0.023 % (maximum) and 0.018 % (minimum). from table 3, the average value of tkc is taken as 0.022 % / 10 k. the data from test 3.2.2 were collected and compared as shown in table 4. as mentioned, tkc is 0.022 % / 10 k for this transducer according to test 3.2.1. using this value, its performance at a temperature of 11.49 °c is corrected to an expected performance at a temperature of 19.53 °c, with the results given in table 5. table 4: deviation from test 3.2.2 f mv/v mv/v deviation kn at 11.49 °c at 19.53 °c % 30 0.200 061 0.200 094 0.016 50 0.333 434 0.333 492 0.017 100 0.666 842 0.666 969 0.019 150 1.000 236 1.000 422 0.019 200 1.333 607 1.333 859 0.019 250 1.666 944 1.667 253 0.019 300 2.000 219 2.000 583 0.018 table 5: corrected value f mv/v mv/v kn at 11.49 °c corrected to 19.53 °c 30 0.200 061 0.200 096 50 0.333 434 0.333 493 100 0.666 842 0.666 960 150 1.000 236 1.000 413 200 1.333 607 1.333 843 250 1.666 944 1.667 239 300 2.000 219 2.000 573 now the corrected and measured values were compared, with the calculated deviations given in table 6. the two curves are also shown in figure 6. table 6: deviations between corrected and measured values f mv/v mv/v deviation kn corrected to 19.53 °c measured at 19.53 °c % 30 0.200 096 0.200 094 -0.001 50 0.333 493 0.333 492 0.000 100 0.666 96 0.666 969 0.001 150 1.000 413 1.000 422 0.001 200 1.333 843 1.333 859 0.001 250 1.667 239 1.667 253 0.001 300 2.000 573 2.000 583 0.000 figure 6: deviation before correction vs after correction http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 136 the value of after correction is nearly the same as the measurement at 19.53 °c (the deviation is -0.001 %), meaning that the value of tkc is reliable. other corrections have also been performed between different temperatures. for a correction at 15.24 °c based on the measurement value at 11.49 °c, the deviation between the corrected value and the measured value is around 0.003 %. 5. summary this special transducer helps to prove the description in iso 376 about enough time: around eight hours should be suitable. indeed, the chinese calibration guideline jjg 144 [4] also suggests that the time which the transducer put at the laboratory is “more than eight hours”. for the transducer of high accuracy class, any time less than eight hours is not recommended. the temperature behaviour of each force transducer is given by its value of tkc. therefore, the data by the manufacturer should be considered by the user during the calibration. at different room temperatures, the value of tkc can be used to make corrections. the deviation between the value after correction and measurement directly is same or less than the value of repeatability. 6. references [1] iso 376, metallic materials — calibration of force proving instruments used for the verification of uniaxial testing machines, 2011. [2] d. röske, “the influence of temperature and humidity on the creep of torque transducers”, imeko 23rd tc3, 13th tc5 and 4th tc22 international conference, helsinki, finland, 30 may to 1 june 2017. [3] min-seok kim, “simultaneous determination of temperature and humidity sensitivity coefficients of torque transfer standards in ambient conditions”, imeko 23rd tc3, 13th tc5 and 4th tc22 international conference, helsinki, finland, 30 may to 1 june 2017. [4] jjg 144, verification regulation for standard dynamometers, saqsiq, china, 2007. http://www.imeko.org/ calibration procedures for torque measuring devices by using a reference type torque calibration machine at nmij acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 179 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 179 183 calibration procedures for torque measuring devices by using a reference type torque calibration machine at nmij m. kiuchi1, a. nishino1, k. ogushi1 1 national metrology institute of japan (nmij), aist, tsukuba, japan, kiuchi-misaki@aist.go.jp abstract: nmij / aist has been disseminating the national torque standard to japanese industry by using deadweight type torque standard machines (dwtsms). in general, dwtsms can generate more precise torque than other types of tsms. on the other hand, the calibration takes much longer time than others. one possible solution is to use a reference type torque calibration machine (rtcm). rtcms have been developed in some national metrology institutes (nmis). we have started the development of the first rtcm in the range of 100 mn·m to 10 n·m. in this study, we developed the automatic calibration system of the rtcm and investigated the calibration procedures for a low nominal capacity torque measuring device (tmd). it was found that the calibration could be realised by the rtcm, compared with the reliable dwtsm of rated capacity of 10 n·m at nmij. keywords: torque; torque standard; reference torque calibration machine; calibration 1. introduction the national metrology institute of japan (nmij), part of the national institute of advanced industrial science and technology (aist), has developed three deadweight type torque standard machines (dwtsms) [1-3], and has used them for the calibration of torque measuring devices (tmds) for accredited laboratories of the first grade [4] with the relative expanded uncertainty order of 10-5. figure 1 shows the si traceability system proposed by nmij. dwtsms can generate more precise torque than other types of torque standard machines (tsms). the calibration using dwtsms, however, takes more extended time than other types of tsms. besides, the calibration procedure is restricted to the step-by-step calibration, where it needs loading and unloading weights at the end of the moment arm and maintaining the moment arm at the horizontal position. figure 1: si traceability system of torque measurement in japan (proposed) [4] http://www.imeko.org/ mailto:kiuchi-misaki@aist.go.jp acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 180 most of tmds did not need to be calibrated with such a small uncertainty (10-5 order of relative uncertainty). a kind of efficient or economic calibrations are more preferable for such tmds in the industry than high-accurate one. so far, some nmis or accredited laboratories have already been developed and investigated reference type torque calibration machines (rtcm), which are the secondary calibration equipment [5-11]. rtcm generally arrays a reference tmd (r-tmd) with a calibrated tmd (a calibration item: c-tmd) on the same axis. a driving system like as an electrical motor loads the torque on both of r-tmd and c-tmd, then loaded torque could be obtained from the measurement value of r-tmd. r-tmd has been calibrated by using upper level of dwtsm beforehand. in japan, an rtcm has been introduced for the calibration service of tmds but from the range of 10 n·m to 10 kn·m at the japan quality assurance organization (jqa) [12]. according to our survey of japanese industry on torque measurement, demands for small range torque calibration is still increasing. then, we decided to develop a rtcm with lower nominal capacity of less than 10 n·m [13]. in addition, the rtcm is possibly able to shorten the calibration time and to investigate new calibration procedures or methods other than the classical style of the step-by-step method. in this study, first, the authors describe the completed automatic new rtcm. then, the trial calibration experiment in the range from 100 mn·m to 1 n·m was conducted with various waiting time and motor speed conditions. finally, we compared the calibration results obtained by the new rtcm to that obtained by the dwtsm with the rated capacity of 10 n·m (10 n·m dwtsm). here, we cannot still name the new rtcm with the capacity although the aimed calibration range will be 10 mn·m to 10 n·m or 100 mn·m to 10 n·m. 2. experiment 2.1. experimental setup 10 n·m dwtsm figure 2 shows a photograph of the 10 n·m dwtsm. the 10 n·m dwtsm consists of; (1) a double-side moment arm, (2) a weight loading component, (3) a counter bearing drive, (4) installation components for a torque transducer, (5) a pedestal, and (6) a windshield. the calibration range of it is from 10 mn·m to 10 n·m. the calibration and measurement capabilities (cmcs) are 2.0 × 10-4 from 10 mn·m to 100 mn·m, and 7.0 × 10-5 from 100 mn·m to 10 n·m [3, 14]. the dwtsm was used for the calibration of not only the r-tmd but also the c-tmd in the new rtcm, as described below. figure 2: photograph of the 10 n·m dead weight type torque standard machine at nmij figure 3: photograph of the new rtcm double-side moment arm counter bearing drive torque transducer servo motor reduction gear system couplings r-tmd aerostatic bearing linear motion guide c-tmd http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 181 figure 4: schematic view of the experimental setup rtcm figures 3 and 4 show a photograph and a schematic diagram of the new rtcm. the new rtcm is set up vertically (the measurement axis is vertical). the new rtcm consists of (1) a servo motor, (2) reduction gear system, (3) couplings, (4) an r-tmd, (5) an aerostatic bearing, (6) a c-tmd and (7) a linear motion guide. the torque transducer for the r-tmd and another torque transducer for the c-tmd are installed in series. here, again, tmd is defined as a complete set of a torque transducer, a cable, and an amplifier/indicator [15]. a motor simultaneously twists the two transducers. in this study, we used tp-1n-0302 as the torque transducer, which was developed by nmij in cooperation with showa measuring instruments co., ltd., as both transducers for the r-tmd and ctmd. the capacity of the tp-1n-0302 is 1 n·m. the new rtcm can calibrate the c-tmd by comparing the output of it with the output of the rtmd. besides, a hygrometer, a barometer, and thermometers are connected to a pc through a highresolution data acquisition system (daq). the servo motor is also connected to a pc through a servo amplifier and the motion controller. the velocity values of the servo motor is appropriately able to be controlled by using the pc while monitoring the output of the r-tmd. 2.2. experimental procedures calibration procedures for the 10 n·m dwtsm a tsm of a higher-level must calibrate the r-tmd inside the new rtcm beforehand. first, we calibrated the r-tmd by using the 10 n·m dwtsm according to a guideline of jmif-015 [15]. the japan measuring instruments federation published this guideline after discussion among nmij, the japanese industry, and universities. the c-tmd was also calibrated by the 10 n·m dwtsm in order to evaluate the calibration capability of the new rtcm later. both tmds were calibrated separately for the clockwise (cw) and counterclockwise (ccw) directions. at the rotational mounting position of 0°, the torque transducer was preloaded three times in the direction to be calibrated, applying the maximum torque tmax, after which, the calibration loading cycles were repeated twice. during one cycle, loading was performed either in eight steps of 10 %, 20 %, 30 %, 40 %, 50 %, 60 %, 80 %, and 100 % of tmax. both the two cycles consisted of increasing and decreasing torques. for successive mounting positions of 120° and 240°, the same calibration cycles as for 0° were used (after one preloading cycle). the calibration was conducted as precisely as possible according to a given timetable, in order to http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 182 eliminate the influence of creep in the torque transducers. each indicated value was acquired after the predetermined waiting time (30 s as the normal condition) from the achievement to each torque step. the above procedure is classically used for the calibration with dwtsms, so-called “step-by-step” calibration. calibration procedures for the rtcm we calibrated the c-tmd by using the new rtcm. for the confirmation that the calibration with the new rtcm just can be conducted with the step-by-step procedure as same as the 10 n·m dwtsm, some calibration experiments with different parameters were carried out. however, only the results of one condition for the motor speed are described in section 3. the motor speed was gradually changed as the torque get closer to the the target value of the next step. whereas, the waiting time was set to 30 s (condition no. 1) and 60 s (condition no. 2). the other calibration procedures and conditions were the same as those described in the previous subsection. 3. results and discussion figure 5 shows characteristic curves of calibration of the r-tmd obtained by using the 10 n·m dwtsm. it can be seen that a transducer with very good characteristics was used as r-tmd. figure 6 shows characteristic curves of calibration of the c-tmd obtained by using the 10 n·m dwtsm. characteristic relative deviations were calculated according to eqs. (33) in reference [4] as deviations from the linear lines from zero to the maximum value of the calibration result. from the figure, the c-tmd could be calibrated in the uncertainty level within 10-4 order. the characteristic curves are smooth, and the dispersion of each curve is small. the calibration capability of the 10 n·m dwtsm has no problem. however, all the calibration (including both cw and ccw calibration) took more than eight hours. figure 7 also shows characteristic curves of calibration of the c-tmd obtained by using the new rtcm in the condition of no. 1 as the best result. the results obtained with other conditions were not good in a few torque steps because out-of-control might occur in the motion control of the servomotor. even in figure 7, it was found that the characteristic curves were not smooth and not successive. that suggests the control of the torque at each step was not stable. further investigation will be required, especially for the motor-control of the new rtcm. nevertheless, the scattering of data was within 0.04 % at around the 1 n·m step. we could achieve the completion of the new rtcm with the classical step-by-step torque calibration, at least. moreover, all the calibration could successfully take less than four hours. the relative deviations of calibration results obtained by using the new rtcm in the condition of no. 1 and no. 2, from the values obtained by using the 10n·m dwtsm were expressed in figures 8 and 9. the relative deviations were within approximately 0.05 %, except at the step of 100 mn·m. they seem to include a kind of systematic error. the remarkable differences of two conditions could not be found. therefore, the waiting time did not affect the calibration results obtained by the new rtcm so much. the future investigation will solve massive deviations. large deviations at the steps of 100 mn·m would probably come from the out-of-control of the servo-motor. figure 5: characteristic curves of calibration of r-tmd by using the 10 n·m dwtsm figure 6: characteristic curves of calibration of c-tmd by using the 10 n·m dwtsm figure 7: characteristic curves of calibrationof c-tmd by using the rtcm http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 183 figure 8: relative deviation of calibration results using rtcm with condition no. 1, from calibration results using 10 n·m dwtsm figure 9: relative deviation of calibration results using rtcm with condition no. 2, from calibration results using 10 n·m dwtsm 4. conclusion the new reference type torque calibration machine (rtcm) was developed at nmij. the calibration capability of this machine was investigated at the range of 100 mn·m to 1 n·m, compared with the deadweight type torque standard machine with the rated capacity of 10 n·m (10 n·m dwtsm). in some experimental conditions, scattering due to changing the mounting position was within 0.04 % at around the 1 n·m step. the relative deviations from the value obtained by 10 n·m dwtsm were approximately 0.05 % except the step of 100 mnm. the authors could achieve the completion of the new rtcm with the classical step-by-step torque calibration, at least. in the future, the problem of motion-control should be improved. the possible systematic error in the relative deviations between the calibration results by using the new rtcm and the 10 n·m dwtsm should be minimized. the estimation of uncertainty for the calibrations would be next research topics. 5. references [1] k. ohgushi, a. nishino, t. ota, k. ueda, “expansion of the calibration range and improvement of the uncertainty in the 1 kn·m deadweight torque standard machine”, in proc. of imeko 20th tc3 int. conf., merida, mexico, 27-30 november 2007. [2] k. ohgushi, t. ota, k. ueda, “uncertainty evaluation of the 20 kn·m deadweight torque standard machine”, measurement, vol. 40, no. 7-8, pp. 797-802, 2007. [3] a. nishino, k. ogushi, k. ueda, “uncertainty evaluation of a 10 n·m deadweight torque standard machine and comparison with a 1 kn·m deadweight torque standard machine”, measurement, vol. 49, pp.77-90, 2014. [4] k. ogushi, t. ota, k. ueda, d. peschel, d. röske, h.-j. tambach, d. mauersberger, “international comparison of torque standards in the range of 0.5 to 20 kn·m between ptb and nmij”, aist bulletin of metrology, vol. 6, no. 3, pp. 133-139, 2007. [5] d. peschel, “the state of the art and future development of metrology in the field of torque measurement in germany”, in proc. of xiv imeko world congress, tampere, finland, 1997. [6] d. peschel, a. brüge, “calibration of torque measuring devices – step-by-step or continuous procedure”, in proc. of 16th imeko tc3 conf., taejon, south korea, 1998. [7] a. brüge, d. röske, d. mauersberger, k. adolf, “influence of cross forces and bending moments on reference torque sensors for torque wrench calibration”, in proc. of xix imeko world congress, lisbon, portugal, 2009. [8] a. brüge, “creep measurements in reference torque calibration machines”, in proc. of 21st imeko tc3 int. conf., pattaya, thailand, 2010. [9] n. saenkhum, t. sanponpute, “a comparison of purely static and continuous torque calibration procedure”, in proc. of imeko 22nd tc3 int. conf., cape town, south africa, 2014. [10] n. saenkhum, t. sanponpute, “the optimization of continuous torque calibration procedure”, measurement, vol. 107, pp. 172-178, 2017. [11] h. j. fraiss, l. stenner, d. röske, “development of a 400 kn·m torque calibration machine”, in proc. of xxi imeko world congress, prague, czech republic, 2015. [12] y. toda, t. kurokawa, h. teshigawara, k. ogushi, “calibration of torque measuring devices using the reference type torque calibration machine in jqa”, in proc. of asia pacific measurement forum on mechanical quantities, t-1, 2019. [13] a. nishino, m. kiuchi, k. ogushi, “development of a low nominal capacity reference torque calibration machine”, in proc. of asia pacific measurement forum on mechanical quantities, t-3, 2019. [14] a. nishino, k. ogushi, “re-evaluation of sensitivity limit in a 10 n·m deadweight torque standard machine” (in japanese), in proc. of 56th annual conference of the society of instrument and control engineers of japan, 2017. [15] jmif-015, “guideline for accredited laboratories of torque measuring devices (pure torque)” (in japanese), japan measuring instruments federation, 2004. http://www.imeko.org/ microsoft word article 15 jena paper 3.docx acta imeko  august 2013, volume 2, number 1, 74 – 78  www.imeko.org    acta imeko | www.imeko.org  august 2013 | volume 2 | number 1 | 74  new demands on inspection planning and quality testing for  micro‐ and nanostructured components  karina weißensee  department of quality assurance and industrial image processing, ilmenau university of technology,pb 100565, 98684 ilmenau, germany      keywords: inspection planning; measuring strategy; micro‐ and nano metrology   citation: karina weißensee, “new demands on inspection planning and quality testing for micro‐ and nanostructured components”, acta imeko, vol. 2,  no. 1, article 15, august 2013, identifier: imeko‐acta‐02(2013)‐01‐15  editors: paolo carbone, university of perugia, italy; gerhard linß, ilmenau university of technology, germany  received march 27 th , 2013; in final form april 4 th , 2013; published august 2013  copyright: © 2013 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: the presented work is the result of the close interdisciplinary research within the sfb 622, which is located at the tu ilmenau, germany, and  funded by the german research foundation (dfg)  corresponding author: karina weissensee, e‐mail: karina.weissensee@tu‐ilmenau.de    1. introduction  the dimensional characterisation of structures on industrial products is fundamental for the assessment of the functionality and for the quality control. measuring tasks on microand nanostructured components are very complex and different due to their high variety on raw materials, functions and geometries. the industrial production of microand nanostructured parts and systems expedites the development of adequate metrology for quality control. furthermore there is the industrial request for measuring instruments solving all measuring tasks on the measuring object preferably fully automated. this means all measuring tasks should be solved using only one instrument. multisensor coordinate measuring machines are the metrological basis for using more sensors to solve various measuring tasks on one measuring object without fixing changes. these machines offer considerable advantages e. g. high flexibility, short measuring times and ways. but for an adequate usage there is the requirement of a high knowledgelevel on the parts of the user [1]. a first way to improve this situation is to develop tools which give assistance in inspection planning to the user. in general, inspections of microand nanostructured components have to cope with a huge number of inspection features on one measuring object only. typically very small features for example 100 nm wide structures are distributed over a large area of several square millimetres. this is a challenging task. with the nanopositioning and nanomeasuring machine developed at ilmenau university of technology, subnanometre resolution and nanometre uncertainty in a measuring volume of 25 × 25 × 5 mm3 have been demonstrated in the last few years. in practice, however, there are too many different requirements for sensing surfaces or for detecting structures [2]. in order to perform 3d-coordinate measurements within the microand nanometre range a combination of different sensors must be utilised [3]. these sensors should be different in relation to their physical measuring principle and their measuring scale. there are several physical principles according to which probes can be realized. every probe, either non-tactile or tactile, shows specific advantages and disadvantages. solving all these various measuring tasks is not possible by means of only optical or only tactile probes. it is very important to be able to select the optimal sensor for a specific measurement task [2]. in order to enable the automatic execution of inspection plans for microand nanostructured components the measuring machine must include a cascaded multisensor system. a cascaded multisensor system consists of multiple probing abstract  the  development  and  control  of  more  and  more  complex  and  extensive  technical  systems  yields  to  measurement‐technology  requirements  in  an  increasing  degree.  these  requirements  can  not  be  met  with  the  exclusive  operation  mode  of  a  single‐sensor  measuring  instrument because  in most  instances multifarious and multistructured measuring quantities are existing on one device  under test. the aim of this paper is to discuss new problems of inspection planning arising from the improvement in measurement  technology. essential demands, ideas and conceptual approaches to multistructured quality inspections will be presented.  acta imeko | www.imeko.org  august 2013 | volume 2 | number 1 | 75  sensors with very different measuring range and very different measuring resolution (figure 1). it is characterised by the internal information processing between the different sensors and enables the stage to stage accuracy-dependent inspection of microand nanostructured 3d features. those specifics must be taken into account at inspection planning and at the execution of inspection plans [3]. in section 2 the main problems of inspection planning in the microand nanoscale will be summarised. furthermore general tasks of inspection planning will be described. the topic of section 3 is the conceptual approach to an assistance tool for inspection planning. in section 4 possible concepts rules for measuring strategies will be discussed. finally in the concluding section the major results are summarised. 2. statement of current problems  the main tasks of inspection planning can be presented as answers of the questions illustrated in figure 2. the question: “what should be inspected?”, yields to the measuring object and its inspection features. the question: “who is to be responsible for the inspection?, or “who should realise the measurement?”, depends on the used measuring instruments and yields to the operators. “with which instrument should the inspection be executed?”, yields to the measuring instruments especially adequate sensors. “how should the measurement be executed?”, yields to the measuring strategy and is one of the most complicated questions. and finally “when should the inspection be executed?”, yields to the course of action in the production chain and depends on the measuring time. furthermore there are interdependencies between the most parts of inspection planning. for example the measuring time depends on the measuring strategy and the selection of an adequate measuring instrument depends on the given tolerances and specifications of the inspection feature. the determination of a potential inspection strategy poses a big challenge for the inspection planner, especially if the sensor selection and the inspection strategy are correlated. all these elements of inspection planning can not be established without considering all the others. therefore often control loops are used for inspection planning [3]. figure 3 illustrates the typical course of action of the planning process in a simplified way. the backward arrow represents the iterative process of sensor selectionand strategy planning based on the given tolerances of the inspection feature. due to the large number of inspection features on multi-scaled devices under test, the realisation of the sensor selection in a manual way is very costly in terms of time, is based on the inspection planners know-how and can cause rough errors. the selection and assignment of capable sensors to the different inspection features of a microand nanostructured device under test is actually realised manually by the inspection planner. the inspection planner has to be well schooled in the technical characteristics of all available measuring instruments/sensors. for measurements on microand nanostructured components different approaches are established. but not only the geometry of the measuring feature determines the sensor selection. also structures in the work environment, which influence the accessibility, as well as materialand surface properties are crucial for the sensor selection [5]. furthermore, the bandwidth of possible application areas is not exhaustively investigated yet for some of the newer sensors. for simplifying the process of inspection planning and to reduce quality cost, it should be worked out if the selection of a capable sensor can be figure 1. setup of a cascaded multisensor system [3].  figure 2. general tasks of inspection planning.  figure 3. typical course of action for inspection planning [4].  acta imeko | www.imeko.org  august 2013 | volume 2 | number 1 | 76  realised automatically. in a first step an assistance tool for sensor selection should be prepared to check the user’s acceptance, too [6]. secondly, concepts for measuring strategies will be developed. additionally in consequence of the large number of inspection features in the microand nanoscale huge measurement data will be generated. this requires adequate hardand software systems for the data handling. 3. assistance tool for sensor selection  the main parts of inspection planning are especially the selection of a capable measuring instrument respectively sensor and the design of adequate inspection strategies. therefore, extensive interrelations between the sensors and possible inspection features and also detailed information on sensor parameterisation have to be implemented. for general measurements the automated sensor selection would require an immense database with regularities for the assignment of a capable sensor and for the sensor parameterisation. only for reiterative measurements, as usual in series production, the fully automated inspection planning is efficient and conceivable within custom-made measuring systems [4]. in a first step the methodology of sensor selection should be fragmented to prepare an assistance tool for inspection planning. from these items general rules should be deduced. the previous knowledge of sensors as well as materials and surfaces of devices under test should be provided by the assistance tool. if all information on available sensors is implemented, the sensor selection is a complex decision problem. in [7] an assistant system is presented which is divided into a hardware and a software system. the hardware assistant system uses general rules of thumb and expert knowledge whereby the software assistant system is based on simulations and uses neural networks and generic algorithms for the optimisation of the measurement. 3.1. acquisition of sensor‐specific properties  in dependence on the functional principle of the used sensors all for the inspection task relevant sensor-specific information should be determined and systematically classified. therefore the inspection task based measurement uncertainty should be analysed for the determination of operation area limits. operation areas of several sensors should be confined in relation to their application. assets and drawbacks of several sensors should be experimentally analysed by means of selected samples of inspection features. the technological prerequisites of measurements on micro and nanostructured quality features have to be defined. therefore material properties of the surface, technological manufacturing processes and the geometry of the inspection feature have to be taken into account. additional intrinsic properties of the sensor have to be determined for the selection of an adequate inspection equipment respectively sensor. within a database criteria for possible inspection features should be assigned to available sensors considering the specifications of the inspection feature. this database is the foundation of the automated sensor selection of microand nanostructured inspection features. often a main part of necessary information on the inspection feature can be taken from the cad-data. the sensor selection can be realised using an inference method based on the information about the measuring spectrum of each sensor stored in the database. compulsory selection criteria are in addition to the technical feasibility, the measurement range of the available sensors and the maximum permissible measurement uncertainty corresponding to the specification of the quality feature. the sensor selection should be realised comprehensible and reproducible according to logical defined rules. 3.2. knowledge‐based method  knowledge-based principles of decision theory are used for assigning inspection features and capable sensors. thereby constructive algorithms should be used for the concentration of information and for the realisation of automation. it is the aim to reduce the influence of the inspection planner insofar as equal measuring tasks yield to the same sensor selection. at the moment inspection planning is effected in dependence on the state of the planner’s knowledge. for this reason it is not enough to upgrade the accuracy of the sensors. a multitude of measurement-specific parameters has to be determined for predefining the measuring strategy, which is an essential part of inspection planning. the parameter setting has a considerable effect on the measuring result. on that account it is needed to develop approaches to inspection planning in which the preparation of the inspection plan is less dependent on the individual measurement technician’s state of knowledge. the sensor selection takes up an essential position within the knowledge-based inspection planning. the automation of sensor selection would reduce the measurement times, the measurement uncertainty and would advance the usability as well as reduce the planning effort. experiments to the measurability of different multistructured inspection features are realised as a first step to the central aim. in the following the stepwise procedural method of automated sensor selection using measurability experiments is itemised: a) definition, characterisation and classification of the inspection feature b) experiments to measurability on the available sensors/sensor systems c) evaluation of experimental results:  feasibility of the measuring task  documentation of capable parameter settings and inspection strategies  measuring condition requirements  expression of the measurement uncertainty  measuring time d) transfer of results in the database. based on the measurement experiments a systematic structure has been developed for automated sensor selection, which depends on possible criteria e. g. physical measuring principle, dimensionality of the inspection structure (direction sensitivity) and surface properties. figure 4 demonstrates one of the experiments, especially the measurement of a microlens. the multi-scaled approach becomes apparent. some inspection features especially the surface structure (figure 4d)) are measurable only with highresolution sensors. the external dimensions of the lens are measurable with standard image sensors, whereas special properties, for example the edge (figure 4b) and c)) are analysable only with high-resolution 3d-sensors. a software tool with the implemented systematic structure for sensor selection has been developed for the inspection acta imeko | www.imeko.org  august 2013 | volume 2 | number 1 | 77  planning support. the systematic of the software tool was verified based on test measurements. the software tool distinguishes between typical devices under test and non-established inspection features. for typical or rather established mechanical components e. g. silicon wafers, ball-grid arrays or injectors varied experiences for capable measuring strategies are available. for this, current literature researches into new measuring methods and measuring results were executed and experts were consulted. after selecting an established device under test, in the most cases, further properties for selection are offered. for nonestablished inspection features a systematic goal-oriented questioning-system is implemented, which is based on the principle of the decision-tree and yields to capable sensor selection. furthermore, the software tool includes an interactive help function, which illustrates the operation instruction of the tool and describes the principles of all available sensors and gives definitions of used technical terms. basically, both ways should yield to the same result in the proposal of capable sensors for the respective measuring tasks. 4. discussion of strategy concepts  the fundamental strategy of measurements with coordinate measuring machines is based on the analysis of single measurement points. these points are used to calculate geometry elements which approximate the inspection feature or serve as foundation for the calculation of the inspection feature. dimensional measurements are always associated with fitting algorithms for the appropriate geometry elements. therefore within the framework of inspection planning the coordinates of touch points are defined. in this connection the number and the position of the touch points are very decisive (radiusmeasurement in figure 2). this general inspection strategy is well-established. the most producers of coordinate measuring machines offer software applications for the inspection planning. the familiar software applications (calypso, quindos) support the import of cad-data for the definition of measuring points. using this kind of software, inspection planning can be realised automatically. in principle, this is common practise for tactileand image sensors and it is used for the most dimensional measurement tasks in the macroscale. that approved approach can not be translated by implication into the microand nanoscale. even if tactileand image sensors can be used, cad-data of the measuring object are not always available or there are excessive deviations between these data and the real unit under test. however, if the topography or also the structures are not known from the available cad-data, this can turn out to be a real challenge. here again, image processing may be helpful. the essential difference is that sensors with other measuring principles are used in the microand nanoscale. in the last few years scanning probe techniques have been rapidly developed. the advantage of scanning sensors is the very high flexibility for adaptation to different scanning areas. these probing systems can acquire measuring data only by scanning the surface sequentially and point-by-point. for measurements on different microand nanostructured step height standards uncertainties of less than 1 nm were reported [2]. the result of measurements with these sensors on smallscale coordinate measuring machines are huge 3d point cloud data. scanning sensors are e. g. the atomic force microscope (afm), the white-light interferometer and the laser scanning microscope. scanning force microscopes are able to measure surfaces with a very high vertical (< 1 nm) and a high lateral resolution (< 10 nm). when inspecting nanostructured features surface metrology and dimensional metrology melt together. exemplary at afm measurements the recorded raw measuring data have to be interpreted respectively deconvoluted according to the existing physical as well as geometrical interactions between tip and sample. otherwise wrong measuring results will be attained [3]. one great challenge of measurements with scanning sensors is to analyse the high volume of measurement data. the second problem is that the strategy used for tactile and image sensors is not capable [8]. whereas in the macroscale measuring points are defined before the measurement, by using scanning sensors the complete scanning area is measured. the result, a 3d point cloud has to be analysed (figure 4c)). initially it is unknown which points should be used for evaluating the measuring feature. and it would be hard going to select each point based on the image of the point cloud in particular. the available inspection planning software applications do not have the ability to handle such amount of data. the following approaches are possible ways to analyse the measurement data of scanning sensors.  segmentation: o a segmentation software delivers segmented point clouds with fitted geometry elements. figure 4.  examples of measurements on a micro lens; a) illustration of the lens (length = 9 mm); b) topography view of an edge on the lens; c) 3d‐view  of the edge; d) surface measurement with laser scanning microscope [4].  acta imeko | www.imeko.org  august 2013 | volume 2 | number 1 | 78  o relevant for the quality of the measurement is to answer the questions: “how good was the fitting process?”, respectively “which points are used for the final fitting?”.  comparative measurement: o state of the art is the comparative measurement by insertion of measuring lines in the reconstructed image of the point cloud. o the disadvantages of this method is that the measuring process can be realised only manually and based on this yields a high measurement uncertainty and there are no performance test results for the determined measurements of geometry elements. o the advantage is that there are numerous software applications mostly supported by the producers of the sensor.  cascaded measurements: o this method is based on overview images of the measuring scene as alternative to cad data. o local relevant scan areas are selected on the basis of the measuring tasks. o a complete scan is not necessary. hence there is a smaller number of measuring points for analysing. if all these ways will not be successful than functional tests which are usually executed after the assembly of the whole micromechanical product are an alternative way. the problem of inspection planning is, to solve the special measuring tasks directly connected with the quality of the part to be tested. however, every sensor presents various advantages and disadvantages. the selection of the best probe and the best strategy depends on several parameters of the measuring task [4]. 5. conclusions  there has been a constant improvement of measuring machines and sensors as well as of manufacturing processes. an essential contribution to the improvement in efficiency and usability can be fulfilled by the preparation of knowledge and rules for inspection planning especially for assigning capable sensors to current inspection tasks. the well known methods and procedures for inspecting macroscopic features respectively the working principles they stand for, was investigated regarding their applicability in inspecting purposeful features at microand nanostructured components. many of the known inspection strategies in dimensional metrology are not likely to be of use under these conditions but some may prove being very useful. the main result of this paper is the statement that automation of inspection planning yields to objectification of the measurement, reduction of the measurement uncertainty and the measuring time as well as the increase of usability and the reduction of the planning effort. topics of further research include methodologies for optimising the measurement strategy and sensor parameter setting using simulations. thereby the expression of the measurement uncertainty is essential for the inspection planning especially the sensor selection. acknowledgement  the presented work is the result of the close interdisciplinary research within the sfb 622, which is located at the tu ilmenau, germany, and funded by the german research foundation (dfg). references  [1] j. fleischer, j. peters, i. buchholz, “bewertungssystem zum vergleich und zur auswahl von sensoren in der multisensorkoordinatenmesstechnik“, proc. of mikrosystemtechnik kongress 2007, oct. 15-17, 2007, dresden. [2] e. manske, t. hausotte, r. mastylo, t. machleidt, k.-h. franke, g. jaeger, “new applications of the nanopositioning and nanomeasuring machine by using advanced tactile and non-tactile probes”, meas. sci. technol. 18 (2007) pp. 520-527. [3] s.c.n. toepfer, g. linss, u. nehse, “automatic execution of inspection plans for knowledge-based dimensional measurements of microand nanostructured components”, proc. of 18th imeko world congress, sept. 17-22, 2006, rio de janeiro, brazil. [4] k. weissensee, “knowledge-based inspection planning for multiscaled quality testing”, proc. of joint int. imeko tc1 + tc7 + tc13 symposium, aug. 31-sept. 2, 2011, jena, germany. [5] j. fleischer, et al. “erfassung von standardgeometrieelementen im mikrometerbereich“, techn. messen 75 (2008) 5 / doi 10.1524/teme.2008.0875, oldenbourg publishing pp. 327-338. [6] i. buchholz, “strategies for quality assurance of micromechanical components with multi-sensor coordinate measurement technique”, shaker, 2008. [7] a. burla, t. haist, w. lyda, m. h. aissa, w. osten, “assistance systems for efficient multiscale measurement and inspection”, proc. spie 8082, 808202 (2011), doi:10.1117/12.889344. [8] e. manske, g. jaeger, t. hausotte, “combination of multisensor technology and multiple measurement strategies in micro and nanometrology”, proc. of ismqc-2010 10th international symposium on measurement an quality control, sept. 5-9, 2010, osaka, japan. acta imeko  september 2014, volume 3, number 3, 28 – 32  www.imeko.org    acta imeko | www.imeko.org  september 2014 | volume 3 | number 3 | 28  imaging interference microscope for nanometrology   igor malinovsky 1 , iakyra b. couceiro 1  , ricardo s. franca 1 , mauricio s. lima 1 , carlos l. s. azeredo 1 ,  clara m. s. almeida 1 , jean p. von der weid 2   1  national metrology institute of brazil (inmetro), av. n.s. graças, 50, xerém, rio de janeiro, 25250‐020, brazil  2  telecommunication center (cetuc), catholic university (puc‐rio), marquês de s.vicente, 225 – gávea, 22453‐900, rio de janeiro, brazil      section: research paper   keywords: nanometrology; interference microscopy; afm; gauge block; step height  citation: igor malinovsky, iakyra b. couceiro, ricardo s. franca, mauricio s. lima, carlos l. s. azeredo, clara m. s. almeida, jean p. von der weid, imaging  interference microscope for nanometrology, acta imeko, vol. 3, no. 3, article 7, september 2014, identifier: imeko‐acta‐03 (2014)‐03‐07  editor: paolo carbone, university of perugia   received april 1 st , 2013; in final form march 28 th , 2014; published september 2014  copyright: © 2014 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: this work was supported by the national research council of brazil (cnpq/ prometro) and also by faperj research project, brazil  corresponding author: igor malinovsky, e‐mail: imalinovski@inmetro.gov.br    1. introduction  traceable nanometrology of high accuracy is a necessary condition for the quality control basis of nanotechnology. in its broad understanding, nanometrology is the science and technology of measurements of the artifacts with nanometric accuracy. since nanotechnology plays a more and more important role in the industry, it is the task of the metrology to provide better measurements to support the productive activity. in this respect, nanometrology at the national metrology institute (nmi) of brazil, inmetro, optical division (diopt) has started with the development of high resolution gauge block interferometry (gbi) about 10 years ago [1]. the gbi instrument was successfully characterized and used in several international comparisons. the next obvious step now is to apply the knowledge in interferometry for measurements of smaller objects like step heights. interference microscopy (im) is commonly used for this task being quite accurate [2]. im is preferable to several known techniques of nanometrology because it provides direct traceability to wave length standards. im can perform measurements directly related to wave length standards such as frequency stabilized lasers. consequently, im can be used for calibration of the secondary standards such as step heights. after calibration of secondary standard, it can be used further to calibrate the vertical axis of the afm. therefore, the final goal of this work is to provide afm measurements being traceable to the primary standards of length. 2. experimental set up and samples   the optical scheme of the constructed linnik type interference microscope based on a frequency stabilized laser as a reference wave-length source is shown in figure 1. all components were placed on an optical breadboard which was placed on the top of a granite table. most of the optomechanical units that are used are commercially available components manufactured by newport and newfocus. an optical beam-splitter of high quality (lambda over 100, from bernard hole) with very low beam distortion was used to provide the highest possible accuracy of the interferometer. abstract  here we report the development of the primary nanometrology capacity at the national metrology institute of brazil (inmetro). an  interference  microscope  of  the  linnik  type  has  been  developed  and  subjected  to  optimization  and  characterization  studies.  the  recording  of  the  fringes  is  done  by  an  automated  ccd  system  with  2  possible  processing  approaches:  interferometric  pattern  processing and the phase stepping technique. current progress in the development of the hardware and software adequate for sub‐ nanometer  resolution  of  the  instrument  is  reported.  a  study  of  systematical  errors  of  the  interference  microscope  has  been  performed. the instrument is aimed for international key comparisons of step height standards. acta imeko | www.imeko.org  september 2014 | volume 3 | number 3 | 29  the employed laser source was a he-ne frequency stabilized laser (spectraphysics or agilent. all used lasers were calibrated with respect to the primary he-ne iodine stabilized laser (primary standard), verified via bipm intercomparison. fringe patterns were collected by a high quality scientific grade digital ccd 1.5 megapixel camera of 12 bits resolution from pco, germany. a sophisticated zoom eyepiece from hirox, japan was used to magnify the output interferometric images. the frames were taken by a pc and processed with specially developed and tested dedicated fringe pattern processor software (sw). the reference mirror was located on a 3-way tilt-move phase shifting unit. the main measuring method is the interferometric fringe image processing. we also have included into the instrument the phase stepping module which allowed for an additional comparative study of both methods. from our previous experience with common gbi, the phase stepping unit can be reliably calibrated “on-flight” during the measurement if multiple steps and respective fringes are available (see details below). during the measurement we perform phase shifting and simultaneous recording of the ccd output in series of the interferograms (typically 200 or more images). each pixel in the image corresponds to a certain region (point) on the sample (or on the reference mirror). the frame-by-frame analysis of each pixel of interest provides the corresponding sinusoidal signal that carries the phase information of this particular point (pixel). this output is the so called multiple-step phase shift signal [3]. we fit this signal with the model to extract the phase at each point of interest. we apply the simplest method of phase shifting, i.e. the movement of the reference mirror. however, in case of the linnik interferometer, it is not really the best solution, because by moving the reference mirror the optimum identical optical path in both arms of the im is disturbed. we plan to use an optical wedge for this purpose in future studies. in here we consider that both the image processor and the phase step processor have their own advantages and disadvantages. in this work we discuss the advantages and disadvantages of the image processor and phase step processor. having both at one´s disposal it is possible to perform more detailed investigations. we used the mitutoyo triple step gauge block (step master) as the secondary z height standard. the step master is a master secondary standard used for the z-axis (vertical direction) calibration of optical and stylus instruments. the standard is made of three interconnected gauge blocks of different heights as shown in figure 2. the choice of this particular standard was carefully considered from the point of view that it is the only artifact that can be measured by both classical michelson type gb interferometry and microscopic methods such as afm and im. the materials division of inmetro (dimat) is in the process of production of more common step height standards similar to those used in [2]. they use the electron microscope lithography method. we have got quite satisfactory results with secondary standards from dimat/inmetro. preliminary studies of the reference secondary step master standard, performed with a zeiss interferometer, have shown that the artifact exhibits quite flat surfaces of all three gauge blocks. among the other advantages of this type of secondary standard we should mention high long time stability and negligible difference in surface roughness (known as the phase change correction) between the blocks produced from the same material and polished to the same texture finish. the nominal height differences between gauge blocs 1 and 2 (gb step), and between gauge blocs 2 and 3 were 10 µm and 2 µm respectively. 3. fringe pattern processing   we have developed a set of software tools suitable for fringe image processing, phase shifting interferometry and post processing data visualization. the main software (sw) module of fringe pattern processing is based on a multi-parametric iterative fit of the digitized pattern along the vertical line (or several neighboring lines). prior to the fit, it is possible to perform a direct / reverse fft combined with a gaussian   figure 2. interferogram of triple gb standard taken with standard zeiss gbi.  on the bottom, is the layout of the triple gb step master standard.    hene stabilized laser anti‐speckle telescope isolator bs mirrors objectives ref mirror object ccd phase shifter, move‐tilt pzt variable zoom ccd eyepiece figure  1.  set  up  of  the  linnik  type  interferometer,  where  bs  is  the  high quality  50/50  beam  splitter;  object  is  the  step  height  or  master  height  standard; objectives are the conformal pairs of x10, x20, etc. microscopic objectives; ref is the reference mirror  located on a phase shifting tilt‐move  pzt module; eyepiece is the up to x20 variable zoom optical element.  acta imeko | www.imeko.org  september 2014 | volume 3 | number 3 | 30  filter. this filter does not perturb the phase and it is used to remove pixel noise from the interferometric pattern. in addition, pixel noise was removed by averaging of several frames to produce the final interferogram used for processing. the model sine function has the following fit parameters: amplitude, offset, phase, frequency, phase modulation and amplitude modulation. a minimization criterion is least square difference between measured data and a model function. by analyzing the fringe pattern along several lines instead of just two, one can find out the topography of the master step gb object. previously it has been shown that using our fringe processing algorithm the resolution of the interferometer can be as high as 0.1 nm (about λ/6000) or better. the accuracy of the measurement is determined by the quality of the gb and it can be as good as 1 nm [1, 4]. figures 3 and 4 show the quality of the fringe pick-up and the fit using our algorithm. it is worth noting here that pixel and intensity (bits) resolution as well as stability/uniformity of the ccd is quite essential for obtaining good results. in order to compare measurement with atomic force microscopy (afm) we have measured the same secondary master standard using a commercial witec alpha 300 afm. afm measurements cannot cover the large xy area of the gb step (the maximum area in our case was 100x100 µm). in addition, because of a slight curvature of blocks, it is desirable to compare several measurements performed in different places of the standard along the step direction. figure 5 shows a typical comparative plot of two data sets obtained in two different locations. for detailed comparison of the different measurements we have developed dedicated 3d software that allows a visualization of several surfaces in one screen with interactive virtual reality style rotation, pan and zoom. several mouse driven measurement tools are available for the analysis of the intersection areas and volumes altogether, the proposed approach to data analysis allows one to examine in depth the obtained data in a comparative and selective way, as exemplified in figure 5. 4. phase shifting method  we have developed procedures suitable for a detailed analysis of the phase shifting technique. the phase shifting unit is based on a comprehensive 3-dof tilt-move stage. each element is independently programmed via a 24 bits digital-toanalog converter (dac) generating signals from -10 v to +10 v, and amplified with a low-noise 3-channel hv amplifier. after amplification, the signal is suitable for pzt operation (100 v to +100 v). in this way, we can program independently all kind of movements aiming at compensation of nonlinear effects of the pzt. we move the reference mirror with 100 or more (typically 200) equal distance intervals. at each mirror position the full figure 4. variation of the experimental (solid) and calculated (dash) fringes intensity as a function of pixel number. typical relative differences between the experimental and calculated intensities are less than 1%.  figure  3.  the  fringe  pattern  of  10  µm  gb  step  taken  with  the  linnik interferometer.  vertical  dashed  lines  are  the  eye  guides  of  the  regions (trajectories) used to digitize the interferometric pattern.  figure 5. comparison of the data with afm in 3d mode. an interactive cube is used to select the area of interest for parameter calculation. planes are  set to define the cross‐section of the 3d afm image and the respective 2d  plots (upper figure). the lower image shows the z intersection plane which is  used  for  detailed  comparison  of  surface  topography  of  two  measurements (left and right surfaces in the lower picture).  acta imeko | www.imeko.org  september 2014 | volume 3 | number 3 | 31  interferogram is taken and saved into the memory of the pc. after having finished this process, it is possible to process the interferometric image stack at each surface point in order to reconstruct the phase for the whole measured surface. further, to remove vibration related noise, we normally take several interferograms, averaged at each position of the phase-shifter. to improve the exactness of the phase step we can optionally make several back-forward mirror positionings to average the interferograms. these improvements resulted in the best scans obtained so far. each individual scan typically produces several interferometric fringe lines that correspond to a pixel (or area of pixels) position. each line is fitted with an algorithm similar to the one shown in figure 4. normally, we do not use a single pixel, but rather a few neighboring pixels to average out the pixel-to-pixel noise [5]. alternatively, it is possible to process individual interferograms in the way described hereinabove. consequently, one has an opportunity to compare both algorithms: the fringe pattern and the pure phase shifting method. the results obtained show that after all above precautions the signal to noise ratio permits a 0.1 nm (or even better) resolution of the system. this conclusion has been confirmed by multiple repeatability tests (see examples in figure 7). 5. study of the systematic errors  one of the most important and difficult tasks of the primary comparators is a detailed characterization of the instrument for the subsequent uncertainty evaluation. this task implies measurement of the systematic errors of the instrument. while the carl zeiss interferometer was already studied in this respect [4], the new linnik interferometer investigation and characterization is ongoing work of our laboratory. the important information concerning possible drifts of the interferometer read out can be obtained by automatic continuous measurements of the length during changes in environmental conditions such as temperature. our interferometer is fully automated and permits this type of measurements. the systematic drift is shown in figure 8. the correlation was expected and observed. we attribute temperature induced drift to possible changes in optical path of the measuring or reference part of the interferometer. dilatation of the standard itself can create similar drift. the most serious errors of the linnik type interferometers is known to be misalignment of the reference and measuring arms [2]. as our interferometer software permits to control misalignments, we have developed a simple but very efficient self-calibration procedure. in this procedure we measure the gb step and immediately after that calibrate the fringe pattern on the reference flat surface of the gauge block. we can optionally use flat surfaces of both gauge blocks in the master standard to perform such self-calibration. if the surface of the gauge block is flat this, in the ideal case, will remove misalignment errors. the idea of self-calibration of the interferometer relative to the flat part of the surface is valid for both the image processing and the phase step method. we applied this idea in both methods to obtain the final results of this work. we have performed several measurements on an interferometer with deliberately slight misalignment. some of the results are presented in figure 9. the typical spread of misaligned data was about 100 nm. while after applying the correction according to the above selffigure.7.  reproducibility  of  the  corrected  phase  step  readings.  data  were taken  during  a  period  of  about  4  weeks.  the  data  sets  correspond  to different  alignments  of  the  interferometer.  lines  are  the  means  of  each series. standard deviation between means in the series is about 30 pm. figure 6. fringe pattern of 100 nm aluminium step height taken with the linnik im. the xy size of the square height is about 30x30 µm. gaussian 2d filtering was applied to the whole image. the horizontal dashed line is the eye guide for the line area used to provide the measurement.   0 0.2 0.4 0.6 0.8 1 1.2 0 50 100 150 200 250 300 l e n g th d e v ia ti o n , n m 20.06 20.08 20.1 20.12 20.14 20.16 0 50 100 150 200 250 300 t e m p e ra tu re , d e g c º mesurement number figure  8.  correlation  of  the  temperature  induced  drift  with  measured  length.  the  data  are  composed  of  about  300  measurements  performed during several hours of continuous read out.  acta imeko | www.imeko.org  september 2014 | volume 3 | number 3 | 32  calibration procedure the data spread is within 1 nm. this shows the benefit of the self-calibration method and the prospects of the instrument under construction. the obliquity or aperture error was estimated theoretically from the geometry of the optical set-up of our im. with our nikon plan x10 long focus microscopic objectives the maximum angle of incidence on the sample is about 20 degrees, which is significantly decreased by the camera lens x20 eyepiece. the eye-piece restricts the aperture; as a result, the angles of incidence decrease correspondingly. estimations of the effective angle of incident give values of about 3 to 4 degrees. this means that the possible obliquity error, as calculated from standard interferometry formulae, is about 80-90 pm on a 100 nm height step. in order to check experimentally these estimations, we have restricted the entrance aperture of the im down to about 2.5 degrees using a variable diaphragm at the entrance of the im interferometer. the measurements of the length of the 100 nm step artifact have been performed according to the procedure described above with and without diaphragm. the result shows that the obliquity error is nondetectable within 100 pm uncertainty. therefore, we consider that for k=2 the uncertainty associated to this particular error type is not higher than 0.2 nm. this is well within the targeted final im uncertainty. 6. results and discussion  in general, a comparison of the results of the different types of instruments is not straightforward because each instrument has its own advantages and restrictions. phaseshifting method has itself certain problems in terms of errors [5]. in case of the step gb, the main concern is the measured area. while the step gb is quite flat on the probed areas recommended by mitutoyo, it is not that flat close to edges of the block which are exactly the only regions where microscopic instrumentation can be used. by using the carl zeiss interferometer, closest approximation to the edge cannot be less than a certain distance because: i) a line thickness of several pixels is required for the analysis, ii) diffraction at the edge of the gb distorts the interferogram making interpretation uncertain. in our case, the minimum approximation to the edge preserving quality of the fringe was about 0.3 mm. reliable results from afm were achieved with an xy range of about 50 µm. usually, the repeatability of all instruments was better than 1 nm, which indicates that all of them can be, in principal, calibrated or studied for systematic errors to even better accuracy. interesting results were obtained with the phase step method. here we have observed much better reproducibility of the output, as compared with the interferometric pattern processor (see figure 7). study of systematic errors as, for example, due to nonlinear effects in the pzt is currently the ongoing research activity in our lab. the results obtained were in agreement within 2-3 nm between the data obtained by the mitutoyo gbi, the linnik interferometer and the zeiss interferometer. nevertheless, the difference between the interferometric and the afm measurements was a bit higher, up to 20 nm in the worst case. we attribute these discrepancies, for the moment, to possible problems of afm instrument. one should recall that the systematic errors of the afm are significant for a large step height. metrological afm is used to remove those errors. due to the nonlinearity of the pzts the error increases with increasing range (z range in this case). in general, we were satisfied with the obtained results. obviously, a commercial afm of this kind is by itself not an easy instrument and should be studied for systematic errors, especially in large scan ranges. acknowledgement       we acknowledge the help and technical assistance of dr. alexei kuznetsov (inmetro). we appreciate technical assistance of miguel a. c. torres, natacha c. e. pereira, maicon s. bessa. this work has been supported by the national research council of brazil (cnpq/ prometro) and also by faperj research project. references  [1] a. titov, i. malinovsky, c. massone, gauge block measurements with a nanometer uncertainty, metrologia 37 (2000) pp.121-130. [2] l. koenders, r. bergmans, j. garnaes, j. haycocks, n. korolev, t. kurosawa, f. meli, b. c. park, g. s. peng, g. b. picotto, e. prieto, s. gao, b. smereczynska, t. vorburger, g. wilkening, comparison on nanometrology: nano 2—step height, metrologia 40 (2003) tech. suppl., 04001. [3] p. jia, j. kofman, c. english, “multiple-step triangular-pattern phase shifting and the influence of number of steps and pitch on measurement accuracy, applied optics 46 (2007) pp.3253-3262. [4] a. titov, i. malinovsky, h. belaïdi, r.s.frança, c. massone, advances in interferometric length measurements of short gauge blocks, metrologia 37 (2000) pp.155-164. [5] k. creath, errors in phase-measuring interferometry, proc. of spie 1720 (1992) pp. 428-435.   figure  9.  misalignment  error  of  the  interferometer.  non‐corrected  measurements  (diamonds)  are  shown  together  with  the  respective  data  after  correction  (squares).  standard  deviation  of  the  corrected  values  is about 0.7 nm.   this particular data was obtained with the  interferometric image processing method.  a practical approach to evaluate the measurement uncertainty budget for calibration of strain gauge based high-precision carrier frequency amplifiers acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 210 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 210 215 a practical approach to evaluate the measurement uncertainty budget for calibration of strain gauge based high-precision carrier frequency amplifiers marco m. schäck1 1 hottinger brüel & kjær gmbh (hbk), darmstadt, germany, marco.schaeck@hbkworld.com abstract: for high-precision measurements of strain gauge-based transducers, 225 hz carrier frequency measuring amplifiers are primarily used [1]. the benefits of this carrier frequency method were discussed in previous publications. this publication shows the measurement uncertainty that can be achieved by calibrating an amplifier based on this method. possibilities for improving the measurement uncertainty and the physical limit from the user's point of view are shown. keywords: measurement uncertainty, strain gauge, carrier frequency, physical limit, autoand background-calibration 1. introduction the true value of something cannot be measured because every measurement contains errors. a measurement is therefore only complete when an estimation of the measurement uncertainty is assigned to each measured value [2]. this is essential, especially when calibrating high-precision measuring amplifiers. it is therefore important to understand the measurement uncertainty related to a calibration of a strain gauge-based amplifier. in this paper, a practical approach is shown to determine the measurement uncertainty for calibrating a high-precision strain gauge-based carrier frequency amplifier. by using a bn100a bridge standard, the calibration of an mx238b precision amplifier of the hbm quantumx product family with a specified class accuracy of only 0.0025 (25 ppm) is considered in detail [3]. the influencing parameters for the measurement uncertainty calculation are listed. the exact order of magnitude and probability distribution are determined, to obtain a resulting overall measurement uncertainty. finally, the largest uncertainty contributions in the overall uncertainty budget are considered in order to identify possible optimisations. 2. calibration setup before calibrating a measuring amplifier, it is necessary to precisely define the measuring setup. if possible, influencing factors regarding the measurement uncertainty should be eliminated or reduced as much as possible. the most accurate bridge calibration standard currently commercially available worldwide, the hbm bn100a, is therefore used to calibrate the high-precision amplifier mx238b. figure 1shows the calibration setup with the used components. first, a calibration certificate is required for the traceability for the individual bridge calibration figure 1: setup of measurement for calibration of the mx238b http://www.imeko.org/ mailto:marco.schaeck@hbkworld.com acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 211 standard. a direct traceability to a national reference standard at a national metrology institute (nmi) generally provides the best available uncertainty [4]. alternatively, the used bridge standard can also be traced back to a calibration laboratory (e.g. dkd german calibration service), which is directly connected to the national metrological institute. the possible measurement uncertainty is in this case greater with respect to the uncertainty of the nmi. if the calibration of the bridge calibration standard also includes the necessary measuring cable (1-kab0238b), then this is also included in the specified measurement uncertainty in the calibration certificate. the calibration certificate is in this case valid for the end of the measuring cable. the used bridge standard in the measurement setup is connected to the measuring amplifier with an adapter (1-kab144). boundary conditions of the measurement, including the ambient temperature and humidity, must also be defined. for this exemplary consideration, default workplace conditions at hbm are assumed. it is assumed that the temperature can vary between 19 °c and 26 °c during a complete year. these environmental conditions are continuously monitored. the measurement is only carried out after all boundary conditions are met and a steady state has been reached within the specified tolerances. 3. considered uncertainty components after defining the calibration setup, the factors that may cause unwanted changes or variations to the measurement results must be identified. figure 2 shows the influencing uncertainty contributions divided into three main groups, namely calibration standard, calibration object and calibration method. figure 2: grouping of the uncertainty contributions the process to define the uncertainty model is not always easy, because any influences can affect a measurement result. a useful way to visualise how different sources of measurement error contribute to the overall measurement result is the ishikawa diagram. a first step is to evaluate the information in the technical data sheets of the devices involved in the calibration [5][6]. if some influencing factors cannot be found in the data sheets, they must be measured individually or requested from manufacturers. influencing factors must never be neglected. figure 3 shows a simplified diagram with the influencing parameters when calibrating a high-precision measuring amplifier. figure 3: ishikawa diagram for the measurement uncertainty 3.1 calibration standard the calibration standard is the reference for the calibration which is traced back to national standards. regarding the calibration of a high-precision strain gauge-based amplifier, one main influencing factor will be the measurement uncertainty of the calibration certificate of the nmi for the bridge calibration standard. the expanded measurement uncertainty of the calibration certificate for a bn100a carried out by the ptb (physikalisch-technische bundesanstalt, ratio and sampling techniques, working group 2.12, germany) is indicated with 11 nv/v (4.4 ppm) for a nominal value of 2.5 mv/v and an excitation voltage of 5 vrms. it is mentioned that the measurement uncertainty specified in calibration certificates is usually including an expansion factor 𝑘 of 2 and corresponds to a normal distribution. if there are explicit calibration values in the calibration certificate for all used steps and polarity combinations of the bridge calibration standard, a step and polarity error has not to be considered. likewise, the measurement cable has also not to be considered in the measurement uncertainty, if the calibration certificate is valid at the end of the measurement cable. in this way, these uncertainty factors can be eliminated. if the measuring cable is not considered in the calibration, the influence can be estimated based on a previous publication [7]. another influencing group regarding the uncertainty of the calibration standard is the drift and stability. without explicit specification in the data sheet, the hbm class accuracy of the bridge calibration standard bn100a of 0.0005 applies to zero point and full-scale values (in this case http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 212 < 5 ppm / 10 k each). regarding the humidity and long-term stability, for which the error is depends on the calibration interval, no information is given in the data sheet. the humidity influence can be found in publications from the ptb [8]; internal hbm measurements show a very similar behaviour. the influence of the humidity from 35 % to 80 % at 22 °c is very low (non-condensing) and assumed to be less than 6 nv/v (< 2.4 ppm) the maximum relative humidity and temperature effect is considered to be very low, which means that the device is very stable under relative humidity or temperature changes. the outstanding long-term stability of the bn100a is assumed to be less than 5 nv/v (< 2 ppm) per year. long-term stability data can be found in previous publications [9][10][11]. the influences of the bridge excitation voltage and the source and sink impedances are negligible and mentioned only for completeness [12]. for example, a slight dc component has no influence on the measurement result due to the carrier frequency method [13][14][15], and a variation of the bridge excitation voltage by a few percent has no measurable influence (< 2.8 nv/v per volt of excitation voltage) on the output signal (mv/v) of the bridge calibration standard. due to the active regulation of the individual divider stages of the bridge calibration standard the frequency response is relatively flat and the phase nearly zero over a very wide frequency range. figure 4 shows a rough overview of the amplitude and phase response of the bn100a on a logarithmic scale in db. figure 4: frequency response of the bn100a to determine the exact frequency dependency in the ppm range around the carrier frequency, measurements were carried out with a modified carrier frequency measuring amplifier mx238b. with this modified amplifier, the carrier frequency can be changed in fine frequency increments. slight possible frequency variation of the bridge excitation voltage around the carrier frequency of 225 hz has no effect to the calibration signal output, even in the ppm range. for 225 hz carrier frequencies outside the normal frequency range of the carrier frequency (other carrier frequencies e.g. 600 hz) the bn100a must be individually modified and at least be tuned to resonance (bn100a is only specified for 225 hz). a final important aspect is the noise of the bridge calibration standard. the noise of the bn100a, which is very low by design, is not specified in the data sheet and it is very difficult for users to measure it (separate the noise of the bridge calibration standard and the noise of the amplifier). a comparison of a passive 350 ω full bridge transducer with the bn100a for a supply of 5 vrms and a detuning of 2.5 mv/v shows no difference on the mx238b in terms of noise. in any case, it is not important for the measurement uncertainty of the calibration whether the noise comes from the bridge calibration standard or from the measurement amplifier. the total noise is of importance. this can simply be determined in the complete measuring chain. therefore, the total noise is not considered as individual components, but is assigned later to the calibration process as a sum. 3.2 calibration object in comparison to the bridge calibration standard, the number of parameters to be considered for the measuring amplifier is significantly lower (only if the amplifier is the object to calibrate). the temperature and humidity influence on the mx238b measuring amplifier is very low. the temperature and humidity are not important for the calibration uncertainty and are in the calibration certificate only for documentation. because of the excellent data from the measuring amplifier, due to the low sensitivity coefficient (partial derivative), an uncertainty in the measurement of temperature and humidity (external sensor, traced back) would not make a meaningful contribution to the measurement uncertainty. the long-term stability of the measuring amplifier is also not important for its calibration. the measuring amplifier must be stable during the calibration measurement, which is guaranteed by the integrated and patented background calibration of the mx238b after the running in phase [16][17]. however, it must be checked whether the input impedance of the measuring amplifier influences the calibration signal of the bridge calibration standard. taking into account the impedances (output impedance of the bridge standard, input impedance of the measuring amplifier), the conclusion is that the input impedance has no influence to the calibration signal, due to the very high-impedance of the measuring amplifier input [18]. the noise of the amplifier specified in the data sheet is irrelevant, because the noise is assigned to the calibration process as total noise. -60.0 -30.0 0.0 30.0 60.0 -35.0 -30.0 -25.0 -20.0 -15.0 p h a se [ °] a tt e n u a ti o n [ d b ] frequency [khz] frequency response bn100a attenuation phase http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 213 3.3 calibration method the final group, the calibration method parameters, includes the total noise, consisting of the noise of the calibration standard and the measuring amplifier. because no information for the noise is available for this special measuring chain, the total noise must be determined for this application by separate measurement. it is easy to determine the noise for the worst calibration point and then use this result as an uncertainty contribution for all other calibration points. the worst case is the largest calibration value of the measuring range. in this case, the noise contains not only the noise of the measuring amplifier input, but also noise of the bridge excitation voltage, the reference voltage of the analogue-digital converter of the amplifier and the noise of the active circuit electronics of the bridge calibration standard. ideally, these additional contributions are not significant and the noise is independent of the calibration value (constant over the complete measuring range). figure 5 shows the noise over time for the calibration setup. figure 5: noise measurement for a combination of mx238b and bn100a to determine the noise, a measurement is recorded over a sufficiently long interval with a defined low-pass filter (e.g. bessel 1 hz -3 db). this is the filter used for calibration of the amplifier. the standard deviation σ is to be determined from the recorded measured values and is in this case 2.3 nv/v (0.9 ppm) [19]. the standard deviation corresponds to the effective noise of the complete measuring chain and can be used for the measurement uncertainty analysis. the hypothesis is made that the noise is normally distributed. this hypothesis must be checked by determining a histogram or by a statistical test for normal distribution. due to the low cut-off frequency of the filter and the associated very low noise of the measured value the measured value has to be filtered digitally to get a higher amplitude resolution in a post process to have a higher bin resolution in the histogram. otherwise, the noise cannot be displayed with a sufficient bin resolution for the histogram. figure 6 shows the histogram of the noise for a modulation of the bridge calibration standard of 2.5 mv/v. figure 6: noise histogram for a combination of mx238b and bn100a similar to the case of noise, there is no information for the reproducibility for this special application. the considered mx238b precision measuring amplifier uses the proven 225 hz carrier frequency method and a six-wire circuit. the advantage of this method has been explained in a previous publication [20]. influences such as thermal/contact voltages due to different materials of the pins, other offset effects and contact resistances on the connectors are suppressed. the carrier frequency method also enables the use of temperature-stable and long-term-stable inductive bridge standards, such as the bn100a bridge calibration standard. even with the best 225 hz carrier frequency technology and the use of the six-wire circuit, there must be a check of how reproducibly identical measurements are. for this, several identical measurements are repeated under the same boundary conditions (in this case fifty independent measurements). comparable to the noise measurement, the type of distribution must also be determined for the reproducibility measurement. figure 7 shows the distribution of the repeatability. this uncertainty contribution results in a normal distribution with an uncertainty of 1 nv/v (0.4 ppm). the repeatability test is performed once, not for every calibration. for a generalised sampling repeatability data are collected and the result is used for all similar subsequent calibrations. 2,500000 2,500010 2,500020 2,500030 a m p li tu d e [ m v /v ] time [s] noise measurement of mx238b and bn100a (2.5 mv/v @ 5.0 vrms for 1hz filter) 0.0 0.2 0.4 0.6 0.8 p ro b a b il it y d e n si ty [ % ] amplitude [mv/v] noise histogram / distribution http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 214 figure 7: repeatability distribution of calibration results as previously mentioned, the calibration certificate for the bridge standard is valid at the cable end of the bridge standard. adapter cables are often necessary for the current plug-in system of the measuring amplifier. the influence of the adaptation must be determined for this. this can be estimated by measuring several cascaded adaptations. measurements in this scenario showed no significant effects for the required adaptation (1-kab144). 4. combined measurement uncertainty the complete measurement uncertainty parameters have been collected in a table with the corresponding distribution type. the considered measurement uncertainty components are uncorrelated, therefore the geometric addition of these measurement uncertainty components gives the total measurement uncertainty (combined all individual a and b types of uncertainties), as shown in equation (1). 𝑈 = 𝑘 ∙ √∑ 𝑈calibrator 2 + ∑ 𝑈object 2 + ∑ 𝑈method 2 = 16.8 nv v =̂ 6.7 ppm (1) if this is expanded by the factor 𝑘 of 2, the expanded measurement uncertainty is obtained. a coverage factor of 𝑘 equal to 2 would indicate a confidence of approximately 95 %. 5. conclusions in this paper, all the main factors influencing the calibration uncertainty of a 225 hz precision carrier frequency measuring amplifier were discussed. mainly due to the carrier frequency method and the six-wire circuit, only a few factors can (significantly) influence the measurement uncertainty. figure 8 shows the main influencing factors previously explained based on the measurement uncertainty amounts. the largest uncertainty contributions are of a very similar order of magnitude. this includes the temperature effects. the temperature and humidity effects can be further reduced by a more precise climatic environmental condition, as in calibration laboratories. the long-term stability used in a measurement uncertainty analysis can be based on the class accuracy of the bridge standard bn100a. this effect of the long-term stability of the bridge calibration standard bn100a could also be determined for an individual device and, if necessary, a recalibration could be done in a smaller interval. this measurement uncertainty contribution could thus be reduced in the uncertainty analysis. measurements have shown that the long-term stability of this bridge calibration standard is typically significantly lower than the value specified in the data sheet. figure 8: parts of the measurement uncertainty the noise of the entire measuring chain can also be further reduced by changing the digital low-pass filter. the noise is expected to reduce with the square root of the bandwidth (valid for white noise). since all influencing factors of the measurement uncertainty are uncorrelated and therefore are added geometrically, possible optimisations do not lead to a significant reduction in the overall measurement uncertainty. a large reduction is not possible due to the given measurement uncertainty in the calibration certificate of the bridge standard bn100a. if it would be possible to eliminate all influencing factors except for the measurement uncertainty of the calibration certificate of the bridge calibration device, the measurement uncertainty could only be reduced by a maximum factor of around 1.5. optimisations only marginally improve the overall measurement uncertainty. this shows that the 225 hz precision carrier frequency measuring amplifier mx238b from hbm measures at close to the achievable limit. a new measurement uncertainty analysis with other influencing factors must be created to consider a measuring chain out of a transducer and measuring amplifier. 6. references [1] k. hoffmann, an introduction to measurements using strain gages, hbm, darmstadt, germany, pp. 171-179, 1989. 0,0 10,0 20,0 30,0 40,0 p ro b a b il it y d e n si ty [ % ] amplitude [mv/v] repeatability distribution of calibration results for 2.5 mv/v bn100a calibr. certificate (nmi) bn100a humidity error bn100a temperature (zero drift) bn100a temperature (full-scale drift) noise (compl. chain) bn100a long time drift (per year) 0.0 5.0 10.0 s ta n d a rd u n ce rt a in ty [ n v /v ] uncertainty component parts of the measurement uncertainty (calibration point 2.0 mv/v 5vrms 1 hz bessel filter) http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 215 [2] joint committee for guides in metrology (jcgm/wg 1), “guide to the expression of uncertainty in measurement” (gum), 2008. [3] m. kreuzer, “kalibrieren des digitalen präzisionsmeßgeräts dmp39 mit einem speziellen brückennormal”, messtechnische briefe (hbm) issue 17, darmstadt, germany, pp. 67-73, 1981. [4] g. ramm, “kalibrieren von brückennormalen für die dehnungs-messstreifen-messtechnik”, messtechnische briefe (hbm) issue 24, darmstadt, germany, pp. 10-16, 1988. [5] hottinger baldwin messtechnik gmbh, datasheet bn100a. [6] hottinger baldwin messtechnik gmbh, datasheet mx238b. [7] m. schäck, “underestimated impact of measuring cables on high-precision carrier frequency amplifier results and compensation methods therefor”, proceedings of xxi imeko world congress, prague, czech republic, 2015. [8] k. khaled, d. röske, (eds.), “humidity and temperature effects on torque transducers, bridge calibration”, measurement; 2015. [9] a. schäfer, h. kitzing, “dmp41 a new chapter of ultra-precision instrument for strain gauge transducers”, proceedings of xx imeko world congress, busan, rep. of korea, 2012. [10] a. schäfer, “a novel type of precision amplifier for strain gauge based transducers”, proceedings of tc4 international conferences imeko congress, budapest; hungary, 2016. [11] a. schäfer, “the ultra-precision instrument dmp41 first experiences & appropriate filter settings“; proceedings of tc22 international conferences imeko congress, cape town, republic of south africa, 2014. [12] l. rafflenbeul, m. schäck, r. werthschützky, “optimization of the input impedance of a lownoise unipolar powered amplifier”, proceedings of eurosensors xxv, athens, greece, 2011. [13] m. schäck, “high-precision measurement of strain gauge transducers at the physical limit without any calibration interruption”, proceedings of tc3 international conferences imeko congress, cape town, republic of south africa, 2014. [14] e. heringhaus, “trägerfrequenzund gleichspannungs-meßverstärker für das messen mechanischer größen ein systemvergleich aus anwendungstechnischer sicht teil 1: arbeitsweisen und vergleich charakteristischer eigenschaften”, messtechnische briefe (hbm) issue 18, darmstadt, germany, pp. 42-49, 1982. [15] e. heringhaus, “trägerfrequenzund gleichspannungs-meßverstärker für das messen mechanischer größen ein systemvergleich aus anwendungstechnischer sicht teil 2: verhalten gegenüber externen störeinflüssen und praktische auswahlhilfen”, messtechnische briefe (hbm) issue 18, darmstadt, germany, pp. 70-73, 1982. [16] m. schäck, h. kitzing, “messverstärker mit hintergrundjustierung und verfahren dafür“, de 10 2013 014 876 b3, german patent and trade mark office, munich, germany, 2014. [17] m. schäck, “long term proven and optimized high-precision 225 hz carrier frequency technology in a modern and universal data acquisition system”, proceedings of tc3 international conferences imeko congress, helsinki, finland, 2017. [18] m. kreuzer, “die schnittstelle zwischen aufnehmer und messverstärker bei gleichspannung und trägerfrequenzspeisung”, messtechnische briefe (hbm) issue 26, darmstadt, germany, pp. 42-48, 1990. [19] m. kreuzer, “ein programmierbares präzisionsmeßgerät der ge-nauigkeitsklasse 0,0005 und seine anwendungen”, messtechnische briefe (hbm) issue 16, darmstadt, germany, pp. 41-46, 1980. [20] m. schäck, “improving the characteristics of a high-precision measuring amplifier by a powerful digital signal processing”, proceedings of xxii imeko world congress, belfast, united kingdom, 2018. http://www.imeko.org/ progress in development of an interferometric oil manometer acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 334 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 334 337 progress in development of an interferometric oil manometer s. ehlers1,2, w. sabuga1,3 1 physikalisch-technische bundesanstalt, bundesallee 100, germany, 2 sven.ehlers@ptb.de , 3 wladimir.sabuga@ptb.de abstract: at ptb, a new interferometric oil manometer for the measurement of pressures below 2 kpa, is in development. this paper describes how the main quantities, length and liquid density, are determined, both in-situ, the latter for the first time in a primary pressure standard. the key features of both methods are presented. keywords: primary pressure standard; low pressure; liquid column manometer; interferometric length measurement; liquid density measurement 1. introduction low pressures, between 1 pa and a few kpa, are measured in high-tech industries, such as pharmaceutical productions, clean room applications e.g. in semi-conductor and solar cell production, and the aviation and the space industry. to guarantee reliability, efficiency and quality of those processes, pressure standards are used. national metrology institutes as integral part of the worldwide quality system use standards, ideally, that are primary ones, then offering smallest uncertainties, because of being directly traceable to the international system of units (si). in dependence on the pressure mode, typical instruments in the described pressure range are, expansion systems, pressure balances with conical or spherical non-rotating piston, force-balanced piston gauges, diving-bell or liquid column manometers. also, new developments based on optical methods, e.g. pressure measurement by determining the refractive index and, from this, density of the pressure transmitting medium at a certain temperature level should be remarked [1], even though, at this time, no examples of practical application of such standards as reference in calibrations are known. among all those instruments, pressure balances with non-rotating piston, such as force-balanced piston gauges, when using techniques recently made available [2], could be characterized as primary standards, by determining the dimensions of the piston-cylinder geometry in combination with the description of the flow conditions inside the pistoncylinder gap. however, in the most customeroriented calibrations of such force-balanced piston gauges, they are calibrated as secondary standards against another pressure standard, such as a deadweight pressure balance at pressures above few kilopascals, or liquid-column manometers in the lower pressure range. as discussed below, liquid-column manometers (lcms) present a preferable alternative to pressure balances for a primary realisation of the pressure unit in the pressure range below few kilopascals. the lcms use fundamental, well-known physics, and, through the measurement of input quantities: length, density and gravity acceleration, they are directly traceable to the si. figure 1: u-tube main assembly consisting of the differential interferometers (1), viewports (2), pool floats (3) and the density standard (4). the liquid column manometer in development at ptb, its u-tube assembly is presented in figure 1, is characterized by the first time integration of an in-situ density measurement, which enables the use of new liquids, like vacuum oils, offering, in comparison to mercury, another common liquid used for lcms, an up to factor 16 lower density, and, therefore, 1 3 4 2 http://www.imeko.org/ mailto:sven.ehlers@ptb.de mailto:wladimir.sabuga@ptb.de acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 335 increasing the pressure sensitivity significantly. in combination with an interferometer that uses the direct reflection of the laser light on the liquids free surface for the length measurement, the height difference of the two columns can be measured with a high accuracy. 2. liquid column manometry making use of the physical connection between the measured height difference of two liquid columns in a u-shaped tube and the applied pressure difference, has a long history in pressure metrology going back till torricelli’s invention of the barometer in 1643. several national metrology institutes have developed liquid column manometers. typically, those instruments cover the barometric pressure range, but especially in the last decades, they were also characterized as owning a measurement range starting from around 1 pa and ending at a few kilopascals or lower [3–6]. the pressure difference ∆𝑝 and the height difference of the liquid columns ∆ℎ stand in connection with the gravitational acceleration 𝑔 and the liquid density 𝜌𝑙. as for 𝑔, it is easily possible to determine it once, onsite, using transportable gravimeter standards, on a relative uncertainty level of 10-7. so that the most important factor for the measurement characteristics of the instrument is the choice of a manometric liquid. indeed, from (1) it can be seen that the lcm sensitivity to pressure changes is higher, the lower the liquid density is. ∆𝑝 = 𝜌𝑙 𝑔 ∆ℎ. (1) among manometric liquids, mercury is quite common [7–10] because of its elemental purity and stable density, but the accuracy of the pressure measurement suffers from the high mercury vapour pressure of around 0.2 pa, and applicability of mercury manometers is nowadays restricted primarily due to mercury's poisonousness. therefore, in the ptb approach, vacuum oils were considered as the best candidates as a manometric liquid inside the new interferometric oil manometer. based on the results of the study of their properties [11], an alpha-olefin, namely edwards 45, usually used for vacuum pumps, was chosen. when being used in the manometer, a thermal control by a thermostatic bath is needed due to the oil's thermal expansion coefficient of around 0.7 ppm/mk (1 ppm ≙ 10-6). in addition, the liquid has a very low vapour pressure of 10-5 pa and a density of around 830 kg/m³, 16 times lower than that of mercury, which together offer a much better measurement capability of low pressures compared with that of mercury manometers. the instrument uses two homodyne laser interferometers to detect the displacement of the liquid columns. the light of the ne-he laser of the interferometer system is directly reflected from the liquid surface, built at the liquid-gas interface in each tube. this has advantage of an accurate detecting the oil surface's height position, without any disturbance which takes place in some alternative systems with a mirror carrier on the oil surface. however, the direct interferometric detecting of the oil surface is challenging because of a low oil reflectivity of about 4%. moreover, to prevent liquid surface waves, originating from the tube walls, reaching the surface’s centre, where the laser beam is reflected, a so-called pool-float is used in each column. 3. length measurement to measure the displacement of the liquid’s surfaces in each arm of the liquid column manometer, two individual differential interferometers, of type sp 2000 di produced by sios meßtechnik gmbh, germany are used. the differential principle – each laser interferometer sends two beams, one to the oil surface and the other to the tube window – allows for minimising the influence of ambient air's refractive index changes figure 2. furthermore, with the use of two interferometers, it is possible to measure the displacement of the oil surface in each column individually and independently of the liquid exchange between both legs of the lcm. the measurement always starts from the zero height difference, when both pressures in the two arms of the manometer are equalised. in the interferometers used, all parts involved in the displacement measurement are thermally passive elements, which is achieved by means of a fibre coupling of the he-ne laser system. figure 2: interferometer setup containing the granite structure (1), stainless steel interferometer carrier (2), differential interferometers (3), viewports (4), reflective coating (5). the interferometer system delivers a 0.1 nm resolution in length measurement, which, in an ideal case, would result in a 1 µpa pressure resolution. additional influences impact the real pressure resolution, e.g. mechanical vibrations, so, in the practical operation of the instrument, the resolution will be lower. experiments were carried out to test the resolution and the stability of the interferometric measurement of the oil surface's height. in the experiments, a test setup was used, arranged on top of the heavy granite structure of the lcm, and thermally shielded by foam plates, and the vacuum 32 145 http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 336 oil’s surface was used as a mirror for the measurement beam of the interferometer. in figure 3, the measurement signal of the laser interferometer recorded over the time of 20 s is shown. there, a scatter in the signal of about 2 nm and signal's temporal fluctuations with an amplitude of up to 5 nm can be identified – the low-frequent fluctuations are most likely related to the light structure of the aluminium test setup. further tests, with the interferometers in their final position, coupled to the granite main structure of the lcm, will be performed. nevertheless, by conservatively comparing the minimum and maximum values of the interferometer signal during the time of 20 s, the difference between two signals is never bigger than 10 nm, resulting in a resolution of lower than 0.1 mpa in the unit of pressure. considering that the measurement beam, along the tube, is passing also the pressure transmitting media, which is usually nitrogen, a correction for the influence of the refractive index [6] needs to be applied. this correction can be calculated very accurately, as the tube assembly is temperature controlled by a thermostatic bath and the pressure inside the manometer can be easily estimated with an accuracy of a few pascals. figure 3: length signal of a differential interferometer, when the measurement beam is reflected from the vacuum oil’s surface, recorded with a sampling rate of 1 khz. as the measurement beam of the interferometer is reflected from the vacuum oil’s liquid surface, a potential signal loss caused by waves that scatter the beam, especially when the height of the column changes, needs to be prevented. therefore, a special pool float made of polytetrafluoroethylene (ptfe) is used in each tube leg to stabilise the oil surface. the pool float, whose sketch is presented in figure 4, is supported by a hollow stainless-steel bouncy body, which defines such a position of the pool float at which its pool bottom lies by few millimetres beneath the liquid free surface. to enable the formation of a free liquid surface inside the pool at the height corresponding to column's hydrostatic pressure, the inner pool volume is always in connection to the surrounding liquid, even though, surface waves being built at the liquid – tube wall boundary are prevented to reach the inner pool. to get the oil surface shape within the pool as flat as possible, the inner walls of the pool are machined with the angle corresponding to the oil-ptfe wettability angle. herewith, the oil surface is nearly horizontal at the wall contact perimeter, and the flatness of the oil layer in the pool is guaranteed. figure 4: pool float setup consisting of the ptfe pool float (1), free surface of the vacuum oil (2) and buoyancy body (3). 4. liquid density determination by the use of vacuum oils inside the liquid column manometers, on one hand, one receives an excellent pressure resolution and optimal conditions for absolute pressure measurements, as the liquid’s vapour pressure is negligibly small. on the other hand, the system needs stable spatial and temporal thermal conditions, and, in addition, vacuum oils easily absorb gases, which influence the liquid density. in order to achieve a relative uncertainty of a few ppm for the liquid density, in spite of the effect of the gas absorption and other instability effects, the idea came up to measure the density, for the first time in a liquid column manometer, in-situ, using the temperature-of-flotation method [11]. for determining the liquid density with this method, a solid density standard, whose density is precisely known and similar to that of the liquid, is immersed in the liquid. then, the temperature of the whole setup, with the help of a thermostatic bath, is changed in a way, that the solid density standard comes in hydrostatic equilibrium with the surrounding liquid freely floating in a certain position in it. under this condition, and especially this temperature, the density of the standard and those of the liquid are the same, and, therefore, known. the practical realisation of this method is possible because, in the case of the chosen vacuum oil, the volumetric thermal expansion coefficient of the oil [11] is by about factor 29 higher than that of the solid density standard, so that the oil density reacts much more sensitively to temperature changes. for the application in the liquid column manometer, a hollow titanium sphere was manufactured as the density standard, which is nominally 100 cm³ in volume. titanium was chosen as a sphere material because of its low density and low thermal expansion coefficient. 1 3 2 http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 337 for the density standard, two thin-walled halfspherical shells where manufactured, which then were laser welded. after that, a final production step was undertaken in ptb’s department "scientific instrumentation", which has instrumentation and great experience in polishing spherical objects, so that the density of the sphere – solid density standard – was adopted to the density of the liquid. figure 5 presents the titanium density standard after its final processing. figure 5: ptb’s titanium density standard ds1, owning a nominal volume of 100 cm³. at ptb, in the department "mass", the density of solids, like the titanium density standard built, could be determined by hydrostatic weighing in comparison to silicon density standards, with a relative standard uncertainty at 20 °c of below 4 × 10-6. with this measurement technique, the density of the titanium sphere will be determined with a comparable uncertainty, which fully satisfies the requirement for the oil density measurement in the interferometric oil manometer. 5. summary in the development of the new interferometric liquid column manometer of ptb, designed for the measurement of low absolute and gauge pressures below 2 kpa, progress was made. the instrument capability in measuring the main relevant quantities – length of the liquid columns and the density of liquid, both directly traceable to the si – was evaluated. the unique feature of the new manometer is its capability to measure the density of the liquid in-situ. with special emphasis on the key issues, such as using the direct reflection of the laser light from the free surface of the oil under low reflectivity conditions, stabilisation of the oil free surface with the help of a pool float, and also the first-time integration of a liquid density measurement, pressure measurement with a standard uncertainty below 1 mpa + 18 ppm appears feasible. 6. references [1] k. jousten, j. hendricks, d. barker, k. douglas, s. eckel, p. egan, j. fedchak, j. flügge, c. gaiser, d. olson, j. ricker, t. rubin, w. sabuga, j. scherschligt, r. schödel, u. sterr, j. stone, and g. strouse, “perspectives for a new realization of the pascal by optical methods”, metrologia, vol. 54, no. 6, pp. 146–161, nov. 2017. [2] a. s. hashad, s. ehlers, o. jusko, and w. sabuga, “characterization of a force-balanced piston gauge as a primary pressure standard”, measurement, vol. 131, pp. 723–729, 2019. [3] m. ueki and a. ooiwa, “a heterodyne laser interferometric oil manometer,” metrologia, vol. 30, no. 6, pp. 579–583, jan. 1994. [4] k. f. poulter and p. j. nash, “an interferometric oil micromanometer,” journal of physics e: scientific instruments, vol. 12, no. 10, pp. 931–936, oct. 1979. [5] y. li, y. yang, j. wang, and j. sun, “a new primary standard oil manometer for absolute pressure up to 10 kpa”, metrologia, vol. 52, no. 1, pp. 111–120, jan. 2015. [6] i. v. sadkovskaya and a. i. eichwald, “a laser interferometric oil manometer as the primary standard for absolute pressure in the range 1 – 1000 pa”, journal of physics: conference series, vol. 100, no. 9, p. 092006, mar. 2008. [7] i. v. sadkovskaya and a. i. eykhval’d, “laser interferometric mercury manometer of state primary standard get 101–2011”, measurement techniques, vol. 57, no. 11, pp. 1238–1244, feb. 2015. [8] j. jäger, “use of a precision mercury manometer with capacitance sensing of the menisci,” metrologia, vol. 30, no. 6, pp. 553–558, jan. 1994. [9] a. p. miiller, c. r. tilford, j. h. hendricks, “a low differential-pressure primary standard for the range 1 pa to 13 kpa”, metrologia, vol. 42, no. 6, pp. s187– s192, nov. 2005. [10] p. l. m. heydemann, c. r. tilford, r. w. hyland, “ultrasonic manometers for low and medium vacua under development at the national bureau of standards”, journal of vacuum science and technology, vol. 14, no. 1, pp. 597–605, jan. 1977. [11] s. ehlers, j. könemann, o. ott, h. wolf, j. šetina, a. furtado, and w. sabuga, “selection and characterization of liquids for a low pressure interferometric liquid column manometer”, measurement, vol. 132, pp. 191–198, 2019. http://www.imeko.org/ digital surveying and 3d modelling structural shape pipelines for instability monitoring in historical buildings: a strategy of versatile mesh models for ruined and endangered heritage acta imeko issn: 2221-870x march 2021, volume 10, number 1, 84 97 acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 84 digital surveying and 3d modelling structural shape pipelines for instability monitoring in historical buildings: a strategy of versatile mesh models for ruined and endangered heritage sandro parrinello1, raffaella de marco1 1 dicar – department of civil engineering and architecture, university of pavia, via ferrata 3, 27100 pavia, italy section: research paper keywords: endangered heritage; reality-based modelling; digital survey; structural modelling; cultural heritage routes citation: sandro parrinello, raffaella de marco, digital surveying and 3d modelling structural shape pipelines for instability monitoring in historical buildings: a strategy of versatile mesh models for ruined and endangered heritage, acta imeko, vol. 10, no. 1, article 12, march 2021, identifier: imeko-acta10 (2021)-01-12 editor: eulalia balestrieri, university of sannio, italy received june 1, 2020; in final form november 23, 2020; published march 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this project has received funding from the european union’s horizon 2020 research and innovation programme under the marie skłodowska-curie grant agreement no 821870. corresponding author: raffaella de marco, e-mail: raffaella.demarco@unipv.it 1. introduction: the documentation of endangered heritage both the analysis of cultural heritage and the development of communitarian guidelines for its protection [1] are determining a growing scientific approach to european and worldwide heritage sites [2]. the variety of built heritage (historical buildings, monuments, historical centres, sites, and territorial landscapes) requires a wider field of knowledge and intervention in terms of both physical structures and cultural policy. this leads to the attendant difficulties in protection and preservation due to a fragmented reality of separated protocols and documentation, which results in difficulties in the sharing of information and data integration, thus hindering the entire approach to an intervention programme. this is especially the case for the so-called ‘endangered heritage’ sites (world heritage convention, list of world heritage in danger 1972), the class of built heritage particularly affected by proven or potential threats that pose a high level of risk for its preservation [3]. in fact, the revision of the sites officially listed as endangered heritage has been achieved in terms of the conditions of relevance and correspondence with the specified risk criteria and indicators (territorial, political, or social dangers). however, the statistics in terms of monitoring parameters and protocols remain non-uniform, highlighting a lack of scientific and technical documentation and a lack of connection between intervention activities and ‘reality-based’ updated analysis. the framework highlights the coexistence of a dual type of emergency issue related to built heritage, with the classification abstract cultural heritage and the attendant variety of built heritage demands a scientific approach from european committees: one related to the difficulties in its protection and management. this is primarily due to the lack of emergency protocols related to the structural knowledge and documentation pertaining to architecture and its ruins, specifically in terms of the goals of protection and intervention for endangered heritage affected by mechanical instabilities. here, we focus on a rapid and reliable structural documentation pipeline for application to historical built heritage, and we introduce a case study of the church of the annunciation in pokcha, russia, while we also review the incorporation of integrated 3d survey products into reality-based models. this practice increases the possibility of systematising data through methodological phases and controlling the quality of numerical components into 3d polygonal meshes, with millimetric levels of detail and triangulation through the integration of terrestrial laser scanner and unmanned aerial vehicle survey data. these models are aimed at emphasising morphological qualities related to structural behaviour, thus highlighting areas of deformation and instability of the architectural system for analysis via computational platforms in view of obtaining information related to tensional behaviour and emergency risks. mailto:raffaella.demarco@unipv.it acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 85 of the site as ‘endangered’ for the stability and integrity of its physical structure on the one hand, and the growing demand for specific parameters and protocols for the analysis and quantification of the emergency value of the building, thus determining the effective intervention. thus, there is a growing demand for the identification of such sites [4], both in geographical and typological terms, expanding the dissemination and application of appropriate monitoring and knowledge practices prior to intervention [4], with the aim of triggering a process of safeguarding policies [6]. the topic of implementing safeguarding methodologies for existing heritage is receiving a great deal of attention among researchers, with new forms of digital products emerging in terms of a survey-based phase of digital documentation related to cultural heritage, which is crucial for an accurate and complete understanding of the characteristics and parameters of the units and contexts of built heritage. this is being combined with computer-based cognitive and interactive models, with the elaboration of 3d digital products for investigations and simulations related to shapes and structures. these new representation systems are resulting in new expectations related to digital communication, changing the objectives and constantly renewing the demand in analytical terms for cognitive requirements, in response to the requirements linked to the computational nature of interaction within the models themselves, which are now capable of providing both quantitative and qualitative answers. the difficulties in the development of the reliable diagnosis of historical structures can be realigned with the need for new methods of analysis that can exploit the computational opportunities through specific methodologies and cognitive practices. these practices relate to the visual and graphic aspects of the documentation pertaining to architecture [7], reliably representing the present state of stability and linking it to the constructive and safeguarding rules. 2. historical structures and instabilities in the digital documentation procedures historical buildings are traditionally founded on and remain linked to their original structural system, from the construction phase to later procedures. the demolition of bearing systems and elevations, the reconfiguration of horizontal levels and vaults, and the construction of expansion blocks thus enrich the mechanical experience of the monument (figure 1). the consideration of structural diagnosis applied to historical built heritage in terms of the knowledge of the stress behaviours and the prevision of damage mechanisms plays a central role in the development of the documentation protocols for safeguarding. a review of the risks, the on-going kinematics, and the priorities of stability related to endangered buildings [8] places the focus on the aspect of robustness, that is, the capacity of the building and its elements to withstand a certain level of stress imposed by a combination of degradation and the changes in both the materials and the environments [9]. thus, considering the deep impact that stress-altering phenomena have on the geometrical and material aspects of buildings (e.g. deformations, drifts, cracks) is part of a comprehensive diagnosis that can be achieved through an investigation into the structural shape [10]. as such, it is possible to focus the evaluation of the stress instabilities on the target of a ‘static morphology' that reflects the mechanical and stress traces left by the adaptation schemes of the system during its physical history. it also highlights the stability principles related to the class and type of resistant geometries (figure 2). nowadays, the analysis of the structural aspects of historical buildings involves simplified typological schemes transformed into numerical structures, selected to be mechanically figure 1. examples of sites along the upper kama route characterised by ongoing risks of abandonment and decay, proving the basis for a classification as endangered heritage. from the top, church of the exhaltation of the holy cross in bondjug, rubezhskaya church in usolye, church of st. nicholas in uzhginskay, church of paraskeva friday in saltanovo. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 86 controllable in defined levels of physical regularisation and abstraction [11]. however, the confidence in the standardisation of structural computing has led to underestimating the direct ‘experience’ of the building, visible in the singular imperfection of its shape. in fact, structural documentation must be both 'comparative', with reference to the archetypes, and 'specific', highlighting the accurate variation in formal detail. the testing of specific intervention procedures for existing components aimed at assessing their instability performance is highlighting the need for more precise models for the creation of reliable simulations [12]. at the same time, the area of digital documentation has developed to include morphometric 3d databases with more dense information, which allows for measuring and analysing space in terms that is closer to the continuous form [13]. the relationships between the shape and mechanics of structural systems can determine new descriptive products through realitybased numeric surfaces in terms of high-poly mesh models, which can be morphologically assessed to extract specific quantitative parameters that can guide the mechanical interpretation. thus, the reality-based mesh model, the numerical component of which reliably determines and characterises the geometric aspects, can become an adaptable tool for the management of the computational and analytical processes related to robustness. these considerations are encouraging strategies of data integration for the comprehensive adoption of structural realitybased models related to architectural heritage. on a scientific level, the experimentation of reality-based models for structural diagnosis will result in a multidisciplinary and implementable methodology, one capable of guaranteeing a standardised product, that is, the polygonal mesh model, involving different levels of detail and integration for the management of the existing built heritage. this type of model is aimed at determining both emergency and long term interventions, in the calibration of their procedural computing in both a scientific and practicaloperational direction through the capacity for the shapes to be transformed into morphological and computational platforms of analysis, such as finite element analysis (fea) platforms. here, the methodological process must be as fast, as extendable, and as replicable as possible to facilitate interchange and to allow for a complete knowledge and management capacity through 3d models related to buildings that are in a state of emergency. 3. an experimental case study: the church of the annunciation in pokcha (russia) 3.1. the site and its historical/structural characterisation the constructive features of built structures relate to the technological and material traditions pertaining to specific continents, where the stylistic influences and source availability of certain elements have determined the specific characteristics of the architectural shape. this is especially the case with the built sites and monuments along the european cultural heritage routes, the architectural value of which is strongly permeated by specific structural typologies. these architectural structures have thus acquired their own identity, characterisation, and specificity over time, evolving specific spatial distributions and constructive apparatuses in their historical development and functional change. resilient structures combine a wide framework of morphological and static relationships that are reflected in their own state of conservation and which influence the robustnessrelated approach to their restoration and preservation for security and stability. the case of upper kama, russia, already the subject of international research related to the digital documentation of the complexes [14], presents an emblematic example of the cultural heritage route and the coexistence of historical structures and various phenomena of instability caused by the functional experience of the building, reflecting a complex framework of diagnosis for the safety and integrity of the various built sites. in these terms, the upper kama sites form a rich morphological abacus of technological modules and elements of structure, along with the related pathologies of degradation and conservation [15] [16]. the cultural tradition of the upper kama settlements, more stabilised by commercial requirements than most villages of the russian countryside, has determined a global phenomenon of transformation regarding the built structural systems, from the typical wooden architecture to the constructive solutions of brick and stone masonry. therefore, these architectural structures are characterised by a morphological configuration involving multiple expansion blocks. during the soviet period, the demand for the conversion of infrastructures for energy or food production resulted in the decline of these religious buildings. the consequent interventions and the alteration of the architectural structures, forcibly adapted to the new functions, led to a prevalent abandonment characterised by eventual ruin and collapse. figure 2. the state of neglect and the lack of conservation present critical situations of widespread instability among the masonry systems, clearly exposing the structure to the damage affecting its elements. robustness instabilities in the structural complex of the church of the transfiguration, pyskor, defining the increasing frameworks of damage that will destroy these buildings without specific lines of intervention. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 87 the case of blagoveshchenskaya church, or the church of the annunciation of the blessed virgin, in the village of pokcha, represents a key monumental site of the cherdyn district, which synthesises various different historical-functional evolutions in the stratification of its structures and walls, which today lie in ruin [17], figure 3. the original wooden complex was replaced in 1785 with a new structure composed of stone and brick masonry, subdivided into multiple structural blocks, which include the main body, with a quadrilateral planimetry, constituting a nucleus for the refectory, the southern and northern chapels, the bell tower, and the entrance narthex. in 1910, a reconstruction intervention resulted in various structural modifications, especially in terms of the bell tower, which was entirely replaced, and the eastern section of the central vault and the altar, reconstructed with the insertion of a five-headed chapter. meanwhile, the interiors formed in plastered stone, with paintings and ornaments dating from 1870, remain largely preserved. the face of the building, which features an additional red brick facing, contributes to the strengthening of the external envelope and allows for the possibility of inserting additional devices of tension resistance into the stratified walls. the history of the site involves attempted restoration works dating from 1920 to the complete abandonment in 1940 and to the re-conversion into a power central. the energy issues related to the new function resulted in the partial collapse of the main pavilion vault and the bell tower roof in the 1990s, following repeated lightning strikes attracted by the electrical system. following this extensive damage, the church was excluded from the list of architectural monuments of interest, precluding any new interventions or restoration works, and leaving the site to suffer virtual collapse. 3.2. the current state of the structural ruins in 2018, the architectural complex was in a clear state of neglected conservation. the wooden roofs had been almost entirely destroyed, and the main masonry structure was damaged in various areas, particularly in the main vaulted system. meanwhile, the vaulted structure of the spans, the loading on the pillars based on a counter-balance scheme involving vault– vault and vault–support relationships, has been distorted by the collapse of the secondary vaults. in terms of the large central pavilion vault, only around half of its total span has been preserved, with the head brick structure left visible along the detachment edge. both the brick vaults and the wooden roof have collapsed into the central environment of the nave, with the debris submerging part of the loading pilasters of the vaults, which cannot be inspected in their ground basement and have been covered by soil and vegetation, creating a natural hill that both reduces the access and is the cause of the degradation of the preserved supports. the connection with the bell tower, once provided by the central nave through the gallery and the refectory, has been demolished and thus prevents the direct documentation of the state of conservation of the elevated structures. following the collapse of the roof, the complex was deprived of the main source of protection from atmospheric agents, particularly pertinent during the winter months, and has therefore suffered a rapid degradation of the remaining portions, affected year on year by localised collapses. the site is also totally lacking in control services to prevent access to both people and animals, who often occupy it and damage the spaces and structures. here, the narthex environments have been particularly affected by the frequent presence of herds in transhumance, often housed by the shepherds inside the church during the summer season (figure 4). 3.3. objectives and targets of the documentation process the objective of the documentation of the blagoveshchenskaya church, in addition to the wider mapping and reconnaissance approach for the sites in the upper kama area, is to encourage a rapid but reliable diagnosis of its structural crisis. this is aimed at directing the organisation of possible intervention operations, and to safely preserve and promote the historical ruins. while the local administration is gradually recognising the historical and cultural value of these sites along the kama, the ruined conditions present a clear disadvantage for the commitment, both technical and economic, to implementing conservation procedures. in this sense, rapid survey and quantification methods for recognising and evaluating the preserved structural systems and the corresponding risk constitute an excellent preliminary tool for accurately determining the qualitative and quantitative planning of interventions that could halt the exponential cycle of damage. the documentation of the current state of conservation of the structure of the blagoveshchenskaya church has thus highlighted the need for structural diagnosis using 3d digital survey methods. this involves applying specific integrated approaches for the acquisition and modelling of the structural shape, one that ensures a descriptive quality of its resistant architectural structure through the data collected through a figure 3. the complex of blagoveshchenskaya church in pokcha. the original architectural structure of the church before the revolution in 1956 (above) and the ruins as of 2018 (below). acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 88 practical and fast on-site survey. the required knowledge is largely related to the shape alteration of the structural ruins, ensuring that it can be comprehensively and reliably documented and allowing for analysing the degree of safety and determining the recovery intervention in relation to the overall mechanical scheme. 4. strategies of analysis for the digitisation of the ruined shape the documentation of the state of conservation of blagoveshchenskaya church as of 2018 involved experimenting with integrated approaches in digital surveying to focus on the static instabilities of the ruins for recovery purposes [18]. here, the morphological analysis of masonry structures was organised during the acquisition processes, semantising the spatial constructive ruins and linking them to the global volumetric macrosystem. furthermore, the internal inspection of the masonry sections in terms of the fractured or collapsed portions has allowed for the integration of information related to the structural envelope, reliably supporting the digitisation procedure [19]. the ruined shape, the central element of the analysis process, has thus become the basis of direct knowledge, supported by the repertoire of historical/constructive analysis data, with the dual purpose of completing the technical stratigraphic knowledge framework and revising it using integrated research in relation to the behavioural diagnosis of the structural components. the ruined complex of blagoveshchenskaya church, for reasons attributable to the collapse of the structures and the lack of site regulation, has been deprived of the original spatial design of the wall system. the absence of the main masonry portions hinders the analysis of the current structure in terms of creating defined schemes of interpretation in relation to the architectural instabilities, and this indicates that the specific evaluation of the figure 4. photographic gallery of the blagoveshchenskaya church in pokcha. on the left, the state of conservation in 2018, the overall structural system and the specific static units that characterise the statics of the entire complex. on the right, evolution of the damage to the main vault from 2006 to 2019, which increased rapidly due to the lack of intervention. details of the constructive system that influences the robustness evaluation of the complex. indication of interventions for the functional conversion into a power central, stratigraphic technology of the masonry walls and distribution of a steal structure inside the masonry. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 89 reality-based digital shape is a crucial step for a reliable diagnosis of the static robustness [20]. the ruined complex has been subjected to an integrated, extensive, rangeand image-based documentation campaign. the attendant approach involved calibrating methodologies of acquisition and representation in terms of the morphological properties of the structural elements, especially in terms of the correspondence and integration of spatial information. the dimensions of the structure (specifically the elevation of the damaged portions of the main vault at over 15 m in height) have oriented the morphometric digitisation strategy toward the use of multi-instrumental close-range measuring systems, aimed at guaranteeing a comprehensive coverage and a high formal detailed resolution comparable to the complexly shaped geometry of the masonry surfaces and the typological damage of the masonries (widely covering the order of dimensions from 5 to 10 cm). at the same time, the colorimetric information has proven to be a key aspect in the diagnostics of the structures, evaluating the pathologies of mechanical influence on the masonry and plastered portions, described through tonal variations in the surface colours. the on-site campaign was organised on a double acquisition level, at the ground level using a terrestrial laser scanner (tls) and at an aerial level using drones for photogrammetry, with the aim of obtaining comprehensive morphological data on the elevations of the structure (over 15 m) in a single 360 ° point cloud database [21]. 4.1. terrestrial laser scanning acquisition campaign and products in the acquisition using a tls faro focus s150, the integrated photographic camera was also enabled, ensuring a red, green, blue (rgb) information quality for each point (x, y, z) that is suitable for surface mapping. each scan was performed in medium or high mode according to the distance of the shooting range from the object and provided an additional acquisition time of 4.00 minutes to allow for the shooting of 16 frames for each camera location (coverage of 360 ° × 320 ° for the scan angle). prior to each photographic shot, the instrument was set with an average balance of colour and lighting while considering only the horizontal level for the balance of light conditions in the outdoor scans, or the entire panoramic space for the interior scans. these measures significantly affected the acquisition campaign in terms of duplicating the scanning times but were still more advantageous for the documentation purposes than a parallel photographic campaign from the ground, which requires further actions to integrate the photographic material into the spatial database. a total of 73 tls scans were realised to collect all external surfaces and to spatially connect the complex distribution of the internal environments. the scans were performed at an approximate 2-mm laser spot spacing up to a height of 5 m, and close to 5 mm in the upper surfaces. the quality of tls acquisition for the bell tower and the central dome was ensured by the presence of the inner natural hill over the ruins of the roof, which allowed for a higher instrument positioning above ground level. the tls metric survey conducted from the ground guaranteed a coverage of 80 % of the morphological data related to the structure (figure 5). the processing of the seven source scans on the vaulted structure provided an alignment procedure to obtain the unified tls point cloud. rotation and translation matrixes were then applied in semi-automatic mode via the cloud software (scene), using a cloud-to-cloud alignment set with values of 0.05 m in subsampling and n° 150 iterations for the alignment algorithm. the resulting values of alignment characterising the final integrated database are presented in table 1. 4.2. unmanned aerial vehicle acquisition campaign at the same time, the aerial photogrammetric acquisition was developed using an unmanned aerial vehicle (uav, dji phantom 4 pro), with the flight plan focused on the points of interest of the architectural complex. in fact, the flight plan mission was set from the top centre of the complex at a level of 50 m from the ground in the mode ‘point of interest’. here, a photogrammetric campaign was conducted using the aerial camera with a conical acquisition surrounding the monumental complex, descending to a height of 10 m above ground and developing 329 shots in 10 min of flight. the guaranteed camera angle and the adjustable inclination of the gimbal allowed for total coverage of the top surfaces, with a quality of photographic data suitable for the clear photogrammetric reconstruction of the edges of the components (21 mpixel). in addition, specific flight plans for the tower blocks and the central vault were planned with greater photographic detail, largely due to the close distance of shooting. the gps coordinates defined by the uav for each shot better addressed the alignment algorithms, set to ‘high’ precision, simplifying the alignment of the photogrammetric structure from motion (sfm) reconstruction. in terms of the overall sfm point cloud, there was an error of 0.004 pixels due to the camera alignment (figure 6). 4.3. post-processing data and products the gps information in the source data obtained via the acquisition campaign provided a preliminary overlapping between the tlsand uav-acquired point clouds, both in terms of vertical and horizontal built surfaces, which was used to optimise the referencing in an integrated sparse database of morphometric characters. to optimise the gps data, the alignment procedure for the tls and uav data was set with reference to the tls point cloud, defining six markers from key morphological features that were recorded on the uav point cloud. the resulting values of alignment characterising the final integrated database are presented in table 2. the higher errors of reference corresponded to the markers with less camera projection in terms of photogrammetric alignment. the final database featured 51.889.557 and 49.853.646 points from the tls survey on the internal surfaces of the masonry structure, and 2.035.911 points from the uav survey on the external surface and the ruins of the vaulted structure. the average density in the overall integrated point cloud was 1 point/3 mm. table 1. alignment values in cloud-to-cloud reference for tls data. scan morpho feature iterations point error iterations in mm average point error in mm min overlapping in % scan 39 4 2.5 2.3 35.5 scan 40 12 2.6 1.8 25.8 scan 41 7 1.9 1.6 29.5 scan 42 6 1.5 1.2 29.5 scan 43 5 1.2 1.0 49.4 scan 46 6 1.3 1.0 33.8 scan 48 4 2.2 2.0 26.3 cluster report 44 1.8 1.5 32.8 acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 90 from the joint processing of the collected archives, useful reference drawings, sections, and plans were produced to obtain comprehensive knowledge of the architectural complex and to serve as guidance for the intensive mapping of the pathologies of degradation and instability documented in the photographic repertoire. however, the 2d representation barely described the quality of distribution and the volumetric development of the main structural elements, rendered even more complex by the ruined condition of the site and, as such, by the loss of many of the main shapes conventionally identifiable for the classification of the elements (intrados of the vaults, opening profiles and local collapses, integrity of the elevation structures). in addition, the attempt at processing for elevation maps, as a standard analysis of deformation mechanisms using the point figure 5. morphometric database from on-site tls acquisition campaign: the single scans, completed via gps, compass, and inclinometer information, were referenced and registered to control the overall alignment to verify the final point cloud. table 2. alignment values in tls and uav data morphometric reference. marker morpho feature correspondence projections error in m point 1 vault springer 1 89 0.0383 point 2 vault springer 2 130 0.0472 point 3 vault springer 3 114 0.0483 point 4 vault springer 4 85 0.0439 point 5 vault key 1 (damaged border) 87 0.0314 point 6 vault key 2 (damaged border) 77 0.0169 control points 6 582 0.0392 figure 6. morphometric database from the on-site photogrammetric uav acquisition campaign: the photos were geo-referenced and aligned to define the main point cloud on the architectural complex and the surrounding landscape. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 91 cloud displacement from a reference plane [22], did not satisfy the knowledge requirements in terms of the deformation of the structural surfaces. the high surface processing of the structural envelope, due internally to the collapse conditions and externally to the decorative brick structure, resulted in difficult-to-read colour maps, where the excessive fragmentation of the level indicators failed to provide an overall interpretative picture of the deformations. 5. opportunities for 3d integrated modelling for the diagnosis of the structural shape the aim of the structural representation was focused on the development of a continuous mesh modelling protocol, enriched with reality-based qualities, achieved as part of the objective of digitally surveying detailed and imperfect shapes as confirmed by the instability and mechanical features. these shapes, controlled and certified within the envelope of the structural object, necessarily introduce a need to manage the polygonal geometric mesh that defines them, the processes of which are, however, improved by the representation of a morphological detail that is itself a source of diagnostic reflections on the mechanisms of analytical systems [22] [24], figure 7. in this sense, the obtained 3d models not only pursue the necessary analysis of the deviation between static surfaces but also consolidate a certified practice that defines them both as outputs, directly applicable for the quality of structural instabilities, and as input data for further implementation, specifically on computational platforms. as such, a digital data processing was established that will exponentially led to the exploitation of the potential of the 3d discrete database, namely, the point cloud, and the continuous 3d object defined by vertex and edges for quantifiable evaluations, namely, the model. the application opportunities, from fem platforms to information systems that enhance the virtual fruition, can thus provide the grounding for involving the models using common languages, thus adapting them appropriately [25] [26], figure 8 and figure 9. the developed research on reality-based mesh models has allowed for accurately delineating the causes and effects produced by the structural survey in the digitisation and modelling processes related to morphometric data, validating the compatibility and correspondence of the produced models via a multi-instrumental comparison of the measurement practices normally applied in the structural field. the integration of the products of digital survey protocols applied on the site [26], from both terrestrial and aerial image-acquisition tools, was complementarily completed through the differentiated visual stations, ensuring the capacity to obtain information related to both the exterior and the interior parameters, as well as to monitor the data related to the roof components and elevation units. focusing on the central pavilion vault, which was largely destroyed during the electrical accident, the documentation, finalised to the reinstatement of a complete structural shape, involved the detection of the vault both from the intrados level, also visible in terms of its constructive thickness, and the extrados level, dominated by the ruins of the octagonal masonry tholobate at the base of the wooden roof, figure 10. 5.1. modelling pipeline the pipeline for 3d reality-based modelling was assessed in relation to the research objectives regarding the diagnosis of the stability of the blagoveshchenskaya church in pokcha. the figure 7. drawings and elaborations for the 2d description of the main structural system of the church. the presence of noise data for elements such as the vegetation and collapsed ruins is highlighted. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 92 choice was oriented both toward a spatial configuration of the complex's elements, articulated on several levels and environments, and for a correspondence of the actual state of ruin of the building, which highlighted various critical issues to be transposed automatically in terms of a digitised model through non-uniform rational basis spline (nurbs) or parametric modelling. first, the data collected from the tls and uav acquisition campaigns were tested using a compatible procedure for the integration of the processed point clouds, also supported by common format extensions for their inclusion in the modelling platforms. during this phase, various issues emerged in the specific management of the vertex and polygon objects. corrections and uniformities of the normal vertex data the first aspect related to the difficulty of transferring the ‘vertex normal’ information for each point of the photogrammetric cloud acquired via the uav on the mesh modelling platforms. the compatible formats (mainly .ptx and .pts) for the reverse modelling provided a different ascii code figure 8. quality of processed morpho-metric tls and uav data on the external surfaces of the masonry structure. the point cloud from the uav acquisition shows a resolution and density of data comparable with the tls acquisition, supported by the higher level and resolution of the camera shooting. figure 9. quality of processed morpho-metric tls and uav data on the internal surfaces of the masonry structure. the point cloud from the uav acquisition shows a worse resolution in the discrete definition of the main profiles, geometries, and surfaces, largely due to the corner and shadow shooting. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 93 in the uav photogrammetric processing environment, and the normal information was corrupted within the different matrixes of conversion. following various attempts at comparisons on the multiple modelling platforms suitable for the managing of architectural scale data, an additional phase of the re-calculation of the normal vertex in the uav point cloud was adopted and performed directly in the modelling environment, so as to correctly orient the mesh triangulation and to avoid anomalies of flipped polygon regions that are not suitable for the mesh management as a whole. due to the high density of points and their unstructured distribution, further complicated by the geometric articulation of the complex, the automatic normal calculation tools did not return uniform and coherent results, fragmenting the orientation of the points and, consequently, the features of the triangulated mesh. as such, only 70 % of the points in the uav-based point cloud (1.426.256 points) were automatically processed for the normal feature. to resolve this issue, a single-step triangulation process was performed with the tls and uav integrated point cloud such that the normal vertex data defined in the tls point cloud could mediate the orientation of the entire mesh surface. the topological meshing procedure was performed using an hd adaptive triangulation set for the following parameters: geometry capture accuracy: set over 75 % of the details for shape accuracy. scanner accuracy: this related to the overall accuracy guaranteed by the source point cloud. given the source tls and uav data, and the quality/noise of the integrated aligned database, the accuracy was set to within 3 mm. filtering and selection of source data for the integration procedure. the second aspect related to the different point spacing interval of the individual point clouds (on average, 1-3 mm from the tls, and 5-10 mm from the uav) and its influence on the triangulation of an overall polygonal mesh, where a uniform reading of structural irregularities was ensured. the two discontinuous datasets were refined with an overall filtering performed on the point clouds and were checked via a ‘watertight remesh’ process on the final mesh, assuming an average edge length of 5 mm to define a final model compatible with the main standards of deformation analysis in the area of structural diagnosis. this range did not compromise the relative dimensions of the structural damage quantifiable as a whole and related to the geometry of the construction materials (mainly bricks and metal chains of larger specific dimensions) and to the tolerance threshold set for the analysis (5 mm, also compatible with the standards for elastic safety assessments), figure 11. the finalisation of the integrated 3d survey database related to the pokcha complex defined a virtual system of the preserved shape, directing the attention toward the metric-spatial correspondence of information obtained from the tls database and the uav photogrammetry, calibrated at different reliability values of space reconstruction from the features of the instruments applied in the acquisition phase. specifically, a morphological reference and registration was developed in terms of the scale of each structural unit of the built complex. here, for the pavilion vault, the two types of data were aligned in terms of perimetrical boundaries and façades while considering the deviation accuracy of the discreate surfaces and target control points. following this, a segmentation of the overlapped point cloud was performed, deleting the overlapped areas and maintaining the quality of the tls data on the intrados surfaces and both the tls and uav data on the extrados surfaces. finally, the set of points was subjected to standard filtering procedures, both automatic and manual, which were aimed at cleaning all the spatial information not pertaining to the envelope of the structural surface, specifically constituted by the infiltrated vegetation both at the base and at the top of the ruins. further attention was paid to the window openings, which were manually cleaned for a complete reconstruction of the wall thickness. the triangulation phase of the final integrated database highlighted certain pieces of missing morphological information, which was due to the building masonry areas being covered by vegetation during the survey campaign (removed in the point cloud during the filtering process). these parts were subsequently integrated via a fitting of mesh holes according to the geometric primitives derived from the mesh model. the mesh model obtained by triangulating and remeshing the surfaces verified the expected target of morphological detail, conserving both the main structural profiles and the specific geometry qualities of the masonry elements, both on the surface and on the edges along the collapsed region. figure 10. alignment and integration of multi-instrumental point clouds for the static unit of the central pavilion vault. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 94 5.2. non-uniform rational basis spline comparison and deviation analysis the investigation into the structural instabilities was focused on the vaulted unit in the central span, with the aim of comparing the specific imperfect geometry of the vault (affected by mechanical deformations) with a reliable reconstruction of its original shape from the in-site dimensions of the ruins. using the final mesh model, various section planes were set with an established pitch of 30 cm along all the surfaces of the unit (vault and vertical walls), extracting 3d sketch profiles that adhered to the specific morphological surface and importing them into a nurbs modelling environment. using extrusion techniques, the structural volumes of the pavilion vault were geometrically reconstructed according to the regular project profiles and imported into the reverse modelling environment to allow for a comparison with the irregular mesh surface, figure 12. the comparison between the nurbs model and the mesh model of the vault was performed using the query tool of deviation available on the platform to compute the discrepancy between the models’ surfaces. the deviation comparison (made possible by the same coordinate system maintained between the models) indicated an overall failure phenomenon that characterised the entire system of the resistant unit, possibly attributable to the accidental impact that caused its collapse. within it, various specific portions affected by instability could be observed since their colour map obtained via the deviation computing was characterised by a colour scale ranging from green to red, highlighting the attendant on-course kinematic deformation that is increasing over time. these areas are located both on the vault and on the supporting walls, within deformations that are on a scale of centimetres and that are affecting the main loaded parts of the vaulted system, figure 13. specifically, much like with the collapsed part, the remaining structure of the vault exhibits a discharging arch not in line with the overall static configuration. in fact, this shape reached 40 mm of deformation along entire height, it is subdivided into three plastic hinges and presents a serious risk of collapse. the main bearing walls also exhibit a buckling deformation that reached a maximum of 70 mm, thus increasing the instability of the structure in the basement area. as such, the reliable mapping of evolving structural instabilities using direct survey and mesh models is aimed at determining the intervention practices involving targeted consolidation actions for the safety and conservation of ruined structures. 6. conclusions the modelling performed in terms of the church of the annunciation in pokcha followed an integrated approach involving morphometric data from different instrumental point clouds. an overall mesh triangulation strategy was devised to produce a reality-based model capable of preserving the structural irregularity through the mediation of numerical polygonal surfaces. specific methodological considerations were developed for the mesh triangulation of the integrated tls and uav sparse databases. in order to perform an hd mesh construction, a correct alignment of the points was required as well as uav point cloud processing to support the optimisation of the poly-face orientation in the mesh. other filtering processes were implemented in terms of the point cloud, specifically regarding the presence of opening grids and extensive vegetation, figure 11. final reality-based mesh models following the triangulation and optimisation processing of the hd mesh. the mesh preserved the specific details of the structural shape of the structural unit. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 95 to better expose the surface of the structural domain affected by the decay and the natural elements of the ruin site. the need for a formal approach to the re-drawing analysis and intervention for endangered historical sites was the motivation behind the operational experimentation of morphologicalstructural representation in terms of two specific research targets: 1) the planning of a documentary strategy aimed at acquiring the totality and particularity of the architectural details in terms of all typological variants (masonry, metal parts, wall coverings) and collocations (main environments, underground, in elevation, coverage levels). 2) the convenience of transferring these detailed systems into suitable morpho-metric products capable of providing information and analytical opportunities in relation to historical masonries through graphic representations. these objectives determined the preference for a 3d approach to the documentation and visualisation of structures directly from the data obtained via digital surveys. the shape representation is preserved in both qualitative and quantitative structural assessments, aware of the interactions that historical architecture can exhibit between its individual preserved components and, in terms of restoration, with the intervention design, figure 14. due to its complex nature in terms of compatibility and format size, the type of morphometric data represented by highpoly mesh models is currently affected by the continuous developments in the field of big data management in relation to 3d databases pertaining to architectural systems and urban aggregates. this possibility for reverse modelling in terms of historical architectural heritage allows for revising the documentation protocols in a field of application of representative systems for structural diagnostics, to aspire to technical-professional adoption in the interface with mesh-to figure 12. nurbs modelling of the pavilion vault from the sections extracted by the mesh surface. the geometric vault was referenced to the mesh model and was imported in the same workspace. figure 13. deviation computing on the shape features between the models of the pavilion vault, from the hd mesh and from the nurb modelling. the colour map defines a quantitative reading of the deformed portions from the static configuration, highlighting the kinematic mechanisms that are affecting the vault. from the comparison, it was possible to identify the areas of instability and to derive diagnostic considerations in relation to the overall static schemes of the architectural complex as a whole. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 96 fea and mesh-to-bim protocols. despite the national risk, the lines applied in the digital documentation demonstrate an attempt to adapt to the provisions of the digital agenda 2020 of the european commission (also included in the agenda culture 2030), regarding the digitation of cultural heritage initiatives for the planning of intervention, restoration, and conservation. with the possibility of experimenting virtually within a simulated but reliable space, the methods and effects of a subsequent physical implementation of the intervention for heritage buildings emphasises the opportunities for digital preservation and egovernance, expanding the ideas of innovation in relation to the type and quality of structural survey products in terms of the curation of digital assets and advanced digitisation. the action of 'structural' documentation, including the knowledge of stress behaviours and the prediction of damage mechanisms in buildings, can thus be identified within the european guidelines for heritage at risk by insisting on the characteristics of robustness. acknowledgements the editorial authorship is granted to sandro parrinello for sections 1, 3 and the conclusions, and to raffaella de marco for sections 2, 4 and 5. the documentation of the upper kama region is part of a wider program of activities carried out, since 2013, by dadalab university of pavia (coordinator prof. s. parrinello) and the perm national research polytechnic university (coordinator prof. s. maximova). the digital survey campaigns of 2018 and 2019 were conducted by parrinello s., picchio f., de marco r., dell’amico a. part of the architectural survey documentation data was processed as part of the activities related to the course entitled ‘architectural survey & restoration’ (prof. s. parrinello, prof. g. minutoli) in the double degree italian/chinese course in building engineering and architecture at the university of pavia. the surveying, documentation and modelling of blagoveshchenskaya church are part of the eu project, prometheus. this project is funded by the eu program horizon 2020-r&i-rise-research & innovation staff exchange marie skłodowska-curie and involves the collaboration between three universities (university of pavia, italy, polytechnic university of valencia, spain, perm national research polytechnic university perm national polytechnic university research, russia) and two companies (ebime, spain, sisma, italy). this project has received funding from the european union’s horizon 2020 research and innovation programme under marie skłodowska-curie grant agreement no 821870. references [1] r. de koster (eds), guidelines on cultural heritage. technical tools for heritage conservation and management, jp eu, 2012, online [accessed 26 march 2021] https://rm.coe.int/16806ae4a9 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| 48 a proposal to minimize the dispersion on primary calibration results of single-ended accelerometers at high frequencies gustavo p. ripper, giancarlo b. micheli, ronaldo s. dias inmetro-instituto nacional de metrologia, qualidade e tecnologia, av. n.s. das graças 50, xerém, d. caxias, 25250-020 r.j., brazil section: research paper keywords: primary calibration; vibration; accelerometer; metrology; interferometer citation: gustavo p. ripper, giancarlo b. micheli, ronaldo s. dias, a proposal to minimize the dispersion on primary calibration results of single-ended accelerometers at high frequencies , acta imeko, vol. 2, no. 2, article 9, december 2013, identifier: imeko-acta-02 (2013)-02-09 editor: paolo carbone, university of perugia received february 20th, 2013; in final form april 27th, 2013; published december 2013 copyright: © 2013 imeko. this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited funding: this work was supported by the national institute of metrology, quality and technology – inmetro, brazil corresponding author: gustavo p. ripper, e-mail: gpripper@inmetro.gov.br 1. introduction primary interferometric calibrations of accelerometers are usually performed at national metrology institute (nmi) level in accordance with the international standard iso 16063-11 [1]. the determination of the sensitivity of an accelerometer is based on the measurement of its electrical output while a given acceleration input is measured at its reference surface. in the case of single-ended accelerometers (se), the interferometric measurement of the input motion quantity is usually made on the moving table of the vibration exciter, close to the accelerometer’s mounting base, since its reference mounting surface is directly not accessible. cipm key comparisons [2] are the best way currently available to establish a reference value and to assess the degree of equivalence between different realizations of primary calibration systems developed by the nmis worldwide. for the first cipm vibration key comparison ccauv.vk1, two accelerometers, one back-to-back and one singleended, were circulated for primary calibration of the sensitivity magnitude between 40 hz and 5 khz. unfortunately, no key comparison reference value could be determined above 2 khz for the se accelerometer. this was caused by a large dispersion observed on the results reported by the participants at high frequencies [3]. considering that several nmis have updated their calibration systems and improved their measurement capabilities, the following cipm key comparison ccauv.vk2 was designed to cover a broader scope of measurement. its technical protocol [4] requests the calibration of both magnitude and phase shift of the complex sensitivity of two accelerometers between 10 hz and 10 khz. during the preliminary stage used for monitoring the stability of the accelerometers, the pilot laboratory (ptb/germany) needed to replace its high frequency vibration exciter with another model of different armature material. ptb has then observed considerable differences between the sensitivities determined using their new exciter in comparison to the ones obtained with the prior one. this has been reported by dr. thomas bruns in 2009, during the imeko tc 22 technical meeting in lisbon. later on, two articles related to this problem were published in metrologia [5][6]. abstract primary calibrations of single-ended accelerometers by laser interferometric methods can be strongly affected at high frequencies by the vibration exciter used to generate the mechanical motion. a study was carried out at the vibration laboratory of inmetro aiming to better understand the causes of this dependence and to find a way to reduce its effect. a simple solution is proposed by the authors to reduce the dispersion on the sensitivity results evidenced when different vibration exciters are used. this proposal can be very helpful for calibration laboratories willing to compare results obtained in different measurement systems associated with low measurement uncertainties. therefore, it might benefit processes, which includes for example the validation of calibration systems and interlaboratory comparisons. acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 49 the same behavior has also been observed at the vibration laboratory of inmetro during the validation stage of our primary quadrature interferometric calibration system, when different vibration exciters were tested. it was realized that this problem could be the major source of error and a limiting factor for the future extension of calibration and measurement capabilities up to 20 khz. the authors have then decided to study this problem closer and carry out a research aiming to reduce the systematic deviation observed at high frequencies. a proposal to minimize this effect is presented herein, which can certainly benefit other laboratories that want to carry out internal validations through comparisons between their own different calibration systems. it can also be helpful to future interlaboratory key and supplementary comparisons in vibrations. in the next section 2 we will present the level of dispersion normally observed in calibrations at high frequencies. in section 3 and 4 the mathematical model of an ideally mounted se accelerometer is presented and the effect of the mounting surface quality is discussed. the use of a mechanical adapter with high stiffness is proposed in section 5 and it is illustrated how it can help to maintain a fixed reference measuring condition in section 6. some experimental results are presented in section 7 to demonstrate the level of improvement that can be achieved and in sequence, conclusions are summarized. 2. dispersion observed in different exciters bruns et al. [5] have reported the comparison of charge sensitivity results of a single-ended (se) accelerometer b&k 8305-001 on two different vibration exciters: a ptb modified version of the bouche shaker (ve) and a spektra se-09 shaker. the results obtained exhibited a systematic deviation which increased with rising frequency, starting at 5 khz. relative deviations up to 2 % at 10 khz and 4 % at 20 khz were evidenced. it was also observed that instabilities of this order could have drawn the use of the se artifact for the key comparison ccauv.v-k2 into question. according to this ptb study the cause of this deviation can be a material dependency of the measured magnitude of sensitivity. the ve and se-09 shakers analyzed in their study use armatures made of beryllium and ceramics, respectively. probable causes of this effect can be either differences in contact stiffness or modal deformation of the armature, where the acceleration input is interferometrically measured. this was justified by results of optical measurements on the base of a “naked” accelerometer (without case) and on the shaker armature close to the accelerometer. fem analysis and scanning laser vibrometry measurements on the surface of the armatures have shown radial relative acceleration deviations. this proves that the armatures are not rigid bodies as they are normally assumed to be. bruns et al. have considered in a second publication [6] the inclusion of additional spring and damping elements in the mathematical modeling for a primary calibration of an se accelerometer, in order to consider finite contact stiffness between the accelerometer and the armature. it was concluded that there are strong evidences that the cause of the problem is a non-negligible relative motion between the se transducer and the shaker armature at high frequencies. these studies carried out at ptb were restricted to calibration shakers models, which include lightweight armatures made of high stiffness materials. most of these calibration exciters use air-borne bearings and present limited maximum force ratings. other nmis have different models of shakers in use, which can produce higher levels of deviations. these shakers are usually older models not initially designed for primary calibration purposes, which can be preferred in some specific conditions. for example, this might be the case when higher acceleration levels are required at high frequencies, when the fringe disappearance or minimum point methods are used [1]. the problem regarding low stiffness of the armature was already presented by the authors of this paper in a former publication [7]. it was shown that the charge sensitivity values obtained for a standard single-ended accelerometer mounted on three different vibration exciters presented large deviations above 1 khz due to modal deformations. the results obtained at 3 khz for two aluminium-armature shakers were much higher than the results obtained with an air-bearing guided beryllium-armature model. the authors have also introduced that using a high stiffness adapter it was possible to get equivalent results using aluminium and beryllium made armatures. 3. mathematical model and simulations piezoelectric accelerometers can be represented as a lightly damped single degree-of-freedom spring-mass system. in order to simplify the analysis, it will be considered here that the accelerometer is ideally mounted with infinite stiffness to a rigid body type armature. in this case, accelerometers can be modelled by the classical second order differential equation whose solution yields on the following frequency-dependent sensitivity equations 2 2 22 0 41 )(                       nn f f f f s fs  (1)                             2 1 2 arctan)( n n f f f f f   (2) where fn is the undamped natural frequency in hz, f is frequency in hz, s( f ) is the sensitivity magnitude at frequency f, s0 is the low frequency reference sensitivity magnitude, δφ( f ) is the sensitivity phase shift at frequency f, ζ is the damping factor. if a high stiffness mechanical connection between the accelerometer and the armature is not achieved, a reduced mounted resonance frequency might be observed in the measured frequency response function. in this case, an abnormal increase of the accelerometer sensitivity occurs at high frequencies and significant effects can be seen above 5 khz. most common accelerometers and reference grade accelerometers have resonances in the 20 khz to 60 khz range. table 1 presents the nominal mounted resonance frequencies of single-ended reference accelerometers from different manufacturers. for practical use and for the simulations acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 50 presented in sequence, fn will be approximated by the accelerometer mounted resonance frequency. in order to evaluate the effect of the change in fn relative to the nominal value specified by the manufacturer, some simulations were performed using eq. (1) and (2). measurements carried out at inmetro have shown that the damping factor of an accelerometer model b&k 8305-001 is approximately ζ = 5 x 10-3. this value will be used here for the simulations. figure 1 presents simulation results considering an accelerometer b&k 8305-001. it shows the difference between sensitivity values obtained for different mounted resonance frequencies and the values relative to a nominal mounted resonance frequency of 34 khz. it can be seen that a change in the mounted resonance frequency of a se accelerometer affects considerably its sensitivity response. if the mounted resonance frequency is reduced, an increase in the magnitude of the sensitivity and a phase shift reduction occur. this effect increases with the vibration frequency. for example, in the case of a mounted resonance frequency 5 % lower than 34 khz, the sensitivity magnitude is increased by 1 % at 10 khz and 5.7 % at 20 khz. the influence on phase shift is reduced by approximately 0.01° at 10 khz and 0.06° at 20 khz. 4. mounting surface conditions as already mentioned, one important factor that affects the mounting stiffness of an accelerometer is the quality of the matting surfaces. it is important to assure that both the se accelerometer base and the surface which it will be attached to are free of scratches and present proper flatness and roughness. the use of a proper lubricant as light oil or silicon grease helps to assure a high mounting stiffness and obtain the highest possible mounted resonance frequency. during the torque application to stud mounted accelerometers it is usual to appear circular scratches on its base. this might cause an increasing degradation of the mounting quality condition with time. one possible way to overcome this problem is to submit the accelerometer to relapping in order to recover its original condition. figure 2 shows the effect of the quality of mounting base on the sensitivity of a se accelerometer measured by a primary laser interferometric system at inmetro. this figure includes both the measurement results obtained with the accelerometer as received from a customer and after a re-lapping of its base was performed. the increase of the mounted resonance after improvement of the mounting surface condition is clearly noted in frequencies above 5 khz. the relative difference on the measured sensitivity in this case was -1.1 % at 10 khz and 4.5 % at 20 khz. during a typical vibration interlaboratory key comparison [3] it is usual to have more than ten participating laboratories performing around five repeated series of calibrations. considering that each series includes two mountings to minimize the coupling effect between transverse accelerations generated by the vibration exciter and the transverse sensitivity of the accelerometer, this would require a minimum of fifty mountings. if in addition ten preliminary measurements are carried out by the pilot laboratory to access the stability of the artifact and another ten monitoring measurements are carried out along the circulation period, it is easy to reach a number over one hundred mountings. this high number of mountings might require several re-lappings of the accelerometer base during the period of the comparison in order to assure that every participant receives the dut with approximately the same mechanical conditions. however, the re-lapping process requires special technical skills; it might pose risks to the integrity of the artifact and is based on the removal of material. if several re-lappings are necessary, some change in the sensitivity can be expected. if the number of mountings is reduced considerably, the relapping process can be eliminated and the accelerometer would probably maintain its original characteristics with minimum risks. 5. proposal of adapter a very simple solution has been used at inmetro to reduce the number of mountings and the dispersion between the results obtained using different vibration exciters. the accelerometer is stud mounted onto the top surface of a difference of sensitivity magnitude x resonance frequency (reference condition fn = 34 khz) -10 -5 0 5 10 15 20 0 5000 10000 15000 20000 frequency [hz] r el at iv e di ffe re nc e [% ] fn=30 khz fn=31 khz fn=32 khz fn=33 khz fn=34 khz fn=35 khz fn=36 khz diffe re nce of se nsitiv ity phase x re sonance fre que ncy (re fe re nce condition fn = 34 khz) -0.20 -0.15 -0.10 -0.05 0.00 0.05 0.10 0 5000 10000 15000 20000 frequency [hz] d iff er en ce o f p ha se s hi ft [ o ] fn=30 khz fn=31 khz fn=32 khz fn=33 khz fn=34 khz fn=35 khz fn=36 khz (a) (b) figure 1. difference of the sensitivity (a) magnitude and (b) phase of a se accelerometer from a reference mounted resonance frequency of 34 khz. table 1. nominal mounted resonance frequencies of se reference accelerometers obtained from their technical specifications. manufacturer model base material frequency brüel & kjaer 8305-001 316 stainless steel 34 khz endevco 2270 m8 17-4 ph stainless steel 55 khz kistler 8002 k 303 stainless steel 40 khz acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 51 mechanical adapter, which is then fixed to the armature of the different shakers available in our vibration laboratory. this adapter can be attached directly to the shaker armature or via a secondary adapter to fit it to the different fixture patterns. these additional adapters can also be used to allow rotation of the set at specific angles. figure 3 shows the mechanical design of an ansi 304 stainless steel adapter. its top surface is polished for direct optical reflection of the laser beam. it includes a # 10-32 unf threaded through hole for stud mounting of the accelerometer and a transverse 3 mm diameter hole to help providing the required mounting torque. the 17 mm diameter neck is to reduce mass and allow the mounting on the armature of a b&k calibration head type 4815. figure 4 shows a single-ended reference accelerometer b&k 8305-001 mounted on the top surface of this stainless steel adaptor. the red lines illustrate laser beams reflecting onto its polished top reference surface. this adapter allows direct mounting to armatures of many different vibration exciters. this is exemplified in figure 5, which shows the accelerometer plus adapter set mounted on the following models: (a) b&k high g shaker head model 4811; (b) b&k shaker model 4809; (c) endevco model 2901; and (d) pcb shaker model 396c10. an additional adapter might be necessary to mount the system shown in figure 4 to some exciters like the b&k calibration head model 4815, which presents a recessed mounting plane. figure 6 (a) presents a square shaped aluminium adapter to allow proper fitting to the mounting hole pattern of this shaker and to raise its mounting plane. this aluminium adapter also allows rotation of the set in 90 angle increments. the final mounting of the set onto the shaker armature is shown in figure 6 (b). it should be noted that despite the lower stiffness of this inseries aluminium adapter, it has negligible influence on the calibration results. the original shaker armature and adapter provided by b&k are also made of the same material and its 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 0 5000 10000 15000 20000 frequency [hz] n or m al iz ed s en si tiv ity data before relapping data after relapping fit (before relapping) fit (after relapping) figure 2. normalized sensitivity magnitude measured before and after relapping the base of a se accelerometer. (a) (b) (c) (d) figure 5. se reference accelerometer and adapter mounted on top of the moving element of different vibration exciters: (a) bk 4805 + bk 4811 high g head; (b) bk 4809; (c) endevco 2901; (d) pcb 396c10. figure 4. se reference accelerometer mounted on top of the stainless steel mounting adapter. figure 3. mechanical design of the mounting adapter for se accelerometers. acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 52 low weight helps to reduce the total mass loading mounted on the top of exciter. finite elements modal analyses have shown that the proposed adapter presents resonance frequencies far beyond the frequency range of interest. the two lower resonance frequencies refer to tilting modes near 30 khz and to a rotational mode at 32 khz, approximately. these modes are shown in figure 7 (a), (b) and (c) respectively. the first flexural mode of the top reference surface, which occurs above 58 khz is shown in figure 7 (d). this flexural deflection mode would very probably introduce errors on the acceleration determined by primary laser interferometric methods and could additionally induce stress in the piezoelectric discs inside the accelerometer under test. 6. maintenance of a fixed reference condition figure 8 illustrates how (a) modal deformation of the shaker armature and (b) finite mounting stiffness can generate errors to the calibration of accelerometers by laser interferometric measurements. changes in the distance between the accelerometer reference plane (i.e. the se accelerometer base) and the laser measurement plane (i.e. the armature surface) can cause large systematic errors to the determination of the accelerometer sensitivity. the use of the adapter proposed by the authors helps to maintain a fixed measurement condition, with very good independence to the shaker used. the distance between the accelerometer reference plane and the interferometric measurement plane is kept constant even if the armature presents flexural modes within the frequency range of measurement and a poor mounting stiffness is obtained between the adapter and the shaker armature. these two conditions are exemplified in figure 9 (a) and (b), respectively. 7. measurement results in order to exemplify the application of the proposed adapter, and to illustrate the level of improvement achieved, some experimental results are presented in this section. 7.1. measurements on an air bearing shaker the frequency response of a se standard accelerometer b&k 8305-001 mounted with the stainless steel adapter on the beryllium moving element of a vibration exciter endevco 2901 is shown in figure 10. the acceleration measurements were performed at two diametrically opposed points (m1p1 and m1p2) on the surface of the adapter by a laser vibrometer polytec model vdd-660. the calculated mean sensitivity curve (mean m1) presents a peak value at approximately 33.5 khz, which is only 1.47 % lower than the nominal mounted resonance frequency given in the technical specifications of the accelerometer. this level of agreement provides an evidence of the good quality of the mounting stiffness. the knowledge of the mounted resonance frequency allows the calculation of the magnitude and phase responses of an accelerometer using equations (1) and (2) respectively for comparison with the experimentally determined sensitivities. even with the use of the adapter, the presence of some glitches on the frequency response functions obtained was observed. the cause of these glitches was investigated and apparently they are motivated by internal resonances of the accelerometer. in order to verify the feasibility of this assumption, the authors have carried out some finite element analyses on a simplified model of the accelerometer under test. the first six vibration modes obtained from this analysis are presented in figure 11. figure 11 shows the presence of five vibrational modes before the first axial longitudinal resonance. two tilting modes of the seismic element occur around 7.6 khz, while one rotational mode shows up at 15.5 khz. flexural and longitudinal case modes appear close do 24 khz. the main longitudinal mode obtained with our simplified model was (a) (b) figure 6. (a) se reference accelerometer and stainless steel adapter mounted on an additional aluminium adapter and this set (b) mounted on the moving element of a vibration exciter bk 4805 + bk 4815 calibration head. (a) (b) figure 8. mismatch between the se reference accelerometer plane and the interferometric measurement plane due to (a) modal deflections on the shaker armature and (b) poor mounting stiffness. (a) (b) (c) (d) figure 7. first four vibrational modes of the adapter: (a) mode 1 tilt mode at 29657 hz; (b) mode 2 – tilt mode at 30032 hz; (c) mode 3 – torsional mode at 32198 hz; (d) mode 4 – flexural mode at 58317 hz. accelerometer reference plane interferometric measurement plane laser beam shaker armature accelerometer k acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 53 observed close to 34 khz. given the good match between the frequencies of the glitches on the measured frf and the resonance frequencies obtained by the fe analysis, it is clear to the authors that the remaining causes of errors are due to internal resonances in the accelerometer. figure 12 also shows that the averaging of the results measured at different points improve the final frf obtained. the major effect of the rotational mode can be explained by the characteristics of the shaker used for these measurements. since the armature is guided by air-bearings and is suspended by rubber o-rings, it does not offer strong mechanical constrains to rotations. this might be the reason that rotational modes cause larger glitches on the frf than the tilting and the flexural case resonances. 7.2. comparison between results obtained in different shakers figure 13 presents a comparison between the results of charge sensitivity measured using a flat-spring-suspension exciter model b&k 4815 against the results obtained using an air-bearing guided beryllium-armature, exciter model pcb 396c10. the sensitivities presented are normalized by the reference value measured at 160 hz. these measurements were carried out with the b&k exciter using the adapters shown in figure 6(a) and with the pcb shaker with and also without the stainless steel adapter. the results obtained using the accelerometer mounted directly onto the beryllium armature of the pcb shaker present a systematic increase of the sensitivity at high frequencies. deviations of 0.9 % and 3.5 % are observed at 10 khz and 20 khz, respectively. using the stainless steel adapter on both shakers, a very good agreement is obtained. the deviations at these same frequencies are then reduced to 0.04 % and 0.3 %. it should be noted that the residual effect on the sensitivity seen at 17 khz is caused by an internal resonance of the accelerometer. apart from this frequency, very smooth curves (a) (b) figure 9. maintenance of a fixed reference condition between the se accelerometer and the interferometric measurement plane with the use of an adapter even in case of (a) modal deflections on the shaker armature and (b) poor mounting stiffness between the adapter and shaker armature. accelerometer + adapter on shaker endevco 2901 10 100 1000 10000 0 5000 10000 15000 20000 25000 30000 35000 40000 frequency [ hz] m ag ni tu de o f v ol ta ge s en si tiv ity [m v /(m /s 2 ) ] m1p1 m1p2 mean m1 33562 hz figure 10. magnitude of the frf of an accelerometer b&k 8305-001 + adapter mounted on the moving element of a vibration exciter endevco 2901. (a) modes 1 and 2 seismic mass tilting modes, 7575 hz and 7682 hz (b) mode 3 seismic mass rotational mode, 15570 hz (c) mode 4 case flexural mode, 24514 hz (d) mode 5 case longitudinal mode, 24682 hz (e) mode 6 sensor longitudinal mode, 34127 hz figure 11. vibrational modes obtained by fea applied to a simplified model of an accelerometer b&k 8305-001. adapter k accelerometer reference plane interferometric measurement plane acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 54 are obtained. a similar behaviour has also been observed when using a vibration exciter endevco model 2901 both with and without the stainless steel adapter. this other shaker also includes an air bearing guided beryllium-armature. the reduction in the dispersion at high frequencies is evidenced in figure 14, which presents the relative difference between sensitivity magnitude results obtained using three different shakers with and without the adapter proposed. the relative values are calculated in reference to the sensitivity results obtained employing a shaker b&k 4815 with the mounting configuration shown in figure 6. in order to facilitate visualization of the improvement achieved, this graph includes polynomial curves fitted to the measured data and excludes some few spurious frequencies contaminated by internal resonances of the accelerometer. the agreement between the sensitivity magnitude results obtained using the adapter on these three different shakers is within ± 0.3 % between 1 khz and 10 khz, excluding 5.5 khz where a deviation of 0.4 % is observed. 8. conclusions a study of the dispersion on primary calibration results of se accelerometers has been presented in this paper. it was confirmed that the results obtained at high frequencies can differ significantly depending on the vibration exciter used. this effect might have different causes, including the different mounting stiffness conditions and modal deformations on the armature of vibration exciters. considering the fact that se transfer accelerometers are intended to calibrate btb standards or calibration shakers with built-in reference accelerometers, it is important to calibrate them in the same condition or as close as possible to its end use. currently, exciters with armatures made of aluminium, beryllium, and ceramics are available at nmis for use in calibration of accelerometers. this variability of mounting conditions might yield in different mounted resonance frequencies, which then affects the sensitivity results obtained by each laboratory at high frequencies. this paper focuses on a proposal that can minimize the dispersion problem. it was shown that the use of a stainless steel mounting adapter with high contact stiffness properties and no vibration modes within the frequency range of interest can improve considerably the agreement between the results obtained. some experimental results have been presented to demonstrate that substantial improvement is obtained and the dispersion can be significantly reduced. this reduction of the effect of the vibration exciter can help laboratories to obtain better comparability between different calibration systems. this is an important issue during the stage of validation of different calibration systems or even comparison between different calibration methods. an important impact of the use of an adapter can be related to primary interlaboratory comparisons. if future comparisons consider this proposal, the dispersion caused by variation of the mounting stiffness due to the use of different exciters by the participants can be minimized. in consequence, a better agreement might be achieved and then a comparison reference value with lower uncertainty can be obtained. figure 12. phase of the frf of an accelerometer b&k 8305-001 + adapter mounted on the moving element of a vibration exciter endevco 2901. 0.95 1.05 1.15 1.25 1.35 1.45 1.55 1.65 1.75 5000 10000 15000 20000 frequency [hz] n or m al iz ed s en si tiv ity b&k 4815 with adapter pcb 396c10 with adapter pcb 396c10 without adapter figure 13. normalized sensitivity results obtained using an exciter bk 4815 with adapter and an exciter pcb 396c10 with and without adapter. acta imeko | www.imeko.org december 2013 | volume 2 | number 2 | 55 considering this, the authors are proposing the inclusion of an adapter together with the accelerometers to be circulated for the future regional key comparison sim.auv.v-k2. the use of a similar adapter by different laboratories could provide a better comparability between their calibration capabilities. additionally, comparing results with and without the common adapter, would be possible to determine correction factors between the results obtained with different vibration exciters. references [1] iso, international standard 16063-11, methods for the calibration of vibration and shock transducers – part 11: primary vibration calibration by laser interferometry, international organization for standardization, geneva, 1999. [2] bipm, measurement comparisons in the context of the cipm mra, cipm mra-d-05 version 1.3, october 2012. [3] h.j. von martens, c. elster, a. link, a. täubner, w. wabinski, final report on key-comparison ccauv.v-k1, metrologia vol. 40, tech. suppl. 09001, 2003 doi:10.1088/00261394/40/1a/09001. http://www.bipm.org/utils/common/pdf/final_reports/auv/ v-k1/ccauv.v-k1.pdf. [4] t. bruns, technical protocol of the cipm key comparison ccauv.v-k2, 2009, http://kcdb.bipm.org/appendixb/appbresults/ccauv.vk2/ccauv.v-k2_technical_protocol.pdf. [5] a. täubner, h. schlaak, m. brucke, t. bruns, the influence of different vibration exciter systems on high frequency primary calibration of single-ended accelerometers, metrologia vol. 47, no.1, 2010, pp.58-64, doi:10.1088/0026-394/47/1/007. [6] t. bruns, a. link, a. täubner, the influence of different vibration exciter systems on high frequency primary calibration of single-ended accelerometers: ii, metrologia vol. 49, no.1, 2012, pp.27-31, doi:10.1088/0026-1394/49/1/005. [7] g.p. ripper, r.s. dias, g.a. garcia, primary accelerometer calibration problems due to vibration exciters, measurement vol. 42, no.9, 2009, pp. 1363-1369, doi:10.1016/j.measurement.2009.05.002. -0.5 0.0 0.5 1.0 1.5 1000 10000 frequency [hz] r el at iv e se ns iti vi ty d iff er en ce [% ] endevco without adapter pcb without adapter endevco with adapter pcb with adapter fit to endevco without adapter fit to pcb without adapter fit to endevco with adapter fit to pcb with adapter figure 14. difference of sensitivity results using exciters endevco 2901 and pcb 396c10 relative to the ones obtained using a b&k 4815 with adapter. a transportable referactometer for assessment of pressure in the kpa range with ppm level precision acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 287 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 287 292 a transportable refractometer for assessment of pressure in the kpa range with ppm level precision c. forssén1,2, i. silander1, d. szabo2, g. jönsson2, m. bjerling2, t. hausmaninger3, o. axner1, m. zelan2 1 department of physics, umeå university, sweden, clayton.forssen@umu.se, isak.silander@umu.se, ove.axner@umu.se 2 measurement technology, rise research institutes of sweden, borås, sweden, david.szabo@ri.se, gustav.jonsson@ri.se, martin.bjerling@ri.se, martin.zelan@ri.se 3 national metrology institute vtt mikes, tekniikantie, finland, thomas.hausmaninger@vtt.fi abstract: a transportable refractometer for assessment of kpa pressures with ppm level precision is presented. it is based on the gas modulation refractometry (gamor) methodology, making it resistant to fluctuations and drifts. at the national metrology institute at rise, sweden, the system assessed pressures in the 4.3 8.7 kpa range with sub-ppm precision (0.5 0.9 ppm). the system was thereafter disassembled, packed, and transported 1040 km to umeå university, where it, after unpacking and reassembling, demonstrated a similar precision (0.8 2.1 ppm). this shows that the system can be disassembled, packed, transported, unpacked, and reassembled with virtually unchanged performance. keywords: refractometry; pressure; gamor; transportable 1. introduction good knowledge of gas abundance (molar density as well as pressure) is of importance in a variety of scientific situations and industrial processes, from basic and applied research (including laser spectroscopy) to process monitoring and feedback control in industry. for this, reliable measurement systems or gauges, traceable to primary standards, that can assess molar density or pressure, are needed. in the si-system of units, the realization of the pascal relies on mechanical primary devices such as pressure balances and liquid manometers that realize pressure as a force per area [1-2]. although current conventional techniques demonstrate excellent performance under optimal conditions and handling [1, 3, 4], their performance have remained nearly unchanged for decades. they are often limited in terms of working range, in particular pressure balances, which have a lower limit given by the mass of the piston, and they need to be firmly stationed in a highly stabilized laboratory, requiring either the system to be characterized to be transported or the use of transportable secondary standards to calibrate systems out in scientific laboratories or industrial facilities. an alternative path to realize pressure of a gas is by the use of an equation of state, in which pressure is given as a function of density and temperature [5, 6], and the density is related to refractivity by the lorentz-lorenz equation [7-9]. this, in combination with the revision of the si-system [10], in which the boltzmann constant is fixed, allows for a primary standard for pressure in terms of refractivity and temperature [6]. although there are a variety of means to assess refractivity [11-15], the most sensitive realizations are refractometry based on a fabry-pérot (fp) cavity [11, 16-17]. however, ordinary fp-based refractometers are often limited by the stability of the length of the cavity [5, 18, 19]. hence an exceptionally good mechanical and thermal stability of the system is required. to remedy this, a novel refractometric methodology, based on gas modulation, referred to as gamor (gas modulation refractometry), has recently been developed [20-21]. correctly done, it can provide pressure assessments that are insensitive to both length drifts (and relaxations) of the cavity spacer as well as gas leakages and outgassing in the reference cavity [20, 22]. furthermore, it has the potential to provide a dynamic range of at least 8 orders of magnitude, i.e. from the low mpa region to atmospheric pressures and above. these properties make a refractometer system based on the gamor methodology ideal for use outside well-controlled laboratories, including industrial environments, and thereby also suitable for transportation. this implies that, whenever characterized, and when shown to retain the characterization during transportation, a gamor-based refractometry system can be used as a travelling standard http://www.imeko.org/ mailto:clayton.forssen@umu.se mailto:isak.silander@umu.se mailto:ove.axner@umu.se mailto:david.szabo@ri.se mailto:gustav.jonsson@ri.se mailto:martin.bjerling@ri.se mailto:martin.zelan@ri.se mailto:thomas.hausmaninger@vtt.fi acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 288 by which other pressure measuring systems directly can be characterized. even if not properly characterized, due to its high precision and large linear dynamic range, such a transportable system can be used in combination with a local primary standard for calibration / characterization of other pressure systems, including those that do not have a measurement range that overlap with the local primary standard. this work presents a transportable refractometer for assessment of pressure in the kpa range with ppm level precision. it describes the instrumentation and the experimental method used in some detail, summarizes the results from a successful transportation, identifies shortcomings of the system, and comments on upgrades that are currently being implemented. 2. theory the gamor-methodology is described in detail elsewhere where extensive descriptions and explanations of its theory, procedures, and abilities of the method are given [20-22]. in short, the refractivity of the gas let into the measurement cavity, 1n − , can be expressed as 1 1 f q n f   +  − = − + . (1) here f = 0 /f  , where, f is the shift in the beat frequency between the two laser fields when the measurement cavity, after being evacuated, is filled with the gas to be characterized, formally given by 0gf f− where gf and 0f are the filled and evacuated measurement cavity beat frequencies respectively, while 0  is the frequency of the cavity mode addressed in the empty measurement cavity (denoted 0 q ). q represents 0 /q q , where q is the number of modes the measurement laser jumps when the measurement cavity is filled with gas.  is the refractivity-normalized deformation coefficient of the measurement cavity, defined by ( 1)n − = 0 /l l , where l is its change in length when it is filled with gas (which includes effects both from an altered length of the cavity spacer and distortion of the mirrors), while 0 l is its length when being empty.1 the conversion of a given refractivity, 1n − , to gas density,  , is being performed through the extended lorentz-lorenz equation, which implies that the density can be assessed from the refractivity by 1 2 ( 1)[1 ( 1)], 3 n r n b n a  − = − + − (2) 1 note that no knowledge of the length of the empty cavity is needed since it is only the change in optical length of the cavity between an empty and a filled cavity that is needed for the assessment of pressure. where r a and 1n b − are the molar dynamic polarizability and a series expansion coefficient, respectively, where the latter is given by 2 (1 4 / ) / 6 r r b a− + , where, in turn, r b is the second refractivity virial coefficient in the lorentz-lorenz equation [7, 20]. the corresponding pressure of the gas, p , can thereafter be obtained from the density as [1 ( ) ]p rt b t   = + , (3) where r is the ideal gas constant, t is the temperature of the gas, and ( )b t is the second density virial coefficient [5]. 3. experimental setup and method 3.1. experimental setup the transportable refractometer is based on the gamor system described in ref. [20]. as is shown in fig. 1, it can schematically be divided into six modules, designed to allow for easy packing, transportation, and installation. the central part of the instrumentation, the gas assessment module, module 1 in the figure, includes the dfpc which consists of two cavities drilled in a 190 mm long zerodur block with a 10 cm2 square cross section (190 × 100 × 100 mm). one of the cavities acts as the reference cavity while the other serves as the measurement cavity. the cavities are made of high reflective (> 99.97%) concave mirrors with a radius of curvature of 500 mm attached to the zerodur spacer by the use of a surrounding aluminum frame, yielding a finesse of ~10 000 and a free spectral range (fsr) of ~800 mhz. to maintain a temperature stability of 1 mk, the cavity spacer is placed inside an aluminum enclosure whose temperature is controlled by four peltier elements. figure 1: schematic illustration of the experimental setup organized in separate modules; module 1: dfpc block http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 289 together with the optical, acousto-optical, and electro optical components; module 2: electronic control and data acquisition system; module 3: gas distribution system; module 4: gas inlet system; module 5: the device under test (dut), represented by a pressure balance; module 6: gas evacuation system. to assess the frequency of the cavity modes, the system encompasses two er-doped fiber lasers, emitting light at 1.53 µm, that are locked to longitudinal modes of the cavities using the pound-dreverhall (pdh) technique [23]. the light from the two lasers are combined in a fiber coupler and sent to a high-bandwidth photo detector, which detects their beat frequency. as is shown in fig. 2, all parts in module 1 are placed on an enclosed optical breadboard that easily can be attached and detached from the other modules. this allows the dfpc, which together with the optical and electro-optical components are the central and most sensitive parts of the system, to be packed and shipped as one congregated unit. figure 2: picture of module 1. the zerodur spacer is placed inside a temperature stabilized aluminum frame that also holds the cavity mirrors in place. the temperature probes were placed in holes bored in the spacer. the main parts of the vacuum system, which constitute module 3, comprise valves and pressure gauges that are mounted on a separate optical breadboard for easy transportation. module 3 is connected to, in addition to the dfpc, the modules 4 6 2. module 4 represents the gas inlet system, which consists of a needle valve to set a suitable flow and a solenoid valve connected to a locally available n2 supply (at either rise or umu). module 5 comprises, at both sites, the device under testing (dut), in this case a pressure balance (ruska 2465-754). module 6 represents the gas evacuation system, here a turbo molecular pump. module 2 comprises electronic control and a data acquisition system. the control unit is a standard 2 the modules 4-6 are not necessarily parts of the transportable system and can be chosen depending on what is available at different sites. 3 the total time for each cycle and the times for each state also depend on the capacity of the gas supply and the vacuum system. in this work the times were chosen to ensure sufficient time for both the stabilization laptop running custom-made labview software that measures and regulates the cavity spacer temperature, controls the valves, and collects the beat frequency. the temperature is measured with a data acquisition module and feedback is applied to the peltier elements. the valves are controlled through a digital output module and the beat frequency is collected from the frequency counter through a usb interface. the labview panel displays a data overview, performs real-time preliminary data analysis, and saves all data to a binary file. the final data analysis is thoroughly performed using matlab. 3.2. experimental method the transportable refractometer utilizes the gamor-methodology, which is described in detail elsewhere [20]. in short, it comprises a number of recurrent measurement cycles that each provides an independent measurement point. at the end of each cycle, to acquire a zero-pressure reference point, the measurement cavity is evacuated. the measurement cycle, which is divided into two states whose lengths were chosen to be suitable for the dut, was taken as 200 s. the first, representing the gas filling and stabilization state, lasted 180 s, while the second, which encompassed gas evacuation, was maintained for 20 s3. the lengths of the filling and stabilization states were found to be sufficient for the gas in the fp-cavity to thermalize with the cavity walls. the dut, which also provided a stable pressure, was, in this work, at both sites, represented by a dead-weight pressure balance. for each cycle, during the evacuation state, the piston in the pressure balance drops to its resting position and consequentially resets for each measurement point 4. 3.3. transportation of the system the modules are constructed so they can be packed and shipped individually (depending on what is available at the host laboratory). to assess to which extent the performance of the system (primarily its precision) is affected by disassembling, packaging, transportation (in this case withstand a rugged road-haulage), unpacking, and reassembling it, measurements were carried out at three different set pressures, both before and after the transportation. to accomplish this, five of the modules (all modules except module 5, the dut) were packed and shipped together in a custom-built wooden box (see fig. 3). the transportation was performed during sub 0° c temperatures, partly on uneven nordic winter-roads, and carried out with no extra care taken to protect the equipment. of the pressure balance and evacuation of the gas. the procedure was thus not optimized for speed. 4 in practice, the dut can be separated from the refractometer and the pressure can be assessed using a suitable high-performance differential pressure measurement instrument. http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 290 unpacking and reassembling of the system (including minor optical alignments) took approximately 4 hours. however, to minimize the amount of contaminations in the system from its exposure to atmosphere during the transportation, the system was thereafter continuously pumped down for 24 hours before any measurements were attempted. figure 3: picture of the box in which the refractometer was transported. the picture also manages to capture a rare proof of a last author performing practical work. 4. results 4.1. precision a set of measurements, comprising three measurement series, representing three different set-pressures of the pressure balance, was first performed at rise before the system was transported to umu, where a similar set of measurements was performed. since dissimilar pressure balances were used at the two premises, the set-pressures at the two premises were slightly dissimilar. at rise they were 4303.36(63), 5766.47(67), and 8691.73(76) pa, whereas at umu, they were 4306.60, 5769.79, and 8695.23 pa. the pressure balance at rise is, by calibration, traceable to the si-system, and provides, according to the cmc values of rise, a pressure dependent uncertainty of 6 0.5 30 10 p − +  pa (k = 2). 5 although expected to have a similar performance, the pressure balance at umu was last calibrated in 2010, whereby no trustworthy uncertainty can be assigned for this work. in order to evaluate performance in terms of precision of the transportable system, a two-hour window of each measurement series was selected by the criteria that a steady-state of the system had been reached, indicated by a stable reading of the mean of the three temperature sensors (±1 mk).6 the selected data for the six measurement series are shown in fig. 4 with their corresponding standard deviations, which for the three set-pressures (ca. 4300, 5 data taken from https://kcdb.bipm.org/appendixc/m/se/m_se.pdf. 6 the difference in time it took to reach a steady temperature state at rise and umu can be attributed to a difference in performance in the climate control of the laboratories. 5775, and 8700 pa) were assessed to 3.7 mpa (0.9 ppm), 5.1 mpa (0.9 ppm), and 4.3 mpa (0.5 ppm) for the measurements performed at rise, and 4.1 mpa (1.0 ppm), 12 mpa (2.1 ppm), and 7.3 mpa (0.8 ppm) for those made at umu, respectively. this shows that the system can be disassembled, packed, figure 4: two-hour windows of data selected from three measurement series performed at each location. each dot represents an independent measurement. transported, unpacked, and reassembled with a remained precision at the subor low ppm-level.7 4.2. linearity it is of interest to investigate the linearity of the system and to assess to which extent this can be affected by transportation. figure 5 shows, by the individual markers, the assessed values of each measurement cycle (taken under quiescent conditions with the deformation coefficient,  , being zero) as a function of set pressure of the pressure balance (black points, data taken at rise; red points, data from umu). in the main window in panel (a), all data points from a given set value (4300, 5775, and 8700 pa) are, with the scale used, overlapping within the size of the markers. the inset displays an enlargement of a fraction of the data in the main window. this shows that, even on this scale, all data points taken at each of the premises still merge to a single data point, indicating the high precision of the system. a scrutiny of the linearity of the system was performed both before and after transportation. linear functions of the type y = kx + m were fitted to the data both before and after the transportation. the deviations of the various data points with respect to the fits are displayed in panel (b). since these deviations are smaller than the uncertainties of the pressure assessments provided by the duts, this indicates that the linearity of the system (in the pressure range investigated here, i.e. 4.3 – 8.7 kpa) was not compromised by the transportation. 7 the slightly higher values of the standard deviation of the assessments at umu are presumed to originate from differences in laboratory conditions, predominantly drifts of the surrounding temperature. http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 291 figure 5: panel (a): assessed pressure by the refractometer plotted versus the nominal value of the pressure balance, (rise: black, umu: red). solid lines: linear fits to the data from each measurement location. inset: a three orders-of-magnitude zoom of the data around 5768 pa. panel (b): the residuals to the fit. 4.3. accuracy since the assessments presented here consistently were performed with no account taken regarding the pressure-induced cavity deformation (i.e. with  = 0), it is expected that the slopes of the fits in fig. 5 should deviate from unity. the results also show this. for the data taken at rise, k = 1.0014371(8) and m = -0.215(5), while for the data taken at umu, k = 1.0015004(9) and m = 0.039(5). the average deviation from unity of the two k parameters is 0.1469 % while their difference is only 63 ppm. the major reason for the large average deviation of the response comes from the fact that the deformation was not considered. other possible reasons are uncertainties in the temperature determination of the gas, the molecular polarizability, r a , the local gravity, and the gas purity. the fact that the pressure independent offsets of the fits were found to be -0.215 and 0.039 pa, which agree well with the uncertainties of the pressure balances, which amount to 0.5 pa8. 5. outlook and recent upgrades since the measurements in this work were made, we have implemented some upgrades. the system has been made more compact. the vacuum system has been upgraded with components utilizing ¼” swagelok tubes and moved to module 1, electronics have been custom built in a much more compact format, fiber optics have been moved to a separate module together with the aoms and eoms, all placed on a single 300 × 450 mm breadboard. the zerodur spacer has been replaced with an invar cavity spacer for better thermal stabilization [24, 25]. all these upgrades have enabled the possibility of placing the entire system in a single rack cabinet. 8 they can predominantly be attributed to uncertainties in the set values of the pressure balances (possibly by an offset in the pressure gauge assessing the hood pressure). hence, we are presently conducting a project with the goal of being able to ship the complete system without any need of disassembly or reassembly, so as to further simplify transportation and setup. the idea is that the system simply needs to be connected to a gas supply, vacuum and a dut at a site on which the system arrives. furthermore, a promising procedure for characterizing a cavity (in terms of deformation, ε) has been developed [26]. 6. conlcusion a first version of a transportable refractometer for pressure assessment is presented. the refractometer was constructed around robust and exchangeable modules, which allows for easy transportation as well as a choice of different types of equipment depending on availability at various premises (e.g. pressure balances, vacuum pumps, and gauges). it was shown that the system assessed pressures in the 4.3 – 8.7 kpa range at the national metrology institute at rise, sweden with sub-ppm precision (0.5 – 0.9 ppm). the system was thereafter disassembled, packed and transported 1 040 km to umeå university, where it, after unpacking and reassembling, demonstrated a similar precision (0.8 – 2.1 ppm). this shows that the system can be disassembled, packed, transported, unpacked, and reassembled with virtually unchanged performance. the results indicate that this type of system is suited for a variety of applications. for example, it can be used as a travelling standard by which various pressure measuring systems at dissimilar premises can be characterized with respect to each other. it can also be used, in combination with a local primary standard, for calibration of pressure systems, including those that do not have a measurement range that overlap with the local primary standard. it is also possible to use the system for benchmarking or characterization of other refractometry systems with respect to pressure induced cavity deformation and wavelength dependence of refractivity. based on this work, several upgrades of the system can be envisioned, both to increase the performance of the system (in particular its accuracy) and to improve on its transportability. some of these are under implementation. following these, a more rigorous measurement campaign involving the transportable refractometer is planned, using fully calibrated pressure balances, in the form of a circle comparison involving multiple laboratories, which should provide information about the repeatability and the accuracy of the system. parallel to the improvement of the transportable system, a number of actions are presently being pursued to address the accuracy of the technique, e.g.; http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 292 assessment of the cavity deformation [26]; improved assessment of temperature [24]; and the use of (or assessment of) updated gas coefficients with less uncertainty, parts of which are scheduled to be carried out within the ongoing empir initiative quantumpascal (no. 18sib04). this project (quantumpascal, 18sib04) has received funding from the empir programme co-financed by the participating states and from the european union's horizon 2020 research and innovation programme. the authors acknowledge support from the swedish research council (vr) (project no. 621-2015-04374); the umeå university industrial doctoral school (ids); the vinnova metrology programme (project nos. 2018-04570 and 201905029); and the kempe foundations (project no. 1823, u12). 7. references [1] c. r. tilford, ”3 and a half centuries later the modern-art of liquid-column manometry”, metrologia, vol. 30, pp. 545-552, 1994. [2] j. w. schmidt, k. jain, a. p. miiller, w. j. bowers, d. a. olson, “primary pressure standards based on dimensionally characterized piston/cylinder assemblies”, metrologia, vol. 43, pp. 53-59, 2006. [3] j. h. hendricks, d. a. olson, “1-15,000 pa absolute mode comparisons between the nist ultrasonic interferometer manometers and non-rotating force-balanced piston gauges”, measurement, vol. 43, pp. 664-674, 2010. [4] j. ricker, j. hendricks, t. bock, p. dominik, t. kobata, j. torres, i. sadkovskaya, “final report on the key comparison ccm.p-k4.2012 in absolute pressure from 1 pa to 10 kpa”, metrologia, vol. 54, p. 07002, 2017. [5] p. f. egan, j. a. stone, j. h. hendricks, j. e. ricker, g. e. scace, g. f. strouse, ”performance of a dual fabry-perot cavity refractometer”, opt. lett., vol. 40, pp. 3945-3948, 2015. [6] k. jousten, j. hendricks, d. barker, k douglas., s. eckel, p. egan, j. fedchak, j. flugge, c. gaiser, d. olson, j. ricker, t. rubin, w. sabuga, j scherschligt., r. schodel, u sterr., j. stone, g. strouse, “perspectives for a new realization of the pascal by optical methods”, metrologia, vol. 54, pp. s146-s161, 2017. [7] a. d. buckingham, c. graham, “density dependence of refractivity of gases”, p. roy. soc. lond. a mat., vol. 337, pp. 275-291, 1974. [8] h. j. achterman, t. k. bose, m. jaeschke, j. m. starnaud, ”direct determination of the 2nd refractivity virial-coefficient of methane, nitrogen, and 5 of their mixtures”, int. j. thermophys., vol. 7, pp. 357-366, 1986. [9] h. j. achtermann, g. magnus, t. k. bose, ”refractivity virial-coefficients of gaseous ch4, c2h4, c2h6, co2, sf6, h2, n2, he, and ar”, j. chem. phys., vol. 94, pp. 5669-5684, 1991. [10] m. stock, r. davis, e. de mirandes, m. j. t. milton, “the revision of the si-the result of three decades of progress in metrology”, metrologia, vol. 56, p. 022001, 2019. [11] m. andersson, l. eliasson, l. r. pendrill, “compressible fabry-perot refractometer”, appl. opt., vol. 26, pp. 4835-4840, 1987 [12] h. fang, p. juncar, “a new simple compact refractometer applied to measurements of air density fluctuations”, rev. sci. instrum., vol. 70, pp. 31603166, 1999. [13] h. fang, a. picard, p. juncar, “a heterodyne refractometer for air index of refraction and air density measurements”, rev. sci. instrum., vol. 73, pp. 1934-1938, 2002. [14] d. mari, m. bergoglio, m. pisani, m. zucco, “dynamic vacuum measurement by an optical interferometric technique”, meas. sci. technol., vol. 25, p. 125303, 2014. [15] l. p. yan, b. y. chen, e. z. zhang, s. h. zhang , y. yang, “precision measurement of refractive index of air based on laser synthetic wavelength interferometry with edlen equation estimation”, rev. sci. instrum., vol. 86, p. 085111, 2015. [16] p. egan, j. a. stone, “absolute refractometry of dry gas to +/3 parts in 109”, appl. opt., vol. 50, pp. 3076-3086, 2011. [17] i. silander, m. zelan, o axner, f. arrhén, l. pendrill, a. foltynowicz, “optical measurement of the gas number density in a fabry-perot cavity”, meas. sci. technol., vol. 24, pp. 105207, 2013. [18] t. rubin, y. yang, “simulation of pressure induced length change of an optical cavity used for optical pressure standard”, journal of physics: conf. series, vol. 1065, p. 162003, 2018. [19] r. w. fox, “temperature analysis of low-expansion fabry-perot cavities”, opt. express, vol. 17, pp. 15023-15031, 2009. [20] i silander, t. hausmaninger, m. zelan, o. axner, “gas modulation refractometry for high-precision assessment of pressure under non-temperature-stabilized conditions”, j. vac. sci. technol. a, vol. 36, p. 03e105, 2018. [21] m. zelan, i. silander, c. forssén, j. zakrisson, o. axner, “recent advances in fabry-perot-based refractometry utilizing gas modulation for assessment of pressure”, acta imeko 2020 [22] i. silander, t. hausmaninger, c forssén., m. zelan, o. axner, “gas equilibration gas modulation refractometry for assessment of pressure with subppm precision”, j. vac. sci. technol. b, vol. 37, pp. 042901, 2019. [23] r. w. p. drever, j. l. hall, f. v. kowalski, j. hough, g. m. ford, a. j. munley, h. ward, “ laser phase and frequency stabilization using an optical resonator”, appl. phys. b, vol. 31, pp. 97-105, 1983. [24] i. silander, c. forssén, j. zakrisson, m. zelan, o. axner, “an invar-based fabry-perot cavity refractometer with a gallium fixed-point cell for assessment of pressure”, acta imeko 2020. [25] i. silander, c. forssén, j. zakrisson, m. zelan, o. axner, "invar-based refractometer for pressure assessments", opt. lett., vol. 45, pp. 2652-56, 2020. [26] j. zakrisson, i. silander, c. forssén, m. zelan, o. axner, “procedure for robust characterization of fabry perot based refractometer”, j. vac. sci. technol. b, vol 38, 2020. http://www.imeko.org/ characterisation of corrosion products on copper-based artefacts: potential of ma-xrf measurements acta imeko issn: 2221-870x march 2021, volume 10, number 1, 136 141 acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 136 characterisation of corrosion products on copper-based artefacts: potential of ma-xrf measurements elisabetta di francia1, sabrina grassini1, giovanni e. gigante2, stefano ridolfi3, sergio a. barcellos lins2,4 1 politecnico di torino, corso duca degli abruzzi 24 – 10129, turin, italy 2 la sapienza università di roma, via antonio scarpa 14/16 – 00161, rome, italy 3 ars mensurae, via vincenzo comparini 101 – 00188, rome, italy 4 surface analysis laboratory infn roma tre, via della vasca navale 84 – 00146, rome, italy section: research paper keywords: cu-based alloys; artificial corrosion products; fesem; µxrd; ma-xrf citation: elisabetta di francia; sabrina grassini; giovanni ettore gigante; stefano ridolfi; sergio augusto barcellos lins, characterisation of corrosion products on copper-based artefacts: potential of ma-xrf measurements, acta imeko, vol. 10, no. 1, article 18, march 2021, identifier: imeko-acta10 (2021)-01-18 editor: carlo carobbi, university of florence, italy received may 11, 2020; in final form september 8, 2020; published march 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: this project has received funding from the european union’s horizon 2020 research and innovation programme under the marie skłodowska-curie grant agreement no. 766311 corresponding author: sergio a. barcellos lins, e-mail: sergio.lins@roma3.infn.it 1. introduction the study of corrosion products, their morphology and chemical, physical and microstructural characterisation, is of considerable interest to heritage and conservation sciences. metallic objects are unique artefacts that can be considered a witness to the past and a record of a society’s technological prowess. therefore, it is of the utmost importance to study and preserve such objects. to achieve this goal, a transdisciplinary approach is always preferable; measurement, instrumentation and material sciences find a point of commonality in conservation science. in fact, metrological and material characterisation are combined in measuring and quantifying the conservation state of artefacts and the aggressiveness of the surrounding environment with a common final goal, the preservation, restoration and safeguarding of objects [1]-[6]. numerous studies on ancient and historical copper (cu)-based artefacts have tried to establish the chemical characteristics and structure of the natural patinas that grow on objects that have been buried in soil for long periods. the long-time corrosion of cu-based alloys leads to structural alloy transformations and to the growth of surface corrosion product layers with complex and stratified structures, the investigation of which allows important information on degradation mechanisms to be obtained [7], [8]. corrosion in soil leads to the formation of different patinas and corrosion product layers, whose chemical composition and microstructure depend not only on the alloy composition but also on the environmental conditions and soil chemico-physical properties (for example, ph, organic and inorganic particles, aggressive ions and oxygen content). usually, an inner layer of cuprite (cuo2) forms on the metal surface, and then, an outer layer, composed of different copper oxides and abstract the use of macro-x-ray fluorescence (ma-xrf) scanners is now widespread in cultural heritage applications. however, its use for the characterisation of metallic works of art is still limited. in this study, a novel portable ma-xrf scanner prototype was tested on artificially corroded copper samples to assess its analytical capabilities on corroded metals, yielding information on the spatial distribution of the corrosion products grown on the metal’s surface. a multi-analytical approach was used to thoroughly characterise the copper samples and compare the obtained results to verify the reliability of the ma-xrf data. the prototype was able to obtain distribution maps of different elements, such as sulphur and chlorine, which can be directly correlated to different corrosion products. with the use of imaging filtering techniques, it was possible to investigate the stratification of the corrosion product layers and observe gradients in the distribution of certain elements. mailto:sergio.lins@roma3.infn.it acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 137 hydroxychlorides, sulphates and carbonates, such as malachite, brochantite, atacamite and paratacamite, develops [9]-[12]. since the external layers are generally quite porous, a continuous interaction between the metallic surface and the environment occurs. in particular, when reactive copper chloride (cucl, nantokite) is formed during burial, it can react with oxygen and humidity, leading to a cyclic copper corrosion process, known as bronze disease [13]-[15], in which chloride ions undergo a catalytic action towards further degradation. to study the corrosion phenomenon and extract as much information as possible, multi-analytical strategies are often employed. within the analytical techniques available, those that are non-destructive are commonly preferred when dealing with historical and archaeological artefacts, and of these, x-ray fluorescence (xrf) is a staple for evaluating chemical composition [16]-[18]. as a matter of fact, the most important preliminary action in the study of the conservation state of cubased artefacts is to assess the presence of aggressive agents, such as chloride ions, and unstable corrosion products that can be dangerous for the preservation of the object, which, therefore, needs immediate stabilisation. in this context, the development of methodologies and portable instruments for quantitative analyses and elemental map distribution, which could also be employed in the field, can be important tools in conservation. moreover, copper and other alloy components and soil element distribution maps could help in obtaining information on degradation mechanisms and be very useful for developing tailored and suitable cleaning and stabilisation treatments before artefact restoration. in the past few years, macro-x-ray fluorescence (ma-xrf) has been extensively used to study paintings, frescoes and stained glass [19]-[21], having the obvious advantage over traditional xrf analysis of yielding an immediate and visual interpretation of the spatial distribution of chemical elements. this advantage can be fully explored in the study of inhomogeneous samples, such as corroded bronzes and other metallic archaeological artefacts. therefore, ma-xrf can be considered an efficient and powerful measuring tool for detecting the presence of active corrosion products, which are particularly dangerous for the long-lasting preservation of the artefact. using ma-xrf scanning, it is possible to provide a better, macro understanding of the corrosion product multi-layer structure by simply detecting the depth elemental distribution from the surface towards the bulk metal. in this study, a portable ma-xrf scanner prototype, built by the sapienza university of rome, istituto nazionale di fisica nucleare (infn) – roma tre division, and ars mensurae, was used to characterise a set of artificially corroded copper reference specimens simulating real archaeological corrosion products grown on cu-based alloys. due to the stratification and heterogeneity of real corrosion product layers formed on cubased artefacts, reference samples were chosen for the optimisation of the proposed analytical methodology and for the preliminary assessment of the developed prototype performance in order to verify whether the obtained results were meaningful [22]. the second section of this article describes the reference sample and the analytical techniques used to characterise them, with particular attention to the description of the main features of the ma-xrf scanner prototype. in the following section, the results obtained on the reference samples are illustrated and discussed. finally, the concluding section summarises the key findings. 2. materials and methods 2.1. reference samples copper reference samples (cu 99.96 wt.%; 45 × 15 × 5 mm3) were polished with 500 to 4000 grit sic paper, rinsed in ethanol in an ultrasonic bath for 5 minutes and dried well. two types of artificial corrosion layer were then produced by chemical synthesis. a set of cu reference samples were immersed in an aggressive solution, according to the astm d1384 standard method (2.8∙10-2 m nacl, 0.01 m na2so4, 16.1∙10-2 m nahco3; modified by constantinides [23]), for three months at room temperature (sample a). the other sample set was artificially aged by immersion in a sulphate-containing solution (0.1 m na2so4) for three months at room temperature (sample b). none of the solutions were stirred or aerated by bubbling. 2.2. ma-xrf scanning the ma-xrf prototype instrument developed in this study is composed of an x–y motorised stage with a sample holder and a detachable scanning head. a scheme of the experimental setup is shown in figure 1. the head comprises a collimated, low-power ta-target moxtek® x-ray tube and a lightweight x-123sdd detector from amptek® with a 125-ev resolution at the mn kα emission line. the scanning head remains fixed while the sample is translated in the stage with a maximum step resolution of 100 μm. the system is controlled by a custom-made interface programmed in labview™ with a near-live display of the counts (intensity) map. an earlier version of the prototype system, using a larger translation stage, has been used previously to study archaeological painted and gilt leathers [24], while an improved, more recent iteration of the portable version was recently used to study a gilded cu-based artefact [25]. the portable instrument is described in more detail elsewhere [26]. the data was collected with a step resolution of 1 mm, dwelltime of 5 seconds and a total mapped area of 43 × 13 mm2 per sample, which was enough to cover the surface. tube voltage was set at 35 kv, the current at 17 μa and each scan took about 54 minutes. elemental distribution maps were generated with an earlier version of the open-source data analysis software xismus [27], embedded with low and high thresholding and 3 × 3 iterative smoothening imaging filters. figure 1. ma-xrf prototype: the x-ray tube, the detector and the sample holder with the x–y motorised stage. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 138 2.3. reference sample characterisation the artificially corroded cu reference samples were first characterised by means of different analytical techniques, such as high-resolution digital photography, confocal microscopy, micro-x-ray diffraction (μxrd) and field emission scanning electron microscopy (fe-sem), coupled with energy dispersive spectroscopy (eds) to collect information on the chemical composition of the corrosion products and on surface roughness and morphology. all these data are necessary for the optimisation of the ma-xrf measurement setup and to identify the main experimental parameters that could affect the obtained results. high resolution digital photographs were acquired using a digital camera (4000 × 3000 pixel2, panasonic lumix g2) equipped with a stand with a 3000 k lamp. roughness evaluations were performed on the acquired profiles by means of a sensofar plµ 2300 confocal microscope (cm) using a 20x objective. the images were processed in the free octave software and surface standard deviation was computed on about 100,000 points, which compose the confocal pictures. µxrd spectra were collected by a d-max rapid rigaku instrument equipped with rint/rapid software. this instrument presents a 2d-curved imaging plate detector (ip, a flexible sheet coated with white -or light-blue-photoluminescent material consisting of bafbr: eu2+), and a very brilliant rotating anode (cu kα) with a spot size of ≈ 70 µm. diffractions were acquired with the x-ray source at 40 kv and 20 ma with a 100 µm collimator in the range of 10° to 160° 2θ. the elaboration and qualitative identification of the diffraction patterns were achieved using x’pert highscore plus v3.0e software and comparison with the pdf and icsd database. fe-sem (zeiss, supra40) images were collected in in-field emission mode by varying the acceleration voltage in the range of 1.5 kv to 15 kv. eds analyses were performed using the eds inca x-sight (oxford instruments) with an acceleration voltage of 15 kv at a working distance of 8.5 mm. finally, the cu reference samples were characterised by means of the ma-xrf analysis using the scanner prototype, as discussed above, to collect information on the chemical element distribution to identify the presence of different corrosion products on the sample’s surface. 3. results and discussion the accelerated corrosion procedures applied to the copper reference samples led to the growth of different artificial corrosion product layers. 3.1. reference sample a looking at the surface and the high-resolution image, the corrosion product layer formed by exposing the cu reference samples to the astm d1384 solution (sample a) appears uniformly green in colour, with a corrosion product layer that is quite homogenous and well adhered to the metal surface (figure 2a). the cm image acquired showed a relatively smooth surface. the cm mainly allows the estimation of the standard deviation of the height, which is about 2.7 μm (figure 2b); therefore, a rather uniform surface is observed. at microscopic level, the fe-sem image (figure 2c) shows a surface well covered by tabular crystals of 1 μm in length and grouped in circular shapes. eds analyses, performed on the crystals, identified the presence of o, c and cu, with a typical composition of ≈ 54 wt.% of cu, ≈ 34 wt.% of o and ≈ 12 wt.% of c. the µxrd patterns (figure 2d) mainly reveal the presence of metallic copper (cu, icsd code: 53757), cuprite (cu2o, icsd code: 52043) and malachite (cu2(co3)(oh)2, icsd code: 15384). the presence of malachite seems confirmed also by the eds analyses; the weight percentage of cu-o-c are close to those detected on malachite reported in the literature [28]. based on the eds and μxrd analyses, it is reasonable to assume that the corrosion product layer is structured in an external layer composed mainly of malachite and an inner layer of cuprite. the metallic copper comes from the bulk, which is evidence of the low thickness of the corrosion product layer. in summary, the morphological and microstructural analyses highlighted that sample a presents a homogenous corrosion product layer characterised by the presence of relatively small crystals, visible on the surface and composed mainly of malachite. the presence of an inner layer of cuprous oxide is also detected. the chemical and microstructural characterisation of the corrosion products do not provide evidence of the presence of copper chlorides, probably due to the hydrolysis of nantokite figure 2. sample a: (a) high-resolution surface picture; (b) confocal microscope image; (c) fe-sem image showing the crystals grouped in circular shapes; (d) µxrd pattern identification. figure 3. sample b: (a) high-resolution surface picture; (b) confocal microscope image; (c) fe-sem image showing the corrosion product morphology; (d) µxrd pattern identification. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 139 (cucl) in the solution, which leads to the formation of cuprite on the metal surface. 3.2. reference sample b observing the high-resolution image, the surface of sample b appears quite variable in colour, with green-to-brown corrosion products mixed together on the copper surface (figure 3a). the corrosion product layer formed in the sulphate-containing solution appears well adhered to the metal. compared to sample a, the surface of sample b presents a more heterogeneous morphology, with a standard deviation of the flat part of about 6.1 μm (figure 3b). a high surface roughness is therefore evidenced. microscopically, fe-sem observation confirmed the presence of a heterogeneous corrosion product layer, showing the presence of two different types of area (figure 3c). one area was characterised by the absence of identifiable crystals and a composition of ≈ 56 wt.% of cu, ≈ 36 wt.% of o and ≈ 8 wt. % of s (figure 3c, dark area), whereas the other area was composed of well-defined and identifiable cubic crystals of less than 2 μm in size (figure 3c, bright area). the crystals presented a typical composition of ≈ 89 wt.% of cu and ≈ 11 wt.% of o, which relates to the cuprite composition [29]. the µxrd diffractograms, performed on several points of the surfaces (figure 2d), consistently highlighted the presence of metallic copper (cu, icsd code: 53246) (possibly coming from the bulk) and cuprite (cu2o, icsd code: 173982). the occurrence of brochantite (cu4(so4)(oh)6, icsd code: 97703) and posnjakite (cu4(so4)(oh)6(h2o), icsd code: 100276 – not shown in the figure) were also detected. the chemical, morphological and microstructural analyses highlighted that sample b presents a heterogeneous corrosion product layer, characterised by the presence of areas of small cuprite crystals and of other areas where other compounds, mainly copper sulphates, are spread around cuprite crystals. 3.3. scanner characterisation and performance the artificially corroded reference samples were analysed by ma-xrf scanning, and illustrative pictures of the analysed areas were taken with a dslr camera with no white balance correction and no external light source. 3.3.1. reference sample a figure 4 shows the total counts (sum) map alongside the elemental maps and a picture of the surface of sample a after being immersed for a long-time in a carbonate and chloridecontaining solution. the sum map is created by summing every count in the pixel spectrum and attributing that value to a pixel, in a ‘flattening’ fashion, transforming the n-dimension pixel (where n represents the number of spectrum channels) to a 1-dimension pixel. therefore, the higher the pixel value, the more counts collected. figure 4 indicates the presence of chlorine in sample a. however, when verifying the unprocessed chlorine map, the signal appeared unusually concentrated in the bottom right of the image. the image was processed and filtered to enhance the darker parts and verify if any signal was being supressed by the higher intensity signal. this may happen given the 0–255 dynamic range of the grayscale maps. the processed image is shown in figure 4e, where a very low-signal map can be observed, obtained by applying a low-pass and smoothening filter. this process cuts off the higher intensity values (brighter pixels) and smooths the image by taking the nearest-neighbours average, weighted at the centre. only then could a portion of the higher intensity signal be cleared. even though a highly manipulated image could be generated, in this case it only represented noise and could not be considered an indication of chlorine. it is very likely that the detected signal came from the sample holder (made from polyvinyl chloride) and not from the sample itself. this has been confirmed by the diffractogram collected on sample a (figure 2d), which demonstrates that the corrosion compounds present on the sample’s surface are mainly composed of copper oxide and malachite. moreover, the copper distribution map did not show any gradients (figure 5), confirming the presence of a homogeneous corrosion product layer. if chloride compounds were present, their detection would be almost impossible. constantinides et al. [23] demonstrated, through artificially corroded cu alloys, that chlorine is preferably deposited as an inner layer, in between the alloy’s bulk and the outer corrosion layer. this preferential deposition is created by a mechanism originating from mechanical stress or chemical reaction, in which cracking occurs on the surface and promotes pitting corrosion. this process causes a further dissolution of the alloy, producing more cu+ cations that react with the clanions present in the solution. as a result, cucl and cucl2 are formed and build up in a layer internal to the corrosion surface. even though the formation of chloride compounds is propelled in this mechanism, the resulting weight percentage concentration of chlorine is still relatively low (when compared to the alloying element concentration and the system detection limit), commonly about 2 wt.%. moreover, if chloride compounds are to be deposited underneath a cu-oxide layer, the cl detection by the ma-xrf system is further hindered by attenuation effects, thus making its detection extremely difficult. chlorine has a low atomic number (z = 17) and, therefore, its fluorescence line energies are very low. heavier atoms, such as copper, present in any given overlapping layer, can easily absorb chlorine’s signal coming from underneath. figure 4. sample a surface (a) and scans: (b) sum map; (c) copper map; (d) filtered copper map; (e) highly enhanced noise map obtained at chlorine k-α energy range. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 140 3.3.2. reference sample b figure 6 shows the total counts (sum) map alongside the elemental maps and a picture of the surface of sample b after being immersed in the sulphate-containing solution. as a first approach, sample roughness and scanner response must be interpreted, as they can play a significant role as major geometrical variables that may affect the scanner performance. therefore, a comparison between roughness and the sum map can show the presence of a geometrical factor that creates a sort of distortion to the scanner response. if the counts per pixel and cu-peak signal-to-noise ratio are relatively high, it is possible to determine, through the total counts (sum) map, if there are any roughness contributions to the final images. in fact, when applying an average-threshold filter (cutting off pixels with a centre-weighted average lower than the threshold input value) to the copper maps, a certain gradient in the intensity can be appreciated. initially, the reasons can be three-fold: (a) the formation of a variable in the thickness corrosion product layer that attenuates (proportionally to its thickness) the signal from the copper atoms underneath; (b) a selective/preferential corrosion of copper; or (c) a distortion caused merely by the surface roughness. after carefully inspecting the elemental maps of copper and sulphur from sample b, the sample’s aspect (figure 6) and the relatively smooth surface presented by the profiles obtained, it is reasonable to assume that what caused the gradient in the copper intensity is the presence of a thicker corrosion product layer area (whiteish region, figure 6a) in the upper part of the sample surface. the corrosion product layer thickness can vary greatly, depending on the nature of the products present and the alloy. as an approximation, for bronzes, its thickness can vary from 40 to 138 μm in the case of tin bronzes and from 14 to 146 μm in the case of quaternary bronzes [30]. therefore, even though a pure copper corrosion product layer thickness is not completely comparable to that of bronzes, it is still unlikely that, in this case, roughness would cause any significant distortion in the elemental distribution maps. concerning the corrosion compounds distributed across the sample surface, the chemical elements that can be associated with any corrosion product formed are copper, sulphur and chlorine (since oxygen cannot be detected by the instrument). in addition, cl and s are light (low z) elements and their count statistics are very low (figure 5). the presence of chlorine (although detected by the instrument), probably belongs to the pvc sample holder instead of the copper surface itself (sample b), as observed in sample a. sulphur was detected across the sample b surface (as expected) and formed what seemed to be a gradient that spread from the upper-middle portion towards the lower region (figure 3e). μxrd analyses on sample b suggest the presence of sulphur in the form of posnjakite and brochantite, so it is reasonable to assume the presence of sulphur. 4. conclusions this study allows a better understanding of the potential and capabilities of the prototyped ma-xrf scanner for the characterisation of cu-based artefacts. the scanner is able to identify the presence and surface distribution of the different elements, which is linked to the presence of different corrosion products on the metal surface. even though an in-depth corrosion assessment requires a multi-analytical approach to characterise the corrosion products from the microstructural and morphological perspective, maxrf measurements can be successfully employed for a preliminary investigation, which can also be carried out in the field, highlighting the presence of stable and reactive corrosion products grown on the metal surface due to its interaction with the environment. future developments within the prototyped ma-xrf framework, apart from its application to different archaeological materials, will comprise its continuous assessment on real stratified corroded archaeological metals to increase its resolving capabilities and performance. acknowledgement the authors would like to thank the european commission for financially supporting this research in the frame of the european union’s horizon 2020 research and innovation programme under the marie skłodowska-curie grant agreement no. 766311. figure 5. sum spectra comparison of sample a and sample b. arrows point to s-kα and cl-kα lines. figure 6. sample b surface (a) and scans: (b) sum map; 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historical bowed musical instruments giacomo fiocco1,2, silvia grassi3, claudia invernizzi1,4, tommaso rovetta1, michela albano1,5, patrizia davit2, monica gulmini2, chiaramaria stani6,7, lisa vaccari6, maurizio licchelli1, marco malagodi1,8 1 arvedi laboratory of non-invasive diagnostics, cisric, università degli studi di pavia, via bell’aspa 3, 26100 cremona, italy 2 department of chemistry, università di torino, via pietro giuria 7, 10125, torino, italy 3 department of food, environmental, and nutritional sciences, università degli studi di milano, via g. celoria 2, 20133 milan, italy 4 department of mathematical, physical and computer sciences, università di parma, parco area delle scienze 7/a, 43124 parma, italy 5 department of physics, polytechnic of milan, piazza leonardo da vinci 32, 20133, milano, italy 6 elettra-sincrotrone trieste s.c.p.a., s.s. 14 km 163.5, 34194 basovizza, trieste, italy 7 ceric-eric, s.s. 14 km 163.5, 34194 basovizza, trieste, italy 8 department of musicology and cultural heritage, università degli studi di pavia, corso garibaldi 178, 26100 cremona, italy section: research paper keywords: chemometrics; cultural heritage; ftir spectroscopy; musical instruments; coatings citation: giacomo fiocco, silvia grassi, claudia invernizzi, tommaso rovetta, michela albano, patrizia davit, monica gulmini, chiaramaria stani, lisa vaccari, maurizio licchelli, marco malagodi, chemometric tools to investigate complex synchrotron radiation ftir micro-spectra: focus on historical bowed musical instruments, acta imeko, vol. 10, no. 1, article 27, march 2021, identifier: imeko-acta-10 (2021)-01-27 editor: ioan tudosa, university of sannio, italy received april 29, 2020; in final form october 23, 2020; published march 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: monica gulmini, e-mail: monica.gulmini@unito.it 1. introduction during the last decade, an increasing number of historical bowed string instruments have been investigated, with a large suite of analytical techniques being employed by the researchers to discover the secrets of these masterpieces of craftsmanship [1],[2]. here, the research was mainly focused on the nature of the precious varnish [3]-[5] and of the other materials involved in the finishing treatments used by the cremonese violin makers [6],[7]. generally, multiple thin varnish layers (a few microns of thickness) were applied on the wood, which was previously treated with a sealer and covered by a ground coat to prevent varnish penetration [8]. in addition, micrometric inorganic particles were often dispersed in the coatings [9],[10]. the most common materials involved in the finishing processes were abstract the investigation of the coating systems used on historical bowed string musical instruments is generally highly complex due to the coatings’ reduced thickness and multi-layered structure. furthermore, sampling is rarely feasible, and non-invasive approaches do not always allow researchers to undertake a thorough characterisation. thus, in the rare cases of availability, the opportunity must be taken to investigate the best micro-samples in detail using a suite of analytical spectroscopic techniques that allow for obtaining various informative spectra. their subsequent interpretation should lead to the characterisation of the finishing layers, the preparation of which involves a careful selection of organic and inorganic compounds. in the present work, synchrotron radiation and micro-fourier-transform infrared spectroscopy were combined in terms of reflection geometry and chemometrics to investigate six cross-sectioned micro-samples detached from four bowed string instruments produced by antonio stradivari, francesco ruggeri, and lorenzo storioni. various chemometric tools enabled us to perform a preliminary exploration of the entire collected infrared dataset, while a classification model based on partial least squares–discriminant analysis was used to discriminate the materials through the characteristic signals. high model specificity (> 0.9) was achieved in the prediction, providing the groundwork for the application of a fast and rigorous methodological approach. mailto:monica.gulmini@unito.it acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 202 siccative oils, natural resins, casein or animal glue, inorganic fillers (e.g. calcium carbonate, gypsum, silicates), and pigments (both organic and inorganic) [11]-[13]. the scientific investigations in this field, generally highly challenging due to the intrinsic complexity of the coating systems, are rendered even more arduous due to the large variety of unknown restoration materials that could have been laid on top of the original materials over time [14]. in addition, given the high value of the historical musical instruments, sampling is rarely allowed. in the rare case where a micro-sample can be taken from the surface, it is generally embedded in epoxy resin and then cut as a cross section [15]. here, to obtain the maximum amount of information, researchers must collect a large amount of data using a suite of analytical techniques. while a non-invasive approach is generally preferred [16], a combination of nonand micro-invasive spectroscopic techniques is often used to comprehensively characterise the historical materials, generally in conjunction with various imaging, tomographic, and chromatographic techniques [17]-[21]. in recent years, the use of chemometrics in the field of heritage science has been tested for an in-depth elaboration of large datasets [22]. these analytical tools are largely employed to support a preliminary interpretation of the spectroscopic results and to improve the visual representation of the information carried by the spectra [23]. in this work, six micro-samples mounted in various cross sections were analysed using synchrotron radiation (sr)–fourier transform infrared (ftir) micro-spectroscopy at elettra sincrotrone trieste (source for imaging and spectroscopic studies in the infrared [sissi] beamline, chemical and life sciences branch) [24]. the samples were detached from four historical bowed string instruments made in cremona by antonio stradivari (1644–1737), francesco ruggeri ‘il per’ (1630–1698), and lorenzo storioni (1744–1816) [25]. to preserve the surface of these unique cross sections such that further analyses can be developed in the future, a microattenuated total reflection mode was excluded [26], with the spectra collected in reflection geometry ensure that the objective did not come into contact with the samples. the sr technique increased the lateral resolution and signal-to-noise ratio [27],[28], allowing us to set the analytical spot up to the minimum layer thickness of 10 µm. despite the constraints imposed by the sampling geometry and by the reduced layer thickness, numerous complex spectra were derived from the cross sections. while this analytical approach is promising for achieving the characterisation of the coating system, a huge amount of effort is required to obtain a reliable and rigorous preliminary picture of the information hidden in the entire infrared (ir) dataset. consequently, to support the data-processing step in view of extending the use of this analytical technique to a larger number of samples, and to obtain as much information as possible from the analyses, it was decided to process the spectra using a multivariate approach. in fact, various chemometric tools are generally used to elaborate the data from a multivariate point of view, which allows for unravelling the relevant information carried by the spectroscopic signals in terms of, for example, sr–ftir spectra. specifically, through the use of an unsupervised exploratory procedure, namely, principal component analysis (pca), it is possible to understand the relationship between all the variables and to extract the sample patterns according to the weight of the variables in a new reduced space defined by the pc components [29],[30]. moreover, supervised classification methods (e.g. linear discriminant analysis, partial least squares–discriminant analysis [pls–da], support vector machines, artificial neural networks) enable the definition of rules aimed at distinguishing objects into classes, such as different materials, which allows for material classification and for skipping the visual inspection of the large number of spectra. within this context, the present work aimed to (i) develop a multivariate methodological framework for managing and interpreting large ir datasets and subsequently (ii) compare and describe the spectra collected for the six micro-samples using the chemometric tools, namely, pca and pls–da. the remainder of the paper is organised as follows. in section 2, the musical instruments considered in the research, the microsampling, and the embedding method are described, while the procedures used during the sr–ftir micro-spectroscopic analyses are explained together with the chemometrics approach and the attendant procedures. in section 3, the expected ir bands and the results obtained via pca from the ir dataset exploration are presented and discussed alongside the classification results obtained via pls–da modelling. finally, the findings are summarised and potential future research directions are outlined in section 4. 2. materials and methods the experimental plan encompassed the analysis of six submillimetric samples collected from four different bowed string instruments (table 1): a fragment of a cello made by francesco ruggeri during the 17th century (fr_c), the toscano violin made by antonio stradivari in 1690 (as_v), and the bracco small violin (ls_sv1, ls_sv2 and ls_sv3) and a small privately owned violin (ls_v), which were made by lorenzo storioni in 1793 and 1790, respectively. the samples were collected under high magnification, employing a disposable blade scalpel on selected areas of the musical instruments. following the sampling, the fragments were embedded into epoxy resin (epofix struers and epofix hardener, 15:2), and then cut into cross sections. the surface was then dry-polished using silicon-carbide fine sandpaper (500–8000 mesh) to obtain a flat surface. at least two layers of organic binders with a minimum thickness of 10 µm table 1. list of the violin makers, musical instruments and samples considered in this project. period instrument type instrument name date instrument part sample name francesco ruggeri “il per” 1630 – 1698 cello 17th century back plate fr_c antonio stradivari 1644 – 1737 violin toscano 1690 soundboard as_v lorenzo storioni 1744 – 1816 small violin 1790 rib ls_v lorenzo storioni 1744 – 1816 small violin bracco 1793 soundboard ls_sv1 lorenzo storioni 1744 – 1816 small violin bracco 1793 soundboard ls_sv2 lorenzo storioni 1744 – 1816 small violin bracco 1793 back plate ls_sv3 acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 203 were observed through an optical microscope in the coating systems of the five selected samples. the micro-samples were analysed at the sissi-bio beamline (chemical and life sciences branch) at elettra sincrotrone trieste (italy) [31]. measurements were performed on the polished samples in terms of reflection geometry via infrared synchrotron radiation (irsr) using a bruker vertex 70v interferometer coupled with a hyperion 3000 microscope (bruker optik gmbh) and a single point mercury–cadmium– telluride (mct) detector. a total of 512 scans were averaged in the acquisition spectral range of 4000–750 cm−1, with a spectral resolution of 4 cm−1 and a scanner speed of 120 khz. the size of the acquisition points was set at 10 × 30 µm by closing the knife-edge apertures of the vis-ir microscope according to the sample stratigraphy. the acquisition of 97 spectra was carried out in single-point mode and linear map mode with a vertical step size of 10 µm. for each sample, the background was acquired on a gold substrate using the same acquisition parameters. the reflection ir spectra were then transformed into absorbance spectra (as required for the interpretation of organic compounds) by applying kramers–kronig (kk) transformations using opus 7.5 software. the preliminary pca data exploration was performed with a selection of 63 ir spectra (collected in linear map mode) from the ir dataset. here, various spectral pre-treatments, namely, smoothing (savitzky–golay, 11 wavelengths gap size), first derivative (savitzky–golay, 11 wavelengths gap size and secondorder polynomial), and mean centre, were applied. to develop the pls–da classification model, the entire ir dataset (97 spectra) was divided into a calibration set and a test set containing 71 (approx. 80 %) and 26 (20 %) spectra, respectively. the calibration set contained spectra referred to as samples as_v, fr_c, ls_sv1, and ls_sv3, while the spectra collected on ls_sv2 and ls_v were used as the test set. meanwhile, the models were cross-validated using the ‘venetian blinds’ procedure with eight splits. the classification via pls–da involved the application of pls regression to a y dummy, completing a rotation of the projection to latent variables to obtain the maximum separation among the classes [32]. all the data analyses were performed in matlab (v. 2016a, mathworks, inc., natick, ma, usa) using the pls toolbox software package (ver. 8.5, eigenvector research, inc., 130 usa). 3. results and discussion to obtain the maximum amount of information from the data and to reduce the impact of the non-diagnostic spectral features (e.g. signal noise and different baselines) the spectral range was initially reduced to a range of 3500–1000 cm−1 before being divided into six selected regions: 3500–3000 cm−1, 3000–2700 cm−1, 1800–1550 cm−1, 1550–1450 cm−1, 1460–1260 cm−1, and 1250–1000 cm−1. the reflection ir bands in the 3500–1000 cm−1 range of the most common organic materials documented in the finishing layers of historical violins are summarised in table 2. in the 3500–3000 cm−1 region, bands attributed to oh around 3500 cm−1 and to nh near 3300 cm−1 are generally present. however, the first overtone signal of the amide ii is generally centred around 3080 cm−1 [35],[36]. characteristic sharp, and often intense, signals produced by ch2 and ch3, can occur in the range of 3000–2700 cm−1. these diagnostic bands are related to the tens of organic compounds variously used in painting and finishing wood surfaces, such as siccative oils, natural resins (vegetal and animal) [33],[34], and proteins, as well as the wood itself [34],[37]. at lower wavenumbers, the bands in the region from approximately 1730 to around 1690 cm−1 are attributed to the carbonyl stretching vibration of esters (e.g. from oils) and carboxylic acids (e.g. from resins) [33],[34], while those falling in the range of 1700–1600 cm−1, mostly centred at around 1650 cm−1, derive from the amide i of the proteinaceous materials (e.g. animal glue, casein) [35],[36]. other marker bands related to these organic compounds can be identified in the fingerprint region, between 1450 and 1000 cm−1, as shown in table 2. the presence of the epoxy resin used to embed the samples is clearly highlighted by the band at 1610 cm−1, which is related to c=c (aromatic ring), the intense and sharp band at 1510 cm−1 c-c (aromatic ring), and by the bands between 1250 and 1000 cm−1 [38]-[40]. signals from the epoxy resin can be expected in the spectra obtained from acquisition areas falling close to the upper or lower boundaries of the embedded sample. table 2. wavenumber values between 3500 and 1000 cm−1 taken from the literature, together with their assignment, of the ftir reflection bands produced by the materials identified in cross-sectioned samples. for the derivative bands, the value refers to the maximum of the band after the application of kk transformations. * siccative oil and natural terpenic resins; ** non-treated wood. material wavenumber (cm−1) band shape assignment proteins 3330 der vasnh [35],[36] 3080 der overtone of amide ii 2960, 2875 der vasch3, vsch3 2935, 2850 der vasch2, vsch2 1650 der vc=o (amide i) 1550 der δnh + vc-n (amide ii) 1450, 1400 der δch 1350–1200 der δnh + vc-n, δch, δnh (amide iii) 1200–1000 der c-o oil-resin varnish * 2950, 2870 der vasch3, vsch3 [33],[34] 2930, 2850 der vasch2, vsch2 1720–10 der vc=o 1465–55, 1380 der δsch2, δasch3, δsch3 1250, 1170, 1100 der vc-o wood ** 3450 abs voh [34],[37] 2940, 2900, 2840 abs vasch2, vasch3, vsch2, 1735 abs vc=o 1650 abs δoh 1598, 1505 abs v(aromatic ring) 1465, 1430, 1380 abs δch2 1330 abs δoh 1280–1240 abs guaiacyl ring vib., syringyl ring vib. 1157 der vasc-o-c 1115 der vas (glucose ring) 1060, 1035 der vasc-o, vsc-o epoxy resin 2960–2850 der vch (arom. and aliph.) [38]-[40] 1610 der vc=c (aromatic ring) 1510 der vc-c (aromatic ring) 1250 der vc-o-c (oxirane group) 1180 der phenyl vib. 1035 der vc-o-c (ether) acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 204 in addition to the signals at high wavenumbers, the wooden substrate produces certain characteristic frequencies in the region between 1750 and 1550 cm−1 and in the fingerprint region, as shown in table 2. 3.1. pca investigation figure 1 presents the results obtained via pca in the six different selected spectral regions. each object (i.e. each single spectrum) is classified and coloured in accordance with the position of the analytical spot in the coating system: epoxy resin (e), varnish (v), ground coat (g), and wood (w). therefore, their assignment mainly reflects the distribution of the organic compounds in the samples. it is worth noting that the ir dataset was composed of spectra clearly related to a single organic material, indicating significant marker bands, together with spectra carrying signals produced by multiple layers (e.g. varnish + ground coat). these expected ‘mixed’ profiles originated from the acquisition areas at the interface between two adjacent layers with different compositions and were labelled as mix 1 (varnish + epoxy resin), mix 2 (varnish + ground coat), and mix 3 (wood + ground coat). the pc1 vs. pc2 scores plot, which related to the range at higher frequencies (figure 1a), accounted for 86 % of the total variance. it is clear that the data exploration in this spectral region did not lead to a clear separation of the objects according to single organic materials. in the pc1 vs. pc2 scores plot (figure 1b), which related to the range between 3000 and 2700 cm−1 (85 % of total variance), the objects were scattered in the four quadrants. nevertheless, it is possible to observe a cluster formed by objects related to the spectra labelled as wood (w), which were grouped in the fourth quadrant. figure 1. pca results obtained in the six spectral regions selected in the range of 3500–1000 cm−1. the scores and loadings plots are related to the following spectral regions: (a) 3500–3000 cm−1, (b) 3000–2700 cm−1, (c) 1800–1550 cm−1, (d) 1550–1450 cm−1, (e) 1460–1260 cm−1, and (f) 1250–1000 cm−1. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 205 the pc1 vs. pc2 scores plot related to the 1800–1550 cm−1 range (figure 1c) accounted for 74 % of the total variance, and this appeared to be the most promising range for discriminating the objects. the spectra identified as v were mostly grouped in the pc1 negative area of the plot, with the spectra identified as e mainly in the first quadrant. furthermore, the spectra collected at the interface between the v and e adjacent layers (i.e. mix 1) assumed intermediate scores. the objects corresponding to the w layer were well grouped in the first quadrant and partly overlapped with the e objects. on the contrary, the g elements did not form a coherent cluster; rather they were heterogeneously scattered. meanwhile, the objects identified as mix 2 (i.e. the interface between v and g) were heterogeneously distributed in the negative pc2 portion, albeit that some were close to the v cluster, thus suggesting a higher influence of the varnish signature in the corresponding ir spectra. this object distribution was well explained by the corresponding loadings. the loadings plot highlighted the significant contribution of the negative signals between 1750 and 1700 cm−1, which mainly characterised the pc1 and pc2 values of the v and mix 1 groups. the positive signals between 1700 and 1600 cm−1, respectively in the pc1 and pc2 loadings plot, characterised the objects labelled as e and w. the scores plot of pc1 vs. pc2 related to the 1550–1450 cm−1 (figure 1d) accounted for 89 % of the total variance. in this case, the objects that mostly differed along pc1 were v and e, albeit that the latter did not form a compact group but were distributed in the first and fourth quadrants. the spectra identified as w were close to each other and were distributed around the axes’ origin. all the mixtures (i.e. the objects identified as spectra collected in the interface between two layers) did not exhibit a specific distribution. the pc1 values of the e objects were positive and higher than those related to other materials. this was due to the intense and sharp marker band of the epoxy resin centred at 1510 cm−1 (c-c, aromatic ring), which was confirmed by the strong signals observed in the region around 1500 cm–1 for both pc1 and pc2 loadings. in the pc1 vs. pc2 scores plot in the range of 1460–1260 cm−1 (54 % of total variance), the picture was more chaotic (figure 1e). here, the objects did not form separate clusters, with the exception of e, which was mainly grouped in the second quadrant. the pc1 vs. pc2 scores plot related to the range of 1250–1000 cm−1 (figure 1f) accounted for 63 % of the total variance. in this case, a partial separation of the objects according to the position of the analytical spot in the coating system was identifiable, with most of the e, v, and w in the fourth, second and third quadrant, respectively. on examining the different examined regions, the object distribution in the ranges of 1800–1550 cm−1 and 1550–1450 cm−1 appeared to be the most promising for discriminating the layers related to one single organic material and exhibiting significant marker bands. meanwhile, the spectra acquired at the interface between two adjacent layers with different compositions did not always exhibit a clear trend. following this, further pca was performed on the entire dataset (97 spectra) while considering the 1800–1400 cm−1 range and grouping all the ‘mixed’ profiles in a separate class, labelled as u (undefined). on examining the pc1 vs. pc2 scores plot (figure 2a), it was clear that the objects associated with the varnish (v) spectral profiles were mostly grouped in the bottom left quarter of the plot, as these objects had both negative pc1 and negative pc2 scores. meanwhile, most of the spectra identified as epoxy resin (e) corresponded to the objects grouped in the bottom right quarter, resulting from a positive pc1 combined with negative pc2 values. the objects corresponding to the spectra collected on the ground coat (g) did not form a sharp cluster in the pc1 vs. pc2 scores plot; however, all these objects were characterised by positive pc2 scores and most of them were well separated from the other layers. wood (w) groups formed around the origin of the pc1 and pc2 axes close to the undefined layers (u). in addition, the third pc (accounting for 11 % of the variance) was investigated. as figure 2b shows, the previously identified groups were confirmed, even though some were more scattered or formed sub-groups. from the loadings plot (figure 2c), the signals corresponding to the bands used to detect epoxy resin, varnish, and ground coat (table 2) – largely composed of proteins – appeared to be those that mostly influenced the spectra distribution in the groups according to the different materials constituting the layers. here, it should be kept in mind that the spectra were transformed according to the first derivative, meaning the maximum of the diagnostic peaks was lost, while it did correspond to the inflection point of the loading profiles. as loadings can assume values from −1 to +1, the variables approaching extreme values in figure 2c were those with a greater influence in constituting the pcs and, thus, were responsible for the spectra distribution in the score plots (figure 2a,b). here, pc1 effectively discriminated the varnish (v) from the epoxy (e) spectra, mainly due to the signals around 1700 and 1510 cm−1 , while pc2 allowed for the discrimination of objects figure 2. pca results obtained in the region between 1800 and 1400 cm−1: a) pc1 vs. pc2 scores plot; b) pc1 vs. pc3 scores plot; c) pc1, pc2, and pc3 loadings plot. e = epoxy resin, v = varnish, g = ground coat; w = wood; u = undefined. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 206 related to ground coat (g) due to the amide i signal (1665–1645 cm−1). 3.2. pls–da classification model the materials identified through the observation of each layer position in the stratigraphy and confirmed via pca were used as classes (e, v, g, w and u), thus constituting the a priori information (y) to build the pls–da classification model (table 3) that has the capacity to predict the predominant materials in the layers based on the obtained spectral data (x). the pls–da model was first calibrated, that is, a classification rule (equation) was established based on a representative set of samples. following this, the model was internally validated via an iterative exclusion of part of the calibration set, that is, one out of the eight groups of samples (selected via the ‘venetian blinds’ procedure) served as an internal test set, while the remaining data were used for the calibration. the results of the eight tests were then averaged and the constituent strategy that achieved the highest accuracy was selected. the prediction ability of the optimised model was then tested using an external test set. the three steps of the model development (calibration, crossvalidation, and prediction) were evaluated in terms of sensitivity and specificity. here, sensitivity relates to the model’s capacity to correctly recognise samples belonging to a specific class, while specificity relates to the model’s capacity to correctly reject the samples belonging to all other classes. the internal validation (cross-validation) of the model performed well for most of the considered classes, achieving a sensitivity of over 0.90 and a specificity of over 0.85. however, with the g class, the sensitivity was 0.75, as four out of the 12 samples were misclassified as a (3) and w (1). this misclassification was expected since the a priori assigned classes related to the most present component in the layer; however, it is unrealistic to assume that each single layer is made up by one pure substance, which means each spectrum would potentially contain signals from different compounds. the model’s prediction ability was optimal for the e and v classes, with a sensitivity of 1.00 and a specificity of over 0.78. however, while the specificity of the g and w classes reached the maximum level (1.00), the attendant sensitivity was poor. here, one out of the two samples defined as ground coat was classified as undefined, while ls_v.25 and ls_v.26, which were defined as wood, were assigned to the epoxy class. the low performance of the prediction phase was largely related to the low number of spectra constituting some of the classes, mainly u and g. in fact, the misclassification of only one spectrum resulted in a specificity of 0.50 for the g class. however, the prediction phase presented the greatest strength of the developed model since this phase is missing in most heritage classification cases due to the difficulties in collecting data from different samples. 4. conclusions the investigation of precious and brittle micro-samples is a challenging task when thin layered systems such as those encountered in musical instruments are considered. in fact, to precisely characterise the attendant materials, an in-depth microinvasive – if not micro-destructive – wide analytical campaign is generally needed. as such, how to employ sr–ftir microspectroscopy to obtain hundreds of informative spectra on micro-samples without damaging the samples is a crucial aspect. using sr at the sissi-bio beamline, it was possible to adopt reflection geometry with increased lateral resolution to take advantage of irsr brightness to obtain better spectra (with higher signal-to-noise ratios) while avoiding contact with the cross sections and preserving the surfaces for further analyses. a preliminary elaboration of the large sr–ftir reflection dataset was performed using chemometric tools to obtain a rigorous picture of the ir results and to distinguish different material classes, previously selected in accordance with the position within the coating system of the analytical spot. three additional mixed classes were identified at the interface between the layers. the pca, which was performed on different spectral regions, confirmed the preliminary material assignment by highlighting clear sample groupings for varnish, ground coat, wood, and epoxy resin on the basis of their spectral features. moreover, the explorative analysis confirmed that the spectra collected at the interfaces between different layers exhibited coherent signals produced by different materials. the pls–da classification model revealed the feasibility of the proposed methodological approach aimed at discriminating the constituent materials of bowed string instruments in a fast and rigorous way without the need to visually inspect a large number of spectra. further case studies are planned to expand the number of investigated historical instruments while considering other materials with characteristic spectral features. in addition, the method developed in this work will be tested on other datasets collected with different non-invasive analytical techniques (e.g. xrf, ftir in reflection geometry) in relation to a large number of historical cremonese musical instruments. acknowledgements the authors would like to thank the fondazione arvedibuschini, the fondazione bracco, the fondazione museo del violino, the international school of violin making of cremona, and the accademia of santa cecilia. a special acknowledgement goes to andrea zanrè and elisa scrollavezza. the authors also acknowledge the ceric-eric consortium for access to the experimental facilities. references [1] f. caruso, d.f.c. martino, s. saverwyns, m.v. bos, l. burgio, c. di stefano, g. peschke, e. caponetti, micro-analytical identification of the components of varnishes from south italian historical musical instruments by plm, esem–edx, microftir, gc–ms, and py–gc–ms, microchem. j. 116 (2014), pp. 31-40. doi: 10.1016/j.microc.2014.04.002 [2] s.l. lämmlein, d. mannes, b.v. damme, f.w.m.r. schwarze, i. burgert, the influence of multi-layered varnishes on moisture protection and vibrational properties of violin wood, sci. rep. 9 table 3. figure of merit of the pls–da model related to the calibration (cal), cross-validated (cv) and prediction (pred) steps. sens = sensitivity; spec = specificity. e = epoxy resin, v = varnish, g = ground coat; w = wood; u = undefined. e v g w u n (cal) 17 20 12 4 18 n (pred) 7 10 2 5 2 sens (cal) 1.00 0.90 0.83 1.00 0.94 spec (cal) 0.98 1.00 0.98 0.97 0.91 sens (cv) 1.00 0.90 0.75 0.75 0.94 spec (cv) 0.98 1.00 0.98 0.97 0.85 sens (pred) 1.00 1.00 0.50 0.60 1.00 spec (pred) 0.78 0.91 1.00 1.00 1.00 https://doi.org/doi:10.1016/j.microc.2014.04.002 acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 207 (2019), 18611. doi: 10.1038/s41598-019-54991-5 [3] j.p. echard, l. bertrand, a. von bohlen, a.-s. le hô, c. paris, l. bellot-gurlet, b. soulier, a. lattuati-derieux, s. thao, l. robinet, b. lavédrine, s. vaiedelich, the nature of the extraordinary finish of stradivari’s instruments, angew. chem. int. ed. engl. 49 (2009), pp. 197-201. doi: 10.1002/anie.200905131 [4] s. tirat, i. degano, j.p. echard, a. lattuati-derieux, a. lluverastenorio, a. marie, s. serfaty, j.y. le huérou, historical linseed oil/colophony varnishes formulations: study of their molecular composition with micro-chemical chromatographic techniques, microchem. j. 126 (2016), pp. 200-213. doi: 10.1016/j.microc.2015.11.045 [5] a. von bohlen, s. röhrs, j. salomon, spatially resolved element analysis of historical violin varnishes by use of μpixe, anal. bioanal. chem. 387 (2006), pp. 781-790. doi: 10.1007/s00216-006-0777-7 [6] f. poggialini, g. fiocco, b. campanella, s. legnaioli, v. palleschi, m. iwanicka, p. targowski, m. sylwestrzak, c. invernizzi, t. rovetta, m. albano, m. malagodi, stratigraphic analysis of historical wooden samples from ancient bowed string instruments by laser induced breakdown spectroscopy, j. cult. herit. 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delagrammatikas2, olga papadopoulou2, charalampos titakis2, panayota vassiliou2 1 school of mechanical engineering, university of california, berkeley, 2521 first ave. ca 94709, usa 2 school of chemical engineering, national technical university of athens, 9 iroon polytechniou st. 15780, athens, greece section: research paper keywords: finite elements analysis; parthenon; steel corrosion; marble; restoration citation: zacharias vangelatos, michail delagrammatikas, olga papadopoulou, charalampos titakis, panayota vassiliou, finite element analysis of the parthenon marble block-steel clamp system response under acceleration, acta imeko, vol. 10, no. 1, article 21, march 2021, identifier: imeko-acta10 (2021)-01-21 editor: carlo carobbi, university of florence, italy received may 15, 2020; in final form august 25, 2020; published march 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. corresponding author: michail delagrammatikas, e-mail: mdel@mail.ntua.gr 1. introduction the temple of the parthenon on the acropolis of athens is considered to be the opus magnum of classical antiquity. this is mainly due to its ideological impact and its architectural and artistic value, which have influenced human civilisation, probably more than any other ancient monument, to the point that it provides the inspiration for the logo of the un cultural organisation, unesco [1], [2]. the building of the parthenon, designed by the architects iktinos and kallikrates, began in 447 bce, and the structure was completed by 438 bce, while its sculptural decoration, designed by the sculptor phidias, was completed in 432 bce. what is probably less familiar to the general public is the technology that allowed this monumental structure to be built, sustain its structural integrity for 21 centuries and be able to remain, even in today's ruinous state, in a very good and recognisable condition almost 2,500 years after its construction. the latter is owed mostly to the laborious restoration programmes, almost constant during the most recent nearly 200 years of the existence of the modern greek state, which are currently conducted by the acropolis restoration service (ysma) [3]. the durability of the monuments of the acropolis of athens is owed to the combination of natural materials of the highest quality, selected and worked not only by skilful artisans but by people who knew how to exploit their properties to their advantage, manmade materials comprising the most advanced technology of the era and elaborate masonry methods and techniques, which incorporated knowledge of previous experience both in the fields of aesthetics and of structural resilience in a zone of high seismicity. the present study aims to highlight the importance of the conservation state of the original and restoration materials in terms of the response of the reinforced marble block system to abstract finite element analysis is employed to investigate the mechanical behaviour and failure scenarios of the marble block–steel clamp ancient masonry system utilised in the parthenon (athens acropolis) under static loading analysis. the input data for the model are acquired by the laboratory testing of early 20th century restoration steel clamps, such as through tensile strength measurements and metallography, as well as bibliographic sources from various scientific fields (i.e. material properties, archaeometry, restoration, structural engineering and geology). two different embedding materials (portland cement mortar and lead), used for the nesting of the clamps, are examined under bending or stretching, induced by acceleration forces. the conservation status of the materials is taken into account by employing an intrinsic stress, as is the case when corrosion products build up in a confined space. the aim of this work is to provide a tool for the assessment of the conservation potential of the marble blocks in parts of the monument that require specific attention. simulation results indicate the resilience of the parthenon’s structural system under most examined scenarios and highlight the importance of intrinsic stresses, the existence of which may lead to the fracture of the marble blocks under otherwise harmless loading conditions. mailto:mdel@mail.ntua.gr acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 156 earthquake-induced stresses. the approach of this work is interdisciplinary, mainly addressing conservation issues in relation to the monument materials rather than structural engineering. the paper consists of six sections. section 2 provides a general presentation on the structural system of the parthenon and its history, emphasising the causes of the monument’s structural decay and describing past restoration interventions, the consequences of which are the main subject of the paper. most of the data for this section have been collected from the ‘study for the restoration of the parthenon’, edited by korres and bouras [4]. section 3 consists of a brief bibliographic presentation of the seismicity in the attica region and previous works on computer-assisted research into the mechanical performance of the acropolis monuments using finite elements analysis. section 4 presents the experimental setup for the laboratory testing of restoration steel clamps and for the finite elements analysis, while section 5 presents the results and discussion. the concluding section summarises the main points of this study and suggests future work. 2. the parthenon’s structural system and history 2.1. the structural system of the parthenon the structural system of the parthenon's load-bearing masonry comprises pentelic marble blocks, interconnected by a system of steel clamps and dowels, embedded in lead without the use of any mud, mortar or cementitious binding material (figure 1); thus, it can be described as a reinforced marble system. each individual marble block has been cut to the correct size and shape (a quasi-rectangular parallelepiped), tailored to satisfy the refinements of the architectural design and compensate any visual distortions. each block has been smoothed to achieve tight contact between the entire surface of the contact areas of the blocks. the contact between the marble blocks is so tight that, even today, water cannot penetrate the non-disturbed parts of the masonry. the ancient marble cutters, where possible, took advantage of the anisotropy of the natural material and oriented the longest axis of the marble crystallites in the direction where the strongest mechanical strength was required [4]. the blocks were anchored to each other using steel joints nested in specially carved cavities and embedded in molten lead, which provided a means of mechanical continuity and a ductile buffer material between the steel and the marble, while also acting as a protective environment against steel corrosion. ‘double-t’-shaped clamps were used to join the marble blocks belonging to the same horizontal level, while smaller dowels were used to join the marble blocks to the ones above and below. the steel-clamp anchoring system did not connect all parts of the monument uniformly but was designed to create rigid areas with similar loadbearing characteristics, while allowing for different areas to perform independently [4]. the steel clamps were produced by cladding and folding layers of low-carbon hypoeutectoid steel and ferritic iron with a very low impurities content, which allowed for better corrosion resistance. the metal joints were not designed to bear the static loads but to contribute to the response of the system in case of deformation due to subsidence or an earthquake. the thickness of the steel clamps (of typical crosssection dimensions of between 0.3 x 3.0 cm to 0.5 x 5.0 cm) was small compared to the width of the nesting cavity and, thus, to the thickness of the moulted lead as well as to the thickness of the surrounding marble. this design ensured that if the marble blocks moved apart, it would be the clamps , not the marble, that would fail [4]. similar structural design and reinforcement technology for stone masonry has also been adopted in several other monuments around the world. corrosion analyses on ancient iron clamps from the gupta temple at eran in madhya pradesh (india), dating from the 5th century ce [5], as well as metallurgical analyses on iron crampons from masonry complexes in a palace and two temples in angkor (cambodia), dating from the khmer empire between the 10th to 13th centuries ce [6], offer valuable information on the properties and preservation state of ancient iron-based architectural elements. 2.2. structural decay of the parthenon until the 20th century this masonry system proved to be very resilient to local seismicity, ensuring the structural integrity of the monument for more than 2,000 years. there have been, in fact, several severe earthquakes in the attica region, and the first documented damage to the parthenon occurred during an earthquake in 426– 5 bce, during which the marble blocks in the north-east corner were shifted about 2 cm northwards [4]. regardless, all the severe damage to the structural elements of the monument is due to anthropogenic causes. the first case of severe damage to the monument is believed to have happened between the 4th and 5th centuries ce, and even though there are no historical records of this event, the pathology of the building materials testifies to thermal damage caused by a fire, which deprived the monument of its original roof and many of the internal architectural elements. nevertheless, the structural integrity of the parthenon was preserved until 1687 ce, when a mortar bomb, launched during the siege of the ottoman guard residing on the acropolis by the troops of the venetian captain-general, and later, doge, francisco morosini, penetrated the roof of the parthenon, causing the ammunition stored inside to explode. this explosion resulted in the catastrophic collapse of most of the long side walls and peristyles. the lower-level marble blocks of the parthenon, and of the other classical monuments on the acropolis, suffered a different catastrophic decay in the course of the greek revolution. during the siege of the acropolis (1821–1822), the besieged ottoman garrison broke off the corners of many of the marble blocks in order to gain access to the lead to use it as ammunition. the created voids and cracks allowed the penetration of water, which reached the steel joints and induced corrosion on the steel clamps. the aforementioned events were not the only cases of severe damage being caused to the parthenon, but they had the most figure 1. (a) a schematic presentation of a wall of the athens parthenon; (b) a schematic presentation of the application of steel double t’s and dowels, embedded in cast lead, to connect the marble blocks [7]. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 157 impact on its structural integrity. acts of vandalism by the early christians and acts of looting during the period between the 17th and early 19th centuries by visiting art collectors had already deprived the monument of a significant amount of sculptural ornamentation, while the looting by lord elgin of large parts of the frieze, which was sculpted on structural marble blocks, caused further structural damage. in addition to this accidental or malicious damage, it should be noted that the building had undergone several restorations and alterations corresponding to its different uses, mainly as a place of worship. the parthenon (i.e. the house of the virgin) served as a pagan temple to the goddess athena, as a christian church dedicated, most of the time, to the virgin mary ‘athiniotissa’ and as a muslim mosque. after the 1687 catastrophe, a new, small mosque was built inside the ruins of the temple, using much of the original debris as building material, while many other such structures were erected on the acropolis. 2.3. the restoration of the parthenon as an ancient monument this was the situation of the athens acropolis monuments when the newly founded greek state began the task of restoration according to the restoration trends of the era. these mainly consisted of the purification of the classical monument from all later modifications. the first large important restoration programme of the parthenon took place between 1842 and 1845 and was part of a large-scale excavation plan that included the demolition of later construction, partly to reclaim the incorporated ancient building material but also serving the ideological context of advocating ancient greek civilisation, which constituted a state policy. fortunately, the restoration principles that were employed by the greek state never followed the fashion of complete reconstruction but aimed to restore the ruins using as much of the original material as possible. this policy can be attributed to the fact that the acropolis monuments were no longer considered as a functional monumental complex, but as a ruinous ancient monument serving as a symbol of the rebirth of the nation. the largest restoration works in the past took place between 1898 and 1933 under the supervision of an engineer, nikolaos balanos, who, against recommendations to use brass reinforcement elements, used restoration double-t or squarebracket-shaped steel clamps of different qualities, mostly embedded in portland cement mortar, along with large iron beams and elements of reinforced concrete, where convenient. the technique of moulding lead was sporadically used, while ancient clamps that were in a good state were reused. the restoration clamps were significantly thicker than the ancient clamps, and their metallurgical characteristics varied significantly during the course of the works. the choice of embedding the joints in portland cement mortar proved to be another unsuccessful choice, as just a few years after the application, the steel showed severe corrosion. unlike the lead, the porous mortar did allow the water to reach the iron clamp, while the restored marbles did not have perfect contact between them, allowing the water to penetrate the interface. as a result, iron corrosion products, mostly oxides and hydroxides, which have larger crystal volumes than the iron alloy, grew producing stresses, which led to the excessive cracking of the marble blocks. the restoration works under balanos are considered to be below the standards of their era, lacking in both design and documentation, resulting in extensive damage to the monument’s original material. however, from a strictly structural point of view, these interventions allowed the monument to withstand the strong 1981 earthquake with minimal damage [4]. the next large-scale restoration project on the acropolis of athens started in 1975, when the interdisciplinary scientific committee (esma) undertook the task of providing scientific support to the acropolis restoration service. one of the measures proposed by esma was the substitution of steel by titanium alloy joints [8]-[10], which have better physicochemical characteristics and are not prone to corrosion in natural environmental conditions. the surrounding material that binds the joints to the marble is a cement mortar comprised of aalborg white cement and quartz sand. hydraulic grouts based on aalborg white cement and finely ground natural pozzolan are being used for the consolidation of broken parts [11]. although, by the end of the current restoration works, most of the previous restoration clamps will have been removed, the parts of the monument that are still standing in their original position will continue to be connected by the authentic steel clamps embedded in lead. 2.4. emergence of atmospheric pollution in the mid-1950s and the acceleration of structural element decay additional causes of anthropogenic damage that emerged in the 20th century are related to atmospheric pollution as a result of the industrialisation of athens in the mid-1950s and the unsound use of materials during the restoration on the monuments before 1933. both marble and steel joints (original and restoration) were, inevitably, exposed to heavy atmospheric pollution during the period 1955–1985 due to the rapid industrialisation of athens and the extensive use of high sulphur content fuels. between 1984 and 1985, the main atmospheric pollutants, according to the monitoring data, were soot (elemental c), particulate matter, no2, so2, co2, co, o3, cfcs (chloro-fluoro-carbons) and pans (peroxy-aketyl-nitrates). all these species were detected at high levels. the dry and wet pollutant precipitation, in the form of particulate matter and acid rain, caused extensive sulphation of the marble surface and also accelerated the corrosion rate of the steel clamps. two different sub-groups of restoration clamp samples, which were collected from 1952– 1954 from demolished buildings and during interventions at the erechtheion from 1979–1980, and which were examined by founding esma member, prof. skoulikides, testified to an increase in the corrosion rate of 25–30 %, attributed to the impact of industrial pollution [10]. more recent studies involving quantitative measurements of atmospheric pollutants at the centre of athens were published in the 1990s. aerosol species (anions, such as cl-, so42-, no3and nh4+, and cations, including na+, k+ and ca2+) were detected at moderate levels, and gaseous pollutants (mainly hcl and hno3 acids) were rather high [12], [13]. cleaner fuels and pedestrianisation in the vicinity of the acropolis hill have significantly reduced the impact of atmospheric pollutants on the monuments since the early 2000s. 3. seismic activity in the attica region and finite element analysis of the parthenon’s structural system response greece lies on the collision plate boundary between the african and the eurasian tectonic plates; thus, it is a region of high seismicity. according to the greek seismic regulation, attica, the province in which athens is situated, is ranked in the acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 158 second of four zones on a scale of increasing seismic activity. thus, modern structures are designed to be able to withstand an effective peak acceleration (epa) of 0.16 g, where g is the gravitational acceleration [14]. during the last severe earthquake that hit athens in september 1999, the peak ground acceleration (pga) recorded varied between 0.044 and 0.511 g, while amplification analysis has estimated values as high as 0.7 g [15]. the earthquake response of many classical monuments in greece, including the parthenon, has been a growing field of study over the last decades. many research groups have conducted computer-aided simulations, based on finite element models, in order to assess the local and overall vulnerabilities of the marble structures (either intact or damaged) during repetitive loading and to predict potential failure conditions [16]-[20]. according to the relevant literature, the mechanical performance of complex marble structures requires the design of both simplified models (sub-assemblies of the system) and structural models of the whole structure [17]. the discontinuity of masonries due to multiple structural marble elements along with iron clamps and joints and other original or restoration materials (lead, cement-based mortars, etc.) at their interface, enhances the challenge of realistic simulations. the effect of past restoration interventions on the monument’s mechanical performance as well as the evaluation of new materials to be used in on-going restoration works (titanium clamps and white cement mortar) has also been studied in some cases [18], [19]. psycharis et al. [17] investigated the dynamic earthquake impact on the parthenon’s marble masonry via a simplified sub-assembly model (section of a wall) and a general structural model of the partially restored monument. these two numerical models were run with and without taking into consideration the metal reinforcing elements. among the most important findings of this work were (a) the overestimation of the maximum displacements by the simplified model and (b) the lower residual deformation in the presence of the metal clamp connectors between the marble blocks, but the significantly higher rocking response (larger maximum displacement). pasiou et al. [19] studied the stress fields that developed after static shearing loading on the parthenon’s restored epistyles, using a numerical model that describes a small system of marble walls–titanium connectors–cement mortar. high mechanical impact was observed around the interface of the two epistyles in contact, near the metal clamps, and the effect of the relieving space between the marble and the metal clamp was also investigated. another finite element analysis, performed on the restoration of the parthenon’s detached epistyle parts by ganniari-papageorgiou [18], focused on the role of the cementitious layer between the marble and the bolted titanium bars during bending, which is found to significantly reduce the strain discontinuity that was observed locally near the titanium clamp. toumbakari analysed the failures of the northern wall orthostate and focused on the role of the connection elements (clamps and dowels) in the observed pathology [20]. 4. experimental 4.1. laboratory testing for the scope of this research, three steel clamps, two doublet shaped (s.200, s.214) and one square-bracket shaped (s.245), all previously used in restoration works executed under the supervision of balanos, were provided to the authors by ysma. the clamps were submitted to a series of chemical analyses, mechanical tests, microscopic evaluation and electrochemical corrosion testing. elemental chemical analyses were conducted using an arl3460 automatic optical emission spectrometer (oes), and tensile tests were performed with a roell amsler system mfl upn 1000 apparatus at the halyvourgiki inc. laboratory on specimens cut out of the three clamps (s.200, s.214 and s.245). the assessment of the experimental data for the calculation of the mechanical properties of the clamp specimens was performed in accordance with the en 10080 (2005) standard, taking into consideration the actual geometric characteristics of the specimens. micro-hardness tests were performed using an instron wolpert gmbh v-testor 4021. fracture surfaces were examined by means of scanning electron microscopy (sem), using a fei 200 sem equipped with a tungsten filament and an everhart-thornley detector. metallographic observations were performed on cross sections, polished to 0.25 μm and etched by nital 2 % ethanol reagent for 30 s using a leitz aristomet optical microscope under crossed nicols. the potentiodynamic electrochemical measurements on the corroded surfaces of the three steel clamps under investigation were conducted using a typical three-electrode cell configuration in 350 ml electrolyte volume, connected to a gamry potentiostat. the clamp specimen was employed as the working electrode, an ag/agcl (ase3.5m kcl) electrode as a reference and a pt wire as the auxiliary electrode. the control of all test parameters and the subsequent analyses/modelling of the experimental data were performed using cms 100 software. the specimen’s free corrosion potential was recorded for 30 minutes under open-circuit conditions, and then a tafel curve was acquired at a scan rate of 1 mv/s. the potential range of the anodic sweep was set at open-circuit potential (ocp) ± 200 mv. the city of athens exhibits environmental conditions typical of an urban–industrial–marine environment. for that reason, a synthetic electrolyte representative of the wet precipitation in athens city centre was produced in the lab, based on atmospheric pollution data collected in this particular geographical area in 1998 [13]. this aqueous electrolyte mixture contained dissolved sulphates, nitrates, chlorides and carbonates at suitable concentrations. the ph of rainwater in athens was measured as being between 4.5 and 5.5 during the period 1995–2000 [10]. the synthetic electrolyte ph is on the upper limit of this range. the molar concentration of all ionic species and properties of the electrolyte are presented in table 1. 4.2. computational analysis all the finite element analysis simulations were performed with the multi-purpose finite element ansys software (workbench 18.0). ansys enables the design of complex components to depict the geometry of a system (cads), utilising ansys design modeller. in addition, it discretises the structure to finite elements, estimating the mechanical behaviour of the system. it can be utilised to estimate the deformation of the structure under static loading and modal analysis. it was table 1. molar concentrations of ionic species and properties of the synthetic electrolyte. so42(mol/l) 0.050 no3(mol/l) 0.025 cl(mol/l) 0.008 hco3(mol/l) 5 ·10-5 total salt concentration (% w/v) 0.97 ionic conductivity (ms/cm at 24 ˚c) 11.29 ph (at 24 ˚c) 5.5 acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 159 employed for the modelling of the mechanical behaviour of the clamp that was representative of those that were used in the acropolis of athens monuments restoration between 1898 and 1933. in our case study, ansys was used for static analysis under acceleration effects caused by loading conditions that emulated an earthquake. a characteristic discretised cad of the system is presented in figure 2. the system was discretised by 10-node, tetrahedral finite elements with 45,924 elements and 76,025 nodes. the various parts comprising the clamp (steel-lead or steel-cement mortar) were 8-node surface elements assigned to the interfaces of the constituent parts. the potential contact between the clamp and the marble block was modelled as simple friction without sliding. the high density of the mesh grid was designed to accurately predict the generated stresses. it must be noted that we focused on the mechanical behaviour of an individual structural block of the system. while in other reports the whole monument was modelled, this significantly decreases the precision of the numerical simulations due to the high computational cost. therefore, a symmetric unit cell of the masonry system was modelled, comprised of two successive half marble blocks that were in contact with each other and connected by a clamp and the embedding material in the nest. three different physical models were tested. the first two consisted of a non-corroded steel clamp (7.6 mm thickness) embedded in the marble nesting using portland cement mortar or lead as binding material. the third physical model consisted of a corroded steel clamp on which the growth of the corrosion products had induced a tensile stress in the lead and the half marble blocks. this last model is more challenging since the determination of the actual mechanical properties of corroded steel reinforcement continues to be an open challenge in the fields of corrosion science and structural engineering [21]-[23]. the aim was to investigate the effects of the corrosion phenomena on the mechanical behaviour of the system under bending loading. the physical model for this simulation was designed by equating the space that would become available as a result of the elastic deformation of the surrounding materials under a stress of 5 mpa, plus the reduction of the metal core of the clamp to the corresponding thickness of a dense layer of lepidocrocite with a density of 4,000 kg/m2. the stress of 5 mpa was chosen as corresponding to the lowest limit of the ultimate tensile stress of pentelic marble. any more stress could lead to the cracking of the marble caused by the crystallisation pressure of the corrosion products without any external loading action, causing irreversible damage, at least from the perspective of monument conservation. the imposed loading consisted of a one-way acceleration enforced for 1 s, corresponding to a 0.25 hz frequency, which is in the range of local earthquakes. the finite element analysis was performed for three acceleration values: 0.3 g, 0.6 g and 1.2 g, corresponding to a typical local pga and its twoand four-fold products, respectively. the direction of the acceleration was simulated for two different scenarios. in the first scenario, the direction of the acceleration was parallel to the axis of the clamp (parallel to the z axis of figure 2). in this scenario, referred to as stretching, the left half marble block was fixed, while the right was free to move. in the second scenario, referred to as bending, the acceleration was perpendicular to the clamp axis (parallel to the x axis) and imposed on the interface of the two marble blocks. the outer unit cell surfaces on the xy plane, which correspond to the middle of the integral marble block, as well as the dowel fittings, were only allowed to rotate; otherwise, the system would sustain a rigid body motion, prohibiting the effective simulation of stresses and strains. the mechanical properties required for the model are density, young's modulus of elasticity, poisson's ratio, tensile yield strength and ultimate tensile strength, as derived from the stress–strain curves. each simulation took approximately 8 hours of computer time. 5. results and discussion 5.1. chemical composition & mechanical properties the oes results, briefly presented in table 2, indicate that all three samples have a carbon concentration between 0.008 and 0.085 %; the s.214 clamp has the highest c content (0.085 %). according to the thermodynamic data on steels (metastable fe figure 2. marble block mesh grid for the finite element analysis. table 2. measured chemical composition and mechanical properties of the three steel clamps in comparison with the ancient clamps. composition (% w/w) / properties clamp s.200 clamp s.214 clamp s.245 ancient clamps* carbon 0.019 0.085 0.008 0.017–0.582 manganese 0.454 0.039 0.007 0.001–0.064 sulphur 0.022 0.010 0.003 0.006–0.007 chromium 0.024 0.003 0.002 0.004–0.017 other minor elements 0.169 0.141 0.076 0.14*** iron balance 99.312 99.722 99.904 99.75*** vickers hardness (hv) 121.5 117.8/110.5** 92.4 max. 107 tensile yield strength (mpa) **** 240 248 ultimate tensile strength (mpa) 410 328 322 max. 380 young’s modulus e (gpa) **** 50 40 strain % 17.1 11.4 18.9 *reported by varoufakis [6]; **zone a/ zone b; ***average based on estimated mode c value of 0.061 % w/w; ****not recorded/calculated acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 160 c diagram [24]), s.214 can be identified as hypoeutectoid plain carbon steel because its c content is between 0.025 and 0.8 %. clamps s.200 and s.245, with a c content below 0.025 %, although typically low-alloy steels, should be categorised as irons due to their ferrite matrix [24]. these results are in accordance with the ancient low-carbon t-clamps analysed by varoufakis [25]. all the examined reinforcement clamps exhibit very low hardness, and their stress–strain curve exhibits the typical patterns of ductile fracture. the measured ranges of hardness, tensile strength and % strain are given in table 2. these values approximate relevant measurements by varoufakis [25] on ancient t-clamps. the reduced hardness is attributed to the low c and si concentrations. in the case of clamps s.214 and s.245, the tensile yield strength was also recorded as well as the young's modulus. upon fracture, the clamps are more or less prone to a ductile fracture mechanism. of the three examined steel clamps, s.214 was selected for more detailed examination because of its extensive microstructure heterogeneity and its unique resemblance to the ancient material characteristics [26]. as can be observed on a polished cross section (figure 3), the manufacturing process resulted in a composite material. it is evident that hypoeutectoid steel and the low-carbon iron-alloy sheets were cladded and folded. the metallographic observations after chemical etching reveal the various microstructure characteristics of the distinct zones a, b and c, marked on figure 4. the recrystallised grains of all the zones testify to the absence of intense cold working processes during the manufacturing process, which is in accordance with the low-hardness values measured. the grain orientation of the thin superficial layer above zone a, not presented in the two optical microscope photographs, indicates a final mild work hardening process of the surface. the fractography of the s.214 clamp provides a variety of ductile fracture patterns, which is characterised by dimples (figure 5). some areas exhibit a more brittle fracture type. the electrochemical tests served the purpose of assessing the relative electrochemical reactivity of these restoration metals when exposed in an electrolyte that approaches the wetness conditions of their original corrosion environment and could enable corrosion reactivation. in addition, these measurements enable a more complete characterisation of the restoration metals and their performance. the electrochemical behaviour of the corroded surfaces of the three steel clamps is in good agreement with the findings of the chemical and metallurgical characterisation and the available literature data. higher carbon and sulphur content in the iron figure 3. macrophotographic documentation of the clamp s.214 cross section. marked areas a and c correspond to external and internal hypoeutectoid steel zones, while b corresponds to the low-carbon iron zone. figure 4. optical microscope photograph (x 100) of the s.214 clamp cross section, chemically etched with 2 % nital reagent. (a) interface of the hypoeutectoid steel zone a (top) and the low-carbon iron alloy zone b (bottom), inside which mns inclusions are visible. (b) hypoeutectoid steel zone c interposed between two low-carbon iron alloy b zones. figure 5. secondary electron micrographs of clamp s.214 fracture surfaces (a) x 1000 and (b) x 2000, acquired by sem. characteristic ductile fracture dimples cover most of the fracture surface. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 161 matrix contribute to a higher susceptibility to corrosion [25]. clamp s.214’s surface, in its as-received state, was covered with some typical iron corrosion products. this corrosion layer delayed the uniform wetting of the metal surface, and this is evident by the fluctuations observed in the ocp curve during the initial 20 minutes of immersion (figure 6). the final equilibrium potential corresponds to an electrochemically active state, i.e. the corrosion process is re-activated. clamps s.200 and s.245 exhibit similar trends, and in both cases, the free corrosion potential is stabilised very quickly at more positive (noble) potentials. the 5-minute and 30-minute ocp values are presented in table 3. the results of the subsequent linear polarisation tests show that s.214’s surface, which is of higher c concentration and metallurgical heterogeneity, is more prone to further corrosion, exhibiting the highest corrosion current density (icorr) and annual corrosion rate and the lowest polarisation resistance (rp) (table 3). s.214’s corrosion rate is two times higher than that of s.245, the pure iron clamp, and nearly four times higher than s.200. the best electrochemical properties relating to corrosion are observed for clamp s.200 and are attributed to the addition of chromium in the alloy. 5.2. computational modelling computational modelling has been performed using as input parameters the experimental values obtained for the s.214 clamp and bibliographic data for all other materials (table 4). these simulations aim to investigate the effect of either lead or portland cement mortar as well as stresses generated by corrosion-product growth in the clamp assembly under either bending or stretching caused by the acceleration force of the marble blocks. regarding the loading conditions, the stresses that develop on the metal clamp–marble block system during earthquakes depend on their orientation in the walls of the structure. thus, for a north–south direction earthquake, the walls that are parallel to this direction will be affected by tensile stresses, while the perpendicular ones will be subjected to shearing or bending stresses. when a body is stressed by tensile and shearing stresses, these types of stress can be converted to what is called equivalent von mises stress, which is considered to be a tensile stress. this is the measure that determines whether a part of the structure has either reached the plastic zone (e.g. ductile) or has surpassed the yield point and will fracture (e.g. brittle). in any case, when the von mises stress is higher than the tensile yield strength, failure occurs. a representative deformed configuration of the simulation results under bending, presented in figure 7, is indicative of the morphology of the generated stress. the different colours represent variations in the stress field of both the clamps and the marble blocks. even though the stresses that develop on the marble blocks in the simulation presented in figure 7 are significantly lower than the tensile strength of the marble, their distribution reaches its maximum near the dowels. this observation is in accordance with the damage caused to the lower parts of the parthenon and to the models published by toumbakari [20]. the fact that the presented results are congruous with the reported ones shows the validity of our model. figure 6. (a) ocp recording for steel clamp s.214 for 30 minutes, before the linear polarisation test, compared to the two low-carbon clamp samples, s.200 and s.245; (b) tafel curve of steel clamp s.214 and fitted model by linear extrapolation method compared to the two low-carbon clamp samples, s.200 and s.245. the tests were conducted in the 0.97 %w/w synthesised atmospheric pollutants electrolyte. table 3. free corrosion potential values recorded under oc conditions before the potentiodynamic tests, and tafel extrapolation results calculated for the three clamp samples after linear polarisation in the 0.97 %w/w atmospheric pollutants electrolyte. measured property s.200 s.214 s.245 5 min ocp (mv/ ase) -578 -743 -573 30 min ocp (mv/ ase) -602 -736 -586 ecorr (mv/ ase) -685 -457 -625 icorr (10-5 a/cm2) 2.28 8.19 4.38 rp (102 ohm cm2) 10.94 3.03 9.85 betac (mv/decade) 85.9 108.3 254.7 betaa (mv/decade) 173.6 121.2 0.953 corrosion rate (mm/year) 0.266 0.953 0.509 table 4. model input data. input property steel clamp lead pentelic marble cement mortar corrosion products density d (kg/m3) 7.85·103 1.13·104 2.71·103 2.40·103 4.00·103 young’s modulus e (gpa) 50 14 49 48 9.0 poisson ratio v 0.30 0.42 0.26 0.33 0.20 tensile yield strength σy (mpa) 2.5·102 18 ultimate tensile strength σy (mpa) 3.5·102 27 15 5.0 1.5 ultimate compressive strength σy (mpa) 114 37.5 15.0 acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 162 table 5 summarises the maximum von mises stress that was obtained for each whole system and for the marble in each system under each loading condition. the first part of the table presents the maximum stress values that develop in the unit cell regardless of the exact material that these are enforced on, and the second part presents the maximum stress that develops on the marble blocks. the third part of the table presents the linear coefficients between maximum stress developed and acceleration imposed and the coefficients of variation, calculated by regression analysis, while the fourth part presents the estimated maximum stress values for the epa of 0.16 g, according to the linear coefficients. the results predict that the most catastrophic loading condition for the system is stretching. the reason is that during this deformation, the marble block applies high stress to the edges of the clamp, leading to high stress concentration. however, it should be pointed out that the free movement of the marble block presupposes the absence of other supports for this particular block, such as is the case in corner blocks or corner groups of blocks where the accelerated mass would be manifold. this scenario could describe the pathology induced by the 426– 5 bce earthquake, during which the stresses were relieved by the displacement of the marble blocks, allowed due to the thin cross section of the original clamps. bending does not cause such high stress concentration because the inertia of the marble block is significantly higher during bending, which obstructs the relative motion of the blocks. before discussing the results further, it should be noted that the main criteria of failure in the simulations is the possibility of inducing damage to the marble blocks. the embedding materials (portland cement mortar or lead) are considered as sacrificial, while stresses on the steel clamp do not reach magnitudes near the tensile yield strength. this occurs even at the significantly lower young’s modulus that was experimentally measured (50 gpa) for the steel. to put this value into perspective, typical values of young’s modulus for stainless steel are in the region of 200 gpa. however, pentelic marble is a natural anisotropic material and, as such, its properties, such as tensile strength and young's modulus, depend on the direction of the imposed loading or the defects that naturally occur. thus, while the ultimate tensile strength of the marble was fed as input into the simulations under a typical value of 15 mpa, weak points in the marble can fail under stresses as low as 5 mpa, as has already been mentioned in section 4.2. figure 8 and figure 9 convey the simulated stress field, total strain field and total deformation field. the areas of high stress concentration and high strains are magnified to elucidate how the loading conditions manifest themselves in the clamp specimens. for illustration purposes, the marble blocks are hidden from the images. as can be deduced from figure 8 and figure 9, as well as from the data presented in table 5, the maximum stress in the system develops in the steel clamp/embedding material assembly, and only a fraction of this stress, roughly varying from 0.2 to 0.3, is passed on as a maximum stress to the marble. it is also important to note that the stress that develops in the system is distinctly larger when lead has been used as an embedding material, though the fraction of the stress that is passed on to the marble is significantly lower, yet still larger than the values predicted when cement mortar has been used. the reduction in stress fraction passing to the marble is more prominent in the bending scenarios, which are more representative of probable actual loading conditions. the calculated development of higher stresses on the system due to embedding the clamps in lead could be misleading as far as the actual physical system is concerned. one of the assumptions of the model was that the entire space between the marble and the clamp would be filled by lead. if one takes into consideration the physicochemical properties of the system and the moulding techniques used, under atmospheric pressure, it is highly unlikely that voids will not be present in that figure 7. marble block–steel clamp system. equivalent von mises stress: bending. table 5. maximum stress calculated by finite element analysis of the discretised cads for the two types of embedding material and the two directions of imposed acceleration leading to stretching and bending motions. the last column presents the results of the simulation for the corroded steel clamp, on which the growth of corrosion products has induced an intrinsic stress of 5 mpa on the surrounding materials. embedding material (intrinsic stress): acceleration direction / type of motion: cement mortar bending cement mortar stretching lead bending lead stretching lead (5 mpa) bending max stress in system; 0.3 g (mpa) 0.45 22.04 1.78 27.98 9.01 max stress in system; 0.6 g (mpa) 0.90 44.40 4.61 56.22 17.98 max stress in system; 1.2 g (mpa) 1.80 88.89 5.89 113.47 33.30 max stress in marble; 0.3 g (mpa) 0.15 4.94 0.28 6.23 8.87 max stress in marble; 0.6 g (mpa) 0.30 9.98 0.49 12.49 17.98 max stress in marble; 1.2 g (mpa) 0.60 19.77 1.27 25.19 22.21 linear coefficients & coefficients of variation max stress in system (mpa·m-1·s2) 1.50 (r2 = 1) 74.03 (r2 = 1) 5.49 (r2 = 0.90) 94.33 (r2 = 1) 22.73 (r2 = 0.98) max stress in marble (mpa·m-1·s2) 0.50 (r2 = 1) 16.50 (r2 = 1) 1.01 (r2 = 0.98) 20.95 (r2 = 1) 15.66 (r2 = 0.92) estimated stress values for epa of 0.16 g according to the linear coefficients max stress in system; 0.16 g (mpa) 0.24 11.84 0.88 15.09 8.64 max stress in marble; 0.16 g (mpa) 0.08 2.64 0.16 3.35 7.51 acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 163 space. the very high surface tension of the molten lead would lead to non-wetting of the marble surfaces; thus, voids should be created along the edges of the nesting. considering the high ductility of lead, the presence of empty spaces in the system could lead to dramatically lower values of developed stresses. another interesting observation is the linear relationship between the enforced acceleration and the maximum stress that developed in the system. the linear coefficients and the coefficients of determination are presented in the second part of table 5. unity r-squared values in all stretching scenarios and in bending scenarios with cement mortar as an embedding material indicate a linear correlation between the input and the output of the simulations. this is expected since the acceleration force over the area provides the applied stress; thus, the relationship between the applied stress and the acceleration force should be linear. in the case of the lead-metal-embedded clamp bending scenarios (both for non-corroded and for corroded steel) the simulation output deviates significantly from linearity when plotted against acceleration. this must be associated with the mechanical properties of the materials. since the system is comprised of different layers of material, this discontinuity suggests a nonlinear relationship between stresses and forces. as can be observed in table 4, the steel and the mortar have approximately the same young’s modulus, whereas the steel and the lead have a significant variance in the young’s modulus. therefore, the nonlinearity will be alleviated in the case of the mortar, and it will be substantial for the lead. this explains the nonlinear relationship that is observed in the reported results on the bending of the lead. finite element calculations performed taking into account the development of steel corrosion products and the intrinsic stress induced have led to a large increase in the generated stress as a result of the acceleration stimulus. it is also notable that this is the only case, of those examined, in which, for an acceleration of 0.6 g, the maximum stress induced on the system is applied to the marble block. the fraction of stress that passes as maximum stress to the marble in this case is high, varying from 0.6 to 1. the system was simulated during bending, the most realistic scenario, and the results indicate that apart from the initial intrinsic 5 mpa stress, the stress induced in the marble by the acceleration is more than 13-fold higher than the values calculated in the absence of the initial stress. therefore, even during bending, corrosion can cause failure and crack formation in the marble, even before the thickness of the corrosion products formed can induce damage by mere crystallisation pressure. it is, again, only in the case of the corroded steel clamps that the maximum stress on the marble block is larger than 5 mpa when 0.16 g epa is used as a stimulus. this point emphasises the significance of intrinsic stress to the modelling of the structure. 6. conclusions the examination of the three early 20th century double-t clamps confirms that even though different types of fe alloys (carbon concentration range: 0.008–0.085 %) and metallurgical processing has been used, all joints exhibit low values of microhardness, tensile strength and young's modulus of elasticity. during the fracture process, all the examined clamps are susceptible to a ductile fracture mechanism. clamp s.214, in particular, is a composite material produced by the cladding and folding of a hypoeutectoid steel and a lowcarbon iron alloy, which has very similar metallurgical characteristics to the original ancient clamps. clamp s.214 exhibits a higher thermodynamic tendency towards corrosion when immersed in a synthetic electrolyte simulating atmospheric pollutants. this particular clamp, when submitted to potentiodynamic electrochemical tests, undergoes more severe corrosion compared to the plain iron clamp and to a low-c steel clamp with added cr. these findings are associated with the clamp's higher carbon content and to its more complex figure 8. characteristic marble block–steel clamp system with lead under bending at 1.2 g. equivalent von mises stress, equivalent strains and total deformation are presented. for this case, the maximum stress concentration is observed at the core of the clamp, close to the intersection of the edges. figure 9. characteristic marble block–steel clamp system with lead under stretching at 1.2 g. the von mises stress, total strain and deformation field are presented. high stress concentration is observed at the edges of the clamp. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 164 metallurgical structure, which explains its corrosion state after its removal from the monument. the finite element analysis simulations elucidated the differences induced by different materials and their state of conservation in relation to the response of the system during an earthquake. the main conclusions that should be highlighted are as follows: the use of lead as an embedding material reduces the fraction of the system maximum stress that is passed on as a maximum stress to the marble. the larger values of maximum stress calculated should significantly decrease if the physical model takes into consideration the voids that would be created during the moulding of molten lead. the intrinsic stresses that develop due to the formation of corrosion products drastically change the response of the system so that an otherwise harmless stimulus could lead to the fracture of the marble blocks, even for small thicknesses of corrosion product. in general, this work demonstrates that it is possible to predict potential local damage with detailed information on the nature and conservation status of the materials. finite element analysis can be performed on discretised cads for unit cells, representative of specific existing parts of a monument, taking into consideration the physical dimensions, the materials and their state of conservation or pathology as well as existing intrinsic stresses, either due to corrosion or gravitational forces. utilising as a stimulus the acceleration forces, as predicted by structural models for specific parts of the monument and combined with the appropriate boundary conditions, we can predict the mechanical response through finite element analysis modelling. this can be a valuable tool for tailored intervention for parts of the monument that require specific attention. authors contribution statement z. vangelatos designed the discretised cad of the system, performed the finite element analysis in ansys and interpreted the results according to the static theory of beams. m. delagrammatikas designed the physical models and correlated the simulation and linear regression analysis results, undertook most of the bibliographic research and performed part of the experimental work. o. papadoloulou designed the electrochemical testing, conducted the related bibliographic research and performed most of the experimental work. c. titakis carried out bibliographic research on the anticipated corrosion-product properties and performed part of the experimental work. prof. p. vassiliou had the original idea for this work, advised on every step of the design of the physical models and provided guidance on the interpretation of the results. acknowledgement the authors would like to thank the staff of ysma for giving permission and providing the historic materials for the execution of the measurements presented in this short paper. special gratitude should be expressed to the staff of chalyvourgiki s.a quality control laboratory for performing the elemental analyses and the mechanical testing of the clamps. further acknowledgement is due to michail kyritsisspinoulas, who performed the vickers micro-hardness testing and calculated the young's modulus of elasticity as part of his master's thesis. references [1] s. kondaratos, the parthenon as cultural ideal, in: the parthenon and its impact in modern times. p. tournikiotis (editor). melissa publishing house, athens, 1994, isbn 960204019x, pp. 19-53. 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[18] e. ganniari-papagerorgiou, joining fragmented epistyles using threaded titanium bars: a numerical analysis, strain 45 (2009), pp. 445-455. doi: 10.1111/j.1475-1305.2008.00507.x [19] e. d. pasiou, s. k. kourkoulis, mechanical response of marble epistyles under shear: numerical analysis using an experimentally validated model, journal of mechanical behaviour of materials 27 (2018), 20180023. doi. 10.1515/jmbm-2018-0023 [20] e. toumbakari, analysis and interpretation of the structural failures of the orthostate in the northern wall of the athens parthenon, strain 45 (2009), pp. 456-467. doi: 10.1111/j.1475-1305.2008.00529.x [21] d. chicot, j. mendoza, a. zaoui, g. louis, v. lepingle, f. roudet, j. lesage, mechanical properties of magnetite (fe3o4), hematite (α-fe2o3) and goethite (α-feo·oh) by instrumented indentation and molecular dynamics analysis, mater. chem. phys. 129 (2011), pp. 862-870. doi: 10.1016/j.matchemphys.2011.05.056 [22] a. ouglova, y. berthaud, m. françois, f. foct, mechanical properties of an iron oxide formed by corrosion in reinforced concrete structures, corros. sci. 48 (2006), pp. 3988-4000. doi: 10.1016/j.corsci.2006.03.007 [23] s. caré, q. t. nguyen, v. l'hostis, y. berthaud, mechanical properties of the rust layer induced by impressed current method in reinforced mortar, cement and concrete research 38 (2008), pp. 1079-1091. doi: 10.1016/j.cemconres.2008.03.016 [24] c. conophagos, physical metallurgy, book ii alloys, selfpublication, athens, 1965, p. 157. in greek. [25] g. varoufakis, the iron clamps and dowels from the parthenon and erechtheion, the journal of historical metallurgy society 26 (1992), pp. 1-18. [26] g. varoufakis, the rapid development of technology in making iron clamps of three ancient temples in the archaic and classical period, j. chem. eng. 6 (2012), pp. 1136-1141. online [accessed 28 march 2021] http://www.davidpublisher.org/index.php/home/article/index ?id=16422.html doi: 10.17265/1934-7375/2012.12.012 https://doi.org/10.1111/j.1475-1305.2008.00507.x https://doi.org/10.1515/jmbm-2018-0023 https://doi.org/10.1111/j.1475-1305.2008.00529.x https://doi.org/10.1016/j.matchemphys.2011.05.056 https://doi.org/10.1016/j.corsci.2006.03.007 https://doi.org/10.1016/j.cemconres.2008.03.016 http://www.davidpublisher.org/index.php/home/article/index?id=16422.html http://www.davidpublisher.org/index.php/home/article/index?id=16422.html https://doi.org/10.17265/1934-7375/2012.12.012 diaphragmless shock tube for primary dynamic calibration of pressure meters acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 310 acta imeko issn: 2221-870x december 2020, volume 9, number 5, 310 314 diaphragmless shock tube for primary dynamic calibration of pressure meters a. svete1, j. kutin2 1 university of ljubljana, faculty of mechanical engineering, ljubljana, slovenia, andrej.svete@fs.uni-lj 2 university of ljubljana, faculty of mechanical engineering, ljubljana, slovenia, joze.kutin@fs.uni-lj abstract: in conventional shock tubes with a diaphragm many effects related to the burst of the diaphragm can influence the shock formation and thus prevent an ideal pressure step change predicted by the shock tube measurement model being generated. this paper presents a newly developed diaphragmless shock tube, in which a diaphragm is replaced with a quick-acting pneumatic valve. the developed shock tube has a capability to generate pressure steps calculable from its measurement model with a relative expanded uncertainty of less than 0.025, which can be used as the input signal in primary calibrations of pressure meters. keywords: time-varying pressure; primary calibration method; diaphragmless shock tube; frequency characteristics 1. introduction a shock tube is only known time-varying pressure generator capable of providing sitraceable high-frequency time-varying pressure calibrations [1]. it consists of two straight gas-filled tubes with the same circular cross-section that are initially separated: a high pressure driver section and a lower pressure driven section. an instantaneous opening of the connection between the two sections generates a shock wave that propagates through the driven gas. the reflection of the shock front off the end-wall of the driven section, where the pressure meter to be calibrated is mounted, causes a rapidly rising step change in the pressure at this point. by assuming a calorically perfect gas and an adiabatic flow, the amplitude of the pressure step upon reflection of the initial shock front can be calculated using traceable measurements of the shock wave velocity w and the initial, stationary absolute pressure p1 and temperature t1 of the gas in the driven section as [2]: ( ) ( )2s 1 121 1 s2 21 s 1 3γ 1 3 γγ δ 2 1 2(γ 1) (γ 1) m p p m m    − + −  = − −   + −  , (1) where 1 is the adiabatic index, s 1m w a= is the shock wave mach number, 1 1 1 1 γa r t= is the speed of sound and r1 is the specific gas constant. after the shock wave has been reflected, the pressure remains constant until the arrival of a second shock front, resulting from the initial shock front’s partial reflection from the contact surface between the driver and the driven gases. the extremely rapid rise in pressure (of the order of 1 ns [3]) can excite frequencies in the mhz range with a low frequency limit that is proportional to the reciprocal of the period of time during which the post-shock pressure remains constant. in the conventional shock tubes the shock waves are generated by bursting the diaphragm, usually induced by increasing pressure in the driven section. the characteristics of the diaphragm material and the way in which it bursts influence the shock formation, which causes large uncertainties associated with the repeatability of the generated pressure step [4]. another major disadvantage of the diaphragm shock tubes is that the diaphragm has to be replaced after each test, which requires the shock tube to be completely disassembled and reassembled before each test. this influences the reproducibility of the generated pressure steps. in order to improve the calibration and measurement capability of the shock tube method, the shock tube with a quick-acting pneumatic valve has been developed. 2. diaphragmless shock tube the diaphragmless shock tube developed for primary dynamic calibrations of pressure meters is shown in figure 1. the shock tube consists of 1 m long driver section and 6 m long driven section, both manufactured from stainless steel tube with a http://www.imeko.org/ mailto:andrej.svete@fs.uni-lj mailto:joze.kutin@fs.uni-lj acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 311 7.5 mm thickness wall. in order to eliminate the imperfections at the junctions that could generate secondary shock waves, the driver and the driven section of the same inner diameter as the effective flow-section diameter of the quick-acting valve of 40 mm are connected to the valve and the end-walls with socket weld flanges. the integrated quickacting pneumatic valve (ista pneumatics, kb-40-70) allows driver pressures up to 7 mpa, which enables generating pressure steps at the end-wall of the driven section of magnitude up to approximately 1.3 mpa when using nitrogen. the shock tube is rigidly attached with five massive mounting blocks along its length in order to eliminate the vibrations of the tube generated by the opening of the valve and the motion of the shock waves, which could affect the more acceleration-sensitive pressure meters during calibration. figure 1: developed diaphragmless shock tube with a quick-acting valve in order to obtain the pressure steps with the shock tube measurement model (1), the initial atmospheric driven pressure p1 is measured with a barometric pressure transducer (mensor, cpr6000, measuring range 75 kpa to 115 kpa, expanded measurement uncertainty (k = 2) 50 pa). the initial, stationary driver gauge pressure p4,g is measured with a pressure transducer (mensor, cpr6000, measuring range 0 to 10 mpa, expanded measurement uncertainty (k = 2) 0.01% of reading but not less than 400 pa) connected to the shut-off valve integrated in the end-wall of the driver section shown in figure 2. the shut-off valve is also used to vent the gas from the tube to the atmosphere. the initial temperature of the gas in driven section t1 is inferred from the temperature of the driven section wall measured with pt100 resistance temperature sensor (tetratec instruments, wit-s) mounted in the middle of the driven section length. the temperature sensor is connected to the digital transmitter (pico technology, pt-104, cal. measuring range 18 °c to 28 °c, expanded measurement uncertainty (k = 2) 0.2 °c, usb pc connection). measured initial absolute pressure p1 and temperature t1 of the gas in the driven section are used to obtain its adiabatic index 1 and the specific gas constant r1 using the nist refprop database. figure 2: schematic view of the driver section the shock wave velocity along the driven section w is determined by measuring time delays between the shock front detections using four identical piezoelectric pressure transducers (kistler, 603caa, nominal sensitivity ‒50 pc mpa‒1, measuring range 0 to 100 mpa, natural frequency > 500 khz, acceleration sensitivity ≤ 1.4 pa m‒1 s2), which are flush mounted in the side wall of the driven section (see figure 3) at evenly spaced distances of 1.5 m from 0.75 m to 5.25 m downstream of the quick-acting valve. the output signals of all four pressure transducers are connected in series to the same input of the charge amplifier (dewetron, daqp-charge-b, sensitivity 0.05 v pc‒1, full scale output ‒5 v to 5 v, frequency range 0.07 hz to 200 khz (‒3 db)). figure 3: cross-sectional view of the side wall of the driven section with a flush mounted pressure transducer the developed shock tube was used to determine frequency characteristics of the piezoelectric pressure measurement system. the piezoelectric pressure measurement system under test consists of pressure transducer (kistler, 7031, nominal sensitivity ‒534 pc mpa‒1, measuring range 0 to 25 mpa, natural frequency approximately 80 khz, acceleration sensitivity < 1 pa m‒1 s2) and charge http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 312 amplifier (kistler, 5018a, measuring range 2 pc to 2200000 pc, full scale output ‒10 v to 10 v, frequency range 0.16  10‒5 hz to 200 khz (‒3 db)). the voltage outputs from the charge amplifiers are acquired by a digitizer (national instruments, ni 9775, set input range −5 v to +5 v, resolution 11 bit, set sampling frequency 1 mhz, analog filter bandwidth 13.9 mhz (‒3 db)). the dataacquisition platform (national instruments, compactdaq) also employs a digital output module (national instruments, ni 9474, output signal 24 v dc) to enable the computer-controlled opening and closing of the quick-acting pneumatic valve using the analog voltage signal supplied to the electro-pneumatic valve. the signal acquisition and processing, and the actuation of the opening and closing of the valve are realized in the labview programming environment. 3. determination of the frequency characteristics assuming that the pressure measurement system under calibration, consisting of a piezoelectric transducer, a charge amplifier and a digitizer, exhibits linear behavior, its frequency response can be determined as the ratio of the discrete fourier transforms (dfts) of the output signal fout() and known input signal fin(). the dft of a signal on a finite interval implies the approximation of the fourier series of its infinite periodic extension. due to the fact that the step signal has unequal values at the interval end points, the periodic extension has discontinuities resulting in significant errors [5]. therefore, prior to applying the dft, both the input and output signals are truncated after constant poststep value of the output signal p is reached and then windowed using the gans-nahman technique [6]. this technique involves the extension of the original, truncated step signal p(nt), into the signal p * (nt) as: ( ) ( ) ( ) * δ , for 0, , 1 δ δ , for , , 2 1 p n t n n p n t p p n n t n n n  = − =  − − = − k k , (2) which satisfies ( ) ( )* *0 2 δ ,p p n t= where n is the number of samples of the truncated signal and t is the sampling period. although the technique doubles the length of the time signals, the frequency resolution of the original signal is preserved as the dft solution is zero at the even harmonics. further, the amplitude-frequency characteristic of the measurement system under test is determined as: ( ) ( )g  =  , (3) where ( ) ( ) ( )out in ,g f f =   and phasefrequency characteristic as: ( ) ( )( ) ( )( )( )atan im re ,g g  = −   (4) where π ω δ k n t = for k = 0, 1, 3, 5 … , 1.n − 4. results the tests were performed using dry nitrogen (5.0) at the atmospheric initial driven pressure and at the initial driver gauge pressures of 3 mpa, 5 mpa and 7 mpa. before the tests, the air was evacuated from the shock tube through the open end of the driven section by blowing through the whole shock tube with nitrogen. after closing the end of the driven section, the quick-acting valve and the shut-off valve were held open for a few minutes to provide atmospheric pressure of the nitrogen in the driven section. after closing the quick-acting valve, the driver section was pressurized to the required pressure. before the actuation of the opening of the quick-acting valve, the initial temperature and absolute pressure (atmospheric pressure) of the gas in the driven section were measured. table 1 presents measured initial absolute pressures (atmospheric pressure) p1 and temperatures t1 of the gas in the driven section, the adiabatic index 1, the speed of sound a1 and measured end-wall shock wave velocities w for the tests conducted at driver gauge pressures 3 mpa, 5 mpa and 7 mpa. table 1: measured p1, t1, 1, a1 and w at initial driver gauge pressures p4,g of 3 mpa, 5 mpa and 7 mpa p4,g mpa p1 mpa t1 k 1 a1 m/s w m/s 3 0.09893 295.5 1.4013 351.4 612.6 5 0.09891 295.1 1.4013 351.1 661.7 7 0.09890 295.4 1.4013 351.3 696.1 using the measurement model of the shock tube (1), the amplitudes of the input pressure signals provided to the transducer under test were determined. a relative expanded uncertainty of the amplitudes of the pressure steps generated within the developed shock tube has been estimated to be less than 0.025 (for the detailed uncertainty analyses see [7]). figure 4 presents the input signals and the output response signals, which were obtained by considering the sensitivity of the piezoelectric pressure measurement system specified by its manufacturer. the signals are presented in the interval from ‒0.1 ms to 1 ms, where the arrival time of the shock front at the end-wall, which was determined with the help of the measured shock wave velocity along the driven section, is set to t = 0. http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 313 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 input output g a u g e p re ss u re / m p a time / ms 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.5 1.0 1.5 2.0 2.5 input output g a u g e p re ss u re / m p a time / ms 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.5 1.0 1.5 2.0 input output g a u g e p re ss u re / m p a time / ms (b) (a) (c) figure 4: pressure steps generated at the end-wall of the driven section at: (a) p4,g = 3 mpa, (b) p4,g = 5 mpa, (c) p4,g = 7 mpa figure 5 presents the amplitude-frequency characteristics of the piezoelectric pressure measurement system in the frequency range up to nyquist frequency of 500 khz. the obtained amplitude-frequency characteristics were normalized with the static measurement sensitivity of the pressure measurement system ( )0 . an average static sensitivity of the pressure measurement system obtained with the calibration using the shock tube differs from that specified by the manufacturer of the pressure measurement system by less than 3%. from the figure it is seen that the amplitude-frequency characteristics of the pressure measurement system exhibit two peaks, one at the frequency of approximately 82.5 khz and one at the frequency of approximately 92.5 khz. 0 100 200 300 400 500 0 2 4 6 8 10 12 14 16 18 a m p li tu d e r a ti o frequency / khz (b) (a) (c) 0 100 200 300 400 500 0 2 4 6 8 10 12 14 16 18 a m p li tu d e r a ti o frequency / khz 0 100 200 300 400 500 0 2 4 6 8 10 12 14 16 18 a m p li tu d e r a ti o frequency / khz figure 5: amplitude-frequency characteristics obtained at: (a) p4,g = 3 mpa, (b) p4,g = 5 mpa, (c) p4,g = 7 mpa figure 6 presents the phase-frequency characteristics of the piezoelectric pressure measurement system. while the characteristics are similar up to the frequency of 200 khz, they start to differ at higher frequencies. the natural frequency of the pressure measurement system under test determined as the frequency at the phase lag  = 90°, which was obtained with the use of a linear interpolation between the two most adjacent values of the phase lags, is approximately 81 khz. http://www.imeko.org/ acta imeko | www.imeko.org december 2020 | volume 9 | number 5 | 314 0 100 200 300 400 500 -600 -500 -400 -300 -200 -100 0 p h a se l a g / ° frequency / khz 0 100 200 300 400 500 -600 -500 -400 -300 -200 -100 0 p h a se l a g / ° frequency / khz 0 100 200 300 400 500 -600 -500 -400 -300 -200 -100 0 p h a se l a g / ° frequency / khz (b) (a) (c) figure 6: phase-frequency characteristics obtained at: (a) p4,g = 3 mpa, (b) p4,g = 5 mpa, (c) p4,g = 7 mpa 5. summary the developed diphragmless shock tube has a capability to act as a primary time-varying pressure calibration standard. in the future, the shock tube will be improved by employing additional side-wall pressure transducer near the end of the tube in order to enable to describe nonlinear decelerations of the normal shock waves along the tube and therefore better predict the generated pressure steps and the arrival times of the shock front at the end-wall. better accuracy of the obtained characteristics of the pressure measurement systems under test could be further improved also by acquiring longer post-step signals with higher sampling frequency. 6. references [1] j. hjelmgren, dynamic measurement of pressure a literature survey. borås: sp swedish national testing and research institute, 2002. [2] d. w. holder, d. l. schultz, on the flow in a reflected-shock tunnel. london: her majesty’s stationery office, 1962. [3] h. j. pain, e. w. e. rogers, “shock waves in gases”, reports on progress in physics, vol. 25, no. 1, pp. 287-336, january 1962. [4] j. n. de souza vianna, a. b. de souza oliveira, j. p. damion, “influence of the diaphragm on the metrological characteristics of a shock tube”, metrologia, vol. 36, no. 6, pp. 599-603, december 1999. [5] w. l. briggs, v. e. henson, the dft: an owner’s manual for the discrete fourier transform. philadelphia: society for industrial and applied mathematics, 1995. [6] w. l. gans, n. s. nahman, “continuous and discrete fourier transforms of steplike waveforms”, ieee transactions on instrumentation and measurement, vol. im-31, no. 2, pp. 97-101, june 1982. [7] a. svete, j. kutin, “characterization of a newly developed diaphragmless shock tube for the primary dynamic calibration of pressure meters”, metrologia, accepted manuscript, 2020. http://www.imeko.org/ non-destructive pole-figure measurements on workshop-made silver reference models of archaic objects acta imeko issn: 2221-870x march 2021, volume 10, number 1, 250 256 acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 250 non-destructive pole-figure measurements on workshopmade silver reference models of archaic objects máté sepsi1, márton benke1, valéria mertinger1 1 institute of physical metallurgy, metal forming and nanotechnology, university of miskolc, miskolc-egyetemváros h3515 hungary section: research paper keywords: non-invasive; non-destructive crystallographic texture; x-ray diffraction; archaic silver model object citation: máté sepsi, márton benke, valéria mertinger, non-destructive pole-figure measurements on workshop-made silver reference models of archaic objects, acta imeko, vol. 10, no. 1, article 33, march 2021, identifier: imeko-acta-10 (2021)-01-33 editor: ioan tudosa, university of sannio, italy received june 1, 2020; in final form october 26, 2020; published march 2021 copyright: this is an open-access article distributed under the terms of the creative commons attribution 3.0 license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. funding: the present paper was supported through the national research, development and innovation office – nkfih k119566 project and the efop3.6.1-16-00011 ‘younger and renewing university – innovative knowledge city – institutional development of the university of miskolc aiming at intelligent specialisation’ project, implemented in the framework of the szechenyi 2020 programme. the realisation of this project is supported by the european union, co-financed by the european social fund. this work was supported by the janos bolyai research scholarship of the hungarian academy of sciences. corresponding author: valéria mertinger, e-mail: femvali@uni-miskolc.hu 1. introduction crystalline metallic materials are typically polycrystalline. the orientation of the polycrystals can be random (isotropic) or oriented (anisotropic). crystallographic anisotropy (often called crystallographic texture or texture) can be caused by many effects, such as unidirectional crystallisation with controlled cooling [1]-[5]. in this case, the deviation of the crystals from the random orientation distribution is caused by the directionality of heat dissipation. in the case of complex-shaped objects, the crystallisation texture can also be very complex. furthermore, this texture is non-stable because, in most cases, it is modified during the subsequent stages of processing. in contrast, plastic deformation, especially the cold forming process, produces a characteristic crystalline arrangement and texture. however, notable changes are caused by annealing, which follows the forming process. one of the great challenges for today's engineers is to influence and design the texture of metals using appropriate technology. the nature of the existing texture is clearly dependent on the metal crystalline lattice, thus on the nature and degree of plastic deformation. depending on the temperature of the heat treatment, so-called recrystallisation processes can also cause characteristic changes in the texture of a given type of alloy. the recrystallised texture is generally dependent on the crystal lattice and material properties, but several process parameters cause typical modifications. measuring, controlling and designing crystallographic texture is a very well-known technique in the industry, as texture formation abstract based on the knowledge of crystallographic texture, the parameters of the metal-forming heat treatment of metallic objects can be reconstructed well when conventional technologies (e.g. rolling, deep drawing, etc.) are applied. the characterisation of texture has been possible only by using destructive techniques, apart from neutron diffraction. recently, a non-destructive texture measurement method has been developed for centreless diffractometers, providing a new dimension to the examination of archaic objects. in the present study, two types of stresstech g3r centreless diffractometer were used with this new method, which proved it to be applicable to both the tabletop and robotic arm-assisted versions of the diffractometer. although the texture of archaic objects can be revealed using this method, the production of these objects cannot be directly deduced from the results, since their manufacturing steps are not identical to the metal-forming operations applied today. in this study, workshop-made silver reference samples were produced with the help of three silversmiths. wrinkling, metal spinning and intermediate annealing were applied to the rolled silver sheet with the aim of making real-sized silver cups. the workshop-made reference cups were then subjected to non-destructive texture examinations. the results reveal the textures developed during the conventional manufacturing steps of silver cups. the obtained information greatly assists future research in understanding the pole figures of archaic objects and the reconstruction of their manufacturing technology. mailto:femvali@uni-miskolc.hu acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 251 can be controlled well by the applied technological processes [6][9]. the most common representation of texture is the pole figure, which is a two-dimensional stereographic projection in which the positions and intensities of specific crystallographic orientations are plotted in relation to the specimen geometry. stereographic projection is a two-dimensional projection of a three-dimensional crystal such that the angular relationships between different planes, different directions, and between crystalline planes and directions in the crystal can be easily identified from the projection. thus, a stereographic projection is an ‘angle-true’ projection in just the same way as a geographic atlas is a two-dimensional ‘area-true’ projection of a threedimensional globe. the conventional techniques for measuring crystallographic texture are based on neutrons [10], x-ray [1]-[3] or electron backscatter diffraction [1]-[3], which involve the determination of several pole figures and odfs (orientation distribution function) for the investigated materials. the main difference between these methods is the size of the examined volume and, thus, the number of crystals involved in the examination. based on reverse engineering theory, we assume that the characterisation of the texture can be used to reconstruct the applied technological operation for archaic objects. this logical theory has not been widely used in archaeometry because texture characterisation, apart from neutron diffraction, could only be used in a way that was destructive until last year [11]-[13]. neutron diffraction is an excellent test method, especially for tomography. there is no other method that provides as much information about the inside of objects without causing destruction. therefore, it is also particularly popular in the field of archaeometry [14]-[16]. when examining metal objects, internal defects often provide information about the manufacturing technology [17]-[20]. examination of bronze coins is widespread mainly due to the small layer thickness and size of the coins, which allows multiple coins to be illuminated simultaneously. it is also easy to detect compositional inhomogeneity, which can also be a useful tool in originality (genuine or fake) testing. kockelmann [21] examined 16thcentury silver/copper coins from the vienna museum with neutron diffraction to determine whether they were original or fake. the rolling texture of the tested coins was the original coin in the silver and copper phase, while the fake coins differed not only in their composition but also in their texture, as they were produced primarily by casting. the problem with using neutrons is that they have limited access to the source and that they also activate the object. furthermore, especially in the case of silver, an isotope with a long half-life is formed. [20]. in other cases, isotopes can be very useful when used to determine the location of a metal ore [23]. despite the limitations of the neutron source, neutron texture studies have been performed on archaic finds. in the case of bronze axes and other objects, neutron texture studies have provided additional information about the age of manufacturing based on the different characteristic forming methods of different ages [23]-[26]. the texture formed by different manufacturing steps, such as shaping and heat treatment, can also be distinguished [27]. in 2007, artioli [28] studied 20 copper axes from the neolithic to the early bronze age from different sites using neutron diffraction. based on this work, the axes can be divided into the following groups in terms of production: cold forming and subsequent annealing, pure annealed, cast and very slow cooling during casting. with the synthesis of odf (showing the orientation of every unit cell within the examined volume), details (which had been preserved for more than five thousand years) were identified, such as that the cold forming had taken place in one or two directions and that it had been applied before the annealing heat treatment or directly after casting. in contrast, there are few test results relating to the texture of gold [29]. due to the previously mentioned activation problem, there are even less examples relating to the examination of the texture of silver [21], [30]. the undoubted advantage of the x-ray diffraction test method over the neutron source is that the devices are easily accessible, and there are even mobile devices available on the market; there is also no activation problem. conventional diffractometers, however, are only capable of accommodating small samples. parallel beam devices are alone in being capable of examining smaller objects. an example of x-ray diffraction is found for small objects where parallel beam alignment with the göbel mirror can be a solution [31]. chiari et al. [32] used a mobile xrf-xrd (x-ray fluorescence spectrometry combined with x-ray diffraction) device to determine the age and production steps of museum objects with similar considerations. their article highlights the mobility of their x-ray diffraction equipment and its benefits, as well as the growing requirement for non-destructive testing using these mobile devices in situ. our experience is similar in the field of archaeometry, namely that the laboratory testing of objects is often very difficult, and museums prefer on-site measurements primarily because moving and monitoring high-value objects requires armed security personnel. because of this need, transportable equipment may have advantages over neutron diffraction, in which the neutron source is stationary, operates with long exposure times and activates the objects. in the last two years, a new non-destructive pole-figure measurement method for centreless x-ray portable diffractometers in modified chi mode has been developed and patented [11] (modified chi mode refers to the position and tilting of the x-ray source relative to the sample). the maths of the method, comparison of the centreless measurements with the conventional ones and validation process on rolled aluminium sheets have been presented in detail in our earlier paper [13]. the method has been further developed over the past year, and it has been demonstrated that it is also possible to construct a semi-full pole figure from data originally acquired during the residual stress measurement (reverse modified chi mode) [13]. a special version of centreless diffractometers is robot armed, which is also available on the market. in the case of a texture measurement, this configuration means that the object does not have to be rotated, even on a rotation table, because the measuring head performs all the necessary movements. a further advantage is that the beam path of conventional diffractometers can be achieved (chi mode) by using a measuring arm originally developed for the measurement of retained austenite, and due to the centreless configuration, sampling is still not necessary. this further expands the freedom of measurement, which is especially advantageous when examining unique archaic objects. in this paper, the model objects were also measured using a robot diffractometer in modified chi mode, and this measurement setup has also been validated. a collection of archaic objects, the seuso treasure [33], was first subjected to an onsite residual stress measurement in the hungarian national museum to identify the production technique. it then became clear that by measuring a pole figure, we could get considerably more relevant information. the stress acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 252 measurement data was analysed to describe the anisotropy (specific section of the pole figure) of the perfume box [35]. after developing the method, we measured each piece of the seuso treasure and determined incomplete pole figures. the pole figures obtained are not similar to the pole figures of metal produced by any of the known industrial metal-forming technologies; therefore, model silver objects were produced by well-known silversmiths, tamás zoltán, miklós varga and istván kéry. they have a lifetime’s experience, especially working on silver. they applied two different methods, metal spinning and wrinkling, to produce silver cups, and the pole figures were determined from the model objects. in the present study, the production of the model objects and their non-destructive pole figures are presented. the measured and calculated pole figures of the model objects, which have been subjected to welldocumented production methods, will help to interpret pole figures of archaic silver objects where the production technology is not yet clearly described. 2. preparation of workshop-made silver reference model objects 2.1. wrinkling cup since silver has excellent formability, most metal-forming techniques can be applied to it. however, our metal-forming experience based on modern industrial techniques does not necessarily help us understand the production technology of archaic objects. fortunately, silver and coppersmiths continue to use techniques now that are suspected to have been used in the production of archaic objects. following the silversmiths’ suggestion, as a first step, two model cups were produced using wrinkling and metal spinning techniques. in both cases, pure 2 mm-thick silver sheets were used for the model objects with dimensions of 120 x 120 mm. first, the silversmith annealed the sheets using a gas jet, rolled them on a hand-driven rolling mill and cut a disk with scissors (figure 1). the wrinkling process was the following: the base of the cup was marked off on a wooden mould with a steel hammer, in the same way as the wrinkles on the cylinder. after creating contours and wrinkles, the wrinkles were shrunk and hammered from the bottom to the top in overlapping circles (figure 2). after hammering, the hardened silver was annealed again using a gas jet and cleaned with a hot borax solution, and the creation and shrinking of the wrinkles and the annealing was repeated twice more to decrease the angle between the cylinder and base of the cup. the final hammering was performed on a metal anvil (figure 3). in the last two pictures of figure 3, the finished wrinkled silver cup and the initial sheet can be seen. 2.2. metal spinning cup the other silver model cup was produced by metal spinning. the initial material was the same as in the previous case, a circlelike disk from a rolled and annealed sheet. generally, metal spinning is performed on a turning-like machine. to mould the disk, it is pressed against a formed block on the driver by a tool called a ‘spoon’ while the disk is spinning. in our case, the spoon was a round-ended steel bar, which pressed the disk from the middle to the edge on the formed block. during the metal spinning process, intermediate annealing was applied because of the extended hardening process of silver. the method for annealing was the same as that used during the wrinkling procedure. before removing the prepared cup, the bottom, which was conical, was smoothed using a flat steel tool (figure 4). the figure clearly shows that the rim of the cup is heavily wrinkled. this is because the machining pad was originally not optimised for metal spinning. this wrinkled rim was cut off with a lathe knife to prevent interference during further examinations. the initial sheet with the rolling direction (rd) indicated and the model objects prepared by metal spinning and wrinkling can be seen in figure 5. the blue numbered dots mark the measured locations of the subsequent pole-figure measurements. 3. pole-figure measurements the new, reverse modified chi method was introduced in [13], [13] using a stresstech g3r-type centreless diffractometer with a crkα source, operating with 30 kv tube voltage and 9 ma figure 1. annealing, rolling and cutting the initial silver sheet. figure 2. initial steps of preparing a wrinkling cup. creating contours and wrinkles and the hammering of the wrinkles. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 253 tube current. the  values used were -59° + 59°, with 5° increments, and  was chosen as -90…+ 90°, with 5° increments. a stresstech xstress robot centreless diffractometer was also used to extend the theta limitation and verify the setup. crkα radiation was used at 30 kv and 9 ma. the measurements were performed using both the {222} and the {311} silver plane systems, and defocus correction was carried out on the ag powder. the absorption-corrected pole figures were plotted by origin software. the tests were performed on the initial metal sheet (figure 5a) at two locations on the cylinder of the metal spinning cup (lower cylinder no. 1 and upper cylinder no. 2) and at three locations on the wrinkling cup (bottom no. 1, lower cylinder no. 2 and upper cylinder no. 3). figure 6 shows the {222} and {311} pole figures of the objects, where the texture number increases from blue to green to red, and figure 7 shows the measurements. the initial sheet’s {222} pole figure shows a characteristic texture. the same character and the homogeneous distribution of {311} can also be seen on the bottom of the wrinkling cup, but there is only a slight angular rotation between the two poles because the test site and the starting point of the pole figure are randomly selected. this is, of course, an expected result, as the operation, in contrast to the metal spinning, has left the base of the vessel almost unaffected. it is important to remember that during the operation, the annealing process was carried out twice, which was necessary to further deform the object, but it did not result in recrystallisation. recrystallisation is a diffusioncontrolled process that can completely eliminate the effect of the previous deformation in the crystal structure if the process is fully completed. the lower part of the cylinder further enhances the shaping character, while the upper part shows a homogeneous character, which may be the result of strong multiple hammering alone or combined with softening. the rate of recrystallisation is proportional to the degree of shaping, so that recrystallisation begins at the top of the cup because of the intermediate softening. the metal spinning cup’s pole figure shows an image of a completely different character. the pole figure of {222} is reminiscent of a fibre texture at both the bottom and the top of the cylinder. the difference between the two textures may indicate the same shaping behaviour at different strains. the metal spinning vessel received an intermediate annealing during the forming of the metal. comparing the results of the two measurement methods, we can conclude that the pole figures obtained with the robot diffractometer and the g3r diffractometer are identical. figure 3. subsequent steps for preparing the wrinkling cup. annealing twice, cleaning and hammering from the bottom to the top. figure 4. preparing a silver cup by metal spinning. figure 5. a) the initial sheet and the reference objects: b) metal spinning cup, c) wrinkling cup. blue numbered dots mark the pole-figure measured points, while the red arrow indicates the rolling direction (rd). acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 254 {222} {311} {222} {311} g3r robot diffractometer initial silver sheet g3r robot diffractometer bottom of the wrinkling cup g3r robot diffractometer lower cylinder of the wrinkling cup g3r robot diffractometer upper cylinder of the wrinkling cup g3r g3r lower cylinder of the metal spinning cup upper cylinder of the metal spinning cup. figure 6. pole figures determined by the g3r and the robot diffractometers in modified psi mode at different locations on the model objects. acta imeko | www.imeko.org march 2021 | volume 10 | number 1 | 255 4. conclusions silver reference cups were produced using metal-spinning and wrinkling techniques by silversmiths. the pole figures were measured with two types of non-invasive x-ray diffractometer setups in reverse modified chi mode. it was concluded that the non-destructive pole-figure method is an efficient way to distinguish between metal objects formed in different ways. the specific forming modes result in specific pole figures. by producing and examining a sufficient number of reference materials, the mode of production of archaic objects can also be reconstructed. the pole figures obtained by the robot diffractometer are identical to the figures of the previously validated g3r diffractometer. however, further analyses on other materials are necessary if the robot diffractometer can be considered as validated in modified chi mode for general texture analysis. acknowledgement the present paper was supported through the national research, development and innovation office – nkfih k119566 project and the efop-3.6.1-16-00011 ‘younger and renewing university – innovative knowledge city – institutional development of the university of miskolc aiming at intelligent specialisation’ project, implemented in the framework of the szechenyi 2020 programme. the realisation of this project is supported by the european union, co-financed by the european social fund. this work was supported by the janos bolyai research scholarship of the hungarian academy of sciences. the authors are very grateful to tamás zoltán, miklós varga, istván kéry, silversmith experts, for their contribution in the production of the reconstruction reference material. many thanks to mária tóth totyi, who contributed her constructive ideas to the development of the measurement methods. references [1] s. suwas, r. ray kumar, crystallographic texture of materials, springer-verlag, london, 2014, isbn 978-1-4471-6314-5. 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https://doi.org/10.1023/b:jmsc.0000026933.28906.19 https://doi.org/10.1016/j.talanta.2008.02.025 https://doi.org/10.1007/s00339-016-0521-x https://iopscience.iop.org/article/10.1088/1757-899x/375/1/012036 https://iopscience.iop.org/article/10.1088/1757-899x/375/1/012036 acta imeko  september 2014, volume 3, number 3, 15 – 21  www.imeko.org    acta imeko | www.imeko.org  september 2014 | volume 3 | number 3 | 15  analyses of direct verification data of rockwell diamond  indenters by iterative regression method  satoshi takagi  national metrology institute of japan, aist, aist tsukuba central 3, 1‐1‐1, umezono, tsukuba 305‐8563 japan      section: research paper   keywords: rockwell diamond indenter; tip radius; cone angle; laser probe 3d profile measurement instrument; least square circle fitting  citation: satoshi takagi, analyses of direct verification data of rockwell diamond indenters by iterative regression method, acta imeko, vol. 3, no. 3, article  5, september 2014, identifier: imeko‐acta‐03 (2014)‐03‐05  editor: paolo carbone, university of perugia   received april 1 st , 2013; in final form august 20 th , 2014; published september 2014  copyright: © 2014 imeko. this is an open‐access article distributed under the terms of the creative commons attribution 3.0 license, which permits  unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited  funding: (none reported)  corresponding author: satoshi takagi, e‐mail: satoshi.takagi@aist.go.jp    1. introduction  it is generally recognized that the geometrical error of diamond indenters affects significantly the indicated hardness values in rockwell hardness measurements and therefore many efforts regarding indenter verification has been made for many years. various types of instruments have been used for the geometrical verification by many researchers, e.g., the stylus-type surface profiler [1], a micro-range coordinate-measuring machine (µcmm) [2], specific designs for spherical and conical parts of rockwell diamond indenters in combination with interferometric optics and precise positioning mechanics [3, 4, 5], etc. recently, observation techniques in small objects have been remarkably improved and 3d geometry of small objects can be obtained easier and more accurately. a 3d optical surface profiler with a white light interferometer [6], a confocal laser scanning microscope [7] and a very wide range atomic force microscope [8] were used to observe precise 3d images of rockwell diamond indenters. along with those investigations, the effects of geometrical errors to hardness values are also studied by multiple regression analysis [3, 9] or a finite element analysis method [8,10]. with those backgrounds, a pilot study to investigate international metrological equivalency of rockwell diamond indenters is now in progress under the framework of the international committee for weights and measures (cipm) [11]. the verification of the indenter geometry is important to confirm how accurately the indenter geometry follows its defined shape. it is commonly believed that a smaller geometrical error brings hardness values with higher certainty. it is, however, still difficult to improve the quality of indenters because the manufacturing technique is not much changing as the measuring techniques. for this reason, the correction of the hardness value for each indenter has been considered [12]. yano et al. proposed a unique technique to estimate the bias of abstract  for the verification of rockwell diamond indenters exactly compliant to the international definition, an iterative method with the least  square circle fitting was introduced. this method was applied to the analysis of verification data obtained with a laser probe 3d profile  measurement  instrument.  the  geometry  of  a  rockwell  diamond  indenter  was  verified  with  the  equipment  as  an  example.  conventional analyses on cross sections of an indenter is demonstrated as well as three dimensional analysis of indenter geometry.  the  analyses  on  cross  sections  showed  that  the  technique  can  be  used  to  express  the  geometrical  parameters  described  in  the  definition of cipm/ccm/wgh properly, whereas three dimensional analysis enables to express the  imperfection of geometry with  fewer parameters. in addition, this technique can be applicable to determine equivalent geometrical parameters obtained with the  optical measurement system currently used at the national metrology  institute of japan (nmij) to establish the national standard  indenters. it is shown that the proposed method describes the geometry of indenter better than currently used method. these results  suggest that the uncertainty of the national standard indenter could be improved through this high resolution geometry measurement  and the multiple regression analysis nmij is using.  acta imeko | www.imeko.org  september 2014 | volume 3 | number 3 | 16  each indenter from the hardness value which would be realized by an ideally shaped indenter, by means of multiple regression analysis between geometric parameters of indenter and hardness [3] and it is the formal procedure to establish the national rockwell hardness standard at the national metrology institute of japan (nmij) until now. in order to utilize this technique, it is important for nmij to evaluate appropriate geometric parameters from the verification results of indenters. in this paper the analytical method to obtain desirable geometric parameters from the measurement data of a rockwell diamond indenter obtained with a laser probe 3d profile measurement instrument is introduced. it allows us to verify an indenter exactly compliant to the definition of cipm and also compatible with currently used methods at nmij. the efficiency of the proposed analytical method was demonstrated with obtained verification results. 2. measurement of indenter geometry  the geometry of each rockwell diamond indenter is measured with a laser probe 3d profile measurement instrument shown in figure 1 (model nh-3sp, mitaka koki, co. ltd., japan) [13]. this instrument can detect the z-axis coordinate with an auto-focus mechanism with a probe laser beam through the lens during scanning of a sample on the movable sample stage. the resolution is 0.001 µm along the zaxis with a 100× objective lens and 0.01 µm along the xand yaxes. one of the advantages of this instrument is that the measuring range is not limited in the field of view of the microscope. in addition, it has the capability to set an arbitrary coordinate system for the measurement independently from its mechanical layout. it enables us to set the datum plane for the indenter measurement. in this study, the datum plane was set on the seating face of the indenter. due to this function, it is also possible to evaluate the tilt of the indenter axis. the setup of a rockwell diamond indenter on the xy stage of the instrument is shown in figure 2. in order to ensure the fitting between the datum plane and the indenter seating face, an indenter is pressed on the datum plane via an o-ring as shown in figure 3. profiles of a rockwell diamond indenter are measured in eight cross sections parallel to the axis in every 22.5°. the result is graphically shown in figure. 4. in this case, the tilt of the datum plane was 0.03° and this was automatically corrected by the instrument. 3. 2d (cross sectional) analysis  the geometry of rockwell diamond indenters was defined by the working group on hardness (wgh), consultative committee for mass and related quantities (ccm), cipm as table 1 [14]. this implies that the indenter geometry should be verified by measurement of its cross section. in order to verify an indenter according to the definition, it is important to determine the blend points between cone and sphere from the measured profile properly. the author developed an analytical method to determine the coordinate of these points with an iterative calculation. its flowchart is datum plane indenter fixing plate figure 2. setup of a rockwell diamond indenter for the datum plane based  profile measurement.   figure  1.  the  laser  probe  3d  profile  measurement  instrument  used  to  measure  the  cross  section  of  rockwell  diamond  indenters  (courtesy  of  mitaka koki, co. ltd.).   indenter fixing plate o‐ring datum plane figure 3. the detail of the sample fixture.   table 1. the definition of the geometry of rockwell diamond  indenter by  cipm/ccm/wgh.   parameters reference value start measurement stop measurement cone angle, m 120° ±30° ±400 µm tip radius, ra 200 µm -30°† +30°† † from the axis 0 ‐0.3 ‐0.5 0 0.5 ‐0.5 0 0.5 x, mm y, mm z, mm figure 4. measurement result of a rockwell diamond indenter.   acta imeko | www.imeko.org  september 2014 | volume 3 | number 3 | 17  illustrated in figure 5. 3.1. extraction of the dataset to be analysed in the spherical part  of indenter  for the verification of the spherical part of an indenter, the dataset of corresponding part should be extracted from the whole profile data in the central angle range between t 2 (p1(m)) and t 2 (p2( m)), where t  60° in the wgh/iso compliant verification (see figure 6). the symbol m between the parentheses represents the number of the iteration. in the beginning of the iterative calculation, the tentative centre of curvature (p0(0)) should be determined. 3.2. least square circle fitting  there are several methods to fit a circle to data [15]. the simplest and the most primitive way is chosen for this analysis. in order to fit the data to the equation (x a(m) )2  (z b(m) )2  r (m)2 , (1) the following function can be taken as a measure of the fitting: ss(a( m), b( m), r ( m) )  r ( m)  (xi  a ( m) )2  (zi  b ( m) )2  2 i1 n  (2) following the least square principle, the constants a(m), b(m) and r(m) are determined when equation (2) is minimized. 3.3. calculation of the central angle of the spherical part  after the least square circle fitting analysis, different points of the centre of the spherical part are obtained from the previous iteration or the initially estimated position. it brings a different central angle expressed by the following equation:  ( m)  tan1 zn (m)  b( m) xn ( m)  a(m)  tan1 z1 ( m)  b( m) x1 (m)  a( m) . (3) where n is the number of data points in the selected dataset. 3.4. iteration  if  ( m) t , pick a different dataset according to the procedure described in 3.1 with parameters a(m) and b(m) and carry out the analysis. when (m+1) agrees with t within the resolution of the dataset, the best evaluation of the spherical part of an indenter is obtained. 3.5. convergence criterion  an iterative calculation does not always bring the right answer but it works when the convergence criterion is satisfied. let the deviation of the calculated central angle (m) from the target angle t be f(( m) ) ( m) t . (4) when the solution is found, equation (4) goes to zero. as seen in equation (3), (m) is a function with 6 parameters, i.e.:  ( m)  f (x1 ( m), z1 (m), xn ( m), zn ( m), a( m), b( m) ) . (5) then the convergence criterion of this iterative calculation can be written as f( (m) )t  d( m) 1 (6) where f( (m) )  f( (m) ) x1 (m) f((m) ) z1 (m)  f( (m) ) b(m)               (7) and d(m)  x1 (m)  x1 (m1) z1 (m)  z1 (m1)  b(m)  b(m1)               . (8) unfortunately, the function f( (m) ) is too complicated because it includes results of the least square analysis and it is not easy to show its convergence property with equations. in the analyses the author carried out, however, the iterative calculation converged in all cases. figure 7 shows the convergence property of the iterative calculation. four cross sections were analysed with the proposed method and in all cases the central angle (m) converged to the target value t = 60° with the number of iterations between 6 and 8. it suggests empirically that the algorithm satisfies its convergence criterion. start set blend points p 1 (m) and p 2 (m) regression analysis (least square circle fitting) center p 0 (m+1), central angle (m+1) and radius of curvature r(m+1) (m+1) =  t tentative center p 0 (0) target central angle,  t pick the dataset to be analyzed no yes center p 0 (m+1) exact blend points and r end   figure 5. flowchart for the determination of the tip radius ra and the  positions of blend points.  r p 1 (m) (x 1 (m), z 1 (m)) p 2 (m) (x 2 (m), z 2 (m)) p 0 (m) (a(m), b(m)) (x-a(m))2+(z-b(m))2=r2  t figure 6. extraction of the dataset of the spherical part from whole profile  data corresponding to the central angle ft.  acta imeko | www.imeko.org  september 2014 | volume 3 | number 3 | 18  3.6. analysis of the conical part  once the blend points between the sphere and cone, p1 and p2, are determined, the end points of verification of the generatrices, p3 and p4 can be determined as points 400 µm distant from p1 and p2, respectively. if the angle between the generatrices and the indenter axis are called 1 and 2, the coordinates of the end points can be expressed as p3 ( z1  400 m  cos(90°-1 ) , z3 ) (9) and p4 ( z2  400 m  cos(90°-2 ) , z4 ). (10) where 1  2  m 2  60°. (11) the values of 90°-1 and 90°-2 can be calculated from the slope of the linear regression of generatrices z p1x q1 (12) and z p2 x q2 (13) in the respective ranges, i.e., the ranges between p1 and p3, and between p2 and p4. in equations (9) and (10), z3 and z4 should be picked from the actual profile data. the included angle of the cone m is calculated from the equation m  1 2 . the straightness of the generatrices can be evaluated as the deviation range from the regression lines, equations (12) and (13), in the normal direction to those lines. 4. 3d analysis  in the iso standard [17], it is mandated that the indenter geometry shall be verified with at least eight axial section planes, equidistant to each other. it means that every cross section is verified independently. however, it seems to be reasonable if all measured data are verified at once as a 3d surface because the indenter geometry is actually three dimensional. in this section, a verification method by which the indenter tip is assumed as a sphere is presented. the equation of a sphere is expressed as (x a( m) )2  (y b(m) )2  (z c( m) )2  r ( m)2 . (14) in this case, the measure of fitting is ss(a( m), b( m), c( m), r ( m) )  r ( m)  (xi  a ( m) )2  (yi  b ( m) )2  (zi  c ( m) )2  2 i1 n  . (15) following the same algorithm as with the 2d analysis described in the previous section, the regression sphere corresponding to the target central angle can be obtained. when the indenter tip geometry is evaluated as a single sphere, it is necessary to verify the imperfection of the geometry in any way. one of the measures of imperfection is its sphericity. it can be evaluated by the deviation of measured data from the regression sphere, (xi , yi )  (xi (m)  x0 (m) )2  (yi ( m)  y0 ( m) )2  (zi (m)  z0 ( m) )2  r ( m) (16) and the difference between the maximum and minimum value of (xi , yi ) represents the sphericity of the indenter tip. 5. results of the analysis and discussions  in this paper, two kinds of analytical results are shown. one is the verification compliant to the specification of wgh/iso, based on the 2d (cross sectional) analyses. another is 3d analysis including a compatible method to the calibration method developed and actually used in nmij to establish the national standard indenters. 5.1. wgh/iso compliant verification (2d analyses)  as an example, eight cross sections of a rockwell diamond indenter were analysed according to the procedure introduced in the previous section with the target central angle t  60° . the centre of curvature p0, the blend points p1 and p2, the end points for the evaluation of generatrices p3 and p4 are illustrated with the fit circle on the profile data in figure 8 for the 0° section. the calculated average tip radius ra and the circularity are shown in table 2. the circularity is calculated on the basis of the minimum zone circles, i.e., it is the separation of two concentric circles that just enclose the circular section of the profile. the average tip radius of eight sections is 0.19621 mm 110 100 90 80 70 60 50 987654321 0° 45° 90° 135°  t = 60° number of iteration c en tr al a n g le  (m ) , ° figure 7. convergence property of the iterative calculation.  -0.4 -0.2 0 0.2 0.4 23.65 23.70 23.75 23.80 23.85 23.90 23.95 24.00 x, mm z, mm p 0 p 1 p 2 p 3 p 4 figure 8. graphical representation of a result of cross sectional analysis in  the 0° section. figure 9. graphical representation of the result of sphere fitting with target  angle of 60°.  acta imeko | www.imeko.org  september 2014 | volume 3 | number 3 | 19  and its standard deviation is 0.00082 mm. the circularity is between 0.00058 mm and 0.00182 mm. these results satisfy the requirement of iso part 3 grade specifications [17]. an apparent relationship between tip radius and its circularity is not observed in these results. in table 3, the analytical results for the conical part, i.e., the angle between the axis and generatrices m/2 (1, 2), included angles of cone m and the straightness of generatrices are listed. the average of m/2 is 59.95° whereas its standard deviation is 0.04°. therefore, the average cone angle m is 119.91°. the straightness of generatrices is between 0.00032 mm and 0.00137 mm. these results also satisfy the requirement of iso part 3 grade requirement. an apparent relationship between angles and straightness is also not observed. as a conclusion of the analyses, this indenter satisfies iso part 3 grade requirement in a completely compliant way of verification to the cipm/ccm/wgh definition. 5.2. 3d analysis  in figure 9, the result of sphere fitting with target angle of 60° is illustrated. the radius of the regression sphere is 0.19544 mm and the sphericity is 0.00329 mm. comparing this result with the results in the cross sectional analyses shown in the previous subsection, the tip radius is smaller by 0.00077 mm than the average radius in the cross sectional analyses and the sphericity is greater by 0.00329 mm than the average circularity in the cross sectional analyses. the reason of these differences may be that the centres of curvature were determined independently in every cross section analysis whereas the centre of sphere is a single point in 3d analysis. in figure 10, the contour plot of the deviation from the regression sphere of the spherical part of the indenter is illustrated. it suggests that the indenter tip is not perfectly spherical but has a complicated geometry. in nmij, every standard indenter is characterized by the multiple regression analysis and has an individual correction value to determine the true hardness [3, 9]. the geometrical parameters used in this analysis are average tip radii in five different ranges set by an aperture in the microscopic measuring device and the cone angle. this aperture works to change the effective numerical aperture (na) of the optical system. therefore, the range under evaluation is expressed by nas (figure 11). as seen in figure 11, the na relates to the indentation depth hc. therefore it can be regarded that these parameters are equivalent to the relation between the average radii of the indenter tip with respect to different indentation depth. the relation between na, central angle and contact depth is calculated for the indenter being shaped exactly according to the iso standard, and are shown in table 4. the range for na = 0.65 is wider than the spherical part of the ideally shaped indenter. the reason why the radius of the range beyond the blend point is included is to evaluate imperfections of the geometry around the blend point. in order to keep the continuity between the currently used verification results and the newly introduced ones, it is useful for us to express the geometry of the spherical part of the indenter as the function of the central angle. the measuring method used in this investigation doesn’t have the function to change the measuring range like the currently used instrument of nmij, but the same parameters can be obtained if the iterative calculation described in section 3 is used, i.e., carrying out the iterative regression analysis with the target central angle t which corresponds to the na of the currently used ‐100x10 ‐3 ‐50 0 50 y , m m 50x10 ‐30‐50 x, mm 0.00 08 0.0006 0.0006 0 .0 0 0 6 0.0004 0.0004 0.00 02 0.000 2 0 .0 0 0 2 0 0 0 0 ‐ 0 .0 0 0 2 ‐ 0 .0 0 0 2 ‐0.0002 ‐ 0 .0 0 0 2 ‐ 0 .0 0 0 4 ‐0.0004 ‐0.0004 ‐ 0 .0 0 0 6 ‐0.0006 ‐0.000 6 ‐0.0006 ‐0.0008 ‐0.000 8 ‐0.001 4 ‐0.0016 figure 10. deviation from the regression sphere of the spherical part of the  indenter.  table 3. verification results of the conical part of the indenter through cross  sectional analysis.  position angle between the axis and generatrix, m/2 cone angle, m straightness of generatrix, mm 0° 60.01° 119.97° 0.00052 22.5° 59.93° 119.91° 0.00032 45° 59.89° 119.85° 0.00062 67.5° 59.86° 119.82° 0.00102 90° 59.90° 119.87° 0.00036 112.5° 59.98° 119.98° 0.00057 135° 59.96° 119.93° 0.00103 157.5° 59.96° 119.93° 0.00050 180° 59.96° — 0.00042 202.5° 59.98° — 0.00048 225° 59.97° — 0.00040 247.5° 59.96° — 0.00087 270° 59.97° — 0.00111 292.5° 60.00° — 0.00080 315° 59.98° — 0.00137 337.5° 59.98° — 0.00070 average 59.95° 119.91° 0.00069 standard deviation 0.04° 0.06° — table 2. verification result of the spherical part of the indenter through  cross sectional analysis.   position tip radius ra, mm circularity, mm 0° 0.19515 0.00058 22.5° 0.19621 0.00069 45° 0.19726 0.00115 67.5° 0.19602 0.00169 90° 0.19496 0.00182 112.5° 0.19659 0.00153 135° 0.19707 0.00133 157.5° 0.19640 0.00104 average 0.19621 0.00123 standard deviation 0.00082 — acta imeko | www.imeko.org  september 2014 | volume 3 | number 3 | 20  verification instrument. the result of this technique is shown in figure 12 as open dots. as seen in this chart, the average radius between na = 0.20 to 0.50, i.e., the whole spherical part, is not constant for this indenter. in other words, the deviation from the definition (ra=200 µm) depends on the indentation depth. it suggests that the deviation of hardness of this indenter from true hardness would be different for different hardness levels of the test materials. in figure 12, the average radii evaluated in every 5° of central angle are shown as solid dots to demonstrate the advantage of the proposed analytical method. those values are obtained from only one measurement but it shows that the proposed method brings more information than the currently used optical method. this result describes the geometry of this indenter in more detail, i.e., the radius of this indenter is almost constant in most of the range but smaller just in the vicinity of the tip. 6. conclusions  the geometry of a rockwell diamond indenter was measured with a laser probe 3d profile measurement instrument. the result was analysed according to the definition by cipm/ccm/wgh and the method compatible to the one currently employed in nmij for the national standard indenters. in order to carry out those verifications, the iterative regression method is proposed to determine the desirable range for the verification. it helps us to determine the exact positions of the blend of sphere and cone or the specified range necessary for the characterization of indenters. results of the analysis of a rockwell diamond indenter are shown as an example for the both purposes. the example demonstrates that the method can determine exact blend points of the cone and sphere and it enables us to verify indenters in a fully compliant way to the definition of wgh. it is also shown that the same result with the currently used method in nmij can be obtained even though different principles and instruments are used for the current and newly proposed procedures. the newly proposed method suggests the capability to describe the geometry in more detail than the currently used method. it is expected that the characterization of standard indenters, i.e., specific bias of each standard indenter from the true hardness value, will be carried out in better accuracy and minimizing the uncertainty of the national standard indenters. references  [1] j. song, s. low, a. zheng and p. gu, “geometrical measurements of nist srm rockwell hardness diamond indenter”, proceedings of imeko 2010: tc3, tc5 and tc22 conferences, pp. 137-140, 2010. [2] o. kruger, l. mostert, “the use of a µcmm in the calibration of hardness indenters”, proceedings of hardimeko 2007, pp. 103-105, 2007. [3] h. yano, h. ishida and t. kamoshita, “characteristics of the standard rockwell diamond indenters and method of establishing standard indenters”, proceedings of the roundtable discussion on hardness testing, 7th imeko, london, 16, 1976. [4] t. narumi, t. nakamura, y. ichihara, y. saruki and j. miyakura, “measuring system for micro radius”, optomechatronic systems iii, proceedings of the society of photo-optical instrumentation engineers (spie), pp. 262-269, 2002. [5] a. liguori, a. germak, g. gori, e. messina, “galindent: the reference metrological system for the verification of the geometrical characteristics of rockwell and vickers indenters”, vdi-berichte, vol. 1685, pp. 365-371, 2002. [6] y.-l. chen and d.-c. su, “a method for measuring the geometrical topography of a rockwell diamond indenter”, measurement science and technology, vol. 21, pp. 1-7, 2010. [7] a. germak and c. origlia, “investigations of new possibilities in the calibration of diamond hardness indenters geometry”, measurement, vol. 44, pp. 351-358, 2011. [8] g. dai, j. zhao, f. menelao, k. herrmann and u. brand, “improved methods for accurately calibrating the 3d na=0.2 na=0.3 na=0.4 na=0.5 na=0.65 indenter hc figure 11. evaluation of geometry of spherical part of indenter adopted by nmij.  220 210 200 190 180 100806040200 central angle , ° selected by na (current condition) every 5° 0.20 0.30 0.40 0.50 0.65 na conesphere 5 10 15 20 25 contact depth h c , µm 3 a v er ag e ra di u s in t h e ar ea u n d er e v al u at io n r , µ m figure 12. average tip radius in the different evaluation area in the spherical part of indenter. table 4. relation between the central angles and the contact depths of the  ideally shaped indenter shown with the numerical apertures (nas) of the  optical measurement system of nmij.   na central angle contact depth, mm 0.20 23.07° 0.00404 0.30 34.92° 0.00921 0.40 47.16° 0.01670 0.50 60.00° 0.02679 0.65 81.08° 0.04540† † including the conical part of the indenter acta imeko | www.imeko.org  september 2014 | volume 3 | number 3 | 21  geometry of rockwell indenters”, proceedings of imeko 2010: tc3, tc5 and tc22 conferences, pp. 141-144, 2010. [9] s. takagi, h. ishida, t. usuda, h. kawachi and k. hanaki, “direct verification and calibration of rockwell diamond indenters”, hardimeko 2004, pp. 149-154, 2004. [10] l. ma, s. low, j. zhou, j. song and r. dewit, “simulation and prediction of hardness performance of rockwell diamond indenters using finite-element analysis”, journal of testing and evaluation., vol. 30, no. 4, pp. 265-273, 2002. [11] a. germak and s. low, summary report of the consultative committee for mass and related quantities (ccm) working group on hardness (wgh) 13th meeting, 21 sep., 2011. [12] d. schwenk, k. herrmann, g. aggag and f. menelao, “investigation of a group standard of rockwell diamond indenters”, proceedings of hardimeko 2007, pp. 106-111, 2007. [13] k. miura and m. okada, “three-dimensional measurement of wheel surface topography with a laser beam probe”, advances in abrasive technology iii, society of grinding engineers (tokyo), pp. 303308, 2000. [14] a. germak and s. low, summary report of the consultative committee for mass and related quantities (ccm) working group on hardness (wgh) 8th meeting, 16 and 21 sep. 2006. [15] d. umback and k. n. jones, “a few methods for fitting circles to data”, ieee transactions on instrumentation and measurement, vol. 52, issue 6, pp. 1881-1885, 2003 [16] iso 6508-2: 2005, “metallic materials — rockwell hardness test — part 2: verification and calibration of testing machines (scales a, b, c, d, e, f, g, h, k, n, t) [17] iso 6508-3: 2005, “metallic materials — rockwell hardness test — part 3: calibration of reference blocks (scales a, b, c, d, e, f, g, h, k, n, t)